# Oxidative Dehydrogenation of Propane on Hydroxyapatite-Supported Cobalt Catalysts

Corentin Reynaud, Cyril Thomas, Dalil Brouri, Josefine Schnee, Sandra

Casale, Guylène Costentin

► To cite this version:

Corentin Reynaud, Cyril Thomas, Dalil Brouri, Josefine Schnee, Sandra Casale, et al. Oxidative Dehydrogenation of Propane on Hydroxyapatite-Supported Cobalt Catalysts. ChemCatChem, 2025, 17 (10), pp.e202402100. 10.1002/cctc.202402100. hal-05370167

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# Oxidative Dehydrogenation of Propane on Hydroxyapatite-Supported Cobalt Catalysts

Corentin Reynaud,[a] Cyril Thomas,[a] Dalil Brouri,[a] Josefine Schnee,[a] Sandra Casale,[a] and Guylène Costentin*[a]

Hydroxyapatite (HAp)-supported cobalt catalysts have been evaluated in the oxidative dehydrogenation of propane to propene (ODHP). The morphology of the HAp supports and the cobalt immobilization mechanisms strongly influence the surface accessibility of cobalt species and their dispersion. A calibration curve obtained from NO-TPD experiments for the cobalt-exchanged materials allowed us to derive their cobalt surface densities. These values were used to better compare the evolution of propane conversion and propene selectivity, as well as to analyze the parameters governing these performances. Co₃O₄ nanoparticles formed by the strong electrostatic

adsorption process are poorly selective in propene compared to isolated Co²⁺ cations immobilized by the cationic exchange mechanism. The isolated Co species exhibit a turnover frequency per Co atom that is two orders of magnitude higher than that reported for Co-based systems in previous studies. This much better performance is attributed to (i) the promotion of HAp basicity by cobalt incorporation as shown by the model MBOH conversion, (ii) the complete surface accessibility of the isolated Co²⁺ cations, and (iii) the close proximity between the Co²⁺ redox centers and the basic sites, as investigated by DFT.

## 1. Introduction

Propene is an essential platform molecule,[1] but there is an increasing gap between its production by steam reforming and fluid catalytic cracking of the oil fractions[2] and its demand on the world market, however.[2d,3] Given the significant amount of propane in the shale gas resources, there has recently been a renewed interest in the valorization of propane to propene. While propane direct dehydrogenation (PDH) selectively produces propene[4] and has been industrialized,[4,5] it requires high operating temperatures due to the endothermic nature of the dehydrogenation reaction.[6] In addition, this process suffers from deactivation and requires periodic regeneration steps of the catalyst.[7] The oxidative dehydrogenation of propane (ODHP) appears to be an attractive alternative to PDH as its thermodynamics allow for lower temperature operating conditions, and the presence of oxygen in the reactant feed makes it less sensitive to coke deposition than the PDH process.[1,2] However, the main challenge for the ODHP reaction lies in the development of catalysts capable of activating the C–H bond of propane at a temperature low enough to avoid its overoxidation to CO and CO₂ by oxygen.[2a,5] The ODHP reac-

tion is a structure-sensitive reaction,[1,8] which is governed by the Mars and van Krevelen mechanism.[9] Metal oxide catalysts have been extensively studied for the ODHP reaction and vanadium-based systems have been found to be among the most efficient catalysts.[2a,b,d] It is generally accepted that highly polymerized oxide-like species are detrimental to the control of propene selectivity. This has been attributed to the ease with which electron exchanges occur within adjacent metal species and/or the rapid mobility of oxygen ions, resulting in the overoxidation of adsorbed products to CO and CO₂. In contrast, low nuclearity or monomeric metal species have been claimed to favor the selectivity in mild oxidation reactions, as described by Grasselli as the concept of active site isolation.[10] Highly dispersed species such as single sites for heterogeneous catalysis (SSHc) have been studied for the ODHP reaction,[2d,11] and it was concluded that the turnover frequency (TOF) for the production of propene catalyzed by monomeric tetrahedral vanadium species, incorporated into the silanol nests of dealuminated BEA zeolite, was much higher than that of polynuclear oxide-like entities present as extra-framework species.[11b] In addition to the transition metal environment, its nature is also important. Recently, several cobalt-based systems have been explored in the ODHP reaction,[12] and promising performance has been reported for isolated cobalt species incorporated into the nest of dealuminated BEA zeolite with a TOF at 400 °C, about 8 times higher than that measured for isolated vanadium species in dealuminated BEA.[13] In addition to the local environment of the transition metal acid-base properties have been reported to influence the alkene selectivity by playing an important role in the desorption of the alkene.[14] Basic oxygen species are also likely to be involved in the activation of the C–H bond of propane, which has been reported to be rate-determining in the ODHP reaction.[1,2c] Finally, the heterolytic dissociation of

<footnote>[a] C. Reynaud, C. Thomas, D. Brouri, J. Schnee, S. Casale, G. Costentin Sorbonne Université, CNRS, Laboratoire de Réactivité de Surface, LRS, 4 place Jussieu, Paris F-75005, France
E-mail: guylene costentin@ sorbonne-universite.fr</footnote>

<footnote>Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.202402100</footnote>

<footnote>© 2025 The Author(s). ChemCatChem published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>
<figure><img src="image_3.png" /></figure>

the C—H bond of propane has been found to be promoted by alkali doping, confirming the beneficial influence of basic oxygen species.[15]

In this context, transition metal-modified hydroxyapatite (TM-HAp) systems are of particular interest for selective oxidation reactions.[16] This environmentally friendly calcium phosphate material known to be the major mineral component of bones and teeth has emerged in heterogeneous catalysis because its adjustable composition allows fine tuning of its acid-base properties[17] and its flexible framework allows substitution of both cationic and anionic species.[18] In addition, HAp materials can already be prepared on a large scale for their numerous industrial applications in the dental field[19] or as an attractive sorbent for water remediation.[20] While under low temperature reaction conditions, the basic sites have been identified as hydroxyl groups emerging from the OH channels running along the c-axis,[21] Sugiyama et al. suggested that “oxygen species” derived from these hydroxyl groups may be involved in the activation of alkanes.[22] In a recent work on vanadium-modified hydroxyapatite (V-HAp), we have shown by operando impedance spectroscopy that anisotropic conduction of protons along the c-axis could assist the in situ generation of O<sup>2-</sup> species under ODHP operating conditions.[23] These basic sites have been suggested to be strong enough to achieve the abstraction of a proton from the propane molecule.[23] However, the poor accessibility of isolated vanadate groups of the V-HAp catalysts resulted in a rather limited propene selectivity, below 30%. The formation of a Ca<sub>4</sub>V<sub>4</sub>O<sub>14</sub> phase epitaxially grown on the V-HAp terminations resulted in a synergistic effect with the close proximity between strong basic sites and tetrameric V<sub>4</sub>O<sub>14</sub> units, in which vanadium species were more easily reducible, ultimately leading to a significant improvement in both propane conversion and propane selectivity.[24] Given the much higher ODHP performance reported for isolated cobalt species incorporated in the dealuminated BEA zeolite (Co-Siβ) than for vanadium species in V-Siβ[13] and the critical role of HAp basic sites for propane activation,[23] cobalt-modified HAp may be of interest for the ODHP reaction. Carvalho et al. reported that Co-containing hydroxyapatite catalysts have a high affinity for CO<sub>2</sub> adsorption and concluded to the presence of relatively strong basic sites.[25] Sugiyama et al. have reported on ODHP activity for cobalt ions introduced into Sr-HAp (Co<sub>x</sub>Sr<sub>10-x</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>2</sub> solid solution) by coprecipitation.[12f] Yet, the unexpected activity of Sr-HAp (Sr<sub>10</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>2</sub>) and the incorporation of most of the cobalt species into the bulk of Sr-HAp made it challenging to clearly determine the intrinsic role of cobalt, although the addition of Co enhanced the propene production. The aim of the present work is to combine the basic properties of HAp with the redox properties of cobalt for the catalytic performance of cobalt-modified HAp in the ODHP reaction. In order to minimize the cobalt content and optimize its surface accessibility, a two-step preparation method was carried out, with post-immobilization of cobalt on HAp s in an excess of solution. HAp materials are currently used as sorbents for the remediation of heavy metals present in wastewater[20] resulting in highly dispersed and stable metal species. In a recent work, we have described how the cobalt concentration in the aqueous solution, the pH condi-

tions and the morphology of the HAp particles influence the Co immobilization processes.[26] It was shown that the surface termination of HAp, either crystalline (100) planes for HAp nanorods or hydrated nonapatitic surfaces for HAp platelets, as well as the applied pH conditions allowed to vary the amount of immobilized cobalt. Four immobilization mechanisms were identified, – Co<sup>2+</sup> cationic exchange, Co<sup>2+</sup> strong electrostatic adsorption (SEA) on phosphate species, Co<sup>2+</sup> diffusion in the hydrated layer and HAp dissolution and reprecipitation to form a Co-HAp solid solution surface layer -, and were found to be critical for the thermal stability and ultimate accessibility of the cobalt species.[26] The catalytic performance of these Co-HAp materials for the ODHP reaction is discussed in terms of the surface organization of the active sites and the surface basicity (as revealed both by NO-FTIR and the model 2-methyl-3-butyn-2-ol (MBOH) conversion reaction). The cobalt surface density was monitored by temperature-programmed desorption of NO (NO-TPD) and the local environment of exchanged surface Co<sup>2+</sup> species was investigated by DFT calculations.

## 2. Experimental Section

### 2.1. Synthesis of the Co-HAp

Three different HAp supports with different textural properties were prepared at 80 °C using the co-precipitation procedures described elsewhere[26] and using an automated reactor (Optimax 1001 synthesis workstation, Mettler Toledo). Briefly, the calcium and phosphorus precursor solutions ([Ca]<sub>0</sub> = 0.216 mol L<sup>-1</sup> and [P]<sub>0</sub> = 0.130 mol L<sup>-1</sup> corresponding to [Ca]<sub>0</sub>/[P]<sub>0</sub> = 1.67) were prepared by dissolving Ca(NO<sub>3</sub>)<sub>2</sub>·4H<sub>2</sub>O (Sigma-Aldrich, purity ≥ 99%) and (NH<sub>4</sub>)<sub>2</sub>HPO<sub>4</sub> (Acros Organics, purity ≥ 99%) in ultrapure water. Based on previous studies[27] two precipitation stationary pH were chosen to obtain HAp particles of different morphologies. Platelet-like (P) and rod-like (R) HAp morphologies were obtained at pH 6.5 and pH 9.0, respectively. The precursor solutions were previously adjusted to pH 10 at room temperature with ammonia (except for the precipitation performed at pH 6.5 for which the calcium solution did not require any basification). The addition of one precursor solution to the other (4.4 mL min<sup>-1</sup> with continuous stirring, 400 rpm, volume of precursor solution added so as to obtain a final [Ca]/[P] ratio of 1.67) was performed at 80 °C and automated addition of concentrated ammonia (Sigma-Aldrich, 28%) was carried out to counterbalance the protons released during the precipitation reaction, while keeping the pH constant at 9.0 or 6.5. The precipitates were washed with distilled water and recovered by centrifugation (8500 rpm) and then dried overnight at 120 °C in a laboratory oven at ambient atmosphere.

