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A new generation of
hierarchical structured
materials with high adsorption
capacity and selectivity
Devriese L.I.a, Martens J.A.b, Aerts A.b, Baron G.V.a, Denayer J.F.M.a
a CHIS, Vrije Universiteit Brussel
b COK, Katholieke Universiteit Leuven
i-SUP 2008 - Brugge, België 1
Introduction: Porous Adsorbents
Zeolites
• Micropores < 1 nm
MOR:
+ Shape selectivity
+ Good stability
- Diffusion limitations
- Pore obstruction
- Limited access
i-SUP 2008 - Brugge, België
Pag.2
Introduction: Porous Adsorbents
Zeolites Mesoporous Silica
• Micropores < 1 nm • Mesopores 2 - 30 nm
MOR: MCM-41 & MCM-48:
+ Shape selectivity + Fast diffusion
+ Good stability + Good accessibility
- Diffusion limitations - No shape selectivity
- Pore obstruction - Inferior stability
- Limited access - Weaker acidity
i-SUP 2008 - Brugge, België
Pag.3
Introduction: Porous Adsorbents
Biporous Hierarchical Material
• Micropores AND Mesopores
• Mesoporous Material
+ Fast diffusion
+ Good accessibility
• Zeolite Silicalite-1 frameworkLinear channel
5.6 x 5.3 Å
6.65 Å
+ Shape selectivity 4.5 Å
+ Good stability Sinusoidal channel
+ Large number of sites 5.5 x 5.1Å
ZSM-5
Micropores: 0.55 nm
Zeolite nuclei
• Nanoslabs: 1.3 x 1.3 x 4.0 nm³
First level of porosity
Kirschhock et al., Chem.Eur.J., 2005
i-SUP 2008 - Brugge, België
Pag.4
Materials
Zeogrid
Half-nanoslab suspension
• Ultra-micropores: 0.55 nm
CTMABr template
• Rectangular mesopores: 3.0 nm
Zeotile-2
Half-nanoslab suspension
• Ultra-micropores: 0.55 nm
CTMABr template
• Cubic mesopores: 2.7 nm
Stirred & Heated
Double nanoslab suspension Zeotile-4
• Ultra-micropores: 0.55 nm
P123
• Hexagonal mesopores: 7.3 nm
HCLaq solution
Kremer et al., Adv.Funct.Mater, 2002 Kremer et al. Adv.Mater, 2003 Kremer et al., C.R.Chimie, 2005
Kremer et al. Solid State Sciences, 2005 Kirschhock et al., Chem.Eur.J., 2005
i-SUP 2008 - Brugge, België
Pag.5
Goal
Study of the Gas phase Adsorption properties
Low and High surface coverage
Compare to Zeolites and Mesoporous solid
i-SUP 2008 - Brugge, België
Pag.6
Low Coverage: Pulse gas chromatography
TRACER
INERT GAS DETECTOR
He
Chromatographic Column
t
µ
µ = [(ε ext + ε macr ) + (1 − ε ext − ε macr )RT ρ cK ′]
K i' =
qi L
pi
vf
Saturation
Henry Constant K’
Henry Adsorption Enthalpy ∆H0
region
Adsorption Entropy ∆S0
i-SUP 2008 - Brugge, België
Pag.7
High Coverage: Gravimetric Technique
qi
K i' =
pi
microbalance
Saturation
vent Henry
region
He Adsorption Isotherm:
oven
Amount adsorbed
Sample
Sample
holder
holder
Adsorbate (vl)
Thermostatic bath
Pi
i-SUP 2008 - Brugge, België
Pag.8
Results: Overview
Low Coverage: Pulse gas chromatography
Aspecific interactions: n-alkanes
Shape Selective Properties: n- and iso-alkanes
Specific interactions: 1-alkenes & aromatics
High Loading: Gravimetric experiments
Aspecific interactions: n-octane
Specific interactions: 1-alkenes & aromatics
i-SUP 2008 - Brugge, België
Pag.9
Low Coverage, Aspecific interactions
Henry constants K’ at 160°C
Zeolite ZSM-5
1,E-01 Zeolite HY
K' (mol/kg/Pa)
1,E-02
1,E-03 Zeotile-4
1,E-04 MCM-48
1,E-05 Zeotile-2
1,E-06 Zeogrid
4 5 6 7 8 9
Carbon number
- Exponential increase
- K’ZEOLITES >> K’BIPOROUS MATERIALS = K’MCM-48
i-SUP 2008 - Brugge, België
Pag.10
Low Coverage, Aspecific interactions
Van ‘t Hoff plot (50-250°C):
n-octane n-pentane
Ln K' (mol/kg/Pa)
-7 Zeotile-2
Zeotile-4 Zeotile-4
-9 Zeogrid
-11
-13 Zeotile-2
0.0018 0.0021 0.0024 0.0027 0.003 0.0033
1/T (K)
Zeogrid
- Highly linear for all alkane components
- Differences in K’ related to:
Adsorption enthalpy ∆H0
Adsorption entropy ∆S0
number of adsorption sites
i-SUP 2008 - Brugge, België
Pag.11
Low Coverage, Aspecific interactions
Adsorption Enthalpy K ' = K ' 0 . exp − ∆H 0 / RT
90 Zeolite ZSM-5
-∆ H0 (kJ/mol)
80 Zeotile-4
70
Zeotile-2
60
50 MCM-48
40 Zeogrid
30
4 5 6 7 8 9
Carbon number
- Microporous Zeolites: highest adsorption enthalpy
