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Cite This: Chem. Mater. 2018, 30, 2193−2197
Room Temperature Synthesis of an 8‑Connected Zr-Based Metal− Organic Framework for Top-Down Nanoparticle Encapsulation Hyunho Noh,† Chung-Wei Kung,† Timur Islamoglu,† Aaron W. Peters,† Yijun Liao,† Peng Li,† Sergio J. Garibay,† Xuan Zhang,† Matthew R. DeStefano,† 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 21589, Saudi Arabia
‡
S Supporting Information *
M
etal−organic frameworks (MOFs) are composed of metal ions/clusters interconnected by multitopic organic linkers, yielding a periodic three-dimensional network with a high internal surface area.1−6 Through a modulation of either building block, judicious design of the physical and/or chemical environment within the pores can be achieved.7−10 Additional functionalization has been realized through various postsynthetic modification techniques.5,11−18 Given the versatility in fine-tuning the confined environment within their structure, MOFs have been widely exploited in applications such as, but not limited to, gas storage,1,2,8,19 liquid/gas-phase separations,18,20,21 and heterogeneous catalysis.5,11−15,22,23 MOFs that consist of Zr6-based inorganic nodes are of particular interest given their high chemical and thermal stability.24 Since the discovery of the Zr6-based MOF, UiO-66 and its isoreticularly expanded analogues,7 numerous other Zr6based MOFs with various organic linkers have been reported.25 We, and others, have exploited their high stability by utilizing them as a heterogeneous catalysts4,26,27 as well as scaffolds for postsynthetically introduced catalytic species.11−16,28 Syntheses of almost all of Zr6-based MOFs involve solvothermal crystallization.7,17,29−32 From a techno-economic perspective, minimizing the amount of heat used in a synthesis, and therefore minimizing the energy input, is desirable as a significant decrease in the effective cost of a MOF synthesis can be expected. An ancillary benefit to room temperature synthesis of MOFs can be realized through top-down (or bottle-aroundship) encapsulation of thermally sensitive materials, such as nanoparticles (NPs), which could lead to the development of composite materials useful for catalysis.33,34 Composite material via room temperature syntheses of Zn2+- or Cu2+-based MOFs, such as ZIF-8 or HKUST-1 has been realized;35,36 however, the aforementioned stability of Zr6-based MOFs24 make them a preferred support for an encapsulation for catalytic applications. Yet, as of today, UiO-66 and a few derivatives are the only Zr6MOFs with an established synthetic protocol at room temperature.37 Thus, substrates are restricted to those significantly smaller than the 6 Å aperture size of UiO-66 for ready diffusion to the active sites within the MOF crystals.7 Herein, we report the synthetic procedure of a microporous MOF with tetratopic organic linkers, NU-901, at room temperature. NU-901 consists of eight-connected Zr6(μ3O)4(μ3-OH)4(H2O)4(OH)4 nodes and tetratopic 1,3,5,8-(pbenzoate)pyrene linkers (TBAPy4−). The two components are aligned to form microporous diamond-shaped channels (Figure 1). We note that the only reported synthesis of this MOF is © 2018 American Chemical Society
Figure 1. Structure of NU-901 viewed through the c-axis (left) and the inorganic node and the organic linker (right). Hydrogen atoms are omitted for clarity.
performed at 120 °C.30 Exploiting the room temperature synthesis of NU-901, we additionally employed a top-down encapsulation of Pt NPs in NU-901. The obtained composite material, designated as Pt@NU-901, was further utilized as an alkene hydrogenation catalyst to demonstrate that the composite can allow efficient diffusion of the substrate while simultaneously preventing the agglomeration of NPs. The NU-901 synthesis involves two steps: (1) synthesis of the Zr6 node capped with benzoate ligands and (2) addition of H4TBAPy to a solution containing the node. Synthesis of the node has been reported previously (see the Supporting Information (SI) for details).38 The purity of the benzoatecapped node was confirmed by comparison of the powder Xray diffraction (PXRD) pattern (Figure S1) with a simulated pattern. A solution of 4 mL of acetic acid in 7 mL of dimethylformamide (DMF) was prepared, of which 10 mL was used to solvate 200 mg of the fully dried Zr6 precursor. Separately, 100 mg of H4TBAPy, synthesized according to the reported procedure,39 was solvated in 10 mL of DMF. The H4TBAPy solution was added dropwise at room temperature at a rate of 0.05 mL/min to the Zr6 solution with vigorous stirring and left overnight. The slow titration of the linker solution is crucial otherwise an immediate addition leads to a rapid precipitation of the organic linker due to its low solubility in the Received: January 30, 2018 Revised: March 13, 2018 Published: March 14, 2018 2193
DOI: 10.1021/acs.chemmater.8b00449 Chem. Mater. 2018, 30, 2193−2197
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of defects has been observed with UiO-66.37 Intriguingly, synthesis temperature (ranging from 30 to 130 °C) seems to alter MOF topology rather than defect density. Even a slight increase of reaction temperatures to 40 °C resulted in a material with an observed peak at 2θ = 2.5° in its PXRD pattern (Figure S5). However, at higher temperatures (>110 °C) this peak is no longer observed in the PXRD patterns (Figure S5). This general trend is reflected in the cumulative mesopore volumes (i.e., total N2 uptake from P/P0 = 0.2−0.26 in Figure 3) where contribution from the mesopore maximizes at 50−60 °C then gradually decreases.
