# Rational engineering of hydroxyapatites for sustainable chemicals, H 2 , biofuels and CO 2 conversion

Kang Hui Lim, Ming Hui Wai, Keyu Cao, Sonali Das, Ange Nzihou, Sibudjing

Kawi

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Kang Hui Lim, Ming Hui Wai, Keyu Cao, Sonali Das, Ange Nzihou, et al.. Rational engineering of hydroxyapatites for sustainable chemicals, H 2 , biofuels and CO 2 conversion. Energy Materials, 2025, 5 (10), 10.20517/energymater.2025.63. hal-05238652

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# Rational engineering of hydroxyapatites for sustainable chemicals, H₂, biofuels and CO₂ conversion

Kang Hui Lim<fnref n="1" />,#, Ming Hui Wai<fnref n="1" />,#, Keyu Cao<fnref n="1" />, Sonali Das<fnref n="2" />, Ange Nzihou<fnref n="3" />,<fnref n="4" />,<fnref n="5" />, Sibudjing Kawi<fnref n="1" />,*

*Correspondence to: Prof. Sibudjing Kawi, Department of Chemical and Biomolecular Engineering, National University of Singapore, 1 Engineering Drive 3, Singapore P.O. Box 117585, Singapore. E-mail: chekawis@nus.edu.sg

**How to cite this article:** Lim, K. H.; Wai, M. H.; Cao, K.; Das, S.; Nzihou, A.; Kawi, S. Rational engineering of hydroxyapatites for sustainable chemicals, H₂, biofuels and CO₂ conversion. *Energy Mater.* **2025**, *5*, 500131. https://dx.doi.org/10.20517/energymater.2025.63

**Received:** 31 Mar 2025 **First Decision:** 7 May 2025 **Revised:** 14 May 2025 **Accepted:** 27 May 2025 **Published:** 26 Jun 2025

**Academic Editor:** Yuhui Chen **Copy Editor:** Fangling Lan **Production Editor:** Fangling Lan

## Abstract

Chemical and fuel production through conventional catalytic processes requires significant improvement to reduce CO₂ emission levels. Sustainable, potentially direct, chemical production is in critical need, such as methane coupling to olefins, propane dehydrogenation to propylene, biodiesel production, and greenhouse gas emissions modulating reactions, including CO₂-utilizing dry reforming of methane, partial oxidative of methane, CO oxidation, CO₂ methanation are some of the emerging technologies to address sustainability challenges. These technologies remained constrained due to the lack of stable and efficient catalysts. Hydroxyapatite (HAP), a highly functional versatile material, offers great potential due to its flexible tunability, and several studies have outlined the synthesis protocol of HAP and design modifications for catalytic applications. However, a comprehensive understanding of connecting reaction-specific demands to tailor HAP catalyst designs is limited. In this review, we bridge that gap, highlight key challenges in catalytic reactions, and propose the necessary HAP catalyst modifications, including acid-base tuning, defects-induced lattice oxygen or vacancies, mesoporosity modulation, and catalyst active metal species dispersion, to improve catalytic performance by limiting catalyst deactivation from absorbates surface poisoning, sintering, and coking. Finally, future research areas for improvement for HAP catalysts are suggested to advance the maturity of catalytic technologies.

**Keywords:** Catalysis, dry reforming, CO₂ utilization, renewable energy, cation substitution, anion substitution

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© The Author(s) 2025. **Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, sharing, adaptation, distribution and reproduction in any medium or format, for any purpose, even commercially, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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<footnote n="1">¹Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore P.O. Box 117585, Singapore.</footnote>

<footnote n="2">²Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai 400076, India.</footnote>

<footnote n="3">³Centre RAPSODEE, UMR CNRS 5302, IMT Mines Albi, Université de Toulouse, Campus Jarlard, Albi F-81013, France.</footnote>

<footnote n="4">⁴School of Engineering and Applied Science, Princeton University, Princeton, NJ 08544, USA.</footnote>

<footnote n="5">⁵Andlinger Center for Energy and the Environment, Princeton University, Princeton, NJ 08544, USA.</footnote>

<footnote n="6">⁶Authors contributed equally.</footnote>
## INTRODUCTION

Attenuating CO₂ concentration is crucial for mitigating climate change, the key challenge of the 21st century. Creating sustainable, long-term, and economically viable methods for CO₂ capture and valorization is crucial, with heterogeneous catalysts becoming increasingly vital to contemporary sustainable energy strategies[¹]. The application of heterogeneous catalysts enables fine chemical and energy production to occur more sustainably. The development of heterogeneous catalysts requires an intimate and fundamental understanding of the catalyst surface at the nano-scale to drive desired catalytic reactions selectively and actively with stable performance. A significant amount of literature has been dedicated to materials for heterogeneous catalysts such as carbons[²,³], metal-organic framework (MOF)[⁴⁻⁶], and metal oxides[³,⁷,⁸]. Other than these classes of catalyst support, hydroxyapatite (HAP)-based catalysts have received increasing research attention in the past few decades, as evidenced by an increasing number of publications in Figure 1A.

HAP is a bio-compatible material that constitutes the main mineral component in bone and teeth and is responsible for hardness and strength. This bio-compatible material stirs great interest in many fields due to its remarkable structure and inherent properties. The surface of HAP material contains both acidic and basic elements such as P and Ca, respectively[⁹]. HAP has often been used as a catalyst or catalyst support for liquid-phase organic reactions, namely, the Suzuki reaction, alcohol oxidation and furfural amination reactions[¹⁰,¹¹]. Surface engineering strategies, based on a rational understanding of HAP material properties, are required to unlock the potential of HAP materials for enhanced catalytic performance, product selectivity, and catalytic stability. This is particularly important when HAP materials are to be used as heterogeneous catalysts. Ibrahim *et al.* had written a comprehensive review on the usage of HAP for air and water pollution control[¹²]. Kaneda and Mizugaki comprehensively covered HAP as a heterogeneous catalyst for organic synthesis reactions and pointed out the possibility of using HAP to solve environmental and energy issues[¹³]. The synthesis route of HAP material for liquid phase reaction has been critically covered by Fihri *et al.*[¹⁴] Sadat-Shojai *et al.* covered HAP synthesis using various methods for different morphologies at length[¹⁵]. Recently, a comprehensive textbook on the use of HAP as a catalyst has been covered by Pham Minh *et al.* The improvements in catalytic performance and product selectivity in using HAP catalysts have motivated further research and development to better understand and tune their catalytic properties for the following catalytic reactions for sustainable fuel and chemical production[¹⁶]. To conduct a holistic understanding of the role of HAP-based catalysts, we first introduce the advantages and gaps of HAP in sustainable chemicals processes (i.e., oxidative CH₄ coupling to higher order hydrocarbons (OCM), oxidative propane dehydrogenation to propylene (PDH), and biodiesel production), hydrogen production processes (i.e., dry reforming of methane (DRM), partial oxidation of methane (POM), and water gas shift (WGS), CO oxidation coupled with H₂ fuel cell), and CO₂ utilization processes (i.e., CO₂ hydrogenation to CO or CH₄). A fundamental understanding of the HAP material will be subsequently discussed. Subsequently, a focused review of the various surface engineering strategies for HAP-based catalysts and consequential surface effects will be presented [Figure 1B]. A research perspective and outlook for the various surface engineering strategies will be discussed at the end of these sections.

## FUNDAMENTAL UNDERSTANDING OF HAP FRAMEWORK

### HAP structure

HAP has a hexagonal (P63/m space group) crystal cell with lattice parameters a = 9.37 Å and c = 6.88 Å[¹⁷]. In the HAP crystal structure, two distinct types of calcium (Ca) atoms occupy different positions within the unit cell [Figure 2A][¹⁸]. Consequently, the chemical formula is expressed as Ca<sub>(I)</sub>Ca<sub>(II)6</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>2</sub>. The HAP framework is composed of a dense network of PO₄ tetrahedra, with each PO₄ group contributing to a
<figure><img src="image_6.png" /><figcaption>**Figure 1.** (A) Number of publications using keywords "hydroxyapatite" and "catalyst" from the year 2000 to the year 2025 from Web of Science as of 28 March 2025. (B) Schematic of the various surface engineering strategies for HAP.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 2.** (A) HAP hexagonal crystal structure unit cell with two Ca sites, Ca(II) and Ca(III) sites. Reproduced from Ref.[18] under CC BY 4.0 license. (B) Product selectivity at 50% ethanol conversion: (▲) total Guerbet alcohols, (●) 1,3-butadiene (■), ethylene. Reproduced from Ref.[19] with permission from Elsevier. (C) Moles of Ca²⁺ released to the solution as a function of moles of Pb²⁺. Reproduced from Ref.[20] with permission from Elsevier. (D) ⁴³Ca MAS spectra of PbₓCa₁₀₋ₓ(VO₄)(PO₄)₅(OH)₂ (x = 0, 2, 4). Reproduced from Ref.[21] with permission from American Chemical Society.</figcaption></figure>
column structure that defines two types of discrete channels. The first channel has a diameter of 2.5 Å with adjacent Ca²⁺ ions. The adjacent Ca²⁺ ions are in the Ca(II) location and these Ca²⁺ play a significant role in modulating the acid-base and electrical properties of HAP material. HAP allows large variations in composition and can be a highly non-stoichiometric solid. Stoichiometric HAP has the chemical formula of Ca₁₀(PO₄)₆(OH)₂ where the ratio Ca/P is 1.67. Non-stoichiometric HAP has Ca/P < 1.67 with chemical formula of Ca₁₀-x(HPO₄)ₓ(PO₄)₆-x(OH)₂-x where 0 < x < 1. The OH⁻ groups are arranged along the c-axis in the HAP unit cell to balance the positive charge of the matrix. The substitution of cations or anions within the lattice can lead to vacancy defects at calcium or OH⁻ sites. Alternatively, such substitutions may induce structural disorder, resulting in the formation of an amorphous phase.

The stoichiometric HAP crystals can exist in two forms: hexagonal (lattice parameters a = b = 9.432 Å, c = 6.881 Å, and γ = 120°) and monoclinic (lattice parameters a = 9.421 Å, b = 18.840 Å, c = 6.881 Å, and γ = 120°)[²²⁻²⁴]. To maintain charge balance in the lattice and surface, the two polymorphs exhibit different surface OH orientations. In the monoclinic form, all the OH groups face in the same direction. As for the hexagonal HAP, the adjacent OH points in opposing directions[²⁴]. The transformation from the monoclinic phase to the hexagonal phase can occur due to synthesis methods, temperature change, and cation exchange[²²⁻²⁴]. The transition from a hexagonal structure to a monoclinic structure is found to be reversible so long the annealing temperature does not go beyond 200 °C[²⁵]. The poly-morphology of metal oxide catalysts can influence catalytic activity and product selectivity as reported by several authors[²⁶,²⁷]. For example, when Ru is deposited onto different phases of TiO₂ - rutile, anatase, or a mix of both, the crystal facets of the TiO₂ support can affect the extent of metal-support interaction and modify the nature and activity of Ru species for CO₂ methanation[²⁸]. In a similar analogy for the HAP catalyst support, different crystal structures exhibited by the HAP catalyst support can influence catalytic activity and product selectivity - we elaborate on this in later sections. Hence, this unique property of the HAP structure can be used to engineer the HAP surface for enhanced catalytic activity and product selectivity.

## Importance of the Ca/P ratio

As the apatite material consists of calcium cation and phosphate anion in the unit cell, the surface properties of the apatite material are intimately connected to the apatite surface composition. When the Ca/P is at a stoichiometric ratio of 1.67, there are more basic sites than acidic sites, which results in HAP displaying basic catalyst behavior. HAP surface displays acidic behavior when the Ca/P = 1.50, which constitutes a non-stoichiometric HAP. The mix of basic and acid sites imbues the catalyst surface with amphoteric properties if the Ca/P is between 1.5-1.67. However, it is also important to note that the bulk Ca/P ratio is observed to be greater than the surface Ca/P ratio[¹⁹]. Thus, the HAP surface can be non-stoichiometric in nature. HAP crystal tends to grow along the c-axis (i.e., rod-like HAP) and expose more of the a-faces, which are rich in Bronsted acid phosphate groups[¹⁹].

