Faujasites, sorption

Usually, the zeolite inner surface characteristics are rather complex as a consequence of the (3D) character of the porous topologies of most of the zeolite types. The porous framework is a (3D) organization of cavities connected by channels. Inner surfaces are composed of several sorption sites characterized by their local geometry and curvature. Illustrative examples of such inner surface complexity are represented on Figures 1 and 2 they concern the Faujasite and Silicalite-I inner surfaces respectively. 

Figure 1. Schematic representation of a faujasite super-cage. Its developed inner surface, which is composed of two types of sorption sites, has been also represented.

Pfeifer et al. (263) conclude from their measurements of T, and T2 versus temperature in samples with controlled water contents that the lifetime of sorption complexes of water is 3.5 x 10-9 sec at 50°C with nonlocalized cations and at - 10°C with localized ones. Water was found to be bound more strongly in faujasites with higher Si/Al ratios, which agrees with model calculations by Dempsey (282) of the electrostatic fields around cations. At higher coverages the mobility of H20 is independent of the Si/Al ratio and is two orders of magnitude lower than in bulk water. 

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. 

Highly selective sorption of aromatic compounds from paraffin-containing solutions has been reported for the faujasites NaX, NaY, and HY under equilibrium conditions (2). Thus, benzene is preferentially adsorbed relative to n-hexane or n-decane, p-xylene relative to n-octane, and naphthalene relative to n-decane. The measured separation factors in these systems are so large (K>700) that essentially only one species exclusively occupies the internal volume of the zeolites. 

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. 

This behavior of ZSM-5 differs considerably from that reported for the large-pore zeolites, mordenite ( ) and faujasite (2 ). With those zeolites, preferential sorption of the cycloparaffin was generally observed. 

Chemical analysis and sorption data. Chemical analyses (ICP-AES) of highly crystalline and pure ZSM-20 materials give Si/Al ratios in the range of 3.7 (Table 2) to 4.7 (Table 1). Na/Al ratios of 0.7 are typical, as are (Na+TEA)/A1 ratios of about 1.1. Single point n-hexane sorption capacities at 40 torr and 23°C, after burn off of the TEA at 550°C in air for 3 hours, are invariably in the range of 18% to 20% wt.- values typical for high silica directly synthesized faujasite type product (24.) ... 

Molecular sieve zeolites have become established as an area of scientific research and as commercial materials for use as sorbents and catalysts. Continuing studies on their synthesis, structure, and sorption properties will, undoubtedly, lead to broader application. In addition, crystalline zeolites offer one of the best vehicles for studying the fundamentals of heterogeneous catalysis. Several discoveries reported at this conference point toward new fields of investigation and potential commercial utility. These include phosphorus substitution into the silicon-aluminum framework, the structural modifications leading to ultrastable faujasite, and the catalytic properties of sodium mordenite. 

Metal atom clusters in the 26-hedra could (as with water molecules) contain many metal atoms. An example is the fourteen atom Agg+8Ag+ cluster in Y-irradiated zeolite Ag-A[25]. Saturation is achieved for mercury sorbed into silver-exchanged faujasites and other zeolites [26]. The Ag+ is reduced to Ag atoms and then at an approximate critical pressure of mercury vapour there is nucleation of mercury clusters which fill all the pore volume as the pressure of Hg vapour increases further. Mercury-zeolite systems are the oily ones in which sorption isotherms have been investigated quantitatively. Hcwever other metal atoms introduced into zeolites (by ion exchange and reduction, or as metal carbonyls and their decomposition) all show, on heating, a strong tendency to form clusters by migration of atoms, which can aggregate both within and outside the crystals. 

Comprehensive review papers on the sorption properties of natural zeolites can be found in literature [72,73]. Referring in particular to the main sedimentary zeolites, the last two columns in Table 2 show some structural features of interest for sorption applications. Chabazite, clinoptilolite, faujasite and mordenite, which couple reasonably large to large window sizes with wide inner volumes (except mordenite), appear the most suitable materials for adsorption processes. 

Of course, lattice parameters can also be used to study the effects of post-synthesis treatment (e.g. ion exchange, calcination, dealumination, sorption, etc.), to estimate Si/Al ratios in well-calibrated systems such as faujasite, to monitor a phase transition as a function of temperature, or to begin the structural characterization of a new material. Indexing a pattern can also serve to establish whether or not a phase is pure. If all lines can be indexed on a single unit cell, there is probably only one crystalline phase present. However, if there are unindexed lines, either the indexing is incorrect or there is a crystalline impurity present. 

More recently, a detailed study of diffusion of the xylene isomers in large crystals of NaX and natural faujasite was undertaken by both sorption rate and tracer exchange.(11-14) The data obtained by both these techniques using several different crystal sizes were entirely consistent but the diffusivities were much smaller than the values derived for the same systems by NMR PFG measurements. In an attempt to resolve this discrepancy we have developed a new chromatographic technique (zero length column or ZLC) which is less sensitive than conventional sorption methods to the intrusion of external heat and mass transfer resistances and which is therefore useful for following relatively rapid diffusion processes. The method has now been applied to study the diffusion of a range of different hydrocarbons in both A and X zeolite crystals and the results of these studies are summarized here. 

Figure 2. Arrhenius plot showing comparison of NMR and sorption diffusivities for benzene and o-xylene in NaX zeolite crystals. NMR data from (1) Germanus et al. (19) and (2) Karger and Ruthven (10). Uptake (corrected diffusivity) and tracer exchange data of Goddard (11-13) (50 pm and 100 pm NaX, 250 pm faujasite). ZLC data of Eic (15,16).

Bounds can be developed to bracket the increase in sorption time as a function of the number N of participating cavities when zeolite A is replaced by faujasite. Data on (n) (for A up to A = 3375) for finite d = 3 structures of coordination v = 6 can be represented by the formula, Eq. (4.23), whereas for coordination v = 4, the Eq. (4.24) pertains. Assuming a common jump time r for both zeolite structures, the following ratio can be constructed ... 

In obtaining the above estimates of the maximum sorption time, it was assumed that sorption occurred at one cavity only, viz., the one farthest removed from the surface of the zeolite crystal. The question arises whether and to what extent the above picture changes if sorption is permitted in any of the polyhedral cavities defining the assembly. Let si be the probability of sorption of the diffusing species in cavity i. In the case, all St = s > 0, the consequences of sorption at the satellite sites i are displayed in tables 3 and 5 of ref. 14 for faujasite-type structures for both ambient conditions noted earlier, and in table VI of ref. 9 for zeolite A-type structures. It is plain from these results that distinctions between different zeolite channel patterns or between different boundary conditions become inconsequential once the satellite sorption probability 5 > 0.10. 

Zeolites of type Y are prepared by either primary or secxindary synthesis. Structures include zeolite Y in t)oth the cubic and hexagonal forms, SAPO-37 and faujasitic frameworks containing Ga or Zn. These materials are characterised using solid state NMR, X-ray powder diffracticai, infrared jectrosccpy, surface aneilysis and sorption. Catalysts are then evaluated for the conversion of n-hexane, cyclohexane and gas-oil. Results are interpreted in terms of the effectiveness of catalytic sites in alkane activaticxi and in the effect of both density and distribution of active sites. 

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