SAPO-5分子筛的晶化过程研究

采用X射线衍射、 X射线荧光光谱、 扫描电子显微镜和固体核磁共振波谱等方法对SAPO-5分子筛的晶化过程进行了研究. 结果表明, SAPO-5分子筛的生成遵循液相晶化机理. 在晶化前期首先生成一种具有较为规整形貌的无定形磷酸铝(记为Am), 晶化温度大于200 ℃后开始出现SAPO-5, 同时无定形物质Am的量也明显增加, 随后Am逐渐溶解并贡献于SAPO-5的生长; 硅原子在晶化初期直接参与晶化进入SAPO-5分子筛骨架, 随着晶化时间延长, 分子筛晶体中硅含量逐渐增加; SAPO-5骨架仅能容纳少量的Si(4Al)物种, 在样品含有较低硅含量时骨架中即出现硅岛. XPS分析进一步揭示SAPO-5分子筛存在表面富硅现象, 硅在晶体中从核到壳含量递增. 研究结果也表明, 分子筛的拓扑结构和硅源选择影响晶体表面富硅程度.

Study on Crystallization Process of SAPO-5 Molecular Sieve

The crystallization process of SAPO-5 molecular sieve（AFI type） was studied by X-ray diffraction， X-ray fluorescence spectroscopy， scanning electron microscopy and solid-state MAS NMR. The results show that the formation of SAPO-5 molecular sieve follows a liquid-mediated mechanism. In the early stage of crystallization， amorphous aluminophosphate particles with relatively regular morphology（named substance Am） were first formed. SAPO-5 began to appear after the crystallization temperature reached 200 ℃， and the amount of amorphous substance Am also increased significantly. Subsequently， substance Am gradually dissolved and contributed to the growth of SAPO-5. The Si atoms directly participated in the formation of SAPO-5 framework since the early stage of crystallization. As the crystallization proceeded， the Si content in the mole-cular sieve crystals gradually increases. The SAPO-5 framework can only accommodate a small amount of Si（4Al） species， and the Si islands started to appear at low Si content. XPS analysis further revealed that SAPO-5 molecular sieve has surface Si enrichment phenomenon， implying that the Si content in the crystal increases from the core to the shell. The topology of the molecular sieve and the choice of Si source affect the degree of Si enrichment on the crystal surface.