Fabrication of Oriented Mixed-Matrix Metal-Organic Framework Membranes for Molecular Separation

Summary

Here we describe a protocol for fabricating mixed matrix membranes (MMMs) by integrating highly selective molecular sieve fillers (here, MOFs) into the easily processable polymer matrix, which offers a promising approach to combine the best properties of both materials and fabricate advanced hybrid membranes with outstanding gas separation performance.

Abstract

The quest for energy-efficient separations is of prime importance. Membrane-based separation can alleviate excessive energy penalties associated with separations. Mixed-matrix membranes (MMMs), combining easily processable polymers and highly selective adsorbents, offer great potential to translate remarkable adsorbent separation properties into the processable matrix. This manuscript describes the detailed methodology for the fabrication of oriented mixed-matrix metal-organic framework (MMMOF) membranes and their subsequent application in various industrially important and highly challenging gas mixture separations. The oriented MMMOF membrane was fabricated based on three essential and interlocked criteria: (i) a MOF molecular sieve filler possesses optimal pore size and shape, functionality, and host-guest interaction that selectively facilitated diffusion of the desired gas molecule over others; (ii) a mastered synthesis control of MOF crystal morphology in a predefined crystallographic direction into high-aspect-ratio nanosheets that ensure maximum accessibility of channels/pores and enhances nanosheets-polymer interface compatibility, enabling high nanosheets loading; and (iii) a conceivable controlled assembly (in-plane alignment) of MOF nanosheets in the polymer matrix to translate the distinct molecular sieving properties of MOF nanosheets into the processable matrix as a form of MMMOF membrane. The membrane exhibited unparalleled hydrogen sulfide and carbon dioxide separation from natural gas under high pressure and high temperature. The methods we presented here have a high potential for the development of various high-performing membranes to address key industrial separations.

Introduction

Chemical separations are highly energy-intensive and consume around 50% of global industrial energy ^1,2. Membrane-based separation processes are highly energy-efficient compared to conventional cryogenic distillation and/or adsorptive separation. Pure polymer membranes commonly suffer from a trade-off behavior (a relationship between permeability and selectivity) known as Robeson's upper bound^3,4. Mixed-matrix membranes (MMMs), combining the benefits of selective adsorbents (molecular sieving effects and enhanced gas diffusion properties) and polymers (easy solution processability), are regarded as prospect-separating agents towards energy-efficient and environmentally sustainable technologies^5,6,7. Nevertheless, effective translation of adsorbent distinct separation properties into the processable matrix as a form of mixed-matrix membranes remains a significant challenge because of severe agglomeration and sedimentation of fillers inside the polymer matrix and the encountered incompatibility issue between filler and polymer interface. As a result of these challenges, the attainment of highly selective membranes is hampered as well as the membrane's mechanical properties are lessened⁸.

To address the compatibility issue, a porous adsorbent comprising organic moieties is suited to promote adhesion with the polymer matrix. Metal-organic frameworks (MOFs), a tunable class of hybrid solid-state materials consisting of metal nodes or metal clusters coordinated to multifunctional organic linkers^9,10,11, are expected to offer better compatibility with the polymer matrix in contrast to pure inorganic zeolites¹². It is to be noted that MOFs have received considerable attention as excellent adsorbents for selective gas separation^13,14,15.

Various MMMs have been reported using isotropic or near-isotropic fillers (e.g., nanoparticles). The fillers used in addition to MOFs include zeolites, mesoporous silica, and covalent-organic frameworks (COFs)^5,6,7. Nevertheless, only a limited MMMs exhibit a concurrent improvement in selectivity and permeability^16,17,18.