The implications of the Ca/P ratio on the surface properties of the HAP support material can be observed from the differences in catalytic activity and product selectivity as noted by several authors[¹⁹,²⁹,³⁰]. HAP had been used as a catalyst for Guerbet alcohol synthesis, wherein the presence of more acidic sites in HAP catalyst at varying Ca/P has shown a clear influence in favoring ethylene selectivity while more basic sites favor 1-butanol selectivity as shown in Figure 2B[¹⁹]. If an active transition metal such as nickel is incorporated into the HAP support, the Ca/P can also be varied to induce CaO to exsolve from the HAP surface which can help to tune metal-support interaction effects[³¹,³²]. Furthermore, the decrease of Ca/P can induce the presence of more acidic sites which can help to anchor the nickel nanoparticles onto the HAP support, thus improving catalyst stability.
Cationic substitutability

The calcium cation site in the apatite unit cell is flexible enough to accommodate transition metal cations that are similar to its ionic radii. The calcium cations in Ca<sub>(I)</sub>Ca<sub>(II)6</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>2</sub> HAP are situated in two different places in the framework, Ca<sub>(I)</sub> and Ca<sub>(II)</sub>. The coordinating number of the Ca<sub>(I)</sub> is 9, while the one for Ca<sub>(II)</sub> is 7. Such a difference suggests that Ca<sub>(I)</sub> would be preferentially occupied by other cations larger than Ca<sup>2+</sup>[33]. Transition metal cations can be incorporated into the HAP framework using coprecipitation or ion-exchange methods. In coprecipitation, two aqueous solutions are typically prepared, one with Ca<sup>2+</sup> and cation, and another with the phosphate precursor. The pH of both solutions should be similar for concurrent precipitation of Ca<sup>2+</sup> and P.

The calcium sites in the HAP lattice are also amenable to substitution by another cation from transition metal elements or lanthanide elements[34]. The calcium sites can be substituted by monovalent (e.g., Na+, K+, Ag+, etc.), divalent (e.g., Cu2+, Ni2+, Zn2+, etc.), and trivalent (e.g., Ce3+, Co3+, etc.) metals. However, the extent of difficulty in substituting the Ca sites could be dependent on the electronegativity of the element, assuming the same ionic radius[35]. For monovalent cations such as Na+, K+, and Li+, Na+ has a better affinity for cation substitution into the HAP lattice due to similar ionic radii as Ca2+ (0.9 Å). Thus, the order of affinity decreases in the following manner: Na+ (1.0 Å) > K+ (1.4 Å) > Li+ (0.7 Å)[36]. The preference for cation exchanges due to differences in ionic radii relative to the Ca2+ ionic radii is similar for divalent components. Takeuchi et al. observed that Pb2+ was exchanged faster than Cu2+ and Cd2+ due to the same ionic radii as Ca2+ in the single-component adsorption study[37]. In a dual-component adsorption study, the Cu2+ was more difficult to exchange with Ca2+ and was attributed to the bigger ionic radii of Cu2+ (1.3 Å) as compared to Ca2+ (1.1 Å). However, Cd2+ (1.1 Å), being of the same ionic radii as Ca2+, was also more difficult to exchange because of lower electronegativity than Pb2+. Furthermore, the hydrated Cu2+ and Cd2+ ions need to be dissociated before exchanging with Ca2+. Thus, there is a possible additional rate-determining step that slows the rate of ion exchange. Further observation by Bailliez et al. noted that the Pb2+ cations have largely occupied the Ca<sub>(II)</sub> position to form Pb<sub>10-x</sub>Ca<sub>x</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>2</sub> solid solution[20]. In Figure 2C, the gradient of the slope is close to unity, which indicates an equimolar exchange of Pb2+ and Ca2+ cations. The Pb2+ cations are first adsorbed onto the HAP surface, especially the POH group, via surface complexation, which occurs very quickly. After complexation, Pb concentration saturates and slows down. Adsorption continues due to the high surface area of HAP powder but at a slower rate as the surface adsorption approaches equilibrium. The HAP surface may be heterogeneous, meaning some areas have lower affinities for Pb2+ cations[20]. The Langmuir-Freundlich model fits better than the Langmuir or Freundlich models, suggesting a heterogeneous HAP surface with varying affinities for cationic adsorption. This indicates two things: first, cations adsorb onto the HAP surface as a monolayer through an ion-exchange mechanism. Second, multiple active sites participate in cation adsorption and integration into the HAP lattice via cation substitution, explaining the Langmuir-Freundlich fitting results[38].

Various cations with differing ionic radii can alter unit cell dimensions when replacing calcium. For example, substituting Ca²⁺ with Mg²⁺ decreases the lattice constant c by 0.33% and increases constant a by 0.1%, leading to significant lattice disturbances and possibly more defects in HAP surfaces[39]. Introducing metal cations smaller than Ca shrinks the lattice unit cell and increases the X-ray diffraction (XRD) Bragg angle, resulting in smaller HAP size and greater irregularity shape. Cu²⁺ and Zn²⁺ preferentially substitute at the Ca<sub>(II)</sub> to form a 4 to 5-fold coordination with the HAP support[40]. Solid-state nuclear magnetic resonance (NMR) can be used to determine whether the cation substitutes at the Ca<sub>(I)</sub> or Ca<sub>(II)</sub> site. Pizzala et al. used solid-state NMR and observed chemical shift in ⁴³Ca NMR, as compared to pure HAP phase, for different concentrations of substituting cations of Pb and V, as shown in Figure 2D[21]. Another technique to confirm cationic substitution is Raman spectroscopy. The characteristic peak belonging to PO₄³⁻ can be shifted to a lower Raman shift if the Ca cation is substituted with another metal cation. For divalent cations with larger
ionic radii, such as Pb²⁺, the substitution increases both lattice constant a and c by around 0.05% and 0.04%, respectively[³⁵]. These changes in the crystal lattice parameters often cause changes in crystallinity, thermal stability, and morphology and may influence product selectivity in DRM and thermal catalytic CO₂ hydrogenation reactions[⁴¹].

Ionic radii depend on valence electron density and the effective nuclear charge of cations. For the same nuclear charge, higher valence charge leads to smaller ionic radii due to stronger electrostatic attraction. Conversely, decreased valence charge results in larger ionic radii. Structural defects may form to accommodate other metal ions and maintain charge neutrality. HAP uniquely preserves the crystallographic symmetry of P63/m even when Ca/P drops below 1.67. Density functional theory (DFT) computational studies provide insights into HAP structure, indicating how different cations substitute in Ca(I) or Ca(II) sites[⁴²]. Ellis et al. combined DFT with experimental methods such as X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) to identify the preferred Ca site for Pb substitution, demonstrating that Pb²⁺ favors the Ca(II) sites[⁴³].

For trivalent cations such as Ce³⁺, Fe³⁺, and Al³⁺, which have ionic radii slightly larger than Ca²⁺, the extent of cation substitution into the HAP lattice is again dependent on the electronegativity of the cations. The coprecipitation method was utilized to synthesize the cation-substituted materials. The authors observed morphological changes by adding different trivalent components[⁴⁴]. The ones added with Fe³⁺ had a longer nanorod morphology than those of Al³⁺ and La³⁺. Furthermore, the addition of such trivalent elements into the HAP lattice brought about reduced crystallinity, as observed from the XRD data. This indicates that ion-exchanged HAP, be it through coprecipitation or immersion, becomes either more amorphous or has reduced particle size. Although La³⁺ has similar ionic radii to Ca²⁺, the order of ionic substitution goes in the following order: Al³⁺ = Fe³⁺ > La³⁺. The charge density goes in the following order: Al³⁺ > Fe³⁺ > La³⁺. Hence, the extent of ion exchange with such trivalent cations appears to be dependent on the component charge density. Cerium can also be substituted into the HAP lattice to create highly amorphous HAP phases[⁴⁵]. Furthermore, as the ionic radius of Ce is larger than Ca²⁺, the lattice constants a and c are observed to increase with a corresponding increase in Ce concentration[⁴⁶]. A possible reason behind the amorphousness of the Ce-HAP could be the presence of mixed Ce³⁺ and Ce⁴⁺ cations in the HAP lattice, which creates a higher concentration of oxygen vacancies on the HAP surface. The presence of oxygen vacancies that would be beneficial in several catalytic reactions will be discussed in later sections.

## Anionic substitutability

The HAP structure contains two anionic sites located at the OH⁻ group and the PO₄³⁻ group. The most common anionic substitution is the carbonate anionic substitution. In such anionic substitution, the surface hydroxyl (OH⁻) is substituted with the carbonate group (CO₃²⁻), also known as A-site substitution. The A-site substitution results in the HAP chemical formula in the following manner: Ca₁₀(PO₄)₆(OH)₂·2k(CO₃)ₖ. Another form of anionic substitution is the substitution of the phosphate group. The phosphate group (PO₄³⁻) in the HAP structure can also be substituted by other anions such as F⁻, Cl⁻, SiO₄³⁻, and CO₃²⁻. This is known as B-site substitution. This results in the HAP chemical formula being written in the following order: Ca₁₀₋ₗ(PO₄)₆₋ₗ(CO₃)ₗ(OH)₂₋ₗ, where 0 < y < 2. The CO₃²⁻ substitution in the B-site can cause an expansion of the c-axis and contraction of the a-axis as the ionic radius of the CO₃²⁻ group is larger than the phosphate group[⁴⁷]. The location of CO₃²⁻ substitution can be observed from Fourier Transform Infrared Spectroscopy (FTIR) spectra as the peak assigned to OH⁻ will be diminished if the CO₃²⁻ group substitutes the surface OH⁻[⁴⁸]. Furthermore, CO₃²⁻ substitution has been observed to cause a reduction in crystallinity and resultant smaller particle size. In addition, the partial substitution of the phosphate group with the carbonate group enables more facile cationic substitution due to more electrons withdrawn from the Ca cations[⁴⁰].
Anionic substitution at either of these two sites can tune surface acidity. CO₃²⁻ substitution at the A site can create two types of acid sites: strong acid sites from the ring of Ca²⁺ cations and very strong acid sites ascribed to proton contribution from the HPO₄²⁻ group[⁴⁸]. CO₃²⁻ substitution can create lattice mismatch as CO₃²⁻ is larger than OH⁻ but smaller than PO₄³⁻[⁴⁹]. As the surface acidity is altered by the presence of anionic substitution of the apatite cell structure, the catalytic activity and product selectivity can be altered. If F⁻ is substituted into Pb-HAP, the catalytic activity for oxidative coupling of methane can be favored towards ethylene as more electrons are donated to stabilize methyl intermediate[⁴¹]. Further discussion on the effect of anionic substitution on catalytic activity will be provided in the later sections.

# SUSTAINABLE CHEMICAL AND ENERGY PRODUCTION

## Sustainable chemical production

### Oxidative CH₄ coupling to higher order hydrocarbon

Higher order hydrocarbon (OCM) has been a “holy grail” for economically viable methane conversion into ethylene and ethane for decades since Lunsford’s seminal paper[⁵⁰]. Traditional OCM catalyst includes promoted silica-based systems and rare-earth oxides to give ca. 20%-30% C₂ yield and around ~80 C₂ selectivity[⁵¹,⁵²]. Importantly, basicity is one key strategy to stabilize methoxy radicals with optimal binding strength for C₂, hydrocarbon formation[⁵²,⁵³]. In recent years, HAP-based catalysts, especially Pb-substituted HAP, with appropriate synthesis procedures and treatment, have been demonstrated to be active for OCM to produce ethane and ethylene [Table 1], to achieve around 22% C₂ yield (35% CH₄ conversion and 62% C₂ selectivity) at 775 °C[⁵⁴]. This performance is about five-fold higher selectivity than the undoped HAP, highlighting the promotional effect of Pb. Pb²⁺ sites doped in HAP were found to moderate active oxygen species [i.e., higher surface basicity confirmed by CO₂-temperature-programmed desorption (TPD) study] and aid in stabilizing key intermediate methoxy radicals, preventing their premature oxidation to COₓ. In addition, Pb improves structural stability; for example, in Ca-Pb HAP, the intact apatite structure is preserved during reaction, preventing deactivation and maintaining active O- sites[⁵⁵]. Other dopants have also been considered, including Sr, Ba, and Ca, but studies revealed that mono-dopants yield poor OCM activity, with the exception of Pb-substituted HAP[⁵⁶,⁵⁷]. The strategy is therefore shifting toward co-doping, where a secondary dopant (e.g., Pb, Cl⁻, or Zr) is introduced to balance surface basicity, enhance methyl radical coupling, and stabilize the apatite framework under high-temperature oxidative conditions.

