86 87 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 Soft CondensedMatter Group Beamline B22 Development of amesoporeMetal-Organic Framework catalyst Related publication: Kang X., Lyu K., Li L., Li J., Kimberley L.,Wang B., Liu L., ChengY., FrogleyM., Rudić S., Ramirez-Cuesta A., Dryfe R., Han B.,Yang S. & Schröder M. Integration of mesopores and crystal defects inmetal-organic frameworks via templated electrosynthesis. Nat. Commun. 10 , 4466 (2019) DOI: 10.1038/s41467-019-12268-5 Keywords: MOFs; Electrosynthesis; Defects; Mesopores; Catalysis M etal-organic frameworks (MOFs) are compounds made by linking together organic and inorganic units with strong bonds. Since their discovery over 20 years ago, scientists have synthesised tens of thousands of unique structures. MOFs have an exceptionally porous structure, and by synthesising structures with differing pore sizes, researchers can tune MOFs to filter, trap, or transport molecules. MOFs have a wealth of potential applications, in areas such as hydrogen storage, catalysis, drug delivery and carbon capture. Mesoporesarebetween2and50nminwidth.IncorporatingmesoporesandactivesitesintoMOFsischallenging,butcanleadtothediscovery of efficient new catalysts. An international team of researchers has created a mesoporous MOF, within 100 seconds at room temperature, by templated electrosynthesis using an ionic liquid as both electrolyte and template. This material incorporates crystal defects with uncoupled Cu(II) centres as evidenced by confocal fluorescence microscopy and by Fourier transform infrared and electron paramagnetic resonance spectroscopy. FTIRmeasurements were collected on the Multimode Infrared Imaging And Microspectroscopy (MIRIAM) beamline (B22). Prepared in this way, the MOF shows exceptional catalytic activity for the aerobic oxidation of alcohols to produce aldehydes in near quantitative yield and selectivity under mild conditions. It also displays excellent stability and reusability over repeated cycles. Incorporation of mesopores and active sites into metal-organic framework (MOF) materials to uncover new efficient catalysts is a highly desirablebutchallengingtask.AmesoporousMOFhasbeenobtainedwithin100secondsatroomtemperaturebytemplatedelectrosynthesis usingan ionic liquidas bothelectrolyteand template.Thismaterial incorporates crystal defectswithuncoupledCu(II) centers as evidencedby confocal fluorescence microscopy and by fourier transform infrared and electron paramagnetic resonance spectroscopy. MFM-100 prepared in this way shows exceptional catalytic activity for the aerobic oxidation of alcohols to produce aldehydes in near quantitative yield and selectivity under mild conditions, as well as having excellent stability and reusability over repeated cycles. The application of metal-organic frameworks (MOFs) in catalysis is often restricted by the hinderedmass transport of substrates throughmicropores and the lack of accessible active sites.Significant efforts have been devoted to the synthesis of mesoporous MOFs to facilitate substrate transport for catalysis and creating crystal defects to serve as active sites 1 . Many MOF systems are built frommetal centres with saturated coordination environments and thus do not incorporate labile sites. Also, the removal of coordinated solvents from metal centres to generate open sites can have a detrimental effect on the overall stability of the framework in activated MOFs. Open metal sites are readily saturated by solvent molecules used in catalytic reactions, and thus remain inaccessible to target substrates. Defects can be produced intentionally during synthesis or via post-synthetic treatment. 2 Integration of mesopores and active sites within crystal defects in MOFs via readily scalable synthetic routes could in principle greatly facilitate their applications in catalysis. Additionally, electrosynthesis can be used for materials production at varying scales 3 . The Mesoporous MOF MFM-100 has been successfully obtained by rapid (<100 seconds) electrosynthesis at room temperature. The structure, morphology and porosity of MFM-100 can be readily controlled by choice of energy input and ionic liquid (IL) in the synthesis. These MFM-100materialscontaincrystaldefects [ e.g. , uncoupled Cu(II) ions as Lewis acid sites], which have been characterised by confocal fluorescence microscopy (CFM), inelastic neutron scattering (INS), electron paramagneticresonance(EPR)andFourier transform infrared (FTIR) spectroscopy. Significantly, the integration of mesopores and active sites within the MFM-100 scaffold has endowed the resultant MOF with exceptional activity for the aerobic oxidation of a range of aldehydes and alcohols in high yield and selectivity. The MFM-100 catalysts also exhibit excellent stability and reusability. Four samples (denoted as a, b, c and d) of MFM-100 (also known as NOTT- 100 4 ) have been synthesised. MFM-100a was synthesised in a solvothermal reaction. MFM-100(b,c,d) were obtained by electrosynthesis at different temperature with different IL concentration. The microphotographs and CFM images reveal a stark comparison between MFM-100a and MFM-100d in furfuryl alcohol (Fig. 1). Large crystallites of MFM-100a only show fluorescence response at the crystal boundaries ( i.e. , the edge and gaps), while