water vapor adsorption and selectivity of novel C-PDA adsorbents with high uptakes of benzene and toluene

Chemical Engineering Journal 335 (2018) 970–978

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Benzene/toluene/water vapor adsorption and selectivity of novel C-PDA adsorbents with high uptakes of benzene and toluene ⁎

Xingjie Wanga, Chen Maa, , Jing Xiaoa, Qibin Xiaa, Junliang Wuc, Zhong Lia,b, a b c

T

⁎

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China State Key Lab of Subtropical Building Science of China, South China University of Technology, Guangzhou 510640, PR China Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, Guangzhou 510640, PR China

G RA P H I C A L AB S T R A C T

A R T I C L E I N F O

A B S T R A C T

Keywords: Adsorption TPD DIH equation C-PDA Water vapor VOCs

A series of novel porous carbon-based material C-PDA (Carbonized Polydopamine Adsorbent) was prepared using one-step synthesis method for VOCs adsorption, and then characterized. The BET surface area and pore volume of C-PDAs can reach as high as 3291 m2/g and 1.78 cm3/g, respectively. FTIR spectra suggested the presence of N/O functionalities on the surfaces of C-PDAs, and its contents decreased with the increasing KOH/C ratio at which the samples were prepared. Adsorption capacity of C-PDAs for VOCs at low pressure increased with the surface N and O contents of C-PDAs. And the adsorption capacity under a high relative pressure depends on the pore volume of the materials. Adsorption capacities of C-PDA for C6H6 and C7H8 reached as high as 19.1 and 15.8 mmol/g (1491.9 and 1455.8 mg/g) at P/P0 of 0.6, much higher than many other adsorbents. Isotherms of C-PDAs for water vapor exhibited S-shaped type of isotherms, indicative of weak adsorption of water vapor on the sample surfaces at low relative humidity. TPD experiments revealed adsorption mechanism of VOCs on CPDAs. The isosteric heats of benzene and toluene adsorption on C-PDA-4 were significantly higher than that of water vapor, which made C-PDA-4 possess a characteristic of favorable adsorption of benzene or toluene over water vapor. The adsorption selectivities of the C-PDA-4 for C6H6/H2O(g) and C7H8/H2O(g) were estimated by means of DIH (Difference of Isosteric Heats) equation, which reached as high as 99 and 13.6, respectively, much higher than those of MIL-101(Cr). The resultant C-PDA-4 is a promising adsorbent for adsorption of VOCs.

⁎

Corresponding authors at: School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China. (Z. Li) E-mail addresses: [email protected] (C. Ma), [email protected] (Z. Li).

http://dx.doi.org/10.1016/j.cej.2017.10.102 Received 7 September 2017; Received in revised form 16 October 2017; Accepted 17 October 2017 1385-8947/ © 2017 Elsevier B.V. All rights reserved.

Chemical Engineering Journal 335 (2018) 970–978

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1. Introduction

that the resultant PDA-derived carbon SMSs possessed high BET surface area of 2006 m2/g and high-level electroactive N. Wu et al. [20] synthesized a series of PDA-functionalized sorbents, and then used them for enrichment of low concentration (parts per billion) uranium in unpurified seawater. These adsorbents exhibited great potential for adsorption of low concentration uranium in seawater. Wang [21] fabricated a novel PDA and chitosan (CS) hybrid nano-biosorbent by assembling biomimetic polymer PDA and CS onto magnetic nanoparticles (NPs) and the nano-absorbent showed highly effective adsorption ability for some heavy metals and dyes. It was reported that its maximum adsorption capacities for Hg(II), Pb(II), Cr(VI), methylene blue and malachite green were up to 245.6 mg/g, 47 mg/g, 151.6 mg/ g), 204 mg/g) and 61 mg/g, respectively. Xian [22] and Wang [23] also prepared polydopamine-based carbon materials with both abundant porosity and N-functionalities. The resultant polydopamine-based carbon materials showed super-high CO2 adsorption capacity of 30.5 mmol/g at 30 bar and 298 K, and besides its C2H6 and C2H4 adsorption capacities reached as high as 7.93 and 6.61 mmol/g, being fully comparable to MOFs. Hu et al. [24] used PDA as N source to manufacture N-coordinated UiO-66(Zr) by Mechano-Chemical method, which showed high CHO-/Cl-VOCs uptakes of 4.94 and 9.42 mmol/g at 298 K and good hydrophobicity. Zhu et al. [25] prepared PDA-derived porous carbon (MGBC) with BET surface area of 2085 m2/g and its adsorption capacities for benzene and toluene were around 6 and 5.8 mmol/g at 298 K and P/P0 < 0.12, respectively. Interestingly, MGBC6 showed a hydrophobic character and a higher working capacity for VOCs than several MOFs under a low relative pressure. In another work, Zhu [26] reported that dopamine-derived N-PCs can selective adsorption towards p-xylene under humid conditions and the high amount of sp2 C/N played an important role in the adsorption. Due to N-rich functionalities of PDA and the stable structure of carbon materials, as well as tunable textural properties during carbonization and activation, it is worthy of utilizing PDA as the precursor to prepare stable and high-porosity carbon materials for VOCs adsorption. In this work, we proposed a “one-step” synthesis method to prepare Carbonized Polydopamine Adsorbents (C-PDAs) with super-high surface area and high VOCs adsorption capacity. The resultant samples were characterized by nitrogen adsorption and desorption, scanning electron microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Kinetic TPDs with the adsorption time as the variation were carried out to exam the adsorption sites. Benzene, toluene, water vapor and nitrogen isotherms at different temperatures of C-PDAs were separately measured. And the isosteric heats of benzene, toluene, and H2O(g) adsorption over C-PDAs were estimated. The benzene/H2O(g) and toluene/H2O(g) adsorption selectivities were calculated by using DIH (Difference of the Isosteric Heats) equation on the basis of their isosteric heats of adsorption on CPDAs. Furthermore, the same experiments were also conducted on MIL101 and the results were in comparison with the C-PDAs. Finally, the adsorption mechanism and selectivity of benzene and toluene on CPDAs were also discussed and reported here.

