OPEN An environmentally friendly deep eutectic solvent for carbon dioxide capture

OPEN An environmentally friendly deep eutectic solvent for carbon dioxide capture

A leading cause of global warming is the increase of carbon dioxide emissions due to anthropogenic activities which prompts an urgent need for substantial reduction. Recently, carbon dioxide absorption in deep eutectic solvents has attracted scientific attention, because of their adaptability compared to traditional ionic liquids and aqueous amine solutions. This study employs the heating method to synthesize deep eutectic solvents using tetrapropylammonium bromide and formic acid with molar ratios of tetrapropylammonium bromide-formic acid one to one and tetrapropylammonium bromide-formic acid one to two. Absorption experiments by static method quantified carbon dioxide solubility in the deep eutectic solvents under varied pressures and temperatures. Tetrapropylammonium bromide-formic acid one to two at twenty-five point zero degrees Celsius was the most efficient with the carbon dioxide solubility of zero point two one eight. Thermodynamic modeling was performed by employing the nonrandom two liquids activity coefficient model and the Peng-Robinson equation of state for the liquid and gas phases, respectively. The Henry's law constant was determined from experimental data. Carbon dioxide physical absorption was confirmed via nuclear magnetic resonance and Fourier-transform infrared analyses. Tetrapropylammonium bromide-formic acid one to two, as the superior deep eutectic solvent, exhibited regeneration efficiency of ninety-nine percent after five absorption/desorption cycles.

Consumption of fossil fuels as the primary energy source significantly affects air pollution and the adverse consequences of climate change. Carbon capture and storage has emerged as a viable approach to mitigate these detrimental effects, including but not limited to the greenhouse effect, global warming, acidification of the oceans, and the spread of diseases and pests. The established methods for carbon capture include adsorption, absorption, membrane separation, and chemical capture. The absorption of carbon dioxide is a promising method due to its effectiveness from an economic and operational point of view. Absorption has better long-term performance and a large processing capacity on the industrial scale. Aqueous amine solutions are the most frequently used reversible solvents for carbon dioxide capture in industrial processes. Monoethanolamine aqueous solution is extensively utilized in contemporary industries for carbon dioxide absorption because of its low cost, notable reactivity, high carbon dioxide capture capacity and significant absorption rate. However, these solvents have some inherent significant drawbacks, such as amine loss due to volatility, environmental issues, high corrosion effects, high energy consumption for the desorption process and degradations at high temperatures. Therefore, it is crucial to find environmentally friendly alternatives to aqueous amine-based solvents.

Ionic liquids have been the subject of considerable research on carbon dioxide absorption. This is primarily due to their tunable chemical structure, low vapor pressure, nonflammability, high solvation capacity, thermal stability and potential for utilization at ambient temperature. Ionic liquids have been found to have applications in various fields, such as organic synthesis, catalysis, separation during extraction and electrochemistry. Numerous subsequent efforts have been devoted to investigating the solubility of carbon dioxide in ionic liquids. Blanchard et al. conducted the first investigation of carbon dioxide absorption by one-butyl-three-methylimidazolium hexafluorophosphate using a high-pressure cell. One-butyl-three-methylimidazolium hexafluorophosphate absorbed a mole fraction of zero point six carbon dioxide at a temperature of twenty-five degrees Celsius and a pressure of eight megapascals. Bates et al. suggested a new kind of amino group-functionalized ionic liquid with a zero point five molar uptake of carbon dioxide per mole of ionic liquid at a pressure of one atmosphere and temperature of two hundred ninety-five kelvin. Huang et al. documented the presence of a chemical reaction between carbon dioxide and a basic ionic liquid. This reaction usually leads to the absorption of carbon dioxide with capacities ranging from zero point five to one mol of carbon dioxide per mole of ionic liquid at a temperature of twenty-five degrees Celsius and various partial pressures of carbon dioxide. However, ionic liquids encountered several limitations that have precluded them from emerging as an optimal candidate for green solvents, including complex and expensive synthesis and the requirement for high purity because the presence of impurities can significantly impair the physicochemical properties of ionic liquids.

More research is also needed to determine if these solvents are environmentally friendly. To overcome these drawbacks, while maintaining the beneficial characteristics of ionic liquids, a novel class of solvents called deep eutectic solvents has been developed.

