High Catalytic Efficiency of Nanostructured Molybdenum Trioxide in the Benzylation of Arenes and an Investigation of the Reaction Mechanism

Abstract

The synthesis and characterization of nanostructured MoO₃ with a thickness of about 30 nm and a width of about 450 nm are reported. The composition formula of the MP (precipitation method) precursor was estimated to be [(NH₄)₂O]_0.169⋅MoO₃⋅ (H₂O)_0.239. The calcination of the precursor in air afforded nanostructured pellets of the α-MoO₃ phase. The nanostructured MoO₃ catalyst exhibited high efficiency in catalyzing the benzylation of various arenes with substituted benzyl alcohols, which were strikingly different to common bulk MoO₃. Most reactions offered >99 % conversion and >99 % selectivity to monoalkylated compounds. MoO₃ is a typical acid catalyst. However, the benzylation reaction over nanostructured MoO₃ does not belong to the acid-catalyzed type or defect site-catalyzed type, since the catalyst has no acidity and defect site on surface. Characterization with thermal, spectroscopic, and electronic techniques reveal that the catalyst contains fully oxygen-coordinated MoO₆ octahedrons on the surface but partially reduced species (Mo⁵⁺) within the bulk phase. The terminal oxygen atoms of MoO bonds on the (010) basal plane resemble oxygen anion radicals and act as active sites for the adsorption and activation of benzyl alcohols by electrophilic attack. Such sites are indispensable for catalytic reactions since the blocking of these sites by electron acceptors, such as tetracyanoethylene (TCNE), can greatly decrease catalytic activity. This work represents a successful example of combining a heterogeneous catalysis study with nanomaterial synthesis.