Supported catalyst for gas-phase oxidation reactions

The invention relates to a supported catalyst for gas-phase reactions having an inert support body and a surface coating comprising PA0 a) at least 5% by weight of silicon carbide, PA0 b) from 5 to 90% by weight, calculated as oxide, of one or more titanium dioxide or zirconium oxide components or mixtures thereof, PA0 c) from 1 to 50% by weight, calculated as V.sub.2 O.sub.5, of one or more vanadium oxide components, PA0 d) from 0 to 10% by weight, calculated as oxide, of one or more compounds of elements of the 1st and 5th main groups of the Periodic Table, and also a process for its preparation and its use.

BACKGROUND OF THE INVENTION 
1). Field of the Invention 
The invention relates to supported catalysts for gas-phase oxidation 
reactions, a process for their preparation and also their use in gas-phase 
oxidation reactions. 
2). Background Art 
In the gas-phase oxidation of hydrocarbons, a mixture of the hydrocarbons 
with air or with oxygen-containing gases is customarily passed over a 
catalyst which is located, for example, in a multitube reactor which 
comprises a plurality of tubes filled with catalyst. For the purpose of 
heating them to the required reaction temperature, and also for cooling 
these strongly exothermic reactions, the tubes are surrounded by a salt 
melt. However, in these (exothermic) oxidation processes, increasing 
loading of the air with hydrocarbons to be oxidized results in increased 
formation of by-products as a result of total oxidation. This is because 
of temperature peaks occurring in the catalyst bed, known as hot spots, 
which are higher the greater the loading of the air with the hydrocarbons 
to be oxidized. These hot spots can become so great as to result in 
runaway behavior of the reactor, which leads to damage to or deactivation 
of the catalyst. 
In the past, there have been many attempts to increase the selectivity of 
gas-phase oxidation by use of specific catalysts. 
Supported catalysts for the gas-phase oxidation of hydrocarbons to give 
carboxylic anhydrides, the catalytically active surface coating of which 
catalysts consists essentially of titanium dioxide (TiO.sub.2) , 
preferably in the anatase modification, and divanadium pentoxide (V.sub.2 
O.sub.5), have been known for some time and, in comparision with 
support-free (homogeneous) catalysts of corresponding composition, have a 
series of advantages such as higher selectivity and better operating 
behavior in such exothermic reactions as the oxidation of o-xylene and/or 
naphthalene to give phthalic anhydride (PA). 
Thus, U.S. Pat. No. 2,157,965 describes the oxidation of naphthalene to PA 
using catalysts which are prepared by spraying a mixed precipitation from 
an aqueous solution or suspension containing ammonium metavanadate and 
titanyl sulfate onto supports such as pumice and subsequent calcination. 
The disadvantages of these catalysts are the initially high proportion of 
total oxidation and their short lifetime. 
Use of inert, nonporous supports such as magnesium silicate or porcelain 
and direct spraying on of TiO.sub.2 -containing suspensions containing 
vanadyl oxalate as vanadium component (De-C 14 42 590=U.S. Pat. No. 
3,509,179) and use of two different TiO.sub.2 components of which an 
anatase component has a BET surface area of 7-11 m.sup.2 /g and a titanium 
dioxide hydrate component has a BET surface area of above 100 m.sup.2 /g 
(DE-C3 21 06 796=U.S. Pat. No. 3,799,886) enabled the selectivity of the 
oxidation reaction and the lifetime of the catalysts to be improved. 
The use of two-bed packages in the multitube reactor also enabled the 
maximum loading to be only slightly increased. For this purpose, the 
catalytic activity of the upper bed was damped by addition of alkali 
metals while the lower bed was activated by phosphorus doping. For this 
method of operation, DE-B 25 46 268 (U.S. Pat. No. 4,077,984) describes 
loading increases in the oxidation of o-xylene to phthalic anhydride from 
42 to merely 60 g/standard m.sup.3 of air. Similar results were observed 
in naphthalene oxidation. 
DE-C 29 48 163 (U.S. Pat. No. 4,284,571) describes a catalyst whose 
catalytically active composition comprising V.sub.2 O.sub.5, anatase 
TiO.sub.2 and various promoters such as oxides of phosphorus, niobium, 
cesium and/or thallium is applied to a porous support comprising at least 
80% of silicon carbide. The advantage of this supported catalyst is the 
lowering of the hot spot temperature by use of a TiO.sub.2 component 
derived from ilmenite (BET surface area: 10-60m.sup.2 /g). 
