Synthesis of nanosized offretite crystals

Zeolites are crystalline microporous aluminosilicates with periodic arrangements of cages and channels that find extensive industrial uses as catalysts, adsorbents, and ion exchangers .Colloidal zeolite crystals can be useful in many ways. They can be used directly in various processes, which may result in better mass transport since the internal diffusion path is reduced as the particle size is decreased. For instance, the activity may be increased in catalytic processes. Besides the benefits of colloidal zeolites over conventional zeolites related to their small crystal size, the major interest in colloidal zeolite suspensions is due to their use for preparation of zeolite films and membranes as well as composites and hierarchical structures[1]. The colloidal size of the crystals employed in such preparations brings unique properties to the structures prepared and expand the area of zeolite applications toward, e.g., optoelectronics, chemical sensing, and medicine.Zeolite Offretite was discovered in the basalt of Mont Siminouse (France)by Gonnard in 1890 and it was first synthesized by Aiello and Barrer about 30 years ago[2] . The offretite structure is related to erionite and often is found intergrowth with that zeolite. Stacking faults or intergrowth of these two zeolites in their synthetic as well as natural forms are very common, such as zeolite T and ZSM-34 . Both offretite and intergrowth-type zeolites have been reported to be very attractive materials as catalysts and adsorbents in several chemical processes such as, (a) conversion of methanol to C2-C5 olefins ; (b) selective catalytic dewaxing ; (c) molecular sieving of n-butenes. The offretite framework shows two types of channels, one of them of 6.4 Å diameter constituted by a 12- membered oxygen ring, located parallel to the c axis. The other type of channel is built by 8-membered oxygen rings, located perpendicular to the former with a size of 3.8 * 5.2 Å.[3] Offretite/erionite type zeolites are normally synthesized in the presence of tetramethylammonium(TMA) ions and other nitrogen containing organic templates (NCOTs). The amount of structural faults known as stacking faults produced by intergrowth should depend strongly on the ratio of TMA/SiO2 in the initial solution precursor of TMA-offretite since the TMA controls the erionite intergrowth.TMA cations proved to be the best template for preventing the formation of erionite during the synthesis of offretite[4]. At present work, The colloidal offretite crystals with a maximum size of up to 250 nm were prepared by hydrothermal synthesis at 100°C and application of TEOS as silica source.