Structure and Surface Reactivity of WO₄^2–, SO₄^2–, PO₄^3– Modified Ca-Hydroxyapatite Catalysts and Their Activity in Ethanol Conversion

Abstract

In the current study, we have synthesized calcium hydroxyapatite (CaHAP) from different phosphorus sources namely, NH 4 H 2 PO 4 and H 3 PO 4 . The structure of CaHAP was confirmed by XRD, FT-IR, and Raman characterization methods. The CaHAP was further modified with various anions such as WO 4 2− , SO 4 2−, and PO 4 3− with fixed content of 10 wt %. To understand the textural and structural properties, these samples were thoroughly characterized by N 2 physisorption, X-ray diffraction, Fourier transform infrared, Raman and thermogravimetry−differential thermal analysis methods. Ethanol adsorption at various temperatures was studied in detail using diffuse reflectance Fourier transform spectroscopic to unravel the formation and stability of surface species and the interaction of ethanol with CaHAP. The temperature programmed desorption of ethanol (ethanol-TPD) was performed to understand the stability, surface reactivity, and product distribution. The catalytic activity of the above catalysts was tested in ethanol conversion over a temperature range of 300−450 °C.

Molecular Understanding of the Bulk Composition of Crystalline Nonstoichiometric Hydroxyapatites: Application to the Rationalization of Structure–Reactivity Relationships

“…Raman and 31 P NMR spectroscopic techniques are commonly used to discriminate between the different calcium phosphates phases . From the Raman spectra reported in Figure (A), all the samples exhibit the typical band at 960 cm –1 assigned to the ν 1 vibration mode of phosphate groups (PO 4 3– ) of the hydroxyapatite structure . In the same way, all 31 P NMR spectra reported in Figure (B), reveal a main intense line centered at 2.9 ppm, which is also characteristic of bulk PO 4 3– of hydroxyapatites…”

“…Beyond phase identification and the crystallinity description provided by XRD, techniques such as IR, Raman, and NMR spectroscopy are useful tools to investigate local organization of the various species building up the HAp structure. Indeed, these characterization techniques are used in a complementary manner to provide qualitative and semiquantitative data not only on the structural hydroxyl (OH – ) and phosphate (PO 4 3– ) groups, which are the main components of the apatitic framework, but also on the presence of additional species possibly substituted within the structure, such as hydrogenphosphate (HPO 4 2– ) or carbonate (CO 3 2– ) groups,[11a], as illustrated by the typical IR DRIFT spectrum recorded at 623 K at the end of the in situ thermal pretreatment that is reported in Figure . Notably, compared with reported FTIR spectra, which are usually recorded at room temp., the band positions observed in the DRIFT spectra recorded at 623 K (see Exp.…”

“…The ν OH area contains (i) an intense and sharp band at 3566 cm –1 assigned to the stretching vibration of column OH groups present inside the channels of the hydroxyapatite[17a] (vs. 3572 cm –1 in FTIR spectrum recorded at room temperature), (ii) bands with low intensity and weak resolution located in the 3800–3600 cm –1 area that were previously assigned to PO‐H groups, and (iii) a low‐intensity, broad contribution at 3500–3200 cm –1 ascribed to residual adsorbed water. The 2200–1900 cm –1 region contains weak contributions associated with the overtone and combination modes of the P–O bands,, and the 1550–1250 cm –1 area contains a broad multicomponent band assigned to the stretching mode of the carbonates groups and the P–O vibrator (below 1350 cm –1 ).…”

Mechanism and Kinetics of Ethanol Coupling to Butanol over Hydroxyapatite

“…Attempts to identify the catalytically active sites on HAP have relied largely on spectroscopic studies. 26,27,28,29 Diallo-Garcia et al have observed the existence of basic OH -, O 2and PO4 3groups through FTIR of adsorbed CO2. 26,27 Further studies by these authors using acetylene as a probe have revealed an additional acid-base (POH-OH) pair.…”

The Guerbet Coupling of Ethanol into Butanol over Calcium Hydroxyapatite Catalysts

“…These cations are coordinated to 6 phosphate oxygens and 1 oxygen belonging to a hydroxyl group [43,47]. ) [42,56], and OHreplacement (F -, Cl -) [43].…”