Introducing Nonstructural Ligands to Zirconia-Like MOF Nodes to

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Introducing Nonstructural Ligands to Zirconia-Like MOF Nodes to Tune the Activity of Node-Supported Nickel Catalysts for Ethylene Hydrogenation Jian Liu, Zhanyong Li, Xuan Zhang, Ken-ichi Otake, Lin Zhang, Aaron W. Peters, Matthias J. Young, Nicholas M Bedford, Steven Letourneau, David J. Mandia, Jeffrey W. Elam, Omar K. Farha, and Joseph T. Hupp ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04828 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 1, 2019

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ACS Catalysis

Introducing Nonstructural Ligands to Zirconia-Like MOF Nodes to Tune the Activity of Node-Supported Nickel Catalysts for Ethylene Hydrogenation Jian Liu,† Zhanyong Li,† Xuan Zhang,† Ken-ichi Otake,† Lin Zhang,† Aaron W. Peters,† Matthias J. Young, ‖ Nicholas M. Bedford,§ Steven P. Letourneau,‖ David J. Mandia,‖ Jeffrey W. Elam,‖ Omar K. Farha,*,†, ‡ and Joseph T. Hupp*,† Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States †

‡ Department ‖ Applied § School

Materials Division, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, Illinois 60439, United States

of Chemical Engineering, University of New South Wales, Sydney, NSW, 2052

ABSTRACT: Previous work has shown that introduction of hexafluoroacetylacetonate (Facac) units as nonstructural ligands for the zirconia-like nodes of the eight-connected metal−organic framework (MOF), NU-1000, greatly alters the selectivity of nodesupported oxy-nickel clusters for ethylene dimerization vs. oligomerization. Here we explore a related concept: tuning of support/catalyst interactions, and therefore, catalyst activity, via parallel installation of organic modifiers on the support itself. As modifiers we focused on para-substituted benzoates (R-BA−; R = –NH2, –OCH3, –CH3, –H, –F and –NO2) where the substituents were chosen to present similar steric demand, but varying electron-donating or electron-withdrawing properties. R-Benzoateengendered shifts in the node-based aqua O-H stretching frequency for NU-1000, as measured by DRIFTS (diffuse-reflectance infrared Fourier-transform spectroscopy), together with systematic shifts in Ni2p peak energies, as measured by X-ray photoelectron spectroscopy, show that the electronic properties of the support can be modulated. The vibrational and electronic peak shifts correlate with the putative electron-withdrawing vs. electron-donating strength of the para-substituted benzoate modifiers. Subsequent installation of node-supported, oxy-Ni(II) clusters for ethylene hydrogenation yield a compelling correlation between log (catalyst turnover frequency) and the electron donating or withdrawing character of the substituent of the benzoate units. Single crystal X-ray diffraction measurements reveal that each organic modifier makes use of only one of two available carboxylate oxygens to accomplish grafting. The remaining oxygen atom is, in principle, well positioned to coordinate directly to an installed Ni(II) ion. We believe that the unanticipated direct coordination of the catalyst by the node-modifier (rather than indirect modifierbased tuning of support(node)/catalyst electronic interactions) is the primary source of the observed systematic tuning of hydrogenation activity. We suggest, however, that regardless of mechanism for communication with active-sites of MOF-supported catalysts, intentional elaboration of nodes via grafted, nonstructural organic species could prove to be a valuable general strategy for fine-tuning supported-catalyst activity and/or selectivity. KEYWORDS: metal−organic framework, heterogeneous catalysis, ligand modification, zirconia-like node, ethylene hydrogenation, Hammett constant

Heterogeneous catalysis is an enabling science1-2 for enormous segments of the petrochemical industry – segments based both on petroleum and, increasingly, shale gas.3 Other prominent applications include emission control for petrol(gasoline)-powered vehicles (e.g., via ceria-promoted platinum, rhodium, or palladium)4 and fuel-cell based catalysis of O2 reduction (e.g., via carbon-supported platinum in PEM (polymer electrolyte membrane) cells).5-7 The best and most useful heterogeneous catalysts typically excel in terms of stability, but lack atomically well-defined and uniform structures. These shortcomings complicate the identification of catalyst active-sites and can limit the ability of experimental structure/activity studies and computational modeling studies to facilitate mechanistic understanding and enable rational

catalyst design. In part with these concerns in mind, model studies often employ, as catalyst supports, structurally welldefined materials such as zeolites.8-13 With such supports, precise structural information about the catalyst itself can often be obtained via a combination of X-ray absorption, Xray scattering, and other spectroscopic methods, together with ultra-high-resolution electron microscopy. In turn, the obtained structural information, especially if gathered in operando, can render corresponding computational modeling powerful for both explanation and prediction of catalyst activity and selectivity. Notably, the number of experimentally realizable zeolite topologies is sizable, exceeding 200.14 Strictly speaking,

