OXIDATION OF METHANOL OVER MANGANESE DIOXIDE-MOLYBDENUM TRIOXIDE CATALYST R .
S. M A N N AND K . W . H A H N
Department of Chemical Engineering, University of Ottawa, Ottawa 2, Ontario The vapor phase catalytic air oxidation of methanol to formaldehyde over manganese dioxide-molybdenum trioxide was investigated in an integral fixed bed tubular flow reactor between 250’ and 46OoC., at atmospheric pressure, and space velocities of 9.6 x lo3 to 8.4 x lo4 hr-’. By using gas chromatographic technique, accurate analyses of the products were obtained. The data for product distribution as a function of the operating conditions are presented.
THE selective air oxidation of methanol to formaldehyde is of considerable industrial importance. Two types of catalysts, silver metal, metallic oxides, and their mixtures, are used in the manufacture of formaldehyde by methanol oxidation. While silver catalysts employ a rich mixture of methanol with air with low conversion, oxide catalysts employ a lean methanol-air mixture and operate a t high conversions and yield of formaldehyde. Several patents (Arnold, 1943, 1948; Bailey and Craver, 1921; Craver, 1932; Jaeger, 1929; Meharg and Adkins, 1933) have used oxides of vanadium, iron, molybdenum, and tungsten either alone or mixed with each other, for the catalytic oxidation of methanol to formaldehyde. Most of the patent literature since 1955 has been devoted chiefly to improvements in the methods of preparation, use, and recovery of these catalysts, especially based on ironmolybdenum oxide mixtures (Allyn et al., 1957, 1958; Hodgins and Shelton, 1961; “Montecatini,” 1959; Shelton and Barrentine, 1957, 1958; Walker et al., 1957; Warner ct al., 1958). More recently, the kinetics of methanol oxidation over oxides of iron and molybdenum has been reported (Dente et al., 1964; Jiru et al., 1964). I n course of our studies on methanol oxidation t o formaldehyde (Hahn, 1968) we tried molybdenum trioxide, vanadium pentoxide, manganese dioxide, and several combinations of molybdenum trioxide and manganese dioxide. A mixture of 20 weight CI: manganese dioxide and 80 weight k molybdenum oxide gave the best conversions, compared to the other catalysts or combinations of oxides of manganese dioxide and molybdenum trioxide. This paper reports the effect of a number of variables on the conversion of methanol to formaldehyde in presence of this catalyst. Experimental
Apparatus and Procedure. The experimental apparatus is shown schematically in Figure 1. Quarter-inch standard copper tubings were used in the apparatus throughout except for portions of the lines in contact with the reactants and the products, where bs-inch stainless steel tubing was used. All fittings were Swagelok standard stainless steel, and the valves, unless specified, were bellow valves. T h e carrier and reactant gases (helium, nitrogen, and air) were obtained from high pressure cylinders through Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 1, January 1970
a series of pressure-regulating devices and drying tubes, and metered over Brook’s glass rotameters. Spectroscopic grade methanol was placed in containers M C , partially compressed with nitrogen, and a needle valve to adjust the flow rate. T h e rate of flow of methanol was measured by counting the number of drops per unit time passing through the sight glass into the evaporating chamber. T h e reactor made of 304 stainless steel tubing, 6 inches long and 0.5 inch in o.d., with a Swagelok connection a t the top, through which two thermocouples, each inside a %e-inch protection tube, were inserted to measure the temperature of the catalyst, was heated with a fluidized sand bath, the temperature of which was controlled to within & 0.1OC. The ends of the thermocouple inside the protection tube were 1 inch apart, the first connected with the temperature controller, for controlling the temperature inside the reactor, and the second to a Honeywell Model 2745 potentiometer for recording the temperature during the reaction. The difference between the temperature a t the two ends was found to be negligible and the axial temperature gradient was less than 0.5” C. The exit gases from the reactor were led to a liquid trap, where heavier products (formaldehyde, methanol, and water) were condensed. The noncondensed gases were passed through a sampling valve leading t o a Model 25 Fisher gas partitioner and vented. Polymerization of formaldehyde in the gas stream was prevented by maintaining the lines from methanol sight glass to the liquid trap including the last 2 feet of the air line immediately preceding the evaporating chamber itself a t a temperature of 100°C. by means of heating tapes, insulating with cloth tape. Polymer formation in the liquid trap was avoided by the presence of 1%methanol. For the start of the run, air was slowly passed through the reactor, while the catalyst was brought to the required temperature. When the required temperature was attained, the methanol-air ratio was adjusted to the desired value, and the constituents were maintained a t specified rates for 30 minutes, while the reactor was brought to steady state. The steady-state run was continued for 1 t o 2 hours, during collection and analysis of the samples. Gas liquid samples were analyzed alternately. Catalyst. The catalyst containing 20 weight ‘C manganese dioxide and 80 weight “c molybdenum trioxide 43
I I
m
F2
I
C. Water condenser
1
D I , D2, Di, Da, Di. Fi, F 2 . GC. GP. IT. MC. N V. PR. R. R!.