Evidently, introducing fillers with non-isotropic morphologies like nanosheets with a high-aspect-ratio is essential for the construction of high-performance MMMs¹⁹. The relatively high external surface area and reduced micropore diffusion length of nanosheets directionally promote gas diffusion with preserved molecular discrimination, affording a considerable increase in both permeability and selectivity²⁰. Importantly, the high external surface areas of nanosheets would proffer a significant enhancement of the nanosheet-polymer interface compatibility compared to nanoparticles. It is to be noted that Cu-BDC^21,22,23 and NH₂-MIL-53(Al)²⁴ MOF nanosheets have been used as fillers in MMMs, atypically these membranes displayed a moderate selectivity improvement at the expense of permeability, pinpointing the vital importance of a proper alignment of nanosheets within the polymer matrix. Along with the production of high-aspect-ratio nanosheets of high-performing MOFs, it is essential to develop a suitable methodology that can offer the key in-plane alignment of nanosheets inside the polymer matrix. Particularly, the quest for a new synthetic protocol to render selected MOF structures as nanosheets is of prime importance, as many contracted pore MOF structures have unique adsorption and diffusion properties^25,26, but they are not compatible with the conventional exfoliation methods²⁷.

In addition to the MOF's nanosheet morphology and proper in-plane alignment of MOF nanosheets inside the polymer matrix, the judicious selection of MOF filler and polymer pair is also an extremely important parameter to construct a high-performing MMM. It should be noted that these two constituents can have very different intrinsic permeability and selectivity. Therefore if a MMM is fabricated using a relatively low permeable MOF filler with a high permeable polymer, then the MOF will have a minimal impact on gas separation because of gas molecules will more likely diffuse/permeate through the polymer phase. In contrast, a MOF with high permeability, however, the fabricated MMM with very low permeable polymer will lead to the polymer-dominated gas separation.

This manuscript describes a concept and construction of an oriented mixed-matrix metal-organic framework (MMMOF) membrane based on three essential criteria: (i) AlFFIVE-1-Ni [NiAlF₅(H₂O)(pyr)₂] as an advanced molecular sieve filler with a suitable pore structure that facilitates the transport/diffusion of targeted gases (CO₂ and H₂S) while impeding the diffusion of other gases (CH₄ or bigger molecule); (ii) tailoring the AlFFIVE-1-Ni MOF crystal morphology along a defined crystallographic direction to maximize the exposure of pore/channel system (high-aspect-ratio nanosheets); (iii) in-plane alignment of nanosheets inside the polymer matrix and translate the remarkable molecular separation/sieving properties of discrete nanosheets into a macroscopic continuous membrane²⁸.

Protocol

1. Preparation of (001) oriented AlFFIVE-1-Ni MOF nanosheet (Figure 1)

NOTE: (001) oriented AlFFIVE-1-Ni nanosheet can be synthesized by combining two different precursor-containing solutions (i) metal and ligand solution and (ii) pillar-containing solution under solvothermal reaction²⁸.

1. Prepare metal and ligand solution by dissolving 1.80 g of pyrazine (22.03 mmol) and 2.20 g of nickel acetate tetrahydrate (8.84 mmol) in 9 mL of 2:1 (v/v) mixture ethanol-H₂O by sonication for 3 min.
2. Prepare [AlF₅(H₂O)]^2- pillar solution by dissolving 0.32 g of aluminum hydroxide (4.10 mmol) in 4 mL of 3:1 (v/v) mixture HF-H₂O by sonication for 2 min at room temperature (RT; 25 °C).
3. Combine metal, ligand, and pillars containing solutions and sonicate the mixture for 3 min.
   NOTE: The sonication frequency for steps 1.1-1.3 is 20 kHz.
4. Perform solvothermal reaction at 50 °C for 3 days under a static condition.
   NOTE: Solvothermal reaction can be performed in a 50 mL conical tube in a preheated oven.
5. Collect AlFFIVE-1-Ni MOF nanosheet with a centrifugal force of 1789 x g for 5 min.
6. Wash the MOF nanosheet with 300 mL of water, and wash it with 60 mL of ethanol.
7. Prepare AlFFIVE-1-Ni nanosheet stock solution by dispersing the nanosheet in the required amount of chloroform and adjusting the nanosheet concentration to about 100 mg of MOF/mL solution.
   NOTE: To prevent agglomeration, do not dry the nanosheet before membrane preparation.
8. Prepare AlFFIVE-1-Ni nanoparticles by using 8.85 mmol of [AlF₅(H₂O)]^2- pillar solution instead of 4.10 mmol [AlF₅(H₂O)]^2- pillar solution²⁸.

