Characterization of 12-tungstophosphoric acid and related salts using photoacoustic spectroscopy in the infrared region: II. Interactions with pyridine

Abstract

Photoacoustic spectroscopy in the infrared region (PAS-FTIR) has been used to study the interactions of pyridine with 12-tungstophosphoric acid, H 3PW 12O 40, and the respective NH + 4, Al 3+, Na +, and pyridinium salts, heteropoly compounds which show interesting trends in activity and selectivity in the conversion of methanol to hydrocarbons. At room temperature, the parent acid readily sorbs up to 6 pyridine(py) molecules per Keggin unit, i.e., PW 12O 40 3−, to form the dimer ion salt (py 2H) 3PW 12O 40, and exchange studies have revealed that the pyridine molecules in the dimer ion are equivalent. Above 100 °C, pyridine is desorbed to yield the pyridinium salt, (pyH) 3PW 12O 40. Sorption of pyridine into the salts is not as facile as NH 3, showing marked dependence on the cation and temperature. Steric factors are evidently important in controlling the diffusion rate of this larger probe molecule into their bulk structures. The existence of the salts in partial (Brønsted) acid form has been confirmed, and the results of pyridine desorption studies indicate qualitative differences in their acid strengths.