Hydrothermal synthesis of a continuous zeolite Beta layer by optimization of time, temperature and heating rate of the precursor mixture

Highly crystalline zeolite Beta coatings in a range of Si/Al ratios of 12–23 were synthesized on a surface-modified molybdenum substrate by hydrothermal synthesis. The average thickness of the coatings was ca. 2 µm corresponding to a coverage of 2.5 g m-2. The coatings were obtained from a viscous Na, K, and TEAOH containing aluminosilicate precursor mixture with silica sol as reactive silicon source. A mechanism for the in situ growth of zeolite Beta coatings is proposed. According to this mechanism, the deposition of an amorphous gel layer on the substrate surface in the initial stage of the synthesis is an important step for the crystallization of continuous zeolite Beta coatings. The heating rate of the precursor mixture and the synthesis temperature were optimized to control the level of supersaturation and to stimulate the initial formation of a gel layer. At a Si/Al ratio of 23, fast heating and a temperature of 150 °C are required to obtain high coverage, while at a Si/Al ratio of 15, hydrothermal synthesis has to be performed with a slow initial heating rate at 140 °C.