Structure and Acidic Properties of Phosphate-Modified Zirconia

Abstract

Phosphate-modified zirconia was prepared by impregnation of Zr(OH) 4 and of m-ZrO 2 with an aqueous solution of (NH 4 ) 2 HPO 4 . Similar to sulfate and tungstate, phosphate shows a strong stabilizing effect on the specific surface area and the tetragonal phase of zirconia when using Zr(OH) 4 as the precursor material. The adsorbed phosphate species were characterized by Raman, DRIFT, and 31 P-MAS-NMR spectroscopies as pyrophosphates, chelate-bonded orthophosphates, and oligophosphates. Only a distortion of the geometry of the adsorbed phosphate species was found after dehydration of the materials in contrast to ZrO 2 /SO 4 . FT-IR spectroscopy with CO as a probe molecule was used to characterize the acidic properties of ZrO 2 /PO 4 . New hydroxyl groups, probably P-OH groups with enhanced protonic acidity as compared to pure zirconia but lower than that of ZrO 2 /SO 4 , were observed. In addition basic Zr-OH groups were observed. Furthermore, strong Lewis acidic centers (cus Zr 4+ ), comparable to ZrO 2 /SO 4 , are formed by phosphate modification of zirconia.

31P Solid State NMR and ESR Investigation of Trimethylphosphane Adsorption on Zirconia and Sulfated Zirconia

“…10,11 CO adsorption had also detected the coexistence of Lewis and Brønsted sites. [3][4][5] The observation of the signal at +65.7 ppm, which must be attributed to a protonated OTMP species, 12 is unexpected, since the presence of O 2 was excluded during the preparation procedure to the best of our knowledge.…”

“…10,11 CO adsorption had also detected the coexistence of Lewis and Brønsted sites. [3][4][5] The observation of the signal at +65.7 ppm, which must be attributed to a protonated OTMP species, 12 is unexpected, since the presence of O 2 was excluded during the preparation procedure to the best of our knowledge.ESR was applied to further elucidate this oxidation process. As shown in Figure 2, ZrO 2 -S shows an intense, nearly isotropic line at g iso ) 2.0038, which may be attributed to a SO 3 -species or an oxygen ion vacancy.…”

“…1,2 The acid properties of ZrO 2 -S have been studied by probe molecules such as CO and FTIR spectroscopy [3][4][5] and 1 H MAS (magic angle spinning) NMR gave proton chemical shifts that were more positive than those reported for H-ZSM-5. 5,6 However, O-H stretching frequency shifts induced by adsorbed CO were significantly lower than those observed for H-ZSM-5, 4 thus suggesting that the Brønsted acid strength is at least not higher than that of the zeolite.…”

Uptake properties of phosphate on a novel Zr–modified MgFe–LDH(CO3)

“…Many researchers have investigated phosphate adsorption mechanisms on trivalent and tetravalent metal hydrous oxides using Fourier transform infrared (FTIR) spectroscopy [7,[26][27][28][29][30] and solid-state NMR spectroscopy [31,32]. The success of FTIR technique relies on the fact that phosphate has strong mid-infrared bands originating from P-O(H) vibrations which can be detected even at low phosphate concentrations on surfaces [26].…”

Zirconium Phosphate Catalysts in the XXI Century: State of the Art from 2010 to Date

“…They postulated that the Brønsted acid sites possibly arise from geminal P(OH) groups, while the Lewis acid centers could be attributed to Zr 4+ [26,27]. Moreover, they found that the total acidity decreased with increasing the calcination temperature to the range 450-550 • C.…”