Emission of VOCs (Volatile Organic Compounds) has been recognized as one of the major contributors to air pollution. The main sources of VOCs are petroleum refineries, fuel combustions, chemical industries, pharmaceutical plants, automobile industries, solvents processes, cleaning products and so on. Many VOCs are classified as known or possible human carcinogens, irritants, and toxicants, and VOCs exposure has been associated with asthma and other respiratory symptoms/diseases [1]. The release of these anthropogenic toxic pollutants into the atmosphere is a worldwide threat of growing concern [2]. More and more countries and regions have proposed stringent legislations to impose stringent standards on VOC emissions from industries. Therefore, it is urgently needed to develop more safe and efficient systems for the removal of VOCs from polluted air. There are many techniques available to abate the emission of VOCs, such as adsorption [3,4], catalytic oxidation [5], condensation [6], and membrane separation [7]. Among diversity technologies, adsorption is considered as one of the most cost-effective and environmentally friendly technologies for the removal of VOCs, especially at low concentration. And adsorbents play a key role in the adsorption technique. There mainly are two types of adsorbents. One is conventional adsorbents such as activated carbon (AC), silica gel, activated alumina, zeolites and so on, these adsorbents possessed advantages of structure stability and low cost, and have been applied in practice. However, low adsorption capacity of VOCs is its disadvantage, limiting their wider application. The other is metal-organic frameworks (MOFs). MOFs as a new class of porous materials has been rapidly developed for VOCs adsorption due to their ultrahigh surface area, sturdy, open crystalline structure and adjustable chemical functionalities [8,9]. Barea et al. [8] reviewed performances of MOFs in environmental remediation processes, and indicated that some of MOFs and modified MOFs exhibited remarkable adsorption capacities and good selectivities of VOC molecules. Zhao et al. [10,11] reported that the maximum capacities of MIL101 for benzene and p-xylene reached as high as 16.5 and 10.9 mmol/g at 288 K, respectively. Shi et al. [12] reported that the adsorption capacity of MIL-101 for ethyl acetate was up to 10.5 mmol/g at 288 K and 54 mbar. Zhao et al. [13] studied the competitive adsorption and selectivity of benzene and water vapor on HKUST-1(Cu-BTC), and reported that the benzene adsorption capacity of HKUST-1 was up to 9.62 mmol/g and its benzene/H2O(g) adsorption selectivity was about 8.32 at 318 K. Lv et al. [14] synthesized MOF-5-B with BET area of 3465.9 m2·g−1 and its adsorption capacities of the alkanes (C3–nC7) were much higher than those of conventional activated carbons and zeolites. These studies above showed that the capacities of MOFs for VOCs were much higher than traditional materials such as activated carbons and zeolites, showing a great application prospect in the fields of environmental remediation and protection. Unfortunately, up to now, none of MOFs can be used as an effective adsorbent applied in the actual cases. They are facing the challenge of high cost and steam instability, which needs to be overcome further. On the other hand, in recent years, some researchers were trying to use new carbon sources such as dopamine and asphalt to prepare novel porous carbon materials with excellent adsorption performance for adsorption of trace pollutants in water and air. It was reported that asphalt can be used to prepare novel porous carbon with high surface area and pore volume, showing excellent adsorption performance for olefin/paraffin adsorption and separation [15] and high adsorption capacity toward CO2 adsorption [16]. Besides, dopamine hydrochloride and dopamine monomer, which can be prepared on scale separately by organic synthesis of opening loop of the pepper ethylamine and by decarboxylation reaction of the L-3,4-dihydro-xyphenylalanine [17,18], are nontoxic, widespread, and sustainable resource [19]. Therefore, some researchers used the polydopamine (PDA) to prepare novel porous carbon materials. Lu et al. [19] used PDA to prepare porous carbon materials by using a two-step synthesis method, and reported

2. Experimental section 2.1. Materials Dopamine hydrochloride (C8H11NO2·HCl, 98%) was obtained from Shanghai Jingchun Sci-Tech Co. Ltd. Ethanol (C2H5OH), ammonia aqueous solution (NH4OH, 25%), potassium hydroxide (KOH) and concentrated hydrochloric acid (HCl) were purchased from Guangzhou Guanghua Sci-Tech Co. Ltd. For the adsorption tests, benzene and toluene (> 99.0%, Tianjin) were used as received from vendors without further purification. N2 with purity of 99.99% was purchased from Guangzhou ZhuoZheng and He (99.999%, Guangzhou ZhuoZheng) was used as a backfill gas during the adsorption tests. 971

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2.2. Preparation of C-PDAs

for 6 h prior to the measurements.

