Potassium Carbonate as a salt for Deep Eutectic Solvents

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Potassium Carbonate as a salt for Deep Eutectic Solvents

Introduction

The scientific and industrial communities have developed great interest on the room temperature ionic liquids (ILs). Their interests have been prompted by their ability to solve the economic and environmental challenges that is because of their broad chemical and physical properties. Their sustainable room temperature properties make it to be used in most industries. Its low volatility reduces environmental degradation (Earle and Seddon 1394). Its low density makes it to have high sensitivity to impurities and additives. The ionic liquids also have wide liquid temperature range, selectivity, immiscible with many organic solvents and high solubility. The high selectivity makes it hard for gases to escape making it advantage over other convectional absorbents.

            One of the major disadvantages in the use of ILs in the industries comes from the high synthesis costs. The green alternative for the traditional ILs involves the incorporation of biodegradable constituents that contain high thermal and chemical stabilities such as the deep eutectic solvents. The deep eutectic solvents (DESs) are non-biodegradable, have low vapor pressure, toxicity and non-flammable. DESs are synthesized under room temperature reducing cost. Conventionally they are produced from ammonium or phosponium based salts. The synthesis involves the combination of hydrogen bond donors such as carboxylic acids alcohol, nitrates, and acetates. There were also the metal halides based DESs.

            Potassium carbonate (K2CO3) also known as pearl ash was not among the ingredient for the synthesis of DESs. K2CO3 is highly soluble in water to form a basic solution. It is used in fertilizer industry to remove carbon dioxide during ammonia production. K2CO3 solution has low heat capacity making it an effective promoter for primary or secondary amines for capture and absorption of CO2 .Ethylene Glycol (C2H6O2) is a non-toxic, colorless, syrupy (viscous) liquid at room temperature. Ethylene Glycol has been used as a hydrogen bond donor (HDB) with chloride for the formation of DES.

            The study conducted involved the use of different ratio of potassium carbonate and ethylene glycol to make new DES system. The physical properties such as density, surface tension among others were investigated at different temperatures. The main purpose of conducting such studies is to develop economical green solvent that can be used for various purposes such as gas absorption, liquid-liquid extraction among others. For this to be achieved evaluation on the new class DESs physical properties have to be conducted.

Experimental Method

Potassium carbonate and ethylene glycol were used to synthesize the DES samples. However, different molar ratio was used. The DES samples synthesized were six, and are shown in Table 1, which also indicates the different ratio used. The samples were kept in glass vials after calculation of the amount of potassium carbonate and ethylene glycol. The samples were heated and stirred at 80 ̊ C but each took different time to form. The more the ethylene glycol used, the more time a sample takes to form. After the heating process, four DESs were formed. The first and second DES did not form to completion while DES 3-6 was placed in an oven for 24 hours. The purpose of this was to ensure they dried by removing all the water. Samples were taken out using a U Tube viscometer. A water path was also used to measure the viscosities of the different DESs.

Results and Discussion

The table below shows the ratios, the abbreviation, time taken to form and the observations that were made with the DESs.

TABLE I: Molar Ratios and Abbreviations of DESs Prepared

       
Ratio Abbreviation Appearance Time to form
       
 1:6 DES 1 dark brown with some solids 2 hours
 1:6.5 DES 2 brown with little solids 1 hour 45 min
 1:7 DES 3 brown with yellow 1 hour 26 min
 1:7.5 DES 4 brown with yellow 1 hour 15 min
 1:8 DES 5 brown with yellow 1 hour 10 min
 1:8.5 DES 6 brown with yellow 59 min

The amount of potassium carbonate used was constant; however, the ethylene ratio used was different with each sample DES. The synthesis of DES 1 and DES 2 were not successful as there were brown solids and the products were dark brown. This indicates an excess amount of salt corresponding to hydrogen bond donor. Therefore, this inhibits the formation of hydrogen bonds.

The required ratio to form DES is 1:7 which was used in DES 3. It took 1hour and 26 minutes to form at 80 ̊ C, and the sample was yellow. DES 4 took 1 hour and 15 minutes to form at 80 ̊ C and was yellow in color. DES 5 formed a yellow sample and took 1 hour and 10 minutes to form, at 80 ̊ C. DES 6 took 59 minutes to form, and was yellow in color. It can be observed that samples took a shorter time to form with less ethylene glycol used.

  • Viscosity

There are very limited experiments on ILs viscosity as a function of temperature (Kareem et al. 4634). Establishing a viscosity database, which is reliable, is vital. This is because of the design of equipment and calculations of fluid flow. The amount of energy required to process the fluids can be greatly reduced by establishing the effect of temperature on the viscosity of the fluids (Hayyan et al. 74). The viscosity of the DES decreases with increasing temperature. This is shown in Fig. 1

Fig.1. Viscosity of DESs as function of temperature

DES 6 had the highest viscosity at low temperatures. An increase in temperature resulted to a difference in viscosity of the four samples. At 70 ̊C, the samples had the same viscosity.

The results obtained from the experiment show a clear relationship between the salt to the HBD molar ratio. The viscosity values dropped as the molar amount of HBD was increased. More ethylene glycol used results to a DES that has a viscosity value almost equal to that of pure ethylene glycol.

Conclusion

Deep Eutectic Solvents have become increasingly important in the chemical and scientific industries. The above experiment was conducted to form a DES using different compounds, which are potassium carbonate and ethylene glycol. Throughout the experiment, it was observed that the more ethylene glycol used, the more the DES formed at a shorter time, and the lower the viscosity values. It is important to establish the effect of temperature on the viscosity of fluids. Fluids that have lower viscosity values heat up faster than those with higher values do. Therefore, it was established that synthesizing a DES with a low viscosity would be economical, as the energy costs in production would be greatly reduced. The purpose of this experiment was to produce a DES that is a green alternative and economical. The properties of the DES produced in the experiment make it very efficient for industrial use. This is because of its low volatility, low viscosity and hence, can perform some industrial functions such as gas absorption and liquid-liquid extraction. Therefore, this experiment was successful as it was established that using more ethylene glycol in the production of DES would produce a less viscous DES, which is ideal for industrial use.

Works Cited

Earle, Martyn J., and Kenneth R. Seddon. ‘Ionic Liquids. Green Solvents for the Future’. Pure and Applied Chemistry 72.7 (2000): n. pag. Web.

Hayyan, Adeeb et al. ‘Fruit Sugar-Based Deep Eutectic Solvents and Their Physical Properties’. Thermochimica Acta 541 (2012): 70-75. Web.

Kareem, Mukhtar A. et al. ‘Phosphonium-Based Ionic Liquids Analogues and Their Physical Properties’. Journal of Chemical & Engineering Data 55.11 (2010): 4632-4637. Web.

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