Adsorption of CO2 on biphasic and amorphous calcium phosphates: An experimental and theoretical analysis

Accepted Manuscript Research paper Adsorption of CO2 on biphasic and amorphous calcium phosphates: an experimental and theoretical analysis F.S. Souza, M.J.S. Matos, B.R.L. Galvão, A.F.C. Arapiraca, S.N. da Silva, I.P. Pinheiro PII: DOI: Reference:

S0009-2614(18)30907-2 https://doi.org/10.1016/j.cplett.2018.10.080 CPLETT 36066

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Chemical Physics Letters

Received Date: Revised Date: Accepted Date:

9 August 2018 30 October 2018 31 October 2018

Please cite this article as: F.S. Souza, M.J.S. Matos, B.R.L. Galvão, A.F.C. Arapiraca, S.N. da Silva, I.P. Pinheiro, Adsorption of CO2 on biphasic and amorphous calcium phosphates: an experimental and theoretical analysis, Chemical Physics Letters (2018), doi: https://doi.org/10.1016/j.cplett.2018.10.080

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CO2 adsorption in calcium phosphates

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Adsorption of CO2 on biphasic and amorphous calcium phosphates: an experimental and theoretical analysis F. S. Souza*, M. J. S. Matos#, B. R. L. Galvão*, A.F.C. Arapiraca* S. N. da Silva*, I. P. Pinheiro* *

Centro Federal de Educacão Tecnológica de Minas Gerais, CEFET-MG,

Av. Amazonas 5253, 30421-169, Belo Horizonte, Minas Gerais, Brazil #

Departamento de Física, Universidade Federal de Ouro Preto,

Ouro Preto, CEP 35400- 000 Minas Gerais, Brazil

The total number of words of the manuscript, including entire text from title page to figure legends: 5050 The number of words of the abstract: 199 The number of figures: 8 The number of tables: 4

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Abstract Calcium phosphates are suggested as a CO2 adsorbent via pressure swing adsorption. Amorphous calcium phosphate (ACP) and biphasic calcium phosphate (BCP) (composed of hydroxyapatite and beta-tricalcium phosphate) were investigated for the capture/immobilization of the gas. A fluidized bed was set up to assess the levels of CO 2 adsorption by ACP and BCP. A gaseous mixture was synthesized, mimicking the conditions for possible industrial use. The results show a significant reduction in CO2 concentrations. Using DFT calculations, we show that CO2 adsorption increases the stability by reducing the surface energy. The energies involved and preferential adsorption sites were also theoretically predicted.

Calcium phosphate; CO2 adsorption; DFT calculations

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1. Introduction Carbon dioxide is a major greenhouse gas and contributor to global warming, due to the large amount of it constantly released to the atmosphere. Therefore, alternative ways to collect and prevent its release are a relevant topic. There are several methods to separate and capture the gases produced in industrial chimneys during power generation processes, such as distillation, absorption, adsorption, gas/solid reaction, membrane, electrochemical pump, and formation of hydrates. However, according to Liu et al.1, most of these alternative methods are rejected due to the low levels of absorption and/or the high operational costs. Most of the gas separation processes can be performed via adsorption techniques, such as pressure swing adsorption (PSA), temperature swing adsorption (TSA) and vacuum swing adsorption (VSA). These are often performed with commercially available adsorbents, such as zeolites, activated carbon and aluminophosphates, although the use of other substances has also been suggested, such as calcium oxide, carbonates, urea, ammonia, and calcium phosphates2–4. Deitz et al.5 studied the adsorption of CO2 using natural materials, such as coconut charcoal, acid-washed bone char, and ash of bone char, where it was shown that basic calcium phosphate provided an initially fast adsorption, which could continue slowly for several days. Ishikawa and coworkers6 investigated the incorporation of CO2 into hydroxyapatites (HA) during its synthesis (by precipitation reactions from Ca(OH)2 and H3PO4) using FTIR spectroscopy and have shown that it increases with an increase in solution pH. They have further elucidated the adsorption mechanism by performing adsorption tests under pure CO2 at different pressures and using deuterated HA7,8. More recently, the surface of HA has been studied by its interactions with different molecules. Diallo-Garcia et al.9 have used CO2 and acetylene as a probe to explore the basic sites that may be involved in catalytic reactions. On the other hand, theoretical calculations have been performed to explore the surface of HA through its interaction with hydroxyl 10. To the best of our knowledge, no theoretical calculations regarding the CO2 interaction with HA have been performed. The aim of the present work is to investigate the CO2 adsorption under atmospheric pressure using gas mixtures that mimic chimney conditions, thus assessing the viability of this process to reduce industrial pollution. To contribute to understanding the mechanism of adsorption, we have also performed the first-principles calculations of CO2-HA interactions.

