I n d . Eng. Chem. Res. 1991, 30, 18-21
18
The penetration depth of the poison front (calculated from the model with injection at t=O) is shown in Figure 10. In the smallest particles, the nitrogen front reaches the center quickly. For the largest particles, only a small fraction of the active material is depleted. In the larger crystals, the diffusive barrier causes most of the activity loss, while in the smallest particles, the loss of active material is the predominant cause of deactivation. This is shown in Figure 11 where the model has been used to calculate conversions with D,/D, 2.5 (the same as Figure 1) and with D,/D, = 1 (dashed line). Acknowledgment We gratefully acknowledge the experimental work of George F. Killmer and the secretarial patience of Diane M. Corradetti. Nomenclature A = reactor cross-sectionalarea (em2) Bi = Biot number for the reactant (based on D,), kfR,/D, Bi, = dimensionless parameter, kfpRc/D, C, C,, C, = reactant concentration in the reactor (bulk), poisoned shell, and unpoisoned catalyst core (mol/cm3 of gas) C, C,,Cg = poison concentration in the bulk and at the surface and core positions (mol/cm3gas) C; = saturation of adsorbed poison in the catalyst (mol/cm3 of catalyst) D,, D, = effective reactant diffusion coefficient in the crystal (cm3of gas/(cm of catalyst-s)) D, = effective diffusion coefficient of poison through the shell (cm3of gas/(cm of catalyst-s)) Da, = dimensionless parameter, kJ?,/D, k , = intrinsic rate constant (cm3of gas/(cm3 of catalyst-s)) k f = film coefficient for the reactant (cm3 of gas/(cm2 of catalystd) k f p = film mass-transfer coefficient of the poison (cm3 of gas/(cm2 of catalystd) k , = rate constant for poison deposition (cm3of gas/(cm3of cata1yst.s)) L = reactor length (cm) n = parameter in coke deactivation function N , = parameter in ( l l ) , 3C$,/C;R,2 (l/s) Q = local gas flow rate (cm of gas/s) Qo = inlet gas flow rate (em3of gas/s) F , = unpoisoned core radius (cm)
R, = catalyst radial position and radius (cm) t N = time for nitrogen front to reach the core (h) V = catalyst volume (cm3),AL X = reactant conversion, 1 - QC/QoC(0) y = fractional reactor length, z / L t = reactor axial position (cm) CY = parameter in coke deactivation function 1 - z = catalyst fraction (cm3of catalyst/cm3of reactor volume) A = diffusivity ratio, DJD, tA = molar expansion coefficient $, = Thiele modulus of core, R,(kc/D,)'/2 @, = Thiele modulus of the shell, R,(k,/D,)'/* r) = effectiveness factor A, = catalyst deactivation function due to coking &(t) = dimensionless position of the core from ( l l ) , rc/R, 0 = dimensionless parameter in (16) and (17); defined in text Registry No. Dodecane, 112-40-3;5,6-benzcquinoline,85-02-9. F,
Literature Cited Carberry, J. J.; Gorring, R. L. Time-Dependent Pour Mouth Poisoning of Catalyst. J . Catal. 1966,5,529-535. Chang, C. D. Hydrocarbons from Methanol; Marcel Dekker: New York, 1983. Chen, N. Y.; Gorring, R. L.; Ireland, H. R.; Stein, T. R. New Process Cuts Pour Point of Distillates. Oil Gas J . 1977, 75 (23), 165-170. Chen, N. Y.; Carwood, W. E.; Dwyer, F. G. Shape Selectiue Catalysis in Industrial Applications; Marcel Dekker: New York, 1989. Froment, G. F.; Bischoff, K. B. Chemical Reactor Design and Analysis; John Wiley and Sons: New York, 1979. Haag, W. 0.;Lago, R. M.; Weisz, P. B. Transport and Reactivity of Hydrocarbon Molecules in a Shape-Selective Zeolite. Faraday Discuss. 1982, 72, 317-330. Herrmann, C.; Haas, J.; Fetting, F. Effect of Crystal Size on the Activity of ZSM-5 Catalysts in Various Reactions. Appl. Catal. 1987,35, 299-310. Kokotailo, G. T.; Lawton, S. L.; Olson, D. H.; Meir, W. M. Nature 1978,272,437-438. Namba, S.; Nakanishi, S.; Yashima, T. Behavior of Quinoline Derivatives as Poisons in Isomerization of p-Xylene. J . Catal. 1984, 65,505-508. Olson, D. H.; Haag, W. 0.;Lago, R. M. Chemical and Physical Properties of the ZSM-5 Substitutional Series Structure of Synthetic Zeolite ZSM-5. J . Catal. 1980,61,390-396. Olson, D. H.; Kokotailo, G. T.; Lawton, S. L.; Meier, W. M. Crystal Structure and Structure Related Properties of ZSM-5. J . Phys. Chem. 1981,85,2238-2243. Smith, K. M.; Starr, W. C.; Chen, N. Y. New Process Dewaxes Lube Base Stocks. Oil Gas J . 1980, 78(21), 75-84.
