Cascade conversion and kinetic modeling of glucose transformation into mixed-biofuels via lignin-derived Lewis-Brønsted acid biochars

Biochar catalysts having Lewis acid (Al) and Brønsted acid (-SO3H) functionalities have wide use in sustainable production of chemicals and energy from renewable resources. Functionalized biochars were prepared by isothermal ball-milling of lignin-derived biochar with aluminum diacetate and thiomalic acid and then applied to single-step synthesis of 5-ethoxymethylfurfural (EMF) and ethyl levulinate (EL) from glucose substrate in ethanol. Lewis and Brønsted acid sites of the biochars could be controlled by the ratio of aluminum diacetate (x) to thiomalic acid (y) used in ball-milling. Al:S-Lig(x = 2.7:y = 1.3), which had the lowest Lewis acidity (38 μmol/g) and Brønsted acidity (14 μmol/g), gave 62.1% yields of etherified products made up of 32.6% EMF and 19.6% EL for 4 h reaction time at 150 °C. Al:S-Lig(2.7:1.3) had higher activation energy for EL formation (99 kJ/mol) than EMF formation (72 kJ/mol) and thus suppressed EMF ring-opening, whereas Al:S-Lig(2:2) promoted EMF ring-opening and gave EL yields of 69% for 6 h reaction time at 150 °C. Simulation of reaction kinetics showed that high Lewis acidity (93 μmol/g) was detrimental to formation of the HMF intermediate, while high temperatures (ca. 170 °C) and short reaction times (<10 min) increased EMF selectivities for Al:S-Lig(2.7:1.3). Recycle experiments confirmed that biochar catalysts were stable; additive co-solvents (ethyl acetate, THF) suppressed EMF ring-opening, but did not afford higher EMF yields over those observed for ethanol solvent alone, whereas DMSO inhibited HMF etherification.