Experimental and theoretical exploration of gas permeation mechanism through 2D graphene (not graphene oxides) membranes
Introduction
Membrane technology is one promising option for cost-effective gas separations to replace the old cryogenic distillation method. In this field, polymeric membranes are limited by the Robeson upper bound, a trade-off phenomenon between selectivity and gas permeability [1,2]. Molecular sieving membranes (MSMs) can break this limit owing to their super-high selectivity and excellent permeability in case of a defect-free and ultrathin separating layer is successfully deposited on the porous supports [3,4]. Typical MSM membranes are made from zeolite, silica, and metal organic frameworks (MoFs). One common technique to grow these MSM membranes is via the bottom-up method where the zeolite, silica or MOF are growing from the precursor solution during the hydrothermal treatment at high temperatures [[5], [6], [7], [8], [9], [10], [11], [12]]. Via this hydrothermal reaction method, these molecular sieving membranes have a strong adhesion with the substrate due to the MSM layers interlocking with the porous support. However, two shortcomings are hindering the further development of MSMs for scaling up. One is the high cost of the complex synthesis procedure sometimes involving many growth repetitions to reduce the defect like zeolite and silica membranes [8,[13], [14], [15], [16], [17]]. Another problem is the presence of none-selective defects formed by the thermal stress during the heat treatment, in particular for the preparation of membranes with large area, which is the main reason hindering MSMs for gas separation in large scale application [[18], [19], [20]]. Recently, a novel method has been widely reported by using these two-dimensional (2D) layered nanosheets to assemble molecular sieving membranes at room temperature [[21], [22], [23], [24], [25], [26]]. Such MSM membranes are in laminar structure where the stacked layers are held by van der Waals force and hydrogen bonds [[27], [28], [29], [30], [31], [32], [33], [34]]. One typical example attracting much enthusiasm is the graphene based membrane. To prepare these graphene membranes, graphene oxide (one of the graphene derivatives) flakes - have to be used as the building block because the pure graphene nano-sheet is impermeable to gases even small molecules like He or H2 due to the narrow interlayer space between two neighboring single-atom graphene layers and strong electron density from the aromatic rings to repel atoms or molecules. By contrast, interlayer-spacing distance of the graphene oxide (GO) nano-sheet is expanded due to the presence of some surface oxygen-containing functional groups and becomes the perfect channels to enable smaller gas (He or H2) transport [[35], [36], [37], [38], [39], [40]]. These GO membranes are commonly prepared by vacuum/pressure-assisted filtration method where the GO nanosheets are homogeneously suspended inside the aqueous solution [25,26,28]. There is still a challenge to fabricate or maintain these GO nanosheets in monolayer thus most of these GO sheets are in multilayer structure as the building block to form the membrane [33,41].
As displayed in Fig. 1, the GO nanosheets are deposited on the porous substrate to form the supported GO membrane by controlling the GO dispersion and filtration conditions. In case of helium (He) permeation, there are possibly two kinds of channels to pass through these laminar layers. The first type of channels is the interlayer space (type-I in Fig. 1) provided by the same GO flake in multilayer structure, which can be referred as the inherent pores from the building block. The second type of channels is the space (type-II) in Fig. 1 between two neighboring GO flakes. Of course, some defects are easily formed during the GO assembling process thus the GO membrane with a certain thickness is required to sufficiently cover these defects (large pores) to achieve a high selectivity. Basically, the gas transport is via the tortuous path along the laminar direction as indicated in Fig. 1b. However, at this stage it is not very clear about the existence of type-II channels and how does the gas transport through both types of laminar channels. We are trying to understand the microstructure of such stacked graphene membranes and their transport mechanism.
In this work, the graphene (not GO) flake with thickness of 7 nm was used as the building block to form the graphene membrane under the hydraulic pressure. Two kinds of graphene flakes with width of 5 and 25 μm were applied as the membrane starting materials to investigate the size influence. Noteworthy that, in a more strict sense, these graphene flakes should be referred as the thin graphite nanosheets and thus Type-I channels for gas transport does not exist and only type-II channels would be created to possible gas permeation. Our results demonstrate that the pressed graphene membranes in thickness of 0.75 mm displayed excellent molecular sieving properties with high selectivity of He/CO2 (65) and He/N2 (22) and He permeance 2.02 × 10−8 mol s−1 m−2 Pa−1.
Section snippets
Materials and Chemicals
Commercially available graphene nanoflakes were purchased from Sigma-Aldrich and manufactured by XGnP®. Two different types of graphene were used. Grade M with flake sizes of 5 and 25 μm with surface area 120–150 m2 g−1 and 6–8 nm thickness. For the gas permeation tests, helium (99.99% purity), nitrogen (99.98% purity) and carbon dioxide (99.98% purity) were supplied by BOC. Zeolite powder 13X powder with particle size of 2 μm and pore size 10 Å manufactured by Fluka Analytical and purchased
Pure graphene membranes
Fig. S4 shows the XRD patterns of the prepared graphene membranes. Both of the XRD patterns show basal reflection peak at 2θ of 26.4° with high intensity due to the characteristic graphitic materials. These graphene flakes (GF) with in thickness of 6–8 nm are actually thin graphite nanosheets. Fig. 2 a&b clearly shows the respective graphene flakes with size 5 and 25 μm. Fig. 2c is the SEM image of graphene membrane from cross sectional views. Fig. 2c shows the regular and well-packed 2D
Conclusions
In this work, instead of GO, thin graphene nanoflakes with size 25 or 5 μm and average thickness 7 nm were used as the building blocks to prepare the 2D laminar GF membranes to verify the hypothesis that molecular sieving properties can be derived from the tortuous interspace between the building blocks but not from the inherent nanochannles of the graphene nanoflake itself as a perfect graphene (not GO) is impermeable to any gases. Modeling and gas permeance measurement from single gas or gas
CRediT authorship contribution statement
Fatemeh A. Nezhad: Conceptualization, Formal analysis, Writing - original draft. Ning Han: Formal analysis, Writing - review & editing. Zhangfeng Shen: Conceptualization, Formal analysis, Writing - original draft. Yun Jin: Conceptualization, Formal analysis, Writing - review & editing. Yangang Wang: Conceptualization, Formal analysis, Writing - review & editing. Naitao Yang: Conceptualization, Formal analysis, Writing - review & editing. Shaomin Liu: Conceptualization, Formal analysis, Writing
Acknowledgements
S. Liu acknowledges the financial support provided by the Australian Research Council (DP180103861 and IH170100009). Y. Jin acknowledges the research funding provided by the National Natural Science Foundation of China (No. 21878179).
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