Comparison of hyper-cross-linked polystyrene/polyacryldiethylenetriamine (HCP/PADETA) interpenetrating polymer networks (IPNs) with hyper-cross-linked polystyrene (HCP): structure, adsorption and separation properties

Abstract

Novel hyper-cross-linked polystyrene/polyacryldiethylenetriamine (HCP/PADETA) interpenetrating polymer networks (IPNs) were prepared, characterized and evaluated for their adsorption and separation properties using hyper-cross-linked polystyrene (HCP) as the reference. In spite of its much lower Brunauer–Emmett–Teller surface area and less pore volume, HCP/PADETA IPNs possessed a preferable pore structure and appropriate polarity, resulting in a larger equilibrium capacity, faster adsorption rate, higher dynamic breakthrough and saturated capacity towards salicylic acid than HCP. In particular, HCP/PADETA IPNs had a higher enrichment factor for salicylic acid over phenol, leading to the high-efficiency separation of salicylic acid from phenol in their mixed solution.

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1. Introduction

In 1969, Davankov proposed a novel approach for producing a novel polymer called “hyper-cross-linked polystyrene (HCP)”.¹ HCP is recognized by its high Brunauer–Emmett–Teller (BET) surface area, predominant micro/mesopores and unprecedented adsorption property, and therefore it is considered as the most efficient polymeric adsorbent for the adsorptive removal of non-polar and weakly polar aromatic compounds.^2,3 HCP has also attracted considerable attention as an excellent adsorbing material for high-performance liquid chromatography (HPLC), size-exclusion chromatography and solid-phase extraction.^4–7 HCP is synthesized frequently from linear polystyrene (PS) or low cross-linked poly(styrene-co-divinylbenzene) using monochloromethylether (MCME), p-dibenzenylchloride (DBC) or p-dichloromethylbenzene (DCMB) as a post-cross-linking reagent via the typical Friedel–Crafts reaction, and the Friedel–Crafts catalysts include anhydrous zinc chloride and iron(III) chloride and stannic (IV) chloride.^1,3 In addition, it can also be prepared from macroporous low cross-linked chloromethylated polystyrene (CMPS) by consuming its own benzyl chloride on the surface.^8–10

Owing to its hydrophobic surface, HCP possesses low adsorption towards polar aromatic compounds.¹¹ To improve the surface hydrophilicity and increase the adsorption capacity of HCP towards polar aromatic compounds, HCP is usually modified by introducing polar monomers into the copolymers using polar compounds as the cross-linking reagent and adding polar compounds in the Friedel–Crafts reaction.^2,9 On the other hand, due to the randomness of the Friedel–Crafts reaction, an excessive cross-linking for the benzyl chloride of CMPS is inevitable at the beginning of the reaction. The cross-linking bridges formed at the initial stages of the reaction are much more rigid than those formed later; therefore, the pore structure of HCP is not uniform. The formed rigid region is called “the magic area”.^12,13 The magic area of HCP is significant for the adsorption, storage and separation of gases, such as N₂, H₂, CO₂ and CH₄,^14,15 whereas it is useless for the adsorption of aromatic compounds because the limitation of the molecular size of aromatic compounds makes it difficult to diffuse into the narrow pores of HCP.^12,16,17 Avoiding the excessive cross-linking for the benzyl chloride of CMPS during the Friedel–Crafts reaction may lead to an improvement of the pore structure of HCP and the adsorption capacity towards aromatic compounds.

The excessive cross-linking for the benzyl chloride of CMPS will be relieved as inserting another polymer networks in the pores of CMPS via a typical interpenetrating polymer networks (IPNs) technology. The introduced polymer networks in the IPNs will separate the benzyl chloride of CMPS effectively, leading to a preferable pore structure over HCP after the Friedel–Craft reaction. Therefore, the preferable pore structure of the modified HCP will bring a favorable adsorption performance towards aromatic compounds. In particular, this study focused on improving the pore structure of HCP and used the novel HCP obtained for adsorption and separation of salicylic acid from aqueous solutions. For this purpose, methylacrylate (MA) was polymerized in situ in the pores of CMPS with the help of the initiator and chloromethylated polystyrene/poly(methylacrylate) (CMPS/PMA) IPNs were prepared by a typical IPNs technology. The Friedel–Crafts reaction was then employed for CMPS networks of the IPNs, followed by an amination reaction for the PMA networks, resulting in the synthesis of a novel hyper-cross-linked polystyrene/polyacryldiethylenetriamine (HCP/PADETA) IPNs. After characterization of the HCP/PADETA IPNs, the adsorption and separation performance of the novel hyper-cross-linked IPNs was investigated from an aqueous solution in detail using salicylic acid and phenol as the adsorbates.

