A Microporous Graphitized Biocarbon with High Adsorption Capacity

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A microporous graphitized biocarbon with high adsorption capacity towards benzene VOCs from humid air at ultra-low pressures Meiping Zhu, Zhang-Fa Tong, Zhongxing Zhao, Yuzhao Jiang, and zhenxia zhao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00056 • Publication Date (Web): 15 Mar 2016 Downloaded from http://pubs.acs.org on March 19, 2016

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Industrial & Engineering Chemistry Research

A microporous graphitized biocarbon with high adsorption capacity towards benzene VOCs from humid air at ultra-low pressures Meiping Zhu, Zhangfa Tong, Zhongxing Zhao, Yuzhao Jiang, Zhenxia Zhao* Guangxi Colleges and Universities Key Laboratory of New Technology and Application in Resource Chemical Engineering, Guangxi University, Nanning 530004, China

KEYWORDS. Microporous graphitized biocarbon, Benzene/toluene vapor adsorption, High capactiy, Breakthrough curves, Humid air and ultra-low pressures.

ABSTRACT. Adsorption studies of vaporous benzene and toluene on a novel microporous graphitized biocarbon material (MGBC) were investigated in this study. The MGBC has high surface area (> 2080 m2/g) and uniform micro-porosity (6.8-8.8 Å). The adsorbed amounts of benzene and toluene on the MGBC are ~3 to 5 times higher than those on zeolites and some MOFs adsorbents (i.e. MIL-101, HKUST-1 and UIO-66) at 80 Pa. Results of the desorption activation energy and isosteric heat indicate a strong affinity of the MGBCs for benzene/toluene molecules due to its uniform micropores size and graphitic N/C in MGBCs structure. Moreover, the MGBC possesses a hydrophobic character, and thus leads to a preferential adsorption for toluene over H2O. Therefore, the MGBC shows a much higher working capacity for toluene than MIL-101(Cr) and commercial ACs in moderate humidity. 1

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1. INTRODUCTION Benzene series volatile organic compounds are known as a set of major outdoor/indoor air pollutants1 from industrial solvents, stored fuels as well as environmental tobacco smoke. They not only have been considered as harmful compounds to human health,2,3 but also can induce some serious environmental problems due to their toxic, mutagenic and carcinogenic natures during long term exposure.4 Recently, adsorption is a well-established and effective technique for the removal and recovery of benzene VOCs from polluted air, especially for trace amounts of VOCs.5,6 Up to now, more than 10% of the industrial abatement units have been based on adsorption, and it will continue to grow in the near future to meet the stricter controls on emissions of VOCs promulgated by government.7 In adsorption process, the crucial aspect is the selection of suitable adsorbents with high capacity and sufficient efficiency for VOCs capture. Some commercial porous adsorbents, such as activated carbon,8 silicon9 and zeolites,10 have been often used for vaporous benzene VOCs capture. However, their sorption abilities as well as selectivity are not very high, and thus cannot satisfy the request for application. During two decades, a great deal of effort has been expended on the synthesis of novel metal organic frameworks (MOFs) for applications in gas storage and removal of VOCs.11,12 Some reports showed that some MOFs have very high adsorption capacity for VOCs molecules and have exceeded some commercial ACFs, activated carbon and zeolite.13,14 For example, Zhou X prepared a novel composite GrO@MIL-101 material and its acetone adsorption capacity reached 20.1 mmol/g at 288 K and 16.2 kPa.14 The adsorbed amount of benzene on the HKUST-1 (MOF-199) was up to 9.5 mmol/g at 298 K and 1.0 kPa.15 Benzene uptake on the MIL-141(Cs) was about 2.6 mmol/g at 303 K and 0.69 kPa.13 The amount of toluene vapor adsorbed on the SCUTC-18 was 170 mg/g (1.848 mmol/g) at P/Po = 0.90 (T=298 K and P=3.405 Kpa).16 They all showed very interesting values of VOCs sorption ability 2


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compared with other commercial porous materials. Nevertheless, many MOFs possess some weak points such as poor chemical and water stability17 that badly impede the use in their longterm development prospects. Recently, a novel porous biocarbon material (MGBC) with adjustable pore size has been reported by using bio-molecule dopamine as the carbon resource.18 Compared to other porous carbon materials, dopamine as its carbon resource is nontoxic and sustainable. More importantly, it has been confirmed to contain rich sp2 C/N after carbonization at high temperature.19 This structure with rich sp2 C/N can enhance π-π interaction20 and selective sorption for benzene series. Meanwhile, the MGBC materials possess uniform micropores size and show excellent chemical/thermal stability. But so far there have been no research on adsorption properties of benzene VOCs on this porous biocarbon material. In this work, adsorption equilibrium of benzene/toluene on the synthesized MGBC was separately measured gravimetrically. Their isosteric heats as well as activation energies of desorption were also estimated from the adsorption isotherms and TPD spectra, respectively. Adsorption isotherms of gaseous VOCs showed high adsorption capacities of benzene/toluene on the MGBC materials at ultra-low pressures. The values were about 3-5 times higher than those on zeolites and some MOFs adsorbents (i.e. MIL-101, UIO-66 and HKUST-1) at 80 Pa and moist condition. All the results suggested that the MGBC materials might be a potentially promising candidate for VOCs capture. 2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation Synthesis of Polydopamine Spheres (PDA): The PDA spheres were synthesized by using optimized procedures.18 Typically, ammonia aqueous solution (0.40 mL, NH4OH, 25-28 wt%, 3



