Abstract
A composition or material, in particular a catalytically active composition or material, a process for producing the composition or material, a composition or material obtained or obtainable by the process, and uses of the composition or material. The composition or material includes a permanently polarized hydroxyapatite and a brushite and/or a brushite-like material.
Claims
1. A composition or material comprising: a permanently polarized hydroxyapatites and brushite and/or a brushite-like material.
2. The composition or material according to claim 1, wherein the composition or material is a multi-phase catalyst, wherein the permanently polarized hydroxyapatite forms a phase of the multi-phase catalyst and the brushite and/or brushite-like material forms a further phase of the multi-phase catalyst.
3. The composition or material according to claim 1, wherein the composition or material has a wide angle x-ray scattering pattern as shown on FIG. 1(a).
4. The composition or material according to claim 1, wherein the composition or material has a Raman spectrum as shown on FIG. 1(b).
5. The composition or material according to claim 1, wherein the permanently polarized hydroxyapatite has a proportion which is larger than a proportion of the brushite and/or the brushite-like material.
6. The composition or material according to claim 1, wherein the permanently polarized hydroxyapatite has a proportion of 50% by weight to 99.9% by weight based on a total weight of the composition or material.
7. The composition or material according to claim 1, wherein the brushite and/or the brushite-like material has a proportion of 0.1% by weight to 35% by weight based on a total weight of the composition or material.
8. The composition or material according to claim 1, wherein the brushite and/or the brushite-like material has a crystallinity, determined via wide angle x-ray scattering, from 65% to 99.9%.
9. The composition or material according to claim 1, wherein the brushite and/or the brushite-like material has a crystallite size, determined via wide angle x-ray scattering, from 20 nm to 500 nm.
10. A process for producing a composition or material according to claim 1, comprising the following steps: (a) providing a sample of hydroxyapatite and/or amorphous calcium phosphate; (b) sintering the sample of hydroxyapatite and/or amorphous calcium phosphate provided in step (a); (c) applying one of: a constant or variable DC voltage between 250 V and 2500 V to the sample of hydroxyapatite and/or amorphous calcium phosphate after step (b) or to a shaped body obtained from the sample of hydroxyapatite and/or amorphous calcium phosphate after step (b), or an equivalent electric field between 1.49 kV/cm and 15 kV/cm to the sample of hydroxyapatite and/or amorphous calcium phosphate after step (b) or to a shaped body obtained from the sample of hydroxyapatite and/or amorphous calcium phosphate after step (b), or an electrostatic discharge between 2500 V and 1500000 V to the sample of hydroxyapatite and/or amorphous calcium phosphate after step (b) or to a shaped body obtained from the sample of hydroxyapatite and/or amorphous calcium phosphate after step (b), or an equivalent electric field between 148.9 kV/cm and 8928 kV/cm to the sample of hydroxyapatite and/or amorphous calcium phosphate after step (b) or to a shaped body obtained from the sample of hydroxyapatite and/or amorphous calcium phosphate after step (b); and (d) cooling the sample of hydroxyapatite and/or amorphous calcium phosphate after step (c), wherein for performing step (c), the sample of hydroxyapatite and/or amorphous calcium phosphate obtained in step (b) or the shaped body obtained from the sample of hydroxyapatite and/or amorphous calcium phosphate obtained in step (b) is arranged between a positive electrode and a negative electrode that are used for applying the constant or variable DC voltage, equivalent electric field or electrostatic discharge during step (c), such that the sample of hydroxyapatite and/or amorphous calcium phosphate obtained in step (b) or the shaped body obtained from the sample of hydroxyapatite and/or amorphous calcium phosphate obtained in step (b) is spaced from one of the positive electrode and the negative electrode.
11. The composition or material according to claim 1, obtained or obtainable by a process comprising the following steps: (a) providing a sample of hydroxyapatite and/or amorphous calcium phosphate, (b) sintering the sample of hydroxyapatite and/or amorphous calcium phosphate provided in step (a), (c) applying one of: a constant or variable DC voltage between 250 V and 2500 V to the sample of hydroxyapatite and/or amorphous calcium phosphate obtained in step (b) or to a shaped body obtained from the sample of hydroxyapatite and/or amorphous calcium phosphate obtained in step (b), or an equivalent electric field between 1.49 kV/cm and 15 kV/cm to the sample of hydroxyapatite and/or amorphous calcium phosphate obtained in step (b) or to a shaped body obtained from the sample of hydroxyapatite and/or amorphous calcium phosphate obtained in step (b), or an electrostatic discharge between 2500 V and 1500000 V to the sample of hydroxyapatite and/or amorphous calcium phosphate obtained in step (b) or to a shaped body obtained from the sample of hydroxyapatite and/or amorphous calcium phosphate obtained in step (b), or an equivalent electric field between 148.9 kV/cm and 8928 kV/cm to the sample of hydroxyapatite and/or amorphous calcium phosphate obtained in step (b) or to a shaped body obtained from the sample of hydroxyapatite and/or amorphous calcium phosphate obtained in step (b); and (d) cooling the sample of hydroxyapatite and/or amorphous calcium phosphate obtained in step (c), wherein, for performing step (c), the sample of hydroxyapatite and/or amorphous calcium phosphate obtained in step (b) or the shaped body obtained from the sample of hydroxyapatite and/or amorphous calcium phosphate obtained in step (b) is arranged between a positive electrode and a negative electrode that are used for applying the constant or variable DC voltage, equivalent electric field or electrostatic discharge during step (c), such that the sample of hydroxyapatite and/or amorphous calcium phosphate obtained in step (b) or the shaped body obtained from the sintered sample of hydroxyapatite and/or amorphous calcium phosphate obtained in step (b) is spaced from one of the positive electrode and the negative electrode.
12. A method for synthesizing organic molecules, comprising the step of using the composition according to claim 1 as a catalyst in a reaction for synthesizing organic molecules.
13. The method according to claim 12, wherein the composition is used in a reaction for synthesizing amino acids.
14. The method according to claim 12, wherein the composition is used in a reaction for synthesizing carboxylic acids.
15. The method according to claim 12, wherein the composition is used in a reaction for synthesizing aldehydes and/or ketones.
