Heterometallic dimer catalyst stabilised in a metal-organic framework with methylcysteine and use thereof in acetylene semi-hydrogenation

Abstract

The invention relates to a heterobimetallic dimer catalyst stabilised by thioether residues of the methylcysteine of a metal–organic framework (MOF) and represented as X1-Y1-MOF, which has the general chemical formula: X1-Y1@{MIICuII 6[(S,S)–Mecysmox]3(OH)2(H2O)}. nH2O, where: - X and Y are metals, where X is different from Y; - X is selected from Pd, Pt and Ni; - Y is selected from Pd, Fe, Ni, Cu, Co, Au, Pt, Sn, Rh and Ag; - Mecysmox means bis[S-methylcysteine]oxalyl diamide; - M is selected from Ca, Sr and Ba; and - n = 0-20, both values inclusive. The invention further relates to the use thereof in hydrogenation reactions, particularly in acetylene semi-hydrogenation.

Description

[0001]HETEROMETALLIC DIMER CATALYST STABILIZED IN A METALORGANIC NETWORK WITH METHYLCYSTEINE AND ITS USE IN THE SEMI-HYDROGENATION OF ACETYLENE Field of the invention The present invention describes a heterobimetallic dimer catalyst stabilized by the thioether residues of methylcysteine ​​​​in a metalorganic network and its use especially in hydrogenation reactions, and more particularly, in a process for the semi-hydrogenation reaction of acetylene under industrial conditions ("front-end"). Background The selective semi-hydrogenation reaction of acetylene in ethylene flows is one of the highest volume processes in the petrochemical industry. Ethylene needs to be purified before polymerization, in order to avoid poisoning of the polymerization catalyst by acetylene.Polyethylene is one of the most in-demand chemicals worldwide, with an estimated annual production of >100 million tons, so ethylene purification moves ~140 million tons of ethylene per year globally [ED Slack, CM Gabriel, and BH Lipshutz. “A palladium nanoparticle-nanomicelle combination for the stereo-selective semihydrogenation of alkynes in water at room temperature”, Angewandte Chemie - International Edition 53 (51 2014), pp. 14051–14054. doi: 10.1002 / anie.201407723]. Ethylene purification is mainly carried out in so-called “front-end” reactors, where acetylene is hydrogenated to ethylene.Although this hydrogenation reaction is thermodynamically favored (ǻH°298 = -172 kJ / mol), a catalyst is needed not only to overcome kinetic barriers but also to control the selectivity of the process, since other undesired reactions such as ethylene hydrogenation to ethane and acetylene dimerization are also thermodynamically favorable (ǻH°298 = - 136, -308 and -173 kJ / mol, respectively). The current industrial catalyst for the semi-hydrogenation of acetylene in ethylene streams is composed of Pd and Ag species, along with other additives (such as Na, Ca), and the resulting output stream must contain < 1 ppm acetylene and < 2% ethane to be industrially acceptable [N. Szesni et al. “Catalyst composition for selective hydrogenation with improved characteristics: WO2013057244A1”]. In addition, a reaction temperature < 60 ºC and a space velocity (WHSV) > 50,000 ml^g are required. -1 ^h -1to avoid over-hydrogenation reactions and unwanted polymerizations (formation of green oil and coke, for example), which deactivate the catalyst. It is difficult to find a catalyst in the open literature that meets the industrial requirements mentioned above. A bibliographic survey shows a large number of solid catalysts with different metallic compositions, supports, and diverse operating conditions (feed composition, temperature, space velocity), but none of them fully meet the industrial requirements mentioned above. A review [T.D. Shittu and O.B. Ayodele. “Catalysis of semihydrogenation of acetylene to ethylene: current trends, challenges, and outlook,” Frontiers of Chemical Science and Engineering 16 (72022). doi: 10.1007 / s11705-021-2113-3 and S. Zou et al. “Grafting nanometer metal / oxide interface towards enhanced low-temperature acetylene semi-hydrogenation.”In: Nature Communications 12 (12021). doi: 10.1038 / s41467-021-25984-8, summarize the most relevant data for >100 catalyst systems. Unfortunately, the catalysts are in almost all cases far from any industrial use. For example, acetylene conversions >99% are generally reported, but are often accompanied by low selectivity, and the specification of <1 ppm acetylene in the final stream (99.999% conversion) is rarely achieved. Furthermore, studies frequently employ an inert diluent gas or eliminate ethylene from the stream (using only acetylene) to better control reactivity, which is, of course, not a realistic approach. “Tail-end” reaction conditions (only a slight excess of H2) are also employed to minimize ethylene hydrogenation, at the expense of promoting undesired polymerizations.Furthermore, reaction temperatures are typically >100 °C to increase productivity, and in fact, reactions occurring below this temperature are called "low" temperature reactions. As mentioned above, the catalyst currently used in industry consists of Pd and Ag aggregates, with some additional additive to regulate selectivity. Various Pd alloys have been described that catalyze the reaction with improved activity compared to their unalloyed counterparts, and, in particular, individual Pd1 atoms supported on different metal clusters and nanoparticles (NPs) have shown good catalytic activity. It is accepted in the literature that Pd1 can dissociate H2, so Pd aggregates are no longer strictly necessary.To our knowledge, no dimer alloys containing a single Pd atom have been experimentally described on solid catalysts, much less for the semi-hydrogenation reaction of acetylene. Metal-organic frameworks (MOFs) are porous microstructured solids with extremely high surface areas, capable of acting as nanoreactors and producing unprecedented metallic chemical species [M. Viciano-Chumillas et al. “Metal-Organic Frameworks as Chemical Nanoreactors: Synthesis and Stabilization of Catalytically Active Metal Species in Confined Spaces”, in: Accounts of Chemical Research 53 (2020). doi: 10.1021 / acs.accounts.9b00609].A