ES2977942B2 - 3D STRUCTURED CATALYST BASED ON ACTIVATED CARBON - Google Patents

Description

DESCRIPTION

3D structured catalyst based on activated carbon

The present invention relates to a self-supporting and highly porous catalyst comprising a printed three-dimensional (3D) structure of activated carbon (CA) or oxidized activated carbon (CA-OX) and aluminum oxide (AI2O3), coated by palladium (Pd) nanoparticles. The invention also relates to its production process by direct ink printing (DIW) and to its use as a catalyst in the production of carbon monoxide (CO)-free hydrogen (H2) from the catalytic dehydrogenation of formic acid (FA).

BACKGROUND OF THE INVENTION

Activated carbon (AC) powder has high porosity and specific surface area, so it is used as an adsorbent and catalytic support, properties that would be increased if it is structured as a 3D material with high porosity from a cellular printing pattern.

One of the most common additive manufacturing or 3D printing techniques of AC is stereolithography (SLA), where a porous polymeric 3D structure is manufactured from a liquid photoresin that is transformed into activated carbon through a high-temperature pyrolysis process and subsequent activation in carbon dioxide (CO2) (H. Steldinger et al., “Activated Carbon in the Third Dimension—3D Printing of a Tuned Porous Carbon”, Adv. Sci. 6 (2019) 1901340).

Direct ink writing (DIW) or robocasting is another of the most widely used additive manufacturing techniques. Its advantages over SLA include being more versatile, cheaper, and with no limitation on the compositions to be printed. AC electrodes have been printed using DIW from an organic aerogel that has been pyrolyzed in nitrogen and subsequently activated in CO2 (US2018345598A1 and S. Chandrasekaran et al., “Direct ink writing of organic and carbon aerogels,” Mater. Horiz. 5 (2018) 1166). However, these processes using polymeric materials involve numerous steps, including the pyrolysis and conversion of the printed 3D structure into carbon, and its activation, steps that do not always allow 100% yield in AC formation.

Mendes et al. (Mendes et al., “3D-printed hybrid zeolitic/carbonaceous electrically conductive adsorbent structures”, Chem. Eng. Res. Des. 174 (2021) 442-453) have developed two types of porous 3D AC-based structures for CO2 adsorption using DIW, containing bentonite and/or zeolite. The procedure they describe includes a pyrolysis step in nitrogen at 5500C, and the structures they obtain do not have a high porosity, so their catalytic activity is compromised.

Other authors have developed 3D AC cellular materials by DIW from AC composites. For example, Verougstraete et al. (Verougstraete et al., “Electrical swing adsorption on 3D-printed activated carbon monoliths for CO2 capture from biogas”, Sep. Purif. Technol. 299 (2022) 121660) have fabricated 3D structures for CO2 capture containing 50 wt% AC and 50 wt% potassium silicate. Acuña-Bedoya et al. (Acuña-Bedoya et al., “Integration of the adsorption and electro-oxidation process using 3D printed activated carbon monoliths for the degradation of pharmaceutical compounds”, J. Environ. Chem. Eng. 10 (2022) 108203) have fabricated 3D structures for pharmaceutical compound degradation by adsorption-electrooxidation containing 32 wt% AC and 68 wt% cellulose. Regufe et al. (Regufe et al., “Electrical conductive 3D-printed monolith adsorbent for CO2 capture”, Micropor. Mesopor. Mat. 278 (2019) 403-413) have fabricated 3D structures for CO2 capture containing 30 wt% AC and 70 wt% zeolite. All these papers describe 3D structures with relatively low AC contents.

On the other hand, ES2829958B2 describes the process for obtaining structured CA monoliths from a polyvinyl acetate mold previously made by 3D printing. This mold is filled with a mixture composed of CA and a polymer precursor solution based on resorcinol, formaldehyde, a basic catalyst, and a naturally occurring starch polymer. The ground, sieved, and packed mixture in the 3D mold is then gelled and cured. Finally, the mold is removed by carbonization, leaving a 3D CA structure. 3D CA structures are obtained by an indirect mold replication method.

Demand for hydrogen has grown considerably, especially as a fuel capable of producing green energy. Document CN114192148A describes the 3D printing of a support based on metal alloys that are subsequently dealloyed and reduced to obtain the 3D catalyst, which is used in the production of H2 by methanol reforming. The reaction also produces CO, which is a serious drawback since, on the one hand, it deactivates the catalyst and, on the other, small amounts of CO (> 10 ppm) included in the H2 poison the electrodes of fuel cells where H2 is used.

Methods for obtaining hydrogen from AF have been described. Thus, CN110479256 (A) describes the preparation of a powder catalyst based on a mixture of CA, palladium salt (Pd) and silver salt (Ag) for obtaining hydrogen from AF. On the other hand, Caiti et al. (Caiti et al., Continuous Production of Hydrogen from Formic Acid Decomposition Over Heterogeneous Nanoparticle Catalysts: From Batch to Continuous Flow, ACS Catal. 9 (2019) 9188-9198) have used a commercial powder catalyst composed of CA containing 5% by weight of Pd for the production of hydrogen from AF. The AF conversion is 81% at a temperature of 500C, and 100% from temperatures of 900C. In these cases, the disadvantage is that the deactivation of the catalyst is exponential from the beginning of the reaction.

Therefore, it would be desirable to have a highly porous 3D catalyst for obtaining H2 from AF that allows its production free of CO, i.e., with 100% selectivity, and that maintains the conversion capacity and selectivity over prolonged periods of use.

