Abstract
A molecular description of the structure and behavior of water confined in aluminosilicate zeolite pores is a crucial component for understanding zeolite acid chemistry under hydrous conditions. In this study, we use a combination of ultrafast two-dimensional infrared (2D IR) spectroscopy and ab initio molecular dynamics (AIMD) to study H2O confined in the pores of highly hydrated zeolite HZSM-5 (∼13 and ∼6 equivalents of H2O per Al atom). The 2D IR spectrum reveals correlations between the vibrations of both terminal and H-bonded O-H groups and the continuum absorption of the excess proton. These data are used to characterize the hydrogen-bonding network within the cluster by quantifying single-, double-, and non-hydrogen-bond donor water molecules. These results are found to be in good agreement with the statistics calculated from an AIMD simulation of an H+(H2O)8 cluster in HZSM-5. Furthermore, IR spectral assignments to local O-H environments are validated with DFT calculations on clusters drawn from AIMD simulations. The simulations reveal that the excess charge is detached from the zeolite and resides near the more highly coordinated water molecules in the cluster. When they are taken together, these results unambiguously assign the complex IR spectrum of highly hydrated HZSM-5, providing quantitative information on the molecular environments and hydrogen-bonding topology of protonated water clusters under extreme confinement.
| Original language | English |
|---|---|
| Pages (from-to) | 10203-10213 |
| Number of pages | 11 |
| Journal | Journal of the American Chemical Society |
| Volume | 143 |
| Issue number | 27 |
| DOIs | |
| State | Published - Jul 14 2021 |
| Externally published | Yes |
Funding
This work was supported as a part of the Advanced Materials for Energy Water Systems (AMEWS) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. X.M., C.L., and G.A.V. gratefully acknowledge the computational resources provided on the Midway cluster operated by the University of Chicago Research Computing Center (RCC). The authors acknowledge Dr. Saikat Banerjee for the H MAS NMR studies included in this work, Johnson Matthey PLC for providing the zeolite samples free of charge, and Halocarbon, LLC, for the PCTFE oils—excluding Fluorolube oils—free of charge. This work made use of the Integrated Molecular Structure Education and Research Center (IMSERC) at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205), the State of Illinois, Northwestern University, and International Institute for Nanotechnology (IIN), the EPIC facility of Northwestern University’s NUANCE Center, which has received support from the SHyNE Resource (NSF ECCS-1542205), the IIN, and Northwestern’s MRSEC program (NSF DMR-1720139). Elemental analysis was performed at the Northwestern University Quantitative Bioelement Imaging Center. We acknowledge the Reactor Engineering and Catalyst Testing (REACT) core facility at Northwestern University for use of the Micromeritics 3Flex instrument. 1