Scalable preparation of monodisperse micron-sized carbon microspheres and their application in anion-exchange chromatography

Abstract

Polyacrylic acid sodium (PAAS), which is an anionic water-soluble polymer, is widely utilized as a thickener in the food industry and flocculant in water treatment owing to its high anion density, low cost and nontoxicity. Herein, we report for the first time the synthesis of monodisperse micron-grade carbon microspheres (CMSs) through the hydrothermal carbonization of sucrose with the assistance of trace PAAS, and their potential application as a green stationary phase for ion chromatography. An appropriate amount of PAAS efficiently inhibits the crosslinking of the formed CMSs, which results in monodispersity. The hydrothermal method was proven effective at a high sucrose concentration and for scalable experiments. The average size of the CMSs could be facilely regulated from 1.2 μm to 5.0 μm via hydrothermal time, temperature and sucrose concentration. The hydrothermal CMSs were further modified with quaternary ammonium groups based on the polycondensation of methylamine and 1,4-butanediol diglycidyl ether. Utilizing the quaternized CMSs as an ion chromatography stationary phase and potassium hydroxide solutions as the mobile phase, common inorganic anions were well separated with good symmetrical peak shapes and high stability.

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Introduction

Carbon materials have attracted considerable attention for decades due to their distinct physical and chemical properties.^1–4 Among them, carbon microspheres (CMSs) have been studied and broadly employed in catalyst carriers,^5,6 adsorbents^7,8 and energy storage.^9–11 Specifically, CMSs with a large size (≥3 μm) are also promising candidates for liquid chromatography stationary phases due to their spherical shape, pressure resistance and chemical stability.^12–14

Various techniques, such as chemical vapor deposition,¹⁵ templating method,^16,17 high pressure carbonization¹⁸ and reduction of carbides,¹⁹ have been developed to prepare CMSs. However, their high cost and low sustainability restrict the application of these methods in industrial fabrication. Recently, much attention has been paid to the hydrothermal carbonization (HTC) of carbohydrates, which is considered as an environmentally friendly and sustainable way to obtain carbonaceous spheres.^20–22 Since Wang et al. first reported the synthesis of CMSs through the HTC of sucrose,²³ many researchers have focused on the production and characterization of CMSs with a specific morphology via the HTC process.^24–29 However, HTC-derived CMSs are always cross-linked.^30–32 To obtain monodisperse and uniform CMSs, the HTC process is often conducted at a low sugar concentration,^33–37 which dramatically decreases the output of CMSs and restricts the sustainability of the reaction conditions. Furthermore, the reported monodisperse CMSs are usually submicro-sized,^4,21 which limits their application in some areas such as chromatography. To the best of our knowledge, no literature has been reported on the scalable production of monodisperse CMSs with a large particle size (≥3 μm) through the HTC process.

As a common polyelectrolyte, polyacrylic acid sodium (PAAS) is widely used as a dispersant or flocculant to tune the stability of particle suspensions.^38–40 It has good thermal tolerance and could be stable up to 300 °C.⁴⁰ Due to its high anion density, PAAS could stabilize metal ions and react with particulate matter to change their surface property.^38,41 Herein, monodisperse and micron-sized CMSs are successfully generated by the HTC of sucrose with the assistance of small amounts of PAAS. With the control of reaction conditions, such as sucrose concentration, reaction temperature and time, the particle diameters obtained range from 1 to 5 μm. The obtained CMSs were facilely quaternized, based on methylamine (MA) and 1,4-butanediol diglycidyl ether (BDDE) as a reactant to construct polymers containing quaternary ammonium groups. Owing to the anion-exchange capacity of quaternary ammonium groups, quaternized CMSs were used as a green stationary phase in IC for the first time.

Experimental

Instruments

Fourier transform infrared (FTIR) spectra were obtained on a Bruker Vector 22 spectrometer (Bruker, German). Elemental analysis was performed with a Flash EA 1112 elemental analyzer (Thermo Fisher, USA). Thermogravimetric analysis (TGA) was carried out on a DSCQ1000 differential scanning calorimeter (TA, USA). Scanning electron microscopy (SEM) images were obtained using a HITACHI S-4700 field emission scanning electron microscope (Hitachi, Japan). X-ray photoelectron spectra (XPS) were obtained on an ESCALAB_250Xi instrument using a magnesium anode (Mg 1253.6 eV) X-ray source. The particle size of CMSs was analyzed on a Nano Measurer 1.2. The column was packed by a pneumatic-pump K-1900 (Knauer, German).