Receiued for reuiew April 14, 1990 Accepted July 19, 1990
Combustion of Formaldehyde on Ruthenium/Cerium( IV) Oxide Catalyst Seiichiro Imamura,* Yasuo Uematsu, and K a z u n o r i Utani Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan
Tomoyasu I t o Department of Chemistry, Faculty of Science, Tokyo Metropolitan Uniuersity, Setagaya-ku, Tokyo 158, Japan
Catalytic combustion of formaldehyde was carried out on supported precious metals. Ruthenium supported on CeOa was the most active, and it completely oxidized formaldehyde at 200 OC without producing formic acid. The IR analysis indicated that formaldehyde was decomposed on this catalyst even at room temperature t o produce intermediate species. However, as the desorption of these intermediates from the catalyst surface was rather difficult at room temperature, it required higher temperatures t o attain the steady-state combustion of formaldehyde. Ruthenium oxide easily lost its lattice oxygen by interaction with formaldehyde, which explained the high activity of this supported ruthenium catalyst. We are exposed to formaldehyde in daily life. It is contained in cigarette smoke and can be found as a residual 0888-5885/91/2630-0018$02.50/0
in a large variety of consumer products such as formaldehyde-based resins in construction materials or cos0 1991
American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991 19 metics. However, the toxicity of formaldehyde was recognized recently. The EPA listed formaldehyde as a priority chemical for regulatory assessment under the Toxic Substances Control Act in 1984 because of its carcinogenicity (Heileman, 1984). Moreover, formaldehyde is the main exhaust from methanol-fueled vehicles, which have attracted attention these days because they produce less emission of pollutants than gasoline-fueled vehicles (McCabe and Mitchell, 1986). The formaldehyde concentration resulting from the emission of methanol-fueled vehicles and its behavior were investigated because of the health concern (Chang and Rudy, 1990; Dunker, 1990). Therefore, the detoxification of formaldehyde is an important subject. Gesser and Fu (1990) reported on the removal of formaldehyde from indoor air by absorption using polymeric amines. Catalytic oxidation of formaldehyde has also begun to be studied recently on Pt (Jaske et al., 19851, Ni (Foster and Masel, 1986), Cu-Ce-Ag (Suzuki et al., 1986), Mn (Solov'ev et al., 1987), and so forth. Previously, one of the authors found that precious metals supported on Ce02were effective in decomposing formaldehyde in the aqueous phase (wet oxidation) and ruthenium exhibited the highest activity (Imamura et al., 1988). The present paper deals with the vapor-phase combustion of formaldehyde on ruthenium supported on Ce02 a t relatively low temperatures. When ruthenium forms higher oxides (Ru03,Ru04),it becomes volatile and toxic (Lowenheim, 1973). Therefore, it is not used as a combustion catalyst at high temperatures. It is expected, however, that ruthenium is durable enough when used in low-temperature combustions.
