“Giant” Nitrogen Uptake in Ionic Liquids Confined in Carbon Pores

Ionic liquids are well known for their high gas absorption capacity. It is shown that this is not a solvent constant, but can be enhanced by another factor of 10 by pore confinement, here of the ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate (EmimOAc) in the pores of carbon materials. A matrix of four different carbon compounds with micro- and mesopores as well as with and without nitrogen doping is utilized to investigate the influence of the carbons structure on the nitrogen uptake in the pore-confined EmimOAc. In general, the absorption is most improved for IL in micropores and in nitrogen-doped carbon. This effect is so large that it is already seen in TGA and DSC experiments. Due to the low vapor pressure of the IL, standard volumetric sorption experiments can be used to quantify details of this effect. It is reasoned that it is the change of the molecular arrangement of the ions in the restricted space of the pores that creates additional free volume to host molecular nitrogen.


Experimental Section
Synthesis of STC and NDSTC materials and loading of EmimOAc. 5 g Sucrose and 5 g ZnCl 2 salt template were dissolved in 30 mL water. Then, 0.53 g concentrated sulfuric acid was added to the solution and the mixture was then treated for 6 h at 100°C and for another 6h at 160°C in a petri dish under air atmosphere for polycondensation of the sucrose. The polymerized carbohydrate was carbonized at 900°C for 2 h under N 2 flow in a horizontal furnace (heating rate 150°C h -1 ). In order to remove the remaining salt template, the carbonized product was stirred in 1 L of 1 M aqueous HCl solution for 3 days. The solution has been exchanged every day. In the final step, the carbons were washed several times with water by vacuum filtration.
The dried carbon powder is labeled as STC-1. Following the same synthesis route, STC-8 was also synthesized by changing just the amount of used salt template (the mass of ZnCl 2 was 40g for STC-8).
For nitrogen-doping of STCs, cyanamide (CH 2 N 2 ) was impregnated into the carbons with a carbon:cyanamide weight ratio of 1:2. An aqueous cyanamide solution was prepared with respect to total pore volumes (TPV) of the STCs. This aqueous solution was added to the carbon powders by incipient wetness impregnation in mortar. The loaded powder was dried in an oven at 60°C overnight. Cross-condensation of the cyanamide was carried out at 900°C for 2 h under N 2 flow in a horizontal furnace (heating rate 120°C h -1 ).
Structural Characterization and Adsorption Measurements. N 2 sorption measurements were carried out with a Quadrasorb apparatus (Quantachrome Instruments) at 77 K (under liquid nitrogen), 298 K (ambient temperature) and 273 K (ice water bath). Before the all measurements, 40-60 mg of sample were outgassed for 20 h under vacuum at 150°C (non ILloaded samples) and room temperature (IL-loaded samples). In order to calculate the specific surface area of the samples (SSA BET ) from the physisorption isotherms at 77 K, the multipoint BET (Brunauer-Emmett-Teller) model was applied (p/p 0 = 0.05-0.2). The total pore volumes were determined at p/p 0 = 0.95. Moreover, by using the QSDFT method (quenched solid density functional theory, adsorption branch kernel), pore size distributions (PSD) were determined for N 2 adsorbed on carbons. Gas uptakes of IL-loaded samples are normalized to the nominal content of ionic liquid or carbon after mixing.
Transmission electron microscopy studies were carried out using a TEM-EM-912 Zeiss Omega with a LaB 6 cathode operated at 120 kV. Scanning transmission electron microscopy and electron energy spectroscopy data were collected on a double Cs corrected JEOL JEM-ARM200F (S)TEM operated at 80 kV and equipped with a cold-field emission gun and a Gatan Quantum GIF spectroscopy system.
The scanning electron microscopy was carried out with a Zeiss Gemini LEO 1550 microscope equipped with a FEG gun. The images were collected at 3kV using In-Lens and ETD secondary electron detectors. EDX data were collected at 10 kV using an Oxford Instruments MAX 80mm 2 SDD detector.
The carbon and nitrogen content of the samples was investigated by elemental analysis with a vario MICRO cube CHNOS Elemental Analyzer (Elementar Analysensysteme GmbH, Langenselbold). Oxygen contents were not specifically analyzed. Besides carbon and nitrogen, traces of sulfur were also detected in the STC and NDSTC materials (between 2 and 6.3 wt.%).