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Introduction to Hydroxyapatite-based Materials in Heterogeneous Catalysis

Doan Pham Minh

Université de Toulouse, IMT Mines Albi, UMR CNRS 5302, Centre RAPSODEE, Campus Jarlard, F-81013 Albi, Cedex 09, France

1.1 Generality

As recently reviewed by Wisniak [1] and summarized in the book of Zecchina and Califano [2] on the history of the catalysis, the first known work seemed to be carried out in 1552 by Valerius Cordus, for the synthesis of ether from alcohol catalyzed by sulfuric acid. During the next centuries, several examples were recorded [1], such as the hydrolysis of potato starch in water catalyzed by potassium hydrogen tartrate or by acetic acid (1781); hydrogen production from alcohol and concentrated sulfuric acid catalyzed by alumina, silica, or clay without external heating (1796); starch conversion into gum, dextrin, and raisin sugar catalyzed by inorganic acids (1811); and starch fermentation into alcohols via sugar formation (1816). However, the term catalysis was only introduced for the first time by Berzelius in 1835 [1,3], which means "the property of exerting on other bodies an action which is very different from chemical affinity. By means of this action, they produce decomposition in bodies, and form new compounds into the composition of which they do not enter." In 1836, Berzelius stated "the catalytic power seems actually to consist in the fact that substances are able to awake affinities, which are asleep at a particular temperature, by their mere presence and not by their own affinity" [2,4].

The first large-scale industrial application of the catalysis could be assigned to the production of sulfuric acid over platinum catalyst at the end of the nineteenth century [2]. This process was later improved (around 1920) by replacing platinum catalyst by vanadium pentoxide, which is still used today for sulfuric acid production. The beginning of the twentieth century was also marked by the industrialization of the ammonia oxidation over platinum catalyst to produce nitric acid (Ostwald-Brauer process), ammonia production from direct hydrogenation of molecular nitrogen over iron-based catalysts (Haber-Bosch process), and liquid fuel synthesis from syngas over iron, cobalt, or ruthenium catalysts (Fisher-Tropsch process) [2]. Then, the catalysis passed through its golden period with the petroleum and polymer eras, with the discovery and industrialization of many important catalytic processes such as catalytic cracking, isomerization, alkylation, reforming, hydrodesulfurization, and polymerization [2]. Nowadays, the catalysis has a pivotal role in the modern society, and it is present in ca. 85% of manufacturing processes (Figure 1.1a), as recently reviewed by Thomas and Harris [5].

Figure 1.1 Pie charts showing: (a) the percentages of catalytic processes versus non-catalytic processes; (b) and the percentages of all industrial processes that entail the use of heterogeneous, homogeneous and bio-catalysis.

Source: Adapted from Thomas and Harris [5] with permission of the Royal Society of Chemistry.

Generally, three categories of catalysts are distinguished: heterogeneous catalysts that are usually solid materials, homogeneous catalysts that are usually soluble salts or complexes, and enzymatic catalysts. Among them, heterogeneous catalysts occupy an important place in comparison with the counterparts, mostly because of their easy separation from products and their applicability in large ranges of temperature and pressure. Thus, among catalytic processes, heterogeneous catalysts represent up to 80%, much higher than the parts of homogeneous and enzymatic catalysts (Figure 1.1b) [5].

More specifically, other catalyst classifications have been also proposed in the literature. For example, by key properties responsible for their catalytic behavior, the following catalyst families can be classified: redox catalysts, acid-base catalysts, or polyfunctional catalysts [6]. Misono [7] classified catalysts by their main components. Thus, different families of catalysts are classified such as metals, metal oxides, metal salts, metal coordination compounds, organic molecules, organic polymers, and biocatalysts.

