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Flexible MOFs for Gas Separation ‒ A Case Study Based on Static and Dynamic Sorption Experiments
Dr. Robert Eschrich1 Christian Reichenbach1, Andreas Möller1, Jens Möllmer2, Marcus Lange2, Hannes Preißler2, Roger Gläser2, Matthias Thommes3
1 3P INSTRUMENTS GmbH & Co. KG 2 INC Leipzig e.V. 3 Quantachrome Instruments
2018-05-08 CPM8
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N2 / 77 K
VP, theor. = 0.42 cm3 g-1 VP, exp = 0.03 cm3 g-1
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CO2 / 273 K
VP, theor. = 0.42 cm3 g-1 VP, exp = 0.38 cm3 g-1
Desorption
Adsorption
J. Lincke, PhD-Thesis, Universität Leipzig, 2012. J. Lincke, D. Lässig, H. Krautscheid, Tetrahedron Letters 2010, 51, 653.
H2(Metrz-ia)
Accessible in a multigramm scale X-ray cristallography 47 % porosity VPore,theor. = 0.42 cm3 g-1
Relative pressure p / p0
Load
ing
n /
mm
ol g
-1
Relative pressure p / p0
Load
ing
n /
mm
ol g
-1
3 www.dynamicsorption.com S. Kitagawa, K. Uemura, Chem. Soc. Rev. 2005, 34, 109, S. Kitagawa, R. Kitaura, S. Noro, Angew. Chem. 2004, 116, 2388-2430; Angew. Chem. Int. Ed. 2004, 43, 2334-2375. S. Horike, S. Shimomura, S. Kitagawa, Nat. Chem. 2009, 1, 695-704.
1. Generation
2. Generation
Type I
Type II
Type III
Reversible network collapse
Guest-induced renewal
Guest-induced transformation
gate opening
pressure
load
ing
3. Generation
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1. Pure Component
Sorption
2. In situ PXRD
Conclusions on Separation Effects and the
Influence of the Structural Flexibility
3. Mixture Sorption
Interpretation of stepped Isotherms with C4-Hydrocarbons
TCI Deutschland GmbH, http://www.tcichemicals.com/eshop/de/de/category_index/03900/. A. Schneemann, V. Bon, I. Schwedler, I. Senkovska, S. Kaskel, R. A. Fischer, Chem.Soc.Rev. 2014, 43, 6062.
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pressure p / kPa pressure p / kPa
load
ing
n /
mm
ol g
-1
load
ing
n /
mm
ol g
-1
n-Butane Isotherms
• stepped isotherm with strong hysteresis in low pressure region (< 5 kPa) • hysteresis dependent on pressure and temperature Structural change?
Linear Logarithmic
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Gleichgewichtsdaten von n- bzw. Isobutan
• VP, exp = 0.31 cm3 g-1 (n-Butane) and 0.34 cm3 g-1 (Isobutane)
• Isobutane adsorption is much slower than n-Butane adsorption!
pressure p / kPa pressure p / kPa
load
ing
n /
mm
ol g
-1
load
ing
n /
mm
ol g
-1
n-Butane n-Butane
Isobutane Isobutane
Comparison of n-Butane and Isobutane at 298 K
Linear Logarithmic
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Phase 1
pressure p / kPa
load
ing
n /
mm
ol g
-1
Uptake Curves – an indication for the rate of adsorption
time t / min
rela
tive
up
take
n-Butane
Isobutane
n-Butane, T = 298 K
Phase 1, T = 298 K,
P < 0.1 kPa
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Phase 2
pressure p / kPa
load
ing
n /
mm
ol g
-1
Uptake Curves – an indication for the rate of adsorption
time t / min
rela
tive
up
take
n-Butane
Isobutane
n-Butane, T = 298 K
Phase 2, T = 298 K, P ≥ 30 kPa
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gate opening
• gate opening has influence on rate of adsorption • Can this be utilized for a kinetic separation?
pressure p / kPa
load
ing
n /
mm
ol g
-1
Uptake Curves – an indication for the rate of adsorption
time t / min
rela
tive
up
take
n-Butane
Isobutane
n-Butane, T = 298 K
gate opening, T = 298 K,
0.1 kPa < p < 1.1 kPa
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gate opening
• gate opening has influence on rate of adsorption • Can this be utilized for a kinetic separation?
pressure p / kPa
load
ing
n /
mm
ol g
-1
Uptake Curves – an indication for the rate of adsorption
time t / min
rela
tive
up
take
Isobutane n-Butane
t0.5 = 14 min
n-Butane, T = 298 K
t0.5 = 667 min
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Coupling of sorption experiments with powder x-ray diffractometry structural changes oberservable?
