Communication pubs.acs.org/IC
Elaboration of a Highly Porous RuII,II Analogue of HKUST‑1 Wenhua Zhang,† Kerstin Freitag,‡ Suttipong Wannapaiboon,†,‡ Christian Schneider,‡ Konstantin Epp,‡ Gregor Kieslich,‡ and Roland A. Fischer*,‡ †
Lehrstuhl für Anorganische Chemie II, Organometallics & Materials, Ruhr-Universität, Bochum, Germany Department of Chemistry, Technical University Munich, Lichtenbergstrasse 4, D-85748 Garching, Germany
‡
S Supporting Information *
obtained from the single-crystal X-ray diffraction (SCXRD) measurements of the dinuclear complex [Ru2(OOCCH3)4](THF)2 [crystallized from a hot tetrahydrofuran (THF) solution] are in a good agreement with the corresponding values communicated in the literature (Table S1).32 The as-prepared RuII,II complex is air-sensitive and was handled under argon. It loses its crystallinity after solvent decoordination to yield [Ru2(OOCCH3)4]. Nonetheless, the Fourier transform infrared (FT-IR) data of the symmetric and asymmetric COO− vibrations at 1425 and 1541 cm−1, respectively, match well with the reported values (Figure S1).25,30 Subsequently, degassed solvents (H2O and acetic acid) and inert conditions (argon atmosphere) were employed to synthesize RuII,II-BTC. The powder X-ray diffraction (PXRD) patterns of the activated mixed-valence RuII,III-BTC sample obtained from [Ru2(OOCCH3)4Cl] (SBUa) in our earlier reports25,30 exhibit shifts of the peaks with respect to those of the parent compound HKUST-1. However, it is important to mention here that the overall indexing of the peaks and therefore the structural integrity are fully preserved. The observed shift comes from substitution of the small CuII ions by larger RuII,III ions. The obtained lattice parameters from Pawley analysis agree well with our expectations; see Figure 1a. Similarly, the PXRD pattern and Pawley analysis of the activated sample RuII,II-BTC is in good agreement with the respective PXRD data of RuII,III-BTC (Figure 1). This not only provides evidence for phase purity but also indicates the successful preparation of a Ru2+−Ru2+ paddle wheel. Furthermore, the IR spectra of the RuII,II-BTC sample matches well with that of the RuII,III-BTC reference (Figure S2). The bands at 1359 cm−1 for νs(COO) and 1429 cm−1 for νas(COO) suggest coordination of the carboxylate groups of the framework BTC. The 1H NMR spectrum of the quantitatively acid-digested, activated sample RuII,II-BTC displays peaks at 8.60 and 1.86 ppm (Figure S3). The former resonance is assigned to the aromatic protons of BTC, while the latter is attributed to the protons of acetate, suggesting the presence of AcO(H). In order to get detailed information about the oxidation states of the Ru centers in RuII,II-BTC, X-ray absorption near-edge structure (XANES) spectra on RuII,II-BTC, RuII,III-BTC, Ru precursors used (SBU-a and -a′), and RuCl3 were collected. The dependence of the edge jump from the Ru oxidation state in XANES spectra allows the assignment of the oxidation state of the Ru centers in the MOF structures by analyzing the position of
ABSTRACT: When the dinuclear RuII,II precursor [Ru2(OOCCH3)4] is employed under redox-inert conditions, a RuII,II analogue of HKUST-1 was successfully prepared and characterized as a phase-pure microcrystalline powder. X-ray absorption near-edge spectroscopy confirms the oxidation state of the Ru centers of the paddle-wheel nodes in the framework. The porosity of 1371 m2/mmol of RuII,II-HKUST-1 exceeds that of the parent compound HKUST1 (1049 m2/ mmol).
