Green Polyelectrolyte-Functionalization of Carbonaceous

Nov 28, 2016 - Carbonaceous nanospheres (CNSs) synthesized from hydrothermal carbonization of glucose were facilely modified with quaternary ...
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Research Article pubs.acs.org/journal/ascecg

Green Polyelectrolyte-Functionalization of Carbonaceous Nanospheres and Its Application in Ion Chromatography Qiming Zhao,† Shuchao Wu,‡ Peimin Zhang,† and Yan Zhu*,† †

Department of Chemistry, Xixi Campus, Zhejiang University, 148 Tianmu Road, West Lake District, Hangzhou, Zhejiang 310028, P. R. China ‡ Zhejiang Institute of Geology and Mineral Resources, 508 Stadium Road, West Lake District, Hangzhou, Zhejiang 310007, P. R. China S Supporting Information *

ABSTRACT: Carbonaceous nanospheres (CNSs) synthesized from hydrothermal carbonization of glucose were facilely modified with quaternary ammonium polyelectrolytes (QAPs) through a green and high-efficiency strategy, and they were successfully applied in anion-exchange chromatography (AEC). Methylamine and 1,4-butanediol diglycidyl ether were utilized as monomers to start the polymerization for constructing QAPs. The entire synthesis was achieved in water without damage to the monodispersity of CNSs. The water-dispersibility of CNSs was significantly increased after QAP-modification. The QAP-grafted CNSs (QAP-CNSs) were characterized by FTIR, XPS, SEM, TGA, and zeta potential measurement. The functionalization methodology was further extended to produce octadecylaminemodified CNSs, which demonstrated its versatility in preparation of functional carbons. For application in AEC, the QAP-CNSs were adhered on sulfonated poly(styrene-divinylbenzene) (PS-DVB) microspheres by electrostatic interaction. Using the PSDVB/QAP-CNS composite as anion-exchange phase and potassium hydroxide solution as eluent, seven common anions were efficiently separated with superior stability. KEYWORDS: hydrothermal carbonization, carbon materials, polyelectrolyte, surface modification, ion chromatography

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INTRODUCTION The hydrothermal carbonization (HTC) of biomass to fabricate carbon spheres (CSs) has captured broad attention owing to its sustainability, economy, and high-efficiency.1−6 The hydrothermal CSs have presented promising applications in multiple areas, such as catalysis,7,8 lithium-ion battery,9−11 and adsorbents.12,13,14 To achieve the desired properties for a specific use, chemical functionization of CSs was usually inevitable. There were two basic approaches: one-pot synthesis and postsynthesis modification.8 The synthesis of CSs with nitrogen-containing groups was achieved through one-pot hydrothermal method using nitrogen-rich reagents, such as ammonia,15−17 glucosamine,18,19 and ammonium sulfate20 as initiative products. Titirici and co-workers successfully incorporated water-soluble monomers in CSs during the HTC of glucose to obtain carboxylated CSs21 and imidazolefunctionalized CSs.22 A similar strategy was also employed to prepare sulfur-containing CSs via introducing L-cysteine or (S)(2-thienyl)-L-cysteine into the glucose HTC,23,24 and CSs with phosphorus-containing groups using phosphoric acid or tetraphenylphosphonium bromide as hydrothermal reactant.25,26 Compared with in situ functionalization, postmodification of the formed CSs provided a milder and controllable © 2016 American Chemical Society