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Scheme 1. (a) Simplified representation of the structure of an isolated cluster, Zr6(μ3–O)4(μ3–OH)4(benzoate)12, that is a model for nodes. Capping of each of the twelve benzoates with a second carboxylate, and binding of the carboxylates to other hexazirconium(IV) centers (and continuation of these in like fashion) yields an idealized, i.e. defect-free and fully twelve-connected, version of UiO-66. Removal and replacement of the red benzoate ligands by aqua and hydroxo ligands, together with extension the green benzoate ligands to form TBAPy4-, that in turn link to other eight-connected hexa-zriconium(IV) centers, yields NU-1000. (b) Schematic representation of elaboration of a node of NU-1000 via SALI. Red ovals indicate the non-structural ligands that are displaced during SALI. Green ovals highlight bridging-hydroxo sites that potentially can serve as metal-ion-reactive sites on otherwise coordinatively saturated nodes. however, zeolites are limited to alumino-silicates; thus, chemical and structural tunability are likewise limited. For example, chemical tunability based on ion-exchange is feasible, but reticular expansion or contraction of the net defining a particular topology is, at present, not routinely achievable. In contrast, reticular size-tunability and enormous chemical tunability are readily achievable with a related class of porous materials, metal−organic frameworks (MOFs).15-21 Of the many thousands of reported MOFs, those based on group-four metal ions (Ti4+, Zr4+ and Hf4+) stand out for their thermal and chemical stability22-26 – clearly desirable properties for applications in heterogeneous catalysis.27-35 As inorganic nodes, many of these MOFs feature structurally well-defined, hexa-metal(IV)oxy clusters. Such nodes can be viewed as ultrasmall and precisely configured pieces of zirconia (or other potential catalyst support materials), uniformly separated and isolated by organic linkers. The linkers help to define MOF porosity, as well present chemical functionalities that may favorably interact with guest molecules, e.g. chemical reactants, intermediates, and/or products. Shown in Scheme 1a is a simplified representation of an isolated cluster, Zr6(μ3–O)4(μ3–OH)4(benzoate)12, that can be viewed as a model for various nodes, with the benzoates as truncated versions of linkers. Thus, the cluster offers a reasonable representation of nodes for idealized, fully twelveconnected, UiO-type MOFs. In this form, the node is saturated with respect to linker binding, at least when binding involves both carboxylate oxygens. Remaining and potentially available for single-metal-ion binding (via deprotonation and coordination), however, are four μ3–OH ligands (circled in green in Scheme 1b) .22, 36 The carboxylate-covered nodes can be unveiled by omitting linkers (i.e., introducing defects), or by turning to topologies requiring fewer than twelve connections. Indeed, examples exist for ten-, eight-, six-, and even four-connected M6-oxy nodes.37 Of relevance here is the eight-connected MOF, NU1000; it consists of Zr6(μ3–O)4(μ3–OH)4(H2O)4(OH)48+ nodes and tetratopic 1,3,6,8-(p-benzoate)pyrene (TBAPy4-) linkers.38

Removal and replacement of the red benzoate ligands in Scheme 1a, by aqua and hydroxo ligands, make the cluster a reasonable model for coordination of the nodes in NU-1000. As illustrated in Scheme 1b, the twelve O-H presenting ligands of NU-1000 and related MOFs (e.g., MOF-545,23 PCN-521,39 and DUT-6740) define zirconia-like sites where new chemical moieties may be grafted. Available methods include SALI (solventassisted ligand incorporation,41-42 which makes use of terminal aqua and hydroxo ligands (Scheme 1b, red ovals)); AIM (ALDlike installation of metal-ion/main-group clusters in MOFs, where ALD is atomic layer deposition),43-45; and SIM (the solutionphase analogue of AIM).44, 46-48 These methods have proven effective for: a) installing and supporting inorganic catalysts,43-44, 46 b) tethering redox-active molecules for transport of electrical charges via hopping,49-51 c) anchoring tri-peptides for recognition and capture of carbon dioxide,52 and d) immobilizing lightabsorbing molecules for sensitization of singlet oxygen production,53 among many other moieties and potential applications. It is well-documented for molecular catalysts that changing the chemical environment proximal to a catalytically active site can greatly affect the site’s catalytic performance, even if direct contact between the active site and the species defining the relevant environment is absent.54-56 Examples include environment-based: a) reactant and/or transition-state spatial confinement,57-58 b) strategic positioning of proton donors or acceptors,59-60 and c) introduction of electric fields.61 Environmental tuning of heterogeneous catalyst sites is also well known – most notably in the form of direct catalyst/support interactions or direct interaction of promotors (typically metal ions) with catalysts.62-67 While supports or promotors are most often inorganic,63, 68-69 examples involving organic species (e.g., calixarenes, or tetraalkyl ammonium cations) are not without precedent.70-71 We reasoned that SALI-based functionalization of MOF nodes might offer a general way of defining the environment around a subsequently installed catalytic cluster. Alternatively, node functionalization with nonstructural

