Infrared spectroscopy, thermoprogrammed desorption, and nuclear magnetic resonance study of the acidity, structure, and stability of zeolite MCM-22

Zeolite MCM-22 has been studied by infrared (i.r.) spectroscopy, thermoprogrammed desorption (t.p.d.) of NH3, and27Al n.m.r. with magic angle spinning (MAS). It has been found that zeolites MCM-22 and ZSM-5 both have framework i.r. bands at about 1,245 and 550 cm−1, and zeolites MCM-22 and Y both have two pore opening i.r. bands at about 380 and 317 cm−1. In MCM-22 there are two kinds of bridged hydroxyls (3,620 and 3,575 cm−1) and two kinds of internal silanols (3,500 and 3,700 cm−1). The latter two i.r. frequencies are similar to those in zeolites ZSM-5 and Y, respectively.27Al n.m.r. reveals two kinds of framework tetrahedral Al. The spectroscopic similarities and the dual appearance of the relevant i.r. bands/n.m.r. peaks seem to indicate that MCM-22 probably has two distinct pore systems containing 10- and 12-member rings, which is in accordance with our recently published results of catalytic tests. Quantitative results on Brønsted and Lewis acidity are reported from the i.r. study of pyridine adsorption and from t.p.d. of NH3. The Brønsted sites are strongly acidic and accessible for pyridine. MCM-22 is very sensitive to the calcination conditions, being more dealuminated on heating in air than in a vacuum or N2. The dealumination occurs even during grinding in a mortar with a concomitant decrease in the acidity of the final sample.