Catalytic Combustion of Volatile Organic Compounds over a

(1, 2, 8, 9) Supported transition metal (such Cu,(1) Mn,(8, 9, 11) and Co(2, 16)) .... diffractometer using Cu Kα radiation (40 kV, 40 mA) with 2θ r...
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Catalytic Combustion of Volatile Organic Compounds over a Structured Zeolite Membrane Reactor Huanhao Chen, Huiping Zhang, and Ying Yan* School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong510640, PR China S Supporting Information *

ABSTRACT: A novel structured zeolite membrane reactor based on a Cu−Mn (1:6)/ZSM-5/PSSF catalyst was developed for catalytic combustion of volatile organic compounds (VOCs). First, the ZSM-5 membrane/PSSF (paper-like sintered stainless steel fibers) composite was fabricated by the wet layup papermaking/sintering process and secondary growth process. The copper/manganese binary oxides modified ZSM-5 membrane catalyst was synthesized by an incipient wetness impregnation method. Catalytic combustion of VOCs in single and binary components was investigated over these structured zeolite membrane reactors using different gas hourly space velocity (GHSV) (3822−11466 h−1). Results showed that the complete destruction of isopropyl alcohol or ethyl acetate in a single component can be achieved below the temperature of 300 °C. The study exhibited that the presence of toluene in the binary mixture had a slight effect on the conversion of isopropyl alcohol or ethyl acetate. Isopropyl alcohol and ethyl acetate were found to be more reactive than toluene in the binary mixture. A higher temperature above 300 °C was needed to totally destroy toluene in the binary mixture. The structured zeolite membrane reactor exhibited a relatively lower bed pressure drop, excellent reaction stability, reasonable mass/heat transfer efficiency, and excellent contacting efficiency as well as relatively lower diffusion resistance.

1. INTRODUCTION Volatile organic compounds (VOCs) have always been hazardous to human health and environment safety, emitting from outdoor sources (such as various transport vehicles and industrial processes) and indoor sources (such as household products).1−4 Several useful techniques have been developed for the removal of VOCs, such as adsorption5 and catalytic oxidation 6 as well as thermal incineration. 7 Catalytic combustion has been regarded as one of the most promising techniques for the VOCs removal because of relatively lower reaction temperature and high efficiency properties.1,2,7−11 It is well-known that the catalyst is the key factor in determining the effectiveness of this technique.7 Various catalytic materials have been widely used in the catalytic combustion of VOCs, such as supported noble metals (such as Pt12 and Pd7,13) and metal oxides10,14,15as well as supported metal oxides.1,2,8,9 Supported transition metal (such Cu,1 Mn,8,9,11 and Co2,16) oxides have been intensively studied due to their relatively cheaper properties and excellent catalytic activity as well as reasonable resistance to poisoning. Among these transition metal oxide supported catalysts, manganese and copper oxides have been reported to exhibit the sufficient catalytic activity for the oxidation of VOCs.8,17 Porous zeolite materials have been widely used as the catalyst support or catalysts for the catalytic combustion of VOCs because of their high surface area, uniform pore structures, and high hydrothermal stability, such as meso-porous materials (MCM-41, SBA-15) and microporous materials (beta, ZSM5).7,18,19 However, the phenomena of relatively lower mass/ heat transfer efficiency and lower contacting efficiency as well as a higher bed pressure drop exist in a traditional fixed bed reactor because of the use of pellet shaped or powder catalysts.20 In order to improve the reactor performance, © 2013 American Chemical Society

several structured catalytic materials have been intensively used to design a structured reactor due to their relatively lower bed pressure drop and higher contacting efficiency as well as excellent mass transfer ability, such as monolithic20 and zeolite membrane/SMF (sintered metal fiber)21 as well as honeycomb material.13 Zeolite membranes offer several outstanding advantages of high surface area and uniform pore structure, nonflammability, and reasonable mechanical strength as well as chemical and hydrothermal stability.22 Considering the improvement of the reactor performance, zeolite membrane reactors have been widely applied in the VOCs removal, offering great advantages of a low bed pressure drop and excellent mass transfer characteristics as well as reasonable contacting efficiency. For example, Nikolajsen et al.21 applied a structured fixed bed based on zeolite/sintered metal fiber in the low concentration VOC removal. A structured fixed bed reactor based on Pt zeolite coated cordierite foams for completely catalytic combustion of toluene was also implemented by Ribeiro and co-workers.20 In a previous work reported by Aguado and co-workers,23 ZSM-5 zeolite membrane reactors have been used in the combustion of VOCs at low concentrations. Navascues et al.24 have also explored the use of interesting zeolite coated microreactors in the catalytic combustion of VOCs at trace concentration levels. It can be found that these structured reactors have outstanding potential in the removal of VOCs. It is well-known that industrial emissions usually possess VOCs mixtures, consisting of different components such as Received: Revised: Accepted: Published: 12819

