Ind. Eng. Chem. Prod. Res. Dev. 1984, 2 3 , 384-388
304
CATALYST SECTION Dehydrogenation of Methanol to Methyl Formate over Copper Catalysts Stephen P. Tonner, David L. Trlmm, and Mark S. Walnwrlght' School of Chemical Engineerlng and Industrial Chemistry, Universky of New South Wales, Kensington, New South Wales 2033, Australla
Noel W. Cant School of Chemistty, Macquarle University, North Ryde, New South Wales 2 113, Australla
The suitability of a number of copper catalysts for the dehydrogenation of methanol to methyl formate has been assessed via the use of specific rate constants, based on metal surface areas, for the dehydrogenation reaction, and the major side reaction, decarbonylation. Copper chromite catalysts, which combine high bulk densities with high metal areas, were particularly effective desptte the existence of a negative contribution of the support to specific activity which was low when compared with copper on silica catalysts. Raney copper catalysts gave very high initial activity, but rapid deactivation took place. This was attributed to polymerlzation of a formaldehyde intermediate in the reaction scheme.
Introduction
The dehydrogenation of methanol to produce methyl formate 2CH30H
HCOOCH3
+ 2Hz
(1)
is important as a first step in the synthesis of acetic acid, N,N-dimethylformamide, and hydrogen cyanide (Chono and Yamamoto, 1981). The reaction was first reported by Ivannikov and Zherko (1933), and copper-based catalysts and Raney copper have been found to be most active and selective for the reaction (Chono and Yamamoto, 1981; Charles and Robinet, 1950). More attention has been paid to the general problem of alcohol dehydrogenation in the literature (Bond, 19621, with the dehydrogenation of ethanol to acetaldehyde and ethyl acetate (Franckaerts and Froment, 1964) and of secondary alcohols to ketones (Kawamoto, 1961) receiving particular attention. The corresponding dehydrogenation of methanol to formaldehyde over copper-zinc catalysts has been reviewed by Chono and Yamamoto (1981), and this reaction has been suggested to be the first step in the decomposition of methanol to carbon monoxide and hydrogen (Lawson and Thomson, 1964). Studies have now been completed of the copper-catalyzed dehydrogenation of methanol as part of an overall study of the production of methyl formate. Comparisons of the kinetics of reactions over various catalysts have been combined with catalyst characterization studies to throw light on the nature of the active species, the activity and selectivity of different catalysts, and the factors governing the choice of a catalyst for the reaction. 0196-432118411223-0384$01.50/0
Experimental Procedure Catalysts. Commercial copper chromite catalysts 1808 (catalyst A) and 0203 (B)were obtained from Harshaw and G-22 (C) from Girdler. Supported copper catalysts were
prepared by impregnating silica (catalysts F, G, H), magnesia (E), or chromia (D) supports with copper nitrate solutions followed by drying at 110 "C for 16 h. Decomposition was carried out over 3 h in air at 450 "C (catalysts E, F, G, H) or over 16 h in hydrogen at 220 "C (catalyst D). A commercial silica support (300 m2g-l) was obtained from the Davison Chemical Division of W. R. Grace and Company. Magnesia was prepared by decomposition of magnesium carbonate at 400 "C in air and chromia by decomposition of chromium(II1) hydroxide at 400 "C in hydrogen. Catalyst I was prepared from copper tetraammine nitrate solution deposited on silica by the ion-exchange method of Hirose et al. (1982). Decomposition was at 500 "C in air. All catalysts were sieved to produce particles in the size range 300 to 420 pm before use. A Raney copper catalyst was prepared by leaching 300 to 420-pm particles of a 50 wt % Cu:50 wt % A1 alloy with 20 wt % NaOH (aq) at 50 "C for 4 h (Marsden et al., 1980). Copper oxide powder (catalyst J) was also investigated as a physical mixture with silica. All catalysts were reduced in hydrogen at 220 "C for 16 h prior to use. Catalyst Characterization. The amount of copper in the catalysts was determined by atomic adsorption spectroscopy. The BET surface areas of the catalysts were obtained from nitrogen adsorption at 77 K using a Micromeritics High Speed Surface Area Analyser or a single 0 1984 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23,No. 3, 1984 385 Table I. Properties of Reduced Catalyst Samples cat. code A
B C
D E F G H
I J
K a
type Harshaw 1808 copper chromite Harshaw 0203 copper chromite Girdler G-22" copper chromite copper on chromia copper on magnesia copper on silica high loading copper on silica moderate loading copper on silica low loading copper on silica ion exchange copper oxide powder Raney copper
surface area, ma g"
Cu content, wt %
crystallite diam, A
SBET
SCU
35.5
28.4
6.8
60
59.4
16.3
10.8
100
33.7
45.1
15.4
90
5.5 20.7 15.6
83.1 44.9 227
4.0 12.3 4.8
166 230
9.1
240
1.6
180
4.4
261
0.94
190
5.0
233
65
99.0
0.93
99.3
280
100
18.1
18.57
Contains 10%BaO.
