Unsaturated Zn–N2–O active sites derived from hydroxyl in graphene oxide and zinc atoms in core shell ZIF-8@ZIF-67 nanocomposites enhanced CO2 adsorption capacity

In order to form strengthened active sites for superior CO 2 adsorption on core shell ZIF-8@ZIF-67 nanocomposites , graphene oxide (GO) with oxygen-containing functional groups was used as support to synthesize hybrid adsorbents with unsaturated Zn–N 2 –O active sites. The results of XPS O 1s and FTIR analysis revealed that the Me–O functional group content in the 0.5 wt% graphene oxide (GO)-supported core shell ZIF-8@ZIF-67 nanocomposite (38.9 at.%) was much higher than that in the 0.5 wt% reduced graphene oxide (RGO)-supported nanocomposite (19.4 at.%). This was because hydroxyl functional groups in the GO formed coordination bonds with zinc atoms in core shell ZIF-8@ZIF-67. XPS N 1s spectra indicated that the incorporation of oxygen-containing functional groups in GO resulted in the transition of binding form from Me–N 4 to Me–N 2 . The CO 2 adsorption capacity of GO-supported core shell ZIF-8@ZIF-67 gradually decreased from 2.15 to 1.30 mmol/g as GO weight percentage increased from 0.5 to 4 wt%, which was consistent with the decrease in BET surface area from 1378 to 585 m 2 /g and increase in peak pore diameter from 7.7 to 8.2 Å. Density functional theory (DFT) calculations demonstrated that unsaturated Zn–N 2 –O active sites in GO-supported core shell ZIF-8@ZIF-67 nanocomposites resulted in a decreased O C⋯Zn interaction distance from 4.65 (Zn–N 4 sites in core shell ZIF-8@ZIF-67) to 3.46 Å and an increased binding energy from 34.02 to 38.87 kJ/mol. Therefore, 0.5 wt% GO-supported core shell ZIF-8@ZIF-67 nanocomposites exhibited the highest CO 2 adsorption capacity of 2.15 mmol/g (at 273 K and 1 bar), 1.34 times higher than that of core-shell ZIF-8@ZIF-67.