Biodiesel is synthesized via the transesterification of lipid feedstocks with low molecular weight alcohols. Currently, alkaline bases are used to catalyze the reaction. These catalysts require anhydrous conditions and feedstocks with low levels of free fatty acids (FFAs). Inexpensive feedstocks containing high levels of FFAs cannot be directly used with the base catalysts currently employed. Strong liquid acid catalysts are less sensitive to FFAs and can simultaneously conduct esterification and transesterification. However, they are slower and necessitate higher reaction temperatures. Nonetheless, acid-catalyzed processes could produce biodiesel from low-cost feedstocks, lowering production costs. Better yet, if solid acid catalysts could replace liquid acids, the corrosion and environmental problems associated with them could be avoided and product purification protocols reduced, significantly simplifying biodiesel production and reducing cost. This article reviews some of the research related to biodiesel production using acid catalysts, including solid acids.

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The energy life cycle assessment for the production of biodiesel from rendered lipids in the United States is presented in this study. Three different scenarios were found eligible for analysis: (I) conversion to biodiesel, (II) rendering and conversion, and (III) farming, rendering, and conversion. The amounts of energy required in farming, meat processing, and baseline conversion to biodiesel were reviewed from the literature. The thermal energy and electricity used in rendering were surveyed from the U.S. rendering industry. For animal fats, scenario III resulted in a net energy ratio (NER, ratio of energy outputs to energy inputs) much lower than 1. In contrast, the NERs for scenarios I and II were both found to be >1. For scenario I, the NER was found to be >3.6, larger than the value typically reported for soybean oil (SBO) biodiesel. As for the waste SBO grease, the NER was found to be >1 for both applicable scenarios (I and II). To a limited extent, sensitivity analysis was used to evaluate changes in assumptions with respect to the type of fuels employed in the generation of thermal energy as well as the method for biodiesel production.

A comprehensive kinetic investigation of the esterification of acetic acid with methanol in both the liquid phase (21-60 °C) and the gas phase (100-140 °C) was carried out using tungstated zirconia (WZ) as the catalyst. The goal of this study was to derive rate law expressions and to propose a reaction mechanism that would support reaction rate data for esterification on WZ. Upon increasing the concentration of acetic acid, an increase in the rate of esterification was obtained at all reaction temperatures. In contrast, the reaction order with respect to methanol evolved from positive to negative as the reaction temperature increased. Using a model discrimination procedure, we found that a single-site (Eley-Rideal) mechanism with the adsorbed carboxylic acid reacting with the methanol from the gas/liquid phase successfully describes these reactions. One important conclusion of this study was that, even though there were significant differences in the power law exponents for gas-and liquid-phase esterifications, the same reaction mechanism can successfully describe both situations. We propose that the adsorption of the carboxylic acid becomes the rate-determining step (RDS) as the reaction temperature increases. At lower esterification temperatures (liquid phase), the ratedetermining step appears to be the nucleophilic attack of the alcohol on the adsorbed/protonated acetic acid molecules. At higher reaction temperature (gas phase), the adsorption of the carboxylic acid becomes rate determining. It was also found that the catalytic activity of WZ was inhibited by water similarly to other strong acid catalysts.