Zeolite catalysis Brpnsted acid sites

Zeolites are not typically used in Lewis acid type catalysis due to the absence of Lewis acid centers in zeolites. This is due to the coordination of the Al-site to four lattice-oxygens in a perfect zeolite framework. It has, however, been shown for zeolite Beta that the aluminum atom can reversibly move between a framework Brpnsted acid site and a framework-grafted Lewis-acid site.70 Accordingly, Creyghton et al. showed that zeolite Beta is active in the Meerwein-Ponndorf-Verley reduction (MPV) of ketones (scheme 4).71 In this reaction a hydrogen hydride transfer reaction between an alcohol and a ketone takes place. 

When zeolites are dealuminated by steam-calcination part of the framework A1 is extracted and generates extra-framework species (EFAL) that can be cationic, anionic or neutral. Some of these EFAL species can act as Lewis acid sites [19] or can influence the Brpnsted acidity, by either neutralizing Brpnsted acid sites by cation exchange, or by increasing the acidity by a polarization effect and/or by withdrawing electron density from lattice oxygens [20-22]. However, under mild steaming the A1 can also become partially, and reversibly, disconnected from the lattice [23]. This opens the way to Lewis acid catalysis by the A1 [24]. 

Xu, B., et al., 2006. Catalytic activity of Brpnsted acid sites in zeolites intrinsic activity, rate-limiting step, and influence of the local structure of the acid sites. Journal of Catalysis 244 (2), 163-168. 

The acidic properties of zeolitic materials are of considerable importance with respect to catalyzed reactions in heterogeneous catalysis. It is vital to know the concentration, strength and accessibility of the Brpnsted and Lewis acid sites and the details of their interaction with adsorbed species (12). For zeolites, for example, 29si MAS NMR plays a crucial role in the determination of the amount of aluminium which is part of the zeolite lattice as well as the... 

The reader is referred the recent book by Bell and Pines [2] for a more complete overview of the various methods and objectives in NMR studies of solid acids and other heterogeneous catalysis. In the present contribution we illustrate the application of H, and MAS NMR to two archetypal solid acids, Brpnsted sites in zeolites and solid metal halides such as aluminum chloride and bromide powders which exhibit "Lewis superacidity". An important characteristic of the more recent work is the integration of quantum chemical calculations into the design and interpretation of the NMR experiments. 

This area also includes the introduction of additional catalytic sites in zeolites by organometallic precursor compounds. Both the adjustment of Brpnsted/Lewis acidity and the directed fixation of active transition metal centers inside zeolite cages and channels seem possible, but the size and structures of these precursors will be crucial. The molecular modification appears all the more important as many catalytic processes will remain heterogeneous in nature, especially when the shape-selectivity of sodalites, zeolites, pentasils, etc. is exploited. Once again, the possible versatility of organolanthanoid complexes deserves emphasis, since these metals verify to the above-mentioned acidity and catalysis effects [92]. 

Nevertheless, solid-state NMR can be usefiil for active site studies, which is nowadays predominantly employed for acid catalysis, e.g., with zeolites [32]. Chemical shifts obtained are related to the electronic state of the atom H-NMR data have been, for instance, used to determine acidity of Brpnsted sites in zeolites, NMR can discriminate in various environments in supported catalysts [33]. As long as the sites are sufficiently dilute, NMR intensity is directly related to concentration. Structural information is accessible by echo techniques which allow determining distances between spin sites. By analogy with IR methodology, adsorption sites have been investigated with probe molecules also in NMR studies. While work with N bases may require isotopic enrichment of the probe, there are opportunities to detect subtle structural features by cross-polarization and doubleresonance (e.g., " N/ A1) techniques [34]. 

The external surface of protonic zeolites can be relevant in acid catalysis. Several data suggest that nonshape-selective catalysis can occur at these sites, like in the case of alkylaromatics conversions over H-MFI [225,226]. On the other hand, H-zeolites also catalyze reactions of molecules, which do not enter the cavities due to their bigger size. Therefore, the external surface of zeolites is certainly active in acid catalysis. Additionally, the bulk versus surface Si/Al composition of a zeolite could be different and different preparation procedures can allow to modify this ratio [226]. Corma et al. [84] reported data on the accessibility of protonic sites of different zeolites to 2,6-di-ter-butyl-pyridine (DTBP). This molecule has been considered selective for Brpnsted sites, due to its impossible interaction with Lewis sites for steric hindrance. According to these authors, however, the interpretation of the data is not straightforward, for several reasons such as the presence of different cavities and the big size of the probe itself. Surprisingly, Corma et al. found a complete accessibility of the sites of beta zeolite to DTBP. This contrasts the data of Trombetta et al. [197], who showed that protonic sites exist also on the smaller channels of beta, whose access to DTBP seems very unlikely. In a more recent publication, Farcasiu et al. [227] reported an accessibility of 90% of the protons of H-BEA to DTBP, much higher than the 36% for H-MOR and 31% for H-USY. 

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