Process of Methanol Production from Carbon Dioxide

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Process of Methanol Production from Carbon Dioxide

April 4, 2025

There are various methods for producing methanol from carbon dioxide, some of which are examined in this section. Generally, the methods for producing methanol from carbon dioxide include:
  • Direct hydrogenation of carbon dioxide
  • Indirect hydrogenation of carbon dioxide
  • Photoelectrochemical method
  • Bioelectrochemical method
  • Thermochemical method
  • Electrochemical method
  • Photocatalytic conversion
In the methanol production method using carbon dioxide, carbon dioxide and hydrogen are obtained from non-fossil fuel sources, which is why the produced methanol is referred to as green and renewable methanol. Currently, the hydrogen production method involves water electrolysis, during which hydrogen and oxygen are produced.
Direct Hydrogenation of Carbon Dioxide
In this method, carbon dioxide and hydrogen are directly converted into methanol. According to the following reactions, the ratio of carbon dioxide to hydrogen is 1:3. This mixture is directly fed into the methanol production tank. During this process, a significant amount of carbon monoxide is produced, so the selectivity of the methanol reaction over carbon monoxide is higher. Other byproducts such as dimethyl ether and methane are also generated. First, the output stream is cooled and then sent to a separation column. In this process, unreacted raw materials like carbon dioxide and water remain. The methanol is separated, and the water and carbon dioxide are recycled back to the methanol production column. Hydrogen is separated from the excess gas using an adsorption process, and the remaining hydrogen is used for combustion.
Two-Step Method for Methanol Production from Carbon Dioxide (RWGS)
In the two-step or indirect method of converting carbon dioxide to methanol, the Reverse Water-Gas Shift (RWGS) reaction is used. The first step involves the hydrogenation of carbon dioxide via the RWGS reaction to produce syngas, which is then transferred to a reactor in the second step as a raw material for methanol production. The CAMERE process for methanol synthesis includes an RWGS reactor and a methanol synthesis reactor. The required raw materials for this method are carbon dioxide and hydrogen. The first step, syngas production from carbon dioxide, is an endothermic equilibrium reaction performed at high temperatures. Therefore, catalysts such as ZnO/Al2O3 are used, and the reaction can reach equilibrium at temperatures above 650°C. Similar to the single-step method, the output stream from the first reactor is cooled and sent to a separation column, where water is removed. The remaining components, including carbon monoxide, carbon dioxide, and hydrogen, along with some excess hydrogen, are sent to the second column where methanol synthesis occurs. The second reactor also operates at high temperatures. Byproducts such as dimethyl ether and methane are produced alongside methanol. After the second step, the output stream is cooled to 30°C and sent to a separator to separate the gas and liquid phases. The separated gas is recycled back to the second reactor, while inactive components are removed from the methanol production loop for combustion. Before combustion, the hydrogen is adsorbed. The liquid phase is sent to a distillation column to separate the methanol. The resulting methanol is highly pure, with a concentration of approximately 99.8%. Figure 7 provides a schematic overview of the methanol production process using the RWGS method.
Photoelectrochemical Method
The increase in greenhouse gases and atmospheric carbon dioxide has drawn more attention to converting this gas into fuel. Photoelectrochemical reduction of carbon dioxide is used to produce materials such as methanol and methane. In this method, a semiconductor catalyst is used along with light. This catalyst is placed on electrodes, and upon absorbing light energy, it generates electron-hole pairs. As the reaction progresses, the produced electrons and protons facilitate the conversion of carbon dioxide. Key steps in this process include:
  1. First step: Light irradiation on the catalyst, leading to the generation of electron-hole pairs.
  2. Second step: Initiation of the reaction of adsorbed materials.
  3. Third step: Initiation of intermediate reactions.
  4. Final step: Formation of the product and its release from the reaction surface.
Bioelectrochemical Method
In the bioelectrochemical method, microorganisms are used as catalysts in oxidation and reduction reactions. Microorganisms can act as electron acceptors. The advantages of this method include the reproductive properties of microorganisms, their effective performance at neutral pH and low temperatures, and the affordability of carbon electrodes. These factors have increased the use of the bioelectrochemical method for converting carbon dioxide to methanol. This method is also used for producing hydrogen, hydrogen peroxide, and ethanol. Factors such as the type of microorganisms, their reactions, and the number of microorganisms on the electrode surface affect the quality of carbon dioxide conversion.
Thermochemical Method
In the thermochemical method, thermal energy is converted into chemical energy. To convert carbon dioxide to syngas using this method, heat transfer tubes resistant to high temperatures (up to 1100°C) are used. To produce fuel, oxygen is first generated through the thermal reduction of metal oxides, and then the reduced metal oxides are re-oxidized by carbon dioxide or water to produce carbon monoxide and hydrogen. The first step, oxygen production, is fast, while the subsequent reaction is slow. Increasing the temperature in the heat transfer pathways improves the efficiency of syngas production from water and carbon dioxide. The produced syngas is used as a raw material for liquid fuel production.
Electrochemical Method
One of the methods for converting carbon dioxide into alcohols is the electrochemical method. This method involves the electrochemical reduction of carbon dioxide to methanol. It is a way to store electrical energy without increasing carbon dioxide levels. The electrochemical reaction occurs between the electrode and the electrolyte. At the anode, water is oxidized, and at the cathode, electrons are released, which reduce carbon dioxide to methanol. This method is also used for producing hydrocarbons through carbon dioxide reduction. It is recognized as an efficient method for converting carbon dioxide under mild conditions.
Photocatalytic Conversion
One of the methods for converting carbon dioxide into fuel and valuable materials is photocatalytic conversion. This method uses sunlight and a semiconductor. With the use of nano-photocatalysts, carbon dioxide emissions can be prevented, and useful products can be produced at room temperature. Semiconductor catalysts such as titanium dioxide, zinc oxide, and cadmium sulfide are used for the photocatalytic conversion of carbon dioxide to methanol and formaldehyde. Photoelectrochemical and photocatalytic methods are highly significant because they use sunlight as the energy source and operate at low temperatures. In contrast, hydrogenation of carbon dioxide requires high temperatures and pressures, demanding substantial energy. In 1979, Honda and his colleagues first converted carbon dioxide to methanol through photocatalytic reduction using semiconductor particles of metal and non-metal oxides in water. Despite their high potential, photocatalytic methods have drawbacks, such as the mismatch between the semiconductor’s absorption ability and the light spectrum, inability to separate charge carriers, low solubility of carbon dioxide in water, and competitive water reduction reactions. This method involves the following processes:
  1. Generation of electron-hole pairs after absorbing photons with suitable energy.
  2. Separation and transfer of charge carriers.
  3. Chemical reactions between surface species and charge carriers.
Reactions after photon absorption on the catalyst surface are crucial. Excitation across the bandgap generates conduction band electrons and valence band holes, which act as sites for photo-oxidation and photoreduction, respectively. Photo-oxidation on the semiconductor catalyst surface forms hydroxyl radicals and H⁺ ions in the presence of valence band holes. The hydroxyl radicals further produce oxygen, while H⁺ ions generate hydrogen with the help of conduction band electrons. From a thermodynamic perspective, carbon dioxide is a stable molecule, making its oxidation and reduction processes challenging. The photocatalytic process for methanol production from carbon dioxide is complex. Research has shown that multiple electron transfers occur in the photocatalytic reduction of carbon dioxide.

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