Recovery of HMF from aqueous solution by zeolitic imidazolate frameworks

Chemical Engineering Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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Recovery of HMF from aqueous solution by zeolitic imidazolate frameworks Hua Jin a,b, Yanshuo Li a,n, Xinlei Liu a,b, Yujie Ban a,b, Yuan Peng a,b, Wenmei Jiao a,b, Weishen Yang a,n a b

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China University of Chinese Academy of Sciences, Beijing 100049, China

H I G H L I G H T S








The uptake of HMF decreased following the order of ZIF-84ZIF-90 4 ZIF-93. The hydrophobicity of ZIFs is the key feature governing the HMF adsorption capacity. The loaded ZIFs can be fully regenerated by solvent assistant regeneration. ZIF-8 exhibits selective adsorption of HMF from HMF/fructose/water mixtures. The ZIF-8 packed column shows excellent reusability for HMF recovery.

art ic l e i nf o

a b s t r a c t

Article history: Received 1 April 2014 Received in revised form 28 June 2014 Accepted 11 July 2014

Three zeolitic imidazolate frameworks (ZIF-8, ZIF-90 and ZIF-93) were investigated for adsorption of 5-hydroxymethylfurfural (HMF) from aqueous solution. The equilibrium uptake of HMF decreased following the order of ZIF-8 (465 mg g  1)4 ZIF-90 (307 mg g  1)4 ZIF-93 (279 mg g  1), in accordance with the hydrophobicity of the frameworks. The adsorption equilibrium data were found to follow Langmuir adsorption model. The sorption kinetics could be well described by the pseudo-second-order kinetic equation. The HMF-loaded ZIFs could be fully regenerated with ethanol as the desorption solvent. Selective adsorption of HMF from HMF/fructose/water mixtures was also carried out. The uptake of HMF remained unchanged, with no observable adsorption of fructose. In liquid phase breakthrough experiments, the uptake of HMF on ZIF-8 in column was consistent with the batch study. Furthermore, the ZIF-8 packed column could be regenerated effectively and recycled with no discernible loss of adsorption capacity. These findings confirm that ZIF-8 can be employed as an effective and reusable adsorbent for HMF recovery from aqueous solution. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Zeolitic imidazolate frameworks 5-Hydroxymethylfurfural (HMF) Fructose Liquid adsorption Breakthrough curve

1. Introduction Biorefinery, a facility that integrates biomass conversion processes and equipments to produce fuels, chemicals and materials from biomass feedstocks (Fernando et al., 2006; FitzPatrick et al., 2010), has attracted worldwide attention due to the limitation of fossil resources and the critical environment concerns (Cherubini, 2010). Chemical building blocks (Bozell and Petersen, 2010) derived from biomass are very important as intermediates for the production of biofuel and bio-based chemicals. 5-Hydroxymethylfurfural (HMF), called “a sleeping giant” (Bicker et al., 2005), is considered to be an important intermediate due to its rich chemistry and n

Corresponding authors. E-mail addresses: [email protected] (Y. Li), [email protected] (W. Yang).

potential availability (Rosatella et al., 2011). HMF, generally obtained from acid-induced dehydration of fructose, glucose and cellulose (Zhao et al., 2007; Kamm, 2007), can be converted into versatile chemicals of high industrial potential (Corma et al., 2007) or liquid alkanes used as transportation fuel components (Huber et al., 2005). In most of the studies on HMF synthesis, the HMF was obtained in solutions with low concentration. For example, the HMF concentration was only 1.5 mg mL  1 (Chheda et al., 2007) in the product using glucose as feedstock and hydrochloric acid (HCl) as catalyst in pure water at 170 1C. Therefore, an efficient recovery method needs to be developed for large scale applications of HMF. Adsorptive separation is widely regarded as one of the most competitive separation methods because of its wide application scope, energy-saving and environmentally friendly (Li et al., 2012; Qureshi et al., 2005).

http://dx.doi.org/10.1016/j.ces.2014.07.017 0009-2509/& 2014 Elsevier Ltd. All rights reserved.

Please cite this article as: Jin, H., et al., Recovery of HMF from aqueous solution by zeolitic imidazolate frameworks. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.017i

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Fig. 1. Crystal structures of ZIFs used in this study: (a) ZIF-8, (b) ZIF-90 and (c) ZIF-93.

