Hydrogenation using magnetic catalysts

Catalysts are composed of PA0 a) a magnetizable core, PA0 b) which may be coated with a binder and PA0 c) which carries catalytically active metals or metal compounds on its surface.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to catalysts composed of 
a) a magnetizable core having a diameter of from 5 to 100 nm, 
b) which may be coated with a binder and 
c) which carries catalytically active metals or metal compounds on its 
surface. 
The present invention furthermore relates to a process for the preparation 
of these catalysts, their use and a method for removing these catalysts 
from reaction solutions. 
2. Description of the Related Art 
Reactions in solution with homogeneous or suspended catalysts are widely 
encountered in the chemical industry. However, the removal and, if 
required, recycling of these catalysts presents difficulties in many 
cases. If catalysts dissolved in the reaction solution, ie. homogeneous 
catalysts, are used, the products are usually separated from the catalysts 
by distillation and the catalysts remain in the bottom product of the 
distillation. The catalysts are frequently deactivated during this 
procedure and can be reused only after being worked up by a complicated 
process. In the case of thermally unstable products, this method of 
removal by distillation leads to a deterioration in the product quality 
owing to decomposition reactions. In these cases, the catalyst may 
alternatively be removed, for example, by extraction or by adsorption, for 
example on active carbon. Attempts have also been made to achieve 
separability of product and catalyst by heterogenization, ie. by binding 
the catalysts to finely divided substances which are insoluble in the 
reaction mixture. However, the filterability of such heterogenized 
catalysts is often poor. 
In the case of suspension catalysts, the highest catalytic activity is 
frequently achieved with small catalyst particles having a high specific 
surface area. Owing to the particle size, these particles, too, are 
difficult to remove from the reaction mixture by simple filtration, so 
that the technically complicated separation methods described above have 
to be used. 
A possible method for removing metallic suspension catalysts containing the 
magnetic elements iron, cobalt and nickel entails removal in a magnetic 
field (magnetic filter) (Journal of Magnetism and Magnetic Materials 85 
(1990), 285). However, this method is limited to magnetic metals and, in 
the case of larger particles, also has the difficulty that the particles 
agglomerate to form larger particles as a result of permanent 
magnetization. 
The immobilization of enzymes by binding to magnetic particles is 
described, for example, in EP-A 125995. However, the use of these enzymes 
is limited to mild reaction conditions under which the enzymes bound in 
this manner are chemically stable. 
SUMMARY OF THE INVENTION 
EP-A 21 854 describes magnetic catalyst particles having a size of about 
800 .mu.m for a fluidized-bed process for a reformer process. The 
catalytically active metal compounds such as hexachloroplatinic acid can 
be applied, for example by impregnation, to spray-dried mixtures of 
Al.sub.2 O.sub.3 and stainless steel. 
EP-A 115 684 concerns magnetizable iron particles having a size of from 150 
to 300 .mu.m which can be used, without application of a coating or being 
silvercoated, for catalysis in fluidized beds.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
It is an object of the present invention to provide metal-containing 
catalysts for liquid-phase reactions, which catalysts can be removed from 
the reaction mixture in a technically simple manner. 
We have found that this object is achieved by the catalysts described 
above. We have furthermore found a process for their preparation, their 
use and a method for removing these catalysts from reaction mixtures. 
The core of the novel catalysts consists of a magnetizable particle. 
Magnetizable particles are to be understood as being those particles which 
are magnetic in an external magnetic field. In general, such substances 
have a saturation magnetization of from 20 to 200, preferably from 30 to 
100, nTm.sup.3 /g. The size of the magnetizable particles can be chosen 
within wide limits. If, however, permanent magnetization of these 
particles by an external magnetic field is to be avoided, particles having 
a diameter of from 5 to 100 nm are used. The size of particles having such 
diameters is determined by known methods, for example by electron 
microscopy procedures, the values being average values for the particular 
sample. 
Specifically, the following substances are suitable as magnetizable cores: 
Iron, nickel, cobalt, chromium dioxide, iron oxides and cubic and hexagonal 
ferrites, such as ferrites doped with manganese, zinc and cobalt ions and 
with magnesium, calcium, strontium and barium ions. Such substances are 
obtainable in a manner known per se, for example by precipitation 
reactions of corresponding metal salts. Thus, magnetite Fe.sub.3 O.sub.4 
can be prepared from solutions of Fe.sup.2+ /Fe.sup.3+ chlorides by 
precipitation with sodium hydroxide solution (cf. for example DE-A 36 19 
746) and chromium dioxide by hydrothermal synthesis (cf. for example EP-A 
27 640). The metallic cores can be prepared by thermal decomposition of 
metal carbonyls (cf. for example U.S. Pat. No. 3,228,881). Preferred 
substances for the magnetizable cores are magnetite, .gamma.-Fe.sub.2 
O.sub.3, chromium dioxide and manganese zinc ferrite. 
