synthesis of biodiesel

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Synthesis of Biodiesel via Acid Catalysis

Edgar Lotero, Yijun Liu, Dora E. Lopez, Kaewta Suwannakarn, David A. Bruce, andJames G. Goodwin, Jr.*

Department of Chemical Engineering, Clemson University, Clemson, South Carolina 29634-0909

Biodiesel is synthesized via the transesterification of lipid feedstocks with low molecular weightalcohols. Currently, alkaline bases are used to catalyze the reaction. These catalysts requireanhydrous conditions and feedstocks with low levels of free fatty acids (FFAs). Inexpensivefeedstocks containing high levels of FFAs cannot be directly used with the base catalysts currentlyemployed. Strong liquid acid catalysts are less sensitive to FFAs and can simultaneously conductesterification and transesterification. However, they are slower and necessitate higher reactiontemperatures. Nonetheless, acid-catalyzed processes could produce biodiesel from low-costfeedstocks, lowering production costs. Better yet, if solid acid catalysts could replace liquid acids,the corrosion and environmental problems associated with them could be avoided and productpurification protocols reduced, significantly simplifying biodiesel production and reducing cost.This article reviews some of the research related to biodiesel production using acid catalysts,including solid acids.

Introduction

Biodiesel is a nonpetroleum-based fuel that consistsof alkyl esters derived from either the transesterificationof triglycerides (TGs) or the esterification of free fattyacids (FFAs) with low molecular weight alcohols. Theflow and combustion properties of biodiesel are similarto petroleum-based diesel and, thus, can be used eitheras a substitute for diesel fuel or more commonly in fuelblends.1 As a point of comparison, pure biodiesel (B100)releases about 90% of the energy that normal dieseldoes, and hence, its expected engine performance isnearly the same in terms of engine torque and horse-power. Biodiesel, however, is made from renewableresources, is biodegradable and nontoxic, and has ahigher flash point than normal diesel. In addition,

biodiesel increases lubricity (even at blends as low as3% or less), which prolongs engine life and reduces thefrequency of engine part replacement. Another signifi-cant advantage of biodiesel is its low emission profileand its oxygen content of 10-11%. Biodiesel is calledthe environmentally friendly biofuel since it provides ameans to recycle carbon dioxide. In other words, bio-diesel does not contribute to global warming.

Table 1 shows a brief comparison of the ASTMstandards for diesel and biodiesel. As can be seen,biodiesel exhibits characteristics that are comparableto traditional diesel fuel. Table 2 summarizes the typicalemission profiles of biodiesel and one of its blends, B20,which consists of 20% biodiesel and 80% diesel, usingpetroleum-derived diesel emissions as the reference. The

information in the table shows how biodiesel signifi-cantly reduces emissions compared to diesel even whenused as the minor component of a fuel blend. Inaddition, the amount of sulfur in biodiesel is quite low,which can significantly contribute to helping meetcurrent sulfur emission standards in diesel vehicles.

Commonly, biodiesel is prepared from TG sourcessuch as vegetable oils, animal fats, and waste greases

such as yellow and brown greases. Oils and fats belongto an ample family of chemicals called lipids. In general,lipids are found in animals and plants. Typically, fatscome from an animal source and oils from a plantsource. Fats and oils are primarily formed of TGmolecules. A TG molecule is basically a triester of glycerol (a triol) and three fatty acids (long alkyl chaincarboxylic acids; see Table 3 for TG compositions of somecommon vegetable oils, animal fats, and greases). Mono-and diglycerides can be obtained from TGs by substitut-ing two and one fatty acid moieties with hydroxyl

* To whom correspondence should be addressed. Phone: 864656 2621. Fax (864) 656 0784. E-mail: [email protected].

Table 1. Values for the American Society for Testing andMaterials (ASTM) Standards of Maximum Allowed

Quantities in Diesel and Biodiesel

property diesel biodiesel

standard ASTM D975 ASTM D6751composition HCa (C10-C21) FAMEb (C12-C22)Kin. viscosity (mm2 /s)

at 40 °C1.9-4.1 1.9-6.0

specific gravity (g/mL) 0.85 0.88flash point (°C) 60-80 100-170cloud point (°C) -15 to 5 -3 to 12pour point (°C) -35 to -15 -15 to 16water (vol %) 0.05 0.05carbon (wt %) 87 77hydrogen (wt %) 13 12oxygen (wt %) 0 11sulfur (wt %) 0.05 0.05cetane number 40-55 48-60

HFRRc ( µm) 685 314BOCLEd scuff (g) 3600 >7000

a Hydrocarbons. b Fatty acid methyl esters. c High-frequencyreciprocating rig. d Ball-on-cylinder lubricity evaluator.

Table 2. Average B100 and B20 Emissions (in %)Compared to normal Diesel63

emission B100 B20

carbon monoxide -48 -12total unburned hydrocarbons -67 -20particulate matter -47 -12nitrogen oxides +10 +2sulfates -100 -20air toxics -60 to -90 -12 to -20mutagenicity -80 to -90 -20.0

10.1021/ie049157g CCC: $27.50 © xxxx American Chemical SocietyPAGE EST: 10.3Published on Web 00/00/0000



groups, respectively. Lipid feedstocks used in biodieselproduction may contain a mixture of all these glyceridespecies plus some FFAs. The chemical transformationof lipid feedstocks to biodiesel involves the transesteri-fication of glyceride species with alcohols to alkyl esters,as shown in Figure 1. Among possible alcohols, metha-nol is normally favored due to its low cost. The reactionshown in Figure 1 uses methanol as a co-reagent fortransesterification. As can be seen, three consecutivereactions are required to complete the transesterifica-tion of a TG molecule. All types of glyceride speciesparticipate in the reaction. If allowed to go to comple-tion, the net reaction produces 3 mol of alkyl esters and1 mol of glycerol for each mole of TG transformed.