2. Materials and methods

2.1. Materials

CMPS was purchased from Langfang Chemical Co. Ltd. (Hebei province, China), its degree of cross-linking was 6%, chlorine content was 17.3% (w/w), BET surface area was 13.1 m² g⁻¹, and average pore size was 25.2 nm. Methyl acrylate (MA) was supplied by Gray West Chengdu Chemical Co. Ltd. (Sichuan Province, China), and washed with 5% of NaOH (w/v), followed by de-ionized water, and then dried with anhydrous magnesium sulfate prior to use. Benzoyl peroxide (BPO) was obtained from Fuchen Chemical Reagents Factory (Tianjin, China) and methanol was used as the solvent to recrystallize BPO. Butyl acetate, triallylisocyanurate (TAIC), n-heptane, 1,2-dichloroethane (DCE), anhydrous ferric(III) chloride and diethylenetriamine (DETA) were analytical grade reagents purchased from Yongda Chemical Company (Shandong province, China), and used without further purification. Salicylic acid and phenol as the adsorbates were analytical grade reagents obtained from Xiya reagent Chemical Company (Sichuan province, China) and used as received.

2.2. Preparation of HCP/PADETA IPNs

As shown in Scheme 1, three continuous processes, interpenetration, Friedel–Crafts reaction and amination reaction, were applied for HCP/PADETA IPNs. According to the sequential interpenetration method in ref. 18 and 19, 20 g CMPS was first swollen by a mixture of MA, TAIC, butyl acetate, n-heptane and BPO for 12 h. Subsequently, 18 g MA was used as the monomer and 2 g TAIC was applied as the cross-linking reagent. Butyl acetate and n-heptane were used as the porogens and they were 250% relative to CMPS (w/w); the mass ratio of butyl acetate to n-heptane was set to 4 : 1. The swollen CMPS was then filtered and added to 200 mL 0.05% of a polyvinyl alcohol (PVA) aqueous solution (w/v). At a moderate stirring speed, the temperature of the mixture increased to 358 K and maintained at this temperature for 12 h. MA was polymerized in situ in the pores of CMPS, resulting in CMPS/PMA IPNs. CMPS/PMA IPNs was further cross-linked by the Friedel–Crafts reaction,^8,9 which was the same as the synthetic procedure for HCP from CMPS. The first CMPS networks of the IPNs were post-cross-linked and HCP/PMA IPNs were produced accordingly. Thereafter, HCP/PMA IPNs was aminated by excess DETA at 393 K for 12 h, and amide and amino groups on the surface chemically modified the second PMA networks of the IPNs and HCP/PADETA IPNs were synthesized accordingly.

Scheme 1 Synthetic procedure of HCP/PADETA IPNs.

2.3. Characterization

The spectra of the IPNs were obtained using Fourier transform infrared (FT-IR) spectroscopy carried out on a Nicolet 510P Fourier transform infrared instrument in the region of 500–4000 cm⁻¹ with a resolution of 1.0 cm⁻¹. The chlorine content of the IPNs was measured by the Volhard method²⁰ and the weak basic exchange capacity of the IPNs was determined according to ref. 21. The surface morphologies of the IPNs were observed by scanning electron microscopy (SEM, FEI Nova Nano SEM 230, 10 kV). The pore structure parameters of the IPNs were determined by N₂ adsorption isotherms at 77 K using a Micromeritics Tristar 3000 surface area and porosity analyzer, and the BET surface area, t-plot surface area, pore volume, t-plot pore volume and pore size distribution were obtained. The BET surface area and pore volume were calculated according to the BET model, while the t-plot surface area, t-plot pore volume and the pore size distribution were calculated by applying the Barrett–Joyner–Halenda (BJH) method to the N₂ desorption data.

