Sustainable routes for acetic acid production: Traditional processes vs a low-carbon, biogas-based strategy

Review Sustainable routes for acetic acid production: Traditional processes vs a low-carbon, biogas-based strategy Juan Luis Martín-Espejo a , Jesús Gandara-Loe a , José Antonio Odriozola a,b , T.R. Reina a,b , Laura Pastor-Pérez a,b, ⁎ a Department of Inorganic Chemistry and Material Sciences Institute of Seville, University of Seville-CSIC, Seville 41092, Spain b Department of Chemical and Process Engineering, University of Surrey, Guildford GU2 7XH, United Kingdom HIGHLIGHTS •Biogas conversion to acetic acid represents a circular economy route for chemicals manufacturing. •Two new BIO-strategies are proposed to obtain acetic acid from CO 2 and CH 4 . •The implementation of plasma technology in dry reforming represents a step-ahead on carbon-neutral processes. •The state-of-the-art of lab-scale nonthermal plasma dry reforming to valueadded products has been reviewed. GRAPHICAL ABSTRACT ABSTRACTARTICLE INFO Editor: Huu Hao Ngo The conversion of biogas, mainly formed of CO 2 and CH 4 , into high-value platform chemicals is increasing attention in a context of low-carbon societies. In this new paradigm, acetic acid (AA) is deemed as an interesting product for the chemical industry. Herein we present a fresh overview of the current manufacturing approaches, compared to potential low-carbon alternatives. The use of biogas as primary feedstock to produce acetic acid is an auspicious alternative, representing a step-ahead on carbon-neutral industrial processes. Within the spirit of a circular economy, we propose and analyse a new BIO-strategy with two noteworthy pathways to potentially lower the environmental impact. The generation of syngas via dry reforming (DRM) combined with CO 2 utilisation offers a way to produce acetic acid in a two-step approach (BIO-Indirect route), replacing the conventional, petroleum-derived steam reforming process. The most recent advances on catalyst design and technology are discussed. On the other hand, the BIO-Direct route offers a ground-breaking, atom-efficient way to directly generate acetic acid from biogas. Nevertheless, due to thermodynamic restrictions, the use of plasma technology is needed to directly produce acetic acid. This very promising approach is still in an early stage. Particularly, progress in catalyst design is mandatory to enable low-carbon routes for acetic acid production. Keywords: Acetic acid Biogas Dry reforming Catalysis Low-carbon chemicals Non-thermal plasma Contents 1. Introduction................................................................ 2 2. Aceticacid:conventionalroutesandmarketreview............................................... 3 2.1. Applicationsinsightandmarketoverview................................................ 3 2.2. Conventionalroutesforaceticacidproduction.............................................. 3 2.2.1. Oxidationofacetaldehyde .................................................. 3 2.2.2. Oxidationofhydrocarbons.................................................. 3 Science of the Total Environment 840 (2022) 156663 ⁎Corresponding author at: Department of Inorganic Chemistry and Material Sciences Institute of Seville, University of Seville-CSIC, Seville 41092, Spain. E-mail address: [email protected] (L. Pastor-Pérez). http://dx.doi.org/10.1016/j.scitotenv.2022.156663 Received 24 February 2022; Received in revised form 9 May 2022; Accepted 9 June 2022 Availableonline13June2022 0048-9697/© 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv 2.2.3. Methanolcarbonylation ................................................... 5 2.3. Researchprogressandalternativerouteproposed ............................................ 6 3. BIO-Indirectapproach............................................................ 7 3.1. Biogasdryreforming......................................................... 7 3.2. ThermocatalyticDRM ........................................................ 8 3.3. PhotocatalyticDRM......................................................... 9 3.4. Plasma-catalyticDRM ....................................................... 10 3.4.1. HybridNTPDRM..................................................... 10 4. BIO-Directapproach ........................................................... 12 4.1. State-of-the-artofplasma-assistedaceticacidproduction........................................ 12 5. Concludingremarksandoutlook...................................................... 14 CRediTauthorshipcontributionstatement .................................................... 14 Declarationofcompetinginterest........................................................ 15 Acknowledgements .............................................................. 15 References.................................................................. 