Hydrophobic and moisture-stable metal–organic frameworks

Abstract

Metal-organic frameworks (MOFs) have proved to be very attractive for applications including gas storage, separation, sensing and catalysis. In particular, CO2 separation from flue gas in post-combustion processes is one of the main focuses of research among the scientific community. One of the major issues that are preventing the successful commercialization of these novel materials is their high affinity towards water that not only compromises gas sorption capacity but also the chemical stability. In this paper, we demonstrate a novel post-synthesis modification approach to modify MOFs towards increasing hydrophobic behaviour and chemical stability against moisture without compromising CO2 sorption capacity. Our approach consists of incorporating hydrophobic moieties on the external surface of the MOFs via physical adsorption. The rationale behind this concept is to increase the surface hydrophobicity in the porous materials without the need of introducing bulky functionalities inside the pore which compromises the sorption capacity toward other gases. We herein report preliminary results on routinely studied MOF materials [MIL-101(Cr) and NiDOBDC] demonstrating that the polymer-modified MOFs retain CO2 sorption capacity while reducing the water adsorption up to three times, with respect to the un-modified materials, via an equilibrium effect. Furthermore, the water stability of the polymer-functionalized MOFs is significantly higher than the water stability of the bare material. Molecular dynamic simulations demonstrated that this equilibrium effect implies a fundamental and permanent change in the water sorption capacity of MOFs. This approach can also be employed to render moisture stability and selectivity to MOFs that find applications in gas separations, catalysis and sensing where water plays a critical role in compromising MOF performance and recyclability.

Full text

Table S1. Contact angle values for unmodified
and P1-modified MOFs
SUPPORTING INFORMATION
Hydrophobic and Moisture-Stable Metal-Organic
Frameworks
Carlos A. Fernandez,* Satish K. Nune,* Harsha V. Annapureddy, Liem X. Dang, , B. Peter McGrail,
Feng Zheng, Evgueni Polikarpov, David L. King, Charles Freeman and Kriston P. Brooks
Number of tables: 1
Number of Figures: 8
No. of videos: 2 (Attached Separately)
Sample
name
Contact angle
at t=0
Droplet life
time, s
MIL101
0
0
P1-MIL101
~25°
~35
Ni-MOF-74
~7°
~1
P1-Ni-MOF-74
~22°
~30
Electronic Supplementary Material (ESI) for Dalton Transactions.
This journal is © The Royal Society of Chemistry 2015

Figure S1. High pressure RT CO2 sorption in unmodified and P1-modified MIL101(Cr)

Figure S2. RT water sorption on unmodified as well as P1-
treated MIL101(Cr) showing no change on capacity after
three activation/exposure cycles.
Figure S3. Top. SEM micrographs of unmodified (top left) and P1-modified NiDOBDC (top right).
Bottom. SEM micrographs of unmodified MIL101(Cr) and P1-modified MIL101(Cr).

Figure S4. Nitrogen isotherms at 77 K and BET surface area calculated for unmodified NiDOBDC
(left) and P1-modified NiDOBDC (right).
Area= 912 m2/g
Slope= 3.94
Y intercept= -0.130
Correlation coefficient= 0.998
C= -29.3
Area= 831 m2/g
Slope= 4.33
Y intercept= -0.1340
Correlation coefficient= 0.998
C= -31.2

Front View Side View
Figure S6: Simulation snapshot of a NiDOBDC fragment and 20 molecules of P-123 solvated in chloroform
during the process of MOF functionalization with P-123.
Figure S5. Water sorption change rate (wt%/t) as a function of time on unmodified NiDOBDC (right) and on
P1-modified NiDOBDC (left) showing the times at which water sorption at a new %RH takes place. Note the
higher rates observed for unmodified NiDOBDC at low %RH and the very high rates observed in the first few
minutes of a new RH% condition followed by exponential rates decay in both materials.

