Cu/CuMgAlO_x-Catalyzed Guaiacol Hydrodeoxygenation in Supercritical Methanol: A Modeling Mechanistic Insight for Lignin-Derivatives Upgrading

Abstract

The development of an economic and sustainable catalytic system was crucial for lignin-based biorefinery. Herein, we reported a low-cost Cu/CuMgAlO x catalyst with promising activity toward lignin hydrodeoxygenation (HDO) through a H2-free method. Supercritical methanol was used as the hydrogen donor, solvent, and reactant simultaneously. Guaiacol was employed as a representative lignin model compound to reveal the HDO mechanism of lignin derivatives. HDOs of guaiacol performed at 250, 275, 300, and 350 °C with durations ranging from 15 to 120 min indicated a high HDO efficiency of the catalytic system. The obtained liquid products were categorized to oxygen-containing unsaturated products (OUPs), oxygen-containing saturated products (OSPs), and cycloalkanes. A kinetic model based on a simplified reaction process containing the three following conversion steps was established: guaiacol transformed to OUPs through the initial HDO, then hydrogenated to OSPs (medium HDO), and eventually turned to cycloalkanes by the deep HDO. The deep HDO was the rate-determining step, and the apparent activation energies of the three steps were all lower than those in the literature. Phenol, 1,2-cyclohexanediol, anisole, and veratrole were the major intermediates, the HDOs of which were programed for pathway verification. Remarkably, catechol (the culprit of condensation) was not produced in this system. Overall, a detailed reaction network of guaiacol HDO was established, and the veil of Cu/CuMgAlO x -catalyzed lignin-derivatives HDO in supercritical methanol was revealed. This work paved the way for the application of Cu/CuMgAlO x catalyst in lignin-derivatives upgrading.

Boosted Hydrodeoxygenation of Lignin-Derived Phenolics to Cycloalkanes with a Complex Copper Acid Catalyst

“…The catalytic performances of the catalysts were initially evaluated by guaiacol HDO in supercritical methanol as a model reaction. It has been indicated that guaiacol undergoes an initial HDO–hydrogenation–deep HDO procedure to produce cyclohexanes, where the deep HDO of cyclohexanols is the rate-limiting step and is mainly controlled by the catalyst acidity. Therefore, the facilitation toward the HDO of cyclohexanols to cyclohexanes by the SA modification was focused.…”

“…Figure shows the total yields of monomers as well as yields of cyclohexanols and cyclohexanes resulting from guaiacol HDO with the SA-Cu/CuMgAlO x catalysts as a function of SA dosage. The reactions were performed at 300 °C for 2 h to give certain yields of cyclohexanes as the activity indicator . The detailed product distributions were given in Table S1 as a reference.…”

“…The SA-Cu/CuMgAlOx catalyst was synthesized by a coprecipitation–impregnation–reduction process. The copper, magnesium, and aluminum species were coprecipitated with the sodium carbonate chelating agent, followed by calcination to give the CuMgAlO x precursor. − SA was then anchored onto the surface of CuMgAlO x by excessive impregnation, followed by recalcination to give the SA-CuMgAlO x . The moderate acidity of SA was intended to minimize the corrosion of the metal oxides during the catalyst modification.…”

“…Meanwhile, the Cu species could simultaneously catalyze the self-reforming and water–gas shifting reactions of alcohols (methanol and ethanol) for the production of activated hydrogen, an alternative of gaseous H 2 as the hydrogen source. − The catalytic performance of the Cu catalyst has been tested with a variety of biosubstrates, including raw biomass, lignin, − and lignin model compounds. , However, the experimental results demonstrated that full HDO of the lignin derivatives was difficult to achieve with the Cu catalyst and that cyclohexanols were generally observed as the dominant products under different conditions (240–350 °C, 2–12 h). A reductive pretreatment of the catalyst was found to facilitate the hydrogenation of aromatic compounds but did not contribute to the further deoxygenation of cyclohexanols. − The mechanistic studies carried out by our group revealed that an initial HDO–hydrogenation–deep HDO process was involved in the upgrading of lignin derivatives over the reduced Cu catalyst (Cu/CuMgAlO x ), during which the deep HDO (dehydroxylation) of cyclohexanols to cyclohexanes was regarded as the rate-limiting step. On the basis of the above discussion, the key to accelerating the HDO activity of the Cu catalyst for lignin derivative upgrading lies in strengthening the deep HDO of the intermediate cyclohexanols.…”

“…Recently, the utilization of a Cu-doped porous metal oxide (CuPMO or CuMgAlO x ) for lignin depolymerization and upgrading has attracted substantial attention and has been intensively exploited. − The Cu catalyst was synthesized by a coprecipitation method, presenting advanced thermal stability and better-distributed active sites compared to those of conventional supported catalysts. Meanwhile, the Cu species could simultaneously catalyze the self-reforming and water–gas shifting reactions of alcohols (methanol and ethanol) for the production of activated hydrogen, an alternative of gaseous H 2 as the hydrogen source. − The catalytic performance of the Cu catalyst has been tested with a variety of biosubstrates, including raw biomass, lignin, − and lignin model compounds. , However, the experimental results demonstrated that full HDO of the lignin derivatives was difficult to achieve with the Cu catalyst and that cyclohexanols were generally observed as the dominant products under different conditions (240–350 °C, 2–12 h).…”