Formic Acid as Renewable Reagent and Product in Biomass Upgrading

Tetrahedron Green Chem 2 (2023) 100020 Available online 15 July 2023 2773-2231/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/). Formic acid as renewable reagent and product in biomass upgrading Mahdi Achour a , b , D´ ebora ´ Alvarez-Hern´ andez a , Estela Ruiz-L´ opez a , Cristina Megías-Sayago c , Fatima Ammari b , Svetlana Ivanova a , * , Miguel ´ Angel Centeno a a Departamento de Química Inorg´ anica e Instituto de Ciencia de Materiales de Sevilla, Centro mixto CSIC-Universidad de Sevilla, 41092, Sevilla, Spain b Laboratoire de G´ enie des Proc´ ed´ es Chimiques- LGPC, D´ epartement de G´ enie des Proc´ e´ es, Facult´ e de Technologie, Universit´ e Ferhat Abbas Setif-1, 19000, Setif, Algeria c Departamento de Química Inorg´ anica, Instituto de Investigaciones Químicas and Centro de Innovaci´ on en Química Avanzada (ORFEO-CINQA), Centro mixto CSIC- Universidad de Sevilla, 41092, Sevilla, Spain ARTICLE INFO Keywords: Biomass upgrading Formic acid H-donor Catalytic transfer reaction (CTH) Hydrodeoxygenation (HDO) reaction Levulinic acid ABSTRACT The problems associated with the use of molecular hydrogen (transportation, storage and high cost) have pushed scientists to the pursuit of efficient hydrogen donors, able to reduce chemical bonds in the presence of catalysts through catalytic transfer hydrogenation (CTH) reactions. In this sense, formic acid stands up as one of the most important and safest chemical molecules for H 2 generation under mild conditions. It can be obtained from biomass through different catalytic transformations and used as well to upgrade biomass to platform chemicals. This review summarizes the recently published studies dealing with formic acid production from biomass (using glucose as representing molecule) along with its use in hydrogen involved reactions of different groups of platform chemicals upgrading. 1. Introduction The global energy demand associated with the continuous population growth starts alarming society on the unsustainable use of nonrenewable energy sources such as natural gas, coal, and crude oil [1]. In the last years, the risk of supply interruption has become even more important referring to the geopolitical conflicts and restriction policies. What is more, the uncontrolled energy consumption causes constant environmental pollution problems, that appeal to limit the fossil sources utilization in favor to a new, clean and sustainable sources for fuels and chemicals production [2,3]. In the last decades, biomass has become the most abundant, versatile and renewable source for fine chemicals and biofuels production in the world. Much efforts have been made to develop new technologies for biomass upgrading into high value-added chemicals [4]. The principal low-cost and sustainable biomass that can be used as feedstock is the lignocellulosic (LCB) one with its three main components: cellulose, hemicellulose and lignin. These macromolecules could originate after hydrolysis a fan of platform molecules, useful to synthesize a wide spectrum of valuable chemicals (sugars, acids and aromatics) [5,6]. The key processes to transform these molecules into chemicals and biofuels are hydrogenation and hydrogenolysis reactions. The former increases the ratio of hydrogen to carbon by using hydrogen from some external sources and the latter cleaves carbon-carbon or carbon-heteroatom bonds in presence of H 2 . In the particular case of biomass we often refer to hydrodeoxygenation (HDO) as a hydrogenolytic reaction in connection to the reduction of oxygen content of all LCB derived molecules [7,8]. The main challenge of the HDO mediated processes is to remove as much oxygen as possible while using the least possible amount of hydrogen [9]. Molecular hydrogen (H 2 ) has been frequently reported as hydrogen donor for HDO processes due to its beneficial activation potential on a wide range of metal surfaces and on its availability [8]. Nevertheless, its use presents some considerable issues owing to the high pressure H 2 over-utilization, and its high cost and safety hazards during transportation, storage, and use. It also presents a low solubility in most solvents, being required a high H 2 pressure to achieve satisfactory conversions and yields without the production of high amounts of by-products [10]. In this sense, some organic molecules have demonstrated to act as hydrogen donors for the reduction of chemical bonds in presence of catalysts through a catalytic transfer hydrogenation (CTH) reaction. Its use avoids the need for complicated installations and harsh conditions [11] being also environmentally friendly and economically viable [12, 13]. Several hydrogen donors, such as alcohols, organic acids and * Corresponding author. E-mail addresses: [email protected] (M. Achour), [email protected] (D. ´ Alvarez-Hern´ andez), [email protected] (E. Ruiz-L´ opez), [email protected] (C. Megías-Sayago), [email protected] (F. Ammari), [email protected], [email protected] (S. Ivanova), [email protected] (M.´ A. Centeno). Contents lists available at ScienceDirect Tetrahedron Green Chem journal homepage: www.journals.elsevier.com/tetrahedron-green-chem https://doi.org/10.1016/j.tgchem.2023.100020 Received 30 March 2023; Received in revised form 11 July 2023; Accepted 12 July 2023 Tetrahedron Green Chem 2 (2023) 100020 2 hydrazine have been used in the CTH process [8,14] and, unlike molecular hydrogen, they may interact effectively in liquid phase thus enhancing the reactivity of the generated hydrogen [11]. Within the group of organic molecules, one of the most interesting hydrogen sources is formic acid (FA) [15,16], which quickly and selectively decomposes into H 2 and CO 2 under mild conditions [17–19]. FA has been recently identified as one of the most important and safest chemical molecules for H 2 storage, having a high volumetric hydrogen content, 26.5 M of H 2 (53 g H 2 /L) reserved in 1 L of FA. For comparison, 1 L of gaseous H 2 is equivalent to 9.8 M and must be stored at a pressure of 22 MPa [17]. All these properties convert FA in an efficient hydrogen carrier with a high flash point (69 ◦C vs. 11 ◦C for methanol and −23 ◦C for gasoline), low inflammability [20], and biodegradability. Currently, FA is industrially produced using a fossil-based process, i.e., carbonylation of methanol [21], although its large-scale production from biomass is also available [22]. Up to now, continuous attempts have been made to obtain FA from renewable sources through green routes, particularly starting with biomass or CO 2 [23,24], being the former feedstock summarized later in this review. Hydrogen is generated from FA via dehydrogenation (Scheme 1) but, depending on the reaction conditions and the catalyst’ nature, the dehydration reaction can also occur with release of CO. In general, this side reaction must be completely avoided not only because CO can be very poisonous for the