All chromatographic tests were carried out on a Thermo Fisher Scientific (Waltham, MA, USA) ICS 2000, which includes a dual-piston serial pump, a column heater, and six-port valve fitted with a 25 μL sampling loop. The mobile phase was achieved by a KOH eluent generator. Suppression was achieved with a Thermo Fisher Scientific ASRS-4 mm suppressor in the recycle mode. Data were collected with a Chromeleon 6.80 chromatogram workstation (Thermo Fisher Scientific).

Reagents

Sucrose (99.5%), MA (40% in H₂O, v/v) and BDDE (60% in H₂O, v/v) were purchased from Aladdin Chemical Co., Ltd (Shanghai, China). Standard solutions containing 1000 mg L⁻¹ of common inorganic anions were prepared by dissolving appropriate amount of salt that contained ions of interest in 100 mL deionized water.

Preparation of micron-sized CMSs

In a typical procedure, 12 g of sucrose and 6 mg of PAAS were dissolved in 30 mL of deionized water. Then, the solution was transferred to a Teflon-lined autoclave, sealed and treated at desired temperature for a defined time. The obtained carbonaceous materials were washed with water and ethanol and then dried in an oven.

Synthesis of quaternized CMSs

Quaternized CMSs were prepared through a multistep reaction, which was inspired by our previous works.^42–44 A 60 mL mixture of MA (5%, v/v) and BDDE (7.5%, v/v) was added to 3.5 g CMSs with magnetic stirring, then the suspension was heated at 60 °C for 1 h, and later filtered and rinsed with deionized water. Then, 60 mL BDDE (10%, v/v) was added to the obtained products and heated at 60 °C for 1 h. Furthermore, 60 mL MA (4%, v/v) was added to the abovementioned CMSs and heated at 60 °C for 1 h. In each step, the black solid obtained was filtered and rinsed with deionized water. The final two steps were repeated four times to bond four layers of polymer on the CMSs. Thus, Q-CMSs were obtained.

Procedure of column packing

The columns were packed using an A QP 6000 packing pump (Chuang Xin Tong Heng Science and Technology Co., Ltd, Beijing, China). A slurry of 3.5 g Q-CMSs in 50 mL deionized water was sonicated for 10 minutes and packed into the stainless steel column (150 mm × 4.6 mm i.d.) by pressing with deionized water as the packing solvent under a working pressure of 40 MPa. The volume of the packing solvent passing through the column should be more than 400 mL. Then, the column was flushed with 10 mM KaOH at 0.2 mL min⁻¹ for at least 24 h before connecting to the IC instrument.

Results and discussion

Preparation of CMSs by PAAS-assisted HTC of sucrose

The different morphologies of the HTC products of sucrose with and without PAAS are shown in Fig. 1. In the traditional HTC of sucrose, a high temperature and concentration usually leads to irregular and highly crosslinked carbonaceous clusters, which was also proven in our experiments (Fig. 1a). However, the addition of PAAS to the HTC of sucrose could efficiently suppress the crosslinking of adjacent spheres, thus resulting in monodisperse micron-sized CMSs (Fig. 1c). The CMSs surface was rough with some ravines, as shown in the high-magnification image, which might be attributed to the heterogeneous absorption of PAAS by the formed carbonaceous spheres.

Fig. 1 SEM images of the products prepared from the HTC of sucrose without PAAS (a) and with 3 mg (b), 6 mg (c) and 12 mg (d) of PAAS at 200 °C.

The amount of PAAS in the sucrose solution plays a crucial role in controlling the morphology of the HTC products. Fig. 1b–d show the SEM images of the changed morphology with different amounts of PAAS. The obtained CMSs transformed from nonuniform and slightly crosslinked to monodisperse when the PAAS dosage was increased from 3 mg to 6 mg. However, a further increase in the PAAS amount to 12 mg caused non-uniformity. Therefore, 6 mg of PAAS was preferable in our reaction system.

The FTIR spectrum of the CMSs obtained by the HTC of sucrose with PAAS was similar to that without PAAS (Fig. 2). The broad peak at 3430 cm⁻¹, which corresponds to the stretching vibration of O–H groups, proves that there are rich O–H groups on the surface of CMSs.²⁵ The two absorption peaks at 2950 and 2890 cm⁻¹ are attributed to the stretching vibration of the aliphatic C–H.²⁴ The bands in the region between 1700 and 1600 cm⁻¹ are assigned to C=O and C=C stretching vibrations.³⁰ The region between 950 cm⁻¹ and 800 cm⁻¹ results from the bending vibration of C–H on the aromatic ring. The similarity of the two IR spectra implies that the PAAS-assisted HTC of sucrose undergoes the same reaction chemistry as the HTC without PAAS.

