Process for producing phthalic anhydride and catalyst therefor

A process for producing phthalic anhydride, which comprises packing a catalyst into a multitube fixed bed converter, said catalyst comprising a catalytically active material composed of 1 to 20 parts by weight as V.sub.2 O.sub.5 of vanadium oxide, 99 to 80 parts by weight of anatase-type titanium oxide being porous and having a particle diameter substantially of 0.4 to 0.7 micron and a specific surface area of 10 to 60 m.sup.2 /g, and per 100 parts by weight of the sum of said two components, 0.01 to 1 part by weight as Nb.sub.2 O.sub.5 of niobium oxide, 0.05 to 1.2 parts by weight as an oxide of at least one ingredient selected from the group consisting of potassium, cesium, rubidium and thallium, and 0.2 to 1.2 parts by weight as P.sub.2 O.sub.5 of phosphorus, and a porous carrier having an alumina content of not more than 10% by weight, a silicon carbide content of at least 80% by weight and an apparent porosity of at least 10% supporting said catalytically active material thereon, wherein the total volume of pores having a diameter of 0.15 to 0.45 micron present in the layer of the catalytically active material on the carrier is at least 50% of that of pores having a diameter of not more than 10 microns present in said layer of the catalytically active material; and passing o-xylene or naphthalene and oxygen or a molecular oxygen-containing gas through the catalyst layer at a temperature of 300.degree. to 450.degree. C., thereby catalytically oxidizing the o-xylene or naphthalene; and a catalyst suitable for performing said process.

This invention relates to a process for producing phthalic anhydride by the 
catalytic vapor-phase oxidation of o-xylene or naphthalene with a gas 
containing molecular oxygen. More specifically, it relates to a process 
for producing phthalic anhydride by the catalytic vapor-phase oxidation of 
a molecular oxygen-containing gas containing o-xylene or naphthalene in a 
high concentration, and to a catalyst suitable for performing this 
process. 
According to this invention, there are provided a process for producing 
phthalic anhydride stably with high productivity by the catalytic 
vapor-phase oxidation of a molecular oxygen-containing gas containing 
o-xylene or naphthalene in a high concentration of, for example, more than 
60 g/Nm.sup.3 in the presence of a catalyst containing vanadium oxide 
while avoiding a danger of explosion both at a gas inlet and a gas outlet 
of a converter; and a catalyst suitable for the process. 
Phthalic anhydride has been produced previously by the catalytic 
vapor-phase oxidation of o-xylene or naphthalane using air as a molecular 
oxygen-containing gas. To avoid the danger of explosion, it is usual in 
the conventional method to maintain the concentration of the starting gas 
below the lower limit of explosion during the reaction. For example, in 
the production of phthalic anhydride from o-xylene, the concentration of 
the starting gas should be maintained at below 40 g/Nm.sup.3. With 
technological advances made in the selectivity and heat resistance of the 
catalyst and in reaction engineering, operations within an explosive range 
have been attempted for the past several years to increase productivity 
per unit converter and save energy. According to these operations, the 
concentration of o-xylene in the air is increased to more than 40 
g/Nm.sup.3 in the aforesaid catalytic vapor-phase oxidation process. 
Suggestions relating to such a process are disclosed in Japanese Laid-Open 
Patent Publications Nos. 40539/75 (West German Laid-Open Patent 
Publication No. 2,417,145), 4051/75 (West German Laid-Open Publication No. 
2,330,841), and 134618/74 (West German Laid-Open Publication No. 
2,309,657). 
The actual operation of these high gas concentration processes involves 
using an o-xylene/air ratio of at most 60 g/Nm.sup.3, as is described in 
detail in Chemical Engineering (March 1974, page 82), and I. Chem. E. 
Symposium Series (1976, Vol. 50, p. 4). The reason for this is not 
entirely clear, but presumably the purpose of it is to keep the 
composition of the product gas from the outlet of the converter outside 
the explosive range. The composition of the starting gas at its inlet 
portion is within the explosive range. But as described in U.S. Pat. No. 
3,296,281, by increasing the linear velocity of the gas between the 
material charging section and the catalyst layer in the converter, the 
danger of explosion can be avoided at a gas concentration of up to a 
certain point even if the gas composition in the stationary state is 
within the explosive range. After the gas has left the gas outlet of the 
converter, however, it is impossible, in view of the operation of 
recovering the resulting phthalic anhydride, to narrow the apparent 
explosive range of the gas composition by increasing the linear velocity 
of the gas. 
