Nanotechnology & Catalysis Research Centre (NANOCAT), IPS Building, University Malaya, Kuala Lumpur, Malaysia
AbstractGlycerol, a byproduct of biodiesel process, further adds more bioderived feedstocks to the scenario. Catalytic approach becomes interested in glycerol conversion to modify its rate, ther-modynamic, and time scale reaction. Many examples of catalytic applications of metal, metal oxide, bimetallic, acid, base, ionic liquid, and the enzyme are documented, but development of the field has been hampered by the lack of a conceptual approach and understanding of the real conversion mechanism. The main core of this paper is to highlight catalytic reactivity on different kinds of catalysis in oxidation, dehydration, acetylation, etherification, esterification, acetalization, ammoxidation, and enzymatic process of glycerol conversion. The productions of different types of chemical value-added of citric acid, lactic acid, 1,3-dihydroxyacetone, 1,3- propanediol, dichloro-2-propanol, acrolein, hydrogen, and ethanol are consequently dem-onstrated. The key aspect, characterization, and synthesis mechanism of each glycerol conver-sion is highlighted, which consequently demonstrate the synthesis strategy on controlling of product selectivity and yield.
The biodiesel industries became one of the most interests due to its clean burning, abundance, ability on a reduction of 41% greenhouse gas emissions during the com-bustion process, potential to reduce the number of pollutants in a closed environment, require no modifications to be made for diesel engine usage, low pricing and renew-able, which make it as an environmentally friendly diesel replacement (Ragauskas et al., 2006; Searchinger et al., 2008; Stephens et al., 2010; Tilman, Hill, & Lehman, 2006). Biodiesel is most commonly produced by transesterification of oil by adding methanol and sodium hydroxide (Ragauskas et al., 2006; Searchinger et al., 2008; Tapasvi, Wiesenborn, & Gustafson, 2005) (Figure 13.1).
310 Handbook of Composites from Renewable Materials
However, the biodiesel production produces a lot of raw glycerol (1,2,3-propane-triol). As the main byproduct, 100 kg of glycerol can be obtained with the production of 1 ton of biodiesel, according to the current process (Gelfand et al., 2013; Stephens et al., 2010; Zilberman, Hochman, Rajagopal, Sexton, & Timilsina, 2012). It can be estimated as well, that in 2015, around 1 Mt of glycerol has been produced as the byproduct of biodiesel and 0.8 Mt of them would be excessive. In short, the amount of raw glycerol produced as a byproduct of biodiesel manufacturing by weight is almost 10% of the entire biodiesel production (Carriquiry, 2007). Production of glycerol is continuously increasing as United States require the alternative of 5.75% of petroleum fuels with biodiesel (Math, Kumar, & Chetty, 2010; Searchinger et al., 2008). The excess of crude glycerol on the market has caused the price of glycerol to plummet and forced many of the biodiesel production industries to shut down (Carriquiry, 2007; Hanh, Dong, Okitsu, Nishimura, & Maeda, 2009a, 2009b; Kalscheuer, Stölting, & Steinbüchel, 2006; Math et al., 2010; Teixeira, 2005). Even though the most traditional applications of glyc-erol have been related to its use as additive in food, tobacco, pharmaceuticals, and med-icine, and for the synthesis of trinitroglycerine, alkidic resins and polyurethanes, one of the most attractive alternatives for glycerol utilization is as a feedstock for production added-value compounds such as bioplastic, platform chemicals, and fuels (Table 13.1; Figure 13.2).
However, in order for glycerol to be incorporated into consumer products, it must be refined and purified (Bell et al., 2013; Larrouy-Maumus, Kelly, & de Carvalho, 2014; Shatalebi & Rafiei, 2014). Indeed, the purification process of glycerol [multi-step distillation (up to 200 °C), ion exchange, alkali removal process or activated car-bon adsorption] is highly expensive, as a result, a good fraction of crude glycerol is disposed either as a waste or burned for energy with low heat value, high auto-ignition temperature (370 °C), and energy density (16 MJkg–1) (Maiti, Bapat, Das, & Ghosh, 2014; Vlasov et al., 2012). In addition, the composition of glycerol is not uniform and highly depends on both the family of used feeding materials and the biodiesel pro-cess conditions (Table 13.2). This fact occurs due to the chemical compositions of the feeding materials used for biodiesel production could change significantly. In addi-tion, crude glycerol contains many impurities (e.g., traces of methanol) which makes the purifi cation process became complicated (Gok, Emami, Shen, & Reaney, 2013; KoohiKamali, Tan, & Ling, 2012; Z.-H. Li et al., 2013; Mendow, Veizaga, & Querini, 2011; Mythili, Venkatachalam, Subramanian, & Uma, 2014; Tapah, Santos, & Leeke,
Figure 13.1 Glycerol by-product in the transesterification of fatty acids with methanol.
O
O
OO
R3
O
Tringlyceride
+ 3CH3OHCatalyst
R1—COOCH3
R2—COOCH3 +
OH
OH
OH
Glycerol
R3—COOCH3O
R1
R2
Biodiesel-derived Raw Glycerol to Value-added Products 311
Table 13.1 List of glycerol application based on its natural characteristics.
i. Nonionic kosmotropeii. Able to form strong hydrogen bonds
with H2O moleculesiii. Able to disrupts the crystal lattice
formation of iceiv. Freezing point = -37.8 °C
(70% glycerol in water)v. Not toxic
vi. Formation of ice-crystals in the cellvii. Maintaining stability and vitality of the
cell wall during the freezing process
22–24
(Continued)
312 Handbook of Composites from Renewable Materials
Application Glycerol characteristics References
Chemical intermediates• Nitroglycerin (ingredient of
various explosive)• Soap making (glycerin)• Synthesis of resin and ester• Sub-lingual tablets• Ally iodide (block polymer,
preservatives, organometallic, catalysts and Pharmecuticals)
i. Ethylene glycol functional groupsii. Non toxic
25–26
Waste water treatment• Denitrification
i. Abundant carbon contentii. Porosityiii. Absorption ability
27–28
Figure 13.2 Overview of glycerol applications.
Pharmaceutical
Plastic
Tobacco
Cosmetic
Paint
Food
Low and high value added chemicals
HO
HO
HO
Adhesive
2014). Discarding glycerol can lead to environmental concerns due to the contami-nations. In contrast, this lets the flood of glycerol presents challenges to traditional glycerol, on the other hand, offer the excellent opportunities in the production of value-added chemicals (Echeverri, Cardeño, & Rios, 2011; Ginting, Azizan, & Yusup, 2012; U Rashid et al., 2012). Thus, it is important to totally utilize the glycerol and make it more economical at larger scale. Glycerol is very versatile due to its unique combina-tion of chemical, physical, and biochemical properties; it is typically easily compatible with other substances as well as easy and safe to be used (Moser, Knothe, & Cermak, 2010; Umer Rashid, Anwar, & Knothe, 2009). From a chemical point of view, glycerol
Table 13.1 Cont.
Biodiesel-derived Raw Glycerol to Value-added Products 313
is a highly versatile molecule with two primary OH groups and a secondary OH group, which in turn can offer different reaction possibilities and stable under the reaction of alcohol (Mendow et al., 2011). Some physical properties of glycerol are water soluble, colorless, odorless, specific gravity of 1.261 gmL-1, melting and boiling of 18.2 °C and 290 °C temperature, respectively (Z.-H. Li et al., 2013; Maiti et al., 2014; Mythili et al., 2014; Tapah et al., 2014).
Meanwhile, from a biochemical view of point, the glycerol molecule is abundant in nature in the form of triglycerides (a chemical combination of glycerol and fatty acids) which might be the main constituents of nearly all animal fats and vegetable oils (Gok et al., 2013; Jham et al., 2009). This high functionality and occurrence in nature of glycerol allow it transformed by a chemical route or the fermentative ways (Kenar & Knothe, 2008; Marchetti, Miguel, & Errazu, 2008; Thompson & He, 2007). Thus, it has been noted that glycerol is currently serving as a highly versatile feedstock for the generation of a variety of high-value fuels, polymers, and chemicals with regard to lactic acid, citric acids, 1,3-PD, DHA, DCP, hydrogen, acrolein, ethanol, and additives (Mahajan, Konar, & Boocock, 2006; W. Zhou & Boocock, 2006). This in turn can make a lot of saving in biodiesel production cost and gain more advantages toward biodiesel industries (Table 13.3; Figure 13.3).
Indeed, with the development of heterogeneous and enzymatic catalyst, the purity, selectivity, and total yield of these value-added products will significantly improve.
13.2 Glycerol
13.2.1 Production of Glycerol
A number of techniques in production of biodiesel such as blending (dilution of hydro-carbon), pyrolysis (thermal cracking), emulsification, and catalytic transesterification with methanol have documented. Glycerol is among the key byproduct of the pro-duction of biodiesel via transesterification, rather than soap, excess of alcohol and a trace amount of water (Larrouy-Maumus et al., 2014). Triglycerides found in oil are by
Table 13.2 Chemical composition of raw glycerol with and type of catalyst utilized in biodiesel production.
Catalysis used in glycerol production Composition of products ReferencesMethanol Glycerol: 38 to 96 %
Figure 13.3 Conversion of glycerol to value-added chemicals: in general.
Bio-resources
Esteri�cation with carboxylation
O O
Glycerol carbonateO
OHHO
HO O R RO O
HOO R
O
OR
O DAG
Acrolein
O
OHHO
O
Dihydroxyacetone
Glyceraldehyde
HO OH
DehydrationOH
OH
OH
Esteri�cation
Etheri�cationO
GTPE
Acrylonitrile Acrylonitrile
R
OO
HO+
O
OHCyclic �ve- and six- membered ring
CO2 H2O+
Syngas
HOHO
1,2-propanediol
1,3-propanediol
HO OH
R
O
+N N
OO
Enzymatic
Reforming
Acetalization
AmmoxidationOxidation
Reduction
(Hyd
rogenolys
is)
O
OO
O
RR
OHO OH1,3-propanediol
TAG
O
MAG
Esteri�cation with carboxylic acids
+ MethanolCatalyst
Biodiesel
Glycerol
10%by-product
definition esters of glycerol with long-chain carboxylic acid (Gok et al., 2013; Lourenço & Stradiotto, 2009). The hydrolysis or transesterification of these triglycerides produces stiochiometric quantities of glycerol. Thus, glycerol separation exploited the density differences between biodiesel and glycerol (Ito, Nakashimada, Senba, Matsui, & Nishio, 2005; Johnson & Taconi, 2007). Glycerol derived from biodiesel productions gener-ally contains an estimated 50% impurities such as water, inorganic salt, methanol, free
Table 13.3 Cont.
316 Handbook of Composites from Renewable Materials
fatty acid, methyl ester, unreacted mono-, di-, and triglycerides (Haas, McAloon, Yee, & Foglia, 2006; Thompson & He, 2007). Conventionally, the purification process is based on the chemical and physical treatment, which generally involves neutralization steps and impurities steps. In neutralization process, glycerol had been acidified through adding highly concentrated acid substance (e.g., H2SO4) to pH = 1, which resulted in three specific layers, which is the top free fatty acid layer, the middle glycerol rich layer along with the bottom inorganic salt rich layer (G. P. Da Silva, Mack, & Contiero, 2009; F. Ma & Hanna, 1999). The impurities and unreacted glycerol ester from the produced crude glycerol would then remove by activated carbon- and alkali-based materials, respectively (Arechederra, Treu, & Minteer, 2007; Meher, Sagar, & Naik, 2006).
13.2.2 Applications of Glycerol
Currently glycerol has a large number of applications in varied fields included cos-metics, pharmaceuticals, food, and beverage (Homann, Tag, Biebl, Deckwer, & Schink, 1990; Pagliaro, Ciriminna, Kimura, Rossi, & Della Pina, 2007). However, glycerol could not used for direct food, fuel, and cosmetic applications. One of the possibilities for large-scale consumption of glycerol would be to utilize glycerol as fuel for the transpor-tation sector (Marchetti et al., 2008). However, due to its low solubility, high viscosity, and instability to high temperature discourage its use as an additive in a combustion engine (Cerrate et al., 2006; Pyle, Garcia, & Wen, 2008). As consequence of the over-production of biodiesel-derived glycerol exceeds, thus new technologies on conver-sion and transformation of glycerol to other chemical compounds would be possible to adjust its properties in order to fill the market demand (Abbott, Cullis, Gibson, Harris, & Raven, 2007; Hájek & Skopal, 2010). It believed that, a highly effective consumption or conversion of glycerol to specific product would reduce the total production cost of biodiesel (Bondioli & Della Bella, 2005; Hayyan, Mjalli, Hashim, & AlNashef, 2010; Mittelbach, 1996). Some research works have reported on blending/incorporating of glycerol with other chemical compounds to yield other useful products (Abeynaike et al., 2012; Groesbeck et al., 2008; Su et al., 2010). For example, the combination of glycerol with ethylene glycol can apply as a solvent for alkaline treatment of poly fabrics (Spangler & Davies, 1943). Meanwhile, glycerol could mix with gasoline as an alter-native fuel (Bagheri, KG, & Hamid, 2013; Kiatkittipong, Suwanmanee, Laosiripojana, Praserthdam, & Assabumrungrat, 2010).
