Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Vanadium Catalyst on Isostructural Transition Metal, Lanthanide, and Actinide Based Metal−Organic Frameworks for Alcohol Oxidation Xingjie Wang,†,‡ Xuan Zhang,‡ Peng Li,⊥,‡ Ken-ichi Otake,‡ Yuexing Cui,‡ Jiafei Lyu,‡ Matthew D. Krzyaniak,‡ Yuanyuan Zhang,‡ Zhanyong Li,‡ Jian Liu,‡ Cassandra T. Buru,‡ Timur Islamoglu,‡ Michael R. Wasielewski,‡ Zhong Li,† and Omar K. Farha*,‡,§ Downloaded via UNIV OF WISCONSIN-MADISON on May 14, 2019 at 13:27:37 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, P. R. China International Institute of Nanotechnology and Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ⊥ Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, 2005 Songhu Road, Shanghai 200438, China § Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ‡
S Supporting Information *
ABSTRACT: The understanding of the catalyst−support interactions has been an important challenge in heterogeneous catalysis since the supports can play a vital role in controlling the properties of the active species and hence their catalytic performance. Herein, a series of isostructural mesoporous metal−organic frameworks (MOFs) based on transition metals, lanthanides, and actinides (Zr, Hf, Ce, Th) were investigated as supports for a vanadium catalyst. The vanadium species was coordinated to the oxo groups of the MOF node in a single-ion fashion, as determined by single-crystal X-ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy, and diffuse reflectance UV−vis spectroscopy. The support effects of these isostructural MOFs were then probed using the aerobic oxidation of 4-methoxybenzyl alcohol as a model reaction. The turnover frequency was found to be correlated with the electronegativity and oxidation state of the metal cations on the supporting MOF nodes, highlighting an important consideration when designing catalyst supports.
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INTRODUCTION Supported heterogeneous catalysts have been extensively used in various industrially important processes, such as hydrogenation,1 hydroformylation,2 and oxidation.3 Solid supports play significant roles in impacting the oxidation states, binding modes, and distribution of the active metals, thus affecting the catalytic activity and selectivity,4 collectively known as “support ef fects”. As such, the study of support effects for heterogeneous catalysis has gained ever-increasing attention.5,6 However, most of the conventional supports such as metal oxides4a,5,7 and activated carbon8 suffer from inhomogeneous catalyst binding sites and hence ill-defined catalyst structure.9 Consequently, detailed understanding of the support effects on certain metal catalyst remains a challenge due to the difficulties in the unambiguous determination of the active species structures.10 Recently, metal−organic frameworks (MOFs), a class of crystalline porous materials constructed from organic linkers and inorganic nodes which can be rationally designed and constructed,11 have drawn much attention as supporting materials in heterogeneous catalysis.12 Compared to conven© XXXX American Chemical Society
tional supports, the atomically ordered structure of the MOFs allows for the precise structural characterization and detailed study of the correlations between the supports and the installed catalytic active sites.13 The high accessible surface area, variable functional groups, and the uniform porosity make MOFs ideal supports for the investigation of structure−property relationships in heterogeneous catalysis.13a,14 Furthermore, the catalytic sites can be spatially isolated in a MOF to prevent agglomeration and deactivation during the catalytic reaction.15 More importantly, the unique reticular chemistry allows for realization of isostructural MOFs based on various metal ions, a feature that is highly desirable for the systematic investigation of support effects.16 MOFs based on Zr6O8 nodes (Figure 1) have been widely studied as supports in both gas-phase12b,17 and liquidphase18 heterogeneous catalysis owing to their high thermal and chemical stability.19 More recently, efforts have been devoted to the synthesis of MOFs based on hexanuclear Hf(IV), Ce(IV), Received: March 8, 2019
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DOI: 10.1021/jacs.9b02603 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Journal of the American Chemical Society
Figure 1. Structure of M-NU-1200 with the four 8-connected M6 nodes and the tritopic linker TMTB. The terminal hydroxyl groups are pointing toward the center of the mesopores.
Figure 2. (a) PXRD patterns and (b) volumetric N2 isotherm of isostructural M-NU-1200; DRIFT spectra of Zr-NU-1200 and V-ZrNU-1200 showing (c) the decreasing hydroxyl groups on the Zr node and the appearance of VO−H upon V loading, and (d) the VO, Zr− O−V stretching regions of the materials.
and Th(IV) oxide cluster nodes,16,20 which are isostructural to the Zr(IV) oxide cluster nodes. Therefore, support effects on metal catalysts can be systematically investigated using a series of these stable MOFs based on these tetravalent hexanuclear metal cluster nodes. Herein, a series of isostructural mesoporous MOFs, M-NU1200 (M = Zr, Hf, Ce, and Th), have been synthesized and employed as supporting materials for a vanadium catalyst (Figure 1). Their catalytic activity for the model reaction, namely alcohol oxidation, was found to be correlated with the electronegativity and oxidation state of the metal cations of the supporting MOFs.