functionalization process, where the functionality of added molecule could be preserved. By post-treatment of CSs with amine,27 cysteine,28 4-amino benzamide,29 or urea,30 nitrogenbearing CSs were facilely fabricated. The CSs grafted with salicylidene imine and 5-azacytosine were also prepared and showed excellent selective absorption toward uranium(VI) from aqueous solution.31,32 Liu et al. introduced functional double bonds onto the CSs’ surface via postmodification to improve CSs’ dispersibility and compatibility in the polymer matrix.33 The surface Diels−Alder reaction was also developed as a facile and effective methodology to synthesize CSs with functional groups.34,35 However, the traditional postmodification was usually performed in ungreen organic solvent.22,30,34−36 Furthermore, the reported functionization methods often had potential damage to the structure of CSs, which led to the formation of irregular and cross-linked carbonaceous clusters.20,23,24,37 To the best of our knowledge, grafting polyelectrolytes onto HTC material surfaces has seldom been researched. Poly(diallyl dimethylammonium chloride) was bonded as polyelectrolyte brushes on the surface Received: April 18, 2016 Revised: November 20, 2016 Published: November 28, 2016 112

DOI: 10.1021/acssuschemeng.6b00802 ACS Sustainable Chem. Eng. 2017, 5, 112−118

Research Article

ACS Sustainable Chemistry & Engineering of hydrothermal CSs to enhance their dispersibility in water.37 However, the approach utilized anhydrous toluene as solvent and required the addition of prepared 4,4′-azobis (4cyanovaleric acyl chloride) as initiators and triethylamine as catalyst, which was tedious and environmentally unfriendly. Furthermore, the amount of bonded polyelectrolyte was limited by the surface hydroxyl groups of hydrothermal CSs. Herein, we reported a green, high-efficiency, and tunable surface modification of HTC-derived carbonaceous nanospheres (CNSs) with quaternary ammonium polyelectrolytes (QAPs), and the successful application of QAP-grafted CNSs (QAP-CNSs) in anion-exchange chromatography (AEC). The functionization was conducted in water at 333 K without any initiator or catalyst, using methylamine and 1,4-butanediol diglycidyl ether (BDDGE) as reagents to activate the fasculation and epoxy-amine reactions. Based on the existing epoxy groups of QAP-CNSs, octadecylamine-functionalized CNSs were also facilely produced through the simple ringopening addition. Thus, this modification of HTC carbon via the use of methylamine, BDDGE, and subsequent functionalization reactions could be further extended as an attractive methodology to produce more novel carbon-based materials.

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EXPERIMENTAL SECTION

Instruments. The Fourier transform infrared (FTIR) characterization was achieved by a Bruker Vector 22 spectrometer (Bruker, German). The elemental and thermogravimetric analysis (TGA) were performed with a Flash EA 1112 elemental analyzer (Thermo Fisher Scientific, U.S.A.) and SDT-Q600 analyzer (TA, U.S.A.), respectively. The samples’ morphologies were observed using a HITACHI SU-8010 field-emission scanning electron microscope (SEM). The nitrogen adsorption analysis was conducted with a Micromeritics ASAP 2020 at 77 K. The X-ray photoelectron spectra (XPS) information was obtained by an ESCALAB_250Xi device with a Mg anode (1.2536 keV) as X-ray source. All chromatographic experiments were fulfilled through ICS-2000 system (Thermo Fisher Scientific, U.S.A.). Materials. Poly(styrene-divinylbenzene) (PS-DVB) microspheres were fabricated based on our previous literature.38 The monodisperse CNSs were produced via HTC of glucose.39 Methylamine (40 vol %) and BDDGE (60 vol %) were bought from Shanghai Aladdin Chemical Co., Ltd.. Glucose (99.0%), acetic acid (99.7%), sulfuric acids (98%), and dichloromethane (99.5%) were supported by Hangzhou Huipu Chemical Reagent Co., Ltd. Deionized water was generated by a GenPure ProUV/UF purifier (Thermo Fisher Scientific, U.S.A.). Synthesis of QAP-CNSs. The QAP-CNSs were prepared via a multistep condensation as displayed in Figure 1. Specifically, 0.4 g of CNSs was added to 50 mL of water solution of methylamine (4 vol %) and BDDGE (7.5 vol %) under magnetic stirring, and the mixture was treated with water bath at 333 K for 1 h. Surface carbonyl groups of hydrothermal CNSs could be reacted with BDDGE and methylamine to form the tertiary amine groups through ring cleavage reaction of BDDGE (Figure 1A).40 Thereafter, 50 mL of methylamine (4 vol %) was added to the above CNSs and treated with a water bath at 333 K for 1 h, and then 50 mL of BDDGE (10 vol %) was added to the obtained products and treated at 333 K for 1 h. In this process, the epoxy-amine addition reaction resulted in generating one QAP layer on CNSs’ surface (Figure 1B,C), and the terminal amines of QAP functioned as active sites for next epoxy-amine reaction (Figure 1C, D).38 Therefore, the CNSs modified with more QAP layers could be facilely gained by repeating the last epoxy-amine additions. The products in all above procedures should be filtered and washed by deionized water after reaction. Preparation of PS-DVB/QAP-CNS Anion Exchangers. The sulfonation of PS-DVB microspheres was carried out on the basis of reported work.36 Then, 20 mL of QAP-CNSs colloidal solution was added into the water suspension of sulfonated PS-DVB microspheres

