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Section 6. Chemistry
Nguyen Thi Huong, Vo Hoang Phuong, Institute for Chemistry and Material, Vietnam Academy of Military Science and Technology, VN E-mail: Nguyenhuong0916@gmail.com Tran Hong Con, Le Thanh Son, Faculty of Chemistry, Hanoi University of Natural Science, VNU
Dang Thi Uyen,
Military Institute of Chemistry and Environment, VietNam
REMOVAL POSIBILITY OF BENZENE VAPOUR IN THE AIR BY ZEOLITE HY COMPOSITE
Abstract: The zeolite composites were synthesized by seft assembly trom tributyl phosphate or tricresyl phosphate and zeolite HY in n-hexane at 70 oC for 3 hours contacting. The structural characterization and the porosity of the material were studied by Field Electronic Scanning Emission Microscope (FESEM), X-Ray Difraction (XRD) and Brunauer-Emmett-Teller (BET) adsorption method. Chemical bondings were characterized by Fourier Transform Infrared Spectroscope (FT-IR). The synthesized materials were studied on their adsorption ability of benzene in the air. Adsorption of benzene vapor in fixed-bed columns of zeolite composite materials was investigated at low vapor concentrations. The effects of empty bed contact time (EBCT), factors influencing on the dynamic equilibrium absorption capacity of the zeolite composties, such as inlet concentrations ofbenzene vapor (20 ppmv to 60 ppmv), working temperature (30o C and 40o C), gas flow rate (0.15 L/min to 0.60 L/min) were studied. The results showed that these composite absorbents were suitable for application of benzene vapour treatment in the air environment.
Keywords: Tributylphosphate, Tricresylphosphate, Benzene, Zeolite HY, Zeolite Composite.
1. Introduction nahan, 2003); Suleeporn Sangrajang, 2008); Robert
Benzene is one of highly toxic agents of volatile Snyder, 1993); Ernest Hodgson, 2004). So, there are
organic compounds (VOCs) upon human and ani- many studies concerning to adsorption of benzene
mal. When the human body is benzene poisoned, vapor in the air used zeolite HY as a benzene adsor-
it will directly affect the structure of cells, or alter bent, showing the benzene adsorbed on zeolite HY
the development of cells and hold-back the enzymes formed a fairly durable compound of HY-benzene.
which are related to the process of forming blood However, this adsorption process is mostly physi-
cells. The above processes would cause diseases re- cal with low binding energy. In this process there
lated to anemia, lack of leukemia and related to bone are two types of binding between benzene and H marrow diseases, as well as cancer (Stanley E. Ma-
Van der Waals forces of benzene inside 12-T or 4-T of zeolite. According to Shuai Ban, the process of benzene adsorption of zeolite HY started at 10 kPa, and rising fast when the pressure increases, the adsorption is less variable at high pressure. With low pressure, the adsorbability ofbenzene on zeolite HY depends greatly on temperature (Shuai Ban, 2009). The benzene adsorption on the zeolite HY mainly depends on the interaction of benzene with 4 hy-droxyl groups of 4 faces in a zeolite cell unit, normally these H groups can be determined through IR spectrum measurement (Shuai Ban, 1995). However, the studies showed that the number of hydrogen group types in the cell unit is different. According to Jirak, in a cell unit were found 21.1 ± 5.5 H (1), 30.9 ± 7.0 H (3), without the presence of H (2), H (4). Czjzek et al. found 28.6 ± 1.0 H (1), 15.0 ± ± 1.0 H (3), 9.5 ± 1.0 H (2) and also did not find the presence of H (4) (Mirjam Czjzek et al., 1992). Meanwhile, Vitale with his group found that all four types of hydrogen were found in the cell unit of the zeolite HY (G. Vitale et al., 1995). The study results of Fabien Jousse et al. have launched an interactive model of benzene in zeolite HY with Si/Al ratio of 2.43; where are 4 types of hydrogen interaction with benzene appeared, including H(2) - Benzene; H(1) - Benzene; 2 H(1) - Benzene and W - Benzene (Fabien Jousse et al., 2000). According to Siricharn S. Jirapongphan et al., benzene adsorption on zeolite HY occurs in the following 6 types of H1, H2, 2H1, W and two types of U6 and U4 (Siricharn S. Jirapongphan et al., 2006).
