Alcohol Production from Carbon Dioxide: Methanol as a Fuel and Chemical Feedstock

Abstract

As scientists and engineers studying CO2 conversion technologies, it is important to understand the CO2-derived methods of production for critical basic chemicals, and renewable alcohol production is one of the few CO2 utilization technologies deployed at industrial scale. Alcohols are building blocks for materials that we encounter every day; and among alcohols, methanol is a critical chemical intermediate for the production of polymers, plastics, fibers, and resins and holds promise as a potential renewable liquid fuel. At present, most methanol is produced using natural-gas-derived syngas. Its alternative production using CO2, water, and renewable electricity presents an opportunity to advance entire industries toward carbon neutrality. By analyzing its current stage of development, this perspective presents research and development goals to further utilize CO2 conversion to methanol technologies. Working together toward these goals, we can advance humanity toward the implementation of carbon-neutral alcohols and low-carbon fuel. Production of renewable alcohols from air, water, and sunlight present an avenue to utilize captured carbon dioxide for the production of basic chemicals and store renewable energy in the chemical bonds of liquid fuels. Of the technologies that utilize CO2 directly, CO2 electrolysis, as well as CO2 hydrogenation coupled with H2O electrolysis, have the benefit of requiring only CO2, H2O, and renewable electricity as inputs with O2 as a sole byproduct. Among alcohols, renewable methanol has seen the most development and analysis in the chemical industry because it is currently a syngas-derived product that could be adapted for direct CO2 utilization. In this perspective, we compare renewably powered CO2 electrolysis and CO2 hydrogenation with the incumbent methanol production method from syngas from a cost and CO2 life cycle perspective by analyzing recent literature to identify the research goals that enable further scale-up. Survey of the industry shows that CO2 hydrogenation is among the closest CO2 utilization technologies to large-scale deployment. We further discuss these CO2 hydrogenation systems and the catalysts that drive them, with recommendations to drive further development and scale-up. Production of renewable alcohols from air, water, and sunlight present an avenue to utilize captured carbon dioxide for the production of basic chemicals and store renewable energy in the chemical bonds of liquid fuels. Of the technologies that utilize CO2 directly, CO2 electrolysis, as well as CO2 hydrogenation coupled with H2O electrolysis, have the benefit of requiring only CO2, H2O, and renewable electricity as inputs with O2 as a sole byproduct. Among alcohols, renewable methanol has seen the most development and analysis in the chemical industry because it is currently a syngas-derived product that could be adapted for direct CO2 utilization. In this perspective, we compare renewably powered CO2 electrolysis and CO2 hydrogenation with the incumbent methanol production method from syngas from a cost and CO2 life cycle perspective by analyzing recent literature to identify the research goals that enable further scale-up. Survey of the industry shows that CO2 hydrogenation is among the closest CO2 utilization technologies to large-scale deployment. We further discuss these CO2 hydrogenation systems and the catalysts that drive them, with recommendations to drive further development and scale-up. The rate at which carbon dioxide (CO2) is being emitted into the atmosphere is outpacing its rate of absorption into terrestrial carbon sinks by a wider margin than ever before in recorded history.1Solomon S. Plattner G.K. Knutti R. Friedlingstein P. Irreversible climate change due to carbon dioxide emissions.Proc. Natl. Acad. Sci. USA. 2009; 106: 1704-1709Crossref PubMed Scopus (1504) Google Scholar By adopting sustainable manufacturing practices worldwide, including those that utilize CO2 to make liquid products that sequester more greenhouse gases (GHGs) than they emit, we can make a substantial contribution to limiting the increase in global average temperature to 1.5°C above pre-industrial levels. Limiting the increase in global average temperature was highlighted in the Paris Agreement under the United Nations Framework Convention on Climate Change2Rogelj J. den Elzen M. Höhne N. Fransen T. Fekete H. Winkler H. Schaeffer R. Sha F. Riahi K. Meinshausen M. 