Considering the periodical table of elements, the most well-known catalysts are found within the elements of the columns 1-14, which are principally metals and metal-based compounds. Among the most available non-metallic elements (e.g. C, Si, N, P, O, S, F, Cl, Br, I), O is usually present in catalytic materials; C constitutes a large family of catalyst supports such as activated carbon, carbon nanotube, carbon nanofiber, and graphene; Si also builds different types of catalysts (zeolites) or catalyst supports (SiO2-based materials); F, Cl, Br, I, N, and S have been applied as acid catalysts (in the form of their inorganic acids) or catalyst additives (in the form of their inorganic acids or salts) to modify acid-base properties of another material. On the other hand, P is more particular in comparison with the other non-metallic elements, since it provides a large number of water soluble and insoluble compounds, of simple or complex oligomer or polymer structures, e.g. orthophosphates (), pyrophosphates (), cyclophosphates, and polyphosphates [8]. Among them, phosphate is the most popular form of phosphorus compounds and also constitutes a large number of materials that have potential applications in the catalysis of various chemical processes such as dehydration and dehydrogenation of alcohols, hydrolysis reactions, oxidation and aromatization reactions, isomerization, alkylation, Knoevenagel reaction, Claisen-Schmidt condensation, epoxidation, nitrile hydration, cycloaddition, Michael addition, water splitting, and conversion of biomass-derived monosaccharides [9-13]. For example, Vieira et al. [13] studied direct conversion of glucose and xylose to HMF (5-hydroxymethylfurfural) and furfural over the niobium phosphates as efficient bifunctional catalysts. The acidity of the catalysts, expressed as the ratio of Lewis to Brønsted acid sites (L/B) could be tuned by varying the molar ratio of P to Nb, which allows controlling the catalyst performance. Thus, both the conversion of monosaccharides (glucose and xylose) and the formation of furans linearly increases with the L/B ratio; the later increases by decreasing the P to Nb molar ratio. In another example, Kim et al. [14] successfully synthesized cobalt pyrophosphate () and cobalt phosphate () based materials as new electrocatalysts for water splitting reaction. High electrocatalytic activity and stability during 100 cycles were observed, explained by the structural stabilities of the investigated materials.

Stoichiometric calcium phosphate hydroxyapatite (chemical formula: Ca10(PO4)6(OH)2), denoted thereafter HA, belongs to the family of apatites, which is a category of phosphate compounds. A given apatite has the general chemical formula of M10(XO4)6Y2, where M is generally a bivalent metal cation (Ca2+, Mg2+, etc.), XO4 is generally a trivalent anion (, , etc.), and Y is typically a monovalent anion (OH-, F-, Cl-). However, other cations and anions with different valences can also be present in an apatitic compound. For example, carbonate anion () can partially replace both Y and XO4 anions, while Na+, K+, Al3+, etc., can partially replace M cations. More details on the apatite composition and structure will be discussed in Chapter 3 of this book.

In heterogeneous catalysis, HA-based materials are still considered as a new potential family of catalysts for various applications, as recently reviewed by Gruselle [15], and Fihri et al. [16]. Gruselle [15] focused his review on the catalytic performance of HA-based materials in the organic synthesis, while Fihri et al. [16] enlarges their review to other processes including the photocatalysis, hydration, hydrogenation, hydrogenolysis, transesterification, and multicomponent reactions. On the one hand, HA can be used itself as a solid catalyst. For example, in the Knoevenagel condensation reaction conducted by Sebti et al. [17], undoped HA allowed obtaining high yields (e.g. 80-98%) at room temperature and atmospheric pressure. In many cases, HA-based materials were usually found to be competitive in terms of catalytic performance in comparison with conventional catalysts. On the other hand, HA can be used as a catalyst support to disperse a catalytically active phase, such as metal nanoparticles, or metal cations. For instance, Phan et al. [18] demonstrated the superior catalytic activity of HA supported bimetallic Co-Ni catalysts in the dry reforming of methane for syngas production. As will be discussed in Chapter 3 of this book, the first publication on HA as catalyst appeared around 1940s, but, research on HA catalysts has only accelerated during the last two decades, with an exponential increase of the annual number of publications. The Section 1.2 will focus on the main reasons making HA-based materials as new promising candidates in the heterogeneous catalysis.

1.2 Hydroxyapatite: A New Family of Catalytic Materials in the Heterogeneous Catalysis

Well-known for their application in the field of biomaterial...