Two-dimensional MXene membrane for ethanol dehydration

Highlights

• MXene membranes stacked by Ti₃C₂T_x MXene nanosheets were successfully prepared.

• Azeotrope of ethanol and water was successfully separated through MXene membranes.

• The MXene membrane showed a better anti-swelling property in alcohol.

Abstract

Two-dimensional MXene membranes have been successfully used in separation technology. With controllable interlayer nanochannels, abundant surface-terminating groups and hydrophilicity, MXene membranes also exhibit the potential in alcohol dehydration. Here, 2-μm-thick MXene membranes stacked by Ti₃C₂T_x nanosheets were fabricated and applied in ethanol dehydration for the first time. Effects of the feed ethanol concentration and operating temperature on the ethanol dehydration performance of the MXene membrane were investigated through a pervaporation process. The water/ethanol separation factor of the MXene membrane increased with increasing feed ethanol concentration. Additionally, the MXene membrane exhibited better ethanol dehydration performance at room temperature compared with that at elevated temperature. It gave a water/ethanol separation factor of 135.2 with a total flux of 263.4 g m⁻² h⁻¹ at room temperature for the azeotrope dehydration (95% ethanol mixed with 5% water). MXene membranes are promising in applications of pervaporation dehydration and solvent separation.

Introduction

Considering that bioethanol is regarded as a kind of promising green energy and sustainable energy for replacing fossil fuels [1], where a certain amount of water would be generated during its production. So ethanol dehydration is a crucial process to obtain ethanol with high purity for energy applications [2]. Distillation is a conventional method to concentrate ethanol, but it is an energy-intensive process [3]. Moreover, once the concentration of ethanol reached 95.6% (by weight), an azeotrope with water would be formed, which made it impossible to separate ethanol-water by traditional distillation. In terms of energy efficiency, membrane separation technology is a promising alternative for ethanol dehydration [4,5].

In recent years, two-dimensional (2D) lamellar membranes have attracted worldwide attention due to their easy processing [6], tunable pore size [7], mechanical robustness [8] and high performance [9]. Plenty of 2D nanomaterials have been successfully fabricated to 2D lamellar membranes, such as graphene [10], molybdenum disulfide [11] and hexagonal boron nitride [12]. These 2D membranes have shown promising performance in gas separation [13], ion sieving [14], water purification [15] etc. For example, Yang et al. reported a kind of ultrathin graphene oxide membrane, which exhibited fast water and organic solvent permeation with high rejection towards ions and dye molecules [9]. Chen et al. reported an amino functionalized boron nitride based membrane, which performed a high water flux and good rejection for dye molecules in both aqueous and organic solvents [12]. Huang et al. present an integrated and continuous graphene oxide membrane by vacuum filtration method, which exhibited high separation performance for dimethyl carbonate dehydration [16]. Wang et al. assembled a g-C₃N₄ membrane with novel self-supported structure, which showed an excellent separation performance in water purification [17,18].

Apart from these 2D materials mentioned above, 2D transition metal carbides, carbonitrides and nitrides (MXenes), as a kind of young 2D material, was first synthesized by Gogotsi and Barsoum [19]. MXene nanosheets with atomic thickness can be obtained by selectively etching the layers of A-element atoms from the laminar bulk precursor of MAX ternary phase, where M refers to the early transition metal elements, like Ti, Ni, Mo and so on; X represents nitrogen and/or carbon; A-elements are mostly in the group 13 and 14 of the periodic table. There are more than 19 different MXene compositions successfully synthesized [20] and Ti₃C₂T_x is the most studied one. Prepared by selectively etching method using HF or the combination of LiF and HCl, MXene has abundant surface terminations (represented as T in Ti₃C₂T_x) like –F, =O, –OH etc [21]. Previous reports have demonstrated that MXene possesses excellent conductivity, mechanical stability and hydrophilic surface, making them applicable for lithium ion battery [22], supercapacitor [23] electromagnetic shielding devices [24] and NH₃ electrosynthesis [25]. Moreover, stacked MXene nanosheets also showed obvious edge when used in membrane separation. First of all, MXene nanosheets have great dispersibility in water and other solvent [26], which means they can be easily fabricated to 2D membrane by vacuum filtration or coating and so on. In addition, MXene membranes have hydrophilcity and tunable interlayer spacing, which make them available for water permeation and molecule rejection. MXene membranes have been successfully applied in gas separation [27,28], ion sieving [29] and water purification [30,31]. For example, Ding et al. reported a kind of MXene membrane which showed favorable rejection for dye molecules in water [30]. Recently, ultrathin MXene membranes were implemented for pervaporation desalination and they can effectively reject NaCl at 65 °C with a high water flux [32]. Besides, MXene has also been used to optimize chitosan (CS) membrane for solvent dehydration, where the separation factor of the CS membrane was notably improved with 3% addition of MXene [33].

However, to the best of our knowledge, there was no report on the pristine MXene membrane used for alcohol dehydration till now. In this work, a kind of 2-μm-thick MXene membrane stacked by monolayer Ti₃C₂T_x nanosheets was fabricated and applied to separate ethanol-water mixture through a pervaporation process. Effects of the feed concentration and operating temperature on the ethanol dehydration performance were investigated in detail. With the concentration of ethanol in feed solution changing from 75% to 95%, the water/ethanol separation factor increased. The MXene membrane exhibited a water/ethanol separation factor of 135.2 with a total flux of 263.4 g m⁻² h⁻¹ at room temperature using 95% ethanol concentration as feed solution. This performance indicates a promising future of MXene membranes for the dehydration of ethanol and other solvents via pervaporation process.