13.3 Catalytic Conversion of Glycerol to Value-added Products
The main approach of green and eco-friendly chemistry is providing the simplicity of product’s separation and catalyst, while removing the need for separation through extraction or distillation (Siegel et al., 2010; Sivakumar, Sivakumar, Anbarasu, Mathiarasi, & Renganathan, 2014; H. Wang, Jusys, & Behm, 2004). The hazardous cata-lyst is currently replaces with more environmentally friendly such as clay and zeolite (Ibáñez et al., 2014; Tonbul, Zahmakiran, & Özkar, 2014). Catalyst should tailor by controlling the size, spatial distribution, surface composition, thermal/chemical sta-bility, shape, and electronic structure to reach the maximum selectivity on glycerol
Biodiesel-derived Raw Glycerol to Value-added Products 317
conversion process (Figure 13.4) (Bhandari, Kumar, Bellmer, & Huhnke, 2014; Z. Li et al., 2014; Lu, Biel, Wan, de Leon, & Arco, 2014). Catalyst research and catalyst-based technologies have been hard on the glycerol conversion process. Metal, metal oxides, and metal sulfides have been the first batch of catalyst to be developed for the hydrocarbon-based conversion, included partial oxidation and combustion reaction (Table 13.4).
The development of supported noble catalyst aimed at reducing costs for large commercial applications. There are numerous inorganic supports, available for prepar-ing the support catalyst, such as silica, alumina, carbon (notably charcoal), montmo-rillonite clays, zeolites, and other aluminosilicates, as well as more complex materials such as heteropolyacids (Feng et al., 2014; D.-W. Kim et al., 2014; Pan et al., 2014).
Figure 13.4 Catalytic strategies for converting glycerol into value-added chemicals.
OH
OHGreen processes
Catalysis
Low value added chemicalsAcetals, mono-ester, etc…
High value added chemicalsGlycerol building blocks for organic synthesis
HO
Table 13.4 Series of glycerol conversion with heterogeneous catalyst
Etherification Acid, heteropolyacid, silica, zeolite, ion exchange resin 114–116
Ammoxidation Bimetallic oxides, Acid 117–118
Enzymatic Mirage, yeast 119–120
318 Handbook of Composites from Renewable Materials
The main requirement of these support materials are, highly porous, high surface area forms, heavily hydroxylated, easily functionalized, and has pore diameter ranging from microporous (>0.3 nm) to macroporous (<100 nm) (Feng et al., 2014; Pan et al., 2014; Shisodia, Auricchio, Citterio, Grassi, & Sebastiano, 2014; Skrzyńska et al., 2014). The materials can be subclassified with respect to its crystalline with regular pore struc-ture and a very narrow pore size distribution and flexible layered structure (Rajan, Rao, Pavankumar, & Chary, 2014).
13.3.1 Catalytic Oxidation of Glycerol
An innovative green technology entailing hydrothermal electrolytic decomposition of glycerol using a continuous flow reactor equipped with metallic catalyst as electrodes have been recently developed (Gil, Cuenca, Romero, Valverde, & Sánchez-Silva, 2014; Padayachee, Golovko, Ingham, & Marshall, 2014). In general, oxidation reactions include all chemical reactions in which atoms have their oxidation state changed; that involves the transfer of electrons between species. This can be either a simple redox pro-cess, such as the oxidation of carbon to yield carbon dioxide (CO2) or the reduction of carbon by hydrogen to yield methane (CH4), or a complex process such as the oxidation of glucose (C6H12O6) in the human body through a series of complex electron transfer processes. This consequently overcomes the technical barrier brought by oxidation of glycerol; selective catalytic oxidation engineering that may operate on a polyfunctional molecule and a simple oxidant to carry out the respective conversion (Fashedemi & Ozoemena, 2014; Kostecka, Kowalska, Kozłowska, & Kowalski, 2013; Rodrigues, Pereira, Chen, Delgado, & Orfao, 2013). The main derived oxygenated products from glycerol are dihydroxyacetone, hydroxypyruvic acid, glyceric acid, tartaric acid, oxalic acid, and mesoxalic acid; moreover, some intermediates like glyoxylic acid, glyceral-dehydes, and glycolic acid could be served as compounds of new-branched nylons or polyester and also as new chemical intermediates (Figure 13.5) (Table 13.5).
The most studied metallic catalysts are Pd, Pt, and Au, although the main dis-advantage of Pt and Pd are their deactivation with the high reaction time (Angelucci, Varela, Tremiliosi-Filho, & Gomes, 2013; Bianchi, Canton, Dimitratos, Porta, & Prati, 2005; Fernández et al., 2013; Janaina F Gomes, Gasparotto, & Tremiliosi-Filho, 2013; Jin, Zhang, Chen, & Chen, 2013; Z. Zhang, Xin, Qi, Chadderdon, & Li, 2013). To overcome such of limitation, support materials have incorporated into metal catalyst and produce a hybrid system (Caliman, Santos, & Ribeiro, 2012; Ishiyama, Kosaka, Shimada, Oshima, & Otomo, 2013; S.-K. Liu & Lin, 2012). It has documented as well that the major product of glycerol oxidation within the existence of Pt/C or Pd/C cata-lyst is glyceric acid with the selectivity up to 70% (Table 13.6). It was also reported that, the selectivity on the oxidation process of the secondary hydroxyl group of glycerol has been significantly improved by promoting Pt with another type of metal like Bi and resulted in 30% yield of hydroxyacetone at 60 wt % conversion (W. Hu, Knight, Lowry, & Varma, 2010). With that, Pt supported on activated carbon and promoted with Bi has been aggressively studied, and resulted in oxidation of the primary and secondary hydroxyl group of glycerol, which in turn induced the production of tartonic acid up to 83 wt % on selectivity and 90 wt % of glycerol conversion at pH of 9–11 (Matsumoto, Ueno, Wang, & Kobayashi, 2008). Rather than pure metal, metallosilicates also found
Biodiesel-derived Raw Glycerol to Value-added Products 319
Figure 13.5 Main reaction products of glycerol oxidation.
Table 13.5 List of derivatives derived from oxidation of glycerol and its applications.
Glycerol derivatives Applications ReferencesDihydroxyacetone Synthon in organic chemistry, starting material in
D, L-serin synthesis, tanning agent in cosmetics132–133
Hydroxypyruvic acid
Flavor components, starting material in D, L-serin synthesis
134–136
Mesoxalic acid Complexing agent, precursor in organic synthesis, anti HIV agents
137–139
Oxalic acid Cleaning or bleaching, removal of dust, mordant in dyeing processes, baking powder
140–141
Tartonic acid Oxygen scavenger 142Glycolic acid Chemical peels performed by a dermatologist, skin care
products143–144
to give selective oxidation to glyceraldehydes, dihydroxyacetone, and glyceric acid, with the change in pore size (W. Hu et al., 2010).
The reactivity on oxidation process, selectivity, and stability could improve by incorporation of heavy metal promoters. For example, the deposition of Bi on Pt orien-tates selectivity toward secondary OH groups, which in turn increase the selectivity of dihydroxyacetone up to 50% with 70% on the degree of conversion is achieved (W. Hu,
320 Handbook of Composites from Renewable Materials
Lowry, & Varma, 2011; J Zhou, Zhao, Xiao, Wei, & Sun, 2012). This motivated another study on selective oxidation of glycerol to dihydroxyacetone with Pt–Bi bimetallic cata-lyst in the semi batch reactor, and found that, at 80 °C, pH 2 and 0.2 MPa, almost 48% and 80% selectivity and conversion of glycerol is achieved (Kwon, Birdja, Spanos, Rodriguez, & Koper, 2012; A. Termehyousefi et al., 2015). However, fixed-bed reac-tor causes some reduction on the degree of conversion and selectivity up to 5% even the process parameters are maintained. It has recorded that, at pH 11 and 50 °C, Pt/C yields glyceric acid with selectivity more than 70% (Villa, Wang, Veith, & Prati, 2012). Indeed, electrochemical oxidation methods with Pt and Au (as electrodes) modified with bimetallic Pt–Pd and Ru nanoparticle has reported, and shown a significant posi-tive result in oxidation of glycerol. Doping Pt/C catalyst with Bi documented to provide the best selectivity to dihydroxyacetone (Lakshmanan et al., 2013; Roucoux, Schulz, & Patin, 2002). Change in the direction of the reaction pathway toward to the second-ary alcoholic group. Furthermore, selective liquid-phase oxidation route to produce of hydroxypyruvic acid over Bi-modified Pt catalyst is reported using air as the terminal oxidant (Kwon et al., 2012).
Numerous studies have shown that supported noble metal catalyst is able to catalyze the oxidation of glycerol, and the selectivity controlled by the nature of the catalyst as well as the pH, temperature, and pressure of the reaction (Janaina F Gomes et al., 2013; Xianfeng LIU et al., 2013; Miedziak et al., 2013; Ntho, Aluha, Gqogqa, Raphulu, & Pattrick, 2013; Yu & Xi, 2012). By carefully controlling, the reaction parameters, either the primary or the secondary OH group could selectively oxidize which in turn could produce various types of oxidation products. For example, CeO2-supported platinum catalyzes the oxidation of both primary hydroxyl groups to give tartronic acid with 40% yield of production (Gil et al., 2014; C. Xu, Zeng, Shen, & Wei, 2005). Supported gold catalyst can give more than 90% on the selectivity of glyceric acid at 100% of glycerol conversion (Janaina F Gomes et al., 2013). Bimetallic catalyst composition of Pd, Au, and Pt supported on carbon resulted a greater than the monometallic catalyst, which is concluded that a synergistic effect between the two involved metals (Ntho et al., 2013). The oxidation on both primary OH groups of glycerol is successfully catalyzed by car-bon-supported Au with 100% conversion and 95% selectivity in production of sodium glycerate (Cheong et al., 2013; Climent, Corma, Iborra, & Martínez‐Silvestre, 2013; Padayachee et al., 2014). Selectivity on 1,3-dihydroxylacetone has ranged from 10% to 80% of glycerol conversion of 80% under the oxidation of aqueous glycerol solu-tion over charcoal-supported Pt within pH 2 to 3 with the incorporation of Bi and Pt (J. Zhou et al., 2012). Furthermore, even under mild conditions (60 °C, 3 hours, water
Table 13.6 Comparative on glyceric acid production with different heterogeneous support catalyst.
Heterogeneous catalyst Selectivity of glyceric acid (wt %) References
Pd/activated charcoal 30 153
Pt/activated charcoal 55 154–155
Bi/activated charcoal 77 156
Biodiesel-derived Raw Glycerol to Value-added Products 321
as solvent), 1% of Au-supported charcoal or 1% Au-supported graphite gave 100% selectivity toward glyceric acid (Tongsakul, Nishimura, & Ebitani, 2013). This is in line with the oxidation of glycerol catalyzed with Pt- and Pd-supported carbon whereby the selectivity of the glyceric acid reached at 55 and 77%, respectively, with glycerol conver-sion of 90% (Heck, Janesko, Scuseria, Halas, & Wong, 2013). This fact allows preventing the products over-oxidation on the metal surface by supported material, avoiding the degradation of products until total oxidation to CO2, also promoters favor the second-ary alcohol oxidation (Brandner, Lehnert, Bienholz, Lucas, & Claus, 2009; Janaina F Gomes et al., 2013).
However, in the absence of promoter with the presence of Pt–Bi bimetallic catalyst, primary alcohol is oxidized to carboxylic acids and produced the series of intermedi-ates (dihydroxyacetone, hydroxypyruvie acid, oxalic acid, tartaric acid, mesoxalic acid, glyceric acid, glyceraldehyde, glycolic acid, and glyoxylic acids) (Angelucci et al., 2013; Worz, Brandner, & Claus, 2009). Studies on the selective glycerol oxidation toward mono or bimetallic catalyst of Pd, Pt, and Au, using oxygen as oxidizer agent recently carried. It has recorded that Pd and Au are much more selective toward glyceric acid under basic conditions as compared to Pt (Gil et al., 2013; Nunes & Guerreiro, 2013).
