2.1. Production of Green Hydrogen

About 95% of global hydrogen (H₂) is produced from fossil fuel sources, such as natural gas, coal and oil. The production of H₂ has been classified into grey, blue and green H₂, based on source and production strategy. Grey H₂ is produced by steam reforming of fossil fuel and termed as “grey” due to substantial GHGs emissions during the process [89]. The source of blue H₂ is also fossil fuel, but the production process must be equipped with efficient CO₂ capturing facility (~100%) and have minimal GHGs impact on environment [90]. Green H₂ is a totally environmentally-friendly process with zero GHGs emissions [91,92], produced by water electrolysis. The electrolyzer must be powered with renewable energy sources to avoid any GHG emission into the atmosphere during its entire production process [93]. This green H₂ is the key approach towards achieving zero emission target of 2050 set by global climate communities. However, despite of the zero emissions from electrolysis operation, an electrolyzer manufacturing process could inevitably lead to GHG emissions, particularly if critical way materials are used, which should be taken into account for net zero emission target [94].

Solid oxide electrolysis (SOEL) is performed in a solid oxide electrolysis cell (SOEC) which works on the reverse principle of a solid oxide fuel cell, used for electricity production from pure H₂ and O₂ [102]. In SOEC, steam is fed at cathode and it undergoes reduction at high temperatures of 600–1000 °C to produce H₂ and oxide ions [103]. Due to the high temperature, oxide ions can travel from cathode to anode via some oxide-conducting electrolyte medium, where it captures an electron from an outer circuit and generates oxygen gas. This process of H₂ production has a good efficiency of more than 80% [102], however, it has the drawback of high-temperature operation [104]. To overcome this issue, researchers have made efforts to develop electrolytes for SOEC which can transfer ions at comparatively at low temperatures. Ishihara et al. performed steam electrolysis at 600 °C in SOEC having LaGaO₃-based electrolytes and successfully achieved H₂ production rate of 70 L/min at 1.8 V of applied potentia [105]. 

2.1.1. Solar to Hydrogen

-holes (h+) pairs under sunlight irradiation. These e−/h+ pairs migrate towards the active sites of respective electrodes to produce H₂ and O₂ [112,113,114]. In spite of direct exposure to sunlight in PC technology, the drawbacks include slow kinetics, unfavorable thermodynamics and some safety issues [115]. For instance, the separation of H₂/O₂ from the PC cell is needed to avoid H₂ explosion in contrast with photovoltaic-electrocatalysis (PV-EC) and PEC [116]. Subsequently, research efforts have been focused to overcome these issues for better efficiency of converting solar energy to H₂ by improving the semiconductor materials. For example, Wang et al. have developed Pt co-catalyzed BaTaO₂N photocatalyst that produced 100 times more efficient H₂ production compared to previous studies [117].

2.1.2. Wind to Hydrogen

Owing to the huge electricity generation capability of wind farms, researchers are exploring the possibility of alternative H₂ storage. For instance, Dinh et al. developed a model to assess the viability of H₂ production using electricity from wind farm and found a balance H₂ price of about 3.22 $/kg of H₂ [143]. Subsequently, Douak et al. performed the on-spot H₂ production from wind farm and estimated the cost of product as low as 1.214 $/kg of H₂ [144]. Ziazi et al. presented a techno-economic analysis of H₂ production in alkaline water electrolysis powered with wind electricity using different types of wind turbines. It was shown by the authors that General Electric 1.5 MW wind turbine (General Electric Company, Boston, MA, USA; Model: GE 1.5sle) has highest efficiency but with the drawback of a high investment cost [145]. Hence, it was shown that wind-to-H₂ is a feasible and clean H₂ production strategy using suitable electrolyzer scheme with zero hazardous emissions to the atmosphere.

2.1.3. Biomass to Hydrogen

Lignocellulose is the most abundant biomass on earth with annual production of 181.5 billion tons, as reported by Dahmen et al. in 2019 [150]. Currently, the waste biomass comes from different sources such as residue of food crops, forest plants, marine life and waste generated by bio-products manufacturing industries. Historically, solid biomass was subjected to combustion for generating energy; however, new routes have been discovered in using solid biomass to obtain useful products [151,152]. H₂ is one of the valuable products which can be extracted from this biomass by many established thermochemical, biological and electrochemical processes [153].

