Effect of Mg/Al Molar Ratio on the Catalytic Performance of Cu-MgAlO Mixed Oxide Catalysts in the Hydrodeoxygenation of Benzyl Alcohol

Abstract

The catalytic hydrodeoxygenation (HDO) of lignocellulose-derived pyrolysis oil is a critical process for producing high-quality biofuels. This study investigates the effect of the Mg/Al molar ratio on the catalytic performance of CuMg(Al)O mixed oxide catalysts in the HDO reaction of benzyl alcohol as a model oxygenated compound. They were synthesized by coprecipitation with a fixed Cu content of 15 at. %, with respect to cations, and different Mg/Al molar ratios (0/1, 1/1, 3/1, 5/1, and 1/0). The catalysts were characterized using X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), UV–vis spectroscopy, nitrogen adsorption–desorption isotherms, temperature-programmed reduction with hydrogen (H₂-TPR), and temperature-programmed desorption (TPD) of CO₂ and NH₃. It has been shown that the Mg/Al molar ratio strongly influences the physicochemical characteristics of the CuMg(Al)O mixed oxides and, hence, their catalytic performance. Catalytic tests were conducted in a stainless-steel autoclave reactor and the obtained results indicated that the systems with Mg/Al molar ratios of 3/1 and 5/1, issued from layered double hydroxide precursors, exhibited the highest activity, with yields to toluene higher than 85%. This superior performance is attributed to the well-dispersed copper species on the catalyst surface combined with appropriate acid–base properties. As the CuMg(Al)O system with Mg/Al molar ratio of 5/1 was the best in terms of benzyl alcohol conversion, i.e., ca. 98% at 230 °C, under 5 atm of H₂, for 3 h of reaction time, with high selectivity to toluene of ca. 87%, the influence of the reaction time, temperature and reusability over multiple reaction cycles on its performance were investigated.