In the Laboratory
Preparation, Characterization, and Postsynthetic Modification of Metal-Organic Frameworks: Synthetic Experiments for an Undergraduate Laboratory Course in Inorganic Chemistry Kenji Sumida and John Arnold* Department of Chemistry, University of California, Berkeley, California 94720, United States *
[email protected] The design and synthesis of materials that exhibit desirable chemical and physical properties for specific applications is a goal of high importance in materials chemistry. Metal-organic frameworks (MOFs) are crystalline materials that are composed of an infinite array of metal nodes (single ions or clusters) linked to one another by polyfunctional organic compounds (Figure 1) (1). One promising feature of MOFs is the ability to judiciously select both the metal and organic components (modular synthesis), which affords a high level of control over the properties of the resultant materials. Many MOFs that display surface areas significantly higher than zeolites and porous carbons have been prepared (2), and this has spurred intense investigation of these materials as sorbents for industrial applications, such as hydrogen storage in automobile applications, CO2 capture, gas separation, and catalysis (3). Despite the excitement toward these materials in the academic and industrial communities, we are unaware of any suitable experiments for undergraduate chemistry students that provide a basic exposure to the field. We introduce synthetic techniques vital to the preparation of these materials and also give the student a taste of the characterization that is required to elucidate the composition and performance of these solids. Typical solution-based characterization techniques that are commonplace in synthetic laboratories, such as solution UVvisible and NMR spectroscopy, are largely not applicable to MOFs because the extended crystalline network renders them insoluble in all solvents. Solubilizing these materials would lead to the loss of the network connectivity that would also destroy the physical properties we are interested in. Therefore, we introduce a number of characterization methods that are often not employed in traditional inorganic chemistry laboratory courses, including powder X-ray diffraction and thermogravimetric analysis (TGA). We also introduce students to the concepts of gas adsorption and surface area, including the Brunauer-Emmett-Teller (BET) model for experimentally determining the surface area of the material. In addition, through their reading of the cited literature, students gain an increased understanding of current global and industrial challenges, such as environmental issues related to CO2 emissions and more detailed knowledge of the requirements for materials used in gas storage and purification. Experimental Overview MOF chemistry exhibits excellent versatility in the design of the materials resulting from the almost infinite combinations of metals and ligands that can be proposed. This allows key properties of the material to be tuned according to the properties 92
Journal of Chemical Education
_
_
Figure 1. Formation of a crystalline and porous metal-organic framework (MOF) from solvothermal combination of a metal ion or cluster, and an organic linker.
desired for the application, for example, the pore size and shape can be controlled by selecting organic ligands of varying lengths and geometries. The network connectivity can be changed by adopting metal ions that have a preference for certain coordination geometries, for example, preferentially octahedral ions such as Fe(II) or square planar ions such as Pd(II). The synthesis of MOFs generally involves the solvothermal reaction of the components in a high-boiling polar solvent such as N,N-dimethylformamide (DMF), sometimes with the addition of a cosolvent such as an alcohol. The reaction mixtures are usually heated either on a hotplate or in an oven under static (vibration-free) conditions, inducing self-assembly of the metal and organic components to yield the extended framework, which is observed as single-crystals or microcrystalline powders after a period of 1-3 days. The formation of high-quality crystals can depend heavily on the reaction parameters, such as the reaction temperature and duration, concentration, and stoichiometry of the reactants, solvent composition, and solution pH. In the case of a porous structure, the voids within the crystal are initially filled with solvent molecules that must be removed to activate the material for accommodation of guest molecules. This is usually achieved by submerging the crystals in a volatile noncoordinating solvent such as methylene chloride over 1 or 2 days, followed by evacuation of the crystals under a dynamic vacuum to remove the methylene chloride from the pores. The evacuated materials are usually air- and moisture-sensitive and storage within a nitrogenfilled glovebag or glovebox is essential if the compounds are to be kept over a long period of time. The present experiment consists of three parts. The first part involves the synthesis and characterization of three MOF materials: Zn4O(1,4-benzenedicarboxylate)3 (MOF-5, 1) (2a); Zn4O(2-amino-1,4-benzenedicarboxylate)3 (IRMOF-3, 2) (4);
_
Vol. 88 No. 1 January 2011 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed100001s Published on Web 10/22/2010
In the Laboratory
Scheme 1. Postsynthetic Modification of the Amine Groups in 2 Using Acetic Anhydride
Figure 2. The ligands used to synthesize 1-3: (left) 1,4-benzenedicarboxylic acid (H2BDC); (center) 2-amino-1,4-benzenedicarboxylic acid (H2ABDC); and (right) 1,3,5-benzenetricarboxylic acid (H3BTC).
