Chapter 1, Introduction _
1
CHAPTER 1
INTRODUCTION
Chapter 1, Introduction _
2
1.0 Introduction
1.1 Overview of Malaysia’s Petrochemical Industry
Oil and Gas sector is a key component in Malaysia‘s economic growth. About 30%
of the manufacturing income and roughly 8% of annual gross of domestic product comes
from the oil and gas sector (Kok, 2009). As at 2010, the petroleum and petrochemical
industry investments in Malaysia totals up to RM58 billion. Malaysia comprises the
world‘s 14th
largest natural gas reserves and the 23rd
largest crude oil reserves (MIDA,
2011). As of January 2010, Malaysia held 83 trillion cubic feet (Tcf) of proven natural gas
reserves, mostly in offshore Sarawak (EIA, 2010).
Malaysia‘s potential as an investment location for petrochemical industries is
distinguished by the presence of world renowned petrochemical companies‘ such as Shell,
BASF, Dow Chemical, BP, Toray, Mitsubishi, Idemitsu, Polyplastics and Eastman
Chemicals. Many of these petrochemical companies are in collaboration with PETRONAS,
Malaysia‘s national petroleum company. The industries rapid growth is attributed to the oil
and gas feedstock availability, infrastructure development, strong supporting services, cost
competitiveness, strategic location within ASEAN and major markets in the Far East
(AHK, 2010; MIDA, 2004).
The adequate supply of petrochemical feedstock are made certain by the six gas
processing plants in Kerteh, Terengganu and Malaysia‘s Peninsular Gas Utilisation (PGU)
pipelines that channel gas to industries around Malaysia. Table 1.1 shows the location of
the Oil Refineries in Malaysia (MIDA, 2004; PETRONAS, 2011).
Chapter 1, Introduction _
3
Table 1.1: Location of Oil Refineries in Malaysia
Oil Refineries Location
Petronas Penapisan (Terengganu) Sdn Bhd Kertih, Terengganu
Petronas Penapisan (Melaka) Sdn Bhd Tangga Batu, Melaka
Malaysia Refining Company Sdn Bhd Tangga Batu, Melaka
Shell Refining Company (FOM) Bhd Port Dickson, Negeri Sembilan
Esso (Malaysia) Bhd Port Dickson, Negeri Sembilan
Source:(MIDA, 2004)
By continuing to improve the incentives and policies, Malaysia is on the right track
to remain as a competitive location for services and manufacturing activities.
Petrochemicals are targeted in the manufacturing sector (resource based) in the Third
Industrial Master Plan (IMP3), 2006-2020, for future development. Among the strategic
measures include expanding and enhancing value-added existing capacities and broadening
the range of petrochemicals produced. Others include enhancing linkages with downstream
industries, in particular plastics and oleo chemicals, intensifying the development of
technologies in materials and product applications and improving chemical process
technologies and the application of catalysts to increase yields (AHK, 2010; MITI, 2006).
1.2 Propene and Its Derivatives
Propene, also known as propene is the second most important starting product in the
petrochemical industry after ethylene. It is one of the key building block petrochemicals
utilized as feedstock for various polymers and intermediates. The main usage for propene is
for the production of polypropylene (PP), which accounts for nearly two-thirds of global
propene consumption. Other propene derivatives include acrylonitrile, propylene oxide,
Chapter 1, Introduction _
4
cumene, acrylic acid, oxo alcohols, isopropyl alcohol, and oligomers as seen in Figure 1.1
(ICIS, 2010; Nexant, 2009)
Source: (Nexant, 2009)
Figure1.1: Global Propene Demand by Derivative
PP has made an impact in a wide range of consumer and industrial products as it is
one of the most versatile bulk polymers due to good mechanical and chemical properties.
Among the application of PP are electronic and electric appliances, packaging, pipes, wire,
cables, toys, and tapes. Acrylonitrile, the second largest derivative is used in various
elastomeric polymers and fibre applications. Acrylic fibres are used in clothing, home
furnishing and bedding such as socks, sports wear, carpets, upholstery, and blankets.
Acrylonitrile are also used as a chemical intermediate in the production of nitrile rubber,
acrylonitrile-butadiene-styrene (ABS)/styrene acrylonitrile (SAN), acrylamide and carbon
fibers (ICIS, 2010; Wakefield, 2007).
Chapter 1, Introduction _
5
Propylene Oxide is a starting material to make propylene glycol which is used in
antifreeze, aircraft de-icing fluids, unsaturated polyester resins, propylene glycol ethers, and
polyether polyols (ICIS, 2010). Cumene is the main feedstock for the production of phenol
and acetone (ICIS, 2010; Zakoshansky, 2009). Acrylic acid is used in the production of
acrylic esters and resins used in paints, coating and printing applications. Oxo-alcohol and
Isopropanol are also used in resins, paints and adhesives application (ICIS, 2010).
1.3 Propene Production Technology
The primary source (88%) of propylene is as a by-product of ethylene production in
steam cracking of liquid feedstock‘s (naphtha and LPGs) and from off gas produced in
refinery fluid catalytic cracking (FCC) streams. Steam cracking is carried out at high
temperature and the predominant co-product is propene. Typically the propene to ethylene
ratio varies from 0.4:1 to 0.75:1. By altering the feedstock choice and cracking severity
propene output can be enhanced. Coke formation and deposition is a major problem since it
reduces products yield and energy efficiency of the process (Chan et al., 1998; ICIS, 2010;
Nexant, 2010).
Propene are also recovered from FCC operations which cracks heavy gas oils by
breaking the carbon bonds in large molecules into multiple smaller molecules (Veoliawater,
2009). However, refinery propene needs to be purified. Among the companies using this
technology are Kellogg Brown & Root (SUPERFLEX and ACO process) and Honeywell
UOP (UOP PetroFCC and UOP RxPro) (ICIS, 2010; Nexant, 2009; Tallman & Eng, 2008;
UOP, 2011).
Chapter 1, Introduction _
6
Other routes leading to propene production technology are;
i. Propane Dehydrogenation (PDH) – Converts propane to propene at high
temperatures (500-800 ºC) in resulting in an endothermic equilibrium reaction. Used
commercially and readily available from licensors such as Sud-Chemie (CATOFIN
process) and UOP (C3 Oleflex Process). Problems include high capital cost due to
high price of propane, and needs continuous supply of propane and coke deposition
on catalyst bed due to its high temperature reaction (ICIS, 2010; Nexant, 2009; Süd-
Chemie, 2011; Tallman and Eng, 2008; UOP, 2011).
ii. Olefin Metathesis – Catalytic conversion of olefins (ethylene and 2-butene) to
propene. Can be added to steam crackers to boost propene production via the cracking
exchange reaction. This technology is used by companies such as BASF (BASF-Fina
cracker) and ABB Lummus Global (Philip triolefin process). Problems associated
with this technology include the need for large C4 streams and is not economically not
favourable as ethylene production as feedstock needs to be improved, thus increasing
total capital expenditure (ICIS, 2010; Mol, 2004; Nexant, 2009; Tallman and Eng,
2008).
iii. Olefin Interconversion – Catalytic conversion of C4 and C5 olefins into propene and
ethylene using fixed or fluidized bed reactor. Compatible with FCC and ethylene
crackers and does not consume ethylene. Lurgi and ExxonMobil (MOI Technology)
are among few companies that are using this technology (ICIS, 2010; Nexant, 2009).
Chapter 1, Introduction _
7
iv. Methanol to Olefins (MTO) Conversion – Natural Gas (Methane) has to be
converted to methanol, and then it‘s converted to propene. This technology increases
the propene production and catalyst consumption is low. UOP Advanced MTO
Process combines MTO with cracking process uses alternative feedstock such as coal.
High capital requirement depends on the price of natural gas feed (ICIS, 2010;
Nexant, 2009; UOP, 2011).
v. Deep catalytic cracking (DCC) – Produces light olefins from vacuum gas oil and de-
asphalted oils using fluid catalytic cracking principles. Developed by Sinopec
(CHINA) and employed by Stone & Webster (ICIS, 2010; Nexant, 2009).
1.4 Propene Market and Demand
Most of the world‘s propene monomer are exported by Japan, Malaysia, South
Korea, Taiwan, Canada and US, while among the importers are West Europe, US,
Colombia, Egypt, China, South Korea, Taiwan and Indonesia. Propene demand is
anticipated to grow an estimated 5% annually from 2007-2015, which accumulates to more
than 100 million tons by 2015. Propene consumption by region is shown in Figure 1.2
(Eramo, 2005; Nexant, 2009).
However, propene supplies are constrained by co-product production from steam
crackers and FCC refinery operations. Steam cracker extension and its additions are unable
to keep up with the propene demand growth. Propene produced by propane
dehydrogenation (PDH) is significantly lower as compared to other production technology
even though PDH are more economically favourable. Therefore propene demand continues
Chapter 1, Introduction _
8
CH3CH3CH3CH2+ O2
CO + CO2
+ H2O
to outpace ethylene demand and there is increasing interest in developing alternatives
sources of propene without adversely affecting ethylene availability (ICIS, 2010; Nexant,
2009).
Source: (Nexant, 2009)
Figure 1.2: Global Propene Consumption Trends
1.5 Oxidative dehydrogenation (ODH) of propane
Catalytic Oxidative dehydrogenation of propane is an attractive alternative to
accommodate the world propane demand although this technology has not been
commercialised. The mechanism of propane ODH reaction is shown in Figure 1.3.
Propane Propene
Figure 1.3: Propane ODH Reaction Network
Chapter 1, Introduction _
9
This synthesis route appears to be far from realization because of some difficulties.
For instance, propene oxidizes more easily than propane, hence reducing selectivity rapidly
with conversion. At temperatures above 700 ºC, propane cracking increases, thus producing
a variety of other products. Selective oxidation catalysts are needed to increase propene
selectivity at temperatures below 700 ºC. Catalytic ODH reaction of propane requires
oxidant molecules that transform the eliminated hydrogen from propane to water thus
converting the highly endothermic reaction into an exothermic process. The reaction
temperature decreases hence reducing deactivation caused by coking as well as side
reactions such as parallel or consecutive oxidation reactions giving carbon monoxide (CO)
or carbon dioxide (CO2) as non-selective products (Bhasin et al., 2001; Khan et al., 2010;
Meunier et al., 1997; Nexant, 2010).
Over the years a variety of catalysts have been studied to improve the ODH reaction
efficiency. However, existing ODH catalyst have limited activity and poor selectivity. The
most studied catalytic systems for ODH reaction are transition metal oxides catalysts such
as molybdenum-based systems and vanadium-based systems with supports such as
niobium, magnesium and nickel (Bhasin et al., 2001; Khan et al., 2010; Meunier et al.,
1997; Nexant, 2010).
Chapter 1, Introduction _
10
1.6 Molybdenum Based-Catalyst
Molybdenum based-catalyst use in petrochemical industry is extensive. The various
application and reaction of the catalyst are shown in the Table 1.2. Molybdates are mostly
used as selective oxidation catalyst (IMAO, 1998, 2005)
Table 1.2: Molybdenum compounds in catalysis
Source: (IMAO, 2005)
In propane ODH reaction, molybdenum is implicated in the activation of C-H bond
of propane and the water formation. Molybdenum can exist between oxidation states
transitions of +4, +5 and +6. The capability to do so creates high structural diversity of the
mixed transition metal oxides. Vanadium also has rich coordination geometries and
oxidizing state ranging from +3 to +5. By incorporating vanadium as a support into the
Catalyst Application Reaction Importance
Sulfided Co-Mo or
Ni-Mo on alumina
Hydrotreating,
hydrodesulfurisation
Remove sulphur
from crude petroleum
Oil and petroleum
refining
Bi-Mo oxides Propene selective
oxidation, ammoxiation
Synthesis acrolein,
acrylonitrile
Making polymers
and plastics
Mo-V oxides Acrolein oxidation Synthesis acrylic
acid
Making polymers
and plastics
Fe-Mo oxides Methanol oxidation Synthesis
formaldehyde Making formalin, polymers, resins
Mo oxide on
alumina Olefin metathesis
Propene to ethene
and butene Olefin synthesis
Mo complexes Epoxidation Olefin to epoxide Polyether
synthesis
Heteropolyacids-
phosphomolybdate Propene hydration Propene to alcohol
Alcohols
synthesis
Chapter 1, Introduction _
11
molybdenum oxide system, the structural motif can be enhanced (Cavalleri et al., 2007;
Khan et al., 2010; Rödel et al., 2007).
In acquiring an enhanced catalyst system performance, structural-activity
relationship understanding is necessary. This can be done by gaining an insight of the
kinetics of structural evolution under variation condition through in-situ studies such as in-
situ X-Ray Diffraction (XRD) and in-situ Differential Scanning Calorimetry (DSC)
(Ressler et al., 2000).
1.7 Project Design Motivation
This project was designed based on the fact that propene demand had outpaced
ethylene demand therefore an alternative technology is needed to increase production of
propene. Catalytic propane oxydehydrogenation (ODH) reaction is a viable technique with
great potential. ODH transforms the highly endothermic dehydrogenation (DH) reaction
into an exothermic process, hence reducing coke formation and side reaction products.
Although ODH is thermodynamically favourable, selectivity to propene is low due to low
reactivity of propane. Subsequent reaction also can occur which leads to prooane cracking
products and combustible products (CO, CO2). Suitable catalyst such as molybdenum-
based-catalyst is needed to activate the propane methyl C-H bond as enormous energy is
needed. One of the key ways to improve on the above limitation is to optimise and control
the molybdenum phase structure, particularly during catalyst precursor activation stage. In
order to enhance catalytic performance, studies of structure-activity relationship were
essential to optimise the catalyst system striving for improved activity and selectivity to
propene.
Chapter 1, Introduction _
12
1.8 Objectives
The objectives of this research project are:
1) to develop methodology for the synthesis of molybdenum oxides based-catalyst
precursors for ODH reaction.
2) to study the structural dynamics of the said precursor during activation and to conduct
preliminary reactivity studies using in-situ techniques.
Chapter 2, Literature Review _
13
CHAPTER 2
LITERATURE REVIEW
Chapter 2, Literature Review _
14
2.0 Literature Review
2.1 Selective Oxidation
Light alkanes (C2-C4) are of natural abundance and the best utilization is to convert
them to desired petrochemical products and feed stocks for valuable chemicals. To fulfil the
worldwide demand, great focus is given to selective oxidation studies which is not only
economical but also of lower environmental impact. The key points in selective oxidation
reactions are the activation of the highly stable C-H bond of light alkanes, further oxidation
suppression of formed products to undesired products and also to reduce C-C bond
breaking to CO and CO2 as alkenes are less stable than alkanes (D‘Ippolito et al., 2008;
Lin, 2001; Vitry et al.,2004).
2.1.1 Propane Selective Oxidation
Various studies have been carried out on the selective oxidation of propane. Figure
2.1 shows the propane oxidation pathways and the standard reaction enthalpies. Propane as
a saturated hydrocarbon is less reactive as compared to the selective oxidation products.
This is due to the bond strength of C-H especially at the methyl group as requires
significant amount of energy to activate it. Temperature control is essential to avoid total
oxidation to CO and CO2 and also to avoid cracking of the C-C bond of propane to give
lower alkanes. To overcome this limitation, catalyst is needed not only to selectively
activate the propane C-H bond but also to hinder C-C bond breaking. (Lin, 2001).
Chapter 2, Literature Review _
15
In a review by Bettahar et al. (1996), propane conversions includes direct oxidation
or via two step reaction (via propylene) to propanol, acrolein, acrylic acid, acetaldehyde,
methanol and formaldehyde. A great deal of attention has been given to catalytic selective
oxidation. Equally the catalytic oxidative dehydrogenations of propane studies are done to
lower the propene cost compared to the non-catalytic and non-oxidative processes as the
demand propene derivatives increases.
Source: (Lin, 2001)
Figure 2.1: Propane reaction pathways and reaction enthalpies
Chapter 2, Literature Review _
16
2.2 Oxydehydrogenation of Propane
Catalytic Oxydehydrogenation occurs with the existence of molecular oxygen
(hydrogen acceptor) over heterogeneous catalyst in the reaction medium. This reaction
process helps to overcome the problems of dehydrogenation that is subjected to
thermodynamics limitations (endothermic reaction) and rapid degenerative catalyst
(coking). The catalytic ODH of propane is a better choice reaction as it is an exothermic
reaction (H= -117 kJ/mol). This is because with the presence of oxygen in the reaction
which leads to water formation. However, many problems still exist such as reaction
mixtures flammability, selectivity control where propene selectivity is small due to the high
reactivity of propene towards further oxidation giving undesired oxygenated by-products
and carbon dioxides. For example propane is more thermodynamically favourable to
convert to 2-propanol (H= -168kJ mol-1
). (Bettahar et al., 1996; Cavani & Trifirò, 1995;
Lin, 2001; Viparelli et al., 1999; Vitry et al., 2004; Watson & Ozkan, 2003).
Catalyst development has to be designed to be capable in activating only C-H bonds
of light alkane molecules under oxygen flow. Focuses have been given to vanadium mixed
oxides and catalysts based on molybdates, ferrites and antimonates for light alkanes ODH
reaction. Studies are done to determine the correlation of catalysts structure to catalytic
activity and selectivity. The control of catalyst phase composition and structure in large
scale preparation are also investigated widely. The nature of support is another important
variable in determining the catalytic properties of supported catalyst (Mamedov & Cortés
Corberán, 1995).
Chapter 2, Literature Review _
17
Buyevskaya & Baerns (1998) have summarized three mechanisms that may be
involved in oxidative dehydrogenation of alkanes on oxides based on the surface species
character which is redox system, adsorbed oxygen state and via strongly bound lattice
oxygen. For the redox system at temperature less than 500 ºC, propene are selective but low
propane conversion observed even at high degree of catalyst. Adsorbed oxygen of rare
earth oxides on reaction surface leads to propylene, COx and significant amount of
ethylene, methane and C4-hydrocarbons caused by C-C cleavage. For propane activation on
lewis acid sites using B2O3/Al2O3, the oxygen participates in product formation leading to
propyl radicals and significant selectivity‘s to olefins and oxygenates.
Liu et al. (2010) also has discussed the mechanism of propane ODH. Propane
conversion to propene (C3H8) is by eliminating two hydrogen atoms on the active oxygen
from the metal oxide catalyst. Consecutively, the propene reacts with the adsorbed/lattice
oxygen which is supplied by the neighbouring electrophilic surface to produce majority
COx thus decreasing the selectivity of propene. Addition of anion doped metal oxide such
as fluoride enhances oxygen mobility and insulated the neighbouring electrophilic activity
oxygen, leading to propene deep oxidation termination.
The selectivity of propane ODH needs control of the active oxygen abundance and
also the adsorption/desorption quality to avoid re-adsorption of the produced propylene.
The surface of the oxides used has a combined influence of redox and acid-base properties.
The important requirements for selective propane ODH are weak Lewis acid centres,
intermediate reducibility and oxygen mobility. The oxygen needs to be able to bind
strongly enough to the surface to comprise attenuated oxidizing strength but weak enough
Chapter 2, Literature Review _
18
to oxidize the reactant molecule selectively. By using transition metal oxides catalyst, the
species of interest are M=O, M-O-M and M-O-support bonds where M is the transition
metal. The characteristic of the active oxygen depends on the transition metal loading,
support, dispersion and addition of modifiers (Watson & Ozkan, 2003). Among the
catalytic systems identified for the ODH reaction are metal molybdates, vanadates and
niobium pentoxides (Vitry et al., 2004).
