Effect of Keggin polyoxometalate on Cu(ii) speciation and its role in the assembly of Cu3(BTC)2 metal-organic framework

Abstract

HKUST-1 is one of the popular metal-organic frameworks (MOFs). The formation of this MOF is significantly accelerated by adding Keggin polyoxometalate anions to the synthesis solution. In this paper the chemistry behind this observation was investigated. Upon addition of Keggin type H 3PW12O40 heteropolyacid the speciation of Cu2+ cations in ethanol:H2O mixture drastically changes. Combining EPR and XANES measurements with accurate pH measurements and prediction of Cu2+ hydrolysis provides strong evidence for surface induced hydrolysis and consequent dimerisation of monomeric Cu2+ species on Keggin ions in acidic conditions. This enables paddle wheel formation, hence explaining the instantaneous precipitation of Cu 3(BTC)2 at room temperature and the systematic encapsulation of Keggin ions in its pores.