Fig. 2 FTIR spectra of traditional HTC carbon (a) and monodisperse CMSs (b).

Effect of HTC conditions on the CMSs morphology

Fig. 3 shows the CMSs morphology and diameter distribution under different reaction conditions, and the effect of HTC conditions on the average sizes and yields of the obtained CMSs is accordingly summarized in Table 1. Sucrose concentration, hydrothermal time and temperature were the main influence factors. The effect of the hydrothermal reaction temperature was studied by maintaining a constant hydrothermal reaction time (12 h) and carbohydrate concentration (12 g per 30 mL water). The average diameters of the CMSs were 2.0, 3.5 and 5.0 μm at 180 °C, 200 °C and 250 °C, respectively, and the CMSs yields were also increased from 29.8% to 42.3% with an increase in the hydrothermal temperature (entries 2, 4, 7, Table 1). A few slightly crosslinked CMSs and wider distribution of particle size were observed at 250 °C, as shown in Fig. 3g, which indicate that the hydrothermal reaction at 250 °C is faster than that at 180 °C or 200 °C.

Fig. 3 SEM images and size histograms of the carbon microspheres obtained by the HTC of sucrose in 30 mL of deionized water. (a) Sucrose (12 g) hydrothermally carbonized at 180 °C for 12 h. (b) Sucrose (6 g) hydrothermally carbonized at 200 °C for 8 h. (c) Sucrose (12 g) hydrothermally carbonized at 200 °C for 8 h. (d) Sucrose (12 g) hydrothermally carbonized at 200 °C for 12 h. (e) Sucrose (12 g) hydrothermally carbonized at 200 °C for 24 h. (f) Sucrose (20 g) hydrothermally carbonized at 200 °C for 8 h. (g) Sucrose (12 g) hydrothermally carbonized at 250 °C for 12 h.

Table 1 The effect of hydrothermal reaction conditions on the yield and size of the CMSs ^a

Entry	Temperature (°C)	Time (h)	Csuc (g)	Size (μm)	Yield^b (%)	Yield^c (%)
a Reaction conditions: 6 mg PAAS, 30 mL deionized water.b Yield is defined as: g of hydrochar per 100 g of sucrose.c Yield by HTC of sucrose without PAAS.
1	200	8	12	2.6	30.1	25.3
2	200	12	12	3.5	38.4	35.1
3	200	24	12	4.1	44.5	40.8
4	180	12	12	2.0	29.8	25.9
5	200	8	6	1.2	26.2	20.3
6	200	8	20	3.6	36.6	32.5
7	250	12	12	5.0	42.3	39.2

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For a constant hydrothermal treatment temperature of 200 °C and sucrose concentration (12 g per 30 mL water), the mean diameter of the CMSs significantly increased from 2.5 μm to 4.1 μm with the extension of the reaction from 8 h to 24 h (entries 1–3, Table 1), which conformed with the growth mechanism of the CMSs obtained by HTC. Moreover, the product yield also increased from 30.1% to 44.5% when the hydrothermal time was changed from 8 h to 24 h. These facts illustrate that prolonging the reaction time is an efficient way to increase the size of CMSs and improve product yields. The average sphere diameter increased from 1.2 μm to 2.5 μm and 3.6 μm, when the sucrose amounts in 30 mL water were improved from 6 g to 12 g and 20 g, respectively, with a fixed reaction temperature of 200 °C and reaction time of 8 h (entries 1, 5 and 6, Table 1). In most previous HTC of sucrose, monodispersity was only realized at either a low sucrose concentration or short reaction time, which gives rise to a low yield of carbon spheres.^{30,32,46–48} In our experiments, monodisperse micron-sized CMSs could be facilely formed at sucrose concentrations as high as 667 g L⁻¹ (entry 6, Table 1).

