Synthesis and Small Molecule Exchange Studies of a Magnesium

Laboratory Experiment pubs.acs.org/jchemeduc

Synthesis and Small Molecule Exchange Studies of a Magnesium Bisformate Metal−Organic Framework: An Experiment in Host− Guest Chemistry for the Undergraduate Laboratory Jeffrey A. Rood*,† and Kenneth W. Henderson‡ †

Department of Chemistry and Biochemistry, Elizabethtown College, Elizabethtown, Pennsylvania 17022, United States Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States

‡

S Supporting Information *

ABSTRACT: Experiments are described that introduce students to the important concepts of host−guest chemistry and size exclusion in porous metal−organic frameworks (MOFs). The experiment has been successfully carried out in both introductory and advanced-level inorganic chemistry laboratories. Students synthesized the porous MOF, αMg3(O2CH)6, and examined its ability to take up a variety of small organic molecules into its pores using 1H NMR spectroscopy. The MOF exhibited size exclusion and students rationalized this behavior based on the structures of the organic molecules and the pore size of the MOF.

KEYWORDS: Separation Science, Solid State Chemistry, Materials Science, Second-Year Undergraduate, Upper-Division Undergraduate, Inorganic Chemistry

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experiment focuses on the host−guest chemistry of porous materials. Host−guest studies have been the topic of a variety of contributions to this Journal, yet none have focused specifically on such properties in MOFs.5 As many institutions do not have direct access to some of the instrumentation common to MOF characterization, such as X-ray diffraction and gas sorption equipment, this system uniquely provides a straightforward investigation of small molecule uptake and size selectivity of the MOF using NMR spectroscopy. Guest uptake by the MOF can be determined by dissolving the material in D2O and obtaining a 1H NMR spectrum. Students that engaged in this experiment gained experience in high-temperature synthesis and crystallization and learned about the properties of extended materials. This experiment was carried out in an introductory-level inorganic chemistry course at Elizabethtown College over the past three years and was completed by 82 students. Additionally, the experiment was part of the laboratory curriculum for an advanced-level laboratory course at the University of Notre Dame for five years and was completed by 80 students. At Notre Dame, this experiment was coupled with other solid-state experiments dealing with zeolites6 and superconductors7 that have been reported in this Journal, and elsewhere, as a unit on solid-state chemistry. An introduction to MOF chemistry not only provided students with exposure to a new area of inorganic

esearch into the design and synthesis of extended materials with specific properties has received extensive interest in recent times. Of the many classes of extended materials, metal−organic frameworks (MOFs) have received tremendous attention. MOFs are hybrid materials, composed of metal ions and organic molecules that are linked into an extended array by what is often referred to as a node-and-linker approach. The most successful synthesis of these materials uses metal−carboxylate systems.1 In many cases, metal carboxylates form frameworks that enclatherate solvent molecules. In some instances, these materials are robust to solvent removal, generating a porous framework. The porous nature of many MOFs makes them attractive for numerous applications involving the uptake and exchange of small molecules.2 MOFs are ideal for such purposes as the size and shape of the pores may be controlled through judicious choice of metal ions and organic linker molecules. Despite the high level of interest in MOF materials in the academic setting, undergraduate students rarely receive exposure to such cutting-edge applications of inorganic chemistry. Recently, Sumida and Arnold published a MOF laboratory designed for the undergraduate inorganic chemistry laboratory that gives students excellent examples of synthesis and characterization of such materials.3 The laboratory experiment described herein provides an 1H NMR-based study of the uptake properties of the MOF α-magnesium formate, α-Mg3(O2CH)6.4 While students will gain experience in the synthesis of MOF materials, the novelty of this © XXXX American Chemical Society and Division of Chemical Education, Inc.

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dx.doi.org/10.1021/ed3000099 | J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Scheme 1. Crystal Structures of α-Mg3(O2CH)6⊃DMF (left) and α-Mg3(O2CH)6 (right)a

a

The coordination environment around each magnesium center is shown as a yellow octahedron to help illustrate the pores within the material. Hydrogen atoms have been omitted for clarity.

usually evident due to the presence of dimethylamine from the decomposition of DMF. Third, after isolating α-Mg3(O2CH)6⊃DMF, samples were heated under reduced pressure at 130 °C using a vacuum pump or vacuum oven. After 36 h, the guest-free, porous phase αMg3(O2CH)6 was isolated. The crystal structures of αMg3(O2CH)6⊃DMF and α-Mg3(O2CH)6 are shown in Scheme 1. Confirmation of the guest-free phase was evident in the 1H NMR spectrum in D2O, where only the formate proton was observed. Fourth, students were tasked with investigating the solventuptake properties of the porous phase and examining the material for any selective uptake. This study focused on the inclusion of THF, acetone, benzene, and cyclohexane; however, many other compounds are suitable for investigation. Uptake studies were carried out by soaking α-Mg3(O2CH)6 in each of the pure solvents listed previously. Selectivity for guest molecules was studied by soaking the MOF in a stoichiometric mixture of the solvents under study. In all cases, the soaking experiments were carried out between laboratory periods. During the final laboratory session, students isolated the solids by vacuum filtration and prepared 1H NMR samples of each trial in D2O. Spectra were obtained for each uptake study, which allowed students to determine whether any new guest molecules where enclatherated into the MOF. Over the course of the five years this experiment has been run, the uptake studies were posed to students in one of two ways. Originally, students were told directly how to investigate the uptake properties of the MOF, as detailed in the Supporting Information. Alternatively, and likely more challenging, students were asked to develop 1H NMR experiments to investigate small-molecule uptake of α-Mg3(O2CH)6 by only being told that the MOF is soluble in D2O. This approach forced students to think more critically about experimental design and steered them away from the “cookbook” approach to completing laboratory experiments.

