Investigation on the porosity of zeolite NU-88 by means of positron annihilation lifetime spectroscopy

Seven well characterized zeolites were investigated by positron annihilation lifetime spectroscopy. The lifetime spectra were analysed in four discrete components. The third one was associated with ortho-positronium annihilation in the channels, framed in terms of infinite cylinders. Differences between the radii determined from the positron annihilation technique and X-ray diffraction data were found and explained in terms of the physical structure of the channel. An analogous study on a high-silica NU-88 zeolite gave a value of 0.33 nm for the corresponding radius, in agreement with At and N(2) adsorption data as well as with the catalytic behaviour of this zeolite in several acid catalyzed reactions. The longest lifetime component in NU-88 reveals the existence of mesopores, with average radius of about 1.8 nm, which could explain the importance of hydrogen transfer reactions in this zeolite.