DESCRIPTION OF THE INVENTION

In a first aspect, the present invention relates to a 3D Pd/CA catalyst characterized in that it comprises:

• a printed three-dimensional structure of filaments, where the filaments comprise pores, where each filament is formed by

activated carbon (AC) and/or oxidized activated carbon (AC-OX); and

aluminum oxide (AI2Ü3); and

• a heterogeneous coating of palladium (Pd) nanoparticles over the entire surface of the three-dimensional filament structure,

where:

The weight percentage of the printed three-dimensional structure is between 90% and 99% of the total weight of the catalyst;

The amount of Pd coating is between 1% and 10% by weight relative to the total weight of the catalyst, and

where the 3D printed structure comprises at least two two-dimensional filament structures, a first two-dimensional structure and a second two-dimensional structure, where the first two-dimensional structure and the second two-dimensional structure are stacked, are parallel and are rotated relative to each other between 86° and 94°, preferably between 89° and 91°, where the three-dimensional structure in turn comprises open channels forming a pattern of interconnected macropores

where each two-dimensional filament structure comprises at least 3 filaments, a first filament, a second filament and a third filament, all 3 with a straight path and in the same direction parallel to each other;

where the filament has a final diameter of between 225 pm and 1,080 and a distance between filaments of between 0.65 and 0.75 mm;

where the printed three-dimensional structure is a frameless structure; and where the printed three-dimensional structure has a density of between 0.34 g cm-3 and 0.46 g cm'3 and a total porosity of between 84% and 87%.

The 3D Pd/CA catalysts of the present invention are highly porous CA/AI2O3 or CA-OX/AI2O3 structures, with total porosities of up to 87% obtained from different aqueous inks with weight contents between 90% and 99% in CA or CA-OX, plus between 1% and 10% by weight of boehmite (ink solids content of 40% by weight with respect to the total ink), which is transformed into AI2O3 with a subsequent heat treatment to give mechanical consistency to the 3D structure. These CA or CA-OX contents (90%) in the 3D structure considerably increase the porosity of the monolith, which allows improving its performance as a support or catalytic material. Other developed inks containing 70% by weight of CA or CA-OX and 30% boehmite (solids content in the ink of 42% by weight with respect to the total ink) as initial material which in the final product will be in the form of AI2O3 due to the heat treatment to which it is subjected, as explained above.

The 3D Pd/CA catalysts of the invention have the advantage of maintaining conversion for extended periods, up to 30 h of reaction time, and always with 100% selectivity to hydrogen due to the synergistic effect between the Pd catalyst and the activated carbon support. These advantages would not be achieved if the catalyst and/or support were different.

In the catalyst structure defined above, the characteristic of the twist between 86° and 94° between the two two-dimensional filament structures generates the pattern of interconnected open channels forming macropores.

The lack of a defined outer frame indicates that no outer filament is printed per layer to enclose the structure. The advantage of not having an outer frame is that it increases the surface area of the 3D structure for subsequent catalyst infiltration.

All measurements of distances and dimensions are made by direct optical and/or scanning electron microscopy. The geometric density (pgeo) has been calculated from its weight and dimensions. The total porosity (m) has been determined from the following expression: m = 100 - (pgeo x 100/pt), where pt is the theoretical density of CA/AI2O3 using the rule of mixtures and as densities of CA (and CA-OX, which is the same) and AL03 of 2.10 gcm -3 and 3.98 gcm -3, respectively.

On the other hand, aluminum oxide (AI2O3) comes from the transformation of boehmite contained in the inks to obtain the printed structures after heat treatment at 13000C in a vacuum atmosphere.

In the catalyst of the invention defined above, the Pd nanoparticles are supported on the printed three-dimensional structure of filaments or the monolith of CA/AI2O3, CA-OX/AI2O3 or the mixture of CA and CA-OX/AI2O3.

Throughout the invention, printed three-dimensional structures will also be referred to as monoliths.

Throughout the present invention, the term “3D Pd/CA catalyst” refers to the catalyst object of the invention as defined in the previous paragraph.

In another embodiment of the invention, the Pd is additionally located inside the pores of the filaments.

In another embodiment, the invention relates to the 3D Pd/CA catalyst defined above, where the AI2O3 is present in crystalline phases a, 5, 0 and/or any of their combinations.

The crystalline phases at,5 and 0 of AI2O3 are determined by X-ray diffraction.

In another preferred embodiment, the catalyst further comprises additional filaments perpendicular to the 3 rectilinear filaments, joining said filaments two by two, the first with the second and the second with the third.

In another preferred embodiment, the catalyst comprises 2 additional two-dimensional structures, a third two-dimensional structure and a fourth two-dimensional structure, with the same two-by-two spatial orientation as the two-dimensional structures described above, more preferably the first and third structures are in the same spatial orientation, and the second two-dimensional structure and the fourth two-dimensional structure are in the same spatial orientation.

In another embodiment the invention relates to the 3D Pd/CA catalyst defined above where the printed three-dimensional structure is cylindrical, preferably the printed 3D structure is cylindrical with a diameter of between 10 mm and 30 mm, more preferably 16.2 mm, and where the cylindrical printed three-dimensional structure has a height of between 1 mm and 30 mm, even more preferably 7.9 mm in height, and even more preferably it comprises at least two two-dimensional filament structures, preferably 12 two-dimensional filament structures, where the two-dimensional structures are stacked, are parallel and are rotated relative to each other between 86° and 94°, preferably between 89° and 91°, more preferably 90°, where it comprises open channels forming a pattern of interconnected macropores.

wherein each two-dimensional structure comprises 3 or more rectilinear filaments per layer in the same direction, more preferably 3 to 60 filaments, and even more preferably 9 rectilinear filaments per layer in the same direction and parallel to each other; with a final filament diameter of between 225 pm and 1,080 pm, even more preferably 760 pm; a distance between filaments of between 0.65 and 0.75 mm, even more preferably 0.71 mm; a structure without an outer frame; and a density of between 0.34 g cm-3 and 0.46 g cm-3, more preferably 0.36 g-cm-3 and a total porosity of between 84% and 87%, more preferably 86%.

In another embodiment, the invention relates to the catalyst defined above, wherein the catalyst has a total porosity of up to 87%.

