Scholarly article on topic 'Study of the Influence of Pore Width on the Disposal of Benzene Employing Tunable OMCs'

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Academic research paper on topic "Study of the Influence of Pore Width on the Disposal of Benzene Employing Tunable OMCs"



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Study on the influence of pore width to the disposal of benzene by employing tunable OMCs

Gang Wang, Zhongshen Zhang, Junhui Wang, Na Li, and Zhengping Hao

Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie5037325 • Publication Date (Web): 06 Jan 2015

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Study on the influence of pore width to the disposal of benzene by employing tunable OMCs

Gang Wang^ Zhongshen Zhang^ Junhui Wang^, Na Li^ and Zhengping Hao*^

^Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

Corresponding author

*Phone: +86-10-62923564. E-mail:

area indicated that the adsorbing behaviors were strongly influenced by the pore

4 ABSTRACT: By employing SBA-15 as a hard template and boric acid as a pore

6 expanding agent, a series of tunable ordered mesoporous carbon (OMC) with high 8

9 specific surface area and large pore volume was synthesized. The synthesized OMCs

11 were used as adsorbents for benzene disposal to explore the influence of pore width

14 on the adsorption of benzene. The XRD and nitrogen adsorption results revealed that

16 the pore sizes of OMCs were contracted at certain values, within the scope of 3.4 18

19 -7.7 nm. The adsorption results of benzene on OMCs showed that there was a

22 gradual decrease of adsorption potential as the pore size of adsorbing material

24 increased. In addition, dynamic adsorption evaluation of benzene per unit surface

29 width of adsorbing material.

35 adsorption; tunable pore size

Key words: Benzene; adsorption; ordered mesoporous carbon; isosteric heat of


5 Volatile organic compounds (VOCs) are kind of special gas pollutants emitted from

8 different sources and processes. " Considering the inherently hazardous properties

10 on environment and human health, the control of VOCs emission have attracted

13 much attention in recent years.4-6 Many technologies, such as adsorption, absorption,

15 catalytic oxidation and biofiltration have been used in practical application for the

18 removal of industrial VOCs.6 Among these methods, adsorption technology has

20 always been a prior choice for efficiency and relatively low cost, especially in

23 adsorption procedure for recovering high-value adsorbates.5'7-9 One of the most

25 important issue in adsorption technology is the choice of proper adsorbent. Until now,

28 carbon materials such as activated carbon and activated carbon fiber are widely used

30 as efficient adsorbents for various kinds of VOCs.1,10 However, the irregular-shaped

33 pores and relatively small pore size of such microporous adsorbents would cause

35 pore blocking and decrease of adsorption/desorption rate during adsorption

38 procedure, which limits the further improvement of their adsorption

40 performances.11,12 Beside, in order to prepare adsorbents with high adsorption

43 efficiency, further researches and studies associated with adsorption mechanism are

45 still needed.

48 Ordered mesoporous carbon (OMC) is a kind of special carbon material that

50 possesses regular pore shape, large specific surface area and pore volume, and it is

53 quite different from traditional carbon adsorbents. ' Besides, the pore size

55 distributions of OMC can be easily tuned by adding some pore expanding agents or

57 15,16

58 selecting different surfactant templates. , Benefiting from these advantages, OMC

4 can be used as efficient adsorbent and model material in adsorption field. Hu et

6 al.18 investigated the adsorption properties of CMK-3 towards three different 8

9 commercial anionic dyes, the results indicated that CMK-3 can be used as an

11 efficient adsorbent for the removal of anionic dyes, with the adsorption capacities

14 being 90-200% higher than traditional activated carbons. Saha and Deng adopted

16 OMC doped with metals as potential adsorbents for hydrogen storage, and the 18

19 adsorption capacity of hydrogen at 25 °C and 300 bar on nickel doped OMC could

21 achieved to 2.14 wt %. Recently, Deng et al. 20 studied the adsorption and separation

24 properties of CO2, N2 and CH4 on OMC. The results revealed that OMC owned high

29 CH4/N2, and CO2/N2 mixtures simultaneously. All the relevant research results

34 knowledge, seldom research has been performed on the adsorption property of

39 the urgent requirement for their disposal. Besides, in view of adsorbent characters

44 whereas few study considered the effect of pore size distribution of adsorbent.21'22

49 main purpose of this study is to investigate the adsorption property of benzene on

54 template method, using SBA-15 as a hard template and sucrose as carbon precursor.

