Generation of Acidic Sites in Metal Salts of Heteropolyacids

Article abstract of DOI:10.1021/j100236a033

The effects of water and hydrogen on the generation of acid sites in the metal salts of dodecatungstophosphoric acid (H3PW12O40, HTP) are studied by means of catalysis amd infrared spectroscopy.The silver salt has no activity for o-xylene isomerization and pyridinium ion was not formed by adsorption of pyridine.By the reaction of the salt with hydrogen at 300 deg C, the activity for xylene isomerization appeared and the development of acid sites is confirmed by infrared spectroscopy.The scheme of the generation of acid sites is expressed as Ag⁺ + 1/2 H2 --> Ag^o H⁺.The features of the copper(II) salt are similar to those of the silver salts.The high acidity of the copper(II) salt is demonstrated by the inhibition of the catalytic activity for dehydration of 1-butanol by the sorption of pyridine.For the aluminium salt, hydrogen has no effect on the isomerization activity.But the activity and the capacity for pyridinium ion formation are enhanced by treatment with water.A plausible scheme for acid-site formation in aluminium salt is expressed as Al³⁺ + H2O --> ²⁺ + H⁺.