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ACS Catalysis

Table 1. Ni(II) loading in Ni-AIM-R-BA-NU-1000 samples, R-BA ligand loading and BET surface area before and after the Ni AIM process, and TOF using Ni-AIM-R-BA-NU-1000 samples in ethylene hydrogenation reactions. –NH2

–OCH3

–CH3

–H

–F

–NO2

Ni Loadinga

3.5±0.1

4.0±0.3

2.9±0.4

2.8±0.5

3.0±0.1

3.2±0.5

R-BA Loading before Ni AIMb

2.3

3.5

3.5

3.8

3.7

4.2

R-BA Loading post Ni AIMb

2.1

3.2

3.3

3.7

3.7

3.7

0.2

0.6

0.2

0.5

0.2

0.1

0.1

0.2

0.1

0.3

0.2

0.1

1660

1715

1900

1880

1865

1870

1300

1200

1240

1375

Formate Loading before Ni

AIMb

Formate Loading post Ni AIMb BET Area before Ni AIM BET Area post Ni AIM

(m2/g)

(m2/g)

TOF * 10-3 (S-1) 1.5±0.2 5.2±0.1 2.6±0.3 2.3±0.3 a, from ICP-OES data; on a per-Zr6-node basis; b, from 1H-NMR data; on a per-Zr6-node basis

1250

860

1.7±0.1

0.2±0.0

Figure 1. Representations of single-crystal X-ray structure of F-BA-NU-1000 (100 K). (a) view along the [001] plane; (b) view along the [110] plane showing one oxygen atom from the carboxylate group binding to a Zr atom. Hydrogen atoms are omitted for clarity. (Zr, green; O, red; C, grey; atoms in F-BA, blue).

organic ligands could provide an indirect means of fine-tuning the electronic properties and/or the Bronsted acid character of a node-supported catalyst. In Scheme 1b, nonstructural organic ligands are grafted to the zirconia-like node via terminal hydroxo and aqua ligands in red ovals; catalytic metal ions are subsequently coordinated to the node via reaction of the support’s bridging hydroxo ligands in green ovals. As sketched, a node of this sort might be expected to bind each of four metal ions as isolated monometallic entities. As detailed below, however, the specific MOF-node-modification chemistry explored yields multi-metal(II) clusters. Here we describe the SALI-based introduction of a series of benzoate ligands onto the nodes of NU-1000. We then examine the consequences in terms of catalytic activity (ethylene hydrogenation) for subsequently installed oxy-Ni(II) clusters, where AIM is the method of cluster installation. Candidate para-substituted benzoates (R-BA-) were selected such that substituents (R = –NH2, –OCH3, –CH3, –H, –F and – NO2) would span a broad range of electron-donating or electron-withdrawing strength, as indicated, for example, by values of the Hammett constant (σ). In previous work, NiAIM-NU-1000 has been shown to be an effective catalyst for ethylene hydrogenation and also for ethylene oligomerization.43 Elsewhere we have observed and reported

that introduction of hexafluoroacetylacetonate units as nonstructural ligands can alter the selectivity of the oxy-Ni(II) cluster, shifting it from broad oligomerization to exclusively dimerization, i.e. butene formation.72 Here, we envisioned that altering the functional group on the coordinated benzoate would systematically tune the electronic environment provided by the Zr6 nodes to the subsequently installed oxy-Ni(II) clusters and that this tuning, albeit indirect, would be manifested in systematic variation of catalytic activity for hydrogenation. We find that activity indeed is systematically altered. Consistent with these observations, we find that the electronic character of the Ni(II) cluster is altered as evidenced by systematic shifts in XPS (X-ray photoelectron spectroscopy) peaks. We additionally find, however, that the nonstructural ligands likely accomplish their tuning not only by indirect electronic communication through the zirconia-like support, but also by direct coordination of Ni(II) by benzoate oxygens. Regardless of the specifics, the findings point to an appealing approach for systematically modulating the properties of MOF-based supports and associated inorganic catalysts. SALI relies on acid−base chemistry and substitution chemistry involving terminal hydroxo and/or aqua ligands on


