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UC Irvine UC Irvine Electronic Theses and Dissertations Title Steps Toward CO2 Reduction to Methanol via Electrochemical Cascade Catalysis Permalink https://escholarship.org/uc/item/4wx6g1vm Author Mercer, Ian Patrick Publication Date 2020 License https://creativecommons.org/licenses/by/4.0/ 4.0 Peer reviewed|Thesis/dissertation eScholarship.org Powered by the California Digital Library University of California UNIVERSITY OF CALIFORNIA, IRVINE Steps Toward CO2 Reduction to Methanol via Electrochemical Cascade Catalysis THESIS submitted in partial satisfaction of the requirements for the degree of MASTER OF SCIENCE in Chemistry by Ian Patrick Mercer Dissertation Committee: Professor Jenny Y. Yang, Chair Professor William J. Evans Professor Andy S. Borovik 2020 © 2020 Ian Patrick Mercer ii Table of Contents Acknowledgements iv Abstract v Chapter 1: Methyl Formate and Formaldehyde Reduction by Metal Hydrides 1 Chapter 2: Synthesis of Group 8 Metal Complexes of LDMA for the Study of Outer Sphere Interactions 25 Conclusion 33 Experimental 34 References 37 iii Acknowledgements I would like to express my sincere appreciation to my committee, especially my chair, Professor Jenny Y. Yang. Her commitment to science, mentoring, and ‘doing the right thing’ will forever inspire me. Financial support was provided by the University of California, Irvine and US Department of Energy, Office of Science, Office of Basic Energy Sciences Awards DE-SC0012150 and DE- 0000243266. iv Abstract of the Thesis Steps Toward CO2 Reduction to Methanol via Electrochemical Cascade Catalysis by Ian Patrick Mercer Master of Science in Chemistry University of California, Irvine, 2020 Professor Jenny Y. Yang, Chair Research on two independent projects is presented: (1) Homogeneous cascade catalysis has been used for the hydrogenation of CO2 to methanol as it allows for rational tuning of catalyst reactivity and lower reaction temperatures compared to heterogeneous catalysis. Still, temperatures and pressures far above ambient conditions are required for hydrogenation. Performing cascade CO2 reduction electrochemically could result in milder reaction conditions. To this end, a series of bis(diphosphine) metal hydrides was synthesized. Their reactivity towards the reduction of methyl formate and formaldehyde, the final two intermediates in a proposed reaction pathway between CO2 and methanol, was investigated in alcohol solvents and acetonitrile (or acetonitrile substitutes). In alcohol solvents, transesterification + of methyl formate and protonation of the metal hydride [HPt(dmpe)2] outcompetes any possible hydride transfer to methyl formate. Because the hydride donor ability, or hydricity, of the metal hydrides tested is known in acetonitrile, studying reactivity in acetonitrile allowed for the bracketing of previously unreported methyl formate and formaldehyde hydride affinity values. The hydride affinity of methyl formate and formaldehyde were bracketed between 26.4 – 34.0 kcal/mol v and 49.9 – 57.6 kcal/mol, respectively. This result represents an important step in the rational construction of an electrochemical cascade pathway for CO2 reduction to methanol. (2) Concerted proton-electron transfer (CPET) pathways can sometimes lead to lower energetic pathways for small molecule oxidation reactions. There is evidence to suggest that incorporating hydrogen bond donors/acceptors in the outer sphere of catalysts can promote CPET. To investigate this idea, syntheses of complexes containing the ligand N,N′-bis((6-(dimethylamino)pyridin-2- yl)methyl)-N,N′-dimethylethane-1,2-diamine (LDMA) were explored. This ligand features pendant dimethyl amine groups that appear to promote CPET water oxidation in previous work with cobalt. Substituting cobalt with group 8 metals (Fe, Ru, Os) may impart enhanced water oxidation reactivity and/or stability of oxidized products (key for verifying a CPET mechanism for water oxidation). Thus, the attempted syntheses of group 8 complexes of LDMA are discussed with only DMA DMA DMA the iron L complex, with purported structure [FeL (CH3CN)2][OTf]2 or FeL (OTf)2, yielding clean product and spectroscopic characterization by 1H NMR. vi Chapter 1 Methyl Formate and Formaldehyde Reduction by Metal Hydrides 1.1 Introduction Growing global energy demands continue to spur increased extraction and consumption of fossil fuels.1,2 Due to the environmental impact of these resources, production of chemical fuels using renewable energy sources is highly desirable to promote a sustainable carbon-neutral energy economy. Thus, there is intense interest in “closing the loop” on CO2, where the byproduct of fuel combustion (CO2) is utilized as the carbon source for fuel formation and the production of industrially