- Email: [email protected]
Hydrophobic design of adsorbent for VOC removal in humid environment and quick regeneration by microwave
Microporous and Mesoporous Materials xxx (xxxx) xxx
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso
Hydrophobic design of adsorbent for VOC removal in humid environment and quick regeneration by microwave Yuting Lv 1, Jing Sun 1, Guanqun Yu, Wenlong Wang *, Zhanlong Song, Xiqiang Zhao, Yanpeng Mao National Engineering Laboratory for Coal-fired Pollutants Emission Reduction, Shandong University, Jinan, 250100, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Adsorption Zeolites VOC removal Hydrophobic modification Microwave regeneration
Absorption is an effective method for the removal of volatile organic compounds (VOCs). However, humidity can sharply decrease the adsorption capacity because part of the adsorbed VOCs will be replaced by water molecules. A hydrophobic adsorbent combined with rapid regeneration has considerable advantages in industrial applica tions. In this study, NaY zeolite, modified by hydrothermal dealumination, acid treatment and silanization, was selected as the adsorbent for toluene adsorption. The adsorption curves revealed that modifications improved the adsorption capacity of toluene from 8 mg/g to 46 mg/g in a humid environment (RH ¼ 50%). The used adsorbent was instantly regenerated by exposing to microwave irradiation, mainly because the water adsorbed in the adsorbent can be efficiently heated and thereby pump the toluene out through evaporation. The adsorption capacity of the modified zeolite keeps well after performing absorption and desorption for five cycles. Moreover, as a good VOC adsorbent and microwave absorber, hydrophobic activated carbon can be added with zeolite to enhance the adsorption capacity and microwave regeneration rate. The adsorption capacity was improved to 95 mg/g when 30% activated carbon were added. Importantly, the composite adsorbent can be efficiently re generated with few losses in adsorption capacity. Therefore, this work can provide an important reference for effective adsorption of VOCs in humid environment.
1. Introduction Volatile organic compounds (VOCs) are a group of carbon-based chemicals that are easily evaporated at room temperature [1]. Their high reactivity enables them to react with various substances in the at mosphere, possibly resulting in photochemical smog, particulate matter, stratospheric ozone depletion, etc [2]. Besides, most VOCs are toxic, posing a serious threat to human health [3,4]. Industrial activities, including coal-fired power plant, pharmaceutical manufacturing, transportation, packaging and printing and furniture decoration, are the main sources of VOC emissions [5,6]. Different approaches can be used to capture or destruct specific types of VOCs under diverse conditions [7]. Adsorption has been considered as an effective method because of its high removal efficiency and low cost [8], and the VOCs recovered by desorption can be commercially valuable [9]. Adsorption can also be combined with plasma, oxidation, and other technologies to improve treatment efficiency [10,11].
Activated carbon and zeolites are the two commonly used adsorbents for removing VOCs [12,13]. Activated carbon is relatively cheap and can be obtained from many sources [14], but its application is restricted owing to its flammability under thermal treatment [15]. Zeolites have excellent adsorption capability and are more hydrothermally stable than activated carbon, and thereby have been widely used in the industry [16–20]. However, in actual applications, the moisture present in exhaust gases interferes with the adsorption technologies used for VOC removal, which could lower the adsorption capacity of adsorbents [21, 22]. There are several modification methods to improve the water resistance of zeolites. These methods mainly include hydrothermal dealumination, silanization, and cation modification [23–25]. Because these treatments are usually conducted separately, the adsorption ca pacity of modified zeolites is not sufficiently high in a humid environ ment. Therefore, it is necessary to further improve the adsorption capacity of VOCs and decrease the water adsorption capacity of zeolites by combining different modification methods.
* Corresponding author. E-mail address: [email protected] (W. Wang). 1 These authors contributed equally. https://doi.org/10.1016/j.micromeso.2019.109869 Received 20 August 2019; Received in revised form 18 October 2019; Accepted 2 November 2019 Available online 6 November 2019 1387-1811/© 2019 Elsevier Inc. All rights reserved.
Please cite this article as: Yuting Lv, Microporous and Mesoporous Materials, https://doi.org/10.1016/j.micromeso.2019.109869
Y. Lv et al.
Microporous and Mesoporous Materials xxx (xxxx) xxx
The adsorption-desorption experimental setup is presented in Fig. 1. The synthetic air (99.9%, Jinan Deyang Special Gas Co., Ltd.) consisting of 79% N2 and 21% O2, was used as carrier gas in the adsorption and desorption process, which was more in line with industrial conditions than nitrogen. The air flow rate was controlled by a mass flow controller (Shenzhen Flowmethod Measure & Control System Co., Ltd.). In the adsorption system, two microinjection pumps separately injected toluene and water into the heat-tracing pipe (150 � C) to simu late the humid VOC airstream. The buffer bottle was set before the adsorbent bed and VOCs detector (PID, PV6001-VOC, HNRIKE) to sta bilize the airstream. Hence, the adsorption system could produce a VOC airstream at a certain concentration and humidity. The desorption system achieved VOC desorption of the adsorbent by a modified microwave oven (Model M3-L233C; 2.45 GHz, Midea) at a power of 900 W. Color-changing silica gel was used to remove the steam at the outlet since the adsorbates contained both steam and VOCs. The adsorbent mass used in each experiment was 1 g, the inner diameter of the quartz adsorption column is 1 cm. The airspeed (F) was 600 mL/min in all the adsorption and desorption process, and the injected volume of toluene was 1 μL/min, so the toluene concentration at the inlet (C0) was 1443 mg/m3. The injected volume of water was 0.31 mL/h. All the ex periments were conducted at room temperature. The saturated adsorption capacity of toluene is calculated by the adsorption curve, and the calculation formula is as follows: 2 3 Zts F � C0 � 10 9 4 Ci Ci 5 q¼ � ts dt Eq. 1 W C0 C0
Fig. 1. Schematic diagram of the adsorption and desorption system.
Adsorbent regeneration is also vital because it prevents environ mental pollution caused by waste adsorbents and minimizes the demand for virgin adsorbents [26]. Conventional thermal regeneration requires a large amount of energy to increase the whole bed temperature to a high level and takes a long period to fully achieve adsorbent regenera tion [27,28]. However, microwave thermal regeneration can directly interact with polar VOC molecules or water molecules [29], which can be instantly heated up as “hot spots”, and thereafter, expand rapidly in volume and then desorbed through evaporation [30], finishing the desorption process in a short time with low energy consumption [31, 32]. Additionally, some microwave absorbers, especially some micro porous materials such as activated carbon, can be used to promote the microwave heating and regeneration process as well as enhance the adsorption capacity [33]. Although there are other regeneration tech nologies such as biological and chemical methods, they may generate other hazardous substances and also require longer time than micro wave regeneration [34]. With an aim to control the emission of VOCs from humid flue gas, this study selected an excellent NaY zeolite as the parent adsorbent and conducted hydrophobic modifications on it. The zeolite modifications mainly relied on hydrothermal dealumination to increase the siliconaluminum ratio (Si/Al) and silane modification to reduce the zeolite surface energy [35]. X-ray diffractometer (XRD), Brunauer-Emmett-Teller measurements (BET), scanning electron mi croscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) were used to characterize the modification effects on the modified zeolite. Besides, recycling efficiency of the zeolite was tested after 30 min microwave radiation. Furthermore, we physically mixed the modified zeolite with commercial hydrophobic activated carbon as a new adsorbent and also tested its recycling performance after micro wave regeneration..
0
where W is the weight of adsorbent, g; Ci is concentration at the outlet, mg/m3; ts is the time of saturation, s; q is the saturated adsorption ca pacity, g/g. 2.2. Hydrothermal reaction To start with, the NHþ 4 form of the NaY zeolite (NH4Y) was obtained by ion exchange between the solid NaY zeolite and a 1 mol/L NH4Cl solution at 80 � C for 3–4 h, in which the solid-liquid ratio (W/V) was 1–20. Then the zeolite suspension was centrifuged, washed with deionized water and dried at 110 � C overnight. The whole process was repeated three times. After that, the NH4Y was calcined in a tube furnace (SLGL-1200, Shanghai Jvjing Precision Instrument Manufacturing Co., Ltd.) with nitrogen protection at a rate of approximately 5 � C/min to the maximum temperature of 600–650 � C, at which point saturated vapor was fed into the furnace for 2 h or 6 h. The thermal treatment converts NaY zeolites into USY (Ultra-stable Y) zeolites by changing the frame work structure. For USY zeolites obtained at different temperatures and reaction times, samples are referred to as USY-temp-time. For instance, USY-600-2 means that this USY zeolite was obtained by hydrothermal reaction at 600 � C for 2 h, and similarly as to the others.
