Deep Eutectic Solvents Feasibility in Oil and Gas Processing Field for Contaminated CO2 Control

Carbon dioxide capture and storage (CCS) is highly expected to be mitigating the discharges of carbon dioxide in a surrounding environment. Solvents are an integral part of CCS. So far, several solvents have been explored the interest of meeting the requirements such as accessibility, non-harmfulness, biocompatibility, recyclability, and inexpensiveness. However, most solvents face failure in fulfilling the requirements due to many factors, so, this review paper gives a brief discussion about another category of solvent, an analogue of ionic liquids (ILs) named deep eutectic solvent (DES). Extensive research has been done on DES in recent years because of their various attractive advantages, i.e., non-poisonousness, biodegradability, cheap cost and easy preparation, that make them as a promising green solvent for many industrial procedure and application, for instance, polymer synthesis, biodiesel treatment, green chemistry, electrochemistry etc. T h e r e f o r e , this manuscript mainly focusses on CO2 capturing through DES in oil and gas field. In addition, the preparation and chemical structures of this novel solvent (DES) is also discussed. Moreover, a detailed study based on experimental solubility of CO2 in DESs is also reported in this article.


Introduction
As the requirements of modern civilisation have been increasing rapidly, the industries continuously produce to meet the demands. As a result, a large amount of CO2 is released from industries, for example, the production of cement and lime, sugar from sugarcane, electrochemistry, analytical chemistry, physical chemistry and CO2 capturing technology [22][23][24][25][26]. There are enormous articles have been published on DESs contribution in the given fields since 2003. The goal of this review is to exhibit the possible and beneficial usage of DESs for contaminated CO2 capturing in petroleum and gas industry and also the investigations of DESs regarding their physical properties, chemical properties, applications, chemical structure, preparation and lastly experimental solubilities CO2 were accounted and compared in this work, which showed DESs feasibility over conventional solvents. It is hoped that in future, DES will show better performance in oil and gas fields.

Deep eutectic solvents
The expanding requests for environmentally friendly procedures inside the system of green chemistry and acknowledgment of phenomenal and favourable features of DESs have led, in the previous twenty years, to a developing enthusiasm for utilisation of these blends as substitutes to conventional organic solvents and their procedure ionic liquids (IL) in various fields. Based upon green chemistry, deep eutectic solvent may be assumed as an adaptable substitute to ILs because of their excellent properties, for instance, non-volatility, reliability, cheap cost and high biocompatibility [20][21][22][23][24][25][26][27]. DESs are usually prepared by mixing of two or three cheap components, that may have the ability to make hydrogen interactions to form a eutectic mixture, because of interaction, the melting point lower than that of every individual component. DES is supposed to be an aqueous state at a temperature lower than 100°C [27]. The concept of deep eutectic solvents was first elucidated by Abbot et al. [28]. Based on mixing of choline chloride (HBA) and urea (HBD) with 1:2 molar ratio and the melting point was 12°C [28] ILs vary from DESs in three major perspectives, firstly, ionic liquids are costly, and some of them exhibit high poisonousness, just as having poor biodegradability, feasibility. On the other hand, DES are inexpensive, biocompatible and environmentally friendly. Another embodiment is the preparations procedures of ILs. Also, DESs are varied somewhat, the preparations of DESs are achieved by blending of two or more constituent, which produce no waste and not needed to be purified, while ILs are tough and costly as well. Lastly, ILs are comprised totally of ions, which interact by ionic bonds; however, DESs contain both ionic and neutral species [29].
Having set themselves up over the most recent few decades, as helpful and regarded as a producing biodiesel from acidic crude palm oil [32]. Also, the proficiency of deep eutectic solvents in CO2 Capture technology has been explored. Not long ago, García et al. revealed a density functional theory simulations report on moderation of CO2 through modern DESs at the molecular level [21]. Moreover, the vast number of experiments and hypothesis were done in the applications of DESs in CO2 capturing technology [33][34]. The physiochemical characteristics of DESs were investigated by various researchers all around the world, because of, their favourable applications are good industrially [20][21][22][23][24][25][26][27] Sinensis. ChCl and urea-based DESs, it exhibited that DESs could be less damaging as compared to each component [37]. DESs are getting a viable option for both ionic liquid and conventional solvents. It depends upon many factors, but the most significant is the physicochemical properties.

