Ethanol derived from the fermentation of biomass can be catalytically converted into higher molecular weight hydrocarbons and oxygenates to produce advanced biofuels and lubricants. The Guerbet reaction, which involves dehydrogenation, aldol addition, dehydration, and hydrogenation, is one pathway for this process. Cascades of Guerbet reactions, similar to step-growth polymerization, produces broad distributions of heavier products. Successful production of higher carbon number (Cn) products relies on high selectivities for dehydrogenation reactions, which occur on metal clusters, and the deprotonation and nucleophilic attack of enolates onto vicinal aldehydes both of which must be chemisorbed on oxide surfaces.
Here, we show how bifunctional catalysts, comprised of metal clusters and amphoteric metal oxides, promote these networks of reactions using ethanol consumption rates and product distributions measured over wide-ranges of alcohol-aldehyde pool conversions and temperature (523 - 623 K). The selectivity with which ethanol is incorporated into growing chains is limited by the selectivity of both catalytic functions, while the isomeric structure of the products is determined by the changes in deprotonation energy of C-atoms α to the carbonyl of the aldehydes reacting on oxide surfaces. Chain growth termination occurs by unimolecular deoxygenation reactions and by coupling steps, which generate ketones and ethers that are unable to undergo further aldol condensation reactions. Consequently, changes in the reaction temperature and the identity and composition of the oxide catalyst influence the isomeric structure of the products by favoring the deprotonation of specific aldehydes. These same parameters also impact the Cn distribution of the final products, achieved as all alcohol-aldehyde reactants are deoxygenated, because reaction steps required for chain propagation and chain termination have different activation enthalpies.
These findings show the aspects of the Guerbet reaction network which are catalyst dependent and those which are catalyst independent, and indicate where changes can be made that may increase selectivity towards specific groups of products. This knowledge is need to guide further research for upgrading alcohols to longer chain oxygenates and hydrocarbons.