Supercritical Nitrogen Processing for the Purification of Reactive Porous Materials

The overall goal of the following experiment is to purify magnesium burrow, hydride, a porous, complex hydride, and high density hydrogen storage material using supercritical nitrogen. This is achieved by first soaking a sample of magnesium burro hydride in compressed liquid nitrogen in the near critical region of the phase diagram at minus 163 degrees Celsius and 20 bar where its high fluid density is indicative of a high solvent power toward target impurities, such as Dior and end butyl species. As a second step, the liquid nitrogen is heated and compressed into the dense supercritical region of the phase diagram at minus 123 degrees Celsius and approximately 100 bar.

Next, the supercritical nitrogen and any dissolved species can be extracted from the sample by slowly reducing the pressure to vacuum at minus 123 degrees Celsius, thereby circumventing the liquid to gas phase transition where surface tension effects can lead to poor extraction of the target species. The results show that after iterative supercritical nitrogen drying treatments, the gas is decomposition. Products of magnesium burrow hydride only consist of hydrogen based on temperature programmed decomposition coupled with infrared spectroscopy.

The focus of our research activities is the material science connected to the storage of renewable energy. Since the beginning of our pioneering research on complex hydrides as hydrogen storage materials, the synthesis of borohydride compounds has been a challenge. Pure compounds are difficult to produce, but essential for scientific investigations.

Purification methodologies can help answer key questions in the field of solid state hydrogen storage, such as what role impurities play in the decomposition reactions of complex hydrates. The answer remains complicated and the topic of dispute even today. The idea to pursue this strategy was inspired by the work of hopin colleagues who reported the effective use of supercritical carbon dioxide in the activation of metal organic frameworks in 2009.

Here we use nitrogen in a similar way for porous complex hydrides by exploiting the high density supercritical region of its phase diagram at much lower than EBR temperatures. The main advantage of supercritical nitrogen processing over existing methods like supercritical, carbon dioxide drying is that nitrogen is in towards strongly reducing compounds, such as alienates and bo hydrides, while still exhibiting many advantages of an ideal supercritical solvent. All four.

This particular method has shown to be effective in treating porous magnesium bore hydride. It can also be applied to other materials where the reactivity of carbon dioxide prohibits its use As a drying or extraction agent, the cold temperatures and modest pressures required make it almost universally applicable. Use a basic super critical drying apparatus comprised of a gas supply dosing manifold, a vacuum system, temperature and pressure sensors, and the sample environment all connected by high pressure gas tubing load 0.1 to 0.5 grams of magnesium borow hydride into the sample holder under an iner atmosphere such as an Argonne glove box at ambient temperature.

Close the sample holder with a filter gasket and then close the valve. After removing the sample holder from the glove box, attach it to the apparatus following this, open the dosing manifold to vacuum via V two and evacuate. After opening V three and evacuating, perch the apparatus with nitrogen via V one and evacuate via V two.

Evacuate and purge a few times. Then open V four and evacuate the sample at room temperature for up to 24 hours. To reach the minimum pressure of the system, install the sample bath around the sample holder by raising the bath into position on a scissor lift.

Following this, set the heater to the desired future liquid temperature of minus 163 degrees Celsius and continue to evacuate the apparatus until the temperature equilibrates For nitrogen supercritical drying processing of magnesium boro hydride, select a liquid temperature of minus 163 degrees Celsius, corresponding to an intermediate density of 0.6 grams per milliliter. Next, close off the system to vacuum by closing V two throttle open V one slowly allowing the pressure to increase into the liquid region of the phased diagram above 20 bar. After equilibrating the system at 20 bar and minus 163 degrees Celsius, allow the sample to soak in liquid nitrogen for four hours.

Set the heater to minus 123 degrees Celsius with a ramp of less than or equal to two degrees per minute. Allow the pressure to increase no higher than the maximum rated pressure of the apparatus after e equilibrating the system at the maximum rated pressure and minus 123 degrees Celsius. Allow the sample to soak in supercritical nitrogen for one hour or more at this point.

Close V one and then carefully crack the system to vacuum by throttling V two, allowing the pressure to decrease as slowly as possible. Remove the sample bath and fully open V two to completely evacuate the sample equilibrating at room temperature and high vacuum after treatment. Close valves V three and V four and remove the sample holder from the apparatus.

Transfer the sample holder to an inert environment for handling such as an argonne filled glove box. Finally, remove the sample from the sample holder and store in a sealed container at ambient temperature. The supercritical nitrogen drying methodology described herein was successful for the purification of porous magnesium boro hydride.

The target species of extraction were D boring and an unspecific end butyl impurity, which were reduced to negligible quantities after iterated treatments with supercritical nitrogen drying has shown by infrared spectroscopy of the gaseous species evolved during decomposition after three treatments, no detected impurities remained. Once a set of successful protocol conditions has been established for a particular application, this technique can be scaled up to purify large quantities of as synthesized material. The main limitation is that the sample should have adequate thermal contact with the bath and therefore an appropriately designed sample environment.

When preparing an appropriate apparatus for this procedure, the most important consideration is low temperature bath. Because this procedure does not demand exceptionally low temperatures, we opted for simple design involving a typical bench drop heater, immersed in leak nitrogen as a coolant Following this procedure. Analytical methods like x-ray photo electron spectroscopy, infrared spectroscopy, or Raman spectroscopy can be performed on the resulting sample.

Without the unwanted influence of residual impurities, the sample can easily be processed immediately before characterization. Super critical extraction techniques are already used in a diverse variety of applications. From the decaffeination of coffee beans to advanced methods for catalyst deposition in porous supports using nitrogen as an extracting agent, we simply extend the applicability of these techniques to materials that are reactive to what other more common supercritical solvents.

The development of new techniques allows scientists to answer questions relevant to emerging materials. For hydrogen energy applications, supercritical nitrogen processing may bring next generation complex hydros, a step closer to serving as mobile hydrogen storage materials in the sustainable energy landscape of the future.