catalysts [25], but also because its conversion to CO 2 and H 2 is conditioned by the water gas shift equilibrium [26]. A DFT study [27] resumes the FA decomposition into two pathways (Scheme 2):i) the carboxyl pathway (COOH), involving C–H bond splitting and followed by O–H break and ii) the formate pathway (HCOO), where the decomposition starts with O–H bond cleavage. In the case of the carboxyl route, the final products selectivity relies on the O–H or C–OH dissociations, leading to dehydrogenation and dehydration products in the first and second case, respectively. Regarding the formate pathway, it is possible to form a stable bidentate (HCOO B ) or monodentate (HCOO M ), resulting both in CO 2 and H 2 formation after C–H bond splitting. The strength of the metal(catalyst)-oxygen coordination would determine the decomposition pathway, being particularly important bidentate (HCOO B ) to monodentate (HCOO M ) complex transition in the final products distribution [27–29]. The employed catalysts have a major impact on the reactivity and selectivity of these routes [30]. In the last decade, various catalytic systems [31] have been reported for in situ hydrogen generation: i) homogeneous non-noble transition metal complexes based on cobalt [32], iron carbonyl [33], manganese [34], nickel [35,36] and copper [37,38]; ii) noble metal complexes based on ruthenium [39–41], iridium [42,43] or palladium [44] for which the control of activity and stability relied on a careful design of the ligand [45]; and iii) heterogeneous catalysts based on monometallic Pd [46–49], Au [50,51], Ru [52] and bimetallic PdAu [53], PdCo [54] formulations. At this point we must specify that a great variety of the catalysts mentioned above for in situ hydrogen production can also serve for CTH/HDO reactions and they are the object of this review. Without pretending to summarize all reported information, we have tried to regroup the recently published studies of formic acid production from biomass (using glucose as representing molecule) and its use in CTH/HDO reactions of various groups of platform chemicals. 2. Direct production of formic acid from biomass Formic acid has multiple uses across industries (agricultural, food or chemical mainly) and its popularity has noticeably increased with the development of scaled-up methods for its production [55]. These methods can be grouped in three categories: methyl formate hydrolysis after methanol carbonylation; carbon dioxide hydrogenation and biomass treatment [56]. As a general drawback of the processes, we can mention i) the use of fossil resources, ii) the formation of undesirable products or iii) the use of harsh conditions. That is why, there is a renewed scientific interest for adopting sustainable and green chemistry practices able to reduce the environmental impact of the production processes [57–60]. Thus, all new technologies should include a renewable feedstock, reduce wastes and emissions, and develop more efficient and less toxic chemicals and processes. FA production from biomass meets all these criteria [61]. Glucose (GLC) is often used as a model molecule for LCB because of its prevailing concentration. Nevertheless, its oxidation selectivity is conditioned by the complexity of its molecule and the nature of the used catalysts. The selectivity to formic acid is often decreased by the presence of products of isomerization and retro-aldol reaction and different products of C–C cleavage (Scheme 3). Thus, the nature of oxidant and catalyst (acidity/basicity or redox behavior) plays a primordial role for achieving good selectivity during the process. 2.1. Hydrogen peroxide as the oxidant Aluminum triflate (Al(OTf) 3 ) is a well-known Lewis acid catalyst, particularly active in reactions (oxidation included) that involve water as reactant and solvent [62]. Kong et al. [63] used that catalyst to convert glucose to formic acid in mild conditions (70 ◦C and reaction time of 12 h) using GLC to oxidant (H 2 O 2 ), ratio of 1:10, and acetonitrile as additive. They achieved a high conversion rate of glucose (85%) but a very low FA yield (6.8%). The main product, glycolic acid, indicated that Lewis acid materials catalyzed glucose to fructose isomerization prior to its oxidation thus significantly decreasing the FA selectivity. Takagaki and co-workers [64] found that alkaline metallic oxides possess a good catalytic activity in glucose oxidation to formic acid and related it to solid’ basicity. CaO appears as the most active with 21% FA yield with fructose as a trace product. Reaction parameter optimization indicates glucose/H 2 O 2 molar ratio of 1/5 in water, 90 ◦C temperature and 2 h time as optimal conditions to achieve full conversion and 45% FA yield. During the reaction the catalyst was gradually transformed into CaCO 3 diminishing the activity in the second cycle but recovering it after calcination at 800 ◦C. Wu et al. [65] investigated the use of MgO as catalyst and found high FA yield (78%) and conversion (90%) under mild conditions (50 ◦C, 4 h) with a stoichiometric GLC/H 2 O 2 ratio. However, the catalyst also produced undesired products such as lactic Scheme 1. Formic acid decomposition reaction in gas phase. M. Achour et al. Tetrahedron Green Chem 2 (2023) 100020 3 acid and arabinose. The catalytic activity of MgO catalyst was maintained through multiple cycles (up to 5) although a calcination between cycles was required. Kılıç et al. [66] investigated the use of hydrotalcite-like materials as catalysts for the oxidation of GLC to FA in ethanol and evaluated the influence of Mg/Al ratio. No significant differences were found for the bare materials (hydrotalcites without calcination) but after treatment higher conversion was observed at higher Mg/Al ratios. The highest conversion (38.7%) with complete selectivity towards FA was achieved for Mg/Al ratio of 1/3. This study also reported a decrease in activity in the 4th catalytic cycle. In a recent publication, the same authors used bimetallic oxides as catalysts in similar conditions [67]. The highest conversion of 29% was achieved with Sr–Fe oxides calcined at 200 ◦C in comparison to the highest GLC conversion of 62% with FA yield of 10% reported for the Mg–Al oxides. The change of the solvent from ethanol to water decreases GLC conversion for the Mg–Al catalyst to 30% but increases the FA selectivity to 60%. A novel silica-encapsulated heteropoly acid (HPAs) catalyst was studied by Yuan et al. [68] for the oxidation of several biomass platform molecules. Even though HPAs showed great redox properties, their low surface area and high solubility in polar solvents are considered as disadvantages for this reaction. That is why its encapsulation was proposed in this study and resulted in conversions close to 25% with increased FA selectivity (67%). 