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2. Materials and methods 2.1 Synthesis For the production of amorphous calcium phosphate (ACP), we have used the synthesis method proposed by Palmer11, based on the following acid-base neutralization: 10Ca(OH)2 + Ca(H2PO4)2.H2O → Ca10(PO4)6(OH)2

(1)

The two solutions were mixed and stirred until homogenization. Biphasic calcium phosphate (BCP) is obtained from a heat treatment of the ACP powder in muffle furnaces at different temperatures. We have explored temperatures from 500 to 1100ºC, up to 4 hours in each test, and characterized each separately. The best result was obtained at 950ºC over 4 hours. After cooling, the resultant material was composed of two phases, with the majority being HA, and 10% of beta-tricalcium phosphate (β-TCP). Both ACP and BCP passed through a sieving process using sieves of 270 and 400 in the Tyler mesh. This method selects the particle size distribution by regulating the openings in the sieve, with granulometry chosen between 38 μm and 53 μm. The minimum size was chosen to avoid humid sieving (to prevent dissolution of the particulates) and larger particles were avoided to obtain a high surface area. "Powder" here means a gathering of numerous solid particles with no agglomerates. The dry sieving methods are highly dependent upon the proper dispersion of the powder. This requirement may be hard to achieve if the method is used at the lower end of the sieving range (i.e., below 37 µm), when the particles tend to be more cohesive, and especially if there is any tendency for the material to develop an electrostatic charge. The goal was to optimize the adsorption capacity at room temperature and 70ºC, aiming for the selective decontamination of industrial effluents.

2.2 Characterization of ACP and BCP X-ray diffraction (XRD) was used to analyze both ACP and BCP powders before and after adsorption. A Shimadzu X-ray diffractometer, model XRD 7000 operated with a Cu tube (λ = 1.5418 Å), 30 kV voltage and 30 mA current were used. The data from the XRD curve were collected at scan intervals of 4 to 850 with a 0.020 step every 40 s at a rate of 20 per min, using a Ni filter. The obtained diffractions were compared with the International Center for Diffraction Data (ICDD) for phase identification and crystallinity index calculation. The X-ray fluorescence (XRF) spectroscopy technique was used for elemental chemical analysis. The XRF equipment, EDX-720 model, provided by Shimadzu was employed in the tests. { PAGE }

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The morphology and the topography of the adsorbents (ACP and BCP) were analyzed by scanning electron microscopy (SEM). They were coated with gold before inserted in Shimadzu's Superscan model SSX-550. This device is coupled with energy-dispersive X-ray spectroscopy (EDS) for the elemental analysis of these samples. The particle size distribution was assessed by laser particle size analyzers CILAS' model 1064 and compared to the SEM results. The surface area and the pore distribution were measured using a gas sorption analyzer, model NOVA 2200e provided by Quantachrome Instruments, using the BET and BJH methods.

