Standardization of catalyst preparation using reference catalyst: Ion exchange of mordenite type zeolite, Remarkable dealumination accompanying ion exchange

A joint study was organized to establish a standard set of conditions for the ion exchange of zeolite from sodium into proton-form. The sodium-form mordenite with Si/Al₂ = ca. 15 was ion exchanged into an NH₄-form, followed by calcination according to various recipes. Then various advanced techniques for characterization (ICP, TG, XPS, SEM, XRD, N₂ adsorption, ²⁹Si and ²⁷AlNMR, benzene diffusion, IR of OH and adsorbed pyridine and CO, ammonia TPD) were applied, and some test reactions (cracking of cumene, isopropylation of biphenyl, oligomerization of propene and chichibabin condensation of acetaldehyde and ammonia into picoline) were conducted. The ion exchange (removal of sodium) proceeded as expected, but remarkable differences were observed in physicochemical and catalytic properties of the thus-prepared proton-form samples. Exceptionally high temperature (383 K) for the ion exchange resulted in the structural degradation, while most samples exchanged at 333-353 K maintained the crystallinity and pore volume. The use of NH₄Cl slightly changed the crystal morphology. Extra-framework aluminum species was formed on most samples after calcination above 773 K. In contrast, calcination at 673 K maintained the framework aluminum. Therefore, the dealumination is considered to proceed after complete removal of ammonia from the ammonium-form zeolite. However, the proton-form zeolite was stable under dry conditions, so it is speculated that the dealumination was induced by the contact of the proton-form zeolite to atmosphere with humidity. The total and Brønsted acidity decreased with the dealumination, while Lewis acidity increased. On the other hand, rapid heating of the ammonium type zeolite caused narrowing of the micropores. These structural changes seriously affected the catalytic activities for various reactions.