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Scheme 2. Node-centric, schematic representation of formation of Ni-AIM-R-BA-NU-1000 from NU-100 via sequential SALI and AIM steps. (The sphere marked Ni here represents an NixOyHz cluster with average metal nuclearity close to four) the nodes and the carboxylic acid presented by R-BAH species.73 The details of R-BA-NU-1000 formation from the parent MOF and substituted benzoic acid are presented in the Supporting Information (SI Material Synthesis). The number of R-BA- ligands installed here on NU-1000 was quantified by 1H-NMR spectroscopy, where the R-BA- signals were inte grated against that of the TBAPy4- linker following dissolution of R-BA-NU-1000 in a mixture of D2SO4/DMSO (Figure S1). (Each node is ligated by eight TBAPy4- units, and each TBAPy4- connects to four nodes, yielding a linker-to-node stoichiometry of 2-to-1.) From Scheme 1, we anticipated that each node ideally would accommodate four nonstructural benzoate ligands. Table 1 shows that in nearly every instance, the benzoate loading is close to, but less than four per node. (An exception is NH2-BA-; for reasons that are not clear, only about two NH2-BA- per node are taken up.) For benzoates other than NH2-BA-, the competing presence of formate (Table 1) appears to account for most of the shortfall. (Formate here comes from the decomposition of dimethylformamide (DMF) during prolonged treatment (heating) of crude NU-1000 in a mixture of DMF and aqueous HCl.) Single-crystal X-ray diffraction was used to determine how the R-BA species bind to the oxy-Zr6 nodes. Prior to transfer to the X-ray diffractometer, single crystals of F-BA-NU-1000 were subjected to CO2 supercritical drying and moved to an ALD chamber. There they were heated to 125 °C under high vacuum and held overnight to mimic conditions experienced during AIM-based installation of catalysts (although no catalysts were installed at this stage). From the single-crystal X-ray structure, two of the ca. four F-BA- ligands were located. As illustrated in Figure 1, they reside in the c-pore (the interstitial space between adjacent Zr6 nodes along the crystallographic c-axis) of NU-1000. (Four benzoates are depicted in the figure, but each is associated with 50% occupancy; thus, any given c-pore contains only two identifiable fluoro-benzoates.) The remaining pair of benzoates could not be located, presumably due to disorder. Similar results (not shown) were obtained for a single-crystal of NO2-BA-NU-1000. Closer inspection of the structure shows that each benzoate employs just one carboxylate oxygen for node coordination (via a single Zr atom). This unexpected mode of binding can be contrasted with a chelating motif involving both carboxylate oxygens and one zirconium ion, or a bridging motif like that used by structural ligands (linkers) in both NU1000 and UiO-66. Thus, the ester-like benzoate anchoring

scheme leaves available a carboxylate oxygen for interaction (potentially) with a second species – for example, an added metal ion. With this new structural information in hand, we have redrawn Scheme 1b as shown in Scheme 2. We speculate that the crystallographically unobservable benzoate ligands are similarly “ester-coordinated” and that they are directed toward mesopores. Ni(II) ions were introduced to R-BA-NU1000 via the vaporphase AIM method as detailed in previous reports.47, 72 Ni loadings, which were determined from inductively-coupled plasma optical emission spectra (ICP-OES), were carefully controlled to be essentially the same across the set of R-BA-functionalized compounds; see Table 1. Control over loading was achieved by adjusting the number of Ni precursor pulses (A cycle) during the AIM process (Table 1). Notably, little change in R-BA- loading accompanied nickel installation (Table 1 & Figure S1), underscoring the comparatively strong interaction between R-BA- ligands and the Zr6 node. To assess the influence of the incorporated R-BA- ligands on the electronic properties of nodes, we first recorded diffuse reflectance infrared Fourier transform (DRIFT) spectra. In previous work involving benzoate-free NU-1000 (see also Figure 2) we observed a sharp peak at 3674 cm−1 and a shoulder at 3672 cm−1, and assigned these to O-H stretches of the terminal + bridging hydroxo ligands and the terminal aqua ligands, respectively.38 As shown in Figure 2, functionalizing nodes with nonstructural benzoates changes relative-peakintensities such that the intensity of the stretch due to terminal/bridging hydroxo ligands is less than that of the aqua ligand, consistent with benzoate attachment via selective displacement of terminal hydroxo ligands, as sketched in Scheme 2. (An outlier is NH2-BA-, for which the changes are less pronounced, consistent with attachment of only about two NH2-BA- units per node.) Comparing the DRIFT spectrum of the R-BA-NU-1000 sample featuring the most strongly electron-donating substituent (–NH2) with that for the ligand featuring the most strongly electron-withdrawing substituent (–NO2), we can see that the frequency of the O-H stretch attributable to the aqua ligand is higher for the latter; see Figure 2. The shift implies a very slight weakening of the aqua O-H bond in the R = NO2 version of R-BA-NU-1000 and thus points to remote electronic communication between the benzoate substituent and the zirconia-like node (potential catalyst support).74 Electronic effects were further interrogated by recording Xray photoelectron spectroscopy (XPS) data in the region