Zeolites and porous carbons have been investigated for selectively separating HMF from aqueous or dimethyl sulfoxide (DMSO) system. For instance, Ranjan et al. (2009) have demonstrated the potential of recovering HMF by preferential adsorption on appropriately chosen zeolites (e.g., MFI). The results indicated that higher Si/Al ratio (i.e., more hydrophobic framework) was desirable for efficient removal of HMF. A recent report showed that solution mixtures (e.g., mixtures of water and DMSO) would affect the uptake of HMF onto hydrophobic zeolites (Xiong et al., 2014). Apart from zeolites, commercial activated carbons (e.g., BP2000 and Norit1240) were also reported for selective adsorption of HMF from HMF/fructose/DMSO mixtures (Rajabbeigi et al., 2012). For carbon materials, both microporosity and oxygenate functionality (i.e., carbon polarity) are key features for governing the adsorption behavior (Yoo et al., 2014). Despite the promising properties of these materials reported, the demand for novel adsorbents with more attractive adsorption performance is urgent. Metal organic frameworks (MOFs) are a new class of hybrid materials consisting of metal-containing units connected with organic linkers by coordination bonds (reticular synthesis) (Furukawa et al., 2013). Owing to their extra-high porosity and adjustable chemical functionality (Millward and Yaghi, 2005), MOFs have attracted enormous interest from both academia and industry. Zeolitic imidazolate frameworks (ZIFs), a subset of MOFs, consist of tetrahedral metal ions (e.g., Zn, Co) bridged by imidazolate ligands (Phan et al., 2010; Hayashi et al., 2007). ZIFs are known to have permanent porosity (Banerjee et al., 2008) and high thermal and chemical stability (Park et al., 2006), which endow them with promising application properties as catalysts (Tran et al., 2011; Zakzeski et al., 2011), adsorbents (Thompson et al., 2013; Gücüyener et al., 2010; Ban et al., 2013), and separation membranes (Li et al., 2010a, 2010b, 2010c; Liu et al., 2010). When ZIFs serve as adsorbents, the most charming advantage is their tuneable pore size (Deng et al., 2012) and adjustable internal surface properties (Cohen, 2012) compared to other traditional nanoporous materials such as zeolites and activated carbons (Chen et al., 2013). Nowadays, many reports about ZIFs on liquid phase adsorption have been given. For example, Liu et al. (2011), Cousin Saint Remi et al. (2011) and Zhang et al. (2013) pointed out that ZIFs can be served as high-efficiency adsorbents for recovery of bio-alcohols. Moreover, selective adsorption and separation of xylene isomers (Peralta et al., 2013), furfural (Liu et al., 2013) and benzotriazoles (Jiang et al., 2013) over ZIFs were also reported. In this work, selective adsorption of HMF from aqueous system was investigated on three different ZIFs, namely ZIF-8 [Zn(2methylimidazole)2, SOD topology (Park et al., 2006)], ZIF-90 [Zn(imidazole-2-carboxaldehyde)2, SOD topology (Morris et al., 2008)] and ZIF-93 [Zn(4-methylimidazole-5-carboxaldehyde)2,

RHO topology (Morris et al., 2010)], respectively. The crystal structures of the three ZIFs were illustrated in Fig. 1. Adsorption experiments were performed in both batch and fixed-bed column systems. The large adsorption capacity, fast adsorption kinetics, excellent chemical stability, and good reusability make ZIF-8 a highly potential adsorbent for separating HMF from aqueous solution.

2. Materials and methods 2.1. Materials Chemicals and solvents were used as received: Zn(NO3)2  6H2O (Z99%, Sigma-Aldrich), Zn(AC)2  2H2O (Z98%, Sigma-Aldrich), ZnCl2 (Z98%, Sigma-Aldrich), 2-methylimidazole (Hmim, 99%, Sigma-Aldrich), imidazole-2-carboxaldehyde (ICA, 97%, AlfaAesar), 4-methylimidazole-5-carboxaldehyde (almeIm, 99%, Sigma-Aldrich), sodium formate (Z98%, Sigma-Aldrich), HMF (99%, Sigma-Aldrich), fructose (BR, Sinopharm Chemical Reagent Co., Ltd.), N,N-dimethylformamide (DMF, Z99.5%, Tianjin Bodi Chemical Co., Ltd.), methanol ( Z99.5%, Tianjin Bodi Chemical Co., Ltd.), ethanol (Z 99.5%, Tianjin Bodi Chemical Co., Ltd.), deionized water (H2O, home-made).

