Achieving
effective and reversible capture of ammonia (NH3) is an
important task in the chemical industry, to avoid the air
pollution potentially induced by NH3 emission, and recycle
the NH3 resource for value-added productions. In this work,
deep eutectic solvents (DESs) comprising ethylamine hydrochloride
(EaCl) and glycerol (Gly) were designed as the media for NH3 absorption, by making use of the protic ionic nature of EaCl and
multiple hydroxyl groups in Gly, which enable strong hydrogen-bonding
interaction with NH3. The absorption amounts of NH3 in prepared EaCl+Gly mixtures at various temperatures and
pressures were experimentally measured. It is found that the NH3 capacities of EaCl+Gly mixtures are quite impressive, with
the highest value of 9.631 mol/kg at 298.2 K and 106.7 kPa, surpassing
those of most absorbents/adsorbents previously reported. The absorption
of NH3 in EaCl+Gly mixtures is also highly reversible,
with almost negligible decrease in NH3 capacities during
absorption–desorption cycles. The mechanism for interaction
between EaCl+Gly and NH3 was validated by spectroscopic
characterizations. Furthermore, the NH3 solubility data
were fitted by the Krichevsky–Kasarnovsky equation to obtain
the Henry’s constants of NH3 in EaCl+Gly mixtures,
and estimate the enthalpy changes, Gibbs free energy changes and entropy
changes, to evaluate the thermodynamic parameters of NH3 absorption process.
Solvents with appropriate acidity are required to simultaneously realize efficient and reversible absorption of NH 3 . To achieve this goal, a series of 1-ethyl-3-methylimidazolium ([emim]Cl)-based DESs with different weak acids as the hydrogen-bond donors (HBDs) were prepared and characterized. The acidities of these weak acids cover a wide range, with the pK a values in DMSO ranging from 8.2 to 18.6. The NH 3 absorption and desorption performance of DESs were investigated systematically and correlated with the acidities of HBDs. The different behavior of NH 3 absorption was analyzed by spectroscopic characterizations and thermodynamic calculations. By manufacturing the acidities of HBDs in DESs, solvents having not only high capacities but also good reversibility for NH 3 absorption were identified. The optimal DESs were then selected as representatives to investigate the effects of HBA to HBD molar ratios and temperatures on NH 3 absorption, as well as the selective absorption of NH 3 from other gases.
It is found that ethylamine hydrochloride (EaCl) and phenol (PhOH) can form a new type of deep eutectic solvent (DES) with quite low viscosity, and two active sites for chemical absorption of NH3.
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