Mechanistic analysis of the role of metal oxophilicity in the hydrodeoxygenation of anisole

Abstract

A combined experimental and theoretical comparative study of the hydrodeoxygenation (HDO) of anisole was conducted over Pt, Ru, and Fe metals. In the experimental part, an inert silica support was used to directly compare the catalytic activity and selectivity of the three metals at 375°C under H2 flow at atmospheric pressure. In parallel, for density functional theory (DFT) calculations the close-packed Pt(111), Ru(0001), and Fe(110) surfaces were employed to compare the possible mechanisms on these metals. It was observed that over Pt/SiO2 and Ru/SiO2 catalysts, both phenol and benzene were the major products in a phenol/benzene ratio that decreased with the level of conversion. By contrast, over the Fe/SiO2 catalyst, no phenol formation was detected, even at low conversions. The DFT results show that over all the three metal surfaces the dehydrogenation at the CH3 side group occurs before the CO bond breaking. This removal of H atoms from the CH3 group facilitates the activation of the aliphatic CalkylO bond. Therefore, it can be concluded that a common intermediate for the three metals is a surface phenoxy and the significant differences between the three metals are related to the reactivity of this surface phenoxy. That is, over Pt(111) and Ru(0001) the phenoxy intermediate is hydrogenated to phenol, which in turn, can undergo further HDO to form benzene. This result is in agreement with the experiments over Pt/SiO2 and Ru/SiO2 catalysts. Over these catalysts, both phenol and benzene are major products, with the selectivity to benzene increasing with conversion at the expense of phenol. In contrast, over the Fe(110) surface, the strong metal oxophilicity makes the direct cleavage of the CO bond in the surface phenoxy easier than hydrogenation to phenol. Thus, it is predicted that phenol is not formed over iron, but only benzene should be observed as HDO product at all conversion levels, which is also in agreement with the experimental observations.