Simultaneous Catalytic Hydrolysis of Carbonyl Sulfide and Carbon

Jul 22, 2012 - Adsorption-oxidation of hydrogen sulfide on Fe/walnut-shell activated carbon surface modified by NH 3 -plasma. Ping Ning , Sijian Liu ,...
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Simultaneous Catalytic Hydrolysis of Carbonyl Sulfide and Carbon Disulfide over Modified Microwave Coal-Based Active Carbon Catalysts at Low Temperature Ping Ning,† Kai Li,† Honghong Yi,*,† Xiaolong Tang,† Jinhui Peng,‡ Dan He,† Hongyan Wang,† and Shunzheng Zhao† †

Faulty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming, 650500, People's Republic of China ‡ Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650500, People's Republic of China ABSTRACT: A series of microwave coal-based active carbon catalysts loaded by metal oxides were prepared by a sol−gel method and tested for the simultaneous catalytic hydrolysis of carbonyl sulfide (COS) and carbon disulfide (CS2) at relatively low temperatures of 50−70 °C. The influences of preparation conditions on catalytic activity were studied, which were the kinds and amount of additive, calcination temperatures, and types and content of alkali. The results show that catalysts with 5.0% Fe2O3 after calcining at 300 °C have superior activity for the simultaneous catalytic hydrolysis of COS and CS2. It also indicated that the catalytic hydrolysis activity increased with the basic intensity, following the order of KOH > K2CO3 > Na2CO3 > NaHCO3, and the optimum amount of KOH was 13%. The structure and surface properties were characterized by Xray diffractometry (XRD), Brunauer-Emmett-Teller measurements (BET), Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS). The XRD and BET results revealed that the calcination temperature controlled the crystalline phase and generation of Fe2O3 and affected the properties of specific surface area and pore structure. The XPS results showed that most of COS and CS2 hydrolysis products were sulfate ion (SO42−) species, which accumulated on the active carbon’s surface and had a negative effect on the hydrolysis activity.

1. INTRODUCTION COS and CS2, which emit from chemical gas produced from coal, natural gas, petroleum, crude oil, and industrial streams,1,2 are the major components of organic sulfur compounds. As COS and CS2 in various gases not only pollute the environment but also poison the catalysts,3,4 many methods have been developed to remove COS and CS2 from Claus tail gas and other process gas streams. CS2 is much more difficult to convert than COS, so it is difficult to remove it.5 The main technologies for COS and CS2 removal include catalytic hydrolysis, oxidation conversion, and hydrogenation conversion, etc.;6−9 among these methods, the catalytic hydrolysis was recognized as the most promising process due to the mild reaction condition, cheapness, and higher conversion efficiency.5 Recently, most studies concentrate on separate catalytic hydrolysis of COS and CS2 in excess of 200 °C. There are various studies on COS and CS2 separate hydrolysis, especially aiming at the development of new catalysts, particularly supported metal oxides (where the support is typically alumina and titania) at relatively high temperatures.10−13 The application of activated carbons (ACs) for air purification has become more popular due to their large surface area and porous structure. The physical adsorption of © 2012 American Chemical Society