bibliographic search on catalysts for the efficient and selective semi-hydrogenation of acetylene, based on dimeric bimetallic alloys, reveals that no such catalyst exists yet. It is true that a certain number of relevant studies have included Pd as an active species in their catalysts, but with higher nuclearities (i.e., a greater number of Pd atoms) than the dimers of the present invention and with results very far from industrial application. For example, an alloy of isolated Pd atoms in Cu NPs [G.X. Pei et al. “Performance of Cu-Alloyed Pd Single-Atom Catalyst for Semihydrogenation of Acetylene under Simulated Front-End Conditions”, in: ACS Catalysis 7 (2017), pp. 1491-1500. doi: 10.1021 / acscatal.6b03293.] requires a reaction temperature of 160 ºC to achieve complete conversion of acetylene, with acceptable selectivity to ethylene (85%), and this only occurs when the CuPd0 alloy is used.006, since higher amounts of Pd result in very poor selectivity. Although the authors work with a stoichiometry of 20:1 H2:acetylene, which could be understood as 'front-end' conditions, their reaction stream contains 60% He. Therefore, the actual product flow rate is 40% lower. In fact, the inert co-feed is a constant throughout the literature, and a pure ethylene stream is not used at the reactor inlet, as occurs under industrial conditions and according to the present invention. The same authors report materials with single Pd atoms alloyed in Au NPs [GX Pei et al. “Promotional effect of Pd single atoms on Au nanoparticles supported on silica for the selective hydrogenation of acetylene in excess ethylene”, in: New Journal of Chemistry 38 (52014), pp. 2043-2051. doi: 10.1039 / c3nj01136d.] and Ag[GX Pei et al.“Ag Alloyed Pd Single-Atom Catalysts for Efficient Selective Hydrogenation of Acetylene to Ethylene in Excess Ethylene”, in: ACS Catalysis 5 (2015), pp.3717–3725. doi: 10.1021 / acscatal.5b00700] for the selective hydrogenation reaction of acetylene under the same temperature and flow conditions mentioned above. The authors obtained an 85.9% conversion of acetylene with a 56.4% selectivity to ethylene with the AuPd0.01 / SiO2 material, and a 92.6% conversion with a 92.3% selectivity to ethylene with AgPd. 0.005 / SiO2. Other authors use Pd alloys to selectively catalyze the hydrogenation of acetylene, with Sn [R. Li et al. “Selective hydrogenation of acetylene over Pd-Sn catalyst: Identification of Pd2Sn intermetallic alloy and crystal plane-dependent performance”, Applied Catalysis B: Environmental 279 (July 2020), pp. 1-11. doi: 10.1016 / j.apcatb.2020.119348] outperforming other alloying agents in terms of selectivity and conversion, and Pd-Zn [M. Hu et al. “MOF-Confined Sub-2 nm Atomically Ordered Intermetallic PdZn Nanoparticles as High-Performance Catalysts for Selective Hydrogenation of Acetylene” in: Advanced Materials 30 (33 2018); doi: 10.1002 / adma.201801878], improving the results obtained in Pd-Ag [A. Pachulski, R. Schodel and P. Claus. “Performance and regeneration studies of Pd-Ag / Al2O3catalysts for the selective hydrogenation of acetylene”, Applied Catalysis A: General 400 (1-22011), pages. 14-24, doi: 10.1016 / j.apcata.2011.03.019.] with a similar WHSV.In these cases, however, half or more of the reaction inlet is again inert gas, resulting in a lower product yield compared to the reaction conditions of the present invention. Pd supported on carbon nanodiamonds has also been used to catalyze the selective hydrogenation of acetylene [F. Huang et al. “Atomically Dispersed Pd on Nanodiamond / - Graphene Hybrid for Selective Hydrogenation of Acetylene”, Journal of the American Chemical Society 140 (412018), pp. 13142–13146. doi: 10.1021 / jacs.8b07476] but only by atomically isolating the Pd within these nanodiamonds were 100% acetylene conversions achieved with high selectivities. In addition to Pd, other metals have been studied as candidates for the semi-hydrogenation reaction of acetylene. For example, with an Fe-based MOF [M. Tejeda-Serrano et al.[“Isolated Fe(III)-O Sites Catalyze the Hydrogenation of Acetylene in Ethylene Flows under Front-End Industrial Conditions”, Journal of the American Chemical Society 140 (2018), pp.8827-8832.] and, working at 100 ºC, the authors achieve total conversion of acetylene and a 90% selectivity towards ethylene during the first minutes of the reaction, before losing efficiency. The amounts of H2 are well above those used in the industry (ethylene + acetylene (1.2%): 1 mL / min, 2 bar and H2: 2 mL / -min, 4 bar), corresponding to a high H2:acetylene ratio of approximately 300:1, which arises as a disadvantage of using a less active species compared to Pd in ​​hydrogenation reactions. A catalyst consisting of Pt layers formed on Au nanorods has also been used for this reaction [J. Wang et al. “Au-Pt Nanotubes within CoZn-Based Metal-Organic Framework for Highly Efficient Semi-hydrogenation of Acetylene”, Science 23 (62020), page 101233.doi: 10.1016 / j.isci.2020.101233], with high WHSV (Weight Hourly Space Velocity) values ​​but much lower conversion and selectivity than the present invention. In contrast, a Cu dendrite catalyst [J. Bu et al. “Selective electrocatalytic semihydrogenation of acetylene impurities for the production of polymer-grade ethylene”, Nature Catalysis 4 (72021), pp. 557-564. doi: 10.1038 / s41929-021-00641-x] achieved exceptional WHSV values ​​and equally exceptional conversion and selectivity. In this case, an electrochemical cell with water as the hydrogen source was used. Single-atom Pd-on-titania (TiO2) catalysts (SACs) have been prepared to catalyze the reaction under photothermal conditions [Y. Guo et al. “Photo-thermo semi- hydrogenation of acetylene on Pd1 / TiO2 single-atom catalyst”, in: Nature Communications 13 (1 Dec 2022), doi: 10.1038 / s41467-022-30291-x].Photoinduced electrons transferred from TiO2 to adjacent Pd atoms help activate acetylene, while strong metal-support interactions (SMSIs) render isolated atoms more active than coexisting Pd clusters and nanoparticles. Al2O3 microrods with different acidic properties have also been employed as Pd supports [Z. Yang et al. “Al2O3 microrods supported Pd catalysts for semi-hydrogenation of acetylene: Acidic properties tuned reaction kinetics behaviors”, in: Chemical Engineering Journal 445 (Oct. 2022). doi: 10.1016 / j.cej.2022.136681]. The semi-hydrogenation of acetylene over these supported Pd catalysts proceeds at low temperatures (up to 70 ºC) but with low selectivity (<60%). A Pd-Bi intermetallic compound has been used to catalyze the semi-hydrogenation of pure acetylene without ethylene [W. Zhang et al.