Figure 1. Illustration of CNSs functionization strategy. and stirred for 4 h. QAP-CNSs were electrostatically coated onto the surface of PS-DVB microspheres. After that, the yellowish solid could be observed at the bottom of the flask. The formed PS-DVB/QAPCNS compound was applied as stationary phases for AEC. Procedure of Column Packing. Slurry of 2.7 g of PS-DVB/QAPCNSs powders in 45 mL of deionized water was sonicated for 5 min, and packed into the stainless steel column (150 × 4.6 mm i.d.). Deionized water (0.5 L) was used to press the above slurry into the column at 40 MPa. The prepared column was washed with 0.01 mol L−1 KOH at 0.2 mL min−1 for 40 h, and it was then connected to AEC for the following study.

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RESULTS AND DISCUSSION Characterizations of CNSs and QAP-CNSs. The CNSs functionalized with varied QAP layers were synthesized through a simple reaction strategy as illustrated in Figure 1. Figure 2a reflected that the hydrothermal CNSs were monodisperse with a mean diameter of 330 nm. The CNSs bonded with 10 QAP layers were still monodisperse without cross-linked carbonaceous clusters, and the particle size was significantly increased compared by that of raw CNSs (Figure 2b,c). The brown CNSs powder turned light yellow after QAP-functionalization (Figure 2d). The N2 adsorption measurements showed the surface areas of CNSs and QAP-CNS with 10 layers were less than 5 m2 g−1, which indicated that the produced CNSs and QAPCNSs had poor porosity. These facts illustrate that our modification strategy has little effect on the morphology and monodispersity of CNSs. The FTIR and XPS spectra of CNSs and QAP-CNSs are exhibited in Figure 3 and Figure 4, respectively. The characteristic peak at 3470 cm−1 resulted from the O−H stretching vibration (Figure 3), which was indicative of the existence of rich hydroxyl groups on both CNSs and QAPCNSs. The two peaks at 2920 and 2840 cm−1 were designated 113

DOI: 10.1021/acssuschemeng.6b00802 ACS Sustainable Chem. Eng. 2017, 5, 112−118

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Figure 2. SEM images of raw CNSs (a), CNSs modified with 10 QAP layers (b),(c), and their different macroscopic morphologies (d).

Figure 4. XPS analysis of QAP-CNSs with five layers.

