NMR Characterization of Zeolitic Systems

This chapter does not cover the most common aspects of the solid-state NMR techniques employed in the study of heterogeneous catalysts such techniques are described in Chapter 4. Since this chapter emphasizes the surface characterization of silica and alumina systems and silica aluminas by NMR methods, only those technical aspects highly relevant to surface characterization and not otherwise emphasized in this volume are explicitly discussed here. NMR studies of zeolites and clays are treated in separate chapters, and the bulk structures of silica and alumina systems are covered by Eckert. Unavoidably this chapter is also concerned with dynamics at the surface, although the amount of detailed work on that subject to date is limited. With the increasing availability of variable-temperature solid-state NMR equipment, however, one can expect that attention devoted to dynamics at surfaces will increase markedly during the next few years. 

The technique of solid-state NMR used to characterize supported vanadium oxide catalysts has been recently identified as a powerful tool (22, 23). NMR is well suited for the structural analysis of disordered systems, such as the two-dimensional surface vanadium-oxygen complexes to be present on the surfaces, since only the local environment of the nucleus under study is probed by this method. The nucleus is very amenable to solid-state NMR investigations, because of its natural abundance (99.76%) and favourable relaxation characteristics. A good amount of work has already been reported on this technique (19, 20, 22, 23). Similarly, the development of MAS technique has made H NMR an another powerful tool for characterizing Br 6nsted acidity of zeolites and related catalysts. In addition to the structural information provided by this method direct proportionality of the signal intensity to the number of contributing nuclei makes it a very useful technique for quantitative studies. 

With the advent of sohd-state NMR, another powerfiil tool for the characterization of zeohtes and related materials emerged. Similarly and, in many respects, complementarily to infrared spectroscopy, solid-state NMR spectroscopy enabled investigations to be carried out of the zeolite framework, extra-frame-work cations, hydroxyl groups in zeolites, pore structure, and zeolite/adsorbate systems. The contributions of solid-state NMR to molecular sieves research is reviewed by M. Hunger and E. Brunner in Chapter 2. 

One further, particularly informative experiment, involves the detection and characterization of the proton sites within the zeolite structure by NMR as pioneered by Freude and Pfeiffer (20). Even when they are not major contributors to the overall lattice structure, they may be central to the catalytic reactions. Since they are relatively dilute in the lattice, a simple MAS experiment yields spectra of sufficient resolution to identify the different functionalities as shown in Figures lA and 15. Spectra of this type will be critical in the probing of the catalytic activities of these systems and optimization of techniques for their activation. The protons detected in these experiments may also be used as magnetization sources for crossipolar-ization to Si and Al. These will yield spectra where the relative intensities of the resonances are weighted by their proximity to the proton source. Experiments of this t3 e have been reported (21). 

E.F. Rakiewicz, A.W. Peters, R.F. Wormsbecher, K.J. Sutovich, K.T. MueUer, Characterization of acid sites in zeolitic and other inorganic systems using solid-state P NMR of the probe molecule trimethylphosphine oxide, J. Phys. Chem. B 102 (1998) 2890-2896. 

The stepwise conversion of N-cyclohexanone oxime into 6-caprolactam via the vapour-phase Beckmann rearrangement on a number of modified zeolite systems was studied by a variety of different solid-state NMR techniques, including N and CP MAS NMR spectroscopy. The zeolite materials are characterized by H, Si, and N MAS NMR... 

In the preceding chapter it had already been discussed that it is less the synthesis itself which may be the bottleneck in high-throughput zeolite science but rather the analysis of the solids formed in a high-throughput program. There are several standard characterization techniques which are typically employed to characterize zeolitic materials. These include powder XRD for phase identification, X-ray fluorescence analysis (XRF) or atomic absorption spectrometry to analyze elemental composition, sorption analysis to study the pore system, IR-speclroscopy, typically using adsorbed probe molecules to characterize the acid sites, NMR spectroscopy and many others. For some of these techniques parallelized solutions have been developed and described in the literature, other properties are more difficult to assess in a parallelized or even a fast sequential fashion. 

In the meantime, new characterization tools such as NMR, STEM, EXAFS, FTIR, and synchrotron XRD, etc., have allowed us to peer into the very heart of the zeolite and discern the nature and orientation of the active tetrahedral sites as well as determine the dimensions and connectivity of the zeolite pore systems. 

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