2. Materials and methods 2.1. Materials, reagents and setup
2.3. Acid treatment
The parent NaY zeolites were purchased: one from Nankai University Catalyst Co., Ltd. (NK), and two from Tianjin Yuanli Chemical Co., Ltd. (YK and YL, respectively). The SiO2/Al2O3 of NK, YK and YL is 5.4, 4.8 and 4.5, respectively. The hydrophobic activated carbon was produced by South China University of Technology, and it was physically mixed with the zeolite to obtain a new adsorbent. All adsorbents used in adsorption experiments were sieved through a 40–60 mesh. Trime thylchlorosilane (TMCS, 98%) was purchased from Shanghai Xian Ding Biological Technology Co., Ltd. HCl (37%) and toluene (99.8%) were purchased from Sigma-Aldrich. Isopropanol (IPA, �99.5%), ethanol (99.7%) and NH4Cl (99.5%) was purchased from Macklin. All reagents were used without further purification. Deionized water, obtained from an ultrapure water system (UPD-II-10T, Chengdu Ultrapure Technology Co., Ltd.), was used in all the experiments.
In a typical experiment, 2.56 g USY zeolite was added into a roundbottomed flask with 64 mL HCl (0.1 mol/L), heated at 80 � C and stirred for 1 h. Then the samples were centrifuged, washed with deionized water and dried at 110 � C overnight. Acid treated zeolites are referred to as HUSY zeolites. 2.4. Silanization In a typical experiment, 2g HUSY zeolite was dissolved into 100 mL toluene and dispersed by ultrasonication for 5 min. After the solution was transferred into a three-necked flask and purged with nitrogen, 1 mL TMCS was added dropwise into the flask under magnetic stirring. Next, the solution was heated and stirred under reflux for 6 h in a 60 mL/min 2
Y. Lv et al.
Microporous and Mesoporous Materials xxx (xxxx) xxx
Fig. 4. Breakthrough curves of toluene on USY and HUSY zeolites (RH ¼ 50%).
Fig. 2. Breakthrough curves of toluene on different NaY zeolites (RH ¼ 0).
Fig. 2. There were a few differences in toluene adsorption capacity be tween YL (102 mg/g) and YK (104 mg/g), while NK (111 mg/g) had a higher adsorption capacity under the same conditions than the other two zeolites. Therefore, NK was chosen as the parent NaY for further modification and evaluation. 3.1. Hydrothermal dealumination and acid treatment In addition to airspeed and concentration, the moisture in the VOC airstream is another dominant factor that affects the adsorption efficacy of the adsorbent. Fig. 3 shows the competitive adsorption between water and toluene on zeolites. Both water and toluene can be adsorbed in the early stage of adsorption process when the relative humidity is 50%. However, as the adsorption continued until the zeolite was nearly saturated, water molecules replaced the previously adsorbed toluene so that the toluene concentration at the outlet reached more than three times the concentration at the inlet. At the end of this adsorption pro cess, only a small amount of toluene was still adsorbed in the zeolite micropore. After the dealumination in the hydrothermal reaction, the hydro phobicity of the zeolites increased because of the improvement of Si/Al ratio in framework. However, different reaction temperatures and du rations can lead to different levels of hydrophobicity. USY zeolites that had been treated at a higher temperature and longer reaction times adsorbed more toluene than USY zeolites treated for shorter times at lower temperatures. As presented in Fig. 3, USY-600-2 was penetrated earlier by toluene than USY-650-2 even though they were both exposed to saturated water vapor for 2 h. At the same time, USY-650-6 adsorbed more toluene than the other zeolites. It is important to remark that only USY-650-6 solved the problem of competitive adsorption between water and toluene because the adsorbed toluene was not replaced by water (C/ C0�1). Although the hydrothermal reaction contributed to a higher adsorption capacity of zeolites in a humid environment, this adsorption capacity is still significantly lower compared to the adsorption capacity in a dry airstream. It is therefore necessary to further modify the USY zeolites in order to improve their adsorption performance in humid environment. The dealumination process also caused the detached aluminum to remain in the USY zeolite pores. Treating the USY zeolites with acid removed the detached aluminum from the zeolite pores, which could bring about more space for the toluene adsorption. The adsorption performance of USY before and after acid treatment can be seen in Fig. 4. Comparing the performance of USY-600-2 and HUSY-600-2 indicated that the acid treatment not only improved the adsorption capacity but also lessened the competitive adsorption of water and toluene, which was due to the increased hydrophobicity of the adsorbent. Similarly, HUSY-650-6 had a better adsorption performance than USY-650-6 in 50% relative humidity. The results in Fig. 4 demonstrate the necessity of
Fig. 3. Breakthrough curves of toluene on NaY and USY zeolites.
nitrogen gas flow. The final product was obtained by centrifugation, washed with IPA three times and then dried at 80 � C; these zeolites are referred to as CH3-HUSY zeolites. 2.5. Characterization The adsorbents were detected using an X-ray diffractometer (XRD, Panalytical Aeris) with Cu K α radiation (λ ¼ 1.54056 Å). The XRD operated at a voltage of 40 kV and an electric current 15 mA in the 2θ scan range from 5� to 35� with a step size of 0.02� . Scanning electron microscopy (SEM) was performed on Hitachi S4800 electron microscope with an accelerating voltage of 5 kV. The N2 sorption measurements were carried out at 196 � C on physisorption (ASAP 2020, Micro meritics). The samples were degassed under vacuum at 90 � C for 1 h and then at 250 � C for another 6 h. The specific surface area (SBET) was calculated using the Brunauer-Emmett-Teller (BET) method while the micropore area (Smicro), external surface area (Sexternal) and micropore volume (Vmicro) were calculated by the t-plot method. The total pore volume (Vtotal) was calculated from the adsorption amount of nitrogen at a relative pressure of 0.995. Fourier transform infrared spectroscopy (FTIR) was performed on a Nicolet 6700 FTIR spectrometer (Thermo Nicolet). 3. Results and discussion NaY zeolite has a large specific surface area and a suitable pore size for adsorbing most VOCs and are widely used for VOC removal [23,36]. To find an excellent parent NaY zeolite, a set of comparative tests on different NaY zeolites were conducted first. The results are shown in 3
Y. Lv et al.
Microporous and Mesoporous Materials xxx (xxxx) xxx
Fig. 5. XRD patterns of different Y zeolites.
Fig. 7. FTIR spectra of zeolites before and after modification.
Fig. 6. Breakthrough curves of toluene on zeolites before and after silanization.
Fig. 8. The nitrogen adsorption/desorption isotherms of different zeolites.
acid treating the USY zeolites, but the improvement in the structural stability and hydrophobicity of the zeolites was based on changing its structure, in other words, increasing the Si/Al in the zeolite framework. However, the Si/Al cannot be increased infinitely and the cost will be high. The XRD patterns (Fig. 5) have two evident reflections of the (311) and (533) crystal faces at 15� and 24� , respectively, which indicate the NaY zeolite structure. After the hydrothermal reaction and acid treat ment, the peaks shifted to a higher angle, which suggests that both these modifications caused a structural shrinkage of the zeolite. In other words, aluminum was successfully detached from the zeolite frame work, and the hydrophobicity of the zeolite increased owing to an in crease in Si/Al [37]. However, a decrease in peaks intensity after each step of modification suggested that zeolite crystallinity decreased and uniformity deteriorated.
airstream. Compared with the parent NaY, a zeolite modified by hy drothermal dealumination, acid treatment as well as silanization demonstrated a great improvement in VOC adsorption in humid conditions. In order to figure out the effect of surface silanization by TMCS on structure, FTIR was conducted on different zeolites and the results are presented in Fig. 7. There is no obvious difference between the zeolites before silane modification, which indicates that no impurities were introduced into the samples by the hydrothermal reaction or acid treatment. However, some new bands were detected in CH3-HUSY-6506. The band that obviously different with others appears at 2960 cm 1 and 2870 cm 1 due to the presence of C-H, which presents the sym metric CH3 stretch. The weaker peaks that appear near 1438 cm 1 represent band of methoxy (O-CH3). This indicates that the H in the silicon hydroxy group (-OH) on the surface of the original zeolite was replaced by the methyl group (-CH3) in TMCS during the silanization process. Consequently, the FTIR results demonstrate that the silane modification succeeded in replacing the hydrophilic -OH group with a hydrophobic -CH3 group such that the hydrophobicity of the zeolite increased. Fig. 8 shows the N2 adsorption/desorption isotherm curves of different zeolites. The adsorption isotherms of NaY, USY, HUSY, and CH3-HUSY all belong to type I isotherm. At lower relative pressure, N2 adsorption capacity increases rapidly with the increase of relative pressure because of the filling of N2 in the micropores. The hysteresis loops of modified zeolites are more obvious than the parent NaY,
3.2. Silanization and characterization In order to further enhance the performance of the adsorbent, surface of HUSY-650-6 was modified by silanization. The specific increase in the hydrophobicity can be seen in Fig. 6. Fig. 6 shows that the zeolite has a higher hydrophobicity and adsorbs more toluene after the surface modification. This is because, on the surface of zeolite, the originally hydrophilic Si-OH was changed to hydrophobic Si-O-CH3 by silaniza tion, which hinders the adsorption of water. Moreover, the break through time of CH3-HUSY-650-6 increased significantly and restored almost half of the adsorption capacity of the parent NaY in a dry toluene 4
Y. Lv et al.
Microporous and Mesoporous Materials xxx (xxxx) xxx
Table 1 Specific surface area and pore volume of different zeolites. Sample
SBET(m [2]/g)
Smicro(m [2]/g)
Sexternal(m [2]/g)
Vtotal(cm [3]/g)
Vmicro(cm [3]/g)
Vmeso(cm [3]/g)
NaY USY-650-6 HUSY-650-6 CH3-HUSY-650-6
935.12 732.53 782.66 658.98
775.98 564.78 620.91 493.28
159.14 167.75 161.75 165.70
0.58 0.50 0.70 0.53
0.30 0.21 0.24 0.19
0.28 0.29 0.46 0.34
Fig. 9. SEM patterns of the parent NaY zeolite (a) and CH3-HUSY-650-6 (b).
