Catalyst system for catalytic heaters

A catalyst system for a catalytic heater comprises a support of a refractory material upon which is deposited by cocrystallization at least one oxide from a Group VI (B) element and at least one oxide from a Group VII (B) or a Group VIII (4th Period) element in addition to a promoter from the platinum-metal group.

FIELD OF THE INVENTION 
The present invention relates to an oxidation catalyst system for the 
oxidation of hydrocarbons such as propane, butane, and their homologs, 
particularly for use in catalytic (flameless) heaters. 
BACKGROUND OF THE INVENTION 
Generally so-called catalytic heaters, which effect catalytic oxidation of 
hydrocarbons on contact, have an active phase constituted by the noble 
metals of group VIII (Periods 5 and 6) of the PERIODIC TABLE OF THE 
ELEMENTS (see the Periodic Chart of the Elements at pages 448 and 449 of 
the Handbook of Chemistry and Physics, 41st Edition 1959/60). The most 
commonly used of these platinum-group elements is platinum itself because 
of its relatively high catalytic activity. 
There are several disadvantages to using platinum, firstly, even when used 
in small quantities it is an expensive metal. Secondly, it is highly 
susceptible to poisoning and to coating with deposits such as carbon which 
develop rapidly if the support for the catalyst contains iron oxide and 
the gas being oxidized contains high proportions of ethylenic 
hydrocarbons. 
OBJECTS OF THE INVENTION 
It is an object of the present invention to provide an improved catalyst 
for the contact oxidation of hydrocarbons in which the aforedescribed 
disadvantages are obviated. 
Another object is to provide a support for the improved catalyst. 
A further object is the provision of a catalyst system particularly usable 
in so-called catalytic or flameless devices. 
DESCRIPTION OF THE INVENTION 
These objects are attained according to the present invention in a catalyst 
which is formed principally of semiconductor oxides. More particularly at 
least two oxides chosen from groups VI(B), VII(B), VIII(Fourth period) of 
the PERIODIC TABLE OF THE ELEMENTS (see the Periodic Chart of the Elements 
at pages 448 and 449 of the Handbook of Chemistry and Physics, 41st 
Edition 1959/60) are employed, the two oxides being selected from 
different groups, at least one of which is group VI(B). 
For the purpose of the present disclosure, the metals of the platinum group 
or family are considered to be those found in Periods 5 and 6 of Group 
VIII of the PERIODIC CHART OF THE ELEMENTS (pages 448 and 449 of the 
Handbook of Chemistry and Physics, 41st Edition, 1959/1960) and to be 
constituted by the elements platinum, palladium, iridium, rhodium, osmium 
and ruthenium. 
When reference is made herein to the elements of Group VI(B) of the 
PERIODIC TABLE it should be understood that these elements are chromium, 
molybdenum and tungsten. The elements of Group VII (B) for the purposes of 
the present invention are manganese, technetium and rhenium. The elements 
of Group VIII (4th Period) which are referred to herein are the so-called 
transition metals consisting of cobalt, iron and nickel. 
While I have pointed out above that the basic catalyst system of the 
present invention comprises a refractory support upon which at least two 
semiconductive oxides are deposited by cocrystallization from a common 
solution and which also includes one of the platinum-group metals, the 
system can be more particularized as follows: 
The support may be preferably silica, alumina or an alumino silicate in any 
desired form permeable by the gases traversing the catalytic mass. At 
least one oxide of a Group VI(B) metal is always present and at least one 
other oxide from Group VII(B) or Group VIII (4th Period) is likewise 
always present. The system further comprises at least one element from 
Group VIII(5th and 6th Periods) as a reaction promoter. 
In the system, the support may constitute up to 90% by weight and 
preferably at least 25% by weight while the balance of 75% by weight to 
10% by weight is constituted by the active catalyst components. A promoter 
as described below may be present in an amount ranging between 0 and 10% 
by weight, the metal of the platinum group is present in an amount of 0.01 
to 10% by weight but preferably no more than 5% by weight, the oxide of 
the Group VI(B) metal should be present in an amount ranging from 0.5 to 
15% by weight and the oxides of the Group VII(B) or Group VIII (4th 
period) metal should be present in an amount ranging between 0.5 and 15% 
by weight. Preferably a chromium oxide is present with either or both of a 
cobalt oxide and a manganese oxide, with platinum being the activator. 
Such oxide catalysts have certain inherent advantages over catalysts which 
are reducing metals. Their thermal stability is much greater and they are 
less sensitive to poisons and to coating. In addition such oxide catalysts 
are of substantially lower cost then the hitherto used catalysts from the 
platinum group, such as rhodium, palladium, and iridium. 