2.2. Synthesis of adsorbents ZIF-8 nanoparticles were synthesized according to the recipe reported by Cravillon et al. (2009). A solution of Zn(NO3)2  6H2O (0.879 g) in 60 mL of methanol was rapidly poured into a solution of Hmim (1.776 g) in 60 mL of methanol, and the resulting mixture was stirred at room temperature for 1 h. The obtained nanoparticles were collected by centrifugation and washed three times with methanol. Two types of ZIF-8 microcrystals with different sizes were synthesized after the reported recipes (Cravillon et al., 2011; Bux et al., 2009). In a typical synthesis of ZIF-8-M1, a solution of Hmim (8.106 g) and sodium formate (6.715 g) in 500 mL methanol was poured into the solution of Zn(NO3)2  6H2O (7.344 g) in 500 mL methanol under stirring. Stirring is stopped after 20 min. After a reaction time of 24 h at room temperature, the precipitate is recovered by centrifugation and washed with methanol. ZIF-8-M2 was prepared by dissolving ZnCl2 (0.809 g), Hmim (0.729 g) and sodium formate (0.405 g) in 60 mL of methanol. The mixture was then heated in a microwave oven at 100 1C for 4 h. The product was recovered by filtration and washed with plenty of methanol. The synthesis of ZIF-90 nanocrystals was carried out at room temperature. Specifically, ICA (0.824 g) was dissolved in 100 mL

Please cite this article as: Jin, H., et al., Recovery of HMF from aqueous solution by zeolitic imidazolate frameworks. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.017i

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DMF under heating at 60 1C for 6 h. After the ICA solution had been cooled to room temperature, a Zn(AC)2  2H2O (0.228 g) solution in 20 mL DMF was rapidly poured into the ICA/DMF solution and stirred for 0.5 h to form a stable nanoparticle

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suspension. The as-synthesized ZIF-90 nanoparticles were collected by means of centrifugation and then washed with fresh methanol. ZIF-93 nanoparticles were synthesized with the protocol reported by Liu et al. (2013). A solution of Zn(NO3)2  6H2O in

Fig. 2. Characterization of the as-prepared ZIF-8, ZIF-90 and ZIF-93: (a) PXRD patterns, (b) TGA curves, (c, e, g) SEM images and (d, f, h) N2 adsorption/desorption isotherms. Insets show corresponding pore size distribution calculated by HK method.

