Finding methods to utilize CO2 has become a focal point for reducing carbon emissions. A long-standing method involves direct hydrogenation and transfer hydrogenation (TH) using isopropanol as a hydrogen donor to convert CO2 to value-added commodity chemicals such as formic acid. Less explored is the use of glycerol, a low value byproduct of biodiesel synthesis made from trans-esterification of triglycerides, as a substitute for isopropanol due to thermodynamic and kinetic challenges. Using CO2 as a substrate and glycerol as a hydrogen donor for TH yields formate and lactate under basic condition following a Cannizzaro reaction. Herein we explored the use of ruthenium and iridium N-heterocyclic carbenes (NHC) homogeneous catalysts immobilized on a tunable hydrotalcite (HT) supports for CO2 TH, converting CO2 to potassium formate and glycerol to potassium lactate in a high pressure batch reactor and a packed bed flow reactor. Comparison of the heterogenous catalysts to their respective homogenous catalysts indicated that the HT is non-innocent during the reaction. The use of a flow reactor allowed for continuous data collection on kinetic parameters as well as catalytic efficiencies within a single reaction setup and demonstrated that immobilized catalysts are stable for over 300 hours on stream with no sign of deactivation in the presence of 0.25 - 2.0 M potassium hydroxide (KOH) at 150 - 225°C. Precise control over the gas-to-liquid ratio within the reactor provided insight into the efficiency of catalysts at utilizing CO2, giving scale-up details on required gas stream purity and operating pressure requirements. Upon depletion of accessible carbonate in the solution, the formate yield stabilizes while lactate/lactic acid yield continues to form under acid conditions at a reduced rate. Due to similar turn-over-frequencies (TOF) in 0.25 - 2.0 M KOH exceeding 4 K and 1.3 K per hour for lactate and formate, respectively, using lower concentrations of KOH with the use of a recycling loop may decrease reactor construction cost.

Glycerol transfer hydrogenation of CO2 using Ir and Ru carbene organometallics immobilized on hydrotalcites

254th National Meeting (2017)

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