Glycerol as additive for fuels - A review

M.O. Ferreira^†, L. Cardozo Filho^‡, C. Silva^‡ and E.M.B.D. Sousa^†§

^† Departamento de Engenharia Química, Universidade Federal do Rio Grande do Norte-UFRN-Campus Universitário, 3000, CEP: 59078-970, Natal-RN, Brazil
^‡ Universidade Estadual do Maringá-DEQ-UEM-Av. Colombo, 5790, Bloco D-90 ,Maringá-PR, Brazil, CEP: 87020-900
^§ Correspondent autor: Tel.:+55 84 32153769; Fax: +55 84 32153770; Email: elisa@eq.ufrn.br

Abstract — Growing environmental concerns, along with the search for renewable energies that emit less CO2, put biodiesel in a favorable position. Biodiesel is commonly produced by the transesterification reaction between vegetable oils and methanol or ethanol, in the presence of a catalyst. With government incentives for biodiesel production in Brazil, there was an exaggerated increase in their production and hence the concern about the disposal of surplus glycerol. Therefore research is being developed with respect to the use of the surplus of glycerol. In this context, emerged as the alternative use of fuel additives, from glycerin to contribute to the development of new applications for innovative and environmentally-friendly processes. The bibliographic review presented here starts with describing the different glycerol production processes, characterization and analysis methods. Next, Glycerol-based processing methods are described, evaluating the main reaction parameters that interfere in process optimization.

Keywords — Glycerol; Additive; Fuel; Review.

I. INTRODUCTION

Modern society is still very dependent on fossil fuels such as oil and natural gas, however, the search for renewable energy sources has been significantly intensified (Dowaki et al., 2007). Among the various sources of renewable energy available, such as wind energy, hydroelectricity, solar energy and biomass, biodiesel has emerged as a promising alternative, evidenced by its increased production in many countries around the world (Hill et al., 2006). Growing environmental concerns, along with the search for renewable energies that emit less CO₂, put biodiesel in a favorable position. This is because studies have demonstrated that the mineral diesel oil substitution by biodiesel reduces sulfur emissions by 20%, carbonic anhydride by 9.8%, unburned hydrocarbons by 14.2%, particulate matter by 26.8% and nitrogen oxide by 4.6%. Environmental benefits may also generate economic advantages, given the possibility of including biodiesel in the Kyoto Protocol and in Clean Development Mechanism guidelines, the sale of carbon quotas through the Prototype Carbon Fund, the reduction in pollutant gas emissions as well as the sale of sequestered carbon credits (Ferrari et al., 2005).

Biodiesel is commonly produced by the transesterification reaction between vegetable oils and methanol or ethanol, in the presence of a catalyst. Biodiesel can be defined as a mixture of mono alkyl long chain esters derived from vegetable or animal oils with similar burning properties to those of petroleum diesel and greater efficiency than gasoline (Graboski and Maccormick, 1998). Transesterification of vegetable and animal oils produces fatty acid alkyl esters and glycerol. Stoichiometry of the reaction generally consists of 3 mols of alcohol for 1 mol of triglyceride, but in practice, excess alcohol is used to increase the yield of ester formation. For each 90 m³ of biodiesel produced by the transesterification of vegetable oils, approximately 10 m³ of glycerol is generated (Behr and Obendorf, 2001, Mota et al., 2009).

Several countries are launching incentive programs and norms for biodiesel production. In mid-2005, for example, Brazil inaugurated a program for the use and production of biodiesel, establishing the addition of 2% biodiesel to conventional diesel (B2) by 2008 and 5% by 2013 (B5) (Abiquim, 2007). In 2013, when this percentage to increase to 5% biodiesel, the surplus glycerin should be 250 thousand tons (www.biodieselbr.com, accessed December 5, 2012).

The substantial growth in worldwide biodiesel production caused an imbalance between supply and demand, including in Europe and the United States, reducing its price to the lowest level in history. Plant and animal-based glycerin have the same market value, which is determined by its purity. The use of this co-product also has the significant advantage of originating from 100% renewable raw materials. However, until the emergence of new technologies for glycerin use, this situation will likely persist, that is, an imbalance between supply and demand. This scenario indicates that the biodiesel production program should be directly linked to the commercialization of glycerin as a subproduct, although there is currently no market for all the glycerin produced (Claude et al., 2000).

Some ether derivatives and glycerin esters have a diversity of applications, from the cosmetic (Nabeshima and Ito, 1997) to the petrochemical industry (Taguchi et al., 2000), where it is used as a fuel additive (Blake, 1960; Hofmann, 1986; Wessendorf, 1995). With respect to the last item, it is important to underscore that the primary additives used today worldwide are ETBE, and TAME, together with ethanol or methanol. However, because of environmental questions, the use of others oxygenated additives have been studied.

In this context, the development of new applications and/or products derived from glycerol is strategic to the development of renewable fuel-based technology (Behr and Obendorf, 2001; Zhou et al., 2008).

