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Enhanced Adsorption Performance of Aromatics on a Novel Chromium-Based MIL-101@graphite oxide Composite Xuejiao Sun, Daofei Lv, Yongwei Chen, Ying Wu, Qi-Hui Wu, Qibin Xia, and Zhong Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02665 • Publication Date (Web): 16 Nov 2017 Downloaded from http://pubs.acs.org on November 19, 2017

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Enhanced Adsorption Performance of Aromatics on a Novel Chromium-Based MIL-101@graphite oxide Composite Xuejiao Sun†, ‡, Daofei Lv‡, Yongwei Chen‡, Ying Wu‡, Qihui Wu†, Qibin Xia *,‡, Zhong Li‡ †

School of Chemical Engineering and Materials Science, Quanzhou Normal University, Quanzhou 362000, China

‡

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China

ABSTRACT: Chromium-based MIL-101 and graphite oxide (GO) composite (MIL-101@GO) was synthesized and applied for adsorption performances towards a series of aromatics (benzene, toluene and ethylbenzene). Desorption activation energies of aromatics on this composite were evaluated based on temperature programmed desorption (TPD) experiments. The results indicated that the aromatic uptakes on the composite increase with the carbon number at low pressure. In contrast, at high pressure, their adsorption capacities exhibit an opposite trend. The composite possesses high uptakes of the aromatics, which are approximately 1.8-6.0 times higher comparing with conventional adsorbents. The desorption activation energies of the aromatics increase with the carbon number of aromatics. Additionally, the adsorption of ethylbenzene on the composite is highly reversible. More importantly, the MIL-101@GO composite exhibits enhanced thermal conductivity up to 0.369 W/mK

*

Corresponding Author. Email: [email protected] (Q.B. Xia).

1


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at 303 K. MIL-101@GO composite with high adsorption capacity, enhanced thermal conductivity and efficient recyclability provided a promising candidate for practical applications in field of volatile organic compounds adsorption. Keywords: Graphite oxide; MIL-101; Composite; Aromatics adsorption

1. INTRODUCTION Volatile organic compounds (VOCs) were regarded as detrimental contaminants owing to the toxic, malodorous, mutagenic and carcinogenic nature.1 Among various VOCs, aromatics (e.g., benzene, toluene and ethylbenzene) are commonly used as industrial solvents for the manufacture of paints, chemicals, pharmaceuticals and rubber.2 The emissions of aromatics have caused serious waste of energy, severe environmental pollution and health issues including cancer and death. Currently, increasing environmental consciousness and health awarenesses have promoted the tight control of the aromatic emissions. Adsorption is a well-established and energy-efficient technique for the collection and recovery of the aromatics,3, 4 whose crucial aspect is to select suitable adsorbents. Some commercial porous adsorbents such as activated carbon, silica gel and zeolites have been often used for aromatics adsorption.5, 6 However, their sorption abilities are not very optimal, and thus cannot satisfy the request for real application. 5, 6 Metal-organic frameworks (MOFs) have been explored in applications of storage and removal of VOCs owing to their high specific areas, adjustable pore sizes and chemical functionalities.7, 8 Zhao et al.9 reported that Cu-BTC possessed the high 2


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uptake of 10.2 mmol/g for benzene at 288 K. Zhou et al.10 reported that the uptake of acetone on MIL-101 was up to 13.9 mmol/g at 288 K. However, the low atomic density of MOFs fails to form strong dispersive forces to efficiently adsorb small gas molecules.11-13 To solve the problem, an efficient strategy is to introduce some materials with dense arrays of atoms into MOFs. Graphite oxide (GO) has received considerable attention because of its dense arrays of atoms, rich oxygen functionalities and unique structure. Endeavours have been made to study MOFs@GO composites.14, 15 For example, Xu et al.16 synthesized the composites based on HKUST-1 and GO which exhibited about a 32% increase in CO2 adsorption capacity. Bandosz et al.11,17 synthesized MOF-5/GO and HKUST-1/GO materials, and found that their uptakes for NH3, NO2 and H2S were greatly enhanced comparing with their parent MOF.