In thermochemical treatment of biomass, it is passed through a gasification process to convert it into gaseous form with different compositions of CO₂, CO, CH₄, H₂ and other hydrocarbons, depending upon the nature of the fed biomass [154]. The gasification process can be performed in numerous ways such as dry and steam gasification, partial oxidation, pyrolysis and combination of these processes [155]. A gasification route is selected based upon the C/H ratio required in gas production and post-processing. If the subject matter is H₂ production, the produced biogas is subjected to steam reforming to produce H₂ rich syngas followed by carbon capture to produce H₂ [153,156]. In the case of methanol production, the produced syngas is directly fed to a methanol production plant after a necessary adjustment in its composition [157].

2.2. Recycled Carbon Dioxide

The recycled CO₂ from direct air capture (DAC) or industrial emission can complete the carbon cycle through methanol production. Based on an analysis report from NOAA’s Global Monitoring Laboratory, the global average atmospheric CO₂ reached 414.72 ppm in 2021. It has reached a new high-level record amount of CO₂ in spite of decelerated human activities due to COVID-19 pandemic [165]. Presently, the average value of CO₂ is 417.2 ppm which is more than 50 percent above pre-industrial levels [166]. The increasing concentration of CO₂ depicts that its natural sinks (that removes CO₂ from atmosphere), which are plants and oceans, are not taking out an equivalent amount as that being emitted. This imposes the need of the installation of other CO₂ capturing points and at the same time reducing the further emissions to balance the atmospheric concentration. Methanol production from captured CO₂ will reduce further emissions in the atmosphere [17,167].

2.2.1. CO₂ from Direct Air Captured

CO₂ from the atmosphere is directly captured from air using well-developed DAC technology. Currently, there are 19 DAC plants operational worldwide, capturing around 0.01 Mt CO₂/year [172]. In comparison with the available CO₂ in atmosphere and net zero target of 2050, this installed capacity is too low and needs to increase up to 60 Mt CO₂/year by the end of 2030. To meet this target, several large capacity DAC plants are under development and will be operational in the near future, including a 1.0 Mt capacity plant in US which will be operational in mid 2020s [173].

Technologies used in DAC are based on suitable solid (s-DAC) or liquid (l-DAC) sorbent to selectively extract CO₂ from the ambient air [174]. Ambient air is brought in contact with these sorbents at certain temperature and pressure, depending upon the physiochemical nature of the sorbent. CO₂ is attracted towards the sorbent while leaving behind the rest of the gases. The CO₂ rich sorbent is then processed under reverse operating conditions to desorb the attached CO₂ and regenerated for the next cycle. The sorption of CO₂ on the sorbent can be either chemical (chemisorption) or physical (physiosorption). In chemisorption, CO₂ is attached to the sorbent via chemical bonds which require a considerably high level of energy for their regeneration [175].

2.2.2. CO₂ Captured from Industrial Emissions

Currently, there are three main approaches available to capture CO₂ from large-scale fossil fueled industrial sectors: (1) post-combustion capture; (2) pre-combustion capture; and (3) oxy-fuel combustion capture. Among all three technologies, post-combustion carbon capture is the oldest one, due to its ease of operation and installation without any significant changes in existing plant [168]. This technology is based on removal of CO₂ from combustion flue gases, mainly consisting of CO₂, O₂, N₂, H₂O, SO_x and NO_x, depending upon the nature of the fuel burned. Around 10–15% of the total volume of flue gases subjected to post-combustion treatment is CO₂ [194]. The second technology, pre-combustion carbon capture, refers to the removal of CO₂ from fossil fuels before feeding to combustion process [195]. In this technology, fossil fuel is first passed through a gasification process to convert it into syngas (CO, H₂ and CO₂) which is then subjected to water gas shift reaction to convert CO to CO₂ and H₂. The final mixture of CO₂ and H₂ having 15 to 50% CO₂, is then fed to a CO₂ removal process and clean H₂ is used for combustion purpose, while rejecting clean flue gases [196]. Pre-combustion capture technology utilizes a highly concentrated stream of CO₂ for carbon capture and hence can perform more efficient removal. The third technology, namely oxy-fuel combustion is one of the leading technologies for carbon capture and utilizes pure oxygen instead of air for fossil fuel burning [197]. An air separation unit separates oxygen from air while leaving behind the major portion of N₂. Flue gases produced from this oxy-fuel combustion are more concentrated in CO₂, which is then removed from it in a flue gas processing unit [198].

