ERIONITE INTERGROWTH

ZSM-34 seems to be another excunple of offretite-erionite intergrowth. The highest SiOj/AljOj molar ratio that has been reported for this zeolite is about 15. ... 

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). 

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. 

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). 

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. 

The other synthetic samples comprised laths. In electron diffraction patterns, spots with 1 odd were always diffuse and often streaked along c (e.g., Figure 5). Assessment of the proportion of erionite in such intergrowths is technically important, as it affects diffusion rates and catalytic properties [Robson et al. (12)]. The 10.1, 20.1, and 21.1 x-ray powder lines are quite strong for ordered erionite, but they were either undetectable or very weak and diffuse for the other 2 Esso samples. 

ZSM-22 (TON) can be produced from potassium based 1,6 hexamethylenediamine synthesis mixture, while, ZSM-34 (an OFF/ERI intergrowth) is synthesized from the same synthesis mixture with the addition of sodium. NMR analyses of the as-synthesized zeolite samples indicated the presence of a carbonyl species in all of the ZSM-34 samples, but not in any of the preparations that produced ZSM-22. The carbonyl species was present only after the synthesis mixture was heated above ambient temperature. Molecular modeling studies calculated a favorable fit of the carbamic species within the pore system of the Erionite suggesting a reason for the formation of the OFF/ERI intergrowth. 

In 2008, Xiao and coworkers also synthesized aluminosilicate zeolite ZSM-34 from a zeolite L seed solution without organic templates [36]. ZSM-34 is an intergrowth of offretite (OFF) and erionite (ERI) zeolites. Structurally, as with zeolite L, both OFF and ERI contain cancrinite cages. Therefore, they used zeolite L seed solution to directly induce and accelerate the formation of ZSM-34. ZSM-34 with heteroatom substitution by B, Ga, and Fe can also be synthesized by this organic-template-free route [37]. 

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