Medium-pore Zeolites

Medium-pore zeolites can be generally described as crystalline molecular sieves consisting of linked silica- and alumina-tetrahedra forming 10-membered oxygen ring channels. The dimensions of these medium-size pores are 0.5 to 0.6 lun. With respect to the conversion of methanol to olefins, only ZSM-5 or its isostructural analogs and, to a much less extent, ZSM-11 and ZSM-48 have been studied. 

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The effect of crystal size of these zeolites on the resulted toluene conversion can be ruled out as the crystal sizes are rather comparable, which is particularly valid for ZSM-5 vs. SSZ-35 and Beta vs. SSZ-33. The concentrations of aluminum in the framework of ZSM-5 and SSZ-35 are comparable, Si/Al = 37.5 and 39, respectively. However, the differences in toluene conversion after 15 min of time-on-stream (T-O-S) are considerable being 25 and 48.5 %, respectively. On the other hand, SSZ-35 exhibits a substantially higher concentration of strong Lewis acid sites, which can promote a higher rate of the disproportionation reaction. Two mechanisms of xylene isomerization were proposed on the literature [8] and especially the bimolecular one involving the formation of biphenyl methane intermediate was considered to operate in ZSM-5 zeolites. Molecular modeling provided the evidence that the bimolecular transition state of toluene disproportionation reaction fits in the channel intersections of ZSM-5. With respect to that formation of this transition state should be severely limited in one-dimensional (1-D) channel system of medium pore zeolites. This is in contrast to the results obtained as SSZ-35 with 1-D channels system exhibits a substantially higher... 

These microporous crystalline materials possess a framework consisting of AIO4 and SiC>4 tetrahedra linked to each other by the oxygen atoms at the comer points of each tetrahedron. The tetrahedral connections lead to the formation of a three-dimensional structure having pores, channels, and cavities of uniform size and dimensions that are similar to those of small molecules. Depending on the arrangement of the tetrahedral connections, which is influenced by the method used for their preparation, several predictable structures may be obtained. The most commonly used zeolites for synthetic transformations include large-pore zeolites, such as zeolites X, Y, Beta, or mordenite, medium-pore zeolites, such as ZSM-5, and small-pore zeolites such as zeolite A (Table I). The latter, whose pore diameters are between 0.3... 

HZSM-5 was used, the starting material (12) was recovered in 69% yield, a result most probably attributable to its more confined-pore structure. However, the highest regioselectivity for the furanoside/pyranoside form (14 1) was obtained with this medium-pore zeolite. It could also be concluded that the external surface area of the zeolites and the Lewis acid sites (Table II), do not have any effect on yield/ regioselectivity of this reaction. 

Only large-pore zeolites exhibit sufficient activity and selectivity for the alkylation reaction. Chu and Chester (119) found ZSM-5, a typical medium-pore zeolite, to be inactive under typical alkylation conditions. This observation was explained by diffusion limitations in the pores. Corma et al. (126) tested HZSM-5 and HMCM-22 samples at 323 K, finding that the ZSM-5 exhibited a very low activity with a rapid and complete deactivation and produced mainly dimethyl-hexanes and dimethylhexenes. The authors claimed that alkylation takes place mainly at the external surface of the zeolite, whereas dimerization, which is less sterically demanding, proceeds within the pore system. Weitkamp and Jacobs (170) found ZSM-5 and ZSM-11 to be active at temperatures above 423 K. The product distribution was very different from that of a typical alkylate it contained much more cracked products trimethylpentanes were absent and considerable amounts of monomethyl isomers, n-alkanes, and cyclic hydrocarbons were present. This behavior was explained by steric restrictions that prevented the formation of highly branched carbenium ions. Reactions with the less branched or non-branched carbenium ions require higher activation energies, so that higher temperatures are necessary. 

MCM-22, with a larger pore volume than ZSM-5, revealed behavior intermediate between what was observed for large- and medium-pore zeolites (126). Unverricht et al. (141) also examined MCM-22 at 353 and 393 K, it was found to produce mainly cracked products and dimethylhexanes and to deactivate rapidly. MCM-36 gained considerable interest that is evidenced by the patent literature (171-174). MCM-36 is a pillared zeolite based on the structure of MCM-22. Ideally, it should contain mesopores between layers of MCM-22 crystallites. This structure was found to be much more active and stable than MCM-22 (175). Alkane cracking experiments with zeolites having various pore dimensions evidenced the preference of monomolecular over sterically more demanding bimolecular pathways, such as hydride transfer, in small- and medium-pore zeolites (146). 

The location of 2-MN acetylation over HBEA zeolites was largely debated. Some authors claimed that the bulkier isomer (1-AMN) could be formed only on the external surface, and the linear one (2-AMN) both within the micropores and on the external surface, while other than that both isomers were essentially formed within the micropores. The second proposal seems to be more likely indeed, adsorption experiments showed that 1-AMN could enter the micropores of HBEA and even those of MFI, a medium-pore zeolite. ... 

Pieterse, J.A.Z., Veefkind-Reyes, S., Seshan, K and Lercher, J.A. (2000) Sorption and ordering of dibranched alkanes on medium-pore zeolites ferrierite and TON. /. Phys. Chem. B, 104 (24), 5715-5723. 