Fig. 11. Recycling adsorption curves of CH3-HUSY-650-6.
Fig. 10. Desorption curves of toluene on HUSY-650-6 under microwave irradiation.
of the zeolite modified by TMCS was grafted with hydrophobic groups (-CH3), which occupied part of the pores, and had a nanometer-sized rough surface, which was densely distributed, but the overall unifor mity was not as good as the parent NaY zeolite (Fig. 9b). In summary, the three modifications (thermal treatment, acid treatment and silanization) did limited damage to the framework structure of the zeolite.
indicating that these modifications increase the proportion of meso porous in zeolites. The specific pore structural parameters of the zeolites are listed in Table 1. After the hydrothermal reaction, the specific sur face area and pore volume decreased sharply, because the hydrothermal dealumination caused cell shrinkage and skeleton collapse. In addition, the detached aluminum also can block part of the pores inside the ze olites. Nevertheless, acid treatment removed the residual aluminum and resulted in the larger micropore area and total pore volume, especially the mesopore volume (Table 1). The acid treatment is crucial for the final step of silane modification because it recreated more pore sites and larger pore structures to achieve silanization. As the hydroxyl groups (-OH) on the surface of the zeolite were replaced by methyl groups (-CH3), silane modification also causes a decrease in specific surface area and pore volume. This decrease also indicates that the degree of silane modification cannot be too high because this form of modification improved the zeolite adsorption capacity in humid conditions by damaging part of its pore structure. The images in Fig. 9 show the microscopic morphology of the parent zeolite and the final zeolite after the modifications. The parent NaY zeolite had a uniform particle distribution with a continuous nano topology, and its framework structure was orderly (Fig. 9a). The surface
3.3. Microwave regeneration After the adsorption reached saturation, the adsorbent bed was transferred into the microwave oven to conduct the desorption and regeneration procedure. Taking HUSY-650-6 as an example, the desorption processes of the adsorbent used in a dry airstream (RH ¼ 0) and the one used in a humid airstream (RH ¼ 50%), respectively, are shown in Fig. 10. Similar to the competitive adsorption, the ratio of toluene concentration at the outlet to that at the inlet (C/C0) is greater than 1 because the microwave heating is instant and efficient and the adsorbent can be heated from both inside and outside at the same time. The desorption curve of the adsorbent used in dry conditions was higher than that used in humid conditions, which suggests that there was more toluene being adsorbed in dry conditions and this adsorbent needed more time for the desorption process to complete. On the other hand, 5
Y. Lv et al.
Microporous and Mesoporous Materials xxx (xxxx) xxx
Fig. 12. Adsorption-desorption curves of toluene on a 70% CH3-HUSY-650-6 with 30% modified activated carbon mixed adsorbent (RH ¼ 50%). Table 2 Adsorption performance of different adsorbents. Adsorbent
Relative humidity
Breakthrough (C/C0 ¼ 5%) min
Saturation (C/C0 ¼ 95%) min
Adsorption capacity mg/g
NaY (NK) NaY (NK) USY-600-2 HUSY-600-2 USY-650-2 USY-650-6 HUSY-650-6 CH3-HUSY-650-6 Mixed adsorbent
0 50%
89 13 26 28 29 32 38 41-42-41-43-43-42b 72-75-77-77b
104 16/36a 38/60a 47/68a 44/66a 70 65 66-68-63-69-63-64b 148-150-152-151b
111 8 19 26 28 38 41 46-46-43-48-47-46b 95-97-98-97b
a b
Two adsorption saturation times due to competitive adsorption between water and toluene. Adsorption performance after several microwave regeneration cycles.
Fig. 10 shows that the desorption is much faster when the zeolite adsorbed both water and VOCs, which demonstrates that the adsorbent applied in humid conditions can be regenerated by microwave irradia tion much more efficiently. The performance of the zeolites, after being recycled, should also be considered. A series of recycling tests were conducted, and the results are presented in Fig. 11. CH3-HUSY-650-6 exhibited the best adsorption performance in humid conditions and still maintained a similar adsorption capacity after five cycles of saturated adsorption and 30 min of microwave regeneration. Furthermore, the process from toluene breakthrough to saturation also remained the same, which indicates that the microwave irradiation can fully desorb the adsorbate without damaging the pore structure and surface modification. Therefore, the modified zeolite has the potential to become a promising catalyst carrier that can be adapted to severe environment and can be efficiently re generated with microwave irradiation. To further enhance the adsorption capacity of adsorbent, 70% CH3HUSY-650-6 with 30% modified activated carbon were mixed to test the adsorption-desorption performance and the results are shown in Fig. 12. It reveals that the mixed adsorbent also has good regeneration perfor mance. Besides, the adsorption capacity from breakthrough to satura tion also improved as a result of the mixing. Although more toluene was adsorbed, the mixed adsorbent can be completely desorbed within 30 min under microwave irradiation because of the enhanced heating performance provided by the activated carbon [38]. Three recycling regeneration iterations did not affect the adsorption capacity of this mixed adsorbent. Generally, the activated carbon would burn violently along with discharge under microwave irradiation [39]. However, being mixed with fire-resistant zeolites, the activated carbon avoids this issue successfully. In the end, the main information in each breakthrough curves are listed in Table 2 for a better comparison of adsorbents performance. Of the three modifications, the hydrothermal dealumination contributed the most to the hydrophobicity of the NaY zeolite, which
enhanced the adsorption capacity of toluene from 8 mg/g to a maximum of 38 mg/g. Although insufficient hydrothermal reaction also led to the improvement of adsorption capacity, it cannot tackle the problem of competitive adsorption between water and VOC. The acid treatment and silanization further improved the adsorption capacity to 46 mg/g, which is better than other technique trying to strengthen the hydrophobicity of zeolites [40]. Moreover, the mixed adsorbent of zeolites and activated carbon also exhibited excellent adsorption performance, maintaining the adsorption capacity above 95 mg/g after three cycles of microwave regeneration. 4. Conclusions NaY zeolite had good adsorption performance on toluene when used in a dry environment, while a humid airstream could decrease its adsorption capability because of the competitive adsorption between toluene and water. After hydrothermal reaction, acid treatment and surface silane modification, the adsorption capacity of the zeolite on toluene in humid conditions improved from 8 mg/g to 46 mg/g. How ever, XRD, BET, and SEM characterization indicated that the modifica tions enhanced hydrophobic performance at the cost of damaging a part of the zeolite framework structure. Although moisture plays an adverse role in the adsorption process, it could be utilized as a catalyst to accelerate the microwave desorption process of adsorbent. The modified zeolite and hybrid adsorbent exhibited good recycling performance after several rapid microwave regenerations. Moreover, the mixed use of activated carbon and zeolite abates carbon loss caused by oxidation under thermal treatment when activated carbon is used alone. Overall, this study demonstrates that the combination of hydro phobic adsorbents and microwave regeneration is a potential way to remove VOCs in humid environment. Moreover, the hydrothermal sta bility, hydrophobicity as well as the excellent microwave renewability suggest that the modified zeolite could be used as a catalyst carrier in the development of catalytic technology in severe conditions. 6