M
etal−organic frameworks (MOFs) as intriguing porous materials have attracted more and more attention because of their ability to form diverse structural designs1−5 and their application in various fields.6−16 HKUST-1, [Cu3(BTC)2]n, where BTC = 1,3,5-benzenetricarboxylate, is one of the wellinvestigated, canonical MOFs that shows good thermal stability and coordinatively unsaturated metal sites (CUSs) upon removal of water molecules or other neutral donor ligands bound to the axial positions of the Cu dimers.17 Those CUSs turn out to be one of the key factors for enhanced catalytic13,18 and gas sorption8,19 properties. Hence, some isostructural analogues of HKUST-1 have been explored by several groups.20−27 In particular, the thermally robust mixed-valence RuII,III analogues continue to receive attention owing to the catalytic redox-active Ru2-CUSs.26,28,29 However, the compositions of these materials of the general formula [Ru3(BTC)2Yy·Gg]n (Y = counterion, with 0 < y ≤ 1.5; G = neutral strongly adsorbed guest, such as solvents or AcOH, H3BTC, etc.) reveal rather a complicated scenario.28,30 Owing to the charge balance of the mixed-valence Ru 2 paddlewheel units, the presence of counterions (Y = Cl−, F−, OH−, AcO−, H3−aBTCa−, etc.) in the frameworks is inevitable, which obstructs the porosity of the Ru-MOFs to some extent and in which strong CUS blocking leads to difficulty in rational property design. The catalytic activity and sorption properties of these Ru-MOFs will improve if the Ru-CUSs were more available by avoiding additional coordination (“blocking”) by counterions Y. Herein, we report on the synthesis of the RuII,II analogue of HKUST-1, abbreviated as RuII,II-BTC, by directly utilizing a controlled secondary building unit (CSA) approach with a preformed RuII,II precursor under inert conditions (argon atmosphere) to exclude the otherwise favored redox chemistry during MOF formation, leading to RuII,III mixed-valence nodes. The utilized RuII,II precursor [Ru2(OOCCH3)4] (SBU-a′) was prepared by reducing ruthenium(III) chloride to the “blue solution” according to a reported method.31 The cell parameters © XXXX American Chemical Society
Received: August 22, 2016
A
DOI: 10.1021/acs.inorgchem.6b02038 Inorg. Chem. XXXX, XXX, XXX−XXX
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SBU-a, indicating its mixed-valence state, as documented earlier.25 Only RuII was found in SBU-a′.32 Thus, the same edge-jump position observed in the XANES spectra of the sample RuII,II-BTC suggests the exclusive presence of RuII centers in the framework structure (Figure 2b). Consequently, the obtained solid should feature the exact Ru2 paddle wheels as those in HKUST-1 without any significant counterions around. In fact, elemental analysis (EA) results do not reveal the presence of any Cl in RuII,II-BTC, ruling out the existence of Cl− and RuCl3 in both SBU-a′ employed during the synthesis and the obtained solid Ru-MOF. A compositional formula of [Ru3(BTC)2· (AcOH)2.3]n assigned to the samples RuII,II-BTC matches reasonably well with the obtained NMR and EA results, which could be another sign of the absence of unwanted counterions (note: a rigorous exclusion of traces of coordinated hydroxide ions is not possible by our data set). In comparison with mixedvalence RuII,III-BTC documented in earlier reports, the obtained composition herein turns out to be simpler in the absence of any counterions, although the notorious presence of acetic acid cannot be avoided. The thermal stability of the obtained solid RuII,II-BTC is quite substantial, and decomposition occurs around 523 K (Figure S4). The mass fraction of the obtained residue amounts to 45.5%, which matches well with the theoretical residual mass fraction of RuO2 (46.6%). Thermogravimetry−mass spectrometry/differential scanning calorimetry data reveal that acetate is leaving the framework at the decomposition temperature of the framework, which indicates the presence of an unsubstitued acetate of SBU-a′ rather than a physiosorbed acetate within the framework. Standard N2 sorption isotherms collected at 77 K for activated RuII,II-BTC demonstrate type I isotherms (Figure 3), indicating microporosity. A considerably higher Brunauer−Emmett−Teller (BET) surface area (SBET) of RuII,II-BTC (1371 m2/g) was found compared to the previously reported mixed-valence RuII,III-BTC samples (704−1180 m2/g).25,26,28,30 Given the higher bulk density of Ru-BTC materials than Cu-BTC (i.e., HKUST-1), SBET is also calculated in units of m2/mmol (based on the
Figure 1. PXRD pattern (colored curves), Pawley fits (orange curves), and difference curves (black) of the activated samples (a) RuII,III-BTC and (b) RuII,II-BTC. The obtained lattice parameters are consistent with the size differences of Cu2+, Ru2+, and Ru3+ compared with the reference Cu-BTC (a = b = c = 26.343 Å). The refined sizes of the diffraction domains, 93 nm for RuII,II-BTC and 128 nm for RuII,III-BTC, reflect the difficulty of crystallizing Ru-based coordination frameworks.