2.3 Catalyst
Catalyst as defined by Wilhelm Ostwald is described as a compound that increases
the rate of a chemical reaction, but which is not consumed by the reaction. Key issues
associated with catalyst are kinetics, mechanism of catalyst reaction, elementary reaction,
reactants adsorption on solid surface, reactivity of surface and materials, enzymes and
organometallics synthesis and structure (Murzin & Salmi, 2005). Catalysts are used widely
in the productions of industrial chemicals, fine chemicals, pharmaceuticals, and to avoid
and clean up pollutants. The effectiveness of a catalyst is evaluated based on its catalytic
activity, reaction selectivity, catalyst stability, and the catalyst regenerability (Clark &
Rhodes, 2000; Heinemann, 1997). The different catalysts used are homogeneous catalyst
dissolved in the reaction solution, heterogeneous catalyst in the form of porous solids and
biological catalyst in the form of enzymes (Murzin & Salmi, 2005).
2.3.1 Heterogeneous catalyst
Heterogeneous catalyst is a solid catalyst getting into contact with a gaseous/liquid
phase reactant medium in which the catalyst is insoluble. This reaction is alternatively
known as contact catalysis. The majority of catalytic process taken place in the
Chapter 2, Literature Review _
19
petrochemical industry uses solid catalysts with gaseous substrates. The advantages using
heterogeneous catalyst over homogeneous catalyst are that solid catalysts are less corrosive,
a vast variety of temperature and pressure can be applied to accommodate strong
exothermic and endothermic reactions, and it is easier and cheaper to separate the substrates
and products from catalysts. The catalytic mechanism using heterogeneous catalyst
involves consecutive substrate diffusion, substrate adsorption, surface reaction, product
desorption and product diffusion (Clark & Rhodes, 2000).
In designing a new selective heterogeneous oxidation catalyst, seven fundamental
principals highlight the importance to it. The seven principles are lattice oxygen, metal–
oxygen bond strength, host structure, redox, multifunctional active sites, site isolation, and
phase cooperation. Graselli (2002) describes the principals as the seven pillars of selective
heterogeneous oxidation catalysis. By applying the seven principles and achieving proper
knowledge of the structural, surface and dynamic properties of the metal oxide catalysts on
atomic levels, a new, more efficient and environmentally friendlier selective oxidation
catalyst.
2.3.2 Catalyst Support
A catalyst support or carrier are used to higly disperse catalyst small particles to
enhance catalytic activity. Supported metals helps in handling thus minimises metal loss.
By using support, incorporation of modifiers such as promoters is also easier. Mostly used
supports are silica and alumina because of the oxide capability to become microporous
(Bond, 2005b). To synthesize a supported catalyst, 2 stages are involved. Firstly, support
material is dispersed using impregnation, co-precipitation, deposition or adsorption from
Chapter 2, Literature Review _
20
solution and the next step is calcination or reduction by thermal treatment. Supported
catalysts is more effectively utilised in precious-metal catalyst as compared to bulk-metal
system. However, in base-metal catalysts, support is used to improve catalyst stability
(Acres et al., 1981).
2.3.3 Nanostructured Catalyst
Nanostructured catalysts are used in a wide array of applications such as
hydrogenation, oxidation and photocatalysis. The size of the deposited metal particles and
their distribution on sizes and surface area are essential to be determined as it affects the
catalytic activity (Sergeev, 2006b). Nanostructured catalysts are usually developed by
modifying chemical and physical properties by size effects. This can occur either through
nano-sized bulk particles or via a nonporous matrix hosting the catalytic species.
Nanostructured bulk catalyst features inherently different adsorption and surface reactivity
which leads to extraordinary surface defects, morphology electronic sites. Commonly used
nanostructured catalysts are carbon nanotubes, rare earth oxides, nobel metals and
transition metal oxides (Sergeev, 2006a, 2006b).
2.3.4 Bulk Catalyst
Bulk materials are samples that contain nanoscale grains with appropriate geometry
and not particulates, thin films or nanoscale wires (Koch, 2009). Bulk nanostructured
materials also known as consolidated materials are defined as bulk solids with nanoscale or
partly nanoscale microstructures (Koch, 1999). Bulk metals mostly display the
characteristic physical properties of metallic state such as hardness, ductility, strength,
lustre, malleability, high electrical and thermal conductivity (Bond, 2005a). Bulk properties
Chapter 2, Literature Review _
21
that are affected when the microstructure is nanoscale are the mechanical properties and the
ferromagnetic materials (Koch, 2009).
The synthesis and assembly can be done by 2 different techniques. The first is
―bottom-up‖ assembly where the nanostructured building blocks are formed first then
consolidated into the final bulk material. Examples are gas condensation, and
powder/aerosol compaction including electrodeposition. This approach are applied mostly
for nanopowders and structural composite materials production and applied for catalysts,
films, coatings, cosmetics, electronic devices, paints, lubricants, rocket fuels and
reinforcements for nanocomposites.
The other technique is the ―top-down‖ which starts with a bulk solid then
structurally decompose to obtain nanostructures. Examples are mechanical attrition (ball
milling) and lithography/etching. In ball milling technique, the powdered particles of all
elements are refined to nanoscale grain size. The particulates of nano building blocks are
then subsequently assembled into a new bulk material (Hu & Shaw, 1999; Koch, 2009).
However lately, it is learnt that ―bottom-up‖ research are to replace ―top-down‖, a strategic
move in nanoscience (Astruc, 2008).
2.4 Catalyst Synthesis
Factors contributing to catalytic performances are crystalline structure, purity,
surface enrichment, particle size, morphology, preparation method and thermal treatment.
Catalyst components self organization in a structure is important in creating new active
sites on a catalyst. Inorganic soft synthesis methods such as micro emulsion method, sol-gel
Chapter 2, Literature Review _
22
method, and hydrothermal method are needed to be employed to gain and increase catalytic
activity (Oshihara et al., 2001).
2.4.1 Hydrothermal
Hydrothermal method is commonly used to synthesise transition metal oxides such
as TiO2 and ZrO2. This method also known as solvothermal method is done by heating the
reaction mixture containing the solution of catalyst components above the boiling point of
water using a closed system such as autoclave and it is exposed to steam at high pressures.
The reaction can be done in water (hydrothermal) or in solvent such as ethanol
(solvothermal). Powders acquired from hydrothermal synthesis have varied microstructure,
morphology and phase composition as determined by parameters such as temperature,
pressure, reaction time, concentration and pH solution. By increasing the reaction
temperature, accelerated crystal growth and better crystalline material with narrow particle
size distribution are obtained (Kolen‘ko et al., 2003; Yu et al., 2007).
2.4.2 Sol-Gel
Sol gel process is used to produce different types of materials such as powders,
films, fibres and monoliths. This method can be used to prepare pure dense, stoichiometric,
equiaxed, nanostructured and consistent particle size of metal oxides such as TiO2. Sol gel
process occur first with the formation of sol which is a liquid suspension of solid particles
(1nm-1micron) produced by precursor hydrolysis or partial condensation. Next, it is
followed by formation of a gel which is a dysphasic material with solid encapsulating
solvent produced by destabilizing a solution of preformed sols. Controlled parameters in
this process are solvent, precursors, catalysts, temperature, pH, additives, and mechanical
Chapter 2, Literature Review _
23
agitation to influence the kinetics, reaction growth, hydrolysis and condensation reaction.
Advantages are high purity can be maintained, physical characteristics (pore size
distribution & pore volume) changing ability, capable of varying compositional
homogeneity at molecular level and at low temperatures and also capable to introduce
several components in a single step. (Ko, 1997; Yu et al., 2007)
2.4.3 Impregnation
This technique is used to introduce the support into the catalyst system. There are
two types of impregnation method; dry impregnation and wet impregnation. In dry
impregnation, appropriate amount of liquid solution is used to fill up the pore volume of the
support material. The maximum loading of the support is determined by the catalyst
precursor solubility and the support pore volume. Dry impregnation can also be used when
several catalyst precursors are available simultaneously in the impregnating solution and
this is known as co-impregnation. In wet impregnation, support is dipped into an excess
quantity of catalyst precursor solution. Different profiles of the active phase over the
support are obtained depending on the process condition such as pH, temperature, and
percentage of support loading (Mul & Moulijn, 2005).
2.4.4 Precipitation
Precipitation method is technically more demanding than other methods since
having to deal with product separation. This method used especially for catalyst support
material. Among advantages of this process are, the process flexibility and very pure
material can also be obtained. Problems occurring in this method are the constant product
quality maintenance throughout the precipitation process and solid nucleation making it
Chapter 2, Literature Review _
24
hard to study the crystallization and precipitation process of solution. Precipitate production
from homogeneous solution involves the formation of a nucleus which is governed by the
free energy of the solution agglomerates (Schüth & Unger, 1997). This is based on the
control of parameters such as temperature, pH, reactant concentration, and time which is
correlated with factors such as supersaturation, nucleation and growth rates, surface energy,
and diffusion coefficients of the precipitate (Schüth & Unger, 1997; Yu et al. 2007).
Rodriguez-Paez et al. (2001) reported that controlled precipitation method allows
the control of the different stages of ceramic powder processing, has reproducible
properties and also larger quantities of ceramic powder can be obtained. Zinc oxide
nanoparticles agglomerates with different morphologies have been obtained by controlling
different parameters of the precipitation process such as solution concentration, pH and
washing medium.
Similar observation was reported by Oliveira et al. (2003), by precipitating zinc
oxide particles at room temperature. The precipitate formation mechanism involves the
nucleation of ZnO particles inside the matrix and then the particles aggregate into star-like
particles. Addition of additives such as sodium sulphate or sodium dodecyl sulphate leads
to synthesis of smaller particles and particles morphology also are affected. This maybe
caused by the adsorption of additives on crystallites which limits the nanocrystallite
aggregation into bigger particles.
Chapter 2, Literature Review _
25
As reported by Abd Hamid et al. (2003), by controlling the precipitation of
molybdenum oxide in aqueous solution, structurally complex solids can be synthesized.
The solid precipitate nucleation can be determined by the pH response immediately before
precipitation. At low rate of addition, several buffering equilibria of polyoxomolybdates are
attained. However at fast addition, buffering equilibrium is not reached leading to reaction
sequence delay thus at lower pH. It is found that at different pH levels, different
distributions of molecular species are obtained. Nanostructuring can be achieved from the
transition result from catalyst preparation to catalyst synthesis thus the surface morphology
(roughness) and the geometric surface area of the unsupported oxide can be optimized.
However, it is still recommended to precipitate at a constant pH and the mother liquor
should be removed quickly to preserve the initial structure during thermal treatments.
2.4.4.1 Co-precipitation
Co-precipitation is described as simultaneous precipitation of more than one
component with products solubility differing possibility. This method is best used for
homogeneous distribution of catalyst components or to synthesise precursors with definite
stoichiometry which will be converted to active catalyst hence good dispersion. This
method can also be used to incorporate support material by co-precipitating metal ions with
support ions to produce an uniform dispersion of active component throughout the surface
and the bulk when catalyst is calcined Co-precipitation technique is mostly used to
synthesise mixed metal oxides even at low temperature and the morphology and particle
size can be controlled to achieve nanostructuring (Acres et al., 1981; Schüth & Unger,
1997).
Chapter 2, Literature Review _
26
Blangenois et al. (2004) has described the effects of precipitation parameters such
as pH, temperature, stirring and addition rate in the preparation of vanadium-aluminium
oxide catalysts which is a potential propane ODH reaction catalyst. The co-precipitation pH
strongly influences texture, surface area, chemical composition, reducibility and the nature
of vanadium oxide species in V-Al-Ox catalyst hence the activity of the catalyst. The
reducibility are affected by the degree of polymerization and the state of coordination, V=O
group position in the chain and the interaction of the vanadate support. At higher pH,
polymerization degree of the tetrahedral vanadium species ([VOx]nn-
) become low.
Catalytic activity also are significantly affected by temperature whereby the lower the
reduction temperature, the higher the activity of the catalyst for propane ODH reaction.
2.5 Transition Metal Oxides (TMO)
Transition metal especially in nanoparticles form are important in catalysis as the
properties are similar to metal surface activation and nanoscale catalysis thus improving
heterogeneous catalysis selectivity and efficiency. The most active catalysts are only a few
nanaometers of diameters in size. Nanoparticle catalysts also are selective, efficient, and
recyclable which is suitable as a green catalyst. Transition metal nanoparticles are able to
self assembly via nucleation to form clusters stabilized by ligands, surfactants and polymers
(Astruc, 2008).
In order to attain high metal surface area, the transition metal catalysts are dispersed
onto oxide support. Supported transition metal oxides such as vanadium pentoxide (V2O5),
molybdenum trioxide (MoO3) and chromic acid (Cr2O3) are studied as catalysis for ODH
reactions (Shiju & Guliants, 2009). A study conducted by Bell (1995), has shown the
Chapter 2, Literature Review _
27
transition metal catalysts activity and selectivity, are significantly affected by metal oxide
moieties presence on the catalyst surface. The effectiveness of the oxides promoter is better
in correlation with higher Lewis acidity. When the metal surface is covered by half oxide
monolayer, the concentration of oxide perimeter is maximized thus promoter is more
effective.
Witko & Tokarz-Sobieraj (2004) have investigated the surface oxygen existence in
transition metal oxides catalyst and used Density Functional Theory (DFT) calculation to
examine the catalytic behaviour and correlating with the electronic properties of different
surface O atoms in several V-O-X systems. Different surface oxygen and local vacancies
reveals different possible sources. Different catalytic properties are influenced by the
different electronic sites resulting from structurally non-equivalent surface oxygen sites.
Adsorbed molecular oxygen can replace the surface oxygen which occupies specific lattice
site, thus big amount of oxygen species are incorporated into oxide surface which may
impact the catalytic behaviour of the V-O/Mo-O systems.
2.5.1 Mixed Metal Oxide (MMO)
An example of MMO is Mo-V-Te(Sb)-Nb-O and Mo-V-Nb-Te catalyst which have
been studied extensively for propane oxidation and ammoxidation The performance of
these catalysts expecially to acrylic acid is reported to be significantly better than other
multi-component metal oxide. In a structural view, Mo and V are essential to form the
desired orthorhombic network which gives the highest conversion of propane and propene
and catalyst containing Te has a better selectivity to arylic acid suggesting the importance
in oxygenated products formation (Vitry et al., 2004).
Chapter 2, Literature Review _
28
For ODH reaction, a study conducted by Li et al. (2010) shows that Ni containing
oxide system are active and selective especially at low temperature reaction. CeNbNiO
nanosize catalysts were prepared by modified sol-gel method and uses citric acid as the
ligand. Pure NiO catalyst exhibits low propane conversion and selectivity. Propane
conversion increases above 350 ºC but propene was not produced. 1.5CeNbNiO catalyst
demonstrates higher propane conversion and selectivity at lower temperature (250 ºC). Nb
holds back the reduction of bulk NiO and also decreases the surface acidity and catalyst
stoichiometric oxygen thus increasing propene selectivity. Ce helps to improve the bulk
nonstoichiometric oxygen reducibility and this enhances the catalyst activity for low
temperature propane conversion.
2.5.2 Molybdenum Oxides
The rate determining step of ODH reaction is the C-H bond breaking. Molybdenum
oxides are identified in activating C-H bonds of alkanes including light olefins and
catalysing selective partial oxidation which actually requires extremely high temperatures.
Structural complexity, the ability to occur at +4 to +6 oxidation states and oxygen
coordination geometry variation of molybdenum oxides give rise to good catalytic
performances. MoxOy units are comprising with structural arrangements of shared corners,
edges and faces forming tetrahedral, octahedral, pentagonal bipyramids and square
pyramids (Cavalleri et al., 2007). For example, Bismuth molybdates and vanadomolybdates
are normally used in propane partial oxidation and ammoxidation (Cavani & Trifirò, 1995).
Chapter 2, Literature Review _
29
Divalent metal molybdates (AMoO4) supported on SiO2 are studied by Stern &
Graselli (1997) for propane ODH reaction and the reaction has been catalytic not gas phase
radical initiated reaction. The reaction is also known to be first order in propane which
involves the C-H bond breaking. Highest propene yield obtained with NiMoO4/SiO4 and
Ni0.5Co0.5MoO4/SiO2. Binary Ni-Co molybdate are a more stable catalytic system due to its
lesser sensitivity to molybdenum level. Redox elements addition such as V, Fe, Ce and Cr
improves the molybdates activity thus allowing reaction at lower temperature (<560 ºC)
and improving product yield.
Harlin et al. (1999) prepared molybdenum oxide impregnated on alumina support
for dehydrogenation reaction of n-butane. It is identified that the active oxidation state of
molybdenum was either Mo(V) or Mo(IV) which were formed by Mo(VI) reduction by
hydrogen of the n-butane. Selectivity increases with addition of magnesium via slow
oxidative dehydrogenation reaction.
Chen et al. (2001) have examined the effects of structure and properties of Al2O3
supported MoO3 catalyst on propane ODH reaction. The catalyst were prepared by
impregnating -Al2O3 into molybdenum solution. The structure of MoOx species dispersed
on Al2O3 depends highly on Mo surface density. Two-dimensional MoOx oligomers formed
preferentially for Mo surface densities lower than polymolybdate monolayers. At this
surface density, the propane ODH rates increases with Mo surface density due to the higher
reducibility of larger MoOx domain. Higher surface density decreases the propane ODH
reaction due to the formation of the three-dimensional MoO3 which makes Mo species
inaccessible. However, the presence of Mo-O-Al surface sites helps the adsorption of
Chapter 2, Literature Review _
30
propene and also the combustion of alkoxide intermediates resulting in undesired COx
products.
To obtain a better understanding of MoOx catalytic activity, Watson & Ozkan
(2003) investigated the induced effects of low-level alkali promotion. This is done by
doping the MoOx-based-catalyst with potassium and correlating with the adsorption and
reactivity of propane and propene. The catalysts were prepared by sol-gel/co-precipitation
technique. Addition of potassium will decrease the amount of lattice oxygen. This leads to
the inhibition of desorbable oxygen species that maybe related with the propane or propene
unselective conversion. However, alkali addition also decreases the reducibility of the
catalyst that may cause the propane activation to be suppressed. Potassium not only affects
the propene transformation over MoOx catalysts, but also modifies the catalyst interaction
with propane therefore altering propane activation pathway.
High purity of hexagonal MoO3 can be done by using precipitation method
followed by hydrothermal treatment. Concentrated acid, in this study HCl once again was
used to precipitate the desired structure of the material. Based on the characterization
results of XRD diffractograms and DTA profiling, metastable h-MoO3 are formed at 300-
350 ºC. At temperature higher than 450 ºC, thermodynamically stable -MoO3 are obtained
(Song et al., 2007).
Chapter 2, Literature Review _
31
2.5.2.1 Polyoxomolybdates
The advance development in the X-ray crystallographic enables crystal structure
determination with highly complicated compounds in a reasonable time. Polyoxometalates
can be classified according to the structure for example, isopoly and heteropoly vanadates,
molybdates and tungstates (Bielański et al., 2003).
Cronin et al. (2000) have described some fundamental principles in controlling the
growth of polyoxomolybdates from discrete molybdenum oxide synthons. [Mo8] is mostly
the building blocks of many polyoxometalates structure. [Mo8] unit itself is built up by a
central MoO7 pentagonal bipyramid sharing edges with five MoO6 octahedra and two
MoO6 sharing corners with pentagon atoms. To allow self aggregation, large molecular
fragments must be functionalized with groups that allow linking through certain reactions.
The linking of the giant-spherical clusters into a proper solid-state layer structure can also
occur at room temperature. Therefore, it is concluded that the nanostructured building
blocks can be isolated according to the stability. The nanostructured cavities and well
defined properties make it possible to construct materials based on characteristic synthons.
