Cavity zeolites

Mcntasty el al. [35] and others [13, 36] have measured methane uptakes on zeolites. These materials, such as the 4A, 5A and 13X zeolites, have methane uptakes which are lower than would be predicted using the above relationship. This suggests that either the zeolite cavity is more attractive to 77 K nitrogen than a carbon pore, or methane at 298 K, 3.4 MPa, is attracted more to a carbon pore than a zeolite. The latter proposition is supported by the modeling of Cracknel et al. [37, 38], who show that methane densities in silica cavities will be lower than for the equivalent size parallel slit shaped pore of their model carbon. Results reported by Ventura [39] for silica xerogels lead to a similar conclusion. Thus, porous silica adsorbents with equivalent nitrogen derived micropore volumes to carbons adsorb and deliver less methane. For delivery of 150 V./V a silica based adsorbent would requne a micropore volume in excess of 0.70 ml per ml of packed vessel volume. 

The presence of the zeolite cavity dramatically lowers the activation energy for the protonation of toluene. It is mainly due to screening of the charges in the transition state due to the polarizable lattice oxygen atoms. In the transition state, a positive charge develops on protonated toluene. 

As long as there are no important steric contributions to the transition-state energies, the elementary rate constant of Eq. (1.22) does not sensitively depend on the detailed shape of the zeolite cavity. Then the dominant contribution is due to the coverage dependent term 9. 

In the present study, we synthesized in zeolite cavities Co-Mo binary sulfide clusters by using Co and Mo carbonyls and characterized the clusters by extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and high resolution electron microscopy (HREM). The mechanism of catalytic synergy generation in HDS is discussed. 

A series of CoSx-MoSx/NaY catalysts was synthesized by intoducing Co(CO)3NO into MoSx/NaY evacuated at 673 K for 1 h, followed by second programmed sulfidation procedures. MoSx-CoSx/NaY catalysts were prepared in the reversed order of the metal sulfide accommodations into the zeolite cavities. When Co2(CO)g was used as the Co precursor, MoSx/NaY was impregnated with COj(CO)g dispersed in n-hexane, followed by evacuation at room temperature to remove the solvent. Co2(CO)g/MoSx/NaY was subsequently sulfided at 673 K to give CoSx/MoSx/NaY. The catalyst composition was determined by AAS and ICP. 

It was shown by EXAFS that the structure and dispersion of the Mo sulfide species in MoSx/NaY were unaltered by a prolonged sulfidation of 20-h at 673 Kina stream of H,S/Hj. This fact indicates that highly dispersed Mo sulfide species are thermally stable. On the other hand, with MoSx/AljOj prepared by using Mo(CO) [12], a considerable agglomeration of highly dispersed Mo sulfide species was observed at a shorter treatment time. It is considered that highly dispersed Mo sulfide clusters are thermally stabilized in zeolite cavities. 

The location or distribution of the Mo sulfide species, that is, inside or outside the zeolite cavities, was examined by HREM, XRD [17], and pore volume measurements by using benzene as adsorbate [18]. HREM observations for MoSx/NaY possessing 2Mo/SC obviously demonstrated that no Mo sulfide spiecies were formed on the outside of the zeolite and that the framework structure of the zeolite was not destroyed at all on the accommodation of Mo sulfide species. The XRD and pore volume measurements confirmed the HREM observations. It is concluded that highly dispiersed intrazeolite Mo sulfide species are produced by using Mo(CO),. 

CoSx-MoSx/NaY exhibited doublet bands at 1867 and 1807 cm, accompanying a weak shoulder peak at ca. 1880 cm. These signals are apparently assigned to those of NO molecules adsorbed on Co sulfides. No peaks ascribable to e NO adsorption on Mo sulfide sites were detected at all. What is important in Fig.7 is that in CoSx-MoSx/NaY, coordinative unsaturation sites are present only on the Co sites in spite of the coexistence of the same amount of Mo sulfide species in the zeolite cavities. These results clearly support that the Co sites in CoSx-MoSx/NaY play major roles in the HYD and HDS reactions. 

Recently techniques have been developed for x02 oxidations in zeolite cavities.174 The photosensitizer is absorbed in the zeolite and generation of l02 and reaction with the alkene occurs within the cavity. The reactions under these conditions show changes in both regiochemistry175 and stereoselectivity. The cis effect is reduced and there is a... 