15 1. Introduction In the last two centuries, greenhouse gas (GHG) emissions have significantly increased due to anthropogenic sources. Both CO 2 and CH 4 are considered key contributors of these GHG emissions, representing >90% of the total anthropogenic emissions to the atmosphere (International Energy Agency, 2018). In particular, CO 2 discharges add up to three-fourths of the total anthropogenic GHG emissions. Furthermore, the adverse impact of releasing CH 4 should also be taken into account since its global warming potential (GWP) is 25 times higher than CO 2 (Boucher et al., 2009). Emissions are increasing year by year and some scenarios still project net increments in the near future (International Energy Agency, 2015). Therefore, there is much to be done to meet the target agreed on “Glasgow Climate Pact”about global emissions and temperature rise (United Nations Framework Convention on Climate, 2021). Effective measures, such as CO 2 capture and utilisation (CCU) strategy, are needed to mitigate the environmental impact of GHG emissions. Most of the GHG emissions come from the consumption of fossil fuels for energy. Notwithstanding, it is less known the relevant contribution of the agriculture, livestock and other organic waste sector, as observed in Fig. 1. Organic waste (e.g., sewage sludge, manure, organic industrial/municipal waste, etc.) greatly contributes to the GHG emissions to the atmosphere due to anaerobic digestion (IPCC Fourth Assessment Report, 2014). These emissions, mainly in the form of biogas (CH 4 and CO 2 ), were considered a waste rather than a value, but this trend is changing since biogas is now considered a renewable asset. Nowadays, the biogas industry business model is based on two main strategies: (i) the direct conversion into calorific energy and (ii) the production of renewable fuels. These two approaches are not completely sustainable since upgrading of biogas is imperatively required to remove part of the CO 2 , thus not achieving the desired carbon neutrality (Le Saché et al., 2019;Navarro Puyuelo et al., 2017). In addition, biogas plants rely on government subsidies due to their lack of competitiveness in the market (Brémond et al., 2021). As an alternative to the current biogas industry business model, new opportunities are blooming through the carbon cycle fixation strategy. The reforming of biogas to fine, value-added chemicals (e.g., acetic acid, methanol, olefins, ammonia, etc.) is deemed a promising way to mitigate GHG emissions, offering an interesting alternative to simply burn it (Liu et al., 2020;Navarro Puyuelo et al., 2017). Biogas upgrading via chemical transformation is an underexplored field which may provide many possibilities for the chemical industry to tackle GHG emissions (Navarro Puyuelo et al., 2017;Puliyalil et al., 2018). Acetic acid (AA) can be an interesting intermediate product generated from biogas which can be later transformed into other valuable products (Tu et al., 2021;Wang et al., 2017a). Traditionally, acetic acid is industrially produced via an indirect route, carbonylation of methanol, using syngas (primarily formed by CO and H 2 ). Globally, three main environmental drawbacks are found in this synthesis pathway: a) it is an energy-intensive process, b) syngas is mainly produced from fossil fuels (e.g., carbon and natural gas) and c) environmental impacts coming from many chemical waste discharges (Cheung et al., 2000;Kalck et al., 2020). Research efforts are now concentrated on promoting alternatives to this process. Furthermore, the current tendency in manufacture is claiming a shift to more sustainable chemical routes to lessen the environmental impacts to the atmosphere and hydrosphere of these large-scale industrial processes. Therefore, the alternative BIO-routes proposed, using biogas as source, could be considered attractive. The general comparison of the traditional route and the alternatives proposed is shown in Fig. 2. Dry reforming of methane (DRM) can be used to produce syngas from biogas. This process, however, faces a series of disadvantages: it is an energy-intensive process (Bouchoul et al., 2021) and suffer for catalyst's deactivation. Despite the considerable positive environmental potentials, DRM cannot be considered an industrially mature process yet since the development of a robust catalyst is still a challenge (Abiev et al., 2020; Bouchoul et al., 2021;George et al., 2021). Even though, the incorporation of biogas-DRM in acetic acid route represents an innovative low-carbon path taking the edge over the traditional process in terms of sustainability. On the other hand, chemically and stoichiometrically, the direct, one-pot transformation of CH 4 and CO 2 into acetic acid (Eq. 1) is a very appealing atom-efficient process (BIO-Direct route). Nonetheless, this one-step process proposed is quite unfavourable thermodynamically (Liu et al., 2001; Tu et al., 2021). CO2þCH4!CH3COOH ΔG 298 K¼71:0kJmol1(1) Fig. 1. Global GHG emissions by economic sector, based on the latest IPCC Fourth Assessment Report (2014). J.L. Martín-Espejo et al. Science of the Total Environment 840 (2022) 156663 2 An ingenious way to overcome these thermodynamic limitations is the use of a non-conventional technology, i.e., non-thermal plasma (NTP). Currently, NTP is considered an enabling green technology for the near future of energy sector inasmuch as it may allow chemical processes to be based on renewable energy sources instead of fossil fuels (Abiev et al., 2020). Indeed, the combination of NTP and a robust catalyst, known as plasmacatalysis or hybrid plasma catalysis, offers a great potential to optimise reaction conditions for the production of acetic acid (Abiev et al., 2020; George et al., 2021;Li et al., 2020). In this regard, this review frames the new potential opportunities emerged from this cutting-edge strategy herein proposed, the BIO-routes, in contrast to the current position of the chemical industry to produce acetic acid with a high greenhouse gases emission penalty. The historical advances of the industrial production of acetic acid are scrutinised to put in context the current situation. In opposition to this, the BIO-strategy is presented due to the increased environmental concern, focusing on the catalytic performance of the most recent, relevant lab-scale catalytic systems. Specifically, the most promising catalysts are revised in this review. As part of the BIO-strategy, the core of this review, non-thermal plasma is raised to be an enabling technology in order to produce acetic acid, reviewing the most innovative catalytic systems implemented up to now. 2. Acetic acid: conventional routes and market review Prior to understanding the main chemical routes for synthesising acetic acid, it is relevant to comprehend the importance of this intermediate chemical compound in the chemical industry. 