Selective CO₂ hydrogenation into methanol in a supercritical flow process

Highlights

• Efficient supercritical flow process for selective valorisation of CO₂ into methanol.

• Catalyst ReO_x/TiO₂ composed of rhenium oxide nanoparticles supported on P25.

• High pressure, low temperature process which does not require any solvents or additives.

• High selectivity towards methanol (98%) at ∼20% conversion of CO₂.

• Supercritical CO₂ improved CH₃OH selectivity, and maintained reaction mixture homogeneous.

Abstract

Methanol plays a crucial role in the novel cycle of carbon capture, recycling and valorisation of anthropogenic carbon dioxide (CO₂). Even though hydrogenation of CO₂ to methanol has favourable thermodynamics, catalyst and processes development is needed for improving stability, reaction rate and selectivity to higher values than of the currently used copper oxide on zinc oxide (CuO/ZnO) catalysts. Here we report an efficient supercritical flow process for the selective valorisation of CO₂ into methanol. At optimized conditions, rhenium oxide on titanium dioxide (ReO_x/TiO₂) catalyst converts CO₂ into methanol with 98% selectivity and at 18% CO₂ conversion rate at 200 °C, 100 bar and CO₂/H₂ ratio of 1/4. A higher conversion of 41% can be achieved at 250 °C, but the selectivity towards methanol decreases to 64%. This strategy has enabled the development of an efficient high-pressure flow process without compromising methanol selectivity.

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 (ReO_x/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 ReO_x/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.