57 SBA-15 was a kind of mesoporous silica material with ordered concentrated pores,

adsorption capacities for CO2 and CH4, and displayed high selectivity for CO2/CH4,

certified the superior adsorption property of OMC. However, to the best of our

VOCs on OMC until now, considering the inherent hazardous property of VOCs and

that influencing the adsorption property, most papers focused on the pore volume

In this research, benzene was selected as a typical adsorbate of VOCs, and the

OMCs with different pore size. OMCs could be synthesized through the hard

4 and it was widely used as a hard template since the silica could be removed by HF or

6 NaOH solutions.23 Furthermore, during synthetic process, boric acid was added as a 8

9 pore expanding agent to attain OMCs with tunable pore size distributions without

11 changing the primary pore structure. During the carbonization process, the

14 boric acid is expected to form boron oxide and borosilicate nanolayers between the

16 silica and carbon frameworks. Removal of these compounds leads to the 18

19 enlargement of pore size for the resultant OMC materials. The adsorption

22 properties of benzene on OMC under static and dynamic conditions were

24 systematically studied. Meanwhile, the effect of pore size on the adsorption property

26 of benzene was also discussed.


33 2.1 Materials and chemicals. Poly(propylene oxide)-b-poly(ethylene

35 oxide)-b-poly(propylene oxide) triblock copolymer Pluronic 123 was purchased

38 from Sigma-Aldrich Company. Ethyl silicate (SiO2, 28.4%+), hydrochloric acid

40 (36.0-38.0%), ethanol (99.7%+), boric acid (99.8%+), sucrose (99.0%+), sulfuric

43 acid (95.0-98.0%), hydrofluoric acid (40.0%+) were purchased from Sinopharm

45 Chemical Reagent Company. All chemicals were used as received without any

48 purification process. Distilled water was used in the whole experiments.

50 2.2 Synthesis of OMC. The hard template method was adopted to synthesize

53 OMC, and the synthetic procedure of OMC was according to the relevant research

55 with some changes modified by us.23,24 During the synthetic procedure, SBA-15 was

58 selected as the hard template and sucrose was used as carbon precursor. First, soft

template synthesis method was adopted to synthesize siliceous template SBA-15. A triblock copolymer P123 and ethyl silicate were employed as sources of template and silica, respectively. Typically, 32 g of Pluronic 123 was first dissolved in 1000 g distilled water and 200 g HCl, and stirred under 40 °C for 4 hours to form a template solution. Then 68.8 g of ethyl silicate was added into the solution with vigorously stirring under 40 °C for 15 minutes. After that, the solution was aged under 40 °C and 100 °C for 24 hours separately. Afterward, the sediment was washed by ethanol thoroughly, and SBA-15 was obtained by calcination under 550 °C in air for 2 hours.

The synthesized mesoporous siliceous SBA-15 material was then used as template for the synthesis of OMC. Sucrose and boric acid were used as carbon source and pore expanding agent, respectively. In a typical synthesized process, 0.141 g sulfuric acid, 1.25 g sucrose and a certain amount of boric acid were dissolved into 5 ml distilled water, then 1 g SBA-15 was added into the solution and dispersed by supersonic method. The resultant pasty sample was dried under 70 °C for 1 hour, and then pre-carbonized under 100 °C and 160 °C for 6 hours, separately. The obtained dark-brown solid sample was then grinded into powder and repeated the above immersion and carbonization processes. The quality of the impregnation liquor for the second time is 66 % of the first time, with the mass ratio of sulfuric acid, sucrose, boric acid and water being the same. The obtained composites were carbonized under 900 °C for 3 hours under nitrogen atmosphere with the heating rate being 5 °C/min. After cooled down to room temperature, the composite was washed by 10 % hydrofluoric acid to remove the SBA-15 template, then continued washing by

4 distilled water until the neutral pH. Finally, the OMC materials were obtained after

6 dried at 60 °C overnight and the OMCs were named as OMC-x%, where x denotes 8

9 the molar ratio of boric acid, x%= n (boric acid)/[ n (boric acid)+ n (sucrose)], and

11 the x value being x=20, 35, 45, 55, 75.