Currently, in order to increase the selective adsorption of volatile organic compounds agents, as well as benzene of zeolite, some research groups have modified zeolite by organosilicone agents, with the aim to bring more aromatic cyclic groups on zeolite surface and to create interaction between the electron n ofbenzene with en agents of the aromatic cyclic group (Qin Hu et al., 2009).
This article presents the method of preparation and properties of materials based on organic phos-
phorus and zeolite HY and study on their adsorbability of benzene vapour in the air.
2. Material and methods
2.1. Zeolite composite preparation
The typical synthesis of a zeolite composite is according to procedure as below: To a 1 g of zeolite HY (HS-320, Wako-Japan; SiO2/Al2O3: 5,5 mol/ mo/; crystal size: 0,3 ^m; pore aperture: 0,8 nm) in the reflux reactor, was added 50 ml of n-hexane and the mixture was continuously stired; then 50 ml of n-hexane containing 0.1 g tributyl phosphate (TBP 0.979 g/ml, Aldrich) or tricresyl phosphate (TCP 1.16-1.175 g/ml, Aldrich) was slowly added and stirring for 3 hours at 70 °C. The synthesis process was performed in closed reaction system to avoid evaporation of n-hexane. The n-hexane (the solvent) in the product sample was then removed by vacuum distillation and finaly, the sample was dried at 110 ° C for one hour, cooled and stored in a desiccator. The zeolite composite samples were signed as CYTBP and CYTCP for tributylphosphate and tricre-sylphosphate respectively.
The chemical bonding characteristics of CY-^ and materials were evaluated by Fourier
IBP TCP '
Transform Infrared Spectroscope (FT-IR, GX-Per-kin Elmer, USA), the materials structure was determined by Field Emission Scanning Electron Microscope method (FESEM, Jeol 6610LA, Japan). Nitrogen adsorption — desorption isotherms were performed at 77 K in Tristar 3000-Micromeritics equipment, USA, using static adsorption proceduce. Samples were degassed at 150oC and 10-6 Torr for minimum 12 h prior to analysis. BET surface areas were calculated from the linear part of BET plot according to IUPAC recommendation. Pore size distributions of the samples were calculated via the conventional BJD model.
2.2. Adsorption of benzene vapor
2.2.1. Adsorption column preparation:
— Quartz sand in mixture with zeolite composite samples: Quartz sand with diameter of 0.4-0.5 mm was washed and soaked in HCl of 0.1 M for 24 hours.
Then the sand was washed again to pH 7, dried and heated at 550-600oC for 3 h to completely remove organic impurity.
— Adsorption column: 0.5 g of zeolite composite and 2.0 g of sand were mixed well together then filled into 1.0 cm diameter quartz column. The length of adsorption material columns after filling was 3.5-3.7 cm. In two ends of the column were blocked by lay-
ers of glass fiber. For experimental performance, the column was inserted into the themostate room as shown in Figure 1.
* The research equipment:
— Figure 1 is diagram of the equipment for organic solvent vapor adsorption investigation with themostate room seting temperature with vibration of ± 0.1o C.
Figure 1. Diagram of vapor adsorption research equipment
* Benzene concentration determination (Abulrah-man Bahrami et al., 2011; L. Zoccolillo et al., 2001): The inlet and outlet benzene vapor concentrations were determined through analysis of benzene adsorbed in acetonitrile — water mixture with volume ratio of 3:1 before and after adsorption respectively by the HPLC Mode HP 1100, using column C18-Zobax and DAD detector series. The content of adsorbed benzene vapro on the column over the time was calculated by the formular:
mB = m - mt (mg);
In which: mI is the amount of input benzene vapor (mg); mf is the amount of output benzene vapor (mg) in the time t.
