Bioinspired Catalysts for Biofuels: Challenges and Future Directions

Abstract

Bioinspired catalysts have been developed for several reactions relevant to biofuels production: acyl transfer, ester and glycosidic bond hydrolysis, aldol condensations, ketonization, dehydration, and lignin depolymerization. Amongst these disparate reactions, however, exist general conceptual approaches for recreating the remarkable catalytic prowess of enzymes. A hallmark feature of bioinspired catalyst design is the desire to emulate the intricate multifunctional environments suspected to be of critical importance in enzymatic catalysis. Two of the main challenges in doing so, however, are the difficulties associated with satisfying the stringent spatial requirements of successful bifunctional catalysts and maintaining the ability to function in water, which is typically present in significant amounts in bio-based feedstocks. The latter has been dealt with by designing catalysts such as cyclodextrins which consist of a hydrophobic binding pocket to exclude water from the active site. However, even in water in the absence of a hydrophobic binding site, some intramolecular systems have proven to exhibit remarkable activity simply by virtue of having exactly the correct active site configuration. The key to bifunctional catalysis is therefore in having the correct distances between the substrate and the appropriate catalytically active functionalities. The current challenge is to translate the demonstrated hydrolysis activity of the intramolecular systems, which function at mild pH, to catalysts capable of acting intermolecularly. Combinatorial approaches such as catalytic antibodies and molecularly imprinted polymers have been successful, to varying degrees, in this application because they produce a multitude of active sites. This increases the probability that some of the sites will have the correct positioning, obviating the practically prohibitive requirement of synthesizing every aspect of the bifunctional catalyst with the precision required for success. Another promising perspective on the combinatorial approach is to provide a continuum of distances between reactive groups so that at least one is bound to be effective. This has been illustrated both with proof-of-concept systems employing one-dimensional organic polymers and two-dimensional inorganic oxide surfaces, which are shown to be versatile bifunctional catalysts.