From Transition Metals to Lanthanides to Actinides: Metal-Mediated

Oct 9, 2018 - Synopsis. Thanks to the reticular chemistry offered by metal−organic frameworks, crystalline porous solids composed of metal nodes lin...
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From Transition Metals to Lanthanides to Actinides: Metal-Mediated Tuning of Electronic Properties of Isostructural Metal−Organic Frameworks Timur Islamoglu,*,†,∥ Debmalya Ray,‡,∥ Peng Li,† Marek B. Majewski,† Isil Akpinar,† Xuan Zhang,† Christopher J. Cramer,‡ Laura Gagliardi,‡ and Omar K. Farha*,†,§ †

Department of Department of Pleasant Street § Department of

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 10/09/18. For personal use only.

‡

Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States Chemistry, Chemical Theory Center and Minnesota Supercomputing Institute, University of Minnesota, 207 Southeast, Minneapolis, Minnesota 55455, United States Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

S Supporting Information *

ABSTRACT: Isostructural metal−organic frameworks (MOFs) have been prepared from a variety of metal−oxide clusters, including transition metals, lanthanides, and actinides. Experimental and calculated shifts in O−H stretching frequencies for hydroxyl groups associated with the metal−oxide nodes reveal varying electronic properties for these units, thereby offering opportunities to tune support effects for other materials deposited onto these nodes.

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INTRODUCTION Self-assembled, atomically ordered porous materials composed of metal ions/clusters and organic linkers, known as metal− organic frameworks (MOFs), have attracted great attention in the past two decades owing, in part, to their high surface areas and structural tunability.1,2 With these properties, MOFs have been identified as platform materials for many potential applications such as catalysis,3,4 gas storage/separation,5−8 chemical sensing,9,10 energy storage,11 and drug delivery.12,13 Of these potential applications, utilizing MOFs as catalysts or catalyst supports has garnered special attention. While the metal ions/ clusters or organic linkers that form the structural backbone of MOFs have been shown to behave as isolated catalytic sites,14−16 MOFs have also been employed as catalyst supports,17,18 and the resulting hybrid catalytic materials have often been shown to exhibit superior performance in activity compared to analogous catalysts on conventional supports such as silica, alumina, and zirconia, all of which present O−H groups at their surfaces. The long-range order intrinsic in MOFs permits installation of catalysts with single-atom precision, something that is almost impossible in the case of conventional supports due to © XXXX American Chemical Society

the limited synthetic control in the preparation of such materials. This often leads to a broad distribution of acidities and densities of surface hydroxyl groups, and a concomitant diversity of binding modes for deposited atoms and clusters. Additionally, it is well documented that the morphology and facet orientation of the support, two aspects that are also challenging to control in conventional porous bulk materials, can profoundly affect the selectivity and conversion to the desired product in a catalytic process.19 Ultimately, quantifying and comparing the “support ef fect” among different materials while keeping consistent variables such as connectivity, facet orientation, and morphology becomes difficult. In this respect, MOFs, which permit isoreticular control over their structural composition, offer a unique advantage over most conventional support materials. More specifically, and as illustrated in this work, MOFs afford a platform where systematic changes can be made to the core of the material (in this case the identity of the metal in the secondary building unit) while the environment surrounding any active site remains constant. Received: June 24, 2018

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DOI: 10.1021/acs.inorgchem.8b01748 Inorg. Chem. XXXX, XXX, XXX−XXX

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nature of the secondary building unit (SBU) structure in Zr6based MOFs. Importantly, Hf(IV), Th(IV), and Ce(IV) have also been shown to form hexa-nuclear [M6O4(OH)4]12+ clusters which can be employed to build fcu and spn topology MOFs among others (Figure 1).38−42 In general, MOFs enable

