Efficient adsorption separation of methane from C2-C3 hydrocarbons in a Co(II)-nodes metal-organic framework

Abstract

Methane (CH4) as a substitute for other mineral fuels plays a crucial role in reducing energy consumption and preventing environmental pollution. The present study employs a solvothermal method to fabricate a porous framework Co-metal-organic framework (Co-MOF) containing two distinct secondary building units (SBUs): an anionic [Co2(μ2-OH)(COO)4(H2O)] and a neutral [CoN2(COO)2]. Notably, within the anionic SBUs, the coordinated water molecules induce the generation of divergent unsaturated Co(II) centers in the unidirectional porous channels, thereby creating open metal sites. The adsorption performance of Co-MOF towards pure component gases was systematically investigated. The results demonstrated that Co-MOF exhibits superior adsorption capacity for C2-C3 hydrocarbons compared to CH4, which offers the potential for efficient adsorption and separation of CH4 from C2-C3 hydrocarbons. The gas selectivity separation ratios of Co-MOF for C2H6/CH4 and C3H8/CH4 were calculated using the ideal adsorbed solution theory method at 273/298 K and 0.1 MPa. The results revealed that Co-MOF achieved remarkable equilibrium separation selectivity for CH4 and C2-C3 hydrocarbon gases among non-modified MOFs, signifying the potential of the synthesized Co-MOF for efficient recovery and purification of CH4 from C2-C3 hydrocarbons. Breakthrough experiments further demonstrate the ability of Co-MOF to purify methane from C2-C3 hydrocarbons in practical gas separation scenarios. Additionally, molecular simulation calculations further substantiate the propensity of anionic SBUs to interact with C2-C3 hydrocarbon compounds. This study provides a novel paradigm for the development of porous MOF materials in the application of gas mixture separation.

1. Introduction

Mineral fuels, formed through complex transformations of ancient organisms' remains, hold significant importance in achieving industrialization through their efficient development and application. Unfortunately, the combustion of fossil fuels emits copious amounts of greenhouse gases, intensifying the greenhouse effect and they are non-renewable resources. Therefore, clean methane gas (CH4) serves as a promising alternative to fossil fuels, effectively reducing energy consumption and mitigating environmental pollution. Moreover, due to the persistent growth in global energy demand, natural gas (NG) has emerged as a strategic energy resource [1,2]. Primarily consisting of CH4, natural gas also contains trace amounts of ethane (C2H6) and propane (C3H8) etc. which must be eliminated to enhance its quality [3]. Moreover, pyrolysis gas (PG), a significant source of light hydrocarbons, comprises abundant methane along with varying quantities of C2-C3 hydrocarbon compounds, including C2H6, C2H4 and C3H8 etc. [4]. To be effectively utilized, all these hydrocarbon compounds must undergo separation from one another as basic raw materials. Consequently, the current research focuses on achieving efficient separation of CH4 and C2-C3 hydrocarbon compounds.

In addition, the low-temperature distillation process under high pressure remains the most advanced technology for separating light hydrocarbon mixtures, given the small differences in boiling points among gas components and their similar sizes. However, this energy-intensive process leads to increased costs and energy consumption during separation. To address these challenges, traditional porous materials like activated carbon, zeolite, and silica have been extensively investigated for their selective adsorption properties in hydrocarbon separation [5,6]. Nonetheless, the practical application of zeolite materials is limited due to their structural drawbacks, including low specific surface area and a lack of active sites seen in activated carbon materials [7].

Metal-organic framework (MOF) materials, as a class of emerging crystalline organic-inorganic hybrid materials, have potential applications in various fields such as catalysis [8], gas separation [9], fuel storage [10], carbon capture [11], and energy devices [12]. In the past decades, research on the separation of hydrocarbon compounds using porous MOFs has gained increasing attention.