Synthesis of polydopamine (PDA): Synthesis of polydopamine as carbon precursor was required before C-PDAs were prepared. Polydopamine can be obtained by self-polymerization reaction of dopamine. A dopamine molecule consists of a catechol structure (a benzene ring with two hydroxyl side groups) with one amine group attached via an ethyl chain. Synthesis of polydopamine can be carried out according to a typical synthesis method [19]. Firstly, NH4OH (0.75 mL, 25 ∼ 28%) was mixed with a liquid mixture of ethanol (40 mL) and deionized water (90 mL). Then the mixture was stirred mildly for 30 min. Secondly, dopamine hydrochloride solution (0.5 g dopamine hydrochloride in 10 mL deionized water) was injected into the mixture solution. After that, the reaction was allowed to proceed for 30 h at room temperature, and thus polydopamine was obtained. The resulting products were filtered, washed with distilled water and ethanol, followed by dried at 353 K under vacuum. Finally, PDA spheres were obtained. Preparation of carbonized PDAs (C-PDAs): The prepared PDA spheres as carbon precursor can be used to prepare C-PDAs by two-step synthesis method [19], which can prepare porous carbon materials with BET area about 2000 m2/g. However, in this work we propose one step synthesis method for preparation of the porous C-PDA to obtain the adsorbents with higher BET area. First, the PDA spheres were mixed with KOH at varied KOH/C ratios (2:1, 3:1, and 4:1), and then the solid mixture were activated at 700 °C in a tube furnace under flowing Ar (60 mL/min, 99.9%) for 1 h with a heating rate of 5 K/min, thus the CPDAs were obtained. After the samples were cooled naturally to room temperature, the resulting C-PDAs were washed with HCl solution(1M) to remove the excess KOH, and then washed with hot distilled water for several times until the pH reached 7 to remove KOH residue. Finally, the obtained C-PDAs were dried in vacuum at 353 K for at least 8 h, and the resulting C-PDAs activated with KOH/C ratios of 2:1, 3:1 and 4:1 were denoted as C-PDA-2, C-PDA-3 and C-PDA-4, respectively.

2.4. Measurement of adsorption isotherms of benzene and toluene vapor Adsorption isotherms of benzene and toluene vapor on C-PDAs samples were measured by using a standard static volumetric method on the Micromeritics 3flex (Micromeritics Instrument Corporation, USA) at 288–308 K. A stainless steel chamber with a hard seal and manual cutoff valve, which was attached in place of the Psat tube, was used to generate the VOC vapor. The adsorption temperature was achieved by putting sample cell into a circulating water bath. 60 mg sample was needed for each run. And before the measurement, all the samples needed to be degassed under vacuum at 423 K for at least 6 h. 2.5. Measurement of adsorption isotherms of water vapor Isotherms of water at 298 K–318 K were measured on gravimetric water sorption analyzer (AQVADYNE DVS, Quantachrome Instruments, USA) equipped with a microbalance with an accuracy of 1 μg. The system for measuring water vapor adsorption was consists of flow controllers chamber, humidifier chamber and microbalance chamber. And the samples, about 10 mg, also needed to be degassed at 423 K for at least 6 h before measurement. 2.6. Temperature programmed desorption experiments Temperature programmed desorption (TPD) is an effective technique of surface analysis. In this work, TPD experiments were conducted to estimate the binding force between an adsorbate and an adsorbent, and kinetic TPD experiments were carried out to explore the adsorptive sites for the VOCs. TPD experiments were carried out on gas chromatography workstation (GC-7890, Agilent, USA) with an accuracy of 0.1 K for measurement of temperature. TPD experiments were conducted at a fixed heating rate of 8 K/min. For each run, 5 mg samples were weighed to adsorb the VOCs vapor for different length of time. And then they were packed in a stainless steel reaction tube with inner diameter of 0.3 cm and packed length of about 0.5 cm. The stainless tube was placed in a reaction furnace and heated in the high-purity N2 flow at a constant rate of 30 mL/min. The desorbed VOCs were measured on line by GC7890 chromatograph with a flame ionization detector at the outlet of the reaction tube and effluent curves (TPD curves) were recorded.

