I think this paper is the first review article on supercritical alcohol transesterifications.

“Supercritical methanol is believed to solve the problems associated with the two-phase nature of normal methanol/oil mixtures by forming a single phase as a result of the lower value of the dielectric constant of methanol in the supercritical state.”

In general, methyl and ethyl alcohols are used in supercritical alcohol transesterification. In the conventional transesterification of animal fats and vegetable oils for biodiesel production, free fatty acids and water always produce negative effects, since the presence of free fatty acids and water causes soap formation, consumes catalysts, and also reduces catalyst effectiveness, each of which result in a low conversion.

The transesterification reaction may be carried out using either basic or acidic catalysts, but the processes required are relatively time consuming and complicated in the separation of the product and the catalyst, which results in higher production costs and energy consumption.

In order to overcome these problems, Saka and Kusdiana (2001) and Demirbas (2002, 2003) have firstly proposed that biodiesel fuels may be prepared from vegetable oil via non-catalytic transesterification with supercritical methanol (SCM).

A novel process of biodiesel fuel production has been developed by a non-catalytic supercritical methanol method. Supercritical methanol is believed to solve the problems associated with the two-phase nature of normal methanol/oil mixtures by forming a single phase, directly as a result of the lower value of the dielectric constant of methanol in the supercritical state. As a result, the reaction was found to be completed in a very short time.

Compared with catalytic processes under barometric pressure, the supercritical methanol process is non-catalytic and the purification of products is much simpler, resulting in a lower reaction time that is more environmentally friendly and requires lower energy use. However, this reaction requires temperatures of 525–675 K and pressures of 35–60 MPa (Demirbas, 2003; Kusdiana and Saka, 2001).