The transesterification reaction requires a catalyst inorder to obtain reasonable conversion rates. The natureof the catalyst is fundamental since it determines thecompositional limits that the feedstock must conformto. Furthermore, the reaction conditions and postsepa-ration steps are predetermined by the nature of thecatalyst used. Currently, most biodiesel is prepared

using alkaline catalysts, such as sodium and potassium

methoxides and hydroxides. Industrially, NaOH andKOH are preferred due to their wide availability andlow cost. Nonetheless, from a chemical standpoint theactive species with both types of catalysts are methoxideions. Methoxide ions form by dissociation of methoxidesalts in one case or when methanol reacts with hydroxylions from added alkaline hydroxides in the secondsituation (Figure 2). Once formed, the methoxide ionsare strong nucleophiles and attack the carbonyl moietyin glyceride molecules to produce the alkyl esters.

Even though transesterification is feasible usinghomogeneous base catalysts, the overall base-catalyzedprocess suffers from serious limitations that translateinto high production costs for biodiesel. Strict feedstockspecifications are a main issue with this process. In

particular, the total FFA content associated with thelipid feedstock must not exceed 0.5 wt %. Otherwise,soap formation seriously hinders the production of fuelgrade biodiesel. Soap forms when the metal hydroxidecatalyst reacts with FFAs in the feedstock (Figure 3a).Soap production gives rise to the formation of gels,increases viscosity, and greatly increases product sepa-ration cost.2 The alcohol and catalyst must also complywith rigorous specifications. The alcohol as well as thecatalyst must be essentially anhydrous (total watercontent must be 0.1-0.3 wt % or less).3 This is requiredsince it is assumed that the presence of water in thefeedstock promotes hydrolysis of the alkyl esters toFFAs (Figure 3b) and, consequently, soap formation. Toconform to such demanding feedstock specificationsnecessitates use of highly refined vegetable oils whoseprice can account for 60-75% of the final cost of biodiesel.4 Other less expensive sources of TG feed-stocks, such as yellow grease, can be used to counteractthe high price tag associated with biodiesel producedfrom refined oils, but this requires additional processsteps.

The demanding feedstock specifications for base-catalyzed reactions have led researchers to seek cata-lytic and processing alternatives that could ease thisdifficulty and lower production costs. Methodologiesbased on acid-catalyzed reactions have the potential toachieve this since acid catalysts do not show measurablesusceptibility to FFAs. For this reason, and since there

Table 3. Typical Fatty Acid Compositions of Vegetable Oils and Animal Fats

fatty acid composition, wt %

fatty acidmyristic14:00a

palmitic16:00

palmitoleic16:01

stearic18:00

oleic18:01

linoleic18:02

linolenic18:03

sat.(%)

rapeseed oil64 3.5 0.9 64.4 22.3 8.2 4.4 virgin olive oil65 9.2 0.8 3.4 80.4 4.5 0.6 12.6sunflower oil5 6.0 4.2 18.7 69.3 10.2safflower oil5 5.2 2.2 76.3 16.2 7.4soybean8 0.1 10.6 4.8 22.5 52.3 8.2 15.5palm oil17 1.2 47.9 4.2 37 9.1 0.3 53.3

choice white grease66 23.3 3.5 11.0 47.1 11 1.0 37.8poultry fat66 22.2 8.4 5.1 42.3 19.3 1.0 35.7lard67 1.7 17.3 1.9 15.6 42.5 9.2 0.4 34.6edible tallow68 4.8 28.4 14.8 44.6 2.7 52.0yellow grease8 2.4 23.2 3.8 13.0 44.3 7.0 0.7 38.6brown grease8 1.7 22.8 3.1 12.5 42.4 12.1 0.8 37.0

a 14:00, the alkyl chain contains 14 carbons and zero double bonds.

Figure 1. Transesterification reactions of glycerides with metha-nol.

Figure 2. Formation of the active species in transesterificationreactions using a base catalyst.

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are no previous reviews in the area of biodiesel synthesisthat primarily tackle the issue of acid-catalyzed reac-tions, the development of acid-catalyzed methodologiesis the focus of this paper. In an attempt to be complete,the second part of this paper also includes a review of work relating to heterogeneously acid-catalyzed reac-tions due to the great potential that solid acids have inthis field.

The Feedstock Issue

As previously mentioned, biodiesel can be prepared

from a variety of sources including vegetable oils, animalfats, and waste greases. Currently, refined vegetable oilsare a major feedstock for biodiesel production. However,waste greases, such as yellow grease from used cookingoils and animal fats, can also be employed because of their availability and low cost. It must be mentionedthat using waste greases to produce biodiesel allows fora convenient route to recycle waste oils that otherwisecould only be sold as a low-value animal feed additive.However, waste greases and animal fats are consideredlow-quality feedstocks compared to refined vegetable oilsin terms of FFA content. For instance, yellow grease,obtained from rendered animal fats and restaurantwaste oil, has FFA levels up to 15 wt %, and its price varies from $0.09 to $0.20/lb.4 Another low-cost waste

grease, brown grease, obtained mainly from trapsinstalled in commercial, municipal, or industrial sewagefacilities, has a free fatty acid level greater than 15 wt% and sells for $0.01-0.07/lb less than yellow grease.Because of their price, use of waste grease feedstockshas been proposed as a way to lower biodiesel produc-tion costs. In fact, some plants in the United States arealready producing biodiesel from yellow grease.5

Predictably, low-cost feedstocks must undergo someform of pretreatment before they can be used forbiodiesel production. Animal fats, for instance, oftencontain meat and bone particles, as well as other typesof organic matter. Commonly, the particulate matter inthese greases is removed using a cellulose filter. Otherpretreatment steps include water removal, steam distil-lation, and bleaching.6 Water removal via gravityseparation in a horizontal tank is normally sufficientto reduce the concentration of water in the waste greaseto the acceptable level of 0.5 wt %.7 This drying step isessential since yellow grease from used cooking oils maycontain up to 3 wt % water. In addition to waterremoval, the steam distillation process denatures anddegrades residual proteins in the grease, and bleachingremoves spoiled proteins.

