(315g) Synthesis of Polycrystalline ZIF-8 Membranes in Few Minutes for CO2/N2 and CO2/CH4 Separation

The superior performance of ZIF membranes in gas separation has propelled intensive efforts to synthesize polycrystalline submicron-thick ZIF films in a scalable way [1]. The traditional solvothermal crystallization route is disadvantageous compared to recently reported vapor-phase crystallization routes [2,3] as the synthesis time for membranes in the former case is often several hours or days. This is mainly because the crystallization in the solvothermal route (dipping a substrate in a solution containing growth precursor solution) is difficult to control with the precursor concentration dropping as a function of time.

In this presentation, I will discuss novel crystallization using sustained precursors (CUSP) route that maintains a high precursor concentration in the growth step, hindering the undesired Ostwald ripening observed in the late stage of growth [4-5]. As a result, well-intergrown polycrystalline MOF films hosting a uniform grain size and a thickness of a few hundred nanometers could be obtained at room temperature in just 8 minutes, yielding attractive H₂/C₃H₈ selectivity (2433) and C₃H₆/C₃H₈ (30) selectivities.

Further, I will discuss post-synthetic treatment, which shrinks the ZIF-8 lattice by ca. 1%, drastically improves the molecular-sieving performance of these films [6]. Lattice stiffening is confirmed by the appearance of a temperature-activated transport, attributed to a stronger interaction of gas molecules with the pore aperture, with activation energy increasing with the molecular size (CH₄ > CO₂ > H₂). High CO₂/CH₄, CO₂/N₂, and H₂/CH₄ selectivities of up to 32.5, 29.3, and 175, respectively, and complete blockage of C₃H₆ is achieved.

References

1. Brown et al., Science. 345, 72–75 (2014).
2. Ma et al., Science. 361, 1008–1011 (2018).
3. Li et al., Commun. 8, 1–8 (2017).
4. He et al., Funct. Mater. 28, 1707427 (2018).
5. Hao et al., Mater. Chem. A, 2020, doi:10.1039/c9ta12027k.
6. Babu et al., Mater. 31, 1900855 (2019).