Recent Advances in the Technologies and Catalytic Processes of Ethanol Production

Abstract

On the basis of its properties, ethanol has been identified as the most used biofuel because of its remarkable contribution in reducing emissions of carbon dioxide which are the source of greenhouse gas and prompt climate change or global warming worldwide. The use of ethanol as a new source of biofuel reduces the dependence on conventional gasoline, thus showing a decreasing pattern of production every year. This article contains an updated overview of recent developments in the new technologies and operations in ethanol production, such as the hydration of ethylene, biomass residue, lignocellulosic materials, fermentation, electrochemical reduction, dimethyl ether, reverse water gas shift, and catalytic hydrogenation reaction. An improvement in the catalytic hydrogenation of CO₂ into ethanol needs extensive research to address the properties that need modification, such as physical, catalytic, and chemical upgrading. Overall, this assessment provides basic suggestions for improving ethanol synthesis as a source of renewable energy in the future.

1. Introduction

Nowadays, scientists, academics, policymakers, and environmental non-governmental organizations focused on global warming or climate change effect due to the impact of greenhouse gas emissions (GHG). Data show increment concentration patterns of methane (CH₄), carbon dioxide (CO₂), chlorofluorocarbons, and nitrous oxide in the environment every year, thus prohibiting the reradiation of solar heat and increasing the temperature of the surface of the earth. Global GHG emissions are from human activities, such as burning fuels for electricity generation systems [1,2,3,4], transportation [5,6], industry [7,8], and agriculture [9,10,11]. CO₂ is also known as the most important anthropogenic GHG for global warming or climate change that is associated with human activities [12,13,14]. The Global Carbon Budget 2021 reported that the global atmospheric CO₂ emissions growth is 5%. The global average amount of atmospheric CO₂ emissions in 2021 is 36 billion metric tons or equivalent to 415 ppm [15]. The increasing patterns of CO₂ concentration globally in the atmosphere are predicted to increase due to excessive industrialization, which leads to the development of heat retention since the introduction of the Fourth Industrial Revolution.

CO₂ is proven to be a recyclable, nonpoisonous, and inexpensive C₁ building block in the synthesis of high-value chemicals and fuels [16,17,18,19]. However, CO₂ is fully oxidized, chemically inert, and thermodynamically stable (∆_fG_298K = −396 kJ·mol⁻¹). Thus, its conversion into chemicals requires large energy and enormous H₂ resources [20,21,22]. Many CO₂ transformation approaches, e.g., hydration of ethylene, biomass residue, lignocellulosic materials, fermentation, electrochemical reduction, dimethyl ether (DME), reverse water gas shift (RWGS), catalytic hydrogenation, and other related processes, have been rapidly studied. Catalytic hydrogenation has been deemed a promising technology for generating a variety of products, such as hydrocarbons [23,24], alcohols [25,26], carboxylic acids [27,28], and aldehydes [29,30].

Ethanol is an ecological fuel that has an important advantage compared with conventional gasoline as a transportation fuel due to its properties of nontoxicity, accumulation of high oxygen content to promote improved combustion with reducing exhaust emissions, and high octane rating to give a high resistance to engine knock [31,32,33,34]. Hence, the establishment technology of fuel ethanol is needed in reducing environmental pollution problems [35,36]. Ethanol is produced from agricultural feedstock, such as corn (United States) and sugarcane (Brazil), and the European Union produces ethanol from wheat and sugar beet. The Renewable Fuels Association in 2021 reported that the main ethanol producer in the world is the United States, estimating ethanol production of more than 13,000 million gallons per year, which is more than half of the global ethanol production. Approximately 8000 million gallons per year of ethanol is produced in Brazil [37]. The use of agricultural feedstock especially corn in ethanol production is being criticized because of their importance as food. Food shortages and rising food prices will occur if agricultural feedstock is used as the raw material in ethanol production. Furthermore, the grain from corn will create environmental pollution problems, such as soil erosion, biodiversity loss, nitrogen oxide pollution, and emission of volatile organic compounds. The major constraint in commercial ethanol production is the disadvantage in energy balance and the area required for plantations [38,39]. Given these considerations, nations continue to look for new technologies and processes in reducing the cost of ethanol production without causing adverse effects on the environment.

2. Conventional Processes to Produce Ethanol

2.1. Ethanol Synthesis Based on the Hydration of Ethylene

In the petrochemical industry, the catalytic hydration of ethylene for ethanol production is a reversible exothermic reaction and is used commercially by Shell Oil Company in 1947 [40]. The process of reaction can be expressed by the following equation:

C₂H₄ + H₂O ⇌ C₂H₅OH.

(1)

The hydration of ethylene comprises three stages, i.e., reaction, recycling, and purification. Mohsenzadeh et al. [41] suggested that this process occurs in a fixed-bed catalytic reactor when ethylene is mixed with steam at a molar ratio of 0.6 at 250–300 °C, 70–80 bar, and the presence of a phosphoric acid catalyst (H₃PO₄/SiO₂) based on silica gel. The ethylene conversion is 4–25% with ethanol selectivity of 98.5 mol.%. A diagram of the hydration of ethylene is shown in Figure 1.

A byproduct of this process, acetaldehyde, can be used directly in a cosmetic product or promoted to form ethanol through the hydrogenation process. The high-pressure separator is used to separate the unreacted reactants from the outlet stream mixture. Unreacted reactants are scrubbed with water to remove impurities before dissolution with ethanol. The molar ratio of ethylene to water is maintained at 1:0.6, and the mixture is combined with a recycle stream. When the bottom streams of the scrubber and the separator are fed to the hydrogenator, the nickel-packed catalyst promotes the formation of an ethanol mixture from acetaldehyde. The unreacted acetaldehyde in the separator column is removed and recycled in the hydrogenator. The ethanol concentration increases when the bottom stream is fed to light and heavy (purifier) columns [42].

The cost of a plant for the hydration of ethylene increases due to the formation of the ethanol-water mixture that will form an azeotrope mixture, which requires a special distillation process. The production of ethylene based on hydrocracking petroleum feedstocks, which are nonrenewable, is not economically feasible because of the market price of ethanol. The phosphoric acid catalyst that is used in this process is prone to leaching via vaporization, which causes the deactivation of catalysts and corrosion of the equipment. A solid acid catalyst, such as WO₃/ZrO₂ [43] and WO₃/TiO₂ [44], is introduced to overcome leaching issues; this route is still not favorable for the large-scale production of ethanol due to the price of ethylene and the rapid development of sugar fermentation during the hydration of ethylene.

The research on the gas-phase hydration of ethylene by using impregnated metal phosphates for catalytic activity at tin(IV) hydrogen phosphate (Sn[HPO₄]∙2H₂O) showed that the weight-based rate is 0.94 µmol∙min⁻¹∙gcat⁻¹, which is approximately seven times higher than that at commercial H₃PO₄/SiO₂ catalyst [45]. The vapor-phase hydration of ethene has recorded a 93% selectivity for the tungsten trioxide (WO₃) monolayer loaded with titania. The co-presence of Brønsted and Lewis acid sites on the monolayer of WO₃ generated high selectivity for ethanol but the reaction process has issues from the environmental protection perspective [46]. The combination of vapor–liquid equilibria and the chemical stage equilibrium for the ethylene–water–ethanol ternary arrangement demonstrates that the crucial point of an azeotrope at 200 °C and 155 atm shows the active catalysis of H-pentasil zeolite for the maximum hydration of ethylene [47]. The ethanol production plant’s simulated modeling was developed using the HYSYS software and the results of optimization over the catalytic hydration using zirconium-tungsten catalysts operating at 299 °C with column configuration for the extractive distillation which produced ethanol concentration of 99.7% and then linked to the Aspen Plus software [48].

The hydration process corresponds to petroleum-derived alkene over solid acid catalysts, which are limited by the low single-pass conversion (<5%), poor long-term stability, and strong dependence on crude oil. The chemical equilibrium conversion of the hydration of ethylene decreases when using high temperatures to increase the rate of reaction. As a result, the temperature setting of the reactor is needed to accommodate between thermodynamics and kinetics [46]. A high amount of energy to heat gases generates high pressure and uses crude oil, a nonrenewable resource. The purification method uses benzene to separate the azeotropes of ethanol and water that produce hazardous ethanol.

2.2. Ethanol Synthesis Based on Biomass Residues

Biofuel is produced from biomass residues and wastes for energy purposes, such as transportation fuels, renewable electricity, and thermal energy [49,50,51]. The three types of biomass residual resources are primary, secondary, and tertiary (Figure 2). Corn stalks, husks, stems, roots, leaves, cob, bagasse, and straw make up the main residue, which is described as outcomes of the cultivation of certain food crops and agroforestry in the agriculture sector. Then, the secondary residue is obtained by the processing of crops into the final form of a product or the production of other biomass-based materials. Examples of agricultural and food processing wastes are sawdust, wood chips, nutshells, palm kernel cake, fruit bunches, coffee husks, rice hulls, bark, and scrap wood. The tertiary residue consists of sewage sludge or wastewater derived after the consumption of biomass-based products, such as municipal solid waste. Human, animal, and industrial wastes have been identified as the main source of municipal solid waste [52,53,54].

A study showed that wood processing residues, such as discarded logs, sawdust, and wood chips produced from sawmill and lumber processing, can be utilized as steamer fuels and feedstock for ethanol synthesis [53,55]. Tropical countries have been shown to apply the concept of economic utilization by using sugarcane residues, such as sugarcane bagasse and leaf residue, for ethanol production and value-added commercial product [56,57,58]. According to the research on delignified coconuts waste and cacti, an ethanol yield of 89.15% is recorded by utilizing the semisimultaneous saccharification and fermentation configuration, and this yield is higher compared with that obtained by simultaneous saccharification and fermentation (SSF) configuration [59]. The pretreatment mixture of 0.06 g·g⁻¹ hydrogen peroxide to green liquor and furfural residues pretreated to cassava residue saccharification liquid with a ratio of 1:1 recorded an advantageous pretreatment method by producing a 93.6% yield of ethanol. This study shows that the increment of a high ethanol yield and lower byproduct concentrations occur when the proportion of lignocellulosic substrates was enhanced in the SSF of the substrate mixture of cassava residue and furfural residues [60]. The investigation of the carnauba straw residue by the SSF configuration process in a single reactor in the presence of Kluyveromyces marxianus ATCC-36907 and observed that cultivation at 45 °C results in the maximum ethanol concentration of 7.53 g·L⁻¹ [61]. The mangosteen pericarp waste that has undergone popping pretreatment and enzymatic hydrolysis in the separate hydrolysis and fermentation (SHF) configuration method can achieve 75% ethanol [62]. The ethanol produced from the combination of Salacca zalacca and coconut sewage shows that the energy required for coordination obtained at 85 °C is 346.32 W, and the resulting ethanol is obtained at 40% and mass flow rate of 0.0655 kg·s⁻¹ [63]. Rahman et al. [64] developed a green biorefinery concept to produce ethanol by integrating the pretreatment of fermentation and ethanol-assisted liquefaction in the presence of Nannochloropsis sp. The process increases the lipid content of fermented microalgae by 40%, whereas 10% of the required ethanol is produced through liquefaction. The utilization of wet algae increases the crude biodiesel yield threefold compared with the liquefaction of microalgae. The fermentation of over-ripened Indian blueberry at 33 °C, pH 5.2, and specific gravity of 0.875 obtain 6.5% ethanol [65].