Li et al.18 prepared the

composite Cu-BTC@GO and found that toluene uptake of Cu-BTC@GO increased by 47% comparing with Cu-BTC. Ahmed et al.14,19 reported that MIL-101@GO hybrid material showed the improved adsorption capacity of nitrogen-containing compounds than pristine MIL-101. Therefore, the incorporation of GO into MOFs could enhance gas adsorption performance. Recently, we also prepared the hybrid materials of MIL-101 and GO, and found that the MIL-101@GO composites had higher adsorption capacities for n-alkanes comparing with pristine MIL-101.20, 21 It is extremely likely that the new composite possesses good absorption property for aromatic hydrocarbons, which is worthy of investigation. In addition, the thermal conductivity of porous MOFs is unusually low 3


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due to their low density of atoms.22, 23 The low conductivity will restrict the property of MOF adsorption systems. For example, most adsorption processes are exothermic and the liberated heat during the adsorption process could cause the decease of the adsorption rate and capacity or even local overheating, which will have destructive consequences for the adsorption bed if the heat is not removed efficiently.24-26 To promote the engineering applications of MOFs and their composites for aromatics adsorption, it is critical to understand and improve their thermal properties. Up to now, the thermal properties of the MOFs@GO composites have not been reported yet. In this study, aromatics adsorption performances and thermal conductivity of a new composite MIL-101@GO were researched. The adsorption isotherms of aromatics on the composite were tested. The interaction between aromatics and the composite MIL-101@GO was evaluated by temperature program desorption (TPD) experiments. The influence of aromatic properties on their adsorption performance on the composite would be studied. Interestingly, the MIL-101@GO composite exhibited enhanced thermal conductivity and good adsorption performance of aromatics. So far as we know, this is the first report of MOF@GO composite with enhanced thermal conductivity and significant aromatics capacity. 2. EXPERIMENTAL SECTION 2.1. Materials The synthesis details of MIL-101 and MIL-101@GO composite could be found in our previous reports.20, 21 5 wt% of GO of the initial material weight was added in the preparation of the composite. The synthesis process and characterization of the 4


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composite were described in Supporting Information. Benzene and toluene (99.5%) were obtained from Guangdong Guanghua Sci-Tech Co. Ltd (Guangdong, China). Ethylbenzene (99.8%) were purchased from Aladdin Reagent (Shanghai, China). 2.2. Adsorption measurements The adsorption of aromatics (e.g. benzene, toluene and ethylbenzene) was measured at 298 K using the Micromeritics 3Flex Surface Characterization Analyzer equipped with a stainless steel vapor vessel. Before the adsorption measurements, the vessel was filled with the aromatic and then residual gases were removed by vacuum pumping. After that, the vessel was kept at 313K to form the aromatic vapor. Each sample was outgassed at 423 K for 8h under vacuum before testing. 2.3. TPD experiments TPD experiments were performed using gas chromatography workstation (GC-9560) and conducted at different heating rates. The samples with adsorbed aromatic vapor were packed in a stainless steel column having inner diameter of 0.3 cm and packed length of 0.5 cm. The column was placed in gas chromatograph and heated by temperature programming in the N2 flow. The desorbed aromatics were detected by FID and then TPD curves were obtained. 2.4. Thermal conductivity Powder samples of MIL-101 and MIL-101@GO were pressed under a pressure of 8 M Pa using 769YP-15A tablet machine to form pellets having a diameter of 16 mm and a thickness of 1 mm. Bulk densities were obtained based on the mass and physical dimensions of each pellet. Thermal diffusivity (α) was performed on disk 5


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samples by an LFA447 light flash system (NETZSCH, Germany) at 303 K. Specific heat was measured by a Netzsch differential scanning calorimeter (DSC 204 F1). Then thermal conductivities (λ, W/mK) can be calculated according to the following equation:22 λ= α* Cp * ρ

