Fluid Phase Equilibria 362 (2014) 227–234 Contents lists available at ScienceDirect Fluid Phase Equilibria j ourna l ho me page: www.elsevier.com/locate/fluid Adsorption and separation of CO 2 /CH 4 mixtures using nanoporous adsorbents by molecular simulation Linghong Lu a,* , Shanshan Wang a , Erich A. Müller b , Wei Cao a , Yudan Zhu a , Xiaohua Lu a , George Jackson b a College of Chemistry and Chemical Engineering, Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, PR China b Department of Chemical Engineering, Imperial College London, South Kensinton Campus, London SW7 2AZ, UK a r t i c l e i n f o Article history: Received 15 July 2013 Received in revised form 4 October 2013 Accepted 7 October 2013 Available online 16 October 2013 Keywords: Adsorption CO2/CH4 Separation Simulation a b s t r a c t A grand canonical Monte Carlo-simulation (GCMC) study is presented focussing on the adsorption of CO 2 /CH 4 mixtures in different nanopore models, including pristine mesoporous carbons, carbon foams, carbon nanotubes (CNTs), and nanopore models modified with hydrophilic carboxylic groups. We also report and discuss the selectivity of the different adsorbent surfaces under a wide range of temperature and pressure. Our results show that foam structures have the highest adsorption capacity of all the pristine structures studied because of its special architecture. The selectivity markedly enhanced after modification, especially at low pressures, and modified CNTs are found to have the highest selectivity among all the models tested. The effect of temperature and pressure is evaluated and the change in the selectivity trends of modified nanopore models are in contrast to that of the pristine models. The results suggest that the separation performance in carbon nanopores is greatly affected by the nature of the architecture and the heterogeneity of the materials. These findings could be beneficial in conventional pressure swing adsorption processes and the nanoporous structures could be used as parts of mixed polymer membranes. The results of this work present some guidelines for the designing nanoporous structures in order to achieve optimal separation of CO 2 /CH 4 mixtures. © 2013 Elsevier B.V. All rights reserved. 1. Introduction At present, there is growing interest in identifying an energy- efficient process for separating a gaseous mixture of carbon dioxide and methane (CO 2 /CH 4 ). On one hand, controlling global warm- ing in the short term relies on the capture and sequestration of CO 2 in viable underground reservoirs. The pumping of CO 2 into unmineable coal seams is therefore an option; however, coal seams are saturated with CH 4 and it is therefore necessary to under- stand the mechanism of selective adsorption of CO 2 /CH 4 in order to perform large-scale modeling of these processes that are con- sidered achievable near-term methods to reduce emissions of this greenhouse gas into the atmosphere [1]. On the other hand, shale and marsh gases are evolving as a replacement for more conventional fossil fuels after separation (methane removal) and purification. The injection of CO 2 into coal seams to enhance the desorption of coal-bed methane [a process called enhanced coal- bed methane recovery (ECBM)] has been reported to be a feasible * Corresponding author. Tel.: +86 25 83172251. E-mail address: linghonglu@njut.edu.cn (L. Lu). process in order to achieve this goal [2,3]. Among the possible low-energy-consumption alternatives for the separation of the CO 2 /CH 4 mixture are adsorption on carbons and purification using nanoporous carbon-based membranes [4]. In the latter case, the selectivity and flux of membrane are important factors related to the pore architecture and chemical composition of the membrane materials in a very non-intuitive way. Separation of the CO 2 /CH 4 mixture could feasibly be performed using pressure swing adsorption (PSA), a technique widely used in industry due to its high removal efficiency, low-energy consump- tion and compact footprint. PSA processes employ a wide range of porous adsorbents, such as carbons, zeolites, metal-organic frame- works (MOFs) all of which have been proposed and studied for CO 2 and CH 4 adsorption. The development of adsorbents with high capacity, high selectivity, rapid uptake, easy recycling, and suitable thermal and mechanical properties is a challenging task. Research on metal-organic frameworks (MOFs) has rapidly developed in the past decade because of the high surface area of these materials [5–9], but their high price, low thermal and water stability has hindered their commercial application. Some of these drawbacks can be remediated by incorporating hydrophobic functional groups [10]. Siliceous zeolites are another kind of promising sorbents for 0378-3812/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fluid.2013.10.013