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Cite this: Energy Environ. Sci., 2012, 5, 7393
www.rsc.org/ees REVIEW
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Biomass as renewable feedstock in standard refinery units. Feasibility,opportunities and challenges
Juan Antonio Melero,*a Jose Iglesiasb and Alicia Garciaa
Received 30th January 2012, Accepted 30th March 2012
DOI: 10.1039/c2ee21231e
Within the present contribution we highlight the feasibility of standard refinery units for the production
of biofuels from different biomass-derived feedstock. The energy densification of biomass, as well as it’s
logistics and incorporation within the refinery supply chain is thoroughly discussed. Likewise, special
attention is focused on the catalytic cracking and hydrotreating of triglyceride-rich biomass feedstock,
which is probably the most suitable one for co-processing in conventional refinery conversion units.
However, the opportunities of other highly oxygenated feedstocks such as pyrolysis oils and sugars are
also discussed. Conversion of different feedstocks into conventional liquid fuels by coupling of aqueous
phase reforming (APR) with catalytic systems typical of standard petroleum refineries is also evaluated.
Thus, here we review the chemistry, catalysis and challenges involved in the production of biofuels from
biomass in conventional refineries.
1. Introduction
One of the most important challenges to face in this 21st century
is the reduction of global warming whilst satisfying growing
aDepartment of Chemical and Environmental Technology, ESCET,Universidad Rey Juan Carlos, C/Tulipan s/n, E28933 Mostoles, Spain.E-mail: [email protected] of Chemical and Energy Technology, ESCET, UniversidadRey Juan Carlos, C/Tulipan s/n, E28933 Mostoles, Spain
Juan Antonio Melero
Juan Antonio Melero studied
chemistry in Complutense
University of Madrid (1988–
1993) and received his PhD in
1998 working on the synthesis
and applications of zeolitic
materials for redox and acid-
catalyzed reactions. He holds
a position as Full Professor in
Chemical Engineering at the
Department of Chemical and
Environmental Technology of
Rey Juan Carlos University in
Madrid. He has written around
75 high-impact scientific articles
and several book chapters
focused on the synthesis and characterization of porous materials
and their catalytic application in refining, fine chemistry and
environmental catalysis. Currently, he is leader–researcher of
several projects related to the processing of biomass feedstock.
This journal is ª The Royal Society of Chemistry 2012
energy demands. Nowadays, most fuels and energy come from
fossil energy resources, but environmental concerns together
with the depletion of crude oil resources, and consequent
increasing prices of this raw material, are becoming important
driving forces encouraging the search for new feedstocks, as
alternative to crude oil, to meet the increasing energy demand.
Many different possibilities have been reported in the literature.
However, such a substitution involves important requirements,
like the renewable nature of the raw material to ensure
Jose Iglesias
Jose Iglesias was born in Bena-
vente (Spain) in 1976. He
received his M. Eng. from Uni-
versidad Complutense, Madrid,
(1999), and his PhD in Chem-
ical Engineering from Uni-
versidad Rey Juan Carlos
(2005) working in the develop-
ment of new catalysts for asym-
metric oxidation reactions.
Since 2008 he is associate
professor in the Department of
Chemical and Energy Tech-
nology in Universidad Rey Juan
Carlos. His main research
interests are focused on the
rational design of heterogeneous catalytic systems for green and
fine chemistry, biofuels production and selective oxidation.
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a sustainable supply. Nevertheless, a very high availability seems
to be another one of the most important requirements, bearing in
mind the high quantities of feedstock needed to ensure a full
coverage of the current demand of automotive fuels. Biomass is
one of the few resources showing high potential to meet the
challenges of sustainable and green energy, and hence its use is
expected to grow in the foreseeable future. Indeed, several
governments have implemented mandatory legislation in order
to increase gross domestic energy from renewable resources,
especially biomass. The U.S. Department of Energy set ambi-
tious goals to derive 20% of the transportation fuel from biomass
by 2030. The European Union has set a mandatory target of 20%
for renewable energy’s share of energy consumption by 2020 as
well as a mandatory minimum target of 10% of renewable energy
sources in transport. Fortunately, the worldwide production
capabilities for renewable and sustainable biomass production
are, in comparison, enormous and thus it exceeds the current and
foreseeable energy demand.
To help reach the ambitious goals established by the United
States and European Union, a system similar to a petroleum
refinery called a ‘‘biorefinery’’ has been proposed in the future.1–5
According to the National Renewable Energy Laboratory
(NREL), a biorefinery is a facility that integrates biomass
conversion processes and equipment to produce fuels, power,
and chemicals from biomass. The biorefinery concept is analo-
gous to today’s petroleum refineries, which produce multiple
fuels and products from petroleum. Industrial biorefineries have
been identified as the most promising route to the creation of
a new domestic bio-based industry. By producing multiple
products, a biorefinery can take advantage of the differences in
biomass components and intermediates and maximize the value
derived from the biomass feedstock. The main goal of a bio-
refinery is to produce high-value low-volume (HVLV) chemicals
products and low-value high-volume (LVHV) biofuels using
different unit operations, while generating electricity and process
heat for its own use and perhaps enough for sale. The operations
must be designed to maximize the mass and energy efficiency and
minimizing the waste streams. The high-value products enhance
profitability, the high-volume fuel helps meet national energy
Alicia Garcia
Alicia Garc�ıa studied Chemical
Engineering in Complutense
University of Madrid (1993–
1998) and received her PhD in
2005 in Rey Juan Carlos
University (Madrid). She has
occupied several teaching posi-
tions in the Department of
Chemical and Environmental
Technologies, starting in 1999
as teaching assistant, becoming
assistant professor in 2001 and
lecturer since 2008. Her main
research lines are focused on
developing new heterogeneous
catalytic systems for green and
fine chemistry, feedstock recycling of plastic wastes and more
recently biodiesel and hydrogen production.
7394 | Energy Environ. Sci., 2012, 5, 7393–7420
needs, and the power production reduces costs and avoids
greenhouse-gas emissions. Fig. 1 illustrates the elements of
a biorefinery in which biomass feedstock are used to produce
various useful products such as fuel, power, and chemicals using
biological, chemical, and thermochemical conversion processes.
Nevertheless, the chemical and energy integration of biomass
transformations in biorefineries is still in the early beginnings
and, in a short- to medium-term, biorefinery development will
likely incorporate existing petroleum refinery infrastructure to
circumvent high capital costs. Hence, one promising alternative
for the production of biofuels is the co-processing of biomass in
conventional oil refineries.6 This alternative involves the co-
feeding of biomass-derived feedstock with typical petroleum
feedstock in conventional refining units. This strategy has
significant advantages as compared with conventional processes
of biofuels production, such as installations that are already built
and hence their use would require little capital investment.
Furthermore, the production process does not require either
secondary reagents or yield by-products, which should have
a market share. Finally, a wide range of biofuels could be
obtained, not only in the range of gasoline and diesel, but also in
the range of LPG, kerosene or fuel oil. All these advantages have
boosted the investigation on this possibility, and some oil
companies have already developed industrial processes for
biomass transformation into fuels.7,9
The purpose of this review is to identify economically attrac-
tive opportunities for biofuels production using petroleum
refinery processes and the challenges for the future integration of
biomass as feedstock for refineries. The study will be focused on
the production of fuels and hydrogen from a high variety of
biomass, including triglyceride7,8 to lignocellulosic-based feed-
stock.6 Many different opportunities for integrating bio-
renewable feeds and products in existing petroleum refineries as
Fig. 1 Main elements in future biorefineries.
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well as new potential refining approaches will be reviewed and
discussed.
2. Biomass feedstock
2.1 Diversity and availability
Biomass is one of the most promising candidates that fulfil the
requirements to substitute crude oil as the main source for
chemical energy, since it is readily available in high quantities,
and it is renewable.10 Biomass feedstock can be grouped into two
wide categories: oleaginous feedstock and carbohydrates.
Triglyceride molecules are the main component of oleaginous
feedstock. These molecules consist of glycerine coupled in ester
form to long alkyl-chain fatty acids, ranging their length between
C8 and C20 (but 16, 18 and 20 carbons are the most common).
The empirical formula of these compounds could be assigned
that corresponding to oleyl triglyceride (C57H114O6) since this is
the most abundant fatty acid in nature. However, triglyceride
molecules display one of the very highest H/C atomic ratios
combined with a low oxygen content, considering the different
chemicals present in biomass feedstock.11 Another important
fraction of oleaginous feedstock is that formed by free fatty acids
(FFA), which could account for most of the composition of
certain materials. The FFA fraction finds its origin in the
hydrolysis of triglyceride molecules and thus, they are usually
present in processed and low-grade oleaginous materials.
Currently, the main use of oleaginous feedstock as crude-oil-
substituting raw materials is the production of biodiesel by
reacting the substrates with methanol. Oils and fats sources
include a wide variety of vegetable and animal raw materials.
Soybean, palm, rapeseed and sunflower oil are the most impor-
tant in terms of worldwide production. However, other sources
of vegetable oils for biofuel conversion can be found in waste
streams of different industries, such as the food industry, animal
waste rendering and many other sources; these last types are
much more stable supplies, and their transformation into fuels
does not create competition with other important sectors. A
source of triglycerides which is currently receiving much atten-
tion can also be found in algae.12–14 However, unlike vegetable
oils and animal fats, which can be easily produced in large scale
with already-built infrastructure, algae oleaginous biomass
production is only on its beginning, and much effort has still to
be invested in order to design and create effective production
processes based on algae.
Carbohydrates are molecules formed of carbon, hydrogen and
oxygen and these are, by far, the most abundant component
found in biomass. There are different families of carbohydrates,
which could be divided into two groups, mono- and poly-
saccharides. The first group is the less abundant in nature, but it
can be found in different plants like sugarcane or sugar beet,
which are quite plentiful in certain areas. Its use in energy
processes is usually assigned to ethanol production by fermen-
tation. Simple monosaccharides include 6-carbon sugars
(C6H12O6) like glucose, galactose and mannose, and 5-carbon
sugars (C5H10O5) like xylose and arabinose. Polysaccharides are
now awakening much interest as source for monosaccharides,
because the former can be turned into simple sugars by hydro-
lysis, though not always easily. Polysaccharides include a whole
This journal is ª The Royal Society of Chemistry 2012
collection of different substances, like starch, cellulose or hemi-
cellulose. Starch (C6H10O5)n is a polysaccharide composed of
a-glucose molecules linked through a-1,4 bondings with
branches formed as a result of a-1,4 linkages. Starch is mostly
produced from several crops like cereals (wheat and corn) and
tubers. Lignocellulose is the most common form in which poly-
saccharides are present in nature. It is not a substance itself but
a mixture of three major components: cellulose (40–50 wt%) and
hemicellulose (25–40 wt%), which are different forms of poly-
saccharides, and lignin (10–25 wt%). Though cellulose
(C6H10O5)n is, like starch, a polysaccharide of glucose, it differs
from the first in the configuration of the link between adjacent
hexose units, b-linkages forming cellobiose, and the absence of
branches to lead a linear polymer of D-glucopyranose. This linear
configuration facilitates the interaction of cellulose polymer
chains through hydrogen bondings leading to the formation of
rigid bundles of cellulose chains. In this way, the hydrolysis of
starch can be easily promoted by enzymes or by acids, while
cellulose is much more difficult to be hydrolyzed, mainly due to
its crystallinity and the relatively hindered access to the ether
bondings between the monomeric units. On the other hand,
hemicellulose is a relatively amorphous component, easier to be
broken down with chemicals and/or heat than cellulose. Finally,
lignin is the glue that provides the overall rigidity to the structure
of plants and trees and its composition is very complex, though it
could be described as a kind of highly condensed polymer (M z10 000) of coniferyl alcohol, so that, it is not a source for simple
sugars. The empirical formula describing the composition of
lignin is C9H10O2(OCH3)n, where n varies from 0.94 for softwood
to 1.40 for hardwood, all along 1.18 for grasses.15 Lignin is a by-
product of little to no commercial value in the pulp and paper
industry, except for its use as a low grade fuel in Kraft pulping,
the dominant pulping process. Large amounts of cellulosic
biomass can be produced via dedicated crops like perennial
herbaceous plant species, or short rotation woody crops. Other
sources of lignocellulose biomass are wastes and residues, like
straw from agriculture, wood waste from the pulp and paper
industry and forestry residues.
The overall biomass composition is estimated to consist of
roughly 75% carbohydrates (sugars), 20% lignin and 5% of other
substances in minor amounts such as oils, fats, proteins,
terpenes, alkaloids, terpenoids and waxes. Fortunately, the
worldwide production capabilities for renewable and sustainable
biomass production are, though quite difficult to estimate,16,17
enormous in quantity. For instance, in the U.S. over 408 million
dry tons of non-used lignocellulosic materials can be harvested
from forest, agriculture and urban and industrial activities.18
Similarly, large lignocellulosic production capacity is available in
Europe, which could yearly produce around 190 millions of dry
tones of lignocellulose biomass.19 With regards to the production
of waste lipids and fats, including rendered fats,20,21 and yellow
and brown grease,22–24 the potential production is much lower,
being of 4.9 and 4.4 millions of tones the estimated yearly
potential for the US and EU25, respectively.
Fig. 2 displays the C, H, O mass composition of several
biomass-derived feedstocks, such as lipids, cellulose and lignin,
together with pyrolysis bio-oil and crude oil, used as reference. In
general terms, biomass feedstocks display, in molar terms, much
lower carbon and hydrogen content than conventional crude oil.
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Fig. 2 Ternary diagram showing the mass composition of usual
biomass-derived raw materials.
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Nevertheless, one of the main differences between the renewable
biomass-derived feedstock and crude oil, is the large amount of
oxygen present in the former, which is one of the causes of the
much lower energy density of these raw materials. Only lipids,
with rather small oxygen content, display a composition close to
that showed by crude oil, though their relatively low availability
makes it necessary to also count on the use of lignocellulosic
feedstock for the substitution of the fossil raw materials. Unlike
lipids, lignocellulosic materials display a high oxygen content,
making its removal necessary to enhance the energy density of
the products. In this way, the overall challenge with biomass
conversion is how to remove efficiently oxygen from the biomass
feedstock and produce a molecule that has a high energy density
and good combustion properties as current petroleum derived
fuels (eqn (1)). Catalytic cracking and hydrotreating are very
effective at removing oxygen from the biomass-derived feed-
stock. However, oxygen is not always removed through the
optimal pathway, and undesired products such as coke or
oxygenated by-products are usually formed during the process.
CxHyOz / aCx�b�d�eHy�2cOz�2b�c�d + bCO2
+ cH2O + dCO + eC (1)
In this sense, Chen et al.25 have defined the effective hydrogen
index (H/C)eff as the amount of hydrogen in the fuel which is
available for energy production, where H, C, O, N and S
correspond to the moles of hydrogen, carbon, oxygen, nitrogen
and sulfur present in the feed (eqn (2)).
ðH=CÞeff ¼ H � 2O� 3N � 2S
C(2)
This index is clearly lower than 1 for highly oxygenated
biomass feedstock, which means that this feedstock is mainly
formed by hydrogen-deficient molecules. In the case of a mixture
of hydrocarbons the (H/C)eff index ranges from 2 (liquid alkanes)
to 1 (for benzene). In contrast, triglyceride-based biomass show
7396 | Energy Environ. Sci., 2012, 5, 7393–7420
hydrogen index of ca. 1.5, a much more energy-dense feedstock
and closer to that showed by hydrocarbons.
2.2 Conditioning and energy-densification
Apart from the composition, there are also other important
differences between crude oil and biomass feedstock which
explain the different energy densities between both kinds of raw
materials. Thus, most of the biomass-derived materials display
a rather low density (80–100 kg m�3 for grasses, to 150–200 kg
m�3 for woody materials), and thus, these are characterised by
a poor energy content. This is not a trouble affecting oleaginous
feedstock because it is the concentrated form of a processed
biomass raw material, whereas lignocellulose is usually found in
the form it appears in nature. However, properties of ligno-
cellulosic materials can be enhanced by biomass densifica-
tion.26,27 This can be achieved by means of physical procedures,
like different mechanical compressing techniques, mainly adap-
ted from other applications, including pellet mills, briquette
presses or screw extruders, among others. Compacting is prob-
ably the simplest way to enhance the energy-density of these
renewable materials. However, other important transformations,
involving chemical modifications, could be considered for a pre-
treatment step focused on the enhancement of, not only the
physical density of the materials, but mostly of the energy density
of the biomass feedstock. Several of the treatment steps are
dedicated to reduce the moisture content of lignocellulose, one of
the reasons of the low energy density of these raw-materials.28
These transformations not necessarily preserve most of the initial
properties of the biomass feedstock, as steam explosion29 or
torrefaction30,31 do, but they can also modify the form in which
the raw materials are presented.
2.2.1 Lignocellulose to bio-oils. Bio-oils can be considered as
an energy-dense form of biomass, produced by a pyrolysis
treatment.32 Bio-oils are produced by direct thermal decompo-
sition of biomass feedstock in the absence of oxygen, or at least in
presence of significantly less oxygen than required for complete
combustion. The gaseous product (mainly carbon oxides,
methane and some higher hydrocarbons) is formed together with
a solid carbonaceous residue and a liquid phase (bio-oil).32,33 The
properties of bio-oils, as well as their composition, depend both
on the specific starting feedstock and the conversion conditions.34
From a compositional point of view, bio-oils consist of two
phases: an aqueous phase, comprising 15–30 wt% of the total
bio-oil, in which several low molecular weight oxygenated
organics are dissolved (acetic acid, methanol, acetone.), and
a non-aqueous phase (35–50 wt%) comprising different oxygen-
containing structures (aliphatic alcohols, carbonyls, acids,
phenols, sugars, hydroxyaldehydes, hydroxyketones.), and
aromatic hydrocarbons (benzene, toluene, indene, naphtha-
lene.).34 Yields of bio-oil from biomass vary in the range�60 to
95%, depending on the initial feedstock: lower for high-lignin
content lignocellulosic biomass, since the presence of lignin
depresses the production of liquids; and higher for more cellu-
losic materials.35 Due to the high oxygen content, bio-oil is
characterised by a low heating value, though this depends on the
initial composition of the starting material, since a high lignin
feedstock leads to bio-oils with higher heating values, because of
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the lower oxygen content of this substance (see Fig. 2). Bio-oil is
rather immiscible with hydrocarbon fuels because of the high
polarity of oxygenated compounds, it is chemically unstable, and
it displays low volatility, high viscosity and corrosiveness.36
Nevertheless, the liquid nature of this intermediate makes easier
to handle biomass-derived feedstock than on their initial form, as
solids. However, bearing in mind all the properties of bio-oils,
they cannot be directly used as a replacement for diesel and
gasoline fuels, but can be considered as an energy-dense form of
lignocellulosic biomass, showing suitable properties to be fed to
a refinery production facility.
2.2.2 Lignocellulose to platform molecules. An even more
desirable option to that showed by pyrolysis conversion of
lignocellulosic biomass into bio-oils is the controlled chemical
transformation of these feedstocks into simple, well-defined
molecules (Fig. 3), called platform molecules, easily separated
from non-interesting substances.37 In this sense, the depolymeri-
sation of lignocellulosic materials into simple sugars and their
transformation into versatile, valuable molecules able to be fed
to a refining process could be considered as the most advanced
way to provide energy-dense biomass-derived refinery-process-
able feedstock. In this approach, lignocellulosic materials have to
be separated into their constituents (lignin, cellulose and hemi-
cellulose) and depolymerised to the corresponding building
blocks. The building blocks of lignin are aromatic alcohols, but
controlled lignin depolymerisation is rather difficult on a tech-
nical scale and this problem has not yet been properly solved.
Controlled cellulose depolymerisation results in glucose whereas
the hemicelluloses are depolymerised to a mixture of different
sugars, mostly pentoses. These simple sugars are the key mole-
cules for the production several important platform molecules,
useful for the production of chemicals and fuels. Conversion of
biomass into functionalized, targeted platform molecules is
unique to hydrolysis based methods and it allows the production
of a wide range of fuel components. Among different platform
molecules, furfural (2-furaldehyde), 5-hydroxymethylfurfural (5-
HMF), levulinic acid (LA) and gamma-valerolactone (GVL) are
of special interest. Fig. 3 schematizes the production of platform
molecules from sugar-based raw materials. Starting from the
hydrolysis of cellulose and hemicellulose into simple sugars, these
are then transformed into 5-HMF or furfural (depending if
Fig. 3 Platform chemical molecules derived from cellulose materials and
reactions involved in their production.