2.2.3. CO₂ Produced from Biomass

The gasification of biomass is mainly performed to convert it into clean H₂ fuel along with a side product, CO₂, which is generated in very abundant amount during this process due to the organic nature of subjected biomass. In this scenario, CO₂ is captured from biogas in order to produce value-added H₂ fuel [207,208]. Biomass usage is not only limited to H₂ production but also used for producing syngas, bio-methane and other light hydrocarbons fuels [209]. In any gasification process, excess CO₂ must be removed via either in situ or post process techniques, as it imparts negative value to any produced fuel. Dinca et al. produced syngas by biomass gasification while capturing the CO₂ by using liquid absorption method [210]. Dashtestani et al. used novel Ca/Fe based sorbents for efficient removal of CO₂ via calcination-carbonation looping from biomass gasification [211]. They also studied the effectiveness of sorbent material over number of regeneration cycles, which is very important stability criteria for a newly developed sorbent.

In view of this, CO₂ captured from its different point sources or directly from air can contribute to the circular economy by producing synthetic fuels such as methanol [216], as shown in Figure 4.

Figure 4. Opportunities for recycling CO₂ into methanol synthesis [216].

3. Production of Clean Methanol

An important method for the production of e-methanol by utilizing these sources, is nameed heterogeneous catalysis. E-methanol is produced by reacting green H₂ with captured CO₂ from industrial sources or ambient air [42]. Because of the green H₂ from renewable energy and captured CO₂, e-methanol is an alternative and net-carbon-neutral fuel. There are several processing routes for producing methanol from captured CO₂, including a well mature catalytic hydrogenation of CO₂ with green H₂, which is responsible for e-methanol production [217]. In catalytic hydrogenation of CO₂, both gases are processed over Cu/Zn based bimetallic or other multi-component catalysts at certain operating conditions. The operating conditions vary depending upon the nature and composition of catalyst employed. The normal operating conditions were in the range of 200–250 °C temperature and 3–10 MPa of pressure. The following reaction scheme is followed during catalytic hydrogenation of CO₂ [218],

CO₂ + 3H₂ → CH₃OH + H₂O   ∆H_298K = −49.5 kJ/mol

(9)

CO₂ + H₂ → CO + H₂O    ∆H_298K = +41.2 kJ/mol

(10)

CO + H₂ → CH₃OH     ∆H_298K = −90.6 kJ/mol

(11)

As shown from the above reaction scheme, the nature of this reaction is overall exothermic, which is more favorable at low temperatures. At low temperatures, it is difficult to energize the stable CO₂ molecule, which decreases the reaction speed, while at high temperatures it is difficult to achieve good conversion. Therefore, well-optimized operating conditions are required for good conversion and high process yields. Other than operating conditions, several other parameters such as catalyst development and reactor designs also play a role for high process throughput [218,219]. Initially, the same commercially available Cu/ZnO/Al₂O₃ (CZA) catalyst, specialized for syngas to methanol conversion was deployed to CO₂ hydrogenation. Due to poor selectivity of CZA for methanol from CO₂, several other catalytic approaches were developed incorporating other metal oxides on a Cu/ZnO base, including ZrO₂ as one of the best additions [218,220,221]. Recently, Zhang et al. performed a detailed analysis of catalytic conversion of CO₂ into e-methanol and addressed other challenges associated with this route [219].