On the other hand, the production of intermediates is dependent on the pH of the reaction. For example, glyceraldehyde and tartaric acid obtained within basic pH, hydroxypyruvie acid obtained under mild acid pH, and dihydroxyacetone and hydroxypyruvic acid obtained under very strong acid pH (Villa et al., 2013). Using acidic conditions, secondary alcoholic groups, hydroxypyruvic acid and dihydroxyac-etone anticipated to produce, while at basic conditions, the primary alcoholic groups are favored oxidized and glyceric acid is acquired (Yongprapat, Therdthianwong, & Therdthianwong, 2012; Zope & Davis, 2011). This also brought the conclusion that, at the same process parameters, almost 70% and 35% on the selectivity of glyceraldehyde and hydroxypyruvic acid obtained, respectively (Bambagioni et al., 2009). Application of Pt–Bi bimetallic catalyst also obtained for the selectivities of 83%, 74%, 37%, and 39% on tartaric acid, hydroxypyruvic acid, dihydroxyacetone, and mesoxalic acid, respectively (Worz et al., 2009). Some evidence has documented that, the redox pro-cess with Pd and activated carbon as a support shown 5% higher in terms of selectivity compared to the one with Pt catalysis (Prati et al., 2011). The selectivity would further increased with the particle diameter diminished as Au used to catalyze the oxidation reaction (Figure 13.6) (Rodrigues, Pereira, Delgado, Chen, & Orfao, 2011). In this case, Au-supported activated carbon demonstrated the production on glyceraldehyde as a major product with selectivity and conversion of 82% and 60%, respectively (Brett et al., 2011). Both of selectivity and conversion values would further improve with incre-ment in pH and pressure in the conversion process. The same phenomenon observed in the oxidation process of glycerol with Pd–Au as the bimetallic catalyst (Bagheri, Chandrappa, & Hamid, 2013). Au catalyst supported on carbon-based materials has been investigated by many works and concluded that carbon nanoparticle showed the most chemically active support material, and this confirmed the relationship between selectivity to total yield of glyceraldehyde and its particle size (Dhital & Sakurai, 2011; Gil, Muñoz, Sánchez-Silva, Romero, & Valverde, 2011; Rodrigues, Pereira, Chen, Delgado, & Orfao, 2011). Other studies have focused on the application of Au–Pd as bimetallic catalyst and documented that, the greatest turnover frequency found for Au
322 Handbook of Composites from Renewable Materials
single metallic catalysts and the maximum selectivity for glyceraldehyde obtained by incorporation of Pd, while the selectivity of conversion much more related to the quan-tity of Au (Janaina Fernandes Gomes & Tremiliosi-Filho, 2011; Ntho et al., 2013). In order to improve the oxidation process, bimetallic catalyst of Au–Pd has supported by carbon material and its effect on process parameters on the oxidation of glycerol with the oxygen molecule through the liquid phase has studied in details (Janaina Fernandes Gomes & Tremiliosi-Filho, 2011). The presence of Au-Pd/C catalyst in the conversion of glycerol results in a low-temperature reaction with a conversion reached up to 50% (Ntho et al., 2013).
Meanwhile, men of the oxidative dehydrogenation mechanisms on the metal surface to produce a variety of chemical derivatives (Padayachee, Golovko, & Marshall, 2013; Yu & Xi, 2012) carry out glycerol oxidation on metal oxides. For example, Glycerol oxidation on metal oxide afforded (2,2-dimethyl-1,3-dioxolan-4-yl) methyl acetate, glyceric, glycolic, dihydroxyacetone, tartronic, and oxalic acids as a derivative com-pound (Ang, Tan, & Lee, 2014). This fuel additive is produced via the oxidation process of glycerol and commonly used as a biodiesel additive due to its ability to improve biodiesel viscosity and could meet the flash point and oxidation stability requirement needed for diesel and biodiesel fuels. Due to the potential complexity of product dis-tribution, the selectivity control of the oxidation process is the main key to optimize the whole reaction. Thus, the optimum condition on production of (2,2-dimethyl-1,3-dioxolan-4-yl) methyl acetate can be achieved as several works have focused on various conditions (e.g., amount of feeding glycerol, oxidation rates) and reported that, the glycerol was only found to be more stable under high-pressure aqueous and high-temperature conditions during hydrothermal treatment in the presence of cataly-sis (Carrettin, McMorn, Johnston, Griffin, & Hutchings, 2002; E. Garcia, Laca, Pérez, Garrido, & Peinado, 2008; Yang, Hanna, & Sun, 2012). The use of bimetallic catalyst has also reported in continuous flow oxidation of glycerol. Under these conditions, glycoal-dehyde and glycolaldehyde, formic acid, and lactic acid were obtained as major products with low formation of hydrogen, glyceraldehyde, glycolic, and acetic acids (Carrettin et al., 2003; Gallo et al., 2012; R. Garcia, Besson, & Gallezot, 1995; Porta & Prati, 2004; Yang et al., 2012). For example, Au–Pt/TiO2 catalyst and oxygen in glycerol conversion to lactic acid have used as reaction parameters of 90 °C with NaOH: glycerol ratio of
Figure 13.6 Reaction pathway for glycerol oxidation using supported Au catalysts.
OH
OHAu, OH
O
OHAu, O2, OH
Oxidation
HOO
OH
OH
OHO
OH
HO
HO O
Oxalic acid
O
Glycolic acid
Glyceric acid
HOO
OH
O
Tartronic acid
HO
Oxidation(C-C Cleaqvage)
OH
Glyceraldehyde
OH
Glycerol
Biodiesel-derived Raw Glycerol to Value-added Products 323
4:1, to reduce the reaction temperature while achieves the higher glycerol conversion (Kimura, 2001; Y. Shen, Zhang, Li, Ren, & Liu, 2010). In order to do so, the remaining of the steps from glyceraldehyde to lactic acid requires alkaline conditions; or other-wise further oxidation would take place on the production of glycerol into glyceric acid (Kimura, 2001). Even though the reaction can reach to the high conversion of glycerol (up to 100%) and lactic acid selectivity (85%) at lower temperatures, the initial glycerol concentration was only 0.22 M, which in turn induces lower lactic acid production. To overcome such limitation, concentration of feed glycerol has increased up to 2.5–3.5 M and this in consequence increased the selectivity of lactic acid to 89.9% (Y. Shen et al., 2010). Noted that, some of the super solid base catalyst included CaO, Al2O3, NaOH, and Na are not conveniences to promote the production of lactic acid due to corrosion to reactor low productivity and restriction solubility in water medium (Anastopoulos, Dodos, Kalligeros, & Zannikos, 2013; Kouzu, Kasuno, Tajika, Yamanaka, & Hidaka, 2008; Xuejun Liu, He, Wang, Zhu, & Piao, 2008). For example, the highest yield on the production of lactic acid has been recorded at 40.8 mol % selectivity with 97.8% conversion for the glycerol oxidation with CaO at 290 °C and 150 min (A. L. Da Silva & Müller, 2011; Thota & Trudell, 2013; Wei, Li, & Ma, 2012). High water content in the reactant would affect the total yield of lactic acid (A. L. Da Silva & Müller, 2011). These series of super solid basis has a good tendency to interact with water first, which in turn reduce its catalytic activity (Thota & Trudell, 2013). Although NaOH has a good poten-tial as a catalyst of the super solid basis for the production of lactic acid, with recorded almost 100% on conversion of glycerol, but a high initial concentration of NaOH will cause severe corrosion to the stainless steel reactor (Jo, Kim, & Moon, 2007; Onwudili & Williams, 2011). Besides, some of the Brosted acids such as Na2O, K2O, MgO, and BaO are also not a good catalyst candidate for the production of the lactic acid (C.-H. C. Zhou, Beltramini, Fan, & Lu, 2008). The Bronsted site of the catalyst generated a strong interaction with the oxygen ions and makes this group vulnerable to CO2 and H2O contamination that in turn enlarged its usage in aqueous media (Gallezot, 1997; Kimura, Tsuto, Wakisaka, Kazumi, & Inaya, 1993). Supported Pt-Bi catalysts (5 wt % Pt, 5.4 wt% Bi) on active charcoal were employed in continuous flow experiments (120 °C, 1 bar oxygen) using the trickle bed reactor to record up to 50% on the selectivity of dihydroxyacetone (Carrettin et al., 2004). Monometallic-supported Au nanoparticles on carbon and Titania have also recently reported in the continuous flow oxidation of glycerol, at a temperature of 60 °C and 11 bar of oxygen gas pressure (Katryniok et al., 2011).
Meanwhile, a green catalyst such as CaO has extensively used to catalyze the glyc-erol conversion for the production of lactic acid (Wei et al., 2012). Under the reaction condition of 290 °C, 15 min with a molar ratio of glycerol: CaO = 0.7, almost 97.8 mol % and 40.8 mol % on conversion and selectivity of glycerol is obtained which in turn indicated the high lactic acid production (3.35 g/min. L). Meanwhile, the application of Na2(SiO2)nO as a green catalyst has been proved with the high selectivity of lactic acid production (90.7%) at reaction parameters of 300 °C and 90 min (Ramírez-López et al., 2010). It has recorded as well that production of lactic acid derived from glycerol can also catalyze with NaOH. With the reaction condition of 300 °C, 220 min, and 1.1 M glycerol, the production of pure lactic acid can be reached to 80.5 mol % with 92.8% of the glycerol conversion (Chieregato et al., 2014). Furthermore, the presence of Cu
324 Handbook of Composites from Renewable Materials
and/or Cu2O-based catalyst decreased the reaction temperature to 240 °C with NaOH catalyst.
13.3.2 Catalytic Dehydration of Glycerol
The increase in the need for a useful and practical ways to convert glycerol to value-added products via a dehydration process with much more efficient on separation, selectivity and yield is currently in demand. In general, dehydration (hypohydration) is the excessive loss of body water with an accompanying disruption of metabolic pro-cesses. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within the microorganism.
However, this not an easy approach since on one hand the hydrogenation of the C=C bond is actually thermodynamically more favorable as compared to the C=O bond (free reaction enthalpy by 35 kJ.mol–1 more negative) (Konaka, Tago, Yoshikawa, Nakamura, & Masuda, 2014; Miranda et al., 2014; Montes et al., 2014) (Figure 13.6). It also recorded that, C=C is kinetically more active than C=O bonds (Montes et al., 2014). Furthermore, the boiling point of glycerol is 290 °C; thus, it is thermally unstable at high temperature, whereby the catalytic glycerol dehydration normally required heat-ing temperature of 250–350 °C (Dalla Costa, Peralta, & Querini, 2014; Viswanadham, Pavankumar, & Chary, 2014; Yue, Gan, Gu, & Zhuang, 2014). This in consequence makes catalyst deactivation during the dehydration process due to the formation of glycerol byproduct, coke deposition and acrolein polymerization (Yue et al., 2014).
Of all above-mentioned issues, the ideal dehydration process of glycerol is suggested to be occurred in the solid (180–340 °C) and liquid (250–340 °C) phase (Haider et al., 2014; Konaka et al., 2014; Yan & Suppes, 2009). Heterogeneous catalyst of H3PO4/Al2O3 or H3PO4/TiO2 are a normal catalyst used for solid-phase conversion, while the liquid-phase catalysts such mordenite, montmorillonite, acidic zeolite, oxide, mixed oxide, and heteroplyacid as being much more predominant (Y. Y. Lee, Am Lee, Park, & Kim, 2014; Rao, Rajan, Pavankumar, & Chary, 2014; Stošić et al., 2014). The catalyst life-time can improve by using diluted glycerol than pure glycerol (Y. Y. Lee et al., 2014). This is greatly concerned for the dehydration of glycerol in liquid phase; indeed, the water dilution is generally applied to reduce the formation of coke during vapor-phase dehydrations. Furthermore, the selectivity of glycerol dehydration in the production of acrolein could optimize with sufficient temperature and partial vacuum condition (Chai, Wang, Liang, & Xu, 2007b). The reaction generally catalyzed by acids and might be occurred in either gas or liquid phase. For example, 66.8% of acrolein yield and 84% of the glycerol conversion could achieve at 260 °C with 0.85 bars in the presence of H3PO4/activated carbon catalyst (Stošić et al., 2014). Indeed, more than 70% of total yield of acrolein could achieve with Hammett acidity constant between –10 and –16 (Chai, Wang, Liang, & Xu, 2007a; Katryniok, Paul, Capron, et al., 2010).
