Base-Free Production of H2 by Dehydrogenation of Formic Acid Using An Iridium-bisMETAMORPhos Complex

Abstract

Hydrogen holds the potential to be one of the major energy carriers for the future. However, a hydrogen-based economy requires technology that allows efficient and safe storage and release of H2. In this light, the reversible storage of dihydrogen in the form of formic acid (HCOOH) provides an interesting H2 storage-release system. Formic acid can be obtained from the catalytic hydrogenation of CO2 [1a–c] or by direct hydrothermal oxidation of biomass.[1d–f] Since the early nineties, substantial developments in the production of HCOOH have stimulated interest in the reverse reaction for hydrogen release on demand.[2] Important recent breakthroughs are the highly active Fe-based dehydrogenation catalyst reported by Laurenczy, Beller, and coworkers[ 2h] and work by Fujita and co-workers on an iridium catalyst that is highly active for CO2 hydrogenation or HCOOH dehydrogenation, depending on pH.[2k] The dehydrogenation of HCOOH to H2 and CO2 is typically performed in the presence of substoichiometric amounts of base, which drastically decreases the hydrogen content (from the theoretical maximum of 4.4 wt% in pure HCOOH to 2.3 wt% based on the typical 5:2 HCOOH/ NEt3 mixture). In the context of developing efficient hydrogen- based fuel cells, HCOOH/base mixtures were recently optimized for H2 release using a ruthenium-based catalysts.[3] Ideally, catalytic dehydrogenation should be performed in the absence of base and other additives, thereby maximizing the overall hydrogen storage capacity (4.4 wt%). Besides this, hydrogen produced from HCOOH/NEt3 mixtures requires purification to remove traces of volatile amines prior to application in fuel cells.[