Hierarchical clinoptilolite zeolite-template carbon for highly selective separation of CO2 and CH4

Abstract

BackgroundSeparating CO2 from natural gas before it is released into the atmosphere is a significant challenge in the present times due to its greenhouse effect. MethodsIn this study, a series of zeolite-template carbons (ZTCs) had been synthesized from acetylene as a carbon source using clinoptilolite as a template, through hydrofluoric acid etching and potassium hydroxide (KOH) activation processes. Significant findingsThe deposition of carbon on the zeolite-template was confirmed with different characterization techniques and their CO2 adsorption capacity and selectively toward methane (CH4) was evaluated at 25 °C, over a 1‒10 bar pressure range. The CO2/CH4 selectivity of the adsorbents was optimized with various synthesis parameters including chemical vapor deposition (CVD) reaction time, acetylene flow rate as well as activation temperature and KOH to carbon (KOH:C) mixing ratio. According to the findings, medium SBET (334.06 m2 g‒1), high Vtot (0.71 cm3 g‒1), high Vmeso (0.68 cm3 g‒1) and a large Dave (8.55 nm) of the ZTC-120–12.5–800–3 adsorbent (CVD reaction time of 120 min and acetylene flow rate of 12.5 cc min−1 as well as activation temperature of 800 °C and KOH:C ratio of 3:1) results in the highest CO2 capacity of 4.81 mmol g‒1 at 25 °C and 10 bar. Moreover, regarding the optimization goal of this study, the highest CO2/CH4 separation at 25 °C and 10 bar is the result of a long CVD reaction time (180 min), a high KOH:C ratio (4:1), a low acetylene flow rate (50 cc min‒1), and a low activation temperature (700 °C). In this regard, ZTC-180–50–700–4 showed the highest CO2/CH4 (15:85) selectivity of 13.35 with a high SBET (549.31 m2 g‒1), Vtot (0.718 cm3 g‒1), Vmicro/meso (15.6014), Vmeso (0.6211 cm3 g‒1) and a medium Dave (5.22 nm). The amine-functionalization of this sample increases the quadrupole moment between CO2 and nitrogen groups, and not only was the F-ZTC-180–50–700–4 sample's CO2 adsorption capacity increased by nearly 41% (4.11 mmol g‒1) compared to the ZTC-180–50–700–4 (2.91 mmol g‒1), but it also showed 58% higher CO2/CH4 selectivity (21.12). Based on the selectivity results of the functionalized ZTCs, their potential use in the separation of CO2 over CH4 at high pressures is demonstrated.