Homogeneous, Heterogeneous, and Heterogenized-Homogeneous Catalytic Hydrogenation for the Cascade Conversion of Carbon Dioxide to Methanol

Abstract

This thesis focuses on the catalytic cascade conversion of carbon dioxide to methanol. Two cascade pathways were studied involving either an amide or ester intermediate. In the amide cascade pathway carbon dioxide is converted to formic acid. The formic acid undergoes an amidation reaction with a dimethylamine to produce an amide, which is ultimately hydrogenated to give methanol. In the ester cascade system, carbon dioxide is hydrogenated to formic acid which undergoes an esterification reaction with an equivalent of alcohol to generate a formate ester. Finally, the ester is hydrogenated to methanol. Chapter 2 of this thesis focuses on finding improved homogeneous amide hydrogenation catalysts for ultimate application in the amide cascade system. Five Ru-PNPR catalysts with varying substitutions on the phosphorus of the PNP were studied (R= Cy > iPr > Ph > tBu > Ad). A combination of batch reactions and in situ Raman monitoring showed that the three best catalysts gave high yields (>95%), selectivity (C-N bond cleavage), and exhibited fast rates (reactions complete in <6 hours under various conditions). Additionally, Ru-PNPCy and its 1st row analog, Fe-PNPCy were directly compared to one another. It was found that Ru was superior to its Fe analog, but Ru was only 1.7 as fast as Fe. This was surprising as computational studies have suggested 1st row analogs to be orders of magnitude slower than their 2nd counterpart. Using the information from Chapter 2, Chapter 3 revolves around applying new homogeneous catalysts to the amide cascade system. Two new catalysts, Ru-PNPCy and Ru-PNPiPr were applied to the amide cascade system and under certain conditions, outperformed the original catalyst used, Ru-PNPPh. Notably, Ru-PNPCy gave the highest CO2 conversions while Ru-PNPPh produced the highest turnovers of methanol. Cooperative cascade catalysis wherein Ru-PNPCy or Ru-PNPiPr was coupled with Ru-PNPPh in the same pot yielded more methanol than the sum of the two catalysts individually, suggesting a synergistic effect between the catalysts. Variability in the amide cascade system was seen and many routes were attempted to eliminate it. Chapter 4 focuses on coupling a homogeneous and heterogeneous catalyst in the ester cascade pathway. Three heterogeneous catalysts (Cu/Mo2C, Mo2C, CZA) were studied to find a superior ester hydrogenation catalyst in order to generate an improved second-generation ester cascade system. The heterogeneous catalysts were active for ethyl formate hydrogenation at low temperatures (80–135 °C) and pressures (≤40 bar). Despite the excellent reactivity of the heterogeneous catalysts, when coupling a heterogeneous catalyst with any one of five homogeneous catalysts, inhibition rather than synergy was seen between the two. Post-catalysis characterization suggested that the homogeneous catalyst was deposited on the heterogeneous catalyst. Chapter 5 investigates heterogenizing a homogeneous ester hydrogenation catalyst inside of a metal-organic framework (MOF) via ionic interactions. The MOF utilized was MIL-101-SO3 which contains an anionic linker and the cationic homogeneous complex used was [IrCp*Bpy(H2O)][OTf]2. The heterogenized-homogeneous catalyst (Ir@MIL) was active for ester hydrogenation, and even outperformed the homogeneous analog. Excitingly, the homogeneous catalyst could be ion-exchanged out of Ir@MIL either pre- or post-catalysis to straightforwardly study the catalytic active site and confirm that the Ir had maintained both its Cp* and bpy ligands. Ultimately, Ir@MIL is not a good candidate for the ester cascade system due to Ir leaching from Ir@MIL during catalysis, which likely the result of the cationic hydrogenation mechanism.