Comparison of Unimolecular Decomposition Pathways for Carboxylic Acids of Relevance to Biofuels

Quantum mechanical molecular modeling is used [M06-2X/6-311++G(2df,p)] to compare activation energies and rate constants for unimolecular decomposition pathways of saturated and unsaturated carboxylic acids that are important in the production of biofuels and that are models for plant and algae-derived intermediates. Dehydration and decarboxylation reactions are considered. The barrier heights to decarboxylation and dehydration are similar in magnitude for saturated acids (∼71 kcal mol^-1), with an approximate 1:1 [H ₂O]/[CO₂] branching ratio over the temperature range studied (500-2000 K). α,β-Unsaturation lowers the barrier to decarboxylation between 2.2 and 12.2 kcal mol^-1 while increasing the barriers to dehydration by ∼3 kcal mol^-1. The branching ratio, as a result, is an order of magnitude smaller, [H₂O]/[CO₂] = 0.07. For some α,β-unsaturated acids, six-center transition states are available for dehydration, with barrier heights of ∼35.0 kcal mol ^-1. The branching ratio for these acids can be as high as 370:1. β,γ-Unsaturation results in a small lowering in the barrier height to decarboxylation (∼70.0 kcal mol^-1). β,γ-Unsaturation also leads to a lowering in the dehydration pathway from 1.7 to 5.1 kcal mol^-1. These results are discussed with respect to predicted kinetic values for acids of importance in biofuels production.