It may be taken for granted that the oxidation reaction at an increased 
concentration of the starting gas is safer than ordinary operations 
outside the explosive range, because the concentration of residual oxygen 
in the product gas decreases. This applies, however, to an o-xylene/air 
ratio of up to about 60 g/Nm.sup.3, and it is difficult to apply this 
principle to higher gas concentrations. When the reaction is carried out 
simply at an increased o-xylene/air concentration of more than 60 
g/Nm.sup.3, the concentration of residual oxygen necessarily decreases 
further. But since the concentrations of combustible phthalic anhydride, 
maleic anhydride, carbon monoxide, etc. increase, the composition of the 
product gas falls within the explosive range. 
Another possible reason for this is the restriction in regard to the 
performance of catalyst, as described in I. Chem. E. Sym. Series (1976, 
Vol. 50, p. 4). The catalytic vapor-phase oxidation of o-xylene to form 
phthalic anhydride is very exothermic. When the concentration of the gas 
is increased abnormal heat generation called "hot spot" is liable to occur 
locally in the catalyst layer. This induces excessive oxidation reaction, 
which results in a decrease in the yield of phthalic anhydride and in a 
marked degradation of the catalyst at the hot spot sites. It has been 
found that when o-xylene is catalytically oxidized with air in the vapor 
phase at a concentration of 80 g/Nm.sup.3 using catalysts described, for 
example, in Japanese Patent Publication No. 41271/74 (U.S. Pat. No. 
3,926,846), and Japanese Laid-Open Patent Publications No. 42096/76 (West 
German Laid-Open Patent Publication No. 2,238,067) and 49189/76 (West 
German Laid-Open Patent Publication No. 2,436,009), for example the 
catalyst described in Japanese Patent Publication No. 41271/74, the 
temperature of the hot spots exceeds 500.degree. C., and side-reactions to 
form maleic anhydride, benzoic acid, carbon dioxide, etc. increase, and 
the yield of phthalic anhydride cannot reach 100% by weight. It is 
difficult therefore to obtain phthalic anhydride in a high yield even when 
the ratio of o-xylene/air is simply increased to more than 60 g/Nm.sup.3 
in a conventional known conventional method in using a known conventional 
catalyst. 
The present inventors searched for reaction conditions under which the 
temperature of hot spots is low and side-reactions are reduced even at a 
high gas concentration, and suitable catalysts for achieving these 
reaction conditions. As a result, they found that the composition of the 
gas in the reaction system can be always maintained outside the explosive 
range by recycling a part of the exhaust gas left after the recovery of 
phthalic anhydride to a converter for reuse. They succeeded in developing 
a process capable of affording phthalic anhydride by the catalytic 
vapor-phase oxidation of o-xylene safely and stably even at an o-xylene 
concentration of more than 60 g/Nm.sup.3, and a catalyst suitable for 
performing this process. Specifically, the inventors found that if the 
temperature of the gas at the inlet of the catalyst layer is maintained at 
not more than 150.degree. C. and the concentration of oxygen in the 
starting gas is maintained at not more than 12% by volume, the danger of 
explosion is completely removed and therefore, any desired concentration 
of o-xylene can be employed, and that under these gas conditions, the gas 
completely falls outside the explosive range even at a site subsequent to 
the outlet of the reactor. Incident to this, the present inventors 
discovered a vanadium-titanium oxide type catalyst comprising anatase-type 
titanium oxide which is porous and has a particle diameter of about 0.4 to 
0.7 micron and a specific surface area of 10 to 60 m.sup.2 /g as a 
catalyst which does not lose catalytic activity over a long period of time 
even within a low range of oxygen concentration. 
