54Eclética Química, 36 (4), 54-61, 2011. genated glycerol products (glycerol ethers12, glycerol acetals13, glycerol esters14, and alcohols15, e.g.) show noticeable physico-chemical properties for petroleum fuels, improving its combustion and reducing its harm- ful environmental impact. Table 1 – Physicochemical characteristics of crude glycerol Crude glycerol in Brazil According to the National Agency of Petro- leum, Natural Gas and Biofuels (ANP), in 2009, the 57 Brazilian power plants has produced approximately 1.6 million m3 biodiesel (Table 2), releasing 178,700 m3 crude glycerol to the internal market16. Unfortunately, only a small part of this co-product is promptly com- mercialized, being the remain portion stored in metal Introduction In consequence of the several environmen- tal problems caused by petroleum use, a large number of nations have adopted policy strategies based on re- newed energy sources as eolic, photovoltaic and bio- mass (bioethanol, biodiesel e.g.). The Brazilian Gov- ernment, through the National Program for Biodiesel Production and Use, made it mandatory the blend of 5% (v/v) of biodiesel to the petroleum-based diesel oil, for its commercialization in all territory1. Besides the economic, environmental and so- cial advantages, biodiesel manufacturing results in a large amount of crude glycerol (co-product) with low profitable prices due to its extension in the local market 2,3,4. Nowadays, crude glycerol has been used as com- bustive5, composite6, anaerobic digestive7 and nutritious feed for animals (pigs8, poultries9 and laying hens10). In general, the crude glycerol properties slightly vary with its manufacture and refine69 (Table 1). However, applying crude glycerol as fuel addi- tive is not technically promising hence its hygroscopic properties and high polarity. In addition, glycerol can polymerizes at high temperatures, causing clogging of internal combustion engines, or partially oxidize into acrolein, being potentially toxic11. Conversely, oxy- Due to the excessive production of crude glycerol by the Brazilian biodiesel industries, the commercial interest for glycerol derivatives has widely increased. Therefore, several methodologies have been proposed for such purpose, combining the reuse of this abundant raw material with new applications. In such context, this review summarizes a number of works focused on oxygenated glycerol derivatives production, regarding the reactions mechanisms (etherification, acetalization, esterification, and fermentation) and the physicochemical chemical properties and application. Thus, this work aims to contribute to future studies in chemical modification of crude glycerol; hence most of the scientific publications have discussed additives yielded from pure glycerol. Key words: glycerol, etherification, acetalization, esterification, fermentation OXYGENATED GLYCEROL DERIVATIVES AS AN ALTERNATIVE SOURCE OF ENERGY: A REVIEW Kiany S. B. Cavalcante1,2, Mitchell G. S. da Silva1, Francisco S. M. Sifrônio1, Renata R. S. Valois1, Adeilton P. Maciel1, Antonio G. Souza2, Fernando C. Silva1*1 1 Universidade Federal do Maranhão, Dep. de Química, CCET, Campus do Bacanga, São Luis/MA 2 Universidade Federal da Paraíba, Dep. de Química, CCEN, Campus I, João Pessoa/PB 1 e-mail: fcs.ufma@gmail.com 55Eclética Química, 36 (4), 54-61. 2011. tertiary alkyl groups (C4-C5) or C1-C10 alkyl groups24,71,72. Figure 1 – Conversion of glycerol into oxygenated derivatives. Glycerol alkyl ethers can be synthesized as de- scribed by Williamson21,22, though a bimolecular nucle- ophilic substitution of alkyl halide with alkoxide ions (SN2). Another acceptable process for ether formation is etherification with an alcohol or olefin in the pres- ence of an acid catalyst23. The first commercial patent (US1968033) to define the formation of ether from glycerol used isobutylene reagent and sulfuric acid as catalysis25. Aiming to improve the percentage of glyc- erol ethers, Karinen and Krause (2003)26 studied the ideal conditions to expand the selectivity for ethers by modifying the pure glycerol/isobutene molar ratio and temperature. Etherification of glycerol with isobutene results in the substitution of some or all three hydroxyl groups in the glycerol molecule. Thus, up to five ether isomers can be formed depending on the degree of etherification: two mono-substituted ethers (3-tert-bu- toxy-1,2-propanediol and 2-tert-butoxy-1,3-propane- diol), two di-substituted ethers (2,3-di-tert-butoxy-1- propanol and 1,3-di-tert-butoxy-2-propanol), and