Fine-Tuning the Activity of Metal–Organic Framework-Supported

Oct 4, 2017 - Few-atom cobalt-oxide clusters, when dispersed on a Zr-based metal–organic framework (MOF) NU-1000, have been shown to be active for ...
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Article Cite This: J. Am. Chem. Soc. 2017, 139, 15251-15258

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Fine-Tuning the Activity of Metal−Organic Framework-Supported Cobalt Catalysts for the Oxidative Dehydrogenation of Propane Zhanyong Li,† Aaron W. Peters,† Ana E. Platero-Prats,‡ Jian Liu,† Chung-Wei Kung,† Hyunho Noh,† Matthew R. DeStefano,† Neil M. Schweitzer,§ Karena W. Chapman,‡ Joseph T. Hupp,*,† and Omar K. Farha*,†,∥ †

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439-4858, United States § Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ∥ Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

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S Supporting Information *

ABSTRACT: Few-atom cobalt-oxide clusters, when dispersed on a Zr-based metal−organic framework (MOF) NU-1000, have been shown to be active for the oxidative dehydrogenation (ODH) of propane at low temperatures ( Zn(II) > Al(III) > Ti(IV) > Mo(VI), opposite to the Lewis acidity of the promoter ions. For bulk metal-oxides, the relative Lewis acidity, either of a promoter ion or the active site, is well-known to affect propane ODH primarily via effects on the basicity of the lattice O atom involved in the C−H bond activation step.13,57,58 Since, under our experimental conditions, the various M-SIM+NU-1000 materials are inactive for propane ODH, the effect of the promoter ions on catalytic activity must stem from their influence on the subsequently installed cobalt-AIM sites. Previous DFT-based computational modeling of propane ODH catalysis of a hypothetical single-cobalt-atom version of the MOF-supported catalyst pointed to the H-atom abstraction by a Co(III)−O• moiety as the rate-determining step. The energy of the transition-state for Co(III)−O• abstraction of a second H atom to yield Co(II)−OH is significantly lower than that for the isopropyl migration from cobalt to ligated oxygen, serving to steer the reaction toward propene formation and completion of the proposed catalytic cycle.47 As noted above, we expect the Lewis acidity of proximal promoters to modulate the energy of the crucial Co(III)−O• containing moiety, where we are agnostic about the number of bridging oxo ligands over which the oxyl radical character may be distributed. We are also agnostic about which metal ions partner with Co(III) to stabilize the seven-valence-electron oxygen radical. It seems clear, however, that the influence of promoter metal ions, M, will be much greater for a Co(III)−O•−M configured active site than for one configured, for example, as Co(III)−O•− Co(II)−O−M. We note again that the variations in ODH catalyst turnover frequency are in the fine-tuning, rather than gross modulation, regime and are equivalent to a ca. 7 kJ/mol range of variations in transition-state energy.

microcrystallites; see Supporting Information, including Figures S10−S14. The relationship between propane conversion and propene selectivity was examined at 230 °C by systematically changing the molar space velocity of the reactants (propane + O2); results for all five versions of the promoted catalysts are presented in Figure 3d. At a fixed propane conversion, the propene selectivity for the supported bimetallic-oxide cluster catalysts is only marginally different than that of promoter-free Co-AIM+NU-1000, presumably due to the active sites being the Co species in all samples. Also, similar to the reported behavior of Co-AIM+... and Co-SIM+NU-1000, the propene selectivity increases as the propane conversion decreases at 230 °C and approaches 100% at very low propane conversion. On the basis of these observations, we conclude that the CO2 formation is mainly from the total oxidation of the in situ generated, more reactive propene rather than from the direct combustive oxidation of propane (i.e., k2 ∼ 0). In situ X-ray absorption spectra at the Co K-edge were collected to gain further electronic and structural information about the Co sites in the as-synthesized materials as well as to identify changes associated with initial catalyst activation. In the X-ray absorption near edge structure (XANES) spectral region, a low-intensity pre-edge peak at ∼7709.4 eV, corresponding to the 1s → 3d transition, is present in all samples; see Figures 4a and S15. Along with the observation that the post-edge minima are at ∼7749.5 eV, the Co ions in all the as-synthesized samples are concluded to be divalent.54 Upon activation in O2, a slight increase in the pre-edge peak intensity and a decrease in the white-line intensity occur in the XANES spectra for all samples, pointing to the formation of Co(III) ionsa finding that is corroborated by postcatalysis X-ray photoelectron spectroscopy data measured (Figures S10−S14). In the Fourier-transformed extended X-ray absorption fine structure (EXAFS) spectra for all samples, a peak at ∼1.56 Å (phase-uncorrected distance) is evident and is associated with the first shell Co−O singlescattering path, the fitting of which gives an average Co−O bond distance of ∼2.04 Å. (See Figure 4b for CoAIMNi(II)SIM+NU-1000 and Figures S16−S25 for all five samples before and after catalysis.) Consistent with conversion of at least some of the cobalt(II) ions to cobalt(III), this peak shifts slightly with activation and yields a slightly shorter average Co− O bond distance at ∼2.00 Å. The fitting results also suggest that the average coordination number (CN) of Co decreases slightly ( Zn(II) > Al(III) > Ti(IV) > Mo(VI). The active species, in all cases, remains cobalt, with initial catalyst activation (ligand loss + cobalt oxidation) being achieved by heating in O2. Previous computational and experimental work, in the absence of promoters, pointed to an under-coordinated cobalt(III)-oxyl species, of unspecified metal nuclearity, as the catalytically active component. We propose that the role of the promoter is to alter the energetic accessibility of Co(III)−O• moieties, and, more importantly, the energy of the transition-state for rate-limiting H atom abstraction step. The observed rate modulation is interpreted as tuning of the energy of the transition state over a range of ca. 7 kJ/mol. The salient features of the promoter in altering the energy of the relevant transition state are the promoter’s Lewis acidity and its siting relative to the oxy-cobalt active site(s). These features yield a correlation of catalyst TOF with the pKa of the promoting ion, when the ion is examined in aqueous solution as a single-metal-center aqua complex. (Thus, pKa values for the