Gas Supply
Detector X-ray source
Cu-Ka
Experimental Setup
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inte
nsi
ty
pressure p / kPa
load
ing
n /
mm
ol g
-1
n-Butane
Adsorption of n-Butane
• Structural Change observable during n-Butane adsorption
vacuum
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pressure p / kPa
load
ing
n /
mm
ol g
-1
n-Butane
Desorption of n-Butane
• Closed structure is retained after desorption. Open structure is retained after resolvatization • Monoclinic crystal structure before and after gate-opening • With n-Butane Guest-induced transformation 3. Generation – Type II
inte
nsi
ty
vacuum
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High experimental effort – continous mixing of the gas phase
GC analysis before and after each experiment partial loadings
Calculation of the mixture isotherm with the IAST
thermodynamically ideal behaviour
molar fraction of n-Butane in gas phase yn
mo
lar
frac
tio
n o
f n
-Bu
tan
e in
ad
sorb
ate
x n
load
ing
n /
mm
ol g
-1
molar fraction of n-Butane in gas phase yn
Static Volumetric-Gravimetric Measurements with n-Butane/Isobutane Gas Mixtures
ptotal= 40 kPa
ptotal= 40 kPa
total
n-Butane
Isobutane
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• faster adsorption of n-Butane opens pore structure • change in selectivity over time can be observed
time t / min time t / min
mo
lar
frac
tio
n o
f n
-Bu
tan
e x n
an
d Is
ob
uta
ne
xis
o in
ad
sorb
ate
sele
ctiv
ity
for
n-B
uta
ne
Static Volumetric-Gravimetric Measurements with n-Butane/Isobutane Gas Mixtures
n-Butane
Isobutane
T = 313 K, ptotal= 40 kPa
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• Equilibrium times for different gas mixtures
• Partial pressure of n-Butane determines the time until equilibrium
more n-Butane = faster equilibrium time
n:iso=75:25 50:50 25:75
t0.5 = 14 min 55 min 391 min
time t / min
rela
tive
up
take
Static Volumetric-Gravimetric Measurement: Uptake Curves
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• Sorption takes place in open system
• Pressure is constant
• Gas Mixtures only
• Outlet composition is recorded over time
• Fixed Bed Adsorber: Gas must not pass without interaction
𝑛adsorbed = 𝑛 in(𝑡)d𝑡 − 𝑛 out(𝑡)d𝑡
𝑛adsorbed = 𝑉 in(𝑡)𝑦in(𝑡)
𝑉md𝑡 − 𝑉 out(𝑡)
𝑦out(𝑡)
𝑉md𝑡
0.0 0.2 0.4 0.6 0.8 1.00
20
40
60
80
100
rel. b
reakth
rou
gh
/ %
time-on-stream / s
Breakthrough Curve Experiment
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• BTC from n-Butane and Isobutane
Combination of equilibrium and kinetic effects
• Adsorption of n-Butane is favored in the dynamic measurement
n : iso = 25:75 n : iso = 50:50 n : iso = 80:20
C4 mixtures in N2: 313 K, Flow: 3 cm3 min-1
ptotal = 100 kPa, pC4 = 40 kPa
specific time t / min g-1
rela
tive
co
nce
ntra
tio
n
Breakthrough Curves
Rel
ativ
e co
nce
ntra
tio
n
Rel
ativ
e co
nce
ntra
tio
n
specific time t / min g-1 specific time t / min g-1
total
total
total
n-Butane
n-Butane n-Butane
Isobutane
Isobutane
Isobutane
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eff. selectivity bn/iso
(dynamic)
3.9
4.7
1.8 • Calculating the partial loadings by
integrating over the Breakthrough curves
Determining effective selectivity β
• Values are very different from
thermodynamic selectivity α
• Gate opening influences
selectivity in dynamic processes
~0.5
ideal selectivity an/iso
(equilibrium)
mo
lar
frac
tio
n o
f
n-B
uta
ne
in a
dso
rbat
e x n
molar fraction of n-Butane in gas phase yn
Comparison of Selectivities: Dynamic vs. Static
T = 313 K, ptotal= 40 kPa
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• Enrichment of Isobutane on the surface in equilibrium
• Sorption-induced structural changes determined with XRD
• gate opening dependent on n-Butane partial pressure
• Stepwise breakthrough curves for n-Butane; spontaneous Breakthrough for Isobutane
• Enrichment of n-Butane on the surface in dynamic measurements
• Kinetics of gate-opening determine selectivities interesting for gas separation applications
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