Insight into the experiment and extraction mechanism for separating carbazole from anthracene oil with quaternary ammonium-based deep eutectic solvents

Abstract

Carbazole is an irreplaceable basic organic chemical raw material and intermediate in industry. The separation of carbazole from anthracene oil by environmental benign solvents is important but still a challenge in chemical engineering. Deep eutectic solvents (DESs) as a sustainable green separation solvent have been proposed for the separation of carbazole from model anthracene oil. In this research, three quaternary ammonium-based DESs were prepared using ethylene glycol (EG) as hydrogen bond donor and tetrabutylammonium chloride (TBAC), tetrabutylammonium bromide or choline chloride as hydrogen bond acceptors. To explore their extraction performance of carbazole, the conductor-like screening model for real solvents (COSMO-RS) model was used to predict the activity coefficient at infinite dilution (γ∞) of carbazole in DESs, and the result indicated TBAC:EG (1:2) had the stronger extraction ability for carbazole due to the higher capacity at infinite dilution (C∞) value. Then, the separation performance of these three DESs was evaluated by experiments, and the experimental results were in good agreement with the COSMO-RS prediction results. The TBAC:EG (1:2) was determined as the most promising solvent. Additionally, the extraction conditions of TBAC:EG (1:2) were optimized, and the extraction efficiency, distribution coefficient and selectivity of carbazole could reach up to 85.74%, 30.18 and 66.10%, respectively. Moreover, the TBAC:EG (1:2) could be recycled by using environmentally friendly water as antisolvent. In addition, the separation performance of TBAC:EG (1:2) was also evaluated by real crude anthracene, the carbazole was obtained with purity and yield of 85.32%, 60.27%, respectively. Lastly, the extraction mechanism was elucidated by σ-profiles and interaction energy analysis. Theoretical calculation results showed that the main driving force for the extraction process was the hydrogen bonding ((N–H...Cl) and van der Waals interactions (C–H...O and C–H...π), which corresponding to the blue and green isosurfaces in IGMH analysis. This work presented a novel method for separating carbazole from crude anthracene oil, and will provide an important reference for the separation of other high value-added products from coal tar.

1. Introduction

Carbazole is an indispensable organic chemical raw material and intermediate, mainly derived from crude anthracene oil [1]. Due to its special rigid molecular structure, carbazole exhibits numerous unique properties, making it highly valuable and extensively utilized in various industries, including dyes [2], pharmaceuticals [3], synthetic resins [4], pesticides [5], and photoelectric materials [6]. With the further expansion of its application, the demand for carbazole in the market has increased rapidly. Therefore, developing carbazole production technology with a low-cost and environmentally friendly has become a hot topic for many researchers.

Although carbazole can be chemically synthesized [7], more than 90% of carbazole is still obtained by separation and purification of crude anthracene oil due to its lower cost. The structure and many physicochemical properties of components in anthracene oil are very similar, which poses a major challenge to the separation of carbazole. The common methods for carbazole separation include sulfuric acid and potassium fusion methods, distillation, emulsion liquid membrane [8], zone melting [9], supercritical fluid extraction [10], and solvent crystallization [11,12]. Generally, sulfuric acid and potassium fusion methods have been abandoned due to the use of strong acids and alkalis, which result in the generation of significant amounts of industrial wastewater. Additionally, separating carbazole by distillation is time and energy-consuming due to the close boiling points of components in crude anthracene oil. The new supercritical fluid extraction technique requires rigorous conditions and specialized equipment. The zone melting technology demands strict temperature control to prevent crystal agglomeration. The emulsion liquid membrane method is limited by the poor stability of the membrane system. These methods mentioned above are immature and still in the laboratory research stage. In chemical industry, solvent crystallization is the most frequently used method for separating carbazole. However, the organic solvents employed in this process exhibit poor performance for isolating the target component, leading to significant problems in the current process. These problems include the need for multiple washing and crystallization, low product yield, high solvent usage, and environmental pollution, etc. Therefore, in order to address these challenges, the development of efficient and environmentally friendly separation methods is urgently needed.