ganic Frameworks via Combined Transmission Electron Microscop

C.; Gándara, F.; Nguyen, P. T. K., A Series of Metal – Organic. Frameworks for Selective Co2 Capture and Catalytic Oxidative. Carboxylation of Olef...
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Interrogating Kinetic versus Thermodynamic Topologies of Metal-Organic Frameworks via Combined Transmission Electron Microscopy and X-ray Diffraction Analysis Xinyi Gong, Hyunho Noh, Nathan C. Gianneschi, and Omar K. Farha J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b01789 • Publication Date (Web): 30 Mar 2019 Downloaded from http://pubs.acs.org on March 30, 2019

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Interrogating Kinetic versus Thermodynamic Topologies of Metal-Organic Frameworks via Combined Transmission Electron Microscopy and X-ray Diffraction Analysis Xinyi Gong,§ Hyunho Noh,§ Nathan C. Gianneschi,* and Omar K. Farha* International Institute of nanotechnology and Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States

Supporting Information Placeholder ABSTRACT: Synthetic protocols that preferentially re-

sult in metal–organic framework (MOF) crystallization of one topology over another remains an elusive, empirical process. This is primarily because of a lack of fundamental insights into MOF crystal growth and the effect of various experimental parameters on the resulting topologies. In this paper, we demonstrate the temperature-topology relationship of MOFs constructed from hexanuclear oxozirconium cluster nodes and tetrakis(4-carboxylphenyl)porphyrin linkers via a combined transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD) study. Synthesis at room temperature led to a mixed phase consisting of twelve-connected (assuming no defects) MOF-525 and six-connected PCN-224, possessing ftw and she topologies respectively. When the temperature was raised to 145 °C, eight-connected PCN222 (csq topology), with a possible concurrence of another eight-connected NU-902 (scu topology) and twelve-connected PCN-223 (shp topology), were found in addition to MOF-525 and PCN-224. With an increase in reaction time at 145 °C, a change in product distribution was observed where PCN-222 remained the major crystal phase after seven days, while the contribution from MOF-525 and PCN-224 decreased. These data suggest that MOF-525 and PCN224 are the kinetic, while PCN-222 is the thermodynamic product.

Metal–organic frameworks (MOFs) have emerged as candidate materials in variety of applications, including gas storage,1-3 gaseous/condensed-phase separation,4-8 catalysis,9-16 and drug delivery.17-20 This has been enabled by exploiting their generally high porosity and crystallinity. These applications heavily rely on

the geometry of the chemical environment within the MOF pores, which are ultimately determined by their crystal topologies.4,21-23 Additionally, the computational screening of “pure phase” structures of MOFs for identifying the top performing systems for a given application has been instrumental in the field.24 Therefore, the development of synthetic protocols that induce complete crystallization yielding only the desired topology is crucial. In the process of MOF crystal growth, identical constituents often result in various crystal topologies (polymorphs), dictated by experimental parameters such as the reaction temperature,25-27 identity and/or concentration of modulator(s),28-30 and substrate identities (i.e. metal salts, solvent), ratios, and concentrations.11,31-32 To date, only a few experimental33-34 and computational35-37 studies have attempted to determine correlations between experimental parameters and the resulting MOF topologies constructed from identical constituents. As such, the development of synthetic protocols remains empirical and the fundamental rationale behind such topological complexities are yet to be unambiguously determined. Topological complexity is at its height in MOFs with hexanuclear oxozirconium inorganic nodes and free base tetrakis(4-carboxyphenyl)porphyrin (TCPP) organic ligands, from which at least six crystallographically unique topologies have been reported to date (Figure 1).21,38 These MOFs are all distinct in their node connectivity (i.e. number of linkers coordinated to each Zr6 node)21 and/or their linker conformation such as the torsion angles of four carboxyphenyl groups relative to the central porphyrin.36 While highthroughput screening efforts reported by Kelty et al.33 highlight the importance of the modulator pKa and

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Figure 1. Crystal structures of six structurally distinct MOFs with Zr6 nodes and TCPP4--based organic linkers. Corresponding nets and node connectivity are given for each structure.