DE-C 30 45 624 (U.S. Pat. No. 4,356,112) discloses catalysts having 
improved heat stability, which catalysts lower the hot spot temperature in 
the oxidation of o-xylene or naphthalene. The catalysts are supported 
catalysts which have an active layer based on TiO.sub.2 /V.sub.2 O.sub.5 
containing Nb.sub.2 O.sub.5, Cs.sub.2 O, K.sub.2 O, P.sub.2 O.sub.5 and 
Sb.sub.2 O.sub.3 on a porous support based on silicon carbide, with the 
heat stability being increased by the Sb.sub.2 O.sub.3 content. A similar 
catalyst for preparing pyromellitic dianhydride (PMDA) is described in 
EP-B 163 231, The last-mentioned catalysts are calcined before use and 
accordingly have relatively low abrasion resistance which can lead to loss 
of catalytically active surface coating on charging into industrial 
reactors. To improve the adhesion of the active layer to the porous SiC 
support, it is recommended that the active layer be mixed with metallic or 
ceramic whiskers having defined geometric dimensions. All these catalysts 
based on porous SiC supports have the disadvantage of the high price and 
the difficult reutilization of the support. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to find improved supported 
catalysts for the air oxidation of, especially, aromatic hydrocarbons, in 
particular for the oxidation of naphthalene or a mixture of naphthalene 
and o-xylene to give PA or for the oxidation of 1,2,4,5-tetramethylbenzene 
(durene) to give PMDA, which catalysts are suitable for industrial 
reactors and give the desired reaction product in high yields. A 
particular focal point of the object is to develop catalysts which can 
react high hydrocarbon loadings of the reaction air, if possible even in 
the initial phase, without damage to the catalyst, so that even at high 
loadings a hot spot temperature (highest temperature in the catalyst bed) 
of 500.degree. C. is not exceeded. 
Surprisingly, this object can be achieved by means of a supported catalyst 
comprising a nonporous, inert support body and a surface coating in which 
part of the TiO.sub.2 active component is replaced by SiC powder. 
The invention provides a supported catalyst for gas-phase reactions having 
an inert support body and a surface coating comprising 
a) at least 5% by weight of silicon carbide, 
b) from 5 to 90% by weight, calculated as oxide, of one or more titanium 
dioxide or zirconium oxide components or mixtures thereof, 
c) from 1 to 50% by weight, calculated as V.sub.2 O.sub.5, of one or more 
vanadium oxide components, 
d) from 0 to 10% by weight, calculated as oxide, one or more compounds of 
elements of the 1st and 5th main groups of the Periodic Table, 
where the figures in percent by weight are each based on the total weight 
of the active compounds and add up to 100% by weight. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In principle, the support bodies can have any shape and surface structure. 
However, preference is given to regular-shaped, mechanically stable bodies 
such as spheres, rings, half rings, saddles, or honeycomb supports or 
supports provided with channels. The size of the support bodies is 
determined primarily by the dimension, especially the internal diameter, 
of the reaction tubes if the catalyst is used in multitube or tube-bundle 
reactors. The support diameter should then be between 1/2 and 1/10 of the 
internal diameter. Suitable materials are, for example, steatite, 
duranite, stoneware, porcelain, silicon dioxide, silicates, aluminum 
oxide, aluminates or mixtures of these materials. Preference is given to 
spheres or rings of support materials such as duranite or steatite. 
The proportion of the active surface coating is 1-30% by weight, preferably 
2-15% by weight, based on the total mass of the supported catalyst. The 
thickness of the surface coating is preferably from 10 to 120 .mu.m. A 
suitable component a) of the surface coating is commercial silicon carbide 
powder having a particle size of preferably up to 100 .mu.m. with very 
fine SiC powder being preferred. Very good results are obtained, for 
example, using SiC having a particle size of from 10 to 50 nm. 
Preference is given to a proportion of from 10 to 75% by weight, in 
particular from 30 to 75% by weight, of silicon carbide, based on the 
total weight of the active components. 