The bibliographic review presented here starts with describing the different glycerol production processes, physical-chemical characterization and standard analysis methods. Next, glycerol-based processing methods are described, evaluating the main reaction parameters that interfere in process efficiency and optimization. Reaction mechanisms of the main processes of transforming glycerol into additives are then presented, describing the state of the art of each process.

II. GLYCEROL

Glycerol or 1,2,3-propanotriol, an organic compound belonging to the alcohol function, contains three hydroxyls with the molecular formula C₃H₈O₃. Some authors use the term glycerin to purified commercial products, normally consisting of at least 95% of glycerol (Felizardo et al., 2003). Carl Wilhelm Scheele first discovered glycerol in 1779, after heating a mixture of lead oxide and olive oil. He named it "the sweet principle of oils" and it became known as Scheele's sweet principle. Even when present in different species, including unicellular protists and mammals, it is difficult to find glycerol in its "free" form in these organisms, since it is generally found as a triglyceride combined with different fatty acids such as oleic, palmitic and stearic, etc (Brisson et al., 2001). Until 1949, all the glycerol produced worldwide came from the soap industry. Currently, 70% of glycerol production in the United States derives from glycerides present in oils and natural fats. The rest is synthetic, obtained from the manufacture of polyethylene and as a subproduct of the transesterification reaction of vegetable oils (Kirk-Othmer, 2007).

Glycerol can be produced by microbial fermentation or by chemical synthesis. The traditional production of glycerol is via saponification from oils, fats, or tallows, using alkaline lyes, being obtained as a by-product in soap manufacture. However, this process has been discarded industrially, due to the replacement of soap by detergents (Rehm, 1988; Lópes et al., 1999; Wang et al., 2001).

Glycerol can also be obtained from petroleum derivatives by high -temperature chlorination, but this method declined due to the formation of environmentally-harmful products (Rehm, 1988; Hester, 2000). Another way of obtaining glycerol is through sucrose hydrogenation in catalytic medium, under severe temperature and pressure conditions (Lópes et al., 1999). The number of studies focusing on glycerol production using fermentation from renewable sources has been increasing due to the wide range of potential industrial applications of glycerol and its occurrence in a number of vital physiological processes in both prokaryotes and eukaryotes (Morrison, 1994; Wang et al., 2001). Additionally, different microorganisms such as bacteria, yeasts, algal fungi and some protozoa are mentioned in the literature as glycerol producers (Wang et al., 2001).

Another process used to obtain glycerol is biodiesel synthesis by transesterification, which after phase separation, leads to a denser phase corresponding to the glycerol to be purified (glycerin) .This is currently the most widely used process for biodiesel production, consisting of three consecutive, reversible reactions, where diglycerides and monoglycerides are the intermediate products (Karmee and Chadha, 2005; Marchetti et al., 2003; Demirbas, 2005). After the transesterification phase, a final reaction mass consisting of two phases separable by decantation or centrifugation is obtained.

The densest phase is composed of raw glycerin, water, alcohol in excess and impurities inherent to the raw material. The least dense phase consists of a mixture of methyl and ethyl esters, according to the nature of the alcohol originally adopted, also impregnated with reaction excesses of alcohol and impurities (Parente, 2003). Glycerol has also been produced commercially by propene synthesis. The initial step of the process is high-temperature chlorination, involving free radicals as intermediaries, in order to form allyl chloride. This then reacts with hypochlorous acid, with halohydrin as a product. Finally, treating halohydrin with base excess leads to glycerol (Mota et al., 2009).

A. Physical-chemical properties and applications of glycerol

Glycerol is an oily, colorless, viscous, sweet-tasting liquid that is soluble in water and alcohol in all proportions. It is little soluble in ether, ethyl acetate and dioxane and insoluble in hydrocarbons (Lópes et al., 1999). It is hygroscopic and odorless (Kirk-Othmer, 2007).

There is risk of explosion when in contact with other strong oxidizing agents such as chromium trioxide, potassium chlorate and potassium permanganate (Kirk-Othmer, 2007). A number of other physical-chemical properties of this compound are shown in Table 1. Due to the combination of these physical-chemical properties, glycerol is a substance with a wide range of applications. In addition to its use as a lubricant and/or raw material in the textile, food, pharmaceutical and cosmetic industry, it can also be used to produce esters, polyglycerols and chlorohydrins (Ullmann, 1989).

Table 1. Physical-chemical properties of glycerol

Reference: Morrison (1994); Lopes et al. (1999)

Figure 1 illustrates the distribution of glycerol consumption in different Brazilian industrial sectors. The greatest distribution is in the pharmaceutical and cosmetic industries. With the large surplus of glycerol resulting from biodiesel production, new glycerin applications must be developed in both Brazil and around the world (Mota et al., 2009).

