Advances in production of bio-based ester fuels with heterogeneous bifunctional catalysts

Dwindling oil reserves, concerns over climate changes related to CO2 emissions, and increasing demands on energy have intensified the interest and efforts in the utilization of bio-renewable resources for biofuels and value-added chemicals. In line with these, promising bio-based ester fuels including biodiesel (fatty acid methyl ester, FAME) and alkyl levulinate (AL) have been manufactured directly from sustainable biomass resources through the environmentally-friendly process using functional catalysts. In order to comply with the principles of green and sustainable chemistry, heterogeneous catalysts rather than the conventional homogeneous catalysts have been primarily considered. Furthermore, in comparison with mono-functional catalysts, heterogeneous bifunctional catalysts can not only enhance the selectivity toward the target products via tandem/sequential-type reactions, but also integrate the correlative catalytic transformations and product isolation into a one-pot process. The present work overviews recent advances in the catalytic valorization of non-food oils and liquid biomass for biodiesels via simultaneous transesterification and esterification, as well as catalytic transformation of solid cellulosic biomass into AL over solid bifunctional catalysts. Particular attention is paid on the performance of various bifunctional acid-base and Brønsted-Lewis acidic catalysts. Plausible reaction mechanisms are also discussed in this review involving various reaction pathways, which provide guidance for the design of appropriate heterogeneous bifunctional catalysts for biodiesel and AL production.