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Synthesis, adsorption and regeneration of nanoporous silica aerogel and silica aerogel-activated carbon composites
CHERD-1691; No. of Pages 10
ARTICLE IN PRESS chemical engineering research and design x x x ( 2 0 1 4 ) xxx–xxx
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Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Synthesis, adsorption and regeneration of nanoporous silica aerogel and silica aerogel-activated carbon composites Akbar Mohammadi, Jafarsadegh Moghaddas ∗ Transport Phenomena Research Center, Faculty of Chemical Engineering, Sahand University of Technology, P.O. Box 51335/1996, Tabriz, Iran
a b s t r a c t Usage of aerogels as an adsorbent has become more widespread because of its specifications such as high porosity and specific surface. Nanometer silica aerogel and silica aerogel-activated carbon composites were synthesized using a water glass precursor by ambient pressure drying method. Then, the adsorption capacity of synthesized adsorbents was studied in terms of benzene and ethyl benzene adsorption by chromatography method for continuous and batch testing. Results showed that silica aerogel and silica aerogel-activated carbon composites had high tendency for benzene and ethyl benzene adsorption. Silica aerogel showed maximum adsorption capacity of 2.3 g g−1 and 0.7 g g−1 in static adsorption of benzene and ethyl benzene respectively. Also, in dynamic adsorption of benzene and ethyl benzene, silica aerogel had maximum equilibrium adsorption capacity of 0.954 g g−1 and 0.219 g g−1 respectively. Minimum equilibrium adsorption capacity in benzene and ethyl benzene static adsorption was related to activated carbon with 0.7 g g−1 and silica aerogel–2 wt% activated carbon with 0.25 g g−1 respectively. After adsorption process, silica aerogel and silica aerogel–0.5 wt% activated carbon composite were regenerated by solvent extraction–thermal treatment method and, after at least 15 adsorption/desorption cycles, their adsorption capacity became fixed. © 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Adsorption; Equilibrium adsorption capacity; Regeneration; Silica aerogel; Pore size distribution; BET surface area
1.
Introduction
Volatile Organic Compounds (VOC) are the most common air pollutants emitted from chemical, petrochemical, motor vehicle exhaust, industrial paints, allied industries and use ˇ of organic solvents (Standeker et al., 2009; Song et al., 2005; Carrasco et al., 2009). Contamination caused by volatile organic compounds in the environment is mainly the result of the historic disposal practices of industrial wastes containing these solvents, most of which have been considered dangerous materials since the early 1970s, when the first environmental laws were enacted (Health Effects Fact Sheet, 2000). Atmospheric emissions of these volatile organic compounds cause serious environmental problems and financial losses. ˇ They are toxic and carcinogenic to human health (Standeker
et al., 2009; Song et al., 2005) and cause serious environmental problems such as the destruction of the ozone layer (Song et al., 2005), global warming (Dou et al., 2011), and formation of tropospheric ozone and other oxidants that cause photochemical smog (Carrasco et al., 2009; Atkinson, 2000). Therefore, controlling VOC emissions according to increasingly stringent environmental regulations is necessary. There are many separation processes to address this problem [B] and prevent VOC ˇ et al., 2009; Anita effluence such as condensation (Standeker and Wilfreid, 1999), catalytic oxidation (Carrasco et al., 2009; Ruddy and Carroll, 1993; Patkar and Laznow, 1992), absorption (Song et al., 2005) membrane-based recovery process (Semenova, 2004; Khan and Ghoshal, 2000), and adsorption (Dou et al., 2011; Crittenden and Thomas, 1998). One of the most widely used methods for controlling VOC emissions and
∗
Corresponding author. Tel.: +98 412 3459155; fax: +98 412 3444355. E-mail address: [email protected] (J. Moghaddas). Received 10 March 2014; Received in revised form 21 August 2014; Accepted 3 September 2014 http://dx.doi.org/10.1016/j.cherd.2014.09.003 0263-8762/© 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Mohammadi, A., Moghaddas, J., Synthesis, adsorption and regeneration of nanoporous silica aerogel and silica aerogel-activated carbon composites. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.09.003
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the removal of them in gaseous streams is adsorption, for which adsorbents with high specific surface areas and the ˇ possibility to be used in many cycles are needed (Standeker et al., 2009; Song et al., 2005; Abril et al., 2010). Small pores, such as micropores and mesopores, result in large specific surface area responsible for adsorption. Pore size, pore distribution and surface area, as well as pore surface chemistry, are the major factors in the adsorption process (Yang, 2003). Silica aerogels are extremely porous (Bhagat et al., 2008; Alnaief and Smirnova, 2010), nanostructured (Bangi et al., 2008), and mesoporous (Ciolek, 2006) and have high specific surface areas (Gurav et al., 2009a; Rao et al., 2006) that are easily recovered. They are stable even after many adsorption/desorption cycles with no loss of efficiency, they are easily recovered and also exhibit capacities which enormously exceed those of commonly used adsorbents such as activated carbon and ˇ et al., 2009; Song et al., 2005). In 2009, silica gel (Standeker ˇ et al. showed that silica aerogels were excellent Standeker adsorbents for the adsorption of BTEX1 vapors from polluted gas stream in comparison to activated carbon and silica gel. Despite their good potential applications, the usage of aerogels as an adsorbent has been restricted due to their weak and fragile mechanical strength. One of the appropriate methods to overcome and strengthen the mechanical fragility of aerogels is the preparation of silica aerogel composites (Rao et al., 2006; Kim et al., 2008). In 2003, Coleman et al. found that silica aerogel-granulated activated carbon was superior in terms of removing uranium from a stock solution compared to granulated activated carbon alone for batch testing. In 2011, Dou et al. showed that silica aerogel-activated carbon composite has high affinity towards aromatic molecules and fast adsorption kinetics. Excellent performance of dynamic adsorption and desorption observed on silica aerogel-activated carbon composite is related to its higher adsorption capacity (Dou et al., 2011). Nanostructured silica aerogels are synthesized by a two-step sol–gel route and drying of wet gel to remove the trapped solvent from the pores of the gel (Bhagat et al., 2006; Gurav et al., 2009b). Aerogels, based on water glass precursor, are hydrophilic and become wet with atmospheric moisture or water, but with appropriate chemical modification the surface of the aerogel can be converted to hydrophobic so that the ˇ et al., 2009). The water molecules will be repelled (Standeker main reason for hydrophilicity of silica aerogels is the presence of Si OH groups in aerogel structure as they promote the adsorption of water. By replacing Si OH groups with Si CH3 groups, hydrophobic silica aerogels are obtained, because the adsorption of water is prevented. Preparation of silica aerogels using water-glass precursor followed by an ambient pressure drying (APD) is the cheapest and safest method for aerogel production (Gurav et al., 2009b; Shi et al., 2006). In order to make the use of silica aerogels economically feasible on a large scale, the exhausted adsorbents must be regenerated and reused (Abril et al., 2010; Garcia-oton et al., 2005). Silica aerogels are easily regenerated as a result of having an ˇ et al., open pore structure and being mesoporous (Standeker 2007). Regeneration of the silica aerogel can be performed using different methods such as thermal, extractive, chemical, electrochemical and biologic ones. Among these methods, thermal regeneration and solvent extraction are considered two of the most effective (Abril et al., 2010). The production of hydrophobic silica aerogels has become an important and
1
Benzene, Toluene, Ethylbenzene and Xylenes.
intensive area of research, like many of the scientific and technological applications of the aerogels (Rao and Kulkarni, 2002). Therefore, in this study, silica aerogel and its composites with activated carbon were synthesized using water-glass precursor by a two-step sol–gel process followed by ambient pressure drying. Performance of synthesized adsorbents was studied under static and dynamic conditions for benzene and ethyl benzene adsorption in fixed beds. So, for the first time, exhausted adsorbents were regenerated by solvent extraction–thermal treatment combined method.
2.
Experimental procedure
2.1.
