Benzylation of Aromatic Compounds with Different Crystallites of MgO

Abstract

Magnesium oxide nanocrystals are shown to be selective catalysts for the benzylation of aromatics with benzyl chloride, in the order xylene > toluene ≫ benzene. Normal polycrystalline MgO is not as effective as the nanomaterials. However, at the highest surface area, normally most reactive MgO samples (aerogel prepared AP−MgO) were poorer catalysts than a hexagonal platelet form (CP−MgO) of lower surface area. During the catalysis, the MgO is partially converted to MgCl₂, but crystal shape differences are not washed out. Rate data, differential thermal analysis, and consideration of crystal shapes and faces suggest that the CP−MgO form is most catalytically active because of optimal adsorption, molecular trafficking, and desorption energies. The AP−MgO apparently adsorbs the reactants too strongly, and thereby this hinders the catalytic process. These results serve as another example of where nanocrystalline shape plays an important role.