The equilibrium adsorption capacity of material is calculated according to the formula:
q = -!L (mg/g)
mz
In which m is the amount of adsorbed benzene
B
vapor (mg), mZ is the adsorption material mass (g). 3. Results and discussion 3.1. Characteristics of zeolite composite materials
The infrared spectrum of CYTBP and CYTCP is
shown in Figure 2.
tjjfi §
3.5 ■
3.0 -
2.5 ; fl
2.0 ■
1.5 ■
1.0 ■ \ i : ;
0.5 -0 ■x if? I \ll ST" \iJ
Figure 2. Infrared spectrum
The infrared spectrum diagram of samples showed the presence of the peak at wave number from 3448 cm-1 to 3460 cm-1 with wide base, characterized the vibration of the OH group of the zeolite. However, the peak intensity is weak due to the presence of physical adsorbed H2O on the surface
of CYTCP, (a) and CYTRP (b)
TBP
of zeolite, so the hydrogen bridge bonds between oxygen in water and the OH group covered the O-H bond of the zeolite surface. It was also demonstrated where presence of peak at wave number 1638 cm-1 (CYTBP) and 1634 cm-1 (CYTCP) (H. Robsin, 2001). The peaks at wave number 2879 cm 1 and
2968 cm-1 characterize the vibration of C-H bond in CH3 group in the tributylphosphate in CYTBP sample, while in CYTCP sample there is no appearance of these peaks at these wave numbers. And the CYTCP has peaks appeared at wave number 1491 cm-1 and 1401 cm-1 with stronger intensity than peak at wavenumber 1468 of CYTBp. This shows that in the CYTCP there is a characteristic vibration of aromatic cyclic structure. Notably, both samples have the appearance of peaks at wave number from 1178 cm-1 to 1163 cm-1 and from 1077 cm-1 to 1050 cm-1, with the strong to moderate intensity characterized the vibration of Si-O-Si and Si-O-C bonds. The above characteristics of the material show that there was interaction between zeolite surface and organic components.
In the wave numbers from 1200 cm-1 to 400 cm-1, there appeared characteristic peaks for frame structural bonds of zeolite, both samples have the appear-
ance of three smaller peaks around wave number 600 cm-1, characterized the vibration of frame structure and distortion vibration of group O-T-O of zeolite. In which, the presence of the peak at wave number 583 cm-1 (CYTCB) and 593 m-1 (CYTCB) characterize asymmetrical and symmetrical vibration of the four-sided ring, which is also the characteristic of distortion vibration of the double ring in the frame structure of zeolite type Faujasite (H. Robsin, 2001; Wtodzimi-erz Mozgawa et al., 2011). In addition, both materials have the appearance of two peaks at wave number characterized the valence vibration of group P-O aliphatic for CYTCB (1150 cm-1; 816 cm-1) and P-O aromatic for CYTCB (1163 cm-1; 1037 cm-1). These two groups indicate the presence of phosphate functional group in the structure of the zeolite composite samples. Surface morphology of samples HY, CYTBP, CYTCP is expressed in FE-SEM image (Figure 3).
Figure 3. The FE-SEM image of sample HY, CYTBP and CYTCP
As what is seen at the FE-SEM images, the original zeolite structure consists of almost unconected grains with the form of rectangular blocks and the size of several ^m to dozens ^m. While the composite of organophosphorous and zeolite showed that in the grains surface likely appeared covered sticky layer. However, the surface and grains size of all three materials are almost the same as that of original zeolite. But the materials covered organic agents hopefully increase ability of VOCs separation and absorption of materials, because organic agents have
the role of both absorber and conductor in interaction with VOCs.
For better understanding the changes in the structure of the material, proceed to determine the structure of capillary and the surface area of sample by the the nitrogen isothermal adsorption method and size distribution of capillary is implemented. The results are presented in Figure 4.
