Incineration of wastes and combustion of chlorine-rich coal are the major sources of local and global environmental pollution related to hydrogen chloride (HCl) emission. At the same time, HCl is a common tail gas in chemical industry such as PVC and Freon production. HCl is regarded as the most significant substance to destroy the ozone layer, moreover it is harmful, corrosive and can contribute to the production of chlorinated unburned hydrocarbons and dioxins. In order to capture HCl for reducing the environmental risk and utilizing HCl as chloride resource, the investigation of efficient methods for HCl removal and recovery is critically important in recent decades.
Although there are some commercial processes for flue gas dechlorination, such as hydrated lime, ion-exchange resin, condensation, alkali absorption, water absorption etc., the disadvantages of these processes are obvious, such as low efficiency, absorbent waste, secondary pollution and so on. Therefore, a novel solvent that could facilitate the separation of HCl from flue gas without concurrent loss of the absorbent into the gas stream is highly required. In this regard, ionic liquids (ILs) show great potential as an alternative for such applications. In recent years, significant progress has been made in the application of ILs as alternative solvents due to their unique properties such as negligible vapor pressure, a broad range of liquid temperatures, excellent thermal and chemical stabilities, tunable physicochemical characteristics and selective absorption of certain organic and inorganic materials, such as CO2, SO2 , NH3 and so on.
In order to develop a novel process for HCl absorption, it is necessary to have the knowledge of HCl absorption in ILs. In this work, 1-octyl-3-methylimidazolium chloride ([Omim]Cl) was synthesized, and the HCl absorption was examined at 273 K, 293 K, 313 K, 333 K, and 353 K under atmospheric pressure. The results are presented in Figure 1. It was found that the HCl absorption properties of these ILs at atmospheric pressure are quite high. For example, the mole fraction of HCl in water is only 0.2886 at 273 K, while in [Omim] Cl is 0.7697. And the saturated absorption of HCl decreases when the temperature increases. The recycle experiments were repeated for four times at 293 K and atmospheric pressure. It is found that the decreasing of the absorption ability was very little. The recovery rates in four times were 99.5 %, 99.1 %, 98.3 %, and 97.7 %, which indicate that ILs can be recycled and used repeatedly. 1H NMR spectra of HCl-absorbed [Omim] Cl were compared with those of HCl-free samples. It is shown that in the 1H NMR spectra of HCl-absorbed samples, there is a new absorption peak at about 13.32 ppm (specific absorption peak of ClHCl-). The results are presented in Figure 2. As a result, the high absorption performance of HCl in [Omim] Cl could be ascribed to three factors: the formation of ClHCl-; the hydrogen bonds between C-2 active hydrogen on the imidazol ring and HCl; and the high asymmetry between anions and cations.
Comparing with the conventional absorbents, the [Omim] Cl have several powerful advantages: higher ability of HCl absorption; recyclable and reusable; the absorbed HCl can be recovered and utilized. Therefore, it shows great potential as alternative solvents for HCl absorption in the chemical industries, and more effective ionic liquids for absorbing HCl are expected to be researched in future.
The author gratefully acknowledges the support of Ministry of Science and Technology of China (973 program 2009CB219901), the Natural Science Foundation (20803075).
See more of this Group/Topical: Topical G: Innovations of Green Process Engineering for Sustainable Energy and Environment