Affinity-based elimination of aromatic VOCs by highly crystalline multi-walled carbon nanotubes

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Talanta 74 (2008) 1265–1270

Affinity-based elimination of aromatic VOCs by highly crystalline multi-walled carbon nanotubes Hiroaki Sone a , Bunshi Fugetsu a,∗ , Takayuki Tsukada b , Morinobu Endo c,∗ a

Graduate School of Environmental Science, Hokkaido University, Sapporo 060-0810, Japan b Nano Carbon Technologies Co. Ltd., 600 Tanaka-cho, Akishima, Tokyo, Japan c Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano-shi 380-8553, Japan

Received 6 August 2007; received in revised form 29 August 2007; accepted 29 August 2007 Available online 7 September 2007

Abstract Carbon nanotubes (CNTs) are capable of adsorbing pollutant chemicals. Their adsorptive capacities and adsorbing mechanisms, however, are not fully understood. As-grown CNTs often contain both crystalline and amorphous carbon, and the ratio of carbon types can affect adsorption. In this study, highly crystalline multi-walled carbon nanotubes (HC-MWCNTs) were used as the adsorbent for volatile organic compounds (VOCs) in contaminated air samples. Air containing 23 added VOCs (1,1-dichloroethylene, dichloromethane, trans-1,2dichloroethylene, cis-1,2-dichloroethylene, chloroform, 1,1,1-trichloroethane, carbon tetrachloride, 1,2-dichloroethane, benzene, trichloroethylene, 1,2-dichloropropane, bromodichloromethane, cis-1,3-dichloropropene, toluene, trans-1,3-dichloropropene, 1,1,2-trichloroethane, tetrachloroethylene, dibromochloromethane, m-xylene, p-xylene, o-xylene, bromoform, and p-dichlorobenzene) was used for model samples. Adsorptive experiments were carried out by passing the air samples through a cartridge packed with HC-MWCNTs. Initial results showing high selectivity and high affinity for adsorbing aromatic VOCs (benzene, toluene, m-xylene, p-xylene, o-xylene, and p-dichlorobenzene) have provided new insight into the adsorption mechanisms. Data suggest that the HC-MWCNTs, unlike conventional carbon materials, adsorb aromatic compounds according to Fukui’s frontier theory, which is based on the interactions between the HOMO and LUMO of the aromatic VOCs and those of the HC-MWCNTs. © 2007 Elsevier B.V. All rights reserved. Keywords: Highly crystalline multi-walled carbon nanotubes (HC-MWCNTs); Volatile organic compounds (VOCs); Affinity-based adsorption; LUMO–HOMO interaction

1. Introduction Volatile organic compounds (VOCs) have received a great deal of attention in the field of environmental control for a number of reasons. First, VOCs are ubiquitous in the environment, workplace, and consumer products [1–4]. Humans can, therefore, be easily exposed to VOCs through skin contact, breathing, and eating. Second, VOCs are suspected to have primary and secondary harmful effects [5,6] due to prolonged exposure. These include eye and throat irritation, liver damage, and damage to the central nervous system. Third, VOCs can form photochemical smog, which contains ozone and other toxic byproducts,

∗

Corresponding authors. E-mail addresses: [email protected] (B. Fugetsu), [email protected] (M. Endo). 0039-9140/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2007.08.041

by reacting with other atmospheric chemicals such as nitrogen oxides [7]. Moreover, VOCs are emitted by landfill leachate and industrial effluent. Leakage into groundwater can cause serious contamination problems [8]. Among VOCs, the aromatic compounds, especially benzene, toluene, xylene and dichlorobenzene, are of particular interest. All of the aromatic VOCs are known or suspected to be carcinogenetic [9]. Furthermore, these compounds have no safety threshold dose for carcinogenic effects. Thus, even low-level exposure poses a finite risk. In fact, benzene, toluene, and xylenes, known as BTX, are the markers for human exposure to VOCs [10]. In the not-so-distant past, adsorptive treatment using activated carbon as the adsorbent was deployed for VOCs elimination [11–13]. However, the inability of activated carbon to selectively adsorb aromatic compounds results in an unpredictable lifespan for the carbon; the selectivity problem needs