This journal is ª The Royal Society of Chemistry 2012
starting from an hexose or a pentose, respectively) by dehydra-
tion. If hydration progresses from HMF, levulinic acid (a sugar-
derived g-keto acid platform molecule) is then produced, though
at the expense of losing a carbon atom in the form of formic acid.
Levulinic acid, on the other hand, can be transformed into GVL
by hydrogenolysis.38 Among all these chemicals, HMF and LA
are probably the most interesting ones as platform molecules,
displaying a higher functionalization degree and thus a higher
reactivity. HMF also displays several advantages over LA, such
as a higher carbon economy (if its preparation from an hexose
sugar is considered) or the good versatility to be transformed not
only into biofuels, but also into several interesting chemical
intermediates. Nevertheless, HMF is quite unstable under several
conditions, and it easily evolves to levulinic acid by acid-
promoted hydrolysis, LA being a much more stable chemical.
However, both types of platform molecules have been used in the
preparation of several chemicals useful as fuel and fuel additives.
Dehydration of sugars to furan compounds: furfural and 5-
HMF. Furfural is obtained by dehydration of C5 sugars like
xylose in a well-developed industrial process.39 Most of them
make use of concentrated sulfuric acid as catalyst, which is
extremely corrosive and highly toxic and suffers from serious
drawbacks concerning homogeneous catalytic processes, such as
difficult separation and recycling of the mineral acid and product
contamination. Other major drawbacks of these processes are
extensive side reactions, resulting in loss of furfural yield due to
long residence times, and the need for significant waste disposal.
Several attempts have been made to develop heterogeneous
catalytic processes for the transformation of pentosans/pentoses
into furfural offering environmental and economic benefits.
Unfortunately, the catalytic performances achieved up to now
have been unsatisfactory for industrial implementation.
Dehydration of hexoses in acid media leads to the formation of
5-(hydroxymethyl)furfural (5-HMF). This process has been
carried out using a great variety of different catalytic systems:
homogeneous organic acids (p-toluenesulfonic acid, H2SO4 and
HCl) as well as heterogeneous catalysts (ionic exchange resins,
H-form zeolites, vanadyl phosphate and ZrO2) and in presence of
different solvents. Reaction conditions range from temperatures
between 100 and 200 �C using conventional heating (reaction
times up to 48 hours depending on the catalytic system) as well as
microwave (shorter reaction times in the range of minutes).40 In
principle, solid acid catalysts are more desirable for this reaction
and display several advantages in comparison with liquid acid
catalysts. These heterogeneous catalysts make separation from
the product and recycling easier; they also allow working under
higher temperatures, thus shortening the reaction time and
avoiding 5-HMF decomposition due to prolonged reaction time;
and finally the adjustment of their surface acidity might allow
controlling 5-HMF selectivity.
Several reaction media have been used in the dehydration of
hexoses. The water medium is a suitable candidate from an
ecological point of view but unfortunately, 5-HMF undergoes
several reaction under aqueous acid conditions to form undesired
side products such as levulinic and formic acids or even self-
condenses to form both soluble polymers and insoluble humins.
In order to minimize these secondary reactions and increase the
yield towards 5-HMF, the use of high-boiling organic solvents
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has been described in literature. For instance, pure dime-
thylsulfoxide (DMSO) in presence of acid resins gave a 5-HMF
yield over 90%, starting from fructose (in water media the yields
are hardly over 60%).41 Likewise, biphasic systems (water–
organic solvent) have also been used in the synthesis of 5-HMF
with the purpose of solving the low solubility of sugars in organic
solvents whilst the continuous extraction of the evolving 5-HMF
from the aqueous phase prevents its degradation. These systems
have been deeply studied by Dumesic and coworkers in order to
improve the selectivity of HMF formation from fructose.42,43
Both homogeneous and heterogeneous catalysts have been
assayed with this biphasic system, including HCl, H2SO4,
H3PO4, ion-exchange resins and niobium phosphate catalysts.
For instance, fructose and glucose were dehydrated to 5-HMF
with selectivities of 89% and 53%, respectively, with high sugar
conversions in a system comprised of a reactive aqueous phase
modified with DMSO, combined with an organic extracting
phase (mixture of methyl isobutyl ketone (MIBK) and 2-
butanol) at 130 �C.Another relatively new catalytic system to produce 5-HMF
involves the use of ionic liquids (ILs). Several works have been
reported in literature combining ILs with homogeneous and
heterogeneous catalysts.44 However, the main drawbacks asso-
ciated with the use of ILs are the need for purification after
recycling, potential sensitivity to moisture and impurities, as well
as the cost for commercial applications.
The dehydration process is more efficient and selective for 5-
HMF when starting from fructose than from glucose. Thus, the
most efficient method for the preparation of 5-HMF is the acid-
catalyzed dehydration of fructose, which can be obtained by
acid-hydrolysis of sucrose and inulin or, as alternative, by means
of glucose isomerization. However, glucose is more abundant
and readily available and hence more appealing feedstock for the
production of 5-HMF. Thus, there is an important incentive to
transform glucose into 5-HMF with high yields. In this sense,
several works have described a new strategy based on the
combination of a basic catalyst (Al/Mg hydrotalcite) and an acid
catalyst (Amberlyst-15).45,46 The basic catalyst is responsible for
the isomerization of glucose into fructose, whereas the acid
catalyst promotes the subsequent dehydration to 5-HMF. The
authors reported a 5-HMF selectivity of 76% with a glucose
conversion of 60% using this mixture of catalytic systems.
Finally, the direct use of polyssacharides, cellulose and
lignocellulose as feedstock for the production of 5-HMF is
more appealing from a commercial point of view. Processing
these highly functionalized polysaccharides, that are inexpen-
sive and abundant, eliminates the need for simple carbohydrate
molecules. This approach has been poorly described in the
literature but there are some works that deserve to be
mentioned. Chheda and co-workers43 achieved good selectivities
for 5-HMF at high conversions from sucrose, starch, cellobiose
and xylan, using a mineral acid as catalyst and a biphasic
reactor. More recently, McNeff et al.47 have described the
continuous production of 5-hydroxymetylfurfural from simple
and complex carbohydrates using a fixed bed porous metal
oxide-based catalytic process (ZrO2 and TiO2) and using methyl
isobutyl ketone as solvent. For instance, they obtained a cellu-
lose conversion of 87% and 5-HMF selectivity of 35% using this
catalytic system.
7398 | Energy Environ. Sci., 2012, 5, 7393–7420
Nevertheless, the large-scale production of 5-HMF from C6
sugars is still far from the industrial implementation and some
challenges must be addressed in the future: utilization of inex-
pensive and high available glucose as a sugar feedstock and the
implementation of solid acid catalysts highly resistant and
selective in appropriate solvent systems.
Synthesis of 5-HMF derivates: levulinic acid and g-valero-
lactone. Levulinic acid (LA, 4-oxopentanoic acid) is an impor-
tant biomass derivative that can be obtained by hydrolysis of
lignocellulosic wastes, such as paper mill sludge, urban waste
paper, and agricultural residues, through the Biofine process.48,49
This process starts by treating the biomass feedstock with
sulfuric acid (1.5–3 wt%) in an initial plug-flow reactor where the
hydrolysis of carbohydrates to intermediates (HMF) takes place
at 483–493 K and 25 bar with a short residence time (12 s) to
minimize the formation of degradation products. Subsequently,
in a second reactor, the intermediates are converted into levulinic
acid and formic acid at 463–473 K and 14 bar, with a residence
time around 20 min. These conditions have been optimized to
remove formic acid, as well as to remove the furfural arising from
dehydration of the C5 sugars present in biomass. Yields towards
levulinic acid are close to 70–80% which corresponds to 50% of
the C6 sugars initial mass, being the rest collected as formic acid
(20%) and a solid insoluble residue (humins). Levulinic acid can
be subsequently converted to g-valerolactone (GVL) by catalytic
hydrogenation. This reaction is carried out at relatively low
temperatures (373–543 K) and high pressures (50–150 bars) and
using both homogeneous and heterogeneous catalysts. The
reduction usually uses external hydrogen, but the achievement of
hydrogen from formic acid, produced as by-product together
with levulinic acid is a promising alternative. Recently, several
works report a simple process for the production of GVL, which
integrates hydrolysis/dehydration of carbohydrates to form LA
and the subsequent hydrogenation to GVL in a single step.50
2.3 Logistics and refinery supply chains
In 2011, the total world crude oil consumption aroused to
89.0 million barrels per day.51 Considering the substitution of
even a small fraction of this huge amount of energy resources by
biomass-derived feedstock involves the use and feeding of large
amounts of renewable raw-materials to refineries. Although the
availability of biomass seems not to be a problem, at least in
a worldwide perspective, there are other practical troubles. Thus,
unlike crude oil, which is collected in a concentrated manner
from large deposits, biomass is produced in a low-energy-density
form. Enhancing the energy density of biomass, as already
stated, can be overcome, but there is still a critical issue, from
a practical point of view, in the incorporation of biomass-derived
feedstock into a fuel-producing scheme of a refinery: How to
supply enough quantities of this renewable resource to a plant so
demanding of raw-materials as a refinery.52–54 Thus, very large
harvesting areas are needed to provide useful quantities of the
raw material containing an appreciable amount of energy. In
addition, several other features, differing in biomass logistics
from those corresponding to other industries, have to be
considered.55,56 Some of the most important characteristics
determining a logistic chain for biomass are the discontinuous
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production (determined by the harvesting season, weather
effects, frequency, etc.), the variety of biomass feedstock to be
supplied to the refinery (involving composition and quality
variation, etc.),57,58 the transportation chains,59–61 the storage of
biomass feedstock (capacity, conditions, etc.)55 and/or the
required pretreatments to accomplish energy densification,62,63
among many others. All these considerations are very important
in the design of a logistic system for biomass feedstock, but the
efficient and already-built infrastructure for the production of
the oleaginous feedstock64 largely simplifies its case in compari-
son to lignocellulosic feedstock.
Several studies have dealt with the optimization of supply
chain systems for the provision of lignocellulosic materials. Most
of them involve the insertion of economic parameters, together
with the rest previously described, in mathematical models with
the aim to build an efficient logistic system by optimising the cost
structure of the same.55,65,66 For this purpose, several alternatives
have been proposed, including the use of small-sized bio-
refineries66 instead of large-capacity crude oil refining facilities,
which seems not to be a feasible option in the foreseeable future.
The location of these facilities close to biomass producing areas
would reduce the transportation costs. However, lower produc-
tion capacities would have to be compensated by building a large
number of biorefineries. As alternative, the building of satellite
storage locations (SSLs) has also been proposed for temporary
storage and loading of biomass, which would collect the renew-
able feedstock from the producing areas before being trans-
ported to refineries.67,68 However, the most interesting option
seems to be the consequence of combining the use of a similar
structure based on the use of SSL with energy densification
treatments,69 like those described in the previous section, to
condition the starting lignocellulosic biomass feedstock, saving
transportation costs from SSL to refineries. In this way, the
supply of biomass-derived feedstock to conventional refineries
can be accomplished as pre-treated streams which can be directly
processed into the already-built infrastructure of a conventional
refinery, by using existing catalytic technology.70 Fig. 4 displays
Fig. 4 Combined supply chain structure for co-feeding biomass and
crude oil as starting raw-materials for conventional refineries.
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the logistic infrastructure for collecting and pretreatment steps to
condition biomass feedstock and their insertion in a refinery
production process as raw materials, together with other
biomass-derived substances like oleaginous feedstock and crude
oil.
3. Processing biomass feedstock in conventionalrefineries
Fig. 5 briefly outlines the possibilities for the production of
biofuels in standard refinery units, which will be further dis-
cussed in the review. Thus, the properties of the starting biomass
raw material, mainly described by the effective hydrogen index,
not only conditions the energy density of the starting feedstock,
but also induce the distinct chemistry involved in the catalytic
process, resulting in a different product distribution. Different oil
companies are already investigating the possibility for develo-
ping some new industrial processes in which biomass-derived
feedstock is treated in conventional refineries. Thus, Neste Oil
has developed a hydrotreating process (NExBTL technology),
which allows flexible use of any vegetable or waste oil in the
production of renewable diesel fuel.71 Recently, a US Depart-
ment of Energy funded collaboration between UOP, the
National Renewable Energy Laboratory, and the Pacific
Northwest National Laboratory completed an evaluation of the
economics of biofuels integration in petroleum refineries.9 Many
economically attractive opportunities were identified for inte-
grating biorenewable feeds and products in existing or new
refining operations,72 particularly for two feedstocks: vegetable
oils/greases to produce green diesel, gasoline or chemicals; and
pyrolysis oil to produce green gasoline.
3.1 FCC and hydrotreating units as the core of conventional
refineries
Several options are available for converting biomass–derived
feedstocks into biofuels in a petroleum refinery: (1) Thermal
Fig. 5 Integration of biomass feedstock in conventional refinery
processes.
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(visbreaking and cocker units) and catalytic (FCC unit) cracking
(2) hydrotreating and (3) hydrocracking.
Cracking reactions in a conventional refinery can be carried
out in presence of catalyst (FCC unit) and in its absence (thermal
units). Thermal units are not considered of interest for the
production of biofuels since the resulting organic liquid product
contains a high content of oxygenated compounds, indepen-
dently from the composition of biomass feedstock, reducing its
interest as fuel transport. In contrast, catalytic cracking is faster
and more selective than thermal cracking and it allows working
under milder reaction conditions, minimizing the yield towards
by-products such as gases, coke and heavy fractions while
maximizing the production of the liquid fraction suitable for use
as transport fuel, showing an effective hydrogen index in the
range from 1 to 2. Fluid catalytic cracking (FCC) is the most
widely used process for the conversion of crude oil into gasoline
and other hydrocarbons because of its flexibility to changing the
feedstock and product demands. The feedstock for the catalytic
cracking process at the petroleum refinery has traditionally been
gas oil with an average molecular weight in the range of 200–600
or higher, though processed bio-oils and triglycerides are being
studied as co-feeds. The reactions occurring in the FCC process
include cracking reactions (cracking of alkanes, alkenes, nap-
thene and alkyl aromatics to lighter products), hydrogen trans-
fer, isomerisation, and coking reactions.73 A variety of process
configurations and catalysts have been developed for the FCC
process. FCC catalysts usually contain mixtures of a Y-zeolite
within a silica–alumina matrix, a binder, a clay, and some
additives.
Hydrotreatment is an indispensable unit operation in
conventional refineries and the hydrotreatment catalysts are,
together with the cracking and three-way exhaust gas catalysts,
the most important industrial catalysts. The Mo and W transi-
tion metal sulfides promoted with Co and/or Ni metal sulfides
have been the active components of hydrotreatment catalysts
from the early beginnings of the process. The objective of
hydrotreating in a petroleum refinery is to remove sulfur
(hydrodesulfurization, HDS), nitrogen (hydrodenitrogenation,
HDN), metals (hydrodemetalation, HDM) and oxygen (hydro-
deoxygenation, HDO) from the heavy gas oil feedstock.
Hydrogen is necessary to perform these transformations and it is
added with the heavy gas oil feed. Typical reaction conditions
employed are temperatures of 300–450 �C, pressures of 35–170bar H2 and liquid hourly space velocities (LHSVs) of 0.2–10 h�1.
Hydrogen based processes are typically more expensive than
cracking because these require hydrogen. Likewise, its
consumption is higher when biomass feedstock is processed, due
to its higher oxygen content in comparison to conventional fuel
sources. On the contrary, hydrotreating processes display
a higher selectivity towards the liquid fraction, minimizing gas
and coke production as compared with cracking units, and also
providing high energy-dense products such as green diesel, which
shows a high (H/C)eff index.
Though cracking and hydrotreating units are already built-up
in conventional refineries, as previously noted, most of the
available biomass feedstock is present in the form of sugars and
polymers. These raw materials display a very low effective
hydrogen index and thus, these have to be processed in more
complicated units than those previously described. The energy
7400 | Energy Environ. Sci., 2012, 5, 7393–7420
upgrading of these chemicals also makes necessary the provision
of large quantities of hydrogen. Fortunately, recent develop-
ments in processing sugar-rich biomass allows production of the
required hydrogen from the same sugar feedstock (APR process)
for the transformation of the own biomass raw material into
hydrocarbons, leading to more energy-dense products.
3.2 Oleaginous biomass feedstock processing
Oleaginous raw materials, such as fats and vegetable oils are
primarily water insoluble, hydrophobic substances that are
comprised almost completely of triglycerides and small amounts
of mono- and diglycerides.74 Triglycerides can be easily con-
verted into liquid transportation fuels because of their low
oxygen content.6 A variety of feedstocks for the production of
biofuel from triglyceride based agriculture-derived fats and oils
can be divided into four different types: crude vegetable oil (palm
oil, rapeseed, soybean), used vegetable oil (waste cooking oil),
animal fats (lard, tallow) and non-edible oil (castor oil, tall oil,
Jatropha curcas, Cynnara cardunculos.).3 Waste oils and fats is
one of the most economical choices to produce biofuel. Large
quantities of these feedstock are available throughout the world,
especially in developed countries,75 though not in quantity
enough to completely replace crude oil.76
This feedstock possesses similar properties (density, viscosity,
hydrogen/carbon ratio.) to those found in vacuum or hydro-
treated gas oil usually fed to the refinery conversion units.
Indeed, co-processing these renewable feedstocks together with
conventional feeds, can lead to a lower content in metals (such as
nickel or vanadium) and heteroatoms (such as sulfur or nitrogen)
in the final products because of the lower content of these
impurities on the composition of the renewable feedstock.
3.2.1 Catalytic cracking of oleaginous feedstock. Catalytic
cracking of vegetable oils, animal fats, and waste oleaginous
feedstock can be used to produce automotive fuels.6 A huge
amount of different studies have been reported dealing with the
catalytic cracking of oleaginous feedstock over different acid
catalysts: zeolites (H-ZSM-5, H-Y, H-mordenite.),77–88
aluminium-containing mesostructured materials (Al-MCM-41,
Al-SBA-15),3,84,88–93 amorphous materials (aluminosilicates,
pillared clays, alumina.).81,82,84,94,95
Typical products obtained from the catalytic cracking of
oleaginous feedstock include gaseous products (hydrocarbons
C1–C5, CO, CO2), organic liquid products (OLP), water and
coke.96 The organic liquid product is composed of hydrocarbons
corresponding to the gasoline, kerosene, and diesel boiling point
ranges. The oxygen initially present in the feedstock is removed
as water, easily isolated, CO and CO2. Therefore, there is no
remarkable presence of oxygenated hydrocarbons in the final
organic cracking products. A general reaction pathway of the
catalytic cracking of a triglyceride molecule over an acid catalyst
is proposed in Fig. 6.96 Once the triglyceride molecule has been
primarily decomposed to less bulky compounds such as free fatty
acids, ketones, aldehydes and esters, their transformation into
different products starts by breaking the C–O and C–C bonds by
b-scission reactions. This transformation follows two competi-
tive routes: (i): decarboxylation and decarbonylation reactions
followed by C–C bond cleavage of the resulting hydrocarbon
This journal is ª The Royal Society of Chemistry 2012
Fig. 6 Chemical reactions taking place in the thermal and catalytic
cracking of triglycerides.
