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POROUS PERMANENTLY POLARIZED HYDROXYAPATITE, A PROCESS FOR ITS PRODUCTION AND USES THEREOF

20250099946 · 2025-03-27

    Inventors

    Cpc classification

    International classification

    Abstract

    A porous permanently polarized hydroxyapatite has a mean pore diameter from 10 nm to 10.000 nm. A process is used for obtaining the porous permanently polarized hydroxyapatite. A composition or material includes the porous permanently polarized hydroxyapatite. The porous permanently polarized hydroxyapatite or the composition or material can be used as a catalyst. A composition is used for preparing the porous permanently polarized hydroxyapatite.

    Claims

    1.-16. (canceled)

    17. A process for obtaining a porous permanently polarized hydroxyapatite comprising the steps of: (a) providing a mixture comprising or consisting of hydroxyapatite powder and a pore-forming material; (b) shaping the mixture provided in step (a) to obtain a shaped body comprising or consisting of the hydroxyapatite powder and the pore-forming material; (c) sintering the shaped body obtained in step (b) to remove the pore-forming material and to obtain a shaped body comprising or consisting of porous hydroxyapatite; (d) applying: (i.) a DC voltage that is constant or variable, the DC voltage being between 250 V and 2550 V, or an equivalent electric field between 1.49 kV/cm and 15 kV/cm, or (ii.) applying an electrostatic discharge between 2500 V and 1500000 V or an equivalent electrical field between 148.9 kV/cm and 8928 kV/cm to the shaped body obtained in step (c) to obtain a shaped body comprising or consisting of porous permanently polarized hydroxyapatite; and (e) cooling: (i.) the shaped body obtained in step (d) maintaining the DC voltage or the equivalent electric field applied in step (d), or (ii.) cooling the shaped body obtained in step (d) maintaining the electrostatic discharge or the equivalent electric field applied in step (d), or (iii.) cooling the shaped body obtained in step (d) without maintaining the DC voltage or the equivalent electric field applied in step (d), or (iv.) cooling the shaped body obtained in step (d) without maintaining the electrostatic discharge or the equivalent electric field applied in step (d), wherein step (d) is carried out at a temperature of at least 900 C.

    18. The process according to claim 17, wherein the pore-forming material comprises a polymer selected from the group consisting of poloxamer, polyolefines, polyesters, polyamides, polyimides, polyvinyl alcohols, polyurethanes, polycarbonates, polyalkylterephthalates, polyarylterephthalates, polyaryletherketones, polyhydroxyalkanoates, proteins, extracellular proteins, globular proteins, enzymes, antibodies, blood clotting factors, polysaccharides and mixtures of at least two of the aforementioned polymers.

    19. The process according to claim 18, wherein the polymer is a poloxamer.

    20. The process according to claim 19, wherein the poloxamer comprises a polyoxypropylene core having a molecular mass of 1 g/mol to 500,000 g/mol.

    21. The process according to claim 19, wherein the poloxamer has a polyoxyethylene content from 1% by weight to 90% by weight.

    22. The process according to claim 18, wherein the polymer has a proportion of 1% by weight to 90% by weight, based on a total weight of the pore-forming material.

    23. The process according to claim 18, wherein the pore-forming material is a hydrogel.

    24. A porous permanently polarized hydroxyapatite obtained or obtainable by the process according to claim 17.

    25. The porous permanently polarized hydroxyapatite according to claim 24, further comprising a mean pore diameter from 10 nm to 500,000 nm.

    26. The porous permanently polarized hydroxyapatite according to claim 24, wherein the porous permanently polarized hydroxyapatite has nanometric pores and submicrometric pores or nanometric pores, submicrometric pores and micrometric pores.

    27. The porous permanently polarized hydroxyapatite according claim 26, wherein: the nanometric pores have a mean pore diameter from 1 nm to 100 nm, and/or the submicrometric pores have a mean pore diameter from 101 nm to 999 nm, and/or the micrometric pores have a mean pore diameter from 1 m to 500 m.

