Preparation of Carbon-Supported Zinc Ferrite and Its Performance in

Nov 6, 2012 - ... butyl mercaptan removal efficiency (222.29 mg/g), fast catalytic degradation rate, stable catalytic activity, and outstanding regene...
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Preparation of Carbon-Supported Zinc Ferrite and Its Performance in the Catalytic Degradation of Mercaptan Qihan Li,† Linzhou Zhuang,† Shuixia Chen,*,†,‡ Jianqiao Xu,† and Haichao Li† †

PCFM Lab, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, People's Republic of China ‡ DSAPM Lab, Materials Science Institute, Sun Yat-Sen University, Guangzhou 510275, People's Republic of China ABSTRACT: Zinc ferrite supported on porous carbon (ZFPC) was prepared by thermal conversion of a mixture of ferric nitrate, zinc chloride, and novolac resin and was used as a catalyst for the catalytic oxidation of mercaptan under alkali-free conditions. ZFPC exhibited a high butyl mercaptan removal efficiency (222.29 mg/g), fast catalytic degradation rate, stable catalytic activity, and outstanding regenerative ability. It is believed that the effective butyl mercaptan removal was achieved by the synergism between adsorption of porous carbon and catalysis of zinc ferrite. The mechanism of catalytic oxidation of mercaptan into disulfides by ZFPC was further discussed based on the contrast experiment. The results indicated that oxidation of mercaptan in the presence of ZFPC followed the Merox process, in which Zn(II) acted as basic sites and Fe(III) acted as oxidation sites.

1. INTRODUCTION

carbon was prepared for butyl mercaptan oxidation, and the mechanism of catalysis was further studied.

Organosulfur compounds presented in gases for energy applications either are deliberately added as a safety precaution due to their low odor thresholds or naturally exist in gases. However, mercaptans are a strong poison for reformer and fuel cell materials, and also corrosive to metals because of their acidity on the condition that a high quality of gases is required as feedstock in applications such as hydrogen production for fuel cells. Therefore, it is necessary to remove these compounds either by extracting with porous material1−3 or by transforming them to innocuous disulfides.4−7 This transformation process is usually referred to as sweetening in the petroleum industry.8 Many different sulfur tolerances have been reported as the materials during the sweetening process.9−12 Among these materials, two varieties of materials have been of catholic concern on the development of the mercaptan oxidation under adequate conditions. One is the cobalt phthalocyanine composite materials13,14 that have been widely studied and used in both the petroleum industry for mercaptan removal and the commercial-scale synthesis of disulfides. Besides the disadvantage of the high price, these disulfide oxidation reactions require strongly alkaline conditions, which may result in corrosion and pollution problems. The other materials are copper-based materials. These kinds of materials have received a lot of attention during recent years.15−17 However, the copper-based materials are less efficient and sulfur is often observed to leak from this type of guard bed.18,19 ZnFe2O4 is an excellent material for hot gas desulfurization with a H2S removal efficiency down to the milligram per liter level, high sulfur loading capacity, and high reactivity.20,21 Its catalytic performance of mercaptan oxidation at ambient temperature in the liquid phase is rarely studied, though it exhibits marvelous catalytic properties at high temperature in the gaseous phase. In this study, ZnFe2O4 supported on porous © 2012 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Material Preparation. A mixture of novolac resin (2123, Shanghai Qinan Viscose Rayon Material Factory) and polyethylene glycol (abbr. PEG, Mw = 4000, Guangdong Xilong Chemical Co., Ltd.), zinc chloride (98+%, Guangzhou Chemical Reagent Factory), and ferric nitrate (99+%, Guangzhou Chemical Reagent Factory) were dissolved in ethanol. Glass fiber mats (R-93, Changzhou Changhai Glass Fiber Reinforced Plastic Products Co., Ltd.) were then dipcoated with the obtained solution, dried, and preheated to 200 °C for 4 h in an air oven. The stabilized mats were then heated with a rate of 10 °C/min to a preset temperature in a N2 atmosphere and kept for a certain period for carbonization and activation. After being cooled in flowing N2, each sample was thoroughly washed with dilute ammonia and distilled water to remove ZnO derived from ZnCl2, and then dried in vacuum at 110 °C for 24 h. The obtained product, ZnFe2O4 supported on porous carbon, was denoted as ZFPC. For structure comparison, the ZFPC composite was soaked in HCl for 24 h and washed; the resulted product was denoted as ZFPC-AW. 2.2. Characterization. Powder X-ray diffraction (XRD) profiles were collected in the 2θ angle between 10° and 70°, at a step width of 0.02° and by counting 10 s at each step with an X-ray diffractometer (D8 ADVANCE, BRÜ CKNER Textile Technologies GmbH & Co. KG), which was equipped with a copper-monochromatized Cu Kα radiation (λ = 0.15406 nm) under the accelerating voltage of 40 kV and the current of 40 mA. X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was employed to assess the chemical state and surface composition of the materials. Elemental composition of Zn and Fe was determined with a Hitachi Z-2000 flame atomic absorption spectrometer (AAS). Pore structure parameters of the materials, including Brunauer− Emmett−Teller (BET) specific surface area and Barrett−Joyner− Received: September 7, 2012 Revised: November 6, 2012 Published: November 6, 2012 7092

