Key words

biodiesel; biorefineries; fuels; glycerol; green chemistry

12.1 Introduction

The development of fuels and chemical products based on renewable resources is the aim of biorefineries. The world is still dependent on oil, but this dependence has led to critical changes in the climate of the planet. Global warming is a reality and may lead to great environmental, economic and social impacts in the coming years, if nothing is done to stop or slow down this process. Greenhouse gases, particularly CO₂ from fossil fuels, are mainly responsible for the global warming process. Thus, it is imperative to develop new processes for the production of fuels and chemicals, without impacting the environment, especially in terms of greenhouse gases.

The use of biofuels is spreading all over the world. Bioethanol and biodiesel will share a significant part of the fuel market, contributing to the control of global warming in the future. Biodiesel is produced mainly through the transesterification of vegetable oils or animal fat, the triglycerides. In this process, methanol reacts with the triglyceride in the presence of a basic or acidic catalyst to afford fatty acid methyl esters, the biodiesel themselves, and glycerol (Fig. 12.1). Roughly, for each 100 m³ of vegetable oil processed, about 10 m³ of glycerol is produced. In recent years, the surplus of glycerol coming from biodiesel fabrication has enormously increased, representing, today, about 65% of the world’s glycerol production.

Glycerol can also be obtained from algae (Muscatine, 1967). This is another potential renewable source of glycerol, alternative to the production from biodiesel that could be used in biorefineries processes. Some species of the unicellular algae Dunaliella possess outstanding adaptability and tolerance toward a wide range of salinities from seawater. The capability of the cell to thrive in high salt concentrations depends on its unique ability to produce intracellular glycerol (Chitlaru and Pick, 1991). Dunaliella salina and Dunaliella viridis grow in media containing different salt concentrations. Algae growth, as well as glycerol production, increased as the salinity of the medium increased (Hadi et al., 2008)

Glycerol or glycerin is traditionally used in cosmetics, soaps and pharmaceuticals. However, these sectors cannot drain the enormous amount of glycerol that comes from biodiesel production. Thus, the glycerol byproduct of biodiesel production must find new applications, capable of making use of the increasing output of this chemical and adding value to the biodiesel chain. The purpose of this chapter is to highlight some potential applications of glycerol in the fuel sector and in the chemical industry.

12.2 Composition and purification of glycerol produced from biodiesel

Glycerol is 1,2,3-propanetriol. It was first identified by Carl Scheele in 1779, upon heating olive oil with litharge (PbO). The viscous, transparent liquid that separated from the oil phase was named glycerol due to its sweet taste (from the Greek: glykos = sweet). The term glycerin applies to commercial products, which are rich (95%) in glycerol. However, with the increasing production of biodiesel, there are many commercial glycerin products with different glycerol contents and other impurities, such as water, salts and organic compounds.

Glycerol has a boiling point of 290°C and a viscosity of 1.5 Pa.s at 20°C. Therefore, purification procedures are normally time-consuming and costly. The most traditional process involves distillation at reduced pressure. A product with a glycerol content of at least 99.5 wt% can be obtained by thin film distillation and meets the requirements of the United States Pharmacopeia (USP). Other purification processes such as extraction, ionexchange, adsorption, crystallization and dialysis can also be applied to the glycerin phase. The commercial utilization of the glycerin will be impacted by the costs and purity of the product.

During biodiesel production, two phases are produced at the end of the transesterification process. The upper ester phase contains the main product (biodiesel). The lower phase consists of glycerol and many other substances. The exact composition of the raw glycerol phase depends on the method of transesterification and the separation conditions of biodiesel production (Hájek and Skopal, 2010). When the lower glycerol layer is removed, the ester phase is washed to remove the residual glycerol, base catalyst and soaps formed during the reaction. Then, methanol and water present in the ester phase can be removed by distillation (Van Gerpen, 2005).

The raw glycerin phase has different compositions: glycerol, soaps, inorganic salts, water, methanol and esters. Table 12.1 shows the typical composition of the glycerin phase obtained in a Brazilian biodiesel plant. The minimum glycerol content is 80 wt% and there is about 10 wt% water. The remaining 10 wt% is mainly methanol and dissolved salts, such as NaCl, formed upon acid neutralization of the homogenous basic catalyst.

