Process for the preparation of formaldehyde

Details Process for the preparation of formaldehyde Process for the preparation of formaldehyde

1998-03-13

1999-11-23

Wilson, James O.

Organic compounds -- part of the class 532-570 series

Organic compounds

Oxygen containing

568471, 568472, C07C 4500

Patent

active

059903589

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a process for preparing formaldehyde by oxidative dehydrogenation of methanol.
2. Description of the Background
Processes for preparing formaldehyde by oxidative dehydrogenation of methanol over a catalyst fixed bed made up of silver crystals are generally known (cf. Ullmanns Enzyklopadie der technischen Chemie, 3rd Edition, Urban und Schwarzenberg, Munich/Berlin, 1956, Volume 7, pages 660 to 663).
Advantageous effects which occur when using phosphorus compounds as promoters for the oxidation of methanol to give formaldehyde in the presence of a silver catalyst are also known from CN-A-85100530, DE-A-4022603 and JP-A-38227/83.
EP-A-0 467 169 describes the preparation of formaldehyde by oxidative dehydrogenation of methanol over a catalyst fixed bed which is made up of layers of silver crystals containing a pulverulent phosphorus-containing salt as promotor. This phosphorus-doped silver catalyst fixed bed is prepared by bringing it into contact with a phosphorus compound prior to the commencement of the formaldehyde preparation.
However, these processes are still capable of improvement because the catalyst continuously loses activity during use for the formaldehyde preparation, which is reflected in a decreasing yield. For this reason, the catalyst has to be replaced and regenerated after a relatively short time, for which purpose the formaldehyde preparation process has to be interrupted. Since the replacement of the catalyst fixed bed and the measures associated with the resumption of the formaldehyde preparation process are time-consuming and laborious, the economics of the process depend on how frequently this replacement has to be carried out.


SUMMARY OF THE INVENTION

It is an object of the present invention to develop a process in which the time between the required replacement of the catalyst is prolonged.
We have found that this object is achieved by a process for preparing formaldehyde by oxidative dehydrogenation of methanol, which comprises passing a gas mixture (i) comprising methanol, oxygen, oxide and fixed bed (a) and applying from 0.01 to 100 ppm by weight of phosphorus, based on the phosphorus-doped silver catalyst fixed bed, 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) to the phosphorus-doped silver catalyst fixed bed (a) per kg of methanol which is passed through in the form of the gas mixture (i) per cm.sup.2 of the cross-sectional area of the phosphorus-doped silver catalyst fixed bed (a).


DETAILED DESCRIPTION OF THE INVENTION

These data are based on a pressure of 1 bar.
The gas mixture generally contains from 0.25 to 0.60, preferably from 0.35 to 0.50, mol of oxygen per mol of methanol and from 0.2 to 3.0, preferably from 0.67 to 1.75, mol of water per mol of methanol and from 0.9 to 2.3, preferably from 1.3 to 1.8, mol of nitrogen per mol of methanol.
Phosphorus-doped silver catalysts suitable for the process of the present invention are, for example, those obtainable by of silver from an aqueous silver salt solution to form a starting silver catalyst fixed bed (a), and subsequently from 1 to 20,000 ppm by weight of phosphorus, based on the silver, in the form of the phosphorus compound (P) before the gas mixture (i) is passed at from 150 to 800.degree. C. through the fixed bed.
The preparation of the silver crystals described in step I is generally known (cf. Ullmann's Enzyklopadie der technischen Chemie, 3rd Edition, Urban und Schwarzenberg, Munich/Berlin, 1956, Volume 7, pages 660 to 663). Particularly good results can be achieved using the starting catalyst fixed beds described in DE-A-2322757.
Suitable silver crystals are obtained particularly when the electrolysis is carried out according to the process described in the German Patent 1166171.
As electrolyte, preference is given to using an aqueous silver nitrate solution. This silver nitrate solution generally has a pH of from

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