2.3 Adsorption tests To test the adsorption of CO2 by the synthesized materials, we have conducted CO2 capture tests in fluidized beds containing ACP or BCP in duplicates, using the pressure swing adsorption (PSA) technique. An apparatus consisting of a fluidization column and a gas heating system for adsorption tests at higher temperatures (simulating chimney conditions) was developed and coupled with a gas analyzer (by Bridge Analyzers, Inc.) capable of measuring CO2 levels, as schematically shown in Figure 1. The injection of gas was performed once by simultaneously injecting carbon dioxide (99.8% pure) and dry air (consisting of 78% of N2 and 21% of O2). Their pressure was tuned to yield a mixture containing 13% of CO2, again mimicking chimney conditions, and the total flow rate was set to 16.7 L/min, which guaranteed the fluidization of the adsorbent material. To optimize the creation of a gas vortex at the entrance of the column, an acrylic plate with 6.35 mm holes was added at the bottom, inclined at 40º. Polypropylene filters were added at the base and the top of the column to avoid losing the material in the tests.

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Figure 1 – Fluidized bed apparatus. 1) gas analyzer; 2) air inflow; 3) thermometer; 4) CO 2 inflow; 5) gas mixer and flow measurement

The gas analyzer allows for the measurement of the CO2 concentrations in the column as a function of time, as the adsorption is taking place. When depressurizing the bed, the retained component is desorbed, and the adsorbent is regenerated, returning to the initial condition.

2.4 Theoretical calculations

The interaction between CO2 and the hydroxyapatite surface was modeled theoretically to give a more in-depth description of the mechanism of the adsorption studied here. Density functional theory (DFT)12,13 calculations were performed, including van der Waals interactions in the exchange-correlation functional (vdW-DF), as implemented in the SIESTA package14–17. We have employed norm-conserving pseudopotentials of Troullier-Martins18, in the separable (Kleinman-Bylander) form19. The basis set was the standard double-zeta plus polarization orbitals (DZP)20. The convergence criterion for the self-consistent calculation is achieved when the maximum difference between the output and the input of each element of the density matrix, in a self-consistent field cycle, is less than 10-4. The real-space grid was defined by a mesh-cutoff of 450Ry and we have used a k-point sampling in Brillouin zone given for a k grid-cutoff of 20 Å21⁠. All geometries were optimized so that the maximum force on any atom is less than 10 meV/Å. { PAGE }

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3. Results 3.1. X-ray diffraction (XRD) Figure 2 shows the XRD patterns of these materials before the adsorption tests.

Figure 2 - XRD patterns of ACP and BCP before adsorption. The patterns of pure HA and β-TCP are also shown for comparison.

The diffractogram in Figure 2 confirmed that the BCP product is crystalline, presenting the characteristic peaks from the hydroxylapatite (ICDD/PDF2 number 9.432) and β-tricalcium phosphate (ICDD/PDF2 number 9.0169) phases. The percentage of each phase was estimated from the XPowder software, yielding approximately 85% of HA, 10% of β-TCP and 5% of amorphous phase. The size of the smallest particle was calculated as 16 nm from the Scherrer equation. As expected from the synthesis route, the ACP powder exhibits a very low crystallinity (less than 15%) but was used throughout this work due to its high reactivity (active sites, open structure, and high surface area) and low cost of production. The size of the smallest particle was estimated as 14 nm. XRD analyses were also performed on both BCP and ACP after the adsorption tests, and the results are shown in Figure 3. It is observed that the peaks related to carbonated apatite appeared, indicating chemical adsorption in the case of ACP but not for BCP, which could be expected given the high reactivity of ACP. The XRD analysis can only detect such peaks if large amounts are present, and for a more in-depth discussion of the structure of HA after the CO2 adsorption, refer to the infrared analysis of Ref.7.

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Figure 3 – XRD patterns of ACP and BCP after adsorption.

3.2 Scanning Electron Microscopy (SEM) and Laser Granulometry Figure 4 shows the morphological aspects of the surface of the particles (ACP and BCP), with these powders having irregular or pointed shapes.

Figure 4 - SEM photomicrography showing the details of the surface of ACP (left ) and BCP (right) powders.

