Catalysis in Biodiesel Production by Transesterification Processes-An Insight

Biodiesel is the mono-alkyl esters of long chain fatty acids derived from renewable lipid feedstock, such as vegetable oils and animal fats, for use in compression ignition (diesel) engines. The conversion of component triglycerides in oils to simple alkyl esters with short chain alcohols like methanol and ethanol amongst others is achieved mainly by transesterification. The transesterification reaction, a reversible process proceeds appreciably by the addition of catalysts, which can be acidic, basic or organic in nature, usually in molar excess of alcohol. The economy of the process depends on the type and quantity of catalyst used among other factors. The catalyst can be homogeneous or heterogeneous depending on whether it is in the same or different phase with the reactants; oils and alcohols. This paper attempts to give an insight into some of the various types of catalysts that have been used to effect the transesterification of vegetable, waste and animal oils in biodiesel production.


Introduction
Chemical transesterification or alcoholysis of triglycerides or the esterification of free fatty acids using short-chain alcohols in the production of alkyl esters or biodiesel was first reported 1 on August 31 st 1937 in a Belgian Patent by Chavanne of the University of Brussels 2 .In the early 1940s researches that did not actually anticipate the production of alkyl esters as fuel were patented [3][4][5][6][7][8] .The original objective of the work was to develop a simplified method for extracting glycerol during soap production 9 .The glycerol was needed for wartime explosives production.The glycerol could be separated since it is insoluble in the esters and has a much higher density that makes settling or centrifugation a choice process in its removal.
Transesterification reaction of vegetable oils can be represented as in Scheme 1.The reaction does not proceed to any appreciable extent in the absence of catalysts or supercritical condition.Various homogeneous and heterogeneous catalysts, ranging from base, acid to enzyme 10,11 as well as carbon catalysts produced from sugar starch and cellulose have been developed for use in biodiesel production.carbonates 22,32 .Alkaline metal alkoxides (as CH 3 ONa for the methanolysis) are the most active catalysts.They give yields greater 98% in a relatively short reaction time of 30 min.even at low molar concentrations of about 0.5 mol%, but their requirement of the absence of water makes them inappropriate for typical industrial processes in which water cannot be avoided completely 33 Step 2: Where R" = CH 2 -CH -OCOR 1 CH 2 -OCOR 1 R 1 = Carbon chain of fatty acid, R = Alkyl group of alcohol Scheme 2. Mechanism for base catalysed transesterification process.Alkaline metal hydroxides (e.g KOH and NaOH) are cheaper than metal alkoxides, but less active.Nevertheless, they are a good alternative since they can give the same high conversions of vegetable oils just by increasing the catalyst concentration to 1 or 2 mol%.However, even if water-free alcohol/oil mixture is used, some water is produced in the system by the reaction of the hydroxide and the alcohol.The presence of water gives rise to hydrolysis of some of the produced ester (Scheme 3), with consequent soap formation 33 .In 2003, metal complexes of the type M(3-hydroxy-2-methyl-4pyrone) 2 (H 2 O 2 ), where M = Tin(Sn), Zinc (Zn), Lead (Pb) and Mercury (Hg) were used for soybean oil methanolysis under homogeneous conditions 34 .The Sn complex at a molar ratio of 400:100:1 methanol:oil:catalyst gave 90% conversion in 3 h.while the Zn complex gave only 40% conversion under the same conditions.This undesirable saponification reaction reduces the ester yields and considerably makes difficult the recovery of the glycerol due to the formation of emulsions, increase in viscosity and greatly increased product separation cost.

R''COOR
R' = carbon chain of fatty acid R = alkyl group of the alcohol Scheme 3. Hydrolysis of esters and formation of soap by the presence of water.Potassium carbonate, used in a concentration of 2 or 3 mol% gives high yields of fatty acid alkyl esters and reduces the soap formation 35 .This can be explained by the formation of bicarbonate instead of water (Scheme 4), which does not hydrolyze the esters.There are other heterogeneous base catalysts which have been tested with promising results 36,37 .Also, solid bimetallic Sn-Ni 38 , exchange resins and zeolites 39,40 , organometallic compounds 41 and mixed oxides [42][43][44] have been developed and used in transesterification reactions.In addition, P(RNCH 2 CH 2 ) 3 N 45 , multifuntionalized 46 as well as organosulphonic acid functionalized mesoporous silicas 47

Shortcomings of homogeneous alkali catalyzed processes
Reports already reviewed showed that base-catalyzed transesterification of vegetable oils results in good yields of the esters.Nevertheless, there are obvious problems encountered by their use.Some of these problems have been identified to include: • High energy demand • Post-reaction treatment to remove the catalyst from the product-biodiesel • Interferences occasioned by the presence of free fatty acid and water during the reaction • Difficulty in the recovery of glycerol after the reaction and • Post-reaction treatment of the alkaline waste-water to obviate the environmental effects of its disposal The development of acid and heterogeneous catalyst systems, some of which run in continuous reactors, have addressed many of these problems which ordinarily meant higher production costs and less economic viability relative to petroleum-based diesel.