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Figure 1. Schematic of the structured zeolite membrane reactor based on Cu−Mn (1:6)/ZSM-5/PSSF catalysts used in this work for VOCs combustion.

process and secondary growth process according to our previous reports.31,32 The Cu−Mn(1:6)/ZSM-5/PSSF catalysts were prepared by an incipient wetness impregnation method. It involved the impregnation of ZSM-5/PSSF composites with the proper amount of Cu(NO3)2 and Mn(NO3)2 with the Cu/ Mn molar ratio of 1/6. Finally, the impregnated ZSM-5 membrane/PSSF samples were dried at 100 °C for 12 h and calcined at 500 °C for 6 h. 2.3. Catalytic Test. Experiments on the catalytic combustion of VOCs were carried out under atmospheric pressure in a conventional continuous-flow experimental system consisting of a stainless steel tube (10 mm i.d., 450 mm length). A structured zeolite membrane reactor for VOCs combustion was employed in our present work, as schematically depicted in Figure 1, consisting of a 10 mm i.d stainless steel reactor located inside an electrical furnace. As can be seen in Figure 1, the structured zeolite membrane reactor based on Cu−Mn (1:6)/ZSM-5/PSSF catalysts with a bed height of 1−3 cm was designed. The temperatures in the catalyst bed and the tubular electrical furnace were monitored automatically with Etype thermocouples. The vapor of VOCs was generated by passing air at a certain flow rate through the generator. The flow rates of the streams exiting the membrane reactor were checked by using a mass flow controller, which was located in the inlet of reactor. Each reaction temperature was kept 20 min until reaching the steady state of system, and the data were determined by using gas chromatographs (Agilent 7890A, Palo Alto, CA) equipped with FID detectors for the quantitative analysis of the reactants and products. The data were the average of at least three measurements. In the present work, only the final products were CO2 and H2O, and no other byproducts were observed. The conversion of VOCs (XVOCs, %) based on the VOCs consumption was calculated as follows

aromatic hydrocarbons (toluene), alkanes, and oxy-derivatives (isopropyl alcohol, ethyl acetate).25,26 However, most of the studies have been reported to investigate the catalytic behavior of catalysts for the oxidation of VOCs in a single component. Therefore, it is so important to investigate the catalytic behavior of catalysts for the oxidation of multicomponent mixture VOCs.26 Much literature has been reported to explore the catalytic behavior of catalysts for the removal of VOCs in a mixture.19,25−30 For example, Abdullah et al.27 investigated the catalytic combustion of the binary mixture (ethyl acetate and benzene) over Cr-ZSM-5 catalysts. It can be noted that ethyl acetate was found to be more reactive than benzene over CrZSM-5 catalysts, inhibiting the conversion of benzene in the binary mixture. Catalytic oxidation behavior of VOCs in a binary mixture (isopropyl alcohol and o-xylene) over the NaA zeolite catalyst has been also studied by Beauchet and coworkers.19,28 It has been confirmed that o-xylene had an inhibiting effect on the isopropyl alcohol destruction; however, the isopropyl alcohol had no effect on the conversion of oxylene. The main objectives of current work were to (1) design a novel structured zeolite membrane reactor based on Cu−Mn (1:6)/ZSM-5 membrane/PSSF (paper-like sintered stainless steel fibers) catalysts and (2) study the catalytic combustion of VOCs alone or in mixtures over this structured zeolite membrane reactor. Isopropyl alcohol, toluene, and ethyl acetate were chosen as the representatives of VOCs.