The catalyst of this invention for the production of phthalic anhydride 
comprises a catalytically active material composed of 1 to 20 parts by 
weight as V.sub.2 O.sub.5 of vanadium oxide, 99 to 80 parts by weight as 
TiO.sub.2 of anatase-type titanium oxide being porous and having a 
particle diameter substantially of 0.4 to 0.7 micron and a specific 
surface area of 10 to 60 m.sup.2 /g, and per 100 parts by weight of the 
sum of these two components, 0.01 to 1 part by weight as Nb.sub.2 O.sub.5 
of niobium oxide, 0.05 to 1.2 parts by weight as an oxide of at least one 
ingredient selected from the group consisting of potassium, cesium, 
rubidium and thallium and 0.2 to 1.2 parts by weight as P.sub.2 O.sub.5 of 
phosphorus, said catalytically active material being supported on a porous 
carrier having an alumina (Al.sub.2 O.sub.3) content of not more than 10% 
by weight, a silicon carbide (SiC) content of at least 80% by weight, and 
an apparent porosity of at least 10%, wherein the total volume of pores 
having a diameter of 0.15 to 0.45 micron present in the layer of the 
catalytically active material on the carrier is at least 50%, preferably 
at least 70%, of that of pores having a diameter of not more than 10 
microns present in said layer of the catalytically active material. 
In one embodiment of this invention, phthalic anhydride is produced by 
passing a gaseous mixture containing naphthalene or o-xylene and air or 
another molecular oxygen-containing gas through a converter packed with 
the aforesaid catalyst, catalytically oxidizing naphthalene or o-xylene in 
the vapor phase at an elevated temperature, conducting the resulting 
phthalic anhydride-containing gas to a switch condenser, cooling the gas 
at a temperature higher than the dew point of water in the reaction 
product gas and recovering phthalic anhydride, and recycling a part of the 
exhaust gas from the condenser without removing water therefrom and then 
mixing it with the starting gas. 
In another embodiment, a stacked catalyst layer composed of a layer of a 
"first-stage catalyst" and a layer of a "second-stage catalyst" is used. 
The first-stage catalyst comprises a catalytically active material 
composed of 1 to 20 parts by weight of V.sub.2 O.sub.5, 99 to 80 parts by 
weight of anatase-type TiO.sub.2 being porous and having a particle 
diameter substantially of 0.4 to 0.7 micron and a specific surface area of 
10 to 60 m.sup.2 /g, and per 100 parts by weight of the sum of these two 
components, 0.01 to 1 part by weight of Nb.sub.2 O.sub.5, 0.05 to 1.2 
parts by weight of at least one ingredient selected from K.sub.2 O, 
Cs.sub.2 O, Rb.sub.2 O and Tl.sub.2 O, and 0.2 to 0.4 part by weight of 
P.sub.2 O.sub.5, and a carrier having an alumina content of not more than 
10% by weight, a silicon carbide content of at least 80% by weight and an 
apparent porosity of at least 10%, supporting said catalytically active 
material thereon, wherein the total volume of pores having a diameter of 
0.15 to 0.45 micron present in the layer of the catalytically active 
material on the carrier is at least 50%, preferably at least 70%, of that 
of pores having a diameter of not more than 10 microns present in said 
layer of the catalytically active material. This first-stage catalyst 
occupies 30 to 70% of the total height of the catalyst layer in the 
reaction tube from the inlet for the starting gas. The second-stage 
catalyst comprises a catalytically active material composed of 1 to 20 
parts by weight of V.sub.2 O.sub.5, 99 to 80 parts by weight of 
anatasetype TiO.sub.2 being porous and having a particle diameter 
substantially of 0.4 to 0.7 micron and a specific surface area of 10 to 60 
m.sup.2 /g, and per 100 parts by weight of the sum of these two 
components, 0.01 to 1 part by weight of Nb.sub.2 O.sub.5, 0.05 to 1.2 
parts by weight of at least one of K.sub.2 O, Cs.sub.2 O, Rb.sub.2 O and 
Tl.sub.2 O, and 0.4 to 1.2 parts by weight of P.sub.2 O.sub.5, and a 
carrier having an alumina content of not more than 10% by weight, a 
silicon carbide content of at least 80% by weight and an apparent porosity 
of at least 10% supporting said catalytically active material thereon, 
wherein the total volume of pores having a diameter of 0.15 to 0.45 micron 
present in the layer of the catalytically active material on the carrier 
is at least 50%, preferably at least 70%, of that of pores having a 
diameter of not more than 10 microns present in said layer of the 
catalytically active material. The second stage catalyst occupies 70 to 
30% of the total height of the catalyst layer in the reaction tube from 
the outlet gas. 
A combination of the use of this stacked catalyst layer with the recycling 
of the exhaust gas described above is the most preferred embodiment of 
this invention. 
In the present invention, various processes such as (1), (2) and (3) 
described below can be used in recycling the exhaust gas to the converter. 