2.1. Applications insight and market overview Acetic acid is considered a commodity chemical for a wide range of enduser industries (textile, fibre, pharma, foods, etc.) (Cheung et al., 2000; Deshmukh and Manyar, 2021). The broad spectrum of applications is outlined in Fig. 3. More than 65% of acetic acid production participates in the production of polymers derived from the vinyl acetate monomer (VAM) or cellulose acetate. Poly (vinyl acetate) produced is mainly used as a precursor for paints and coatings, or the production of plastics. VAM is one of the principal additives in the polymer industry, employed as an emulsifier, resin, or intermediate for surface coating, acrylic fibre and polymer wires production. On the other hand, cellulose acetate is used to produce acetate fibres (Budiman et al., 2016;Pal and Nayak, 2017). Another prominent application is the production of acetic anhydride (Cheung et al., 2000;Deshmukh and Manyar, 2021). According to a recent study (Mordor Intelligence, 2020), the 2020 updated global market share by application shows that VAM is the main application of acetic acid, followed by terephthalic acid (TPA), acetate anhydride and acetate esters. On the other hand, the global demand for virgin acetic acid was estimated to be 16.1 million tonnes in 2020, and it is projected to reach 19.6 million tonnes by 2027. Global market analysis forecast a compound annual growth rate (CAGR) around 3% for the period 2020–2027 (IMARC Group, 2021;Zion Market Research, 2021). The champion EU producer of acetic acid is BP, producing 2.5 million tonnes/year in multiple facilities worldwide. Other relevant producers are BASF, Chiyoda, Celanese, Dow Chemical, INEOS, LyondellBasell and China Petrochemical (Budiman et al., 2016). All these players produce acetic acid based on different technologies, depending on the route followed. The conventional routes for acetic acid synthesis should therefore be reviewed. 2.2. Conventional routes for acetic acid production Different chemical processes have been developed in order to produce acetic acid. Traditionally, thermocatalytic routes have been predominant in the chemical industry, prevailing methanol carbonylation (Cheung et al., 2000;Kalck et al., 2020). Nevertheless, there are other routes for large-scale production of acetic acid and are briefly discussed. 2.2.1. Oxidation of acetaldehyde In this process, petroleum stock-derived acetaldehyde is oxidised to produce acetic acid in the presence of heterogeneous manganese or cobalt acetate catalysts (Eqs. 2 and 3). This process, whose yield exceeds 90%, was widely extended before the discovery of carbonylation of methanol. The use of an organo‑mercury catalyst produced an enormous environmental impact due to the toxicity of this material, limiting this route to other alternatives (Pal and Nayak, 2017). C2H4þ1=2O2!CH3CHO (2) CH3CHO þ1=2O2!CH3COOH (3) 2.2.2. Oxidation of hydrocarbons The direct synthesis of acetic acid by oxidation of hydrocarbons is recognised as a viable route. It is reported that approximately 9% of acetic acid produced worldwide is synthesised by the oxidation of hydrocarbons. Nevertheless, downstream separation is a complex and energy-consuming process due to its low selectivity. Despite that, direct oxidation processes have been commercialised, for instance, using naphtha, by BP, or using nbutane, by Celanese (Sano et al., 1999). Direct liquid-phase catalytic oxidation of butane is one of the most preferred routes to acetic acid owing to the low cost of these hydrocarbons. This process undergoes at the critical point of butane (152 °C and 38 bar) (John Wiley &Sons, Inc., 2000), but it fails at given selectively acetic acid. Even though, it is an alluring process for some applications (Budiman et al., 2016;Cheung et al., 2000). On the other hand, direct vapour-phase catalytic oxidation of ethylene has been of interest to produce acetic acid due to the small generation of non-toxic wastes. This one-step process comprises a single main reaction (Eq. 4) and two side reactions (Eqs. 5and6). Its energy-intensive purification is due to the large amount of water required and the number of byproducts formed (Budiman et al., 2016). To solve the main problems, Showa Denko K.K. has patented a water-free process by combining a palladium-based catalyst with heteropolyacids. An energy-saving strategy by combining extraction and distillation is included in the intellectual property. This chemical process meets both sustainability requirements and competitiveness. It is reported that little non-toxic waste is generated Fig. 2. Traditional acetic acid production (right-sided) versus alternative BIO-routes to acetic acid production from biogas (left-sided). J.L. Martín-Espejo et al. Science of the Total Environment 840 (2022) 156663 3 thanks to the high selectivity of the reaction, and small amounts of wastewater are produced. Another great advantage of this process is the use of standard materials since no corrosive compounds are treated except for acetic acid. This route is believed to be competitive with methanol carbonylation for small plants (100–250 kT year −1 ), subject to the prices of ethylene (Sano et al., 1999). C2H4þO2!CH3COOH (4) Fig. 3. Applications of acetic acid. [Adapted with permission from Cheung et al., 2000. Copyright 2002, Wiley-VCH Verlag GmbH &Co. KGaA]. J.L. Martín-Espejo et al. Science of the Total Environment 840 (2022) 156663 4 C2H4þ3O2!2CO2þ2H2O(5) C2H4þ1=2O2!CH3CHO (6) Another route is the partial oxidation of ethane (Eq. 7). Molybdenum‑vanadium-based catalysts, at 220–300 °C and 12–15 bar, have extensively been researched (Pal and Nayak, 2017). In spite of the research progress, conversion and selectivity are constrained due to oxygen limits in the process for safety reasons, since oxygen should be kept below the explosion limit (Budiman et al., 2016;Cheung et al., 2000). 