Selective CO2 hydrogenation into methanol in a supercritical flow process
Graphical abstract
Introduction
The global warming as a result of anthropogenic emissions of CO2 is by far the greatest challenge of mankind. In 2018, emissions of carbon dioxide from anthropic sources of combustion reached the historic value of 33.1 GtCO2 [1]. At this point, without efficient abatement measures and actions, the goals established by the Paris agreement [2] will be virtually impossible to attain. Therefore, a variety of efforts has been placed into developing economically feasible strategies to enable the capture of CO2. This concept can be achieved by following two main strategies: Carbon Capture and Sequestration (CCS) or Carbon Capture Utilization (CCU). The latter is a rather novel concept of carbon footprint reduction and it is based on converting carbon dioxide into fuel or valuable chemicals. Several strategies have been proposed to accomplish this. They can be chemical, electrochemical, photochemical or biochemical catalytic processes [[3], [4], [5], [6], [7], [8], [9], [10], [11]]. A myriad of products can be obtained from CO2 such as organic or inorganic carbonates, amides, urea, salicylic acid, syngas, fuel hydrocarbons or fuel alcohols [12].
Despite the numerous catalytic concepts developed for the valorisation of CO2, heterogeneous catalysis has been one of the most studied strategies for that purpose, especially due to the industrial know-how of operation and scale-up of such catalytic processes. Methanol (CH3OH) is considered one of the most promising platform molecules obtainable directly from CO2, since it can be integrated in numerous upgrading processes, making a closed carbon cycle economy possible [13]. The direct substitution of petroleum based fuels by methanol in internal combustion engines has also been shown to be feasible [14,15]. Moreover, unlike CO2 derived methanol, the use of fossil fuels causes urban air pollution, which leads to an increase in the occurrence of respiratory diseases and deaths from cardiovascular and respiratory ailments due to alveolar inflammation, as demonstrated by epidemiological studies [16]. Hence, a methanol economy can aid the improvement of urban pollution and public health, as well as help mitigating the greenhouse effect, if it is derived from CO2. Methanol consumption and demand has been growing intensely worldwide, and that growing trend is even more evident in China, where the government has set goals to tackle air pollution [17]. In light of this context, methanol as fuel or platform molecule is rising as one of the most promising energetic transition solutions. Therefore, the development of novel catalytic materials can be focused on accomplishing the clean and selective production of methanol from CO2. So far, the only large-scale process of CO2 conversion into methanol is the Vulcanol® process, developed by Carbon Recycling International at the George Olah plant in Iceland. This process uses copper oxide (CuO), zinc oxide (ZnO) and/or aluminium oxide (Al2O3) as heterogeneous catalysts [18].
Various studies have been performed in order to improve the activity of the CuO/ZnO catalysts by modifying or combining the oxides used to support Cu. In one example, a zirconium dioxide (ZrO2) support was added for its lower affinity towards water and showed better activity than other supports due to its synergy with Cu. With these improvements, a methanol selectivity of 45.8% was achieved at 240 °C [19]. Also with a CuO/ZnO/ZrO2 catalyst, a high selectivity of 97% towards methanol was reported by Arena et al., in a pressurized flow process at 50 bar and 180 °C, although CO2 conversion was merely 2.7% [20]. Notwithstanding the efforts towards improvement of catalyst support, Cu based catalysts have historically and currently carried the disadvantage of significant carbon monoxide (CO) formation and low methanol selectivity at higher conversion rates [[21], [22], [23]]. There was one variable which has enabled the classic CuO/ZnO/Al2O3 catalyst to present a 98% selectivity towards methanol whilst still converting 95% of CO2, which was the CO2/H2 molar ratio of 1/10 and pressure of 360 bar [24]. Despite the great results observed at these conditions, hydrogen gas has been shown to be the main economic cost at an industrial process of CO2 conversion into methanol [25], even if H2 is produced from water electrolysis and used in a 1/3 ratio [26], meaning a process employing a 1/10 ratio of CO2 to H2 would hardly be economically feasible. Thus, different research groups aim to develop a process to selectively hydrogenate CO2 into methanol, with high CO2 conversion rates.