However, biomass residue requires an energy-intensive process from large and specific machinery due to different types of biomass, thus increasing the cost of operation because of expensive machinery and the fuel needed for operation. The low density of biomass also influences the cost of operation by occupying increased volume and needing increased transportation for space. The other challenge is developing effective pretreatment technologies that cover physical, chemical, and biological pretreatments. The ideal pretreatment increases the rate of enzyme hydrolysis and decreases the amount of enzyme needed to convert the biomass into sugars in the presence of the microorganism. Issues regarding environmental pollution also need to be addressed because the conversion of biomass residue into ethanol produces a huge amount of CO₂.

2.3. Ethanol Synthesis Based on Lignocellulosic Materials

Lignocellulosic materials, which comprise nonedible feedstock from various agricultural and forestry residues, are abundantly available without geographical limitation and have a low cost. Extensive research showed the production of ethanol and value-added chemical by using different types of lignocellulosic sources, including waste paper [66,67], orange peel [68,69], sugarcane straw [70,71], corn stover [72,73], sugarcane bagasse [74,75], rice straw [76,77], wheat straw [78,79], sweet sorghum [80,81], oil palm empty fruit [82,83], and banana waste [84,85]. Figure 3 shows the composition of lignocellulosic materials that consist of three major fragments of cellulose (40–50%), hemicellulose (25–35%), and lignin (15–20%), which always exist beside other extracts and mineral traces [86,87,88,89]. However, the composition differs on the basis of the type of biomass, cultivation, and atmospheric conditions. The complex and rigid structure of lignocellulosic materials is made by noncovalent interactions with covalent cross-linkages [90].

The conversion of lignocellulosic materials into ethanol involves pretreatment, enzymatic hydrolysis, fermentation, and distillation [92,93,94,95]. During the pretreatment process, various cutting-edge technologies are utilized to open the structure of lignocellulosic materials by physical, chemical, physicochemical, and biological techniques and separate complex interlinked structures among hemicellulose and lignin from the matrix [87,96,97,98]. Chemical pretreatment uses a variety of chemicals such as acid and alkaline chemicals to break down the structures present in the lignocellulosic biomass at a constant ambient temperature which subsequently enhances the biomass surface availability to enzymatic hydrolysis, permitting the cellulose and hemicellulose for further conversion of fermentable sugars into biofuels [99,100,101]. The research on the bioconversion of lignocellulosic by-product corn stover into the value-added fermentative product l-lactic acid using the furfural tolerant Enterococcus mundtii WX1 and Lactobacillus rhamnosus SCJ9 showed that corn stover pretreated with 1% (v/v) sulfuric acid was selected for l-LA fermentation and shows the highest efficacy of fermentable sugar with the optimal conditions achieved for the release of glucose and xylose at 24.5 g/L and 11.2 g/L, respectively, from 100 g/L pretreated corn stover at 121 °C for 30 min [102]. A similar result was presented by other researchers reported in the study of tobacco stem waste [103], palm kernel shell [104], sugarcane bagasse [105], and oil palm frond bagasse [106] that the dilute acid for chemical pretreatment is effective to attain high reactivity and generates protons that have a quick diffusion which substantially enhances the hydrolysis of amorphous cellulose chains and the solubilization of hemicellulose.

Dilute acid pretreatment has received wide attention due to its cost-effective, non-toxicity, lower degradation products, corrosive, and hazardous processes that do not require as much corrosion-resistant equipment, making it easier to scale-up the operation process [107,108,109]. Alkaline pretreatment leads to the delignification of agricultural biomass by cleaving the intermolecular ester linkages between hemicelluloses and lignin fragments, increases the amorphous surface area of the cellulose as well as the porosity of the biomass, reduces the degree and crystallinity of the polymerization rate at low temperature and pressure, resulting in an enhanced hydrolysis and fermentation yield and a high amount of sugars [110,111,112]. In a comparison of various alkaline pretreatment techniques, alkaline hydrogen peroxide (AHP) pretreatment is the most effective as it increases the fermentation yield at mild conditions effectively by solubilizing lignin from the complex recalcitrant structure of the macromolecules because H₂O₂ could degrade to oxygen and H₂O without any residues left and increases the enzyme digestibility and fermentation efficiency of the feedstock required for subsequent processing [99,113,114,115]. The primary advantages of AHP pretreatment are environmentally friendly chemicals and reagent reusability, high effectiveness for various biomass concentrations providing high efficiency of enzymatic hydrolysis, high lignin, and hemicellulose solubilization values for the liquid fraction without a loss of carbohydrates retention, low energy consumption, less formation of toxic byproducts, no need for special reactors, compatibility with high solid loadings, and sterility conditions provided by alkaline H₂O₂ without a need to use antibiotics [116,117,118].

Organosolv pretreatment with aliphatic organic solvents is among the most promising pretreatments compared to acidic or alkaline pretreatment by producing a very distinctive separation of high-purity cellulose content from the remaining lignocellulosic constituents, such as lignin and hemicellulose at relatively low temperatures (below 180 °C), while preserving the integrity of the hemicellulose structure from thermal degradation kinetics [119,120,121]. Organosolv pretreatment using ethanol has also some advantages over other methods such as low toxicity and environmentally friendly nature, high delignification rates, high reaction stability, good solubility of lignin, miscibility with water, complete restoration of ethanol solvent due to its low boiling point and potentially provides substantial economic benefits [122,123,124,125].

The hydrolysis process breaks down the hemicellulose and cellulose components in the presence of cellulolytic enzymes or acids to form monosaccharides [126,127,128]. The conversion of sugars into ethanol by using a variety of potential microorganisms occurs during the fermentation process. Saccharomyces cerevisiae, a microorganism that is commonly used as baker’s yeast on a large scale at the industrial level, has been identified to have a tolerance for ethanol production, robust ethanol dehydrogenase, and potential good resistance against inhibitors generated during the process. The use of this strain results in high ethanol productivity and efficient conversion of most of the sugars into ethanol rather than other byproducts [129,130]. The last step is distillation where the purification of the fermentation broth occurs. Distillation is an effective and favorable separation technique as the preferred choice for industrial application due to high alcohol recovery of 99.5% v/v purity, sufficient energy efficiency at moderate feed concentrations, and the ability to simulate the process using process simulation software which makes the integration of mass and energy in other processes easier to accomplish [131,132,133]. All formed byproducts and other impurities are removed during this process, and only imprints remain. Most energy-intensive units have remarkable effects on the gross energy demand that takes place during the distillation process. The cost of plant operation also increases due to the formation of an ethanol–water mixture that produces an azeotrope mixture where the simple distillation method cannot be used to change its composition [134,135].

The SHF process has been studied for ethanol production from waste paper. The pretreatment process of waste paper is applied using 0.5% (v/v) hydrogen peroxide at 121 °C for 30 min. The office paper that has been pretreated by hydrolytic enzymes produces 24.5 g·L⁻¹ sugar equivalent to 91.8% hydrolysis efficiency. Then, the fermentation process that uses S. cerevisiae through hydrolysate obtains 11.15 g·L⁻¹ ethanol with ethanol productivity of 0.32 g·L⁻¹·h⁻¹ [67]. Oil palm trunk chips are introduced into a two-stage pretreatment, i.e., steam explosion and alkaline extraction. The steam explosion pretreatment shows that the reduction and isolation of hemicellulose occur in biomass recalcitrance. The alpha-cellulose content has been improved from 40.83% to 87.14% with alkaline extraction pretreatment at the conditions of 15% (w/v) NaOH at 90 °C for 60 min. By using S. cerevisiae, the ethanol concentration at SSF (44.25 g·L⁻¹) is prominent compared with that at prehydrolysis SSF (31.22 g·L⁻¹) [136].

The SHF process of the pretreatment of rice straw using Saccharomyces tanninophilus produces 9.45 g·L⁻¹ ethanol with 83.5% yield. The saccharification of pretreated rice straw with A. fumigatus by using 1.0% NaOH in 200 FPU·mL⁻¹ crude enzyme for 20 h of reaction obtains 22.15 g·L⁻¹ limiting sugars, demonstrating high lignin-degrading manganese peroxidase activity and the activity of laccase enzymes [76]. The effectiveness of SHF and SSF techniques for the synthesis of ethanol originating from oil palm empty fruit bunch (OPEFB) with the conditions of 10% (w/v) loading of the substrate, pH 5, 1% (v/v) K. marxianus at 37.50 °C for 48 h of reaction is compared. SHF and SSF obtained 25.80 and 28.10 g·L⁻¹ ethanol, respectively. The acid–alkali pretreatment of OPEFB is conducted by the loading of the substrate at 12.50% w/v with 0.2 M concentration of H₂SO₄ at 121 °C for 53 min followed by 5% (w/v) NaOH at 121 °C for 20 min. This result demonstrates that the acid–alkali pretreatment increases the cellulose yield to 72.10 wt.% and this process is a feasible method for eliminating hemicellulose and lignin from lignocellulosic biomass [82].

The cellulosic ethanol production by using Issatchenkia orientalis KJ27-7 in 90% wheat straw hydrolysate media for 24 h has obtained 10.3 g·L⁻¹ ethanol corresponding to 0.50 g·g⁻¹ glucose (97% of efficiency relative to the theoretical yield). The correlation of ethanol production with wheat straw hydrolysate concentrations is observed [78]. Studies on the effect of varying lignocellulosic feedstocks on technical performance for ethanol production that use the dilute acid pretreatment show that the switchgrass produces 46.2% energy efficiency of feedstock LHV, which is the highest carbohydrate content with the lowest forest residues compared to the Eucalyptus globulus., Birch sp. residues, Spruce sp. residues, Miscanthus, corn stover, and wheat straw [137]. Cunha et al. [138] reported that the direct production of ethanol by using non-detoxified hemicellulose liquor by S. cerevisiae using hydrothermally pretreated corn cob without external hydrolytic catalysts results in 11.1 g·L⁻¹ ethanol titer correlated with the ethanol yield of 0.328 g·g⁻¹ potential sugar. The consolidated bioprocessing (CBP) of pretreated corn cob with the addition of commercial hemicellulases is more efficient than SSF in hemicellulosic ethanol production.

Diverse sources and seasonal lignocellulosic biomass affect the chemical characteristics of ethanol produced due to different harvesting times, resulting in the inconsistent composition of lignocellulosic biomass components. Pentose sugars are not fermented by the brewer’s yeast, i.e., S. cerevisiae, during the hydrolysis of hemicellulose, thus compromising the ethanol production from total sugars in lignocellulosic materials. Hence, the energy consumption during the distillation process for ethanol recovery and treatment of a large amount of stillage increases due to the subsequent reduction of discharge. The pretreatment process produces lignin from inhibitors that act as limiting agents for high biomass loading and do not react productively with lignocellulosic materials. As a result, the ethanol production from cellulose is lower compared with that from grains. Other limitations of ethanol production from lignocellulosic materials, such as high capital, operational expenditure, dwindling price of gasoline, process uncertainty, low growth, and product yield, have also been identified.

