Plants have already been used for ‘phytomining’ metal from waste, including making gold, silver and copper nanoparticles, though existing processes are long-winded. ‘Normally you’d burn off the biomass, solubilise and purify the metal,’ Clark tells Chemistry World. ‘Then if you were going to use it as a catalyst, you’d have to convert it into a catalytic form, typically absorbing it again onto some suitable matrix.’ But previous work developing processes that might feature in a ‘biorefinery’ helped the York scientists develop a much more direct approach.
Team members Helen Parker and Liz Rylott left Arabidopsis seedlings in a solution of K2PdCl4 for 24 hours. Palladium-containing nanoparticles up to 32nm in diameter grew in their leaves, which the York researchers then washed, dried and ground to a powder.
Parker and Rylott then pyrolysed the powder at 300°C in nitrogen to make catalyst nanoparticles. Using their nanoparticles to couple phenylboronic acid and various aryl halides – including hard-to-react chlorides and bromides – they got better yields than with Pd(OAc)2 or palladium-on-carbon. ‘Taking the material and turning it directly into something useful, to the best of our knowledge, is original,’ Clark says.
‘These findings are extremely important,’ enthuses Jenny Edwards, who develops metal catalysts at Cardiff University, UK. ‘They demonstrate that an extremely valuable source of “waste” precious metals, which are usually remediated using quite complex methodologies, can be recovered quickly and effectively using an environmentally benign vector – the plant.’
With metal scarcity and waste processing becoming increasingly pressing global issues, these results have secured backing from the G8 Research Councils Initiative on Multilateral Research Funding. Researchers from York, Yale University, US, and the University of British Columbia, Canada, are working together in a collaboration called Phytocat. The other groups’ knowledge will help tailor the approach to relevant mining situations, Clark explains.