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radicals or (ii): C–C bond cleavage within the hydrocarbon
section of the oxygenated hydrocarbon molecule followed by
decarboxylation and decarbonylation of the resulting short chain
molecule.83 The occurrence of these different reaction routes
depends on the presence of double bonds in the initial oxygen-
ated hydrocarbon. Whereas C–C bond breaking in a and
b position is favoured in presence of unsaturated hydrocarbon
molecules, decarboxylation and decarbonylation reactions take
place before C–C bond cleavage for saturated oxygenated
hydrocarbons, since the least endothermic bonding in a saturated
hydrocarbon chain is the one associated with the b position of the
carbonyl group.97 Different subsequent cracking reactions finally
yield CO, CO2 and water, as the main oxygenated compounds,
and a mixture of hydrocarbons produced by different reactions
such as b-scission, hydrogen transfer, isomerization, cyclization,
or aromatization, some of them possible because there is an acid
catalyst present in the reaction system. Furthermore, coke is
formed by means of polymerization reactions.98
The choice of the catalyst is crucial in processes driven by
heterogeneous catalysts, due to its role in controlling the yield
and selectivity towards the different products present in the
biofuel. The properties of catalysts are governed by acidity, pore
shape and size. Furthermore, the reaction conditions (tempera-
ture, pressure, space velocity, presence of steam, type of reactor,
etc.) and the nature of feedstock also exert a significant influence.
Generally, the presence of zeolite catalysts increases the yield
towards the OLP fraction whereas amorphous catalysts
predominantly produce a higher amount of gases.82,84,99 Co-
feeding steam during the reaction helps to increase both the olefin
yield as well as the durability of the catalyst, since the presence of
steam diminishes the coke formation and thus the catalyst
deactivation.81 The use of a fluidized bed usually reduces the
selectivity towards the OLP fraction in contrast to that found for
a fixed bed. This fact arises from the shorter contact time in
a fluidized bed which diminishes the extent of the oligomerization
reactions taking place between C2–C5 olefins.100 From all the
different studies, it can be concluded that most of the triglyceride
feedstock is converted (>80%), leading to an OLP fraction
mainly composed of a high amount of aromatics (�50%) with
a null presence of oxygenated hydrocarbons.82,86,89,100 The
This journal is ª The Royal Society of Chemistry 2012
different authors have shown that the initial stages in the trans-
formation of the triglyceride molecule are thermally driven
processes, which are followed by the subsequent secondary
cracking reactions (hydrogen transfer, isomerization, oligo-
merization, b-scission, aromatization) in which the acid catalyst
plays a crucial role.89
Table 1 summarizes the most relevant works dealing with the
catalytic cracking of triglyceride molecules indicating the type of
feedstock, the reaction conditions, and the catalyst. Typically
these studies have been performed in fixed bed reactors, in
a temperature range between 300 and 500 �C, with liquid space
velocities ranging from 2 to 4 h�1. Although the cracking of
vegetable oils into liquid fuels has been much studied, the
cracking of triglycerides molecules under realistic FCC condi-
tions is scarcely described in the literature. However, certain
number of authors have performed studies about the processing
of vegetable oils93,101–107 and animal fats104,106,107 under conditions
which try to simulate the operating conditions of the FCC unit.
In these studies, the reaction system employed is usually based in
a riser reactor and a FCC catalyst. After the catalytic cracking
reactions, conversion is usually over 75%.85,96,107,108 Furthermore,
there are no remarkable amounts of oxygenated hydrocarbons in
the final cracking products, since almost all the initial oxygen
present in the triglyceride molecule is transferred to water or
carbon gases (CO and CO2).101,104,107
Co-processing renewable raw materials in an FCC refining
unit has also been studied by several research groups, demon-
strating the technical feasibility of co-processing several mixtures
of vegetable oils (palm, rapeseed, soybean or sunflower oils),
waste cooking oil and animal fats and vacuum gasoil under FCC
conditions.104,113–117 Not only the operation conditions registered,
but also the final products obtained after the catalytic cracking
reactions, are perfectly compatible with the conditions and
products usually related to the FCC unit. However, there is
a strong effect of the feedstock composition in the cracking
products’ distribution.
The main differences in the processability of the typical feed-
stock (oil derived) and its mixtures with vegetable oils and animal
fats are the production of oxygenated compounds coming from
the presence of oxygen in the starting triglycerides. However, the
usual conditions of the FCC unit are severe enough to decom-
pose the heavy oxygenated hydrocarbons by means of decar-
boxylation, decarbonylation or dehydration reactions to yield
a mixture of different hydrocarbons. Nevertheless, beyond the
production of non-valuable oxygenated compounds (which are
easily separated from the valuable ones), it is possible to find
several differences between the cracking of a hydrocarbon and
a vegetable oil in FCC conditions. Triglyceride-based biomass
enhances the gas production and always reduces the yield
towards the liquid fraction, especially liquid cycle oil (LCO) and
mainly towards decanted oil (DO)104 (Fig. 7A). These results are
associated with the higher crackability of vegetable oils and
animal fats in comparison with the petrol feedstock, since the
later present a lower concentration of aromatic rings, which tend
to be refractory and more difficult to be cracked. Hence, the
gasoline content in the organic liquid product (GLN + LCO +
DO) is always enhanced as the percentage of vegetable oil is
increased in the initial feedstock.113,116 Bormann et al.114 indicate
that the percentage of gasoline in the liquid products rises from
Energy Environ. Sci., 2012, 5, 7393–7420 | 7401
Table 1 Catalysts and reaction conditions used in the catalytic cracking of several oleaginous feedstock
Catalyst Reaction conditions Feedstock References
SiO2 & silicalite MCM-41 SBA-15 Fixed bed reactor T ¼ 375–500 �C.WVSH: 1.8–15.4 h�1
Rapeseed oil, palm oil 82, 84, 88 and 91
Al2O3 gamma bauxite Fixed bed reactor T ¼ 400–500 �CWVSH: 0.6–15.4 h�1
Rapeseed oil, coconut oil 84 and 94
SiO2–Al2O3 Al-MCM-41 Al-SBA-15 Fixed bed and raiser reactors. T:200–600 �C. WVSH: 1.8–15.4 h�1
Rapeseed oil, palm oil, canola oil 84, 90, 108 and 109
H-ZSM-5 Fixed bed reactor T: 340–500 �CWVSH: 1–4 h�1
Rapeseed oil, corn oil, WCO 78, 79, 82, 83, 85–88, 91 and 109–112
H-Mordenite H-Y Fixed bed reactor T: 200–600 �CWVSH: 1.8–3.6 h�1
Rapeseed oil, canola oil 82 and 109
FCC catalyst Fixed & fluidized bed, micro riser &FCC pilot plant T: 380–585 �Ccontact t: 50 ms �30 min
Rapeseed oil, cottonseed oil, palmoil, soybean oil, Calopris pocera
101–105
Fig. 7 Reaction results from the catalytic cracking of palm oil–VGO
mixtures. Data adapted from ref. 104. Reaction conditions: Temperature
¼ 565 �C; Catalyst to oil mass ratio ¼ 4.Publ
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60.3% to 61.1%, when using rapeseed oil instead of vacuum
gasoil in cracking experiments. Similar results were obtained by
Carlos deMedeiros et al.,116 when cracking soybean oil instead of
the typical vacuum gasoil, achieving a yield to gasoline in OLP
8.6 points higher than under normal feeding conditions. This
better crackability of triglyceride-based biomass is also clearly
confirmed by the gasoline distribution obtained by Melero
et al.104 The medium (MN; 90–140 �C) and heavy (HN; 140–
221 �C) naphtha yields are reduced 4.7% and 6.8%, respectively,
in the catalytic cracking of palm oil in comparison to vacuum gas
oil cracking, whereas light naphtha (LN; C5–90�C) production is
favoured when increasing the amount of renewable feedstock. In
addition, the decanted oil fraction in the catalytic cracking of
triglyceride-based materials is produced by means of polymeri-
sation reactions, in contrast to the petrol feedstock, which is the
remaining unconverted fraction.
Another important difference found when co-processing
vegetable oil in an FCC unit is the higher yield produced for
aromatic compounds (Fig. 7B). These aromatic compounds are
mainly monoaromatics, which would finally be incorporated to
the gasoline (GLN) fraction (raising its octane number) and
7402 | Energy Environ. Sci., 2012, 5, 7393–7420
diaromatic compounds. The presence of renewable feedstock
reduces the yield towards polyaromatic compounds, since these
are not present in the starting raw-material. In this way, the high
aromaticity of the liquid fraction when cracking renewable raw
materials comes from the formation of olefins, which have a big
trend to end as aromatic compounds,101 during the removal of
hydrogen from the triglyceride molecules. This transformation is
rather important and several authors, like Adjaye et al.,118 have
described liquid products with more than a 95 wt% of aromatics,
for instance by treating rapeseed oil in presence of ZSM-5. In the
same way, if excessive hydrogen elimination from hydrocarbons
continues, an increase in coke formation is produced.101,104
Therefore, coke production is enhanced with the increase of
triglyceride-based biomass in the feedstock.104,113,116,119
A recent report by Universal Oil Products (UOP) Corporation
discussed how biofuels might be economically produced in
a FCC unit,7,72 starting from vegetable oils and greases to
produce gasoline and liquid cycle oil (LCO). Nevertheless,
a pretreatment unit to remove catalytic poisons such as alkali
metals and other problematic components, such as water and
solids, has been considered as a crucial step. Then, the pretreated
feed can be introduced as a co-feed with vacuum gas oil (VGO) to
yield gasoline and other products. A modified catalytic cracking
process was also proposed to yield high-value products such as
ethylene and propylene (severe conditions to maximize olefin
production).
3.2.2 Hydrotreating of oleaginous feedstock. A different
option to the catalytic cracking in the use oleaginous feedstock in
conventional crude-oil refinery plants is their hydrotreatment to
produce straight chain alkanes ranging from C12 to C18. These
alkanes have high cetane numbers (80–100) and good fuel
properties,120 which can be used in producing diesel-like fuels.
The advantages of hydrotreating over transesterification are that
the former is compatible with current treatment infrastructure,
products are fully usable in existing engines, and there is some
flexibility with respect to the feedstock.121 Hydrotreating,
because of the need for hydrogen, is much more expensive than
catalytic cracking, but the products achieved through this
pathway (green diesel) are pure hydrocarbons indistinguishable
from their petroleum counterparts. In fact, the high cetane
number displayed by green diesel makes it possible to meet the
current specifications for petroleum-derived diesel-like fuels. In
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this sense, several studies reveal a meaningful decrease of
hydrocarbons, carbon monoxide and nitrogen oxides in diesel
engine emissions when using green and conventional diesel
mixtures instead of petroleum-derived diesel as fuels.121–123
A general reaction pathway for hydrotreating vegetable oils is
shown in Fig. 8. The total hydrogenation of triglycerides leads to
the formation of n-alkanes and propane as main reaction prod-
ucts, and water, CO and CO2 as by-products. This process
involves several reactions which can be lumped into two different
reaction pathways:124,125 hydrodecarboxylation (HDC) and
hydrodeoxygenation (HDO). The former involves the loss of
a carbon atom, coming from the carboxylic group at the fatty
acid chain. In this way, the main n-alkanes obtained as products
display one less carbon atom than the original fatty acid alkyl
chain, this carbon being lost as carbon monoxide or dioxide.
Nevertheless, hydrodeoxygenation leads to the formation of long
n-alkanes showing the same number of carbon atoms as the
original fatty acid alkyl chain as well as propane, coming from
the hydrodeoxygenation of glycerine. The proportion of hydro-
deoxygenation and hydrodecarboxylation pathways at the total
conversion of triglycerides to hydrocarbons is decisive for the
overall hydrogen consumption of the process. Based on the
stoichiometry of the hydrodeoxygenation mechanism, the mass
balance indicates the requirement for 12 moles of hydrogen per
mole of triglyceride plus an additional mole of H2 per double
bond present in the fatty acid alkyl chains, which is the faster
transformation, and thus occurs in first term when hydrotreating
triglycerides.6 Hence, the total hydrodeoxygenation of rapeseed
oil (4 double bonds per mole) involves the requirement for 16
moles of hydrogen per mole of triglyceride, leading to a mixture
of water (six moles), propane (one mole) and a mixture of n-C18
and n-C22 (three moles) –rapeseed oil fatty acid profile is mainly
composed of oleic, linoleic and linolenic acids (C18 z 35 wt%)
and erucic acid (C22 z 40 wt%). Nevertheless, only 3 moles of
hydrogen are needed to process a mole of triglyceride by the
hydrodecarboxylating mechanism, plus the additional H2 mole-
cule required for reducing each double bond, leading to
Fig. 8 Proposed mechanism for triglyceride hydrotreating reactions.
This journal is ª The Royal Society of Chemistry 2012
a hydrogen consumption of 7 moles per mole of rapeseed oil.
This fact suggests that hydrodecarboxylation should be favoured
over hydrodeoxygenation to reduce hydrogen consumption,
a critical issue in the profitability of hydrotreating units.
However, the mere presence of CO2 in the reaction system leads
to the existence of methanation or at least the partial reduction of
the same to CO, as well as the water gas shift reaction,126 though
to a minor extent. Hence, these reactions should be considered
when evaluating the process economy, since they lead to the
consumption/production of hydrogen. The extent of these reac-
tions depends on the catalyst and reaction conditions. Therefore,
if all CO2 was to be converted into methane, or its trans-
formation into carbon monoxide and subsequent methanation,
12 additional moles of hydrogen should be added to the three
already considered from the HDC transformation – plus the
hydrogen consumed in reducing hydrocarbon bonds. Conse-
quently, rapeseed oil hydrodecarboxylation would need 19 moles
of hydrogen per mole of triglyceride, leading to n-C17 and n-C21
as the main products (three moles), water (six moles), propane
(one mole) and methane (three moles). Hence, considering
hydrogen consumption, the HDO pathway may be more
attractive than HDC, though some authors have reported
experimental results contrary to these calculations.127 Neverthe-
less, such a low difference between H2 consumption by both
mechanisms and the similarities observed in the final hydro-
carbon products do not allow the determination of which is the
best option, depending on the process and on the catalyst used in
the hydrotreating unit.124
Because of the similarity between hydrodesulfurization and
hydrodeoxygenation, oxygen removal from triglycerides by
HDO seems to be a rather easy task to implement on conven-
tional refinery hydrotreating units used for hydrodesulfurization
of petroleum-derived streams.128 Hydrodesulfurization (HDS)
usually co-exists with HDO and hydrodenitrogenation (HDN)
for the removal of sulfur, oxygen and nitrogen, respectively. The
most important industrial catalysts used for HDS conventionally
involve alumina-supported molybdenum and tungsten sulfides as
the main catalytic species, usually promoted with cobalt and/or
nickel.129 However, Co–Mo and Ni–Mo are the most widespread
catalysts in hydrotreating units, due to their higher catalytic
activity.130 These catalysts are employed because of their high
resistance against sulfur poisoning, in contrast with noble metals,
which display a much higher hydrogenating activity but a lower
resistance against deactivation with sulfur. Nevertheless,
important different behaviors are found for the HDS catalysts
when hydrotreating triglyceride-containing feedstock (HDO
process) in comparison with petroleum-derived streams (HDS
process). These differences are associated with the nature of the
feedstock and the final form of the heteroatoms to be removed
(oxygen and sulfur, respectively), even though the reaction
pathways and mechanisms are rather similar in HDS and HDO.
Thus, hydrodeoxygenation leads to the formation of water
(H2O) whereas hydrodesulfurization leads to the formation of
H2S, both of them interacting with the surface of sulfided cata-
lysts.131 It is well known that H2S promote the acid catalytic
activity of these catalysts,132 enhancing the reaction rate of the
acid-driven transformations. On the contrary, the interaction of
water with hydrogenating sites leads to the inhibition of the
hydrogenation reactions.133 Sxenol et al.134 described the
Energy Environ. Sci., 2012, 5, 7393–7420 | 7403
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hydrogenation of model aliphatic methyl esters in the presence of
alumina-supported sulfided CoMo and NiMo catalysts as
a sequence of three reaction pathways: the formation of alcohols
which evolve towards hydrocarbons by dehydration; the de-
esterification between the alcohol and the carboxylic acid func-
tionality; and the hydrogenation of the carboxylic acid towards
hydrocarbon, either passing through the alcohol or not. Thus,
dehydration, hydrolysis and hydrogenation reactions are present
at the same time when hydrotreating these triglyceride-contai-
ning feedstocks, the two first reactions being promoted by acid
catalysis whereas the last one is driven by the hydrogenating
activity. In fact, some decarboxylating activity134 has also been
found to be present, which could not be avoided, at least under
the employed reaction conditions. This last reaction is also
driven by the acid sites of these catalysts, whose presence is
associated with sulfhydryl acid groups.131 A proof of this
behavior is the low hydrodecarboxylating activity of the same
catalysts when used as non-sulfided metal oxides.134
The HDO reactions carried out in the presence of H2S might
avoid the deactivation caused by the evolving water molecules,
but the enhancement of acid activity modifies the behaviour of
the catalyst leading to a meaningful increase of the extent of
hydrodecarboxylation reactions.131,132 On the contrary, hydro-
genolysis and hydrogenation reactions are due to the presence of
sulfur vacancies associated with molybdenum atoms.135
The reaction pathways observed when treating more realistic
feedstocks like sunflower136 or waste cooking oils137,138 were
essentially the same to those previously described for model
compounds, so that the catalytic performance of conventional
hydrodesulfurization catalysts seems not to be affected by the
nature of the feedstock. However, increasing the reaction
temperature leads to a much higher extent of the removal of
oxygen, though this enhancement of triglyceride conversion is
accompanied by a much higher rate in HDC, isomerization and
even hydrocracking reactions, which are more important when
NiMo/g-Al2O3 was used as catalyst.139–141 Thus, though the
triglycerides conversion increases with the operation tempera-
ture, the yield towards green diesel decreases. A proof of this fact
is the decreasing of the cetane number in the final product or the
increase of the bromine index when operating at high tempera-
ture (�400 �C). In this way, it seems preferable to operate at
lower temperatures and recirculate the heavy fraction and the
residue to maximize the yield towards biodiesel.
In order to increase the catalytic performance of hydro-
desulfurization catalysts in triglyceride HDO treatments, some
modifications of conventional catalysts have been assayed. The
main improvements lie in the modification of the catalytic
support, mainly tackled through the enhancement of the support
surface area142 or the assay of different supports like silica or
silica alumina mixed oxides.143,144 However, the conversion of
triglycerides was lower than that achieved over conventional
hydrodesulfurization catalysts. Hence, further research is needed
to fully understand the influence of the catalyst-related proper-
ties on the triglyceride HDO process.
The use of conventional HDS catalyst for treating mixed
blends composed by triglyceride-containing oils and petroleum-
derived oils is now gaing more and more interest by petroleum
refineries. This strategy is readily applicable in conventional
refineries without the needing to implement great modifications
7404 | Energy Environ. Sci., 2012, 5, 7393–7420
on existing hydrotreating units. Several advantages have been
found when treating the mixture of renewable and conventional
raw-materials in comparison to the single processing of both
feedstocks. Thus, Huber et al.145 reported the treatment of
sunflower oil together with heavy vacuum oil (HVO) in presence
of a conventional sulfided NiMo/g-Al2O3 catalyst. Interestingly,
the treatment of the mixture led to a higher amount of straight
alkyl chains in the range C15–C18 than if treating pure sunflower
oil, as a consequence of the dilution of free fatty acids (FFA),
which inhibit polymerization and hydrocracking reactions.146
Moreover, since hydrodesulfurization is a much slower reaction
than alkane production from the vegetable oils, the use of feed-
stock mixtures does not affect the rate of desulfuration. Similar
results have also been obtained using CoMo/g-Al2O3 catalyst in
the hydrotreatment of cottonseed oil.147 Besides this fact, quality
enhancement of several properties of the final products can also
be found, like the cetane index,148 which shows higher values
compared to those achieved when treating only petroleum frac-
tions. In this way, the direct application of the existing infra-
structure in petroleum refineries for treating petroleum streams
and vegetable oils mixtures makes possible extensive industrial
application in the near future. However, blending vegetable oils
with VGO dilutes the latter. Consequently, contact time has to be
adjusted to maintain high rates of conversion of sulfur and
nitrogen. This change may cause the catalysts to deactivate faster
and decrease the catalyst cycle length.72
Therefore, the hydrotreatment of triglycerides for the
production of hydrocarbons on an industrial scale can be
implemented either by HDO of triglycerides in stand-alone units
or by co-processing of triglyceride feedstock with crude-oil-
derived fractions (Fig. 9). Co-processing offers the advantage of
low implementation cost due to the possibility of using the
existing hydrotreatment equipment. However, several potential
risks have been identified, including: (i) the need for a pretreat-
ment reactor to remove contaminants from the vegetable oil; (ii)
revamping the gas recycling systems to deal with the deoxygen-
ation products (CO2, CO, H2O); (iii) the competition of HDO
reactions with HDS leading to lower desulfurization efficiency;
and (iv) the cold-flow properties of the combined diesel product
may limit the quantity of vegetable oil that can be processed as n-
paraffin – the primary product from hydrotreating vegetable oil –
and will impact the cloud point.7 After considering all the
potential issues, it seems to be more cost-effective to build
a dedicated unit specifically designed for vegetable oil processing
(Fig. 10) than using a hydrotreatment unit for co-processing
renewable and conventional feedstock.