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ACS Catalysis

associated with Zr3d excitation. Shown in Figure 3a are spectra for the series of modified MOFs, where all spectra are referenced to the C1s peak at 284.8 eV. From the figure, small variations (a total spread of about 0.2 eV) are evident. The differences are small enough, however, that it is difficult to say with certainty that the XPS peak energies vary

4. A d-PDF peak at ∼ 2.03 Å is assigned as Ni−O, while one at ∼ 3.03 Å is associated with Ni···Ni and O···O in edgeshared NiO6 octahedra (where, for simplicity, and because we lack more specific information, we do not differentiate between oxo, hydroxo, and aqua ligands in the NiO6 units). A peak at 3.26 Å can be attributed to corner-shared Ni···M pairs,

Figure 2. DRIFT spectra of R-BA-NU-1000. (The diamond and star symbols stand for terminal/bridging –OH and terminal –OH2, respectively.)

systematically with benzoate substituent electronic properties. Nevertheless, the strongly electron-withdrawing NO2 group clearly shifts Zr binding energies higher relative to the electron-donating OCH3 group. (Notice that for zirconia-like catalyst supports consisting of hexa-zirconium(IV)-oxy, hydroxy clusters within NU-1000, fully two-thirds of the zirconium atoms interact directly with the support’s organic modifiers, i.e. functionalized, nonstructural benzoates.) We next turned to catalyst-loaded (oxy-Ni(II)-loaded) samples that had been pre-functionalized with benzoate derivatives. Following Ni installation, the energy of the Zr3d binding energy of samples shifts to a slightly lower, but clearly different energy, i.e. from 182.8 eV to 182.7 eV; see Figures 3 and S6. As illustrated in Figure 3b, the tuning imparted on the 2p binding energy of nickel by support-sited benzoate ligands is more substantial, tuning the energy from ~856.8 eV (Ni-AIM-NH2-BA-NU-1000) to ~857.4 eV (NiAIM-NO2-BA-NU-1000). While our initial premise (Scheme 1) was that modification of the MOF’s zirconia-like support units (nodes) by nonstructural organic ligands could be used to fine-tune catalyst/support interactions and thereby indirectly modulate the electronic properties of the catalyst, the available crystallographic data for R-BA-NU-1000 (Figure 1) suggests instead that tuning is accomplished mainly by direct coordination of one or more nickel atoms by benzoate oxygen atoms. Scheme 2 illustrates the revised proposal. To determine whether the AIM-installed oxy-nickel(II) species consists of isolated Ni ions or NixOyHz clusters, we performed high energy X-ray diffraction (HE-XRD) measurements, with subsequent pair-distribution-function (PDF) analysis. Differential analysis of the PDF data (d-PDF) performed by subtracting the PDF of bare NU-1000 from PDFs for Ni-AIM-R-BA-NU-1000 clearly shows atom pairs at distances corresponding to nickel oxy-hydroxide;75 see Figure

Figure 3. XPS spectra in the (a) Zr3d region of as-synthesized RBA-NU-1000 and (b) Ni2p region of as-synthesized Ni-AIM-RBA-NU-1000.

which likely include both corner shared Ni···Zr and Ni···Ni.75 By evaluating the peak areas highlighted in Figure 4, and assuming a Ni–O coordination number (CN) of 6, we are able estimate, as summarized in Table S2, the Ni···Ni and Ni···Zr CN for each benzoate variant of Ni-AIM-R-BA-NU-1000. From this analysis the Ni···Ni CN for Ni-AIM-R-BA-NU1000 samples varies from 0.89 to 1.44, while the value for the benzoate-free version of Ni-AIM-NU-1000 is 1.44. (This last value is slightly smaller than what we have previously reported; the disparity presumably reflects slightly lower Ni loading here.) The values for the Ni···Ni CN for the range of Ni-AIM-R-BA-NU-1000 samples examined are arguably “more similar than different,” suggesting that the various samples contain NixOyHz clusters of similar size. To complete the initial characterization of candidate catalysts and supports, we examined samples by scanning electron microscopy-energy dispersive spectroscopy (SEMEDS). By employing the technique in line-scan mode we ascertained that the distribution of Ni ions within the porous MOF crystallites is uniform; see Figure S2. Powder X-ray diffraction (PXRD) patterns of Ni-AIM-R-BA-NU-1000 (Figure S3) indicate retention of framework crystallinity after both ligand (SALI) and metal-oxide (AIM) modification of the


