Adsorption Behavior of Toluene on Modified 1X Molecular Sieves

Abstract

In this paper, the toluene adsorption/desorption properties of modified 13X molecular sieves (M-13X) are discussed. M-13X molecular sieves were prepared by acidic and steam treatments of 13X molecular sieves. The structural parameters of M-13X were evaluated and compared with those of other molecular sieves (HY, HZSM-5, Cs7NaMOR, and a commercial 13X). The results show that the specific surface area, average pore diameter, and pore volume of M-13X were 414.17 m²/g, 2.98 nm, and 0.31 mL/g, respectively. The pore size distribution of M-13X was 1.8–3.0 nm. Because of its larger Si/Al ratio (Si/Al = 6.77), the hydrophobicity of M-13X is much higher than that of 13X (Si/Al = 1.28), indicating that it is particularly well suited to toluene control applications. The saturation adsorption capacity of M-13X was 0.045 g/g for simulated toluene at a temperature of 293 K and a relative humidity of 50%. The optimal regeneration temperature of M-13X was 473 K for 120 min with a hot air flow rate of 140 L/min.

Implications:

The modified 13X molecular sieves (M-13X) are adsorbents with a high adsorption capacity and great hydrophobicity, suitable for the treatment of VOCs. The purpose of the present investigation is to provide a practical guide for their design.

Introduction

Volatile organic compounds (VOCs) are organic compounds such as aliphatic hydrocarbons, aromatic hydrocarbons, esters, ethers, and halohydrocarbons that have a high vapor pressure under normal room-temperature conditions (Harper, 2000; Li et al., 2009; Tseng et al., 2001). Sources of VOC emissions include fuel combustion, transport, industrial processes, solvent use, and storage and distribution of fuels (Chang et al., 2002; Li, 1996). In China, manmade VOC emissions are expected to increase from 19.4 Tg in 2005 to 25.9 Tg in 2020, along with an increase in VOCs attributed to industrial production from 17% to 24% (Wei et al., 2011; Zhao, 2009). The development of an efficient and safe technology for the reuse of VOCs is therefore needed (Wei et al., 2010).

During the past decade, many efficient and practical technologies for VOC abatement have been developed (Guieysse et al., 2008; Gupta et al., 2002; Monneyron et al., 2008), including adsorption, photocatalytic destruction, biological processes, plasma decomposition, and catalytic combustion (Borwankar et al., 2010; Chinte et al., 2009; Christian et al., 2010; Cui et al., 2009; Koziel et al., 2010; Toshiaki et al., 2010; Xue et al., 2011). Adsorption, combined with other techniques such as condensation and catalytic combustion, is considered to be a promising method for VOC removal or solvent recovery. In practical VOC removal processes, the primary requirements of the adsorbent include (1) a high adsorption capacity (large accessible pore volume), (2) uniform channels, (3) hydrophobic properties, and (4) ease of regeneration (Douglas, 2011. Polyakov et al., 1993). Activated carbon is an excellent adsorbent for nonpolar compounds; it has a large number of cavities and a large surface area (600–1500 m²/g). However, activated carbon also has some disadvantages; for example, it is a fire risk, and it is hygroscopic (Makowski et al., 2007), which might have a negative effect on the adsorption of humid gas streams containing VOCs. Molecular sieves with crystalline aluminosilicate frameworks, another type of adsorbent, have recently attracted considerable attention (Chun et al., 2011; Kresge et al., 1992). Furthermore, as well as a porous structure, molecular sieves have other excellent properties, such as the potential for shape-selective catalysis and fire resistance, which might be of benefit in the treatment of VOCs in gas streams and enable them to overcome some of the disadvantages associated with activated carbon (Li, 2009). Bao et al. (2011) and Hu et al. (2010) studied the adsorption performances of MCM-41and silicalite-1 molecular sieves for benzene under both dry and wet conditions, and found that water vapor reduced the benzene-adsorption capacity.

In the present study, a new modified 13X molecular sieve (M-13X) was investigated. The structural properties of M-13X, such as pore ​​volume, average pore size, specific surface area, pore distribution, molecular composition, and Si/Al ratio, were measured. Water isotherms for M-13X and 13X were used to investigate the water adsorption behavior on sample adsorbents. Toluene was used as a probe molecule, and the adsorption/desorption properties of M-13X under different relative humidities and temperatures were studied.