Synthesis of silica aerogel
The chemicals used for the synthesis of silica aerogel were water-glass (Na2 SiO3 , 1.35 g ml−1 , Merck Co., Germany), trimethylchlorosilane (TMCS) (C3 H9 ClSi, 0.86 g ml−1 , Merck Co., Germany), isopropyl alcohol (IPA), ammonium hydroxide solution (1.0 M), and n-hexane. The ion exchange resin, Amberlite IR 120 H+ (Merck Co., Germany), was used for replacing the Na+ ions present in the water-glass with the H+ ions. Hydrogels were prepared using water-glass by a two-step sol–gel process. The water-glass solution was first diluted with the deionized water (water-glass: deionized water (volume ratio) = 1:4) so as to obtain the desired weight percentage of silica in the starting material. In the next step, the ion exchange was carried out by mixing the diluted water-glass solution with the ion exchange resin in 1:1 volume proportion. The mixture was stirred for 5 min and, consequently, pH of the ion exchanged solution changed from 13 to 2, clearly showing the removal of unwanted Na+ ions from the water-glass solution and resulting in the formation of silicic acid (SA). Then, 1.0 M NaOH solution was added to the silica sol to raise its pH to 4 for gelation. The obtained silica sols were stirred for 1 min and then transferred to a plastic beaker with 30 mm diameter and 60 mm height, where the sols aged into hydrogels within about 180 min at 60 ◦ C. The next step was to immerse the hydrogels in isopropyl alcohol and normal hexane and then subsequently age them for 18 h at 60 ◦ C in order to extrude pore water (exchanged by n-hexane) and also strengthen the networks of the gels. After solvent exchanging and aging, the wet gels were immersed in TMCS/n-hexane solutions for 12 h at 60 ◦ C in order to adequately modify the surface and discharge the pore water, mainly containing H2 O/n-haxane solutions. Volumetric ratio of n-hexane/TMCS was fixed at 5, because less shrinkage was observed at this ratio. Finally, the modified gels were dried at room temperature for 24 h and then at 60, 80, 120, and 180 ◦ C for 2 h, respectively, in the oven to reduce shrinkage during drying. In the synthesis of silica aerogel-activated carbon composite, powdery activated carbon (Iran, 200 mesh, 1.6 g cm−1 , 300 m2 g−1 ) was immersed in silica sol after the ion exchange step.
2.2.
Characterizing aerogels
Apparent density of the aerogel was measured by mass to volume ratio ( = mv−1 ) of the aerogel, in which mass was measured by the microbalance with the accuracy of 10−5 g and volume of the aerogel was measured by filling aerogel in the measuring cylinder of the known volume. The microstructure and morphology of silica aerogels were observed by scanning electron microscopy (SEM: JSM-6700F). Specific surface area and pore size distribution of aerogels were determined
Please cite this article in press as: Mohammadi, A., Moghaddas, J., Synthesis, adsorption and regeneration of nanoporous silica aerogel and silica aerogel-activated carbon composites. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.09.003
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2.5.
Fig. 1 – Scheme of dynamic adsorption system for elimination of VOC.
by Brunauer–Emmitt–Teller (BET) method (Belsorb mini II). Fourier transform infrared spectroscopy (FTIR, 4600 Unicam) was employed to investigate the chemical bonding state of aerogels which gave information about various chemical bonding such as –OH, Si OH, Si O Si, Si C, and C H.
2.3.
Measuring static adsorption
Static adsorption equilibrium measurements of the aerogels were recorded using an intelligent gravimetric analyzer (Model AT261, Mettler) with sensitivity of 10−5 g. Before the measurement, each sample was degassed at 120 ◦ C for 4 h. All experiments were replicated with four adsorbents for two pollutants at room temperature.
2.4.
Measuring dynamic adsorption
Dynamic adsorption was studied using the parameters of contaminated flow rate (at 2 levels of 55 and 119 ml min−1 ), concentration of pollutant (at 2 levels of 839 × 10−6 and 1390 × 10−6 g ml−1 ), height of adsorption bed (at 2 levels of 1 and 3 cm), and type of adsorbent (4 blocks) by statistical analysis method: 2 levels of full factorial design + centre point with four time replications of centre point and one time replication of corner points. Investigations of the dynamic adsorption of benzene and ethyl benzene for silica aerogel, silica aerogelactivated carbon composite, and activated carbon were carried out by a flow method on an experimental set-up (Dou et al., 2010). Fig. 1 shows the experimental set-up used in dynamic adsorption process. Concentrations of benzene and ethyl benzene before and after the adsorption process were tested using gas chromatograph (GC) equipped with a flame ionization detector (FID). Before the measurement, each sample was degassed at 120 ◦ C for 4 h. All the experiments were carried out at room temperature.
Regenerating adsorbents
The most limiting step of the adsorption process is often regeneration step, which is time and energy consuming. In this context, the use of solvent extraction–thermal treatment method appears to offer a potential solution. Thus, in this paper, a combination of two methods (solvent extraction–thermal treatment) was used. Saturated adsorbents were first immersed in ethanol for 1 h. Then, liquids of the adsorbent surface were vacated by syringe and wet adsorbents were placed in an oven at 80 ◦ C for 1 h in order to dry and to remove ethanol and adsorbates from adsorbent pores. Adsorption–regeneration cycle was repeated for the static adsorption of benzene over silica aerogel and silica aerogel–0.5 wt% activated carbon composite at 25 ◦ C.
3.
Results and discussion
3.1.
Properties of synthesized samples
Table 1 shows the properties of silica aerogel and silica aerogel-activated carbon composites synthesized with water glass and surface modification agent (TMCS). Specific surface area of silica aerogel is more significant than that of the silica aerogel-activated carbon composites. Specific surface area relies on density and pore size distribution, which has a reverse relationship with density and pore size distribution (Rao and Rao, 2008). Therefore, it is logical that silica aerogel has higher specific surface area. Density of silica aerogel-activated carbon composite is greater than that of pure silica aerogel and this characteristic increases with the increment of activated carbon in silica aerogel matrix, since the density of activated carbon is higher than that of pure silica aerogel. Fig. 2 shows the SEM morphology of silica aerogel and silica aerogel-activated carbon composites obtained by using TMCS for modification of the wet gels. The synthesized samples exhibit porous network structure which contains spherical solid clusters and pores below 100 nm and the particle distribution of samples are uniform. The particles size of composites is greater than that of pure silica aerogel. This figure denoted that surface modification reaction in the composites was completely done. The FTIR analysis also shows this fact. Fig. 3 illustrates the pore size distribution (PSD) profiles of the silica aerogel and silica aerogel-activated carbon composites synthesized by an ambient pressure drying (APD) method. A noticeable change has been observed in the PSD profiles of the silica aerogel and silica aerogel-activated carbon composites. It is known that the synthesized samples contained mesopores (2–25 nm pores) and macropores (above 50 nm) from pore size distribution graphs. Most pores of silica aerogel known to be mesopores are below 20 nm and the peak of the smallest pore was 8 nm. Whereas most pores of silica aerogel composites known to be mesopores are below about 30 nm and their peak of smallest pore was 16 nm. In surface
Table 1 – Properties of synthesized samples. Samples Silica aerogel Silica aerogel–2 wt% activated carbon composite Silica aerogel–0.5 wt% activated carbon composite
BET surface area (m2 g−1 )
Average pores diameter (nm)
Volume of total pores (cm3 g−1 )
Density (g cm−3 )
427.16 358.4
19.72 29.51
2.11 2.64
0.16 0.22
360.1
32.38
2.91
0.18
Please cite this article in press as: Mohammadi, A., Moghaddas, J., Synthesis, adsorption and regeneration of nanoporous silica aerogel and silica aerogel-activated carbon composites. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.09.003
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Fig. 2 – SEM photographs of silica aerogel (a) silica aerogel–0.5 wt% activated carbon composite (b) and silica aerogel–2 wt% activated carbon composite.