### Oxidative propane dehydrogenation

Commercial propane dehydrogenation is an endothermic catalytic reaction that produces propylene as a valuable chemical intermediate. Thermal energy input is required to drive the catalytic reaction, incurring CO₂ emissions[⁶¹]. Although the CO₂ emission for commercial propane dehydrogenation is lower than steam cracking of naphtha, there is still a need to lower the CO₂ emission by making the process more exothermic using oxidants such as O₂ and CO₂[⁶²,⁶³]. This makes the process more sustainable and commercially viable. However, when oxidants such as O₂ are used, the propylene selectivity becomes greatly reduced and the catalyst becomes easily oxidized and sintered; thereby, deactivating the catalyst over time[⁶⁴]. Hence, it is crucial to explore alternative materials for catalyst development for this catalytic reaction.

For oxidative propane dehydrogenation catalyst, V/SiO₂ exhibited a reasonably good performance of 5% propane conversion and 66% propylene selectivity[⁶⁵]. HAP-based catalysts displayed around 5% propane conversion and 26.4% propylene selectivity as seen in Table 2. HAP facilitates C-H bond activation through its inherent basic sites, particularly surface OH⁻ groups and phosphate units, which promote the initial hydrogen abstraction from propane. Additionally, the incorporation of metal ions (e.g., Co²⁺ and Cr³⁺) into the HAP lattice modifies its redox and acid-base properties. For example, Co-doped HAP facilitates hydrogen abstraction via surface OH⁻ group[⁶⁶], whereas Cr-substituted HAP enhances release of lattice
<table><caption>Table 1. Catalytic performance of HAP-based catalysts for oxidative coupling of methane (OCM)</caption><thead><tr><th>Catalyst</th><th>WHSV (mL/gcat.h)</th><th>T (°C)</th><th>Pressure (bar)</th><th>Feed composition</th><th>CH<sub>4</sub> conversion (%)</th><th>C<sub>2</sub> and (CO<sub>2</sub>) selectivity (%)</th><th>Stability (mins)</th><th>Reference</th></tr></thead><tbody><tr><td>Pb<sub>2</sub>Ca<sub>8</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>0.5</sub>Cl<sub>1.5</sub></td><td>6,000</td><td>740</td><td>1</td><td>P(CH<sub>4</sub>) = 21.6 kPa<br>P(O<sub>2</sub>) = 10.8 kPa</td><td>35</td><td>62(n.r.)</td><td>3,600</td><td>[54]</td></tr><tr><td>Ca9.5Pb0.5</td><td>6,186</td><td>800</td><td>1</td><td>P(CH<sub>4</sub>) = 21.8 kPa<br>P(O<sub>2</sub>) = 10.9 kPa</td><td>40</td><td>55(n.r.)</td><td>2,400</td><td>[55]</td></tr><tr><td>SrC1ZrP(13/4)</td><td>6,000</td><td>750</td><td>1</td><td>P(CH<sub>4</sub>) = 16.5 kPa<br>P(O<sub>2</sub>) = 8.3 kPa</td><td>25</td><td>52(n.r.)</td><td>3,000</td><td>[56]</td></tr><tr><td>CaHAP-PbHAP</td><td>3,600</td><td>700</td><td>1</td><td>P(CH<sub>4</sub>) = 28.7 kPa<br>P(O<sub>2</sub>) = 4.1 kPa</td><td>18</td><td>80(n.r.)</td><td>1,440</td><td>[57]</td></tr><tr><td>Pb-HAP-CO<sub>3</sub></td><td>9,200</td><td>700</td><td>1</td><td>P(CH<sub>4</sub>) = 27.1 kPa<br>P(O<sub>2</sub>) = 11.0 kPa</td><td>-25</td><td>35(65)</td><td>600</td><td>[58]</td></tr><tr><td>Pb-HAP (0.1)</td><td>3,000</td><td>700</td><td>1</td><td>P(CH<sub>4</sub>) = 29.0 kPa<br>P(O<sub>2</sub>) = 4.0 kPa</td><td>-14.5</td><td>-20(20)</td><td>180</td><td>[59]</td></tr><tr><td>Pb-HAP (c-surface)</td><td>34,300</td><td>700</td><td>1</td><td>P(CH<sub>4</sub>):P(O<sub>2</sub>) = 4:1, unspecified<br>N<sub>2</sub> dilution</td><td>-5.0</td><td>-35(35)</td><td>600</td><td>[60]</td></tr></tbody><tfoot><tr><td colspan="9">Remarks: The c-surface on Pb-HAP enables the more facile formation of hydroxyl vacancies which can absorb molecular O<sub>2</sub> and make the stabilization of methyl radicals more energetically favorable. This results in higher C<sub>2</sub> selectivity in OCM</td></tr></tfoot></table>

WHSV: Weight hourly space velocity; n.r.: not reported.

oxygen for hydrogen abstraction[67]. The propylene selectivity is still lower than the V/SiO₂ benchmark catalyst. Hence, more research effort is required to enhance propylene selectivity through a more rational understanding of the catalyst surface.

## Biodiesel production

Diesel is derived from petroleum distillation and is widely used in vehicle transportation. Biodiesel production from renewable sources is strongly considered a more sustainable pathway than oil-derived diesel. Biodiesel comprises fatty acid alkyl esters (FAME) which can be synthesized from vegetable oil or animal fats
<table><caption>Table 2. Catalytic performance of HAP-based catalysts for oxidative propane dehydrogenation</caption><thead><tr><th>Catalyst</th><th>WHSV (mL/gcat.h)</th><th>T (°C)</th><th>Pressure (bar)</th><th>Feed composition</th><th>C<sub>3</sub>H<sub>8</sub> conversion (%)</th><th>C<sub>3</sub>H<sub>6</sub> selectivity (%)</th><th>Stability (mins)</th><th>Reference</th></tr></thead><tbody><tr><td>Co55SrHAP</td><td>3,600</td><td>450</td><td>1</td><td>P(C<sub>3</sub>H<sub>8</sub>) = 14.5 kPa<br>P(O<sub>2</sub>) = 4.1 kPa</td><td>22.1</td><td>48.6</td><td>360</td><td>[66]</td></tr><tr><td>Cr(3.7)/CaHAP</td><td>90,000</td><td>550</td><td>1</td><td>P(C<sub>3</sub>H<sub>8</sub>) = 6.08 kPa<br>P(O<sub>2</sub>) = 3.04 kPa</td><td>-20.0</td><td>-34.0</td><td>300</td><td>[67]</td></tr><tr><td>V-CaHAP (V/P = 15) re-oxidized</td><td>3,600</td><td>450</td><td>1</td><td>P(C<sub>3</sub>H<sub>8</sub>) = 14.5 kPa<br>P(O<sub>2</sub>) = 4.1 kPa</td><td>15.2</td><td>55.2</td><td>360</td><td>[68]</td></tr><tr><td>Ca<sub>10</sub>(PO<sub>4</sub>)<sub>0.78</sub>(VO<sub>4</sub>)<sub>5.22</sub>(OH)<sub>1.32</sub>O<sub>0.34</sub></td><td>7,500</td><td>450</td><td>1</td><td>P(C<sub>3</sub>H<sub>8</sub>) = 7.5 kPa<br>P(O<sub>2</sub>) = 2.13 kPa</td><td>-5.0</td><td>-27.0</td><td>90</td><td>[69]</td></tr></tbody><tfoot><tr><td colspan="9">Remarks: The formation of vanadium oxy-HAP created basic sites that are essential for activating the C-H bonds in propane molecules</td></tr></tfoot></table>

WHSV: Weight hourly space velocity.

via a transesterification reaction with methanol as the reactant[70]. Waste cooking oil can be used as an alternative feedstock which lowers biodiesel production costs[71]. Basic homogeneous catalysts such as NaOH and KOH are effective for biodiesel production but pose challenges such as corrosiveness, difficult separation, and wastewater generation[72], while solid CaO offers recyclability and high yields, but suffers from deactivation by CO₂/H₂O and leaching due to reaction with glycerol[73,74]. HAP-based catalysts demonstrate several innovations that address the key limitations of conventional CaO-based systems in biodiesel production. Notably, HAP provides excellent surface basicity and structural stability, enabling a high biodiesel yield of 95% for eight cycles with negligible leaching as seen in Table 3; hence, HAP-based catalysts deserve closer research attention[75].

## Hydrogen production

### Dry reforming of methane (DRM)

Catalytic transformation of greenhouse gases such as CO₂ and CH₄ to produce syngas is highly desirable as syngas can be used as feedstock for the Fischer-Tropsch process, methanol synthesis, and one-step dimethyl ether (DME) synthesis process[83]. Thus, it can be considered a sustainable path for syngas production. Syngas can also be used as a source of hydrogen or directly to produce electricity[84,85]. DRM, which uses CO₂ as a feedstock, is a desirable alternative to current syngas generation processes from an environmental and sustainability perspective[86]. A major technical challenge in DRM is rapid catalyst deactivation induced by severe coke formation during the reaction, which hinders the commercial development of the DRM process[87]. The last decade has thus seen vigorous research in catalyst development for this process to increase activity, stability, and resistance to coking[88].
<table><caption>Table 3. Catalytic performance of HAP-based catalysts for biodiesel production</caption><thead><tr><th>Catalyst</th><th>T (°C)</th><th>Feed condition</th><th>Conversion (%)</th><th>No of cycles</th><th>Reference</th></tr></thead><tbody><tr><td>30K/HAP-600</td><td>65</td><td>30 g of palm oil and an appropriate amount of methanol</td><td>96.4</td><td>8</td><td>[75]</td></tr><tr><td>Ni-Ca-HAP</td><td>70</td><td>The waste fish scale has a high acid value</td><td>59.9</td><td>2</td><td>[76]</td></tr><tr><td>11 wt.% dosage of 30%CaO-CeO<sub>2</sub>/HAP-650</td><td>65</td><td>30 g of palm oil with a 9:1 methanol to palm oil molar ratio</td><td>91.84</td><td>8</td><td>[77]</td></tr><tr><td>5%CaO-HAP</td><td>60</td><td>100 g of vegetable oil</td><td>95.18</td><td>4</td><td>[78]</td></tr><tr><td>50-NaHAP-800</td><td>100</td><td>Methanol to oil molar ratio of 6:1</td><td>99</td><td>5</td><td>[79]</td></tr><tr><td>HAP-γ-Fe<sub>2</sub>O<sub>3</sub></td><td>65<br>(methanol reflux temperature)</td><td>Methanol/soybean oil molar ratio of 25:1</td><td>94</td><td>5</td><td>[80]</td></tr><tr><td>Marble slurry-based HAP</td><td>65</td><td>Methanol/soybean oil molar ratio 9:1</td><td>94</td><td>5</td><td>[81]</td></tr><tr><td>HAP</td><td>65</td><td>The canola oil, rapeseed oil, and waste cooking oil to methanol molar ratio was taken as 1:12</td><td>89.4%, 96.7% and 91.7% were achieved for canola oil, rapeseed oil, and waste cooking oil, respectively</td><td>-</td><td>[82]</td></tr></tbody><tfoot><tr><td colspan="6">Remarks: The synthesized catalyst showed high total basicity after calcination</td></tr></tfoot></table>

WHSV: Weight hourly space velocity.

