## 1 Introduction
In contrast, the use of solid acids (e.g., sulfonic acid resins, sulfonic acid-modified silica, _etc._), or solid bases (e.g., BaO, SrO, MgO, KNO\({}_{3}\)/Al\({}_{2}\)O\({}_{3}\), _etc._), as heterogeneous catalysts permits their easy separation from reaction mixture by simple filtration, enabling recycling [3,4]. Biocatalysts represent a completely different approach. As far as all catalyzed reactions are concerned, Nature has always been a source of inspiration for chemists. To transfer the exquisite efficiency shown by enzymes in Nature to chemical processes may well constitute the most powerful toolbox for developing a clean and sustainable chemical industry. Biodiesel synthesis by transesterification and/or esterification using immobilized lipase catalysis is applicable to both refined and raw plant oils, free fatty acids, waste fats from frying, tallow and other waste fats. A small excess of alcohols (e.g., methanol, ethanol or propanol) provides a high biodiesel yields under mild conditions (20-60 \({}^{\circ}\)C) [5]. Low water concentrations in the reaction medium can have a positive impact on biodiesel production. In the same way, the conversion of free fatty acids (FFAs) can be much higher than in base catalyzed processes. In this case, both triacylglyceride and FFAs can simultaneously be converted to biodiesel since lipase efficiently catalyzes both transesterification and esterification. Finally, upon completion of the transesterification process, the glycerol (lower phase) is simply separated from the biodiesel (upper phase) and neither product deodorization nor neutralization is required [6]. Probably, the most intriguing feature in the catalytic synthesis of biodiesel is the phase behaviour of the reaction system during the transformation process. The starting materials, triacylglyceride and methanol, are non-miscible, as well as the final products, glycerol and biodiesel (see Figure 1). This phenomenon leads to the low efficiency of immobilized enzymes and their deactivation due to poisoning as results of the adsorption of the continuously formed glycerol by-product onto the support. In this context, both reactor design and reaction medium engineering (e.g., the use of cosolvents,different reaction conditions, _etc._) are essential approaches to improve the performance of the heterogeneous (bio)catalysts. However, the most important drawback to apply biocatalysts in biodiesel synthetic processes is the high cost of enzymes, making necessary to develop stable and reusable biocatalytic systems for extended operation times [5, 6, 7]. Several strategies have been developed to overcome these constraints. One example is the sequential addition of methanol in three different doses [8], or its storage by adsorption onto silica gel particles, acting as \"microreservoirs\" that slowly release the alcohol needed for the transesterification process. In this way, up to 90% biodiesel yield, together with improved catalyst recycling was obtained after 18 h of reaction [9]. Alternative approaches for enzyme immobilization (e.g., encapsulation by sol-gel methods [10], covalent attachment onto magnetic nanoparticles [11], _etc._) have also been described as improving the biocatalytic efficiency of biodiesel synthesis. Enzyme efficiency (up to 97% yield after 24 h at 50 \({}^{\circ}\)C) is also improved by dissolution of the substrates in an organic solvent of medium polarity (e.g., _tert_-butanol), resulting in a one-phase reaction medium and avoiding the direct interaction between the enzyme and pure methanol [12]. Despite being key elements in all chemical processes (e.g., mass-transport, reactions, product separation, _etc._), volatile organic solvents are also responsible for a not incosiderable part of the environmental impact of the chemical industry. The search for new environmentally benign non-aqueous solvents, or green solvents, which can easily be recovered/recycled, allowing (bio)catalysts to operate efficiently, is a priority for the development of an integral green process of biodiesel synthesis.