Mechanistic studies of zeolite synthesis from silicate gels containing quaternary ammonium cations.

摘要

The synthesis of purely siliceous zeolites from silicate mixtures containing alkali and quaternary ammonium cations is studied using a variety of analytical techniques; including X-ray diffraction (XRD), scanning electron microscopy (SEM), elemental analysis, ion-exchange studies, and several types of nuclear magnetic resonance (NMR) spectroscopy. Attention is focussed on the role(s) of quaternary ammonium cations towards the direction of the formation of specific zeolite structures, and how the structure and composition of the quaternary ammonium cation affect the kinetics of zeolite nucleation and crystallization.; Silicalite synthesis from sodium silicate gels containing tetraalkylammonium (TAA+) cations proceeds via a hydrogel-solid transformation mechanism. TAA+ cations are encapsulated by the gelatinous silica, forming cage-like silicalite precursors in the gel. These silicate moieties form through coulombic and hydrophobic interactions between the TAA+ and the anionic silicalite gel. Silicate crystallizes through the rearrangement of the precursors in the gel, where the TAA+-containing moieties become the silicalite channel intersections. Tetrapropylammonium (TPA+) cations promote the formation of silicalite-1, whereas tetrabutylammonium (TBA+) promotes the formation of an intergrowth silicalite-1 and silicalite-2. However, tetraethylammonium (TEA+) cations are not hydrophobic enough to promote silicalite formation. Additionally, a neutral organic (tripropylamine) also does not promote silicalite formation.; Zeolite synthesis from silicate mixtures containing alkali and N,N,N-trimethyl-1-adamantammonium hydroxide (TMAAOH) shows that the structure of the zeolite products depends strongly on the type of alkali cation. SSZ-24 forms from K+-containing mixtures, and SSZ-31 forms from Na+-containing mixtures, whereas unknown silicates form from mixtures containing Rb+ and Cs+ cations. Zeolite precursors are not observed in the gel prior to the onset of crystallization in the TMAA+ systems. This may be due to a different silica source and a higher pH than in the TAA+ systems.; Two different quaternary ammonium cations (N-isopropylquinuclidinium (IPQ+) and 2-isobutyltrimethylammonium (IBMA+)) are used in the synthesis of the clathrasil nonasil. Even though the synthesis results are not reproducible, the structure of the organic cation is observed to have a strong influence on the rates of nonasil nucleation and crystallization.