Effects of calcination temperature on the catalytic performance of Ti(SO₄)₂/CS for DME direct oxidation to polyoxymethylene dimethyl ethers

A series of Ti(SO₄)₂/activated carbon spheres (CS) bifunctional catalysts were designed and prepared by impregnation method, and the effect of calcination temperature of the catalysts on direct oxidation of dimethyl ether (DME) to polyoxymethylene dimethyl ethers (DMM_x) was investigated. The results showed that the performance of Ti(SO₄)₂/CS catalysts was closely related to the calcination temperature. The 30% Ti(SO₄)₂/CS catalyst calcined under O₂ atmosphere at 280 ℃ exhibited excellent activity over which the conversion of DME reached 11.7% with the selectivity of DMM₁₋₃ up to 75.8%, wherein, the selectivity of DMM₂₋₃ was over 30%. The texture and surface properties of the catalysts were characterized by SEM, XRD, Raman, TG, NH₃-TPD and XPS. The suitable amount of weak acid sites and redox sites of the Ti(SO₄)₂/CS were beneficial to the direct oxidation of DME to DMM_x. The calcination temperature changed the distribution of functional groups on the surface of CS which then affected the dispersion form of Ti(SO₄)₂. The type and amount of acid centers especially the ratio of weak acid and medium strong acid could also be adjusted, which can lead to different gradients of the surface acidity of the catalyst. The reasonable matching of the acidic and redox sites on the catalyst can evidently promote the growth of C−O chain.