Incorporation of vanadium into the framework of hydroxyapatites: importance of the vanadium content and pH conditions during the precipitation step

Abstract

Even though vanadium-modified hydroxyapatite (V-HAp) samples are very promising systems for oxidative dehydrogenation of propane, the incorporation of vanadium into the hydroxyapatite framework was reported to be limited and to lead to over-stoichiometric compounds. Here, the synthesis of a Ca(PO)(VO)(OH) stoichiometric solid solution using a co-precipitation method is monitored in the whole composition range (0 ≤ x ≤ 6) by controlling the pH of the precipitation medium, with continuous (the first series of samples) or periodic (the second series of samples) addition of NHOH during the precipitation step or during the maturation step, respectively. It is demonstrated that the changes in pH conditions result in materials of a substantial difference in terms of the final composition. From XRD patterns and Rietveld refinements, a solid solution V-HAp phase was found to be exclusively obtained for the first series of samples for x varying from 0 to 6. This also occurred in the second series of samples but only for x lower than 4. For 4 ≤ x ≤ 5.22, the materials were composed of a mixture of V-HAp and CaVO, whereas for a x value of 6 only CaVO was formed. The predominance of polymeric V species in solution at a high vanadium concentration deduced from the diagram of speciation of vanadium accounts for the preferential formation of CaVO under these particular conditions. However, provided that a higher pH value was maintained, isolated VO(OH) species are predominant, which accounts for the incorporation of isolated vanadates into the hydroxyapatite framework and for the well-controlled stoichiometry with Ca/(P + V) ratios found to be close to 1.67. Such a very good accommodation of vanadium in the hydroxyapatite framework is illustrated by the characterization of the local surrounding of phosphorus and vanadium species using P andV NMR, Raman and UV-vis spectroscopies.

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.

“…In addition, the bulk Ca/(P+V) molar ratios were found to be close to 1.67 indicating that stoichiometric hydroxyapatites have been successfully prepared for the samples with x <6. The higher Ca/(P+V) molar ratio obtained for the fully V‐substituted sample ( x =6) is attributed to carbonate incorporation that is favored during the precipitation step under high pH conditions …”

“…It was also suggested that the full substitution of phosphates by vanadate groups in the apatite structure might result in the preferential stabilization of vanadate oxyapatite (Ca 10 (VO 4 ) 6 O) . Recently, we succeeded in synthesizing Ca 10 (PO 4 ) 6‐ x (VO 4 ) x (OH) 2 solid solutions (0≤ x ≤6) (Figure S1), while maintaining the targeted stoichiometric Ca/(P+V) molar ratio to a value of 1.67 over the whole composition range (except for the fully substituted sample with x =6) . This was achieved thanks to a one pot co‐precipitation synthesis carried out with continuous adjustment of the pH of the precipitation medium to 9 during the whole precipitation step at 353 K .…”

“…Recently, we succeeded in synthesizing Ca 10 (PO 4 ) 6‐ x (VO 4 ) x (OH) 2 solid solutions (0≤ x ≤6) (Figure S1), while maintaining the targeted stoichiometric Ca/(P+V) molar ratio to a value of 1.67 over the whole composition range (except for the fully substituted sample with x =6) . This was achieved thanks to a one pot co‐precipitation synthesis carried out with continuous adjustment of the pH of the precipitation medium to 9 during the whole precipitation step at 353 K . Yet the catalytic properties of such highly‐loaded vanadium‐modified HAps have not been investigated to our knowledge.…”

“…The influence of the incorporation of increasing vanadium contents into the bulk of HAps on the surface composition of the as‐prepared catalysts will be discussed on the basis of XPS and CO‐FTIR data. In the absence of nucleophilic vanadyl bonds in this system, the involvement of other peculiar oxygen species in the activation of propane will be investigated. Given the possible influence of vanadium on the stabilization of oxyapatite, and the high temperatures applied in the reaction, the structural organization of the OH groups stacked along the channels was investigated both in the as‐prepared samples and in the thermally‐activated ones.…”

Synergism of wt-p53 and synthetic material in local nano-TAE gene therapy of hepatoma: comparison of four systems and the possible mechanism

“…However, that safety is only observed in bone tissue and nonparenchymal cell. In the present study, the unmodified nHAP showed comparable cytotoxicity both to HepG2 and L02 cells, mostly due to its surface properties as well as high negative zeta-potential, whose inner expulsion also induce nHAP precipitation and congregation [18, 21, 24]. As surface coating is a primary determinant of cytotoxicity, nHAP was surface-modified by utilizing Ca (2+) and Pll, representing popular strategies of inorganic and organic respectively.…”