Chemical Reactions in Zeolites

There are several interesting attempts of in-situ studying chemical reactions in zeolites. 

Olefin reactions were, among other reactions, also studied in situ by DRIFT spectroscopy as described in an review article by Maroni et al. [883], where a cell was used similar to that mentioned in Sect. 4.2 (cf. [176]). Salzer et al. [884] described in-situ DRIFT experiments of activation of zeolite catalysts, for instance, NH4-erionite, where they also employed the commercially available, heatable DRIFT cell mentioned in Sect. 4.2. 

Ethylbenzene disproportionation catalyzed by add zeolites was studied by Karge et al. [887, 888] and recommended as a versatile test reaction for acid (monofunctional or bifunctional) catalysts such as add zeolites or related materials and is frequently used for this purpose. Also, this reaction was studied in situ by IR spectroscopy in combination with gas chromatography for determination of conversion and selectivity [888]. In these experiments, a flow-reactor quartz glass cell as described in Ref. [152] was used, which could be operated imder ultra-high vacuum during the pretreatment of the thin catalyst wafers of pressed zeolite powder at, e.g., 670-870 K and 10 Pa. After pretreatment, the cell was used as a differential fixed-bed micro-flow reactor (cf. also [ 152,158]). Results are illustrated by Fig. 52. 

Alkylation of benzene with methanol over H, Na-Y via reaction with methoxy groups at 533 K was monitored through in-situ FTIR experiments carried out by Rakoczy and Romotowski [889]. 

Another instructive example of a reaction monitored in situ by transmission IR spectroscopy is the alkylation of the toluene ring by methanol reported by Lercher and coworkers [820,890,891 ]. This work was related to the investigation of adsorption and co-adsorption of toluene, methanol and ammonia on H-ZSM-5 and H, Na-ERI by the same group (vide supra and [127,652,814-821]). The authors used a continuously stirred tank reactor [892-895]. Some conclusions drawn by the authors from their results of combined IR spectroscopic and gas chromatographic experiments were reported as follows. 

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In zeolites Na X and ZSM-5, the self-diffusion coefficients were found to decrease with increasing concentration while for zeolite NaCa A they are essentially constant The highest diffusivities were observed in zeolite Na X. This is in agreement with the fact that due to the internal pore structure the steric restrictions of molecular propagation in zeolite Na X are smaller Aan those in Na Ca A and ZSM-5 (94). Mass transfer and chemical reaction in zeolite channels in which the individual molecules cannot pass each other (single-file... 

It seems that the zeolites have been well screened in a qualitative sense, for their catalytic properties. This paper is concerned with the quantitative aspects of catalytic reaction rates in zeolites. The question whether the model of coupled surface adsorption and reaction is still meaningful in the case of zeolite catalysis was already raised by Weisz and Frilette (4) when they wrote In conventional surface catalysis the termination of a three-dimensional solid structure is considered to be the locus of activity. For these zeolites the concept of surface loses its conventional meaning.. . It is the purpose of the present article to examine critically some possibile models representing equilibrium and rate phenomena in gas-zeolite systems, in order to obtain an understanding of the kinetics of chemical reactions in zeolites. Sorption equilibria, on the one hand, and rates of sorption/desorption, exchange, and catalytic reaction on the other hand are closely related and therefore have to be represented in terms of the same model. 

The application of PFG NMR to chemical reactions in zeolite catalysts is not necessarily confined to the observation of the reactant and product molecules. By monitoring the diffusivity of an inert molecule it is also possible directly to trace any changes in the transport properties of the catalyst during the reaction. As an example of this procedure, in [236] tetrafluoro-methane is used to follow the transport inhibition within ZSM-5 during ethene conversion. In these studies, an H-ZSM-5 zeolite was loaded with 4 CF4 and 12 ethene molecules per unit cell and kept at a temperature of 343 K. The representation of the results in Fig. 22 shows that the mobility of the probe species drops by a factor of 6 during the first 3 hours of reaction and then remains relatively constant. Obviously, ethene conversion leads to the formation of larger compounds, which more effectively reduce the mobility of the probe molecules than is the case with the ethene molecules. 

In the last decade, in situ NMR spectroscopy has found an increasing application for the investigation of chemical reactions heterogeneously catalyzed by acidic and basic zeolites. Using this method, it is possible to study the formation of adsorbate complexes, intermediates, and reaction products inside the zeolite crystals. An excellent review of in situ NMR spectroscopic studies of chemical reactions in zeolites is given by Haw [335]. Furthermore, the reader is referred to papers published by Kfinowski [54, 58], Ivanova and Derouane [336], and Pfeifer and Ernst [4]. 

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. 

Wenhui, Zhou and Clennan, Edward L. (1999) Organic reactions in zeolites. 1. photooxidations of sulfides in methylene blue doped zeolite Y. Journal of the American Chemical Society, 121 (12), 2915-2916. 

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. 

Let us assume that the chemical transformations in zeolite catalyst systems occur vnthin the high surface area intracrystalline volumes. Then, for a reaction within a zeolite particle it is apparent that both the entry pores and the channel-cavity system must be open enough to allow transport of reactant molecules from the bulk phase to the active sites (and vice versa). Thus, any crystalline sieve that could sorb simple organic molecules such as w-hexane might conceivably have catalytic potential. Factors pertinent to these processes are discussed below. 

Quantitative and semiquantitative results in terms of relative rates and product distributions have been obtained for many reactions in zeolites [see Venuto and Landis (10) andTurkevich (ii)]. These results have mostly been discussed under the assumption that the rate of chemical reaction in the solid is rate limiting. 

