Sulfonic Acid-Functionalized Solid Polymer Catalyst from Crude Cashew Nut Shell Liquid: Synthesis of Tetra(indolyl)methanes and Bis(indolyl)methanes from Xylochemicals

Xylochemistry presents a sustainable solution to the depletion of petroleum resources, contributing to the success of the circulatory economy. The development of reusable carbonaceous materials as heterogeneous acid catalysts has garnered significant attention in both research community and industry. Catalysis research has an intrinsic connection with low-cost synthetic routes, sustainable raw materials, and chemical and thermal stability. We designed and made a solid acid catalyst that can be used more than once from cheap, naturally occurring, crude cashew nut shell liquid (CNSL). Identification of practical applications for waste biomass is a component of the objectives of sustainable development. We treated the black-colored crude CNSL with varying amounts of formaldehyde and further sulfonated the resulting crude resins with chlorosulfonic acid. The solid with the most sulfonic acid groups was used as a Bronsted acid catalyst (CNSLF-SO₃H) for the Friedel-Craft reactions of indoles and furfuraldehydes. We synthesized 15 novel di[bis(indolyl)methane] derivatives from secondary xylochemical 2,5-diformylfuran (DFF) and 15 bis(indolyl)methanes from 5-hydroxymethylfurfural (5-HMF).