Process for preparing pyromellitic dianhydride

The invention relates to a process for preparing pyromellitic dianhydride (PMDA) by heterogeneously catalyzed oxidation in the gas phase by means of a gas containing molecular oxygen. The process involves oxidizing benzaldehydes which are 2,4,5-trialkylated by C.sub.1 - to C.sub.3 -alkyl groups or mixtures of benzaldehydes which are 2,4,5-trialkylated by C.sub.1 - to C.sub.3 -alkyl groups and benzenes which are 1,2,4,5-tetraalkylated by C.sub.1 - to C.sub.3 -alkyl groups in the presence of a catalyst. The catalyst contains as active components 5% to 95% by weight of one or more transition-metal oxides of sub-group IV of the Periodic Table of the Elements, from 1% to 50% by weight of one or more transition-metal oxides of sub-group V of the Periodic Table of the Elements. The catalyst also contains from 0% to 10% by weight of one or more oxides of elements of main group I of the Periodic Table of the Elements and/or from 0% to 50% by weight of one or more oxides of elements of main groups III, IV and V of the Periodic Table of the Elements and of elements of sub-groups VI and VII of the Periodic Table of the Elements. The indicated percentages by weight are based in each case on the total weight of the active components and add to 100% by weight.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for preparing pyromellitic 
dianhydride (PMDA) by heterogeneously catalyzed oxidation in the gas phase 
by means of a gas containing molecular oxygen and to catalysts to be used 
in this process. 
2. The Prior Art 
PMDA has, up to now, been obtained on a large scale mainly by liquid-phase 
oxidation of 2,4,5-trimethylbenzaldehyde with atmospheric oxygen, in a 
process analogous to the process described in DE-A 1,943,510 (GB-A 
1,282,775) for preparing terephthalic acid from p-toluylaldehyde, in which 
the pyromellitic acid thus obtained is dehydrated to PMDA. The 
2,4,5-trimethylbenzaldehyde is prepared by carbonylation of 
1,2,4-trimethylbenzene (pseudo-cumene) (DE-A 2,422,197=GB-A 1,422,308). 
The use of acetic acid as a solvent and heavy-metal salts in combination 
with a bromide source (Chem-Systems Report: PERP 1987-T-4, 16-40) as 
catalysts necessitate, in this process, the use of high performance and 
therefore very expensive alloys (Hastelloy C) for the reactor. Besides the 
batchwise operation, a further disadvantage of this process is that the 
pyromellitic acid obtained by liquid-phase oxidation must be dehydrated to 
PMDA in a very energy-intensive step (&gt;200.degree. C.). 
A further process for preparing PMDA employing the principle of 
liquid-phase oxidation is the Amoco process (U.S. Pat. No. 4,719,311). 
Using a similar catalyst (Co-Mn-Br), 1,2,4,5-tetramethylbenzene (durene) 
is oxidized with atmospheric oxygen to pyromellitic acid, which likewise 
still has to be dehydrated to PMDA. In addition to the disadvantages 
described for the above-mentioned process, this process has the further 
disadvantage that durene is above five times more expensive than 
pseudo-cumene. 
A third PMDA process operates in the gas phase. Analogous to the oxidation 
of o-xylene to phthalic anhydride, durene is oxidized directly to PMDA 
over a heterogeneous catalyst in a multiple-tube reactor. U.S. Pat. No. 
4,665,200 discloses V.sub.2 O.sub.5, TiO.sub.2, P.sub.2 O.sub.5, Nb.sub.2 
O.sub.5, Sb.sub.2 O.sub.3, K.sub.2 O and Cs.sub.2 O as catalyst 
components. Particular advantages of the gas-phase oxidation are 
continuous operation and the easy handling of the catalysts, as a result 
of which the use of expensive materials in plant construction can be 
dispensed with. In principle, it is possible to inexpensively retrofit 
existing plants for PMDA production. The energy-intensive dehydration of 
the liquid-phase process is eliminated, since the anhydride is 
desublimated directly from the reaction gas. A PMDA purity of 99% is 
achieved by means of suitable desublimation processes (DE-A 3,730,747=U.S. 
Pat. No. 4,867,763). 