2.4. Adsorption and separation experiments

For the equilibrium adsorption, about 0.1 g of the IPNs was mixed with 50 mL of a series of salicylic acid aqueous solution with a concentration of about 200, 400, 600, 800 and 1000 mg L⁻¹ (w/v). The series of mixtures were shaken in a thermostatic oscillator at the desired temperature (298, 308 or 318 K) until the equilibrium was reached. The absorbance of salicylic acid was analyzed using a Shimadzu UV-2450 spectrophotometer at a wavelength of 296.5 nm and the equilibrium adsorption capacity of salicylic acid on the IPNs, q_e (mg g⁻¹), was calculated based on the equation as follows:

q_e = (C₀ − C_e)V/W (1)

where C₀ is the initial concentration of salicylic acid (mg L⁻¹), C_e is the equilibrium concentration of salicylic acid (mg L⁻¹), V is the volume of the solution (L) and W is the mass of the IPNs (g). The equilibrium adsorption capacity of phenol on the IPNs was determined via an UV-2450 spectrophotometer at a wavelength of 269.0 nm using the similar method for salicylic acid.

For the kinetic adsorption, about 1.0 g of the IPNs was mixed with 250 mL of a salicylic acid solution at the initial concentration of 604.3, 804.3 or 1003.8 mg L⁻¹. The flasks were then continuously shaken until the adsorption equilibrium was reached. In this process, 0.50 mL of the solution was withdrawn at set intervals and the concentration of salicylic acid was determined, the adsorption capacity at a contact time t was calculated using the equation as follows:

q_t = (C₀ − C_t)V/W (2)

where q_t (mg g⁻¹) and C_t represent the adsorption capacity and the concentration at contact time t (mg L⁻¹), respectively.

For the column adsorption and desorption, 10 mL of the wetted IPNs was packed densely in a glass column with an inner size of 16 mm. Salicylic acid aqueous solution at an initial concentration of 1017.3 mg L⁻¹ was passed through the resin column at a flow rate of 1.0 mL min⁻¹. The concentration of salicylic acid in the effluent from the column exit, C (mg L⁻¹), was recorded continuously until it approached the initial concentration. After column adsorption, the given desorption solvent passed through the resin column at a flow rate of 0.6 mL min⁻¹, the concentration of salicylic acid in the effluent was determined until it was close to zero.

For the dynamic separation experiment, a mixed solution containing 513.1 mg L⁻¹ of salicylic acid and 489.5 mg L⁻¹ of phenol was passed through the wetted IPNs (10 mL) at a flow rate of 1.5 mL min⁻¹, and the concentration of salicylic acid and phenol in the effluent from the column exit was recorded continuously until it nearly reached the initial concentration. The desorption solvent was used for the desorption of IPNs and the concentration of salicylic acid and phenol was determined.

3. Results and discussion

3.1. Characterization

After the interpenetration of PMA in the pores of CMPS, two strong vibrations at 1736 and 1701 cm⁻¹ (ref. 22–24) were observed in the FT-IR spectrum of CMPS/PMA IPNs (Fig. 1). These two peaks were assigned to the C=O stretching of the ester carbonyl groups of MA and the amide carbonyl groups of TAIC, respectively. After the Friedel–Crafts reaction, the chlorine content decreased to 3.2%, two strong vibrations of the –CH₂Cl groups at 1263 and 675 cm⁻¹ were weakened greatly, whereas the other vibrations were similar with HCP. Li et al.,²⁵ Pan et al.²⁶ and Huang et al.^27,28 reported similar results. After amination, the vibration at 1736 cm⁻¹ was reduced sharply, whereas a new strong band appeared at 1650 cm⁻¹. This band was assigned to the C=O stretching of the amide carbonyl groups.²⁹ In particular, the weak basic exchange capacity of HCP/PADETA IPNs was 2.107 mmol g⁻¹. These results suggest that the interpenetration, Friedel–Crafts reaction and amination reaction were carried out successfully and HCP/PADETA IPNs were synthesized accordingly. Fig. S1† shows SEM images of CMPS, CMPS/PMA IPNs, HCP/PMA IPNs and HCP/PADETA IPNs. Interestingly, the surface of CMPS/PMA IPNs and HCP/PMA IPNs was much smoother than that of the CMPS, while the surface of HCP/PADETA IPNs was rougher than that of HCP/PMA IPNs, which is in accordance with the reaction processes.

Fig. 1 FT-IR spectra of CMPS, CMPS/PMA IPNs, HCP/PMA IPNs, HCP/PADETA IPNs and HCP.

