Adsorption Separations

Transcription

1 Molecular Modeling and Design of Metal-Organic Frameworks for CO 2 Capture Randy Snurr Department of Chemical & Biological Engineering Northwestern University, Evanston, IL Adsorption Separations PSA, TSA, VSA Adsorption separations are widely used in processes such as air separation can be more energy efficient than traditional distillation separations can be based on differences in adsorption thermodynamics (more common) or rates of diffusion (less common) A key issue is the choice of the adsorbent Novel adsorbent Nanotechnology for Carbon Dioxide Capture, R.R. Willis, A.I. Benin, R.Q. Snurr, A.O. Yazaydin, in Nanotechnology for the Energy Challenge, J. Garcia-Martinez, Ed., Wiley-VCH, 2010.

2 A Building-block Approach to Materials Synthesis Use specific transition-metal coordination chemistry: + Metal corners and organic linkers can be chosen to yield a wide variety of porous, crystalline structures Pores or cavities of controlled sizes Wide variety of chemical functionalities Metal-Organic Frameworks 3D frameworks Potential for adsorption applications crystalline Snurr, Hupp, Nguyen, 2004 very open structures potential applications in energy storage, sensing, adsorption separations, and catalysis

3 Example: IRMOFs IsoReticular Metal-Organic Frameworks O O O O O O = = O O Zn 4 O O O O O O O IRMOF-1 IRMOF-4 IRMOF-10 Omar Yaghi, Univ. California, Los Angeles IRMOFs

4 Mixed-Ligand MOFs Pyridine-only frameworks are generally unstable against channel collapse. However, a family of mixed-ligand MOFs shows permanent microporosity. (1) Carboxylate paddle-wheel-type coordination of Zn(II) pairs 2D sheets (2) Pyridine/Zn linkages open frameworks [Ma, Mulfort, Hupp, Inorg. Chem Diversity of MOFs MOF-177 MIL-53 MOF-177 MIL-103 HKUST-1

5 Molecular Tinker Toys Ultra-High Surface Area Extended linker lengths MOF-210 has the largest specific surface area reported to date: 6240 m 2 /g BET. These large-pore MOFs have very high capacity for CO 2 at higher pressures. H. Furukawa, N. Ko, Y.B. Go, N. Aratani, S.B. Choi, E. Choi, A.Ö. Yazaydin, R.Q. Snurr, M. O Keeffe, J. Kim, O.M. Yaghi, Ultra-high porosity in metalorganic frameworks, Science, in press.

6 Materials Design? Can tune material properties via synthesis pore size linker functionality open-metal sites extraframework cations or anions Can also modify MOFs after their synthesis Outline Screening of MOFs for CO 2 Capture with Molecular Modeling Model development Test of model versus experiment Screening results Identify candidate MOFs What do we learn? Post-Synthesis Modification of MOFs for Improved CO 2 Uptake Summary and Outlook

7 Simulation Model Atomistic representation of MOFs MOF atoms are held fixed at their crystallographic coordinates. Lennard-Jones parameters taken from the DREIDING force field. Charges on framework atoms from quantum chemical calculations. Atomistic representation of guest CO 2 molecules CO 2 /CO 2 parameters taken from TraPPE force field that matches bulk vapor/liquid equilibria * Lennard-Jones + Coulomb 1.16 Å * Potoff, Siepmann, AIChE J., Molecular Simulation of Adsorption A phase equilibrium problem At equilibrium: T I = T II P I = P II µ I i =µ II i for all species i (or f ii = f II i for all species i) Fluid phase, I MOF phase, II

8 Grand Canonical Monte Carlo (GCMC) adsorbed phase in equilibrium with bulk fluid µ, V, T constant as in adsorption experiments number of molecules fluctuates µ T random moves insertions deletions of molecules CO 2 Adsorption in MOFs HKUST-1 ZIF-8 Yazaydin, Snurr, Park, Koh, Liu, LeVan, Benin, Jakubczak, Lanuza, Galloway, Low, Willis, J. Am. Chem. Soc., 2009.