Conventionally, Ni/MgAl₂O₃ catalysts were commonly used as a benchmark, which exhibited 90% CH₄ conversion and approx. 92% CO₂ conversion at 750 °C[⁹⁹]. HAP-based catalysts have been shown to outperform the benchmark catalyst with 90% CH₄ conversion and approx. 95% CO₂ conversion for 24 h at 750 °C[⁹⁹]. One Ni-HAP catalyst prepared using the one-pot synthesis method can produce a stable catalytic performance of 200 h[⁹⁰]. HAP-based catalysts for
DRM reaction present opportunities to enhance catalytic performance and stability by adjusting surface basicity, porosity, and metal-support interaction. Strategies such as coprecipitation, incorporation of secondary metals (e.g., Co, Ce), or use of structure-directing agents have been shown to enhance Ni dispersion, prevent sintering, and promote single-atom configurations, leading to improved resistance against coking [Table 4]. Moreover, non-stoichiometric HAP and doped HAP variants allow for oxygen vacancy formation and lattice substitution, which are crucial for redox-driven activation of CO₂ and stabilization of reactive intermediates.

### Partial oxidation of CH₄ (POM)

The current technology for syngas production, methane steam reforming ($\Delta H°_{298} = 206.0$ kJ/mol), requires a tremendous amount of energy to break the C-H bond in methane and the O-H bond in steam. As a result, the CO₂ emission per hydrogen produced (i.e., 11.5 tons CO₂ emitted per ton H₂) is the highest among the available technologies, such as natural gas-fired methane pyrolysis and electric-arc furnace methane pyrolysis for hydrogen production and creates a strong motivation to move towards a more sustainable hydrogen production[¹⁰⁰].

POM is an attractive alternative to the commercial steam reforming of methane reaction as it is an exothermic reaction, and a high H₂/CO ratio (~2) can be attained. Higher H₂/CO is advantageous for Fischer-Tropsch, solid oxide fuel cells, and methanol synthesis from CO hydrogenation[¹⁰¹]. The technical issues with POM are that full methane combustion can occur as a side reaction, which can reduce the H₂/CO ratio and metal sintering due to exothermic heat from the reaction. HAP had been used as a catalyst support for POM and exhibited reasonable catalytic performance. The benchmark catalyst for POM, Ni-Mg-Al hydrotalcite, showed 88.4% CH₄ conversion and 86.2% H₂ yield at 750 °C. The HAP-based catalyst (as seen in Table 5) can attain up to around 90% CH₄ conversion and 90% H₂ yield at 750 °C, which is very similar to the hydrotalcite catalyst[¹⁰²]. The similarity in catalytic performance justifies the effort in further exploration of HAP-based catalysts for POM.

### CO oxidation

CO oxidation is an integral reaction to eliminate CO in hydrogen fuel applications and emission control in automobile applications to meet government emission standards[¹¹³]. Hence, it is an important reaction that can speed up the use of hydrogen fuel cells and make transportation more sustainable. Typically, noble metals such as Au have shown to be effective in creating highly active sites for CO oxidation due to the formation of cationic Au and metallic Au, and this may enhance O₂ dissociation at the metal-support interface[¹¹⁴⁻¹¹⁶]. The HAP-based catalyst is seven times more active than the benchmark catalyst (Au/Fe₂O₃) (i.e., 0.071 mol CO/g Au·h vs. 0.011 mol CO/g Au·h) [Table 6] [¹¹⁷]. The excellent catalytic performance indicates that HAP-based catalysts deserve more careful investigation to develop an active and stable catalyst for this reaction

### Water gas shift reaction

Currently, the WGS reaction is employed to increase the H₂/CO ratio of syngas produced from the steam methane reforming (SMR) process. Thereby enhancing the sustainability of the SMR process. The commercial WGS catalysts used are the Fe-Cr mixed oxide catalyst and the Cu-Zn mixed catalyst used at a high-temperature range (350-450 °C) and a lower temperature range (190-250 °C), respectively[¹²⁴]. As the WGS reaction is restricted by thermodynamic equilibrium, the commercial catalytic process typically performs the reaction at high temperatures, followed by a lower temperature range to increase CO conversion and H₂ yield, thus approaching thermodynamic limits. Although the Fe-Cr mixed oxide catalyst displayed moderate catalytic performance suitable for industrial purposes, it is important to note that there
<table><caption>Table 4. Catalytic performance of HAP-based catalysts for dry reforming of methane</caption><thead><tr><th>Catalyst</th><th>WHSV (mL/gcat.h)</th><th>T (°C)</th><th>Pressure (bar)</th><th>Feed composition</th><th>CH<sub>4</sub> conversion/CO<sub>2</sub> conversion (%)</th><th>H<sub>2</sub>/CO ratio</th><th>Stability (h)</th><th>Carbon formation (%wt.)</th><th>Reference</th></tr></thead><tbody><tr><td>2Ni<sub>0.5</sub>/HAP-Ce</td><td>60,000</td><td>750</td><td>1</td><td>20 mol%CH<sub>4</sub><br>20 mol%CO<sub>2</sub><br>60 mol%He</td><td>-95/80</td><td>-0.8</td><td>100</td><td>-0</td><td>[89]</td></tr><tr><td>Ni<sub>0.5</sub>/HAP-OP</td><td>30,000</td><td>800</td><td>1</td><td>10 mol%CH<sub>4</sub><br>10 mol%CO<sub>2</sub><br>80 mol%N<sub>2</sub></td><td>93.3/98.8</td><td>1.0</td><td>200</td><td>-0</td><td>[90]</td></tr><tr><td>Ca-HAP2</td><td>15,900</td><td>700</td><td>1.6</td><td>20 mol%CH<sub>4</sub><br>20 mol%CO<sub>2</sub><br>60 mol%N<sub>2</sub></td><td>78/84</td><td>n.r.</td><td>90</td><td>n.r.</td><td>[91]</td></tr><tr><td>Co-HAP-N</td><td>31,764</td><td>700</td><td>1.6</td><td>20 mol%CH<sub>4</sub><br>20 mol%CO<sub>2</sub><br>60 mol%N<sub>2</sub></td><td>75/55</td><td>-0.75</td><td>50</td><td>-10</td><td>[92]</td></tr><tr><td>Ni(4)/CaHAP</td><td>38,400</td><td>600</td><td>1</td><td>3 mol%CH<sub>4</sub><br>3 mol%CO<sub>2</sub><br>94 mol%N<sub>2</sub></td><td>78/78</td><td>-0.70</td><td>4</td><td>-50</td><td>[93]</td></tr><tr><td>Ni-<br>Co/HAPSIWI</td><td>8,823</td><td>750</td><td>1.6</td><td>20 mol%CH<sub>4</sub><br>20 mol%CO<sub>2</sub><br>60 mol%N<sub>2</sub></td><td>73/79</td><td>-0.90</td><td>160</td><td>&lt; 13</td><td>[94]</td></tr><tr><td>Pt-HAP</td><td>8,400</td><td>700</td><td>1</td><td>20 mol%CH<sub>4</sub><br>20 mol%CO<sub>2</sub><br>60 mol%N<sub>2</sub></td><td>40/30</td><td>-0.81</td><td>50</td><td>n.r.</td><td>[95]</td></tr><tr><td>0.5Ni<sub>0.5</sub>/HAP-</td><td>60,000</td><td>750</td><td>1</td><td>20 mol%CH<sub>4</sub></td><td>-75/85</td><td>-0.9</td><td>16</td><td>-0</td><td>[96]</td></tr></tbody></table>
<table><tr><td>SAC-PVP</td><td colspan="8">20 mol%CO<sub>2</sub><br/>60 mol%He</td></tr><tr><td colspan="9">Remarks: The Ni-HAP assisted by PVP addition exhibited good catalytic performance over time due to the greater prevalence of Ni single atoms. The authors did not further expound on the role of PVP in enhancing the catalytic performance and stability of the Ni single atom on the HAP catalyst surface</td></tr><tr><td>Ni/HAP-80</td><td>24,000</td><td>750</td><td>1</td><td>50 mol%CH<sub>4</sub><br/>50 mol%CO<sub>2</sub></td><td>44/55</td><td>-0.77</td><td>-12</td><td>0.99</td><td>[97]</td></tr><tr><td colspan="10">Remarks: The ratio of mesopores and macropores of HAP can be controlled when the heating temperature varies in the presence of cetyltrimethylammonium bromide (CTAB). This ratio can influence the Ni particle size which can affect catalytic performance and stability</td></tr><tr><td>Ni/Ca-HA1_S</td><td>15,882</td><td>700</td><td>1</td><td>50 mol%CH<sub>4</sub><br/>50 mol%CO<sub>2</sub></td><td>50/60</td><td>0.8-0.9</td><td>30</td><td>&lt; 10</td><td>[98]</td></tr><tr><td colspan="10">Remarks: The regeneration property of the Ni-HAP catalysts in different oxidizing atmospheres (O<sub>2</sub> and CO<sub>2</sub>) was investigated. There is a very small decrease in the CH<sub>4</sub> conversion of the Ni-HAP catalyst over three cycles. The reason for the decrease in catalytic activity could be Ni nanoparticle sintering during the reaction</td></tr></table>

WHSV: Weight hourly space velocity; n.r.: not reported.

is strong regulatory pressure to replace the Fe-Cr mixed oxide catalyst as Cr<sup>6+</sup> is highly carcinogenic and can cause immense harm to human life and the natural environment[125]. These concerns have motivated intense research to explore alternative materials that exhibit high catalytic performance and yet are less harmful to human life and the natural environment. A work by Miao *et al.* demonstrated that HAP-based catalysts can display better catalytic performance than CeO<sub>2</sub>-based catalysts in WGS, as a consequence of the unique HAP structure where H<sub>2</sub>O could be polarized by Lewis acidic Ca<sup>2+</sup> ions and H bonding to basic O atoms of PO<sub>4</sub><sup>3-</sup> units[126]. Thus, the HAP-based catalyst deserves detailed investigation to improve WGS catalytic performance.

## CO<sub>2</sub> utilization

### CO<sub>2</sub> hydrogenation to CH<sub>4</sub>

As the mitigation of CO<sub>2</sub> emissions becomes a pressing issue globally, CO<sub>2</sub> utilization technologies via thermal and photothermal means have gathered momentum in recent years[127]. CO<sub>2</sub> hydrogenation to methane (i.e., CO<sub>2</sub> methanation) has gained increasing attention as an alternative method to produce sustainable methane while reducing CO<sub>2</sub> emissions at the same time[128-130]. HAP has gained traction as an alternative support in CO<sub>2</sub> methanation[131]. As photothermal CO<sub>2</sub> reduction is still in a nascent stage of development, significant effort is required to develop an active, stable, and selective catalyst before scaling-up efforts can be undertaken[132]. The Cu/HAP catalyst demonstrates catalytic performance on par with other metal oxide-based catalysts, indicating that HAP-based catalysts warrant further investigation for photothermal CO<sub>2</sub> reduction[133]. The catalytic performance of the HAP-based catalysts for CO<sub>2</sub> methanation can be seen in Table 7.

