LEWIS-ACID SITES AND SURFACE ALUMINUM IN ALUMINAS AND ZEOLITES - A HIGH-RESOLUTION NMR-STUDY
LEWIS-ACID SITES AND SURFACE ALUMINUM IN ALUMINAS AND ZEOLITES - A HIGH-RESOLUTION NMR-STUDY
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DOI：
10.1021/j100075a024
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发表时间：
1994-06-16
影响因子：
--
通讯作者：
FRIPIAT, JJ
中科院分区：
文献类型：
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作者：
COSTER, D;BLUMENFELD, AL;FRIPIAT, JJ
表面铝原子在不同配位态间的分布是理解氧化铝和沸石酸性的关键。单脉冲Al-27 MAS核磁共振光谱不能区分表面和主体物质。当羟基是质子源时，质子-铝交叉极化（CP-MAS）光谱给出了表面拓扑结构的偏置表示。实际上，由于化学吸附是解离的，因此水的化学吸附涉及未知数量的重建层。另一方面，当CP从化学吸附的氨中操作时，观察到表面Al的分布，它代表了活性催化表面的状态。本文的主要目的是研究各种催化剂（氧化铝和沸石）在不同温度的可控气氛下的路易斯酸度性质。采用同位素富集氨(NH3)-N-15化学吸附法探测表面活性位点，并允许磁化转移到表面和近表面Al核。采用一种特殊的反褶积方法获得了部分分辨光谱中不同Al位的Al-27光谱参数。该程序解释了电场梯度张量分量在特定位置的正态分布。结果表明，氧化铝上存在两种Lewis位，即各向同性位移约为58 ppm，四极耦合常数QCC接近或等于6 MHz的四面体位和各向同性位移约为40 ppm， QCC略小的五边形位。在沸石中，非骨架铝碎屑中也存在相同类型的位点，此外，Bronsted位点与骨架铝相关。
The distribution of surface aluminum atoms among different coordination states is a key to the understanding of the acidity of aluminas and zeolites. Single-pulse Al-27 MAS NMR spectra do not distinguish the surface from the bulk species. Proton-aluminum cross-polarization (CP-MAS) spectra, when hydroxyls are the source of protons, give a biased representation of the surface topology. Indeed, water chemisorption involves an unknown number of reconstructed layers because chemisorption is dissociative. On the other hand, when CP is operated from chemisorbed ammonia, the distribution of the surface Al is observed, and it represents the state of the active catalytic surface. It was the main purpose of this work to study the nature of Lewis acidity in various catalysts (aluminas and zeolites) pretreated under controlled atmosphere at different temperatures. Isotopically enriched ammonia (NH3)-N-15 chemisorption was employed to probe the surface active sites and to allow for magnetization transfer to surface and close-to-surface Al nuclei. A special deconvolution procedure was applied to obtain Al-27 spectral parameters for different Al sites in partially resolved spectra. The procedure accounted for the normal distribution of electric field gradient tenser components at a particular site. It is shown that there are two kinds of Lewis sites on aluminas: namely, a tetrahedral site with isotropic shift of about 58 ppm and quadrupolar coupling constant QCC similar or equal to 6 MHz and a pentagonal site with a isotropic shift of about 40 ppm and a slightly smaller QCC. In zeolites the same kinds of sites exist in the nonframework aluminum debris, and in addition, the Bronsted sites are associated with framework aluminum.