By comparison, such systematic variations can be very challenging in most common support materials (such as metal oxides) that are often composed of multiple phases that may interconvert (e.g., the thermally induced conversion of the metastable anatase or brookite phases of TiOx to the rutile phase).20 Furthermore, the atomically precise structure of MOFs presents a distinct advantage with respect to the prospects for theoretical investigations to greatly assist in building an understanding of structural and reactivity details on an atomic scale, thereby accelerating progress toward solutions to challenging problems from catalysis to gas separation.21−23 Among the many MOFs that have been employed as catalyst supports, zirconium(IV)-based MOFs have been extensively explored due to their chemical and hydrothermal stability, these properties being crucial for practical catalytic applications.24 Spatially oriented −OH and −H2O groups on the metal nodes of Zr-MOFs have been exploited to install welldefined catalytically active metal centers.25,26 Recently, we have shown that installing metal centers on organic linkers via pendant −OH (e.g., catechol) groups yields a metal center that exists in a measurably different chemical environment compared to metal centers installed on the node, resulting in decreased catalytic activity for oxidation of cyclohexane.27 This observation afforded the possibility to compare the nature of the support effect on catalytic activity while demonstrating the importance of employing the structural diversity of MOFs to obtain key information about structure−property relationships in hybrid catalytic materials. Importantly, the ability to keep the connectivity, surface area, and pore size comparable while changing the identity of the active metal center in MOFs can elicit useful information for designing next generation catalysts. In this work, we demonstrate that the infrared stretching frequency of hydroxyl groups on the nodes, a key indicator of their electronic character when serving as catalyst binding sites in the UiO-66 and MOF-808 MOF families, can be tuned by substituting the metal in the node (from zirconium to hafnium, thorium, or cerium). We expect this range of tuned MOFs will be useful for future informed catalyst design.

Figure 1. Representative structures of M-UiO-66 and M-MOF-808, where M = Zr, Hf, Ce, and Th. Hydrogens are omitted for clarity. MOF-808 is shown on the (110) direction.

the study of the intrinsic effects associated with the identity of the metal in metal-catalysts or catalyst supports as opposed to the effect of crystallographic surfaces where different sites may bear different reactivity. To elucidate the potential role of the metal node in catalysis, we have prepared a series of isostructural MOFs and collected diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra in order to compare the stretching of hydroxyl groups on the node. In this study, we focus on the stretching frequencies of bridging μ3-OH groups in the node core, as other OH vibrational bands can show different intensities based on varying defect densities and/or the presence of other capping ligands on the node. UiO-66 and MOF-808, two Zr6-based MOFs with fcu and spn topologies, respectively, were prepared according to literature procedures, as were M-UiO-66 and M-MOF-808 [M = Zr(IV), Hf(IV), Ce(IV) and Th(IV)] analogues.32,38−43 Th-MOF-808 is reported here for the first time, and the details of the synthesis are provided in the Supporting Information. Volumetric nitrogen uptakes for these series of MOFs showed comparable porosity (Figure 2), and powder X-ray diffraction (PXRD) patterns confirmed the formation of both isostructural MOF series (Figure 3). The hydroxyl region of DRIFTS spectra for the M-UiO-66 and M-MOF-808 series is shown in Figure 4 and tabulated in Table 1 along with the μ3-OH stretches computed using density functional theory (DFT). For the M-UiO-66 series, we tested different structural models (cluster44 and periodic45−48) and a variety of density functionals (M06-L,49 M06-L-D3,50 PBE,51 BLYP,52,53 and B3LYP53−56) as discussed in the Computational Details section in the Supporting Information. Both the cluster and periodic models give consistent trends in predicted IR frequencies for the M-UiO-66 series, as do all of the tested functionals. Thus, for the MOF-808 series, we used exclusively M06-L cluster calculations to compute IR spectra. We note that, since MOF-808 has only a six-connected SBU, compared to the 12-connected UiO-66 node, every MOF-808 node has six known “defect” sites (i.e., six nonstructural ligands or ligand sets per node), ruling out variations in spectra, if any, associated with the number of defects as is the case in the M-UiO-66 series.

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RESULTS AND DISCUSSION Under carefully controlled reaction conditions, ideal (nearly defect free) UiO-66 with each Zr6 node connected to six benzene-1,4-dicarboxylate (bdc) can be prepared.28 However, introducing defects (or missing linkers and/or nodes) in the MOF is of some interest for catalytic applications due to the Brønsted acidity of the protons on defect sites and the Lewis acidity of the exposed metal sites.24,29,30 We and others have demonstrated that the UiO series of MOFs can be made intentionally defective by using a modulator method or changing the reaction temperature.31−34 In these processes, defect sites can be capped with a labile hydroxyl and water pair behaving in a charge compensating capacity. Residual modulators or formate anions formed from the decomposition of DMF during synthesis can also cap such defect sites.35 Known amounts of open sites in other Zr6-based MOFs can also be introduced by design, e.g., by controlling the connectivity around the node through the targeting of specific topologies.36 Reactive capping groups on the node can be reacted with metal-containing reagents (in vapor or condensed phase) to initiate the formation of metal clusters on the node (or coordinatively unsaturated zirconium on the node can act as a Lewis acid to catalyze many important reactions).37 Thus, a growing amount of attention is being diverted to understanding the B

DOI: 10.1021/acs.inorgchem.8b01748 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. N2 isotherms of (A) M-UiO-66 and (B) M-MOF-808 series MOFs. M = Zr, Hf, Ce, and Th.

