Catalyst for selective oxidation reactions

Silicate catalysts which can be employed for selective oxidation reactions of organic substances, a process for the preparation of these catalysts and their use are described. The catalyst is distinguished by high long-term activity, by the reaction temperature required for the reaction being lower in comparison with catalysts composed of pure silver and by higher conversions and selectivities in the oxidative dehydrogenation of alkanols in comparison with pure silver or silver on supports.

DESCRIPTION 
The invention relates to silicate catalysts which can be employed for 
selective oxidation reactions of organic substances, to a process for the 
preparation of these catalysts and to their use. 
Skeleton silicate catalysts, for example zeolites, are already employed on 
a large industrial scale as catalysts for the non-oxidative conversion of 
hydrocarbons, for example in catalytic cracking. 
It is also known that organic compounds can be oxidized selectively by 
means of zeolite catalysts, as is the case, for example, in the 
manufacture of acetone from propene. In this case a selectivity of 90% is 
achieved at a conversion of 50%. No statements are made concerning the 
long-term activity and the aging behavior of the catalysts used. 
It is also assumed that, because of their special structure, zeolites tend 
to accelerate the total oxidation of organic compounds to CO.sub.2 and 
water. 
The preparation of certain catalysts based on zeolites is described in East 
German Patent 113,173. In this case certain metals, such as vanadium 
and/or titanium, are incorporated into the zeolites by ion exchange. 
A "silver-on-pumice" or "silver-on-Al.sub.2 O.sub.3 " catalyst is 
frequently used for the oxydehydrogenation reactions of C.sub.1 -C.sub.4 
-alcohols, for example of methanol to give formaldehyde, which are of 
particular industrial importance. The preparation of a silver catalyst 
synthesized by impregnating the support, composed of Al.sub.2 O.sub.3 and 
SiO.sub.2 in the form of cristobalite, with silver nitrate solution is 
discussed in German Patent 3,037,536. A catalyst composed of a metal or 
ceramic support which has been coated or impregnated with metallic copper, 
silver, gold or iron (German Offenlegungsschrift 2,816,471) is also 
described for the preparation of formaldehyde. 
However, the catalysts mentioned, which are employed in processes for the 
preparation of carbonyl compounds by the oxidative dehydrogenation of 
C.sub.1 -C.sub.4 -alcohols, give only relatively low yields of end 
product, relative to the throughput and conversion of raw material; in 
addition the degree of conversion of the raw material and the quality of 
the products is still unsatisfactory in most cases. Formic acid is also 
frequently formed as an undesirable by-product in the known processes. 
It was therefore the object to find other catalyst substances by means of 
which the process for the preparation of oxo compounds is improved 
further. 
The invention therefore relates to a complex silicate catalyst having the 
formula M .multidot.v Ag.sup.0 .multidot.w AgX.multidot.Al.sub.2 O.sub.3 
.multidot.x SiO.sub.2 .multidot.y H.sub.2 O (1) 
in which M denotes a metal atom of valence n, Ag.sup.0 denotes elementary 
silver, X denotes a halogen atom and v, w, x and y denote stoichiometric 
coefficients. Elements of the first and second main group of the periodic 
system, preferably sodium, potassium, calcium and barium, are used as the 
metals M. The H.sup.+ or NH.sub.4.sup.+ forms can also be present in the 
silicate instead of the metals M.sup.n+. The NH.sub.4.sup.+ form is 
converted into the H.sup.n+ form at high reaction temperatures. The 
halogen atom X employed is bromine, but preferably chlorine. The 
coefficients v, w, x and y represent numbers from 0.1 to 7.5, preferably 
to 4.6. A particularly preferred silicate complex has the formula 
EQU Na.sub.2 O.multidot.1.4 Ag.multidot.0.4 AgCl.multidot.Al.sub.2 O.sub.3 
.multidot.2.47 SiO.sub.2 .multidot.3.9 H.sub.2 O (2) 
The invention also relates to a process for the preparation of the complex 
silicate catalyst, in which (a) a skeleton silicate is reacted with a 
silver salt, (b) the skeleton silicate containing silver ions is converted 
by means of reducing agents into a skeleton silicate complex containing 
elementary silver, (c) this complex is converted by partial re-oxidation 
with oxidizing agents into a silicate complex containing silver(I) ions 
and elementary silver and (d) the complex is then converted into a 
silicate complex of the formula (1) by treatment with a solution 
containing halide ions in an alkaline medium. 
