Intracrystalline channels

There are two types of stmctures one provides an internal pore system comprising interconnected cage-like voids the second provides a system of uniform channels which, in some instances, are one-dimensional and in others intersect with similar channels to produce two- or three-dimensional channel systems. The preferred type has two- or three-dimensional channel systems to provide rapid intracrystalline diffusion in adsorption and catalytic apphcations. 

Palygorskite and sepioHte minerals are 2 1 layered phyUosiHcates that differ from the above mentioned clays because the octahedral sheets have significant intracrystalline void space caused by discontinuous octahedral layers. The basal tetrahedral unit is connected to an adjacent inverted basal tetrahedral creating a void space or channel. Charge deficits are balanced by hydrated cations in the intracrystalline space. 

Figure 1 shows a sharp decrease of low-pressure hysteresis loop when introducing copper in S-l, pointing to the formation of (CuO)n nanoclusters into the S-l intracrystalline channels and supermicropores. The adsorption data analysis (see Table 1) shows a decrease of both the total (BET) surface area and micropore volume of the CuS-1 sample with respect to the S-l matrix. 

The best correlation of the observed isomerization selectivities was found in terms of the diameter of the intracrystalline cavity, determined from the known crystal structure (9) of these zeolites, as shown in Figure 2. While faujasite, mordenite and ZSM-4 all have 12-membered ring ports and hence should be similar in their diffusion properties, they differ considerably in the size of their largest intracrystalline cavity both mordenite and ZSM-4 have essentially straight channels, whereas faujasite has a large cavity at the intersection of the three-dimensional channel system. 

A good example for reactant shape selectivity includes the use of catalysts with ERI framework type for selective cracking of linear alkanes, while excluding branched alkanes with relatively large kinetic diameters from the active sites within the narrow 8-MR zeolite channels [61, 62]. Here molecular sieving occurs both because of the low Henry coefficient for branched alkanes and because of the intracrystalline diffusion limitations that develop from slow diffusivities for branched alkane feed molecules. 

Zeolite crystal size can be a critical performance parameter in case of reactions with intracrystalline diffusion limitations. Minimizing diffusion limitations is possible through use of nano-zeolites. However, it should be noted that, due to the high ratio of external to internal surface area nano-zeolites may enhance reactions that are catalyzed in the pore mouths relative to reactions for which the transition states are within the zeolite channels. A 1.0 (xm spherical zeolite crystal has an external surface area of approximately 3 m /g, no more than about 1% of the BET surface area typically measured for zeolites. However, if the crystal diameter were to be reduced to 0.1 (xm, then the external surface area becomes closer to about 10% of the BET surface area [41]. For example, the increased 1,2-DMCP 1,3-DMCP ratio observed with decreased crystallite size over bifunctional SAPO-11 catalyst during methylcyclohexane ring contraction was attributed to the increased role of the external surface in promoting non-shape selective reactions [65]. 

Unlike ordinary zeolites, silicalite is organophilic and hydrophobic and can remove from water a variety of dissolved organic compounds. Both ZSM-5 and silicalite display remarkable shape selectivity because of the geometry of the channels only certain reactants may enter and diffuse through the crystals, and only certain products may diffuse out of the intracrystalline space. 

The open three-dimensional nature of zeolite structures permits diffusion of reactant molecules into the interior voids in the crystal and accounts for the high effective surface area of these materials. Faujasitic zeolites have channels of about 8-A diameter connecting cavities of 13-A diameter (supercages) in a three-dimensional network. The zeolite mor-denite has parallel channels with a diameter of about 7-A. The intracrystalline surface of the zeolite is, therefore, accessible to molecules with kinetic diameters equal to or smaller than the channel diameters. 

The intracrystalline pore volume of the catalysts was evaluated by n-hexane sorption as shown in Fig. 6. Sorption capacities for samples SI to S3 are comparable to that of the zeolite before Ga impregnation and correspond to the value expected for an unaltered ZSM-5 type material (S10). Sorption capacity decreases for samples S3, S4, S5, and S6, because of intracrystalline volume blockage by coke deposits and possibly also (silica)-alumina debris [6] in the aged catalyt S6. In addition, the sorption rate for S6 is about twice the rate observed for the other samples, suggesting that adsorption occurs mostly at the external surface of the S6 catalyst crystallites. Thus, it appears that coke deposited on S6, probably as polyaromatic species, has almost blocked the channel pore mouths and/or practically occupied the whole intracrystalline pore volume. It explains the poor catalytic performance of S6. 

It has been concluded that, in most cases, catalytic reactions over zeolites occur within their intracrystalline cages and channels. Zeolite catalysts can therefore be considered as a succession of nano or molecular reactors. The consequence is that the activity, selectivity, but also the stability of all the reactions carried out over zeolite catalysts, depend (slightly or significantly) on the shape and size of cages, channels and of their apertures, hence that shape selectivity is a general characteristic of zeolite catalyzed reactions. 

Large differences exist between the xylene disproportionation/isomerization ratios (D/I) found with acid catalysts. With zeolites the size of the space available near the acid sites was shown to play a determining role (2). The smaller the size of the intracrystalline zeolite cavities, the lower the ratio between the rate constants of disproportionation and isomerization 0.05 at 316°C with a FAU zeolite (diameter of the supercage of 1.3 nm), 0.014 and 0.01 with MOR and MAZ (0.08 nm). Steric constraints which affect the formation of the bulky bimolecular transition states and intermediates of disproportionation (Figure 9.4) would be responsible for this observation. However, the very low value of D/I (0.001) obtained with MFI (2), the channel intersection of which has a size of 0.85 nm, is also due to other causes limitations in the desorption of the bulky trimethylbenzene products of disproportionation from the narrow pores of the zeolite ( 0.6 nm) and most likely the low acid site density of the used sample (Si/Al=70 instead of 5-15 with the large pore zeolites). 

