SAPO-34分子筛合成中的主客体电荷平衡与硅分布

doi:10.1016/S1872-2067(15)61025-7

摘要/Abstract

摘要：

SAPO-34分子筛的硅磷酸铝组成与菱沸石笼结构孔道促进了甲醇高效转化为乙烯、丙烯(MTO)的反应, 以其为催化剂的工业过程不断取得进步. 然而, MTO反应过程中SAPO-34分子筛的迅速失活成为困扰该过程的重要问题. 研究发现, 反应中形成的多环芳烃阻塞了SAPO-34分子筛晶粒表面的笼结构孔道, 甲醇分子难以向晶体内部扩散, 致使分子筛在其活性中心未曾充分利用之前便已失活. 为此, 纳米晶粒SAPO-34的合成引起了人们的广泛兴趣, 通过提高分子筛晶体的利用率有限地延长了催化剂的MTO反应寿命. 但反应后催化剂的结焦量明显增加, 说明引起SAPO-34结焦的因素不因晶粒的减小而受到抑制. 我们曾研究了无硅AlPO₄-34分子筛的MTO反应, 发现初始活性良好的催化剂在未结焦的情况下却因为活性中心的缺失而失活. 显然, SAPO-34的MTO反应活性及其结焦均与其酸性密切相关, 而酸性取决于其结构中的硅含量与硅分布. 我们通过单晶结构解析获得了二乙胺(DEA)与哌嗪(PIPZ)导向合成SAPO-34的晶体结构, 并从文献获得了四乙基氢氧化铵(TEAOH)、三乙胺(TEA)、吗啉(MOR)为模板剂合成SAPO-34的晶体结构. 按照SAPO-34结构中菱沸石笼内形成的模板剂阳离子的电荷数(R)对不同模板剂导向合成的SAPO-34加以分类: TEAOH, TEA (R⁺); DEA, MOR (2R⁺); PIPZ (2R²⁺). 它们不因SAPO-34合成条件的变化而改变. 依据分子筛骨架负电荷与SAPO-34菱沸石笼内模板剂阳离子正电荷平衡的原则(主客体电荷平衡), 模板剂的类型决定SAPO-34骨架负电荷数, 即引入硅形成的酸中心密度; 酸中心的强度则取决于合成体系中硅的加入量. 由此可以给出不同类型模板剂导向合成SAPO-34骨架中的最低硅含量(形成隔离硅原子). R⁺型模板剂SiO₂/Al₂O₃为0.11; 2R⁺型模板剂为0.22; 2R²⁺型模板剂为0.44.
合成体系中硅加入量的增加, 会在SAPO-34骨架中形成"硅岛"结构. 然而, 我们的研究表明, "硅岛"的形成同样受到主客体电荷平衡原则的制约. 通过结构分析发现, 一个独立的"五硅岛"至少为6个相邻的菱沸石笼所分享, 而每个"五硅岛"仅能形成3个负电荷. 以R⁺型模板剂导向合成的SAPO-34为例, 需要6个骨架负电荷来平衡6个菱沸石笼中的R⁺电荷, 因此, 除了"五硅岛"的3个负电荷, 还要在这些笼骨架上形成3个"孤立"硅原子. 该类模板剂导向合成SAPO-34, 形成"五硅岛" 的最低SiO₂/Al₂O₃摩尔比(硅含量)为0.45. 进一步分析可知, 在相邻的7个菱沸石笼之间可以分别形成"八硅岛"、"十一硅岛", 而"十四硅岛"只能形成于相邻的10个菱沸石笼之间. 同理, 每种"硅岛"的形成都要伴随相应数目的"孤立"硅原子共同来平衡菱沸石笼内的R⁺电荷. 对于以2R⁺型模板剂导向合成的SAPO-34, 仍然是6个相邻的菱沸石笼分享"五硅岛", 但是, 每个菱沸石笼内的正电荷阳离子数增加了一倍, 为满足主客体电荷平衡, 需要形成更多"孤立"硅原子. 由此可见, 随着菱沸石笼内模板剂电荷数的增加, 骨架的负电荷密度增大, 给SAPO-34带来了更多酸中心, 无论硅以"孤立"硅原子形式分布, 还是形成"硅岛". 同时, "硅岛"伴随"孤立"硅原子的共同存在也使我们理解了²⁹Si MAS NMR中的一个独特现象：在硅含量很高时形成了"硅岛", 可是却存在着很强的属于"孤立"硅原子的谱峰.

关键词: SAPO-34, 硅分布, 合成, 硅岛, 甲醇制烯烃

Abstract:

The distribution of Si atoms in the SAPO-34 framework determines its acidity and catalytic effects. This was investigated using the charge balance between the inorganic framework and trapped template ions. Three types of templates, which yielded R⁺, 2R⁺ and 2R²⁺ positive charges in the cages of SAPO-34, were obtained from single crystal data and they were used to direct the synthesis of SAPO-34 with different Si contents and formation of isolated Si atoms and Si islands in the lattice. The concentration limits of SiO₂ in the gel for constituting isolated Si atoms were calculated and verified experimentally. Si islands, including 5-Si, 8-Si, 11-Si, 14-Si island were described on the basis of host-guest charge compensation. An overall view of the distribution of Si atoms in SAPO-34 was given and a criterion for the strength and density of acid sites in SAPO-34 for it to be an efficient catalyst for MTO was made available.

Key words: SAPO-34, Si distribution, Synthesis, Silicon island, Methanol to olefins

赵慧茹, 史淑美, 吴金雄, 丁月, 李牛. SAPO-34分子筛合成中的主客体电荷平衡与硅分布[J]. 催化学报, 2016, 37(2): 227-233.

Huiru Zhao, Shumei Shi, Jinxiong Wu, Yue Ding, Niu Li. Charge compensation dominates the distribution of silica in SAPO-34[J]. Chinese Journal of Catalysis, 2016, 37(2): 227-233.

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