Role of ball milling during Cs/X catalyst preparation and effects on catalytic performance in side-chain alkylation of toluene with methanol

球磨在制备Cs/X型催化剂中的作用以及对甲苯甲醇侧链烷基化反应的影响

Ball milling modification was performed on Cs/X catalysts before or after cesium ion exchange.Multiple characterization results(such as pyridine-FTIR,XPS,and solid-state NMR)demonstrated that ball milling played a distinct role in these two different preparation procedures of the catalyst.Ball milling performed after the cesium modification has a strong influence on the Cs/X structure and acid-base properties,which results in the enhancement of the catalytic performance for side-chain methylation of toluene with methanol.Detailed studies revealed that ball milling intensified the interactions between oxides and molecular sieves,which not only increased the dispersion of the Cs species but also generated some weaker basic centers.It is proposed that the new basic centers could be Si-O-Cs and Al-O-Cs,which are produced by breaking of the Si-O-Al bonds of the zeolite framework under the synergetic effect of ball milling and alkali treatment.These new active sites may help to promote the side-chain methylation reaction.However,excessive ball milling will lead to the vanishing of zeolite micropores,thus deactivating side-chain methylation activity,which indicates that microporosity plays a key role in side-chain methylation and individual basic centers cannot catalyze this reaction.

苯乙烯是重要的化工中间体,可以用来生产聚苯乙烯以及其他多聚物化学品.工业上苯乙烯主要通过苯和乙烯烷基化生成乙苯,再将乙苯脱氢制得.该方法存在能耗高、工艺流程复杂、生产成本高等问题.甲苯与甲醇在碱性分子筛上可以发生侧链烷基化反应直接生成苯乙烯,在各种碱性分子筛上中,Cs离子交换的X型分子筛催化剂(CsX)表现出较为突出的催化性能.尽管如此,低的甲苯转化率和苯乙烯选择性仍然是限制甲苯甲醇侧链烷基化工艺在工业上应用的主要因素.为了进一步提高甲苯甲醇侧链烷基化反应的催化性能,向CsX催化剂中添加其他化合物或助剂的化学改性方法被广泛研究.本文采用一种物理方法—球磨法对CsX进行改性,并对比了13X分子筛在球磨前和球磨后担载Cs对甲醇甲苯侧链烷基化反应的影响.研究发现,CsX分子筛在经过球磨之后,其催化性能大幅提高.通过各种表征手段进一步阐释了球磨法在分子筛改性过程中的作用机理.CO2-TPD结果表明,Cs X在球磨之后碱量增加而碱强度降低.XPS结果显示,CsX-mill上Cs元素主要以离子型的Cs离子形式存在,而CsX上的Cs元素主要以氧化物形式存在.Cs阳离子是路易斯酸中心,可以与甲苯的苯环发生作用,从而吸附稳定甲苯.离子型Cs比例的增加意味着增加了活性路易斯酸中心.27Al和29Si MAS NMR结果表明,CsX经过球磨之后,Si和Al的配位环境也产生了些许变化,Cs X-mill上有非骨架铝出现.综上,我们认为催化效果的显著差异归因于在球磨过程中分子筛上的Cs2O与分子筛载体发生了强相互作用,进而改变了催化剂的酸碱性质并形成了Si-O-Cs和Al-O-Cs等新的碱性中心.这些新的碱性中心有别于传统的Cs X上的碱性中心,且可能对反应有促进作用.然而,长时间球磨会导致催化剂结构彻底坍塌和微孔消失,这也从侧面证明了微孔在甲苯甲醇侧链烷基化反应中的重要性.球磨法的优势在于不必改变CsX催化剂的Cs含量就可以调节催化剂的酸碱性质,进而提高侧链烷基化反应的催化效果.