Application of temperature-programmed desorption to the study of heteropoly compounds: Desorption of water and pyridine

The behavior of a series of heteropoly compounds during temperature-programmed desorption (TPD) before and after exposure to pyridine has been examined and correlated with structural changes and variations in sorptive capacities of these materials. Representative heteropoly acids and their metallic salts were chosen such that the primary Keggin structure (XM₁₂) was present in each but the series examined included variations in peripheral element, M as Mo or W, in hetero element, X as P or Si, and in cation, H⁺, Na⁺ or Mg²⁺. Heteropoly acids of Mo and W desorbed water in two temperature regions; one between 423 and 473 °K was assigned to water held in the solid structure by hydrogen bonding and one between 623 and 773 °K was assigned to deprotonation of the acid with concurrent nonreductive loss of lattice oxygen, but with retention of the primary Keggin structure. Pyridine sorption by H₃PW₁₂O₄₀ · nH₂O was strongly dependent upon pretreatment temperature. Pyridinium ion formation was maximized after pretreatment between 463 and 593 °K. In static conditions sorption of pyridine was not inhibited but pyridinium ion formation was when hydrogen-bonded water was still present in the structure. Conversely removal of protons by thermal treatment above 593 °K reduced the number of sorption sites. Pyridine was desorbed into the gas phase only in conditions such that more than ca. 1 pyridine molecule had been sorbed per Keggin unit. When smaller quantities were sorbed at 298 °K decomposition products only were detected. A model is presented based on correlations between TPD peaks and DTA-measured decompositions.