CO2 separation using surface-functionalized SiO2 nanoparticles incorporated ultra-thin film composite mixed matrix membranes for post-combustion carbon capture
Graphical abstract
Introduction
The enormous CO2 emissions induced by human activities, including the combustion of fossil fuels, have been recognized as one of main contributors to climate change, with the atmospheric CO2 concentration increasing by 40% since the industrial revolution [1]. To address this global problem, carbon dioxide capture and storage (CCS) technologies including oxy-combustion, pre-combustion and post-combustion carbon capture have been developed [2]. Post-combustion capture in particular, has shown significant near-term potential for reducing CO2 emissions through retrofits to existing coal fired power plants, as these account for 40% of the total CO2 released to the atmosphere [3]. The use of membrane technology for this application shows significant potential [4]. However, the low CO2 partial pressure (typically 13% CO2 at 1 atm) and massive flue gas volumes (11,000 t CO2/day) means that large compression energies and large membrane surface areas will be required [4].
The use of high performance polymer membranes for CO2 separation increase the capacity to achieve economic carbon capture without loss of product purity [5]. However, more permeable polymeric materials generally show lower gas selectivity and vice versa. This results in an upper bound to performance, usually presented within a log-log plot between gas permeability in Barrer (1 Barrer = 10−10 cm3 (STP) cm−2 s−1 cmHg−1) and selectivity.[6] Nevertheless, ultimately it is the permeance, expressed in GPU (1 GPU = 10−6 cm3 (STP) cm−2 s−1 cmHg−1), which dictates the membrane performance. Cost-efficient CO2 capture requires a CO2 permeance over 1000 GPU and a CO2/N2 selectivity over 20 [4]. Permeance is a product of permeability and membrane thickness and thus can be increased by either developing high permeability polymeric materials or by reducing the thickness of the selective layer without defects [7], [8], [9], [10], [11].
Thin film composite (TFC) membranes consist of a thin selective layer (less than 0.5 µm), supported by a porous substrate to provide mechanical strength [12]. The use of a highly gas permeable gutter layer between the selective layer and the porous substrate has also been widely adopted. This prevents the penetration of the selective layer into the open structure of porous substrate, leading to a reduction in pinhole defects when the thin selective coating layer is applied [13]. At present though, it is challenging to fabricate an ultra-thin film composite (UTFC) membranes with a selective layer less than 100 nm. Such membranes would allow not only improved gas permeance due to the diminished membrane resistance but also cut the membrane fabrication cost and required surface area [13], [14].
Defect-free TFC membranes can be prepared by several methods such as dip-coating, spin-coating and interfacial polymerization [15], [16], [17]. In contrast to these conventional coating methods, surface-initiated polymerization is an emerging technology with a broad range of applications including biomaterials, nanofiltration and gas separation [18], [19], [20]. The recent continuous assembly of polymers (CAP) approach has been developed to allow the preparation of such films from tailor-made polymers with a range of functionalities [21]. The CAP approach uses end-functionalized polymers to form a cross-linked film grown only from an initiating surface. It enables film fabrication form the surface of diverse substrate such as planar substrates, hollow capsules and nanoparticles [22].
To provide high gas permeability, inorganic fillers have been incorporated as a dispersed phase within the polymer matrix to form mixed matrix membranes (MMMs) [23]. MMMs have shown their potential to overcome the trade-off limitation of polymeric membranes by combining the high performance of inorganic fillers and the easy processability of polymer membranes [24]. However, most MMMs are dense membranes with thickness more than 10 µm due to the difficulty in preparing fillers of this dimension and in avoiding their aggregation during membrane manufacture [25], [26], [27]. Moreover, poor adhesion of the inorganic fillers to the polymer matrix can produce non-selective voids at the polymer-filler interface, resulting in reduced CO2 separation performance [28].
In our previous work, we reported the first example of nano-engineered PEG-based ultra-thin film composite membranes via the CAP process [29]. The defect-free cross-linked PEG-based thin selective layers showed the potential to achieve high CO2 permeance as well as good gas selectivity over the mechanically and chemically stable ultra-thin CAP films. Building on this earlier work, in this study, we add SiO2 nanopaticles in the top layer of the CAP UTFC membrane. A SiO2 nanoparticle size of ~10 nm, less than the thickness of a PEG-based selective layer (~ 55 nm) was selected. In order to adjust the interface interaction between the SiO2 NP dispersed phase and the PEG continuous phase, we develop nano-film coatings of polyethyleneimine (PEI), polydopamine (PDA) and their mixtures onto the porous SiO2 nanoparticles. Then, novel UTFC-MMMs using ATRP-mediated CAP approach were prepared through incorporation of the surface-functionalized SiO2 nanoparticles (SFSNPs) within the PEG matrix of a selective layer. The surface-confined and cross-linked selective layer is less than 100 nm thick and the surface functionalized SiO2 nanoparticles are incorporated well within the PEG-based selective layer. The CO2 separation performance of the composite membranes was controlled by tuning the morphology of the SiO2 nanoparticle interface with the different PEI/PDA ratios.
Section snippets
Materials
α-bromoisobutyryl bromide (BIBB, 98%), poly(ethylene glycol) dimethacrylate (PEGDMA9, Mn = 550 Da), 1,3,5-benzenetricarbonyl trichloride (TMC, 98%), dopamine hydrochloride (DA), polyethyleneimine (PEI, oligomer mixture, Mn = 423 Da), silica nanoparticle (SiO2 NP, spherical, porous, 5–15 nm particle size), tris(hydroxyethyl)aminomethane (tris base, ≥ 99.8%), sodium ascorbate (NaAsc, ≥98%), tris(2-aminoethyl)amine (96%), formic acid (98%), formaldehyde (25%) and ammonium hydroxide (NH4OH, 28–30%)
Results and discussion
In this study, the CAP-ATRP approach was used to prepare PEG-based ultra-thin film composite mixed matrix membranes (UTFC-MMMs) incorporating surface-functionalized SiO2 nanoparticles (SFSNPs). There are three steps to this process, as illustrated in Scheme 1. At first, a cross-linked PDMS gutter layer was prepared via spin-coating of a solution of amino-terminated poly(dimethylsiloxane) (PDMS1) and 1,3,5-benzenetricarbonyl trichloride (TMC) onto a porous polyacrylonitrile (PAN) substrate (Step
Conclusion
Thin film composite mixed matrix membranes with a PEG-based ultra-thin selective layer (thickness ~55 nm) incorporating surface-functionalized SiO2 nanoparticles were developed via the CAP nano-coating technology. The CO2 separation properties of the resulting UTFC-MMMs varied as a result of differing adhesion between the nanoparticles and the PEG matrix. Specifically, functionalization of the nanoparticles with PEI rich coatings led to better adhesion with the polymer matrix due to increased
Acknowledgement
The authors wish to acknowledge the funding provided by the Australian Government through the CRC program to support this research. The authors also acknowledge the Australian Research Council under the Future Fellowship (FT110100411, G.G.Q.). Q. Fu is the recipient of an Australian Research Council Super Science Fellowship (FS110200025) and the Australia-China Emerging Future Leaders in Low Emissions Coal Technology Fellowship.
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