Acid-catalyzed processes
The mechanism of the acid-catalyzed transesterification of vegetable oils is as shown in Scheme 5 for a monoglyceride.The protonation of the carbonyl group of the ester leads to the carbocation which after a nucleophilic attack of the alcohol produces the tetrahedral intermediate.This in turn eliminates glycerol to form the new ester, and to regenerate the catalyst.The mechanism can be extended to diand triglycerides 48 .Carboxylic acids can be formed by reaction of the carbocation with water present in the reaction mixture.This suggests that an acid-catalyzed transesterification should be carried out in the absence of water, in order to avoid the competitive formation of carboxylic acids, which reduce the yield of alkyl esters.
R / = carbon chain of fatty acid, R = alkyl group of the alcohol Scheme 5. A typical mechanism of acid catalyzed transesterification of vegetable oils.The transesterification process in biodiesel production is catalyzed by Bronsted acids like HCl, BF 3 , H 3 PO 4 , H 2 SO 4 and sulphonic acids 49,50 .Preferably, sulphonic and sulphuric acids are mostly used.These catalysts give very high yields in alkyl esters, but the reactions are slow, requiring typically, temperatures above 100 o C and from 3-48 h to reach complete conversion [51][52][53][54][55][56] .Freedman et al showed that the methanolysis of soybean oil, in the presence of 1 mol% of H 2 SO 4 , with an alcohol/oil molar ratio of 30:1 at 65 o C, takes 50h to reach complete conversion of the vegetable oil (>99%), while the butanolysis (at 117 o C) and ethanolysis (at 78 o C) using the same quantities of catalyst and alcohol take 3 h and 18 h, respectively 27 .
Peter et al. 57 studied the methanolysis of palm oil in a 6:1 molar ratio of methanol to oil using the following metal salts of amino acids; cadmium, cobalt, copper, iron, lanthanum, nickel and zinc.Arginate of zinc was shown to result in the highest yield and the reasonable rate of reaction estimated to be obtained at temperatures higher than 130 0 C. Report also indicated that soybean oil can be transesterified in methanol using sulphated zirconia-alumina and tin oxide as well as tungstated zirconia-alumina acid catalysts, though the latter was adjudged most effective as it gave 90% conversion in 20 h at 250 0 C 58 .Other sulphated compounds of zirconium have also been studied with varying results [59][60] .New solid acid/base catalysts as well as metal oxides have also been used in the transesterification process [61][62][63][64] .
Reaction rates in acid-catalyzed processes may be increased by the use of larger amounts of catalyst.Typically, catalyst concentrations in the reaction mixture have ranged between 1 and 5 wt % in most academic studies using sulphuric acid 27 .Canakci and Van Gerpen 65 used different amounts of sulphuric acid (1, 3 and 5 wt %) in the transesterification of grease with methanol.In these studies, a rate enhancement was observed with the increased amounts of catalyst and ester yield went from 72.7 to 95.0% as the catalyst concentration was increased from 1 to 5 wt%.The dependence of reaction rate on catalyst concentration has been further verified by the same authors and other groups [66][67] .A further complication of working with high acid catalyst concentration becomes apparent during the catalyst neutralization process, which precedes product separation.Since CaO addition during neutralization is proportional to the concentration of acid needed in the reactor, high acid concentration leads to increased CaO cost, greater waste formation, and higher production cost.
The liquid acid-catalyzed transesterification process does not enjoy the same popularity in commercial applications as its counterpart, the base-catalyzed process.The fact that the homogeneous acid-catalyzed reaction is about 4000 times slower than the homogeneous base-catalyzed reaction has been one of the main reasons 68 .However, acid-catalyzed transesterification holds an important advantage with respect to base-catalyzed ones; the performance of the acid catalyst is not strongly affected by the presence of free fatty acids in the feedstock.Thus, a great advantage with acid catalysts is that they can directly produce biodiesel from low-cost feedstocks, generally associated with high free fatty acid concentrations.A two step esterification process in which the free fatty acid is converted to fatty acid methyl esters in an acid-catalyzed treatment followed by base-catalyzed process has been proposed 66 .
In the transesterification of triglyceride feedstock using acid catalysts, Mittelbach et al. 69 compared the activities of a series of layered aluminosilicates with sulphuric acid for the transesterification of rapeseed oil.These researchers used an initial molar ratio of 30:1 alcohol-to-oil and 5wt% catalysts.Among the catalysts tested, sulphuric acid showed the highest activity.The solid catalysts showed varied activities depending on reaction conditions.The most active catalysts were activated by sulphuric acid impregnation.For instance, activated montmorillonite KSF showed a 100% conversion after 4 h of reaction at 220 o C and 52 bar.However, leaching of sulphate species compromised the re-usability of this clay.Thus, to maintain clay activity at constant values, sulphuric acid re-impregnation had to be carried out after each run.It is also likely that some degree of homogeneous catalysis was taking place due to sulphuric acid leaching, they concluded.
Kaita et al. designed aluminum phosphate catalysts with various metal-to-phosphoric acid molar ratios and used these materials for the transesterification of kernel oil with methanol 70 .According to the authors, durable and thermostable catalysts were obtained with good reactivity and selectivity to methyl esters.However, the use of these materials still needed high temperatures (200 o C) and high methanol to-oil molar ratios (60:1) in order to be effective.
In a related study, Waghoo et al. 71 reported on the transesterification of ethyl acetate with several alcohols over hydrous tin oxide to obtain larger esters.Linear and aromatic alcohols were tested in a temperature range of 170-210 o C. All reactions were completely reflective for transesterification.In particular, this catalyst presented an appreciable activity for reactions involving nbutyl alcohol, n-octyl alcohol and benzyl alcohol.
Amberlyst-15 has also been studied for transesterification reactions.However, mild reaction conditions are necessary to avoid degradation of the catalyst.At a relatively low temperature (60 o C), the conversion of sunflower oil was reported to be only 0.7% when carrying out the reaction at atmospheric pressure and a 6:1 methanol-to-oil molar ratio 72 .Also, hydrochloric, organic, sulfonic, formic, acetic and nitric acids have been investigated by other authors 33,[73][74][75] .

