Study on hydrogenation of oxygenated aromatic hydrocarbons via water-gas shift reaction in a subcritical CO-H2O system

Abstract

The hydrogenation properties of dibenzyl ether (an oxygenated model compound of lignite) via the water–gas shift reaction (WGSR) were studied in a subcritical CO-H2O system. The result showed during the hydrogenation process, the C-O bond was broken, and the fragments of dibenzyl ether were stabilized by the hydrogen generated by WGSR, generating benzene, toluene, benzyl alcohol and benzaldehyde. The presence of excess water inhibited the hydrogen transfer, largely reducing the conversion of dibenzyl ether. However, the inhibition effect was substantially reduced when the reaction environment reached the critical temperature and subcritical pressure of water. The hydrogen and deuterium generated by H2O and D2O via WGSR were utilized to label and analyze the hydrogenation position of the product, respectively. The results showed that the hydrogen generated by WGSR was added to the methylene after the break of dibenzyl ether, and this hydrogen appeared on the benzene ring structure via hydrogen transfer. The bond dissociation energy and energy required for the hydrogenation process of dibenzyl ether under aqueous condition were calculated by utilizing density functional theory (DFT). First, hydrides (H−) are formed by the WGSR under alkaline conditions. Then, the formation energy (140.81 kJ/mol) of hydrides and the energy (15.54 kJ/mol) of hydrides attacking the dibenzyl ether to form the transition state were lower than the bond dissociation energy (259.26 kJ/mol) of the dibenzyl ether. That is, the hydrogenation of dibenzyl ether via WGSR prefers to be a nucleophilic reaction rather than a pyrolysis reaction. In the coal liquefaction process, the hydrides produced by WGSR under the catalysis of alkali is conducive to the hydrocracking of oxygen containing functional groups.