Bielański et al. (2003) have compared the thermal behaviour of hydrated ring
polyoxomolybdates. In the thermal analysis using TG/DTG and DTA two stages of
dehydration are observed. Between room temperature and 200 ºC, crystallization water is
released and from in-situ FTIR, no large destruction of polyoxomolybdates occurs. In the
second stage of heating, strongly bonded water is released which can be observed from the
TG/DTG thermogram curve as there is a fast decrease of weight along with exothermic
peaks. The exothermic peaks observed are caused by the heat evolution of the
Chapter 2, Literature Review _
32
recrystallization of strongly disordered dehydrated reaction into a new solid crystalline
phase.
Polymolybdates were obtained from self-assembly processes controlled by
oxidation-reduction reactions and also from MoVI
condensation-polymerisation at pH lower
than 3. Isopolymolybdate [Mo36O112(OH2)16]8-
have been found in pH 1 solution. A new
crystalline solid consisting of open framework was synthesized using (bipy) as the organic
component. The assembly of the [Mo36O112(OH2)16]8-
and H2bipy2+
builds up a mesoporous
H-bonded organic-inorganic hybrid material that has large cavities and exhibits water
sorption behaviour and still maintains its crystal properties (Atencio et al., 2004).
2.5.3 Vanadium Oxides
Vanadium systems such as vanadium oxides, vanadium-antimony, vanadium-
molybdenum and vanadium-phosphorus are constantly studied in olefins dehydrogenation
since are able to activate propane at lower temperature due to the metalloradical character
(Stern & Graselli, 1997). Vanadium pentoxide (V2O5) catalyst is not very promising for
olefins ODH reaction but by spreading the oxides onto support, a more complex structure
will lead to a more selective catalyst. However the selectivity are not higher than 40%, even
at low temperature (350-450 ºC) (Cavani & Trifirò, 1995).
Viparelli et al. (1999) have reported that vanadium supported on TiO2 have high
activity and low selectivity in propane ODH reaction but addition of niobium enhances the
catalytic performances at low V/Nb ratio. The catalytic activities were correlated with the
redox and acid properties. Propane can be activated by V/Ti binary catalysts since redox
Chapter 2, Literature Review _
33
sites increases as vanadium content increases. However, by increasing vanadium content,
the propene selectivity are lowered due to consecutive CO formation. 6Nb/Ti catalysts are
more selective but have low activity due to lower redox sites. By combining vanadium and
niobium species on TiO2 surface affects the catalytic properties in propane ODH reaction.
Vanadium and niobium interaction in ternary catalysts changes the acidity of the sample
surface creating different active centres as compared to the binary catalyst. Increasing
surface acidity has a promoting effect on propene selectivity.
The catalytic activity in propane ODH reaction is influenced by the reducibility and
vanadium species surface structure. Supported V2O5 on Al2O3 and TiO2 prepared by wet
impregnation disperses the vanadium species highly. The most active catalyst is V2O5/TiO2
while the most selective in propene is V2O5/Al2O3. Propene selectivity also increases by
doping the catalyst with alkali metals but catalytic activity is reduced (Lemonidou et al.,
2000).
The structural influence of active sites in Me-V-O (Me=Mg, Zn, Pb) on propane
ODH catalytic performance was studied by Rybarczyk et al. (2001). The most active
catalysts are Mg1V4 and Mg9V1 having the lowest and highest fraction of V(IV) in total
vanadium content. Propane ODH is catalysed by both valence states of vanadium, V(V)
and V(IV). However, due to the lower oxidation potemtial, V(V) is more active but less
selective than V(IV). Vanadium sites in octahedral or square pyramidal coordination are
more active but less selective than VO4 tetrahedra. Metal ions catalytic properties (Mg(II),
Zn(II), Pd(II)) are influenced by oxidation potential, acid-base properties, crystal size and
structural disorder. It is concluded that for propane ODH reaction, the promising catalyst is
Chapter 2, Literature Review _
34
supported vanadia catalyst with highly dispersed, tetrahedrally coordinated, V5+
sites for
higher selectivity and to achieve higher activities, high surface area supports with low
surface acidity catalyst is needed.
Ballarini et al. (2004) also have investigated the catalytic activity of supported
Vanadium oxides for ODH reaction but under co-feed and cyclic, redox-decoupling
conditions. In this study, the supported catalysts over alumina, titania, alumina-titania cogel
and silica were prepared by ion exchange, wet impregnation and co-gelation. The
mechanism on still oxidized catalysts under cyclic condition is either ODH or DH which is
followed by hydrogen combustion and has higher conversion in short reaction times. In co-
feed reaction, the catalysts are less selective as it is DH mechanism. Under cyclic condition,
vanadium oxide are highly dispersed when silica is used leading to higher propane
conversion and propane selectivity.
Khan et al. (2010) have recently reported that Vanadium oxides based materials
shows potential as ODH catalyst by investigating the effect of heterometallic centre (Mn,
Co, Fe) of the linkers in the catalyst clusters of framework. Catalyst interconnected with Fe
is however is the most active but least selective. Mn containing catalyst is the least active in
propane conversion. The best performance was exhibited by catalyst with Co linker with
the highest propane conversion and selectivity at 350 ºC. The catalysts are all active at low
temperature as they are easily reduced as indicated by TPR analysis. When treated under
400 ºC in 20% O2/Ar, DRIFTS studies shows structural changes of catalysts framework
with the broadening of V=O stretching bands. However, more work has to be put into the
material design and development for commercialization.
Chapter 2, Literature Review _
35
Vanadia supported on SBA-15 have shown promising catalytic activity in the ODH
of propane. Vanadia dispersion role on SBA-15 for propane ODH reaction was investigated
by Gruene et al. (2010). Mesoporous silica SBA-15 was synthesised using automated
laboratory reactor. Incipient wetness impregnation and grafting/ion-exchange are used to
disperse the vanadia on silica. Structural characterization shows mixture of monomeric and
oligomeric tetrahedral (VOx)n species depending on the loading quantity. However, at
higher vanadia loading (13.6 V/nm2), tetrahedral (VOx)n along with substantial amount of
three-dimensional, bulk-like V2O5 exists. At higher vanadia loading, the activation energy
changes with reaction condition unlike lower loading reflecting structural changes between
amorphous, bulk-like vanadia and two-dimensional highly dispersed vanadia spesies. It is
therefore concluded that polymeric species rather than monomeric species are more
important in propane activation.
2.5.4 Molybdenum Vanadium Oxides
An example is molybdenum with vanadium as the other basic catalyst component is
reactive in reactions such as propane dehydrogenation (Cavalleri et al., 2007). Various
combinations of molybdenum and vanadium oxides have been studied for ethane ODH.
The catalysts however are rarely reproducible though the optimum atomic ratio of V/Mo
seems to be between 0.25 and 0.5 which gives ethylene selectivity around 80% for low
conversion. The VMoO catalyst consists of several highly crystalline materials. Both mixed
oxides phases Mo4V6O25 and Mo6V9O40 obtained are not the most active and the surface
composition is close to bulk material (Mamedov & Cortés Corberán, 1995).
Chapter 2, Literature Review _
36
Meunier et al. (1997) had studied the molybdenum loading effect in ODH catalyst.
The catalytic performance of supported molybdena on oxides such as niobia, zirconia,
alumina, silica, magnesia and titania are compared for propane ODH reaction. TiO2 is the
most effective support to disperse the molybdenum. Titania coverage has to be higher than
monolayer coverage which is around 2.4 monolayers coverage to prevent any side reaction
from bare support. Catalytic activity increased with vanadium addition but less selective at
isoconversion. Addition of niobium leads to slight decrease in activity but selectivity is
improved. Therefore mixture of molybdenum, vanadium and niobium oxides supported on
titania (Mo+V+Nb)/TiO2 catalyst is more selective and the activity is good but still less
efficient as compared to NiMoO4. Modification by adding alkalies or varying the atomic
ratio of Mo/V/Nb might further improve the activity.
As reported by Cindric et al. (1999), polyoxoanions surfaces are similar to
heterogeneous metal oxides, thus new catalyst can be designed from organic derivatives of
polyoxoanions study. Polyoxomolybdates containing oxygen and nitrogen donating ligands
have been extensively prepared and characterised. Even though many types of
polyoxometalates exist, pH, range, temperature and metal ions concentration strongly
influence the formation by crystallisation caused by insolubility. Molybdovanates
(NH4)6[Mo6V2O24(C2O4)2].6H2O and (NH4)4[H2Mo2V2O12(C2O4)2].2H2O are prepared
where different species were obtained by successive precipitation at different concentration
of metal ions and pH but solubility dependant. The first molybdovanate anion consists of
6MoO6 and 2VO6 edge-sharing octahedral with two bidentate oxalate ligands bonded at V
ions, octahedral coordination of all metal ions. The bond lengths of M-O differ depending
on the metal ions repulsion. The second molybdate anion is centrosymmetric tetranuclear
Chapter 2, Literature Review _
37
molybdovante anion (M4O16)n-
consisting of two MoO6 and two VO6 edge-sharing
octahedral with two oxalate ligands bonded to the vanadium ions. However,
molybdovanadate anions are interconnected by H-bonds through NH4+ ions and water
molecules.
Mougin et al. (2000) synthesised Mo-V-O catalyst that has a metastable hexagonal
phase. This is done using soft chemistry method by precipitating Mo-V precursor using
65% nitric acid and varying the AMV and AHM composition while maintaining at pH1.
Proper crystallised hexagonal h-MoO3 with needle like structure and a hexagonal cross
section are obtained. Two criteria‘s that influences the structural formation are the
vanadium insertion filling the molybdenum vacancies of the h-MoO3 structure and
followed by vanadium substituting molybdenum following the formula (NH4)xVxMo1-XO3.
The addition of vanadium not only affected the morphology but also the lattice parameters
of the crystal structure. Based on the solubility limit, the final composition of the solid
solution is (NH4)0.13V0.13Mo0.87O3. Beyond the limit, during precipitation (V9Mo6O40) is
formed due to the decreasing vanadium content in the hexagonal structure.
Molybdenum oxide based-catalyst such as Mo5O14 usually incorporates transition
metals such as tungsten (W) and vanadium (V) which acts as a promoter to stabilize the
structure. Mixed oxides such as (Mo0.92V0.08)5O14 and (Mo0.75W0.25)5O14 are close to
industrial catalysts. In (MoVW)5O14, V and W promoting effect may be able to localize the
oxygen defects as V5+
prefers five-fold coordination while the redox stable W confines the
lattice deformations. Based on Raman studies, catalyst action may be determined by the
metal oxygen bond where the shorter M-O bond, the more basic is the oxygen functionality.
Chapter 2, Literature Review _
38
Promoters (V and W) stabilize the intermediate Mo oxides with proper M-O bonds for
selective C-H activation (Dieterle et al., 2001).
Three series of Vanadium molybdenum mixed oxides (VxMoyOz, x+y=1) were
prepared by Adams et al. (2004) for partial oxidation of acrolein to acrylic acid. The
methods used are pure oxide melting, crystallisation and spray drying which are all
followed by calcination. The samples acquired are mostly hexagonal h-(V,Mo)O3 and
(V,Mo)2O5 phases which has similar structure to vanadium pentoxide. Temperature
programmed reduction (TPR) was used to study the catalytic activity and selectivity. The
most promising composition of vanadium to molybdenum ratio is 3:7 at low temperature.
At temperature higher than 500 ºC, the catalyst decomposes into thermodynamically stable
phase and has no significant catalytic activity.
Similar studies have been conducted by Kunert et al. (2004) to investigate the
correlation of drying method with structural composition and catalytic performance. The
synthesized of Mo/V mixed oxides were dried via spray drying and crystallisation. From
TPR studies, the spray dried samples are more active. This is due to the fact that spray dried
samples yield the desirable hexagonal MoO3-type structure but crystallisation method
yields V2O5-type structure. Spray drying technique has many advantages as compared to
crystallisation. Among them is spray drying technique exerts influence on structural aspects
during the sample preparation thus enabling a promising understanding on the genesis of
desired, mostly metastable structure parts. Other advantages are spray drying can be used
for continuous preparation as it is rapid method, no loss of specific metal ion through
Chapter 2, Literature Review _
39
filtration, metal ions homogeneous distribution and also reproducible structurally
depending on the exact parameter variations.
Mo and V interaction on alumina for propane ODH was studied by Bañares and
Khatib. (2004). It is possible to disperse Mo and V on alumina by controlling the Mo and V
loading. During redox cycle, Mo-V-(Al)-O phase stability is dependent on the Mo + V
coverage because of the oxygen mobility. Overall, the alumina support is shared by Mo and
V thus contributes to the reaction with its own reactivity. No distinctive cooperation was
observed between the dispersed V and Mo oxides in the reaction but vanadium dominated
the catalytic performance.
The catalytic activity of MoOx catalyst doped with vanadium has been studied by
Haddad et al. (2009) for the ODH of ethane to ethylene reaction. MoOx is less active at
relatively low temperatures. By doping with a small amount of vanadium, catalytic activity
is enhanced and even better when phosphorus is added. This is because of Mo(VI) is
stabilized by the vanadium content in Mo5O14 and in MoO3. The calcination of the catalyst
creates well dispersed phosphate groups and also improves Mo and V species interactions.
These synergetic effects make Mo11VPOx catalytic performance for ethane ODH the best.
A different approach was taken by Sidochuk et al. (2010) in preparing vanadium
and molybdenum composition by mechanochemical technique of V2O5/(NH4)2Mo2O7
(V/Mo = 0.7/0.3) composition in air, ethanol, and water followed by thermal treatment
from 300 ºC to 700 ºC. The catalyst precursor was subjected to thermal analysis and the
data were correlated with powder XRD. After mechanochemically treated, the sample was
Chapter 2, Literature Review _
40
heated at 300 ºC and 350 ºC and ammonium hexavanadate is obtained. By heating to
400ºC, reflectance intrinsic to vanadium pentoxide and orthorhombic molybdenum trioxide
appears while ammonium hexavanadate disappears completely. At 450ºC, V2MoO8 appears
along with V2O5 and MoO3 reflection. Continuous heating leads to the growth of V2MoO8
reflection intensity. This final phase have enough high specific surface areas and
considerable mesopore and macropore volumes which is useful as a catalyst for
hydrocarbon oxidation catalysts.
2.6 Structure-Activity Relationship
Molybdenum-oxide based clusters provide a molecular model for catalytically
active metal-oxides used in especially industries. In order to exploit and control the solid-
state structures incorporating metal clusters, designed synthesis helps and also new
properties emergence can be predicted that may form due to the structural size and
complexity. Figure 2.2 shows the {Mo36} polyhedral cluster contains {Mo8} unit as one of
the building blocks (Cronin et al., 2000).
Source: (Cronin et al., 2000).
Figure 2.2: Polyhedra representations of {Mo36} and a {Mo8} unit.
Chapter 2, Literature Review _
41
Abd Hamid et al. (2003) have described how protonation of polyoxomolybdates,
reoligomerisation to precursors, and polyoxomolybdates hydrolysis can form either
supramolecular (Mo36O112) structure of hexagonal phase (h-MoO3) depending on the
reaction temperature. The polarity of O-H bond of the terminal oxygen species at
uncoordinated corners of MoO6 octahedra are the main reason for these transformation as
well as the occurrence of stable intermediates hence differently reactive to protonation.
Restructuring or condensation also is influenced by the structural motif steric constraints
variation caused by the increasing octahedra per polyoxomolybdate.
The source of molybdenum to synthesis hexagonal MoO3 sometimes contains
monovalent cations such as NH4+, K
+, Rb
+, Cs
+ where the cations exists in the tunnels of
the hexagonal structure as can be seen in Figure 2.3 (Mougin et al., 2000).
Source: (Mougin et al., 2000)
Figure 2.3: h-MoO3 (xy) projection
Chapter 2, Literature Review _
42
In Molybdenum Vanadium Oxides, the cation presence in hexagonal structure
tunnels is essential in order for the hexagonal h-MoO3 type structure to exist with needle
morphology. The cation nature also affects the lattice parameter but only in the a
parameter. When vanadium is introduced, the molybdenum vacancies are filled and a
increases. When all vacancies are filled, vanadium atoms substitutes molybdenum and
ammonium cation in the tunnels increases and a decreases. Finally the system is not
monophased and vanadium content in hexagonal needles decreases to form V9Mo6O40 and
parameters a and c increases. Vanadium also shows a stabilizing effect of the solid oxide
(Mougin et al., 2000).
It is hard to prepare pure oxides of Mo5O14 as the synthesis usually yields Mo4O11,
Mo17O47 and also MoO2. The Mo5O14 structure is deduced from the MoO3. The pentagonal
tunnels formed by the variation of an ordered array of rotational axes are incorporated into
the framework. Transition metal cation occupies the formed pentagonal tunnels in each
rotation group which consists of four corner-sharing octahedral. Therefore, the Mo5O14
structure are portrayed as a network consisting of MoO6 polyhedra and MoO7 pentagonal
bipyramids connected mutually by corner sharing and edges (Dieterle et al., 2001).
Mo5O14 structure as seen in Figure 2.4 is the best final product formed under
reduced oxygen partial structure by the grouping of oligo anions mixtures generated in the
solution. The active phase is metastable unti crystallization and the oxidative
decomposition under high oxygen partial pressure forms binary oxide phase (Knobl et al.,
2003).
Chapter 2, Literature Review _
43
Source: (Dieterle, et al., 2001)
Figure 2.4: Crystal structure of Mo5O14
2.7 In-situ Structural Technique Studies
The reduction of MoO3 mechanism and the existence of molybdenum suboxides
were determined by in-situ structural studies using a combination of in-situ XRD (long-
range order) and in-situ XAS (short-range order). The experiments were conducted under
different H2 partial pressure with isothermal and temperature-programmed reduction
conditions while the elucidating phase composition and the evolution with time were
observed. At temperature below 698 K, MoO3 reduction only elucidate MoO2 while
reduction above 773 K (with 10% H2), Mo metal is the final product in a two step reduction
process via MoO2 and also forming Mo4O11 above 698 K. Overall results obtained
highlight the importance of the effect of reactant concentration, reaction temperature and
reaction time on reaction products (Ressler et al, 2000).
Chapter 2, Literature Review _
44
Schlogl et al. (2001) has demonstrated the importance of in-situ analysis in
understanding the atomistic details of chemical reaction. The solid state phase
transformation of the reduced and oxidised forms of molybdenum oxides takes place
rapidly that the suboxides are unidentified using ex-situ analysis. The reaction rate, active
structure and the electronic properties of the catalyst especially when it is in bulk can be
identified by in-situ studies
Topsøe (2003) have described the importance of in-situ characterization in the
research and development of heterogeneous catalysts. Before new and improved
characterization techniques were introduced, many problems exists such as lacking surface-
sensitive techniques that provides spectroscopic information at pressure relevant to the
catalysis and also the difficulty of obtaining a specified understanding of structural insight
in nanostructure complexes which often presents in heterogeneous catalysts. These
problems are known as ‗materials gap‘ and ‗pressure gap‘. In-situ studies are described as a
technique that gives detailed structural and chemical atomic scale insight in complex
heterogeneous catalyst. In-situ studies have been used to explain studies of individual
adsorption/desorption process, catalytic behaviour in a controlled environment after
quenching reaction and also catalytic performance under high pressure. Combination of
several in-situ techniques along with theoretical calculations and surface science studies are
essential to have a complete understanding of the catalyst behaviour.
The structural phase formation and transformation kinetics in molybdenum during
ion implantation and post-annealing treatment were studied using in-situ XRD by Bohne et
al. (2005). Ion implantation technique is used to incorporate a controllable concentration of
Chapter 2, Literature Review _
45
oxygen in host matrix to synthesize molybdenum oxide with thin buried layers.
