Intermediate pore

In this appHcation, ZSM-5 acts as a strong, soHd acid, and may be viewed as supported on the surfaces of the crystalline zeoHte stmcture. The older, Friedel-Crafts aluminum chloride catalyzed process for ethylbenzene produces considerably more by-products and suffers from the corrosivity of the catalyst system. Because of the intermediate pore size of ZSM-5, those reactions that produce coke from larger molecules that cannot enter the ZSM-5 pore stmcture are significantly reduced, which greatly extends catalyst lifetime. 

Recent work [4] showed that EU-1, an intermediate pore size (10MR) monodimensional zeolite, leads to very high isomerization selectivity during EB conversion. This results from the blockage by carbonaceous deposits of the access to the inner sites of micropores. 

Intermediate pore zeolites typified by ZSM-5 (1) show unique shape-selectivities. This has led to the development and commercial use of several novel processes in the petroleum and petrochemical industry (2-4). This paper describes the selectivity characteristics of two different aromatics conversion processes Xylene Isomerization and Selective Toluene Disproportionation (STDP). In these two reactions, two different principles (5,j6) are responsible for their high selectivity a restricted transition state in the first, and mass transfer limitation in the second. 

The effect of different zeolite structures and pore systems is also reflected in the data of Table II. With the intermediate pore ZSM-5, xylene is apparently much less reactive than ethylbenzene, both as an alkyl donor and acceptor, than it is with the large pore zeolites, ZSM-4 and synthetic mordenite. 

A.B. "Shape Selective Conversion Over Intermediate Pore Size Zeolite Catalysts" Am. Inst, of Chem. Eng. 72,... 

CHA (-34, -44, -47), ERI (-17), GIS (-43), LEV (-35), LTA (-42), FAU (-37) and SOD (-20). Also shown is the pore size and saturation water pore volume for each structure type. The structures include the first very large pore molecular sieve, VPl-5, with an 18-ring one-dimensional channel with a free pore opening of 1.25 nm [29], large pore (0.7-0.8nm), intermediate pore (0.6nm), small pore (0.4 nm) and very small pore (0.3 nm) materials. Saturation water pore volumes vary from 0.16 to 0.35cm /g, comparable to the pore volume range observed in zeolites (see Chapter 2 for detailed structures). 

It is our intention to present strategies based on chemically induced phase separation (CIPS), which allow one to prepare porous thermosets with controlled size and distribution in the low pm-range. According to lUPAC nomenclature, porous materials with pore sizes greater than 50 nm should be termed macroporous [1]. Based on this terminology, porous materials with pore diameters lower than 2 nm are called microporous. The nomination mesoporous is reserved for materials with intermediate pore sizes. In this introductory section, we will classify and explain the different approaches to prepare porous polymers and to check their feasibility to achieve macroporous thermosets. A summary of the technologically most important techniques to prepare polymeric foams can be found in [2,3]. 

The shape-selectivity of ZSM-5 is particularly remarkable. Active centres at the inner walls of the catalyst readily release protons to organic reactant molecules forming carbonium ions, which in turn, through loss of water and a succession of C—C forming steps, yield a mixture of hydrocarbons that is similar to gasoline. The feedstock can be methanol, ethanol, corn oil or jojoba oil. The shape-selectivity of this catalyst is particularly striking, as can be seen from the product distribution obtained for the dehydration of three different alcohols (Table 8.2). The product distribution can be understood in terms of the intermediate pore size of ZSM-5, which can accommodate linear alkanes and isoalkanes as well as monocyclic aromatic hydrocarbons smaller than 1, 3, 5-trimethyl benzene. In Table 8.3, we list some of the recent innovations in catalysis, to highlight the important place occupied by molecular-sieve catalysts. 

One shortcoming of the Fischer-Tropsch synthesis is its lack of selectivity in giving complex product mixtures. In an attempt to improve the selectivity of syngas-based hydrocarbon synthesis, Mobil researchers developed a process consisting of converting methyl alcohol (itself, however, produced from syngas) to gasoline (or other hydrocarbons) over a shape-selective intermediate-pore-size zeolite catalyst (H-ZSM-5) 22 78... 

Quite apart from this molecular sieving effect, zeolites are also effective in selectively sorbing particular components from a mixture of molecules all individually capable of penetrating the entire zeolite. Some liquid phase sorption equilibria studies have been reported for both the small-pore 5A molecular sieve (1 ) and the large-pore faujasite NaY zeolite (2). With the recent synthesis of intermediate pore sTze zeolites such as ZSM-5 and ZSM-11(3), a study of the selective sorption properties of these zeolites was initiated. 

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... 