(1) All of the exhaust gas from the condenser is passed through a catalytic 
combustion system packed with a platinum or palladium type catalyst, and 
then water is removed from the gas. A part of the resulting gas is 
recycled to the converter and mixed with the starting gas. 
(2) All the exhaust gas from the condenser is sent to a tower adapted for 
recovering maleic anhydride. A part of the exhaust gas saturated with 
steam at the tower top temperature is recycled to the converter and mixed 
with the starting gas, and the remainder is conducted to a catalytic 
combustion system. 
(3) A part of the exhaust gas from the condenser, without removing water 
from it, is recycled to the converter and mixed with the starting gas, and 
the remainder of the exhaust gas is conducted to a catalytic combustion 
system. 
The process (3) is very simple and economical as compared with the 
processes (1) and (2). It is impossible however to apply a conventional 
V.sub.2 O.sub.5 -TiO.sub.2 supported catalyst having a high selectivity to 
process (3). In the process (3), the concentration of steam at the inlet 
of the converter reaches 5 to 15% by volume although it varies according 
to the amount of o-xylene fed. Usually, in the production of an organic 
acid from a hydrocarbon compound by catalytic oxidation, the entraining of 
steam in the reaction gas is advantageous because steam acts as an 
accelerator for the desorption of the product from the catalyst surface 
and inhibits excessive oxidation reaction. The use of the conventional 
V.sub.2 O.sub.5 -TiO.sub.2 type catalyst in the presence of steam is 
disadvantageous, however, because when naphthalene or o-xylene is oxidized 
with molecular oxygen in the presence of steam using the conventional 
V.sub.2 O.sub.5 -TiO.sub.2 type supported catalyst, steam extremely 
accelerates the degradation of the catalyst as the time passes. For 
example, when the catalyst described in Example 1 of Japanese Patent 
Publication No. 4538/77 (U.S. Pat. No. 4,046,780) was packed to a height 
of 2.5 meters into a tube having an inside diameter of 20 mm, the tube was 
dipped in a molten salt bath at 370.degree. C. and a gaseous mixture 
composed of 10% by volume of steam, 10% by volume of oxygen, 83 g/Nm.sup.3 
(the ratio of o-xylene/molecular oxygen) of o-xylene, and nitrogen was 
passed through the catalyst layer at a space velocity of 2,500 hr.sup.-1, 
phthalic anhydride was obtained in a yield of more than 112% by weight in 
the early stage of reaction after the initiation, and the difference 
(abbreviated .DELTA.T) between the temperature of the hot spot and the 
temperature of the molten salt was about 60.degree. C. In about 2 months 
from the initiation of the reaction, however, .DELTA.T decreased to 
20.degree. C., and the yield of phthalic anhydride decreased to 108% by 
weight. 
The reason for this was extensively sought, and the following conclusions 
were obtained. When o-xylene is oxidized at an o-xylene/molecular 
oxygen-containing gas ratio of more than 80 g/Nm.sup.3 at an oxygen 
concentration of as low as less than 12% by volume, both the concentration 
of o-xylene as a material to be oxidized and the concentration of oxygen 
exert a great load on the catalyst. To increase active sites in this case, 
a high loading catalyst could be produced by increasing the specific 
surface area of TiO.sub.2 as one catalytically active substance. 
Specifically, the use of anatase-type TiO.sub.2 having a specific surface 
area of at least 10 m.sup.2 /g, preferably about 15 to 40 m.sup.2 /g, i.e. 
anatase-type TiO.sub.2 having a primary particle diameter of about 0.05 to 
0.2 micron, is preferred. The use of this finely divided anatase-type 
TiO.sub.2 as a material for catalyst is effective in obtaining a high 
loading catalyst, but has been found to suffer from the defect that the 
speed of degradation of the catalyst is high. Various physical analyses 
have shown that as the primary particle diameter of TiO.sub.2 is smaller, 
the crystal growth of TiO.sub.2 in the catalyst layer, especially at hot 
spots, increases, and the catalyst activity is decreased accordingly, and 
that when o-xylene is oxidized in the presence of steam, the steam 
accelerates the crystallization of V.sub.2 O.sub.5 to needle-like crystals 
and aggravates the state of dispersion of V.sub.2 O.sub.5 as an active 
site on the catalyst surface, and consequently, the activity of the 
catalyst is reduced. 