Figure 1. Distribution of glycerol consumption in the main Brazilian industrial sectors

New applications using glycerin are being studied. These applications include aviation fuels, bioremediation (as hydrogen donor) and other industrial applications that do not require USP standards. They can also be used as an energy source, in alcohol production and as a catalyst (Bonnardeaux, 2006). Another proposal for using this residue is in a methane digester to produce clean-burning and efficient biogas (Amon and Boxberger, 2001). Another very important market that will likely develop with a greater supply of glycerol is its application in synthesizing high-added value molecules, as substrate for bacterial fermentations, in order to obtain products such as biodegradable polymers, rhamnolipids and biosurfactants, among others (Brisson et al., 2001). Some derivatives of glycerin, such as esters and ethers, have a wide range of applications in the petrochemical industry (Nabeshima and Ito, 1997; Taguchi et al., 2000), in addition to its use as fuel additive (Blake, 1960; Hofman, 1986; Wessenford, 1995). This alternative is characterized by the etherification reaction of glycerol with alcohols (for example, methanol, ethanol or t-butanol) or alkenes (for example, isobutylene), leading to the formation of branched oxygenated compounds with adequate properties for use as additives, solvents or fuels (Wessenford, 1995).

III. FUEL ADDITIVES

An additive is a substance that helps clean engines, carburetors and injection valves, avoiding incomplete combustion. It prevents fuel from freezing as it passes through the valves, thereby averting clogged fuel injectors, and protects parts against corrosion, resulting in better engine performance (Norhasyimi et al., 2010).

Fuel that contains between 10 and 25% (v/v) of oxygenated compounds can significantly reduce particle emissions. The oxygenated fuels used are generally alcohol, ethers and esters from the functional group (Marchetti et al., 2003). In aviation, the introduction (between 1 and 70% by weight) of additive to fuel can improve thermal stability and reduce jet engine deposits (Forester et al., 2003). Oxygenated compounds can contribute to increase octane levels and fuel quality, reducing particle emissions and carbon monoxide production (Norhasyimi et al., 2010).

In addition to the application of oxygenated additives such as MTBE (methyl-t-butyl-ether), ETBE (ethyl-t-butyl-ether) and TAME (methyl-t-amyl-ether) in gasoline, the use of oxygenates in diesel fuels (diesel, biodiesel and their mixtures) has become a priority owing to strict laws in force to decrease air pollution. The mixtures of t-butyl ethers containing high proportions of diethers and triethers have long been known to be potential additives for diesel fuels and capable of reducing emissions and particulate matter (Klepácova et al, 2005). In 1979, methyl tertiary butyl ether (MTBE) was widely used as an octane enhancer in gasoline. It was manufactured by the chemical reaction between methanol and isobutylene (Franklin et al., 2000). For several years it was used in gasoline mixing tanks, since it was considered the most economical additive in terms of production costs. However, this compound was considered a health risk, according to the International Agency for Research on Cancer (IARC) and the Environmental Protection Agency (EPA).

Ethanol, known as bioethanol, has also been amply used as an oxygenated additive in gasoline, given that it significantly reduces CO₂ emissions and increases the engine's thermal efficiency (Wheals et al., 1999; Hu et al., 2006). It can be produced biologically from fermentation and hydrolysis of renewable energy derivatives such as beet root, corn, wheat, sugar cane and straw (Wheals et al., 1999). However, ethanol vaporizes easily and may be potentially harmful to the ozone layer and increase aldehyde and acetic acid emissions. Methanol, although more available than ethanol, is less preferred because it is more damaging to the environment and much more inflammable on hot surfaces, which could melt the engine (Demirbas, 2008). Methanol can be synthesized from natural gas and biomass through a partial oxidation reaction, but this is very costly. Studies conducted prove its efficiency in engine performance and in the assessment of emissions from mixtures of methanol and oxygenated additives in a compression-ignition engine (Huang et al., 2005). The physical properties of fuel additives, obtained from different reactions such as acetylation, esterification and etherification, are very important and normatized by standard methods (ASTM, EN and AOCS) used to characterize these physical properties.

IV. TRANSFORMATION OF GLYCEROL INTO ADDITIVES

Without undergoing any modification, glycerol cannot be added directly to fuel, due to the former's decomposition and natural polymerization, which could cause engine problems at higher temperatures. However, when converted into ethers, ketals, or esters of glycerol, these compounds showed satisfactory results in the use as additives for fuels, with improving the fluidity reducing particulate emissions (Delfort et al., 2003; Dubois, 2008). These compounds (additives from the glycerin) have demonstrated excellent performance in engines in addition to meeting environmental standards. The main parameters evaluated in the transformation of glycerol into fuel additives are operational conditions such as temperature, pressure, solvent addition, reaction mechanisms, among others (Norhasyimi et al., 2010).