Even though animal fats and waste greases constitutesources of inexpensive feedstocks for biodiesel, theirhigh concentrations of FFAs make them inappropriatefor the conventional direct base-catalyzed transesteri-fication route to biodiesel due to soap formation. How-

ever, an alternative multistep process allows the use of feedstocks having high FFA concentrations by firstcarrying out the acid-catalyzed pre-esterification of theFFAs prior to the base-catalyzed TG transesterifica-tion.8,9 The combination of acid-catalyzed FFA pre-esterification followed by base-catalyzed TG transes-terification is commonly called the integrated process.Despite the added cost of production, the integratedprocess is being increasingly applied to produce biodieselfrom low-cost but high-FFA feedstocks with good re-sults.

There are characteristics of biodiesel that are a direct

consequence of the long alkyl chains associated with thealkyl esters that constitute the biofuel and stronglydepend on the feedstocks used. For instance, dependingon feedstock composition, biodiesel and its blends in coldclimates can suffer significantly from high pour points,cloud points, and cold filter plugging points.10 In par-ticular, these problems are more apparent when thesources of biodiesel feedstocks are animal fats. Here,the more saturated character of the fatty acid composi-tion of animal fats (Table 3) is transferred to the alkylesters (biodiesel) obtained. In contrast, when biodieselis prepared from vegetable oils, it mainly contains estersof oleic and linoleic acids, which have unsaturated alkylchains. In general, the unsaturation degree of the alkylchains correlates well with the cold flow performanceof biodiesel. This means that the more double bonds onthe alkyl chain, the lower the melting point of the estersobtained and, therefore, the colder it can get withoutexhibiting undesirable high viscosity or solidifying.

The problems outlined above could severely limit theuse of biodiesel prepared from animal fats. However,the use of additives to improve the flow properties of biodiesel at low temperatures has been proposed as acounter measure for this problem.11 This topic will beexpanded later in the heterogeneous-catalyzed glyceroletherification section.

Homogeneous Acid-Catalyzed Reactions

The liquid acid-catalyzed transesterification processdoes not enjoy the same popularity in commercialapplications as its counterpart, the base-catalyzedprocess. The fact that the homogeneous acid-catalyzedreaction is about 4000 times slower than the homoge-neous base-catalyzed reaction has been one of the mainreasons.12 However, acid-catalyzed transesterificationshold an important advantage with respect to base-catalyzed ones: the performance of the acid catalyst isnot strongly affected by the presence of FFAs in thefeedstock. In fact, acid catalysts can simultaneouslycatalyze both esterification and transesterification.Thus, a great-advantage with acid catalysts is that theycan directly produce biodiesel from low-cost lipid feed-stocks, generally associated with high FFA concentra-

Figure 3. (a) Base catalyst reaction with FFAs to produce soap and water, both undesirable byproducts. (b) Water promotes the formationof FFAs. These, then, can deactivate the catalyst and produce soap, as in (a).

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tions (low-cost feedstocks, such as used cooking oil andgreases, commonly have FFAs levels of g6%). Recently,it has been shown how acid-catalyzed production of biodiesel can economically compete with base-catalyzedprocesses using virgin oils, especially when the formeruses low-cost feedstocks.13,14 A simplified block flowdiagram (BFD) of a typical acid-catalyzed process isshown in Figure 4 illustrating the most important stepsin biodiesel production.13

As already mentioned, the transesterification of TGs,catalyzed by either bases or acids, consists of three

consecutive, reversible reactions. In the reaction se-quence, TG is converted stepwise to diglyceride, mono-glyceride, and finally glycerol accompanied with theliberation of an ester at each step (Figure 1). Thetransesterification chemical pathway shown in Figure5, for an acid-catalyzed reaction, indicates how in thecatalyst-substrate interaction the key step is the pro-tonation of the carbonyl oxygen. This in turn increasesthe electrophilicity of the adjoining carbon atom, makingit more susceptible to nucleophilic attack. In contrast,base catalysis takes on a more direct route, creating firstan alkoxide ion, which directly acts as a strong nucleo-phile, giving rise to a different chemical pathway forthe reaction (Figure 6). This crucial difference, i.e., theformation of a more electrophilic species (acid catalysis) versus that of a stronger nucleophile (base catalysis),is ultimately responsible for the observed differences inactivity.

To date, most studies have focused on base-catalyzedtransesterification of TGs. For acid-catalyzed systems,sulfuric acid has been the most investigated catalyst,but other acids, such as HCl, BF3, H3PO4, and organicsulfonic acids, have also been used by different research-ers.15 In a pioneering work, Freedman et al. examinedthe transesterification kinetics of soybean oil withbutanol, using sulfuric acid as the catalyst.16 They foundthat the rate-limiting step varied over time, and threeregimes, in accordance with the observed reaction rate,could categorize the overall reaction process. Initially,

the reaction was characterized by a mass-transfer-controlled regime that resulted from the low miscibilityof the catalyst and reagents; i.e., the nonpolar oil phasewas separated from the polar alcohol-acid phase. Asthe reaction proceeded and ester products acted asemulsifiers, a second-rate regime emerged, which waskinetically controlled and characterized by a suddensurge in product formation. Finally, the last regime wasreached once equilibrium was approached near reactioncompletion. In addition, these authors established thata large molar ratio of alcohol-to-oil, 30:1, was needed tohave acceptable reaction rates. In this way, for the

Figure 4. Simplified BFD of the acid-catalyzed process. The diagram includes the following: feedstock pretreatment (1), catalystpreparation (2), transesterification and esterification (3), alcohol recycle (4), acid catalyst removal (5), and biodiesel separation andpurification process (6).

