The Nature of Zeolite Catalytic Sites

Abstract

The nature of zeolite catalytic sites was studied by observing their interactions with sorbants. In situ FTIR studies of a range of sorbants on H+ZSM-5, H+mordenite and H+Y showed that the zeolite Bronsted proton was transferred towards the sorbant. Sorbants could be placed in three classes depending on the type of hydrogen bond formed. "Class A" sorbants were alcohols, alkanes, ammonia, amines and carboxylic acids and showed a single v0-H band shifted from the vO-H of the zeolite. The shift in vO-H increased with increasing proton affinity of the sorbant. "Class B" sorbants were alkenes and aromatics and showed a resultant broad, flat vO-H due to bonding through the t electrons of the double bond or aromatic ring. "Class C" sorbants included water, ethers, ketenes, aldehydes, nitriles and carboxylic acids (also Class A). They showed extremely broad hydroxyl bands from -3700 to -1200 cm-1 with several maxima. Bonding was through oxygen or nitrogen lone electron pairs. A novel, low temperature (-400°C), reaction of acetic acid to ketene was observed over alkali-exchanged zeolites. Thermal desorption/mass spectrometry, mini-reactor mass spectrometry and in situ FTIR techniques were used to investigate the products obtained by varying the carboxylic acid and the catalyst, and the reaction mechanism.