Refining glycerol from biodiesel production begins with an acid treatment to split the soaps into free fatty acids and salts. Fatty acids are not soluble in glycerol and are separated from the top and recycled to the process. Excess methanol can be recovered by distillation. The salts remain in the glycerol phase and may be one of the most deleterious impurities that limit the use of this raw material in chemical processes (da Silva and Mota, 2011). Purification is required to achieve a product with the necessary purity.

The crude glycerol phase can be purified by ion exchange resins to remove the sodium from glycerol/water solutions with a high salt concentration (Carmona et al., 2008, 2009). Adsorption and membrane technologies can also be used. Many purification methods are based on the distillation of the glycerol phase to strip alcohol contaminants from glycerol (Potthast et al., 2010).

Ion exchange purification is not considered economically viable when high concentrations of salts are present in the crude glycerol. Distillation and membrane technologies are commonly used to obtain ultra-pure glycerol, but membrane technologies are more cost-effective than distillation, provided that some form of prior purification that reduces salts and organic matter has taken place (Manosak et al., 2011).

Purification of crude glycerol from biodiesel production with phosphoric acid, obtaining a product with a final purity of over 86%, has been reported (Hájek and Skopal 2010; Javani et al., 2012). Potassium phosphate obtained as byproduct could potentially be used as fertilizer. Glycerol of high purity can be obtained upon the extraction of crude glycerol with ethanol (Kongjao et al., 2010). Salts and fatty acids were separated through filtration and decantation, respectively.

Thus, there are many physical and chemical methods of glycerol purification. It is important that dissolved salts are reduced to the lowest possible level, because they may affect the catalysts used in further glycerol processing.

12.3 Applications of glycerol in the fuel sector

The complete combustion of glycerol (Fig. 12.2) generates 4195 kcal per kg, but there are many difficulties associated with this process. Incomplete burning may generate acrolein, which is highly toxic to humans. The salts present in the crude glycerol from biodiesel production may deteriorate the equipment, leading to corrosion and other problems. All these drawbacks make the direct combustion of glycerol economically less attractive than the chemical or biochemical transformation.

Glycerol can also be used in the production of ethanol, an important biofuel used worldwide. Historically, ethanol has been produced mainly from sugars and carbohydrates via microbial fermentation. Speers and co-workers (2012) developed a microbial co-culture for the conversion of glycerol into ethanol and electricity in bioelectrochemical systems. Ethanol can be used as a feedstock for the transesterification of vegetable oil for biodiesel production and the electricity can be used to partially offset the energy needs of the biodiesel plant. The platform includes a glycerol-fermenting bacterium, Clostridium cellobioparum, which produces ethanol and other fermentative byproducts (lactate, acetate, formate, and H₂), and Geobacter sulfurreducens, which can convert the fermentative byproducts into electricity. Both organisms were adaptively evolved for tolerance to industrially relevant glycerol concentrations and co-cultivation of these strains stimulated microbial growth and resulted in ethanol and complete conversion of fermentative byproducts into electricity.

The yeast Saccharomyces cerevisiae utilizes the general glycolytic pathway for the majority of its energy production. The carbohydrates are converted to pyruvic acid, which is decarboxylated to acetaldehyde, and then to ethanol. Starting from glycerol, the pathway involves formation of di-hydroxy-acetone (DHA) and pyruvic acid (Fig. 12.3). To further increase ethanol production and evaluate fermentative performance, the genes involved in the conversion of pyruvate to ethanol were overexpressed (Yu et al., 2012). These genes included pyruvate decarboxylase, which is involved in the decarboxylation of pyruvate and thus controls the first step in the production of ethanol from pyruvate, and alcohol dehydrogenase, which is the enzyme involved in the ethanol production pathway from acetaldehyde.

There have been intensive efforts to describe methods for the efficient conversion of glycerol to ethanol via metabolic pathway engineering of E. coli, to minimize byproducts. The engineered E. coli strain produced 21 g/L of ethanol from 60 g/L of pure glycerol, with a volumetric productivity of 0.216 g/L/h under anaerobic conditions (Yazdania and Gonzalez, 2008).