    28. The porous permanently polarized hydroxyapatite according to claim 24, wherein the porous permanently polarized hydroxyapatite has: a wide angle X-ray scattering (WAXS) pattern as showing peaks at 2=25.9, 31.7, 32.1, 32.8, 34.0 and 39.8; and/or a Raman spectrum showing peaks at v1=962 cm.sup.1, v2=400 cm.sup.1-480 cm.sup.1, v3=570 cm.sup.1-625 cm.sup.1, and v4=1020 cm.sup.1-1095 cm.sup.1, and a v-OH at 3574 cm.sup.1 for hydroxyapatite and polarization peaks at 878 cm.sup.1, 848 cm.sup.1 and 794 cm.sup.1; and/or a .sup.31P-NMR spectrum having a peak between 2.3 ppm and 2.9 ppm corresponding to phosphate groups of the hydroxyapatite.

    29. A composition or material comprising a porous permanently polarized hydroxyapatite according to claim 24.

    30. A catalyst comprising the composition or material according to claim 29.

    31. A pasty or ink composition for preparing a porous permanently polarized hydroxyapatite according to claim 24, the pasty or ink composition comprising hydroxyapatite powder and a pore-forming material.

    32. A catalyst comprising the porous permanently polarized hydroxyapatite according to claim 24.

    Description

    BRIEF DESCRIPTION OF THE DRAWINGS

    [0140] The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

    [0141] In the figures, the following is schematically displayed:

    [0142] FIG. 1. Structural characterization of as prepared HAp, s-HAp and s/50-HAp samples: (a) WAXD spectra, (b) Raman spectra in the region of the four characteristic vibrational modes (v1-V4), and (c) Stacked Raman spectra of the samples in the region of 3550-3600 cm.sup.1 corresponding to the v(OH) vibrational mode.

    [0143] FIG. 1 (d). A .sup.31P-NMR spectrum of a porous permanently polarized hydroxyapatite.

    [0144] FIG. 2 SEM micrographs of s-HAp and s/x-HAp samples obtained using different pluronic hydrogel mass percentages. Porosity was clearly observed when the hydrogel was introduced into the blend.

    [0145] FIG. 3. Dependence of (a) the pore size and (b) the porosity of s/x-HAp samples on the pluronic hydrogel wt %.

    [0146] FIG. 4. (a) HAp ink loaded with a 60 wt.-% of pluronic hydrogel extruded with a syringe. (b) Effect of sintering on the structure of the same HAp ink, as modeled with different shapes. (c) SEM inspection of porosity the outer face (c1) and the interior (c2) of a HAp ink.

    [0147] FIG. 5. Geometrical comparison of (a).sub.60-HAp/c cube, and (b) HAp/c disks.

    [0148] FIG. 6. (a) Comparison of the ethanol yields obtained from the CO.sub.2 and CH.sub.4 fixation reaction as catalyzed by HAp/c and 60-HAp/c. Reactions were performed using an initial CO.sub.2: CH.sub.4 mixture (3 bar each), 1 mL of water, at 140 C. without UV irradiation for 48 h. (b) Variation of the ethanol production as a function of the initial water content (expressed in mmol/gcat instead of mol/g.sub.cat). The rest of the reaction conditions were identical to those described for (a).

    [0149] FIG. 7. Comparison of the ammonium yields obtained from the N.sub.2 fixation reaction as catalyzed by HAp/c and 60-HAp/c. Reactions were performed using an initial N.sub.2 pressure (6 bar), 20 mL of water, at 120 C. with UV irradiation for 24 h.

    [0150] FIG. 8. Pore size distribution histograms for s/x-HAp samples.

    DETAILED DESCRIPTION

    Experimental Section

    1. Materials

    [0151] Calcium nitrate Ca(NO.sub.3).sub.2, diammomium hydrogen phosphate [(NH.sub.4).sub.2HPO.sub.4; purity>99.0%], ammonium hydroxide solution 30% [NH.sub.4OH; purity: 28-30% w/w], zirconyl chloride (ZrOCl.sub.2.Math.8H.sub.2O; ZC), aminotris(methylenephosphonic acid) (ATMP) and the initial Pluronic F-127 polymer (C.sub.3H.sub.6O.Math.C.sub.2H.sub.4O).sub.x, BioReagent powder) were purchased from Sigma Aldrich. Ethanol (purity>99.5%) was purchased from Scharlab. N.sub.2, CH.sub.4 and CO.sub.2 gases with a purity of >99.995% were purchased from Messer. All experiments were performed with milli-Q water.