## SURFACE ENGINEERING OF HYDROXYAPATITE CATALYSTS

### Active metals deposition

The active center affects the reaction rate, selectivity, and stability of a catalytic reaction. Active centers can be introduced to HAP support through several
<table><caption>Table 5. Catalytic performance of HAP-based catalysts for partial oxidation of methane (POM)</caption><thead><tr><th>Catalyst</th><th>WHSV (ml<sub>CH4</sub>/gcat.h)</th><th>T (°C)</th><th>Pressure (bar)</th><th>Feed composition</th><th>CH<sub>4</sub> conversion (%)</th><th>CO selectivity (%)</th><th>Stability (mins)</th><th>Reference</th></tr></thead><tbody><tr><td>Ca90NiP(25)</td><td>4,800</td><td>800</td><td>1</td><td>P(CH<sub>4</sub>) = 16.2 kPa<br>P(O<sub>2</sub>) = 8.1 kPa</td><td>-95.0</td><td>-95.0</td><td>n.r.</td><td>[102]</td></tr><tr><td>HAP(1.61)</td><td>3,000</td><td>600</td><td>1</td><td>P(CH<sub>4</sub>) = 29.0 kPa<br>P(O<sub>2</sub>) = 4.0 kPa</td><td>-6.0</td><td>n.r.</td><td>180</td><td>[103]</td></tr><tr><td>SrHAP</td><td>3,600</td><td>600</td><td>1</td><td>P(CH<sub>4</sub>) = 28.7 kPa<br>P(O<sub>2</sub>) = 12.3 kPa</td><td>-10</td><td>-90.0</td><td>5,050</td><td>[104]</td></tr><tr><td>Pb12HAP</td><td>3,600</td><td>600</td><td>1</td><td>P(CH<sub>4</sub>) = 28.7 kPa<br>P(O<sub>2</sub>) = 4.3 kPa</td><td>-11.0</td><td>-30.0</td><td>180</td><td>[105]</td></tr><tr><td>BaHAP</td><td>3,600</td><td>700</td><td>1</td><td>P(CH<sub>4</sub>) = 28.7 kPa<br>P(O<sub>2</sub>) = 4.1 kPa</td><td>-8.0</td><td>-10.0</td><td>6</td><td>[106]</td></tr><tr><td>HAP</td><td>3,600</td><td>700</td><td>1</td><td>P(CH<sub>4</sub>) = 28.7 kPa<br>P(O<sub>2</sub>) = 4.1 kPa</td><td>-4.0</td><td>-10.0</td><td>6</td><td>[107]</td></tr><tr><td>Ni-Sr-HAP</td><td>6,000</td><td>750</td><td>1</td><td>P(CH<sub>4</sub>) = 16.1 kPa<br>P(O<sub>2</sub>) = 8.2 kPa</td><td>-90.0</td><td>-90.0</td><td>n.r.</td><td>[108]</td></tr><tr><td>Ca95NiP(25)</td><td>4,800</td><td>750</td><td>1</td><td>P(CH<sub>4</sub>) = 16.1 kPa<br>P(O<sub>2</sub>) = 8.2 kPa</td><td>-80.0</td><td>-88.0</td><td>84</td><td>[109]</td></tr><tr><td>Ce<sub>0.1</sub>Ni<sub>2.5</sub>Ca<sub>10</sub></td><td>32,000</td><td>600</td><td>1</td><td>P(CH<sub>4</sub>) = 16.1 kPa<br>P(O<sub>2</sub>) = 8.1 kPa</td><td>-65.0</td><td>-60.0</td><td>20</td><td>[110]</td></tr></tbody></table>
**Remarks:** Ce, added as a promoter after the coprecipitation was completed, could enhance CH₄ conversion and improve catalyst stability as Ce can provide oxygen vacancies to promote carbon gasification and so reduce carbon formation

<table><tr><td>Ni-Ca-HAP</td><td>n.r.</td><td>750</td><td>1</td><td>P(CH₄) = 16.2 kPa<br>P(O₂) = 8.1 kPa</td><td>-90.0</td><td>-85.0</td><td>n.r.</td><td>[111]</td></tr></table>

**Remarks:** The Ni-Ca-HAP catalyst was subjected to N₂O pulse chemisorption. The adsorbed CH₄ is converted into CO₂ due to lattice oxygen on the HAP surface. When lattice oxygen depletes, CO formation and, subsequently, carbon formation become more favored. Partially oxidized Ni species are required for high CH₄ conversion and stable performance. The catalyst was deposited onto monolith walls, and it was found that at higher temperatures, the temperature change decreases due to higher heat loss at the reactor walls

<table><tr><td>Rh(1)-HAP</td><td>19,200</td><td>700</td><td>1</td><td>P(CH₄) = 10.1 kPa<br>P(O₂) = 5.1 kPa</td><td>-76.0</td><td>-85.0</td><td>n.r.</td><td>[112]</td></tr></table>

**Remarks:** Three Rh species were observed on the HAP surface from characterization techniques: surface Rh particles, RhOx with good interaction with the support, and Rh incorporated into the HAP lattice structure. Although the dispersion of Rh (1)-HAP appears relatively low, the catalyst stability remains well-preserved over 30 h due to better metal-support interaction which makes it difficult to sinter and oxidize. Rh (1)-HAP catalyst appears comparable to industrial Rh-Al₂O₃ catalyst in CH₄ conversion and H₂ yield

WHSV: Weight hourly space velocity; n.r.: not reported.

methods, with incipient wetness impregnation as the commonly used strategy. Alternatively, electrostatic adsorption using ammonia can create dispersed active sites on the HAP surface. Additionally, coprecipitation and ion-exchange methods can enhance surface basicity and create dispersed active sites on the HAP catalyst surface[16].

The wetness impregnation method not only deposits the active center onto the catalyst surface but also may perform partial cation exchange as the cations in the impregnation solution may swap with the Ca²⁺ cation during the impregnation process[137]. This creates two different sites with different sizes: one with nanoparticles and the second one with cluster or single atom size. The active center of two different sizes may affect product selectivity. Domínguez et al. used HAP as catalyst support, impregnated with Au, for CO oxidation. This catalyst has demonstrated near complete CO conversion at room temperature due to the presence of structural defects by eliminating surface carbonates which alter the Au oxidation state[119]. Wang et al. reported that the Au/HAP catalyst calcined under reaction conditions is more active than in air[138]. Higher Au loading resulted in lower CO activity. Further characterization using X-ray photoelectron spectroscopy (XPS) revealed that higher HAP crystallinity resulted in a lower Au³⁺/Au⁰ ratio due to lower Ca/P. This indicates that the surface becomes more acidic, thereby withdrawing more electrons from the Au species and creating Au³⁺. Further investigation by Huang et al. has shown that the calcination temperature and calcination environment can influence CO oxidation activity and selectivity at room temperature due to the formation of very small Au nanoparticles[118]. Although very small Au nanoparticles, thought to contribute to high initial activity, have been observed in TEM images, SMSI is believed to underlie this influence. Tang et al., through an in-situ Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) study of catalyst samples prepared at different temperatures, have indeed shown that the SMSI effect has a detrimental effect on CO oxidation activity at high calcination temperature as the active sites were encapsulated by HAP support under an oxidative environment[120]. However, such SMSI effect can be tuned and manipulated through partial encapsulation to prevent metal sintering and yet allow selective and adequate catalytic activity to occur on the active sites. In one such demonstration, Fe was introduced into the HAP lattice through cation substitution using the coprecipitation method; the Fe-HAP surface with abundant
<table><caption>Table 6. Catalytic performance of HAP-based catalysts for CO oxidation</caption><thead><tr><th>Catalyst</th><th>WHSV (mL/gcat.h)</th><th>T<sub>50%</sub> (°C)</th><th>Pressure (bar)</th><th>Feed composition</th><th>Stability (h)</th><th>Reference</th></tr></thead><tbody><tr><td>Au/FH-400</td><td>20,000</td><td>--25</td><td>1</td><td>1.0 mol%CO<br>1.0 mol%O<sub>2</sub><br>98 mol%He</td><td>300</td><td>[117]</td></tr><tr><td>Au/HAP-O<sub>2</sub></td><td>100,000</td><td>14</td><td>1</td><td>1 mol%CO<br>20 mol%O<sub>2</sub><br>79 mol%He</td><td>24</td><td>[118]</td></tr><tr><td>Au/HAP</td><td>31,500</td><td>35</td><td>1</td><td>3.4 mol%CO<br>21 mol%O<sub>2</sub><br>75.6 mol%He</td><td>n.r.</td><td>[119]</td></tr><tr><td>Au/HAP(400)</td><td>20,000</td><td>-0</td><td>1</td><td>1.0 mol%CO<br>1.0 mol%O<sub>2</sub><br>98 mol%He</td><td>n.r.</td><td>[120]</td></tr><tr><td>Au-FH(400)-17.5%</td><td>20,000</td><td>--30</td><td>1</td><td>1.0 mol%CO<br>1.0 mol%O<sub>2</sub><br>98 mol%He</td><td>n.r.</td><td>[121]</td></tr><tr><td>Au-Cu-HAP</td><td>10,890</td><td>-30</td><td>1</td><td>10 mol%CO<br>90 mol%air</td><td>14</td><td>[122]</td></tr><tr><td>Cu(4)-HAP</td><td>80,000</td><td>-175</td><td>1</td><td>1.0 mol%CO<br>1.0 mol%O<sub>2</sub><br>98 mol%He</td><td>n.r.</td><td>[123]</td></tr></tbody><tfoot><tr><td colspan="7">Remarks: The Cu-HAP catalysts with lower loading (0.8% and 3.6%) exhibited better catalytic performance. The Cu lattice micro-strain correspondingly increases with Cu loading and appears to contribute catalytic activity for CO oxidation</td></tr></tfoot></table>

WHSV: Weight hourly space velocity; n.r.: not reported.
OH⁻ groups and/or PO₄³⁻ groups was speculated to anchor the Au nanoparticles, akin to dispersing Au on FeOₓ[¹³⁹,¹⁴⁰]. Consequently, the Au/Fe-HAP (Au/FH) catalyst is significantly more active and stable than that of the conventional Au/Fe₂O₃ (Au/F) catalyst, as shown in Figure 3A[¹¹⁷].

Bimetallic nanoparticles on the catalyst surface often induce two catalytic phenomena: electronic and geometric effects[¹⁴¹]. Guo et al. synthesized Au-Cu/HAP for CO oxidation and observed that Cu addition causes a reduction of Au nanoparticle size which resulted in greater Au dispersion. XPS characterization indicated the presence of Cu⁺ and Cu²⁺ which appeared to indicate strong interaction with Au[¹²²]. Further characterization using O₂-TPD shows that more oxygen sites are present in the Au-Cu system as compared to the monometallic Au catalytic system[¹²²]. Other than Au, Cu has been used as an active metal for CO oxidation. Different Cu loadings had been impregnated onto stoichiometric HAP for CO oxidation and preferential CO oxidation. The lowest Cu loading at 0.8%wt Cu shows the highest catalytic activity among the other catalyst samples. Surface characterization using XPS and H₂-TPR revealed the presence of Cu⁺ species and Cu²⁺ species that might be integrated into the HAP framework. Higher Cu dispersion on the lowest loading catalyst resulted in better CO chemisorption[¹²³].

A noble metal component, such as Pd, has been used for full methane oxidation. In the presence of strong metal-support interaction - induced by increasing the calcination temperature, the Pd coordination changes from tetrahedral to square planar geometry while containing a mixture of Pd⁰ and Pd²⁺[¹⁴²]. Stronger metal-support interaction can help stabilize the Pd nanoparticles in successive oxidation cycles and provide better catalytic conversion via improvement in PdO reduction[¹⁴²]. Nickel has been added as a bimetallic component to enhance the catalytic performance but the monometallic nickel displays reasonable performance for full methane oxidation without carbon formation[¹⁴³].

The presence of Pb appears to increase the ethylene selectivity, which transforms POM into OCM. Other than lead (Pb), barium (Ba) is also used as an active component for the POM reaction. The addition of Ba produces better methane conversion than Ca or Pb, and the resulting trend is similar to what is reported[¹⁰⁶].