Thus, V.sub.2 O.sub.5 -TiO.sub.2 type supported catalysts for the 
production of phthalic anhydride by catalytic oxidation of o-xylene in the 
presence of steam under very high loading conditions would be industrially 
insignificant if the specific surface area of the anatase-type TiO.sub.2 
as a raw material is simply increased. 
The present inventors therefore made various investigations in order to 
improve the durability of the catalyst under high loading conditions. 
These investigations have led to the discovery that the use of 
anatase-type TiO.sub.2 being porous and having a particle diameter 
substantially of 0.4 to 0.7 micron and a specific surface area of 10 to 60 
m.sup.2 /g, preferably 15 to 40 m.sup.2 /g, as a TiO.sub.2 source leads to 
a marked improvement of the heat durability, especially steam resistance, 
of the catalyst, and therefore that the process (3) described hereinabove 
can be operated in the presence of the resulting catalyst. The catalyst in 
accordance with this invention operates even when steam is present only in 
an amount of about 0 to 5% by volume in the starting gas, and can be 
applied also to the processes (1) and (2) described hereinabove or to an 
ordinary one-pass process. 
The catalyst of this invention consists basically of a catalytically active 
material composed of V.sub.2 O.sub.5, anatase-type TiO.sub.2 (to be 
referred to simply as TiO.sub.2) being porous and having a particle 
diameter of 0.4 to 0.7 micron and a specific surface area of 10 to 60 
m.sup.2 /g, Nb.sub.2 O.sub.5, P.sub.2 O.sub.5, and at least one of K.sub.2 
O, Cs.sub.2 O, Rb.sub.2 O and Tl.sub.2 O, and a porous carrier composed 
mainly of SiC supporting said catalytically active material thereon. 
In the best mode of using the catalyst in an actual operation, the filling 
of the catalyst into a reaction tube is done in two layers (first-stage 
and second-stage), and the catalyst having a specified P.sub.2 O.sub.5 
content is filled in the starting gas inlet portion (first-stage), and the 
catalyst having a higher P.sub.2 O.sub.5 content than the catalyst used in 
the starting gas inlet portion is filled in the product gas outlet portion 
(second-stage). According to this embodiment, the formation of hot spots 
in the catalyst layer is inhibited, and therefore, the high loading 
ability of the catalyst is increased. 
The catalytically active material of the catalyst at the gas inlet portion 
(to be referred to as the first-stage catalyst) is composed of 1 to 20 
parts by weight of V.sub.2 O.sub.5, 99 to 80 parts by weight of TiO.sub.2, 
and per 100 parts by weight of the sum of these two components, 0.01 to 1 
part by weight of Nb.sub.2 O.sub.5, 0.2 to 0.4 part by weight of P.sub.2 
O.sub.5, and 0.05 to 1.2 parts by weight of at least one of K.sub.2 O, 
Cs.sub.2 O, Rb.sub.2 O and Tl.sub.2 O. 
The catalytically active material of the catalyst at the gas outlet portion 
(to be referred to as the second-stage catalyst) is composed of 1 to 20 
parts by weight of V.sub.2 O.sub.5, 99 to 80 parts by weight of TiO.sub.2, 
and per 100 parts by weight of the sum of these two components, 0.01 to 
1.0 part by weight of Nb.sub.2 O.sub.5, 0.4 to 1.2 parts by weight of 
P.sub.2 O.sub.5, and 0.05 to 1.2 parts by weight of at least one of 
K.sub.2 O, Cs.sub.2 O, Rb.sub.2 O and Tl.sub.2 O. 
Anatase-type TiO.sub.2 being porous and having a particle diameter of 0.4 
to 0.7 micron and a specific surface area of 10 to 60 m.sup.2 /g, 
preferably 15 to 40 m.sup.2 /g, is used as a TiO.sub.2 source. The use of 
anatase-type TiO.sub.2 having a particle diameter of less than 0.4 micron 
and a specific surface area of 15 to 40 m.sup.2 /g is undersirable for the 
reason stated hereinabove. Anatase-type TiO.sub.2 having the unique 
property of possessing a high specific surface area despite its large 
particle diameter is produced by mixing ilmenite with 70-80% conc. 