Experimental Section

Experimental adsorbents and reagents

Adsorbents

Granular commercial 13X molecular sieves were provided by the Shanghai Zeolite Molecular Sieve Co., Ltd, China. Granular M-13X molecular sieves were prepared by acid and steam treatments of 13X molecular sieves according to the methods described by Gola et al. (2000). The 13X molecular sieves were steamed using inlet water vapor (0.6 g/g molecular sieve) at 923 K for 4 hr. Acid treatment was carried out using nitric acid (0.02 mol/L) at 373 K for 2 hr. The suspension was then filtered, washed with distilled water, and dried at 393 K. The M-13X and 13X molecular sieves were first outgassed under a vacuum at 378 K (a temperature suitable for pore water removal) until no weight loss was observed using an analytical balance.

Reagents

Analytically pure toluene (Quhua Reagent Co., Ltd., Hangzhou, China) was used as the probe molecule. Analytically pure sodium citrate (Sinopharm Chemical Reagent Co., Ltd., Beijing, China) was used to absorb toluene remaining in the off-gas (Lan et al., 2008).

Experimental analyzers

Chemical–physical adsorption measurements

The specific surface area, average pore diameter, and pore volume were measured using a chemical–physical adsorption instrument (ASAP2010, Micromeritics, Norcross, GA). Surface areas were calculated using the Brunauer–Emmett–Teller (BET) model. The pore size distributions were obtained using the Barrett–Joyner–Halenda (BJH) model. Total pore volumes were measured at a relative pressure of 0.998.

Scanning electron microscopy and x-ray diffraction

The surface morphology of the adsorbents was determined using scanning electron microscopy (SEM; XL-30-ESEM, Philips, Eindhoven, The Netherlands) and by x-ray diffraction (XRD; D/MAX-2500PC. XRD, Rigaku, Tokyo, Japan) using Cu Kα radiation at 40 kV and 40 mA, with scanning from 10° to 60°.

X-Ray fluorescence

Molecular compositions were determined using x-ray fluorescence (XRF; ARL ADVANT'X IntelliPowerTM 4200, Thermo Scientific, Waltham, MA). XRF was performed with Rh Kα radiation at 60 kV and 140 mA, and UniQuant software was used to analyze the samples quantitatively.

Gas chromatography

Toluene concentrations were determined using a gas chromatograph (GC; Agilent 6980, Agilent, Santa Clara, CA) with flame ionization detectors and a capillary column of dimensions 30.0 m × 320 μm × 0.5 μm. The temperatures of the vaporization chamber, detector, and capillary column were 483 K, 473 K, and 363 K, respectively. The column flow rate was 1 mL/min, and the injection volume was 800 μL. The flow rate of nitrogen, which was used as the carrier gas, was 33.4 mL/min, and the split ratio was 30:1. The flow rates of hydrogen and air were controlled at 30 mL/min and 400 mL/min, respectively.

Experimental apparatus

The experimental fixed-bed reactor is shown in Figure 1. Adsorption columns with Φ 30 mm × 600 mm were connected using stainless-steel pipes (Φ 15 mm). A stream of toluene diluted with dry air was obtained by passing air through a gas generator containing toluene. The diluted toluene gas entered an adsorption column packed with adsorbent (100 g). The small amount of gaseous toluene remaining in the off-gas from the adsorption column was absorbed using 5% sodium citrate. The experiment was stopped when the outlet toluene concentration reached the same level as that of the inlet stream.

Figure 1. Experimental flow in adsorptive fixed-bed reactor: (1) air compressor; (2) gas generator; (3) humidifier; (4) gas mixture buffer; (5) adsorption column; (6) heater; (7), (8), (9), (10), (11), and (12) sampling ports; (13) absorber; and (14) dehumidifier.

This investigation simulated practical conditions as follows. The concentration of toluene gas was controlled at 1000 ± 50 mg/m³, the gas flow rate was 17 L/min, and the temperature was kept constant at 293 K. The flow rate of hot air for regeneration was about 140 L/min, periodically; the toluene concentrations from the inlet and outlet were measured quantitatively using GC.

