Apatites based catalysts: A tentative classification

Hydroxyapatite from Mollusk Shells: Characteristics, Production, and Potential Applications in Dentistry

Modern dentistry is turning towards natural sources to overcome the immunological, toxicological, aesthetic, and durability drawbacks of synthetic materials. Among the first biomaterials used as endosseous dental implants, mollusk shells also display unique features, such as high mechanical strength, superior toughness, hierarchical architecture, and layered, microporous structure. This review focusses on hydroxyapatite-a bioactive, osteoconductive, calcium-based material crucial for bone healing and regeneration. Mollusk-derived hydroxyapatite is widely available, cost-effective, sustainable, and a low-impact biomaterial. Thermal treatment coupled with wet chemical precipitation and hydrothermal synthesis are the most common methods used for its recovery since they provide efficiency, scalability, and the ability to produce highly crystalline and pure resulting materials. Several factors, such as temperature, pH, and sintering parameters, modulate the size, purity, and crystallinity of the final product. Experimental and clinical data support that mollusk shell-derived hydroxyapatite and its carbonated derivatives, especially their nanocrystaline forms, display notable bioactivity, osteoconductivity, and osteoinductivity without causing adverse immune reactions. These biomaterials are therefore highly relevant for specific dental applications, such as bone graft substitutes or dental implant coatings. However, continued research and clinical validation is needed to optimize the synthesis of mollusk shell-derived hydroxyapatite and determine its applicability to regenerative dentistry and beyond.

Tuning Mg Doping and Features of Bone-like Apatite Nanoparticles Obtained via Hydrothermal Synthesis

Nanocrystalline apatites have been intensively studied for decades, not only for their well-known mimesis of bone apatite but also for applicative purposes, whether as biomaterials for skeletal repair or more recently for a variety of nanomedical applications enabled by their peculiar surface characteristics. Particularly, ion-doped apatites are of great interest because the incorporation of foreign ions in the composition of apatite (nano)crystals alters the bulk and surface properties, modifying their ability to interact with the external environment. This is clearly seen in the physiology of bone tissue, whose mineral phase, a low crystallinity apatitic phase, can dynamically exchange ions with cells, thus driving bone metabolism. Taking bone mineral as a model, the present work describes the development of Mg-doped hydroxyapatite nanoparticles, exploiting hydrothermal synthesis to achieve extents of Mg2+ doping hardly achieved before and using citrate to develop stable apatite colloidal dispersions. Morphological and physicochemical analyses, associated with in-depth investigation of ions populating the apatitic lattice and the nonapatitic surface layer, concurred to demonstrate the cooperative presence of Mg2+ and citrate ions, affecting the dynamic ion retention/release mechanisms. Achieving high Mg2+ doping rates and understanding how Mg doping translates into surface activation of apatite-based nanoparticles is expected to foster the design of novel smart and tunable devices, to adsorb and release ionic species and cargo molecules, with potential innovations in the biomedical field or even beyond, as in catalysis or for environmental remediation.

Highly dispersed Co anchored on Ce-doped hydroxyapatite as a dual-functional catalyst for selective hydrogenolysis of 5-hydroxymethylfurfural

Hydrodeoxygenation (HDO) is an indispensable approach to produce renewable biofuels and value-added chemicals using natural biomass and its derivatives. 2,5-Dimethylfuran (DMF) is considered to be a very promising liquid biofuel, and it can be fabricated by HDO of the biomass derivative 5-hydroxymethylfurfural (HMF). Herein, a highly efficient bifunctional catalyst, Co/HAP(Ce), was fabricated by anchoring highly dispersed Co on Ce-doped hydroxyapatite (HAP(Ce)). Co/HAP(Ce) displayed excellent HDO catalytic activity to convert HMF to DMF, and 99% HMF conversion and 96% DMF selectivity can be obtained under 150 °C, 2 MPa H2 conditions for 5 h. Density functional theory calculations revealed that H2 can be more easily activated by Co/HAP(Ce). Systematic studies confirmed that the high activity of Co/HAP(Ce) can be ascribed to the desired acid-alkali properties, highly dispersed cobalt species and strong metal-support interactions. This research provides a cost effective approach for designing efficient catalysts for HDO of biomass and its derivatives.

The Mutual Incorporation of Mg2+ and CO32- into Hydroxyapatite: A DFT Study

Hydroxyapatite (HA) with a stoichiometry composition of Ca₁₀(PO₄)₆(OH)₂ is widely applied for various biomedical issues, first of all for bone defect substitution, as a catalyst, and as an adsorbent for soil and water purification. The incorporation of foreign ions changes the acid-base relation, microstructure, porosity, and other properties of the HA materials. Here, we report the results of calculations of the density functional theory and analyze the possibility of two foreign ions, CO₃^2- and Mg²⁺, to be co-localized in the HA structure. The Na⁺ was taken into account for charge balance preservation. The analysis revealed the favorable incorporation of CO₃^2- and Mg²⁺ as a complex when they interact with each other. The energy gain over the sole ion incorporation was pronounced when CO₃^2- occupied the A position and Mg²⁺ was in the Ca(2) position and amounted to -0.31 eV. In the most energy-favorable complex, the distance between Mg²⁺ and the O atom of carbonate ion decreased compared to Mg…O distances to the surrounding phosphate or hydroxide ions, and amounted to 1.98 Å. The theoretical calculations agree well with the experimental data reported earlier. Understating the structure-properties relationship in HA materials varying in terms of composition, stoichiometry, and morphology paves the way to rational designs of efficient bio-based catalytic systems.