Many companies investigate and plan to apply hydrotreat-
ment technologies for the production of renewable diesel fuel
from vegetable oils and animal fats. Some of these developments
consist on modified gasoil hydrotreating processes to which
a blend of gasoil and triglyceride-containing mixtures is fed.149–151
However, industrial experience has been gained in building
trygliceride hydrotreatment stand-alone units. Thus, Neste Oil
OYJ, a Finnish oil refining company, has licensed a new process –
the NExBTL process – for the production of green diesel from
pure vegetable oils and fats.126,152 NexBTL renewable diesel is
currently produced at three renewable diesel plants in: Porvoo
(Finland) with a total capacity of 190 000 metric tons per year, at
the world largest renewable diesel refinery in Singapore (800 000
This journal is ª The Royal Society of Chemistry 2012
Fig. 9 Option for co-processing triglycerides with crude oil-derived
fractions in a hydrotreating unit.
Fig. 10 UOP/ENI Ecofining process flow diagram.
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metric tons per year) and at the Europe largest renewable diesel
refinery in Rotterdam, Netherlands (800 000 metric tons per
year).71 A similar process is that licensed by UOP/ENI – the
Ecofining process153,154 – which involves the same two different
reaction stages as in the NExBTL process. Thus, the two distinct
stages comprise the hydrodeoxygenation step in which trigly-
ceride-based biomass and hydrogen are fed to. Obviously, not
only HDO reactions occur during this step, but also HDC and
subsequent methanation takes place in this first reaction. The
second stage involves the hydroisomerisation of the deoxygenated
product to improve its cold properties. Light ends can be used to
produce hydrogen, which is then recycled to the reaction stages to
increase the profitability of the processes.
Essentially, both processes, NExBTL and Ecofining, are rather
similar and the final products coming from the process are the
same: light naphtha and propane and green diesel. This last is the
major component, comprising more than 80 wt% of the final
This journal is ª The Royal Society of Chemistry 2012
products. The final properties of this green diesel are rather
similar to those achieved through the Fischer–Tropsch liquid fuel
from syngas. Table 2 lists the properties calculated for diesel-like
fuels, including green diesel and its blend with conventional
diesel fuel. These fuels are featured by a high cetane number
(involving very good engine efficiency), low oxygen content
(makes green diesel more similar to petroleum derived fuels), and
very low sulfur content. The boiling range of green diesel is rather
similar to that of conventional diesel fuel, preventing vapor-
ization problems in the combustion chamber. Besides, green
diesel displays very high stability in contrast with biodiesel.
Finally, the low specific gravity of this fuel makes possible to
upgrade to low value and highly dense refinery streams.
The economical and environmental impact of the Eco-
fining153,154 and the NExBTL155 processes in conventional
petroleum refineries has been studied by means of lifecycle
assessments. The results of these studies indicate that these
processes are competitive with biodiesel when operating
moderately sized units. However, the profitability of the
processes depends on the differential price between crude
petroleum and renewable plant oils. From an environmental
point of view, the production processes for green diesel are
clearly superior to conventional diesel fuel production, both in
terms of energy consumption and greenhouse gas emissions.
Nevertheless, differences with conventional diesel fuels can
become negligible depending on the starting triglyceride-con-
taining oil or even the farming practices.
As an alternative to these NExBTL and Ecofining
processes,153–155 a process for producing an isoparaffinic product
useful as jet fuel from renewable feedstocks – the Bio-Synfining�process – has been also developed by the Syntroleum Corpora-
tion.156–158 Syntroleum’s Bio-Synfining� process is capable of
processing a wide range of renewable feedstock including vege-
table oils, fats and greases into a broad slate of synthetic ultra-
clean fuels, including jet fuel. The patent-pending process is
schematically represented in Fig. 11. This process catalytically
converts the fatty acids into paraffinic hydrocarbon fractions
containing virtually no residual oxygenates. The fatty acid chains
are first hydrogenated and deoxygenated into straight chain
paraffins, which are then hydrocracked into shorter straight and
branched paraffins (iso-paraffins). The hydrocracker product
consists of a broad distribution of mainly C3–C18 hydrocarbons
that may be fractionated into LPG, naphtha and jet fuel frac-
tions. In a first step, triglycerides are converted into a LPG,
a naphtha fraction and a kerosene or gas oil fraction, whose
relative amounts depend on the conditions used in the hydro-
cracker. The steam cracking of the renewable naphtha in a pilot
plant revealed that high olefin yields are obtained, being this
option useful as an alternative route for the production of light
olefins.159
3.2.3 Catalytic deoxygenation of oleaginous feedstock.
Although, previous studies on the hydroprocessing of trigly-
ceride-based feedstocks have forwarded considerable informa-
tion, there are still some weak points that would benefit from
more attention. From an economics and environmental view-
point, minimizing unnecessary hydrogen consumption is
important. The amount of hydrogen consumed for triglyceride
conversion by the HDO route (breaking the C–O linkage with no
Energy Environ. Sci., 2012, 5, 7393–7420 | 7405
Table 2 Properties of different diesel-like fuels obtained through different techniques. Data from ref. 6, 123, 149 and 150
Property Diesel fuel Biodiesel Green diesel Blenda
Cetane number 53 50 70–90 57.8Oxygen content (wt%) 0 11 0 0Sulfur content (wt ppm) <10 <10 <1 4.7Distillation/�C 180–360 340–355 265–320 249–341Lower heating value/MJ kg�1 35.7 38 44 36.5Cloud point/�C �5 �5 �10 to 20 �4.1Stability Baseline Low Baseline BaselineSpecific gravity/kg m�3 835 883 780 827
a Effluent coming from the HDS unit when co-feeding VGO and palm oil (10 wt%).
Fig. 11 Bio-Synfining process flow diagram.
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release of CO/CO2) is considerable higher than that by the
decarboxylation route (breaking of a C–C bond with release of
CO or CO2). However, as previously mentioned, the total
hydrogen consumed by the decarboxylation route could theo-
retically exceed the HDO route through secondary reactions such
as water gas shift and methanation.124 The extent of hydrogen
consumption via the different mechanism is expected to be
sensitive to the process conditions and catalysts, and thus, more
studies need to minimize unnecessary hydrogen consumption
should be undertaken.
Recently, Murzin et al. have investigated the catalytic deoxy-
genation of long-chain free fatty acids and derivatives by the
decarboxylation route under low hydrogen pressures or without
hydrogen.126,160–164Awide variety of supported noble metals were
used as catalysts for catalytic deoxygenation of stearic acid under
He.160 Reaction experiments indicate that supported metal
carbon-based catalysts display a much higher selectivity towards
decarboxylated products than analogue catalysts based on
different supports. Considering the active metal species, palla-
dium and platinum display a much better catalytic performance
than other metals like ruthenium, rhodium or iridium. However,
palladium mainly drives decarboxylation reactions, whereas
platinum produces decarbonylation. Apart, from deoxygenation
reactions, other reactions, such as hydrogenation, dehydroge-
nation, cyclization, ketonization, dimerization and cracking were
observed to various extents depending on the catalysts. Never-
theless, the preparation conditions used for these Pd/C materials
7406 | Energy Environ. Sci., 2012, 5, 7393–7420
exert a dramatic influence on their catalytic behavior, being the
particle size of the final supported metal species one of the most
important parameter conditioning the catalytic activity.161
Moreover, the Pd/C catalyst studied deactivated rapidly,
apparently because of coking.161,164 However, the FFA deoxy-
genation reactions carried out under low H2 partial pressures
might avoid catalyst deactivation and with and enhancement of
the catalyst lifetime and deoxygenation rate while maintaining
high selectivity to the H2-neutral decarboxylation pathway.165,166
Apart from FFA, these catalysts have also been used for
treating different feedstock such as alkyl esters and triglycerides.
Independent of the feedstock, the major products were hydro-
carbons – both saturated and unsaturated – formed by an alkyl
or alkenyl chain with one carbon atom less than the original fatty
acid chain.162 However, FFA deoxygenation seems to be rather
effective and fast, being more difficult than the deoxygenation of
alkyl esters163 and even more complicated in the case of triglyc-
erides. These differences seem to be caused by a different reaction
mechanism. Thus, whilst the deoxygenation of free fatty acids
follows a decarboxylating pathway, in the case of fatty acid alkyl
esters it mostly proceeds via decarbonylation,164 though both
types of reactions coexists in both cases,126 and the dominant one
can be tuned depending on the catalyst and the reaction
conditions.
Based on the results obtained in the deoxygenation of
tristearin, triolein and soybean oil over Ni/C, Pd/C and Pt/C
catalysts, Morgan et al. suggested a generalized scheme for
triglycerides deoxygenation (Fig. 12).167 The formation of
hydrocarbons from triglyceride can proceed via different path-
ways. In the bottom route, liberation of a fatty acid occurs via
a b-elimination process, the co-products being an unsaturated
glycol difatty ester. The acid subsequently undergoes de-
carboxylation in a separate step to give an alkane containing one
carbon less than the acid. In the case of the unsaturated glycol
difatty ester formed from the triglyceride, further reaction by b-
elimination is not possible and other pathways must be involved
to explain the formation of additional hydrocarbons. Alternative
pathways for hydrocarbon formation involve either scission of
the C–C bond between the ester carbonyl carbon and a carbon of
the hydrocarbon chain, or scission between the b and g carbon
atoms. These processes may lead to alkanes or terminal alkenes
depending on whether hydrogen abstraction occurs during bond
scission.
In order to avoid Pd/C catalyst under decarboxylation
conditions, the modification of the catalytic support has been
This journal is ª The Royal Society of Chemistry 2012
Fig. 12 Suggested general reaction scheme for triglyceride
deoxygenation.167
Fig. 13 Process flow diagram of Avjet Biotech’s Red Wolf Refining
System.175
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investigated for different authors. Ping et al. have reported the
synthesis, characterization and application of well-dispersed
palladium nanoparticles supported on a mesocellular silica
support in the catalytic decarboxylation of stearic acid.168,169 The
different catalysts synthesized were active in the batch decar-
boxylation of stearic acid under nitrogen atmosphere at 300 �C,reaching conversions of 85–90% after 6 h with complete selec-
tivity to the decarboxylation product n-heptadecane. However,
lower conversions (<15%) were observed in the decarboxylation
of ethyl stearate. After one use, the catalysts activity was
dramatically reduced, achieving less than 5% conversion to n-
heptadecane in the second use, as a consequence of carbonaceous
deposition. Nevertheless, contrary to previous assumptions, the
majority of these carbonaceous deposits were found not to be
traditional coke, but instead residual reactants, solvent and
product. Detrimental coke formation was verified to be absent,
as extraction of surface-deposited organic species yields nearly
complete recovery of the total surface area, pore volume and
active palladium surface area. Moreover, regenerated catalyst
exhibits a significant recovery of decarboxylating activity.
Berenblyum et al. have studied the catalytic decarboxylation
of stearic acid over palladium supported over acid solids.127,170
Palladium compounds were deposited onto WO3/ZrO2 support
(3 : 7) followed by the reduction of these compounds into
metals.170 It was found that the reaction proceeds quite effec-
tively on the catalyst with a palladium loading of 0.5% in an inert
atmosphere. However, it has been proved hydrogen promotes the
reaction, although the stoichiometry of the decarboxylation
reaction does not require the introduction of hydrogen from an
external source. The conversion of stearic acid has been also
carried out in the presence of palladium on alumina, being
heptadecane the main product of the reaction at a relatively low
H2 pressure (10 atm or less).127
High activity and selectivity toward diesel-like hydrocarbons
from both model compound (methyl octanoate) and real vege-
table oil feedstock (methyl stearate) can be successfully achieved
with 1%Pt/g-Al2O3.171 When feeding methyl octanoate,
a mixture of C7 alkane and alkenes, which results from decar-
bonylation or decarboxylation of the ester, acid and other 1-
oxygen-containing compounds are dominant products. C8
products are also formed but in much smaller quantities.
Nevertheless, if the reaction is carried out under hydrogen
atmosphere, formation of heavy products are much suppressed
compared to the reaction carried out under inert gas. Regarding
to methyl stearate, its decarboxylation produces C17 with
This journal is ª The Royal Society of Chemistry 2012
high selectivity, though both the conversion and selectivity
towards paraffin are higher under hydrogen than under an inert
gas.
Since the precious metals are rare and expensive, some tran-
sition metal oxide and metal catalysts have been studied for the
catalytic decarboxylation process. Hydrotalcites with various
MgO contents172 and Ni-containing hydrotalcites173 have been
tested as catalysts in the decarboxylation of oleic acid instead of
precious metals. Hydrotalcite is a layered double hydroxide
composed of MgO and MgAl2O4 and is used as absorbents for
CO2 and catalyst supports. Hydrotalcites showed decarboxyl-
ation activity without using hydrogen and could produce pure
hydrocarbon streams. However, selectivity of heptadecene, the
product obtained by direct decarboxylation of oleic acid, was not
very high, implying that the cracking and decarboxylation
occurred simultaneously during the reaction over hydrotalcites.
Ni catalysts supported on MgO–Al2O3 have also shown catalytic
activity for decarboxylation of oleic acid, but heptadecene
selectivity was lower than 15%.174
The Red Wolf Refining System, developed by Avjet Biotech
Inc. is already harnessing this technology to convert the fatty
acids coming from the hydrolytic conversion of triglycerides to
aviation biofuels on an industrial scale.175 As depicted in Fig. 13,
the process comprises the following steps: (1) thermal hydrolysis
of a lipidic biomass to yield a product stream comprising free
fatty acids and a by-product stream comprising glycerol; (2)
catalytic deoxygenation of the free fatty acid stream to form
a product stream comprising n-alkanes; and (3) reforming step of
the n-alkane stream to form a product stream comprising
a mixture of hydrocarbon compounds selected from the group
consisting of n-alkanes, isoalkanes, aromatics and cycloalkanes.
Hydrogen consumption during the process is minimal. Most of
the hydrogen consumed is used to saturate any unsaturated fatty
acids during the deoxygenation process (a necessary step in
making drop-in fuels). Likewise, this hydrogen is not actually
consumed, but becomes part of the fuel molecule, increasing the
energy density of the fuel. In contrast, hydrogenation processes
where the hydrogen is used to deoxygenate the fat/oil/lipid result
in more water than energy-dense fuel.
Energy Environ. Sci., 2012, 5, 7393–7420 | 7407
Fig. 14 Reactions and mechanisms taking place in the thermal and
catalytic cracking of bio-oils.
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3.3 Lignocellulosic based feedstock processing
The use of lignocellulose as a refinery feedstock is a promising
alternative to conventional crude oil because of the large abun-
dance and variety of this raw material.176 Lignocellulosic-derived
feedstock accounts for more than 95% of the total biomass
produced by the whole planet every year. There are a huge
variety of lignocellulosic materials which could be considered as
feedstock for biorefineries:177 agricultural residues (i.e. corn
stover, cereals straw and bagasse.), woody materials including
hardwood (poplar, aspen.) and softwood (pine), herbaceous
materials (switchgrass, sorghum) and wastes from paper
industry, among others. Despite only certain lignocellulose
sources seem to lead to an optimal combination of biomass-
starting raw materials as the entrance point to a refinery, both
from an environmental and energy point of view, any kind of
lignocellulose source can be used as substitutes for crude oil in
a biorefinery.10 Furthermore, the pseudo-homogeneous compo-
sition of cellulosic materials, polysaccharides, are all built from
simple sugars, which facilitates their processing, making easier
their inclusion as starting raw material of a continuously oper-
ating production process. The use of lignocellulosic biomass in
a refinery can be accomplished in different ways, however, it
seems rather difficult to establish a biomass supply chain con-
sisting of the lignocellulosic material in the form it is collected, as
it was previously established, due to the low energy density these
feedstocks show. Quite effective options are the transformation
lignocellulosic biomass into a fluid by pyrolytic treatment,
leading to the production of a liquid called bio-oil, or by de-
polymerising the constituents of lignocellulosic biomass into
simple sugars and subsequently transforming them into platform
molecules.
3.3.1 Bio-oils to hydrocarbon fuels. Bio-oil upgrading to
hydrocarbon fuels can be accomplished by means of different
procedures,178 like the treatment of bio-oils in presence of zeolite
catalysts, which allows a reduction the oxygen content and
improvement of the thermal stability of the feedstock.179Thermal
treatment at 350–500 �C and atmospheric pressure leads to the
production of hydrocarbons (both aromatic and aliphatic),
water-soluble organics, water, oil soluble organics, gases (CO2,
CO, light alkanes) and coke. The chemical pathway through
which these chemicals are produced is very complex,35 and
several types of reactions take place simultaneously, including
dehydration, cracking, polymerization, deoxygenation and
aromatization reactions. Nonetheless, a poor hydrocarbon yield
and high production rates of coke limit the usefulness of zeolite
upgrading.
Adjaye and Bakhshi180 explored the possibility to feed bio-oil
to a FCC unit by means of a deep study of the reactions taking
place when feeding different bio-oil model compounds to a fixed-
bed reactor loading a HZSM-5-based catalyst. These authors
concluded that the bio-oil became separated into volatile and
non-volatile fractions in the early stages of the reaction while
reaching the reaction temperature. The heavy fraction (which
accounts for 37 wt% of the total bio-oil) undergoes a cracking
pathway, yielding volatile compounds and a solid residue
produced by means of polymerization reactions and coke
formation. The resultant volatile fraction suffers deoxygenation
7408 | Energy Environ. Sci., 2012, 5, 7393–7420
(both through dehydration, leading to water formation, and
through decarbonylation and decarboxylation reactions,
producing carbon oxides), secondary cracking (leading to
hydrocarbon gases), oligomerization, olefin formation, hydrogen
transfer, cyclation, disproportionation, alkylation and isomeri-
sation reactions to form the hydrocarbon-rich product. Some of
the forming hydrocarbons also polymerize and condense to form
more residue. Other types of catalysts181–184 have also been tested
in the upgrading of bio-oils, including HZSM-5, H-Y, morde-
nite, silicalite and silica alumina. Acid zeolite-based catalysts
have revealed to be highly effective in the conversion of bio-oil to
hydrocarbons, and thus the HZSM-5 catalyst produced the
highest amount (34 wt% of feed) of organic liquid products
among the tested catalysts, while providing the lowest coke
formation. On the other hand, the less acidic silica-alumina
catalyst minimizes char formation, whereas the H-Y catalyst
leads to a minimum tar formation and maximum production of
aliphatic hydrocarbon. From these experiments, authors have
proposed several reaction pathways for bio-oil when treated in
presence of acid zeolites (Fig. 14). Two different reaction stages
were proposed in the reaction mechanism, consisting of an initial
thermal step followed by a thermocatalytic one. The former leads
to the separation of bio-oil into light and heavy organic
compounds, while inducing char formation. The thermocatalytic
step produces coke, tar, gas, water and the desired organic
distillate fraction. Deoxygenation, cracking, cyclation, aromati-
zation, isomerization and polymerization reactions take place
during this step. This double stage transformation allows sepa-
ration of the different reaction steps in different reactors by using
an installation consisting of two reaction units. Thus, in a first
empty reactor the thermal transformations can be accomplished,
whereas in a second reactor, loaded with the catalyst, the ther-
mocatalytic transformations are promoted. In this way, a longer
catalyst life is achieved, since the coke formed during the first
stage is not deposited on the surface of the catalyst,185 avoiding
its poisoning.