“Bismuth-Modulated Surface Structural Evolution of Pd3Bi Intermetallic Alloy Catalysts for Selective Propane Dehydrogenation and Acetylene Semihydrogenation,” ACS Catalysis 12 (September 17, 2022), pp. 10531–10545. doi: 10.1021 / acscatal.2c00642.] It was found that, even without ethylene in the feed, only up to 80% ethylene selectivity could be achieved, and only when adjacent Pd atoms in the Bi3Pd are sufficiently separated to give catalytically isolated active sites. However, at higher Bi loadings, the BiOx overcoating leads to a greater loss of activity. Unsaturated penta-coordinated aluminum-coordinated Pd on Al2O3 (meso-alumina) has been used as a catalyst for the semi-hydrogenation reaction, showing an ethylene selectivity of ~83%, at full acetylene conversion (>99%) [L. Xu et al.“Anchoring Pd species over defective alumina to achieve high atomic utilization and tunable electronic structure for semi-hydrogenation of acetylene”, Applied Catalysis A: General 642 (Jul. 2022). doi: 10.1016 / j.apcata.2022.118690]. Pd agglomeration is suppressed by SMSIs. A Zr-based MOF with isolated Pd atoms was used to catalyze the semi-hydrogenation reaction of acetylene with a high turnover frequency (0.132 s). -1) [ W. Ru et al. “Control of Local Electronic Structure of Pd Single Atom Catalyst by Adsorbate Induction”, Small 18 (2022). doi: 10.1002 / smll.202103852]. The supported electron-deficient single Pd atoms are critical for the high intrinsic activity and 93% selectivity observed in long-time stability experiments (1500 min). PdCu alloy NPs on a zeolite are a highly active and selective catalyst for the semi-hydrogenation reaction, due to the particular structural features of the solid catalyst. [ Q. Luo et al. “ Alloyed PdCu Nanoparticles within Siliceous Zeolite Crystals for Catalytic Semihydrogenation”, ACS Materials Au (2021). doi: 10.1021 / acsmaterialsau.1c00080]. Furthermore, the typical oligomerization reaction triggered by zeolite acid is avoided by using siliceous zeolite.A solid Pd-Bi2O3-TiO2 containing hybrid clusters catalyzes the selective hydrogenation of acetylene with >90% conversion and selectivity on a metal / oxide basis [S. Zou et al. “Grafting nanometer metal / oxide interface towards enhanced low-temperature acetylene semi-hydrogenation”, Nature Communications 12 (1 2021). doi: 10.1038 / s41467-021-25984-8.239]. The clusters feature Pd-Pd bonds and intracluster electron transfer, allowing weak ethylene adsorption without compromising H2 activation. In terms of the catalytic process, the present invention has the following advantages over the state of the art: • It achieves 99.9999% acetylene conversion (less than 1 ppm remaining). • It reaches one of the highest WHSVs known to date, while maintaining total conversion over long times. • Does not use any inert flux. • It achieves these specifications by operating at a 10:1 H2:acetylene ratio, considered optimal “front-end” conditions.Description of the invention The present invention shows how a particular MOF with cysteine ​​groups in its structure is capable of generating heterometallic dimeric catalysts X1-Y1, where X is Pd, Pt or Ni and Y can be Pd, Fe, Ni, Cu, Co, Au, Pt, Sn, Rh or Ag, with X and Y being different. As an example, in the present invention, we present three catalysts (Pd1-Au. 1, Pt1-Au1 and Pd1-Ag1) fully characterized and tested in the semi-hydrogenation reaction of acetylene. The present invention relates firstly to a heterobimetallic dimer catalyst X1-Y1, where X is Pd, Pt or Ni and Y can be Pd, Fe, Ni, Cu, Co, Au, Pt, Sn, Rh or Ag, X and Y always being different, stabilized by the methylcysteine ​​thioether residues of a metal-organic network (MOF). The resulting catalyst has the general chemical formula: X1- Y1@{M II Cu II6[(S,S)–Mecysmox]3(OH)2(H2O)} . nH2O, where X and Y have the meaning above, Mecysmox stands for bis[S-methylcysteine]oxalyl diamide, M = Ca, Sr or Ba and n = 0-20, inclusive. The term “dimer” means a metal cluster containing only two metals. The symbol @ corresponds to a terminology frequently used in MOFs and expresses that what appears to the left of @ are the guest species housed in the MOF channels (in this case the dimers) while the chemical formula that appears to the right of @ is that of the MOF used. The amount of metal X (Pd, Pt or Ni) in the catalyst of the present invention may be between 0.01% and 40% by weight, relative to the total weight of the catalyst, preferably between 0.05 and 20% and more preferably between 1 and 7%, even more preferably between 3 and 6% relative to the total weight of the catalyst.The amount of Y (Pd, Fe, Ni, Cu, Co, Au, Pt, Sn, Rh or Ag), in the material of the present invention, may be between 0.01% up to 40% by weight, relative to the total weight of the catalyst, preferably between 0.05 and 20% and more preferably between 1 and 7%, even more preferably between 3 and 6% relative to the total weight of the catalyst. According to additional particular embodiments, X is palladium. According to additional particular embodiments, the amount of palladium is between 0.01% up to 40% by weight, relative to the total weight of the catalyst, preferably between 0.05 and 20% and more preferably between 1 and 7%, even more preferably between 3 and 6% relative to the total weight of the catalyst. According to additional particular embodiments, Y is gold. According to additional particular embodiments, the amount of gold is between 0.01% up to 40% by weight, relative to the total weight of the catalyst, preferably between 0.05 and 20% and more