As component b), preference is-given to using pulverulent TiO.sub.2 in the 
anatase modification and having a BET surface area of from 5 to 200 
m.sup.2 /g. Preference is also given to using a titanium dioxide hydrate 
(hydroxyl-rich, microcrystalline anatase) having a BET surface area of 
more than 100 m.sup.2 /g or a mixture of anatase having a BET surface area 
of 5-11 m.sup.2 /g and titanium dioxide hydrate in a mixing ratio of 
preferably from 1:3 to 3:1. Preference is given to using a proportion of 
from 10 to 75% by weight, in particular from 20 to 65% by weight, of 
component b), calculated as oxide and based on the total weight of the 
active components. 
Components c) which can be used are vanadium oxide or vanadium compounds 
which are converted into vanadium oxide on heating in air, individually or 
in the form of their mixtures. Preference is given to using V.sub.2 
O.sub.5 or NH.sub.4 VO.sub.3. Preference is given to using a proportion of 
from 5 to 30% by weight of vanadium oxide component, calculated as V.sub.2 
O.sub.5 and based on the total weight of the active components. 
Suitable components d) are, for example, alkali metal compounds such as 
K.sub.2 O, Cs.sub.2 O, Cs.sub.2 CO.sub.3 in an amount of preferably from 
0.01 to 1.0% by weight, in each case based on the total weight of the 
active components. Also suitable are compounds of phosphorus, antimony, 
bismuth, preferably their oxides, in an amount of preferably from 0.1 to 
10% by weight, in each case based on the total weight of the active 
components. Particularly, preferred examples of the last-named group are 
P.sub.2 O.sub.5, (NH.sub.4).sub.2 HPO.sub.4, Sb.sub.2 O.sub.3. 
To prepare the supported catalysts, the support bodies are preferably 
coated with an aqueous slurry of a mixture of the active components or 
else the individual components, and dried, for example in a rotary tube 
furnace at 200.degree.-300.degree. C. Supported catalysts having excellent 
adhesion of the coatings, which is particularly important for transport 
and charging of the catalyst into the reactor, are obtained, for example, 
by applying an aqueous suspension containing the mixture or the individual 
components as well as, if desired, an organic binder uniformly to the 
support bodies. Preferred organic binders are copolymers, advantageously 
in the form of an aqueous dispersion, of vinyl acetate/vinyl laurate, 
vinyl acetate/acrylate, styrene/acrylate, vinyl acetate/maleate or vinyl 
acetate/ethylene. Binder amounts of 10-20% by weight, based on the solids 
content of the suspension, are quite sufficient. After the catalyst is 
charged into the reactor, these copolymers burn out quantitatively in the 
stream of air within a short time. 
The supported catalysts are suitable, for example, for use as oxidation 
catalysts in the oxidation of aromatics or alkylaromatics or mixtures 
thereof for preparing the corresponding acid anhydrides, preferably for 
preparing phthalic anhydride (PA) by catalytic gas-phase oxidation of 
o-xylene or naphthalene or mixtures of o-xylene and naphthalene. A further 
preferred application is an oxidation catalyst in the preparation of 
pyromellitic dianhydride (PMDA) by catalytic gas-phase oxidation of 
1,2,4,5-tetraalkylated benzenes (for example 
durene=1,2,4,5-tetramethylbenzene). 
In the preparation of PA and PMDA, the respective starting materials are 
reacted with an oxygen-containing gas in the presence of the catalyst of 
the invention, preferably in fixed-bed reactors. Typical fixed-bed 
reactors are, for example, reaction tubes which are collected into 
tube-bundle reactors and are surrounded by a heat exchange medium. The 
reaction tubes are arranged vertically and the reaction mixture flows 
through them from the top. They comprise a material inert toward the heat 
exchange medium, catalyst, starting materials and products, generally 
steel, and have a length of from 2000 to 6000 mm, an internal diameter of 
from 10 to 30 mm and a wall thickness of from 1 to 4 mm. Heat exchange 
media which have been found to be useful in practice are salt mixtures, 
for example, a chloride-free melt of potassium nitrate and sodium nitrite. 
The catalyst is introduced into the reaction tubes from the top and is 
fixed by means of holders fitted in the vicinity of the lower ends of the 
tubes. The bed depth can be between 900 and 3300 mm. The reaction tubes 
can, if desired, be charged in layers with support bodies of different 
shape and size and also different concentration and composition of the 
active components. 
