Mapping the evolution of adsorption of water in nanoporous silica by in situ solid-state 1H NMR spectroscopy

A recently developed technique for carrying out in situ solid-state NMR studies of adsorption processes in nanoporous materials is applied to map the evolution of the adsorption of water on the widely used siliceous nanoporous host material MCM-41. The technique allows the very earliest stages of the adsorption process to be probed, and using solid-state H-1 NMR, the time resolution is of the order of a few hundred seconds. This work reveals that different water environments are populated at different stages of the adsorption process and reveals insights regarding both the sequence in which these different environments become populated and the exchange of water molecules between these environments. The results also provide access to information on the kinetics of the adsorption process, revealing an initial regime of rapid water adsorption up to ca. 1 wt % water, followed by a regime of slower water adsorption. In both regimes, the amount of water adsorbed increases linearly with time. As demonstrated by these results, the approach employed in this work creates new opportunities for investigating, in unprecedented detail, adsorption processes in nanoporous materials.