Through the SEM photomicrographs, it can be seen that the powders of both ACP and BCP are constituted by small (colloidal) particles and plates forming aggregates with some cohesion between them. In both cases, it is observed in SEM that the granules are formed by a cluster of submicrometric particles grouped together, possibly increasing its surface area. The aspect ratio varies on average from 1.5 to 2.5, indicating deviations from a spheroidal shape. Table 1 gathers the numerical results, where it is observed that the range of 38 μm to 53 μm aimed in the sieving process was approached. The Ca/P ratio could also be predicted from the EDS coupled in the SEM device. Table 1. SEM granulometric distribution of the ACP and BCP powders. ACP BCP Size measurement (μm) 10-45 20-50 Ca/P ratio 1.65 1.65 The stoichiometric Ca/P ratio obtained here, 1.65±0.15, was also confirmed by the XRF analysis, which yielded 1.71±0.13. In the laser granulometry technique, the particle size distribution was obtained and is given in Table 2. Table 2. Laser granulometry particle size distribution { PAGE }

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Range

ACP

BCP

10%

3.0 μm

4.3 μm

50%

12.8 μm

17.7 μm

90%

29.5 μm

39.8 μm

average

14.8 μm

20.3 μm

These results help to provide a better idea of the number of particles in each size range. As seen, the results deviate considerably from the expected results in the sieving range proposed. This deviation is due to the dispersion of the particle aggregates promoted by the surfactants and the ultrasonication in the laser granulometry measurement.

3.3 Surface area and porosity by gas adsorption

Table 3 shows the results of the nitrogen physisorption. Table 3. Surface area and pore size distribution. ACP BET Surface Area (m2/g) 66±2 3 Micro- and mesoporous volumes (10³ cm /g) 13±1 Mean diameter of micro- and mesopores (angstroms)

17±1

BCP 4.0±0.2 9±1 31±1

The surface area was calculated using the BET method22⁠, in which the adsorbed amount N is plotted against the relative pressure (P/P0) according to { SHAPE }

(2)

where the parameter C is related to the heat of adsorption and liquefaction, and N0 is the number of atoms needed to cover the substrate with a monolayer, from which the surface area can be obtained. The average of three separate measurements was employed. The pore volume and the pore size distribution were calculated using the BJH method23⁠. According to this model, in the capillary condensation region, each pressure increment leads to an increase in the thickness of the layer adsorbed on the pore walls, and the capillary condensation in pores with a core size of rc follows the Kelvin equation: { SHAPE }

(3)

where { SHAPE } stands for the surface tension, { SHAPE } stands for the molar volume, and { SHAPE } is the contact angle. Assuming a cylindrical shape, it is possible to calculate the { PAGE }

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contribution of the thickness of the adsorbed film to the total adsorption and then the core volume. It is shown in Table 3 that the adsorbent has a high surface area and a large total pore volume. The positive slope of the curve for relative pressures higher than 0.5 bar suggests the presence of mesopores in the structure of this material, leading to the classification of the isotherm as type I + IV. These results are relevant for the interpretation of the CO2 adsorption below. The high surface area in the samples reinforces the SEM observation that the powders are composed of colloidal aggregates, which certainly increases the average surface area, due to the contribution of several particles that compose the clusters.

3.4 Adsorption tests

The adsorption tests in the fluidized bed were operated with a continuous flow rate of 16.7 L/min at room temperature, in a fluidization column containing 50 g of the material. They were conducted over 180 min in duplicates, and the results are shown in Figure 5.

Figure 5: CO2 removal as a function of time from a gas mixture initially containing 13% of CO2. It can be seen that both ACP and BCP can significantly reduce the concentration of CO 2 in the gas flow. ACP is shown to perform better than BCP, achieving the total removal after 2 h and maintaining it up to the tested time. Although BCP was not able to completely remove CO2 from the gas flow at the tested flowing rate, it has shown a faster adsorption than ACP. Indeed, after only 30 min of exposure, the CO2 percentage decreased three times, but increased after 80 min. { PAGE }

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This is due to a saturation of the reactive sites, which occurs faster for BCP due to a more closed structure, as revealed from the smaller pore volume indicated in table 3. The total amount of CO2 adsorbed per gram of material (ACP/BCP) was estimated by calculating the total amount of the gas that passed through the apparatus and taking the data from Figure 5 to assess the fraction removed. It is found that the total amount of CO2 adsorbed during the experiment was 982 g, which corresponded to 19.6 g of CO2 per gram of ACP. BCP shows a similar performance, adsorbing 15.9 g CO2 per gram of BCP.

