Methane Emissions Abatement by Multi-Ion-Exchanged Zeolite A

Aug 26, 2008 - The performance of multimetal-(Cu, Cr, Zn, Ni, and Co)-ion-exchanged zeolite A prepared from both a commercial-grade sample and one ...
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Environ. Sci. Technol. 2008, 42, 7392–7397

Methane Emissions Abatement by Multi-Ion-Exchanged Zeolite A Prepared from Both Commercial-Grade Zeolite and Coal Fly Ash K. S. HUI AND C. Y. H. CHAO* Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

Received April 23, 2008. Revised manuscript received July 21, 2008. Accepted July 22, 2008.

The performance of multimetal-(Cu, Cr, Zn, Ni, and Co)-ionexchanged zeolite A prepared from both a commercial-grade sample and one produced from coal fly ash in methane emissions abatement was evaluated in this study. The ion-exchange process was used to load the metal ions in zeolite A samples. The methane conversion efficiency by the samples was studied under various parameters including the amount of metal loading (7.3-19.4 wt%), reaction temperature (25-500 °C), space velocity (8400-41 900 h-1), and methane concentration (0.5-3.2 vol %). At 500 °C, the original commercial-grade zeolite A catalyzed 3% of the methane only, whereas the addition of different percentages of metals in the sample enhanced the methane conversion efficiency by 40-85%. Greater methane conversion was observed by increasing the percentage of metals added to the zeolite even though the BET surface area of the zeolite consequently decreased. Higher percentage methane conversion over the multi-ion-exchanged samples was observed at lower space velocities indicating the importance of the mass diffusion of reactants and products in the zeolite. Compared to the multi-ion-exchanged zeolite A prepared from the commercial-grade zeolite, the one produced from coal fly ash demonstrated similar performances in methane emissions abatement, showing the potential use of this low cost recycled material in gaseous pollutant treatment.

Introduction Methane is a potent greenhouse gas with a global warming potential (GWP) 23 times greater than carbon dioxide (mole basis, 100 year time frame) (1). It is estimated that 60% of global methane emissions are related to human-related activities (2). Low-concentration methane (0.5-3.2 vol %) is commonly found in emissions from biological waste, wastewater treatment facilities, coal mine ventilation systems, and natural-gas-fueled vehicles (3). Reducing methane emissions would lead to substantial environmental and economic benefits. Metal-oxides of Cu, Cr, Ni, Co, and Zn are effective in methane emission abatement (4). Studies have also shown that the catalytic activity of multicomponent samples is superior to that of single-component samples (5). One study * Corresponding author phone: +852 2358-7210; Fax: +852 23581543; e-mail: [email protected]. 7392

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(6) reported that multi-ion-exchanged zeolite X materials were 2-34 times more active in methane conversion than the single-ion-exchanged and the original zeolite X materials under the conditions tested. The reaction mechanism of methane oxidation with zeolite can be briefly described in a few steps (6). Oxygen molecules are first adsorbed on the ions sites which can be alkali ions, alkaline earth metal ions, transition metal ions, or hydrogen ions. Dissociations of the adsorbed oxygen to form atomic oxygen then occur. Methane molecules are then adsorbed on the atomic oxygen. Finally, reactions between the adsorbed methane and the atomic oxygen proceed to form carbon dioxide and water. The methane oxidation activity is mainly influenced by the type of ions in the zeolite. Different types of ions in the zeolite can lead to different activation energies in the reaction (7). Studies have shown that zeolite A, having spherical cages with 11.4 Å diameters, can be used to adsorb methane (molecular dimension 3.9 Å) (8). In theory, zeolite A should be a good material for loading active metal species in its framework for methane emissions abatement because it has the highest cation exchange capacity (4 meq/q) among other zeolites (zeolite X, Y, mordenite, ZSM-5, -11, and -48; 2-3 meq/g) (9). However, multimetal-ion-exchanged zeolite A materials have not yet been studied for their methane removal capability. This is the first study to explore the use of multi-ion-(Cu, Cr, Zn, Ni, and Co)-exchanged zeolite A materials in methane emissions abatement. These five metals were chosen because of their high activity for methane oxidation at low concentrations (4, 7). Understanding the potential of these materials in methane oxidation will allow its use for methane emissions control from some sources in which methane cannot be easily captured for use as an energy source, e.g., natural-gas-fueled vehicles. Both a commercial-grade sample and one produced from coal fly ash were used in preparing the multi-ionexchanged zeolite A materials. The annual production of coal fly ash by coal-fired power plants throughout the world is about 800 million tons (10). This amount is predicted to increase in the future. At present, recycling coal fly ash has become a very important issue in waste management (11). The ion-exchange capacity or uptake ability of zeolites is 2-4 times higher than that of coal fly ash (fly ash has the uptake capacity of 1 meq/g) (12). Thus, the production of zeolites from coal fly ash has attracted attention (13, 14) due to its practical applications in removing metal ions or ammonia from water or in adsorption of SO2 (15). However, these studies reported long conversion time (10-72 h or more) in producing zeolites from the ash. Moreover, the zeolites thus produced contained certain amounts of residual fly ash, which limited their effectiveness. A novel preparation method has recently been reported to produce zeolite A (without residual fly ash) from coal fly ash with a short conversion time (6.5 h) (16). The zeolite samples produced from this method can be used to generate multi-ionexchanged zeolite A for methane removal. This study began by investigating the effect of the amount of metal loading in a commercial-grade zeolite A on methane conversion efficiency at various reaction temperatures, which is a critical parameter for catalytic performance. The best performing multi-ion-exchanged zeolite A sample and the most effective reaction temperature at which the percentage of methane conversion was high were then selected for the subsequent experiments. The methane conversion efficiency from the best multi-ion-exchanged zeolite A sample was compared with that of each single-ion-exchanged sample. Influences of space velocity and methane concentration on 10.1021/es801099y CCC: $40.75