Depending on their diameters, which as a rule are small, the magnetizable 
cores generally have surface areas (determined according to DIN 66 132) of 
from 1 to 300 m.sup.2 /g. 
The magnetizable cores can be reacted directly with metal compounds which 
bind by adsorption or chemically to the surface of the core. However, the 
magnetic core is preferably first coated with a binder. This binder should 
bind by adsorption or chemically to the magnetizable core and at the same 
time make it possible to bind, by adsorption or chemically, metal 
compounds which are catalytically active either directly or, if required, 
after chemical modification. 
Preferred novel binders are organic polymers which are water-soluble or 
water-dispersible, ie. in general at least 1 g can be dissolved or 
dispersed in 1 l of water. Examples of monomers for Such polymeric binders 
are olefinically unsaturated compounds containing acid groups, such as 
unsaturated carboxylic acids, eg. acrylic acid or methacrylic acid, 
unsaturated sulfonic acids, eg. vinylsulfonic acid, and unsaturated 
phosphonic acids, such as vinylphosphonic acid, as well as unsaturated 
anhydrides, such as maleic anhydride, amino-carrying monomers, such as 
vinylamine, amido-carrying monomers, such as acrylamide, and 
vinylpyrrolidine, vinylpyrrolidone, vinylpyridine and vinylbipyridyl. 
Homo- and copolymers of the stated monomers may also be used. The polymers 
may contain further copolymerizable monomers, but the amount of the stated 
monomers is preferably at least 50% by weight. Homo- or copolymers of 
these compounds are commercially available or are obtainable, for example, 
by free radical polymerization. Polyesters, such as polylactide, are also 
suitable binders. 
Polyacrylic acids having an average molecular weight of from 1000 to 
1000000 are preferred. Polymers which contain up to 20% by weight of 
further monomers in addition to acrylic acid are also suitable. Such 
products are commercially available. 
The amount of the binder is as a rule such that at least one monolayer of 
the binder can form on the magnetizable core. In general, from 0.1 to 5 
mg, preferably from 0.3 to 1.5 mg, of polymer per m.sup.2 of specific 
surface area of the magnetizable core are required. 
In order to coat the magnetizable core with a binder, a solution of the 
binder in a polar solvent, such as water, may be added to the magnetizable 
material in a polar solvent, such as water. The temperature is, as a rule, 
from 10 to 100.degree. C. The components are generally stirred for from 10 
to 60 minutes, and the solid thus obtained is isolated, for example by 
filtration or by applying a magnetic field, and, if required, is dried. It 
may be isolated and the content of bound polymer may be determined by 
elemental analysis. 
It is furthermore possible further to treat the resulting solids in a 
manner such that reactive groups which are present in the polymer are 
subjected to a reaction. For example, polar carboxyl groups may be 
liberated from ester groups present in the binder in a conventional manner 
by hydrolysis with a mineral base. 
The novel catalysts carry metals or metal compounds on their surface. These 
are preferably the platinum metals, ruthenium, rhodium, palladium, osmium, 
iridium and platinum, or copper, silver, gold and rhenium, or compounds of 
these metals. Ruthenium, palladium and platinum are particularly 
preferred. For the preparation of the novel catalysts, metal compounds 
which are dissolved or finely suspended in a polar solvent may be bound 
chemically or adsorptively to the magnetizable cores, which, if required, 
are coated with a binder. These metal compounds are, for example, salts of 
the metals, such as acetates and nitrates. Examples are ruthenium chloride 
trihydrate, ruthenium oxide hydrate, palladium acetate, palladium 
chloride, rhodium chloride, platinum chloride, gold chloride, osmium 
tetraoxide, copper chloride, copper nitrate, silver nitrate and rhenium 
chloride. The solvents comprise C.sub.1 -C.sub.4 -alcohols, such as 
methanol, ethanol, isopropanol and tert-butanol, ethers, such as 
tetrahydrofuran, water, pyridine or mixtures of these solvents, preferably 
water. Depending on the polar groups present in the binder, the metal 
compounds may be bound by interactions with, for example, carboxyl groups, 
amino groups or oxo groups. In the case of (colloidal metal compounds,) 
such as ruthenium oxide hydrate, adsorptive binding may be effected. As a 
rule, from 0.1 to 20% by weight, based on the magnetizable core, of the 
metal compound in solution are reacted with the magnetic particles which 
may carry a binder. In general, the reaction is carried out at room 
temperature but may also be effected at from 0 to 100.degree. C. It is 
complete as a rule after from 1 to 6 hours. 
