Environ. Sci. Technol. 2007, 41, 4754-4760
Methanol Oxidation Using Ozone on Titania-Supported Vanadia Catalyst CATHERINE B. ALMQUIST,† E N D A L K A C H E W S A H L E - D E M E S S I E , * ,‡ SRIDARA CHANDRA SEHKER,‡ AND JULIA SOWASH† Paper Science and Chemical Engineering Department, Miami University, 246 Gaskill Hall, Oxford, Ohio 45056 and Office of Research and Development, National Risk Management Research Laboratories, U.S. Environmental Protection Agency, 26 W. Martin Luther King Drive (MS 443), Cincinnati, Ohio 45268
Ozone-enhanced catalytic oxidation of methanol has been conducted at mild temperatures of 100-250 °C using a V2O5/ TiO2 catalyst prepared by the sol-gel method. The catalyst was characterized using XRD, surface area measurements, and temperature-programmed desorption of methanol. The oxidation of methanol with ozone in the absence of a catalyst gave about 30% conversion at 100 °C. Methanol oxidation over a V2O5/TiO2 catalyst at 100 °C gave very little conversion with oxygen, whereas the conversion increased to 80% with ozone. Methanol, having an inlet stream concentration of 15 000 ppmv, can be completely oxidized to COx with an ozone-to-methanol ratio of 1.2, a temperature of 150 °C, and a gas hourly space velocity (GHSV) of 60 000 h-1. The apparent activation energy with ozone was calculated to be ca. 40 kJ/mol, which is much lower than that calculated with oxygen (60 kJ/mol). At low methanol conversion methyl formate was the main product, whereas higher conversions favored oxidation to COx. The results imply a consecutive reaction of adsorbed methanol species, favoring selectivity toward methyl formate at lower temperatures and ozone-to-methanol ratios and COx at higher temperatures and ozone-to-methanol ratios. LangmuirHinshelwood kinetics was used to model the reaction with and without ozone in the feed. The model parameters were obtained using least-squares fit to a selected set of experimental data, and the model was subsequently compared to all experimental data obtained in this study.
Introduction Existing and anticipated air pollution laws have forced the pulp and paper industry to reduce toxic pollutant releases. The combined air and water “cluster rule” provides individual mills with incentives to adopt control technologies that will result in reductions in toxic pollutant discharges beyond current discharge limits of volatile organic compounds (VOCs) by nearly 50% (1-3). Methanol from the recovery boiler, delignification systems, and wastewater treatment systems accounts for 70-80% of the total volatile organic emissions from these types of mills. Some sulfur compounds, such as CH3SH, CH3SCH3, and CH3SSCH3, are present in the * Corresponding author phone: (513) 569-7739; fax: (513) 5697677; e-mail:
[email protected]. † Miami University. ‡ U.S. Environmental Protection Agency. 4754
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 13, 2007
stream in the 1-5% range, as are low levels of terpenes, pinenes, and related plant oils (3-5). Integrated Kraft mills typically incinerate non-condensable gases (NCGs) using on-site boilers. This process requires extensive ductwork and energy. Significant energy savings could be realized by point-of-source treatment alternatives, specifically for the high-volume, low-concentration (HVLC) emissions of NCGs. Catalytic oxidation is a low-temperature alternative to incineration for the control of VOC emissions because of minimal formation of nitrogen oxides (NOx) and lower energy requirements. Potential challenges to implementing catalytic technologies are high cost and susceptibility to physical and chemical changes. Ozone is a strong oxidant in the gas phase and catalytic oxidation has opened new low-temperature reaction means with much higher rates and lower activation energies than catalytic oxidation with molecular oxygen. The goal of this study was to assess the feasibility of ozoneenhanced catalytic oxidation of low-concentration methanol emissions from pulp and paper mills. Ozone reduces the conversion temperatures for the oxidation of various VOCs, resulting in a cost-effective alternative technology (6, 7). Lowtemperature oxidation of benzene and aliphatic alcohols, using ozone over SiO2 and Al2O3, has been reported (6, 8). Commercial vanadia/titania (V2O5/TiO2) catalysts have been developed for the selective catalytic reduction (SCR) of NOx, primarily for application in power plants. These catalysts also have been found effective in the destruction of VOCs (9-11), extending to the oxidation of methanol on V2O5/ TiO2 catalysts (11-17). Supported V2O5/TiO2 catalysts also exhibit resistance against SO2 poisoning (18), which is important since there is a significant amount of total reduced sulfur emissions in HVLC emission streams from a Kraft mill (3, 4). This work presents a systematic investigation of the effects of ozone-enhanced oxidation of methanol on a V-Ti-O catalyst. Kinetics modeling and product distribution studies were conducted in attempt to gain deeper insight to the reaction mechanism.
