Tuning the Properties of Zr6O8 Nodes in the Metal Organic Framework

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Tuning the properties of Zr6O8 nodes in the metal organic framework UiO-66 by selection of node-bound ligands and linkers Ruiping Wei, Carlo Alberto Gaggioli, Guozhu Li, Timur Islamoglu, Zhuxiu Zhang, Ping Yu, Omar K. Farha, Christopher J. Cramer, Laura Gagliardi, Dong Yang, and Bruce C. Gates Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b05037 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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Chemistry of Materials

Tuning the properties of Zr6O8 nodes in the metal organic framework UiO-66 by selection of node-bound ligands and linkers Ruiping Wei,a Carlo Alberto Gaggioli,b Guozhu Li,a Timur Islamoglu,c Zhuxiu Zhang,d Ping Yu,e Omar K. Farha,c Christopher J. Cramer,b Laura Gagliardi,b* Dong Yang,a* Bruce C. Gatesa* Department of Chemical Engineering, University of California, Davis, California 95616, United States Department of Chemistry, Chemical Theory Center, and Minnesota Supercomputing Institute, University of Minnesota, Minneapolis, Minnesota 55455, United States c Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States d College of Chemical Engineering, Nanjing Tech University, Nanjing 210009, P.R. China a

b

e

NMR Facility, University of California, Davis, California 95616, United States

ABSTRACT: The metal organic framework (MOF) UiO-66, which incorporates Zr6O8 nodes, exhibits high stability under a wide range of conditions that commends it for potential applications. The MOF properties can be tuned by choice of the groups bonded to the node defect sites and to the linkers. We report data for UiO-66 incorporating formate, acetate, benzoate, and trifluoroacetate on the nodes and -NH2, -OH, -NO2, and phenyl substituents on the benzene rings of the benzene-1,4-dicarboxylic acid-derived linkers. The MOFs were characterized by infrared and 1H NMR spectroscopies, thermal gravimetric analysis, N2 adsorption, X-ray diffraction crystallography, scanning and transmission electron microscopy, and electronic structure calculations. The ligands on the nodes were identified and quantified by 1H NMR spectra of the MOFs digested in NaOH/D2O solutions. The effects of the node and linker groups on the electronic properties of the nodes have been quantified with IR spectra of the node µ3-OH groups and by DFT calculations, which are in good agreement with one another.

Introduction Metal organic frameworks (MOFs) are crystalline porous structures with high internal surface areas that can be synthesized with an enormous variety of node/linker combinations and therefore have high degrees of tunability and offer excellent opportunities for tailoring of their catalytic properties.1-4 MOFs with Zr6O8 and Hf6O8 clusters as nodes (e.g., UiO-665 and NU-10006) offer especially good prospects as catalysts because they have high thermal and chemical stabilities and high densities of metal-oxide nodes that are catalytic sites for many reactions as well as sites for anchoring catalytically active metal complexes.7-9 The Zr6O8/Hf6O8-containing MOFs are typically synthesized in reactions involving (a) node precursors (zirconium or hafnium salt), (b) organic linkers, (c) solvents (mostly dimethylformamide (DMF) and diethylformamide (DEF) and more recently water)and (d) monocarboxylic acids or inorganic acids as modulators which slow down the MOF formation and therefore help to form high-quality MOF crystals.7, 10-11 Zr6O8/Hf6O8 nodes can be combined with di-, tri-, tetra-, and hexa-topic linkers to form a wide variety of topologies.7 In the work described here, we focus on UiO-66 and its derivatives, which are made with dicarboxylic acid linkers. Although these MOFs can be made near defect-free under controlled conditions, they have often been reported to incorporate defects where linkers and/or nodes are missing.12-13 Such defects, which may be present in high densities, dictate surface area, pore volume, crystallinity, stability, and reactivity of the MOF. And defect sites may be exposed Zr or Hf sites that

are themselves catalytically active for reactions catalyzed by Lewis acids8, 14-15 and such sites can be used for anchoring metal complex catalysts.16-17 Besides structural linkers, UiO-66 also has non-structural ligands that cap defect sites. These non-structural ligands initially present on the MOFs are typically derived from modulators and solvents used in the syntheses, which compete with linkers in bonding to the nodes and thereby lead to the formation of defect sites. Examples of these ligands include chloride,14 formate10 and acetate.14 They have been characterized by infrared (IR) spectroscopy and density functional theory (DFT) calculations14 and resolved and quantified by NMR spectroscopy of digested MOFs.14 These ligands are significant not only because they influence the reactivities of the MOFs, but because they can contribute to uncertainties in analysis of the MOFs—especially when a nondefective structure is assumed in modelling the reactivity. Therefore, the identification and quantification of these defects as well as the capping ligands have received great attention from MOF community.10, 12-13, 18 Progress has also been made in the manipulation of these ligands, for example, by replacement of carboxylate with alkoxy by reaction with an alcohol14 and replacement of alkoxy with hydroxo by reaction with water.19 Moreover, it has been shown recently that electronic properties of the metals in the nodes and therefore the µ3-OH IR stretching frequencies can be tuned in M6O8 containing MOFs dictated by the choice of the metal.20 However, much remains to be learned about the node ligand chemistry, with questions about the dynamics of the bond-breaking and bond-forming reactions

