Production of indoles via thermo-catalytic conversion and ammonization of bio-derived furfural

Furfural, an abundant biomass derived compound, was converted into indoles via thermo-catalytic conversion and ammonization with zeolites. The effect of reaction temperature, catalyst, weight hourly space velocity (WHSV) and ammonia to furfural molar ratio on the product distribution was investigated systematically. They all played important roles on indoles production. The maximal carbon yield of indoles (20.79%) was achieved by using HZSM-5 (Si/Al = 25) catalyst at 650 °C with WHSV as 1.0 h−1 and NH3 to furfural molar ratio as 2. The HZSM-5 (Si/Al = 25) catalyst was recycled for five times and showed slight deactivation. Three possible aldehyde group of furfural conversion pathways were proposed and one possible reaction pathway from furfural to indoles was verified based on the experimental and quantum calculation results, that is, furfural firstly reacted with ammonia to form furfural-imine, and then furfural-imine underwent a cracking reaction to form furan. Thereafter, furan was converted to pyrrole and finally to indoles.