A cationic porous organic polymer for high-capacity, fast, and selective capture of anionic pollutants
Graphical abstract
Introduction
In the past decades, on the basis of rational molecular design, tremendous porous materials with high surface area and accessible pore channels have been prepared [1], such as metal–organic frameworks (MOFs) [[2], [3], [4]], covalent organic frameworks [5], [6,7], conjugated microporous polymers [8,9], polymers of intrinsic microporosity [10], and porous aromatic frameworks (PAFs) [11,12]. These porous materials have been explored in a wide range of applications including sensing [3], gas adsorption or separation [13], and heterogeneous catalysis [14]. Only in the very recent years, some emerging studies suggest that the charged porous materials would bring about more advantages over the neutral analogues [15,16]. These advantages include that (1) charged skeleton of the porous materials can strengthen the host–guest repulsion or attraction that is beneficial for the applications in selective separation [17], (2) counter-ions of the charged skeleton can be easily exchanged for the adjustment of porosity [18], enhancement of adsorption behaviors [19], and introducing new functionalities (such as catalysis, ions conduction) [20,21]. Therefore, the scope of the applications of porous materials can be further extended [20,22,23].
In order to endow the porous materials with charged character, there are basically two strategies to achieve the goal. One is the post-synthesis modification (PSM) to the as-obtained porous materials. For instance, Ma and co-workers functionalized a series of PAFs with NR3+X− (XOH or Cl) groups via this strategy to achieve the removal of perrhenate ions or other toxic substance [22,24]. Chan and coworkers reported the incorporation of ionic polymers into MOFs to prepare the ionic MOFs as ion exchange materials [25]. However, the above-mentioned PSM strategy may suffer from the blocking of pore channels or only partial functionalization of the porous skeleton, which may weaken their application performance [16,26,27]. In addition, the complexity and high cost for preparing PAFs, and the stability concerns for MOFs may also limit the potential for their practical applications.
The other strategy is the direct polymerization of molecules with target-specific charged functional groups [16],28,29]. Currently, there are some pioneering studies on the direct polymerization of ionic monomers into charged porous materials. Early in 2009, Dai and co-workers reported a series of imidazolium-based networks through the trimerization of cationic monomers with nitrile groups, in the presence of ZnCl2 at very high temperature [30]. Wang and co-workers prepared a type of imidazolium-based porous polymer through Suzuki–Miyaura reaction using expensive palladium catalyst [31]. Yuan and co-workers reported that non-porous poly(ionic liquid)s can be crosslinked into porous networks by organic acids via simple ion exchange process. However, the as-obtained material is not stable due to its ionic bonds [32]. Therefore, from the perspective of methodology, a direct, facile, and catalyst-free polymerization method is still necessary to facilitate the development of charged porous materials [15,33].
Water pollution is a global concern, since a great amount of waste water is generated from the industrial activities. The waste water contamination includes charged compounds, such as organic dyes and toxic metal ions, which mostly are very toxic and put much threat to both environment and human health [34,35]. In the very recent years, porous organic polymers (POPs) show enormous potential for the water remediation application. Due to the designable and easy-modification merits, different POPs have been prepared by incorporating task-specific functional groups into the porous network. The as-prepared porous materials display powerful removal capability to the pollutants in aquatic systems [[36], [37], [38]]. Among the various removal methods, ion exchange is considered to be very efficient to capture those ionic types of pollutants [39]. However, most traditional ion exchange resins suffer from low capacity or low efficiency [22]. Ionic porous materials are capable of treating the ionic pollutants, however, the applications of the recently reported ionic porous organic polymers mainly focus on the carbon dioxide capture and heterogeneous catalysis [31,[40], [41], [42]], there is rare study on using these ionic materials for the removal of pollutants from water [24,43,44]. Imidazolium is a cationic functional group and it will therefore endow the POPs with charged character if it is incorporated into the porous networks, there are some imidazolium-based polymers proved to be very powerful in removing ionic pollutants [39,45].
Herein, we adopt a facile and direct polymerization method to prepare a cationic porous organic polymer. Firstly, a cationic molecule with aldehyde groups as polymerizable sites was designed and synthesized. The cationic monomers were then successfully crosslinked with benzidine into cationic porous organic polymers, via the facile Schiff base reaction without using any catalyst. Considering the cationic character of the porous material, we performed a series of experiments to verify its capability to remove anionic pollutants in aqueous solution. Impressively, the cationic porous organic polymer (ImPOP-1, Scheme 1) exhibits high adsorption capacity, excellent selectivity, and fast kinetics to remove anionic pollutants including organic dyes, such as orange G, and inorganic metal ions, such as PdCl42–.
Section snippets
Materials
N,N-Dimethylfomamide (DMF), toluene, dichloromethane, and methanol, sodium chloride, and sodium sulfate were purchased from Beijing Chemical Works, China. 1,3,5-Tribromobenzene (98%), 1-bromo-4-(bromomethyl)benzene (99%), diisobutylaluminum hydride (1.0 M in hexane), and benzidine (98%) were purchased from J&K Chemical, China. Methylene blue (MLB+) and methyl violet (MV+) from Beijing Chemical Reagent Company were both analytically pure. Amaranth (AMR3–) was commercially available from Ourchem
Results and discussion
A facile and catalyst-free synthesis method is always attractive to promote the development of charged porous organic materials. Inspired by the easy synthesis of porous materials via Schiff base reaction achieved by Müllen and coworkers [46], we applied this type of reaction to constructing ionic porous organic polymers. The general strategy to prepare ImPOP-1 is illustrated in Scheme 1. We started with designing a molecule that contains dual functional groups, the imidazolium part for the
Adsorption mechanism
In order to elucidate the sorption mechanism for anionic pollutants, we firstly conducted the SEM energy dispersive spectrometer (EDS) element mapping characterization. As seen from Fig. 7(a), the decrease in Br– anions and the appearance of the Pd featuring for PdCl42– and S featuring for AO7– suggest that the sorption process is dominantly caused by the ionic exchange that is driven by the electrostatic force. Furthermore, the solid-state NMR result shows that the peak featuring for
Conclusions
In summary, we have successfully designed and prepared an imidazolium-based cationic porous organic polymer, ImPOP-1, via the facile Schiff base reaction without use of any catalyst. ImPOP-1, which possesses porous and cationic character, is one of the rare examples that ionic porous organic polymers serves as the ion exchange materials for capturing ionic pollutants. It exhibits high adsorption capacity, superfast kinetics, and great selectivity towards the anionic pollutants, including
Conflict of interest
The authors declare no competing financial interest.
Acknowledgement
The financial support from the National Natural Science Foundation of China (Grant no. 21574032) is acknowledged.
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