Acid catalysis is simultaneously performing esterification of FFAs and TAGs. In this way, it is more economical to use low-quality feedstocks and lower processing costs.

The reaction mechanism using solid Brpnsted acids catalyzed esterifications is similar to that of the homogeneously catalyzed process. The reaction involves a nucleophilic attack of the adsorbed carboxylic acid with the free alcohol in the rate-determining step. The formation of a more elecrophilic intermediate is also occurring with solid Lewis acids. The rate-determining step is dependent on acid strength. If the strength of the acid sites is too high, the desorption of the ester is decreased. This mechanism is valid for both homogeneous and heterogeneous catalyst (Bonelli et al., 2007).

Many studies for the heterogeneous acid esterification have been carried out, mainly using acid resins (Lotero et al., 2006).

Lopez (2006) has tested the activity of various acid catalysts in the transesterification of triacetin at 60°C. The flowing order of activity was observed: H2SO4 > Amberlyst-15 (polystryrenesulphonic acid resin) > sulfated zirconia > Nafion NR-50 (perfluorinated alkanes resin sulfonic acid) > tungstated zirconia B > supported phosphoric acid > zeolite B.

The remarkable low activity is due to diffusion limitations in the zeolite pores of the bulky TAGs. At low temperatures the transesterification activity is slow and in order to obtain high reaction rates the temperature has to be increased above 170°C. However, many sulphonic acid catalysts are unstable at these high temperatures and therefore lower temperatures have to be used (120°C). Esterification is an equilibrium reaction and a nearly complete ester formation can only be reached after stripping off the water and adding additional methanol (Pasias et al., 2006).

An industrial process for the conversion of FFAs into FAMEs using heterogeneous catalyst (e. g. acid Amberlyst™ BD20), called FACT (Fatty Acid Conversion Technology) has been described by Soragna (2008).

The process involves a continuous, counter-current, multiple-step esterification using a solid catalyst in fix bed reactors at 90°C and 3.5 bar with intermediate methanol recovery. Production of biodiesel is performed by direct conversion as ‘stand-alone process’ where the quality of the FAMEs are increased by distillation or by an ‘integrated process’ where the ester content is increased by transesterifica­tion of the residual acylglycerols. A schematic representation of these two processes has been discussed later in this book (Chapter 22). High acidity feedstocks such as animal fats, used cooking oils, fatty acid distillates and high acidity vegetable oils can be used.

Esterification of FFAs in waste cooking oils was studied by Ozbay et al. (2008). The highest FFA conversion (46%) was obtained over a strong acidic macroreticular ion-exchange resin A-15 at 60°C with two per cent catalyst. Conversion of FFAs increased with increasing temperature and catalyst amount.

A comparative study of different heterogeneous catalyst (Dowex Monosphere 550A and zeolites NaY, VOx over USY) and different alcohols with oleic acids show FFA conversion of 51%. Enzymatic esterification is looking more promising (Marchetti and Errazu, 2008).

Superiority of physical properties of resins may be a dominant factor for high activity. Other acid catalyst A-16, A-35 and Dower HCR-W2 are less active.

Similar results have been obtained by Marchetti et al. (2007), showing that reuse of the catalyst results in low conversion rates. A general overview of the production from acidulated soapstock (acid oil) has been described by Luxem and Mirous (2008) emphasizing various processes using homogeneous and heterogeneous catalyst, mainly converting FFAs to FAME (87-92%) with 20% catalyst, a ratio of methanol to FFA of 3.8:1 and 3.5 hours.

Tin (Sn2+) complexes using the ligand 3-hydroxy-2-methyl-4-pyronate (maltolate) have been used to convert various vegetable oils into FAME at 80°C using a molar ratio 400:100:1 of methanol:oil:catalyst.

Yields up to 90% can be obtained but methanolysis is dependent on the nature of acid chain favoring the presence of unsaturation and chain length. Technological potential is rather low as the complexes remain dissolved in the reaction medium. Attempts have been made to immobilize the complex (Suarez et al., 2008).

A combined acid esterification and alkaline transesterification using a base and acid functionalized mesoporous silica nanoparticles has been proposed by Huang et al. (2008). These nanoparticles contain base (primary amines) and sulfonic acids inside the porous channels and are employed for one-pot reaction cascades.