Preparation of Hydrophobic Metal-Organic Frameworks via Plasma Enhanced Chemical Vapor Deposition of Perfluoroalkanes for the Removal of Ammonia

The overall goal of the following experiment is to prepare a hydrophobic metal organic framework to increase its water stability. This is achieved by first synthesizing a metal organic framework through VO thermal methods as a second step. The metal organic framework is treated by plasma enhanced chemical vapor deposition of a per fluoro acan to add hydrophobic groups to the inner and outer surfaces.

Next, the material is aged and the ammonia capacity of the resulting material is evaluated using a micro breakthrough apparatus. The results demonstrate that the HD plasma treated samples show an enhancement in the uptake of ammonia. The implications of this technique extend toward the creation of hydrophobic metal organic frameworks in various other porous materials that can be stable to a variety of ambient conditions.

These materials have various applications in gas storage, filtration, catalysis, and even drug delivery where structural stability is paramount. The main advantage of this technique over existing post synthetic modification methods using wet chemistry is that plasma enhanced chemical vapor deposition of per fluoro alkanes can be applied to a variety of materials regardless of the functional groups present. Furthermore, this technique is greener than other methods as no solvent is used and therefore there is minimal waste production.

First, stir 12.5 milliliters of deionized water and 12.5 milliliters of dimethylformamide in a 100 milliliter screw cap jar for approximately five minutes, add 0.87 grams of copper, two nitrate hydrate, followed by 0.5 grams of trius acid to the jar, and stir the resulting blue solution for approximately five minutes. Then close the jar and place it in a preheated oven at 120 degrees Celsius for approximately 24 hours. After allowing the jar to cool to room temperature, recover the copper BTC crystals via vacuum filtration using filter paper rated to recover crystals greater than or equal to 2.5 micrometers.

Then rinse the resulting crystals with 10 milliliters of di chloro methane three times, and transfer the resulting material to a 20 milliliter vial. Next, add enough di chloro methane to submerge the crystals and then seal the vial. Exchange the solvent every 24 hours with fresh di chloro methane for the next three days to assist in the removal of the less volatile solvents from the copper BTC pores.

After filtering the copper BTC crystals heat gradually to 170 degrees Celsius in a vacuum oven to remove any residual guest molecules from the material when finished. Confirmed the structure and chemical makeup, the copper BTC crystals via powder X-ray diffraction and Fourier. Transform infrared spectroscopy prior to each experiment.

Clean the plasma reactor and any glassware to be used in the plasma treatment with air plasma at 50 watts for at least 30 minutes. When finished, place 0.5 grams of activated copper BTC in a 250 milliliter Pyrex bottle and spread it throughout the bottle to ensure a homogenous treatment. Then place a permeable cloth around the neck of the bottle and secure it with a rubber band to minimize sample loss upon applying a vacuum.

Next, place the bottle in the plasma chamber. Apply a vacuum until the chamber reaches a pressure of less than or equal to 0.2 millibar for at least 30 minutes to remove any water that may have absorbed onto the sample. Connect the HEXA fluoro, ETH ethane gas to the instrument.

Then adjust the mass flow controller to fill the reaction chamber with the appropriate amount of HEXA fluoro ETH ethane gas. To maintain the desired pressure of the experiment, rotate the bottle within the P-E-C-V-D device to create a more homogenous treatment of the powder. Light the plasma at the desired power with a 13.56 megahertz RF generator, and tune the radio frequency with the lc matching unit to maximize the power while minimizing the reflectance.

Once the treatment is complete, evacuate the chamber of any residual hexa fluoro et ethane gas, and then vent with dry air to atmospheric pressure. Remove the sample from the P-E-C-V-D apparatus and recover the treated material from the sides of the bottle into a 20 milliliter vial. Place the open vial in a preheated oven at 120 degrees Celsius for two hours to remove any unreactive hexa fluoro ethane gas.

After removing the treated material from the oven, transfer it to a desiccate to prevent absorption of water from the atmosphere. Rinse the residual material left in the bottle with water and filter to recover the waste for proper disposal. Characterize the treated copper BTC with solid state fluorine, nuclear magnetic resonance Fourier, transform infrared spectroscopy and x-ray photo electron spectroscopy.

Following this, set the desired temperature and relative humidity of an environmental chamber to 45 degrees Celsius and 100%respectively. After equilibration, spread the sample out evenly in an open container and place it in the chamber for three days. Once the aging process is complete, characterize the copper BTC sample with x-ray diffraction and a nitrogen isotherm at 77 kelvin to determine the degree of degradation.

