Heterogeneous acid-catalysed transesterification of TGs has not been investigated as much as the base-catalysed reaction (see Table 8.18). Solid — acid catalysis as an active research area for biodiesel synthesis has been largely ignored due to pessimistic expectations in terms of reaction rates and undesired side reactions. Even though solid-acid catalysts have been applied effectively in the esterification of carboxylic acids, the use of these catalysts to obtain high conversion of triglycerides to biodiesel generally necessitates much higher reaction temperatures than base catalysts because of their lower activity for transesterification [198]. Amberlyst®15 is a notable exception. Fundamental studies dealing with the reaction of model compounds of TGs on solid acids are lacking.

One important problem with acid catalysts in general is their lower reaction rates compared to base catalysts. Even lower rates can be expected for solid acids as compared to homogeneous acid catalysts on the same weight basis. Thus, generally, solid-acid catalysts need high temperatures, high catalyst loading, and/or large reactors in order to attain satisfactory biodiesel production rates. The reaction mechanism for heterogeneous acid-catalysed esterification is in principle similar to the homogeneously catalysed one [199].

Another main concern expressed has been the possibility of unwanted side reactions, in particular double dehydration of glycerol to form acrolein and water. However, this acid-catalysed reaction becomes important only above about 523 K [20]. On the other hand, attractive aspects are the ability of solid acids to catalyse both esterification (see Section 9.2) and transesterification reactions simultaneously and the possibility of reuse. Solid-acid catalysts eliminate the corrosion problems and environmental threat of liquid acids. Solid acids have the advantage of being easily removable by filtration and can be used for large-scale production [200]. Rohm & Haas has recently developed a commercial esterification/transesterification process for the conversion of high-acid feedstocks using Amberlyst™ BD20 solid-catalyst technology (see Section 15.3.1, pp. 732-6).

General requirements for a good solid-acid catalyst for biodiesel synthesis by (trans)esterification are appropriate acid site strength, high acid concentration (to secure an acceptable reaction rate), an interconnected system of large pores

aWithout co-solvent. bWith co-solvent (DMSO). cBabassu, CRO, PKO, PMO, SBO. dEsterification. ePre-esterification step. fPalmitic acid. gHigh FFA content. hMicrowave-enhanced.

(in order to minimise diffusion problems), hydrophobic surface (to promote preferential absorption of oily species and avoid deactivation by polar by­products, such as water and glycerol) and low cost. Factors promoting organic reactions catalysed over solid acids comprise the strength of the acid sites, the Br0nsted or Lewis nature, and active site location [211]. In comparison to liquid acids, solid acids encompass different populations of acid sites varying in nature and strength. In a large number of solid acids the acidity varies from mild to superacid level. Conflicting reports in the literature about the strength and nature of the acid sites intervening in catalysis may derive from poor material’s characterisation or from the use of different reaction conditions. The acidity can be tuned and adapted to the particular reaction thus largely avoiding undesirable reactions from occurring.

The main problems for solid-acid catalysts concern the diffusivity of large molecules through pores and cavities of solid materials and the deleterious effect of water (and some polar compounds such as methanol and glycerol) on acid sites, which affects the acid strength of these sites. The solid acid must show water resistance in order to avoid adsorption of the water by­product that will lead to deactivation. Water covering the surface of solid acids prevents adsorption of organics. The hydrophobicity of the catalyst surface and the density of the acid sites are therefore of paramount importance in determining the catalyst’s activity and selectivity. With the possibility of tuning the acidity and controlling the pore size, the selectivity in organic reactions can be improved.

Various classes of solid-acid catalysts are potentially available with sufficient acid strength to be effective in transesterification, such as ion — exchange resins, pillared clays, zeolites in acid form, mesoporous molecular sieves, Lewis acids, solid superacids such as promoted (mixed) metal oxides, carbon-based polysulphonic acids, functionalised polymers and others. Some typical examples of solid-acid catalysts that have been investigated as potential replacements of mineral acids in esterification and transesterification reactions are tungsten oxides [209, 212-214], sulphated zirconia [202, 206, 210, 212, 215-217], sulphonated saccharides [218-221], organosulphonic functionalised mesoporous silicas [10, 191, 222, 223], Amberlyst®15 [201, 202, 212], and Nafion® resins [202, 212, 217, 224-227]. The majority of publications on transesterification catalysed by solid acids have focused on the reaction of yS-ketoesters [194, 228-232] using H-MCM-41, TBD/MCM-41, HP, HY, H-mordenite, H-ZSM-5, H-ZSM-12, kaolin, STO, Amberlyst®15, Amberlyst®16 using C4-C12 alcohols. Larger pore zeolites and dealuminated zeolites showed higher yields, suggesting that catalytic activity depends on reagent intra-pore diffusion, acid strength and surface hydrophobicity. Various catalysts active for transesterification of P-ketoesters, such as Envirocat EPZG, natural kaolinite clay, B2O3/ZrO2, sulphated SnO2 and zeolites, could be effective in the transesterification of triglycerides and deserve testing.