2. EXPERIMENTAL SECTION 2.1. Materials. Stainless steel fibers with an average diameter of 6.5 μm were purchased from Huitong Advanced Materials Company (Hunan, China). Tetrapropylammonium hydroxide (TPAOH, 25% in H2O) was purchased from SigmaAldrich. Tetraethoxysilane (TEOS, >99%), isopropyl alcohol (>99.5%), and toluene were purchased from Guangzhou Chemical Reagent Factory (Guangzhou, China). Ethanol (C2H5OH, >99.8%), ethyl acetate, and sodium aluminate (NaAlO2, Anhydrous) were all purchased from Sinopharm Chemical reagent Co., Ltd. (Beijing, China). The copper nitrate (Cu(NO3)2·3H2O) and manganese nitrate (Mn(NO3)2·4H2O) were obtained from Guangzhou Chemical Reagent Factory (Guangzhou, China). 2.2. Catalyst Preparation. Novel gradient porous ZSM-5/ PSSF (paper-like sintered stainless steel fibers) composites were prepared by the traditional wet layup papermaking

XVOCs =

C VOCs(in) − C VOCs(out) C VOCs(in)

× 100% (1)

where CVOCs(in) (mg/L) and CVOCs(out) (mg/L) are the concentrations of VOCs in the inlet and outlet gas, respectively. Long-term reaction stability of a structured zeolite membrane reactor for VOCs combustion was performed at the temperature of 260 °C and the Gas Hourly Space Velocity (GHSV) of 7643 h−1, and the long-term stability experiment of structured zeolite membrane reactor was evaluated for 50 h. 2.4. Bed Pressure Drop Measurements. The sintered stainless steel fibers support and ZSM-5 membrane/PSSF 12820

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Figure 2. Photograph (a), SEM images (d−f), XRD patterns (b), and H2-TPR profile (c) of the Cu−Mn (1:6)/ZSM-5/PSSF catalyst.

composites as well as fine ZSM-5 particles (40−60 mesh) were employed to measure the bed pressure drop in the same stainless steel reactor at room temperature. The thickness of the test beds was 20 mm, and the cross-sectional area of the reactor was 0.785 cm2; the gas flow rate was increased from 32 to 3020 mL/min in order to vary the face velocity from 0.69 to 70 cm/s. Meanwhile, the bed pressure drop over the test beds was measured by using a microdifferential pressure gauge with 5000-50 PA (Foshan, China). 2.5. Characterization of Catalysts. The morphologies of the samples were observed by scanning electron microscopy (SEM, Hitachi S-3700N). All of the samples were coated with an ultrathin film of gold to make them conductive before analysis. X-ray diffraction (XRD) patterns of samples were recorded on a D8 Advance (Bruker Co.) diffractometer using Cu Kα radiation (40 kV, 40 mA) with 2θ range of 5−80°. The X-ray photoelectron spectra (XPS) results were obtained using a Kratos Axis Ultra (DLD) spectrometer with an Al Kα (1486.6 eV) radiation source operated at 15 kV and 10 mA. The binding energy (BE) of the C1s peak at 284.6 eV was taken as a reference. Temperature programmed reduction (TPR) tests were performed on Quantachrom Automated Chemisorption Analyzer. 50 mg of each sample was loaded into the reactor and purged with 30 mL/min of helium at 300 °C for 1 h to eliminate contaminants and then cooled down to room temperature. The temperature was increased to 700 °C at a heating rate of 10 °C/min with flowing of 10% H2 and 90% Ar.

