Reversible interconversion between methanol-diamine and diamide for hydrogen storage based on manganese catalyzed (de)hydrogenation

Reversible interconversion between methanol-diamine and diamide for hydrogen storage based on manganese catalyzed (de)hydrogenation

Abstract

The development of cost-effective, sustainable, and efficient catalysts for liquid organic hydrogen carrier systems is a significant goal. However, all the reported liquid organic hydrogen carrier systems relied on the use of precious metal catalysts. Herein, a?liquid organic hydrogen carrier system based on non-noble metal catalysis was established. The Mn-catalyzed dehydrogenative coupling of methanol and?N,N’-dimethylethylenediamine to form?N,N’-(ethane-1,2-diyl)bis(N-methylformamide), and the reverse hydrogenation reaction constitute a hydrogen storage system with a theoretical hydrogen capacity of 5.3 wt%. A rechargeable hydrogen storage could be achieved by a subsequent hydrogenation of the resulting dehydrogenation mixture to regenerate the H2-rich compound. The maximum selectivity for the dehydrogenative amide formation was 97%.

Introduction

Hydrogen has long been regarded as one of the most promising sustainable energy carriers because it has a high mass energy density, can be efficiently transformed into electricity by fuel cells, and generates only water during combustion. In recent years, significant progress has been made in the generation of hydrogen from renewable energy sources and the development of efficient hydrogen-powered fuel cells. However, hydrogen has not been widely used as an energy source because its storage is challenging. There are economic and safety concerns associated with the compressed and cryogenic liquid hydrogen, therefore reversible hydrogen storage in chemical bonds via catalytic hydrogenation/dehydrogenation reactions is a promising technique. Formic acid, formaldehyde, and methanol?have been extensively studied as molecular hydrogen carriers. However, H2?release from these compounds involves generation of CO2, and H2?cannot be readily reloaded because the liquid carriers are consumed. The need to use stoichiometric amounts of bases and the low hydrogen capacity of formic acid (4.3 wt%) further limit such approaches. To develop more efficient hydrogen storage systems, liquid organic hydrogen carriers (LOHCs) have emerged as a powerful strategy, in which a pair of H2-rich and H2-lean liquid organic compounds can repeatedly release and unload H2. Early LOHCs studies focused on the dehydrogenation of cycloalkanes and the reverse hydrogenation of aromatics, but harsh reaction conditions (usually >250 °C) were required. To lower the endothermicity, LOHCs systems based on nitrogen-containing organic hydrides, e.g.,?N-heterocycles, were developed. These have high H2?capacities, in the range 5.3–7.3 wt%. These systems can be promoted by various homogeneous and heterogeneous catalysts?under relatively mild conditions (Fig.?). Besides, the Milstein and Prakash groups reported LOHCs systems via Ru-catalyzed amide bond formation and hydrogenation (Fig.?). Notably, widely available and inexpensive amines and alcohols can be used as hydrogen carriers in these systems. Despite the favorable thermodynamics, usually <80% selectivity for dehydrogenative amide bond formation was reached.