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

Podsumowanie

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.

Streszczenie

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.

Wprowadzenie

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^1,3,4,5,6.

Numerous MOFs have proven successful in selectively adsorbing volatile organic compounds and greenhouse gases or to catalytically degrade chemicals that may prove harmful to human health or the environment. In particular, MIL-96 (Al) has shown to selectively adsorb nitrogenous volatile organic compounds (VOCs) due to the availability of lone pair electrons in the nitrogen groups to coordinate with the weak Lewis acid Al present in the metal clusters⁷. MIL-96 has also been shown to adsorb gases such as CO₂, p-xylene, and m-xylene^8,9. MOF adsorption selectivity is dependent on both the Lewis acid of the metal cluster, as well as pore size. The pore size of MIL-96 increases with temperature, resulting in increased adsorption capacity of trimethylbenzene with increased temperature, and presents the opportunity of tuning selectivity with adsorption temperature⁹.

The second MOF of focus here, UiO-66-NH₂ has been shown to catalytically degrade chemical warfare agents (CWAs) and simulants. The amine group on the linker provides a synergistic effect in degrading nerve agents, while preventing agent degradation products from binding irreversibly to the zirconium clusters and poisoning the MOF¹⁰. UiO-66-NH₂ has catalytically hydrolyzed dimethyl p-nitrophenylphosphate (DMNP) with a half-life as short as 0.7 minutes in buffered conditions, nearly 20 times faster than its base MOF UiO-66^11,12.

While these adsorption and catalytic properties are promising, the physical form of the MOFs, primarily bulk powder, can be difficult to incorporate into platforms for gas capture and filtration without adding significant bulk, clogging pores, or reducing MOF flexibility. An alternative is to create MOF functionalized fabrics. MOFs have been incorporated into fabrics in myriad ways, including electrospinning MOF powder/polymer slurries, adhesive mixes, spray coating, solvothermal growth, microwave syntheses, and a layer-by-layer growth method^{13,14,15,16,17,18}. Of these, electrospinning and polymer adhesives can result in blocked functional sites on the MOF as they are encapsulated in the polymer, significantly decreasing adsorption capacity and reactivity. Additionally, many of these techniques fail to create conformal coatings on the fibers due to line of sight difficulties or poor adhesion/nucleation and the reliance on purely electrostatic interactions. An alternative method is to first coat the fabric with a metal oxide to allow for stronger surface interactions with the MOF^18,19.

One method of metal oxide deposition is atomic layer deposition (ALD). ALD is a technique for depositing conformal thin films, controllable to the atomic scale. The process utilizes two half reactions that occur only at the surface of the substrate to be coated. The first step is to dose a metal containing precursor, which reacts with hydroxyls on the surface, leaving a metallated surface while excess reactant is purged from the system. The second reactant is an oxygen-containing reactant, typically water, which reacts with the metal sites to form a metal oxide. Again, excess water and any reaction products are purged from the system. These alternating doses and purges can be repeated until the desired film thickness is achieved (Figure 1). Atomic layer deposition is particularly useful because the small-scale vapor phase precursors allow for conformal films on every surface of substrates with complex topology, such as fiber mats. Additionally, for polymers such as polypropylene, the ALD conditions can allow the coating to diffuse into the fiber surface, providing a strong anchor for future MOF growth²⁰.

The metal oxide coating allows for increased nucleation sites on the fibers during traditional solvothermal synthesis by increasing functional groups and roughness^18,20. Our group has previously shown the ALD metal oxide base layer is effective for UiO-6X, HKUST-1, and other syntheses through various routes of solvothermal, layer-by-layer, and hydroxy-double salt conversion methods^{13,17,18,21,22,23}. Here we demonstrate two synthesis types. The MIL materials are formed by converting the Al₂O₃ ALD coating directly to MOF by diffusion of the organic linker. By submerging an Al₂O₃ ALD coated fiber mat in trimesic acid solution and heating, the organic linker diffuses into the metal oxide coating to form MIL-96. This results in a strongly adhered, conformal MOF coating on every fiber surface. The second synthesis approach calls for typical UiO-66-NH₂ hydrothermal synthesis using metal and organic precursors, but adds a metal oxide coated fiber mat on which the MOF nucleates. For both synthesis approaches, the resulting products consist of conformal thin films of MOF crystals strongly adhered to the supporting fabric. In the case of MIL-96, these can be incorporated into filters for adsorption of VOCs or greenhouse gases. For UiO-66-NH₂ these fabrics can be easily incorporated into lightweight protective clothing for military personnel, first responders, and civilians for continuous defense against CWA attacks.