Combination of XRD technique and ion implantation provides In-situ observation during
implantation at different temperatures and also obtains kinetic information of structural
phase formations. This gives an understanding of the nucleation and growth process during
implantation and/or post annealing treatment. Implantation temperature effects the crystal
growth kinetics and oxide phase formation. At 160 ºC, MoO3 and/or Mo4O11 precipitates
were formed while at 700 ºC, the precipitate grain grew to coalescence and finally to the
buried MoO2 layer. During annealing, a continuous transformation from MoO3 to MoO2 is
observed by in-situ XRD from 600 ºC to 700 ºC.
The techniques for in-situ XRD powder diffraction studies of hydrothermal and
solvothermal synthesis have matured and it is now possible to design experiments and to
study the crystallization of microporous materials under various conditions such as the
crystallization process of zeolite. XRD Diffractogram mainly provides information about
crystalline materials thus it is an efficient way of studying crystallization kinetics.
However, important processes, as reactions occurring in solution, gel formation stages and
nucleation, cannot be directly probed by this technique. This is one of the reasons for the
current emphasis on combined in-situ studies using complementary techniques
simultaneously in the same experiment, e.g. XRD/DTA, SAXS/WAXS or XRD/DLS.
Considerable challenges still remain in interpreting the results and elucidating nucleation
and crystallization mechanisms in detail (Norby, 2006).
Chapter 2, Literature Review _
46
Rodel et al. (2007) employed in-situ XRD and in-situ XAS techniques combined
with gas phase analysis to explore the structural evolution of single phase Mo5O14-type
materials which is (MoVW)5O14 and (MoV)5O14 under different reaction condition. These
techniques can provide an insight to the long-range order of materials and also the local
structure around the metal centres in the mixed oxides. The bulk properties of the two oxide
system appears to be different under reducing (propene), oxidizing (oxygen), atmosphere
and isothermal redox condition. From in-situ XRD analysis, both catalysts were initially
heated under helium to 773 K and the Mo5O14-type structure appears stable. Under
reducing condition (10% propene) at 773 K, both catalysts are transformed to monoclinic
MoO2-type structures. However the lattice constants of (MoV) dioxide have an increased
cell volume with a-axis expansion, b-axis and c-axis shortening. Temperature-programmed
XRD shows the reduction of (MoVW)5O14 starts at 723 K while (MoV)5O14 is at 673 K.
Under oxidizing condition (20% oxygen) at 773 K, the (MoVW) dioxide is re-oxidized to
the initial structure but (MoV) re-oxidized to orthorhombic MoO3-type structure. These
findings were backed up by in-situ XAS analysis. Therefore, the structure stabilizing effect
of tungsten was determined and also structure-directing effect towards re-oxidation to
Mo5O14-type structure.
Chapter 3, Methodology _
47
CHAPTER 3
METHODOLOGY
Chapter 3, Methodology _
48
3.0 Methodology
3.1 Chemicals and Gases
All of the chemicals and gases used in the catalyst preparation, activation and
reactivity studies are listed in Table 3.1.
Table 3.1: List of Chemicals & Gases Used
No Chemicals/Gases Supplier Description
1 Propane MOX
2 Purified Argon MOX 99.99%
3 Purified Helium MOX 99.99%
4 Purified Oxygen MOX 99.8%
5 Synthetic Air MOX 21% Oxygen in Nitrogen
6
Ammonium heptamolybdate
tetrahydrate
((NH4)6Mo7O24 . 4 H2O)
Merck
99%
7 Ammonium Metavanadate
(NH4VO3) Fluka 99%
8 Nitric Acid
(HNO3)
Friendemann
Schmidt 65%
9 Oxalic acid dihydrate
(COOH)2.2H2O Sigma Aldrich 99%
10 Vanadium (V) Oxide
(V2O5) Sigma Aldrich 98+%
Chapter 3, Methodology _
49
3.2 Synthesis of Mo Based-Catalyst Precursors
3.2.1 Synthesis of MoOx Catalyst
All the synthesis was done using controlled precipitation method to obtain
reproducible properties of the synthesized materials. For Molybdenum Oxides (MoOx)
precursor, the method used follows the controlled precipitation technique established by
Abd Hamid et al. (2003). In this study, the method is further optimized by adding more
variations to the controlled parameters. This was done to establish a set of relationship
between the parameters and investigate the effects of each variable on the molybdenum
oxide structure. Metal molybdate source used was Ammonium Heptamolybdate
Tetrahydrate (AHM), while Nitric Acid (HNO3) was used as the precipitating agent. Nitric
Acid (HNO3) was used since it is readily available and will not poison the catalyst as it is
thermally decomposed. The parameters varied were temperature (30 °C and 50 °C),
molybdate solution concentration (0.07 M, 0.10 M and 0.14 M), precipitating agent
concentration (1 M, 2 M, and 5 M), and rate of addition (1 mL/min, 3 mL/min, and 5
mL/min). The varying parameters of the experiments are shown in Table 3.2.
The precipitating agent (HNO3) was added to 200 mL AHM solution with a fine
control of the rate of addition using an autotitrator (Mettler Toledo DL50). The solution
was stirred and the pH changes were monitored throughout the titration process as the
termination point was set at pH 1. Equation (3.1) shows the reaction for this process.
OHNHMoOHOMoNH 24324764 3676 _ _ _ _ _ _ _ _ _ _ (3.1)
All precipitate obtained were vacuum filtered using a Buchner flask and vacuum pump at
-20 bar. The precipitates were then dried using vacuum dessicator for 3 days at 30 °C.
.
Chapter 3, Methodology _
50
Table 3.2: MoOx precursors synthesized using various conditions
No Sample No Cation [MoO4]
2-
Mol/l
[H]+
Mol/l
Temperature
°C
Rate of
Addition
mL/min
1 M014 NH4+ 0.07 1 30 1
2 M033 NH4+ 0.10 1 30 1
3 M064 NH4+ 0.10 1 30 3
4 M065 NH4+ 0.10 1 30 5
5 M035 NH4+ 0.14 1 30 1
6 M043 NH4+ 0.10 2 30 1
7 M021 NH4+ 0.10 5 30 1
8 M039 NH4+ 0.10 1 50 1
3.2.2 Synthesis of MoVOx Catalyst
For the synthesis of Molybdenum Vanadium Oxides (MoVOx) precursor, two
methods with different vanadium sources were used to achieve the desired catalyst
structure. The first synthesis method was achieved by modifying the formula of the method
established by Rodel et al. (2007) to obtain desired catalytic phase. Firstly Vanadyl oxalate
(VO(C2O4)) solution was prepared by dissolving the Vanadium (V) Oxide in Oxalic acid.
This was then added to a solution of Ammonium Heptamolybdate (AHM) with the rate of
addition of 1 mL/min using an autotitrator at 80 °C. After stirring at this temperature for
one hour, the solution was spray-dried by atomizing with compressed air at 6 bar and dried
with hot air at 200 °C using a mini spray-dryer (Buchi). The spray dried powder was then
collected for activation.
Chapter 3, Methodology _
51
The second method used was by mixing AHM and Ammonium Metavanadate
(AMV) (Adams et al., 2004). The synthesis was done in a precipitation reactor (LabMax) at
30 ºC. 200 mL of AMV solution varying from 10-90 % loading of vanadium content was
added to 200 mL AHM solution and the pH values were maintained at 5 by adding
appropriate amounts of nitric acid during the synthesis. The solution was then stirred for 90
min at 80 ºC. The final precursor solution was then spray dried by atomizing with
compressed air at 6 bar and dried with hot air at 200 °C. The samples with varying
parameters were shown in Table 3.3.
3.2.2.1 MoVOx Activation
The MoVOx spray dried precursors were calcined under different environments.
Samples from the first method using vanadyl were calcined under static air using muffle
furnace (Barnstead Thermolyne) at 500 ºC for 4 hours and another batch under Helium
using temperature-programmed-Reduction (TPR1100) pre-treatment port at 500 ºC for
4 hours.
Table 3.3: MoVOx precursors synthesized using various conditions
Sample
No
Vanadium
Source
[VO]2+
Mol/l
[VO3]4-
Mol/l
[MoO4]2-
Mol/l
Temperature
°C Activation
M038 Vanadyl Oxalate 0.1 - 0.05 80 Helium
M040 Vanadyl Oxalate 0.01 - 0.005 80 Helium
M042 Vanadyl Oxalate 0.05 - 0.025 80 Helium
M044 AMV 0.01 0.1 30 Nitrogen
M047 AMV 0.02 0.1 30 Nitrogen
M045 AMV 0.05 0.1 30 Nitrogen
M056 AMV 0.07 0.1 30 Nitrogen
Chapter 3, Methodology _
52
However, the obtained spray dried powders of MoVOx precursor synthesized using
Vanadates were calcined under Nitrogen using temperature-programmed-Reduction (TPR)
pretreatment port at 500 ºC for 4 hours.
3.3 Structural and Elemental Characterization
3.3.1 X-Ray Diffractogram (XRD)
The synthesized catalyst precursors were subjected to structural and compound
analysis using Bruker‘s XRD. The diffractograms were obtained using a Theta/2theta
goniometer and a Scintillation counter detector. For MoOx precursors, the data sets were
collected in reflection geometry in the range of 2° ≤ 2θ ≤ 60° with a step size of Δ2θ =
0.02° while for MoVOx precursors, the data sets were collected in in the range of 2° ≤ 2θ ≤
80° with a step size of Δ2θ = 0.02°. Phase analysis was done and phase purity was
determined using EVA software version 2002.
3.3.2 Scanning Electron Microscope (SEM) Imaging
FEI quanta 200F Field Emission Scanning Electron Microscope (FESEM) was used
to investigate the microstructure and surface structural defects of the precursors and the
catalyst. The morphology observation was carried out under low vacuum and accelerating
voltage of 5.0 HV. The images were captured under magnifications between ranges of 1000
to 60000.
Chapter 3, Methodology _
53
3.3.3 Energy Dispersive X-ray (EDX)
Spot analysis was done using Energy Dispersive X-ray (EDX) with INCA energy
400 to quantitatively analyze the local metal content. The weight percentage of elements in
the precursors and catalyst were determined. Elemental mapping were done to verify the
metal homogeneous distribution in the MoVOx precursors and catalyst.
3.4 Thermal Analysis
3.4.1 Thermogravimetric (TG)
The calcination temperature were determined by a Mettler Toledo TGA/SDTA851e
analyser coupled with a Thermostar Mass Spectrometer (MS) to analyze the evolved
decomposition gas as it measures the weight changes with respect to temperature. MoOx
precursors were heated from 30 °C to 700 °C while for MoVOx precursors were heated
from 30 °C to 500 °C at a heating rate of 5 °C/min under synthetic air flow rate at 50
mL/min. The thermograms were evaluated and the weight loss percentages were calculated
using the STARe software (V9.00).
3.4.2 Differential Scanning Calorimetry (DSC)
The crystallization and melting properties of the precursors were analyzed using
Differential Scanning Calorimetry (DSC) technique. The DSC instrument used was a
Mettler Toledo DSC822e equipped with a measuring cell with ceramic sensors which
measures the heat flow to detect endothermic and exothermic effects (Wagner, 2009). All
samples were heated from 30 °C to 500 °C at a heating rate of 10 °C/min under synthetic
air flow rate at 50 mL/min. The heat flow/energy of samples was calculated and evaluated
using STARe software V.800.
Chapter 3, Methodology _
54
3.5 Reactivity Studies
3.5.1 In-situ XRD
The structural phase changes with temperature were monitored through an in-situ
X-Ray Diffractometer (Bruker) (Figure 3.1) which was equipped with Flowbus mass flow
controllers and a Thermostar mass spectrometer (MS). Selected synthesised samples
(M033, M039, and M038) were heated from 30 °C to 500 °C at a heating rate of 5 °C/min
under Helium and synthetic air at 100mL/min. The diffractograms were obtained using a
Theta/theta goniometer and a position sensitive detector (PSD). For MoOx precursors, the
diffractogram data sets were collected using a position sensitive detector (PSD) in
reflection geometry in the range of 2° ≤ 2θ ≤ 60° with a step size of Δ2θ = 0.02° while for
MoVOx precursors, the data sets were collected in in the range of 2° ≤ 2θ ≤ 80°. The XRD
diffractograms were obtained at 50 °C & 25 °C intervals.
Figure 3.1: In-situ X-Ray Diffractometer (XRD)
X-ray Source PSD Detector
Reaction cell
Chapter 3, Methodology _
55
3.5.2 In-situ DSC
The energy changes of the catalyst precursors throughout the reaction were
investigated using High Pressure Differential Scanning Calorimeter, Model HPDSC827e
(Mettler Toledo) (Figure 3.2) attached to mass flow controllers (Bronkhorst) and a mass
spectrometer (Thermostar). Selected samples (M033, M039, and M038) were heated from
30 °C to 500 °C at a heating rate of 5 °C/min under inert (Argon) at 50mL/min. For the
propane ODH reaction, gas mixture of propane: oxygen: inert of 46:2:2 was mixed by the
mass flow controllers(MFC) and flowed at 50 mL/min into the chamber as the samples
were heated from 30 °C to 500 °C at 5 °C/min. The heat/energy required to transforms the
catalyst precursor structurally and the active phases were determined.
Figure 3.2: In-situ Differential Scanning Calorimeter (DSC)
Furnace
Mass Flow
Controller
HPDSC
Chapter 4, Results and Discussion _
56
CHAPTER 4
RESULTS AND
DISCUSSION
Chapter 4, Results and Discussion _
57
0 20 40 60 80 100
1
2
3
4
5
6
M033 (30°C)
M039 (50°C)
pH
Vol of HNO3
0 20 40 60 80 100
-0.12
-0.08
-0.04
0.00
M033 (30°C)
M039 (50°C)
Fir
st
De
riv
ati
ve
pH
Vol of HNO3
4.0 Results and Discussion
PART A
4.1 Synthesis and Characterization of MoOx Catalyst
4.1.1 Titration Curves
Table 3.2 shows a summary of the samples synthesized and their experimental
conditions. The differences from varying temperature of the AHM solution were
investigated by looking at samples M033 and M039. The samples were synthesized at
30 ºC (M033) and 50 ºC (M039) while maintaining all other parameters as constant.
Figure 4.1: Titration of 0.10 M AHM with 1.0 M HNO3 at different temperature
Figure 4.1 shows the titration curve and pH first derivatives comparison for M033
and M039. Based on the titration curve for M039, the starting pH is lower where the curve
starts at pH 5, implying that heating the AHM solution increases the solubility and
dissociation of AHM in water giving rise to H+ ions thus increasing acidity (Zhang et al.,
2011). The pH drop for M039 was less steep at the beginning of the titration and becomes
almost parallel to M033 at around pH 4.4 to pH 2.6. Based on the first derivative curve,
Chapter 4, Results and Discussion _
58
M033 exhibits 2 inflection points. At the first minimum point at 49.4 mL corresponding to
pH 2.4, no obvious precipitate was observed but a cloudy suspension was noticed.
However, at the maximum inflection point at 71.9 mL which corresponds to pH 1.7,
spontaneous white precipitate was observed. The building blocks consisting of {MoO7}
units were formed at the first inflection point. As protonation continues, solution
supersaturation was reached and the particles nucleation takes place. The bridging of
oxygen atoms networks of the MoO7 leads to the nuclei growth of nuclei and assembles
into a new bulk material of polyoxymolybdate (Cronin et al., 2000, Hu and Shaw, 1999).
M039 titration curve had a minimum point at 51.6 mL corresponding to pH 1.9 but
did not exhibit a maximum point as compared to M033. When heated, the solution
supersaturation decreases as {MoO7} unit particles solubility was increased in the solution
and therefore no spontaneous precipitation was observed (Feng et al, 2007a). Significantly
lesser amount of acid was needed for M039 synthesis to reach the end point. Only 74 mL of
HNO3 is needed for M039 to reach the final pH compare to M033 where 100 mL of HNO3
is needed. This coincides with the increase of proton consumption when temperature was
raised (Duc et al., 2008). The titrated solution was then further heated to 70 ºC and fine
white precipitates were observed. This was because temperature changes the MoO7
solubility which then affects the morphology of the structure (Feng et al. 2007a).
The second experiment was conducted by varying the molybdate source (AHM)
concentration. Figure 4.2 shows the titration curve and pH first derivatives comparison of
samples M014, M033, and M035 with AHM concentrations of 0.07 M, 0.10 M, and 0.14 M
respectively. Similar pattern of titration curve is observed for all three samples.
Chapter 4, Results and Discussion _
59
0 20 40 60 80 100 120 140
1
2
3
4
5
6
M014 (0.07M)
M033 (0.10M)
M035 (0.14M)
pH
Vol of HNO3
-20 0 20 40 60 80 100 120 140 160
-0.12
-0.08
-0.04
0.00
M014 (0.07M)
M033 (0.10M)
M035 (0.14M)
Fir
st
De
riv
ati
ve
pH
Vol of HNO3
Figure 4.2: Titration of AHM at different concentration with 1.0 M HNO3 at 1 mL/min
All three titration curves starts at pH 5.3 and as acid was added M014 curve shows
steeper drop in pH followed by M033 and M035. The minimum point of the derivative
curve gradually shifts as the concentration increases from 34.6 mL corresponding to pH 2.8
for M014, 50.2 mL corresponds to pH 2.4 for M033 and to 70.2 mL and 78.6 mL
corresponding to pH 2.5 and pH 2.1 respectively for M035. This was because more acid
was needed to reach the buffering equilibrium. The two minimum points observed in M035
titration curve indicates heterogeneous nucleation process takes place at different pH. The
unstable colloidal distribution of the precipitate consisting of MoOx species cannot be
maintained and the fragment interlinks and progressively grew forming larger units (Yu et
al., 2007) As Supersaturation was reached spontaneous precipitation creating white
precipitate were observed at the titration curve maximum points. These points also
gradually shift from 55.5 mL corresponding to pH 1.9 for M014, 72.1 mL corresponding to
pH 1.7 for M033 and at 90.3 mL corresponding to pH 1.8 for M035.
Chapter 4, Results and Discussion _
60
0 20 40 60 80 100 120
1
2
3
4
5
6
pH
Vol of HNO3
M033 (1mL/min)
M064 (3mL/min)
M065 (5mL/min)
0 20 40 60 80 100 120
-0.10
-0.08
-0.06
-0.04
-0.02
0.00
0.02
M033 (1mL/min)
M064 (3mL/min)
M065 (5mL/min)
Fir
st
De
riv
ati
ve
pH
Vol of HNO3
The third controlled precipitation experiment was conducted by varying the rate of
addition of the precipitating agent (HNO3). Figure 4.3 shows the titration curve and pH first
derivatives comparison for M033, M064 and M065 with titration rate of 1 mL/min, 3
mL/min, and 5 mL/min respectively.
Figure 4.3: Titration of 0.10 M AHM with 1.0 M HNO3 at different rate of addition
All three pH first derivatives curves exhibit one minimum point and one maximum
point. The pH curves become less steeper indicating that the continuous addition of acid
was consumed for the precipitation therefore contributes lesser to the pH changes (Behrens
et al., 2011). Almost the same amount of acid was needed to reach the first inflection point
for all curves. For M033 the minimum was observed at 49.4 mL corresponding to pH 2.4
while for M064 the minimum was observed at 51.5 mL corresponding to pH 2.3 and for
M065 the minimum was observed at 51.9 mL corresponding to pH 2.6. The maximum
inflection point however differs for all curves. Buffering equilibrium was not reached
during fast addition (5 mL/min) resulting in delay of reaction sequence. Therefore, M065
had the smallest maximum point followed by M064 and M033 (Abd Hamid et al., 2003).
