P-31 AND AL-27 NMR INVESTIGATIONS OF HIGHLY ACIDIC, AQUEOUS-SOLUTIONS CONTAINING ALUMINUM AND PHOSPHORUS

P-31 and Al-27 NMR spectroscopies have been used to characterize acidic, aqueous solutions of orthophosphoric acid, aluminum chloride, and tetramethylammonium (TMA) hydroxide. The final compositions of the solutions range from 0.1 to 1 mol % P, 0.0 to 20 mol % HCI, P/Al = 0.1 to 20, and P/(TMA)20 = 2 to 20. Soluble aluminophosphate cations form reactions of hexaaqua Al monomeric cations, [Al(H20)6]3+, with phosphate ligands (i.e., H3PO4, H2PO4-, and acid dimers H6P2O8 and H5P2O7-). P-31 NMR spectra of aluminophosphate solutions reveal five distinct chemical shift regions. Free phosphate molecules and ions resonate at 3 to 5 ppm, P nuclei in acid dimers bonded to Al monomers resonate at -3.5 to -5.5 ppm, P nuclei in H2PO4- ions bonded to Al monomers resonate at -8.5 to -11 ppm, P nuclei in H3PO4 molecules bonded to Al monomers resonate at -11 to -13 ppm, and P nuclei bridging two Al monomers resonate at < 15 ppm. A correlation between the partial charge on P nuclei (calculated using semiemperical molecular orbital theory, MNDO [modified neglect of differential overlap]) and the P-31 NMR chemical shift confirms the assignments of the P-31 NMR resonances. Regardless of the total charge of the complex or the connectivity of the Al to -0-P units, the MNDO calculations show that P atoms in the same type of phosphate ligand bonded to Al monomers have similar partial charges, and, therefore, exhibit nearly identical P-31 chemical shifts. Al-27 NMR spectra of aluminophosphate solutions display four resonances: A peak at 0 ppm assigned to free Al monomers, a peak from -3 to -5 ppm assigned to Al monomers bonded to phosphate acid dimers, a peak from -6 to -7.5 ppm assigned to Al monomers bonded to H3PO4 molecules, and at peak from -7.5 to 9 ppm assigned to Al monomers bonded to H2PO4- ions. The assignment of the Al-27 NMR resonances is based on the manner in which the Al-27 and P-31 NMR peaks change in intensity with pH, P, and Al concentration. Estimates of the number of Al-O-P bonds are calculated independently from P-31 and Al-27 NMR spectra and agree quite closely for a range of pH's and both Al and P concentrations.