Experimental Section Material and Catalyst Preparation. The methanol (AR grade, 99% Aldrich) and ultrahigh purity substrates for the synthesis of the catalyst were obtained from Aldrich. The nitrogen and oxygen gases were high purity (Matheson, USA 99.8%) and ultrahigh purity (Air Products >99.9%); no moisture traps were needed. Samples consisting of V2O5 coprecipitated with TiO2 were prepared by sol gel synthesis using 8.93 g of V2O5 (Aldrich USA >99%) dissolved in 300 mL of 10% H2O2 and precipitated with 56.97 g of Ti(i-Pr)4 (diluted in propanol) under continuous stirring at room temperature. The gel was washed with deionized water and vacuum-dried for 6 h. Then it was calcined at 350 °C for 3 h in oxygen. Catalyst Characterization. X-ray diffraction (XRD) analyses were performed to characterize the bulk properties of the catalyst. Powder diffraction patterns were recorded on a Siemens D5000 diffractometer using a Cu KR radiation source. Scans were performed over the 2θ range from 20° to 60° using a resolution of 0.017° and count time of 2 s at each point. The XRD was equipped with voltage and current stabilizers and a computer with necessary software to record the diffraction patterns. The surface area of the catalyst, fresh and used, was measured using nitrogen adsorption at 77 K and the Brunauer-Emmett-Teller (BET) method using a Micromer10.1021/es062518u CCC: $37.00
2007 American Chemical Society Published on Web 06/06/2007
etics (AutoChem 2920) surface area analyzer and a Beckman Coulter BET surface area analyzer. Prior to analyses, 0.2 g of sample catalysts were loaded into a quartz reactor and degassed at 120 °C with a helium purge for 60 min. A fivepoint nitrogen (Air Product >99.9%) adsorption isotherm was used to determine the BET surface area of the sample. Temperature-programmed desorption of methanol was performed on a sorption unit by a Micrometric Autochem 2920 Instrument. The catalyst (0.2 g) was charged in a stainless steel reactor and adsorbed by methanol at 80 °C under reaction conditions for 1 h and then charged into the U tube reactor for thermal desorption studies using helium as a carrier. Scanning electron microscopy and energy dispersive spectroscopy (EDS) X-ray microanalyses were performed on the catalyst sample to measure the mass and atomic ratios of vanadium and titanium in the catalyst. Generation and Quantification of Ozone. An annular gas-phase corona reactor was custom built (Ceramatec Inc., Salt Lake City, UT) for the generation of ozone. Flow of dry oxygen gas was controlled through the reactor, which can produce from 3 to 25 g/h of ozone. The corona reactor was cooled using a water jacket, and the maximum temperature in the ozone generator was less than 50 °C. The ozone dose to the reactor and residual amounts were measured using the iodometric titration method. The results were confirmed with a UV absorption technique ozone analyzer (model-OLA, Ozone Services, Yanco Insustries LTD) (19). The concentration of ozone produced at the different oxygen flow rates decreases slightly as the flow increases, however, the total amount of ozone produced increased as O2 increased. Ozone-Enhanced Catalytic Oxidation Experiments. A steady flow catalytic reactor test system was used during this study (Figure S-1, Supporting Information). Oxygen gas flow to the corona discharge ozone generator was maintained by mass flow controllers (model C100, Sierra Instruments Inc.). The ozone generation was also controlled using the potentiometer ozone generator. Methanol vapor was generated by purging nitrogen through a reservoir of liquid methanol at constant temperature (23 °C) and flow rate. The methanolsaturated nitrogen stream was diluted with a clean nitrogen stream to prevent condensation of the methanol and mixed with the ozone/oxygen stream just prior to the reactor. The total flow was directed either to the reactor or to a bypass line. The reactor was controlled at a pre-set temperature (100-350 °C). It was constructed of 0.9 cm i.d. stainless steel tubing and was 25 cm long. Two grams of catalyst mixed with glass beads (3 mm) were packed in the center of the reactor and secured with plugs of glass wool. The reactor was mounted vertically in an electrically heated split-furnace (Supleco). The gas hourly space velocity (based upon the catalyst volume) was varied between 18 000 and 160 000 h-1. The ozone to methanol mole ratio was varied from 0.06 to 2. Prior to the activity studies, the catalyst was activated at 150 °C in oxygen (50 cm3/min) for 30 min. The bypass line and effluent stream temperatures were also kept at 100 °C to allow for analysis of the gas stream prior to the reactor and to minimize the condensation of water and organics formed in the reactor. All VOCs were analyzed using an Agilent 6890 gas chromatograph GC/FID. Inorganic reaction products, such as CO and CO2 (COx) were quantified using GC/FID with a methanizer that uses a nickel catalyst (HP G2747A). Samples were introduced to the GC/FID either manually by taking syringe samples of the gas stream before and after the reactor and injecting them into the GC/FID, or automatically via gas sampling valves on the GC/FID. The product stream was separated with a Supleco-Wax-10 capillary column (30 m × 0.52 mm). Tedlar bag samples were collected at the exit of reactor for each set of experiments for qualitative analysis of reaction products using Thermo Finnegan Trace GC/MS
equipped with Supleco-Q plot column (30 m, 0.32 mm). Carbon mass balances were conducted for each set of experimental conditions and the methanol conversion and selectivity to oxidation byproducts were calculated. Model Development to Assess Experimental Results. The experimental data were further studied using a model based on Langmuir-Hinshelwood kinetics. The following steps were used to derive the kinetic expressions:
Step (1)
k1
2O3 + heat 98 3O2
(1)
k2
Step (2) R + O3 98 Products
(2)
k3
Step (3) R + S 798 RS
(3)
k4
Step (4)
RS + O3 98 Products
Step (5)
RS + O2 98 Products
(4)
k5
(5)
where R represents the organic pollutant, S represents an active site on the catalyst, and RS represents an active site occupied by the pollutant. Step 1 represents the degradation of ozone with temperature; step 2 represents the reaction of the organic pollutant with ozone in the absence of a catalyst; step 3 represents the adsorption and desorption of the organic pollutant on the catalyst; and steps 4 and 5 represent the degradation of the adsorbed pollutant by ozone and oxygen, respectively. The assumptions that were used in this model include the following: pseudo-steady-state (the adsorbed pollutant concentration is constant on the catalyst at each temperature); the number of active sites on the catalyst, ST, remains constant; and the oxygen concentration is constant since it is present in large excess of the pollutant. The equations used to develop the kinetic model include
d[R] ) k3[R][S] - k-3[RS] + k2[R][O3] dt
(6)
d[O3] ) k1[O3]2 + k2[R][O3] + k4[RS][O3] dt
(7)
-
-
-
d[RS] ) 0 ) - k3[R][S] + k-3[RS] + k4[RS][O3] + dt k5[RS][O2] (8) [ST] ) [S] + [RS]
(9)
The effects of temperature were assessed using the Arrhenius equation
ki ) Ai exp
(
)
- Eo,i RT
(10)
where Ai is the pre-exponential factor, Eo,i is the apparent activation energy of the reaction, T is the absolute temperature of the reaction, and R is the universal gas constant. This equation has considerable empirical justification since a linear relationship is most commonly observed between ln k and 1/T. The activation energy (Eo,i) can be evaluated from the slope of this linear relationship, -Eo,i/R. Noted in eq 7 is that the degradation of ozone on the catalyst in the absence of methanol was not included. The model was developed to predict the degradation of methanol with and without ozone in our test system. Therefore, it was assumed methanol would always be present in the inlet stream. VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4755
The final model for which the data were fit and for which the model parameters were calculated is comprised of two equations: an expression for the destruction of the organic (methanol), and an expression for the destruction of ozone.
