Of zeolite ZSM

Figure C2.7.12. Stmcture of zeolite ZSM-5 (a) framework, (b) schematic representation of pores.

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. 

Figure 6. Digital x-ray imaging of zeolite ZSM-5 (Si/Al 49,5) thin section a) bright-field STEM image, b) A1 x-ray image smoothed by averaging each pixel with its 8 nearest neighbors. The darker shading within the particle indicates higher A1 content. The circular field is due to the image of the selected area diffraction aperture.

An extremely versatile catalyst for a variety of synthetically useful oxidations with aqueous hydrogen peroxide is obtained by isomorphous substitution of Si by Ti in molecular sieve materials such as silicalite (the all-silica analogue of zeolite ZSM-5) and zeolite beta. Titanium(IV) silicalite (TS-1), developed by Enichem (Notari, 1988), was the progenitor of this class of materials, which have become known as redox molecular sieves (Arends et al., 1997). 

The liquid-phase hydration of cyclohexene is carried out by a Japanese company with a slurry of zeolite ZSM-5 as the catalyst. Here, the product separates into two layers and cyclohexano leaves in the organic cyclohexene phase and the catalyst stays in the aqueous phase, which is recycled. The two-phase strategy, therefore, has special significance in this case. A recent publication by Ogawa et al. (1998a) gives some details of this system. 

Figure 2. 29Si MAS NMR spectrum of zeolite ZSM-4 recorded at 11.7 T with 4.0 kHz MAS and (inset) the previously proposed peak assignments.

For the non-oxidative activation of light alkanes, the direct alkylation of toluene with ethane was chosen as an industrially relevant model reaction. The catalytic performance of ZSM-5 zeolites, which are good catalysts for this model reaction, was compared to the one of zeolite MCM-22, which is used in industry for the alkylation of aromatics with alkenes in the liquid phase. The catalytic experiments were carried out in a fixed-bed reactor and in a batch reactor. The results show that the shape-selective properties of zeolite ZSM-5 are more appropriate to favor the dehydroalkylation reaction, whereas on zeolite MCM-22 with its large cavities in the pore system and half-cavities on the external surface the thermodynamically favored side reaction with its large transition state, the disproportionation of toluene, prevails. 

MOG [Mobil olefins to gasoline] A process for converting dilute streams of C2- to C4-hydrocarbons to gasoline, using a fluidized bed of zeolite ZSM-5 catalyst. Developed by Mobil Research Development Corporation and piloted in 1990. 

Combined Physical Techniques in the Characterization of Zeolite ZSM-5 and ZSM-11 Acidity and Basicity... 

Home, P.A., Williams, P.T., The effect of zeolite ZSM-5 catalyst deactivation during the upgrading of biomass-derived pyrolysis vapours, J. Anal. Appl. Pyrolysis, 1995, 34, 65. 

Synthesis of Zeolite ZSM-48 with Different Organic and Inorganic Cations... 

Zeolitic materials have been prominent amongst those so far studied by high resolution powder diffraction using synchrotron X-rays [36]. High definition synchrotron PXD data has been helpful in a number of framework structure determinations and has facilitated studies of planar faulting (see below). Successful Rietveld refinements of the framework structures of zeolite ZSM-11 [37, 38] and silica-ZSM-12 [39], and of the complete structures of zeolite Y containing cadmium sulfide [40] and cadmium selenide [41] clusters have been described. 

The alkylation of naphthalene and 2-methylnaphthalene with methanol and their ammoxidation were investigated by F r a e n k e 1 et al. [22-25] on zeolites ZSM-5, mordenite and Y. In the alkylation over HZSM-5 - unlike on H-mordenite or HY - the slim isomers, namely 2-methylnaphthalene as well as 2,6- and 2,7-dimethylnaphthalene, again clearly predominated. These authors suggest that such shape selective reactions of naphthalene derivatives occur at the external surface of zeolite ZSM-5, in so-called "half-cavities" [22, 24, 25]. D e r o u a n e et al. [26,27] went even further and generalized the concept of shape selectivity at the external surface. Based, in part, on Fraenkel s experimental results, Derouane [26] coined the term "nest effect". This whole concept, however, is by no means fully accepted and has recently been severely questioned in the light of results obtained in catalytic studies with a much broader assortment of ten-membered ring zeolites [28]. 

