Process for oxidation of methanol to formaldehyde with nitrous oxide

Formaldehyde is synthesized by continuously passing a gaseous mixture of methanol and nitrous oxide over a metallic silver-containing catalyst bed at a temperature of about 475 to about 675.degree. C.

TECHNICAL FIELD 
This invention relates to an improved process for the synthesis of 
formaldehyde by the oxidation of methanol over a silver catalyst at 
elevated temperatures. 
BACKGROUND ART 
Formaldehyde is commercially manufactured at the present time by passing 
methanol vapors in the presence of oxygen (air) over a silver catalyst, 
for example, over fine silver gauze, or granules, at temperatures in 
excess of 500.degree. C. or an oxide catalyst such as iron molybdate at 
lower temperatures. Formaldehyde may also be obtained in small yield by 
the partial oxidation of hydrocarbons such as ethane, ethylene and natural 
gas or by the oxidation of dimethylether. 
DISCLOSURE OF INVENTION 
According to this invention, there is provided a process for the 
preparation of formaldehyde involving contacting, for a period of time 
ranging between about 0.001 and about 0.2 second, a gaseous mixture of 
methanol and nitrous oxide in a methanol to nitrous oxide mole ratio 
ranging between about 0.5 and about 4.0 with a metallic silver-containing 
catalyst bed maintained at a temperature of about 475.degree. to about 
675.degree. C. The principle advantage of this process is that it results 
in a higher conversion of methanol and a higher selectivity to 
formaldehyde than prior art processes utilizing oxygen or air rather than 
nitrous oxide. 
As previously mentioned, formaldehyde is presently prepared by contacting 
methanol vapors with air or oxygen over a silver catalyst at elevated 
temperatures. The crux of the present invention lies in utilizing nitrous 
oxide instead of air or oxygen as the oxygen source with improved yeilds 
of formaldehyde. At the preferred operating temperatures, nitrous oxide is 
converted over silver, yielding gaseous nitrogen and atomic oxygen species 
adsorbed on the catalyst. The use of oxygen in the atomic form is believed 
to result in higher conversion of methanol and higher selectivity to 
formaldehyde. The principal advantage of this process lies in combining 
the quantitative decomposition of nitrous oxide with highly selective 
conversion of methanol to formaldehyde. 
In a preferred mode of operation, the process of the present invention 
involves the synthesis of formaldehyde by contacting a gaseous mixture of 
methanol and nitrous oxide with a bed of silver catalyst kept at the 
desired operating temperature in a continuous flow reactor. 
The feed contains a gaseous mixture of methanol and nitrous oxide in which 
the mole ratio of CH.sub.3 OH/N.sub.2 O ranges between about 0.5 and about 
4.0, with the range of about 1.0 to about 2.5 being preferred. In a 
preferred embodiment, the gaseous mixture of methanol and nitrous oxide is 
diluted with an inert gas. The term "inert gas" is intended to mean gases 
which are not reactive with methanol, formaldehyde, nitrous oxide or the 
silver catalyst under the reaction conditions, and includes helium, neon, 
argon and nitrogen, as well as mixtures thereof. 
The catalyst employed must contain metallic silver. It can be used as pure 
metal, for example, silver gauze or granules, or as silver supported on an 
inert material such as .alpha.-alumina, or as an alloy of silver with 
other metals, such as gold and copper. Pure silver catalysts, unsupported 
or supported on .alpha.-alumina, are preferred over the alloy catalysts. 
Supported catalysts should contain a minimum of at least about 3% by 
weight of silver. Alloys should contain at least 2.5 mole percent of 
silver. 
The catalyst bed can be operated at temperatures between about 475.degree. 
and about 675.degree. C. with the preferred operating temperature ranging 
between about 550.degree. and about 650.degree. C. The contact time of the 
gaseous reactants with the catalyst should be short enough to avoid 
further conversion of formaldehyde and recombination of the atomic oxygen 
species, but long enough to maximize methanol conversion. Contact times 
ranging between about 0.001 and about 0.2 second are satisfactory, with 
contact times of about 0.001 to about 0.1 second being preferred. 
Experience has shown that the shorter the contact time, the better the 
selectivity achieved. In commercial operations contact times of about 
0.001 to about 0.005 are most preferred. These short contact times, 
however, are difficult to achieve under laboratory conditions. 
