(650d) Glycerol Transfer Hydrogenation of CO2 Using Ir and Ru Carbene Organometallics Immobilized on Active Hydrotalcites in a Packed Bed Flow Reactor
Utilization of 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 packed bed flow reactor. HT was synthesized using 3:1 ratio of Al:Mg in flow, and composition was varied by incorporating Cu or Fe in place of Al up to a total of 15 wt. %. 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 of 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 150-225ËC. 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. Reactions were initially carried out in 45 bars of CO2 in a batch reactor and then scaled to a flow reactor using less than 35 bar CO2. 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 4.4 K and 1.5 K per hour for lactate and formate, respectively.