Figure 3. FTIR spectra of CNSs and CNSs with five QAP layers.

contain rich quaternary ammonium groups and methylene groups. Owing to the weak thermo stability of most functionalized polymers on carbon materials, the TGA analysis could provide valuable information about polyelectrolytes bonded on CNSs. The TGA results of CNSs and CNSs with 3 layers and 10 layers of QAPs are displayed in Figure 5. The experiments were conducted under nitrogen atmosphere with temperature increasing speed of 10 K min−1 and the same amount of samples (10 mg). CNSs sample began to lose weight at about 563 K because of the decomposition of surface oxygencontaining groups such as −COOR and −OH (Figure 5a), and remained 66 wt % of the total until 773 K. As revealed in Figure 5b,c, the quite distinct curves from that of CNSs were received for QAP-CNSs. The lower weight-loss temperature (463 K) was attributed to the easier decomposition of QAPs. Apparently, QAP-CNSs with 10 layers lost considerably more

to the nonaromatic C−H stretch. The absorption peaks between 1750 and 1550 cm−1 could be ascribed to the CO and CC stretching vibrations. However, it should be noted that the absorption strength of carbon hydrogen bonds in QAPCNSs spectrum was markedly heightened, while the carbonyl bond stretch was significantly weakened in contrast to that of CNSs. Meanwhile, a distinctly different and sharp peak at 1116 cm−1 appeared in FTIR spectrum of QAP-CNSs, which was linked to C−N stretch on QAPs. To further confirm the formation of QAPs, XPS was also carried out to characterize the nitrogen-containing groups of QAP-CNSs (Figure 4). Figure 4 manifests the existence of two kinds: quaternary-N denoted by the peak of 402.1 eV, and secondary-N denoted by the peak of 399.2 eV. The quaternary-N amount was much more than that of secondary-N. These facts indicate that carbonyl groups are reacted with methylamine and BDDGE to form QAPs, which 114

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groups (Figure 6a). However, observation of the QAP-CNSs slurry with the naked eye only exhibited little solid deposit after 1 week, whereas the CNSs were totally precipitated (Figure 6b). The results indicate that QAP-CNSs have much better dispersing performance in water-based system than CNSs. The different stabilities of QAP-CNSs and CNSs suspensions are related to their surface potentials. Figure 7 presents the zeta

Figure 5. TGA results of raw CNSs (a), CNSs with 3 QAP layers (b), and 10 QAP layers (c).

weight than that with 3 layers during the TGA investigation. The above facts imply that our functionalization strategy can effectively graft QAPs on CNSs, and the QAP mass is augmented with grafted layer counts. The polyelectrolytes grafted on CNSs’ surface are nitrogenous, whereas raw CNSs do not have nitrogen at all. Therefore, the percentage of nitrogen in QAP-CNSs reflects the amount of QAPs grafted on CNSs. Elemental analysis presented that the N-content was 0.27%, 0.63%, 0.98%, 1.56%, and 1.84% for QAP-CNSs with 1, 3, 5, 10, and 15 layers, respectively. The N-content in QAP-CNSs was scaled up with QAP layer counts, whereas the increment speed was inferior to that of exponential increase as theory predicted. That could originate from high stereohindrance effect of ideal reactions in Figure 1. The results proved that the QAP-modification of CNSs was successfully performed and could be simply tuned by our strategy. Comparation of Water-Dispersibilities of QAP-CNSs and CNSs. The valid QAP-modification was also illustrated by the different water-dispersibilities of CNSs and QAP-CNSs. This parameter was investigated by treating same amount (2.5 g L−1 herein) of CNSs and QAP-CNSs with three layers in pure water for 15 min and comparing with the sedimentation of CNSs and QAP-CNSs after 7 days. Figure 6 demonstrates the different dispersedness of CNSs and QAP-CNSs water-based suspension after they were ultrasonicated and allowed to stand for 7 days. The CNSs could be well-dispersed like QAP-CNSs at first, owing to the surface hydrophilic −OH and −COOH

Figure 7. Zeta potentials of CNSs with different QAP layers in water.

