Offretite, structure

The measured pore volumes are consistent with the offretite structure. Water appears to occupy the total void volume (0.24 cm3/gram), oxygen and n-CJHio appear to occupy the main channels and gmelinite-type cages (0.21 cm3/gram), and neopentane can occupy only the main C-axis channel (0.13 cm3/gram). Nitrogen is anomalous (discussed below). 

Erionite has been synthesized at i00°-I50°C from a (Na,K) aluminosilicate gel with Si02/AUOs = 10. X-ray and electron diffraction results on the product show intergrowths of the related offretite structure, which is a large-pore zeolite. Adsorption capacity for n-hexane is consistent with the density but adsorption rates are far slower than for zeolite A. Adsorption rates for n-octane are even slower but still better than for natural erionite. Hydrocracking tests on a C /Cq naphtha show strong selectivity for converting normal paraffins to Cf gas, particularly propane. As temperature is increased, other components of the naphtha feed are cracked and selectivity decreases. 

Such an effect is understandable in view of the distinction between erionite and offretite structures published by Bennett and Card (2, 9). The designated lines are forbidden for the offretite structure. Card has examined our synthetic erionite product by electron diffraction and found disordered intergrowth with widely varying proportions of erionite and offretite structures (8). 

The offretite structure is known (13). Offretite has a two-dimensional pore system, large 12-ring channels interconnected by an eight-ring network. It is in the detailed structure that evidence for templating... 

Fig. 24. Gmelinite cage, hexagonal prism and cancrinite cage of the offretite structure and indication of crystallographically different oxygen atoms and corresponding OH groups therein [487]...

M.L. Occelli, H. Eckert, C. HudaUa, A. Auroux, P. Ritz, P.S. Iyer et al., Acidic properties of gaUiosilicate molecular sieves with the offretite structure. Stud. Surf Sci. Catal. 105, 1981-1987(1997)... 

One of the earliest direct bonuses of imaging zeolitic catalysts by HRTEM was the discovery (10) that the nominally phase-pure ZSM-5 (structure code MFI) contained sub-unit-cell coherent intergrowths of ZSM-11 (MEL). It soon became apparent (46) that, depending on the mode of synthesis of these and other pentasil (zeolitic) catalysts, some specimens of ZSM-5 contained recurrent (regular) intergrowths of ZSM-11. It also emerged that intergrowths of offretite and erionite are features of both nominally phase-pure erionite and of pure offretite and of many members of the so-called ABC-6 family of zeolites (47). 

There is no systematic nomenclature developed for molecular sieve materials. The discoverer of a synthehc species based on a characteristic X-ray powder diffraction pattern and chemical composihon typicaUy assigns trivial symbols. The early syn-thehc materials discovered by Milton, Breck and coworkers at Uruon Carbide used the modem Lahn alphabet, for example, zeoHtes A, B, X, Y, L. The use of the Greek alphabet was inihated by Mobil and Union Carbide with the zeoHtes alpha, beta, omega. Many of the synthetic zeoHtes which have the structural topology of mineral zeoHte species were assigned the name of the mineral, for example, syn-thehc mordenite, chabazite, erionite and offretite.The molecular sieve Hterature is replete with acronyms ZSM-5, -11, ZK-4 (Mobil), EU-1, FU-1, NU-1 (ICI), LZ-210, AlPO, SAPO, MeAPO, etc. (Union Carbide, UOP) and ECR-1 (Exxon). The one pubHcaHon on nomenclature by lUPAC in 1979 is Hmited to the then-known zeoHte-type materials [3]. 

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]. 

The isomorphous replacement of aluminum by gallium in the framework structure of zeolites (beta, MFI, offretite, faujasite) offers new opportunities for modified acidity and subsequently modified catalytic activity such as enhanced selectivity toward aromatic hydrocarbons [249,250]. The Ga + ions in zeolites can occupy tetrahedral framework sites (T) and nonframework cationic positions. 

Microcalorimetry experiments with NH3 and pyridine as probe molecules indicated that insertion of Ga into the offretite aluminosilicate structure increased the overall acid sites strength of the crystals while decreasing its acid sites density [255], The observed heterogeneity of acid site strength distribution of H,Ga,Al-offretites was attributed to some extra-framework Al(Vl) and Ga(Vl) species generated during the ion exchange and calcination procedures used to prepare H-offretite crystals. 