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Chem. 2018; 6: 446Crossref PubMed Google Scholar which is one of the largest and most mature CO2 utilization technology demonstrations. Production of methanol from direct electrolysis is in earlier stages, with hurdles around catalyst selectivity and current density. Methanol has a unique advantage when compared with CO and CH4 in that it is a room-temperature liquid with high energy density and CO2 sequestration potential (Table S1). It also has a large demand as both a fuel and a basic feedstock chemical. This is exemplified by global methanol consumption, which was 64 million metric tons in 2015, and of which 23 million metric tons were used for fuel-related applications.11Younas M. Loong Kong L. Bashir M.J.K. Nadeem H. Shehzad A. Sethupathi S. Recent advancements, fundamental challenges, and opportunities in catalytic methanation of CO2.Energy Fuels. 2016; 30: 8815-8831Crossref Scopus (0) Google Scholar As methanol and its derivatives are already well established in the global chemicals and transportation market, infrastructure and logistics do not present a significant challenge. More than 90 methanol plants offer a combined maximum capacity of nearly 110 million metric tons per year, generating $55 billion in economic activity.13Methanol InstituteThe methanol industry.https://www.methanol.org/Date: 2020Google Scholar These plants use a mixture of CO and H2 (syngas) as reagents. For a CO2-derived process to displace this legacy one, it is imperative that researchers understand the current syngas process and the ways that CO2 utilization technologies can improve upon it. Syngas is obtained by reforming or partial oxidation of carbonaceous materials such as coal, coke, natural gas, petroleum and heavy oils, or biomass. Economic considerations, the long-term availability of raw materials, energy consumption, and environmental aspects determine the choice of raw material. Natural gas is currently the most widely used feedstock, which is transformed into syngas by steam methane reforming (SMR, Equation 1) and the water-gas shift (WGS) reaction to optimize CO:H2 ratios (Equation 2), which is followed by methanol formation (Equation 3).14Aresta M. Caroppo A. Dibenedetto A. Narracci M. Life Cycle Assessment (LCA) applied to the synthesis of methanol. Comparison of the use of syngas with the use of CO2 and dihydrogen produced from renewables.in: Maroto-Valer M.M. Song C. Soong Y. Environmental Challenges and Greenhouse Gas Control for Fossil Fuel Utilization in the 21st Century. Springer, 2002: 331-347Crossref Google Scholar Each of these reactions is reversible and thus limited by thermodynamic equilibrium depending on the reaction conditions, including temperature, pressure, and composition of the syngas.CH4+H2O↔CO+3H2ΔH298K=205.43kJmol.(Equation 1) CO+H2O↔CO2+H2ΔH298K=−41kJmol.(Equation 2) CO+2H2↔CH3OHΔH298K=−90.7kJmol.(Equation 3) Equations 2 and 3 are exothermic and cumulatively result in a decrease of volume as the reaction proceeds, thus the conversion into methanol is favored by increasing pressure and decreasing temperature.14Aresta M. Caroppo A. Dibenedetto A. Narracci M. Life Cycle Assessment (LCA) applied to the synthesis of methanol. Comparison of the use of syngas with the use of CO2 and dihydrogen produced from renewables.in: Maroto-Valer M.M. Song C. Soong Y. Environmental Challenges and Greenhouse Gas Control for Fossil Fuel Utilization in the 21st Century. Springer, 2002: 331-347Crossref Google Scholar The WGS reaction results in a small amount of CO2 production that is hydrogenated during methanol synthesis, making this a process that already utilizes some CO2 though very minor compared with a direct CO2-fed approach.15Mikkelsen M. Jørgensen M. Krebs F.C. The teraton challenge. A review of fixation and transformation of carbon dioxide.Energy Environ. Sci. 2010; 3: 43-81Crossref Scopus (1321) Google Scholar To build viable systems that convert CO2 to methanol to effectively replace the present natural-gas-fed systems, we must understand the differences between the two. One major difference is the pathway for feedstock production. The existing method uses SMR to produce syngas, which usually is carried out using