Process for preparing phosphorus-doped silver catalysts

A process for preparing phosphorus-doped silver catalysts from silver contaminated with phosphorus compounds, which comprises PA1 a) heating the silver which is contaminated with phosphorus compounds to temperatures at which the silver is liquid and subsequently cooling to temperatures below the melting point, PA1 b) preparing silver crystals from the silver obtained in stage a) by anodically oxidizing it to silver ions in an electrolysis cell with an aqueous electrolyte, and cathodically reducing the silver ions again to elemental silver, and PA1 c) contacting the silver crystals with a finely divided phosphorus compound with a melting point or decomposition temperature above 500.degree. C. (phosphorus compound P).

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 stream of 
gaseous starting materials to be preheated to at least 700.degree. C. for 
the reaction to start up. 
The preheating of the stream of gas to such a high temperature is very 
costly industrially. The equipment necessary for this is not available in 
the plants normally used for industrial production of formaldehyde. 
Gmelins Handbuch der anorganischen Chemie, Verlag Chemie GmbH 1970, 8th 
Edition, Part A2, System Number 61, pages 34 to 35, discloses the 
preparation of pure silver by concentrating crude silver by a melt process 
(cupellation) as far as possible and subsequently further purifying the 
silver by electrolysis. In this case, the silver is dissolved anodically 
and deposited cathodically in a solution of silver nitrate in nitric acid. 
The cupellation in which crude silver is heated in the presence of air at 
from 900.degree. to 1100.degree. C. (cf. loc. cit. pages 7 to 9) is, 
however, recommended only for crude silver which is contaminated with Pb, 
Cu, As, Sb, Zn, Se and Te. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a process for preparing 
phosphorus-doped silver catalysts which does not have the prior art 
disadvantages. 
We have found that this object is achieved by a process which comprises 
a) heating the silver which is contaminated with phosphorus compounds to 
temperatures at which the silver is liquid and subsequently cooling to 
temperatures below the melting point, 
b) preparing silver crystals from the silver obtained in stage a) by 
anodically oxidizing it to silver ions in an electrolysis cell with an 
aqueous electrolyte, and cathodically reducing the silver ions again to 
elemental silver, and 
c) contacting the silver crystals with a finely divided phosphorus compound 
with a melting point or decomposition temperature above 500.degree. C. 
(phosphorus compound P). 
DETAILED DESCRIPTION OF THE INVENTION 
A particularly suitable starting material for the process according to the 
invention is silver which contains about 0.01 to 1000 ppm, preferably 1 to 
100 ppm, phosphorus or phosphorus compounds, calculated as elemental 
phosphorus, but is virtually free of impurities such as Pb, Cu, As, Sb, 
Zn, Se, Te and noble metals. Silver contaminated in this way results, for 
example, when silver is used as catalyst together with 
phosphorus-containing promoters for oxidizing methanol to formaldehyde. 
Processes which result in catalysts which are contaminated in this way and 
require regeneration are disclosed, for example, in EP-A 0104666, CN-A 
85100530 and JP-A 38227/83. 
The catalysts used in these processes are obtained, for example, by silver 
being impregnated with solutions of phosphorus or organic phosphorus 
compounds, eg. triphenylphosphine, or inorganic phosphorus compounds, eg. 
orthophosphoric acid, or exposed to phosphorus vapor. 
The phosphorus-contaminated silver which is particularly preferably 
employed is a used phosphorus-doped silver catalyst as results from the 
preparation of formaldehyde by oxidative dehydrogenation of methanol by 
the process described in EP-A 0467169. 
In this process, an initial phosphorus-doped silver catalyst which is 
obtainable by contacting silver crystals which are virtually free of 
phosphorus compounds and other impurities with a phosphorus-containing 
salt, eg. phosphates or polyphosphates of alkali metals or alkaline earth 
metals, by a generally known process (cf. Ullmann's Enzyklopadie der 
technischen Chemie, 3rd Edition, Urban und Schwarzenberg, Munich-Berlin, 
1956, Volume 7, pages 660 to 661) is used for the oxidative 
dehydrogenation of methanol to formaldehyde. 