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methanol was rapidly poured into a solution of almeIm in methanol under stirring at room temperature with a molar ratio of Zn2 þ /almeIm/methanol ¼1:8:1000. After 20 min, the nanoparticles were centrifugally separated and washed with ethanol for three times. 2.3. Characterization Powder X-ray Diffraction (PXRD) patterns were obtained on Rigaku D/MAX 2500/PC instrument using Cu Kα radiation (λ ¼0.154 nm at 40 kV and 200 mA). Morphological features were observed by Scanning Electron Microscopy (Quanta 200 FEG, FEI Co., 30 kV). Nitrogen physisorption isotherms were measured at 77 K, on a Quantachrome Autosorb Automated Gas Sorption instrument. Thermogravimetric analysis (TGA) was performed on a PerkinElmer Pyris Diamond TG instrument at a heating rate of 5 1C min  1 under nitrogen atmosphere. Infrared spectra were acquired from a Nicolet 6700 FTIR-ATR (Fourier Transform Infrared Spectroscopy-Attenuated Total Reflectance) spectrophotometer. 2.4. Adsorption experiments 2.4.1. Liquid phase batch experiments A typical adsorption experiment was conducted by mixing of 0.050 g ZIFs with 0.50 mL aqueous solution containing different amount of HMF under stirring for a fixed time (1 min to 24 h) at 25 1C. A blank experiment was also performed at the same time. After adsorption for a pre-determined time, the supernatants were separated from the adsorbent by centrifugation and the HMF concentration of the obtained supernatants was determined by high performance liquid chromatography (HPLC, Agilent 1260) with refractive index (RI) detector. The detection of HMF was performed on a Shodex SC1211 column using pure water as the mobile phase at a flow rate of 1.0 mL min  1. The uptake was determined using the mass balance equation q ¼ V ðC initial  C f inal Þ m  1, where q is the amount adsorbed by the adsorbent (mg g  1), V is the volume of the solution used in the adsorption experiment (mL), m is the mass of adsorbent (g), and Cinitial and Cfinal are the initial and final concentrations of the adsorbate (g L  1), respectively. The adsorption of HMF from HMF/fructose/water mixtures was studied using solutions containing HMF and fructose with 1:1 mass ratio. 2.4.2. Liquid phase breakthrough experiments ZIF-8 nanoparticles were packed into a stainless–steel column (150 mm length and 2.1 mm inner diameter). The feed solution was subsequently pumped through the column with a flow rate of 0.05 mL min  1 at 25 1C. Samples were collected at the column outlet and analyzed by HPLC. Each data on the breakthrough curve represents the average concentration over the collection period. 2.5. Desorption experiments 2.5.1. Desorption in batch experiments Pre-adsorbed ZIFs were obtained by mixing 0.050 g of ZIFs with 0.50 mL of 50 g L  1 HMF solution for 1 h. The suspension was then centrifuged and the solid was used as the pre-adsorbed ZIFs. Desorption experiments were carried out using two different methods. The first method is thermal regeneration, targeting temperature swing adsorption (TSA) applications. The preadsorbed ZIFs were heated under vacuum. The second method is solvent assistant desorption, targeting simulated moving bed (SMB) applications. 2 mL of ethanol was used for desorption of

HMF from the HMF-loaded ZIFs (0.050 g). The mixtures were stirred for 1 h at 25 1C. Then the solid obtained by centrifugation was dried at room temperature. 2.5.2. Desorption in column experiments Ethanol was chosen as the eluent. For HMF removal, ethanol passed through adsorption column with a flow rate of 0.05 mL min  1 at 25 1C. Samples were collected at the column outlet and analyzed by HPLC.

3. Results and discussion 3.1. Characterization of the as-synthesized ZIF adsorbents The prepared ZIF-8, ZIF-90 and ZIF-93 were characterized by PXRD, TGA, SEM, and N2 adsorption experiments (Fig. 2). ZIF-8, ZIF-90 and ZIF-93 particles were successfully prepared with pure phase as demonstrated by PXRD patterns [Fig. 2(a)]. The TGA data [Fig. 2(b)] reveals that the ZIFs are stable up to 600 1C (ZIF-8), 320 1C (ZIF-90) and 395 1C (ZIF-93), respectively. The SEM images of the prepared ZIFs are shown in Fig. 2(c, e, g). The average particle size of ZIF-8, ZIF-90 and ZIF-93 were approximately 47 nm, 85 nm and 42 nm, respectively. The BET surface area (pore volume) of each framework was calculated with N2 adsorption data [Fig. 2(d, f, h)] and found to be 1339 (0.598), 1387 (0.623) and 1679 m2 g  1 (0.678 cm3 g  1) for ZIF-8, ZIF-90 and ZIF-93, respectively. The pore size distribution (PSD) analyzed with HK method demonstrated the existence of microporous in each ZIF framework. 3.2. Batch studies 3.2.1. Adsorption isotherms The adsorption isotherms of HMF on the ZIF-8, ZIF-90 and ZIF93 are presented in Fig. 3(a). The adsorption amount for HMF increased with the increasing equilibrium concentration, as a result of the increased driving force. In the range of our investigation, the maximum adsorption capacities for HMF on ZIF-8, ZIF-90 and ZIF-93 were 465 mg g  1, 307 mg g  1 and 279 mg g  1, respectively. The adsorption capacity of ZIF-8 for HMF is three times more than the reported value measured on zeolites (Ranjan et al., 2009). The adsorption isotherms were fitted with the Langmuir model. The equation is expressed as: Ce Ce 1 ¼ þ qe Q 0 Q 0 b

