Supercritical Nitrogen Processing for the Purification of Reactive Porous Materials

Summary

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.

Abstract

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 Å. 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.

Introduction

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....

Protocol

1. Apparatus

1. 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.
2. For nitrogen ScD (NScD) treatments, ensure that the gas supply is research purity (>99.999%) nitrogen gas fitted with a pressure regulator for outl....

Results

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.......

Discussion

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 .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Compressed Nitrogen Gas	Messer Schweiz AG		50 L bottle, purity > 99.999%, <3 ppmv H2O
Liquid Nitrogen	Pan Gas AG		Bulk storage, on site
Custom Supercritical Drying Apparatus	Empa		Swagelok (compression fitting and VCR) components
Custom Cryogenic Furnace Bath	Empa
Custom Labview Interface	Empa