the ammo nium form of the zeolite was obtained by repeatedly ion exchanging it with a 1.0 M aqueous solution of NH4NO3 at room temperature mal treatment inside an IR cell, as described below. Fig. 1 shows the powder XRD pattern (CuKa radiation) of H-McM-22. For CuKa radiation, the main fingerprint region of the MWW group of zeolites comes in the range 5-10 (20): lines (100), (101) and (102). Clear separation of (101) and (102) diffraction peaks, together with the absence of the (001) and (002) peaks, are the main indicators of an ordered 3D structure (44.45 All of the diffraction peaks in Fig. 1 could be assigned to the MWW structure type, and their sharpness proves a high degree of crystallinity 2.2. Variable temperature IRMIR) spectroscopy For IR spectroscopy, a thin self-supported wafer of NH4-MCM 22 was prepared and outgassed by heating at 673 K for 4 h under a dynamic vacuum (residual pressure smaller than 10-4 mbar) inside a home-made IR cell 146] that allowed on-line thermal treat ment of the zeolite wafer, gas dosage and variable-temperature IR spectroscopy to be carried out. After sample outgassing, the cell was dosed with 0.5 mbar of helium (to improve thermal contact between the cell body and the sample wafer) and cooled with liquid nitrogen. After that, the background spectrum of the sam ple wafer was obtained: this spectrum showed no IR absorption bands corresponding to ammonium ions, thus proving complete