Drying tubes Funnels for methanol input Gas chromotograph Gas partitioner Liquid trap Liquid methanol containers Needle volve for adjusting methanol flow rate Porous plate Reactor and fluidized sand both Rotometers Sight glass Sample valve Thermocouple Temperature controllers
u,;@ TO VENT
,'
sc. s v. 11.
TC,, TC?, TCI.
Figure 1. Flow diagram for oxidation of methanol to formaldehyde
was prepared in a manner similar to the one used earlier (Klissourski and Bliznakov, 1965). A paste, obtained from the mixture of manganese dioxide and molybdenum trioxide in the required amounts, was subjected to 6-hour drying a t room temperature, 12-hour a t 40°C., and 6-hour a t 150°C. The catalyst was subsequently activated by successive 1-hour heatings a t 200", 250", 300", and 35OoC., and a 6-hour calcination a t 420°C. Manganese dioxide and molybdenum trioxide were obtained by the thermal decomposition of Analar grade manganese nitrate, M n ( N 0 J 2 , and ammonium molybdate, (NH4)6Mo-024. 4 H 2 0 ,supplied by the Fisher Scientific Co. The catalyst was activated by passing air over it for 24 hours before any experimental run was made. The catalyst was very active, and showed constant activity for more than 2 months from run to run. The surface area of the fresh catalyst determined by the B E T method was found to be 7.8 sq. meters per gram. The average diameter of the catalyst particle was 0.525 mm. and a bulk density was 4.6 grams per ml. Analytical Procedure. ACIDS.The total acid content was obtained by titration of the condensate from the traps with 0.1N KOH. GASES.The inlet feed gases and the product gases were analyzed for Cog, CO, 0 2 ,and Nz by periodic injection of 0.5 ml. of sample into a Fisher gas partitioner containing a 6-foot column of hexamethylphosphoramide and a 13-foot 13X molecular sieve column, connected in series. LIQUID PRODUCTS. Aliquots were withdrawn when required and analyzed by gas chromatography. A gas chromatograph assembled in the department, incorporating a Gow-Mac, Model TR I11 A, 4W2 temperatureregulator thermal conductivity cell and Model 405 C 1 power supply control unit, was used for analyzing the products. A &meter 15 weight 5% sucrose octaacetate on Columpak T was used in separating the liquid products (CHBOH, HCHO, H 2 0 , HCOOH, and C4HiOH), as it proved to be the most efficient column for such separations (Mann and Hahn, 1967). The products were separated a t 100"C. Results and Discussion
Flow rates were calculated on the basis of the rotameter readings and the flow rate of methanol. The effluent rates were computed on the basis of the total flow rate and the composition of the product stream. All the experiments 44
were carried out a t atmospheric pressures. Though air was used for oxidation, the calculations are based on oxygen, so that the effect of oxygen on the course of the reaction can be visualized. Table I (Hahn, 1968) shows typical results obtained from the various runs, where feed compositions (percentage methanol in air), temperature, and catalyst-feed ratios were varied. The mass of the catalyst in grams employed for each experiment is denoted by W , the rate of flow of the total feed in gram moles per hour by F , and oxygen-methanol ratio by R. While conversion is referred to as the moles of methanol consumed (reacted) per hour to the moles of methanol fed per hour, the rate of formation is referred t o as the moles of various products formed per hour per gram of catalyst. The ratio of moles of formaldehyde produced per hour to the moles of methanol reacted is defined as selectivity, S. The yields are based upon the moles