Figure 5. Homogeneous acid-catalyzed reaction mechanism forthe transesterification of triglycerides: (1) protonation of thecarbonyl group by the acid catalyst; (2) nucleophilic attack of thealcohol, forming a tetrahedral intermediate; (3) proton migrationand breakdown of the intermediate. The sequence is repeatedtwice.

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transesterification reaction (Figure 1), the forwardreactions followed pseudo-first-order kinetics, while thereverse reactions exhibited second-order kinetics.

In a continuing effort to decrease biodiesel productioncosts, researchers have attempted to ascertain whatoptimum ratio of alcohol-to-TG should be used. Two

factors must be considered during this optimizationprocess. First, increasing the molar ratio of alcohol-to-TG increases as well alcohol recovery and productseparation costs. Second, the acid-catalyzed transes-terification achieves greater and faster conversions athigh alcohol concentrations. To help identify an opti-mum alcohol-to-TG ratio, Canakci and Van Gerpenstudied how the reagent molar ratio affected reactionrates and product yield in the transmethylation of soybean oil by sulfuric acid.7 Five different molar ratios,from 3.3:1 to 30:1, were studied. Their results indicatedthat ester formation increased with increasing the molarratio, reaching its highest value, 98.4%, at the highestmolar ratio used, 30:1. However, the benefits fromhigher alcohol-to-TG molar ratios became limited withincreasing ratio, ester formation increased sharply from77% at 3.3:1 to 87.8% at 6:1 and ultimately reaching aplateau value of 98.4% at 30:1. In a related study,Crabbe et al. also determined the effect of molar ratiowithin the range of 3:1-23:1 and concluded that thehighest molar ratio required for complete transmeth-ylation could be found between 35:1 and 45:1 byextrapolation.17 However, neither group determined anoptimum alcohol-to-TG molar ratio based on esterformation and separation efficiency.

Alcohols used in acid-catalyzed transesterificationhave included methanol, ethanol, propanol, butanol, andamyl alcohol. Methanol and ethanol are used mostfrequently in both laboratory research and the biodiesel

industry.18,19 As already mentioned, the low cost of methanol makes it the first choice for the transesteri-fication reaction. Ethanol, however, is derived fromagriculture products (renewable sources) and biologi-cally less objectionable to the environment than metha-nol; thus, ethanol is the ideal candidate for the synthesisof a fully biogenerated fuel.18 Nonetheless, working withhigher molecular weight alcohols, such as butanol,brings about interesting advantages. For instance,butanol has better miscibility with the lipid feedstockthan smaller alcohols, contributing to a less pronouncedinitial mass-transfer-controlled regime. Additionally, theelevated boiling points of larger alcohols enable theliquid reaction system to be operated at higher temper-atures while maintaining moderate pressures. This isan important issue with the acid-catalyzed transesteri-fication since higher reaction temperatures are oftenrequired to achieve faster reaction rates. However, onlya limited number of papers have actually addressed thesubject of the alcohol characteristics. Nye et al. com-pared the series of linear alcohols from methanol tobutanol to find the most suitable for the transesterifi-cation of waste frying oil.20 All reactions were carriedout using 0.1% sulfuric acid at reflux temperatures with

all other conditions kept constant. Butanol yielded thehighest reaction rates and conversion followed by 1-pro-panol and then ethanol. Even though acid-catalyzedreactions carried out with methanol proved to be theslowest, the reverse was true for the base-catalyzedprocess. These results suggest that initial reagent phasemiscibility was more critical in acid catalysis than inbase catalysis. Two factors could have affected phasemiscibility: the increased hydrophobicity associatedwith larger alcohols and the higher reaction tempera-ture that was applied as the alcohol molecular weightincreased.

Thus, temperature plays an important role in theacid-catalyzed synthesis of biodiesel.15,21 For instance,

in the butanolysis of soybean oil catalyzed by 1 wt %H2SO4, five different temperatures from 77 to 117 °Cwere examined.16 Increasing temperature had a markedeffect on reaction rate with near complete conversionof TGs requiring only 3 h at 117 °C, while comparableconversions at 77 °C required 20 h. At higher temper-atures, the extent of phase separation decreases andrate constants increase, due to the higher temperatureas well as improved miscibility, leading to substantiallyshortened reaction times. The effect of temperature iseven more noticeable at higher temperatures and pres-sures. In particular, at 240 °C and 70 bar, using 1.7 wt% H2SO4, ester conversions greater than 90% could beobtained in only 15 min.22 Under such conditions, high-FFA feedstocks (e.g., 44 wt % FFAs) could easily be

transformed with continuous water removal. However,side reactions such as alcohol etherification could alsobe observed under such harsh conditions.

Reaction rates in acid-catalyzed processes may alsobe increased by the use of larger amounts of catalyst.Typically, catalyst concentrations in the reaction mix-ture have ranged between 1 and 5 wt % in mostacademic studies using sulfuric acid.16,21,23 Canakci and Van Gerpen used different amounts of sulfuric acid (1,3, and 5 wt %) in the transesterification of grease withmethanol.7 In these studies, a rate enhancement wasobserved with the increased amounts of catalyst, andester yield went from 72.7 to 95.0% as the catalystconcentration was increased from 1 to 5 wt %. The

Figure 6. Homogeneous base-catalyzed reaction mechanism forthe transesterification of TGs: (1) production of the active species,RO-; (2) nucleophilic attack of RO- to carbonyl group on TG,forming of a tetrahedral intermediate; (3) intermediate breakdown;(4) regeneration of the RO- active species. The sequence isrepeated twice.

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dependence of reaction rate on catalyst concentrationhas been further verified by the same authors and othergroups.9,17 It is known, however, that large quantitiesof acid catalyst can promote ether formation by alcoholdehydration.24 A further complication of working withhigh acid catalyst concentrations becomes apparentduring the catalyst neutralization process, which pre-cedes product separation. Since CaO addition duringneutralization is proportional to the concentration of acid needed in the reactor, high acid concentrations leadto increased CaO cost, greater waste formation, andhigher production cost.