BEST MOLD 
The best mode contemplated for carrying out the invention involves 
contacting, for a period of time ranging between 0.03 and 0.1 second, a 
gaseous mixture of methanol and nitrous oxide diluted with an inert gas, 
in which the mole ratio of methanol to nitrous oxide is within the range 
of 1.0 to 2.5, with a pure metallic silver catalyst maintained at a 
temperature of 550.degree. to 650.degree. C.

The following examples more fully illustrate the invention. 
EXAMPLE 1 
A. A mixture of CH.sub.3 OH and N.sub.2 O in helium, in which the mole 
ratio of CH.sub.3 OH to N.sub.2 O was 2, was passed over 0.5 g. of silver 
granules having an average particle diameter of 2.times.1 mm. at 
599.degree. C. with a contact time of 0.1 second. After 59 minutes on 
stream, an 86% conversion of methanol and a 63% yield of formaldehyde was 
achieved. 
B. The above process was repeated with the exception that the concentration 
of N.sub.2 O in the feed was increased by a factor of 2 so that the mole 
ratio of CH.sub.3 OH to N.sub.2 O was 1. The result was a 92% methanol 
conversion to give a 56% yield of formaldehyde. 
C. For comparison, part A above was repeated with the exception that 
molecular oxygen was substituted for N.sub.2 O. In order to equate the 
oxidizing power of the oxygen to that of nitrous oxide, the mole ratio of 
methanol to oxygen was 4.0. After 71 minutes at 595.degree. C., the 
conversion of methanol was 50% with a 54% yield of formaldehyde. 
EXAMPLE 2 
A mixture of CH.sub.3 OH and N.sub.2 O in helium, in which the mole ratio 
of CH.sub.3 OH to N.sub.2 O was 1.57, was oxidized over the metallic 
silver catalyst described in Example 1 at a contact time of 0.08 second. 
When the temperature was 550.degree. C. 85% methanol conversion and 66% 
formaldehyde yield were observed. At 650.degree. C. the methanol 
conversion was 81% and the formaldehyde yield was 71%. 
EXAMPLE 3 
A mixture of CH.sub.3 OH and N.sub.2 O, in which the mole ratio of CH.sub.3 
OH to N.sub.2 O was 1.82, was oxidized over a 0.5 g. bed of metallic 
silver at a contact time of 0.05 second. When the catalyst bed was 
maintained at 550.degree. C., an 89% methanol conversion and a 62% 
formaldehyde yield were observed. At 600.degree. C., the methanol 
conversion was 78% and the formaldehyde yield was 75%. 
EXAMPLE 4 
In a flow reactor, 0.5 g. of a catalyst containing 17% Ag supported on 
.alpha.-Al.sub.2 O.sub.3 (Harshaw Co.) was prereduced in situ in a stream 
of H.sub.2 /He at 300.degree. C. 
At temperatures of 600.degree. C. and higher, N.sub.2 O is quantitatively 
converted when feeds containing 20 to 40 mole percent N.sub.2 O and 30 
mole percent CH.sub.3 OH are passed through the bed of catalyst. At 
600.degree. C., conversion of CH.sub.3 OH is of the order of 75%, with the 
yield of HCHO in the range of 72-75%. At 625.degree. C., conversion of 
CH.sub.3 OH is 91% (after one hour on stream at that temperature) with the 
yield of HCHO being 52%. The contact time averaged 0.05 second. 
When oxygen was substituted for N.sub.2 O in the feed over the same 
catalyst, the conversion of methanol was lower than 50% and the 
formaldehyde yield was below 30% after two hours on stream at 600.degree. 
C. 
EXAMPLE 5 
A copper-silver alloy containing 2.5 mole % of silver was prepared in the 
form of shavings about 2 mm. long. The alloy catalyst was oxidized during 
its preparation at high temperatures; its performance varied with aging in 
the reactor as a function of the extent of reduction in the feed mixture 
which contained 29.5 mole percent CH.sub.3 OH and 38.5 mole percent 
N.sub.2 O. 
Conversion of methanol was high on the fresh catalyst, while selectivity to 
formaldehyde was initially low. For example, there was an 87% CH.sub.3 OH 
conversion at 600.degree. C. (contact time averaged 0.05 second), and a 
35% formaldehyde yield. After 3 hours on stream, conversion dropped to 
58%, but the yield of formaldehyde rose to 72%. Further aging of the 
catalyst resulted in still lower conversions. At the same time, the 
unreacted N.sub.2 O level in the off-gas increased with further aging of 
the catalyst, i.e., with increased reduction of the catalyst. 