Vanadium was chosen as the active metal due to its multivalence and high catalytic activity in oxidation reactions.18a,23 Vanadyl acetylacetonate (V(O)-(acac)2) was used as the metal precursor to prepare the NU-1200-supported catalysts (denoted as V-M-NU-1200 (M = Zr, Hf, Ce, Th)) by solvothermal deposition in MOFs (SIM) under similar conditions (see details in Methods). The vanadium loadings were controlled to be around 1 vanadium atom per node, as confirmed by the inductively coupled plasma optical emission spectroscopy (ICP-OES) (Table S4). Scanning electron microscope (SEM) images exhibited no morphological change, while the energy dispersive spectroscopy (EDS) line scans confirmed uniform V distribution through the crystals (Figure S5). The structural integrity of V-M-NU-1200 was further confirmed by PXRD (Figure S6), N2 isotherms (Figure S7), and thermogravimetric analysis (TGA) (Figure S8), as their crystallinity, porosity, and thermal stability were retained after the vanadium deposition. NMR signals for formate and trifluoroacetate disappeared after V-SIM (Figure S3b and S4b), concomitant with appearance of 1 acetylacetonate (acac) ligand per node (Figure S3c). After vanadium deposition, density functional theory (DFT) pore size distribution (PSD) curves revealed that the 1.4 nm micropore stayed the same while the 2.2 nm mesopore displayed an obvious decrease in incremental pore volume, indicating that the vanadyl species were mainly located in the mesopores (Figure S9). To get a better understanding of the structure of the active species, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of the MOFs and the catalysts was carried out (Figure 2c, 2d and S10), and the peaks at 3673 cm−1 (Zr-NU-1200), 3678 cm−1 (Hf-NU-1200), 3646 cm−1 (CeNU-1200), and 3654 cm−1 (Th-NU-1200) represented the non-hydrogen-bonded −OH stretches on the nodes.24 The peak intensity decreased as vanadium was deposited onto the MOFs, manifesting that the vanadium species were chemically bonded with the M6 nodes through oxo-bridges. The peaks around 3653 cm−1 (Figure 2c) and 1010 cm−1 for V-Zr-NU-1200 (Figure 2d) can be assigned to VO−H and VO stretches, respectively, while the peak around 900 cm−1 was related to the V−O− M(Zr/Hf) vibration band.22 Similar peaks can also be found in
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RESULTS AND DISCUSSION Zr-NU-1200 was synthesized according to the reported procedure with some modifications (see Methods for details).21 The MOF is comprised of 8-connected Zr6 nodes and tricarboxylic acid linkers (4,4′,4″-(2,4,6-trimethylbenzene1,3,5-triyl) tribenzoic acid, TMTB), which crystallizes in the topology and possesses mesoporous channels of 2.2 nm as well as 1.4 nm cages. Note, all terminal hydroxyl groups, which will serve as isolated grafting sites for metal species on the nodes,22 face into the mesopores (Figure 1). The detailed synthesis of three new MOFs, Hf-NU-1200, CeNU-1200, and Th-NU-1200, are described in Methods. All the M-NU-1200 were synthesized using trifluoroacetic acid (TFA) as the modulator to minimize the structural differences, and the powder X-ray diffraction (PXRD) patterns matched well with the simulated patterns (Figure 2a). The N2 adsorption− desorption measurements of M-NU-1200 (Figure S1a) indicated a structure comprised of both 1.4 nm micropores and 2.2 nm mesopores (Figure S1b). The normalized Brunauer−Emmett−Teller (BET) volumetric areas and total pore volumes for these four isostructural MOFs are almost identical (Figure 2b and S2c, Table S3). To minimize the effect of the diffusion on the initial rate of the catalysts, the particle sizes of the four M-NU-1200 were controlled to be around 1 μm with Zr-NU-1200, Hf-NU-1200, and Th-NU-1200 showing cubic shapes while Ce-NU-1200 exhibits irregular morphology likely due to fast precipitation of this MOF (Figure S2). The amounts of coordinated modulator on the nodes in M-NU-1200 were determined by 1H (Figure S3a) and 19F NMR spectroscopy (Figure S4a). B
DOI: 10.1021/jacs.9b02603 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Journal of the American Chemical Society the other V-M-NU-1200 materials (Figure S10). Raman spectra of the catalysts were also attempted, but the peak around 993 cm−1 belonging to the VO moiety was absent, as well as the peak at 200−300 cm−1 belonging to the V−O−V bond, which may be due to the low vanadium loading (Figure S11).25 Moving forward, diffuse reflectance UV−vis spectrum (DRUV vis) of each sample was then obtained (Figure S13), and the edge energies for these four catalysts were calculated to be 3.51, 3.54, 3.66, and 3.64 eV for V-Zr-NU-1200, V-Hf-NU1200, V-Ce-NU-1200, and V-Th-NU-1200, respectively (Figure S14), which suggest that the vanadium structures on these MOFs are mainly monomeric VOx species.26 The oxidation state of the vanadium in all the catalysts was confirmed to be V5+ by X-ray photoelectron spectroscopy (XPS), indicating that the V4+ precursor was oxidized during the SIM process (Figure S15). Further structural insight of the vanadium species was explored by single-crystal X-ray diffraction (SCXRD) analyses. V-Zr-NU-1200 and V-Hf-NU-1200 single crystal structures were obtained as shown in Figures 3 and S16. Both crystals
Scheme 1. Reaction Scheme for the Oxidation of 4Methoxybenzyl Alcohol
V-Zr-NU-1200 had the highest conversion (78%) after 5 h, followed by V-Th-NU-1200 (58%) and V-Hf-NU-1200 (51%), with V-Ce-NU-1200 showing the lowest performance of only 7% (Figure 4a). Using V-Zr-NU-1200, the reaction reaches
Figure 3. Crystal structures of (a) V-Zr-NU-1200 and (b) V-Hf-NU1200, showing the vanadium binding motif onto the node. For clarity, the hydrogen atoms are omitted.
