Basicity in Zeolites

Photoelectron spectroscopy (XPS) and other surface analysis XPS of zeolites Acidity in zeolites Basicity in zeolites... 

Barthomeuf, D. (1991) "Acidity and basicity in zeolites", to be published. Stud. 

Some of the previous results discussed so far describe efforts to tailor basicity in zeolite catalysts by the incorporation of non-zeolitic phases. 

Another NMR spectroscopic method for the determination of basicity in zeolites was suggested by Bosacek [183]. This method is based on measurement of the NMR chemical shift of surface methyl groups. Surface methyl... 

Figure 2.17. Diagram showing nitroside and nitrosonium ways of NO activation upon coordination to the transition-metal ions in zeolites along with basic characteristics of the NOs+ and NO5-species.

Kustov, L.M. (1997) New trends in IR-spectroscopic characterization of acid and basic sites in zeolites and oxide catalysts, Top. Catal., 4, 131. 

Probing the strength, concentration and environment of basic sites in zeolites by IR spectroscopy... 

This study permits to discuss the efficiency of new probe molecules for the characterization of basic sites in zeolites. For MBOH and for methylacetylene, the observed frequencies shift account for the variations in the basic strength of the zeolitic framework oxygen atoms. Interestingly, methylacetylene also informs on the environment of the basic sites and H2S dissociation brings information on the amount of strong basic sites. These results show the high potential and the complementarity of these protic probes to describe the strength, concentration and environment of basic sites. 

Another possibility for characterizing zeolite acid sites is the adsorption of basic probe molecules and subsequent spectroscopic investigation of the adsorbed species. Phosphines or phosphine oxides have been quite attractive candidates due to the high chemical shift sensitivity of 31P, when surface interactions take place [218-222]. This allows one to obtain information on the intrinsic accessibility and acidity behavior, as well as the existence of different sites in zeolite catalysts. 

The pore types FAU, MFI, MOR, and LTA discussed above contain basically all micropore types possible in zeolites. There are accessible and nonaccessible cages (FAU and LTA) and straight or meandering channels in one, two, or three dimensions, which may be either isolated or connected to each other (MFI and MOR). 

The rich variety of active sites that can be present in zeolites (i) protonic acidic sites, which catalyze acid reactions (ii) Lewis-acid sites, which often act in association with basic sites (acid-base catalysis) (iii) basic sites (iv) redox sites, incorporated either in the zeolite framework (e.g., Ti of titanosHicates) or in the channels or cages (e.g., Pt clusters, metal complexes). Moreover, redox and acidic or basic sites can act in a concerted way for catalyzing bifunctional processes. 

Introductory Chapter 1 provides a historical overview of molecular sieve materials. Chapter 2 covers the definition of a zeolite and describes their basic and composite building units and how they are linked in zeolite frameworks. It defines pores, channels, cages and cavities and it gives references for finding detailed... 

Alkoxyl species form very readily from the reaction of alkyl halides on alkali, alkaline earth, transition metal, and lanthanide exchanged zeolites (128, 129). The more basic the zeolite, the more readily the reaction proceeds. Alkyl halides have been used to generate methoxyl, ethoxyl, isopro-poxyl, and ferf-butoxyl species on metal-exchanged zeolites. The mechanistic significance of alkoxyl species in zeolite acid catalysis is not in general clear in some reactions they may be true intermediates, and in others mere spectators. 

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