Chapter 4, Results and Discussion _
61
0 20 40 60 80 100
1
2
3
4
5
6
M033 (1.0M)
M043 (2.0M)
M021 (5.0M)
pH
Vol of HNO3
0 20 40 60 80 100
-0.4
-0.3
-0.2
-0.1
0.0
0.1 M033 (1.0M)
M043 (2.0M)
M021 (5.0M)
Vol of HNO3
Fir
st
De
riv
ati
ve
pH
The last experiment was conducted by varying the precipitating agent concentration
which in this experiment is Nitric Acid (HNO3) at 1.0 M, 2.0 M and 5.0 M.
Figure 4.4: Titration of 0.10 M AHM with HNO3 at different concentration
Figure 4.4 shows the titration curve and pH first derivatives comparison of M033,
M043 and M021. The higher the concentration of HNO3 used, the lesser amount of acid
needed to reach the termination point at pH1. The titration curve of M021 was the steepest
followed by M043 and M033. Based on the derivatives, sharp pH change was observed for
M021 which causes huge error in the curve where the inflection point is uncertain.
However for titration using lower acid concentration, small but unambiguous pH change
was observed.
Chapter 4, Results and Discussion _
62
4.1.2 Structural and Elemental Characterization
4.1.2.1 X-Ray Diffractogram (XRD) Analysis
Structural and phase purity was determined by XRD Analysis. For MoOx based
precursors, two phases was observed, ‗supramolecular‘ phase and hexagonal phase.
Figure 4.5: XRD Diffractograms of MoOx showing ‗Supramolecular‘ structure peak
characteristics.
Figure 4.5 shows the XRD patterns of some of the samples consisting of
supramolecular structures. The diffractograms appear to have a similar pattern where a
sharp high peak appear at around 7° and low intensity peaks around 11°-12°. This is
consistent with the ‗supramolecular‘ structure of isopolyanion (Mo36O112) properties where
high intensity peaks are observed at lower angle (7°) and peaks are roughly resolved with
low intensity at slightly higher diffraction angle (11°-12°).
0 10 20 30 40 50 60
Supramolecular
M021
M043
M064
M065
M033
In
ten
sit
y (
a.u
)
2
M014
Chapter 4, Results and Discussion _
63
Table 4.1: X-Ray Data of M014 (MoOx) Table 4.2: X-Ray Data of M033 (MoOx)
Table 4.3: X-Ray Data of M064 (MoOx) Table 4.4: X-Ray Data of M065 (MoOx)
Table 4.5: X-Ray Data of M043 (MoOx) Table 4.6: X-Ray Data of M021 (MoOx)
2 Intensity (%) dexp
6.916 100 12.7718
9.717 42 9.0949
11.848 38 7.4636
12.979 32 6.8157
19.647 31 4.5149
25.866 32 3.4417
26.427 28 3.3699
* All interplanar distances were reported in Angstrom (Å)
2 Intensity (%) dexp
6.904 100 12.7929
9.247 34 9.5565
9.712 36 9.0999
11.320 36 7.8103
11.843 52 7.4666
2 Intensity (%) dexp
6.916 100 12.7714
9.492 29 9.3097
11.836 44 7.4709
12.06 21 7.3328
12.936 28 6.8378
19.577 27 4.5310
26.25 21 3.3923
2 Intensity (%) dexp
4.639 25 19.0342
6.995 100 12.6267
8.398 65 10.5208
10.739 35 8.2316
11.788 54 7.5015
12.624 25 7.0063
2 Intensity (%) dexp
4.586 24 19.2531
6.010 26 14.6930
6.950 100 12.7079
8.345 48 10.5870
9.341 66 9.4603
11.823 52 7.4793
2 Intensity (%) dexp
5.998 38 14.7229
6.943 100 12.7211
7.258 52 12.1701
8.374 66 10.5501
9.334 61 9.4674
11.253 67 7.8568
11.987 55 7.3775
25.844 48 3.4446
27.122 48 3.2852
Chapter 4, Results and Discussion _
64
The lower intensity peaks observed at higher angle indicates the synthesized
molybdenum oxides contain nanostructured building blocks (Abd Hamid et al., 2003).
Diffractograms of samples synthesized using lower concentration of AHM appears to be
more crystallized. The intensity of the prominent peak at 7º is different with M033 having
the highest intensity among all the other supramolecular structured catalyst indicating a
boost of oxygen influence on the structure (Bohne et al., 2005). The supramolecular
structure involving a 36-molybdate ion was self-assembled, involving an intricate system of
interlinking components of two 18-molybdate sub units via four common oxygen atoms.
The units of {Mo}18 were constructed from repetitive arrangement of sixteen {MoO6}
pseudo edge sharing octahedra and two {MoO7} distorted pentagonal bipyramids (Atencio
et al., 2004; Hu and Shaw, 1999; Paulat-Boschen, 1979). The powdered structure was
compacted as it was needed to form bulk materials and therefore protecting the
microstructures of nanoscale structure (Koch, 1999).
Table 4.1-4.6 shows some experimental data of the prominent XRD peaks of M014,
M033, M064, M065, M043 and M021 respectively. The interplanar spacing (d value) of the
most prominent peak at nearly 7º is slightly different. The interplanar distances of
molybdenum oxides samples synthesized using lower molybdate concentration are larger.
The interplanar spacing differences were also observed by comparing the precipitating
agent (HNO3) concentration where at higher concentration at 5 M (M021), the d value is
larger as compared to 1 M HNO3 (M033) and 2 M HNO3 (M043) which are similar. The
large interplanar spacing observed indicates the molybdenum oxide catalysts have smaller
crystallite size consistent with the nanostructural units (Ahmad and Bhattacharya, 2009).
Titration processes involving higher acid concentration creates precipitate faster thus lesser
Chapter 4, Results and Discussion _
65
0 10 20 30 40 50 60
Hexagonal
Supramolecular
M035
Inte
nsi
ty (
a.u
)
2
M039
time to agglomerate therefore smaller crystallite size (Song et al., 2007). However, as can
be seen in Figure 4.4, higher acidic concentration leads to indecisive particles structures
and can be seen in the diffractogram as there were more unidentified peaks compared to the
others.
Figure 4.6: XRD Diffractograms of MoOx showing hexagonal structure peak
characteristics.
Figure 4.6 shows the diffractogram obtained for both M035 and M039 and the
peaks exhibit high crystallinity. The diffractogram obtained for M039 which was
synthesized at 50 ºC has hexagonal phase properties and matches to the compound of
Ammonium Molybdenum Oxide Hydrate, (NH4)0.15MoO3.0.5H2O (PDF-File 29-0115)
with unit cell parameter a = 6.09 Å., b = 6.09 Å., c = 9.14 Å although the intensity of the
sample was higher as compared to the reference material indicating higher degree of
crystallization (Dieterle et al., 2001). As reported in Table 4.7, the interplanar spacing of
Chapter 4, Results and Discussion _
66
the M039 is smaller compared to the reference and this can be seen in the diffractogram
with the peaks shifting slightly. The sample peaks are shifted to higher angles relative to the
reference molybdenum oxides as the unit cell volume is smaller. The small peak shifts
suggests internal stress due to stacking faults of particles and this can be seen in the SEM
images which will be discussed later ( Abrishami et al., 2011; Ungár, 2004).
M035 XRD analysis shows characteristics of mixed phases of ‗supramolecular‘ and
hexagonal structure which matches Ammonium Molybdenum Oxide, (NH3 (MoO3)3) (PDF-
File 78-1027) which belongs to space group P63/m (176) with unit cell parameter a =
10.568 Å, b = 10.568 Å, and c = 3.726 Å. The supramolecular characteristics exist as
described earlier at lower angle 7º and 11º - 12º in the diffractogram but with a much lower
intensity. As reported in Table 4.7, the interplanar spacing of the sample is smaller
compared to the reference and this can be seen in the diffractogram with the peaks shifting
slightly to higher angle indicating sample particle unit cell volume to be smaller (Abrishami
et al., 2011; Keijser et al., 1991). This shows molybdate concentration does influence phase
structure of molybdenum oxide.
The diffraction peak for M035 with the highest intensity was at 9.77º angle indexed
at (110) plane as compared for M039 was at 26.03º angle indexed at (102) plane. This trend
can be attributed as the synthesized material still hold crystallized water which was
supported by the diffractogram match to Ammonium Molybdenum Oxide Hydrate. For
M039, there was also crystallized water (which will be discussed in thermal analysis) but
was possibly held in the supramolecular phase structure (Ilkenhans et al., 1995).
Chapter 4, Results and Discussion _
67
Table 4.7: X-Ray Data of M039 (MoOx)
2 Intensity (%) hkl dexp dref Phase
9.920 55 001 8.9092 9.1400 Hexagonal
17.027 12 100 5.2032 5.2741 Hexagonal
19.641 25 002 4.5162 4.5700 Hexagonal
26.030 100 102 3.4204 3.4538 Hexagonal
29.574 55 110 3.0181 3.0450 Hexagonal
35.640 33 112 2.5171 2.5340 Hexagonal
45.642 15 210 1.9861 1.9934 Hexagonal
49.103 16 - 1.8539 1.8601 Hexagonal
56.249 19 302 1.6341 1.6408 Hexagonal
Table 4.8: X-Ray Data of M035 (MoOx)
2 Intensity (%) hkl dexp dref Phase
6.971 31 - 12.6709 - Supramolecular
9.779 100 100 9.0378 9.1522 Hexagonal
16.906 17 110 5.2401 5.2840 Hexagonal
19.521 34 200 4.5438 4.5761 Hexagonal
25.899 82 101 3.4374 3.4510 Hexagonal
29.458 26 111 3.0297 3.0451 Hexagonal
35.514 18 121 2.5258 2.5351 Hexagonal
45.535 13 410 1.9905 1.9972 Hexagonal
* All interplanar distances were reported in Angstrom (Å)
Chapter 4, Results and Discussion _
68
Table 4.9: Crystallite size of MoOx catalyst precursors
Sample 2 d value (Å) FWHM Crystallite size (nm)
M014 6.932 12.7413 0.238 6.095
M033 6.923 12.7580 0.103 14.085
M064 6.985 12.6450 0.113 12.839
M065 6.956 12.6971 0.098 14.803
M035 9.781 9.0358 0.119 12.213
M043 6.916 12.7703 0.129 11.246
M021 6.954 12.7007 0.169 8.584
M039 26.031 3.4203 0.157 12.978
The X-ray crystallite size resembles the average of the smallest undistorted volumes
in the crystal. Table 4.9 shows the size differences of all samples calculated using the
Scherrer equation. The Full Width at Half Maximum (FWHM) values varies based on the
degree of crystallization and also the intensity of the diffractograms peak. Overall, the
crystallite sizes do vary indicating the titration parameters do play a major role in
controlling the degree of crystallization as suggested above. M033 synthesized at 30ºC has
a larger crystallite size compared to M039 at 50 ºC as temperature alters the morphology.
The crystallite size of M014 which is shown in Table 4.9 is smaller compared to M033 and
M035 which were prepared with higher molybdate concentration. This shows the
crystallinity of the molybdenum oxide particles increases with molybdate concentration,
hence larger crystallite size (Mahajan et al. 2008). The rate of precipitating agent addition
does not influence greatly on the crystallite size of the molybdenum oxide particles as the
sizes are very similar to each other. The crystallite size of both hexagonal structures of
M039 and M035 are similar.
Chapter 4, Results and Discussion _
69
4.1.2.2 Scanning Electron Microscope (SEM) Imaging
The effects of all the synthesis parameters on the morphology of the particles
structure were observed by SEM imaging. Figure 4.7 shows the SEM images for M014
which was synthesized using lower molybdate concentration (0.07M) M014 at 1000, 8000
and 15000 times magnification At lower magnification (1000 X), the image shows
aggregates clusters of particles. At higher magnification (15000 X) it is observed that there
are long blocks with smooth edges and no particular shape. Similar observation was seen in
Figure 4.8 for M033 but at lower maginification (1000 X), M033 shows large fraction of
needle-like agglomerates (Wagner et al., 2004). This result coincides with the XRD results
where the most crystalline peak among all the ‗supramolecular‘ structures was in M033.
Figure 4.9 and Figure 4.10 shows the catalyst structure SEM images for M064 and
M065 synthesized with fast addition of precipitating agent (HNO3) at 3 mL/min and 5
mL/min respectively. Both catalyst particles exhibits undefined particle structures, where
both do not have smooth edges and appears to have structural collapse at higher
magnification (15000 X). This is because the nanoparticles nucleation takes place quickly
due to the fast addition. Equilibrium is not reached at supersaturation point thus leading to
incomplete catalyst structure.
The morphology of M021 seen is Figure 4.12 also have undefined particle structure
as the nuclei growth of the catalyst becomes rapid due to the high precipitating agent
concentration (5 M). The morphology of M043 catalyst structure as can be seen in Figure
4.11 is similar to the morphology observed for M014 (Figure 4.7) where the cross section
Chapter 4, Results and Discussion _
70
average length is the same at (2.9 0.9) µm. The structure similarity also can be seen from
the XRD diffractograms of both catalysts (Figure 4.5).
Figure 4.14 shows the SEM image for M039 at 1000, 8000 and 15000 times
magnification. These images show clear hexagonal structure further confirming the XRD
analysis. The lower magnification image shows a cluster of the long rods with hexagonal
cross section (Song et al., 2007).
Figure 4.13 shows the SEM images for M035. The XRD analysis implies a phase
mixture of supramolecular structure and hexagonal. Here at lower magnification (1000X),
the agglomerates are like long blocks with no particular shape similar to Figure 4.8(a) but at
higher magnification (Figure 4.13 (b)), the hexagonal cross section of the rods are clearly
seen. However, at 15000 times magnification (Figure 4.13 (c)), hexagonal plates are
observed and appears to be larger than M039. This is in good agreement with the influence
of supersaturation on the morphology of the precipitate. At lower supersaturation, the
particles formed are small and nicely shaped while at higher supersaturation level larger
particles are formed but in a controlled way (Chow and Kurihara, 2002; Yu et al., 2007).
71
a) b) c)
Figure 4.7: SEM Imaging of M014 (Supramolecular structure)
a) b) c)
Figure 4.8: SEM Imaging of M033 (Supramolecular structure)
72
a) b) c)
Figure 4.9: SEM Imaging of M064 (Supramolecular structure)
a) b) c)
Figure 4.10: SEM Imaging of M065 (Supramolecular structure)
73
a) b) c)
Figure 4.11: SEM Imaging of M043 (Supramolecular structure)
a) b) c)
Figure 4.12: SEM Imaging of M021 (Supramolecular structure)
74
a) b) c)
Figure 4.13: SEM Imaging of M035 (Mixed Hexagonal and Supramolecular structure)
a) b) c)
Figure 4.14: SEM Imaging of M039 (Hexagonal structure)
Chapter 4, Results and Discussion _
75
4.1.2.3 Energy Dispersive X-ray (EDX)
Table 4.10: EDX Analysis of MoOx catalyst precursors
Sample Elements (Weight %)
Molybdenum (Mo) Oxygen (O)
M014 (supramolecular) 57.77 42.23
M033 (supramolecular) 56.01 43.99
M064 (supramolecular) 56.20 43.80
M065 (supramolecular) 57.06 42.94
M035 (hexagonal & supramolecular) 58.39 41.61
M043 (supramolecular) 56.07 43.93
M021 (supramolecular) 65.01 34.99
M039 (hexagonal) 53.51 46.49
Table 4.10 shows the morphological differences in terms of weight percentage
composition of molybdenum and oxygen in the synthesised MoOx catalyst precursor
samples. The molybdenum to oxygen bulk composition ratio of all samples appears to be
similar except for M021 catalyst precursor supporting the argument presented by XRD and
titration curve where the synthesis creates undefined particle structure therefore disrupts the
oxygen dispersion on the supramolecular framework.
Chapter 4, Results and Discussion _
76
4.1.3 Catalytic Thermal Analysis
Three catalyst samples of MoOx were chosen for thermal analysis due to the
structural diversity. Catalyst M033 (supramolecular), M035 (hexagonal) and M039
(hexagonal and supramolecular mixed phase structure) were subjected to
Thermogravimetric Analysis (TGA) accompanied with Mass Spectrometer (MS) and the
results were correlated with Differential Scanning Calorimetry (DSC) results during
temperature programmed analysis.
Figure 4.15 shows the thermogram of M033 catalyst (supramolecular) where the
analysis were done under synthetic air with flow rate at 50 mL/min while heated from 30
°C to 500 °C at a heating rate of 5 °C/min. Based on the evaluation, four mass changes
recorded and the rate of changes can also be seen clearly from the DTG curve. The mass
losses were correlated with the Mass Spectroscopy analysis in Figure 4.16 and linked to
DSC analysis in Figure 4.17.
Figure 4.15: TG/DTG Analysis of M033 from 30 °C to 500 °C
100 200 300 400 500
88
90
92
94
96
98
100
-0.08
-0.06
-0.04
-0.02
0.00
We
igh
t/ %
Temperature/C
DT
G
Chapter 4, Results and Discussion _
77
100 200 300 400 500
m/e=44
m/e=16
m/e=17
m/e=18
m/e=43
Temperature/C
Figure 4.16: MS Evaluation of M033 from 30 °C to 500 °C
Figure 4.17: DSC Analysis of M033 from 30 °C to 500 °C
100 200 300 400 500-4
-3
-2
-1
0
Temperature/C
mW
Chapter 4, Results and Discussion _
78
The first mass loss of 3.20% was recorded at temperature below 95 °C. At this
point, loose water was eliminated which can be identified in the MS graph as mass number
(m/e), 16 and 17. Since the DSC822e is a heat flux system, this reaction is considered
endothermic (Gabbott, 2008). Another two endothermic peaks were observed at the second
mass loss range at 95 °C to 215 °C. This temperature range recorded the highest mass loss
of 4.18% as the structure melting intensifies with crystallized water desorption. At the third
mass loss of 2.29% at temperature 215 °C to 320 °C, fragments of nitrogen dioxides
(m/e=44), and ammonium nitrates (m/e=17,44) are released as the catalyst restructures
from metastable hexagonal molybdates (h-MoO3) as can be seen from the exothermic peak.
The existence of ammonium fragments such as NH3, N2, and NO2 proves the existence of
ammonium in the hexagonal structure tunnels (Mougin et al., 2000).
The final weight loss was 1.6% at 320 °C to 500 °C is where the remaining
ammonium and nitrates corresponding to mass number 16 and 17, are evolved. At this
range, exothermic peak designating crystallization is observed as the catalyst decomposes
into the stable orthorhombic phase (o-MoO3) at 430 °C (Mougin et al., 2000). No more
changes is recorded after this temperature suggesting that 430 °C is the most
thermodynamically temperature. This transformation trend will be studied more deeply
using in-situ XRD in the following Part C.
Chapter 4, Results and Discussion _
79
100 200 300 400 500
94
96
98
100
-0.08
-0.06
-0.04
-0.02
0.00
We
igh
t /
%
Temperature/C
D
TG
100 200 300 400 500
m/e=19
m/e=20
m/e=17
m/e=44
m/e=18
Temperature/C
m/e=43
Figure 4.18: TG/DTG Analysis of M039 from 30 °C to 500 °C
Figure 4.19: MS Evaluation of M039 from 30 °C to 500 °C
Chapter 4, Results and Discussion _
80
.