2.4. Ethanol Synthesis Based on Fermentation

The major steps in ethanol production via the fermentation process are the treatment of a solution containing fermentable sugars, the formation of ethanol from sugars through fermentation, and distillation for the separation and purification of ethanol [134,139,140]. The main metabolic route involved in ethanol production by fermentation is glycolysis, which converts glucose into pyruvate that is further reduced to produce ethanol and CO₂ under anaerobic conditions [65,141]. Based on the stoichiometric equivalence, 1 mol glucose creates 2 mol CO₂, which is then expelled from the reactor as a weight loss and is proportional to ethanol yield.

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.

(2)

The fermentation process can be produced in different systems, such as a batch, fed-batch, or continuous bioreactor. The batch bioreactor fermentation is a simple method with a closed culture system where both biomass and substrates are added to the fermenter in a single step of the procedure in which nothing is added or removed during the process and the products are only removed at the end of the process [142,143]. The system operation produces high cell densities, of which almost 99.5% is recycled in subsequent fermentation. The closed-loop design system that uses a high concentration of sugars generates a high concentration of ethanol [143]. In the conventional batch fermentation process at an ideal temperature and under anaerobic conditions, S. cerevisiae is used to convert glucose into ethanol. However, this process only occurs in hexose sugars but not in pentose sugars [97,129]. Although equipped with multiple vessels, the batch fermentation system is considered the simplest operation system due to its ability to complete the sterilization process, resulting in a low risk of contamination, low operation costs due to no labor required, easy control of feedstock processes, and flexibility for various product specifications. The disadvantage of this process is solvent inhibition, time consumption, difficulty in maintaining the sterilization of bioreactors, major downtime, long lag phase, and low productivity.

The fed-batch bioreactor process is a semicontinuous or partly open system that allows the addition of fermentation medium gradually or consistently during the process after the initial substrate has been used, overcoming the difficulties of substrate constraint in the batch bioreactor process. This process enables the overall proportion of substrate uptake to increase and sustain a low concentration of substrate within the fermentation vessel, thus decreasing the negative influence of osmotic pressure or rheology-related limitation linked with highly viscous substrates [143,144]. Knudsen and Rønnow [145] reported the highest ethanol production from wheat straw by using S. cerevisiae in the co-fermentation stage at the C5/C6 fermenting yeast, where glucose and xylose are fermented simultaneously. With the addition of urea and a primary yeast pitch of 0.2 g·L⁻¹ completed broth in at least five fermenter volumes, the fed-batch fermentation process is stable, yielding an ethanol yield >90% during the experiment. The fed-batch fermentation process by using the mixture of sugarcane and molasses based on the Central Composite Design evaluates the effect of temperature at 27 °C, the concentration of sugar at 300 g·L⁻¹, and the concentration of cells at 15% (v/v) in the presence of S. cerevisiae for 30 h of reaction. This process has obtained ethanol concentration, productivity, and yield of 135 g·L⁻¹, 4.42 g·L⁻¹·h⁻¹, and 90%, respectively [146]. The fed-batch process is a cost-effective operation with an efficient cultivation strategy, short fermentation time, high dissolved oxygen concentration in the medium, and low toxic efficacy of medium constituents. However, the ethanol production in fed-batch fermentation is minimal; the concentration of cell mass and feed rate of the reaction thus provided a point of ingress for contamination and allowed the buildup of inhibitory agents and toxins. The outcome of high cell density numbers and product yields are difficult to deal with downstream, creating bottlenecks in the whole process.

The continuous fermentation process is carried out by continuously feeding substrates, new media, and nutrients into a bioreactor containing active microbes. This process concurrently harvests the used medium and cells, removes toxic metabolites, and replaces the consumed nutrients from the culture. As a result of the equivalency process of addition and removal, the culture volume in this process remains constant. Then, the maximum working volume of the vessel does not limit the amount of fresh medium or feed solution which can be added to the culture in the course of the process. However, the long cultivation period increases the risk of contamination and genetic changes in the cultures. This process also difficult to keep a constant population density over prolonged periods and the products of a continuous process cannot be neatly separated into batches for traceability. The production of high residual sugar and ethanol in this process is caused by the continuous exposure of yeast cells that may affect cell growth until biomass washout [147]. Margono et al. [148] developed the uncontrolled continuous fermentation process equipped with an integrated aerobic–anaerobic baffled reactor (IAABR) to study molasses in the presence of S. cerevisiae and generated 92.55 g·L⁻¹ ethanol with a productivity of 4.63 g·L⁻¹·h⁻¹ for a residence time of 19.2 h. The ethanol productivity with IAABR is 3.4% higher compared with that through the industrial batch process, and the maximum operation reaches 14 days of fermentation without contamination.

The cassava supernatant subjected to continuous ethanol production with a high cell density strategy at the dilution rate of 0.092 h⁻¹ generated 104.65 g·L⁻¹ ethanol and ethanol productivity of 9.57 g·L⁻¹·h⁻¹. The ethanol yield of this system is 96.96%, which is approximately 4.2% higher compared with that obtained by traditional fermentation with free cells. This research shows that cells sustain optimum condition activity by switching the flow direction in the in-series bioreactors and extend the long-term stability of continuous fermentation without any possibility of a contamination effect [149]. The continuous fermentation with a high cell density recycle operation demonstrates a better result compared with typical molasses-based batch fermentation by obtaining 0.44 g·g⁻¹ ethanol from xylose and glucose and ethanol productivity of 3.4 g·L⁻¹·h⁻¹ [150]. The continuous fermentation method yields an improved output in minor bioreactor volumes, has low operational costs by lowering production times, is cost-effective, allows for growth control via nutrient supply management, and is scale-up friendly. Some limitations of the continuous fermentation process, such as low product concentration, complicated downstream processes, difficulty in maintaining sterilization conditions, high risk of contamination with the extended culture time, limited yeasts’ capacity to create ethanol and periodic handling, which may also increase the costs of operation, are observed.

Syngas fermentation is a biological carbon fixation process that uses a gaseous feedstock, primarily composed of a mixture of CO, CO₂, and H₂ which is obtained from biomass, coal, animal or municipal solid waste, and industrial CO-rich waste gases, that is a promising approach converted into valuable chemicals and fuels by microorganisms through a hybrid thermo/biochemical process [151,152]. Several Clostridium species are known to produce different bioproducts, but only a few of them use syngas as the sole carbon and energy source [153,154]. Clostridium carboxidivorans are acetogenic bacteria that are known to grow autotrophically with syngas and chemoorganotrophically with a wide range variety of sugars [155,156,157]. It is able to ferment these carbon sources to produce volatile fatty acids and alcohols that can be employed as platform chemicals or as feedstock for liquid fuel production qualifying it as an interesting microorganism for industrial production [152,158,159]. However, the issues that must be addressed in order to incorporate the syngas fermentation into an industrial-scale process include the gas-liquid mass transfer limitation brought to the low aqueous solubilities of the gaseous substrates that occur when cells have the capacity to process more gas than the bioreactor can supply. The resistance of gaseous substrate diffusion at the gas-liquid interface has been identified as the limiting step in syngas fermentation [160,161,162,163]. The other challenge identified as low carbon fixation yield, high production cost, and the effects of gaseous impurities such as NH₃, H₂S, and NO_x even at low concentrations by limiting microbial growth, enzyme activities or by changing physiochemical conditions led to the unintended accumulation of organic acids and decreased alcohol formation [152,153,154,159,164,165].

The SSF method combines enzymatic hydrolysis and fermentation in a single phase to produce value-added products. This method involves hydrolyzing cellulose and extracting sugars by using an enzymatic complex. These sugars are then used by microbes and transformed into value-added compounds [166]. The combination of the semicontinuous fermentation of sugarcane bagasse and SSF system produces 9.07% (v/v) ethanol with <1% residual glucose at the optimum conditions of 1% (w/v) NaOH, 160 °C, and 20 min of reaction. This study shows no remarkable variation throughout the whole process and that the system achieves a constant state [167]. Compared with SHF, SSF has several advantages. These advantages include the use of an individual vessel for fermentation and saccharification, which reduces the residence period and capital expenditure, and the reduction of the inhibitory composite from enzymatic hydrolysis, which enhances inclusive operational achievement. SSF has been intensively studied for the manufacture of ethanol from lignocellulosic and starchy raw materials because of these benefits. The optimal temperature for enzymatic hydrolysis is often higher than the fermentation temperature, and the SSF reaction is limited by the pH and temperature of the operation.

Meanwhile, in the pre-hydrolysis simultaneous saccharification and fermentation (PSSF) configuration, the pretreated material is pre-hydrolyzed at the optimum temperature of the cellulolytic enzyme, and the temperature is then lowered for further inoculation with no other additional step [168,169]. The main advantage of PSSF is significantly reduced overall fermentation time, environmentally friendly, increased initial velocity (V₀) of enzymes, and provided the optimum conditions for both the enzyme and yeast to utilize the substrate sufficiently that also reduces the production cost and favors the distillation process for high ethanol yields [170,171]. The banana peel with 25% (w/v) of high solid loading using commercial S. cerevisiae at 64 h of fermentation has been demonstrated as the promising feedstock for ethanol production by PSSF by achieving a maximum ethanol concentration of 32.6 g/L [172]. The bioconversion of barley straw to bioethanol was carried out by PSSF where the kinetic model was used as guidance in the choice of pre-hydrolysis time step. The highest ethanol concentration reached in the present study was 46.62 g/L at a high solid loading of 20% (w/v) of barley straw by applying 16 h of pre-hydrolysis. The mass balance of PSSF showed that the reduction in ethanol yield when solid loading increases could be attributed to the decrease in cellulose enzymatic conversion [173]. Under the PSSF strategy in the development of a process using Sargassum biomass at high pretreated solid loading 13% (w/v) was subjected to high-pressure technology for biomass fractionation recorded the maximum ethanol concentration of 18.14 g/L after 12 h of fermentation [174]. The ethanol production from potato peel waste subjected to the PSSF process allowed for reaching a maximum ethanol concentration of 104.1 g/L at high productivity with 54 h of fermentation [175].

The simultaneous saccharification and co-fermentation (SSCF) method breaks down cellulose into sugars called hexoses by using an enzyme complex. Specialized microorganisms with the capacity to ingest substrates consume these sweeteners generated in situ along with pentoses following a pretreatment to acquire a product of significance. The research on the corn stover subjected to temperature-profiled SSCF at 12% of glucan loading eliminated sugar accumulation and alleviated ethanol repression by process optimization, 59.8 g·L⁻¹ ethanol. It suggested that the high-temperature resistant strain helped the xylose-utilizing strain maintain cell viability in SSCF at high temperatures (42 °C) which are higher compared with the threshold concentration for the economic distillation process [176]. Xylose utilization in the study of sugarcane bagasse by using SSCF with a thermotolerant S. cerevisiae at 40 °C demonstrated 99% xylose in the hydrolysate during the co-fermentation process, generating 36.0 g·L⁻¹ ethanol [177]. The differences in pH, temperature, and other parameters required for the enzymatic hydrolysis and co-fermentation process have been recognized as limiting factors in the SSCF.