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the methane conversion efficiency over the sample were then investigated. Finally, the methane conversion efficiency of the multi-ion-exchanged zeolite A sample prepared from coal fly ash was studied, which was compared with the performance of the sample prepared from the commercial-grade zeolite A.

Experimental Methods Loading of the Multi-Ions in the Commercial-Grade Zeolite A. Loading of the multi-ions in the commercial-grade zeolite A with the mean particle size of 2 µm in diameter (Valfor 100 from PQ Chemicals Limited Company), denoted as CZA (commercial-grade zeolite A), was achieved through an ionexchange method (17). The ion-exchange experiments, simultaneously performed in multi-ion solutions, were based on the information from our earlier study (18) that the selectivity sequence and also the metal-loading capacity of the ions in zeolite A were dependent on the initial concentrations of the metal ions and the initial pH of the solution. Briefly, the selectivity sequence was Cu2+ > Cr3+ > Zn2+ > Co2+ > Ni2+ by zeolite A at the initial pH of 3. In the ionexchange experiments, 0.5 g of zeolite A was mixed with 100 mL of the multi-ion solution containing 0.8, 1.7, or 5 mM of Cr3+, Cu2+, Zn2+, Ni2+, and Co2+ ions. These solutions were prepared by dissolving analytic-grade nitrate salts in deionized water. The initial pH of the mixture was adjusted to 3 by adding a few drops of 2% HNO3. The mixture was mechanically stirred at 600 rpm at 25 °C for 24 h. No significant adsorption of the metal ions was found after 24 h of stirring. After the ion-exchange process, the final pH values of the solutions were in the range of 4.7-5.6. This is because the zeolite samples neutralized the solutions by acting as a proton acceptor (18, 19). No precipitation of metal hydroxides was found. The ion-exchanged zeolite A powder was then filtered and washed by stirring in 1 L deionized water for 30 min. The zeolite A was then air-dried at 110 °C for 12 h. The multi-ion-exchanged zeolite A prepared using CZA containing 0.8, 1.7, or 5 mM of each ion was labeled as CZA1, CZA2, and CZA3, respectively. Production of Zeolite A from Coal Fly Ash. This study adopted a novel conversion method to produce a pure form, single-phase, and high-crystalline zeolite A from coal fly ash (FA) (16). Compared with the zeolite A prepared under a constant synthesis temperature, the adopted method reduced the total conversion time by half-without compromising its quality. Briefly, a mixture of 30 g of FA and 300 mL of 2 M NaOH solution in a 1 L sealed polypropylene bottle was kept in an oil bath at 100 °C for 2 h under stirred conditions (300 rpm). Then, the solution was separated from the mixture by a filtration process. The molar ratio of SiO2/Al2O3:Na2O/SiO2: H2O/Na2O in the solution was adjusted to 1.64:8.09:56.51 by adding 100 mL of aluminum solution. With the addition of the aluminum solution, single-phase, and high-crystalline zeolite A could be easily produced from the solution compared to the case without the addition of the aluminum solution. The solution was then stirred (500 rpm) for 30 min at room temperature (25 °C) and kept at its first reaction temperature of 90 °C for 1.5 h and subsequently at the second reaction temperature of 95 °C for 2.5 h. The precipitated sample was separated from the mixture by a filtration process and washed with deionized water until the pH of the solution was around 10. The sample was kept in an oven and dried at 100 °C for 12 h. In this study, it took 6.5 h to produce 7.5 g of zeolite A powder from FA, denoted as FAZA (fly ash zeolite A). Detailed information about the production of zeolite A from FA can be found in our previous work (16). Moreover, another type of zeolite, Na-P1, was found on the outer surface of the residual FA which showed good performance in wastewater metal ions removal (18).