and Cu3(1,3,5-benzenetricarboxylate)2 (HKUST-1, 3) (Figure 2) (5). Compounds 1 and 2 show a network topology wherein oxo-centered tetranuclear [Zn4O]6þ tetrahedra are coordinated to six ligand carboxylate groups. Because the ligands have two carboxylate groups, the metal clusters are bridged such that the inorganic and organic components form a cubic extended network. In 3, the metal building-unit is a dinuclear copper paddlewheel unit, for which molecular analogues are well known. These frameworks have been selected because of the high level of interest that these materials have received for gas storage and separation applications. Under certain preparative conditions, compound 1 exhibits a BET surface area of up to 3800 m2/g (almost the area of a football field!) (2a). As a result, it is one of the best cryogenic hydrogen-storage MOF materials, storing 10 wt % and 66 g/L of H2 at 100 bar. Furthermore, although 2 exhibits an identical network structure, the amine substituents on the organic linkers project into the pores of the framework, allowing chemical tuning of the pore surface. Compound 3 has also been the focus of many studies owning to the presence of open coordination sites on the Cu(II) centers of the paddlewheel units following removal of the axial solvent molecules. This allows the framework to have strong interactions with gas molecules, such as hydrogen and carbon dioxide (6). Finally, from a practical viewpoint, this experiment is attractive because these compounds can be easily prepared from inexpensive materials that are commercially available. Following the preparation of the three materials, the surface area of an activated sample of 1 is determined in the second part of the experiment using a basic sorption apparatus to collect a nitrogen isotherm. In the third part of the experiment, postsynthetic modification of the ligand groups in 2 is performed (7), showing that organic reactions, which proceed in molecular systems, can also be performed on the surface of the MOF pores. Hazards Students should be made aware of the oxidizing ability of metal nitrates, and the hazards associated with each of the solvents used. Special care should be taken when handling acetic anhydride as it is corrosive, causes burns to any area of contact, is a flammable liquid, and is water reactive. The benzenedicarboxylic acid linking compounds may cause irritation to skin, eyes, and respiratory tract and may be harmful if swallowed or inhaled. Preparation of the reaction mixtures in each of the syntheses described can be safely performed in a fume hood with standard laboratory protective equipment, such as a laboratory coat, safety glasses, and nitrile gloves. Summary of Procedure The experiment is suitable for an upper-level undergraduate course in synthetic inorganic chemistry, particularly in courses where there is some flexibility in the experiment schedule. The reagents are relatively inexpensive and can be easily obtained
r 2010 American Chemical Society and Division of Chemical Education, Inc.
_
from commercial vendors, and the preparation of the materials can be performed within standard variable temperature ovens. Prior to beginning the experiment, students set up a nitrogen-filled glovebag as a means for storing their products and dry solvents. The glovebag should be stocked with 200 mL of anhydrous DMF and 200 mL of methylene chloride prior to the syntheses. Approximately 5 mL of acetic anhydride and 100 mL of anhydrous chloroform should also be prepared in the glovebag prior to the postsynthetic modification of 2. The syntheses of 1-3 can be easily completed within one laboratory session, although the washing and activation requires exchange of the solution approximately once per day for several days. Once synthesized, three laboratory sessions should be devoted to the characterization of the frameworks, surface area measurement of 1, and postsynthetic modification of 2. The preparation of 1 and 2 is carried out in the air using a tightly capped 100 mL vial in an oven (80-100 °C) for a period of 10-18 h, after which time high-quality cubic crystals are observed on the walls of the jar. Previous studies on 1 have shown that the method of preparation has a significant impact on the quality of the sample (2a), thus, N,N-diethylformamide (DEF) is preferred as reaction solvent. However, depending on the availability of DEF, DMF can be used as a substitute to obtain an equivalent phase but of a lower quality. Similarly, 2 can also be conveniently synthesized in DMF. Compound 3 is prepared by refluxing the starting materials in ethanol, and a blue crystalline product is obtained upon cooling of the solution to room temperature. Prior to activation, powder X-ray diffraction patterns, IR spectra, and TGA should be performed on 1-3 to ensure the desired products have been obtained. Following these characterization steps, the samples should be transferred to the glovebag for washing with DMF, followed by washing with methylene chloride. The characterization methods should ideally be run again at this point to ensure no change to the basic framework structure upon exchange of the solvent. The surface area measurement of 1 can be performed by taking a small (ca. 100-200 mg) sample