DMF/acetic acid mixture. The resulting material after slow titration, named NU-901-RT, was washed extensively with DMF to remove excess reagents, solvent-exchanged to acetone, and dried at 120 °C under vacuum. As revealed in its N2 adsorption−desorption isotherm, the activated NU-901-RT shows high porosity with a Brunauer− Emmett−Teller (BET) surface area of 2130 ± 50 m2/g (Figure 2a). Along with the expected 12 Å micropore, the density
Figure 2. (a) N2 isotherm at 77 K and (b) DFT-calculated pore size distribution of NU-901-RT.
functional theory (DFT)-calculated pore size distribution (Figure 2b) shows a 27 Å mesopore with a smaller pore volume compared to the micropore.40 The presence of this mesopore suggests two possibilities: (1) the obtained NU-901RT is phase impure, where a small fraction of NU-1000, a MOF constructed from identical building blocks but with large mesoporous channels (∼31 Å), is present17 or (2) the crystals have defect sites (i.e., missing linkers/nodes) creating larger mesopores, analogous to the case of UiO-67.41 The PXRD pattern of NU-901-RT (Figure S2) does not exhibit a detectable peak characteristic of the (100) plane in NU-1000 (2θ = 2.5°). The cumulative pore size distribution of NU-901RT (Figure S3b) suggests a minimum of 16 mol % NU-1000 impurity if all the apparent mesopores are from NU-1000. Yet, the PXRD pattern of a physical mixture of 16/84 mol % NU1000/NU-901 shows a sharp peak at 2θ = 2.5° (Figure S2). Thus, we conclude that NU-901-RT may contain defect sites, presumably creating significantly larger pores due to a missing Zr6 node making the resulting mesopore comparable in dimensions to that of NU-1000 (Figure 1).42 It is worth noting that a similar phenomenon has been observed for UiO66 synthesized at room temperature.37 Supporting evidence accrues from the diffuse reflectance infrared Fourier transform spectra (DRIFTS) of NU-901-RT, nondefective NU-901, and the H4TBAPy organic linker (Figure S4). The CO stretches at 1700 and 1736 cm−1 observed for NU-901-RT suggest the presence of some carboxylic groups on the linker that are not coordinated to the Zr6 node. Note that whereas the same stretches were observed for the H4TBAPy linker, that of nondefective NU-901 did not exhibit the CO stretches. We sought to determine how manipulation of reaction temperature induces changes in defect density given that an inverse correlation between synthesis temperature and number
Figure 3. N2 isotherms of NU-901 synthesized at (a) 40−80 °C with a peak at 2θ = 2.5° and (b) 90−130 °C with no detectable peak at 2θ = 2.5° in their PXRD patterns (Figure S5). For the DFT-calculated pore size distribution of all samples, see the SI.
Room temperature synthesis of MOFs is ideal for encapsulating NPs as thermally induced aggregation of nanoparticles can effectively be mitigated. Pt NPs, synthesized according to a reported procedure,43 were dispersed in 10 mL of the acetic acid/DMF/Zr6 node solution under the same conditions as listed previously. The concentration of Pt NPs in this solution is ∼700 ppm. The remaining protocol of the MOF synthesis is identical to that without the NPs. The resulting composite (Pt@NU-901) possesses a slightly lower BET surface area (1650 ± 40 m2/g) when compared to NU-901-RT 2194
DOI: 10.1021/acs.chemmater.8b00449 Chem. Mater. 2018, 30, 2193−2197
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Intriguingly, the PXRD pattern of Pt@NU-901 after catalysis showed a very small additional peak at 2θ = 2.9° (Figure S21). This is attributed to an increase in density of the aforementioned defect sites during the catalysis, and hence, is detectable via PXRD. Indeed, a simulated PXRD pattern of a defective structure with a missing Zr6 node (Figure S21) also exhibits this peak. The resulting mesopore has dimensions of ca. 25× 26 Å, consistent with the mesopore size returned through the aforementioned DFT calculation of pore size distribution (Figure 2b). To conclude, we have shown the room temperature synthesis of NU-901, a Zr6-based MOF constructed from tetratopic pyrene-based organic linkers. Incorporation of Pt NPs in the reaction solution during synthesis renders the resulting composite an effective cis-stilbene hydrogenation catalyst. Without the NU-901 framework, rapid agglomeration of Pt NPs is observed under reaction conditions, indicating that both the MOF and the NPs are crucial in order for the catalysis to be effective. With this facile room temperature synthetic protocol, we envision further application of the material toward the encapsulation of more complex systems. As control over exact crystal facets and/or morphologies of the NPs is difficult to achieve through bottom-up impregnation in the framework, the top-down encapsulation of preformed NPs in this work may lead to development of composite materials with controllable functionalities.