Other metal oxide catalysts studied for the SCR-NH3 reaction include iron, copper, chromium and manganese oxides supported on various oxides, introduced into zeolite cavities or added to pillared-type clays. Copper catalysts and copper-nickel catalysts, in particular, show some advantages when NO—N02 mixtures are present in the feed and S02 is absent [31b], such as in the case of nitric acid plant tail emissions. The mechanism of NO reduction over copper- and manganese-based catalysts is different from that over vanadia—titania based catalysts. Scheme 1.1 reports the proposed mechanism of SCR-NH3 over Cu-alumina catalysts [31b],... 

Methanol played a role not only on selectivity but also on activity. With the methanol-lean solution the conversion was higher than with the commercial, methanol-rich solution. This confirms that methanol competes with formaldehyde for adsorption on active sites in the zeolite cavities. The ratio between isomers was not very different from that achieved at pH 2 under homogeneous conditions. 

Figure 1 shows the H2-TPR profiles of Co- and Co/Pd-HFER catalysts. The H2-TPR profile of Co-HFER shows the presence of two peaks at 340 °C and 670 °C corresponding to the reduction peaks of particles of cobalt oxides (Co304 and CoOx respectively). Normally, Co304 are on the external surface while CoOx is inside the zeolite cavities [11-13], At 960 °C, the reduction of the cationic species Co2+ occurs [14]. 

Interestingly, Co(ll) exchanged NaY zeolite is also operative under the F-T conditions (26) However, large amounts of methane and higher hydrocarbons are produced. This behaviour is reminiscent of "classical" F-T catalysts ( ,25) and suggests the occurence of cobalt crystallites outside the Y zeolite cavities. 

Corma, A., Fornes, V., Garcia, H., Miranda, M.A., Primo, J. and Sabater, M.-J. (1994). Photoinduced electron transfer within zeolite cavities eiv-Stilbcnc isomerization photosensitized by 2,4,6-triphenylpyrylium cation imprisoned inside zeolite Y. J. Am. Chem. Soc. 116, 2276-2280... 

The currently available quantum chemical computational methods and computer programs have not been utilized to their potential in elucidating the electronic origin of zeolite properties. As more and more physico-chemical methods are used successfully for the description and characterization of zeolites, (e.g. (42-45)), more questions will also arise where computational quantum chemistry may have a useful contribution towards the answer, e.g. in connection with combined approaches where zeolites and metal-metal bonded systems (e.g. (46,47)) are used in combination. The spectacular recent and projected future improvements in computer technology are bound to enlarge the scope of quantum chemical studies on zeolites. Detailed studies on optimum intercavity locations for a variety of molecules, and calculations on conformation analysis and reaction mechanism in zeolite cavities are among the promises what an extrapolation of current developments in computational quantum chemistry and computer technology holds out for zeolite chemistry. 

However, this hypothesis does not explain the higher distribution of the cyclobutyl bromide as compared to cyclopropylcarbinyl bromide, since a distribution near 1 1 would be expected, if nucleophilic attack to the bicyclobutonium occurs in the same way as in solution. The different distribution, favoring the cyclobutyl bromide, may suggest that the bromide ion is not uniformly dispersed on the zeolite cavity, preferentially occupying certain positions on the zeolite surface, where it can better attack the bicyclobutonium at one of the three positions. 

Due to the confined environment and cation-jr interactions within the zeolite cavities as well, it is expected that the regioselectivity in the photooxygenation of trisubstituted... 

Among three-dimensional framework hosts forming intercalation compounds, zeolites (see Section 1.5) have attracted considerable attention because of their technical importance (Derouane, 1982 Dyer, 1984). Intracrystalline voids in anhydrous zeolites provide a strongly polar environment that can be filled with polar or ionic species to increase the crystal energy. Treatment of zeolites (e.g. Na-Y zeolite) with vapours of sodium results in the formation of a red product consisting of Na " intercalated into the zeolite cavities (Thomas, 1983). The locations of these clusters inside the zeolites are not yet known, but they seem to be buried within the rather inaccessible cavities inside zeolites, with the result that ordinary solvents do not reach them. Such intercalation of ionic species in zeolites may have implications in nuclear-waste treatment and storage. 

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