As previously mentioned, biodiesel production cost canbe reduced by using low-cost feedstocks with high FFAcontents. However, the acid-catalyzed conversion of low-cost feedstocks leads to the formation of significantquantities of water, which has a negative effect onbiodiesel production since water can hydrolyze the esterproducts, producing (again) FFAs. Though ester hy-drolysis can occur by either acid or base catalysis, acidcatalysis is generally believed to be more tolerant of moisture and high FFA levels in the starting feedstockand, hence, more suitable for low-grade fats andgreases.15,19,21 In an effort to quantify the effect of water

on sulfuric acid-catalyzed formation of biodiesel, Canak-ci and Van Gerpen investigated the sensitivity of esterforming reactions to water and the FFAs levels presentin soybean oil.7 Ester production was affected by as littleas 0.1 wt % water concentration and was almost totallyinhibited when the water level reached 5 wt %. Theseauthors established that water content has to be keptunder 0.5 wt % to achieve a 90% ester yield under theirreaction conditions. Recently, Kusdiana and Saka stud-ied the effect of water on methyl ester formation by thetransesterification of rapeseed oil with methanol usingdifferent catalysts.25 Reactions were conducted at thesame conditions except for the catalyst amount (1.5 wt% NaOH, 3 wt % H2SO4) and reaction time (1 h for the

alkali, 48 h for the acid). These studies found that waterconcentration was more critical in acid catalysis thanin base catalysis. In the acid-catalyzed reaction, esterconversion was reduced to 6% when 5 wt % water wasused in the starting reagent mixture. In contrast, thealkali-catalyzed reaction was only slightly affected bythe presence of water, and ester conversion was ap-proximately 70% with an equivalent amount of waterin the reaction mixture, in agreement with the resultsof Canakci and Van Gerpen.7,9 Kusdiana and Saka alsostudied the esterification of FFAs.25 As expected, thealkaline catalyst was completely saponified, but theacid-catalyzed esterification showed excellent toleranceto water, maintaining an almost constant ester yield atthe levels of water added. Unfortunately, neither Kus-

diana and Saka nor Canakci and Van Gerpen addressedthe differences in the observed sensitivity to water fortransesterification and esterification.

In related work, Sridharan and Mathai noticed thatthe transesterification of small esters was retarded bythe presence of spectator polar compounds.26 Accordingto their findings, the presence of polar compoundsduring acid-catalyzed alcoholysis reactions significantlyreduced reaction rates. These authors attributed thisretardant effect to the interference that polar com-pounds pose to the reaction by competing for hydrogenions, hindering the availability of these ions for cataly-sis. Water exerts similar effects in the transesterifica-tion of TGs. Considering the strong affinity that sulfuric

acid has for water, it is likely that the acid will interactmore strongly with water molecules than with alcoholmolecules. Thus, if water is present in the feedstock orproduced during the reaction, the acid catalyst willpreferentially bind to water, leading to a reversible typeof catalyst deactivation.

There are, therefore, two aspects that affect catalystaccessibility to the TG molecules. The first has beenpreviously discussed and has to do with macroscopic

phase separation evidenced by the initial mass-transfer-controlled regime that can seriously hinder reactionrate.16 In fact, when cosolvents, such as tetrahydrofuran(THF), are used to counteract the miscibility problem,the methanolysis rate is known to dramatically in-crease.27,28 The other less understood aspect that seemsto affect acid catalysis is the effect that water has oncatalyst activity. Water not only can bind to acid speciesin solution (H+) more effectively than the alcohol (givingrise to a weaker acid) but also, with increasing waterconcentrations, can give rise to water-rich clustersaround protons.29 These extra molecules of water aroundthe catalyst could shield it from the hydrophobic TGmolecules, inhibiting the reaction. Hydration of the acidcatalyst should have a more pronounced effect in

transesterification than in esterification, given thelarger and more hydrophobic compounds involved in theformer (TGs vs FFAs). The polar carboxylic groups inFFAs and their capacity to easily interact throughhydrogen bonds with water molecules facilitate theFFAs-catalyst interaction and, therefore, esterification.

As mentioned earlier, the acid-catalyzed pre-esteri-fication of FFAs is more commonly used to decrease FFAlevels in high-FFA feedstocks, before they can be usedin the base-catalyzed transesterification. In fact, someacid catalysts, i.e., BF3 (a strong Lewis acid), have suchesterifying capacity that a method combining the alka-line hydrolysis of feedstocks and the subsequent BF3-catalyzed esterification of FFAs has been proposed to

prepare biodiesel in relatively short times.15

In practice,esterification is subjected to thermodynamic limitations,and water removal is desirable to obtain good conversionrates and push the reaction to completion. Unfortu-nately, calculations of average equilibrium constants inthese type of systems have not been reported given thepresence of a wide range of free fatty acids (C8-C24).

Canakci and Van Gerpen recently applied a two-stepsulfuric acid-catalyzed pre-esterification process of FFAsin used cooking oil to reduce FFA levels to below 1 wt%.9 The low FFA concentration was required for thesubsequent alkaline-catalyzed transesterification. TheFFA pre-esterification step was preceded and followedby a relatively slow (∼24 h) water removal process thatemployed large settling tanks as splitters. The use of gravity separation for water removal is lengthy andother methodologies have also been proposed for thistask.13,30,31 However, in most cases gravity separationin settling tanks is preferred due to economical reasonsarising from lower process utility costs.