2C2H6þ3O2!2CH3COOH þ2H2O(7) Despite some of these routes being interesting for acetic acid production, the market production, as mentioned before, is mainly focused on carbonylation of methanol, which represents approximately 80–90% of the total production (Shah, 2014). 2.2.3. Methanol carbonylation The liquid-phase methanol carbonylation to acetic acid, an exothermic reaction, shown in Eq. 8, is performed using methanol with an excess of carbon monoxide, coming from syngas. Thermodynamically, this route yields acetic acid under ambient conditions. However, an increase in temperature is needed due to the process kinetics, which obliges the operation at medium-high pressures to get high yields. CH3OH þCO !CH3COOH ΔH 298 K¼138:6kJmol1(8) Several homogeneous metal-organic complexes have been used industrially for the carbonylation of methanol, such as cobalt, rhodium, ruthenium or iridium-based homogeneous catalysts. Such materials operate at temperatures ranging 150–300 °C and pressures below 60 bar (Budiman et al., 2016;Cheung et al., 2000). In general, the presence of high concentrations of water is needed since the catalyst solubility is enhanced, but this situation also facilitates the side water-gas shift (WGS) reaction, which competes with the carbonylation, thus resulting in lower CO uptake for reaction of interest (Haynes, 2006). Acetic acid was firstly commercialised via methanol carbonylation in 1960. BASF discovered that the presence of methyl iodide was necessary to convert methanol into acetic acid (Ludwigshafen and Friderich, 1957). In its absence, CO tends to insert in the O\\H bond of methanol, instead of the C\\Obond(Thomas, 2003). Eqs. 9, 10 and 11 show the main reaction steps in the process. CH3OH þHI !CH3IþH2O(9) CH3IþCO !CH3COOI (10) CH3COOI þH2O!CH3COOH þHI (11) This first approach was improved by the Monsanto process (Schultz, 1973) and the Cativa process (David et al., 1997). Monsanto, in the middle 60s, improved BASF process, obtaining excellent acetic acid selectivity (99%) under milder conditions, around 30–60 bar and 150–200 °C. The Monsanto process involves the use of homogeneous methyl iodidepromoted rhodium-based catalyst in the system, which improves the catalytic activity obtained with the cobalt-based catalyst of BASF. In this process, the most energy-demanding step is the production of CO and H 2 (syngas), used as an intermediate (Thomas, 2003). This step has been traditionally accomplished via steam reforming of natural gas, leading to a heavy carbon fingerprint which is a major environmental burden. The catalytic reaction mechanism of Monsanto process was proposed to be composed by six cyclic steps, as depicted in Fig. 4. Within the six main steps, the oxidative and selective addition of methyl-iodide to the rhodium complex [Rh(CO) 2 I 2 ] − (step 1) seemed to be the rate-determining step, which is consistent with the observed kinetics. This route operates under excess of water of 14–15 wt%, which is needed to (a) enhance the catalytic activity and (b) maintain the stability of the Rh-based catalyst. Indeed, this optimum water concentration avoids the formation and accumulation of inactive [Rh(CO) 2 I 4 ] − from the unstable intermediate [Rh(CO) 2 I 3 H] − and the posterior precipitation of inactive RhI 3 salts (Haynes, 2006;Kalck et al., 2020). In the late 60s, Monsanto also discovered that iridium was an effective catalyst for methanol carbonylation (Paulik and Roth, 1968). However, this catalytic system was not commercially developed by BP until the early 1990s, under the name of the Cativa process. Iridium clusters act as an active centre using ruthenium species as promoters in the presence of methyl iodide (Ren et al., 2020). The main advantages of this process are its superior stability and the operation using a lower amount of water (~5 wt%) (Sunley and Watson, 2000;Tu et al., 2021). Indeed, compared to rhodium, iridium precipitation IrI 3 is less common due to the stronger metal-ligand bonding for the third-row metal, which inhibits CO loss from the Ir centre (Haynes, 2006). The added [Ru(CO) 3 I 2 ] − complex can react efficiently the iodide from [MeIr(CO) 2 I 3 ] − to fasten the carbonyl coordination step, enhancing the reaction of methyl acetate and hydrogen iodide to acetic acid and methyl iodide. Cativa process is considered a more efficient and greener process than the preceding Monsanto process (Kalck et al., 2020;Ren et al., 2020). In general, the chemistry of BASF, Monsanto and Cativa processes is reported to be similar since the catalyst is promoted by a halogen promoter, methyl iodide or iodide acid (Cheung et al., 2000). Differently, Chiyoda/Universal Oil Products (UOP) and KBR have commercialised a process called Acetica process. Heterogenised homogeneous rhodium-based catalyst is used, showing high activity as well as long-term stability and insignificant rhodium leaching. The key to the improved features of this process is the use of a novel polymer resin, polyvinyl pyridine (PVP) resin, to immobilise the catalyst, which prevails and also facilitates the separation of the reaction mixture. It is reported