Variables affecting the acid-catalyzed processes
The acid-catalyzed process is thought to be more suitable for the production of biodiesel from low feedstocks (used frying oil, waste animal fat), mainly because of the fact that these feedstocks contain greater amounts of free fatty acids (FFAs) 76 .The base-and acidcatalyzed transesterification processes were compared with respect to the FFAs content of the feedstock.The greater tolerance of an acid catalyst to the FFA content compared to an alkaline catalyst was confirmed in a report by Canakci and Van Gerpen 65 .They also showed that acid catalyzed reactions are more susceptible to water content of the feedstock than the base-catalyzed process and that the presence of more than 0.5% water in the oil will decrease the ester conversion to below 90% 66,[77][78] .The fact that the water content is more crucial in acid catalysis than in alkaline catalysis is mainly caused, according to Siakpas et al 76 , by the greater affinity of water by sulphuric acid, which will lead to the acid catalyst preferentially interacting with water rather than alcohol with the consequent deactivation of the catalyst.Also, there is evidence that large quantities of acid catalyst in biodiesel production may lead to ether formation by alcohol dehydration 79 and the consequent high use of calcium oxide in the acid neutralization after production with its attendant high production cost and waste generation.It has been suggested that acid-catalyzed transesterification achieves greater and faster conversions at high alcohol concentrations 74 .
Schuchardt 80 inferred that according to the observed results and to the mechanism of the base-catalyzed transesterification, it seemed that the good performance of TBD, when compared to BEMP and Me 7 P, is related to its kinetic activity.The catalytic site (unshared electron pair of the SP 2 N) of TBD is practically unhindered allowing an easy access of the methanol for proton transfer, while the steric hindrance shown by the triamino (imino) phosphoranes is so significant that they are practically inert to alkylating agents, such as isopropyl bromide, as well as extremely resistant to react with thionyl chloride and thiophosgene 91 .Structures of some non-ionic bases 80 .
In a second series of studies 80 , the catalytic activity of TBD was compared to that observed for typical industrial catalysts (e.g NaOH and K 2 CO 3 ).The reaction yields obtained with TBD were close to those observed with NaOH and no undesirable by-products such as soaps (easily formed when alkaline metal hydroxides are used) were observed.When compared to potassium carbonate, TBD was always more active, even at low molar concentrations.Although TBD is less active than sodium methoxide (at only 0.5%, CH 3 ONa produces more than 98% of methyl esters after 30 min), its use does not require any special condition.Due to the excellent performance of TBD in the transesterification of vegetable oils, the catalytic activity of other alkylguanidines was also investigated, in order to establish and understand all factors that may affect their catalytic properties.
Results obtained in the transesterification of soybean oil with methanol show that 1, 2, 3, 4, 5-pentacyclohexyl biguanidine (PCBG) is even more active than TCG, as an 82% yield of methyl esters is obtained with PCBG after 1 h, compared to 69% with TCG under the same conditions 94 .

