Packed silver-catalyst bed doped with phosphorus

Details Packed silver-catalyst bed doped with phosphorus Packed silver-catalyst bed doped with phosphorus

1998-08-05

1999-11-16

Bell, Mark L.

Catalyst, solid sorbent, or support therefor: product or process

Catalyst or precursor therefor

Phosphorus or compound containing same

502 34, 502 38, 502 56, 502344, 502347, 502353, B01J 2714, B01J 2034, B01J 2302, B01J 2348

Patent

active

059857862

DESCRIPTION:

BRIEF SUMMARY
THE PRESENT INVENTION RELATES TO A PHOSPHORUS-DOPED SILVER CATALYST FIXED
BED OBTAINABLE BY from an aqueous silver salt solution to form a starting silver catalyst fixed bed, silver catalyst fixed bed by passing through the latter, at from 150 to 800.degree. C., a gas mixture comprising methanol and oxygen (gas mixture M), and from 1 to 20,000 ppm by weight of phosphorus, based on the silver, in the form of a finely divided phosphorus compound having a melting point or decomposition temperature of more than 500.degree. C. (phosphorus compound P).
The invention further relates to a process for producing this and also a process for preparing formaldehyde using a phosphorus-doped silver catalyst fixed bed according to the present invention.
Silver catalysts which are suitable for the oxidation of methanol to give formaldehyde are generally known (cf. Ullmann's Enzyklopadie der technischen Chemie, 3rd edition, Urban und Schwarzenberg, Munich-Berlin, 1956, Volume 7, pp. 660 to 663). According to this process, silver is oxidized anodically in an electrolysis cell to form silver ions and these are again cathodically reduced to give silver. The coarsely crystalline silver formed at the cathode is suitable as catalyst for the synthesis of formaldehyde from methanol.
Advantageous effects which occur when using phosphorus compounds as promoters for the oxidation of methanol to formaldehyde in the presence of a silver catalyst are also known from CN-A-85 100 530, EP-A-0 467 169 and JP-A-38227/83.
EP-A-0 467 169 describes the production of a catalyst fixed bed built up of layers of silver crystals containing a pulverulent phosphorus-containing salt as promoter. In this process, the phosphorus-containing salt is applied to the silver catalyst before the silver catalyst is brought into contact with a hot gas mixture comprising oxygen and methanol.
However, the catalyst fixed beds described in that reference are in a modification of low activity immediately after application of the phosphorus-containing salt and transform only relatively slowly into a modification of high activity by means of which formaldehyde can be prepared in high yield and at a high conversion. This is associated with the following problems:
A catalyst fixed bed described in EP-A-0 467 169, which is used for the industrial production of formaldehyde in a generally customary, continuously operated plant, loses activity with time so that the yield drops. Therefore, to be able to operate the plant economically the used catalyst fixed bed has to be replaced by a fresh one after a certain operating time, for which purpose the synthesis process has to be interrupted. In order to subsequently restart the synthesis process, it is necessary to preheat the catalyst fixed bed to at least 360.degree. C. so as to make available the activation energy required for the reaction. However, owing to the fact that the heat is rapidly given off because of the high surface area of the catalyst it is technically not simple to achieve such high temperatures. In practice, the catalyst fixed bed is heated, for example, by passing a hot inert gas stream (for example nitrogen or carbon dioxide) through the catalyst fixed bed and, when the catalyst has reached the required temperature, adding the gaseous reactants to the gas stream. Once the reaction has started and the catalyst has reached its full activity the heating of the catalyst fixed bed is no longer necessary since the oxidation of the methanol to formaldehyde is exothermic and the reaction zone, ie. the catalyst fixed bed, heats up to from 500 to 700.degree. C. and thus has the minimum temperature required for the activation of the reaction. Since the catalyst fixed bed does not yet have its full activity immediately after the reaction starts, only small amounts of the reactants may be passed through the catalyst fixed bed at the beginning, since otherwise the reaction proceeds with little selectivity or there is even the danger of the catalyst fixed bed assuming a modification in which it is inactive. Only after abou

REFERENCES:
patent: 4235823 (1980-11-01), Dudeck et al.
patent: 5854163 (1998-12-01), Diercks et al.

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