Development Of Coconut Shell Activated Carbon Tethered Ionic Liquids For Biogas Upgrading

Ionic liquids (ILs) were successfully in-situ prepared on coconut shell activated carbon (AC) granules for biogas upgrading in a packed bed. The adsorbents were characterized by N2adsorption-desorption, Fourier Transform Infrared Spectrometer (FT-IR), X-ray photoelectron spectroscopy (XPS), and thermogravimetry (TG) analysis. The decomposition temperature of AC-[N3444][Br] was330℃, and the decomposition temperature of both AC-[BPIM][Br] and AC-[EPIM-NH2][Br] were350℃. Comparing with the pure ionic liquids, the immobilized ionic liquids were more stable. The BET surface area of the raw AC is-551m2/g, and the average pore size and pore volume were1.64nm and0.363cm3/g, respectively. After imobilizing ionic liquids, the BET surface areas, average pore sizes and pore volumes were slightly decreased, and also indicated that AC successfully immobilized ionic lizuids. The contents of N element were determined by elemental detector to calculate the mass of ionic liquids immobilized in the carrier.Anions play a key role in biogas upgrading, the CO2adsorption capacity increased with the order [Br-]<[BF4-]<[PF6-]<[Tf2N-].[Tf2N"] displayed better than other anions, and [Tf2N"] modified ionic liquids were the best choice for biogas upgrading. The saturated CO2capture capacities of AC-[N3444][Tf2N], AC-[BPIM][Tf2N] and AC-[EPIM-NH2][Tf2N] were2.40,2.58and2.95mmol/g at20℃and0.5MPa, respectively. Without considering the adsorption capacity of the AC carrier, per molar ILs on the AC can adsorb0.63and0.91mol of CO2for AC-[N3444][Tf2N] and AC-[BPIM][Tf2N], respectively. While for AC-[EPIM-NH2][Tf2N], there exhibited both physical and chemical reaction betwen the ionic liquid and CO2, and the CO2adsorption capacity was increased to1.5mmol/g. After immobilizing ionic liquids, the selectivities of CO2in the biogas were improved apparently. The adsorption heat was23-28KJ/mol for AC-[N3444] and AC-[BPIM] based adsorbents for phisycal adsorption, and it was increased to65-71KJ/mol for AC-[EPIM-NH2] based adsorbents for the chemical interaction between primary amino and CO2.Since a CO2concentration less than3%is required for biogas to be used in vehicle, an adsorption capacity at which3%of CO2is reached in the upgraded gas could be more important. The fow rate of biogas play a key role in this process. When the flow rate of the inlet gas was50mL/min, the adsorption capacity(3%CO2) was1.34,1.44and1.95mmol/g for AC-[N3444][Tf2N], AC-[BPIM][Tf2N] and AC-[EPIM-NH2][Tf2N], respectively, at20℃and0.2MPa. The adsorption selectivities of CO2over biogas were also increased. The regenerability of the adsorbents were tested in a packed bed. The immobilized ILs exhibited a very stable cyclability for biogas upgrading after ten adsorption-desorption cycles.