(141f) Evaporative Separation of Ethanol-Water Mixtures Using Functionalized Nanoporous Graphene: A Sustainable Strategy for Azeotropic Mixtures

Separation processes are a cornerstone of many industrial applications, serving a pivotal role in both product purification and waste reduction. A particularly significant example is the separation of ethanol-water mixtures, a process integral to industries such as biofuels, pharmaceuticals, and beverages. Despite its importance, the conventional methods for this separation, such as distillation, are often energy-intensive and inefficient, underscoring the need for innovative alternatives. In this study, we explore one such alternative by employing molecular dynamics simulations to investigate the efficacy of functionalized nanoporous graphene [1, 2] for the evaporative separation of ethanol-water mixtures of varying compositions. Under ambient conditions and without external hydraulic pressure, we discovered that hydrophilic OH-terminated nanopores with a diameter of 4.54 Š(Figure 1A) were impermeable to ethanol molecules (Figure 1B). In contrast, the hydrophobic H-terminated nanopores of diameter 5.49 Š(Figure 1C) exhibited separation factors, α, between 110 and 180 for ethanol concentrations ranging from 25-75 mol%, outperforming most conventional polymeric and mixed matrix membranes (Figure 1D). Our observations indicate that the evaporation of ethanol from graphene nanopores is a multi-step process, involving permeation through the nanopores, adsorption on the vapor side of the nanoporous graphene, and finally, desorption into the vapor region (Figure 1E, F). Across all compositions and nanopore types, ethanol demonstrated a greater affinity for the nanopores, significantly reducing water evaporation and resulting in the high separation factors. The potential of mean force profiles revealed a significantly higher energy barrier for water permeation through the nanopores and subsequent evaporation from the adsorbed layer on the vapor side. Additionally, the atomistic insights showed faster hydrogen bond dynamics and translational dynamics associated with the ethanol molecules, leading to a higher evaporation flux of ethanol compared to water, thus resulting in a spontaneous separation of two species. We extended these findings to the separation of azeotropic mixtures and found that OH-terminated pores enrich the liquid phase in ethanol, while the H-terminated pores enrich the liquid phase in water. This is achieved simply by placing the appropriate nanoporous graphene at the liquid-vapor interface, without the application of external pressure and at the ambient conditions. Our study not only paves the way for revolutionizing ethanol-water separation processes, but also suggests a more efficient and energy-conservative approach that could have far-reaching implications for various separation processes, contributing to energy conservation and process efficiency across industries.

References

[1] Ronghe, Anshaj, and K. Ganapathy Ayappa. "Graphene Nanopores Enhance Water Evaporation from Salt Solutions: Exploring the Effects of Ions and Concentration." Langmuir 39, no. 25 (2023): 8787-8800.

[2] Lee, Wan-Chi, Anshaj Ronghe, Luis Francisco Villalobos, Shiqi Huang, Mostapha Dakhchoune, Mounir Mensi, Kuang-Jung Hsu, K. Ganapathy Ayappa, and Kumar Varoon Agrawal. "Enhanced water evaporation from Å-scale graphene nanopores." ACS nano 16, no. 9 (2022): 15382-15396.