2.3. Adsorbent characterizations The textural properties of the synthesized materials were determined by N2 adsorption and desorption at 77 K using a Micromeritics ASAP 2020 apparatus. The surface area was calculated by using the Brunauer-Emmett-Teller (BET) equation applied to nitrogen adsorption data in the relative pressure (P/P0) range of 0.05–0.35. The total pore volume was determined from the amount of nitrogen adsorbed at P/ P0 = 0.989. The porosity distribution was determined by using the original density functional theory (DFT) model. FTIR spectra of all samples in KBr pellet were recorded on a Bruker Vector33 spectrometer. Thermogravimetric analysis (TGA) was performed on a NETZSCH STA449C in nitrogen atmosphere at a heating rate of 5 °C/ min. The morphology was observed from a scanning electron microscope (SEM, Hitachi S-4800). And the XPS was performed on a Thermo ESCALAB 250. All the samples were degassed under vacuum at 150 °C 5

(a) 3

50

Differential Pore Volume(cm /g)

Quantity Adsorbed (mmol/g)

60

40 30 A-C-PDA-2 D-C-PDA-2 A-C-PDA-3 D-C-PDA-3 A-C-PDA-4 D-C-PDA-4

20 10 0 0.0

3. Result and discussion 3.1. Sample characteristics Fig. 1a shows N2 adsorption isotherms at 77 K of the samples CPDAs. These isotherms exhibited typical type-I profiles with significant adsorption amount below relative pressure P/P0 of 0.1, corresponding to the typical microporous structure. Fig. 1b presents the DFT pore-size

(b)

C-PDA-4 C-PDA-3 C-PDA-2

4

3

2

1

0

0.2

0.4

0.6

Relative Pressure (P/P0)

0.8

1.0

0.5

1

2

4

Pore Width(nm)

972

8

16

32

Fig. 1. (a) N2 adsorption-desorption isotherms of samples at 77 K; (b) DFT Pore-size distribution of C-PDAs.

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20

that the shape of C-PDAs was uniformly spherical at low activated KOH/C ratio and transferred to amorphous form gradually with an increase in KOH/C ratio at which C-PDAs were prepared due to the collapse of the carbon sphere surface, being etched by the KOH. Besides, the particle sizes of the samples were all in the range of 500–600 nm and they turned smaller by degrees.

Quantity Adsorbed (mmol/g)

Benzene 16

12

8

3.2. Isotherms of benzene and toluene on C-PDAs

C-PDA-2 C-PDA-3 C-PDA-4

4

0 0.0

0.1

0.2

0.3

0.4

0.5

Fig. 2 presents the isotherms at 298 K of benzene and toluene on the samples. It was noticed that at pressure below P/P0 of 0.08, the amounts adsorbed of benzene on the samples followed the order: CPDA-2 > C-PDA-3 > C-PDA-4, corresponding to the order of the pore sizes of the samples, which was ascribed to the smaller pore sizes resulting in the stronger attraction forces acting on the adsorbate molecule because of the overlapping potentials from the surrounding wall. With an increase in pressure from P/P0 = 0.17, the benzene adsorption capacity of the samples followed the order: C-PDA-4 > C-PDA-3 > CPDA-2, which is consistent with the order of the BET surface area and pore volumes of the samples. The reason was that higher surface area and pore volume could supply more adsorptive sites and larger volume to accommodate more benzene molecules at high relative pressure. Such similar phenomenon was observed for toluene adsorption on the samples. For comparison, Table 2 lists the adsorption capacities of C-PDA-4 and some other materials for benzene and toluene at 298 K and P/P0 of 0.2. The data in Table 2 indicated that the adsorption capacities of CPDA-4 for benzene and toluene were higher than those of MOFs, zeolites and activated carbons. Besides, Table S1 also lists VOCs adsorption capacities under P/P0 of 0.6 for comparison. It is noteworthy that the adsorption capacities of C-PDA-4 for benzene and toluene reached as high as 19.1 and 15.8 mmol/g (1491.9 and 1455.8 mg/g), respectively, at 298 K and P/P0 of 0.6.

0.6

Relative Pressure (P/P0) Fig. 2a. Adsorption isotherms of benzene on C-PDAs at 298 K.

20

Toluene Quantity Absorbed (mmol/g)

16

12

8

C-PDA-2 C-PDA-3 C-PDA-4

4

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

Relative Pressure (P/P0) Fig. 2b. Adsorption isotherms of toluene on C-PDAs at 298 K.

distribution of C-PDAs. It was observed that the pore-size distribution became wider and the mesopore portion (> 2 nm) became more with an increase in KOH/C ratio (from 2 to 4) at which the samples were activated. The reason was that when the KOH/C ratio increased, the activation reaction of carbon became severer, and thus the transformation of some micropores to larger pores (mesopores) occurred, resulting in more mesopore formation in C-PDAs. Table 1 lists the BET surface area and pore volume of the C-PDAs prepared with varied KOH/C ratio. The surface area and pore volume of C-PDAs gradually increased with the KOH/C ratio, and at the KOH/C ratio of 4, the BET surface area and total pore volume of the resultant CPDA-4 separately reached as high as 3291 m2/g and 1.78 cm3/g, and the micropore volume of C-PDA-4 showed a very large value of 0.883 cm3/g, all of these parameters were much higher than many reported activation carbons and templated carbon materials, and were also comparable to many MOFs. Combined with all these conditions, it is believed that C-PDAs can be excellent adsorbents for VOCs adsorption not only under low pressure but also high relative pressure. Fig. S2 presents the SEM image of the samples. It was clearly visible

Table 2 The adsorption capacities of C-PDA and some other materials for benzene and toluene at P/P0 of 0.2.