(1)

where α is the thermal diffusivity (mm2/s), ρ is the compact density (g/cm3), and Cp is the specific heat capacity, J/(g·K). 3. RESULTS AND DISCUSSION 3.1. Physical characteristics Figure 1a presents PXRD patterns of MIL-101 and MIL-101@GO composite. It is noticed that MIL-101@GO shows the similar XRD patterns as that of MIL-101， suggesting that GO did not destroy the crystallization of MIL-101. Figure 1b shows the N2 isotherms of the MIL-101 and the composite at 77 K, which have been discussed in our previous work.20 Their BET surface areas were respectively 2920 m2/g for MIL-101 and 3439 m2/g for the composite. Figure 1c and 1d show the SEM images of MIL-101 and the composite. MIL-101@GO presents smaller particle size and less defined crystalline features compared with MIL-101 due to the interaction of GO with MIL-101. 3.2. Adsorption isotherms of aromatics Figure 2 presents the isotherms of aromatics (benzene, toluene and ethylbenzene) on MIL-101@GO composite. It is noticed that at low pressure the uptakes of the aromatics on the composite increase with the carbon number of aromatics following 6


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the order: ethylbenzene > toluene > benzene. In contrast, at high pressure, their adsorption capacities exhibit an opposite trend: benzene > toluene > ethylbenzene. Similar phenomena were also reported for n-alkanes adsorption.21 Generally speaking, the adsorption behavior is largely affected by the physical properties of adsorbates. 27, 28 Therefore, relevant physical properties of the aromatics are presented in Table 1. It could be observed that the polarizability and dipole moment increase with the carbon number of aromatics. In addition, the molecular cross-sectional area and length also increase with the carbon number of aromatics, indicating that the molecule sizes of aromatics follow the order: ethylbenzene > toluene > benzene. At low pressure, mono-adsorption is dominant and adsorption capacity is managed by the interaction between aromatics and MIL-101@GO. In general, the interaction between gas molecules and MOFs increases with the polarizability and dipole moment of gas molecules.29 Therefore, the interaction between gas molecules and MIL-101@GO follow the order: ethylbenzene > toluene > benzene, which will be further confirmed by TPD experiments latter. As a result, the uptake of MIL-101@GO obeys the order: ethylbenzene > toluene > benzene. However, multilayer adsorption or pore filling will occur at high pressure, and consequently the number of molecules diffusing to the pores becomes smaller as the molecular size increases owing to the limited pore volume. Therefore, molecules with larger sizes present lower adsorption capacity at high pressure and the adsorption capacity obeys the order: benzene > toluene > ethylbenzene, which are 20.0, 16.6 and 13.6 mmol/g, respectively, as shown in Table 1. 7


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Figure 3 presents the isotherms of the aromatics on MIL-101. A comparison of Figure 2 and Figure 3 shows that the shapes of aromatics isotherms on MIL-101 and MIL-101@GO are semblable, suggesting that the adsorption mechanism of the two adsorbents for aromatics are also semblable. For MIL-101, it shows four adsorption sites near (a) supertetrahedra, (b) the pentagonal window, (c) the edge of small cages and (d) the edge of large cages.30 Based on these results, we analyzed the slight several steps (inset Figure 2 and Figure 3) of these aromatics isotherms at low relative pressure in detail. The isotherms of benzene exhibited four steps as follows: (1) benzene molecules are located only the tetrahedral pores owing to the metal−π interactions between benzene molecules and the metal cation; (2) benzene molecules begin to populate the edges of the large cages, small cages and pentagonal windows due to the π−π stacking and electrostatic interactions between benzene molecules and the organic ligand; (3) benzene molecules start clustering around the molecules, thus filling small cages and large cages owing to the intermolecular π−π interactions of benzene; (4) after filling small cages completely, the benzene molecules continue to fill the large cages owing to the intermolecular interactions. These adsorption interactions were analyzed based on the recent reports by Trens et al.31 and Wu et al.32 However, the isotherms of toluene and ethylbenzene exhibit three steps. Compared with benzene adsorption isotherms, their major difference is that the second and third steps mentioned above merge into one step. That means that toluene and ethylbenzene molecules begin to populate the edges of the large cages, small 8


