Process for preparing phosphorus-doped silver catalysts

Details Process for preparing phosphorus-doped silver catalysts Process for preparing phosphorus-doped silver catalysts

1997-09-15

1998-12-29

Lewis, Michael L.

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

Regenerating or rehabilitating catalyst or sorbent

By heat

502208, 502347, C01J 2034, C01J 3802, C01J 2714, C01J 2350

Patent

active

058541632

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a process for preparing phosphorus-doped silver catalysts from silver contaminated with phosphorus compounds.
2. Description of the Background
Processes for preparing silver catalysts suitable for oxidizing methanol to formaldehyde are generally known (cf. Ullmann's Enzyklopadie der technischen Chemie, 3rd Edition, Urban und Schwarzenberg, Munich-Berlin, 1956, Volume 7, pages 660 to 663). In this process, silver is anodically oxidized in an electrolytic cell to silver ions, and cathodically reduced again to silver. An aqueous solution of silver nitrate and nitric acid is recommended as electrolyte. The coarsely crystalline silver formed at the cathode is suitable as catalyst for synthesizing formaldehyde from methanol.
According to DE-A 1166171, particularly high yields can be obtained in the oxidation of methanol to formaldehyde on use of a silver catalyst prepared by the following process: silver which has, where appropriate, initially been heated at 500.degree. to 600.degree. C. in a stream of oxygen-containing gases is subjected to electrolysis at least twice, with the current density in the first electrolysis or the initial electrolyses being above 250, in particular from 300 to 500, A/m.sup.2 of cathode area and in the last electrolysis being below 250, in particular from 100 to 200, A/m.sup.2 of cathode area.
EP-A 0104666 discloses that the selectivity in the preparation of formaldehyde from methanol using a silver catalyst can be increased by using small amounts of phosphorus compounds as promoters in addition to the silver catalyst.
Advantageous effects occurring on use of phosphorus compounds as promoters for the oxidation of methanol to formaldehyde in the presence of a silver catalyst are furthermore disclosed in CN-A 85100530, EP-A 0467169 and JP-A 38227/83.
EP-A 0467169 describes the preparation of a fixed bed catalyst which is composed of layers of silver crystals which contain a phosphorus-containing salt in powder form as promoter. Particularly high yields and conversions can be obtained in the preparation of formaldehyde from methanol using this phosphorus-doped fixed bed silver catalyst.
The commercial use of the processes for preparing formaldehyde using phosphorus compounds as promoters has hitherto been impeded by the fact that no economic processes are known for regenerating the used silver catalysts which result in these processes and which are contaminated with phosphorus compounds. A process for regenerating these catalysts is important for the economics of the preparation of formaldehyde because the catalysts have lost so much activity after about 8 weeks of use that the space-time yield is unsatisfactory.
Regeneration of silver catalysts contaminated with phosphorus compounds by one of the previously described processes for preparing silver catalysts results in silver catalysts which have little suitability for economic preparation of formaldehyde on the industrial scale for the following reasons:
When a silver catalyst used in a conventional continuously operated plant for the industrial production of formaldehyde loses its activity, it is necessary to stop the production process in order to be able to remove the catalyst from the plant and replace it by a regenerated catalyst with higher activity. When the synthetic process is started up again, it is necessary to preheat the stream of gaseous starting materials which is passed over the catalyst. Temperatures of about 300.degree. to 350.degree. C. are necessary for this in the case of regenerated catalysts which have been used without phosphorus compounds as promoters. Once the reaction has started, it is unnecessary for the stream of gas to be preheated further because the reaction zone (catalyst) heats up to about 700.degree. C. because of the heat liberated in the reaction for the formaldehyde synthesis.
On use of regenerated silver catalysts which have been used together with phosphorus compounds as promoters, it is necessary for the str

REFERENCES:
patent: 4235823 (1980-11-01), Dudeck et al.

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