16. The method according to claim 12, wherein the composition is used in a reaction for synthesizing alcohols.
Description
BRIEF DESCRIPTION OF THE FIGURES
[0148] In the figures, the following is schematically displayed:
[0149] FIG. 1(a): WAXS diffraction patterns in the characteristic 2 range from 20 to 60 for multiphasic HAp-brushite catalysts (hereafter named C-2);
[0150] FIG. 1(b): Raman spectra of the PO.sub.4.sup.3 internal modes (v.sub.1, v.sub.2, v.sub.3 and v.sub.4) for C-2;
[0151] FIG. 1(c): WAXS diffraction patterns in the characteristic 2 range from 20 to 60;
[0152] FIG. 1(d): Raman spectra of the PO.sub.4.sup.3 internal modes (v.sub.1, v.sub.2, v.sub.3 and v.sub.4) for single-phase hydroxyapatite (HAp) catalysts (hereafter named C-1) and C-2. Characteristic reflections and vibrations for HAp and brushite are labelled in FIGS. 1(a)-1(d);
[0153] FIG. 2: Stacked Raman spectra obtained from a 74 array with a spacing of 1 m for the C-1 and C-2 catalysts. Scale bar refers to the relative intensity of the peaks;
[0154] FIG. 3: In-depth analyses of Raman spectra of C-1 and C-2 of the zones of interest: (a) v1 principal mode; (b) lattice modes; and (c) OH stretching vibrational mode in the region of 3500 to 3700 cm.sup.1 (acquired with a 532 nm laser);
[0155] FIG. 4: XPS spectra of Ca 2p obtained for C-1 and C-2 catalysts;
[0156] FIG. 5: 1H-NMR spectra of the regions at: (a) 0.5 to 4.0 ppm, (b) 5.5 to 8.5 ppm;
[0157] FIG. 6: Comparison of catalytic performance of C-1 and C-2 catalysts: (a) ratio of products/ethanol, (b) C-2/C-1 ratio of all the products detected;
[0158] FIG. 7(a): Unit cell of HAp crystalline structure;
[0159] FIG. 7(b): OH.sup. channels along the c axis of HAp crystalline structure;
[0160] FIG. 8(a) Unit cell of brushite;
[0161] FIG. 8(b): unit cell of monetite;
[0162] FIG. 9(a): Crystalline structure of brushite. The atoms involved in the hydrogen bonds have been labeled;
[0163] FIG. 9(b): Crystalline structure of brushite. The atoms involved in the hydrogen bonds have been labeled;
[0164] FIG. 10: Raman spectra of: (a) crystalline hydroxyapatite (hereafter named cHAp)(T.sub.h), and (b) thermally stimulated polarized crystalline (hereafter named cHAp/tsp)(T.sub.h) prepared using different hydrothermal temperatures for 24 h;
[0165] FIG. 11: Raman spectra in the: (a) 770-910 cm.sup.1, and (b) 100-350 cm.sup.1 intervals of cHAp(150 C.) and cHAp/tsp(T.sub.h100 C.);
[0166] FIG. 12: (a) Raman spectra of cHAp(150 C.) and cHAp/tsp(150 C.) prepared using different times for the HT: 10 h and 24 h. (b) Raman spectra of cHAp/tsp(150 C.) prepared by applying different DC voltages to cHAp(150 C.). (c) Raman spectra of cHAp/tsp(150) prepared by applying a voltage of 500 V and maintaining the steel electrodes in contact or not with the cHAp(150 C.) samples. cHAp(150 C.) were prepared applying the HT for 24 h in both (b) and (c);
[0167] FIG. 13: (a) Raman spectra at different depths of cHAp/tsp(150 C.) obtained applying the HT for 24 h and a polarizing voltage of 500 V at 1000 C., (b) Magnification of the spectra displayed in (a) for the 100-350 cm.sup.1region.
[0168] FIG. 14: Low magnification SEM micrographs of cHAp(T.sub.h) with T.sub.h=50, 100, 150, 200 and 240 C. In all cases the HT was applied for 24 h. High magnification micrographs of cHAp(150 C.) are also displayed;
[0169] FIG. 15: (a) X-ray diffraction patterns corresponding to cHAp(T.sub.h) with T.sub.h=50, 100, 150, 200 and 240 C. In all cases the HT was applied for 24 h. Reflections attributed to cHAp and brushite are marked by filled diamonds and empty diamonds, respectively, in the diffractogram of cHAp(150 C.), while those associated with TCP (-tricalcium phosphate) are indicated by filled circles in the diffractogram of cHAp(50 C.). (b) Distribution of cHAp and brushite phases in cHAp(T.sub.h) with T.sub.h100 C. This was obtained from the (211) reflection of cHAp and the (141) reflection of brushite;
[0170] FIG. 16: For sample obtained after reaction (95 C. and 48 h) using a chamber pressure of 6 bar (i.e. 2 bar of each feeding reaction gas) and by the catalyst prepared using cHAp(150 C.): (a) 1 H NMR spectrum of the solution obtained after extraction of the amino acids from the catalyst by dissolving the sample in deuterated water containing 100 mM of HCl and 50 mM of NaCl; and (b) solid state 13 C NMR spectra of the catalyst with the synthesized amino acids;
[0171] FIG. 17: Yield (in % per cm.sup.2 of catalyst) of alanine and glycine in the electrophotocatalytic fixation of N.sub.2, CO.sub.2 and CH.sub.4 as a function of T.sub.h. In all cases reactions (in triplicate) were performed at 6 bar and 95 C. for 48 h; and
[0172] FIG. 18: .sup.31P-NMR spectrum of permanently polarized hydroxyapatite.
EXPERIMENTAL SECTION
1. Materials
[0173] 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 (ZC; ZrOCl.sub.2.Math.8H.sub.2O) and aminotris(methylene phosphonic acid) (ATMP) were purchased from Sigma Aldrich. Ethanol (purity>99.5%) was purchased from Scharlab. All experiments were performed with milli-Q water. N.sub.2, CH.sub.4 and CO.sub.2 gases with a purity of >99.995% were purchased from Messer.
2. Synthesis of Crystalline Hydroxyapatite (cHAp)
[0174] 15 mL of 500 mM (NH.sub.4).sub.2HPO.sub.4in de-ionized water (pH adjusted to 10.50.2 with ammonium hydroxide) were added drop-wise (2 mL/min) and under gentle agitation (100 rpm) to 25 mL of 500 mM Ca(NO.sub.3).sub.2 in ethanol. The mixture was stirred for 1 h (100 rpm) at room temperature resulting in a suspension. Hydrothermal (HT) treatment was applied to the suspension with temperature (T.sub.h) ranging from 50 C. to 240 C. using an autoclave Digestec DAB-2 for 24 h unless otherwise is specified. The autoclave was allowed to cool down before opening. The white precipitates were separated by centrifugation and washed sequentially at 8000 rpm for 5 minutes with water and a 60/40 v/v ethanol/water mixture (twice). After freezedrying for 3 days the powder obtained was sintered at 1000 C. during 2 h at an air atmosphere, in particular using a Carbolite ELF11/6W/301 furnace. Hereafter, samples obtained using this procedure have been denoted cHAp(T.sub.h), where T.sub.h refers to the temperature used for the HT of hydroxyapatite (HAp).
3. Thermally Stimulated Polarization Process (TSP)
[0175] According to a first approach, 150 mg of sintered cHAp powder were uniaxially pressed at 620 MPa for 10 minutes (i.e. 5 tons of applied weight) to obtain a disc of 10 mm of diameter and 1 mm of thickness. The disc was placed in between two stainless steel (AISI 304) plates separated at 4 cm and heated at 1000 C. in air atmosphere. Then, a DC voltage of 100 V, 500 V or 1000 V was applied during 1 h. Hereafter, samples obtained by applying the TSP process to cHAp(T.sub.h) are denoted cHAp/tsp(T.sub.h), the applied voltage being explicitly indicated in each case.