Similar studies, but feeding model bio-oil compounds (alde-
hydes, ketones, acids, alcohols, phenols and their mixtures)
instead of real bio-oil186–188 revealed differences between the
reactivity of the assayed families of chemicals. However, most of
them produced large amounts of coke, and caused catalyst
deactivation. In this way, the previous stabilization of the bio-oil,
for instance through a specific thermal step,189 was required
before being treated in presence of the acid catalyst. Co-feeding
This journal is ª The Royal Society of Chemistry 2012
Fig. 15 Pretreatment options for bio-oil in its co-feeding with vacuum
gas oil to FCC units.
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methanol (�70 wt%) resulted to be another option to minimize
coke deposition on the catalyst particles.190
Model bio-oil compounds, such as acetic acid, hydroxyacetone
and phenol, added to a standard gasoil FCC feed, lead to
increasing overall raw material conversion, reduced coke yields
(except for phenol) and higher fuel gas, LPG and gasoline
production, when treated in presence of an industrial FCC
equilibrium catalyst (E-CAT).191 However, the bio-oil model
compounds suppress the effect of the ZSM-5 catalyst additive,
suggesting a preferential interaction of such compounds with the
ZSM-5 additive, leading to its inactivation.
The direct use of bio-oils in refineries, either as the feedstock of
FCC unites or mixed with conventional petroleum streams, is
conditioned by the high concentration of water, oxygen and
metals, particularly potassium and calcium. Metals can be
removed by retention in guard beds by ion exchange. On the
contrary, the low thermal stability, high water content and very
high oxygen content of bio-oils make difficult their blending with
common refinery intermediate streams such as VGO. A
maximum amount of 10 wt% of oxygenated compounds, referred
to gasoil, has been estimated to be possibly fed to a FCC unit
without major problems,191 though the amount of benzene in the
product stream can be rather high due to the presence of
phenolics in the feed stream. A different drawback of bio-oils to
be treated in standard refinery units, but nonetheless highly
important, lies in its high acid number which makes the standard
refinery vessels unsuitable to treat such acid feedstocks. The
industry standard supports acid numbers on the streams to be
treated below 1.5 mg KOH g�1. Bio-oils can probably be pro-
cessed using 317 stainless steel cladding, which is not standard in
refinery units. Therefore bio-oils would require to be pre-pro-
cessed in 317 stainless steel-made pre-conditioning units to
reduce their acid number before being fed to typical refinery
units.72 Since the FCC is the biggest unit and the heart of most
refineries, much development work is required before such an
approach would become viable. Hence, the direct feeding of bio-
oils into standard refinery (see Fig. 15a) does not appear as
a straightforward task.
A different option to tackle the insertion of bio-oils as feed-
stock in a conventional refinery is by first hydrotreating this
biomass-derived raw material, prior to feeding to the FCC unit
(Fig. 15b). In fact, a hydrotreatment conditioning step is rather
usual, depending on the feedstock, in conventional refineries,
prior to the catalytic cracking treatment. Thus, acid numbers in
bio-oils can be reduced by hydrodeoxygenation (HDO) together
with the oxygen content. This treatment allows stabilizing of the
bio-oil as well as increasing its energy density, leading to a stream
able to be blended with a petroleum-derived feedstock. The
reactions involved when hydrotreating bio-oils are rather similar
to those taking place in petroleum fractions. Furimsky192 estab-
lished the following order of reactivity of oxygen-containing
groups: alcohols > ketones > alkyl ethers > carboxylic acids zm- and p-alkyl–substituted phenols z naphthol > phenol >
diaryl ethers z o-alkyl-substituted phenols z alkylfurans >
benzofurans > dibenzofurans. Bearing in mind that the most
abundant compounds in bio-oils are phenols, acids and esters,
their reactivities are the most relevant. Phenols, which may
account for up to 25 wt% of liquids obtained by pyrolysis of
lignocellulosic materials, especially if lignin is abundant, are
This journal is ª The Royal Society of Chemistry 2012
refractory oxygenates. The overall mechanism for the HDO of
o-substituted phenols can be split into two main HDO reactions:
direct HDO and HDO via hydrogenated phenol, both taking
place at the same time.193 In the latter case, H2O elimination may
result in the formation of intermediate methylcyclohexene
species, which are quickly hydrogenated. The formation of
cyclohexene, alkyl-cyclohexenes and methyl-cyclopentanes can
also be present, though these are only minor products. Guaiacol
(GUA) and its derivatives are also interesting compounds, since
they are present in high quantities in bio-oils. The hydrotreat-
ment of guaiacol has also been deeply studied132,194 to enhance
the stability of bio-oil, since these compounds are quite unstable.
The proposed mechanism through which this compound is
converted considers the hydrogenolysis of the methoxy group of
guaiacol to catechol and methane as the first stage, followed by
the dehydroxylation of catechol in the second stage to produce
phenol. At the same time coke can be formed from both guaiacol
and catechol.
HDO of bio-oils has been carried out at moderate tempera-
tures (300–600 �C) under high H2 pressure in presence of
heterogeneous catalysts. Most of these studies have been focused
on the use of conventional hydrotreating catalysts like sulfided
Co–Mo- and Ni–Mo.132,194 However, as previously stated in the
case of hydrotreatment of oleaginous feedstock, these catalysts
have been developed for hydrodesulfurization (HDS), and thus,
in this case too, sulfur must be added to avoid catalyst deacti-
vation. Elliott and co-workers developed a two-step bio-oil
hydrotreating process based on the use of Al2O3-supported,
sulfided Co–Mo and Ni–Mo catalysts.195,196 The first step con-
sisted on a low-temperature (270 �C, 136 atm H2) catalytic
treatment to promote the hydrogenation of the thermally
unstable bio-oil compounds, which would otherwise undergo
thermal decomposition to form coke and plug the reactor. The
second step consisted of a catalytic hydrogenation but at a higher
temperature (400 �C, 136 atm H2). During this process, 20–30%
of the total carbon in the starting bio-oil is converted into
gaseous compounds, decreasing the overall liquid yield. Catalyst
stability and formation of gums in the lines were identified as
points of major uncertainty of the process, and future work is
needed to develop improved hydrotreating catalysts. However,
treating bio-oil at higher pressures (142–178 atm) and longer
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contact times,72 allowed these troubles to be overcome, since
hydrodeoxygenation prevails, though this approach is unlikely to
be commercially viable because of the high hydrogen require-
ment and the high capital cost of the hydrotreatment step.
Different types of catalysts, like Pt/SiO2-Al2O3,197 vanadium
nitride,198 and ruthenium/C,199 have also been used for hydro-
deoxygenating bio-oils. These catalysts do not need to co-feed
sulfur together with the feedstock to avoid catalyst deactivation,
and besides, they display a much higher hydrogenating capability
than sulfided metal oxides. Nevertheless, these catalysts are also
known to drive decarboxylation reactions, which could not be
interesting due to the lower atom efficiency of this reaction
pathway in comparison to HDO, though much more efficient in
terms of hydrogen consumption. Furthermore, their cost is also
much higher than that corresponding to the sulfided metal
oxides.
Hydrodeoxygenation, as a conditioning step to pre-treat bio-
oils before being fed to a FCC unit, has been tested by different
authors (Fig. 15c). For instance, de Miguel Mercader et al.199
hydrotreated bio-oils in batch conditions (230–240 �C; 290 bar
H2; 5 wt% Ru/C catalyst), looking for the required product
properties necessary for successful FCC co-processing (misci-
bility with FCC feed and good yield structure: little gas/coke
production and good boiling range liquid yields). The product
resulted in three well-separated phases consisting of: gases
(mainly CO2, from the decarboxylation reactions, and CH4,
coming from the methanation of carbon oxides); an aqueous
phase; and an oil phase. Increasing the reaction temperature does
not led to meaningful variations with regards to the yield towards
the aqueous and oil phases, though some compounds (hydro-
deoxygenated sugars) were transferred from the aqueous to the
oil phase, increasing the carbon recovery in the oil product (up to
70 wt% of the carbon in the staring bio-oil). The resultant oil
fraction was, despite the relatively high oxygen content (from 17
to 28 wt%, on dry basis), miscible with the long residue (20 wt%),
being co-processable, at least in a lab scale catalytic cracking unit
(MAT reactor). The assays yielded near normal FCC gasoline
(44–46 wt%) and Light Cycle Oil (23–25 wt%) products without
an excessive increase of undesired coke and dry gas formation, as
compared to the base feed. Samolada et al.200 reported a similar
process consisting of a two-step procedure in which bio-oil is
hydrotreated before being fed to the catalytic cracker. The
thermal hydrotreatment produced the liquid products able to be
upgraded in a refinery. The heavy liquid product of this process,
mixed with light cycle oil (LCO) (15/85 w/w), was considered as
a potential FCC feedstock. Commercially available cracking
catalysts were found to have an acceptable performance in the
transformation of this mixed feedstock, leading to a bio-gasoline
similar to that obtained solely from VGO but in lower yields
(�20 wt%). A similar study to this previous one, accomplished by
Lappas et al.,146 allowed the reduction of the oxygen content
below 6.5 wt% for a hydrodeoxygenation conversion around
85%. These results in the HDO step allowed the accomplishment
of the separation of the hydrotreated bio-oil by distillation into
two different fractions: the light and the heavy ones. The former
mainly contains hydrocarbons lying in the gasoline and diesel
ranges and thus, it could be directly blended with the corre-
sponding petroleum fractions. The heavy fraction was instead
quite similar to conventional VGO, being processable as co-feed
7410 | Energy Environ. Sci., 2012, 5, 7393–7420
with vacuum gas-oils (VGO) in the catalytic cracker. This
process was developed by Viba oil.146 The co-processing of VGO
with the heavy fraction showed that the presence of the bio-oil
fraction favours gasoline and diesel production but increases the
coke yield. However, depending on the concentration of biomass
liquids, it was shown that this option is technically feasible for
FCC units running with good quality feedstock, that is the FCC
unit with excess coke burning capacity.
Fogassy and co-workers increased the amount of HDO-oil to
20 wt% in the feeding stream, composed of VGO, which was co-
fed to a fixed-bed reactor,201 simulating FCC conditions, and to
a lab-scale FCC reactor.202 In both cases, an industrial equili-
brium FCC catalyst was tested, whereas in the FCC reactor HY
and HZSM-5 zeolites were also used, for comparison purposes.
In both cases, the results were almost the same when treating the
mixed HDO/VGO feeds, no differences relating to the gasoline
fraction were observed in comparison to those achieved when
VGO was treated. Nevertheless, the higher amount of oxygen in
the mixed feedstock when adding HDO-bio-oil leads to a higher
hydrogen consumption during dehydroxylation, resulting in
a poor hydrogen reaction media which finally leads to a product
richer in aromatics and olefins, as well as producing higher
amounts of coke and not totally converting the phenolic fraction.
This phenolic fraction mainly comes from lignin polymers, which
undergo cracking reactions in the external surface of the catalyst
followed by deoxygenation. Since oxygenated compounds
strongly interact with acid sites, large amounts of coke are also
produced, leading to the blockage of the catalyst pores, hindering
the access to the zeolite catalytic sites. In this way, the structural
parameters of the zeolite catalysts have to be tuned in order to
maximize the phenolics’ conversion whilst keeping the coke
deposition low.
Hence, the best approach for the processing of bio-oils in
refining units, is that schematized in Fig. 15d, which is composed
of three different stages: the first one is a hydrogenating step in
which the feed is conditioned to preserve the activity of the
catalyst in the second reaction stage, the hydrodeoxygenation.
The aim of HDO is thus, to extend the catalyst life in the third
stage, the FCC. This strategy allows a better catalytic perfor-
mance of the HDO and FCC, and a good quality of the final
products, though the hydrogen requirements are much higher.
3.3.2 Sugar derivatives as refining raw materials. Fuels from
sugars can be obtained by conventional fermentation, mainly
resulting in the formation of bioethanol, an excellent alternative
to gasoline. However, this conversion involves drawbacks,
especially in view of atom economy, the low energy density of
ethanol and its relative low boiling point. Alternatively, via
a number of direct chemical catalytic conversion processes, fuel-
type molecules could be synthesized by controlled trans-
formation of sugars into several important platform molecules
and subsequent conversions (dehydration, reforming, hydroge-
nation, hydrogenolysis, aldol condensations and olygomeriza-
tion) (see Fig. 16). However, these processes involve the
controlled degradation of polymers and the development of
highly selective transformations of sugars to target molecules in
aqueous solutions. Approaches to solve these challenges may
have to involve novel catalytic materials as well as novel reaction
systems. Moreover, the conversion of renewable feedstock with
This journal is ª The Royal Society of Chemistry 2012
Fig. 17 Preparation of oxygenated bio-fuels and bio-fuel additives from
cellulose-derived platform molecules.
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heterogeneous catalysts provides new challenges in inorganic
catalyst research widely used in the oil industry.203 These unique
challenges include the need to convert selectively highly func-
tionalized molecules and to develop catalytic liquid–solid inter-
faces, in which the liquid phase is commonly aqueous, to control
phase effects and to develop novel reaction systems.
Sugars to oxygenated fuels. One interesting alternative to use
sugar-derived platform molecules is their transformation into
second-generation biofuels, keeping the original carbon skeleton
of the platform molecules but enhancing the energy density and
fuel properties of the same. This way to produce biofuels from
lignocellulosic materials is superior, for instance, to the
production of ethanol by fermentation, in the sense of the
preserved atom efficiency avoids the intrinsic energy loss while
helping to reduce the CO2 emissions. Among the different
possibilities, much effort has been centred on the transformation
of these platform molecules into oxygenated fuels (Fig. 17).
One of the easiest ways to prepare a biofuel from sugar
derivatives is that patented by Avantium, which have recently
described two procedures to take advantage of the hydroxyl
group in 5-HMF by means of etherification and esterification
reactions. In particular, final chemicals consist of ethers204 and
esters205 and these are prepared from a glucose-rich raw material
and an alcohol or an organic acid, depending on the final desired
type of HMF derivative. Both reactions can be either accom-
plished in presence of an acid catalyst, which, if strong enough, it
is able to promote the interetherification between two 5-HMF
molecules,206,207 or by reaction with stoichiometric reactants such
as anhydrides206 or acyl chlorides.208,209 The resultant compounds
resulting from both the etherification and esterification reactions,
usually called furanics, display high energy density, making them
interesting compounds to be used as biofuels. Thus, the energy
density of ethoxymethylfurfural (EMF), a chemical liable to be
produced by etherification between 5-HMF and ethanol,
contains 8.7 kWh L�1. This is as good as regular gasoline
(8.8 kWh L�1), as good as diesel (9.7 kWh L�1) and significantly
more energy-dense than ethanol (6.1 kWh L�1).
Fig. 16 Transformations required for the conversion of platform
molecules into bio-fuels and fuel additives.
This journal is ª The Royal Society of Chemistry 2012
A different option in the preparation of biofuels from 5-HMF
is its conversion by hydrogenation/hydrogenolysis, yielding 2,5-
dimethylfuran (DMF) and 2-methylfuran (2-MF).210 For this
purpose hydrogenation has to be performed in presence of Cu-
Ru/C catalysts40,210–214 to drive the transformation towards the
reduction of the formyl and the hydroxyl groups, instead of the
reducing the furan heterocycle to tetrahydrofuran, which can be
easily accomplished in presence of Pt, Pd, or Ni.214,215 The
resultant products display high octane number, limited oxygen
content, high efficient hydrogen index and thus high energy
density. Physically, they are miscible with gasoline, all these
properties being good reasons to use these furans as biofuels.
However, initial estimates about the cost for the production of
DMF and the low performance of the production process make
it, at the moment, non-viable for commercial applications.215
Levulinic acid (LA) constitutes another good starting point for
the production of oxygenated biofuels216,217 (Fig. 17), for instance
methyltetrahydrofuran (MTHF), a gasoline-soluble fuel
extender.218 MTHF can be blended up to 30% with gasoline
without modification of current internal combustion engines.
Though the heating value of MTHF is lower than that of gaso-
line, this drawback is compensated by the higher specific gravity
of the former, leading to very similar mileage for both fuels.
Though the direct conversion of levulinic acid to MTHF is
possible,219 improved MTHF yields can be achieved through
indirect routes, for instance by means of firstly hydrogenating
LA to 1,4-pentanediol, followed by subsequent dehydration to
MTHF,216 with a total consumption of 3 moles of external H2 per
mole of LA.220 Nevertheless, this is not the only way to take
advantage of LA for the synthesis of biofuels. Esters of this
platform molecule,221–223 produced from either methanol or
ethanol, have significant potential as blending components in
diesel formulations.224,225 LA esters display similar properties to
fatty acid methyl esters (FAME) that are used in some low-sulfur
diesel formulations, whereas they lack their principal drawbacks
– mainly related to cold flow properties and gum formation. In
this way, the formulation of FAME and alkyl levulinates
mixtures alleviates these troubles, which makes these chemicals
acceptable diluents for biodiesel fuels, even if they contain a high
saturated fatty acid content.225 In this sense, the formulation of
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ethyl levulinate (EL) with conventional diesel fuel, has been
studied,226 demonstrating several advantages can be attained
when using the mixture in contrast to the use of conventional
fuel. Thus, these mixtures lead to a low-smoke diesel formulation
(the high oxygen content of EL, 33%, helps to provide clean
burning), with low sulfur emissions (EL does not supply sulfur to
the mixture) but improved lubricity in comparison to low sulfur
diesel (because of the excellent lubricity provided by EL).
Furthermore, this oxygenated additive does not lead to a signif-
icant decrease of the engine efficiency, and thus, a similar mileage
per volume can be achieved with the diesel/EL mixture compared
with conventional diesel. The synthesis of alkyl levulinates
usually takes place starting from cellulose undergoing a multistep
process. However, a dual catalytic system for the one-step
synthesis of methyl levulinate from cellulose has been recently
reported.227 In this work, the combination of two homogeneous
catalysts, metal triflates (Lewis catalyst) and sulfonic acids
(Br€onsted acids) yielded up to 75% methyl levulinate.
As a different option, GVL can also be converted into a huge
variety of chemicals and fuels, though its use as an oxygenated
fuel or as a fuel additive itself has been scarcely reported.38,228
Nonetheless, blending GVL with gasoline leads to a composition
displaying similar properties to those achieved from gasoline–
ethanol mixtures, though the lower vapour pressure of GVL also
provides improved performance. GVL can be prepared by
hydrogenation of LA, either by using H2 or by means of
hydrogen transfer from formic acid, in the presence of noble
metal-based catalysts,50,229–231 especially ruthenium-based cata-
lysts. Likewise, GVL can be hydrogenated to valeric acid fol-
lowed by esterificationwith alcohols to yield alkyl valerate esters –
valeric biofuels232 (Fig. 17). Gasoline blended with 10 and 20%
of ethylvalerate (EV) largely complies with the European gasoline
specification and even EV blends show an enhancement of some
gasoline properties – increasing the octane number and lowering
aromatics, olefins and sulfur contents. Likewise, EV also offers
the advantages of a higher energy density and lower blending
volatility than ethanol. Heavier esters, such as butyl and pentyl-
valerates, showed polarity, volatility, and ignition properties that
are suitable to be mixed with diesel, which could be another
possibility for their use as biofuels.
Sugars to hydrocarbon fuels: APR and related processes. A
very interesting alternative to the production of oxygenated
biofuels from sugar-derived platform molecules is the direct
transformation of these compounds into a mixture of hydro-
carbons. This option involves numerous advantages, like the
highest stability of the final fuel products, their higher intrinsic
energy density and the perfect compatibility with conventional
fuels, such as gasoline or diesel fuels. However, transforming
highly functionalized compounds, such as HMF, LA or furfural,
into hydrocarbons, involves severe treatments directed towards
the removal of the oxygen atoms from the starting molecules.