The enzymes that hydrolyze cellulose in the production of ethanol require high-temperature conditions, i.e., thermotolerant yeast and bacteria, to produce high enzyme contents. The thermotolerant microorganisms are beneficial in terms of efficiency improvement of processes by obtaining higher yields in saccharification, reducing costs associated with a cooling system while reducing the risk of bacterial contamination [178,179,180,181]. Many studies have examined various thermotolerant yeasts with their optimized temperature for ethanol production. The isolate Pichia kudriavzevii at 28 °C achieved the optimal ethanol concentration of 10.10 g/L and a productivity of 0.21 g/L/h in monoculture fermentation [182]. The optimal conditions of Meyerozyma guilliermondii at 45 °C using sugarcane bagasse as a substrate achieved the maximum ethanol concentration of 11.12 g/L and a productivity of 0.23 g/L/h [183]. The optimum fermentation conditions for ethanol production from sweet sorghum juice with the thermotolerant yeast S. cerevisiae at 37 °C revealed a maximum ethanol concentration of 99.75 g/L and a productivity of 2.77 g/L/h was achieved [184]. K. marxianus at 42 °C also effectively utilized biopretreated elephant grass hydrolysate and produced the maximum ethanol concentration of 14.65 g/L and a productivity of 0.62 g/L/h [185]. Fermentation or hydrolysis can be achieved under ideal specifications, and the microorganism must be specialized for both substrates and only applicable at high-temperature conditions. The process requirements also point to the necessity for the creation of specialized microorganisms. With the use of genetic engineering, the SSCF process offers several advantages, including the utilization of minimal equipment, short processing times, reduced contamination risk, and high ethanol production efficiency.

The development of consolidated bioprocess (CBP) of lignocellulosic biomass is the most integrated process for the bioconversion approach, where the process of hydrolysis, fermentation, and enzyme production occurs in a single reactor. The conversion of pretreated lignocellulose employs genetically modified single microorganisms or a microbial consortium capable of hydrolyzing biomass with enzymes produced on its own and fermenting monosugars into value-added products could provide the environmentally friendly, economically competitive by reducing costs for infrastructure, raw materials, and enzyme production [117,186,187]. The effective fermentation of monosugars obtained from lignocellulosic biomass is the next bottleneck in bioethanol production reaching an industrial scale. Several factors might affect its low conversion efficiency, including low enzyme concentrations at the start of the fermentation, temperature, time, pH, inoculum size, solid-to-liquid ratio, agitation rate, oxygen content, and rotation speed [188,189,190]. The isolated bacterium of Hangateiclostridium thermocellum in the study of pre-treated Nannochloropsis gaditana biomass converted into ethanol through CBP was investigated. In this study, the hemicellulose removal of dilute H₂SO₄ treatment was found to be best for the pretreatment of biomass at the concentration of 2.5% under 100 °C for 60 min, effectively disrupting a complex matrix of the holocellulose sample and removing the hemicellulose. The optimized conditions of the medium components and process parameters yielded a maximum ethanol concentration of 12.90 g/L [191]. The investigation on the potential of the fungus Trichoderma asperellum to produce ethanol and the physicochemical parameters required for paddy straw waste conversion via CBP using the numerical optimization was statistically validated by comparing the volume of ethanol produced, to the volume analyzed via Response Surface Methodology (RSM). The investigation proved that the fungus is a potential organism for on-site enzyme production with a maximum ethanol concentration of 0.94 g/L [192]. The maximal ethanol production capability of Fusarium moniliforme integrated biodelignification and CBP of Napier grass at solid-state conditions is 10.5 g/L by feeding the fungus with a surplus of glucose for fermentation. These results demonstrated the characteristics of a fungus for potential ethanol production from cellulose, mixed sugars, and lignocellulosic materials [193]. The study of the bioconversion of Sargassum wightii via CBP using the bacterial isolate, Lachnoclostridium phytofermentans shows excellent growing ability in the optimized production medium conditions with the maximum ethanol concentration of 13.75 g/L [194].

The pretreatment of feedstocks minimizes their size and makes following the procedures easy. Cellulose and hemicellulose are hydrolyzed into sugars that may be fermented. These carbohydrates are fermented into ethanol by using yeasts. S. cerevisiae is widely utilized as a yeast strain for the ethanol fermentation of lignocellulosic hydrolysates due to fast growth, high tolerance, efficient glucose anaerobic metabolism, high selectivity, cost-effective process for high ethanol yield, high rate of fermentation, low accumulation of byproducts, and use of a broad scale of disaccharides (e.g., sucrose and maltose) and hexoses (e.g., glucose, mannose, and galactose) [195,196].

Table 1. Ethanol production by S. cerevisiae from different types of feedstock at varying treatments.

Feedstock	Parameters			Ethanol Concentration (g/L)	Ref.
	Temperature (°C)	Agitation Speed (rpm)	Incubation Time (h)
Galactose	30	200	28	96.90	[197]
Rice husk	43	150	96	15.63	[198]
Oil palm frond	30	152	15	4.79	[199]
Cellulose and sucrose/xylose	30	200	96	4.30	[200]
Papaya peels	30	200	48	0.51	[201]
Pineapple leaf	30	150	72	9.75	[202]
Pomegranate peel	30	100	24	5.58	[203]
Sweet sorghum	30	150	18	97.54	[204]
Sugarcane distillery waste	30	150	48	49.77	[205]
Corn starch	30	300	192	98.13	[206]
Rice straw	30	150	72	18.07	[207]
Sugarcane molasses	30	200	56	114.71	[208]
Corn stover	34	150	48	21.47	[209]
Oil palm trunk	30	150	18	44.25	[171]
Microalgae biomass	30	150	48	52.10	[210]
Sugar beet molasses	30	140	112	79.60	[211]
Cassava starch	30	200	72	81.86	[212]
Suweg starch	37	80	78	99.52	[213]
Frond Waste	50	150	96	33.15	[214]

shows ethanol production by S. cerevisiae from a different type of feedstock at varying treatments.

Industrial ethanol production is efficiently produced from lignocellulosic hydrolysates by yeast strains with high hexose and pentose fermentation. This result is due to the high xylose and glucose contents in lignocellulosic biomass [215,216]. The modest acid stress caused by lignocellulosic materials also inhibits yeast fermentation. The presence of weak acids in deficient concentrations can boost ethanol synthesis through cellular division. In S. cerevisiae, weak acids are shown to increase glucose consumption, ethanol synthesis, and tolerance to 5-hydroxymethylfurfural and furfural [217,218]. Despite the fermentation route having been commercially realized, the cost is expensive due to the energy-intensive distillation steps and low yield to meet the market demand. Other remarkable obstacles to ethanol generation, such as excessive temperatures, prominent ethanol concentrations, and capacity to ferment pentose sugars remain in yeast fermentation. The main disadvantage is that yeasts grown in anaerobic states for an extended period lose the capacity to manufacture ethanol. Furthermore, at high dilution rates, which allow for high productivity, the substrate is not entirely utilized, resulting in low yield. The rate of development and metabolism of yeasts increases as the temperature rises until the optimal level is reached. The inhibition of microorganism expansion and viability can occur when ethanol concentrations rise during fermentation. The difficulty of S. cerevisiae growing on a medium with a high concentration of alcohol causes ethanol production to be inhibited. The limitation of S. cerevisiae is the inefficient fermentation of glucose and xylose. As a result, yeast strains that can ferment glucose and xylose or utilizing two separate yeast strains that can utilize these sugars individually should be found.

3. Future Directions of CO₂ Conversion into Ethanol

3.1. Ethanol Synthesis Based on Electrochemical Reduction

A carbon-neutral energy cycle is through the transformation of sunlight towards energy-dense fuels via electrolytic CO₂ reduction to fuels [17,219,220]. Based on the Nernst equation, electrochemical reduction potentials are translated into the reversible hydrogen electrode (RHE) range:

E_RHE = E_Ag/AgCl + 0.059 × pH + E⁰_Ag/AgCl,

(3)

where the potential, E_RHE vs. RHE, E⁰_Ag/AgCl = 0.198 V at 25 °C, and the potential measured, E_Ag/AgCl vs. the reference electrode, Ag/AgCl.

The conversion of electrochemical CO₂ into ethanol involves a set voltage flow with steady or unnoticeable current conditions. This process determines the best voltage to convert CO₂ into ethanol via electrochemical synthesis easily. The conversion of CO₂ into ethanol is a nonspontaneous reaction (E⁰ = negative) that requires an external voltage source from the power supply. A predetermined voltage flow with stable or imperceptible current states is used to convert electrochemical CO₂ into ethanol. This process makes identifying the appropriate voltage for the conversion of electrochemical CO₂ into ethanol easy. The conversion of CO₂ into ethanol is a nonspontaneous reaction (E⁰ = negative) that needs a power supply voltage source. Splitting the process into two separate electrochemical stages is used to pursue ethanol synthesis. The intermediate product in the assembly cascade technique should be a stable species that can be easily isolated from the initial electrolyte. CO is chosen as the stable intermediate product from the start, as evidenced by CO₂ electroreduction at excessive faradaic efficiencies:

CO₂ + H₂O + 2e⁻ → CO + 2OH⁻   E⁰ = −0.10 V vs. RHE

(4)

The poor solubility of CO causes the easy separation of intermediate products for transfer to the second-stage electrolyzer, leading to excessive current density due to the difficulty of the CO reduction process. Han et al. [221] concluded a feasible approach to solve the restricted CO coverage and deficiency in CO solubility in the catalytic position is to develop a cocatalyst for the formation of CO and reduction of CO₂ in the electrocatalytic reaction. Thus, the catalyst system can be prepared by coupling two sites, where one site efficiently reduces CO₂ to CO, which further distributes to the construction of C–C coupling in the formation of long carbon chain species that occur on the other sites of coupling. Yuan et al. [222], Kou et al. [223], and Ramírez-Valencia [224] reported that the CO-producing site’s pyridinic N-doped carbon species components show excellent performance selectivity and high catalytic accomplishment for the reduction of CO₂ to CO. The development of N-doped porous carbon components influences the electronic order and size of the Cu catalyst and further improves the gas transport for enhanced availability to pyridinic N during the process of adsorption of CO₂ and reduction of CO. The electrocatalyst reaction for the direct transformation of CO₂ into ethanol demonstrates competitive faradaic efficiencies but prefers high current densities, low overpotentials, and poor selectivity with the long-term stability of the operation [225,226,227].

