Experimental study on graphene-based nanocomposite membrane for hydrogen purification: Effect of temperature and pressure
Graphical abstract
Introduction
In last two decades, membranes for gas separation are increasingly becoming a significant and convenient technology for the hydrogen recovery from gas mixtures, oxygen enrichment, gas sweeting of natural gas, CO2 capturing, vapour-vapour separation and air dehydration [1]. It should be noted that the main reason of this affinity is that the membrane technology can successfully separate gas mixtures under low pressure, obviously reducing the required industry area and minimizing the necessary energy consumption with relatively low contamination, compared to traditional separation technologies [[2], [3], [4]]. Up till now, numerous papers about gas separation membranes are focused on achieving high flux and surprising selectivity [see for example: [[5], [6], [7]].
Recently, considerable interest has been observed by the emerging two-dimensional structural materials, such as MoS2 [8,9], Phosphorene [10], ZIF-7 [11], and graphene based material [12], due to their ultra-thin thickness and unique separation property. Utilizing two-dimensional intriguing materials to fabricate thin membranes is considered as a useful and effective way to overcome the current performance (permeability/selectivity) [13,14] which often occurred in traditional polymer membranes [14,15]. Among the mentioned options, there is no doubt that graphene based material, a two-dimensional monolayer of sp2 hybridized carbon atoms arrayed in a honeycomb pattern, shows the most outstanding prospect, owing to a series of unique physical/chemical properties, such as good chemical stability, excellent thermal conductance and strong mechanical strength [12,[16], [17], [18], [19]]. Hitherto, some exciting and encouraging works have been achieved [20,21]. However, because of the complicated membrane preparation process [[22], [23], [24]], it is very hard to transfer their membrane to the practical application. On the other hand, the attractive ultrathin membrane structure brings another critical issue: how to bear complex and harsh long-term operation environments in the real industry (such as high pressure and unstable gas flow). Hence, the remarkable interest in graphene based material must be complemented by insistent efforts to develop new membrane materials with much higher performance (permeance and selectivity) for more energy-efficient membrane processes.
In the last five years, some of theoretical and experimental papers on porous graphene and graphene oxide (GO) nanocomposite membranes have been presented for gas separation applications. Recently, the progress on graphene-based membranes has been nicely reviewed [[25], [26], [27]]. In general, extremely careful manipulations are needed for few-layers GO membranes preparation [28,29]. Therefore, it is still necessary to design GO membranes which can satisfy the practical requirements and explore their gas separation.
In order to make up above shortages, in this work, we try to prepare porous γ-alumina supported GO membranes, because alumina substrates not only can decrease thickness of membranes to realize high flux obviously, but also can offer a good mechanical strength for composite membranes [30,31]. Moreover, it can be forecasted good matching of alumina substrate with GO layer. Therefore, in this study, the porous modified γ-alumina tube was selected as the substrates due to its characteristic configuration (low mass transfer resistance and high-packing density) and good thermal and chemical stability [32].
Here, a convenient and rapid vacuum dip-coating method to prepare GO membranes on the modified γ-alumina tubular substrate, which exhibited excellent Knudsen permeation mechanism [30]. The special configuration makes GO membranes very easy to be scaled-up. Moreover, the GO nanosheets stacked to form a cylinder shell around the ceramic tubular substrate, keeping it more stable than flat GO membranes.
Herein, as a first approach, this work reports our finding that can fulfill the details about high-quality GO nanocomposite membranes on modified γ-alumina tubular substrate with excellent hydrogen separation. In particular, effects of operating parameters such as pressure and temperature on the performance of synthesized GO nanocomposite membrane were analyzed in terms of hydrogen purification.
Section snippets
Material
In this experimental study, the material sources used as follows: nitric acid (HNO3, 65%, Merck) as catalyst for boehmite sol preparation, and Poly vinyl alcohol (PVA, Merck, MW: 72,000) as stabilizer, aluminum-tri-sec-butoxide (ATSB) (97%, Sigma Aldrich) as source of γ-alumina, graphite as source of GO solution, NaNO3 (98%, Merck), H2SO4 (98%, Merck), H3PO4 (99%, Sigma Aldrich), H2O2 (30%, Merck), HCL (37%, Merck) and KMnO4 (99%, Sigma Aldrich) for GO solution preparation, the high purity
Characterization analysis
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γ-alumina intermediate layer:
In order to prepare microporous composite membranes, the quality of the support is very effective on the membrane layer integrity. The surface roughness and homogeneity of the support determines not only the integrity of the membrane layer, but also the minimal thickness of the membrane layer for complete surface coverage [35]. The use of thin intermediate layers is an attractive alternative which can be used: a) to generate a smooth surface; b) to improve the
Conclusion
The GO nanocomposite membrane with high quality was synthesized by vacuum dip coating method showing a temperature and pressure dependency flux of molecular sieve mechanism. During synthesis of GO nanocomposite membrane a new successful strategy was used for surface modification of α-alumina support, in which a particles size control of boehmite sol was applied. The permeance test results strongly show that the higher quality of surface modification of supports can affect directly the GO
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