The transesterification reaction of TGs with dimethyl carbonate (DMC),49,50 methyl acetate51,52 or ethyl acetate53 produces a mixture of FAMEs (or FAEEs) and cyclic glycerol carbonate esters of fatty acids (FAGCs) [or glycerol triacetate (triacetin)] (Fig. 7.3).

DMC-BioD® is a biofuel, patented by Notari et al.,54 that integrates glycerol as glycerol carbonate in a process that can be developed by enzymatic technology,55 but conventional basic catalyst (sodium methoxide — the same biodiesel obtained by vegetable oils and methanol, MeOH-biodiesel) can also be used.

The main problem of an enzymatic process is the inactivation of the enzyme (in this case of lipases) by some short-chain alcohol acyl acceptors such as methanol. In order to enhance the stability of lipases, the short-chain alcohols could be substituted by methyl acetate as acyl acceptors. But this solution needs a great amount of enzyme (three times more than in a normal alcoholysis) and an excessive amount of methyl acetate (1:12 of oil/methyl acetate) to obtain good conversion values. These drawbacks could be the main limitations for a potential industrial application of methyl acetate as acyl acceptor in the transesterification reaction of vegetable oils.

In this context, it is worthwhile exploring novel reagent as acyl acceptors to prepare esters from lipids. DMC is a potential candidate as a reagent for the transesterification of oils due to its eco-friendliness, chemical reactivity and physical properties.56 DMC is neutral, odourless, cheap, non-corrosive, non-toxic and exhibits good solvent properties. Pioch et al. were the first researchers that reported ethyl oleate production by ethyl carbonate and oleic acid reaction catalyzed by an immobilized lipase.57 The enzymatic transesterification of oil with DMC, as acyl acceptor, catalyzed by lipase, results in an irreversible reaction due to the decomposition of carbonic acid monoacyl ester into carbon dioxide and an alcohol, and consequently, the reaction is favoured towards its completion. Moreover, the DMC gives higher conversion than those of conventional acyl acceptors such as methanol or methyl acetate.

Different lipase sources and various vegetable oil feedstocks have been investigated. Some key parameters were explored to determine the optimal transesterification conditions, first of all the stability of the immobilized enzyme, in view of a potential scaling-up to industrial processes.55 The main results concerning lipase sources and vegetable oils are summarized in Table 7.5.

From the screening results shown in Table 7.5, it is noticeable that Novozyme 435 (immobilized Candida antarctica) shows better activity towards all selected

vegetable oils (81.2%, highest conversion with olive oil). Other lipases showed very little or no activity. Further results show that this lipase also exhibited high conversions in non-polar solvents (with the best performance using petroleum ether) and high activity with the optimum molar ratio of 1:4.5 for oil/DMC, using a DMC one-step addition. Concerning the optimum temperature reaction and the enzyme amount, Novozyme 435 strongly increases its activity with increasing quantities of the enzyme (optimum quantity was found to be 10% based on oil weight). Its performance gradually decreases above 50°C. Finally, concerning the more important parameter for an industrial application, the enzyme reusability, Su et al. showed that Novozyme 435 preserves up to 80% of its initial activity after five reaction cycles, if washed with acetone between each batch use.

The principal difference between DMC-BioD® and biodiesel produced from vegetable oil and methanol (MeOH-biodiesel) was the presence of FAGCs in addition to FAMEs. However, the mixture (FAMEs + FAGCs) has relevant physical properties to be employed as a fuel.54,58 Flow and combustion properties of DMC-BioD®, relevant for its applications as a biofuel, are reported in Table 7.6.

Differences with respect to conventional biodiesel can be attributed to the presence of the FAGCs, which have a molecular weight larger than those of the corresponding FAMEs (see flash point and density). Nevertheless, the cetane number is almost the same but always lower than that of fossil diesel. DMC — BioD® has a higher viscosity than MeOH-biodiesel, but if blended with petroleum diesel, for example in a ratio of 20:80 v/v, the kinematic viscosity decreases to

3.3 cSt, a value closer to that of conventional diesel.

Moreover, the addition of DMC-BioD® at 20% level to diesel not only does not affect the fuel performance but also improves the lubricity of the diesel blend, which is a crucial factor for low-sulfur petroleum diesel. The lubricity value does not change significantly between MeOH-biodiesel and DMC-BioD®.

Last, but not least, from an economical point of view, the use of DMC in the transesterification reaction of vegetable oils will bring a minor impact on the overall biofuel costs: a large fraction of glycerol (> 65%) is incorporated into the biofuel in the form of FAGCs and a minor fraction is converted into glycerol carbonate and dicarbonate. These latter compounds could find utilization as additive and chemical intermediates, while, introducing into the market, glycerol carbonate and its derivatives (characterized by a low toxicity) can mitigate the problem of glycerol overproduction due to the increasing biodiesel utilization.58