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from Sigma, USA) was mixed with ethanol (40 mL, 99.7%, from Kelong, China) and deionized water solution (100 mL). Then dopamine hydrochloride (0.5 g, > 98%, from Sigma, USA) was added into the above ammonia ethanol/water solution (see Scheme 1, step I). The obtained PDA spheres were filtered, washed and dried for further preparation. Synthesis of MGBC Samples: The MGBC samples were synthesized by using carbonizationactivation method. Firstly, the obtained PDA spheres were carbonized under N2 atmosphere at 800 °C for about 2 h with a heating rate of 5.0 °C/min (see Scheme 1, step II). Then, the carbonized PDA powders were mixed with KOH at the weight ratio of 1:4 and 1:6, respectively. The mixture were then heated to 700 °C for 1 h under N2 flow with a heating rate of 5.0 °C/min (see Scheme 1, step III). After that, the black powder was then washed with HCl (1.0 M) solution to remove the residual KOH until pH ~7. Finally, the MGBC samples were filtered and dried under vacuum at 150 °C overnight. The obtained powders activated with PDA/KOH ratio of 1/4 and 1/6 were denoted as MGBC4 and MGBC6, respectively. The SY-1 (activated carbon) and MIL-101 (MOFs) used for comparison were purchased from Letai Chem. Co. (Tianjin) and HWRK Chem. Co. (Beijing), respectively. 2.2. Materials Characterizations The morphology and structure of the PDA, MGBC4 and MGBC6 samples were analyzed by scanning electron microscopy (SEM, S-3400N, Hitachi), powder X-ray diffraction (XRD, Rigaku, Smart Lab diffractometer using Cu Kα radiation at 30 kV, Japan) and Raman spectrometer (LabRAM HR-800, Horiba), respectively. X-ray photoelectron spectroscopy (XPS) measurements were performed with a Kratos Axis Ultra spectrometer using a focused monochromatized Al Kα radiation (λ=1486.6 eV). Their textural characterization was performed by N2 adsorption at 77 K using a surface characterization analyzer (3Flex, Micromeritics). Prior 4


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to each adsorption experiment, the samples were out-gassed at 383 K for 8 h and a vacuum of < 0.05 Pa. The pore size distribution was calculated by using the Horvath-Kawazoe model. 2.3. Measurement of Adsorption Isotherms of VOCs Vapor The adsorption isotherms of toluene were measured at the temperatures of 283.2, 298.2 and 313.2 K by using Micromeritics 3Flex. The highest adsorption pressure was up to their saturated vapor pressure at the measured temperature. The adsorption uptake of VOCs vapor on the MGBC was calculated as follows Eq. (1):21

Qe 

1000(We  Wa ) Wa M voc

Eq. (1)

where Mvoc (g/mol) is the molecular weight of VOC molecule, We (g) is the adsorbed amount of adsorbent at equilibrium, Wa (g) is the initial weight of the adsorbent, and Qe (mmol/g) is the adsorbed amount per gram at equilibrium. 2.4. Temperature Programmed Desorption Experiments Temperature programmed desorption (TPD) experiment was carried out to estimate binding energy between the adsorbate and adsorbent,4,22 and the detailed analysis was carried out in the same way as previously described.15 The MGBC samples were put into a desiccator to adsorb vaporous VOC (benzene or toluene) at 298.2 K and ambient pressure for a short time (~5 min). After that, the MGBC samples adsorbed VOCs were transferred to a copper tube and sealed with silica wool plugs. After purging in nitrogen for about 20 min at 313.2 K, the heating program immediately started with different heating rates from 6 to 14 K/min, respectively. The adsorbed benzene/toluene vapors were desorbed from the MGBC samples, which were detected and recorded by using a GC (Agilent, 7820A) with hydrogen flame detector (FID) as the TPD curves. 5



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Desorption activation energies ( Ed , kJ/mol) of VOCs (benzene and toluene) vapors on the MGBC samples were estimated from their TPD curves. In this case, desorption was assumed to follow Arrhenius-like behavior with rate law for the desorption reaction described by the PolanyiWigner equation Eq. (2):23

 ln(

H RT

2 p

)(

Ed E )  ln( d ) RT p ko

Eq. (2)

where  H is the heating rate, K/min; Tp is peak temperature of desorption, K; The desorption activation energy can be calculated from the slope of the plot of  ln(  H / RT p2 ) vs 1/ Tp .

2.5. Isosteric Heat and Change in Free Energy of Adsorption The isosteric heat ( H s , kJ/mol) usually can be used to evaluate the interaction between the adsorbate molecules and the adsorbent surface. The isosteric heats of benzene/toluene adsorption on the MGBC samples were calculated from the corresponding adsorption isotherms at different temperatures by using Eq. (3):24,25 H S  ln( P)  R (1/ T )

Eq. (3)

The change in free energies ( G o ) associated with the adsorption process was calculated by using following Eq. (4):26

G o  RT ln

P PO

Eq. (4)

2.6. Breakthrough Experiment Breakthrough experiment27,28 was used to assess the effect of relative humidity on the adsorption of toluene on the MGBC6 sample with higher surface area. The breakthrough 6