ACS Catalysis parent material. Consistent with our previous reports on postsynthetically modified NU-1000,43 N2 adsorption isotherms, recorded at 77 K (Figures S4 & S5, Table 1), show that BET gravimetric surface areas of both R-BA-NU-1000 (1,660 to 1,800 m2 g−1) and Ni-AIM-R-BA-NU-1000 (860 to 1,300 m2 g−1) are less than that of the unmodified material (2200 m2 g−1). (In part, the decreases reflect increases in sample mass with ligand and metal addition.) DFT-derived pore-size distribution (based on N2 isotherm measurements) show that the widths of the MOF’s triangular micropores are essentially unchanged by support modification and metal-oxide catalyst installation. In contrast, pore-size analysis shows that the diameter of the hexagonal mesopore of NU-1000 (~30 Å) decreases by a few angstroms upon benzoate installation. These finds are qualitatively consistent with the single-crystal X-ray structure representations shown in Figure 1. (See also Figures S4 & S5). Thus, while the identifiable support modifiers (fluorobenzoates) clearly are anchored in the MOF c-pore (panel b), the modifiers extend into the hexagonal channels (panel a).

Ni-O Ni···Ni Ni···Zr Ni-NO2

Ni-F Ni-H

G(r) (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ni-CH3 Ni-OCH3 Ni-NH2

Ni-AIM -4r 0

1

2

3

r (Å)

4

5

Figure 4. Differential pair distribution function (d-PDF) data of as-synthesized Ni-AIM-R-BA-NU-1000 obtained by subtraction of the PDF of pristine NU-1000 from the PDF of each postsynthetically modified sample.

We selected gas-phase ethylene hydrogenation as a model reaction to assess the dependence of the catalytic activity of MOF-supported oxy-Ni(II) clusters on the chemical identity of the benzoate. As with Ni-AIM-NU-1000, Ni-AIM-R-BA-NU1000 compounds become active for ethylene hydrogenation after treatment with flowing H2 (5% in Ar) at 200 °C for 2 h, a step that appears to remove a nickel-coordinated water molecule that otherwise blocks ethylene binding.43 (Computations indicate that ultimately two coordination sites must be opened up.43) Turnover frequencies (TOFs) were determined from the slopes (linear fits) of plots of extent of ethylene conversion versus reciprocal space velocity (W/F

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(unit: second (s)) is termed the residence time for the reactant with the catalyst, where W is the amount (unit: mol) of the Ni catalyst and F is the flow-rate of ethylene (unit: mol/s));76 see Figure 5a and Table 1. All of the Ni-AIM-R-BA-NU-1000 samples proved to be functionally stable during the 12-hour time-on-stream (TOS) (Figure 5b). Post-catalysis characterization, including SEM-EDS line scans, powder Xray diffraction patterns and N2 isotherms at 77 K, of the NiAIM-R-BA-NU-1000 samples indicate the retention of crystallinity and high surface area (Figures S7−S9). From Table 1, the TOFs of Ni-AIM-R-BA-NU-1000 compounds featuring benzoates having electron-donating substituents, such as OCH3, are noticeably faster than analogous compounds featuring electron-withdrawing groups, such as NO2.77 Figure 5c shows that TOF variations can be correlated with the Hammett constant (σ) for para substituents; the Hammett constant is a well-known empirical parameter that is intended to report on the relative electron-donating or electron-withdrawing strength of chemical substituents appended to organic molecules. An outlier in this regard is NiAIM-NH2-BA-NU-1000 which shows a lower TOF than anticipated from the correlation – behavior that conceivably could be due to the anomalously low loading of NH2-BA-; see Table 1. Again, ignoring the data point for Ni-AIM-NH2-BA-NU1000, a somewhat stronger correlation is evident when kinetic data are replotted as log(TOFR) (normalized in Figure 5d by dividing by the value (TOF0) for substituent-free benzoate (i.e., R = H) as the catalyst and support modifier). The improved correlation suggests that the effective activation barrier for ethylene hydrogenation may be the physical parameter tuned by varying substituent electronic properties, as the TOF should depend exponentially (negatively) on the magnitude of the effective barrier. A simplified, DFT-based computational investigation based on a single MOF-nodesupported nickel site pointed to migratory insertion of a hydride from nickel hydride to adsorbed/ligated ethylene as the rate-determining step for catalytic hydrogenation of ethylene.43 Benzoate-sited electron-withdrawing groups would be expected to stabilize the nickel hydride intermediate relative to the subsequent transition-state for the migratory insertion, thereby slowing the reaction. Electron-donating groups would be expected to preferentially destabilize the energy of the nickel hydride intermediate relative to the subsequent transition-state, thereby accelerating catalytic reaction. Together these effects ought to yield a correlation between log (TOFR) and substituent Hammett parameter akin to that shown in Figure 5d, i.e. decreasing activity with less negative (or more positive) value of the empirical σ parameter.77 It will be interesting to ascertain (computationally) the likely minimum-energy arrangement of the node-supported catalytic cluster + benzoate-based modifiers, and the extent to which this arrangement or others can reproduce the experimental activity/substituent correlation presented in Figure 5d. The findings conceivably could inform us about the comparative utility of substituted benzoates for modulating the rate and chemical selectivity of closely related reactions such as ethylene oligomerization and