Reference is made herein to French Patent No. 31, 1966 1,466,195 of January 
31, 1966 and British Patent No. 862,771 which have investigated the 
characteristics of platinum-type catalysts of the character previously 
described as constituting the prior art. 
According to a further feature of this invention the oxides of the catalyst 
include one from each of groups VI(B) and VIII (fourth period) of the 
PERIODIC TABLE OF THE ELEMENTS (see the Periodic Chart of the Elements at 
pages 448 and 449 of the Handbook of Chemistry and Physics, 41st Edition 
1959/60). More particularly chromium oxides and cobalt oxides have been 
found particularly useful. These oxide catalysts have an especially high 
catalytic activity when they are employed in a particularly homogeneous 
cocrystallization (i.e. crystals formed by coprecipitation). It may be 
necessary in certain cases to activate the oxide catalyst so that the 
temperature at which the catalytic reaction starts should be as low as 
possible and thereby limit the time necessary for the catalytic mass to 
function.

Thus in accordance with the present invention the oxide catalysts are doped 
by introducing into the active component an oxidation catalyst which is 
more active and which can be based on a metal or a group of metals from 
the platinum group, platinum being particularly suitable. This doping 
operation uses relatively small quantities of noble metals relative to the 
total active mass. Much less of these expensive metals is needed than in 
the cases where they alone constitute the active mass. 
Such a catalyst can oxidize completely a large number of organic compounds, 
especially the hydrocarbons mentioned previously. 
In accordance with a further feature of this invention the support for such 
a catalyst can be any form of asbestos, silica, alumina, or any other 
refractory such as metallic sheet or foil, sintered metal, and the like. 
Any fibrous material or agglomerate inert at the working temperature of 
the catalytic composition can be used. 
Silica in accordance with this invention is particularly suited for use as 
the support. It can be used in the form of a fiber mat or layer for 
receiving the catalyst as described in French Patent No. 1,505,615. 
In accordance with a further feature of this invention it has been 
discovered that two elements of groups VI(B) and VIII(fourth period) of 
the PERIOD TABLE OF THE ELEMENTS (see the Periodic Chart of the Elements 
at pages 448 and 449 of the Handbook of Chemistry and Physics, 41st 
Edition 1959/60) are particularly suited for forming the active mass of 
oxide catalysts, namely chromium and cobalt. The activation or doping 
element usable with these elements is platinum. The salts (E.G. Nitrates) 
of thee elements which are soluble or decomposable by heat are brought 
into solution, in a common solvent e.g. watery then deposited on silica 
fibers as described in the above-mentioned patent. Subsequent calcination 
giving the following products: platinum sponge, chromium (III) oxide and 
the oxides of Co.sup.2 Co.sup.3, all being perfectly homogeneous in a 
cocrystallization system. 
According to yet another feature of this invention there is introduced into 
the active mass a recrystallization inhibitor serving primarily to 
stabilize the reduced platinum in its most active crystalline form. This 
promoter can be a refractory oxide obtained during the thermal treatment 
of a salt deposited on the support, the salt of the element constituting 
the promoter base having been previously mixed and dissolved with other 
salts. The elements selected are of high electronic density whose oxides 
have melting points considerably higher than that of the active element 
which is to be stabilized. 
The proportions of base elements going into the catalytic composition are 
carefully defined so that there is obtained a level of efficiency close to 
100% for a space-time rate on the order of 125 for butane for example. 
This means that in 1 hour 125 volumes of butane are treated on 1 volume of 
the catalyzer and its support, the volume of the contact mass being in 
this case defined by the formula M/da wherein da equal the apparent 
density of the contact mass and M its mass. 
The ratio: catalysts components/mass of butane oxidized in one hour can be: 
EQU CoO+ Co.sub.2 O.sub.3 = 42.5.times.10.sup.-.sup.4, 
EQU Cr.sub.2 O.sub.3 = 44.times.10.sup.-.sup.4, 
EQU P.sub.t = 12.times.10.sup.-.sup.4, and 
EQU Promoter= 12.times.10.sup.-.sup.4. 
In accordance with yet another feature of this invention two supports are 
used. The first support serves to disperse the active mass, the second 
support serves to allow the active mass to be placed in a condition for 
use on the first support. The first support is chosen so as to stabilize 
the catalytic oxide mainly by inhibiting recrystallization due to the 
temperature at which the contact mass is used. The first support is formed 
of a refractory oxide powder such as SiO.sub.2, Al.sub.2 O.sub.3 MgO and 
ThO.sub.2. Preferably a refractory oxide is used having a large specific 
surface, for example alumina. 