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experiment was carried out on the laboratory designed GC workstation and conducted in a steel column of 3.0 mm diameter and ~2.8 cm length. The tested sample was treated at 130 C for 3 h under N2 purge of 40 mL/min. The mixture of toluene and water vapors (60 RH%) was introduced into the fixed bed by using inert carrier gas of N2 and the toluene concentration were fixed at 1.43 g/m3 (ppm) (15.5×10-6 mmol/mL). The effluent toluene streams from the adsorption chamber section were detected and recorded by gas chromatography (GC). 3. RESULTS AND DISCUSSION 3.1. Physical Characteristics Fig. 1 depicts the scanning electron microscopy (SEM) images of the PDA spheres and the synthesized MGBC samples. The PDA spheres present the spherical morphology with smooth surface and uniform diameter of approximately 750-800 nm, which is similar to previous report of the PDA SMSs.18 After being carbonized and activated, the resultant biocarbon materials (MGBC4 and MGBC6) appear to preserve the spherical morphology and sphere diameter. Also, the N content of the biocarbon materials were detected by using energy-dispersive X-ray analysis. The value of N content was about 8-10 wt% for the MGBC4 and MGBC6 samples (seen in Fig. 2), which is consistent with the calculation result from the dopamine formula. The X-ray diffraction (XRD) patterns of the as-prepared MGBC4 and MGBC6 samples are shown in Fig. 3. It can be found that the general pattern of MGBC4 and MGBC6 are very similar, indicating a similar crystal structure of the MGBC samples. The major identified peaks of the MGBC samples are located at approximately 26 and 44°, corresponding to the graphitic (002) and (100) planes, respectively. It is inferred that some graphitic carbon/nitrogen appear in the MGBC samples. The partially graphitic character is believed to enhance the surface non-polarity of the biocarbon framework, which will lead to an increased electrostatic interaction with non7



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polar and weak polar molecules (e.g., benzene and toluene VOCs). Additionally, the peak intensity of the MGBC4 is stronger than that of the MGBC6, suggesting a higher degree of graphitization of the MGBC4. The different level of graphitization in carbon framework may be affected by their pore structure. Fig. 4 shows the Raman spectra of the PDA spheres and the MGBC samples. Two main peaks are seen in the spectra taken from the MGBC samples, the G peak (1580 cm-1) and the D peak (1300 cm-1). The D band is related to the defects and disordered amorphous carbons within the carbon structure, while the G-band feature is the in-plane vibration of sp2 carbon atoms.30,31 The coexistence of the two bands in the spectra indicates a partial graphitization of the MGBC structure with some defects and disorders.29 Table S1 lists the relative intensity ratio of G band and the D band (IG/ID) of each sample, which can be used to assess their degree of graphitization. As shown, the IG/ID values of the MGBC samples (IG/ID: > 0.48) are obviously larger than that of the PDA spheres (IG/ID: ~ 0.14). In general, the higher the ratio of the IG/ID is, the higher is the degree of graphitization.31 Thus, the MGBC4 and MGBC6 samples have much higher degree of graphitization than the PDA spheres, suggesting more sp2-hybridized carbons in the MGBC samples when being carbonized at high temperatures. By comparison, the MGBC4 possesses higher IG/ID value than the MGBC6, which are about 0.66 and 0.48 for the MGBC4 and MGBC6, respectively. The discrepancy of their IG/ID values may arise from their porous texture. From Fig. 5 and Table S2, it can be seen that the MGBC6 has more micropores than the MGBC4 after being activated. The formed pores may destroy the graphitic structure in their carbon matrixes to some extent. To better insight the surface properties of the MGBC samples, we next employed X-ray photoelectron spectroscopy (XPS) analysis to investigate C component of the MGBC samples. Fig. 6 shows the C1s XPS spectrum of the MGBC4 material, and we observed a remarkable 8


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graphitic character of the MGBC4 material. The C1s XPS spectrum of MGBC was curve-fitted into three peaks with binding energies at 284.8, 285.2 and 287.7 eV, which were corresponding to sp2 C, sp3 C & C=N and -C-O-C & C-N components, respectively.32 Clearly, the peak intensity for the sp2 C is much stronger than the sp3 C & C=N and -C-O-C & C-N components, suggesting that the MGBC contains rich sp2 C after carbonization at high temperatures. The high content of sp2 C was mainly attributed to the PDA, which possessed C that was entirely located within its rings.18 Fig. 5 shows the N2 adsorption/desorption isotherms and pore size distribution curves of the assynthesized samples (PDA spheres, MGBC4 and MGBC6), and their textural parameters are summarized in Table S2. As seen in Fig. 5a, the PDA sample shows a very low adsorbed amount of N2 and gives a low specific surface area (Langmuir: 13.1 m2/g) as well as total pore volume (0.028 m3/g). This suggests that the polymerization of dopamine (PDA) has almost no pore structure. However, the MGBC samples show a typical type I isotherm with no hysteresis, characteristic of a microporous material with uniform pores size after being carbonized and activated. By comparison, the MGBC6 has higher adsorbed amount of N2 than the MGBC4, indicating more pores of the MGBC6. As a result, the MGBC6 has higher surface area and pore volume, which are 2085.2 (Langmuir, m2/g) and 0.792 cm3/g, respectively. Additionally, the micropore area and micropore volume of the MGBC6 are 1330.9 (Langmuir, m2/g) and 0.532 cm3/g, which are also higher than those of the MGBC4. Fig. 5b shows the pore sized distribution of the MGBC4 and MGBC6 samples. As shown in this figure, both the MGBC4 (~ 6.75 Ǻ) and MGBC6 (~ 8.78 Ǻ) samples exhibit a very narrow super-microporosity. The narrow super-micropores (size smaller than 1.0 nm) favor the adsorption of benzene series VOCs under very low concentrations. Meanwhile, the formation of