At lower-acidity condition, the yield of acrolein recorded to be as low as 60%, and the catalyst is comparatively simply deactivated (Lili et al., 2008). The reaction mostly accompanied by side reactions resulting in the formation of acetaldehyde, hydroxy-propanone, propanaldehyde, adducts, acetone, and polyaromatic compounds, which consequently form a coke on the catalyst (Katryniok, Paul, Capron, & Dumeignil, 2009; Lili et al., 2008). This leads to deactivation of catalyst, a decrease of yield and
Biodiesel-derived Raw Glycerol to Value-added Products 325
the selectivity of the acrolein (Figure 13.7). The presence on the series of byproducts with acrolein like hydroxypropane and propanoldehyde, which are moreover to iso-late, necessitates purification and separation steps, which result in higher recovery cost (Dalla Costa et al., 2014). Moreover, it is necessary to generate the catalyst regularly in order to establish the satisfactory catalytic activity. Therefore, some research has focused on application of solid acid catalyst ZSM-5 zeolite catalyst. ZSM-% catalyst used to enhance the product separation, maintain the regeneration of catalyst over the large period, induce availability for wide range of glycerol concentration; co-solvent is not necessary to carry out the reaction even with polluted feeds and less valuable heavy byproducts of the reaction can burn in the process (Carriço et al., 2013; Y. Gu, Cui, Yu, Li, & Cui, 2012; Kubička, Kubičková, & Čejka, 2013; Lin et al., 2013; Pestana, Guerra, Ferreira, Turci, & Mota, 2013).
Due to its crystalline nature composed of the SiO4 and [AlO4]− tetrahedra, constant
electroneutrality, cation characteristics of [AlO4]−, highly acidic charge compensation,
multi-dimensional microporosity, shape selectivity and valorization of hydrocarbon streams in refineries make zeolites one of the promising catalyst in the dehydration of glycerol (Carriço et al., 2013). For example, with the presence of zeolites, the dehy-dration of glycerol at 330–360 °C can reach to 100% of the glycerol conversion with acrolein selectivity more than 70%. The selectivity figure significantly increases with reaction temperature, which the highest is recorded at 83% at 500 °C without signif-icant on the formation of coke (Kubička et al., 2013). Chemical analysis proved on the formation of a good interaction revealed between secondary OH groups of glyc-erol and zeolite, resulting to acrolein being formed selectively (Y. Gu et al., 2012). In another case, zeolite-based catalyst has been used in the dehydration process of glycerol to focus on the production of light olefins (Y.-L. Gu, Shi, Cui, & Li, 2011). In order to enhance the acid sites for the glycerol dehydration, zeolite has impregnated with metals (Lin et al., 2013). The metal loading was responsible for the physical changes that can recorded from the micropore area, surface area and pore volume of the synthesized catalyst, which at the end resulted almost 16.3% on the selectivity of the light olefins (Katryniok, Paul, Bellière-Baca, Rey, & Dumeignil, 2010). However, the acrolein selec-tivity found on zeolite considered lower as compared to other type of catalyst (Yoda & Ootawa, 2009). The lower selectivity contributed due to higher acidity of the zeolite compared to other heterogeneous catalyst and that assisted in simultaneously coke and hydrocarbon formation (Mészáros, Halász, & Kiricsi, 2008).
Consequently, application of others heterogeneous catalyst and super critical water as a reaction medium for conversion of glycerol to acrolein have obtained some interest (Table 13.7). Incorporation of support materials documented to increase the selectivity and conversion of the glycerol dehydration process. For example, 75% on selectivity
Figure 13.7 Continuous flow glycerol dehydration to acrolein.
OH
OHAcid catalyst O
OH +
O
Acetol Acroline
DehydrationOH
Glycerol
326 Handbook of Composites from Renewable Materials
and almost 100% on conversion of glycerol to acrolein observed as Al2O3 and SiO2/Al2O3 used as a support for the silicotungstic acid catalyst (Atia, Armbruster, & Martin, 2011; Shiju, Brown, Wilson, & Rothenberg, 2010). Rather than both aluminas-based supports, TiO2 and SiO2, have also been used for the H2SO4, H3PO4 and H3Mo12O40P support, with almost 58 mol % on conversion of acrolein to allyl alcohol at reaction tem-perature of 300 to 320 °C is recorded (Alhanash, Kozhevnikova, & Kozhevnikov, 2010; F. Wang, Dubois, & Ueda, 2010). Acrolein can produce with mixed oxide such as Bi–Mo and Nb2O5 as a catalyst under gas-phase oxidation (L. Shen et al., 2012; Yoshikawa et al., 2011). Continually, there is also some effort to synthesize the ZrO2-FeOx mixed oxide catalyst and study its performance toward the production of ally alcohol derived from crude glycerol (Konaka et al., 2013; Kurosaka, Maruyama, Naribayashi, & Sasaki, 2008). Two main related pathways suggested: the production of ally alcohol and pro-pylene initially obtains another involves the carboxylic acid from acetol production before being followed by their ketonization (Kurosaka et al., 2008). The components of ZrO2–FeOx are highly resistance to the conversion of glycerol from both of mixed metal oxide categorized as alkaline metal (Konaka et al., 2013).
Besides, it also found that, conversion into acrolein is much more convenient at lower reaction temperature, while high temperature is only suitable for conversion into acetyldehyde. High selectivity (almost 75%) for acrolein from oxidation of glycerol at 275 °C is also attained as [Sin +W12O40]
8-n.xH2O has been used as a catalyst (F. Wang, Dubois, & Ueda, 2009). Meanwhile, there is some effort in using series of VOPO4 to catalyze the reaction. During the initial observation, it has screened the VOPO4.2H2O, VOHPO4. ½H2O and (VO) 2P2O7; that shown a good catalytic activity and selectiv-ity (Beneke & Lagaly, 1983; Beneš, Melánová, Svoboda, & Zima, 2012; Di Serio et al., 2007; Sádaba, Lima, Valente, & Granados, 2011). In this case, VOHPO4.½H2O giving up to 66% acrolein at 100% on glycerol conversion (Rajan et al., 2014). Consequently, it has been discovered that the performance of (VO)2P2O7 is strongly dependent on the activation temperature and that catalyst calcined at 800 °C gave a selectivity to useful products of 95% at 100% on glycerol conversion (Akizuki & Oshima, 2013).
Table 13.7 List of heterogeneous catalyst used in different dehydration parameters of glycerol.
Heterogeneous Catalyst
Process Parameters Conversion of glycerol
(%)
Selectivity of acrolein
(mol %) ReferencesTemperature
(°C)Pressure
(MPa)
Zinc sulfate 360 25 50 75 252
Sulfuric acid 400 34.5 90 80 253
Silicotungstic acid 275 101 100 80 254
Zeolite 350 101 100 100 255
Phosphoric acid/activated carbon
260 0.85 85 67 256
Tungsten oxide/titanium oxide
300 25 100 85 257
Biodiesel-derived Raw Glycerol to Value-added Products 327
The double dehydration of glycerol could be achieved under sub and supercritical water as reaction media. It can conclude that, lower pressure is most effective for quickly eliminating the more volatile products from the catalyst site, achieving an extended catalyst service life (Akizuki & Oshima, 2012; L. Cheng, Liu, & Ye, 2013; Lehr, Sarlea, Ott, & Vogel, 2007; Ott, Bicker, & Vogel, 2006; Ramayya, Brittain, DeAlmeida, Mok, & Antal, 1987). The decomposition of glycerol in supercritical water without catalysis addition is recorded at 349–475 °C, pressures of 250, 350, or 450 bar under reaction time of 32–165 seconds with different derivative compounds are obtained (L. Cheng et al., 2013). The reaction then continued in a tubular reactor with varying parameters and derivative products such as formaldehyde, allylic alcohol, propionaldehyde, acetal-dehyde as well as acrolein (Lehr et al., 2007). The maximum yield of acrolein recorded at 27% with selectivity more than 38% with reaction parameters of 356 °C, 450 bar, and 50 seconds. The reaction can be enhanced the catalyst’s selectivity of glycerol to acro-lein since high initial glycerol content can result in glycerol polymerization and allow on degradation under corrosion which occurred on the catalyst sites (Ott et al., 2006).
It has recently studied on the influence of acid catalyst on the selectivity of the dehy-dration reaction of glycerol to acrolein. There are some interests to conduct the conver-sion of glycerol under supercritical water in the presence of sulfuric acid as catalysis (L. Cheng et al., 2013). It recorded that production of acrolein has a linear relationship with the amount of glycerol feeds and concentration of sulfuric acid (Cavani, Guidetti, Trevisanut, Ghedini, & Signoretto, 2011; Randy Latayan Maglinao & He, 2009; Ulgen & Hoelderich, 2009; Zhao et al., 2013). Optimized results could afford acrolein with up to 74% of yield under reaction conditions of 400 °C, 345 bar, and 12 seconds (Randy Latayan Maglinao & He, 2009). The rate constant of glycerol decomposition recorded to be greater than that of process in the absence of sulfuric acid catalysis. Besides, almost 72% on selectivity of acroelin from glycerol has obtained on acid solid catalyst (Ulgen & Hoelderich, 2009) (Table 13.8).
It is believed that, the addition of an acid catalyst under supercritical conditions could whereby induce the kinetics toward more selectivity and yield on production of acrolein, and become one of the promising technique for the liquid-phase continu-ous flow synthesis of acrolein from glycerol (Randy Latayan Maglinao & He, 2009). Another process of dehydration of glycerol is implemented with liquid raw glycerol is directly added into a fluidized bed reactor, vaporized, and perform a reaction to gen-erate acrolein over a W-doped Zr catalyst (Cavani et al., 2010; Lauriol-Garbey et al., 2011). Current process gave minimum accumulation of salt, once the glycerol evapo-rated; abandoning salt crystals that were only loosely bound to the surface and utilizing
Table 13.8 Different heterogeneous catalyst on the selectivity of acrolein from the dehydra-tion process of glycerol
Heterogeneous catalyst Selectivity (%) References
WO3/ZrO2 75 283
Zeolite 75 284
Zeolite 67 250
328 Handbook of Composites from Renewable Materials
mechanical agitation could separated from catalyst (Cavani et al., 2010). To overcome such of limitation and further promote glycerol dehydration process, some research works have focused on utilization of ZnS2 as an electrolyte in the reaction (Bañares & Guerrero-Pérez, 2014). With this approach, acrolein with 38% of yield and almost 75 mol % of selectivity is produced at parameters of 360 °C, 250 bar and 60 seconds. It was noted that, the presence of ZnS2 electrolyte in reaction is essential to reduce the pressure of the whole reaction.
Furthermore, the advanced glycerol dehydration process recorded in the produc-tion of acrylonitrile with ammonia as a precursor (Guan, Wang, Wang, & Mu, 2013). Glycerol is catalyzed dehydrated with metal oxide catalyst (e.g., Al, V, Sb and Nb oxides) before being grafted with ammonia to form a C–N bond (Mane, Yamaguchi, Malawadkar, Shirai, & Rode, 2013). In other research works, conversion of glycerol to propanediol via a combination of dehydration and hydrogenation process has high-lighted. The process initiated with the dehydration of glycerol to acetol with acid-based catalyst and followed by hydrogenation of acetol to propanediol with the metal cata-lyst (Ang et al., 2014). Dehydration process is much more convenient at lower pres-sure while hydrogenation normally insisted of higher pressure due to the kinetics and thermo dynamic consideration.
13.3.3 Catalytic Acetylation of Glycerol
In general acetylation refers to the process of introducing an acetyl group (resulting in an acetoxy group) into a compound, namely the substitution of an acetyl group for an active hydrogen atom. A reaction involving the replacement of the hydrogen atom of a hydroxyl group with an acetyl group (CH3 CO) yields a specific ester, the acetate. The catalytic acetylation of glycerol is actually another alternate path to enhance the profitability of biodiesel production plants in order to produce of acetins (mono-, di-, and tri-esters of glycerol) (Costa et al., 2013; Silva, Gonçalves, & Mota, 2010; J. Zhang & He, 2014). These series of acetins mainly used as transport fuel additives (Table 13.9). Mineral acid catalyst commonly used to catalyze the glycerol acetylation process (Kim, Kim, & Lee, 2014; Liao, Zhu, Wang, & Li, 2009). However, the application of this type of catalyst induced some of environmental problems such as excessive catalyst usage, toxic, no recyclability and serious corrosion of equipment. To overcome such limita-tions, some attempt made using solid acid catalysts such as zeolites, amberlyst, sulfonic acid-functionalized mesostructured materials, montmorillonite, niobic acid, hetero-polyacids, and metal oxide as a catalyst in the glycerol acetylation process (Balaraju et al., 2010; De Canck, Dosuna-Rodríguez, Gaigneaux, & Van Der Voort, 2013; Dosuna-Rodríguez, Adriany, & Gaigneaux, 2011; Dosuna-Rodríguez & Gaigneaux, 2012; Gonçalves, Pinto, Silva, & Mota, 2008; Reddy, Sudarsanam, Raju, & Reddy, 2010, 2012; Silva et al., 2010; Testa, La Parola, Liotta, & Venezia, 2013; L. Zhou, Al-Zaini, & Adesina, 2013).
