Structure-property relationships that influence platinum stability in all-silica or highly siliceous zeolites

Abstract

In this study we examine the role of alkali cations and siloxy/silanol nests in stabilizing platinum clusters on all-silica zeolites. This work complements a recent study by the Corma group at Instituto de Tecnología Química (ITQ) that investigated platinum stability within Pt–K/MFI zeolites prepared by one-pot methods [1]. Here we demonstrate that stable platinum particles are likewise produced either by impregnating platinum onto an alkali-cation-containing all-silica MFI or by deboronating a borosilicate to create silanol nests and then co-impregnating with potassium and platinum sources. In this context, stability is assessed by resistance to sintering as measured by hydrogen chemisorption. Platinum impregnation alone onto a deboronated or borosilicate MFI does not produce stable platinum clusters after reduction. Furthermore, a one-pot synthesis of a Pt–K/B-MFI (borosilicate) or exchange of Pt onto a K-containing borosilicate does not lead to stable Pt particles. This further demonstrates the necessity of internal silanol/siloxy nests in stabilizing platinum clusters. In summary, silanols and alkali cations together create silanol/siloxy nests that stabilize Pt clusters. Hence, stable Pt clusters can be achieved by 1) a one-pot synthesis of the all-silica zeolite in the presence of Pt and alkali cations, 2) platinum impregnation on an all-silica sample that has occluded a high enough concentration of an alkali cation during the synthesis, or 3) by co-impregnating platinum and alkali metal cations together onto a calcined zeolite that has a high density of internal silanol nests. These nests can be generated by removal of boron from the framework of the zeolite. In the current study, we have examined these systems by 11B and 29Si NMR spectroscopy.