Experimental Research Of Deep Eutectic Solvents For NH₃ Uptake

Ammonia?NH₃?is an environmentally harmful gas causing the oxidation of SO₂in the atmosphere to form a gaseous sulfate gel,which in turn forms haze.Inhalation of NH₃ by the human body can cause respiratory illnesses,which even can lead to death in severe cases.Ammonia is also an important raw material in chemical industry and can be used as a fluorine-free refrigerant and a carbon-free hydrogen storage material.The recovery of ammonia possesses economic and environmental significance.Washing or pickling by acidic water suffers from the disadvantages of equipment corrosion and high cost in the separation of pure ammonia.Therefore,it is desirable to develop renewable,inexpensive,and environmentally friendly absorbents for NH₃.As new generation of green solvent,deep eutectic solvent?DES?is composed of hydrogen bond donor?HBD?and hydrogen bond acceptor?HBA?.Compared with ionic liquids,DES has similar excellent properties and superiority in terms of production cost and synthesis technology.In present work,several kinds of of DESs were constructed and investigated as NH₃ absorbents from absorption performances as well as rules.The absorption mechanism was also analyzed using spectroscopy and quantum chemistry calculation.Choline chloride?ChCl?was paired with various HBDs of glycerol,ethylene glycol,and N-methylurea)to prepare ChCl-based DESs and used as NH₃ asorbents.The solubility data were determined by isochoric saturation method at 303.15³33.15K and 0⁵50.0 kPa.By correlating the experimental data,Henry's constant H_m and the dissolved standard Gibbs free energy?_disG,the dissolved enthalpy?_disH and the dissolved entropy?_disS were obtained.Results showed that choline chloride-glycerol demonstrated the highest absorption capacity for NH₃,and the NH₃ absorption was a spontaneous and exothermic process.A series of azole-glycerol DESs were prepared and used to uptake NH₃ due to the fact that amino group-containing azoles and the hydroxyl group-containing glycerin easily form hydrogen bond with NH₃.The influences of azole structure,absorption temperature and NH₃ partial pressure on the absorption performance of DES for NH₃ were further investigated.It was shown that the Tri-Gly systems demonstrated efficient and reversible absorption performance for NH₃,with the satisfactory absorption capacity of 0.111 g NH₃/g DES at atmospheric pressure and313.15 K for Tri-Gly?1:2?.and high NH₃/CO₂ selectivity up to 216.By introducing electron withdrawing groups into the ring of azoles or hydroxyl group into HBDs,the obtained DESs presented higher absorption capacity for NH₃.The density linearly decreased along with the absorption of NH₃ for Tri-Gly,while the viscosity first increased and then decreased.In order to further enhance the NH₃ absorption capacity,NH₄SCN with weak acidity was selected as HBA and paired with glycerol?Gly?,ethylene glycol?EG?,urea?Ur?,acetamide?AT?and caprolactam?CL?to construct protic functionalized DESs as new NH₃ absorbents.The results showed that NH₄SCN-based DESs possessed good thermal stability and low viscosity.As far as NH₄SCN-Gly?2:3?DES,the viscosity was only 71.18 mPa·s at 313.15 K,the absorption capacity was 0.223 g NH₃/g DES for atmospheric NH₃ at 303 K,which is higher than all ionic liquids and DESs reported in the literatures.The performance was still stable after 25absorption/desorption cycles.The selectivity of NH₃/CO₂ in NH₄SCN-Gly?2:3?was as high as 609.¹H-NMR spectroscopy and quantum calculations indicated that the ammonium cation and hydroxyl group played major NH₃ absorption sites in the NH₄SCN-based DESs.