Methanol
The starting point of catalytic conversion is the addition of hydrogen to CO2 creating methanol. Methanol is a key platform chemical widely used in applications such as fuel additives, and as a precursor for the production of complex fuels, textiles, paints, and various chemicals. Traditionally, industrial methanol production relies on syngas (a mixture of CO and H2) and employs copper/zinc oxide-based catalysts, typically under high-pressure (50–100 bar) and moderate temperature (200–300 °C) conditions.
In recent years, the synthesis of methanol from CO2 has gained significant attention as a sustainable alternative. Unlike many CO2 utilization routes, this process has already reached commercial viability.
The fundamental electrochemical reaction for CO2 reduction to methanol is:
CO2 + 6H+ + 6e- → CH3OH + H2O
Copper (Cu) is one of the most promising electrocatalysts for this CO2 reduction reaction (CO2RR). Among copper-based systems, Cu-Ni alloys have demonstrated the highest selectivity towards methanol production.
Conducting the reaction at elevated CO2 pressure is beneficial due to factors as high CO2 concentrations accelerate the reaction rate due to increased reactant availability; elevated pressure enhances CO2 adsorption on the catalyst surface, which increases the number of active sites for the reduction reaction, and higher CO2 pressures also suppress the competing hydrogen evolution reaction (HER) by promoting competitive adsorption of CO2 over protons [3].
Dimethyl Ether (DME)
Dimethyl ether (DME) is increasingly recognized as a sustainable alternative fuel due to its non-toxicity, and similarity to liquefied petroleum gas (LPG). Other than that, DME also serves as an important intermediate in the production of chemicals such as methyl acetate, light olefins, and synthetic gasoline.
Traditionally, DME is produced from methanol derived from syngas, which is primarily obtained via fossil fuel-based processes. In contrast, synthesizing DME from CO2 and green hydrogen presents a far more sustainable route, with estimated greenhouse gas (GHG) emission reductions of 82–86% compared to conventional petroleum-based fuels [4].
The CO2-to-DME synthesis typically follows a two-step reaction:
CO2 + 3H2 → CH3OH + H2O
2CH3OH → CH3OCH3 + H2O
Copper-based catalysts are considered as the most efficient for the hydrogenation of CO2 to methanol, while zeolite-based catalysts, particularly ZSM-5, have shown strong performance in the methanol dehydration step.
To simplify the process, a one-pot catalytic system using bifunctional catalysts that combine metal oxides (e.g., Cu/ZnO/Al2O3) with acidic zeolites (such as ZSM-5) has been developed, offering high selectivity and conversion efficiency.
The overall reaction is favoured under high pressure (≥30 bar) and moderate temperatures (≤260°C) [5].
Dimethyl Carbonate (DMC)
Dimethyl carbonate (DMC) is used in many different applications such as polycarbonate synthesis, electrolyte in lithium-ion batteries, fuel additives, and green solvents for carbonylation and transesterification reactions.
Traditionally, DMC is synthesized from phosgene and methanol, a route associated with significant toxicity and safety risks.
An environmentally friendly alternative is the direct synthesis of DMC from CO2 and methanol:
2CH3OH + CO2 → C3H6O3 + H2O
This reaction offers a sustainable, non-toxic pathway for DMC production by utilizing CO2 as a feedstock. However, the process is thermodynamically challenged by water formation, which limits yield and can deactivate the catalyst.
To address this, dehydrating agents are typically added. Among them, 2-cyanopyridine has demonstrated excellent performance, effectively removing water and shifting the reaction equilibrium toward DMC formation.
Heterogeneous catalysts such as cerium oxide (CeO2) and zirconium oxide (ZrO2) have shown promising activity and stability, enabling moderate to high yields under controlled conditions.
It is found generally that the increase in pressure during the reaction to certain extent results in higher DMC yield and selectivity [6].
For instance, the use of near-supercritical CO2 conditions (e.g., 93 bar) significantly enhances the reaction, achieving 100% selectivity to DMC at 32°C [7].
An LCA study suggests that GHG emission in DMC synthesis from methanol and CO2 over cerium oxide and 2-cyanopyridine would result in emission of 0.39 kg-CO2-eq per kg-DMC [8].
Formic Acid
Formic acid has gained significant interest as a potential hydrogen carrier due to its low flammability and high hydrogen density. It can store approximately 53 grams of H2 per litre [9], offering a potential alternative to compressed hydrogen gas, which requires storage at 22 MPa to achieve an equivalent molarity of 9.8 M in just 1 litre.
Beyond its potential in hydrogen storage, formic acid plays a vital role across various industries. It is widely used in leather tanning, as a preservative, a chemical intermediate in pharmaceutical and fine chemical synthesis, and as an acid catalyst.
Currently, industrial production of formic acid is based on the reaction of methanol and carbon monoxide. However, the carbon monoxide feedstock is primarily derived from syngas, which is typically generated through steam reforming of natural gas or partial oxidation of fossil fuels. This conventional route results in substantial emissions, around 2.2 kg of CO2 per kg of formic acid produced [10].
A more sustainable alternative lies in the direct electrochemical synthesis of formic acid from CO2. This pathway not only eliminates upstream emissions but also offers potential carbon savings between 1.43 and 1.95 kg CO2-equivalent per kg of formic acid [11].
The electrochemical reduction of CO2 to formic acid is typically carried out under alkaline conditions to suppress the hydrogen evolution reaction (HER), which competes with CO2 reduction. However, in alkaline media, formic acid readily dissociates to formate, complicating product downstream processing.
Recent advancements have demonstrated the feasibility of direct formic acid synthesis from CO2 under low pH and elevated pressure conditions.
For example, using a Sn-based cathode and K2SO4 as the electrolyte in a bipolar membrane configuration, a study has achieved efficient conversion at pressures up to 50 bar [12].
Syngas
Carbon monoxide (CO) is a valuable precursor in various chemical syntheses, including formic acid and DMC. When combined with hydrogen, it forms syngas, a critical intermediate used in processes such as Fischer-Tropsch synthesis for e-fuels, as well as the production of methanol and DME. Currently, the majority of syngas is derived from fossil fuel combustion, which contributes significantly to greenhouse gas emissions. As a sustainable alternative, CO2 gasification offers a promising route for syngas production. One advanced method involves the use of solid oxide electrolyser cells (SOECs) to co-electrolyse CO2 and H2O, producing CO and H2.
This high-temperature process (typically 600–900°C) is thermodynamically favourable and well-suited for integration with renewable energy sources or waste heat recovery systems, such as modified supercritical CO2 Brayton cycles [13].
Copper-based catalysts are among the most studied for this reaction due to their activity and cost-effectiveness. However, selectivity remains a challenge, as side products like ethylene, methane, and formate are often formed.
As a result, ongoing research focuses on catalyst modification to enhance selectivity and overall efficiency of CO production from CO2 electrolysis [14].
Classically, syngas was produced from coal or better biochar offering an alternative to produce it from biowaste materials. Using either the classical pyrolysis or the torrefaction method.This can be used in combination with steam and oxygen to produce syngas or “town gas” as has been used before the introduction of natural gas. This route is well known and explored for over hundred years.