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cages and pentagonal windows due to the π−π and electrostatic interactions between the molecules and organic ligand, meanwhile they started to fill small cages and large cages owing to the intermolecular π−π interactions. This behavior could be explained by the similar strength of the interactions in the two adsorption steps. Therefore, the isotherms of toluene and ethylbenzene exhibit only three steps: (1) toluene and ethylbenzene molecules are located only the tetrahedral pores; (2) toluene and ethylbenzene molecules begin to populate the edges of the large cages, small cages and pentagonal windows, meanwhile they start to fill small cages and large cages; (3) small cages are filled completely, and the large cages continue to be filled. Moreover, MIL-101@GO composite also exhibit higher uptakes of aromatics than MIL-101, which was ascribed to the increases in the surface area and the surface dispersive forces due to the incorporation of GO.20 For comparison, the aromatic uptakes of some adsorbents are shown in Table 2. It shows that the aromatic uptakes of the composite are almost 13%-23% higher in comparison with the MIL-101, and approximately 1.8-6 times higher than that on the conventional adsorbents (e.g., activated carbon and zeolites). In addition, the aromatic uptakes of the composite are much higher than that of other MOFs (e.g., HKUST-1 and UiO-66). This implies that MIL-101@GO exhibits excellent aromatic adsorption performance, which could be used as a promising material for adsorption of aromatics. The reversibility of MIL-101@GO for aromatics adsorption was also detected in this study. Ethylbenzene was selected as the probe molecule, since it can form strong binding force with MIL-101@GO. After the adsorption experiment of ethylbenzene, 9


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the composite was degassed at 298 K for 8 h under high vacuum, and then was reused as sorbent for next isotherm of ethylbenzene adsorption. Figure 4 presents the five adsorption isotherms of ethylbenzene on the composite during five cycles experiments at 298 K. One may notice that the fifth adsorption isotherm of ethylbenzene on MIL-101@GO is still in agreement with the first one, suggesting that ethylbenzene adsorption on the composite is highly reversible. 3.3．Desorption activation energies of aromatics Desorption activation energies of aromatics on the composite were evaluated based on TPD experiments, which analyse surface interaction between the aromatics and the composite. Desorption activation energy Ed of aromatics on the composite can be obtained from the following equation3

 β   E  E  ln  H2  = −  d  − ln  d   RT   RT   k0   p   p

(2)

where βH is the heating rate (K/min), Tp is the peak desorption temperature (K), k0 is the desorption rate coefficient (s-1), Ed is the desorption activation energy (kJ/mol) and R is the gas constant (8.314 J/ (K·mol )). Figure 5 shows TPD spectra of these aromatics on the composite. It could be clearly noted that each TPD spectrum exhibits an obvious peak owing to the aromatic desorption. Meanwhile, we observe that the peak temperature Tp gradually increases with heating rate βH. The linear dependences between ln[(Tp2R)/βH] and 1/Tp are presented in Figure 6. Ed can be obtained according to the straight slope. Table 3 presents the desorption activation energies of aromatics on the composite. It is observed that the desorption activation energies of aromatics follow the order: 10


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benzene < toluene < ethylbenzene. This suggests that the interaction between aromatics and the composite becomes stronger with increasing the carbon number of aromatics. 3.4. Thermal Conductivity The heat transfer parameters of MIL-101 and the composite are presented in Table 4. The MIL-101@GO composite shows enhanced thermal conductivity up to 0.369 W/mK at 303 K, which increases by 60% comparing with that of MIL-101, and is far above other porous MOFs, such as MOF-5(0.10 W/mK) and ZIF-8(0.165 W/mK). 36,37 In order to identify the reason for the significant enhancement of thermal conductivity and clarify the synthetic process, we subjected GO alone to the same synthesis process as for the composite, and then the product was labelled as GN. Figure S1 and Figure S2 presents the PXRD patterns and FTIR spectra of GO and GN. It indicates that GO is reduced to graphene with little oxygen functional groups in the hydrothermal synthesis process. For MIL-101@GO composite, the oxygen functional groups on the basal planes of GN coordinate with the Cr3+ mental sites and replace the BDC ligand in MIL-101, as shown in Figure 7. The inside graphene layers of GN show extremely high intrinsic thermal conductivity and could function as highly conductive channels for thermal transport.38 Thus, graphene could act as thermal conductivity enhancers for the composite and the composite could exhibit the improved thermal conductivity. 4. CONCLUSION 11