[0176] According to a second approach, catalytic activation was (also) successfully achieved applying the thermal stimulation polarization (TSP) treatment, which consisted in exposing the HAp disks at 500 V and 1000 C. for 1 h. For obtaining single-phase HAp catalysts (hereafter named C-1), the electrodes (two stainless steel AISI 304 plates) were placed in contact with the HAp disk. Instead, multiphasic HAp-brushite catalysts (hereafter named C-2) were attained by separating the positive electrode 4 cm from the HAp disk, which was left in contact with the negative electrode.
4. Characterization
[0177] Structural characterization studies were conducted using X-ray photoelectron spectroscopy (XPS), micro-Raman spectroscopy and wide angle X-ray scattering (WAXS). From WAXS spectra, crystallinity (.sub.c) and crystallite sizes L.sub.hkl were determined.
[0178] Further, the structural fingerprint of the samples was studied using the inVia Qontor confocal Raman microscope (Renishaw), equipped with a Renishaw Centrus 2957T2 detector. All measurements were performed with a 532 and a 785 nm laser. In order to achieve representative results, all the spectra presented in this study are the result of the average of a 10590 m grid with 42 points. Depth profiles were also obtained using the same equipment.
[0179] Morphological characterization has been performed by scanning electron microscopy (SEM) using a Focused Ion Beam Zeiss Neon40 microscope equipped with a SEM GEMINI column with a Shottky field emission. Samples were sputter-coated with a thin layer of carbon to prevent sample charging problems.
[0180] As mentioned, crystallinity (.sub.c) was obtained by wide angle X-ray scattering (WAXS) using a Brucker D8 Advance model with Bragg-Brentano 2 configuration and Cu K.sub. radiation (=0.1542 nm). A one-dimensional Lynx Eye detector was employed. Measurements were performed in a 2 range of 20-60 in steps of 0.02, and scan speed of 2 s. The .sub.c value was obtained using the following expression:
[00001]
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. The crystallite size, L.sub.hkl, was calculated using the Debye-Scherrer equation
[00002]
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 .sub.hkl is the peak diffraction angle that satisfies the Bragg's law for the (hkl) plane.
5. Carbon Fixation
[0181] The reaction was carried out in an inert reactor chamber with a 3 bar CO.sub.2 and 3 bar CH.sub.4 atmosphere at 95 C. for 72 h under UV irradiation (GPH265T5L4, 253.7 nm). Before sealing, 0.5 mL of de-ionized liquid water were added to the reactor. The products of the reaction from the catalyst surface were analyzed by .sup.1H-NMR spectroscopy (Bruker Avance III-400). Yields of the reaction were obtained using commercial products with controlled concentration as a reference.
6. Effect of TSP Treatment on the Coexistent Brushite Phase
[0182] FIG. 1(a), which compares the WAXS spectra recorded for C-1 and C-2, reveals the unambiguous presence of highly crystalline HAp in both catalysts. Thus, the peaks identified at 2=31.8, 32.2, and 33.0 correspond to the (211), (112) and (300) reflections of HAp, respectively (JCPDS card number 9-0432). The most significant peaks of the co-existent brushite appear at 2=29, 31, 35, 42, and 51, which have been attributed to the (141), (22.sup.1), (121), (15.sup.2) and (14.sup.3) reflections, respectively (JCPDS card number 72-0713). As it can be seen, although the (22.sup.1), 2=31.0, is the only distinguishable reflection for brushite in FIG. 1a, comparison of the relative intensities obtained for C-1 and C-2 supports the presence of this co-existing apatitic phase in the latter. One of the most relevant differences is observed at 2=51.3, which can be attributed to the (410) reflection of HAp or to the (14.sup.3) of brushite. For C-1 the intensity of the peak at 2=51.3 relative to the one at 2=52.1, which corresponds to the (402) HAp reflection, is lower than one (I.sub.2=51.0/I.sub.2=52.1=0.96), indicating that the intensity of the former peak is slightly lower than that of the latter. For C-2, I.sub.2=51.0/I.sub.2=52.1 increases to I.sub.51/52.1=1.14, evidencing that the peak at 2=51.3 becomes more intense than the one at 2=52.1.
[0183] The (112) and (300) peaks were also used to determine the crystallinity (.sub.c; Eq S1), whereas the (211) reflection was used to calculate the crystallite size (L.sub.211; Eq S2). The crystallinity is very high and similar for the two catalysts, .sub.c=0.950.03 and 0.920.03 for C1- and C-2, respectively. Crystallite sizes are also comparable for the two catalysts, the obtained values (L.sub.211=75.22.4 and 82.73.72 nm for C-1 and C-2, respectively), being in agreement with those reported in the literature. Overall, these observations indicate that the differences applied during the TSP treatment do not affect the predominant HAp phase.
[0184] Raman studies on C-1 and C-2 catalysts are presented in FIG. 1b. The spectra of apatitic phases are mainly dominated by their characteristic PO vibrations. The four characteristic regions of HAp, which correspond to the PO.sub.4.sup.3 internal modes, can be seen in the spectra of the catalysts, being: v.sub.1=962 cm.sup.1, v.sub.2=400-900 cm.sup.1, v.sub.3=570-625 cm.sup.1 and v.sub.4=1020-1095 cm.sup.1. Both samples present a slight splitting of the vi mode in another two peaks at 970 and 949 cm.sup.1, which has attributed to the different PO stretching vibrations of the three crystallographic non-equivalent PO.sub.4.sup.3 tetrahedra found in the -tricalcium phosphate (TCP) apatitic phase. Accordingly with the literature, the presence of TCP is not related to the TSP treatment but to small variation in the conditions during the HT synthesis and the sintering applied afterwards. Moreover, previous studies showed that the TSP treatment increases the crystallintiy and reduces TCP phase by imposing crystallographic specific orientations. Besides, the fact that the two samples present approximately the same relative amount of TCP (i.e. the ratio of the intensities of the Raman shift at 970 cm.sup.1 and the main peak at 962 cm.sup.1, I.sub.970/962, is 0.12 and 0.10 for C-1 and C-2 respectively) evidences that differences in the conditions applied during the TSP treatment are not the precursors of the TCP generation. However, the effect of separating the positive electrode from the mineral disc during the TSP treatment is clearly manifested in FIG. 1b. More specifically, the presence of brushite in C-2 sample is confirmed by the peaks at 878, 848 and 794 cm.sup.1, which correspond to the normal vibration mode of HPO.sub.4.sup.2, the POH deformation mode and the POH rotation mode, respectively. Other differences found between the Raman spectra of C-1 and C-2 catalysts are discussed below.
