Offretite crystallization

The adsorption microcalorimetry has been also used to measure the heats of adsorption of ammonia and pyridine at 150°C on zeolites with variable offretite-erionite character [241]. The offretite sample (Si/Al = 3.9) exhibited only one population of sites with adsorption heats of NH3 near 155 kJ/mol. The presence of erionite domains in the crystals provoked the appearance of different acid site strengths and densities, as well as the presence of very strong acid sites attributed to the presence of extra-framework Al. In contrast, when the same adsorption experiments were repeated using pyridine, only crystals free from stacking faults, such as H-offretite, adsorbed this probe molecule. The presence of erionite domains in offretite drastically reduced pyridine adsorption. In crystals with erionite character, pyridine uptake could not be measured. Thus, it appears that chemisorption experiments with pyridine could serve as a diagnostic tool to quickly prove the existence of stacking faults in offretite-type crystals [241]. 

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. 

Because of the high surface free energy at the liquid-solid interface, it is suggested that the stages of nucleation, transport of species by surface diffusion, and crystallization occur at the interface in the boundary layer. Culfaz and Sand in this volume (48) propose a mechanism with nucleation at the solid-liquid interface. This mechanism should be most evident in more concentrated gel systems where interparticle contact is maximized for aggregation, coalescence, or ripening processes. The epitaxy observed by Kerr et al. (84) in cocrystallization of zeolites L, offretite, and erionite further supports a surface nucleation mechanism. 

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... 

The occurrence of intergrowths of zeolite Y and ZSM-20, the cubic and hexagonal forms, is analogous to similar intergrowths in SiC and ZnS crystals. Intergrowths in zeolite Y and ZSM-20 do not block channels, as is the case in the erionite-offretite family, where rotation of cancrinite layers blocks the 12MR channels, but are more like intergrowths in the ZSM-5/ZSM-11 family, which modify the channel system. 

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 1. Kinetics of crystallization of zeolite X, o (37), zeolite L, e (38) and offretite, A (43), 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.

Upon seeding, ZSM-20 appears better stabilized as no traces of zeolite Beta were found after 27 days heating. Our observation is in agreement with other recent works that showed the high efficiency of the addition of very small and freshly formed zeolite nuclei as seeds to batches giving zeolites TMA-Offretite 1481 or TMA-Omega 149). Both materials were formed more rapidly and selectively, free from stable side phases like Analcime or Mordenite that usually co-crystallize in absence of seeds. 

Thermograms of(Rb,TMA) offretite crystals in oxygen and nitrogen. Sample weight 15.15 mg. Heating rate 10 °C/min. Gas flow rate ... 

Infrared spectra of Rb-offretite before and after room temperature sorption of pyridine. Outgassing temperatures (a) 100, (b) 200, (c) 400 °C. Curve (d) refers to the Rb-offretite crystals used... 

The spectra in Figure 9 is somewhat different from the spectra of offretite crystals synthesized in the NaOH-KOH-TMAOH mixed base system. Wu et al.(ll), in addition to our observed band at 3615 cm , reported the appearance of acidic bands at 3690 and 3550 cm. In contrast, Mirodatos et al.(12) reported the appearance of only one, weakly acidic band at 3660 cm . The TMA content and the concentrations of different charge compensating cations may be responsible for these differences. 

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. 

The question remains why the other components, principally branched paraffins, are converted at all. Several explanations can be offered, none completely satisfactory. Not all the palladium is inside the zeolite cages but may be partially on external surfaces and nonzeolite components, amorphous material which is either the residue of incomplete crystallization or the product of zeolite decomposition in subsequent treatments. Since x-ray crystallinity is uniformly high, the amorphous component should be quite small. Branched paraffins can penetrate the zeolite surface far enough to be cracked. High temperature alters the selective adsorption properties of the zeolite, which were observed at low temperature. Offretite intergrowths provide enough surface in larger diameter pores partially to convert branched and cyclic molecules. There is some truth in all of these but we prefer the latter. 

Chemical Properties Si/Al ratio (used to be Si/ Al>2.4 Si/Al >3.0 Si/(A1- -Pe)>2.9 for erionite) is no longer a criteria for discrimination between erionite and offretite, because of the extensive compositional overlap that exists between the two species. However, the Si-Al content in the tetra-hedra framework is the major control on the unit cell volume dimensions in erionite. In addition, Mg cation is a major factor in controlling the crystallization of the mineral species. 

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