The BET surface area and pore volume decreased after the interpenetration of PMA in the pores of CMPS (Table 1), while they increased significantly after the Friedel–Crafts reaction, Ahn et al.,⁷ Fontanals et al.³⁰ and Urban et al.³¹ reported similar results. In addition, the BET surface area and pore volume of HCP/PMA IPNs were much lower than those of HCP, implying that the introduced PMA networks separated the benzyl chloride of CMPS networks successfully and the excessive cross-linking of CMPS was effectively controlled. Specifically, 240.3 m² g⁻¹ of the t-plot micropore surface area and 0.3350 cm³ g⁻¹ of the t-plot micropore volume were measured for HCP/PMA IPNs, while that of HCP was predicted to be 570.9 m² g⁻¹ and 0.5980 cm³ g⁻¹. In addition, the amination reaction decreased the BET surface area and pore volume, which may be from the increased polarity.^9,19,26

Table 1 Structural parameters of CMPS, CMPS/PMA IPNs, HCP/PMA IPNs, HCP/PADETA IPNs and HCP

	CMPS	CMPS/PMA IPNs	HCP/PMA IPNs	HCP/PADETA IPNs	HCP
BET surface area/(m^2 g^−1)	13.1	3.4	521.6	330.7	948.2
t-Plot micropore surface area/(m^2 g^−1)	—	—	240.3	150.3	570.9
Pore volume/(cm^3 g^−1)	0.0240	0.0080	0.3350	0.2030	0.5980
t-Plot micropore volume/(cm^3 g^−1)	—	—	0.154	0.117	0.336
Average pore size/(nm)	7.43	9.52	2.68	2.77	2.45

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Fig. 2(a) indicates that the macropores are the dominant pores for CMPS and CMPS/PMA IPNs, whereas mesopores ranging from 2 to 5 nm are predominant for HCP/PMA IPNs and HCP/PADETA IPNs. The average pore size of the CMPS/PMA IPNs is 9.52 nm, whereas that of HCP/PADETA IPNs is 2.77 nm, confirming that many micropores are produced. Compared to the pore size distribution of HCP/PADETA IPNs with HCP (Fig. 2(b)), the HCP/PADETA IPNs had a much lower pore volume, whereas it had a peak in 4 nm, and its mean pore size was slightly larger than HCP (2.45 nm). This indicates that the introduced PMA networks partially control the excessive cross-linking of CMPS in the Friedel–Crafts reaction and the pore structure of HCP/PADETA IPNs is better than HCP.

Fig. 2 Pore size distribution of (a) CMPS, CMPS/PMA IPNs, HCP/PMA IPNs and HCP/PADETA IPNs and (b) HCP and HCP/PADETA IPNs.

3.2. Equilibrium adsorption

The equilibrium adsorption of CMPS, CMPS/PMA IPNs, HCP/PMA IPNs and HCP/PADETA IPNs was studied comparatively using salicylic acid as the adsorbate and the results are displayed in Fig. 3(a). The adsorption of CMPS and CMPS/PMA IPNs is very weak, which may be due to their low BET surface area. After the Friedel–Crafts reaction, HCP/PMA IPNs have a much larger equilibrium capacity, which may be due partly to the sharply increased BET surface area and the predominant micro/mesopores.^5,32 In addition, the HCP/PMADETA IPNs showed enhanced adsorption compared to HCP/PMA IPNs despite its decreased BET surface area and pore volume. The uploaded amide and amino groups on the surface should contribute to the increased adsorption.^28,32

Fig. 3 Equilibrium isotherms of salicylic acid on (a) CMPS, CMPS/PMA IPNs, HCP/PMA IPNs and HCP/PADETA IPNs; (b) HCP and HCP/PADETA IPNs from aqueous solutions at 298 K.

In addition, Fig. 3(b) indicates that HCP/PADETA IPNs is superior to HCP considering the adsorption of salicylic acid. Although the BET surface area of HCP/PADETA IPNs is 65.1% lower than that of HCP, the pore structure of HCP/PADETA IPNs is better than HCP. Furthermore, the uploaded amide and amino groups on the surface of the HCP/PADETA IPNs enhance the adsorption performance due to polarity matching and there may be possible hydrogen bonding between the amide and amino groups of HCP/PADETA IPNs and the phenolic hydroxyl and carboxyl groups of salicylic acid.