Experimental Section Ce02was obtained by precipitating aqueous cerium(II1) nitrate and calcinating the resultant precipitate at 450 OC for 3 h in air. Its BET surface area (S,) was 135.7 m2/g. yA120, (S, = 138.5 m2/g), NaY zeolite (S, = 509.8 m2/g), Zr02 (S, = 15.2 m2/g), Ti02 (S, = 3.8 m2/g), ruthenim(II1) chloride, rhodium(II1) nitrate, palladium(I1) nitrate, iridium(II1) chloride, hydrogen hexachloroplatinate(IV), and other reagents were used as obtained commercially. The catalysts were prepared as follows. A known amount of precious metal salts ( 5 wt % loading as metal on the supports) and an excess amount of formaldehyde over the precious metals were dissolved in deionized water in the presence of dispersed supports, and the solution was heated to 90 "C. Sodium hydroxide (3 N) was added until the pH of the solution was 12, and the solution was kept standing for 1 h with stirring. The resultant composite material was filtered and was washed several times with water until the pH of the filtrate was below 9. It was dried overnight and was calcined a t 450 OC for 3 h in air. The Ru supported on cerium(1V) oxide was identified as Ru02 by X-ray and ESCA analyses (Imamura et al., 1988). This catalyst was designated as Ru/Ce02 hereafter. The supported catalysts were mixed with a 9-fold volume of finely ground silica gel and were molded into a disk under a pressure of 20 MPa/m2. They were cut to 10-14-mesh size before use. Reactions were carried out under an atmospheric pressure with a tubular flow reactor made of silica glass (outer diameter 8 mm, inner diameter 6 mm). One milliliter of the catalysts was charged in the reactor, and the reactor was heated with an electric furnace. Aqueous formaldehyde containing methanol was fed into the reactor by use of an automatically driven syringe under a flow of air. The concentrations of formaldehyde, methanol, and water were 900, 160, and 180000 ppm, respectively. The space
Table I. Combustion of FormaldehydeD catalyst T60,b"C Ru/Ce02 4150 Pd/CeO, C150 Rh/Ce02 C150 Pt/Ce02 C150 Ir/CeOz 207 Ru/ ZrOz 188 Ru/A1203 198 Ru/ zeolite 210 Ru/TiOz 212 CeO, 238
T W t b "C
150 181 204 304 281 276 239 320 364 297
DIFormaldehyde]= 900 ppm, [methanol] = 160 ppm, and [H20] = 18% in air; catalyst, 1 mL;SV = 20000 h-I. bTemperatures at which the conversion of formaldehyde reached 50% and 90%.
velocity (SV) of the reaction gas was 20000 h-l. The conversion of formaldehyde was determined at 150, 200, 250,300, and 350 "C. The reaction below 150 "C was not carried out because the temperature of the catalyst bed fluctuated due to condensation of water contained in the reaction gas. Oxidation of formic acid and methanol was carried out with the same reactor. Air passed through a saturator of formic acid or methanol was introduced into the reactor. The concentrations of formic acid and methanol were 11000 and 7200 ppm, respectively. Formaldehyde which escaped oxidation was absorbed in a cold water trap for 30 min and was analyzed by iodometric titration as follows. An aliquot of the above formaldehyde solution was added with 2 N NaOH and an excess amount of l/lo N aqueous I2and was kept standing for 10 min in the dark. After the solution was acidified with 2 N HCl, the remaining I2 was titrated with l/lo N aqueous thiosulfate. Formic acid, methanol, and carbon dioxide were determined with a Shimadzu GC-12A gas chromatograph equipped with a flame ionization detector at 110 "C. The column packings were Chromosorb 101 (1 m) for formic acid and methanol, and activated charcoal (1m) for carbon dioxide and carbon monoxide, respectively. Carbon dioxide and carbon monoxide were converted to methane with a Shimadzu MTN-1 methanizer before the analysis with the gas chromatograph. The adsorption species on Ce02 and Ru/Ce02 formed from formaldehyde were analyzed with a JASCO FTIR-5MP infrared spectrophotometer equipped with a diffuse reflectance cell which enables measurements at high temperatures under various atmospheres. The ESCA spectra of the catalysts were obtained with a Shimadzu ESCA 750 spectrophotometer, and EDX analysis was carried out with a Hitachi EMAX 3000 energy-dispersive X-ray spectrometer.