of formaldehyde formed in proportion to the moles of methanol fed per hour. T h e reaction products a t temperatures up to 365" C. were formaldehyde, methanol, water, oxygen, and nitrogen only. Above 365"C., small amounts of carbon monoxide and carbon dioxide were formed, indicating further oxidation of formaldehyde. N o other aldehyde or any acid besides formaldehyde was detected in the effluent stream. The temperature and partial pressure gradients between the flowing fluid and the exterior surface of the catalyst were evaluated by the method of Yoshida et al. (1962). The maximum temperature difference across the film thus calculated was of the order of about lac. The highest partial pressure gradient thus calculated was of the order of 0.0005 atm., which showed that the pressure drop across the gas film was insignificant and the effect of mass transfer was negligible. Diffusional effects were kept to a minimum by using a high velocity of the gas through the catalyst bed. The fair constancy of conversion obtained by changing the velocity of the reactants, while keeping W /F constant, suggested that the diffusion of the gases was not rate-controlling. The internal diffusional resistance in the catalyst particle was negligible, since a change in particle size from 0.20 to 1.65 mm. did not vary the reaction rate to any measurable degree. Figure 2 shows the effect of temperature on methanol oxidation between 250" and 460°C. a t a catalyst-feed ( W I F ) ratio of 16.3, and an oxygen-methanol ratio ( R ) Ind. Eng. Chem. Process Des. Develop., Val. 9,No. 1, Januory 1970
Table 1. Effect of Process Variables on Conversion'
Run Temp., N O . 'C. W / F 396 390 360 330 300 301 305 311 317 323 329 326 327 328 330 331 214 215 216 217 a
250 16.3 319 16.3 326 16.3 365 16.3 422 16.3 460 16.3 365 13.3 365 13.3 365 13.3 365 13.3 365 13.3 365 2.5 5.0 365 8.8 365 365 16.3 365 22.0 326 13.3 326 13.3 326 13.3 326 13.3
Feed, MoleslHr. R CH30H 02 NZ 2.42 2.42 2.42 2.42 2.42 2.42 5.04 3.61 3.02 2.79 2.42 2.42 2.42 2.42 2.42 2.42 2.42 2.42 2.42 2.42
0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.030 0.035 0.040 0.045
0.0724 0.0724 0.0724 0.0724 0.0724 0.0724 0.157 0.108 0.091 0.084 0.0724 0.0724 0.0724 0.0724 0.0724 0.0724 0.0788 0.0919 0.1050 0.1181
0.273 0.273 0.273 0.273 0.273 0.273 0.593 0.407 0.341 0.315 0.273 0.273 0.273 0.273 0.273 0.273 0.2963 0.3456 0.3450 0.4444
Analysis of Products, MoleslHr. HCHO CHZOH HzO CO COz 02 0.0021 0.0165 0.0182 0.0246 0.0114 0.0015 0.0154 0.0195 0.0203 0.0209 0.0219 0.0042 0.0059 0.0169 0.0246 0.0251 0.0159 0.0185 0.0213 0.0238
0.0279 0.0135 0.0118 0.0054 0.00352 0 0.0146 0.0105 0.0097 0.0092 0.0081 0.0258 0.0201 0.0135 0.0054 0.0049 0.0141 0.0165 0.0187 0.0331
0.0021 0.0165 0.0182 0.0246 0.0416 0.0585 0.0154 0.0195 0.0209 0.0209 0.0219 0.0042 0.0059 0.0169 0.0246 0.0251 0.0159 0.0185 0.0213 0.0238
0 0 0 0 0.00302 0.00142 0 0 0 0 0 0 0 0 0 0 0
0 0 0
0 0 0 0 0.01280 0.02708 0
0 0 0 0 0 0 0 0 0 0 0 0 0
0,0777 0.0705 0.0697 0.0665 0.0537 0.0453 0.150 0.0980 0.0809 0.0733 0.0678 0.0767 0.0738 0.0705 0.0665 0.0662 0.0709 0.0827 0.0944 0.1062
Nz 0.273 0.273 0.273 0.273 0.273 0.273 0.593 0.407 0.341 0.315 0.273 0.273 0.273 0.273 0.273 0.273 0.2963 0.3456 0.3450 0.444
ConuerSelecsion, Yield, tiuity, Y S X 0.06 0.54 0.61 0.84 0.88 1.00 0.513 0.650 0.677 0.697 0.730 0.140 0.330 0.500 0.820 0.837 0.530 0.528 0.532 0.529
0.06 0.54 0.61 0.84 0.38 0.05 0.513 0.650 0.677 0.697 0.730 0.140 0.330 0.500 0.820 0.837 0.530 0.528 0.532 0.529
1.00 1.00 1.00 1.00 0.43 0.05 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
Complete data available from Hahn (1968).
