Solid State MAS NMR of High-Valent Cation Exchanged H-MFI
EMSL Project ID
10306
Abstract
Since the discovery of Mo-based H-ZSM-5 catalysts for CH4 pyrolysis, many studies have addressed improvements in catalytic rates, selectivity, and stability. Many efforts involved the exchange of various cations onto the acid form of zeolites. V, Ti, Cr, and W have been introduced into H-ZSM5, in most cases without structural evidence for exchange, but with varying degrees of catalytic improvements over unmodified H-ZSM5. This class of materials provides a unique setting to prepare isolated, single-site catalysts within size-constrained environments. Typically, cation exchange is performed using aqueous exchange or impregnation methods that fail with high-valent cations because of the size and solvation characteristics of aqueous species. We have prepared Zr- and V-ZSM-5 using chemical vapor deposition with volatile precursors, ZrCl4 and VOCl3, which avoid the formation of hydrated ionic species and penetrate channels in ten-ring medium-pore zeolites. Solid-state NMR has not been used extensively to characterize cation exchanged H-ZSM-5 because of limited resolution, availability of high fields, long spectra acquisition times, and high level of expertise required for analysis and interpretation of spectra. 27Al MAS NMR tends to underestimate some forms of Al with distorted tetrahedral or octahedral symmetry. Many of these problems have been overcome in recent years by using high-field NMR. Exchanged Zr- and V-ZSM-5 contain zeolitic framework tetrahedral Al in different environments: acid sites, Si-OH-Al, and high valent cation exchanged sites, Si-ZrOx-Al or Si-VOx-Al. To our knowledge, these two forms of Al have never been detected using NMR. The use of high-field (18.7 T) NMR may allow us to distinguish these two forms of Al and to unambiguously establish the presence of exchanged cations. Moreover, if successful, this technique could then be applied to other cation-exchanged zeolite materials to probe Al coordination with the resolution required to distinguish Al-centers coordinated to high valent cations or protons. We will also use high field 1H-27Al CP MAS NMR as part of this proposal in conjunction with 27Al MAS NMR to measure the number of tetrahedral Al coupled to protons. Finally, we will perform 51V-NMR experiments at high field on samples with a range of V loading in order to determine the structure of isolated vanadia with unprecedented detail
Project Details
Project type
Capability Research
Start Date
2005-01-15
End Date
2006-11-21
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members