Results and Discussion

Characterization of M-13X

Figure 2 and Figure 3 show that M-13X and 13X have similar x-ray diffraction patterns, indicating that crystalline frameworks were not destroyed during the modified process (Cheng et al., 2008). The sharp peaks labeled in the Figure 3, revealing the presence of a highly organized crystalline structure in 13X (Alonso-Vicario et al., 2010), were also observed in M-13X. However, some new diffraction peaks appeared in M-13X, indicating that different crystalline phases were formed. In addition, the lower strengths of related peaks also indicated that the sizes of crystalline grains turned smaller during the modified process. Crystallite details of M-13X and 13X were observed further by SEM, as illustrated in Figure 4. The crystals of 13X and M-13X were grainy, but the crystal size in M-13X was less regular than in 13X. In M-13X the intercrystalline pores were blocked by small crystals, which would reduce the adsorption capacity (Yao et al., 2010). An increase in the Si/Al ratio resulted in shrinkage of the unit cell, leading to a decrease in the particle size, because the Al–O bond length is 0.171 nm, whereas the Si–O bond length is only 0.164 nm (Lloyd, 2011). The pore size distributions of M-13X and 13X, shown in Figure 5, demonstrate that the pore sizes of M-13X were mainly in the range 1.8–3.0 nm, and they were a little more uniform than those of 13X. Furthermore, the average micropore widths of both M-13X and 13X were observed at around 1.9 nm, so they could be easily accessed by pollutant macromolecules.

Figure 2. XRD pattern of M-13X; triangle represents the 13X crystalline phase.

Figure 3. XRD pattern of 13X; triangle represents the 13X crystalline phase.

Figure 4. SEM images of (a) M-13X and (b) 13X.

Figure 5. Pore size distributions of 13X and M-13X.

The specific surface area, pore volume, and average pore diameter of M-13X, which were measured using chemical–physical adsorption, were 414.17 m²/g, 0.31 mL/g, and 2.98 nm, respectively. The specific surface area of M-13X, listed in Table 1, was similar to those of HZSM-5 and Cs₇NaMOR, but the pore volume was about twice those of HZSM-5 and Cs₇NaMOR. A comparison of 13X and M-13X shows that their porous structural parameters were very similar. The XRF data for M-13X and 13X are shown in Table 2; they indicate that the Si/Al ratio of M-13X was much improved. The Si/Al ratio of M-13X was 6.77, which was almost 5.3 times that of 13X (1.28). The reason was that cations (Na⁺, Ca²⁺, Mg²⁺, etc.) in 13X were exchanged by H⁺ from solution, forming hydroxyl bridges; the bridging hydroxyls were then converted to Si–OH as a result of the reaction of H⁺ and framework Al, so Al was extracted, which improved the Si/Al ratio (Xu et al., 2004; Yang et al., 2004,). A high Si/Al ratio would decrease the polarity of the molecular sieve and greatly improve the hydrophobicity.

Table 1. Structural parameters and adsorption properties of adsorbents

				Toluene saturated capacity/(g/g)
Samples	SBET/(m2/g)	VTotal/(mL/g)	Average pore diameter/ (D/nm)	Dry	RH = 50%	Carrier gas	References
HY	681	0.47	—	0.077	—	N2	Baek et al., 2004
HZSM-5	410	0.13	—	0.046	—	N2	Serra et al., 2012
Cs7NaMOR	344	0.14	—	0.082	—	N2	Serra et al., 2011
13X	415.17	0.30	2.91	0.055	0.037	Air	—
M-13X	414.36	0.31	2.98	0.048	0.045	Air	—

Table 2. Molecular composition of adsorbents (%)

Adsorbent	Si	Al	Ca	Mg
13X	21.68	16.97	7.47	2.10
M-13X	38.93	5.75	0.27	0.126

Breakthrough curves

Breakthrough curves, a common technique for evaluating the adsorption times of adsorbents, were used in this study. A ratio of outlet toluene concentration to inlet concentration of 5% meant that the adsorption bed had been broken through. When the outlet toluene concentration reached the same level as that of the inlet stream, this meant that the adsorbents were saturated.

However, in practical situations, the VOC stream usually contains a large amount of water vapor, about 80% relative humidity (Zhao et al., 1998). It is therefore necessary to observe the effects of humidity on the adsorption process. The adsorption isotherms of water vapor on 13X and M-13X are shown in Figure 6. As expected, M-13X, which had a low water loading, was much more hydrophobic than 13X. In addition, the adsorption isotherms showed that capillary condensation occurred at 0.5–0.7 for 13X, showing that 13X was unsuitable as an adsorbent for relative humidity above 50%. The breakthrough curves of M-13X and 13X under different relative humidities are shown in Figure 7. For 13X, the breakthrough time decreased from 50 min to 20 min, and the saturation time was reduced from 110 min to 80 min, as the relative humidity increased from 50% to 80%; this indicated that the moisture resistance of 13X was poor. In contrast, the breakthrough curve at 50% relative humidity on M-13X almost coincided with that at 80% relative humidity, which demonstrated that M-13X had excellent hydrophobicity, in accordance with its high Si/Al ratio.

