双模板剂法SAPO-34分子筛的合成及其MTO催化性能调变

以四乙基氢氧化铵（TEAOH）和二乙铵（DEA）为混合模板剂,在低投料硅铝比n (SiO₂) ∶n (Al₂O₃)=0.2]及低模板剂用量n (模板剂) ∶n (Al₂O₃)=1.9]下,考察了两种模板剂比例的调变对合成的SAPO-34分子筛物化性能及其催化甲醇制烯烃反应（MTO）催化性能的影响。研究发现,通过改变两种模板剂比例,可以明显调变SAPO-34分子筛晶粒尺寸、硅分布（晶粒表面和体相的硅分布）、硅原子的配位环境,从而影响其MTO催化反应的效果。在低模板剂用量制备的SAPO-34产品中,晶粒尺寸是影响其催化寿命的最主要因素,小晶粒分子筛因其扩散路径短有利于延长催化寿命。此外,硅分布也是影响催化寿命的因素之一,表面富硅的分子筛导致外表面积炭程度大于晶内积炭,积炭趋势由外向内发展,加速分子筛“假性”失活。硅分布还影响MTO反应产物分布,表面富硅分子筛外表面更易发生非择形催化,显著提高C₄～C₆等产物的选择性,不利于目标产物双烯（乙烯+丙烯）选择性的提高。

Synthesis of SAPO-34 with dual-template method and its MTO catalytic performance

SAPO-34 molecular sieve was synthesized by using a dual-template (TEAOH and DEA), under the conditions of low silica content n(SiO₂)∶n(Al₂O₃)=0.2] and low template contentn(template) : n(Al₂O₃)=1.9] in the starting gel. The effect of TEAOH/DEA ratio on the physicochemical properties of SAPO-34 molecular sieve and its catalytic performance in the methanol to olefin (MTO) reaction were investigated. It has been found that the ratio of TEAOH and DEA have significant effect on the crystal size, Si distribution and coordination environment of framework Si. The crystal size and Si distribution are the major factors to determine the catalytic lifetime of MTO. Due to its short diffusion pathway, small crystals prolong the catalytic lifetime, while SAPO-34 with Si-rich surface leads to more carbon deposition on the surface than interior, which causes the “fictitious” deactivation of molecular sieves. Si distribution also determines the product profiles of MTO reaction, for example, the crystals with Si-rich surface would favor the non-shape selectivity catalysis, leading to more C₄～C₆ hydrocarbon products and thus suppressing the selectivity of ethylene + propylene.