modification reaction, silanol groups are replaced with nonpolar hydrolytically stable Si (CH3 )3 groups. A repulsive force acts between the Si (CH3 )3 groups, therefore the gels start to increase in volume, “spring back” and strengthen the pores (Rao et al., 2007). The FTIR analysis denotes that surface modification reaction in the samples containing activated carbon was completely done and the number of Si (CH3 )3 groups
Fig. 3 – Pore size distribution profiles of the water-glass based silica aerogel (a) silica aerogel–0.5 wt% activated carbon composite (b) and silica aerogel–2 wt% activated carbon composite dried at ambient pressure.
in composite are more than that of pure silica aerogel. So the average pore diameter of composites is greater than that of pure silica aerogel. Fig. 4 shows the FTIR analysis of synthesized samples. The FTIR spectrum represents a broad band at around 3500 cm−1 and a peak at around 1600 cm−1 , which can be attributed to O H groups. The peaks at around 1100
Please cite this article in press as: Mohammadi, A., Moghaddas, J., Synthesis, adsorption and regeneration of nanoporous silica aerogel and silica aerogel-activated carbon composites. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.09.003
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Fig. 4 – FTIR analysis of silica aerogel and silica aerogel-activated carbon composites. and 800 cm−1 are due to asymmetric and symmetric modes of SiO2 (Rao et al., 2007). The absorption peaks at 2963, 1256 and 846 cm−1 corresponding to CH3 terminal groups are quite visible which are attributed to surface modification of wet gel by TMCS (Shi et al., 2006). Intensity of OH peak (1600 cm−1 ) in silica aerogel–2 wt% activated carbon composite is less than that of silica aerogel–0.5 wt% activated carbon composite and silica aerogel. Therefore, the hydrophobicity of silica aerogel–2 wt% activated carbon composite is more than that of the other two samples. This analysis indicated that surface modification reaction in the samples containing activated carbon was completely done and the hydrophobicity of composites increased with increment of activated carbon in silica aerogel matrix. Also, Table 1 shows that the synthesized samples are nanostructured and mesoporous (2–50 nm).
3.2.
Static adsorption behaviour
Fig. 5 illustrates the equilibrium adsorption capacity of four types of adsorbents. It is evident that equilibrium adsorption capacity of benzene changes in the order of silica aerogel > silica aerogel–2 wt% activated carbon composite > silica aerogel–0.5 wt% activated carbon composite > activated carbon. This result can be explained by the fact that the static VOC adsorption capacity is proportional to the total pore volume (Dou et al., 2010) and available surface for adsorbate. Since pure silica aerogel had the highest surface area (427.16 m2 g−1 ) and lowest pore volume (2.11 cm3 g−1 ), it is obvious that its equilibrium adsorption capacity would be significant. Interaction between activated carbon and silica is important in the benzene adsorption over silica aerogelactivated carbon composites. Adsorption capacity of silica aerogel–2 wt% activated carbon composite was more than that of silica aerogel–0.5% activated carbon composite because of the interaction between activated carbon and silica and the extra adsorption of benzene by activated carbon in silica aerogel–2 wt% activated carbon. Also, the amount of equilibrium adsorption capacity in ethyl benzene adsorption changes in the order of silica aerogel > silica aerogel–0.5 wt% activated carbon composite > silica aerogel–2 wt% activated carbon composite > activated carbon. As a result of having high specific surface area, the equilibrium adsorption capacity of silica aerogel was more than that
Fig. 5 – Equilibrium adsorption capacity of (a) benzene and (b) ethyl benzene over 0.01 g of silica aerogel, silica aerogel-activated carbon and activated carbon at ambient temperature and pressure. of other samples. Since the pore volume of silica aerogel–5 wt% activated carbon composite was more than that of the other composite, it had more equilibrium adsorption capacity. It is evident that silica aerogel and its composites had a major tendency to VOCs adsorption, especially benzene, in comparison to conventional adsorbents such as activated carbon. Silica aerogel was able to adsorb benzene by more than twice its own weight.
3.3.
Dynamic adsorption behaviour
A breakthrough measurement is a direct method designed to investigate the dynamic performance of VOC adsorption (Kosuge et al., 2007). The breakthrough curves for benzene adsorption in different conditions of the experiment are shown in Fig. 6. It is found that silica aerogel and silica aerogel–2 wt% activated carbon composite traversed the same procedure in different conditions of the experiment in spite of having different adsorption capacities and breakpoints. Slope of activated carbon breakthrough curve was greater than that of silica aerogel and its composites. Therefore, its mass transfer rate was higher than that of other adsorbents.
Please cite this article in press as: Mohammadi, A., Moghaddas, J., Synthesis, adsorption and regeneration of nanoporous silica aerogel and silica aerogel-activated carbon composites. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.09.003
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Table 2 – Equilibrium adsorption capacity and breakpoint obtained by different experiment in benzene dynamic adsorption. No. experiment
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Blocks
4 4 4 4 4 4 2 2 2 2 2 2 1 1 1 1 1 1 3 3 3 3 3 3
Activated carbon
Silica aerogel–2 wt% activated carbon composite
Silica aerogel
Silica aerogel–0.5 wt% activated carbon composite
Rate (ml min−1 )
Concentration (ppm)
85 85 55 119 85 85 85 119 55 85 85 85 85 119 55 85 85 85 119 85 85 55 85 85
1115 1115 839 1390 1115 1115 1115 1390 839 1115 1115 1115 1115 839 1390 1115 1115 1115 839 1115 1115 1390 1115 1115
Equilibrium adsorption capacity and breakpoint obtained from different experiments in benzene adsorption are listed in Table 2. It is evident that equilibrium adsorption capacity changes in the order of silica aerogel > silica aerogel–2 wt% activated carbon composite > silica aerogel–0.5 wt% activated carbon composite > activated carbon. The introduced reasons
Height of adsorption bed (cm) 2 2 1 3 2 2 2 1 3 2 2 2 2 1 3 2 2 2 3 2 2 1 2 2
Breakpoint (min)
2 2 2 2 2 2 6 2 11 <6 6 <6 7 6 7 6 <6 >6 <4 4 4 2 <4 <4
Equilibrium adsorption capacity (g g−1 ) × 103 260 261 307 310 258 258 870 875 867 868 868 870 954 957 958 951 953 954 548 550 552 561 543 550
for the explanation of variation in equilibrium adsorption capacity in Section 3.2 could be introduced in this section for the interpretation of equilibrium adsorption capacity in dynamic adsorption. The average breakthrough time of samples changes in order of silica aerogel > silica aerogel–2 wt% activated carbon composite > silica aerogel–0.5 wt% activated
Table 3 – Equilibrium adsorption capacity and breakpoint obtained by different experiments in ethyl benzene dynamic adsorption. No. experiment
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Blocks
4 4 4 4 4 4 2 2 2 2 2 2 1 1 1 1 1 1 3 3 3 3 3 3
Activated carbon
Silica aerogel–2 wt% activated carbon composite
Silica aerogel
Silica aerogel–0.5 wt% activated carbon composite
Rate (ml min−1 )
85 85 55 119 85 85 85 119 55 85 85 85 85 119 55 85 85 85 119 85 85 55 85 85
Concentration (ppm)
1115 1115 839 1390 1115 1115 1115 1390 839 1115 1115 1115 1115 839 1390 1115 1115 1115 839 1115 1115 1390 1115 1115
Height of adsorption bed (cm) 2 2 1 3 2 2 2 1 3 2 2 2 2 1 3 2 2 2 3 2 2 1 2 2
Breakpoint (min)
2 <2 2 2 2 2 1 2 8 1 1 1 2 <2 2 2 2 2 4 2 2 <2 2 2
Equilibrium adsorption capacity (g g−1 ) × 103 260 261 307 310 258 258 870 875 867 868 868 870 954 957 958 951 953 954 548 550 552 561 543 550
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Table 4 – Estimated effects and coefficients for equilibrium adsorption capacity of benzene. Terms
Effect
Constant Block 1 Block 2 Block 3 Rate (ml min−1 ) Concentration (ppm) Height (cm) Rate (ml min−1 ) × Concentran (ppm) × height (cm) Ct Pt
Coef 819.2 295.7 212.7 −111.2 −0.9 −0.1 −40.2 0.0 −15.5
−1.8 −0.1 −80.5 0.0
T
P
53.79 213.04 148.68 −82.51 −10.19 −7.45 −11.14 11.41 −9.41
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Table 5 – Analysis of variance for equilibrium adsorption of benzene. Source
DF
Adj SS
Adj MS
F
P
Significant
Blocks Main effects 3-Way interactions Residual error Lack of fit Pure error Total
3 3 1 15 3 12 23
1309098 1878 1886 217 154 64
436366 626 1886 14 51 5
30099.66 43.17 130.11
0.000 0.000 0.000
Yes Yes Yes
9.70
0.002
adsorption capacity, since adsorption mechanism was different in dynamic and static adsorption. This mechanism in static adsorption was mono layer with capillary condensation, whereas in dynamic adsorption it was mono layer. Obtained results from data analysis by the experiment design for benzene and ethyl benzene adsorption are listed in Tables 4–7, respectively. These tables denote that basic parameters of height of adsorption bed, rate of polluted stream, pollutant concentration, 3-way interactions of these parameters, and type of adsorbents were effective in adsorption process and enhanced equilibrium adsorption capacity. In each adsorbent (block), equilibrium adsorption capacity increased with increment of each of the three parameters from low to high level.