This property can be utilized to load transition metals, such as Ni, Co, etc., with strong interaction with the HAP support. Depending on the metal loading, the distribution of various types of transition metal species interacting with different support sites is obtained in HAP-based catalysts. For example, in a study by Boukha et al.[⁹³], the Ni loading on HAP catalysts varied from 1% to 10% and it was observed that Ni was mostly present as Ni²⁺ ions by ion exchange with Ca²⁺ at lower Ni loadings (< 1 wt%). The progressive increase of Ni loading from 1% to 10% led to the formation of small NiO nanoparticles at less than 2%-4% loading and large NiO nanoparticles at higher loadings. The strong interaction of the metal with the support is highly desirable for DRM to prevent metal sintering at the high reaction temperature and subsequent formation of coke catalyzed by large metal ensembles[¹⁴⁴,¹⁴⁵]. However, while the ion-exchanged Ni²⁺ has a very strong interaction with HAP support, they exhibited low activity for DRM (as evidenced by poor conversion on the 1% Ni loading catalysts). Thus, the Ni/HAP catalyst with an intermediate Ni loading of 4% and small NiO nanoparticles showed the best performance. The poor activity of the ion-exchanged Ni species may be caused by the cationic state of Ni. Metallic Ni has traditionally been recognized as the active site for DRM, which is why a pre-reduction step before the reaction increases catalytic effectiveness activity. Ion-exchanged Ni²⁺ on HAP is difficult to reduce, possibly leading to lower activity than NiO nanoparticles on HAP, as also reported by other studies[⁹¹,⁹⁴]. Rêgo De Vasconcelos et al. compared three loading techniques - ion exchange, incipient wet impregnation, and liquid phase impregnation for Pt and Ru on HAP support and also observed that incipient wetness impregnation resulted in better catalytic performance in DRM[⁹⁵]. This broadly agrees with the higher activity of smaller metal nanoparticles on HAP
<table><caption>Table 7. Catalytic performance of HAP-based catalysts for CO₂ hydrogenation to CH₄</caption><thead><tr><th>Catalyst</th><th>WHSV (mL/gcat.h)</th><th>T (°C)</th><th>Pressure (bar)</th><th>Feed composition</th><th>CO₂ conversion (%)</th><th>CH₄ selectivity (%)</th><th>Stability (h)</th><th>Reference</th></tr></thead><tbody><tr><td>NiHAP (0.5)</td><td>30,000</td><td>350</td><td>1</td><td>8 mol%CO₂<br>32 mol%H₂<br>60 mol%He</td><td>75-80</td><td>97.0</td><td>70</td><td>[134]</td></tr><tr><td>Ni/La-(6.6)</td><td>30,000</td><td>350</td><td>1</td><td>16 mol%CO₂<br>64 mol%H₂<br>20 mol%He</td><td>70</td><td>98.0</td><td>100</td><td>[135]</td></tr><tr><td>10Ni₆CeHA</td><td>12,000</td><td>325</td><td>1</td><td>8 mol%CO₂<br>32 mol%H₂<br>60 mol%He</td><td>92.5</td><td>100</td><td>n.r.</td><td>[136]</td></tr></tbody><tfoot><tr><td colspan="9">Remarks: La acts as a dual-function material: the first purpose is to chemisorb CO₂ while the second purpose is to create highly dispersed Ni nanoparticles supported on HAP. The presence of such nanoparticles provides active sites for active and stable catalytic CO₂ methanation<br>Remarks: Ce was added onto the Ni/HAP catalyst surface and increased the catalytic activity and methane selectivity. The improved catalytic performance can be attributed to increased oxygen vacancy and higher dispersion of the Ni nanoparticles</td></tr></tfoot></table>

WHSV: Weight hourly space velocity; n.r.: stands for not reported.

support with intermediate metal-support interaction than strongly interacting ion-exchanged cationic metal species or weakly interacting larger metal nanoparticles. Similar results were also reported for Pd loaded on HAP in DRM[146].

To further increase methane conversion for POM, Ni was used as an active metal and the CH₄ conversion for Ni-HAP catalyst was comparable to hydrotalcite catalyst as aforementioned[102]. Upon increasing reaction temperature, NiO nanoparticles on the HAP surface were partially externally reduced to Ni⁰ in the presence of reductive gases (i.e., H₂, CO, CH₄), along with Ni exsolution from the HAP structure [Figure 3B]. Upon cooling, the catalyst surface reoxidizes, forming a layer of NiO, and the exsolved Ni remains exsolved. When heated again, it returns to the reduced state, allowing for repeated cycles of temperature changes [Figure 3C]. The interesting catalytic performance is attributed to SMSI, where HAP support decorates Ni nanoparticles, enhancing their sintering resistance. During the reaction, larger nickel nanoparticles oxidize, while smaller ones maintain their metallic character. Fresh samples do not show nickel nanoparticles, but they appear post-reaction, suggesting that smaller nanoparticles are key for CH₄ activation and H₂ production[102]. In another example, Ni nanoparticles are deposited onto HAP support using oleic acid. Through the use of *in-situ* DRIFTS and *ex-situ* XANES, the amount of surface formate species is related to the chemical nature of Ni on the catalyst surface, as seen in Figure 3D[134]. A reaction mechanism for the CO₂ methanation was proposed and validated using non-linear kinetic modeling. Although the use of HAP-based material can bring about improvement in CO₂ conversion and CH₄ selectivity, the use of more basic promoters can bring about even greater improvement in catalytic performance.
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<figure><img src="image_14.png" /><figcaption>**Figure 3.** (A) Stability curves in CO oxidation of various catalysts calcined at 400 °C or dried at 60 °C. CO:O₂:He = 1:1:98, GHSV = 200,000 mL/gₐₜ·h for Au/F-60 and Au/FH-60 and 40,000 mL/gₐₜ·h for the others to avoid 100% CO conversion. Reproduced from Ref.[117] with permission from the Royal Society of Chemistry. (B) Proposed scheme of changes of Ni state in partial oxidation of methane (POM) reaction. Reproduced with permission from Ref.[102] with permission from Elsevier. (C) CH₄ conversion for Ca90NiP (20) catalyst was tested two times under increasing and decreasing temperatures. Reproduced from Ref.[102] with permission from Elsevier. (D) HAP structure and influence of Ni-Ni coordination number on formate integrated area at 350 °C and CH₄ formation. Adapted from Ref.[134] with permission from Wiley-VCH GmbH. (E) Activity and product distribution over reduced Rh (0.5)/HAP vs. time on stream in the POM reaction. Reaction conditions: 19,000 mL CH₄/gₐₜ·h, mₐₜ = 0.25 g, T = 973 K. Feed composition: 10%CH₄/5%O₂/N₂. Reproduced from Ref.[112] with permission from Elsevier.</figcaption></figure>

Noble metals such as Rh were also used as active metals for the POM. Three different types of Rh species were observed from H₂-TPR: Rh₂O₃ on the HAP catalyst surface and small RhOₓ particles with SMSI and Rh²⁺ cations integrated into the HAP framework. The metal-support interaction of the active metal
component determines the number of metallic sites available for POM; thus, optimum metal-support interaction is required for good methane activation and catalytic stability. Furthermore, the Rh/HAP catalyst at low Rh metal loadings (i.e., 1 wt.%) gave comparable performance to Rh/Al₂O₃ catalyst due to the abundance of basic hydroxyl sites and higher pore size, as shown in Figure 3E.[112]. In summary, active metal deposition, the resulting metal-support interactions, and the dispersion state of metal species on HAP significantly influence catalytic performance, product selectivity, and stability across various catalytic conversion reactions.

## Cationic substituted catalysts

The substitution of the Ca cations with different cations can affect catalytic activity, product selectivity, and catalyst stability for a myriad of catalytic reactions. The active sites in cation-substituted HAP are more sintering-resistant, as these active sites have better metal-support interaction as compared to the catalyst prepared by the conventional incipient impregnation method[90]. The free metal oxide nanoparticles on HAP support tend to sinter into larger particles during reduction and catalysis, leading to catalyst deactivation and side products. Substituting a transition metal into the HAP lattice improves active site dispersion, reducing side product formation[147]. The partial substitution of the Ca cations has been demonstrated for PDH. Partial cationic substitution using Co²⁺ has led to greater improvement in propane conversion and propylene selectivity as compared to Sr³⁺ and Ba²⁺. The partial Co substitution in the cation site altered the surface property by making OH⁻ groups more amenable to hydrogen abstraction from propane, thus creating O²⁻ species[66].

Alternatively, chromium (Cr) can be cation-substituted into the HAP framework via the ion-exchange method and has shown interesting catalytic activity for oxidative propane dehydrogenation. The Cr species changed to Cr⁶⁺ after air calcination due to oxidation. However, the initial high catalytic activity decreases over time due to the reduction of Cr⁶⁺ species to Cr⁴⁺[67]. However, as chromium can be carcinogenic, other active metals can be explored as an alternative to chromium-based HAP catalysts for oxidative propane dehydrogenation.

CO₂ reduction via the photothermal route has attracted attention for utilizing sunlight to drive catalytic reactions. Photons generate electron-hole pairs or high-energy carriers through localized surface plasmon resonance, which can also induce local heating and enhance catalytic activity through thermal effects[148]. Guo *et al.* have exploited the ability of HAP for partial cation substitution to create highly dispersed Cu species on the HAP surface for photothermal CO₂ reduction to CO by using the coprecipitation method for catalyst synthesis[133]. The activation energy of the CO₂ reduction reaction is lowered by 27.5% and CO formation during the photothermal reaction appears to be dependent on the Cu loading as observed in Figure 4A and B. The effectiveness of the photothermal approach can be attributed to the presence of photogenerated electrons and holes that could be trapped at the Lewis-acidic Cu²⁺ and Lewis-basic PO₃⁻₄ sites of surface frustrated Lewis pair (SFLPs) upon absorption of solar light. This consequently lowered the activation barrier of the CO₂ reduction reaction[149].

Trivalent La³⁺ has been successfully incorporated into HAP up to 7 wt.% due to the close similarity in ionic radius between La³⁺ and Ca²⁺ (i.e., 101 and 100 Å, respectively)[150]. Subsequently, Ni was impregnated, and the HAP with La³⁺-substitution was found to exhibit better activity in DRM [Figure 4C]. In addition, La promotes carbon nanotube formation instead of parasitic amorphous carbon coating to ensure nickel active sites are able to interact with the reactants and prolonged catalytic lifetime, as illustrated in Figure 4D.
<figure><img src="image_15.png" /><figcaption>**Figure 4.** (A) Capillary flow reactor results for the calcined 10 mol% Cu-HAP catalyst. The conversion rates and activation energies, recorded under dark and light conditions, for the selective hydrogenation of CO₂ to CO are plotted. Reproduced from Ref. [133] with permission from American Chemical Society. (B) Photocatalytic reverse-WGS activity of calcined Cu-HAP samples as a function of Cu content in a batch reactor. Reaction conditions: H₂/CO₂ ratio = 1:1, light intensity = 40 suns, no external heating, and measurement time = 1 h. Reproduced from Ref. [133] with permission from American Chemical Society. (C) CH₄ conversion for Ni/xLa-HAP catalysts. Reaction condition: CH₄:CO₂ = 1:1, catalyst weight = 100 mg, total flow rate = 40 mL/min, reaction temperature = 750 °C, GHSV = 24,000 mL/g·h. Adapted from Ref. [150] with permission from Elsevier. (D) Schematic of possible reaction pathways in DRM for La-substituted and pristine Ni/HAP. Adapted from Ref. [150] with permission from Elsevier.</figcaption></figure>

## Anionic substituted catalysts

The HAP surface property can be tuned via anion substitution at the B-site by substituting the phosphate group (PO₄) with the vanadate group (VO₄). Calcium vanadate apatite [Ca₁₀(PO₄)₆-x(VO₄)ₓ(OH)₂] was first prepared using coprecipitation of calcium and vanadate aqueous solution with ammonium phosphate dibasic for oxidative PDH[⁶⁸].

The addition of vanadium to the HAP catalyst induces the formation of lattice oxygen during the reaction due to the structure of the vanadate group. Sugiyama *et al.* reported that the presence of a surface hydroxyl group is beneficial for propane conversion as compared to one without a surface hydroxyl group and that was observed on a partially substituted vanadate HAP catalyst[⁶⁸]. Partial substitution of the phosphate group with the vanadate group enables the presence of a surface hydroxyl group which maintains the acid-base properties and enables propane activation[⁶⁸]. Lattice oxygen abstraction creates a redox cycle whereby the vanadium valence charge is reduced from V⁺⁵ to V⁺⁴[¹⁵¹]. This redox behavior is an important characteristic of the oxidative dehydrogenation of propane to propene. Recently, Petit *et al.* have tried to explain this phenomenon by employing *in-situ* DRIFTS and *operando* ionic conductance characterization comprehensively[⁶⁹]. The partial substitution of the vanadate group into the phosphate group creates more defects and results in fewer OH groups. This causes charge balance on the HAP surface, which results in the
creation of O²⁻ species on the surface at a high temperature (723 K) that was shown to be active in propane activation and conversion to propene, as shown in the proposed mechanism in Figure 5A. Hydroxyl groups in the channels may play a part in activating propane molecules through the creation of O²⁻ groups. The anion exchange between the PO₄³⁻ and CO₃²⁻ anions may also occur through the ball-milling method, which can increase oxygen vacancy concentration on the PO₄³⁻ site[¹⁵²].

