Zeolites thermal reduction

When the temperature is increased at 550 °C, the water removed from the solid reduces Ni2+ to Ni+. The formation of the monovalent nickel cations by thermal reduction of Ni2+-exchanged Y zeolite has been detected using the IR spectroscopy of CO adsorption [9]. The NH3-TPD measurements have evidenced that the acid sites strength strongly... 

Another important zeolite catalyst is the so-called bifunctional catalyst. The thermal reduction of zeolites previously exchanged with metals is the method currently used for the preparation of bifunctional catalysts for hydrocarbon conversion. The bifunctional zeolite catalysts are composed of both acidic sites and metal clusters. The preparation methods of these catalysts encompass three steps ion exchange, calcinations, and reduction, (Section 3.2.1.4) [123,127],... 

Cations of noble metals in zeolites are easily reduced by H2, and this is the method most commonly applied. Proper activation and reduction treatments are essential to obtain the highest metal dispersions. Empirically determined treatment conditions are usually employed to obtain well dispersed metals, and generalizations about them are difficult. For example, prior to the final reduction in H2, it is usually necessary to eliminate any NH3 produced during the thermal decomposition of an amine complex or a NH4 ion in the zeolite. The reduction of metal ions in the presence of evolving NH3 can easily lead to the formation of agglomerated metal. Furthermore, Dalla Betta and Boudart [129] pointed out... 

Szanyi et al., using again in situ IR-TPD coupled skills, studied the effect of acid sites on the catalytic activities of a series of H+-modified Na-Y zeolites in the non-thermal plasma assisted NO reduction reaction using a simulated diesel engine exhaust gas mixture. The acid sites were formed by NH ion exchange and subsequent heat treatment of a NaY zeolite. The catalytic activities of these H+- modified NaY zeolites... 

Kwak, J.H., Peden, C.H.F. and Szanyi, J. (2006) Non-thermal plasma-assisted NO reduction over Na-Y zeolites The promotional effect of acid sites, Catal. Lett., 109, 1. 

The development of composite micro/mesoporous materials opens new perspectives for the improvement of zeolytic catalysts. These materials combine the advantages of both zeolites and mesoporous molecular sieves, in particular, strong acidity, high thermal and hydrothermal stability and improved diffusivity of bulky molecules due to reduction of the intracrystalline diffusion path length, resulting from creation of secondary mesoporous structure. It can be expected that the creation of secondary mesoporous structure in zeolitic crystals, on the one hand, will result in the improvement of the effectiveness factor in hydroisomerization process and, on the other hand, will lead to the decrease of the residence time of products and minimization of secondary reactions, such as cracking. This will result in an increase of both the conversion and the selectivity to isomerization products. 

Durable changes of the catalytic properties of supported platinum induced by microwave irradiation have been also recorded [29]. A drastic reduction of the time of activation (from 9 h to 10 min) was observed in the activation of NaY zeolite catalyst by microwave dehydration in comparison with conventional thermal activation [30]. The very efficient activation and regeneration of zeolites by microwave heating can be explained by the direct desorption of water molecules from zeolite by the electromagnetic field this process is independent of the temperature of the solid [31]. Interaction between the adsorbed molecules and the microwave field does not result simply in heating of the system. Desorption is much faster than in the conventional thermal process, because transport of water molecules from the inside of the zeolite pores is much faster than the usual diffusion process. 

Methane has also been used as the reducing agent in the catalytic conversion of NO to N2 over Co-ZSM-5 zeolites [75] in the presence of oxygen. The high NO conversions (>70%) were achieved by microwave irradiation at 250-400 °C, whereas under similar conditions thermal runs failed to convert either NO or methane in significant amounts. The high activity and selectivity of the reduction of NO by methane achieved with microwave irradiation was probably because of the activation of methane to form methyl radicals at relatively low reaction temperatures. 

A considerable reduction in pumping speed and failure to reach the ultimate pressure which is normally attainable in spite of thermal regeneration having been carried out indicates that the zeolite being used has become contaminated by outside substances. It does not make good sense to attempt to rejuvenate the contaminated zeolite with special thermal processes. The zeolite should simply be replaced. 

Modification of zeolites, based on chemisorption of silane or diborane and subsequent hydrolysis of the chemisorbed hydride groups can also be applied for encapsulating gas molecules in zeolites. For example, krypton and xenon can be encapsulated in mordenite combining the modification process with a physical adsorption of the noble gases at moderate pressures and temperatures (e.g. 100 kPa, 300 K). The encapsulates are homogeneous and stable towards acids, mechanical grinding and y-irradiation. By controlling the pore size reduction however, the thermal stability can be controlled. 

USY (ultra-stable type Y) is a good material which has served us well but which has probably been pushed to its limit (10). In simplified terms, as Al3 is eliminated from the T-positions in the structure by thermal treatment in the presence of H2O, they are replaced by Si4 from some other portion of the crystal. Table n compares a typical USY (LZ-Y82) to the parent material, NaY. The Si02/Al203 ratio (5.77) probably understates the transformation because of non-framework alumina retained in the structure. Reduced crystallinity is evidence of structural damage this same effect would be expected to reduce the zeolite character of its sorption properties. The reduction in cation content (0.38 Na/Al) renders it unsuitable for an alkaline application such as the ELF-Aquitaine aromatization catalyst... 

It is unlikely that the asymmetric photoreduction procedure described here will be able to compete with the available thermal methods for the reduction of carbonyl compounds. Generality of the zeolite-based method even with respect... 

In several cases, such as the MPV reduction, the specific pore structures of zeolites lead to selectivities not realizable with other catalysts. Nevertheless, in some cases, a zeolite shows no advantage in activity over other solid acids, except for the superior regenerability provided by their thermally stable inorganic matrix. 

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