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In summary, we further investigated aromatics adsorption performance on a novel Cr-based MIL-101composite. The adsorption capacities of aromatics on the composite increased with the carbon number at low pressure, and in contrast the order was converse at high pressure. The adsorption uptakes of aromatics on the composite were approximately 1.8-6 times higher comparing with the previously reported conventional activated carbons and the zeolites. The desorption activation energies of aromatics increased with increasing the carbon number of aromatics. The adsorption of ethylbenzene on the MIL-101@GO composite was highly reversible. Moreover, the MIL-101@GO composite exhibited enhanced thermal conductivity up to 0.369 W/mK at 303 K, which increased by 60% in comparison with that of MIL-101. The enhanced thermal conductivity, high adsorption uptake and outstanding reversibility of aromatics would make MIL-101@GO composite as an interesting material for practical applications in field of VOCs adsorption. ASSOCIATED CONTENT Supporting Information Synthesis and characterization of MIL-101@GO composites, PXRD patterns and FTIR spectra of GO and GN (PDF). The Supporting Information is available free of charge on the ACS Publications website at DOI: AUTHOR INFORMATION Corresponding Authors *Telephone/Fax: +86-20-87113513. E-mail: [email protected]. Notes 12


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The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos.21576092, 21276092, 21436005 and 21606144). REFERENCES (1) Lee, H. J.; Seo, H. O.; Kim, D. W.; Lim, D. C.; Kim, J. W. Chem. Commun. 2011, 47, 5605−5607. (2) Zhu, M. P.; Tong, Z. F.; Zhao, Z. X.; Jiang, Y. Z.; Zhao, Z. X. Ind. Eng. Chem. Res. 2016, 55, 3765−3774. (3) Zhao, Z.; Li, X.; Huang, S.; Xia, Q.; Li, Z. Ind. Eng. Chem. Res. 2011, 50, 2254−2261. (4) Trens, P.; Belarbi, H.; Shepherd, C.; Gonzalez, P.; Ramsahye, N. A.; Lee, U. H.; Seo, Y. K.; Chang, J. S. Microporous Mesoporous Mater. 2014, 183, 17−22. (5) Zhao, X.; Ma, Q.; Lu, G. Energy Fuels 1998, 12, 1051−1054. (6) Russo, P. A.; Carrott, M.; Carrott, P. J. M. New J. Chem. 2011, 35, 407−416. (7) Li, Y.; Wang, L. J.; Fan, H. L.; Shangguan, J.; Wang, H.; Mi, J. Energy Fuels 2015, 29, 298−304. (8) Salehi, S.; Anbia, M. Energy Fuels 2017, 31, 5376−5384. (9) Zhao, Z. X.; Wang, S.; Yang, Y.; Li, X. M.; Li, J.; Li, Z. Chem. Eng. J. 2015, 259, 79−89. (10) Zhou, X.; Huang, W.; Shi, J.; Zhao, Z.; Xia, Q.; Li, Y.; Wang, H.; Li, Z. J. Mater. Chem.A 2014, 2, 4722−4730. 13


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Tables and Figures Table 1．Physical properties and the maximum adsorption capacities of selected aromatics on MIL-101@GO Table 2．Adsorption capacities of selected aromatics on different adsorbents Table 3．Desorption peak temperatures of aromatics at different heating rates and desorption activation energies of aromatics on MIL-101@GO Table 4．Heat transfer parameters of the MIL-101 and MIL-101@GO composite at 303 K