In another embodiment, the invention relates to the 3D Pd/CA catalyst, where the printed 3D structure is present in an amount of 95% by weight relative to the total weight of the catalyst.

The amount of Pd nanoparticles present in the catalyst has been determined by total reflection X-ray fluorescence spectroscopy.

In another embodiment, the invention relates to the 3D Pd/CA catalyst where the Pd nanoparticles have a size between 4 ± 0.4 nm and 3.8 ± 0.7 mm; and preferably less than or equal to 2 nm.

The size of Pd nanoparticles has been estimated by direct transmission electron microscopy.

In another embodiment the invention relates to the 3D Pd/CA catalyst, where:

The 3D printed structure is present in an amount of between 90% and 99%, preferably in an amount of 95% by weight relative to the total weight of the catalyst; and

Pd nanoparticles are present in an amount of between 1% and 10% by weight relative to the total weight of the catalyst, preferably 5% by weight relative to the total weight of the catalyst.

In another embodiment, the invention relates to the 3D Pd/CA catalyst, where the printed 3D structure comprises between 55% and 90.9% of CA and/or CA-OX and between 9.1% and 45% of AI2O3, preferably between 70% and 90.9% of CA and/or CA-OX and between 9.1% and 30% of AI2O3, more preferably the printed 3D structure comprises 90.5% of CA or CA-OX and 9.5% of AI2O3.

Another aspect of the invention relates to a process for obtaining the 3D Pd/CA catalyst defined above, comprising the steps of:

a) direct printing of aqueous inks (DIW) based on mixtures of CA or CA-OX and boehmite gel to obtain the 3D structure;

b) heat treatment of the 3D structure obtained in step (a) at a temperature between 1200 0C and 1400 0C, preferably at 1300 0C, in vacuum, and for a time between 0.4 h and 2 h, preferably for 0.5 h, to transform boehmite into AI2O3, and preferably with crystalline phases a, 5, 0 and/or any of the combinations;

c) coating the 3D structure after the heat treatment of step (b) with Pd nanoparticles obtained from PdCh dissolved in an aqueous solution of HCl at a concentration of between 0.2 M and 0.6 M, preferably dissolved in an aqueous solution of HCl at a concentration of 0.30 M, previously heating, for a time of between 0.8 h and 3 h, preferably for 1 h, at a temperature of between 35 0C and 75 0C, preferably at a temperature of 65 0C, to then carry out complete evaporation of the aqueous solution of HCl at a temperature between 90 and 980 C, preferably at a temperature of 950 C;

d) washing the 3D Pd structure obtained in step (c) with milli O water and then drying at a temperature between 50 0C and 70 0C, preferably at a temperature of 600C, for a time between 10 h and 24 h, preferably for 12 h;

e) treatment of the 3D Pd structure obtained in step (d) with a reducing gas H2/N2 (100:50 mL-min'1) at a temperature between 2000C and 350 0C, preferably at 250 0C, for a time between 1.8 h and 4 h, preferably for a time of 2 h, to obtain the 3D Pd/CA catalyst defined above.

The process for obtaining the catalyst of the present invention is a direct method for obtaining 3D CA structures, where only the mixing of the original CA or CA-OX powder with a boehmite gel, DIW printing and a heat treatment for its consolidation are required, which reduces the number of steps with respect to the procedures described in the state of the art, the raw materials used, and the waste generated, and is, therefore, a more sustainable manufacturing method.

Thus, the present invention includes the fabrication of 3D CA structures (monoliths), which have been impregnated with a Pd precursor to finally obtain 3D Pd/CA catalysts with a Pd content of 5% by weight. The 3D structures with high porosity and specific surface area favor the synthesized Pd nanoparticles to have a smaller size (~2 nm), observed by transmission electron microscopy, to have a better distribution of catalytic active centers, measured by selective chemisorption of CO, and to have an adequate ratio of electrodeficient and metallic Pd species, quantified by X-ray photoelectron spectroscopy. In addition, the uniform 3D structures allow the hydrogen gas and CO2 bubbles generated by the catalyst to escape much more efficiently than in traditional powder nanoparticle catalysts compacted in reactors.

In the present invention, "aqueous ink" refers to inks prepared for direct printing (or DIW) that comprise water in a weight percentage with respect to the total ink of at least 60% and in solution the solids, where said solids are CA or CA-OX plus boehmite.

In an embodiment of the process of the invention where the aqueous ink of step (a) comprises between 55% and 90.9% of CA or CA-OX and between 9.1% and 45% of boehmite and where in the aqueous ink the solids, CA or CA-OX plus boehmite, have a weight percentage of between 40% and 42% with respect to the total ink, preferably between 70% and 90.9% of CA or CA-OX and between 9.1% and 30% of boehmite and where in the aqueous ink the solids, CA or CA-OX plus boehmite, have a weight percentage of between 40% and 42% with respect to the total ink, more preferably it comprises 90.5% of CA or CA-OX and 9.5% of boehmite with a weight percentage of solids of 40% with respect to the total ink.

Another aspect of the invention relates to the use of the 3D Pd/CA catalyst defined above to produce CO-free H2 from the catalytic dehydrogenation of formic acid (FA) in liquid phase, preferably at atmospheric pressure between 0.8 atm and 1.2 atm and at a temperature between 10 and 950 C, more preferably at a temperature between 25 0 C and 55 0 C, even more preferably at 55 0 C. At atmospheric pressure of around 1 atm, it works as a catalyst in the entire temperature range that allows the FA to be in liquid phase. Furthermore, the Pd/CA catalysts in powder form, which deactivate more quickly from 550 C onwards and complete regeneration of the activity is not possible, therefore the catalytic action with the 3D catalysts at a temperature greater than 550 C has fewer advantages, but it could be done up to 950 C and H2 production is observed. Below this temperature and above the AF melting point and up to 55 0C the catalyst is completely regenerated, with 100% conversions and up to more than 30 hours of activity.