properties of Cu−Mn (1:6)/ZSM-5/PSSF catalysts are all presented in Figure 2. As can be seen in Figure 2(b), besides the diffraction peaks attributed to the stainless steel fibers, there are diffraction peaks appearing at the ranges of 2θ = 7−9° and 2θ = 23−25°, which match well with the standard pattern of the ZSM-5 zeolite according to the literature.33 The reduction properties of fresh and used Cu−Mn (1:6)/ZSM-5/PSSF catalysts were also investigated by H2-TPR, and the results are shown in Figure 2(c). It is obviously observed in Figure 2(c) that there is a wide peak in the range of 150−350 °C for the fresh and used Cu−Mn (1:6)/ZSM-5/PSSF catalysts, indicating that the catalysts possess excellent reduction properties. The SEM images in Figure 2 (d−f) clearly show that the stainless steel fibers were completely sintered together to form many three-dimensional network structures, and continuous ZSM-5 membranes were covered on the surface of stainless steel fibers; the copper/manganese binary oxides catalysts were well dispersed on the surface of the ZSM-5 membrane. In addition, SEM and XPS results are also presented in the Supporting Information. As can be seen in Figure S2 (A), there is an asymmetrical Cu 2p3/2 signal (BE = ca. 933 eV) for fresh Cu−Mn(1:6)/ZSM-5/PSSF (a) and used Cu−Mn(1:6)/ZSM5/PSSF (b), which could be deconvoluted into two components at BE = 933.5 (Cu1+ species) and 955.0 eV (Cu2+ species). In the XPS spectra of Mn 2p shown in Figure S2 (B), there are two main asymmetrical peaks (Mn 2p3/2 and Mn 2p1/2 peaking at about 641.1 and 652.4 eV), which can be deconvoluted into two components at BE = 641.5 or 652.9 eV (Mn3+ species) and 643.0 or 654.5 eV (Mn4+ species). It can be observed in Figure S2 (C) that the peak at lower binding energy (about 529.8 eV) is assigned to lattice oxygen, and the

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. The surface morphologies and structural and textural properties as well as reduction 12821

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peak at higher binding energy (532.5 eV) is assigned to the chemisorbed oxygen. 3.2. Bed Pressure Drop Measurements. For the design of the structured zeolite membrane reactor, the bed pressure drop is also an important consideration. As shown in Figure 3,

Figure 4. Catalytic combustion performances for isopropyl alcohol over the structured zeolite membrane reactor based on Cu−Mn(1:6)/ ZSM-5/PSSF catalysts with different bed heights (4.7 mg/L of isopropyl alcohol in the feed gas, GHSV of 7643 h−1 in all cases).

ZSM-5/PSSF catalysts increased. Although the T50% for isopropyl alcohol over the structured zeolite membrane reactor with a bed height of 3 cm was lower than that of the structured zeolite membrane reactor with a bed height of 2 cm, the T90% for isopropyl alcohol over the structured zeolite membrane reactor with a bed height of 3 cm is higher. The structured zeolite membrane reactor with a bed height of 2 cm showed the best catalytic activity for isopropyl alcohol combustion. Therefore, the structured zeolite membrane reactor with bed height of 2 cm was selected for measuring the catalytic performances of VOCs combustion. 3.3.2. Combustion of Single VOCs (Isopropyl Alcohol, Ethyl Acetate). The catalytic performance for single VOCs (isopropyl alcohol, ethyl acetate) combustion over the structured zeolite membrane reactor were investigated using different inlet concentrations (isopropyl alchohol: 3.7−6.8 mg/L, ethyl acetate: 3.1−11.1 mg/L) at a constant GHSV of 7643 h−1. Figure 5 shows that the T50% and T90% for VOCs (isopropyl alcohol, ethyl acetate) over the structured zeolite membrane reactor decrease slightly as the initial concentrations of VOCs decreased. The possible reasons are that the treated VOCs quantity in a unit catalysts bed increases at a higher inlet concentration of VOCs. However, the unit catalysts bed in a structured zeolite membrane reactor possesses limited active sites for VOCs combustion, which leads to a decrease of catalytic combustion conversion of VOCs (isopropyl alcohol, ethyl acetate). Both conversions of isopropyl alcohol and ethyl acetate over a structured zeolite membrane reactor can reach 100% below the temperature of 300 °C. To investigate the effects of the flow rate on the catalytic performance of a structured zeolite membrane reactor, the conversions of single VOCs (isopropyl alcohol, ethyl acetate) over a structured zeolite membrane reactor at different temperatures were measured using different flow rates (3822−11466 h−1) but at constant inlet concentration (isopropyl alcohol: 4.7 mg/L, ethyl acetate: 5.3 mg/L). Plots of conversion versus temperature for VOCs (isopropyl alcohol, ethyl acetate) are presented in Figure 6. Figure 6 shows that the T50% and T90% for VOCs (isopropyl alcohol, ethyl acetate) over a structured zeolite membrane reactor decrease slightly as the flow rate decreased. The possible explanations are that the

