Integrated CO<sub>2</sub> Capture and Conversion to Formate and Methanol: Connecting Two Threads (2019)

The capture of CO<sub>2</sub> from concentrated emission sources as well as from air represents a process of paramount importance in view of the increasing CO<sub>2</sub> concentration in the atmosphere and its associated negative consequences on the biosphere. Once captured using various technologies, CO<sub>2</sub> is desorbed and compressed for either storage (carbon capture and storage (CCS)) or production of value-added products (carbon capture and utilization (CCU)). Among various products that can be synthesized from CO<sub>2</sub>, methanol and formic acid are of high interest because they can be used directly as fuels or to generate H<sub>2</sub> on demand at low temperatures (<100 °C), making them attractive hydrogen carriers (12.6 and 4.4 wt % H<sub>2</sub> in methanol and formic acid, respectively). Methanol is already produced in huge quantities worldwide (100 billion liters annually) and is also a raw material for many chemicals and products, including formaldehyde, dimethyl ether, light olefins, and gasoline. The production of methanol through chemical recycling of captured CO<sub>2</sub> is at the heart of the so-called "methanol economy" that we have proposed with the late Prof. George Olah at our Institute. Recently, there has been significant progress in the low-temperature synthesis of formic acid (or formate salts) and methanol from CO<sub>2</sub> and H<sub>2</sub> using homogeneous catalysts. Importantly, several studies have combined CO<sub>2</sub> capture and hydrogenation, where captured CO<sub>2</sub> (including from air) was directly utilized to produce formate and CH<sub>3</sub>OH without requiring energy intensive desorption and compression steps. This Account centers on that topic. A key feature in the combined CO<sub>2</sub> capture and conversion studies reported to date for the synthesis of formic acid and methanol is the use of an amine or alkali-metal hydroxide base for capturing CO<sub>2</sub>, which can assist the homogeneous catalysts in the hydrogenation step. We start this Account by examining the combined processes where CO<sub>2</sub> is captured in amine solutions and converted to alkylammonium formate salts. The effect of amine basicity on the reaction rate is discussed along with catalyst recycling schemes. Next, methanol synthesis by this combined process, with amines as capturing agents, is explored. We also examine the system developments for effective catalyst and amine recycling in this process. We next go through the effect of catalyst molecular structure on methanol production while elucidating the main deactivating pathway involving carbonylation of the metal center. The recent advances in first-row transition metal catalysts for this process are also mentioned. Subsequently, we discuss the capture of CO<sub>2</sub> using hydroxide bases and conversion to formate salts. The regeneration of the hydroxide base (NaOH or KOH) at low temperatures (80 °C) in cation-conducting direct formate fuel cells is presented. Finally, we review the challenges in the yet unreported integrated CO<sub>2</sub> capture by hydroxide bases and conversion to methanol process.