The HAP framework is also flexible towards chlorine storage via anion exchange with the surface hydroxyl groups, also known as A-site substitution, to form chlorapatite. The introduction of chlorine into the HAP framework can be done using tetrachloromethane (TCM). Pre-treatment of the Sr-HAP catalyst with TCM causes the formation of SrCl₂ and better ethylene yield can be observed as complete transformation to Sr₃(PO₄)₂, an inactive phase, is hindered[¹⁰⁵].

Combining cationic and anionic substitution can be done with HAP. Oh *et al*. exploited the tunable nature of the cationic and anionic sites in the HAP framework by using the Pb²⁺ cation and CO₃²⁻ anion to form Pb-HAP-CO₃[⁵⁸]. The product selectivity and CH₄ conversion were tuned as described in Figure 5B as Pb²⁺ ions can stabilize methyl radicals and facilitate the coupling of methyl radicals to form C₂ by forming covalent bonds with carbon[⁵⁹]. The presence of the CO₃²⁻ group appeared to maintain the HAP structure under harsh OCM conditions. However, the role of the CO₃²⁻ group in stabilizing the HAP structure remains unclear. As different crystal facets can affect the catalytic activity and product selectivity, Oh *et al*. synthesized two types of HAP catalyst support: one with basal-faceted plane c-surface and prism-faceted a-surface as shown in Figure 5C(i-ii)[⁶⁰]. The HAP and Pb-HAP with a c-surface performed better than the ones with the a-surface as oxide vacancies and OH⁻ vacancies found on the c-surface facilitate the formation of methyl or methylene group, which are important to ethylene or ethane from the radical coupling [Figure 5C(iii)]. Thus, there is room for improvement in catalytic activity through a better understanding of the structure-activity relationship of the HAP catalyst through *operando* or *in-situ* characterization of the HAP surface structure.

## Use of promoters in HAP catalysts

Alkali, alkaline earth, and noble metal promoters are frequently used in heterogeneous catalysis for positive benefits in catalytic activity and selectivity. In general, there are two types of promoters: structural and textural. Structural promoters enhance the desired product selectivity for a certain reaction by making the desired reaction pathway more energetically favorable[¹⁵³]. Textural promoters attenuate catalyst nanoparticle agglomeration due to metal sintering; thereby, increasing catalytic stability[¹⁵⁴,¹⁵⁵].

In several reports on CO oxidation, the active site is frequently poisoned by the formation of carbonate, which leads to reduced catalytic activity. Basic elements such as Na can be added to the catalyst to decrease the number of basic sites, enabling better desorption of CO₂ and resulting in less poisoning of the active sites[¹⁵⁶]. Noble metal promoters are used for CO oxidation. Other than Au, Ag is also used as a cheaper alternative for CO oxidation in recent years. Silver doping in the HAP framework produced stable and active CO oxidation catalyst at high temperatures. The silver-doped HAP catalyst also demonstrated stable catalytic performance despite multiple cycles. However, the silver-doped HAP catalyst shows favorable room for improvement[¹⁵⁷].

For the WGS reaction, the increase in surface basicity by the addition of alkaline earth or basic elements such as Sr, Na, and K helps to strengthen CO adsorption and favors water dissociation[¹⁵⁸⁻¹⁶⁰]. Iriarte *et al.* used calcined pork bone with HAP as the main phase and a minute amount of potassium as the support to be impregnated with Ni, Co, Cu and Fe[¹⁶¹]. The Ni-impregnated catalyst is found to be more active and selective to H₂ formation than the other catalyst due to the presence of trace amounts of potassium, which
<figure><img src="image_16.png" /><figcaption>**Figure 5.** (A) Schematized representation of the catalytic transformation of propane to propylene via non-oxidative and oxidative dehydrogenation routes on V-HAP samples with Ca²⁺-O²⁻ surface acid-base pairs generated due to proton migration process close to VO₄ tetrahedra. The square box represents oxygen vacancy and the squiggly arrow refers to the proton migration process. Reproduced from Ref.[69] with permission from Wiley-VCH GmbH. (B) Product selectivity for OCM reactions over HAP-based catalysts at 23% conversion under 973 K and 101 kPa pressure conditions and a space velocity of 8,800 mL·gcat⁻¹·h⁻¹. Reproduced from Ref.[58] with permission from Elsevier. (C) c-surface HAP (i), a-surface HAP (ii), the CH₄ conversion rate of the Pb-HAP catalyst with different orientations for OCM reaction. Reproduced from Ref.[60] with permission from American Chemical Society.</figcaption></figure>

suppresses methane formation. K is postulated to block the active site for CO methanation reaction. This study shows that a minute amount of alkaline promoter in naturally sourced material such as bones can be beneficial towards WGS reaction. It also highlights bones as a rich and accessible source of HAP when properly treated.

The addition of Lanthanum Oxide (La₂O₃) as a promoter has been explored in HAP. The increased La loading at fixed Ni loading enables higher CO₂ conversion and CH₄ selectivity in CO₂ methanation, as shown in Figure 6A and B[135]. The increased addition of La₂O₃ introduces strong basic sites at greater concentrations, helping to strengthen CO₂ chemisorption to the catalyst surface, which aids in CO₂ conversion and eventual CH₄ formation, as observed in Figure 6C. Additionally, La₂O₃ can enhance the
<figure><img src="image_17.png" /><figcaption>**Figure 6.** (A) CO₂ conversion and (B) CH₄ selectivity of the Ni/La-(x) catalysts in the CO₂ methanation reaction. Reaction mixture conditions: 16% CO₂ and 64% H₂, balanced in He (WHSV = 30,000 mL/g.h). Reproduced from Ref. [135] with permission from Elsevier. (C) CO₂-TPD (Left) and CH₄ formation during TPSR experiments (Right) with pre-adsorbed CO₂ on the reduced Ni/La-(x) catalysts. Reproduced from Ref. [135] with permission from Elsevier. (D) Reusability studies of catalysts with leaching content of K⁺ ions. Reproduced from Ref. [75] with permission from Elsevier.</figcaption></figure>

dispersion of Ni nanoparticles, thereby making the catalyst more reducible. Other metals can also enhance the dispersion of Ni nanoparticles, such as alloying strategy with Co[162] or Ca as a monolayer[163]. In addition, the Co was demonstrated to inhibit graphitic carbon formation in DRM, thereby improving catalytic stability.

Ceria is usually added as a promoter to increase the presence of oxygen species. The presence of oxygen species from Ce helps to alleviate carbon formation by promoting the Bouduoard reaction (C + CO₂ → 2CO) which will increase CO selectivity[110]. As the presence of oxygen vacancy promotes the CO₂ methanation reaction, ceria was added as a promoter onto Ni/HAP catalyst in a highly dispersed fashion. The resultant catalytic performance was comparable to other metal oxide catalysts which demonstrated the suitability of HAP support for CO₂ methanation. Nguyen *et al.* added ceria to Ni/HAP catalyst for CO₂ methanation and observed enhanced CO₂ conversion and CH₄ selectivity as compared to the undoped sample[136]. The enhancement in catalytic performance can be attributed to higher Ni nanoparticle dispersion and higher Ni reducibility. In addition, the presence of CeO₂ introduces more medium basic sites. The introduction of medium basic sites may originate from the facile redox property of ceria to transit from Ce⁴⁺ to Ce³⁺ and *vice versa*. The redox property also creates oxygen vacancies which
enhances CO₂ chemisorption. The stronger CO₂ chemisorption leads to the formation of monodentate formate which is beneficial to methane formation and, subsequently, higher methane selectivity[¹⁶⁴]. Hence, CeO₂ acts more as a structural promoter in this work.

Other than CO oxidation and WGS reaction, the transesterification reaction requires a highly basic or alkaline element to accelerate the reaction towards FAME formation. To enhance the alkalinity of the HAP catalyst surface, K₂CO₃, Sr, and Li were impregnated onto the bone-derived HAP catalyst support, respectively. The FAME yield increases in the following order: Li/HAP (93.2%) < Sr/HAP (94.7%) < 30K/HAP-600 (96.4%)[⁷⁵]. The better catalytic performance exhibited by the 30K/HAP-600 catalyst is due to the higher strength of the basic sites and greater concentration of surface basic sites. The FAME yield was found to be not just dependent on the K₂CO₃ loading but also on the calcination temperature. However, if the calcination temperature is too high, the FAME yield is observed to decrease, which may be attributed to K₂CO₃ sintering and lower Brunauer-Emmett-Teller (BET) surface area. The authors also studied the extent of potassium leaching as catalytic stability is an important factor in commercial applications and potassium salt tends to leach during the reaction[¹⁶⁵]. The extent of potassium leach was found to be reduced with increasing calcination temperature due to the creation of the KCaPO₄ phase that is more active and stable for biodiesel production [Figure 6D]. The Na-doped HAP catalyst can attain high rapeseed oil conversion and high selectivity to FAME, thereby attaining high FAME yield at 99%. The produced biodiesel is comparable to EU biodiesel testing standard EN 14214. Catalyst leaching did not occur due to strong Na⁺ integration into the HAP framework. Impregnating the bone-derived HAP catalyst with K can result in a higher FAME yield at 96.1%[¹⁶⁶]. Furthermore, the produced biodiesel can satisfy ASTM biodiesel specifications.

## Single atom and cluster on HAP catalyst

Single-atom catalysts (SAC) have received extensive research attention due to their promise of high atom efficiency and favorable metal-support interaction[¹⁶⁷]. Ion exchange method is most commonly used to synthesize SAC using HAP as catalyst support. The seminal work by Yamaguchi et al. used the ion-exchange method to produce single-atom Ru³⁺, which substitutes Ca²⁺ in the HAP framework[¹⁶⁸]. The presence of Ru³⁺ had been evidenced using XANES and EXAFS, as observed in Figure 7A. The Ru³⁺ species are observed to be coordinated to the Cl atom, which produces a highly active catalyst for aerobic alcohol oxidation. The same ion exchange method has been demonstrated for single atomic Pd[¹⁴³], and Co[¹⁶⁹], on HAP. Theoretical and computational study was conducted on single atomic transition metals (i.e., Fe, Co, Ni, and Cu) on HAP, revealing their suitability in direct POM to methanol[¹⁷⁰]. The ion-exchange method can also be used to deposit clusters on HAP support as well. Tounsi et al. reported that the time taken for ion exchange to occur plays an important role in influencing the nature of Cu species and Cu loading as Cu²⁺ species will first chemisorb onto the HAP surface, followed by a slower reaction to be integrated into the HAP framework[¹⁷¹]. Although Tounsi et al. claimed that CuO was present on the HAP, no TEM and XANES characterization were shown to support this claim[¹⁷¹]. Thus, the effect of ion-exchange time on cluster formation requires further investigation. Lu et al. performed DFT on Fe-exchanged HAP for oxygen reduction reaction (ORR)[¹⁷²]. The first principles study indicated that Fe species exchanged at Ca(II) site is more favorable. The Fe species at that site is favorable towards oxygen chemisorption which is the highest energy barrier to overcome to start ORR. The electrostatic adsorption method was used to create highly dispersed Ni-active sites on Ce-doped HAP [Figure 7B], which was characterized by higher activity and stability in DRM[⁸⁹]. Although active sites in the form of single atoms and clusters can be deposited onto HAP support, the mesoporosity of the HAP support can be increased to enhance active site dispersion.
<figure><img src="image_18.png" /><figcaption>**Figure 7.** (A) FT magnitude of *k*³-weighted EXAFS of RuHAP with an inset showing a proposed surface around Ru³⁺ of the RuHAP catalyst (Left) and inverse FT of the peaks with the 0.8 < R/Å < 2.8 range (Right). Adapted from Ref. [168] with permission from American Chemical Society. (B) Electron microscopy images and size distribution with evidence of single-atom Ni in 0.5Ni₃/HAP-Ce samples. Adapted from Ref. [89] under CC BY 4.0 license.</figcaption></figure>
## Mesoporosity in HAP-based catalyst