A. Glycerin acetal production

Acetals and cetals can be formed from the reaction of alcohols with aldehydes or ketones, under the influence of acid catalysts, producing five or six-member cyclic compounds. In the acetalization reaction, these compounds are generally formed at a molar ratio of 50:50. To change this molar ratio, a number of parameters have been investigated, such as temperature, glycerol/ aldehyde or ketone ratio and solvent exchange (Behr et al., 2008).

Glycerol acetals and cetals have several applications, ranging from their use in fuel additives, improving fluidity and reducing particulate emissions (Delfort et al., 2003; Dubois, 2008), and as surfactants (Piasecki et al., 1997), flavorings (Climent et al., 2002) and solvents used in medicine (Sari et al., 2004). Acetals derived from the reaction of glycerol with acetone and formaldehyde can be added to gasoline, improving its burning properties (Wessendorf, 1995). They were also tested in mixtures with biodiesel, showing that they decrease the flash point of biodiesel to below the limit established by the European Union (Garcia et al., 2008).

The condensation of glycerol with benzaldehyde was evaluated by Deutsch et al. (2007), using Amberlyst-36 resin, H-Beta zeolite and K10 clay as catalyst and benzene, toluene, chloroformium and dichloromethane as solvents. The acetalization reaction is reversible and water must be removed from the medium to increase reaction yield. A yield of 80 to 94% was achieved in acetals two hours after reaction onset. The reaction of glycerol with acetone and formaldehyde was also evaluated in this study, but a slightly lower yield was obtained due to excess water formed in the reaction medium. The acetals 1,3-dioxan-5-ol and 1,3-dioxolan-4-methanol were obtained from the reaction with benzaldehyde.

Mota and Gonçalves (2007) studied the reaction of glycerol with formaldehyde using a high Si/Al ratio of a zeolite beta without the use of solvents, where they obtained a conversion above 90%. H-beta zeolites obtained better conversion, compared to other acid catalysts, due to the fact that their pores are hydrophobic (the high Si/Al ratio that prevents water in the reaction medium from diffusing to the interior of the pore, thereby preserving the strength of acid sites) (Okuhara, 2002). USY zeolite is hydrophilic and has a much lower Si/Al ratio, possibly leading to lower acidity from excess water in the reaction medium (Mota et al., 2009). ZSM-5 has a high Si/Al ratio and smaller pores, hindering product formation, an effect known as shape selectivity (Smit and Maesen, 2008).

The reaction between glycerol and acetic acid was studied by Melero et al. (2007). In this reaction, glycerol conversion increased with a rise in acetic acid, and optimal operational conditions (higher glycerol conversion) were obtained with high acetic acid/glycerol molar ratios and mean temperature between 100 and 125°C. It was observed that no monoacetin formation occurred at high molar ratios and elevated temperatures. There was a tendency to diacetin formation at an intermediate molar ratio and high temperatures. Molar ratio is probably the most influential parameter for mono-, di-and triacetin formation of glycerol. It was also observed that the acid strength of sulfonic acid incorporated to mesoporous acid is an important factor that affects catalytic performance. Experiments involving specific activity of acid sites showed that increased catalytic activity is dependent on increasing acid strength of the sulfonic<propylsulfonic<aerosulfonic<fluorosulfonic group.

Luque et al. (2008) investigated glycerol acetylation with irradiation using microwaves. A higher percentage of monoacetin formation occurred at low potency and short reaction time, while high potency, longer reaction time and a greater amount of catalyst favored triacetin formation.

Garcia et al. (2008) were the first to characterize the additive resulting from the reaction between glycerol and acetone, denominated acetal-1. However, its properties did not obey ASTM and EN standard guidelines. For this reason, acetal was modified with anhydrous acetic acid, producing mixtures of acetal products, denominated acetal-2 and triacetins, obtaining excellent results with respect to physical properties. Acetal-2 produced better viscosity, better flash point and enhanced stability to oxidation.

B. Glycerin ester production

Mono-and diacylglycerols occur naturally in fats that were partially hydrolyzed and are widely used as surfactants. Triacylglycerols are primary components of fats and vegetable oils. Glycerol monoesters can be prepared by glycerol esterification with carboxylic acids as well as by esterification or transesterification with their methyl esters (Fig. 2).

Figure 2. Selective synthesis of monoglycerides.

Cho et al. (2006) studied the synthesis of glycerol mono and diesters by acid catalysis in the reaction with aliphatic carboxylic esters, varying the molar ratio of the substrate and reaction temperature to optimize product yield. After 8 hours of reaction, the monoester was formed with 60% yield when a glycerol/ester molar ratio of 4:1 was used, and it was observed that the increased molar ratio promoted a decrease in yield. Temperature variation also influenced reaction yield. higher temperatures the formation of diacylglycerol occurred more rapidly. The esterification reaction is reversible, and for this reason, the amount of water also influences product yield.