The overall cathode half-reaction for ethanol formation is as follows:

2CO₂ + 9H₂O + 12e⁻ → C₂H₅OH + 12OH⁻   E⁰ = 0.09 V vs. RHE

(5)

The advantage of the electrochemical synthesis of ethanol from CO₂ is the product selectivity generated on each electrode terminal. Then, the equipment and substance used are basic and have a low cost. The process is controllable and flexible with a safe and mild operating background and empowers the nonfossil energy from renewable energy sources with environmentally friendly coupling [224]. The kind of metal utilized on the electrode affects the electrochemical synthesis efficacy of converting CO₂ into ethanol. The electrocatalytic characteristics of metals employed as electrodes affect the transformation percentage of CO₂ and the distribution in overall compounds. The category of catalyst, reaction potential, properties of electrolyte solution, cell design, pH value, and reaction circumstances, such as temperature and pressure, influence the outcomes of electrochemical synthesis [228,229,230]. The variety of alkaline electrolytes ranging in pH from neutral to alkaline has shown the potential to improve C₂ products [231,232,233]. Some studies in the electrochemical synthesis operation for converting CO₂ into other chemicals revealed that the category of electrode used and the sensor preparation have a remarkable effect on the results produced [234,235].

In the electrochemical synthesis process, where the water oxidation reaction occurs, carbon is an inert electrode attribute that does not react when utilized as an anode. Carbon is not affected during electrochemical production because of its inert characteristics. Water is oxidized and becomes a source of protons and electrons because carbon is an inert compound, and the bicarbonate anion (HCO₃⁻) does not oxidize in water. The mechanism of the reaction of ethanol synthesis at the cathode involves 12 protons and electron transfer, which aid the process of ethanol formation at the cathode [17,232,236,237,238]. In the electrolytic CO₂ reduction, normal metallic electrocatalysts only generate the C₁ building block but copper (Cu) elements have been identified to catalyze the manufacturing of low hydrocarbons at reasonable excessive faradaic efficiency (FE). The transformation of CO₂ into multicarbon alcohols via multiple electron transfer reactions facilitates C–C coupling reactions to produce C₂ products, resulting in decreased system’s energetic competency and poor selectivity [221,239,240]. Various factors of the selectivity and activity, such as catalyst size, catalyst surface structure, catalyst oxidation state, structural morphology, crystallographic orientation, composition, type of electrolyte ions, pH, pressure, temperature, design of the electrochemical cell, and the existence and number of deficiencies (i.e., point fault, contamination, unorganized location, grain limits), enhance the catalytic performance of CO₂ electroreduction towards multicarbon products [230,239,241,242]. Nanocatalyst morphologies, supporting materials, nanograin boundaries, and catalyst surface changes can all have an impact on contrary reaction pathways.

Zhu et al. [243], Zhou and Yeo [244], and Chen et al. [245] believed that Cu-based catalyst arrays’ structure elevates local pH, which favors CO generation and C–C coupling to generate C₂ products. Zhang et al. [246] agreed that on the Cu surface, the conversion of CO₂ to form C–H is difficult because it requires several electron reductions, protonation, and C–C coupling reactions. The restructuring of Cu facet coordination has stabilized facets on metal surfaces under electrolysis conditions, promoting the production of hydrogen. Compared with a traditional H-cell, a gas-fed flow cell improves FE toward CO₂ reduction products [239,247]. Jung et al. [248] also highlighted the importance to generate C₂ products selectively, which is critical to regulate and maintain the morphology of amorphous Cu nanoparticles. Density functional theory (DFT) calculation has shown that Cu is the preferable electrocatalyst for the formation of C₂ products [249,250,251]. CO₂ activation and CO dimerization to form C₂ products are remarkably improved by the linkage between the functional surfaces of Cu. The outcome also reveals that using the Cu complex as a precursor is critical for excellent performance because the Cu catalyst generated via direct electrodeposition has substantially low efficiency. The combination of copper with other metals produces higher catalytic activity for converting CO₂ into ethanol than pure copper metal. This finding is proven by the combination of Pd–Cu nanoparticles [252], copper-modified boron-doped diamond [253], copper–cuprous oxide [249], copper surface with a family of porphyrin-based metallic complexes [254], copper–silver composites [255], and cuprous oxide nanocubes with silver (Ag) nanoparticles [256].

Table 2. Summary of the experimental procedures applying copper-based catalysts in the transformation of CO₂ into ethanol.

Catalyst	Electrolyte	Cell Configuration	Current Density (h)	Overpotential	Faradaic Efficiency (%)	Total Current Density (mA cm−2)	Reference
Cu	1.0 M KOH	Electrochemical flow cell	4	−0.58 V vs. RHE	46	200	[257]
	0.5 M KHCO3
Cu	0.1 M KBr	H-type glass cell	3	−1.10 V vs. RHE	23	170	[258]
Cu	1.0 M KOH	Two compartment electrochemical H-cell	16	−0.95 V vs. RHE	32	126	[259]
Cu/Ag	1.0 M KOH	Electrochemical flow cell	not available	−0.70 V vs. RHE	25	300	[260]
Ce(OH)x-doped-Cu	0.1 M KCl	Three-electrode electrochemical cell	6	−0.70 V vs. RHE	43	128	[261]
Cu	0.1 M KHCO3	Flow cell reactor	6	−0.60 V vs. RHE	40	200	[262]
Cu/Ag	1.0 M KOH	Flow cell reactor	2	−0.67 V vs. RHE	41	250	[263]
	1.0 M KHCO3
Cu	1.0 M KHCO3	MicroFlow® cell	20	−0.97 V vs. RHE	89	300	[264]
Cu	1.0 M KOH	Electrochemical flow cell	65	−0.71 V vs. RHE	90	520	[265]
FeTPP[Cl]/Cu	1.0 M KHCO3	Electrochemical flow cell	12	−0.82 V vs. RHE	41	124	[254]
N-C/Cu	1.0 M KOH	Flow cell reactor	15	−0.68 V vs. RHE	52	156	[266]
zCu/Ni-N-C	1.0 M KOH	Electrochemical flow cell	103	−0.70 V vs. RHE	62	415	[267]
Cu2O	1.0 M KHCO3	Gas diffusion electrode flow cell	10	−0.85 V vs. RHE	76	300	[268]
np-Cu/VO2	1.0 M KOH	Electrochemical flow cell	12	−0.80 V vs. RHE	38	102	[269]
ZnO/4Cu2O	1.0 M KOH	Electrochemical flow cell	not available	−1.0 V vs. RHE	50	140	[270]
Cu50/PTFE15	1.0 M KOH	Gas diffusion electrode flow cell	2	−1.85 V vs. RHE	47	200	[271]
Cu2O/Ag	1.0 M KOH	Gas diffusion electrode flow cell	not available	−1.18 V vs. RHE	73	243	[256]
Cu/C/PTFE	1.0 M KOH	Gas diffusion electrode flow cell	2	−1.0 V vs. RHE	76	250	[272]

shows a summary of the experimental procedure applying copper-based catalysts in the transformation of CO₂ into ethanol.

Despite advancements in the electrochemical CO₂ reduction process, creating highly operative and selective nanocatalysts for the electrochemical CO₂ reduction reaction remains a major issue. The Cu-based catalyst of the nanostructure is chemically unstable, which demonstrates various catalytic performances via different procedures of operations due to the uncontrolled facet of oxidation and is related to the alternate in facet chemistry. The local reaction environment is further altered by electron reduction and protonation, making it challenging to stabilize the nanocatalyst. Facet coatings are a common way to improve the strength of nanocatalysts but affect the Cu’s facet chemistry and its capability to convert CO₂ to form C–H hydrocarbon. MOFs are favorable support materials in stabilizing and improving the catalysts due to their electrical conduction. MOFs require pressured reactant supply and an outcome separation mechanism due to their porous nature. More experimental research should be conducted to stabilize Cu nanocatalysts for the discovery of their catalytically functional area and improved activity/selectivity. Other challenges include the effect of the poor transfer current densities causing the ineffective electron exchange rate of kinetics, the deactivation of electrodes, enormous overpotential (or low energy performance), restricting practical use, deficient selectivity of the product, which necessitates expensive separation processes, and technological commercialization.

3.2. Ethanol Synthesis from DME

The thermochemical approach to the production of ethanol from CO₂ via DME has two phases of reaction. The initial step is to make DME from CO₂ and H₂. Methanol synthesis and dehydration are the two phases in the traditional commercial DME synthesis from syngas, which includes CO and CO₂. However, a one-step experimental procedure on multifunctional catalysts is favorable due to its thermodynamic stability and operational cost-effectiveness [273,274]. DME is a cheap and bulk chemical with environmental acceptability, has high quality, and is an excellent replacement fuel for use in diesel engines [275,276,277]. DME is also used as a critical intermediary to bulk chemicals in the industrial sector by producing acetic acid, olefins, and hydrocarbons [278,279,280]. The commercialized process reaction of the DME synthesis reactor is based on the equation below.

CO Hydrogenation:

CO + 2H₂ ⇌ CH₃OH      ∆H⁰₂₉₈ = −90.8 kJ·mol⁻¹

(6)

CO₂ Hydrogenation:

CO₂ + 3H₂ ⇌ CH₃OH + H₂O    ∆H⁰₂₉₈ = −49.5 kJ·mol⁻¹

(7)

RWGS:

CO₂ + H₂ ⇌ CO + H₂O    ∆H⁰₂₉₈ = +41.2 kJ·mol⁻¹

(8)

Methanol dehydration:

2CH₃OH ⇌ CH₃OCH₃ + H₂O     ∆H⁰₂₉₈ = −23.4 kJ·mol⁻¹

(9)

Methanol catalysts, e.g., Cu/ZnO/Al₂O₃ (CZA), catalyze the reactions at Equations (6)–(8), whereas catalysts with acidic properties such as HZSM-5, zeolites or γ-alumina catalyze reactions at Equation (9) [281,282,283,284]. CZA is a dominant conventional catalyst in the DME reaction due to its capability to improve the catalytic performance and selectivity toward methanol production. Studies on the CZA catalyst for CO₂ hydrogenation to methanol for 720 h time-on-stream of the reaction demonstrated that the space–time output of methanol is reduced to 34.5% during long-term testing [285]. The addition of Zr in the CZA catalyst, which forms CuO/ZnO/ZrO₂/Al₂O₃ (CZZA) with HZSM-5, shows improvement stability with methanol production by reduction from 18.5% to 14.1% with more than 58.7% selectivity after 100 h of DME reaction [286]. The DME reaction by using CuZn/Al₂O₃ catalyst recorded optimum conditions at 250 °C and 40 bar, resulting in a methanol selectivity of 58% [287]. The optimum reaction condition for DME synthesis requires temperatures and pressures ranging from 200 °C to 300 °C and from 20 bar to 50 bar, respectively [288,289,290].