It is worth noting that the product yield of the PAAS-assisted HTC process was larger than that obtained by the traditional HTC methodology without PAAS under the same hydrothermal condition (Table 1). This could be attributed to the fact that PAAS inhibits the crosslinking of CMSs, which reduces side reactions and promotes the formation of more CMSs. Inspired by the abovementioned experiential results, a scaled experiment with 120 g of sucrose and 60 mg of PAAS for 8 h at 200 °C was also conducted, as shown in Fig. 4. Monodisperse CMSs were successfully produced with the average size of 2.4 μm, which indicates that the methodology is effective for the scalable preparation of monodisperse CMSs. Therefore, the PAAS-assisted HTC of sucrose could be a potential strategy for the production of micron-sized CMSs on a large scale.

Fig. 4 SEM image of the product synthesized with 120 g sucrose and 60 mg PAAS at 200 °C over 8 hours.

Suggested growth mechanism of CMSs obtained by PAAS-assisted HTC of sucrose

The formation of traditional HTC-based CMSs is basically consistent with the crystal growth theory, in which carbonaceous seeds are firstly generated and gradually grow to CMSs with hydrothermal time.^24,45 The abovementioned experimental results demonstrate that PAAS has a significant influence on the monodispersity of prepared CMSs (Fig. 1). The zeta potential and PAAS absorption of carbonaceous seeds were investigated to help understand the generation mechanism of monodisperse CMSs. Carbonaceous seeds were obtained by the traditional HTC of sucrose at 200 °C for 1 h. Colloidal solutions containing the same amount of carbonaceous seeds and different amounts PAAS were treated at 200 °C for 1 h, and then rapidly cooled with ice-water bath for zeta potential measurements. PAAS absorption was measured by the weight difference of the carbonaceous seeds before and after hydrothermal treatment.

Fig. 5a indicates that PAAS could be absorbed by the carbonaceous seeds, and the adsorption amount of PAAS increases with PAAS concentration. This indicates that the absorbed PAAS layer on the carbonaceous seeds becomes thicker with an increase in the amount of PAAS. Fig. 5b shows that the zeta potential value increases with the amount of PAAS, which demonstrates that the addition of more PAAS could introduce more negative charges onto the surface of the carbonaceous seeds. Based on these results, a plausible generation mechanism for monodisperse CMSs is illustrated in Fig. 6. When 6 mg PAAS was used, sucrose could facilely penetrate the PAAS layer and continue to react with the carbonaceous seeds. Moreover, the repulsion between electronegative carbonaceous seeds inhibited the crosslinking of adjacent particles. Therefore, the seeds could grow into monodisperse CMSs, which are presented in Fig. 6. In contrast, a large amount of PAAS such as 12 mg led to a thick PAAS layer, which hindered the smooth movement of sucrose to the seed surface. Accordingly, the growth of the carbonaceous spheres was suppressed, which led to the occurrence of new seeds. Since the dissociative PAAS amount is rather less than the initial amount in a solution owing to the absorption of previous carbonaceous seeds, these formed seeds could develop into bigger spheres. Therefore, the obtained CMSs were not monodisperse, as shown in Fig. 1d.

Fig. 5 Absorbed PAAS amounts (a) and zeta potentials (b) of carbonaceous seeds with different concentrations of PAAS.

Fig. 6 Suggested formation mechanism of monodisperse CMSs.

Characterizations of Q-CMSs and their application in ion chromatography

The hydrothermal CMSs with the average size of 5 μm, which were produced by the PAAS-assisted HTC of sucrose at 250 °C for 12 h (Fig. 3g), were functionalized with quaternary ammonium based on the polycondensation of MA and BDDE.^42,44 The element analysis results indicate that the nitrogen content of Q-CMSs is 0.99%, and the carbon content of Q-CMSs increased, whereas the hydrogen content decreased compared with CMSs (Table S1†). Fig. S1† shows the FTIR spectra of the raw CMSs and prepared Q-CNSs. Compared with the raw CMSs, the spectrum of the Q-CMSs shows a different and strong absorption peak at 1160 cm⁻¹, which could be assigned to the stretching vibration of C–N groups.⁴⁹ Moreover, the strength of the C–H peaks in the Q-CNSs spectrum is enhanced due to the rich methylene groups of the formed polyelectrolytes. XPS characterization confirmed that there are two nitrogen-containing species contained in Q-CMSs (Fig. S2†). The peak at 402.0 eV is assigned to quaternary ammonium groups, and the peak at 399.2 eV is related to secondary amine groups.^50,51 The quaternary-N makes up 68% of the total nitrogen in the N-containing groups. The TGA results of CMSs and Q-CMSs are shown in Fig. S3.† The weight of the CMSs sample started to decrease at about 290 °C due to the decomposition of the functional groups such as carboxylic acid and hydroxyl groups on the CMS's surface. A different and steep curve was observed in TGA results of Q-CMSs in the temperature range of 180–430 °C, which is attributed to the decomposition of polymers grafted on the Q-CMSs surface. The weight loss of Q-CMSs was much more than that of CMSs.