The melting of the silver in step (a) of the process according to the 
invention preferably takes place in the presence of from 1 to 100% by 
weight, based on the silver, of a compound or of a mixture of compounds 
selected from a group of alkaline earth metal oxides, preferably calcium 
oxide, silicon dioxide or aluminum oxide. 
Particularly suitable for this purpose are ternary and binary mixtures of 
silicon dioxide, aluminum oxide and calcium oxide, which are particularly 
preferably used as eutectic mixtures in order to keep the melting point as 
low as possible. 
The silver is generally heated in the presence of oxygen for from 0.5 to 10 
min at temperatures at which the silver is liquid under normal conditions, 
preferably at from 960.8 to 1500.degree. C. If the silver is melted in the 
presence of additives, the temperature is beneficially above the melting 
point of these additives. 
The silver is then cooled. It is advantageous to do this by pouring the 
molten silver into water in order to remove any water-soluble impurities. 
In the next step (b), the silver is subjected to electrolysis. In this, the 
silver is anodically oxidized to silver ions in an electrolysis cell with 
an aqueous electrolyte, and is cathodically reduced again to elemental 
silver. Processes suitable for this purpose are those generally known for 
the purification of silver by electrolysis (cf. Hollemann-Wiberg, Lehrbuch 
der anorganischen Chemie, 91st-100th Editions, Verlag Walter de Gruyter 
1985, pages 1012-1013). 
Elemental silver in the form of silver crystals which are particularly 
suitable as catalysts for formaldehyde synthesis are obtained, in 
particular, when the electrolysis is carried out by the process described 
in German Patent 1166171. 
An aqueous silver nitrate solution is preferably employed as electrolyte. 
This silver nitrate solution generally has a pH of from 1 to 4 and 
contains from 1 to 5% by weight of silver. The pH is beneficially adjusted 
with nitric acid. 
The electrodes used as those normally employed in the electrolysis of 
silver. Suitable anodes are bags into which the silver to be oxidized has 
generally been introduced as granules or powder. Silver plates are 
particularly suitable cathodes. 
The electrolysis is beneficially carried out at current densities of from 
80 to 500 A/m.sup.2 of cathode area and electrolyte temperatures of from 
10 to 30.degree. C. 
In order to reach these current densities, voltages of from 1 to 15 volts 
are necessary in most electrolysis cells. 
It is advisable continuously to remove the silver crystals from the cathode 
as they are formed. Silver crystals with a size of from 0.2 to 5 mm are 
generally obtained. 
In most cases a single electrolysis is sufficient to obtain usable silver 
crystals. 
The silver crystals are contacted in step (c) with a finely divided 
phosphorus compound with a melting point or decomposition temperature 
above 500.degree. C. (phosphorus compound P). Phosphorus-containing salts 
in powder form which have a melting point or decomposition temperature 
above 800.degree. C. are preferred. 
Examples of suitable salts are inorganic phosphates of alkali metals and 
alkaline earth metals, heavy metals, eg. Ag, Zn and Fe or of boron and 
ammonium. 
Phosphates or pyrophosphates of alkali metals or alkaline earth metals, eg. 
Na.sub.4 P.sub.2 O.sub.7, Li.sub.3 PO.sub.4, Mg.sub.3 (PO.sub.4).sub.2, 
Ca.sub.3 (PO.sub.4).sub.2, are preferred. 
The particle size of the compounds (P) is not critical and is, in general, 
from about 1 mm to 1 .mu.m. 
The phosphorus-doped silver catalyst preferably contains from 0.1 to 0.0001 
g, particularly preferably from 0.01 to 0.001 g, of compound (P), 
calculated as elemental phosphorus, per cm.sup.3 of silver crystals. 
The general procedure for preparing the phosphorus-doped silver catalysts 
from the silver crystals is as described in EP-A 0467169. In this process, 
the silver crystals are arranged to give a fixed bed catalyst which 
consists of several layers of the silver crystals, as disclosed, for 
example, in DE-A 2322757, and compound (P) in the form of a powder is 
applied to one or more of these layers, resulting in a phosphorus-doped 
fixed bed silver catalyst. 