The ability to control the catalyst pore size can have a profound effect on product selectivity[173]. The catalyst pore size can be tuned by two methods: soft-template and hard-template. The effect of pore structure in the HAP support on the DRM performance of Ni/HAP catalysts was reported by Li *et al.*[97]. The mesopore/macropore ratio of the support can be altered by adjusting the urea-hydrolysis precipitation temperature. This ratio dictates Ni dispersion and the size and interaction of NiO nanoparticles with the support. A high fraction of macropores results in poor Ni dispersion and low metal-support interaction. As mesopores increase, Ni preferentially disperses along mesopore channel walls, resulting in smaller nanoparticles with enhanced metal-support interaction and improved activity. However, too small pore sizes cause blockage by Ni nanoparticles, leading to suboptimal DRM activity. In another study, the specific surface area and pore structure of the HAP support were modified using various surfactants in a soft-templating synthesis approach. Soft templating by a combination of two surfactants was reported to significantly increase the surface area and enhance reactant conversion in DRM[146]. The use of surfactants such as CTAB during the synthesis stage can help to increase BET surface area. Essamlali *et al.* utilized CTAB to increase the synthetic HAP surface area, in addition, to increasing surface basicity by impregnating with Na[79]. The hard templating method first involves the hydrothermal treatment of SBA-15 in sucrose solution to produce carbonized sucrose in the SBA-15 pores[174]. The SBA-15 SiO₂ hard template was removed by washing with concentrated NaOH to obtain carbon nanorods. Those carbon nanorods were added to the coprecipitation solution for HAP nanoparticle synthesis. The BET surface area of the resultant HAP after air calcination is 242.2 m²/g~cat~, which is among the highest BET surface areas reported for the HAP material.

The HAP BET surface area can be increased using various treatment methods. Bone-derived HAP catalyst underwent two preparation methods: (1) hydrothermal treatment after calcination, and (2) without hydrothermal treatment after calcination[175]. The hydrothermal-treated sample showed a higher BET surface area and FAME yield, indicating improved access to active sites due to increased surface area and mesopores, as shown in the BET results. However, no relationship was found between hydrothermal duration and catalytic activity. Ghanei *et al.* used waste bone and reported low catalytic activity[78]. Their procedure indicated an average particle size of 180-300 µm, resulting in a lower BET surface area of 3-5 m²/g. Therefore, the preparation method and type of bone significantly influence catalytic performance.

## FUTURE RESEARCH DIRECTIONS AND OUTLOOK

The HAP unit cell structure accommodates various cations due to similar ionic radii and valence charges. Cations integrate into HAP through surface complexation, which involves liquid-solid mass transfer that can create resistance. Therefore, it is crucial to explore strategies to speed up the substitution. The Ca/P ratio also significantly affects the HAP surface basicity or acidity. For DRM, the presence of basic sites generally helps to alleviate carbon filament formation, thereby extending the longevity of the catalytic performance. For other catalytic reactions such as WGS reaction, higher surface basicity helps with enhancing water dissociation; thereby, higher H₂ formation[160]. To better understand the surfaces of HAP-based catalyst surfaces, *in-situ* or *operando* spectroscopic techniques such as DRIFTS, XPS, and X-ray absorption spectroscopy (XAS) can be used to gain insight into the active metal (e.g., Ni)-Ca coordination, which may be important to understand metal-support interaction.

According to the literature examined in the review manuscript, Ca(II) is observed to be the site of choice for cation substitution. Limited studies attempted to investigate the concentration of Ni²⁺ substituted into the Ca(I) or Ca(II) sites[90]. However, the structure-activity relationship between DRM activity and the concentration of Ni²⁺ substituted into the Ca(I) or Ca(II) remains unclear. Using DFT and microkinetic modeling to clarify the structure-activity relationship for DRM would be interesting, involving examining
how catalytic activity improves with varying $Ni^{2+}$ concentrations in $Ca_{(I)}$ or $Ca_{(II)}$ sites. In fact, DFT studies have been employed in other reaction studies, such as the ORR using Fe-doped HAP, where DFT uncovered superiority in reducing $O_2$ to $H_2O$ with less kinetic and thermodynamic barrier in the OH hydrogenation or 2OH self-dehydration in Fe-substituted HAP (i.e., Fe@HAP) compared to Fe/HAP[172]. DFT and microkinetic studies were employed to uncover how the unique combination of acidic and basic properties of HAP facilitates different elementary reactions in ethanol-to-butanol biofuels production[176], or how defects in HAP can stabilize intermediate reaction species[177]. Importantly, DFT is not only a tool for understanding cationic substitutability as earlier elucidated[42,43], but can reveal key mechanistic insights not limited to preferential reaction pathways or adsorption affinity on different active sites (i.e., active metals, or defects on HAP)[132,178]. Substitution with foreign anion at either the $OH^-$ group or $PO_4^{2-}$ group can affect surface properties of the HAP surface, especially surface acidity and basicity. Currently, there is a scarcity of literature on HAP-based catalysts examining the effect of anionic substitution on catalytic activity, except for the ones on oxidative PDH and OCM. More experimental studies on the effect of anionic substitution on catalytic activity and product selectivity for $CO_2$ methanation, WGS reaction, CO oxidation, and DRM can be examined in greater detail. For example, studying how $CO_2$ can be absorbed in HAP through anionic substitution as carbonated HAP[179,180] or possibly with exsolved calcium ions after cationic substitution may be an interesting avenue of research for identifying efficient, low-cost carbon capture or chemical looping materials[181].

Core-shell nanoarchitectures involving the HAP nanoparticles were primarily used for gas sensors and non-catalytic applications[182-184]. HAP-based catalysts have shown promise in high-temperature reactions such as DRM and OCM. Their catalytic stability can be improved by encapsulating HAP nanoparticles with inert shells (e.g., $SiO_2$) using methods such as Stöber synthesis or reverse microemulsion. Promoters play a critical role by blocking undesired reaction pathways, enhancing reactant adsorption, and improving product selectivity. Alkaline or basic promoters facilitate water dissociation, aiding WGS reaction rates, and also enhance the reducibility of active metal centers.

In $CO_2$ capture and utilization, such promoters enable dual-function materials that both adsorb and convert $CO_2$[185]. During flue gas exposure, $CO_2$ is captured at basic sites; when switched to $H_2$, the adsorbed $CO_2$ is converted into products such as $CH_3OH$ or $CH_4$ at metal sites. These promoters also increase surface basicity, which improves biodiesel yields. To fully understand their role, it is important to examine whether they also influence catalyst structure and texture, using both *ex-situ* and *in-situ* characterizations on spent catalysts.

Literature reports that the BET surface area of HAP-based catalysts can vary widely, from as low as 5 up to 242.2 $m^2$/g. The highest values are typically achieved using hard templating methods, such as SBA-15-derived carbon nanorods. However, this approach is complex and time-consuming. Despite the promise of mesoporous HAP, its actual use in catalysis remains limited. Alternatively, poly(methyl methacrylate) (PMMA) can serve as a hard template that burns off easily, potentially saving synthesis time and reducing costs. Although natural bones can still be used as a source of HAP, the BET surface area is still very low (~3-5 $m^2$/g), which implies that the HAP particles are in the micron range. Thus, in addition to physical pulverization, the size of natural bone-derived HAP may be further reduced using a ball-milling technique. Although increasing the ball-milling speed can result in smaller particle sizes, the phase of the material may change. Therefore, it is important to optimize the ball-milling speed to achieve a minimum particle size without significantly altering the material phase.
<figure><img src="image_19.png" /><figcaption>**Figure 8.** (A) Piezoelectric effect of HAP. Reproduced from Ref. [191] with permission from Elsevier. (B) Alcohol production at different CH₄:O₂ ratios for HAP via piezoelectric catalysis. Reproduced from Ref. [191] with permission from Elsevier.</figcaption></figure>

More recently, piezoelectric catalysis has gained increasing research attention to generate reactive oxygen species (ROS) - hydroxyl radicals (OH·) can activate C-H bond in methane at milder conditions to facilitate C-C coupling to form hydrocarbon compounds and oxygenates[186,187]. Piezoelectric materials, upon application of mechanical stress, can induce the asymmetrical distribution of surface charge and start an electrochemical reaction[188]. HAP is found to possess piezoelectric properties, which can be exploited to generate ROS[189,190]. Zhou *et al.* recently studied the use of HAP as a potential material for piezoelectric methane oxidation[191]. Interestingly, the HAP material, upon ultrasonic-induced stresses, produces hydroxyl radical (·OH) to attack the methane C-H bond and negative surface charges to drive methanol dehydration into methyl carbene as a C1 precursor for upgrading into C₂-C₃ alcohols, as shown in Figure 8. Further examination of the surface engineering strategies discussed in this review on HAP as piezoelectric catalysts could be an interesting future research focus.

We have covered sustainable chemical production via OCM, PDH, biodiesel production, and H₂ production and/or CO₂-modulating reactions, including CO₂-utilizing DRM, POM, CO oxidation, and CO₂ methanation. However, other reactions relevant to sustainable chemical and fuel production include CO₂ hydrogenation to methanol, CO₂ hydrogenation to olefins and aromatics, and the upgrading of alcohol (ethanol) to higher alcohols[192], which can benefit from HAP-based catalysts. However, these are explored only to a limited extent. Additionally, to the best of our knowledge, there seems to be a scarcity of literature regarding the economics of HAP-based catalyst production. According to Guo *et al.*, the cost to produce the copper HAP catalyst is approximately USD 1.0 per kilogram[133]. However, this calculation does not account for key factors such as operating costs and bulk raw material pricing. HAP is often viewed as a green and sustainable catalyst, especially because it can be derived from bio-based waste sources such as animal bones and fish scales. This not only reduces environmental impact but also improves the overall sustainability of HAP-based catalytic systems. That said, the feasibility of scaling up HAP - whether to pilot[193] or commercial levels - still requires further investigation, particularly in terms of supply chain and catalytic performance.

## CONCLUSION

The propensity of HAP material for cation and/or anion substitution enhances surface basicity, creates active sites, and introduces lattice oxygen species. As a result, new active sites can be formed for catalytic reactions that are crucial for chemical and fuel products such as DRM, CO₂ hydrogenation, POM, CO
oxidation, and biodiesel production. The basicity of the HAP surface can be adjusted by adding basic and alkaline elements such as K, Na, and Sr. Incorporating these elements helps improve product selectivity and reactant conversion. For certain reactions that rely on precious metals as active sites, metal loading can be reduced by employing single-atom or cluster catalysts. Cationic substitution in HAP can be utilized to generate such single atoms and clusters, which may be catalytically active, selective, and stable for reactions essential to developing more sustainable fuels and chemical products such as CO₂ hydrogenation and DRM. Enhancements in these catalytic properties would lead to higher product yields, thereby lowering manufacturing costs. Improvements in catalytic performance using HAP-based catalysts can be achieved through a deeper understanding of the HAP surface, utilizing synergistic surface characterization techniques such as XAS, XPS, and FTIR in either *in-situ* or *operando* mode.

## DECLARATIONS

### Authors' contributions

Wrote the manuscript: Lim, K. H.; Wai, M. H.

Reviewed the manuscript: Cao, K.; Das, S.; Nzihou, A.; Kawi, S.

### Availability of data and materials

Not applicable.

### Financial support and sponsorship

The authors acknowledge the financial support by the National Research Foundation, Singapore, and A*STAR under its Low-Carbon Energy Research (LCER) Funding Initiative (FI) Project (U2102d2011, WBS: A-8000278-00-00).

### Conflicts of interest

Kawi, S. is an Editorial Board Member of the journal *Energy Materials* but is not involved in any steps of editorial processing, notably including reviewer selection, manuscript handling, or decision-making, while the other authors have declared that they have no conflicts of interest.

### Ethical approval and consent to participate

Not applicable.

### Consent for publication

Not applicable.

### Copyright

© The Author(s) 2025.

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