The acid-catalyzed esterification follows a mechanisticscheme similar to transesterification. Accordingly, in-stead of starting with a TG molecule, as in the trans-esterification reaction (Figure 5), the starting moleculeis a FFA. Again, the key catalyst-FFA interaction isthe protonation of the carboxylic moiety in the FFA. Inthis case, the important factor influencing the reactionrate at the molecular scale is the steric hindranceinherent to both the carboxylic acid and the alcohol

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species.30 For example, in the esterification of palmiticacid with either methanol or ethanol, ethanol was foundto be less efficient than methanol.9

Heterogeneous Acid Catalysis

Homogeneous catalysts, although effective, lead toserious contamination problems that make essential theimplementation of good separation and product purifi-cation protocols, which translate into higher production

costs. To be economically viable and to compete com-mercially with petroleum-based diesel fuel, processes forthe synthesis of biodiesel need to involve continuousprocessing in a flow system, have as few reaction stepsas possible, limit the number of separation processes,and ideally use a robust heterogeneous (solid) catalyst.The appropriate solid catalysts could be easily incorpo-rated into a packed bed continuous flow reactor, sim-plifying product separation and purification and reduc-ing waste generation. In this section, three aspects of solid acid catalysis for biodiesel production will bereviewed. The first section deals with the transesteri-fication by solid acid catalysts of esters with alcohols,the second topic relates to the solid acid-catalyzed

esterification of FFAs, while the third deals with solidacid-catalyzed etherification of glycerol.1. Transesterification. Research on the direct trans-

esterification of lipid feedstocks into biodiesel by solidacid catalysts has not been explored as one might expectgiven the amount and variety of catalytic materials nowavailable. Actually, less than a handful of studies haveexplored the topic and without decisive conclusionsabout its potential. Even fundamental studies dealingwith the reaction of model compounds of TGs on solidacids are missing. Low expectations in terms of reactionrates and probable undesired side reactions have dis-couraged more significant research efforts into thissubject to date.

In one of the few published works dealing with the

transesterification of TG feedstocks using solid acidcatalysts, Mittelbach et al. compared the activities of aseries of layered aluminosilicates with sulfuric acid forthe transesterification of rapeseed oil.32 These research-ers used an initial molar ratio of 30:1 alcohol-to-oil and5 wt % catalyst. Among the catalysts tested, sulfuricacid showed the highest activity. The solid catalystsshowed varied activities depending on reaction condi-tions. The most active catalysts were activated bysulfuric acid impregnation. For instance, activatedmontmorillonite KSF showed a 100% conversion after4 h of reaction at 220 °C and 52 bar. However, leachingof sulfate species compromised the reusability of thisclay. Thus, to maintain clay activity at constant values,sulfuric acid re-impregnation had to be carried out aftereach run. It is also likely that some degree of homoge-neous catalysis was taking place due to sulfuric acidleaching.

Attempts to prepare other solid acid catalysts withhigh activities for transesterification have been re-ported, as well. In particular, Kaita et al. designedaluminum phosphate catalysts with various metal-to-phosphoric acid molar ratios (1:3-1:0.01) and used thesematerials for the transesterification of kernel oil withmethanol.33 According to the authors, durable andthermostable catalysts were obtained with good reactiv-ity and selectivity to methyl esters. However, the useof these materials still needed high temperatures (200°C) and high methanol-to-oil molar ratios (60:1) in order

to be effective. In a related study, Waghoo et al. reportedon the transesterification of ethyl acetate with severalalcohols over hydrous tin oxide to obtain larger esters.Linear and aromatic alcohols were tested in a temper-

ature range of 170-210 °C. All reactions were com-pletely selective for transesterification.34 In particular,this catalyst presented an appreciable activity for reac-tions involving n-butyl alcohol, n-octyl alcohol, andbenzyl alcohol.

Amberlyst-15 has also been studied for transesteri-fication reactions. However, mild reaction conditions arenecessary to avoid degradation of the catalyst. At arelatively low temperature (60 °C), the conversion of sunflower oil was reported to be only 0.7%, whencarrying out the reaction at atmospheric pressure anda 6:1 methanol-to-oil initial molar ratio.35

On a related subject, Bronsted solid acids have alsobeen proposed for the transesterification of β-ketoestersto produce precursors for pheromones36 and othernatural products (Figure 7).37 Among the catalysts usedwere Amberlyst-15,38 Envirocat EPZG,39 natural ka-olinite clay,39,40 B2O3 /ZrO2,36 sulfated SnO2,41 and zeo-lites.42 Even though β-ketoesters usually show a higherreactivity than simple esters for transesterification,under the right reaction conditions, a catalyst active forthe transesterification of β-ketoesters could be effectivein the transesterification of other types of esters as well.

2. Esterification. The esterification of carboxylicacids by solid acid catalysts is important consideringthat low-cost lipid feedstock contains high concentra-tions of FFAs. Therefore, it is expected that a good solidacid catalyst must be able to carry out simultaneouslyboth esterification and transesterification. Since esteri-

fication and transesterification share a common molec-ular pathway, evidence about catalyst reactivity foresterification also provides evidence about transesteri-fication and vice versa. Thus, works cited in this sectionabout esterification catalyzed by solid acids support theidea that heterogeneously acid-catalyzed transesterifi-cation of TGs should also be possible with these typesof catalysts.

In addition, the reader should be aware that, althoughthe objectives of most works cited here have not beenespecially concerned with biodiesel synthesis, the factthat all carboxylic acids (in our specific case FFAs) sharethe same chemical functionality makes these resultsrelevant to biodiesel synthesis via esterification of FFAsusing solid acids.

Currently, the esterification of fatty acids with alco-hols is commercially achieved using liquid catalysts,such as sulfuric acid, hydrofluoric acid, and p-toluene-sulfonic acid. The scientific literature contains a goodnumber of reports about the use of heterogeneous acidcatalysts for esterification. For instance, esterificationhas been carried out using ion-exchange resins such as Amberlyst-15 and Nafion with good results.43,44 Ingeneral, when using organic resins, the catalytic activitystrongly depends on their swelling properties. Resinswelling capacity is fundamental since it controls sub-strate accessibility to the acid sites and, therefore,affects its overall reactivity. Once swelled, the resinpores usually become macropores. This means that big

Figure 7. Transesterification of β-ketoesters.

