Positron lifetimes in pores of some low-silica zeolites: influence of water content, crystal size and structural type

Analysis of the Chemical State in Y-zeolite Pores by Positron Annihilation Lifetime Spectroscopy

“…The presence of more negative-charged (AlO4) 5units can increase the Ps pick-off annihilation rate and, therefore, reduce the lifetime. The lifetime of the sample with lowest Si/Al ratio agrees well with earlier PALS measurements on low-silica Y zeolites 34. The lifetime value calculated using the Tao-Eldrup equation from the known diameter of the β-cage or sodalite cage (0.66 nm) is ~2.5…”

“…This result suggests that this lifetime component stems from o-Ps diffusing through the micropore framework and undergoing pick-off annihilation around the α-cage apertures, as supported by earlier work. 34 Although o-Ps tends to diffuse and annihilate in larger spaces, given that the lifetime of the sample with Si/Al ratio = 2.6 is close to 2.5 ns, it cannot be excluded that some o-Ps also annihilates in the β-cages. The scatter in the measured lifetimes at different Si/Al ratios is simply an acidity effect, as explained above.…”

“…5 under the influence of water can be ascribed to the strong affinity of low-silica zeolites to adsorb a large amount of water molecules in their pores and channels, which has a critical influence on the adsorptive and catalytic properties. 34 The adsorption and desorption of water molecules inside the zeolite framework induces the displacement of cations and changes the distribution of electron density of the crystalline backbone structure. 66,67 In turn, these competing active sites are expected to more greatly affect the Ps formation probability in low-silica rather than high-silica zeolites.…”

“…17,18,27,28,[19][20][21][22][23][24][25][26] In general, the catalytic activity was found to increase and the action as a polar adsorbent (hydrophilicity) became stronger with the number of Brønsted acid 4 sites. The specific surface area, [29][30][31] pore size distributions, 30,32,33 crystallinity, [30][31][32][33][34] and Al distribution within and outside the framework 31,[35][36][37][38] in numerous zeolites, including the Y zeolites subject of the present study, have been extensively studied both through empirical measurements and modelling. As a result, the influence of these properties on the acid strength is well characterized.…”

“…(11 MPa). Based on earlier work, 34 the samples were dehydrated in vacuum at 185°C for 2 h to remove any moisture absorbed in the zeolites during the preparation process. The samples were transported in an evacuated glass vial into a glovebox filled with Ar gas to avoid exposure of the zeolites to the atmospheric humidity.…”