The direct production of DME in a one-step process involves the simultaneous completion of two stages of reactions, i.e., methanol generation (via CO₂ hydrogenation) and methanol dehydration to DME, in the same reactor by using hybrid/bifunctional catalysts in a closed system, avoiding the need for intermediate purification steps and transportation units to minimize the cost of operation [273,291,292]. The hybrid/bifunctional catalysts needed for direct DME production require the combination of metal sites with redox function properties for the selective CO₂ hydrogenation to methanol and acidic function for the transformation of methanol dehydration to produce DME. Based on the Le Chatelier principle, the high water content limits the production of methanol that occurs in the hydrogenation of CO₂. The dehydration reaction contributes to the production of water. If the reaction area is divided through the core-shell structure, the presence of water on metallic sites can be significantly limited [291,293,294]. Water molecules tend to be adsorbed on the surface of catalysts which deactivation of catalyst function by metal oxidation of the catalytic phase and constructs a faster metal sintering and destroys the structure of the acid catalysts blocking the production of methanol on the hydrogenation sites. Hence, an investigation is conducted to increase the stability of the catalyst implemented in the hydrogenation reaction. A remarkable improvement in catalytic stabilities has been recorded in In₂O₃/ZrO₂ [295], interlinkage of CuO–ZnO–ZrO₂ on the surface of the zeolite [296], zirconium-modified CZA [286], gallium nitride [297], Cu–Ho–Ga/$\gamma$-Al₂O₃ [298]_, Cu/ZnO/ZrO₂//H-FER 20 [299], and PdZn/TiO₂-ZSM-5 hybrid [300]. The introduction of membrane reactor technologies [292,301] and adsorbent material [288,289,302] has been proposed to limit the effectiveness of water in DME production. Additionally, stable acid sites are needed in DME production due to the presence of water because strong acidic sites catalyze secondary dehydration reactions that deposit carbon and form hydrocarbon [273,276].

The Cu-MOR@SiO₂ core−shell microcapsules catalyst in tandem with the ternary oxide CZA catalyst recorded the catalytic activity of DME conversion at 83.8% and ethanol selectivity at 48.7% over 50 h at 220 °C of reaction [303]. On the reaction of CZ@Cu-MOR microcapsule catalyst for 50 h at 400 °C, DME carbonylation converted to methyl acetate on the active sites of the zeolite subsequently hydrogenated the syngas to DME conversion, and ethanol selectivity of about 26.8% and 45.8%, respectively, was achievable [304]. In the optimal reaction condition of 24 h at 220 °C, the proximity effect in the two components of NMOR zeolite and CZA tandem catalysts exposed to syngas achieved a DME conversion of 66% along with ethanol selectivity of 43.4% [305]. The most challenging aspect of the direct production of DME from CO₂ by utilizing hybrid/bifunctional catalysts is ensuring the correct ratio with regulated metal and acid interaction that is required for methanol production and dehydration. The detriment of utilizing DME as an alternative fuel to diesel is due to its low viscosity, which causes a leak and component damage. Furthermore, DME has low heating point than diesel. Therefore, despite its higher energy performance, DME still requires fuel insertion every cycle of the reaction. DME also has low combustion enthalpy, low modulus of elasticity, and fuel tanks with low energy content. These disadvantages counteract the features of DME’s low boiling point, and a pressured system must be used to keep the fuel in a liquid condition.

3.3. Ethanol Synthesis Based on RWGS

One of the most promising methods for CO₂ consumption as a renewable system delivering feedstock for nonfossil fuel synthesis and important chemical processes is the RWGS procedure [306,307,308,309]. The hydrogenation of CO₂ is converted into hydrocarbons via the RWGS reaction, which is catalyzed in tandem and subsequently modified by the Fischer–Tropsch synthesis (FTS) mechanism, where the intermediate product of the reaction is CO before hydrocarbons or alcohols are formed [310,311].

CO₂ hydrogenation:

nCO₂ + (3n+1)H₂ → C_nH_2n+2 + 2nH₂O → ∆_RH_573K = −128 kJ·mol⁻¹

(10)

RWGS:

CO₂ + H₂ → CO + H₂O → ∆_RH_573K = 38 kJ·mol⁻¹

(11)

FTS:

nCO + (2n+1)H₂ → C_nH_2n+2 + nH₂O → ∆_RH_573K = −166 kJ·mol⁻¹

(12)

This process is also known as the CO₂-FTS mechanism, where the CO produced from the RWGS reaction is inserted into the *CH₃ or *CH₃(CH₂)_n generated from the CO-FTS mechanism to form methanol, ethanol, or other higher alcohol synthesis [312,313]. The RWGS reaction is an endothermic process favored at high temperatures, resulting in a high equilibrium conversion of CO₂ and performed at relatively low contact times [307,314]. The hydrogenation of the CO₂ reaction path is an inherent drawback because the FTS mechanism is an exothermic process that favors low temperatures [311,315]. Active sites for dissociating hydrogen and adsorbing CO₂ should be present in RWGS catalysts. The excellent performance of the precious metal-based catalyst in RWGS reactions has been recorded in a few studies due to their ability to dissociate hydrogen at low-temperature catalytic activities [316,317,318]. However, these catalysts are not suitable for industrial-scale promotion and application due to limitations of high prices and rare resources. In the RWGS reaction, Ni and Cu-based catalysts have demonstrated good activity and selectivity but are prone to sintering deactivation at high temperatures [318,319,320,321]. Transition-metal carbides are also favorable in RWGS reactions due to their dual functionality for dissociating hydrogen and C=O bond cleavages [322,323]. In ethanol production, Fe-based catalysts with the right combination of promoters, organized additives, or assistance form the active area of reaction. Alkali metals particularly Na and K elements have been identified as the most effective promoters of the catalytic performance of Fe-based by producing highly active for the FTS mechanism in improving the selectivity and CO₂ conversion during ethanol production [324,325]. Carbon support substances are natural support materials for Fe-based catalysts and show outstanding catalytic achievement for ethanol generation by enhancing selectivity and expanding the active dispersion phase in the FTS mechanism [326].

The catalytic performance of carbon support materials is based on the surface area, pore size, distribution, and pore structure [327,328]. Unfortunately, the use of Fe-based catalysts for CO₂ hydrogenation reactions produces highly toxic precursors and needs a long time of carbonization [326]. Cu-based catalysts in RWGS reactions have been extensively investigated due to their high stability, low cost, high-performance atmospheric pressure at very low temperatures, and excellent selectivity for CO [283,316,329,330]. When a considerable quantity of CO₂ in the feed is adsorbed on the surface, the oxidation and reduction processes of Cu-based catalysts demonstrate strong activity with a minimal number of undesirable products [283,331,332]. In the RWGS reaction, the hydrogenation of the CO₂ mechanism happens by using a Cu-based catalyst, with CO as an intermediate product before forming hydroxyl species on the surface, thus constraining the operative area for alcohol synthesis when CO₂ decomposes into water molecules. This process is the redox mechanism with the Cu-based catalyst involving CO₂ reduction, the rate-determining step, and active sites in RWGS reactions. CO₂ oxidizes into Cu⁰ to produce Cu⁺, which improves the CO selectivity by 10%, whereas H₂ reduces the Cu⁺ to form Cu⁰ to generate H₂O [306,333]. The investigation of the morphological effect shows that Cu/CeO₂ nanorods exhibit the highest CO₂ conversion compared with Cu/CeO₂ nanocubes and favor the strong link of metal–support interaction in generating a high density of oxygen vacancies under reducing conditions [334,335]. The rod-like morphology of CuO/CeO₂ demonstrates the highest catalytic activity and stability and achieves the thermodynamic equilibrium conversion at 350 °C [336]. In the RWGS reaction, the oxygen vacancies on the spinel oxide surfaces are vital in the adsorption and activation of CO₂ [337]. Based on the activation method, the adsorption of CO₂ on oxygen vacancies is the initial step of RWGS, which involves C=O bond cleavages under a high-temperature energy-driven process [338]. A study on the role of copper as a promoter has shown an indirect effect on catalyst activity. The study reported that the addition of Cu to the Mo₂C catalyst enhances the selectivity of CO yield [333]. The presence of Cu in MoO₃/FAU zeolite catalysts influences the reduction step of MoO₃ to MoO₂, thus improving the CO yield [332]. However, the major drawback of the Cu-based catalyst, which undergoes deactivation during the RWGS reaction because of poor thermal stability due to the fractional oxidation of the Cu metal, leads to the reduction in the surface area of the active sites and copper particle agglomeration at high temperatures. The effective metal stage and/or coke deposition are hampered by material sintering, which lowers the CO₂ transformation degree by restricting catalyst activity.

Some thermodynamic limits in RWGS reactions are present. The CO₂ reactant has the potential to damage the CO hydrogenation catalyst, and the water that is inevitably retained in the end product, generally 20–45% of the whole product, decreases product selectivity and catalytic activity. The endothermic nature of the RWGS reaction uses sophisticated catalysts that are frequently necessary to customize the cascade reactions, and a high temperature, typically above 300 °C, is required to drive these processes. Although methanol is used as an intermediary to make liquid hydrocarbon from CO₂ hydrogenation at high temperatures on some occasions, the end product has remarkable CO byproducts. The low activity and unstable C–C coupling formation in the FTS mechanism is another challenge in the CO₂ hydrogenation process that usually produces light hydrocarbons, particularly methane. Catalyst deactivation has been identified in the FTS mechanism by poising the catalyst in the presence of sulfur and nitrogen compounds, inactive metal support compound, hydrothermal sintering, and the formation of inactive catalytic phases as oxides.

3.4. Ethanol Synthesis Based on Catalytic Hydrogenation

Catalytic hydrogenation is one of the promising approaches to overcoming the obstacle in the chemical reduction activation of CO₂ [316,339,340]. The hydrogenation of CO₂ yields useful alcohols, such as methanol, ethanol, and higher alcohol, that have impressive energy density with broad applications to value-added chemicals, such as neat fuels, fuel additives, and raw chemicals [16,17,297,341]. However, due to a shortage of effective catalysts with excellent stability, the effective cleavage of the C–O bond, excessive strength barrier of C–C coupling, and generation of water as a byproduct in the process can simply inactivate several catalysts for CO₂ transformation, and the direct synthesis of ethanol via CO₂ hydrogenation is substantially more difficult than methanol synthesis [16,342,343]. The most efficient catalytic technique for producing ethanol directly from CO₂ should encourage partial CO₂ reduction, hydroxylation, and C–C bond formation at the same time [232]. According to theoretical investigations, minor catalyst effects improve CO₂ hydrogenation catalytic performance. The link between the structure and catalytic performance is established by regulating the catalyst structure of active sites, and constructing optimum catalysts is the most effective technique in managing carbon chain expansion with controlled alcohol arrangement [341,344]. The calcination process to synthesize the catalyst has also been identified to affect the performance of the catalyst. The maximum product yield of the CO₂ hydrogenation reaction is obtained from nickel(II) oxide supported on alumina and calcined at 700 °C with rod-like morphology and tiny crystallite size of nickel(II) oxide nanoparticles (12.7 nm) at facet (111) [345].