For example, a series of zirconia heterogeneous catalyst such as ZrO2/SiO2/ME, ZrO2/SiO2/SG, HClSO3/ZrO2, and S-ZrO2 toward acetylation of glycerol have been recently reported (De Canck et al., 2013; Gonçalves et al., 2008). The influence of reaction parameters like catalyst dosage, reactant ratio, pressure, temperature and reaction time on the catalyst selectivity and activities investigated toward the acetylation process. It
Biodiesel-derived Raw Glycerol to Value-added Products 329
can found that, HClSO3/ZrO2 exhibits the highest catalytic activity with 100% selectiv-ity in action, followed by S/ZrO2 (91%) and H2SO4/ZrO2 (50%) (Gonçalves et al., 2008). In contrast, the acetylation reaction catalyzed with ZrO2/SiO2/ME and ZrO2/SiO2/SG exhibit the lowest selectivity with 29% and 27%, respectively. It can be concluded that acetylation could be catalyzed by not only Bronsted acid sites, but also Lewis acid sites, thus sulfated ZrO2 catalyst indicate higher activity than non-sulfated ones (Reddy et al., 2010). This attributed to their diversity of acid sites amount, acid strength and robust-ness properties of the catalyst. However, the usage of ZrO2-based catalyst reports on some drawbacks such as high pressure of reaction, highly diffused, acid site deactiva-tion and high molar ratios of acetic acid to glycerol (Silva et al., 2010).
With that in mind, some researchers have investigated the potential of Amberlyst (ion exchange resin) to catalyze the glycerol acetylation process (Ferreira, Fonseca, Ramos, Vital, & Castanheiro, 2009; L. Zhou, Nguyen, & Adesina, 2012). Lately, 100% selectivity of triacetin was reached in high molar ratio (acetic acid: glycerol 24:1) at high pressure (200 bar) using Amberlyst 15 as a catalyst. This encourages the investiga-tion of performance of different types of ion exchange resins included Amberlyst 15, Amberlyst 36, Dowex 50 Wx2, Dowex 50 Wx4, and Dowex 50Wx8 toward acetylation of glycerol, and found that, the best performance at 6.25 g of catalysts were exhibited by Dowex 2 and Amberlyst 36 (Ferreira et al., 2009). It noted that, the catalytic per-formance of the resins was unaltered after washing with distilled water, indicated that sulphonic species were not removed by leaching make it convenience of recycling and reusability up to five times of the cycle (Reddy et al., 2012). However, such high molar ratio and ineffective in nonpolar media, desulfonation due to high working conditions, deactive by metal ions/cations, the difficulty of the separation of unreacted reactant and subproducts make application of resin catalyst is not applicable to the industrial scale
Table 13.9 Acetylene derivatives of acetylation of glycerol and its industrial applications.
Triacetin • Antiknock additives for gasoline• Improve the cold and viscosity properties of
biodiesel• Production of photographic films• Perfumery industry
104;291
Diacetin • Solvent for various dyes• Softening agents• Printing ink• Plasticizer
292
Monoacetine • Manufacture of dynamite• Tanning leather• Cryogenics• Raw materials for production of biodegradable
polyester• Food additives• Explosive and smokeless powder
293
330 Handbook of Composites from Renewable Materials
(Dosuna-Rodríguez & Gaigneaux, 2012). This sort of catalyst is only catalytically active in the reaction media where solvent or the reactants are prepared for inducing the swell-ing process. Additionally, both type of catalyst are suffered of poor thermal stability, highly soluble in polar media, poor regeneration ability and low specific surface area (L. Zhou et al., 2012). Supported materials such as silica or activated carbon were then applied to increase the surface area of the catalyst, even though the accessibility and efficiency of the catalyst is reduced concurrently (Testa et al., 2013). For example, in case of alkylated sulfonic acid (propyl-, arene-, and perfluoro-sulfonic acid) supported by siliceous mesoporous, the formation of di and tri-acetin linearly increased with the acid strength while mass transport property was concomitantly offered by mesopo-rous as the support material (Balaraju et al., 2010). In contrast, in case of niobic acid supported with heteropoly tungstate with the Keggin structure, the analysis revealed on well-dispersed Keggin ion on niobia at lower supports content and this in turn makes the acetylation activity occurred within short reaction time (30 min) with 90% of glycerol conversion (Cordeiro, Arizaga, Ramos, & Wypych, 2008). A new catalytic acetylation process based on metal oxide (e.g., CeO2/ZrO2, CeO2/Al2O3, SO4
2–/CeO2 and SO4
2–/CeO2–Al2O3) was then introduced due to its stability, inexpensive, regener-able, and being 100% active over a wide range of temperature and pressure. It recorded that almost 100% on conversion of glycerol with 90% on the selectivity of triacetin was observed at reaction parameters of 120 °C for 40 hours.
13.3.4 Catalytic Esterification of Glycerol
In general, the esterification process occurred as the π bond of the carbonyl group can act as a base to a strong inorganic acid due to the distortion of the electrons from the electronegativity difference between the oxygen atom and the carbon atom and also the resonance dipole. The cation produced in the reaction with sulfuric acid will have resonance stabilization. From the kinetic point of view, the glycerol esterifi-cation can conduct in processes catalyzed by heterogeneous/homogeneous acids and bases. It noted that, the use of homogeneous acids/bases catalysis results in separation difficulties, promotes product contamination and limits the recycling of the catalyst (Bournay, Casanave, Delfort, Hillion, & Chodorge, 2005; Y. Liu, Lotero, & Goodwin, 2006; Marchetti, Miguel, & Errazu, 2007). These difficulties can be overcome by the use of heterogeneous catalysis, allowing a better separation of solid/liquid phase, the reuse of catalysis several times, reducing costs while still improving the quality of the prod-ucts (Y. Liu et al., 2006; Marchetti et al., 2007; Patel & Singh, 2014). The esterification of glycerol normally carried out with basic, acid, multivalent metal salt, resins, zeolite, heteropolyacids, and sulfonic acid as the heterogeneous catalyst (Figure 13.8) (Barrault, Bancquart, & Pouilloux, 2004; P. Guo, Danish, Du, Kong, & Guan, 2014; Luo & Li, 2014; Okitsu, Maeda, & Bandow, 2014).
The basic heterogeneous catalysis is arguably the most used especially in cases of esterification. The catalysts used in the basic catalysis, the alkali metal hydrox-ides (NaOH, KOH), and metal alkoxides (NaOCH3, KOCH3) and a combination of both hydroxides and alkali metal alkoxides (alkaline catalysts) (Bancquart, Vanhove, Pouilloux, & Barrault, 2001; Díaz, Márquez-Alvarez, Mohino, Pérez-Pariente, & Sastre, 2000; Márquez-Alvarez, Sastre, & Pérez-Pariente, 2004). This process is also eligible for
Biodiesel-derived Raw Glycerol to Value-added Products 331
other alkoxides butoxides and propoxidos catalysts. The heterogeneous acids used in the glycerol esterification include H2SO4, RSO2OH, PO(OH)3, or HCl, among others (Devi, Reddy, Lakshmi, & Prasad, 2014; Xiumei Liu et al., 2011). Among else, H2SO4 is the most commonly used. Some research works have reported the synthesis of mono-glyceride by esterifiction of glycerol with oleic and lauric acid with functionalized ordered mesoporous materials containing R-SO3H groups as a catalyst and its effect on alkyl chain length of HSO3-R-MCM-41 on the esterification with a fatty acid. The distance between R-SO3H groups and its porosity balanced the nature of its organic groups (Xiumei Liu et al., 2011; Sakthivel, Nakamura, Komura, & Sugi, 2007). On the other hand, there is some interest to conduct the production process of monoglycerides it alkaline catalyst under a nitrogen atmosphere, or aluminium and zirconium-contain-ing mesoporous molecular sieves in supercritical carbon dioxide medium. With the above-mentioned methods, highly glycerol conversion with a great selectivity to three
Figure 13.8 Main reaction products in the esterification of glycerol.
332 Handbook of Composites from Renewable Materials
esters of monoglycerides achieved (Jagadeeswaraiah, Balaraju, Prasad, & Lingaiah, 2010; Y. Wang et al., 2012; Zhu et al., 2013). Esterification of glycerol also has been con-ducted with acid catalyst such as dodecamolybdphosphoric acid engaged in the zeolite, tungstophosphoric acid supported on silica/activated carbon, niobic acid-supported ZrO2, sulfonic acid groups linked to mesostructured materials and activated carbon treated with H2SO4 (Kulkarni, Gopinath, Meher, & Dalai, 2006; Lopez, Suwannakarn, Bruce, & Goodwin, 2007).
Some research works have conducted the esterification process of glycerol with a series of multi-valet metal salt in production of mono to dilaurins and found that, chlo-ride-based catalysts such as ZrOCl2.8H2O and AlCl3.6H2O is the most active in the formation of the monolaurin (Das, Das, & Thakur, 2012; Gao & Gao, 2013; Nakamura, Komura, & Sugi, 2008). While, in case of dilaurin, sulfate-based catalyst such as Fe2(SO4)n.H2O and Zr(SO4).H2O indicated the most convenience and selective esteri-fication process (X. Wang, Jin, Wang, Huang, & Wang, 2013). It has demonstrated that, the use of basic catalysts allows obtaining reaction rate almost triple times higher rather those obtained with the same amount of catalysts.
Production of glycerol acetate is one of the examples under esterification of glycerol with resin catalyst; Amberlyst resin (Fukumura et al., 2009; Paterson, Issariyakul, Baroi, Bassi, & Dalai, 2013). As a strong acid ion exchange resin, Amberlyst resin expected two large-pore zeolites H-Y and H-Beta were used as catalysts in etherification of glycerol with isobutylene or tert-butyl alcohol (Pouilloux, Abro, Vanhove, & Barrault, 1999). The continuous flow synthesis of glycerol acetate was documented under supercritical CO2 conditions (110 °C, 200 bar, 120 min, CO2 flow: 0.2 mL.mn–1) using acetic acid presence of Amberlyst 15 as the catalyst (Fukumura et al., 2009). The ratio of glycerol to Aberlyst 15 recorded to play a major role rather than other insignificant contributions (e.g.: pressure, time, CO2 flow rate). The stability of Amberlyst 15 under those kinds of harsh conditions not commented, particularly after reuse, in terms of structure, acidity and surface properties. Comparatively, the catalytic synthesis of glycerol monoacetate was reported using a continuous bed column reactor (temperature: 50 °C, residence time: 30 min) packed with Amberlyst 16 (5 g) as the exchange resin catalysis (Pouilloux et al., 1999). The reaction ended with a good selectivity of correspond monoacetate. Rather than that, Amberlyst 35 found to be an excellent resin catalyst for esterification of glycerol with acetic acid (D. A. Sánchez, Tonetto, & Ferreira, 2014). Under conver-sion conditions of 105 °C, 0,5 g of catalyst and 9:1 ratio of acetic acid to glycerol, 65% of selectivity and almost 100% of glycerol conversion is recorded.
In other cases, biocatalyst (e.g., Novozyme 435 lipase) production of acetate based has also been highlighted. Starting from ethyl acetate and vinyl acetate as acyl donors, the selectivity toward acetins found to be related on residence time and temperature (D.-J. Hu, Chen, & Xia, 2013; J. Huang et al., 2013; Zhong et al., 2013). However, both parameters are insignificantly affecting the distribution of mono-, di, and tri-acetine (J. Huang et al., 2013). The production of monoacetine strongly depends on flow rate (Zhong et al., 2013). Comparatively, the use of vinyl acetate at retention time is signifi-cantly provided diacetine as a main reaction product with high conversion and selec-tivity, which brought by the acyl donor of the vinyl acetate. Consequently, up to 84% on production of triacetin obtained at low flow rate of 0.5 ml. min–1, retention time of 4.8 min at 60 °C, with only minor quantities of diacetines (Franchini, Aranzaez,
Biodiesel-derived Raw Glycerol to Value-added Products 333
de Farias, Pecchi, & Fraga, 2014). However, the diacetine production increased almost 70% at the retention time increased to 28.8 min.