synthesis temperature in the resulting crystal topologies, the fundamental mechanism behind preferred crystallization in these porphyrinic MOF crystallizations remains to be elucidated. Herein, we focus on thermal modulation of Zr6-porphyrinic MOF synthesis from room temperature (22 °C) to 145 °C (i.e. just below the boiling point of the organic solvent) to systematically analyze the effect of synthesis temperature on the resulting topologies while keeping other variables constant. We previously have observed a non-linear correlation between the reaction temperature and the resulting topology in a scu-net Zr6-based MOF, NU-901, with tetratopic pyrene-based linkers.27 Examining the crystal structures of the Zr6-based porphyrinic MOFs with more available topologies as to the pyrene-based MOFs, we hypothesized the torsion angle of the four carboxyphenyl groups on the linker relative to the central porphyrin and relative alignment (i.e. parallel or antiparallel) to the neighboring carboxyphenyl groups control the resulting topologies, given the previous results on tetratopic pyrene-based MOFs, NU-901 and NU1000.29,37 In PCN-222, the carboxylphenyl groups reach a ~60 torsional angle and are aligned antiparallel (typical of a csq net),16 significantly reducing the -delocalization, thus suggests an energetically less

stable intermediate. In contrast, at room temperature, a more planar linker36-37 may lead to a ftw net. We note that, given a similar pyrene-based linker but with almost no torsional angle, MOFs with a ftw net have been reported.39 The systematic examination described is critical to our fundamental understanding of which crystal phases are preferred under given reaction conditions, allowing for predictable approaches to more readily prepare targeted MOF topologies. Synthesis of the Zr6-TCPP-based MOFs at various temperatures involves two steps, 1) crystallization of benzoate-capped Zr6-cluster nodes and 2) controlled addition of free base TCPP to the solvated node solution.27,40 MOF syntheses were performed via syringe titration of 0.5 mL/min of a TCPP linker solution into the Zr6 node solution with formic acid as the modulator at various temperatures (see Supplementary Information (SI) for details). The material synthesized at room temperature is described as Zr6-TCPP-RT. All resulting materials are described below as Zr6-TCPP-ST, where ST defines the synthesis temperature. As evident from the N2 adsorption-desorption isotherm at 77 K, Zr6-TCPP-RT exhibited a high porosity with a Brunauer–Emmett– Teller (BET) area of 2400 m2/g ; see Figure 2a. The

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density-functioanl theory (DFT)-calculated pore size distribution (Figure 2b) derived from the isotherm presents three distinct pores of sizes 13, 15, and 20 Å. By comparing the pore size distributions (Figure 2b) of all reported MOFs solvothermally synthesized as a reference (Figure S1; see the SI for details), the 13 and the 15 Å pores are attributed to those from MOF-525 and/or PCN-224 with she and ftw topologies, respectively). This is also evident from a comparison of the powder X-ray diffraction (PXRD) patterns of the material to pure-phase MOFs (Figure 2c). Returning to Figure 2b, the largest 20 Å pores, not evident in any MOFs synthesized as a reference, are attributed to the defect sites, which the two other reported Zr6-based MOFs synthesized at room temperature also show a high density of.27,41 The scanning electron microscopy (SEM) images revealed the cubic morphology of the crystals (Figure S3), in agreement with the previous reports of MOF-525 and PCN-224.42-43

Figure 2. a) N2 isotherms (where filled and unfilled points indicate adsorption and desorption, respectively) and b) DFT-calculated differential pore size distributions of Zr6-TCPP-ST synthesized at RT-145 °C, and c) their PXRD patterns plotted with simulated ones of purephase MOFs.

The aforementioned non-linear temperature-topology relationship in the tetratopic pyrene linker-based MOF27 inspired us to expand this concept to porphyrinic MOF-based system with more available topologies. An increase in synthesis temperature to 60 or 90 °C resulted in minor increases in pore volume (Figure 2b) and negligible differences in the porosity (see Figure 2a and 2b) and the PXRD pattern (Figure 2c). Yet, Zr6-TCPP-130 shows a peak at 2θ = 2.5°, corresponding to the (100) plane of a csq-topology MOF, PCNACS Paragon Plus Environment

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222.44 The diffraction peaks of PCN-222 become even more prominent upon further increase in synthesis temperature. Specifically, the peaks at 2θ = 2.5°, 5°, and 7.5° for Zr6-TCPP-145 have significantly enhanced intensities compared to those peaks for materials synthesized at lower temperatures (Figure 2c). The apparent crystal phase distribution change is well correlated with the DFT-calculated pore size distribution (Figure 2b). Namely, Zr6-TCPP-145 has a slightly smaller micropore of 12 Å, associated with the triangular micropore of PCN-222 along with the 32 Å mesopore attributed to the hexagonal pore of PCN-222 (Figure 1). Simultaneously, the cumulative pore volume of 20 Å pore (i.e. pores due to defects of MOF-525 and PCN-224) decreased with the increasing temperature (see Table 1). We note that the inverse correlation between the temperature and the defect sites, again, has been observed in two other Zr6-based MOFs synthesized at various temperatures.27,41 We further note due to similar crystallographic d-spacings, NU902 (scu net) and PCN-223 (shp net) may be present in the PCN-222 containing Zr6-TCPP-130 and Zr6TCPP-145. 11,45 The observed micropores in the aforementioned two samples (Figure 2b) can be a sum of contributions from PCN-222, NU-902, and PCN-223, due to their remarkably similar sorption behavior.11,45

to PCN-222) emerging from the surface of cubic crystals (attributed to MOF-525 and/or PCN-224). Yet, in the SEM images of those after three and seven days (Figure S5), the rod-like morphology crystals are concurrently observed. Thus, we believe the initially crystallized MOF-525 and/or PCN-224, under elevated temperature and given substantial reaction time, can undergo a phase change(s) via linker and node rearrangement.