9 CO 2 Adsorption in MOFs 298 K IRMOF-1 CO 2 Loading, mg/g MOF-177 IRMOF-3 GCMC Pressure, kpa CO 2 Loading, mg/g K 208K 218K 233K 273K GCMC Pressure, kpa Walton, Millward, Dubbeldam, Frost, Low, Yaghi, Snurr, J. Am. Chem. Soc., Systematic Comparison with Experiment IRMOF-1 IRMOF H simulation / KJ.mol N 2 CH 4 Kr Xe CO 2 i-c 4 H 10 n-c 4 H 10 H simulation / KJ.mol N 2 Kr CO 2 CH4 Xe H experimental / KJ.mol H experimental / KJ.mol -1 Farrusseng, Daniel, Gaudillere, Ravon, Schuurman, Mirodatos, Dubbeldam, Frost, Snurr, Langmuir, 2009.

10 Screening MOFs for CO 2 Capture Given the large number of possible MOF topologies, linkers, and metal nodes, there are an almost unlimited number of MOFs that could be synthesized. Screening and understanding of the fundamental structure/function relationships are, thus, very important for developing new processes based on MOFs. Choose a diversity of materials for screening to help improve our understanding of CO 2 capture in MOFs. CO 2 /MOF Screening Collaboration Team Approach Synthesis Characterization Testing Modeling Team Members Richard Willis, Annabelle Benin, Syed Faheem, John Low at UOP LLC, Des Plaines, IL Adam Matzger at University of Michigan, Ann Arbor Douglas LeVan at Vanderbilt University Stefano Brandani at University of Edinburgh Ozgur Yazaydin at Northwestern University

11 Screening MOFs for CO 2 Capture 14 MOFs Mg\DOBDC Ni\DOBDC Co\DOBDC Zn\DOBDC M\DOBDC (1D Channels) UMCM-1 (High surface area) Pd(2-pymo) 2 (Narrow pores) HKUST-1 (Side pockets) Pd(2-pymo) 2 HKUST-1 UMCM-150(N) 2 UMCM-150 MIL-47 ZIF-8 IRMOF-1 IRMOF-3 UMCM-1 MOF-177 Screening MOFs for CO 2 Capture Experimental CO 2 uptake at 0.1 bar and 298 K M\DOBDC MOFs perform particularly well. MOFs with large free volume perform the worst at low pressure. MOFs having coordinatively unsaturated metal sites (open-metal sites) demonstrate the best performance. Yazaydin, Snurr, Park, Koh, Liu, LeVan, Benin, Jakubczak, Lanuza, Galloway, Low, Willis, J. Am. Chem. Soc., 2009.

12 Screening MOFs for CO 2 Capture No correlation with SA There is a strong correlation between CO 2 uptake and heat of adsorption at low pressure. No correlation with free volume Yazaydin et al., J. Am. Chem. Soc., Screening MOFs for CO 2 Capture Why do M\DOBDCs perform better than other MOFs which also have open-metal sites? CO 2 density profile in Mg\DOBDC at 0.1 bar from simulations. Location of adsorbed CO 2 molecules in Ni\DOBDC from X-ray diffraction data and IR spectroscopy. Dietzel, Johnsen, Fjellvag, Bordiga, Groppo, Chavan, Blom, Chem. Comm

13 Screening MOFs for CO 2 Capture Why do M\DOBDCs perform better than other MOFs which also have open-metal sites? Surface and free volume density of metal atoms in MOFs with open-metal sites MOF Sites / nm 2 Sites / nm 3 Mg\DOBDC Ni\DOBDC Co\DOBDC Zn\DOBDC HKUST UMCM UMCM-150(N) Simulation versus Experiment This diverse set of MOFs is a stringent test of simulation. Ranking from simulation is very close to that from experiment. The top 5 MOFs are correctly identified by the simulations. Experiment GCMC Mg-MOF Ni-MOF Co-MOF Zn-MOF Pd(2-pymo) HKUST UMCM-150(N 2 ) 7 9 UMCM MIL ZIF IRMOF UMCM MOF IRMOF

14 Simulation versus Experiment Simulation vs. experiments at room temperature There is generally good agreement between predicted and measured adsorption, R 2 = One exception is the M\DOBDC samples, particularly at 0.1 bar (open blue circles). If the M\DOBDC data at 0.1 bar are excluded, R 2 = Yazaydin et al., J. Am. Chem. Soc., The simulations perform well, with a level of agreement that is satisfactory for screening purposes. Limitations of the Model Model underpredicts adsorption on the open-metal sites. Since classical model does not include orbital interactions, this is expected.

