gas adsorption in cu-btc metal-organic frameworks
TRANSCRIPT
3-6-2010
Challenge the future
DelftUniversity ofTechnology
Gas adsorption in Cu-BTCMetal-Organic FrameworksA molecular simulation studyJuan Manuel Castillo, Delft University of Technology, Process&Energy Laboratory, Leeghwaterstraat 44. 2628CA Delft. [email protected]
S. Calero, T.J.H. Vlugt, E. García-Pérez, A. Martín-Calvo
2Gas adsorption in Cu-BTC
Metal organic frameworks
Large surface area, tailor-made designMOF
metal center
+
organic linker
IRMOF-1
Applications: gas storage, gas separation, catalysis…
3Gas adsorption in Cu-BTC
Metal organic frameworks
Cu-BTCAdsorption sites
I I’
IIIII
Central pore + ‘side pockets’ I. By the open metal centersI’. At the central poreII. At the side pocket centerIII. At the side pocket windowRemember!!
4Gas adsorption in Cu-BTC
Molecular simulations
Statistical basis
Statisticalmechanics
Microscopic view(microstate)
Microscopic view(microstate)
Adsorption isotherms → we need µVT ensemble
5Gas adsorption in Cu-BTC
Molecular simulations
Monte Carlo
How to ‘measure’ property A in µVT ensemble?
∑∑
−
−⋅
=
jiji
jiji
EN
ENAA
,
,
)exp(
)exp(
ββµ
ββµ Impossible to calculate:. Too many microstates. Most microstates give zero contribution
Importance sampling: the Metropolis method
Generate a sequence of microstate configurations. If o is an initial configuration with N particles, the probability of accepting a new configuration n with N+1 particles is:
Similar rules for changing from N+1 to N particles, and between microstates with the same N
⎟⎟⎠
⎞⎜⎜⎝
⎛∆−
+Λ=→ )exp(
)1(,1min)( 3 U
NVnoacc ββµ
6Gas adsorption in Cu-BTC
Simulation model
Molecular model – force field
Pre-existing models for adsorbed molecules
Non-bonded interactions:
⎪⎩
⎪⎨⎧
><−
=cutoffij
cutoffijcutoffLJ
ijLJ
ijLJshift rr
rrrUrUrU
0)()(
)( Lennard-Jones: Mixing rules
∑∑= <
=N
j
N
ji ij
jiCoulomb
rqq
U1 04
1πε
Ewald summation
Cu-BTC: quantum chemistry calculations
Dreiding + UFF
7Gas adsorption in Cu-BTC
Water adsorption
Adsorption isotherms
● New set
▲ 2% increase
♦ 6% increase▼ 4% increase
□ Experimental
Charge fitting
Electrostatic interaction is dominant!
Castillo, J.M.; Vlugt, T.J.H.; Calero, S. J. Phys. Chem. C 112 (2008) 15934-15939
8Gas adsorption in Cu-BTC
Water adsorption
Adsorption sites (low loading)
Very strong preference for site I (by the metal centers)
9Gas adsorption in Cu-BTC
Excess adsorption
Nitrogen isotherm
77 K 295 K
The computed adsorption has to be scaled by a 0.8 factor
10Gas adsorption in Cu-BTC
Adsorption isotherms
Other gasses
oxygen methane
Preferential adsorption at site is II (at the side pockets)García-Pérez, E.; Gascón, J.; Morales-Flores, V.; Castillo, J.M.; Kapteijn, F.; Calero, S.
Langmuir 25(2009) 1725-1731
11Gas adsorption in Cu-BTC
Adsorption sites
Other gasses (low pressures)
carbon dioxide methane
Preferential adsorption at site II (at the side pockets)
12Gas adsorption in Cu-BTC
Adsorption isotherms
Hydrocarbons
ethane propane
We can not reproduce propane isotherms . Why?
13Gas adsorption in Cu-BTC
Framework flexibility
X-ray patterns
298 K – 373 K 373 K – 473 K
Negative thermal expansion: effect on the pocket windows
14Gas adsorption in Cu-BTC
Possible applications
Gas separation: natural gas
Adsorption selectivity in Cu-BTCCO2 / CH4 mixtures
♦ 50/50 mixture▲ 10/90 mixture■ 50/50 other sim
open symbols:
Other MOFs
CO2 can be separated from CH4 efficiently in Cu-BTCReason: competitive adsorption at site IIMartín-Calvo, A.; García-Pérez, E.; Castillo, J.M.; Calero, S. Phys. Chem. Chem. Phys. 10 (2008)
7085-7091
15Gas adsorption in Cu-BTC
Conclusions
Gas adsorption in Cu-BTC
. Molecular simulations are a suitable tool to study gas adsorption in porous media
. The adsorption behavior of molecules in Cu-BTC is conditioned by their dipole/quadrupole moment
. Cu-BTC is a promising material for gas separation
16Gas adsorption in Cu-BTC
Thank you!