Effect of Pillar Modules and Their Stoichiometry in 3D Porous Frameworks of Zn(II) with [Fe(CN)6]3–: High CO2/N2 and CO2/CH4 Selectivity
journal contribution
posted on 2013-10-07, 00:00 authored by Arpan Hazra, Satyanarayana Bonakala, Sandeep K. Reddy, Sundaram Balasubramanian, Tapas Kumar MajiWe report the synthesis, single-crystal
structural characterization,
and selective gas adsorption properties of three new 3D metal–organic
frameworks of Zn(II), {[Zn3(bipy)3(H2O)2][Fe(CN)6]2·2(bipy)·3H2O}n (1), {[Zn3(bipy)][Fe(CN)6]2·(C2H5OH)·H2O}n (2), and {[Zn3(azpy)2(H2O)2][Fe(CN)6]2·4H2O}n (3) (bipy = 4,4′-bipyridyl
and azpy = 4,4′-azobipyridyl), bridged by [Fe(CN)6]3– and exobidentate pyridyl-based linkers.
Compounds 1–3 have been successfully
isolated by varying the organic linkers (bipy and azpy) and their
ratios during the synthesis at RT. Frameworks 1 and 3 feature a biporous-type network. At 195 K, compounds 1–3 selectively adsorb CO2 and
completely exclude other small molecules, such as N2, Ar,
O2, and CH4. Additionally, we have also tested
the CO2 uptake capacity of 1 and 3 at ambient temperatures. By using the isotherms measured at 273
and 293 K, we have calculated the isosteric heat of CO2 adsorption, which turned out to be 35.84 and 35.53 kJ mol–1 for 1 and 3, respectively. Furthermore,
a reasonably high heat of H2 adsorption (7.97 kJ mol–1 for 1 and 7.73 kJ mol–1 for 3) at low temperatures suggests strong interaction
of H2 molecules with the unsaturated Zn(II) metal sites
and as well as with the pore surface. Frameworks 1 and 3 show high selectivity to CO2 over N2 and CH4 at 273 K, as calculated based on the IAST model.
The high values of ΔHCO2 and ΔHH2 stem from
the preferential electrostatic interaction of CO2 with
the unsaturated metal sites, pendent nitrogen atoms of [Fe(CN)6]3–, and π-electron cloud of bipyridine
aromatic rings as understood from first-principles density functional
theory based calculations.