the highest peak in the plot of derivative normalized absorption versus energy. As illustrated in Figure 2a, RuII,III-BTC shows an edge-jump position between RuCl3 and its starting precursor
Figure 2. (a) XANES spectra of RuII,III-BTC and RuII,II-BTC in comparison with the employed Ru precursors (SBU-a = [Ru2(OOCCH3)4Cl] and SBU-a′ = [Ru2(OOCCH3)4]) and RuCl3. (b) XANES spectra of the obtained RuII,II-BTC and the employed RuII,II precursor SBU-a′. The vertical line stands for the same position of the edge jump.
Figure 3. N2 adsorption (solid symbols) and desorption (open symbols) isotherms recorded at 77 K for activated RuII,III-BTC (orange triangles) and RuII,II-BTC (violet circles). Monolayer saturation adsorption capacity (Vm): RuII,II-BTC, 315 cm3/g (270 cm3/mmol); RuII,III-BTC,33 229 cm3/g (221 cm3/mmol); HKUST-1,26 ∼425 cm3/g (257 cm3/mmol). B
DOI: 10.1021/acs.inorgchem.6b02038 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry empirical formula assigned to RuII,II-BTC). Thus, SBET of RuII,IIBTC amounts to 1173 m2/mmol, which matches well with the reported value of Cu-BTC (1049 m2/mmol),26 indicating a similar porosity, unless significant adsorption of acetic acid at RuII,II-BTC exists. Similarly, RuII,II-BTC shows that CO2 uptake at 298 K can reach 3.4 mmol/g (ca. 15.0 wt %) at 1 bar (Figure S5). This value is also comparable with that reported for HKUST-1 (15.2−18.4 wt %).34−36 To recall, CO2 uptake at 298 K and 1 bar for RuII,III-BTC was revealed to be somewhat less with 2.7 mmol/g (11.88 wt %).33 These good adsorption data of RuII,II-BTC matching with the reference material HKUST-1 are encouraging. When the pore-size distribution was further examined on the basis of density functional theory calculations (Figure S6), 88.6% of the total pore volume can be accounted for by two micropores at 5.9 and 7.9 Å. An additional micropore at 14.3 Å might point at the presence of structural defects generated by the inclusion of acetic acid into the framework. However, this is a matter of further studies. Interestingly, notorious persistence of acetic acid was also observed for the mixed-valence RuII,IIIBTC materials. The use of acetic acid (or acetate) cannot be avoided and is a consequence of the controlled SBU approach for solvothermal synthesis of the samples, the situation of which we have discussed in detail earlier.30 Simple coordination of AcOH to the apical RuII centers via the O-donor function of the carboxylate group would not easily explain the observed high N2 uptake. An only partially affective substitution of all acetate ligands of SBU-a′ by BTC linkers may be taken into account, alternatively. Ligand substitution at Ru centers is kinetically hindered and slow compared to substitution at labile Cu centers. If this is the true case, the current samples RuII,II-BTC may be considered as a kind of defect-type or mixed-component variant of the ideal still-not-realized composition [Ru3(BTC)2]n. However, from FT-IR data, there is no indication of noncoordinated free carboxylate groups or any kind of