Facet-Specific Stability of ZIF -8 in the Presence ... - EFRC · Scientific Achievement . ZIF-8...
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Scientific Achievement ZIF-8 metal-organic frameworks were examined for stability in the presence of mild acidic conditions. It was found that material stability is controlled by the exposed surface facets. Significance and Impact This study represents the first comprehensive investigation of differences between exposed surface facets of a MOF and provides guidelines for synthesizing sulfur resistant ZIFs. Pang, S. H.; Han, C.; Sholl, D. S.; Jones, C. W.; Lively, R. P. “Facet-Specific Stability of ZIF-8 in the Presence of Acid Gases Dissolved in Aqueous Solutions” Chem. Mater., Accepted (2016). DOI: 10.1021/acs.chemmater.6b02643 (L) Representation of crystallographic planes examined to model different surface facets of ZIF-8 and (R) microscopy images of facet-specific ZIF-8 crystals before exposure to SO 2 . Facet-Specific Stability of ZIF-8 in the Presence of Acid Gases Dissolved in Aqueous Solutions Research Details – ZIF-8 particles were synthesized to yield either {110} rhombic dodecahedra (RD) facets or {100} truncated cubes. – Electron microscopy, BET surface area, XPS, FTIR, and diffusion studies were performed to characterize differences in stability between exposed surface facets after exposure to aqueous SO 2 solutions. – Studies revealed the {110} RD facet was more susceptible to degradation; it is hypothesized that the mechanism for degradation follows a shrinking-core model, with surface imidazolates being replaced by hydroxyls. – Computational investigations support this degradation mechanism and suggest that the reaction is more favorable on the {110} facet than the {100} facet due to steric constraints.
Transcript of Facet-Specific Stability of ZIF -8 in the Presence ... - EFRC · Scientific Achievement . ZIF-8...
Scientific Achievement ZIF-8 metal-organic frameworks were examined for stability in the presence of mild acidic conditions. It was found that material stability is controlled by the exposed surface facets. Significance and ImpactThis study represents the first comprehensive investigation of differences between exposed surface facets of a MOF and provides guidelines for synthesizing sulfur resistant ZIFs.
Pang, S. H.; Han, C.; Sholl, D. S.; Jones, C. W.; Lively, R. P. “Facet-Specific Stability of ZIF-8 in the Presence of Acid Gases Dissolved in Aqueous Solutions” Chem. Mater., Accepted (2016). DOI: 10.1021/acs.chemmater.6b02643
(L) Representation of crystallographic planes examined to modeldifferent surface facets of ZIF-8 and (R) microscopy images offacet-specific ZIF-8 crystals before exposure to SO2.
Facet-Specific Stability of ZIF-8 in the Presence of Acid Gases Dissolved in Aqueous Solutions
Research Details– ZIF-8 particles were synthesized to yield either {110} rhombic
dodecahedra (RD) facets or {100} truncated cubes.– Electron microscopy, BET surface area, XPS, FTIR, and
diffusion studies were performed to characterize differences in stability between exposed surface facets after exposure to aqueous SO2 solutions.
– Studies revealed the {110} RD facet was more susceptible to degradation; it is hypothesized that the mechanism for degradation follows a shrinking-core model, with surface imidazolates being replaced by hydroxyls.
– Computational investigations support this degradation mechanism and suggest that the reaction is more favorable on the {110} facet than the {100} facet due to steric constraints.
