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Hydrodechlorination of trichloroethylene over MoP/␥-Al2O3 catalyst with high surface area Qinghui Guo, Lili Ren ∗ School of Chemistry and Chemical Engineering, Southeast University, Nanjing, 211189, PR China
a r t i c l e
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Article history: Received 25 May 2015 Received in revised form 11 September 2015 Accepted 15 September 2015 Available online xxx Keywords: MoP/␥-Al2 O3 Sol–gel Hydrodechlorinaton Trichloroethylene
a b s t r a c t Alumina-supported molybdenum phosphide (MoP/␥-Al2 O3 ) with high surface area was successfully synthesized by combining sol–gel and temperature-programmed reduction (SG-TPR) method and was firstly used to study the hydrodechlorination (HDC) of trichloroethylene (TCE). The properties of the catalysts were evaluated by several different characterization techniques, such as Braunner–Emmet–Teller (BET) surface area, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The results prove that a well-dispersed and high BET surface area catalyst can be obtained by SG-TPR method, which shows higher catalytic activity than those prepared by impregnation and mechanical mixing method for the HDC of TCE. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Trichloroethylene (TCE) is one of the halogenated organic compounds that can be easily found in the emission from chemical processing, ground water, and soils due to its extensive uses in industries [1–3]. It has been pointed out that TCE is a volatile organic compound with potent carcinogenicity and resistant to natural degradation leading to long-term environmental and health risks [4]. Also, the emission of TCE into the environment is now strictly regulated [5]. As a result, there is substantial interest in the development of efficient, economic methods for remediating ground water, and soils contaminated with TCE. It has been recognized that the traditional remediation technologies, such as incineration and solidification, are not entirely satisfactory, because they require high capital, operation cost, and extremely long-time [5,6]. In addition, they can lead to the formation of carcinogenic byproducts [6]. Compared with the conventional methods, the catalytic hydrodechlorination (HDC) for treating TCE is a viable and nondestructive low-energy alternative, which transforms the chlorinated waste streams into nontoxic, easily degradable, or useful raw materials [7]. Therefore, as a safe method, the catalytic HDC of TCE has attracted more and more attention.
Over the past years, studies on HDC of TCE compounds have been carried out mainly using supported metal, notably Pt, Pd [4,8–10]. Although they exhibit good catalytic activities, they are expensive and easily deactivated due to HCl poisoning, metal sintering or carbon deposition, which limits their application in industry [11]. Recently, transition metal phosphides have been reported as a new class of high activity hydroprocessing catalysts that have substantial promise as next-generation catalysts [12]. The group of Nozaki first reported the catalytic activity of phosphides in 1980 [13]. In 1998, Oyama group found that MoP could be easily synthesized by temperature-programmed reduction (TPR) of a metal phosphate precursor [14], transition metal phosphides began to be widely used in catalytic reaction. Especially, they exhibited excellent reactivity in hydrodenitrogenation (HDN) [15] and hydrodesulfurization (HDS) [16]. Considering the similarity of HDN, HDS, and HDC, we synthesized the MoP catalysts used for HDC of TCE. However, the low BET surface area limits the performance of the catalysts [16]. In this paper, well-dispersed MoP/␥-Al2 O3 with high surface area was prepared by SG-TPR method for the first time, and the application of the catalyst in the HDC decomposition of TCE was compared with those prepared by impregnation and mechanical mixing methods. 2. Experimental 2.1. Preparations
∗ Corresponding author. Tel.: +86 25 83358312; fax: +86 25 83358312. E-mail address:
[email protected] (L. Ren).