Microporous and Mesoporous Materials xxx (xxxx) xxx
Y. Lv et al.
Author contributions
[14] J.Y. Oh, Y.W. You, J. Park, J.S. Hong, I. Heo, C.H. Lee, J.K. Suh, Adsorption characteristics of benzene on resin-based activated carbon under humid conditions, J. Ind. Eng. Chem. 71 (2019) 242–249. [15] G. Zhang, Y. Liu, S. Zheng, Z. Hashisho, Adsorption of volatile organic compounds onto natural porous minerals, J. Hazard Mater. 364 (2019) 317–324. [16] W.W. Zhang, Z.P. Qu, X.Y. Li, Y. Wang, D. Ma, J.J. Wu, Comparison of dynamic adsorption/desorption characteristics of toluene on different porous materials, J. Environ. Sci. China 24 (3) (2012) 520–528. [17] J. Janchen, D. Ackermann, H. Stach, W. Brosicke, Studies of the water adsorption on Zeolites and modified mesoporous materials for seasonal storage of solar heat, Sol. Energy 76 (1–3) (2004) 339–344. [18] K. Zhang, R.P. Lively, M.E. Dose, A.J. Brown, C. Zhang, J. Chung, S. Nair, W. J. Koros, R.R. Chance, Alcohol and water adsorption in zeolitic imidazolate frameworks, Chem. Commun. 49 (31) (2013) 3245–3247. [19] H.W.B. Teo, A. Chakraborty, W. Fan, Improved adsorption characteristics data for AQSOA types zeolites and water systems under static and dynamic conditions, Microporous Mesoporous Mater. 242 (2017) 109–117. [20] Y.H. Xu, T. Nakajima, A. Ohki, Adsorption and removal of arsenic(V) from drinking water by aluminum-loaded Shirasu-zeolite, J. Hazard Mater. 92 (3) (2002) 275–287. [21] Y.F. Yu, L.W. Zheng, J.D. Wang, Adsorption behavior of toluene on modified 1X molecular sieves, J. Air Waste Manag. 62 (10) (2012) 1227–1232. [22] D.-H. Park, N. Nishiyama, Y. Egashira, K. Ueyama, Separation of organic/water mixtures with silylated MCM-48 silica membranes, Microporous Mesoporous Mater. 66 (1) (2003) 69–76. [23] D.G. Lee, J.H. Kim, C.H. Lee, Adsorption and thermal regeneration of acetone and toluene vapors in dealuminated Y-zeolite bed, Separ. Purif. Technol. 77 (3) (2011) 312–324. [24] X.L. Han, L. Wang, J.D. Li, X. Zhan, J. Chen, J.C. Yang, Tuning the hydrophobicity of ZSM-5 zeolites by surface silanization using alkyltrichlorosilane, Appl. Surf. Sci. 257 (22) (2011) 9525–9531. [25] H.N. Tran, P.V. Viet, H.P. Chao, Surfactant modified zeolite as amphiphilic and dual-electronic adsorbent for removal of cationic and oxyanionic metal ions and organic compounds, Ecotoxicol. Environ. Saf. 147 (2018) 55–63. [26] I.K. Shah, P.P. Pre, B.J. Alappat, Steam regeneration of adsorbents: an experimental and technical review, Chem. Sci. Trans. 2 (4) (2013). [27] E. Gabrus, J. Nastaj, P. Tabero, T. Aleksandrzak, Experimental studies on 3A and 4A zeolite molecular sieves regeneration in TSA process: aliphatic alcohols dewatering-water desorption, Chem. Eng. J. 259 (2015) 232–242. [28] C.O. Ania, J.B. Parra, J.A. Menendez, J.J. Pis, Effect of microwave and conventional regeneration on the microporous and mesoporous network and on the adsorptive capacity of activated carbons, Microporous Mesoporous Mater. 85 (1–2) (2005) 7–15. [29] Q.B. Meng, G.S. Yang, Y.S. Lee, Preparation of highly porous hypercrosslinked polystyrene adsorbents: effects of hydrophilicity on the adsorption and microwaveassisted desorption behavior toward benzene, Microporous Mesoporous Mater. 181 (2013) 222–227. [30] J. Sun, W.L. Wang, Q.Y. Yue, Review on microwave-matter interaction fundamentals and efficient microwave-associated heating strategies, Materials 9 (4) (2016). [31] M. Meier, M. Turner, S. Vallee, W.C. Conner, K.H. Lee, K.S. Yngvesson, Microwave regeneration of zeolites in a 1 meter column, AIChE J. 55 (7) (2009) 1906–1913. [32] I. Polaert, L. Estel, R. Huyghe, M. Thomas, Adsorbents regeneration under microwave irradiation for dehydration and volatile organic compounds gas treatment, Chem. Eng. J. 162 (3) (2010) 941–948. [33] F. Salvador, N. Martin-Sanchez, R. Sanchez-Hernandez, M.J. Sanchez-Montero, C. Izquierdo, Regeneration of carbonaceous adsorbents. Part I: thermal regeneration, Microporous Mesoporous Mater. 202 (2015) 259–276. [34] Y. Zhang, F. Jin, Z. Shen, F. Wang, R. Lynch, A. Al-Tabbaa, Adsorption of methyl tert-butyl ether (MTBE) onto ZSM-5 zeolite: fixed-bed column tests, breakthrough curve modelling and regeneration, Chemosphere 220 (2019) 422–431. [35] U. Khalil, O. Muraza, H. Kondoh, G. Watanabe, Y. Nakasaka, A. Al-Amer, T. Masuda, Robust surface-modified Beta zeolite for selective production of lighter fuels by steam-assisted catalytic cracking from heavy oil, Fuel 168 (2016) 61–67. [36] D.G. Lee, Y.J. Han, C.H. Lee, Steam regeneration of acetone and toluene in activated carbon and dealuminated Y-zeolite beds, Korean J. Chem. Eng. 29 (9) (2012) 1246–1252. [37] S.Y. Kang, J.Z. Ma, Q.M. Wu, H. Deng, Adsorptive removal of dichloromethane vapor on FAU and MFI zeolites: Si/Al ratio effect and mechanism, J. Chem. Eng. Data 63 (6) (2018) 2211–2218. [38] H.L. Cao, F.Y. Cai, W.B. Jiao, Y. Wang, C. Liu, N. Zhang, J. Lu, Microwave-induced decontamination of mercury polluted soils at low temperature assisted with granular activated carbon, Chem. Eng. J. 351 (2018) 1067–1075. [39] J. Sun, Q. Wang, W.L. Wang, Z.L. Song, X.Q. Zhao, Y.P. Mao, C.Y. Ma, Novel treatment of a biomass tar model compound via microwave-metal discharges, Fuel 207 (2017) 121–125. [40] S. Wang, P. Bai, Y. Wei, W. Liu, X. Ren, J. Bai, Z. Lu, W. Yan, J. Yu, Threedimensional-printed core-shell structured MFI-type zeolite monoliths for volatile organic compound capture under humid conditions, ACS Appl. Mater. Interfaces (2019), https://doi.org/10.1021/acsami.9b13819 [EPUB AHEAD OF PRINT].
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding sources This work was generously supported by the National Key Research and Development Program of China (2018YFB0605200), Young Scholars Program of Shandong University (2018WLJH75), the Funda mental Research Funds of Shandong University (2017GN009), and Natural Science Foundation of Shandong Province (ZR2019MEE035). Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Abbreviations VOCs RH TMCS IPA NH4Y USY HUSY XRD SEM FTIR
volatile organic compounds relative humidity trimethylchlorosilane isopropanol NHþ 4 –zeolite complex Ultra-stable Y zeolite acid treated USY zeolite X-ray diffractometer scanning electron microscopy Fourier transform infrared spectroscopy
References [1] N.S. Sanchez, S.T. Alarcon, R.T. Santonja, J. Llorca-Porcel, New device for timeaveraged measurement of volatile organic compounds (VOCs), Sci. Total Environ. 485 (2014) 720–725. [2] X. Zhang, B. Gao, A.E. Creamer, C. Cao, Y. Li, Adsorption of VOCs onto engineered carbon materials: a review, J. Hazard Mater. 338 (2017) 102–123. [3] O. Hwang, S.R. Lee, S. Cho, K.S. Ro, M. Spiehs, B. Woodbury, P.J. Silva, D.W. Han, H. Choi, K.Y. Kim, M.W. Jung, Efficacy of different biochars in removing odorous volatile organic compounds (VOCs) emitted from swine manure, ACS Sustain. Chem. Eng. 6 (11) (2018) 14239–14247. [4] L. Zhang, Y. Peng, J. Zhang, L. Chen, X. Meng, F.-S. Xiao, Adsorptive and catalytic properties in the removal of volatile organic compounds over zeolite-based materials, Chin. J. Catal. 37 (6) (2016) 800–809. [5] M. Schiavon, V. Torretta, A. Casazza, M. Ragazzi, Non-thermal plasma as an innovative option for the abatement of volatile organic compounds: a review, Water Air Soil Pollut. 228 (10) (2017). [6] B.W. Li, S.S.H. Ho, S.L. Gong, J.W. Ni, H.R. Li, L.Y. Han, Y. Yang, Y.J. Qi, D. X. Zhao, Characterization of VOCs and their related atmospheric processes in a central Chinese city during severe ozone pollution periods, Atmos. Chem. Phys. 19 (1) (2019) 617–638. [7] Bouchaala, Volatile organic compounds removal methods: a review, Am. J. Biochem. Biotechnol. 8 (4) (2012) 220–229. [8] C.T. Yang, G. Miao, Y.H. Pi, Q.B. Xia, J.L. Wu, Z. Li, J. Xiao, Abatement of various types of VOCs by adsorption/catalytic oxidation: a review, Chem. Eng. J. 370 (2019) 1128–1153. [9] V.K. Saini, J. Pires, Development of metal organic fromwork-199 immobilized zeolite foam for adsorption of common indoor VOCs, J. Environ. Sci. (China) 55 (2017) 321–330. [10] S.K.P. Veerapandian, N. De Geyter, J.M. Giraudon, J.F. Lamonier, R. Morent, The use of zeolites for VOCs abatement by combining non-thermal plasma, adsorption, and/or catalysis: a review, Catalysts 9 (1) (2019). [11] G. Swetha, T. Gopi, S. Chandra Shekar, C. Ramakrishna, B. Saini, P.V.L. Rao, Combination of adsorption followed by ozone oxidation with pressure swing adsorption technology for the removal of VOCs from contaminated air streams, Chem. Eng. Res. Des. 117 (2017) 725–732. [12] Y.T. Liu, T. Tian, Fabrication of diatomite/silicalite-1 composites and their property for VOCs adsorption, Materials 12 (4) (2019). [13] M.S. Kamal, S.A. Razzak, M.M. Hossain, Catalytic oxidation of volatile organic compounds (VOCs) - a review, Atmos. Environ. 140 (2016) 117–134.