    2. Synthesis of Hydroxyapatite (HAp)

    [0152] 15 mL of 0.5 M (NH.sub.4).sub.2HPO.sub.4 in de-ionized water were added at a rate of 2 mL/min to 25 mL of a 0.5 M of Ca(NO.sub.3).sub.2 solution in ethanol with pH previously adjusted to 11 using ammonium hydroxide solution. The mixture was left aging for 1 h under gentle agitation (150 rpm) at room temperature. Hydrothermal treatment at 150 C. was applied using an autoclave Digestec DAB-2 for 24 h. The autoclave was allowed to cool down before opening. The precipitates were separated by centrifugation and washed with water and a 60/40 v/v mixture of ethanol/water (twice). After freeze-drying it for 3 days, a white powder was obtained.

    3. Synthesis of Pluronic Hydrogel

    [0153] 25 g of distilled water was mixed with 25 g of Pluronic F-127 polymer using a FlackTek SpeedMixer at 3500 rpm for 5 minutes. After that, 50 g of Pluronic polymer were added and vigorously stirred using the same conditions. The resultant hydrogel was stored at 4 C.

    4. Synthesis of Porous HAp Inks

    [0154] HAp powder, hereafter denoted as prepared HAp, was obtained from a hydrothermal route and freeze-dried for 72 h to eliminate the water content. Pluronic hydrogel was prepared from a water and Pluronic F-127 mixture.

    [0155] HAp inks with desired weight percentage of pluronic hydrogel were obtained from the slow addition of half of the weighted pluronic hydrogel to HAp powder, followed by rigorous stirring at 2500 rpm for 2 minutes using a Fisherbrand Digital Vortex Mixer. This process was repeated again adding the rest of hydrogel to achieve the homogeneous mixture. All the procedure was carried out at low temperature (i.e. in a cold room <4 C.). The white paste obtained was left aging at 4 C. for 24 h to ensure that the pluronic hydrogel became homogeneously distributed. Finally, HAp inks were modeled at low temperatures to obtain the desired 3D HAp scaffolds and sintered at 1000 C. using a muffle Carbolite ELF11/6B/301 for 2 h. Hereafter, this product is denoted as s/x-HAp, where s refers to sintered and x corresponds to the mass percentage (%) of pluronic hydrogel. For terms of completion, sintered grains based on as prepared HAp powder were also prepared using the same temperature and time conditions (s-HAp).

    5. Catalytic Activation

    [0156] s-HAp disks, which were obtained by pressing 150 mg of s-HAp powder at 620 MPa for 10 min, and s/x-HAp cubes were catalytically activated by placing the samples between two stainless steel plates (AISI 304) and applying a constant DC voltage of 500 V for 1 h with a GAMMA powder supply, while temperature was kept at 1000 C. Samples were allowed to cool down maintaining the applied electric potential for 30 minutes, and finally, all the system was powered off and left to cool overnight. Hereafter, catalysts derived from s-HAp and s/x-HAp have been denoted HAp/c and x-HAp/c.

    6. Characterization

    [0157] Structural characterization was performed using wide angle X-ray diffraction (WAXD), Raman microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses. Water absorption capabilities were obtained by means of a contact angle measuring equipment.

    [0158] More specifically, structural analyses were performed by means of a inVia Qontor confocal Raman microscope (Renishaw) equipped with a Renishaw Centrus 2957T2 detector and a 532 nm laser. In order to obtain representative data, 32 single point spectra were averaged.

    [0159] Wide angle X-ray scattering (WAXS) studies were conducted using a Brucker D8 Advance model with Bragg-Brentano 2 configuration and Cu K.sub. radiation (=0.1542 nm). Measurements were performed in a 2 range of 20-60 in steps of 0.02 and scan speed of 2 s, using a one-dimensional Lynx Eye detector. The crystallinity (.sub.c) was obtained using the following expression:

    [00001] c = 1 - V 112 / 300 I 300 ( S1 ) [0160] where I.sub.300 is the intensity of the (300) reflection and V.sub.112/300 is the intensity of the hollow between the (112) and (300) reflections, which disappears in non-crystalline samples. The crystallite size, L.sub.hkl, was calculated using the Debye-Scherrer equation

    [00002] L hkl = 0.9 .Math. B .Math. cos hkl ( S2 ) [0161] where is the wavelength of the monochromatic X-ray beam, B is the full width at half maximum of the peak at the maximum intensity, and Oki is the peak diffraction angle that satisfies the Bragg's law for the (hkl) plane.

