Mordenite pore structure

It would seem that it is the mordenite pore structure influences the ordering of the simple gases adsorbed. The ordering of the adsorbed phase is slightly influenced by the polar character of the individual molecules. It would seem that the more polarizable or more polar the molecule, the more the adsorption phenomena are enhanced. 

The mordenite pore structure (Fig. 3.62) consists of elliptical and noninterconnect-ed channels parallel to the r-axis of the orthorhombic structure. Their openings are limited by twelve-membered rings (0.6 —0.7 nm). ZSM-5 zeolite (Fig. 3.63) shows a unique pore structure that consists of two intersecting channel systems one straight and the other sinusoidal and perpendicular to the former (Fig. 3.63). Both channel systems have ten-membered-ring elliptical openings (ra. 0.55 A in diameter)... 

Commercially significant zeolites include the synthetic zeolites type A (LTA), X (FAU), Y (FAU), L (LTL), mordenite (MOR), ZSM-5 (MFI), beta ( BEA/BEC), MCM-22 (MTW), zeolites E (EDI) andW (MER) and the natural zeolites mordenite (MOR), chabazite (CHA), erionite (ERl) and clinoptiloUte (HEU). Details of the structures of some of these are given in this section. Tables in each section lists the type material (the common name for the material for which the three letter code was established), the chemical formula representative of the unit cell contents for the type material, the space group and lattice parameters, the pore structure and known mineral and synthetic forms. 

Zeolites are crystalline aluminosilicates with a regular pore structure. These materials have been used in major catalytic processes for a number of years. The application using the largest quantities of zeolites is FCC [102]. The zeolites with significant cracking activity are dealuminated Y zeolites that exhibit greatly increased hydrothermal stability, and are accordingly called ultrastable Y zeolites (USY), ZSM-5 (alternatively known as MFI), mordenite, offretite, and erionite [103]. 

In order to avoid the acid-catalyzed dehydration of sorbitol to isosorbide, Corma et al. reported a new innovative three-step cascade route involving (1) an acetaUza-tion of sorbitol, (2) an esterification of sorbitol, and (3) a deacetalization of sorbitol esters (Scheme 14) [131]. In this case, esters of sorbitol were obtained. At 408 K, between 70 and 90% of conversion of sorbitol was obtained. Activity of the zeolites employed (H-Beta, mordenite, and dealuminated zeolite ITQ-2) was dependent on both their acidity and pore structure. The modemite catalyst has emerged as the most efficient. 

Of course, certain features of overall kinetics are inaccessible via a cluster model method, such as the influence of pore structure on reactivity. The cluster model method cannot integrate reaction rates with concepts such as shape selectivity, and an alternative method of probing overall kinetics is needed. This has recently been illustrated by a study of the kinetics of the hydroisomerization of hexane catalyzed by Pt-loaded acidic mordenite and ZSM-5 (211). The intrinsic acidities of the two catalysts were the same, and differences in catalyst performance were shown to be completely understood on the basis of differences in the heat of adsorption of hexene, an intermediate in the isomerization reaction. Heats of adsorption are strongly dependent on the zeolite pore diameter, as shown earlier in this review (Fig. 11). 

Since the mordenite type zeolite has two dimensional pore structure with nonintersecting parallel channels, the internal surface area of the catalyst may be easily blocked by the adsorption of reactants as well as by the deposition of deactivating agents. To confirm the speculation that the adsorbed reactants can block the pores, the change in surface... 

Mordenite has a channel-like pore structure in which the basic building blocks consist of five-membered rings. A view of the mordenite structure perpendicular to the main channels is shown in Fig. 3. 

Y-zeolite, ZSM-5 and mordenite are three important industrial zeolites because of their pore structure and surface acidity. The coke extraction from the strongly coked HYZ catalyst has already been investigated under supercritical conditions [4,5], However, investigations on ZSM-5 and mordenite under supercritical conditions can not be found in literatures. 

D DM catalysts have a new pore structure consisting of crystalline domains of 8- and 12-ring pores connected by mesopores (5-10 nm). The presence of the latter enhances accessibility to the micropore regions without seriously compromising the shape-selective character of the catalyst. This combination of changes in acidity and pore structure transforms synthetic mordenites into highly active, stable and selective alkylation catalysts. 

The mode of coke deposition is closely related to the pore structure of the zeolite (5-8). Figure 1 shows how coke deposits on typical zeolites. In the case ofZSM-5, coke deposits at intersections of the straight and zigzag channels, and also on the outer surface of the crystal. Whereas, Y type zeolites and mordenites have supercages whose sizes are almost equal to the molecular sizes of aromatic compounds composed of a few benzene rings, and coke is easily formed in the supercages. These differences in the manner of the coke formation reflect on mode of the deactivation... 