In another embodiment, the invention relates to the use of the 3D Pd/CA catalyst defined above, where the catalytic dehydrogenation of AF is carried out in a fixed bed reactor loaded with one or more 3D Pd/CA structures, preferably with 5 3D Pd/CA structures.

In another embodiment, the invention relates to the use of the 3D Pd/CA catalyst defined above, where the catalytic dehydrogenation of AF is carried out with a flow rate of aqueous AF solution of between 0.125 mL-min'1 and 1 mL-min'1, preferably 0.25 mL-min'1.

In another embodiment, the invention relates to the use of the 3D Pd/CA catalyst defined above, where the catalytic dehydrogenation of AF is carried out in a space time of between 40 gcarh-L'1 and 320 gcarh-L'1, preferably 160 gcat h-L'1.

In another embodiment, the invention relates to the use of the 3D Pd/CA catalyst defined above where the catalytic dehydrogenation of AF is carried out at an initial AF concentration of between 0.25M and 2M, preferably 1M.

In another embodiment, the invention relates to the use of the 3D Pd/CA catalyst defined above where the catalytic dehydrogenation of AF is carried out with a helium (He) flow rate of between 5 mL-min'1 and 30 mL-min'1, preferably 17 mL-min'1.

Thus, with the 3D Pd/CA catalysts of the present invention, the AF dehydrogenation reaction has been carried out in the liquid phase at temperatures up to 55 0C, with AF conversions of 80% at 250C and 95% at 550C, calculated from the AF concentration in the liquid phase quantified by UV-VIS spectrophotometry, and producing a CO-free H2 gas stream, measured by gas chromatography.

As mentioned above, the reaction can be maintained for several hours of use, up to 30 h, without any decrease in catalyst activity. Furthermore, the 3D Pd/CA catalyst recovers its activity after simple drying at 60 °C in an oven in an air atmosphere for 24 h. Furthermore, very mild dehydrogenation reaction temperatures, well below those used in the state of the art, between 25 °C and 55 °C, and at atmospheric pressure, are used, making the process more energetically cost-effective and scalable. In this way, conversions of up to 95% have been achieved with 100% selectivities in the production of H2 and CO2, thus without producing CO.

Throughout the invention, the term “direct ink writing (DIW)” or “robocasting” refers to an additive manufacturing technology that allows obtaining 3D materials with complex shapes and composed of a wide variety of materials from the extrusion of filaments based on concentrated aqueous inks with pseudoplastic behavior.

“Pattern(s) of interconnected open channels forming macropores” is understood to mean the set of porous channels formed, on the one hand, by the vertical channels that cross the structure in the direction perpendicular to the plane of the filaments and their growth directions by deposition of the material using the additive technique of forming the structure and, on the other hand, by the narrower orthogonal open channels that are generated by not including a frame in the printed structure in the plane of the deposition directions of the filaments that form the structure.

In the present invention, activated carbon and chemically oxidized AC were used as the starting carbonaceous material. In this case, the AC was treated with different reagents, such as HNO3, H2O2 and NH4OH, using mechanical stirring and varying the temperature between 250°C and 80°C and the reaction time between 2 h and 8 h.

Therefore, in another embodiment, the invention relates to the process for obtaining the 3D Pd/CA catalyst defined above, which also comprises a step of treating the starting activated carbon to chemically oxidize it to oxidized CA (CA-OX) by treating CA with a reagent selected from HNO3, H2O2 and NH4OH, preferably with HNO3; at a temperature between 25 0C and 80 0C, preferably at 80 0C; and for a reaction time between 2 h and 8 h, preferably, for a time of 4 h, giving rise to CA-OX with an oxygen content between 20 and 30%, preferably 26%, and a nitrogen content between 0.7 and 1.1%, preferably 0.8%.

Thanks to the treatment described, aqueous inks of activated carbon (CA or CA-OX) and boehmite with pseudoplastic behavior are prepared without the need to include organic additives, which contain between 40% and 42% by weight of solids and which are composed of between 55% and 90% by weight of CA or CA-OX and between 45% and 10% by weight of boehmite, and preferably by 70% by weight of CA or CA-OX and 30% by weight of boehmite.

The ink viscosity decreases from 104 to 101 Pas in the shear rate range of 10.1 to 102 s-1. The storage and loss shear moduli are approximately 106 and 105 Pa, respectively. The yield strength varies depending on the ink composition, ranging from 1000 to 3500 Pa, although a value of -1500 Pa is preferred.

Thus, in another embodiment the invention relates to the process for obtaining the 3D Pd/CA catalyst defined above, where the viscosity of the ink decreases from 104 to 101 Pa s in the shear rate range between 10'1 to 102 s-1.

In another embodiment, the invention relates to the process for obtaining the 3D Pd/CA catalyst defined above, where the elastic storage and loss shear moduli are in the range of 105 to 106 Pa and 104 to 105 Pa, respectively.

In another embodiment, the invention relates to the process for obtaining the 3D Pd/CA catalyst defined above, where the yield stress is between 1,000 Pa and 3,500 Pa, and preferably is -1500 Pa.

Using a computer-aided design (CAD) program, a cylindrical structure with dimensions between 16.2 and 17.2 mm in diameter and 7.9 mm in height is created, which contains a pattern of interconnected open channels forming macropores defined by: i) 9 linear filaments per layer in the same direction, ii) 12 stacked layers, iii) 90o rotation between layers, iv) printing tip diameter of 840 pm and v) structures without an external frame. A 3 cm3 syringe is filled with the CA/boehmite or CA-OX/boehmite aqueous ink and printed in air and at room temperature using robocasting equipment and an X-Y plane speed of 10 mm-S'1.

