Dual-layer hollow carbon fiber membranes for gas separation consisting of carbon and mixed matrix layers
Introduction
In the area of membranes for gas separation, intensive efforts have been focused on novel membranes having the potential to overcome the productivity/selectivity trade-off limit of commercially available polymeric membranes [1]. In recent years, much attention has been given to carbon molecular sieve membranes (CMSMs) [2], [3], [4], [5], [6]. Owing to the molecular sieving mechanism, CMSMs have effective size and shape discrimination between gas molecules and can obtain extremely high selectivity [7], while the porous structure of the CMSMs allows significantly high flux. Additionally, the inorganic CMSMs show great thermal and chemical stability in harsh environments [8]. Thus, CMSMs appear to be a promising candidate in the field of membrane-based separation.
The preparation of CMSMs is a process involving many steps like material selection for precursor, membrane precursor formation, carbonization in controlled thermal and chemical environments, and sometimes pre/post-treatments [7]. Many carbon-containing materials such as thermosetting resins [7], [9], polyimides [10], [11], [12], [13], [14], [15], [16], [17], [18] and others [19], [20] have been employed as precursors for carbon molecular sieves. Polyimides are among the most stable polymers which often do not go through a melting stage and can maintain their shape during the whole pyrolysis process. Therefore they are most widely applied in the production of CMSMs. The properties of the CMSMs also depend significantly on the pyrolysis conditions like pyrolysis temperature, heating rate, pyrolysis atmosphere (e.g., inert gas purge or vacuum) and final soaking duration [21], [22], [23], [24]. Soffer and Koresh were among the first to study the influence of high temperature on the development of the pore structure. Vu et al. found that increasing the final pyrolysis temperature could dramatically increase the CO2/CH4 selectivity (>600) of the CMS membranes but significantly decrease the CO2 permeance [24]. Pretreatment of precursors and the post-treatment of produced hollow carbon fibers are also helpful in adjusting CMSMs’ property to meet desired separation requirements [25], [26].
To ensure high efficiency of a membrane in a given separation process, the geometry of the carbon membrane and the way it is installed are also very important [27]. The most preferable configuration for commercial application is hollow fiber because of its high effective separation area per unit volume [28]. For polymeric hollow fibers developed by precipitation, variations in the spinning conditions may alter the properties of the precursors, and consequently the carbon membranes [29], [30], [31], [32].
Previous research involving hollow fiber CMSMs has confined to single-layer hollow fibers; in addition, much effort has gone into the preparation of precursor fibers with defect free skin layers, because defects in the skin layer may be considered as the cause of decreased selectivities in the resultant CMSMs. This has strict requirements on choosing a suitable yet complicated process from dope preparation to fiber spinning. However, real applications require more flexibility in preparing high performance hollow carbon fibers. Therefore, in this work, a novel dual-layer hollow fiber consisting of neat Matrimid polyimide as the inner support layer, while polysulfone-zeolte beta (PSF-beta) mixed matrix structure as the outer layer is applied for producing hollow carbon fibers by pyrolysis. The advantages of dual-layer technology have been demonstrated by many works. It can largely save the material cost, employ brittle but high performance material as the outer layer to form the composite hollow fiber membrane with ultra-high pressure resistance [33], [34], [35], [36]; additionally, Jong et al. for the first time produced multilayer inorganic hollow fibers by using the dual-layer spinning followed by sintering [37].
It is expected in this study that the dual-layer composite structure containing two layers with different thermal stability may experience a pyrolysis process different from that when only one of the two layers is involved. Therefore, the preparation process and performance of the resultant carbon hollow fibers may show some new properties different from that when only single-layer polymer precursor is involved.
Section snippets
Materials
Udel® Polysulfone (PSF) from Amoco and Matrimid® 5218 from Vantico were the polymers used in this study. The polymers were dried overnight in a vacuum oven at 110 °C–120 °C before dope preparation; N-methyl-pyrrolidinone (NMP) and Ethanol (EtOH) from Merck were used as received. Zeolite beta particles were added to the spinning solution for mixed matrix outer layer of the dual-layer hollow fibers. The particles had an average diameter of about 0.4 μm. All particles were dehydrated at 350 °C for 2 h
Results and discussion
Typical morphology of the polymeric dual-layer hollow fiber precursor is shown in Fig. 1. As can be seen, the ratio of the outer layer over inner layer in thickness is around 1:10. The outer layer cross-section is porous and zeolite particles are uniformly dispersed in PSF matrix; the inner layer cross-section has a graded porous structure with big macrovoids distributed near the lumen side. The outer surface of the outer layer is roughly dense as illustrated in Fig. 1 (B1); however, the
Conclusions
A novel approach has been demonstrated to produce hollow carbon fibers with excellent separation properties. These hollow carbon fibers have been produced from PSF-beta zeolite/Matrimid dual-layer hollow fibers. The performance of the resultant dual-layer hollow carbon fibers is much superior over that of single-layer hollow carbon fibers. TGA experiments indicate that the enhancement in the separation factor of the dual-layer hollow carbon fiber is probably due to uneven pyrolysis dynamics and
Acknowledgements
The author would like to thank Mr. Y. Li, Dr. P. S. Tin, and Mr. Y. C. Xiao for their suggestions and help in this work. Appreciation also goes to A-Star for funding this research with the account R-398-000-029-305 (052-101-0014 (A-Star)) and R-279-000-184-112.
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