Integration of heterogeneous and biochemical catalysis for production of fuels and chemicals from biomass.

The past decade has seen significant government and private investment in fundamental research and process development for the production of biofuels and chemicals from lignocellulosic biomass-derived sugars. This investment has helped create new metabolic engineering and synthetic biology approaches, novel homogeneous and heterogeneous catalysts, and chemical and biological routes that convert sugars, lignin, and waste products such as glycerol into hydrocarbon fuels and valuable chemicals. With the exception of ethanol, economical biofuels processes have yet to be realized. A potentially viable way forward is the integration of biological and chemical catalysis into processes that exploit the inherent advantages of each technology while circumventing their disadvantages. Microbial fermentation excels at converting sugars from low-cost raw materials streams into simple alcohols, acids, and other reactive intermediates that can be condensed into highly reduced, long and branched chain hydrocarbons and other industrially useful compounds. Chemical catalysis most often requires clean feed streams to avoid catalyst deactivation, but the chemical and petroleum industries have developed large scale processes for C-C coupling, hydrogenation, and deoxygenation that are driven by low grade heat and low-cost feeds such as hydrogen derived from natural gas. In this context, we suggest that there is a reasonably clear route to the high yield synthesis of biofuels from biomass- or otherwise derived-fermentable sugars: the microbial production of reactive intermediates that can be extracted or separated into clean feed stream for upgrading by chemical catalysis. When coupled with new metabolic engineering strategies that maximize carbon and energy yields during fermentation, biomass-to-fuels processes may yet be realized.