In this paper, s-MoP/␥-Al2 O3 represents the catalyst prepared by sol–gel method. i-MoP/␥-Al2 O3 represents the catalyst prepared
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by impregnation method. m-MoP/␥-Al2 O3 represents the catalyst prepared by mechanical mixture method. The detailed preparation methods are as follows: For s-MoP/␥-Al2 O3 catalyst, firstly, a mixture of 1.25 g (NH4 )6 Mo7 O24 ·4H2 O (AR, Hefei Kehua Fine Chemical Research Institute) and 0.94 g (NH4 )2 HPO4 (AR, Xilong Chemical Co., LTD., China) was dissolved in 15 mL deionized water and stirred for 3 h under 80 ◦ C. Secondly, a solution was made by mixing AlCl3 . 6H2 O (24.43 g, 0.1 mol) (AR, Sinopharm Chemical Reagent limited corporation), 24 mL water with 56 mL ethanol, and heated to 80 ◦ C, while stirred for 0.5 h. After that, the mixture prepared in the first step was poured into the solution. Then, propylene oxide, as gelation inducing agent, was added to the solution, cooled in ice water bath. Gel formation typically occurred within 40 min at the temperature of 30 ◦ C. The gel parts were then soaked in a bath of absolute ethanol for 24 h to exchange the water and reaction byproducts from the pores of the material. After the dryness at 120 ◦ C, the sample was calcined in air at 500 ◦ C for 5 h, pressed, and sieved to particles between 420 and 840 m in diameter for use. The particles obtained above were reduced by heating from room temperature to 300 ◦ C at a rate of 10 ◦ C/min, then from 300 to 650 ◦ C at a rate of 1 ◦ C/min, and kept at 650 ◦ C for 2 h in flowing H2 (50 mL/min), followed by cooling to room temperature. Bulk MoP, i-MoP/␥-Al2 O3 , m-MoP/␥-Al2 O3 , and Mo/␥-Al2 O3 catalysts were prepared as the procedure reported references [14] and [17]. Bulk MoP catalyst was prepared as the procedure: a mixture of 4.0 g ammonium heptamolybdate (NH4 )6 Mo7 O24 ·4H2 O and 3.0 g diammonium hydrogen phosphate (NH4 )2 HPO4 was dissolved in 30 mL deionized water. A white solid obtained following evaporation of the water. Then the sample was calcined in air at 773 K for 5 h to give a dark blue solid, subsequently pressed, broken, and sieved to particles between 420 and 840 m in diameter for use (mesh size 20–40). And then the sample was reduced by H2 as the same as the preparation method of s-MoP/␥-Al2 O3 catalyst.iMoP/␥-Al2 O3 catalyst was prepared as the procedure: a commercial aluminum oxide was used as the support. Impregnating solutions were prepared by dissolving 2.45 g (NH4 )6 Mo7 O24 ·4H2 O and 1.83 g (NH4 )2 HPO4 in approximately 60 mL water. Then 10 g Aluminum oxide was added to it. The mixture was stirred 12 h at the room temperature. After evaporation of the water at 80 ◦ C, the following operation was the same as the preparation method of unsupported MoP. Mo/␥-Al2 O3 catalyst was prepared as the same method as i-MoP/␥-Al2 O3 catalysts except for the absence of P. MoP loadings for all the MoP/␥-Al2 O3 catalysts in this paper equal to 15 wt%. 2.2. Characterizations BET surface areas were determined from the nitrogen adsorption-desorption curves by the conventional multipoint technique with a Belsorp Mini 2. XRD patterns of the samples were acquired on a Bruker D8 Discover X-ray diffractometer using Cu K␣ ˚ at a respective voltage radiation and a nickel filter ( = 1.5406 A)
of 40 kV and current of 40 mA. TEM was carried out on a FEI-G202010 electron microscope operated at an acceleration voltage of 200 kV. XPS measurements were carried out using a Thermo Scientific K-Alpha instrument. Binding energies were corrected for sample charging using the C (1s) peak at 284.6 eV for adventitious carbon as a reference. 2.3. Catalysts activity measurement Trichloroethylene HDC reactions were carried out using a continuous fixed-bed quartz reactor (id. = 10 mm) at 400–600 ◦ C and atmospheric pressure. A total of 8 mL catalyst was put in the middle of the quartz reactor. The hydrogen flow rate was 100 mL/min. Using a bubbler for TCE supply, the amount of evaporated TCE corresponds to the weight loss of the bubbler. The flow ratio of H2 to TCE was adjusted approximately 14:1. The produced HCl was trapped in a water bubbler and the amount of formed HCl can be determined very accurately by NaOH titration with a pH-indicator (e.g. phenolphthalein). So the decomposition rate of C Cl bonds, which reflects the catalyst activity, can be calculated as the following formula: R C − Cl bonds decomposition =