A further way of obtaining PMDA from the reaction gas is gas scrubbing with 
an anhydrous solvent, a technology which is state of the art in, for 
example, the preparation of maleic anhydride (SRI International, PEP 
Report 46C, 1989). Further examples of the preparation of PMDA via 
gas-phase oxidation in the presence of vanadium- or titanium-containing 
catalysts are described in EP-A 405,508 and EP-A 330,195. A disadvantage 
of the gas-phase oxidation when compared with the previously known 
processes is the lower selectivity in comparison with liquid-phase 
oxidation. 
For the gas-phase oxidation, starting materials which have been described 
are, in addition to 1,2,4,5-tetraalkylated benzenes, functionally 
substituted benzene derivatives which are prepared from trisubstituted 
benzenes, for example pseudo-cumene. Functional groups described are 
chloromethyl and alkoxymethyl (AT-PS 169 330). For ecological reasons, 
chlorine-containing aromatics are questionable, especially at such high 
reaction temperatures as occur in the gas-phase oxidation. 
Alkoxymethylbenzenes are likewise produced via a chloromethylation and are 
to be avoided for the same reason. 
In summary, the most serious disadvantages of the processes known from the 
prior art are, for the liquid-phase oxidation processes, the expensive 
reactor materials because of the corrosive catalysts, the long down-times 
as a result of batchwise operation, the energy-intensive dehydration of 
the acid to the anhydride and, for the gas-phase oxidation processes, 
their expensive raw material base and low selectivity. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a process which 
combines the advantages of the liquid-phase oxidation, namely the 
favorable raw material base, with the advantages of the gas-phase 
oxidation, namely economical reactor materials, continuous operation, 
avoidance of the dehydration step by desublimation of the anhydride, and, 
at the same time, gives the desired product with very high selectivity. 
It has been found that especially the use of alkylated benzaldehydes in the 
gas-phase oxidation is particularly advantageous. In comparison with 
durene oxidation, aldehyde oxidation is substantially more selective, so 
that the PMDA yield increases. This was not to be expected since, at such 
high temperatures as occur in a gas-phase reactor, aromatic aldehydes very 
easily undergo decarbonylation or decomposition reactions and should 
therefore be unsuitable as starting materials for the gas-phase oxidation 
to PMDA. A further desirable effect is that the formation of by-products, 
particularly trimellitic anhydride (TMSA), is suppressed. Advantage can 
thus be taken of the price advantage of the pseudo-cumene base, since, in 
analogy with the first step of the liquid-phase oxidation, 
trimethylbenzaldehyde (TMBA) can be prepared relatively simply by 
carbonylating the inexpensive pseudo-cumene. 
The invention provides a process for preparing pyromellitic dianhydride 
(PMDA) by heterogeneously catalyzed oxidation in the gas phase by means of 
a gas containing molecular oxygen, which comprises oxidizing benzaldehydes 
which are 2,4,5,-trialkylated by C.sub.1 - to C.sub.3 -alkyl groups or 
mixtures of benzaldehydes which are 2,4,5-trialkylated by C.sub.1 - to 
C.sub.3 -alkyl groups and benzenes which are 1,2,4,5-tetraalkylated by 
C.sub.1 - to C.sub.3 -alkyl groups in the presence of a catalyst which 
contains as active components 5% to 95% by weight of one or more 
transition-metal oxides of sub-group IV of the Periodic Table of the 
Elements, from 1% to 50% by weight of one or more transition-metal oxides 
of sub-group V of the Periodic Table of the Elements, from 0% to 10% by 
weight of one or more oxides of elements of main group I of the Periodic 
Table of the Elements and/or from 0% to 50% by weight of one or more 
oxides of elements of main groups III, IV and V of the Periodic Table of 
the Elements and of elements of sub-groups VI and VII of the Periodic 
Table of the Elements, where the indicated percentages by weight are based 
in each case on the total weight of the active components and add up to 
100% by weight. 
PMDA is obtained by catalytic gas-phase oxidation starting from 
2,4,5-trialkylated benzaldehydes, in which the alkyl groups may be methyl, 
ethyl, propyl or isopropyl radicals, or starting from a mixture 
2,4,5-trialkylated benzaldehydes, in which the alkyl groups may be methyl, 
ethyl, propyl or isopropyl radicals, and 1,2,4,5-tetraalkylated benzenes, 
in which the alkyl groups may likewise be methyl, ethyl, propyl or 
isopropyl radicals. If these mixtures are used, the weight ratio of 
2,4,5-trialkylated benzaldehydes to 1,2,4,5-tetraalkylated benzenes is 
preferably from 10:1 to 1:10. Preferably, 2,4,5-trimethylbenzaldehyde or a 
mixture of 2,4,5-trimethylbenzaldehyde and 1,2,4,5-tetramethylbenzene 
(durene) is used. The particularly preferred starting material is 
2,4,5-trimethylbenzaldehyde alone. 