7
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso
Hydrophobic design of adsorbent for VOC removal in humid environment and quick regeneration by microwave Yuting Lv 1, Jing Sun 1, Guanqun Yu, Wenlong Wang *, Zhanlong Song, Xiqiang Zhao, Yanpeng Mao National Engineering Laboratory for Coal-fired Pollutants Emission Reduction, Shandong University, Jinan, 250100, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Adsorption Zeolites VOC removal Hydrophobic modification Microwave regeneration
Absorption is an effective method for the removal of volatile organic compounds (VOCs). However, humidity can sharply decrease the adsorption capacity because part of the adsorbed VOCs will be replaced by water molecules. A hydrophobic adsorbent combined with rapid regeneration has considerable advantages in industrial applica tions. In this study, NaY zeolite, modified by hydrothermal dealumination, acid treatment and silanization, was selected as the adsorbent for toluene adsorption. The adsorption curves revealed that modifications improved the adsorption capacity of toluene from 8 mg/g to 46 mg/g in a humid environment (RH ¼ 50%). The used adsorbent was instantly regenerated by exposing to microwave irradiation, mainly because the water adsorbed in the adsorbent can be efficiently heated and thereby pump the toluene out through evaporation. The adsorption capacity of the modified zeolite keeps well after performing absorption and desorption for five cycles. Moreover, as a good VOC adsorbent and microwave absorber, hydrophobic activated carbon can be added with zeolite to enhance the adsorption capacity and microwave regeneration rate. The adsorption capacity was improved to 95 mg/g when 30% activated carbon were added. Importantly, the composite adsorbent can be efficiently re generated with few losses in adsorption capacity. Therefore, this work can provide an important reference for effective adsorption of VOCs in humid environment.
1. Introduction Volatile organic compounds (VOCs) are a group of carbon-based chemicals that are easily evaporated at room temperature [1]. Their high reactivity enables them to react with various substances in the at mosphere, possibly resulting in photochemical smog, particulate matter, stratospheric ozone depletion, etc [2]. Besides, most VOCs are toxic, posing a serious threat to human health [3,4]. Industrial activities, including coal-fired power plant, pharmaceutical manufacturing, transportation, packaging and printing and furniture decoration, are the main sources of VOC emissions [5,6]. Different approaches can be used to capture or destruct specific types of VOCs under diverse conditions [7]. Adsorption has been considered as an effective method because of its high removal efficiency and low cost [8], and the VOCs recovered by desorption can be commercially valuable [9]. Adsorption can also be combined with plasma, oxidation, and other technologies to improve treatment efficiency [10,11].
Activated carbon and zeolites are the two commonly used adsorbents for removing VOCs [12,13]. Activated carbon is relatively cheap and can be obtained from many sources [14], but its application is restricted owing to its flammability under thermal treatment [15]. Zeolites have excellent adsorption capability and are more hydrothermally stable than activated carbon, and thereby have been widely used in the industry [16–20]. However, in actual applications, the moisture present in exhaust gases interferes with the adsorption technologies used for VOC removal, which could lower the adsorption capacity of adsorbents [21, 22]. There are several modification methods to improve the water resistance of zeolites. These methods mainly include hydrothermal dealumination, silanization, and cation modification [23–25]. Because these treatments are usually conducted separately, the adsorption ca pacity of modified zeolites is not sufficiently high in a humid environ ment. Therefore, it is necessary to further improve the adsorption capacity of VOCs and decrease the water adsorption capacity of zeolites by combining different modification methods.
* Corresponding author. E-mail address: [email protected] (W. Wang). 1 These authors contributed equally. https://doi.org/10.1016/j.micromeso.2019.109869 Received 20 August 2019; Received in revised form 18 October 2019; Accepted 2 November 2019 Available online 6 November 2019 1387-1811/© 2019 Elsevier Inc. All rights reserved.
Please cite this article as: Yuting Lv, Microporous and Mesoporous Materials, https://doi.org/10.1016/j.micromeso.2019.109869
Y. Lv et al.
Microporous and Mesoporous Materials xxx (xxxx) xxx
The adsorption-desorption experimental setup is presented in Fig. 1. The synthetic air (99.9%, Jinan Deyang Special Gas Co., Ltd.) consisting of 79% N2 and 21% O2, was used as carrier gas in the adsorption and desorption process, which was more in line with industrial conditions than nitrogen. The air flow rate was controlled by a mass flow controller (Shenzhen Flowmethod Measure & Control System Co., Ltd.). In the adsorption system, two microinjection pumps separately injected toluene and water into the heat-tracing pipe (150 � C) to simu late the humid VOC airstream. The buffer bottle was set before the adsorbent bed and VOCs detector (PID, PV6001-VOC, HNRIKE) to sta bilize the airstream. Hence, the adsorption system could produce a VOC airstream at a certain concentration and humidity. The desorption system achieved VOC desorption of the adsorbent by a modified microwave oven (Model M3-L233C; 2.45 GHz, Midea) at a power of 900 W. Color-changing silica gel was used to remove the steam at the outlet since the adsorbates contained both steam and VOCs. The adsorbent mass used in each experiment was 1 g, the inner diameter of the quartz adsorption column is 1 cm. The airspeed (F) was 600 mL/min in all the adsorption and desorption process, and the injected volume of toluene was 1 μL/min, so the toluene concentration at the inlet (C0) was 1443 mg/m3. The injected volume of water was 0.31 mL/h. All the ex periments were conducted at room temperature. The saturated adsorption capacity of toluene is calculated by the adsorption curve, and the calculation formula is as follows: 2 3 Zts F � C0 � 10 9 4 Ci Ci 5 q¼ � ts dt Eq. 1 W C0 C0
Fig. 1. Schematic diagram of the adsorption and desorption system.
Adsorbent regeneration is also vital because it prevents environ mental pollution caused by waste adsorbents and minimizes the demand for virgin adsorbents [26]. Conventional thermal regeneration requires a large amount of energy to increase the whole bed temperature to a high level and takes a long period to fully achieve adsorbent regenera tion [27,28]. However, microwave thermal regeneration can directly interact with polar VOC molecules or water molecules [29], which can be instantly heated up as “hot spots”, and thereafter, expand rapidly in volume and then desorbed through evaporation [30], finishing the desorption process in a short time with low energy consumption [31, 32]. Additionally, some microwave absorbers, especially some micro porous materials such as activated carbon, can be used to promote the microwave heating and regeneration process as well as enhance the adsorption capacity [33]. Although there are other regeneration tech nologies such as biological and chemical methods, they may generate other hazardous substances and also require longer time than micro wave regeneration [34]. With an aim to control the emission of VOCs from humid flue gas, this study selected an excellent NaY zeolite as the parent adsorbent and conducted hydrophobic modifications on it. The zeolite modifications mainly relied on hydrothermal dealumination to increase the siliconaluminum ratio (Si/Al) and silane modification to reduce the zeolite surface energy [35]. X-ray diffractometer (XRD), Brunauer-Emmett-Teller measurements (BET), scanning electron mi croscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) were used to characterize the modification effects on the modified zeolite. Besides, recycling efficiency of the zeolite was tested after 30 min microwave radiation. Furthermore, we physically mixed the modified zeolite with commercial hydrophobic activated carbon as a new adsorbent and also tested its recycling performance after micro wave regeneration..
0
where W is the weight of adsorbent, g; Ci is concentration at the outlet, mg/m3; ts is the time of saturation, s; q is the saturated adsorption ca pacity, g/g. 2.2. Hydrothermal reaction To start with, the NHþ 4 form of the NaY zeolite (NH4Y) was obtained by ion exchange between the solid NaY zeolite and a 1 mol/L NH4Cl solution at 80 � C for 3–4 h, in which the solid-liquid ratio (W/V) was 1–20. Then the zeolite suspension was centrifuged, washed with deionized water and dried at 110 � C overnight. The whole process was repeated three times. After that, the NH4Y was calcined in a tube furnace (SLGL-1200, Shanghai Jvjing Precision Instrument Manufacturing Co., Ltd.) with nitrogen protection at a rate of approximately 5 � C/min to the maximum temperature of 600–650 � C, at which point saturated vapor was fed into the furnace for 2 h or 6 h. The thermal treatment converts NaY zeolites into USY (Ultra-stable Y) zeolites by changing the frame work structure. For USY zeolites obtained at different temperatures and reaction times, samples are referred to as USY-temp-time. For instance, USY-600-2 means that this USY zeolite was obtained by hydrothermal reaction at 600 � C for 2 h, and similarly as to the others.
2. Materials and methods 2.1. Materials, reagents and setup
2.3. Acid treatment
The parent NaY zeolites were purchased: one from Nankai University Catalyst Co., Ltd. (NK), and two from Tianjin Yuanli Chemical Co., Ltd. (YK and YL, respectively). The SiO2/Al2O3 of NK, YK and YL is 5.4, 4.8 and 4.5, respectively. The hydrophobic activated carbon was produced by South China University of Technology, and it was physically mixed with the zeolite to obtain a new adsorbent. All adsorbents used in adsorption experiments were sieved through a 40–60 mesh. Trime thylchlorosilane (TMCS, 98%) was purchased from Shanghai Xian Ding Biological Technology Co., Ltd. HCl (37%) and toluene (99.8%) were purchased from Sigma-Aldrich. Isopropanol (IPA, �99.5%), ethanol (99.7%) and NH4Cl (99.5%) was purchased from Macklin. All reagents were used without further purification. Deionized water, obtained from an ultrapure water system (UPD-II-10T, Chengdu Ultrapure Technology Co., Ltd.), was used in all the experiments.
