The direct coupling of carboxylic acids with biomass derived species is important for the production of renewable fuels and specialty chemicals.
For fuels production, this reaction has the potential to significantly increase yields to liquid products from biomass by directly coupling light acids into the gasoline/diesel fuel range.
This reaction also has also gained attention recently for its potential to form specialty commodity chemicals such as renewable surfactants. Here we discuss the mechanism of direct activation of carboxylic acids with renewable furanic and phenolic species over zeolites.
Through a combination of detailed vapor phase reaction kinetics, isotope labeling experiments, temperature programmed experiments, and DFT calculations we reveal conditions necessary for selective dehydration of acids to form surface acyl groups over H-ZSM-5 and H-Beta zeolites.
In the presence of only carboxylic acids, the rate limiting step is the carbon-carbon coupling step, with dehydration to form surface acyl species requiring a lower barrier. We then evaluate the nature of the kinetically relevant transition state required for coupling surface acyl species a variety of substrates. The barrier for acylation is contrasted with coupling with a second carboxylic acid. For several renewable furanic and phenolic species, the barrier for the C-C coupling step is reduced to a value comparable to or lower than that required for dehydration to form surface acyls, making dehydration the rate determining step. This significantly shifts the dependence of the acylation reaction rate with respect to water approaching an order of -1. By comparing a variety of furanic, phenolic, and aromatic substrates, a general correlation is developed between the electronic structure of the substrate and the net barrier for C-C coupling, reveling the shift in degree of rate control for the dehydration vs. C-C coupling steps for the various substrates.
 A. Gumidyala, B. Wang, S. Crossley, Science Advances 2016, 2, e1601072.
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