TEMPERATURE
"C
Figure 2. Effect of temperature on conversion, yield, a n d selectivity
of 2.42. With increasing temperature, both conversion and yield increased up to 365"C., after which conversion continued to increase, reaching almost 100% a t 460" C., while yield decreased. The selectivity was nearly 100% up to 365" C., and decreased at higher temperatures. The effect of oxygen-methanol ratio in the feed on the conversion of methanol and the yield for a W I F ratio of 13.3 at 365°C. are shown in Figure 3. The ratio of oxygen to methanol in the feed was varied between 2.42 and 5.04. While with increasing reactant ratios, the conversion of methanol and the yield of formaldehyde decreased rapidly, selectivity remained constant a t nearly 100%. Figure 4 shows the effect of W / F ratios in the range 2.5 to 22 on the conversion of methanol to formaldehyde at 365'C., for methanol-air mixtures (4 to 8% methanol in air). The conversion increased with increased W!F ratios and increased methanol in methanol-air mixtures. At temperatures over 365" C., formaldehyde underwent Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 1, January 1970
further oxidation to form Con, water, and small amounts of CO. The increased rate of formation of C 0 2 and CO supports this contention, which is in agreement with earlier findings (Jiru et al., 1965). Conclusions The catalytic air oxidation of methanol over manganese dioxide-molybdenum trioxide was investigated between 250" and 46OoC., a t atmospheric pressure for WIF ratios of 2.5 and 22 gram/ (moles) (hr.), oxygen-methanol ratios of 2.42 to 5.04, and space velocities of 9.6 x lo3 to 8.4 x lo4 hr.-' The optimum values determined were 8 mole % methanol in air at 365" C. for a W F ratio of 22 gram/ (mole) (hr.). The highest yield and conversion under these conditions were 84% with a selectivity of almost 100%. At highest temperatures, though the conversion increased with temperature, yield and selectivity were adversely affected. 45
SELECTIVITY
University of Ottawa for a fellowship to one of the authors (K. W. H.).
?
Nomenclature
F = moles of feed per hour
R = moles of oxygenimoles of methanol in feed r = rate of formation, moles of product formed per gram of catalyst per hour s = selectivity T = temperature, C. weight of catalyst, grams x = conversion y = yield O
w =
literature Cited 0 VI
0
-
CD
0
CONVERSION a n d YIELD Figure 3. Effect of oxygen-methanol ratio on conversion, yield, and selectivity
CONVERSION 0
(X) 0
Figure 4. Effect of catalyst-feed ratio on conversion
Acknowledgment
The authors are indebted to the Xational Research Council of Canada for financial support of the project and to University Affairs, Ontario Government, and the
46
Allyn, C. L., Barrentine, E. M., Hodgins, T. S., Rawson, R . L., Shelton, E. J. (to Reichhold Chemicals), U. S. Patents 2,812,309 (Xov. 5, 1957), 2,849,492 (Aug. 26, 1958). Arnold, H. R . ( t o E. I. du Pont de Nemours & Co.), U. S. Patents 2,320,253 (May 25, 1943), 2,439,880 (April 20, 1948). Bailey, G. C., Craver, A. E. (to Barrett Co.), U. S. Patent 1,383,059 (June 28, 1921). Craver, A. E. (to Weiss and Downs), U. S. Patent 1,851,754 (March 29, 1932). Dente, M., Poppi, R., Pasquon, J., Chim. e ind. ( M i l a n ) 46, 1326 (1964). Hahn, K. W., Ph.D. thesis, University of Ottawa, 1968. Hodglans, T. S., Shelton, E. J. (to Reichhold Chemicals), U. S. Patent 2,973,326 (Feb. 28, 1961). Jaeger, A. 0. (to Selden Co.), U. S. Patent 1,709,853 (April 23, 1929). Jiru, P., Wichterlova, B., Tichy, J., Proc. Intern. Congr. Catalysis, 3rd., Amsterdam, 1964, 1, 199 (1965). Klissourski, D., Bliznakov, G., Compt. Rend. Acad. Bulgare Sei. 18,549 (1965). Mann, R. S., Hahn, K. W., Anal. Chem. 39, 1314 (1967). Meharg, V. E., Adkins, H. (to Bakelite Corp.), U. S. Patent 1,813,405 (June 13, 1933). “Montecatini” Societa Generale per 1’Industria Mineraria e Chimica, Ital. Patent 589,718 (March 12,1959). Shelton, F. J., Barrentine, E. M. (to Reichhold Chemicals), U. S. Patents 2,812,308 (Nov. 5, 1957), 2,849,493 (Aug. 26, 1958). Walker, R . B., Warner, H. O., Hodgins, T. S. (to Reichhold Chemicals), U. S. Patent 2,812,310 (Nov. 5, 1957). Warner, H. O., Hodgins, T. S., Hagen, G. L. (to Reichhold Chemicals), U. S. Patent 2,852,564 (Sept. 16, 1958). Yoshida, F., Ramaswami, D., Hougen, 0. A., A . 1 . C h . E . J . 8 , 5 (1962). RECEIVED for review October 23, 1968 ACCEPTED August 8, 1969
Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 1, January 1970