This case study clearly illustrates the usefulness of the ZLD-TEOM technique in determining intracrystalline diffusivities in zeolites, provided that effects of other transport resistances such as the surface barrier are eliminated by varying the crystal size of the zeolites. The measured steady-state diffusivity can be directly used for predicting effects of diffusion in reactions catalyzed by zeolites. More important, the TEOM makes it possible to distinguish the deactivation caused by blockage of the active sites and by increased diffusion resistance caused by blockage of cavities or channels by coke. 

As Fig. 25 shows, the intracrystalline self-diffusion coefficient of methane in ZSM-5 is between coefficients in zeolites NaCaA and NaX (5,71,114,115,). This order can be interpreted in terms of the minimum apertures of the zeolite channels, which are approximately 0.45, 0.55, and 0.75 nm for 5A, ZSM-5, and X-type zeolites. Due to the hydrophobic nature of ZSM-5, the mobility of water in ZSM-5 considerably exceeds the mobility in zeolites NaA and NaX. A change in the Si02/Al203 ratio of ZSM-5 does not alter the self-diffusion coefficient of methane. On the contrary, for water in ZSM-5 an increase in the self-diffusion coefficients with decreasing A1 concentrations in the framework is indicated. 

Commentary on Intracrystalline channels in levynite and some related zeolites,... 

INTRACRYSTALLINE CHANNELS IN LEVYNITE AND SOME RELATED ZEOLITES... 

Anionic frameworks have been proposed for chabazite. gmelinite 5 and erionite. In this paper we suggest an anionic framework for levynite, and compare, on the basis of the proposed frameworks, the ease and degree of anisotropy of molecule diffusion, and the possible molecular sieve behaviour, for the four zeolites. It has already been shown that diverse intracrystalline channel systems can arise in structures such as analcite, nosean-sodaUte minerals, cancrinite, faujasite, and Linde Sieve A.9> 10... 

The present survey has outlined some properties of porous crystals as diffusion media. It has shown some aspects which have been studied insufficiently, such as concentration dependence of D, which may be very important for catalysis at intracrystalline sites. Many more accurate determinations of differential diffusion coefficients are required to give added understanding of the role of concentration, chain length (of paraffins), polarity, exchange ions, channel geometry, and chemical dam-... 

However, despite a significant decrease in the micropore volume, the organic chain content obtained with the non-mesoporous zeolite whatever the silylating agent, is very low with respect to the intracrystalline surface. This could be explained by a channel blockage occurring near the pore entrance. This hypothesis could also explain the incomplete transformation of the amino group previously linked to the mineral structure. 

ZSM-11 zeolite is controlledj at least partially, by intracrystalline diffusion. Indeed, the zeolite channel diameter (5.6 A) is i aller than the critical diameter of m-xylene, mesitylene and naphthalene ( 7.4, 8.4 and 7.4 A, respectively). In the case of m-xylene and mesitylene (Fig lb) the first peak at low temperature is mostly due to the desorption of m-xylene and mesitylene adsorbed on the external surface [9], with Tm values (temperature corresponding to peak maximum of the TPD curve) similar to the Tm value for naphthalene sorbed on HZSM-11 zeolite. The second peaks are mostly due to the interaction of m-xylene and mesitylene which partially penetrated in the channels of the zeolites. The TPD of naphthalene sorbed on HZSM-11 zeolite (N1 in Fig. Ic) shows that N did not interact above 298 C, with Tm value close to 198 C. Because the zeolite channel diameter in H-ZSM-11 type zeolite is smaller than the critical size of N, the interaction is only possible with the external sites. Thus, the desorption of N would be mostly due to the N adsorbed on the external surface of the zeolite crystallites. In the case of H Y zeolite, the TPD result for naphthalene (N2 in Fig.lc) shows two peaks. The second one, at high temperatine (high Tm value), corresponds to the desorption of N sorbed in the intracrystalline voids of the large pore HY zeolite, which may easily accommodate naphthalene molecules. 

In the decompositions of some particularly stable crystalline materials, the reactant stracture does not undergo recrystallization or disintegration, although there may be modification of lattice parameters following the loss of a small stable molecule, such as HjO or NHj, from the reactant phase. Such molecules diffuse outwards between structural components that are sufficiently stable to survive unmodified. Reaction rates are controlled by Fick s laws, ease of movement being determined by the dimensions of the intracrystalline channels. The participation of diffusion control is often recognized by the appearance of the characteristic A term in the rate equation estabhshed. Theoretical aspects of diffusion control have been discussed by Okhotnikov el uf/. [53-57]. 

The key structural feature of the molecular sieves is the narrow, uniform, continuous channel system that becomes available after the zeolitic water has been driven off by heating and evacuation. Great thermal stability after dehydration has been observed in the rigid lattices of X- and Y-type faujasites, zeolite A, mordenite, and chabazite. The geometry of the internal channel and cavity system is characteristic of the individual zeolite. Entrance to the intracrystalline volume is through orifices (ranging from 3 to 9 A in the various zeolites) located periodically throughout the structure. It is thus apparent that access to the intrazeolitic environment is limited to molecules whose dimensions are less than a certain critical size. 

Let us assume that the chemical transformations in zeolite catalyst systems occur vnthin the high surface area intracrystalline volumes. Then, for a reaction within a zeolite particle it is apparent that both the entry pores and the channel-cavity system must be open enough to allow transport of reactant molecules from the bulk phase to the active sites (and vice versa). Thus, any crystalline sieve that could sorb simple organic molecules such as w-hexane might conceivably have catalytic potential. Factors pertinent to these processes are discussed below. 

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