Bremus et al. (1983) created a process to produce triacetin continuously in three steps. In the first step, glycerol is partially converted with acetic acid. In the second step, acetic anhydride is added until complete conversion. In the first step, triacetin is separated and purified by distillation. Triacetin can be used, for example, as a textile auxiliary. Esterification or acetylation of glycerol with acetic acid forms mono-, di-and triacetins. Triacetin (glycerol triacetate) was tested as a fuel additive, especially biodiesel, since it improves viscosity and fluidity (Garcia et al., 2008; Delagado, 2002).

Mesostructured functionalized acid materials demonstrated excellent catalytic behavior in glycerol acetylation with acetic acid to produce acetylated compounds with interesting bioadditive properties for fossil fuels. Their activity and selectivity were comparable or better than other conventional acid catalysts (homogeneous and heterogeneous). It was necessary to use large excess of acetic acid to obtain an increase in glycerol conversion and selectivity using di-and triacetylated derivatives. Optimal reaction conditions were established at a temperature of 125°C and molar ratio of 9:1. Under these conditions and after 4 hours of reaction with modified sulfonic acid, glycerol conversions above 90% and combined di-and tri-acetylglycerol selectivity of 85% were achieved. Satisfactory catalytic results were obtained, in both glycerol conversion and product selectivity, when mesostructured catalysts modified by aerosulfonic and fluorosulfonic acid containing stronger acid sites were used. The catalytic performance of these materials was recovered after regeneration by solvent washing (Ribeiro, 2009).

Synthesis of triacetin by glycerol esterification with acetic acid was studied by Gelosa et al. (2003). At each step of the reaction a water molecule was obtained as subproduct and monoacetin and diacetin as intermediate products (Fig. 3). A new chemical synthesis method using chromatographic reactors was evaluated, which allowed the formation of significant amounts of intermediate products that can be used in new applications. Triacetin production was hampered as a function of reaction steps in series and equilibrium conditions. Amberlyst-15 acid resin obtained the best result as catalyst, with a conversion of 97%, followed montmorillonite K-10 clay, niobic acid and HSM-5 and HUSY zeolites. Zeolites exhibited the worst results, likely due to deactivation of active sites by water formed in the reaction medium. Selectivity varied as a function of the catalyst used, but a gradual formation of di-and triacylated products occurred with an increase in conversion and reaction time. In all cases acetal (α hydroxyacetone) formation from dehydration of the terminal hydroxyl of glycerol occurred (Gelosa et al., 2003).

Figure 3. Glycerol acetylation (Norhasyimi et al., 2010).

C. Glycerin ether production

Glycerol etherification leads to the production of low-polarity compounds, lower viscosity and greater volatility, resulting in some applications for glycerol ethers as fuel and solvent additives. A viable alternative is the reaction of glycerol etherification with alcohols (for example, methanol, ethanol or t-butanol) or alkenes (for example, isobutylene), leading to the formation of branched oxygenated compounds with adequate proper-a ties for use as additives, solvents or fuels (Wessendorf, 1995).

When glycerol is etherified with isobutylene (Fig. 4), some or all of the hydroxyl groups in the glycerol molecule react. Thus, depending on the extent of etherification, up to five isomer ethers can be formed, two monosubstituted monoethers (3-tert-butoxy-1,2-propanediol and 2-tert-butoxy-1,3-propanediol), two disubstituted diethers (2,3-di-tert-butoxy-1-propanol and 1,3-di-tert-butoxy-2-propanol) and one trisubstituted triether (1,2,3-tri-tert-butoxy-propane). Because of the two remaining hydroxyl groups, monoethers are still soluble in polar solvents, while di-and triethers, the so-called "higher ethers", are soluble in hydrocarbons. This property allows the application of these ethers as fuel additives, in order to reduce particulate matter, carbon monoxide and carbonylated compound emissions (Behr et al., 2001).

Figure 4. Etherification reaction of glycerol with isobutylene.

Williamson synthesis is the most common method for obtaining ethers, symmetrical or not. The reaction is characterized by displacement of alkyl halides by alkoxide ions (obtaining an alcohol or phenol) and they can be used to prepare glycerol ethers (March, 1992). The typical Williamson reaction is conducted at 50-100°C and is complete in 1 to 8 hours. The complete elimination of raw materials is often difficult to achieve and collateral reactions are common. Yields of 50-95% are generally obtained in laboratory syntheses and extrapolatable to industrial processes (Boyd and Morrison, 1992).

