CHARACTERIZATION BY INFRARED AND NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPIES OF CALCINED BETA-ZEOLITE

A beta-zeolite with template has been calcined in air at 550-degrees-C using two different experimental procedures. IR and Si-29 magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopies showed that the extent of framework dealumination and the number of Bronsted acid sites were greatly affected by the conditions of treatment. With shallow-bed calcination, framework-Al extraction was limited but reversible openings of framework Si-O-Al linkages were observed. The amount of extra-framework species increased with deep-bed calcination. Al-27 NMR results confirmed that the appearance of octahedral Al in protonic beta-zeolite does not necessarily involve framework-Al extraction. Acid strengths were evaluated by pyridine adsorption-desorption cycles. The splitting of the band of Lewis-bonded pyridine at 1455 cm-1 at temperatures higher than 350-degrees-C was shown to be due to adsorbate modifications rather than irreversible structural changes of the solid.