Figure 1. (a) PXRD patterns, (b) N2 isotherms at 77 K of the MIL-101 and MIL-101@GO composite, and their SEM images: (c) MIL-101 and (d) MIL-101@GO. Figure 2. Adsorption isotherms of aromatics on MIL-101@GO at 298 K. Figure 3. Adsorption isotherms of aromatics on MIL-101 at 298 K. Figure 4. Cycle performance of ethylbenzene adsorption on MIL-101@GO at 298 K. Figure 5. TPD profiles of aromatics on MIL-101@GO at different heating rates as follows: (a) benzene, (b) toluene and (c) ethylbenzene. Figure 6. Linear dependences between ln[(Tp2R)/βH] and 1/Tp for TPD of the various aromatics on MIL-101@GO. Figure 7. Schematic structures of (a)MIL-101 and (b) MIL-101@GO composite.

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Table 1．Physical properties and the maximum adsorption capacities of selected aromatics on MIL-101@GO

Adsorbates

Polarizability ×1025(cm3)

Dipole moment (D)

Molecular cross-sectional area (nm2)

Molecular length (Å)

Maximum capacity (mmol/g)

Benzene Toluene Ethylbenzene

100-107.4 118-123 142

0 0.375 0.59

0.305 0.344 0.368

7.337 8.252 9.361

20.0 16.6 13.6

Table 2．Adsorption capacities of selected aromatics on different adsorbents

Adsorbates

Adsorbents

BET surface area (m2/g)

Benzene

BPL(AC) Zeolite Y HKUST-1 UiO-66 MIL-101 MIL-101@GO SBA-15 MCM-48 PCH HKUST-1 MIL-101 MIL-101@GO PCH MIL-101 MIL-101@GO

923 692 1568.5 2920 3439 829 1126 740 907 2920 3439 740 2920 3439

Toluene

Ethylbenzene

Maximum capacity (mmol/g)

Temperature (K)

Ref

4.9 3.5 10.0 6.5 16.3 20.0 7.8 9.3 2.9 6.6 13.6 16.6 4.3 12.0 13.6

295 295 298 313 298 298 298 298 298 298 298 298 298 298 298

5 5 9 33 present work present work 6 6 34 35 present work present work 34 present work present work

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Energy & Fuels

Table 3．Desorption peak temperatures of aromatics at different heating rates and desorption activation energies of aromatics on MIL-101@GO The peak temperatures Tp at different heating rates (K) Aromatics

8 K/min

Desorption activation energy Ed(kJ/mol)

4 K/min

5 K/min

6 K/min

7 K/min

Benzene

397.0

403.6

408.8

412.8

416.0

43.0

Toluene

401.1

406.0

411.6

414.3

418.8

48.0

Ethylbenzene

405.1

410.4

413.9

417.3

421.4

55.2

Table 4．Heat transfer parameters of the MIL-101 and MIL-101@GO composite at 303 K Sample

ρ (g/cm3)

α (mm2/s)

Cp (J/(g·K))

λ (W/mK)

MIL-101 MIL-101@GO

1.26 1.32

0.151 0.223

1.224 1.252

0.233 0.369

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Figure 1. (a) PXRD patterns, (b) N2 isotherms at 77 K of the MIL-101 and MIL-101@GO composite, and their SEM images: (c) MIL-101 and (d) MIL-101@GO.

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Energy & Fuels

Figure 2. Adsorption isotherms of aromatics on MIL-101@GO at 298 K.

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Figure 3. Adsorption isotherms of aromatics on MIL-101 at 298 K.

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Energy & Fuels

Figure 4. Cycle performance of ethylbenzene adsorption on MIL-101@GO at 298 K.

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Figure 5. TPD profiles of aromatics on MIL-101@GO at different heating rates as follows: (a) benzene, (b) toluene and (c) ethylbenzene. 23


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Energy & Fuels

Figure 6. Linear dependences between ln[(Tp2R)/βH] and 1/Tp for TPD of the various aromatics on MIL-101@GO.

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Figure 7. Schematic structures of (a)MIL-101 and (b) MIL-101@GO composite.

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Enhanced Adsorption Performance of Aromatics ... - ACS Publications

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