2.2. Molecular oxygen as the oxidizing agent Albert [69] reported a series of heteropolyacids (HPA) with different vanadium/molybdenum ratios within the Keggin structure and found that the highest FA yield of 26% was given by the HPA-4 (H 7 PV 4 Mo 8 O 40 ) sample at 90 ◦C and 20 bars of O 2 pressure after 8 h of reaction. The authors conclude that at least two vanadium atoms are needed within the polyoxometalate structure to produce some formic acid. Within the same group of catalysts, Reichert and Albert [70] compared HPA-5 (H 8 PV 5 Mo 7 O 40 ) to HPA-2 (H 5 PV 2 Mo 10 O 40 ) structures and found that the difference in activity arises from vanadium content, highest the content highest the glucose conversion. In this sense, higher vanadium content in HPA-5 results in more VO 2+ liberation, presumably the active species. Nevertheless, the selectivity towards formic acid remains the same for both catalysts at around 68%. Maerten et al. [71] also studied HPA-5 catalysts in water at 90 ◦C and 20 bars of O 2 and reported a 55% FA yield and 45% carbon dioxide (CO 2 ) yield. The change of the solvent to ethanol allowed a full yield towards methyl formate. The results of 13 C labelled glucose study confirmed that the formic acid unit is produced from this molecule and not from oxidation and esterification of the solvent. This study also Scheme 2. Formic acid decomposition via carboxyl (top) and formate (bottom) intermediates, adapted from Ref. [27] with permission from the Royal Society of Chemistry. Scheme 3. Formic acid production from glucose and related reactions. M. Achour et al. Tetrahedron Green Chem 2 (2023) 100020 4 proposed a mechanism for glucose oxidation in ethanol that involves C–C cleavage producing erythrose on first step and glyoxal and glycolaldehyde after additional C–C breaking. These 2-carbon intermediates are then oxidized to formic acid forming rapidly methyl formate through esterification (Scheme 4). The study did not observe any other intermediates or CO 2 . Albert et al. [73] continued to clarify the role of vanadium species in GLC transformation by using two commercial catalysts: NH 4 VO 3 (V 5+ source) and VOSO 4 (V 4+ source) and two heteropolyacids HPA-1 (H 4 PV 1 Mo 11 O 40 ) and HPA-5 at 90 ◦C and 20 bar O 2 . At shorter reaction times (3 h) only HPA-5 showed good activity (around 34% FA yield). NMR and EPR spectroscopy were used to monitor the V 5+ and V 4+ species, respectively, before and after the reaction. They observe that the HPA-5 solution (catalyst dissolved in the continuous phase) before reaction contained HPAs with higher and lower levels of vanadium substitution. However, after the reaction, only HPA with lower substitutions were detected. The authors concluded that the active species in HPA-5 are the V 5+ containing HPAs with higher levels of substitution. On the other hand, Lu et al. [72] studied the impact of solvents on GLC oxidation at 160 ◦C, 30 bar O 2 and 10 min of reaction time using HPA-2 as catalyst. They found a total selectivity to FA in aqueous solution and 45% of CO 2 in gas phase. Adding 50% v/v of methanol to water reduces CO 2 production to around 4% but also decreases FA yield and selectivity due to the formation of dimethoxymethane and methyl formate. The catalyst behavior was studied by NMR spectroscopy, and it was found that methanol stabilizes the HPA structure, preventing vanadium over-oxidation due to lower VO 2+ release. The catalyst showed excellent stability in 4 reaction cycles. A comparative study between several types of HPA was also reported by Voβ and co-workers [74] at 160 ◦C, 20 bar of O 2 and 1 h of reaction time. The best performance was obtained with HPA-3 catalysts (H 6 PV 3 Mo 9 O 40 ) with 36% FA yield and 27% CO 2 yield. Even though FA was the main product in liquid phase, acetic acid and glyceraldehyde formation also took place. Some iron and manganese compounds were also studied for this reaction as an alternative to the vanadium compounds. A mixture of iron (III) chloride and sulfuric acid (FeCl 3 –H 2 SO 4 ) was proposed as catalyst by Hou et al. [75]. The reaction carried out at 170 ◦C and 30 bar O 2 for 50 min, results in full glucose conversion with 53% FA yield in liquid phase and carbon dioxide production. Acetic and glycolic acids were detected as secondary products. The proposed mechanism involves the oxidation of glucose with the reduction of Fe 3+ to Fe 2+ deoxidized later by the molecular oxygen. All the systems described up to now include a reaction in homogeneous phase. However, the use of manganese oxides as alternatives orients the systems toward heterogeneous catalysis. Li et al. [76] studied the effectiveness of 3 manganese oxides. The highest yield of FA 81% was found for MnO x -100 sample, prepared hydrothermally at 100 ◦C and tested at 160 ◦C in 30 bar O 2 and 2.5 h reaction time. The main proposed mechanism of conversion includes glucose α -scission to arabinose as intermediate. However, the catalyst deactivated after three successive runs due to the increased leaching of Mn and decreased surface-adsorbed oxygen. Xu et al. [77] found that bimetallic Mn–Ce oxides provide better activity in comparison to their monometallic oxides attained to the increase of the surface area and corresponding surface-adsorbed oxygen for the mixed samples. The best results were obtained at 160 ◦C, 30 bar O 2 after 3.5 h, with 63% FA yield over Mn 4 Ce 0.05 O x catalyst. Another mixed oxide containing manganese and molybdenum was studied by Guo et al. [78]. The highest FA yield of 79%, under reaction conditions (160 ◦C, 30 bar O 2 for 1.5 h) was achieved for 5% Mo added to MnO x . A continuous increase of Mo did not result in further improvement due to the decrease of the overall proportion of Mn 2+ and Mn 3+ species, suggested as active sites for oxidation. A summary of all described studies including catalysts, solvents and reaction conditions is listed in Table 1. The oxidation of glucose to formic acid is still a very challenging process from a heterogeneous catalysis point of view. Up to date stable heterogeneous catalysts are still missing, but the nature of the active species oscillates around vanadium, manganese and molybdenum species. The exigency of the direct oxidation process (high oxygen pressure and moderate temperatures) makes difficult the stabilization of the catalytic systems causing severe leaching and giving the main future direction for the process: discovering of a stable catalytic system. Formic acid is also produced as a byproduct during the synthesis of levulinic acid from biomass. All involved processes and catalysts are frequently reviewed [79–81] and are not the subject of this review. Nevertheless, we will consider the levulinic acid in the next section as a principal substrate for CTH or HDO reactions using formic acid as a natural continuation of the biomass upgrade of co-produced platform molecules. Scheme 4. Glucose oxidation in methanol over polyoxymethylene (POM) based catalysts, adapted from Ref. [72] with permission from Elsevier. M. Achour et al. Tetrahedron Green Chem 2 (2023) 100020 5 3. CTH reactions