carbon composite > activated carbon (see Table 2). This result can be explained by the fact that longer breakthrough time results in higher dynamic adsorption capacity. Fig. 7 shows the breakthrough curve of ethyl benzene adsorption in different conditions of the experiment. It is evident that all adsorbents traversed the same procedure in different conditions of the experiment in spite of having different adsorption capacities and breakpoints. Slope of the breakthrough curves of silica aerogel and its composites was more than that of activated carbon. Therefore, their mass transfer rate was more than that of activated carbon. This major mass transfer rate was due to the presence of mesopores in the structure of aerogels. Table 3 shows the equilibrium adsorption capacity and breakpoint obtained from different experiments in ethyl benzene adsorption. Silica aerogel showed the highest adsorption capacity and mass transfer rate among the adsorbents. These features decreased with the addition of activated carbon in silica aerogel matrix. Dynamic adsorption capacity of benzene and ethyl benzene over adsorbents was less than static
3.4.
Regenerating adsorbents
To test adsorption properties of silica aerogels and silica aerogel-activated carbon composite repeated adsorption/desorption cycles of benzene on silica aerogel and
Table 6 – Estimated effects and coefficients for equilibrium adsorption capacity of ethyl benzene. Terms
Effect
Constant Block 1 Block 2 Block 3 Rate (ml min−1 ) Concentration (ppm) Height (cm) Rate (ml min−1 ) × Concentran (ppm) × height (cm) Ct Pt
−1.36 −0.08 −63.04 0.00
Coef
T
P
290.06 46.36 −56.37 −18.49 −0.68 −0.04 −31.52 0.00 −11.72
19.40 34.02 −40.14 −13.97 −7.91 −5.03 −8.89 8.86 −7.23
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Table 7 – Analysis of variance for equilibrium adsorption of ethyl benzene. Source
DF
Adj SS
Adj MS
F
P
Blocks Main effects 3-Way interactions Residual error Lack of fit Pure error Total
3 3 1 15 3 12 23
36421.9 1197.7 1095.9 209.6 159.8 49.8
12140.6 399.2 1095.9 14.0 53.3 4.1
868.97 28.57 78.44
0.000 0.000 0.000
12.85
0.000
Significant Yes Yes Yes
Please cite this article in press as: Mohammadi, A., Moghaddas, J., Synthesis, adsorption and regeneration of nanoporous silica aerogel and silica aerogel-activated carbon composites. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.09.003
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Fig. 6 – Breakthrough curve of (a) silica aerogel, (b) silica aerogel–2 wt% activated carbon composite, (c) silica aerogel–0.5 wt% activated carbon composite and (d) activated carbon in benzene adsorption at ambient temperature and pressure.
Fig. 7 – Breakthrough curve of (a) silica aerogel, (b) silica aerogel–2 wt% activated carbon composite, (c) silica aerogel–0.5 wt% activated carbon composite and (d) activated carbon in ethyl benzene adsorption at ambient temperature and pressure.
Please cite this article in press as: Mohammadi, A., Moghaddas, J., Synthesis, adsorption and regeneration of nanoporous silica aerogel and silica aerogel-activated carbon composites. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.09.003
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Fig. 8 – Static adsorption capacity of benzene over silica aerogel–0.5 wt% activated carbon composite in sequential adsorption/desorption cycles at ambient temperature and pressure.
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Fig. 10 – FTIR analysis of regenerated adsorbents via solvent extraction–thermal treatment method after 17 times adsorption/desorption cycles.
demonstrated that, there was no significant variation in the FTIR analysis of regenerated adsorbent and the adsorbents maintained their hydrophobicity after regeneration. It was found that regeneration using this method did not change the structure of adsorbents and also the intensity of Si O Si, and Si C peaks was equal in regenerated and new adsorbents.
4.
Fig. 9 – Static adsorption capacity of benzene over silica aerogel in sequential adsorption/desorption cycles at ambient temperature and pressure.
silica aerogel–0.5 wt% activated carbon composite were performed. In the repeated adsorption/desorption experiments the adsorption steps were performed after complete desorption (regeneration) by solvent extraction–thermal treatment method. The regeneration of silica aerogel and silica aerogelactivated carbon composite was achieved by extraction of adsorbed benzene and drying of samples at 80 ◦ C. As shown in Figs. 8 and 9, the adsorptivity of silica aerogel and silica aerogel–0.5 wt% activated carbon composite was not influenced by desorption steps and remains more or less stable for at least 17 cycles because the porous texture of adsorbent did not change (Pei and Zhang, 2012). It is found that adsorption capacity of the new adsorbent (cycle 1) was much less than that of cycle 2. Since the new adsorbent was used without degassing in the first cycle, the adsorbent was regenerated for the first time as a degassing stage for the second cycle. For this reason, adsorption capacity was a major issue in the second cycle.
Conclusions
In this study, silica aerogel and silica aerogel-activated carbon composites were synthesized using water glass precursor by ambient pressure drying method. Hydrophobicity of the synthesized samples was obtained using surface modification reagent (TMCS) and replacement of hydrophilic OH groups with hydrophobic CH3 groups. Hydrophobic silica aerogel with surface area (427.16 m2 g−1 ), pore diameter (19.72 nm), and density (0.16 g cm−3 ) were obtained. The synthesized samples were nanometer and had pore size distribution between 2 and 50 nm with an average pore size of 8 and 16 nm for silica aerogel and silica aerogelactivated carbon composites respectively. In benzene static adsorption, silica aerogel and activated carbon had maximum equilibrium adsorption capacity (2.3 g g−1 ) and minimum equilibrium adsorption capacity (0.7 g g−1 ), respectively. Also, in ethyl benzene static adsorption, silica aerogel and silica aerogel–2 wt% activated carbon composite had maximum equilibrium adsorption capacity (0.7 g g−1 ) and minimum equilibrium adsorption capacity (0.25 g g−1 ), respectively. The highest equilibrium adsorption capacity in benzene and ethyl benzene dynamic adsorption capacity was related to silica aerogel with 0.9545 g g−1 and 0.219 g g−1 , respectively. Regeneration of the adsorbents did not change adsorption capacity and the physical and structural properties of adsorbents and intensity of Si OSi Si and Si C peaks were equal in regenerated and new adsorbents.
List of symbols 3.5.
