Can the addition of carbon nanoparticles to a polyimide membrane reduce plasticization?
Introduction
Membrane technology is an attractive approach to gas and vapor separation due to its ease of operation, energy efficiency and cost-effectiveness, relative to other techniques such as solvent absorption, adsorption, and cryogenic distillation [1]. Polymer membrane-based separation technologies have been applied in industrial applications including natural gas sweetening, hydrogen recovery and oxygen enrichment. They are also seen to have potential for post combustion capture of carbon dioxide. However, there is a well-known trade-off in dense polymer membranes between gas permeability and selectivity for any given pair of components [2]. One of the effective ways to overcome this trend is to use mixed matrix membranes (MMMs) which combine the benefits of both polymer substrates and organic and/or inorganic fillers [3], [4]. To date, a large variety of mixed matrix membranes using carbon nanotubes, carbon molecular sieves [5], [6], activated carbon [7], [8], [9], zeolites [10], silica [11], [12], [13], and metal organic frameworks (MOFs) [14], [15], [16] have been investigated for gas separation applications. These systems are attractive because of their enhanced thermal, chemical, and mechanical stability in addition to their improved gas permeation, when compared to homogeneous polymer membranes.
Some workers have shown that the addition of fillers such as metal organic frame works (MOFs) [17], mesoporous silica [18] and zeolites [19] can suppress high pressure CO2-induced plasticization. This is a phenomenon often observed in glassy polymers where the polymer matrix expands rapidly beyond a critical penetrant pressure, referred to as the plasticization pressure. It is claimed that the fillers used in mixed matrix systems enhance the interaction between polymer and fillers and thus act as pseudo-crosslinkers. However, other workers find the addition of ZIF-8 nanoparticles ineffective in reducing CO2 plasticisation, unless a cross-linkable moiety is specifically added [20].
Carbon membranes have also been widely investigated as homogeneous systems derived from polymer precursors [21], [22], [23]. For example, we have previously reported the performance of microporous (diameter, d < 2 nm) carbon membranes for carbon dioxide (CO2) capture [24]. Gas adsorption and molecular simulation experiments suggested that CO2 is more readily adsorbed on microporous carbon than methane (CH4) and nitrogen (N2) [25]. Further, it has been shown that the selectivity of carbon membranes for more strongly adsorbed molecules such as CO2 can be enhanced beyond that in a polymeric system by surface diffusion [26]. Indeed, in some cases, more of the diffusing species can migrate in the surface layer than through the pore volume [27]. Importantly, we have also shown that carbon membranes are resistant to the impact of a range of gas impurities, including water, which is the most common contaminant in post-combustion capture [28].
In our more recent work, we have also shown that carbon nanoparticles can be similarly effective in improving the gas separation performance in mixed matrix systems [29], [30]. Further, these particles are likely to be available at more moderate cost than additives such as metal organic frameworks (MOFs). In this work, we expand on these results to provide a more detailed analysis of the effectiveness of carbon-based mixed matrix systems across a range of operating pressures and water vapor humidities. A specific focus of the manuscript is to determine whether the addition of these nanoparticles can reduce plasticization of the membrane by both water vapor and CO2, thus providing a material that provides for more stable operation in the field, with no loss in separation performance.
Section snippets
Materials
The base polymer material used as host matrix in this research is Matrimid®5218 (3,3,4,4-benzophenone tetracarboxylicdianhydride-diaminophenylidane) polyimide purchased from Huntsman Advanced Materials Americas Inc, America, in a powder state. This polyimide was purified by dissolution in dichloromethane (DCM, Analytical reagent, Chem-Supply, Australia) and re-precipitation using methanol (Analytical reagent, Chem-Supply, Australia) to remove impurities. Carbon nanoparticles were purchased from
Characterization
The membrane density of the base Matrimid is measured as 1.223 ± 0.019 g/cm3, which is in good agreement with the literature [40]. The density of the carbon nanoparticle is given by the manufacturer as 1.887 g/cm3. Fig. 1 presents the effect of filler loading on the membrane density of MMMs formed from the mixing of these two materials. The difference between the experimental and theoretical densities can be characterized by defining a void volume fraction (ϕvoids) as expressed in Eq. (3) [33].
As
Conclusion
We have prepared dense polyimide-carbon MMMs containing up to 30 wt% loading by a casting method. The experimental densities of the MMMs were only slightly lower than those theoretically predicted, suggesting that interfacial gaps between the polyimide and particles were small. The CO2 solubility for the MMM system was readily predicted from the sorption isotherms of the individual components, indicating that the availability of sorption sites was not reduced by their interactions. The CO2
Acknowledgements
This research was supported by the Scientific and Industry Endowment Fund in Australia (SIEF Grant ID RP02-035) and the Sasakawa Scientific Research Grant from The Japan Science Society (28-228). Specialist gas infrastructure was funded by the Australian Research Council (LE120100141) and by the Australian Government Education Investment fund and this support is also gratefully acknowledged.
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