The 3D printed structures are treated at temperatures between 1200-13000C for 0.5-2 h to mechanically consolidate the structures, eliminating water, preventing CA degradation, and transforming boehmite into AI2O3, which crystallizes in phases including α, β, and β. Treatment at 13000C for 0.5 h in vacuum is preferably selected. These structures, hereinafter monoliths, have a specific surface area, determined by nitrogen adsorption using the B.E.T. method, ranging from 150 to 454 m2 g-1, geometric density between 0.34-0.46 gcm-3, and total porosity of 84-87% by volume. The diameter of the filaments forming the monolith is -760 pm and the width between channels is -710 pm. The compressive strength of the monolith is 0.5 MPa, calculated by compression tests with a load cell of up to 5 kN and a displacement speed of 0.5 mm-min-1.

The 3D Pd/CA catalyst comprises palladium (Pd) supported on the 3D structures described above. The amount of Pd present in the catalyst will range from 1 to 10% by weight, preferably 5% by weight, so that the size of the palladium nanoparticles is -2 nm. The Pd is incorporated into the monolith by impregnation according to the following process:

- The appropriate amount of PdCL is dissolved in a given volume of a 0.30 M HCI solution and heated to 650C.

- The appropriate number of monoliths is immersed in the precursor solution at 65 0C and kept under stirring for 1 h in a rotating and oscillating roller shaker.

- Subsequently, the precursor solution is divided into aliquots of adequate volume to immerse a monolith in each aliquot, and they are introduced into an oven at the appropriate temperature so that the precursor solution reaches the temperature of 950C.

- When half of the precursor solution has evaporated, the monolith is turned over to promote the homogeneous distribution of palladium over its entire surface, and the water is allowed to evaporate completely.

- The Pd-impregnated monoliths are washed with Milli Q water and dried in an oven at 600C for 12h.

- The Pd impregnated monoliths are treated with a reducing gas H2/N2 (100:50 mLmin'1) at 2500C for 2 h.

Alternatively to PdCL, other Pd precursors such as palladium(II) acetate or palladium(II) nitrate have been used. In these cases, the impregnation procedure is the same, but the appropriate amount of precursor salt is dissolved in a 0.001 M aqueous solution of acetone or Milli Q water, respectively.

3D Pd/Ca catalysts have been used in the catalytic dehydrogenation of AF in liquid phase to produce hydrogen. The reactor is operated in a fixed bed at temperatures between 25 and 550C, with aqueous AF solution flow rates between 0.125 and 1 mL min'1, space times between 40 and 320 gcarh L'1, initial AF concentrations of 0.5 to 2 M and optionally a variable inert gas flow rate, between 0 and 20 mL min'1 of helium, preferably 17 mL min'1, and at atmospheric pressure.

The catalytic dehydrogenation of AF of the present invention is carried out in a fixed bed reactor and according to the following procedure: the reactor is loaded with one or more 3D Pd/Ca structures (monoliths), preferably with 5 structures; the reactor is heated, preferably by means of hot water circulating through the reactor jacket, until the selected reaction temperature is reached, between 25 0 C and 55 0 C, and measured in the thermostatic water bath; until the selected temperature is reached, the He stream is introduced into the reactor and in an upward direction at the selected flow rate, between 5 mL min'1 and 30 mL min'1, preferably 17 mL min'1; once the reaction temperature is reached, the AF aqueous solution stream is introduced into the reactor and in an upward direction at the selected flow rate, between 0.125 mL min'1 and 1 mL min'1, preferably 0.25 mL min'1; During the course of the reaction, liquid and gas samples are taken at the outlet of the gas-liquid separator located at the reactor exit. The liquid samples are analyzed using a UV-VIS spectrophotometer to quantify the AF concentration, and the gas samples are measured using a gas chromatograph to identify and quantify the gaseous products produced during the reaction, which are exclusively H2 and CO2.

AF conversions between 80% and 97% have been obtained at high space times, starting from 160 gCathL"1, and depending on the operating conditions used, with temperature and initial AF concentration being the most influential. For example, at a temperature of 550C, initial AF concentration of 1 M and space time equal to 160 gCathL"1, AF conversions of 92% and gas flow rates of 12 mL min'1 are obtained; at a temperature of 350C, initial AF concentration of 0.5 M and space time equal to 320 gCathL"1, AF conversions of 97% and gas flow rates of 4 mL min'1 are obtained.

The selectivity to the AF dehydrogenation reaction is always 100%. 3D Pd/CA catalysts allow the production of CO-free H2. Furthermore, the 3D structure allows H2 and CO2 gas bubbles generated at the Pd active sites to be released into the reaction medium more efficiently than powdered Pd catalysts packed in reactors when operating at liquid phase flow rates greater than 0.125 mL min'1.

The initial activity of the catalyst can be kept constant for 3.5 to 35 h of use, depending on the operating conditions used, the most influential being the flow rate of the AF solution and the initial AF concentration. For example, at a temperature of 55 0C, initial AF concentration of 0.5 M and space time equal to 320 gcarh L '1, the activity is maintained for 35 h of use. At a temperature of 250 C, initial AF concentration of 1 M and space time equal to 160 gCat h L'1, the activity is maintained for 4 h of use. The 3D Pd/CA catalyst always recovers its activity after being dried at 60 0C in an oven in an air atmosphere for 24 h.

All preferred embodiments of the present invention are combinable with each other whenever such combination is possible.

Throughout the description and claims, the word "comprise" 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 be apparent partly from the description and partly from the practice of the invention. The following examples and figures are provided for illustrative purposes only and are not intended to limit the present invention.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 shows (a) a 3D printed structure from an aqueous ink composed of 70 wt% CA and 30 wt% boehmite and a (b) top and (c) sectional image of the monolith heat-treated at 13000C for 0.5 h.

Fig. 2 shows (a) a scanning electron microscopy image of a section through a palladium-impregnated CA/AI2O3 monolith and (b) the same field in (a) analyzing the composition of the filaments by energy-dispersive X-ray spectroscopy. The dark areas correspond to CA/AI2O3 and the light areas to palladium.

