Mingcan Xu

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 1H 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.

In situ solid-state 1H NMR studies of hydration of the solid acid catalyst ZSM-5 in its ammonium form

Hydration of the ammonium form of the solid acid catalyst ZSM-5 is investigated by applying a technique that has been developed recently for carrying out in situ solid-state NMR studies of adsorption processes. From (1)H MAS NMR spectra recorded as a function of time and temperature during the hydration process, insights are established on the nature of the interaction between the adsorbed water molecules and the ammonium cations in the ZSM-5 material. The change in isotropic chemical shift for the ammonium cations is consistent with the formation of N-Hcdots, three dots, centeredO hydrogen bonding with the water molecules. Studies of the adsorption of deuterated water, and dehydration of the hydrated material, are also reported.