[0185] The evident presence of HPO.sub.4.sup.2 and POH vibrations allow tracking the variations in the amount of brushite, depending on the TSP conditions. In this sense, the synergistic activity between the HAp and brushite phases is strongly dependent on their exposed surfaces and, therefore, characterization of their superficial distribution is of major interest. FIG. 2 depicts 28 Raman spectra of C-1 and C-2 samples, which have recorded from a 74 array with a spacing of 1 m. For clarity purposes, all spectra have been stacked together, the scale bar referring to the relative intensity of the peaks. As expected, the C-1 spectra do not show any trace of the brushite peaks aforementioned, in comparison with C-2 sample. Surprisingly, the 878 cm.sup.1 peak of C-2, which corresponds to the HPO.sub.4.sup.2 normal vibration mode, presents variation in its relative intensity up to 90%, indicating that the coexisting brushite phase is distributed very heterogeneously. Results derived from FIG. 2 suggest that the geometry of the set-up used to apply the electric field plays an important role, defining the heterogeneity of the two co-existing phases. In order to support this conclusion, the inventors conducted additional experiments repeating the TSP treatment with the separated positive electrode. More specifically, the plate was removed, leaving only the cooper cable acting as the positive electrode. Examination of the recorded Raman spectra confirmed the dependence of the brushite phase with the geometry of the applied field. Not only the peaks HPO.sub.4.sup.2 and POH vibration peaks appeared more intense (i. e. I.sub.878/962=0.22 and I.sub.878/962=0.73 for the C-2 catalysts prepared without and with plate at the positive electrode, respectively), but also the sample was more homogenous, the peak at 878 cm.sup.1 presenting a relative variation of 21% only. Overall, results demonstrate that the HAp/brushite ratio and heterogeneity of the catalyst can be regulated by varying the distance between the plates and the geometry of electric during the TSP treatment.
7. Structural Characterization of the C-2 Catalyst
[0186] In order to understand the synergistic effects occurring in the brushite-containing catalyst, exhaustive structural characterization is crucial. In the case work, the inventor's efforts have been focused on discerning how brushite is integrated into the HAp crystal lattice as distortion on its boundaries, generating new active edge sites and increasing locally the electric conductivity. The most common polymorph of HAp is the hexagonal lattice (see FIG. 7) with the space group P6.sub.3/m (a=b=9.432 , c=6.881 ; ==90, =120) PO.sub.4.sup.3 groups are ordered in equivalent tetrahedra while Ca' ions occupy two different crystallographic positions. In this crystalline phase, the OH.sup. groups are ordered in columns along the c-axis but with disordered orientations because of electrostatic forces. Upon application of the TSP treatment, OH.sup. groups tend to orient along the specific direction of the electric field, thus introducing crystallographic stress in the crystal lattice. This crystallographic stress is shown for C-1 in FIG. 3a, which displays a magnification of the Raman spectrum in the region of the v1 principal mode. Whereas the main vi vibration mode of HAp of C-2 sample coincides with its theoretical value at 962 cm.sup.1, C-1 main peak is located at 963 cm.sup.1, reflecting the tensile strain of the PO.sub.4.sup.3 tetrahedra. The fact that C-1 is the only sample presenting such shift indicates that the brushite found in the C-2 catalyst compensates the local tensile stress tension induced by re-arrangement of the OH.sup. chains.
[0187] One of the most surprising aspects of the C-2 catalyst is that the attainment of the brushite phase is apparently contradictory with the fact that the TSP is carried out at high temperatures. At around 160 C., brushite dehydrates to monetite (CaHPO.sub.4) for further deprotonation to the , and different forms of calcium pyrophosphate (Ca.sub.2P.sub.2O.sub.7) when the temperature increases to 320, 700 and 1200 C., respectively. The experimental evidence obtained from the HPO.sub.4.sup.2 and the POH vibration modes at 878, 848 and 794 cm.sup.1, as well as the absence of the POP vibration at around 732 cm.sup.1, allow to discard the presence of Ca.sub.2P.sub.2O.sub.7. The discrimination of brushite from monetite is clearly visualized by analyzing the lattice vibration modes since such structures crystallize in different space groups, as reflected in FIG. 8. Brushite consists on a monoclinic structure with an Ia space group symmetry (a=5.799 , b=15.126 , c=6.184 ; ==90, =116.428), while monetite exhibits a triclinic unit cell with a P.sup.1 space group symmetry (a=6.916 , b=6.619 , c=6.946 ; =96.180, =103.82, =88.34). The lattice modes of the C-1 and C-2 catalysts are compared in FIG. 3b. C-1 presents the characteristic lattice modes of HAp at: 140 and 155 cm.sup.1 (attributed to the transitional vibrations of Ca.sub.1+Ca.sub.2 and Ca.sub.2, respectively); 193 and 205 cm.sup.1 (translational vibration of PO.sub.4.sup.3); 235 and 288 cm.sup.1 (librational vibrations of PO.sub.4.sup.3); 270 cm.sup.1 (transitional vibration of Ca.sub.1); and 332 cm.sup.1 (translational vibrations of OH.sup.).
[0188] In comparison with C-1, the spectrum recorded for C-2 displays much more intense peaks at 111, 142 and 270 cm.sup.1, which have been attributed to the contribution of the Ca.sup.2+ unique crystallographic sites in brushite. Moreover, the PO.sub.4.sup.3 translational mode at 205 cm.sup.1 is enhanced in C-2 with respect to C-1. Is it worth noting that, while the peak at 142 cm.sup.1 (assigned to the transitional vibrations of Ca.sub.1+Ca.sub.2 of HAp and Ca of brushite) is much more intense and presents a red shift of 2 cm.sup.1 with respect to C-1, the peak at 155 cm.sup.1 remains unaltered, as it has been attributed to the HAp transitional vibration of Ca.sub.2, which is inexistent in the brushite phase. The red shift of Ca.sup.2+ transitional vibration from 140 cm.sup.1 in C-1 to 142 cm.sup.1 in C-2 confirms the presence of brushite in the latter, instead of monetite. Thus, due to group symmetries, the raman shift attributed to the Ca.sup.2+ transitional vibration should be ordered as follows: monetite<HAp<brushite. Overall, the results shown in FIG. 3a-b not only confirms the presence of brushite but also highlights that the C-2 catalyst presents locally Ca.sup.2+ with higher mobility and less tensile stress, which could cause a synergistic effect responsible of an enhance of its catalytic performance.
[0189] As shown in FIG. 9, the crystal structure of brushite can be understood as a layered system formed by zig-zag Ca.sup.2+ and PO.sub.4.sup.3 alternated chains parallel to the a-axis and growing along the c-axis. These chains are bond together along the b-axis through hydrogen bonds, provided by H.sub.2O molecules which form an intermediate layer. However, this water intermediate layer has not been detected in the Raman spectrum of C-2. Characteristic stretching modes for water should be detected in form of two duplets at 3539 and 3483 cm.sup.1 for v.sub.1, and 3270 and 3163 cm.sup.1 for v.sub.2. FIG. 3c shows the characteristic HAp peak corresponding to the OH.sup. stretching mode of HAp at 3574 cm.sup.1. Moreover, neither the water liberations modes, which are typically located at 678 cm.sup.1, can be appreciated in FIG. 1b.