In general, a phosphorus-doped fixed bed silver catalyst consists of from 4 
to 9 layers of silver crystals and has a total layer thickness of from 5 
to 50 mm. 
The amount of phosphorus compound (P), calculated as elemental phosphorus, 
is generally from 0.1 to 100 mg, preferably 0.5 to 50 mg, per cm.sup.2 of 
cross-sectional area of the phosphorus-doped fixed bed silver catalyst. 
A phosphorus-doped fixed bed silver catalyst can be prepared particularly 
simply by installing a fixed bed catalyst consisting of the silver 
crystals in a tubular reactor in such a way that the layers are arranged 
perpendicular to the direction of flow of a gas which is passed through 
the tubular reactor, and by subsequently distributing the compound (P) on 
the topmost layer of silver crystals. It is expedient for the 
cross-sectional areas of the tubular reactor and of the phosphorus-doped 
fixed bed silver catalyst to be approximately the same. 
These tubular reactors equipped with a phosphorus-doped fixed bed silver 
catalyst are suitable for preparing formaldehyde by oxidative 
dehydrogenation of methanol by passing methanol and oxygen through the 
tubular reactor at a temperature of, normally, from 500.degree. to 
750.degree. C. This process is disclosed, for example, in Ullmann's 
Enzyklopadie der technischen Chemie, 3rd Edition, Urban und Schwarzenberg, 
Munich-Berlin, 1956, Volume 7, pages 660 to 661, or in EP-A 0467169. 
The phosphorus-doped silver catalysts prepared in this way are particularly 
distinguished by it being sufficient to preheat the reactants in the 
presence of the catalysts to relatively low temperatures in order to start 
the formaldehyde preparation process, with which formaldehyde can be 
prepared with high conversion and high selectivity.

EXAMPLES 
A. Preparation of the phosphorus-doped silver catalysts 
Example 1 
A silver catalyst which had been prepared according to the example with the 
experiment no. 3 from EP-A 0467169 (charged with Na.sub.4 P.sub.2 O.sub.7) 
was operated for 2000 hours to synthesize formaldehyde. 
For workup, 50 g of the catalyst were initially washed with distilled water 
for 1 h. The content of phosphorus compounds was, measured as elemental 
phosphorus, 27 ppm. 
The catalyst was heated at 1200.degree. C. for 15 min and then poured into 
distilled water, after which the content of phosphorus compounds was less 
than 1 ppm (detection limit). 
A new silver catalyst was then prepared from the silver by the Mobius 
electrolytic process (cf. Ullmann's Enzyklopadie der technischen Chemie, 
3rd Edition, Urban und Schwarzenberg, Munich-Berlin, 1956, Volume 7, pages 
660 to 661). Electrolysis conditions: current density 120 A/m.sup.2 ; 
electrolyte temperature 27.degree. C.; aqueous silver nitrate solution 
with a pH of 2.5 and a silver concentration of 3 g/l. 
Example 2 
The silver catalyst was prepared as described in Example 1, but the used 
catalyst was melted in the presence of 25% by weight, based on the 
catalyst, of a mixture of aluminum oxide and calcium oxide in the ratio 
1:1 by weight. 
Comparative Example 
The silver catalyst was prepared as in the example in German Patent 
1166171. 
B. Properties of the silver catalysts 
The silver catalysts which had been prepared as in Examples 1 and 2 and the 
comparative example were used as described in the example in EP-A 0467169, 
together with Na.sub.4 P.sub.2 O.sub.7 as promoter, in the experimental 
reactor described therein. The experimental conditions were otherwise 
chosen as in experiment no. 3. It was possible to reproduce the result of 
experiment no. 3 with the catalysts of Examples 1 and 2. The reaction 
started when the starting materials were at 330.degree. C. 
When the catalyst from the comparative example was used, the reaction did 
not start despite heating the starting materials to 550.degree. C.