The intermediate pore-size ZSM-5 is also effective in selectively sorbing paraffins from cycloparaffins. Certain cycloparaffins such as decalin and cyclooctane appear to be totally excluded from the interior of the zeolite at room temperature. Selectivity factors for n-hexane relative to cyclohexane and for n-heptane relative to cycloheptane, all of which are capable of being sorbed individually, were about 100 in favor of the normal paraffin. Even isoparaffins such as 3-methylpentane were selectively sorbed relative to their cyclic isomers (selectivity factor = 4). 

The unique intermediate pore size channel struc-... 

Patent (6) claims the desirability of intermediate pores (100-1000 A) plus channels (>1000 A) "to take up preferentially adsorbed large molecules without causing blockage, so that the smaller size pores can desulfurize smaller molecules." Patent (7) also prefers the open structure for collection of coke and metals. It specifies 0.3 cc/g of pore volume in diameters larger than 150 A and "many pores from 1,000-50,000 A."... 

Finely microporous Intermediate pore-flow ceramic/carbon solution-diffusion... 

The possibility of selective reactions on the external surface of zeolites, more exactly at the pore mouth, was recently addressed by Martens et al. (56, 57) to explain the unusual selectivity of several intermediate pore size zeolites and especially of ZSM-22 (TON) in long chain n-alkanes isomerization. Over PtHTON, this isomerization is very selective towards monobranched isomers even though... 

Anisole acetylation, which was one of the main reactions investigated, was first shown to be catalysed by zeolite ten years ago by Bayer (13), which was confirmed by Harvey et al. (14), then by Rhodia (15). Large pore zeolites and especially those with a tridimensional pore structure such as HBEA and HFAU were found to be the most active at 80°C, in a batch reactor with an anisole/acetic anhydride molar ratio of 5 and after 6 hours reaction, the yield in methoxyacetophenone (MAP) was close to 70% with HBEA and HFAU zeolites, to 30% with HMOR and 12% with HMFI. With all the zeolites and also with clays and heteropolyacids, the selectivity to the para-isomer was greater than 98%, which indicates that this high selectivity is not due to shape selective effects but rather to the reaction mechanism (electrophilic substitution). The lower conversion observed with HMOR can be related to the monodimensional pore system of this zeolite which is very sensitive to blockage by heavy secondary products. Furthermore, limitations in the desorption of methoxyacetophenone from the narrow pores of HMFI are probably responsible for the low activity of this intermediate pore size zeolite. 

The pores carbon phase indicate hydrophobic properties, responsible for the affinity for particles of organic compounds. Due to low specific surface of macropores (0.5 - 2 mVg), their adsorption capacity is considered negligible. However they play the role of the transportation routes, enabling the difiusion of adsorbed substances to narrower pores. The similar function can be attributed to intermediate pores - mesopores with the specific surface in the range of 10 - 400 m /g. 

An interesting related study introduced the concept of inverse shape selectivity in molecular sieves [99]. Relative computed adsorption heats for n-hexane and 2,2-dimethylbutane in a series of zeolites with 1-dimensional channels were compared with corresponding experimental adsorption data and data for the relative selectivity to production of these two C6 isomers in hydrocracking of n-Ci6H34. A peak in the relationship between 2,2-dimethylbutane n-hexane selectivity and channel diameter at intermediate pore sizes indicated a channel size domain in which the branched isomer was... 

Complete pore blockage Intermediate pore blockage Cake filtration Pore constriction... 

N.y. Chen, W.E. Garwood, W.O. Haag, and A.B. Schwartz, "Shape Selective Conversion over Intermediate Pore... 

For further considerations it is useful to distinguish between systems with (a) small micropores (pore diameter dp < 0.5 nm) and (b) wide micropores (dp = 1.0-2.0 nm) with a transition region in between for intermediate pore diameters. 

This group is mainly formed by silica or carbon membranes. For silica in the small pore and intermediate pore region very good combinations of selectivity and fluxes are reported. The porosity of the membrane seems to be too low however (note good measurement methods for supported microporous membranes do not exist). Porosities of at least 20% of theoretical density should give considerable improvement in the permeance. A strategy to overcome this is. 

POROUS REACTANT (intermediate pore-diffusion resistance) An example of this case would be a solid reactant formed by compressing nonporous particles into a porous pellet, as shown in Fig. 14-2c. The pores surrounding the particles are supposed to be small enough that the fluid reactant concentration decreases significantly toward the center of the pellet. 

Along an intermediate branch of pore widths, that is, for 8 < < 16, p is somewhat smaller than for the tightest pores (z < 8). An inspection of a prototypical plot of the local densities for z = 12 reveals that the confined phase now consists of a locally equimolar mixture. Hence, for intermediate pore sizes, the confined phase is a mixed hquid. 

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