Figure 5 presents a plot of catalyst TOF versus relative Lewis acidity of the promoter, as inferred-from/represented-by pKa

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DOI: 10.1021/jacs.7b09365 J. Am. Chem. Soc. 2017, 139, 15251−15258

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Journal of the American Chemical Society

Pinos fellowship (BP-DGR 2014) from the Ministry of Economy and Knowledge from the Catalan Government. C.W. K. acknowledges support from the Postdoctoral Research Abroad Program (105-2917-I-564-046) sponsored by Ministry of Science and Technology (Taiwan). This work made use of the J.B. Cohen X-ray Diffraction Facility supported by the MRSEC program of the National Science Foundation (DMR1121262) at the Materials Research Center of Northwestern University. This work made use of the EPIC, 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 DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. Use of the Advanced Photon Source is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract DE-AC0206CH11357. Materials Research Collaborative Access Team (MRCAT, 10-BM-B) operations are supported by the Department of Energy and the MRCAT member institutions.

solution-phase complexes appear to be good proxies for the relative Lewis acidities of the promoters.) Notably, departures from the correlation are paralleled by structural differences for specific catalyst/promoter/support (MOF node) assemblies. These differences are readily observable in the aforementioned DED maps of catalyst cluster siting and in comparative EXAFS assessments of cobalt−cobalt proximity (or isolation) within clusters. The wide variety of candidate promoter ions, their ease of installation and their effectiveness in tuning catalytic activity, coupled with the structurally well-defined features of the support, render MOF-supported bimetallic-oxide clusters wellsuited for mechanistically relevant investigation of factors determining catalyst activity and selectivity. The findings here, specifically for propane ODH, underscore the value of synthesizing arrays of heterogeneous catalyst/promoter/support assemblies featuring close-to-single-atom specificity for both chemical composition and spatial configuration. We suggest that these kinds of studies, in combination with computational modeling, can be exploited to identify relevant catalyst design principles and, ideally, to guide the synthesis of improved catalysts. Our current ODH-specific work is focused on (a) melding experimental findings with DFT-centered computational modeling of increasingly sophisticated models of ODH catalyst active sites, and (b) exploiting the findings already in hand to predictively expand the experimental range of tunability of the catalytic activity of these intriguing MOFsupported, bimetallic-oxide clusters.

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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.7b09365. Additional materials synthesis and characterization data, including N2 isotherms, powder X-ray diffraction patterns, SEM-EDS, XPS, X-ray absorption spectra and analysis, and cyclic voltammetry diagrams (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Zhanyong Li: 0000-0002-3230-5955 Ana E. Platero-Prats: 0000-0002-2248-2739 Chung-Wei Kung: 0000-0002-5739-1503 Matthew R. DeStefano: 0000-0002-2201-9808 Karena W. Chapman: 0000-0002-8725-5633 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 We thank Prof. Christopher J. Cramer for useful discussions. This work was supported by the Inorganometallic Catalyst Design Center, an EFRC funded by the DOE, Office of Basic Energy Sciences (DE-SC0012702). A.W.P. and M.R.D. were supported by the Department of Defense (DoD) through the National Defense Science and Engineering Graduate Fellowship (NDSEG) program. A.E.P.-P. acknowledges a Beatriu de 15257

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