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involving non-structural ligands, including the linkers. Further, how these non-structural ligands affect the electronic properties of the nodes is yet to be explored. Investigations of these properties are of central importance for understanding the reactivities and catalytic properties of Zr6O8- and Hf6O8containing MOFs. In the research described here, we investigated the node ligand chemistry of MOFs with Zr6O8 nodes, namely, UiO-66 and its derivatives with various ligands on the nodes (Figure 1). To

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assess the effects of the ligands on the node chemistry, we took advantage of the strong IR signature in the O–H stretching region representing µ3-OH groups on the nodes. The results show how the linkers and other ligands bonded to the nodes influence the electronic properties of the nodes, where catalysis takes place and/or catalytic groups can be anchored. We demonstrate the effectiveness of density functional theory in predicting these properties.

Figure 1. Schematic representation of UiO-66 and its structure modification; BDC is benzene-1,4-dicarboxylic acid. Experimental Section Synthesis of UiO-66 with Modulators. ZrCl4 (0.120 g, 0.515 mmol) and one of the modulators (formic acid, acetic acid, trifluoroacetic acid, or benzoic acid) were dissolved in 20 mL of DMF in an 8-dram vial using ultrasound for 5 min. The linker precursor (benzene-1,4-dicarboxylic acid or a functionalized BDC linker precursor) was then added to the solution and dissolved by ultrasound for ∼15 min. The vials were kept under static conditions in a pre-heated oven at 393 K for 24 h. MOF precipitates formed, and they were isolated by centrifugation after cooling to room temperature. The solids were washed with DMF (30 mL) three times in a day to remove unreacted precursors and with acetone (30 mL) six times in two days to remove DMF. Then, the powder was dried at room temperature and activated at 393 K under vacuum for 18 h prior to characterization. The amounts of all the reactants used in the syntheses are summarized in the Supporting Information (SI), Table S1. Synthesis of Standard UiO-66 with Few Defects.20 Benzene1,4-dicarboxylic acid (45 mg, 0.27 mmol), triethylamine (7.0 L, 0.05 mmol), glacial acetic acid (65 mL, 1.135 mol), and 63 mL of dimethylformamide (DMF) was charged into a 1-L glass jar. Benzene-1,4-dicarboxylic acid was dissolved completely by sonication for 15 min followed by heating at 373 K for 15 min. In the meantime, an 8-dram vial was loaded with ZrCl4 (63 mg, 0.27 mmol) and 4.5 mL of DMF followed by sonication until the solids were fully dissolved. The ZrCl4 solution was added into the glass jar containing the linker solution then placed back into the 373 K pre-heated oven for 24 h. After the solution had cooled to room temperature, the product was collected by centrifugation at 7500 rpm for 5 min, washed three

times with the DMF (30 mL), and then washed three times with acetone (30 mL). Then the product was soaked with acetone overnight to ensure complete removal of DMF and activated at 393 K under vacuum for 18 h prior to characterization. IR Spectroscopy. A Bruker IFS 66v/S spectrometer with a spectral resolution of 2 cm−1 was used to collect transmission IR spectra of MOF powder samples. Approximately 5 mg of a solid sample in an argon-filled glovebox was sealed between two KBr disks, and after transfer to the spectrometer, the IR spectrum was recorded with the sample under high vacuum. Each spectrum is the average of 64 scans. Powder X-ray Diffraction (PXRD). PXRD patterns of the MOFs were obtained with a Panlytical X-ray Diffractometer Model X’Pert Pro MRD instrument. Measurements were made over a range of 5° < 2θ < 30° in 0.05° steps at a scan rate of 1°/min. Scanning Electron Microscopy (SEM). SEM images were collected with a ThermoFisher Environmental Quattro SEM instrument. Transmission electron microscopy (TEM). TEM images were collected with a FEI Tecnai G2 T20 TEM instrument. BET surface area measurements. N2 adsorption isotherms were collected on a Micromeritics Tristar II 3020 instrument with each sample at 77 K. Samples were heated at 393 K under high vacuum for 12 h prior to recording of the isotherms. Thermal gravimetric analysis (TGA) measurements. Thermogravimetric analyses (TGA) were carried out with a TA Instruments Q500 thermogravimetric analyzer with an evolved gas analysis furnace. Samples in flowing O2 were heated from 293 to 1073 K at a rate of 10 K/min.