    [0162] SEM studies were carried out using a Focused Ion Beam Zeiss Neon40 microscope operating at 5 kV equipped with an EDX (20 kV) spectroscopy system. The latter technique was used to estimate the composition of the distribution of the ATMP/ZC/ATMP coatings.

    [0163] Water absorption capability was computed by means of a contact angle measuring equipment OCA 15EC (Data-Physics Instruments). To dispense the water droplet, a 500 L DS500/GT glass syringe and a needle SNS 021/011 were used. The absorption flow rates were calculated by dropping 1.5 L water droplets onto the surface of the samples while recording with a fps camera. Then, the photograms were analyzed.

    7. Structural Characterization

    [0164] In order to achieve optimal catalytic activation, as prepared HAp powder was calcined at 1000 C., enhancing the crystallinity, surface charge accumulation and OH.sup. vacancies through a dehydration process. Moreover, exposure of HAp inks at such high temperature is also necessary to confer mechanical stability and to eliminate the pluronic hydrogel (which could interfere in the catalyst performance, masking the catalytic activity). In this section, the structure and crystallinity of s-HAp and s/x-HAp are compared with the original as prepared HAp.

    [0165] WAXD spectra of as prepared HAp, s-HAp and s/50-HAp samples are displayed in FIG. 1a. All samples presented the characteristic peaks of HAP at 20=25.9, 31.7, 32.1, 32.8, 34.0 and 39.8, which correspond to the (002), (211), (112), (300), (202) and (310) reflections, respectively (JCPDS card number 9-0077). Representative crystallographic parameters (see below table 1) were obtained to determine whether the suppression of pluronic hydrogel affected the dehydration process.

    TABLE-US-00001 TABLE 1 Representative crystallographic parameters obtained for as prepared HAp, s-HAp and s/50-HAp samples. Hap S-HAp s/50-Hap .sub.c 0.78 0.01 0.81 0.02 0.84 0.02 L.sub.221 [nm] 24 2 24 1 20 1 L.sub.002 [nm] 54 3 39 2 24 2 I.sub.002 0.585 0.02 0.438 0.02 0.238 0.02 I.sub.112 0.728 0.04 0.717 0.03 0.598 0.05

    [0166] The expected crystal refinement was observed for the two samples treated at high temperatures, slightly increasing their crystallinity (.sub.c; Eq S1) from 0.780.01 for HAp to 0.840.02 for s/50-HAp and 0.810.02 for s-HAp. Nevertheless, some structural differences arose when the crystallite size of the (211) main reflection (L211) was analyzed (Eq S2). A reduction of 4 nm was observed for the s/50-HAp, suggesting that the presence of the hydrogel restricted the crystal growth during the crystal refinement process. Accordingly, this observation together with the fact that the crystallinity was higher for s/50-HAp than for s-HAp, indicates porosity promoting the formation of crystallization nuclei and, therefore, crystals are more abundant but slightly smaller for the former than for the latter. On the other hand, the intensities of the (002) and (112) reflections normalized by the intensity of the (211) main reflection (I.sub.002 and I.sub.112, respectively) were considerably lower for s/50-HAp, supporting such hypothesis. I.sub.002 is commonly used to study the growth anisotropy through the c-axis of the HAp crystal lattice, also affected by sintering of HAp grains (I.sub.002.sup.HAp>I.sub.002.sup.s-HAp>I.sub.002.sup.8/50-HAp). These results allow to conclude that the pluronic hydrogel acts as a template, directing the shape and size of HAp grains that results in the generation of porosity.

    [0167] Generation of other calcium phosphate phases, such as B-tricalcium phosphate (-TCP), can be easily produced during the dehydration process. Although the characteristic reflection (021) at 2 =31.5 of -TCP (JCPDS card number 9-0432) was not observed in the WAXD spectra, the formation of other salts was not completely discarded as most of their reflections share peak positions with HAp. In addition, Raman microscopy measurements were performed in order to obtain more detailed information about the phase distribution of the sintered samples.