[0190] Detailed analysis of the lattice modes in FIG. 3b revealed the presence of a strong new peak at 323 cm.sup.1 for C-2, which has been assigned to the translational vibration modes of OH.sup. groups. This peak can be clearly deconvoluted into two different peaks located at 332 cm.sup.1, which matches the translational vibration of HAp obtained in the C-1 spectrum, and at 323 cm.sup.1. The fact that this new peak presents an important blue shift indicates the existence of a different crystallographic OH.sup. with less mobility, and thus, somehow bonded. Due to their participation in hydrogen bonding interaction, H.sub.2O molecules of pure brushite are distorted from that of free molecules, generating two distinct crystallographic sites for water molecules (labeled in FIG. 9 as H.sub.1O1 and H.sub.2OO2). The external layers are bond together via the O3.Math..Math..Math.H2 (of H.sub.2OO1) and OH1.Math..Math..Math.H4 (of H2OO2) hydrogen bond interactions, presenting short distances of 1.81 and 1.90 and almost linear angles of 167.3 and 175.7 respectively. Taking into account the theoretic aspects aforementioned and in agreement with the experimental results obtained, hydroxyl groups occupying the crystallographic positions of water molecules appear to be responsible for stabilizing the crystallographic brushite or brushite-like phase through hydrogen bonding interactions. The exchange of proton and hydroxyl groups at high temperatures, causing modifications in the crystalline structure of HAp, has been widely studied. However, substituting H.sub.2O molecules by OH.sup. may cause other crystallographic distortions since the remaining water proton, which is hydrogen bonded to other oxygen of the lattice, is suppressed. For to case of H.sub.2OO2 water molecule this is not crucial for the stability of brushite, as H5 is weakly bonded to the H.sub.2OO1 water molecule, with a H5.Math..Math..Math.H.sub.2OO1 distance of 2.16 . Accordingly, the alternating Ca.sup.2+ and PO.sub.4.sup.3 ions form weaker interaction, and thus have more mobility. This hypothesis is supported by the drastic increment in the intensity of the lattice modes for such ions in the C-2 catalyst (FIG. 3b).
[0191] On the other hand, H.sub.2OO1 water molecule is also hydrogen bond to the O3 of a PO.sub.4.sup.3 group, with a distance of 1.78 . In this case, determining which hydrogen bond is substituted by the OH.sup. is harder, as they are energetically very similar. However, hypothesizing a combination of OH.sup. ions pointing in both directions (O3.Math..Math..Math.H2 and H3.Math..Math..Math.O3) might be reasonable since the TSP treatment imposes a specific OH.sup. orientation, whereas OH1.Math..Math..Math.H4 and O3.Math..Math..Math.H2 are pointing backwards.
[0192] In order to quantify the enhancement in the lattice mobility of Ca.sup.2+ ions highlighted by Raman spectroscopy, XPS measurements were conducted to capture the surface electron binding states of Ca. FIG. 4 compares the XPS Ca2p of C-1 and C-2 catalysts. Both spectra present the characteristic Ca 2p.sub.3/2 and Ca 2p.sub.1/2 peaks of HAp located at 346.9 and 350.5 eV for C-1 and 346.4 and 350.0 eV for C-2. This observation is in good agreement with recent XPS studies on HAp after TSP treatment. The binding energy of the Ca 2p peak is mainly related with the Ca.Math..Math..Math.PO.sub.4.sup.3 bonds. The standard value of Ca 2p.sub.3/2 is usually measured at 347.2 eV, even though shifts at higher binding energies are observed in some cases, for example when RCOO.sup. groups are adsorbed (i.e. Ca.Math..Math..Math.COO.sup. bonds are stronger than Ca.Math..Math..Math.PO.sub.4.sup.3 bonds). Instead, shifts to smaller binding energies are appreciated for C-1 and C-2, which has been associated to the OH.sup. vacancies generated by the TSP treatment. In agreement with the Raman spectra, the shift is more pronounced for the C-2 sample (0.8 eV) than for the C-1 sample (0.3 eV). This 0.5 eV difference has been attributed to the presence of the brushite co-existent phase in the C-2 catalyst.
8. Effect of the Presence of Brushite on the Selectivity of the Catalyst
[0193] The exhaustive analysis accomplished in previous sections allowed discerning the structural differences between the C-1 and C-2 catalysts, highlighting some possible synergies between the two phases detected in the latter. More specifically, the effects of the coexisting brushite phase on the catalytic properties of HAp can be explained as follows: 1) Ca.sup.2+ ions have smaller binding energy and, thus, can act as new catalytic or adsorption sites; and 2) even though the presence of OH.sup. is maintained in both phases, the symmetry of ordered OH.sup. columns may be broken in the boundaries between phases, creating regions with lower electron conductivity but with higher accumulated charge. In order to ascertain the possible synergies contributed by the brushite phase, carbon fixation reactions have been conducted using the C-1 and C-2 catalysts for comparison. More specifically, reactions have been catalyzed in an inert reaction chamber (120 mL) using a CO.sub.2 and CH.sub.4 gas mixture (3 bar each) and liquid water (1 mL) under irradiation of an UV lamp illuminating directly the catalysts at 95 C.
[0194] FIG. 5 presents the .sup.1H-NMR spectra obtained after 72 h reaction from the C-1 and C-2 catalysts dissolved in deuterated water containing 100 mM HCl and 50 mM NaCl, which allowed us to identify the reaction products formed on the surface of the catalysts. Three reaction products are clearly identified for both catalysts by inspecting the spectra at the low frequency region (FIG. 5a): ethanol (CH.sub.2 quartet the CH.sub.3 triplet at 3.50 and 1.06 ppm, respectively), acetone (CH.sub.3 singlet at 2.08 ppm), and acetic acid (CH.sub.3 singlet at 1.85 ppm). The OH peak of ethanol, which is the predominant product in both cases (i.e. 13.133.75 and 15.014.62 mol per gram of catalyst for C-1 and C-2, respectively), overlaps the intense water peak at 4.65 ppm (not shown). In any case, the two spectra displayed in FIG. 5a are relatively similar, indicating that the coexistence of brushite does not play any significant effect on the yield of ethanol, acetone and acetic acid.
[0195] In contrast, analysis of the spectra .sup.1H-NMR at the high frequency region (FIG. 5b) reveals a very important and noticeable difference. More specifically, formic acid (singlet at 8.28 ppm) and a peak at 5.81 ppm, which has been attributed to traces of double bonded carbon compounds are detected for C-1 sample only. This is an important achievement since the yield of formic acid, 8.061.89 mol, from C-1 is very high, reaching 36% of the total yields (vs. 59% for ethanol). Table 1 summarizes the yields of the reactions using C-1 and C-2:
TABLE-US-00001 TABLE 1 Yields (in mol of product per gram of catalysts) for all the products of the reactions catalyzed by C-1 and C-2. Catalyst Acetone Acetic Acid Ethanol Formic Acid C-1 0.9 0.24 0.26 0.05 13.13 3.75 8.06 1.89 C-2 1.31 0.29 0.65 0.13 15.01 4.62 0
[0196] To further understand the selectivity differences between C-1 and C-2 catalysts, yields of the reaction with respect to ethanol production (FIG. 6a) and C-2/C-1 product ratios (FIG. 6b) have been analyzed. Firstly, the total catalytic activity ratio, which is defined by the C-2.sub.total/C-1.sub.total ratio (where C-2.sub.total and C-1.sub.total refers to all the products obtained using the C-2 and C-1 catalysts, respectively), is 0.8. The fact that this value is close to 1 indicates that tuning the catalyst by introducing a new phase does not have any important effect on the total catalytic activity, but only the selectivity. Thus, the acetone, acetic acid and ethanol yields are higher for C-2 than for C-1, compensating the amount of formic acid generated. Furthermore, the results evidence that the coexistence of the HAp and brushite phases facilitates the incorporation of .Math.CH.sub.3, favoring the conversion of formic acid to acetic acid is favored. Consistently, the yield of acetone is higher for C-2 than for C-1.