2. Fabrication of [001]-oriented mixed-matrix metal-organic framework (MMMOF) membrane

1. Use chloroform as a dispersant medium for in-plane alignment of nanosheets inside the polymer matrix (Figure 2).
   NOTE: Dispersant mediums have a critical role in fabricating a defect-free mixed-matrix membrane.
2. Perform solution-casting method to fabricate [001]-oriented MMMOF membranes with different thicknesses.
3. Prepare polymer solution by dissolving 150 mg of dried polyimides (6FDA-DAM) in 2 mL of chloroform by shaking on a mechanical shaker for 3 h at RT.
4. Take the required amount of aliquot from AlFFIVE-1-Ni nanosheets stock solution (step 1.7.) (e.g., 1.02 mL for 40 wt%, 1.50 mL for 50 wt%, and 2.25 mL for 60 wt% MMMOF membranes) in a 25 mL glass vial and add a required amount of chloroform to make a total volume of 2.5 mL.
5. Add polymer solution dropwise into the MOF suspension and stir the mixture at 150 rpm for at least 150 min at 35 °C.
   NOTE: This MOF and polymer mixture is called a dope solution.
6. Pour the dope solution into a glass Petri dish or polytetrafluoroethylene (PTFE) dish on a leveled surface.
   NOTE: The glass Petri dish or PTFE dish must be placed in a glove bag pre-saturated with chloroform vapor for at least 10 min.
7. Keep the dope solution-containing dish in the glove bag overnight at RT (25 °C).
8. Allow slow evaporation of chloroform solvent to self-assemble MOF nanosheets inside the polymer matrix (Figure 3).
   NOTE: MOF nanosheets' in-plane alignment is performed by the slow evaporation of solvent during the membrane preparation²⁸.
9. Peel off the freestanding [001]-oriented MMMOF membrane from the Petri dish using a spatula (Figure 4).
10. Dry the membrane in a vacuum oven at 200 °C for 20 h to remove any residual chloroform.
11. Fabricate AlFFIVE-1-Ni nanoparticles containing mixed-matrix membrane using the same procedure (steps 2.1-2.10) but use AlFFIVE-1-Ni nanoparticles instead of AlFFIVE-1-Ni nanosheets.

