CuCr2O4 derived by the sol–gel method as a highly active and selective catalyst for the conversion of glycerol to 2,6-dimethylpyrazine: a benign and eco-friendly process

Abstract

Vapour phase dehydrocyclization of crude glycerol in conjunction with 1,2-propanediamine (1,2-PDA) was examined over CuCr₂O₄ obtained by different preparation methods. A high proportion of copper species interacted with Cr₂O₃ in CuCr₂O₄ derived from the sol–gel route with a low ratio of Cu²⁺/Cu⁰ demonstrating higher dehydrocyclization activity and 2,6-dimethylpyrazine (2,6-DMP) selectivity. X-ray photoelectron spectroscopy analysis of the reduced CuCr₂O₄ revealed a lower fraction of ionic Cu and a high percentage of metallic Cu in the near surface region. The HCOOH and pyridine adsorbed DRIFT spectra of CuCr₂O₄ revealed that strong basic and moderate Lewis acid sites are responsible for the selective formation of 2,6-dimethylpyrazine which is consistent with the catalyst poisoning studies on CuCr₂O₄ co-feeding with pyridine as both Brønsted and Lewis acid site blocker and 2,6-lutidine as a selective Brønsted acid site blocker during the dehydrocyclization reaction. The presence of isolated CuO and Cr₂O₃ species led to a high selectivity for 2,6-dimethylpiperazine. The high intrinsic activity of CuCr_sol–gel was also concomitant with the Cu metal surface areas of the catalysts. The fresh, reduced and some of the used catalysts are characterized by BET – surface area, powder XRD, FTIR, XPS, TEM, H₂-TPR, TPD of NH₃, pyridine, 2,6-dimethylpyridine and HCOOH adsorbed DRIFT spectroscopy.