Figure 4.20: DSC Analysis of M039 from 30 °C to 500 °C
Two mass losses of M039 catalyst (hexagonal structure) were recorded and
displayed in the thermogram at Figure 4.18. The first weight loss was 3.28% at temperature
below 235 °C and according to the mass spectroscopy evaluation (Figure 4.19), adsorbed
water (m/e=18) was released as the hexagonal structure starts melting and was seen as the
endothermic peak in Figure 4.20. The second mass loss was 2.80% recorded at temperature
from 235 °C to 500 °C. The fragments of nitrates, ammonium and oxides (m/e=17, 18, 44)
were evolved at this point. As all the ammonium ions were released from the metastable
hexagonal molybdates (h-MoO3) tunnels, the catalysts decomposed into its
thermodynamically stable state which was seen as sharp exothermic DSC peak in Figure
4.20. Unlike described by Song et al. (2007) the catalyst recrystallizes into the
thermodynamically stable orthorhombic MoO3 at 400 °C and not 450 °C (Mougin et al.,
2000; Song et al., 2007). No more change takes place after 420 °C. Hence this temperature
is used as the reference temperature or the activation of the catalyst.
100 200 300 400 500
-3
-2
-1
0
1
2
mW
Temperature/C
Chapter 4, Results and Discussion _
81
Figure 4.21: TG/DTG Analysis of M035 from 30 °C to 500 °C
Figure 4.22: MS Analysis of M035 from 30 °C to 500 °C
100 200 300 400
93
94
95
96
97
98
99
100
101
-0.05
-0.04
-0.03
-0.02
-0.01
0.00
0.01
We
igh
t/ %
Temperature/C
DT
G
100 200 300 400 500
m/e=19
m/e=20
m/e=17
m/e=44
m/e=18
m/e=43
Temperature/C
Chapter 4, Results and Discussion _
82
Figure 4.23: DSC Analysis of M035 from 30 °C to 500 °C
Figure 4.21 thermogram displays two major mass losses for M035 catalyst
(hexagonal and supramolecular mixed phase structure) similar to sample M039. The first
loss is at temperature below 225 °C with mass loss of 3.06%. Based on the mass
spectroscopy analysis (Figure 4.22), desorption of crystallized water (m/e=17,18) takes
place at this temperature range which were also accompanied by the shallow broad
endothermic DSC peak in Figure 4.23 indicating moisture loss. The second mass loss was
at 225 °C - 500 °C with 3.51%. At this point, fragments of nitrates, ammonium and oxides
(m/e=17, 18, 44) were evolved at this point. The metastable hexagonal molybdates (h-
MoO3) decomposes into its thermodynamically stable state of orthorhombic molybdenum
oxides (o-MoO3). However, the DSC peak in Figure 4.23 was not as sharp as Figure 4.20.
During crystal structure rearrangement, the catalyst undergoes degradation as the material
did not reach the energy state equilibrium, thus resulting in non-uniform crystallization
(Gabbott, 2008).
100 200 300 400 500
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
mW
Temperature/C
Chapter 4, Results and Discussion _
83
PART B
4.2 Synthesis and Characterization of MoVOx Catalyst
4.2.1 Structural and Elemental Characterization of MoVOx
4.2.1.1 X-Ray Diffractogram (XRD) Analysis
(a)
(b)
Figure 4.24: XRD Diffractograms of MoVOx spray dried precursors synthesized
using (a) vanadyl oxalate (b) ammonium metavanadate
0 20 40 60 80
Inte
nsity
(a.u
)
2
M040
M042
M038
0 20 40 60 80
M056
M045
M047Inte
nsity
(a.u
)
2
M044
Chapter 4, Results and Discussion _
84
The first method which is using vanadyl oxalate as the vanadium source shows that
all spray dried precursors samples exhibits similar XRD pattern which is amorphous phase
as can be seen in Figure 4.24 (a). The amorphous material was obtained since the solute in
the precursor solution was in a distorted phase; there was not enough time to form
crystallization as it is dried rapidly. The highest reflection which was the amorphous hump
observed between 8º and 15º shows poorly crystalline oxides and a second broad shorter
hump observed around 24º 2 angles. This suggests that the precursors may be
nanostructured (nanocrystalline) material (Knobl et al., 2003). The halo intensity of all
oxide precursors were the same indicating modification on the concentration ratio of
molybdenum and vanadium does not affect the structural properties.
The other method in synthesizing MoVOx precursors was done using ammonium
metavanadate and was later calcined under Nitrogen flow using TPR pretreatment port.
Similar XRD diffractogram patterns of the spray dried precursors were observed in Figure
4.24(b) as compared to Figure 4.24(a). Vanadium loading at 10% (M044) and 20% (M047)
does not affect the intensity of the highest intensity amorphous halo which is between 7 º
and 14 º. However, higher vanadium loading at 50% (M045) and 70% (M056) increases the
intensity of the broad peak. This was because molybdenum and vanadium atomic ratio is
affected as more vanadium was deposited in precursor oxide thus increasing the intensity of
the amorphous halo.
Chapter 4, Results and Discussion _
85
0 20 40 60 80
Hexagonal
Orthorhombic
(iii)
(ii)
Inte
nsi
ty (
a.u
)
2
(i)
* Tetragonal
***
**
**
*
****
0 20 40 60 80
Orthorhombic
Hexagonal
Monoclinic
(iii)
(ii)
Inte
nsi
ty (
a.u
)
2
(i)
* Tetragonal
*****
*
*
**
Figure 4.25: XRD Diffractograms of M038 before and after calcined (i) spray Dried
(ii) calcined under static air (iii) calcined under Helium.
Figure 4.26: XRD Diffractograms of M042 before and after calcined (i) spray Dried
(ii) calcined under static air (iii) calcined under Helium
Chapter 4, Results and Discussion _
86
The spray dried amorphous precursor of M038 and M042 were activated under two
conditions. The first activation condition was under static air using a muffle furnace at
500 °C. The XRD diffractogram observed in Figure 4.25 showed the changes of the spray
dried M038 amorphous phase into crystalline peaks after activation under both conditions.
Figure 4.25(ii) shows the transformation into highly crystalline structure as can be observed
by the rapid increase of intensity at 10°. The XRD diffractogram exhibits mixed phase with
hexagonal structure matching Vanadium Molybdenum Oxide, (V0.12Mo0.88) O2.94, PDF File
81-2414, which belongs to space group P63 (173) with unit cell parameter a = 10.593 Å, b
= 10.593 Å, and c = 3.6944 Å. The other phase that exist is orthorhombic phase that
correlates to the thermodynamically stable Molybdenum Oxide, (MoO3), PDF-File 65-
2421, which belongs to space group Pnma (62) with unit cell parameter a = 13.825 Å, b =
3.694 Å, and c = 3.954 Å. This was because of exposure to oxidizing medium such as air
thus causing the complex hexagonal oxides breakdown to the o-MoO3 phase (Knobl et al.,
2003).
Table 4.11 shows the X-ray data of M038 calcined in air. The hexagonal phase was
more prominent in the agglomerated crystal structure. The interplanar spacing of the
hexagonal reference material of Vanadium Molybdenum Oxide is smaller but the
orthorhombic reference material of Molybdenum Oxide is larger compare to M038
interplanar spacing. This indicates that the hexagonal Vanadium Molybdenum Oxide
crystallite size is smaller than the reference material but the crystallite size of the
orthorhombic phase that exists is larger than the reference material. This may have been
due to the incorporation of Vanadium into the framework. Highest intensity was recorded
at 9.6º at (100) plane.
Chapter 4, Results and Discussion _
87
Table 4.11: X-Ray Data of M038 (calcined in air)
2 Intensity (%) hkl dexp dref Phase
9.651 100 100 9.1574 9.1738 Hexagonal
12.768 14 200 6.9278 6.9125 Orthorhombic
16.760 13 110 5.2856 5.2965 Hexagonal
19.347 18 200 4.5842 4.5869 Hexagonal
23.367 12 101 3.8038 3.8016 Orthorhombic
25.691 62 210 3.4648 3.4674 Hexagonal
27.392 18 210 3.2534 3.2580 Orthorhombic
29.441 28 111 3.0315 3.0301 Hexagonal
33.825 8 220 2.6479 2.6483 Hexagonal
35.457 19 121 2.52966 2.5283 Hexagonal
Table 4.12: X-Ray Data of M038 (calcined in He)
2 Intensity (%) hkl dexp dref Phase
7.719 26 200 11.4433 11.4195 Tetragonal
8.657 21 210 10.2056 10.2139 Tetragonal
12.255 15 310 7.2165 7.2223 Tetragonal
16.459 21 330 5.3815 5.3832 Tetragonal
22.263 100 001 3.9899 3.9900 Tetragonal
23.372 43 600 3.8031 3.8065 Tetragonal
24.951 71 540 3.5659 3.5669 Tetragonal
26.146 31 630 3.4056 3.4046 Tetragonal
27.612 17 550 3.2279 3.2299 Tetragonal
28.146 17 640 3.1679 3.1672 Tetragonal
31.565 36 740 2.8321 2.8328 Tetragonal
33.711 18 541 2.6566 2.6592 Tetragonal
* All interplanar distances were reported in Angstrom (Å)
Chapter 4, Results and Discussion _
88
The M038 precursor which was activated under Helium flow using TPR
pretreatment port, the diffractogram obtained (iii) is very different from (ii). The
diffractogram is also crystalline and matches Vanadium Molybdenum Oxide (V0.07Mo0.93)5
O14, PDF File 31-1437 corresponding to the tetragonal phase with unit cell parameter a =
22.839 Å, b = 22.839 Å, and c = 3.99 Å. Between 8º and 15º angle as observed in the spray
dried amorphous diffractogram, the broad halo transforms to sharp peaks corresponding to
the Mo5O14 reflections of (210) and (310). The second halo around 24º transforms sharp
peaks which correspond to plane (540) also corresponding to the tetragonal phase as
displayed in Table 4.14 (Knobl et al., 2003; Zenkovets et al., 2007). The interplanar
spacing of M038 catalyst activated under Helium is smaller when the focus is on prominent
peaks.
The highest intensity of the catalyst diffractogram is recorded at (001) plane which
was the reflection for Mo5O14 therefore was in good agreement with the literature (Knobl et
al., 2003). The tetragonal structure of Mo5O14 was regarded to be a suitable catalyst
because of the structure and channel network that can accommodate heteroatom dopants
and oxygen for reaction. The structure does not have edge sharing octahedra but instead
clusters of octahedra around a fivefold bipyramid containing Mo5+
and V4+
atoms which are
interconnected by a corner-sharing octahedra network consisting of [Mo8O26]4-
(Knobl et
al., 2003; Werner et al., 1997). In the tetrahedra V4+
containing groups have oxygen ions
which were known to be easier to remove from the lattice during reaction thus promoting
the oxygen-containing products. The stability of this catalyst with oxygen vacancies was
essential for high dehydrogenation selectivity (Mamedov & Cortés Corberán, 1995). The
dynamics of this structural transformation will be shown in in-situ XRD analysis in part C.
Chapter 4, Results and Discussion _
89
Table 4.13: X-Ray Data of M042 (calcined in air)
2 Intensity (%) hkl dexp dref Phase
12.809 100 001 6.9056 6.8917 Monoclinic
23.406 20 10-1 3.7976 3.7559 Monoclinic
25.738 81 210 3.4585 3.4674 Hexagonal
27.409 45 011 3.2514 3.2510 Monoclinic
33.792 10 220 2.6504 3.6483 Hexagonal
39.013 37 400 2.3069 2.2935 Hexagonal
Table 4.14: X-Ray Data of M042 (calcined in He)
2 Intensity (%) hkl dexp dref Phase
7.752 16 200 11.3954 11.4195 Tetragonal
8.684 20 210 10.1746 10.2139 Tetragonal
22.300 100 001 3.9833 3.9900 Tetragonal
23.385 24 600 3.8010 3.8065 Tetragonal
24.972 84 540 3.5629 5.6685 Tetragonal
26.263 31 630 3.3907 3.4046 Tetragonal
27.632 20 550 3.2257 3.2299 Tetragonal
28.204 21 450 3.1615 3.1618 Orthorhombic
28.450 21 630 3.1348 3.1428 Orthorhombic
31.582 41 740 2.8306 2.8328 Tetragonal
33.724 26 750 2.6556 2.6550 Tetragonal
36.975 19 711 2.4292 2.4198 Orthorhombic
* All interplanar distances were reported in Angstrom (Å)
Chapter 4, Results and Discussion _
90
Figure 4.26 shows the diffractogram changes of the spray dried M042 (i) when
calcined using two different conditions. Figure 4.26 (ii) shows that when calcined in static
air the diffractogram obtained reveal highly crystalline structure of mixed phases but is
different as compared to M038 calcined under air diffractogram. The diffractogram also
displays hexagonal structure matching Vanadium Molybdenum Oxide, (V0.12Mo0.88) O2.94,
PDF File 81-2414. However, the other phase that exist was the monoclinic phase that
attributes to Molybdenum Oxide, (MoO3), PDF File 47-1320, which belongs to space
group P21/m(11) with unit cell parameter a = 3.954 Å, b = 3.687 Å, and c = 7.095 Å.
Table 4.13 shows the X-Ray data of M042 (calcined in air). The most prominent
peak with the highest intensity was recorded at 12.8º 2 angle and unlike M038 (calcined in
air), it is at monoclinic (001) plane. The interplanar spacing of the existing monoclinic
phase is larger compare to the reference but for the hexagonal phase that was present, the d
values are smaller as compared to the PDF-File reference material values.
M042 catalyst activation under Helium flow gives the same result as M038
(calcined He). The crystalline diffractogram mostly matches Vanadium Molybdenum
Oxide (V0.07Mo0.93)5 O14, PDF File 31-1437 corresponding to the tetragonal phase.
However the diffractogram obtained also reveals orthorhombic phase matching
Molybdenum Oxide (Mo17O47), PDF File 71-0566 corresponding with unit cell parameter a
= 21.531 Å, b = 19.534 Å, and c = 4.001 Å belonging to space group Pba2 (32). These
traces were of partial collapse of (MoV)5O14 oxide phase and this was not observed in
M038 catalyst precursor which uses higher concentration of both vanadyl and molybdate
Chapter 4, Results and Discussion _
91
0 20 40 60 80
Orthorhombic
Monoclinic
Triclinic
Unknown
*
*
*
M056 (70%V)
M045 (50%V)
M047 (20%V)
Inte
ns
ity
(a
.u)
2
M044 (10%V)
*
*
salt (Knobl et al., 2003). For both phases, as in Table 4.14, the d values of the catalyst are
smaller compared to the reference material.
Figure 4.27: XRD Diffractograms of MoVOx synthesized using ammonium
metavanadate calcined under Nitrogen
Figure 4.27 shows the diffractograms of M044, M047, M045 and M056 (Vanadium
varies from 10%-70%) after activation under Nitrogen with a rate of 5 ºC/min. M044
(10%V), M047 (20%V) and M045 (50%) all shows similar diffractogram with varying
intensity. All the diffractograms shows mixed phase of monoclinic phase of Vanadium
Molybdenum Oxide, (MoV2O8), PDF-File 20-1377 with space group C2 (5) and unit cell
parameter a = 19.398 Å, b = 3.629 Å, and c = 4.117 Å. The other phase coexisting is
orthorhombic phase of Molybdite,syn (MoO3), PDF-File 89-5108 with space group Pbnm
(62) and unit cell parameter a = 3.962 Å, b = 13.855 Å, and c = 3.701 Å.
Chapter 4, Results and Discussion _
92
Table 4.15: X-Ray Data of M044 (calcined in Nitrogen)
2 Intensity (%) hkl dexp dref Phase
12.739 25 020 6.9433 6.9275 Orthorhombic
21.003 18 - 4.2264 - -
22.159 35 001 4.0085 4.1169 Monoclinic
23.350 57 110 3.8065 3.8093 Orthorhombic
25.696 41 040 3.4641 3.4638 Orthorhombic
27.356 100 021 3.2575 3.2644 Orthorhombic
33.641 30 111 2.7187 2.6945 Monoclinic
33.820 22 510 2.6483 2.6502 Monoclinic
38.960 15 060 2.3099 2.3092 Orthorhombic
49.286 25 002 1.8474 1.8531 Orthorhombic
Table 4.16: X-Ray Data of M047 (calcined in Nitrogen)
2 Intensity (%) hkl dexp dref Phase
12.771 29 020 6.9263 6.9275 Orthorhombic
21.002 12 - 4.2265 - -
22.191 28 001 4.0027 4.1169 Monoclinic
23.393 52 101 3.7997 3.8016 Orthorhombic
25.680 40 040 3.4662 3.4638 Orthorhombic
27.342 100 021 3.2593 3.2644 Orthorhombic
32.914 31 111 2.7191 2.6945 Monoclinic
33.659 28 510 2.6606 2.6502 Monoclinic
38.958 24 060 2.3100 2.3092 Orthorhombic
49.290 29 020 1.8473 1.8470 Orthorhombic
* All interplanar distances were reported in Angstrom (Å)
Chapter 4, Results and Discussion _
93
Table 4.17: X-Ray Data of M045 (calcined in Nitrogen)
2 Intensity (%) hkl dexp dref Phase
12.801 31 020 6.9097 6.9275 Orthorhombic
21.003 26 - 4.2264 - -
22.236 51 001 3.9947 4.1169 Monoclinic
23.418 63 110 3.7957 3.8093 Orthorhombic
25.703 43 040 3.4633 3.4638 Orthorhombic
27.379 100 021 3.2549 3.2644 Orthorhombic
32.899 31 111 2.7203 2.6945 Monoclinic
33.651 33 510 2.6612 2.6502 Monoclinic
39.013 21 060 2.3069 2.3092 Orthorhombic
49.306 24 002 1.8467 1.8505 Orthorhombic
Table 4.18: X-Ray Data of M056 (calcined in Nitrogen)
2 Intensity (%) hkl dexp dref Phase
9.695 31 200 9.1156 9.6988 Monoclinic
22.289 100 001 3.9853 4.1169 Monoclinic
23.470 26 -201 3.7874 3.7978 Monoclinic
25.052 39 001 3.5517 3.5570 Triclinic
25.743 35 -110 3.4579 3.3771 Triclinic
29.539 26 200 3.0216 3.0236 Triclinic
31.692 33 - 2.8211 - -
45.341 19 202 1.9986 2.0112 Monoclinic
* All interplanar distances were reported in Angstrom (Å)
Chapter 4, Results and Discussion _
94
Table 4.15 – 4.17 shows some of the experimental data of all three diffractograms.
As observed, the most prominent peak of all diffractograms with the highest intensity was
at 27.4º corresponding to (021) plane of the orthorhombic structure. At this plane, the
interplanar spacing of all three synthesized catalyst is smaller as compared to the reference
material. The monoclinic phase existence contradicts the findings by Kunert et al. (2004),
where only hexagonal phase was detected in the spray dried material.
M056 (70%) however exhibits a different diffractogram compare to samples
containing lower amount of Vanadium. The diffractogram shows mixed phase but mostly
matches Vanadium Molybdenum Oxide (V0.95 Mo0.97O5), PDF-File 77-0649, attributing to
a Triclinic system with space group P1 (1) and unit cell parameter a = 6.334 Å, b = 4.0463
Å, and c = 3.7255 Å. The other phase that coexists is the monoclinic phase of Vanadium
Molybdenum Oxide, (MoV2O8), PDF-File 20-1377 which is the same phase in the other
three synthesized catalyst. Higher dispersion of vanadium provides ‗site-isolation‘ effect
which is important to avoid olefins transforming to neighboring oxidized sites (Ballarini et
al., 2004). Table 4.18 shows the X-ray data of M056 (calcined). The prominent peak with
the highest intensity unlike the other catalyst is at 22.2º at plane (001) of the monoclinic
phase. The shifting to lower angle can be deduced as the catalyst having a larger unit cell
volume (Abrishami et al., 2011; Keijser et al., 1991).