Figure 4. Experimental DRIFTS spectra M-UiO-66 (top) and M-MOF-808 (bottom) series of MOFs showing bridging μ3-OH stretch.

Table 1. Experimental and Calculated Bridging μ3-OH Vibrational Modes in the M-UiO-66/MOF-808 Series of MOFs wavenumbers (cm1) MOF

experimental

calculated

Hf-UiO-66/MOF-808 Zr-UiO-66/MOF-808 Th-UiO-66/MOF-808 Ce-UiO-66/MOF-808

3679/3679 3674/3674 3653/3650 3646/3645

3661/3652−3669 3656/3648−3665 3644/3640−3649 3635/3636−3640

follow the same trend as that observed in the experimental spectra. While defect free M-UiO-66 MOFs should nominally have a single peak in the hydroxyl region corresponding to the μ3-OH group, new bands can appear near the μ3-OH signals upon the introduction of −OH/−OH2 pairs capping missing linker sites, and indeed shoulders or distinct peaks appear in the experimental spectra (Figure 4). We have performed DFT calculations on defected UiO-66 cluster models to assess the effects of increasing defects (missing linkers) in such spectra (Figures S23−S26). From our DFT calculations, we observe that terminal O−H stretching frequencies appear at higher frequencies than μ3-OH frequencies in the pristine M-UiO-66 (using the M06-L functional). However, the μ3-OH group nearest the defect site has lower frequencies compared to other μ3-OH groups in the M-UiO-66 nodes. This red shift in IR frequency near the defect site is consistent with the red-side shouldering observed in the experimental DRIFTS spectra (Figure 4). Notably, this spectral region for Th-UiO-66 does not show much shoulder on the μ3-OH stretch (Figure 4), suggesting a low defect formation during MOF synthesis, which is also evident from the single peak observed in the pore size distribution of Th-UiO-66 (Figure S3).

Figure 3. Experimental PXRD patterns of M-UiO-66 and M-MOF808 series MOFs. M = Zr, Hf, Ce, and Th.

Notably, experiment observes and theory computes a distinct red shift in the μ3-OH hydroxyl stretching band across the M-UiO-66 series when the metal is varied from Hf(IV) to Zr(IV), Th(IV), and Ce(IV), with values measuring 3679, 3674, 3653, and 3646 cm−1, respectively (Figure 4, Table 1). To further confirm that the observed sharp peaks belong to bridging μ3-OH groups on the nodesas opposed to being contributions from nonstructural capping ligandswe synthesized a nearly defect-free version of Zr-UiO-66 (Figure S4). The DRIFTS spectrum of the hydroxyl region shows a single sharp peak centered at the same location as for the more typically defective UiO-66 (Figure S10), securing the band assignment. Table 1 also lists μ3-OH hydroxyl stretching frequencies predicted for pristine M-UiO-66 models, which C

DOI: 10.1021/acs.inorgchem.8b01748 Inorg. Chem. XXXX, XXX, XXX−XXX

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obtain water binding strength in this isostructural M-UiO-66 series. Our calculations suggest that removal of a terminal H2O molecule from Hf-, Zr-, Th-, and Ce-UiO-66 nodes requires 84.8, 79.7, 70.1, and 55.7 kJ mol−1 of energy, respectively. A correlation between the computed water binding affinities and μ3-OH stretching frequencies further confirms alteration of the electronic properties of the metal node according to the identity of the metal (Figure 5). An energy difference of ca. 25 kJ mol−1 associated with water binding to a Ce(IV) node compared to a Zr(IV) node suggests higher water lability from the former. In this regard, we note that our recent finding59 of enhanced catalytic activity of Ce-UiO-66 compared to Zr-UiO66 for the hydrolysis of phosphate ester-based nerve agent simulant, DMNP, may well be attributed to the difference in water binding affinity (ca. ∼55.7 vs 79.7 kJ mol−1 for Ce(IV) and Zr(IV), respectively), as theory has computed that dehydration of Zr-based MOFs can significantly enhance the catalytic hydrolysis rates.37,61−64