dangling OH species (no band in the region 4000−3000 cm−1; Figure S2). Unravelling the situation requires more sophisticated spectroscopic investigations involving FT-IR under controlled UHV conditions including probe molecules (CO) and temperatureprogramed desorption studies, similar to our previous work on Ru-BTC materials.29 In summary, replacing the mixed-valence RuII,III-SBU by the uniform valence RuII,II-SBU and conducting synthesis under redox-inert conditions allowed access to the uniform-valence RuII,II analogue of [Cu3(BTC)2]n (HKUST-1) with the compositional formula [Ru3(BTC)2·(AcOH)2.3]n. XANES spectra confirm the RuII,II framework sites. The N2 BET surface area of the RuII,II-BTC material is the highest (1173 m2/mmol) among all of the so-far-reported Ru-BTC MOFs, and it is also quite comparable with that of HKUST-1 (1049 m2/mmol). Thus, RuII,II-BTC holds perspectives for various applications in sorption and catalysis. The results are important for ongoing studies on related but microstructurally more complex, defectengineered, and mixed-valence Ru-MOFs featuring even more reduced Ruδ+ sites (δ < 2).
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TGA, and pore-size-distribution analysis data for the obtained Ru-MOF. (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: roland.fi
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS W.Z. is grateful for a Ph.D. fellowship from the China Scholarship Council and also thanks the Research School at Ruhr University Bochum for support of her Ph.D. project with international travel grants. The authors further thank the team at DELTA synchrotron facility at the Technical University Dortmund for support in the XANES data collection at beamline BL8 and the PXRD data collection at beamline BL9.
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REFERENCES
(1) Fang, Z.; Bueken, B.; De Vos, D. E.; Fischer, R. A. DefectEngineered Metal-Organic Frameworks. Angew. Chem., Int. Ed. 2015, 54, 7234−7254. (2) Schneemann, A.; Henke, S.; Schwedler, I.; Fischer, R. A. Targeted Manipulation of Metal-Organic Frameworks To Direct Sorption Properties. ChemPhysChem 2014, 15, 823−839. (3) Deria, P.; Mondloch, J. E.; Karagiaridi, O.; Bury, W.; Hupp, J. T.; Farha, O. K. Beyond post-synthesis modification: evolution of metalorganic frameworks via building block replacement. Chem. Soc. Rev. 2014, 43, 5896−5912. (4) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. The Chemistry and Applications of Metal-Organic Frameworks. Science 2013, 341, 1230444. (5) Bunck, D. N.; Dichtel, W. R. Mixed Linker Strategies for Organic Framework Functionalization. Chem. - Eur. J. 2013, 19, 818−827. (6) Herm, Z. R.; Bloch, E. D.; Long, J. R. Hydrocarbon Separations in Metal-Organic Frameworks. Chem. Mater. 2014, 26, 323−338. (7) Yoon, M.; Suh, K.; Natarajan, S.; Kim, K. Proton Conduction in Metal-Organic Frameworks and Related Modularly Built Porous Solids. Angew. Chem., Int. Ed. 2013, 52, 2688−2700. (8) Mason, J. A.; Veenstra, M.; Long, J. R. Evaluating metal-organic frameworks for natural gas storage. Chem. Sci. 2014, 5, 32−51. (9) Li, J.