3.5 Theoretical results

To help elucidate the mechanism involved in the adsorption process shown in this work, such as the preferred bonding sites in the solids and the energies involved, we have carried out first principles calculations within the density functional theory (DFT). To better understand the interaction between HA and CO2, we choose the exchange-correlation functionals that include van der Waals interactions, and in view of the wide variety of flavors of functionals available, we choose the one that best reproduces the crystalline structure of HA. For this, we have calculated its lattice constants with several GGA and vdw-DF functionals available16,24–28, and compared them to the experimental data. The results are given in Table 1. These calculations were performed by optimizing the atomic positions and the lattice vectors with a stress convergence criterion of 0.1 GPa, and the geometries have been optimized until the force under each atom was less than 10 meV/Å. The maximum deviation from the experimental results is 4.1% (average of) for all functionals tested, with the c parameter being the one with the largest errors. The best agreement with the experimental results was obtained by vdW-DF/BH25, and therefore this functional will be used for studying the CO2-HA interaction henceforth. Table 4: Lattice constants of hydroxyapatite in Å and degrees. GGA/PBE vdW-DF/BH vdW-DF/DRSLL vdW-DF/LMKLL vdW-DF/KBM vdW-DF/VV Exp. [16]

a 9.52 9.41 9.62 9.40 9.46 9.49 9.430

b 9.52 9.41 9.62 9.64 9.46 9.49 9.430

c 7.13 7.08 7.17 7.16 7.09 7.09 6.891

c/a 0.749 0.7.52 0.746 0.743 0.750 0.748 0.7308

α 90.00 90.00 90.00 90.00 90.00 90.00 90

β 90.00 90.00 90.00 90.00 90.00 90.00 90

γ 119.99 119.99 119.97 119.95 119.96 119.96 120

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To model the adsorption of CO2, we have used the periodic hydroxyapatite surfaces obtained from a slab model of a finite thickness. Among the possible HA surfaces, (0001) is the dominant and often selected for studies of this type29⁠. We have then focused on this specific face, with the termination (Ca)4(PO4)3OH. The slab thickness is 28.3 Å and corresponds to four repetitions in the direction of the c vector and a single unit cell in the direction of the a and b vectors (a=b=9.41 Å). The vacuum region employed was of 37.7 Å (see supplementary materials). Starting from the atomic positions and the optimized lattice vectors, the calculations on the HA crystal were carried out using a supercell corresponding to the slab of this surface. For this, we have imposed a constraint of fixed atomic positions at the center of the slab, corresponding to two unit cells. We calculate the surface energy using: Esurf = (Eslab − nEbulk)/2A

(4)

where Esurf is the surface energy, Eslab is the energy of the slab supercell, Ebulk is the energy per unit cell of the bulk, with n being the number of unit cells used in the model, and finally A is the total surface area per unit cell30,31⁠. For the present study, we have obtained Esurf = 1.075 J/m2, which is in good agreement with similar calculations from Ref. 32,33.

Finally, the CO2 adsorption in HA is analyzed. To determine the preferential CO2 site of attack, we have first used a single CO2 molecule as a probe. We have performed a grid of 122 calculations (without geometry optimization) for 4 different values of the HA-CO2 height (totaling 488 energy points), keeping the molecule parallel to the surface, and after that, keeping it perpendicular to the surface with another grid totaling 363 energy points. Considering all calculated energy points, the one with the lowest binding (most stable) energy corresponds to a parallel configuration. All results are given as supplementary materials. To calculate the energy stability in each surface site, we have calculated the binding energy as: Eb = EHA+CO2 − EHA − ECO2

(5)

where EHA and ECO2 are calculated in the same geometries as EHA+CO2 but using ghost atoms to correct for basis set superposition errors. To obtain ECO2, the lattice vectors were chosen to represent an isolated molecule. Figure 6 shows the results for the parallel orientation, where it is observed that when the molecule is 4 Å from HA, the minima on the energy landscape are very { PAGE }

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shallow, corresponding to long range interactions. The lowest energy was obtained for the closest CO2-HA distance, indicating the formation of chemical bonds. From these graphs, we can see that there is a preference for the bonds between an oxygen atom from CO2 and a surface Ca atom, while the other oxygen interacts with the OH. The same preferential site has also been obtained theoretically but for the calculations of a hydroxyl molecule interacting with HA10.

