Accelerated synthesis of Sn-BEA in fluoride media: effect of H₂O content in the gel

Tin-containing zeotypes, particularly Sn-BEA, are promising heterogeneous catalysts for a number of important industrially relevant reactions. However, the direct hydrothermal synthesis of these materials requires unfavourably long times, which is an obstacle for their industrial application. In the present study we show that up to 4-fold reduction of the crystallization time can be achieved by the decreasing of the H₂O/SiO₂ ratio in the synthesis gel from 7.5 to 5.6. The crystallization kinetics has been studied for five series of gels containing 1.0 SiO₂ : 0.27 TEA₂O : x SnO₂ : 0.54 HF : y H₂O, for which y was fixed to 5.6, 6.8 and 7.7 at x = 0.005 and to 5.6 and 6.8 at x = 0.010. The crystallization time was varied within 0.5 – 60 days. The intermediate and final products obtained were investigated using XRD, FTIR, XRF, SEM, UV-Vis, MAS NMR spectroscopy and nitrogen adsorption-desorption techniques. The products obtained with lower water content are shown to have the same structure, textural properties and morphology as materials synthesized with higher water content. Although the size of the crystals is found to decrease with water content in the gel, it does not affect the Sn coordination and environment as confirmed by ¹¹⁹Sn MAS NMR.