Next, prepare a 14.6 liter ballast of ammonia at five milligrams per meter cubed by first injecting an empty ballast with 210 milliliters of neat ammonia. Then fill the ballast with zero air to a pressure of 15 PS.I connect the ballast in line with the micro breakthrough apparatus. Run a blank tube in the micro breakthrough apparatus.

Set the mass flow controllers for ammonia and dry air to eight and 12 milliliters per minute respectively to create a flow of 20 milliliters per minute of 2000 milligrams per meter cubed of ammonia. Run a programmed method to control the gas chromatograph and photo ionization detector to determine the feed signal of ammonia. In the effluent place a small amount of glass wool below the glass pritt in a nominal four millimeter inner diameter glass tube.

Fill the sample to four millimeters in height and weigh the material. Then heat the sample to 150 degrees Celsius for one hour while flowing dry air through the glass tube to remove any absorbed water. After weighing the dried sample, place the tube in line and secure it upright in a water bath set to 25 degrees Celsius.

Set the mass flow controllers for ammonia and dry air to eight and 12 milliliters per minute respectively to create a flow of 20 milliliters per minute at 2000 milligrams per meter cubed of ammonia while bypassing the sample to the fill lines with the feed gas. Following this, flow the ammonia stream through the sample and run a programmed method to control the gas chromatograph and photo ionization detector to monitor the concentration of ammonia in the effluent. Once the effluent concentration has reached the feed concentration, switch off the ammonia stream and allow the sample to off gas any ammonia that is not strongly absorbed to the sample.

Remove the sample from the water bath for post-exposure analysis via x-ray diffraction and four eea, transform infrared analysis. Finally, integrate the gas chromatograph signal versus time data to determine the ammonia loading for the sample moths treated with a per fluoro acan plasma under adequate conditions should display enhanced hydrophobicity. This can be demonstrated by placing the powder on top of liquid water and determining if the sample floats or by measuring the water contact angle on a pressed pellet.

As seen here, the contact angle for the copper BTC and the HEXA fluoro eth ethane plasma treated copper BTC pellets were measured to be 59 degrees and 123 degrees respectively. The presence of CFX groups on the surface of the pores adds to the hydrophobicity of the material causing the material to repel water. The presence of CF bonds are indicated by spectral bands between 1300 and 1, 140 inverse centimeters.

In the attenuated total reflectance Fourier transform infrared spectroscopy results as can be seen here, the degree of Fluor nation and confirmation of CFX species type can be done with fluorine magic angle spinning nuclear magnetic resonance as shown here, or x-ray photo electron spectroscopy. The two main fluorine species observed in this sample are CF two groups at about minus 87 PP M and CF groups at about minus 152 PP M.There is a small peak at about minus 80 PP M representing the CF three groups. All other significant peaks represent spinning side bands at approximately nine kilohertz intervals from the parent peak.

The CFX groups are likely a combination of groups that have reacted with the inner surfaces of the moth, as well as an amorphous coating on the outside of the moth crystal. The large size and quantity of spinning side bands for the CF two and CF species indicate that these CFX groups are tightly bound to the copper BTC structure and relatively immobile the x-ray diffraction patterns of the copper BTC and HOF fluoro ETH ethane plasma treated copper. BTC samples are pictured here after aging at 45 degrees Celsius and 100%RH for three days.

The patterns show a near complete change in the structure of the untreated sample. However, the plasma treated sample shows minimal changes in the structure. The results are indicative of enhanced structural stability even under harsh humidity conditions.

Micro breakthrough testing of aged copper BTC and HOF fluoro eth Ethan treated copper BTC samples for ammonia at a concentration of 2000 milligrams per meter cubed is presented here. Integration above the breakthrough curves yields capacities of 1.1 millimole of ammonia per gram of copper, BTC and 5.3 millimoles of ammonia per pergram of hexa fluoro, ETH ethane plasma treated copper BTC. The enhanced ammonia uptake of the plasma treated copper BTC sample after aging is due to the retention of the original copper BTC crystal structure when compared to the aged copper BTC sample.

While attempting this procedure, it is important to remember that in any per fluoro alcan plasma, there is a potential to form hydrogen fluoride and other corrosive gases. Special care must be taken to protect the user and the instruments from these harmful species, including making sure that all tubing valves, mass flow controllers, connections, seals in the vacuum pump are made of corrosion resistant materials and are inspected regularly. Following this procedure, many new microporous materials can be developed with fluorinated species on their surfaces.

These functional groups not only affect the we properties and hydrophobicity of the material, but also alter the absorption properties as the inner surfaces of the modified material are highly polar and chemically inert.