showed the same vanadium binding mode with single-ion vanadium species bridging two of the oxo groups on the nodes. The V atom is coordinated in a tetragonal pyramidal geometry to four singly bonded oxygen atoms (two from the node and two pointing to the mesopore) and a doubly bonded O1 (O1′ being the positional disorder of O1). O2 and O3 have the same bond length toward V at 2.124 Å, therefore, corresponding to V−OH and V−OH2 bonds, respectively. For V-Hf-NU-1200 (Figure 3b), two O1 positional disorder was also found while the two oxygen atoms pointing toward the mesopore were not located due to disorder. Acac ligands were also observed coordinated to the node in a chelating mode with around one per node which is consistent with the NMR result (Figure S3c). Note, in line with our PSD curve (Figure S9), the postsynthetically installed groups were mainly located in the mesopore. As vanadium can exist in multiple oxidation states and is highly active in oxidation reactions, 4-methoxybenzyl alcohol oxidation was chosen as a model reaction to compare the support’s effect (Scheme 1). With 1 atm of oxygen as the oxidant and triethylamine as a basic additive, 4-methoxybenzyl alcohol can be selectively oxidized to 4-methoxybenzyl aldehyde in toluene at 105 °C without overoxidation to the carboxylic acid. Catalyst amounts were normalized to the number of vanadium active sites.
Figure 4. Reaction conversion comparison with one standard deviation for all V-M-NU-1200 (a) in 5 h with the inset showing the reaction profiles in the first 30 min. (b) Recycling catalytic performances on VM-NU-1200.
100% conversion in 11 h (Figure S17). V-Hf-NU-1200 showed similar initial conversion (∼9.0% at 30 min) as V-Th-NU-1200 (Figure 4a), while V-Zr-NU-1200 still possessed the highest conversion (∼13%). Blank controls with M-NU-1200 showed negligible activity after 5 h of reaction. The reaction was proven to be heterogeneous in nature by (1) ICP-OES, where there was no detectable V signal in the supernatant postcatalysis (Table S3), and (2) leaching test, where no substrate conversion occurred after the catalyst was filtered out (see Methods for details). The stability of the materials was further tested by recycling the catalysts and running the same reaction 3 times. The decrease in substrate conversion from run to run (Figure S18) can be attributed to sample loss during washes between C
DOI: 10.1021/jacs.9b02603 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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small amount of paramagnetic vanadium was present in the fresh V-Zr-NU-1200 while there was no signal in the fresh V-Ce-NU1200 (the signal was assigned to be V4+), indicating that there are mainly +5 vanadium in these MOFs (the rather low content of V4+ in V-Zr-NU-1200 is hard to deconvolute from XPS). The oxidation of V4+ to V5+ during the SIM for V-Ce-NU-1200 resulted in the reduction of Ce4+ to Ce3+; Ce XPS results confirmed that there was around 51% of Ce3+ forming after vanadium deposition (Figure S19). However, attempts to observe the Ce3+ from EPR at room temperature and 10 K did not work well, likely due to fast spin relaxation times and the mixed valence character of the Ce nodes in a mixture of +3 and +4 oxidation states. Given that the Ce ions are only bridged through oxygen bonds, the spin−spin coupling between paramagnetic centers would be strong, and the spins are likely delocalized across the cluster, leading to enhanced spin relaxation, potential line broadening, and ill-defined or resolved EPR spectrum. After the oxidation reaction proceeded for 5 h, the V-Zr-NU-1200 exhibited much more pronounced V4+ EPR signal compared to fresh V-Zr-NU-1200 (Figure 5e), indicating the presence of more V4+ species during the reaction was catalyzed by V-Zr-NU-1200. In contrast, Ce-NU-1200 supported vanadium species showed weak EPR signal (Figure 5f), which is consistent with the XPS results showing that no V4+ appeared even after 5 h of reaction. EPR spectra obtained at 2 and 5 h showed no obvious increase in the amount of V4+ on VZr-NU-1200 as the reaction proceeds further, while only a tiny peak appeared on V-Ce-NU-1200, showing the ratio of V5+/V4+ can reach an equilibrium state during the reaction (Figures S20 and S21). A control experiment was carried out under an inert atmosphere (N2) with V-Zr-NU-1200 as catalyst. The material after reaction was collected and saved in a glovebox after N2 purging to dryness. The vanadium XPS of this catalyst showed there was a mixture of V4+/V5+ on the catalyst (Figure S22a and S22b). Since different mechanisms were proposed in the literature for vanadium-catalyzed alcohol oxidation in the presence or absence of an organic base,7,27 a control reaction with V-ZrNU-1200 was also carried out without using triethylamine to elucidate the role of the base in our system. A conversion of ∼30% was observed after 3 h which is lower than the conversion in the presence of triethylamine (ca. ∼55%). This indicated that