Despite catalytic development for enhanced CO₂ conversion and process efficiency, another big challenge is the downstream removal of an essential by-product, water. Water separation is the last step of any methanol production facility to get high purity product, which is mainly achieved by distillation technique. Numerous research studies have been conducted in the distillation for efficient removal of methanol-water mixture by consuming less energy [222,223].

4. Post-Synthesis Methanol/Water Separation

4.1. Challenges and Improvement in Methanol/Water Separation

In distillation operation, re-boiler and condenser are major energy consumers, and it accounts for approximately 40% of total energy requirement of a chemical industry [225,226]. To have better process efficiency and energy economy, several improvements were made in existing distillation process including heat integrated distillation, multi-effect distillation and heat pump assisted distillation [227,228,229,230,231]. Liang et al. worked on making distillation process energy efficient by combining preconcentration and entrainer recovery columns instead of 3-column configuration of conventional extractive distillation which is good energy and capital cost saving [232].

4.2. Membrane Based Methanol/Water Separation

Membrane technology has a great potential to replace any distillation process by incorporating suitable membrane materials. Different organic and inorganic membrane materials have been proposed and tested for methanol-water dehydration following vapor-permeation or pervaporation mechanisms [20,21]. In vapor-permeation method, the feed is evaporated and enters in the membrane module in vapor form. In this case, both species (methanol and water) are in vapor form when they come in contact with membrane, which allows one vapor to pass through [234]. Conversely, in the pervaporation method, the feed is preheated to a required temperature and passes through the membrane module in liquid form. Vacuum is generated on the permeate side (either by condensing the incoming vapors or by a vacuum pump) for in situ evaporation of selective species from feed and pass-through membrane [235]. For most of the methanol-water separation studies using membranes, a pervaporation scheme is used, along with organophilic or hydrophilic membrane material.

4.2.1. Zeolite Membranes for Methanol/Water Separation

Zeolites are porous crystalline aluminosilicates having successful applications in ion exchange, adsorption and catalysis [238]. Zeolite membranes being hydrophilic in nature has been widely used for selective water removal applications, especially from aqueous-organic mixtures [236,239]. Due to fine crystalline porous structure, the zeolite-based molecular sieves were used for separating species based on size such as separation of ethanol/water or iso-propanol/water mixtures [240,241]. As for methanol-water separation, the simple molecular sieving mechanism itself is not enough for effective separation due to comparable sizes of both components [234]. Therefore, structural tuning and modifications were performed to make zeolite applicable for methanol-water separation.

4.2.2. Synthesis of Zeolite Membranes

In membrane separation processes, the thickness of the selective layer has a great influence on the mass transfer resistance to the permeating species. During fabrication of zeolite membranes, the thickness is kept at a few microns in order to minimize the mass transfer resistance. This thin layer can be very fragile; therefore, these membranes are prepared on a porous supports of different materials (alumina or stainless steel) in different configurations such as tubular, flat, and hollow fiber [254,255,256].

Several methods have been developed for synthesizing zeolite membranes on these supports using in situ crystallization and secondary-growth [257,258,259,260]. Both processes work on the principle of hydrothermal crystallization of zeolite particles on a well-prepared support material. A synthesis solution is prepared by mixing required amounts of aluminate and silicate solutions along with other structural directing agents. These agents upon post calcination are removed from inner cavities of zeolite leaving behind porous crystalline structure [261]. For in-situ crystallization, the support is directly immersed in synthesis solution/gel contained in a Teflon lined stainless steel autoclave and kept at a certain temperature. The zeolite crystals grow on support surface by thermally driven crystallization followed by in situ nucleation [257,258]. This method produces non-uniform zeolite layers and is not reproducible, as nucleation and crystal growth processes happen simultaneously.