-
k3k4[R][O3]ST + k3k5[R][O2]ST d[R] ) k2[R][O3] + (11) dt k-3 + k4[O3] + k5[O2] + k3[R]
d[O3] ) k1[O3]2 + k2[R][O3] + dt k3k4[R][O3]ST k-3 + k4[O3] + k5[O2] + k3[R]
(12)
Equations 11 and 12 were further simplified by using the literature and by making the following assumptions. (i) The thermal degradation of ozone follows the expressions cited by Oyama (7), and those expressions in eq 19 were used instead of the first term in eq 12. k1′,k-1′
O3 + M 798 O + O2 + M
(13)
k2 ′
O3 + O 98 2O2
(14)
where:
k1′ ) 4.6 × 1012 exp(-12 000/T) L mole-1 s-1 (15) k-1′ ) 2.2 × 108(T/300)-2.8 L2 mol-2 s-1
(16)
k2′ ) 4.8 × 109 exp(-2060/T) L mol-1 s-1
(17)
[M] ) [O3] + 0.44[O2] + 0.41[N2] + 1.06[CO2] + 0.34[He] (18) and the first term in eq 12 was changed from k1[O3]2 to:
-rozone,thermal )
2k1′k2′[O3]2
[
k-1′[O2] 1 +
k2′[O3] k-1′[O2][M]
]
(19)
(20)
(iii) The oxygen concentration is in large excess, and assumed constant. (iv) Assuming low concentrations of ozone and a constant oxygen concentration in large excess of the organic concentration [R], the second and third terms in eq 11 and the third term in eq 12 were simplified:
4756
9
-
-
d[R] ) ksurface[R][O3] + kcatalyst,ozone[R][O3] + dt kcatalyst,oxygen[R][O2] (23) d[O3] ) dt
2k1′k2′[O3]2
[
k-1′[O2] 1 +
k2′[O3]
]
+ ksurface[R][O3] +
k-1′[O2][M] kcatalyst,ozone[R][O3] (24)
The constants ksurface, kcatalyst,ozone, and kcatalyst,oxygen were assessed as functions of temperature using Arrhenius’ equation (eq 10) and a set of experimental data. Since the degradation of both ozone and methanol occurs as soon as they are mixed at the top of the reactor, and the catalytic oxidation of methanol, with and without ozone, occurs only on the catalyst surfaces, a discrete finite difference method was used to assess the degradation in the reactor. Several assumptions were used to set up the model as follows: (i) Constant temperature in the reactor; heats of reaction were not considered in the model. (ii) The reaction occurs only in the reactor, starting from the very top and ending at the bottom. No reaction outside of the reactor was considered. (iii) [M] in the thermal degradation of ozone was calculated considering only methanol, oxygen, nitrogen, and ozone. No reaction byproducts were considered. (iv) No reactions of partial oxidation products with ozone or on the catalyst surface were considered. An example of the model predictions for methanol degradation with and without ozone and with and without catalyst at 125 °C, ozone-to-methanol ratio ) 1.2, and gas hourly space velocity (GHSV) ) 60 000 h-1 is shown in Figure 1. In Figure 1, the methanol degradation is shown as a function of reactor length, and the catalyst was assumed to occupy 1.4 cm of reactor length in the center of the reactor. The model predictions at the end of the reactor were compared to all experimental data obtained during this study under various experimental conditions.