This objective involves the use of dehydration reaction pathways
to remove hydroxyl functionalities, which finally leads to C]C
bonding, that have to be reduced by hydrogenation to enhance
the chemical stability of the final products. As an alternative
option, hydrogenolysis and hydrogenation pathways can also be
employed to remove oxygen, A second important difficulty,
apart from the severe treatments when directly transforming
7412 | Energy Environ. Sci., 2012, 5, 7393–7420
sugar-derived platform molecules into automotive fuels, lies in
the small size of these sugar-derived compounds. Thus, platform
molecules like furfural, HMF or LA display five and six atom
carbon-skeletons, whereas conventional automotive fuels are
composed of chemicals containing up to 10 carbon atoms, in the
case of gasoline, and up to 20 carbon atoms for diesel fuels. In
this way, not only the removal of oxygen from the starting
molecules is needed, but also the oligomerization of the raw
materials is mandatory to achieve fuel products showing carbon
chain lengths in the range of those showed by conventional
hydrocarbon fuels.
The process developed by Dumesic and co-workers233–235 for
the conversion of sugar-derived compounds is, up to now, the
most successful in performing the required transformation for
converting sugar-derivatives into hydrocarbons. This process has
attracted much attention during the last years, as it is inferred
from the huge amount of references reported on the matter. This
process is based on a reaction pathway in which the carbohy-
drate-derived compounds undergo condensation reactions to
form C–C bonds whereas oxygen is removed by dehydration,
hydrogenation and hydrogenolysis reactions, which are con-
ducted in aqueous phase. An important drawback of this tech-
nology is the need for large quantities of hydrogen, which
involves high operation costs which have to be compensated
through higher prices for the obtained fuel products, making less
favorable the competition of these renewable feedstock-derived
fuels in comparison to those obtained from petroleum.
One alternative for the production of hydrogen to be used in
such a hydrogen demanding process as the biomass to hydro-
carbon fuels, is the use of the same biomass from which derived
platform molecules are produced, as a substrate for the
production of hydrogen. This option can be accomplished by
means of a process named Aqueous Phase Reforming
(APR).234–239 This procedure allows obtaining large quantities of
hydrogen from several reactions conducted in the liquid phase,
taking advantage of the favoured water gas shift reaction under
these conditions. This procedure allows the production of a H2
stream in a single step, with very low formation of carbon
monoxide, in contrast to conventional processes used for the
production of hydrogen from non-renewable sources of hydro-
carbons. The starting point of aqueous phase reforming is the
availability of oxygenated hydrocarbons, which can be obtained
from a huge variety of biomass feedstock. Biomass resources
such as ethanol,240,241 ethylene glycol,242 polyol compounds (such
as glycerol,243–247 sugars248 or sorbitol249), cellulose (either as
a pure compound250 or coming from waste paper or wood251),
woody bio-oil252 or even waste-water,253 have been reported to be
transformed through APR under moderate temperature
conditions.
The reactions taking place in hydrogen production through
APR involve the cleavage of C–C, C–H and O–H bonds. Carbon
monoxide is simultaneously produced together with hydrogen,
though CO is readily removed from the surface of the catalysts
by its transformation into CO2 through the water gas shift
reaction.254 This reaction allows the increase of the formation of
hydrogen while reducing the amount of CO present in the
product stream, producing a ‘cleaner’ hydrogen stream.
Depending on the starting oxygenated hydrocarbon, the reaction
mechanism is more or less complicated, but in any case, these can
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be simplified as C–C and C–H bonds cleavage reactions
accompanied by dehydration, hydrogenation and dehydrogena-
tion reactions, as is shown in Fig. 18.
Bearing in mind the complex and multiple reaction pathways
taking place during aqueous-phase reforming of oxygenated
hydrocarbons to hydrogen, discerning requirements are needed
for the catalyst to be used in APR transformation. First, as
already mentioned, the catalyst must promote C–C, C–H and
O–H bonds cleavage. These reactions are needed because they
lead to the formation of hydrogen and carbon monoxide.
Second, the water gas shift must be promoted because the
evolved carbon monoxide from the previous step depresses the
activity of the catalysts, and thus hydrogen production via bond
cleavage is reduced. Moreover, the water gas shift increases the
yield towards hydrogen.
Simultaneously to these determining factors, the catalyst
should present very low activity in C–O cleavage and CO/CO2
methanation reactions, because both of them lead to hydrogen
consumption. Bearing in mind all these considerations, several
metal species such as Pd, Pt, Ru, Rh or Ir,254 or their mixtures,
could promote C–C, C–H and O–H bond cleavage reactions.
Most of the reported catalysts used in APR involve one or more
of these expensive metal species, though it has been shown that Pt
and Pd had higher selectivity towards H2 production. However,
their high price is a very important drawback to extend their use
to an industrial scale, though their intrinsic catalytic activity and
Fig. 18 Main types of reactions taking place in the aqueous phase
reforming of oxygenated compounds.
This journal is ª The Royal Society of Chemistry 2012
selectivity can be enhanced by using bimetallic catalysts incor-
porating Ni, Co or Fe species,254 or by selecting the proper
catalyst support.255
Alternatively, Sn-promoted Ni Raney� catalysts have
revealed to be an interesting option233,256 to the noble-metal
based catalysts. This inexpensive catalyst displays several
advantages over other APR catalysts such as Pd/C or Pt/Al2O3,
its lower price being the most important one from an economic
point of view. Furthermore, Ni–Sn Raney catalysts display
a very high resistance against deactivation, being reusable for
several days on stream.256 An important feature of this catalyst
over analogous Ni-Raney catalysts lies in the effect of the tin
species, which promotes hydrogen-producing reactions to reac-
tion rates comparable to those achieved with Pt/Al2O3 catalysts,
whilst depressing the intrinsic catalytic activity of Ni in the
methanation of CO and CO2.256 Optimal reaction conditions for
this catalytic system usually involve mild temperatures and low
pressures, so that the required equipment to perform the aqueous
phase reforming of biomass-derived compounds is not greatly
demanding, being possible to conduct these transformations in
conventional hydrotreating refinery systems.
A key feature of the APR process, which makes it even more
interesting, is that, while hydrogen evolves from the reaction
media, intermediate oxygenated hydrocarbons are produced too.
These chemicals include alcohols, acids, ketones and aldehydes,
though the exact composition depends on both the APR condi-
tions and the starting biomass-derived substrate. One possibility
to take advantage of these compounds is their transformation, by
means of different pathways (condensation, hydro-
deoxygenation, dehydration, oligomerization.), into larger
alkanes which can be used as fuels (diesel, gasolines or kerosene).
This is the origin of the process licensed by Virent Energy
Systems, the Bioforming process,238 whose main steps are
depicted in Fig. 19.257–259
Thus, Virent’s BioForming process combines the APR
procedure for hydrogen production with several hydrogenation,
dehydration and base-catalyzed condensation reactions to
prepare saturated hydrocarbons, suitable for the formulation of
liquid fuels. The most impressive fact of this process is the nature
of the raw material, since this can be accomplished starting from
renewable biomass derived carbohydrates such as sugars.
However, since the reaction routes to achieve the alkane fraction
as the main product are different to those required for maxi-
mizing the hydrogen production, a different catalyst is required
for the production of alkanes in the BioForming process. Thus,
not only hydrogenation, C–C and C–O bond cleavage are
required (in contrast to those preferred for hydrogen produc-
tion), but dehydration reactions are also needed, so that the
employed catalysts should present a bifunctional activity. On one
hand, the metal species are needed to promote the already-
described reactions for H2 production, on the other hand, acid
activity is needed to drive the dehydrogenation reactions.
Finally, preliminary economic analysis suggests that, since
approximately 90% of the total energy contained in the starting
carbohydrate and H2 feed is transferred to the final products, this
technology may be competitive at crude oils prices greater than
60 $/bbl.
As in the case of the APR-evolving light oxygenated hydro-
carbons, other compounds, such as furfural or HMF, can also be
Energy Environ. Sci., 2012, 5, 7393–7420 | 7413
Fig. 19 Virent Bio-forming Process for the transformation of glucose-
based biomass into hydrocarbon fuels.
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used for the production of hydrocarbon fuels directly in the
range of gasoline, but also in the range of jet fuel or even diesel
fuel. However, in these cases, oligomerization reactions are
needed to increase the molecular weight of the carbon skeleton
from the starting furfural or HMF to the final hydrocarbonated
fuels. These oligomerization reactions can be accomplished in
many different ways,260 though the cross-condensation of either
furfural (self-aldol condensation), or HMF with other carbonyl
compounds are the most-reported one.261–263 These reactions are
conducted in presence of base catalysts, which promote several
aldol-condensations increasing the size of the resulting
compound carbon chain. On the other hand, the dehydration,
hydrogenation and hydrogenolysis reactions take place through
aqueous-phase dehydration/hydrogenation (APD/H) reactions,
involving the use of bi-functional metal-acid catalysts. A large
variety of different combinations of active species for aldol-
condensations, dehydrations and hydrogenation–hydrogenolysis
reactions, has been proposed. Thus, from the homogeneous
water-soluble NaOH to solid base catalysts, such as Mg–Al
oxides, have been described to be effective catalysts in the
promotion of C–C bond formation. In a similar way to APR, Pd,
Pt, Ni, Ru, Rh or Ir,254 and their mixtures and combinations,
have been described as active species in hydrogenation–hydro-
genolysis reactions, whereas, different solids, including solid
acids like alumina, silica-alumina, several metal oxides or
carbon, have been proposed as supports.264,265 Among the
reported results in hydrocarbon production from furfural/HMF,
the most promising results have been achieved when using
bifunctional catalysts. These catalytic systems promote the
7414 | Energy Environ. Sci., 2012, 5, 7393–7420
production of liquid hydrocarbon fuels with targeted chain
lengths (C9–C15 for HMF and C8–C13 for furfural). Some
examples of these bi-functional catalysts are the Pt/NbPO4
catalyst reported by Serrano-Ruiz,266 which leads to an overall
carbon yield of 60%, or the more complex, less expensive Pd/
MgO–ZrO2 catalyst,267 which provides an overall carbon yield of
about 80%.
The bulk chemicals produced during condensation reactions
lead to the separation of an organic layer from the aqueous
reaction media because of the low polarity of the forming
compounds. This fact involves a great advantage, since the
separation of the products from the starting raw materials and
reaction media becomes energetically non-demanding. Further-
more, the use of organic solvents and biphasic systems allows the
enhancement of this spontaneous separation, making the sepa-
ration and product recovering from the reaction media even
easier, as well as the reaction equilibrium is shifted towards the
formation of higher quantities of products. Nevertheless, bearing
in mind that two different liquid layers are produced, and that
most of the catalysts used for catalyzing the condensation reac-
tions are heterogeneous in nature, at least three different phases
coexist inside the reactor. Furthermore, if oxygen removal by
hydrogenolysis–hydrogenation reactions is considered too, the
complexity of the reaction system is even higher, since a fourth
phase has to be added to the three already-considered ones,
because hydrogen plays the central role in APD/H. This high
complexity must be considered in the design of both the catalytic
systems and the reaction equipment in order to control the mass
transfer between the different phases taking place during the
transformation of furans into hydrocarbons.267
Levulinic acid, like HMF, is the other starting raw-material
useful as an intermediate in the transformation of sugars into
hydrocarbons, since the de-oxygenation of LA can lead to
energy-dense chemicals easily upgradable to hydrocarbon fuels.
One of these options passes through the obtention of GVL by
hydrogenolysis of LA,215 followed by lactone ring opening by
acid isomerisation, leading to different isomers of pentenoic acid.
Two different options have been described for taking advantage
of these pentenoic acid isomers: the decarboxylation route268 and
the hydrogenation/ketone condensation to form larger hydro-
carbon structures (see Fig. 20). The decarboxylation route,
applied to pentenoic acids, leads to the formation of a mixture of
butene isomers. In this transformation, acid catalysts, such as
silica-alumina, can be used to promote the decarboxylation
reaction, even starting from GVL,268 a catalytic activity also
found when treating other lactones. Nevertheless, the resultant
olefins can then be processed in an alkylation unit, for instance in
a butamer process, using the same procedure used to transform
butenes into gasolines in a conventional refinery fed with crude
oil. Alternatively, the use of a strong solid acid catalyst, such as
HZSM-5 or a sulfonic acid-functionalized resin, as suggested by
Dumesic and co-workers,269 can promote the oligomerization of
the resultant butenes into larger i-alkanes, allowing the produc-
tion of both gasoline and jet fuel, though at a substantial lower
production rate than the commercial process. Anyhow, the real
advantage of this option is the low H2 consumption, which
makes the decarboxylation pathway economically desirable. A
proof of this is that the techno-economic analysis of this option
(GVL decarboxylation followed by oligomerization of butenes)
This journal is ª The Royal Society of Chemistry 2012
Fig. 20 Conversion of GVL into hydrocarbon fuels.
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reveals it as a cost-effective route to hydrocarbon fuels from
lignocellulosic biomass.270 The second option to take advantage
of GVL as key molecule in the transformation of LA into
hydrocarbon fuels is the production of pentanoic acid (PA) (by
hydrogenation of pentenoic acids coming from GVL isomeriza-
tion). Later, PA is submitted to base-catalyzed ketone oligo-
merization. The hydrogenation of pentenoic to pentanoic acids
can be accomplished in presence of a water stable Pd/Nb2O5
catalyst, whereas ketonization can be either accomplished with
Ce0.5Zr0.5O2271,272 or with the same Pd/Nb2O5
273 catalyst used for
hydrogenation, but with longer contact times, compared to the
basic catalyst. The resultant 5-nonanone spontaneously sepa-
rates from water, and it is subsequently hydrogenated into the
corresponding alcohol. The C9 alcohol can be transformed into
n-nonane or nonene by hydrogenation/dehydration. The last
part of the process is the isomerisation of the resultant C9
hydrocarbons to achieve the required properties for the desired
fuel.
4. Conclusions and final remarks
There are potential analogies between the 20th century petroleum
refinery and the 21st century biorefinery. In the beginning, the
petroleum refinery made few products and incorporated little
chemical and energy integration. Development of the petroleum
refinery took considerable effort to become the highly efficient
and integrated system that it is nowadays, and most of the
breakthroughs that allowed this remarkable transformation
involved enhanced catalytic technologies. Current biorefineries,
which are still in their tender infancy, produce relatively few
chemicals and fuels and most of the processes involve little
chemical and energy integration. In analogy to the history of
petrol industry, with the development and updating of different
chemical and biochemical conversion technologies, the bio-
refinery can also become an efficient and highly integrated system
to meet the chemical and energy requirements of the 21st century.
In order to materialize this system, serious efforts must be
addressed to the development of new infrastructures dealing with
the setting-up and optimisation of new logistic chain systems able
to provide huge amounts of biomass-derived feedstock to cen-
tralised transformation centres like conventional refineries. This
is a crucial step in the insertion of biomass raw-materials into the
production schemes of current refineries. Furthermore, efficient
biochemical processes, integrating different conversion technol-
ogies and industrial scaling-up has to be considered in the feeding
of biomass-derived feedstock to the current units in refineries.
This journal is ª The Royal Society of Chemistry 2012
Nevertheless, a significant percentage of the chemical conversion
technologies available in a petroleum refinery can also be used in
biomass transformation into valuable fuels and chemicals. In this
review different economically attractive options have been
identified for the integration of biorenewable feedstock and
biofuels in petroleum refineries.
Low quality vegetable oils and greases are likely to be promi-
sing in a short-to-medium term to yield green diesel and jet-fuel
by means of hydroprocessing of triglyceride-based feedstock. In
fact, there are several countries where these fuels are already
commercialised. Moreover, these are synthesized using
commercially available refining technology, though their
production is limited to a small fraction of liquid transport fuels,
mainly due to the limited availability of the feedstock. However,
some aspects of these hydrotreatments still need more attention.
From an economical and environmental point of view, mini-
mizing unnecessary hydrogen consumption is crucial and further
insights must be addressed. In this sense, the development of
active and stable catalysts for hydrogen-free catalytic deoxy-
genation is presented as a good alternative to the expensive
hydrodeoxygenation–hydrodecarboxylation conversion routes.
Likewise, to ensure optimal co-processing of triglycerides with
petroleum fractions in existing refinery units, it is critical to fully
understand the effect of triglycerides (feed and conversion
products, especially water) on the processing of the petroleum
fraction. In this sense, the use of a separate modular unit where
processing conditions are optimized for the triglyceride based
feedstock is an attractive approach.
Pyrolysis oil processing requires larger efforts in commercial
development, since the commercial production of this substance
is still in its infancy, apart from the numerous drawbacks found
in its upgrading to biofuels. Due to the poor quality of the bio-
oils, the conventional hydrotreating catalysts are expected to
have a considerably lower catalyst life in bio-oil upgrading
operations than that observed with petroleum feedstock. While
the current generation commercial catalysts are excellent
hydroprocessing catalysts, they are optimized for petroleum
feedstock. Since the bio-oils have significantly different proper-
ties than petroleum feedstock, it would be worthwhile to dedicate
efforts to developing catalysts specifically designed for upgrading
bio-oils. From a widespread commercial application viewpoint,
an ideal catalyst for bio-oil upgrading should have the attributes
as follows: high activity for deoxygenation; ability to withstand
large quantities of coke and/or minimize coke formation; high
tolerance to water and poisons; and high availability with
a competitive cost. We honestly think that, in the long term, the
huge potential volume of pyrolysis oil coming from large amount
of available lignocellulosic wastes might replace shortages in
petroleum fuel and thus, developing this technology would be
one of the milestones, in the medium term, in substituting crude
oil as energy source.
Ethanol, the main biomass-derived fuel used today, is widely
used in refineries for the formulation of gasoline. However,
ethanol suffers from important limitations as a fuel (e.g., low
energy density, high solubility in water, etc.) that can be over-
come by designing strategies to convert non-edible lignocellulosic
biomass into liquid hydrocarbon fuels chemically similar to those
currently used in internal combustion engines. The present
review has described the main routes available to carry out such
Energy Environ. Sci., 2012, 5, 7393–7420 | 7415
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deep chemical transformation with particular emphasis on those
pathways involving aqueous-phase catalytic reactions. These
routes offer the opportunity to selectively carry out a variety of
reactions to achieve the deep chemical transformations and C–C
coupling reactions required when converting sugars into liquid
hydrocarbons. Nonetheless, aqueous-phase routes require
lignocellulosic biomass to be subjected to pretreatment/hydro-
lysis steps to yield water-soluble sugars (and platform chemicals
derived from them). Unfortunately, the economic trans-
formation of lignocellulosic biomass into soluble sugar and
platform molecules represents a major challenge as a conse-
quence of the complex nature of the biomass and the presence of
non-cellulose components. As described in this review, aqueous-
phase routes for the transformation of sugars and derivatives
into hydrocarbons is carried out with multi-step processes and
requires the use of fossil fuel-based hydrogen sources. In this
sense, the insertion of lignocellulosic derived platform molecules
as raw materials in the supply chain of conventional refineries
seems not to be probable in the near future and further
improvements in the existing technology for their transformation
into fuels have to be firstly addressed. However, it must also be
pointed out that most of these catalytic transformations might be
achieved by using conventional heterogeneous catalysts, though
their improvement and the development of more selective cata-
lysts remains a key technical barrier which seems to be
unavoidable for the future.
Most of the biomass conversion processes carried out in
a refinery need a high amount of hydrogen in order to remove
oxygen and yield high energy density fuels. It is likely that, in the
future, hydrogenwouldhave tobeproducedbymeansof renewable
energy sources such as the sun, wind, or biomass. Hopefully, the
transformation of carbohydrates towards hydrogen using APR
processes might be a good alternative to current H2 sources,
supplying renewable hydrogen in future refineries.
Although biomass valorization can be performed on current
commercially available petroleum-based technology, it should be
considered that petroleum and biomass feedstocks are very
different from a chemical point of view. It seems that heteroge-
neous catalysis, which has made it possible to convert efficiently
petroleum-derived resources to fuels, will also be able to provide
the necessary technology to get similar fuels starting from
biomass feedstock. However, it is most likely that applying the
‘‘old’’ catalyst technology, developed for petroleum refining, to
renewable biomass-derived substrates will not be enough.
Intensified efforts would have to be applied in the development
of new heterogeneous catalytic materials, based on a specific
design considering the nature of the renewable feedstock, a fact
which also constitutes new catalytic opportunities.
Acknowledgements
Financial support from the Spanish Ministry of Economy and
Competitiveness and from the Regional Government of Madrid
through the projects CTQ2011-28216-C02–01 and S2009-
ENE1743, respectively, is kindly acknowledged.
