Phosphorus-31 and aluminum-27 NMR investigations of the effects of pH on aqueous solutions containing aluminum and phosphorus

Abstract

[sup 31]P and [sup 27]Al 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)[sub 2]O = 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 [ge] 6), a medium-pH range (2 [le] pH [le] 10) in which stable solid phases exist, and a low-pH region (pH [le] 2). In the low-pH region, soluble aluminophosphate complexes form between P species (H[sub 3]PO[sub 4] acid dimers, H[sub 3]PO[sub 4] molecules, and H[sub 2]PO[sub 4][sup [minus]] ions) and Al species (Al monomers as [Al(H[sub 2]O)[sub 6]][sup 3+] cations, and Al dimers as [Al[sub 2](OH)[sub 2](H[sub 2]O)[sub 8]][sup 4+] cations). Specific [sup 31]P NMR and [sup 27]Al 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 HPO[sub 4][sup 2[minus]] ions and Al species. Also at high pH, [sup 31]P NMR spectra displaysmore » a series of sharp resonances (from [minus]2 to 5 ppm) assigned to P atoms in aluminophosphate complexes. [sup 27]Al NMR spectra of the same solutions display broad features centered at 5 ppm assigned to octahedrally coordinated Al bonded through oxygen to P, at [approx]45.5 ppm to pentacoordinated Al bonded through oxygen to P, and at [approx]65 ppm to tetrahedrally coordinated Al bonded through oxygen to P. Independent Al-O-P bond counts from [sup 31]P and [sup 27]Al NMR spectra of aluminophosphate solutions of high pH ([ge]6) indicate that each octahedrally coordinated, pentacoordinated, and tetrahedrally coordinated Al nucleus is bonded primarily to only one phosphate ligand. 21 refs., 13 figs., 2 tabs.« less