smaller particles show no fluorescence response. In contrast, all particles of MFM-100d exhibit strong fluorescence that is distributed evenly across the entire particle, directly confirming the presence of homogenous active Cu(II) sites as Lewis acid sites at defects within MFM-100d. MFM-100(a,b) are more crystalline than MFM-100(c,d) as measured by PXRD (Fig. 2a) and the INS features of MFM-100c and MFM-100d are heavily convoluted and broadened (Fig. 2b) suggesting that the latter have substantial crystal defects leading to reduction in long-range order of the sold-state lattice. Powder X-ray diffraction (PXRD) and INS give the average property of samples, while FTIR from Beamline B22 offers detailed information on a single particle. FTIR spectra show notable decrease in intensity and increase of band broadening on going from MFM-100a to MFM-100d (Fig. 2c). These results are consistent with the reduced crystallinity and presence of crystal defects in MFM-100(c,d) and indicates that every particle exhibits defects. A small increase in intensity at 1574 cm -1 (assigned to the C=C vibration of the imidazole ring) is observed in the FTIR spectra of MFM-100c and MFM-100d, indicating the presence of trace IL cations. The EPR spectrum confirms that the defects are uncoupled Cu(II) centres (Fig. 2d) 5 , which can be considered as active centers for oxidation of alcohols. The presence of mesopores is another important factor for catalysis.The highly crystallineMFM-100a shows high surface areas (1586m 2 g -1 ) with the pore size distribution centred at 6.5 Å. There is thus an absence of mesopores in MFM- 100a. In contrast, mesopores centred at 4.6 nm are observed in MFM-100d (V meso = 1.17 cm 3 g -1 and S meso /S total = 0.65), which shows a total BET surface area of 1353 m 2 g -1 . The Cu(II)-based MOFs, HKUST-1 and MOF-2, have also been prepared by solvothermal synthesis and by templated electrosynthesis. Analysis of the structure and porosity of these samples confirms that the electrosynthesised MOFs show reduced crystallinity and the presence of significant amounts of mesopores compared with the samples obtained by solvothermal synthesis. This result indicates that the strategy based upon templated electrosynthesis developed here has general applicability to the synthesis of mesoporous, defective Cu(II)-MOFs. Oxidation of alcohols over Cu(II)-based catalysts is an efficient method to synthesise aldehydes, and we therefore tested the catalytic activity of MFM- 100, HKUST-1 and MOF-2 for the oxidation of a range of primary and secondary alcohols. Crystal defects consisting of uncoupled Cu(II) ions in MFM-100(c,d) play a positive role in the observed catalytic activity, leading to the quantitative conversion of benzyl alcohol with MFM-100(c,d). Similarly, electrosynthesised HKUST-1 and MOF-2 show better catalytic performance than catalyst materials synthesised by solvothermal methods. Mesoporous MOFs are particularly beneficial for oxidation of large molecules such as 3,3’,5,5’-tetrakis(trifluoromethyl)benzhydrol. Thus, MFM- 100(c,d) exhibit notably higher activity than MFM-100(a,b), and MFM-100d affords quantitative production of the corresponding aldehyde. We have found that the co-presence of crystal defects and mesopores greatly promotes catalytic oxidation, with exceptional catalytic performance being observed for MFM-100d.Stronger interactionbetweenMFM-100dandalcoholwasobserved by INS spectroscopy, indicating the defective structure plays a significant role in enhancing catalytic activity.The methodology developed here paves a new and easy-scalable pathway to synthesise mesoporous and defect MOFs as efficient catalysts. References 1. Drake T. et al. Site isolation in metal-organic frameworks enables novel transition metal catalysis. Acc. Chem. Res. 51 , 2129-2138 (2018). DOI: 10.1021/acs.accounts.8b00297 2. Shearer G. C. et al. Tuned to perfection: ironing out the defects in metal- organic framework UiO-66. Chem. Mater. 26 , 4068-4071 (2014). DOI: 10.1021/cm501859p 3. Li M. et al. . Reductive electrosynthesis of crystalline metal-organic frameworks. J. Am. Chem. Soc. 133 , 12926-12929 (2011). DOI: 10.1021/ja2041546 4. Lin X. et al. High H 2 adsorption by coordination-framework materials. Angew. Chem. Int. Ed. 45 , 7358-7364 (2006). DOI: 10.1002/ange.200601991 5. EL Mkami, H. et al. EPR and magnetic studies of a novel copper metal organic framework (STAM-I). Chem. Phys. Lett. 544 , 17-21 (2012). DOI: 10.1016/j.cplett.2012.06.012 Funding contribution: Diamond Light Source; ISIS Facility; University of Manchester; Institute of Chemistry, Chinese Academy of Sciences; Royal Society. Corresponding authors: Dr. Sihai Yang, University of Manchester,
[email protected] Prof. Martin Schröder, University of Manchester, M.Schroder@manchester. ac.uk Figure 1. Comparison of the micrographs and CMF images of MFM-100a (a, b) and MFM-100d (c, d). The scale bar is 5 μm in all images. The fluorescence (red colour) indicates crystal defects determined by the oligomerization of furfuryl alcohol. Figure 2. Characterisations of samples of MFM-100 samples obtained by solvothermal reactions and by electrosynthesis. (a) PXRD patterns; (b) INS plots; (c) FTIR spectra; (d) solid state EPR spectra at same sensitivity and concentration of material.