that the catalyst activity is maintained after 7000 h and the water content is greatly reduced to 3–8 wt% (Haynes, 2006). A simplified flowsheet example for the aforementioned processes commercialised by BASF, Monsanto, Cativa and Acetica can be observed in Fig. 5. These processes are designed in three main sections: a liquidphase slurry reaction, a flash separation tank for the catalyst separation and a separation unit where pure acetic acid is finally obtained. The catalyst remains in the liquid phase and is recycled to the reactor. For all these processes, separation issues of homogeneous catalyst arise (Haynes, 2006; Thomas, 2003). More recently, in 2010, BP patented a new multi-step vapour-phase process for acetic acid production at a large scale, the so-called SaaBre process. Using the new BP strategy, the only raw material of the process is syngas (Budiman et al., 2016;Deshmukh and Manyar, 2020), industrially produced by steam reforming of natural gas (Cheung et al., 2000). This three-step scheme resembles the one proposed elsewhere by HaldorTopsøe (Joensen et al., 1997, 1998). The first step is a methanol synthesis by CO hydrogenation using syngas (Eq. 12), which can be further transformed into DME (Eq. 13). The second step consists of DME carbonylation (Eq. 14), obtaining methyl acetate (MA). Finally, intermediate methyl acetate is dehydrated and hydrolysed to an equal molar acetic acid and methanol (Eq. 15). The advantages reported by BP, compared with the other carbonylation routes, are the elimination of the homogeneous precious metal separation and, consequently, lower energy consumption and easier separation, lessening the environmental damage of the process (Budiman et al., 2016). Even though, continuous development of alternative routes, catalysts and strategies is needed in order to reduce the cost production at a large scale as well as the reduction of the environmental burden. CO þ2H2!CH3OH (12) 2CH3OH !CH3OCH3þH2O(13) CH3OCH3þCO !CH3COOCH3(14) J.L. Martín-Espejo et al. Science of the Total Environment 840 (2022) 156663 5 CH3COOCH3þH2O!CH3OH þCH3COOH (15) The reaction conditions, catalysts and by-products generated in the main conventional routes presented herein can be consulted in Table 1 and Table 2. All the raw materials of these industrial reaction pathways for acetic acid production are derived from petroleum, such as methanol, acetaldehyde, ethylene, butane or syngas. In addition, direct discharges of chemical waste leads to severe environmental issues. Furthermore, most of the catalysts used in these routes are expensive and need continuous regeneration (Pal and Nayak, 2017). Owing to the high operation conditions and the amount of energy needed to synthesise acetic acid, alternatives need to be researched to achieve a novel, greener, environmentallyfriendly process by reducing the energy consumption and improve the integrity &safety of the process with milder conditions. 2.3. Research progress and alternative route proposed Despite the high efficiency of the homogeneous methanol carbonylation, the issues abovementioned (ecological impact of wastewater discharges and separation) pose strong motivation to shift the research lines into the development of a new generation of catalysts for this process. The focus is on improving the performance of the existing catalysts by adding ligands or on devising new strategies to immobilise the homogeneous systems (Kalck et al., 2020;Ren et al., 2020). On the other hand, heterogenisation is also a direct research line to partially overcome these Fig. 4. Catalytic reaction mechanism proposed by Monsanto to produce acetic acid. [Adapted with permission from Forster, D. Mechanistic Pathways in the Catalytic Carbonylation of Methanol by Rhodium and Iridium Complexes. In Advances in Organometallic Chemistry; Academic Press, 1979; Vol. 17, pp. 255–267. Copyright 1979, Elsevier]. Purge CO Methanol Acec acid Catalyst recycle stream Heavy endproducts column Drying column & “lights” removal separaon system Flash tankReactor Fig. 5. Simplified process flow diagram (PFD) of a typical liquid-phase methanol carbonylation process for acetic acid production. [Adapted with permission from Haynes, 2006. Copyright 2006, Springer Nature]. J.L. Martín-Espejo et al. Science of the Total Environment 840 (2022) 156663 6 problems. Even though, its potentiality lays on the simplification of the separation procedure and process intensification (Budiman et al., 2016;Tu et al., 2021). Improvements in methanol carbonylation using heterogeneous catalysts are still under investigation. For instance, Ni et al. (2017) developed a selective, halide-free, noble-metal-free catalyst based on pyridine-modified Hmordenite zeolite for the direct synthesis of acetic acid on vapour phase. Their promising results showed methanol conversions up to 100% while selectivity towards acetic acid is 95% at 250 °C. A recent study by Qi et al. (2020) demonstrated that heterogeneous catalysts based on Rh\\Re/SiO 2 are effective in gas-phase carbonylation of methanol. In this catalyst, two different phases of rhenium are found. Atomically-dispersed ReO 4 active sites enhance acetic acid selectivity since they promote CO insertion into the methoxy species, whereas ReO x cluster promotes DME. However, the presence of dispersed Rh also formed Rh-ReO x pair-sites which further promote the stable production of acetic acid up to 60 h. The volumetric reaction rate was comparable to the homogeneous processes. Interestingly, other bimetallic catalysts have also demonstrated to enhance the conversion and selectivity due to the synergic effect of both metals. On this premise, Zhang et al. (2020) developed Rh\\Ru