FTIR analysis of regenerated adsorbents
Fig. 10 shows the FTIR analysis of regenerated adsorbents after 17 times of adsorption. Comparison of FTIR analysis of the regenerated adsorbents with that of new adsorbents (Fig. 2)
Symbols used mass [g] m volume [cm−3 ] v
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Abbreviations Benzene, Toluene, Ethyl benzene, Xylene BTEX TMCS Trimethylchlorosilane Brunauer–Emmitt–Teller BET Fourier transform infrared spectroscopy FTIR Flame ionization detector FID Greek letters density [g cm−3 ]
Table of contents Silica aerogel and silica aerogel-activated carbon composites synthesized in this paper were nanostructured and found to be superior in the adsorption of benzene and ethyl benzene. These materials were regenerated by solvent extraction–thermal treatment method. Regenerated adsorbents were stable even after many adsorption/desorption cycles with no noticeable loss of efficiency.
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Contents lists available at ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Synthesis, adsorption and regeneration of nanoporous silica aerogel and silica aerogel-activated carbon composites Akbar Mohammadi, Jafarsadegh Moghaddas ∗ Transport Phenomena Research Center, Faculty of Chemical Engineering, Sahand University of Technology, P.O. Box 51335/1996, Tabriz, Iran
a b s t r a c t Usage of aerogels as an adsorbent has become more widespread because of its specifications such as high porosity and specific surface. Nanometer silica aerogel and silica aerogel-activated carbon composites were synthesized using a water glass precursor by ambient pressure drying method. Then, the adsorption capacity of synthesized adsorbents was studied in terms of benzene and ethyl benzene adsorption by chromatography method for continuous and batch testing. Results showed that silica aerogel and silica aerogel-activated carbon composites had high tendency for benzene and ethyl benzene adsorption. Silica aerogel showed maximum adsorption capacity of 2.3 g g−1 and 0.7 g g−1 in static adsorption of benzene and ethyl benzene respectively. Also, in dynamic adsorption of benzene and ethyl benzene, silica aerogel had maximum equilibrium adsorption capacity of 0.954 g g−1 and 0.219 g g−1 respectively. Minimum equilibrium adsorption capacity in benzene and ethyl benzene static adsorption was related to activated carbon with 0.7 g g−1 and silica aerogel–2 wt% activated carbon with 0.25 g g−1 respectively. After adsorption process, silica aerogel and silica aerogel–0.5 wt% activated carbon composite were regenerated by solvent extraction–thermal treatment method and, after at least 15 adsorption/desorption cycles, their adsorption capacity became fixed. © 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Adsorption; Equilibrium adsorption capacity; Regeneration; Silica aerogel; Pore size distribution; BET surface area
1.
Introduction
Volatile Organic Compounds (VOC) are the most common air pollutants emitted from chemical, petrochemical, motor vehicle exhaust, industrial paints, allied industries and use ˇ of organic solvents (Standeker et al., 2009; Song et al., 2005; Carrasco et al., 2009). Contamination caused by volatile organic compounds in the environment is mainly the result of the historic disposal practices of industrial wastes containing these solvents, most of which have been considered dangerous materials since the early 1970s, when the first environmental laws were enacted (Health Effects Fact Sheet, 2000). Atmospheric emissions of these volatile organic compounds cause serious environmental problems and financial losses. ˇ They are toxic and carcinogenic to human health (Standeker
et al., 2009; Song et al., 2005) and cause serious environmental problems such as the destruction of the ozone layer (Song et al., 2005), global warming (Dou et al., 2011), and formation of tropospheric ozone and other oxidants that cause photochemical smog (Carrasco et al., 2009; Atkinson, 2000). Therefore, controlling VOC emissions according to increasingly stringent environmental regulations is necessary. There are many separation processes to address this problem [B] and prevent VOC ˇ et al., 2009; Anita effluence such as condensation (Standeker and Wilfreid, 1999), catalytic oxidation (Carrasco et al., 2009; Ruddy and Carroll, 1993; Patkar and Laznow, 1992), absorption (Song et al., 2005) membrane-based recovery process (Semenova, 2004; Khan and Ghoshal, 2000), and adsorption (Dou et al., 2011; Crittenden and Thomas, 1998). One of the most widely used methods for controlling VOC emissions and
∗
Corresponding author. Tel.: +98 412 3459155; fax: +98 412 3444355. E-mail address: [email protected] (J. Moghaddas). Received 10 March 2014; Received in revised form 21 August 2014; Accepted 3 September 2014 http://dx.doi.org/10.1016/j.cherd.2014.09.003 0263-8762/© 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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the removal of them in gaseous streams is adsorption, for which adsorbents with high specific surface areas and the ˇ possibility to be used in many cycles are needed (Standeker et al., 2009; Song et al., 2005; Abril et al., 2010). Small pores, such as micropores and mesopores, result in large specific surface area responsible for adsorption. Pore size, pore distribution and surface area, as well as pore surface chemistry, are the major factors in the adsorption process (Yang, 2003). Silica aerogels are extremely porous (Bhagat et al., 2008; Alnaief and Smirnova, 2010), nanostructured (Bangi et al., 2008), and mesoporous (Ciolek, 2006) and have high specific surface areas (Gurav et al., 2009a; Rao et al., 2006) that are easily recovered. They are stable even after many adsorption/desorption cycles with no loss of efficiency, they are easily recovered and also exhibit capacities which enormously exceed those of commonly used adsorbents such as activated carbon and ˇ et al., 2009; Song et al., 2005). In 2009, silica gel (Standeker ˇ et al. showed that silica aerogels were excellent Standeker adsorbents for the adsorption of BTEX1 vapors from polluted gas stream in comparison to activated carbon and silica gel. Despite their good potential applications, the usage of aerogels as an adsorbent has been restricted due to their weak and fragile mechanical strength. One of the appropriate methods to overcome and strengthen the mechanical fragility of aerogels is the preparation of silica aerogel composites (Rao et al., 2006; Kim et al., 2008). In 2003, Coleman et al. found that silica aerogel-granulated activated carbon was superior in terms of removing uranium from a stock solution compared to granulated activated carbon alone for batch testing. In 2011, Dou et al. showed that silica aerogel-activated carbon composite has high affinity towards aromatic molecules and fast adsorption kinetics. Excellent performance of dynamic adsorption and desorption observed on silica aerogel-activated carbon composite is related to its higher adsorption capacity (Dou et al., 2011). Nanostructured silica aerogels are synthesized by a two-step sol–gel route and drying of wet gel to remove the trapped solvent from the pores of the gel (Bhagat et al., 2006; Gurav et al., 2009b). Aerogels, based on water glass precursor, are hydrophilic and become wet with atmospheric moisture or water, but with appropriate chemical modification the surface of the aerogel can be converted to hydrophobic so that the ˇ et al., 2009). The water molecules will be repelled (Standeker main reason for hydrophilicity of silica aerogels is the presence of Si OH groups in aerogel structure as they promote the adsorption of water. By replacing Si OH groups with Si CH3 groups, hydrophobic silica aerogels are obtained, because the adsorption of water is prevented. Preparation of silica aerogels using water-glass precursor followed by an ambient pressure drying (APD) is the cheapest and safest method for aerogel production (Gurav et al., 2009b; Shi et al., 2006). In order to make the use of silica aerogels economically feasible on a large scale, the exhausted adsorbents must be regenerated and reused (Abril et al., 2010; Garcia-oton et al., 2005). Silica aerogels are easily regenerated as a result of having an ˇ et al., open pore structure and being mesoporous (Standeker 2007). Regeneration of the silica aerogel can be performed using different methods such as thermal, extractive, chemical, electrochemical and biologic ones. Among these methods, thermal regeneration and solvent extraction are considered two of the most effective (Abril et al., 2010). The production of hydrophobic silica aerogels has become an important and
1
Benzene, Toluene, Ethylbenzene and Xylenes.
intensive area of research, like many of the scientific and technological applications of the aerogels (Rao and Kulkarni, 2002). Therefore, in this study, silica aerogel and its composites with activated carbon were synthesized using water-glass precursor by a two-step sol–gel process followed by ambient pressure drying. Performance of synthesized adsorbents was studied under static and dynamic conditions for benzene and ethyl benzene adsorption in fixed beds. So, for the first time, exhausted adsorbents were regenerated by solvent extraction–thermal treatment combined method.