Characterization of the Sample. The elemental composition of the sample was determined by an X-ray reflective fluorescence spectrometer (XRF, JSX 3201Z). The particle size measurement of the sample was performed by a laser beam scattering technique (Coulter LS230). The type of zeolite was characterized by powder X-ray diffraction (XRD) using a Philips PW 1830 diffractometer with Cu KR radiation (1.5406 Å). SEM images of the sample were obtained using a scanning electron microscopy (JEOL-6300). The sample was gold coated prior to the measurements. The BET surface of the sample was measured using the Coulter SA3100 nitrogen physical adsorption apparatus. Prior to the experiments, the sample was dehydrated at 150 °C for 2 h. Catalytic Combustion of Methane. A schematic diagram of the experimental setup for this study is shown in Figure S1 of the Supporting Information (SI). The methane conversion was evaluated in a 3.9 mm inner diameter and 600 mm long cylindrical quartz tube reactor heated by a temperaturecontrolled chamber, as used in our previous study (6). Samples, in powder form (bed length of 1.5 cm), were loaded in the middle of the tube that was plugged with quartz wool at the two ends. The tube was heated by infrared heaters placed above and under the tube. A K-type thermocouple was fixed to the middle of the sample bed to measure the reaction temperature and to control the reaction temperature to the set-point temperature. Before being loaded into the reactor, the sample was pretreated to ensure that its catalytic activity was representative, not due to the fact that it was “fresh”. First, the sample was exposed to an air stream at a space velocity of 41 900 h-1 at 500 °C for 1 h (heated from room temperature to 500 °C at a rate of 10 °C/min). The space velocity was defined as the total volume flow at the test temperature divided by the volume of the sample bed. Our previous works (17, 20) have shown that 500 °C was high enough to desorb any chemically adsorbed species in the sample. The sample was then aged at a space velocity of 41 900 h-1 with 1 vol % methane for 6 h at 500 °C. In all our tests, the relative humidity in the reactant stream was around 65%. Methane conversion percentage was assessed on the basis of the percentage of carbon dioxide produced from the equation (CH4 + 2O2 f CO2 + 2H20) under steady-state conditions. The reported values are the average of two sets of independent experiments using two different batches of pretreated sample. No methane conversion was detected in any of the tests using the empty reactor. No CO was detected in any of the tests.

Results and Discussion Removal of Methane with Multi-Ion-Exchanged Commercial-Grade Zeolite A Samples. Table 1 shows the elemental composition and the BET surface area of the CZA and the multi-ion-exchanged zeolite A samples (CZA1-CZA3). The metal loading capacity in the sample increased with the concentration of the ion-exchange solution. A clear sorption sequence of metals in the CZA3 sample was observed under high concentration of metal ions. Factors such as free energy of hydration, hydrated radii, and concentration of the metal ions as well as sorption mechanism of the ions in the sample are responsible for the observed sorption sequence, as reported in our previous study (18). The decrease in the BET surface area with the amount of metal loading was due to the blockage of the pore structure of zeolite A by the exchanged metal ions. Effect of Reaction Temperature on Activity of Methane Conversion. Figure 1 shows the influence of the reaction temperature on methane conversion with all samples (each sample had a mass of 0.18 g). The tests were carried out with an inlet methane concentration of 0.5 vol%, a fixed inlet volume flow rate of the reactant gases (the corresponding VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Elemental Composition of the Samples and Their BET Surface Areas sample wt % Si Al Na Cu Cr Zn Ni Co Fe

CZA

FA

FAZA FAZA3

20.9 20.1 19.3 17 23.4 20.3 18.5 18.3 17.7 15.8 13.2 18.9 14.9 9.3 5.1 0.9 0.1 14.6 n.d.a 1.5 2.5 7.4 n.d. n.d. n.d. 1.4 2.6 7.2 n.d. n.d. n.d. 1.5 2.6 3.1 n.d. n.d. n.d. 1.5 2.6 0.7 n.d. n.d. n.d. 1.5 2.8 1 n.d. n.d. n.d. n.d. n.d. n.d. 5.3 0.6

BET surface area (m2 g-1) 71 a

CZA1 CZA2 CZA3

44

40

38

1.4

66

16.4 16.1 0.6 6.7 6.4 3.8 1.4 1.6 0.5 37

n.d. ) not detected.