the oxidation can be catalyzed by V-Zr-NU-1200 without a base, but the base can promote the catalytic performance. XPS of the catalyst used under base-free conditions showed ∼40% V4+ (Figure S22c). For the base-free mechanism (Figure S24a), it is proposed that a hydrogen atom from the hydroxyl group will be transferred to the VO bond, forming a HO−V(d1) and 4methoxybenzyl alcohol coordinated complex, while the HO− V(d1) will combine with one H+ from the methylene to desorb a H2O, forming V(d2) that featured an oxygen vacancy and releasing 4-methoxybenzyl aldehyde. The catalytic cycle is finally completed by a reoxidation step while one more substrate will take part in and produce one more product (Figures S23 and S24). Since obtaining a hydrogen from the substrate is the ratedetermining redox step in the whole reaction,7 the metal nodes with stronger electron-withdrawing ability allow the VO on the catalysts to more easily initiate the reaction, resulting in a higher initial rate. In the base-assisted mechanism (Figure S24b), the hydroxyl group on the V−OH was first substituted by a substrate to form a coordinated benzyloxide. The base, triethylamine, then abstracted a hydrogen at the benzylic position and a V(d 2 ) intermediate was formed. After
cycles rather than catalyst deactivation, as the TOFs of all these reactions remained constant (Figure 4b). Further insights into the support effects of M-NU-1200 were investigated by using V-Zr-NU-1200 and V-Ce-NU-1200 as examples since they had the most extreme difference in catalytic performance. XPS and electron paramagnetic resonance (EPR) spectra of these catalysts (Figure 5) were collected prior to and after 5 h of reaction without exposure to any oxidants (i.e., air/ O2) (see detailed information in Methods).
Figure 5. V-XPS spectra of (a) fresh V-Zr-NU-1200, (b) V-Zr-NU1200 after 5 h reaction, (c) fresh V-Ce-NU-1200, and (d) V-Ce-NU1200 after 5 h reaction. EPR spectra of (e) V-Zr-NU-1200 and (f) VCe-NU-1200 before and after catalysis (without being exposed to air).
Vanadium 2p XPS spectra showed the binding energies were slightly lower for V-Zr-NU-1200 than for V-Ce-NU-1200 (Table S5). The higher oxygen binding energy of V-Zr-NU1200 (∼530.9 eV) than that of V-Ce-NU-1200 (∼530.4 eV) indicated that the VO oxygen on Zr-NU-1200 will have a stronger electron-withdrawing ability than that on Ce-NU1200. This result is further supported by the XPS and EPR data collected after the reaction but before being exposed to air/ oxygen. The oxidation state of vanadium in V-Zr-NU-1200 changed from +5 to a combination of +5 and +4, as the binding energy of ∼523.2 (2p 3/2) and ∼516.4 (2p 1/2) eV matched well with the reference oxidation state of V4+ (Figure 5a and 5b); the V4+ amount was estimated to be 48%. This result showed that a large amount of the V5+ active sites on V-Zr-NU-1200 was reduced during the reaction,7 which is in accordance with the good catalytic performance of the V-Zr-NU-1200. However, only V5+ was found on V-Ce-NU-1200 (Figure 5c and 5d), corresponding to the low catalytic activity of this material. The XPS binding energy of the vanadium (Tables S5 and S6) shows that the values for V 2p band were identical regardless of the oxidation states. Continuous wave EPR spectra under similar conditions are consistent with XPS data (Figure 5e and 5f). A D
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the identity of the supports has a major impact on the activity of the catalyst. Postcatalysis characterization indicated that the NU-1200 based vanadium catalysts are stable under the reported reaction conditions. PXRD patterns showed that the crystallinity was retained (Figure S25). The N2 isotherms were collected and BET areas exhibited only minor decreases (Figure 7), while the
restructuring, one 4-methoxybenzyl aldehyde will be released, together with TEA. The cycle was then also completed by a reoxidation step. For this catalytic cycle, the deprotonation at the benzylic position is reported to be the rate-determining step,27d which suggested that with a stronger withdrawing ability of the metal nodes, V5+ will more readily obtain electrons from oxygen on the aldehyde and get reduced to V3+, resulting in the release of the coordinated benzaldehyde. Overall, the electronegativities of the metals or cations were then assumed to have a relationship with the catalytic performance, to be specific, the TOFs, which were calculated to be 3.86 ± 0.14 (V-Zr-NU-1200), 2.5 ± 0.09 (V-Hf-NU1200), 2.44 ± 0.13 (V-Th-NU-1200), and 0.2 ± 0.04 h−1 (VCe-NU-1200) in the first half hour. As the Mulliken electronegativity scale is the combination of ionization energy and electron affinity, the electronegativity of four metal ions by Mulliken scale was calculated and the relationship between the catalytic performance and electronegativity was then plotted in Figure 6 (the fitting line shown was fitted with Zr4+, Hf4+ and
Figure 7. N2 isotherms of M-NU-1200 and V-M-NU-1200 before and after catalysis.