the pores sizes and volumes is important and intense, and surface reaction and chemisorption surface chemistry also play a significant role.14−16 The conventional AC heating treatment methods do not ensure a uniform temperature for different shapes and sizes of samples and result in long activation times and higher energy consumption. However, microwave heating pretreatment is being used in various technological and scientific fields for a variety of applications. Energy transfer is not by conventional heating, so it is easily transformed into heat inside the particles with dipole rotation and ionic conduction. Thus, the tremendous temperature gradient from the inside of the char particle to its cool surface, which allows the microwave-induced reaction to proceed more quickly and effectively.17,18 In the present study, the separate catalytic hydrolysis of COS or CS2 is investigated using AC-supported catalysts the low temperature (50 nm). Fourier transform infrared spectroscopy (FT-IR) with the Varian 640-IR was used to determine sulfur-containing species on the catalyst surface. The FTIR spectra were recorded using the KBr pellet technique on a Thermo spectrometer in the 4000−500 cm−1 wavenumber range. Before the analyses, the sample was ground, mixed, and made into pellets with KBr. The analysis temperature was at room temperature, and the analyses done were ex situ. 2.3. Catalytic Activity Measurement. Desulfurization tests were performed in a fixed-bed quartz reactor (3 mm inside diameter × 100 mm length) under atmospheric pressure. COS and CS2 from a gas cylinder (1% COS in N2; 0.3% CS2 in N2) were diluted with N2 (99.99%) to the required concentration (COS, 350−400 ppm; CS2, 10−20 ppm). The overall flow rate was controlled using calibrated mass flow controllers, and the overall gas hourly space velocity (GHSV) of the reaction mixture was standardized at 6000−10 000 h−1. The water was introduced using a saturator system. The temperature of this reactor was controlled to 50−70 °C by a water bath with a circulating pump, with an accuracy of ±0.1 °C; the relative humidity (RH) was 49%. The total COS and CS 2 concentrations of the gaseous feed and effluent from the reactor were analyzed by an online HC-6 sulfur phosphorus microscale analyzer with an FPD detector. The conversion of COS or CS2 is determined by analyzing the inlet and outlet concentrations of CS2:

research focuses on the simultaneous catalytic hydrolysis of COS and CS2 at low temperature ( ≈Al > Co > Ni > Zn. Only the Fe(5)-HP(5)/MCAC(400) catalyst could effectively enhance the CS2 conversion. From the results of activity measurement, for sample Fe(5)-HP(5)/ MCAC(400), 100% COS conversion and 100% CS2 conversion are observed for about 270 and 180 min, respectively, whereas no H2S was detected during the initial reaction period of 270 min. The reason may be that, as a transition metal, Fe can promote the catalyst to have the highest hydrolysis activity for the removal of COS and CS2. From these studies, it is apparent that an Fe−S binding energy of intermediate strength (−4.5 eV) provides the optimum catalyst performance.9 Besides, the outer shell of the Fe ion is a kind of unfilled structure, and it has a more effective nuclear charge. Therefore, this kind of texture is propitious to generating coordination compounds in the process of reaction. As an intermediate product, the coordination compounds can provide coordination catalysis action and pertinent surface reaction to promote the catalysis reaction. In addition, this kind of ion structure can make Fe2O3 react with H2S to generate polysulfide. Meanwhile, desulphu17056

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Figure 1. Effect of different metal oxides on the simultaneous catalytic hydrolysis of COS and CS2 (reaction conditions: 400 ppm of COS; 19 ppm of CS2; GHSV = 6000 h−1; reaction temperature = 70 °C; RH = 49%). (a) Effect of different metal oxides on COS conversion. (b) Effect of different metal oxides on CS2 conversion.

Figure 2. Effect of calcination temperatures on the simultaneous catalytic hydrolysis of COS and CS2 (reaction conditions: 400 ppm of COS; 15 ppm of CS2; GHSV = 6000 h−1; reaction temperature = 70 °C; RH = 49%). (a) Effect of calcination temperatures on COS conversion. (b) Effect of calcination temperatures on CS2 conversion.

rization by Fe2O3 has a higher sulfur capacity and can enhance the desulphurization precision.25 3.2. Effect of Calcination Temperature on Simultaneous Catalytic Hydrolysis of COS and CS2. As an important process of catalyst preparation, the calcination temperature has a great effect on the activation, grain distribution, and formation of the catalysts. The calcination temperature can influence the decomposition of metal salts and relate to the redistribution and aggregation of products on the MCAC surface. The crystallinity and oxidation states can be changed at different calcination temperatures. The active sites of most catalyst precursors can only form after calcination at moderate temperatures. Figure 2 shows the effect of calcination temperatures on the simultaneous catalytic hydrolysis of COS and CS2 over the Fe(5)-HP(5)/MCAC catalyst. The results show that the optimal calcination temperature is 300 °C for the simultaneous catalytic hydrolysis of COS and CS2. When the calcination temperature was above 400 °C, the COS and CS2 conversions were decreased sharply. The calcinations temperature can affect the surface oxidation of transition metals. At low temperature (below 300 °C in this work), the oxidizing