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to be addressed. Over the last decade, growing effort has been devoted to developing and utilizing new carbon materials for VOCs elimination. The so-called carbon nanotubes (CNTs) are especially promising because of their unique properties. CNTs can be divided into multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs) based on the carbon atom layers in the walls of the tubes. MWCNTs have been known for more than 30 years [14] and SWCNTs for 14 years [15,16]. The hexagonal arrays of sp2 -like carbon atoms in the tubular graphene sheets on the surfaces of CNTs provide a favorable morphological structure for adsorbing aromatic compounds. Long and Yang [17] have used MWCNTs as an adsorbent for the removal of dioxin from model air samples. They found the adsorptive capability of CNTs to be much higher than that of conventional activated carbon, especially in the Henry’s law region. Li et al. [18] employed MWCNTs as an adsorbent for solid-phase extraction of VOCs from model water samples. The MWCNTs exhibited excellent extraction capability and high recoverability [18,19]. These studies have provided experimental evidence that CNTs are capable of adsorbing pollutants. Nevertheless, none of these studies has explored the possibility of using CNTs as an adsorbent for separating aromatic VOCs from other types of VOCs. This study presents the first data on the use of CNTs as an adsorbent for selectively eliminating aromatic VOCs. Model air samples containing six aromatic VOCs and 17 aliphatic VOCs were passed through a CNT-packed cartridge, and the amount of VOCs retained by the CNTs was quantitatively analyzed using GC–MS. The CNTs were found to be capable of selectively adsorbing the aromatic VOCs. This is also the first study to describe the possible mechanism for the adsorption of aromatic compounds by CNTs using an orbital framework (Fukui’s frontier orbital theory). 2. Materials and methods 2.1. Preparation of the model air samples containing VOCs A commercially available methanol solution (Wako Pure Chemicals) containing six aromatic VOCs (benzene, toluene, m-, p-, o-xylene, and p-dichlorobenzene) and 17 aliphatic VOCs (1,1-dichloroethylene, dichloromethane, trans-1,2-dichloroethylene, cis-1,2-dichloroethylene, chloroform, 1,1,1-trichloroethane, carbon tetrachloride, 1,2-dichloroethane, trichloroethylene, 1,2-dichloropropane, bromodichloromethane, cis-1,3-dichloropropene, trans-1,3-dichloropropene, 1,1,2-trichloroethane, tetrachloroethylene, dibromochloromethane and bromoform) was used for preparing the model air samples. Most compounds were present at a concentration of 1.0 mg/mL, except for m-xylene and p-xylene (m-xylene + p-xylene = 1.0 mg/mL). The model air samples were generated by using a syringe to inject 100 ␮L of the VOCs mixture into a 10-L tetra-bag that was previously filled with purified air under ambient pressure. The injection valve was immediately switched to the closed position and the tetra-bag was then heated at 80 ◦ C in an oven for 1 h. Samples prepared in this manner were used for the adsorptive studies.