As described previously[26] excess solution was used to deposit cobalt on the HAp supports. The subsequent steps can be described as follows: i) 100 mL of ultrapure water was equilibrated under ambient conditions by magnetic stirring (800 rpm) either at a pH of ~ 5.4 (due to CO<sub>2</sub> dissolution) or at a pH of about 9.0, for which basification was achieved by the addition of 2–5 mL of ammonia (0.1 mol L<sup>-1</sup>), ii) 1.00 g of the dried HAp support was then added and the resulting suspension was stirred for 2 h, iii) an appropriate amount of Co(NO<sub>3</sub>)<sub>2</sub>·6H<sub>2</sub>O (Sigma-Aldrich, purity ≥ 98%) was added to obtain a variable cobalt loading and the suspension was kept under stirring for 1 h, iv) the wet sample was recovered by centrifugation (8500 rpm) and washed three times with acetone. After drying for 12 h at room temperature, the powdered samples were
finely ground in an agate mortar. The samples were then subjected to thermal annealing under an argon flow (50 mL<sub>NTP</sub> min<sup>-1</sup>) up to 500 °C (2.5 °C min<sup>-1</sup>) for 90 min. This temperature was set to match the maximum ODHP reaction temperature.

## 2.2. Characterization Techniques

### 2.2.1. X-ray Fluorescence Spectroscopy

The bulk Ca/P ratio of the HAps supports and the deposited Co loadings (wt%) were determined following a quantitative XRF protocol. To avoid matrix and grain size effects, all samples were melted into "beads" and analyzed using an X-ray fluorimeter Epsilon 3xl (P analytical) equipped with an Ag X-ray tube operating under three conditions in a He atmosphere: 12 kV-25 µA for 120 s for the analysis of Ca, 10 kV-30 µA for 120 s for the analysis of P and 20 kV - 15 µA for 120 s for that of Co. The beads were prepared by mixing approximately 0.114 g of sample, 1.231 g of fluxing agent LiBO₂/Li₂B₄O₇ and 0.0019 g of nonwetting agent (LiBr) in a platinum crucible. This mixture was heated at 1050 °C for 25 min in a melting device (LeNeo fluxer, Claisse). Calibration curves were obtained from beads of mechanical mixtures of Ca(NO₃)₂·4H₂O, (NH₄)₂HPO₄ and Co₃O₄. Their relative proportions were adjusted in order to prepare reference samples with Ca/P ratios close to those of the prepared HAps (1.5 < Ca/P < 1.8) and Co loadings close to the nominal loadings (from 0.25 to 4 wt%). The calibration curves were found to be perfectly linear over the whole concentration range. To validate the protocol, the agreement of the XRF data with the ICP-OES data (after mineralization of the samples with HNO₃) was also checked on selected representative samples.

### 2.2.2. NO-FTIR

The adsorption-desorption steps of NO on the materials were monitored by Fourier transform infra-red spectroscopy (FTIR) using a Vector 22 (Bruker) FTIR spectrometer equipped with an MCT detector (resolution 2 cm<sup>-1</sup>, 128 scans per spectrum). All the spectra were recorded at RT from self-supported wafers (15-20 mg) placed in a quartz cell equipped with CaF₂ windows and connected to a vacuum line allowing thermal treatments and adsorption-desorption experiments to be carried out in situ. Prior to NO-adsorption at RT, the wafers were heated at 500 °C (3 °C min<sup>-1</sup>) for 2 h under Ar flow (25 mL<sub>NTP</sub> min<sup>-1</sup>) and outgassed under vacuum at 500 °C for 1 h before being cooled down to RT, for which a residual pressure of ~ 1.33 10<sup>-4</sup> Pa was obtained. NO was then introduced into the cell and contacted with the wafers for 2 h. Using these static adsorption conditions, it was found that a NO partial pressure of 10 Torr was required to favor NO adsorption and obtain recordable FTIR spectra. Spectra were recorded before and after the introduction of NO. The cell was then degassed under dynamic vacuum at RT before being heated at increasing temperatures to monitor NO desorption.

### 2.2.3. NO-TPD

NO Temperature-Programmed Desorption (NO-TPD) experiments were carried out in a U-shaped quartz reactor (15 mm i.d.). Samples (approximately 300 mg) were held on a plug of quartz wool and the temperature was controlled by a Eurotherm 2408 temperature controller using a K-type thermocouple. Reactant gases, used as received, were supplied from independent gas cylinders (purchased from Air Liquide) via mass flow controllers (Bronkhorst F-201CV-200-RAD-11-V and Brooks 5850TR) at a total flow rate of 230 mL<sub>NTP</sub> min<sup>-1</sup>. The NO<sub>x</sub> (NO and NO<sub>2</sub>) concentrations at the reactor outlet were

continuously monitored by an FTIR detector (MKS2030 instrument). Prior to the NO-TPD experiments, the samples were annealed in situ at 500 °C (3 °C min<sup>-1</sup>) for 2 h at a flow rate of 100 mL<sub>NTP</sub> min<sup>-1</sup> of He. The samples were then exposed to the adsorption mixture consisting of 387 ppm of NO diluted in He for about 2 h until the outlet NO<sub>x</sub> value was close to the inlet value. The latter parameter was set to ensure that each sample was saturated under the present experimental conditions. Prior to the NO-TPD step, the samples were flushed in He at RT for about 1 h to remove weakly chemisorbed species until the NO and NO<sub>2</sub> concentrations detected at the reactor outlet were found to be negligible. NO-TPD experiments were carried out from RT to about 500 °C at a heating rate of 3 °C min<sup>-1</sup> under a He flow rate of 230 mL<sub>NTP</sub> min<sup>-1</sup>.

## 2.3. Catalytic Reactions

### 2.3.1. MBOH Model Reaction

The gas phase model reaction of 2-methyl-3-butyn-2-ol (MBOH) conversion was carried out using a differential flow micro-reactor. The catalyst (20 mg) was held on a fritted disc in a U-type quartz reactor with an internal diameter of 10 mm. The temperature of the catalyst bed was controlled by a thermocouple located near the wall of the quartz reactor. The samples were first pre-treated under a He flow (20 mL<sub>NTP</sub> min<sup>-1</sup>) up to 500 °C (3 °C min<sup>-1</sup> for 2 h and then cooled down to the reaction temperature of 140 °C. The catalyst was then exposed to the reaction feed obtained by bubbling He (100 mL<sub>NTP</sub> min<sup>-1</sup>) in liquid MBOH (Fluka, 99.9%) at 20 °C (MBOH partial pressure of 1.73 kPa). The reaction products were analyzed on-line using a μ-gas chromatograph (μ-GC 990 Agilent) equipped with a CPWAX 52 CB column.

### 2.3.2. ODHP Catalytic Experiments

The ODHP experiments were carried out in a U-type quartz reactor (10 mm i.d.). The samples were held on a plug of quartz wool, and the temperature was controlled by a Eurotherm 2408 temperature controller using a K type thermocouple. The reactant gases (Air Liquide, used as received) were supplied by independent mass flow controllers (Brooks 5850TR and Bronkhorst F-201CV-200-RAD-11-V). Prior to the catalytic runs, 20 mg of the samples (diluted in 600 mg of α-Al₂O₃ (125-200 µm, Aldrich) to avoid hot spots) were calcined in situ in O₂ (2.2%)–He at a total flow rate of 69.4 mL<sub>NTP</sub> min<sup>-1</sup>, at 500 °C (3 °C min<sup>-1</sup>) for 2 h. The temperature was then reduced to 400 °C. The typical composition of the reaction mixture was: 8.2% C₃H₈ and 2.1% O₂ in He with a total gas flow rate of 75.6 mL<sub>NTP</sub> min<sup>-1</sup>. The catalyst was contacted with the reaction mixture at 400 °C for 1 h 30 and the temperature was increased by steps of 25 °C (5 °C min<sup>-1</sup>) up to 500 °C for about 1 h 30 at each reaction step at atmospheric pressure. After the reaction at 500 °C, the catalyst was cooled down to 425 °C and then to 400, 375, 350 and 325 °C and allowed to stand for a further 1 h 30 at each step before being heated to 400 °C for a further 1 h 30. The composition of the reactor effluent was analyzed on-line using a μ-GC (Varian, CP4900) with three different columns running in parallel. A 5A molecular sieve column (80 °C, 150 kPa He, 200 ms injection time, 30 s backflush time) was used to separate H₂, O₂, CH₄ and CO. The Poraplot Q column (100 °C, 150 kPa He, 50 ms injection time, 6 s backflush time) was used for the separation of CO₂, C₂H₄, C₂H₆, H₂O, C₃H₆, and C₃H₈. The CP-SILS column (45 °C, 150 kPa He, 50 ms injection time) was used to separate C₃H₆O compounds (acetone and propenal) from C₃H₄O (acrolein).
The conversion of C₃H₈ and the product selectivity and yield were calculated as follows:

$$C_3H_8 \text{ conversion (\%)} = \left( \sum n_i [A_i] / (n_{C_3H_8} \times [C_3H_8]_{\text{inlet}}) \right) \times 100$$

$$A_i \text{ selectivity (\%)} = (n_i [A_i] / \sum n_i [A_i]) \times 100$$

$$A_i \text{ yield (\%)} = C_3H_8 \text{ conversion} \times A_i \text{ selectivity} / 100$$

where nᵢ, [Aᵢ], and [C₃H₈]ᵢₗet are the number of carbon atoms in the Aᵢ products, the concentrations of the Aᵢ products at the reactor outlet and the inlet concentration of propane, respectively.

Under the present experimental conditions, it was verified that the reactor loaded with 0.6 g of α-Al₂O₃ did not contribute significantly to the conversion of C₃H₈ at 400 °C. Furthermore, both C₃H₈ and O₂ conversions measured at 400 °C varied proportionally with the amount of sample loaded into the reactor indicating the absence of external limitations. Carbon and oxygen balances were found to be above 98% and 90% respectively.

## 2.4. Surface Modelling

DFT calculations were performed using the VASP software (version 5.4.1)[28] and the PBE functional in the generalized gradient approximation.[29] Atomic positions and cell parameters were optimized by reducing the total energy of the considered system. In the minimization process, self-consistent cycles were converged to 10⁻⁴ eV and ionic relaxation to 5 × 10⁻² eV Å⁻¹ using the quasi-Newton algorithm. The reciprocal space was sampled with an automatic generation scheme for the bulk structure and limited to the gamma point for the surface calculations, requiring large unit cells including 15 Å vacuum for the creation of the surfaces. The cut-off energy for the plane waves was fixed at 400 eV for all the calculations. However, it was increased to 520 eV in the cell parameter optimization in order to avoid Pulay stress effects. The bulk structure of hydroxyapatite is usually described by the P6₃/m crystallographic space group,[30] as reported in the 4317043.cif file from the crystallographic open database.[31] Since the two OH groups in the P6₃/m crystallographic space group have a half occupancy rate, a derived structure was generated with nonfractional occupancy rates in the P6₃ space group, where the OH groups are all oriented in the same direction. According to the Ca₁₀(PO₄)₆(OH)₂ formula, each unit cell contains 44 atoms. The atomic positions as well as the a- and c-cell parameters were relaxed for bulk optimization. From a previous study,[26] the most stable HAp surface (100) can be described as a zigzag surface combining both surface calcium deficiency (Ca/P ratio lower than that of the bulk structure) and OH groups. This modelled (100) surface reproduces the composition of the experimental surface measured for crystalline rod-shaped particles that preferentially expose (100) faces.[26]

In the context of Co deposition in aqueous solution, an explicit solvent model was considered: water molecules were progressively added considering a 1 × 1 × 3 supercell and the structure was optimized until saturation was reached with 5 water molecules per elementary cell, as detailed in the Supporting Information and Figure S1. Calcium cationic exchange for cobalt cation was then studied on the corresponding hydrated surface. Two-unit cells (one unit containing a hydrated surface in addition to a bulk unit) were extracted from the model of the optimal (100) surface containing 5 H₂O molecules (S1l). These two units were extended in the two directions perpendicular to the slab axis, resulting in a 2 × 2 × 2 super-cell containing a (100) surface with 20 Ca²⁺ sites and 20 H₂O

molecules. The four-unit cells exposing the surface were subjected to a relaxation of the ionic positions during the DFT calculations, while the four-unit cells below the surface were not relaxed.