Many applications of AFM to pillared clays or zeolites have not specifically addressed the porosity characteristics, but rather the occurrence of adsorbed surface Al species in, e.g., pillared montmorillonite [41], or the crystal growth processes, adsorption on porous surfaces and the surface structure of natural zeolites [42]. Sugiyama et al. [43] succeeded to reveal the ordered pore structure of the (001) surface of mordenite after removal of impurities that clogged the pores. The authors indicated that resolution in AFM imaging of zeolites is significantly affected by the magnitude of the periodical corrugation on the crystal surface, so that if the surface contains deep pores only the pore structure, but not the atomic structure, can be resolved. 

For the alkylation of pofyaromatics and biphenyls Mordenite with high sihca to alumina ratios seems to be the prrferred catalyst. Lee at d. [64] observed that dealumination of Mord e by add wadiing with 6 N HNO3 modified the pore structure of Mordenite resulting in an increase in the total pore volume and e edalfy an increase in the volume of pores a diameter between 20 and 1000 A. In the isopropylation of b henyl an increase in the yield of diisopropylb henyl was obtained which mi t be ascribed to either the enhanced difiiision of the reactants and produds via the newly created meso-pores or the decrease in the rate of deactivation during the alkylation reaction. 

Another consequence of the rather limited number of molecular sieves that are used, is a high emphasis is being placed on post synthesis modification of molecular sieves. Especially, the introduction of a secondary pore structure (such as achieved for mordenites [263]), the deactivation of outer crystal surface and the adjustment of the acid strength by selected ion exchange procedures are examples for that approach. 

Zeolite ZSM-S has a three-dimensional pore structure with interconnected channels that have dimensions of 0.53 x 0.57 and 0.55 nm (162). Y zeolite and mordenite have wider openings of their principal channel networks, and these zeolites thus show less molecular shape selectivity than ZSM-5 zeolite. 

A wide range of reactions of polar compounds was claimed to be catalysed by zeolites of high Si02/Al203 ratio, such as de-aluminated mordenite and HZSM-5. The liquid-phase dehydration of t-butanol catalysed by H-mordenite was studied by Ignace and Gates.Reaction took place within the pore structure and was hindered by the difficulty of transport of reactant, product, and diluent molecules. Barrer and Oei found that ethylvinyl, n-butylvinyl, and isobutylvinyl ethers all react readily over H-mordenite near room temperature... 

H-Mordenite catalyzes the smooth conversion of simple aldehydes and alcohols to form acetals at 30° in the liquid phase. From the examples in Table XXVII, it is apparent that in these heterogeneous catalytic systems, acetal formation is dependent on the structures of both the aldehyde and the alcohol involved. Thus, for a given aldehyde, yields of acetal decreased in the order primary > secondary > tertiary that is, branching at the a-carbon of the alcohol reduced the equilibrium conversion to acetal. In the isobutyraldehyde reactions, an extremely sharp drop in conversion was observed upon changing from isopropanol to fert-butanol as reactant. This observation suggests that, in addition to the increased steric interactions between organic reactants encountered in the tert alcohol system, molecular sieving-type interactions within the narrow mordenite pore system are operative. 

To determine the effect of zeolite pore structure on coke removal by SCF regeneration a series of wide pore zeolites, which included acidic Ys, betas, and mordenite, were fully deactivated under a flowing isobutane/butene mixture and then regenerated under flowing SC isobutane for 60 min (77). The spent catalysts were examined ex-situ by TPO, DRIFTS, and UV-Vis spectroscopy both before and after regeneration. These analyses demonstrated that although most adsorbed hydrocarbons were removed in some catalysts none of the catalysts were completely free of hydrocarbon deposits after SC isobutane regeneration. 

Preliminary results for the hydrolysis of aromatic amides indicate that HY zeolites are the most active catalysts, probably due to their tridirectional pore structure and large pore apertures, while other tridirectional HBeta zeolites and especially unidirectional mordenites were less performant. 

Mordenite is a large pore zeolite with elliptical pores defined by 12 oxygen atoms and major and minor axes of 7.1 A and 5.9 A, respectively.35 Figure 9.17 provides a schematic for mordenite s structure.34 These dimensions mean that n-and isoparaffins can enter the pores where they react under the acidic conditions. However, these pore dimensions have also been interpreted to mean that larger molecules can also enter the pores36 and react to form coke,37 limiting catalyst life. 

Deactivation bv Coke. - The channels of large-pore molecular sieves can accommodate carbonaceous residues that lead to loss of catalytic activity. The deactivation by coke is rather rapid with mordenite, due to the blockage of the uniform noninterconnecting channels by coke. SAPO-5, MAPO-5, and MeAPO-5 have also a onedimensional porous structure and can be deactivated in a similar way. Faujasite-like zeolites, such as zeolite X and Y, can initially accommodate some coke in the large cavities without blockage of the pore structure. 

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