Active Response of Six-Coordinate Cu2+ on CO2 Uptake in Cu(dpa)2SiF6-i from in Situ X-ray Absorption Spectroscopy

Abstract

Interpenetrated metal organic framework (MOF) Cu(dpa)2SiF6-i (dpa = dipyridyl acetylene) with 6-coordinate copper takes up CO2 with isosteric heat of sorption Qst ≅ 40 kJ/mol, comparable to MOFs designed to have specific, strong chemisorption interactions. Although small pore size contributes to this high Qst, any preferred microscopic physisorption mechanisms were incompletely understood. We present in situ X-ray absorption spectroscopy results sensitive to local electronic structure around copper, fluorine, and nitrogen to look for signatures of CO2 interaction with and near ionic species in Cu(dpa)2SiF6-i. No changes in F or N K edge spectra imply no structural or electronic distortions to SiF62– or dpa molecules on CO2 uptake. Cu L2,3 edge spectra confirm Cu2+ valence and show weak systematic CO2-induced effects that result from distortions of the copper coordination shell. We suggest that electrostatic interaction between Cu2+ and axially aligned CO2 quadrupole moments contributes to the large Qst.