Synthetic offretite

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. 

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... 

Molecular sieve effects and their influence on catalytic selectivity offer important possibilities. Chen (48) showed that for a given reaction synthetic offretite, with its 12-membered rings of oxygen ions, exhibited no selectivity where the presence of small amounts of erionite (3%) resulted in an effective blocking of the large openings and the creation of selectivity. This emphasizes the possible influence of impurities on the practical uses of zeolite catalysts. 

Figure 4. Synthetic offretite (A) Electron diffraction of the hOl zone odd-l reflections are absent. (B) Typical micrograph of sausage-shaped particles with c-elongation. (C) Dark-field image using electrons diffracted by the bottom particle, showing that the outer layer is crystalline.

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]. 

Fig. 24. 29Si MAS NMR spectra at 79.80 MHz (ahove) of zeolite Y, zeolite omega (synthetic mazzite), offretite, and mordenite, and their dealuminated forms (below).

Twenty-eight kinetics of crystallization of different types of zeolites (A (2,12,13,35,36), X (2,6,37), L (38), P (3 0, ZSM-5 (39-41), synthetic mordenite (2,42) and offretite (43)), synthetized by various authors under various experimental conditions, have been analysed by using Equations (1) and (5). 

Figure 2. Kinetics of crystallization of ZSM-5, A (39), zeolite L, o (38), offretite, e (43) and synthetic mordenite, A (42), correlated by Equation (1) (solid curves in Figure A) and by Equation (5) (solid curves in Figure B), respectively, using the corresponding values of K, q, K0 and Ka from Table I.

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). 

Since offretite is a large-pore structure, intergrowth of offretite in the erionite phase would be expected to affect the adsorption properties. Table II compares adsorption capacities for natural and synthetic erionite with Zeolite A (Ca) and synthetic faujasite (Na) (4.8 Si02/Al203). As expected, the more dense erionite structure shows lower capacity (5). There is substantial agreement between natural and synthetic erionite capacity the difference shows in adsorption rates (D/r ). The low apparent diffusivity of n-parafBns in erionite is somewhat a mystery since there does not appear to be that much difference in pore dimensions between erionite and zeolite A as predicted from their structures (6). The difference cannot be attributed to crystallite size since the natural erionite sample (laths, 0.5 /x diameter or less) has finer crystallite size than any of the synthetic materials (1-5 /x). 

The difference is more notable in n-octane adsorption which is shown in the last 2 columns of Table II. Zeolite A shows substantially the same capacity and adsorption rate for n-octane as for n-hexane. But for erionite, both natural and synthetic, n-octane capacities, and particularly the adsorption rates are substantially reduced. Here the difference between synthetic and natural erionite adsorption rate is quite large. It is possible that this is an effect of residual cations. However, simple exchange of Na" and for H" showed little change. We believe the more probable explanation is the intergrowth of offretite in the erionite crystal. The large offretite channels could give more rapid distribution of the sorbate molecule within the synthetic erionite crystal. 

Synthetic zeolites which have been resolved include the zeolite type L (14) and the zeolite Q (13), both of which have open frameworks and should have important catalysis application. Ion exchange studies (71) have contributed to an understanding of the synthetic zeolite T (19), which appears to be structurally related to the minerals offretite and erionite. 

The framework Si/Al ratio of zeolite Q (synthetic mazzite) was increased by treatment with SiCl4 at 500 °C from 4.24 to 6.00 without significant loss in crystalHnity [177]. The dealumination reaction was accompanied by a sfight increase in the hexagonal lattice parameter a while c remained unaffected. This unusual phenomenon, i.e., cell expansion upon isomorphous substitution of silicon for aluminum, as well as AL MAS NMR spectroscopic results, pointed to a redistribution of aluminum on at least two crystallographically different framework T-sites. Si MAS NMR spectra of offretite, erionite and zeofite Q all dealuminated with SiCl4 were presented in [178]. 

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