P-HZSM-5分子筛的一步法直接合成及其MTP催化性能的研究

以正硅酸乙酯、偏铝酸钠、磷酸、四丙基氢氧化铵为原料,采用水热法,一步直接合成了P-HZSM-5分子筛,着重考察了磷酸对其结构、酸性、粒径和甲醇转化制烯烃(MTO)催化性能的影响。结果表明,随着磷酸用量的增加,其晶体对称性由正交晶系转变为单斜晶系,合成的P-HZSM-5分子筛的结晶度降低;同时,进入分子筛骨架的铝含量减小,分子筛酸性降低,粒径逐渐变大,形貌由球形变为长条形。这些变化显著影响甲醇转化制烯烃反应的选择性。n(H3PO4)/n(Al2O3)投料比分别为2和4时,乙烯与丙烯总的收率相近(分别为52.30%和52.63%);然而产物的丙烯/乙烯比(P/E)有明显差异(分别为7.02和4.12);通过调节磷酸用量,可以调控丙烯(或乙烯)的选择性。

One-step synthesis of P-HZSM-5 zeolite and its catalytic performance in MTP reaction

P-HZSM-5 zeolite was successfully synthesized in one step under conventional hydrothermal conditions by using tetraethyl orthosilicate (TEOS), sodium metaaluminate, phosphoric acid (85%), and tetrapropylammonium hydroxide (TPAOH, 25%) as raw materials. The influence of phosphate acid content on the structure, acidity, particle size, and catalytic performance in methanol conversion to propene (MTP) of P-HZSM-5 obtained was investigated. The results indicated that the crystallinity, acidity and particle size of PZSM-5 zeolites are decreased with the increase of phosphoric acid amount used in the synthesis; meanwhile, the crystal symmetry is changed from orthorhombic to monoclinic and the morphology is transformed from spherical into strip. When P-HZSM-5 is used as the catalyst in MTP, these changes exhibited evident influence on the product selectivity. Over the P-HZSM-5 samples obtained with a molar ratio of H₃PO₄/Al₂O₃ being 2 and 4, the total yields of ethylene and propylene are close (52.30% and 52.63%, respectively), but the ratio of propene/ethene in the product are quite different (7.02 and 4.12, respectively); it suggests that the product selectivity of MTP can be adjusted by the phosphoric acid content.