Table 1 Textual properties of the C-PDAs. Materials

C-PDA-2 C-PDA-3 C-PDA-4

Textual Properties SBET (m2/g)

SLangmuir (m2/g)

Vtotal (cm3/g)

Vmicro (cm3/g)

2715.42 3160.35 3291.03

3301.88 4410.96 4637.78

1.16 1.51 1.78

0.514 0.725 0.883

973

VOC

Materials

Q (mmol/g)

Q (mg/g)

T (K)

Reference

Benzene

C-PDA-4 MIL-101 Activated carbon SBA-15 HZSM-5 MFOF-1a Cu2L Ni(bpb) Zn(bpb) MAF-2 FMOF-1 MOF-177 Zeolite L

15.57 15.02 7.01 1.75 1.74 4.56 2.04 4.65 2.0 2.45 1.89 8.01 1.15

1216.17 1173.21 547.55 136.69 135.91 356.18 159.34 363.21 156.22 191.37 147.63 625.66 89.83

298 298 303 303 303 298 298 303 303 298 298 298 298

Present work Present work [27] [27] [27] [28] [29] [30] [30] [31] [32] [33] [34]

Toluene

C-PDA-4 MIL-101 Beaded activated carbon MOF-177 Activated carbon PCH HKUST-1 Cu-BTC@GO Pine-activated carbon Silica gel PAC3.0 PWC3.0 CPOP-15 Zeolite L

13.62 4.98 5.38

1254.95 458.86 495.71

298 298 298

Present work Present work [35]

3.2 1.92 1.25 2.82 5 8.96

294.85 176.91 115.18 259.83 460.7 825.57

298 293 298 298 298 298

[33] [37] [38] [36] [36] [39]

3.84 8.96 6.14 5.12 1.08

353.82 825.57 565.74 471.76 99.51

298 298 298 298 298

[40] [41] [41] [42] [34]

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Fig. 3. Comparision of adsorption capacity for two kinds of VOCs on C-PDA-4.

10

15

10

5

Benzene Toluene 0 0.0

0.1

0.2

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0.5

0.6

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Quantity Adsorbed (mmol/g)

Quantity Adsorbed (mmol/g)

20

8

6

4

2

0 0.00

Benzene Toluene 0.02

0.04

0.06

0.08

0.10

Relative Pressure (P/P0)

(a)

(b)

3.3. Adsorption mechanism of benzene and toluene

3.4. TPD curves of benzene and toluene on C-PDA-4

Fig. 3 presents isotherms of benzene and toluene on C-PDA-4. It was observed that at pressure below P/P0 = 0.04 the adsorption capacity of C-PDA-4 for toluene was higher than its benzene adsorption capacity, implying that the interaction of toluene with the surfaces of C-PDA-4 was stronger than benzene at low coverage. For a given adsorbent, the adsorbate–adsorbent interaction potential mainly depends on the properties of the adsorbate. The polarizability of an adsorbate, is one of the most important properties, determines the interactions between the adsorbates and an adsorbent, which increases with the molecular weight because more electrons are available for polarization. Table S2 lists physical parameters of benzene and toluene. The data in Table S2 indicated that the polarizability of toluene is higher than benzene, and thus it would make the interaction of toluene with the polar adsorbent surface become stronger than benzene. In order to get well-understanding of the interaction between the VOCs and the surfaces, the binding energies (BE) were calculated by Density Functional Theory (DFT) using DMol3 code. And the calculation results are shown in Fig. 4. Both benzene and toluene interacted dominantly through π-π stacking conjugation with carbon material CPDA. Besides, apart from the π system, the presence of the methyl group in toluene acted as electron donor to strengthen the π conjugation, and thus, toluene exhibited relatively more exothermic binding energy with the sample C-PDA (58.56 kJ/mol) than that of benzene (54.38 kJ/mol). On the other hand, it was noticed that the benzene adsorption capacity of C-PDA became higher than toluene at pressure above P/ P0 = 0.04, as shown in Fig. 3a. The reason may be that VOCs adsorption capacities in the higher pressure range are dominated mainly by the pore volume. For a given adsorbent, its pore volume is constant, and thus the number of molecules with smaller kinetic diameter entering the pores becomes more compared to the molecules with larger kinetic diameter. Since kinetic diameter of benzene is smaller than toluene, the C-PDA can accommodate more benzene molecules than toluene in the region of the higher pressure, exhibiting higher molar adsorption capacity of benzene compared to toluene.

