Hydrophobic–hydrophilic post-cross-linked polystyrene/poly (methyl acryloyl diethylenetriamine) interpenetrating polymer networks and its adsorption properties
Graphical abstract
The CMPS_pc/PMADETA IPNs had a relatively enhanced adsorption to p-hydroxybenzoic acid.
Introduction
Aromatic compounds (i.e. p-hydroxybenzoic acid, gallic acid, tannic acid, tyrosol, etc.) are widespread species in the industrial wastewater generated from food-processing, paint, pesticide, petroleum, and other chemical industries, and they are considered as prior pollutants because they are harmful to organisms even at low concentrations [1], [2], [3], and hence efficient removal of them from aqueous solution is of great importance. Various alternative methods and technologies including catalysis [4], membrane separation [5], oxidation [6], extraction [7] and adsorption [8], [9], [10] are available for removal of aromatic compounds, and among which adsorption is proven the most simple and efficient method and has attracted many attentions in recent years [11], [12], [13].
As compared with some common and novel adsorbents such as activated carbon, molecular sieving membrane, metal-doped microporous materials and nanoporous metal organic frameworks (MOFs) [14], [15], [16], synthetic polymeric adsorbents are increasingly employed for efficient removal and recovery of aromatic compounds from wastewater due to their stable physicochemical structure, diverse chemical structure, controllable pore structure, feasible regeneration property and a high priority to the molecular structure of the adsorbate [17], [18], [19]. Interpenetrating polymer networks (IPNs) are composed of two or several chemically independent cross-linked polymers, and they present an important family of polymeric materials [20]. During the past three decades, the IPNs are one of the most efficient polymeric materials applied in reinforced rubbers, toughened plastics, damping materials, coatings and functional materials because of its unique forced compatibility [21], [22]. Moreover, the IPNs technology is proven to be an outstanding method for stable integration of two polymer networks with different properties or functions by physical entanglements [23], [24], [25]. However, the Brunauer–Emmett–Teller (BET) surface area of the IPNs is low, resulting in a low equilibrium adsorption capacity toward aromatic compounds. If post-cross-linking technology can be introduced for one network of the IPNs, the obtained IPNs would give a higher adsorption capacity because of the higher BET surface area after the reaction. In a previous paper, we prepared post-cross-linked polystyrene/polyacryldiethylenetriamine (PST_pc/PADETA) IPNs by performing a Friedel–Crafts reaction for the first highly cross-linked polystyrene [12]. The residual pendent vinyl groups in the first highly cross-linked polystyrene were further cross-linked using the Friedel–Crafts catalysts, and the BET surface area of the obtained IPNs increased, inducing a greater adsorption toward salicylic acid (equilibrium adsorption capacity: 230.7 mg/g). However, the quantity of the residual pendent vinyl groups was much less than that of the benzenyl chloride of chloromethylated polystyrene (CMPS), and the increased BET surface area of the obtained PST_pc/PADETA was not obvious.
In 1970s, Davankov et al. [26] synthesized a kind of post-cross-linked polystyrene by introducing a post-cross-linking technology, which leads to a creation of novel polymeric materials with extraordinary structure and remarkable properties [27], [28], [29], [30], [31], [32], [33]. In general, the post-cross-linked polystyrene is prepared from a linear polystyrene or a low cross-linked polystyrene by adding some bi-functional cross-linking reagents such as 1, 4-bis-(chloromethyl)-diphenyl (CMDP), p-xylylene dichloride (XDC), 1, 4-bis-(p-chloromethylphenyl)-butane (DPB) and 1, 3, 5-tris-(chloromethyl)mesitylene (CMM) and the Friedel–Crafts catalysts such as AlCl3, FeCl3 or SnCl4 [32], [33], [34]. The post-cross-linked polystyrene can also be prepared from a CMPS via intramolecular Friedel–Crafts reaction [35]. After the Friedel–Crafts reaction, many intensive bridges with conformationally rigid links are formed, leading to a major shift of the pore diameter distribution from predominately mesopores to mesopores–micropores bimodal distribution, and hence bringing about a sharp increase of the BET surface area [36]. The post-cross-linked polystyrene is proven to be an efficient polymeric adsorbent for adsorptive removal of aromatic compounds from aqueous solution [33], [34], [35], [36]. In 1988, Ando et al. [37] prepared post-cross-linked polystyrene without adding any bifunctional cross-linking reagents. They found that a considerable number of residual pendent vinyl groups were located in the dense cores in the highly cross-linked polystyrene, and these residual pendent vinyl groups could be further cross-linked with the neighboring phenyl groups by the Friedel–Crafts reaction [5], [31], [38], [39], [40]. However, the quantity of the residual pendent vinyl groups of the highly cross-linked polystyrene was relatively low, and hence the increased BET surface area of the obtained post-cross-linked polystyrene was unconspicuous.