one tri-substituted ether (1,2,3-tri-tert-butoxy-propane), known as glycerol tert-butyl ether (GTBE). The mono- and di-ethers are the main prod- ucts when the molar ratio of pure glycerol/isobutene is less than 1:3. However, tri-tert-butyl-glycerol ethers are produced at significant proportions when molar ratios higher than 1:4 are used. In both cases, secondary prod- ucts of isobutene oligomerization are detected at the beginning of the reaction, which tend to increase when the temperature exceeds 80 ºC. This might occur when the activation energy of the oligomerization reaction is higher than that of etherification26. Noureddini et al. (1998) investigated the in- fluence of impurities in glycerol on the etherification reaction. The authors found that the presence of meth- anol and water residue from the oil transesterification tanks under inappropriate conditions. Table 2 – Production of crude glycerol in Brazil Contaminants traces originated by the bio- diesel synthesis (catalyst, unconverted substances and secondary product) often remains in the crude glycerol phase, influencing on the overall quality of its oxygen- ated compounds. In the main, the Brazilian biodiesel industries use homogeneous alkaline catalysts (sodium and potassium hydroxide) due to its reasonable low cost and high reactivity4. However, the yielded glycerol re- quires certain purification treatment, such as removal of the applied catalyst and fatty acid salts, being time consuming and expensive processes17,18. Day (2008) de- scribes one of these processes by adding inorganic acid (sulfuric, hydrochloric or nitric acid) to glycerol under heating (40 - 90 ºC) and low stirring. The remained wa- ter and alcohol traces are then removed under room or reduced pressure, being followed by the precipitated salts19. In contrast, industries that apply heterogeneous catalysts (inorganic oxides, modified zeolites etc.) do not have technical restrictions for triacylglycerides sa- ponification or catalyst removal. Contrariwise, they of- ten lead to problems attributed to corrosion of the oper- ation unities2. 3. Glycerol derivatives A series of processes are being explored by researchers worldwide for the catalytic conversion of glycerol into potential fuel additives70, such as etherification12, acetalization13, acetylation14, and fermentation15 of glycerol (Figure 1). 3.1 Glycerol ethers By definition, glycerol ethers contain three oxygen-bound carbons derived from glycerol and R1, R2 and R3 radicals, being hydrogen atoms or alkylated groups. Preferentially, R1 and R3 are Etherification Acetalization Acetylation Fermentation (R1 and R2 = hydrogen or alkylated groups and R3 = alkylated groups) (R1, R2 and R3 = hydrogen or alkylated groups) R1O OR2 OR3 Acetal and Ketal R1 R1 R2 R2 R3 R3 Ether AcO OAc OAc Ester 1,3-Propanodiol e Other alcoholsR – OH (Ac = CH3COO-) (R = alkylated groups) 56Eclética Química, 36 (4), 54-61. 2011. Etherification of glycerol produces compounds of lower polarity and viscosity, hence they are often volatile30. GTBEs are excellent additives for diesel and biodiesel fuels. Studies have shown that glycerol ethers, i.e., a mixture of 1,3-di-, 1,2-di- and 1,2,3-tri-tert-butyl glycerol, can be incorporated into standard diesel fuel that contain about 30 - 40% of aromatic compounds. This addition leads to a significant reduction in the emission of particle matters, hydrocarbons, carbon monoxide, and aldehydes. Also, GTBE can increase the octane number of fuel but, since it is a branched molecule, lowers its cetane number24,31. Table 4 shows some of the physicochemical properties of biodiesel and diesel oil containing 400 ppm sulfur and 31% aromatic compounds, blended with 5% (v/v) GTBEs, with composition 70:10:2024 and 24:62:1431. Table 4 – Parameters of diesel-glycerol ether blend Karas et al. (1994) studied the emission of pollutant gases from a diesel engine. The addition of 5% (w/w) GTBEs to certified diesel (0.25% sulfur, 43% aromatic compounds, cetane number: 39) substantially reduces the emission of carbon monoxide and hydrocarbons32. The ethers leads to a slightly reduction in particle matter, but it frequently goes with an increase of the NOx emission (Figure 2). process can consume the etherification agent, reacting with isobutylene and forming methyl-tert-butyl ether and tert-butyl alcohol, respectively27. Furthermore, the presence of sodium hydroxide deactivates the catalyst and reduces the reaction yield, suggesting that the ab- sence of crude glycerol pretreatment