Molecular thermodynamic and dynamic insights into gas dehydration with imidazolium–based ionic liquids

Highlights:

• Structural effects on ILs suitability as gas dehydration agents were revealed;

• Dehydration performances of various ILs were evaluated by free energy calculations;

• The interaction mechanism of IL-H₂O at the molecular level was deeply unraveled;

• The relationship of dehydration performance and interactions was explored.

Abstract

Structural effects on various imidazolium–based ionic liquids (ILs) (i.e., [C][A], [C] = [EMIM]⁺, [BMIM]⁺, and [OMIM]⁺; [A] = [BF₄]^–, [PF₆]^–, and [Tf₂N]^–) suitability as drying agents for gas dehydration processes are explored from thermodynamic and dynamic insights by quantum chemistry (QC) calculations and molecular dynamics (MD) simulations. It is found that [EMIM][BF₄] is regarded as the most promising drying agent because it exhibits the lowest Henry’s law constant and highest diffusion coefficient of H₂O among all ILs. The microscopic mechanism at molecular level is revealed based on QC calculations and MD simulations, and the results demonstrate that the IL (i.e., [EMIM][BF₄]) simultaneously with the smallest cation and anion size corresponds to both the strongest hydrogen bond (HB) interaction of H₂O–anion and the strongest HB together with van der Waals interactions of H₂O–cation. This work provides a valuable guidance from viewpoint of thermodynamics and dynamics for developing and screening novel ILs for gas dehydration.

Introduction

Water contained in gases have always been a persistent problem in industry, since it can readily form the hydrate leading to plug the pipelines or even destroy the machines [1], [2], [3]. Moreover, the presence of H₂O in industrial gas can damage the catalysts (e.g., zeolites and molecular sieves) and affects subsequent catalytic reactions associated with gases. Thus, the gas dehydration technology is of great significance to avoid the hydrate formation and corrosion in pipelines especially in presence of acid gases, and ensure the safe and efficient operation of processing equipment. Glycols (e.g., triethylene glycol and ethylene glycol) are traditionally used as liquid desiccants, but there are two main disadvantages associated with them: 1) the high energy consumption during the regeneration process (due to the high regeneration temperatures), and 2) glycol loss and contamination in dried gas production [4].

As environment–friendly solvents, ionic liquids (ILs), possessing ultralow vapor pressure, good thermal and chemical stability, and tunable and designable properties [5], [6], have an increasing application in the field of chemical, environmental and energy engineering processes[7], [8], [9], [10], [11]. In chemical or environmental separation processes like extraction [12], [13], [14], [15], [16], distillation [15], [17], absorption [18], [19], [20], [21], [22], [23], ILs show more excellent behaviors such as efficient separation performance, no solvent loss, no product contamination, simplified flowsheet, and low energy consumption for solvent regeneration than traditional solvents [24], [25]. In the field of gas dehydration, the dehydration processes of CO₂, syngas, natural gas, air and chlorine gas with ILs as absorbents have been studied in previous works [22], [26], [27], [28], [29], [30], [31]. In these studies, the H₂O content in various gases can be reduced to the ppm level to accomplish both energy saving and emission reduction.

However, the high cost of ILs is a critical problem to limit the further industrial application. One of the most effective ways to overcome this problem is developing and synthetizing the high efficient ILs to save the amount of ILs used in gas dehydration processes. Thus, identifying and recognizing the mechanism of gas dehydration with ILs is an important prerequisite for developing more novel and efficient ILs. In the previous studies [26], [28], [29], [30], it was found that the reason why ILs can be used as absorbents to dry the industrial gases (e.g., CO₂, CO, H₂, CH₄, and Cl₂) effectively and efficiently is attributed to the significant distinctness between the solubility of H₂O and other gases in ILs, leading to the large selectivity. However, this insight is still not enough to provide valuable guidance for designing new IL molecules. It is required to understand the nature of gas dehydration with ILs at the microscopic molecular level. Although it is known that the hydrogen bond (HB) between the anion in IL and H₂O plays an important role in gas dehydration, the effects of different anions and cations on the HB strength, the microscopic structural distribution in different IL + H₂O systems, and the relationship between microscopic structural distribution and separation performance are still unclear so far. In this regard, this work aims to identifying the structural effect of ILs on gas dehydration performance based on the Henry’s law constants of H₂O ranging from thermodynamics to molecular insights.

This work is dedicated to solving the following important scientific issues: (i) investigating the relationship between IL structures and Henry’s law constants based on the solvation free energy calculation; (ii) identifying the interaction strength and type between anions (i.e., [BF₄]^–, [PF₆]^–, [Tf₂N]^–) and H₂O as well as between cations ([EMIM]⁺, [BMIM]⁺, [OMIM]⁺) and H₂O at the molecular cluster level by quantum chemistry (QC) calculation; and (iii) exploring the microstructural distribution of anions and cations with H₂O, and the effect of different types of anions and cations on the structural distribution and diffusion behavior in IL + H₂O systems at the bulk molecule level by molecular dynamic (MD) simulation. This study serves as a deep understanding on the relationship between IL structure and thermodynamic interactions together with dynamic diffusions of H₂O in ILs at the molecular level, and further provides a theoretical guidance for developing and screening novel ILs for efficient gas dehydration. Particularly, [EMIM][PF₆] is not investigated in this work because it is in solid state at the room temperature (25 °C) with a melting of 333 K at 1 bar [32], resulting in poor industrial potential of gas drying.