The products thus obtained can be used directly in solution as catalysts 
but may first be isolated, if necessary purified and then added to the 
reaction to be catalyzed. 
The novel catalysts permit the catalysis of a large number of different 
reactions, for example 
hydrogenation of aromatic nuclei in the presence of further reducible 
groups using ruthenium-containing catalysts, 
hydroformylation of olefins using rhodium-containing catalysts, 
transvinylidations of vinyl ethers using palladium-containing catalysts and 
selective hydrogenation of carbon-carbon triple bonds to double bonds using 
palladium-containing catalysts. 
The catalysts may be used in general at up to 250.degree. C., and the 
reactions may be carried out at any pressure. The novel catalysts can be 
separated from the reaction mixtures by a magnetic filter, as described, 
for example, in Journal of Magnetism and Magnetic Materials 85 (1990) 285. 
A simple bar magnet has proven useful for separating off small amounts of 
the catalyst. 
The novel catalysts furthermore have the advantage that, after removal of 
the magnetic field used for separation, they are not permanently magnetic 
and therefore do not agglomerate. This considerably facilitates the 
recycling of the catalysts to the reaction and their uniform distribution 
in the reaction mixture. 
EXAMPLES 
Preparation of the Magnetizable Particles 
1.1 Magnetite Fe.sub.3 O.sub.4 was prepared as follows 
The magnetite was prepared according to DE-A 35 00 471 by a precipitation 
reaction, by adding a stoichiometric solution of Fe(.sup.2+)/Fe(.sup.3+) 
chlorides in water dropwise to a solution of sodium hydroxide in water. 
The precipitated magnetite was filtered off and washed chloride-free. A 
filter cake having a magnetite content of 26% by weight was formed. The 
dried pigment was characterized by the following measured values: The 
specific BET surface area was measured according to DIN 66 132. It was 51 
m.sup.2 /g. The magnetic properties were determined using a vibrating 
sample magnetometer. The saturation magnetization was 85 nTm.sup.3 /g. 
1.2 Preparation of Ruthenium Oxide Hydrate as a Catalytically Active Metal 
Compound 
The reaction was carried out according to Example 1 of DE-A 2132547. The 
ruthenium oxide hydrate was used in the form of a moist filter cake 
containing 8.4% by weight of ruthenium, for the following experiments: 
1.3 Preparation of the Novel Catalyst 
230 g of filter cake containing 60 g of the magnetizable particles prepared 
according to Example 1.1 and 71.3 g of the filter cake prepared according 
to Example 1.2 were dispersed in 200 g of water in the course of 15 
minutes by vigorous stirring. A solution of 1.8 g of polyacrylic acid 
having an average molecular weight of 250,000 was then added to 3.3 g of 
water. The pH was brought to 7.9 by adding 13.4 g of a 5% strength by 
weight sodium hydroxide solution. After 15 minutes, the solid was filtered 
off and washed chloride-free with water. The filter cake was then washed 
with tetrahydrofuran THF and the water was substantially exchanged for 
THF. The filter cake thus obtained was characterized as follows: Ruthenium 
content: 2.1% by weight. Solids content (determined by drying at 
70.degree. C. under reduced pressure): 22% by weight, BET surface area 86 
m.sup.2 /g. 
1.4 Testing of the Catalytic Properties 
In an autoclave, 70 g of bisphenol F bisglycidyl ether (prepared by 
condensation of formaldehyde and 2 equivalents of phenol and subsequent 
reaction with epichlorohydrin) and 10 g of the ruthenium magnetite 
suspension obtained according to Example 1.3 (corresponding to 3 o/oo by 
weight, based on the bisglycidyl ether, of Ru) were made up to a total 
weight of 150 g with THF. Heating was then carried out to 50-67.degree. C. 
at a hydrogen pressure of 100 bar in the course of 7 hours. The total 
conversion was 94%. The epoxide equivalent value was 192. 
This value corresponds to epoxide equivalent values as obtainable according 
to the prior art, for example EP-A 402 743. 
The catalyst could be readily and completely separated from the reaction 
mixture by means of a magnet.