ð1Þ

where Ce (mg L  1) is the equilibrium concentration of the adsorbate, qe (mg g  1) is the equilibrium adsorption amount, Q0 is the maximum adsorption amount (mg g  1), and b is the Langmuir constant (L mg  1). The fitting curves of all the adsorption data to Langmuir model are shown in Fig. 3(b). The correlation coefficients of Langmuir isotherm model were 0.9972, 0.9749 and 0.9953 for ZIF-8, ZIF-90 and ZIF-93, respectively, indicating that the adsorption of HMF follows a Langmuir model. The value of b calculated from the intercept of ZIF-8 (3.33  10  4L mg  1) was larger than that of ZIF-90 (3.5  10  5L mg  1) and ZIF-93 (7.1  10  5L mg  1), indicating higher strength of adsorption of HMF on ZIF-8. Material texture and surface property are the two recognized material characteristics governing adsorption capacity. Although the BET surface area and pore volume increased in the order of ZIF-8o ZIF-90oZIF-93, the HMF uptake was totally in opposite. One possible reason should be that the hydrophobic/hydrophilic properties of the ZIF frameworks played an important role in the HMF adsorption. A simple tool to estimate the hydrophobic or hydrophilic nature of the surface of a ZIF structure has been

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Fig. 3. (a) Adsorption isotherms of HMF on ZIF-8, ZIF-90 and ZIF-93, respectively. (b) The fitted adsorption isotherms of HMF on ZIF-8, ZIF-90 and ZIF-93 by the Langmuir equation, respectively. Table 1 Adsorption of HMF on ZIF-8 from HMF/fructose/water mixtures. Adsorbates Equilibrium concentration (g L Adsorbed amount (mg g  1)

HMF 1

)

5.80 399

Fructose 27.6 453

51.2 466

24.8 0

50.3 0

72.1 0

Fig. 4. (a) Plots of the amount of HMF adsorbed versus time, initial HMF concentration¼ 70 g L  1. (b) Plots of pseudo-second-order kinetics. (c,d) SEM images. (e,f) Size distribution.

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Fig. 5. FTIR-ATR spectra and PXRD patterns for ZIFs after desorption of HMF via thermal regeneration and solvent assistant regeneration: ZIF-8 (a, b), ZIF-90 (c, d) and ZIF-93 (e, f), respectively.

Fig. 6. Experimental (filled squares) and simulated (line) breakthrough curves of HMF on ZIF-8 packed column at 25 1C.

Fig. 7. Regeneration profile with ethanol on ZIF-8 packed column.

proposed (Amrouche et al., 2012). According to their results, ZIF-8 is highly hydrophobic [which was also confirmed by experimental measurements (Küsgens et al., 2009)] followed by ZIF-90, while

ZIF-93 is hydrophilic. It can be concluded that a ZIF framework with hydrophobic nature is favorable for adsorption of HMF. This conclusion agrees with the results obtained on zeolitic adsorbents

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(i.e., a decrease in sorption amount occurs with decreased Si/Al ratio) and carbon-based adsorbents (i.e., increasing hydrophobicity leads to an increase in HMF capacity). The adsorption selectivity and capacity of ZIF-8 for HMF in the presence of fructose was also investigated. The results (Table 1) revealed that the adsorbed amount of HMF remained the same, with no observable adsorption of fructose. This means that the ZIF-8 exhibits large potential for HMF recovery from fermentation broth. 3.2.2. Adsorption kinetics It is difficult to measure the adsorption kinetic of HMF on ZIF nanoparticles because of the rapidly reaching equilibrium within 10 min. Thus, we investigated the time-dependent adsorption of HMF on ZIF-8 microcrystals, considering that ZIF-8 has the highest adsorption capacity. The initial concentration of HMF for the adsorption kinetic study was 70 g L  1.The results are shown in Fig. 4(a). The average particle sizes of the two types of ZIF-8 microcrystals were 4.1 μm (ZIF-8-M1) and 13.4 μm (ZIF-8-M2), respectively [see SEM images and size distribution curves in Fig. 4 (c–f)]. The adsorption of HMF on ZIF-8 microcrystals is well fitted with the pseudo second-order kinetic model (Eq. (2)), with R2 of 0.9982 for ZIF-8-M1 and of 0.9944 for ZIF-8-M2, respectively. t batch 1 t ¼ þ batch qt qe k qe 2