The incremental energy to exchange each nᵗʰ additional Ca²⁺ species for Co²⁺ species was calculated for each non-equivalent surface calcium site as follows:

$$\Delta E_{\text{incr.exch.}} = E(\text{HAp\_}(n)) - E(\text{HAp\_}(n-1)) + E(\text{Ca}) - E(\text{Co})$$

where n is for the number of cobalt cations inserted in the supercell structure, E(HAp\_n) and E(HAp\_(n-1)) are the energies of the supercell system with n and (n-1) cobalt cations, respectively, E(Ca) and E(Co) correspond to the energies of “free” Ca²⁺ and Co²⁺ cations, respectively, calculated under the same conditions.

The cumulative energy required to exchange n calcium cations for cobalt cations according to the following equation:

$$\begin{aligned} \text{Ca}_{20}(\text{PO}_{4})_{12}(\text{OH})_{4} \cdot 20\text{H}_{2}\text{O} + n\text{Co}^{2+} = & \text{Co}_{n}\text{Ca}_{20-n}(\text{PO}_{4})_{12}(\text{OH})_{4} \\ & \cdot 20\text{H}_{2}\text{O} + n\text{Ca}^{2+} \end{aligned}$$

is calculated as follows:

$$\Delta E_{\text{cum.exch.}} = E(\text{HAp\_}(n)) - E(\text{HAp}) + nE(\text{Ca}) - nE(\text{Co})$$

For each substitution step, the nature of the optimal Ca²⁺ site to be substituted was determined by minimizing the ΔEᵢₘₐₗₐₖ. The additional energy that would be required for substitution at other calcium sites is given as ΔEᵃ₉ₐ₉:

$$\Delta E^{\text{add}} = E_{\text{site opt.}}(\text{HAp\_}(n)) - E_{\text{site}}(\text{HAp\_}(n))$$

where E-site opt.(HAp\_n) is the lowest energy of the supercell system with n cobalt cations, while E-site(HAp\_n) is for the energy corresponding to the other investigated sites for the nᵗʰ Co²⁺ cations.

The structures were visualized using VESTA Software.[32]

# 3. Results and Discussion

## 3.1. Description of the Co-Modified-HAp Catalysts

It has previously been shown that the order of introduction of one precursor solution into the other has a significant effect on the final textural properties of the HAp support.[27] The Ca → P addition route was used for the synthesis of the two HAp supports prepared at pH 9.0 and 6.5. These syntheses resulted in the production of HAp s with specific surface areas of 88 and 37 m² g⁻¹ for the materials prepared at pH 9.0 (rod-shaped particles, R) and 6.5 (platelets, P), respectively.[26] The P → Ca addition route led to the production of rod-shaped HAp particles at pH 9.0 with a specific surface area of 37 m² g⁻¹.[26] According to their final morphologies (M = P or R) and specific surface areas (SSA), the three HAp supports are referred to as HAp-MSSA (HAp-R₃₇, HAp-R₈₈ and HAp-P₃₇). Immobilization of Co²⁺ cations on the three supports in an excess of solution (basified or not) resulted in Co-modified samples with different measured cobalt loadings (wt%), hereafter referred to as HAp-MSSA-Co(wt%) and HAp-MSSA-Co(wt%)-b, where b indicates a basification of the aqueous Co²⁺ solution prior to HAp introduction. The cobalt immobilization mechanisms and the Co loadings depend on
<figure><img src="image_4.png" /></figure>

<table><caption>**Table 1.** Co-modified HAp samples (HAp-M<sub>SSA</sub>-Co(wt%)(-b)): XRF bulk (Ca + Co)/P molar ratios, Co immobilization mechanisms resulting in full or partial surface accessibility of cobalt species on the top surface, [Co]<sub>s</sub> cobalt surface densities, either calculated for Co species fully dispersed on the top surface of the thermally treated samples or deduced from NO-TPD experiments.<sup>a)</sup></caption><thead><tr><th>Sample Name</th><th>(Ca + Co)/P (XRF)<sup>[26]</sup></th><th>Co Immobilisation Mechanism<sup>[26]</sup></th><th>Co Surface Accessibility</th><th>Co Surface Density [Co]<sub>s</sub> (Co nm<sup>-2</sup>)</th></tr></thead><tbody><tr><td>HAp-R<sub>37</sub>-Co(0.22)</td><td>1.77</td><td rowspan="5">Surface cationic exchange</td><td rowspan="5">Complete</td><td>0.61</td></tr><tr><td>HAp-R<sub>37</sub>-Co(0.37)</td><td>1.77</td><td>1.02</td></tr><tr><td>HAp-R<sub>37</sub>-Co(0.53)</td><td>1.76</td><td>1.46</td></tr><tr><td>HAp-R<sub>37</sub>-Co(0.78)</td><td>1.75</td><td>2.15</td></tr><tr><td>HAp-R<sub>88</sub>-Co(0.48)</td><td>1.70</td><td>0.56</td></tr><tr><td>HAp-R<sub>88</sub>-Co(0.88)</td><td>1.69</td><td rowspan="2">Surface cationic exchange</td><td rowspan="2">Complete</td><td>1.02</td></tr><tr><td>HAp-R<sub>88</sub>-Co(1.67)</td><td>1.68</td><td>1.94</td></tr><tr><td>HAp-R<sub>37</sub>-Co(1.13)-b</td><td>1.79</td><td>SEA then sintering to Co<sub>3</sub>O<sub>4</sub> NPs</td><td>Partial</td><td>1.1<sup>a)</sup></td></tr><tr><td>HAp-P<sub>37</sub>-Co(0.86)</td><td>1.58</td><td>Subsurface Diffusion</td><td>Partial</td><td>0.4<sup>a)</sup></td></tr><tr><td>HAp-P<sub>37</sub>-Co(1.20)-b</td><td>1.61</td><td>Dissolution-reprecipitation</td><td>Partial</td><td>0.4<sup>a)</sup></td></tr></tbody></table>

both the surface properties of the HAp supports and on the pH conditions (Table 1).<sup>[26]</sup> The immobilization of Co<sup>2+</sup> cations on the two rod-shaped HAp supports (HAp-R<sub>SSA</sub>) has been shown to occur via a cationic exchange mechanism in which the Ca<sup>2+</sup> cations of the HAp supports are replaced by Co<sup>2+</sup> cations.<sup>[26]</sup> As previously reported, the deposited cobalt loading remains below the target and could be varied from 0.22 to 0.78 wt% for the HAp-R<sub>37</sub>-Co(wt%) series and from 0.48 to 1.67 wt% for the HAp-R<sub>88</sub>-Co(wt%) series, which is consistent with the different surface areas of the two HAp-R<sub>37</sub> and HAp-R<sub>88</sub> supports. In line with the cationic exchange immobilization mechanism, the bulk (Ca + Co)/P ratios remained essentially unchanged for each series (Table 1). This mechanism resulted in the immobilization of isolated cobalt ions located at surface crystallographic sites of the (100) crystalline surfaces and prevented them from sintering upon thermal annealing at 500 °C. Such an optimal Co dispersion allowed us to estimate the cobalt surface density for a given Co loading and specific surface area. The Co surface density was found to vary from 0.56 to 2.15 Co nm<sup>-2</sup> (Table 1). In contrast, a strong electrostatic adsorption (SEA) mechanism appeared to be favored when the suspension containing the HAp support was basified prior to the incorporation of the cobalt salt.<sup>[26]</sup> As shown in the TEM and HRTEM micrographs (Figure S2), the weaker interaction of the Co cations with the HAp surface when immobilized by SEA compared to that of Co cations immobilized by the cationic exchange mechanism did not prevent sintering upon thermal treatment, which eventually resulted in the formation of Co<sub>3</sub>O<sub>4</sub> nanoparticles (NPs). Contrary to the deposition of Co by a cationic exchange mechanism, the complete dispersion of cobalt is not achieved by a SEA mechanism, which prevents the direct estimation of the cobalt surface density.

For the HAp-P<sub>37</sub> support, despite a similar SSA to that of HAp-R<sub>37</sub> and the bulk stacking of (100) planes in the thin platelets formed, the different terminations of the HAp support platelets compared to the rod-shaped particles resulted in a the completely different Co immobilization mechanism.<sup>[26]</sup> Ion dif-

fusion was indeed favored by the presence of a non-apatitic outer hydrated surface layer surrounding the platelet particles. The ionic composition of the latter was found to be close to that of octacalcium phosphate (OCP, Ca<sub>8</sub>(HPO<sub>4</sub>)<sub>2</sub>(PO<sub>4</sub>)<sub>4</sub>·5H<sub>2</sub>O) and enriched in HPO<sub>4</sub><sup>2-</sup> ions.<sup>[27]</sup> The high lability of the ions within this hydrated layer resulted in a rather high cobalt loading located in the subsurface of HAp-P<sub>37</sub>-Co(0.80). When operated in a basified medium, the cobalt ion immobilization appeared to be even more promoted (HAp-P<sub>37</sub>-Co(1.20)-b). In the latter sample, a Co-substituted HAp deep layer was precipitated due to the presence of Co<sup>2+</sup> cations and to the increase in the OH<sup>-</sup> concentration in the medium, which favored the hydrolysis of the surface non-apatitic layer.<sup>[26,27]</sup> For this HAp-P<sub>37</sub> support, both ion diffusion and dissolution-reprecipitation mechanisms are responsible for the lack of accessibility of a fraction of the immobilized cobalt species, making direct estimation of the cobalt surface density based on Co loading and specific surface area hazardous. To overcome this difficulty, NO-TPD experiments were performed to investigate the accessible cobalt cations, as described in Section 3.3.2.

## 3.2. ODHP Reaction

After annealing at 500 °C, the ODHP catalytic activity was recorded from 400 to 500 °C in increments of 25 °C before being recorded from 425 to 325 °C and finally at 400 °C (Figure 1). O<sub>2</sub>, C<sub>3</sub>H<sub>6</sub>, CO<sub>2</sub>, CO, and H<sub>2</sub>O concentrations are plotted in Figure 1 as a function of time on stream and reaction temperature (red line) for HAp-R<sub>37</sub>-Co(0.78). As expected, the O<sub>2</sub> concentration decreased with increasing reaction temperature (corresponding to an O<sub>2</sub> conversion of about 80% at 500 °C) due to the increased formation of the oxidized products. Stationary conditions are reached very rapidly at each temperature, indicating no deactivation of the catalyst. The absence of catalyst deactivation is confirmed by the similar product concentrations measured

<footnote>a) Estimated from the NO uptake of the low temperature desorption peaks of the NO-TPD profiles of the corresponding samples and the calibration line of the surface-exchanged HAp-R<sub>37</sub>-Co(wt%) and HAp-R<sub>88</sub>-Co(wt%) samples, see Section 3.3.2.</footnote>
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 1.** Evolution of the concentrations in C<sub>3</sub>H<sub>6</sub> (●), O<sub>2</sub> (○), CO<sub>2</sub> (●), CO (●), and H<sub>2</sub>O as a function of time on stream and reacting temperature for HAp-R<sub>37</sub>-Co(0.78), (20 mg diluted in 600 mg α-Al<sub>2</sub>O<sub>3</sub>). The reaction mixture was made of 8.2% C<sub>3</sub>H<sub>8</sub> and 2.1% O<sub>2</sub> in He with a total flow rate of 75.6 mL<sub>NTP</sub> min<sup>-1</sup>.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 2.** Influence of the reaction temperature on propane conversion (●, ○), propene selectivity (●, ○) and propene yield (■ for HAp-R<sub>37</sub>-Co(0.78) (20 mg) under 8.2% C<sub>3</sub>H<sub>8</sub>, 2.1% O<sub>2</sub> in He, 75.6 mL<sub>NTP</sub> min<sup>-1</sup> total flow rate. The dark (black and brown) and light (grey and orange) dots correspond to the measurements recorded when increasing and decreasing reaction temperatures, respectively. The grey-blue and yellowish dots correspond to the last measurement at 400 °C.</figcaption></figure>

within the three reaction steps recorded at 400 °C. However, it should be noted that at 500 °C the CO<sub>2</sub> concentration increased very slightly upon time on stream at the expense of a decrease in the CO concentration, which is consistent with the observed decrease in the O<sub>2</sub> concentration. The evolution of the C<sub>3</sub>H<sub>8</sub> conversion and C<sub>3</sub>H<sub>6</sub> selectivity as a function of reaction temperature (Figure 2) is found to be typical of this reaction, with an increase in propane conversion and a decrease in propene selec-

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 3.** Influence of the cobalt loading on (a) propane conversion, (b) propene selectivity, and (c) propene yield; 20 mg catalyst under: 8.2% C<sub>3</sub>H<sub>8</sub>, 2.1% O<sub>2</sub> in He, 75.6 mL<sub>NTP</sub> min<sup>-1</sup> total flow rate at 400 °C. Black symbols correspond to Co deposition at natural pH on the HAp-R<sub>37</sub> (●), HAp-R<sub>88</sub> (▲), and HAp-P<sub>37</sub> (◆) supports, while blue symbols correspond to Co deposition prepared in a basified medium. The green symbols correspond to the C<sub>3</sub>H<sub>8</sub> conversion and C<sub>3</sub>H<sub>6</sub> selectivity measured for the bare HAp-R<sub>37</sub> support.</figcaption></figure>

tivity with increasing temperature. However, the loss in propene selectivity remained limited (from 65% to 59%), resulting in an increase in propene yield with increasing temperature. The back points recorded at 400 °C once again highlight the remarkable stability of the catalyst.