    [0168] Raman spectra, which are shown in FIG. 1b, present the characteristic vibrational fingerprint of HAp (corresponding to the PO.sub.4.sup.3 internal modes), enabling to discard both the pluronic hydrogel residues (since no peaks related to pluronic identity were detected) and the presence of other calcium phosphate salts. Thus, the latter would be unequivocally identified by a characteristic splitting or a shifting of the main peak at v.sub.1=962 cm.sup.1, which corresponds to the PO symmetric stretching mode. Moreover, such peaks become sharper and better defined after sintering, indicating that crystallinity increases. Additionally, the peak areas at 3574 cm.sup.1, which are associated to the OH stretching mode and normalized by the area of the v.sub.1 (A.sub.3574), were evaluated to control the proper generation of OH vacancies during the dehydration process (FIG. 1c). The obtained results clearly show that the presence of pluronic hydrogel does not affect the final amount of generated vacancies, as long as A.sub.3574=0.1600.003 and 0.1670.002 for s-HAp and s/50-HAp are almost 33% times lower than the value obtained for as prepared HAp. Therefore, it can be concluded that the sintering process successfully suppresses the hydrogel from the HAp scaffold without affecting the final composition of the mineral.

    8. Influence of Pluronic Hydrogel on the Final HAp Scaffolds

    [0169] FIG. 2 compares SEM micrographs of s-HAp and s/x-HAp, which were obtained using different pluronic hydrogel mass percentages (i.e. 50%, 60%, 73% and 78%). The generation of spherical nano- and submicrometric pores (from 12073 to 400122 nm) was clearly observed in all s/x-HAp samples. Nano- and submicrometric pores consistently presented normal distributions for all samples (FIG. 8), even though a new family of micrometric cavities appeared when the mass percentage of pluronic hydrogel increased from 50% to 73% wt. and 78% wt (FIG. 2). This phenomenon has been attributed to the effect of residual hydrogel that was not properly dispersed among the HAp powder. Due to the fact that their sizes presented a poor dependence on the mass percentage of pluronic hydrogel (6.22.0 m and 5.42.8 m for s/73-HAp and s/78-HAp, respectively), both types of pores coexisted independently.

    [0170] The porosity of samples associated to nano- and submicrometric pores (i.e. discarding micrometric cavities), which was obtained considering the percentage of void space observed in SEM images, ranged from 6% to 14%. As is illustrated in FIG. 2 for s/60-HAp, high magnification images allow discerning how pluronic hydrogel droplets were the precursors of the resulting pores, favoring the location of HAp sintered grains in their surroundings. FIG. 3 shows that the pore size and the porosity of s/x-HAp depend on the size and number of hydrogel droplets, evidencing that the former can be controlled through the mass percentage of hydrogel added to the initial mixture.

    [0171] 3D printed HAp scaffolds were achieved by extruding the pluronic hydrogel-HAp mixture, which exhibited good rheological conditions, as the hydrogel bound the HAp powder, when it was maintained at low temperatures (<4 C.). In order to obtain the optimum paste properties for 3D printing, different HAp inks were preliminary examined by varying the mass percentage of the hydrogel. Finally, a pluronic hydrogel mass percentage of 60% was considered for further studies due to its optimal printable properties, good mechanical stability and control of pore size and porosity (see below). FIG. 4a-b shows that such HAp ink was viscous enough to be introduced into a syringe but sufficiently dry to maintain the desired shape even at cold temperatures, when pluronic hydrogel was still liquid. Moreover, sintered samples maintained the desired shape without showing observable cracks or shrinkage, and with good structural stability with fair bonding between filaments (FIG. 4b).

    [0172] SEM images acquired from extruded filaments revealed that the porosity at the exposed surface decreased to 4% (FIG. 4c1), which has been attributed to the friction with the walls and suggesting that such behavior could be a drawback. However, focusing on a broken region of the rod where the inner part (or the cavity) can be observed, the porosity recovers the standard value of 12% (FIG. 4c2), which is in agreement with that was found for s/60-HAp samples (FIG. 3b). Even though the surface porosity could be enhanced by using coated syringes and/or more viscous HAp inks, together with a precise control of the temperature, the inventors' results indicate that the utilization of a syringe presents significant handling advantages.