nickel-based catalysts at high temperatures of 800°C–1,000°C and pressures of 290–580 psi.16Gangadharan P. Kanchi K.C. Lou H.H. Evaluation of the economic and environmental impact of combining dry reforming with steam reforming of methane.Chem. Eng. Res. Des. 2012; 90: 1956-1968Crossref Scopus (0) Google Scholar Another is reaction conditions; the production of methanol from syngas is much more exothermic than from CO2 and H2, though the latter still occurs at moderate temperatures and pressures. Electrolysis offers a route that occurs at lower temperature and pressure but as such faces challenges with production rate and cost of capital equipment. These key differences, however, help to identify use cases where the CO2-derived approaches bring additional value propositions. Both electrolysis and direct hydrogenation of CO2 to methanol have been studied widely in literature, toward improving catalysts as well as technoeconomic and life cycle aspects. It has been shown that the CO2-based methanol production processes can have lower global warming impacts compared with the fossil-based process, as long as the electricity is provided renewably.17Artz J. Müller T.E. Thenert K. Kleinekorte J. Meys R. Sternberg A. Bardow A. Leitner W. Sustainable conversion of carbon dioxide: an integrated review of catalysis and life cycle assessment.Chem. Rev. 2018; 118: 434-504Crossref PubMed Scopus (527) Google Scholar, 18Goeppert A. Czaun M. Jones J.P. Surya Prakash G.K. Olah G.A. Recycling of carbon dioxide to methanol and derived products - closing the loop.Chem. Soc. Rev. 2014; 43: 7995-8048Crossref PubMed Google Scholar, 19Hoppe W. Thonemann N. Bringezu S. Life cycle assessment of carbon dioxide–based production of methane and methanol and derived polymers.J. Ind. Ecol. 2018; 22: 327-340Crossref Scopus (0) Google Scholar, 20Kim J. Henao C.A. Johnson T.A. Dedrick D.E. Miller J.E. Stechel E.B. Maravelias C.T. 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Table 1 gives a high-level overview of the energy cost and life cycle GHG emissions of these processes compared with the production of methanol from syngas, assuming all electricity used comes from photovoltaics or a source with equivalent lifecycle GHG emissions per kWh. It is worth noting that converting natural gas to methanol is an efficient (as high as 75%) process, but the energy comes from fossil fuels and, therefore, has a high carbon intensity.Table 1Comparison Showing Ranges of CO2 Emissions and Normalized Energy Use from CO2 Electrolysis, H2O Electrolysis Coupled with CO2 Electrolysis, and Conventional Methanol Production in Idealized CasesProcessSubcomponentEnergy Use (kWh/kg MeOH)Net GHG Emissions (kg CO2e/kg MeOH)CO2 electrolysis–9.3–11.5−0.72 to −0.82CO2 hydrogenation with H2O electrolysis–9.9–12.4−0.68 to −0.84H2O electrolysis9.6–11.1+0.43 to +0.48CO2 hydrogenation0.3–1.3−1.11 to −1.32Natural gas to methanol–8.5–12.7+0.77 to +1.6 Open table in a new tab Electrochemical reduction of CO2 has attracted substantial interest in the last decade as a CO2 conversion pathway because of its potential for renewable electricity utilization and high overall energy efficiency.31Chen C. Khosrowabadi Kotyk J.F.K. Sheehan S.W. Progress toward commercial application of electrochemical carbon dioxide reduction.Chem. 2018; 4: 2571-2586Abstract Full Text Full Text PDF Scopus (89) Google Scholar CO2 electrolysis proposes efficient, on-site chemical production, provided reactor and catalyst combinations with suitable selectivity, stability, overpotential, and capability to sustain commercially relevant current densities are found.32Burdyny T. Smith W.A. CO2 reduction on gas-diffusion electrodes and why catalytic performance must be assessed at commercially-relevant conditions.Energy Environ. Sci. 2019; 12: 1442-1453Crossref Google Scholar If CO2 electrolysis follows in the path of water electrolysis for hydrogen production, the latter of which now achieves overall thermal efficiencies greater than 70% in modular systems,33Ayers K. The potential of proton exchange membrane–based electrolysis technology.Curr. Opin. Electrochem. 2019; 18: 9-15Crossref Scopus (14) Google Scholar this potential may be realized. Cathodic reduction of CO2 to saturated straight-chain alcohols, including methanol, ethanol, and n-propanol requires six protons and six electrons provided by water oxidation, as shown in Equation 4:nCO2+(6n)H++(6n)e−→CnH2n+1OH+(2n−1)H2O.