may result in un- desired compositions and low conversion rates. In order to minimize these effects, the authors neutralized crude glycerol with Amberlyst-15, preventing the formation of sodium salts. The etherification reaction of crude glycerol pretreated with Amberlyst-15 showed the same behavior as that of pure glycerol. Di Serio et al. (2010) proposed a new pro- cess in which GTBE were extracted with biodiesel as extraction agent68. The final product was a mixture of biodiesel and GTBEs with glycerol content below the required level, which can be used directly as diesel ad- ditive. The applied reaction settings were: isobutene/ glycerol molar ratio of 2; 1.1% (m/m) Amberlyst-15; temperature of 92 ºC; pressure of 15 bar, and reaction time of 480 min. The performance of the process was evaluated by means of preliminary kinetic analysis, demonstrating satisfactory results. Klepacova et al. (2003, 2006)28,29 evaluated various catalysts for the etherification of glycerol with isobutene: ion-exchange resins Amberlysts (A) and large-pore zeolites (HY and H-BEA). The conversion results suggested that Amberlysts were the most active catalysts due to the pore size and high degree of cross- linking structure that allows the formation tert-butyl- ether of glycerol. Table 3 summarizes some reaction methodologies (molar ratio of etherification agent/glyc- erol, temperature, reaction time, and catalyst) used for the conversion of glycerol into ethers using alkenes and alcohols. Table 3 – Etherification of glycerol 57Eclética Química, 36 (4), 54-61. 2011. Delfort et al. (2003) adding 1-40% (v/v) of acetals to diesel oil, observed that the blend solubility was favorable to the separation of compounds during its storage36. By means of compression ignition engine test, Nord and Haupt (2005) detected that glycerol acetals/ diesel mixture didn’t alter the physicochemical charac- teristics of fossil fuel (Table 6) and it has lower number of particle materials; however, a high quantity of hydro- carbons and carbon monoxide, 4.9 and 3.8% respective- ly, are emitted (Figure 3)37. According to the authors, such behavior is attributed to a reduction in cetane num- ber that delays the onset of combustion and reduces the efficiency of the engine. The lower cetane number may also explain the reduced formation of NOx. In contrast, a moderate increase of formaldehyde and acrolein for- mation was observed, in comparison to acetaldehyde, probably due to incomplete fuel combustion, decompo- sition and/or oxidation of the acetal. Table 6 – Parameters of glycerol acetal-diesel blend Figure 2 – Emissions from GTBE-fuel blends. PM: particle matter; NOx: nitrogen oxides; CO: carbon monoxide; HC: hydrocarbons. Source: Karas et al. (1994). Glycerol acetals and ketals Acetals and ketals derived from glycerol are obtained by acetalization or transacetalization of glyc- erol in acid medium with aldehydes and ketones under the action of acid catalysts. Their structure contains R1 and R2 radicals, which generally are hydrogen atoms or a methyl, ethyl or propyl radical, and a methyl or ethyl radical at R3 36. According to Mota et al. (2009)30, acetalization of glycerol with ketones almost exclusively results in the formation of a ketal. The ketal contains a five-mem- ber ring due to dehydration of the hemiketal and the for- mation of a tertiary carbocation that is rapidly attacked by the central hydroxyl group, forming the ring. The re- action with aldehydes forms two acetals by dehydration of the hemiacetal through an SN2 mechanism. The first study on the acetalization of pure glyc- erol was published in 1958 by Piantadosi et al. Table 5 shows some other reaction settings for the formation of glycerol acetals and ketals. Table 5 – Acetalization of glycerol 0.433 1.979 4.170 0.174 0.291 1.588 4.235 0.154 0 1 2 3 4 5 HC CO NOx PM Emission of atmospheric pollutants (g/bhp-hr) Diesel + 5% GTBE Diesel 58Eclética Química, 36 (4), 54-61. 