The reaction gas comprising starting hydrocarbons and an oxygen-containing 
gas, preferably air, is passed over the catalyst at a space velocity of 
from 800 to 8000 h.sup.-1, preferably from 1000 to 6000 h.sup.-1. The 
mixing ratio is here from 10 to 150 g of hydrocarbon per standard cubic 
meter of oxygen-containing gas. 
After the reaction, the product formed is isolated from the reaction gas in 
a manner known per se by desublimation or by appropriate gas scrubbing 
using a suitable solvent. 
The supported catalysts of the invention are distinguished from the 
oxidation catalysts known hitherto and based on TiO.sub.2 /V.sub.2 O.sub.5 
by part of the TiO.sub.2 component being replaced by SiC. 
It has been found that mixing SiC powder into the catalytically active 
surface coating of catalysts has a particularly advantageous effect on the 
operating behavior and the selectivity of catalysts. This applies 
particularly to the oxidation of naphthalene or of a mixture of 
naphthalene and o-xylene to give PA and to the oxidation of durene to give 
PMDA. SiC-containing catalysts give higher carboxylic anhydride yields at 
higher hydrocarbon loadings both in the naphthalene and/or o-xylene 
oxidation and in the durene oxidation. 
Completely unexpectedly, replacement of part of the TiO.sub.2 active 
component by SiC, which was hitherto known only as an inert filler, gives 
catalysts which are superior to the SiC-free catalysts used hitherto in 
respect of selectivity and maximum loading. The catalysts of the invention 
make higher loadings possible. The yields are significantly improved. The 
supported catalysts of the invention are insensitive to short-term 
stressing at temperatures above 600.degree. C.

The following examples serve to illustrate the invention. 
Catalyst Preparation 
The amounts indicated in Table 1 of the active components were suspended in 
400 ml of deionized water and stirred for 18 hours o as to achieve 
homogeneous dispersion. Before application of the mixture to the steatite 
support bodies indicated in Table 1, the organic binder, a copolymer of 
vinyl acetate and vinyl laurate, was added in the form of a 50% strength 
aqueous dispersion to the suspension. The s was subsequently applied to 
the support with evaporation of the water. 
TABLE 1 
__________________________________________________________________________ 
Composition of the catalysts: 
Catalyst 
A B C D E 
__________________________________________________________________________ 
Support 
7 .times. 4 .times. 4 mm 
7 .times. 4 .times. 4 mm 
7 .times. 4 .times. 4 mm 
8 mm 
8 mm 
rings rings rings spheres 
spheres 
1225 g 1225 g 1225 g 1000 g 
1000 g 
V.sub.2 O.sub.5 
15.33 g 
15.12 g 
15.12 g 
19.6 g 
19.6 g 
TiO.sub.2 hydrate, 
22.29 g 
23.67 g 
23.67 g 
40.27 g 
40.27 g 
BET surface 
area: 
&gt;150 m.sup.2 /g 
SiC -- 94.69 g 
94.69 g 
-- 16.00 g 
Particle dia- 4 .mu.m .phi. 
30 mm .phi. 
-- 4 .mu.m .phi. 
meter 
Anatase, BET 
96.00 g 
-- -- 16.00 g 
-- 
surface area: 
&lt;10 m.sup.2 /g 
Ground steatite 
-- -- -- -- -- 
Particle 
diameter 
Ca.sub.2 CO.sub.3 
322.7 mg 
393.6 mg 
393.6 mg 
-- -- 
(NH.sub.4).sub.2 HPO.sub.4 
-- -- -- 6.35 g 
6.35 g 
Dispersion 
42 g 42 g 42 g 30 g 
30 g 
__________________________________________________________________________ 
To test the suitability of the supported catalysts as oxidation catalysts, 
they were tested in the oxidation of naphthlene to give phthalic anhydride 
(Examples 1 and 2) and in the oxidation of durene to give PMDA (Example 
3). Conventional catalysts based on TiO.sub.2 /V.sub.2 O.sub.5 were used 
for comparison (Comparative Examples 1, 2). 