13.3.5 Catalytic Reforming of Glycerol
In general, catalytic reforming is a chemical process used to convert hydrocarbon-based chemicals, typically having low octane ratings, into high-octane liquid products called reformates which are components of high-octane gasoline (also known as high-octane petrol). The process re-arranges or re-structures the hydrocarbon molecules in the naphtha feedstocks as well as breaking some of the molecules into smaller molecules. H2 or syngas raw glycerol confirmed to be a practical alternative for producing H2 or syngas via gasification technique (Ciftci, Ligthart, & Hensen, 2014; Dou, Song, Wang, Chen, & Xu, 2014; Manfro & Souza, 2014) (Figure 13.9). In situ TGA analysis showed that the thermal decomposition mechanism of raw glycerol principally involved four phases of degradation with CO2, H2, CH4, and CO were the major gas products (Ciftci et al., 2014). The optimal conditions for H2 production of glycerol were found to be at 600 to 700 °C and water/glycerol ratio of 9–12 at atmospheric pressure, which conse-quently can produce almost 6.2 moles of H2 per mole of glycerol. This condition was supported by other studies and confirmed that CH4 production is minimized and the carbon formation is thermodynamically inhibited (Byrd, Pant, & Gupta, 2008; Ciftci et al., 2014).
Supercritical water is another gasification technique in the production of H2 with NaOH is commonly used as catalysis (Bühler, Dinjus, Ederer, Kruse, & Mas, 2002; Chakinala, Brilman, van Swaaij, & Kersten, 2009; D. Xu et al., 2009). It has been recorded that, highly selectivity H2 production is obtained (up to 90 vol %) with no char was produced. Reaction by protons or OH groups derived from alkali catalyst can per-form under super critical conditions. In this case, water not only acted as solvents, but also a catalyst due to the self-dissociation that takes to the formation of hydroxyl ions and protons. Thus, conversion of glycerol with super critical water condition can con-sider to occur with two consequence steps. Initially, the ionic reactions would occur at high pressure and/or high temperature, followed by a degradation reaction of free radi-cals at low pressure and/or high temperature. With that, the reaction rate significantly increased with an increment of temperature until critical temperature is obtained, then its consequently reduced related to subcritical conditions (S. Guo et al., 2012; May, Salvadó, Torras, & Montané, 2010; Voll et al., 2009; Watanabe et al., 2003).
Furthermore, some reports claimed that, the quantity of inorganic alkaline catalyst in the raw glycerol affected the concentration of H2 produced by a record of 2.7 wt % on NaOH produced 42 vol % of H2. However, the long-chain fatty acids are hardly
Figure 13.9 Hydrogen generation via catalytic aqueous-phase reforming of glycerol.
OH
OHCatalyst
CO2 H2O+
SyngasOH
334 Handbook of Composites from Renewable Materials
reformed and more likely to form carbon rather than desired product (Onwudili & Williams, 2010). Therefore, methanol, acrolein, propionaldehyde, ethanol, allylic alco-hol, formaldehyde, carbon dioxide, carbon monoxide, and hydrogen are the series of products obtained from these types of reactions (Valliyappan, Ferdous, Bakhshi, & Dalai, 2008; Yuksel, Koga, Sasaki, & Goto, 2010). Production of acetyldehyde and formaldehyde increased by pressure, which indicates that both products are mainly formed by ionic reaction, while methanol and allylic alcohol formation, decrease with the pressure, which indicates that these compounds are formed by the free radical way (Adhikari, Fernando, & Haryanto, 2009). The free radical mechanism also occurs in the formation of gas products occurred at high temperature and low pressure. The production of H2 from glycerol, ethylene glycol and sorbitol at temperature of 225 °C and 227 °C under high pressure in a single-reactor aqueous-phase reforming process assisted by Pt/Al2O3 is normally performed (Randy L Maglinao & He, 2011). The total H2 yield with this kind of reaction reached up to 64.8 mol % and 57 mol % at 225 °C and 265 °C, respectively. In the presence of Sn promoted Raney-Ni catalyst, the H2 production by aqueous phase reforming at a lower heating temperature of 227 °C and a pressure of 2.58 to 5.14 MPa (E. A. Sánchez, D’Angelo, & Comelli, 2010). It has been documented as well that, incorporation of Sn increased the formation of H2 (up to 66 mol %) with decrease the CO2 production (32 mol %). The atomization of glycerol is strongly assisted by the presence of Ni-based catalyst at superheated steam condition (60 °C to 80 °C) (Dou et al., 2014). In this case, H2 production went up to 77 wt % and could be linearly increased with steam temperature. However, the disadvantages of this type of H2 production are that CO produced that requires purification of the H2 steam to prevent fuel cell poisoning. This technique also requires the use of large amount of O2 that would increase the production of CO while reducing the H2 yield obtained (Iriondo et al., 2010). A wide range of selectivities and conversions have been reported based on the operational conditions like pressure, temperature and glycerol concentra-tion as well as the impurities of glycerol (CH2OH and KOH). The reaction with low concentration of glycerol and high temperature resulted in high CO2 concentration with most products is still in the liquid phase (C. K. Cheng, Foo, & Adesina, 2011; Iriondo et al., 2010; E. A. Sánchez et al., 2010). With that, many researchers investigated the effect of heterogeneous catalyst on the steam reforming of glycerol for H2 produc-tion with different kind of catalyst and support to achieve almost yield stoichiometric conversion of glycerol to H2 (C. K. Cheng, Foo, & Adesina, 2010). To selectivity pro-duce of syngas, inert materials such as carbon-based materials much better used as a catalyst support instead of using metal oxide to increase the activation of the water. A combination of chemical inertness also reported to prevent ionic-catalyzed polymer-ization reaction from occurring and hydrophobicity of reactions that consequently pro-vides the stability of reaction under the aqueous-phase processing of glycerol to syngas (Barbelli, Pompeo, Santori, & Nichio, 2013; Bobadilla et al., 2012; Nichele et al., 2012).
The H2 production has taken place via steam reforming process of glycerol at high temperature endothermic. The viability of Pt/Al2O3 heterogeneous catalysts in the pro-duction of hydrogen from glycerol also confirmed by other researchers to the conclusion that, optimum reformer performance achieved at 880 °C, flow rate of 0.12 mols. min–1 per kg of catalyst (B. Liu & Greeley, 2011). It believed that, the presence of Pt favors the cleavage of C–C bonds over C–O bonds, especially under gas-phase conversion
Biodiesel-derived Raw Glycerol to Value-added Products 335
(225–275 °C) (Idesh, Kudo, Norinaga, & Hayashi, 2013). In this case, adsorbed CO mol-ecules predominately cover Pt surface, which inhibits catalyst performance (Carrera Cerritos et al., 2010). In advance, Pt/Al2O3 catalyst has doped with La2O3 or CeO2 and concluded that, the addition of metal oxides to Pt/Al2O3 catalysts found to consider-ably enhance the glycerol steam reforming with high H2 and CO2 production due to the greater surface and distribution of Pt (Manfro, Ribeiro, & Souza, 2013). With that, a better catalytic stability obtained by composition of Pt/La2O3/Al2O3 at working system of 350 °C, while the Pt/CeO2/Al2O3 catalyst strongly deactivates after 20 hours under the same conditions. Meanwhile, PtRu and PtRe have identified as alloys that could offer the reforming activity of Pt but that bind CO less strongly, thus mitigating reaction inhibition in the presence of desired products. Both of these alloys were active in the production of syngas from glycerol with less susceptibility to adsorb more CO. Besides, the production of syngas can be tuned by modification of the introduced pressure and temperature; at lower pressure and temperature, the water gas is not equilibrated thus the system produce more CO and H2 gasses (B. Zhang, Tang, Li, Xu, & Shen, 2007). Summary on Ni, Ce and Ru heterogeneous catalyst supports provided in Table 13.10.
In the meantime, glycerol aqueous reforming is one of the promising techniques to grant access on production of relevant chemicals (e.g., propanediols). The effect of various heterogeneous catalysts under different suitable conditions for production of propanediol derived from glycerol summarized in Table 13.11. Some studies have combined dehydration and hydrogenation process (hyrogenolysis process) of glycerol catalyzed with solid acid catalysts (sulfated zirconia, zeolites, ion exchange resin, and
Table 13.10 Series of catalyst and their support for re-forming process of glycerol.
Heterogeneous catalyst Support Function of support References
Ni MgO Increase stability of the catalyst under reaction conditions
362
CeO2 364
TiO2 368
Al2O3 363;365; 366;372
La Increase hydrogen selectivity 345
Ce 369
Mg Increase surface catalyst concentration
373
Zr Improve capacity to activated stem 364
Ce Ir Increase selectivity 375
Co Activate catalyst 376
Ni Reduce formation of coke 367
Ru Y2O3 Increase efficiency of the catalyst 377
336 Handbook of Composites from Renewable Materials
tungstic acid) (Balaraju et al., 2009; L. Guo, Zhou, Mao, Guo, & Zhang, 2009; Maris & Davis, 2007). In this case, the conversion process starts with selectively transform the middle OH groups of glycerol into a tosyloxyl group before being removed the transformed group by catalytic hydrogenolysis. It can be concluded that, the hydroge-nolysis process generally involves three main steps; acetalization, tosylation and deto-syloxylation. Production of propanediol also could perform at mild condition (358 K; 5 MPa) of sulfolane with Ru as catalysts (Miyazawa, Koso, Kunimori, & Tomishige, 2007; Amin TermehYousefi et al., 2014). This process scarified on reaction activity, total yield and selectivity degree of the propanediol. The combination of Ru/C with ion exchange resin indicated the highest activity even under mild conditions (tem-perature: 393 K; pressure: MPa) rather than other solid acid catalysts mentioned above (L. Ma & He, 2010).
This encourages other research studies to move forward in the application of Ru catalyst with SiO2 with expected that combination of Ru/SiO2 as an effective catalyst in the conversion of glycerol under higher pressure of H2 conditions (Xi et al., 2010).
This normally developed in a continuous flow fixed-bed reactor (at temperature up to 235 °C, pressure of 31 bar) with the addition of ZnO and NiMo (in ratio of ZnO: NiMo 2:1) to catalyze the reforming reaction (Jiye et al., 2013). This reaction typically carried out over ZnO and/or NiMo catalyst due to the ability on both heterogeneous catalysts to achieve the formation of C–O breaking reaction and formed a light hydro-carbon rather than the C-C breaking reaction that could lead to CO, H2, and CO2 pro-duction (Liang, Ma, Ding, & Qiu, 2009; Srivastava, 2013). The several co-catalysts such as SiO2, MgO, HZSM-5, TiO2, Al2O3, CeO2, and ZrO2 also reported protocol to pro-vide better selectivity (almost 53%) and reduction of reaction pressure (Chaudhari, Torres, Jin, & Subramaniam, 2013; Guan et al., 2013; Hosgün, Yıldız, & Gerçel, 2012). It is believed that, the synergistic effect between physically mixed of skeletal NiMo and ZnO would enhance the Lewis acidity of ZnO by chemisorbed CO2 from the reforming process of glycerol, which in turn assist ZnO to promote further dehydration of glycerol to acetol as well as the NiMo-catalyzed hydrogenation of actual to 1,2-propanediols. In the other hand, production of 1,2-propanediols via hydrogenation of glycerol can be
Table 13.11 Production of propanediol with different catalyst under different synthesis conditions.
Reaction
Condition
ReferencesTemperature (°C) Pressure (MPa)
Zinc and copper catalyst along with sulfided Ru catalyst
240–270 15 378
Raney Cu, Cu-Pt, Cu-Ru and Cu/C 220–240 1–4 379
Cu, Co, Mo, Mn and an inorganic polyacid
259 25 380
Homogeneous catalyst containing W and group VII transition metal
200 32 381
Biodiesel-derived Raw Glycerol to Value-added Products 337
achieved using metabolically engineered microorganisms (e.g.: Clostridium acetobu-tylicum) (Bagheri, Chekin, & Hamid, 2014; X. Guo et al., 2009). Reduction of glycerol to 1,2-propanediol has been catalyzed with Co/MgO and indicated a low conversion of glycerol (below than 55%) with selectivity as low as 42% (X. Guo, Li, Song, & Shen, 2011; Yuan et al., 2010; Jinxia Zhou, Guo, Guo, Mao, & Zhang, 2010). However, as Cu/Al2O3 used as catalysts, the glycerol conversion in 190–200 °C under 0.1 MPa partial hydrogen pressure will reach up to 100% with selectivity is more than 75% (S. Wang & Liu, 2007).
