论文题目：Spectroscopic Signature of Lewis Acidic Framework and Extraframework Sn Sites in Beta Zeolites

论文作者：Weili Dai, Qifeng Lei, Guangjun Wu, Naijia Guan, Michael Hunger, Landong Li*

发表期刊：ACS Catalysis, 10(23), 14135-14146, 2020   

Abstract： 

A variety of Sn-containing Beta zeolites possessing framework or  extraframework Sn sites were prepared by a two-step postsynthesis  procedure. The nature of these Sn species was systematically  investigated by several approaches, including DRIFT, XPS, UV-vis, and  solid-state Sn-119 MAS NMR spectroscopy. The subtle distinctions of the  Lewis acidic features between the framework and extraframework Sn sites  were identified via acetone-2-C-13, TMPO, and NH3 adsorption, studied  via C-13, P-31, and H-1 MAS NMR spectroscopy, respectively. Two types of  Lewis acid sites (LAS) have been observed and attributed to (i)  framework Sn sites and (ii) extraframework SnOx clusters confined inside  the pores or cages of [Si]Beta zeolite, both exhibiting Lewis acid  properties. The Lewis acidic characteristics of the extraframework Sn  sites were found to depend on the cluster sizes. The confined SnOx  clusters with sizes of 0.5-2 nm possess Lewis acidic characteristics  similar to those of the framework Sn species and also exhibit a  comparable catalytic activity in the Baeyer-Villiger (B-V) oxidation of  cyclohexanone and cyclohexene oxide hydration. With an increase in size  of the SnOx clusters to over 2 nm, the Lewis acidic characteristics of  the extraframework Sn species gradually weaken or even disappear.