Fig. 3 shows the AF conversion(a) and the gas flow rate produced in the catalytic dehydrogenation of AF (b) as a function of the run time and reaction conditions: temperature of 55 0C, initial AF concentration of 1 M and space time of 160 gcarh L"1 is the black solid curve with black squares representing values; temperature of 25 0C, initial AF concentration of 1 M and space time of 160 gcarh L -1 is the dark grey dashed curve with dark grey circles representing values; temperature of 550C, initial AF concentration of 0.5 M and space time of 320 gcat h L-1 is the light grey dotted-dash curve with light grey triangles representing values, all at a helium flow rate of 17 mL-min-1.

EXAMPLES

The invention will now be illustrated by tests carried out by the inventors, which demonstrate the effectiveness of the product of the invention.

Example 1. 3D structured catalyst based on activated carbon/aluminium oxide and 5% by weight of palladium for hydrogen production

An aqueous CA/boehmite ink with pseudoplastic behavior has been prepared containing 42% by weight of solids and preferably composed of 70% by weight of CA and 30% by weight of boehmite.

The ink viscosity decreases from 104 to 101 Pa s in the shear rate range of 10.1 to 102 s_1. The storage and loss shear moduli of elasticity are around 106 and 105 Pa, respectively. The yield strength is -1500 Pa.

A cylindrical structure of 16.2 mm diameter and 7.9 mm height is designed containing a pattern of interconnected open channels forming macropores defined by: i) 9 linear filaments per layer in the same direction, ii) 12 stacked layers, iii) 90o rotation between layers, iv) 840 pm printing tip diameter and v) structures without external frame. A 3 cm3 syringe is filled with the aqueous CA/boehmite ink and printed in air and at room temperature using robocasting equipment and an X-Y plane speed of 10 mm-s-1.

The 3D printed structures (Figure 1a) are treated at 13000C for 0.5 h in vacuum and the boehmite transforms into 5- and 0-Al2O3, obtaining the structure shown in Figure 1b and c. The monoliths have a specific surface area of 454 m2 g-1, a geometric density of 0.36 g cm'3 and a total porosity of 86% by volume. The diameter of the filaments forming the monolith is -760 pm and the width between channels is -710 pm. The compressive strength of the monolith is 0.5 MPa.

The 3D Pd/CA catalyst comprises 5% by weight of palladium (Pd) supported on the CA monolith (Figure 2). The size of the palladium nanoparticles is -2 nm. The Pd is incorporated into the monolith by impregnation according to the following process:

- 0.267 g of PdCl is dissolved in 42.5 mL of a 0.30 M HCl solution and heated to 650C.

- 6 3D Pd/CA monoliths, with a total weight of around 3 g, are immersed in the precursor solution at 650°C and kept under stirring for 1 h in a rotating and oscillating roller shaker.

- Subsequently, the precursor solution is divided into aliquots of adequate volume to immerse a monolith in each aliquot, and they are introduced into an oven at the appropriate temperature so that the precursor solution reaches the temperature of 950C.

- When half of the precursor solution has evaporated, the monolith is turned over to promote the homogeneous distribution of palladium over its entire surface, and the water is allowed to evaporate completely.

- The Pd-impregnated monoliths are washed with Milli Q water and dried in an oven at 600C for 12h.

- The Pd impregnated monoliths are treated with a reducing gas H2/N2 (100:50 mLmin'1) at 2500C for 2 h.

3D Pd/Ca catalysts are used in the catalytic dehydrogenation of AF in liquid phase to produce H2. The reactor is loaded with 5 monoliths, operated at a temperature of 55 0C, AF aqueous solution flow rate of 0.25 mL min'1, space time 160 gcarh L'1, initial AF concentration of 1 M and helium flow rate of 17 mL min'1, and at atmospheric pressure (solid black curve with black squares in Figure 3).

An AF conversion of 91.6% is obtained (solid black curve with black squares in Figure 3a) and a gas flow rate of 12 mL min'1 (solid black curve with black squares in Figure 3b). The selectivity to the dehydrogenation reaction is 100%. Therefore, H2 and CO2 are obtained as the only reaction products. The activity is maintained for 3.5 h of use, after which the catalyst begins to progressively deactivate until its activity is completely lost after 24 h of use.

The 3D Pd/CA catalyst regains activity after being dried at 600C in an oven in an air atmosphere for 24 h.

Example 2. 3D structured catalyst based on oxidized activated carbon/aluminium oxide and 5% by weight of palladium for hydrogen production

An aqueous CA-OX/boehmite ink with pseudoplastic behavior has been prepared containing 42% by weight of solids and preferably composed of 70% by weight of CA-OX and 30% by weight of boehmite.

The ink viscosity decreases from 104 to 101 Pa s in the shear rate range of 10.1 to 102 s_1. The storage and loss shear moduli of elasticity are of the order of 106 and 104 Pa, respectively. The yield strength is -3500 Pa.

A cylindrical structure of 17.2 mm diameter and 7.9 mm height is designed containing a porous channel pattern defined by: i) 9 linear filaments per layer in the same direction, ii) 12 stacked layers, iii) 90o rotation between layers, iv) 840 pm printing tip diameter and v) structures without external frame. A 3 cm3 syringe is filled with the CA-OX/boehmite aqueous ink and printed in air and at room temperature using robocasting equipment and an X-Y plane speed of 10 mm-S'1.

The 3D-printed structures are treated at 1,3000C for 0.5 h in vacuum, and the boehmite is transformed into 5- and 0-Al2O3. The monoliths have a specific surface area of 434 m2 g'1, a geometric density of 0.36 g cm'3, and a total porosity of 86% by volume. The diameter of the filaments forming the monolith is -760 pm, and the channel width is -710 pm. The compressive strength of the monolith is 0.6 MPa.

