AFI-type zeolites

Significant Parameters in the Synthesis of Large Alkaline-Free MFI-Type Zeolites and AFI-Type Aluminophosphates... 

When zeolites are grown as films, zeolite membranes are formed. Efforts to prepare polycrystalline zeolite membranes started in the late 1980s, but not until the early 1990s were MFI-type zeolite membranes (ZSM-5 and silicalite-1) successfully prepared with very good permeation and separation properties [3]. Since then, zeolite membranes have constantly attracted considerable attention because of their unique properties in terms of size uniformity, shape selective separation behavior, and good thermal/chemical stabilities. So far, more than 20 different types of zeolite membranes have been prepared - such as LTA, FAU, MOR, FER, MEL, CHA, DDR, and AFI - with significant separation interest [4, 5]. Table 3.1 lists a few typical zeolite membranes and their potential applications for separation of fluid mixtures. 

Without any doubt, the zeolite framework porous characteristics (micropores sizes and topology) largely govern the zeolite properties and their industrial applications. Nevertheless for some zeolite uses, as for instance, host materials for confined phases, the zeolite inner surface characteristics should be precised to understand their influence on such low dimensionality sorbed systems. In that paper, we present illustrative examples of zeolite inner surface influence on confined methane phases. Our investigation extends from relatively complex zeolite inner surface types (as for MOR structural types) to the model inner surface ones (well illustrated by the AFI zeolite type). Sorption isotherm measurements associated with neutron diffraction experiments are used in the present study. 

Determination of zeolite closed porosity in (ID) channel systems (AFI and MTW types ). 

We have observed large variations in the sorption capacities of zeolite samples characterized by (ID) channel systems, as for instance AFI (AIPO4-5 zeolite) and MTW (ZSM-12 zeolite) architectural framework types. Indeed, for such unconnected micropore networks, point defects or chemisorbed impurities can annihilate a huge number of sorption sites. Detailed analysis, by neutron diffraction of the structural properties of the sorbed phase / host zeolite system, has pointed out clear evidence of closed porosity existence. Percentage of such an enclosed porosity has been determined. 

BEA (Si/Al2 = 30), FAU (Si/Al2 = 8.6) and EMT (Si/Afi = 8.6) framework types were compared for i-butane/2-butylene alkylation. During the lifetime of the catalyst the butylene turnover number (TON) was approximately the same for each of the three zeolites and the acid sites were equivalent from the standpoint of stability in each case. With EMT the lowered selectivity to consecutive reaction products 2,2,4-TMP -I- 2,3,4-TMP relative to 2,2,3-TMP -i- 2,3,3-TMP and the lowered selectivity to heavies relative to BEA was interpreted as higher hydride transfer activity. 

Nitric oxide is rapidly desorbed on evacuation. The number of adsorption sites for this reaction was found to increase from 1.5/100 A2 after pretreatment at 25°C to only 2.3/100 A2 after pretreatment at 800°C for afi-Al203 (Degussa). These values are about an order of magnitude higher than those obtained by pyridine adsorption on the same type of alumina (121) and by NH3 adsorption on a y-Al203 (168). The number of a sites as determined by C02 adsorption by Peri (157) is still lower. The chemisorption of N02, therefore, seems to be less specific than that of pyridine, NH3, and C02. The weak absorptions at wave numbers above 2000 cm-1 were tentatively assigned by Parkyns to N02+ species. These may be comparable to the species found in zeolites by Naccache and Ben Taarit (242). 

The PBUs are built from smaller units composed of a limited number of T-atoms, by applying a simple operation to the smaller unit, e.g. translation, rotation.[4] The zeolite framework types can be analysed in terms of these component PBUs. The infinite PBUs, such as (multiple) chains, tubes, and layers, and the finite PBUs, such as (double) 4-rings, (double) 6-rings, and cages are far from unique. However, they are common to several zeolite framework types and allow an easy description of the frameworks to be made. Infinite PBUs and finite PBUs can be used to build the zeolite frameworks (see details in the database of zeolite structures Schemes for Building Zeolite Structure Models).141 Here only an example from AFI is presented to show the building of zeolites. 

Conventional model building methods have, however, recently enabled elucidation of the framework stroctures of Montesommaite [46], ZSM-18 [47], ZSM-57 [48], A1P04-52 [49] and VPI-5 [50]. The stracture of ZSM-18 had perplexed zeolite crystallographers for many years. Its solution by the model building approach, like the related structures of beryllophosphate-H (BPH) and Linde type Q (BPH), was made possible by the determination of the related APS and AFY framework stractures by ctmventional difiiaction teclmiques. 

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