mesopores of the catalysts were also maintained (Figure S26). XPS showed that the oxidation state for the vanadium was still mainly +5 after the catalysis (Figure S27 and Table S8) and the oxidation state of the supporting metals were unchanged during the process (Figure S28), further proving the good recycling ability of all these catalysts.
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CONCLUSIONS In summary, four isostructural mesoporous MOFs M-NU-1200 (M = Zr, Hf, Ce, Th) were synthesized and used as supports for the systematic investigation of their effect on vanadium catalysts, using 4-methoxybenzyl alcohol oxidation as a model reaction. Our results showed that the supports are of great importance in the catalysis, as V-Zr-NU-1200 showed ∼20 times higher TOF than V-Ce-NU-1200. The catalytic activities are correlated to the electronegativity of the support metals, while the oxidation state of the metal also showed significant effect on the performance. This work provides rational insights into the MOF-based support effects and will pave the way for the design and synthesis of supported heterogeneous catalysts with controlled activities.
Figure 6. Relationship between the catalytic performance and electronegativity of the metal ions in the supporting material. The fitting line was based on only Zr4+, Hf4+, and Th4+, while the scales for Ce3+, Ce4+, and the mixture of Ce3+/Ce4+ were shown as well to make an arbitrary comparison since a catalyst with pure Ce3+ or Ce4+ cannot be obtained.
Th4+ since a catalyst with pure Ce3+ or Ce4+ cannot be obtained).25e,28 The plot exhibited a rough linear relationship, suggesting that the electronegativity of the metals on the nodes may influence the catalytic performance of the MOF-supported catalysts. Detailed values of the electronegativity were listed in Table S7, and the results indicated that a better catalytic performance is correlated with a higher electronegativity of the metal ions on the node. However, for V-Ce-NU-1200, due to the mixed valence of +3 and +4 in the Ce6 node which possesses different electronegativity, reporting either of the electronegativity in the fitting line will not be representative of what the system is actually composed of. Since both oxidation states can contribute to the performance, both should be considered in the discussion. Therefore, an average electronegativity value was arbitrarily calculated based on the ratio of Ce3+ and Ce4+ (based on XPS results) and included in the figure just for comparison. These findings support that the electronegativity as well as the oxidation state of the metals on the nodes can influence the catalytic performance of the MOF-supported catalysts. Hence,
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METHODS
NU-1200 Powder and Single Crystal Synthesis. Zr-NU-1200 Powder. In a 4 mL vial, TMTB linker (0.02 mmol, 10 mg) was dissolved in 2 mL of DMF and then 17 mg of ZrOCl2·8H2O (0.07 mmol) were added into the solution followed by sonicating for 10 min. Afterward, 150 μL of TFA, as the modulator, was added into the resulting solution. The solution was then placed in the 120 °C oven for 24 h. After the vials cool down to room temperature, MOF crystals were isolated by centrifuge and then DMF was used to wash the crystals (2 × 12 mL soaking for 10 min each time). And then the material was transferred to a solution of 12 mL of DMF and 500 μL of 4 M HCl. The solution was then placed in the 100 °C oven for 24 h to remove the residual modulator. Afterward, DMF was used to wash the material twice every 10 min. Acetone was used to exchange the DMF once, and then the material was soaked in acetone overnight; acetone was used to E
DOI: 10.1021/jacs.9b02603 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Journal of the American Chemical Society wash the material 3 times. After the solvent was poured out, the material was dried at 80 °C in a vacuum oven for 2 h to give ∼80% product based on the linker. Zr-NU-1200 Single Crystal. The procedure for the crystal synthesis was similar to the powder synthetic method but with 16 mg of ZrCl4 (0.07 mmol) and 400 μL of TFA in DEF in the 120 °C oven for 3 days. V-Zr-NU-1200 Single Crystal. The Zr-NU-1200 crystal was washed with DMF (2 × 12 mL soaking for 10 min each time). And then the crystal was transferred to a solution of 12 mL of DMF and 500 μL of 4 M HCl. The solution was then placed in the 100 °C oven for 24 h to remove the residual modulator. Afterward, DMF was used to wash the crystal twice. Methanol was used to exchange the DMF (3 × 12 mL soaking for 10 min each time), and then vanadyl acetylacetonate (around 4 V/M6 compared to the crystal amount) was dissolved into 2 mL of methanol which was then added into the crystal. The mixture was put in the 60 °C oven for 2 days to yield the V-Zr-NU-1200 single crystal. Hf-NU-1200 Powder. In a 4 mL vial, TMTB linker (0.02 mmol, 10 mg) was dissolved in 2 mL of DMF and then 17 mg of HfCl4 (0.05 mmol) was added into the solution with sonication for 10 min. Afterward, 300 μL of TFA were added into the mixture. The solution was then put in the 120 °C oven for 24 h. The purification method for the Hf-NU-1200 was the same as that for the Zr-NU-1200 and gives ∼90% yield. Hf-NU-1200 Single Crystal. The synthetic procedure was similar to the powder synthetic method but with 17 mg of HfOCl2·4H2O and 150 μL of TFA in DEF in the 120 °C oven for 2 days. V-Hf-NU-1200 Single Crystal. The