Deep Eutectic Solvent Based Membranes for Gas Separation

Abstract

The aim of this research was to synthesize the novel supported liquid membrane using DESs. Based on various combinations of HBA and HBD, different novel DESs were synthesized and immobilized into PVDF membrane for the application in CO2 separation. The investigations were made on the betaine (Be), choline chloride (ChCl) and potassium carbonate (PC) (HBAs) in combination with malic acid (Ma), tartaric acid (Tart), oxalic acid (Oxa), glycerol (G), ethylene glycol (EG), and urea (U) (HBDs).

In the first study, two types of NADESs were synthesized by mixing betaine (hydrogen bond acceptor-HBA) with malic acid and tartaric acid (hydrogen bond donors-HBD) respectively. FTIR and Raman spectroscopy were studied to confirm the synthesis and purity of the NADESs. The thermal strength of the NADESs was investigated using thermogravimetric analysis. The gas permeation results of the fabricated NADES based supported liquid membranes (NADES-SLMs) showed that the permeability of CO2 increased from 25.55 Barrer to 29.33 Barrer on substitution of hydrogen bond donor from tartaric acid to malic acid. Similarly, the ideal CO2/CH4 selectivity varied from 51.1 to 56.4 as tartaric acid was replaced by malic acid as the HBD. The performance of NADES-SLMs was compared with the competing imidazolium based supported ionic liquid membranes and has shown high selectivity and comparable permeability on Robeson Upper bond plot that lies above the plot.

In second study, two types of DESs were synthesized by mixing and heating Potassium carbonate (PC) with glycerol (G) or ethylene glycol (EG) separately. The mechanism of interaction was inferred from the spectral analysis (FTIR) whereas thermal analysis (TGA) was performed to analyze the stability of the prepared membrane. Experiments were performed to analyze the permeability and selectivity of the membranes. The results showed that PC-Glycerol based SLM exhibited permeability of 34 Barrer and ideal selectivity of 59 while PC-Ethylene Glycol based SLM exhibited permeability of 20 Barrer and ideal selectivity of 34 under similar operating conditions. Systematic analysis was made for some of the important operating parameters affecting the separation performance such as temperature and feed composition. Comparison was made between the performance of PC-DES-SLMs and conventional imidazolium based SILMs. The results have shown PC-G- DES-SLMs exhibit high selectivity and comparable permeability and lie above the well-known Robeson’s upper bound plot while PC-EG lies very close to the Robeson plot.

In the third study, three types of DESs were prepared by mixing and heating of hydrogen bond acceptor (choline chloride) with either malic acid, tartaric acid or oxalic acid as hydrogen bond donor. The Fourier transform infrared spectroscopy confirmed the hydrogen bond interactions in the resulting DES. Thermal gravimetric analysis (TGA) was used to evaluate the thermal stability of the prepared membranes. The DES based supported liquid membranes were investigated systematically to determine the permeability and selectivity for the mixture of gases at both ambient and elevated temperatures. The results showed that Choline Chloride-Malic acid, Choline Chloride-Tartaric acid and Choline Chloride-Oxalic acid. DES-SLMs exhibited CO2 permeability of 30.32 Barrer, 34.00 Barrer and 37.30 Barrer, respectively while ideal selectivity of these membranes was found 51.39, 55.74 and 60.16 for CO2/CH4, respectively. The fabricated membranes were compared with some of the imidazolium supported ionic liquid membranes (SILMs) and exhibited highest selectivity and comparable permeability and lies above the Robeson upper bound plot.

In fourth study, three types of DESs were prepared by mixing and heating of betaine as hydrogen bond acceptor (HBA) in combination with either glycerol, ethylene glycol and urea as hydrogen bond donors (HBD) in 1:3 stoichiometric mole ratio. The Fourier transform infrared (FTIR) spectroscopy were performed to confirm the formation of DESs. The gas permeation results showed that permeability of CO2 increased from 31.23 to 35.67 Barrer on substitution of HBD from glycerol to urea. The selectivity of CO2/CH4 increased from 43.32 to 57.53 for the substitution of HBD from ethylene glycol to urea. These intriguing results are ascribed by the high interaction of CO2 with DES in which solubility of CO2 is directly influenced by the HBDs. The membranes were also tested under conditions relevant to industrial feed gas and the results were compared with imidazolium supported ionic liquid membranes (SILMs) on the Robeson upper bound plot. the result has shown high selectivity and comparable permeability and lies slightly above the Robeson plot. The comparative studies revealed that that ChCl-Oxa based membranes exhibited highest selectivity and permeability that shows ChCl-Acid based system has the highest potential for the separation of CO2 from CH4 as compared to other membrane system studied