Notes and references
1 F. Cherubini, Energy Convers. Manage., 2010, 51, 1412.2 A. Demirbas, Appl. Energy, 2009, 86, 151.
7416 | Energy Environ. Sci., 2012, 5, 7393–7420
3 A. Demirbas, Energy Convers. Manage., 2009, 50, 2782.4 S. Octave and D. Thomas, Biochimie, 2009, 91, 659–664.5 S. Fernando, S. Adhikari, C. Chandrapal and N. Murali, EnergyFuels, 2006, 20, 1727.
6 G. W. Huber and A. Corma, Angew. Chem., Int. Ed., 2007, 46,7184.
7 J. Holmgren, C. Gosling, R. Marinangeli, T. Marker, G. Faraci andC. Perego, Hydrocarbon Process., Int. Ed., 2007, 9, 67–71.
8 S. Lestari, P. M€aki-Arvela, J. Beltramini, G. Q. Max Lu andD. Y. Murzin, ChemSusChem, 2009, 2, 1109.
9 J. Holmgren, R. Marinangeli, T. Marker, M. Mc Call, J. Petri,S. Czernik, D. Elliott and D. Shonnard, Hydrocarbon Eng., 2007.
10 B. Brehmer, R. M. Boom and J. Sanders, Chem. Eng. Res. Des.,2009, 87, 1103.
11 M. Crocker and R. Andrews, The Rationale for Biofuels, inThermochemical Conversion of Biomass to Liquid Fuels andChemicals, ed. M. Crocker, Royal Society of Chemistry, 2010.
12 H. C. Greenwell, L. M. L. Laurens, R. J. Shields, R. W. Lovitt andK. J. Flinn, J. R. Soc. Interface, 2009, 7, 703.
13 R. H. Wijffels, M. J. Barbosa and M. H. M. Eppink, Biofuels,Bioprod. Biorefin., 2010, 4, 287.
14 N. H. Tran, J. R. Bartlett, G. S. K. Kannangara, A. S. Milev,H. Volk and M. A. Wilson, Fuel, 2010, 89, 265.
15 Feedstock Composition Glossary, http://www.1.eere.energy.gov/biomass/feedstock_glossary.html.
16 M. Hoogwijk, A. Faaij, R. van den Broek, G. Berndes, D. Gielenand W. Turkenburg, Biomass Bioenergy, 2003, 25, 119.
17 J. Heinim€o and M. Junginger, Biomass Bioenergy, 2009, 33, 1310.18 A. Milbrandt, A Geographic Perspective on the Current Biomass
Resource Availability in the United States, Tech. Rep NREL/TP-560–39181, National Renewable Energy Laboratory, 2005.
19 A. Nikolau, M. Remrova and I. Jeliazkov, Biomass Availability inEurope, Appendix to: Lot 5: Bioenergy’s Role in the EU EnergyMarket, 2003.
20 National Renderers Association, US Production, Consumption andExport of Rendered Products for 2004–2009, http://nationalrenderers.org/economic/statistics/, 2010.
21 European Comission statistics database, Eurostat, http://epp.eurostat.ec.europa.eu/portal/page/statistics/search_database,2011.
22 K. S. Tyson, Brown Grease Feedstocks for Biodiesel, NationalRenewable Energy Laboratory, 2002.
23 K. S. Tyson, Brown Grease to Biodiesel, Oak Ridge NationalLaboratory, 2008.
24 SEAI, A Resource Study on Recovered Vegetable Oil and AnimalFats, Sustainable Energy Authority of Ireland, 2003.
25 N. Y. Chen, J. T. F. Degnan and L. R. Koening, Chem. Tech., 1986,16, 506.
26 S. C. Bhattacharya, S. Sett and R. M. Shrestha, Energy Sources,1989, 11, 161.
27 J. S. Tumuluru, C. T. Wright, J. R. Hess and K. L. Kenney, Biofuels,Bioprod. Biorefin., 2011, 5, 683–707.
28 A. Demirbas, J. Anal. Appl. Pyrolysis, 2004, 71, 803.29 P. S. Lam, S. Sokhansanj, X. Bi, C. J. Lim and S. Melin, Energy
Fuels, 2011, 25, 1521.30 M. J. C. van der Stelt, H. Gerhauser, J. H. A. Kiel and
K. J. Ptasinski, Biomass Bioenergy, 2011, 35, 3748.31 J. J. Chew and V. Doshi, Renewable Sustainable Energy Rev., 2011,
15, 4212.32 T. L. Chew and S. Bhatia, Bioresour. Technol., 2008, 99, 7911.33 A. Demirbas, Energy Convers. Manage., 2000, 41, 633.34 S. Yaman, Energy Convers. Manage., 2004, 45, 651.35 D.Mohan, C. U. Pittman Jr and P. H. Steele, Energy Fuels, 2006, 20,
848.36 J. D. Adjaye, R. K. Sharma and N. N. Bakhshi, Fuel Process.
Technol., 1992, 31, 241.37 H.-J. Huang, S. Ramaswamy, U. W. Tschirner and B. V. Ramarao,
Sep. Purif. Technol., 2008, 62, 1.38 I. T. Horv�ath, H. Mehdi, V. F�abos, L. Boda and L. T. Mika, Green
Chem., 2008, 10, 238.39 K. J. Zeitsch, The Chemistry and Technology of Furfural and its
Many By-products, in Sugar Series, Elsevier, Amsterdam, 1st edn,2000, vol. 13, pp. 34–69.
40 A. A. Rosatella, S. P. Simeonov, R. F. M. Frade andC. A. M. Alfonso, Green Chem., 2011, 13, 754.
This journal is ª The Royal Society of Chemistry 2012
Publ
ishe
d on
30
Mar
ch 2
012.
Dow
nloa
ded
by U
nive
rsity
of
Bel
grad
e on
04/
11/2
014
10:1
3:41
. View Article Online
41 S. Morikawa and Y. Nakamura, Bull. Chem. Soc. Jpn., 1980, 53,3705.
42 Y. Roman-Leshkov, J. N. Chheda and J. A. Dumesic, Science, 2006,312, 1933.
43 J. N. Chheda, Y. Roman-Leshkov and J. A. Dumesic, Green Chem.,2007, 9, 342.
44 M. E. Zakrzewska, E. Bogel-Lukasik and R. Bogel-Lukasik, Chem.Rev., 2011, 111, 397.
45 A. Takagaki, M. Ohara, S. Nishimura and K. Ebitani, Chem.Commun., 2009, 6276.
46 M. Ohara, A. Takagaki, S. Nishimura and K. Ebitani, Appl. Catal.,A, 2010, 383, 149.
47 C. V. McNeff, D. T. Nowlan, L. C. McNeff, B. Yan and R. L. Fedie,Appl. Catal., A, 2010, 384, 65.
48 S. W. Fitzpatrick, US Pat., No. 4,897,497, 1990.49 S. W. Fitzpatrick, US Pat., No. 5,608,105, 1997.50 H. Heeres, R. Handana, D. Chunai, C. B. Rasrendra, B. Girisuta
and H. J. Heeres, Green Chem., 2009, 11, 1247.51 IEA, Oil Market Report, International Energy Agency, 2011.52 H. An, W. E. Wilhelm and S. W. Searcy, Biomass Bioenergy, 2011,
35, 3763.53 S. Gold and S. Seuring, J. Clean. Prod., 2011, 19, 32.54 D. Inman, N. Nagle, J. Jacobson, E. Searcy and A. E. Ray, Biofuels,
Bioprod. Biorefin., 2010, 4, 562.55 S. Sokhansanj, A. Kumar and A. F. Turhollow, Biomass Bioenergy,
2006, 30, 838.56 X. Zhu, X. Li, Q. Yao and Y. Chen, Bioresour. Technol., 2011, 102,
1344.57 M. Ebadian, T. Sowlati and S. Sokhansanj, Biosyst. Eng., 2011, 110,
280.58 A. A. Rentizelas, A. J. Tolis and I. P. Tatsiopoulos, Renewable
Sustainable Energy Rev., 2009, 13, 887.59 P. P. Ravula, R. D. Grisso and J. S. Cundiff, Bioresour. Technol.,
2008, 99, 5710.60 J. D. Stephen, W. E. Mabee and J. N. Saddler, Biofuels, Bioprod.
Biorefin., 2010, 4, 503.61 Z. Miao, Y. Shastri, T. E. Grift, A. C. Hansen and K. C. Ting,
Biofuels, Bioprod. Biorefin., 2012, DOI: 10.1002/bbb.62 P. C. Badger and P. Fransham, Biomass Bioenergy, 2006, 30, 321–
325.63 P. L. eranki, B. D. Bals and B. E. Dale, Biofuels, Bioprod. Biorefin.,
2011, 5, 621.64 C. Sheng-Goh and K. Teong-Lee, Renewable Sustainable Energy
Rev., 2010, 14, 2986.65 J. R. Hess, C. T. Wright and K. L. Kenney, Biofuels, Bioprod.
Biorefin., 2007, 1, 181.66 S. D. Eksxio�glu, A. Acharya, L. E. Leightley and S. Arora, Comput.
Ind. Eng., 2009, 57, 1342.67 Y. Shastri, A. Hansen, L. Rodr�ıguez and K. C. Ting, Biomass
Bioenergy, 2011, 35, 2961.68 J. D. Judd, S. C. Sarin and J. S. Cundiff, Bioresour. Technol., 2012,
103, 209.69 T. L. Richard, Science, 2010, 329, 793.70 M. Hara, Energy Environ. Sci., 2010, 3, 601.71 Neste Oil, http://www.nesteoil.com, accessed December, 2011.72 R. Marinangeli, T. Marker, J. Petri, T. Kalnes, M. McCall,
D. Mckowiak, B. Jerosky, B. Reagan, L. Nemeth, M. Krawcyk,S. Czernik, D. Elliott and D. Shornnar, Opportunities forBiorenewables in Oil Refineries, Report No. DE-GD36–05GO15085, UOP, 2006.
73 A. Corma, P. J. Miguel and A. V. Orchilles, J. Catal., 1994, 145,171.
74 N. O. V. Sonntag, Structure and Composition of Fats and Oils, JonhWiley & Sons Inc., New York, 1979.
75 A. B. Chhetri, K. C. Watts and M. R. Islam, Energies, 2008, 1, 3.76 J. Iglesias and J. Morales, Biodiesel Production fromWaste Oils and
Fats, in Advances in Biodiesel Production: Processes andTechnologies, Woodhead Publishing Limited, Cambridge, ISBN:978-0-85709-117-8, 2012.
77 Y. S. Prasad and N. N. Bakhshi, Appl. Catal., 1985, 18, 71.78 Y. S. Prasad, N. N. Bakhshi, J. F. Mathews and R. L. Eager, Can. J.
Chem. Eng., 1986, 64, 278.79 Y. S. Prasad, N. N. Bakhshi, J. F. Mathews and R. L. Eager, Can. J.
Chem. Eng., 1986, 64, 285.80 T. A. Milne, R. J. Evans and N. Nagle, Biomass, 1990, 21, 219.
This journal is ª The Royal Society of Chemistry 2012
81 S. P. R. Katikaneni, J. D. Adjaye and N. N. Bakhshi, Energy Fuels,1995, 9, 599.
82 S. P. R. Katikaneni, J. D. Adjaye and N. N. Bakhshi, Can. J. Chem.Eng., 1995, 73, 484.
83 S. P. R. Katikaneni, J. D. Adjaye, R. O. Idem and N. N. Bakhshi,Ind. Eng. Chem. Res., 1996, 35, 3332.
84 R. O. Idem, S. P. R. Katikaneni and N. N. Bakhshi, Fuel Process.Technol., 1997, 51, 101.
85 S. Bhatia, J. K. Heng,M. L. Lim andA. R.Mohamed, Production ofBio-fuel by Catalytic Cracking of Palm Oil: Performance ofDifferent Catalysts, in Proceedings of the Biofuel PORIMInternational Biofuel and Lubricant Conference, Malaysia, 4–5 May1998.
86 T. Y. Leng, A. R. Mohamed and S. Bhatia, Can. J. Chem. Eng.,1999, 77, 156.
87 F. A. Twaiq, N. A. M. Zabidi and S. Bhatia, Ind. Eng. Chem. Res.,1999, 38, 3230.
88 Y. S. Ooi, R. Zakaria, A. R. Mohamed and S. Bhatia, Energy Fuels,2005, 19, 736.
89 F. A. Twaiq, A. R. Mohamed and S. Bhatia, MicroporousMesoporous Mater., 2003, 64, 95.
90 F. A. Twaiq, A. R. Mohamed and S. Bhatia, Fuel Process. Technol.,2003, 84, 105.
91 Y. S. Ooi, R. Zakaria, A. R. Mohamed and S. Bhatia, Appl. Catal.,A, 2004, 274, 15.
92 Y. S. Ooi and S. Bhatia,Microporous Mesoporous Mater., 2007, 102,310.
93 S. Bhatia, A. R. Mohamed and N. A. A. Shah, Chem. Eng. J., 2009,155, 347.
94 D. G. B. Boocock, S. K. Konar, A. Mackay, P. T. C. Cheung andJ. Liu, Fuel, 1992, 71, 1291.
95 E. Vonghia, D. G. B. Boocock, S. K. Konar and A. Leung, EnergyFuels, 1995, 9, 1090.
96 J. A. Melero, A. Garcia and M. Clavero, Production of Biofuels viaCatalytic Cracking, in Handbook of Biofuels Production: Processesand Technologies, Woodhead Publishing Limited, Cambridge,2010, pp. 390–419.
97 A. Osmont, L. Catoire, I. Gkalp and M. T. Swihart, Energy Fuels,2007, 21, 2027.
98 K. D. Maher and D. C. Bressler, Bioresour. Technol., 2007, 98,2351.
99 D. Pioch, P. Lozano, M. C. Rasoanantoandro, J. Graille, P. Genesteand A. Guida, Oleagineux, 1993, 48, 289.
100 S. P. R. Katikaneni, J. D. Adjave, R. O. Idem and N. N. Bakhshi,Dev. Thermochem. Biomass Convers., 1997, 1, 633.
101 X. Dupain, D. J. Costa, C. J. Schaverien and M. Makkee, Appl.Catal., B, 2007, 72, 44.
102 K. V. Padmaja, N. Atheya, A. K. Bhatnagar and K. K. Singh, Fuel,2009, 88, 780.
103 H. Li, P. Yu and B. Shen, Fuel Process. Technol., 2009, 90, 1087.104 J. A. Melero, M. Clavero, G. Calleja, A. Garcia, R. Miravalles and
T. Galindo, Energy Fuels, 2010, 24, 707.105 P. Bielansky, A. Reichhold and C. Sch€onberger, Chem. Eng.
Process., 2010, 49, 873.106 P. Tamunaidu and S. Bhatia, Bioresour. Technol., 2007, 98, 3593.107 H. Tian, C. Li, C. Yang and H. Shan, Chin. J. Chem. Eng., 2008, 16,
394.108 T. L. Chew and S. Bhatia, Bioresour. Technol., 2009, 100, 2540.109 P. B. Weisz, W. O. Haag and P. G. Rodewald, Science, 1979, 206, 57.110 W. O. Haag, P. G. Rodewald and P. B. Weisz, Catalytic Production
of Aromatics and Olefins from Plant Materials, in Abstracts ofPapers of the American Chemical Society, Las Vegas, 24–25August 1980.
111 L. Dandik and H. A. Aksoy, Converting used oil to fuel andchemical feedstock through a fractionating pyrolysis reactor, inProceedings of the World Conference on Oilseed and Edible OilProcessing, Istanbul, Turkey, 1998.
112 S. M. Sadrameli, A. E. S. Green and W. Seames, J. Anal. Appl.Pyrolysis, 2009, 86, 1.
113 A. Buchsbaum, K. Hutter, F. Danzinger and J. Lichtscheidl, TheChallenge of the Biofuels Directive for a European Refinery, inOMW Refining and Marketing, ERTC 9th Annual Meeting,Prague, Czech Republic, 2004.
114 K. Bormann, H. Tilgner and H. J. Moll, Erdoel, Erdgas, Kohle, 1993,4, 172.
Energy Environ. Sci., 2012, 5, 7393–7420 | 7417
Publ
ishe
d on
30
Mar
ch 2
012.
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nloa
ded
by U
nive
rsity
of
Bel
grad
e on
04/
11/2
014
10:1
3:41
. View Article Online
115 K. Bormann and H. Tilgner, Erdoel, Erdgas, Kohle, 1994, 2, 75.116 J. C. Carlos de Medeiros, J. M. Fusco and J. A. R. Cabral, Brasil
Pat., No. 8,304,794, 1985.117 A. D. R. Pinho, M. Silva, A. P. Da Silva Neto and J. A. R. Cabral,
Catalytic Cracking Process and Catalysts for Production of DieselFuel from Pure and Waste Vegetable Oils, P.B.S.A. (Petrobras),Brazil, US 2007/0007176, 2007.
118 J. D. Adjaye, S. P. R. Katikaneni and N. N. Bakhshi, Can. J. Chem.Eng., 1995, 73, 484.
119 C. Ramakrishan, Umfassende Untersuchungen zur KatalytischenKonversion von Bio€olen in einer volkontinuierlichen FCC–Technikumsalange, in Institute of Chemical Engineering, ViennaUniversity of Technology, Vienna, 2004.
120 M. J. McCall, T. L. Marker, J. Petri, D. Mackowiak, S. Czernik,D. Elliott and D. Shonnard, Abstracts of the 230th AmericanChemical Society National Meeting (Washington DC, USA),2005.
121 M. Stumborg, A.Wong and E. Hogan,Bioresour. Technol., 1996, 56,13.
122 D. W. Soveran, M. Sulatisky, K. Ha, W. Robinson andM. Stumborg, Symposium on Impact of Environmental Constraintson Fuel Composition. Meeting of the ACS Division of FuelChemistry, San Francisco, US, 1992.
123 H. Aatola, M. Larmi, T. Sarjovaara and S. Mikkonen, SAE Int. J.Engines, 2008, 1, 2500.
124 B. Donnis, R. G. Egeberg, P. Blom and K. G. Knudsen, Top. Catal.,2009, 52, 229.
125 Q. Semejkal, L. Smejkalov�a and D. Kubi�cka, Chem. Eng. J., 2009,146, 155.
126 M. Sn�are, P. M€aki-Arvela, I. L. Simakova, J. Myllyoja andD. Y. Murzin, Russ. J. Phys. Chem. B, 2010, 3, 1035.
127 A. S. Berenblyum, V. Y. Danyushevsky, E. A. Katsman,T. A. Podoplelova and V. R. Flid, Pet. Chem., 2010, 50, 305.
128 T. V. Choudhary and C. B. Phillips, Appl. Catal., A, 2011, 397, 1.129 D. Kubi�cka, Collect. Czech. Chem. Commun., 2008, 73, 1015.130 I. V. Babich and J. A. Moulijn, Fuel, 2003, 82, 607.131 M. Ferrari, R. Maggi, B. Delmon and P. Grange, J. Catal., 2001,
198, 47.132 E. Laurent and B. Delmon, Appl. Catal., A, 1994, 109, 97.133 O. I. Sxenol, T. R. Viljava and A. O. I. Krause, Catal. Today, 2005,
100, 331.134 O. I. Sxenol, T. R. Viljava and A. O. I. Krause, Catal. Today, 2005,
106, 186.135 R. G. Leliveld, A. J. van Dillen, J. W. Geus and
D. C. Koningsberger, J. Catal., 1998, 175, 108.136 M. Kr�ar, S. Kov�acs, D. Kall�o and J. Hancs�ok, Bioresour. Technol.,
2010, 101, 9287.137 S. Bezergianni, A. Dimitriadis, A. Kalogianni and P. A. Pilavachi,
Bioresour. Technol., 2010, 101, 6651.138 S. Bezergianni, A. Dimitriadis, T. Sfetsas and A. Kalogianni,
Bioresour. Technol., 2010, 101, 7658.139 J. Gusmao, D. Brodzki, G. Dj�ega-Mariadassou and R. Frety, Catal.