Hf-NU-1200 crystal was washed with DMF (2 × 12 mL soaking for 10 min each time). And then the crystal was transferred to a solution of 12 mL of DMF and 500 μL of 4 M HCl. The solution was then placed in the 100 °C oven for 24 h to remove the residual modulator. Afterward, DMF was used to wash the crystal twice. Methanol was used to exchange the DMF (3 × 12 mL soaking for 10 min each time), and then vanadyl acetylacetonate (around 4 V/per node compared to the crystal amount) was dissolved into 2 mL of methanol which was then added into the crystal. The mixture was put in the 60 °C oven for 2 days to yield the V-Hf-NU1200 single crystal. Ce-NU-1200 Powder. In a 4 mL vial, TMTB linker (0.1 mmol, 50.9 mg) was added into 1.2 mL of DEF, and then 600 μL of 0.533 M Ce(NH4)2(NO3)6 solution were added into the mixture with sonication for 10 min. Afterward, 40 μL TFA were added into the mixture. The solution was then put in the 100 °C heating block and stirred for 15 min. When the vials cooled down, the solution was centrifuged and the upper clear solvent was poured out; DMF was used to wash the material several times, and acetone was used to exchange the DMF. The material was soaked in the acetone overnight; acetone was then used to wash completely for 3 times. After the solvent was poured out, the material was dried at 80 °C in a vacuum oven for 2 h to give ∼50% product based on the linker. Th-NU-1200 Powder. Two solutions were made before synthesis. Solution A was made by dissolving 1 g of thorium nitrate (1.8 mmol) in 20 mL of DMF. Solution B was made by dissolving 100 mg of linker (0.2 mmol) in 10 mL of DMF. Afterward, 400 μL of solution A were combined with 400 μL of solution B, followed by addition of 80 μL of H2O and 20 μL of TFA. Finally, the solution was put into a 120 °C oven for 24 h. When the vials cooled down, the solution was centrifuged and the upper clear solvent was pour out; DMF was used to wash the material several times, and acetone was used to exchange the DMF. The material was soaked in the acetone overnight, and then acetone was used to wash totally for 3 times. After the solvent was poured, the material was dried at 80 °C in a vacuum oven for 2 h to give ∼60% product based on the linker. Catalysts Synthesis. M-NU-1200 and vanadyl acetylacetonate were added into a 4 mL vial with 2 mL of methanol; the amount of the vanadyl acetylacetonate used was controlled to be 4 V/M6 node (that is 1 mmol of M-NU-1200, 4 mmol precursor were needed). The mixture was then sonicated for 10 min to totally dissolve the precursor. Afterward, the mixture was put in a 60 °C oven for different times to control the loading amount on every MOF to be similar. In this work,
The SIM time is 6 h for the Zr-NU-1200, 24 h for the Hf-NU-1200, 10 h for the Ce-NU-1200, and 30 h for the Th-NU-1200. Afterward, hot methanol was used to wash the catalysts until the solution is clear and the acetone was used to wash the catalysts several times. Finally, the powder was dried in an 80 °C vacuum oven for 2 h to give a yellow powder. All catalysts need to be activated under vacuum at 120 °C (100 °C for Ce-NU-1200) for at least 8 h before use for catalysis. Oxidation of 4-Methoxybenzyl Alcohol. The catalysis procedure was similar to the literature with some modification.18 4Methoxybenzyl alcohol (38 μL, 0.3 mmol), 750 μL of anhydrous toluene, 5 μL of triethylamine, 28 μL of 1,2-dichlorobenzne (0.25 mmol), and catalysts (2.2 × 10−5 mol V) were added into a 2−5 mL Biotage microwave process vial; to be specific, the mass of the V-ZrNU-1200, V-Hf-NU-1200, V-Ce-NU-1200, and V-Th-NU-1200 were calculated to be ∼46.8, 58.3, 53.2, and 65.4 mg, respectively. The mixture was purged with O2 for 20 min, and then an O2 balloon was connected to the vial. The mixture was then put in the 105 °C oil bath for catalysis for 5 h. The reaction conversions were monitored by measuring the components of aliquots taken at given times by GC-FID. Blank reactions were also carried out with bare MOFs without the V loading under the same conditions. To obtain a better understanding of the catalytic performance, a full conversion of the reaction was also run by using V-Zr-NU-1200. Air-Free Catalysts Collection. To understand the underlying mechanism of the reaction, catalysts collected after 2 and 5 h of reaction but before getting oxidized were used to do further characterization. The procedure was as follow: The reaction was run by the same procedure as above but stopped at 2 or 5 h. After running for 2 or 5 h, the reaction was stopped by cooling in an ice bath with purging N2 into the microwave vial to get rid of the oxygen. Then N2 was used to continuously flow into the vial under a high temperature overnight so as to gradually evaporate the solvents. Afterward, the catalysts were saved in the glovebox which was filled with Ar for future characterization. Leaching Test. For the leaching experiment, after the first reaction was carried out (5 h), the supernatant was filtered from the catalysts and then the supernatant was used to do a longer time reaction for about 2 h; no higher conversion was observed. Stability Test. For the stability test, after the first reaction (5 h), the MOF catalysts were washed multiple times with acetone. And then the materials were activated again under a 120 °C vacuum for 12 h. Afterward, a second reaction was then performed under the same conditions. The third time also followed the same procedure.