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pores brings some changes in graphitic structure of the MGBC samples. Characterization results show that MGBC6 with higher porosity presents lower degree of graphitization than the MGBC4. 3.2. Adsorption Isotherms of Benzene and Toluene Vapor on the MGBC Materials Fig. 7 gives the adsorption isotherms of benzene and toluene on several porous adsorbents, such as MOFs (MIL-101,4,33 HKUST-115,34 and UIO-6635), zeolites36 and resin37 at relative pressures up to 0.3. Clearly, the saturated adsorbed amounts of benzene and toluene on the synthesized MGBC4 and MGBC6 are much lower than that on the MIL-1014,33 and HKUST115,34 at higher P/Po (> 0.02, 0.05). The lower adsorption capacity for the MGBCs is attributed to the lower specific surface area and total pore volume than those of the selected MOFs. However, in the low-pressure region, they present much higher benzene/toluene adsorption capacities than any of the other selected adsorbents. The result indicates that the synthesized MGBC samples have higher affinity for benzene VOCs in comparison to MOFs. Therefore, tailoring MGBCs shows higher uptake of VOCs at ultra-low pressures.38 More interestingly, the adsorption capacities of benzene and toluene on the MGBC materials are still much higher than the microporous HKUST-115,34 with similar pore size. Unlike HKUST1, the MGBCs have partially graphitized biocarbon, which can give a more non-polar surface and form stronger π-π interactions towards benzene series.20 Hence, the MGBCs exhibit much stronger adsorption sites for benzene/toluene than HKUST-1. Moreover, there is another advantage of the graphitic C/N in the MGBCs. The graphitic surface of the MGBCs shows more hydrophobic property than polar surface of MOFs with saturated ions. Thus, it will enhance the preferential adsorption for benzene series rather than H2O molecules in humid environment. 3.3. Adsorption Behaviour and Isosteric Heats of Toluene Vapor on the MGBC Materials

10


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Fig. 8 shows the adsorption isotherms of toluene vapor on the MGBC4 and MGBC6 at 283.2, 298.2 and 313.2 K measured experimentally. The shapes of all three adsorption isotherms on these two MGBCs display type I isotherms. The adsorbed amounts of toluene on the MGBCs increase rapidly with pressure, and then reach the maximum uptakes at around 100 Pa. Clearly, the type I adsorption isotherms indicate a strong interaction between toluene and the MGBCs. The strong interaction is due to the uniform micropores size as well as the surface graphitic character of the MGBCs. In addition, their adsorption capacities of toluene decrease with increasing temperature obviously. These isotherms are typical for physical adsorption and show the adverse trend of adsorption capacities with temperature. As temperature rises, the adsorption interaction between toluene and the pores in the biocarbon become weaker. Therefore, some of the adsorption sites were closed off, leading a decrease adsorption capacity for toluene with temperature. Compared with some irreversible VOCs adsorption, physical adsorption on the MGBCs has the great advantages of exhibiting completely reversible.39 Therefore, the MGBC samples can be regenerated by heating or pressurizing to release the adsorbed VOCs. 3.4. Isosteric Heat and Change in Free Energy of Toluene Adsorption on the MGBC Materials Fig. 9 shows the trend of the estimated isosteric heat as a function of surface coverage for the MGBCs. Note that the isosteric adsorption heats are nearly invariable with increasing surface coverage for both MGBC4 and MGBC6. It is verified a constant energy of surface property for MGBC4 and MGBC6 samples40 at moderate surface coverage. The constant heat is the result of compensation between MGBC-toluene interaction and toluene-toluene interaction. With increasing surface coverage, MGBC-toluene interaction gradually decreases, while toluenetoluene interaction increases. Herein, the total adsorption heats are nearly unchanged with surface coverage. In addition, as compared to the MGBC6, the MGBC4 shows a higher isosteric

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adsorption heat. It indicates that the MGBC4 showed higher physical electrostatic interaction with toluene than the MGBC6 due to its smaller pore sizes.38 Gibbs free energy ( G o ) of toluene adsorption on the MGBC4 and MGBC6 as a function of adsorbed amount is calculated from Eq. (4) and shown in Fig. 10. The negative value of Gibbs free energy ( G o ) indicates that the adsorption of toluene onto MGBC samples is feasible and spontaneous thermodynamically.41 In addition, G o value gradually increases with the increase in adsorbed amount of toluene at moderate loading, and then rises sharply when approaching saturation condition at higher pressure. At very low-pressure region or low-coverage region, by comparison, - G o value of the MGBC4 is clearly higher than that of the MGBC6 sample. This result reveals that the adsorption of toluene onto the MGBC4 with smaller pores is more favourable than that onto the MGBC6 with larger pores, due to a stronger interaction of toluene with carbon framework of MGBC6.42 This result is very consistent with values of their isosteric heat in Fig. 9. 3.5. Desorption Activation Energies of Benzene and Toluene on the MGBC Samples Fig. 11 and Fig. 12 show the TPD spectra of benzene and toluene vapor from the MGBC4 and MGBC6 at the heating rate of 6-14 K/min, respectively. It can be seen that there is only one peak in the corresponding TPD curves of benzene and toluene adsorption on the synthesized MGBC samples. Note that the energetic distribution of adsorption sites is relatively homogeneous for benzene series in the MGBC samples.43 Fig. 13 depicts the corresponding linear dependencies of  ln(  H / RT p2 ) vs 1/ Tp for benzene and toluene desorption from the MGBC samples. From Eq. (2), their desorption activation energies were estimated from the corresponding slopes of their linear plots of  ln(  H / RT p2 ) vs 1/ Tp and listed in Table S3, respectively. By comparison, desorption activation energies of 12