At an equilibrium concentration of 100 mg L⁻¹, HCP/PADETA IPNs possessed an equilibrium capacity of 313.3 mg g⁻¹, which is 87.8 times higher than CMPS, 143.4 times higher than CMPS/PMA IPNs, 4.5 times higher than HCP/PMA IPNs and 1.6 times higher than HCP, respectively. Compared to some other materials, the HCP/PADETA IPNs were superior to the results reported in ref. 33 (43.0 mg g⁻¹ by Duolite S861; 85.1 mg g⁻¹ by Amberlite XAD16), ref. 34 (70.0 mg g⁻¹ by XAD-4; 125.0 mg g⁻¹ by an amino modified hyper-cross-linked resin), ref. 9 (205.0 mg g⁻¹ by an amide-modified hyper-cross-linked resin) and this study (195.6 mg g⁻¹ by HCP).

In addition, the equilibrium isotherms in Fig. S2† shows that the equilibrium capacity increases with increasing temperature, demonstrating that the interaction between HCP/PADETA IPNs and salicylic acid is strengthened at a higher temperatures and the adsorption is an endothermic process.³⁵ Langmuir and Freundlich models were used to describe the equilibrium process^36,37 and the corresponding parameters, such as q_m, K_L, K_F and n as well as the correlation coefficients R², are summarized in Table S1.† Both the Langmuir and Freundlich models appear to be appropriate for fitting the equilibrium data due to the high correlative coefficients (R² > 0.98). The n values of the Freundlich model for the adsorption are greater than 1, implying that the adsorption is a favorable process. In particular, only the Freundlich model can fit the equilibrium data on HCP, which is different from the adsorption performance of the HCP/PADETA IPNs.

3.3. Kinetic adsorption

The kinetic curves of salicylic acid adsorption on HCP/PADETA IPNs and HCP were measured comparatively and the results are illustrated in Fig. 4. The adsorption capacity increased rapidly with increasing adsorption time and the adsorption capacity reached over 75% of the equilibrium capacity within one hour, suggesting that the adsorption is a fast process. Moreover, HCP/PADETA IPNs is more efficient than HCP and it requires much less time to reach equilibrium than HCP. The adsorption capacity reached 96% of the equilibrium capacity within one hour for HCP/PADETA IPNs, whereas only 75% was achieved for HCP, which further confirms that HCP/PADETA IPNs has a more favorable pore structure for the diffusion of salicylic acid in the pores.

Fig. 4 Kinetic curves of salicylic acid on HCP and HCP/PADETA IPNs at 298 K.

The pseudo-first-order rate equation and the pseudo-second-order equation^38,39 were employed to characterize the kinetic data and the corresponding parameters, as summarized in Table 2. Both the pseudo-first-order and the pseudo-second-order rate equations are suitable for fitting the kinetic data on HCP/PADETA IPNs and the predicted equilibrium capacity is much larger than HCP. In addition, the values of k₂ on HCP/PADETA IPNs and HCP were predicted to be 1.29 × 10⁻³ and 3.10 × 10⁻⁴ g (mg⁻¹ min⁻¹), respectively, confirming that the adsorption rate on HCP/PADETA IPNs is much faster than HCP.

Table 2 Correlative parameters of the kinetic data of salicylic acid on HCP and HCP/PADETA IPNs according to the pseudo-first-order and pseudo-second-order rate equations

	Pseudo-first-order rate			Pseudo-second-order rate
	k 1 (min^−1)	q e (mg g^−1)	R ^2	k 2 (g (mg^−1 min^−1))	q e (mg g^−1)	R ^2
HCP	0.0299	116.8	0.9678	0.00031	130.8	0.9926
HCP/PADETA IPNs	0.1342	142.3	0.9899	0.00129	155.0	0.9961

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In addition, the kinetic curves for the adsorption of salicylic acid on HCP/PADETA IPNs with the initial concentration of 604.3, 804.3 and 1003.8 mg L⁻¹ and temperature of 293, 303 and 313 K were measured (Fig. S3†). At a higher temperature, less time is required to reach equilibrium, implying that the adsorption rate is faster at a higher temperature, while more time is needed to reach equilibrium at a higher initial concentration, which may be from the increased collision of salicylic acid in the solution. The pseudo-first-order and pseudo-second-order rate equations were applied to simulate the kinetic data and the corresponding parameters are listed in Table S2.† Both the pseudo-first-order and pseudo-second-order rate equations are suitable for characterizing the kinetic data. The values of k₂ at 293, 303 and 313 K are predicted to be 1.29 × 10⁻³, 1.37 × 10⁻³ and 1.84 × 10⁻³ g (mg⁻¹ min⁻¹), respectively, which is in accordance with the fact that the adsorption rate is faster at a higher temperature.