using formic acid as reagent 3.1. Levulinic acid upgrading Levulinic acid (LA) has been recognized as one of the top 10 biomassderived platform compounds [82] and presents an interesting molecular structure (carbonyl (C – – O) and carboxyl (-COOH) functional groups). The acid is mainly produced via acid-catalyzed hydrolysis of the LCB and a subsequent dehydration reaction in a low-cost process [83,84]. The starting feedstocks are normally cellulose/C6 sugars via 5-hydroxyme- thylfurfural (HMF) intermediate [85] or hemicellulose/C5 sugars via furfural (FF) intermediate [86]. Once obtained, LA can be converted into high valuable bio-chemicals, i.e. hydrogenated to γ-valerolactone (GVL) [87,88], 2-methyltetrahydrofuran (MTHF) [89], 1,4-pentanediol (PDO) [90], valeric acid (VA, pentanoic acid) [91], oxidized to various organic acids like 3-hydroxypropanoic acid [92], succinic acid [93] etc., or condensed with other molecules such furfural [94], phenols [95] etc. (Scheme 5). The hydrogenation of levulinic acid to γ-valerolactone is one of the most interesting reactions, due to its specific physicochemical features and prospective fuel applications [96,97]. GVL is considered a safe, biodegradable and nontoxic compound with broad applications as biofuel and fuel additive [98], food additive [99], green solvent [100] or as a platform molecule for a variety of derivatives, including PDO, alkanes, and MTHF [101]. GVL is produced from LA either by hydrogenation with molecular hydrogen at high pressures or by catalytic transfer hydrogenation (CTH) reaction with H-donor compounds. The CTH of levulinic acid with formic acid as internal hydrogen source has been investigated using both homogeneous and heterogeneous catalysts. Our interest in this review will center principally in the CTH reactions with formic acid over noble and non-noble metal based systems as well as over some bimetallic formulations. The levulinic acid conversion to γ-valerolactone goes through two possible pathways (Scheme 6) [102]. In the first one, LA is hydrogenated to 4-hydroxypentanoic acid (4-HPA) over metallic sites and further transformed to GVL through cyclization (dehydration) over acidic sites. On the other hand, the second pathway, considered predominant, involves LA dehydration over acidic sites to form an intermediate called angelica lactone (AGL) which is subsequently hydrogenated to GVL over metallic sites. No matter the pathway, the presence of acidic sites promote some secondary reactions such as LA polymerization and coke formation [103]. After GVL the upgrade can continue resulting in pentanoic acid after isomerization and hydrogenation or to methyl tetrahydrofuran (2-MTHF) after hydrogenation and dehydration. 3.1.1. Noble-metal based catalytic systems Ru-based catalysts were among the most employed in LA to GVL catalytic transformation. Deng et al. [14] evaluated the performance of RuCl 3 ⋅3H 2 O/PPh 3 mixture (0.2/0.6 M ratio) as catalyst at 150 ◦C and 12 h of reaction time using 1 equivalent of formic acid. The authors compared the effect of various bases (KOH, NaOH, Et 3 N, pyridine, NH 3 , LiOH) on GVL yield and they found that the use of Et 3 N and pyridine resulted in very high yields (94% and 93%, respectively). Similar yields have been obtained after shortening the reaction time (6 h) but at higher temperature (200 ◦C) using the same Et 3 N and pyridine bases. The same reaction, in aqueous media using various ligands (PPh 3 , dppe, tppms, tppts and PCy 3 ) and under the initial reaction conditions (150 ◦C, 12 h and 25 mL of water), showed important GVL yields within the 80–90% range, being the PPh 3 the most beneficial ligand. In another study of the same group [106], RuCl 3 was immobilized on functionalized silica resulting in three different samples (Ru–N/SiO 2 , Ru–S/SiO 2 , Ru–P/SiO 2 with N – – NH 2 , S – – SH, P=PPh 2 ). The highest GVL yield (96% at 150 ◦C, 12 h reaction time) resulted for Ru–P/SiO 2 catalyst performing as bifunctional catalyst for FA dehydrogenation and LA hydrogenation. Nevertheless, the recyclability tests showed an important decrease in GVL yield (43%) after 3 runs attained to the reduction of Ru 2+ species to Ru 0 , according to the XPS analysis. Ruppert et al. [52] investigated the impact of catalyst (Ru/C) preparation method and reduction temperature on its CTH activity. They use two precursors (RuCl 3 and Ru(acac) 3 ), and two reduction temperatures: low 200 ◦C (LR) and high 500 ◦C (HR). The reaction was carried out in Table 1 Overview of the catalytic systems used for formic acid production from glucose. Catalyst Solvent Oxidant Conversion % Yield % Timeh Temp ◦C Ref. Al(OTf) 3 water H 2 O 2 85 6.8 12 70 [63] CaO water H 2 O 2 100 45 0.5 70 [64] MgO water H 2 O 2 90.5 78.6 4 50 [65] HPA@SiO 2 –S–N2 water H 2 O 2 78 – 12 70 [68] hydrotalcite ethanol H 2 O 2 38.7 5 6 70 [66] Mg–Al oxide ethanol H 2 O 2 62 10.2 6 70 [67] HPA-4 water O 2 – 25.6 8 90 [69] HPA-5 water O 2 53.2 37.8 8 80 [70] HPA-5 water O 2 100 55 24 90 [71] HPA-5 water O 2 43 34.3 3 90 [73] HPA-2 50% v/v MeOH O 2 100 25 0.17 160 [72] HPA-3 water O 2 100 36 1 160 [74] FeCl 3 –H 2 SO 4 water O 2 100 53 0.83 170 [75] MnO x -100 water O 2 100 81.1 2.5 160 [76] Mn 4 Ce 0.05 O x water O 2 – 63.4 3.5 160 [77] Mn–Mo oxide water O 2 100 79 1.5 160 [78] Scheme 5. Levulinic acid platform. M. Achour et al. Tetrahedron Green Chem 2 (2023) 100020 6 aqueous phase at 190 ◦C for 2 h, being the Ru/C(Cl)HR sample (prepared from RuCl 3 precursor by incipient wet impregnation and reduced at 500 ◦C) the one that achieve a complete formic acid decomposition, high LA conversion (81%) and medium GVL yield (57%). The authors compared the efficiency of Ru/C, Pd/C and Pt/C and explained the higher activity of Ru/C by the decrease of the reaction energy barrier provided by the oxophilic metals (like ruthenium) in aqueous solutions [107]. Feng et al. [108] reported GVL production from LA over Ru (5 wt %)/C, using FA produced from cellulose as hydrogen source in presence of triethylamine (Et 3 N). The catalyst showed 87% LA conversion and 81% of GVL yield at 160 ◦C after 3 h of reaction using 10 g/mol (LA) catalyst and 150 mL/mol Et 3 N. They found that an increase of Et 3 N leads to an increase in both LA conversion and GVL yield as a consequence of hydrogen formation from the reaction between FA and E t3 N. Later, it was reported that the addition of Et 3 N base facilitates the hydrogen transfer and reduces byproducts formation [14]. However, an excessive amount of Et 3 N provokes a slight decline in both LA conversion and GVL yield, suggesting an inhibitory effect in a strongly alkaline environment. The temperature effect was also studied within the 40–180 ◦C range where the GVL yield slowly increased with the reaction temperature achieving a maximum at 160 ◦C [109]. Further increase of the temperature caused a decrease of GVL yield to 38%, attributed to multiple side reactions. During the reaction, the formic acid rapidly