Effect of acidity and pore diameter of zeolites on detection of base molecules by zeolite thick film sensor

In order to investigate molecular recognition by zeolite thick film gas sensors, the effect of zeolite acidity and pore diameter of zeolites on the detection of base molecules (water, acetonitrile, ammonia, benzonitrile, pyridine, aniline, and triethylamine) was investigated. The impedance spectra of the thick film sensors of H-type zeolites (MFI, MOR and BEA) showed a semicircle attributable to an RC parallel equivalent circuit from 4 Hz to 1 MHz, and resistance R decreased in the presence of 300 ppm of base molecules at 673 K. For smaller molecules having molecular diameter of less than 0.4 nm (water, acetonitrile, and ammonia), a good correlation was observed between the response magnitude of R and the proton affinity of base molecules. It was indicated that acid–base interaction is the major factor when molecular diameter is smaller enough than zeolite pore diameter. As for larger molecules (benzonitrile, pyridine, aniline, and triethylamine) of which the molecular diameter is nearly the pore diameter of zeolites, the response magnitudes were lower than those of smaller molecules. Quantification of adsorbed species by in situ FT/IR revealed that the response magnitude for larger molecules significantly depends on the mobility of migrating species controlled by relative size of molecules to zeolite pore diameter.