3. Characterization of [001]-oriented MMMOF membranes

1. Obtain the cross-section scanning electron microscopy (SEM) image of the membrane to corroborate that the (001) nanosheets are uniformly in-plane aligned (Figure 5A).
   NOTE: Before SEM image measurement, the membrane should be cryogenically fractured in liquid nitrogen to preserve its microstructures. SEM images were obtained using Magellan 400-FEG operating at an acceleration voltage of 1.5 kV and an emission current of 13 pA.
2. Evaluate X-ray diffraction (XRD) of the membrane to ascertain the crystallographic orientation of (001) nanosheet in the polymer matrix (Figure 5B).
   NOTE: [001]-oriented membrane is supposed to display only two major Bragg diffractions from the two crystallographic planes ([002], [004]) of the AlFFIVE-1-Ni structure. The XRD data collection was performed on a diffractometer (Cu K_α = 1.54056 Å) in Bragg-Brentano θ-θ geometry with the following settings: operating power: 40 kV/40 mA; automatic divergence slit (irradiated length = 0.6 mm), sample holder: flat plate sample holder; temperature: RT; method: continuous-count method (3° min^-1); 2 range: 4°-40°.
3. Perform focused ion beam SEM (FIB-SEM) image analyses of the oriented membrane to verify the highly aligned nature of nanosheets and the membrane's internal microstructure on an extensive area (Figure 6).
   NOTE: FIB-SEM is an excellent characterization technique to identify MOF-polymer interface compatibility.
   1. Create about 35 µm wide trench on the top surface of the MMMOF membrane, and mask the remaining area by deposited platinum (Figure 6A).
      1. Sputter coat the membrane with ⁓10 nm of Iridium. Before milling, deposit a protective layer of Pt (0.3-0.4 µm thickness) on the areas of interest of the specimen.
      2. Then, use the gallium (Ga⁺) ion beam FIB working at 5 kV and 75-100 pA to create a trench on the top surface of the membrane.
   2. Slice and mill away the membrane with a minimum thickness of 50 nm by the FIB, operating at about 5 kV and 25 pA (Figure 6B).
   3. Collect between 100 and 300 individual SEM micrographs of the consecutive cross-sections subjected to milling at a magnification range of 12,000x-45,000x, with a secondary electron detector at ~1 kV (Figure 6C).
   4. Align the stacking of images to an external feature on the membrane surface using a cross-correlation algorithm.
4. Analyze the relative viscosity changes of nanosheets-polymer suspension (dope solution) with respect to the stirring time (Figure 7).
   NOTE: Rheological characterization is rather simple yet effective in elucidating the enhanced interaction between MOF nanosheets and polymers.
5. Collect stress-strain curves of membranes at different MOF loadings and calculate Young's modulus and elongation strain from stress-strain curves.
   NOTE: Mechanical properties of the membrane can be assessed using stress-strain analysis.
6. Assess the MOF nanosheet loading in the oriented membrane by thermogravimetric analysis method.
   1. First, activate about 10 mg of nanosheet powder under N₂ purging at 200 °C for 135 min to ensure activation.
   2. Increase the temperature up to 800 °C at a heating rate of 10 °C·min⁻¹ under O₂ flow and hold the temperature at 800 °C for 2 h. Compare the residual metal oxide mass to the initial activated mass of the MOF²⁸.
   3. Use the same protocol (steps 3.6.1 and 3.6.2) for MOF nanosheet-containing membranes to determine the MOF loading.
      NOTE: From the residual metal oxide mass after heating at 800 °C for 2 h under O₂ environment, the content of MOF nanosheets in the membrane can be determined.

4. Assess the single and mixed-gas separation properties of oriented membranes

1. Evaluate single/pure gas permeation measurements using a homemade variable-pressure constant-volume method²⁸.
   NOTE: Single gas permeation condition typically at 35 °C and 2 bar feed pressure for He, H₂, CO₂, O₂, N₂, CH₄, C₂H₄, C₂H₆, C₃H_6, and C₃H₈. A homemade gas separation instrument was used. Refer to our previously published work²⁸ for the details of the setup.
   1. Cut the membrane using a mechanical punch into a circle/coin shape about 2 cm² in size.
   2. Mount the membrane on the permeation cell, mask it with adhesive aluminum tape, and seal it with epoxy resins.
      NOTE: The membrane sealing using epoxy resin should be carefully performed to avoid any undesirable pinholes.
   3. Keep a selected area of the membrane exposed for gas permeation.
   4. Measure the accessible area of the membrane for gas permeation using a scanner and measure the membrane thickness using a depth gauge.
   5. Evacuate the permeation system at 35 °C for 12 h before performing any permeation test.
      NOTE: This step helps to remove any adsorbed moisture during membrane sealing.
   6. Feed the gas at 2 bar (upstream pressure) and wait for 20 min to equilibrate the permeation system temperature.
   7. Calculate the single gas permeability using the slope of downstream pressure vs. time (dp/dt) at the steady state condition by the following equation:
           (1)
      NOTE: P refers to the gas permeability of a membrane in barrer (1 barrer = 10^-10 cm³(STP)·cm^-2·s^-1·cmHg^-1), V is the volume of the downstream chamber (cm³), l is the membrane thickness (cm), p₀ is the upstream pressure (cmHg), A refers to the effective area of membrane (cm²), R is the gas constant (0.278 cm³ cmHg cm^-3(STP) K^-1), T is the operating temperature (K).
   8. Obtain the ideal selectivity (S_A/B) of gas A to gas B by using the following equation:
          (2)
      NOTE: P_A and P_B denote the permeability of gases A and B, respectively.
2. Evaluate mixed-gas permeation measurements.
   1. Use the binary gas mixture (CO₂/CH₄:10/90; 20/80 and 50/50) and/or ternary gas mixture (H₂S/CO₂/CH₄:1/9/90; 2/18/80 and 5/5/90) for mixed-gas separation tests.
   2. Use modified single-gas permeation equipment for mixed-gas permeation measurement, which must be integrated with micro-gas chromatography (Agilent 490 Micro-GC)²⁸.
      NOTE: Mixed-gas feed pressure can be varied from 2-30 bar and temperature 25-100 °C.
   3. Maintain the stage cut (the flow rate ratio of permeate to feed) below 1%.
      NOTE: The gas concentration polarization on the upstream side of the membrane can be circumvented in this way, and it will also maintain the constant driving force across the membrane during the gas permeation study.
   4. Allow the gas mixture to permeate through the membrane until a steady-state permeation rate is reached, typically above 5-time lags.
   5. Evacuate the permeate mixture and then collect the gas mixture under steady-state conditions.
   6. Dilute the permeate gas with He gas and analyze the gas mixture with a Micro-GC.
   7. Calculate the mixed gas permeability of component i (P_i) using the following equation.
          (3)
      NOTE: y and x are the mole fractions in permeate and feed, respectively, and the p₀ is the feed pressure of component i.
   8. Analyze the mixed-gas selectivity (S_i/j) using the ratio of the permeability of two components, i and j.
          (4)

