Seed-assisted, OSDA-free synthesis of MTW-type zeolite and 'Green MTW' from sodium aluminosilicate gel systems

摘要

Recently, the seed-assisted methods have provided a new route for the organic structure-directing agent (OSDA)-free synthesis of industrially useful zeolites under environmentally-friendly conditions. In this paper, the various parameters are investigated in the OSDA-free synthesis of MTW-type zeolite by the hydrothermal treatment of sodium aluminosilicate gel with the aid of calcined ZSM-12 seed crystals prepared using tetraethylammonium hydroxide (TEAOH). More importantly, the green production of MTW-type zeolite referred as "Green MTW" is achieved for the first time, by using the product of OSDA-free synthesis as seeds. The X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and solid-state ~(27)A1 MAS NMR results exhibit better crystallinity of MTW-type zeolite and Green MTW than those of calcined ZSM-12 seeds. MTW-type zeolite is not obtained when seeds are absent in the initial reactant gel while ZSM-5 (MFI-type zeolite) and/or mordenite (MOR-type zeolite) are formed. The results of thermogravimetric (TG) analysis of hydrated samples confirm the enough micropore volume in MTW-type zeolite whereas nitrogen adsorption-desorption measurements of MTW-type zeolite and Green MTW show nitrogen adsorption capacity lower than that of the seeds, which might be attributed to the partial pore blocking in the one-dimensional channel systems of the MTW framework.