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Halenda (BJH) pore size distribution, were determined according to the nitrogen adsorption−desorption isotherms measured at 77 K at the range of relative pressure from 10−6 to 1 using an automatic gas adsorption instrument (ASAP2010, Micromeritics Corp). All samples were degassed at 100 °C for 8 h before the measurement. 2.3. Catalytic Oxidation Experiments. A batch test was developed for easy measurement of catalytic activity. For this test, a synthetic butyl mercaptan solution was prepared through adding butyl mercaptan to hexane to achieve a concentration of 300 mg/L. A certain amount of ZFPC was added into 50 mL of butyl mercaptan solution. The mixture was then stirred at 25 °C in a rotary shaker at a speed of 140 rounds per minute. The residual butyl mercaptan concentration was determined with an Agilent 6820 gas chromatograph (GC) equipped with a flame ionization detection. For continuous catalysis experiments, a certain amount of ZFPC was packed into a glass column with a diameter of 1 cm and a bed depth of around 6 cm. Butyl mercaptan solution was pumped with a peristaltic pump through the fixed bed with a specified flow rate of 2.5 mL/min at 25 °C. Air was also purged into the fixed bed. Effluent was collected at regular intervals, and the concentration of butyl mercaptan was determined with the GC method mentioned above.

Figure 2. Adsorption−desorption isotherms of N2 at 77 K on ZFPC and ZFPC-AW (prepared by carbonizing at 450 °C for 1.5 h).

3. RESULT AND DISCUSSION 3.1. Characterization of ZFPC Composite. The XRD pattern of the ZFPC composite is shown in Figure 1. For the

Table 2. Pore Structure Parameters of ZFPC and ZFPC-AW (Prepared by Carbonizing at 450 °C for 1.5 h) materials ZFPC ZFPC-AW

Stotal (m2/g)a Smicro (m2/g)a 781 452

570 334

Smeso (m2/g)a

Vtotal (cm3/g)

25 33

0.38 0.21

a

Smicro is the specific surface area of micropores calculated through the t-plot method. Smeso and Stotal were calculated through the BJH method and BET method. Vtotal was calculated based on the nitrogen amount adsorbed at p/p0 = 0.95.

Figure 1. Powder X-ray diffraction patterns of the ZFPC and ZFPCAW (prepared by carbonizing at 450 °C for 1.5 h).

Table 1. Elemental Composition of ZFPC and ZFPC-AW Estimated with XPS and AAS (Prepared by Carbonizing at 450 °C for 1.5 h) sample

method

Zn (wt %)

Fe (wt %)

Zn/Fe (mol/mol)

ZFPC ZFPC ZFPC-AW

XPS AAS XPS

15.8 25.0 3.3

5.1 6.6 1.2

2.7 3.3 2.3

Figure 3. Relationship between contact time and butyl mercaptan removal by ZFPC (prepared by carbonizing at 450 °C for 1.5 h).

Table 3. Kinetic Parameters and Correlation Coefficients for ZFPC R2

k

sake of comparison, the pattern of ZFPC-AW is also shown in the same figure. The broad peak at about 2θ = 25° is associated with the C (200) plane. The diffraction pattern of ZFPC showed peaks at approximately 2θ = 30, 35, 43, 56.5, and 62°, which were the main characteristic peaks of crystalline ZnFe2O4. The intensity of the strongest peak corresponding to the diffraction plane (311) of the ZnFe2O4 was measured as high as 900 cps, indicating a large crystal formation. No separate phase for iron oxide or ZnO species was observed in

pseudo-first-order reaction rate equation pseudo-second-order reaction rate equation