Figure 3. Pressure drop versus face velocity for different materials.

with the increase in fiber dosage at a particular velocity, the bed pressure drop increased. It can be found that a higher dosage of stainless steel fibers leads to a lower permeability and a higher bed pressure drop. The ZSM-5 membranes covered on the surface of stainless steel fibers also lead to a higher bed pressure drop. Considering the mechanical strength and bed pressure drop, the selection of the optimum fibers dosage for preparing PSSF was 6 g. Plots of the bed pressure drop versus face velocity of air across the ZSM-5 membranes bed and the traditional packed bed are also shown in the insert of Figure 3. As shown in Figure 3, for the same face velocity (25 cm/s), a pressure drop of 52 Pa was produced over per mm of ZSM-5 membranes bed, being about one second of the pressure drop (125 Pa) which produced over per mm of the traditional packed bed. It can be concluded that a very low bed pressure drop was produced in our ZSM-5 membrane/PSSF composite due to its high permeability, which might be attributed to the large void volume and open structure of three-dimensional sintered stainless steel fibers network. 3.3. Combustion of Single VOCs. Catalytic oxidation of single VOCs (isopropyl alcohol, ethyl acetate) was investigated over a structured zeolite membrane reactor based on Cu−Mn (1:6)/ZSM-5/PSSF catalysts. First, the effects of the bed height of Cu−Mn (1:6)/ZSM-5/PSSF catalysts on the catalytic performance were studied. Second, single VOCs (isopropyl alcohol, ethyl acetate) combustion was carried out over the structured zeolite membrane reactor using different flow rates at three inlet concentrations. Finally, the reaction stability of the structured zeolite membrane reactor for single VOCs (isopropyl alcohol, ethyl acetate) combustion was also measured at the reaction temperature of 260 °C and GHSV of 7643 h−1. 3.3.1. Effect of Bed Height. Catalytic combustion of isopropyl alcohol was studied over the structured zeolite membrane reactor with different bed heights, and all the conversion profiles are presented in Figure 4. It can be observed from Figure 4 that the T50% and T90% (the value of the temperature at conversion approaches 50% and 90%) for isopropyl alcohol decrease as the bed height of Cu−Mn(1:6)/ 12822

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Figure 6. Catalytic performance for a single VOCs combustion over a structured zeolite membrane reactor at different GHSV: (A) isopropyl alcohol (GHSV of 3822−11466 h−1, 4.7 mg/L of isopropyl alcohol in the feed gas in all cases) and (B) ethyl acetate (GHSV of 3822−11466 h−1, 5.3 mg/L of ethyl acetate in the feed gas in all cases).

Figure 5. Catalytic performance for single VOCs combustion over a structured zeolite membrane reactor at different inlet concentrations: (A) isopropyl alcohol (3.7−6.8 mg/L of isopropyl alcohol, GHSV of 7643 h−1 in all cases) and (B) ethyl acetate (3.1−11.1 mg/L of ethyl acetate in the feed gas, GHSV of 7643 h−1 in all cases).

residence time of VOCs molecules in the structured zeolite membrane reactor was extended, and the contacting efficiency was obviously enhanced because of a relatively lower flow rate. In other words, for a higher flow rate, VOCs molecules had insufficient time to contact with active sites of catalyst. Although some differences in conversion of VOCs can be observed at low and intermediate temperatures, complete conversion was achieved essentially for the three values of GHSV at temperatures above 240 °C. The possible reasons are that Cu−Mn (1:6)/ZSM-5/PSSF catalysts possessed a higher contacting efficiency and enhanced mass/heat transfer as well as a shorter diffusion path due to the unique surface and uniform pore properties of the ZSM-5 zeolite membrane. 3.3.3. Reaction Stability of a Structured Zeolite Membrane Reactor. The reaction stability of a structured zeolite membrane reactor for single VOCs (isopropyl alcohol, ethyl acetate) combustion was also investigated, and the results are shown in Figure 7. Figure 7 shows the conversions of single VOCs (isopropyl alcohol, ethyl acetate) as a function of timeon-stream at 260 °C for 50 h. The experimental results indicate that the structured zeolite membrane reactor showed excellent reaction stability for single VOCs (isopropyl alcohol, ethyl acetate) combustion in 50 h, and the conversions of VOCs always remained above 94% at 260 °C. Therefore, the structured zeolite membrane reactor has huge practical application prospects for VOCs elimination.