decomposes to H 2 and CO 2, leading to a reactor pressure increase and system polarity decrease. As a consequence, LA rapidly decomposes in other products since GVL formation remains slow and complex [110]. Gao et al. [111] investigated the performances of Ru/ZrO 2 , Ru/TiO 2 and Ru/Al 2 O 3 catalysts, using an equimolar formate/formic acid mixture in water at 150 ◦C in 12 h reaction time. The catalysts were prepared by sol-gel method to trap the ruthenium nanoparticles within the support. The Ru/ZrO 2 catalyst achieved 73% LA conversion and 73% GVL yield with full formic acid/formate decomposition. Nevertheless, Ru/TiO 2 and Ru/Al 2 O 3 achieved 98% of FA decomposition with 16% and 0% of LA conversion, respectively. The absence of activity in LA hydrogenation for the latter was attributed to the changes produced after an acid attack (LA or FA) on Al 2 O 3 crystallinity [112,113]. Ru-based catalysts supported on various supports (C, SBA-15, Al 2 O 3 , TiO 2 , ZrO 2 ) have been also reported [114] using water as solvent, equimolar FA/LA ratio at 150 ◦C for 5 h. Poor LA conversion (10–31%) and GVL yield (2–22%) for all catalysts were detected. Only 5 wt% Ru/C sample showed higher GVL selectivity (73%). Increasing the FA/LA molar ratio to 3 reflected in full LA conversion, 90% GVL yield, but only a 43% of FA conversion. F´ abos et al. [115] reported full LA conversion and 99% GVL yield at 100 ◦C after 500 min at FA/LA molar ratio =2 with the use of organo-rhutenium Shvo catalyst, {[2,5-Ph 2 -3,4-(Ar) 2 ( η 5 -C 4 CO)] 2 H} Ru 2 (CO) 4 ( μ -H)]} (Ar =p-MeOPh, p-MePh, Ph, respectively) in an open vessel. Ortiz-Cervantes [116] investigated the catalytic activity of ruthenium nanoparticles in water and water/triethylamine mixture at 130 ◦C for 24 h being the LA:Et 3 N:FA molar ratio 1.0:1.6:5.4. An excellent conversion of LA (99%), high selectivity (100%) and yield (100%) to GVL were obtained with low loading of Ru-NPs (particles size ~5 nm and using [Ru 3 (CO) 12 ] as precursor). For the same catalyst, a drastic deactivation was detected, dropping LA conversion to 40% after the 3rd run and with a complete loss of activity in the 4th run. This behavior was attributed to nanoparticles agglomeration. Amenuvor et al. [117] investigated the catalytic performance of a series of pyrazolylphosphite ruthenium (II) complexes in solvent-free condition at Scheme 6. Catalytic hydrogenation of LA to GVL and subsequent products. Adapted from Refs. [104,105] with permission from MDPI and Royal Society of Chemistry. M. Achour et al. Tetrahedron Green Chem 2 (2023) 100020 7 120 ◦C during 16 h with 0.1 mol% catalyst loading, 20 mmol of FA and 20 mmol of KOH. The neutral catalyst complexes showed 96% LA conversion and 100% GVL selectivity while the rest of precursors presented traces of 4-hydroxyvaleric acid (4-HVA) as intermediate with low LA conversion and GVL selectivity. It was suggested that the neutral catalyst precursors may have higher activity than their cationic counterparts due to the higher stability of the cationic complexes (chelate ring formation). The addition of base promotes the formic acid deprotonation and dehydrogenation to CO 2 and H 2 [118] and facilitates the coordination of formate ion to the metal center, thus enhancing H 2 heterolysis and in consequence the hydrogenation reaction [119]. In this sense, Oklu et al. [120] reported the performance of pyridylimine ruthenium (II) complexes as catalyst precursors. The catalytic tests were carried out in presence of potassium hydroxide (KOH) or triethylamine (Et 3 N) as a solvent at 150 ◦C during 12 h. The highest yield was found while using Et 3 N (100% GVL selectivity and 98% LA conversion) and without formation of 4-HVA as intermediate. In contrast, the KOH promotes the production of 4-HVA intermediate decreasing the LA conversion but showing a full GVL selectivity. The catalyst operated 3 runs without any noticeable loss in activity. The authors demonstrated that their catalysts perform better with organic bases (Et 3 N) than with inorganic bases (KOH) and more studies demonstrated that the absence of base lead to low LA conversion with or without catalyst [121]. Another study [122] reported the use of cyclopentadienone Ru 0 complexes with different N- or P- donor ligands for the conversion of equimolar FA/LA mixture at 120 ◦C after 4 h. The pre-catalyst (as synthesized complex) showed 84% LA conversion with 99% GVL selectivity in EtOH solvent using KOH as base. Prolonging the reaction time up to 16 h lead to LA conversion increase (97% with 99% GVL selectivity). Nevertheless, a longer reaction time (24 h) produced traces of MTHF (7%) decreasing the GVL selectivity. The pre-catalyst showed excellent stability during four cycles without important decrease in activity. Noyori’s catalysts (Ru−TsDPEN) were also evaluated in asymmetric transfer hydrogenation (ATH) of LA to GVL with FA [123] in methanol and using N-methylpiperidine (in 1:1 M ratio respect to FA). 98% yield of chiral R-GVL with enantiomeric excess of 93% was obtained at 30 ◦C after 16 h of reaction time. Au-based catalysts are also commonly employed in LA conversion to GVL. Son et al. [114] were able to produce a reasonable GVL yield using 5 wt% Au/ZrO 2 catalyst in water at 150 ◦C during 5 h. The catalyst showed excellent activity and recyclability with full LA conversion, high GVL yield (97%) and complete FA decomposition to CO 2 and H 2 . The catalyst activity remained stable for at least five consecutive runs. The same catalyst was used for one-pot synthesis of GVL from fructose under the same conditions showing 48% GVL yield. Du et al. [124] also prepared gold NPs supported on acid-tolerant zirconia (Au/ZrO 2 -VS). The catalyst showed full LA conversion with selectivity to GVL rounding 99% at 150 ◦C after 6 h of reaction at LA/FA molar ratio =1. The material demonstrated important stability in five runs with an insignificant GVL yield decrease to 95%. The authors evaluated the support nature effect on this reaction. While Au/TiO 2 sample showed moderate GVL yields, Au/SiO 2 and Au/C catalysts were almost inactive, leading to traces of desired product. On the other hand, Li et al. [125] investigated the performance of Au nanoparticles supported on Ce x Zr 1-x O 2 mixed oxides in aqueous media and equimolar FA/LA mixture. The best results were registered for 2 wt% Au/Ce 0⋅4 Zr 0⋅6 O 2 sample with 91% LA conversion and 83.5% GVL yield at 240 ◦C after 2 h. At prolonged reaction times, LA conversion increased slightly but the associated GVL yield and selectivity decreased significantly in favor to methyltetrahydrofuran (MTHF). Activity loss was not observed after five runs (90% LA conversion) attributed to both, improved metal-support interaction in presence of Ce [126], formation of tetragonal zirconia phase [127] stabilizing Au against leaching in acidic conditions. In another study [128], several supported Au NPs (Au/ZrO 2 , Au/C, Au/Al 2 O 3 , Au/SiO 2 , Au/TiO 