Preparation and chemical structure of DESs
In general, the chemical structure of solvents is very essential in deciding their interaction to various solutes. However, when it comes to DESs, minimal investigations have been on the microscopic agents which may influence the structure and characteristics of the DES.
In other words, the connection and intermolecular interaction of hydrogen bond donor and hydrogen bond accepter which have a critical effect on the properties have not been explored [21]. DESs, by applying DFT method to explore the stable key structure of molecules in DESs.
On the hand, the right design of DES and to reasonably grow their application, the complete comprehension of the physical properties is important.
The preparation of DES is simple by the mixing of hydrogen bond donor (HBD) and hydrogen bond acceptor (HBD) at a specific temperature in of two ways, (1) when the HBD and HBA are mixed, the lower melting point constituent begins to melt and then the remaining compound which has a high melting point is put into the liquid and the mixtures are melted collectively, and (2) when both constituents are mixed and melt together, since the first work of Abbot et al. 2003 [28]. For example, the solid beginning material of ChCl and Urea were heated at 1:2 proportional to acquire a blend that was liquid at ambient temperature, numerous DES prepared, as studied in [40][41]. Figure 1 represents components in DES and how it interacts or make the bond with one another during the formation of DES molecule. Moreover, it is found that humidity can significantly impact on the properties of DES. Hence, it is profoundly imperative to keep HBAs, HBDs, and the prepared DES away from the vapour or steams. It is supposed to be noticed that the homogeneous mixture of liquid is the ending stage of the procedure of Deep eutectic solvent preparation. to improve oil recuperation or saline water or coal mine [45,46]. There are different processes through CO2 is captured, but the easiest way which is to be applied as a post-combustion sequester [47]. That incorporates the elimination of pernicious gases like CO2 from exhaust gases [20], that originates from the burning compartment of the generating station [48]. Various other technologies for post-combustion CO2 are used, for example, hydration formation, chemical absorption, membranes, metal frameworks Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 19 October 2020 doi:10.20944/preprints202010.0378.v1 [4][5]. The chemical absorption was considered as economical in post-combustion method [49]. The other techniques are reported to be less suitable in post-combustion [7].
Chemical absorption procedure for CO2 used was bottomed on reacting sorbent exothermically with CO2 in the existence of gas flow at ambient temperature [5]. Chemical absorption processes with liquid alkanol amine solution have been used for decades so that the dismissal CO2 from waste gases stream [50]. Though, applying alkanol amines showed drawbacks industrially, for example, formed salt, corrosiveness, high cost, non-biocompatible etc. [51]. Because of these disadvantageous, the researchers started to investigate novel solvents to overcome the existence issues with low cost.
Therefore, a new solvent called deep eutectic solvents extensively known as potential substitutes to ILs and conventional solvent [52]. Due its viable characteristics, for instance, accessibility, harmless, non-combustibility, cheap cost, recyclability and biocompatibility [33], DES has been deemed suitable to be applied in CCS technology [24].

Investigational dissolubility of CO2 in DESs
Even though there are various surveys which have been convened to find out the dissolving power of CO2 in DESs, the CO2 dissolubility in DESs has been reviewed no long ago [53]. (ChCl-MDEA. This study described the solubility of nominated DES at the various molar ratio, 1:6, 1:8 and 1:10; it is indicated that the solubility of CO2 increased as the molar ratio of amine in DES increase [54]. Furthermore, Kamgar  decreases as the water vapour increase, yet, water vapor can also effect on viscosity of the selected species [59]. On the other hand, carbon dioxide (CO2) separation is assumed significantly as an energy-saving and CO2 emission reduction.
In    Figure 2 is a good sign for the possibility of implementing DES for industrial practices involving contaminated CO2 control in near future. It is highly recommended for the researchers to focus more on DES and utilise the properties for greener applications.