Aqueous-phase catalytic hydrogenation of organic acids and their mixtures.

摘要

Biomass-based organic acids are attractive feedstocks for chemicals production because they are available in large quantities and can undergo a variety of reactions to useful products. Hydrogenation of organic acids yields value-added alcohol products that are important building blocks for pharmaceuticals, foods, agricultural chemicals, and polymers. Carrying out hydrogenation in water over heterogeneous metal catalysts eliminates waste generation associated with traditional hydride reagents, facilitates viable reaction rates at mild conditions, allows easy catalyst separation and reuse, and avoids organic solvents. Further, the mild aqueous environment allows transfer of chirality present in bio-derived organic acids to their alcohol products, giving high-value, optically pure materials in a single step. Development of low-cost, high-efficiency hydrogenation routes could open economically viable pathways from renewable resource-derived materials as alternatives to today's petroleum-based chemicals.; We investigated the aqueous-phase hydrogenation of lactic acid and propionic acid and their mixtures, as well as mixtures of the acids with their alcohol products propylene glycol and 1-propanol, over Ru1C and Ru sponge catalysts in a three-phase stirred batch reactor. The goals of the work are to examine the acids' relative hydrogenation reactivities, the substrates' and products' relative affinities for metal surface sites, and how one substrate's conversion is influenced by other acid or alcohol species in solution. By relating adsorption affinity and reactivity to substrate structure and feedstock composition, we aim to provide a rational basis for design of aqueous-phase heterogeneous hydrogenation catalysts and processes.; For the study of acid hydrogenation over Ru/C catalyst, kinetic data were collected for reactions at 343-423 K, 3.4-10.3 MPa hydrogen pressure and 0.05-5 M acid feed concentrations. Mass-transfer analysis showed that acid conversion rates were not limited by mass-transport resistances over the reaction conditions studied. A two-site Langmuir-Hinshelwood (L-H) kinetic model with a single set of rate and adsorption constants fits the conversion kinetics of both individual and mixed acid hydrogenations over a wide range of condition. Competitive adsorption of acids and their alcohol products strongly affects hydrogenation rates.; Hydrogenation reactions over Ru sponge catalyst were conducted at 403 K, 3.4-7.9 MPa hydrogen pressure and 0.1-1 M acid feed concentrations. The same two-site L-H model with a new set of kinetic constants was used to characterize acid hydrogenations over Ru sponge. Hydrogenation reactivity/selectivity and competitive adsorption of the reacting species on Ru sponge are significantly different from that on Ru/C.; The activated carbon support material facilitates selective adsorption of acids or alcohols into the carbon pore structure, which results in higher local concentrations in the vicinity of the catalyst and thus influences the observed hydrogenation rates. Adsorption of reactant and product species into the activated carbon support was studied for the prediction of local pore concentrations. A global model that incorporates local pore concentrations into the hydrogenation kinetics over Ru sponge was used to predict hydrogenation rates of organic acids over Ru/C catalyst in water.