-R.; Sculley, J.; Zhou, H.-C. Metal-organic frameworks for sperations. Chem. Rev. 2012, 112, 869−932. (10) Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. Metal-organic framework materials as chemical sensors. Chem. Rev. 2012, 112, 1105−1125. (11) Zhao, M.; Ou, S.; Wu, C.-D. Porous Metal-Organic Frameworks for Heterogeneous Biomimetic Catalysis. Acc. Chem. Res. 2014, 47, 1199−1207. (12) Wang, J.-L.; Wang, C.; Lin, W. Metal-Organic Frameworks for Light Harvesting and Photoctalysis. ACS Catal. 2012, 2, 2630−2640. (13) Liu, J.; Chen, L.; Cui, H.; Zhang, J.; Zhang, L.; Su, C.-Y. Applications of metal-organic frameworks in heterogeneous supramolecular catalysis. Chem. Soc. Rev. 2014, 43, 6011−6061. (14) Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.; Hupp, J. T. Metal-organic framework materials as catalysts. Chem. Soc. Rev. 2009, 38, 1450−1459. (15) Meek, S. T.; Greathouse, J. A.; Allendorf, M. D. Metal-Organic Frameworks: A rapidly Growing Class of Versatile Nanoporous Materials. Adv. Mater. 2011, 23, 249−267. (16) Kuppler, R. J.; Timmons, D. J.; Fang, Q.-R.; Li, J.-R.; Makal, T. A.; Young, M. D.; Yuan, D.; Zhao, D.; Zhuang, W.; Zhou, H.-C. Potential applications of metal-organic frameworks. Coord. Chem. Rev. 2009, 253, 3042−3066.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02038. Experimental details, cell parameters and IR spectra for Ru precursors, 1H NMR spectra of acid-digested samples, EA, C
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Organic Frameworks via Occupation of OPen-Metal Sites by Coorindated Water Molecules. Chem. Mater. 2009, 21, 1425−1430. (35) Millward, A. R.; Yaghi, O. M. Metal-Organic Frameworks with Exceptionally High Capacity for Storage of Carbon Dioxide at Room Temperature. J. Am. Chem. Soc. 2005, 127, 17998−17999. (36) Sumida, K.; Rogow, D. L.; Mason, J. A.; McDonald, T. M.; Bloch, E. D.; Herm, Z. R.; Bae, T.-H.; Long, J. R. Carbon Dioxide Caputre in Metal-Organic Frameworks. Chem. Rev. 2012, 112, 724−781.
(17) Chui, S. S. Y.; Lo, S. M. F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D. A chemically functionalizable nanoporous materials. Science 1999, 283, 1148−1150. (18) Corma, A.; García, H.; Llabrés i Xamena, F. X. Engineering Metal Organic Frameworks for Heterogeneous Catalysis. Chem. Rev. 2010, 110, 4606−4655. (19) Murray, L. J.; Dinca, M.; Long, J. R. Hydrogen storage in metalorganic frameworks. Chem. Soc. Rev. 2009, 38, 1294−1314. (20) Wu, Y.; Chen, H.; Xiao, J.; Liu, D.; Liu, Z.; Qian, Y.; Xi, H. Adorptive Separation of Methanol-Acetone on Isostructural Series of Metal-Organic Frameworks M-BTC (M= Ti, Fe, Cu, Co, Ru, Mo): A Computational Study of Adsorption Mechanism and Metal-Substitution. ACS Appl. Mater. Interfaces 2015, 7, 26930−26940. (21) Feldblyum, J. I.; Liu, M.; Gidley, D. W.; Matzger, A. J. Reconciling the Discrepancies between Crystallographic Porosity and Guest Access As Exemplified by Zn-HKUST-1. J. Am. Chem. Soc. 2011, 133, 18257− 