Results and Discussions
(ii) Oxygenates that do not contain an unsaturated carbon-carbon bond are largely unreactive with ozone in the gas phase (7). Therefore, purely gas-phase reactions between ozone and methanol were not considered in the model. However, in the test system, methanol degraded with ozone in the absence of a catalyst; it was assumed that the reaction was occurring on the surfaces. The surface reaction between ozone and methanol was assumed to occur as first order with respect to both methanol and ozone (20). Therefore, the first term in eq 11 and the second term in eq 12 were changed from k2[R][O3] to
-rsurface ) ksurface[R][O3]
Equations 11 and 12 were represented by the following expressions:
-rcatalyst,ozone ) kcatalyst,ozone[MeOH][O3]
(21)
-rcatalyst,oxygen ) kcatalyst,oxygen[MeOH][O2]
(22)
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 13, 2007
Catalyst Characterization. The X-ray diffraction patterns of the V2O5/TiO2 catalyst are shown in Figure 2a. The XRD patterns of the sol-gel catalyst show very weak diffraction signals of anatase (d-values: 2θ ) 25.3, 48.06, 53.9 ASTM 86-1157), corresponding to the diffraction plane (101) of the anatase TiO2 phase in all the calcined and used catalysts. No separate phase was observed in the XRD patterns for V2O5 species. However, the presence of V2O5 crystallites less than 5 nm cannot be ruled out because they are beyond the XRD detection limits. The BET surface area decreased with time-on-stream, from an initial surface area of 190 m2/g for the fresh catalyst to as low as 70 m2/g after several experimental trials. A detailed study of the reduction of BET surface area with time and temperature was not done in this study. Both the pre-set reaction temperatures up to 250 °C, and the localized “hot spots” due to the heat of reaction can result in catalyst sintering. Differences in the extent of sintering are due to differences in temperature and the time histories of the catalyst samples. SEM imaging analysis showed that the catalyst particles are large, ranging from approximately 10 to 50 µm, which indicates that the high surface area is predominantly within the pores (Figure S-2, Supporting Information). EDX was carried out on thin evaporated layer to determine the atomic composition of the catalyst. The catalyst had a V/Ti atomic
FIGURE 1. Model prediction of methanol degradation along length of reactor (GHSV ) 60 000 h-1; ozone-to-methanol ratio ) 1.2, temperature ) 125 °C).
FIGURE 3. Degradation of methanol under various conditions. (GHSV ) 60 000 h-1; ozone-to-methanol ratio ) 1.2; initial methanol concentration ) 15 000 ppm.).
FIGURE 2. Characterization of V2O5/TiO2 catalyst used in this study: (a) XRD spectra, (b) temperature programmed desorption (TPD). ratio of 0.47, which is equivalent to a V/Ti mass ratio of 0.50. This supports the nominal V/Ti mass ratio from
the catalyst synthesis method (Figure S-3, Supporting Information). The desorption behavior of methanol from the V2O5/TiO2 catalyst was studied using a Micrometrics TPD unit (Authochem 2920) to provide additional insight about the surface characteristics (Figure 2b). The TPD measurements of methanol from the catalyst surface yielded well-defined peaks at 84 and 150 °C. The small peak centered at 84 °C may be attributed to the desorption of physisorbed methanol. The peak centered at 150 °C, with an on-set temperature of 130 °C, may be attributed to the desorption of chemisorbed methanol. This corresponds to 150 °C, at which the maximum catalyst activity was observed in the presence of ozone. The TPD profile of methanol also supports the drop in selectivity VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4757
FIGURE 4. Effect of ozone-to-methanol ratio: (a) on methanol conversion at 125 °C, GHSV ) 60 000 h-1; line represents model prediction; (b) on products selectivity.
TABLE 1. Summary of Reaction Rate Constant Parameters Used in Model
ksystem kcatalyst,ozone kcatalyst,oxygen
pre-exponential factor, Ai
apparent activation energy, Eo,i
5.46 × 107 2.12 × 1010 1.17 ×1010
33.8 kJ/mol 38.7 kJ/mol 57.8 kJ/mol