Protokół

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 nitrogen. Turn off manual control.
4. Load the recipe for Al₂O₃ on fabrics. The recipe will alternately dose trimethylaluminum (TMA) for 1.2 s, followed by a 30 s dry nitrogen purge or dose water for 1 s followed by a 60 s dry nitrogen purge. Set the recipe to run 1000 cycles.
5. Set the mass flow controller to 20 cfm and the furnace temperature to 90 °C (84 °C on the furnace interface).
6. Open the manual valve to the TMA and water. Close the pressure gauge. Replace the clasp on the reactor cap. Press Start on the interface.
7. Upon completion of the recipe, open the pressure gauge. Remove the clasp from the reactor cap. Turn on manual control in the system. Close the carrier nitrogen and gate valve on the ALD reactor. Open the vent nitrogen.
8. Remove the reactor cap and sample boat. Re-seal the reactor.
   NOTE: The procedure may be paused at this point.

2. Atomic Layer Deposition (ALD) of TiO₂ on Polyamide-6 (PA-6) Fiber Mats

1. Place a 2.54 x 2.54 cm² PA-6 fabric sample in the reactor boat (a thin, rigid, metal mesh holder).
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 nitrogen. Turn off manual control.
4. Load the recipe for TiO₂ on fabrics. The recipe will alternately dose TiCl₄ for 1 s, followed by a 40 s dry nitrogen purge or dose water for 1 s followed by a 60 s dry nitrogen purge. Set the recipe to run 300 cycles.
5. Set the mass flow controller to 20 cfm and the furnace temperature to 90 °C (84 °C on the furnace interface).
6. Open the manual valve to the TiCl₄ and water. Close the pressure gauge. Replace the clasp on the reactor cap. Press Start on the interface.
7. Upon completion of the recipe, open the pressure gauge. Remove the clasp from the reactor cap. Turn on manual control in the system. Close the carrier nitrogen and gate valve on the ALD reactor. Open the vent nitrogen.
8. Remove the reactor cap and sample boat. Re-seal the reactor.
   NOTE: The procedure may be paused at this point.

3. Solvothermal Synthesis of MIL-96

1. Add 0.0878 g of H₃BTC to an 80 mL glass beaker.
2. Add 12 mL of H₂O and 12 mL of ethanol to the beaker.
3. Stir magnetically for 10 min or until the H₃BTC is fully dissolved.
4. Place the solution in a Teflon lined pressure vessel.
5. Add the Al₂O₃ coated polypropylene to the solution and prop the fabric on a mesh support so it does not lie flat against the bottom of the vessel.
6. Seal the pressure vessel and place it in the furnace at 110 °C for 24 h.
7. After allowing the sample to cool, place the fabric sample in a mesh basket in a beaker. Wash twice with ethanol, each for 12 h.
8. Sample activation requires heating at 85 °C for 6 h under vacuum, followed by heating at 110 °C for 12 h under vacuum.
   NOTE: The procedure can be stopped here. All samples should be stored in a desiccator to maintain sample activation.

4. Solvothermal Synthesis of UiO-66-NH₂

1. Add 0.08 g of ZrCl₄ to a 20 mL glass scintillation vial.
2. Add 20 mL of N,N-dimethylformamide (DMF) to the ZrCl₄ in 5 mL increments. Cap the vial between increments and allow fumes to dissipate.
3. Sonicate the solution for 1 min.
4. Add 0.062 g of 2-aminoterephthalic acid to the vial, and magnetically stir the solution for 5 min.
5. Add 25 µL of deionized water to the vial.
6. Add 1.33 mL of concentrated HCl to the vial.
7. Submerge the TiO₂ ALD coated fabric swatch in the solution and cap the vial.
8. Place the sample in the furnace at 85 °C for 24 h.
9. After allowing the sample to cool, place the fabric sample in a mesh basket in a beaker. Wash twice with 80 mL of DMF, each for 12 h. Wash 3 times with 80 mL of ethanol, each for 12 h.
10. After removing the fabric swatch, filter the residual MOF powder. Wash twice with 80 mL of DMF, each for 12 h. Wash 3 times with 80 mL of ethanol, each for 12 h.
11. Sample activation requires heating at 85 °C for 6 h under vacuum, followed by heating at 110 °C for 12 h under vacuum.
    NOTE: The procedure can be stopped here. All samples should be stored in a desiccator to maintain sample activation.

Wyniki

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, m^2...

Dyskusje

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...

Materiały

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