Aqueous-phase hydrogenation of biomass derived lactic acid to propylene glycol.

摘要

Aqueous phase hydrogenation of biomass-derived lactic acid (L+ 2-hydroxy-propionic acid) to propylene glycol (PG) has been performed using a stirred batch reactor and a continuous trickle bed reactor. In the optimal reaction conditions and catalysts, over 90% PG selectivity with 95% lactic acid conversion can be achieved in both batch and trickle bed reactors. The major side reactions are the formation hydrocarbon (methane, ethane, and propane). When reaction temperature is lower than 170°C, PG is the only liquid product, which makes the product separation very simple. The best active metal is ruthenium and the best catalyst supports are selected activated carbons. In the stirred batch reactor, optimal selectivity and high reaction rate are reached at a temperature of 150°C and high pressure of hydrogen (1500∼2000psi). In the trickle bed reactor, the reaction temperature can be as low as 80°C with a pressure as low as 800psi without significant sacrifice of PG selectivity. The reaction temperature and pressure used in this process are very mild compared to carboxylic acid hydrogenation reported in literature.; In the stirred batch reactor, the measured gas-liquid mass transfer coefficient and theoretical mass transfer analysis have shown that gas-liquid, liquid-solid, and intra-particle mass transfer are negligible at our reaction conditions. The intrinsic kinetics have been analyzed and the activation energy for lactic acid hydrogenation is 96kJ/mole. Lactic acid consumption rate is sensitive to reaction temperature and catalyst loading but insensitive to hydrogen pressure. The performance of laboratory prepared carbon-supported ruthenium catalysts are as good as commercial catalysts in the batch reactor.; Granular carbon supported ruthenium catalysts were prepared and used in the trickle bed reactor to continuously hydrogenate lactic acid to PG. Experiments and calculation have verified that gas-liquid, liquid-solid and intra-particle mass transfers and surface chemical reaction together control the lactic acid hydrogenation reaction in the trickle bed reactor. Gas-liquid mass transfer is the major resistance. Lactic acid conversion increases with temperature at the same pressure and fixed hydrogen to lactic acid molar ratio. Like the reaction in the batch reactor, propylene glycol selectivity increases with hydrogen pressure. The overall activation energy in the trickle bed reactor is only 48kJ/mole, which indicates that mass transfer controls this hydrogenation reaction.; Hydrogen solubility at high pressure and trickle bed dynamic liquid holdup, which are two key parameters in the trickle bed reactor modeling, were measured in the trickle bed reactor. A one-dimensional trickle bed reactor model was derived. This model consists of two differential and two algebraic equations and forms a typical two-point boundary value problem mathematically. With simplification, the model was solved in Mathematica. Liquid phase hydrogen and lactic acid concentration and catalyst surface hydrogen and lactic acid concentration profiles were plotted. The results give us more information about the role of gas-liquid and liquid-solid mass transfer.