ð2Þ

where qe is the amount adsorbed at equilibrium (mg g  1), qt is the amount adsorbed at time t (mg g  1), tbatch is the adsorption time in batch system (h), k is the pseudo-second-order constant (g mg  1 h  1). Plots for the adsorption kinetics fitted using the pseudo-second-order kinetic model are displayed in Fig. 4(b). Good agreement of the calculated equilibrium capacities (qe) is found between ZIF-8-M1 and ZIF-8-M2, indicating that adsorption capacity of ZIF-8 is unrelated with particle size. 3.2.3. Desorption of HMF from ZIFs A commercial adsorbent is expected to exhibit excellent facile regeneration. In order to estimate the reversibility, desorption of HMF from ZIFs using thermal regeneration and solvent assistant regeneration were given. The regenerated ZIFs were examined using FTIR-ATR and PXRD. As proved by FTIR-ATR spectra [Fig. 5(a), (c), (e)], thermal regeneration at 60 1C is effective for ZIF-8. Whereas, higher regeneration temperature for ZIF-90 (120 1C) and ZIF-93 (100 1C) are needed possibly due to the temperaturedependent gate-opening effect. Regeneration with ethanol as the desorption solvent is highly effective for each ZIF. When ethanol was utilized, there was no HMF signals (e.g., C ¼O stretching vibration at 1677 cm  1 or C ¼C double bonds in furan rings stretching vibration at 1518 cm  1) present in spectra of each ZIF (C ¼O stretching vibration also exists in ZIF-90 and ZIF-93), elucidating a complete desorption. In addition, no phase transformation of ZIFs was observed during HMF desorption as characterized by PXRD patterns [Fig. 5(b), (d), (f)]. 3.3. Column studies 3.3.1. Breakthrough experiments and regeneration of ZIF-8 packed column The promising results obtained with ZIF-8 mentioned above inspired us to test the separation of HMF from aqueous system in a continuous-flow setup, using a column packed with ZIF-8 nanoparticles. It was observed that the breakthrough curve exhibited a typical “S” shape (Fig. 6). In general, the steep slope of the curve indicates that the adsorption was very fast, which coincides with the adsorption kinetic study result (3.2.2). The experimental data

Fig. 8. Regeneration profile with water and ethanol on ZIF-8 packed column.

was fitted using Thomas model. The equation is given as: C out; t ¼ Cf 1 þ exp




1  kTH C f t þ kTH QQ fTH m



ð3Þ

where Cout,t is the concentration of the solute in the effluent solution at time t (g L  1), Cf is the initial concentration of the solute in solution (g L  1), t is the time (min), kTH is the kinetic constant in the Thomas model (L g  1 min  1), QTH is the theoretical saturate adsorption capacity in Thomas model (mg g  1), Qf is the inlet feed flow rate (mL min  1). The coefficient of linear regression (R2) was 0.9997, indicating that Thomas model was suitable for the prediction of breakthrough curves. After adsorption experiment, the ZIF-8 column was regenerated by pumping desorption solvent (ethanol in our work) into the system to remove the HMF. Fig. 7 shows the desorption curve of HMF. The mass balance of HMF calculated as ratio of a total amount collected from the regeneration to the amount fed into the column is about 102%. The result is reasonable within the experimental error. To evaluate the adsorption capacity of ZIF-8 for HMF in the fixed-bed column, an experiment was conducted as follows. Firstly, the HMF trapped in the void space of the column was flushed out by water (Liu et al., 2012), and then, ethanol was used to regenerate the adsorbed HMF. The result was shown in Fig. 8. The HMF trapped in the void space of the column was about 56 mg according to water regeneration. Total quantity of HMF adsorbed in the column (packed with 0.159 g ZIF-8) was 126 mg calculated from the breakthrough curve using the Eq. (4) (Lin et al., 2013).  Z ttotal  C out;t qtotal ¼ Q f C f 1 dt ¼ Q f C f S ð4Þ Cf 0 where qtotal is the total adsorbed quantity of HMF in the ZIF-8 column (mg), ttotal is the total flow time (min), and S is the integration of the area above the breakthrough curve. Thus, the uptake of HMF with ZIF-8 was approximately 440 mg g  1, which was in accordance with the batch study. 3.3.2. Reusability of ZIF-8 packed column The reusability for ZIF-8 column was evaluated by comparing the breakthrough curves of HMF on regenerated and fresh ZIF-8 columns (Fig. 9). The Thomas model was applied to describe experimental data obtained. The parameters predicted from Thomas model were given in Table 2. From the coefficient of linear regression (R2), it can be concluded that there was a good agreement between the experimental and the simulated data predicted by Thomas model. No loss of the adsorption capacity (qexp: Equilibrium HMF uptake of ZIF-8 in the fixed-bed column) was observed on the regenerated ZIF-8 for the two times reuse, showing excellent reusability of ZIF-8 for HMF adsorption.