−3

5.83 × 10 8.22 × 10−6

0.9505 0.9972

the XRD pattern. However, the possibility for a trace of iron or zinc oxide, or the presence of an amorphous phase, cannot be ruled out because they are out of the XRD detection limit. Therefore, XPS and AAS analyses were employed for further studies on the composition of the material. 7093

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Table 4. Batch Test Parameters and Butyl Mercaptan Removal Efficiency by ZFPC and ZFPC-AW (Prepared by Carbonizing at 450 °C for 1.5 h)

ZFPC ZFPCAW

mass (g)

initial concn of butyl mercaptan (mg/L)

residual concn of butyl mercaptan (mg/L)

contact time (h)

removal amount of butyl mercaptan (mg/g)

0.05 0.05

300 300

77.71 238.86

12 12

222.29 61.14

Figure 6. Effect of carbonization temperature on butyl mercaptan removal (prepared by carbonizing for 1.5 h).

Figure 4. Dynamic removal behavior of butyl mercaptan by ZFPC (prepared by carbonizing at 450 °C for 1.5 h).

Figure 7. Effect of carbonization time on butyl mercaptan removal (prepared by carbonizing at 450 °C).

Table 5. Comparison of Sulfur Removal Result between ZFPC and Catalysts in References catalyst CuO/AC Cu/AC Co−Fe magnetic composite ZFPC

Figure 5. Regeneration behavior of ZFPC (prepared by carbonizing at 450 °C for 1.5 h).

S removal efficiency (mg/g)

TOFa (min−1)

ref

49.2

24 25 8

28.0 60.8

78.2

55.3

a

TOF: the average turnover frequency, which means the total reactions initiated by individual reactive site per unit of time.

Table 1 shows the composition of Zn and Fe estimated from XPS and AAS results for the ZFPC. Both results reflected that there was more zinc than iron element in the ZFPC composite, which was different from the theoretical Zn/Fe atomic ratio of 0.5 in ZnFe2O4. The result might imply the existence of some other zinc compounds, such as amorphous ZnO, as a separate phase. These zinc compounds, which could not be detected by XRD, could not be removed by NH3 solution thoroughly. The amounts of Fe calculated from the two methods were similar, whereas those of Zn were quite different. It was assumed that most of the Fe grew on the surface of the carbon layer, while

some Zn was embedded inside the carbon. For the ZFPC-AW, most of the zinc and iron were removed by the HCl wash, but there was still some zinc and iron remaining, which could be detected by XPS. It was deduced that some metallic ions might be adsorbed by carbon and some of them may be buried in the inner layers of carbon. The porosity of ZFPC was confirmed by nitrogen adsorption−desorption experiment at 77 K (Figure 2). As depicted in Table 2, the ZFPC is a highly porous material with a higher BET specific surface area (781 m2/g) and a large pore 7094

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of ZFPC decreased after it was treated with HCl, which was different from other studies in ref 11. Therefore, it is assumed that, besides pores in the carbon layer, interstices derived from the stacking of the ZnFe2O4 particles also contributed to the surface area of ZFPC. 3.2. Reaction and Regeneration Behavior of ZFPC Composite in Butyl Mercaptan Removal. The relationship between butyl mercaptan removal by ZFPC and contact time in batch tests is presented in Figure 3, which depicts the tendency of butyl mercaptan removal by the ZFPC composite. At the initial stage, removal of butyl mercaptan ascended sharply with time and achieved 80% of removal efficiency in 7 h, followed by a slow increase. The dynamic butyl mercaptan removal data were fitted by the pseudo-first-order reaction rate equation (eq 1) and the pseudo-second-order reaction rate equation (2) lg(Ce − Ct ) = lg Ce −

Figure 8. Removal mass of butyl mercaptan by ZFPC, ZnOPC, and ZFPC-AW (prepared by carbonizing at 450 °C for 1.5 h).

t 1 t = + 2 Ct Ce k 2Ce

Scheme 1. Reaction Mechanism of Merox Process

k1 t 2.303

(1)