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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/jacs.9b02603. Additional materials synthesis and characterization data, including N2 isotherms, powder X-ray diffraction patterns, SEM-EDS, XPS, DRIFTs, Raman, DRUV vis, TGA, etc. (PDF) Crystallographic data for Hf-NU-1200 (CIF) Crystallographic data for V-Zr-NU-1200 (CIF) Crystallographic data for V-Hf-NU-1200 (CIF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Xingjie Wang: 0000-0002-5802-9944 Xuan Zhang: 0000-0001-8214-7265 Peng Li: 0000-0002-4273-4577 Ken-ichi Otake: 0000-0002-7904-5003 Jiafei Lyu: 0000-0002-0842-7151 Matthew D. Krzyaniak: 0000-0002-8761-7323 F
DOI: 10.1021/jacs.9b02603 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Journal of the American Chemical Society
S. Highly Efficient Heterogeneous Hydroformylation over RhMetalated Porous Organic Polymers: Synergistic Effect of High Ligand Concentration and Flexible Framework. J. Am. Chem. Soc. 2015, 137 (15), 5204−9. (3) (a) Cordoba, M.; Miranda, C.; Lederhos, C.; Coloma-Pascual, F.; Ardila, A.; Fuentes, G.; Pouilloux, Y.; Ramírez, A. Catalytic Performance of Co3O4 on Different Activated Carbon Supports in the Benzyl Alcohol Oxidation. Catalysts 2017, 7 (12), 384. (b) Ye, J. Y.; Liu, C. J. Cu3(BTC)2: CO oxidation over MOF based catalysts. Chem. Commun. 2011, 47 (7), 2167−9. (4) (a) Olthof, B.; Khodakov, A.; Bell, A. T.; Iglesia, E. Effects of Support Composition and Pretreatment Conditions on the Structure of Vanadia Dispersed on SiO2, Al2O3, TiO2, ZrO2, and HfO2. J. Phys. Chem. B 2000, 104 (7), 1516−1528. (b) Comotti, M.; Li, W.-C.; Spliethoff, B.; Schüth, F. Support Effect in High Activity Gold Catalysts for CO Oxidation. J. Am. Chem. Soc. 2006, 128 (3), 917−924. (5) Kon, K.; Onodera, W.; Toyao, T.; Shimizu, K.-i. Supported rhenium nanoparticle catalysts for acceptorless dehydrogenation of alcohols: structure−activity relationship and mechanistic studies. Catal. Sci. Technol. 2016, 6 (15), 5864−5870. (6) Dinse, A.; Frank, B.; Hess, C.; Habel, D.; Schomäcker, R. Oxidative dehydrogenation of propane over low-loaded vanadia catalysts: Impact of the support material on kinetics and selectivity. J. Mol. Catal. A: Chem. 2008, 289 (1−2), 28−37. (7) Kropp, T.; Paier, J.; Sauer, J. Support effect in oxide catalysis: methanol oxidation on vanadia/ceria. J. Am. Chem. Soc. 2014, 136 (41), 14616−25. (8) (a) Aksoylu, A. E.; Madalena, M.; Freitas, A.; Pereira, M. F. R.; Figueiredo, J. L. The effects of different activated carbon supports and support modifications on the properties of Pt/AC catalysts. Carbon 2001, 39 (2), 175−185. (b) Sun, Y.; Zhang, G.; Liu, J.; Zhao, P.; Hou, P.; Xu, Y.; Zhang, R. Effect of different activated carbon support on CH4/CO2 reforming over Co-based catalysts. Int. J. Hydrogen Energy 2018, 43 (3), 1497−1507. (9) (a) Sokolov, S.; Bychkov, V. Y.; Stoyanova, M.; Rodemerck, U.; Bentrup, U.; Linke, D.; Tyulenin, Y. P.; Korchak, V. N.; Kondratenko, E. V. Effect of VOxSpecies and Support on Coke Formation and Catalyst Stability in Nonoxidative Propane Dehydrogenation. ChemCatChem 2015, 7 (11), 1691−1700. (b) Baldychev, I.; Gorte, R. J.; Vohs, J. M. The impact of redox properties on the reactivity of V2O5/ Al2O3 catalysts. J. Catal. 2010, 269 (2), 397−403. (c) Henrich, V. E. Metal-oxide surfaces. Prog. Surf. Sci. 1995, 50 (1), 77−90. (10) (a) Babucci, M.; Fang, C.