2.
Experimental procedure
2.1.
Synthesis of silica aerogel
The chemicals used for the synthesis of silica aerogel were water-glass (Na2 SiO3 , 1.35 g ml−1 , Merck Co., Germany), trimethylchlorosilane (TMCS) (C3 H9 ClSi, 0.86 g ml−1 , Merck Co., Germany), isopropyl alcohol (IPA), ammonium hydroxide solution (1.0 M), and n-hexane. The ion exchange resin, Amberlite IR 120 H+ (Merck Co., Germany), was used for replacing the Na+ ions present in the water-glass with the H+ ions. Hydrogels were prepared using water-glass by a two-step sol–gel process. The water-glass solution was first diluted with the deionized water (water-glass: deionized water (volume ratio) = 1:4) so as to obtain the desired weight percentage of silica in the starting material. In the next step, the ion exchange was carried out by mixing the diluted water-glass solution with the ion exchange resin in 1:1 volume proportion. The mixture was stirred for 5 min and, consequently, pH of the ion exchanged solution changed from 13 to 2, clearly showing the removal of unwanted Na+ ions from the water-glass solution and resulting in the formation of silicic acid (SA). Then, 1.0 M NaOH solution was added to the silica sol to raise its pH to 4 for gelation. The obtained silica sols were stirred for 1 min and then transferred to a plastic beaker with 30 mm diameter and 60 mm height, where the sols aged into hydrogels within about 180 min at 60 ◦ C. The next step was to immerse the hydrogels in isopropyl alcohol and normal hexane and then subsequently age them for 18 h at 60 ◦ C in order to extrude pore water (exchanged by n-hexane) and also strengthen the networks of the gels. After solvent exchanging and aging, the wet gels were immersed in TMCS/n-hexane solutions for 12 h at 60 ◦ C in order to adequately modify the surface and discharge the pore water, mainly containing H2 O/n-haxane solutions. Volumetric ratio of n-hexane/TMCS was fixed at 5, because less shrinkage was observed at this ratio. Finally, the modified gels were dried at room temperature for 24 h and then at 60, 80, 120, and 180 ◦ C for 2 h, respectively, in the oven to reduce shrinkage during drying. In the synthesis of silica aerogel-activated carbon composite, powdery activated carbon (Iran, 200 mesh, 1.6 g cm−1 , 300 m2 g−1 ) was immersed in silica sol after the ion exchange step.
2.2.
Characterizing aerogels
Apparent density of the aerogel was measured by mass to volume ratio ( = mv−1 ) of the aerogel, in which mass was measured by the microbalance with the accuracy of 10−5 g and volume of the aerogel was measured by filling aerogel in the measuring cylinder of the known volume. The microstructure and morphology of silica aerogels were observed by scanning electron microscopy (SEM: JSM-6700F). Specific surface area and pore size distribution of aerogels were determined
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2.5.
Fig. 1 – Scheme of dynamic adsorption system for elimination of VOC.
by Brunauer–Emmitt–Teller (BET) method (Belsorb mini II). Fourier transform infrared spectroscopy (FTIR, 4600 Unicam) was employed to investigate the chemical bonding state of aerogels which gave information about various chemical bonding such as –OH, Si OH, Si O Si, Si C, and C H.
2.3.
Measuring static adsorption
Static adsorption equilibrium measurements of the aerogels were recorded using an intelligent gravimetric analyzer (Model AT261, Mettler) with sensitivity of 10−5 g. Before the measurement, each sample was degassed at 120 ◦ C for 4 h. All experiments were replicated with four adsorbents for two pollutants at room temperature.
2.4.
Measuring dynamic adsorption
Dynamic adsorption was studied using the parameters of contaminated flow rate (at 2 levels of 55 and 119 ml min−1 ), concentration of pollutant (at 2 levels of 839 × 10−6 and 1390 × 10−6 g ml−1 ), height of adsorption bed (at 2 levels of 1 and 3 cm), and type of adsorbent (4 blocks) by statistical analysis method: 2 levels of full factorial design + centre point with four time replications of centre point and one time replication of corner points. Investigations of the dynamic adsorption of benzene and ethyl benzene for silica aerogel, silica aerogelactivated carbon composite, and activated carbon were carried out by a flow method on an experimental set-up (Dou et al., 2010). Fig. 1 shows the experimental set-up used in dynamic adsorption process. Concentrations of benzene and ethyl benzene before and after the adsorption process were tested using gas chromatograph (GC) equipped with a flame ionization detector (FID). Before the measurement, each sample was degassed at 120 ◦ C for 4 h. All the experiments were carried out at room temperature.
Regenerating adsorbents
The most limiting step of the adsorption process is often regeneration step, which is time and energy consuming. In this context, the use of solvent extraction–thermal treatment method appears to offer a potential solution. Thus, in this paper, a combination of two methods (solvent extraction–thermal treatment) was used. Saturated adsorbents were first immersed in ethanol for 1 h. Then, liquids of the adsorbent surface were vacated by syringe and wet adsorbents were placed in an oven at 80 ◦ C for 1 h in order to dry and to remove ethanol and adsorbates from adsorbent pores. Adsorption–regeneration cycle was repeated for the static adsorption of benzene over silica aerogel and silica aerogel–0.5 wt% activated carbon composite at 25 ◦ C.
3.
Results and discussion
3.1.
Properties of synthesized samples
Table 1 shows the properties of silica aerogel and silica aerogel-activated carbon composites synthesized with water glass and surface modification agent (TMCS). Specific surface area of silica aerogel is more significant than that of the silica aerogel-activated carbon composites. Specific surface area relies on density and pore size distribution, which has a reverse relationship with density and pore size distribution (Rao and Rao, 2008). Therefore, it is logical that silica aerogel has higher specific surface area. Density of silica aerogel-activated carbon composite is greater than that of pure silica aerogel and this characteristic increases with the increment of activated carbon in silica aerogel matrix, since the density of activated carbon is higher than that of pure silica aerogel. Fig. 2 shows the SEM morphology of silica aerogel and silica aerogel-activated carbon composites obtained by using TMCS for modification of the wet gels. The synthesized samples exhibit porous network structure which contains spherical solid clusters and pores below 100 nm and the particle distribution of samples are uniform. The particles size of composites is greater than that of pure silica aerogel. This figure denoted that surface modification reaction in the composites was completely done. The FTIR analysis also shows this fact. Fig. 3 illustrates the pore size distribution (PSD) profiles of the silica aerogel and silica aerogel-activated carbon composites synthesized by an ambient pressure drying (APD) method. A noticeable change has been observed in the PSD profiles of the silica aerogel and silica aerogel-activated carbon composites. It is known that the synthesized samples contained mesopores (2–25 nm pores) and macropores (above 50 nm) from pore size distribution graphs. Most pores of silica aerogel known to be mesopores are below 20 nm and the peak of the smallest pore was 8 nm. Whereas most pores of silica aerogel composites known to be mesopores are below about 30 nm and their peak of smallest pore was 16 nm. In surface
Table 1 – Properties of synthesized samples. Samples Silica aerogel Silica aerogel–2 wt% activated carbon composite Silica aerogel–0.5 wt% activated carbon composite
BET surface area (m2 g−1 )
Average pores diameter (nm)
Volume of total pores (cm3 g−1 )
Density (g cm−3 )
427.16 358.4
19.72 29.51
2.11 2.64
0.16 0.22
360.1
32.38
2.91
0.18
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Fig. 2 – SEM photographs of silica aerogel (a) silica aerogel–0.5 wt% activated carbon composite (b) and silica aerogel–2 wt% activated carbon composite.
modification reaction, silanol groups are replaced with nonpolar hydrolytically stable Si (CH3 )3 groups. A repulsive force acts between the Si (CH3 )3 groups, therefore the gels start to increase in volume, “spring back” and strengthen the pores (Rao et al., 2007). The FTIR analysis denotes that surface modification reaction in the samples containing activated carbon was completely done and the number of Si (CH3 )3 groups
Fig. 3 – Pore size distribution profiles of the water-glass based silica aerogel (a) silica aerogel–0.5 wt% activated carbon composite (b) and silica aerogel–2 wt% activated carbon composite dried at ambient pressure.