The oxidation experiments were carried out in a reaction tube replacing an 
industrial scale. The length of the reaction tube was 3.3 m (filling 
height 2.8 m), its diameter was 25 mm. The temperature of the reactor was 
controlled using a circulated salt bath (eutectic, chloride-free melt of 
potassium nitrate and sodium nitrite). The amount of air fed in was 4 
standard m.sup.3 /h. The purity of the starting materials was always above 
99%. 
TABLE 2 
______________________________________ 
Results of the oxidation experiments: 
anatase-SiC comparison 
Compar- Compar- 
ative ative 
Example Example 1 
Example 1 
Example 2 
Example 2 
Example 3 
______________________________________ 
Catalyst 
A B C D E 
(anatase) 
(SiC) (SiC) (anatase) 
(SiC) 
Starting 
N.sup.4) N N D.sup.5) 
D 
material 
MV (max).sup.1) 
52 80 90 26 40 
g/Nm.sup.3 ! 
SBT .degree.C.!.sup.2) 
360 365 364 376 385 
HST .degree.C.!.sup.3) 
470 450 446 482 461 
Pure yield 
98 101 100 80 80 
% by PA PA PA PMDA PMDA 
weight! 
______________________________________ 
.sup.1) MV (max) is the maximum usable hydrocarbon loading of the air in 
of hydrocarbon per standard m.sup.3 of air. 
.sup.2) Salt bath temperature 
.sup.3) Hot spot temperature 
.sup.4) Naphthalene 
.sup.5) Durene 
Examples 1 and 2 show the advantages of SiC in the oxidation of naphthalene 
to give phthalic anhydride. Compared with the anatase-containing catalyst 
A (Comparative Example 1), catalyst B (Example 1) gave a 3% by weight 
improvement in the PA yield. At the same time, substantially higher 
naphthalene loadings with lower hot spot temperatures were possible using 
catalyst B (Example 1). 
Very fine SiC (30 nm particle diameter, Example 2) made possible loadings 
of up to MV 90 with even lower HST. Example 2 evidences the great 
flexibility in the selection of the SiC particle size and the improvement 
in the effect with decreasing particle size. 
In the durene oxidation (Comparative Example 2, Example 3) too, 
significantly higher loadings with lower hot spot temperatures were 
possible. 
TABLE 3 
______________________________________ 
Composition of the catalysts: Anatase-SiC 
comparison for two-bed packings 
Catalyst F G H 
______________________________________ 
Support 7 .times. 7 .times. 4 mm 
7 .times. 7 .times. 4 mm 
7 .times. 7 .times. 4 mm 
rings rings rings 
1000 g 1000 g 1000 g 
Packing Filling 
Upper packing 
Upper packing 
Lower packing 
height 150 cm 150 cm 130 cm 
V.sub.2 O.sub.5 
9.95 g 10.23 g 10.05 g 
Ti hydrate 
17.60 g 16.02 g 19.63 g 
SiC (particle .phi.) 
-- 38.45 g (4 .mu.m .phi.) 
-- 
Anatase, BET 
14.19 g 25.63 g 56.08 g 
&lt;10 m.sup.2 /g 
Ca.sub.2 CO.sub.3 
220 mg 222 mg -- 
(NH.sub.4).sub.2 HPO.sub.4 
-- -- 1.372 g 
Dispersion 
35 g 35 g 35 g 
______________________________________ 
Even in the case of the two-bed packings comprising damped upper bed and 
activated lower bed which are now customary in industry, SiC shows its 
advantages. It can here be sufficient to provide only the upper packing, 
in which the hot spot zone is located, with SiC (catalyst G). This 
catalyst G (upper bed), in which 60% of the low surface area anatase 
component was replaced by SiC, could be loaded at 102 g/standard m.sup.3 
after an operation time of about 4 weeks, without hot spot temperatures 
above 470.degree. C. occuring (Example 4). The same lower bed H was used 
for both Example 4 and Comparative Example 3. 
TABLE 4 
______________________________________ 
Results of the oxidation experiments: 
Anatase-SiC/anatase comparison for two-bed packings 
Comparative 
Example Example 3 Example 4 
______________________________________ 
Catalyst F/H G/H 
(anatase) (SiC/anatase) 
Starting material 
N N 
MV (max) g/Nm.sup.3 ! 
62 102 
SBT .degree.C.!.sup.2) 
360 368 
HST .degree.C.! 
474 462 
PA pure yield 98 99 
% by weight! 
______________________________________