Production of biodiesel (fatty acid alkyl esters) through transesterification of virgin plant oils, as well as low-quality waste oils with short-chain alcohols, has received considerable attention during the past decade for producing a biodegradable and nonpolluting fuel. The conventional method for industrial biodiesel production is a chemical process using acid or base catalysts [1–3]. The chemical process offers a high yield of biodiesel in a short reaction time but has drawbacks, such as the need to remove the catalyst and by-products, and high energy consumption [1–5]. As an alternative, an enzymatic process using lipase as a biocatalyst may overcome these problems because lipases can catalyze a variety of transesterification and esterification reactions relatively efficiently under mild conditions and in non-aqueous environments [1–8]. Thus considerable research has been directed at achieving high yields of biodiesel in short reaction times, for various feedstocks ranging from virgin vegetable oils to low-quality acid oils with high free fatty acid content. Among non-edible oils, Jatropha curcas, which is toxic owing to the presence of carcinogenic phorbol esters, has great potential for biodiesel production [9–13].
Immobilization of lipase is an essential technology in enabling us to perform continuous production of biodiesel using packed-bed bioreactors [14–21]. The immobilized Candida antarctica B lipase commercialized as 'Novozyme 435' has been investigated widely and is reported to exhibit the best performance [22–24]. However, although methanol is the most commonly used acyl acceptor, the enzyme is deactivated in the presence of small droplets of insoluble methanol [15, 22, 25]. Several strategies have been attempted to overcome this adverse effect, including stepwise addition of methanol to the reaction mixture [15, 22, 26], use of alcohols with longer alkyl chains than methanol [24, 26–30], use of methyl or ethyl acetate [23, 31–33] as acyl acceptors, and the introduction of organic solvents, such as ter-butanol, that are not accessible to the lipase [18, 26, 34–38]. Other researchers have shown that lipases from Pseudomonas species are the most promising for biodiesel production [9, 27–29, 39–45]. Immobilization support materials include particles of diatomaceous earth [9, 40, 42], polypropylene [40, 42, 44, 45], mesoporous silica , kaolinite , and silica-polyvinyl alcohol composite [43, 46].
It is now well known that lipases are highly activated and stabilized, owing to conformational change leading to the open-lid structure, when encapsulated in alkyl-substituted silicates by the sol-gel method [47, 48], and the resultant preparations are used effectively for many organic syntheses [49–51]. Several researchers have applied sol-gel immobilized lipases to the production of biodiesel. Hsu et al. [16, 28, 30] prepared immobilized Burkholderia cepacia (formerly Pseudomonas cepacia) lipase in a phyllosilicate sol-gel matrix and used it for the transesterification of tallow and grease. Repeated production of ethyl esters was also performed in a recirculating packed-column bioreactor loaded with particles of immobilized lipase . Noureddini et al.  immobilized B. cepacia lipase within a hydrophobic iso-butyl-substituted sol-gel support and succeeded in producing methyl and ethyl esters yield of 67% and 65%, respectively, from soybean oil in 1 hour. Lipase from the same origin was also immobilized in a methyl-substituted silica aerogel and applied in the transesterification of sunflower seed oil with methyl acetate .
Recently, we demonstrated that a macroporous, non-shrinkable silica monolith could be formed easily from a mixture of methyltrimethoxysilane (MTMS) and tetramethoxysilane (TMOS), and that an enzyme-immobilized silica monolith was applicable as a flow-through microbioreactor for organic syntheses . We also developed a highly efficient bioreactor loaded with a lipase-immobilized silica monolith by adopting a two-step sol-gel method, that is, by preparing an MTMS-based silica monolith coated with butyl-substituted silicates that entrapped lipase . We applied this type of bioreactor to the continuous production of fatty acid methyl esters through methanolysis of rapeseed oil in n-hexane by Rhizopus oryzae lipase . The use of such an enzyme-immobilized silica monolith bioreactor is expected to be useful for the biodiesel production, because it offers several benefits, including very low backpressure, high contacting efficiency and mechanical durability, as compared with conventional packed-bed bioreactors [52–54]. In the present study, we selected B. cepacia lipase as the most promising enzyme, and investigated the production of biodiesel through solvent-free methanolysis of Jatropha oil in batch and continuous bioreactors loaded with lipase-immobilized silica monoliths.