Experimental investigation of carbon dioxide adsorption using functionalized MWCNTs with 1,6-diaminohexane

Abstract

In this study, multi-walled carbon nanotubes (MWCNTs) were functionalized with 1,6-Diaminohexane (DAH) in a two-step process to investigate the CO2 adsorption capacity. For this purpose, MWCNTs were first carboxylated with a mixture of sulfuric acid and nitric acid (5 M HNO3/5 M H2SO4 with a volume ratio of 1:3). Then oxidized MWCNTs were modified with 1,6-Diaminohexane solution. Reflux temperature, reflux time, and amine concentration were identified as three effective parameters in the amine-functionalization process. The response surface methodology (RSM) was used to optimize these synthesis parameters to achieve maximum CO2 adsorption capacity. The CO2 adsorption experiments were carried out at the pressure range of 1.8–17 bar and temperatures of 30, 50, and 70 °C. The results showed that the functionalization of carbon nanotubes by DAH significantly increases the CO2 adsorption capacity via the formation of carbamate ions. The maximum adsorption capacity of MWCNTs-DAH was 253.99 mg/g at 30 °C and 17 bar. The Langmuir and Freundlich isotherms were used to model the experimental data, and the results illustrated the high consistency of the experimental data with the Freundlich model. The uptake of CO2 remained almost constant during the four regeneration cycles, indicating the high stability of the optimal adsorbent. The MWCNTs-DAH can be introduced as a good adsorbent with high CO2 adsorption capacity.