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molecules with long hydrocarbon chains show no diffu-sion limitations and can readily access the acid sites inthe bulk.45 However, most ion-exchange resins are notstable at temperatures above 140 °C, which prohibitstheir application to reactions that require higher tem-peratures. For this kind of application, inorganic acidcatalysts are generally more suitable.

Among the different types of inorganic solids thathave been used to produce esters, the most popular have

been zeolites. Different characteristics make zeolitesexcellent catalysts for organic syntheses in general. Forinstance, zeolites can be synthesized with differentcrystal structures, pore sizes, framework Si/Al ratios,and proton-exchange levels. These characteristics per-mit tailoring important catalytic properties, such as acidstrength. In zeolites, the acid strength can be adjustedsuch that it fits the reaction requirements.46 Too littleacidity and the reaction may not proceed at a reasonablerate under the chosen conditions. Too high acidity anddeactivation by coking may occur or undesirable byprod-ucts might be formed requiring additional and expensiveseparation procedures. For esterification, however, op-timum acidity has yet to be established. In addition,zeolites provide the possibility to choose among different

pore structures and surface hydrophobicity, accordingto the substrate’s size and polarity. For esterification,the zeolite that is more suitable for a specific reactiondepends on the polarity and miscibility of the acid andalcohol reagents. Nevertheless, when using zeolites,mass-transfer resistance becomes a critical issue dueto their microporous nature. In general, zeolite catalysisof reactions using large molecules takes place on theexternal surface of zeolite crystals. Zeolites are activecatalysts for the esterification of large carboxylic acids,but they catalyze the reaction rather slowly. Thus, onlylarge-pore zeolites have been used with any success infatty acid esterifications.47 But even in those successfulcases, the reaction always gives a variety of undesired

byproducts due to the high reaction temperature used.In general, the catalytic activity of zeolites for theesterification of fatty acids increases with increasing Si/ Al ratio, indicating that reactivity is influenced by acidsite strength as well as surface hydrophobicity. Inconclusion, pore size, dimensionality of the channelsystem (related to the diffusion of reagents and prod-ucts), and aluminum content of the zeolite frameworkstrongly affect the zeolite’s catalytic activity for esteri-fication.

Related to zeolites, but with amorphous pore walls,silica molecular sieves, such as MCM-41 mesoporousmaterials, are generally not sufficiently acidic to cata-lyze esterification reactions due to their pure silicastructure. Introducing aluminum, zirconium, titanium,or tin compounds into the silica matrix of these solidscan significantly improve their acid properties. However,metal-doped materials behave more like weak acids andcan only be used for reactions that do not require astrong acid catalyst. For instance, the catalytic activityof Al-MCM-41 in the esterification of oleic acid withglycerol was found to be significantly lower than thatof zeolite beta with a similar Si/Al ratio.48 To enhancethe catalytic activity while keeping the benefits of largepore diameters, strong acid species have been intro-duced to the pore interior of MCM-like solids. Inparticular, MCM-41-supported heteropoly acids (HPAs)have been used as catalysts in the gas-phase esterifi-cation of acetic acid and 1-butanol.49 These catalysts

showed good activity at 110 °C (95% conversion of 1-butanol). As expected, MCM-41-supported HPA wasmore active than pure HPA. The enhanced activity couldbe ascribed to a high dispersion of the HPA on theMCM-41 internal surface, giving rise to a higher popu-lation of available acid sites than in pure HPA. How-ever, the supported catalyst was considerably morehydrophilic than the original HPA. Water formationcaused HPA migration from the MCM-41 pores to theouter surface, facilitating the sintering of HPA species.This was verified by measuring spent catalyst activities,which significantly decreased with catalyst reuse.

Other composite catalysts, made using silica meso-porous materials modified with sulfonic groups, wereused in the pre-esterification of mixtures of FFAs andsoybean oil. Mbaraka et al. found that the activity of these hybrid mesoporous silicas was highly dependenton their pore dimensions and probably on their hydro-phobic character as well.50 Pore dimensions stronglyaffected reagent diffusion, especially problematic formedium and small pore size materials. Acid strengthalso played an important role in these reactions. Forinstance, catalysts prepared from a stronger acid pre-cursor containing benzenesulfonic acid groups were

more active than those containing only propylsulfonicacid groups. Indeed, catalysts with a medium porediameter of 50 Å and benzenesulfonic acid groupsshowed activities comparable to sulfuric acid.

Recently, sulfated zirconia (SO4 /ZrO2) has been shownto have applicability for several acid-catalyzed reac-tions.51 For esterification, SO4 /ZrO2 has shown somepromise as an active catalyst due to its high acidstrength; however, SO4 /ZrO2 deactivates due to sulfateleaching enhanced by hydrolysis.52 Sulfate groups canleach out as H2SO4 and HSO4

-,46 which in turn can giverise to homogeneous acid catalysis, interfering withmeasurements of the heterogeneous catalytic activity.To overcome the susceptibility of SO4 /ZrO2 to water and

to improve its general characteristics, new preparationsof SO4 /ZrO2 have recently been proposed. For instance, Yadav and Murkute published a new route to preparingSO4 /ZrO2 with higher sulfate loadings and resistant tosulfate leaching by hydrolysis.53 This catalyst wasprepared using a chlorosulfonic acid precursor dissolvedin an organic solvent, instead of the conventionalsulfuric acid impregnation. The prepared SO4 /ZrO2exhibited higher catalytic activity for esterification thanthe conventionally prepared SO4 /ZrO2 and no sulfateleaching was observed. Additionally, the catalyst dem-onstrated good retention of its activity for subsequentexperiments.