preferably between 1 and 7%, even more preferably between 3 and 6% with respect to the total weight of the catalyst. According to additional particular embodiments, X is palladium and Y is gold. According to specific additional particular embodiments, X is palladium, Y is gold and M is Sr. According to a particular embodiment, the catalyst has the formula Pd. 0.5 Au 0.5 @{Mr II Cu II6[(S,S)– Mecysmox]3(OH)2(H2O)} .12H2O. According to further particular embodiments, X is platinum. According to further particular embodiments, the amount of platinum is between 0.01% up to 40% by weight, relative to the total weight of the catalyst, preferably between 0.05 and 20% and more preferably between 1 and 7%, even more preferably between 3 and 6% relative to the total weight of the catalyst. According to further particular embodiments, Y is gold. According to further particular embodiments, the amount of gold is between 0.01% up to 40% by weight, relative to the total weight of the catalyst, preferably between 0.05 and 20% and more preferably between 1 and 7%, even more preferably between 3 and 6% relative to the total weight of the catalyst. According to further particular embodiments, X is platinum and Y is gold. According to specific additional particular embodiments, X is platinum, Y is gold and M is Sr. According to a particular embodiment, the catalyst has the formula Pt 0.5 Au0.5 @{AC II Cu II 6[(S,S)– Mecysmox]3(OH)2(H2O)} .8H2O. According to further particular embodiments, Y is silver. According to further particular embodiments, the amount of silver is between 0.01% up to 40% by weight, with respect to the total weight of the catalyst, preferably between 0.05 and 20% and more preferably between 1 and 7%, even more preferably between 3 and 6% with respect to the total weight of the catalyst. According to further particular embodiments, X is palladium and Y is silver. According to specific further particular embodiments, X is palladium, Y is silver and M is Ba. According to a particular embodiment, the catalyst has the formula Pd 0.5 Ag 0.5 @{Ba II Cu II6[(S,S)– Mecysmox]3(OH)2(H2O)}.9H2O. The present invention further relates to a process for obtaining the catalyst defined above, comprising: - synthesizing a base MOF with structure {Cu6M[(S,S)-Mecysmox]3(OH)2(H2O)}^ mH2O (where M = Ca, Ba or Sr), and where m is a variable number between 0 and 20 (for example, 12, 13, 14, 15, 16, 17, 18, 19 or 20) where Mecysmox means bis[S-methylcysteine]oxalyl diamide, from a precursor dimer of formula (Me4N)2{Cu2[(S,S)- Mecysmox](OH)}2.4H2O - introducing a metal salt of X and a different metal salt of Y (“different” means that the metal cation Y is different from the metal cation X), into the channels of the MOF synthesized in the previous step, in equimolar amounts, so as to form, within the channels of the MOF and after the addition of a reducing agent, a dimer formed by a combination of two different metal atoms X and Y, where X and Y have the meanings given above, which are stabilized by the thioether residues of methylcysteine. Any known inorganic salts of Ca can be used as metal salts of element M. 2+ , Mr 2+ or Ba 2+. Examples of salts are BaCl2, CaCl2, Ba(NO3)2 and, according to preferred embodiments, those used are BaCl2, CaCl2 and Ba(NO3)2. According to particular embodiments, the precursor MOF is selected from: {Cu6Sr[(S,S)-Mecysmox]3(OH)2(H2O)}^15H2O {Cu6Ca[(S,S)-Mecysmox]3(OH)2(H2O)}.16H2O and {Cu6Ba[(S,S)-Mecysmox]3(OH)2(H2O)}.13H2O The introduction of the respective salts of X and Y is carried out by successively immersing the precursor MOF for a certain time in each of the salt solutions consecutively. The dissolution of a salt of element M may comprise the use of a solvent such as, for example, alcohol, water, acetonitrile or DMF, or mixtures thereof. The solutions of salts of X and Y (for example, of Au and Pd) are different, and the precursor MOF is immersed in one and the other alternately. According to particular embodiments, the solutions of the salts of X and Y are solutions of water and alcohol, such as methanol.The water and alcohol may be mixed in proportions between 1:99 and 99:1 of water:alcohol. According to particular embodiments, the mixture of water and alcohol is a 1:1 mixture, for example, water / methanol in a 1:1 proportion. According to particular embodiments, the MOF of formula {Cu6M[(S,S)-Mecysmox]3(OH)2(H2O)} ^mH2O is suspended in equimolar solutions of a salt of X and a salt of Y, alternately. The suspension time of the {Cu6M[(S,S)-Mecysmox]3(OH)2(H2O)}^mH2O in each of the salts of X and Y may be variable, for example, for 1 hour. The operation of suspending the intermediate MOF precursor {Cu6M[(S,S)-Mecysmox]3(OH)2(H2O)}^ mH2O in the solutions of the salt of X and the salt of Y can be performed repeatedly to ensure the maximum amount of inserted metal salts of each of them, for example, it can be repeated up to 15 times, depending on the nature of X and Y.The product resulting from the introduction of the salts of X and Y is an intermediate solid material, of formula (X. a+ ) 0.5 (AND b+ ) 0.5 @{Cu6M[(S,S)-Mecysmox]3(OH)2(H2O)}^mH2O, where X, Y, a, b, M, m and Mecysmox have the meanings given above. This intermediate solid material is not isolated. It can be washed, for example, with a water / alcohol solution (such as methanol) in varying proportions, for example, 90:10 to 10:90, such as 1:1. As reducing agent, a conventional reducer can be used. Examples of reducing agents are NaBH4 or a stream of H2(g)) which can be used with similar results. By adding the reducing agent, the metals X and Y, which were in the form of cations in the respective salts are reduced and the final heterobimetallic dimer catalyst is formed. According to particular embodiments, the addition of NaBH4, to the intermediate solid material (X a+ ) 0.5 (AND b+ ) 0.5@{Cu6M[(S,S)-Mecysmox]3(OH)2(H2O)}^mH2O (where a and b are the number of positive charges of X and Y, respectively), is carried out progressively such that the NaBH4 is divided into fractions, allowing each fraction to react for a period of time, for example, one