Results and Discussion Table I shows the results of the oxidation of formaldehyde on the precious metal catalysts. The activity of the catalysts was evaluated by the temperatures a t which and the conversion of formaldehyde reached 50% (TW) 90% (Tw). Ru/Ce02 had the highest activity among the precious metals on Ce02 investigated in this work. The effect of the supports on the activity of Ru was also investigated. Although the BET surface area differed among the supports and the inherent activity of the catalysts waa not evaluated exactly, Ce02 was the most effective for practical purposes. These results clearly coincided with the result obtained in the wet oxidation of poly(ethy1ene glycol) on these catalysts (Imamura et al., 1988). Catalytic action of Ru/Ce02 was investigated hereafter. Figure 1 shows the oxidation of formaldehyde, formic acid, and methanol. Oxidation of formic acid required
20 Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991
I
Ru I CeOz
t
- 400
I d
I '
I
6 1
150
200
1
50%
CeOz
250
Temp ('C)
Figure 1. Oxidation of formaldehyde, formic acid, and methanol on Ru/CeOz. ( 0 )Formaldehyde. The reaction conditions are shown in Table I. (A)Formic acid (11000 ppm in air), SV = 9000 h-l. (0) Methanol (7200 ppm in air), SV = 9000 h-l. Methanol was determined by a gas chromatograph. (0)Decomposition curve for methanol determined on the basis of C 0 2 evolution.
higher temperatures than those of formaldehyde. This phenomenon differed from that observed in the wet oxidation, in which formic acid decomposed more readily than formaldehyde. The product in the oxidation of formic acid was selectively CO,, and no CO was formed. Formic acid was not detected by the gas-chromatographic analysis during the oxidation of formaldehyde. If formic acid had been produced, it would have escaped further oxidation below 150 "C owing to its higher stability and could have been detected. Therefore, formaldehyde was oxidized directly to COPand water without the formation of formic acid as an intermediate. Methanol had an intermediate reactivity between formaldehyde and formic acid. The conversion of methanol determined by the direct analysis of methanol and that estimated from COPevolution differed a t lower temperatures. Formaldehyde (1460 ppm, 20.4% based on the feed methanol concentration of 7200 ppm) was detected at 150 "C, which roughly corresponded to the amount of the missing methanol. A t higher temperatures, the discrepancy between the both conversions became negligible, and only a trace amount of formaldehyde was detected; 91 ppm at 175 "C, 30 ppm at 200 "C, and 0 ppm at 230 "C. Although the reaction conditions employed (concentration and space velocity) differed between the oxidation of methanol and that of formaldehyde, the qualitative result shown in Figure 1 indicates that coexisting methanol can be eliminated simultaneously on this catalyst around the temperatures where combustion of formaldehyde takes place effectively. Intermediate species on the surface of CeO, and Ru/ CeOPformed from formaldehyde were analyzed by an IR technique, and the spectra obtained are shown in Figure 2. Paraformaldehyde was heated under a flow of nitrogen, and the resultant formaldehyde (3.6%) was passed over these catalysts for 30 min at room temperature. Then these catalysts were heated in an oxygen flow at prescribed temperatures for 10 min, and the IR spectra were recorded a t ambient temperature. The Ru/CeOP to which formaldehyde was admitted at room temperature showed three weak absorption bands at 1050-1100, 1250-1380, and 1530-1630 cm-'. When the sample was heated at 120 OC, the band a t 1050-1100 cm-' disappeared, whereas the two bands at higher wave numbers remained unchanged. Upon heating a t 150 OC, all the bands disappeared. The CeO, sample also showed absorption bands a t 1080-1150, 1300-1450, and 1550-1650 cm-'. In addition, an absorption
3000
2500 zoo0 1500 Wave number (cm-1)
1000
Figure 2. IR spectra of the adsorbed species on the catalysts formed from formaldehyde. (a) Room temperature; (b) after heating a t 120 "C in O2for 10 min; (c) after further heating at 150 "C in O2for 10 min; (d) room temperature; (e) after heating a t 150 "C in O2for 10 min; (f) after further heating a t 250 "C in O2for 1 h.
