Zeolites oxide clusters encapsulated

J.E. Readman, I. Gameson, J.A. Hriljac, RR Edwards, and RA. Anderson, Synthesis and Structure of Zinc Oxide Clusters Encapsulated in Zeolite LTA. Chem. Commun., 2000, 595-596. 

Metal Species and Oxide Clusters Encapsulated in Zeolites Basicity and General Aspects... 

The catalytic activity of alkali metal oxide clusters encapsulated in the zeolitic cages of faujasite has been tested in different reactions catalyzed by basic sites, e. g. olefin isomerization [75,77], isopropanol decomposition [67,68], and Knoe-venagel condensation [73,74,78]. 

Instead of cupric or ferric ions, other zeolite-encapsulated oxidants have also been studied for the polymerization of pyrrole. These include small SnOa particles and oxygen-covered Pd clusters residing in potassium L zeolite. The structure of the clusters was elucidated using x-ray absorption spectroscopy, and ESR and IR data as well as the observation that more monomer than oxidant was present led to the conclusion that the pol5unerization reaction might proceed in a catalytic fashion, involving air oxidation. 

Conventional preparation techniques result in non-uniform supported metal species, which are difficult to characterize structurally. Therefore, Gates developed a strategy [266,267] to prepare nearly uniform metal clusters, either on metal oxide support or encapsulated in zeolite cavities. Starting from supported metal carbonyl clusters as precursors, the synthesis proceeded with decarbonyla-tion aiming at a minimal perturbation of the metal frame. However, fragmentation or aggregation of metal clusters may occur. 

Prior to characterization encapsulation must be ensured and clusters formed outside the cavities must be ruled out. Only then can characterization be reliably carried out. A battery of techniques is available for this purpose, such as C,Xe and metal NMR, EXAFS/XANES, XPS, IR and UV-VIS spectroscopy, electron microscopy, ESR, XRD, etc. Among these methods electronic spectroscopy plays an important role. The UV-VIS spectra reflect changes in the oxidation state of the metal as well as structural changes forced by incarceration and so serve as a valuable tool for the ascertainment of intrazeolite complexation. Although vibrational spectroscopy is most frequently applied, sometimes using the IR spectra as fingerprints for identification, it is inadequate to predict the exact structure of the clusters as these spectra maybe different from those in solution or in the sofid state due to interaction with the zeolite matrix. In any case, reliable characterization requires the combined application of complementary analytical methods. 

Chavan, S., Srinivas, D., and Ratnasamy, P. (2001) Selective oxidation of para-xylene to terephthalic acid by //3-oxo-bridged Co/Mn cluster complexes encapsulated in zeolite-Y. Journal of Catalysis, 204 409-419. 

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