Sulfonic Acid-Functionalized\nSolid Polymer Catalyst\nfrom Crude Cashew Nut Shell Liquid: Synthesis of Tetra(indolyl)methanes\nand Bis(indolyl)methanes from Xylochemicals

Xylochemistry presents a sustainable solution to the\ndepletion\nof petroleum resources, contributing to the success of the circulatory\neconomy. The development of reusable carbonaceous materials as heterogeneous\nacid catalysts has garnered significant attention in both research\ncommunity and industry. Catalysis research has an intrinsic connection\nwith low-cost synthetic routes, sustainable raw materials, and chemical\nand thermal stability. We designed and made a solid acid catalyst\nthat can be used more than once from cheap, naturally occurring, crude\ncashew nut shell liquid (CNSL). Identification of practical applications\nfor waste biomass is a component of the objectives of sustainable\ndevelopment. We treated the black-colored crude CNSL with varying\namounts of formaldehyde and further sulfonated the resulting crude\nresins with chlorosulfonic acid. The solid with the most sulfonic\nacid groups was used as a Bronsted acid catalyst (CNSLF-SO<sub>3</sub>H) for the Friedel–Craft reactions of indoles and furfuraldehydes.\nWe synthesized 15 novel di[bis(indolyl)methane] derivatives from secondary\nxylochemical 2,5-diformylfuran (DFF) and 15 bis(indolyl)methanes from\n5-hydroxymethylfurfural (5-HMF).