important chemical precursors, like methanol.3-4 Although there exist numerous examples of selective and highly efficient catalysts for the 5-6 hydrogenation of CO2 to formic acid/formate, direct CO2 reduction to simple alcohols remains a challenge for homogeneous catalysis. Thus, an attractive approach for CO2 conversion to methanol involves indirect hydrogenation, meaning CO2 derivatives serve as an intermediate between CO2 and methanol. In the first example of indirect homogeneous hydrogenation of CO2 to methanol, demonstrated by Huff and Sanford7 in 2011, a three-step cascade catalysis reduction pathway (Scheme 1.1) proceeds as follows: hydrogenation of CO2 to formic acid (step A), followed by Lewis acid-catalyzed esterification to methyl formate (step B), and a final hydrogenation step to yield two equivalents of methanol (step C). Using this approach, they report a 75% yield of methanol with a TON of 21 after 16 hours at 135°C. Other examples of indirect or 8-12 cascade CO2 reduction to methanol have followed in recent years, further improving methanol selectivity and yield and broadening the array of intermediate compounds that can bridge CO2 and 1 methanol. This approach has even been combined with CO2 capture methods as a means of 13-16 utilizing captured CO2. Scheme 1.1. Reaction pathway for conversion of CO2 + 3 H2 to CH3OH + H2O outlined by Huff and Sanford.7 Each catalyst is optimized for a particular reaction step, characteristic of a cascade- style catalysis. Unfortunately, to achieve high yields of methanol, hydrogenation reactions typically require the use of high temperatures (75-160°C), high pressures (upwards of 30 atm H2/CO2), and long reaction times (>16 hours), decreasing the energy efficiency of the process. A more sustainable route to methanol could likely be accomplished via electrochemical reduction of CO2 as reactions could be run at more moderate temperatures and pressures. Operating at ambient pressure also allows for probing of various kinetic and thermodynamic aspects of this pathway, something not as easily accomplished under high pressure. Thus, our new reaction pathway to methanol can be visualized in Scheme 1.2 whereby H2 has been replaced by successive hydride and proton transfer reactions, the former catalyzed by electrochemically-generated metal hydride complexes. Scheme 1.2. Proposed reaction pathway for electrochemical CO2 reduction to methanol. To investigate the thermodynamic landscape of this reaction pathway, I synthesized a series of bis(diphosphine) metal hydride complexes across a wide range of hydricities, or hydride donor strengths (eq 1, ΔGH-). These metal hydride complexes were used to bracket the hydride acceptor 2 ability, or hydride affinity, of intermediates methyl formate and formaldehyde via NMR-scale reactivity studies. To my knowledge, these hydride affinity values have not been reported. Additionally, I discuss the attempted esterification and amidation of tetrabutylammonium formate (Scheme 1.2, Step B), possible reasons why these reactions failed, and how to improve conversion in future experiments. By building the thermodynamic and kinetic groundwork for indirect CO2 reduction to methanol, we can work towards developing a rational approach to catalyst selection and optimization of reaction parameters that improve product yield, selectivity, and reaction rate. 1.2 Results and Discussion 1.2.1 Synthesis of bis(diphosphine) complexes of Pt, Ni, and Rh The syntheses for complexes [Pt(dmpe)2][PF6]2 (dmpe = 1,2- bis(dimethylphosphino)ethane), [Ni(dmpe)2][BF4]2, [Ni(dhmpe)2][BF4]2 (dhmpe = 1,2- bis(dihydroxymethylphosphino)ethane), [Rh(dmpe)2][BF4], and [Rh(dppb)2][BF4] (dppb = 1,2- bis(diphenylphosphino)benzene) have been previously published.17-20 Typically, synthesis n+ involves addition of a stoichiometric amount of diphosphine to either [M(CH3CN)6] or n+ [M(COD)2] (COD = 1,5-cyclooctadiene) in acetonitrile, followed by salt metathesis where appropriate. These complexes were used to synthesize the respective metal hydride complexes [HPt(dmpe)2][PF6], [HNi(dmpe)2][BF4], [HNi(dhmpe)2][BF4], HRh(dmpe)2, HRh(dppb)2, all of 17,18,21-23 which have been previously prepared. Complexes [Pt(dmpe)2][OTf]2 and [HPt(dmpe)2][OTf] were prepared by a method similar to the PF6⁻ analogs but with NH4OTf as the metathesis salt (Scheme 1.3). Details of their synthesis and spectroscopic characterization are listed in the Experimental section. The identity and purity of all complexes listed above were 3 confirmed by 31P{1H} and 1H NMR spectroscopy. The metal hydrides chosen are well studied and have known hydricity values (in acetonitrile) that span a range of 31 kcal/mol.24 Diphosphine-type ligands allow the use of 31P NMR spectroscopy to track the conversion of the metal hydride species during hydride