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benzene and toluene from the MGBCs are clearly higher than those from some activated carbons.44,45 The higher interactions between benzene/toluene and the MGBCs are attributed to two factors:(1) MGBC samples have high surface area and uniform microporous framework, and (2) their graphitic surface exhibits a more hydrophobic character.38,46 Moreover, MGBC4 shows a higher desorption activation energy than the MGBC6, which is consistent with the isosteric heat result in Fig. 9. Meanwhile, the Ed values of toluene are significantly higher than those of benzene on the same MGBC samples. Thus, toluene can be adsorbed on the MGBC samples with higher affinity. On the one hand, toluene has larger molecular size than benzene. And on the other hand, toluene has higher polarity (3.0) and electron density than benzene due to greater electron-releasing properties of its methyl group. Thus, toluene can be adsorbed on the porous graphitic MGBC with higher π-π interaction.20 3.6. Effects of Relative Humidity on Toluene Breakthrough Curves Breakthrough curves of toluene through the fixed bed with samples of the MGBC6, SY-1 (AC) and MIL-101 were tested at 293 K and 60.0 RH% and shown in Fig. 14, respectively. Following the same pattern of the toluene breakthrough experiments, the synthesized MGBC6 depicts much longer breakthrough times (~ 51.0 min, in Table S4) than the MIL-101(Cr) (~ 20.0 min) and SY-1 AC (~ 14.7 min). Clearly, MGBC6 sample requires longer times to approach its saturation and exhibits a higher toluene uptake than the other two materials. The working capacity of its toluene could maintain about 78% of the initial capacity (4.74 mmol/g) in the presence of 60 RH% (seen in Fig. S1). From the adsorption isotherms in Fig. 7b, we can see that the maximum uptake of toluene on MIL-101 is much higher than that of the MGBC6 due to its higher surface area and larger pore volume. MIL-101, however, has large amounts of hydrophilic adsorption sites in 13



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framework (e.g., carboxylic groups and unsaturated metal centers). As a result, in the presence of water, it can dramatically lose their adsorption capacity for toluene due to preferential moisture adsorption. In contrast, MGBC6 is superior to toluene in the competitive adsorption of watertoluene system because of its containing of hydrophobic sites of graphitic carbon/nitrogen. Therefore, MGBC6 shows a high selectivity for toluene over water, and the finial adsorption capacity of toluene on the MGBC6 is much higher than that on MIL-101. Moreover, analyzing the shape of the toluene breakthrough curves (Fig. 14) for a more detailed study, MGBC6 sample displays a narrower mass transfer zones (between the break point and saturation) than MIL-101 and SY-1 materials. It means that mass-transfer for the MGBC6 is faster than that for the MIL-101 and SY-1. Fast mass transfer will improve utilization of the adsorbent bed as well as reduce the energy costs associated with the subsequent regeneration.47 Also, the equilibrium and working capacities of toluene on these adsorbents were all calculated from Eq. (6) and listed in Table S4.

Q

1 t F (Cin - Ceff )dt m 0

Eq. (5)

Integrating Eq. (5) from t  0 to equilibrium time ( te ) when Ceff reaches C in gives the equilibrium capacity of the adsorbents ( Qe , mmol/g); while integrating Eq. (5) from t  0 to breakthrough time ( tb ) when Ceff reaches 5% of C in (that is, ≥ 95% capture efficiency) gives the working capacity of adsorbents ( Qw , mmol/g). From Table S4, the equilibrium and working capacities of toluene on the MGBC6 are much higher than that on MIL-101 and SY-1, showing about 2.5 and 3.5 times of the amount of toluene (based on the working capacities) on MIL-101 and SY-1 at 293 K and 60 RH%. Meanwhile, the ratio of Qw / Qe for the MGBC6 is also much

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higher than the two other adsorbents, which exhibits a higher utilization of the MGBC samples for benzene series adsorption. 4. CONCLUSIONS The adsorption equilibrium and breakthrough experiments of vapor-phase benzene and toluene on a novel graphitic biocarbon spheres (MGBC) possessing micropores are investigated systematically. The MGBC with high surface area (2085 m2/g) and uniform micropore-size (6.88.8 Å) shows a very high adsorption capacity of 5.8 and 5.2 mmol/g for benzene and toluene at 80 Pa and 298 K, respectively. Their VOCs adsorption uptakes are about 4-6 times the amount in previously reported adsorption performance of zeolite and some MOFs adsorbents (i.e. MIL-101, HKUST-1 and UIO-66) at low-vapor pressures. Their single-component adsorption isotherms of benzene and toluene vapors are both type I. The isosteric heats and desorption activation energies of benzene and toluene on the MGBC samples were estimated from their isotherms and TPD curves. Isosteric heat results indicated that a high interaction between the synthesized MGBC samples and benzene series molecules. Meanwhile, breakthrough curves clearly showed much higher working capacity of toluene in the MGBC6 sample than that in MIL-101(Cr) and SY-1. Therefore, MGBC samples exhibited a stronger hydrophobic character and preferential adsorption for toluene over H2O molecules in the moderate humidity. Their high affinity and selectivity towards benzene and toluene molecules were mainly due to their uniform micropores size and high degree of graphitic structure. From this, it can be considered that the MGBC is a promising candidate for the capture of VOCs at ultra-low concentrations as well as in moist conditions. NOMENCLATURE

