Borane compounds for chemical hydrogen storage and carbon dioxide capture

Overview & Synopsis

In the last decade, boron- and nitrogen- based compounds have been considered as potential materials for chemical hydrogen storage due to their unique properties. Ammonia borane (NH3BH3, AB, 19.5 wt. % H) and hydrazine borane (N2H4BH3, HB, 15.4 wt. % H) are two of the most representative examples. However, these materials cannot be used in the pristine state due to different drawbacks (i.e. release of unwanted byproducts during thermolysis, high dehydrogenation temperature…). To overcome these problems, different approaches have been studied. The chemical modification of AB and HB is one of these options, where a protic hydrogen atom of the molecule is substituted by a metal cation. Thereby, the derivatives of AB and HB (amidoboranes and hydrazinidoboranes) have emerged with improved dehydrogenation properties.In this work, the last two members of the alkali derivatives of hydrazine borane are presented. Rubidium hydrazinidoborane (RbN2H3BH3, RbHB, 4.6 wt. % H) and cesium hydrazinidoborane (CsN2H3BH3, CsHB, 3.4 wt. % H) were obtained by a wet synthesis between hydrazine borane and the respective alkali metal. Two new crystalline solids were obtained. The full characterization of RbHB and CsHB was performed in the context of hydrogen storage materials. The successful introduction of the alkali metal cation and the destabilization of the molecule were confirmed. With the family of alkali derivatives of HB complete, we have a better overview of these materials and the findings are discussed.This thesis also pretends to open new perspectives for amidoboranes. Computational works have shown that boron nitride (BN) can uptake hydrogen or carbon dioxide (CO2) at room conditions. In this way, we selected the lithium and sodium amidoboranes to be studied as precursors of boron nitride-like structures in order to test their gas sorption properties. The formation of hexagonal BN has been observed when the materials were treated at 800°C. The capacity of CO2 capture of these materials was investigated. The results are presented for the first time in this work.