chemistry, but also allowed for broader discussion of important topics related to MOFs, such as H2 storage8 and CO2 sequestration.9

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EXPERIMENTAL OVERVIEW This experiment utilized the smallest carboxylic acid, formic acid, in the construction of the open framework αMg3(O2CH)6 from a high-temperature reaction in dimethylformamide (DMF). The decomposition of dialkylformamides to compounds such as carbon monoxide, formic acid, and dialkylamines helps drive the reaction toward the formation of the MOF, allowing for a discussion of the reaction chemistry with students.10 The crystallization of MOFs at elevated temperatures was a new concept to most students, as typical experiences in crystallization usually involve the cooling of saturated solutions. Completion of this laboratory exercise involved four steps and was carried out over multiple class periods. Students first synthesized α-Mg3(O2CH)6⊃DMF (the phase containing DMF molecules in the pores) by the reaction of Mg(NO3)2·6H2O with 2 equiv of formic acid in DMF. The reaction was carried out on a hot plate using a Teflon-capped scintillation vial immersed in silicon oil bath at 110 °C. Alternatively, an oven set to the desired temperature may be used. Suitable quantities of crystalline product deposited on the walls of the vial after 40 h. The material was stable in the mother liquor until students were able to continue their studies. Students isolated the crystalline product by vacuum filtration. Typical yields ranged from 50 to 60% based on magnesium. Second, 1H NMR was carried out on this sample in D2O. It is important to note that although dissolving the material in D2O destroys the integrity of the MOF, this method provides a simple way for examining guest molecules that reside in the pores. Because only a small quantity of solid is needed for the NMR study, the majority of the solid material remained for further studies regarding host−guest chemistry. From the 1H spectrum of α-Mg3(O2CH)6⊃DMF, students were able to identify the formate proton resonance along with the methyl and formyl protons from DMF. An additional peak at δ2.4 was

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HAZARDS In all manipulations, students should wear standard personal protective equipment, including safety goggles and gloves. The B




available as Supporting Information in the original report of αMg3(O2CH)6 in Inorganic Chemistry.4 Once students obtained the .cif files, crude measurements of pore sizes were made using software such as CrystalMaker or Mercury.

flammability of dimethylformamide, tetrahydrofuran, acetone, benzene, and cyclohexane should be discussed with students and these solvents should be handled in a fume hood. Magnesium nitrate is an oxidizing agent and can cause eye and skin irritation. Benzene is a carcinogen. When handling benzene, all manipulations should be carried out in a fume hood. This lab should only require ∼1 mL of benzene per student. Formic acid can cause irritation or burns to the skin, mucous membranes, and respiratory tract and should be handled with care. Additionally, students should use caution around hot silicon oil baths if used as the source of heat for the experiment. In all cases, students should be encouraged to consult MSDS sheets for all chemicals prior to beginning the laboratory exercise.

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SUMMARY This experiment provided a unique investigation of the host− guest properties of MOFs and allowed such studies to be carried out in a straightforward manner using 1H NMR spectroscopy. The goals of this experiment included providing students with experience in solid-state synthesis and, most importantly, illustrating the important host−guest chemistry that such materials exhibit. The latter served as a platform for discussion of broader impacts of MOF materials in areas such as alternative energy. Upon completion of the laboratory, students wrote a formal report in which they carried out a literature review of MOF chemistry in addition to reporting their experimental findings and interpretations of the results they observed. At both the introductory and upper-level course, students found further examples of MOF applications in the literature and were able to discuss their findings in context with the experiment described here. Students found this experiment to be interesting and enjoyable and many ranked it as their favorite laboratory experiment in an informal end-of-semester survey. Students also came to better appreciate the broader applications of inorganic chemistry when MOFs and their potential industrial and environmental uses were discussed. The goals laid forth for this laboratory experiment were achieved in that students were introduced to solid-state inorganic chemistry and provided with examples of the numerous applications for which such materials hold promise.

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RESULTS Representative student 1H NMR spectra for this study are shown in Figure 1. From the spectra, students were able to

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ASSOCIATED CONTENT

S Supporting Information *

A student handout containing detailed experimental instructions for the synthesis of α-Mg3(O2CH)6 and the uptake studies; details for instructors. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Figure 1. Representative student-acquired 1H NMR spectra of αMg3(O2CH)6⊃DMF, α-Mg3(O2CH)6, and each uptake experiment described in this study. All spectra were obtained in D2O.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank students of Chemistry 443 at the University of Notre Dame and Chemistry 242 at Elizabethtown College for suggestions and helpful comments regarding the design of this experiment. J.A.R. would like to thank the National Science Foundation (grant CHE-0958425) for instrument support.

determine that THF, acetone, and benzene were taken up by αMg3(O2CH)6, whereas cyclohexane was excluded, presumably due to size. Furthermore, the spectrum resulting from soaking the MOF in a stoichiometric mixture of the four solvents showed preference for the uptake of acetone and THF over benzene by comparing the integral ratios of these compounds. Students were asked to consider reasons for the size selectivity that is observed by thinking about both the guest molecule size and the size of the pores in α-Mg3(O2CH)6. Students at Notre Dame utilized Chem3D to compare guest molecule size. This program was not available to all students at Elizabethtown and more qualitative comparisons of Lewis structures proved sufficient. Students investigated the crystal structures of αMg3(O2CH)6 and its various inclusion compounds by downloading the crystallographic information files (.cif) from the Cambridge Structural Database. Alternatively, the .cif files are

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Synthesis and Small Molecule Exchange Studies of a Magnesium

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