Humin-sulfuric acid as a novel recoverable biocatalyst for pyrrole synthesis in water.

Humin-sulfuric acid (Humin-SOH) as a novel efficient biobased sulfonic acid was easily prepared by adding chlorosulfuric acid (ClSOH) to Humin and characterized by potentiometric titration and FT-IR spectrum. Humin-SOH is an eco-friendly, heterogeneous biobased, and efficient catalyst for Paal-Knorr and Clauson-Kaas pyrrole synthesis. The catalyst is easily recovered by simple filtration and has excellent turnover efficiency even after 4 cycles. Besides, due to the clearance of the biocatalyst away from the reaction media, the desired highly pure products can be achieved in high to excellent yields. Due to high water dispersibility, Humin-SOH can be utilized as a highly efficient green catalyst for pyrrole synthesis.