nature of peroxy radicals can make the final hydrolysis product of H2S oxidize into sulfate species and sulfur, which damages hydrolysis active sites.22,26 However, MCAC could be oxidatively decomposed to CO and CO2 above 400 °C, which results in the loss of carbon weight, and the decomposition rates increased with increasing temperature. It also suggested that there may be a certain relationship between the activity and the crystallinity of the oxidation state. The phase and crystalline orientation of Fe(5)-HP(5)/ MCAC with different calcination temperatures were determined by XRD and presented in Figure 3. It can be seen that comparatively weak changes were detected at lower calcination temperatures ( K2CO3 > Na2CO3 > NaHCO3. On the other hand, H2S is not detected during the 270 min test for all the samples, indicating 100% adsorption for H2S. Figure 8 shows the catalyst hydrolysis activity of the samples for COS and CS2 simultaneous removal when adding different amounts of KOH. From the experimental results, the COS and CS2 removal efficiency initially increased and then decreased as the KOH content increased. When the mass ratio of KOH was 13%, the catalysts displayed the best activity for simultaneously removing COS and CS2. It indicates that the alkali content greatly influenced the catalytic hydrolysis reaction. When the alkalinity of the catalyst is weak, the promotion efficiency is limited. However, the alkalinity of the catalyst cannot be too strong, because COS and CS2 and their final hydrolysis products (H2S and CO2) are easily irreversibly adsorbed on the surface to restrain the catalytic hydrolysis reaction.22 The FTIR method was used to analyze the reasons for the effects of the kinds of alkali. From Figure 9, we can see that a broad strong absorption band at 3000−3500 cm−1 is noted, which can be attributed to the OH combination stretching vibration of hydroxyl groups (alcoholic extract hydroxyl group or phenolic hydroxyl group) and other forms of OH stretching vibrations.21 It is clear that the broadening degree of Fe(5)HP(5)/MCAC(300) is strongest, which shows that KOH can provide more OH functional groups. As for that at nearby 1400 cm−1, the IR characteristic peaks are NO3−, which were from nitrate (Fe(NO3)3) in the process of catalyst preparation.27 A small quantity of nitrate may not decompose at the calcination temperature of 300 °C, and adding KOH can reduce the NO3− contents. The two factors above are the main reasons why adding KOH can enhance the catalytic hydrolysis activity of the catalysts more effectively than that of adding other alkalis.

investigated, as shown in Figure 6. The COS removal efficiency initially increased and then decreased with the amount of Fe2O3

Figure 6. Effect of Fe2O3 content on the simultaneous catalytic hydrolysis of COS and CS2 (reaction conditions: 400 ppm of COS; 15 ppm of CS2; GHSV = 10 000 h−1; reaction temperature = 50 °C; RH = 49%). (a) Effect of Fe2O3 content on COS conversion. (b) Effect of Fe2O3 content on CS2 conversion.

increasing, and the best content of Fe2O3 was 5%. When the CS2 was removed separately, the variation trend of Fe2O3 was similar to that of COS.22 However, when COS and CS2 were catalytically hydrolyzed simultaneously, the variation trend of Fe2O3 was not regular for CS2 removal. The worst Fe2O3 content was not 10%, but 7.5%, for CS2 removal efficiency, and the optimal one was also 5%. In addition, H2S is not detected in the tail gas at 240 min. The catalysts modified by different contents of Fe2O3 could enhance the catalytic hydrolysis activity of COS compared to fresh MCAC regardless of the Fe2O3 content, but only 5% Fe2O3 addition could improve the CS2 removal efficiency. The improvement of COS removal efficiency occupied the main position when the Fe2O3 content was 1.5−3.0%. COS could react with a small quantity of Fe2O3, but not enough active compounds reacted with CS2. This situation was changed when the Fe2O3 content was 5.0%; the CS2 removal efficiency was enhanced. However, if the Fe content was above 5%, Fe surface loading will occupy adsorption sites of the MCAC 17059

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Figure 8. Effect of KOH contents on the simultaneous catalytic hydrolysis of COS and CS2 (reaction conditions: 400 ppm of COS; 15 ppm of CS2; GHSV = 10 000 h−1; reaction temperature = 50 °C; RH = 49%). (a) Effect of KOH contents on COS conversion. (b) Effect of KOH contents on CS2 conversion.