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MOF Sample Digestion and Characterization by 1H NMR Spectroscopy. Samples were prepared by weighing 10 mg of a MOF into a 1.5-mL vial, and then 1 mL of digestion solution consisting of 1-M NaOH in D2O was added. Each vial was capped and inverted 2−3 times before the samples were allowed to digest over a period of 24 h. This treatment in NaOH dissolved only the organic portion of the MOF (linker, solvent, components formed from modulator, and any other such components), with the inorganic component dropping out as ZrO2. After 24 h, the clear supernatant solution of 0.6 mL was transferred to an NMR tube. Liquid 1H NMR spectra were recorded with a Bruker Avance DPX-500 NMR spectrometer (500 MHz). The relaxation delay (d1) was set to 20 s to ensure that reliable integrals were obtained, allowing for the accurate determination of the relative concentrations of the molecular components. The number of scans per sample was 16. Computational Methods. The UiO-66 MOF was modeled as a finite cluster, formed by extracting one zirconium oxide

node from a periodically optimized structure of UiO-66, using benzoate groups as linkers and freezing the carbon atoms (of the phenyl rings) in para-positions with respect to the carboxylate groups during geometry optimizations (Figure S7, Supporting Information, SI). DFT geometry optimization and frequency calculations were performed by using the M06-L density functional21 and the Gaussian 0922 software package. The def2-SVP basis set was used for H, C, and O atoms; the def2-TZVPP basis set23-24 was employed for the Zr; and def2-TZVP was used for the µ3-OH groups. Moreover, the SDD pseudopotential25 was used to take into account the core electrons in Zr. Numerical integrations were performed using an ultrafine grid. All calculations have been done in the unrestricted formalism. The frequencies reported in this work were obtained by applying a scaling factor of 0.956 to the computed frequencies. This is a standard value for M06L.26

Table 1. Influence of modulators and linkers on the number of non-linker ligands per node on UiO-66 determined by 1H NMR spectroscopy and TGA. Modulator

Molar ratio of modulator/Zr in synthesis

Linker Precursora

BET Surface area, m2/g

Number of formate ligands per node

Number of DMF ligands per node

acetic acid

0

BDC

1222.0

0.20

0.02

6

BDC

1343.0

0.32

0.02

30

BDC

1391.5

0.25

100

BDC

1479.0

Standardc

BDC

1250.5

30

BDC

15

BDC

15

formic acid

benzoic acid trifluoroacetic acid

Total number of nonlinker ligands per node

Number of vacancies per node by TGA

0.22 b

1.5

0.30 (acetate)

0.64

1.0

0.01

0.80 (acetate)

1.06

1.4

0.19

0.01

1.74 (acetate)

1.94

2.4

0.01

0.03

0.40 (acetate)

0.44

1.0

1496.0

1.52

0.02

-

1.54

2.5

1378.0

0.99

0.04

-

1.03

1.3

BDC-OH

612.5

0.93

0.29

-

1.22

-

15

BDC-NH2

1164.0

3.17

0.11

-

3.28

-

15

BDC-NO2

782.0

0.94

0.06

-

1.00

-

15

BDCphenyl

547.5

0.48

0.54

-

1.02

-

30

BDC

1522.0

0.17

0.01

3.57 (benzoate)

3.75

-

30

BDC

1535.0

0.06

0.02

3.44 (trifloroacetate)

3.52

3.1

aBDC

Number of acetate or benzoate ligands per node

is benzene-1,4-dicarboxylic acid. The functionalized BDC linker precursors are 2-hydroxy-1,4-benzenedicarboxylic acid (BDC-OH), 2-amino-1,4-benzenedicarboxylic acid (BDC-NH2), 2-nitro-1,4-benzenedicarboxylic acid (BDC-NO2), 1,4naphthalenedicarboxylic acid (BDC-phenyl). b This number underestimates the total number of defects because a substantial number of vacancies with terminal OH groups were found by IR spectroscopy which cannot be quantified by 1H NMR spectroscopy. c This sample has the lowest density of defect sites among those investigated in this work, and it is used as a standard for comparison with other samples as a basis for determining the electron-donor properties of the various ligands. modulators and linkers (Table 1) to prepare a family having various ligands and linkers at different coverages on the nodes. For example, UiO-66 with acetate ligands on the nodes was Results synthesized with acetic acid used as a modulator. The other MOF synthesis and quantitative characterization of ligands modulators, formic acid, benzoic acid, and trifluoroacetic acid, on nodes gave the corresponding anionic ligands on the nodes. The resulting MOFs were characterized by means of PXRD, MOF samples were synthesized with each of the several