2 , Au/MgO) were evaluated at 210 ◦C during 5 h. The 6 wt% AuNPs/ZrO 2 -D catalyst (prepared by co-precipitation method) showed the best catalytic results, reaching 93% of LA conversion and 85% GVL yield. The authors suggested the existence of Au 3+ species involved in GVL yield improvement. Species leaching caused GVL yield decrease to 69% after three successive runs. On the other hand, Al-Naji et al. [129] studied the catalytic performance of Pt nanoparticles supported on mesoporous zirconia (Pt/MP-ZrO 2 ). The catalyst showed 97% LA conversion and 90% GVL yield at 240 ◦C in 24 h with complete FA conversion to H 2 and CO 2 . However, the efficiency drops importantly after 4 reaction cycles. At high reaction temperature (above 240 ◦C), the reaction equilibrium shifted from GVL to pentanoic acid (PA) formed with 22% yield after 24 h of reaction time. Ortiz-Cervantes et al. [118] investigated the efficiency of palladium complexes. The pre-catalyst [(dtbpe)PdCl 2 ] showed complete LA conversion and GVL yield at 100 ◦C, 1 h, 1 equivalent of FA, 33 mol.% of NEt 3 in water and 0.1 mol.% of Pd complex. The change of the solvent resulted in poor GVL yields, 9% for 1,4-dioxane and 3% for THF. The highest activity in water medium was related to the higher decomposition capability of [(dtbpe)PdCl 2 ] in aqueous media [130]. 3.1.2. Non-noble metal based catalytic systems Nickel, copper and iron-based systems are the most studied nonnoble catalysts. Varkolu et al. [131] studied Ni dispersed on various supports (MgO, Al 2 O 3 and hydrotalcite) as catalysts for the gas phase hydrogenation of levulinic acid. Within the series, Ni/Al 2 O 3 showed the best catalytic results converting 99% of LA to GVL yield up to 90% at 250 ◦C and 1 h of reaction. They found that the main factors related to Ni/MgO deactivation were both, water and coke formation during the hydrogenation and condensation reactions, respectively, as reported also by Mohan et al. [132]. In another study from Varkolu’s group [133], Ni/SiO 2 catalyst prepared by citric acid assisted method achieved complete LA conversion and 93% GVL yield in the reaction conditions reported above. The conversion continuously decreased with the time on stream attributed to catalyst’ coking activity increase in presence of formic acid. The latter acts as Brønsted acid catalyzing the condensation of reaction intermediates and leading to coke formation. Therefore, the CTH reaction in liquid phase must solve the main challenge of preventing the catalyst deactivation. Gundekari et al. [134] studied the catalytic transfer hydrogenation of LA to GVL over Ni/SiO 2 –Al 2 O 3 catalysts in a batch reactor using water as solvent. The catalyst shows 70% LA conversion with 70% GVL yield at 200 ◦C in 10 h of reaction. Guo et al. [135] reported the use of Ni-functionalized carbon (Ni/NiO-FC) in ionic liquid 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) as solvent and super critical CO 2 (scCO 2 -IL) with FA/LA molar ratio of 12. They found 98% of LA conversion and 97% GVL yield at 170 ◦C after 3 h of reaction time. Nevertheless, the use of such FA/LA ratio could produce an excess of hydrogen leading to further GVL hydrogenation into byproducts and lowering lactone selectivity [115,136]. On the other hand, the formation of byproducts lowers the reaction phase viscosity promoting the mass transfer and therefore the hydrogenation reaction. In addition, catalyst recycling showed acceptable stability with 5% of catalytic activity loss after five catalytic cycles. Yuan et al. [13], investigated the performance of Cu/ZrO 2 –OG catalyst (OG stands for oxalate gel preparation) achieving complete LA conversion and GVL yield at 200 ◦C after 5 h of reaction in batch reactor. Some reports argued the use of supported copper catalysts in hydroconversion of carboxylic acids due to Cu leaching [14] and subsequent deactivation, but the 20 wt% Cu/ZrO 2 –OG catalyst showed high stability. No copper ions were detected after catalyst recycling, suggesting strong metal-support interaction between copper and zirconium oxide matrix [137] and complete absence of metal leaching. In addition, the XRD results showed no important change in Cu NPs mean diameter after recycling. Lomate et al. [138] used commercial Cu–SiO 2 -Q6 catalyst (average pore diameter of 2.9 nm) in a fixed bed reactor, obtaining 66% LA conversion and 81% GVL selectivity at 270 ◦C after 1 h of reaction. They suggested that Cu + species act as Lewis’s acid sites, forming angelica lactone (AGL) via dehydration [139]. Another study of the same M. Achour et al. Tetrahedron Green Chem 2 (2023) 100020 8 group [140] evaluated several supported copper catalysts (Cu–SiO 2 , Cu–TiO 2 , Cu-ZSM-5, Cu–Al 2 O 3 and Cu-SAL), with a copper loading of 6 wt% each. Among them, Cu–SiO 2 showed good performance with an increase of LA conversion (48 vs. 56%) and GVL selectivity (80 vs. 87%) with the increase of FA concentration (1:2 to 1:3 LA/FA ratio). Cu–Al 2 O 3 showed a good selectivity toward GVL, but a low LA conversion; while Cu-ZSM-5, Cu–TiO 2 and Cu-SAL catalysts exhibited high selectivity toward AGL attributed to the presence of acidic sites. Cu–TiO 2 showed the highest selectivity to pentanoic acid and the lowest LA conversion and GVL selectivity. The intrinsic magnetic properties of iron oxides have attracted attention, providing an easier catalyst separation step after reaction. Ashoukraju et al. [141] evaluated the performance of Cu/Fe 2 O 3 catalyst under optimized conditions (250 ◦C, 1 h) in down-flow fixed bed glass reactor. The LA conversion and GVL selectivity linearly increased with the copper content, although some decrement in conversion was detected after 8 h of time on stream. More reports over the same system assumed that LA transformation depends on copper and iron oxide particles presence [142–145]. While iron oxides particles decompose selectively FA [146], a moderate LA conversion is observed over pure Fe 2 O 3 catalyst suggesting that the hydrogenation role is played by the copper metal. Yepez et al. [145] evaluated also supported iron oxide nanoparticles on porous silicates. The prepared Fe/Zr-SBA-15 showed a low LA conversion (30%) with 78% GVL selectivity at 200 ◦C after 1 h of reaction in pressure-controlled CEM-Discover microwave reactor. Homogeneous catalysts were employed by Godwa et al. [147], particularly earth-abundant metal carbonyls M −CO (with M: Fe, Co, W, Cr and Mo) nanoparticles. Fe 3 (CO) 12 pre-catalyst achieved 93% of GVL yield at 190 ◦C in aqueous media with FA/Et 3 N molar ratio =6:1 after 36 h of reaction. As an alternative to Ni and Cu based systems, several MnCo oxide catalysts have been reported [148], being the Mn 2 Co 0.1 O x formulation the one that provided the best results (around 79% LA conversion and 77% GVL selectivity at 230 ◦C after 20 h of reaction in aqueous medium and FA/LA molar ration of 10). Byproducts have also been observed, valeric acid being the principal, but also acetone, ethanol, isopropanol and even methane. It was suggested that the lattice distortion of MnCoO 3 phase lowers the overall reaction energy barrier by favoring the formation of H–H bond between two adjacently pre-adsorbed H atoms on the catalyst surface. In that way, the hydrogen desorption step results favored, increasing the rate of hydrogenation reaction [149–151]. The presence of Co promotes the π - π interaction with the ketone group of LA [152], while Mn species enhance the C–H bond cleavage rate thus promoting H 2 generation [153–155]. In addition, MnCo catalysts presented excellent recyclability after five cycles without any significant loss in activity. 