The 3D Pd/CA-OX catalyst comprises 5% by weight of palladium (Pd) supported on the CA-OX monolith. The size of the palladium nanoparticles is 1.9 ± 0.4 nm.

The Pd is incorporated into the monolith by impregnation according to the following process:

- 0.267 g of PdCl is dissolved in 42.5 mL of a 0.30 M HCl solution and heated to 650C.

- 6 3D Pd/CA-OX monoliths, with a total weight of around 3 g, are immersed in the precursor solution at 650°C and kept under stirring for 1 h in a rotating and oscillating roller shaker.

- Subsequently, the precursor solution is divided into 7 mL aliquots to immerse a monolith in each aliquot, and they are introduced into an oven at the appropriate temperature so that the precursor solution reaches the temperature of 950C.

- When half of the precursor solution has evaporated, the monolith is turned over to promote the homogeneous distribution of Pd over its entire surface, and the water is allowed to evaporate completely.

- The Pd-impregnated monoliths are washed with Milli Q water and dried in an oven at 600C for 12h.

- The Pd impregnated monoliths are treated with a reducing gas H2/N2 (100:50 mLmin'1) at 2500C for 2 h.

3D Pd/CA-OX catalysts are used in the catalytic dehydrogenation of AF in liquid phase to produce H2. The reactor is fitted with a fixed bed loaded with 5 monoliths, operated at a temperature of 25 0C, AF aqueous solution flow rate of 0.25 mL min'1, space time 160 gcarh L'1, initial AF concentration of 1 M and helium flow rate of 17 mL min'1, and at atmospheric pressure.

An AF conversion of 83.3% is achieved at a gas flow rate of 10 mL min-1. The selectivity to the dehydrogenation reaction is 100%. Therefore, H2 and CO2 are the only reaction products. The activity is maintained for 6 hours of use, after which the catalyst gradually deactivates, until it loses all activity after 24 hours of use.

The 3D Pd/CA-OX catalyst regains activity after being dried at 600C in an oven in an air atmosphere for 24 h.

Example 3. Effect of operating conditions on the hydrogen production reaction from the catalytic dehydrogenation of formic acid using the 3D structured catalyst based on activated carbon/aluminum oxide and 5% by weight of palladium for the production of hydrogen

An aqueous CA/boehmite ink with pseudoplastic behavior has been prepared containing 42% by weight of solids and preferably composed of 70% by weight of CA and 30% by weight of boehmite.

The ink viscosity decreases from 104 to 101 Pa s in the shear rate range of 10.1 to 102 s_1. The storage and loss shear moduli of elasticity are around 106 and 105 Pa, respectively. The yield strength is -1500 Pa.

A cylindrical structure of 16.2 mm diameter and 7.9 mm height is designed containing a pattern of interconnected open channels forming macropores defined by: i) 9 linear filaments per layer in the same direction, ii) 12 stacked layers, iii) 90o rotation between layers, iv) 840 pm printing tip diameter and v) structures without external frame. A 3 cm3 syringe is filled with the aqueous CA/boehmite ink and printed in air and at room temperature using robocasting equipment and an X-Y plane speed of 10 mm-s-1.

The 3D-printed structures are treated at 13,000°C for 0.5 h in vacuum, and the boehmite transforms into 5- and 0-AlO. The monoliths have a specific surface area of 454 m2 g-1, a geometric density of 0.36 gcm-3, and a total porosity of 86% by volume. The diameter of the filaments forming the monolith is -760 pm, and the channel width is -710 pm. The compressive strength of the monolith is 0.5 MPa.

The 3D Pd/CA catalyst comprises 5% by weight of palladium (Pd) supported on the CA monolith. The size of the palladium nanoparticles is -2 nm. The Pd is incorporated into the monolith by impregnation according to the following process:

- 0.267 g of PdCL is dissolved in 42.5 mL of a 0.30 M HCI solution and heated to 650C.

- 6 3D Pd/CA monoliths, with a total weight of around 3 g, are immersed in the precursor solution at 650°C and kept under stirring for 1 h in a rotating and oscillating roller shaker.

- Subsequently, the precursor solution is divided into aliquots of adequate volume to immerse a monolith in each aliquot, and they are introduced into an oven at the appropriate temperature so that the precursor solution reaches the temperature of 950C.

- When half of the precursor solution has evaporated, the monolith is turned over to promote the homogeneous distribution of palladium over its entire surface, and the water is allowed to evaporate completely.

- The Pd-impregnated monoliths are washed with Milli Q water and dried in an oven at 600C for 12h.

- The Pd-impregnated monoliths are treated with a reducing gas H2/N2 (100:50 mL-min'1) at 2500C for 2 h.

3D Pd/Ca catalysts are used in the catalytic dehydrogenation of AF in the liquid phase to produce H2. The reactor is loaded with five monoliths and is operated under two different operating conditions:

- Group 1: temperature of 25 0C, aqueous solution flow rate of AF of 0.25 mL-min'1, space time 160 gcat h L'1, initial AF concentration of 1 M, helium flow rate of 17 mL-min'1, and at atmospheric pressure (dark grey curve with intermittent traces and dark grey circles in Figure 3).

- Group 2: temperature of 55 0C, aqueous solution flow rate of AF of 0.25 mL-min'1, space time 320 gcat h-L'1, initial AF concentration of 0.5 M, helium flow rate of 17 mL-min'1, and at atmospheric pressure (light grey curve of dash and dot plot and light grey triangles Figure 3).

Under Group 1 operating conditions, an AF conversion of 80.3% is obtained (dark grey curve with intermittent traces and dark grey circles, Figure 3a) and a gas flow rate of 10 mL-min'1 (dark grey curve with intermittent traces and dark grey circles, Figure 3b). The selectivity to the dehydrogenation reaction is 100%. Therefore, H2 and C02 are obtained as the only reaction products. The activity is maintained for 3 h of use, after which the catalyst begins to progressively deactivate until its activity is completely lost after 27 h of use.