for methyl formate at reaction temperatures above 150 °C since there is decreased condensation of methoxy species. Catalytic Oxidation of Methanol. A series of experiments was conducted to isolate the effects of the catalyst and ozone on methanol oxidation as a function of reaction temperature. The total flow rate was maintained at 1 L/min (gas hourly space velocity of 30 000 h-1), and the methanol concentration was 15 000 ppm, resulting in ozone-to-methanol ratio of approximately 1.2. The reaction temperature was increased by 50 °C intervals, from 100 to 350 °C for each set of reaction conditions. All tests were run with 2 g of catalyst (when used) and 1 g of 3 mm glass beads, which were secured in the reactor with plugs of quartz wool. Each set of experiments 4758
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 13, 2007
FIGURE 5. Effects of GHSV at selected temperatures and ozoneto-methanol ratio ) 1.2: (a) methanol degradation, and (b) selectivities to methyl formate and COx. involved four sets of conditions: (1) methanol oxidation using oxygen, (2) an ozone/oxygen mixture, (3) methanol oxidation with no catalyst but with ozone, and (4) both catalyst and ozone. The results of these experiments are shown in Figure 3. The combination of ozone and V2O5/TiO2 catalyst can degrade greater than 80% of the methanol at 100 °C and greater than 98% at 150 °C. Methanol, alone, is stable in O2 up to 250 °C, above which it begins to degrade. The catalyst with only O2 degrades more than 95% of the initial methanol at 250 °C. Ozone without the catalyst can degrade 35% methanol at temperatures as low as 100 °C. Its oxidizing effect appears to reach a plateau at approximately 150 °C. The data in Figure 3 were used to assess the rate constants, ksurface, kcatalyst,ozone, and kcatalyst,oxygen as functions of temperature using Arrhenius’ equation (eq 10). The pre-exponential factor and apparent activation energy for each rate constant were calculated and are summarized in Table 1. The model predictions were compared to the experimental data for various experimental conditions. Effects of Process Variables. Ozone-to-Methanol Ratio. The effects of ozone concentration on the conversion of methanol and product selectivity are shown in Figure 4a and b. The markers represent experimental data and the line represents the model predictions. A higher concentration of
FIGURE 6. Effects of temperature on ozone decomposition for catalytic and noncatalytic condition. Symbols show experimental data and curves represent model predictions for (I) in gas phase, (II) gas phase with methanol, (III) with catalyst and methanol.
FIGURE 7. Selectivity for reaction products as a function of methanol conversion for all the data acquired in this study. the oxidant in the test system resulted in increased and deeper oxidation of methanol. Methyl formate and COx were the
predominant byproducts of methanol oxidation. At ozone deficient conditions (ozone to methanol ratio of less than 0.2) the conversion of methanol is low and the high surface methanol concentration increases formation of methyl formate. As ozone concentrations increase, the catalyst surface is enriched with oxidant O22- and surface methanol concentrations are not high enough high to form methyl formate. The decrease in the selectivity for methyl formate and the excess ozone favors consecutive oxidation steps, leading to deep oxidation products COx (7, 20). Gas Hourly Space Velocity (GHSV). Figure 5a and b show the effects of GHSV on methanol conversion and selectivity to byproducts. GHSV is defined as the volume of gaseous feed (measured at room temperature and 1 atm pressure) passed through a unit volume of the reactor per hour. Figure 5a shows the experimental methanol conversion data (markers) versus the model predictions (line). The conversion of methanol decreases as GHSV increases. At higher temperatures, the selectivity toward the formation of methyl formate increases with GHSV (Figure 5b). These trends are expected since longer reaction times (lower GHSV) typically result in deeper oxidation. At low space velocities methanol conversions are almost constant over temperatures ranging from 100 to 200 °C with high yields of CO2 indicating the reaction is taking place on the surface of the catalyst and higher temperatures are assisting the deep oxidation. At high space velocities the methylformate is observed as a byproduct with a yield of ca. 40%. At increased space velocities the effect of heterogeneous catalysis is less prominent. Ozone Decomposition Studies. Stability and reactivity of ozone was studied as a function of temperature and the presence of methanol or the catalyst. Experimental data (markers) and model prediction (curves) of the decomposition of ozone under various conditions are shown in Figure 6. Thermal