Optimal synthesis parameters and application of Sn-MCM-41 as an efficient heterogeneous catalyst in solvent-free Mukaiyama-type aldol condensation

Abstract

Hydrothermal synthesis of mesoporous Sn-MCM-41 materials was established by optimizing crucial synthesis parameters. Several synthesis compositional variables such as molar TMAOH/SiO 2, CTMABr/SiO 2, H 2O/SiO 2 and SiO 2/SnO 2 ratios in the starting gel, type of silica and/or tin sources and synthesis time whose cause and effect relationship on the properties of Sn-MCM-41 materials have been explored. Gels with molar compositions SiO 2: w Sn O 2 : xCTMABr: yTMAOH: zH 2O where 0.00 < w < 0.01, 0.18 < x < 0.32, 0.25 < y < 0.35, 25 < z < 45 were prepared and subjected for hydrothermal treatment at 383 K. Under identical and judiciously pre-optimized synthesis conditions, the influence of different combinations of silicon sources and tin sources on structural properties, tin configuration and morphology of Sn-MCM-41 has been investigated. Powder XRD, DRUV-vis, AAS, SEM and low temperature N 2 adsorption/desorption isotherms have been employed to characterize the products. Selected Sn-MCM-41 materials prepared using different silica and tin sources were evaluated for their efficiency in Mukaiyama-aldol condensation of methyl trimethylsilyldimethyl ketene acetal and benzaldehyde under solvent-free system. Invariably, the β-hydroxy ester selectivity was found to be 100% irrespective of the conversion levels. Among different source combinations, Sn-MCM-41 prepared with: (1) fumed silica and tin tetrachloride and (2) fumed silica and tin tert-butoxide have shown higher catalytic activity. The differences in the catalytic behavior of the materials were found to depend on tin content, the intrinsic differences in Sn siting and textural properties. Sn-MCM-41 materials have shown noticeable morphological differences upon changing the silica sources.