Similar trends were observed for the other samples. Figure 3 shows the catalytic performance of the Co-modified HAp catalysts as a function of their cobalt loading (Figures 3a–c). The comparison of propane conversion (Figure 3a) and propene selectivity (Figure 3b) at 400 °C was made from experiments where the catalyst weight was kept constant. For the bare HAp
supports, C₃H₈ conversion was found to be very limited and no propene was produced (data shown only for HAp-R₃₇, green dot in Figure 3). Immobilization of Co on all HAp supports clearly leads to improved C₃H₈ conversion (Figure 3a) and C₃H₆ selectivity (Figure 3b), and hence C₃H₆ yield (Figure 3c), but to different extents depending on the nature of the support and the pH conditions used for Co immobilization.

Even if the C₃H₈ conversion remains low for Co-modified HAp-P₃₇ catalysts, C₃H₆ formation could be accurately detected, leading to C₃H₆ selectivities of 15% and 53% for the samples for which cobalt deposition was performed under natural (HAp-P₃₇-Co(0.86), ◆) and basified (HAp-P₃₇-Co(1.20)-b, ◆) conditions, respectively. Much better ODHP catalytic performance was obtained for cobalt immobilized on the rod-like HAp-R supports. When immobilized at natural pH (Figure 3a, ● and ▲) an increase in the Co loading resulted in a progressive increase in C₃H₈ conversion with this tendency appearing to be more pronounced for the HAp-R₃₇-Co(wt%) series (Figure 3a, ●) than for the HAp-R₈₈-Co(wt%) series (Figure 3a, ▲). For these two HAp-RSSA-Co(wt%) series, there is hardly any influence of the Co loadings (Figure 3b) on the propene selectivity, which reaches 62%. Consequently, the trends obtained for the propene yield as a function of the Co loading (Figure 3c) are similar to those described for the propane conversion (Figure 3a,c).

Although the introduction of Co on HAp-R₃₇ under basified conditions allowed a higher cobalt loading to be deposited, there appears to be no beneficial effect on the propane conversion (Figure 3a, ●), as the conversion measured for HAp-R₃₇-Co(1.13)-b remained lower than that measured for the HAp-R₃₇-Co(0.78). This could be related to the formation of Co₃O₄ oxide nanoparticles on the former catalyst, which reduced the accessibility of the reactants to the cobalt species (Table 1). In addition, Figure 3b shows that the deposition of Co in basified media on HAp-R₃₇ is clearly detrimental to the propene selectivity as it drops drastically from 64 (●) to about 36% (●). Consequently, the basification process significantly reduced the propene yield (Figure 3c ●). The formation of Co₃O₄ oxide nanoparticles on HAp-R₃₇-Co(1.13)-b must be responsible for the low propene selectivity because, as described in the concept of active site isolation[10] transition metal oxide particles allow too rapid redox exchange, resulting in the overoxidation of alkanes.

Table 2 lists the turnover frequencies (TOFs) calculated on a per cobalt atom basis for the production of propene in the ODHP reaction at 400, 450, and 500 °C. It can be seen that the TOFs of HAp-R₃₇-Co(0.78) and HAp-R₈₈-Co(1.67), which are representative of their respective series and that of HAp-R₃₇-Co(1.13)-b are found to be one to two orders of magnitude higher than those previously reported for most of the other cobalt-based systems. Recently, the good performance reported for a Co₁.₉-Siβ-pH-2.5 catalyst was attributed to the formation of isolated cobalt species located in the silanol nests of a dealuminated Siβ zeolite.[13] As vanadium is often considered as a benchmark transition element for ODHP reaction, it was highlighted that the TOF value for Co₁.₉-Siβ-pH-2.5 was an order of magnitude higher than that of the corresponding V₀.₂₅-Siβ[¹¹b] vanadium catalyst. Interestingly, the data reported by Sugiyama et al. for a Co-modified strontium hydroxyapatite (Co36SrHAp, Table 2)

sample[¹²f] resulted in a TOF 28 times lower than that found in the present work for HAp-R₃₇-Co(0.78) at 450 °C. Since the latter authors claimed that the synthesis of the Sr₁₀-xCoₓ(PO₄)₆(OH)₂ sample resulted only in the formation of a solid solution, the amount of cobalt exposed on the top surface of such a catalyst can be reasonably questioned and may explain the observed difference in TOF with that measured in the present work on HAp-R₃₇-Co(0.78). Furthermore, surprisingly, these authors also reported on a significant conversion of propane and propene production on a Co-free Sr-HAp material. The much higher ODHP performance found for the HAp-RSSA-Co(wt%)-b series compared to other cobalt catalysts (Table 2) highlights the specific advantage of HAp used as a Co matrix or support. This suggests that Co and HAp act as co-catalysts and their synergy merits further investigation of their surface properties.

### 3.3. Impact of the Co Deposition Mechanism on the Surface Properties of Co-HAp Catalysts

#### 3.3.1. NO-FTIR

As NO is both a Lewis acid and a Lewis base, it can probe Lewis basic and acid surface sites, respectively.[³³] Due to its highest SSA, the study was carried out on the HAp-R₈₈ support and the HAp-R₈₈-Co(1.67) sample, which has the highest Co loading among the HAp-R₈₈-Co(wt%) series.

The contributions already present before the introduction of NO for both HAp-R₈₈ and HAp-R₈₈-Co(1.67) samples (green spectra in Figures 4a,b) can be attributed to the combination bands of the phosphate groups (around 2000–2100 cm⁻¹) and to the ν₅₀ vibrational modes of carbonates (1300–1550 cm⁻¹).[²¹a] The adsorption of incremental doses of NO leads to the progressive appearance of new contributions in the 1700–2000 and 1200–1600 cm⁻¹ regions (black spectra, Figure 4a,b). These contributions are attributed to ν₅₀ vibrational modes of chemisorbed NO on surface cations and basic sites, respectively.[³⁴] The 1700–2000 cm⁻¹ absorption region is much more complex and intense for HAp-R₈₈-Co(1.67) (Figure 4b) compared to that for HAp-R₈₈ (Figure 4a). This can be attributed to the involvement of two types of Lewis acid sites, namely Ca²⁺ and Co⁷⁺ cations, resulting from the vibrational modes of Ca-NO (1900–1910 cm⁻¹), Co-NO (1900–1910 cm⁻¹) and Co(-NO)₂ (1780–1790 cm⁻¹ and 1895–1865 cm⁻¹ for symmetric and antisymmetric modes, respectively).[³⁵] Due to the different cations involved in the adsorption of NO, the different adsorption modes involved (nitrosyl and dinitrosyl) and the lack of molar absorption coefficients for the different species, no quantitative information on the cation content could be derived from these spectra. Note that desorption under vacuum at RT significantly decreases the intensities of both nitrosyl and dinitrosyl contributions (blue spectra in Figures 4a,b), indicating that NO should be weakly bound to such cations.

The 1200–1600 cm⁻¹ region also shows increasing contributions for both samples upon NO introduction due to the formation of NOₓ y⁻ adsorbed species (Figure 4). The lower initial carbonate content in HAp-R₈₈-Co(1.67) compared to the
<figure><img src="image_11.png" /></figure>