point chromatographic method (Evans et al., 1983). The copper surface area was determined by reaction with nitrous oxide following the method of Evans et al. (1983). The surface areas and composition of the catalysts are shown in Table I. X-ray powder diffraction studies were carried out on the catalysts with a Rigaku Geigerflex diffractometer. Copper crystallite sizes of the reduced catalysts were determined from X-ray line broadening measurements using silicon powder as a reference and are reported in Table I. Catalysts E and I contained copper in a largely amorphous state. Apparatus. A simple flow apparatus was used to measure catalyst activities and selectivities. Methanol was metered to a vaporizer/preheater using an Eldex Model E-12043-2 pump to the catalyst bed (0.5 to 2.0 g) contained in a stainless steel U-tube reactor immersed in a vigorously stirred molten salt bath. The bed temperature was controlled to fl OC through a proportional-integral controller and was measured with a chromel-alumel thermocouple. The lines to and from the reactor were electrically heated to prevent condensation of reactants and products. The reaction products were sampled with a heated Valco 6-port valve and were analyzed with a Gow-Mac gas chromatograph fitted with a thermal conductivity detector. Separation of the products (CO, CO,, formaldehyde, water, methanol, and methyl formate) was performed on a 1 m X 1/4 in. column of Porapak T which was maintained at 150 "C with a hydrogen carrier gas flow of 40 cm3 min-l. Mass balances on the results reported were accurate to 100 f 1%. The apparatus was equipped with gas mixing facilities which enabled diluent gases (N, and He) and hydrogen for catalyst reduction to be fed to the reactor. The flows of these gases were controlled by needle valves and a backpressure regulator and were measured with rotameters. All measurements of activity were conducted with operating conditions involving a total pressure of 1 atm and at a temperature of 220 "C with liquid methanol feed rates in the range 0.5 to 2.0 cm3 min-'. Catalyst deactivation was measured by sampling the reaction products at time intervals of 5 to 10 min for a period of up to 3 h. Results Stable activity was exhibited over the period of the measurements (ca. 3 h) for all catalysts except Raney
Table 11. Conversion and Selectivity Data [ 1 g of Catalyst; 220 'C; 101 kPa; 0.55 mL min" CH,OH(l)] conversion, cat.
%
A
22.2 27.7 22.7 3.9 27.1 30.0 16.3 10.3 31.1 6.8
B C D E F G H I J
yMeF
selectivity,
%g
10.42 14.56 10.73 1.10 8.60 13.76 8.09 5.36 12.50 3.40
%
3.04 2.05 3.16 0.90 8.0 5.46 0.59 0.34 5.25 0.30
86.0 85.9 85.9 62.2 66.4 82.7 94.7 94.5 81.4 91.9
y = mole fraction produced, MeF = methyl formate. CO = carbon monoxide.
copper, which underwent significant deactivation. Table I1 summarizes the initial activities of the various catalysts mole fractions of CO, in terms of overall conversion (XJ, methyl formate, and selectivity, SMeF. Equilibrium conversion at 220 "C is ca. 40%. The conversions were calculated on the assumption that CO is produced only by decarbonylationof methyl formate (Higdon et al., 1974). HCOOCHS + CO
+ CH30H
Experiments conducted at long contact times showed that decarbonylation of methanol CO + 2H2
CHSOH
(3)
is not significant below 240 "C. Trace amounts of carbon dioxide (
