0428同步年報-2021-全

Chemical Science 025 Metal-Organic Framework Membranes for Gas Separation An aluminium-carboxylate-based metal-organic framework (MOF), CAU-10-H, was utilized to fabricate a dense MOF membrane for CO 2 capture. The pore aperture size of this MOF is identified as 3.2 Å, which is ideal to separate CO 2 from N 2 and CH 4 . M etal-organic frameworks (MOF) possess adjustable pore topologies and interior functionality, and MOF membranes are known to have a great potential in gas separations. A molecular-sieving effect is the principal mechanism for gas separations with membranes. MOF with a controllable pore-limiting diameter (PLD) can be engineered to achieve high performance for such membrane gas separation. This work led by Dun-Yen Kang (National Taiwan University) aimed to fabricate MOF membranes with excellent gas-permeative selectivity. They focused on an aluminium hydroxide isophthalate MOF, CAU-10-H, with a rigid pore structure (PLD = 3.2 Å) for the adsorption of various gas molecules. This MOF was comprised of aluminium as metal cluster and isophthalic acid as organic linker ( Fig. 1 ). The chemical formula of this compound is Al(OH)(C 8 H 4 O 4 ). Kang’s group conducted time-resolved powder X-ray diffraction (XRD) measurements for various gases to investigate a possible structural change of CAU-10-H using the facilities at TPS 09A . These XRD patterns were obtained in situ with an X-ray source of synchrotron radiation, which allows for a direct comparison of the signal intensities and positions between the patterns. The results from the measurements under CO 2 , N 2 and CH 4 are summarized in Fig. 2 . They noticed a significantly decreased signal intensity in time-resolved patterns of CAU-10-H with partial pressure of CO 2 increasing from 0.005 to 3 bar ( Fig. 2(a) , see next page); this substantial drop in intensity indicated the adsorption of CO 2 in CAU-10-H. The adsorption of gas molecules in the cage of MOF decreased the contrast of electron density inside the microporous materials, resulting in a diminished intensity of the powder XRD patterns. They further identified the structure of CAU-10-H after exposure to CO 2 for 30 minutes using Rietveld refinement. The result showed that the crystal structure of CAU-10-H remained nearly unchanged after exposure to CO 2 even though the signal intensity decreased ( Fig. 2(b) ). The rigid pore structure of CAU-10-H makes it an ideal material for CO 2 separation because of a molecular-sieving effect. In contrast to CO 2 measurements, there was no considerable difference in signal intensity for XRD patterns under N 2 and CH 4 ( Figs. 2(c) and 2(d) ). This condition indicates that the uptakes of N 2 and CH 4 in CAU-10-H were smaller than the uptake of CO 2 . The permeation measurements on a CAU-10-H membrane of single gases for H 2 , CO 2 , N 2 and CH 4 and mixed gas for CO 2 and CH 4 are summarized in Fig. 3 (see next page). These gas-permeance results indicated that there was a permeation cutoff between CO 2 and N 2 ( Fig. 3(a) ). The ideal selectivity, defined as the permeance ratio between two gases, of CAU-10-H for every gas pair (H 2 /CH 4 , CO 2 / N 2 , CO 2 /CH 4 and N 2 /CH 4 ) considerably deviated from the Knudsen selectivity ( Fig. 3(b) ), indicating that the formation of pinholes in a CAU-10-H membrane was successfully suppressed; the transport of gases inside CAU- 10-H was properly controlled by the intrinsic property of the rigid pore structure of CAU-10-H. In summary, Kang’s group proposed that utilizing the rigid pore structure of MOF under a varied gas atmosphere Fig. 1 : Aluminium hydroxide isophthalate MOF for membrane separation. [Reproduced from Ref. 1]