Magic-angle carbon-13 NMR study of CO2 adsorbed on some molecular sieves

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Abstract
Carbon‐13 NMR spin‐lattice relaxation times of isotopically enriched CO2 adsorbed on a variety of channel and cavity molecular sieves have been obtained. At room temperature, the spin‐lattice relaxation of adsorbed CO2 is dominated by paramagnetic impurities in commercial molecular sieves with critical cage dimensions less than about 8 Å, but by spin rotation in some sieves with larger cage dimensions. Asymmetric 13C line shapes are generally observed for CO2 adsorbed in these molecular sieves. This asymmetry is, for the most part, independent of the presence of paramagnetics, but is instead generated by the local microscopic magnetic susceptibility heterogeneities of the host sieve. Substantial narrowing of the asymmetric lines can be achieved by magic‐angle spinning. For CO2 adsorbed on Na+‐mordenite, spinning experiments at 3 kHz reveal only a single symmetric line, consistent with the great majority of CO2 molecules in the system being physi‐ rather than chemisorbed.