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order kinetic equation. HMF-loaded ZIFs could be fully regenerated by solvent assistant regeneration (with ethanol). The liquid phase breakthrough experiments of HMF on a column packed with ZIF-8 nanoparticles were conducted, showing a sharp breakthrough of HMF. The breakthrough data fitted well with the Thomas model. No loss of the adsorption capacity was observed on the regenerated ZIF-8 packed column with ethanol for two times reuse, showing excellent reusability of ZIF-8 for HMF adsorption. Moreover, ZIF-8 exhibits selective adsorption of HMF from HMF/ fructose/water mixtures either in batch adsorption or fix-bed adsorption. As a whole, the high adsorption capacity, fast adsorption kinetics as well as excellent reusability make ZIF-8 promising adsorbent for HMF recovery from aqueous solution. Fig. 9. Experimental (filled dots) and simulated (line) breakthrough curves of HMF on fresh and regenerated ZIF-8 packed columns.

Nomenclature Table 2 Parameters predicted from the Thomas model for HMF adsorption on ZIF-8 in fixed-bed column. Usage

Cf (g L  1) Thomas model kTH (L g  1 min  1) QTH (mg g  1) qexp (mg g  1) R2

Fresh 49.5 1st reuse 54.2 2nd reuse 54.5

0.030 0.020 0.034

777 789 829

786 798 830

0.9993 0.9997 0.9987

q V Cinitial Cfinal m Ce qe Q0 b qt tbatch k Cout,t Cf kTH QTH Qf t qtotal ttotal qexp

adsorption amount of the adsorbate (mg g  1) volume of the adsorbate (mL) initial concentration of the adsorbate (g L  1) final concentration of the adsorbate (g L  1) mass of the adsorbent (g) equilibrium concentration of the adsorbate (mg L  1) equilibrium adsorption amount (mg g  1) maximum adsorption amount (mg g  1) langmuir constant (L mg  1) adsorption amount at time t (mg g  1) adsorption time in batch study (h) pseudo-second-order constant (g mg  1 h  1) concentration of the solute in the effluent solution at time t (g L  1) initial concentration of the solute in solution (g L  1) kinetic constant in the Thomas model (L g  1 min  1) theoretical saturate adsorption capacity in Thomas model (mg g  1) inlet feed flow rate (mL min  1) time in column system (min) total adsorbed quantity of the adsorbate in the fixed-bed column (mg) total flow time (min) equilibrium uptake of the adsorbent in the fixed-bed column (mg g  1)

Fig. 10. Breakthrough curves of HMF and fructose on ZIF-8 packed column.

3.3.3. Breakthrough curves of HMF and fructose on ZIF-8 Liquid phase breakthrough experiments were also performed with equal amounts (50 g L  1) of HMF and fructose as the feed solution. The breakthrough curves in Fig. 10 showed that fructose appeared in the column outlet after a very short time. The breakthrough of HMF was observed after a long time interval. The result presented here proved the potential of ZIF-8 for effective separation of HMF and fructose in aqueous solution.

4. Conclusions In conclusion, a comparative investigation on liquid-phase batch adsorption of HMF with ZIF-8, ZIF-90 and ZIF-93 was carried out in this study. The maximum adsorption amount for HMF on ZIF-8, ZIF-90 and ZIF-93 at 25 1C were determined to be 465, 307, 279 mg g  1, respectively. The hydrophobic/hydrophilic properties of ZIFs are the factor governing the adsorption capacity of ZIFs for HMF. The adsorption isotherms of each ZIF could be fitted well to the Langmuir model. The sorption kinetics of HMF on ZIF-8 microcrystals could be well described by the pseudo-second-

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Please cite this article as: Jin, H., et al., Recovery of HMF from aqueous solution by zeolitic imidazolate frameworks. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.017i