18263. (22) Murray, L. J.; Dinca, M.; Yano, J.; Chavan, S.; Bordiga, S.; Brown, C. M.; Long, J. R. Highly-Selective and Reversible O2 Binding in Cr3(1,3,5-benzenetricarboxylate)2. J. Am. Chem. Soc. 2010, 132, 7856− 7857. (23) Xie, L.; Liu, S.; Gao, C.; Cao, R.; Cao, J.; Sun, C.; Su, Z. MixedValence Iron (II,III) Trimesates with Open Frameworks Modulated by Solvents. Inorg. Chem. 2007, 46, 7782−7788. (24) Kramer, M.; Schwarz, U.; Kaskel, S. Synthesis and properties of the metal-organic framework Mo3(BTC)2 (TUDMOF-1). J. Mater. Chem. 2006, 16, 2245−2248. (25) Kozachuk, O.; Yusenko, K.; Noei, H.; Wang, Y.; Walleck, S.; Glaser, T.; Fischer, R. A. Solvothermal growth of a ruthenium metalorganic framework featuring HKUST-1 structure type as thin films on oxide surfaces. Chem. Commun. 2011, 47, 8509−8511. (26) Wade, C. R.; Dinca, M. Investigation of the synthesis, activation, and isosteric heats of CO2 adsorption of the isostructural series of metalorganic frameworks M3(BTC)2 (M = Cr, Fe, Ni, Cu, Mo, Ru). Dalton Trans. 2012, 41, 7931−7938. (27) Maniam, P.; Stock, N. Investigation of Porous Ni-based MetalOrganic Frameworks Containing Paddle-Wheel Type Inorganic Building Units via High-Throughput Methods. Inorg. Chem. 2011, 50, 5085−5097. (28) Kozachuk, O.; Luz, I.; Llabrés i Xamena, F. X.; Noei, H.; Kauer, M.; Albada, H. B.; Bloch, E. D.; Marler, B.; Wang, Y.; Muhler, M.; Fischer, R. A. Multifunctional, defect-engineered metal-organic frameworks with ruthenium centers: sorption and catalytic properties. Angew. Chem., Int. Ed. 2014, 53, 7058−7062. (29) Noei, H.; Kozachuk, O.; Amirjalayer, S.; Bureekaew, S.; Kauer, M.; Schmid, R.; Marler, B.; Muhler, M.; Fischer, R. A.; Wang, Y. CO Adsorption on a Mixed-Valence Ruthenium Metal-Organic Framework Studied by UHV-FTIR Spectroscopy and DFT Calculations. J. Phys. Chem. C 2013, 117, 5658−5666. (30) Zhang, W.; Kozachuk, O.; Medishetty, R.; Schneemann, A.; Wagner, R.; Khaletskaya, K.; Epp, K.; Fischer, R. A. Controlled SBUApproaches to Isoreticular Metal-Organic Framework RutheniumAnalogues of HKUST-1. Eur. J. Inorg. Chem. 2015, 2015, 3913−3920. (31) Rose, D.; Wilkinson, G. Substitution reactions of the hydridopentamminerhodium(III) ion. J. Chem. Soc. A 1970, 1791− 1795. (32) Lindsay, A. J.; Wilkinson, G.; Motevalli, M.; Hursthouse, M. B. The synthesis, magnetic, electrochemical, and spectroscopic properties of diruthenium(II,II) tetra-μ-carboxylates and their adducts. X-ray Structures of Ru2(O2CR)4L2 (R= Me, L = H2O or tetrahydrofuran; R = Et, L = Me2CO). J. Chem. Soc., Dalton Trans. 1985, 2321−2326. (33) Zhang, W.; Kauer, M.; Halbherr, O.; Epp, K.; Guo, P.; Gonzalez, M. I.; Xiao, D. J.; Wiktor, C.; Liabrés i Xamena, F. X.; Wöll, C.; Wang, Y.; Muhler, M.; Fischer, R. A. Ruthenium Metal-Organic Frameworks with Different Defect Types: Influence on Porosity, Sorption, and Catalytic Properties. Chem. - Eur. J. 2016, 22, 14297. (34) Yazaydın, A. Ö .; Benin, A. I.; Faheem, S. A.; Jakubczak, P.; Low, J. J.; Willis, R. R.; Snurr, R. Q. Enhanced CO2 Adsorption in MetalD
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