2.2. Highly crystallized multi-walled carbon nanotubes (HC-MWCNTs) Highly crystalline multi-walled carbon nanotubes (HCMWCNTs) were used as the adsorbent throughout this study. The HC-MWCNTs were produced using the floating reactant method [20–22]. Ferrocene was used as the catalyst precursor and toluene was used as the carbon feedstock, with hydrogen as the carrier gas. A toluene solution containing a specified amount of ferrocene was fed into a reactor with a temperature of approximately 1200 ◦ C to produce the “as-grown” nanotubes. These nanotubes contain a substantial portion of amorphous carbon. The resultant nanotubes were further thermally annealed at 2600 ◦ C for 30 min in a highly purified argon gas atmosphere to produce the HC-MWCNTs. MWCNTs and SWCNTs containing relatively large amounts of amorphous carbon were also prepared in the laboratory using the CVD method, and these “as-grown” CNTs were used as comparison adsorbents. 2.3. Laboratory-made CNT-packed cartridges CNT-packed cartridges were made by dispersing carbon nanotubes in methanol using an ultrasonic generator (42 kHz) and then loading the CNTs into an empty cartridge (tubular shaped polypropylene: length, 16.5 mm; internal diameter, 10 mm) using a vacuum pump. The final mass of CNTs in each cartridge was approximately 80 mg. Polypropylene filters (10 ␮m) were placed at each end of the cartridges to hold the CNTs in place. 2.4. Adsorption and desorption of VOCs The inlet end of the CNT-packed cartridge was connected to the sample bag. The other end was connected to a Sibata charcoal tube (Sibata, Tokyo, Japan), which is a glass tube filled with activated carbon and is used as the domestic standard cartridge for sampling VOCs in indoor air samples. In these experiments, the charcoal tube was used to trap any VOCs not retained by the CNT-packed cartridge. The outlet end of the tube was connected to a vacuum pump and the air samples were passed through the cartridges at an adjustable flow rate. To measure the amounts of VOCs trapped by the charcoal, the tube was extracted with 5.0 mL CS2 . Analysis was performed with a Shimadzu GCMS-QP5050A system, using toluene D8 as the internal standard. The VOCs adsorbed by the CNT cartridge were estimated by subtraction of the VOCs retained by the charcoal from the initial quantities injected into the sample bag. Adsorptive and desorptive experiments were conducted in triplicate for each sample. 3. Results and discussion 3.1. Characteristics of the highly crystalline multi-walled carbon nanotubes (HC-MWNTs) Fig. 1 shows typical SEM images of the HC-MWCNTs at low (upper photo) and high (lower photo) resolutions. Tubes with

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Table 1 GC–MS conditions for quantitative VOCs determination Chromatograph Column Carrier gas Total flow Split ratio Injector temperature Detector temperature Oven temperature program

Mass spectrometer Interface temperature Electron multiplier voltage Scan mode

DB-624, length: 60 m, φ: 0.25 mm, Film: 1.4 ␮m He 50 mL/min 24 200 ◦ C 230 ◦ C 50 ◦ C (2 min) to (10 ◦ C/min) to 100 ◦ C (5 ◦ C/min) to 130 ◦ C (10 ◦ C/min) to 190 ◦ C (5 min) 230 ◦ C 1.0 kV SIM

the D band (1350 cm−1 ) to that of the G band (1582 cm−1 ). This indicates that the thermally annealed tubes were highly crystalline. Also, the G band, which corresponds to the overtone of 2D at 2700 cm−1 , was very intensive and symmetric for the HC-MWCNTs. 3.2. Adsorptive studies

Fig. 1. SEM images of the HC-MWCNTs at low (upper photo) and high (lower photo) resolutions.

external diameters ranging from 40 to 90 nm were observed, together with a few carbon nano-particles. According to X-ray elemental analysis, the metallic impurity of the HC-MWCNTs was less than 450 ppm. The specific surface area (measured by N2 adsorption based on the BET method) was 25 m2 /g. Analysis using Raman spectroscopy (Fig. 2) revealed an extremely small R-value, which is the ratio of the intensity of