Preferably, the catalyst contains as active components from 10% to 90% by 
weight of an oxide of titanium and/or zirconium, from 5% to 35% by weight 
of an oxide of vanadium and/or niobium and also from 0% to 5% by weight of 
one or more oxides selected from the group consisting of oxide compounds 
of potassium, rubidium, cesium and/or from 0.1% to 10% by weight of one or 
more oxides selected from the group consisting of phosphorus, antimony, 
bismuth, chromium, molybdenum, tungsten, manganese, where the indicated 
percentages by weight are based in each case on the total weight of the 
active components. 
Particularly preferred catalyst compositions have titanium dioxide as the 
oxide of the transition metals of sub-group IV of the Periodic Table of 
the Elements, vanadium pentoxide as the oxide of transition metals of 
sub-group V of the Periodic Table of the elements, which are doped with 
the phosphorus pentoxide, either alone or together with Sb.sub.2 O.sub.3 
and/or Cs.sub.2 O. The most preferred catalyst compositions are those 
containing titanium dioxide in the anatase form having a BET surface area 
of from 5 to 200 m.sup.2 /g. 
The catalyst may be used as a solid catalyst (compacts, extrudates, 
granules) or in the form of catalysts being coated onto a substrate, the 
form depending on the gas-phase oxidation process. For example, in the 
fluidized-bed process, catalysts in granulated form are used and, in the 
fixed-bed process, compacts or rings or beads coated with the active 
catalyst components (coated catalysts) are used. 
The fixed-bed process is preferred; for it the catalytically active 
composition is on inert support or substrate materials. The proportion of 
the active catalyst composition, based on the total weight, i.e., the sum 
of the weights of the support bodies and the active catalyst component 
composition, is from 1% to 30% by weight, preferably from 2% to 15% by 
weight. In principle, the substrate supports may be of any desired shape 
and surface structure. Preferred supports are, however, regularly shaped, 
mechanically stable bodies such as beads, rings, half rings, cylinders, 
saddles, having a smooth pore-free surface. The size of the support bodies 
is primarily determined by the dimension, particularly the internal 
diameter of the reaction tube, if the catalyst is used in a tube or 
multiple-tube reactor. The support diameter should then be between 1/2 and 
1/10 of the reactor internal diameter. Suitable inert materials for the 
supports are, for example, steatite, duranite, silicon carbide, 
earthenware, porcelain, silicon dioxide, silicates, aluminum oxide, 
aluminates or mixtures of these materials. Preferably, beads or rings of 
steatite are used. 
The active components may be applied to the inert supports in conventional 
manner. Hence, the supports may be coated with an aqueous suspension of 
the mixture or else of the individual components in a rotating-tube 
furnace at 200.degree.- 300.degree. C. The active components may be 
applied in the form of the oxides or in the form of compounds which are 
converted to the oxides under the conditions of the gas-phase oxidation or 
in a preceding heat-treatment step. Supported catalysts having coatings 
which adhere extremely well are obtained by applying to the support bodies 
an aqueous suspension which contains the mixture or the individual 
components and an organic binder. Such processes for coating catalysts 
onto supports are described, for example, in DE-B 2,106,796 (U.S. Pat. No. 
3,799,886). 
In the process according to the invention, the starting materials are 
reacted together with an oxygen-containing gas in the presence of the 
oxidation catalyst described above, preferably in fixed-bed reactors. 
Customary fixed-bed reactors are, for example, reaction tubes, which are 
combined to form a multiple-tube reactor and are surrounded by a 
heat-exchange medium. The reaction tubes are arranged vertically and the 
reaction mixture flows through them from the top to the bottom. They are 
made of a material which is inert to the heat-exchange medium, catalyst, 
starting materials and products. In general, they were made of a suitable 
steel, and have a length of from 2000 to 4000 mm, preferably from 2500 to 
3500 mm, an internal diameter of from 10 to 30 mm, preferably from 18 to 
26 mm, and a wall thickness of from 1 to 4 mm. Heat-exchange media which 
have proven suitable in industrial practice are eutectic salt mixtures, 
such as a chloride-free melt of potassium nitrate and sodium nitrite. 