Therefore, rather than varying the test temperature, a much simpler and more accurate procedure is to actuaily perform aii measurements at the standard temperature of 538°C (1000°F), as has been proposed previously [2], By an appropriate choice of F, W and conversion (e.g. 0.3-60%) rate constants differing by over four orders of magnitude can be readily measured. It has been found that even for the medium pore zeolite ZSM-5, of high activity, no diffusion limitations exist even at this relatively high temperature [8,12] except for very large crystals exceeding 40 pm [13]. At 538°C it is also easy to woh<... 

While medium pore zeolites such as ZSM-5 do not deactivate significantly during hexane cracking at 538°C, large pore zeolites usually do. For maximum accuracy of results in these cases we found it advisable to use a low hexane partial pressure of about 10 torr. This not only completely eliminates catalyst deactivation during the test (Fig. 7),... 

The styrene oxide isomerization is known to be an easy reaction due to the carbonium stabilization by the aromatic nucleus. In the case of H-ZSM-5, taking into account the respective size of this medium-pore zeolite (5.5A) and the kinetic diameter of the styrene oxide molecule (5.9A), it was assumed that the weak external acidic sites are active enough to catalyze the reaction (ref. 16). If this were the case for all zeolites, no shape-selectivity could be obtained for any epoxide rearrangement. Nevertheless, for large-pore zeolites, the contribution of all the acidic sites cannot be excluded. 

Many titanium silicates are active for this reaction, Ti-beta and Ti-HMS being the most active. The results demonstrate that in catalysis by TS-1 and by medium-pore zeolites, the reaction is limited by diffusion. TS-48, which has been found to be inactive in other oxidation reactions, is an active catalyst for the oxidation of aniline (Gontier et al, 1994 Sonawane et al., 1994). 

Therefore, it can be concluded that 1- and 2-AMN can be formed within the zeolite micropores of large and even medium pore zeolites, their distribution... 

Medium pore zeolites influence of crystal size and acid site density.. . 127... 

MEDIUM PORE ZEOLITES INFLUENCE OF CRYSTAL SIZE AND ACID SITE DENSITY... 

Expressions 2 and 3 show that, in order to overcome this energy difference, the micropore cavities should be largely filled with adsorbed molecules. As mentioned earlier, in cases where low-alumina-content materials have been directly synthesized, high values for 6 are invariably found. This confirms Barrer s postulation. The dominant interaction that governs narrow- and medium-pore zeolite synthesis is the strong interaction of the occluded organic molecule with the micropore wall. 

Of interest with respect to this hypothesis is the significant difference in heat of paraffin adsorption between the medium-pore zeolite silicalite and large-pore, de—aluminated faujasite. The heat of paraffin adsorption is much smaller in the case of the de-aluminated faujasite, which has so far had to be prepared by an indirect route, than for silicalite, which can be synthesized direct in the presence of an organic molecule. The difference, which increases with chain length, is of the order of 5 kJ/mol per CH2 unit, and may be ascribed to the optimum fit of hydrocarbon and channel in the case of the medium-pore zeolite (H, 12). ... 

Changing the aluminum content has a significant effect on the relative stability of zeolite structures with very different topology. While the heat of formation of the wide-pore zeolite is affected very little, the heats of formation of the medium-pore zeolites fall significantly. 

Isomorphic substitution of cations of a lower valency for Si in the tetrahedral zeolite framework causes large-ring structures to stabilize with respect to dense structures. In small- to medium-pore zeolites, the cations which will have to be introduced in the micropore channels in order to compensate for the negative charge on the zeolite lattice and to maintain charge neutrality will interact with each unfavorably other if the concentration of low-valency cations in the lattice is high. In wide-pore zeolites the repulsion is less moreover, more favorable channel positions may be available than in the more dense zeolites. 

The main objective of the present work was to investigate the possibilities of direct (and selective) n-butane dehydroisomerisation into isobutene over Ga-containing zeolites. Another objective was to evaluate the role played by Ga and acid sites in this reaction. For this work such medium pore zeolites, as ferrierite (FER) and theta-1, were chosen because of their superior performance in n-butene isomerisation reaction.3,7 The modifying metal, Ga, was chosen due to the known high dehydrogenation activity of Ga-ZSM-5 catalysts in propane and n-butane conversions. 10 However, Ga-ZSM-5 catalysts were not used in this study because of their high aromatisation activity,8,9 which would not allow to stop the reaction at the stage of formation and isomerisation of butenes. 

Two medium-pore zeolites, namely, ferrierite (Si/Al = 6.3) and theta-1 (Si/Al = 30) were used in this work. They were used as catalysts in the H-form or were modified by gallium before catalytic experiments. In the latter case, Ga was introduced into the zeolites by an incipient wetness impregnation method, using aqueous solutions of Ga(N03)3. In this work, catalysts with a Ga content of 2.2 wt.% were investigated. 

Isobutene is present in refinery streams. Especially C4 fractions from catalytic cracking are used. Such streams consist mainly of n-butenes, isobutene and butadiene, and generally the butadiene is first removed by extraction. For the purpose of MTBE manufacture the amount of C4 (and C3) olefins in catalytic cracking can be enhanced by adding a few percent of the shape-selective, medium-pore zeolite ZSM-5 to the FCC catalyst (see Fig. 2.23), which is based on zeolite Y (large pore). Two routes lead from n-butane to isobutene (see Fig. 2.24) the isomerization/dehydrogenation pathway (upper route) is industrially practised. Finally, isobutene is also industrially obtained by dehydration of f-butyl alcohol, formed in the Halcon process (isobutane/propene to f-butyl alcohol/ propene oxide). The latter process has been mentioned as an alternative for the SMPO process (see Section 2.7). 

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