[0197] Overall, incorporation of small amounts of brushite phase in HAp-based catalysts enhances the incorporation of .Math.CH3 species in CO.sub.2 and CH.sub.4 fixation reactions. This has been attributed to: 1) charge accumulation favors the dissociation of CH.sub.4 to .Math.CH.sub.3; and 2) Ca.sup.2+ are more susceptible to adsorb species as they are less bonded.
9. Preparation of the cHAp/tsp-Based Catalyst
[0198] The 3-component catalyst was prepared by dropping successively 100 L of 50 mM ATMP, 10 mM ZC and 50 mM ATMP aqueous solutions on a cHAp/tsp disk (diameter: 10 mm; thickness: 1 mm). Before each dropping step, the sample was kept 8 h at room temperature for drying.
10. Synthesis of Amino Acids
[0199] A high pressure stainless steel reactor was employed to perform the synthesis of amino acids. The reactor was also characterized by an inert reaction chamber coated with a perfluorinated polymer (120 mL) where both the catalyst and water 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.
[0200] The reactions were performed at 95 C. for a reaction time of 48 h. Catalyst samples weighed approximately 150 mg and 0.5 mL of de-ionized liquid water were initially incorporated into the reaction chamber. The chamber was extensively purged with the first selected gas in order to eliminate the initial air content. Each selected gas was introduced to increase the reaction chamber pressure (measured at room temperature) to the target pressure. In all cases the chamber pressure was increased up to 6 bar by introducing sequentially 2 bar of each feeding reaction gas.
[0201] The reaction products were analyzed by NMR spectroscopy. All NMR spectra were acquired with a Bruker Avance III-400 spectrometer operating at frequencies of 400.1 and 100.6 MHz for .sup.1H and .sup.13C, respectively. Chemical shifts were calibrated using tetramethylsilane (.sup.1H and .sup.13C) as internal standard. Sixty-four and one thousand scans were recorded for .sup.1H and .sup.13C NMR, respectively. In order to remove the amino acids from the catalyst, samples were dissolved in deuterated water containing 100 mM of HCl and 50 mM of NaCl with the final addition of deuterated water.
11. cHAp: Temperature for the Hydrothermal (HT) Treatment
[0202] The precipitation and HT of HAp are crucial steps to adjust the stoichiometry and avoid the formation of other phases, such as -tricalcium phosphate (TCP) that can be easily formed depending on the conditions, and the sintering at 1000-1200 C. is frequently used to refine the crystal structure.
[0203] FIG. 10a compares the Raman spectra recorded for cHAp(T.sub.h) samples prepared by applying, after the precipitation step, a HT at T.sub.h=50 C., 100 C., 150 C., 200 C. and 240 C. for 24 h. The values of the normal-mode frequencies of the PO.sub.4.sup.3 tetrahedron typically observed from Raman measurements in aqueous solution are .sub.1=938 cm.sup.1, .sub.2=420 cm.sup.1, .sub.3=1017 cm.sup.1 and .sub.4=567 cm.sup.1. In cHAp and TCP, the crystalline field causes not only the shifts but also the splitting of the PO.sub.4.sup.3 normal modes, even though these effects depend on the crystallographic structure. The spectra displayed in FIG. 10a clearly indicates that the splitting of the PO.sub.4.sup.3 normal modes decreases with increasing hydrothermal temperature. For example, a single intense peak arising from the non-degenerate .sub.1 mode of PO.sub.4.sup.3 at 962 cm.sup.1 is detected in the spectra of cHAp(T.sub.h100 C.) samples, whereas several peaks can be observed for cHAp(50 C.). A similar feature is observed for the doubly degenerate .sub.2 and triply degenerate .sub.3 and .sub.4, which span a frequency range of 400-490, 570-625 and 1020-1095, respectively, for cHAp(T.sub.h100 C.). In all cases the PO.sub.4.sup.3 bands appear at frequencies 20-25 cm.sup.1 higher than those corresponding to the free-tetrahedral normal modes (i.e. in aqueous solution).
[0204] cHAp exhibits a P6.sub.3/m space group and the unit cell contains six equivalent PO.sub.4.sup.3 tetrahedrons, whereas TCP crystallizes in the R3c space group and its unit cell contains 42 PO.sub.4.sup.3 tetrahedrons distributed in three non-equivalent types. Consequently, the single intense peak detected at .sub.1=962 cm.sup.1 is observed as two peaks and a shoulder for TCP. This structural difference also affects .sub.2 and .sub.4, which span over a higher frequency range in TCP than in cHAp. Moreover, .sub.2 and .sub.4 are separated by a frequency gap of only 55 cm.sup.1 in TCP, while the gap is of 120 cm.sup.1 in cHAp. As it can be seen in FIG. 10a, the fingerprints of the spectrum recorded for the cHAp(50 C.) sample agree with those expected for TCP, indicating that this is the predominant phase when the HT is performed at low temperature.
[0205] The Raman spectra obtained for cHAp/tsp(T.sub.h), which are displayed in FIG. 10b, show similar features to those described for cHAp(T.sub.h) for the normal modes of PO.sub.4.sup.3 (FIG. 10a). After the TSP treatment, the intensity and width of the peaks associated to the PO.sub.4.sup.3 correspond to the TCP phase for the samples prepared at T.sub.h=50 C., while cHAp is clearly identified in samples hydrothermally treated at T.sub.h100 C. However, some distinctive features can be also identified in cHAp/tsp(T.sub.h) samples as a function of T.sub.h. For example, cHAp/tsp(150 C.) shows weak peaks at around 330 cm.sup.1, whereas no signal is detected for the rest of the samples. Considering that the detection of these peaks depend on the degree of crystallinity of the sample, Raman results suggest that the highest crystallinity is achieved when the HT is performed at T.sub.h=150 C. Furthermore, a peak at 878 cm.sup.1 is detected for cHAp/tsp(T.sub.h100 C.), as is clearly in the evidenced in FIG. 11a. This signal, which is characteristic of the Brushite mineral (i.e. CaHPO.sub.4.Math.2H.sub.2O) that is understood to be a precursor of apatite, has been attributed to the normal mode frequency of HPO.sub.4.sup.2. The intensity of this band is much greater for the sample obtained at T.sub.h=150 C., while for the rest it is observed as a weak peak. In addition, two bands typical of POH rotation and deformation modes appear at 794 and 848 cm.sup.1. Although they are very weak in all cases, POH signals also showed the highest intensity for samples obtained at T.sub.h=150 C.