Glycerol gasification to synthesis gas, a mixture of CO and H₂, has also been studied (Soares et al., 2006). The reaction is endothermic by 83 kcal/mol, but can be carried out at temperatures around 350°C over Pt and Pd catalysts. Synthesis gas is used in many industrial processes, like methanol production and Fischer–Tropsch synthesis of hydrocarbons.

Another approach to use glycerol from biodiesel production in the fuel sector is the development of glycerol ethers, acetals/ketals and esters with potential use as fuel additives. The glycerol molecule has about 50% of its mass in terms of oxygen atoms, which makes it a good platform for the production of oxygenated additives.

The acid-catalyzed reaction of glycerol with isobutene affords tert-butyl-glyceryl ethers (Klepacova et al., 2005), which are considered as an octane booster for gasoline (Wessendorf, 1995). Ethyl glyceryl ethers (Fig. 12.4) can be produced through the acid-catalyzed reaction between glycerol and ethanol (Pariente et al., 2009), being a completely renewable molecule. These ethers are potential additives for biodiesel, improving the cold flow properties (Pinto, 2009). For instance, addition of 0.5 vol% of glyceryl ethyl ethers in the soybean and palm biodiesel led to a reduction of up to 5°C in the pour point, indicating that these ethers can be used in blends with biodiesel.

Glycerol acetals and ketals are another class of derivatives with potential use as fuel additives. They are produced through the acid-catalyzed reaction of glycerol with aldehydes and ketones, respectively. The reaction of acetone with glycerol produces one ketal, known as solketal, whereas reaction with formaldehyde solution affords two acetal isomers (da Silva et al., 2009) (Fig. 12.5). Solketal is a potential additive for gasoline (Mota et al., 2010). Within 5 vol% addition, it improved the octane number and significantly reduced gum formation, without affecting other important properties of the gasoline, such as the vapor pressure. Although acetone is produced today from petrochemical feedstock, it can be produced from sugars, through fermentation procedures (Jones and Woods, 1986), making solketal a completely renewable oxygenated compound with potential to be used in the fuel sector.

Acetals produced in the reaction between glycerol and n-alkylaldehydes have found application as additives for biodiesel, improving the cold flow properties (Silva et al., 2010b). The best results were found with the acetals of glycerol and butyraldehyde. As the aldehyde chain increases, the effect in the pour point is less relevant. In addition, the glycerol conversion decreases with the increase in the hydrocarbon chain. Glycerol acetals can also be used as antioxidants. The acid-catalyzed reaction of glycerol with aromatic aldehydes, such as benzaldehyde, anisaldehyde and furfural, affords acetals with a benzylic C–H bond (Fig. 12.6). These molecules showed antioxidant properties in the diphenylpicrylhydrazyl (DPPH) test, which is a known procedure to estimate the antioxidant activity of a compound (Molyneux, 2004). An antioxidant is a hydrogen donor, forming a delocalized, more stable, free radical. The benzylic C–H bonds of the aromatic glycerol acetals can afford highly delocalized radicals, explaining their antioxidant properties (Fig. 12.7). However, tests of the aromatic glycerol acetals with soybean biodiesel did not lead to significant improvement in the oxidation resistance, measured according to the EN 14112 method, but the concomitant use of a commercial antioxidant, such as butyl-hydroxy-toluene (BHT) and the acetals gives much better results (Table 12.2), indicating a synergistic effect (Soares, 2011).

The acetins or glycerol acetates are useful compounds. Triacetin or glycerol triacetate is important in the tobacco industry and, more recently, has been tested as a fuel additive, especially for biodiesel, improving the viscosity and the pour point (Melero et al., 2007). The most traditional method of preparation of the acetins is the direct esterification of glycerol with acetic acid in the presence of an acidic catalyst (Gonçalves et al., 2008), yielding a mixture of the acetins. To increase the selectivity to triacetin, a large excess of acetic acid should be used. Another approach is to use acetic anhydride (Fig. 12.8) (Liao et al., 2009). Use of zeolite beta or K-10 montmorillonite as catalysts for the acetylation of glycerol with acetic anhydride leads to 100% selectivity to triacetin within 20 minutes of reaction time (Silva et al., 2010a).