(Equation 4) For the production of methanol (n = 1), there are still substantial hurdles in the two major areas of research in CO2 electroreduction: (1) catalyst development, in that there is no stable catalyst yet developed that can sustain both high current densities and selectivity near 100% and (2) reactor design and engineering, which goes hand-in-hand with catalyst development especially when dealing with multi-pass scenarios where side products and unreacted CO2 are recycled through the electrolyzer. The first challenge requires developing a catalyst that stably binds CO to its surface yet retains lability to initiate four-electron, four-proton reduction to methanol. Since this necessitates delicate balance in CO-binding energies, molecular catalysts have shown promise toward driving this reaction.34Wu Y. Jiang Z. Lu X. Liang Y. Wang H. Domino electroreduction of CO2 to methanol on a molecular catalyst.Nature. 2019; 575: 639-642Crossref PubMed Scopus (81) Google Scholar Oxide nanoparticles are also competent catalysts as a component in gas diffusion electrodes when comprisingCu2O/ZnO.35Albo J. Irabien A. Cu2O-loaded gas diffusion electrodes for the continuous electrochemical reduction of CO2 to methanol.J. Catal. 2016; 343: 232-239Crossref Scopus (39) Google Scholar This is a notable development, as the same metal combination is used in methanol synthesis by hydrogenation. Although there has been strong progress developing flow reactors for CO2 electroreduction,36Jeng E. Jiao F. Investigation of CO2 single-pass conversion in a flow electrolyzer.React. Chem. Eng. 2020; 5: 1768-1775Crossref Google Scholar the second challenge, reactor design, still requires significant development to optimize system-level efficiency and mass transport.37Angulo A. van der Linde P. Gardeniers H. Modestino M. Fernández Rivas D. Influence of bubbles on the energy conversion efficiency of electrochemical reactors.Joule. 2020; 4: 555-579Abstract Full Text Full Text PDF Scopus (18) Google Scholar Among room-temperature liquids that can be made by CO2 reduction, methanol has a unique property that may facilitate improved efficiency at a systems level. The boiling point of methanol at atmospheric pressure (65°C) is above room temperature but comparable to the operational temperatures of a polymer electrolyte membrane (PEM) electrolyzer. This presents a unique opportunity for efficient process integration, as shown in Figure 1, by generating methanol in the gas phase while water (which is typically found in the cathode outlet stream from the reduction reaction, humidified CO2 feed, electroosmosis, or a combination of these factors) is removed as a liquid. This enables separation of methanol from gas and liquid recycle streams with a simple glycol-cooled condenser and gas-liquid separator, reducing the need for costlier distillation. Direct CO2 electroreduction is a potentially promising single-step route for methanol production from CO2; however, CO2 hydrogenation combined with electrolysis has proven to be the most scalable and implemented technology for renewable CO2 conversion to alcohols. Electrolysis of water occurs in standard commercial alkaline, PEM, or solid oxide systems that are commercially available on the scale of several megawatts,38Danilovic N. Ayers K.E. Capuano C. Renner J.N. Wiles L. Pertoso M. (Plenary) challenges in going from laboratory to megawatt scale PEM electrolysis.ECS Trans. 2016; 75: 395-402Crossref Scopus (17) Google Scholar whereas exothermic hydrogenation of CO2 (Equation 5) occurs in fixed-bed flow reactors that typically use a catalyst comprising copper oxide, zinc oxide, and alumina (CZA) similar to that of the syngas process.CO2+3H2↔CH3OH+H2OΔH298K=−40.9kJmol.(Equation 5) Experimental data have suggested that the legacy syngas process and CO2 hydrogenation are both promoted by a formate intermediate, which is one explanation why the syngas catalyst is also a competent catalyst for CO2 and H2.39Kuld S. Thorhauge M. Falsig H. Elkjær C.F. Helveg S. Chorkendorff I. Sehested J. Quantifying the promotion of Cu catalysts by ZnO for methanol synthesis.Science. 