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

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 subst...