The kind of alumina which is particularly suitable for use in accordance 
with the present invention is defined by the size of the granules and the 
specific surface but may be that described in French Patent No. 1,367,925. 
An alumina whose granulometry has grains of a diameter between 0.1 and 30 
microns, preferably not greater than 20 microns, and whose specific 
surface B.E.T. lies between 70 and 350 M.sup.2 /g, preferably between 150 
and 250 M.sup.2 /g, is perfectly suitable. It should be noted that too 
large a specific surface generally means that there are open pores having 
very small openings which has the disadvantage that gas will stagnate 
inside the pores due to the irreversible adsorption of the gas. 
The catalytic oxides and the activation element, platinum, deposited on the 
alumina give a powder which is particularly active and whose specific 
surface B.E.T. lies between 80 and 250 M.sup.2 /g according to the alumina 
used. This active mass is then deposited on the second support necessarily 
formed by refractory fibers or any arrangement of fibers, or constituted 
by agglomerates inert at the operation temperature of the catalytic mass. 
Silica fibers deposited in a layer are extremely advantageous for this 
type of second support. 
The catalytic mass obtained in this manner allows a maximum contact area 
between the reactive and active particles. The conversion of hydrocarbon 
is practically total. 
According to yet another feature of the present invention such a contact 
mass is prepared in the following manner: the alumina whose 
characteristics are given above is impregnated in an aqueous solution of 
chromium and cobalt salt, such as the nitrates. After having allowed the 
mixture to stand for several hours the hydroxides of chromium and of 
cobalt are precipitated by an ammonia solution or better by bubbling 
ammonia through them. The wet mass thus obtained is carefully rinsed so as 
to remove the mother liquor, it is dried, then finally calcined slowly 
from 100.degree. to 600.degree. C., then rapidly at this latter 
temperature. The pulverant compound obtained is constituted at Al.sub.2 
O.sub.3, Cr.sub.2 O.sub.3, CoO, Co.sub.2 O.sub.3 and is in its turn mixed 
into a solution containing a salt of a metal having a high catalytic 
activity such as platinum in the form of chloroplatinic acid for example. 
This solution can also contain another element in the form of a salt, 
which serves to form the promoter compound whose characteristics have been 
recited above. The space obtained is dried at 90.degree. C. as it is 
agitated. Then it is calcined slowly from 100.degree. to 600.degree. C. 
while remaining at least fifteen minutes at this latter temperature. This 
produces an extremely active catalytic powder. 
The deposition of the platinum and of its promoter on the chromium or 
cobalt oxides themselves previously on alumina has the advantage of 
producing a very homogenous wide dispersion of the platinum since the 
above-cited oxides also serve as a support for the platinum which itself 
serves as doping agent or activation element as described above. 
The catalytic powder is suspended in a liquid which can advantgeously be 
water. The medium is maintained homogenous by rapid agitation. The 
particles thus suspended are deposited on the silica fiber preferably 
disposed in a layer, and the suspension liquid carries the particles over 
all of the fibers. It is possible to simply dip the fibers or to pour the 
suspension over them. The fibers are then dried at a moderate temperature. 
This produces a contact mass having a considerable catalytic activity. It 
lies within the scope of the invention to employ an adhesive so as to fix 
the particles on the surface of the substrate. This latter operation has 
the advantage of preventing particle loss when the contact mass is being 
installed. It is desirable that this adhesive disappear entirely during 
the first use in service of the contact mass. 
A first support has the following composition 
EQU Al.sub.2 O.sub.3 = 69.5% by weight; 
EQU Cr.sub.2 O.sub.3 = 2.22% by weight; 
EQU CoO+Co.sub.2 O.sub.3 = 11.62% by weight; 
EQU Pt= 3.33% by weight; and 
EQU Promoter= 3.33% by weight. 
Such a catalytic composition is deposited on silica fibers and is capable 
of producing the complete combustion of hydrocarbons such as propane, 
butane, and their homologs with nearly 100% completeness. At the same time 
the ethylene hydrocarbons are perfectly oxidized. This catalytic 
composition can function perfectly for several thousand hours as long as 
catalyst/butane or propane ratio of 0.036 is maintained, or under the 
spatial velocity is held under 125 as indicated above. 
The composition of the invention is particularly suited for use in 
catalytic heating devices.