固相外延生长法制备ZSM-5@Silicalite-1分子筛及其CO2加氢耦合甲苯烷基化反应的影响

Effect of ZSM-5@Silicalite-1 zeolites prepared by solid phase epitaxial growth method on CO2 hydrogenation and toluene alkylation reactions

  • 摘要: 本研究采用固相法在ZSM-5表面外延生长Silicalite-1,制备出ZSM-5@Silicalite-1分子筛。同时制备高活性氧化物ZnZrOx,并与ZSM-5@Silicalite-1物理混合组成ZnZrOx/ ZSM-5@Silicalite-1双功能催化剂,研究了CO2加氢耦合甲苯烷基化催化性能。相比于ZnZrOx/ZSM-5催化剂,分子筛改性后的双功能催化剂提高了对二甲苯(PX)选择性。研究了晶化条件(硅源、晶化过程、晶化次数)对ZSM-5外延生长Silicalite-1的影响,以及Silicalite-1钝化层厚度对CO2加氢耦合甲苯烷基化反应性能的影响。在400 ℃、3 MPa反应条件下,ZZO/1:3.5Z5-Na-SiO2催化剂的甲苯转化率为12.0%,二甲苯选择性为77.4%,在二甲苯中对二甲苯选择性为73.4%。通过SEM、XRD、N2吸附-脱附、XPS、NH3-TPD、Py-FTIR等表征,研究了分子筛的结构和酸性质。结果表明,通过固相外延生长,延长ZSM-5的孔道,增加间二甲苯(MX)、邻二甲苯(OX)的扩散阻力,同时钝化外表面的酸性,可以有效提高对二甲苯(PX)的选择性。固相外延生长法改性ZSM-5分子筛,摒弃了以往堵塞孔以缩小孔口改性分子筛的缺点,在保证催化剂活性的同时提高了产物选择性。

     

    Abstract: CO2 hydrogenation to synthesize high value-added aromatics is of great significance to alleviate the energy climate problem caused by CO2 emission. It is generally believed that the reaction course of CO2 hydrogenation of toluene coupled with alkylation to produce xylenes is as follows: firstly, CO2 reacts with H2 to produce methanol intermediates, and then the methanol intermediates react with toluene on zeolite catalysts to produce para-xylene (PX) by alkylation. According to the reaction pathway, it is necessary to construct a bifunctional catalyst with synergistic matching of the two process conditions to simultaneously realize the hydrogenation of CO2 to methanol intermediate and the alkylation of the intermediate and toluene to generate para-xylene. The ZnZrOx/ZSM-5 catalytic system, in which the ZnZrOx has strong thermal stability and CO2 activation ability, and the ZSM-5 has a good morphology selectivity for PX, is considered to be a promising CO2 hydrogenated toluene coupled alkylation catalyst. However, this system still suffers from low PX selectivity, mainly due to the presence of non-selective acidic sites on the outer surface of the zeolite or near the pore orifice, which leads to the generation of side reactions, such as deep methylation and toluene isomerization, and reduces the selectivity. In this paper, ZSM-5@Silicalite-1 zeolites were prepared by epitaxial growth of Silicalite-1 on the surface of ZSM-5 using solid-phase synthesis. At the same time, the highly active oxide ZnZrOx was prepared and physically mixed with ZSM-5@Silicalite-1 to form a ZnZrOx/ZSM-5@Silicalite-1 bifunctional catalyst to study the catalytic performance of CO2 hydrogenation coupled with toluene alkylation. Compared with the ZnZrOx/ZSM-5 catalyst, the modified zeolite improved the para-xylene (PX) selectivity. The effect of crystallization conditions (silicon source, crystallization process, and number of crystallizations) on the epitaxial growth of Silicalite-1 from ZSM-5 was investigated, as well as the effect of the thickness of the Silicalite-1 passivation layer on the performance of the reaction between carbon dioxide hydrogenation and toluene alkylation. The ZZO/1:3.5Z5-Na-SiO2 catalyst showed a toluene conversion of 12.0%, a xylene selectivity of 77.4%, and a PX selectivity of 73.4% in xylene under 400 ℃ and 3 MPa reaction conditions. The structure and acid properties of the zeolites were investigated in detail by SEM, XRD, N2 adsorption-desorption, XPS, NH3-TPD and Py-FTIR characterization, and the results show that the selectivity of para-xylene (PX) can be effectively improved by solid-phase epitaxial growth to extend the pore channels of ZSM-5, increase the diffusion resistance of m-xylene (MX) and o-xylene (OX), and passivate the acidity of the outer surface at the same time. This method abandons the disadvantage of previous modification of molecular sieves by blocking the pores to narrow the orifice, and upgrades the product selectivity while ensuring the catalyst activity.

     

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