Role of copper and cerium species in Cu/CeZSM catalysts for direct methane to methanol reaction: Insights of structure–activity relationship

Abstract

The direct methane to methanol (DMTM) conversion was studied in a fixed bed reactor under varying reaction parameters including temperature, and weight hourly space velocity (WHSV) and CH₄:O₂ over several CeO₂-ZSM5, CeO₂-SiO₂ and CeO₂-Al₂O₃ supported CuO catalysts prepared by wetness impregnation method and characterized by several techniques including N₂ adsorption desorption, X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), ultraviolet visible spectroscopy (UV–vis), temperature programmed reduction (H₂-TPR), temperature programmed desorption (CO₂-TPD) and CO-diffuse reflectance infrared Fourier transform spectroscopy (CO-DRIFT) studies. The characterization results confirm the formation of bis(µ-oxo) dicopper or mono-(µ-oxo) dicopper species considered as the active sites for the DMTM reaction. Moreover, various copper (Cu²⁺, Cu⁺ and Cu⁰) and cerium (Ce³⁺ and Ce⁴⁺) species are coexisted in a redox cycle equilibrium (Cu⁺ + Ce⁴⁺ ⇌ Cu²⁺ + Ce³⁺) depending on CeO₂ loading. XPS studies indicates the generation of lattice and adsorbed oxygen species on deposition of CeO₂ and their ratio varied with the CeO₂ loading. Moreover, the different cerium species induces charge unbalance, oxygen vacancies, and formation of unsaturated chemical bonds on the catalyst’s surface. The deposition of CeO₂ and CuO incorporates more Lewis’s acid sites in xCu/yCe-ZSM5 composite catalysts and even formation of additional Lewis’s acid sites originated from exchanged copper species. The various copper and cerium species formed in the composite catalysts strongly influenced the methane molecule activation, and selectivity and yield of methanol. The surface Cu²⁺ species promotes the formation of methanol and prevents the methanol overoxidation forming oxygenates and carbon dioxide. In addition to the Cu²⁺ species, the lattice and adsorbed oxygen generated on deposition of CeO₂ also influence the formation and oxidation of methanol. Thus, optimum surface concentration of Cu²⁺ and lattice to adsorbed oxygen maximizes the yield of methanol. The process parameters also the affect the methane conversion and methanol selectivity and yield. The methanol selectivity of 6.34 % with methane conversion of 37.89 % was achieved over 20Cu/15CeZ catalysts at 873 K, 1030 ml hr^-1 gcat⁻¹ and CH₄:O₂ = 2:1. A plausible reaction mechanism of oxidation of methane to methanol based on the activity results and in-situ DRFIT studies of methanol oxidation.