The second method of secondary-growth crystallization is the most reliable for producing a uniform zeolite layer by separately performing the nucleation and crystal growth steps [259,260], as shown in Figure 5. Nucleation is done by depositing zeolite nuclei (seeds) on support by dip coating or rubbing, before doing final crystallization. This step is more crucial in the secondary growth method and must be carried out carefully to ensure evenly distributed crystal seeds. Many factors such as seed concentration, seed size, coating cycles, coating time, pH and temperature of seed solution are optimized to prepare an optimized seeded support [262,263]. This seeded support is then subjected to hydrothermal treatment similar to in situ crystallization, which provides the seeds as active sites for further growth of zeolite crystals and hence results in a smooth and uniform layer. After successful deposition of zeolite layer on the support, the prepared membrane is subjected to high temperature calcination to strengthen and stabilize the zeolite layer on the support [22,32].

Figure 5. Schematic diagram for hydrothermal synthesis of zeolite membranes [264].

5. Zeolite Membrane Reactors for Methanol Synthesis

Currently, thermally stable membrane materials are being studied for direct water removal from methanol synthesis reactors [24]. Clean methanol is produced from high-purity captured CO₂ and green H₂ and at the same time, water is the only impurity emerging from the reaction, with other unreacted gases and product methanol. Therefore, in situ water removal from a reaction mixture of CO₂, H₂, CH₃OH and H₂O can be achieved by putting water extraction arrangement in methanol synthesis reactor as shown in Figure 6 [34,272,273,274,275].

Figure 6. Schematic representation of zeolite membrane reactor for pure methanol synthesis [34].

5.1. Effects of In Situ Water Removal on Process Efficiency

A water-selective zeolite membrane capable of bearing the operating conditions of methanol synthesis reactor was deployed for in situ removal of by-product water [34]. The pores of the zeolite membrane, when filled with water molecules due to its intrinsic hydrophilic nature, can efficiently reject even smaller molecules like H₂ from passing through it [32]. This in situ water removal is a breakthrough to overcome the thermodynamic limitations of CO₂ conversion to methanol. Subsequently, removal of water directly from the reactor has many direct effects on overall process efficiency in terms of increasing CO₂ conversion, increasing methanol yield and purity.

5.1.1. Increasing CO₂ Conversion and Methanol Yield

Catalytic conversion of CO₂ to methanol is an exothermic reaction producing fewer number of moles in products; for example, four moles of reactants give two moles of products [280]. According to Le Chatelier’s principle, there is a need for suitable conditions to shift the reaction equilibrium to forward side, which conditions are low temperature and high pressure. The extreme conditions of these values result in negative effect on large scale production. For instance, low temperature slows down the reaction kinetics of catalytic reaction, while high pressures make the process more expensive. Therefore, the reaction must be performed at moderate conditions to save the processing time, even though at the expense of single pass equilibrium conversion. Borisut et al. found the optimized operating conditions of CO₂ hydrogenation to methanol as 183.6 °C and 57.8 bar along with 40 to 45% CO₂ conversion [281]. The unreacted gases are recycled back into the reactor to save raw material cost, while having higher overall conversions. Instead of regulating the temperature and pressure, an alternate method to increase the equilibrium conversion can be used to remove the resultant product from the reaction mixture. As per Le Chatelier’s principle, removal of product species pushes the reaction equilibrium towards a forward direction and hence increases the reactant conversion [273]. This phenomenon is utilized in several membrane reactors to achieve high conversion by selectively removing one of the product species [282,283].

5.1.2. In Situ Methanol Purification

By removing the water from methanol synthesis reactor, one can decrease a huge load from downstream methanol purification which is mainly done by distillation [32]. To get high purity of methanol with a simple process, it is necessary to choose a membrane reactor that screens out only H₂O molecules. Seshimo et al. reported the production of 95% pure methanol using Si-rich LTA-Zeolite membrane [37].

This in-situ methanol purification by using a water-selective membrane reactor incorporated with zeolite membrane has the advantage of decreasing the load on the final distillation unit. In this way, a relatively smaller distillation unit is required to remove the remaining 5 to 10% of water impurities [32]. This strategy not only saves the plant energy but also cuts down the capital investment by shrinking the equipment footprint. This new technology could be a way forward towards obtaining further high purity of methanol from membrane reactor by developing more efficient membrane materials.