To get further understanding of benzene and toluene adsorption on the sample C-PDA-4, TPD experiments were carried out, in which the samples for TPD were prepared by adsorbing benzene or toluene for 5, 10, 20, 40 and 80 s separately. Fig.5 TPD shows spectrums of benzene and toluene desorption from the samples C-PDA-4 with different amounts adsorbed of benzene and toluene. It can be seen that with the VOCs adsorption time of the sample increased (from 5 s to 80 s), the desorption peak area of benzene and toluene on the sample increased correspondingly since the longer the adsorption time of the sample for VOCs, the more the amount of VOCs was adsorbed, resulting in desorption of more VOCs in TPD experiments. Noteworthy, only one desorption peak was observed from benzene TPD curves, implying that there exists only one type of adsorptive sites for benzene on the C-PDA surfaces, and from toluene TPD curves, two desorption peaks were clearly observed on the samples with adsorption of toluene for 10 and 20 s separately, implying that there were two types of adsorptive sites on the C-PDA surfaces at low surface coverage for toluene. The one was centered at about 500 K, an indicative of toluene desorption from strong adsorptive sites, and the other one was centered at about 469 K, an indicative of toluene desorption from weak adsorptive sites. From TPD curve of toluene on the sample with previous adsorption of toluene for 5 s, one desorption peak centered at about 500 K of toluene was observed. From TPD curve of toluene on the sample with previous adsorption of toluene for 10 s, besides the desorption peak centered at about 500 K, the second desorption peak centered at about 469 K was also observed. This meant that during adsorption process toluene was firstly adsorbed on the strong adsorptive sites of the C-PDA-4 surfaces, and then adsorbed on the weak adsorptive sites. 3.5. Isotherms of water vapor adsorption on C-PDAs Water vapor isotherm of an adsorbent is a very important thermodynamic property for evaluating the affinity of the adsorbent toward water vapor since water vapor is everywhere and thus results in competitive adsorption with other gas molecules in realistic word inevitably. If the affinity of an adsorbent with water vapor is relatively Fig. 4. Binding energy between benzene/toluene and CPDA.

974

9600

24300

7200

16200

Signal (a.u.)

32400

4800

8100

2400

0

0

(s )

tim e

tim e

40s ad so rp tio n

Ad so rp tio n

20s

20s 10s

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5s 300

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650

Benzene

Temperature (K)

(s )

80s

80s 40s

signal (a.u.)

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300

350

400

450

500

550

600

650

Temperature (K)

Toluene

Fig. 5. TPD spectrums of benzene and toluene desorption from the sample C-PDA-4 with different amounts adsorbed of VOCs.

Quantity Adsorbed (mmol/g)

100

even though at very low relative humidity it began to adsorb great deal of water vapor in comparison with C-PDAs, implying that MIL-101(Cr) was more hydrophilic than C-PDAs.

MIL-101 C-PDA-2 C-PDA-3 C-PDA-4

80

3.6. Isotherms of N2 adsorption on C-PDAs

60

Fig. 7 presents the isotherms of N2 on C-PDAs at 298 K. The N2 adsorption capacity of C-PDAs was very low (lower than 0.8 mmol/g at pressure below 100 kPa), an indicative of weak affinity between the adsorbate and the adsorbents.

40

20

3.7. Isosteric heats of benzene, toluene and water vapor adsorption on CPDAs

0 0

20

40

60

80

The isosteric heat of adsorption reflects the interaction between the adsorbate molecules and the adsorbent surfaces at specific surface loading [43], which is a function of the surface coverage. The isosteric heats of adsorption can be calculated from isotherms at different temperatures by using Clausius-Clapeyron equation [44,45], which is expressed as follows:

100

Relative Humitity (%) Fig. 6. The adsorption isotherm of water vapor on C-PDAs at 298 K.

weak, it could effectively reduce competitive adsorption of water vapor in practical applications. Fig. 6 shows the isotherms of water vapor on C-PDAs at 298 K. These isotherms exhibited S-shaped type of isotherms, indicative of weak adsorption of water vapor on the sample surfaces at low relative humidity. The amounts adsorbed of water vapor were very low when the relative humidity was below 40%, then it began to rise obviously as the relative humidity increased, and it began to increase sharply after the relative humidity reached 50 or 60%. It was observed that at the relative humidity below 60%, the adsorption capacity of the samples for water vapor on followed the order of C-PDA-2 > C-PDA-3 > C-PDA-4, which was accordant to the order of O and N contents on the surfaces of C-PDAs. Table S3 lists the surface O and N contents of C-PDAs, which were in the order of C-PDA-2 > C-PDA-3 > C-PDA-4. Generally, H2O molecules are readily to bond oxygen and nitrogen atoms on the surfaces of C-PDAs by hydrogen bonding. Low surface O and N contents would be favorable to weaken competitive adsorption water vapor on the surfaces of C-PDAs, resulting in the lowest amount adsorbed of water vapor on C-PDA-4 among the three samples. On the other hand, it was observed that at high relative humidity the adsorption capacity of water vapor on the samples followed the order as C-PDA-4 > C-PDA3 > C-PDA-2. The reason was that at high relative humidity the adsorption capacity of water vapor was mainly dependent on the pore volumes of the samples. As shown by Table 2, the pore volumes of the samples also followed the order as C-PDA-4 > C-PDA-3 > C-PDA-2. On the other hand, MIL-101(Cr) was one of MOFs with super-high adsorption capacity of VOCs. For comparison, Fig. 6 also shows water vapor isotherm of MIL-101(Cr), exhibiting V type isotherm. MIL101(Cr) possessed a rather high adsorption capacity of water vapor and

lnp = −

ΔHS +C RT

(1)

where ΔHs (kJ/mol) is the isosteric heat, R (kJ/mol·K) is the ideal gas constant, p (Pa) is vapor pressure, and C is an integration constant. After the isotherms of benzene and toluene as well as water vapor and N2 at different temperatures were obtained (See Fig. S3, S6a and S7a in Supporting Information), the isosteric heats of adsorption can be

2.0 CPDA-2 CPDA-3 CPDA-4

Quantity Adsorbed (mmol/g)

N2 1.6

1.2

0.8

0.4

0.0 0.0

0.2

0.4

0.6

0.8

Relative Pressure (P/P0) Fig. 7. The adsorption isotherm of N2 on C-PDAs at 298 K.