hour, up to, for example, 72 hours. The product resulting from the addition of the reducing agent, which is the dimeric catalyst X1-Y1-MOF (where X and Y have reduced their charge), can be washed with a water / alcohol mixture, such as methanol, and filtered, for example, on paper. In the specific case of the catalyst comprising Pd1-Au 1,it is shown how a particular MOF with cysteine ​​groups in its structure is able to generate the Pd1 - Au1 catalyst. A base MOF with structure {Cu6Sr[(S,S)-Mecysmox]3(OH)2(H2O)}^ 15H2O, with methylcysteine ​​groups, is synthesized, and precursor [Pd(NH3)4]Cl2 and AuCl3 groups are introduced into the channels, in equimolar amounts, to form the (Pd1 - Au1) dimer MOF, solving the structure of the catalytic solid by single crystal X-ray crystallography. (Figure 1). According to this particular embodiment, the process for obtaining the catalyst defined above comprises: introducing the salts [Pd(NH3)4]Cl2 and AuCl3 into the channels of a precursor MOF of formula {Cu6Sr[(S,S)-Mecysmox]3(OH)2(H2O)}.15H2O (Mecysmox = bis[S-methylcysteine]oxalyl diamide), and - a chemical reduction, with NaBH4, of the Pd metal cations. 2+ and Au 3+ , inserted into the channels, obtaining the Pd1 - Au1 dimer catalyst, with formula Pd 0.5 Au 0.5 @{MrII Cu II 6[(S,S)–Mecysmox]3(OH)2(H2O)} .12 H2O. The precursor MOF {Cu6Sr[(S,S)-Mecysmox]3(OH)2(H2O)}.15H2O is obtained by dissolving the precursor dimer (Me4 N) 2 {Cu 2[(S,S)-Mecysmox](OH)}2 .4H 2 O, - and adding to the previously obtained solution, a solution of a Sr(II) salt, such as Sr(NO3)2, obtaining {Cu6Sr [(S,S)-Mecysmox]3(OH)2(H2O)}.15H2O as a green polycrystalline powder. The precursor MOF can be prepared as described in: Tiburcio, E.; Greco, R.; Mon- Conejero, M.; Ballesteros-Soberanas, J.; Ferrando-Soria, J.; Lopez-Haro, M.; Hernández-Garrido, JC.. (2021). Soluble / MOF-Supported Palladium Single Atoms Catalyze the Ligand-, Additive-, and Solvent-Free Aerobic Oxidation of Benzyl Alcohols to Benzoic Acids. Journal of the American Chemical Society.143(6):2581-2592. https: / / doi.org / 10.1021 / jacs.0c12367 According to particular realizations, in obtaining Pd 0.5 Au 0.5 @{Mr II CuII 6[(S,S)– Mecysmox]3(OH)2(H2O)} .12 H2O the process comprises: - immersing the {Cu6Sr [(S,S)-Mecysmox]3(OH)2(H2O)}.15H2O (green polycrystalline powder) in solutions of [Pd(NH3)4]Cl2 and AuCl3, alternately and consecutively, obtaining a solid material as an intermediate product, of formula (Pd 2+ ) 0.5 (Au 3+ ) 0.5@{Cu6Sr[(S,S)- Mecysmox]3(OH)2(H2O)}^14H2O - suspend the solid material obtained above in a mixture of water and alcohol, and add NaBH4, giving rise to the dimer catalyst Pd1-Au1-MOF. The {Cu6Sr[(S,S)-Mecysmox]3(OH)2(H2O)}.15H2O (green polycrystalline powder) obtained in the first step, before performing the second step, can be collected, for example, by filtration and dried, for example, with ethanol, acetone and diethyl ether, obtaining a pure and dry precursor MOF. According to particular embodiments, the solutions of [Pd(NH3)4]Cl2 and AuCl3 are solutions of water and alcohol, such as methanol. According to particular embodiments, the mixture of water and alcohol is a 1:1 mixture. According to particular embodiments, the intermediate solid material, of formula (Pd 2+ ) 0.5 (Au 3+ ) 0.5 @{Cu6Sr[(S,S)-Mecysmox]3(OH)2(H2O)}^14H2O .,,obtained as a product resulting from immersing the green polycrystalline powder in a solution of [Pd(NH3)4]Cl2 and in a solution of AuCl3, is suspended in a mixture of water and alcohol, such as ethanol, over a period of time, which may be, for example, one hour. According to particular embodiments, the intermediate solid material, of formula (Pd 2+ )0.5(Au 3+ )0.5@{Cu6Sr[(S,S)-Mecysmox]3(OH)2(H2O)}^14H2O,. is suspended in equimolar solutions of [Pd(NH3)4]Cl2 and AuCl3, alternately, in which the solvent is a water / methanol mixture in 1:1 ratios, for variable periods of time, such as 1 hour. The operation of suspending the intermediate solid material in the [Pd(NH3)4]Cl2 and AuCl3 solutions can be carried out repeatedly to ensure maximum loadings of [Pd(NH3)4]Cl2, for example, it can be repeated up to five times. The intermediate solid material of formula (Pd 2+ ) 0.5 (Au 3+ ) 0.5@{Cu6Sr[(S,S)- Mecysmox]3(OH)2(H2O)}^14H2O , resulting from the introduction of [Pd(NH3)4]Cl2 and AuCl3 can be washed, for example, with a water / alcohol solution (such as methanol) in variable proportions, for example 1:1. According to particular embodiments, the addition of NaBH4 to the intermediate solid material of formula (Pd 2+ )0.5(Au 3+)0.5@{Cu6Sr[(S,S)-Mecysmox]3(OH)2(H2O)}^14H2O, is carried out progressively such that the NaBH4 is divided into fractions, allowing each fraction to react for a period of time, for example, one hour, up to, for example, 72 hours. The product resulting from the addition of NaBH4, which is the dimeric catalyst Pd1-Au1-MOF, can be washed with a water / alcohol mixture, such as methanol, and filtered, for example, on paper. According to a further particular embodiment of the process for obtaining a catalyst comprising Pt1-Au1, a base MOF with the structure {Cu6Ca[(S,S)-Mecysmox]3(OH)2(H2O)}^16H2O is synthesized, with methylcysteine ​​groups, and precursor K2PtCl4and AuCl3 groups are introduced into the channels, in equimolar quantities, to form the MOF of (Pt1-Au1) dimers. According to further alternatives, for this embodiment of the process, the K2PtCl4and AuCl3 salts are dissolved respectively in a mixture of water and alcohol, preferably water and methanol.The molar ratio of the salts to each other is, for example, H2O / CH3OH 1:6. The immersion time in each of the salt solutions can be, for example, 1 hour. According to additional alternatives, for this embodiment of the process, the intermediate solid material of formula (Pt. 2+ )0.5(Au 3+ )0.5@{Cu6Ca[(S,S)-Mecysmox]3(OH)2(H2O)}^10H2O can be washed with a mixture of water and alcohol such as water and methanol, and preferably, with a solution of H2O / CH3OH (1:1). According to additional alternatives, for this embodiment of the process, the intermediate solid material