14 2.3 Material characterization. The small-angle X-ray diffraction (XRD) patterns

16 were recorded on PANalytical X'Per PRO MPD using a Cu-Ka radiation 18

19 (X = 1.540598 A). High-resolution scanning electron microscopy (HRSEM) analysis

22 was conducted on a JEM 2010. Texture properties were measured using nitrogen

24 sorption isotherms at liquid nitrogen temperature on a Micromeritics ASAP 2020 gas

29 110 °C. The BET surface area was calculated using Brunauer-Emmett-Teller theory.

34 The micropore volume was determined by the t-plot method. The pore size

39 on the Barrett-Joyneer-Halenda (BJH) model.

44 OMCs were measured on a HIDEN Intelligent Gravimetric Analyzer IGA-002. In

49 saturation vapor pressure of adsorbates was calculated following the Antoine

52 equation:

54 , . B

55 lo§ P0 = A -

sorption analyzer. Before measurements, all samples were degassed overnight under

The total pore volume was obtained by the nitrogen amount adsorbed at P/Po = 0.99.

distributions were calculated by analyzing the desorption branch of isotherms based

2.4 Adsorption measurements. The static adsorption properties of benzene on

this work, adsorption isotherms under 25 °C, 35 °C and 45 °C were performed. The

where p0 is the saturated vapor pressure (Torr), T is the temperature (°C) and A, B, C are constants defined by the adsorbate; for benzene (8-103 °C), the corresponding parameters are as follows: A=6.90565, B=1211.033, C=220.790.25

The dynamic adsorption properties of benzene on OMCs were performed on a fixed bed equipment described in our previous work.26 The gas concentration of benzene was about 650 ppmv. Before adsorption, all the carbon materials were degassed under 60 °C for 10 hours to remove the physically adsorbed water molecules and other impurities. The inner diameter of adsorbed column is 6 mm, in each experiment, 50 mg OMC was loaded and total gas flow rate was maintained at 50 mlmin-1. During the whole experimental procedure, the concentration of benzene at the outlet of the reactor was determined intermittently using a photoionization VOCs detector.27 The breakthrough time is defined as the time when the outlet concentration is about 5% of the inlet concentration.28 The adsorbed amounts were calculated by the following equation:

q =— (Ct - I" >SC dt) q Mw I t " 0 )

where q is adsorbed amount (mmolg-1); F is volumetric flow rate of carrier gas

(ml min-1); w is the net weight of adsorbent (g); M is molar mass of benzene

(gmol-1); Ci represents the benzene concentration at the inlet (gml-1), while C0 is

the benzene concentration at the outlet (gml-1); ts is the saturation time of adsorbent


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The isosteric heats of adsorption are usually used as a measure of the surface heterogeneity of a solid adsorbate. The values can be calculated by the

Clausius-Clapeyron equation, which can be written in the form,

O =R( SInP \

Qst=-R( ST-1 )e

where, Qst is the isosteric heats of adsorption at a given uptake 0, P is the vapor pressure, T is the temperature and R is the gas constant.


3.1 Characterization of OMC. The XRD patterns of SBA-15 and OMCs were depicted in Fig. 1. The three diffraction peaks in the region 0.5-2° can be indexed to the (100), (110) and (200) reflections of SBA-15 possessing a dimensional hexagonal space group p6mm, implying the successful synthesis of SBA-15.29 All the OMCs possessed diffraction peak at 2 theta 1~1.2°, which corresponds to the two-dimensional hexagonal crystal structure (100) diffraction, indicating the OMCs have reproduced the mesoscopic structure of SBA-15. For these OMCs, there was a slight shift to lower angle for the diffraction peak at 2 theta 1~1.2 This might be due to the crescent pore width of OMCs, which caused by the increase dosage of boric acid. In addition, a small diminution of intensity for the characteristic peaks with the increase of boric acid dosage occurs, indicating the slight decrease of the long range order of OMCs.18 To further illustrate the pore structures of materials, the nitrogen adsorption/desorption measurements were employed. The isotherms of SBA-15 and