Under Group 2 operating conditions, an AF conversion of 94.4% is obtained (light grey curve plotted with dashes and dots and light grey triangles in Figure 3a) and a gas flow rate of 5.8 mL-min'1 (light grey curve plotted with dashes and dots and light grey triangles in Figure 3b). The selectivity to the dehydrogenation reaction is 100%. Therefore, H2 and C02 are obtained as the only reaction products. The activity is maintained for 30 h of use, after which the catalyst begins to progressively deactivate until its activity is completely lost after 50 h of use.

In both cases, the 3D Pd/CA catalyst recovers its activity after being dried at 600C in an oven in an air atmosphere for 24 h.

Claims (15)

1. A 3D Pd/CA catalyst characterized in that it comprises: • a printed three-dimensional structure of filaments, where the filaments comprise pores, where each filament is formed by activated carbon (AC) and/or oxidized activated carbon (AC-OX); and aluminum oxide (AI2O3); and • a heterogeneous coating of palladium (Pd) nanoparticles over the entire surface of the three-dimensional filament structure, where: The weight percentage of the printed three-dimensional structure is between 90% and 99% of the total weight of the catalyst; The amount of Pd coating is between 1% and 10% by weight relative to the total weight of the catalyst, and where the 3D printed structure comprises at least two two-dimensional filament structures, a first two-dimensional structure and a second two-dimensional structure, where the first two-dimensional structure and the second two-dimensional structure are stacked, are parallel and are rotated relative to each other between 86° and 94°, preferably between 89° and 91°, where the three-dimensional structure in turn comprises open channels forming a pattern of interconnected macropores where each two-dimensional filament structure comprises at least 3 filaments, a first filament, a second filament and a third filament, all 3 with a straight path and in the same direction parallel to each other; where the filament has a final diameter of between 225 pm and 1,080 and a distance between filaments of between 0.65 and 0.75 mm; where the printed three-dimensional structure is a frameless structure; and where the printed three-dimensional structure has a density of between 0.34 g cm-3 and 0.46 g cm-3 and a total porosity of between 84% and 87%. 2. The catalyst according to claim 1, wherein the AI2O3 is present in crystalline phases a, 5, 0 and/or any combination thereof. 3. The catalyst according to any of claims 1 or 2, wherein the 3D printed structure is cylindrical. 4. The catalyst according to claim 3, wherein the cylindrical 3D printed structure has a diameter between 10 mm and 30 mm. 5. The catalyst according to claim 5, wherein the cylindrical 3D printed structure has a height of between 1 mm and 30 mm. 6. The catalyst according to any of claims 1 to 5, wherein the printed 3D structure comprises between 55% and 90.9% of CA and/or CA-OX and between 9.1% and 45% of AI2O3. 7. Method for obtaining the catalyst defined in any of claims 1 to 6, comprising the steps of: a) direct printing of aqueous inks (DIW) based on mixtures of CA or CA-OX and boehmite gel to obtain the 3D structure; b) heat treatment of the 3D structure obtained in step (a) at a temperature between 1200 0C and 1400 0C, preferably at 1300 0C, in vacuum, and for a time between 0.4 h and 2 h, preferably for 0.5 h, to transform boehmite into AI2O3, and preferably with crystalline phases a, 5y0; c) coating the 3D structure after the heat treatment of step (b) with Pd nanoparticles obtained from PdCh dissolved in an aqueous solution of HCl at a concentration of between 0.2 M and 0.6 M, preferably dissolved in an aqueous solution of HCl at a concentration of 0.30 M, previously heating, for a time of between 0.8 h and 3 h, preferably for 1 h, at a temperature of between 35 0C and 75 0C, preferably at a temperature of 65 0C, to then carry out complete evaporation of the aqueous solution of HCl at a temperature between 90 and 980 C, preferably at a temperature of 950 C; d) washing the 3D Pd structure obtained in step (c) with milli Q water and then drying at a temperature between 50 0C and 70 0C, preferably at a temperature of 600C, for a time between 10 h and 24 h, preferably for 12 h; e) treatment of the 3D Pd structure obtained in step (d) with a reducing gas H2/N2 (100:50 mL-min'1) at a temperature between 2000C and 350 0C, preferably at 250 0C, for a time between 1.8 h and 4 h, preferably for a time of 2 h, to obtain the 3D Pd/CA catalyst defined above. 8. Method according to claim 7, wherein the aqueous ink of step (a) comprises between 70% and 90.9% of CA or CA-OX and between 9.1% and 30% of boehmite gel and where in the aqueous ink the solids represent a percentage by weight of between 40% and 42% with respect to the total ink. 9. Use of the catalyst defined according to any of claims 1 to 7 for producing CO-free H2 from the catalytic dehydrogenation of formic acid in the liquid phase. 10. Use of the catalyst according to claim 9, wherein the conditions are at atmospheric pressure between 0.8 atm and 1.2 atm and at a temperature between 250C and 550C. 11. The use of the catalyst according to claim 9, wherein the catalytic dehydrogenation of AF is carried out in a fixed bed reactor loaded with at least one 3D Pd/CA catalyst. 12. The use of the catalyst according to any of claims 9 or 10, wherein the catalytic dehydrogenation of AF is carried out with a flow rate of aqueous AF solution of between 0.125 mLmin'1 and 1 mLmin'1. 13. The use of the catalyst according to any of claims 9 to 11, wherein the catalytic dehydrogenation of AF is carried out in a space time of between 40 gcat h-L'1 and 320 gcarh-l_-1. 14. The use of the catalyst according to any of claims 9 to 12, wherein the catalytic dehydrogenation of AF is carried out at an initial AF concentration of between 0.25 My2M. 15. The use of the catalyst according to any of claims 9 to 13, wherein the catalytic dehydrogenation of AF is carried out with a helium (He) flow rate of between 5 mL min'1 and 30 mL min'1.