Ward, J.W, 1976, Infrared studies of zeolite surface and surface reactions, in Zeolite Chemistry and Catalysis, ed. J.E. Rabo, ACS Monograph 171 (American Chemical Society, Washington, DC) pp. 118—284. 

From the above discussion it can be seen that zeolites are unique catalytic materials combining a size and shape selectivity toward organic reactant and product species with high reactivity, as illustrated schematically in Fig. 4 115], and in fact they show many of the characteristics of enzymes. It should be noted that all the chemical reactions in this figure are practical examples. Thus for a complete understanding of the catalytic properties of specific zeolites, a full and detailed description of the lattice frameworks that confer the molecular selec-... 

It is not practical to cover every topic in this review in which radiation chemical techniques have contributed to the understanding of catalyst function or catalytic reactions. With this introduction as a cursory guide to relevant topics, we move on to a discussion of the radiolytic spin labeling technique for analyzing products of catalytic reactions in zeolites, which has been the main thrust of experiments directed at fundamental aspects of catalysis in our laboratory. 

For more practical purposes, therefore, one should take recourse to metal particles as produced by other means, in particular on supports or in matrices. The advantage is the availability of macroscopic amounts of sample the disadvantage is that interaction with the supporting medium must be assessed. A great variety of synthetic methods exists, of which we can mention only a few. Metal clusters can be produced by aerosol techniques, by vapor deposition, by condensation in rare-gas matrices, by chemical reactions in various supports, e.g. zeolites, SiOi, AI2O3, or polymer matrices. Many different metal-nonmetal composites, such as the ceramic metals (cermets) have been obtained with metal particles with sizes varying from nanometers upward. In alternative approaches, metal particles are stabilized by chemical coordination with ligand molecules, as in metal colloids and metal cluster compounds. 

ABSTRACT. Early observations and more recent systematic studies of solid-state ion exchange in zeolites, which is a possible way of zeolite mo fication, are reviewed. Particular attention is paid to the presentation of important experimental techniques which are appropriate to prove solid-state reaction in zeolites and determine the degree of such solid-state ion exchange. Examples are provided for the introduction of alkaline, alkaline earth, rare earth and transition metal cations into zeolites such as A, X, Y, mordenite and ZSM-5. Techniques of investigation are IR, ESR, MAS NMR, XRD, XPS, TPD and chemical analysis. 

Guisnet, M., Magnouoc, P. and Canaff, C. (1986), Coke formation on protonic zeolites Rate and selectivity, in R. Setton (ed.). Chemical reactions in organic and inorganic constrained systems, Reidel Publishing Company, Dordrecht, pp. 131-140. 

In order to aid readers unfamiliar with the subject of zeolites, we start in part II, with a brief description of their structure and properties. Though proton tranfer is a common phenomena in the mechanism of chemical reactions, some aspects specific to reactions in zeolites are recalled in section III. In section IV, the applicability of Quantum Chemistry methods in describing proton transfer involving adsorbed molecules in the vicinity of acid sites in zeolites is examined, as well as the procedures employed. Finally, the success and also failures of these methods in reproducing experimental data or predicting specific interactions, are discussed in part V. Concluding remarks on the future of Quantum Chemistry in this field is provided in the last section (VI). 

The large number of early zeolite studies primarily focused on demonstrating the usefulness of Quantum Chemistry methods in exploring simple reactions in zeolites. Standard methodologies in conjunction with the simplest of model clusters were used to study well known chemical reactions, such as acid-base solution chemistry, observed experimentally. 

In this chapter we present an introductory overview of important theoretical concepts and practical tools essential for computational modeling of catalytic reactions in zeolites using quantum chemical calculations. The power and capabilities of state-of-the-art computational catalysis methodologies will be illustrated by discussing selected relevant examples from the recent literature. 

The soluble Si and Al, present in the final effluent after the TSA, as completed by following the procedures described in Chap. 5 [3], can be altered if pure SiOa and/or AI2O3, in different proportions, are added in the effluent externally. A controlled chemical reaction, in the chemically modified effluent, might be helpfiil to yield improved zeolites, which in turn would facilitate synthesis of comparatively different grades of zeolites as compared to those obtained from the TSA. This endeavor is also expected to have a zero effluent, as an additional advantage of the fly ash zeolite synthesis process. 

Besides stmctural variety, chemical diversity has also increased. Pure silicon fonns of zeolite ZSM-5 and ZSM-11, designated silicalite-l [19] and silicahte-2 [20], have been synthesised. A number of other pure silicon analogues of zeolites, called porosils, are known [21]. Various chemical elements other than silicon or aluminium have been incoriDorated into zeolite lattice stmctures [22, 23]. Most important among those from an applications point of view are the incoriDoration of titanium, cobalt, and iron for oxidation catalysts, boron for acid strength variation, and gallium for dehydrogenation/aromatization reactions. In some cases it remains questionable, however, whether incoriDoration into the zeolite lattice stmcture has really occurred. 

Catalysis of Diels-Alder reaction by zeolites is predominantly physical rather than chemical in nature [19]. The reactants are concentrated internally in cavities... 

Shape-selective zeolites can also be used to discriminate among potential products of a chemical reaction, a property called product shape selectivity. In this case, the product produced is the one capable of escaping from the zeolite pore structure. This is the basis of the selective conversion of methanol to gasoline over... 

Many chemical reactions, especially those involving the combination of two molecules, pass through bulky transition states on their way from reactants to products. Carrying out such reactions in the confines of the small tubular pores of zeolites can markedly influence their reaction pathways. This is called transition-state selectivity. Transition-state selectivity is the critical phenomenon in the enhanced selectivity observed for ZSM-5 catalysts in xylene isomerization, a process practiced commercially on a large scale. 

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