Ion chromatography (IC) is a common technique for the determination of various inorganic and organic ions.⁵² The separation performance of IC relies principally on its stationary phases. Owing to their stability under extreme pH conditions, organic polymers, such as polystyrene divinylbenzene (PS-DVB)⁵³ and ethylvinylbenzene divinylbenzene (EVB-DVB),⁵⁴ are the main separation materials of commercial IC. However, it is worth noting that traditional PS-DVB or EVB-DVB microspheres are usually prepared with the help of toxic initiators in organic solvents through tedious procedures and long reaction times.^44,55 In addition, the surface of raw PS-DVB or EVB-DVB microspheres are hydrophobic and contain rich phenyl groups, and the non-ionic interactions between anion exchangers and polarizable anions, such as nitrate and iodide, could have an adverse effect on the selective separation of these anions.^55,56 Herein, for the first time, highly hydrophilic hydrothermal CMSs modified with quaternary ammonium are used as green and low-cost stationary phases in ion chromatography. Similar to the working principle of commercial anion exchange phases in IC, the quaternary ammonium groups on the surface of the Q-CMSs play the role of anion exchange sites, since they could remain completely ionized under high pH conditions. For the ion chromatographic evaluation, a solution of five common anions, including fluoride, chloride, nitrite, bromide and nitrate, was tested with 18 mmol L⁻¹ hydroxide as the mobile phase. Fig. 7 shows the chromatographic separation performances of these anions on Q-CMSs columns. All the anions were well separated in 12 minutes. The highly hydrophilic surface of the Q-CMSs provided good symmetrical peak shapes for the tested anions, including polarizable anions such as nitrate and nitrate. The Q-CMSs columns presented excellent stability with hydroxide eluents, and the relative standard deviation of the retention times was <0.41% after more than 8 × 10⁴ column volumes. The abovementioned results reveal that the hydrothermal CMSs could be a potential stationary phase for IC due to their green preparation and good chromatographic performance.

Fig. 7 Separation of five common anions on a Q-CMSs stationary phase. Column dimensions: 4.6 mm i.d. by 150 mm; eluent: 18 mM KOH; flow rate: 1.0 mL min⁻¹; injection volume: 25 μL; and conductivity detector. Peaks: (1) fluoride, 2 mg L⁻¹; (2) chloride, 5 mg L⁻¹; (3) nitrite, 10 mg L⁻¹; (4) bromide, 10 mg L⁻¹; and (5) nitrate, 10 mg L⁻¹.

Conclusion

The production of monodisperse micron-sized CMSs through the PAAS-assisted HTC of sucrose was reported for the first time. The absorbed PAAS on the surface of the original carbonaceous seeds led to electrostatic repulsion between the seeds, thus restraining the crosslinking of the particles and monodisperse CMSs were obtained. The preparation methodology still worked well at a high sucrose concentration and for scaled experiments. The effects of PAAS amount, sucrose concentration, hydrothermal time, and temperature on the morphology of the CMSs were studied preliminarily. Furthermore, the quaternization of the prepared CMSs was facilely achieved, which depended on the condensation polymerization of MA and BDDE, and the quaternized CMSs were first applied in IC. By using KOH solution as the eluent, the Q-CMSs IC stationary phase showed a good separation performance for common inorganic anions with high stability. This work provides a sustainable and low-cost approach for monodisperse CMSs with a micron size, which can be produced on a large scale, and preliminarily presents the potential of hydrothermal CMSs in IC. Systematical research about the chromatographic performance of hydrothermal CMSs based ion chromatography stationary phases is in progress.

Acknowledgements

This research was supported by the National Science Foundation of China (No. 21405141), the National Important Project on Science Instrument (No. 2012YQ09022903), the Zhejiang Provincial Natural Science Foundation of China (No. LZ16B050001 LY15B050001, LY12B05003, LQ13B050001) and the Key Laboratory of Health Risk Appraisal for Trace Toxic Chemicals of Zhejiang Province.

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Footnote

† Electronic supplementary information (ESI) available: IR and XPS spectrum, element analysis and TGA results. See DOI: 10.1039/c6ra16939b

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