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Qe

the amounts of adsorbed per gram at equilibrium (mmol/g)

We

the adsorbed amounts of adsorbent at equilibrium (g)

Wa

the initial weight of the adsorbents (g)

Mvoc

the molecular weight of toluene molecule (g/mol)

H

the heating rate (K/min)

R

the universal gas constant (8.314 J/K·mol)

Tp

peak temperature of desorption (K)

Ed

desorption activation energy (kJ/mol)

ko

the constant that depends on the desorption kinetics (s-1)

HS

the isosteric heat of adsorption (kJ/mol)

P

the pressure of bulk gas at equilibrium with the adsorbed phase (Pa)

PO

saturation pressure of the adsorbed phase (Pa)

T

temperature (K)

G

the Gibbs free energy (kJ/mol)

IG/ID

the relative intensity ratio of G band and the D band

bL-F

L-F affinity coefficient (Pa1/n)

16


Page 16 of 35

Page 17 of 35

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1/nL-F

L-F heterogeneity factor

Qm

the saturation capacity (mmol/g)

Qw

working capacity of benzene (mmol/g)

m

the weight of the used adsorbents (g)

F

the flow rate (mL/min)

Cin

the influent concentration of toluene vapor (mmol/mL)

Ceff

the effluent concentration of toluene vapor (mmol/mL)

te

equilibrium time (min)

tb

breakthrough time (min)

Creak letters βH

the heating rate, K/min

FIGURES Fig. 1. SEM images (top) of (a) PDA spheres, (b) MGBC4, and (c) MGBC6 samples with the corresponding magnified SEM images with a scale bar of 3.0 μm. Fig. 2. EDAX spectra of (a) the MGBC4, (b) the MGBC6 and (c) their element distribution. Fig. 3. XRD patterns of the synthesized MGBC4 and MGBC6 samples. Fig. 4. The Raman spectra of the (a) the PDA, (b) the MGBC4 and (c) the MGBC6 samples.

17



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Page 18 of 35

Fig. 5. Plots of log(Q) vs log(P/Po) from the nitrogen adsorption isotherms (a) and pore size distribution curves (b) of the PDA and MGBC samples. Fig. 6. C1s spectrum of the MGBC4. Fig. 7. Adsorption isotherms of (a) benzene and (b) toluene on the MGBC4, MGBC6, MIL101,4,33 HKUST-1,15,34 UIO-66,35 zeolites (NaY, USY, ZSM-5)36 and resin (NDA-201)37 at 298 K and P/Po < 0.12. Fig. 8. Adsorption isotherms of toluene on the (a) MGBC4 and (b) MGBC6 at 283-313 K. Fig. 9. The isosteric heat of toluene adsorption on the (a) MGBC4 and (b) MGBC6 as a function of surface coverage of toluene molecules. Fig. 10. The change in free energy of toluene adsorption on the (a) MGBC4 and (b) MGBC6 as a function of adsorbed amount. Fig. 11. TPD spectra of benzene on (a) the MGBC4 and (b) MGBC6 samples at different heating rates from 6 to 14 K/min (N2 flow rate 30 mL/min). Fig. 12. TPD spectra of toluene on (a) the MGBC4 and (b) MGBC6 samples at different heating rates from 6 to 14 K/min (N2 flow rate 30 mL/min). Fig. 13. Linear dependence between - ln(  H / RT p2 ) vs 1/TP for TPD of (a) benzene and (b) toluene on the MGBC4 and (b) MGBC6 samples. Fig. 14. Breakthrough curves of toluene vapor in the fixed bed filled with the MGBC6 at 293 K and 60 RH% (toluene concentration: 15.5×10-6 mmol/mL, 1.43 ppm). SCHEME 18


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Scheme 1. Synthesis procedure of the MGBC samples AUTHOR INFORMATION Corresponding Author *Tel: 86-771-323 6484. Fax: 86-771-323 6484. E-mail: [email protected] Funding Sources Support from the National Natural Science Foundation of China (No. 21466003 & 21376090), and the Scientific Research Foundation of GuangXi University (Grant No.XGZ130963). Notes The authors declare no competing financial interest. ASSOCIATED CONTENT Supporting Information Supporting material includes breakthrough curves of toluene vapor in the fixed bed filled with the MGBC6，main Raman spectra parameters，and pore structure parameters of the PDA spheres and MGBC samples，activation energy of benzene and toluene vapor desorbed from the MGBC porous samples calculated from their TPD curves, Working and equilibrium capacities of toluene on three adsorbents. ACKNOWLEDGMENT Support from the National Natural Science Foundation of China (No. 21466003 & 21376090), and the Scientific Research Foundation of GuangXi University (Grant No.XGZ130963). We are 19