determination of N2 adsorption isotherms at 77 K, TGA, SEM, and TEM. Further, each MOF was dissolved in 1-M NaOH in D2O solution, and where the 1H NMR spectra of the resultant solutions were integrated to quantify all the non-linker ligands on the nodes—formate, acetate, benzodetermined the numbers of ligands bonded per node and the number of missing linker sites per node. The phase purity of the MOFs made with various amounts of acetic acid is characterized by the PXRD (Figure S1, SI). With an increasing number of acetic acid modulators, an increase in the crystallinity was observed, so do with the N2 uptake, BET surface area and pore volume (Figure S2, Table 1 and Table S2), consistent with reported results.10 SEM and TEM images (Figure S3 and S4, SI) show that the MOF crystals grew larger, and typical octahedral morphology was observed with increasing acetic acid ratio. Results summarized in Table 1 show that variation of the concentration of acetic acid modulator in the synthesis allows tuning of the defect site density. The data are consistent with reported results12 and show that increasing the ratio of acetic acid to BDC linker leads to more such defects. The data also show that an increased defect density is correlated with an increased number of acetate ligands per node. Besides acetate ligands, formate ligands—generated by decomposition of the solvent DMF—were also observed, and as the concentration of acetic acid modulators was increased with other concentrations being held constant, the number of formate ligands per node first increased and then decreased. DMF was also bound as a ligand on the nodes, as expected.14 All these ligands compete with each other and with the linkers for bonding sites on the nodes. The numbers of vacancies on the MOF nodes were also estimated on the basis of TGA data (as has been reported numerous times10, 27), whereby the stoichiometry of the combustion of the organic linkers of MOF is used to count the number of linkers per node (in this experiment, at high temperature and in O2, organic linkers are burned to form CO2 and H2O, leaving ZrO2 as the only solid product, determined gravimetrically), with the number of vacancies calculated by difference. The non-linker ligands, such as formate, acetate, or terminal OH groups, are excluded from the mass balance calculation because they tend to be completely removed at temperatures of about 623 K which is much lower than the temperature at which the linkers start to burn (764 K). However, we note that the method is limited for quantifying each of the organic non-linker ligands on the nodes as they are of more than one type, but the method considers them all as vacancies. Furthermore, in using the TGA method, one usually assumes that there are no missing-node defects and instead only missinglinker defects. Notwithstanding this limitation, our results show a broad consistency of the TGA-determined values with the 1HNMR-determined values, indicating an increasing number of vacancies per node with an increasing ratio of acetic acid to Zr used in the synthesis (Table 1). However, the sample made without added acetic acid (or another modulator) is characterized by a much higher number of vacancies per node determined by the TGA method than by the 1H-NMR method. We emphasize that this sample incorporated a significant number of terminal OH ligands on the nodes, as shown by its

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IR spectrum (3780 cm-1), and these cannot be identified in the NMR spectrum (because they react with NaOH during NaOH/D2O treatments to form water, which could not be quantified accurately because the NaOH and D2O both had substantial water impurities). However, other samples do not include this ligand, as shown by the lack of the 3780 cm-1 band in IR spectra. We thus infer that node structures with acetate or formate ligands are more stable than those with terminal OH ligands (our previous DFT calculations14, 19 show that acetateand formate- ligated nodes have much lower free energies than terminal OH ligand-bonded nodes, in agreement with this result) and that when modulators were present in a synthesis, these organic ligands rather than OH ligands were bonded to the nodes. It is evident that the method for determining vacancy densities by use of NaOH/D2O treatments and 1H-NMR spectroscopy has its drawbacks for quantification of MOF samples with terminal OH ligands on the nodes. The results show clearly that there is competition between modulator acetic acid and BDC linkers to bond on the node sites. Lillerud et al.10 have pointed out that the competition inhibits crystal nucleation and promotes growth, and when the concentration of acetic acid is increased, larger octahedral crystals form and the crystallinity increases. Defects generated by acetate ligands ultimately become part of the crystal, evidenced by the missing node structures in “reo phase/nanodomains” with 2θ peaks at range of ca. 2-7o. IR spectra of these samples in the νOH region (Figure 2A) show that the µ3-OH band characterizing UiO-66 made with no modulator is broad, centered at 3671 cm-1, and accompanied by broad shoulders (ranging in wavenumber from 3550 to 3700 cm-1). When acetic acid was added to the synthesis mixture, the µ3-OH band became sharper and shifted slightly, to 3674 cm-1. Simultaneously, the shoulders gradually decreased in intensity and almost disappeared when 100 mols of acetic acid were added per mol of ZrCl4. In the limiting case, each node should be expected to be [Zr6O4(OH)4]12+, with four µ3-OH groups per node, and the results demonstrate that the µ3-OH groups become nearly uniformly distributed on the nodes as this limit is approached and the MOF becomes increasingly crystalline. The pore size distributions of UiO-66 modulated by acetic acid are all narrow (centered at 10.5–11.0 Å), and the pore sizes become slightly larger with increasing amounts of acetic acid added in the synthesis (Figure S2, SI), which may attribute to the increase number of missing linker defect sites (the missing node defect sites are discussed below). To clarify the influence of ligands on the node vacancies, we sought to synthesize a standard MOF sample with a large crystal size and low number of defects per node. We synthesized this standard UiO-66 sample by using both triethylamine and acetic acid as co-modulators, finding by PXRD, SEM, and TEM that it is highly crystalline and consists of relatively large crystals, with a typical diameter of about 1 µm. The sample was analyzed as stated above with 1H NMR spectroscopy, and the results demonstrate that there were only 0.44 vacancies per node (whereas all the other samples had higher densities of vacancies). Because this sample had a nearly ideal structure, we relied on its IR spectra for comparisons with the other samples and for assessing the effects of the various modulators.


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A

B

AA-100 Standard FA-30 BA-30 TFA-30

Intensity (a.u.)

