不同前驱体制备的SO42-/AC催化剂在二甲醚直接氧化合成聚甲氧基二甲醚中的反应性能

摘要

A series of SO24-/AC bifunctional catalysts,in which SO42-and AC acted as the acid sites and redox sites,respectively,are prepared by impregnation with H2 SO4 and (NH4)2 SO4 as precursors and active carbon as support;their catalytic performance in the direct oxidation of dimethyl ether (DME) to polyoxymethylene dimethyl ethers (DMMx) was investigated.The results show that the catalytic performance of SO24-/AC catalysts is significantly related to the precursor used.Over the 40％ H2SO4/AC catalyst,the selectivity to DMM1-2 reaches 59.7％,with a DME conversion of 8.4％;in addition,there is no COx observed in the products.However,a large amount of COx 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 DMMx from DME oxidation.The modification of AC with SO42-promotes the activation of O2 over the surface of AC support,whereas the introduction of H2 SO4 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.%分别用硫酸、硫酸铵作为前驱体,活性炭(Ac)作为载体,采用浸渍法制备了SO24-/AC双功能催化剂(SO42-离子和AC载体分别提供催化剂的酸性和氧化还原性),考察了其在二甲醚(DME)直接氧化合成聚甲氧基二甲醚(DMMx)反应中的催化性能.结果表明,不同前驱体制备的SO2-/AC催化剂表现出显著的催化活性差异.40％H2SO4/AC催化剂具有较好的反应活性,DME转化率为8.4％,DMM1-2的选择性达到59.7％,并且没有COx的生成;而在40％(NH4)2SO4/AC催化剂上,反应主要生成了COx,DMM选择性仅2.7％,且无DMM2生成.XRD、BET、NH3-TPD及O2-TPD-MS等表征结果显示,H2SO4/AC催化剂中适量的弱酸性位和氧化还原性位有利于DME直接氧化合成DMMx.经过SO42-修饰的催化剂促进了O2在活性炭表面的活化;前驱体H2SO4的加入提高了催化剂表面的弱酸性位数量,而(NH4)2SO4的引入却促使催化剂表面产生中强酸性位.