    [0173] Further, water absorption capability studies comparing s-HAp and s/60-HAp scaffolds were conducted. To enhance their porosity, s/60-HAp scaffolds were shaped as cubes (FIG. 4b), whereas s-HAp consisted of disks made of compressed HAp powder. The below table 2 shows that water flow absorption was almost four times greater for s/60-HAp than for s-HAp, which was mainly attributed to the pores of the former.

    TABLE-US-00002 TABLE 2 Water absorption capability of different samples before (s-HAp and s/60-HAp) and after (HAp/c and 60-HAp/c) catalytic activation. Samples s-HAp s/60-Hap HAp/c 60-HAp/c Water flow 0.93 0.14 3.57 0.17 1.73 0.39 2.60 0.04 absorption [L/s]

    [0174] The same study was performed after catalytic activation of s-HAp and s/60-HAp, which produced the HAp/c and 60-HAp/c catalysts by applying the TSP process. The same experimental conditions (Methods section) were applied to both samples. Although the polarization process is known to affect the hydrophilicity of the samples due to a surface charge induction effect (Rivas, M.; del Valle, L. J.; Armelin, E.; Bertran, O.; Turon, P.; Puiggal, J.; Alemn, C. Hydroxyapatite with Permanent Electrical Polarization: Preparation, Characterization, and Response against Inorganic Adsorbates. ChemPhysChem 2018, 19, 1746-1755), the enhancement of water absorption capability was only observed for HAp/c, which has been attributed to the non-negligible effect of increased roughness. Despite of this, 60-HAp/c still presented much better absorption capability than HAp/c (i.e. 1.5 times greater), which is expected to promote the final catalytic activity of the carbon and nitrogen fixation reactions.

    9. Catalytic Activity of 60-HAp/c

    [0175] In order to elucidate the effect of porosity on the catalytic activity of the HAp-based catalysts, 60-HAp/c cubes were prepared and compared with HAp/c disks (FIG. 5). For this purpose, s/60-HAp cubes and s-HAp disks were polarized using identical experimental conditions. The porosity was calculated in previous sections considering SEM images of grains and comparing different pluronic hydrogel loadings. However, the utilization of specific geometrical parameters, as defined in FIG. 5, instead of grains allows an alternative expression of relative porosity (.sub.rel) measured by gravimetry, which appears to be more appropriate for comparing both samples from a catalytic point of view. Hence:

    [00003] rel = 1 - 60 - HAp / c / HAp / c ( 1 ) [0176] where .sub.60-HAp/c and .sub.HAp/c are the densities of 60-HAp/c cubes and HAp/c disks, respectively, which were obtained using the parameters described in the below table 3. The resulting value, .sub.rel=0.70.03, indicates that the difference between the porosities of the materials is higher than the one obtained by SEM inspection. Indeed, .sub.rel suggests greater porosity with good pore interconnectivity in the bulk, and confirms the fact that external handling of the HAp inks results in a reduction of superficial porosity due to abrasion.

    TABLE-US-00003 TABLE 3 Macroscopic parameters of the 60-HAp/c and HAp/c catalysts (FIG. 5). The exposed area only considers the faces of the geometry that are exposed in the reaction. Exposed Area [mm.sup.2] Volume [mm.sup.3] Weight [mg] 60-HAp/c (cube) 74.3 81.8 132 c-HAp (disk) 94.3 39.3 200

    [0177] The catalytic performance of the 60-HAp/c and HAp/c was compared for two different reactions based on nitrogen and carbon fixation: i) the production of ethanol using mixtures of CO.sub.2 and CH.sub.4 (Sans, J.; Revilla-Lpez, G.; Sanz, V.; Puiggal, J.; Turon, P.; Alemn, C. Permanently Polarized Hydroxyapatite for Selective Electrothermal Catalytic Conversion of Carbon Dioxide into Ethanol. Chem. Commun. 2021, 57, 5163-5166; and ii) the conversion of N.sub.2 to ammonia. As it is shown below, the conversion of CO.sub.2 and CH.sub.4 to ethanol and of N.sub.2 to ammonia improves by around 3000% and 2000%, respectively, in comparison to HAp/c, as discussed below.