In a typical experiment, 2.56 g USY zeolite was added into a roundbottomed flask with 64 mL HCl (0.1 mol/L), heated at 80 � C and stirred for 1 h. Then the samples were centrifuged, washed with deionized water and dried at 110 � C overnight. Acid treated zeolites are referred to as HUSY zeolites. 2.4. Silanization In a typical experiment, 2g HUSY zeolite was dissolved into 100 mL toluene and dispersed by ultrasonication for 5 min. After the solution was transferred into a three-necked flask and purged with nitrogen, 1 mL TMCS was added dropwise into the flask under magnetic stirring. Next, the solution was heated and stirred under reflux for 6 h in a 60 mL/min 2
Y. Lv et al.
Microporous and Mesoporous Materials xxx (xxxx) xxx
Fig. 4. Breakthrough curves of toluene on USY and HUSY zeolites (RH ¼ 50%).
Fig. 2. Breakthrough curves of toluene on different NaY zeolites (RH ¼ 0).
Fig. 2. There were a few differences in toluene adsorption capacity be tween YL (102 mg/g) and YK (104 mg/g), while NK (111 mg/g) had a higher adsorption capacity under the same conditions than the other two zeolites. Therefore, NK was chosen as the parent NaY for further modification and evaluation. 3.1. Hydrothermal dealumination and acid treatment In addition to airspeed and concentration, the moisture in the VOC airstream is another dominant factor that affects the adsorption efficacy of the adsorbent. Fig. 3 shows the competitive adsorption between water and toluene on zeolites. Both water and toluene can be adsorbed in the early stage of adsorption process when the relative humidity is 50%. However, as the adsorption continued until the zeolite was nearly saturated, water molecules replaced the previously adsorbed toluene so that the toluene concentration at the outlet reached more than three times the concentration at the inlet. At the end of this adsorption pro cess, only a small amount of toluene was still adsorbed in the zeolite micropore. After the dealumination in the hydrothermal reaction, the hydro phobicity of the zeolites increased because of the improvement of Si/Al ratio in framework. However, different reaction temperatures and du rations can lead to different levels of hydrophobicity. USY zeolites that had been treated at a higher temperature and longer reaction times adsorbed more toluene than USY zeolites treated for shorter times at lower temperatures. As presented in Fig. 3, USY-600-2 was penetrated earlier by toluene than USY-650-2 even though they were both exposed to saturated water vapor for 2 h. At the same time, USY-650-6 adsorbed more toluene than the other zeolites. It is important to remark that only USY-650-6 solved the problem of competitive adsorption between water and toluene because the adsorbed toluene was not replaced by water (C/ C0�1). Although the hydrothermal reaction contributed to a higher adsorption capacity of zeolites in a humid environment, this adsorption capacity is still significantly lower compared to the adsorption capacity in a dry airstream. It is therefore necessary to further modify the USY zeolites in order to improve their adsorption performance in humid environment. The dealumination process also caused the detached aluminum to remain in the USY zeolite pores. Treating the USY zeolites with acid removed the detached aluminum from the zeolite pores, which could bring about more space for the toluene adsorption. The adsorption performance of USY before and after acid treatment can be seen in Fig. 4. Comparing the performance of USY-600-2 and HUSY-600-2 indicated that the acid treatment not only improved the adsorption capacity but also lessened the competitive adsorption of water and toluene, which was due to the increased hydrophobicity of the adsorbent. Similarly, HUSY-650-6 had a better adsorption performance than USY-650-6 in 50% relative humidity. The results in Fig. 4 demonstrate the necessity of
Fig. 3. Breakthrough curves of toluene on NaY and USY zeolites.
nitrogen gas flow. The final product was obtained by centrifugation, washed with IPA three times and then dried at 80 � C; these zeolites are referred to as CH3-HUSY zeolites. 2.5. Characterization The adsorbents were detected using an X-ray diffractometer (XRD, Panalytical Aeris) with Cu K α radiation (λ ¼ 1.54056 Å). The XRD operated at a voltage of 40 kV and an electric current 15 mA in the 2θ scan range from 5� to 35� with a step size of 0.02� . Scanning electron microscopy (SEM) was performed on Hitachi S4800 electron microscope with an accelerating voltage of 5 kV. The N2 sorption measurements were carried out at 196 � C on physisorption (ASAP 2020, Micro meritics). The samples were degassed under vacuum at 90 � C for 1 h and then at 250 � C for another 6 h. The specific surface area (SBET) was calculated using the Brunauer-Emmett-Teller (BET) method while the micropore area (Smicro), external surface area (Sexternal) and micropore volume (Vmicro) were calculated by the t-plot method. The total pore volume (Vtotal) was calculated from the adsorption amount of nitrogen at a relative pressure of 0.995. Fourier transform infrared spectroscopy (FTIR) was performed on a Nicolet 6700 FTIR spectrometer (Thermo Nicolet). 3. Results and discussion NaY zeolite has a large specific surface area and a suitable pore size for adsorbing most VOCs and are widely used for VOC removal [23,36]. To find an excellent parent NaY zeolite, a set of comparative tests on different NaY zeolites were conducted first. The results are shown in 3
Y. Lv et al.
Microporous and Mesoporous Materials xxx (xxxx) xxx
Fig. 5. XRD patterns of different Y zeolites.
Fig. 7. FTIR spectra of zeolites before and after modification.
Fig. 6. Breakthrough curves of toluene on zeolites before and after silanization.
Fig. 8. The nitrogen adsorption/desorption isotherms of different zeolites.
acid treating the USY zeolites, but the improvement in the structural stability and hydrophobicity of the zeolites was based on changing its structure, in other words, increasing the Si/Al in the zeolite framework. However, the Si/Al cannot be increased infinitely and the cost will be high. The XRD patterns (Fig. 5) have two evident reflections of the (311) and (533) crystal faces at 15� and 24� , respectively, which indicate the NaY zeolite structure. After the hydrothermal reaction and acid treat ment, the peaks shifted to a higher angle, which suggests that both these modifications caused a structural shrinkage of the zeolite. In other words, aluminum was successfully detached from the zeolite frame work, and the hydrophobicity of the zeolite increased owing to an in crease in Si/Al [37]. However, a decrease in peaks intensity after each step of modification suggested that zeolite crystallinity decreased and uniformity deteriorated.
airstream. Compared with the parent NaY, a zeolite modified by hy drothermal dealumination, acid treatment as well as silanization demonstrated a great improvement in VOC adsorption in humid conditions. In order to figure out the effect of surface silanization by TMCS on structure, FTIR was conducted on different zeolites and the results are presented in Fig. 7. There is no obvious difference between the zeolites before silane modification, which indicates that no impurities were introduced into the samples by the hydrothermal reaction or acid treatment. However, some new bands were detected in CH3-HUSY-6506. The band that obviously different with others appears at 2960 cm 1 and 2870 cm 1 due to the presence of C-H, which presents the sym metric CH3 stretch. The weaker peaks that appear near 1438 cm 1 represent band of methoxy (O-CH3). This indicates that the H in the silicon hydroxy group (-OH) on the surface of the original zeolite was replaced by the methyl group (-CH3) in TMCS during the silanization process. Consequently, the FTIR results demonstrate that the silane modification succeeded in replacing the hydrophilic -OH group with a hydrophobic -CH3 group such that the hydrophobicity of the zeolite increased. Fig. 8 shows the N2 adsorption/desorption isotherm curves of different zeolites. The adsorption isotherms of NaY, USY, HUSY, and CH3-HUSY all belong to type I isotherm. At lower relative pressure, N2 adsorption capacity increases rapidly with the increase of relative pressure because of the filling of N2 in the micropores. The hysteresis loops of modified zeolites are more obvious than the parent NaY,
3.2. Silanization and characterization In order to further enhance the performance of the adsorbent, surface of HUSY-650-6 was modified by silanization. The specific increase in the hydrophobicity can be seen in Fig. 6. Fig. 6 shows that the zeolite has a higher hydrophobicity and adsorbs more toluene after the surface modification. This is because, on the surface of zeolite, the originally hydrophilic Si-OH was changed to hydrophobic Si-O-CH3 by silaniza tion, which hinders the adsorption of water. Moreover, the break through time of CH3-HUSY-650-6 increased significantly and restored almost half of the adsorption capacity of the parent NaY in a dry toluene 4
Y. Lv et al.
Microporous and Mesoporous Materials xxx (xxxx) xxx
Table 1 Specific surface area and pore volume of different zeolites. Sample
SBET(m [2]/g)
Smicro(m [2]/g)
Sexternal(m [2]/g)
Vtotal(cm [3]/g)
Vmicro(cm [3]/g)
Vmeso(cm [3]/g)
NaY USY-650-6 HUSY-650-6 CH3-HUSY-650-6
935.12 732.53 782.66 658.98
775.98 564.78 620.91 493.28
159.14 167.75 161.75 165.70
0.58 0.50 0.70 0.53
0.30 0.21 0.24 0.19
0.28 0.29 0.46 0.34
Fig. 9. SEM patterns of the parent NaY zeolite (a) and CH3-HUSY-650-6 (b).
