Zeolites surface defects

Tlhe importance of zeolites in research on heterogeneous catalysis is A based mainly on the fact that the structure of the active surface is a defined part of the crystal structure and does not represent a more or less severe lattice defect as most catalyst surfaces do. The crystal structure, and therefore the structure of the zeolite surface, can be determined by x-ray diffraction. Knowledge of this structure allows the construction of simple models of the distribution of electric fields in the holes of the zeolite by which wide ranges of experimental results can be explained, as is shown by the pioneering work of Barrer 1-5) and Kiselev 6-9) on calculation of the heats of adsorption of various substances. 

The molecular field distribution within the channels must be investigated, taking into consideration the structure of the zeolite, and the calculation of the potential energy of interaction between the zeolite and particular molecules must be made. These investigations would be assisted greatly by spectroscopic studies which would make it possible to establish the nature of the zeolite surface, the presence and the nature of structural defects, and the state of the adsorbed molecules. 

Bessel and Rolison reported the electrochemical behavior of [Co(SALEN)]2"1" and Fe(bpy)3]2+ in zeolite Y.[1571 They prepared an electrode using the complex-zeolite composite and carbon powder, and tested the electrochemical behaviors of the electrode and the composite dispersed in a solution. It was found that the electrochemical behaviors of these two materials differ to a great extent. After several cycles, the former loses all the electrochemical signals, whereas the latter continuously shows the signals. They believed that the electrochemical signals arise from the complex attached onto the zeolite external surface (defects or external supercages), whereas the complex inside the zeolite channel does not participate in electron transfer of the electrochemical process. In fact, there has been dispute on whether the electrochemical signals arise from electron transfer in zeolite channels or from those complexes on the zeolite external surface. Both views can find experimental support.1158 1591... 

In zeolites with defective structures, silanol groups can also be located at the internal surface. In this case, they are slightly FI-bonded and are observed at somewhat lower frequencies (3740-3730 cm ) than free silanols. 

Defect-free membranes comprising zeolites and amorphous glassy perfluoropolymers can be prepared by modifying the surface of the filler. The pure gas permeation experiments of a series of Teflon AF 1600 membranes with various amounts of 80 and 350nm silicalite-1 crystals cannot be interpreted on the basis of the Maxwell model, but are compatible with a model in which a barrier to transport exists on the zeolite surface and a lower density polymer layer surrounds the crystals. With a small zeolite size (80nm) the low density layers around the crystals may coalesce and form percolation paths of lesser resistance and less selectivity. Silicalite-1 crystals improve the CO2/CH4 selectivity of Hyflon AD60X, and drive the N2/CH4 selectivity beyond the Robeson s upper bound. It also turns out that the presence of silicaUte-l crystals, like fumed silica, promote the inversion of the methane/butane selectivity of Teflon AF2400 in mixed gas experiments. 

However, zeolites form problematic aggregates that create defects, especially in glassy polymer matrices. Several attempts of modifying the surface of the zeolites to chemically bond them to the polymer chains and avoid macrovoid formation have been made, which often led to a considerable decrease of the gas permeability due to the rigidification of the polymer chains near the zeolite surface. Also modification of the zeolite surface by using amino functionalized siloxane to induce the formation of hydrogen bonds has been used. ... 

Abstract This review is a summary of supported metal clusters with nearly molecular properties. These clusters are formed hy adsorption or sirnface-mediated synthesis of metal carbonyl clusters, some of which may he decarhonylated with the metal frame essentially intact. The decarhonylated clusters are bonded to oxide or zeolite supports by metal-oxygen bonds, typically with distances of 2.1-2.2 A they are typically not free of ligands other than the support, and on oxide surfaces they are preferentially bonded at defect sites. The catalytic activities of supported metal clusters incorporating only a few atoms are distinct from those of larger particles that may approximate bulk metals. 

It is well know that the zeolite materials synthesized in alkaline systems usually have a high number of silanol groups (=SiOH) named defect groups [10] which possess a moderated Bronsted acidity [11]. Oppositely, Silicalite-1 synthesized in fluorine media are relatively defect-free [12] and the fluorine ions remain in the small cages of the MFI structure even after the calcination process [12]. The 29Si-NMR analyses carried out on samples Na-Silicalite-1 and F-Silicalite-1 confirm the presence of silanol groups only on the SI support surface (results not showed). Delaminated zeolites (ITQ-6) are obtained by exfoliation of as-synthesized lamellar precursor zeolites [13]. After this process, the final structure of the delaminated zeolite results in a completely hydroxylated and well-ordered external surface [13]. 

Besides the 29Si and 27 A1 NMR studies of zeolites mentioned above, other nuclei such as H, 13C, nO, 23Na, 31P, and 51V have been used to study physical chemistry properties such as solid acidity and defect sites in specific catalysts [123,124], 129Xe NMR has also been applied for the characterization of pore sizes, pore shapes, and cation distributions in zeolites [125,126], Finally, less common but also possible is the study of adsorbates with NMR. For instance, the interactions between solid acid surfaces and probe molecules such as pyridine, ammonia, and P(CH3)3 have been investigated by 13C, 15N, and 31P NMR [124], In situ 13C MAS NMR has also been adopted to follow the chemistry of reactants, intermediates, and products on solid catalysts [127,128],... 

In the last few years, computer graphics with colour display are being more commonly used not only to visualize complex structures better, but also to examine unusual structural features, defects and transformations as well as reactions. In Fig. 1.45, we show the presence of a Nal" cluster within the sodalite cage of zeolite Y as depicted by computer graphics the cluster fits well within the cavity bounded by the van der Waals surface (net) of the framework atoms. The immense power of computer graphics has been exploited widely in recent years. Structural transitions in solids and sorbate dynamics in zeolites are typical areas where computer simulation and graphics have been used (Ramdas et al., 1984 Rao et al., 1992). 

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