Study on the surface properties of zeolite composite materials, based on the BET method shows that the nitrogen isothermal adsorption-desorption
line of all three samples was of type IV and has hysteresis loop type H2 (as sorted by IUPAC). The properties belong to materials have medium cap-
illary with hollow both ends cylinders, and corresponding to the Barret-Joyner-Halenda (BJH) hole size distribution.
170-
OL 160-
w
-5? 150
- Desorption- CYTCP Adsorption- CYTCP Desorption- CYTBP Adsorption- CYTBP
140-
o 130-
120-
100-
0.0080.007 -
^ 0.006E
sz "
ró 0.005-
"e ■
Ä 0.004-(1)
E "
J5 0.003-o
> -
¡Ü 0.002-o
CL -
0.001 -
0.000-
- CYTC
CYT,
0.0
0.2
0.4
0.6
0.8
1.0
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 Pore width (nm)
Figure 4. Diagram of the nitrogen isothermal adsorption-desorption line and capillary size distribution of CYTBP and CYTCP
Figure 3 also shows that desorption line of the samples were relatively smooth, in which the two samples has the nitrogen isothermal adsorption-desorption line started condensation in the relative pressure P/Po about 0.5-0.95, demonstrates that the material has a relatively large capillary diameter. As for the two samples CYTBP, CYTCP, they have the condensation in the earlier relatively pressure P/Po, and have a capillary diameter greater than the capillary diameter of original zeolite sample. According to the BJH survey, all three samples have capillary distribution ranging from 0.8 nm to 80 nm, in which the current pore data of the two samples CYTCP, CYTBP are 2.46 nm and 2.42 nm respectively.
Thus, the difference in the average capillary size of samples is result of spatial effects of organic agents used in zeolite composite, as well as their interaction with zeolite created larger capillaries than original zeolite sample. The specific surface area of CYTBP and CYTCP are 409.34 m2/g and 388.17 m2/g respectivelysmaller than those of original HY (550 m2/g). This again confirms that in CYTBP and CYTCP samples, the organophosphorous components were linked and obstructed the spatial part of the pore on the original zeolite surface. However, when com-
paring the results with zeolite composite specimens modified by silicified muscle compounds, it shows that these samples have larger specific surface area (Le Thanh Son et al., 2016).
3.2. Adsorption ability of benzene vapor on zeolite composite materials 3.2.1. Influence of contact time Empty bed contact time is known as the time required for a adsorbate element moving through the length of the adsorbent layer in the case of empty column. Empty bed contact time is one of the important parameters of an adsorption system. Assuming the flow is ideal, the empty bed contact time is calculated by the following formula (William F. Naylor et al., 1995; Chai-
wat Rongsayamanont et al., 2007):
y
EBCT=— f
where EBCT is Empty bed contact time, s V is Empty bed volume of the column, L. f is The gas flow rate through the column, L/s The study was conducted with the gas flow rate from 0.15 L/min to 0.60 L/min at 40oC; the inlet benzene concentration is 21.8 ppmv, the bed volume of the material is 2.9 mL. The results are given in table 1 and Figure 5.
Table 1.- Influence of gas flow rate to the content of adsorption of CY , CY
TCP
No. Gas flow rate (L/min) Contact time (s) Adsorption capacity of CYTBP (mg/g) Adsorption capacity of CYtcp (mg/g)
1 0.15 1.16 1.0038 1.8224
2 0.30 0.58 1.0235 1.8970
3 0.60 0.29 0.9070 1.7446
25 -,
Time (min)
Figure 5. Breakthrough curve of benzene vapor through zeolite composite column at 40oC
As results showed in table 1, with the same inlet benzene concentrations, but with lower contact time (about 0.29 s), the adsorption capacity began decrease. It demonstrates the rate of gas flow really affecting on adsorption capacity, particularly, when the contact time is too short, it shall not guarantee that gas molecules can diffuse through the absorbed material surface. From the above review, we conducted 1.2
1.0
0.8
0.6
cr
0.4
0.2
0.0
the survey of benzene vapor adsorption process of the materials in the two flow rate 0.15 L/min and 0.30 L/min.
