Redox sites, zeolite structures

Once the multi-step reaction sequence is properly chosen, the bifunctional catalytic system has to be defined and prepared. The most widely diffused heterogeneous bifunctional catalysts are obtained by associating redox sites with acid-base sites. However, in some cases, a unique site may catalyse both redox and acid successive reaction steps. It is worth noting that the number of examples of bifunctional catalysis carried out on microporous or mesoporous molecular sieves is not so large in the open and patent literature. Indeed, whenever it is possible and mainly in industrial patents, amorphous porous inorganic oxides (e.g. j -AEOi, SiC>2 gels or mixed oxides) are preferred to zeolite or zeotype materials because of their better commercial availability, their lower cost (especially with respect to ordered mesoporous materials) and their better accessibility to bulky reactant fine chemicals (especially when zeolitic materials are used). Nevertheless, in some cases, as it will be shown, the use of ordered and well-structured molecular sieves leads to unique performances. 

The reactivity of NO on Co +/Co + redox sites in CoAPO-18 was studied by FTIR and UV-Vis spectroscopy in a contribution of Gianotti et al. to the 12th International Zeolite Conference [787]. Dinitrosyls were found absorbing at 1903 and 1834 cm when B-sites were involved, whereas bands at 1900 and 1813 cm appeared when the dinitrosyls were stabilized on structural Co + defects (C-sites). The adsorption of NO and NO-I-O2 on Co-Y was investigated and compared with the adsorption on Co-ZSM-5 in a study by Ivanova et al. [788]. On Co-Y, they observed formation of Co +(NO)2 species with Vjs=1900 and Vs= 1819 cm >, the stability of which was similar to that formed on Co-ZSM-5 with the corresponding bands at 1894 and 1819 cm they were, however, not involved in the selective catalytic reduction. While on Co-ZSM-5 monodentate nitrates appeared, indicated by a band at 1540 cm, which easily interacted with hydrocarbons and, thus, seemed to be the key species in selective catalytic reduction (SCR), no such monodentates were detected on Co-Y. 

With a porous framework structure and large intra-crystalline surface area, zeolites can adsorb and store a considerable amount of hydrocarbons (HCs). At low temperatures, HCs may simply block the active sites for SCR reaction causing an HC inhibition effect. Such an effect is reversible the SCR activity recovers once the HCs are removed from the stream. In addition, the acid sites and redox sites in... 

The importance of XAS spectroscopy in oxidation catalysis research results from its abiUty to provide information on the local structure of active redox sites under reaction-Uke conditions, which can often hardly be obtained by other methods, such as XRD which is restricted to crystaUine material, EPR spectroscopy which only detects species with unpaired electrons, or UV-vis diffuse reflectance and Raman spectroscopy which are less sensitive to strongly absorbing TMI in reduced valence states. Thus, in situ EXAFS has been widely used to determine the local environment and redox behaviour of TMI in nanoporous oxides. A typical example is discussed in 19.3.2.2a for Fe-ZSM-5 zeolites. Another major application is the determination of size and shape of supported metal clusters, especially for particles smaller than... 

Supported metal oxide catalysts are a new class of catalytic materials that are excellent oxidation catalysts when redox surface sites are present. They are ideal catalysts for investigating catalytic molecular/electronic structure-activity selectivity relationships for oxidation reactions because (i) the number of catalytic active sites can be systematically controlled, which allows the determination of the number of participating catalytic active sites in the reaction, (ii) the TOP values for oxidation studies can be quantitatively determined since the number of exposed catalytic active sites can be easily determined, (iii) the oxide support can be varied to examine the effect of different types of ligand on the reaction kinetics, (iii) the molecular and electronic structures of the surface MOj, species can be spectroscopically determined under all environmental conditions for structure-activity determination and (iv) the redox surface sites can be combined with surface acid sites to examine the effect of surface Bronsted or Lewis acid sites. Such fundamental structure-activity information can provide insights and also guide the molecular engineering of advanced hydrocarbon oxidation metal oxide catalysts such as supported metal oxides, polyoxo metallates, metal oxide supported zeolites and molecular sieves, bulk mixed metal oxides and metal oxide supported clays. 

B. Wichterlova, J. Dedeak, and Z. Sobalik, Redox Catalysis over Molecular Sieves. Structure and Function-active Site. Proceedings of the 12th International Zeolite Conference, Part II, ed. M.M.J. Treacy, B.K. Marcus, M.E. Bisher, and J.B., Higgins MRS, Warrendale, PA, 1998 941-973. 

The mechanism for the polymerisation of acetylene is inherently different from that of aromatic monomers such as pyrrole or thiophene. Whereas the polymerisation of pyrrole or thiophene involves a redox reaction,(77,74) the corresponding reaction of acetylene is probably initiated by acidic properties of the catalyst.(22) In the case of polyacetylene evidence has been obtained to suggest that the nature of the cations in the zeolite lattice is also important.(75) Fig. 1 shows a series of Raman spectra which illustrate the influence of various cations upon the extent of polymerisation, demonstrate the effect of elevating the acetylene pressure and indicate a role for Lewis acid sites in the reaction mechanism. Exposure of acetylene (0.1 MPa) to sodium-mordenite (NaM) at 295 K gave the spectrum displayed in Fig. 1(a). Bands at 398 and 468 cm are ascribed to lattice modes of the mordenite structure(2J), whereas the peak at ca. 1958 cm can be attributed to the Vj vibration of adsorbed monomeric acetylene bound in a side-on" manner to cation sites (16,23). Relatively small maxima at 1112 and 1502 cm are characteristic of trans-polyacetylene (5,18,24,25). Exchange of cesium for the sodium ions in mordenite was found to be beneficial for the formation of polyacetylene, as can be seen in Fig. 1 (b). In addition to the noted intensification of bands typical of rra/iy-polyacetylene at 1112 and... 

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