Figure S7: (a) Radial distribution function between the Ni atoms of NiDOBDC and the oxygen atoms
of the P-123. (b) Radial distribution functions between the Oxygen atoms of NiDOBDC and the
terminal Hydrogen atoms of P-123. In inset we show the snapshot illustrating the corresponding
interactions between P-123 and MOF.
02468 10
r, Å
0
20
40
60
80
g(r)
OMOF-HOP-123
0 2 4 6 8 10
r, Å
0
30
60
90
120
150
g(r)
NiMOF-OP-123
NiMOF-CP-123
(a)
(b)

Figure S8: Front and side views of a NiDOBDC fragment with P-123 wrapped round it.
Video 1: Simulation trajectory of P1-modified
NiDOBDC with water
Front view
Side view

Video 2: Simulation trajectory of P1-functionalized NiDOBDC with CO2
Molecular Dynamics (MD) Simulations. MD simulations were performed by using Amber 9 package. The radial
distribution functions (RDFs) was computed to understand the interaction between the NiDOBDC fragments and P-123.
Force field parameters for MOF were taken from previous work.1,2 The SPC/E and the potential parameters for
chloroform were taken from literature.3,4 The molecular formula of polymer (P-123) unit used in our MD simulations is
(HO(CH2CH2O)2(CH2CH(CH3)O)2(CH2CH2O)2H). Charges for the P-123 are computed using am1-bcc method
implemented in the antechamber module of Amber 9 package. Lennard–Jones (LJ) and all the other intramolecular
parameters such as bond, angle and torsion were obtained from general AMBER force field (GAFF).5, 6
A portion of a rigid fragment of MOF was taken from the NiDOBDC crystal structure. The MOF fragment is
then centered in a box of size 80x80x80 Å. To this 20 units of P-123 were added and then solvated with chloroform.
Snapshots of simulation box containing MOF + P123 solvated in chloroform are shown in Figure S9. Equilibration runs
were carried out in a constant NPT (number of particles, pressure and temperature) ensemble to get the correct density,
followed by a 20 ns of production run using canonical ensemble NVT (number of particles, volume and temperature)

using periodic boundary conditions applied in all three directions with a time step of 1 fs. The three-dimensional particle
mesh Ewald summation technique was used to calculate the long-range electrostatic interactions.7
Figure S10 illustrates the radial density functions (RDF) computed between the Ni-atoms of MOF and the
oxygen and carbon atoms of P-123. The NiMOF-OP-123 RDF has sharp and intense peak at ~2.2 Å. As expected we
observed strong interaction between the positive Ni-atoms of the MOF and the partially negative O-atoms of P-123. In
Figure S10 we show the RDF between the oxygen atoms of MOF linker units and the terminal hydrogen atoms of P-123.
This RDF has peak at ~1.7 Å, indicating strong OMOF-HOP-123 interactions. These two kinds of interactions are primary
governing factors for binding of P-123 to MOF material.
Equilibrated MOF fragment with the polymer wrapped around it is taken and placed at the center of a simulation
box of size 60x60x60Å (Figure S11). Two systems were prepared by randomly adding three hundred molecules of water
and carbon dioxide respectively. MD simulations are performed for both the systems in an NVT ensemble at 298 K.
References
(1) Case, D. A.; Darden, T. A.; Cheatham III, T. E.; Simmerling, C. L.; Wang, J.; Duke, R. E.; Luo, R.;
Merz, K. M.; Pearlman, D. A.; Crowley, M. Amber 9. University of California, San Francisco: 2006.
(2) Sun, X. Q.; Wick, C. D.; Thallapally, P. K.; McGrail, B. P.; Dang, L. X. Computational study of
hydrocarbon adsorption in metal-organic framework Ni2(dhtp). Journal of Physical Chemistry B 2011, 115, 2842-2849.
(3) Berendsen, H. J. C.; Griger, J. R.; Straatsma, T. P. T. The missing term in effective pair potentials. J.
Phys. Chem., 1987, 91, 6269-6271.
(4) Jorgensen, W. L., Tirado-Rives, J. (1988). Energy Minimizations for Crystals of Cyclic Peptides and
Crambin. J. Am. Chem. Soc. 110 (6): 1657–1666.
(5). Wang, J. M.; Wolf, R. M.; Caldwell, J. W.; Kollman, P. A.; Case, D. Development and testing of a
general amber force field. J. Comp. Chem. 2004, 25, 1157-1174.
(6). Wang, J. M.; Wang, W.; Kollman, P. A.; Case, D. A. Automatic atom type and bond type perception in
molecular mechanical calculations. J. Mol. Graph Model 2006, 25, 247-260.
(7) Essmann, U.; Perera, L.; Berkowitz, M. L.; Darden, T.; Lee, H. and Pedersen, L. G. A smooth particle
mesh Ewald method. J. Chem. Phys. 1995, 103, 8577- 8593.