The etherification reaction of glycerol and isobutylene was studied by Noureddine et al. (1998) where it was investigate the effect of reaction time. In this study was used commercial pure glycerol and crude glycerol was obtained from the pilot plant in the Biomass Lab at the University of Nebraska. It was observed that the reaction stabilized after 2 hours with a molar ratio of 3:1 (isobutylene /glycerol), temperature of 93°C and 5% by weight of catalyst. They surmised that when equilibrium is reached, free isobutylene will be consumed, forming 1-pentene and 2-pentene. After more than 3 hours of reaction, the formation of 1 and 2-pentene decreased due to reaction reversibility. The physical properties of ethers of glycerin and various mixtures of ethers with diesel fuel or biodiesel were evaluated. In the experiments were used 24% of monoethers, 62% of diethers and 14% of triethers on 100% of diesel, 100% of pure biodiesel and blends of biodiesel-diesel. The best results were obtained for catalyst loading >5 wt%, 1-2 h of reaction time, and a molar ratio glycerol: isobutylene of about 3. Lower temperatures (80 °C) under these conditions resulted in a higher concentration of di-and tri-ethers. Impurities in the crude glycerol appeared to poison the catalyst, which resulted in poor conversions. To minimize these effects, neutralization and methanol removal from the crude glycerol need to be performed prior to its use. Solubility studies determined that these additives are compatible with diesel fuel and biodiesel. A mixture of 20% of additives with biodiesel resulted in a reduction of 5° C cloud point and an 8% reduction in viscosity.

Klepácova et al. (2005) investigated the influence of temperature, the catalytic activity and selectivity of different catalysts in the glycerol etherification reaction with isobutylene. It was concluded that the narrow relationship between acid strength and catalytic activity could only be observed by comparing the same type of catalysts. The 10% difference in the initial reaction rate between ionic exchange catalysts (Amberlyst-35 and Amberlyst-15) is due to the fact that the acid strength of Amberlyst-35 is 10% higher than that of Amberlyst-15. This same conclusion was drawn for H-Beta and HY zeolites. However, when the acid strength of ionic exchange resin and large-pore zeolites was compared, it was concluded that the fact that total ion acidity of ion exchange resin is nearly five times higher than that of zeolite is not responsible for the different initial reaction rates between these two types of catalysts. A conversion of 71.5% at a temperature of 60°C and 180 minutes of reaction time was achieved. At slightly higher temperatures (around 75°C), glycerol conversion was 71.3% at a shorter reaction time. The increase in temperature raised the reaction rate, but undesirable dealkylation of higher ethers (di-tert-butylglycerols (DTBG) and tri-tert-butylglycerols (TTBG)) caused by monoether and isobutylene formation occurred.

Karinen and Krause (2006) optimized the reaction conditions between glycerol and isobutylene, using an isobutylene:glycerol ratio of 3, reaction temperature between 50 and 90°C and pressure of 1.5 MPa. After 7 hours of reaction, 3 tert-butoxy-1 -2-propanediol was the main monoether formed. There was also significant isobutylene oligomerization of C8, C12 and C16 hydrocarbons. In some experiments t-butylic alcohol (TBA) was added to the reaction mixture to prevent the oligomerization reaction and improve selectivity in relation to the triether. On the other hand, glycerol etherification with t-butylic alcohol leads to low conversions, as a function of the water formed, which weakens the acidic sites of the catalyst.

Gonçalves et al. (2006) studied glycerin methylation with iodide and methyl sulfate, with the aim of preparing the trimethylated product. The reactions of glycerin ether preparation were performed in batch regime with reflow and heating in oil bath. The general procedure consisted of reacting glycerin with alkaline solution to remove hydroxyl-linked acid protons, followed by a reaction with iodide or methyl sulfate, under reflow conditions, using sufficient molar amounts for trimethylation. In the reaction with methyl iodide, conversion of glycerin and selectivity to the 1,2,3-trimethoxypropane product was 100% after 10 minutes of reaction, demonstrating high system reactivity. In the reaction with methyl sulfate, glycerin conversion was also 100%, but system reactivity was lower and after 40 minutes of reaction the dimethylated (1,3-dimethoxy-2-propanol) and trimethylated product (1,2,3-trimethoxypropane) still coexisted. In this study it was also concluded that the trimethylated product can be obtained with 100% selectivity after around 90 minutes of reaction.

Mota et al. (2007) studied glycerol methylation with methyl chloride and methyl sulfate, optimizing the preparation of 1,2,3-trimethoxypropane. The general procedure consisted of reacting glycerol with alkaline solution to remove hydroxyl-linked acid protons, followed by the addition of methyl chloride or methyl sulfate, using molar amounts for trimethylation. In this study it was observed that crude glycerol obtained from biodiesel production produced excellent results with the formation of 1,2,3 -trimethoxypropane after around 20 minutes of reaction. Crude glycerol alkalinity is important for the reaction to occur, and the product can be subsequently isolated from the medium by low-pressure distillation. Salts generated in the reaction (potassium chloride or potassium sulfate) can be isolated from the medium and used as fertilizers.

