Increasing energy demands have been met with added combustion of fossil fuels. The massive quantities of carbon dioxide (CO2) given off as a byproduct of these processes have led to environmental and economical ramifications. Consequently, great emphasis has been placed in remediating CO2 emissions through Carbon Capture and Sequestration (CSS) technologies. A limitation of CSS is that it fails to productively use CO2. A complementary approach is to utilize CO2 as a C-1 source. This dissertation discusses several strategies for the valorization of CO2 to methanol (CH3OH) stemming from fundamental hydrogenation studies.
Chapter 2 outlines a facile approach for the in situ generation of ester hydrogenation catalysts. Unlike traditional methods, this simple approach circumvents the use of sub-stoichiometric alkoxide base. Systematic studies of ligand and base effects on the hydrogenation of the esters, are disclosed. Generally, diphenylphosphinoethylamine, was found to form the most active catalyst for the hydrogenation of alkyl and aryl esters with >80% yield for select substrates. Mechanistic studies elucidated the unproductive, base-catalyzed decarbonylation of the formate ester with traditional alkoxide bases. Consequently, alternatives were investigated and K3PO4 was found to be a viable and compatible substitute.
The improved insight from formate ester hydrogenation guided our studies for the one-pot hydrogenation of CO2 to CH3OH. Application of these catalysts and conditions to the cascade hydrogenation of CO2 identified incompatibility with Lewis acids. Chapter 3 focuses on this limitation and discloses a new class of ester hydrogenation catalysts that are compatible with Lewis acids. Application of these half-sandwich ester hydrogenation catalysts to the Lewis acidic cascade system led up to 8 turnovers of CH3OH in a single-pot batch reactor. Further studies implicate labile ligands as a source of inhibition.
In Chapter 4, a conceptually novel approach is disclosed, wherein CO2 is captured using an amine scrubbing agent (NHMe2) and subsequently hydrogenated in a single pot to >500 turnovers of CH3OH. Up to 96% of CO2 was converted to a mixture of CH3OH and N,N-dimethylformamide (DMF). Mechanistic studies of the pathway identify DMF as a key intermediate. This strategy of carbon capture and hydrogenation provides a complementary approach to many industrial carbon capture methods.
In an effort to develop an earth-abundant process for the hydrogenation of CO2 to CH3OH, iron catalysts were investigated as surrogates to the ruthenium catalysts used in Chapter 4. These iron-catalysts demonstrated high activity for the hydrogenation of amides yielding C–N bond scission products with high selectivity. DMF, a key intermediate in the CO2 to CH3OH pathway developed in Chapter 4, was hydrogenated to yield >1000 turnovers of CH3OH and HNMe2. Kinetic studies were performed to compare the activity of the earth abundant iron catalyst to ruthenium. Remarkably, under otherwise identical conditions, the iron and ruthenium catalysts displayed rates within a factor of 2. Application of these catalysts to the CO2-capture and hydrogenation pathway is also discussed.
Finally, with the development of hydrogenation methodologies for C–N bond scission of formamides to yield CH3OH, complementary methods have been disclosed to yield the methylated amine through deoxy-hydrogenation. Fundamental studies were undertaken in Chapter 6 to explore the origin of selectivity for the hydrogenation of amides (C–N vs. C–O bond cleavage). Through these fundamental studies, a proton responsive catalyst was identified that enabled selective access to each product (C–N or C–O bond cleavage).PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/140933/1/nomaanr_1.pd