Results

Oriented nanosheet fillers (Figure 1) intrinsically possess several advantages in MMMs compared to nanoparticles. The relatively large external surface area and reduced micropore diffusion distance of nanosheets directionally promote gas diffusion with preserved molecular discrimination (Figure 2). The relatively large external surface area of nanosheets compared to nanoparticles proffer a greater enhancement of nanosheet-polymer interface compatibility, enablin...

Discussion

The synthesis of oriented MOF nanosheets with a high-aspect-ratio is remarkably challenging. Unlike other applications, the MOF nanosheets for gas separation must be synthesized without surfactant and/or template to avoid any undesired substance on the surface of MOF nanosheets. In addition, the channel/pores of nanosheets should be oriented out of the plane direction to directionally promote gas diffusion. Each MOF structure has a different crystal growth mechanism, therefore same synthesis procedure can not apply to al...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Aluminum (III) hydroxide hydrate	Aldrich	1330-44-5
Bruker D8 Advance	Bruker	NA	X-ray diffractometer
CAP-2000+ Viscometer	AMETEK Brookfield	NA	Rheological characterization
Chloroform (99.8%)	Sigma-Aldrich	67-66-3
CO2/CH4 : 50/50, 20/80, 10/90 and H2S/CO2/CH4 = 1/9/90, 2/18/80, 5/5/90	Air Liquid and AHG	NA
Dichloromethane (99.9%)	Sigma-Aldrich	75-09-2
Ethanol (99.7%)	VWR	64-17-5
H2 (99.999%), N2 (99.999%), CO2 (99.999%), CH4 (99.999%), C3H6 (99.5%), C3H8 (99.5%)	Air Liquid and AHG	NA
Helios G4 UX DualBeam microscope	ThermoFisher	NA	FIB-SEM
Homemade gas separation instruments	NA	NA	See ref. 28 for details setup
Hydrofluoric acid (48 wt% in water)	Sigma-Aldrich	7664-39-3
Magellan 400-FEG	ThermoFisher	NA	SEM images
Nickel (II) acetate tetrahydrate (99%)	Organics	6018-89-9
Polyimide 6FDA-DAM (Mw = 330 kDa, PDI: 2.48)	Akron Polymer Systems, Inc.	NA
Pyrazine (99%)	Aldrich	290-37-9
TA Q-5000 analyzer	TA Instruments	NA	TGA
Tetrahydrofuran (99%)	Sigma-Aldrich	109-99-9