Distribution of Aluminum over the Tetrahedral Sites in ZSM-5 Zeolites and Their Evolution after Steam Treatment
Distribution of Aluminum over the Tetrahedral Sites in ZSM-5 Zeolites and Their Evolution after Steam Treatment
复制标题
DOI：
10.1021/acs.jpcc.8b05277
复制
发表时间：
2018-07-12
影响因子：
3.7
通讯作者：
Skibsted, Jurgen
中科院分区：
文献类型：
--
作者：
Holzinger, Julian;Beato, Pablo;Skibsted, Jurgen
沸石ZSM-5是最广泛使用的沸石之一，特别是在多相催化中。本文利用Al-NMR（22.3T）、Si-29 NMR、X射线衍射和红外光谱等方法，研究了四种不同硅铝比的ZSM-5沸石在水蒸气处理过程中铝在硅骨架中的引入和形态变化。在22.3 T下的Al-27 MQMAS NMR允许鉴定和定量10个不同的四面体骨架共振，其通过由相同样品的粉末XRD图案确定的平均T-O-T角分配给晶体学位点。观察到一个明确的Al位点的偏好，并发现是依赖于母体沸石的Si/Al比。在ZSM-S的框架网站面临的交叉点被认为是最容易脱铝，而铝在直的和正弦形的通道是更稳定的蒸汽处理在高温下。框架外铝物种与5-和6-倍的协调和两个扭曲的四面体AlO 4网站也已确定。光谱数据的支持，从一个测试裂化实验，这是根据在温和的蒸汽和在较高的温度下开始脱铝的框架中的铝的重新插入的催化活性。
Zeolite ZSM-5 is one of the most widely used zeolites, in particular in heterogeneous catalysis. This work investigates the incorporation of Al in the silica framework of monoclinic ZSM-5 and the Al speciation during steam treatment for four ZSM-5 samples with different Si/Al ratios, using ultrahigh field Al-27 NMR (22.3 T), Si-29 NMR, X-ray diffraction, and IR spectroscopy. Al-27 MQMAS NMR at 22.3 T allows identification and quantification of 10 distinct tetrahedral framework resonances which are assigned to the crystallographic sites by their average T-O-T angles, determined from powder XRD patterns for the same samples. A clear Al site preference is observed and found to be dependent on the Si/Al ratio of the parent zeolite. The framework sites facing the intersections in ZSM-S are found to be most prone to dealumination, whereas Al in the straight and sinusoidal-shaped channels are more stable toward steam treatment at high temperatures. Extra-framework Al species with 5- and 6-fold coordination and two distorted tetrahedral AlO4 sites have also been identified. The spectroscopic data are supported by catalytic activities from a-test cracking experiments which are in accordance with a reinsertion of Al in the framework at mild steaming and the onset of dealumination at higher temperatures.