By Laurent Bonneviot, Serge Kaliaguine
Previously, the catalytic use of zeolites was once unique to the sector of acid catalysis. these days, zeolites additionally locate functions as catalysts in a wide range of chemical reactions comparable to: base catalyzed reactions; Redox reactions; and catalytic reactions on transition metals and their complexes in limited environments. The options of Bronsted or Lewis acid-base pairs are thoroughly illustrated within the literature and well-understood by way of structural and digital houses of zeolites. against this, houses of chemically changed silicates, aluminosilicates and aluminophosphates haven't but been totally explored. The checklist of oxydo-reduction reactions played within the presence of those new fabrics in transforming into as tested through the selective catalytic aid of nitrogen oxides or the various oxidations utilizing hydrogen peroxide. a lot attempt is currenty being made to get a greater perception into the character of the websites concerned. The zeolite lattice can also be used as a bunch for encapsulated complexes or steel clusters permitting the keep an eye on of nuclearity of those energetic species and the steric constraints imposed at the reactants. Molecular sieve and form selectivity results have additionally constituted interesting features of zeolite homes. fresh advancements resulting in more and more huge pore sizes with VPI-5, cloverite and extra lately mesoporous molecular sieves have broadened the spectrum of those purposes. certainly, better and bigger reactant and product molecules will be accommodated in those lattices. those new adsorbant/adsorbate structures create extra wishes for experimental facts and theoretical descriptions of delivery homes, particularly of mono- and multi-components diffusion coefficients within the zeolite pore lattice. some of these issues, representing the leading edge and present traits in zeolite study, have been mentioned within the submitted papers to the symposium and are greatly represented within the chosen papers contained during this quantity. A function universal to every one of these contributions is the mixed use of numerous analytical suggestions. a few of these concepts are on the frontier of the newest analytical advancements corresponding to a number of scattering EXAFS advert bidimensional MAS-NMR.
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Additional info for Zeolites: A Refined Tool for Designing Catalytic Sites, Proceedings of the International Zeolite Symposium
Sample text
The 13C MAS NMR measurements were performed at lower temperatures (298 or 387 K) after quenching of the sample cells. After collection of the NMR data, the NMR cells were returned to reaction conditions. Two different reaction protocols were used. In the first one, the temperature was increased in a stepwise manner from 298 to 673 K, the NMR spectra being recorded after heating for 10 min at each temperature step. In the other experiments, the sample was rapidly heated to a final temperature (433 K) and maintained at this temperature for various periods of time (2 - 1400 min).
The second reaction pathway includes toluene alkylation with the fragments formed upon methanol conversion to hydrocarbons and thus leads to various alkyltoluenes. The latter, in turn, undergo dealkylation, fragmentation and transalkylation to yield mainly xylenes, ethyltoluenes and polymethylbenzenes. 1. INTRODUCTION Toluene alkylation with methanol over pentasil type catalysts has been thoroughly studied during the last decade [1-12]. Attention was focused mainly on the application of these catalysts in the selective production of p-xylene [1-7].
13C MAS NMR spectra observed before and after reaction of methanol 13C and toluene over H-ZSM-11 at progressively increasing temperatures. *- denotes spinning sidebands. Isomerization of xylenes begins in the temperature range of 523 - 573 K as evidenced by the appearance of the growing NMR lines at ca. 4 and 130 ppm, corresponding to m-xylene [13]. At this reaction step, the lines corresponding to trimethylbenzenes and tetramethylbenzenes mainly 1,2,4-trimethylbenzene, and 1,2,4,5-tetramethylbenzene (growing shoulder at ca.