FIGURE 2. Methane conversion percentage of the multi-ionand the single-ion-exchanged samples.

FIGURE 1. The influence of reaction temperature on methane conversion %. inlet space velocity was 16 200 h-1 as calculated at 25 °C) and a reaction temperature between 25 and 500 °C. The residence time of the gases in the empty reactor decreased from 0.23 to 0.09 s when the reaction temperature increased from 25 to 500 °C. When no sample was loaded in the reactor, no methane conversion was observed under the gas phase reaction between methane and air. The methane conversion percentage increased with the reaction temperature with all samples even though the residence time was reduced. Below the reaction temperature of 370 °C, no methane conversion with any samples was observed. With the CZA, methane conversion was observed at a reaction temperature of 474 °C and it increased from 0.5 to 3% as the temperature increased from 474 to 500 °C. Compared to the CZA, the multi-ion-exchanged samples showed greater methane conversion. The methane conversion efficiency was 0.5-85% greater with the multi-ion-exchanged CZA under the same reaction temperatures. This suggests that the addition of multi-ions to CZA can significantly improve the methane conversion efficiency compared with the original CZA. This result agrees with our previous findings for zeolite X samples (6). Among the multi-ion-exchanged samples, the results showed that the methane conversion percentage increased from sample CZA1 to CZA3. This observation suggests that methane conversion depends on the metal loading which plays an important role in affecting the activation energy for methane oxidation, the active sites and the BET surface area of the sample. Increasing the amount of metal loading in the sample decreased the activation energy for methane oxidation 7394

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and also the BET surface area of the sample (6). The results showed that the positive effect of metal loading in zeolite A was more dominating than the reduced BET surface area effect on methane conversion as can be seen from the performance of CZA3 which showed the highest methane conversion percentage but the smallest BET surface area among the samples (Table 1). The Effect of Multi-Ion-Exchanged Samples Compared with Single-Ion-Exchanged Samples. From the previous results, the effective temperature of 500 °C was chosen in the subsequent experiments to further investigate the effect of the multi-ion-exchanged samples compared with the effect on single-ion-exchanged samples. In this set of experiments, the inlet methane concentration was fixed at 1.1 vol % and the space velocity was 41 900 h-1. Since there is no study of multi-ion-exchanged zeolite A samples versus single-ionexchanged zeolite A samples in methane conversion published in the literature, we tested the hypothesis that multiion-exchanged samples outperform single-ion-exchanged samples. Five single-ion-exchanged zeolite A samples, each with the same amount of metal loading (17 mol %) as CZA3, were prepared and tested. Figure 2 shows the methane conversion percentage of these samples. Under the same condition tested, no methane conversion was observed over the CZA sample. The single-ion-exchanged CZA samples were effective in methane removal compared to the CZA sample. The multi-ion-exchanged sample, CZA3, outperformed the single-ion-exchanged samples even though Cu and Cr were known to be the most active elements among the tested elements in zeolite X in methane oxidation (7). Both the activity of the metals in methane oxidation and the location of the metal ions within the zeolite structure (which was affected by the preferred coordination of the metal ions in the zeolite) played a role in affecting the methane conversion efficiency (7). Compared to the methane conversion efficiency reported using zeolite X (Cu-X achieved higher methane conversion percentage than Cr-X) (7), the different activity trend observed in the current study (Cr-CZA > Cu-CZA) suggests that the type of zeolite support also affects the methane conversion percentage. The good activity of the multi-ion-exchanged sample in methane oxidation may be due to synergistic effects, which seem to be a general phenomenon in a number of catalytic reactions with mixtures of many different metals and metal oxides in different samples (5, 6). The enhanced removal efficiency was attributed to the differences in the catalytic active sites of the multicomponents in samples. Although no study has systematically investigated the effect of the number of multicomponents added in the sample on the enhanced methane removal efficiency, it is expected that the removal efficiency will be enhanced if more