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Heat of adsorption (kJ/mol)

90

adsorption separation efficiency in the realistic applications. It is wellknown that direct measurement of mixed-gas adsorption equilibrium and selectivity remains one of the most difficult experimental techniques in the adsorption field today, especially for adsorption systems of mixtures containing molecules that vary significantly in size, polarity, or adsorption interactions (e.g., one component adsorbs very strongly over the other species) [46]. Thus, the adsorption selectivity of an adsorbent toward binary mixtures could usually be predicted by means of theoretical or semi-empirical model. In this work, we tried to use DIH (difference of the isosteric heats) equation to estimate VOCs/H2O vapor adsorption selectivities on C-PDA-4 and MIL-101(Cr). DIH-based equation was proposed by Wang and Cao [47], they thought that the adsorption selectivity of an adsorbent toward binary gas mixture is closely related to the difference between the isosteric heats of two gases in binary mixture. DIH-based equation can be expressed as follows [47,48]:

C-PDA-4-Benzene C-PDA-4-Toluene C-PDA-4-Water C-PDA-4-N2

75

60

45

30

0

2

4

6

8

10

12

Quantity Adsorbed (mmol/g) Fig. 8a. Isosteric heats of benzene, toluene, water vapor and N2 adsorption on C-PDA-4.

i Sads ⎛⎜ ⎟⎞ = ⎝ j⎠

estimated. Firstly, these isotherms were converted to their adsorption isosteres. Then the lnp was plotted to 1/T at a given amounts adsorbed of adsorbates on the basis of equation (1), a straight line with a slope of -ΔHs/R was yielded. Finally, the isosteric heats (ΔHs) of this adsorbate adsorption can be calculated directly from the slope -ΔHs/R of the plotted straight line. Fig. 8a shows the isosteric heats of benzene, toluene and water vapor adsorption on C-PDA-4. The isosteric heats of benzene (56.8 kJ/ mol) and toluene (64.5 kJ/mol) adsorption on C-PDA-4 were significantly higher than that of water vapor (47.9 kJ/mol), implying that C-PDA-4 possessed a characteristic of favorable adsorption of benzene or toluene over water vapor. In addition, it was noticed that the isosteric heat of benzene remained constant with an increase in the amounts adsorbed, and was nearly independent on the surface coverage or the amounts adsorbed of benzene on the surfaces of C-PDA-4, indicating that the surface adsorption sites of C-PDA-4 were energetically homogeneous towards adsorption of benzene. In contrast, the isosteric heats of toluene and water vapor adsorption decreased slightly with the amounts adsorbed of toluene or water vapor, indicating that the surfaces of C-PDA-4 were energetically heterogeneous towards adsorption of toluene and water vapor. For comparison, Fig. 8b shows the isosteric heats of benzene, toluene and water vapor adsorption on MIL-101(Cr). It indicated that the isosteric heats of water vapor adsorption on MIL101(Cr) was higher than that on C-PDA-4 after certain adsorption qualities, implying that H2O(g) would form stronger competitive adsorption with VOCs on MIL-101(Cr) than on C-PDAs.

(2)

where i and j present components i and j, respectively. Sideal = Ni(Pi)/ Nj(Pj), Ni(Pi) and Nj(Pj) are equilibrium amounts adsorbed of pure component i and component j at pressure Pi and Pj, separately. According to the different proportion of binary components and total N pressure, two partial pressure, Pi and Pj, can be confirmed. Sideal = Ni , j

and Ni or Nj (same unit with Ni) are static adsorption amounts of pure Δqst0

component i or j at Pi or Pj. S0 = 0.716 RT , and S0 is only related with the Δqst0 . Δqst0 (kJ/mol) is equal to the difference between the isosteric heats of the two components corresponding to Ni and Nj. Δqst0 can represent the difference of the interaction strengths between materials and the two gases. R (=8.314 × 10−3 kJ·mol−1·K−1) is molar gas constant and T(K) presents temperature. In this work, VOCs/H2O adsorption selectivities of C-PDA-4 and MIL-101(Cr) were estimated using equation (2) on the basis of the isosteric heats of C6H6, C7H8, and water vapor on the samples present in Fig.8. Fig. 9a shows C6H6/H2O(g) and C7H8/H2O(g) adsorption selectivities of the samples for adsorption system with initial VOCs concentration of 1000 ppm and relative humidity of 70%. It showed that the toluene/H2O(g) selectivity of C-PDA-4 was much higher than benzene/H2O(g) selectivity. The C7H8/H2O(g) adsorption selectivity was in the range of 3.5–99, and the C6H6/H2O(g) adsorption selectivity was in the range of 2–13.6. It was mainly attributed to the stronger interaction of toluene with C-PDA-4 in comparison with benzene. It was well known that MIL-101(Cr) was an excellent adsorbent for adsorption of VOCs [10–12]. For comparison, Fig. 9b presents C6H6/H2O(g) and C7H8/H2O(g) adsorption selectivities of MIL-101(Cr). It was clearly visible that the adsorption selectivities of MIL-101(Cr) for C6H6/H2O(g) and C7H8/H2O(g) were separately in the range of 0.62–2.04 and in the range of 1.97–5.32, much lower than those of C-PDA-4. It could be attributed to the stronger interaction of MIL-101(Cr) with H2O(g) than that of C-PDA-4 resulting in stronger competitive adsorption of H2O(g) on MIL-101(Cr). The results above indicated that the C-PDA would possess more excellent performance for VOCs adsorption from moist air, and C-PDA would be a potential adsorbent for VOCs adsorption in realistic application.