2011. According to Gelosa (2003), conversion effi- ciency is widely influenced by the reaction mechanism and the equilibrium parameters, being strongly unfavor- able to the dehydration of acetic acid45: Glycerol + acetic acid monoacetin + water (1) Monoacetin + acetic acid diacetin + water (2) Diacetin + acetic acid triacetin + water (3). The author therefore investigated this organic synthesis catalyzed by acid polymer resin using reactive chromatography, which consists of the simultaneous re- action and separation of the products. The reactor per- mitted to obtain high conversion rates of glycerol and high purity of the products45. Later, Mota et al. (2008)46 evaluated the performance of glycerol acetylation us- ing different solid acid catalysts, whom proposed the kinetic mechanism for glycerol into mono-, di- and tri- acetin. In such work, amberlyst-15 was found to be the most active catalyst, converting 97%, being followed by montmorillonite K-10, Nb2O5 and the zeolites HZSM-5 and HUSY. Even expected, the catalyst acidity was not an indicator its catalyst efficiency, with zeolites present- ing low conversion and selectivity for di- and triace- tin despite its high acidity. Such chemical behavior can be explained by diffusion limitations within the zeolite pores. Other similar reaction as summarized in Table 7 using different types of acid catalysts. Delgado (2003) produced glycerol triacetate by the reaction of crude glycerol with methyl acetate and potassium hydroxide. When applied as biodiesel addi- tive the glycerol yielded in a high freezing point and low viscosity of biofuel (Table 8) 47. Figure 3 – Emissions of acetal-fuel blends. PM: particle matter; NOx: nitrogen oxides; CO: carbon monoxide; HC: hydrocarbons. Source: Nord and Haupt (2005). Adding ketals to gasoline, Mota et al. (2010) observed a significant reduction of gum formation in conventional gasoline and ethanol blended gasoline. In contrast, the addition of acetals leads to gum formation. One hypothesis to explain such contradictory behavior is the radical reactions and, consequently, the polymer- ization of olefins present in gasoline38. The acetal obtained for Bruchmann et al. (1999)39, called acetal-1, apparently does not meet the requisite of a low flashpoint and oxidation stability of biodiesel. Garcia et al. (2008)40 synthesized a new ace- tal (2,2-dimethyl-1,3-dioxan-4-yl-methyl acetate) from crude glycerol and acetic anhydride, capable to improve the biodiesel viscosity, flashpoint and oxidation stabil- ity. Glycerol esters Triacetin, triacetate ester of glycerol, is a col- orless fuel oil, being poorly soluble in water43,44. Tria- cetin can be obtained by means of methyl acetate trans- esterification or glycerol esterification with acetic acid, also yielding in di-acetin and mono-acetin (intermediate products). Table 7 – Acetylation of glycerol 0.433 0.373 5.056 6.249 0.410 0.358 4.892 4.124 0 1 2 3 4 5 6 7 HC CO Nox PM Emission of atmospheric pollutants (g/bhp-hr) Diesel + 5% acetal Diesel 59Eclética Química, 36 (4), 54-61. 2011. erol is dehydrated into 3-HPA, followed by hydrogena- tion of 1,3-propanediol by NADH2. On the other hand, glycerol is converted into glyceraldehyde-3-phosphate by DHA and into pyruvate which, in turn, is further metabolized to produce NADH2 and other subproducts (acetate, lactate, butyrate, ethanol, H2 and CO2) 61. Ac- cording to Barbirato et al. (1998), industrial production of 1,3-propanediol by means of Clostridium butyricum presented reasonable efficiency. The Table 9 shows the different types of bacterial species, media and growth conditions used for the production of 1,3-propanedi- ol62,63. Most researchers use the pure glycerin. Rangas- wamy, et al. (2010) used the crude glycerin, byproduct of biodiesel production from jatropha, with about 18- 22% glycerol, 9% of free fatty acids, 29% soap, 10% moisture and volatile impurities, 14.5% methanol, 3% of methyl esters, 2.5% of sediment and pH 3.863. Table 10 – Parameters of alcohol-gasoline blends The use of alcohols derived from glycerol was studied by Fernando and colleagues (2007) in com- mercial gasoline. They evaluated the amount of ener- gy and octane number of the mixtures (Table 10)64. The 14:10:76 ethanol/propanediol/gasoline mixture present- Table 8 – Parameters of glycerol ester-fuel blends Alcohols Glycerol can be catalytically converted into a complex mixture of lower alcohols using inorgan- ic catalysts (supported ZrO2 49 and copper50) bacterium species such as Clostridium51,52,53,54, Klebsiella and Cit- robacter55,56, Enterobacter57, Escherichia coli15, and Sac- charomyces cerevisiae58,59. Table 9 – Fermentation of glycerol Fermentation of glycerol by different microor- ganisms has been reported recently59. The application of glycerol as raw material for ethanol manufacture was evaluated recently, being feasible with recombinant S. cerevisiae strains. 1,3-Pro- panediol, one of the oldest fermentation products, was obtained from glycerol for the first time in 188160. Glyc- 60Eclética Química, 36 (4), 54-61. 2011. 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