    10. Carbon and Dinitrogen Fixation Reactions

    [0178] Catalysts were introduced in an inert reactor and tested for two reported electrothermal catalytic reactions: 1) the synthesis of ethanol from CO.sub.2 and CH.sub.4; and 2) the conversion of N.sub.2 to ammonia. Quantification of the reaction yields was performed using 1H-NMR spectroscopy. Specific details are provided in the following.

    [0179] The reactor comprised an inert reaction chamber coated with a perfluorinated polymer (120 mL), in which both the catalyst and water (1 mL) were incorporated. The reactor was equipped with an inlet valve for the entrance of N.sub.2, CH.sub.4, CO.sub.2 and an outlet valve to recover the gaseous reaction products. A UV lamp (GPH265T5L/4, 253.7 nm) was also placed in the middle of the reactor to irradiate the catalyst directly, the lamp being protected by a UV transparent quartz tube. All surfaces were coated with a thin film of a perfluorinated polymer in order to avoid any contact between the reaction medium and the reactor surfaces, in this way discarding other catalyst effects.

    [0180] The reaction products were analyzed by 1H NMR spectroscopy. All 1H NMR spectra were acquired with a Bruker Avance III-400 spectrometer operating at 400.1 MHz. The chemical shift was calibrated using tetramethylsilane as internal standard. Sixty-four scans were recorded in all cases. In order to remove the products formed on the catalysts from reactions involving CO.sub.2 and CH.sub.4, samples were dissolved in deuterated water containing 100 mM of HCl and 50 mM of NaCl with the final addition of deuterated water.

    [0181] In the case of the dinitrogen fixation reaction to produce ammonia, the catalyst (10 mg) was dissolved in 15 mL of water with pH adjusted to 2.10.2 using 7.6 mM H.sub.2SO.sub.4, to promote the conversion of ammonia in NH.sub.4+, and applying 4 cycles that involved sonication (5 min) and stirring (1 min) steps. Then, for the 1H NMR sample preparation, 500 L of the reacted catalyst solution were mixed with 100 L of DMSO-d.sub.6 instead of solvents with labile deuterons (i.e. D.sub.2O) to avoid the formation of ammonium deuterated analogues, not desired for quantitative analysis. The same treatment was applied to the water supernatant.

    10.1 Production of Ethanol Using CO.SUB.2 .and CH.SUB.4

    [0182] The performance of 60-HAp/c and HAp/c catalysts for the production of ethanol have been compared. The reaction was performed using the same reactor chamber under a CO.sub.2 and CH.sub.4 atmosphere (3 bar each), at 140 C. but without the presence of UV light for 48 h. Initially, CO.sub.2 was used to purge the reactor. The catalyst, HAp/c disks or 60-HAp/c cubes without any coating, and 1 mL of de-ionized liquid water were incorporated into the reaction chamber (reactions were performed separately for each catalyst). Additionally, the effect of the initial water content on the reaction yield was investigated for one of the 60-HAp/c catalysts.

    [0183] The ethanol yields obtained for both 60-HAp/c and HAp/c are compared in FIG. 6a. The ethanol production is around four times higher for the former than for the latter (55.04.9 and 13.30.7 mol per gram of catalyst, respectively), which represents an outstanding increment of the catalytic performance (414%).

    [0184] Previous studies evidenced that water is necessary as proton source, whereas the excess of water content hinders the gas fixation onto the HAp/c surface, diminishing the final yields of the reaction (Sans, J.; Revilla-Lpez, G.; Sanz, V.; Puiggal, J.; Turon, P.; Alemn, C. Permanently Polarized Hydroxyapatite for Selective Electrothermal Catalytic Conversion of Carbon Dioxide into Ethanol. Chem. Commun. 2021, 57, 5163-5166). In this work the examiner have examined if the water content is still a limiting factor for 60-HAp/c, which is characterized by both the generation of pores and the high water absorption in comparison to HAp/c. Accordingly, a series of reactions varying the initial water content from 1 to 80 mL, while the CO.sub.2 and CH.sub.4 pressures (3 bar each), the temperature (140 C.) and the reaction time (48 h) were kept.

