New Catalysts and Reactors for the Synthesis or Conversion of Methanol

Energy storage is a critical issue in the development of an economy based on the use of renewable energy. A suitable method for storing large amounts of energy for long periods of time is to transform electrical energy from renewable sources into fuels, which leads to so-called “e-fuels”. Several processes are currently used to produce e-fuels, mostly on a laboratory or pilot plant scale, although several demonstration projects with capacities of several MW are already under development. These technologies, also a part of the so-called “Power to Liquids” technologies, have been widely studied in Europe, with more than 200 projects planned, in development, or finished [1]. Power-to-fuels would have the following advantages:

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High energy density;

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Applicable for existing technologies;

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Suited for heavy-duty applications;

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Quick deployment since no infrastructural adaptions are required.

Among liquid e-fuels, methanol is considered one of the most promising technologies [2,3]. It is a versatile matter used both for industrial purposes and for various day-to-day life activities, and as it exhibits high effectiveness as an energy carrier, renewable methanol has been proposed by the Nobel Prize Winner G. Olah as a way to close the CO₂ loop [4]. Methanol can be environmentally synthesized from any feedstock, and its reforming reaction does not alter net CO₂ emissions to the atmosphere. Industrial production of methanol from syngas (R1) occurs at temperatures around 220–300 °C and pressures of 50–100 bar, generally occurring in the presence of copper or nickel–alumina-based catalysts. Along the path of methanol synthesis, the reactions that take place are the hydrogenation of carbon monoxide (R1), water–gas shift (R2), and the hydrogenation of carbon dioxide [5]:

CO + 2 H₂ ⇄ CH₃OH ΔH_298K = −90.7 kJ/mol CO

(R1)

CO + H₂O ⇄ CO₂ + H₂ ΔH_298K = −41.2 kJ/mol CO

(R2)

CO₂ + 3 H₂ ⇄ CH₃OH + H₂O ΔH_298K = −49.5 kJ/mol CO₂

(R3)

Methanol formation is exothermic, and therefore, the thermodynamic equilibrium is favored at low temperatures, while kinetics is favored by high temperature. Although reaction (R1) and reaction (R3) both give methanol as a major product, it was confirmed through reagents with isotopes that, under industrial conditions, the first reaction is more favorable, and when reactions are enabled without carbon monoxide as a staring material, methanol is entirely formed through reaction (R1) without passing through the third reaction [6]. In addition, the difference between the number of moles of starting materials and products requires operation at high pressure to obtain a high yield to methanol. Methyl alcohol is exclusively achieved through the direct hydrogenation of carbon dioxide (reaction 1) [7]. CO₂ hydrogenation competes with reverse water-gas shift (reaction 2), which drops the selectivity to methanol. Reverse water–gas shift is endothermic, and thus, its equilibrium is favored at high temperatures. Although methanol is currently obtained from fossil fuels (via synthesis gas), in a future decarbonized economy, it could be obtained via the reaction of hydrogen (coming from water hydrolysis) and carbon dioxide (from carbon capture processes).

On the other hand, methanol is a raw material for the chemical industry. In recent years, alternative processes have been developed to substitute crude oil by methanol. The new route of methanol transformation into light olefins (mainly ethylene and propylene) is known as the Methanol-to-Olefins (or MTO) process [8,9,10,11]. The MTO process is usually carried out over zeolite-based catalysts, such as ZSM-5 or SAPO-34, which, because of their shape selectivity, are able to produce a well-defined range of hydrocarbons [12,13,14].

The MTG (Methanol to Gasoline) process allows us to transform methanol into hydrocarbons within the range of gasoline boiling points [15]. The conversion of methanol to hydrocarbons can be carried out with a wide variety of acid catalysts. Some studies report the use of non-zeolitic catalysts for the MTG reaction [16,17,18]. However, those based on zeolite have offered the best performance. Among them, the HZSM-5 zeolite stands out from the rest [19].

Additional uses for methanol are currently under study [20], so the Special Issue entitled “New Catalysts and Reactors for the Synthesis or Conversion of Methanol” allows readers to gain a complete view of the state of the art for methanol applications based on new catalysts or reactors.

In contribution 1, a review of the utilization of methanol to produce dimethyl carbonate (DMC) is presented. DMC is widely used as an intermediate and solvent in the organic chemical industry. In recent years, compared with the traditional DMC production methods (phosgene method, transesterification method), the methanol oxidation carbonylation method, the gas-phase methyl nitrite method, and the direct synthesis of CO₂ and methanol have made significant progress in the synthesis process and catalyst development.

In contribution 2, the authors use a colloidal synthesis technique to produce MoP nanoparticles (4 nm average particle size) which they deposit on metal oxide supports for a systematic study on interface effects on CO₂ hydrogenation. The results reveal MoP/ZrO₂ as an exceptionally active and selective catalyst for methanol synthesis.

The preparation, characterization, and testing of different catalysts through the modification of the traditional catalyst CuO/ZnO/Al₂O₃ are shown in contribution 3. The authors compare them in terms of their conversion of and electivity to methanol at low pressure.

Different catalysts based on HZSM-5 zeolite for the reaction of Methanol to Gasoline (MTG) are prepared, characterized, and tested in the contribution 4. The three catalysts obtained were active in the reaction and did not suffer severe textural deterioration after use (decrease of approximately 10% in their specific areas).

Contribution 5 studies the advantages of the Powder Bed Fusion by Electron Beam (PBF-EB) additive manufacturing process with the kinetically controlled catalytic activation of Raney-type Cu to induce highly active and selective methanol synthesis. The catalysts thus obtained were evaluated for the hydrogenation of CO.

In contribution 6, the changes that occur in the hybrid photocatalyst during the reaction of hydrogen production from aqueous methanol under UV light are studied. The authors investigate the structure, composition, particle morphology, and surface area of the photocatalyst before and after the photocatalytic reaction, as well as analyzing the composition of the reaction solution during the photocatalytic process.

An experimental and simulation-based investigation is carried out for the selective oxidation of green methanol to the oxygenates dimethoxymethane and methyl formate in contribution 7. It includes an initial catalyst screening, the derivation of a reaction kinetic model, and a feasibility study of a fixed-bed and membrane reactor with oxygen distribution.

Finally, in contribution 8, the authors compare different kinetic models for methanol synthesis under similar conditions with a better-performing model for CO₂ hydrogenation to methanol (CH₃OH) with CO in the feed.

I would like to express gratitude to MDPI Editorial and the Catalysts journal for the opportunity to serve as a Guest Editor, contributing to the current state of the art in environmental catalysis for water remediation, as well as to the Assistant Editor, Ms. Bella Zhang, who worked diligently alongside me to publish this Special Issue. In addition, I would like to thank all the authors who shared their research and the referees for their invaluable contributions.