Commercial cation-exchange organic resins such as Dowex® 2030 (Dow Chemical Company) or Amberlyst® 36 (Rohm & Haas) are strong acids and good esterification catalysts. In general, when using organic resins, the catalytic activity strongly depends on their swelling properties. Resin swelling capacity is fundamental since it controls substrate accessibility to the acid sites and therefore affects its overall reactivity. Once swelled, the resin pores usually become macropores. Big molecules with long hydrocarbon chains (such as TG or FFA) then show no diffusion limitations and can readily access the acid sites in the bulk. At variance to most solid-acid catalysts, some resins such as Amberlyst® 15 catalyse appreciably both esterification and transesterification reactions under mild reaction conditions (see Table 8.18) due to their high concentration of acid sites [212]. However, their stability becomes an issue when resin-type catalysts are used at higher temperatures (for higher reaction rates, reactive distillative applications, or catalyst regeneration). Use of Amberlyst®15 for transesterification thus requires mild reaction conditions to avoid degradation of the catalyst. At 333 K and atmospheric pressure for a molar ratio MeOH/SNO = 6 : 1, a conversion of only 0.7% was achieved (see Table 8.18) [2]. Brazilian vegetable oils (babassu, corn, palm, palm kernel and soybean) were transesterified over ion-exchange resins (Amberlyst®15/31/35/36) [201]. The catalytic activity of the resins depends on the FA composition of the VO employed. The methyl ester yield is higher for palm kernel oil (PKO) and babassu oil than for SBO, probably due to their higher content of shorter-chain FA. A recent commercial solid-catalyst esterification/transesterification technology development is described in Section 15.3.1, pp. 732-6.

Nafion® (a perfluorosulphonated ionomer) is non-porous whereas Amberlyst® has large pores. Although Nafion® has very strong acid sites able to catalyse a great number of reactions occurring through carbocations, its low surface area is a serious handicap. Ion-exchange resins such as Nafion® NR50 are rapidly deactivated due to poor textural properties and also fail on thermal stability. However, recently developed high-surface — area Nafion®-silica composites [233] are expected to substitute other less environment-friendly acids such as HF or H2SO4, and deserve attention as solid-acid transesterification catalysts.

Mittelbach et al. [132] compared the activities of a series of layered aluminosilicates with H2SO4 for alcoholysis (MeOH, EtOH, PrOH, i-PrOH) of RSO (5 wt% catalyst, ROH/RSO = 30 : 1). The sulphuric acid catalyst showed the highest activity. The most active solid catalysts were activated by sulphuric acid impregnation. Activated KSF montmorillonite showed almost total conversion after 6 h of reaction at 493 K and 5.2 MPa. However, at this temperature some side reactions were observed, namely dimerisation of alkyl esters and formation of glycerol ethers. In less severe reaction conditions the side reactions were suppressed but ester yields were affected (70% yield after 8 h). Compared with liquid acids such as H2SO4, clay catalysts produce a cleaner biodiesel product due to their bleaching activity. Unrefined oils or waste cooking oils could be employed as feedstock without pre-treatment. The catalyst had to be reactivated after each run, suggesting that some leaching of sulphuric acid took place. Consequently, a homogeneous reaction mechanism may have contributed to the overall catalytic activity. Reusability of the catalyst was compromised by leaching of the sulphate species. Recently, KSF montmorillonite was also used in microwave-enhanced methanolysis of RSO [205]. The degree of acidity of KSF montmorillonite, as calculated by the Hammett acidity function, ranges between H2SO4 and HNO3.

Pillared clays (PILC) are another class of solid-acid catalysts. Kloprogge et al. [234] have described the use of a variety of pillared clays, which exhibit a 2D porous structure with acidic properties comparable to those of zeolites, to produce biofuels from VOs such as CPO, SNO and canola.

Most ion-exchange resins are not stable at temperatures above 413 K. For reactions requiring higher temperatures, inorganic acid catalysts such as zeolites are generally more suitable. Zeolites such as ZSM-5 (UOP or Zeolyst International) or ultrastable Y-zeolite (USY, Tosoh Corporation) as solid-acid catalysts enable reactions with low acid pretentions (e. g. Beckmann rearrangement), medium strength acid requirements (e. g. acetal formation, Friedel-Crafts alkylations) as well as those reactions demanding strong acid sites (e. g. dehydration of aliphatic alcohols, skeletal isomerisation of hydrocarbons). Strongly acidic zeolites also catalyse esterification of carboxylic acids with alcohols as well as their hydrolysis. In reactions involving bulky reactants (as in the case of transesterification of fatty acids), it is far more indicated to use new long-range ordered mesoporous materials (e. g. MCM-41 or MCM-48) as solid catalysts characterised by large pores, provided that they exhibit the required acidity.

Kaita et al. [140] designed various aluminum phosphate catalysts (molar ratio Al/P = 1 : 3 to 1 : 0.01) for transesterification of PKO with methanol. These durable and thermostable catalysts with good selectivity to FAME (< 0.8% ether formation) required high temperatures (473 K) and a molar ratio MeOH/PKO = 75 : 1. Methyl ester yields (> 50%) were insufficient for industrial application.