FIGURE 3. The influence of space velocity on methane conversion percentage. active metal ions are added in the sample on the conditions that the total metal loading and the BET surface area of the samples are the same. It is also expected that the overall performance in methane removal efficiency will be different if other transition metals such as Mn, Ti, etc. are tested. However, these areas remain poorly understood and deserve more investigations. The Effect of Space Velocity and Inlet Methane Concentration. Experiments were conducted under different space velocities (8400-41 900 h-1) and inlet methane concentration of 1.1 vol % at reaction temperature of 500 °C. Figure 3 shows that higher percentages of methane conversion were achieved at lower space velocities with the CZA3 sample. This may have been due to the increased residence time for reactant molecules to diffuse to the active sites of the zeolite structure, and for diffusion of desorbed carbon dioxide and water from the active sites. This agrees with our previous study (6) showing that the diffusion limitation was predominant at 350-550 °C. Experiments were then conducted to investigate the influence of methane concentration under different space velocities on methane conversion efficiency by the CZA3 sample. The results are summarized in Table 2. The methane reaction rates with the CZA3 sample (a mass of 0.18 g) were calculated based on the amount of carbon dioxide produced under the conditions tested. The methane reaction rates (in terms of mmol of methane converted/(g of sample x s)) increased with the inlet methane

concentration under each space velocity. This was because the methane conversion rate depends on the concentration of the reactants. More methane reacted when the inlet methane concentration was higher. However, methane conversion decreased at higher inlet methane concentrations. This outcome was expected since the total number of active sites in the sample remained unchanged (the same amount of sample was used in the various experiments). Thus, when the amount of methane increased, there were not enough active sites available for the reactions at the same residence times (6). The Zeolite A Produced from Fly Ash (FAZA). The main components of the FA and the FAZA were oxides of Si and Al and various metallic oxides as shown in Table 1. Fe in the FAZA was due to the incorporation of Fe during the crystallization process. Soluble oxide of Fe was dissolved in the alkaline solution during the extraction of the Si from the FA. The FA was obtained from a power plant in the southern part of China with pretreatment at 120 °C for 30 min before each experiment. The FA particles were spherical with smooth surfaces varying from 0.04 to 600 µm in diameter (mean ) 20.7 µm). The XRD pattern of the FA is shown in Figure S2 of the SI. Quartz, mullite and trace amount of calcite, portlandite, anhydrite, hematite, and gehlenite were identified as crystalline substances in the FA. The amorphous phases (a hump from 15 to 35 ° of 2θ) in the FA may come from the amorphous inorganic materials formed during combustion of the coal and/or the organic matter in the unburned coal (fly ashes contain 2-7% unburned coal) (14). The FAZA was identified as a single-phase zeolite A (JCPDS card 43-0142) from the XRD pattern as shown in SI Figure S2. The very weak background hump (from 20 to 35 ° of 2θ) suggests that the FAZA was not completely crystalline. The degree of crystallinity of the FAZA was determined to be 77% which was evaluated with respect to the CZA (16). The SEM results agreed with the XRD analysis that residual FA was not present in the FAZA. The results of the particle size analysis indicated that the mean size of the FAZA particles was around 2.5 µm in diameter. The XRD, SEM and particle size distribution results suggested that the FAZA made from the FA could be used in applications (requiring ion-exchange, adsorption of gas, and catalysis) in which the performance of the FAZA would not be impaired by the presence of nonactive, residual FA. The BET surface area of the FAZA was 66 m2 g-1, which was similar to the value reported in the literature (21).

TABLE 2. Methane Conversion Percentage with the Samples at 500 °C sample space velocity/ residence time (h-1)/(s) 8400/0.43 16 800/0.21 25 100/0.14 41 900/0.09

CZA3

controlb

FAZA3

inlet CH4 conc. (vol%)

con%

reaction ratea

con%

1.1 2.1 3.2 1.1 2.1 3.2 1.1 2.1 3.2 1.1 2.1 3.2

100 92 74 89 61 50 76 48 37 63 33 24

1.1 × 10-3 1.9 × 10-3 2.3 × 10-3 1.9 × 10-3 2.5 × 10-3 3.1 × 10-3 2.5 × 10-3 3.0 × 10-3 3.5 × 10-3 3.4 × 10-3 3.4 × 10-3 3.8 × 10-3

84 68 58 66 53 42 59 49 34 45 22 22

reaction rate 9.1 1.4 1.8 1.4 2.2 2.7 1.9 3.0 3.2 2.4 2.3 3.5

× × × × × × × × × × × ×

10-4 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3

reaction rate 1.1 2.1 3.1 2.2 4.1 6.3 3.2 6.2 9.4 5.4 1.0 1.6

× × × × × × × × × × × ×

10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-2 10-2

heat releasec (W) 5 9 14 10 18 28 14 28 42 24 46 70

a The unit of reaction rate is (mmol methane converted /(g of sample x s). b Assumes a sample achieves 100% methane conversion. c This was calculated assuming a sample achieves 100% methane conversion, and heat of methane combustion is 802 kJ/mol.

