Solvothermal Synthesis of MIL-96 and UiO-66-NH2 on Atomic Layer Deposited Metal Oxide Coatings on Fiber Mats

Summary

Metal-organic frameworks are effective in gas storage and heterogeneous catalysis, but typical synthesis methods result in loose powders that are difficult to incorporate into smart materials. We demonstrate a method of first coating fabrics with ALD metal oxides, resulting in conformal films of MOF on the fabrics during solvothermal synthesis.

Abstract

Metal-organic frameworks (MOFs), which contain reactive metal clusters and organic ligands allowing for large porosities and surface areas, have proven effective in gas adsorption, separations, and catalysis. MOFs are most commonly synthesized as bulk powder, requiring additional processes to adhere them to functional devices and fabrics that risk decreasing the powder porosity and adsorption capacity. Here, we demonstrate a method of first coating fabrics with metal oxide films using atomic layer deposition (ALD). This process creates conformal films of controllable thickness on each fiber, while providing a more reactive surface for MOF nucleation. By submerging the ALD coated fabric in solution during solvothermal MOF synthesis, the MOFs create a conformal, well-adhered coating on the fibers, resulting in a MOF-functionalized fabric, without additional adhesion materials that may block MOF pores and functional sites. Here we demonstrate two solvothermal synthesis methods. First, we form a MIL-96(Al) layer on polypropylene fibers using synthetic conditions that convert the metal oxide to MOF. Using initial inorganic films of varying thicknesses, diffusion of the organic linker into the inorganic allows us to control the extent of MOF loading on the fabric. Second, we perform a solvothermal synthesis of UiO-66-NH₂ in which the MOF nucleates on the conformal metal oxide coating on polyamide-6 (PA-6) fibers, thereby producing a uniform and conformal thin film of MOF on the fabric. The resulting materials can be directly incorporated into filter devices or protective clothing and eliminate the maladroit qualities of loose powder.

Introduction

Metal-organic frameworks are crystalline structures consisting of reactive metal cluster centers bridged by organic molecule linkers to provide large porosities and surface areas. Their structure, porosity, and functionality can be designed by choosing appropriate clusters and linkers, leading to surface areas as high as 7,000 m²/g_MOF^1,2. Their high porosity and surface area have made MOFs diversely applicable in adsorption, separation, and heterogeneous catalysis in fields ranging from energy production to environmental concerns to biological processes^1,....

Protocol

1. Atomic Layer Deposition (ALD) of Al₂O₃ on Fiber Mats

1. Place a 2.54 x 2.54 cm² polypropylene fabric sample in the reactor boat (a thin, rigid, metal mesh holder). A schematic of the reactor is presented in Figure 2.
2. Open the pressure gauge. Remove the clasp from the reactor cap. Turn on manual control in the LabView system. Close the carrier nitrogen and gate valve on the ALD reactor. Open the vent nitrogen.
3. After removing the reactor cap, load the fabric sample into the ALD reactor. Replace the reactor cap and open the gate valve. Close the vent and open the carrier nitrog....

Results

To describe the MOF/fabric materials, we delineate two terms related to measured surface area. First, projected surface area, cm²_projected, refers to the macroscopic size of the fabric swatch as measured with a ruler, i.e., the area of the sample's projected shadow. The second surface area of interest is the BET surface area, calculated from a nitrogen isotherm obtained at 77 K. These values are given in units of m²/g_Fabric, m²

Discussion

The ALD coating strongly influences the adhesion and loading of the MOF. First, depending on the type of substrate and ALD precursor, the ALD layer can either form a distinct outer shell around the fiber, or diffuse into the fiber to create a gradual transition to the metal oxide coating²⁰. The hard shell has been observed on cotton and nylon substrates, while diffusive layers can be observed in polypropylene under proper conditions. Second, the diffusion into the fiber can also be controlled by v.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
trimethylaluminum	Strem Chemicals	93-1360
home-built ALD reactor	N/A
nitrogen cylinder	Arc3	UN1066
trimesic acid	Sigma-Aldrich	482749-500G
ethanol	Koptec	V1001
teflon lined autoclave	PARR Instrument Company	4760-1211
isotemp furnace	Fisher Scientific	F47925
Zirconium (IV) chloride	Alfa Aesar	12104
2-aminoterephthalic acid	Acros Organics	278031000
N,N-dimethylformamide	Fisher Scientific	D119-4
Hydrochloric Acid	Fisher Scientific	A481-212
Polypropylene fiber mats	N/A
Polyamide fiber mats	N/A