In early experiments, only HC-MWCNT-packed cartridges were used as the adsorptive medium. However, difficulties were encountered in extracting VOCs from the cartridge. A small amount of the carbon nanotubes became entrained in the extraction solution and needed to be removed before the samples could be injected into the GC–MS system. Some of the extracted VOCs were lost during this extra step and recoveries were unsatisfactory. This difficulty was overcome by connecting a standard Sibata charcoal tube next to the HC-MWCNT-packed cartridge. The VOCs retained by the charcoal tube were extracted using CS2 and the extraction solution was directly analyzed using GC–MS. Operating conditions for the GC–MS are summarized in Table 1. Table 2 shows recoveries for four groups of standard samples processed using only the charcoal tube (no HC-MWCNT-packed cartridge). The samples were heated in an oven for 1 h at 50, 60, 70, and 80 ◦ C, respectively. Recoveries were slightly dependent on the temperature (Table 2) and the best recoveries (from 93.9% to 106.3%) were obtained at 80 ◦ C. Note that m-xylene and pxylene were unable to be measured separately using this GC–MS system. 3.3. Affinity of HC-MWCNTs for aromatic VOCs

Fig. 2. Raman spectroscopy for the HC-MWCNTs.

Average values for adsorption (␮g VOCs/g HC-MWCNTs) are shown in Fig. 3. The HC-MWCNTs-packed cartridges demonstrated a high capacity for adsorbing p-dichlorobenzene, o-xylene, m-, p-xylene, toluene, and benzene (i.e., the aromatic VOCs). The adsorption capacity of the HC-MWCNTs for adsorbing p-dichlorobenzene was 1048.3 ␮g/g. For o-xylene, m,p-xylene, toluene, and benzene, adsorption capacities were

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Table 2 VOCs recoveries (%) using the domestic standard cartridge (Sibata-charcoal) at the outlet Recovery (%)

50 ◦ C

60 ◦ C

70 ◦ C

80 ◦ C

1,1-Dichloroethylene trans-1,2-Dichloroethylene cis-1,2-Dichloroethylene Chloroform 1,1,1-Trichloroethane Carbon tetrachloride 1,2-Dichloroethane Benzene Trichloroethylene 1,2-Dichloropropane Bromodichloromethane cis-1,3-Dichloropropene Toluene trans-1,3-Dichloropropene 1,1,2-Trichloroethane Tetrachloroethylene Dibromochloromethane m,p-Xylene o-Xylene Bromoform p-Dichlorobenzene

96.65 99.83 100.29 104.47 107.17 107.99 103.05 104.16 106.99 105.43 107.14 101.47 105.04 102.30 104.99 104.88 101.32 104.11 102.58 101.48 87.53

104.08 101.27 102.37 106.36 106.63 103.95 101.62 101.77 102.99 109.92 101.92 105.25 107.66 106.16 108.63 108.92 109.19 104.93 102.88 104.78 88.24

88.64 87.73 90.04 94.32 95.16 94.21 92.89 93.49 95.16 92.99 95.54 94.92 96.08 95.76 97.41 96.72 97.23 97.20 96.08 97.04 90.70

96.51 93.90 95.44 100.56 100.32 98.25 97.17 95.27 99.75 96.29 99.88 96.78 96.86 96.92 99.44 100.63 99.60 95.88 94.99 106.39 91.61

771.7, 740.3, 274.1 and 178.6 ␮g/g, respectively. The HCMWCNTs thus showed an affinity order of p-dichlorobenzene > o-xylene > m,p-xylene > toluene > benzene  the other aliphatic VOCs. 3.4. Effects of temperature on aromatic VOCs adsorption The effect of temperature on adsorption was studied for four aromatic VOCs (m,p-xylene, o-xylene, and p-dichlorobenzene), chosen as representative compounds. Experiments were conducted at 25, 40, 50, 60, and 70 ◦ C, and the adsorptive capacity (␮g VOCs per g HC-MWCNTs) for each compound was plotted against temperature (K) (Fig. 4). The adsorptive capacities for these VOCs decreased as the temperature increased. Moreover, an adsorption hill was observed for all four compounds at approximately 333 K. This feature was also observed for toluene

Fig. 4. Effects of adsorption temperature on the adsorptive capability of the HC-MWCNTs for VOCs.

