Cooperative catalysis of carbon supported zinc salts hybrid for efficient conversion of fructose to ethyl lactate

Abstract

Efficient conversion of biomass sugars to lactate is a great challenge due to the difficulty in regulating the active centers of the catalyst to match the cascade reaction. In this paper, a new process was developed by using a hybrid catalyst composed of two carbon-based catalysts ZnC1 and ZnC2 with different acidic and basic active sites. The catalytic performance of these two catalysts as well as their hybrid with different mixed mass ratio was investigated in fructose conversion to ethyl lactate (EL). Both ZnC1 and ZnC2 could offer abundant medium and strong basic sites, which are effective for the [3 + 3] retro-aldol condensation of fructose to glyceraldehyde (GLA) and 1,3-dihydroxyacetone (DHA), while the H+ protons generated in situ by interaction with the hydroxyl groups in alcohol were responsible for the conversion of GLA and DHA to EL via dehydration, acetalization and rearrangement reactions. For these reasons, the hybrid catalyst afforded EL yield of 78.2% with turnover frequency of 43.4 mmol⋅h−1⋅g−1 and total selectivity of 98.2% reacting in ethanol at 220 °C, which are far higher than the previously reported values. Combined with the catalyst characterization, apparent kinetic modeling and DFT calculation, the catalytic reaction mechanism was clarified. The hybrid catalyst demonstrated good anti-leaching stability, while the deactivation was mainly resulted from coking coverage of the active sites. The research not only helps design and prepare the catalysts with bifunctional acidic and basic sites, but also deepens the insight into the conversion of biomass-derived sugars to value-added chemicals.