3.1.3. Bimetallic catalytic systems Bimetallic catalyst formulations have received important attention in LA CTH reaction due to the presumption that the combination of the properties of two metals could influence both hydrogen generation reaction and following hydrogenation. Drew et al. [156] used bimetallic 15 wt% RuRe/C catalyst achieving 95% LA conversion and 95% GVL selectivity. Complete FA conversion was attained in presence of sulfuric acid at 160 ◦C. Nevertheless, these drastic conditions caused a severe deactivation of the monometallic Ru/C counterpart. Sneka-Platek et al. [26] reported the use of Ag–Pd bimetallic catalysts prepared by different impregnation methods (co-impregnation method (CIMP), subsequent impregnation method (SIMP), co-impregnation method with chemical reduction (CIMPCR) and subsequent impregnation with chemical reduction (SIMP-CR). A 4%Ag–1%Pd/AlOOH catalyst prepared by the four methods reached a complete formic acid decomposition at 190 ◦C after 2 h but the LA conversion and GVL yield remained low, 34% and 32%, respectively. All the other catalysts showed very low LA conversion and GVL yield. Serra et al. [157] investigated the catalytic activity of Ni–Pt mesoporous nanowires (MNWs) at 140 ◦C for 2 h in free-solvent conditions and LA/FA weight ratio of 1/2. Ni–Pt MWs showed an excellent performance with GVL yield up to 99% and complete LA conversion. Notwithstanding, only the Ni 78 Pt 22 showed excellent reusability, with minimal nickel leaching into the reaction mixture. The catalyst operated in 4 successive cycles without loss of activity or need of catalyst reactivation. The increase of the Ni content, however, resulted in an important metal corrosion. Bimetallic 1%Au– 2%Ni/ZrO 2 catalyst have been reported by Zhou et al. [158] using equimolar FA/LA mixture, achieving 89% LA conversion and 88% GVL yield (complete FA decomposition) in aqueous solution at 240 ◦C and 1 h of reaction. Longer reaction time did not appear to impact the process; though increasing the temperature up to 260 ◦C lead to a slight decrease in GVL selectivity due to its further hydrogenation to pentanoic acid. The authors also studied the performance of monometallic 1%Au/ZrO 2 and 2%Ni/ZrO 2 catalysts separately. The former showed complete FA decomposition for 73% LA conversion and 67% of GVL yield, whereas the latter manifested negligible LA (1%) and FA (19%) conversion. Soszka et al. [159] evaluated the influence of Ni doping by low noble metals (Pd, Pt, Rh, Ru) amount using water as solvent at 190 ◦C and 4 h of reaction. The authors found an excellent Ni–Pd activity, 83% LA conversion, full FA decomposition and 79% GVL selectivity. Later work of the same group based on Au–Ni/Al 2 O 3 catalysts [27] studied the impact of the preparation methods (subsequent impregnation (SI), deposition precipitation (DP), co-impregnation (CI) and chemical reduction (CR)) on the reaction. Au–Ni (CI) /γ-Al 2 O 3(C) catalyst reached 89% LA conversion and 86% GVL yield with complete FA decomposition at 190 ◦C in 2 h of reaction. The authors also provided a DFT comparison of the bimetallic Au–Ni alloy with the monometallic Au and Ni formulation and found the existence of strong Au–Ni interaction lowering the energy barrier for FA decomposition. The highest hydrogen production for the bimetallic catalysts resulted in higher GVL yields. The performance of nickel-promoted copper-silica nanocomposite was investigated by Upare et al. [160]. Within the prepared series, Ni 20 –Cu 60 /SiO 2 catalyst showed a complete LA conversion with 98% GVL yield at 265 ◦C after 10 h of reaction using 1,4-dioxane as solvent and FA/LA ratio of 0.5. The catalyst reached 200 h of operation without deactivation. Bimetallic Ag–Ni/ZrO 2 catalysts prepared by co-precipitation were also reported [136]. The 10%Ag–20%Ni/ZrO 2 sample achieved a complete FA conversion, 99% LA conversion and 99% of GVL yield at 220 ◦C after 3 h of reaction in aqueous phase with FA/LA molar ratio of 10. The monometallic formulations (20% Ni/ZrO 2 and 10% Ag/ZrO 2 ) achieved high GVL selectivity but low GVL yield (22% for Ag vs 34% for Ni). As reflected above, the CTH reaction using formic acid as H-donor can occur over different metals formulation. While the use of noble metals requires lower temperatures, the non-noble metals need in general 100 ◦C more and longer times to achieve acceptable GVL yields. The CTH reaction is conditioned by the FA decomposition and higher metal loading is applied in the case of non-noble metals to reach the needed pressure for the subsequent hydrogenation. The variation of the conditions does not allow a proper comparison between the used systems and often very controversial conclusions are made on catalyst stability and deactivation behavior. A summary of the described systems, reaction conditions and results are provided in Table 2. 3.2. Oxygenated furans upgrading via CTH and hydrodeoxygenation reactions Formic acid can be used in the upgrade of other molecules, all belonging to the biomass feedstock spectra and two of the most important being 5-hydroxymethyl furfural (HMF) and furfural (FF), respectively. 3.2.1. HMF upgrading HMF can be used to produce a wide variety of chemicals and M. Achour et al. Tetrahedron Green Chem 2 (2023) 100020 9 biofuels, including levulinic acid [85], 2,5-furandicarboxylic acid (FDCA) [161], ethyl levulinate [162], and 2,5-dimethylfuran (DMF) [163]. Nevertheless, HMF is known as an unstable molecule, participating in unwanted side reactions with the formation of different byproducts besides being transformed by traditional routes [164,165]. That is why, HMF conversion to other value-added products must be undertaken on selective catalyst using formic acid via CTH/HDO/hydrogenolysis reactions. An example of such process is the production of DMF via selective hydrodeoxygenation of renewable HMF [166]. DMF molecule possesses advantageous physical and chemical characteristics to be used as possible liquid transportation biofuel such as a volumetric energy density of 30.0 MJ/L(very close to gasoline), nearly optimal boiling point, miscible with gasoline and immiscible with water. DMF has been already favorably tested in combustion engines [167]. Moreover, DMF can be used as feedstock for the manufacture of