Several supported, mono and bi-metallic transition metal catalysts, including Pt, Cu, Au, Au–Ru, Ni, Ru–Re, and Cu–ZnO have been extensively used in the glycerol reforming process for the 1,3-propanediols production (Bienholz, Schwab, & Claus, 2010; Z. Huang et al., 2008; Zhu et al., 2012). The activities on series of above men-tioned catalyst arranged as follows: Ru ≈ Cu ≈ Ni > Pt > Pd (Z. Huang et al., 2008). Meanwhile, the conversion of glycerol to 1,3-propanediols, an aqueous-phase glycerol degradation protocol reported to use a series of ternary catalyst system Pt/WO3/ZrO2 (at composition of 10 wt % Pt, 10 wt % WO3, 80 wt % ZrO2). The reaction took place in a fixed-bed continuous flow reactor (temperature of 130 °C, a pressure of 40 bars, 24 hours), and manage to produce 70% conversion of glycerol with 32% yield, 46% selectivity of 1,3-propanediols as main products (Leifeng et al., 2009; Terao et al., 1988). Under such condition also, n-proponal and i-propanol reported to be another major product with selectivity more than 50%, which pointed to high secondary deoxygen-ation selectivity as compared to the primary OH groups in glycerol. This deoxygenation mechanism involved proton transfer and hydride transfer steps. Meanwhile, one step reaction on conversion of glycerol to 1,3-propanediol by using a direct chlorination of glycerol. With that, very high glycerol conversion (100% with selectivity of 90%) at the temperature of 90–110 °C produced in batch reactor system (Yin, Guo, Dai, & Fan, 2009). This will further improve by the support materials such as ZnO, C, Al2O3, H2O, sulfolane, dioxane, and H2WO4 have intensively used to improve the selectivity and the degree of glycerol conversion.
13.3.6 Catalytic Reduction of Glycerol
In general, reduction is a reaction that loss an oxygen. The catalyst for glycerol reduc-tion at present are optimized only partially for industrial application for production of commodity chemicals. Several research works have used heterogeneous catalysts of Zn, Cu, Mg, Co, Mo, Pd, Ni, and Pt catalyzes the reduction of glycerol (Johnson & Taconi, 2007; Z. Shen et al., 2009). The main products of this reduction process could be ethyleneglycol, 1,2-propileneglycol, 1,3-propileneglycol, lactic acid, acetol, propa-nol, or even acrolein, with widely reaction parameters on temperature (200–350 °C) and pressure (2000–5000 psi) (Roy, Subramaniam, & Chaudhari, 2011). Amongst all heterogeneous catalysis, Cu reported the highest yield and selectivity on production of propylene glycol while low selectivity to ethylene glycol (Jiang, Zhou, Liang, Liu, & Han, 2009; Mane et al., 2010; Sun & Liu, 2011). In contrast, Ru- and Pd-based catalysts resulted in low selectivities of propylene glycol due to the competition in hydrogenoly-sis process (Nakagawa & Tomishige, 2011; Y. Shen et al., 2010). With that, C–C and C–O bonds are being excess to produce lower alcohols and gases. Otherwise, undesired
338 Handbook of Composites from Renewable Materials
products such as ethylene glycol or even CH4 are suspected to obtain. Some studies have concluded that the degree of reduction is independent of the initial glycerol con-centration, while its strongly support by catalyst types, temperature, and pressure of the process in order to produce ethylene glycol and propyleneglycol with high yield and selectivity (Nakagawa & Tomishige, 2011; Ramimoghadam, Bagheri, & Abd Hamid, 2014) (Figure 13.10).
Furthermore, the selectivity in production of ethyleneglycol not linearly connected with the pH of the reaction, with less production at low basic condition (Wolosiak-Hnat, Milchert, Lewandowski, & Grzmil, 2011). Thus, hydrogenolysis of glycerol has also proposed in the presence of bimetallic and bifunctional catalyst (e.g., PtRu/Ca and AuRU/Ca). The presence of Ru catalyst make the conversion process to occur at mild condition at temperature of 180 °C, hydrogen pressure of 5 MPa and reaction time of 12 hours resulted on high selectivity of ethylene glycol (almost 41%). The same approach also applied by other research groups, but more focused on glycerol concen-tration, pressure, temperature and residence time. It found a significant relationship between those mentioned factors with the conversion and selectivity of ethylene glycol. Other bimetallic system for reduction on glycerol listed in Table 13.12.
Several mechanisms have proposed for the reduction process of glycerol, which included adsorb–desorb process. Another reduction process of glycerol to produce pro-pylene glycol and water has proposed. In this mechanism, hydroxyacetone produced by
Figure 13.10 Catalytic conversion of glycerol to ethylene glycerol, 1,2- and 1,3-propanediols.
OH
OH
1,2-Propanediol Acetol GlycerolOH
OH
OHOHHydrogenation Reduction
OH
OH
1,2-Propanediol
OH
OH
OH
OH
1,3-Propanediol
Ethyleneglycol
O
Table 13.12 Parameters involved in the catalytic glycerol reduction.
Biodiesel-derived Raw Glycerol to Value-added Products 339
dehydrogenation of glycerol as precursor of propylene glycol (Nakagawa & Tomishige, 2011; Y. Shen et al., 2010; Sun & Liu, 2011).
13.3.7 Catalytic Etherification of Glycerol
Etherification defined as the process of making ether from an alcohol. Ethers are com-patible with materials like Grignard reagents and lithium aluminum hydride that react with and destroyed by alcohols that they are generally unreactive. Etherification process of glycerol produced a low polymerization (with lineal, branched, or cyclic chains) and oxygenated compound known as polyglycerols (e.g: glycerol tertiary butyl ether, methyl tertiary butyl ether, 1,3-ditertbutyl glycerol, 1,2-di-tertbutyl glycerol and 1,2,3-tri-tertbutyl glycerol) can be more effectively achieved by the presence of hetero-geneous catalyst (González et al., 2014) (Table 13.13) (Figure 13.11). It recorded that the etherification process of glycerol shows the highest catalytic activity with sulfonic acid (CH–SO3H) as the catalysis due to the presence sulfonic groups (González et al., 2014; Pariente, Tanchoux, & Fajula, 2009).
Such ethers are simply generated from glycerol by treatment with isobutylene in the presence of an acid catalyst (Chakrabarti & Sharma, 1993). Glycerol’s etherifications with isobutylene have extensively investigated over sulfonic mesostructured silicas, strong acid ion-exchange resins and zeolite brought to a complete conversion of glyc-erol with 90% of selectivity (J.-M. Clacens, Pouilloux, Barrault, Linares, & Goldwasser, 1998). Besides, incorporation of homogeneous catalysis such as sodium, potassium or carbonate hydroxide documented to produce polyglycerols with mixture of lineal and cyclic characteristics (Ozbay, Oktar, Dogu, & Dogu, 2011). Furthermore, a posi-tive effect on the selectivity of polyglycerol recorded with alkaline exchange zeolite as catalysis (Serafim, Fonseca, Ramos, Vital, & Castanheiro, 2011). It is worth to evalu-ate robust acid ion exchange resins as an appropriate catalyst for the production of
Table 13.13 Series on catalytic etherification process of glycerol with different heterogeneous catalyst.
Reaction Heterogeneous catalyst References
Etherification of glycerol with ethanol Sulfonic acid 415–417
Zeolites 418
Grafted silicas 419–420
Heteropolyacid 421–422
Mesoporous MoO3/SiO2 423
Etherification of glycerol with butanal Zeolite 424–425
Etherification of glycerol with methyl acetate Sulfonic acid 426–427
Etherification of glycerol with aqueous formaldehyde
Amberlyst 15 428
Zeolites 429–430
P-toluenesulfonic acid 431
340 Handbook of Composites from Renewable Materials
commercial ethers from glycerol. The total yield of etherification process can improve by via two-phase reaction system. The first phase involved a glycerol-rich polar phase (containing the acidic catalyst) and other phase is consisting of an olefin-rich hydro-carbon phase from which the product ethers can be readily separated (Cogan & Koch, 2003).
However, as acid catalysis are applied, the selectivity of the etherification process of glycerol become uncontrollable with a mixture of di- to hexa glycerol (lineal or cyclic) is obtained and consequently produce a series of byproducts (polyglycerol ester and acroleine). Thus, some studies have modified the pseudo-pore size in these mesopo-rous materials with an aim to achieve better selectivity in the first step of the reac-tion. For example, glycerol conversion has improved by incorporation of Na2CO3 as catalysis, and this in turn resulted in low selectivity of di and tri-glycerols (F. Chekin, J. B. Raoof, S. Bagheri, & S. B. A. Hamid, 2012; J. Clacens, Pouilloux, & Barrault, 2000; García-Sancho et al., 2011). Impregnation of inorganic element, such as Al, Mg, and La into the mesoporous catalysis expected to modify both selectivity and activity of glycerol conversion and hold the reaction to almost constant (Klepáčová, Mravec, Kaszonyi, & Bajus, 2007). Amongst all impregnated elements, La and Mg have shown the most active and selectivity results. Thus, it can conclude that, impregnation meth-ods gave results that are more positive in term of activity and selectivity as compared to incorporate methods.
Furthermore, glycerol can converted into branched oxygen-containing components by catalytic etherification with either alkenes (isobutene) or alcohols (methanol or ethanol). It has been demonstrated that the glycerol’s etherification with tert-butanol at 90 °C within 180 min with the existence of catex Amberlyst 15 as catalyst obtained almost 96% conversion (Calvino-Casilda, Guerrero-Pérez, & Bañares, 2009; Ruppert et al., 2009). Consequently, some comparative works have been further carried out on etherification of glycerol with isobutene and tert-butanol without solvent as liquid
Figure 13.11 Main reaction products in the etherification of glycerol.
OH
OH +
OH
Glycerol
Catalyst
O
OH
OH
O
O
O
O
O
OO
OH
O
OH
Di-glycerol ether
Tri-glycerol ether
OH
Mono-glycerol ether
OH
Biodiesel-derived Raw Glycerol to Value-added Products 341
phase, and found that isobutane indicate better conversion over different temperature ranges (Guerrero‐Pérez & Bañares, 2008). This brought the next investigation on appli-cation of isobutane with macroreticular ion exchange and/or sulfonic mesostructured silica as catalyst. It is indicates almost 100% conversion of glycerol. Glycerol etherifica-tion with ethanol investigated using different types of heterogeneous catalyst such as grafted silica, sulfonic resins and zeolites to produce oxygenated diesel additives.
13.3.8 Catalytic Ammoxidation of Glycerol
In chemistry, ammoxidation is an industrial process for the production of nitriles using ammonia and oxygen. The usual substrates are alkenes. It sometimes called the Sohio process. Ammoxidation of alkenes exploits the weak C–H bonds that are located in the allylic position of unsaturated hydrocarbons. Benzylic C–H bonds are also suscep-tible to ammoxidation, reflecting the weakness of their C–H bonds. For this reason, cyanopyridines (e.g. the precursor to niacin) and benzonitriles produced from meth-ylpyridines and toluene, respectively. Dinitriles produced by double ammoxidation, examples being phthalonitriles (precursor to phthalocyanines) and terephthalonitriles, both from xylenes.
Direct production of acrylonitrile via ammoxidation of glycerol with single and mixed oxides such as Mo, Bi, Sb, V, Sn, W, Zr, Ti, Ni, Al, P, G, and Nb as catalyst has described in the literature review [438–439] (Figure 13.12). The catalyst has to select wisely to avoid that the acidic centers of the catalyst blocked by ammonia through the reaction time (Calvino-Casilda, Guerrero-Pérez, & Bañares, 2010; Cavani, 2010; F. Chekin, J. Raoof, S. Bagheri, & S. A. Hamid, 2012). The ammonia/glycerol molar ratio should vary between 1 and 1.5 at an oxygen/glycerol ratio of 0.5 and 10. The total conversion reached to 100% with selectivity more than 48% (Martin & Kalevaru, 2010). Injection of pure aqueous solution of glycerol (10wt % minimum concentration) at reaction temperature of 280 °C and 550 °C at 5 bar is suggested parameters used in the direct ammoxidation process (ten Dam & Hanefeld, 2011). Some research stud-ies have used alumina-supported catalyst that contained V, Sb, and Ni in a continuous fixed-bed reactor (Golinska et al., 2010; Liebig et al., 2013). On the other hand, Nb and Sb oxides supported on alumina are extremely less active than the V-containing cata-lyst. Sb oxide supported on alumina exhibits significant selectivity to a nitrile products
Figure 13.12 Direct and indirect glycerol conversion via ammoxidation process.
OH
OHCatalyst
Direct ammoxilation
Acrylonitrile
N
NH3
OIndirect ammoxilation
Acrolein
OH
Glycerol
Exeter
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342 Handbook of Composites from Renewable Materials
(cetonitrile), while V supported on alumina is the most active, but produces acrolein, propanal, 1,2-propanediol and cracking products. Alumina-supported Sb and Ni oxide catalysts are relatively inactive, but both types of catalyst produce acrolein and most interestingly, alumina-supported Sb exhibits a capacity to from carbon-nitrogen bonds (Guerrero-Pérez & Bañares, 2015). However, these catalytic processes produced CO2 as a sub product to yield more than 50%. Therefore, some efforts have made to apply indirect route, allowing the independent choice of the catalyst and reaction parameters. The indirect route should consider on the compatibility of the catalyst and the condi-tions. For instance, if acid catalyst applied on the first step of dehydration, this might cause problem considering the presence of NH3, which in turn blocked the catalyst active side. Furthermore, most of ammoxidation process required high processing tem-perature (first step: 270–300 °C; second step: 400–500 °C) (Liebig et al., 2014).