of formula (Pt2+ )0.5(Au 3+)0.5@{Cu6Ca[(S,S)-Mecysmox]3(OH)2(H2O)}^10H2O is subjected to reduction with NaBH4. Preferably, it is suspended in H2O / CH3OH, preferably in a 1:1 ratio, and the NaBH4 is divided into fractions (for example, 0.4 mmol of NaBH4 per mmol of MOF each). The reduction time with each NaBH4 fraction can be variable, and for example can be between 1 and 2 hours. According to additional alternatives, for this embodiment of the process, the final catalyst obtained after the reduction can be washed with solvents such as mixtures of water and alcohol, for example, water and methanol, and preferably, with a H2O / CH3OH solution. According to additional alternatives, for this embodiment of the process, a catalyst of formula Pt0.5Au0.5@{Ca II Cu II6[(S,S)–Mecysmox]3(OH)2(H2O)} .8H2O. According to a further particular embodiment of the process for obtaining a catalyst comprising Pd1 - Ag1, a base MOF with the structure {Cu6Ba[(S,S)- Mecysmox]3(OH)2(H2O)}.13H2O is synthesized, with methylcysteine ​​groups, and precursor [Pd(NH3)4]Cl2 and AgNO3 groups are introduced into the channels, in equimolar amounts, to form the (Pd - Ag1) dimer MOF. According to further alternatives, for this embodiment of the process, the [Pd(NH3)4]Cl2 and AgNO3 salts are dissolved in a mixture of water and alcohol, preferably water and methanol. The molar ratio of the salts to each other is, for example, equimolar. The immersion time in each of the salt solutions can be, for example, 1 hour. The immersion process can be repeated successively, for example, 8 times. According to additional alternatives, for this embodiment of the process, the intermediate solid material of formula (Pd 2+ ) 0.5 (Ag +) 0.5 @{Cu6Ba[(S,S)-Mecysmox]3(OH)2(H2O)}^10H2O, can be washed with a mixture of water and alcohol such as water and methanol, and preferably, with a solution of H2O / CH3OH (1:1). According to additional alternatives, for this embodiment of the process, the intermediate solid material of formula (Pd 2+ )0.5(Ag +)0.5@{Cu6Ba[(S,S)-Mecysmox]3(OH)2(H2O)}^10H2O, is subjected to reduction with NaBH4. Preferably, it is suspended in H2O / CH3OH, preferably in a 1:1 ratio, and the NaBH4 is divided into fractions (for example, 0.4 mmol of NaBH4 per mmol of MOF each). The reduction time with each NaBH4 fraction can be variable, and for example can be between 1 and 2 hours. According to additional alternatives, for this embodiment of the process, the final catalyst obtained after the reduction can be washed with solvents such as mixtures of water and alcohol, for example, water and methanol, and preferably, with a H2O / CH3OH solution. According to additional alternatives, for this embodiment of the process, a catalyst of formula Pd is obtained 0.5 Ag 0.5 @{Ba II Cu II6[(S,S)–Mecysmox]3(OH)2(H2O)} .9H2O. Furthermore, the present invention relates to the use of the catalyst defined above as a catalyst, such as in hydrogenation reactions. Furthermore, the present invention relates to the use of the catalyst defined above in a process for the semi-hydrogenation of acetylene under industrial conditions (“front-end”), comprising at least the following steps: - a first step in which the X1-Y1-MOF is contacted with an ethylene:H2:acetylene mixture, - a second step in which the mixture is heated, - a third step in which the product obtained in the second step is collected, - an optional fourth step for recovering the solid catalyst.According to a particular embodiment of the present invention, the amount of X1-Y1-MOF, (e.g. PdAu-MOF) placed in the reactor is between 50-500 mg per ml / min of total flow rate, and preferably between 5-400 mg per ml / min of total flow rate, and more preferably between 10-350 mg per ml / min of total flow rate. According to a particular embodiment of the present invention, the ethylene:H2:acetylene mixture in the first stage may vary between 1-20:99-80:0.05-0.01, and will preferably be 10:89.9:0.1. According to a particular embodiment of the present invention, hydrogen is fed in stoichiometries of 80:1 to 1:30 with respect to the total moles of ethylene fed to the reactor, preferably from 10:1 to 1:30, and preferably in stoichiometries of 1:10 - 1:30. According to a particular embodiment, the reaction can be carried out in batch or continuously, and preferably continuously in a tubular reactor.According to a particular embodiment of the present invention, the spatial velocity of the continuous reaction may be between 1000 and 100000 ml^g. -1 ^h -1 , preferably 20000 and 70000 ml^g -1 ^h -1 and more preferably between 30,000 and 50,000 ml^g -1 ^h -1. According to a particular embodiment of the present invention, the reaction temperature may be between 25 and 160 ° C, preferably between 30 and 90 ° C, and more preferably between 35 and 70 ° C. According to a particular embodiment of the present invention, the reaction may be carried out in reductive atmospheres under pressures between 1 and 20 bars, preferably between 1 to 5 bars, and more preferably 1 to 2 bars. According to another particular embodiment of the present invention, the contact time inside the tubular reactor may vary between 0.1 and 24 h, preferably between 0.1 and 10 hours, and more preferably, between 0.1 and 5 hours. Thus, the product obtained in the semi-hydrogenation reaction of acetylene is a mixture of ethylene:H2:acetylene in proportions typically 9-99-0.0001. Throughout the description and claims, the word "comprises" and its variants are not intended to exclude other technical features, additives, components or steps.For those skilled in the art, other objects, advantages and features of the invention will become apparent partly from the description and partly from the practice of the invention. Brief Description of the Figures Figure 1: X-ray structure of Pd1-Au1-MOF. Pd atoms are represented as dark balls and Au atoms as light balls. Figure 2: Kinetics for the acetylene semi-hydrogenation reaction catalyzed by Pd1-Au1-MOF. EXAMPLES Non-limiting examples of the present invention are detailed below: Example 1: Synthesis of Pd1-Au1-MOF. The precursor