Synthesis of bi- and trifunctional mesoporous silica-based catalyst and application in one-pot cascade reactions

Abstract

This work reports the synthesis and full characterization of bifunctional mesoporous silica supported acid (-SO3H)-base (-NH2) catalysts and trifunctional mesoporous silica supported acid-base-metal catalysts as well as their applications. A series of bifunctional catalysts were synthesized with different site separations, acid loadings, base loadings and types of aminosilane, namely N2SA2, SA0.5N0.5, SA0.5N2, SA1N2, SA2N2, SA0.5N4, SA0.5NN4 and SA0.5NNN4, via a co-condensation and post-synthetic grafting methods and were characterized by XRD, FT-IR, N2 adsorption-desorption, SEM, TEM, TGA and XPS techniques. The well-ordered hexagonal mesoporous structure was preserved throughout the synthesis where the synthesized materials exhibited small kidney-bean-shaped rod and highly ordered hexagonal mesoporous structures. The surface area, acidity, and basicity of the bifunctional catalysts were in the ranges of 609-729 m2 g-1, 0.88-1.10 mmol g-1, and 0.29-0.57 mmole g-1, respectively. These catalysts were tested in the one-pot deacetylation-Knoevenagel of benzaldehydedimethylacetal (A) with malononitrile to produce benzylidenemalononitrile (C) at 90 oC for 5 h. It was found that the conversion of A and the yield of C were influenced by the acidity, basicity and pore characteristics of the catalysts. In particular, The SA0.5N4 catalyst exhibited the highest conversion of A (100%) and the highest yield of C (71%) with good reusability of at least four cycles where no significant loss in catalytic activity was observed.

The best performing bifunctional catalyst was further used for the preparation of trifunctional catalysts, where the latter were prepared by impregnation of palladium metal (0.03, 0.2 and 1 wt.%) on the SA0.5N4 acid-base material. The trifunctional catalysts were tested in the one-pot deacetylation-Knoevenagel-hydrogenation of A to produce benzylmalononitrile (D). The effects of reaction time, hydrogen pressure, Pd loading and type of aminosilanes on the catalytic performance were investigated. Nevertheless, the desired product (D) was not observed in any treatment conditions. The production of D was, on the other hand, achieved by using the physical mixture of 5%Pd/Al2O3 and either of the SA0.5N4, SA0.5NN4, and SA0.5NNN4 catalysts. The SA0.5N4 catalyst continued to perform well for the one-pot production of D, where the highest yield of D (49.4 %) was achieved over the consecutive deacetylation-Knoevenagel-hydrogenation process catalyzed by the physical mixture of SA0.5N4 and 5%Pd/Al2O3.