Presenter
Presentation Notes
Zeolitic imidazolate frameworks (ZIFs) are a subclass of metal-organic frameworks (MOFs) that are composed of tetrahedral metal nodes and imidazolate linkers. ZIF-8 in particular, made with Zn2+ ions and 2-methylimidazolate (2-MeIm) linkers, is a widely studied material due to its thermal and chemical stability relative to other metal-organic frameworks and its potential for use in a number of gas adsorption, separations and catalysis applications. However, most studies focus on characterization of the bulk structure of ZIF-8, ignoring the potential effect of particle shape and the external surface on these applications. The external surface of a porous crystal is the first point of interaction between the crystal and the external fluid; this initial interaction can be particularly problematic when the crystals are exposed to environments that potentially degrade the material. Since the external surface is the first to interact with the material, the external surfaces are likely to degrade first. Thus, understanding the overall stability of the crystal begins with understanding the stability and degradation behavior of the external surface. In this contribution, we examine the stability of two crystallographic facets of ZIF-8 after aqueous SO2 exposure through a variety of microscopic, spectroscopic, and computational techniques. By synthesizing either rhombic dodecahedra (RD) or cubes, which expose purely {110} or {100} facets, respectively, facet-specific stability trends are demonstrated, with the thermodynamically-favored but less sterically hindered {110} facet being less stable to acid exposure. We propose a shrinking-core model of degradation, with surface imidazolates being replaced by hydroxyls, under aqueous acid exposure conditions.
Particles exposing a single facet can be synthesized using crystal engineering techniques
1 μm 2 μm
J. Cravillon, M. Wiebcke, et al. CrystEngComm 2012Y. Pan, Z. Lai, et al. CrystEngComm 2011
10 μm
(100)
(110)
The surfaces of (100)-terminated cubes appear more stable than the surfaces of (110)-terminated RD
1 μm 1 μm 1 μm
500 nm 500 nm 500 nm
cube
RD
as synthesized 3 days 14 days
Bulk textural properties are not significantly affected, suggesting a shrinking-core model of degradation
10-6 10-5 10-4 10-3 10-2 10-1 100
0
100
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600
N2 A
dsor
bed
(cm
3 /g S
TP)
P/P°
RD RD, after SO2
cube cube, after SO2
Pre-SO2(m2/g)
Post-SO2(m2/g)
RD 1440 1290Cube 1420 1220
1 μm
500 nm
XPS studies suggest that imidazole is removed from the surface region and is replaced by hydroxyl
980 985 990 995 10000.0
0.1
0.2
0.3
0.4
0.5
Zn(OH)2
Nor
mal
ized
Inte
nsity
(a.u
.)
Kinetic Energy (eV)
RD RD, after SO2
cube cube, after SO2
Zn(OH)2
ZnO
Zn LMM
ZnO
294 292 290 288 286 284 2820.0
0.2
0.4
0.6
0.8
1.0
Nor
mal
ized
Inte
nsity
(a.u
.)
Binding Energy (eV)
RD RD, after SO2
cube cube, after SO2
C1s
542 540 538 536 534 532 530 528 5260.0
0.1
0.2
0.3
0.4
0.5
Nor
mal
ized
Inte
nsity
(a.u
.)
Binding Energy (eV)
ZnO Zn(OH)2
RD RD, after SO2
cube cube, after SO2
O1sZn(OH)2
174 172 170 168 166 164 162 160
0.00
0.05
0.10
0.15
0.20
Nor
mal
ized
Inte
nsity
(a.u
.)Binding Energy (eV)
RD RD, after SO2
cube cube, after SO2
S2p
1 μm
500 nm
N
NH
IR studies confirm formation of hydroxyl and sulfite species, likely on the external surfaces
3800 3600 3400 3200 3000 2800 2600
Abs
orba
nce
(a.u
.)
Wavenumber (cm-1)
RD RD, after SO2
cube cube, after SO2
1400 1200 1000 800 600 400
Abs
orba
nce
(a.u
.)Wavenumber (cm-1)
ν(NH/OH) ν(SO3)
ρ(SO2)
N N ZnZn
H2O N NH
ZnZn
OH2O SO2HN NH
ZnZn
OHH2OH2SO3
OSHO
OH
N N ZnZn
H2O OSOH
O
H H2O
Proposed sulfite-assisted degradation reactions at ZIF-8 surfaces. Imidazole is removed from the framework, and is replaced by surface sulfites or hydroxyls.