in composite are more than that of pure silica aerogel. So the average pore diameter of composites is greater than that of pure silica aerogel. Fig. 4 shows the FTIR analysis of synthesized samples. The FTIR spectrum represents a broad band at around 3500 cm−1 and a peak at around 1600 cm−1 , which can be attributed to O H groups. The peaks at around 1100
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Fig. 4 – FTIR analysis of silica aerogel and silica aerogel-activated carbon composites. and 800 cm−1 are due to asymmetric and symmetric modes of SiO2 (Rao et al., 2007). The absorption peaks at 2963, 1256 and 846 cm−1 corresponding to CH3 terminal groups are quite visible which are attributed to surface modification of wet gel by TMCS (Shi et al., 2006). Intensity of OH peak (1600 cm−1 ) in silica aerogel–2 wt% activated carbon composite is less than that of silica aerogel–0.5 wt% activated carbon composite and silica aerogel. Therefore, the hydrophobicity of silica aerogel–2 wt% activated carbon composite is more than that of the other two samples. This analysis indicated that surface modification reaction in the samples containing activated carbon was completely done and the hydrophobicity of composites increased with increment of activated carbon in silica aerogel matrix. Also, Table 1 shows that the synthesized samples are nanostructured and mesoporous (2–50 nm).
3.2.
Static adsorption behaviour
Fig. 5 illustrates the equilibrium adsorption capacity of four types of adsorbents. It is evident that equilibrium adsorption capacity of benzene changes in the order of silica aerogel > silica aerogel–2 wt% activated carbon composite > silica aerogel–0.5 wt% activated carbon composite > activated carbon. This result can be explained by the fact that the static VOC adsorption capacity is proportional to the total pore volume (Dou et al., 2010) and available surface for adsorbate. Since pure silica aerogel had the highest surface area (427.16 m2 g−1 ) and lowest pore volume (2.11 cm3 g−1 ), it is obvious that its equilibrium adsorption capacity would be significant. Interaction between activated carbon and silica is important in the benzene adsorption over silica aerogelactivated carbon composites. Adsorption capacity of silica aerogel–2 wt% activated carbon composite was more than that of silica aerogel–0.5% activated carbon composite because of the interaction between activated carbon and silica and the extra adsorption of benzene by activated carbon in silica aerogel–2 wt% activated carbon. Also, the amount of equilibrium adsorption capacity in ethyl benzene adsorption changes in the order of silica aerogel > silica aerogel–0.5 wt% activated carbon composite > silica aerogel–2 wt% activated carbon composite > activated carbon. As a result of having high specific surface area, the equilibrium adsorption capacity of silica aerogel was more than that
Fig. 5 – Equilibrium adsorption capacity of (a) benzene and (b) ethyl benzene over 0.01 g of silica aerogel, silica aerogel-activated carbon and activated carbon at ambient temperature and pressure. of other samples. Since the pore volume of silica aerogel–5 wt% activated carbon composite was more than that of the other composite, it had more equilibrium adsorption capacity. It is evident that silica aerogel and its composites had a major tendency to VOCs adsorption, especially benzene, in comparison to conventional adsorbents such as activated carbon. Silica aerogel was able to adsorb benzene by more than twice its own weight.
3.3.
Dynamic adsorption behaviour
A breakthrough measurement is a direct method designed to investigate the dynamic performance of VOC adsorption (Kosuge et al., 2007). The breakthrough curves for benzene adsorption in different conditions of the experiment are shown in Fig. 6. It is found that silica aerogel and silica aerogel–2 wt% activated carbon composite traversed the same procedure in different conditions of the experiment in spite of having different adsorption capacities and breakpoints. Slope of activated carbon breakthrough curve was greater than that of silica aerogel and its composites. Therefore, its mass transfer rate was higher than that of other adsorbents.
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Table 2 – Equilibrium adsorption capacity and breakpoint obtained by different experiment in benzene dynamic adsorption. No. experiment
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Blocks
4 4 4 4 4 4 2 2 2 2 2 2 1 1 1 1 1 1 3 3 3 3 3 3
Activated carbon
Silica aerogel–2 wt% activated carbon composite
Silica aerogel
Silica aerogel–0.5 wt% activated carbon composite
Rate (ml min−1 )
Concentration (ppm)
85 85 55 119 85 85 85 119 55 85 85 85 85 119 55 85 85 85 119 85 85 55 85 85
1115 1115 839 1390 1115 1115 1115 1390 839 1115 1115 1115 1115 839 1390 1115 1115 1115 839 1115 1115 1390 1115 1115
Equilibrium adsorption capacity and breakpoint obtained from different experiments in benzene adsorption are listed in Table 2. It is evident that equilibrium adsorption capacity changes in the order of silica aerogel > silica aerogel–2 wt% activated carbon composite > silica aerogel–0.5 wt% activated carbon composite > activated carbon. The introduced reasons
Height of adsorption bed (cm) 2 2 1 3 2 2 2 1 3 2 2 2 2 1 3 2 2 2 3 2 2 1 2 2
Breakpoint (min)
2 2 2 2 2 2 6 2 11 <6 6 <6 7 6 7 6 <6 >6 <4 4 4 2 <4 <4
Equilibrium adsorption capacity (g g−1 ) × 103 260 261 307 310 258 258 870 875 867 868 868 870 954 957 958 951 953 954 548 550 552 561 543 550
for the explanation of variation in equilibrium adsorption capacity in Section 3.2 could be introduced in this section for the interpretation of equilibrium adsorption capacity in dynamic adsorption. The average breakthrough time of samples changes in order of silica aerogel > silica aerogel–2 wt% activated carbon composite > silica aerogel–0.5 wt% activated
Table 3 – Equilibrium adsorption capacity and breakpoint obtained by different experiments in ethyl benzene dynamic adsorption. No. experiment
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Blocks
4 4 4 4 4 4 2 2 2 2 2 2 1 1 1 1 1 1 3 3 3 3 3 3
Activated carbon
Silica aerogel–2 wt% activated carbon composite
Silica aerogel
Silica aerogel–0.5 wt% activated carbon composite
Rate (ml min−1 )
85 85 55 119 85 85 85 119 55 85 85 85 85 119 55 85 85 85 119 85 85 55 85 85
Concentration (ppm)
1115 1115 839 1390 1115 1115 1115 1390 839 1115 1115 1115 1115 839 1390 1115 1115 1115 839 1115 1115 1390 1115 1115
Height of adsorption bed (cm) 2 2 1 3 2 2 2 1 3 2 2 2 2 1 3 2 2 2 3 2 2 1 2 2
Breakpoint (min)
2 <2 2 2 2 2 1 2 8 1 1 1 2 <2 2 2 2 2 4 2 2 <2 2 2
Equilibrium adsorption capacity (g g−1 ) × 103 260 261 307 310 258 258 870 875 867 868 868 870 954 957 958 951 953 954 548 550 552 561 543 550
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Table 4 – Estimated effects and coefficients for equilibrium adsorption capacity of benzene. Terms
Effect
Constant Block 1 Block 2 Block 3 Rate (ml min−1 ) Concentration (ppm) Height (cm) Rate (ml min−1 ) × Concentran (ppm) × height (cm) Ct Pt
Coef 819.2 295.7 212.7 −111.2 −0.9 −0.1 −40.2 0.0 −15.5
−1.8 −0.1 −80.5 0.0
T
P
53.79 213.04 148.68 −82.51 −10.19 −7.45 −11.14 11.41 −9.41
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Table 5 – Analysis of variance for equilibrium adsorption of benzene. Source
DF
Adj SS
Adj MS
F
P
Significant
Blocks Main effects 3-Way interactions Residual error Lack of fit Pure error Total
3 3 1 15 3 12 23
1309098 1878 1886 217 154 64
436366 626 1886 14 51 5
30099.66 43.17 130.11
0.000 0.000 0.000
Yes Yes Yes
9.70
0.002
adsorption capacity, since adsorption mechanism was different in dynamic and static adsorption. This mechanism in static adsorption was mono layer with capillary condensation, whereas in dynamic adsorption it was mono layer. Obtained results from data analysis by the experiment design for benzene and ethyl benzene adsorption are listed in Tables 4–7, respectively. These tables denote that basic parameters of height of adsorption bed, rate of polluted stream, pollutant concentration, 3-way interactions of these parameters, and type of adsorbents were effective in adsorption process and enhanced equilibrium adsorption capacity. In each adsorbent (block), equilibrium adsorption capacity increased with increment of each of the three parameters from low to high level.