Zeolites which are generally customary can be used as skeleton silicates 
for the preparation of the catalysts according to the invention. 
Particularly suitable examples of silicates of this type are products 
marketed commercially such as 6 Na.sub.2 O.multidot.6 Al.sub.2 O.sub.3 
.multidot.12 SiO.sub.2 .multidot.12 H.sub.2 O, known as molecular sieve ZA 
(manufacturer: Union Carbide, USA) and also Na.sub.1.8 H.sub.0.2 
O.multidot.Al.sub.2 O.sub.3 .multidot.4.multidot.65SiO.sub.2 
.multidot.10.multidot.4H.sub.2 O, known as Baylith CP 190 (manufacturer: 
Bayer AG, Leverkusen, West Germany). The water content of the zeolites is 
subject to considerable variations. It is preferable to use Na.sub.2 
O.multidot.Al.sub.2 O.sub.3 .multidot.2.47 SiO.sub.2 .multidot.3.9 H.sub.2 
O, known as molecular (manufacturer: Union Carbide, Corp., New York, USA).

Solutions containing silver cations are employed for doping, i.e. for 
inclusion into the skeleton silicates. The molar silver ion concentration 
is 0.001 to 10, preferably 0.01 to 1, mol/liter. The reaction is carried 
out at room temperature, i.e. at 20.degree.-30.degree. C., with stirring 
and with the exclusion of light. The treatment of the skeleton silicate 
with the solution of silver ions can be repeated until the desired degree 
of exchange for the metal ions of the skeleton silicate has been obtained. 
For the subsequent reaction the doped silicate is dried. A similar loading 
can also be obtained, for example, in the impregnation process. 
The silver ions contained in the skeleton silicate are then converted into 
elementary silver by means of reducing agents. This reduction is carried 
out, for example, in a customary manner by means of hydrogen at 
temperatures between 100 and 500.degree. C., preferably between 300 and 
400.degree. C. The partial pressure of the reducing agent is 0.05 to 5.0 
bar, preferably 0.4 to 0.75 bar, the falling partial pressure being a 
measure of the reduction. 
The partial re-oxidation of stage (c) is carried out by means of oxidizing 
agents, for example oxygen or a halogen, such as bromine or chlorine. In 
this reaction the elementary silver is partially converted into silver(I) 
ions or silver halide with the elimination of water or hydrogen halide. 
The reaction takes place at temperatures between 100 and 350.degree. C., 
preferably 250 to 290.degree. C., and in the case of a halogen preferably 
between 100 and 200.degree. C., under a partial pressure of 0.05 to 5.0 
bar, preferably 0.4 to 0.75 bar. The treatment time for the oxidation 
extends from 5 to 60, preferably 10 to 30, minutes. 
In stage (d) the resulting silicate complex is treated with an aqueous 
solution containing halide ions, preferably chloride. The molar 
concentration of this solution is 0.001 to 2, preferably 0.01 to 0.5. It 
is preferable to employ hydrogen halide acids, such as hydrochloric acid 
or hydrobromic acid, for this purpose. The treatment is carried out within 
a period of 2 to 15, preferably 5 to 10, minutes. A silver ion 
complex-former, for example ammonia in a 0.1 to 0.3 molar solution, is 
then added. An excess of a salt containing the metal cation M in equation 
(1), for example NaNO.sub.3, is added at the same time. The suspension is 
then stirred for 20 to 50 minutes. The digestion with the complex-former 
and the salt can be repeated several times. 
If the oxidation is carried out with a halogen, the treatment with a 
solution containing halide ions becomes unnecessary. The after-treatment 
in stage (d) is then only carried out with an alkaline solution of a 
complex-former in the presence of the metal salt. 
