Sorption catalytic processes

So. presented experimental results shows, that at first time we ve found the effect of overselectivity, based on fact, that in oxide s systems of mixed type, passed through the functional-depended thermo-handling, it is formed, clusters center and carboncontaining nanotubes nature, providing the possibility of proceeding of over selective sorption-catalytical process of purification of liquid hydrocarbons from toxic admixtures... 

By irradiating a solid with photons, it is possible to alter in a well controlled way the occupation of the bond orbitals in the surface of the solid. This procedure enables us to influence surface migrations of atoms of the solid, sorption reactions and catalytic reactions, and to reveal the bond changes fundamental to any surface reaction. Several sorption processes have been investigated and may be interpreted to a certain extent. The interpretation of catalytic processes, however, is still too general. One problem in particular remains unsolved whether the adsorption states studied so far are identical with the unstable short-lived transition states which occur in catalytic reactions. 

The chemical behaviour of aluminosilicates in the presence of water or steam at elevated temperatures determines the technical conditions when considering commercial applications of zeolites as molecular sieves in adsorption or catalytic processes. Under hydrothermal conditions zeolites may be transformed into the amorphous state or into new crystalline structures. In both cases the sorption capacity or catalytic activity of the zeolites may be influenced. 

These results demand a reassessment of our basic ideas on sorption kinetics and the role of intracrystalhne diffusion in zeohte-based processes. It seems clear that intracrystalline diffusion can be reliably measured only by microscopic or mesoscopic techniques. In ideal crystals these values should correspond with the values derived from macroscopic measurements of sorption rates, but, since the majority of crystals that have been studied appear to be far from ideal, such a correspondence should not be assumed a priori. Conversely, the role of true intracrystalline diffusion in determining the rates of sorption and catalytic processes may be minimal and we may be forced to conclude that the rates of most large-scale processes are in fact largely influenced or even controlled by surface and internal barriers imre-lated to the ideal zeohte structure. 

Fundamentals of sorption and sorption kinetics by zeohtes are described and analyzed in the first Chapter which was written by D. M. Ruthven. It includes the treatment of the sorption equilibrium in microporous sohds as described by basic laws as well as the discussion of appropriate models such as the Ideal Langmuir Model for mono- and multi-component systems, the Dual-Site Langmuir Model, the Unilan and Toth Model, and the Simphfied Statistical Model. Similarly, the Gibbs Adsorption Isotherm, the Dubinin-Polanyi Theory, and the Ideal Adsorbed Solution Theory are discussed. With respect to sorption kinetics, the cases of self-diffusion and transport diffusion are discriminated, their relationship is analyzed and, in this context, the Maxwell-Stefan Model discussed. Finally, basic aspects of measurements of micropore diffusion both under equilibrium and non-equilibrium conditions are elucidated. The important role of micropore diffusion in separation and catalytic processes is illustrated. 

The predominant importance of the cations in zeolites is that they form so-called active sites for selective interaction with guest molecules in sorption and catalytic processes. From the point of view of advanced material science [47] they play a significant role in the formation of quantum-sized clusters with novel optical or semiconducting properties. As they give rise to cationic conductivity, zeolites can be used as solid electrolytes, membranes in ion-selective electrodes and as host structures in solid-state batteries. Organometallic compounds and coordination complexes can be readily formed on these cations within the larger cages or channels and applied to gas separation, electron-transport relays and hybrid as well as shape-selective catalysis [48]. 

The material 6 showed a remarkable catalytic activity in the oxidation of thioethers 13 to sulfoxides 14 by urea hydroperoxide (UHP) or H O (Scheme 2). Although the conversion and selectivity (for 14 over 15, >90%) was reasonable with UHP for the substrates with smaller substituents, 13a and 13b, the ones with bulkier substrates 13c and 13d failed to produce any measurable conversion. The conversion increases to 100% by changing UHP with H O. The catalytic activity of 6 for selective sulfoxidation remains similar even after 30 cycles. Despite the fact that no asymmetric induction was found in the catalytic sulfoxidations, enantioen-riched sulfoxides were obtained by enantioselective sorption of the resulting racemic mixture by the chiral pores of 6, which occured simultaneously with the catalytic process. Thus, after catalytic oxidation of 13a, (5)-14a was preferentially absorbed by the pore of 6 leaving exactly equal amount of the excess ) -enantiomer in the solution phase (-20% ee). The combination of high catalytic activity and enantioselective sorption property of 6 provides a unique opportunity to device a one-step process to produce enantioenriched products. 

Porous structure of sorbents and catalysts determines basically the adsorption, diffusion and dynamical characteristics of many sorption and catalytic processes. Therefore the determination of pore structure parameters (such as pore volume and surface area) is attracted the attention of many scientists. Traditional methods for the study of porous structure (the Hg-porosimetry and capillary method) required usually the complex set up, which has many disadvantages and limitations (ref. 1, 2). 

The sorption evaluation of Pd(II) micro-amounts by active coals, ACs, from solutions with 50-500-fold excess of accompanying metals compounds was shown [1]. From the other hand catalytic action of Pd(II) in reaction of Mn(III) reduction by Ck is used for Pd(II) micro-amounts determination by catalytic method [2]. The co-operation of soi ption and catalytic detenuination of Pd(II) in one process was investigated. 

In the sorption-enhanced reforming (SER) process, one of the gaseous reaction products (C02) of the catalytic reforming reaction is separated from the reaction zone by sorption. As a result, the equilibrium of the reaction is shifted toward products according to the Le Chatelier s principle. Balasubramanian et al. [18] studied the SMR reaction in the presence of CaO as a C02 acceptor. Thus, in addition to reactions 2.4 and 2.6, the reaction of C02 with the C02 acceptor (CaO) takes place in the reaction zone ... 

The attractive (80) features of MOFs and similar materials noted above for catalytic applications have led to a few reports of catalysis by these systems (81-89), but to date the great majority of MOF applications have addressed selective sorption and separation of gases (54-57,59,80,90-94). Most of the MOF catalytic applications have involved hydrolytic processes and several have involved enantioselec-tive processes. Prior to our work, there were only two or three reports of selective oxidation processes catalyzed by MOFs. Nguyen and Hupp reported an MOF with chiral covalently incorporated (salen)Mn units that catalyzes asymmetric epoxidation by iodosylarenes (95), and in a very recent study, Corma and co-workers reported aerobic alcohol oxidation, but no mechanistic studies or discussion was provided (89). 

Novel manufacturing technologies are needed to address modem separation and reaction challenges. ZeoUtes possess molecular sieving, selective sorption properties and catalytic activity, as well as enhanced thermal and chemical stabiUty which make them good candidates for these applications. Future focus needs to address the processing methods and theoretical understanding of these materials so their potential for commercial appUcation can be realized. 

The discussion above explains why basic information on sorption and diffusion under the reaction conditions, especially at elevated pressures, is required for kinetic and mass- and heat- transfer modelling of catalytic polymerization reactors. If such information is sufficiently available, one should be able, for example, to compare the kinetics of gas-phase and slurry-processes directly by taking into account both gas solubilities in swollen polymers and the hydrocarbons used in slurry processes. 

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