Performance of SO₄^2-/AC catalysts prepared with different precursors in the synthesis of polyoxymethylene dimethyl ethers from dimethyl ether direct oxidation

A series of SO₄^2-/AC bifunctional catalysts, in which SO₄^2- and AC acted as the acid sites and redox sites, respectively, are prepared by impregnation with H₂SO₄ and (NH₄)₂SO₄ as precursors and active carbon as support; their catalytic performance in the direct oxidation of dimethyl ether (DME) to polyoxymethylene dimethyl ethers (DMM_x) was investigated. The results show that the catalytic performance of SO₄^2-/AC catalysts is significantly related to the precursor used. Over the 40%H₂SO₄/AC catalyst, the selectivity to DMM_1-2 reaches 59.7%, with a DME conversion of 8.4%; in addition, there is no CO_x observed in the products. However, a large amount of CO_x by-product is formed over the 40%(NH₄)₂SO₄/AC catalyst; meanwhile, the selectivity to DMM is only 2.7% and no DMM₂ is formed. The XRD, N₂ sorption, NH₃-TPD and O₂-TPD-MS characterization results illustrate that the suitable amount of weak acid sites and redox sites of the H₂SO₄/AC catalyst is beneficial to the formation of long chain DMM_x from DME oxidation. The modification of AC with SO₄^2- promotes the activation of O₂ over the surface of AC support, whereas the introduction of H₂SO₄ improves the weak acid sites of the catalyst. On the contrary, the number of medium-strong acid sites of the catalyst is increased by modifying AC with (NH₄)₂SO₄.