OMCs are depicted in Fig. 2. As shown in Fig. 2, only few amount of nitrogen adsorbed on OMCs in the low relative pressure range is observed, and the adsorbed nitrogen is mainly contributed by mesopores, implying the mesoporous nature of materials. With the increase of relative pressures, typical type IV isotherms appeared according to the IUPAC classification, indicating the existence of concentrated distribution mesopores.30 Besides, all the OMCs demonstrated a H1 type hysteresis loop. With the increase of boric acid dosage, the OMCs' hysteresis loops shifted to higher relative pressure, indicating the gradual increase of concentrated mesopores of OMCs, which agree well with the XRD results. Additionally, the inflection point of each isotherm where capillary condensation occurs gradually grew up to a higher relative pressure value as the dosage of boric acid increased. The pore size distribution curves of OMCs obtained from BJH model are also shown in Fig. 2. They are mainly in the range of 2-12 nm as the larger parts of pore size can be neglected. The pore size distribution of OMCs are concentrated at certain width, the most probable distributed pores are 3.5, 4.0, 4.5, 5.2, 6.0 and 7.0 nm, respectively, with the pore expanding agent increased from 20% to 75% in sequence. It is clear that an increase of the centralized pore diameter occurs as the amount of pore expanding agent rose, indicating the well synthesis of pore width tunable OMCs.

The texture properties of samples are summarized in Table 1. Generally, the pore volumes and micropore volumes of OMCs are almost the same with each other. The micropore volume of SBA-15 accounted for about 7 % of the total pore volume, this value is similar as previous research, confirming the successful synthesis of

4 SBA-15. With the increasing amount of boric acid, a increase of average pore

6 diameter is observed and thus the specific surface area decreased accordingly. 8

9 Corresponding data revealed that as the average pore diameter of OMC-20%

11 increased from 3.4 nm to 7.7 nm of OMC-75%, the specific surface area decreased

14 about 42%.

16 In order to investigate the pore structure of materials more intuitively, TEM 18

19 images of SBA-15 and OMCs were taken and shown in Fig. 3. Clearly, all the

22 samples showed an ordered 2-d hexagonal mesoporous structure. Meanwhile, a

24 slight decrease of long-range order structure can be found, as the amount of boric

29 3.2 Static adsorption of benzene on OMCs. The adsorption isotherms are

34 particular adsorbent/adsorbate system and the data can be used to evaluate the

39 benzene on OMCs at 25, 35 and 45 °C. Similar to the nitrogen adsorption/desorption

44 and a type H1 hysteresis loop. The maximum adsorption amounts of benzene on

49 volumes of OMCs are similar with each other. Under the saturation vapor pressure,

54 amounts obtained. Besides, it is interesting to note that as the temperature raised

57 from 25 to 45 °C, the adsorption amount of benzene on a single adsorbent under the

acid increased, which are consistent with the XRD patterns in Fig. 1.

important as they can be regarded as the primary source of information on a

adsorption property. Fig. 4 (a) - (e) shows the adsorption/desorption isotherms of

isotherms, each of these benzene adsorption isotherms showed a type IV isotherm

OMCs under various temperatures are almost the same, this is because the total pore

all the pores are accessible for the benzene molecule, thus similar adsorption

same vapor pressure decreased, this certified the physical adsorption characteristic of benzene onto OMCs.

For a more intuitive comparison, all the adsorption isotherms performed at 25 °C on OMCs were gathered in Fig. 4 (f). A clear difference can be seen that as the dosage of boric acid increased, the adsorption isotherm of benzene gradually moved to right, i.e. the pore size distributions of OMCs has a great influence on the adsorption property of benzene. Choosing the adsorption amount of benzene at a fixed pressure for comparison, the corresponding value decreased as the pore size increased, except a small deviation occurs between the isotherms of OMC-45% and OMC-55%. Former studies associated with the adsorption property of VOCs onto carbon materials pointed out that, in the case of physical adsorption, the adsorption amount of adsorbate was determined only by the total pore volume instead of pore size distributions.32 However, this result is inconsistent with the previous result that under physical adsorption conditions, the pore size of adsorbent has a great influence on the adsorption amount of adsorbates.

