Low-temperature synthesis and characterization of a stable colloidal TPA-silicalite-1 suspension

A stable colloidal suspension of the high-silica zeolite, TPA-silicalite-1, has been synthesized at below approximately 35 degrees C and 1 atmosphere pressure in a precipitated silica-TPAOH solution aged for 40 months, as confirmed by small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), transmission electron microscopy (TEM), and electron diffraction. Condensation of the aged, TPA-silicalite-l sol by concentration, followed by purification, drying, and calcinination, yielded a clear (in bulk), grassy silicatite-1 ceramic-like material, stable in air below ca. 800 degrees C. Thus, nucleation and growth of TPA-silicalite-l is demonstrated for the first time at relatively low temperatures (<35 degrees C), consistent with predictions of crystallization times based upon the kinetic equations of nucleation and crystallization reported in the literature. TEM, scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDXA), infrared spectroscopy (i.r.) and nitrogen adsorption isotherms (BET; t-plot) were also used to further characterize the purified and unpurified condensed forms of the aged sol. At this relatively tow temperature, precipitated silica probably entered the solution via slow depolymerization. Once available, dissolved silica was templated by the TPA(+)/H2O structure to yield 5-10-nm germ nuclei, most of which fused syntaxiaily with other nuclei; the remainder, if any, grew via a surface-limited reaction tin addition to fusion) to give ca. 55-nm diameter single-crystal colloidal TPA-silicalite-l particles. The ca. 55-nm particles formed are roughly isometric, stabilized in solution by glassy surfaces rich in hydroxyl groups, and have an interfacial area indicative of much smaller constituent subparticles (5-10 nm). (C) Elsevier Science Inc, 1997.