Esterification of the free hydroxyl group of glycerol ketals and acetals has also been reported in the literature (Garcia et al., 2008). The reaction of solketal with acetic anhydride in the presence of triethylamine produces solketal acetate in 90% yield (Fig. 12.9). This product may be used to improve the viscosity of biodiesel, without affecting the flash point. It may also reduce the formation of particulates in diesel.

12.4 Glycerol as raw material for the chemical industry

The chemical industry is still based on oil and gas, with sales of approximately 2 trillion euros, indicative of its huge and powerful economic situation. Naphtha is the main feedstock for the chemical industry. It is initially transformed into light olefins, such as ethene and propene, and aromatics, like benzene, toluene and xylenes. These compounds are then transformed into polymers and other chemicals through complex chemical processes, before being used in everyday life as plastic components, dyes, textiles, paints and other materials. The shortage and price fluctuations of oil in the near future will force the chemical industry to diversify its processes, giving more importance to renewable materials. Bioethanol produced from sugarcane is opening this new era. Recently, the major Brazilian chemical company, Braskem, has started up a plant to dehydrate ethanol to ethylene, which is subsequently polymerized to polyethylene (Braskem, 2010). Glycerol from biodiesel production may follow the same path, being a substitute for propylene-based chemicals.

12.4.1 Glycerol to propanediols

The hydrogenolysis of glycerol over supported metal catalyst can afford 1,2 and 1,3-propanediol (Chaminand et al., 2004) (Fig. 12.10). The 1,2 isomer, also known as propylene glycol, has many uses, including as an anti-freezing agent and in the production of polyurethane foams and other polymers. The present worldwide production of propylene glycol is about 1 million tonnes per year. The traditional process involves the hydrolysis of propylene oxide, which in turn is produced from propene. The reaction can be carried out in batch or continuous flow conditions, in temperatures normally ranging from 180 to 250°C. Copper, palladium and ruthenium supported catalysts have normally been used (Dasari et al., 2005), but other metals like iron, nickel and rhodium can be used as well. The concomitant use of an acidic catalyst, such as sulfonic acid resins, improves the selectivity of the catalyst toward hydrogenolysis, allowing working at lower temperatures and reduced pressures in batch conditions (Miyazawa et al., 2006).

The hydrogenolysis of glycerol to propylene glycol has been industrially implemented by ADM (2012). Dow Chemical also has a technology named Propylene Glycol Renewable (PGR), that is based on glycerol hydrogenolysis. These facts show that the use of glycerol as a renewable feedstock for the chemical industry is now a reality.

In contrast to its 1,2 isomer, 1,3-propanediol has considerably fewer uses, with a global market of approximately 360,000 tonnes per year. The main utilization is in the reaction with terephthalic acid to produce a polyester fiber named poly-trimethylene-terephthalate (PTT) (Fig. 12.11), commercially known as CORTERRA. The traditional route used by Shell involves the reaction of ethylene oxide with CO/H₂ at high pressures. Nevertheless, the product can also be obtained by glycerol hydrogenolysis. With the choice of proper reaction conditions and modifications on the metal catalyst, the ratio between 1,3 and 1,2-propanediol can go up to 5, but at moderate glycerol conversion (Gong et al., 2009).The 1,3-propanediol can also be produced from glycerol through biotechnological processes, using genetically modified bacteria (Emptage et al., 2006). Nevertheless, the long reaction time limits the widespread use of this route.

12.4.2 Glycerol to propene

More severe reaction conditions leads to deeper hydrogenolysis of the glycerol molecule, yielding n-propanol and isopropanol (Casale and Gomez, 1994). The reaction pathway is complex and may involve dehydration steps. This may explain why the concomitant use of acidic catalysts in the medium or as support improves the selectivity. It has been shown that CO and CO₂ can also be formed through decomposition reactions over supported Pt catalyst (Wawrzetz et al., 2010). These results indicate that a complete hydrogenolysis of glycerol to remove all the oxygen atoms is feasible, opening up a technological pathway for the production of propene.