gas-phase decomposition of ozone was slow at low temperatures (Curve I) and decomposition increased with increase in temperature. In the presence of methanol, about 50% decomposition of ozone was observed at 150 °C and molar ozone concentrations of 0.05 and 0.19 mmol/L (Curve II). In the presence of the catalyst nearly complete decomposition of ozone was measured at 150 °C (Curve III). Steady-state decomposition of ozone on supported metal oxide catalyst involves dissociative adsorption of ozone to form an oxygen molecule and an atomic oxygen species (2224). This result supports the experimental observations that the effectiveness of ozone for methanol oxidation is optimal near 150 °C. Reaction Byproducts and Reaction Mechanism. Methanol oxidation can lead to various products, such as formaldehyde (CH2O), methyl formate (HCOOCH3), dimethyl ether (CH3-
FIGURE 8. Proposed reaction mechanism for ozone-enhanced catalytic oxidation of methanol. VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4759
OCH3), methylal ((CH3O)2CH2), and CO2, depending upon the catalyst used, the reaction temperature, and the reactant partial pressures (5, 9-17). In this study, only methyl formate and COx, along with methanol, were quantitatively measured in the effluent stream. A carbon balance (carbon output/ carbon input) was obtained for methanol, methyl formate, and COx. It averaged 87% ( 20% over all samples taken in the study. The decomposition of ozone on the catalyst surface generates active oxygen species that allow the oxidation of organic compounds to occur at lower temperatures (21). The selectivity to methyl formate and COx was plotted as a function of methanol conversion in Figure 7 for all data collected. Note excess concentrations of gaseous methanol favor the formation of dimethoxymethane and high selectivity to methyl formate. At conditions where methanol conversions are greater than 80%, complete oxidation to COx is favored. A relevant reaction mechanism was developed based upon both literature and experimental data to explain the data obtained in this study (Figure 8). A similar reaction mechanism was developed for methanol oxidation on two V2O5/ TiO2 catalysts based on studies that used FT-IR spectroscopy (9). According to this scheme presented, methanol is first activated in the form of methoxy species, which can be oxidized in the presence of V-Ti-O by dissociative adsorption of methanol on dual acid-base sites formed by an accessible cation, a surface oxygen ion, and molecularly chemisorbed formaldehyde (25-27). Further reaction of adsorbed methoxy species with methanol produces adsorbed formate ions such as hemiacetal or methoxymethanol intermediates (CH3OCH2OH) (28, 29). These, in turn, can undergo hydrogen abstraction reactions to form methyl formate or decompose to form CO or CO2. Previous studies have shown that hydrogen abstraction from methoxy group is the rate-determining step in methanol oxidation (8) and V2O5/TiO2 catalysts are expected to speed up the redox cycle (30). This work proved that the use of a V-Ti-O catalyst and ozone for deep oxidation of VOCs afforded a low-temperature, high rate reaction alternative to incineration.
Acknowledgments We are grateful to Felisha Lotspeich and Venugopal Devulapelli for their experimental support.
Supporting Information Available Schematic diagram of test system, SEM micrograph, energy dispersive X-ray, and relationship diagram. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) U.S. Environmental Protection Agency. Fact Sheet: EPA’s Final Pulp, Paper, and Paperboard “Cluster Rule” - Overview; EPA821-F-97-010; Washington, DC, 1997. (2) World Bank Group. Pollution Prevention and Abatement: Pulp and Paper Mills; Technical Background Document; Environment Department: Washington, DC, 1998. (3) Springer, A. M. Industrial Environmental Control: Pulp and Paper Industry, 3rd ed.; TAPPI Press: Atlanta, GA, 2000. (4) U.S. EPA. 2005 Toxic Release Inventory (TRI) Program. http:// www.epa.gov/tri. (5) Jacoby, M. Catalytic Cleanup at the Pulp Mill. Chem. Eng. News 2002, March 25, 39-40. (6) Hunter, P. H.; Oyama, S. T. Control of Volatile Compound Emissions, Conventional and Emerging Technologies; John Wiley & Sons: New York, 2000. (7) Oyama, S. T. Chemical and Catalytic Properties of Ozone. Catal. Rev. - Sci. Eng. 2000, 42 (3), 279. (8) Ping, T. S.; Hua, L. W.; Qing, Z. J. Nan, C. C. Catalytic Oxidation of Sulfosalicylic Acid. Ozone-Sci. Eng. 2002, 24, 117.