Intensity (a.u)

AA-0 AA-6 AA-30 AA-100

3780 3800

3700

3600

3500

Wavenumber (cm -1)

A C

3700

3680

3660

3640

Wavenumber (cm -1)

FA-15 BDC-NH2

D

AA-30 AA-Ethoxy AA-OH

BDC-OH BDC-NO2 Intensity (a.u.)

Intensity (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


BDC-Phenyl

3780

3800

3700

3600

Wavenumber (cm -1)

3500

3700

3680

3660

3640

3620

3600

Wavenumber (cm -1)

Figure 2. IR spectra in νOH region characterizing (A) UiO-66 synthesized with various concentrations of acetic acid as a modulator, AA represents acetic acid as modulator, and the numbers represent the molar ratio of modulator to Zr; (B) UiO-66 synthesized with various modulators, FA, BA, and TFA represent acetic acid, formic acid, benzoic acid, and trifluoroacetic acid as modulator; (C) UiO-66 modulated with acetic acid in a ratio of 30 acetic acid molecules per Zr atom; the same sample after treatment with ethanol at 473 K for 20 h; and the sample further treated with water at 423 K for 8 h; (D) UiO-66 having various substituents on the BDC linker and synthesized by modulation with formic acid. MOFs synthesized with various modulators To test for the role of the modulator, a family of UiO-66 MOFs was made under otherwise the same conditions with modulators other than acetic acid, namely, formic acid, benzoic acid, and trifluoroacetic acid. The data of Table 1 show that for a given modulator/Zr ratio used in the synthesis, the total number of defects per node increased in the order acetic acid < formic acid < benzoic acid ~ trifluoroacetic acid (matching the trend in the N2 uptake and BET surface area data), consistent with reported results.10, 28 We infer that the defects were generated in the syntheses as the modulator and linker precursors (acetic acid, formic acid, benzoic acid, and BDC linker) competed for binding sites on the nodes while leaving some of these sites open. Trifluoroacetic acid (TFA) is a stronger acid (pKa = 0.23) than formic acid (FA, pKa = 3.75) and acetic acid (AA, pKa = 4.75), and it has been concluded10 that increasing acid strength of the modulator favors defect formation in the MOF (more

defects are generated by modulators with strong acidity than by those with weak acidity when the same modulator amount is used). Further, the SEM and TEM images (Figures S3 and S4) show that the UiO-66 particles/crystals became much less uniform when the strong acid TFA was used as a modulator, and the same trend has been observed for UiO-66 synthesized with HCl as a modulator.14 However, the number of defect sites is determined not only by the pKa of the modulator, but also by its molecular size Benzoic acid has a molecular dimension similar to that of the BDC linker and favors the formation of a relatively large number of defects per node, although the pKa of benzoic acid is only 4.20 (because it is larger, it is difficult for BDC to replace it when BA is high in concentration).28 Moreover, BA-modulated UiO-66 has shown the most uniform octahedral shape among the samples



made with all modulators: the particles are the largest among all of them (Figure S3 and S4). Further still, the pore size distributions of UiO-66 are also influenced by the modulators (Figure S2). Besides the major pores with a diameter of approximately 10 Å, larger pores, in the range of 15–20 Å, were also evident for UiO-66 synthesized with modulators having stronger acidity, FA (15 Å) and TFA (15 Å), or with BA (18 Å) with larger size—in contrast to what was observed for the sample synthesized with AA. We infer that these larger pores may be related to missing node defects, inasmuch as they can result in larger pore structures than missing linker defects. Each of the UiO-66 MOFs incorporated ideally 4 µ3-OH groups on each Zr6O8 node, identified by IR spectra.5, 14, 17, 19, 29 A basis for comparison of the effects of the ligands on the nodes of the various samples is provided by the frequencies of the O–H stretches determined by the IR spectra. Considering the standard UiO-66 as a reference material, we see from Figure 2B that the electron-withdrawing strength of the ligands increases in the order acetate ~ formate < BDC linker < benzoate < trifluoroacetate. The IR bands of acetate-modulated UiO-66 and formate-modulated UiO-66 are so close to each other that they could not be distinguished. To make the comparison between formate and benzoate clear, we synthesized NU-1000 (with tetratopic 1,3,6,8-tetrakis(p-benzoate)pyrene (TBAPy) linker) by using benzoic acid as modulator,6 which generated a MOF with four benzoate ligands per node on the structural vacancies of NU-1000, and these were subsequently converted to four formate ligands by treatment of the benzoate-capped NU-1000 with HCl/DMF at 373 K for 24 h in a batch reactor. The distinction referred to above is made clear in the spectrum of NU-1000 for the comparison between formate and benzoate ligands—the electron-withdrawing tendency of benzoate is greater than that of formate, as shown in Figure S5 in the SI. Besides carboxylate ligands, alkoxy and terminal OH groups can be present on the nodes. When the acetate-modulated UiO66 was treated with ethanol vapor at 473 K for 10 h, the ligands on the node vacancies were completely converted to ethoxy, as shown by IR and 1H NMR data. Figure 2C shows the IR spectra of these samples. Reported results14, 19 based on quantitative 1H NMR data show that each node vacancy bonds to only one ethoxy group as a monodentate ligand, leaving open one neighboring Zr site (Lewis acid site, Figure 1). These asymmetric structures cause a splitting of the µ3-OH IR bands (Figure 2C). Thus, two new bands appear when ethoxy groups are formed on UiO-66, at lower frequencies than those characterizing acetate-UiO-66 (at 3565 and 3636 cm-1). DFT calculations confirm the experimental observation of the asymmetry of the ethoxy-UiO-66 sample, showing the split of the µ3-OH band, with one band at a lower frequency than that characterizing defect-free UiO-66. When the node ethoxy ligands were converted to OH by further treatment with water vapor at 423 K for 8 h, giving OH-UiO66, a new band appeared, at 3780 cm-1, which has been assigned to terminal OH groups on the node.19, 29 Further, three new bands appeared, with one at a higher frequency (3693 cm-1) and