Table VII (51). The relevant free dimensions are often similar for zeolite and nonzeolite. Urea (free diameter 5.2 A) is like Sieve A (free diameter of windows 4.3 A) in accommodating n- but not isoparaffins. Thiourea (6.1 A) and offretite (6.3 A) have channels with similar free diameters as do 0-cyclodextrin (7-8 A) and zeolite L (7.1 X 7.8 A). In thiourea the loose fit of n-paraffins in the tunnel appears to destabilize the adducts (85, 36). The same is true of disc-shaped molecules comprising only benzenoid rings. However, if suitably bulky saturated side chains are attached (cyclohexyl-benzene or fertf-butylbenzene), then adduction readily occurs. Heterocy-clics, like unsubstituted aromatics, do not readily form adducts. Thus flat molecules also exert a destabilizing effect upon the tunnels of a circular cross section. Such stability problems do not arise with the robust, permanent zeolite structures, and this constitutes an interesting distinction. Offretite, for example, readily sorbs benzene or heterocyclics with or without alkyl side chains, provided only that they are not too large to permeate the structure.

Range of free diameter of maximum included sphere observed in the structures of L and offretite (3.5 A) and in the structure of cancrinite (5.0 A). Variation reflects the degree of distortion of the ideal unit (60, 88). 

N2 02, neopentane) in the zeolites A, X, L, mordenite, omega, and a synthetic offretite type have been determined from isotherms. These have been compared with the void volumes calculated from the known crystal structures. For most adsorbates the measured and calculated void volumes are in good agreement. However, helium and nitrogen exhibit anomalous behavior. A void volume-framework density relation for zeolites is given. 

Unlike the usual amorphous, microporous adsorbents, it is possible to calculate the theoretical micropore volume of a dehydrated zeolite from the known crystal structure. We have performed these calculations here for several of the better known zeolites including zeolite A, zeolite X, zeolite L, mordenite (Zeolon), (8) zeolite omega, (4) and the zeolite 0 (offretite... 

Offretite Type. The synthetic offretite-type zeolite, TMA-O, consists of a framework structure formed by linked cancrinite-type units in columns and enclosing a large C-axis channel (18). These columns are further joined by gmelinite-type units. The calculated total void space including the cancrinite units is 0.244 cm3/gram. The measured adsorption pore volumes shown in Table VI show that even a hydrocarbon such as n-butane... 

Figure 1. Relationship between the measured adsorption volumes, Fp (measd) and calculated void volume Vp of several zeolites. The dashed line corresponds to Vp (measd) = Vp (calcd). The symbols represent the zeolites as described in Tables I-VI A, X, L, Z (mordenite Zeolon), omega (to), and offretite-type 0. Vertical shaded areas containing plotted values of Vp (measd) correspond to calculated values of Vp for the main pore systems. The narrow area, 0, corresponds to the main c-axis void of zeolite 0. The value of Vp for Zt = Vp for zeolite 0. Symbols with the subscript t (At Xt) etc.) represent values of Vp for the total void volume shown by narrow shaded areas. The neopentane (NP) volumes lie consistently below the dashed line thus showing a paeking effect. In all of these zeolites of varying structure, the H20 and N2 volumes correspond with complete filling of the total voids even though this is not possible in the case of N2 in zeolites A, X, and L.

It was also of interest to compare sorption by the ion-exchanged forms of zeolite L with corresponding forms of erionite and offretite, in view of the structural similarities and differences among the three zeolites (3, 8) and the growing interest in erionite and offretite. Previous sorption... 

The first criterion allows to discriminate between 10-and 12-MR structures (9). MCM-l behaves like a 12-MR zeolite. The second criterion allows to make a ranking according to the size of the windows (10). MCIJ-1 is among the most shape-selective 12-MR zeolites. A Cl pf 2.0 and 1.8 is situated between that of offretite (CI =1.8) and ZSM-12 (CI =2.2) and corresponds, therefore, to a window size of about 0.6 nm. Important differences are found in the values of criterion 3 and 4, indicating that the voids of the different materials are significantly different. In this respect, there seems to be no relation with the Si content. According to Criterion 5 MCM-l is... 

Figure 2. The structures of the zeolites listed in Table I may be regarded as having been derived from various, regular stacking sequences of the single sheet as shown. Thus an AAB sequence yields offretite, AB cancrinite, and so on. Each vertex is a tetrahedral site (T = Si4+ or Al3+ surrounded by four oxygens).10...

In addition, structural similarities can often be determined from careful interpretation of XRD powder patterns. The powder patterns of offretite and erionite look quite different, but are easily understood in terms of the crystallographic consequences of a change in the ordered layer stacking sequence (11), cf. Figure 4. In offretite, the layers are stacked in an AAB sequence, while in erionite, they are ordered in an AABAAC arrangement that doubles one of the crystallographic unit cell parameters. The doubled c-parameter is readily deduced from an analysis of the XRD powder pattern of erionite. Another framework structure effect, isomorphous substitution, can result in changing unit cell sizes, observed as shifts in XRD line positions for such systems as X and... 

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