Propene is one of the major raw materials of the chemical industry. It is used in the production of many polymers and chemicals. The world production of propene is about 40 million tonnes per year, and it is expected to sharply increase in the coming years. Propene is normally produced from the steam cracking of naphtha. More recently, the catalytic cracking of vacuum gas oil has been an alternative source of propene. In contrast to ethene, which can be produced from dehydration of bioethanol, there are few technological routes to propene from renewable feedstock, most of them involving multistep reactions.

Mota and collaborators (2009) showed that the use of supported iron-molybdenum catalysts can be used in the selective hydrogenolysis of glycerol to propene (Fig. 12.12). The selectivity to propene can reach up to 90% (excluding water) over this catalyst at 300°C and continuous flow conditions. It is not clear why this catalyst is so selective to propene under the reaction conditions. X-ray diffraction analysis and temperature programmed reduction have indicated a strong interaction between the metals, which may affect the reducibility of the iron. The mechanism of the reaction seems to involve the participation of metal or metal oxide centers and acid sites of the support. Glycerol is initially dehydrated to acetol over the acid sites, which is then hydrogenated to 1,2-propanediol. Interaction of the diol with the acid sites leads to acetone, which is hydrogenated to produce isopropanol. The final step involves dehydration of the alcohol to propene (Fig. 12.13). Further hydrogenation of propene to propane is slowed down due to the incomplete reduction of the metal catalyst. It is worth mentioning that acetol, propanediol, acetone and isopropanol can all be observed upon varying the reaction temperature, space velocity and hydrogen partial pressure, supporting the proposed reaction pathway.

The search for a ‘green’ propene, produced from renewable materials, is a major goal of many chemical companies. The use of glycerol from biodiesel production may be an alternative for the ‘green’ propene, opening up the possibility of producing plastics from this renewable feedstock.

12.4.3 Glycerol dehydration to acrolein and acrylic acid

Acid-catalyzed glycerol dehydration can follow two pathways: dehydration of the primary hydroxyl group affording hydroxy-acetone, also known as acetol, or dehydration of the secondary hydroxyl group yielding 3-hydroxy-propanal. This latter compound can subsequently be dehydrated to acrolein, which is an important intermediate in the chemical industry. Oxidation of acrolein over Mo- and V-based catalysts produces acrylic acid (Kampe et al., 2007), used in the fabrication of superabsorbent polymers, paints and adhesives, with global annual production of nearly 4.5 million tonnes.

Acrolein and acrylic acid are industrially produced from the oxidation of propene. Initially, the olefin is oxidized to acrolein over Bi-Mo oxide catalyst. Then, in a second reactor, acrolein is oxidized to acrylic acid. Nevertheless, acrolein can also be produced from acid-catalyzed dehydration of glycerol. The reaction runs in liquid phase with the use of mineral acids, such as H₂SO₄. However, dehydration in the gas phase, under continuous flow conditions and using heterogeneous catalysts, has received more attention lately. Different acidic catalysts, such as zeolites, metaloxides, clays and heteropolyacids have been tested (Chai et al., 2007). The conversion and selectivity depend on the catalysts and conditions used. For instance, supported heteropolyacids can present 86% selectivity to acrolein for a glycerol conversion of 98% (Tsukuda et al., 2007), but deactivation of the catalyst is still a major problem.

The one-step synthesis of acrylic acid from glycerol (Fig. 12.14), combining acidic and oxidant properties in the same catalyst, is an interesting approach. The oxidative dehydration of glycerol has been studied over mixed oxide catalysts in the presence of air (Deleplanque et al.,2010).Mixed molybdenum and vanadium oxides, as well as vanadium/tungsten oxides, were active in this reaction. They show 100% glycerol conversion with selectivity to acrylic acid between 24 and 28%. Acetic acid, probably coming from oxidation of acetaldehyde formed upon cracking of the 3-hydroxy-propanal, was also observed as byproduct. Supported tungsten oxide catalyzes the oxidative dehydration of glycerol to acrylic acid (Ulgen and Hoelderich, 2011), but the selectivity to acrylic acid is rather low, being within 5%.