Heteropolyacids (HPAs), characterised by a large number of strongly acidic sites, have occasionally been applied to transesterification [235, 236]. HPAs show opportunities for substituting environmentally unfriendly Br0nsted and Lewis acids, such as H2SO4, AlCl3, BF3 and ZnCl2. Sunita et al. [208] have compared zirconia-supported isopoly — and heteropolytungstates in methanolysis of SNO at 473 K and MeOH/VO molar ratio of 15 : 1. The most active catalyst (WO3/ZrO2) gave 97% oil conversion and also efficiently catalyses methanolysis of mustard and sesame oil. The deactivated catalyst could be regenerated by calcination without appreciable loss in activity.

It appears that other solid superacids promote the transesterification of vegetable oils as well as the esterification of free fatty acids. In contrast to zeolites, sulphated zirconia (SO42-/ZrO2) and related promoted oxides can behave as strong acid to superacid solids. As the acid strength of SO42-/ZrO2 equals that of pure H2SO4 [237] this would imply that it is not really a solid superacid but a strong acid. Sulphated zirconia (SO42-/ZrO2) and tungstated zirconia (WO3/ZrO2) exhibit high catalytic activities for various reactions. The acid strength of SO42-/SnO2 exceeds that of SO42-/ZrO2 [238]. Tungstated zirconia-alumina shows particularly high activity for transesterification of vegetable oils (e. g. 90% yield in methanolysis of SBO at 473-573 K after 20 h) and for esterification of free fatty acids [209]. The activity of the catalyst was maintained for up to 100 h. High sulphur content ZrO2-based catalysts (from chlorosulphonic acid as a source of sulphate ions) exhibit higher catalytic activity than conventional sulphated zirconia, albeit without testing for methanolysis [239]. Jitputti et al. [206] have tested various solid catalysts (ZrO2, ZnO, SO42-/SnO2, SO42-/ZrO2, KNO3/KL zeolite and KNO3/ZrO2) for transesterification of crude palm kernel oil (CPKO) and crude coconut oil (CCNO). The superacid solids SO42-/SnO2 and SO42-/ZrO2 provided both the highest yield of methyl esters (90.3%) for CPKO, whereas SO42-/ZrO2 showed the highest yield (86.3%) for CCNO. ZnO has considerable potential for transesterification catalysis of both CPKO and CCNO. Zirconia, with acidic and basic properties, exhibits the lowest catalytic activity. Recently, Garcia et al. [210] investigated sulphated zirconia as a heterogeneous catalyst in the transesterification of SBO and simultaneous esterification of fatty acids with methanol and ethanol. Although the catalyst is very active under optimised conditions (393 K, 1 h, 5 wt% catalyst, ROH/VO molar ratio = 20 : 1), with conversions of 98.6% (methanolysis) and 92% (ethanolysis), the catalyst is deactivated rapidly. Sulphated zirconia also tends to form volatile sulphur compounds during catalysis [240]. Finally, Lopez et al. [241] described the catalytic performance for (trans)esterification of three thermally robust zirconia catalysts: titanic zirconia (TiZ), sulphated zirconia and tungstated zirconia (WZ). TiZ shows greater transesterification activity than WZ, as opposed to esterification. Heterogeneous catalytic conversions with hydrous SnO2 were used for the transesterification of ethyl acetate at 443-483 K, but not yet for triglycerides [242]. SnO is active for SBO methanolysis [167].

Zn/I2 was found to be a practical and effective catalyst for the transesterification of SBO with methanol [164]. The high catalytic activity is not associated with ZnI2. Tentatively, a Lewis acid mechanism was proposed. Advantages of the use of Zn/I2 (with co-solvent) are operational simplicity, mild reaction conditions and high conversion (96% for 5 wt% catalyst at 338 K with MeOH/SBO molar ratio of 42 : 1 after 26 h in the presence of dimethyl sulphoxide (DMSO)). The catalytic activity was only slightly affected by 1.5 wt% H2O and free fatty acids (acid value of 3 mg

KOH/g), with a decrease in oil conversion to 85%. The system may be a promising alternative catalyst for biodiesel production when the feedstock does not meet the requirements for base-catalysed processes.

Recently, sulphonated incompletely carbonised organic material (such as saccharides) was reported to act as a solid (trans)esterification catalyst [218]. A drawback of the method is the hugh amount of H2SO4 needed for sulphonation, which is significantly higher than the direct use of H2SO4 for esterification [243]. Moreover, reproducibility appears to be problematic.

The catalytic activity of strong Lewis acids such as titanium (IV) alkoxides Ti(OR)4 [169, 182, 183] can provide effective alternatives to traditional Br0nsted base catalysts. However, many of these catalysts suffer from problems of cost, toxicity, water poisoning and/or difficulty of removal from the product.

The synthesis of biodiesel via (solid) acid catalysis was recently reviewed by Lotero et al. [5, 20]. Acid catalysts are in great need for the transesterification of triglycerides with high content of free fatty acids. Rothenberg [244] has described use of an undisclosed solid-acid catalyst in a reactive distillative process for biodiesel production from high-FFA oils (see also Section 11.2). Further development of heterogeneous acid catalysts which esterify and transesterify simultaneously is highly desirable.