Abstract
Nano/microsized MIL-101Cr was synthesized by microwave heating of emulsions for the use as a composite with Matrimid mixed-matrix membranes (MMM) to enhance the performance of a mixed-gas-separation. As an example, we chose CO2/CH4 separation. Although the incorporation of MIL-101Cr in MMMs is well-known, the impact of nanosized MIL-101Cr in MMMs is new and shows an improvement compared to microsized MIL-101Cr under the same conditions and mixed-gas permeation. In order to reproducibly obtain nanoMIL-101Cr microwave heating was supplemented by carrying out the reaction of chromium nitrate and 1,4-benzenedicarboxylic acid in heptane-in-water emulsions with the anionic surfactant sodium oleate as emulsifier. The use of this emulsion with the phase inversion temperature (PIT) method offered controlled nucleation and growth of nanoMIL-101 particles to an average size of <100 nm within 70 min offering high apparent BET surface areas (2,900 m2 g−1) and yields of 45%. Concerning the CO2/CH4 separation, the best result was obtained with 24 wt.% of nanoMIL-101Cr@Matrimid, leading to 32 Barrer in CO2 permeability compared to six Barrer for the neat Matrimid polymer membrane and 21 Barrer for the maximum possible 20 wt.% of microMIL-101Cr@Matrimid. The nanosized filler allowed reaching a higher loading where the permeability significantly increased above the predictions from Maxwell and free-fractional-volume modeling. These improvements for MMMs based on nanosized MIL-101Cr are promising for other gas separations.
Highlights
Starting at the beginning of the 1990s metal-organic frameworks (MOFs) as new porous materials have been constructed from metal-atom or metal-cluster building blocks and bridging organic linkers (Batten and Robson, 1998)
Powder X-ray diffraction (PXRD) patterns were obtained on a Bruker D2 Phaser powder diffractometer equipped with a flat silicon, low background sample holder using Cu–Kα radiation (λ = 1.5418 Å, 30 kV, 10 Ma, ambient temperature)
Nanosized particles of MIL-101Cr can be synthesized by microwave heating alone (Khan et al, 2011), yet in our hands the use of microwave heating alone did not lead to reproducible results
Summary
Starting at the beginning of the 1990s metal-organic frameworks (MOFs) as new porous materials have been constructed from metal-atom or metal-cluster building blocks and bridging organic linkers (Batten and Robson, 1998). Consideration of the geometric and chemical attributes of the secondary building units and linkers leads to prediction of the framework topology (Eddaoudi et al, 2001) It became quickly apparent, how molecular complexes and clusters can be transformed to extended solids (Yaghi et al, 2000). There are various approaches to control the MOF particle size, for example, microwave-assisted routes (Khan et al, 2011), surfactant-mediated syntheses (Huang et al, 2003), reverse microemulsions (Lin et al, 2009), and sonochemistry (Khan and Jhung, 2015) Among these strategies microwave heating and the use of surfactants have been the most advantageous methods (Khan and Jhung, 2015; Huang et al, 2018). In microwave reactions the required temperatures can be reached within seconds (Galema, 1997) and microwave heating is an “instant on/instant off ” energy source, significantly reducing the risk of overheating reactions (Bogdal, 2006)
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