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Page 20 of 35

also thankful to Zhen Tian from Micromeritics Instrument (Shanghai, China) for providing pore structure and vapor adsorption information of the MGBC samples. REFERENCES (1) Wu, Y.; Liu, D. F.; Wu, Y. B.; Qian, Y.; Xi, H. X., Effect of electrostatic properties of IRMOFs on VOCs adsorption: a density functional theory study. Adsorption 2014, 20, 777788. (2) Wolkoff, P., Indoor air pollutants in office environments: Assessment of comfort, health, and performance. Int J Hyg Envir Heal 2013, 216, 371-394. (3) Martin, L.; Ognier, S.; Gasthauer, E.; Cavadias, S.; Dresvin, S.; Amouroux, J., Destruction of highly diluted volatile organic components (VOCs) in air by dielectric barrier discharge and mineral bed adsorption. Energ Fuel 2008, 22, 576-582. (4)

Zhao, Z. X.; Li, X. M.; Huang, S. S.; Xia, Q. B.; Li, Z., Adsorption and Diffusion of Benzene on Chromium-Based Metal Organic Framework MIL-101 Synthesized by Microwave Irradiation. Ind Eng Chem Res 2011, 50, 2254-2261.

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(25) Prabhakaran, P. K.; Deschamps, J., Doping activated carbon incorporated composite MIL101 using lithium: impact on hydrogen uptake. J Mater Chem A 2015, 3, 7014-7021. (26) Sari, A.; Tuzen, M.; Soylak, M., Adsorption of Pb(II) and Cr(III) from aqueous solution on Celtek clay. J Hazard Mater 2007, 144, 41-46. (27) Qi, N.; Appel, W. S.; LeVan, M. D.; Finn, J. E., Adsorption dynamics of organic compounds and water vapor in activated carbon beds. Ind Eng Chem Res 2006, 45, 2303-2314. (28) Delage, F.; Pre, P.; Le Cloirec, P., Mass transfer and warming during adsorption of high concentrations of VOCs on an activated carbon bed: Experimental and theoretical analysis. Environ Sci Technol 2000, 34, 4816-4821. (29) Gao, H.; Song, L.; Guo, W. H.; Huang, L.; Yang, D. Z.; Wang, F. C.; Zuo, Y. L.; Fan, X. L.; Liu, Z.; Gao, W.; Vajtai, R.; Hackenberg, K.; Ajayan, P. M., A simple method to synthesize continuous large area nitrogen-doped graphene. Carbon 2012, 50, 4476-4482. (30) Yao, C.; Shin, Y.; Wang, L. Q.; Windisch, C. F.; Samuels, W. D.; Arey, B. W.; Wang, C.; Risen, W. M.; Exarhos, G. J., Hydrothermal dehydration of aqueous fructose solutions in a closed system. J Phys Chem C 2007, 111, 15141-15145. (31) Yuan, D. S.; Zeng, F. L.; Yan, J.; Yuan, X. L.; Huang, X. J.; Zou, W. J., A novel route for preparing graphitic ordered mesoporous carbon as electrochemical energy storage material. Rsc Adv 2013, 3, 5570-5576. (32) Luo, H. Y.; Gu, C. W.; Zheng, W. H.; Dai, F.; Wang, X. L.; Zheng, Z., Facile synthesis of novel size-controlled antibacterial hybrid spheres using silver nanoparticles loaded with poly-dopamine spheres. Rsc Adv 2015, 5, 13470-13477. (33) Qin, W. P.; Cao, W. X.; Liu, H. L.; Li, Z.; Li, Y. W., Metal-organic framework MIL-101 doped with palladium for toluene adsorption and hydrogen storage. Rsc Adv 2014, 4, 24142420. 23



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FIGURES

Fig. 1. SEM images (top) of (a) PDA spheres, (b) MGBC4, and (c) MGBC6 samples with the corresponding magnified SEM images with a scale bar of 3.0 μm.

25



100

5

6 MGBC4

C 5 4

(c)

MGBC6

C 4

80

3

60

2

40

MGBC4 MGBC6

3 2 1 0

1

O N 0

2

4

6

8

10

0

20

O N

(a)

(b)

0

2

4

6

8

0

10

C

N

O

(100)

Relative Intensity (a.u.)

(002)

Fig. 2. EDAX spectra of (a) the MGBC4, (b) the MGBC6 and (c) their element distribution.

MGBC6

MGBC4

0

15

30

45 60 2 (degrees)

75

90

Fig. 3. XRD patterns of the synthesized MGBC4 and MGBC6 samples.

1.0

1.0

(a)

1.0

(b)

0.8

(c) 0.8

0.6 0.4 0.2

PDA

Intensity (a.u.)

0.8

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 35

0.6 0.4 0.2

800

1000 1200 1400 1600 1800 2000 Raman shift (cm-1)

0.4 0.2

MGBC4

MGBC6 0.0

0.0

0.0

0.6

800

1000 1200 1400 1600 1800 2000 Raman shift (cm-1)

800

1000 1200 1400 1600 1800 2000 -1

Raman shift (cm

)

Fig. 4. The Raman spectra of (a) the PDA, (b) the MGBC4 and (c) the MGBC6 samples.

26


100

Amount of adsorbed N2 (mmol/g) STP

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a) 10

1

PDA MGBC4 MGBC6

0.1

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1

Relative pressure (P/Po) 0.5

(b) dV/dW(cm3·g-1·Å-1)

Page 27 of 35

0.4 6.75Å 0.3 0.2

8.78 Å

MGBC4 MGBC6

0.1 0.0 5

10

15 20 Pore size (Å, H-K model)

25

30

Fig. 5. Plots of log(Q) vs log(P/Po) from the nitrogen adsorption isotherms (a) and pore size distribution curves (b) of the PDA and MGBC samples.

