DE1693092C - Process for the production of formaldehyde by the oxidation of methanol - Google Patents

Description

The invention relates to a process for the production of formaldehyde by oxidation of methanol in the presence of an unsupported catalyst which contains molybdenum oxide and iron oxide in a MoO ₃ : Fe ₂ O ₃ ratio between 3: 1 and 8: 1 and is gradually heated up to 450 ° C has been.

Such catalysts are known per se. These known catalysts are characterized in that they have a specific surface, measured by the BET method (by adsorption of nitrogen at the temperature of liquid nitrogen) of 4 to 15 m ² / g and a pore volume between 0 , 15 and 0 _T 60 cm ³ / g is, the pore volume is calculated on the basis of a DICHTR the crystallized compound of 4.6 g / cm ^3, which corresponds to a specific volume of 0.22 cm ³ / g corresponds.

It has been found that the activity of such a catalyst decreases with use after some time. At the same time, the specific surface area and the pore volume also decrease. It was therefore sought a means to measure the specific surface area and the pore volume of this catalyst to stabilize.

It is already an unsupported catalyst based on molybdenum oxide and iron oxide for manufacture of formaldehyde by oxidation of methanol known (French patent 1239S46), which has a chromium content of 0.1%. This chromium was not considered to be an effective component in the known catalyst, but rather as an impurity which is undesirable in and of itself, but which did not result in any disadvantageous consequences.

The invention is based on the knowledge that an increase in the chromium content of the catalyst leads to a considerable improvement in the properties of the catalyst. According to the invention, the oxidation in the above-mentioned process is therefore carried out in the presence of a catalyst which additionally contains 0.2 to 1% chromium as chromium oxide. It has also proven to be particularly advantageous to carry out the oxidation in the presence of a catalyst whose specific surface area is 4 to 15 m ² / g and whose pore volume. is between 0.15 and 0.60 lnS / g.

It has been found that when a catalyst made from Cr ₂ O ₃ and 3 MoO ₃ precipitated at the same time is used to oxidize the methanol, the specific surface area of the catalyst increases considerably in the course of the first 80 hours of use. Accordingly, the activity of this catalyst also increases considerably during this time. In contrast, its selectivity decreases. Chromium molybdate cannot be used as such for the oxidation of methanol, since its selectivity is too low and is still reduced during use. But it has

ίο reveal that when one is in appropriate proportion the chromium molybdate, the surface area of which increases with use, and the iron molybdate, the Surface decreases with use, combined, a catalyst with improved stability

! 5 is obtained. In addition, you can not only Stabilize the surface by adding chromium oxide, but you can also add some improve other properties.

The role that chrome plays in this depends on the

jo condition of the catalytic converter before use.

1. Resim the fresh catalyst before application has a considerable specific surface area and a considerable pore volume, so the presence an appropriate amount of chromium die

Effect that not only the specific surface, but to a certain extent also that Pore volume is retained. The consequence is that both the activity and the selectivity of the catalyst at a greater height

being held.

2. Was the catalyst manufactured by a process that is proportionate to the catalyst there is a suitable amount

Chromium in the catalyst has the effect that not only the specific surface, but up to the pore volume also increases to a certain extent during use. It follows a significant increase in activity and only

^4a a certain reduction in selectivity. However, this reduction is small compared to the increase in the degree of conversion. During use, the specific surface area approaches a certain maximum,

that of the chromium content and the catalyst temperature depends.

In the above-mentioned known catalyst, the chromium content of 0.1% was too low to achieve the to obtain specific surface and pore volume. The chromium content that was too low couldn't cause the catalyst improvements listed under 1. and 2. above.

The following comparative tests show the effect of the catalyst according to the invention. In all experiments, the catalyst was produced by precipitation from aqueous solutions of ammonium molybdate and iron chloride or iron chloride and chromium chloride, filtering off, washing and drying of the precipitate. 100 cm ^{3 of} the catalyst formed into tablets was placed in a tubular stainless steel reaction vessel of 20 mm inside diameter fitted with an axial tube made of Pyrex glass with an outside diameter of 3.7 mm and placed in a bath of molten sodium and potassium nitrate would. The layer height was 30 cm, in Example 2 58 cm.

Comparison test without chromium oxide

After the catalyst had been roasted for 27 hours at 350 ° C. in a stream of air, a gas mixture of air and methanol vapor was introduced. At a throughput of 12,000 Nm ³ feed gas per cubic meter of catalyst and per hour, at a temperature of the salt bath of 280 ⁰ C and with a gas mixture content of 6.5 percent by volume CH ₃ OH, the overall degree of conversion was 96% and the selectivity 94%, that is, 90% of the methanol introduced was converted into CH ₂ O. The space-time yield was 0.50 kg of CH ₂ O, corresponding to 2.6 kg of 36% strength aqueous formaldehyde solution per liter of catalyst per hour.

At the hottest. Steüe remained throughout the experiment, the catalyst temperature below 440 ⁰ C. The body t'er highest temperature in the catalyst layer was the experiment during the first 300 hours stable, but during the following 400 hours wandered the hottest point approximately 10 cm below the maximum position at 15cm, which indicates that some inactivation of the catalyst occurred in the hottest zone. After about 1200 hours of experimentation, the conversion and the selectivity had remained the same, but the bath temperature had risen by a few degrees.

Before the start of the experiment, the specific surface area of the catalyst was 11.4 m ² / g and after the experiment 3 m * / g in the middle of the layer. The pore volume decreased from 0.31 to 0.13 cm ³ / g during the same time.

example 1
(with chromium oxide)

In this experiment, a catalyst was used with one after radiation chemistry Procedure determined chromium content of 0.2% chromium oxide.