The fabrication of efficient heterogeneous Rh-based catalysts on TiO₂ nanorods should be beneficial in boosting ethanol selectivity due to a synergetic combination of surface hydroxyls and widely dispersed Rh nanoparticles. The use of promoters particularly Li and Fe is generally effective in enhancing the activity and ethanol selectivity by increasing the strength of adsorption of bridged-bond CO species and influencing the electronic condition of Rh [346]. Therefore, the RhFeLi/TiO₂ nanorod catalysts in CO₂ hydrogenation exhibit 30% ethanol selectivity, 15% CO₂ conversion, and stable performance for 20 h of operation. The effect on promotion is associated with the prominent density of the hydroxyl group on TiO₂ nanorods and the excessive distribution of Rh elements. Hydroxyl groups have been demonstrated to equalize the protonation of methanol and formate compounds, which are efficiently detached to form *CH_x. The production of CO from the RWGS reaction is then introduced to construct CH₃CO*, which is hydrogenated to further produce ethanol [347]. Rh-based catalysts may catalyze CO dissociation and CO insertion over their atomically neighboring Rh⁰–Rhⁿ⁺ species, increasing the possibility of coupling between *CO and *CH₃ in the synthesis of C₂₊ oxygenates from syngas, such as ethanol, acetic acid, and acetaldehyde [348]. The strong interaction in Rh/TiO₂ catalysts demonstrates excellent steady-state activity with 40% ethanol conversion at 120 min of reaction. The transformation of TiO₂ nanotubes into the anatase structure due to the acceleration of Rh nanowires has a remarkable impact on catalytic effects. The positive charge on Rh activates the CO₂ hydrogenation and promotes the further decomposition of formate intermediates [349]. Simulations through DFT reveal that the ionic liquid connects to the Rh species on TiO₂ with a binding energy from 0.69 eV to 1.19 eV. The turnover frequency (TOF) of the stabilized single atom Rh/TiO₂ is 800 h⁻¹ in styrene hydroformylation and potentially recycled for five runs under harsh reaction conditions [350].

The quantity of vanadium oxide loaded and promoted on Rh-based catalysts enclosed in mesopore MCM-41 (Rh-0.3VO_x/MCM-41) has been demonstrated as extremely promising for ethanol synthesis, with CO₂ transformation and ethanol selectivity of 12% and 24%, respectively. This result is contributed by the equilibrium amount of CO dissociative adsorbed with nondissociative adsorbed affecting the yield and selectivity of ethanol synthesis. The electrical effect is thought to be responsible for the creation of Rh⁺ species and the construction of interfacial VO_x–Rh active sites, which dissociate CO into *CH_x and aid in the synthesis of ethanol following CO introduction [351]. The 2K20Fe5Rh–SiO₂ catalyst in CO₂ hydrogenation has recorded 16% ethanol selectivity and 18% CO₂ conversion for 6 h of stability. The presence of K as a promoter in this experiment stabilizes the CO intermediate produced and C–H bond formation during CO₂ hydrogenation [346,352]. Rh10Se clusters supported on TiO₂ (Rh10Se/TiO₂) and treated in a fixed-bed reactor at 350 °C show optimized selectivity to ethanol synthesis at 83% and CO₂ conversion of 27%. The strong electrical interaction between Rh10 and Se is thought to hinder methane production and boost ethanol synthesis on Rh sites by encouraging C–C bond formation via CH_x and carbonyl coupling on the surface to produce acetate substances [353]. The scarcity and high cost of Rh-based catalysts restrict further development. Table 3 shows a summary of CO₂ hydrogenation over homogeneous and heterogeneous catalysts.

Pd-based catalysts have shown promise because they aid in C–C coupling, a crucial step in the formation of C₂₊ molecules, and precisely modify nanoparticle composition, nanoparticle structure, and support materials [354,355]. As a result, numerous Pd-based catalysts for direct ethanol synthesis from CO₂ hydrogenation with good selectivity in an autoclave reactor at a very high reaction temperature (>250 °C) and confined to standard disordered architectures have been created [313,347,356]. Ordered catalysts with considerable interaction with active sites have been proposed to increase charge transfer and regulate electronic effects. Other disordered catalysts may be outperformed by such a catalytic system [357,358]. Within 5 h of the experiment, the Pd₂/CeO₂ nanorod catalyst with a unique two-atom geometric feature arrangement enables the facile cleavage of the C–O bond and efficiently contributes to C–C coupling, yielding 99.2% ethanol selectivity, 9.2% CO₂ conversion, and TOF value of 211.7 h⁻¹. DFT results show that Pd dimers bind CO tightly, preventing CO desorption and forming the precursor of ethanol via connecting CO and CH₃ intermediates [343]. Similar to precious metal group materials, the Pd-based catalyst is scarce and expensive, thus hindering its large-scale application.

At 200 °C for 5 h, the Ir₁–N₂O₃ single-atom catalyst displays exceptional efficiency for CO₂ hydrogenation with 99% ethanol selectivity and a TOF value of 481 h⁻¹. The isolated monoatomic Ir atom interacts with the neighboring oxygen vacancy on In₂O₃ to create a Lewis acid-base pair. This phenomenon generates two independent catalytic centers that reduce CO₂ into active intermediate species of carbonyl (CO*) adsorbed on the Ir atom and subsequently contribute to the C–C coupling to ethanol production [344]. However, the scarcity and high cost of In-based catalysts render them unsuitable for practical application. Co-based catalysts may generate a high selectivity of alcohol products due to excellent CO insertion ability and catalyze C–C coupling. The synergistic impact of facets Co⁰ and Co^δ+ provides the remarkable achievement of the Co/La–Ga–O composite oxide catalyst in CO₂ hydrogenation, with 9.8% CO₂ conversion, 74.7% ethanol selectivity, and 88.1% ethanol [359]. The introduction of nickel into a Co-based catalyst exhibits high activity and selectivity in forming ethanol by CO₂ hydrogenation. The optimized Co_0.52Ni_0.48AlO_x catalyst has recorded ethanol synthesis of 85.7% at 200 °C for 12 h on stream. This catalyst also shows high stability by remaining unchanged metal nanoparticle size and composition five times, whereas the selectivity of ethanol is maintained [360].

The study of the catalytic achievement of Na–Co/SiO₂ catalyst at 250 °C, 5 Mpa, gas hourly space velocity (GHSV) of 4000 h⁻¹ for 300 h of reaction recorded 62.8% ethanol selectivity and 18% CO₂ conversion. The CO generated in the Co₂C active phase is injected into CH_x intermediates, leading to the production of ethanol according to in situ DRIFTS data [313]. The interaction between Na and Co₂C produces ethanol efficiently with a CO₂ conversion of 53%. The electronic environment of Na–Co₂C active sites and the effects of CO activation are revealed using DFT calculations. The Bader charge analysis revealed that Na is a cation on the Co₂C surface with a Bader charge of 0.78 e, suggesting electron transfer from Na to Co₂C and the presence of contact between Na and Co₂C (Figure 4a). When CO₂ is adsorbed (Figure 4b), the most charge transferred and the stable adsorption structure on Na–Co₂C active sites reveal that the O–C–O bond of CO₂ is bent from linear in gaseous to 122.4, in which C and two O atoms contact with 2 Co atoms and 1 Na atom, showing that Na–Co₂C allows for easy adsorption and activation [361].

The energy barriers of transition states on the Na–Co₂C surface are higher than that on the Co₂C surface regardless of CO-direct or H-assisted dissociation. A large number of alkyl species (CH_x) formed on the Co₂C surface (formate species or CO hydrogenation) and the adsorption energies of CH_x species are constant on Na–Co₂C functional surface area. The increase in CO non-dissociative adsorption improves the CO/CH_x ratio, which is conductive and allows for easier CO activation and coupling on Na–Co sites into the adjacent CH_x on Co atoms to synthesize ethanol [361]. The use of CoMoC_x catalyst prepared using the ionic liquid method as all-in-one precursors at 800 °C carbonized temperature results in the optimal catalytic performance of 97.4% ethanol selectivity for 6 h of the experiment. This catalyst also shows high stability performance in the seventh run without remarkable deactivation. This study also found that the most electron transfer and the largest shift towards the low binding energy occur at 180 °C and 2 Mpa and detected CO gas as a byproduct of the reaction. Water, a green solvent used in this reaction, is combined with CO₂ to form bicarbonate and accelerate CO₂ conversion [362]. However, the conventional Co₂C phase under H₂ above 220 °C usually suffers from the rate of deactivation in the long-term catalyst performance. The uncontrollable synergism between CO dissociative activation and CO insertion of metallic Co-based catalysts generates the high selectivity of methane in CO₂ hydrogenation.

Fe-based catalysts possess the ability of CO dissociation catalyzed by an operative composition, and the catalytic activity can be constructed in the formation of hydrocarbon products [363,364]. The improvement of the CO insertion procedure and match the alkyl species formed on Fe sites to make alcohol products, a Fe-based catalyst combined with a noble metal is required [347,356,365]. The catalytic activity of FeMnNa catalyst under experimental conditions of 340 °C, 2.0 Mpa, and CO₂/H₂/Ar ratio of 24/72/4 has resulted in 35% CO₂ conversion and 31.7% ethanol selectivity. The temperature-programmed reduction analysis has also recorded excellent performance of the FeMnNa catalyst by promoting the formation of MnCO₃ from MnO in the presence of CO₂ and indicated that the Na component hinders the synergy between Fe and Mn in the reduction of FeMnNa catalyst for selective CO₂ hydrogenation to form ethanol [366]. The monometallic Fe-based catalyst co-modified with Na and S (FeNaS-0.6) achieves 16% ethanol selectivity, 32% CO₂ conversion, and high stability performance of over 100 h of the evaluation with no methanol formation [367]. In the CO₂ hydrogenation process, the sulfur present in the sulfate and its electron-withdrawing effects on Na-assisted Fe sites contribute to CO dissociation, nondissociative CO adsorption, boosting the hydrogenation barrier of *CH_x compound and improving the production of ethanol [23,366].

The development of Mo-based catalysts has opened the door to a new strategy in the hydrogenation of CO₂, which promotes the C–C coupling in ethanol synthesis. The deposition reaction of atomic operative elements Rh and K successfully synthesizes the one-dimensional b-phase of Mo₂C nanowires with specified crystal facets (101). The modification of the K_0.2Rh_0.2/b-Mo₂C complex catalyst results in a prominent production of 33.7 µmol·g⁻¹·h⁻¹ ethanol and ethanol selectivity of 72.1% at 150 °C [368]. Cu-based catalysts have been extensively studied in the hydrogenation of CO₂ to form ethanol products from syngas although their activity and selectivity are highly dependent on the support and promoter. Through local arrangement and fine-tuning of the catalytic centers by alkali promoters, the Zr12-bpdc-CuCs catalyst demonstrates ethanol synthesis with 99% selectivity in a 10 h assessment. With the help of alkali–metal promoters, the Cu-based catalyst facilitates H₂ activation and promotes direct C–C coupling and formyl species to offer an electron-rich environment for Cu-based catalysts and boost the stability and activity of a formyl intermediate [369]. Alkali promoters (K, Rb, and Cs) influence the achievement of CO₂ hydrogenation reactions made of precipitated iron-based catalysts; 1.5 Cs boosted catalyst has the best steady-state conversion stability of all the catalysts. Results show that these promoters have a synergistic impact that may result in improved CO₂ hydrogenation catalysts if balanced [370].

Table 3. Summary of CO₂ hydrogenation over homogeneous and heterogeneous catalysts.

