Of zeolite ZSM-5 and

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. 

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). 

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

Tungstophosphoric acid (HPW) and its Cs and ammonium salts encapsulated into the channels of MCM-41 molecular sieves were useful for the conversion of phenol and acetone to Bisphenol-A. The Cs-HPW/MCM system was more selective to the p,p -isomer than that of zeolites ZSM-5 and H-Y. The Bisphenol-A is useful industrially in the production of polymeric resins. Various other catalysts such as Amberlyst resins were used for this purpose (equation 20). The latter catalyst gave a 90% selectivity for the p,p -isomer . The MCM-41 encapsulated catalyst was shown to have superior thermal characteristics compared to that of the Amberlyst catalyst. 

Novel Pathways and Reactants This is a very broad area. We will thus restrict discussion to few examples. The first regards the important reaction of phenol synthesis and the possibility to realize it in one step directly from benzene using molecular oxygen as the oxidant. Various aspects of direct phenol synthesis from benzene are discussed in Chapter 13. We highlight here only recent results that exemplify how starting from the previously cited activity of Re complexes in the epoxidation in homogeneous phase could lead to investigation of the behavior of Re complexes when inserted into the channels of zeolites (ZSM-5) and in gas-phase selective oxidations. This has opened a new unexpected direction. 

J.C. Vedrine, A. Auroux, G. Coudurier, Combined physical techniques in the characterization of zeolite ZSM-5 and ZSM-11 acidity and basicity, in Catalytic Materials Relationship Between Structure and Reactivity, vol. 248, ASC Symposium Series, ed. by T.W. Whyte Jr, R. A. Dalla Betta, E.G. Derouane, R.T.K. Baker (American Chemical Society, Washington, 1984), pp. 253-273... 

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. 

Diffraction patterns and FTIR spectra of skeletal vibrations of the ZSM-5 and ferrierite zeolites indicated high crystallinity of the analyzed samples. The strong band with a chemical shift of about 55 ppm in the 27Al MAS NMR spectra of hydrated zeolites indicated the presence of more than 97 % Al in the framework in tetrahedral coordination the very low intensity of the peak at 0 ppm indicated less than 3 % rel. of Al in octahedral coordination. 

The shift of the acidic OH-band due to the interaction of the Bronsted acid sites with hexane reactant is clearly accompanied by the appearance of the absorption bands in the vCh region (fig. 2). The Av0h, C6 values listed in table 1 suggest that the apparent acidity of the ZSM-5 and mordenite samples is distinctly different (113-116 and 85-94 cm 1 shifts, respectively) and significantly higher than that of the zeolite Y samples (41-52 cm"1). 

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. 

Zeolites ZSM-5 and ZSM-11 are the most commercially important end-members of a continuous series of intermediate structures belonging to the so-called pentasil family (4,5). The first preparation of ZSM-5 was described in 1972 (6) and since thbn, a number of elaborate synthesis recipes have been reported in the patent literature. Because of the unique and fascinating activity and (shape) selectivity of this material for a variety of catalytic reactions currently processed in chemical industries, increasing attention has been devoted to a better understanding of the various mechanisms that govern the synthesis of ZSM-5 (7-33). 

In the present work ir, esr and microcalorimetry techniques were used to characterize the acid-base properties of acid ZSM-5 and ZSM-11 samples. Complementary studies by TEM, EDX-STEM and XPS were also carried out to determine the size and shape of zeolite particles and the A1 distribution within a particle. Catalytic properties for methanol conversion were also determined. 

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... 

Whereas most effort has been directed toward the zeolite ZSM-5 and its all-silica polymorph silicalite, other frameworks such as zeolite A, faujas-ite, and ferrierite have also been considered. This theoretical interest mirrors the commercial relevance of these zeolites. 

Exclusive ring-nitration occurs with alkylbenzenes. The nitration of toluene in the presence of H-ZSM-5 and molecular oxygen shows a remarkable enhancement of para selectivity (ortho para ratio = 0.08).268 A review is available for the nitration of aromatics by nitrogen oxides on zeolite catalysts.269... 

Among the medium-pore sized zeolites, perhaps the most studied are the pentasil zeolites, ZSM-5 and ZSM-11 (Figure 13). These zeolites also have three-dimensional pore structures a major difference between the pentasil pore structures and the faujasites described above is that the pentasil pores do not link cage structures as such. Instead, the pentasils are composed of two intersecting channel systems. For ZSM-5, one system consists of straight channels with a free diameter of about 5.4 x 5.6 A and the other consists of sinusoidal channels with a free diameter of about 5.1 x 5.5 A. For ZSM-11, both are straight channels with dimensions of about 5.3 x 5.4 A. The volume at the intersections of these channels is estimated to be 370 A3 for a free... 

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. 

The high silica/alumina ratio zeolites ZSM-5 and ZSM-11 both contain two intersecting channel systems composed of 10-membered oxygen rings. The channels in these zeolites are elliptical, with a free cross-section of 5.5 x 5.1 for the linear channels, and a cross-section of 5.6 x 5.4 for the sinusoidal channels in ZSM-5. The channel structures of these two zeolites are shown in Figure 1. 

The hydrogen forms of the ZSM-5 and ZSM-11 samples were prepared from the parent as-synthesized zeolites ( ) by drying at 125°C, calcining in nitrogen... 

In distinct contrast to the faujasites, the intermediate-pore zeolites ZSM-5 and ZSM-11 exhibited a marked preference for n-paraffins relative to aromatics. As can be seen from Table II, both H-ZSM-5 and H-ZSM-11 preferentially sorbed n-nonane from mixtures of nonane and p-xylene dissolved in an inert, non-sorbable solvent, 1,3,5-tri-isopropylbenzene. Selectivity factors greater than 40 were observed, despite the fact that both compounds were readily sorbed when higher zeolite/sorbate ratios were used. Highly selective sorption of n-heptane relative to naphthalene, and n-tetradecane relative to 1-phenyloctane, was also observed with H-ZSM-5. 

Despite the fact that both normal and monomethyl-substituted paraffins readily enter the pores of ZSM-5 and ZSM-11, preferential sorption of the normal isomer is observed under thermodynamic equilibrium, non-kinetically controlled conditions. Whereas small-pore zeolites, such as 5A and erionite, totally exclude branched hydrocarbons, and large-pore zeolites exhibit little preference, the intermediate pore-size zeolites ZSM-5 and ZSM-11 show a marked preference for sorption of the linear paraffin, even under equilibrium conditions. Competitive liquid phase sorption studies at room temperature indicated selectivity factors greater than ten in favor of n-hexane relative to... 

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. 

Results from the over-cracking of the parent gasoline over zeolites ZSM-5 and Y are summarised in Table 3. 

As-synthesized Na-ZSM-5, which possesses an isostructure with TPA-ZSM-5, exhibits a hexagonal-lath-shaped morphology. Liquid phase Sil /A Oo ratio is important in controlling the formation of Na-ZSM-5 and mordenite, and the silica source influences the liquid phase composition. High yield of Na-ZSM-5 is obtained only when a small-sized silica sol is employed. A linear relationship between Na/Al starting atomic ratio and the nucleation rate indicates that charge neutralization of SBU is required for zeolite crystallization. 

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