Caffeic acid complexes over alumina surface: FTIR, TPD MS and DFT calculations

Caffeic acid (CA) is a phenolic, natural, biologically active compound with pronounced antioxidant and antimicrobial properties. It has a great potential for use in medicine and cosmetology and can also be used to obtain a number of other useful chemicals. Therefore, research aimed at improving the technologies for removing caffeic acid from plant raw materials and its processing is relevant. In our work, we have studied the complexes of caffeic acid with nanosized Al2O3, which is usually used in various plant biomass conversion technologies, using FT-IR spectroscopy, temperature-programmed desorption mass spectrometry (TPD MS), and DFT calculations. The FT-IR spectra analysis shows that CA can interact with aluminum oxide via both the carboxyl and phenolic groups. Based on the measured differences between carbonyl symmetric and asymmetrc vibrational peaks (Δν), it was found that carboxylate complexes of CA on the Al2O3 surface can have bidentate and monodentate structures. The mass spectrometric data analysis made it possible to identify compounds of 4-vinyl catechol, pyrocatechol, and phenol, which are decomposition products of the formed carboxylate and phenolic complexes. It was found that on the surface of the studied CA/Al2O3 samples, CA chelate complexes, formed with the participation of both OH groups of the aromatic ring, predominate.