A mixture of air and methanol containing 6.5 percent by volume of methanol was passed through the reaction vessel. The throughput was 20,000 Nm ³ feed gas per cubic meter of catalyst and hour. The temperature was 350 to 420 ° C. The activity of the catalyst, expressed by the average degree of conversion of the methanol, was 98.4 mol percent in 16 experiments, and the selectivity was 92.2 mol percent. As a result, 90.6% of the methanol was converted into CH ₂ O.

The specific surface area of the catalyst was 6.3 m ^{2 / g before it was used and 7.3 m a} / g after 575 hours of use. The pore volume increased from 0.32 to 0.36 cm ³ / g within the same time.

Example 2
(with chromium oxide)

The same catalyst was used as in Example 1. In this experiment, 200 cm ³ catalyst tablets were placed in the reaction vessel, the layer height was 58 cm. A gas mixture of air and methanol vapor containing 6.5 percent by volume of methanol was passed through the reaction vessel. The throughput was 12, 000 Nm ³ feed gas per cubic meter of catalyst per hour, the temperature of the salt bath was 240 ⁰ C. The total degree of conversion was 99.2 mol percent and ^the selectivity 93 1 mole percent i.e., 92 3 mol percent were of the introduced methanol in CH ₂ O converted. After about 2000 hours, the conversion and the selectivity had remained the same.

After this time, the experiment was stopped, but the life of the catalyst is without Doubt much longer.

The pressure drop in the catalyst layer changed from 200 mm Hg at the beginning to 270 mm Hg

ία after 2060 hours of operation at a throughput of 10,000 Nm ³ feed gas per cubic meter of catalyst and hour. This change is relatively small.

The temperature of the hottest point of the catalyst was throughout the experiment below 430 ⁰ C. and its location was determined at the beginning of 25 cm and after 2060 hours of operation with 26cm, ie. The deactivation of the catalyst in the hottest zone was only very slight.

The specific surface area had changed from 6.1 m ² / g before the start of the experiment to 5.5 to 11.5 m ² / g after the end of the experiment, depending on the position of the layer: 5.5 m ² / g in the hottest Zone, 11.5 m ² / g in the coldest zone. The pore volume had changed from 0.32 cm ³ / g to ¹ "0.36 to 0.32 cm ³ / g depending on the position in the layer.

The experiments show that the specific surface and the pore volume when using a Katiilvsators with chromium oxide remained almost constant, while the catalyst without chromium oxide, like itself from the comparative experiment shows a very strong reduction in the specific surface and the Shows pore volume.

B e i s ρ i e 1 3

The catalyst used had a chromium oxide content of 0.43% and at the start of the experiment a relatively small specific surface. This experiment shows the great magnification of the specific surface, during use, and that with a higher chromium content than in Example 1 the Surface is kept at a value that is higher than in example 1.

The experiments were carried out in the same way as in Example 1. Eight analyzes showed an average degree of conversion of the methanol of 99.9 mol percent, while the selectivity was 92.4%, ie 92.3% of methanol was in CH ₂ O convicted.

Before the experiment, the catalyst had a specific surface area of 6.3 m ² / g, and after 372 hours of operation it had a specific surface area of 11.4 m ² / g.

The pore volume remained ^almost constant during the same time. It decreased from 0.55 cm ³ / g before the test to 0.53 cm ³ / g after the test, that is, “the change was less than the error limits of the measurement.

About the superiority and the technical progress of the new catalytic converter over the known one Catalyst, which has a chromium oxide content below 0.1%, to clarify, are in the the following table gives comparative values. The comparison values relate to that in example 1 the known catalyst described in French patent 1 239 546 and in Example 2 new catalyst described in this patent:

Throughput Nm ³ cm ³ ■ h

Total degree of conversion in mole percent

Methanol content in 36 to 37% formaldehyde solution (proportion by weight)

Yield kg

Catalyst hour

CH ₄ O / liter

Known catalyst

after

French patent specification

1 239 (example 1)

8900 90.6

0.7 0.7

Catalyst according to the invention

(Example 2)

12,000 92.3

0.3

As can be seen from the table, the Gesarot-

conversion rate by the new catalyst

increased about 2% at a space velocity

which is 30% higher than that of the well-known

lysators lies. It is particularly noteworthy that

that just the increase in the degree of conversion

91% is associated with particular difficulties.

Also in terms of breaking strength is the new one

Catalyst superior to the known catalyst.

As described in the above-mentioned French patent, the known catalyst has an average breaking strength of 7.4 kg per tablet with a length and height of 3.5 mm. In contrast, test tablets of the new catalyst with the same dimensions have an average breaking strength of 12.5 kg y. · Tä'üicitc.

Claims (2)

Patent claims: 1. Process for the production of formaldehyde by oxidation of methanol in the presence of an unsupported catalyst which contains molybdenum oxide and iron oxide in the ratio MoO ₃ : Fe ₂ O ₃ between 3: 1 and 8: 1 and has been gradually ^{heated up to 450 0} C, characterized in that the oxidation is carried out in the presence of a catalyst which additionally contains 0.2 to 1% chromium as chromium oxide. 2. The method according to claim 1, characterized in that the oxidation in counter-WEr * e! Ne £ iC2t3l ^v SHtOrS dUrC ^ fU ^ ¹¹ ^ Hp «cpn cnp. Specific surface area is 4 to 15 m ² / g and its pore volume is between 0.15 and 0.60 ml / g.