Fig. 11. Recycling adsorption curves of CH3-HUSY-650-6.
Fig. 10. Desorption curves of toluene on HUSY-650-6 under microwave irradiation.
of the zeolite modified by TMCS was grafted with hydrophobic groups (-CH3), which occupied part of the pores, and had a nanometer-sized rough surface, which was densely distributed, but the overall unifor mity was not as good as the parent NaY zeolite (Fig. 9b). In summary, the three modifications (thermal treatment, acid treatment and silanization) did limited damage to the framework structure of the zeolite.
indicating that these modifications increase the proportion of meso porous in zeolites. The specific pore structural parameters of the zeolites are listed in Table 1. After the hydrothermal reaction, the specific sur face area and pore volume decreased sharply, because the hydrothermal dealumination caused cell shrinkage and skeleton collapse. In addition, the detached aluminum also can block part of the pores inside the ze olites. Nevertheless, acid treatment removed the residual aluminum and resulted in the larger micropore area and total pore volume, especially the mesopore volume (Table 1). The acid treatment is crucial for the final step of silane modification because it recreated more pore sites and larger pore structures to achieve silanization. As the hydroxyl groups (-OH) on the surface of the zeolite were replaced by methyl groups (-CH3), silane modification also causes a decrease in specific surface area and pore volume. This decrease also indicates that the degree of silane modification cannot be too high because this form of modification improved the zeolite adsorption capacity in humid conditions by damaging part of its pore structure. The images in Fig. 9 show the microscopic morphology of the parent zeolite and the final zeolite after the modifications. The parent NaY zeolite had a uniform particle distribution with a continuous nano topology, and its framework structure was orderly (Fig. 9a). The surface
3.3. Microwave regeneration After the adsorption reached saturation, the adsorbent bed was transferred into the microwave oven to conduct the desorption and regeneration procedure. Taking HUSY-650-6 as an example, the desorption processes of the adsorbent used in a dry airstream (RH ¼ 0) and the one used in a humid airstream (RH ¼ 50%), respectively, are shown in Fig. 10. Similar to the competitive adsorption, the ratio of toluene concentration at the outlet to that at the inlet (C/C0) is greater than 1 because the microwave heating is instant and efficient and the adsorbent can be heated from both inside and outside at the same time. The desorption curve of the adsorbent used in dry conditions was higher than that used in humid conditions, which suggests that there was more toluene being adsorbed in dry conditions and this adsorbent needed more time for the desorption process to complete. On the other hand, 5
Y. Lv et al.
Microporous and Mesoporous Materials xxx (xxxx) xxx
Fig. 12. Adsorption-desorption curves of toluene on a 70% CH3-HUSY-650-6 with 30% modified activated carbon mixed adsorbent (RH ¼ 50%). Table 2 Adsorption performance of different adsorbents. Adsorbent
Relative humidity
Breakthrough (C/C0 ¼ 5%) min
Saturation (C/C0 ¼ 95%) min
Adsorption capacity mg/g
NaY (NK) NaY (NK) USY-600-2 HUSY-600-2 USY-650-2 USY-650-6 HUSY-650-6 CH3-HUSY-650-6 Mixed adsorbent
0 50%
89 13 26 28 29 32 38 41-42-41-43-43-42b 72-75-77-77b
104 16/36a 38/60a 47/68a 44/66a 70 65 66-68-63-69-63-64b 148-150-152-151b
111 8 19 26 28 38 41 46-46-43-48-47-46b 95-97-98-97b
a b
Two adsorption saturation times due to competitive adsorption between water and toluene. Adsorption performance after several microwave regeneration cycles.
Fig. 10 shows that the desorption is much faster when the zeolite adsorbed both water and VOCs, which demonstrates that the adsorbent applied in humid conditions can be regenerated by microwave irradia tion much more efficiently. The performance of the zeolites, after being recycled, should also be considered. A series of recycling tests were conducted, and the results are presented in Fig. 11. CH3-HUSY-650-6 exhibited the best adsorption performance in humid conditions and still maintained a similar adsorption capacity after five cycles of saturated adsorption and 30 min of microwave regeneration. Furthermore, the process from toluene breakthrough to saturation also remained the same, which indicates that the microwave irradiation can fully desorb the adsorbate without damaging the pore structure and surface modification. Therefore, the modified zeolite has the potential to become a promising catalyst carrier that can be adapted to severe environment and can be efficiently re generated with microwave irradiation. To further enhance the adsorption capacity of adsorbent, 70% CH3HUSY-650-6 with 30% modified activated carbon were mixed to test the adsorption-desorption performance and the results are shown in Fig. 12. It reveals that the mixed adsorbent also has good regeneration perfor mance. Besides, the adsorption capacity from breakthrough to satura tion also improved as a result of the mixing. Although more toluene was adsorbed, the mixed adsorbent can be completely desorbed within 30 min under microwave irradiation because of the enhanced heating performance provided by the activated carbon [38]. Three recycling regeneration iterations did not affect the adsorption capacity of this mixed adsorbent. Generally, the activated carbon would burn violently along with discharge under microwave irradiation [39]. However, being mixed with fire-resistant zeolites, the activated carbon avoids this issue successfully. In the end, the main information in each breakthrough curves are listed in Table 2 for a better comparison of adsorbents performance. Of the three modifications, the hydrothermal dealumination contributed the most to the hydrophobicity of the NaY zeolite, which
enhanced the adsorption capacity of toluene from 8 mg/g to a maximum of 38 mg/g. Although insufficient hydrothermal reaction also led to the improvement of adsorption capacity, it cannot tackle the problem of competitive adsorption between water and VOC. The acid treatment and silanization further improved the adsorption capacity to 46 mg/g, which is better than other technique trying to strengthen the hydrophobicity of zeolites [40]. Moreover, the mixed adsorbent of zeolites and activated carbon also exhibited excellent adsorption performance, maintaining the adsorption capacity above 95 mg/g after three cycles of microwave regeneration. 4. Conclusions NaY zeolite had good adsorption performance on toluene when used in a dry environment, while a humid airstream could decrease its adsorption capability because of the competitive adsorption between toluene and water. After hydrothermal reaction, acid treatment and surface silane modification, the adsorption capacity of the zeolite on toluene in humid conditions improved from 8 mg/g to 46 mg/g. How ever, XRD, BET, and SEM characterization indicated that the modifica tions enhanced hydrophobic performance at the cost of damaging a part of the zeolite framework structure. Although moisture plays an adverse role in the adsorption process, it could be utilized as a catalyst to accelerate the microwave desorption process of adsorbent. The modified zeolite and hybrid adsorbent exhibited good recycling performance after several rapid microwave regenerations. Moreover, the mixed use of activated carbon and zeolite abates carbon loss caused by oxidation under thermal treatment when activated carbon is used alone. Overall, this study demonstrates that the combination of hydro phobic adsorbents and microwave regeneration is a potential way to remove VOCs in humid environment. Moreover, the hydrothermal sta bility, hydrophobicity as well as the excellent microwave renewability suggest that the modified zeolite could be used as a catalyst carrier in the development of catalytic technology in severe conditions. 6