DESs are formed by combining a hydrogen bond donor with a hydrogen bond acceptor. Since DESs can easily be synthesized, they are practical and economical alternatives to ILs. The first DES was synthesized by Abbott et al. using choline chloride and urea with the molar ratio of one to two. Both of these components are biodegradable and non-toxic. Some of recent investigations have focused on the corrosion behavior of DES based on choline chloride, representing ammonium quaternary salts. These studies have recognized the high stability of the ammonium salt, which can maintain its stability without decomposition or deactivation under severe electrochemical conditions and in the presence of negative electrical potentials. Consequently, a considerable body of literature exists on the application of choline chloride-based DESs as corrosion inhibitors in aqueous environments. Ammonium quaternary salts are the most commonly employed hydrogen bond acceptors due to their accessibility, affordability and low toxicity. Another advantage of DESs is that hydrogen bond donor and hydrogen bond acceptor concentrations may be modified to customize their properties for a specific purpose. Also, DESs have emerged as a viable substitute for ILs for carbon dioxide absorption. Li et al. effectively synthesized several choline-based DESs and utilized them for carbon dioxide absorption for the first time. They demonstrated that at a pressure of twelve point five MPa and a temperature of forty degrees Celsius, choline chloride-urea one to two absorbed carbon dioxide with a mole fraction of zero point three zero nine. Leron et al. investigated the impact of varying temperatures and pressures on the carbon dioxide solubility in choline chloride-urea one to two at temperatures ranging from three hundred three point one five to three hundred forty-three point one five Kelvin and pressures of up to six point zero MPa. They demonstrated that carbon dioxide solubility in choline chloride-urea one to two DES increased with increasing pressure and decreased with increasing temperature. Additionally, they examined how the molar ratio of salt and hydrogen bond donor affects the solubility of carbon dioxide. The solubility of carbon dioxide in various ammonium and phosphonium DESs was examined by Sarmad et al. at temperatures of two hundred ninety-eight point one five Kelvin and pressures of up to two MPa. They reported that the carbon dioxide solubility of fifteen synthesized DESs is higher than that of conventional ILs. It should be mentioned that the renewal of the absorbent in practical applications is important in any absorption process including carbon dioxide capture. Zhang et al. demonstrated that after six absorption-desorption cycles, the regeneration efficiency of [TETA]Cl-DG one to two and [TETA]Cl-EG one to three drops from one hundred to ninety-seven point five percent. Yan et al. examined the solubility of carbon dioxide in various superbase IL-based DESs. The one, eight-diazabicyclo-[five, four, zero] undec-seven-ene imidazole/ethylene glycol ([HDBU][Im]/EG) with a mass ratio of seven to three, exhibited the maximum carbon dioxide absorption capacity of zero point one four one grams carbon dioxide per gram DES at one hundred kilopascals and forty degrees Celsius. Additionally, the carbon dioxide absorption capacity of DES remained stable after five absorption and desorption cycles. Recently, several articles have investigated the effect of viscosity on the solubility of carbon dioxide in DESs. The solvent's viscosity is an important physical property that can substantially impact the mass transfer. An enhancement in the solvent's capacity to capture carbon dioxide can result from a reduction in viscosity. Viscosity also impacts the energy needed to manufacture and move materials. Temperature, kind of hydrogen bond acceptor and hydrogen bond donor, and their respective molar ratios all affect the viscosities of DESs. The viscosity of DES increases during absorption, resulting in a decrease in the absorption rate. Some studies have documented the viscosities of amine-based DESs in their pure form and the viscosities of the DESs after carbon dioxide absorption. For a solvent to be considered suitable in the gas absorption industry, multiple factors beyond absorption capacity must be evaluated. For solvents with relatively low toxicity, considerations include the potential for long-term use, the absence of solvent loss, the energy required for solvent recovery, and the regeneration efficiency after multiple absorption and desorption cycles. Aqueous amine solutions which chemically absorb carbon dioxide, present challenges such as low biodegradability, volatility, and high energy requirements for regeneration, which are costly and environmentally detrimental.

Our study is focuses on experimentally investigating the utilization of tetrapropylammonium bromide and a naturally occurring carboxylic acid to form a DES for carbon dioxide capture. To accomplish this goal, two DESs were synthesized employing TPAB as a hydrogen bond acceptor and formic acid as a hydrogen bond donor in molar ratios of one to one and one to two. The presence of hydrogen bond between TPAB and formic acid, and physical absorption of carbon dioxide were confirmed through FT-IR and NMR spectra. Experiments were conducted at the temperatures of twenty-five point zero, thirty-five point zero and forty-five point zero degrees Celsius and pressures of approximately up to thirty-five thousand bar. The impact of variations in pressure, temperature, and viscosity on carbon dioxide absorption was investigated. Using the carbon dioxide solubility data, Henry's law constant and the enthalpy of dissolution were obtained. To model the vapor-liquid equilibrium of the carbon dioxide-DES system, the Peng-Robinson equation of state and the nonrandom two liquid activity coefficient model were employed. Furthermore, five cycles of regeneration experiments were performed on the DES with better performance under vacuum and at sixty-five point zero degrees Celsius condition.

Experimental procedure Materials

The substances used in this study and their corresponding molecular structures, sources, and purities are presented in Table one. They were used as received.