Ethylene glycol as an efficient and reversible liquid-organic hydrogen carrier

Abstract

Hydrogen has long been regarded as an ideal alternative clean energy vector to overcome the drawbacks of fossil technology. However, the direct utilization of hydrogen is challenging, due to low volumetric energy density of hydrogen gas and potential safety issues. Herein, we report an efficient and reversible liquid to liquid organic hydrogen carrier system based on inexpensive, readily available and renewable ethylene glycol. This hydrogen storage system enables the efficient and reversible loading and discharge of hydrogen using a ruthenium pincer complex, with a theoretical hydrogen storage capacity of 6.5 wt%.

Phosphines and N‐Heterocycles Joining Forces: an Emerging Structural Motif in PNP‐Pincer Chemistry

“…Instead, H 2 is released via a H 2 O‐assisted pathway from a PNP‐coordinated Ru(H)(OH 2 ) intermediate . A similar non‐ligand‐cooperative mechanistic scenario was also suggested for the acceptorless dehydrogenative coupling of ethylene glycol catalyzed by 45 . Hofmann and co‐workers also arrived at the conclusion that metal‐ligand cooperative pathways are most likely not involved in their direct aminations of alcohols with ammonia using the PCy 2 ‐substituted catalyst 46 , and extensive DFT calculations revealed lower barriers for an alcohol assisted mechanistic scenario .…”

Homogeneous Reforming of Aqueous Ethylene Glycol to Glycolic Acid and Pure Hydrogen Catalyzed by Pincer‐Ruthenium Complexes Capable of Metal–Ligand Cooperation

“…Very recently, we described a liquid organic hydrogen carrier system based on ethylene glycol, which enabled the efficient and reversible loading and discharge of hydrogen using a single ruthenium pincer complex (Scheme 1 b). [22] Based on this report, we envisioned that ethylene glycol would undergo dehydrogenation to glycolaldehyde ( GAL ) in the presence of a ruthenium pincer complex via MLC, accompanied by hydrogen release (Scheme 1 c). Subsequently, the interception of glycolaldehyde by water and base could liberate a second molecule of hydrogen and enable access to glycolic acid ( GAC ) after acid workup.…”

“…To examine the feasibility of the above‐proposed reforming process, we evaluated the acridine‐type PNP complexes, Ru‐1 and Ru‐2 , that gave excellent results in the dehydrogenative coupling of ethylene glycol to oligoesters (Table 1). [22] The reforming of aqueous ethylene glycol was attempted using 0.5 mol % catalyst loading and 5 equivalents of potassium hydroxide at 115 °C (oil bath temperature) in a 1:1 mixture of water (1.0 mL) and THF (1.0 mL) [23] . Somewhat surprisingly, these catalysts showed only sluggish reactivity (Table 1, entries 1 and 4) giving a maximum of 14 mL of hydrogen.…”

“…Even though complexes Ru‐1 and Ru‐2 were shown to be efficient in the dehydrogenative coupling of ethylene glycol to oligoesters, [22] the PNNH complex Ru‐8 showed superior performance in the reforming reaction. In order to understand the efficiency of the PNNH‐family of complexes, we sought to investigate its possible modes of bond activation.…”

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

“…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 H 2 -rich and H 2 -lean liquid organic compounds can repeatedly release and unload H 2 [24][25][26][27][28] . Early LOHCs studies focused on the dehydrogenation of cycloalkanes and the reverse hydrogenation of aromatics, but harsh reaction conditions (usually >250°C) were required 29,30 .…”