Page 7 of 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 5. Catalytic hydrogenation of ethylene to ethane at 100 °C. (a) Ethylene conversion as a function of ethylene space velocity for a series of activated Ni-AIM-R-BA-NU-1000 samples; (b) Stability test of the activated Ni-AIM-R-BA-NU-1000 samples; (c) TOFs of the different samples as a function of the respective Hammett constant (σ) of the substituents R on the benzoate ligands; (d) Hammett plot depicting log(TOFR/TOF0) as a function of the Hammett constant (σ) of the respective benzoate ligand substituent; TOF0 is the TOF with the parent benzoate (R = H) as the support and catalyst modifier.

dimerization. Elsewhere we have reported that support and catalyst modification with acetylacetonate species can strongly influence the selectivity of Ni-AIM-NU-1000 for ethylene dimerization versus oligomerization.72 Finally, provided that other factors, such as steric constraints, do not contribute, the observed correlation of log(TOFR) with the Hammett parameter obviously should be predictive of hydrogenation TOFs for versions of Ni-AIM-R-BA-NU-1000 not included in this study. To summarize, we have capitalized on the fact that the metal-oxide-cluster based nodes of selected MOFs can function as structurally well-defined supports for model heterogeneous catalysts. For the eight-connected MOF, NU1000, featuring zirconia-like nodes and several displaceable or reactive, nonstructural ligands (aqua and hydroxo ligands), nodes can be functionalized not only with candidate inorganic catalysts (here, oxy-Ni(II) clusters), but also with organic modifiers/promotors. We find that by grafting to NU-1000’s

nodes a series of para-substituted benzoates, featuring various electron-withdrawing or electron-donating functional groups, the electronic properties of the support itself can be subtly tuned, as evidenced, for example, by shifts in O-H stretching frequencies for support-ligated (node-ligated) water molecules, and by changes in Zr3d excitation energies in XPS spectra. We further find that the activity of subsequently installed oxy-Ni(II) clusters for catalytic hydrogenation of ethylene is systematically tunable by the grafted supportmodifiers such that values of log(TOF) correlate with Hammett constants for benzoate substituents – where the constants are empirical measures of electron-donating or electron-withdrawing strength. While it is tempting to interpret the observed modulation of catalytic activity as emblematic chiefly of fine-tuning of catalyst/support interactions via remote modification of the electronic character of the support itself, structural data from single-crystal X-ray measurements point to a more complex


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interpretation. The data show that each of the node’s organic modifiers is linked to a single zirconium ion by a single carboxylate oxygen. (In contrast, the benzoate groups that terminate the MOF’s linkers make use of both carboxylate oxygens, with each carboxylate group binding to a pair of Zr(IV) ions.) The unexpected ester-like linking of modifiers to supports (i.e., MOF nodes) leaves available for direct binding to Ni(II) a second modifier-terminating, carboxylate oxygen. Direct binding can be expected to yield significant benzoatesubstituent-correlated modulations of catalyst activity via effects akin to standard ligand-based tuning of properties of molecular coordination complexes. We suggest, however, that regardless of mechanism for communication with active-sites of MOF-supported catalysts, intentional elaboration of nodes via grafted, nonstructural organic species could prove to be a valuable general strategy for fine-tuning supported-catalyst activity and/or selectivity.