carbon composite > activated carbon (see Table 2). This result can be explained by the fact that longer breakthrough time results in higher dynamic adsorption capacity. Fig. 7 shows the breakthrough curve of ethyl benzene adsorption in different conditions of the experiment. It is evident that all adsorbents traversed the same procedure in different conditions of the experiment in spite of having different adsorption capacities and breakpoints. Slope of the breakthrough curves of silica aerogel and its composites was more than that of activated carbon. Therefore, their mass transfer rate was more than that of activated carbon. This major mass transfer rate was due to the presence of mesopores in the structure of aerogels. Table 3 shows the equilibrium adsorption capacity and breakpoint obtained from different experiments in ethyl benzene adsorption. Silica aerogel showed the highest adsorption capacity and mass transfer rate among the adsorbents. These features decreased with the addition of activated carbon in silica aerogel matrix. Dynamic adsorption capacity of benzene and ethyl benzene over adsorbents was less than static
3.4.
Regenerating adsorbents
To test adsorption properties of silica aerogels and silica aerogel-activated carbon composite repeated adsorption/desorption cycles of benzene on silica aerogel and
Table 6 – Estimated effects and coefficients for equilibrium adsorption capacity of ethyl benzene. Terms
Effect
Constant Block 1 Block 2 Block 3 Rate (ml min−1 ) Concentration (ppm) Height (cm) Rate (ml min−1 ) × Concentran (ppm) × height (cm) Ct Pt
−1.36 −0.08 −63.04 0.00
Coef
T
P
290.06 46.36 −56.37 −18.49 −0.68 −0.04 −31.52 0.00 −11.72
19.40 34.02 −40.14 −13.97 −7.91 −5.03 −8.89 8.86 −7.23
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Table 7 – Analysis of variance for equilibrium adsorption of ethyl benzene. Source
DF
Adj SS
Adj MS
F
P
Blocks Main effects 3-Way interactions Residual error Lack of fit Pure error Total
3 3 1 15 3 12 23
36421.9 1197.7 1095.9 209.6 159.8 49.8
12140.6 399.2 1095.9 14.0 53.3 4.1
868.97 28.57 78.44
0.000 0.000 0.000
12.85
0.000
Significant Yes Yes Yes
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Fig. 6 – Breakthrough curve of (a) silica aerogel, (b) silica aerogel–2 wt% activated carbon composite, (c) silica aerogel–0.5 wt% activated carbon composite and (d) activated carbon in benzene adsorption at ambient temperature and pressure.
Fig. 7 – Breakthrough curve of (a) silica aerogel, (b) silica aerogel–2 wt% activated carbon composite, (c) silica aerogel–0.5 wt% activated carbon composite and (d) activated carbon in ethyl benzene adsorption at ambient temperature and pressure.
Please cite this article in press as: Mohammadi, A., Moghaddas, J., Synthesis, adsorption and regeneration of nanoporous silica aerogel and silica aerogel-activated carbon composites. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.09.003
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Fig. 8 – Static adsorption capacity of benzene over silica aerogel–0.5 wt% activated carbon composite in sequential adsorption/desorption cycles at ambient temperature and pressure.
9
Fig. 10 – FTIR analysis of regenerated adsorbents via solvent extraction–thermal treatment method after 17 times adsorption/desorption cycles.
demonstrated that, there was no significant variation in the FTIR analysis of regenerated adsorbent and the adsorbents maintained their hydrophobicity after regeneration. It was found that regeneration using this method did not change the structure of adsorbents and also the intensity of Si O Si, and Si C peaks was equal in regenerated and new adsorbents.
4.
Fig. 9 – Static adsorption capacity of benzene over silica aerogel in sequential adsorption/desorption cycles at ambient temperature and pressure.
silica aerogel–0.5 wt% activated carbon composite were performed. In the repeated adsorption/desorption experiments the adsorption steps were performed after complete desorption (regeneration) by solvent extraction–thermal treatment method. The regeneration of silica aerogel and silica aerogelactivated carbon composite was achieved by extraction of adsorbed benzene and drying of samples at 80 ◦ C. As shown in Figs. 8 and 9, the adsorptivity of silica aerogel and silica aerogel–0.5 wt% activated carbon composite was not influenced by desorption steps and remains more or less stable for at least 17 cycles because the porous texture of adsorbent did not change (Pei and Zhang, 2012). It is found that adsorption capacity of the new adsorbent (cycle 1) was much less than that of cycle 2. Since the new adsorbent was used without degassing in the first cycle, the adsorbent was regenerated for the first time as a degassing stage for the second cycle. For this reason, adsorption capacity was a major issue in the second cycle.
Conclusions
In this study, silica aerogel and silica aerogel-activated carbon composites were synthesized using water glass precursor by ambient pressure drying method. Hydrophobicity of the synthesized samples was obtained using surface modification reagent (TMCS) and replacement of hydrophilic OH groups with hydrophobic CH3 groups. Hydrophobic silica aerogel with surface area (427.16 m2 g−1 ), pore diameter (19.72 nm), and density (0.16 g cm−3 ) were obtained. The synthesized samples were nanometer and had pore size distribution between 2 and 50 nm with an average pore size of 8 and 16 nm for silica aerogel and silica aerogelactivated carbon composites respectively. In benzene static adsorption, silica aerogel and activated carbon had maximum equilibrium adsorption capacity (2.3 g g−1 ) and minimum equilibrium adsorption capacity (0.7 g g−1 ), respectively. Also, in ethyl benzene static adsorption, silica aerogel and silica aerogel–2 wt% activated carbon composite had maximum equilibrium adsorption capacity (0.7 g g−1 ) and minimum equilibrium adsorption capacity (0.25 g g−1 ), respectively. The highest equilibrium adsorption capacity in benzene and ethyl benzene dynamic adsorption capacity was related to silica aerogel with 0.9545 g g−1 and 0.219 g g−1 , respectively. Regeneration of the adsorbents did not change adsorption capacity and the physical and structural properties of adsorbents and intensity of Si OSi Si and Si C peaks were equal in regenerated and new adsorbents.
List of symbols 3.5.
FTIR analysis of regenerated adsorbents
Fig. 10 shows the FTIR analysis of regenerated adsorbents after 17 times of adsorption. Comparison of FTIR analysis of the regenerated adsorbents with that of new adsorbents (Fig. 2)
Symbols used mass [g] m volume [cm−3 ] v
Please cite this article in press as: Mohammadi, A., Moghaddas, J., Synthesis, adsorption and regeneration of nanoporous silica aerogel and silica aerogel-activated carbon composites. Chem. Eng. Res. Des. (2014), http://dx.doi.org/10.1016/j.cherd.2014.09.003
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Abbreviations Benzene, Toluene, Ethyl benzene, Xylene BTEX TMCS Trimethylchlorosilane Brunauer–Emmitt–Teller BET Fourier transform infrared spectroscopy FTIR Flame ionization detector FID Greek letters density [g cm−3 ]
Table of contents Silica aerogel and silica aerogel-activated carbon composites synthesized in this paper were nanostructured and found to be superior in the adsorption of benzene and ethyl benzene. These materials were regenerated by solvent extraction–thermal treatment method. Regenerated adsorbents were stable even after many adsorption/desorption cycles with no noticeable loss of efficiency.
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