P-31 AND AL-27 NMR INVESTIGATIONS OF THE EFFECTS OF PH ON AQUEOUS-SOLUTIONS CONTAINING ALUMINUM AND PHOSPHORUS

P-31 and Al-27 NMR spectroscopies are used to characterize the distribution of soluble aluminophosphate species in aqueous solutions of tetramethylammonium (TMA) hydroxide, phosphoric acid, and aluminum chloride. Solution compositions range from 0.1 to 1 mol % P, P/Al = 0.1-5, P/(TMA)20 = 0.37-10. For solutions of 1 mol % P, a phase diagram is constructed for various concentrations of TMAOH and Al, The phase diagram is divided into three regions: a high-pH region (pH greater-than-or-equal-to 6), a medium-pH range (2 less-than-or-equal-to pH less-than-or-equal-to 10) in which stable solid phases exist, and a low-pH region (pH less-than-or-equal-to 2). In the low-pH region, soluble aluminophosphate complexes form between P species (H3PO4 acid dimers, H3PO4 molecules, and H2PO4- ions) and Al species (Al monomers as [Al(H20)6]3+ cations, and Al dimers as [Al2(OH)2(H20)8]4+ cations). Specific P-31 NMR and Al-27 NMR peaks are assigned to P and Al nuclei in aluminophosphate complexes which are stable in the low pH range. In the high-pH region, soluble aluminophosphate Complexes form between HPO42- ions and Al species. Also at high pH, P-31 NMR spectra display a series of sharp resonances (from -2 to 5 ppm) assigned to P atoms in aluminophosphate Complexes. Al-27 NMR spectra of the same solutions display broad features centered at 5 ppm assigned to octahedrally coordinated Al bonded through oxygen to P, at almost-equal-to 45.5 ppm to pentacoordinated Al bonded through oxygen to P, and at almost-equal-to 65 ppm to tetrahedrally coordinated Al bonded through oxygen to P. Pentacoordinated Al complexes are observed only when the concentration of P exceeds the concentration of Al and then only in neutral pH. Independent Al-O-P bond counts from P-31 and Al-27 NMR spectra of aluminophosphate solutions of high pH (greater-than-or-equal-to 6) indicate that each octahedrally coordinated, pentacoordinated, and tetrahedrally coordinated Al nucleus is bonded primarily to only one phosphate ligand.