Figure 7. Reaction stability tests for a single VOCs combustion with time-on-stream over a structured zeolite membrane reactor based on Cu−Mn(1:6)/ZSM-5/PSSF catalysts with a 2 cm bed height (4.7 mg/ L of isopropyl alcohol or 5.3 mg/L of ethyl acetate in the feed gas, reaction temperature of 260 °C, GHSV of 7643 h−1).

3.4. Combustion of Binary Mixtures. It has been wellknown that industrial emissions usually contain VOCs mixtures.30 Therefore, it is of interest to investigate the 12823

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catalytic combustion performance of catalysts for simple mixtures of VOCs. In our present work, catalytic combustion behaviors of binary VOCs mixtures (isopropyl alcohol and toluene, ethyl acetate and toluene) over structured zeolite membrane reactor were also carried out. Catalytic combustion of isopropyl alcohol or ethyl acetate with the addition of gradual concentration of toluene was performed using a constant flow rate. The effects of the flow rate on the catalytic performance of structured zeolite membrane reactor for binary VOCs mixtures were also investigated. 3.4.1. Effect of Toluene on Isopropyl Alcohol Combustion. The catalytic combustion of binary mixture (isopropyl alcohol and toluene) over a structured zeolite membrane reactor was investigated using different inlet concentration ratios (6.6 mg/L of isopropyl alcohol, 3.7−14 mg/L of toluene) but at a constant space velocity of 7643 h−1. The catalytic oxidation of a binary mixture (isopropyl alcohol and toluene) over a structured zeolite membrane reactor was also studied using different flow rates (3822−11466 h−1) at a constant inlet concentration (6.6 mg/L of isopropyl alcohol, 6.7 mg/L of toluene). The conversions of isopropyl alcohol and toluene in the mixture as a function of temperature are presented in Figures 8 and 9.

Figure 9. Conversion of binary VOCs mixtures (isopropyl alcohol and toluene) over a structured zeolite membrane reactor (GHSV of 3822− 11466 h−1, 6.6 mg/L of isopropyl alcohol, and 6.7 mg/L of toluene in the feed mixture gas in all cases).

in the channel of the ZSM-5 zeolite membrane (0.5 nm diameter), and the contacting efficiency between isopropyl alcohol molecules and active sites was higher than that of toluene. On the other hand, as can be seen in Figure 9, the T50% and T90% for isopropyl alcohol and toluene in the binary mixture are slightly influenced by enhancing the space velocity from 3822 h−1 to 7643 h−1, but the conversion curve is shifted to higher temperatures by approximately 20 °C at a higher space velocity of 11466 h−1. Similar behavior for toluene combustion in the binary mixture can be observed in Figure 9. 3.4.2. Effect of Toluene on Ethyl Acetate Combustion. The catalytic combustion of a binary mixture (8.9 mg/L of ethyl acetate plus 3.9−13.9 mg/L of toluene) over a structured zeolite membrane reactor was also investigated at a constant space velocity of 7643 h−1. The effects of the flow rate of the binary mixture on the catalytic behavior were also studied by changing the space velocity from 3822 h−1 to 11466 h−1 but at a constant inlet concentration ratio (ethyl acetate: 8.9 mg/L, toluene: 7.3 mg/L). Figure 10 shows that the complete conversion of ethyl acetate in the binary mixture was achieved at 300 °C. It also can be observed in Figure 10 that there are no significant variations of the T50% and T90% for ethyl acetate as the inlet concentration of toluene in the binary mixture increased. These results reveal that the addition of toluene in the binary mixture had a minimal inhibition effect on the conversion of ethyl acetate. Moreover, the conversion curves of toluene are shifted slightly to higher temperatures as the inlet concentration of toluene increased, and a higher temperature above 300 °C was also needed to totally destroy toluene in the binary mixture. Similar phenomenon has been observed by Abdullah et al.27 during catalytic oxidation of a binary VOCs mixture (ethyl acetate and benzene) over the Cr-ZSM-5 catalyst. It can be concluded that the destruction of ethyl acetate in the binary mixture was easier than that of toluene, which should be attributed to its nucleophilic property and smaller kinetic diameter as well as linear molecule type. Moreover, the increasing inlet concentration of toluene (with a constant ethyl acetate inlet concentration) leads to a decrease of the toluene conversion; however, there are no significant changes on the conversion of ethyl acetate, which could be the

Figure 8. Conversion of binary VOCs mixtures (isopropyl alcohol and toluene) over a structured zeolite membrane reactor (6.6 mg/L of isopropyl alcohol and 3.7−14 mg/L of toluene in the feed mixture gas, GHSV of 7643 h−1 in all cases).

