Catalytic conversion mechanism of guaiacol as the intermediate of lignin catalytic pyrolysis on MgO surface: Density functional theory calculation

MgO has better catalytic effect on the alkylation reactions of methoxy phenols in the pyrolysis of lignin to form alkylated phenols, thus, the research on the conversion mechanism of methoxy phenols into alkylated phenols on the MgO surface is necessary. In this study, the catalytic conversion mechanism of guaiacol as a model compound of methoxy phenols on the MgO surface is investigated by density functional theory calculations. The possible reactions for the conversion of guaiacol on the clean MgO(1 0 0) and the H atom pre-adsorbed MgO(1 0 0) surfaces are discussed. The adsorption energies, adsorption structures, electronic structures, and energy barriers of guaiacol on the MgO(1 0 0) surfaces are obtained to illustrate the impact of pre-adsorbed H atom on the adsorption of guaiacol and the conversion mechanism of guaiacol. The results indicated that a hydrogen bond is generated between the O1 atom of guaiacol molecule and the pre-adsorbed H atom, resulting in the adsorption energy of guaiacol molecule on the MgO(1 0 0) surface is lower than that on the H atom pre-adsorbed MgO(1 0 0) surface. The energy barriers for the conversion of guaiacol molecule to form o-cresol molecule on the clean MgO(1 0 0) surface is higher than generate catechol molecule on the H atom pre-adsorbed MgO(1 0 0) surface. Thus, the results indicate that the presence of hydrogen source has a promotion effect on the conversion of methoxy phenols over MgO catalyst into alkylated phenols.