(2)

where t is the contact time; k1 and k2 are the pseudo-first-order reaction rate equation constant and pseudo-second-order reaction rate equation constant, respectively; Ce is the butyl mercaptan concentration (mg/L) in the solution at equilibrium; and Ct is the butyl mercaptan concentration in the solution at time t. The reaction rate equation constants and correlation coefficients based on the simulation results are presented in Table 3. The pseudo-second-order reaction rate equation fitted the data obviously better than the pseudo-first-order reaction rate equation, as a higher R2 in the pseudo-second-order reaction rate equation model. As Table 4 illustrates, the removal amount of butyl mercaptan by ZFPC (222.29 mg/g) was much higher than that by ZFPC-AW (61.14 mg/g). For ZFPC-AW, it was

volume (0.38 cm3/g). On the other side, a much higher micropore surface area (calculated by the t-plot method) compared with the mesopore surface area (by the BJH method) indicated that there existed a considerable amount of micropores in the ZFPC composite. The specific surface area

Figure 9. Suggested reaction mechanism for butyl mercaptan removal over ZFPC composite. 7095

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Figure 10. Retention time of the solution components in GC.

The economic feasibility of the catalyst in practical applications strongly relies on its regeneration ability during the catalysis−regeneration process. The regeneration process was conducted by heating the deactivated ZFPC at 450 °C for 30 min under a N2 flow. The regenerated ZFPC was then subjected to successive catalysis−regeneration cycles to test its recycle ability. The performance is shown in Figure 5. The regenerated ZFPC presented no significant decline on removal efficiency after four cycles, indicating that it had good chemical stability and could be regenerated efficiently. 3.3. Effects of Preparation Condition on the Reaction Activity of ZFPC. Effects of carbonization temperature and carbonization time on the reaction activity were investigated to optimize preparation conditions of the composite. High carbonization temperature played a positive role in reaction performance as the ZnFe2O4 was inclined to form at a higher temperature. However, the carbon layer would be burnt off at high temperature, which lowered the loading amount of ZnFe2O4. That was why the mass of butyl mercaptan removal by ZFPC prepared at 550 °C decreased (Figure 6). A long carbonization time would be advantageous to the growth of ZnFe2O4, thus enhancing the removal capacity for butyl mercaptan (Figure 7). The removal mass of butyl mercaptan by ZFPC increased from 200 to 275 mg/g as the carbonization time was extended from 30 to 150 min. The butyl mercaptan removal efficiency by ZFPC was compared with other types of catalysts, including copper catalysts and a cobalt catalyst. The comparison results are listed in Table 5. From the results presented in Table 5, it is evident that the copper catalysts (metallic copper or copper oxide supported on active carbon) showed lower sulfur removal efficiency compared with the ZFPC. The cobalt catalyst (a Co− Fe magnetic composite) showed no catalytic activity in “nonbasic solvents”; it also had a lower TOF (33−49 min−1) in the “basic solvent” than ZFPC. 3.4. Catalytic Mechanism of ZFPC. As mentioned above, the ZFPC composite consisted of ZnFe2O4 and amorphous ZnO. ZnO supported on porous carbon without iron (ZnOPC) was prepared and subjected to a batch test to clarify the difference on catalysis property between ZnFe2O4 and

assumed that butyl mercaptan was simply removed due to the adsorption effect caused by the microporous structure of the carbon matrix. However, the removal amount of butyl mercaptan by ZFPC under the same condition was almost 4 times as high as that by ZFPC-AW. Therefore, it was concluded that butyl mercaptan might be catalytically degraded by ZnFe2O4, besides adsorption by the porous carbon matrix. In a word, it was conceived that the synergism between adsorption and catalysis of ZFPC achieved the effective butyl mercaptan removal. A dynamic removal experiment was conducted to model the fixed-bed sweetening process, in which butyl mercaptan solution with an initial concentration of 300 mg/L was pumped with a peristaltic pump through the fixed bed at a specified flow rate of 2.5 mL/min. The breakthrough curve is shown in Figure 4. It is evident that the ZFPC composite with 14.2 wt % ZnFe2O4 loading showed an excellent removal efficiency, and it could completely remove all the butyl mercaptan in the first phase, in which the effluent concentration of butyl mercaptan could be kept at near 0 mg/L for 60 min. This is mainly attributed to the adsorption ability of ZFPC. The effluent concentration then rose fast to a concentration of about 200 mg/L, much lower than the initial concentration. The effluent concentration of butyl mercaptan kept constant at this value for over 750 min (over 400 bed volumes), in which the catalytic reaction played an important role and butyl mercaptan was catalytically degraded. Another ZFPC sample with a lower zinc ferrite loading (9.5 wt % ZnFe2O4 loading) also exhibited a fair removal efficiency of butyl mercaptan. However, its adsorption capacity and catalysis efficiency were lower than those of ZFPC with a higher ZnFe2O4 loading. After a shorter complete removal of butyl mercaptan, the effluent concentration of butyl mercaptan rose fast to 240 mg/L; then the effluent concentration of butyl mercaptan kept constant at this value. However, ZFPC-AW with a much lower ZnFe2O4 on the surface almost showed no catalytic activity, and a very low adsorption capacity for butyl mercaptan; the effluent concentration of butyl mercaptan approached its initial concentration in about 50 min. 7096