- Only subtle differences between Mesoporous and Biporous
i-SUP 2008 - Brugge, België
Pag.12
Low Coverage, Shape Selectivity
n- and iso-alkanes
Micropore
- Branched alkanes: less adsorption
- Shape Selective Property
i-SUP 2008 - Brugge, België
Pag.13
Low Coverage, Shape Selectivity
Separation factors at 160°C
α = K’n-alkane / K’iso-alkane
α Zeotile-4 Zeogrid MCM-48 Zeotile-2 ZSM-5
n-C8 / 2-MeC7 1.2 1.2 1.3 1.5 2.1
n-C8 / 2,5diMeC6 1.4 1.4 1.5 2.0 -
n-C8 / 2,2,4-triMeC5 1.8 1.7 2.0 3.1 >100
- α: ZSM-5 >> Zeotile-2 > Zeogrid, Zeotile-4 and MCM-48
- Biporous materials: separating linear from branched alkanes
i-SUP 2008 - Brugge, België
Pag.14
Low Coverage, Shape Selectivity
Zeotile-2 versus MCM-48
Zeotile-2 MCM-48
Cubic structure Cubic structure
Micropores: 0.55 nm NO micropores
Mesopores: 2.7 nm Mesopores: 2.5 nm
i-SUP 2008 - Brugge, België
Pag.15
Low Coverage, Shape Selectivity
Separation factors at 160°C
4,00
Zeotile-2
3,50 MCM-48
3,00
2,50
α
2,00
1,50
1,00
4
5
5
6
6
7
4
5
5
5
22 6
22 5
6
2C
2C
3C
2C
3C
2C
C
C
C
C
C
4C
4C
23
23
24
33
25
- Separation factors: Zeotile-2 > MCM-48
i-SUP 2008 - Brugge, België
Pag.16
Low Coverage, Shape Selectivity
Adsorption enthalpy
65
MCM-48
60
-∆H0 (kJ/mol)
Zeotile-2
55
50
45
40
35
30
5
6
7
4
5
5
6
6
4
4
5
5
C
C
C
2C
2C
3C
2C
3C
C
C
C
C
22
23
23
24
- Interaction energy: Zeotile-2 > MCM-48
- Micropore adsorption
i-SUP 2008 - Brugge, België
Pag.17
Low Coverage, Shape Selectivity
Vapour Pressure
Log K’ = a . Log ps + b
Log K' (mol/kg/Pa)
-3
Log K' (mol/kg/Pa)
-3.5
-3.5 MCM-48 -4 Zeotile-2
-4
-4.5 -4.5
-5 Linear alkanes -5 Linear alkanes
Branched alkanes Branched alkanes
-5.5 -5.5
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 0 0.2 0.4 0.6 0.8 1 1.2 1.4
Log ps (bar)
Log ps (bar)
No Shape Selective Properties Shape Selective Properties !
i-SUP 2008 - Brugge, België
Pag.18
Low Coverage, Specific interactions
Chromatogram:20200°C on Zeotile-2
0 10 30 40 50 60
Time (min)
300000
T: 200°C
n-hexane
n -hexane
250000
benzene
benzene
200000
Signal (a.u.)
1-hexene
1-hexene
150000
100000
50000
0
0 0.5 1 1.5
Time (min)
- Preference in adsorption: aromatics > alkenes > alkanes
i-SUP 2008 - Brugge, België
Pag.19
Low Coverage, Specific interactions
Separation factors at 160°C
α = K’i / K’j
α Zeotile-2 Zeotile-4 Zeogrid
benzene/ n-hexane 2.3 1.8 1.4
1-hexene/ n-hexane 1.9 1.3 1.7
o-xylene/ m-xylene 1.0 1.1 1.0
p-xylene/ m-xylene 1.1 1.0 1.0
- Preference in adsorption: aromatics > alkenes > alkanes
- Xylene isomers: no separation
i-SUP 2008 - Brugge, België
Pag.20
High Coverage, Aspecific interactions
n-octane (70°C)
Zeotile-2 Silicalite
Sun et al., J.Phys.Chem., 1996
40 40
Weight % chg
Weight % chg
30 30
20 20
10 10
n-octane
n-Octane 0
0
0 0.02 0.04 0.06 0 0,005 0,01
Pvap (bar) Pvap (bar)
- Two Step Behaviour
- High adsorption capacities! 40-50 wt%
- Zeolites Max 35 wt%, Typical < 20 wt%
i-SUP 2008 - Brugge, België
Pag.21
High Coverage, Specific interactions
o-xylene, 1-octene and n-octane (70°C)
o-Xylene
Zeotile-2
60
50
Weight % chg
40
1-Octene
30
20
10 n-Octane
0
0 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4
Pvap /Psat
- 1-octene & n-octane: similar adsorption capacities
- o-xylene: stronger adsorption + higher adsorption capacity
i-SUP 2008 - Brugge, België
Pag.22
High Coverage, Specific interactions
Xylene isomers (70°C)
m-xylene
Zeotile-2
60
50
Weight % chg
40
30 o-xylene
20
10
0
0 0,1 0,2 0,3 0,4
Pvap/Psat
- Xylene isomers: no separation
- High capacities
i-SUP 2008 - Brugge, België
Pag.23
Conclusions
• Lower K’ and ∆H0 compared to zeolites
• Significant higher adsorption capacities
Up to 50 weight %
• Selectivity: aromatics > alkenes > alkanes
• Presence of micropores
Shape Selectivity at low coverage: Zeotile-2
i-SUP 2008 - Brugge, België
Pag.24
Acknowledgements
• IWT Vlaanderen ( SBO ‘BIPOM’)
• FWO Vlaanderen (J. Denayer)
i-SUP 2008 - Brugge, België
Pag.25
Thank you
i-SUP 2008 - Brugge, België
Pag.26
