Preparation of sulphonic acid-functionalized heterogeneous catalyst from cashew nut shell liquid and its application for biomass value addition

Although cashew nut shell liquid (CNSL) is a well-known biofuel, feedstock for sustainable chemicals and phenolic resins, its use for preparing heterogeneous catalysts by suitable structural modifications remains virtually unexplored. This work reports the preparation of a sulphonic acid-functionalized catalyst by reacting CNSL with concentrated sulphuric acid, followed by carbonization of the polymeric solid material. The polymerized, sulfonated and carbonized CNSL at 200°C (CNSL-PSC200) demonstrated the best catalytic activity for esterification and dehydration reactions. CNSL-PSC200 was used as a novel heterogeneous acid catalyst for synthesizing ethyl levulinate (EL), a promising fuel additive, from carbohydrate-derived levulinic acid (LA) and angelica lactone (AGL). Under optimized conditions (120°C, 6 h, 8 equiv. EtOH, 50 wt% cat.), EL was isolated in 83% and 87% yield from LA and AGL, respectively. Furthermore, the catalyst was successfully used to dehydrate glucose-derived sorbitol into isosorbide (IS). A 65% isolated yield of IS was obtained under optimized conditions (160°C, 4 h, 1 torr, 50 wt% cat.). The catalyst was successfully recovered and recycled five times without catastrophic loss in activity. The catalysts (fresh and recycled) were characterized by Fourier transform infrared (FTIR), powder X-ray diffraction (PXRD), Brunauer–Emmett–Teller (BET), field-emission scanning electron microscope (FESEM), elemental analysis and N2-adsorption isotherm techniques. This work will diversify the applications of CNSL as an agricultural waste and promote new ideas for developing sustainable catalysts from unconventional sources.