Figure 6: 3D plot of the CO2 potential energy on several positions relative to the HA slab. We have also performed a geometry optimization to obtain a relaxed configuration of the system, where a binding energy of -0.34 eV was found. This optimization is illustrated in Figure 7 from the top and side views, where the chemical bond can be observed.

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Figure 7: Top view (left) and side view (right) of CO2 at the OH site. The color scheme employed is: O (red), Ca (cyan) and P (yellow). For better visualization, the atoms in the CO2 molecule have followed different color schemes: C (green) and O (blue).

To verify these results, we also calculated how the surface energy changes after the adsorption. When molecules are present, the surface energy of eq. 4 is changed to Esurf = (EHA+nCO2-Ebulk − nECO2)/2A

(6)

where n is the number of the molecules adsorbed. For the configuration presented in Figure 7, the surface energy is reduced from 1.08 to 1.00 J/m2. It must be said, however, that this single molecule occupies a considerable fraction of the unit cell. Using experimental values for the area of a single CO2 molecule as 21.8 Ų and 25.3 Ų (Refs.34,35,36), a coverage of 28.4% and 32.9% is obtained. On the other hand, if we consider the percentage of Ca sites, a similar value of 25% coverage is obtained, since there are four Ca atoms (in the surface) per unit cell. In any case, this results shows, qualitatively, the role of CO2 in stabilizing the surface of HA. Since a surface saturation has occurred in our experiments, we have performed further calculations including up to four molecules adsorbed at the Ca sites. The optimized geometries for each case are shown in Figure 8. The surface energy obtained for one, two, three and four CO2 molecules were 1.02, 0.94, 0.85 and 0.77 J/m2, respectively. In the last case, which is close to a 100% coverage, a reduction of 29% in surface energy occurs, thus confirming that CO2 molecules have a stabilizing role in the Ca rich surfaces of calcium phosphates.

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Figure 8: Optimized geometries for the adsorption of (a) one, (b) two, (c) three and (d) four CO2 molecules in HA. The atom colors follow the schemes of previous figures.

4. Conclusions It is shown that both materials synthesized in this work have potential applications in CO2 capture in industrial chimneys, since they both significantly reduce the concentration of CO2 in the gas flow. ACP is shown to perform better than BCP, achieving the total removal after 2 h and maintaining it up to the tested time. Although BCP was not able to completely remove CO2 from the gas flow at the tested flowing rate, it has shown a faster adsorption than ACP. Indeed, after only 30 min of exposure, the CO2 percentage decreased three times, but saturated faster. It is concluded that the total amount adsorbed is 19.6 g of CO2 per gram of ACP, and 15.9 g CO2 per gram of BCP. First principles calculations on the CO2 adsorption on the surface of HA were performed using DFT calculations. Among several tested functionals, vdW-DF/BH was shown to give the best description of the lattice constants of HA, and chosen to study the adsorption process. We have verified that the CO2 molecule shows a stronger interaction at Ca sites of the surface, and the adsorption of multiple molecules largely reduces the surface energy, thus stabilizing the system.

Conflicts of interest There are no conflicts to declare.

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5. Acknowledgements This work was supported by Brazilian agencies CNPq, FAPEMIG and CAPES. M.J.S.M. acknowledge support from UFOP – Grant Custeio 2017. We also acknowledge computational support from LCC–Cenapad–UFMG and CEFET-MG.

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Highlights: It is revealed that easily produced calcium phosphates have high CO adsorptive power Experiments mimicking industrial gas ow show that the materia could be used to reduce CO emission Theoretical calculations provide insights into the adsorption mecha- nism

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