硅源及晶化时间对SAPO-5分子筛结构及性质的影响

EFFECTS OF Si SOURCES AND CRYSTALLIZATION TIME ON STRUCTURE AND NATURE OF SAPO5

分别以硅凝胶和正硅酸乙酯为硅源，通过改变晶化时间合成了系列ＳＡＰＯ５分子筛．用ＸＲＤ，ＭＡＳＮＭＲ和ＳＥＭ对分子筛的结构和形貌进行了表征．结果表明，以硅凝胶为硅源时，凝胶中的铝有３种面体配位状态．加热晶化很容易使凝胶转化为ＳＡＰＯ５分子筛，合成物的晶形更加规则．在４８ｈ内，延长晶化时间有助于硅进入分子筛骨架，使残余的Ａｌ２Ｏ３·Ｈ２Ｏ进一步反应，使骨架硅和铝的面体配位环境更加对称；超过４８ｈ后，硅开始游离出分子筛骨架，并使部分ＳＡＰＯ５转化为ＳＡＰＯ３４．以正硅酸乙酯为硅源时，硅和铝的反应活性均较低，凝胶中ＡｌＰＯ４及Ａｌ２Ｏ３·Ｈ２Ｏ的量均比四面体配位铝的量大．只有当晶化时间超过４８ｈ时，反应物中Ａｌ２Ｏ３·Ｈ２Ｏ才能完全转化．在此反应体系中，延长晶化时间有助于ＳＡＰＯ５的生成，使分子筛骨架硅的取代量更大，晶形更加规则，骨架铝的对称性更好．

A series of SAPO_5 was synthesized by using SiO_2 gel and Si(OEt)_4 as Si sources and by changing the crystallization time. The products were characterized by XRD, MAS NMR and SEM. The results reveal that Al has three kinds of coordination states, the precursor with tetrahedrallycoordinated Al, octahedrallycoordinated AlPO_4 and pseudobeohmite, in the gel mixture when using SiO_2 gel as Si source. Heating converts the gel to SAPO_5 easily, and the crystal shape becomes more regular with prolonging the crystallization time . Within 48 h, prolonging the crystallization time helps the Si in the gel mixture to enter the framework of SAPO_5, promotes further the conversion of Al_2O_3·H_2O, and improves the symmetry of coordination environment of Al and Si. When the crystallization time exceeds 48 h, the Si in the framework begins to leave its sites, and partial SAPO_5 converts to SAPO_34. When Si(OEt)_4 is used as Si source, the reaction activity of both Si and Al is lower than that when SiO_2 gel is used as Si source. In addition, the amounts of AlPO_4 and Al_2O_3·H_2O in Si(OEt)_4 gel mixture are larger than those in SiO_2 gel mixture. Only when the crystallization time exceeds 48 h, the reactant of Al_2O_3·H_2O can completely convert into the framework of SAPO_5. In this system, prolonging the crystallization time helps the formation of SAPO_5, causes the Si to replace more Al and P in the framework, gives more regular morphology of the crystal, and improves the symmetry of the coordination environment of Al.