3000
2500
2000
1500
1000
Wave number (cm-')
Figure 3. IR spectra of C02 and methanol on Ru/CeO,. (a) Room temperature; (b) after heating a t 150 "C in O2 for 10 min; (c) room temperature.
band due to the C-H stretching vibration appeared at 2800-3000 cm-'. When the sample was heated at 150 "C, the band at 1080-1150 cm-' disappeared. However, the three bands remained even after the sample was heated a t 250 "C for 10 min, although their intensity decreased. This suggested that the species giving rise to the band a t 2800-3000 cm-' was also responsible for the other two bands at 1300-1450 and 1550-1650 cm-'. The fact that the adsorbed species disappeared at lower temperatures on Ru/CeO, than on CeOpsuggested the high activity of Ru. When 150 mL of CO, in an oxygen flow was admitted on Ru/CeO, at room temperature, weak absorption bands appeared a t about 1380 and 1540 cm-' (Figure 3). These bands disappeared on heating at 150 "C for 10 min. When methanol in an oxygen flow (3.8%) was admitted on Ru/CeO, a t room temperature, a band due to C-0 stretching vibration was observed at 1000-1070 cm-' in addition to a weak C-H band a t 2900-2960 cm-'. The absorption band at about 1100 cm-' for both Ce02 and Ru/CeO, was probably ascribed as C-0 stretching vibration. Formaldehyde decomposes on many metal ox-
Ind. Eng. Chem. Res., Vol. 30, No. 1, 1991 21 ides below room temperature to produce various species such as methoxide [CH30] poly(oxymethy1ene)[(CH20),], and dioxymethylene [H2C02],which show C-O stretching vibrations around 1100 cm-' (Busca et al., 1987). The present result shows that formaldehyde was also decomposed at low temperatures on Ce02and Ru/Ce02, because, if molecular formaldehyde had been present, it would have given rise to an absorption band a t about 1700 cm-' due to C=O stretching vibration (Busca et al., 1987). The reason why there no C-H absorption band appeared due to the above species (CH,O, etc.) for Ru/Ce02 is not known, but it may be that the amount of the species that gave rise to the band at about 1100 cm-' (and should have presented the C-H band also) was small or that the intensity of this band is inherently small as was indicated by the intensity of the C-H absorption band of methanol on Ru/Ce02 (Figure 3). The bands at about 1350 and 1550 cm-' for both Ru/Ce02 and Ce02were assumed to be due to the stretching vibration of the C02group (Busca et al., 1987). As the species formed on Ru/Ce02 presented no C-H absorption band, it was tentatively assigned as adsorbed carbon dioxide, which is present as carbonato and/or carbonate species (Parkyns, 1969). The spectrum for carbon dioxide on Ru/Ce02 and its behavior on heating supported this assignment. The C 0 2group on Ce02 was assumed to have a C-H bond, because the band at 2800-3000 cm-' behaved in a similar manner as the bands due to this C02group on heat treatment. Therefore, this species seemed to be formate ion (HCOO-). The C-H absorption band due to methoxide, poly(oxymethylene), or dioxymethylene on Ce02may have overlapped the C-H band of this formate ion or may have been absent as in the case of the C-H band on Ru/CeOa. The present results indicated that the adsorption species on Ru/Ce02 were more extensively oxidized than those on Ce02. The binding energy of Ru 3d3 of the fresh Ru/Ce02 was 281.0 eV, which showed that the form of Ru was Ru02 (Imamura et al., 1988). When formaldehyde (3.6% in N,) was admitted on Ru/Ce02 for 30 min and, then, this catalyst was heated for 30 min a t 100 "C, the binding energy decreased to 279.7 eV, corresponding to that of metallic Ru (Kim and Winograd, 1974). No change in the binding energy occurred when the catalyst was heated a t 150 OC in N2 in the absence of formaldehyde. This result indicated that Ru could transfer its lattice oxygen very easily to formaldehyde under the reaction conditions, which explains the high activity of Ru/Ce02. As the result of the IR analysis indicated, formaldehyde decomposes easily into some fragments on Ru/Ce02 a t ambient temperatures. However, it requires higher temperatures for the fragments to suffer further oxidation and desorb from the surface of this catalyst. Therefore, temperatures higher than 100 OC will be needed to maintain the steady-state combustion of formaldehyde. As described before, ruthenium becomes volatile and toxic when oxidized to higher oxides (Ru03, RuO,). Therefore, it is not employed in combustions that usually proceed at high temperatures. However, it is expected that ruthenium is durable enough in the combustion of formaldehyde, which proceeds in relatively mild reaction conditions at low temperatures. Bell and Tagami (1963) investigated the oxidation of Ru02 under an oxygen atmosphere in the temperature range from 802 to 1503 "C. The extrapolation of their data on equilibrium constants for the reactions shown below gave partial pressures of 1.95
RuO~(S)+ 7 2 0 2 * RuOB(g)
+ 02 + RuO,(g) RuO~(S) X and 2.04 X lo4 atm for Ru03 and Ru04 at 300 "C, respectively. Therefore, the oxidation of Ru02 seems negligible. Ru/Ce02 was heated in oxygen at 370 "C for 10 h, and the change in the surface content of Ru was determined by an EDX analysis. The average contents of Ru determined on the three different specimens were 5.57 wt % (based on Ru plus Ce) before heat treatment and 5.59 wt % after treatment, respectively. Therefore, Ru did not escape into the vapor phase, and thus, Ru/Ce02 can be used for the low-temperature combustions as in the present reaction conditions.