bimetallic catalyst with an outstanding performance. The mechanism of Rh(I)/Ru(III) for methanol carbonylation is similar to that of the Monsanto process. The methyl iodide oxidation (the rate-determining step) by the Rh(I)/Ru(III) bimetallic catalyst is reported to reduce the energy barrier by 23.88 kJ mol −1 .Itwas found that the 3D structure of Rh/Ru has a bridged dimer space structure which was responsible for the reduction of the energy barrier and increase of the catalyst stability. Differently, in a research line by Park et al. (Bae et al., 2015;Budiman et al., 2016), the synthesis of acetic acid using a series of heterogenised rhodium immobilised in 3-benzoyl pyridine (3BP) over a graphitic carbon nitride catalyst (Rh-3BP/g-C 3 N 4 ) successfully achieved great performance for liquid-phase carbonylation of methanol at 40 bar and 135 °C. Very high conversions (>97%) and yields (75–93%) were obtained using a CO:methanol ratio of 1.5 due to the good dispersion of Rh3BP complex on the high active surface of carbon nitride. Research is also focused on nickel, copper, gold, cobalt, iridium and ruthenium-based catalyst. However, these homogeneous and heterogeneous catalysts need to be further improved, in general, either in activity, selectivity and stability, to operate under the typical industrial conditions (Kalck et al., 2020;Ren et al., 2020). On the other hand, other routes, e.g., the direct thermocatalytic transformation of syngas to acetic acid, have being studied. Based on premise, direct conversion of syngas to acetic acid is reported to be selective (67.5%) at extreme conditions of 480 bar and 220 °C using a combination of Ru\\Co with promoters, e.g., Zr and Ti (Knifton, 1985). In addition, an unpromoted rhodium-based catalyst over NaY zeolite was used, achieving very low conversions of 1.2% and selectivity of 45–56% (Xu et al., 2000). Using Rh/SiO 2 catalyst, a selectivity of 74.8% to acetic acid and ethyl acetate was reported using H 2 :CO ratio 2 at 30 bar and 220 °C, with a meaningless 0.5% conversion (Chen et al., 2006). This thermocatalytic approach does not seem to be the most adequate route due to the severe thermodynamic restrictions of the direct synthesis. Alternatively, the innovative and original route proposed using biogas as feedstock to generate acetic acid is attractive due to the renewable source consideration of biogas. In this approach, two noteworthy alternatives are set (Fig. 6): •BIO-Direct route. Direct, one-pot production of acetic acid from biogas feedstock. This is a cutting-edge approach to generate acetic acid in one single step. The direct transformation of CO 2 and CH 4 into acetic acid is a 100% atom-efficient process that is thermodynamically unfavourable at ambient conditions. Nevertheless, using a non-equilibrium technology, i.e., NTP, this issue is overcome. •BIO-Indirect route. Herein, via biogas dry reforming (DRM), syngas is generated from a renewable source. Syngas can be further used in another processes, e.g., methanol carbonylation or BP SaaBre process, lessening the environmental impact of these processes. In this context, the next two sections present an overview of the state-ofthe-art of the BIO-Indirect and BIO-Direct route to produce acetic. 3. BIO-Indirect approach Alternatives to traditional synthesis of acetic acid are gaining ground due to in the shift in manufacturing practices towards a greener and more sustainable chemical industry. In this context, the proposed BIO-Indirect route to generate acetic acid seems in accordance with this current tendency. Compared to the conventional routes, two main characteristics are changed. Firstly, the use of a different feed. Conventional routes rely on petroleum-derived raw materials, which are known to suffer from a CO 2 penalty, whereas BIO-Indirect route proposes the use biogas. The use of a renewable source is empowered, thus preventing emission of GHGs to the atmosphere. (ii) What is more, CO 2 is partially used instead of being emitted. This strategy synergises bio-resource utilisation and CO 2 utilisation, promoting the circular carbon economy. This original approach is intended to minimise the environmental damage of this large-scale industrial process to achieve carbon neutrality. Since dry reforming of methane (DRM) is the principal step in the BIOIndirect route approach, recent tendencies using DRM need to be explored. 3.1. Biogas dry reforming Dry reforming of methane (DRM), shown in Eq. 16, is a strongly endothermic reaction (ΔH o298K =247.3kJmol −1 &ΔG o298K =170.0kJ mol −1 ). The forward reaction is favoured at low pressures, as dictated by Table 1 Key features of conventional oxidation catalytic routes for acetic acid production. Process Raw material Catalyst T (°C) P (bar) Yield (%) Main by-products Acetaldehyde oxidation CH 3 CHO, O 2 Co, Mn or Cr-based 150 50–60 90–95 Formic acid, formaldehyde, ethyl or methyl acetate, ethylidene diacetate Butane oxidation C 4 H 10 ,O 2 Co or Mn-based 150–230 50–60 50 Acetone, formic or propionic acid or cracked HC Ethylene oxidation C 2 H 4 ,O 2 Pb, Pb\\Pt or heteropolyacid Pd 160–210 7 87 Water, acetaldehyde or CO 2 Ethane oxidation C 2 H 6 ,O 2 Mo or V-based 220–300 12 <20 Water Table 2 Key features of methanol carbonylation routes for acetic acid production. Process Catalyst T (°C) P (bar) Yield (%) Main by-products BASF Homogeneous Co-based iodide promoted 230–250 60–80 90 Methane, acetaldehyde, ethanol, propionic acid, CO 2 and ethers Monsanto Homogeneous Rh-based iodide promoted 150–200 30–60 99 Not significant reported Cativa Homogeneous Ir-based iodide promoted 190 28 >99 Not significant reported Acetica Heterogenized homogeneous Rh-based 160–200 