<table><caption>**Table 2.** Comparison of the turnover frequencies (TOF, molecule C₃H₆ s⁻¹ Co or V atom⁻¹) reported in previous studies with those measured in the present work for HAp-R₃₇-Co(0.78) and HAp-R₈₈-Co(1.67). The TOF data calculated on a per atom basis were either estimated from the data reported in these works¹⁾ or extracted as is from the corresponding studies.¹⁾</caption><thead><tr><th>Sample</th><th>Co or V<br>(wt%)</th><th>SW<sup>a)</sup><br>(mg)</th><th>[C<sub>3</sub>H<sub>8</sub>]<sup>b)</sup><br>(%)</th><th>[O<sub>2</sub>]<sup>c)</sup><br>(%)</th><th>F<sup>d)</sup><br>(mL min<sup>-1</sup>)</th><th>T<sup>e)</sup> (°C)<br>(%)</th><th>Conv. C<sub>3</sub>H<sub>8</sub><sup>f)</sup><br>(%)</th><th>S C<sub>3</sub>H<sub>6</sub><sup>g)</sup><br>(%)</th><th>TOF<sup>h)</sup><br>(× 10<sup>3</sup> s<sup>-1</sup>)</th><th>Refs.</th></tr></thead><tbody><tr><td>HAp-R<sub>37</sub>-Co(0.78)</td><td>0.78</td><td>20</td><td>8.2</td><td>2.1</td><td>75.6</td><td>400</td><td>1.9</td><td>63.2</td><td>20.9</td><td>This work</td></tr><tr><td>HAp-R<sub>88</sub>-Co(1.67)</td><td>1.67</td><td>20</td><td>8.2</td><td>2.1</td><td>75.6</td><td>400</td><td>2.1</td><td>61.1</td><td>10.4</td><td>This work</td></tr><tr><td>HAp-R<sub>37</sub>-Co(1.13)-b</td><td>1.13</td><td>20</td><td>8.2</td><td>2.1</td><td>75.6</td><td>400</td><td>1.7</td><td>34.3</td><td>7.1</td><td>This work</td></tr><tr><td>HAp-P<sub>37</sub>-Co(1.20)-b</td><td>1.20</td><td>20</td><td>8.2</td><td>2.1</td><td>75.6</td><td>400</td><td>0.2</td><td>53.2</td><td>1.0</td><td>This work</td></tr><tr><td>CoO<sub>x</sub>-MCM-41</td><td>0.49</td><td>—</td><td>16.7</td><td>16.7</td><td>30</td><td>393</td><td>—</td><td>—</td><td>0.70<sup>i)</sup></td><td>[12a]</td></tr><tr><td>CoO<sub>x</sub>-SiO<sub>2</sub></td><td>0.49</td><td>—</td><td>16.7</td><td>16.7</td><td>30</td><td>393</td><td>—</td><td>—</td><td>0.33<sup>i)</sup></td><td>[12a]</td></tr><tr><td>CoMO<sub>x</sub>φy (CoMoO<sub>4</sub>)</td><td>26.9</td><td>200</td><td>29.0</td><td>14.9</td><td>20</td><td>400</td><td>8.8</td><td>46.4</td><td>0.19<sup>i)</sup></td><td>[12b]</td></tr><tr><td>Co<sup>III</sup><sub>0.36</sub>Co<sup>II</sup><sub>0.30</sub>Al<sub>0.34</sub>(PO<sub>4</sub>)<sub>0.11</sub>O<sub>1.19</sub></td><td>50.1</td><td>100</td><td>16.7</td><td>16.7</td><td>50</td><td>400</td><td>27.9</td><td>36.0</td><td>0.73<sup>i)</sup></td><td>[12c]</td></tr><tr><td>Co<sub>1.9</sub>-Siβ-pH-2.5</td><td>2.00</td><td>60</td><td>8.2</td><td>1.9</td><td>74.1</td><td>400</td><td>2.9</td><td>46.8</td><td>3.13<sup>i)</sup></td><td>[13]</td></tr><tr><td>V<sub>0.25</sub>-Siβ</td><td>0.25</td><td>200</td><td>6.6</td><td>2.1</td><td>15</td><td>400</td><td>1.3</td><td>34.1</td><td>0.4<sup>i)</sup></td><td>[11b]</td></tr><tr><td>HAp-R<sub>37</sub>-Co(0.78)</td><td>0.78</td><td>20</td><td>8.2</td><td>2.1</td><td>75.6</td><td>450</td><td>5.1</td><td>60</td><td>53.1</td><td>This work</td></tr><tr><td>HAp-R<sub>88</sub>-Co(1.67)</td><td>1.67</td><td>20</td><td>8.2</td><td>2.1</td><td>75.6</td><td>450</td><td>5.0</td><td>59.3</td><td>26.8</td><td>This work</td></tr><tr><td>HAp-R<sub>37</sub>-Co(1.13)-b</td><td>1.13</td><td>20</td><td>8.2</td><td>2.1</td><td>75.6</td><td>450</td><td>4.5</td><td>33.6</td><td>18.1</td><td>This work</td></tr><tr><td>HAp-P<sub>37</sub>-Co(1.20)-b</td><td>1.20</td><td>20</td><td>8.2</td><td>2.1</td><td>75.6</td><td>450</td><td>0.5</td><td>30.7</td><td>1.7</td><td>This work</td></tr><tr><td>Co<sub>36</sub>SrHAp</td><td>1.42</td><td>500</td><td>14.3</td><td>4.0</td><td>30</td><td>450</td><td>15.8</td><td>46.6</td><td>1.95<sup>i)</sup></td><td>[12f]</td></tr><tr><td>Co<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub></td><td>3.55</td><td>1000</td><td>18.1</td><td>18.1</td><td>83</td><td>450</td><td>15.8</td><td>15.8</td><td>0.46<sup>i)</sup></td><td>[12d]</td></tr><tr><td>CoMgAlO</td><td>9.67</td><td>—</td><td>—</td><td>—</td><td>—</td><td>450</td><td>1.5</td><td>92.5</td><td>0.33<sup>i)</sup></td><td>[12e]</td></tr><tr><td>HAp-R<sub>37</sub>-Co(0.78)</td><td>0.78</td><td>20</td><td>8.2</td><td>2.1</td><td>75.6</td><td>500</td><td>12.2</td><td>59.3</td><td>126.1</td><td>This work</td></tr><tr><td>HAp-R<sub>88</sub>-Co(1.67)</td><td>1.67</td><td>20</td><td>8.2</td><td>2.1</td><td>75.6</td><td>500</td><td>10.8</td><td>56.6</td><td>55.0</td><td>This work</td></tr><tr><td>HAp-P<sub>37</sub>-Co(1.20)-b</td><td>1.20</td><td>20</td><td>8.2</td><td>2.1</td><td>75.6</td><td>500</td><td>1.3</td><td>26.0</td><td>3.7</td><td>This work</td></tr><tr><td>CoWO<sub>4</sub></td><td>19.2</td><td>1000</td><td>15.0</td><td>15.0</td><td>60</td><td>500</td><td>12.6</td><td>38.5</td><td>0.10<sup>i)</sup></td><td>[12g]</td></tr><tr><td>Co<sub>2</sub>B/Co<sub>3</sub>B</td><td>93.2</td><td>235</td><td>30.0</td><td>15.0</td><td>50–160</td><td>500</td><td>13–23</td><td>88.2</td><td>0.36<sup>i)</sup></td><td>[12h]</td></tr><tr><td>P(0.05)CoO</td><td>69.4</td><td>100</td><td>11.8</td><td>9.8</td><td>50</td><td>500</td><td>27.2</td><td>53.8</td><td>0.55<sup>i)</sup></td><td>[12i]</td></tr></tbody></table>

a) Sample weight.

b) C₃H₈ concentration in the feed in vol.%.

c) O₂ concentration in the feed in vol.%.

d) Flow rates.

e) Reaction temperature.

f) Conversion of C₃H₈.

g) Selectivity in C₃H₆.

h) Turnover frequency on a per cobalt atom basis (TOF).

i) Estimated from the data reported in these previous works or

j) extracted as is from the corresponding studies.

pristine HAp-R<sub>88</sub> support (comparison of the green spectra in Figures 4a,b) may explain to some extent for the different final relative intensities for contributions around 1450 and 1300 cm<sup>-1</sup>. The different nature of the NO<sub>x</sub><sup>γ</sup> species, together with their different adsorption modes on the two surfaces, makes it difficult to discuss the differences in the basic sites of these samples.

In order to follow the interaction of the NO<sub>x</sub> species with the different adsorption sites with increasing temperature, the wafers were briefly heated to increasing temperatures (50, 100, 150, 200, 300, 400, 500, and 600 °C) before being transferred back into the IR beam to record the spectra at RT (Figures 4c,d). For both samples, the contributions related to NO adsorbed on surface cations (1700–2000 cm<sup>-1</sup>) completely disappeared after heating up to 150 °C, while those related to the interaction

with basic sites remained almost unchanged at the latter temperature. This indicates that NO interacts more strongly with basic sites than with Lewis acid sites such as surface cations. The intensities of the bands probing basic sites then decreased with increasing desorption temperature, and the NO<sub>x</sub><sup>γ</sup> adsorbed species were completely released at about 600 °C (red spectra in Figures 4c,d) for HAp-R<sub>88</sub> and HAp-R<sub>88</sub>-Co(1.67).

### 3.3.2. NO Temperature-Programmed Desorption Experiments

The TPD profiles recorded on the HAp-R<sub>37</sub> and HAp-R<sub>88</sub> series up to 550 °C showed three peaks in the temperature ranges of 25–175, 150–425, and 375–550 °C (Figures 5a,b). For the HAp-P<sub>37</sub>-based
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 4.** Interaction of NO with HAp-R<sub>88</sub> and HAp-R<sub>88</sub>-Co(1.67) followed by FTIR. Adsorption step of incremental doses of NO up to 10 Torr of NO for 2 h at room temperature on (a) HAp-R<sub>88</sub> and (b) HAp-R<sub>88</sub>-Co(1.67), the green and black spectra correspond to the spectra recorded before and after adsorption of incremental doses of NO, respectively. The desorption step with increasing temperature starts from the last spectra recorded under vacuum for the adsorption step at room temperature (blue spectra) up to the complete desorption (red spectra) after thermal treatment at high temperature (~600 °C) for (c) HAp-R<sub>88</sub> and (d) HAp-R<sub>88</sub>-Co(1.67).</figcaption></figure>

materials, only the low temperature desorption peak could be observed (Figure 5c).

**NO interaction with cationic sites:** Based on the adsorption-desorption experiments monitored by FTIR (Figure 4), the first desorption peak in the 25–175 °C range (peaking at 80 °C) can be attributed to NO adsorbed on surface cations. Recently, it has been shown that NO desorbs at the same temperature from Co<sup>2+</sup> species located in the silanol nests of a dealuminated Siβ zeolite.[13] It can be observed that the first desorption peak is centered around 80 °C regardless of the type of support, Co

content and deposition procedure (Figure 5a–c). Interestingly, low temperature desorption contributions were also observed for the three Co-free HAp supports, but at a much lower intensity than that recorded for the Co-modified HAp materials. In agreement with the FTIR data (Figure 4), this suggests that NO also adsorbs to Ca<sup>2+</sup> cations, but to a marginal extent compared to its adsorption to Co<sup>2+</sup> cations. When cobalt is immobilized on HAp-R<sub>37</sub> and HAp-R<sub>88</sub> under natural pH conditions, the intensity of the low temperature contribution clearly increases with increasing Co content (Figures 5a,b). As a cationic exchange mechanism has been demonstrated at natural pH on these
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>**Figure 5.** NO-TPD profiles recorded under 230 $mL_{NTP} min^{-1}$ He flow for (a) HAp-R$_{37}$, (b) HAp-R$_{88}$, and (c) HAp-P$_{37}$-based materials.</figcaption></figure>

supports,[26] this increase in intensity with the Co loading indicates a higher affinity of NO for $Co^{2+}$ cations than for $Ca^{2+}$ cations, which is consistent with the higher Lewis acidity of $Co^{2+}$ compared to $Ca^{2+}$. In contrast, despite the higher cobalt content in HAp-R$_{37}$-Co(1.13)-b compared to that in HAp-R$_{37}$-Co(0.78), the low-temperature desorption peak recorded for the former sample (blue TPD profile in Figure 5a) is of much lower intensity than that recorded for HAp-R$_{37}$-Co(0.78) (black TPD profile in Figure 5a). Such a discrepancy can be explained by the limited amount of $Co^{2+}$ species accessible for NO adsorption on HAp-R$_{37}$-Co(1.13)-b due to the formation of $Co_3O_4$ NPs after cobalt deposition under basified conditions and thermal treatment at 500 °C.[26] Even though the Co-modified HAp-P$_{37}$ samples show

<figure><img src="image_16.png" /><figcaption>**Figure 6.** Linear correlation between the NO uptake (NO $nm^{-2}$) of the low-temperature peak (25–175 °C in Figures 5a,b) and the Co atom surface density (Co $nm^{-2}$) in the Co surface-exchanged HAp-R$_{37}$-Co(wt%) (●) and HAp-R$_{88}$-Co(wt%) (▲) series. The dotted line corresponds to the linear regression using the data from both series. Blue and orange dashed lines show extrapolations to derive the Co surface density of HAp-R$_{37}$-Co(1.13)-b and HAp-P$_{37}$-Co(wt%)(-b), respectively, where only a limited amount of cobalt is exposed on the outer surface.</figcaption></figure>

significant amounts of Co, the intensity of the NO-TPD peaks at low temperature was also seen to be very weak and similar to that of the Co-free HAp support (Figure 5c). As mentioned in Section 3.1, such a lack of accessible surface cobalt species observed for HAp-P$_{37}$-Co(0.86) and HAp-P$_{37}$-Co(1.20)-b (black and blue TPD profiles in Figure 5c) can be attributed to their respective cobalt immobilization mechanisms, resulting in the incorporation of $Co^{2+}$ species in the subsurface of HAp rather than on its top surface,[26] as reported in (Table 1).