Catalysts	Reactor	Reaction Temperature (°C)	Pressure (MPa)	Mixed Gas Ratio	Time Reaction (h)	CO2 Conversion (%)	Ethanol Selectivity (%)	Ethanol STY (mmol g−1 h−1)	Brief Description	Ref
Na/Co2C	Fixed-bed	250	5	CO2/H2/N2 = 24.6/72.4/3	5	23.8	17.5	0.72	The active sites of Na/Co2C improve the CO2 and CO non-dissociative adsorption, then regulated the surface CO/CHx ratio to accelerate CO insertion in generating ethanol.	[361]
CoMoC	Fixed-bed	180	2	CO2/H2 = 1/3	6	n.d	97.4	0.53	The excellent stability of CoMoCx promotes the activation of H2 and CO2 and C-C coupling which is generated by the HCOO* and DMF species.	[362]
Pt/Co3O4	Fixed-bed	200	8	CO2/H2 = 1/3	15	n.d	82.5	0.42	Water protonates methanol followed by dissociation into CH3*, OH*, and H* (or H2O) species on the Pt/Co3O4 surface that promotes CH3*−CO coupling to form ethanol.	[353]
Cs/CuFeZn	Fixed-bed	330	5	CO2/H2/N2 = 24/72/4	3	36.6	20.7	1.47	The synergetic combination of Cu-Fe dual interfaces sites in the Cs/CuFeZn overrides methanol synthesis through a direct CO2 hydrogenation route via HCOO* intermediates.	[356]
Ir/In2O3	Fixed-bed	200	6	CO2/H2 = 1/5	5	n.d	99.7	0.99	The Ir/In2O3 reduced a Lewis acid−base pair between Ir and adjacent oxygen vacancy to form a distinct catalytic center, which reduces CO2 to active intermediates and facilitates the C−C coupling to form ethanol.	[344]
Cu/Co3O4	Fixed-bed	200	30	CO2/H2 = 1/3	2	13.9	15.2	1.87	The adjacent oxygen vacancy on the surface of CoO promotes the CH3O* intermediate dissociation is the rate-determining step for ethanol synthesis.	[371]
Co3O4	Fixed-bed	200	2	CO2/H2/N2 = 22/66/12	2	28.9	19.2	1.60	The metallic Co reduced from Co3O4 was the main activity site for CO2 hydrogenation by promoting the growth of the C−C coupling for the production of ethanol.	[372]
CoAlO	Fixed-bed	200	4	CO2/H2 = 1/3	15	n.d	92.1	0.44	The CoAlO is attributed to the formation of acetate from formate with the insertion of *CHx which is an important intermediate to produce ethanol from CO2 hydrogenation.	[373]
Pt/Co3O4	Fixed-bed	200	2	CO2/H2/N2 = 22.5/67.5/10	2	44.5	26.7	0.69	The synergic effect of Pt, Co nanoparticles, and oxygen vacancies of Co3O4 improved the adsorption of H2 and CO2 with stable CO2 conversion in the synthesis of ethanol.	[374]

Table 3. Summary of CO₂ hydrogenation over homogeneous and heterogeneous catalysts.

Catalysts	Reactor	Reaction Temperature (°C)	Pressure (MPa)	Mixed Gas Ratio	Time Reaction (h)	CO2 Conversion (%)	Ethanol Selectivity (%)	Ethanol STY (mmol g−1 h−1)	Brief Description	Ref
Na/Co2C	Fixed-bed	250	5	CO2/H2/N2 = 24.6/72.4/3	5	23.8	17.5	0.72	The active sites of Na/Co2C improve the CO2 and CO non-dissociative adsorption, then regulated the surface CO/CHx ratio to accelerate CO insertion in generating ethanol.	[361]
CoMoC	Fixed-bed	180	2	CO2/H2 = 1/3	6	n.d	97.4	0.53	The excellent stability of CoMoCx promotes the activation of H2 and CO2 and C-C coupling which is generated by the HCOO* and DMF species.	[362]
Pt/Co3O4	Fixed-bed	200	8	CO2/H2 = 1/3	15	n.d	82.5	0.42	Water protonates methanol followed by dissociation into CH3*, OH*, and H* (or H2O) species on the Pt/Co3O4 surface that promotes CH3*−CO coupling to form ethanol.	[353]
Cs/CuFeZn	Fixed-bed	330	5	CO2/H2/N2 = 24/72/4	3	36.6	20.7	1.47	The synergetic combination of Cu-Fe dual interfaces sites in the Cs/CuFeZn overrides methanol synthesis through a direct CO2 hydrogenation route via HCOO* intermediates.	[356]
Ir/In2O3	Fixed-bed	200	6	CO2/H2 = 1/5	5	n.d	99.7	0.99	The Ir/In2O3 reduced a Lewis acid−base pair between Ir and adjacent oxygen vacancy to form a distinct catalytic center, which reduces CO2 to active intermediates and facilitates the C−C coupling to form ethanol.	[344]
Cu/Co3O4	Fixed-bed	200	30	CO2/H2 = 1/3	2	13.9	15.2	1.87	The adjacent oxygen vacancy on the surface of CoO promotes the CH3O* intermediate dissociation is the rate-determining step for ethanol synthesis.	[371]
Co3O4	Fixed-bed	200	2	CO2/H2/N2 = 22/66/12	2	28.9	19.2	1.60	The metallic Co reduced from Co3O4 was the main activity site for CO2 hydrogenation by promoting the growth of the C−C coupling for the production of ethanol.	[372]
CoAlO	Fixed-bed	200	4	CO2/H2 = 1/3	15	n.d	92.1	0.44	The CoAlO is attributed to the formation of acetate from formate with the insertion of *CHx which is an important intermediate to produce ethanol from CO2 hydrogenation.	[373]
Pt/Co3O4	Fixed-bed	200	2	CO2/H2/N2 = 22.5/67.5/10	2	44.5	26.7	0.69	The synergic effect of Pt, Co nanoparticles, and oxygen vacancies of Co3O4 improved the adsorption of H2 and CO2 with stable CO2 conversion in the synthesis of ethanol.	[374]

However, remarkable drawbacks to CO₂ hydrogenation for alcohol syntheses, such as CO₂ activation problems, a high energy barrier for C–O bond scission, and the creation of C1 by-products, remain. As a result, designing effective heterogeneous catalysts for ethanol generation is critical. The morphology of catalysts, such as the particle size and dispersion of deposited metal particles, influences the optimization of the metal/oxide interface to improve CO₂ conversion and product selectivity. The collision theory explained by increasing the surface area of a reactant created by high dispersion increases the frequency of collisions and increases the reaction rate. Reducing metal particle sizes leads to a high fraction of low-coordinate surface atoms at locations, such as corners and edges, especially when the particle size is smaller than 2–3 nm [353,375,376,377]. The small particle size content high in the surface area that generates by high dispersion is available for particles to collide, leading to improved catalytic performance of the reaction. Appropriate reducible metal oxide supports, such as TiO₂ and ZrO₂, have been used extensively to tailor the particle size. The Au/TiO₂ catalyst has good selectivity for ethanol from CO₂ reduction in DMF solvent due to the abundance of oxygen vacancies. With the addition of water, the bimetallic Pd₂Cu/P25 catalyst produces a high yield of ethanol. Other studies indicated that by manipulating the particle size, the electronic state of Rh for alcohol production from CO₂ hydrogenation can be adjusted. In CO hydrogenation, a promotion strategy based on hydroxyl groups is an effective way to improve alcohol selectivity at a high conversion rate and a wide range of operating temperatures.

4. Future Direction and Perspective

Ethanol is a fundamental chemical product, an important solvent, an industrial building block, and a promising renewable fuel. The chemical equilibrium conversion of the hydration of ethylene decreases at high temperatures to increase the rate of reaction. The excessive amount of energy to heat gases generates high pressure and utilizes crude oil, which is a nonrenewable resource. Ethanol synthesis from biomass residue produces a huge amount of CO₂ with a high cost of operation because of expensive machinery and fuel. The ethanol production from total sugars in lignocellulosic materials is inhibited by the action of pentose sugars, which are not fermentable by the brewer’s yeast, i.e., S. cerevisiae, during the hydrolysis of hemicellulose. Although the fermentation route has been commercially realized, the cost of operation for this process is expensive due to the energy-intensive distillation steps at high dilution rates, which allow for high productivities, and the incomplete utilization of substrate, resulting in low yields to meet the market demand. Creating highly operative and selective nanocatalysts for the electrochemical CO₂ reduction reaction remains a major issue in the electrochemical CO₂ reduction process. DME is not suitable as an alternative fuel to diesel due to its lower viscosity, which causes leaking and component damage. The RWGS reaction uses sophisticated catalysts that are frequently necessary to customize the cascade reactions and require a high temperature (typically above 300 °C) to drive the processes still experiencing difficulty; low activity and unstable C–C coupling formation in the FTS mechanism is another challenge in the CO₂ hydrogenation process that are usually producing light hydrocarbons, methane in particular. The heterogeneous catalytic CO₂ hydrogenation approach is one of the initiatives to reduce CO₂ emissions as the source of GHG, which prompts the climate change or global warming that currently shows an effect worldwide, into the numerous high economic value-added chemicals and easily marketable fuel additives that have made substantial progress to explore as new concepts and opportunities for industrial manufacture. The hydrogenation of CO₂ into C₂₊ products occurs via a methanol-mediated route or modified FTS mechanism that involves two steps of the reaction, and these routes are distinguished by the intermediate’s product. In the methanol-mediated route, CO₂ is hydrogenated into methanol and then converted into hydrocarbons, whereas CO₂ is reduced to CO via RWGS followed by chain propagation via a modified FTS mechanism. A recent development showed that the direct catalytic hydrogenation of CO₂ into ethanol in a single reactor is one of the promising strategies based on economic potential and energy efficiency. The formation of *CH_x is the rate-determining step in this process because the *HCOO generation and coupling steps are known to be accelerated in the CO₂-to-ethanol transformation. The reaction mechanism is also closely associated with multiple catalytic active sites controlling every elementary reaction over catalysts. Thus, catalysts for optimum active sites should be designed and prepared to improve CO₂ conversion and ethanol selectivity. The formation rate of ethanol increases with increasing reaction temperature because thermodynamic equilibrium is still not reached. However, many challenges, e.g., the control of the ratio of multiple active sites, regulation of interface sites, and device of catalyst structures need to be explored in future research to improve the conversion and selectivity in ethanol production.

5. Conclusions

In this exploration, we discuss current research discoveries in the improvement of technologies and operation procedures in ethanol production. The catalytic hydrogenation of CO₂ promises development direction in the production of ethanol and reduces environmental pollution problems. The limitations of CO₂, i.e., a fully oxidized, chemically inert and thermodynamically stable molecule, should be considered in designing the research because its conversion into chemicals requires large amounts of energy and H₂. The production of ethanol reduces CO₂ emissions globally, but reducing the dependence on conventional gasoline shows a decreasing pattern of production every year because ethanol has been identified as an ecological fuel due to its nontoxicity, accumulation of high oxygen content to promote better combustion with reduced exhaust emissions, and high-octane rating to giving high resistance to engine knock. The outcomes of this review will play a crucial and interesting role in further research development in providing the latest proposed approach for effective CO₂ hydrogenation to promote C–C coupling in ethanol production.