30–60 99 Ester, ether and water J.L. Martín-Espejo et al. Science of the Total Environment 840 (2022) 156663 7 stoichiometry. This reaction offers the advantage of using simultaneously two of the major GHG gases to produce syngas (Price et al., 2020). CO2þCH4!2CO þ2H2(16) As a co-reactant on DRM, CO 2 is used as a soft oxidant and can provide an extra carbon source for the conversion. Theoretically, this process generates syngas with a H 2 /CO molar ratio ≤1, which is suitable for the further generation of oxygenates from syngas (Le Saché et al., 2018;Tu and Whitehead, 2012a). Furthermore, this chemical reaction takes place using biogas without previous separation of CO 2 in biogas feed. On the opposite, CO 2 can be added to the biogas if required (George et al., 2021). CH 4 , having a tetrahedral geometry with four strong C\\Hbonds (434 kJ mol −1 ), is chemically more stable than CO 2 .CO 2 is considered inert since of the nature of the C_O bond. Co-activation of CO 2 and CH 4 , thus, requires high energy input to make the molecules chemically react. Hence, three main ways for the production of syngas using three technologies are presented in Fig. 7: a) thermocatalytic, b) photocatalytic and c) plasma-catalytic DRM (Liu et al., 2020). 3.2. Thermocatalytic DRM In DRM, due to the endothermic nature and high stability of reactants, high temperatures are needed in order to achieve desirable conversion levels and yields of syngas. Indeed, researchers conclude that the main reaction is thermodynamically favoured at temperatures above 700 °C (Aramouni et al., 2018;Bradford and Vannice, 1999;Wang et al., 1996). Nevertheless, at these high temperatures, the activity of the catalysts can be compromised due to (i) coke deposition and (ii) sintering, which blocks the access to the active sites (Le Saché et al., 2019;Tu and Whitehead, 2012b). Therefore, a suitable catalyst should be able to overcome deactivation whilst giving high stability and yields. While performing DRM (Eq. 16), other side reactions can also occur. From all the potential side reactions reported in literature (Aramouni et al., 2018;Nikoo and Amin, 2011), the most representative are illustrated: reverse water-gas shift (RWGS) (Eq. 17); Bourdouard reaction (Eq. 18); CH 4 decomposition (Eq. 19); methanation (Eq. 20); CO and CO 2 reduction (Eqs. 21 and 22) may affect the distribution of products. The thermodynamics of the reactions involved in DRM are explained by Aramouni et al. (2018). CO2þH2!CO þH2OΔH 298 K¼41:2kJmol1(17) 2CO !CþCO2ΔH 298 K¼172:4kJmol1(18) CH4!Cþ2H2ΔH 298 K¼74:9kJmol1(19) CO2þ4H2!CH4þ2H2OΔH 298 K¼165:0kJmol1(20) Fig. 6. Acetic acid BIO-routes approach. Fig. 7. DRM to syngas main approaches. [Adapted with permission from Liu et al., 2020. Copyright 2020, American Chemical Society]. J.L. Martín-Espejo et al. Science of the Total Environment 840 (2022) 156663 8 CO þH2!CþH2OΔH 298 K¼131:3kJmol1(21) CO2þ2H2!Cþ2H2OΔH 298 K¼90:6kJmol1(22) In general, noble metals are very active in DRM, exhibiting great performances compared to transition metals (Aramouni et al., 2018;Price et al., 2020). That is because these catalysts partially inhibit the formation of carbon due to the lower equilibrium constants for methane decomposition and reduced dissolution of carbon in their structure. The difference in the performance of noble metals and transition metals may also be ascribed to the ability of noble metals to better disperse on the supporting structures and retain a small particle size (North et al., 2019;Price et al., 2020). Interestingly, the activity of some noble metals was compared with Ni using a MgAl 2 O 4 support, showing an activity order at 650 °C of Ru >Rh, Ni >Ir >Pt >Pd, whereas coke deposition behave differently: Ni >Pd>>Ir > Pt >Ru, Rh (Rostrupnielsen and Hansen, 1993). Notwithstanding, their scarcity makes them uneconomical for large-scale industrial applications. Transition metals such as Ni, Co, Fe, Mo and Cu are preferred due to their comparable catalytic activity to noble metal, remaining an interesting option concerning manufacturing costs (Le Saché et al., 2021;Stroud et al., 2018). Nickel has especially been researched but, like other transition metal catalysts, suffers from severe coke deposition and sintering, which leads to rapid deactivation. For nickel and the rest of the metals, dispersion is a key parameter favouring stability. Metal particle size is also important to achieve a great catalytic activity and stability as well as avoiding excessive coke formation. Multiple strategies are put into practice in order to engineer brand-new, robust catalysts. The use of a combination of support structures (Al 2 O 3 -CeO 2 ,Al 2 O 3 -La 2 O 5 or Al 2 O 3 -MgO) or other non-typical support structures (K 2 O, ZrO 2 , MoC, CaO) can upgrade the material. Most catalysts used in reforming industry are supported on aluminium oxides, which tends to catalyse carbon formation reactions, hence making mandatory catalyst's regeneration. The acid-basic properties of alumina are not favourable to CO 2 adsorption (Aramouni et al., 2018). Differently, the addition of promoters (lanthanide or alkali elements, e.g., La, K, Ca, Mg, Zr or Ce) may affect the electronic interactions, tuning the redox properties, stability or dispersion (Abdulrasheed et al., 2019;Jang et al., 2019). It is reported that small amounts of alkali and alkali earth elements minimise coke deposition. For instance, Alipour et al. (2014) has reported that adding small amounts of Ba and Ca over Ni/γ-Al 2 O 3 enhances coke resistance due to interaction between the active phase and the support, leading to changes of the basic properties of the catalyst and the adsorption of CO 2 . In addition, bimetallic systems (Ni\\Co, Ni\\Pt, Ni\\Mo, Ni\\Fe, Co\\Mo, etc.) are being explored due to the synergy between two different metals, which provides the ability to fine-tune the properties of the active phases. The combination of transition metals or transition-noble metals seems to be encouraging, demonstrating enhanced activity and stability (Abiev et al., 2020;Yentekakis et al., 2021). Apart from conventional supported-based catalyst formulations, the use of special structures (e.g., core-yolk (Li et al., 2014), sandwich (Zhao et al., 2018), tubular (Coelho et al., 2016), mesoporous (Rodriguez-Gomez et al., 2018), perovskites, fluorites, pyrochlores (Le Saché et al., 2018), hexaaluminates (Gardner et al., 2013) and hydrotalcite (Dębek et al., 2016)) are being investigated as catalytic systems owing to their particular properties, which can enhance the performance of DRM (Abdulrasheed et al., 2019;Bhattar et al., 2021). Some literature reviews are highlighted about the most recent advances (Abdulrasheed et al., 2019;Jang et al., 2019;le Saché and Reina, 2022;Yentekakis et al., 2021). Even though, further investigations are needed in order to solve the shortcomings of the thermocatalytic DRM. Alternative methods, i.e., photo-assisted catalysis and NTP catalysis, are a good choice to promote syngas production while improving the sustainability of the process (Chung et al., 2018). 3.3. Photocatalytic DRM Harnessing solar energy to enable chemical transformations using photocatalysis is an auspicious strategy to produce value-added products (Muhammad et al., 2020). From the thermodynamic perspective, the use of photo-technology activates CO 2 and CH 4 to generate products. Using UV photo-energy, thermodynamic limitations of unfavourable endothermic reactions can be surpassed at lower temperatures (Shi et al., 2004;Tahir et al., 2015;Yuliati et al., 2008). In this scenario, photo-assisted DRM has come to the fore. The main advantages of photocatalytic CO 2 /CH 4 conversion are: (i) the combination of solar energy utilisation and recycling of GHG emissions to value-added products, (ii) low operating temperatures and, consequently, and (iii) the mild, safer operation (Chung et al., 2018;Tahir et al., 2015). The utilisation of light energy and conversion efficiency is primarily determined by the photocatalyst used, which is the core component of a photocatalytic process. The development of energy-active, stable, cheap photocatalysts is demanded (Muhammad et al., 2020;Wang et al., 2021;Yuliati et al., 2008). Conventional photocatalysis takes place at the fluid-solid interface, and the light intensity decreases along with the thickness of the photocatalytic layer. Hence, the surface area and the capability to capture the sunlight are key parameters in order to enhance the kinetics and performance of the catalysts (Atanda, 2021;Cho et al., 2020). Generally, in the field of photocatalysis, semiconductors are the stateof-the-art catalysts since these materials are prone to capture sunlight to generate the energy to effectively activate the reactants, in this case CO 2 and CH 4 (Cho et al., 2020). In this context, several semiconductors can be used as photocatalysts for biogas upgrading, e.g., TiO 2 , ZrO 2 , CdS, ZnO, NbO 5 or CeO 2 (Kulandaivalu et al., 2020). Bimetallic oxides are also under scrutiny, but more research is needed (Zhao et al., 2020). Among all the semiconductor available, titanium dioxide (TiO 2 ) is the champion photocatalytic material due to its noteworthy properties. Takami et al. (2019) studied various metallic catalysts over different supporting structures at a temperature of 200 °C. Ni/Al 2 O 3 showed the highest activity over the other studied systems, Ni/Ga 2 O 3 and Ni/TiO 2 . Metallic Ni was found to be essential in order to achieve the high activity reported to generate syngas due to the plasmonic excitation of Ni active sites. Conversions of 18% and 17.2% were found for CO 2 and CH 4 . Besides, different structures are being studied in photocatalytic DRM systems. A promising study by Muhammad et al. (2020) focused on graphitic carbon nitride nanotubes modified with lanthanum (La/g-CNT). Interestingly, experiments proved effective CO 2 and CH 4 conversion over this catalyst under visible light. The enhancement of the photo-activity reported was due to the efficient transference of electrons over the 1D structure and the separation of charge carriers of La over the supporting g-C 3 N 4 structure. In another study, the improvement of H 2 /CO ratio was reported using Ni/Ga 2 O 3 using photoassisted catalysis (Rao et al., 2021). The light irradiation provided by a 300 W Xenon lamp improve the H 2 /CO ratio from 0.55 to 0.94 while restraining RWGS reaction. The light irradiation boosted the direction of electron transfer to be reversed from Ga 2 O 3 to Ni amid the synthesis of H 2 . Consequently, more hot electrons from the electronic interband transition of Ni simultaneously stimulated the dehydrogenation of CH 4 and the posterior desorption and formation of H 2 .Differently,Chung et al. (2018) assessed a perovskite-type photocatalyst to study its effectiveness. In this study, the combination of plasma-assisted and photo-assisted DRM was overviewed, reporting a promising synergy towards conversion. Plasma promoted photocatalytic activity by (i) reducing the metal oxide cluster size, (ii) increasing the specific active surface area and (iii) the lifetime of the electron-hole pair. Although photocatalysis is currently being investigated, some drawbacks hinder its potential owing to its low conversion and low solar energy utilisation. 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