The low temperature desorption peak of the NO-TPD profiles is therefore essentially due to the desorption of NO from surface $Co^{2+}$ species, as it can be reasonably assumed that the amount of NO released from the surface $Co^{2+}$ species is negligible compared to that released from the surface $Co^{2+}$ species (Figure 5). Figure 6 shows a remarkable linear correlation between the NO uptake, recorded as a low temperature peak from 25 to 175 °C and the Co surface density of the samples in which Co was immobilized by a cationic exchange process with surface $Co^{2+}$ species (HAp-R$_{37}$-Co(wt%) and HAp-R$_{88}$-Co(wt%) series). The slightly positive Y-intercept is consistent with the limited NO adsorption expected on the $Co^{2+}$ cations of the Co-free HAp supports. The much lower NO/Co molar ratio (~0.10, Figure 6) than that expected from nitrosyl and/or dinitrosyl species indicates that only a fraction of the $Co^{2+}$ surface species is titrated with NO. This can be attributed to the weak interaction of NO with such species, in agreement with the NO-FTIR data (Figure 4), and to the flushing step under the He flow performed before the recording of the NO-TPD profiles (see *Experimental Section*), which should result in the release of weakly adsorbed NO molecules already at RT. Under the present experimental conditions, the Co surface densities of HAp-R$_{37}$-Co(1.13)-b, HAp-P$_{37}$-Co(0.86), and HAp-P$_{37}$-Co(1.20)-b,
<figure><img src="image_17.png" /></figure>

which expose only a fraction of Co on their top surface, can be derived from the NO uptake of their low temperature desorption peaks and the linear correlation shown in Figure 6, which can be used as a calibration line. Assuming that NO similarly probes isolated Co²⁺ and the exposed Co⁺ species on the outer surface of the Co₃O₄ NPs, the cobalt surface density of HAp-R₃₇-Co(1.13)-b is estimated to be 1.1 Co nm⁻², while that of HAp-P₃₇-Co(0.86) and HAp-P₃₇-Co(1.20)-b is 0.4 Co nm⁻² (Figure 6 and Table 1). Although the basification of the medium allowed immobilization of significantly higher amounts of cobalt than immobilization at natural pH, the surface cobalt densities remained either lower for the samples prepared in a basified medium (HAp-R₃₇-Co(1.13)-b versus HAp-R₃₇-Co(0.53): 1.1 versus 1.46 Co nm⁻², respectively) or comparable (HAp-P₃₇-Co(1.20)-b versus HAp-P₃₇-Co(0.86): 0.4 Co nm⁻² for both) to those of the HAp-R₃₇ and HAp-R₈₈ series (Table 1). This is consistent with the formation of Co₃O₄ NPs (HAp-R₃₇-Co(1.13)-b) and/or the diffusion of the Co²⁺ species in the subsurface of the HAp-P₃₇ support due to the peculiar lability of the ions in its outer non-apatitic layer (HAp-P₃₇-Co(1.20)-b).[26]

**NO interaction with surface anionic sites:** Based on the FTIR data (Figure 4), the high temperature desorption peaks recorded for the HAp-R₃₇-Co(wt%) and HAp-R₈₈-Co(wt%) series can be attributed to NO adsorption on basic sites. Despite a comparable SSA with HAp-R₃₇, it should be noted that neither HAp-P₃₇-Co(0.86) nor HAp-P₃₇-Co(1.20-b) show high temperature desorption peaks in the temperature range studied (Figure 5c). For the pristine HAp-R<sub>SSA</sub> supports (green traces in Figures 5a,b), the desorption peak at the highest temperature is not fully detected under the present conditions, its maximum being above 550 °C. When cobalt was deposited on HAp-R<sub>37</sub> in a basified medium (HAp-R<sub>37</sub>-Co(1.13)-b), the maxima of the two highest temperature peaks were also shifted compared to those of HAp-R<sub>37</sub>-Co(0.78) (Figure 5a), and the intensity of the highest temperature peak becomes larger than that at intermediate temperature. Several parameters could be responsible for these changes, including different NO adsorption modes depending on the Co deposition mechanisms and the Co surface densities of the samples. The assignment of the basic sites responsible for the high temperature peaks remains unclear and would require further investigation, which is beyond the scope of the present article.

### 3.3.3. Model MBOH Conversion Reaction

The MBOH model reaction was used to investigate the acid-base reactivity of the samples. This model molecule is converted along parallel pathways, leading to the formation of different products depending on the involvement of acidic and/or basic pathways (the basic pathway requires the involvement of acid-base pairs).[36] As previously reported for rod-like hydroxyapatites,[17,37] only the basic pathway is observed for the two HAp-R<sub>SSA</sub> supports, with the formation of acetone and acetylene in equimolar amounts. The nature of the basic sites was investigated in a previous operando DRIFT study and was identified as OH⁻ groups emerging from the apatite channels, which can be converted to O²⁻ species by their dehydration at elevated

<figure><img src="image_18.png" /><figcaption>**Figure 7.** Basic MBOH conversion measured after 1 h on stream at 140 °C (20 mg catalyst, P<sub>MBOH</sub> = 1.73 kPa, He flow: 100 mL<sub>NTP</sub> min<sup>-1</sup>) for the three supports (green rectangles) and the Co-modified HAp series (blue rectangles refer to as samples for which Co deposition was carried out under basified conditions).</figcaption></figure>

temperatures.[21b] It should also be recalled that the basic MBOH conversion is sensitive to both the strength of the basic sites and their surface density. HAp-R<sub>88</sub> shows a higher basic reactivity compared to HAp-R<sub>37</sub> (Figure 7), which can be attributed to some extent to its higher specific surface area. In contrast to the HAp-R<sub>SSA</sub> supports, HAp-P<sub>37</sub> shows no basic reactivity (Figure 7), probably due to the lack of basic OH⁻ groups of its non-apatitic surface termination enriched in HPO₄²⁻ species.[26,27]

The immobilization of cobalt on both HAp-R<sub>SSA</sub> supports was found to improve the basic MBOH conversion for both series, in line with previous data reported by Carvalho et al.[25] Note that the introduction of Co on the HAp-R<sub>SSA</sub> supports resulted in a more pronounced promotion of the basicity of the materials for the HAp-R<sub>37</sub>-Co(wt%) series compared to the HAp-R<sub>88</sub>-Co(wt%) series (Figure 7), although this basic promotion decreases as the cobalt loading increases. Acid-base pairs play a key role in the conversion of MBOH, since the anionic MBO⁻ intermediate, formed by deprotonation at a basic site, is transiently stabilized by an acidic site.[36] The exchange of calcium for cobalt modifies the acid-base balance of the Co-modified HAp materials, with the extent of this change varying with the Co loading. On the one hand, the increase in the intrinsic strength of the basic sites may reflect the greater ability to generate O²⁻ species from OH⁻ groups due to the presence of cobalt, and/or the modification of the local environment of the basic sites in the vicinity of Co species. On the other hand, the Co enrichment should also modify the intrinsic acidic properties of the Co-modified HAp materials: the stronger acidity of Co²⁺ cations compared to Ca²⁺ cations could promote the stabilization/formation of the anionic MBO⁻ intermediate and its transformation into acetone and acetylene products. These assumptions obviously raise the question of the proximity of the cobalt species, introduced
<figure><img src="image_19.png" /></figure>

by cationic exchange, to the basic sites derived from the OH⁻ groups and the nature of the exchangeable Ca²⁺ cations, which will be investigated in the next *DFT section*.

The promotion of the basic reactivity of HAp s by cobalt was also observed to various extents when Co²⁺ immobilization was carried out in a basified medium (Figure 7). These oper- ating synthesis conditions resulted either in the formation of Co₃O₄ NPs on HAp-R₃₇-Co(1.13)-b after thermal treatment or in the incorporation of Co into a cobalt-substituted HAp solid solu- tion embedding the HAp-P₃₇-Co(1.20)-b platelets. If in the latter case the effect remains quite small, the origin of the significant promotion of the HAp basicity on the introduction of Co remains unclear for HAp-R₃₇-Co(1.13)-b (Figure 7), even if confirmed for other Co loadings (not shown).

### 3.3.4. DFT Investigation: Location of Co²⁺ Cations in the Co-Exchanged HAp s

The hydroxyapatite structure shows two different types of crys- tallographic sites for Ca²⁺ cations. The Caᵢ cations, shown as dark blue spheres in Figures 8 and S1, form linear columns along the c-axis, whereas the Caᵢᵢ sites are organized in equilateral triangles alternately rotated by 60°. The Caᵢᵢ cations, belonging to trian- gles with different orientations, can be distinguished in Figures 8 and S1 as light blue and cyan spheres. These Caᵢᵢ cations delimit the so-called apatitic hexagonal channel, which is a linear series of ovoid cavities[38] in which the OH groups are stacked. The characteristic distances between atoms in the HAp structure are given in Supporting Information 1. As expected from the hexag- onal symmetry, crystalline rod-like particles mainly expose (100) faces.[26,39] DFT calculations were performed to identify the sur- face calcium sites involved in the exchange of calcium for cobalt during cobalt immobilization under natural pH conditions. In a previous study, the (100) hydroxyapatite surface termination was modelled by considering a perfect bulk crystal and opti- mizing the surface termination by comparing different cleavage planes.[26] While a flat surface termination was most commonly considered in previous studies,[40] we found that the zigzag ter- mination (Figure S1) was more stable with a lower surface energy (ΔE = -0.48 eV) than that of the flat surface.[26] The better sta- bility of this zigzag (100) surface compared to the flat surface is also supported by its lower Ca/P molar ratio compared to that of bulk HAp, which is consistent with the surface composition determined by XPS (Ca/P molar ratios of 1.27 and 1.24 for HAp- R₃₇ and HAp-R₈₈, respectively) compared to the bulk Ca/P molar ratios of 1.75 and 1.68 determined by XRF.[26] In contrast to the flat termination, this zigzag termination has hydroxyl groups emerg- ing from the tunnels on its surface, which have been shown by in situ and *operando* studies to be crucial for the basic reactivity of HAp s.[21,41]

As the present study aimed to investigate cationic exchange in aqueous solution, such a zigzag surface termination was first fully hydrated by the adsorption of five water molecules per 1 × 1 × 3 supercell (Supporting Information, Figure S1c). To identify the surface location of exchangeable calcium cations for cobalt with increasing cobalt surface density, a correspond- ing surface-hydrated 2 × 2 × 2 supercell structure was built.

As shown in Figure 8, each incremental cationic exchange on the corresponding 2 × 2 surface results in an increase in the cobalt density of 0.38 Co nm⁻². The first six exchanged cobalt atoms therefore increase the cobalt surface density to 2.28 Co nm⁻², which corresponds to the maximum Co surface density observed experimentally (2.15 Co nm⁻², Table 1). The relaxation of this hydrated zigzag surface resulted in a reduction of the surface Caᵢ-Caᵢ and Caᵢᵢ-Caᵢᵢ distances compared to those found in the bulk HAp structure, and a greater dispersion of the Ca–OH distances (Supporting Information 1). For each incre- mental exchange step, the energy minimization of the relaxed structures was achieved by comparing the positions of the exchanged cobalt cation at all of the non-equivalent calcium cation positions. As shown in Figure 8a, where only the surface layer is shown for the sake of clarity, the 2 × 2 surface typi- cally contains 4 OH groups, labelled O₁–₄H in Figures 8b and S3, and 20 calcium cations. Note that these numbers are reduced to only one and five non-equivalent OH and Ca surface posi- tions, respectively, namely three surface Caᵢᵢ cations and two Caᵢ cations for the initial (100) cleaved hydrated zigzag surface. In addition to the Caᵢᵢ calcium cation that is located on the outer- most surface, (Caᵢᵢ 2 in Figure 8a), the other two Caᵢᵢ cations are located in the inner zigzag surface and belong either to the same triangle (Caᵢᵢ 5) as the outer Caᵢᵢ 2 cation, or to a rotated neigh- boring triangle (Caᵢᵢ 3). The two non-equivalent inner Caᵢᵢ cations are located at a height comparable to that of the two non- equivalent Caᵢ cations (Caᵢ 7 and Caᵢ 8). The energies required for the exchange of a calcium cation with the first cobalt cation (Co¹st) and for surface relaxation have been estimated for the five configurations mentioned above. The least energy consuming exchange is associated with the substitution of the most external Caᵢᵢ 2 site (ΔEᵢₘcr. exch. = 1.71 eV, Figure 9). The additional energy required to alternatively exchange the other calcium cations are listed in Table S1. Compared to the other internal calcium cations, the upper position of Caᵢᵢ 2 makes it more accessible for exchange and facilitates relaxation (Figure 8a). As shown in Figure S3 (n = 1), this first cationic exchange resulted in a close proximity of Co¹st to a neighboring O₁H group with a Co¹st–O₁H distance of 1.904 Å compared to an initial Caᵢᵢ 2-O₁H distance of 2.257 Å in HAp (Figure S3, n = 0).