n (HCl) × 100% 3n (C2 HCl3 )
3. Results and discussion The BET surface area, pore volume, and average pore diameter for the s-MoP/␥-Al2 O3 , i-MoP/␥-Al2 O3 , m-MoP/␥-Al2 O3 , bulk MoP catalysts, and ␥-Al2 O3 support are listed in Table 1. The low BET surface area of the bulk MoP ( i-MoP/␥-Al2 O3 > MoP. This order is in accordance with the order of the hydrodechlorination 60
50 40 30 20 10 0
a 1000 2000 3000 4000 5000 6000 Space velocity / h
-1
C-Cl bonds decomposition ratio/%
C-Cl bonds decomposition ratio/%
60
50 40 30 20
b
10 0 0
1
2
3
4
5
6
7
8
Time / h
Fig. 6. (a) C Cl bonds decomposition ratio at different space velocity (keep hydrogen flow rate at 100 ml/min and the catalysts change from 1 to 12 mL) over the 15 wt% s-MoP/␥-Al2 O3 catalysts under 450 ◦ C, (b) C Cl bonds decomposition ratio at 1500 h−1 space velocity (4 mL catalysts under 100 ml/min hydrogen flow rate) over the 15 wt% s-MoP/␥-Al2 O3 catalysts under 450 ◦ C.
Please cite this article in press as: Q. Guo, L. Ren, Hydrodechlorination of trichloroethylene over MoP/␥-Al2O3 catalyst with high surface area, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.019
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activities of catalysts, which hints that except the surface area, the acid amount on catalyst surface also effect the activities of catalysts. Fig. 6a is the curve of the C Cl bonds decomposition ratio at different space velocity over the 15 wt% s-MoP/␥-Al2 O3 catalysts under 450 ◦ C. The activity of this catalyst increased from 500 to 1500 h−1 space velocity and decreased after 1500 until 2000 h−1 . The C Cl bonds decomposition ratio stabilized when the space velocity is beyond 2000 h−1 . We can obtain the highest activity at the 1500 h−1 space velocity. To study the stability of the catalyst, we carried out a long time experiment and the results were presented in Fig. 6b. Fig. 6b is the curve of the C Cl bonds decomposition ratio at 1500 h−1 space velocity over the 15 wt% s-MoP/␥-Al2 O3 catalyst under 450 ◦ C over 7 h. In Fig. 6b, C Cl bonds decomposition ratio of the catalyst keep around 51 ± 3%, and the fluctuations are tend to reduce along the time without decrease of the activity. It indicates that the catalyst will remain high HDC activity for a long time. 4. Conclusions We have successfully prepared MoP/␥-Al2 O3 catalyst with high surface area through SG-TPR method, which shows remarkably HDC catalytic activities. This method can be applied to prepare other phosphide catalysts and solve their common problem-low surface area, and accelerate the industrialization of such kind catalysts. Acknowledgments This work was financially supported by National Nature Science Foundation of China (21206018), Nature Science Foundation of Jiangsu Province of China (BK20151408), and the Chinese Fundamental Research Funds for the Central Universities, (3207045411). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2015.09. 019. References [1] C. Amorim, X.D. Wang, M.A. Keane, Application of hydrodechlorination in environmental pollution control: comparison of the performance of supported and unsupported Pd and Ni catalysts, Chinese J. Catal. 32 (2011) 746–755. [2] N. Barrabes, D. Cornado, K. Foettinger, A. Dafinov, J. Llorca, F. Medina, G. Rupprechter, Hydrodechlorination of trichloroethylene on noble metal promoted Cu-hydrotalcite-derived catalysts, J. Catal. 263 (2009) 239–246. [3] O. Orbay, S. Gao, B. Barbaris, E. Rupp, E. Saez, R.G. Arnold, E.A. Betterton, Catalytic dechlorination of gas-phase perchloroethylene under mixed redox conditions, Applied Catalysis B: Environmental 79 (2008) 43–52.
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