Effects of nitro-functionalization on the gas adsorption properties of isoreticular metal-organic framework-eight (IRMOF-8)

•A novel IRMOF-8-NO2 was prepared by rapid solvothermal synthesis.•H2, CH4 and CO2 adsorb more efficiently on IRMOF-8-NO2 than on IRMOF-8.•IRMOF-8-NO2 has greater sticking factors for gas adsorption than IRMOF-8.•IRMOF-8-NO2 has higher enthalpy of gas adsorption than IRMOF-8.

A study was done to determine the effects of nitro-functionalization on the gas sorption properties of IRMOF-8. The IRMOF-8 and IRMOF-8-NO2 were prepared by a rapid solvothermal method. BET isotherm analysis showed that nitro-functionalization of IRMOF-8 caused the surface area to decrease from 1599 to 832 m2/g while the pore volume decreased from 0.639 to 0.322 cm3/g. However, these large decreases in surface area and pore volume did not result in a proportionate decrease in gas sorption capability. For example, the IRMOF-8 adsorbed 3.2 wt% hydrogen while IRMOF-8-NO2 adsorbed 2.2 wt% hydrogen. This is a relatively small reduction in hydrogen adsorption (∼30%) compared to the large decrease in surface areas (∼50%). The amounts of methane and carbon dioxide adsorbed by the IRMOF-8-NO2 were also disproportionately high. A sticking factor (defined as the percent gas adsorbed per m2 of surface area per gram) was determined for the adsorption of each gas on the two MOFs at various temperatures. In all cases, the sticking factors for gases adsorbed on the IRMOF-8-NO2 were greater than those for gas adsorbed on the IRMOF-8. Enthalpies were also measured for the adsorption of each gas on the two MOFs at various temperatures. It was found that, in all cases, the enthalpies for gas adsorption on IRMOF-8-NO2 were greater than those for IRMOF-8. Thus it appears that nitro-functionalization strengthens the bonding of the gases to the surface thus resulting in greater sticking efficiency and larger than expected gas adsorption.

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