(100c) Mechanistic Insights on the Dehydration of Polyols over Bronsted Acid Sites

Multilayer plastic films provides excellent properties for efficient food packaging. However, difference in chemistry of hydrophilic tied layer and hydrophobic substrate leads to low recyclability of this material. Selective conversion of polar impurities with minimal impact on the hydrocarbon backbone is considered a promising solution for the problem. In accordance with this strategy, it is critical to understand the reaction mechanisms of polar functional groups in such polymeric system over the heterogeneous catalysts. Furthermore, atomic insights into the solvent effects on the reaction kinetics are essential guidelines for improving the target conversion.

In this contribution, we probe the selective conversion of hydroxyl groups in polyethylene-vinyl alcohol (EVOH)–a common co-polymer for oxygen-barrier functionality–using polyalcohols as the model molecules. The DFT calculations are performed to investigate the reaction kinetics of two neighboring hydroxyl groups over propylsulfonic functionalized amorphous silica. The study of elementary steps shows that the dehydration of the first hydroxyl group occurs via four plausible routes, in which the “direct” dehydration reassembling the E₂ mechanism to trans-allylic alcohol is more favored. The “indirect” pathway initiates with the formation of alkoxide intermediate through the S_N1 mechanism as the rate-determining step, which subsequently deprotonates following E₁-like mechanism to form allylic alcohols. Limited by the reaction coordinates, the dehydration of the second hydroxyl group proceeds through the similar “indirect” pathway to form conjugated diene products. The isomerization of allylic alcohols to ketones exhibits higher activation barriers compared to the dehydration. Furthermore, AIMD simulations in the presence of explicit water reveals that the proton-shuttling among solvent molecules and the adjacent acid site can change the energetics of reactions. These results provide guidelines for tuning the rate and selectivity of conversion in the real polymeric system by precisely manipulating the time on stream, acid site density, and solvent concentration.