Figure 7. Effect of different kinds of alkali solutions on the simultaneous catalytic hydrolysis of COS and CS2 (reaction conditions: 400 ppm of COS; 15 ppm of CS2; GHSV = 10 000 h−1; reaction temperature = 50 °C; RH = 49%). (a) Effect of different kinds of alkali solutions on COS conversion. (b) Effect of different kinds of alkali solutions on CS2 conversion.

3.5. Production Analysis of Simultaneous Catalytic Hydrolysis of COS and CS2. Regarding the production of catalytic hydrolysis of COS or CS2, most researchers think that the simple substance S and sulfate can be formed on the catalyst’s surface, and the activity of the catalyst decreased when the S/SO42− species accumulated on the active carbon surface.21,22 To clarify the production of the simultaneous catalytic hydrolysis of COS and CS2, XPS analysis was performed to examine the composition of the element S and Fe in the used catalyst and to determine the valence states of S and Fe on the catalyst’s surface. Figure 10a shows the relevant detailed S 2p XPS spectra of the used Fe(5)-HP(13)/ MCAC(300) catalyst. Deconvolution of the S 2p spectra gives two individual component groups that represent the monosulfide (161.6 eV) and the SO42− (167.3 eV, may be sulfone sulfur28). However, the sulfur (164.6 eV) was not observed in the XPS results. As for Fe 2p3/2 XPS spectra, three individual component groups represent Fe2O3/Fe2+ (708.4 eV), Fe2+/Fe3+ (710.3 eV), and Fe2(SO4)3 (712.2 eV). Therefore, the products may be the mixture of sulfate over the used Fe(5)HP(13)/MCAC(300) samples. In previous studies, the

Figure 9. FT-IR spectra of the catalysts with different kinds of alkali.

accumulation of sulfate on catalysts could lead to poisoning of catalysts in the oxygen atmosphere. On the basis of the 17060

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Figure 10. Detailed S 2p and Fe 2p3/2 XPS spectra of exhausted Fe(5)-HP(13)/MCAC(300) samples: (a) S 2p XPS spectra and (b) Fe 2p3/2 XPS spectra.

superior activity for the simultaneous catalytic hydrolysis of COS and CS2. Meanwhile, alkali is one of the important factors, and the best additive is 13% KOH. The XRD results illustrate that the calcination temperature can control the formation of the crystalline phase of Fe2O3. Besides, the BET results show that the hydrolysis activity of the catalyst is improved with an increase in the SBET and the quantity of micropores (1.5−3.0 nm). In addition, the XPS results indicate that the products of the simultaneous catalytic hydrolysis of COS and CS2 were SO42− species, which accumulated on the active carbon surface and had a negative effect on the hydrolysis activity. Future research will include study of the impact of potential poisons present within the yellow phosphorus tail gas.

above XPS results, it is a fact that the production of H2S can be converted to surface SO42− species with the interaction with Fe oxides during the process of reaction.

4. CONCLUSIONS A series of microwave coal-based active carbon catalysts loaded with metal oxides were prepared by the sol−gel method for the simultaneous catalytic hydrolysis of COS and CS2, and their catalytic hydrolysis performances at the relatively low temperatures of 50−70 °C were investigated. The modified MCAC simultaneously removed COS and CS2 very effectively, and H2S was also removed by activated carbon. The results show that modified MCAC with 5.0% Fe2O3 after calcining at 300 °C has 17061

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AUTHOR INFORMATION

Corresponding Author

*Tel: 86-871-5920508. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (50908110), the National High Technology Research and Development Program of China (2008AA062602), the China Postdoctoral Science Foundation (20090451431), the Young and Middle-aged Academic and Technical Back-up Personnel Program of Yun-nan Province (2007PY01-10), and the Analysis and Testing Foundation of Kunming University of Science and Technology.

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REFERENCES

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