Tunable synthesis of the polar modified hyper-cross-linked resins and application to the adsorption

Abstract

The polar modified hyper-cross-linked resins were synthesized by the suspension polymerization and Friedel-Crafts alkylation reaction, and the effect of the feeding amount of the monomers (N-vinylimidazole and p-vinylbenzyl chloride) on the porosity and polarity were clarified. As the feeding amount of p-vinylbenzyl chloride decreased from 75% to 50% (w/w), the Brunauer-Emmett-Teller surface area of the resins lowered from 1306 to 436 m²/g, and the pore size distribution showed a large population of pores in micropores stretching to a higher percentage of mesopores. As the feeding amount of N-vinylimidazole increased from 5% to 30% (w/w), the nitrogen content of the resins rose from 0.005% to 3.06% (w/w), and the contact angle decreased from 97 ± 4.2° to 28 ± 2.0°. As a result, the porosity and polarity of the resins could be accurately tuned by adjusting the feeding amount of the monomers, and the different porosity and polarity of these resins endowed them with selective adsorption to benzoic acid and Rhodamine B.

Introduction

Because of their high Brunauer-Emmett-Teller (BET) surface area (∼1300 m²/g), predominant micropores and mesopores [1], [2], [3], [4], hyper-cross-linked resins have been attracting increasing attention in recent years. Particularly, they are proven to be the promising polymeric materials in gas storage [5], separation [6], [7], catalysis [8], and energy conversion [9], [10]. The special morphology for the synthesis of the hyper-cross-linked resins is a result of fixing the well swollen polystyrene chains with abundant rigid cross-linking bridges, which are generated by the Friedel-Crafts alkylation reaction [11]. Accordingly, the hyper-cross-linked resins possess narrow pore size distribution (PSD) [12], [13]. In addition, they can be synthesized by copolymerization of p-vinylbenzyl chloride (VBC) with divinylbenzene (DVB) [14], [15]. VBC has pendent chloromethyl groups, which can perform the Friedel-Crafts alkylation reaction directly, leading to the further cross-linking of the initial copolymer. However, the reactivity ratio of VBC is much different from styrene, and VBC is difficult to copolymerize with DVB, resulting in a low yield of the initial copolymer. If a third monomer such as N-vinylimidazole (VIM) can be introduced in the polymerization, the copolymerization between VIM, VBC, and DVB will be favorably carried out, and the initial copolymer will be obtained with a high yield.

On the other hand, due to the extreme hydrophobicity of the skeleton, chemical modification of the hyper-cross-linked resins is another important issue. For this purpose, Bayramoğlu et al. [16], Fu et al. [17], and Özer et al. [18] introduced the polar monomers such as methylacrylate (MA), vinylpyrrolidone (VP), and ethylene glycol dimethacrylate (EGDMA) in the polymerization, and they synthesized polar modified hyper-cross-linked resins. Masoumi et al. [19] used methylmethacrylate (MMA) as the polar monomer and prepared another polar modified hyper-cross-linked resin. Maya et al. [20], Ansari et al. [21], Krajnc et al. [22], and Huang et al. [23], [24], [25] introduced the amino, hydroxyl, amide, and carbonyl groups on the surface of the hyper-cross-linked resins. The results indicated that the polarity of the hyper-cross-linked resins was partially improved after the chemical modification, and the polar modified hyper-cross-linked resins showed higher adsorption to polar aromatic compounds. Moreover, VIM is proven to be an excellent polar monomer to make the initial copolymers polar [26], [27]. Fontanals et al. [26] synthesized VIM/DVB copolymers used VIM as the polar monomer, and Zhang et al. [27] prepared another macroporous poly (VIM-co-DVB). They found that these resins showed higher adsorption to sulfur in oil. However, the copolymers in Refs. [26], [27] are macroporous copolymers, the BET surface areas of these copolymers are relatively low (∼600 m²/g), particularly the porosity of the copolymers is uncontrollable. If VBC is incorporated in the polymerization, and the Friedel-Crafts alkylation reaction is performed after the copolymerization between VIM, VBC, and DVB, the benzyl chloride of VBC can be reacted with the neighboring phenyl groups, which are intensively cross-linked each other, and hence generated hyper-cross-linked resins with remarkable high BET surface area. More importantly, the porosity as well as the polarity of the hyper-cross-linked resins can be carefully tuned by the feeding amount of the monomers. However, to the best of our knowledge, few researches are reported in literature so far [28], [29], [30].

In this work, VIM and VBC were used as the monomers and DVB was applied as the cross-linking agents, and a series of polar modified hyper-cross-linked resins were synthesized by the suspension polymerization and Friedel-Crafts alkylation reaction. In addition, the effect of the feeding amount of the monomers (VIM and VBC) on the porosity and polarity of the resins were investigated in detail, and these resins were applied as the adsorbents for selective adsorption to benzoic acid and Rhodamine B from aqueous solution.