Name: supercritical nitrogen processing for the purification of reactive porous materials
Uploaded: 2015-05-15T14:00:00
Description: Watch this Scientific Journal Video about Supercritical Nitrogen Processing for the Purification of Reactive Porous Materials at JoVE.com

This work was supported by the European Fuel Cells and Hydruogen Joint Undertaking under collaborative project BOR4STORE (Grant Agreement No. 303428) and infrastructure program H2FC (Grant Agreement No. FP7-284522).

1. Apparatus
<ol>
	<li>Use a basic supercritical drying (ScD) apparatus comprised of four primary components connected by high pressure gas tubing: the gas supply, a vacuum system, sensors (temperature and pressure), and the sample environment (which can be submersed in a bath). Ensure that the construction is of high-quality stainless steel valves, fittings, and tubing, pressure-rated to at least 10 MPa within the temperature range between 80-300 K. 
	Note: A schematic is shown in Figure 1.</li>
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<ol>
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Perhaps due to its relatively low critical temperature (126 K), N2 has historically been overlooked as an effective ScD solvent. In earlier reports,3,17,42,43 it has only been alluded to in the context of processing temperatures at or above ambient, where it exhibits only modest solvation power due to its low fluid density in this region of its phase diagram (except at extremely high pressures43). The key step in realizing the practical utility of N2 as a supercritical solvent ...

Supercritical fluid extraction (SFE) and drying (ScD) methods are well established in a wide range of practical applications, especially in the food and petroleum industries, but also in chemical synthesis, analysis, and materials processing.1-6 The use of drying or extraction media at conditions above their critical points is often faster, cleaner, and more efficient than traditional (liquid) techniques, and has the added advantage of being highly tunable with respect to the solvation power of the fluid by slight adjustment of the operating conditions.3,7 A simple ScD method consists of three basic steps. The first step is exposing the solid (or perhaps liquid) starting material which contains the target impurity compound to an appropriately chosen ScD fluid in its liquid (or near-liquid supercritical) phase, where its high density corresponds to a high (and perhaps selective7) solvent power with respect to the target species. The second step is heating and compressing the system above the chosen ScD fluid&#8217;s critical point in a closed container so that the fluid and its dissolved target species do not pass a phase boundary which might result in separation. The final step is slowly reducing the pressure of the ScD fluid to vacuum at a temperature above the critical temperature, allowing the fluid solution containing the target species to escape, again without encountering a phase boundary or any detrimental surface tension effects along the way.
The starting material is left depleted of the target species and may be subjected to iterated treatments if necessary. In cases of supercritical fluid extraction, the target solute species is the desired product, and is collected from solution for further use.8,9 In other cases, the dried or purified starting material is the desired product, and the extracted impurities are discarded. This latter scenario, referred to herein as the ScD approach, was discovered to be an effective strategy for the pretreatment of high surface area, microporous materials such as metal-organic frameworks (MOFs), where traditional heat-treatment methods under vacuum are in many cases not sufficient in clearing the pores of all unwanted guests, or result in pore collapse.10 Carbon dioxide ScD (CScD) processing is now a routine post-synthetic process for MOFs,11 leading to increases in nitrogen-accessible surface areas over untreated materials of up to 1,000%12 and other improvements, such as in catalytic activity.13 Other notable supercritical fluid applications are as a widely tunable medium for chemical reactions,14-16 supercritical fluid chromatography (SCFC)6,17,18 and synthesis of aerogels and advanced composite materials.19-22
For drying applications, a ScD fluid is chosen based on two criteria: a) the proximity of its critical point to ambient conditions (for convenience and to reduce energy costs or process complexity) and b) its solvation power with respect to the target species. Carbon dioxide (CO2) has proven to be a convenient ScD fluid in many applications since it is nontoxic, nonflammable, and cheap, and can be tuned to exhibit a high solvation power toward a number of common organic target species in its near-liquid state (at pressures of &#60;10 MPa and temperatures of 273-323 K).1-3,7-9 Other common supercritical solvents (or co-solvents) include water (spanning a remarkable range of solvent properties between its ambient and supercritical state23), acetone, ethylene, methanol, ethanol, and ethane, covering the spectrum from polar (protic and aprotic) to nonpolar, and having critical points relatively near to ambient conditions.
Carbon dioxide is by far the most common ScD fluid used. In established CScD methods, the reactivity of the starting material is not an inhibitive factor since CO2 is only very weakly reactive at temperatures near its critical point. However, certain classes of materials such as so-called complex hydrides (e.g., alanates and borohydrides) present unique challenges in handling due to their strong reactivity in the presence of water or CO2 in addition to their (perhaps intentionally tailored) instability under heating.24-26 Moreover, there is great international interest in such materials as high-density hydrogen storage compounds,27-30 and therefore also in nanostructured and/or porous varieties31-33. For the effective purification of such reactive, unstable, and nanostructured materials, ScD methods are a promising strategy.34 A ScD fluid must be used which has a small molecular diameter appropriate for penetration into narrow cavities and which also has a high solvation power toward the target impurities, while remaining unreactive toward the starting material itself. Herein, the use of supercritical nitrogen (N2) as an effective fluid for such extraction and especially drying applications is presented. A specific supercritical nitrogen drying (NScD) methodology is described below for the purification of &#947;-phase magnesium borohydride where the target species include both diborane and an n-butyl compound (similar to but not specifically identifiable as n-butane). The following protocol can be easily modified for general extension to other supercritical nitrogen drying or extraction processes.

