The reaction mechanism of polyalcohol dehydration in hot pressurized water

Abstract

The use of high-temperature liquid water (HTW) as a reaction medium is a very promising tech- nology in the field of green chemistry. In order to fully exploit this technology, it is crucial to unravel the reaction mechanisms of the processes carried out in HTW. In this work, the reaction mecha- nism of 2,5-hexanediol dehydration in HTW has been studied by means of three different ab initio simulations: string method, metadynamics and molecular dynamics in real time. It is found that the whole reaction involving the protonation, bond exchange and the deprotonation occurs in a single step without a stable intermediate. The hydrogen bonded network of surrounding water has a vital role in assisting an efficient proton relay at the beginning and at the end of the reaction. It is confirmed that the reaction is energetically most favorable in the SN2 pathway with an estimated barrier of 36 kcal/mol, which explains the high stereoselectivity and the reaction rate observed in experiment. The mechanistic insights provided by our study are relevant for a prominent class of reactions in the context of sustainable biomass processing, namely dehydration reactions of polyalcohol molecules.