The adsorption of formic acid on Y zeolites: A multiple-pulse nuclear magnetic resonance study

A series of multiple-pulse nuclear magnetic resonance (NMR) techniques have been applied to the study of the adsorbed state of formic acid on ammonium-Y (NH4Y) and ultrastable hydrogen-Y (HY) zeolites. It is determined that the formic acid is adsorbed on the zeolites in two different forms: as unidentate and bidentate formate species, which can be distinguished by the symmetry of the 13C chemical shift powder patterns. Cross-polarization NMR techniques exploit differences in the 13C1H dipolar couplings to separate the two groups. The ratio of bidentate to unidentate species is about 52:48 on the NH4Y zeolite and about 83:17 on the ultrastable HY zeolite at a coverage of 0.3 monolayer. The strength of the 13C27Al dipolar interaction suggests that the bidentate formate ions are bonded to the Al atoms of the zeolites. The 1H NMR spectrum of the carbonyl hydrogen, isolated by a dipolar-difference technique, is shifted downfield from that of neat formic acid, indicating that the hydrogen has become more acidic.