potentials of CNSs modified with different QAP layers and CNSs in deionized water. The raw CNSs were negatively charged with the zeta potential of −15.1 eV, which could be attributed to the partial ionization of surface carboxyl groups. After the QAP-functionalizaion, obtained QAP-CNSs possessed positive charges accordingly, and their zeta potentials obviously increased with bonded layer counts, which contributed to their excellent water-dispersibility. The zeta potential reached 58.8 mV when three polyelectrolyte layers were grafted onto the surface of CNSs, which was much larger than that of raw CNSs, thus resulting in better dispersion in water. These facts reveal that CNSs with tunable amounts of positive charges can be facilely produced, and their dispersibility in water could be greatly improved through our modification methodology. Potential of the Modification Strategy in Preparation of Functional Carbons. Owing to the high reactivity of epoxy groups (or amine groups) at the terminal of QAP molecular, established QAP-CNSs could be further modified as required under mild conditions. To prove that, octadecylamine (ODA) was used as the substrate for the ring-opening addition (Figure S1). The elemental analysis (Table S1), FTIR spectrum (Figure

Figure 6. Water dispersions of CNSs and QAP-CNSs after ultrasonication (a) and 7 days of standing (b). 115

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Figure 8. Formation process of the PS-DVB/QAP-CNSs composites.

Figure 9. SEM characterizations of PS-DVB basements (a) and PS-DVB/QAP-CNSs composites (b).

QAP-CNS stationary phases, a mixed solution of seven common anions including fluoride, formate, chloride, nitrite, bromide, nitrate, and sulfate was tested with 0.01 mol L−1 hydroxide as eluent. Figure 10 reflects the chromatographic

S2), and toluene dispersibility (Figure S3) of the ODAfunctionalized CNSs (ODA-CNSs) compared to those of QAPCNSs or CNSs demonstrated the successful modification. After they modified with octadecylamine, the CNSs had much better dispersity in toluene, and the C−H stretching vibration in FTIR spectrum was significantly enhanced because of the rich methylene and methyl groups in octadecylamine. Meanwhile, the increase of C% and H% observed from ODA-CNSs supported the successful ring-opening addition. These confirm that our functionalization strategy can be expanded as a universal approach to sustainably prepare novel functional carbon materials. Application of QAP-CNSs in AEC. QAP-CNSs have abundant quaternary ammonium groups, which can function as anion-exchange positions and be applied in AEC. However, nanosized QAP-CNSs are too small to be chromatographic packing materials, and thus, sulfonated PS-DVB microspheres are chosen for the basements to establish PS-DVB/QAP-CNS composites. As manifested in Figure 8, the composite fabrication depends on the classic electrostatic attractiontriggered agglomeration between positive QAP-CNSs and negative PS-DVB basements. The coverage of five-layer QAPCNSs on PS-DVB could be observed by SEM images (Figure 9). Prepared PS-DVB microspheres are almost monodisperse with an average size of 7.5 μm, and the surface is smooth with only few attached small particles (Figure 9a). By contrast, Figure 9b manifests that PS-DVB/QAP-CNS materials have an obvious pellicular structure, and QAP-CNSs are tightly agglomerated on the surface of sulfonated PS-DVB basements. The obtained PS-DVB/QAP-CNS compound has a similar structure as the commercial agglomerated anion exchange phase.41 The PS-DVB basements are the core, and QAP-CNSs with five layers serve as the agglomerated ion-exchange layer. To evaluate the chromatographic performance of PS-DVB/

Figure 10. Separation of seven common anions on the PS-DVB/QAPCNSs phase. Eluent: 0.01 mol L−1 KOH; flow rate: 1.0 mL min−1; injection volume: 25 μL; conductivity detector.

separation of these anions on a PS-DVB/QAP-CNS column. All the anions were separated by baseline in 13 min. The chromatographic peaks were highly symmetrical without tailing appearance, and the reason could be attributed to the highly hydrophilic surface of QAP-CNSs with little nonspecific absorbance. The anion-exchange capacity was measured to be 0.049 mequiv/column based on the breakthrough curve of nitrite.38 The obtained column presented outstanding stability 116

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to hydroxide eluents, and the RSD of retention times was