Hydrodeoxygenation of anisole over different Rh surfaces

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The cleavage of the alkoxy (Ar–O–R) ether bond present in anisole is an interesting hydrodeoxygenation (HDO) reaction, since this asymmetric group contains two different C–O bonds, C aryl –O or C alkyl –O, which could potentially cleave. Recent work on the HDO of anisole over Pt, Ru, and Fe catalysts has shown that a common phenoxy surface intermediate is formed on all three metals. The subsequent reaction path of this intermediate varies from metal to metal, depending on the metal oxophilicity. Over the less oxophilic Pt, phenol is the only primary product. By contrast, on the more oxophilic Fe catalyst, the sole primary product is benzene instead of phenol. On Ru, with intermediate oxophilicity, both benzene and phenol are primary products. In this contribution, we have investigated Rh catalysts of varying surface nanostructures. A combination of experimental measurements and computational calculations was used to explore the effects of varying metal coordination number, an additional parameter that can be used to control the oxophilicity of a metal. The results confirm that metal oxophilicity is a good descriptor for HDO performance of metal catalysts and it can be controlled via selection of metal type and/or metal extent of coordination. Small Rh metal clusters with low coordination metal sites are more active for the deoxygenation pathway but also quickly deactivated while large clusters with high coordination sites are more active toward hydrogenation and more stable. Manipulating the surface coordination of rhodium atoms can greatly affect the metal oxophilicity and in turn control its activity for C–O bond cleavage. In the hydrodeoxygenation of anisole, a step surface favors deoxygenation to benzene while a smooth terrace prefers hydrogenation to phenol.