In the present study, we developed an efficient approach for increasing the equilibrium adsorption capacity of the IPNs toward polar aromatic compounds. For this purpose, CMPS was used as the raw material and poly (glycidyl methacrylate) (PGMA) was interpenetrated in the macropores of CMPS, and the precursor CMPS/PGMA IPNs was obtained by a typical sequential IPNs technology. Then a typical Friedel–Crafts alkylation reaction was performed for CMPS/PGMA IPNs, the benzyl chloride groups of the first networks CMPS were further cross-linked with the phenyl groups, and the obtained post-cross-linked polystyrene/poly (glycidyl methacrylate) (CMPS_pc/PGMA) IPNs could lead to a much higher BET surface area than its precursor. Afterwards, an amination reaction was carried out for CMPS_pc/PGMA IPNs, and some polar amide and amino groups can be introduced on the surface of the second networks PGMA, and polystyrene/poly (methyl acryloyl diethylenetriamine) (CMPS_pc/PMADETA) IPNs could be prepared accordingly. CMPS_pc/PMADETA IPNs contain hydrophobic networks CMPS_pc as well as hydrophilic PMADETA, and hence it should possess a much larger equilibrium adsorption capacity toward polar aromatic compounds like p-hydroxybenzoic acid.
Section snippets
Materials
CMPS was purchased from Langfang Chemical Co. Ltd. (Hebei, China, the chlorine content was 17.3% (w/w)). GMA was purchased from Gray West Chengdu Chemical Co. Ltd., and the industrial triallylisocyanurate (TAIC) was obtained from Liuyang Chemical Co. Ltd. Benzoyl peroxide (BPO) was refined by recrystallization before use. Butyl acetate, n-heptane, 1, 2-dichloroethane (DCE), anhydrous iron (III) chloride and diethylenetriamine (DETA) were obtained from Yongda Chemical reagents Company and they
Physical properties
The FT-IR spectra of CMPS, CMPS/PGMA IPNs, CMPS_pc/PGMA IPNs and CMPS_pc/PMADETA IPNs are shown in Fig. 1. It is obvious that all of the typical absorption bands related to CMPS (the absorption bands at 1606 and 1450 cm−1, which can be assigned to CC stretching of the benzene ring [40], [41], and the strong CCl stretching vibration of the benzyl chloride at 1261 cm−1) are existent in the FT-IR spectrum of CMPS/PGMA IPNs. Moreover, a strong absorption band with frequency at 1726 cm−1 is also shown
Conclusions
Hydrophobic-hydrophilic CMPS_pc/PMADETA IPNs were synthesized by interpenetration of PGMA in the pores of CMPS, followed by a Friedel–Crafts reaction and an amination reaction. The prepared IPNs had a relatively higher BET surface area and moderate polarity, which endowed them with efficient adsorption toward p-hydroxybenzoic acid from aqueous solution. The equilibrium adsorption data were correlated better by Sips model than the Langmuir and Freundlich models. Furthermore, the adsorption was a
Acknowledgments
The authors are gratefully acknowledged the National Natural Science Foundation of China (No. 21174163, 21376275 and 21446016), a Chinese-Croatian bilateral project and South Wisdom Valley Innovative Research Team Program for the financial support.
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