4760
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 13, 2007
(9) Tatiboue¨t, J. M. Methanol Oxidation as a Catalytic Surface Probe. Appl. Catal., A 1997, 148, 213-252. (10) Hengstum, A. J.; van Ommen, J. G.; Bosca, H.; Gelling, P. J. In Proceedings of the 8th International Congress on Catalysis; Vol. 4; Verlag, Chemie and Dechema: Berlin, 1984; p 297. (11) Busca, G.; Tettarelli, P.; Tronconi, E.; Forzatti, P. Evidence for the formation of an antase-type V-Ti oxide solid sate solution. J. Solid State Chem. 1987, 67, 91. (12) Busca, G.; Elmi, A. S.; Forzatti, N. Mechanism of selective methanol oxidation over vanadium oxide-titanium oxide catalysts: a FT-IR and flow reactor study. J. Phys. Chem. 1987, 91, 5263-5269. (13) Tronconi, E.; Elmi, A. S.; Ferlazzo, N.; Forzatti, P.; Busch, G.; Tittarelli, P. Methyl Formate from Methanol Oxidation over Coprecipitated V-Ti-O Catalysts. Ind. Eng. Chem. Res. 1987, 26, 1269-1275. (14) Ali, M. The Production of Methyl Formate by the Vapor-Phase Oxidation of Methanol. J. Catal. 1982, 77, 279-288. (15) Arora, N.; Deo, G.; Wachs, I. E.; Hirt, A. M. Surface Aspects of Bismuth-Metal Oxide Catalysts. J. Catal. 1996, 159, 1-13. (16) Valente, N. G.; Cadus, L. E.; Gorriz, O. F.; Arrua, L. A.; Rivarola, J. B. Synergy in the Sn-Mo-O catalysts: The selective oxidation of methanol Applied Catal., Al 1997, 153, 119-132. (17) Valente, N. G.; Arrua, L. A.; Cadus, L. E. Structure and Activity of Sn-Mo-O catalysts: Partial Oxidation of Methanol. Appl. Catal., A 2001, 205, 201-214. (18) Castillo, R.; Dewaele, K.; Ruiz, P.; Delmon, B. Mechanical Mixtures of R-Sb2O4 and MoO3 as Highly Selective Catalysts for the Oxidation of Methanol to Formaldehyde. Appl. Catal., A 1997, 153, L1-L8. (19) Gordon, G.; Cooper, W. J.; Rice, R. G.; Pacey, G. E. Disinfectant Residual Measurement Methods, 2nd ed.; AWWARF and AWWA: Denver, CO, 1992. (20) Oyama, S. T.; Li, W.; Zhang, W. A Comparative Study of Ethanol Oxidation with Ozone on Supported Molybdenum and Manganese Oxide Catalyst. Stud. Surf. Sci. Catal. 1999, 121, 105112. (21) Busca, G. Infrared studies of the reactive adsorption of organic molecules over metal oxides and of the mechanisms of their heterogeneously-catalyzed oxidation. Catal. Today 1996, 27, 457. (22) Imamura, S.; Ikebata, M.; Ito, T.; Ogita, T. Decomposition of ozone on a silver catalyst. Ind. Eng. Chem. Res. 1991, 30, 217221. (23) Li, W.; Oyama, S. T. Mechanism of Ozone Decomposition on a Manganese Oxide Catalyst. 2. Steady-State and Transient. J. Am. Chem. Soc. 1998, 120, 9047-9052. (24) Pernicone, N.; Lazzerin, F.; Liberti, G.; Lanzavecchia, G. On the mechanism of CH3OH oxidation to CH2O over MoO3-Fe2(MoO4)3 catalyst. J. Catal. 1969, 14, 293-302. (25) Dhandapani, B.; Oyama, S. T. Gas phase ozone decomposition catalysts, Appl. Catal., B 1997, 11, 129-166. (26) Yang, T. J.; Lunsford, J. H. Partial oxidation of methanol to formaldehyde over molybdenum oxide on silica. J. Catal. 1987, 103, 55-64. (27) Busca, G.; Lamotte, J.; Lavalley, J. C.; Lorenzelli, V. FT-IR study of the adsorption and transformation of formaldehyde on oxide surfaces. J. Am. Chem. Soc. 1987, 109, 5197. (28) Loius, C.; Tatibouet, J. M.; Che, M. Catalytic properties of silicasupported molybdenum catalysts in methanol oxidation: The influence of molybdenum dispersion. J. Catal. 1988, 109, 354366. (29) Titabouet, J. M.; Lauron-Pernot, H. Transient isotopic study of methanol oxidation on unsupported V2O5: Mechanism of methylal formation. J. Mol. Catal., A 2001, 171, 205-216. (30) Mars, P.; van Krevelen, D. W. Oxidations carried out by means of vanadium oxide. catalysts. Chem. Eng., Sci., Suppl. 1954, 3, 41.
Received for review October 19, 2006. Revised manuscript received April 20, 2007. Accepted April 24, 2007. ES062518U
