Postassembly Transformation of a Catalytically Active Composite

Communication pubs.acs.org/IC

Postassembly Transformation of a Catalytically Active Composite Material, Pt@ZIF-8, via Solvent-Assisted Linker Exchange Casey J. Stephenson,† Joseph T. Hupp,*,† and Omar K. Farha*,†,‡ †

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia

‡

S Supporting Information *

imidazolate-based ligand.5 Nanoparticle@ZIF-8 composites have been shown to be effective for both size-selective and regioselective catalysis.4a,c,e As a host framework for a nanoparticle composite, ZIF-8 has the advantages of being free of long-range defects and synthesizable under a range of conditions.6 However, because of the small size of the ZIF-8’s apertures (the largest presents a nominal diameter of 3.4 Å, expandable to ca. Å), only linear organic substrates pass through its apertures and enter its pores. Opening up the narrow apertures of a nanoparticle@ZIF-8 composite would allow larger substrates, potentially with greater complexity, to enter the MOF and react with encapsulated nanoparticles. Solvent-assisted linker exchange (SALE) is a versatile technique for the synthesis and modification of MOF materials.7 Many MOFs that are not accessible de novo have been synthesized indirectly via SALE. SALE is a single-crystal-tosingle-crystal transformation in which the linkers of a parent framework are exchanged with a desired linker.7 To achieve high degrees of exchange, solvothermal conditions and an excess of replacement linkers are typically used.7c In this work, we describe the use of SALE to transform a metal nanoparticle@MOF composite, Pt@ZIF-8, into a new composite material, Pt@ SALEM-2, by substituting 2-methylimidazolate for imidazolate (Scheme 1). To our knowledge, this work constitutes the first report of SALE for a nanoparticle@MOF composite. We anticipate, however, that this approach will prove attractive and useful for synthesizing a broad range of catalytic nanoparticle@ MOF composites for which the aperture size, or composition of

ABSTRACT: 2-Methylimidazolate linkers of Pt@ZIF-8 are exchanged with imidazolate using solvent-assisted linker exchange (SALE) to expand the apertures of the parent material and create Pt@SALEM-2. Characterization of the material before and after SALE was performed. Both materials are active as catalysts for the hydrogenation of 1octene, whereas the hydrogenation of cis-cyclohexene occurred only with Pt@SALEM-2, consistent with larger apertures for the daughter material. The largest substrate, β-pinene, proved to be unreactive with H2 when either material was employed as a candidate catalyst, supporting the contention that substrate molecules, for both composites, must traverse the metal−organic framework component in order to reach the catalytic nanoparticles.

T

he encapsulation of catalytically active nanoparticles within porous materials such as metal−organic frameworks, or MOFs, is of great interest.1 MOFs are crystalline, modular materials composed of inorganic nodes and organic linkers.2 Nanoparticles can be encapsulated within MOFs by a top-down approach, where the MOF is formed first and molecular precursors of the metal nanoparticle precursors are allowed to permeate the MOF and then are reduced.3 Top-down methods typically lack control over the nanoparticle size and distribution, often yielding particles that are larger than the cavities of the host framework. Nanoparticles can also form on the exterior of the MOFan undesirable outcome if the composites are targeted at reactions for which size-selective or regioselective catalysis is required.3b However, procedures do exist for the top-down synthesis of composites where the nanoparticles match the pore diameter of the host framework and remain fully encapsulated.3b−e Alternatives are bottom-up approaches that involve first formation of the nanoparticles and then growth of the MOF around them.4 These methods are capable of yielding, exclusively, fully MOF-enshrouded nanoparticles with no detectable direct exposure of particles to external atmospheres or solutions. Thus, contact between the reactants and a catalytic nanoparticle or between catalytically formed products and an external environment is achievable only via molecular navigation of the channels, cavities, and apertures that define the MOF’s intrinsic porosity. Our group reported a bottom-up procedure for the encapsulation of nanoparticles within zeolitic imidazolate framework 8 or ZIF-8.4a ZIFs are a subset of MOFs with the general formula M(im)2, where M is a metal and im is an © XXXX American Chemical Society

Scheme 1. Representation of Pt@ZIF-8 Undergoing Exchange of 2-Methylimidazolate with Imidazolate To Form Pt@SALEM-2