15 Outline Screening of MOFs for CO 2 Capture with Molecular Modeling Model development Test of model versus experiment Screening results Identify candidate MOFs What do we learn? Post-Synthesis Modification of MOFs for Improved CO 2 Uptake Summary and Outlook Post-Synthesis Modification of MOFs o C Dimethylformamide (DMF) + 5 Zn(NO 3 ) H 2 O Pyridine-CF o C Hypothesis Fluorine groups will increase CO 2 uptake Change in pore size may also play a role in CO 2 selectivity over N 2 1) Soak in CHCl 3 /4-(trifluoromethyl)pyridine 2) Heating at 100C Bae, Farha, Hupp, Snurr, J. Mater. Chem., 2009.

16 Enhancement of CO 2 / N 2 Selectivity CO 2 / N 2 Selectivity Selectivity [-] (with coordinated solvents) 4 (with open metal sites) 5 (with Py-CF 3 ligands) Cavity modification can be used to enhance selectivity. These are among the highest selectivities reported Selectivity Pressure [bar] x x Selectivities predicted from ideal adsorbed solution theory A B / y / y A B Need to increase the capacities. Bae, Farha, Hupp, Snurr, J. Mater. Chem., Effect of Coordinated Water Molecules HKUST-1 (Cu-BTC)* Cubic unit cell 0.5/0.9 nm pores Cu 2 corners Benzene-1,3,5-tricarboxylate linker As synthesized HKUST-1 has one coordinated water molecule per Cu HKUST-1 has been the subject of numerous experimental and modeling studies. *Chui, Lo, Charmant, Orpen, Williams, Science, 1999.

17 Effect of Coordinated Water Molecules CO 2 Simulations CO 2 Experiments 298 K Yazaydin, Benin, Faheem, Jakubczak, Low, Willis, Snurr, Chem. Mater., CO 2 Adsorption in MOFs HKUST-1 ZIF-8 Yazaydin, Snurr, Park, Koh, Liu, LeVan, Benin, Jakubczak, Lanuza, Galloway, Low, Willis, J. Am. Chem. Soc., 2009.

18 Effect of Coordinated Water Molecules Selectivity for CO 2 over N 2 from mixture GCMC simulations Selectivity x x A B / / y y A B Significant increase in selectivity if water molecules are present. Axial ligation of coordinatively unsaturated metal sites by various molecules could open up new possibilities for tuning the adsorption behavior of MOFs for CO 2 capture and other applications. Yazaydin, Benin, Faheem, Jakubczak, Low, Willis, Snurr, Chem. Mater., Summary We have screened a diverse set of 14 metal-organic frameworks for low-pressure CO 2 uptake using a consistent, predictive molecular modeling approach. The model was validated against experiments. Given this validation, the molecular model can aid in selection of MOFs for flue gas separation by screening a large number of materials and providing insight into the mechanism of CO 2 adsorption. Parameters from generalized force fields are usually good enough to predict experimental data. However, the model can be further improved to account for the strong interactions between open-metal sites and CO 2. MOFs with a high density of open-metal sites are good candidates for CO2 capture from flue gas.

19 Outlook Strengths of MOFs Huge variety of potential structures Ready functionalization Heats of adsorption are lower than zeolites Molecular modeling can be used for screening Challenges for MOFs MOFs with high CO 2 uptake tend to adsorb water Stability in flue gas environment Cost uncertainties Acknowledgments Post-docs A. Özgür Yazaydin Krista Walton (Georgia Tech) David Dubbeldam (U. Amsterdam) Screening Collaborators Rich Willis (UOP) John Low (UOP) Annabelle Benin (UOP) M. Doug LeVan (Vanderbilt U.) Stefano Bandani (U. Edinburgh) Adam Matzger (U. Michigan) Other Collaborators Omar Yaghi (UCLA) David Farrusseng (CNRS) Northwestern Collaborators Joseph Hupp SonBinh Nguyen Funding Department of Energy, NETL Department of Energy, Basic Energy Sciences TeraGrid Computing Resources

20 CO 2 Adsorption in MOFs 298 K CO 2 Density, g/cm IRMOF-1 IRMOF-10 IRMOF-16 b, 298K Pressure, bar Walton, Millward, Dubbeldam, Frost, Low, Yaghi, Snurr, J. Am. Chem. Soc., 2008.