Electrophoretic Crystallization of Ultrathin High-performance Metal-organic Framework Membranes

Summary

A simple, reproducible, and versatile approach for the synthesis of intergrown, polycrystalline metal-organic framework membranes on a wide range of unmodified porous and non-porous supports is presented.

Abstract

We report the synthesis of thin, highly intergrown, polycrystalline metal-organic framework (MOF) membranes on a wide range of unmodified porous and non-porous supports (polymer, ceramic, metal, carbon, and graphene). We developed a novel crystallization technique, which is termed the ENACT approach: the electrophoretic nuclei assembly for the crystallization of highly intergrown thin films (ENACT). This approach allows for a high density of heterogeneous nucleation of MOFs on a chosen substrate via the electrophoretic deposition (EPD) directly from the precursor sol. The growth of well-packed MOF nuclei leads to a highly intergrown polycrystalline MOF film. We show that this simple approach can be used for the synthesis of thin, intergrown zeolite imidazole framework (ZIF)-7 and ZIF-8 films. The resulting 500 nm-thick ZIF-8 membranes show a considerably high H₂ permeance (8.3 x 10^-6 mol m^-2 s^-1 Pa^-1) and ideal gas selectivities (7.3 for H₂/CO₂, 15.5 for H₂/N₂, 16.2 for H₂/CH₄, and 2655 for H₂/C₃H₈). An attractive performance for C₃H₆/C₃H₈ separation is also achieved (a C₃H₆ permeance of 9.9 x 10^-8 mol m^-2 s^-1 Pa^-1 and a C₃H₆/C₃H₈ ideal selectivity of 31.6 at 25 °C). Overall, the ENACT process, owing to its simplicity, can be extended to synthesize intergrown thin films of a wide range of nanoporous crystalline materials.

Introduction

Thin molecular sieving membranes offer a high-energy efficiency in the separation of molecules and can reduce the overall cost of fuels, CO₂ capture, water purification, solvent recovery, etc.^1,2. MOFs are a promising class of material for the synthesis of molecular sieving membranes because of the involved isoreticular synthetic chemistry and relatively straightforward crystallization³. To date, MOF membranes comprising of diverse crystalline structures, including that of ZIF-4, -7, -8, -9, -11, -67, -90, and -93, and UiO-66, HKUST-1, and MIL-53 have been reported

Protocol

CAUTION: Read carefully the material safety data sheets (MSDS) of the chemicals involved. Some of the chemicals used in the experiment are toxic. The present method involves the synthesis of nanoparticles. Therefore, take appropriate precautions. The entire synthesis procedure must be performed in a well-ventilated fume hood.

NOTE: The details of the instruments, the chemicals, and the materials involved in the synthesis of the MOF films are listed in Table 1.

1. Preparation of a Metal Salt Solution and Linker Solution for the Precursor Sol

NOTE: For the synthesis of ZIF....

Results

A homemade EPD set-up was used to synthesize the MOF films (Figure 1). Scanning electron microscopy (SEM) images and X-ray diffraction (XRD) patterns were collected for the ZIF-8 nuclei film (Figure 2). SEM was used to image the surface and cross-sectional morphologies of the AAO support, ZIF-8/AAO membrane, PAN support, ZIF-8/PAN membrane, ZIF-8/graphene film, and ZIF-7/AAO membrane (Figure 3). The .......

Discussion

The standout feature of the ENACT method with respect to the existing methods¹⁵ is that the ENACT method enables the synthesis of highly intergrown, ultrathin MOF films on a wide range of porous and nonporous substrates. Any substrate pretreatment is avoided, making this method quite straightforward for the synthesis of MOF films. Although EPD equipment has to be used for the deposition of a nuclei film, the equipment is composed of a power source, a metal electrode, and a beaker, which is quite s.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Zinc nitrate hexahydrate	Sigma-Aldrich	96482-500G	98% purity
2-Methylimidazole	Sigma-Aldrich	M50850-500G	99% purity
Benzimidazole	TCI	B0054-500G	98% purity
Tape	DuPont	KPT-1/8
Epoxy	GC Electronics	19-823
Copper foil	Alfa Aesar	13380.CV	99.9% purity
Power source for EPD	Gamry Instruments	Interface 1000E Potentiostat
Ultrasonic cleaner	MTI corporation	VGT-1860QTD
AAO	GE Healthcare Life Sciences‎	6809-7013
PAN	Shandong MegaVision		The molecular weight cut-off is 100 kDa