Beta分子筛催化剂上甲苯和三甲苯烷基转移反应条件优化及其反应机理研究

Optimization of Reaction Conditions in the Transalkylation of Toluene with 1,2,4-Trimethylbenzene Catalyzed by Beta Zeolite and the Investigation of Its Reaction Mechanism

以Beta分子筛为催化剂,系统考察了反应条件和催化剂酸性变化对甲苯和三甲苯烷基转移反应催化性能的影响.实验结果表明,反应条件、分子筛晶粒尺寸及其酸性对烷基转移催化活性和稳定性都具有显著影响.研究发现:当反应温度为450℃、压力为3 MPa以及反应物甲苯和三甲苯物质的量比为1时,二甲苯收率达到最高;分子筛催化活性随硅铝比升高而降低;催化剂晶粒尺寸减小能够显著提高反应稳定性.同时,还对甲苯和三甲苯烷基转移反应机理进行了深入研究,推导出了可能的反应机理路线图.

The effect of reaction conditions and acidic property of Beta zeolite on the transalkylation reaction between toluene and trimethylbenzene （TMB） was studied systematically. The results indicate that the activity and stability of catalysts are strongly dependent on the reaction conditions including reactant composition, reaction temperature and pressure, and crystal size as well as the acidic property of the zeolite. For TMB disproportionation reaction, the addition of toluene should be helpful to the increase of selectivity of xylene. In the meantime, for the disproportionation reaction of toluene, the mixing of TMB into feedstocks can also increase the xylene yield. Therefore, the higher yield of xylene is obtained at the equal molar ratio oftohiene and TMB as feedstocks： The temperature of reaction plays an important role on the transalkylation of toluene with TMB. The catalytic activity of transalkylation increases gradually with the increasing of reaction temperature until 450 ℃, and then the dealkylation reaction and the formation of coke will be intensified with the further increasing of temperature, which leads to the decrease of catalytic stability. Therefore, the optimum temperature for transalkylation reaction is 450 ℃. The increasing of reaction pressure also has a positive effect on the catalytic activity of transalkylation. Thus, the transalkylation reaction is conducted at the pressure of 3 MPa taking into account the bearing capacity of reactor. The transalkylation catalytic activity is decreased with the increase of Si/Al ratio of Beta zeolite because of the reduction of the amount of acid sites that act as the active sites for transformation of alkylaromatics. The catalytic stability of zeolite could be significantly improved with the decrease of crystal size. In order to investigate the reaction mechanism of transalkylation, gas chromatography-mass spectrometry （GC-MS） technique is used to study the generation and decomposition of intermediate species over Beta zeolite during the transalkylation reaction at the lower experiment temperature of 150 ℃ because the intermediate species is instable at higher reaction temperature. The results indicate that the transalkylation of toluene with 1,2,4-TMB occurs via the bimolecular intermediate mechanism. As the bulky species, the formation and decomposition of these intermediate species require a large zeolite channel. Therefore, the Beta zeolite with 12 member-ring large pore shows higher catalytic activity for transalkylation reaction.