Non-noble metal heterogeneous catalysts for hydrogen-driven deoxydehydration of vicinal diol compounds

Abstract

Noble-metal-free heterogeneous catalysts for deoxydehydration (DODH) using H₂ as a reductant were developed. Among various transition metals examined as additives to modify the MoO_x/TiO₂ catalyst, Cu showed good conversion and selectivity in the transformation of 1,4-anhydroerythritol (1,4-AHERY) to 2,5-dihydrofuran (2,5-DHF). The performance of the MoO_x–Cu/TiO₂ catalysts was comparable to those modified with either Ag or Au instead of Cu. Upon the combination of two Mo precursors, i.e. (NH₄)₆Mo₇O₂₄ and Na₂MoO₄, the selectivity of the catalyst (MoO_x–Cu–Na/TiO₂) was further enhanced to achieve 81% yield of 2,5-DHF. This catalyst also exhibited broad substrate scope including cyclic and linear alkyl vicinal diols and tartaric ester. Furthermore, MoO_x–Cu–Na/TiO₂ was reusable at least three times after its calcination as regeneration. The reaction was almost zero-order with respect to the H₂ pressure and 1,4-AHERY concentration, suggesting that the release of the alkene is the rate-determining step. The comprehensive characterization using STEM, XRD and XAFS provided insights into the surface structure of the catalyst, revealing that H₂ is activated over Cu particles and subsequently transferred to Mo cluster species on the TiO₂ surface via the spillover effect to proceed with the DODH reaction.