Microporous and Mesoporous Materials xxx (xxxx) xxx
Y. Lv et al.
Author contributions
[14] J.Y. Oh, Y.W. You, J. Park, J.S. Hong, I. Heo, C.H. Lee, J.K. Suh, Adsorption characteristics of benzene on resin-based activated carbon under humid conditions, J. Ind. Eng. Chem. 71 (2019) 242–249. [15] G. Zhang, Y. Liu, S. Zheng, Z. Hashisho, Adsorption of volatile organic compounds onto natural porous minerals, J. Hazard Mater. 364 (2019) 317–324. [16] W.W. Zhang, Z.P. Qu, X.Y. Li, Y. Wang, D. Ma, J.J. Wu, Comparison of dynamic adsorption/desorption characteristics of toluene on different porous materials, J. Environ. Sci. China 24 (3) (2012) 520–528. [17] J. Janchen, D. Ackermann, H. Stach, W. Brosicke, Studies of the water adsorption on Zeolites and modified mesoporous materials for seasonal storage of solar heat, Sol. Energy 76 (1–3) (2004) 339–344. [18] K. Zhang, R.P. Lively, M.E. Dose, A.J. Brown, C. Zhang, J. Chung, S. Nair, W. J. Koros, R.R. Chance, Alcohol and water adsorption in zeolitic imidazolate frameworks, Chem. Commun. 49 (31) (2013) 3245–3247. [19] H.W.B. Teo, A. Chakraborty, W. Fan, Improved adsorption characteristics data for AQSOA types zeolites and water systems under static and dynamic conditions, Microporous Mesoporous Mater. 242 (2017) 109–117. [20] Y.H. Xu, T. Nakajima, A. Ohki, Adsorption and removal of arsenic(V) from drinking water by aluminum-loaded Shirasu-zeolite, J. Hazard Mater. 92 (3) (2002) 275–287. [21] Y.F. Yu, L.W. Zheng, J.D. Wang, Adsorption behavior of toluene on modified 1X molecular sieves, J. Air Waste Manag. 62 (10) (2012) 1227–1232. [22] D.-H. Park, N. Nishiyama, Y. Egashira, K. Ueyama, Separation of organic/water mixtures with silylated MCM-48 silica membranes, Microporous Mesoporous Mater. 66 (1) (2003) 69–76. [23] D.G. Lee, J.H. Kim, C.H. Lee, Adsorption and thermal regeneration of acetone and toluene vapors in dealuminated Y-zeolite bed, Separ. Purif. Technol. 77 (3) (2011) 312–324. [24] X.L. Han, L. Wang, J.D. Li, X. Zhan, J. Chen, J.C. Yang, Tuning the hydrophobicity of ZSM-5 zeolites by surface silanization using alkyltrichlorosilane, Appl. Surf. Sci. 257 (22) (2011) 9525–9531. [25] H.N. Tran, P.V. Viet, H.P. Chao, Surfactant modified zeolite as amphiphilic and dual-electronic adsorbent for removal of cationic and oxyanionic metal ions and organic compounds, Ecotoxicol. Environ. Saf. 147 (2018) 55–63. [26] I.K. Shah, P.P. Pre, B.J. Alappat, Steam regeneration of adsorbents: an experimental and technical review, Chem. Sci. Trans. 2 (4) (2013). [27] E. Gabrus, J. Nastaj, P. Tabero, T. Aleksandrzak, Experimental studies on 3A and 4A zeolite molecular sieves regeneration in TSA process: aliphatic alcohols dewatering-water desorption, Chem. Eng. J. 259 (2015) 232–242. [28] C.O. Ania, J.B. Parra, J.A. Menendez, J.J. Pis, Effect of microwave and conventional regeneration on the microporous and mesoporous network and on the adsorptive capacity of activated carbons, Microporous Mesoporous Mater. 85 (1–2) (2005) 7–15. [29] Q.B. Meng, G.S. Yang, Y.S. Lee, Preparation of highly porous hypercrosslinked polystyrene adsorbents: effects of hydrophilicity on the adsorption and microwaveassisted desorption behavior toward benzene, Microporous Mesoporous Mater. 181 (2013) 222–227. [30] J. Sun, W.L. Wang, Q.Y. Yue, Review on microwave-matter interaction fundamentals and efficient microwave-associated heating strategies, Materials 9 (4) (2016). [31] M. Meier, M. Turner, S. Vallee, W.C. Conner, K.H. Lee, K.S. Yngvesson, Microwave regeneration of zeolites in a 1 meter column, AIChE J. 55 (7) (2009) 1906–1913. [32] I. Polaert, L. Estel, R. Huyghe, M. Thomas, Adsorbents regeneration under microwave irradiation for dehydration and volatile organic compounds gas treatment, Chem. Eng. J. 162 (3) (2010) 941–948. [33] F. Salvador, N. Martin-Sanchez, R. Sanchez-Hernandez, M.J. Sanchez-Montero, C. Izquierdo, Regeneration of carbonaceous adsorbents. Part I: thermal regeneration, Microporous Mesoporous Mater. 202 (2015) 259–276. [34] Y. Zhang, F. Jin, Z. Shen, F. Wang, R. Lynch, A. Al-Tabbaa, Adsorption of methyl tert-butyl ether (MTBE) onto ZSM-5 zeolite: fixed-bed column tests, breakthrough curve modelling and regeneration, Chemosphere 220 (2019) 422–431. [35] U. Khalil, O. Muraza, H. Kondoh, G. Watanabe, Y. Nakasaka, A. Al-Amer, T. Masuda, Robust surface-modified Beta zeolite for selective production of lighter fuels by steam-assisted catalytic cracking from heavy oil, Fuel 168 (2016) 61–67. [36] D.G. Lee, Y.J. Han, C.H. Lee, Steam regeneration of acetone and toluene in activated carbon and dealuminated Y-zeolite beds, Korean J. Chem. Eng. 29 (9) (2012) 1246–1252. [37] S.Y. Kang, J.Z. Ma, Q.M. Wu, H. Deng, Adsorptive removal of dichloromethane vapor on FAU and MFI zeolites: Si/Al ratio effect and mechanism, J. Chem. Eng. Data 63 (6) (2018) 2211–2218. [38] H.L. Cao, F.Y. Cai, W.B. Jiao, Y. Wang, C. Liu, N. Zhang, J. Lu, Microwave-induced decontamination of mercury polluted soils at low temperature assisted with granular activated carbon, Chem. Eng. J. 351 (2018) 1067–1075. [39] J. Sun, Q. Wang, W.L. Wang, Z.L. Song, X.Q. Zhao, Y.P. Mao, C.Y. Ma, Novel treatment of a biomass tar model compound via microwave-metal discharges, Fuel 207 (2017) 121–125. [40] S. Wang, P. Bai, Y. Wei, W. Liu, X. Ren, J. Bai, Z. Lu, W. Yan, J. Yu, Threedimensional-printed core-shell structured MFI-type zeolite monoliths for volatile organic compound capture under humid conditions, ACS Appl. Mater. Interfaces (2019), https://doi.org/10.1021/acsami.9b13819 [EPUB AHEAD OF PRINT].
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding sources This work was generously supported by the National Key Research and Development Program of China (2018YFB0605200), Young Scholars Program of Shandong University (2018WLJH75), the Funda mental Research Funds of Shandong University (2017GN009), and Natural Science Foundation of Shandong Province (ZR2019MEE035). Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Abbreviations VOCs RH TMCS IPA NH4Y USY HUSY XRD SEM FTIR
volatile organic compounds relative humidity trimethylchlorosilane isopropanol NHþ 4 –zeolite complex Ultra-stable Y zeolite acid treated USY zeolite X-ray diffractometer scanning electron microscopy Fourier transform infrared spectroscopy
References [1] N.S. Sanchez, S.T. Alarcon, R.T. Santonja, J. Llorca-Porcel, New device for timeaveraged measurement of volatile organic compounds (VOCs), Sci. Total Environ. 485 (2014) 720–725. [2] X. Zhang, B. Gao, A.E. Creamer, C. Cao, Y. Li, Adsorption of VOCs onto engineered carbon materials: a review, J. Hazard Mater. 338 (2017) 102–123. [3] O. Hwang, S.R. Lee, S. Cho, K.S. Ro, M. Spiehs, B. Woodbury, P.J. Silva, D.W. Han, H. Choi, K.Y. Kim, M.W. Jung, Efficacy of different biochars in removing odorous volatile organic compounds (VOCs) emitted from swine manure, ACS Sustain. Chem. Eng. 6 (11) (2018) 14239–14247. [4] L. Zhang, Y. Peng, J. Zhang, L. Chen, X. Meng, F.-S. Xiao, Adsorptive and catalytic properties in the removal of volatile organic compounds over zeolite-based materials, Chin. J. Catal. 37 (6) (2016) 800–809. [5] M. Schiavon, V. Torretta, A. Casazza, M. Ragazzi, Non-thermal plasma as an innovative option for the abatement of volatile organic compounds: a review, Water Air Soil Pollut. 228 (10) (2017). [6] B.W. Li, S.S.H. Ho, S.L. Gong, J.W. Ni, H.R. Li, L.Y. Han, Y. Yang, Y.J. Qi, D. X. Zhao, Characterization of VOCs and their related atmospheric processes in a central Chinese city during severe ozone pollution periods, Atmos. Chem. Phys. 19 (1) (2019) 617–638. [7] Bouchaala, Volatile organic compounds removal methods: a review, Am. J. Biochem. Biotechnol. 8 (4) (2012) 220–229. [8] C.T. Yang, G. Miao, Y.H. Pi, Q.B. Xia, J.L. Wu, Z. Li, J. Xiao, Abatement of various types of VOCs by adsorption/catalytic oxidation: a review, Chem. Eng. J. 370 (2019) 1128–1153. [9] V.K. Saini, J. Pires, Development of metal organic fromwork-199 immobilized zeolite foam for adsorption of common indoor VOCs, J. Environ. Sci. (China) 55 (2017) 321–330. [10] S.K.P. Veerapandian, N. De Geyter, J.M. Giraudon, J.F. Lamonier, R. Morent, The use of zeolites for VOCs abatement by combining non-thermal plasma, adsorption, and/or catalysis: a review, Catalysts 9 (1) (2019). [11] G. Swetha, T. Gopi, S. Chandra Shekar, C. Ramakrishna, B. Saini, P.V.L. Rao, Combination of adsorption followed by ozone oxidation with pressure swing adsorption technology for the removal of VOCs from contaminated air streams, Chem. Eng. Res. Des. 117 (2017) 725–732. [12] Y.T. Liu, T. Tian, Fabrication of diatomite/silicalite-1 composites and their property for VOCs adsorption, Materials 12 (4) (2019). [13] M.S. Kamal, S.A. Razzak, M.M. Hossain, Catalytic oxidation of volatile organic compounds (VOCs) - a review, Atmos. Environ. 140 (2016) 117–134.
7