超临界氮处理的活性多孔材料的净化
Summary
氮气是用于萃取或干燥过程的有效的超临界流体，由于其小的分子尺寸，高密度在近液体超临界状态，和化学惰性。我们提出了一个超临界氮气干燥协议用于净化处理的反应性的，多孔的材料。
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
超临界流体萃取（SFE）和干燥（SCD）方法被很好地建立在广泛的实际应用，特别是在食品工业和石油工业，也可在化学合成，分析和材料的加工。1-6使用干燥或提取介质在高于其临界点条件往往是更快，更清洁和更有效的比传统的（液体）的技术，并具有可通过操作条件稍作调整相对于高度可调的流体的溶剂化功率的额外优点3,7一种简单的方法理学包括三个基本步骤。第一步是露出其包含在其液体中的目标杂质化合物来适当选择理学流体的固体（或者液体）的原料（或接近液体超临界相），在那里它的高密度对应于高（也许选择性7）溶剂功率相对于所述靶物种。 Ť他第二步骤加热和压缩上述所选理学流体的临界点，系统中的封闭容器，以使流体和其溶解靶物种不通过相界这可能导致分离。最后一步是缓慢降低理学流体的真空压力在高于临界温度的温度下，使含有目标物种逃脱，再次没有遇到一个相界或沿途的任何有害的表面张力作用的流体溶液。
起始原料为左耗尽目标物种的，并且可以进行迭代处理，如果有必要的。在超临界流体萃取的情况下，目标溶质种类是所需产物，并从供进一步使用的解决方案。8,9-收集在其他情况下，将干燥的或纯化的起始材料是期望的产物，并将所提取的杂质被丢弃。这后一种情况下，这里所指作为理学方法，被发现是一种有效的策略，高表面积，微孔材料，如金属 – 有机骨架（MOFs），其中，在真空下传统的热处理方法在许多情况下，在清除孔不充分的预处理所有的不速之客，或导致孔隙坍塌。10二氧化碳理学博士（CSCD）处理现在是一个常规的后期合成的过程MOFs材料，11导致增加氮可接触的表面面积比高达1000％未经处理的材料12其他的改进，如在催化活性。13其他值得注意的超临界流体的应用是作为一种广泛可调谐介质进行化学反应，14-16超临界流体色谱（SCFC）6,17,18和合成气凝胶和先进复合材料。19- 22
对于干燥应用，理学博士流体是基于两个标准来选择：一）接近它的临界点到环境的条件（为方便起见，并降低能源成本或处理复杂性）和b）其溶剂化功率相对于所述靶物种。二氧化碳（CO 2）已被证明是在许多应用中方便理学流体，因为它是无毒的，不可燃的，并且价格便宜，并且可以进行调节，以表现出高的溶剂化功率朝向一些普通的有机靶物种在其近液体的状态（在<10兆帕的压力和温度273-323 K）1-3,7-9其他常见的超临界溶剂（或共溶剂）包括水（跨越一个显着的其环境和超临界状态之间溶剂性能范围23），丙酮，乙烯，甲醇，乙醇，和乙烷，覆盖从极性（质子和非质子传递）到非极性的，并具有对环境条件相对临界点附近的频谱。
二氧化碳是迄今为止最常用理学流体。在建立CSCD方法中，反应性的原料不是抑制因子，因为二氧化碳是只有非常弱的温度接近它的临界点的反应。然而，由于其在水或CO 2存在强的反应，除了在加热下其（可能故意量身定做）不稳定某些类的材料，如所谓的复合氢化物（ 例如 ，铝氢化物和硼氢化物）在处理本独特的挑战24-26此外，还有在这样的材料如高密度的储氢化合物，27-30极大国际兴趣，因此，可以在纳米结构和/或多孔的品种31-33。对于这样的反应性的，不稳定的，纳米结构材料的有效的净化，理学的方法是有前途的策略。34所述理学流体必须使用具有适用于穿入窄空腔分子直径小并且还具有高的溶剂化功率朝向目标杂质，WHI勒剩余朝向起始材料本身不反应。这里，利用超临界氮气（N 2）作为用于这种提取和特别干燥应用提供有效的流体呈现。具体超临界氮气干燥（NSCD）方法是下面描述的γ相镁硼氢化其中目标种类包括二硼烷和正丁基的化合物（类似但不专门识别为正丁烷）的纯化。下面的协议可以很容易地修改一般扩展到其他超临界氮气干燥或提取过程。
Protocol
Representative Results
Discussion
这可能是由于其相对较低的临界温度（126 K），N 2 -历来被忽视作为有效理学溶剂。在早先的报告，3,17,42,43它仅被暗示在等于或高于室温，在那里它显示只有少量的溶剂化功率加工温度的范围内，由于其低的流体密度在其相图的这个区域（除在极高的压力，43）。在实现的N 2的实际效用作为超临界溶剂的关键步骤是在维持处理温度在临界点附近，那里的密度（因此?…
Disclosures
The authors have nothing to disclose.
Acknowledgements
这项工作是由欧洲燃料电池和Hydruogen联合执行下的合作项目BOR4STORE（批准号：303428协定）和基础设施项目H2FC（赠款协议号FP7-284522）的支持。
Materials
| 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 |
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