of the evacuated material and collecting a nitrogen adsorption isotherm. The sample tube should be loaded in anhydrous conditions owing to the rapid decomposition that occurs for activated samples of 1 in the presence of air. The BET surface area (8), which is commonly reported as a benchmark for comparing the porosity of two materials, is calculated according to methods that have been recently reported for MOFs (9). An Excel worksheet that details this procedure can be found in the supporting information. Similar experiments can be performed on 2 and 3 if desired by activating and evacuating the crystals of these frameworks using a procedure similar to that of 1. The postsynthetic modification of 2 results in the conversion of the amine groups residing on the aryl groups of the organic linkers to an amide by reaction with acetic anhydride (Scheme 1). Although many anhydride reagents can be used,
pubs.acs.org/jchemeduc
_
Vol. 88 No. 1 January 2011
_
Journal of Chemical Education
93
In the Laboratory
acetic anhydride was readily available in our undergraduate laboratory. In this procedure, a dilute solution of acetic anhydride in chloroform is charged to a vial containing 2, and this solution is replenished once per day for several days. The progress of the reaction can be followed by IR spectroscopy, taking care to thoroughly wash the solid with chloroform prior to collection of the spectrum. At the end of the post-synthetic modification experiment, the proportion of amine substituents converted to the amide can be precisely determined by dissolving the solid within 1 M DCl in D2O (a process known as acid digestion), and collecting an NMR spectrum. Overall, the experiment is expected to have a high pedagogical value owing to the general skills in solid-state characterization that the students acquire. The students routinely use powder X-ray diffraction as a method to probe the level of crystallinity and purity of their samples, TGA to elucidate the thermal stability of the compounds, and IR and NMR spectroscopy to monitor the solvent exchange procedures and the extent of postsynthetic modification in 2. Students are encouraged to download the crystallographic information files (CIF) of 1-3 from the Cambridge Structural Database for viewing in the Mercury Viewer software to increase their understanding of the structures. Questions were included in the student handout to promote the level of overall comprehension and to encourage student-based ideas on directions the experiments could be extended if they desired to study these and other related materials in further detail. Acknowledgment
Literature Cited 1. (a) Ferey, G. Chem. Soc. Rev. 2008, 37, 191. (b) Morris, R. E.; Wheatley, P. S. Angew Chem., Int. Ed. 2008, 47, 4966.
Journal of Chemical Education
3.
4.
5. 6.
7. 8.
We thank the students of Chemistry 108 for their efforts in helping to develop this experiment, in particular Marcus Gibson who initiated this study. We are grateful to the Camille and Henry Dreyfus Special Grant Program in the Chemical Sciences for financial support of this project.
94
2.
_
Vol. 88 No. 1 January 2011
_
9.
(c) Kitagawa, S.; Kitaura, R.; Noro, S.-I. Angew. Chem., Int. Ed. 2004, 43, 2334. (a) Kaye, S. S.; Dailly, A.; Yaghi, O. M.; Long, J. R. J. Am. Chem. Soc. 2007, 129, 14176. (b) Furukawa, H.; Miller, M. A.; Yaghi, O. M. J. Mater. Chem. 2007, 17, 3197. (c) Llewellyn, P. L.; Bourrelly, S.; Serre, C.; Vimont, A.; Daturi, M.; Hamon, L.; De Weireld, G.; Chang, J.-S.; Hong, D.-Y.; Hwang, Y. K.; Jhung, S. H.; Ferey, G. Langmuir 2008, 24, 7245. (d) Koh, K.; Wong-Foy, A. G.; Matzger, A. J. J. Am. Chem. Soc. 2009, 131, 4184. MOFs were recently the theme of a special issue in Chem. Soc. Rev. (a) Czaja, A. U.; Trukhan, N.; Muller, U. Chem. Soc. Rev. 2009, 38, 1284. (b) Murray, L. J.; Dinca, M.; Long, J. R. Chem. Soc. Rev. 2009, 38, 1294. (c) Zacher, D.; Shekhah, O.; Woll, C.; Fischer, R. A. Chem. Soc. Rev. 2009, 38, 1418. (d) Lee, J.-Y.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.; Hupp, J. T. Chem. Soc. Rev. 2009, 38, 1450. (e) Li, J.-R.; Kuppler, R. J.; Zhou, H.-C. Chem. Soc. Rev. 2009, 38, 1477. (a) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O'Keeffe, M.; Yaghi, O. M. Science 2002, 295, 469. (b) Rowsell, J. L. C.; Yaghi, O. M. J. Am. Chem. Soc. 2006, 128, 1304. Poppl, A.; Kunz, S.; Himsl, D.; Hartmann, M. J. Phys. Chem. C 2008, 112, 2678. Bordiga, S.; Regli, L.; Bonino, F.; Groppo, E.; Lamberti, C.; Xiao, B.; Wheatley, P. S.; Morris, R. E.; Zecchina, A. Phys. Chem. Chem. Phys. 2007, 9, 2676. Tanabe, K. K.; Wang, Z.; Cohen, S. M. J. Am. Chem. Soc. 2008, 130, 8508. (a) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309.(b) Do, D. D. Adsorption Analysis: Equilibria and Kinetics; Imperial College Press: London, 1998. Walton, K. S.; Snurr, R. Q. J. Am. Chem. Soc. 2007, 129, 8552.
Supporting Information Available A student handout with detailed experimental instructions; an instructor reference containing sample data from powder X-ray measurements and gas sorption measurements, and additional comments and references. This material is available via the Internet at http://pubs. acs.org.
pubs.acs.org/jchemeduc
_
r 2010 American Chemical Society and Division of Chemical Education, Inc.
