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 technology 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 mechanism of 2,5-hexanediol dehydration in HTW has been studied by means of three different ab initio simulations: the string method, metadynamics and molecular dynamics in real time. It is found that the whole reaction involving protonation, bond exchange and deprotonation occurs in a single step without a stable intermediate. The hydrogen bonded network of the 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 S2 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.

Understanding Competition of Polyalcohol Dehydration Reactions in Hot Water

“…This is also similar to what was found earlier for the dehydration reaction of hexanediol. 32 Therefore, this mechanism should be general for polyalcohol dehydration in HTW under acidic conditions.…”

“…In fact, in the case of hexanediol dehydration the free energy barrier in a microsolvated cluster with only six water molecules was found to have the same value (36 kcal/mol) as that in bulk water with 70 water molecules with periodic boundary conditions. 32 Now we describe the simulation setup in detail. A summary is also shown in Table 1.…”

“…and because reaction mechanisms operating in room-temperature water can drastically differ from those operating at high temperatures and pressures. 25,26 In view of the proven utility of computational chemistry in establishing reaction mechanisms in room-temperature aqueous media, [27][28][29][30][31] it is clear that computational studies of reactivity in hot pressurized water, which are still scarce 25,26,32,33 in comparison with those at ambient temperature, can provide valuable insights into reaction mechanisms, thereby facilitating the design of optimal chemical processes in hydrothermal conditions. Here we elucidate the mechanism and the origin of selectivity of an intramolecular dehydration process that serves as a model system for polyalcohol dehydration reactions in hot pressurized water.…”

“…It should be mentioned that the mechanism of dehydration reactions of dialcohols or glycerol in HTW has been investigated in previous computational works using different simulation techniques. 32,48,49 However, this is the first computational work dealing with selectivity issues when both five-membered and six-membered cyclic ethers can be formed.…”

Refined metadynamics through canonical sampling using time‐invariant bias potential: A study of polyalcohol dehydration in hot acidic solutions

“…[49][50][51] Recently, the reaction rates of polyalcohol dehydration in high-temperature water and carbonated water has been studied systematically by an experimental group of Shirai, Yamaguchi, et al [52][53][54][55][56][57] Accordingly our research group has been working on elucidating the mechanism of this reaction from a standpoint of computational chemistry. [58,59] From the analysis of dehydration of simple polyalcohols, that is, 2,5-hexanediol (HDO) and 1,2,5-pentanetriol (PTO), we proposed that the main reaction is dominated by a protonation-assisted S N 2 pathway. However, we have not yet proved the acidity dependence that has been observed experimentally.…”