The catalyst is introduced into the reaction tubes from the top and fixed 
in place by securing devices fitted near the lower ends of the tubes. The 
bed depth may be between 900 and 3300 mm. The reaction tubes may, if 
required, be packed with layers of support bodies of varying shape and 
dimensions and varying concentration and composition of the active 
components. 
In the process of the invention, the reaction gas containing 
2,4,5-trialkylated benzaldehyde, which may be mixed with 
1,2,4,5-tetraalkylated benzene, with an oxygen-containing gas, preferably 
air, is brought into contact with the catalyst. Preferably, the space 
velocities are from 800 to 8000 h.sup.-1, particularly preferably from 
1000 to 6000 h.sup.-1. The mixing ratio is from 10 to 100 g of starting 
material/Nm.sup.3, preferably from 10 to 40 g of starting 
material/Nm.sup.3. The reaction temperature is from 250.degree. to 
600.degree. C., preferably from 300.degree. to 500.degree. C. 
After the reaction, the pyromellitic dianhydride (PMDA) formed is isolated 
from the reaction gas in a conventional manner by desublimation in a 
downstream separator at from 40.degree. to 80.degree. C. (DE-A 
3,730,747=U.S. Pat. No. 4,867,763) or by corresponding gas scrubbing with 
a suitable solvent. 
The pyromellitic dianhydride obtainable from the process of the invention 
is used as starting material (comonomer) for producing high-temperature 
resistant polymers, as hardener for epoxy resins and as starting material 
for plasticizer components.

Other objects and features of the present invention will become apparent 
from the following detailed description considered in connection with the 
accompanying Examples, which disclose embodiments of the present 
invention. It should be understood, however, that the Examples are 
designed for the purpose of illustration only and not as a definition of 
the limits of the invention. 
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Catalyst Preparation 
55 g of TiO.sub.2 (anatase), 7 g of V.sub.2 O.sub.5 and 3.5 g of 
(NH.sub.4).sub.2 HPO.sub.4 were suspended in 400 ml of deionized water and 
stirred for 18 hours, so as to obtain a homogeneous mixture. Before the 
mixture was applied to 1000 g of 8 mm steatite beads support, 20 g of a 
copolymer of vinyl acetate and vinyl laurate in the form of a 50% by 
weight aqueous dispersion were added to the suspension. Subsequently, the 
suspension was applied to the support with evaporation of the water. After 
a heat-treatment step of 4 hours at 410.degree. C. and an air flow rate of 
0.5 Nm.sup.3 /h, the catalytically active composition had a surface area 
of 95 m.sup.2 /g (measured by BET). 
All the examples below were carried out in a reaction tube reflecting an 
industrial scale. The length of the reaction tube was 3.3 m (bed depth 2.8 
m, corresponding to 1730 g of catalyst), its diameter being 25 mm. The 
reactor was heated by a circulating salt bath (eutectic, chloride-free 
salt melt of potassium nitrate and sodium nitrite). The feed rate of air 
was 4 Nm.sup.3 /h. The mixing ratio of starting material/air was from 12 
to 35 g/Nm.sup.3 of air. The purity of the 2,4,5-trimethylbenzaldehyde was 
between 95% and 98% by weight. The purity of the 
1,2,4,5-tetramethylbenzene (durene) in the comparative examples was from 
97% to 99% by weight. 
The reaction conditions and yields for the two examples and the two 
comparative examples are shown in the table below. 
TABLE 
______________________________________ 
Comparative 
Comparative 
Example 
Example Example Example 
1 2 1 2 
______________________________________ 
Starting TMBA TMBA Durene Durene 
Material 
SBT (.degree.C.) 
375 380 370 375 
PMDA (% 83 90 75 75 
by weight*) 
TMA (% by 
0.8 0.7 5 4 
weight**) 
______________________________________ 
* = based on 100% strength starting material 
** = based on PMDA 
SBT = salt bath temperature 
TMBA = 2,4,5Trimethylbenzaldehyde 
TMA = Trimellitic anhydride 
To enable the TMA content of the PMDA separated out to be analyzed, the 
reaction product was converted to the methyl ester with an H.sub.2 
SO.sub.4 /CH.sub.3 OH mixture (1:3 % by volume) and the TMA content 
subsequently determined by gas chromatography. 
While only a single embodiment of the present invention has been shown and 
described, it is to be understood that many changes and modifications may 
be made thereunto without departing from the spirit and scope of the 
invention as defined in the appended claims.