3.4. Column adsorption and desorption

Fig. 5 shows the column adsorption and desorption properties of salicylic acid on HCP/PADETA IPNs and HCP. The shape of the dynamic adsorption curve was very sharp, meaning that adsorption reaches equilibrium quickly after leakage.⁴⁰ We defined C/C₀ = 0.05 (where C is the concentration of salicylic acid from the effluent, mg L⁻¹) as the breakthrough point and C/C₀ = 0.95 as the saturated point and the volume of the effluent to reach the breakthrough point and the saturated point was defined as V_b and V_s, respectively. Fig. 5(a) indicates that V_b is 109.1 BV (1 BV = 10 mL) and V_s is 163.3 BV for HCP/PADETA IPNs, which is much greater than HCP (81.4 and 116.6 BV, respectively). The dynamic saturated capacities were 137.6 and 92.6 mg mL⁻¹ wet resins for HCP/PADETA IPNs and HCP, respectively.

Fig. 5 Dynamic adsorption and desorption curves of salicylic acid on HCP/PADETA IPNs from aqueous solutions (for the adsorption: 10.00 mL of wet resins, C₀ = 1017.3 mg L⁻¹, flow rate = 1.0 mL min⁻¹. For desorption: 0.01 mol L⁻¹ of NaOH (w/v) and 20% of ethanol (v/v) was used as the desorption solution, flow rate = 0.6 mL min⁻¹).

After the column adsorption, different desorption solvents were employed for the desorption process (Fig. S4†); 0.01 mol L⁻¹ of NaOH (w/v) and 20% of ethanol (v/v) were found to be the best desorption solvent for desorption of the resin column. This was utilized for the column desorption and the results are displayed in Fig. 5(b). At a flow rate of 0.6 mL min⁻¹, only 20 BV of the desorption solvent was sufficient to completely regenerate the resin column, and the dynamic desorption capacity was 1320.0 and 858.6 mg for HCP/PADETA IPNs and HCP, respectively, which is coincident with the dynamic saturated capacity (1376.0 and 926.3 mg) in the column adsorption. Moreover, the desorption of HCP/PADETA IPNs is faster than HCP and less desorption solvent is needed for the desorption, demonstrating that the pore structure of HCP/PADETA IPNs is superior to HCP. HCP/PADETA IPNs was used repeatedly for five cycles in a continuous adsorption–desorption process. In the first run, the recovery rate (the ratio of desorption to adsorption) of salicylic acid was 97.6%, which means HCP/PADETA IPNs can be used as a good adsorbent to enrich salicylic acid. Moreover, the salicylic acid uptakes decreased to approximately 84.5% after five cycles of the adsorption–desorption process, exhibiting good reusability and regeneration behaviors (Fig. S5†).

3.5. Separation of salicylic acid from phenol in their mixed solution

Salicylic acid is generally produced from phenol and therefore phenol may exist jointly with salicylic acid.⁴¹ Therefore, the selective adsorption of salicylic acid on the adsorbent as well as separation of salicylic acid from phenol is of great importance.³³ Compared to phenol, salicylic acid has another carboxyl group and the carboxyl group is at the ortho-position with respect to the phenolic hydroxyl group. Moreover, the neighboring carboxyl group can form intramolecular hydrogen bonds with the phenolic hydroxyl group,⁴² which makes salicylic acid a well-balanced molecule with both the hydrophobic and hydrophilic portions. HCP/PADETA IPNs in this study possessed hydrophobic HCP networks as well as hydrophilic PADETA networks. Therefore, as shown Fig. 3, HCP/PADETA IPNs achieves highly efficient adsorption towards salicylic acid. The hydrophobic HCP networks have relatively strong affinity to the hydrophobic portion of salicylic acid due to the hydrophobic interaction or π–π stacking,^43,44 whereas the hydrophilic PADETA networks are more inclined to approach the hydrophilic portion of salicylic acid by the possible electrostatic interaction or hydrogen bonding.^22,27,40 Moreover, some other molecules such as phenol are not adsorbed effectively and the separation of salicylic acid from phenol is possible. For this reason, a mixed adsorption test was conducted for salicylic acid and phenol and the results are shown in Fig. 6. The enrichment factor was used as a performance parameter to evaluate the adsorption.⁴⁵ The distribution ratio (D) is given by eqn (3):where C₀ is the initial concentration of the adsorbate in solution, C_e is the concentration of the adsorbate in the aqueous phase after adsorption (mg L⁻¹), V is the volume of the aqueous phase (mL), and M is the amount of HCP/PADETA IPNs (g).

