Activation of C−H Bond of Propane by Strong Basic Sites Generated by Bulk Proton Conduction on V‐Modified Hydroxyapatites for the Formation of Propene.

Abstract

Insights into the catalytic transformation of propane to propene on V-apatite catalysts are provided based on structure-reactivity relationships. Substitution of phosphates by vanadates in the hydroxyapatite structure leads to the formation of Ca 10 (PO 4 ) 6-x (VO 4 ) x (OH) 2-y O y V-oxy-hydroxy-apatite solid solutions (x = 0!6). Bulk vanadium incorporation promotes (i) calcium rich terminations (XPS, CO adsorption), (ii) proton deficiency inside the OH À channels ( 1 H NMR) giving rise to O 2À native species, (iii) the thermally-activated formation of additional O 2À species along the OH À channels resulting in H-bonding interaction (in situ DRIFT) and (iv) the proton conduction process that eventually results in the surface exposure of O 2À species (in situ impedance spectroscopy). The exposure of Ca 2 + À O 2À surface acid-base pairs allows the dissociation of hydrogen, emphasizing the strong basicity of the related O 2À species. Whereas an increasing vanadium content is beneficial to propene selectivity, it scarcely impacts propane conversion. The reaction proceeds mainly upon oxidative dehydrogenation, even if the minor dehydrogenation route is also observed. Surface O 2À generated thanks to proton mobility are involved in the CÀ H bond activation, as shown by the synergistic effect between the oxidative dehydrogenation of propane reaction and the bulk proton conduction measured under operando conditions. This puts emphasis on the key role of strong basic sites for propane activation.the temperature was cooled down to 673 K and the conductivity set-up was fed with the reacting feed. The H + ion-conductivity of the working catalyst was measured after 2 hours on stream at 673, 723 and 773 K.

Characterisation and reactivity of oxygen species at the surface of metal oxides

“…Thus, new research areas merged to investigate the reactivity of other anions in light of their coordination chemistry. To exemplified briefly these ongoing studies, we can focus on the work already made on hydroxyapatite, a biocompatible calcium phosphate with generic formula Ca10(PO4)6(OH)2, involving a flexible framework that makes its composition tunable due to substitution ability [186][187][188] and non-stoichiometric properties [189]. This non-stoichiometry composition was long used to describe the tunable catalytic properties ranging from acidic to basic.…”

Synergistic Effect Between Ca4V4O14 and Vanadium‐Substituted Hydroxyapatite in the Oxidative Dehydrogenation of Propane

“…[21] These V x -HAp-pH-9 samples were found to dehydroxylate to Ca 10 (PO 4 ) 6-x (VO 4 ) x (OH) 2-y O y vanadium oxyhydroxy-apatite solid solutions upon thermal treatment. [22] In this series of samples, vanadium was shown to be present as highly-dispersed isolated monomeric tetrahedral species (VO 4 3À : vanadates, Figure 1A, left). These compounds were investigated as a model system for the ODHP reaction as they likely provide increasing amounts of isolated vanadates upon increasing the V loading.…”

“…In that case, a Ca 10 (VO 4 ) 5.22 (PO 4 ) 0.78 (OH) 2 single phase was obtained and will be hereafter referred to as V 5.22 -HAp-pH-9. [21][22]…”

“…Such a finding has already been observed for the pure vanadium-substituted hydroxyapatite solid solution and was accounted for the involvement of Ca-rich terminations of the hydroxyapatite phase. [22] This effect is the more pronounced as the Ca 4 V 4 O 14 fraction becomes predominant (x = 5.22 and 6), which suggests that Ca terminations should be favored also in the lamellar structure of Ca 4 V 4 O 14 . [21,23b] Similarly, the higher surface P/V molar ratios (XPS) compared to those found for the bulk (ICP) in Table 1 indicate that the prepared materials exhibit V-deficient surfaces.…”

“…These compounds were investigated as a model system for the ODHP reaction as they likely provide increasing amounts of isolated vanadates upon increasing the V loading. [22] Although the surface relaxation was found to result in the prevailing exposure of Ca 2 + species on the surface at the expense of that of vanadates, the vanadium oxy-hydroxyapatite solid solutions formed in situ upon heating were found to enable propane activation. This interesting peculiarity was attributed to the intrinsic and thermally-activated proton mobility that eventually generates, in situ, strongly basic O 2À species on the oxy-hydroxy-apatite surface.…”