TY - JOUR
T1 - Transport Diffusion of Linear Alkanes (C5-C16) through Thin Films of ZIF-8 as Assessed by Quartz Crystal Microgravimetry
AU - Audu, Cornelius O.
AU - Chen, David
AU - Kung, Chung Wei
AU - Snurr, Randall Q.
AU - Nguyen, Sonbinh T.
AU - Farha, Omar K.
AU - Hupp, Joseph T.
N1 - Funding Information:
We gratefully acknowledge support of this work by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES), under Award DE-FG02-08ER15967. C.O.A. acknowledges support from the National Science Foundation Graduate Research Fellowship program under grant no. DGE-1324585. This work made use of the J. B. Cohen X-ray Diffraction Facility supported by the MRSEC program of the National Science Foundation (DMR-1121262) at the Materials Research Center of Northwestern University and the EPIC facility (NUANCE Center, Northwestern University) supported by the MRSEC program (NSF DMR-1121262), the International Institute for Nanotechnology (IIN) and the State of Illinois, through the IIN. C.-W.K. acknowledges support from the Postdoctoral Research Abroad Program (105-2917-I-564-046) sponsored by Ministry of Science and Technology (Taiwan). S.T.N. acknowledges support from the Defense Threat Reduction Agency (HDTRA1-14-1-0014).
Publisher Copyright:
© 2021 American Chemical Society.
PY - 2021/8/10
Y1 - 2021/8/10
N2 - We report uptake capacities and transport diffusivities, D, for each of eight linear alkanes (ranging from C5 to C16) in quartz crystal-supported films of solvent-evacuated ZIF-8. Analyses of the alkane uptake profiles revealed that the transport dynamics are governed by guest diffusion through metal-organic framework (MOF) (ZIF-8) crystallites rather than by rates of entry into films at the MOF/vapor interface. The obtained diffusivities range from just over 10-18 m2/s to just under 10-14 m2/s. Notably, minimum cross-sectional widths for all guests exceed the crystallographically measured width of ZIF-8's largest apertures and imply consistently with previous experimental and computational studies that apertures expand to accommodate guest uptake. On average, each additional carbon decreases the transport diffusivity of an alkane by twofold. Closer examination, however, reveals an odd-even effect such that linear alkanes having even numbers of carbons diffuse more rapidly than alkanes featuring one more or one less carbon atom. Thus, ZIF-8's differentiation of transport diffusivities for pairs of alkanes differing in length by only one carbon atom can be significantly greater than the aforementioned factor of 2. Elucidation of the microscopic basis for the odd-even behavior, however, awaits the outcome of molecular dynamics calculations that are beyond the scope of the present study. For compact, solvothermally prepared films, guest transport is dominated by 1D diffusion from the film/vapor interface and toward the underlying quartz crystal. For much lower density, electrophoretically deposited (EPD) films, crystallites behave nearly independently, and guest transport can be adequately modeled by assuming rapid permeation of macroscopic voids between crystallites, followed by entry and rate-limiting radial diffusion into isolated crystallites. One consequence is that EPD films can be much more rapidly infiltrated by molecular guests than can compact, solvothermally grown films. The combined results have potentially favorable implications for the development of kinetic separation schemes for closely related analytes.
AB - We report uptake capacities and transport diffusivities, D, for each of eight linear alkanes (ranging from C5 to C16) in quartz crystal-supported films of solvent-evacuated ZIF-8. Analyses of the alkane uptake profiles revealed that the transport dynamics are governed by guest diffusion through metal-organic framework (MOF) (ZIF-8) crystallites rather than by rates of entry into films at the MOF/vapor interface. The obtained diffusivities range from just over 10-18 m2/s to just under 10-14 m2/s. Notably, minimum cross-sectional widths for all guests exceed the crystallographically measured width of ZIF-8's largest apertures and imply consistently with previous experimental and computational studies that apertures expand to accommodate guest uptake. On average, each additional carbon decreases the transport diffusivity of an alkane by twofold. Closer examination, however, reveals an odd-even effect such that linear alkanes having even numbers of carbons diffuse more rapidly than alkanes featuring one more or one less carbon atom. Thus, ZIF-8's differentiation of transport diffusivities for pairs of alkanes differing in length by only one carbon atom can be significantly greater than the aforementioned factor of 2. Elucidation of the microscopic basis for the odd-even behavior, however, awaits the outcome of molecular dynamics calculations that are beyond the scope of the present study. For compact, solvothermally prepared films, guest transport is dominated by 1D diffusion from the film/vapor interface and toward the underlying quartz crystal. For much lower density, electrophoretically deposited (EPD) films, crystallites behave nearly independently, and guest transport can be adequately modeled by assuming rapid permeation of macroscopic voids between crystallites, followed by entry and rate-limiting radial diffusion into isolated crystallites. One consequence is that EPD films can be much more rapidly infiltrated by molecular guests than can compact, solvothermally grown films. The combined results have potentially favorable implications for the development of kinetic separation schemes for closely related analytes.
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U2 - 10.1021/acs.langmuir.1c00672
DO - 10.1021/acs.langmuir.1c00672
M3 - Article
C2 - 34338528
AN - SCOPUS:85113399308
VL - 37
SP - 9405
EP - 9414
JO - Langmuir
JF - Langmuir
SN - 0743-7463
IS - 31
ER -