Andrew Cooper and coleagues from the University of Liverpool, UK, have proposed a model for a hypothetical porous polymer in which naked fluoride moieties are site-isolated and available for H₂ physisorption. The strategy, which is based on incorporated 'site isolated' fluoride groups with substantial vacant coordination space, might be applicable to a variety of microporous materials for gas sorption applications. Cooper spoke to NJC about the work: 

How did you get interested in this research project?

We have been synthesizing high surface area polymers for hydrogen storage in my group, but known examples only physisorbed hydrogen at very low temperatures. Other studies show that the US Department of Energy gravimetric storage targets for H₂ can be approached or even met by physisorption-at least based on sorbent mass-but the use of cryogenic temperatures is likely to be a problem for practical usage. Modelling seemed very helpful to guide our materials synthesis programme and the EPSRC Portfolio Partnership grant supporting the work gave us the flexibility to do this. 

What is the most important result of the work?

The concept that site-isolated localised non-metal centres might interact strongly with hydrogen and the ideas as to how such entities might actually be realised in porous materials.

What are the implications of these results?

The modelling results suggest higher interaction energies with hydrogen than have been observed in uncharged microporous polymers based on C,H,N-for example, polymers of intrinsic microporosity, hypercrosslinked polymers, and conjugated microporous polyaryleneethynylenes-or in metal organic frameworks. 

The site-isolated "naked" fluorides that we propose are tricky to synthesise because they are very reactive-hence their use, for example, in direct fluorination reactions. Preparing materials to test the hypotheses in the models has proved harder than we first expected. We and other groups are also looking at light transition metals and dihydrogen complexes; this is promising but there are similar issues of high reactivity and stability to address, plus the perennial problem of the high mass of most elements relative to hydrogen.