Studies on gas adsorption in porous polymers via solid-state NMR

概要

学位論文の要約
題目

Studies on gas adsorption in porous polymers via solid-state NMR
（固体 NMR による多孔質高分子中のガス吸着に関する研究）

氏名

姜 偉明 (Jiang Weiming)

序論
Over the past two decades, porous polymers represented by metal organic frameworks
(MOFs) and covalent-organic frameworks (COFs) have attracted great research interest
owing to their outstanding properties. They are porous crystalline polymers constructed by
organic linkers and interconnected by metal clusters (MOFs) or covalent bonds (COFs).
The superior surface area and the designability of the pores in nano-meter scale endow
these two materials with unprecedented performance for adsorption and separation.
Solid-state NMR (SSNMR) spectroscopy is an excellent characterization method with both
nuclide and site specificity. It can extract information about the local environment in the
atomic range without the influence of long-range disorder. Therefore, SSNMR is an ideal
method to study the gas adsorption process of porous materials to obtain insight into the
host-guest interaction and molecular dynamics.
In this thesis, SSNMR spectroscopy was exploited to investigate the gas adsorption in
MOFs and COFs from three aspects utilizing homemade NMR probes, gas loading setups,
and hermetic rotors.

1. Sodium-Ion-Induced Discriminative C2H2/CO2 Adsorption in a Covalent
Organic Framework Studied by Solid-State 23Na NMR
The discriminative adsorption of acetylene (C2H2) over carbon dioxide (CO2) in Py-Na
COF was analyzed using 23Na MQMAS coupled with density functional theory calculations
and molecular dynamics simulations. The Na+ ion sites of Py-Na COF are confirmed to be
interaction sites for both C2H2 and CO2 molecules. The stronger host-guest interaction
developed between the C2H2 and Py-Na COF than that of CO2 is the origin of the
discriminative adsorption.

2. Crystal-size effect on the kinetics of CO2 adsorption in metal organic
frameworks studied by NMR
The crystal-size dependent adsorption kinetics of CO2 in NDMOF was studied by

13C

spectra of CO2 adsorbed in MOF crystals of sizes from μm to mm. The crystal-size
dependence of the 13C NMR lineshape of the guest CO2 molecules reflects the core-surface
nature of the crystallites in gas adsorption. The CO2 exchange process between the motionrestricted core sites and hindrance-free surface sites inside the crystal was revealed and
quantified using an isolated two-site exchange model. The populations of each site, the
thickness of the surface layer, and the apparent exchange rate could be estimated using
this model. The surface-to-volume ratio is the origin of the trend that the smaller the host
crystal is, the faster the exchange is.

3. Probing dynamics of carbon dioxide in a metal organic framework under high
pressure by high-resolution solid-state NMR
The behavior of CO2 molecules in Zn-MOF-74 at high pressure was investigated using
high-resolution in-situ MAS 13C spectroscopy. It is found that the guest-guest interaction
between CO2 molecules at high pressure resulted in abnormally low mobility of CO2
molecules and thereby facilitated the stable adsorption of CO2 in the pore of MOF at high
pressure.

This thesis demonstrates that SSNMR is a powerful and versatile technique for studying
gas adsorption in porous polymers that can probe the host-guest interaction,
dynamics/kinetics, and guest-guest interaction. More and more molecular-scale insights
are looking forward to being unraveled by SSNMR in the future. ...