Table 1. BET area and cumulative pore volume associated with 20 and 32 Å of TCPP-based MOFs synthesized at various temperatures.

Synthesis Temperature (°C)

BET Area (m2/g)

RT

Cumulative Pore Volume (cm3/g)a 20 Å

32 Å

2400

0.54

0.0

60

2700

0.41

0.0

90

2600

0.43

0.0

130

2600

0.26

0.094

145

2300

0.18

0.076

aCumulative pore volumes of individual pores were calculated via difference in ruling out the contributions from uptakes from other pores. For cumulative pore volume plots, see Figure S4.

From our temperature modulation study, changes in the crystal phase(s) distribution suggest that in this system, MOF-525 and PCN-224 exist as the kinetic products, and only upon additional thermal energy can the system yield the thermodynamic product, PCN-222. Indeed, when the reaction time was extended, relative abundance of PCN-222 gradually increased, and at seven days, the PCN-222 became the major product based on PXRD (Figure 3). The ex-situ SEM images of the isolated product after one day (Figure S3) reveal iconic rod-like crystallites (attributed

Figure 3. PXRD patterns of Zr6-TCPP-145 allowed to react for specified amounts of time.

The notably similar sorption behavior and the diffraction patterns of the two possible kinetic products, MOF-525 and PCN-224 (Figure S1-2),11,42-43,45 prompted us to attempt to distinguish them via highresolution transmission electron microscopy (HRTEM) – an increasingly more prevalent characterization applied to MOFs to unravel phase purity and allocations of post-synthetically incorporated guest molecules.46-49 However, under such imaging conditions, the two MOFs with free base linkers are indistinguishable (Figure S6a-b). This lead to our generating MOF-525(Pt) and PCN-224(Pt) using the Pt-metalated TCPP linker (see the SI for the detailed synthesis). These materials exhibit enough image contrast such that for PCN-224(Pt), which is tilted on its crystallographic axis, the 110 distance around 27 Å was observed directly, while no such distance was observed in MOF-525(Pt), revealing the key differences in their linker arrangements (Figure 4).

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and PCN-224, commonly invisible next to a more electron-dense metal nodes under an electron microscope. We propose these approaches for achieving a more comprehensive and fundamental understanding of the correlation between experimental parameters and MOF topologies, ultimately assisting in the facile development of MOF synthetic protocols yielding the desired topology without laborious screening of reaction conditions. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Detailed material synthesis and characterization (PDF)

AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected]

Author Contributions §X.

G. and H. N. contributed equally.

Funding Sources

No competing financial interests have been declared.

ACKNOWLEDGMENT

Figure 4. HRTEM images of a) MOF-525(Pt) and b) PCN-224(Pt). Inset: FFT of the image inside red square, cropped at the predominant lattice fringes.

The synthetic protocol allowed for ready crystallization of porphyrinic MOFs at a desired temperature from room temperature to 145 °C, further aiding in systematic examination of temperature-topology as well as temperature-defect density relationships. Namely, in a porphyrinic MOF-based system, dynamic crystal phase change from MOF-525 and PCN-224 to PCN-222 was observed with increasing temperature. csq-net topology, compared to ftw or she topologies, requires a larger torsion angle of the four benzoates to the central porphyrin.44 Simultaneously, we developed practical methods for characterization by utilizing TEM in combination with metalation of linker ligands, allowing one to distinguish between porphyrinic MOFs that otherwise elude differentiation. Specifically, by modifying organic linkers with Pt, linker arrangements can be distinguished between MOF-525

O.K.F. and N.C.G. gratefully acknowledge support from National Science Foundation's MRSEC program (grant number NSF DMR-1720139v). N.C.G. gratefully acknowledge support from ARO. H. N. gratefully acknowledges support from the Ryan Fellowship and the Northwestern University International Institute of Nanotechnology. This work made use of the J. B. Cohen X-Ray Diffraction and EPIC facilities of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN.