Vanadium-impregnated zeolite beta can also be used in the oxidative dehydration of glycerol to acrylic acid (Pestana, 2009). The catalysts were prepared by wet impregnation ammonium metavanadate on the ammonium-exchanged zeolite beta, followed by air calcination. The selectivity to acrylic acid was around 20% at 70% glycerol conversion. The catalytic activity was associated to the dispersion of vanadium species inside the zeolite pores.

12.4.4 Glycerol oxidation

Glycerol oxidation can produce several important compounds (Fig. 12.15). The selective oxidation of the secondary hydroxyl group leads to 1,3-dihydroxy-acetone (DHA), which is an artificial tanning agent and has global production of more than two thousand tonnes per year. DHA is normally produced from glycerol fermentation (Hekmat et al., 2007), but can also be prepared by electrochemical methods (Ciriminna et al., 2006).

Glyceraldehyde is an intermediate in the carbohydrate metabolism. A good method of preparation involves the oxidation of glycerol over Pt-supported catalysts. Selectivity of 55% in glyceraldehyde with glycerol conversion of 90% can be observed with the use of Pt/C catalyst (Garcia et al., 1995).

Glyceric acid is selectively produced by the oxidation of glycerol over Au/C catalysts in the presence of oxygen (Carrettin et al., 2004). Bimetallic gold-platinum or gold-palladium catalysts show higher turnover frequencies, but are also less selective, yielding C-C cleavage products, such as oxalic and glycolic acids (Bianchi et al., 2005).

Glycerol oxidation with H₂O₂ in the presence of metal-containing silicate catalysts gives formic acid as a major compound, indicating the high activity of the system, which leads to C–C bond cleavage (McMorn et al., 1999). Formic acid is the major observed product.

12.4.5 Other glycerol transformations

Epichloridrin can be produced from glycerol in a process called epicerol (Solvay, 2011). This chemical is mostly used in the production of epoxy resins, as well as in water and paper treatment. The process involves the reaction of glycerol with two moles of HCl in the presence of Lewis acid catalysts, followed by controlled alkaline hydrolysis (Fig. 12.16). The traditional process of epichloridrin production starts with the chlorination of propene at elevated temperatures, forming allyl chloride, which is then reacted with hypochlorous acid to afford 1,3-dichloro-isopropanol. Treatment of this latter product with aqueous sodium hydroxide yields epichloridrin. The epicerol process of producing epichloridrin from glycerol has several advantages. The water consumption is 90% lower compared to the traditional process, as well as chlorinated waste materials, not to mention the use of a renewable feedstock.

Glycerol carbonate has gained increasing applications in recent years. It can be used as solvent and monomer for the production of polycarbonates, polyesters and polyamides. There are many routes for its production, such as the reaction of glycerol with cyclic organic carbonates (Vieville et al., 1998), and through the reaction of glycerol with urea (Hammond et al., 2011). This latter procedure has become popular in the last few years, but involves long reaction times (6–8 hours) and high temperatures (160°C). A simple procedure to produce glycerol carbonate involves the reaction of glycerol with N,N′-carbonyl-diimidazol (CDI) (Mota et al., 2007) (Fig. 12.17). The product can be obtained in quantitative yields in 15 minutes at room temperature. The use of crude glycerin, coming from biodiesel production, does not affect the yield, as the alkaline catalyst dissolved in the glycerin phase accelerates the reaction. The major drawback is the cost of CDI and its production route, which still employs phosgene (COCl₂), a highly toxic gas.

The direct carbonation of glycerol with CO₂ is a promising route for the production of glycerol carbonate, because it uses a greenhouse gas (Fig. 12.18). Organotin compounds (Aresta et al., 2006) have been employed as catalysts, but the yields are still modest, in the range of 5%, and the reaction time is over 12 hours. Metal-impregnated zeolites may be promising catalysts for the direct carbonation of glycerol. Preliminary results indicate that glycerol carbonate can be produced in up to 5.8% yield within 2 hours of reaction time (Ozório, 2012). The use of metal-impregnated zeolites opens the possibility of using heterogeneous catalysts in this reaction, contributing to lowering the costs associated with catalyst recovery, which are always high when homogeneous catalysts are involved.

