An Inner-/Outer-Sphere Stabilized Sn Active Site in β‑Zeolite: Spectroscopic Evidence and Kinetic Consequences

• Published in: ACS catalysis 2016-01, Vol.6 (1), p.31-46
• Main Authors: Dijkmans, Jan, Dusselier, Michiel, Janssens, Wout, Trekels, Maarten, Vantomme, André, Breynaert, Eric, Kirschhock, Christine, Sels, Bert F
• Format: Article
• Language: English
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.5b01822

A highly active Sn site with Lewis acid properties is identified in post-synthetically synthesized Sn/DeAlβ catalyst, prepared by liquid-phase Sn grafting of a dealuminated β-zeolite. Though apparently similar Sn active-site structures have been reported for the post-synthetic and the conventional hydrothermal Snβ, detailed study of the electronic structure and redox behavior of Sn with EXAFS, XANES, DR UV–vis, and TPR clearly reveals dissimilarities in geometry and electronic properties. A model of the active Sn site is proposed using a contemporary interpretation of inner-/outer-sphere coordination, assuming inner-sphere coordination of SnIV with three framework SiO– and one outer-sphere coordination by a distant charge-balancing SiO–, resulting in a separated Lewis acid–base pair. Stabilization of this geometry by a nearby water molecule is proposed. In comparison with active Sn sites in a hydrothermally synthesized Snβ, those in the grafted dealuminated material are sterically less demanding for substrate approach, while the low inner-sphere coordination of Sn leads to a stronger Lewis acidity. Proximate silanols in the active-site pocket, identified by FTIR, 29Si MAS NMR, 1H–29Si CP MAS NMR, DR NIR, and TGA, may impact local reagent concentration and transition states stabilization by hydrogen bonding. The structural dissimilarity of the active Sn site leads to a different kinetic behavior. Kinetic experiments using two Lewis-acid-catalyzed reactions, Baeyer–Villiger and Meerwein–Ponndorf–Verley, show differences that are reaction-type dependent and have different entropic (like sterical demand and hydrogen bonding) and enthalpic contributions (Lewis acid strength). The active-site model, containing both inner- and outer-sphere ligands with the zeolite framework, may be considered as a general model for other grafted Lewis acid single sites.