Acknowledgment We express our gratitude to Dr. S. Ishida of Chubu University for his kind cooperation in the ESCA analysis of the catalysts. Registry No. Ru, 7440-18-8; CeOz, 1306-38-3; Pd, 7440-05-3; Rh, 7440-16-6; Pt, 7440-06-4; Ir, 7439-88-5; ZrOz, 1314-23-4; TiOz, 13463-67-7; formaldehyde, 50-00-0; formic acid, 64-18-6; methanol, 67-56- 1.
Literature Cited Bell, W. E.; Tagami, M. High-temperature chemistry of the ruthenium-oxygen system. J. Phys. Chem. 1963, 67, 2432-2436. 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, 5197-5202. Chang, T. Y.; Rudy, S. J. Roadway tunnel air quality models. Enuiron. Sci. Technol. 1990, 24, 672-676. Dunker, A. M. Relative reactivity of emissions from methanol-fueled vehicles in forming ozone. Enuiron. Sci. Technol. 1990, 24, 853-862.
Foster, J. J.; Masel, R. I. Formaldehyde oxidation on nickel oxide. Ind. Eng. Chem. Prod. Res. Deu. 1986,25,563-568. Gesser, H. D.; Fu, S. Removal of aldehydes and acidic pollutants from indoor air. Enuiron. Sci. Technol. 1990, 24, 495-497. Heileman, B. Formaldehyde: Assessing the risk. Enuiron. Sci. Technol. 1984, 18, 216A-221A. Imamura, S.; Fukuda, I.; Ishida, S.Wet oxidation catalyzed by ruthenium supported on cerium(1V) oxides. Ind. Eng. Chem. Res. 1988,27, 718-721.
Jaske, F. P.; Friedman, R. M.; Delk, F. S.; Bulock, J. W. The oxidation of formaldehyde over platinum/titania. Appl. Catal. 1985, 14, 303-321.
Kim, K. S.; Winograd, N. X-ray photoelectron spectroscopic studies of ruthenium-oxygen surfaces. J . Catal. 1974,35,66-72. Lowenheim, F. A. Platinum Group Metals. In Encyclopedia of Industrial Analysis; Snell, F. D., Ettre, L. s., Eds.; Interscience Publishers: New York, 1973; Vol. 17, pp 220-228. McCabe, R. W.; Mitchell, P. J. Exhaust-catalyst development for methanol-fueled vehicles: 1. A comparative study of methanol oxidation over alumina-supported catalysts containing group 9,10, and 11 metals. Appl. Catal. 1986,27, 83-98. Parkyns, N. D. The surface properties of metal oxides. Part 11. An infrared study of the adsorption of carbon dioxide on y-alumina. J. Chem. SOC.A 1969, 410-417. Solov'ev, S. A.; Vol'fson, V. Ya.; Vlasenko, V. M. Removal of a formaldehyde impurity from air under sorptive-catalytic conditions on manganese-containing catalysts. Zh. Prikl. Khim. 1987, 60,2137-2140. Suzuki, K.; Fujitani, Y.; Yoshimoto, T.; Muraki, H. Oxidation catalyst for aldehyde in air. Japanese Patent 86146348,1986; Chem. Abstr. 1986, 105, 196576. Received for review March 27, 1990 Revised manuscript received June 26, 1990 Accepted July 11, 1990