MOF {Cu6Sr[(S,S)-Mecysmox]3(OH)2(H2O)}.15H2O was prepared from the precursor dimer (Me4N)2{Cu2[(S,S)-Mecysmox](OH)}2.4H2O (4.32 g, 6.0 mmol) dissolved in 50 mL of water, to which another aqueous solution (10 mL) containing Sr(NO3)2 (0.42 g, 2.0 mmol) was added dropwise and under stirring for 2 h at room temperature.A green polycrystalline powder was obtained which was collected by filtration and dried with ethanol, acetone and diethyl ether, to give 2.91 g (83% yield) of a solid, which was immersed (2 g, 1.15 mmol) alternately in H2O / CH3OH (1:1) solutions of [Pd(NH3)4]Cl2 and AuCl3 (1.1 mmol each) for periods of 1 hour each. The process was repeated five times to ensure maximum loading of [Pd(NH3)4]Cl2. The material was washed with a H2O / CH3OH (1:1) solution several times, isolated by filtration over paper and air dried. This solid material (2 g) containing the inserted metal cations (in this case Pd. 2+ and Au 3+ ) before being reduced to form the dimer has the formula: (Pd 2+ )0.5(Au 3+ )0.5@{Cu6Sr[(S,S)-Mecysmox]3(OH)2(H2O)}^14H2O. Said solid material is resuspended in a solution of H2 O / CH 3 CH2OH (1:1) to which NaBH4 was progressively added, divided into 15 fractions (0.4 mmol of NaBH4 per mmol of MOF each), over 72 hours. Each fraction was left to react for 1.5 hours. After this period, the sample was washed with a H2O / CH3OH solution and filtered through paper, to obtain Pd1-Au1-MOF. The formula of this catalyst was Pd 0.5 Au 0.5 @{Mr II Cu II 6[(S,S)– Mecysmox]3(OH)2(H2O)} .12H2O. Example 2: Synthesis of Pt1-Au1-MOF with formula Pt 0.5 Au 0.5 @{AC II Cu II6[(S,S)–Mecysmox]3(OH)2(H2O)} .8H2O. The precursor MOF {Cu6Ca[(S,S)-Mecysmox]3(OH)2(H2O)}.16H2O was prepared by dissolving the precursor dimer (Me4N) 2{Cu2[(S,S)-Mecysmox](OH)} 2.4H2O (4.32 g, 6.0 mmol) in 50 mL of water, to which another aqueous solution (10 mL) containing CaCl2 (0.44 g, 2.0 mmol) was added dropwise and under stirring for 2 h at room temperature. A green polycrystalline powder was obtained, which was collected by filtration and dried with ethanol, acetone and diethyl ether, to give 3.07 g of sample (yield = 89%). This solid (2 g, 1.17 mmol) was alternately immersed in H2O / CH3OH (1:6) solutions of K2PtCl4 and AuCl3 (1.1 mmol each) for 1-h periods. The process was repeated eight times to ensure maximum loading of both metals. The material was washed with a H2O / CH3OH (1:1) solution several times, isolated by filtration over paper, and air-dried. This solid intermediate material of formula (Pt 2+ ) 0.5 (Au 3+) 0.5 @{Cu6Ca[(S,S)- Mecysmox]3(OH)2(H2O)}^10H2O is resuspended in its entirety in a H2O / CH3CH2OH (1:1) solution to which NaBH4 is progressively added, divided into 15 fractions (0.4 mmol of NaBH4 per mmol of MOF each), over 72 hours. Each fraction was left to react for 1.5 hours. After this period, the sample was washed with a H2O / CH3OH solution and filtered over paper, to obtain Pt1-Au1-MOF. The formula of this catalyst was Pt0.5Au0.5@{Ca II Cu II 6[(S,S)–Mecysmox]3(OH)2(H2O)} .8H2O. Example 3: Synthesis of Pd1 -Ag 1-MOF with formula Pd0.5 Ag 0.5@{BaIICuII6 [(S,S)– Mecysmox]3(OH)2(H2O)} .9H2O.The MOF precursor {Cu6Ba[(S,S)-Mecysmox]3 (OH) 2 (H 2 O)}.13H 2O was prepared by mixing, with continuous stirring, the precursor dimer (Me4N) 2{Cu2[(S,S)-Mecysmox](OH)} 2.4H2O (4.32 g, 6.0 mmol) (4.32 g, 6.0 mmol) dissolved in 50 mL of water with another aqueous solution (10 mL) containing Ba(NO3 )2 (0.53 g, 2.0 mmol) was added dropwise over 2 h at room temperature. A green polycrystalline powder was obtained, which was collected by filtration and dried with ethanol, acetone, and diethyl ether to give 3.22 g of sample (yield = 90%). This intermediate solid (2 g, 1.17 mmol), of formula (Pd 2+ ) 0.5 (Ag + ) 0.5 @{Cu6Ba[(S,S)- Mecysmox]3(OH)2(H2O)}^10H2O, was immersed, alternately, in H2O / CH3OH (1:6) solutions of [Pd(NH3)4]Cl2 and AgNO3 (1.1 mmol each) for periods of 1 hour. The process was repeated eight times to ensure maximum loading of both metals. The material was washed with a H2O / CH3OH (1:1) solution several times, isolated by filtration through paper and air-dried. This solid material of formula (Pd2+) 0.5 (Ag+) 0.5 @{Cu 6Ba[(S,S)-Mecysmox]3 (OH) 2 (H 2 O)}^10H2O is again suspended, in its entirety, in a solution of H2O / CH3CH2OH (1:1) to which NaBH4 is progressively added, divided into 15 fractions (0.4 mmol of NaBH4 per mmol of MOF each), over 72 hours. Each fraction was left to react for 1.5 hours. After this period, the sample was washed with a solution of H2O / CH3OH and filtered through paper, to obtain Pd1-Ag1-MOF. The formula of this catalyst was Pd0.5Ag0.5@{Ba II Cu II6[(S,S)– Mecysmox]3(OH)2(H2O)} .9H2O. Example 4: Semi-hydrogenation reaction under flowing acetylene (1%) in ethylene (99%). The reactions were carried out, in independent tests, in a packed bed reactor with 5-35 mg of the corresponding catalysts Pd1Au1-MOF, aPt1Au1-MOF and Pd1Ag1-MOF, with Pd1Au1-MOF, at a total flow rate of 10-150 ml / min, without using any inert carrier gas. The corresponding catalyst was placed in the center of the reactor tube, packed between glass wool. Hydrogen was fed in stoichiometries of 1:10 - 1:30. The reactions were followed by on-line gas chromatography, equipped with a column suitable for the separation of C1 - C6 hydrocarbons. The reactor was purged for 30 minutes with 20 ml / min N 2,after loading the catalyst, and was then heated to the reaction temperature, between 30 and 70 °C. The reaction mixture was flowed through the reactor bypass and the areas were calibrated on the gas chromatograph. After using the Pd1Au1-MOF, Pt1Au1-MOF or Pd1Ag1-MOF to catalyze the acetylene semi-hydrogenation reaction in flow, and once its amount dropped to 1 ppm, the packed bed was recovered from the reactor and sieved to remove the glass wool. The recovered catalyst was placed on a filter paper and introduced into a Soxhlet for extraction with dichloromethane, in a 250 mL flask on a stirrer plate at 700 °C for 1 hour (approximately 10 filling and washing cycles in the Soxhlet). The extraction content was also analyzed by GC-MS.