Today, 1989, 5, 533.140 G. N. da Rocha Filho, D. Brodzki and G. Dj�ega-Mariadassou, Fuel,
1993, 72, 543.141 P. �Sim�a�cek, D. Kubi�cka, G. �Sebor and M. Posp�ı�sil, Fuel, 2009, 88,
456.142 D. Kubi�cka, P. �Sim�a�cek and P. �Zilkov�a, Top. Catal., 2008, 52, 161.143 Y. Liu, R. Sotelo-Boy�as, K. Murata, T. Minowa and K. Sakanishi,
Chem. Lett., 2009, 38, 552.144 D. Kubi�cka, M. Bejblov�a and J. Vlk, Top. Catal., 2009, 53, 168.145 G. W. Huber, P. O’Connor and A. Corma, Appl. Catal., A, 2007,
329, 120.146 A. A. Lappas, S. Bezergianni and I. A. Vasalos, Catal. Today, 2009,
145, 55.147 I. Sebos, A. Matsoukas, V. Apostolopoulos and N. Papayannakos,
Fuel, 2009, 88, 145.148 S. Bezergianni, A. Kalogianni and I. A. Vasalos, Bioresour. Technol.,
2009, 100, 3036.149 V. Mayeur, C. Vergel and L. Mariette, World Pat., No. 12,415,
2008.150 T. Kalnes, T.Marker andD. R. Shonnard, Int. J. Chem. React. Eng.,
2007, 5, A48.151 P. O’Connor, D. Stamires, A. Corma and G. W. Huber, US Pat.,
No. 20, 048, 2008.
7418 | Energy Environ. Sci., 2012, 5, 7393–7420
152 V. Markkanen, P. Lindquist, E. Harlin, P. Aalto, J. Myllyoja andV. Alopaeus, World Pat., No. 934, 2010.
153 T. N. Kalnes, K. P. Koers, T. Marker and D. R. Shonnard, Environ.Prog. Sustainable Energy, 2009, 28, 111.
154 F. Baldiraghi, M. di Stanislao, G. Faraci, C. Perego, T. Marker,C. Gosling, P. Kokayeff, T. Kalnes and R. Marinangeli,Ecofining: New Process for Green Diesel Production fromVegetable Oil, in Sustainable Industrial Catalysis, ed. F. Cabani,G. Centi, S. Perathoner and F. Trifir�o, Wiley–VCH Verlag,Weinheim, 2009, pp. 427–438.
155 S. O. G€artner, G. H. Helms, G. Reinhardt and N. Rettenmaier,Institute for Energy and Environmental Research, Heidelberg,Germany, 2007.
156 R. Abhari, H. L. Tomlinson, P. Z. Havlik and N. L. Jannasch, USPat., No. 0244962, 2008.
157 R. Abhari and P. Z. Havlik, US Pat., No. 8,026,401, 2009.158 R. Abhari, H. L. Tomlinso and E. G. Roth, US Pat., No. 300, 971,
2009.159 S. P. Pyl, C. M. Schietekat, M.-F. Reyniers, R. Abhari, G. B. Marin
and K. V. van Geem, Chem. Eng. J., 2011, 176–177, 178.160 M. Sn�are, I. Kubi�ckov�a, P. M€aki-Arvela, K. Er€anen and
D. Y. Murzin, Ind. Eng. Chem. Res., 2006, 45, 5707.161 I. Simakova, O. Simakova, P. M€aki-Arvela, A. Simakov,M. Estrada
and D. Y. Murzin, Appl. Catal., A, 2009, 355, 100.162 I. Kubi�ckov�a, M. Sn�are, K. Er€anen, P. M€aki-Arvela and
D. Y. Murzin, Catal. Today, 2005, 106, 197.163 M. Sn�are, I. Kubi�ckov�a, P. M€aki-Arvela, D. Chichova, K. Er€anen
and D. Y. Murzin, Fuel, 2008, 87, 933.164 P. M€aki-Arvela, I. Kubi�ckov�a, M. Sn�are, K. Er€anen and
D. Y. Murzin, Energy Fuels, 2007, 21, 30.165 J. G. Immer, M. J. Kelly and H. H. Lamb,Appl. Catal., A, 2010, 375,
134.166 J. G. Immer and H. H. Lamb, Energy Fuels, 2010, 24, 5291.167 T. Morgan, D. Grubb, E. Santillan-Jimenez and M. Crocker, Top.
Catal., 2010, 53, 820.168 E. W. Ping, R. Wallace, J. Pierson, T. F. Fuller and C. W. Jones,
Microporous Mesoporous Mater., 2010, 132, 174.169 E. W. Ping, J. Pierson, R. Wallace, J. T. Miller, T. F. Fuller and
C. W. Jones, Appl. Catal., A, 2011, 396, 85.170 A. S. Berenblyum, T. A. Podoplelova, R. S. Shamsiev,
E. A. Katsman and V. Ya. Danyushevsky, Pet. Chem., 2011, 51,336.
171 P. T. Do, M. Chiappero, L. L. Lobban and D. E. Resasco, Catal.Lett., 2009, 130, 9.
172 J.-G. Na, B. E. Yi, J. N. Kim, K. B. Yi, S.-Y. Park, J.-H. Park,J.-N. Kim and C. H. Ko, Catal. Today, 2010, 156, 44.
173 T. Morgan, E. Santillan-Jimenez, A. E. Harman-Ware, Y. Ji,D. Grubb and M. Crocker, Chem. Eng. J., 2012, 189–190, 346.
174 H.-S. Roh, I.-H. Eum, D.-W. Jeong, B. E. Yi, J.-G. Na andC. H. Ko, Catal. Today, 2011, 164, 457.
175 W. L. Roberts, H. H. Lamb, L. F. Stikeleather and T. L. Turner,USPat., No. 105, 813, 2011.
176 A. Garc�ıa, I. Eg€ues, A. Toledano, M. Gonz�alez, L. Serrano andJ. Labidi, Chem. Eng. Trans., 2009, 18, 911.
177 H. J. Huang, S. Ramaswamy, W. Al-Dajani, U. Tschirner andR. A. Cairncross, Biomass Bioenergy, 2009, 33, 234.
178 P. M. Mortensen, J.-D. Grunwaldt, P. A. Jensen, K. G. Knudsenand A. D. Jensen, Appl. Catal., A, 2011, 407, 1.
179 A. V. Bridgwater, Appl. Catal., A, 1994, 116, 5.180 J. D. Adjaye and N. N. Bakhshi, Biomass Bioenergy, 1995, 8, 131.181 J. D. Adjaye and N. N. Bakhshi, Fuel Process. Technol., 1995, 45,
161.182 J. D. Adjaye and N. N. Bakhshi, Fuel Process. Technol., 1995, 45,
185.183 S. P. R. Katikaneni, J. D. Adjaye and N. N. Bakhshi, Energy Fuels,
1995, 9, 1065.184 R. K. Sharma and N. N. Bakhshi, Energy Fuels, 1993, 7, 306.185 S. T. Srinivas, A. K. Dalai and N. N. Bakhshi, Can. J. Chem. Eng.,
2000, 78, 343.186 A. G. Gayubo, A. T. Aguayo, A. Atutxa, R. Aguado and J. Bilbao,
Ind. Eng. Chem. Res., 2004, 43, 2610.187 A. G. Gayubo, A. T. Aguayo, A. Atutxa, R. Aguado,M. Olazar and
J. Bilbao, Ind. Eng. Chem. Res., 2004, 43, 2619.188 A. G. Gayubo, A. T. Aguayo, A. Atutxa, B. Valle and J. Bilbao, J.
Chem. Technol. Biotechnol., 2005, 80, 1244.
This journal is ª The Royal Society of Chemistry 2012
Publ
ishe
d on
30
Mar
ch 2
012.
Dow
nloa
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by U
nive
rsity
of
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grad
e on
04/
11/2
014
10:1
3:41
. View Article Online
189 A. G. Gayubo, B. Valle, A. T. Aguayo, M. Olazar and J. Bilbao, Ind.Eng. Chem. Res., 2010, 49, 123.
190 A. G. Gayubo, B. Valle, A. T. Aguayo, M. Olazar and J. Bilbao,Energy Fuels, 2009, 23, 4129.
191 I. Graca, F. Ramoa Ribeiro, H. S. Cerqueira, Y. L. Lam andM. B. B. de Almeida, Appl. Catal., B, 2009, 90, 556.
192 E. Furimsky, Appl. Catal., A, 2000, 199, 147.193 E. Furimsky, J. A. Mikhlin, D. Q. Jones, T. Adley andH. Baikowitz,
Can. J. Chem. Eng., 1986, 64, 982.194 E. Laurent and B. Delmon, Appl. Catal., A, 1994, 109, 77.195 D. C. Elliott, E. G. Baker, J. Piskorz, D. S. Scott and Y. Solantausta,
Energy Fuels, 1988, 2, 234.196 D. C. Elliott and G. G. Neuenschwander, Liquid Fuels by Low-
Severity Hydrotreating of Biocrude, in Developments inThermochemical Biomass Conversion, ed. A. V. Bridgwater and D.G. B. Boocock, Blackie Academic and Professional, London, 1996,vol. 1.
197 Y.-H. E. Sheu, R. G. Anthony and E. J. Soltes, Fuel Process.Technol., 1988, 19, 31.
198 S. T. Ramanathan and S. T. Oyama, J. Phys. Chem., 1995, 99, 16365.199 F. de Miguel Mercader, M. J. Groeneveld, S. R. A. Kersten,
N. W. J. Way, C. J. Schaverien and J. A. Hogendoorn, Appl.Catal., B, 2010, 96, 57.
200 M. C. Samolada, W. Baldauf and A. Vasalos, Fuel, 1998, 77, 1667.201 G. Fogassy, N. Thegarid, G. Toussaint, A. C. van Veen,
Y. Schuurman and C. Mirodatos, Appl. Catal., B, 2010, 96, 476.202 G. Fogassy, N. Theragarid, Y. Schuurman and C. Mirodatos,
Energy Environ. Sci., 2011, 4, 5068.203 B. H. Shanks, Ind. Eng. Chem. Res., 2010, 49, 10212.204 G. J. M. Gruter and F. Dautzenberg, Eur. Pat., No. 1,834,950,
2007.205 G. J. M. Gruter and F. Dautzenberg, Eur. Pat., No. 2,050,742, 2007.206 L. Cottier, G. Descotes, L. Eymard and K. Rapp, Synthesis, 1995,
303.207 O. Casanova, S. Iborra and A. Corma, J. Catal., 2010, 275, 236.208 M. K. Jogia, V. Vakamoce and R. T. Weavers, Aust. J. Chem., 1985,
38, 1009.209 R. Bognar, P. Herczegh, M. Zsely and B. Batta, Carbohydr. Res.,
1987, 164, 465.210 Y. Rom�an-Leshkov, C. J. Barrett, Z. Y. Liu and J. A. Dumesic,
Nature, 2007, 447, 982.211 J. B. Binder and R. T. Raines, J. Am. Chem. Soc., 2009, 131, 1979.212 J. B. Binder and R. T. Raines, US Pat., No. 4, 437, 2010.213 G. C. A. Luijks, N. P. M. Huck, F. van Rantwijk, L. Maat and
H. van Bekkum, Heterocycles, 2009, 77, 1037.214 M. Chidambaram and A. Bell, Green Chem., 2010, 12, 1253.215 F. K. Kazi, A. D. Patel, J. C. Serrano-Ruiz, J. A. Dumesic and
R. P. Anex, Chem. Eng. J., 2011, 169, 329.216 J. J. Bozell, L. Moens, D. C. Elliott, Y. Wang,
G. G. Neuenscwander, S. W. Fitzpatrick, R. J. Bilski andJ. L. Jarnefeld, Resour., Conserv. Recycl., 2000, 28, 227.
217 D. W. Rackermann and W. O. S. Doherty, Biofuels, Bioprod.Biorefin., 2011, 5, 198.
218 V. M. Ghorpade and M. A. Hanna, Industrial Applications forLevulinic Acid, in Cereals: Novel Uses and Processes, ed. G. M.Campbell, C. Webb and S. L. McKee, Plenum, 1997.
219 D. J. Hayes, S. W. Fitzpatrick, M. H. B. Hayes and J. R. H. Ross,The Biofine Process: Production of Levulinic Acid, Furfural andFormic Acid from Lignocellulosic Feedstocks, in Biorefineries:Industrial Processes and Products: Status Quo and FutureDirections, ed. B. Kamm, P. R. Gruber and M. Kamm, JohnWiley & Sons, 2006.
220 D. C. Elliott and J. G. Frye, US Pat., No. 5,883,266, 1999.221 R. Le Van Mao, Q. Zhao, G. Dima and D. Petracone, Catal. Lett.,
2010, 141, 271.222 X. Hu and C.-Z. Li, Green Chem., 2011, 13, 1676.223 S. Saravanamurugan and A. Riisager, Catal. Commun., 2012, 17, 71.224 H. Joshi, B. R. Moser, J. Toler, W. F. Smith and T. Walker, Biomass
Bioenergy, 2011, 35, 3262.225 B. C. Windom, T. M. Lovesteda, M. Mascal, E. B. Nikitin and
T. J. Bruno, Energy Fuels, 2011, 25, 1878.226 Texaco/NYSERDA/Biofine, Ethyl Levulinate D-975 Diesel Additive
Test Program, Glenham, NY, 2000.227 K. Tominaga, A. Mori, Y. Fukushima, S. Shimada and K. Sato,
Green Chem., 2011, 13, 810.
This journal is ª The Royal Society of Chemistry 2012
228 I. T. Horv�ath, Proceedings of the COST Action CM0903MeetingUBIOCHEM I – Utilisation of Biomass for Fuels andChemicals, C�ordoba, Spain, 13–15 May 2010.
229 H. Mehdi, V. F�abos, R. Tuba, A. Bodor, L. T. Mika andI. T. Horv�ath, Top. Catal., 2008, 48, 49.
230 P. P. Upare, J.-M. Lee, D. W. Hwang, S. B. Halligudi, Y. K. Hwangand J.-S. Chang, J. Ind. Eng. Chem., 2011, 17, 287.
231 M. Chalid, A. A. Broekhuis and H. J. Heeres, J. Mol. Catal. A:Chem., 2011, 341, 14.
232 J. P. Lange, R. Price, P. M. Ayoub, J. Louis, L. Clarke andH. Gosselink, Angew. Chem., Int. Ed., 2010, 49, 4479.
233 G. W. Huber, J. W. Shabaker and J. A. Dumesic, Science, 2003, 300,2075.
234 G.W. Huber, R. D. Cortright and J. A. Dumesic,Angew. Chem., Int.Ed., 2004, 43, 1549.
235 G.W. Huber, J. N. Chheda, C. J. Barrett and J. A. Dumesic, Science,2005, 308, 1446.
236 R. D. Cortright, R. R. Davda and J. A. Dumesic, Nature, 2002, 418,964.
237 J. W. Shabaker, G. W. Huber and J. A. Dumesic, J. Catal., 2004,222, 180.
238 J. N. Chheda, G. W. Huber and J. A. Dumesic, Angew. Chem., Int.Ed., 2007, 46, 7164.
239 R. R. Davda and J. A. Dumesic, Angew. Chem., Int. Ed., 2003, 42,4068.
240 I. O. Cruz, N. F. P. Ribeiro, D. A. G. Aranda andM. M. V. M. Souza, Catal. Commun., 2008, 9, 2606.
241 S. M. Swami, V. Chaudhari, D.-S. Kim, S. J. Sim andM. A. Abraham, Ind. Eng. Chem. Res., 2008, 47, 3645.
242 G. W. Huber, J. W. Shabaker, S. T. Evans and J. A. Dumesic, Appl.Catal., B, 2006, 62, 226.
243 G. Wen, Y. Xu, H. Ma, Z. Xu and Z. Tian, Int. J. Hydrogen Energy,2008, 33, 6657.
244 D. L. King, L. Zhang, G. Xia, A. M. Karim, D. J. Heldebrant,X. Wang, T. Peterson and Y. Wang, Appl. Catal., B, 2010, 99,206.
245 A. O. Menezes, M. T. Rodrigues, A. Zimmaro, L. E. P. Borges andM. A. Fraga, Renewable Energy, 2011, 36, 595.
246 D. €O. €Ozg€ur and B. Z. Uysal, Biomass Bioenergy, 2011, 35, 822.247 K. Lehnert and P. Claus, Catal. Commun., 2008, 9, 2543.248 R. R. Davda and J. A. Dumesic, Chem. Commun., 2004, 36.249 A. Tanksale, C. H. Zhou and J. N. Beltramini, J. Inclusion Phenom.
Macrocyclic Chem., 2009, 65, 83.250 G. Wen, Y. Xu, Z. Xu and Z. Tian, Catal. Commun., 2010, 11, 522.251 M. B. Valenzuela, C. W. Jone and P. K. Agrawal, Energy Fuels,
2006, 20, 1744.252 T. P. Vispute and G. W. Huber, Green Chem., 2009, 11, 1433.253 X. Li, L. Kong, Y. Z. Xiang, Y. M. Ju, X. Q. Wu, F. Feng,
J. F. Yuan, L. Ma, C. S. Lu and Q. F. Zhang, Sci. China, Ser. B:Chem., 2008, 51, 1118.
254 G. W. Huber and J. A. Dumesic, Catal. Today, 2006, 111, 119.255 R. R. Davda, J. W. Shabaker, G. W. Huber, R. D. Cortright and
J. A. Dumesic, Appl. Catal., B, 2005, 56, 171.256 J. W. Shabaker, D. A. Simonetti, R. D. Cortright and J. A. Dumesic,
J. Catal., 2005, 231, 67.257 R. D. Cortright and J. A. Dumesic, World Pat., No. 112,314,
2007.258 J. A. Dumesic, G.W. Huber, J. N. Chheda and C. J. Barret,US Pat.,
No. 58, 563, 2008.259 J. A. Dumesic and Y. Roman-Leshkov,US Pat., No. 0124839, 2009.260 O. O. James, S. Maity, L. A. Usman, K. O. Ajanaku, O. O. Ajani,
T. O. Siyanbola, S. Sahu and R. Chaubey, Energy Environ. Sci.,2010, 3, 1833.
261 J. N. Chheda and J. A. Dumesic, Catal. Today, 2007, 123, 59.262 D. A. Simonetti and J. A. Dumesic, ChemSusChem, 2008, 1, 725.263 R.M.West, Z. Y. Liu, M. Peter, C. A. G€artner and J. A. Dumesic, J.
Mol. Catal. A: Chem., 2008, 296, 18.264 J. W. Shabaker and J. A. Dumesic, Ind. Eng. Chem. Res., 2004, 43,
3105.265 A. Tanksale, Y. Wong, J. N. Beltramini and G. Q. Lu, Int. J.
Hydrogen Energy, 2007, 32, 717.266 J. C. Serrano-Ruiz and J. A. Dumesic, Energy Environ. Sci., 2011, 4,
83.267 C. Barret, J. Chheda, G. W. Huber and J. A. Dumesic, Appl. Catal.,
B, 2006, 66, 111.
Energy Environ. Sci., 2012, 5, 7393–7420 | 7419
Publ
ishe
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11/2
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. View Article Online
268 J. Q. Bond, D. Martin-Alonso, R. M. West and J. A. Dumesic,Langmuir, 2010, 26, 16291.
269 J. Q. Bond, D. Martin-Alonso, D. Wang, R. M. West andJ. A. Dumesic, Science, 2010, 327, 1110.
270 D. J. Braden, C. A. Henao, J. Heltzel, C. C. Maravelias andJ. A. Dumesic, Green Chem., 2011, 13, 1755.
7420 | Energy Environ. Sci., 2012, 5, 7393–7420
271 D. Martin-Alonso, J. Q. Bond, J. C. Serrano-Ruiz andJ. A. Dumesic, Green Chem., 2010, 12, 992.
272 J. C. Serrano-Ruiz, D. J. Braden, R. M. West and J. A. Dumesic,Appl. Catal., B, 2010, 100, 184.
273 J. C. Serrano-Ruiz, D. Wang and J. A. Dumesic, Green Chem., 2010,12, 574.
This journal is ª The Royal Society of Chemistry 2012