Generally, indirect ammoxidation involved two continuous steps: dehydration of glycerol followed by ammoxidation of acrolein. Acid-based catalysts commonly used for the first step of dehydration [448]. In this case, WO3/TiO2 system proved to be efficient for the glycerol’s dehydration to acrolein, limited production of sub prod-ucts (acetic acid, acetaldehyde and hydroxyacetone) and yield up to 70% of acrolein. The challenges of reaction are much more predominant in the second step of reaction concerning the large amount of impurities/subproducts from the first step of glycerol dehydration (Bañares & Guerrero-Pérez, 2014). Therefore, the selected catalyst must be tolerance for ammoxidation process, especially toward large amounts of water content, but also of which the performances not altered by the existence of organic impurities. Some mixed oxides based on V/Al, VSb/Al, and VSbNb/Al, Sb/Fe, Sn/Sb/Fe/O and Sb/Vd known to work for ammoxidation of acrolein even in water conditions. Some efforts have been done in stream condition. It found that, the conversion rate of acro-lein increased almost six times and better selectivity toward acrylonitrile. The result is more significant as Sb/VO used as the catalyst (Guillon et al., 2013). Meanwhile, at ratio of 0.6 and 1.8 of Sb/FeO catalyst, the catalytic performance in the ammoxidation of acrolein found to be more favorable with selectivity reached to 44% and conversion of acrolein recorded at 81%. XRD studies have revealed that addition of FeSbO4 is corre-lated to enhance in selectivity toward (Gholamrezaei, Salavati-Niasari, Bazarganipour, Panahi-Kalamuei, & Bagheri, 2014; Mikolajska et al., 2011; Soriano et al., 2011) ACN in the first 3 times on stream due to the increment of catalyst surface and formation of FeSbO4 phase. However, the reduction phase of Fe2O3 to Fe3O4 is possible once the reaction time reached to 400 °C. Additionally, ammonia and oxygen content should control for avoiding the destruction of the desirable FeSbO4 phase. It noted that, feed-ing ammonia during the reaction of glycerol may drive the production distribution toward nitriles. Thus, acrylonitrile production would drop by 10% with the increasing of NH3/acrolein ratio that can caused by deactivation of the dehydration catalyst, which resulted in a reduction acrolein concentration in feed to the second step of ammoxida-tion (Khayoon & Hameed, 2013; Ramimoghadam, Bagheri, & Hamid, 2015a).
13.3.9 Catalytic Acetalization of Glycerol
Acetalization is a noun that refers to any reaction that yields an acetal. It generally used in organic chemistry, especially when dealing with chain reactions, and in plural form
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Biodiesel-derived Raw Glycerol to Value-added Products 343
this word changes to acetalizations. The acetalization of glycerol is one of the most important processes for the synthesis of green and cost-effective bio-additive chemical form glycerol. Through the acetalization process, glycerol reacts with simple carbonyl compounds to provide isomeric six- (1,3-dioxane) and five-membered (1,3-dioxolane) cyclic products as novel fine chemical intermediates (Sudarsanam, Mallesham, Prasad, Reddy, & Reddy, 2013; Umbarkar et al., 2009). These additives find a good potential application in fragrances, cosmetic, food, beverage, pharmaceuticals, detergent, and lac-quer and combustion engine. However, there is no such of 50:50 on production of each 1,3-dioxane and 1,3-dioxolane are recorded. This, in turn, encourages some research works to vary on reaction parameters such as the molar ratio of carbonyl compound to glycerol and temperature to modify on the selectivity of the reaction. However, none of the related approach report on a complete conversion of glycerol acetalization with no isomeric six- (1,3-dioxane) is produced. In addition, complete selectivity toward solke-tal and acetal for both isomerix six and five not achieved even a supercritical condition (with hazardous dichmoromethane solvent) is applied. In consequence, the complete isomerix six (1,3-dioxane) is produced with the aldehydes facilitated at the evaluated temperature, which this reaction is considered not environmentally friendly. With that in mind, researchers have worked in using homogeneous acid catalyst such as HCl, divinylbenzene-styrene resin, H3PO4 and p-toluenesulfonic acid (PTSA) for the acetal-ization of glycerol (Amiri, Salavati-Niasari, Farangi, Mazaheri, & Bagheri, 2015; Fan et al., 2012). Since this approach is used toxic reagents, tedious work-up procedures and problem in disposing of the effluents makes the whole process considered environ-mentally unfriendly.
Therefore, there is an intensive effort in using heterogeneous catalyst for acetaliza-tion of glycerol with ketones and/or aldehyde to control the selectivity in production of glycerol solketal and acetal. Indeed, heterogeneous catalyst can easily separate from the reaction mixture either by filtration or by centrifugation and do not require neutraliza-tion procedure. The catalytic acetalization of glycerol with ketones have been intensively reported in yielding the five-membered (1,3-dioxolane) cyclic only. As well, the use of organic solvents included chloroform, benzene and toluene to improve the glycerol conversion is also reported and discussed. This route considered effective, inexpensive and more reliable process on acetalization of glycerol. For example, glycerol has been selectively converted to branched oxygenated compounds of five- and six-membered (1,3-dioxolane) through a solventless acetalization process with acetone catalyzed by mesoporous 5% Ni–1% Zr-supported activated carbon catalyst. The reaction has been performed with only 0.20 g of catalyst, N2 flow at 45 °C. The chromatography result shown a complete conversion process with the selectivity of 26% and 74% in five- and six-membered (1,3-dioxolane), respectively. The catalytic activity mainly attributed to the intercalated NiO and ZrO2 species into the activated carbon structure and to the surface characteristics.
A solid acid catalyst such as MoOx or Wox promoted ZrO2 catalyst have also gained some intention to catalyze the acetalization process of glycerol due to its nature of active sites which can be defined either by the presence of surface proteins (Bronsted acid sites) or by coordinating unsaturated cationic centers (Lewis acid sites). Besides, its environmental factors such as environmentally benign ease in preparation, own-ing a better thermal stability and display strong surface acidity has made this type
344 Handbook of Composites from Renewable Materials
of catalyst as a promising candidate. However, further improvements, however, are still required to meet the increased practical demands. Use of zirconia-based mixed oxides and subsequent impregnation with molybdate ions is one of the efficient ways to enhance the catalytic performance. In general, mixed oxides show superior physi-cochemical and acidic properties than the individual component oxides for a better catalytic activity. Therefore, molybdenum oxide promoted zirconia-based metal oxide catalyst has currently received attention with the catalyzed acetalization of glycerol with different kinds of benzaldehydes under solvent free conditions. For example, ZrO2 and TiO2-ZrO2, and the respective MoO3 promoted catalyst has been prepared by a fac-ile precipitation and wet-impregnation method and found that almost 74% glycerol conversion is observed by 51% is selected on 1,3-dioxane production. However, the conversion of glycerol relatively decreased with substituted benzaldehydes due to the presence of stearic hindrance structure. The similar observation is occurred in the case of p- anisaldehyde with MoOx/TiO2–ZrO2 solid acid catalyst whereby up to 71% on selectivity of 1,3-dioaxane is obtained. The potential on Molybdenum supported with SiO2 and Al2O3 have then extensively investigated due to its catalytic activity in oxida-tion and organic reaction. For example, acetalization of glycerol with benzaldehyde was carried out using series of MoO3/SiO2 catalyst with varying MoO3 loading (1–2 mol %). Among the series, 20 mol % of MoO3/SiO2 catalyst is found to be the most active cata-lyst in acetalization under mild conditions. With that, glycerol conversion has reached almost 72% with 60% selectivity on six-membered acetal. Thus encouraged in using a number of solid acids in acetalization process, including protic acid, Lewis acid (ZnCl), alumina, montmorillonite, zeolite, mesoporous alumina silicates and ion exchange res-ins (Forsberg, 1987; Ramimoghadam, Bagheri, & Hamid, 2015b).
13.3.10 Enzymatic Conversion of Glycerol
The conversion of raw glycerol into value-added products by biological techniques with enzymatic catalytic is another main focus of this review. The conversion of glyc-erol to 1,3-PD is promising, especially when using enzymatic approach by Clostridium acetobutylicum and Clostridium butylicum (González-Pajuelo et al., 2005; Vasconcelos, Girbal, & Soucaille, 1994). Different heterotrophic microorganisms such as microalgae and yeast have also the capability to grow on glycerol (Table 13.14).
Succinic acid, carotenoids, polyhydroxyalkanoates, citric acid, rhammolipids and polyunsaturated fatty acid are the main value-added products obtained by glycerol-based fermentations of Actinobacillus succinogenes (Guettler, Rumler, & Jain, 1999; McKinlay, Zeikus, & Vieille, 2005; Zheng, Dong, Sun, Ni, & Fang, 2009). Basfia
Table 13.14 Fermentation studies on enzymatic conversion of glycerol to 1,3-PD.
Enzyme catalysis Yield (g per g glycerol) References
C.butyricum 0.50 459
Klebiella pneumoniae 0.86 460
K. pneumoniae 0.52 461–463
Biodiesel-derived Raw Glycerol to Value-added Products 345
succinoproducens has demonstrated to be a viable succinic acid producer, with a yield of 1.2 per gram of glycerol (Abad & Turon, 2012). Other reports focused on production of succinic acid up to 1.33 per gram of glycerol under Anaerobiospirillum sp enzyme catalysis in anaerobic fermentation of glycerol, to limit the acetic acid formation and simplify succinic acid downstream purification (P. C. Lee, Lee, & Chang, 2010). This encourages a lot of work to produce citric acid, arachidonic acid and unsaturated fatty acid from glycerol with microorganisms such as Yarrowia lipolytica, Mortieralla alpina and naturally prolific (Bagheri, Muhd Julkapli, & Bee Abd Hamid, 2014; Papanikolaou & Aggelis, 2002, 2003). The production of pigment derived from crude glycerol via Blakeslea trispora enzyme catalysis. It yielded about 15 mg β-carotene/g of dry biomass and has a good potential in food industry as a nutritional supplement. Some other studies have discussed about the production of ethanol and hydrogen from glycerol uti-lizing Enterobacter aerogenes HU-101 enzyme catalysis. The threshold hydrogen pro-ductivity with a viable process achieved with continuous cultures of semi-immobilized cells in a continuous packed reactor.
Several enzymcatalytic approaches have also reported with excellent results for glyc-erolysis in noncontinuous mode yielding 70–99% monoglycerides with the achieve-ment of 70.6% yield for monoglycerides. The reaction took place in continuous mode with immobilized Staphyloccus simulans lipase on CaCO3 in a solvent-free system. Some catalytic enzymatic conversion of glycerol has also applied for the glycerol carbonate synthesis at mild condition with high selectivity. It has recorded that, the procedure of transesterification of renewable dimethyl carbonate and glycerol in the presence of immobilized lipase isolated from Candida antartica (Ruzin & Novick, 2000; Shimada et al., 1999). Glycerol carbonate also can generate on the direct way in high yield from renewable glycerol and dimethyl carbonate in a reaction catalyzed by lipase.
13.4 Conclusion
The improvement of sustainable processes for using glycerol is crucial. Because purified glycerol is a high-value commercial product with a wide range of applications. With that, scientists have devised ways to deconstruct different chemical platform derived from glycerol with regards on simpler and more understood chemistries could be in principle designed to provide similar products. Being a polyol with three hydroxyl groups with different reactivity, multiple chemistries ranging from redox (oxidations and hydrogenolysis) to acid-catalyzed processes (etherifications, esterification), dehy-drations and oligomerisations can be designed and optimized. Chemically, glycerol can catalytically transform to oxidation products on metallic catalysts as Pt, Pd, and Au using promoters as Bi and Pb; glycols by hydrogenolysis on Ru, Cu, and Pt catalysts; polyglycerols by etherification on zeolites and mesoporous materials and; syngas by pyrolysis and gasification. Also, different kind of microorganisms could be metabo-lized glycerol as a sole carbon and energy source, and then this may substitute tradi-tional carbohydrate in some industrial fermentation processes. However, significant challenges are still, however, to be addressed in terms of developing chemical platforms under aqueous processing conditions, design of stable and active catalysts and essen-tially different processing of raw glycerol.
346 Handbook of Composites from Renewable Materials
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