[0206] FIG. 11b compares the weak intensity bands observed between 100 and 350 cm.sup.1 for cHAp(150 C.) and cHAp/tsp(T.sub.h100 C.). These bands have been attributed to translational modes of the Ca.sup.2+ (111, 139 and 154 cm.sup.1), PO.sub.4.sup.3 (287 cm.sup.1) and OH.sup. (331 and 323 cm.sup.1) sublattices, and rotational modes of the PO.sub.4.sup.3 group (205 cm.sup.1). As it can be seen, both the intensity and narrowness of all these bands is maximum when the TSP process is applied to cHAp(150 C.) samples. Moreover, the intensity bands associated to the translational modes of the OH.sup. sublattice yield (331 and 323 cm.sup.1 in FIG. 11b) increases linearly with that of the POH rotation and deformation modes (794 and 848 cm.sup.1 in FIG. 11a), evidencing a structural correlation.
12. Duration of the HT and Characteristics of the TSP Treatment
[0207] The duration of the HT is another factor that affects the structure of the HAp and, therefore, the performance of cHAp/tsp as catalyst. FIG. 12a compares the Raman spectra recorded for samples prepared at T.sub.h=150 C. when the time for the HT was of 10 or 24 h. As it can be seen, the spectra of the cHAp samples obtained after a HT of only 10 h show the peaks described in the previous section for the TCP phase. Thus, the conversion of the TCP phase into cHAp is only completed when the HT is long enough, even when the T.sub.h was the optimum. Interestingly, FIG. 12a shows that the TSP treatment favors the re-arrangement of the surface in samples prepared using both 10 and 24 h, as is proved by the apparition of the Brushite peak at 878 cm.sup.1 and the translational modes of the OH.sup. sublattice at 323 cm.sup.1. Nevertheless, the TCP fingerprints identified for cHAp(150 C.) samples obtained using a treatment time of 10 h, which are associated to the four normal-mode frequencies of the PO.sub.4.sup.3, remain practically unaltered after the permanent polarization process. Accordingly, the TSP process cannot be used to compensate the undesirable structural effects induced by the shortening of the HT.
[0208] The effect of the electric field strength used to induce permanent polarization in cHAp has been examined by applying DC voltages of 100 V, 500 V or 1000 V (25, 125, 250 V.Math.cm.sup.1 respectively) to cHAp(150 C.) samples. Although the Raman spectra obtained for all the resulting samples correspond to cHAp (FIG. 12b), some bands apparently associated to the catalytic activity of cHAp/tsp are slightly influenced by the strength of the DC voltage. More specifically, the peaks attributed to translation modes of the OH.sup. sublattice (323 cm.sup.1), the normal mode of HPO.sub.4.sup.2 (878 cm.sup.1), and the POH rotation and deformation modes (794 and 848 cm.sup.1) are more intense and better defined for samples polarized at 500 V than for those obtained at 100 V and 1000 V. According to these results, the alteration of the voltage used for the TSP process is not expected to annihilate the activity of cHAp/tsp as catalyst but to induce small changes in its effectivity.
[0209] Finally, the influence of the geometry of the electrodes in the TSP treatment has been investigated. For this purpose, cHAp/tsp(150 C.) samples were prepared using a DC voltage of 500 V and two different geometries for the electrodes: i) steel plates separated at 4 cm and, therefore, the cHAp(150 C.) disc was in contact with one electrode only (i.e. the thickness of the sintered mineral discs was 1 mm); and ii) steel plates separated at 1 mm and, therefore, each side of the cHAp(150 C.) disc was in contact with an electrode. The recorded Raman spectra, which are compared in FIG. 12c, reveals that the bands at 794, 848 and 878 cm.sup.1 are only observed when the TSP process is conducted hindering the steel-cHAp contact. Also, the intensity of the band associated to the translational mode of the OH.sup. sublattice (323 cm.sup.1) is much higher when the separation between the electrodes is of 4 cm.
[0210] A depth profiling Raman analysis was conducted to monitor the extent of the changes induced by the TSP treatment. FIG. 13a compares the Raman spectra recorded at different depths (i.e. from the surface to a depth of 95 m) for cHAp/tsp(150 C.) prepared applying the HT for 24 h and using a polarizing voltage of 500 V at 1000 C. Although the spectra show the same fingerprints in all cases, the intensity of the signals decreases with increasing depth. This observation is particularly remarkable for the bands of HPO.sub.4.sup.2 (878 cm.sup.1) and both POH rotation and deformation modes (794 and 848 cm.sup.1). These results indicate that the changes caused by the TSP treatment are predominantly located at the surface of the mineral. This is corroborated in FIG. 13b, which displays the intrinsically weak bands detected in the 110 and 330 cm.sup.1 region. The intensity of the bands associated to the rotational modes of the PO.sub.4.sup.3 group (205 cm.sup.1) and, specially, to the translational modes of the Ca.sup.2+ (111, 139 and 154) cm.sup.1), PO.sub.4.sup.3 (287 cm.sup.1) and OH.sup. (323 cm.sup.1) sublattices decreases with increasing depth.
13. Morphological and Structural Characterization of cHAp Samples
[0211] Raman spectra displayed in FIGS. 10 and 11 indicate that the chemical properties of cHAp(T.sub.h) and cHAp/tsp(T.sub.h) depends on T.sub.h and, therefore, the catalytic activity of the latter. To investigate the implications of T.sub.h in the morphology and structure of cHAp, SEM and XRD have been used.
[0212] Low magnification SEM micrographs of cHAp(T.sub.h) with T.sub.h=50 C., 100 C., 150 C., 200 C. and 240 C. are displayed in FIG. 14. With exception of the sample with TCP (T.sub.h=50 C.), which presents the most compact surface, the rest of the samples presents a morphology made of compact regions alternated with extensive porous zones. Inspection of the high magnification micrographs, shown in FIG. 14 for cHAp(150 C.) as representative sample, indicate that porous zones are constituted by pillars that grow through the aggregation of mineral nanoparticles in a preferential direction. However, the morphology of the porous areas is apparently independent of T.sub.h.