Fig. 6 Equilibrium isotherms of phenol and salicylic acid on HCP/PADETA IPNs from the single or the mixed aqueous solution at 298 K.

The enrichment factor (α) for the adsorption of salicylic acid in the presence of phenol can be obtained from the equilibrium distribution ratio according to eqn (4) as follows:

where D_SA and D_phenol represent the distribution ratios of salicylic acid and phenol, respectively.

In the single solution, the equilibrium capacity of salicylic acid on HCP/PADETA IPNs was 313.3 mg g⁻¹ at an equilibrium concentration of 100 mg L⁻¹, which is much larger than phenol (42.6 mg g⁻¹), and the enrichment factor was 10.3. The enrichment factor of HCP/PADETA IPNs was much higher than HCP. In the mixed solution, the equilibrium capacity of salicylic acid increased to 337.1 mg g⁻¹, whereas that of phenol decreased to 18.6 mg g⁻¹ and the enrichment factor (α) increased to 17.3, suggesting that the adsorption of salicylic acid and phenol is competitive, and the adsorption of salicylic acid from the mixed solution is enhanced. Therefore, it is possible to separate salicylic acid from phenol in a mixed solution by HCP/PADETA IPNs.⁴⁶

As shown in Fig. 7. The V_b was measured to be 255.0 BV for salicylic acid at a flow rate of 1.5 mL min⁻¹, which was much greater than phenol (30.0 BV). The corresponding saturated capacity for salicylic acid was 178.0 mg mL⁻¹ wet resins, which is much greater than phenol (6.4 mg mL⁻¹ wet resins), i.e., only phenol exists in the effluent, ranging from 0 to 255 BV, and the collected effluent is a pure phenol solution, whereas salicylic acid is concentrated on the resin column. In particular, in the range of 127–722 BV, the concentration of phenol is higher than its initial one (C/C₀ > 1.0), confirming that HCP/PADETA IPNs has stronger affinity to salicylic acid than phenol, and the pre-loaded phenol molecules on the resin column are pushed out by the following salicylic acid molecules. The amount of phenol excluded from the resin column was 82.4% (w/w). After desorption of the resin column, both phenol and salicylic acid were desorbed from the resin column; 2122 mg of salicylic acid and 60 mg of phenol were collected and the concentration of salicylic acid increased from 50.1% to 97.2% (w/w).

Fig. 7 Dynamic adsorption–desorption curves of phenol and salicylic acid on HCP/PADETA IPNs.

4. Conclusions

HCP/PADETA IPNs was prepared from CMPS by the interpenetration of PMA networks in the pores of CMPS, self cross-linked reaction of the benzyl chloride of CMPS networks and amination reaction of PMA networks with DETA, and its adsorption and separation properties were evaluated using HCP as the reference. The BET surface area HCP/PADETA IPNs was only 55% of that of HCP, whereas the introduced PMA networks separated the benzyl chloride of CMPS networks successfully and the excessive cross-linking of CMPS was controlled effectively, which induced its preferable pore structure. HCP/PADETA possessed a very large equilibrium capacity towards salicylic acid, and it reached 313.3 mg g⁻¹ at an equilibrium concentration of 100 mg L⁻¹, which was 1.6 times higher than that of HCP. HCP/PADETA IPNs was highly efficient for the adsorption of salicylic acid and required much less time to reach equilibrium than HCP. In addition, the dynamic saturated capacity of salicylic acid was 137.6 mg mL⁻¹ wet resins for HCP/PADETA IPNs, which is much greater than HCP. Moreover, HCP/PADETA IPNs had an enrichment factor of 17.3 for salicylic acid over phenol and it separated salicylic acid from phenol successfully in their mixed solution.

Acknowledgements

The authors thank the National Natural Science Foundation of China (No. 21376275), the Fundamental Research Funds for the Central Universities of Central South University (No. 2015zzts020), the Open-End Fund for the Valuable and Precision Instruments of Central South University and South Wisdom Valley Innovative Research Team Program for the financial support. The authors also thank Prof. Kirin in Ruđer Bošković Institute of Croatia for his helpful suggestion.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01932c

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