Acidic and catalytic properties of H-MCM-22 zeolites - 1. Characterization of the acidity by pyridine adsorption

Three H-MCM-22 zeolite samples with total Si/Al ratios of 10, 14 and 30 (M-10, M-14, M-30) calcined at 550 degreesC were characterized by various techniques; Al-27 MAS NMR, XRD, nitrogen adsorption, pyridine adsorption followed by infrared spectroscopy and by microcalorimetry. Extraframework aluminium species (EFAL) are present in all the samples, mainly under the form of hydroxylated species responsible for an IR band at 3665 cm(-1). With M-10 and M-14 at 150 degreesC, pyridinium ions are formed on all the bridged hydroxyl groups whereas with M-30 a weaker interaction exists with part of them. The distribution of the bridged hydroxyl groups was estimated: 50-70% would be located in supercages, 20-30% in sinusoidal channels and 10% linked to hexagonal prisms between two supercages with M-10 and M-14 and 25% with M-30. Microcalorimetry experiments show the presence of very strong acid sites particularly with M-30: initial value of differential heat of pyridine adsorption of 230 kJ mol(-1). Both calorimetry and IR experiments indicate a significant increase of the acid strength with the Si/Al ratio of the zeolite. (C) 2001 Elsevier Science B.V. All rights reserved.