Although the economic cost of the metals involved in the catalyst is of great importance to the feasibility of any industrial process, catalysts made of cheaper metals such as copper or zinc have an important drawback, which is their incompatibility with water or even moisture present in the gas mixture - they are deactivated if in contact with water, and over high CO2 partial pressures [27]. The hydrogenation of CO2 into methanol has also been catalysed by noble metals, but even then, selectivity remains an issue - particularly at CO2 conversion rates higher than 10% - with the use of noble metals commonly active on hydrogenations, such as Pd, Pt, Ir, Rh or Ag either leading to CO [[28], [29], [30]] or methane (CH4) as the main product [[31], [32], [33], [34], [35]]. A few decades ago, rhenium on cerium oxide (Re/CeO2) and rhenium on zirconium dioxide (Re/ZrO2) were used in the hydrogenation of CO2 to methanol by Hattori et al., although the major products were in fact CO and CH4, respectively [36]. Supported rhenium catalysts have also been tested in the hydrogenolysis of formic esters to methanol, wherein titanium dioxide (TiO2) had the greatest selectivity towards methanol [37]. Rhenium catalysts have often been studied in the hydrogenation of carbonyl and carboxylic groups, both in homogeneous and heterogeneous catalysis, showing great results and selectivity towards the hydroxyl group [[38], [39], [40], [41], [42], [43]]. Those evidences suggest that rhenium oxide supported on titanium dioxide (ReOx/TiO2) could perform well as a catalyst for the hydrogenation of CO2 into methanol. In fact, shortly before this work was completed, Ting et al. have published their work with conversion of CO2 into methanol over a Re/TiO2 catalyst in a batch process, at 150 °C, CO2/H2 ratio of 1/5, total pressure of 60 bar, in 1,4-dioxane as solvent, over a 24 h reaction period. In those conditions, a turn-over-number (TON) of 44 was obtained, with a selectivity of 82% towards methanol and with carbon monoxide being the secondary product [44].
Employing high pressures and temperatures has been one of the most successful approaches to enhance methanol selectivity. These new attempts are based on pushing the thermodynamic equilibrium by removing the products from the gas mixture due to condensation of water and methanol. Nonetheless, these processes still struggle with selectivity, mainly at higher CO2 conversion rates [[45], [46], [47]]. The supercritical condition is attained by carbon dioxide above P = 74 bar and T = 31 °C [48]. The utilization of supercritical carbon dioxide for its hydrogenation reaction has been studied by Evdokimenko et al. [49] and Bogdan et al. [50], who have respectively reported the hydrogenation of CO2 into carbon monoxide and methane in supercritical conditions using different catalysts. Nonetheless, neither author has studied the same reaction below or near the mixture’s critical point, so it was not possible to evaluate the beneficial effect of the supercritical conditions on conversion rate or selectivity. Kommoß et al. [46] have performed the hydrogenation of CO2 to methanol using a CuO/ZnO/Al2O3 catalyst at 150 bar and analysed the possibility of in situ phase separation of reaction products within the reactor, in order to shift the thermodynamic equilibrium by removing methanol. Their density measurements of H2/CO2 mixtures show that an increase in molar ratio of H2/CO2 beyond 1/1 leads to a dramatic decrease of the mixture density, when pressure is maintained at 150 bar. At these conditions, temperature variations from 200 °C up to 400 °C had a negligible effect on mixture density. Additionally, these authors have also performed a view cell experiment in order to analyse phase separation and have observed that, for a mixture of H2−CO2−CH3OH−H2O (62:13:10:15), it occurs at 206 °C for a pressure of 150 bar.
The latest works point to the fact that achieving high conversion and selectivity rates requires a judicious choice of reaction conditions, in order to establish an ideal correlation between phase equilibria and CO2 conversion. It is noteworthy that an increase in temperature helps maintaining a homogeneous system within the reactor at high pressures, but it usually produces a negative impact on reaction selectivity. Furthermore, the use of supercritical fluids, which are known to combine the great diffusibility of gases with the high density of liquids, leads to a high space-time yield, due to a higher density of gaseous reactants being in contact with the catalyst at a given residence time, and it is also an essential condition not only to process large amounts of CO2, but also because most of the CO2 obtained from CCS processes is compressed to high pressures.
Herein we report a high-pressure continuous supercritical flow process with a highly selective and efficient ReOx/TiO2 catalyst for the hydrogenation of CO2 to methanol, designed to maximize the conversion rate without compromising selectivity or phase homogeneity. The high pressure allows this system to convert a large amount of CO2 almost exclusively into methanol, since at 200 °C methanol selectivity was 98% and CO2 conversion was 18%. To our knowledge, this is the best combination of methanol selectivity and CO2 conversion rate reported in literature thus far, since other processes employing CO2/H2 ratios of 1/4 or higher either need to compromise conversion rates in order to increase selectivity or the opposite.
Section snippets
Materials
Rhenium (VII) oxide (Re2O7), iridium chloride hydrate (IrCl3xH2O) and palladium chloride (PdCl2) were purchased from Sigma-Aldrich at ≥99.9% purity, titanium oxide P25 was purchased from Degussa at 99.9% purity and 20 nm particle size, HDK pyrogenic silica was purchased from Wacker at >99.8% purity, and aluminium oxide was obtained from Vetec at 99.5% purity. Hydrogen gas and carbon dioxide were acquired from Special Gases, at 99.5% purity. Carbon monoxide and methane analytical standards were
Catalyst screening in batch reactions
The first goal of this work was to evaluate what was the most suitable combination between metal and support to address the reduction of carbon dioxide. Five catalysts were prepared and used in batch reactions of CO2 hydrogenation, and the activity and selectivity of those catalysts were analysed (Table 1, Table 2).