The substitution of a second cation (Co²ⁿ) was investi- gated by starting from the most favorable Co¹st configuration (Figure 8b). Due to the loss of symmetry caused by the substi- tution of Co¹st, all remaining calcium sites (19) of the supercell were considered. As reported in Table S1, the second exchange is the most favored at the Caᵢᵢ 14 site. This places Co²ⁿ in the inner surface and away from Co¹st (in the next crystallographic cell). In contrast to Co¹st, Co²ⁿ is quite far away from the clos- est O₄H group (3.550 Å, Figure S3, n = 2). The higher Co²ⁿ-O₄H distance compared to Co¹st-O₁H is due to the position of Co²ⁿ in a rotated triangle, which is associated with a long cation-OH distance (Figure S3). Note however that this Co²ⁿ-O₄H is slightly shorter than the initial Caᵢᵢ 14-O₄H distance (3.742 Å, Figure S3, n = 0). As shown in Figure 9, the energy cost for the second Co substitution (n = 2, ΔEᵢₘcr. exch. = 0.75 eV) is lower than that for the first Co substitution (ΔEᵢₘcr. exch. = 1.71 eV), probably due to its location far from Co¹st (9.98 Å in an adjacent unit cell) and/or
<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /><figcaption>**Figure 8.** Representations of the relaxed modelled 2 × 2 supercell of the (100) hydrated zigzag termination of hydroxyapatite with increasing cobalt density (0–2.28 Co nm⁻²): (a) no Ca²⁺ cationic exchanged, (b) CaII 2 exchanged for Co¹st, (c) CaII 14 exchanged for Co²nd, (d) CaII 12 or (d') CaII 11 exchanged for Co³rd, (e) CaII 11 or CaII 12 exchanged for Co⁴th, (f) CaII 6 exchanged for Co⁵th, (g) CaII 1 exchanged for Co⁶th. Cations are labelled from CaI to Ca20, but since CaI, Ca5, and Ca15 are close to the limits of the supercell structure, their CaI', Ca5', and Ca15' equivalents in the next cell are also shown. CaI, CaII in triangles with the same orientation and CaII in rotated triangles are shown as dark blue, light blue and cyan spheres, phosphorus and oxygen atoms are shown as grey and red spheres, respectively (P–O bonds are shown), H atoms of the hydroxyl groups are shown as black spheres and water molecules are shown in blue. The four non-equivalent surface OH groups are identified as OI, O₂, O₃, and O₄, as shown in Figure 8b.</figcaption></figure>

in an inner rotated triangle. In agreement with the last statement, the Co²⁺ exchange for a closer Ca²⁺ cation, such as CaII 4 at 4.76 Å, is indeed significantly less favorable with ΔEadd = 0.16 eV (Table S1).

Two energetically equivalent configurations were obtained for the location of the third Co²⁺ cation (Co³rd) on CaII 11 and CaII 12 sites of the outermost surface (Figures 8d and 8d', respectively). These two configurations for Co³rd have similar energy

costs to that of Co¹st, which was also exchanged on an external CaII cation (Figure 9). Irrespectively of its preferential position on CaII 11 or CaII 12, Co³rd is close to the neighboring OH (1.873 Å from O₄H in Figure S3, *n* = 3 or 1.917 Å from O₃H (configuration not shown), respectively).

The position of the fourth Co²⁺ cation (Co⁴th) was investigated by considering the two possible configurations for Co³rd located on either CaII 12 or CaII 11 and the 17 associated configura-
<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /><figcaption>Figure 9. Calculated incremental ΔE<sub>incr. exch.</sub> (empty symbols) and cumulative ΔE<sub>cum. exch.</sub> (full symbols) energies for the exchange of calcium cations by cobalt cations in the experimental Co surface density range (0–2.15 Co nm<sup>-2</sup>) (black and blue symbols).</figcaption></figure>

tions. Table S2 shows that Co⁴th is preferentially located on either CaII 11 or CaII 12 when not occupied by Co³rd, ultimately leading to an equivalent structure for the surface with 4 Co²⁺ cations (Figure 8e). The three closest Co²⁺ cations, Co²nd, Co³rd, and Co⁴th, form a Co trimer terminated by two different OH groups (Figure S3, n = 4). The position of Co⁵th (16 configurations explored, Table S1) was investigated by starting from the unique configuration found for n = 4. The substitution of a Ca²⁺ cation located at CaII 6 for Co²⁺ was found to be the most favorable (Figure 8f), and again this cationic exchange requires a similar energy cost as that of the two previous substitutions (n = 3 and 4) and the first substitution (n = 1, Figure 9). This is consistent with the fact that the Co⁴th position on CaII 6 was found to be energetically comparable to that on CaII 11 with a difference of only 0.01 eV (Table S1). Note that this Co⁵th cation belongs to the same triangle as that of Co¹st, resulting in an O₁H bridge on a Co¹st-Co⁵th dimer (Figure S3, n = 5). Finally, the position of the sixth Co²⁺ cation (Co⁶th) appeared to be energetically favored as a substituent of a Ca²⁺ cation at the CaII 1 site (Table S1) located on the outermost surface, as it was the case for Co¹st, Co³rd, and Co⁴th (Figure 8g). This Co⁶th cation is far from any other Co²⁺ cations, but is found to be close to an O₂H hydroxyl group with a Co⁶th-O₂H distance of 1.964 Å (Figure S3, n = 6).

### 3.4. Structure-Reactivity Relationships

The different preparation routes of the Co-HAps materials have a strong influence on the ODHP performance of the corresponding catalysts (Figure 3). This shows that the environment of the cobalt cations, which depends on the Co immobilization mechanisms, is a key parameter for the ODHP reaction (Figure 3a-c). The ODHP reaction is known to be a structure sensitive reaction[1] and it has been reported that the activation of

propane is favored by strong basic sites.[1,14b,15a] In particular, it has been shown that the basic O²⁻ species derived from OH⁻ groups, which are formed at high temperature under ODHP conditions, contribute to the activation of the C–H bond of propane in the case of the vanadium modified HAp system.[23] For the Co-modified HAp catalysts, structure-reactivity relationships should be discussed by considering both the influence of the Co²⁺ local environment and its surface accessibility and the role of the basic properties in the ODHP reaction.

#### 3.4.1. Influence of the Co Surface Density on the Catalytic Performance

In order to avoid any bias either due to the different specific surface area of the HAp-R<sub>88</sub>-Co(wt%) series compared to other catalysts, or due to the different surface accessibility of cobalt resulting from the different Co immobilization mechanisms (Table 1), the propane conversion should be better compared by considering the influence of the Co surface density (Figure 10) rather than the Co loading (Figure 3). NO-TPD has proved to be a robust method that allowed us to estimate the Co surface density of samples where Co was not fully accessible (Figure 6, Table 1). Figure 10a shows a close catalytic performance of the Co-modified catalysts prepared with two rod-like supports, consistent with the involvement of a common Co immobilization mechanism: namely, cationic exchange leading to isolated Co cations. The differences in the trends shown in Figure 3a can therefore be essentially attributed to differences in the specific surface areas of the two HAp-R<sub>SSA</sub>(Co wt%) series.

#### 3.4.2. Importance of the Proximity Between the Co Species and the Basic Sites in Co-Exchanged HAps

The DFT study of Co-exchanged HAps (Section 3.3.4) shows that all of the cobalt cations are exchanged exclusively with CaII sites within the experimental range of cationic exchange studied in the present work (up to 6 substituted Co²⁺ cations, Figure 8, Table S1). This may be due to the more open structure close to the hydroxyl groups (Figure 8), which makes it more flexible and probably facilitates the ease with which cationic exchange occurs at the CaII sites. Note that spatial constraints considerations have also been reported as a key parameter favoring type II substitution in the in the bulk HAp lattice.[18] Such a type II cationic site location for cobalt implies a proximity (Figure 11a) to the basic site derived from the O<sub>1.3</sub>H group with a Co-OH distance of ~1.9 Å, except for the incorporation of the second Co²⁺ cation (Figure S3). The latter Co species belongs to a rotated triangle (cyan color for the corresponding Ca²⁺ cations in Figures 8 and S3) and this deeper position results in a significantly longer Co²nd-O<sub>4</sub>H distance of ~3.5 Å. Note, however, that this distance remains compatible with the size of the propane molecule, whose maximum kinetic diameter is 4.3 Å.[42] Such a proximity between isolated surface redox Co species and basic sites leads to the optimal configuration for propene production (Figure 11a).
<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /><figcaption>**Figure 10.** Influence of the cobalt surface density (Co nm⁻²) on (a) propane conversion, (b) propene selectivity, and (c) propene yield; 20 mg under: 8.2% C₃H₈, 2.1% O₂ in He, 75.6 mL·NTP·min⁻¹ total flow rate at 400 °C. Black symbols correspond to Co deposition at natural pH on the HAp-R₃₇(●) HAp-R₈₈(▲) and HAp-P₃₇(◆) supports and blue symbols correspond to Co deposition obtained in a basified medium. The green symbols correspond to the C₃H₈ conversion and C₃H₆ selectivity measured for the bare HAp-R₃₇ support.</figcaption></figure>

HAp-P₃₇-Co(1.20)-b appears to be much less active than HAp-R₃₇-Co(0.22) and HAp-R₈₈-Co(0.48) in ODHP (Figure 10a, ▲), although all of these catalysts have comparable Co surface densities (Table 1). The fact that the Co-substituted surface layer of HAp-P₃₇-Co(1.20)-b results from a dissolution-reprecipitation mechanism[26] strongly suggests that the surface Co cations of this sample should be in a similar environment to that modelled using DFT (Figure 8) on the Co-exchanged HAp sam-

<figure><img src="image_26.png" /><figcaption>**Figure 11.** Schematic representation of the influence of the local environment of Co and the basicity of the Co-HAp materials on the ODHP catalytic performance: (a) isolated Co²⁺ species close to a strong basic site is the most favorable configuration for the selective production of propane from propane, while (b) in the absence of strong basicity, the isolated Co²⁺ species remain selective in propene but are poorly active, and (c) the combination of strong basicity with Co₃O₄ nanoparticles favors the overoxidation of propane to COₓ.</figcaption></figure>

ples (HAp-R₃₇-Co(0.22) and HAp-R₈₈-Co(0.48)), namely isolated Co²⁺ species at type II crystallographic sites of the HAp surface. In addition, this sample remains poorly basic (Figure 7) due to the lack of basic OH⁻ groups in its non-apatitic (OCP-like) precursor. As represented in Figure 11b, the low propane conversion of HAp-P₃₇-Co(1.20)-b is thus more likely due to the low basicity of this catalyst (Figure 7), even if the similar Co environment in HAp-P₃₇-Co(1.20)-b, HAp-R₃₇-Co(0.22) and HAp-R₈₈-Co(0.48) results in a similar propene selectivity (Figure 10b, ▲).