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Figure 11. Mass spectroscopy of the two peaks in GC with retention times of 2.16 and 6.81 min.

mercaptan to mercaptide anion by Zn2+, (step 2) contact between mercaptide anion and Fe3+ site, (step 3) electron abstraction from mercaptide anion to form mercapto radical, (step 4) electron abstraction from one more butyl mercaptan and formation of another mercapto radical, (step 5) formation of dibutyl disulfide and ZnFe2O4 catalyst with oxygen vacancy, (step 6) dissociation of oxygen on the gas phase to make up oxygen vacancy in the catalyst and reoxidation of multivalent cations. The formation of the catalysate BtSSBt was further confirmed by chromatography/mass spectroscopy (GC/MS). Two peaks were observed in the solution after the catalysis reaction, whose retention times were 2.16 and 6.81 min in Figure 10. For the peak with a retention time of 2.16 min, the largest molecular ion (at 90 m/z) was observed, which indicated that it corresponded to butyl mercaptan (Figure 11). The latter peak with a retention time of 6.81 min

amorphous ZnO for mercaptan oxidation (Figure 8). No obvious catalysis of the ZnOPC for the mercaptan oxidation reaction was observed, and the removal of butyl mercaptan from the liquid phase was mainly owed to the adsorption by porous carbon. Although the fundamental catalysis mechanism of butyl mercaptan removal has not been clearly elucidated, most studies agree that the mercaptan oxidation by many catalysts follows the reaction mechanism of the so-called Merox process, as in Scheme 1: It is widely accepted that this mechanism consists of two major steps: first, mercaptan is transformed to mercaptide anion by a base catalyst; second, mercaptide anion is oxidized to disulfide by a high value metal oxide.22,23 In our catalyst system, the Zn(II) offers basic sites and Fe(III) offers oxidation sites for the whole reaction, and the mechanism is conjectured as followed in Figure 9. The reaction mechanism consists of six sequential elementary steps: (step 1) transformation of butyl 7097

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the Science and Technology Project of Guangdong Province (2010B010800040, 2011B090400030).

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Figure 12. Powder X-ray diffraction patterns of the ZFPC catalyst before reaction, after reaction, and after regeneration.

corresponded to the catalysate BtSSBt with the largest molecular ion at 178 m/z. ZnFe2O4 with an oxygen vacancy exhibited strong reducibility and was easily reoxidized in the atmosphere. However, the dibutyl disulfide formed on the surface of the ZnFe2O4 would block the active sides, preventing oxygen permeating and contact between ZnFe2O4 and butyl mercaptan, which led to deactivating of ZFPC. This kind of ZnFe2O4 with an oxygen vacancy could be detected by XRD due to the change of interplanar distance of the crystallite (d) caused by the lattice imperfection. As shown in Figure 12, the peaks of the deactivated ZnFe2O4 shifted to a higher 2θ value, indicating that the interplanar distance of the crystallite decreased, which might be caused by an oxygen vacancy. At the stage of regeneration, dibutyl disulfide was burned off at high temperature. After the elimination of dibutyl disulfide, the ZnFe2O4 with an oxygen vacancy was reoxidized in the atmosphere, which was demonstrated by the reposition of the characteristic diffraction peaks.

4. CONCLUSIONS ZnFe2O4 supported on porous carbon was prepared in situ by the pyrogenation of a mixture of ZnCl2, Fe(NO3)3, and novolac resin. This composite with a highly microporous structure presented a high activity for butyl mercaptan removal from nhexane solution under alkali-free conditions. It is believed that the synergism between adsorption and catalysis of the composite achieved the effective butyl mercaptan removal. XRD analysis results of the composite indicated that oxidation of mercaptan on ZFPC followed the Merox process, in which Zn(II) acted as basic sites and Fe(III) acted as oxidation sites. During that process, butyl mercaptan was oxidized into dibutyl disulfide, which was confirmed by the GC-MS analysis.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-20-84034027. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 7098

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