-Y.; Perez-Aguilar, J. E.; Hoffman, A. S.; Boubnov, A.; Guan, E.; Bare, S. R.; Gates, B. C.; Uzun, A. Controlling catalytic activity and selectivity for partial hydrogenation by tuning the environment around active sites in iridium complexes bonded to supports. Chem. Sci. 2019, 10, 2623. (b) Bainbridge, M.; Clarkson, J. S.; Parnham, B. L.; Tabatabaei, J.; Tyers, D. V.; Waugh, K. C. Evidence for support effects in metal oxide supported cobalt catalysts. Catal. Struct. React. 2017, 3 (3), 128−137. (c) Gates, B. C.; FlytzaniStephanopoulos, M.; Dixon, D. A.; Katz, A. Atomically dispersed supported metal catalysts: perspectives and suggestions for future research. Catal. Sci. Technol. 2017, 7 (19), 4259−4275. (d) Yokota, S.; Okumura, K.; Niwa, M. Support Effect of Metal Oxide on Rh Catalysts in the CH4-CO2 Reforming Reaction. Catal. Lett. 2002, 84 (1), 131− 134. (e) Yang, D.; Gates, B. C. Catalysis by Metal Organic Frameworks: Perspective and Suggestions for Future Research. ACS Catal. 2019, 9 (3), 1779−1798. (11) (a) Chen, Z.; Weselinski, L. J.; Adil, K.; Belmabkhout, Y.; Shkurenko, A.; Jiang, H.; Bhatt, P. M.; Guillerm, V.; Dauzon, E.; Xue, D. X.; O’Keeffe, M.; Eddaoudi, M. Applying the Power of Reticular Chemistry to Finding the Missing alb-MOF Platform Based on the (6,12)-Coordinated Edge-Transitive Net. J. Am. Chem. Soc. 2017, 139 (8), 3265−3274. (b) Jiang, H.; Jia, J.; Shkurenko, A.; Chen, Z.; Adil, K.; Belmabkhout, Y.; Weselinski, L. J.; Assen, A. H.; Xue, D. X.; O’Keeffe, M.; Eddaoudi, M. Enriching the Reticular Chemistry Repertoire: Merged Nets Approach for the Rational Design of Intricate MixedLinker Metal-Organic Framework Platforms. J. Am. Chem. Soc. 2018, 140 (28), 8858−8867. (c) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.;
Yuanyuan Zhang: 0000-0003-4285-2853 Zhanyong Li: 0000-0002-3230-5955 Jian Liu: 0000-0002-5024-1879 Cassandra T. Buru: 0000-0001-6142-8252 Timur Islamoglu: 0000-0003-3688-9158 Michael R. Wasielewski: 0000-0003-2920-5440 Zhong Li: 0000-0001-6354-883X Omar K. Farha: 0000-0002-9904-9845 Notes
The authors declare no competing financial interest. Crystallographic data in CIF format have been deposited in the Cambridge Crystallographic Data Centre (CCDC) under deposition number CCDC-1876021 (Hf-NU-1200), 1899941 (V-Zr-NU-1200), and 1899942 (V-Hf-NU-1200). The data can be obtained free of charge via www.ccdc.cam.ac.uk/data_ request/cif (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.).
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ACKNOWLEDGMENTS This work was supported as part of the Inorganometallic Catalyst Design Center, an EFRC funded by the DOE, Office of Science, Basic Energy Sciences (DE-SC0012702). EPR spectroscopy (M.D.K. and M.R.W.) was supported as part of the Center for Light Energy Activated Redox Processes, an EFRC funded by the DOE, Office of Science, Basic Energy Sciences (DE-SC0001059). X.W. gratefully acknowledges support from China Scholarship Council (CSC) during his visit to Northwestern University (201706150062). This work made use of the J. B. Cohen X-ray Diffraction Facility supported by the MRSEC program of the National Science Foundation (DMR-1720139) at the Materials Research Center of Northwestern University. This work made use of Keck-II facilities of the NUANCE Center at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205); the MRSEC program (NSF DMR-1720139) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. This work made use of the IMSERC at Northwestern University, which has received support from the NSF (CHE-1048773 and DMR0521267); SHyNE Resource (NSF NNCI-1542205); and the State of Illinois and IIN.
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