TY - JOUR
T1 - Investigating the Process and Mechanism of Molecular Transport within a Representative Solvent-Filled Metal-Organic Framework
AU - Wang, Rui
AU - Bukowski, Brandon C.
AU - Duan, Jiaxin
AU - Sheridan, Thomas R.
AU - Atilgan, Ahmet
AU - Zhang, Kun
AU - Snurr, Randall Q.
AU - Hupp, Joseph T.
N1 - Funding Information:
J.T.H and R.Q.S gratefully acknowledge the support from the Defense Threat Reduction Agency (HDTRA1-19-1-0007). This work made use of BIF at Northwestern University, which is supported by Chemistry for Life Processes Institute, the Northwestern University Office for Research and the Rice Foundation. This work made use of IMSERC at Northwestern University, which has received support from the NSF (CHE-1048773), Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205), and the State of Illinois and International Institute for Nanotechnology (IIN). The authors thank Dr. Jessica Hornick for helpful discussion on confocal fluorescence microscopy and thank Yongwei Chen for useful discussion on single crystal synthesis.
Publisher Copyright:
Copyright © 2020 American Chemical Society.
PY - 2020/9/15
Y1 - 2020/9/15
N2 - Effective permeation into, and diffusive mass transport within, solvent-filled metal-organic frameworks (MOFs) is critical in applications such as MOF-based chemical catalysis of condensed-phase reactions. In this work, we studied the entry from solution of a luminescent probe molecule, 1,3,5,7-tetramethyl-4,4-difluoroboradiazaindacene (BODIPY), into the 1D channel-type, zirconium-based MOF NU-1008 and subsequent transport of the probe through the MOF. Measurements were accomplished via in situ confocal fluorescence microscopy of individual crystallites, where the evolution of the fluorescence response from the crystallite was followed as functions of both time and location within the crystallite. From the confocal data, intracrystalline transport of BODIPY is well-described by one-dimensional diffusion along the channel direction. Varying the chemical identity of the solvent revealed an inverse dependence of probe-molecule diffusivity on bulk-solvent viscosity, qualitatively consistent with expectations from the Stokes-Einstein equation for molecular diffusion. At a more quantitative level, however, measured diffusion coefficients are about 100-fold smaller than expected from Stokes-Einstein, pointing to substantial channel-confinement effects. Evaluation of the confocal data also reveals a non-negligible mass transport resistance, i.e., surface barrier, associated with the probe molecule leaving the solution and permeating the exterior surface of the MOF. Permeation by the probe entails displacement of solvent from the MOF channels. The magnitude of the resistance increases with the size of the solvent molecule. This work draws attention to the importance of MOF structure, external-surface barriers, and solvent molecule identity to the overall transport process in MOFs, which should assist in understanding the performance of MOFs in applications such as condensed-phase heterogeneous catalysis.
AB - Effective permeation into, and diffusive mass transport within, solvent-filled metal-organic frameworks (MOFs) is critical in applications such as MOF-based chemical catalysis of condensed-phase reactions. In this work, we studied the entry from solution of a luminescent probe molecule, 1,3,5,7-tetramethyl-4,4-difluoroboradiazaindacene (BODIPY), into the 1D channel-type, zirconium-based MOF NU-1008 and subsequent transport of the probe through the MOF. Measurements were accomplished via in situ confocal fluorescence microscopy of individual crystallites, where the evolution of the fluorescence response from the crystallite was followed as functions of both time and location within the crystallite. From the confocal data, intracrystalline transport of BODIPY is well-described by one-dimensional diffusion along the channel direction. Varying the chemical identity of the solvent revealed an inverse dependence of probe-molecule diffusivity on bulk-solvent viscosity, qualitatively consistent with expectations from the Stokes-Einstein equation for molecular diffusion. At a more quantitative level, however, measured diffusion coefficients are about 100-fold smaller than expected from Stokes-Einstein, pointing to substantial channel-confinement effects. Evaluation of the confocal data also reveals a non-negligible mass transport resistance, i.e., surface barrier, associated with the probe molecule leaving the solution and permeating the exterior surface of the MOF. Permeation by the probe entails displacement of solvent from the MOF channels. The magnitude of the resistance increases with the size of the solvent molecule. This work draws attention to the importance of MOF structure, external-surface barriers, and solvent molecule identity to the overall transport process in MOFs, which should assist in understanding the performance of MOFs in applications such as condensed-phase heterogeneous catalysis.
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U2 - 10.1021/acs.langmuir.0c01999
DO - 10.1021/acs.langmuir.0c01999
M3 - Article
C2 - 32841562
AN - SCOPUS:85091125445
VL - 36
SP - 10853
EP - 10859
JO - Langmuir
JF - Langmuir
SN - 0743-7463
IS - 36
ER -