Phosphotungstic acid modified sulphonic acid functionalised SBA-15: exploring the synergistic effect for thermal and light assisted synthesis of bis(indolyl)methanes

Abstract

Sulphonic acid functionalized mesoporous SBA-15 (SSA) was synthesized by a one-pot hydrothermal method using P123 as the surfactant in acidic media. The basic framework of the SSA material was assembled through the simultaneous hydrolysis and co-condensation of tetraethylorthosilicate (TEOS) and 3-mercaptopropyltrimethoxysilane (MPTMS) and in situ oxidation of the thiol moieties to sulfonic acid functionalities aided by H₂O₂. Phosphotungstic acid (HPW) was incorporated into the SSA matrix via a simple wet impregnation. Successful immobilization of HPW over SSA without disrupting the mesoporous framework could be established by detailed spectroscopic and microscopic characterization. The potential of HPW/SSA systems in catalyzing the formation of bis(indolyl)methanes under ambient conditions is explored. The remarkable performance under light-assisted conditions and a comparative evaluation with conventional thermal synthesis are demonstrated. The novel protocol offers an excellent yield of BIMs in an aqueous medium in a short reaction time, eliminating the need for organic solvents and complex work-up procedures. Sustainable performance upon recycling and broad substrate scope validate the versatility of the catalyst. This present protocol thus provides an operationally simple, scalable, and versatile method for synthesizing BIMs.