Chemical Hydrogen Storage

Due to environmental concerns and the decline in our petroleum reserves, there is currently enormous interest in the development of alternative sustainable energy vectors to replace fossil fuels. H2 is a potential clean energy source, as it can be directly or electrochemically combusted within a proton-exchange membrane fuel cell. However, as a result of problems with the storage and transport of gaseous H2 and its low volumetric energy density, chemical H2 storage (CHS) based on the dehydrogenation of small molecules is an attractive alternative. In principle, both formic acid (FA, 4.4 wt% H2) and methanol (MeOH, 12.6 wt% H2) can be obtained renewably from biomass oxidation and are therefore excellent potential liquid CHS materials. Balanced chemical equations for the dehydrogenation of FA and MeOH to generate H2, along with their corresponding free energies are shown below.

At this stage the production of FA and MeOH from biomass is not commercially viable, in part due to the by-products that are also produced. Another option for FA and MeOH synthesis is the direct hydrogenation of CO2. CO2 is a readily available, non-toxic and inexpensive source of carbon and it is expected that there will be economic and environmental benefits from using CO2 as a feedstock.

Our strategy, in collaboration with Professor Wesley Bernskoetter at the University of Missouri, to facilitate both the dehydrogenation of FA and MeOH and the hydrogenation of CO2 and H2 to FA and MeOH is to utilize a homogeneous transition metal catalyst. To date catalysts for all 4 processes are known, however, more efficient systems that give higher turnover numbers and operate under milder conditions are required. In addition, the best current catalysts for these processes generally use precious metals and it would be more desirable to develop catalysts based on first row transition metals, which are cheaper, more abundant and less toxic. The aim of this project is to develop efficient Fe catalysts for the dehydrogenation of FA or MeOH to CO2 and H2 and the reverse hydrogenation of CO2 to FA or MeOH.

We demonstrated that the five coordinate complexes (RPNP)Fe(CO)H (RPNP = N{CH2CH2(PR2)}2-; R = iPr (1a) or Cy (1b)), and the six coordinate complexes (RPNHP)Fe(CO)H(COOH) (R = iPr (2a) or Cy (2b)), formed through the 1,2-addition of FA to 1, are highly active catalysts for FA dehydrogenation (Eqs 3 & 4). In fact, when these catalysts, are used in the presence of a LA co-catalyst, such as LiBF4, NaCl or CsCl, there was no need to add a base or extra ligand to the catalytic mixture. In the presence of 10 mol% LiBF4, our best system gave a TON of 984,000 (98% conv) and a TOF of 197,000 h-1. In future work, we aim to fully understand the effect of the LA.

Additionally, we have shown 1 and 2 are active catalysts for CO2 hydrogenation to formate, giving TONs of up to 9,000, in the presence of LA co-catalysts. Mechanistic investigations implicated the LA in disrupting an intramolecular hydrogen bond between the PNP ligand N-H moiety and the carbonyl oxygen of a formate ligand in the catalytic resting state. When the secondary amine containing ligand, was replaced with an analogous ligand containing a tertiary amine, hydrogen bonding can no longer occur, and TONs of nearly 60,000 were achieved; the highest activity catalyst based on an earth abundant metal reported to date (Eq 5). A LA is still required to achieve this activity and it was proposed that the LA assists in substitution of a formate ligand for dihydrogen during the slow step in catalysis. Further work will concentrate on trying to modify the ligand framework to improve the TON.

We are studying the mechanism of MeOH dehydrogenation using RPNHP supported Fe complexes and have demonstrated that a stepwise pathway is operative. The reaction is essentially a tandem reaction with 4 steps (Scheme 1): (i) initial dehydrogenation of MeOH to formaldehyde, (ii) reaction of water with formaldehyde to form methanediol, (iii) dehydrogenation of methanediol to release H2 and form FA, and (iv) decarboxylation of FA to release CO2 and the final equivalent of H2. We demonstrated that base was not required for any of the steps and a LA was needed only to catalyze the FA dehydrogenation (step (iv)). As a result, our RPNHP supported Fe complexes can perform base-free MeOH dehydrogenation in the presence of LAs

 Scheme 1. Proposed pathway for MeOH dehydrogenation with water.

A maximum TON of 51,000 was achieved and high conversion of MeOH, with excellent selectivity was also possible (Eq 6). This is the highest TON achieved for a first row metal catalyzed MeOH dehydrogenation, and the first example that does not require a base. However, the stability of our Fe catalysts is low and we are looking to design longer lasting catalysts.