Sulfated tin oxide (SO42- /SnO2), prepared from m-

stannic acid, has shown activity superior to that of SO4 / ZrO2 for the esterification of n-octanoic acid withmethanol at temperatures below 150 °C due to itssuperior acid strength.54 However, a more widespreadtesting of SO4

2- /SnO2 has not been carried out due topreparation inadequacies of the synthetic route initiallyproposed to prepare this solid. New synthetic routes aremaking SO4

2- /SnO2 more accessible and additionalstudies are expected with this promising materialsoon.55

The catalytic activities of hafnium and zirconium saltshave been investigated for the esterification of carboxy-lic acids with primary and secondary alcohol in equimo-lar ratios.56,57 In general, for esterification an equimolarratio of reactants is preferred instead of excess alcohol.

H



Equimolar ratios can reduce waste and simplify productseparation protocols, providing not only environmentalbut also economic benefits. In particular, hydrouszirconium oxide showed good activity and selectivity foresterification. The low acid strength of this catalysthelps to avoid undesirable side reactions, such as alcoholdehydration. In addition, the hydrated oxide was notsensitive to water. Thus, esterification did not requirewater-free conditions, an important characteristic giventhat water is a reaction byproduct.

3. Glycerol Etherification. An issue some authorshave been recently exploring is the use of glycerol-basedadditives to improve the flow properties of biodiesel atlow temperatures. For instance, glycerol ethers havebeen prepared using an olefin such as isobutylene anda solid acid catalyst like Amberlyst-15.58 Furthermore,Noureddini proposed a two-step process comprising thetransesterification of soybean oil using a homogeneousbase catalyst and the separated etherification of glycerolbyproduct with isobutylene or isoamylene using Am-berlyst-15.11 Afterward, the obtained mono-, di-, andtributyl ethers of glycerol were blended back with thepreviously prepared biodiesel, resulting in a final fuelwith lower viscosity, cloud points, and pour points than

standard biodiesel, closely resembling a petroleum-based diesel. This approach not only makes good use of the reaction byproduct glycerol but potentially also couldincrease the fuel yield by approximately 20 vol %.However, a more appealing process would conduct theglycerol etherification in situ, without requiring glycerolseparation and subsequent back blending. The basicproblem with this is that using olefins and an alcohol,such as methanol, in the presence of a strong acidcatalyst could result in methanol olefin etherification.59

Another possible hurdle to in situ etherification of glycerol could be the tendency of olefins to polymerize,giving rise to catalyst deactivation in the case of solidacids and formation of gums when using liquid acids.On the other hand, an olefin may not be needed to carry

out the etherification of glycerol. Ironically, the use of sulfuric acid as the catalyst at temperatures above 100°C has received some criticism due to the formation of byproducts such as ethers of glycerol and methanol.However, formation of such ethers could be taken as apositive side reaction, rather than something thatshould be avoided. Indeed, more detailed studies dealingwith the issue of glycerol ether additives to improve theflow properties of biodiesel and their synthesis arerequired.

Conclusions

Despite all the promising characteristics related tobiodiesel, such as its low emission profile and therenewable character of its feedstocks, biodiesel is notyet the solution to any current fuel crisis. Several factorsplay a role here. On one hand, current biodiesel produc-tion (about 2% of total diesel production in the UnitedStates) is not sufficient to make an important impacton fuel markets. On the other hand, biodiesel productioncosts are still rather high compared to costs for produc-ing petroleum-based diesel fuel (price for biodiesel notcounting tax breaks is about $2.02/gallon depending onthe feedstock used for its preparation. The price forpetroleum-based diesel is about $1.87/gallon).60,61 How-ever, even if biodiesel is not the total solution to anyenergy crisis, it certainly is an important component of a combined strategic approach to decrease our currentdependence on fossil fuels.62

To lower prices and to make biodiesel competitivewith petroleum-based diesel, less-expensive feedstockssuch as waste greases have to be used in its production.Indeed, the homogeneous acid-catalyzed process candeal with high-FFA feedstocks, carrying out simulta-neously both esterification and transesterification, pro- vided that water is removed during processing. Other-wise, increasing water concentrations can seriously hurtreaction conversions. An efficient way to remove wateris achieved by conducting feedstock transformation attemperatures above 100 °C at moderate pressures andwith continuous flow of an inert gas such as nitrogen.This way, with continuous alcohol feed, the in situseparation of water by water/alcohol coevaporation helpsto drive the reaction toward high conversions usingrelatively low-cost means.

Solid acid catalysts have the capacity to replace strongliquid acids, eliminating corrosion problems and theenvironmental threat posed by them. However, researchdealing with the use of solid acid catalysts for biodieselsynthesis has been limited due to pessimistic expecta-tions about reaction rates and unfavorable side reac-tions. In general, the use of solid catalysts to producebiodiesel requires a better understanding of the factors

that govern their reactivity. Thus far, it seems that anideal solid acid catalyst should show some underlyingcharacteristics, such as an interconnected system of large pores, a moderate to high concentration of strongacid sites, and a hydrophobic surface. The former tworequirements, the interconnected pore system of largepores and a high population of strong acid sites, are verymuch evident. Large interconnected pores would mini-mize diffusion problems of molecules having long alkylchains, and strong acid sites are needed for the reactionto proceed at an acceptable rate. The latter requirement,having a hydrophobic surface, is essential to promotepreferential adsorption of oily hydrophobic species onthe catalyst surface and to avoid possible deactivationof catalytic sites by the strong adsorption of polar

byproducts, such as glycerol and water. More research,especially of a fundamental nature using model com-pounds, is required in order to better delineate esteri-fication and transesterification reactions on solid acidcatalysts. The results of such studies would aid in thedesign of more active solid acid catalysts, certainlyimpacting in a positive way biodiesel synthesis tech-nologies.

Acknowledgment

The authors acknowledge financial support for thiswork by the U. S. Department of Agriculture underGrant 68-3A75-3-147 and by the Fats and Proteins

Research Foundation.

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Received for review September 3, 2004 Revised manuscript received November 4, 2004

Accepted November 6, 2004

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