    [0185] The yield of ethanol, which is plotted in FIG. 6b, increased progressively with the increase of initial water introduced in the reactor, even when the initial water content is 80 times greater than that used in the standard reaction for HAp/c (1 mL). This result describes the huge catalytic potential of 60-HAp/c that is able to produce up to 43427 mol per gram of catalyst when the water content is 80 mL, reaching for the first time a production comparable to the one reported for cutting edge Cu-based catalysts (Ma, W.; Xie, S.; Liu, T.; Fan, Q.; Ye, J.; Sun, F.; Jiang, Z.; Zhang, Q.; Cheng, J.; Wang, Y. Electrocatalytic Reduction of CO.sub.2 to Ethylene and Ethanol through Hydrogen-Assisted CC Coupling over Fluorine-Modified Copper. Nat. Catal. 2020, 3, 478-487). Moreover, it is worth noting that Cu-based catalysts require the application of electric potentials, whereas no potential is necessary for 60-HAp/c performance. On the other hand, the ethanol or any other product in the remaining water inside the reactor was found to be null, highlighting the absorption capability of the catalyst and stressing out the scalability of it. Overall, it can be concluded that porosity boosts the catalytical performance of HAp/c for the production of ethanol, obtaining for the first time yields high enough for industrial scalability and feasibility.

    10.2 Conversion of N.SUB.2 .to Ammonia

    [0186] The production of ammonia using 60-HAp/c and HAp/c catalysts was performed at 120 C. and under UV illumination. In order to eliminate the initial air content, the reaction chamber was first purged with N.sub.2 and, subsequently, filled with N.sub.2 until a pressure of 6 bar was reached. A volume of 20 mL of de-ionized water was introduced in the reactor and put in contact with the non-irradiated side of the catalyst. It is worth noting that in this reaction water acted not only as the proton source for the ammonia formation (from water splitting) but also as a medium to facilitate the recovery of the formed product. The product generated on the surface of the catalysts as well as the product collected in liquid water after 24 h of reaction was identified as NH.sub.4 adapting a procedure based on 1H NMR spectroscopy (Hodgetts, R. Y.; Kiryutin, A. S.; Nichols, P.; Du, H.-L.; Bakker, J. M.; Macfarlane, D. R.; Simonov, A. N. Refining Universal Procedures for Ammonium Quantification via Rapid 1H NMR Analysis for Dinitrogen Reduction Studies. ACS Energy Lett. 2020, 5, 736-741).

    [0187] Results, which are depicted in FIG. 7, showed that the total amount of NH.sub.4 formed in presence of the HAp/c was of 6.20.9 mol per gram of catalyst. Although around 25% of the formed NH.sub.4 (i.e. 1.70.3 mol per gram of catalyst) remained adsorbed onto the catalyst surface, the main part of the reaction product was transferred from the catalyst to the water medium (i.e. 4.50.6 mol per gram of catalyst). The influence of the porosity and water absorption capacity on the yield of NH.sub.4 was dramatic, the amount of product formed in presence of 60-HAp/c increasing to 128.822.3 mol per gram of catalyst. Moreover, the total of the yield, which represented an increment of more than 2000% with respect to HAp/c, was collected on the remaining liquid water. This feature, which is fully consistent with the results obtained in the previous reaction, corroborates that 60-HAp/c enhances the synthesis of the reaction products.

    [0188] Porous HAp scaffolds with controlled architecture have been successfully created by mixing HAp powder with pluronic hydrogel. This composition of this mixture allows to regulate the properties of the resulting ink to fulfill 3D-printing requirements, proper mechanical stability being achieved by sintering at high temperatures. Sintered HAp scaffolds exhibit high purity and crystallinity, reflecting a correct dehydration process. Therefore, the addition of pluronic hydrogel for enabling the printable inks, and their posterior generation of pores, does not affect the structure required for preparing polarized HAp-based catalysts.

    [0189] The catalytic activity of the porous samples has been evaluated using different carbon and/or dinitrogen fixation reactions. The 60-HAp/c catalyst showed an outstanding increment of the yields of the reaction. This has been mainly attributed to the enhanced water absorption capability and higher exposed surface. More specifically, the presence of micro-cavities inside the catalyst promotes the heterogeneous catalytic processes used for the production of ethanol and ammonia. Overall, its catalytic activity and huge scalability potential, postulates 60-HAp/c as a solid, cheaper and environmentally friendly alternative to other conventional catalysts.