Session 15C - MOF and COF membranes 2

Metal-organic frameworks (MOFs) are porous hybrid crystalline materials composed of organic ligands with inorganic metal clusters. Because of their tunable pore size and versatile chemical compositions, MOFs are promising membrane materials for molecular separations. In this talk, I will present our group’s studies in the fabrication of water-stable polycrystalline MOF membranes and their applications in liquid separations including water purification, alcohol dehydration, and organic solvent nanofiltration. Through the strategies including postsynthetic defect healing, judicious selection of secondary building units, and adoption of flexible substrates, we have gradually revealed the potential of polycrystalline MOF membranes for industrial liquid separations.

Introduction

Zeolitic-imidazolate frameworks (ZIFs) are candidate materials for the next generation of membranes for cheaper separations. Herein, we demonstrate the effectiveness of molecular-scale modification in the design of new ZIFs useful for the separation of important mixtures such as H₂/CH₄, CO₂/CH₄.

Methods

We construct new ZIFs by replacing the original metal, Zn²⁺, and the original organic linker, mIm, in the basic unit of ZIF-8 (Figure 1(a)), with three different metals (Be²⁺, Cu²⁺, Co²⁺) and one different linker (bIm), to produce ZIF-8 analogues. Unit replacement offers control over the size of the aperture that bridges the cages of the ZIF (Figure 1(b)) and subsequently control over the separation efficiency.^1,2

Figure 1. (a) basic tetrahedra unit in ZIF-8 topology and (b) aperture connecting two cages.

Molecular simulations were employed to estimate the permeabilities of He, H₂, CO₂, N₂ and CH₄ and selectivities of the corresponding mixtures. Details on our simulation approach and the force fields we developed for these calculations can be found in our recent works.^1,2,3

Results

In the case of H₂/CH₄ the new modifications demonstrate a clearly improved performance over ZIF-8 already considerable performance, and they surpass the existing competition (Figure 2(a)). In the case CO₂/CH₄, the modifications offer a considerable improvement over the original ZIF-8 average separation. In both cases, BeIF-1 exhibits an outstanding performance.

Figure 2. (a) H₂/CH₄ and (b) CO₂/ CH₄: Performance of the ZIFs of this work in comparison with membrane data from literature.

Discussion

The results and the comparison with existing membranes acts as a proof of the possibility to develop membranes of unprecedent separation performance.

References

1. Krokidas et al. Phys. Chem. Chem. Phys. 2018, 20 (7), 4879.
2. Krokidas et al. ACS Appl. Mater. Interfaces 2018, 10 (46), 39631.
3. Krokidas et al, under review.

Metal-organic frameworks (MOFs) are a new family of porous materials, are promising fillers for mixed matrix membranes (MMM) due to their unique structural properties like high surface area, high selectivity of gas adsorption, controllable pore sizes, tunable pore surface property, and low densities. Tremendous efforts have been made to develop MOF-based MMMs for highly selective gas separation. Nevertheless, harnessing MOF/polymer interfacial compatibility is still challenging for the current development of MOF-based MMMs. Poor MOF/polymer contact could lead to the formation of nonselective interfacial defects, which are detrimental to the gas selectivity of the membrane.

Here we proposed a post-treatment strategy to heal the interfacial defects by in situ vitrification of meltable MOF crystals within the polymer matrix (Fig. 1), aiming to employ the high-temperature flowing liquid phase of MOF to fix the defects. A family of zeolitic imidazolate frameworks (ZIFs), such as ZIF-62 [Zn(Im)_2-x(bIm)_x] (Im – imidazolate, bIm – benzimidazolate), ZIF-4, ZIF-76 and ZIF-UC-n (n = 1 – 5), can form a molten state with amorphous frameworks before their decomposition temperature in an inert gas atmosphere. ZIF-62 glass, obtained by quenching the liquid ZIF-62, preserves a 3D network with permanently accessible microporosity containing tetrahedral Zn²⁺ linked by Im and bIm ligands.

Fig. 1 SEM images of (a) ZIF-62-bIm_0.05 crystal after ball mill treatment and (b) a_gZIF-62-bIm_0.05 from melting of the ball-milled crystals

Fig. 2 Schematic diagram of the preparation of glass ZIF-based MMMs

We discovered with the transformation of ZIF-62 crystal into a glass phase, 78.9 % of the interfacial defects were filled with compared to the MMM with untreated ZIF crystal. The presence of glass MOF in membrane enhances the difference of solubility and sorption affinity for CO₂ over N₂, which combined with the reduced interfacial voids in the MMM, lead to a 57 % increase in CO₂ selectivity with compared to the pristine polymer membrane.

With rising environmental issues like CO2 emission reduction and establishing a hydrogen economy with components like electrolysers and fuel cells, membranes offer enormous potential to contribute to these requirements due to their inherent energy efficiency. Thus, microporous compounds like zeolites and metal-organic frameworks (MOFs) with defined pore sizes embedded in polymer matrices are appropriate candidates as membranes for the separation of gas molecules by size exclusion. To prepare these mixed matrix membranes, continuous dip coating is an efficient procedure to create thin selective layers on porous hollow fiber membranes.

For this purpose, porous poly(vinylidene fluoride) (PVDF) hollow fiber membranes were prepared via non-solvent-induced phase separation and metal organic framework nanoparticles were prepared via solvothermal synthesis. Those MOFs like ZIF-7 and UiO-66 or SAPO-34 zeolites were dispersed in coating solutions with polymers like poly(vinyl alcohol) (PVA) or poly(ether-b-amide) (PEBA). Afterwards PVDF hollow fiber membranes were coated with these dispersions by a continuous dip coating (Fig. 1). The layer thickness of the mixed matrix layer could be controlled in the range from 500 nm to 5 μm via the viscosity of the dispersion and the coating speed. The use of microporous nanoparticles (< 100 nm) is decisive for the quality of the layers. Additionally, the resulting layer thicknesses were investigated and correlated with theoretical layer thicknesses according to the theory of Landau, Levich and Derjaguin to further optimize the coating process.

The mixed matrix membranes show improved separation properties compared to pure polymer coatings. As examples, the water vapor permeability of polyvinyl alcohol layers increased about almost 100% to 7000 Barrer by adding SAPO-34 nanoparticles. The CO2 permeability of a PEBA coating increased by 350% by addition of UiO-66 nanoparticles. In both cases, the selectivity is not negatively affected.

Figure 1: SEM image of a coated mixed matrix membrane hollow fiber cross-section with ZIF-7 particles.