Figure 8 shows that isopropyl alcohol in a binary mixture was all completely destroyed below 300 °C over a structured zeolite membrane reactor. The result is in agreement with the observation in Figure 5 that the complete destruction of a single isopropyl alcohol was also achieved below the temperature of 300 °C. It seems that the addition of toluene had no effect on the isopropyl alcohol conversion. Figure 5 also indicates that no significant variations of the T50% and T90% for isopropyl alcohol are observed when a gradual concentration of toluene (3.7−14 mg/L) was added. However, the T50% and T90% for toluene are shifted to higher temperatures by 10−20 °C as the inlet concentration of toluene in the binary mixture increased, and a higher temperature above 300 °C was needed to totally destroy toluene in binary mixture. According to the literature reported by Beauchet,19 the kinetic diameter of isopropyl alcohol (0.47 nm) is smaller than that of toluene (0.58 nm). Another possible explanation is that the linear type molecule of isopropyl alcohol enabled it to diffuse much easier 12824

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zeolite membrane reactor were carried out at different inlet concentrations and space velocity. The results demonstrated that the total destruction of single VOCs (isopropyl alcohol or ethyl acetate) was all achieved below the temperature of 300 °C. The catalytic behaviors of binary VOCs mixtures (isopropyl alcohol and toluene, ethyl acetate and toluene) were also investigated over a structured zeolite membrane reactor. Isopropyl alcohol and ethyl acetate were found to be more reactive than toluene in the binary mixture. These can be explained by the fact that isopropyl alcohol and ethyl acetate possess linear molecule type and smaller kinetic diameter as well as nucleophilic property. Moreover, the structured zeolite membrane reactor showed excellent reaction stability for VOCs combustion. Therefore, these structured zeolite membrane reactors possess a huge application potential for eliminating the industrial VOCs emissions because of their relatively lower bed pressure drop, reasonable mass/heat transfer efficiency, and excellent contacting efficiency as well as relatively lower diffusion resistance.

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Figure 10. Conversion of binary VOCs mixtures (ethyl acetate and toluene) over a structured zeolite membrane reactor (8.9 mg/L of ethyl acetate and 3.9−13.9 mg/L of toluene in the feed mixture gas, GHSV of 7643 h−1 in all cases).

ASSOCIATED CONTENT

* Supporting Information S

Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

■

result of a competitive adsorption between ethyl acetate and toluene on the same active sites of the catalyst.27 The effects of the flow rate on the catalytic performance of the binary VOCs mixture (ethyl acetate and toluene) over a structured zeolite membrane reactor were also studied. Results in Figure 11 show that a negative effect of flow rates (3822−11466 h−1) on VOCs in binary mixture conversion is obtained, shifting the conversion curves to higher temperatures by 10−20 °C.

AUTHOR INFORMATION

Corresponding Author

*Phone: +86 2087111975. Fax: +86 2087111975. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS A novel structured zeolite membrane reactor based on the Cu− Mn (1:6)/ZSM-5/PSSF catalyst was developed for VOCs removal. The catalytic combustion performances of single VOCs (isopropyl alcohol or ethyl acetate) over a structured

ACKNOWLEDGMENTS We gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 21176086).

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REFERENCES

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Figure 11. Conversion of binary VOCs mixtures (ethyl acetate and toluene) over a structured zeolite membrane reactor (GHSV of 3822− 11466 h−1, 8.9 mg/L of ethyl acetate ,and 7.3 mg/L of toluene in the feed mixture gas in all cases). 12825

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dx.doi.org/10.1021/ie401882w | Ind. Eng. Chem. Res. 2013, 52, 12819−12826