In contrast, HAp-R₃₇-Co(1.13)-b, whose Co surface species belong to Co₃O₄ nanoparticles,[26] shows significantly higher propane conversion and lower propene selectivity (Figures 10a,b ●) than HAp-R₃₇-Co(0.37) and HAp-R₈₈-Co(1.69), while all of these samples have similar Co surface densities (~1 Co nm⁻², Table 1). These data show that the Coₓ⁺ cations located on the surface of the Co₃O₄ nanoparticles also catalyze the ODHP reaction, but with a much lower selectivity than that of the isolated surface Co²⁺ species of the HAp-R₃₇-Co(0.37) and HAp-R₈₈-Co(0.88) samples (●, ●, ▲, Figure 10b), as represented in Figure 11c. It should also be recalled that HAp-R₃₇-Co(1.13)-b has a high basicity (Figure 7), although the origin of this basicity is still unclear. The fact that the propene yield of HAp-R₃₇-Co(1.13)-b is comparable to that of HAp-R₃₇-Co(0.37) and HAp-R₈₈-Co(1.69) (Figure 10c), may result from a trade-off between the improved basicity of HAp-R₃₇-Co(1.13)-b (Figure 7), which contributes to the activation/conversion of the propane molecule (Figure 10a, ●), and its lower propene selectivity (Figure 10b, ●) due to the overoxidation of the hydrocarbons associated with the rapid electron exchanges in the Co₃O₄ nanoparticles.[10]

As shown schematically in Figure 11, these results highlight that the local cobalt environment plays a key role in controlling propene selectivity, while a combination of the basicity and the cobalt environment is required for the selective propane activation. The much higher TOF data of the Co-HAp-RSSA catalysts of the present study compared to those reported on Co-based catalysts in previous works (Table 2) can be attributed to the absence of any reported basicity and/or the presence of polymeric oxide-like Co species in most of the previously reported Co-based samples.
<figure><img src="image_27.png" /></figure>

## 4. Conclusion

Cobalt-modified hydroxyapatites are very stable catalysts under the operating conditions used and they appeared to be up to two orders of magnitude more active than other cobalt-based systems in the ODHP reaction on a TOF basis. This outstanding catalytic performance can be attributed to the unique combination of i) the basic properties of the materials that contribute to the activation of propane, ii) the peculiar ability of the HAp matrix to favor the immobilization of isolated Co redox centers and iii) the complete accessibility of the latter redox centers on the top surface for propane adsorption. The basic properties of the materials were characterized using the model MBOH conversion reaction, while DFT modelling allowed us to describe the local environment of cobalt species on the HAp and to show that the exchange of Ca²⁺ for Co²⁺ cations occurs preferentially at CaII surface sites in the range of the Co surface densities investigated experimentally. Finally, the Co accessibility was investigated by NO-TPD. The best Co-modified HAp ODHP catalysts were obtained when Co was deposited with a cationic exchange of the surface Ca²⁺ sites in excess solution at natural pH (slightly acidic) on rod-shaped crystalline basic HAp s with high Ca/P molar ratios prepared by precipitation at high pH. In this case, the Co redox centers are expected to be in close proximity to the basic sites as indicated by DFT.

In contrast, the deposition of Co under basic conditions can proceed by an SEA immobilization mechanism, in which the interaction of Co with HAp is weaker than that of the Co species immobilized with the cationic exchange mechanism and leads to the formation of Co₃O₄ nanoparticles after annealing, producing catalysts with more limited cobalt accessibility, as probed by NO-TPD. In the presence of such Co₃O₄ nanoparticles, the transformation of the propane molecule becomes much less selective, illustrating Grasselli's principle of the need for site isolation to promote the olefin selectivity in mild oxidation reactions.[10] When a cobalt-substituted solid solution is formed by a dissolution-precipitation mechanism under basic conditions on HAp platelets, this results in the exposure of isolated surface Co cations whose ODHP performance remains limited due to the limited basicity of the corresponding materials.

Beyond the outstanding performance of such hydroxyapatite-supported cobalt catalysts for the ODHP reaction, this study paves the way for extended applications of these original catalysts. The precise control of both Co dispersion and acid-base properties makes these multifunctional catalysts very attractive for other structure-sensitive reactions catalyzed by Co-based materials, such as the oxidative dehydrogenation of other alkanes,[43] epoxidation reactions,[44] and the upgrading of alcohols.[25]

## Acknowledgements

The authors thank Sorbonne University and CNRS for providing funding (Credits récurrents) and access to the analytical equipment used in this work. C.R. gratefully acknowledges ENS Saclay for financial support (Ph.D. Grant no. 3415/2019). The authors

would also like to express their gratitude to Dr. H. Toulhoat (LRS) for his invaluable recommendations on the DFT calculations to J-M. Krafft (LRS) for his help for the FTIR set-up and to S. Nowak of the Université Paris Cité for the XRF analysis.

## Conflict of Interests

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Keywords:** Basicity · Cation exchange · Co accessibility · Co surface density · DFT · Morphology · NO-TPD · Propene selectivity

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Manuscript received: December 20, 2024

Revised manuscript received: March 14, 2025

Accepted manuscript online: March 18, 2025

Version of record online: April 21, 2025
OXIDATIVE DEHYDROGENATION OF PROPANE ON
HYDROXYAPATITE SUPPORTED COBALT CATALYSTS.

Corentin Reynaud,<fnref n="1" /> Cyril Thomas,<fnref n="1" /> Dalil Brouri,<fnref n="1" /> Josefine Schnee,<fnref n="1" /> Sandra Casale,<fnref n="1" /> and

Guylène Costentin<fnref n="1" />*

<footnote n="1">¹ Sorbonne Université, CNRS, Laboratoire de Réactivité de Surface, LRS, 4 place Jussieu, F-75005 Paris, France</footnote>
**SI1: building of the hydrated (100) surface model.**

The surface model consisted of slabs containing three hydroxyapatite unit cells with two (100) surfaces as shown in **Figure S1** (before (a) and after (b) relaxation of the ions). Hydrated surfaces were built stepwise by adding one H₂O molecule on the two (100) surfaces of the slab. Both molecular adsorption and dissociative adsorption of H₂O, leading to the formation of Ca-OH and (P-OH) species, were studied. All of the Ca²⁺ cation adsorption sites and possible H₂O adsorption modes were considered as starting points for a new optimisation with relaxations leading to the optimal configuration (lowest energy) with one H₂O molecule. This procedure was repeated iteratively with up to five H₂O molecules on each (100) surface, finally resulting in the hydrated surface shown in **Figure S1c**. The distances between neighbouring Ca-I-Ca-I cations (dark blue spheres in **Figure 1**) in the hydrated surfaces were shortened compared to those in the bulk (3.15-3.49 Å vs 3.33-3.36 Å). Similarly, the distances between Ca-II-Ca-II surface cations belonging to the same triangle were found to be 3.90 Å (light blue spheres in **Figure 1**), compared to 4.10 Å in the bulk structure. Finally, the distances between Ca-II-Ca-II cations belonging to two adjacent rotated triangles were found to be 3.93-4.14 Å (4.20 Å in the bulk). The distance between the Ca²⁺ and OH⁻ ions is reduced to 2.26-2.57 Å in the zigzag surface structure compared to the bulk structure (2.38-2.42 Å).
<figure><img src="image_29.png" /><figcaption>**Figure S1:** Zig-zag HAp (100) surfaces hydrated with 5 H₂O molecules built by a) extracting 3 unit cells from the bulk structure and creating 2 surfaces, b) relaxing the ion positions, and c) determining the optimal surface resulting from the successive addition of 5 H₂O molecules. CaI and CaII cations in triangles with the same orientation and CaII cations in rotated triangles are shown as dark blue, light blue and cyan spheres, phosphorus and oxygen atoms are shown as grey and red spheres, respectively (P-O bonds are shown), H atoms of the hydroxyl groups are shown as black spheres and water molecules are shown in blue.</figcaption></figure>
**Electron microscopy.** TEM and HRTEM characterisations were carried out at the *Plateforme de microscopie de l'Institut des Materiaux de Paris Centre*, using a JEOL JEM 2100 microscope operating at 200 kV, emitting with a LaB₆ tip equipped with an X-ray detector (Oxford SDD X-max 80). Micrographs were taken using a 4 K camera (Gatan Orius). Samples were first suspended in ethanol and dispersed in an ultrasonic bath before rapid deposition onto a carbon-coated copper grid.

<figure><img src="image_30.png" /><figcaption>b)</figcaption></figure>

<figure><img src="image_31.png" /></figure>

**Figure S2:** Co₃O₄ nanoparticles on thermally treated HAp-R₃₇-Co(1.13)-b detected in a) TEM (red circles) and b) HRTEM micrographs with fringe indexing at 3.4 Å characteristic of the (002) plane of HAp and at 2.9 and 2.4 Å characteristic of the (220) and (311) planes of Co₃O₄ particles.
<table><caption>**Table S1:** Identification of the most favourable exchangeable Ca²⁺ sites for cobalt cations (grey lines) and the additional energy cost (ΔE<sup>add</sup>) required compared to that required for the most stable configuration. Only the configurations with ΔE<sup>add</sup> ≤ 0.35 eV are shown for *n* > 1.</caption><thead><tr><th>Co<sup>nth</sup> cation</th><th>Calcium sites investigated for substitution</th><th>ΔE<sup>add</sup> <sup>$</sup><br>(eV)</th></tr></thead><tbody><tr><td rowspan="5">Co<sup>1st</sup></td><td>Ca<sub>II</sub>2</td><td>0.00</td></tr><tr><td>Ca<sub>II</sub>5</td><td>0.39</td></tr><tr><td>Ca<sub>II</sub>3</td><td>0.58</td></tr><tr><td>Ca<sub>I</sub>8</td><td>0.69</td></tr><tr><td>Ca<sub>I</sub>7</td><td>0.90</td></tr><tr><td rowspan="3">Co<sup>2nd</sup></td><td>Ca<sub>II</sub>14</td><td>0</td></tr><tr><td>Ca<sub>II</sub>4</td><td>0.16</td></tr><tr><td>Ca<sub>I</sub>7</td><td>0.33</td></tr><tr><td rowspan="4">Co<sup>3rd</sup></td><td>Ca<sub>II</sub>12</td><td>0</td></tr><tr><td>Ca<sub>II</sub>11</td><td>0.0</td></tr><tr><td>Ca<sub>I</sub>17</td><td>0.10</td></tr><tr><td>Ca<sub>I</sub>7</td><td>0.30</td></tr><tr><td rowspan="4">Co<sup>4th</sup></td><td>Ca<sub>II</sub>11</td><td>0</td></tr><tr><td>Ca<sub>II</sub>6</td><td>0.01</td></tr><tr><td>Ca<sub>II</sub>1</td><td>0.25</td></tr><tr><td>Ca<sub>II</sub>5</td><td>0.35</td></tr><tr><td rowspan="3">Co<sup>5th</sup></td><td>Ca<sub>II</sub>6</td><td>0</td></tr><tr><td>Ca<sub>II</sub>1</td><td>0.11</td></tr><tr><td>Ca<sub>I</sub>8</td><td>0.30</td></tr><tr><td rowspan="7">Co<sup>6th</sup></td><td>Ca<sub>II</sub>1</td><td>0</td></tr><tr><td>Ca<sub>I</sub>8</td><td>0.12</td></tr><tr><td>Ca<sub>II</sub>5</td><td>0.15</td></tr><tr><td>Ca<sub>I</sub>7</td><td>0.22</td></tr><tr><td>Ca<sub>I</sub>10</td><td>0.23</td></tr><tr><td>Ca<sub>II</sub>13</td><td>0.33</td></tr></tbody></table>
**Figure S3:** Local environment of the four O₁·₄H surface hydroxyl groups in the 2×2×2 supercell with increasing Co density (n = 0-6). The reported Ca-OH, Co-OH and Co-Co (pink) distances (Å) correspond to those measured after surface relaxation; As n increases, Co-Ca, then Co-Co bridging hydroxyl groups are formed, designing monomeric, dimeric and trimeric Co entities. The color codes and the numbering of calcium atoms and O₁·₄H hydroxyl groups are the same as those used in **Figure 8**, Ca-OH and Co-OH bond distances are shown in black and O-H distances in red.

<figure><img src="image_32.png" /></figure>

* for n = 0, O₁·₄H hydroxyl groups are all equivalent and there are only 5 non-equivalent Ca atoms, but for a sake of simplicity, their labelling is the same as those used when the Co introduction leads to a loss of symmetry.