Claims

Claims 1. A heterobimetallic dimer catalyst stabilized by thioether residues of methylcysteine ​​of a metal-organic network, MOF, represented as X1-Y1-MOF, having the general chemical formula: X1-Y1@{M II Cu II6[(S,S)–Mecysmox]3(OH)2(H2O)} . nH2O, wherein: - X and Y are metals, where X is different from Y, -where X is selected from Pd, Pt and Ni, - Y is selected from Pd, Fe, Ni, Cu, Co, Au, Pt, Sn, Rh and Ag, - Mecysmox means bis[S-methylcysteine]oxalyl diamide, - M is selected from Ca, Sr and Ba and - n = 0-20, both values ​​included.

2. A catalyst according to claim 1, wherein X is present in an amount comprised between 0.01% up to 40% by weight, relative to the total weight of the catalyst.

3. A catalyst according to claim 1, wherein Y is present in an amount comprised between 0.01% up to 40% by weight, relative to the total weight of the catalyst.

4. A catalyst according to claim 1, selected from Pd1–Au1-MOF, Pt1–Au1-MOF or Pd1–Ag1-MOF.

5. A catalyst according to claim 1, having the formula selected from Pd0.5Au0.5@{Sr II Cu II 6[(S,S)–Mecysmox]3(OH)2(H2O)} .12H2O, Pt0.5Au0.5@{Ca II Cu II6[(S,S)–Mecysmox]3(OH)2(H2O)} .8H2O and Pd0.5Ag0.5@{Ba II Cu II 6. A process for obtaining the catalyst defined in one of claims 1 to 5, comprising: - synthesizing a precursor MOF with structure {Cu6M[(S,S)-Mecysmox]3(OH)2(H2O)} ^mH2O, where M = Ca, Ba or Sr, and where m is a variable number between 0 and 20, both values ​​inclusive, where Mecysmox means bis[S-methylcysteine]oxalyl diamide, from a precursor dimer of formula (Me4N)2{Cu2[(S,S)-Mecysmox](OH)}2.4H2O - introduce a salt of X and a different salt, of Y, into the channels of the MOF synthesized in the previous stage, obtaining an intermediate solid material of formula (X a+ ) 0.5 (AND b+ ) 0.5@{Cu6M[(S,S)-Mecysmox]3(OH)2(H2O)}^mH2O, where X, Y, a, b, M, m and Mecysmox have the meanings given above, - and subsequent addition of a reducing agent to obtain the heterobimetallic dimer catalyst.

7. A method according to the preceding claim, comprising successively immersing the precursor MOF in a solution of a salt of X and a solution of a salt of Y alternately.

8. A process according to one of claims 5 to 7, wherein the precursor MOF is selected from: {Cu6Sr[(S,S)-Mecysmox]3(OH)2(H2O)}^15H2O {Cu6Ca[(S,S)-Mecysmox]3(OH)2(H2O)}.16H2O and {Cu6Ba[(S,S)-Mecysmox]3(OH)2(H2O)}.13H2O.

9. The process according to one of claims 5 to 8, wherein the salts of X and Y are added in the form of solutions of water and alcohol, preferably methanol, and more preferably, solutions in which the water and methanol are mixed in a 1:1 ratio. 10.The process according to one of claims 6 to 9, comprising suspending the precursor MOF {Cu6M[(S,S)-Mecysmox]3(OH)2(H2O)}^mH2O in the salt solution of X and in the salt solution of Y repeatedly.

11. The process according to one of claims 6 to 10, wherein the reduction of the intermediate solid material is carried out with NaBH4o with a stream of hydrogen gas.

12. The process according to one of claims 6 to 11, wherein the intermediate solid material of formula (X. a+ )0.5(Y b+ )0.5@{Cu6M[(S,S)-Mecysmox]3(OH)2(H2O)}^mH2O is washed with a water / alcohol solution, preferably methanol, in proportions between 90:10 and 10:

90.

13. The process according to one of claims 6 to 12, wherein the reduction of the intermediate solid material of formula (X a+ )0.5(Y b+)0.5@{Cu6M[(S,S)-Mecysmox]3(OH)2(H2O)} ^mH2O is carried out by suspending it in a mixture of water and alcohol, and adding NaBH4, giving rise to the dimer catalyst.

14. The process according to one of claims 6 to 13, wherein the reduction of the intermediate solid material is carried out with NaBH4 and the addition of NaBH4 is carried out progressively such that the NaBH4 is divided into fractions, allowing each fraction to react for a period of time, obtaining the dimer catalyst.

15. Use of the catalyst defined above in one of claims 1 to 5, in hydrogenation reactions. 16.Use of the catalyst defined above in one of claims 1 to 5, in a process for semi-hydrogenation of acetylene under industrial conditions ("front-end"), comprising at least the following steps: - a first step in which the catalyst is contacted with an ethylene:H2:acetylene mixture, - a second step in which the mixture is heated, - a third step in which the product obtained in the second step is collected.

17. Use of the catalyst according to the preceding claim comprising a fourth step of recovering the solid catalyst.

18. Use according to claim 15, wherein the amount of catalyst placed in the reactor is between 50-500 mg per ml / min of total flow, and preferably between 5-400 mg per ml / min of total flow, and more preferably between 10-350 mg per ml / min of total flow. 19.Use according to claim 16, wherein the ethylene:H2:acetylene mixture in the first stage may vary between 1-20:99-80:0.05-0.01, and is preferably 10:89.9:0.

1.

20. Use according to claim 16, wherein the hydrogen is fed in stoichiometries of 80:1 to 1:30 with respect to the total moles of ethylene fed to the reactor, preferably 10:1 to 1:30, and preferably in stoichiometries of 1:10 - 1:

30.

21. Use according to claim 16, wherein the reaction is carried out in batch or continuous, and preferably continuously in a tubular reactor.

22. Use according to claim 21, wherein the space velocity of the continuous reaction is between 1000 and 100000 ml^g -1 ^h -1 , preferably 20000 and 70000 ml^g -1 ^ h -1 and more preferably between 30,000 and 50,000 ml^g -1 ^h -1.

23. Use according to claim 16, wherein the reaction temperature is between 25 and 160 ° C, preferably between 30 and 90 ° C, and more preferably between 35 and 70 ° C.

24. Use according to claim 16, wherein the reaction is carried out in reductive atmospheres under pressures between 1 and 20 bars, preferably between 1 to 5 bars, and more preferably 1 to 2 bars.

25. Use according to claim 16, wherein the contact time inside the tubular reactor varies between 0.1 and 24 h, preferably between 0.1 and 10 hours, and more preferably between 0.1 and 5 hours.

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