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge via the Internet at http://pubs.acs.org. Detailed material synthesis and characterization, including Figures S1-S10 and Tables S1 and S2 (PDF) X-ray crystallographic data for F-BA-NU-1000.

AUTHOR INFORMATION Corresponding Authors * Omar K. Farha ([email protected]), Joseph T. Hupp ([email protected])

Author Contributions J. L., O. K. F. and J. T. H. developed the concept. J. L. carried out the materials synthesis, characterization, and catalysis reactions under the supervision of J. T. H. and O. K. F.; X. Z. carried out the NMR measurement; K. O. carried out the single crystal X-ray measurement; L. Z. carried out the synthesis of NU-1000; M.J.Y, N.M.B, D.J.M, and S.L. performed HE-XRD measurements; M.J.Y. performed PDF data analysis under the guidance of J.W.E.; J. L. wrote the manuscript first draft, with J.T.H. writing the second and subsequent drafts, and with all authors contributing to editing.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS 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). HE-XRD and PDF analysis in this work was supported by the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. We acknowledge Richard Spence, Olaf Borkiewicz and Yang Ren at Sector 11-IDB at Argonne National Laboratory for assistance with HE-XRD and PDF measurements. This work made use of the J.B. Cohen Xray Diffraction Facility supported by the MRSEC program of the National Science Foundation (DMR-1121262) at the Materials Research Center of Northwestern University. This work made use of the EPIC and Keck-II facilities of the NUANCE Center at Northwestern University, which has received support from the

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Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-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.

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(72) Liu, J.; Ye, J.; Li, Z.; Otake, K. I.; Liao, Y.; Peters, A. W.; Noh, H.; Truhlar, D. G.; Gagliardi, L.; Cramer, C. J.; Farha, O. K.; Hupp, J. T., Beyond the Active Site: Tuning the Activity and Selectivity of a Metal-Organic Framework-Supported Ni Catalyst for Ethylene Dimerization. J. Am. Chem. Soc. 2018, 140, 11174-11178. (73) Deria, P.; Mondloch, J. E.; Tylianakis, E.; Ghosh, P.; Bury, W.; Snurr, R. Q.; Hupp, J. T.; Farha, O. K. Perfluoroalkane Functionalization of NU-1000 via Solvent-assisted Ligand Incorporation: Synthesis and CO2 Adsorption Studies. J. Am. Chem. Soc. 2013, 135, 16801-16804. (74) The unexpected ester-like binding mode of benzoates leaves the possibility of hydrogen bonding between the carboxylate and the node aqua groups. To the extent this interaction is significant, it offers a potential alternative explanation for the observed correlation of nodebased O-H stretching frequency and electronic character of the benzoate-based modifier. (75) Platero-Prats, A. E.; League, A. B.; Bernales, V.; Ye, J.; Gallington, L. C.; Vjunov, A.; Schweitzer, N. M.; Li, Z.; Zheng, J.; Mehdi, B. L.; Stevens, A. J.; Dohnalkova, A.; Balasubramanian, M.; Farha, O. K.; Hupp, J. T.; Browning, N. D.; Fulton, J. L.; Camaioni, D. M.; Lercher, J. A.; Truhlar, D. G.; Gagliardi, L.; Cramer, C. J.; Chapman, K. W. Bridging Zirconia Nodes within a Metal-Organic Framework via Catalytic Ni-Hydroxo Clusters to Form Heterobimetallic Nanowires. J. Am. Chem. Soc. 2017, 139, 1041010418. (76) In view of cluster size and siting, we have assumed that all nickel atoms are accessible to reactants. For simplicity, we further assume that all nickel are catalytically active. If not, the reported TOF values will be greater than reported here. (77) A reviewer has pointed out that the activity of a supported monometallic Ir(I) catalyst for ethylene hydrogenation varies in opposite fashion with electronic properties of the support, i.e. supports that are more strongly electron-withdrawing boost the catalyst’s activity. See: Lu, J.; Serna, P.; Aydin, C.; Browning, N. D.; Gates, B. C. Supported Molecular Iridium Catalysts: Resolving Effects of Metal Nuclearity and Supports as Ligands. J. Am. Chem. Soc. 2011, 133, 16186-16195. According to this study, on supports yielding comparatively high turnover frequencies, ethylene binding to monometallic iridium(I) is thought to be the rate-determining step. Thus, a more strongly electron-withdrawing support ought to stabilize the (intermediate) ethylene-iridium moiety and lower the energy of the transition-state immediately preceding it – thereby accelerating the overall rate of the catalytic reaction.


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Introducing Nonstructural Ligands to Zirconia-Like MOF Nodes to

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