Figure 6. Adsorption isotherms of water on 13X and M-13X.

Figure 7. Breakthrough curves under different relative humidities of adsorbents.

Comparison of saturation capacity on various adsorbents

One of the most important indicators of adsorption properties is the adsorption saturation capacity, which is calculated using the following equation (Chun et al., 2009):

(1)

where q denotes the saturation adsorption amount (g/g), F is the velocity of the gas (mL/min), t is the adsorption time (min), c ₀ and c ₁ are the inlet and outlet concentrations (mg/m³), W is the weight of the adsorbent (g), and t _s is the saturation adsorption time (min).

The adsorption performances of M-13X and 13X were investigated at a temperature of 293 K and a gas flow rate of 17 L/min for the toluene concentration range 900–1100 mg/m³. Based on the data in Figure 7, the saturation adsorption capacities of M-13X and 13X, calculated using eq 1, are shown in Table 1. Obviously, the saturation adsorption capacity of 13X is slightly higher than that of M-13X under dry conditions. However, the saturation adsorption capacity of 13X dramatically decreased from 0.055 g/g to 0.037g/g when the relative humidity was increased to 50%. The saturation adsorption capacity of M-13X was higher than that of 13X; for 13X, competitive adsorption of water decreased the volume available for toluene adsorption. The adsorption properties of other molecular sieves are also listed in Table 1, and it can be seen that the saturation capacities of M-13X and HZSM-5 are quite similar to each other. The data also show that the saturation capacities of HY and Cs₇NaMOR are less than twice that of M-13X.

Desorption properties

The desorption performance has a profound effect on the adsorbent service life. Usually, adsorptive reactions are exothermic (William et al., 1997), and therefore adsorbents can be regenerated using hot air or hot water vapor.

In these experiments, a hot air stream was used for the regeneration of M-13X, and the typical sectional flow rate of the hot air stream was 140 L/min. The desorption properties of M-13X at different temperatures were investigated. Desorption efficiency was defined as the ratio of the amount desorbed by heating to the difference between the amount initially adsorbed and the amount that is reversibly adsorbed (Kim and Ahn, 2012). As shown in Figure 8, all the curves increased dramatically in the first 20 min, which indicated that the externally adsorbed toluene was desorbed quickly. After 50 min, the curves rose slowly, showing that toluene which had diffused into the M-13X channels was released slowly. After 90 min, the desorption efficiencies of M-13X were 55%, 90%, and 88% at regenerating temperatures of 423 K, 473 K, and 523 K, respectively. No more toluene was desorbed at 423 K after 90 min. However, a maximum desorption efficiency of 98% was observed after 120 min at 473 K and 523 K. There was no obvious improvement in the desorption efficiency at 523 K compared with that at 473 K. The regeneration temperature of molecular sieves depends on the properties of adsorbates and the strength of interaction between adsorbates and adsorbent. According to the previous study (Shen et al., 2009), 423 K was not enough for the regeneration since the optimal regeneration temperature of 13X was 473–573 K. Considering the energy utilization, 473 K is a reasonable regeneration temperature of M-13X in the present study.

Figure 8. Influence of temperature on desorption on M-13X.

Conclusion

In conclusion, M-13X is a potential adsorbent for the treatment of VOCs in high-humidity streams. XRD, XRF, chemical–physical adsorption measurements, and BET and BJH models were used to characterize the physical performances of M-13X and 13X. The results revealed that the Si/Al ratio of M-13X was much higher than that of 13X. Other properties, such as specific area, pore volume, and average pore diameter, were unchanged, but the crystal size and array changed, and this needs to be addressed in future research. In addition, the pore size distribution of M-13X was more uniform than that of 13X; this is one of the primary requirements for an excellent adsorbent. Breakthrough curve experiments using toluene indicated that the saturation capacity of M-13X were slightly higher than that of 13X under the tested conditions. This may be because of the high hydrophobicity of M-13X as a result of its high Si/Al ratio. The hydrophobicity was confirmed by investigating the water isotherms and toluene adsorption under different relative humidities; the results indicated that M-13X has super-hydrophobic properties. The desorption efficiency curves demonstrated that the optimal regeneration temperature for toluene adsorption was 473 K, taking account of energy saving.