Chapter 4, Results and Discussion _
95
Table 4.19: Crystallite size of MoVOx samples
Sample 2 d value FWHM Crystallite size
(nm)
M038 (calcined in air) 9.651 9.1567 0.095 15.297
M038 (calcined in He) 22.269 3.9888 0.143 10.321
M042 (calcined in air) 12.806 6.9072 0.097 15.022
M042 (calcined in He) 22.283 3.9864 0.115 12.834
M044 (calcined in Nitrogen) 27.346 3.2587 0.140 10.645
M047 (calcined in Nitrogen) 27.349 3.2584 0.157 9.492
M045 (calcined in Nitrogen) 27.373 3.2556 0.139 10.722
M056 (calcined in Nitrogen) 22.291 3.9850 0.172 8.581
Table 4.19 displays the different crystallite sizes of all the activated catalysts. As
observed, the size which depends on the degree of crystallization varies with different
conditions used not only in synthesizing but also in catalyst activation. The calcined
catalysts were therefore made of nanostructure crystallite which is stabilized by amorphous
matrix by spray drying process (Li et al., 2010). The smallest crystallite size observed is in
sample M056_calc which were synthesized with 70% Vanadium salt precursor. Higher
vanadium loading decreases the crystallite size with more bonding interaction between
Mo-V. For samples synthesized using vanadyl source, the smallest particle size as shown
are in M038 (calcined Helium). Smaller crystallite size shows that under controlled flow of
He, nanostructuring were more refined as compared to the calcination under static air.
Chapter 4, Results and Discussion _
96
4.2.1.2 Scanning Electron Microscope (SEM) Imaging
Figure 4.28 shows the SEM images of M038 (a, b, c) and M042 (d, e, f) spray dried
catalyst morphology at 8000, 15000 and 30000 times magnification. At lower
magnification, the images shows clusters of spherical, smooth ball like particles with no
discrete features which is consistent with any kind of spray dried precursors (Kunert et al.,
2004). At higher magnification, the ball like structure appears to be spherical particles
smooth surface areas indicating the effect of drying process on surface texture (Endres et
al., 2007).
The morphology of M038 catalytic structures after calcination was shown in Figure
4.29. Figure 4.29 (a, b, c) were images after calcination under static air. Here as can be seen
at the lower magnification, the ball like structure appears to be decomposed and no longer
has a smooth surface. At higher magnification, new crystallite structure emerges and looks
like to be made of rough looking hexagonal slices. This confirms the XRD data analysis as
shown in Figure 4.25. The hexagonal plates cross section length measure at an average of
(1.1 0.4) µm.
Figure 4.29 (d, e, f) were images for M038 catalyst precursor after calcination under
Helium. At lower magnification, the ball like structure appears to be uneven but less ‗flaky‘
unlike images (a,b). At higher magnification, a new finely dispersed phase which appears
to be compiling finer crystallite. The newly formed particles morphology have tetragonal
cross sections coexisting with some other morphology indicating mixed phase and further
confirming the XRD analysis. The length of the tetragonal cross section averages around
(0.5 0.1) µm which was half the size of M038 (b) (Sidorchuk et al., 2010).
Chapter 4, Results and Discussion _
97
The morphology of M042 catalytic structures after calcination was shown in Figure
4.30. Figure 4.30 (a, b, c) were images after calcination under static air. Here as can be seen
at the lower magnification (a) the ball like shape seems to have decomposed thus appear to
be made out of aggregates of flat plates. At higher magnification (b), it can be seen clearly
that the flat plates were hexagonal crystallite. The cross section length of the hexagonal
plates measures at an average of (2.0 0.5) µm. There was also a different morphology that
can be seen in the SEM images and this was related to the orthorhombic structure as
discussed in the XRD diffractogram in Figure 4.26.
Figure 4.30 (d,e,f) were images of M042 after calcination under Helium flow.
Similar to M038, the spray dried balls particulates do not exhibit any sorts of plate at lower
magnification. At higher magnification, a mixed phase of tetragonal cross sections
coexisting with some other morphology presumably orthorhombic phase as analyzed from
the XRD data (Table 4.15). The length of the tetragonal cross section averages around
(0.39 0.08) µm.
Figure 4.31 shows the SEM images of M044 before (a,b,c) and after (d,e,f)
calcination under Nitrogen flow. Before calcination, the SEM images appears to be ball
like structures just like the other spray dried catalyst precursors but the ball structures are
not smooth and appears to be made out of clusters of plates. The diameter of the balls were
in the range of 1-10 micrometer while the cross section length of the plates averages around
(0.8 0.2) µm. However after calcination, there seems to have cracking of plates, creating
polycrystalline solid rod like structure with cross sections diameter averages at (0.21
0.07) µm (Petkov, 2008).
Chapter 4, Results and Discussion _
98
Figure 4.32 shows the SEM images of M056 before (a,b,c) and after (d,e,f)
calcination under Nitrogen flow. Before calcination, the SEM images appear to be ball like
structures just like the other spray dried catalyst precursors. After calcination,
polycrystalline needles are formed with the ball like structure still remains intact although
decomposition had taken place. As it is also displayed in M044, these were deduced to be
the thermodynamically stable oxides of monoclinic phase of Vanadium Molybdenum
Oxide, (MoV2O8) (Adams et al., 2004).
99
a) b) c)
d) e) f)
Figure 4.28: SEM imaging for M038 (a,b,c) and M042 (d,e,f) before calcination
100
a) b) c)
d) e) f)
Figure 4.29: SEM imaging for M038 after calcination (a,b,c) in air & calcination (d,e,f) in Helium
101
a) b) c)
d) e) f)
Figure 4.30: SEM imaging for M042 after calcination (a,b,c) in air & calcination (d,e,f) in Helium
102
a) b) c)
d) e) f)
Figure 4.31: SEM imaging for M044 before (a,b,c) & after calcination in Nitrogen (d,e,f)
103
a) b) c)
d) e) f)
Figure 4.32: SEM imaging for M056 before (a,b,c) & after calcination in Nitrogen (d,e,f)
Chapter 4, Results and Discussion _
104
4.2.1.3 Energy Dispersive X-ray (EDX)
Table 4.20: EDX Analysis of MoVOx catalyst precursors
Sample Elements (Weight %)
Molybdenum (Mo) Oxygen (O) Vanadium (V)
M038 52.67 45.21 2.12
M038 (calcair) 56.04 41.73 2.23
M038 (calcHe) 64.27 33.16 2.57
M042 45.16 51.59 3.24
M042 (calcair) 55.71 40.27 4.03
M042 (calcHe) 60.96 34.95 4.09
M044 56.18 43.31 0.50
M044 (calc) 55.20 43.18 1.62
M056 52.01 45.31 2.68
M056 (calc) 54.61 42.82 2.57
Table 4.20 shows the weight percentage composition of Molybdenum, Oxygen and
Vanadium in the synthesised MoVOx catalyst precursor samples. The vanadium
composition increases coinciding with an increase of vanadium addition. Calcination
affects the elemental composition with small changes (Knobl, et al., 2003)
Based on the elemental mapping analysis in Figure 4.33 (M038) and 4.34 (M042),
by employing the method involving vanadyl oxalate as the Vanadium source and after
calcination under helium flow, the elements appears to be homogenously dispersed in the
bulk catalyst.
105
Figure 4.33: Elemental Mapping for MoVOx, M038 (calcination in Helium)
106
Figure 4.34: Elemental Mapping for MoVOx, M042 (calcination in Helium)
Chapter 4, Results and Discussion _
107
4.2.2 Catalytic Thermal Analysis
Four catalyst precursors of MoVOx (M038, M042, M044 and M056(70 %V))
varied in synthesis method and structural properties were chosen for thermal analysis. The
catalyst subjected to Thermogravimetric (TG) Analysis accompanied with Mass
Spectrometer (MS) and the results were correlated with Differential Scanning Calorimetry
(DSC) results during temperature programmed analysis.
Figure 4.35 shows the thermogram of M038 where the analysis was done under air
with flow rate at 50ml/min while the sample was heated from 30 °C to 500 °C. Four steps
of mass loss were recorded. First mass loss of 5.45% was recorded at below 170 °C.
Based on the MS evaluation in Figure 4.36, water (m/e=16, 17 & 18) is released at this
point. Broad endothermic peak is observed from the DSC curve in Figure 4.37. As the
melting process continues, a second endothermic peak is observed which was associated
with a mass loss of 2.71% in the thermogram and at this section desorption of crystallized
water (m/e=16, 17 & 18) takes place. The third mass loss was 4.98% at temperature 230 °C
to 285 °C. DSC curve shows endothermic and exothermic effect with fragments of water
(m/e=16,17), carbon dioxide (m/e=28), ammonia (m/e=17,16) and nitrogen oxides
(m/e=29) released from the catalyst. The oxidation of ammonia and the reduction of
vanadium and molybdenum precursors generate the nitrogen oxides fragments while carbon
dioxide formation results from the decomposition of vanadyl oxalate used as the starting
material (Knobl et al., 2003). The final mass loss of 7.3% was recorded at 285 °C to 500
°C. At this section, the remaining fragments of water, oxides and ammonium were released
as crystallization process to the thermodynamically stable tetragonal phase of the catalyst
was correlated with the exothermic peak observed in the DSC curve.
Chapter 4, Results and Discussion _
108
100 200 300 400 500
80
85
90
95
100
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
Temperature/C
We
igh
t/ %
DT
G
100 200 300 400 500
Temperature/C
m/e=29
m/e=16
m/e=14
m/e=17
m/e=18
m/e=28
Figure 4.35: TG/DTG Analysis of M038 from 30 °C to 500 °C.
Figure 4.36: MS Evaluation of M038 from 30 °C to 500 °C.
Chapter 4, Results and Discussion _
109
Figure 4.37: DSC Analysis of M038 from 30 °C to 500 °C.
100 200 300 400 500
-4
-3
-2
-1
0
Temperature/C
mW
Chapter 4, Results and Discussion _
110
Figure 4.38: TG/DTG Analysis of M042 from 30 °C to 500 °C.
Figure 4.39: MS Evaluation of M042 from 30 °C to 500 °C.
100 200 300 400 500
80
85
90
95
100
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
DT
G
We
igh
t /
%
Temperature/C
100 200 300 400 500
m/e=46m/e=44
m/e=43
m/e=17
m/e=45
m/e=29
Temperature/C
Chapter 4, Results and Discussion _
111
Figure 4.40: DSC Analysis of M042 from 30 °C to 500 °C.
Five steps of mass loss were observed from the Figure 4.38 thermogram. The first
mass loss of 3.68% was recorded at temperature below 150 °C. Based on the MS evaluation
in Figure 4.39, Nitrogen (m/e=29) and loose water (m/e=17) were evolved at this point
which associates with the broad endothermic peak displayed in DSC curve (Figure 4.40).
The second mass loss of 4.05% in temperature range of 150 °C involves the elimination of
crystallized water (m/e=17) which was correlated with the endothermic effect observed.
Third mass loss of 7.00% and fourth mass loss of 3.29% at temperature range of 230 °C to
300 °C and from 300 °C to 370 °C were accompanied with a endothermic and exothermic
effect respectively. Similar to M038, based on the MS evaluation, ammonia oxidation along
with the vanadium and molybdenum reduction releases nitrogen oxides (m/e=44,46) and
ammonium fragments (m/e=17,29) while the decomposition of vanadyl oxalate releases
carbon dioxide (m/e=43,45). The final mass loss is associated with the crystallization
process of the catalyst as it decomposes to the thermodynamically stable tetragonal and
orthorhombic phase which is correlated to the DSC exothermic peak.
100 200 300 400 500
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
Temperature/C
mW
Chapter 4, Results and Discussion _
112
100 200 300 400 500
80
85
90
95
100
-0.0003
-0.0002
-0.0001
0.0000
0.0001
W
eig
ht/
%
Temperature/C
D
TG
Figure 4.41: TG/DTG Analysis of M044 from 30 °C to 500 °C.
Figure 4.42: MS Evaluation of M044 from 30 °C to 500 °C.
100 200 300 400 500
m/e=16
m/e=17
m/e=18
m/e=20
m/e=39
m/e=40
m/e=19
Temperature/C
Chapter 4, Results and Discussion _
113
100 200 300 400 500
-5
-4
-3
-2
-1
0
1
mW
Temperature/C
Figure 4.43: DSC Analysis of M044 from 30 °C to 500 °C.
Six phases of mass loss were observed in the thermogram of M044 (10 %V) in
Figure 4.41. At temperature below 130 °C, 1.84% mass loss were recorded along with the
release of water (m/e=18) which can be correlated to the broad endothermic peak in Figure
4.43 at this temperature range. The second mass loss of 3.17% from temperature 130 °C to
195 °C eludes crystallized water (m/e=16,17,18) according to the MS evaluation in Figure
4.42. At this range also, endothermic effect was observed in the DSC Curve. The following
three mass losses were 7.82%, 5.02% and 0.89% at temperature 195 °C to 275 °C, 275 °C
to 345 °C and 345 °C to 390 °C respectively. At all three temperature range, fragments of
oxides (m/e=39, 40), nitrates (m/e=18, 19) and ammonium (m/e=16, 17) were released and
endothermic effects were observed. The final mass loss of 1.43% at temperature 390 °C to
500 °C attributes to an exothermic peak at 420 °C as the catalyst decomposes to the
thermodynamically stable molybdenum oxides (MoO3) and Vanadium Molybdenum Oxide
(V2MoO8) which is irreversible (Adams et al., 2004).
Chapter 4, Results and Discussion _
114
Figure 4.44: TG/DTG Analysis of M056 from 30 °C to 500 °C.
Figure 4.45: MS Evaluation of M056 from 30 °C to 500 °C.
100 200 300 400 500
75
80
85
90
95
100
-0.20
-0.15
-0.10
-0.05
0.00
We
igh
t/ %
DT
G
Temperature/ C
100 200 300 400 500
m/e=16m/e=44
m/e=43
m/e=17
m/e=18
Temperature/C
Chapter 4, Results and Discussion _
115
100 200 300 400 500
-4
-3
-2
-1
0
1
mW
Temperature/C
Figure 4.46: DSC Analysis of M056 from 30 °C to 500 °C.
Figure 4.44 shows the thermogram of M056 (70 %V). Five steps of mass loss were
observed. Similarly to M044, the first mass loss of 3.30% at temperature below 135 °C and
based on the MS evaluation in Figure 4.45, this loss is attributed to the release of water
(m/e=17, 18) and can be confirmed by the broad endothermic effect in Figure 4.46 DSC
peak. The second mass loss of 6.65% from temperature 135 °C to 250 °C corresponds to
the release of crystallized water (m/e=17,18), ammonium (m/e=16) and nitrates
(m/e=43,44) At this temperature two endothermic effects were observed from the DSC
Analysis. Similar results were obtained at the third mass loss of 4.69% at temperature 250
°C to 300 °C with ion fragments from the decomposition of oxides (m/e=16) and
ammonium nitrates (m/e=43,44) that was formed during synthesis, which correlates to an
endothermic effect (Mougin et al., 2000). The following loss of 5.09% also involved the
removal of nitrogen oxides and the remaining ammonium and towards the end of this range
Chapter 4, Results and Discussion _
116
an exothermic peak is observed indicating restructuring process in the bulk structure mixed
oxide starts to occur (Kunert et al., 2004). This crystallization process takes place at
temperature of 455 °C to 500 °C with mass loss of 0.54%, as the catalyst is decomposed to
the thermodynamically stable phase consisting of mixed phase mostly of monoclinic and
triclinic phase as discussed based on the XRD Data in Table 4.18.
Chapter 4, Results and Discussion _
117
PART C
4.3 Reactivity Studies
4.3.1 In-situ X-Ray Diffractogram (XRD) Analysis
Selected catalyst precursors were subjected to activation using the experimental
obtained from thermal analysis of those samples. Form MoOx precursors, M033 and M039
were chosen. This was done to show comparison of the two different crystalline phase
structure of supramolecular and hexagonal. While for MoVOx precursors, M038 were
chosen based on the calcination analysis seeing that at 500 °C, M038 appears to be more
pure phase as compared to the other samples. All samples were activated under Helium at
100ml/min. The first experiment (M033) were conducted from 50 °C to 500 °C at a heating
rate of 5 °C/min and the diffractograms were obtained using a Position-Sensitive Detector
(PSD) at every 50 °C until 200 °C and every 25 °C from 200 °C until 500 °C.
Figure 4.47 shows the XRD of MoOx precursor (M033) under in-situ activation
program. The final temperature was set at 500 °C, determined based on the thermal
analysis. When heated above 250 °C, the removal of water causes the transformation of
phase to a metastable hexagonal phase and reaches the optimal metastable hexagonal phase
at 300 °C. The phase matches Molybdenum Oxide, (MoO3) (PDF-File 21-0569) which
belongs to space group P(0) with unit cell parameter a = 10.531 Å, b = 10.531 Å, and c =
14.876 Å although the diffractogram shifts a little to the lower angle indicating a larger
hexagonal unit cell. Table 4.21 shows the interplanar spacing of the metastable hexagonal
phase at 300 °C. By comparison, the d values of the sample were larger than the reference
Chapter 4, Results and Discussion _
118
material and this was consistent with the sample having larger unit cell volume (Keijser et
al., 1991).
Figure 4.47: In-situ XRD of MoOx precursor (M033) activation from
50 °C-500 °C under Helium Gas
As the sample was heated continuously, at 375 °C the intensity of the peak at 10°
plane (100) reduces drastically and eventually disappears at 425 °C. The intensity of the
other prominent metastable peaks at 25° and 29° correlated to plane (210) and (300)
respectively were also reduced while new peak was observed at 27° when the temperature
reaches 375 °C. The catalyst phase changes to the final thermodynamically stable
orthorhombic phase (o-MoO3) at above 425 °C although the diffractogram shows a few
10 20 30 40 50 60
500C
475C
450C
425C
400C
375C
350C
325C
300C
275C
250C
225C
200C
150C
100C
50C
Inte
ns
ity
(a
.u)
2
Hexagonal (PDF 21-059)
Orthorhombic (PDF 89-5108)
Chapter 4, Results and Discussion _
119
unidentified peaks. X-Ray data (Table 4.22) at 500 °C shows the orthorhombic phase
matches Molybdite, syn (MoO3), (PDF-File 89-5108) which belongs to space group
Pbnm(62) with unit cell volume a = 3.962 Å, b = 13.855 Å, c = 3.701 Å although there are
few unidentified peaks.
Table 4.21: X-Ray Data of M033 Activation (300 °C)
2 Intensity (%) hkl dexp dref Phase
9.591 100 100 9.2145 9.1201 Hexagonal
16.684 18 110 5.3094 5.2655 Hexagonal
19.294 27 200 4.5967 4.5601 Hexagonal
25.617 93 210 3.4746 3.4471 Hexagonal
29.283 57 300 3.0474 3.0400 Hexagonal
30.877 10 204 2.8936 2.8821 Hexagonal
33.760 11 220 2.6528 2.6328 Hexagonal
35.319 34 310 2.5392 2.5295 Hexagonal
Table 4.22: X-Ray Data of M033 Activation (500 °C)
2 Intensity (%) hkl dexp dref Phase
12.561 17 020 7.0412 6.9275 Orthorhombic
23.249 34 110 3.8229 3.8093 Orthorhombic
23.433 26 - 3.7932 -
25.394 35 040 3.5046 3.4638 Orthorhombic
27.221 100 021 3.2735 3.2644 Orthorhombic
32.735 16 101 2.7336 2.7046 Orthorhombic
33.489 23 111 2.6737 2.6545 Orthorhombic
* All interplanar distances were reported in Angstrom (Å)
Chapter 4, Results and Discussion _
120
Similar as the phenomena observed around 300 ºC, the diffractogram shifts to lower
angle as compared to the reference material and the interplanar spacing as shown in Table
4.22 were bigger compared to the reference material indicating larger unit cell volume. The
arrangement to thermodynamically stable phase of o-MoO3 was facilitated by the oxygen
vacancies in the oxygen deficient intermediate metastable h-MoO3 phase (Giebeler et al.,
2010). The precursor of molybdenum oxides were therefore presumed to grow
topotactically along the (110) plane in reflection to orthorhombic molybdenum oxides.