3.3 Isoteric heats of adsorption. The corresponding Qst of benzene onto OMCs was calculated and depicted in Fig. 5. Sample OMC-20% possesses the highest isosteric heats of adsorption, especially in the beginning of adsorption process. In comparison with OMC-20%, the variation of isosteric heats for other OMCs seems not obvious as the curves are more stable. Considering the pore diameter of OMC-20% is much smaller than other OMCs, it could be concluded that OMCs with narrower pore width are more accessible for benzene. In addition, although the values of

4 isosteric heats for OMC-35%, OMC-45% and OMC-55% are close with each other,

6 an overall result can be concluded that isosteric heat decreased with the increase of

9 pore size, as the values of isosteric heats for OMC-20% are the highest and the

11 corresponding values for OMC-75% are the lowest. Thus a speculation can be

14 obtained that adsorbate with narrower pore diameter owns a higher adsorption

16 potential for the benzene adsorption, i.e. the benzene molecular are more likely to be 18

19 adsorbed onto adsorbate with narrower pore diameter.

21 3.4 Dynamic adsorption of benzene on OMCs. In practical application, the

24 adsorbates were usually loaded in a fix-bed equipment and used for adsorption, thus

29 real situation, and the results were shown in Fig. 6. As can be seen from Fig. 6, there

34 OMCs with bigger average pore diameter, their breakthrough time become

39 dynamic adsorption capacity the adsorbent will get. Thus, the gradually increased

44 amount of benzene in the same sequence. The dynamic breakthrough times and

49 of OMCs increased from 3.4 nm to 7.7 nm, the breakthrough time decreased from 32

54 mmol/g to 0.041 mmol/g.

a dynamic adsorption of benzene vapor onto OMCs were performed to simulate the

is a clear regularity of dynamic breakthrough curves between various OMCs. For the

correspondingly shorter. Generally, the longer the breakthrough time is, the higher

breakthrough time from OMC-75% to OMC-20%, implies the higher adsorption

adsorption amounts of benzene in OMCs were tabulated in Table 2. As the pore size

min to 6 min, and the corresponding adsorption amount decreased from 0.114

It is meaningful to reveal how the microscopic structure of adsorbent is reflected on the macroscopic characteristics of adsorption by employing a single kind of adsorbate. In this study, the prepared OMCs possess similar total pore volume and micropore volume, but the tunable pore size and the BET specific surface area present greater changes(the BET specific surface area decreased from 1120.8 m2/g to 640.9 m2/g). In order to eliminate the influence of the surface area and pore volume, normalized adsorption quantity, i.e. adsorption amounts per unit surface area and pore volume were tabulated in Table 2. The trend that the dynamic adsorption capacity decreases with the enlargement of pore size is not changed. Furthermore, amount adsorbed per unit surface area and per unit pore volumes for different samples were depicted in Figure 7 so as to reveal the relationships between adsorption characteristics and the pore width more intuitively. It helps to conceptualize the relations between pore width and dynamic adsorption capacity. Clearly, the benzene adsorption amount per unit surface area and pore volume decreased with the augment of the average pore diameter of OMCs. Thus it is able to clarify the relationships between the adsorbing characteristics and texture property unequivocally that, in the mesoporous range, narrower pore size would effectively increase the utilization of adsorbents' surface area and pore volume. In a word, the pore size distribution is the dominant factor that influences the adsorbed characteristics of benzene. 4 CONCLUSIONS


4 In this paper, OMC was synthesized via a hard template method, and boric acid

6 was added during synthetic procedure to tune the pore sizes in small mesopore range 8

9 3-8 nm without changing the main texture of OMC. The resultants OMCs were used

11 as adsorbents to evaluate their adsorption properties to benzene. Study on the

14 isosteric heats of adsorption revealed that the OMCs with narrower pore size possess

16 higher adsorption potential. Static and dynamic adsorption amounts of benzene onto 18

19 OMCs indicating that the pore size distribution is a dominant factor that influence

22 the adsorbing amounts of benzene, and in the mesoporous range a narrower

24 average pore size would be preferred for adsorption and result in a higher adsorbed



36 Corresponding author

42 Notes


52 Research and Development Program of China (2012AA063101), the Key Program

57 Research Program (XDB05050200), Science Promotion Program of the Research

*Phone: +86-10-62923564. Email:

The authors declare no competing financial interest.