12.5 Conclusions and future trends

In the few years, glycerol will certainly occupy an important position as a renewable feedstock for the chemical industry. As the worldwide utilization of biodiesel increases, the surplus of bioglycerol will become attractive and new applications will emerge. Purity is still a major concern if chemical transformation is forecasted. The presence of salts dissolved in the glycerol phase of biodiesel production may affect the catalysts used for further chemical transformation. Purification procedures are still time-consuming and costly. On the other hand, the development of biodiesel processes that use heterogeneous catalysts may circumvent this problem, because the purity of the glycerol will be significantly higher.

The hydrogenolysis of glycerol to propylene glycol is an established technology. The process is operated in continuous flow mode and uses metal-supported catalysts, with industrial facilities being in operation. A variation of this reaction is the deeper hydrogenolysis to propene. This is a unique development, but still requires further major developments before being operated at industrial scale. A major concern in the hydrogenolysis of glycerol is the source of hydrogen, which normally limits the economic feasibility of the processes. Location of an industrial plant near a petrochemical complex may be more attractive, because hydrogen is a byproduct of cracking and catalytic reforming of naphtha. Another approach would be the generation of hydrogen from biomass gasification, but such processes are still not employed on a large scale and lack economical feasibility. Thus, the utilization of non-renewable hydrogen would be necessary for the rapid industrial implementation of glycerol hydrogenolysis to propanediols and propene.

The epicerol process, developed by Solvay, offers an opportunity to produce epichloridrin from glycerol. The company reports several advantages of the process in relation to the traditional one, based on propene. This is a good example of how a renewable feedstock process can compete with oil-based processes.

Glycerol dehydration to acrolein and acrylic acid still faces some challenges. Most of the acid catalysts tested deactivate during work conditions, making the development of a competitive industrial process still far away. The oxidative dehydration of glycerol to acrylic acid may be more interesting from a technical and economic point of view. Since the reaction is carried out in air flow, catalyst deactivation is usually a minor problem. However, selectivity is still a major concern. The best catalysts can produce up to 28% of acrylic acid, with acrolein being the main product formed. This may suggest that catalyst activity must be adjusted to perform the two reactions under the same experimental conditions. Apparently, the acidic function is working properly, but the oxidant function requires further improvement. Once the selectivity problem is resolved, the oxidative dehydration of glycerol to acrylic acid may economically compete with the traditional process, which involves two steps starting with propene. The possibility of developing a one-step process from glycerol would be a major achievement, reducing operational and capital costs.

Glycerol may also be used in the fuel sector. There are many studies for the biotechnological transformation of glycerol in ethanol, a major biofuel used worldwide. This achievement may integrate the biodiesel and the bioethanol industries. Another possibility would be the use of ethanol in the transesterification process, producing a completely renewable biodiesel. Glycerol can also be employed in the production of syngas, which can be transformed into hydrocarbons through the Fischer–Tropsch process yielding diesel, gasoline and kerosene.

Glycerol ethers, acetals/ketals and esters show potential of use as additives for gasoline, diesel and biodiesel. Solketal, produced in the reaction of glycerol with acetone, is an interesting additive for gasoline, improving the octane number and reducing gum formation. Glycerol acetals of aromatic aldehydes show antioxidant properties and may be useful in many sector, including fuels, plastics and food.

In summary, glycerol will be one of the most important renewable feedstock in the near future. Together with bioethanol, glycerol will be an important raw material for the production of plastics. Bioethanol is a good option for ethylene-based polymers, and glycerol will be the best option for propylene-based polymers. Some of the developments are already a reality, whereas others still require further developments. As fuel additives, glycerol derivatives can occupy increasing market share, especially in the biodiesel industry. Many biodiesel additives can be made from glycerol, integrating the whole industrial chain and draining huge amounts of the glycerol produced in transesterification.