Table 4.23: Crystallite size of M033 after in-situ XRD activation
Sample 2 d value FWHM Crystallite size (nm)
M033 (50 ºC) 12.116 7.2991 0.234 6.223
M033 (300 ºC) 9.589 9.2157 0.095 15.296
M033 (500 ºC) 27.203 3.2755 0.159 9.370
Table 4.23 shows the crystallite size of the catalyst precursor upon activation
progression. As can be seen, the highest intensity peak changes when the precursor is
heated. The crystallite size also changes drastically where the metastable hexagonal phase
having the largest crystallite size while the hexagonal phase obtained at 50 °C had the
smallest crystallite size according to the Scherrer equation. As the sample was heated,
sample hydration creates a structure directing effect, nanostructuring happens at the
metastable hexagonal phase and the crystal structure growth increases the crystallite size at
300 °C. According to the Wagner et al. (2004), at 500 °C, nanostructuring is lost because of
the o-MoO3 large crystal growth. However, as shown in Table 4.23, the crystallite size
decreases suggesting structural degradation of the large o-MoO3 catalyst which may be
attributed to the unidentified peaks.
Chapter 4, Results and Discussion _
121
The second experiment using MoOx precursor (M039) were conducted from 50 °C
to 500 °C at a heating rate of 5 °C/min and the diffractograms were obtained using a
Position-Sensitive Detector (PSD) at every 50 °C until 200 °C and every 20 °C from
200 °C until 500 °C. The XRD diffractograms of M039 under in-situ activation were shown
in Figure 4.48. The hexagonal Molybdenum Oxide phase obtained at 50 °C becomes more
crystallised as the catalyst precursor was heated to higher temperature. X-ray data obtained
for diffractogram at 240 °C matches Molybdenum Oxide, (MoO3) (PDF-File 21-0569)
corresponding to hexagonal phase same as M033 catalyst activation. This shows as the
temperature increases, all the ammonium ions were released to form metastable hexagonal
molybdates (h-MoO3) as can be seen from the thermal analysis.
Figure 4.48: In-situ XRD of MoOx precursor (M039) activation from
50 °C-500 °C under Helium Gas
10 20 30 40 50 60
500C
480C
460C
440C
420C
400C
380C
360C
340C
320C
300C
280C
260C
240C
220C
200C
150C
100C
50C
Inte
ns
ity
(a
.u)
2
Hexagonal (PDF 21-0569)
Orthorhombic (PDF 89-5108)
Chapter 4, Results and Discussion _
122
Table 4.24: X-Ray Data of M039 Activation (240 °C)
2 Intensity (%) hkl dexp dref Phase
9.591 48 100 9.2139 9.1201 Hexagonal
16.701 19 110 5.3040 5.2655 Hexagonal
19.333 26 200 4.5875 4.5601 Hexagonal
25.713 100 210 3.4619 3.4471 Hexagonal
29.264 46 300 3.0493 3.0400 Hexagonal
35.343 28 310 2.5376 2.5295 Hexagonal
43.024 12 320 2.1007 2.0923 Hexagonal
45.353 16 410 1.9981 1.9902 Hexagonal
46.522 14 404 1.9505 1.9438 Hexagonal
48.837 11 008 1.8634 1.8595 Hexagonal
Table 4.25: X-Ray Data of M039 Activation (500 °C)
2 Intensity (%) hkl dexp dref Phase
9.671 14 - 9.1382 - -
12.403 26 020 7.1309 6.9275 Orthorhombic
23.138 47 110 3.8409 3.8093 Orthorhombic
25.074 24 040 3.5486 3.4638 Orthorhombic
27.159 100 021 3.2808 3.2644 Orthorhombic
33.029 12 101 2.7099 2.7046 Orthorhombic
33.603 21 111 2.6649 2.6545 Orthorhombic
38.108 16 131 2.3596 2.3338 Orthorhombic
45.564 14 200 1.9893 1.9810 Orthorhombic
49.250 19 002 1.8487 1.8505 Orthorhombic
55.150 12 112 1.6640 1.6645 Orthorhombic
* All interplanar distances were reported in Angstrom (Å)
Chapter 4, Results and Discussion _
123
Similar to M033, according to Table 4.24 the interplanar spacing of the catalyst was
larger compare to the reference material, thus having larger unit cell volume and decreasing
crystallite size (Ahmad and Bhattacharya, 2009). Heating the sample above 340 °C reduces
the intensity of the h-MoO3 peak at 26° plane (210). Continuous heating leads to the
formation of peak at 27° plane (021) and eventually the intensity increased with
temperature. Above 400 °C, The catalyst phase changes to the final thermodynamically
stable orthorhombic phase (o-MoO3). X-ray data at 500 °C (Table 4.25) shows the
orthorhombic phase matches Molybdite, syn (MoO3), (PDF-File 89-5108) same as M033
catalyst. The interplanar spacing as seen in Table 4.25 were also larger compared to the
reference material indicating larger unit cell volume and smaller crystallite size.
Table 4.26: Crystallite size of M039 after in-situ XRD activation
Sample 2 d value FWHM Crystallite size (nm)
M039 (50 ºC) 25.690 3.4649 0.137 10.841
M039 (240 ºC) 25.708 3.4625 0.147 10.104
M039 (500 ºC) 27.160 3.2807 0.171 8.712
The crystallite sizes of the M039 catalyst during activation were calculated using
Scherrer equation (Table 4.26). The crystallite size decreases as the catalyst reaches its
thermodynamically stable o-MoO3 phase at 500 ºC. As the hexagonal structure morphology
of molybdenum oxides did not change at 240 ºC, the crystallite size is similar as the starting
material. However, similarly to M033 catalyst, at 500 ºC structural degradation occur thus
creating smaller crystal structure which may also be correlated to the unknown diffraction
peaks. This occurrence indicates that the bulk o-MoO3 catalyst were not stable thus forming
crystallized MoO2 at higher temperature (Dieterle et al., 2001; Ressler et al., 2000).
Chapter 4, Results and Discussion _
124
10 20 30 40 50 60 70 80 90
500C
450C
400C
350C
300C
250C
200C
150C
100C
50C
Inte
ns
ity
(a
.u)
2
Tetragonal(PDF 31-1437)
The third experiment were conducted for MoVOx precursor sample, where M038
were heated from 50 °C to 500 °C under helium at a heating rate of 5 °C/min and the
diffractograms were obtained using a Position-Sensitive Detector (PSD) at every 50 °C.
The XRD diffractograms of M038 under in-situ activation were shown in Figure 4.49. The
amorphous phase of M038 starts to change at 150 °C with the increase of the highest
reflection of the amorphous halo at 12° and 26° region. These broad amorphous peaks
intensities grew with the precursor‘s degree of crystallinity. The precursor therefore grew in
a perpendicular direction along (001) plane which is the reflection of nanocrystalline
Mo5O14 (Knobl et al., 2003).
Figure 4.49: In-situ XRD of MoVOx precursor (M038) activation from
50 °C-500 °C under Helium Gas
Chapter 4, Results and Discussion _
125
These reflections continue to increase drastically until 400 °C. At this point, it is
presumed the water is removed leading to the vanadium expulsion into secondary structure
which corresponds to an intermediate amorphous form (Ilkenhans et al., 1995). At 400 °C,
the diffractogram has hump that is broad and this is presided to be the nanocrystalline phase
of Mo5O14 (Giebeler et al., 2010). Heating the precursor to 450 °C, a sharp reflection is
observed at 27° which was in good agreement with the domain growth in basal plane
(Zenkovets et al., 2007).
The final thermodynamically stable phase obtained is Vanadium Molybdenum
Oxide (V0.07Mo0.93)5 O14, PDF File 31-1437 corresponding to the tetragonal phase with unit
cell parameter a = 22.839 Å, b = 22.839 Å, and c = 3.99 Å. However unlike the ex-situ
XRD analysis during M038 calcination under helium, the prominent peaks of the
diffractogram obtained has as higher intensity suggesting a higher degree of crystallization.
This catalyst can be deduced to having a very high structural stability as temperature does
not affect the particles bulk structure which is Mo5O14 (Zenkovets, et al., 2007). The
interplanar spacing was larger indicating smaller refined crystallite size due to the
confinement of lattice deformations of V5+
five fold coordination (Dieterle et al., 2001)
According to the Scherrer equation which was calculated using the FWHM value
(Table 4.28), the crystallite size is smaller at the nanocrsystalline phase at 450 °C as
compared to the final tetragonal phase structure. This supports the previous argument that
the tetragonal phase growth starts from the nanocrystalline phase at 400 °C and at 500 °C
the crystallite size increases in accordance with particle growth. The crystallite size of the
Chapter 4, Results and Discussion _
126
nanocrystalline phase at 450 °C is also smaller compared to the MoOx samples showing
that Vanadium addition induces nanocrystallite particles.
Table 4.27: X-Ray Data of M038 Activation (500 °C)
2 Intensity (%) hkl dexp dref Phase
16.458 20 330 5.3820 5.3832 Tetragonal
22.017 100 001 4.0340 3.9900 Tetragonal
23.336 28 600 3.8089 3.8065 Tetragonal
23.661 15 610 3.7573 3.7547 Tetragonal
24.920 49 540 3.5703 3.5669 Tetragonal
26.105 61 630 3.4107 3.4046 Tetragonal
27.574 18 550 3.2323 3.2299 Tetragonal
28.128 16 640 3.1699 3.1672 Tetragonal
28.395 18 - 3.1407 - -
31.515 34 740 2.8365 2.8328 Tetragonal
33.485 18 621 2.6740 2.6774 Tetragonal
36.829 19 721 2.4385 2.4662 Tetragonal
Table 4.28: Crystallite size of M038 after in-situ XRD activation
Sample 2 d value FWHM Crystallite size (nm)
M038 (450 ºC) 26.099 3.4115 0.219 6.787
M038 (500 ºC) 22.009 4.0354 0.118 12.502
* All interplanar distances were reported in Angstrom (Å)
Chapter 4, Results and Discussion _
127
100 200 300 400 500
-16
-14
-12
-10
-8
-6
-4
-2
0
2
mW
Temperature/C
4.3.2 In-situ Differential Scanning Calorimetry (DSC) Analysis
Figure 4.50: In-situ DSC of M033 activation from 30 °C -500 °C
Figure 4.51: MS of In-situ DSC of M033 activation from 30 °C -500 °C
100 200 300 400 500
Temperature/C
m/e=17
m/e=19
m/e=18
Chapter 4, Results and Discussion _
128
In-situ DSC analysis was carried out for selected samples (M033 and M038) using
50 ml gas mixture of propane: oxygen: inert (Ar) of 46:2:2. The gases were mixed by the
mass flow controllers and flowed at 50 ml/min into the chamber as the samples were heated
from 30 °C to 500 °C at 5 °C/min. Figure 4.50 shows the in-situ DSC peaks for M033
during the propane ODH reaction. Four phases of catalyst structural transformation was
observed.
Comparing with the structural transformation trend using in-situ XRD, the first
phase was in the region of temperature less than 120 °C. DSC curve in Figure 4.50 shows
endothermic effect with water desorption (m/e=18,19) as shown in Mass evaluation in
Figure 4.51. The second phase at temperature range 120 °C to 250 °C also demonstrates
endothermic effect with the elimination of crystalline water (m/e=18,19). There is a rather
shallow exotherm observed in the third section of the DSC peak at temperature 250 °C to
375 °C. Here, the metastable hexagonal molybdenum oxide catalyst were oxidized and the
structural phase transition is irreversible (Gabbott, 2008; Werner et al., 1997).
However, no changes in MS were detected unlike the ex-situ DSC results in this
temperature range which eluded NH3, N2, and NO2. Ammonium (m/e=17) was only
detectable in the final phase transition of M033 at temperature of 375 °C to 500 °C. The
exothermic effect shows the crystallization to the final thermodynamically stable
orthorhombic phase of molybdenum oxides. Fragments of propene were not detectable
throughout the reaction suggesting either the reaction condition was not suitable or the
catalyst was not effective.
Chapter 4, Results and Discussion _
129
100 200 300 400 500
-40
-30
-20
-10
0
mW
Temperature/C
Figure 4.52: In-situ DSC of M038 activation from 30 °C -500 °C
Figure 4.53: MS of In-situ DSC of M038 activation from 30 °C -500 °C
100 200 300 400 500
Temperature/C
m/e=33
m/e=32
m/e=58
m/e=57
m/e=56
m/e=17
m/e=18
Chapter 4, Results and Discussion _
130
The same reaction condition was applied for catalyst precursor M038. Five regions
of intrinsic structural transformation were observed in Figure 4.52. A broad endothermic
effect was observed at temperature below 100 °C that correlates to the release of water
(m/e=18) which is shown in the mass evaluation in Figure 4.53. A small exothermic effect
was observed in the second temperature range of 100 °C – 350 °C. Fragments of
ammonium (m/e=17) and crystallized water (m/e=18) are eluded. However, fragments of
oxygenates (m/e=56,57,58) also exist indicating propane/propene oxidation process may
have occurred.
At the third temperature range of 350 °C to 400 °C, only water (m/e=18) was
detected from the MS which may have resulted as a side product of the oxidative
dehydrogenation reaction. This however did not have any effect in the catalyst structure as
shown in the DSC peak. An increase of lower hydrocarbon fragments (m/e=56,57,58) and
water (m/e=18) was observed from the MS at the temperature range of 400 °C to 450 °C.
At this section also, an exothermic peak was observed which supports the fact of
nanocrystalline phase growth. At the final temperature range of 450 °C to 500 °C, no
structural changes was observed while the MS continues to show that fragments of
hydrocarbon (m/e=57,58) and water (m/e=18) are released. Thus, this nanocrystalline phase
also is the most reactive.
Chapter 5, Conclusion _
131
CHAPTER 5
CONCLUSION
Chapter 5, Conclusion _
132
5.0 Conclusion
Propylene demand has increased drastically over the past years outpacing the
ethylene demand. Catalytic propane oxydehydrogenation (ODH) provides a better
alternative compared to the dehydrogenation process by reducing the reaction temperature
thus thermodynamically more favorable. The design of ODH reaction catalyst required
fundamental understanding of the structural-activity relationship to gain an insight of the
catalytic facilitated reaction mechanism. Molybdenum and vanadium based-catalyst system
are known to activate the C-H bond of propane which is the reaction mechanism rate
determining step hence making them the most suitable catalytic material for ODH reaction.
A variety of samples have been successfully synthesized for MoOx and MoVOx
based-catalyst via controlled precipitation method. For MoOx based-catalyst, the titration
parameters varied were temperature (30 ºC, 50 ºC), rate of addition (1 mL/min, 3 mL/min,
and 5 mL/min), molybdate solution concentration (0.07 M, 0.10 M and 0.14 M) and
precipitating agent (HNO3) concentration (1 M, 2 M, and 5 M). Temperature played a
major role in establishing the solution supersaturation forming the catalyst structure. At
30 ºC, precipitate formed exhibited supramolecular structure (Mo36O112) properties.
From the XRD analysis, high intensity peaks are observed at lower angle (7°) and
peaks are poorly resolved with low intensity at higher angle (10°-12°). The growths of the
catalytic structure were induced by protonation where Mo7O24 acts as a nucleus creating the
polyoxomolybdates. The bulk structural arrangement of the corner sharing pentagonal
channels of Mo36O112 distorted state gives rise to active lattice oxygens and may facilitate
Chapter 5, Conclusion _
133
the migration of lattice oxygen in the lattice, which is suitable for achieving high and stable
oxidation activity. This is achieved without applying heat to the preparation. At 50 ºC, no
spontaneous precipitation is observed and upon further heating to higher temperature,
supersaturation is reached and the precipitate displayed clear hexagonal phase structure
(h-MoOx), regardless of concentration used. The crystallite size calculated using the
Scherrer equation show evidence of the crystallite particles being nanostructured. The
properties of these catalysts were also analyzed using SEM and EDX analysis.
Thermal Analysis using TG-MS correlated with the DSC were also done. For the
supramolecular structure, endothermic effect was observed from as the catalyst restructures
to the metastable hexagonal molybdates (h-MoO3). At 430 °C, exothermic peak designating
crystallization is observed as the catalyst decomposes into the stable orthorhombic phase
(o-MoO3). For the hexagonal structure, the catalyst recrystallizes from the metastable
hexagonal phase into the thermodynamically stable orthorhombic MoO3 at 400 °C.
In binary oxides, the addition of vanadium promotes catalytic activity thus the need
to increase interactions of Mo and V species. In the synthesis of MoVOx based-catalyst,
amorphous phase was observed for all spray dried precursors. Highly crystalline hexagonal
phase (MoV2O8) and tetragonal phase [(MoV)5O14] were obtained after activation under air
and inert respectively when vanadyl was used as the vanadium source. Mo5O14 with the
tetragonal structure contains the pentagonal ring channels also. Vanadium behaves as
structural promoters thus stabilizing the Mo5O14 phase. The distortion of the dioxide phases
of Mo5O14 increases with increasing Vanadium content. When vanadates were used as the
vanadium source, different phases were observed with the increasing vanadium loading
Chapter 5, Conclusion _
134
where at 10% vanadium, mostly orthorhombic phase was observed form the XRD
diffractogram while at 70% vanadium loading, mixed phase of monoclinic and triclinic
were obtained. All the particles obtained had displayed nanocrystalline properties and had
been verified in the material characterization using SEM, EDX and XRF which also
showed homogeneous dispersion of the loading metals which provides the ‗site isolation‘
effect.
Thermal Analysis using TG-MS correlated with the DSC were also done for
MoVOx catalyst. For MoVOx with tetragonal structure (Mo5O14), endothermic effect were
observed until around 400 ºC where the amorphous phase transforms into the
thermodynamically stable tetragonal phase accompanied with an exothermic peak showing
crystallization process. However, for MoVOx with monoclinic structure (70% V),
crystallisation to the thermodynamically stable mixed phase of monoclinic and triclinic
phase happened only at 450 ºC.
Temperature programmed activation using in-situ XRD were used to study the
dynamics of structural transformation of selected synthesized MoOx bulk catalyst
precursor. The transformation for MoOx precursor took place from supramolecular to
metastable hexagonal phase at 275 ºC. The structure finally transforms to the stable
orthorhombic phase at 450 ºC. For MoVOx catalyst precursor, transformation was from
amorphous to nanocrystalline at 400 ºC. The crystal growth along the (001) plane indicates
the reflection of nanocrystalline Mo5O14. At 500 ºC, the thermodynamically stable
tetragonal phase was achieved with high degree of crystallization.
Chapter 5, Conclusion _
135
By incorporating the in-situ XRD data analysis, the reactivity of the catalyst was
also studied using a recently developed technique, in-situ DSC. For MoOx catalyst
precursor, fragments of propene were not observed thus no catalytic activity was observed
as the catalyst structural morphology transforms. However, the catalyst oxidation did take
place around 275 ºC. For MoVOx catalyst precursor, catalytic activity was observed at the
nanocrystalline phase region at 450 ºC as the catalyst structure transformed with
temperature. Fragments of olefins were also detectable. Hence, the most reactive region is
the nanocrystalline phase but the reaction mechanism remains unclear.
The reaction parameters also play a major role in obtaining the highest activity
besides the catalyst. In future works, parameters such as inert gas flow rate, hydrocarbon
and oxygen ratio can be varied. Moreover, by interfacing a Gas chromatograpy (GC) to the
in-situ XRD and in-situ DSC instrumental setup, the percentage of olefins eluded can be
measured giving a more cohesive activity comparison to the structural-activity relationship
of the catalyst.