We interprete the above effects as conventional product shape selectivity inside the pore system of zeolite ZSM-5 or ZSM-11, and part of our arguments were presented earlier, in a preliminary note [28]. While the catalyst is on stream, coke is gradually formed and deposits, in part, inside the channel system. As a consequence, the diffusion pathways for product molecules increase. Slim molecules, such as 2,6-dimethylnaphthalene are less affected than... 

However, apart from silicalite, the spectra of synthetic zeolites do not reveal signals that may be assigned to nonequivalent silicon sites. In particular, no fine detail is observed in the spectra of zeolite ZSM-5 (Si/Al ratio typically ca. SO), which is isostructural with silicalite, and of similar degree of crystallinity (see Fig. 20). However, studies of highly siliceous... 

Si spin-lattice relaxation in zeolite ZSM-39 (97) and a range of zeolites ZSM-5 (134) have been studied. As-synthesized zeolite ZSM-5 with TPA+ as template has very long Tj (up to 145.1 sec for Si/Al = 5000) in a 300 MHz magnet. When 1,6-hexanediol is used as template, Tl was as short as 2.6 sec. Calcination causes a marked reduction of T (to several seconds) in all samples. Further reduction of was observed when cyclohexane or benzene were adsorbed. 

Fig. 49. 27A1 MAS NMR spectrum at 52.11 MHz of zeolite ZSM-5 dealuminated with silicon tetrachloride vapor (57).

Scholle et al. (220) used 27A1 MAS NMR at 130.3 MHz to study the intermediates of zeolite ZSM-5 containing tetrapropylammonium hydroxide (TPAOH) as template as a function of crystallization time. For the samples... 

Another promising approach to the study of microporosity of zeolites involves the measurement of the isotropic 13C NMR chemical shift which, as has been shown in the studies of the tacticity of polymers, is highly sensitive to the environment of the nucleus. In the first study of this kind, Boxhoorn et al. (329) observed that the C-3 carbon resonance from the tetrapropylam-monium cation enclathrated in the framework of zeolite ZSM-5 in the course of synthesis is split into two components of equal intensity. The reason for this is that the cation is located at the cross-section of the two nonequivalent... 

Zeolites [63] are extensively used as shape-selective solid acid catalysts in many industrial processes [64]. Their acidic properties stem from the presence of trivalent elements, such as Al, in the zeolite framework. The strength of these acid sites is one of the main features that determine the catalytic properties of a zeolite catalyst. Substitution of the Al atoms by other trivalent elements, such as Ga, Fe, and B, alters the strength of these acid sites, and hence also the catalytic properties of a zeolite. The possible effect of the partial substitution of the tetravalent Si atoms (which, in principle, do not create acid sites in zeolites) by Ge atoms (also tetravalent) on the catalytic properties of zeolite ZSM-5 [65] is presented here. The idea is that the different electronic and geometric properties of Ge, compared with Si, may influence the acid sites related to the Al atoms, and thereby the catalytic properties of ZSM-5. 

The aim of the present paper is double. Firstly, we wish to question more precisely the role of TEA+ ions in competition with the Na+ cations and possibly in close relation with other synthesis parameters such as the silica source, or the alumina content, by comparing a series of other physicochemical characterizations(chemical composition, nature of the occluded organics, void volume...) of zeolites ZSM-20 and Beta. In a second step, we conduct a more in depth investigation of the whole synthesis conditions and their modification in order to propose selective preparation routes for both zeolites and to possibly define further favorable conditions for the formation of other potential open phases. 

With Si(OEt)4 as silica source, the reference gel (sample 1) that usually yields ZSM-20 at 100°C after 11 days heating starts to yield an admixture of zeolites ZSM-20 and Beta, after 15-20 days heating at 130°C,while Beta is the only crystalline phase detected after 31 days heating "TableVn . 

Competitive roles of Na+ and TEA+ ions in forming and stabilizing frameworks of zeolites ZSM-20 and Beta... 

The thorough investigation of a whole set of experimental variables that affect the crystallization of zeolites ZSM-20, Beta and FAU-polytype, and their careful optimization enabled us to propose selected recipes to obtain pure materials rapidly in good yield. 

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