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Journal of the American Chemical Society (35) Avila, J. R.; Peters, A. W.; Li, Z.; Ortuño, M. A.; Martinson, A. B. F.; Cramer, C. J.; Hupp, J. T.; Farha, O. K., Atomic Layer Deposition of Cu(I) Oxide Films Using Cu(Ii) Bis(Dimethylamino-2Propoxide) and Water, Dalton Trans. 2017, 46, 5790-5795. (36) Ma, J.; Tran, L. D.; Matzger, A. J., Toward Topology Prediction in Zr-Based Microporous Coordination Polymers: The Role of Linker Geometry and Flexibility, Cryst. Growth Des. 2016, 16, 4148-4153. (37) Liu, W.-G.; Truhlar, D. G., Computational Linker Design for Highly Crystalline Metal–Organic Framework Nu-1000, Chem. Mater. 2017, 29, 8073-8081. (38) We omitted MOF-545 in this list as they are crystallographically identical to PCN-222. (39) Gómez-Gualdrón, D. A.; Wang, T. C.; García-Holley, P.; Sawelewa, R. M.; Argueta, E.; Snurr, R. Q.; Hupp, J. T.; Yildirim, T.; Farha, O. K., Understanding Volumetric and Gravimetric Hydrogen Adsorption Trade-Off in Metal – Organic Frameworks, ACS Appl. Mater. Interfaces 2017, 9, 33419-33428. (40) Kickelbick, G.; Wiede, P.; Schubert, U., Variations in Capping the Zr6o4(Oh)4 Cluster Core: X-Ray Structure Analyses of [Zr6(Oh)4o4(Ooc – Ch=Ch2)10]2( Μ -Ooc – Ch=Ch2)4 and Zr6(Oh)4o4(Oocr)12(Proh) (R = Ph, Cme = Ch2), Inorg. Chim. Acta 1999, 284, 1-7. (41) DeStefano, M. R.; Islamoglu, T.; Garibay, S. J.; Hupp, J. T.; Farha, O. K., Room-Temperature Synthesis of Uio-66 and Thermal Modulation of Densities of Defect Sites, Chem. Mater. 2017, 29, 1357-1361. (42) Morris, W.; Volosskiy, B.; Demir, S.; Gándara, F.; McGrier, P. L.; Furukawa, H.; Cascio, D.; Stoddart, J. F.; Yaghi, O. M., Synthesis, Structure, and Metalation of Two New Highly Porous Zirconium Metal–Organic Frameworks, Inorg. Chem. 2012, 51, 6443-6445.

(43) Feng, D.; Chung, W.-C.; Wei, Z.; Gu, Z.-Y.; Jiang, H.-L.; Chen, Y.-P.; Darensbourg, D. J.; Zhou, H.-C., Construction of Ultrastable Porphyrin Zr Metal – Organic Frameworks through Linker Elimination, J. Am. Chem. Soc. 2013, 135, 17105-17110. (44) Feng, D.; Gu, Z.-Y.; Li, J.-R.; Jiang, H.-L.; Wei, Z.; Zhou, H.C., Zirconium-Metalloporphyrin Pcn-222: Mesoporous Metal – Organic Frameworks with Ultrahigh Stability as Biomimetic Catalysts, Angew. Chem. Int. Ed. 2012, 51, 10307-10310. (45) Feng, D.; Gu, Z.-Y.; Chen, Y.-P.; Park, J.; Wei, Z.; Sun, Y.; Bosch, M.; Yuan, S.; Zhou, H.-C., A Highly Stable Porphyrinic Zirconium Metal–Organic Framework with Shp-a Topology, J. Am. Chem. Soc. 2014, 136, 17714-17717. (46) Zhu, Y.; Ciston, J.; Zheng, B.; Miao, X.; Czarnik, C.; Pan, Y.; Sougrat, R.; Lai, Z.; Hsiung, C.-E.; Yao, K.; Pinnau, I.; Pan, M.; Han, Y., Unravelling Surface and Interfacial Structures of a Metal–Organic Framework by Transmission Electron Microscopy, Nat. Mater. 2017, 16, 532. (47) Zhang, D.; Zhu, Y.; Liu, L.; Ying, X.; Hsiung, C.-E.; Sougrat, R.; Li, K.; Han, Y., Atomic-Resolution Transmission Electron Microscopy of Electron Beam – Sensitive Crystalline Materials, Science 2018, 359, 675-679. (48) Wiktor, C.; Meledina, M.; Turner, S.; Lebedev, O. I.; Fischer, R. A., Transmission Electron Microscopy on Metal–Organic Frameworks – a Review, J. Mater. Chem. A 2017, 5, 14969-14989. (49) Aulakh, D.; Liu, L.; Varghese, J. R.; Xie, H.; Islamoglu, T.; Duell, K.; Kung, C.-W.; Hsiung, C.-E.; Zhang, Y.; Drout, R. J.; Farha, O. K.; Dunbar, K. R.; Han, Y.; Wriedt, M., Direct Imaging of Isolated Single-Molecule Magnets in Metal – Organic Frameworks, J. Am. Chem. Soc. 2019, 141, 2997-3005.

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