Lipase-catalyzed processes
Due to their ready availability and the ease with which they can be handled, hydrolytic enzymes have been widely applied in organic synthesis.They do not require any coenzymes, are reasonably stable, and often tolerate organic solvent 76 .Their potential for regioselective and especially for enantioselective synthesis makes them valuable tools 95 .Immobilized Candida Antarctica lipase has been used for ethyl esterification of docosahexanoic acid 95 and latter used to effect over 98.5% fatty acid methyl ester conversion 96,97 .
Although the enzyme-catalyzed transesterification processes are not yet commercially developed, new results have been reported in recent articles and patents 98 .A solution to the inhibition of enzyme activity by high concentration of methanol as well as water generated during the reaction have been proffered by many authors 96,[99][100][101] to include carrying out the reaction with immobilized enzymes and the use of multiple stages.The common aspects of these studies consist in optimizing the reaction conditions (solvent, temperature, pH, type of microorganism which generates the enzyme, etc.) in order to establish suitable characteristics for an industrial application.However, the reaction yields as well as the reaction times are still unfavorable compared to the base-catalyzed reaction systems, though Shimada and co-workers reported that the stepwise addition of methanol gave 98% conversion of oil to methyl ester with an amazing re-use of the immobilized enzyme for 50 times 98 .Several reports exist on lipase catalyzed transesterification process with extracellular and intracellular lipases as catalyst in either aqueous or non-aqueous systems [99][100][101][102][103][104] .The incubation of Candida antarctica lipase consecutively in methyl ester (oleate) for 30 min and in soybean oil for 12 h has been reported to lead to a dramatic increase in the efficiency of the enzyme, giving almost 100% conversion 109 .While Du et al1 10 reported lipase-catalyzed transformation of soybean oil for biodiesel production with different acyl acceptors, others [111][112][113] , reported the use of other enzymes like Chlorella vulgaris and Candida cylindracae in the conversion of other oils to biodiesel.
The variables affecting the use of enzymes like temperature, solvent, water content, immobilization or free enzyme use as well as pH and time of reaction have been studied with a view to obtaining optimum conditions for improved conversions [114][115][116] .

Use of organic catalyst from carbonized sugar
Japanese researchers have devised a low-cost, ecologically friendly solid catalyst for the production of biodiesel: a carbon catalyst produced from sugar, starch or cellulose 117 .The separation of the liquid catalyst from the reaction mixture is costly and wasteful.Other catalysts such as nafion or sulphonated naphthalene are either expensive or offer less or rapidly diminishing catalytic activity.
The researchers avoided all those issues by devising a mechanism to incompletely sulphonate (treat with sulphuric acid) carbonized natural organic material such as sugar, starch or cellulose to prepare a more robust solid catalyst.Incomplete carbonization of these natural products results in a rigid carbon material.The team found that sulphonation of this material generates a stable solid with a high density of active sites, enabling the inexpensive preparation of a high performance catalyst.The team found that the activity of the solid sulphonated carbon catalyst is more than half that of a liquid sulphuric catalyst 117 .

Conclusion
The industrial homogeneous catalysts will have to be substituted in the near future by heterogeneous catalysts due to environmental reasons.Good strong-base heterogeneous catalysts are still in development.One possibility would be the use of zeolites with strong basic sites.Enzymes, especially lipases, are also becoming catalysts of choice in transesterification reactions in biodiesel production despite their cost.This is because they offer some advantages as far as pretreatment of process streams are concerned.Efforts are being made by scientists to develop novel catalysts that will offer best environmental practices as well as relatively good cost.