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two at lower frequencies (3664 and 3627 cm-1) than those of acetate-UiO-66. MOFs with various linkers To investigate the influence of variations in the linkers, we synthesized a family of MOFs using each of the linker precursors shown in Figure 1 and Table 1; thus, with formic acid as the modulator, we made UiO-66 with -NH2, -OH, -NO2, and phenyl substituents on the benzene ring of the BDC linker. The 1H NMR results show that the resultant UiO-66 samples all incorporated formate ligands, inferred to have formed from the formic acid modulator. The data show that the defect site density depends on the nature of the linker precursor. The MOF with BDC-NH2 linkers has more vacancies per node than the others. This result indicates that the NH2 group has a significant influence on the bonding of linkers to the nodes, in competition with formate. Further, we observed that all the substituent ligands on the linkers led to an increase in the number of DMF ligands bonded to the node vacancies (characterized by an IR band at 1656 cm-1, assigned to C=O vibrations of chemically bonded DMF molecules; and we note that we did not observe any bond characteristic of N– H vibrations (at about 3200 cm-1) from decomposition of DMF), especially so for the BDC–OH and BDC–phenyl samples. This trend is not yet explained. We also noticed that the number of defect sites formed in MOFs with various linkers does not follow the order of linker pKa values, which decrease in the order BDC-OH (pKa1 = 4.79) > BDC-phenyl (pKa1 = 4.04) > BDC-NH2 (pKa1 = 3.95) > BDC (pKa1 = 3.51) > BDC-NO2 (pKa1 = 1.69).30-31 We infer that it is the strong basicity of the -NH2 substituent on the linker that led to the larger number of defects than in the other samples, but the reason for the pattern is still unclear. TGA results (Figure S5 in the SI) show that these UiO-66 samples with substituents on the BDC linkers were more easily decomposed in the presence of O2 than the MOF made with the unsubstituted BDC linker. The temperatures of MOF decomposition indicate that the stability of these MOFs increased in the order UiO-66-NH2 (566 K) < UiO-66-NO2 (641 K) ~ UiO-66-OH (641 K) < UiO-66-phenyl (693 K) < UiO-66 (764 K). We emphasize the numbers of defects on the nodes in these samples could not be determined by TGA because of the low stabilities of the MOFs. The BET surface area and N2 uptake data follow the same trend, with values decreasing in the order FA-15 > BDC-NH2 > BDCNO2 > BDC-OH > BDC-phenyl. Moreover, pore size distributions and pore volumes were also strongly influenced by substituents on the linker. For example, the BDC-NH2 sample was found to have a larger fraction of larger pores (diameter ≈ at 15 Å than the FA-15 sample, and the difference may be attributed to a larger number of missing node defects, leading to a higher pore volume in the former sample than the latter). However, BDC-NO2 and BDC-OH incorporate much lower amount of such large-pore structure; BDC-phenyl has even larger pores (diameter ≈ 18 Å) which could be explained by the larger size of this linker.

Table 2. DFT-calculated and experimental νOH values of µ3-OH groups on MOF nodes in UiO-66. Ligand on the vacancies

Linker

Computed frequencies (average of µ3-OH/difference between four wavenumbers) (cm-1)

Experimental frequencies (cm-1)

None

Benzoate

3682.4, 3681.7, 3681.6, 3681.2 (3681.7/1.2)

3672.6


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

Chemistry of Materials Formate

Benzoate

3673.4

Benzoate

3682.1, 3681.6, 3681.2, 3680.9 (3681.4/1.2) 3684.4, 3681.8, 3681.7, 3681.6 (3682.4/2.9)

Acetate CF3COO

Benzoate

3679.8, 3679.5, 3678.9, 3677.9 (3679.0/1.9)

3666.5

CF3COOa

Benzoate

3679.0, 3677.3, 3677.2, 3676.8 (3677.6/2.2)

3666.5

OH

Benzoate

3707.2 3681.6, 3681.2, 3681.0, 3680.5

OH/H2O

Benzoate

Ethoxy

Benzoate

3686.5 3682.1, 3681.9, 3681.4, 3674.0 3643.8 3681.8, 3681.3, 3680.7, 3653.0

Ethoxy/H2O

Benzoate

Formate

BenzoateNH2

3693.3, 3673.1,3663.1, 3637.4 3693.3, 3673.1, 3663.1, 3637.4 3674.3, 3656.4, 3636.3 3674.3, 3656.4, 3636.3 3672.4

