Abstract
A family of silica-supported H3PW12O40 (HPW) solid acid catalysts was prepared by wet impregnation of mesoporous SBA-15 and investigated for the solventless synthesis of 3-substituted indoles under mild conditions. Mesoporous SBA-15 facilitated a high dispersion of immobilised H3PW12O40, significantly improving the catalytic efficiency of the heteropolyacid for the desired multi-component transformation. The yield of 3-substituted indoles strongly correlated with HPW loading (which spanned 3.2–51.6 wt %) and corresponding acid strength; the 51.6 wt % HPW/SBA-15 delivered 81% yield of 2-[(1H-indol-3-yl)(phenyl)methyl]malononitrile, approximately five times greater than that of the unsupported HPW, and exhibited a broad substrate scope for aromatic aldehydes.
Highlights
Porous solids find widespread application in heterogeneous catalysis as either active components or high-area support architectures, wherein pore dimensions and network connectivity control the attendant internal surface area and accessibility [1]
Successful immobilization of HPW over SBA-15 has been previously demonstrated by a range of bulk and surface characterization techniques, including N2 porosimetry, elemental analysis, X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM) and ammonia and propylamine chemisorption [16]
This coincided with a small decrease in the Brunauer Emmer-Teller (BET) surface area and a more significant fall in the pore volume (Figure S2) likely due to the blockage of micropores within the silica walls
Summary
Porous solids find widespread application in heterogeneous catalysis as either active components or high-area support architectures, wherein pore dimensions and network connectivity control the attendant internal surface area and accessibility [1]. Microporous and mesoporous solid acids, such as zeolites, resins, and sulfonated carbons and silicas, facilitate dehydration, hydrolysis, condensation, and isomerization reactions pivotal to petroleum refining and fine and specialty chemical manufacturing [2]. The relatively large and tunable pore dimensions, thick walls, and high surface area of SBA-15 have rendered it an attractive support for sterically demanding organic transformations including biodiesel synthesis [4] and photodegradation of wastewater pollutants [5]. SBA-15 (HPW/SBA-15) as an effective solid acid catalyst for the solventless synthesis of 3-substituted indoles by the condensation of some aromatic aldehydes with indole and malononitrile under mild. The high HPW dispersion achieved over the mesoporous silica offers a five-fold enhancement in the resulting yield of condensation products
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