Zeolite-catalyzed chemical reactions

Although there are many ways to describe a zeolite system, models are based either on classical mechanics, quantum mechanics, or a mixture of classical and quantum mechanics. Classical models employ parameterized interatomic potentials, so-called force fields, to describe the energies and forces acting in a system. Classical models have been shownto be able to describe accurately the structure and dynamics of zeolites, and they have also been employed to study aspects of adsorption in zeolites, including the interaction between adsorbates and the zeolite framework, adsorption sites, and diffusion of adsorbates. The forming and breaking of bonds, however, cannot be studied with classical models. In studies on zeolite-catalyzed chemical reactions, therefore, a quantum mechanical description is typically employed where the electronic structure of the atoms in the system is taken into account explicitly. 

Another important zeolite-catalyzed chemical reaction is the decomposition of NO. Cu-exchanged zeolites, especially Cu-ZSM-5, have been shown to catalyze the decomposition of NO in the presence of hydrocarbons and excess oxygen. The increasing awareness of the detrimental effects of automobile exhaust has prompted several theoretical studies on the active site and reaction mechanism. ° Cu-ZSM-5 was described using an empirical force field and energy minimization to locate the copper ions in ZSM-5. Isolated copper atoms and copper clusters were found in the micropores, mostly associated with framework aluminium species. A cluster of two copper ions bridged via an OH species not part of the zeolite framework ( extra-framework ) was proposed as the active site. Quantum mechanical cluster calculations were carried out to study the elementary steps in the NO decomposition. A single T-site model was used to represent the zeolite framework. 

Fig. 26. Geometries of the two transition states for zeolite-catalyzed cracking reactions of ethane obtained from a density functional study on a zeolite cluster model. The distances are in angstroms and angles in degrees. (Reprinted with permission from Blaszkowski et al. (134). Copyright 1996 American Chemical Society.)...

Following the successful application of zeolites in the major oil refining processes, zeolites entered the field of bulk chemicals synthesis, e.g. ethylbenzene, cumene and recently caprolactam. Their applications in the areas of fine chemicals arc also growing. An important factor is that zeolites, compared to conventional Brocnsted and Lewis acid catalysts, are low-waste catalysts and are regarded as green . After the early review on zeolite-catalyzed organic reactions by Venuto and Landis [ 1J the field has been reviewed several times [2-11],... 

Svelle S, Tuma C, Rozanska X, Kerber T, Sauer J (2009) Quantum chemical modeling of zeolite-catalyzed methylation reactions toward chemical accuracy for barriers. J Am Chem Soc 131(2) 816-825. doi 10.1021/ja807695p... 

Pentene oxidation over TS-1 catalyst is a fast reaction and hence fulfils a basic requirement for being suited to micro-channel processing [30]. Thus, it can serve as a model reaction to demonstrate the benefits of micro chemical engineering, particularly for zeolite-catalyzed reactions. Apart from this, epoxidations are an important class of organic reactions, also of industrial importance. 

Most mechanistic work has focused on chemical reactions in solution or extremely simple processes in the gas phase. There is increasing interest in reactions in solids or on solid surfaces, such as the surfaces of solid catalysts in contact with reacting gases. Some such catalysts act inside pores of defined size, such as those in zeolites. In these cases only certain molecules can penetrate the pores to get to the reactive surface, and they are held in defined positions when they react. In fact, the Mobil process for converting methanol to gasoline depends on zeolite-catalyzed reactions. 

Zeolite catalysts play a vital role in modern industrial catalysis. The varied acidity and microporosity properties of this class of inorganic oxides allow them to be applied to a wide variety of commercially important industrial processes. The acid sites of zeolites and other acidic molecular sieves are easier to manipulate than those of other solid acid catalysts by controlling material properties, such as the framework Si/Al ratio or level of cation exchange. The uniform pore size of the crystalline framework provides a consistent environment that improves the selectivity of the acid-catalyzed transformations that form C-C bonds. The zeoHte structure can also inhibit the formation of heavy coke molecules (such as medium-pore MFl in the Cyclar process or MTG process) or the desorption of undesired large by-products (such as small-pore SAPO-34 in MTO). While faujasite, morden-ite, beta and MFl remain the most widely used zeolite structures for industrial applications, the past decade has seen new structures, such as SAPO-34 and MWW, provide improved performance in specific applications. It is clear that the continued search for more active, selective and stable catalysts for industrially important chemical reactions will include the synthesis and application of new zeolite materials. 

Fig. 21. A reaction mechanism proposed by Xiong et al. to rationalize the toluene disproportion reaction on zeolites. The benzyl cations and benzenium ions were proposed as reactive intermediates for this zeolite-catalyzed, high-temperature reaction. (Reprinted with permission from Xiong et al. (122). Copyright 1995 American Chemical Society.)...

In addition to their beauty, many of the described nanovessels also show interesting endo/exo chemistry ( inside and outside ). In the interior, species can be bound, and highly reactive intermediates can be stabilized, or chemical reactions supported or catalyzed. In the latter case, unusual reactivity or selectivity might be observed. Thus, container molecules act as homogeneous equivalents of heterogeneous porous materials like zeolites or MOFs. 

The contents of the present contribution may be outlined as follows. Section 6.2.2 introduces the basic principles of coupled heat and mass transfer and chemical reaction. Section 6.2.3 covers the classical mathematical treatment of the problem by example of simple reactions and some of the analytical solutions which can be derived for different experimental situations. Section 6.2.4 is devoted to the point that heat and mass transfer may alter the characteristic dependence of the overall reaction rate on the operating conditions. Section 6.2.S contains a collection of useful diagnostic criteria available to estimate the influence of transport effects on the apparent kinetics of single reactions. Section 6.2.6 deals with the effects of heat and mass transfer on the selectivity of basic types of multiple reactions. Finally, Section 6.2.7 focuses on a practical example, namely the control of selectivity by utilizing mass transfer effects in zeolite catalyzed reactions. 