p-xylene via Diels-Alder reaction [168]. Hence, the production of liquid fuels and chemicals in the future will rely heavily on the reductive upgrading of HMF to DMF. The tests with pure DMF, DMF/gasoline mixtures, and DMF/diesel blends, respectively, are being explored to learn more about their combustion and emission properties for future use in internal combustion engines [169–171]. A complete hydrogenation of HMF can also result in 2,5-Bis(hydroxymethyl)tetrahydrofuran (BHTF) [172], an important compound used in the production of resins, crown ethers, and heat insulating materials [164,173,174]. Furthermore, BHTF is a potential diol that can be used to synthesize memory-shape and self-healing polymers as well as 1,6-hexanediol, another crucial polymer precursor [175]. There are two known pathways for the synthesis of DMF (Scheme 7), the first involves the 5-HMF hydrogenolysis to 5-methylfuran (5-MF) which further hydrogenates to 2-hydroxymethyl-5-methylfuran (HMMF), the latter converted to DMF through hydrogenolysis of its carbonyl group. In the second pathway HMF is hydrogenated to 2,5-bis (hydroxymethyl)furan (BHMF), further converted to HMMF by hydrogenolysis and subsequently transformed to DMF. Therefore, the synthesis of DMF needs hydrogenation/hydrodeoxygenation catalysts able to activate hydrogen production in situ from formic acid. Mitra et al. [172] studied the hydrodeoxygenation of HMF to DMF using FA as hydrogen donor. They used a Pd/C catalyst and achieved 95% HMF conversion and 95% DMF yield, at 120 ◦C after 15 h using dioxane as solvent. The same catalyst was explored in another study in THF/H 2 SO 4 mixture [176], showing complete HMF conversion and 95% DMF yield after 5 h at 110 ◦C. Bifunctional Pd NPs supported on N-doped mesoporous carbon (Pd/ NMC) has also been reported [177] with 61% of HMF conversion and 39% DMF yield. The nitrogen content appears to promote the activity as the N-free mesoporous carbon supported catalyst (Pd/CMC) reached only 21% of HMF conversion and 13% DMF yield at 160 ◦C after 3 h in THF solvent. Both catalysts showed important improvement in activity when the FA was combined with gaseous H 2 . Pd/NMC achieved full HMF conversion and 97% DMF yield, while the Pd/CMC showed 99% HMF conversion and 72% DMF yield. Pd–Au/g-C 3 N 4 has also been investigated [178], showing good activity with an important DMF yield (86%) achieved in water/ethyl acetate mixture after 1 h at room temperature. The same metals supported Table 2 Formic acid as hydrogen donor in catalytic transfer hydrogenation of levulinic acid. Catalysts Solvent a Conversion (%) Yield b (%) Time (h) Temperature (◦C) Ref. Ru/C Et 3 N 87.26 80.75 3 160 [108] Ru/C H 2 O 81 57 2 190 [52] Ru–P/SiO2 H 2 O – 96 12 170 [106] RuCl 3 ,3H 2 O/PPh 3 – – 94 12 150 [14] Ru/ZrO 2 H 2 O 73 73 12 150 [111] Ru/C H 2 O 100 90 5 150 [114] Ru-NPs H 2 O +Et 3 N 99 100 24 130 [116] Pyrazolylphosphite ruthenium – 96 100 16 120 [117] Pyridylimine Ru (II) complexes Et 3 N 98 98 12 150 [120] Shvo catalyst – 100 99 8 100 [115] Ru 0 complexes EtOH 84 99 b 4 120 [122] Ru-TsDPEN MeOH 99 98 16 30 [123] Au/ZrO 2 H 2 O 100 97 5 150 [114] Au/ZrO 2 -VS – 100 99 6 150 [124] Au/Ce x Zr 1x O 2 H 2 O 90.8 83.5 2 240 [125] Au NPs/ZrO 2 -D – 93.3 85 5 210 [128] Au–Ni/ZrO 2 H 2 O 89.1 87.9 1 240 [158] Pt/MP-ZrO 2 – 97 90 24 240 [129] [(dtbpe)PdCl 2 ] H 2 O 100 100 5 100 [118] Ni/Al 2 O 3 – 99 >90 1 250 [131] Ni/SiO 2 – 100 93 1 250 [133] Ni/SiO 2 –Al 2 O 3 H 2 O 70 70 10 200 [134] Ni/NiO-FC scCO 2 -IL 98 97 3 170 [135] Cu/ZrO 2 –OG H 2 O 100 100 5 200 [13] Cu–SiO 2 -Q6 – 66 81 b 1 270 [138] Cu–SiO 2 – 56 87 b 1 250 [140] Cu/Fe 2 O 3 – 100 100 1 250 [141] Fe/Zr-SBA-15 – 30 78 c 1 200 [145] Fe 3 (CO) 12 – – 93 36 190 [147] Mn 2 Co 0.1 O x H 2 O 78.9 76.7 b 20 230 [148] RuRe/C H 2 SO 4 95 95 b – 160 [156] Ag–Pd/ALOOH – 34 32 5 190 [26] Ni–Pt – 100 99 2 140 [157] Au–Ni/Al 2 O 3 H 2 O 89 86 2 190 [27] Au–Ni/ZrO 2 H 2 O 89.1 87.9 1 240 [158] Ni–Pd(Cl)/γ-Al 2 O 3 H 2 O 83 79 4 190 [159] Ni–Cu/SiO 2 DO 100 98 10 265 [160] Ag–Ni/ZrO 2 H 2 O 99 99 3 220 [136] a Et 3 N: Triethylamine, EtOH: Ethanol, MeOH: Methanol, scCO 2 -IL: Super critical carbon dioxide and ionic liquid, H 2 SO 4 sulfuric acid, DO: 1,4-dioxane. b Selectivity. M. Achour et al. Tetrahedron Green Chem 2 (2023) 100020 16 very important; they are not only spectators. Supports, such as functionalized carbons and metal oxides could provide surface oxygen vacancies (functional groups) interacting directly with the oxygen containing functional groups of the biomass molecule and available basic sites that capture directly hydrogen from formic acid, thus facilitating the H donation. Supports like carbon nitride becomes more and more interesting providing the possibility to modify the electronic properties of the dispersed metal via different metal-support interactions. Moreover, the textural properties of the support (surface area, porosity, hydrophilicity … etc.) also play an essential role in catalyst stabilization. Obviously the choice of reaction conditions (temperature, initial concentration, time, solvent nature and amount … etc.) and chemical reactors is another key step when envisaging formic acid use as hydrogen generating agent. In many cases, the use of formic acid instead of molecular hydrogen leads to improved selectivity in CTH/HDO mediated biomass upgrading reactions but still the selectivity and product yield control in some reactions should be well optimized. The operational and safety drawbacks of H 2 transportation, storage, and use along with its high cost, positions the use of liquid hydrogen carriers as an excellent approach to potentiate the use of in-situ generated hydrogen in biorefinery. Among the liquid organic carriers, formic acid needs special attention of being easily producible from biomass. The possibility to use formic acid produced after oxidation of glucose or as a co-product of the levulinic acid allows the decrease of the investment requirement to cost the catalytic systems. So, any future work must be dedicated to propose a stable catalyst formulation at low price able to work in all hydrogen involved biorefinery reactions. In the last decade, the catalytic society has advanced with giant steps toward the development of such systems and the time to increase the industrial production and use of formic acid will come sooner than expected. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Financial support was obtained from Spanish Ministerio de Ciencia e Innovaci´ on (MCIN/AEI/10.13039/501100011033/) and for FEDER Funds una manera de hacer Europa), Project PID2020-113809RB-C32. Also the financial support from Junta de Andalucía via Consejería de Transformaci´ on Econ´ omica, Industria, Conocimiento y Universidades and its PAIDI 2020 program (Grant P18-RT-3405) co-financed by FEDER funds from the European Union is highly appreciated. References [1] L.T. Mika, E. Cs´ efalvay, ´ A. 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