<table border="0" cellpadding="0" cellspacing="0">
	<tbody>
		<tr>
			<td>Compressed Nitrogen Gas</td>
			<td>Messer Schweiz AG</td>
			<td></td>
			<td>50 L bottle, purity &#62; 99.999%, &#60;3 ppmv H2O</td>
		</tr>
		<tr>
			<td>Liquid Nitrogen</td>
			<td>Pan Gas AG</td>
			<td></td>
			<td>Bulk storage, on site</td>
		</tr>
		<tr>
			<td>Custom Supercritical Drying Apparatus</td>
			<td>Empa</td>
			<td></td>
			<td>Swagelok (compression fitting and VCR) components</td>
		</tr>
		<tr>
			<td>Custom Cryogenic Furnace Bath</td>
			<td>Empa</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Custom Labview Interface</td>
			<td>Empa</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
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supercritical nitrogen processing for the purification of reactive porous materials

Supercritical fluid extraction and drying methods are well established in numerous applications for the synthesis and processing of porous materials. Herein, nitrogen is presented as a novel supercritical drying fluid for specialized applications such as in the processing of reactive porous materials, where carbon dioxide and other fluids are not appropriate due to their higher chemical reactivity. Nitrogen exhibits similar physical properties in the near-critical region of its phase diagram as compared to carbon dioxide: a widely tunable density up to ~1 g ml-1, modest critical pressure (3.4 MPa), and small molecular diameter of ~3.6 &#197;. The key to achieving a high solvation power of nitrogen is to apply a processing temperature in the range of 80-150 K, where the density of nitrogen is an order of magnitude higher than at similar pressures near ambient temperature. The detailed solvation properties of nitrogen, and especially its selectivity, across a wide range of common target species of extraction still require further investigation. Herein we describe a protocol for the supercritical nitrogen processing of porous magnesium borohydride.

Alkali and alkaline earth metal borohydrides are potential hydrogen storage materials, which deliver a large content of gaseous hydrogen upon decomposition.27,29 Other decomposition products such as diborane have also sometimes been detected in the gas desorbed, but their origin is not a priori clear; it is possible they are products of the pure phase decomposition, but may also be impurities or products of reactions of impurities leftover from chemical synthesis.35 The porous phase of magnesium bor...

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Supercritical Nitrogen Processing for the Purification of Reactive Porous Materials

Nitrogen is an effective supercritical fluid for extraction or drying processes due to its small molecular size, high density in the near-liquid supercritical regime, and chemical inertness. We present a supercritical nitrogen drying protocol for the purification treatment of reactive, porous materials.

Supercritical fluid extraction and drying methods are well established in numerous applications for the synthesis and processing of porous materials. ...

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Chemistry

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