Formate

BenzoateOH

3682.7, 3679.7, 3679.5,3678.7 (3680.2/4.0)

3669.9

Formate

BenzoateNO2

3634.8, 3613.0, 3609.9, 3603.5 (3615.3/31.4)

3670.6

Formate

BenzoateNO2b Benzoatephenyl

3681.3, 3639.7, 3599.8, 3591.5 (3628.1/89.8)

3670.6

3682.6, 3679.5, 3678.8, 3672.9 (3678.4/9.8)

3671.7

Formate

3682.7, 3681.7, 3681.6, 3666.9 3636.2 3682.7, 3678.9, 3678.6, 3675.7 (3679.0/7.0)

Calculation with three benzoate linkers substituted by CF3COO. respect to the carboxylate groups.

a

DFT calculations In order to corroborate experimental results, we computed the vibrational frequencies for the various ligands on the vacancies and various substituents on the BDC linkers. We modeled the BDC linkers by using benzoate linkers (more details of the model are reported in the SI and Figure S7). The various ligands on the vacancies were modeled by substituting only one benzoate linker (unless otherwise specified) with formate, acetate, trifluoroacetate, OH, or C2H5O linker, respectively (Figure S8). The substituents on the benzoate linkers were modeled by placing one substituent on each phenyl ring in the ortho-position (unless otherwise specified) with respect to the carboxylate group. Furthermore, one benzoate was replaced with a formate linker (Figure S9). In Table 2 we show the DFTcomputed stretching frequencies (scaled with a factor of 0.956; see computational details) of the µ3-OH bond along with the experimental ones. In brackets the average of the four computed µ3-OH stretching frequencies is reported, together with the difference between the highest and the lowest one.

b

3673.8

Two NO2 groups have been placed in the meta-position with

From the DFT analysis, we see that the substitution of one linker with either formate or acetate has a little effect on the µ3OH stretching frequencies, and therefore on the µ3-OH bond lengths. The substitution with trifluoroacetate red shifts the frequency by about 3 cm-1. Because there are about 3 vacancies per node when CF3COO is used as a modulator (Table 1), we also computed the frequencies for UiO-66 in which three benzoates have been replaced with CF3COO, with the results showing only very little difference with respect to UiO-66 having only one vacancy. Regarding the benzoate substitution with an OH ligand, we see that, apart from the µ3-OH stretching frequencies, there is an additional frequency, at 3707.2 cm-1 (as found experimentally), which corresponds to the OH stretching mode. However, the computational model does not produce the experimental red-shifted frequency at 3637.4 cm-1. We then thought that, if there is a small amount of humidity, one could generate some OH/H2O capping. By modeling this particular capping, we noticed that the presence of the water introduces a new OH stretching at 3643.8 cm-1, in agreement with the experimentally observed value. For the ethoxy ligand we



noticed an asymmetry (as in the experiment), with one µ3-OH frequency highly red-shifted by 28.9 cm-1 (with respect to the highest µ3-OH stretching frequency for the ethoxy ligand). And there is an even higher red- shifted frequency found experimentally at 3636.3 cm-1, that can be explained only by invoking a water molecule that is capping the vacant site (see the entry ethoxy/H2O in Table 2). The interpretation of this redshifted shoulder as arising from the water is consistent with a higher intensity of this shoulder for the OH ligand (whereby water is the source of the ligand) than for the ethoxy ligand (Figure 2c). Regarding the substituents on the benzoate, we see that the NH2 and OH substituents present a small red shift (about 3 cm-1) with respect to the UiO-66 with no substitutions, and a wider broadening of the range of frequencies with respect to the broadening caused by ligands on vacancies. On the other hand, the NO2 substituents have a dramatic effect on the µ3-OH stretching frequencies, producing a red shift of about 65 cm-1 with respect to UiO-66 with no substitutions. Accordingly, all the µ3-OH bonds are elongated. Furthermore, the difference between the highest and the lowest frequency is 31.4 cm-1, in agreement with the experimentally observed broad absorption of the NO2-substituted UiO-66. However, the computed frequencies are all highly red-shifted, whereas experimentally there is also absorption in the region where the unsubstituted UiO-66 absorbs (the experimental value is 3670.6 cm-1, Table 2). In any case, the NO2 substitution in the BDC linker can either be in an ortho or meta position with respect to one carboxylate group (Figure 1); we therefore also carried out frequency calculations for the same NO2 substitutions, in which we placed two of the NO2 groups in the meta position with respect to the carboxylate (of the benzoate, see Figure S10). In this molecule, there is one µ3-OH bond that is not now engaged in hydrogen bonding with NO2, and that µ3-OH bond now absorbs at similar frequencies with respect to unsubstituted UiO-66. Correspondingly, this µ3-OH bond has the same length as that in the unsubstituted UiO-66. With this model, we can explain the experimental spectra, with the broad red-shifted absorption (which is now 89.8 cm-1). Regarding the phenyl substituent, we see that the average frequency is red-shifted by about 10 cm-1 with respect to the spectrum of unsubstituted UiO-66, and the frequency range is about 10 cm-1. The results show that the DFT calculations agree well with the experimental trends. Discussion Evaluation of the influence of the linker substituents is challenging because some of the syntheses did not give highquality MOF crystals (especially, BDC-OH and BDC-phenyl), as shown by the PXRD patterns, SEM and TEM images, and the breadths of the IR bands (Figure 2d). Moreover, the µ3-OH bands characterizing these samples are all broader than that of the original UiO-66 sample modulated by formate, because of the asymmetric placement of the substituents on the benzene ring in the linkers. These results are confirmed by DFT calculations (Table 2) showing that the differences between the calculated frequencies of the four µ3-OH bands became greater with these addition of substituents to the benzene ring of the linker.