Klepacova et al. (2007) assessed the influence of the catalyst, solvent and temperature on glycerol etherification with liquid-phase isobutylene, catalyzed by Amberlyst strong acid ion exchange resins (Amberlyst 15 and 35), p-sulfonic acid and two large pore zeolites (HY and H-Beta). Reactions were performed in a 100-ml stain-less steel reactor, at temperatures varying between 50 and 90 °C, with autogenic pressure and the use of solvents (dioxane, dimethyl sulfoxide and sulfolane). Conversion and selectivity results were better when acid resins were used, due to the large pore diameter. Glycerol conversion with H-Y zeolite was 88.7% after 8 h. and the reaction was slower because of its lower acidity. The highest glycerol conversion was obtained in H-Beta, but with this catalyst tri-tert-butyl glycerol was not formed. It was also concluded that zeolites are not suitable as catalysts for the reaction studied, because of their easy deactivation and higher price than that of ion exchange resins. The solvent dioxane was inadequate for glycerol etherification with p-sulfonic acid, because no etherification reaction was observed in this system. It can also be concluded that the best temperature for glycerol etherification was 60 °C. Was observed also the formation of isobutylene dimers after 8 hours of reaction, due to equilibrium, causing the etherification reaction to form lower ethers and glycerol through the removal of isobutylene.

Glycerin etherification by CsHCO₃ in a batch reactor at 260 °C under atmospheric pressure was studied by Richter et al. (2008). Water formed during the reaction was evaporated and separated by a lateral condenser. Aliquots of reagent material (maximum of 0.2 g) were removed after 2, 4, 8, 12 and 24 hours for gas chromatographic analysis. Glycerol etherification for oligoglycerols was catalyzed by CsHCO3 at 260 °C, with total conversion after 24 h of reaction. The amount of catalyst influenced glycerol conversion, as well as product formation, within the first 2 h after onset of reaction, obtaining 10% glycerol conversion with less catalyst applied (0.1% by weight), 15% with 0.2% by weight and 20% with 0.4% by weight of catalyst. The reaction obeyed pseudo first-order kinetics.

Melero et al. (2008) evaluated the influence of temperature, molar ratio (isobutylene/glycerol) and the behavior of silica mesostructures functionalized by sulfonic acid as catalyst in glycerol etherification with isobutylene to produce the following tert-butyl derivatives: di-tert-butylglycerols (DTBG) and tri-tert-butylglycerols (TTBG). Experiments were conducted varying the isobutylene/glycerol molar ratio and reaction temperature. In this study it was concluded that sulfonic acid functionalized silica demonstrated excellent behavior in producing tert-butyl compounds. Its activity and selectivity for high value products used in fuel bioadditives (DTBG and TTBG) were comparable or even superior to commercial acid resins. Under optimized conditions these catalysts had a yield of 90% in complete glycerol conversion to DTBG and TTBG. The selectivity of DTBG and TTBG increased with a rise in molar ratio as a function of temperature. However, for even higher temperatures, there was a decrease in high ether selectivity. The authors concluded that ideal selectivity occurs when a molar ratio of 4:1 and temperatures between 75 and 85°C are used. Low ether selectivity, at low molar ratios and temperatures, was also evaluated as a function of viscosity, where a greater amount of glycerol caused high viscosity in the reaction medium. The comparison between the performance of these silicas and other types of commercial resins showed that moderately strong acid centers of mesostructured silica could produce better glycerol conversion and selectivity results for the desired products, at moderate temperatures and suitable molar ratio.

Gu et al. (2008) studied glycerol etherification with 1-phenyl-1-propanol using different solid acid catalysts. The best results were found using silica functionalized with sulfonic groups (SiO₂-SO₃H), resulting primarily in monoether derivatives. The same catalyst was used in glycerol etherification with other alcohols. In the reaction with 1,3-diphenyl-2-propanol, 96% monoether yield was obtained and with isoborneol, the monoether was isolated with 83% yield. A mixture of 1-alkyl and 2-alkyl-glycerol ether was formed, the former in a larger amount. With primary alcohols yield was significantly lower. It was observed that 2-octen-1-ol reacted with glycerol to form monoethers with 61% yield after 39 h of reaction at 80°C, while lauric alcohol (n-dodecanol) was inactive in the reaction, even after 48 h. The reaction of glycerol with ethanol and methanol was performed in the presence of acidic solids, producing mono-, di-and triethers as a function of the alcohol/glycerol molar ratio. Ethers from the reaction of glycerol with ethanol (Fig. 5) have wide application when added to fuels, since they are formed from renewable raw materials.

Figure 5. Etherification of glycerol with ethanol catalyzed by acidic solids

Phosphomolybdic acid (H₃PMo₁₂O₄₀) impregnated on NaUSY zeolites loaded with different amounts of HPA (heteropolyacid) acid was used by Ferreira et al. (2009) to test the influence of the catalyst on the reaction of glycerin with acetic acid. Catalyst activity increased for a larger amount of HPA up to a certain quantity of H₃PMo₁₂O₄₀-NaUSY. Agnieszka et al. (2008) evaluated glycerol conversion with different basic catalysts and reported that glycerol conversion increased with a rise in catalyst basicity in the following order: MgO<CaO<SrO<BaO.