Received: December 15, 2015

A

DOI: 10.1021/acs.inorgchem.5b02880 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 1. TEM images of (a) a single crystallite and (b) multiple crystallites of Pt@ZIF-8. TEM images of (c) a single crystallite and (d) multiple crystallites of Pt@SALEM-2.

the local chemical environment, is a variable that, if modulated, can impart desirable substrate and/or product selectivity. The Pt@ZIF-8 composite with 3.0 nm platinum nanoparticles was synthesized as previously reported (see the Supporting Information for details).4d Pt@SALEM-2 was obtained by adapting the method reported by Karagiardi et al.7b for ZIF-8 alone. Briefly, 100 mg of Pt@ZIF-8 and 200 mg of imidazole were suspended in 20 mL of n-butanol and heated at 100 °C for 4 days. The solid was isolated by centrifugation and washed several times with methanol. Between washings, the sample was soaked for several hours in methanol. Finally, the dark-gray solid was isolated via centrifugation and dried under vacuum overnight at 120 °C. The sample was characterized by powder X-ray diffraction (PXRD), 1H NMR spectroscopy, inductively coupled plasma atomic-emission spectroscopy (ICP-AES), and scanning transmission electron microscopy (STEM). The PXRD pattern (Figure S1) is in agreement with the pattern reported by our group for SALEM-2 alone. N2 adsorption measurements indicate that Pt@SALEM-2 retained its porosity as well. 1H NMR spectroscopy of digested samples indicates that approximately 90% of the original linker was exchanged, while ICP-AES performed on the same sample both before and after SALE indicates that no leaching of platinum occurred over the course of the reaction. STEM analysis performed on the same sample before and after SALE indicates that the platinum nanoparticles remain essentially unchanged (3.0 ± 0.7 nm for Pt@ZIF-8 vs 2.8 ± 0.4 nm for Pt@SALEM-2; Figure 1). As a proof-of-concept test of aperture expansion, we studied the behavior of Pt@ZIF-8 versus Pt@SALEM-2 as potential catalysts for the hydrogenation of a series of substrates of progressively larger kinetic diameter: 1-octene, cyclohexene, and β-pinene. Catalytic trials were conducted at room temperature using 2000 equiv of substrate under a constant 1 bar of H2 using 0.200 mL of undecane as an internal standard in 3.6 mL of ethyl acetate as the solvent. To determine whether the composite materials are inherently active for hydrogenation, we used a linear substrate with a terminal CC unsaturation, 1-octene, whose hydrogenation is known to be catalyzed by [email protected],d Using Pt@ZIF-8 as the catalyst, n-octane was produced in 20% conversion (Table 1, entry 1). With Pt@SALEM-2 as the catalyst, 1-octene is hydrogenated to n-octene in 30% conversion (Table 1, entry 2), indicating that the platinum nanoparticles of Pt@SALEM-2 are active for catalytic hydrogenation. Given that the platinum nanoparticle size and distribution are essentially identical in the two composites, differences in conversion might reflect faster transport of the substrate through the ca. 6 Å aperture of [email protected] Next, we examined cis-cyclohexene. Consistent with previous work and with modeling of substrate versus aperture size, cis-

Table 1. Catalytic Results for the Hydrogenation of Substrates Using Pt@ZIF-8 and Pt@SALEM-2

a

Performed in duplicate using 2000 equiv of substrate in 3.6 mL of ethyl acetate with 0.200 mL of undecane as an internal standard. Prior to each reaction, the catalyst was heated under vacuum at 150 °C for 2 h followed by reduction at 150 °C under H2 for an additional 2 h. b Conversion determined by time-of-flight gas chromatography.

cyclohexene proved unreactive with hydrogen in the presence of Pt@ZIF-8 as a candidate catalyst (Table 1, entry 3). The absence of reactivity also serves as confirmation that the platinum particles are fully MOF-enshrouded. In contrast, with Pt@ SALEM-2 as the candidate catalyst (Table 1, entry 4), ciscyclohexene is hydrogenated in 7% yield. (The extent of conversion is a function of the reaction time; we intentionally chose 24 h so that the extent of conversion for all reactions would be far from complete, i.e., conditions where dif ferences in the reactivity should be readily observable.) The observed activity supports the contention that SALE indeed does expand the apertures of the composite material and open continuous pathways from the external solution to catalytic particles, where the pathways are navigable by substrates that are too large to permeate the parent material. Finally, β-pinene, a substrate with a larger kinetic diameter than cis-cyclohexene, was found to resist hydrogenation in the presence of either potential catalyst (Table 1, entries 5 and 6). This finding is consistent with a molecular gate-keeping role for the apertures of both composites. Control experiments were run with a candidate catalyst consisting of platinum nanoparticles on the exterior surface of ZIF-8. Here β-pinene hydrogenation was obtained with 15% conversion (Table 1, entry 7), thus confirming that the particles, when accessible, are catalytically competent for hydrogenation of the large substrate. B