Under batch conditions, the ReOx/TiO2 catalyst yielded the largest methanol space time yield of 328 mmol g−1 h−1, while the ReOx/SiO2 catalyst was far behind with 62 mmol g−1 h−1.
Conclusion
A highly selective process for the conversion of CO2 into methanol using a supercritical flow process is reported, without solvents and additives, catalysed by ReOx/TiO2. An unprecedented high selectivity to methanol (98%) at ∼20% conversion rate was achieved at a relatively mild temperature of 200 °C when using 100 bar of pressure (CO2/H2 molar ratio of 1/4). On the other hand, an increase in temperature to 250 °C led to a higher conversion rate, though partly due to an increase in methane
CRediT authorship contribution statement
Maitê L. Gothe: Investigation, Formal analysis, Writing - original draft, Writing - review & editing. Fernando J. Pérez-Sanz: Investigation, Formal analysis, Writing - original draft, Writing - review & editing. Adriano H. Braga: Formal analysis. Laís R. Borges: Formal analysis, Writing - review & editing. Thiago F. Abreu: Formal analysis, Writing - review & editing. Reinaldo C. Bazito: Resources, Writing - review & editing, Project administration, Funding acquisition. Renato V. Gonçalves:
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge the financial support of FAPESP through project 2015/14905-0 and of FAPESP and SHELL Brazil through the ‘Research Centre for Gas Innovation – RCGI’(FAPESP Project 2014/50279-4), hosted by the University of Sao Paulo, and the support given by ANP (Brazil’s National Oil, Natural Gas and Biofuels Agency) through the R&D levy regulation. Maitê Gothe and Thiago Abreu acknowledge RCGI for their PhD grant, Fernando Perez, Adriano Braga and Laís Borges also acknowledge RCGI for
References (80)
- et al.
The use of pure methanol as fuel at high compression ratio in a single cylinder gasoline engine
Fuel
(2011) - et al.
Powering the future with liquid sunshine
Joule
(2018) - et al.
Effects of oxide carriers on surface functionality and process performance of the Cu-ZnO system in the synthesis of methanol via CO2 hydrogenation
J. Catal.
(2013) - et al.
Towards full one-pass conversion of carbon dioxide to methanol and methanol-derived products
J. Catal.
(2014) - et al.
Methanol production from captured CO2 using hydrogenation and reforming technologies - environmental and economic evaluation
J. CO2 Util.
(2019) - et al.
Pd/ZnO catalysts for direct CO2 hydrogenation to methanol
J. Catal.
(2016) - et al.
Recent developments on heterogeneous catalytic CO2 reduction to methanol
J. CO2 Util.
(2019) - et al.
Noble-metal-free and Pt nanoparticles-loaded, mesoporous oxides as efficient catalysts for CO2 hydrogenation and dry reforming with methane
J. CO2 Util.
(2019) - et al.
Bimetallic Pd-Cu catalysts for selective CO2 hydrogenation to methanol
Appl. Catal. B Environ.
(2015) - et al.
Direct hydrogenation of CO2 on deposited iron-containing catalysts under supercritical conditions
Mendeleev Commun.
(2018)
Selective hydrogenation of fatty acids to alcohols over highly dispersed ReOx/TiO2 catalyst
J. Catal.
XPS study of oxidation of rhenium metal on γ-Al2O3 support
J. Catal.
CO2 fixation into methanol at Cu/ZrO2 interface from first principles kinetic Monte Carlo
J. Catal.
Rhenium-promoted selective CO2 methanation on Ni-based catalyst
J. CO2 Util.
High-pressure advantages in stoichiometric hydrogenation of carbon dioxide to methanol
J. Catal.
Methanol formation by CO2 hydrogenation on Au/ZnO catalysts - effect of total pressure and influence of CO on the reaction characteristics
J. Catal.
Platinum-rhenium-alumina catalysts. II. Study of the metallic phase after reduction
J. Catal.
CO2 hydrogenation into CH4 on NiHNaUSY zeolites
Appl. Catal. B Environ.
Catalytic performance of the Pt/TiO2 catalysts in reverse water gas shift reaction: controlled product selectivity and a mechanism study
Catal. Today
Synthetic natural gas production from CO2 over Ni-x/CeO2-ZrO2 (x = Fe, Co) catalysts: influence of promoters and space velocity
Catal. Today
Global Energy & CO2 Status Report
Paris Agreement
Int. Leg. Mater.
