不同方法脱铝八面沸石性质研究——Ⅰ.骨架结构与 Si、Al 分布

STUDY ON FAUJASITES DEALUMINATED WITH DIFFERENT METHODS——Ⅰ.STRUCTURE,COMPOSITION CHARACTERIZATION AND Si,AI DISTRIBUTION

采用XRD、IR、XRF、XPS等多种实验方法研究了不同方法制备脱铝八面沸石的骨架结构及Si、Al 分布差异。结果表明,随着骨架脱铝,所有脱铝沸石的晶胞参数a_0减小,骨架振动频率向高频位移。(NH_4)_2SiF_6脱铝样品由于外部Si 的补入,结晶度很高;而H_2C_2O_4法脱铝样品要保持高的结晶度,需严格操作。H_2C_2O_4和(NH_4)_2SiF_6溶液化学脱铝法都造成了脱铝沸石表面很高的Si/Al 比;而SiCl_4和水热气相脱铝沸石的表面则呈富铝状态。脱铝过程中,骨架外铝可能以铝阳离子形式占据沸石中S_1位;当单胞平均骨架外铝原子数多于16时,开始向表面富集。化学脱铝样品中包含少量骨架外硅,在其进一步水热处理过程中也向表面富集,致使表面Si/Al 比提高。

Dealumination is well known to be one of the most important methods of improving Y-zeolite'(?) hydrothermal stability and catalytic performances.With increasing framework Si/Al ra- tio,(1)the unit cell size decreases and framework stretching vibration frequencies v_1,v_2 increase; (2)(NH_4)_2SiF_6,SiC(?)ealuminated Y-zeolites show more perfect framework structure with a higher crystallinity,while H_2C_2O_4 dealuminated Y-zeolites have a lower crystallinity at more than 25% framework alurninium removal. The outer surface of the NH_4Y crystallite(after three times NH_4^+ exchange)is a little rich in Si.With the Y-zeolites dealuminated chemically by(NH_4)_2SiF_6,H_2C_2O_4 respectively in solution, the Si/Al ratios on outer surfaces are found to be much greater than those in the bulk,even to 7～ 10,which originates from nonuniform framework dealumination by diffusion-limitod effect.On the contrary,in gas-phase SiCl_4 and hydrothermally dealuminated Y-zeolites,the surface Si/Al ratios by XPS are lower than those in the bulk and the framework Si/Al ratios estimated by IR,XRD are greater than those by XRF,chemical analysis.This indicates the existence of a large amount of nonframework aluminium species in the two kinds of dealuminated Y-zeolites,especially on the outer surface.Consequently,for most dealuminated Y-zeolites the concentration gradients of Si/Al are mainly located at 300～500(?) surface layer. At low framework Si/Al levels,no accumulation of aluminium takes place,nonframework alu- minium specifically locates at S I sites,which can be expected for cationic aluminium.When the number of nonframework aluminium atoms per unit cell is more than 16,they start to migrate onto outer surface of crystallite.