As expected, analogous bands corresponding to bridging μ3-OH groups and analogous red shifts with respect to metal identity appear in the DRIFTS spectra of the M-MOF-808 series, emphasizing that this observed shift in the MOF series correlates to a change in the electronic structure of the node associated with variations of the metal center. However, the MOF-808 spectra are complicated by other OH functionality capping open sites intrinsic to this MOF’s topology. Thus, because of the diversity of OH functionality in the M-MOF808 series, qualitatively, we do not see perfect agreement when comparing experiment to theory. However, from our DFT calculations, we do observe that the μ3-OH IR stretching frequencies still follow the same red shifts observed in experimental DRIFTS spectra while going from Hf(IV) to Zr(IV), Th(IV), and Ce(IV) (Table 1 and Table S7). This suggests, despite the complexities arising from the capping of defect sites by organic linkers or OH/H2O functionality, μ3-OH stretching can be used as a descriptor of changing electronic properties with changing metal for both the M-UiO66 and M-MOF-808 series (Table 1, Figures S20 and S22). A similar trend in the shifting of bands associated with bound hydroxyls, from Hf to Ce, has also been reported for these groups on the surfaces of the bulk oxides HfO2, ZrO2, ThO2, and CeO2.57 According to the Sanderson scale, electronegativity values for Hf(IV), Zr(IV), and Ce(IV) are 0.96, 0.90, and 0.41, respectively,58,59 suggesting that the observed shifts in hydroxyl stretching can be correlated with the electronegativity of the metal center in the SBU of the MOF rather than textural/physical properties of the MOF. In general, bands for hydrogen-bonded hydroxyl groups are red-shifted compared to their non-hydrogen-bonded counterparts due to a weakening of the O−H bond attributed to charge transfer from the proton acceptor lone pair to the σ*OH of the proton donor.60 While our DRIFTS experiments showed bands between 2716 and 2753 cm−1 which could be attributed to hydrogen-bonded −H2O/−OH stretching bands (Figures S6−S10), the C−H bands from residual formate can also be observed in the same region and therefore we will not focus on bands in that region due to possible overlaps.35 In order to observe the effect of changing metal in the node, we have also calculated the energetic costs associated with dissociating the H2O ligand in the M-UiO-66 series so as to

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CONCLUSIONS Comparisons of DRIFTS spectra corresponding to nonhydrogen-bonded hydroxyl groups located on the nodes of two series of MOFs, M-UiO-66, and M-MOF-808 [where M = Zr(IV), Hf(IV), Ce(IV), Th(IV)] have shown that modes associated with these groups consistently shift to the red with decreasing electronegativity of the metal center, and that this shift also correlates with the binding affinity of H2O molecules capping node metal atoms that are coordinatively unsaturated from either accidental or designed missing-linker defects. These intrinsic MOF properties are associated exclusively with the nature of the metal center in the node, as densities of defects, local environments, and morphology are effectively unchanged within individual series. Due to the structural variations inevitably found in common amorphous or nanocrystalline support materials, quantification of this “support ef fect” is most readily accomplished in materials such as MOFs, where regardless of the nature of the metal in the node local variables such as the coordination environment can be held the same. This has implications with respect to designing systems for applications in catalysis, where the reactivity of the active site (in these MOFs, for example, the labile hydroxyl/water pair and/or the exposed metal center) greatly influences catalytic activity, and the findings of this work may inform future design of support materials.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01748.

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Experimental details including N2 isotherms, PXRD patterns, and full DRFITS spectra and calculation details including full DRIFTS spectra and tables for important stretching bands and bond distances (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Figure 5. Relationship between the binding affinity of water in M-UiO-66 series MOFs and the IR stretching frequency of hydroxyls. The red solid line is the linear fitting line (R2 = 0.91).

Timur Islamoglu: 0000-0003-3688-9158 Peng Li: 0000-0002-4273-4577 D

DOI: 10.1021/acs.inorgchem.8b01748 Inorg. Chem. XXXX, XXX, XXX−XXX

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Marek B. Majewski: 0000-0002-5190-7193 Xuan Zhang: 0000-0001-8214-7265 Christopher J. Cramer: 0000-0001-5048-1859 Laura Gagliardi: 0000-0001-5227-1396 Omar K. Farha: 0000-0002-9904-9845 Author Contributions ∥

T.I., D.R.: These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

O.K.F. acknowledges the support from the U.S. Department of Energy, National Nuclear Security Administration, under Award Number DE-NA0003763 and Northwestern University for the experimental work. The theoretical work was supported as part of the Inorganometallic Catalyst Design Center, an EFRC funded by the DOE, Office of Basic Energy Sciences (DE-SC0012702). The computations were performed at the Minnesota Supercomputing Institute.

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DOI: 10.1021/acs.inorgchem.8b01748 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b01748 Inorg. Chem. XXXX, XXX, XXX−XXX