Nanoscaled M-MOF-74 Materials Prepared at Room Temperature

Abstract

This paper describes the preparation and characterization of nano-scaled M-MOF-74/CPO-27-M (M = Mg, Mn, Co, Ni and Zn) materials at room temperature. Some of the soformed crystals are the smallest ones of any MOF material (and, to the best of our knowledge, of any microporous material) ever reported. They are in the limit of being able to diffract, particularly these forming the Co-and Ni-MOF-74 samples. Consequently, unequivocal identification as crystalline MOF-74 phase was deduced by combining other characterization techniques rather than powder X-ray diffraction. These small crystals are unstable as isolated ones, so they form steady and robust aggregates, whose mechanical properties strongly depend on the crystal size. The particles that results from the 'fusion' of nanocrystals smaller than 10 nm (more properly denoted as nanodomains) could not be disaggregated by conventional ultrasonic and graining techniques. On the contrary, agglomerates of crystals larger than 10 nm are dissociable in discrete crystals. It allows characterizing Zn-MOF-74 nanocrystals by advanced electron microscopy methods. Cs-corrected STEM provided, for the first time, 'quasi' atomic resolution images of MOFs, which are especially unstable under electronic radiation. The magnitude of the crystal size of M-MOF-74 is tentatively associated to the solubility of the metal source.

Controlling the size and shape of Mg-MOF-74 crystals to optimise film synthesis on alumina substrates

“…In addition, the broad peaks observed in XRD pattern for the synthesis with DMF only (F1) suggests that nano-crystals are produced. The size of the crystals from formulation F1 (DMF-only) formed after 6 h at 125°C was calculated as 8.0 nm, showing good agreement with previously report 38 . When the volume of water and ethanol was increased in the synthesis solutions (from F1 to F4) the intensity of the characteristic XRD peaks (6.7°and 11.7°) for Mg-MOF-74 increased and sharper peaks were obtained.…”

“…Further images of the alumina surface after synthesis using formulation F1 are shown in the supplementary information ( Figure S5). Using only DMF (F1), the crystals formed have a diameter of around 8 nm as reported by Díaz-García et al 38 (confirmed with the Scherrer equation) and are prone to agglomeration, as can also be seen in Figure S1. The nano-sized particles may not readily attach to the surface of the alumina substrate, most likely because the crystals are much smaller than alumina substrate pores (200 nm).…”

“…Mg-MOF-74 crystal synthesis was based on the previously reported method by Díaz-García et al 38 with an elevated temperature to reduce synthesis time. In a typical synthesis, 0.39 to 0.41 g H4DHTP was dissolved in 10 ml DMF using a magnetic stirrer until the ligand had completely dissolved.…”

Doping of metal–organic frameworks towards resistive sensing

“…Note that characterization of MOFs by TEM/STEM is notoriously challenging because of the high susceptibility of the organic linkers to electron beam induced degradation 19–21 . Our high-resolution microscopy data are the first such available on Co-MOF-74 nanocrystals to date 19, 20, 22 .

Figure 2( a ) BF TEM image of agglomerated Co-MOF-74 nanocrystals. ( b ) SAED pattern of such nanocrystals in which the extracted radial intensity profile in the overlay confirms their Co-MOF-74 structure 18 .

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Multi-scale crystal engineering of metal organic frameworks

“…Following a similar procedure, M-MOF-74 nanocrystals with a narrow size distribution (M = Mg, Mn, Co, Ni, Zn) were immediately isolated adding dropwise a DMF solution of 2,5-dihydroxyterphthalic acid (DHTP) over a DMF solution containing the corresponding divalent metal acetate tetrahydrate [197]. On the opposite, by using slow diffusion and long reaction times (more than one week), large crystals of MOF-2, MOF-3 and MOF-5 [198][199][200], have been synthesized from metal salt Zn(NO 3 ) 2 and 1,4-benzenedicarboxylic acid (H 2 BDC).…”