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 polym....

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(H₂....

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