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甲苯在无担体磷酸改性的ZSM-5催化剂上,歧化反应活性急剧下降,但在氧化铝担体存在下,反应活性下降缓慢。研究结果表明,磷酸与氧化铝生成磷酸铝,可起到良好的改性作用,不但对催化剂的歧化活性没有多大损害,而且使甲苯歧化生成二甲苯的选择性,大幅度地提高.如在温度500℃、WHSV5.7小时~(-1)、H_2:甲苯=2∶1(摩尔)时,甲苯转化率、对二甲苯选择性和对二甲苯收率分别为16.0%、95.3%和8.32%,而改性前,在同样条件下,则分别为17.0%,26.8%和2.46%.而且,磷酸与ZSM-5直接作用,大大减少了催化剂的酸中心数。磷酸、氧化铝和ZSM-5三者之间的作用,主要表现在强酸和弱酸中心的比例变化和总酸量的减少。改性后,L酸中心变化不明显,B酸中心变化很大。通过分析和测定,说明甲苯歧化的活性中心主要归因于B酸中心;还说明改性分子筛结构没有变化,即磷原子不能取代骨架中的硅和铝原子.
Toluene disproportionation reaction activity dropped sharply on the aphilic phosphate-modified ZSM-5 catalyst, but the reaction activity decreased slowly in the presence of alumina carrier. The results show that the phosphoric acid and aluminum oxide to produce aluminum phosphate, can play a good role in the modification, not only on the disproportionation activity of the catalyst without much damage, but also the selectivity of toluene disproportionation xylene greatly increased, such as at the temperature The toluene conversion, p-xylene selectivity and p-xylene yield were 16.0%, 95.3% and 8.32% respectively at 500 ℃ and WHSV of 5.7 hours at -1 H 2: toluene = 2:1. , And 17.0%, 26.8% and 2.46% respectively under the same conditions before modification, furthermore, the direct interaction between phosphoric acid and ZSM-5 greatly reduced the acid number of the catalyst. The role of phosphoric acid, aluminum oxide and ZSM-5 is mainly reflected in the change of the ratio of the strong acid to the weak acid and the decrease of the total acidity. After modification, L acid center change is not obvious, B acid center changes greatly. Through the analysis and determination, the active center of toluene disproportionation is mainly attributed to the B acid center; also shows no change in the molecular structure of the modified zeolite, that is, phosphorus atoms can not replace the skeleton of silicon and aluminum atoms.