DOI: 10.1021/acs.inorgchem.5b02880 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Morabito, J. V.; Tsung, C.-K. ACS Catal. 2014, 4, 4409−4419. (e) Rosler, C.; Fischer, R. A. CrystEngComm 2015, 17, 199−217. (2) (a) Ferey, G. Chem. Soc. Rev. 2008, 37, 191−214. (b) Horike, S.; Shimomura, S.; Kitagawa, S. Nat. Chem. 2009, 1, 695−704. (c) Zhou, H.-C.; Long, J. R.; Yaghi, O. M. Chem. Rev. 2012, 112, 673−674. (d) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. Science 2013, 34110.1126/science.1230444. (e) Farha, O. K.; Hupp, J. T. Acc. Chem. Res. 2010, 43, 1166−1175. (f) Allendorf, M. D.; Stavila, V. CrystEngComm 2015, 17, 229−246. (3) (a) Hwang, Y. K.; Hong, D.-Y.; Chang, J.-S.; Jhung, S. H.; Seo, Y.K.; Kim, J.; Vimont, A.; Daturi, M.; Serre, C.; Férey, G. Angew. Chem., Int. Ed. 2008, 47, 4144−4148. (b) Proch, S.; Herrmannsdörfer, J.; Kempe, R.; Kern, C.; Jess, A.; Seyfarth, L.; Senker, J. Chem. - Eur. J. 2008, 14, 8204−8212. (c) Esken, D.; Zhang, X.; Lebedev, O. I.; Schroder, F.; Fischer, R. A. J. Mater. Chem. 2009, 19, 1314−1319. (d) Jiang, H.-L.; Liu, B.; Akita, T.; Haruta, M.; Sakurai, H.; Xu, Q. J. Am. Chem. Soc. 2009, 131, 11302−11303. (e) Esken, D.; Turner, S.; Lebedev, O. I.; Van Tendeloo, G.; Fischer, R. A. Chem. Mater. 2010, 22, 6393−6401. (f) Guo, Z.; Xiao, C.; Maligal-Ganesh, R. V.; Zhou, L.; Goh, T. W.; Li, X.; Tesfagaber, D.; Thiel, A.; Huang, W. ACS Catal. 2014, 4, 1340−1348 For additional references, see the Supporting Information.. (4) (a) Lu, G.; Li, S.; Guo, Z.; Farha, O. K.; Hauser, B. G.; Qi, X.; Wang, Y.; Wang, X.; Han, S.; Liu, X.; DuChene, J. S.; Zhang, H.; Zhang, Q.; Chen, X.; Ma, J.; Loo, S. C. J.; Wei, W. D.; Yang, Y.; Hupp, J. T.; Huo, F. Nat. Chem. 2012, 4, 310−316. (b) Kuo, C.-H.; Tang, Y.; Chou, L.-Y.; Sneed, B. T.; Brodsky, C. N.; Zhao, Z.; Tsung, C.-K. J. Am. Chem. Soc. 2012, 134, 14345−14348. (c) Zhang, W.; Lu, G.; Cui, C.; Liu, Y.; Li, S.; Yan, W.; Xing, C.; Chi, Y. R.; Yang, Y.; Huo, F. Adv. Mater. 2014, 26, 4056−4060. (d) Stephenson, C. J.; Hupp, J. T.; Farha, O. K. Inorg. Chem. Front. 2015, 2, 448−452. (e) Choi, K. M.; Na, K.; Somorjai, G. A.; Yaghi, O. M. J. Am. Chem. Soc. 2015, 137, 7810−7816 For additional references, see the Supporting Information.. (5) (a) Banerjee, R.; Phan, A.; Wang, B.; Knobler, C.; Furukawa, H.; O’Keeffe, M.; Yaghi, O. M. Science 2008, 319, 939−943. (b) Banerjee, R.; Furukawa, H.; Britt, D.; Knobler, C.; O’Keeffe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2009, 131, 3875−3877. (6) (a) Chizallet, C.; Lazare, S.; Bazer-Bachi, D.; Bonnier, F.; Lecocq, V.; Soyer, E.; Quoineaud, A.-A.; Bats, N. J. Am. Chem. Soc. 2010, 132, 12365−12377. (b) Pan, Y.; Wang, B.; Lai, Z. J. Membr. Sci. 2012, 421− 422, 292−298. (c) Tao, K.; Kong, C.; Chen, L. Chem. Eng. J. 2013, 220, 1−5. (d) Liu, X.; Li, Y.; Ban, Y.; Peng, Y.; Jin, H.; Yang, W.; Li, K. Mater. Lett. 2014, 136, 341−344. (e) Shamsaei, E.; Low, Z.-X.; Lin, X.; Mayahi, A.; Liu, H.; Zhang, X.; Zhe Liu, J.; Wang, H. Chem. Commun. 2015, 51, 11474−11477. (7) (a) Cohen, S. M. Chem. Rev. 2012, 112, 970−1000. (b) Karagiaridi, O.; Lalonde, M. B.; Bury, W.; Sarjeant, A. A.; Farha, O. K.; Hupp, J. T. J. Am. Chem. Soc. 2012, 134, 18790−18796. (c) Deria, P.; Mondloch, J. E.; Karagiaridi, O.; Bury, W.; Hupp, J. T.; Farha, O. K. Chem. Soc. Rev. 2014, 43, 5896−5912. (d) Zhang, W.; Liu, Y.; Lu, G.; Wang, Y.; Li, S.; Cui, C.; Wu, J.; Xu, Z.; Tian, D.; Huang, W.; DuCheneu, J. S.; Wei, W. D.; Chen, H.; Yang, Y.; Huo, F. Adv. Mater. 2015, 27, 2923−2929 For additional references, see the Supporting Information.. (8) The given interpretation is reasonable, but stronger confirmation will have to come from more extensive comparative studies, which is beyond the scope of this Communication.