We gratefully acknowledge the financial support from National High Technology

of National Natural Science Foundation of China (21337003), Strategic Priority

Center for Eco-environmental Sciences (YSW2013B05) and the Knowledge Innovation Program of Cooperative Interaction Team of the Chinese Academy of Sciences.


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Legends For Figures and Tables

Table 1. Texture properties of SBA-15 and OMCs.

Table 2. The breakthrough times and dynamic adsorbed amounts of benzene vapor onto OMCs.

Figure 1. XRD patterns of SBA-15 and OMCs.

Figure 2. Nitrogen adsorption/desorption isotherms and pore size distributions curves of SBA-15 and OMCs.

Figure 3. TEM images of SBA-15 (a), OMC-20% (b), OMC-35% (c), OMC-45% (d), OMC-55% (e), and OMC-75% (f).

Figure 4. Adsorption isotherms of benzene vapor on OMC-20% (a), OMC-35% (b), OMC-45% (c), OMC-55% (d) and OMC-75% (e) at 25 °C, 35 °C and 45 °C, and the collective adsorption isotherms of benzene at 25 °C on OMCs (f).

Figure 5. Variation of isosteric heats for benzene adsorption onto OMCs.

Figure 6. Experimental dynamic breakthrough curves of benzene vapor onto OMCs.

Figure 7. The dynamic adsorption capacities of benzene per unit surface area and unit pore volume on different samples.

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Table 1. Texture properties of SBA-15 and OMCs.

Sample Surface area m2/g Pore Volume cm3/g Micropore Area m2/g Micropore Volume cm3/g Average Pore Size nm

SBA-15 844.2 0.98 134.7 0.07 5.7

OMC-20% 1120.8 1.00 199.9 0.09 3.4

OMC-35% 893.5 0.94 210.3 0.10 3.9

OMC-45% 792.7 0.97 294.1 0.15 4.9

OMC-55% 698.8 1.02 200.3 0.10 5.4

OMC-75% 640.9 1.22 223.6 0.11 7.7

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Table 2. The breakthrough times and dynamic adsorbed amounts of benzene vapor

onto OMCs.

Sample OMC-20% OMC-35% OMC-45% OMC-55% OMC-75%

Breakthrough time (min) 32 23 18 10 6

Amount adsorbed (mmol/g) 0.114 0.089 0.073 0.052 0.041

aQarea (^moW) 0.102 0.100 0.092 0.074 0.064

bQvol (mmol/cm3) 0.114 0.095 0.075 0.051 0.034

aQarea- Amount adsorbed per unit surface area (^mol/m ) bQvol: Amount adsorbed per unit pore volume (mmol/cm3)

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Figure 1. XRD patterns of SBA-15 and OMCs.

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i- 700-

O) 600-

■Q 400-

T3 < 300-

c 200-

a 100-

—»—SBA-15 —•— OMC-20%

-A- OMC-35%


OMC-55% . 11 % ? v ilely

—«-OMC-75% /Mjv

0.4 0.6

Figure 2. Nitrogen adsorption/desorption isotherms and pore size distributions curves of SBA-15 and OMCs.

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1 8 ■o

■ - 25 °C

• 35 °C

* 45 °C tfyz i/s/f - ' A

J/// • ¥ * (a)

0 50 100 150 200 250


1 8 -o

-■—25 »C

• 35 °C

* 45 °C

'■/•Vf r (b)

100 150 Pressure(mbar)

■ - 25 °C

• 35 °C

* 45 °C

■ .A'

// / A

////> ^

■ • ▲

100 150 Pressure(mbar)

Figure 4. Adsorption isotherms of benzene vapor on OMC-20% (a), OMC-35% (b), OMC-45% (c), OMC-55% (d) and OMC-75% (e) at 25 °C, 35 °C and 45 °C, and the collective adsorption isotherms of benzene at 25 °C on OMCs (f).

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O in TS re

+-I (0 4> JC

70605040 3020-

— OMC-20%

— •— OMC-35%

OMC-45% -▼- OMC-55% OMC-75%

♦ ♦♦

Amount adsorbed, mmol/g

Figure 5. Variation of isosteric heats for benzene adsorption onto OMCs

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Figure 6. Experimental dynamic breakthrough curves of benzene vapor onto OMCs.

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Figure 7. The dynamic adsorption capacities of benzene per unit surface area and unit pore volume on different samples.

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