EXAMPLE 6 
A silver-gold alloy catalyst in the form of shavings about 2 mm. in length 
and having an Ag:Au atomic ratio of 1:1 was used. After one hour on stream 
at 650.degree. C. (contact time averaged 0.5 second), at 60% methanol 
conversion, a 60% formaldehyde yield was recorded using a feed containing 
51 mole percent CH.sub.3 OH, 29.5 mole percent of N.sub.2 O and the 
remainder helium. Increasing the concentration of N.sub.2 O to 40 mole 
percent resulted in 98% conversion of methanol at 600.degree. C., but 
decreased the formaldehyde yield to about 30%. 
EXAMPLE 7 
The following procedure was used to compare the use of air and nitrous 
oxide as an oxygen source. A 10 mm I.D. quartz tube was filled to a depth 
of 19.05 mm (3/4 inch) with metallic silver catalyst which was in the form 
of irregular or polysurface granules in the 8-60 mesh particle size range. 
The catalyst section was heated externally to initiate the reaction, and 
once initiated, external heat was adjusted to run the process at the 
desired temperature. The bed and wall temperatures were recorded using 
appropriate thermocouples. 
For evaluation of the oxygen source, liquid methanol (0.5 g/min.) was 
vaporized, mixed with enough preheated air or nitrous oxide to furnish the 
desired methanol conversion, and passed through the catalyst bed at 
550.degree.-650.degree. C. The product was quickly cooled to below 
200.degree. C. and analyzed by gas chromatography to determine yield and 
conversion. The following results were obtained: 
______________________________________ 
MeOH Oxygen 
Conversion % 
Source Temp. .degree. C. 
Yield to HCHO % 
______________________________________ 
65.0 N.sub.2 O 
628 94.5 
65.0 Air 620 92.5 
______________________________________ 
EXAMPLE 8 
The following comparative example illustrates a two-stage operation using 
air as the oxygen source in the first stage and nitrous oxide or air as 
the oxygen source in the second stage. Two-stage operation was carried out 
using two 10 mm I.D. quartz tubes filled with metallic silver catalyst as 
described in Example 7 in series. The procedure was similar to that used 
in Example 7 except that the product from the first stage was mixed with 
additional preheated air or nitrous oxide to furnish the desired overall 
methanol conversions. The conditions in the first stage were as follows: 
MeOH conv.=65.0% 
Yield to HCHO=92.5% 
Temp.=620.degree. C. 
The following results were obtained in the second stage. 
______________________________________ 
Overall MeOH 
Oxygen Source 
Second Stage 
Yield to 
Conversion % 
in Second Stage 
Temp. .degree. C. 
HCHO % 
______________________________________ 
99.3 N.sub.2 O 645 86.1 
97.4 N.sub.2 O 625 87.5 
97.4 Air 615 82.2 
______________________________________ 
EXAMPLE 9 
The following comparative example illustrates the yield improvement 
obtained using either air or nitrous oxide as the oxygen source in both 
stages. Two-stage operation was carried out using the reactors described 
in Example 8. The following results were obtained. 
______________________________________ 
MeOH First Stage 
Oxygen Sec. Stage 
Yield to 
Conversion % 
Temp. .degree. C. 
Source Temp. .degree. C. 
HCHO % 
______________________________________ 
95.2 590 N.sub.2 O 
570 89.2 
96.8 590 " 600 89.5 
98.7 590 " 640 88.5 
92.6 565 Air 540 85.8 
93.9 570 " 560 84.2 
97.1 620 " 590 81.8 
97.4 610 " 615 82.2 
______________________________________ 
EXAMPLE 10 
This example illustrates a supported catalyst containing a relatively low 
concentration of silver. 
A 0.5 g. (0.446 cc)-portion of 10-20 mesh catalyst containing 3.5-4% Ag on 
Al.sub.2 O.sub.3 was loaded into a quartz reactor surrounded by a furnace. 
A mixture of CH.sub.3 OH (31.7 mole %) and N.sub.2 O (20.1 mole %), i.e., 
CH.sub.3 OH/N.sub.2 O=1.58, in helium was fed into the catalyst bed for a 
contact time of 0.035-0.030 seconds. After 10 minutes on stream at 
630.degree. C. the conversion of methanol was 83.6% and the yield of HCHO 
was 21.5%. After 93 minutes on stream, the conversion was 60% and the 
yield of HCHO was 67.6%. 
INDUSTRIAL APPLICABILITY 
The process described herein is suitable for commercial production of 
formaldehyde.