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Removal of Methane with Multi-Ion-Exchanged Zeolite A Sample Produced from Coal Fly Ash. Since the CZA3 showed the highest percentage of methane conversion under the conditions tested, the same approach used to prepare the CZA3 was used to load metal ions in the FAZA. The sample was denoted as FAZA3 and its elemental composition is given in Table 1. The results show that the selectivity sequence of the metal ions by FAZA3 was the same as by CZA3. Compared to the metal loading capacity of the metals in CZA3, lower loading of Cr and Cu and higher loading of Zn, Ni, and Co in the FAZA3 were observed. These observations are in agreement with our previous study (18) indicating that the selectivity sequence and the metal loading capacity of the ions in zeolite A depend on the properties of the sample. Experiments were conducted to compare the methane conversion efficiency of FAZA3 with that of CZA3 under the conditions tested in Figure 3 and Table 2. Similar to the CZA3, a higher percentage methane conversion with the FAZA3 was achieved at lower space velocity, as shown in Figure 3. As the inlet methane concentration increased, the reaction rate of methane per unit mass of sample increased but the methane conversion decreased, as shown in Table 2. The results in Table 2 showed that the CZA3 achieved higher methane conversion percentages (2-24%) than the FAZA3 under the conditions tested. These differences were attributed to the different percentages of metals loaded in the sample and the particle size of the sample. Taking into account the FAZA3 achieving a high methane conversion of 84% at low space velocity (such as 8400 h-1), applications of the FAZA3 in low-concentration methane emissions abatement under low space velocity seem to be promising. However, the FA source and the trace impurities contained in recycled zeolite A may limit its performance. The Activation Energy for Methane Oxidation. The activation energy of methane oxidation with the CZA and the multi-ion-exchanged samples was determined following the approach by Neyestanaki et al. (22) and Hui et al. (6). The tests were conducted with conditions (methane concentration of 0.5 vol %, constant space velocity of 16 200 h-1, reaction temperature of 370-540 °C, and sample mass of 0.18 g) intended to limit the methane conversion to less than 10% in order to minimize the mass diffusion effect. The reactor was considered as an integral flow reactor system. Methane oxidation with the samples was assumed to follow pseudofirst-order kinetics with respect to methane and zeroorder kinetics with respect to oxygen and the methane conversion percentage in relation to the temperature is expressed as eq 1 (6, 7). ln[-ln(1 - x)] ) -E/RT + ln(K0m ⁄ F)

(1)

where x, E, R, K0, m and F are the fractional methane conversion, the activation energy, the gas constant, the reaction rate constant, the mass of the catalyst, and the volumetric feed flow rate, respectively. The activation energy, E, was obtained from the linear part of the Arrhenius plot, as shown in Figure S3 of the SI. With the CZA, methane oxidation reaction occurred with an activation energy of 155 kJ mol-1. The activation energies were 119 and 120 kJ mol-1 with the CZA3 and the FAZA3, respectively. These activation energies are comparable to the reported values (60-145 kJ mol-1) in the literature (6, 7, 22). Compared to other hydrocarbons, methane is considered the most difficult one to oxidize (23) and is often chosen as a model compound for evaluating the performance of catalysts tested in many volatile organic compound (VOC) abatement studies (24). It seems that zeolite-based catalysts prepared from FA can be a low-cost VOC abatement option. The price of transition metals ions precursor per gram is cheaper than noble metals ions by 100-1000 times and that of FA recycled zeolite A can be as low as 1 US$/kg (25). The 7396

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cost of the multi-ion-exchanged zeolite A catalyst prepared from FA can then be at least 70% lower than the commercial 2% Pd/Al2O3 catalyst (11 US$/g). The big cost difference indicates potential applications of FA recycled zeolite in environmental industry.

Acknowledgments Funding for this research was provided by RGC Grant 611507 from the Hong Kong SAR Government.

Supporting Information Available A schematic diagram of the experimental setup (Figure S1). XRD pattern of the FA and the FAZA (Figure S2). Arrhenius plots for the methane conversion over the samples (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

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