and benzene, but was absent for the aliphatic VOCs (data not shown). The hills could be attributed to activated entry effects; m,p-xylene, o-xylene, and p-dichlorobenzene are disc-liked molecules [23]. Higher temperatures may facilitate diffusion of the aromatic VOCs, propelling the molecules through the tube/tube networks and enhancing the adsorptive capacity. 3.5. HOMO–LUMO interactions The total energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the aromatic VOCs were calculated based on Fukui’s frontier orbital theory [24,25] with a PM3 program. Table 3 summarizes the HOMO, LUMO, and LUMO–HOMO values. The p-dichlorobenzene had the highest HOMO energy (−9.459 eV), followed by o-xylene (−9.707 eV), m-xylene (−9.716 eV), toluene (−9.811 eV), and benzene (−10.945 eV). This sequence matches the order of the affinities of the aromatic VOCs for the HC-MWCNTs. HOMO and LUMO electron densities for the aromatic VOCs were calculated using the MOPAC program (Fig. 5). The high frontier electron (HOMO) density points of the aromatic carbon atoms were C3 and C6 (0.3392, 0.3392) for p-dichlorobenzene, C1 and C6 (0.5080, 0.5082) for o-xylene, C3 and C6 (0.5663, 0.5663) for p-xylene, and C1 and C3 (0.4849, 0.5078) for m-xylene. The HOMO points for benzene were C1 and C4 (0.5429, 0.5429), and those for toluene were C3 and C6 (0.5960, 0.6118). The resulting HOMO electron density order was pdichlorobenzene < o-xylene < p-xylene < benzene < toluene. Table 3 HOMO, LUMO, and LUMO–HOMO energies of aromatic VOCs

Fig. 3. Capacity of the HC-MWCNT-packed cartridges for adsorbing VOCs, expressed as micro-grams of VOCs per gram of HC-MWCNT.

VOCs

HOMO (eV)

LUMO (eV)

LUMO–HOMO (eV)

Benzene Toluene m-Xylene p-Xylene o-Xylene p-Dichlorobenzene

−10.115 −9.811 −9.716 −9.592 −9.707 −9.459

0.025 −0.069 −0.085 −0.105 −0.070 −0.425

10.139 9.742 9.631 9.487 9.637 9.034

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Fig. 6. Adsorptive studies carried out using the as-grown MWCNT-packed cartridge (white column) and the as-grown SWCNT-packed cartridge (black column) as the adsorbing media. Other experimental conditions were the same as for the HC-MWCNTs experiments.

selectivity/affinity for adsorbing the aromatic VOCs decreased and finally disappeared as the ratio of the crystalline to amorphous carbon decreased. 4. Conclusion

Fig. 5. Electron densities of HOMO and LUMO on the carbon atoms of the aromatic VOCs, calculated using the MOPAC program. Parentheses indicate HOMO values.

These calculations provide new insight into the mechanism of aromatic VOCs adsorption by HC-MWCNTs. The high degree of adsorption is due to the transfer of the ␲electrons from the HOMO of the HC-MWCNTs to the LUMO of the aromatic VOCs. The frontier electrons (HOMO) of the aromatic VOCs, on the other hand, repel the ␲-electrons transferring to the LUMO. Therefore, a compound with a small HOMO–LUMO gap will have a higher affinity towards the HC-MWCNTs. For example, p-dichlorobenzene has the smallest HOMO–LUMO gap and was the most extensively adsorbed by the HC-MWCNTs. The order of the affinities of the aromatic VOCs for the HC-MWCNTs (p-dichlorobenzene > oxylene > m,p-xylene > toluene > benzene) was identical to the order of the HOMO–LUMO gap values. 3.6. Comparison to “as-grown” CNTs Fig. 6 shows the experimental data obtained using the asgrown MWCNTs and SWCNTs as adsorbents. The value of R (ratio of the intensity of the D band to that of G band) for the MWCNTs was 0.92. The specific surface area measured by N2 adsorption (BET method) was 91 m2 /g. For the as-grown SWCNTs, R = 1.86 and the specific surface area was 254 m2 /g. The

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