Cascade Hydrogenation of Carbon Dioxide to Methanol.

Abstract

CO2 is an abundant C1 building block that has the potential to be utilized in the synthesis of many commodity chemicals and fuels that are currently derived from fossil feedstocks. Methanol in particular is produced annually on a multimillion metric ton scale, primarily from CO/H2 at elevated temperatures (240–260 ºC). However, because the hydrogenation of CO2 is entropically unfavorable, the ability to operate at lower reaction temperatures is expected to lead to an overall higher theoretical yield of methanol. Herein we report the use of homogeneous catalysts in tandem for the hydrogenation of CO2 to CH3OH at substantially lower temperatures (135 ºC). Chapter 2 details the first system established for the direct homogeneous hydrogenation of carbon dioxide to methanol. A combination of ruthenium and scandium catalysts is employed to undergo the one pot stepwise reduction of CO2 to formic acid, methyl formate, and finally methanol. Incompatibilities between catalysts and cascade system components are introduced and are further evaluated in detail in later chapters. Chapter 3 describes potential deactivation pathways involving components of the cascade system with the Ru pincer ester hydrogenation catalyst applied in the cascade system. A new mode of activation of CO2 and carbonyl compounds (esters, ketones, and aldehdyes) by this Ru pincer complex is discussed. Additionally, the relevance of these organometallic compounds under cascade catalysis conditions is studied. Chapter 4 explores the idea of using a single catalyst for the cascade conversion of CO2 to CH3OH. A Ru pincer complex is tested for the CO2 conversion to formate salts where the mechanism is investigated and catalytic conditions are established. Furthermore, these conditions are applied to a second-generation cascade system comprised of formate salt and amide intermediates, where the later is reduced to CH3OH using a single catalyst. Chapter 5 describes the application of heterogeneous catalysis for low temperature CO2 conversion to methanol in the ester intermediate cascade system. In order to enhance the rate of the slow step while using heterogeneous catalysts at lower temperatures, homogeneous catalysts are added to the tandem system. Previously reported heterogeneous catalysts are explored, in addition to unprecedented Mo2C based catalysts.