Fig. 6. C1s spectrum of the MGBC4. 27



Amount of adsorbed benzene (mmol/g )

16

(a)

MIL-101

12

HKUST-1 8 MGBC6

MGBC4 4

NDA-201 0

USY 0.00

0.02

0.04

0.06

0.08

0.10

P / Po 8

Amount of adsorbed toluene (mmol/g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 35

(b)

MIL-101

HKUST-1

6

MGBC6

MGBC4

4

ZSM-5

UIO-66

2

NaY 0 0.00

0.02

0.04

0.06

0.08

0.10

0.12

P / Po

Fig. 7. Adsorption isotherms of (a) benzene and (b) toluene on the MGBC4, MGBC6, MIL-101,4,33 HKUST-1,15,34 UIO-66,35 zeolites (NaY, USY, ZSM-5)36 and resin (NDA-201)37 at 298 K and P/Po < 0.12.

28


4.5 4.0

(a)

3.5 4

Amount of adsorbed toluene (mmol/g)

3.0 2.5 2.0 1.5 1.0

3 2

283.2 K 298.2 K 313.2 K

1 0

0.5

0

5

10

15 20 P (Pa)

0.0 0

100

200

300

25

30

400

500

P (Pa)

6

(b)

5 6

4

5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 29 of 35

3 2 1

4 3

283.2 K 298.2 K 313.2 K

2 1 0 0

10

20

30

40

50

P (Pa)

0 0

100

200

300

400

500

P (Pa)

Fig. 8. Adsorption isotherms of toluene on the (a) MGBC4 and (b) MGBC6 at 283-313 K.

29



30

-s (kJ/mol)

25 20 15 10

MGBC4 MGBC6

5 0 20

30

40 50 60 Adsorbed coverage (%)

70

Fig. 9. The isosteric heat of toluene adsorption on the (a) MGBC4 and (b) MGBC6 as a function of surface coverage of toluene molecules.

5

MGBC4 MGBC6

0 G (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 35

-5 -10 -15 -20 -25 0

1

2

3

4

5

6

7

Adsorbed amount of toluene (mmol/g)

Fig. 10. The change in free energy of toluene adsorption on the (a) MGBC4 and (b) MGBC6 as a function of adsorbed amount.

30


(a)

Desorption Rate

H=14 K/min

Tp = 405.41 K

H=12 K/min

Tp = 400.03K

H=10 K/min

Tp = 394.65 K

H = 8 K/min

Tp = 389.15 K

 = 6 K/min

Tp = 381.79 K

300

350

400

450

500

550

Temperature (K) (b) H=14 K/min

Tp = 409.75 K

H=12 K/min

Desorption Rate

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Tp = 403.15K

H=10 K/min

Tp = 396.65 K

H = 8 K/min

Tp = 390.99 K

 = 6 K/min

280

320

Tp = 381.01 K

360

400

440

480

520

560

Temperature (K)

Fig. 11. TPD spectra of benzene on (a) the MGBC4 and (b) MGBC6 samples at different heating rates from 6 to 14 K/min (N2 flow rate 30 mL/min).

(a) H = 14 K/min

Desorption Rate

Page 31 of 35

Tp = 463.15 K

H = 12 K/min

Tp = 457.15 K

H = 10 K/min

Tp = 452.15 K

H = 8 K/min  = 6 K/min

Tp = 446.15 K Tp = 438.15 K

250 300 350 400 450 500 550 600 650 700

Temperature (K)

31



(b)

Desorption Rate

H = 14 K/min H = 12 K/min

Tp = 540.37 K

H = 10 K/min

Tp = 531.55 K Tp = 528.15 K

H = 8 K/min  = 6 K/min

Tp = 517.95 K Tp = 509.35 K 400

450

500

550

600

650

Temperature (K)

Fig. 12. TPD spectra of toluene on (a) the MGBC4 and (b) MGBC6 samples at different heating rates from 6 to 14 K/min (N2 flow rate 30 mL/min).

-7.0

MGBC4 MGBC6

(a) -7.2

-ln(H /Tp2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-7.4 -7.6 -7.8 -8.0 2.44

2.48

2.52

2.56

2.60

1000/Tp (K)

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2.64

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-7.4

MGBC4 MGBC6

-ln(H /Tp2)

-7.6

(b)

-7.8 -8.0 -8.2 -8.4 -8.6 1.8

1.9

2.0

2.1

2.2

2.3

1000/Tp (K)

Fig. 13. Linear dependence between  ln( H / RTP2 ) vs 1/TP for TPD of (a) benzene and (b) toluene on the MGBC4 and (b) MGBC6 samples.

1.6 3

Toluene concentration (g/m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MIL-101(Cr) (3450 m2/g)

1.4

MGBC6 (2085 m2/g)

1.2 1.0 0.8 SY-1 AC (1022 m2/g)

0.6 0.4 0.2

3

Co(toluene)=1.43 g/m

0.0 0

20

40

60 80 time (min)

100

120

140

Fig. 14. Breakthrough curves of toluene vapor in the fixed bed filled with the MGBC6 at 293 K and 60 RH% (toluene concentration: 15.5×10-6 mmol/mL, 1.43 ppm).

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SCHEME

Scheme 1. Synthesis procedure of the MGBC samples.

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Graphic of MGBC samples 108x82mm (150 x 150 DPI)


A Microporous Graphitized Biocarbon with High Adsorption Capacity

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