[0213] Structural characterization of prepared cHAp(T.sub.h) samples was completed by WAXD (FIG. 15a). The characteristic fingerprint of hexagonal crystal symmetry cHAp (a=b=9.421 , c=6.881 , ==90, and =120; JCPDS card number 9-0432) is typically associated to the peaks at 32-34 2, which correspond to the (211), (112) and (300) reflections. These reflections are clearly identified in the diffraction profiles of all samples prepared using T.sub.h100 C. and can be intuited in that of the cHAp(50 C.) sample. Other characteristic reflection peaks of cHAp appear at 2=32, 34, 40, 47 and 49, which correspond to (211), (202) (130) (222) and (214) reflections. The (112) and (300) peaks were also used to determine the crystallinity (.sub.c: Eqn 1), whereas the (211) reflection was used to calculate the crystallite size (L.sub.211; Eqn 2). The .sub.c of cHAp(T.sub.h) is 0.53, 0.82, 0.68 and 0.77 for T.sub.h=100 C., 150 C., 200 C. and 240 C., respectively, which is fully consistent with Raman spectra displayed in FIG. 10a. Thus, the .sub.c of cHAp is maximum when it is prepared at T.sub.h=150 C. followed by that obtained at T.sub.h=240 C. The variation detected in cHAp(200 C.) with a decrease in .sub.c in relation to cHAp(150 C.) and cHAp(240 C.), and which is experimentally reproducible, has been attributed to combined effect of the lyophilization and sintering processes on the crystals, which seems to depend on the value of T.sub.h used. This could not only explain the reduction of .sub.c when T.sub.h increases from 150 C. to 200 C. but also the fact that .sub.c is lower for cHAp(240 C.) than for cHAp(150 C.). Besides, the calculated L.sub.211 values correspond to 69.1, 82.7, 82.6 and 82.7 nm for cHAp(T.sub.h) with T.sub.h=100 C., 150 C., 200 C. and 240 C., respectively.
[0214] On the other hand, the most relevant reflection peaks of brushite (JCPDS card number 72-0713), which has a monoclinic structure with cell parameters a=5.812 , b=15.180 , c=6.239 , ==90 and 62 =116.43, are clearly observed in the X-ray diffraction pattern of cHAp(150 C.) (FIG. 15a). These reflections, which are weaker for cHAp(240 C.) and much weaker for cHAp(100 C.) and cHAp(200 C.), correspond to the (141), (121), (152) and (143) with 2=29, 35, 42 and 51, respectively. It is worth mention that, although the position of some of these reflections, as for example the (143) matches other reflections found in the theoretical diffraction of pure cHAp, the changes in the relative intensities points to co-existence of the two phases, which is in agreement with Raman spectra. The amounts of cHAp and brushite phases in samples prepared at T.sub.h100 were roughly estimated using X-ray diffraction patterns by comparing the (211) reflection of cHAp and the (141) reflection of brushite. Results show that, although the formation of the latter phase is promoted when the HT is conducted at T.sub.h100 C. (FIG. 15b), cHAp is clearly the predominant phase in all cases. However, the content of Brushite increases from 5% for T.sub.h=100 C. and 200 C. to 45% for T.sub.h=150 and 240 C.
[0215] In general the anomalous behaviour of the samples obtained at T.sub.h=200 C. has been attributed to the dehydration process reported for Brushite at such temperature. Thus, the layered structure of Brushite, CaHPO.sub.4.Math.2H.sub.2O, in which mineral layers are held together by hydrogen bonded water molecules, converts into an amorphous phase and Monetite, CaHPO.sub.4. More specifically, although surface water evaporates at around 100 C., the two crystallographic water molecules of Brushite, which are associated by hydrogen bonds with oxygen atoms in phosphate group, remain stable at such temperature, leaving from the system at 200 C. The structural transitions associated to the Brushite dehydration at 200 C. explains the reduction of the .sub.c, which in turn is in detriment of the catalytic activity.
[0216] Finally, inspection of the diffractogram obtained for cHAp(50 C.) (FIG. 15a) allows to recognize the reflection peaks typically reported for TCP (JCPDS card number 09016), which predominate over those associated to the cHAp. Thus, the rhombohedral TCP appears as the predominant crystalline phase in samples HT-treated at the lowest temperature.
14. Performance of cHAp/tsp(T.SUB.h.) as Catalyst for the Synthesis of Amino Acids
[0217] In a recent study, the inventors catalyzed the fixation of nitrogen from N.sub.2 and carbon from CO.sub.2 and CH.sub.4 to obtain Gly (glycine) and Ala (alanine), the two simplest amino acids. The catalyst was prepared by coating cHAp/tsp(150 C.) samples with two layers of ATMP separated by an intermediate ZC layer. For this purpose, the cHAp/tsp(150 C.) disks were sequentially immersed in 5 mM ATMP, 5 mM ZC and 1.25 mM ATMP aqueous solutions at room temperature for 5 h. After each immersion, samples were dried at 37 C. for 3 h. The catalyzed reaction was conducted under UV light irradiation and mild reaction conditions, in an inert reaction chamber starting from a simple gas mixture containing N.sub.2, CO.sub.2, CH.sub.4 and H.sub.2O.
[0218] FIG. 16 proves that change in the application of the ATMP and ZC coatings does not alter the performance of the catalyst. FIG. 16a shows the .sup.1H NMR spectrum of samples obtained by dissolving the catalyst prepared using cHAp(150 C.) and the products of reaction after 48 h at 95 C. The signal corresponding to ATMP methylene group appears as doublet at 3.53-3.56 ppm, while the signals corresponding to the methylene group of produced Gly is singlet at 3.37 ppm and both methine and methyl groups of Ala are the quadruplet at 3.84-3.87 ppm and the doublet at 1.60-1.62 ppm, respectively. The same compounds are also detected in the .sup.13C NMR spectrum displayed in FIG. 16b, where only peaks assigned to the ATMP (53.82 and 52.92 ppm), Gly (172.26 and 41.35 ppm) and Ala (175.26, 50.56 and 16.18 ppm) units are detected.
[0219] It should be noted that the reaction was positive also for catalysts prepared using cHAp(T.sub.h) with T.sub.h100 C. The yield of the reaction was calculated using commercial Gly and Ala (purchased from Sigma-Aldrich) at a controlled concentration to calibrate the .sup.1H NMR peaks. The variation of the yield of the reaction expressed in % per cm.sup.2 of catalyst against T.sub.h is represented in FIG. 17. These values are higher than those found in the inventor's previous study, which was 2.5 after 48 h (i.e. Gly/Ala ratio decreased from 5.4 to 2.2 when the reaction time increased from 2 to 96 h). This feature has been attributed to the presence of the brushite phase, which was not observed in the catalyst prepared in the inventor's previous study. Accordingly, the combination of brushite and cHAp achieved with the process described in this work apparently accelerates the transformation of Gly into Ala.
[0220] On the other hand, the yield correlates with both the .sub.c and the brushite content in the cHAp/tsp(T.sub.h). Thus, the maximum yield of amino acids after 48 h was obtained for the catalysts prepared at T.sub.h=150 C. and 240 C. (i.e. 2.8% and 2.7%, respectively), which displayed not only the highest .sub.c (i.e. 0.82 and 0.77, respectively) but also the highest content of brushite (i.e. 15% and 12%, respectively). In contrast, the total yield decreases one order of magnitude, 0.5-0.6%, for reactions catalyzed by cHAp/tsp(T.sub.h) samples with .sub.c<0.7. This feature clearly reflects the very important role of the spatially translations modes, which facilitates the transport of charge at the surface through oscillation and translational molecular movements upon excitation of the lattice.