In summary, we report the use of SALE with a nanoparticle@ MOF composite material to synthesize a de novo unattainable composite material, Pt@SALEM-2, that features larger pore apertures than the parent compound. PXRD data are in agreement with previously reported results, while N2 isotherms indicate that the porosity is retained. 1H NMR spectroscopy indicates 90% linker exchange, while size-selective reactions confirm that the gate-keeping apertures of the MOF@nanoparticle composite are expanded by SALE and that the nanoparticles remain fully enshrouded. STEM of Pt@SALEM2 indicates that the platinum nanoparticle size and distribution are unchanged by SALE. This initial demonstration of catalysisrelevant linker exchange for an archetypal nanoparticle@MOF composite suggests extension to other linkers, especially linkers that may function as cocatalysts but are too delicate for, or are otherwise incompatible with, the reagents or conditions used for direct synthesis.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02880. Details of the catalytic experiments, nanoparticle statistics, and experimental spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Science Foundation (NSF; Grant DMR-1334928). Acquisition of the data on trace analysis and gas chromatography instruments used in the IMSERC facility of Northwestern University was made possible by support from Northwestern University and NSF Grant CHE-0923236, respectively. This work made use of TEM and STEM instruments located in the EPIC facility (NUANCE Center-Northwestern University), which has received support from the MRSEC program (NSF Grant DMR-1121262) at the Materials Research Center, the Nanoscale Science and Engineering Center (NSF Grant EEC-0647560) at the International Institute for Nanotechnology, and the State of Illinois, through the International Institute for Nanotechnology. PXRD was recorded at the J. B. Cohen X-ray Facility at Northwestern University supported by the MRSEC program of the NSF (Grant DMR-1121262) at the Materials Research Center of Northwestern University. We thank Cassandra Whitford for assistance with STEM.

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REFERENCES

(1) (a) Dhakshinamoorthy, A.; Garcia, H. Chem. Soc. Rev. 2012, 41, 5262−5284. (b) Aijaz, A.; Xu, Q. J. Phys. Chem. Lett. 2014, 5, 1400− 1411. (c) Tian, Y.; Wang, W.; Chai, Y.; Cong, J.; Shen, S.; Yan, L.; Wang, S.; Han, X.; Sun, Y. Phys. Rev. Lett. 2014, 112, 017202. (d) Hu, P.; C

DOI: 10.1021/acs.inorgchem.5b02880 Inorg. Chem. XXXX, XXX, XXX−XXX