The results also showed that at the flow rate of 0.15 and 0.30 L/min, the adsorption capacity of the materials varied not so much. However, adsorption breakthrough curve is different. The breakthrough curve at a flow rate of 0.30 L/min is more sloping than those of 0.15 L/min. However, the difference of adsorption capacity appeared by the different organophosphorous component in the material. The composte material with tributylphosphate showed higher adsorption capacity than those with tricre-sylphosphate.
3.2.2. The influence of temperature
The investigation was carried out at 30 °C and 40 °C; the initial benzene concentration was 12.6 ppmv and 21.8 ppmv; air flow rate 0.30 L/min; the adsorption fractions were measured after every 5 min, the cumulative curves of benzene vapor on the zeolite composite waere given in Figures 6.
2.0-
1.5-
1.0-
0.5-
0.0
10
20
30
40
50
60
Time (min) Time (min)
Figure 6. Cumulative curves of benzene vapor on the zeolite composites at 30oC and 40oC
0
The results showed that, increasing of temperature from 30oC to 40oC, the adsorption capacity of CY™_ and increased, and after maximum 60
IBP TCP
min contacted the adsorption process of all materials reached equilibrium state. The dynamic equilibrium adsorption capacity of CYTBP reached 0.8206 mg/g and 1.0235 mg/g, and those of CYTCP were 1.0822 mg/g and 1.8970 mg/g respectively. This phenomenon seems unaccordant to the adsorption kinetics (temperature increased, decreased the adsorption capacity). In this case, it can be explained by better diffusion of benzene through organophosphorous layer to zeolite core in high temperature and holed in zeolite structure. But actualy, the threshold of higher temperature was not determined yet.
The adsorption capacity of benzene vapor on CYTCP was better than that on CYTBP and original zeolite. The reason can be explained by that, kinetic diameter of benzene is 5.85 A which is much smaller than capillary size of original zeolite and composite system of zeolite and organophosphorous agents which have average capillary diameter determined by the BET method (Figure 3). In particular, the IR study showed that the CYTCP has interaction of benzene ring in the TCP agent with benzene vapor, so the dynamic equilibrium adsorption capacity of this material was greater than that of CYTBP. The results also showed that benzene adsorption process of zeolite HY formed a fairly durable compound like HY-
benzene, however it still belonged to physical adsorption type. There are able two main compounds created by links of benzene with H in zeolite and by Van der Waals interaction of benzene inside 12-T or 4-T of zeolite (Shuai Ban, 2009). According to Qin Hu (Qin Hu et al., 2009), the presence of aromatic cyclic agent in the material, its adsorption ability changed, and this was explained by the interaction of electron-n of benzene or phenyl when studied adsorption of benzene on different materials.
4. Conclusion
Zeolite composite materials (CYTBP and CYTCP) prepared from zeolite HY with tributylphosphate and tricresylphosphate have a high surface area (409 m2/g and 388 m2/g respectively), grain size of materials from several ^m to dozens ^m, the current pore data around 2.46 nm to 2.42 nm. The IR spectra of the zeolite composite showed strong interaction between zeolite base and organophosphate substances.
Zeolite composite materials have high adsorption capacity ofbenzene vapor in quite low concentration (from 20 ppmv to 50 ppmv) and the adsorption time of 0.29 s to 1.16 s. The dynamic equilibrium adsorption capacity of CYTCP was 1.0822 mg/g and 1.8970 mg/g and those of CYTBP was 0.8206 mg/g and 1.0235 mg/g corresponding with 30 oC and 40 oC in the adsorption conditions of gas flow rate of 0.30 L/min; input benzene concentration of 21.8 ppmv and temperature of 30oC and 40oC.
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