due to mass gain from the Pt NPs, but it retains a similar DFT pore size distribution, suggesting that the encapsulation of Pt NPs does not significantly affect the pore structure of the MOF (Figure S3c). Approximately 70% of the volumetric surface area of NU-901-RT was retained in Pt@NU-901 (i.e., from 1500 ± 40 to 1050 ± 30 m2/cm3, see the SI for details), further suggesting the retention of the porous network upon the NP encapsulation. Through an inductively coupled plasma optical emission spectroscopy (ICP-OES) measurement, 11 ± 1 wt % loading of Pt was confirmed in Pt@NU-901. Similar wt % loading was determined through thermogravimetric analysis (TGA, Figure S7). Scanning electron microscopy (SEM) images show no difference in the crystallite morphology of NU-901-RT versus Pt@NU-901 (300−500 nm tear-shaped crystals), and the transmission electron microscopy (TEM) image shows that in Pt@NU-901, most Pt NPs are encapsulated within MOF crystals while small amount are perhaps physisorbed onto the surface of the crystals (Figures S8 and S9). The size distribution of the NPs in the composite was confirmed to be similar to that of bare NPs, with an average diameter of 4.0 ± 0.5 nm (Figures S9 and S10). A diffuse reflectance UV−vis spectrum of Pt@NU-901 shows significant enhancement of extinction in the 500−800 nm region as compared to that of NU-901-RT (Figure S10). This is presumably a result of close packing between the NPs and the framework, resulting in high scattering.43 X-ray photoelectron spectroscopy (XPS) was employed to verify that the oxidation state of Pt in Pt@NU-901 is Pt0 (Figure S11). As Pt NPs are known as effective alkene hydrogenation catalysts,44,45 we chose to test the propensity of Pt@NU-901 to perform hydrogenation reactions (for details in experimental, see the SI). Namely, we chose cis-stilbene as the substrate; given the 7.8 Å kinetic diameter, it should readily diffuse through the pores of NU-901 (ca. 12 × 26 Å). Indeed, when Pt@NU-901 was subjugated to the reaction condition for 1 h, a conversion of 31 ± 1% was confirmed (see Figures S13 and S14 for 1H NMR and GC−MS spectra that were used to identify the substrate/product). PXRD patterns confirm that the framework retained its crystallinity after the NP encapsulation and after the catalysis (Figure S15). N2 isotherm of the composite after catalysis also reveals that much of its porosity has been retained (Figure S16). A TEM image of the composite after catalysis (Figure S17) shows no significant agglomeration of NPs and ICP-OES measurements confirmed no detectable loss of Pt. In comparison, bare Pt NPs undergo rapid agglomeration in the reaction mixture, as shown by SEM and TEM images (Figures S18 and S19). Consequently, after 1 h, no detectable catalysis was observed. When a composite with Pt NPs only on the exterior of the crystals (named Pt/NU-901; for synthesis and characterization, see the SI) was instead used as a catalyst, conversions ranging from 25 to 42% were detected. The high variability is attributed to the uncontrolled MOF crystal agglomeration induced due to the low solubility of polyvinylpyrrolidone (PVP) surfactants on the NPs in the reaction mixture, as shown by the TEM image (Figure S20). We note that the individual NPs retain their 3−4 nm size after catalysis (Figure S20). Pt/NU-901 also retained its crystallinity as shown by the PXRD pattern in Figure S21. Control reactions, without the Pt NPs and without the parent framework, show no catalytic behavior, thus confirming that both the MOF and the Pt NPs are necessary to achieve hydrogenation.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b00449. Detailed material synthesis and characterization data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*O.K.F. E-mail:
[email protected]. ORCID
Hyunho Noh: 0000-0003-3136-1004 Chung-Wei Kung: 0000-0002-5739-1503 Timur Islamoglu: 0000-0003-3688-9158 Aaron W. Peters: 0000-0002-6554-8507 Peng Li: 0000-0002-4273-4577 Xuan Zhang: 0000-0001-8214-7265 Matthew R. DeStefano: 0000-0002-2201-9808 Joseph T. Hupp: 0000-0003-3982-9812 Omar K. Farha: 0000-0002-9904-9845 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS O.K.F. gratefully acknowledges support from the Defense Threat Reduction Agency (HDTRA1-18-1-0003) for the catalysis study and Army Research Office-STTR (W911SR17C-0007) for the linkers and the MOFs syntheses. H.N. gratefully acknowledges support from the Ryan Fellowship and the Northwestern University International Institute of Nanotechnology. C.-W.K. acknowledges support from the Postdoctoral Research Abroad Program (105-2917-I-564-046) sponsored by Ministry of Science and Technology (Taiwan). 2195
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We thank Prof. Justin M. Notestein and his lab for the use of the diffuse reflectance UV−vis spectroscopy. This work made use of the IMSERC, J. B. Cohen X-ray Diffraction, EPIC, and KECK II facilities of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN.
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