Selective iron-catalyzed N-Formylation of amines using dihydrogen and carbon dioxide
ACS Catal.
Highly active and selective hydrogenation of CO2 to ethanol by ordered Pd-Cu nanoparticles
J. Am. Chem. Soc.
Cuo nanoparticles supported on TiO2 with high efficiency for CO2 electrochemical reduction to ethanol
Catalysts
Electrochemical CO2 reduction to formic acid at low overpotential and with high faradaic efficiency on carbon-supported bimetallic Pd-Pt nanoparticles
ACS Catal.
Enzymatic conversion of carbon dioxide to methanol: enhanced methanol production in silica sol-gel matrices
J. Am. Chem. Soc.
Using carbon dioxide as a building block in organic synthesis
Nat. Commun.
Photocatalytic back-conversion of CO2into oxygenate fuels using an efficient ZnO/CuO/carbon nanotube solar-energy-material: artificial photosynthesis
J. CO2 Util.
Photochemically driven reverse water-gas shift at ambient conditions mediated by a nickel pincer complex
Angew. Chemie Int. Ed.
Thermal, electrochemical, and photochemical conversion of CO2 to fuels and value-added products
J. CO2 Util.
Catalysis for the valorization of exhaust carbon: From CO2 to chemicals, materials, and fuels. Technological use of CO2
Chem. Rev.
Beyond oil and gas: the methanol economy
Angew. Chemie Int. Ed.
High efficiency and low emissions from a port-injected engine with neat alcohol fuels
SAE Tech. Pap.
Particulate air pollution and acute health effects
Lancet
Process for Producing Liquid Fuel From Carbon Dioxide and Water, US8198338B2
Catalytic hydrogenation of CO2 to methanol: Study of synergistic effect on adsorption properties of CO2 and H2 in CuO/ZnO/ZrO2 system
Catalysts
P.R. De La Piscina, J. Toyir, N. Homs, CO2 hydrogenation to methanol over CuZnGa catalysts prepared using microwave-assisted methods
Catal. Today
Enhanced activity, selectivity and stability of a CuO-ZnO-ZrO2 catalyst by adding graphene oxide for CO2 hydrogenation to methanol
Chem. Eng. J.
The Role of Metal Oxides in Promoting a Copper Catalyst for Methanol Synthesis
Cited by (26)
Mononuclear Re sites on In<inf>2</inf>O<inf>3</inf> catalyst for highly efficient CO<inf>2</inf> hydrogenation to methanol
2023, Journal of CatalysisCitation Excerpt :The more abundant oxygen vacancies for 3%Re/In2O3 are beneficial to the CO2 activation and thus contribute to the methanol synthesis in CO2 hydrogenation. The Re 4f core-level spectra of fresh catalyst (Fig. S7c) present two peaks at 46.0 eV and 48.4 eV assigned to the 4f7/2 and 4f5/2 of Re (+7) in NH4ReO4[44]. In Fig. 3c, for 3%Re/In2O3 after reaction, three pairs of peaks could be identified through deconvolution processing.
Nanocatalysts as potential candidates in transforming CO<inf>2</inf> into valuable fuels and chemicals: A review
2022, Environmental Nanotechnology, Monitoring and ManagementCitation Excerpt :However, rhenium oxide on titanium dioxide (ReOx/TiO2) catalyst were able to convert CO2 into CH3OH with 98% selectivity (18% CO2 conversion rate at 200 °C, 100 bar and CO2/H2 ratio of ¼). Although, the conversion rate can be raised 41% at 250 °C but it costs selectivity (64%) of the hydrogenation process (Gothe et al., 2020). Furthermore, the synthesis of CH3OH can be achieved at low CO2 pressure using AuNiGa and GaPd2/SiO2 catalyst (Duyar et al., 2020; Fiordaliso et al., 2015).
A sustainable and green route to furan-2,5-dicarboxylic acid by direct carboxylation of 2-furoic acid and CO<inf>2</inf>
2021, Journal of CO2 UtilizationCitation Excerpt :Therefore, it is dramatically essential for investigating 2,5-FDCA production by lignocellulosic biomass-based compounds, such as furfural or its downstream products. It has attracted increasing attention that CO2 is used as a carbon source for green synthesis of fine chemicals because of its abundance, accessibility, non-flammability, low toxicity, and harm for the ecological environment [19–25]. At present, a variety of reaction systems have been initially established to successfully apply CO2 for synthesizing high-molecular polymers and fine chemicals [26–28].
DFT mechanistic study of the chemical fixation of CO<inf>2</inf> by aziridine derivatives
2024, Journal of Computational Chemistry