Among the chemical reactions of interest catalyzed by zeolites, those involving alkanes are specially important from the technological point of view. Thus, some alkane molecules were selected and a systematic study was conducted, on the various steps of the process (diffusion, adsorption and chemical reaction), in order to develop adequate methodologies to investigate such catalytic reactions. Linear alkanes, from methane to n-butane, as well as isobutane and neopentane, chosen as prototypes for branched alkanes, were considered in the diffusion and adsorption studies. Since the chemical step requires the use of the more time demanding quantum-mechanical techniques, only methane, ethane, propane and isobutane were considered. 

We are concerned with the kinetics of zeolite-catalyzed reactions. Emphasis is put on the use of the results of simulation studies for the prediction of the overall kinetics of a heterogeneous catalytic reaction. As we will see later, whereas for an analysis of reactivity the results of mechanistic quantum-chemical studies are relevant, to study adsorption and diffusion, statistical mechanical techniques that are based on empirical potentials have to be used. 

Aromatic gasoline is now produced from methanol in MOBIL s MTG-process, which is catalyzed by the zeolite H-ZSM-5. Much attention has been paid to understanding the mechanistic pathway(s) of the chemical reactions that take place. Although there is still some controversy about the details, the main steps are generally accepted ... 

This new zeolite-catalyzed ring-shift isomerization could provide a new route to anthracene and its derivatives [Song, 1996], which are valuable chemicals in demand, from phenanthrene, which is rich in liquids from coal. Selective partial hydrogenation of phenanthrene would be needed as the first step. Possible applications of sym-OHAn include the manufacturing of anthracene (for dyestuffs and fine chemicals), anthraquinone (pulping agent), and pyromellitic dianhydride (the monomer for polyimides such as Du Pont s Kapton) [Song and Schol rt, 1993]. Fundamental research is needed to clarify the mechanisms and reaction pathways. 

This chapter presents a comprehensive overview of heterogeneously catalyzed MPVO reactions. It includes, apart from the use of various metal oxides, the more recent application of chemically anchored co-ordination compounds, hydro-talcites, mesoporous materials, and zeolites as recyclable solid catalysts. Some remarkable examples of shape-selective conversions resulting in high stereoselectivities illustrate the progress made in this field. 

Owing to the great interest in the argument, minireviews have been published on the use of solid catalysts in Friedel-Crafts acylation. Kouwen-hoven and van Bekkum, in a chapter of the Handbook of Heterogeneous Catalysis, faced the basic problem of the use of zeolites in the reaction. A further essential overview of the same argument was reported by Metivier in Fine Chemicals through Heterogeneous Catalysis Furthermore, Bezouhanova described the synthetic aspects of the zeolite-catalyzed preparation of aromatic ketones. ... 

ZSM-5 zeolites attracted particular attention due to their ability to catalyze various reactions in petrochemistry, oil refining and fine chemistry [1,2,3]. However, for many useful chemical reactions, size exclusion virtually operates, as the steric requirements of the reactants and products are beyond the pore sizes. There are several methods how to improve accessibility of active sites located inside microporous structure [4,5]. The first possibility how to diminish the role of the molecular transport is a creation of secondary mesopores in zeolite ciystals, which can be done by post-synthesis treatments such as desilication [6] or dealumination [7], however, these treatments cannot provide zeolites having mesopores uniform in size and lattice positions [8]. The second... 

Song, Ch., and K. Moffatt, 1993, Zeolite-catalyzed ring shift and conformational isomerization reactions of polycyclic hydrocarbons, in Preprints, Division of Petroleum Chemistry, Vol. 38 (American Chemical Society, Washington, DC) pp. 779-783. 

Under the usual conditions for enzyme-catalyzed (trans)esterification, the composition at equilibrium is often far from optimal. Complete conversion can be achieved by removal of alcohol and/or water by vacuum or chemical means, i.e. by the use of enol or oxime" esters. We endeavoured to fix the water activity at a low level by adding zeolite to the reaction mixture. Immobilized enzymes which are stable in these very dry media have recently become available. ... 

Increasing attention is now on solid catalysts possessing both basic and shape selective properties to perform selective base-catalyzed fine organic chemical reactions [1-9]. Exchanged alkali zeolite demonstrated both these characters, however recent results show that these catalysts possess much less activity than sepiolites or hydrotalcites [10,11]. 

The present world rcscr es of natural gas that contains mainly methane are still underutilized due to high cost of transportation. Considerable interest is therefore presently shown in the conversion of methane to transportable liquids and feedstocks in addition to its previous sole use for heating purposes by combustion. One possible new route for the utilization of methane derived from natural gas or other sources for conversion to more valuable higher hydrocarbons is the methylation of aromatic hydrocarbons. This chapter provides a general overview of the work that has been done so far on the use of methane for catalytic methylation of model aromatic compounds and for direct liquefaction of coal for the production of liquid hydrocarbons. The review is especially focused on the use of both acidic and basic zeolites in acid-catalyzed and base-catalyzed methylation reactions, respectively. The base-catalyzed methylation reaction covered in this discussion is mainly the oxidative methylation of toluene to produce ethylbenzene and styrene. This reaction has been found to occur over basic sites incorporated into zeolites by chemical modification or by changing the electronegative charge of the zeolite framework. 

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