2016; 352: 969-974Crossref PubMed Scopus (265) Google Scholar In model systems, as shown in Figure 2, captured CO2 and H2 from a water electrolyzer are compressed and preheated to up to 280°C to maintain an optimal thermal profile in the reactor. The reactor itself is loaded with the CZA catalyst that is pelletized to ensure that it remains immobile and does not pulverize under the differential pressure present during the reaction, while simultaneously optimizing the catalyst mass to reactor volume ratio. In the reactor, Equation 5 and other side reactions reach equilibrium producing methanol, water, and byproducts such as CO and CH4. Unreacted CO2, H2, and other gaseous products are separated from liquid products after exiting, and the liquids are distilled to separate methanol from water. The unreacted CO2, H2, and product gases are, in many cases, recompressed and recycled to the reactor. The water obtained from the distillation column can be further recycled to the electrolyzer for hydrogen production to minimize the water usage of the process. At a systems level, recycling of both water and gaseous products may increase system cost but are desirable to maximize CO2 utilization and minimize life cycle GHG emissions. The primary advantage of CO2-based methanol is that it can offer lower overall GHG emissions as compared with the legacy syngas process. In a cradle-to-gate analysis, all steps of methanol production and their environmental impact are accounted for, including parameters for the process steps shown in Figure 2 as well as several external factors, including the method of CO2 capture, electricity generation, utilization of side products, and more. To begin with the current syngas process as a comparison point, Aresta and coworkers14Aresta M. Caroppo A. Dibenedetto A. Narracci M. Life Cycle Assessment (LCA) applied to the synthesis of methanol. Comparison of the use of syngas with the use of CO2 and dihydrogen produced from renewables.in: Maroto-Valer M.M. Song C. Soong Y. Environmental Challenges and Greenhouse Gas Control for Fossil Fuel Utilization in the 21st Century. Springer, 2002: 331-347Crossref Google Scholar conducted a life cycle analysis (LCA) on various syngas to methanol routes and found that optimizing process engineering, for example by efficiently capturing and recovering heat, can lower the overall energy required for methanol production by approximately 17%. However, even under optimized conditions, their analysis showed that reacting recovered CO2 with H2 from water electrolysis powered by photovoltaics consumed approximately seven times less energy than the conventional technology. Several studies have been published investigating different process conditions for the production of methanol from CO2 toward minimizing its CO2 equivalent GHG emissions (CO2e); typical boundary conditions for these studies are shown in Figure 3, as compared with the legacy syngas method. As the majority of the global warming impact (GWI) for the direct hydrogenation of CO2 to methanol is due to hydrogen supply,23Sternberg A. Jens C.M. Bardow A. Life cycle assessment of CO2-based C1-chemicals.Green Chem. 2017; 19: 2244-2259Crossref Google Scholar the lowest global warming impacts are achieved if hydrogen is supplied by water electrolysis using wind electricity.22Matzen M. Demirel Y. Methanol and dimethyl ether from renewable hydrogen and carbon dioxide: alternative fuels production and life-cycle assessment.J. Clean. Prod. 2016; 139: 1068-1077Crossref Scopus (79) Google Scholar,40Pérez-Fortes M. Schöneberger J.C. Boulamanti A. Tzimas E. Methanol synthesis using captured CO2 as raw material: techno-economic and environmental assessment.Appl. Energy. 2016; 161: 718-732Crossref Scopus (247) Google Scholar Solar-driven thermochemical process combined with solar heat have also been studied and found to have similar life cycle CO2 emissions.20Kim J. Henao C.A. Johnson T.A. Dedrick D.E. Miller J.E. Stechel E.B. Maravelias C.T. Methanol production from CO2 using solar-thermal energy: process development and techno-economic analysis.Energy Environ. Sci. 2011; 4: 3122-3132Crossref Scopus (65) Google Scholar Different CO2 sources can have an effect on the GWI almost as substantial as the electricity source for the system, which makes modular systems with flexible deployment parameters desirable. Although the conventional method maintains the highest GHG emissions, the GWI of CO2-based methanol can swing between carbon positive and negative depending on carbon source and its heat and electricity requirements.19Hoppe W. Thonemann N.