After drying, the resulting catalyst is processed into pellets measuring 1 
to 5 mm and is then ready for use. 
As a result of the precipitation of the silver, which is partly present in 
the ionic form, as halide and the partial washing out of the latter by 
means of complex-formers, an elementary silver component which is 
surprisingly active and selective in action remains in the silicate 
matrix. The catalyst prepared in this way is distinguished by a high 
long-term activity, by the reaction temperature required for the reaction 
being lower in comparison with catalysts composed of pure silver and by 
higher conversions and selectivities in the oxidative dehydrogenation of 
alkanols, for example methanol to give formaldehyde, in comparison with 
pure silver or silver on supports. 
The catalyst according to the invention is used in oxydehydrogenation 
reactions. It is particularly suitable for the oxydehydrogenation of 
alkanols having 1 to 4 carbon atoms in the alkyl radical and particularly 
in the oxidative dehydrogenation of methanol to give formaldehyde as 
described in German Patent Application P 39 21 452.4, entitled: "Process 
for the preparation of carbonyl compounds", which has been filed on the 
same day and to which reference is hereby made. 
EXAMPLES 
1) 10.71 g of zeolite 13.times. (formula: Na.sub.2 O.multidot.Al.sub.2 
O.sub.3 .multidot.2.47 SiO.sub.2 .multidot.3.9 H.sub.2 O) were put into a 
beaker, 500 ml of 0.1N AgNO.sub.3 solution were added and the mixture was 
stirred vigorously for 4 hours with the exclusion of light. After 
filtration, the residue was washed with 50 ml of water and a further 400 
ml of 0.1N silver nitrate solution were added, the mixture was stirred for 
4 hours with the exclusion of light and then filtered and the resulting 
solid substance was dried in the air. 7.88 g of this substance were freed 
from water under reduced pressure at 350.degree. C. and were then 
subjected to reductive treatment with hydrogen under a pressure of 0.6 bar 
for 20 minutes at the same temperature. The hydrogen was then removed and 
the residue was cooled to 270.degree. C. The solid substance was then 
treated in oxygen (0.67 bar) for 8 minutes at this temperature and, after 
the oxygen had been removed, was cooled to approx. 25.degree. C. 3.5 g of 
this resulting substance were suspended in 200 ml of water, and 5 ml of 
concentrated hydrochloric acid were added with stirring. Five minutes 
later 25 ml of half-concentrated ammonia solution (7 mol/liter) and 17 g 
of sodium nitrate were metered in. This suspension was stirred for 1 hour 
and then filtered, and the residue was washed with a total of 200 ml of 
water. The substance obtained was dried in the air and compressed into 
pellets of 6 mm diameter which were subsequently comminuted. 
2) 8.51 g of zeolite 13.times. (formula: Na.sub.2 O.multidot.Al.sub.2 
O.sub.3 .multidot.2.47 SiO.sub.2 .multidot.3.9 H.sub.2 O) were put into a 
beaker, 400 ml of 0.1N AgNO.sub.3 solution were added and the mixture was 
stirred vigorously for 3 hours with the exclusion of light. After 
filtration, the residue was washed with 50 ml of water and a further 400 
ml of 0.1N silver nitrate solution were added, the mixture was stirred for 
3 hours with the exclusion of light and then filtered, and the resulting 
solid substance was dried in the air. 6.35 g of this substance were freed 
from water under reduced pressure at 330.degree. C. and were then reduced 
for 25 minutes with hydrogen under a pressure of 0.67 bar. The substance 
was then cooled to 170.degree. C. under reduced pressure, oxidized with 
chlorine under a pressure of 0.6 bar for 30 minutes and cooled to room 
temperature under reduced pressure. 4 g of the substance obtained were 
suspended in 200 ml of water, and 20 g of sodium nitrate and 30 ml of 
half-concentrated ammonia solution were metered in. This suspension was 
stirred for 20 minutes and then filtered and the residue was washed with a 
total of 250 ml of water. The substance obtained was dried in the air and 
compressed into pellets of 6 mm diameter, which were subsequently 
comminuted.