Luque et al. (2008) evaluated the use of Starbon 1400-SO₃H in glycerol etherification with isobutylene. This catalyst demonstrated similar activity to that of functionalized silica and better performance than other types of catalysts such as H-Beta zeolite and p-toluenesulfonic acid.

Glycerol etherification with benzyl acid, using different acid catalysts, was also studied by Mota et al. (2009). Catalysts used in this work were niobic oxide, Amberlyst 35 sulfonic resin, β zeolite, mordenite and montmorillonite K10 clay. The best results were obtained with the use of Amberlyst-35 acid resin and H-Beta zeolite, which was highly selective to monoethers. When p-toluenesulfonic acid was used as catalyst in homogeneous medium, glycerol conversion was complete and the formation of the product resulting from mono-, di-and trietherification of glycerol was observed. We can also observe the formation of dibenzyl ether, product of the self etherification of benzyl alcohol, due to the excess used. In reactions catalyzed by acidic solids in heterogeneous medium, glycerol conversion was 100% when Amberlyst-35 resin and montmorillonite K-10 were used. Zeólite β exhibited conversion of 80% and the main product obtained was monoetherified. Etherified products were not observed when niobic acid and modernite were used as catalyst.

Frusteri et al. (2009) assessed etherification of glycerol anhydrous (99.5%) with tert-butylic alcohol (99.7%) using different systems of acidic solids. Liquid phase experiments were conducted in a stainless steel jacketed reactor under different reaction conditions. The reaction occurred under pressure, at temperatures varying between 303 and 363K, alcohol/glycerol ratio of 4 and at different reaction times (1, 2, 6, 24 and 30 hours). A well-defined amount of dry catalyst (Amberlyst 15) was introduced into the reactor and then heated at a prefixed reaction temperature. At the end of the experiment, the reactor was recooled in an ice bath until the steam pressure of the mixture reached atmospheric pressure, allowing all gas-phase compounds to condense. The results show that the influence of reaction temperature on glycerol conversion increased linearly with a rise in temperature (from 10% at 303K to 85% at 343 K). At higher temperatures (363K), glycerol was not totally converted. It can also be observed that an increase in reaction temperature favors the formation of disubstituted ethers, and that a low catalyst/glycerol molar ratio (equivalent to 1.2 %) is sufficient to ensure high glycerol conversion. Pressure also significantly affects reaction kinetics. Glycerol conversion is always higher at 1.0 MPa. The most marked effect of pressure was observed after 2 hours of reaction. Glycerol conversion values increased from 53% (at 0.1 MPa) to 72% (at 1.0 MPa). Additionally, after 6 hours of reaction, no conversion differences were observed, since the system had reached equilibrium. Results also showed that without a catalyst the reaction does not occur, while an increased amount of catalyst causes a significant increase in the formation of glycerol ether. It was also necessary to remove water formed in the reaction medium to allow greater ether formation.

V. FINAL CONSIDERATIONS

The discussion surrounding global warming has been increasing, primarily due to the search for environmentally-friendly renewable energy sources. Modern society still depends significantly on fossil fuels such as petroleum and natural gas, but the search for renewable energy sources has been intensifying exponentially. It is also clear that among the most widely recognized possibilities, biodiesel is currently the most promising renewable energy source, evidenced by its increased production in several countries worldwide. It is also evident that the large volume of glycerol obtained from biodiesel production (worldwide glycerol production is expected to reach 1.2 million tons by 2012) and policies regarding renewable energy sources are problems in search of solutions. There is currently no market for all the glycerol produced. Glycerochemistry offers several opportunities for professionals in the area, such as development of new products, processes and applications or in the synthesis of new, more active and selective catalysts. This motivates research and development of glycerol as raw material for organic compounds in a wide range of applications. The fact that 52% of the molecular weight of glycerin consists of oxygen atoms makes it a potential candidate for use as oxygenated additive in fuels. Therefore, glycerol additives show excellent qualities, making them suitable for use in gasoline, diesel and biodiesel. Although the literature review presented here shows a wide range of viable processes, including transformation of glycerol into ethers, esters and acetals, much remains to be investigated.

Among the several routes used for the production of additives, the etherification with alcohols stands out as the most promising in terms of industrial application. It is also interesting that the research in supercritical medium can increase the selectivity while maintaining high conversions of the desired products and reducing the reaction time (McHugh and Krukonis, 1994). This can be a secure way, and not cause environmental damage and require less investment in the process because the cost of suitable equipment may be offset by the high rates of conversion of desired products. Every effort must be made to combine creativity and knowledge in order to develop economically viable products that benefit all society. Finding solutions to ecological problems means improving quality of life, mainly in large urban centers, and minimizing environmental damage.

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Received: November 23, 2011
Accepted: March 7, 2013
Recommended by Subject Editor: Orlando Alfano