Page 8 of 11

The band frequencies (Table 2) show that for all of these samples, the bands shifted to lower frequencies relative to those of the formate-modulated UiO-66, indicating that these linkers withdraw more electrons from the nodes than the BDC linker. These results too are confirmed by the DFT calculations (Table 2). We emphasize that the influences on the node chemistry of the substituents on the linkers are not as large as those of the ligands bonded directly to the nodes, as expected. Our results demonstrate that the number of defect sites is controlled by the type and amount of modulator present in the synthesis and by the substituents present on the linker. Synthesis of a “defect free” UiO-66 sample is challenging because the modulator used in synthesis always competes with linkers to bind to the node and form defects. The pore size distribution data may include important information to distinguish missing linker and missing node defects. The data show that the number of defects increased markedly with an increasing amount of acetic acid (modulator) in the synthesis, but the pore diameters of these UiO-66 MOFs shifted only slightly to higher values (from 10.5 to 11.0 Å), indicating that the defects in these UiO-66 samples are mostly missing linkers—because these defects do not lead to major changes in pore size. In contrast, larger pores (with diameters of 15–20 Å) were observed for UiO-66 samples synthesized with formic acid, trifluoroacetic acid, and benzoic acid as modulators (the major pores were still those with a diameter of about 10 Å), and these can be attributed to missing node defects, as these defects are associated with larger pores. Our data show these missing node sites are generated by modulators with higher acidity or larger size. On the other hand, the substituent ligands on the linker also have a marked influence on the fraction of missing node structures. We emphasize that missing node and missing linker defects may coexist in all the samples, and it remains a challenge to distinguish them quantitatively. The data taken together show that the ligands on the nodes, and—to a lesser degree—those on the linker, allow tuning of the electronic properties of the UiO-66 nodes, thereby offering means to control the adsorption/catalytic properties of the nodes. Such effects have been reported before; for example, NH2 groups on BDC linkers have been reported32 to significantly improve the activity of the MOF for catalytic hydrolysis of surrogates of the nerve agent Sarin;33 -NO2 groups on BDC linkers also have been reported to increase the reaction rate 56 fold for the cyclization of citronella to isopulegol.34 However, the synergistic effects of the node and these groups are complicated and unresolved. Our data provide the first evidence resolving effects of ligands on the nodes and substituents on the linkers. We stress that the modifications of UiO-66 with various ligands on the node vacancies and linkers not only change the electron-donor properties of the nodes but also the number and type of vacancies (missing linkers or missing nodes), and in turn the pore structures and particle sizes. All these structural characteristics can influence reactivity and catalytic performance. Resolution of these effects requires further work. Many questions remain about these OH groups on the nodes; for example, how do the µ3-OH groups on the node influence/participate reactions (act as Brønsted acids)? How accessible are they to reactants? How effectively can the protons be replaced by metal ions through ion-exchange, and what are the resulting structures? These fundamental questions


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require more work, which may help in predicting the reactivities these and related materials, improve their performance, and expand the prospects of their applications. Conclusions UiO-66, one of the most stable MOFs, has been broadly investigated, with results characterizing its (defective) structure, reactivity, and potential for applications, such as gas separation and catalysis. Our data show that the µ3-OH groups on the Zr6O8 clusters (nodes) are an informative indicator of the electronic properties related to the ligands on the nodes, including the linkers. The MOF crystallinity, the number of vacancies per node, the ligands on the vacancies, the substituents on the linkers, and the symmetry of the structure all influence the µ3-OH IR band. The results of the DFT calculations agree well with the experimental results and may, we anticipate, help guide MOF synthesis for control of MOF properties.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Experimental results and computational details are reported, primarily IR spectra, N2 isotherms, XRD, SEM and TEM images (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] *[email protected] ORCID Dong Yang: 0000-0002-3109-0964 Carlo Alberto Gaggioli: 0000-0001-9105-8731 Christopher J. Cramer: 0000-0001-5048-1859 Laura Gagliardi: 0000-0001-5227-1396 Bruce C. Gates: 0000-0003-0274-4882

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported as part of the Inorganometallic Catalyst Design Center, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (DE-SC0012702).

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Tuning the Properties of Zr6O8 Nodes in the Metal Organic Framework

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