Applied Surface Science 258 (2012) 4819–4825 Contents lists available at SciVerse ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Separation of methane–nitrogen mixtures using synthesis vertically aligned carbon nanotube membranes Neda Gilani a , Jafar Towfighi Daryan a,∗ , Alimorad Rashidi b , Mohammad Reza Omidkhah a a Faculty of Chemical Engineering, Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran b Nanotechnology Research Center, Research Institute of Petroleum Industry (RIPI), P.O. Box 14665-1998, Tehran, Iran article info Article history: Received 11 November 2011 Received in revised form 16 January 2012 Accepted 21 January 2012 Available online 30 January 2012 Keywords: Carbon nanotubes Anodic aluminum oxide Gas separation CH4/N2 mixture abstract In this paper, capabilities of carbon nanotube (CNT) membranes fabricated in cylindrical pores of anodic aluminum oxide (AAO) substrate to separate the binary mixtures of CH 4 /N 2 are studied experimentally. For this purpose, the permeability and selectivity of three CNT/AAO membranes with different growth time as 6 h, 12 h and 18 h are investigated. CNTs are grown vertically through holes of AAO with average pore diameter of 45 nm by chemical vapor deposition (CVD) of acetylene gas. CNT/AAO membranes with the same CNTs’ outer diameters and different inner diameters are synthesized. The AAO are character- ized by SEM analysis. In addition, SEM, TEM, BET N 2 adsorption analysis and Raman spectroscopy are employed to characterize aligned CNTs. Study on permeability and selectivity of membranes for three binary mixtures of CH 4 /N 2 showed that when the CNT inner diameters are 34 nm and 24 nm, viscous flow is the governing mechanism and insignificant selectivities of 1.2–1.24 are achieved. However, the membrane with CNT inner diameter and wall thickness of 8 nm and 16 nm respectively is considerably selective for CH 4 over N 2 . It was also found that CH 4 mole fraction in the feed and upstream feed pressure have major effect on permeability and selectivity. The membrane with 18 h synthesis time showed the selectivity is in the range of 1.8–3.85. The enhancement factor for N 2 single gas diffusivity was also found to be about three times larger than that predicted by Knudsen diffusion model. © 2012 Elsevier B.V. All rights reserved. 1. Introduction As energy costs rise, membrane technology for separation of gasses is likely to play an increasingly important role in reduc- ing the environmental impact and costs of industrial processes. Gas separating membranes offer a number of advantages over the other gas separation technologies. For example, they do not cause a phase change, and their related plants footprint are smaller than that of the other types of technologies. The lack of mechanical complexity in membrane systems is another advan- tage as well. Currently, gas separation membranes are most widely used in industry for hydrogen/nitrogen separation, hydro- gen/hydrocarbon separations and separation of nitrogen from natural gas. There are many types of gas separating membranes. One of them is carbon membrane which is chemically and ther- mally more stable than polymer membranes. Moreover, carbon membranes have superior adsorptive when used for separation of some special gas mixtures [1]. CNT membranes are a kind of carbon membranes, which have recently received much attention due to their high permeance [2]. For example, Hinds et al. [3] ∗ Corresponding author. E-mail address: towfighi@modares.ac.ir (J.T. Daryan). fabricated aligned carbon nanotubes (ACNTs) membrane using CVD method on quartz substrate across a polystyrene film. They pro- duced CNTs with inner diameter of 7.5 nm and membrane pore density of 6 × 10 10 CNT/cm 2 . They also measured nitrogen perme- ance with the flux calculated by Knudsen diffusion. Holt et al. [4] fabricated multiwall carbon nanotubes (MWCNTs) membranes in a silicon nitride matrix. They showed that water flow across a nan- otube membrane, predicted a much higher molar flux, and the nanotubes had a “bamboo” morphology. Nitrogen flow measure- ments gave a membrane permeance of 4.7 × 10 -4 mol/m 2 s Pa at a pore density of 4 × 10 10 CNT/cm 2 . Using a Knudsen diffusion model, the average pore size of this membrane was estimated to be 66 nm. Holt et al. [5] also reported gas and water flow measurements through micro-fabricated membranes in which ACNTs diameters of less than 2 nm serve as pores. It was found that the gas and water permeabilities of nanotube-based membranes were several orders of magnitude higher than those of commercial polycarbon- ate membranes, despite having pore sizes an order of magnitude smaller. Mi et al. [6] grew vertically aligned MWCNTs in the porous -alumina support by multi-step and chemical vapor deposition (CVD) methods. They showed that gas permeance through the membrane, is inversely proportional to the squared root of molec- ular weight, and diffusivity is about four times larger than the Knudsen model. The enhancement in permeability or diffusivity 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.01.126