Physical State of di-Nitrogen in the Presence of SBA and Various Zeolites Probed by 15N Spin–Lattice Relaxation of Pure 15N2 in the 30–300 K Temperature Range

Abstract

15N NMR spin–lattice relaxation times of N2 in the presence of mesoporous SBA-80 and selected zeolites (Y, OMEGA, MORDENITE, ZSM-5, ZSM-11, AlPO-5, RHO) have been measured at 30 MHz and at temperatures ranging from ambient temperature to about 30 K. The interpretation of these experimental data rests on two relaxation mechanisms: spin-rotation (mainly active when N2 molecules reorient rapidly) and dipolar interaction with surface protons (depending essentially on the N2 translational diffusion mobility). Both mechanisms have to be taken into account for all samples except SBA-80 (for which the interpretation of experimental data requires two different spin-rotation interactions), and RHO (for which two different dipolar interactions are required). For all zeolites investigated here it has been indispensable to define a cutoff temperature below which spin-rotation is no longer active, indicating the quasi-disappearance of the gaseous state to the benefit of a condensed state. This cutoff temperature is 77 K for Y, OMEGA, and possibly MORDENITE, whereas it is higher for those zeolites with pore diameters approaching the dinitrogen molecular length. It is thus concluded that these NMR relaxation measurements are able to reveal the physical state of dinitrogen in the presence of SBA-80 and zeolites possessing very different structures.