SOLID PHOSPHORIC-ACID CATALYST - A MULTINUCLEAR NMR AND THEORETICAL-STUDY

The synthesis, structure, and acid function of solid phosphoric acid ( SPA) catalyst were studied in detail. P-31 and Si-29 MAS NMR and X-ray powder diffraction identified the following crystalline silicon phosp hate phases in SPA: Si5O(PO4)(6), hexagonal-SiP2O7, Si(HPO4)(2). H2O, and SiHP3O10. The acidity of SPA is due to a liquid or glassy solution of phosphoric acid oligomers supported on the silicon phosphate phase s. N-15 MAS NMR of adsorbed pyridine-N-15 and C-13 MAS NMR of adsorbed acetone-2-C-13 showed Bronsted acid sites and no Lewis acid sites. H- 1-->N-15-->P-31 and H-1-->C-13-->P-31 double cross polarization MAS NM R of the probe molecules provided a rare opportunity to use NMR to una mbiguously localize chemisorption sites; the probe molecules are compl exed to phosphoric acid and pyrophosphoric acid but not to the silicon phosphate phases. In situ NMR of the oligomerization of propene on SP A suggests that propene quantitatively reacts with phosphoric acid and its oligomers to form isopropyl phosphate, and formation of this very stable intermediate accounts for the lower olefin oligomerization act ivity of SPA relative to acidic zeolites. Theoretical calculations inc luding geometries at B3LYP/6-311+G(d,p) and chemical shifts at GIAO-MP 2/tzp/dz were used to model complexation of acetone or propene to SPA, and these support our conclusions.