3.8. Estimation of VOCs/H2O(g) adsorption selectivity on C-PDA-4 The adsorption selectivity is a significant parameter to evaluate the 90

Heat of adsorption (kJ/mol)

Sideal·S0

Benzene Toluene H2O 60

4. Conclusion

30

A series of dopamine-derived adsorbents C-PDAs were synthesized using one-step synthesis method, then characterized and examined for benzene, toluene and H2O(g) adsorption property. The resulting C-PDA4 exhibited super-high adsorption capacities for benzene and toluene of 19.1 and 15.8 mmol/g (1491.9 and 1455.8 mg/g), respectively at 298 K and P/P0 of 0.6. Isotherms of water vapor on C-PDAs at 298 K showed that the amounts adsorbed of water vapor were very low at the relative

0 0

3

6

9

12

Quantity Adsorbed (mmol/g) Fig. 8b. Isosteric heats of benzene, toluene and water vapor adsorption on MIL-101(Cr).

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Benzene/H2O

MIL-101

Toluene/H2O

Toluene/H2O 6

Selectivity

80

Selectivity

Fig. 9. DIH-equation predicted selectivities for C6H6/C7H8/ H2O mixtures on C-PDA-4 and MIL-101 at 298 K.

8

Benzene/H2O

C-PDA-4

100

60

40

4

2 20

0 1.0

1.5

2.0

2.5

Pressure (kPa)

(a)

3.0

0 0.8

1.6

2.4

3.2

Pressure (kPa)

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humidity below 40%, which is critical for realistic application of CPDA-4. TPD experiments revealed that there were two types of adsorptive sites for toluene while only one type of adsorptive sites for benzene on C-PDA-4. A DIH-equation was used to predict the VOCs/ H2O(g) adsorption selectivity of C-PDA-4, which reached as high as 99 and 13.6 for C7H8/H2O(g) and C6H6/H2O(g), respectively, indicating that C-PDA-4 is a potential adsorbent for adsorption of VOCs under the moist atmosphere. C-PDAs had exhibited excellent adsorption property toward benzene and toluene. Since benzene and toluene are representative compounds of non-polar and polar VOCs, respectively, it can be considered that C-PDAs are suitable to adsorb other non-polar and polar VOCs from multi-component mixture. However, regeneration of the C-PDA needs to be investigated before it can be applied in practical adsorption of VOCs. The application of hot water vapor or vacuuming to regenerate the C-PDA is worthy of evaluation and optimization. In addition, scale production of C-PDAs is also needed to address in future. Acknowledgment This work was supported by Key Program of National Natural Science Foundation of China (No. 21436005), National Natural Science Foundation of China (No. U1662136, 21776097), the Research Foundation of State Key Lab of Subtropical Building Science of China (C715023z), the Guangdong Province Science and Technology Project (No. 2016A020221006) and the Fundamental Research Funds for the Central Universities. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2017.10.102. References [1] I. Paciencia, J. Madureira, J. Rufo, et al., A systematic review of evidence and implications of spatial and seasonal variations of volatile organic compounds (VOC) in indoor human environments, J. Toxicol. Environ. Health-Part B-Crit. Rev. 19 (2) (2016) 47–64. [2] X.B. Zhu, X. Gao, C.H. Zheng, Plasma-catalytic removal of a low concentration of acetone in humid conditions, RSC Advance 4 (2014) 37796–37805. [3] S.J. Zhang, T. Shao, Kose H. Selcen, Tanju Karanfil, Adsorption of aromatic compounds by carbonaceous adsorbents: a comparative study on granular activated carbon, activated carbon fiber, and carbon nanotubes, Environ. Sci. Technol. 44 (16) (2010) 6377–6383. [4] Y.S. Chen, Y.C. Hsu, C.C. Lin, C.Y.D. Tai, H.S. Liu, Volatile organic compounds absorption in a cross-flow rotating packed bed, Environ. Sci. Technol. 42 (7) (2008) 2631–2636. [5] J.C. Fang, X. Chen, Q.B. Xia, H.X. Xi, Z. Li, Effect of relative humidity on catalytic combustion of toluene over copper based catalysts with different supports, Chin. J. Chem. Eng. 17 (5) (2009) 767–772. [6] H. Zaitan, D. Bianchi, O. Achak, T. Chafik, A comparative study of the adsorption and desorption of o-xylene onto bentonite clay and alumina, J. Hazard. Mater. 153

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