sulfuric acid, fllowing them to react fully with each other, diluting the 
reaction product with water to form an aqueous solution of titanium 
sulfate, adding iron fragments, reducing iron in the ilmenite, cooling the 
product to precipitate and separate ferric sulfate and to obtain an 
aqueous solution of highly pure titanium sulfate, blowing heated steam at 
150.degree. to 170.degree. C. into the aqueous solution to hydrolyze it 
and precipitate hydrous titanium oxide, and calcining the titanium oxide 
at a temperature of 600.degree. to 900.degree. C. The specific surface 
area of 10 to 60 m.sup.2 /g corresponds to the particle diameter range of 
0.05 to 0.20 micron of non-porous anatase-type TiO.sub.2 (primary 
particles). Accordingly, the TiO.sub.2 particles used in this invention 
are considered to be aggregated masses of such primary particles. The 
TiO.sub.2 particles, however, can not be crushed by a mechanical means 
such as a hammer mill, and as far as this is concerned, they have such a 
strength as can be regarded as primary particles. 
Depending upon the raw ilmenite ore, TiO.sub.2 may include iron, zinc, 
aluminum, manganese, chromium, calcium, lead, etc. These incidental 
elements are not detrimental to the reaction if their total amount is less 
than 0.5% by weight based on TiO.sub.2. 
Raw materials for V.sub.2 O.sub.5, Nb.sub.2 O.sub.5, P.sub.2 O.sub.5, 
K.sub.2 O, Cs.sub.2 O, Rb.sub.2 O and Tl.sub.2 O can be suitably selected 
from those which can change to oxides upon heating, such as the sulfates, 
ammonium salts, nitrates, organic acid salts, halides, and hydroxides of 
these metals. 
A porous carrier composed mainly of SiC is used in the catalyst of this 
invention. Specifically, the porous carrier has an alumina content of not 
more than 10% by weight, preferably not more than 5% by weight, a silicon 
carbide content of at least 80% by weight, preferably at least 98% by 
weight, and an apparent porosity (to be referred to simply as porosity 
hereinbelow) of at least 10%, preferably 15 to 45%. A typical example of 
the carrier is the one obtained by self-bonding of a powder of SiC having 
a purity of 98% to adjust its porosity to 15-40%. The shape of the carrier 
is not particularly limited so long as its size is 2 to 15 mm in diameter. 
Spherical or circular-cylindrical carriers are suitable for handling. 
The catalytically active material is supported on the carrier by a known 
conventional method. The simplest method comprises placing a fixed amount 
of the carrier in a rotary drum adapted to be externally heated, and 
spraying a liquid (e.g., slurry) containing the catalytically active 
material onto the carrier while maintaining the temperature at 200.degree. 
to 300.degree. C. The suitable amount of the catalytic material supported 
is 3 to 15 g/100 cc of carrier although varying depending upon the size of 
the carrier. 
The titanium oxide used in the catalyst of this invention is essentially 
aggregated masses of primary particles although its mechanical strength is 
so high that it can be substantially regarded as primary particles. The 
particle diameter of the primary particles can be measured by a mercury 
penetration-type porosimeter. Accordingly, in order for both the 
first-stage and second-stage catalysts of this invention to meet the 
requirement that "the total volume of pores having a diameter of 0.15 to 
0.45 micron present in the layer of the catalytically active material on 
the carrier is at least 50%, preferably at least 70%, of that of pores 
having a diameter of not more than 10 microns present in said layer of the 
catalytically active material", it is necessary to adjust the slurry 
concentration according to the particle diameter of the primary particles 
of titanium oxide, as described in the specification of Japanese Patent 
Publication No. 41036/74 (U.S. Pat. No. 3,962,846). When using porous 
titanium oxide consisting of primary particles having a particle diameter 
in the range of 0.005 to 0.05 micron, the concentration of the slurry is 5 
to 25% by weight, preferably 10 to 20% by weight. When using porous 
titanium oxide consisting of primary particles having a particle diameter 
of 0.05 to 0.4 micron, the slurry concentration is 10 to 40% by weight, 
preferably 15 to 25% by weight. 
The catalyst so obtained is then calcined at 300.degree. to 600.degree. C., 
preferably 350.degree. to 550.degree. C., for 2 to 10 hours in a current 
of air. 
The catalyst and process in accordance with this invention are most 
suitable for the catalytic oxidation of o-xylene or naphthalene to form 
phthalic anhydride, but can also be applied to the catalytic oxidation of 
durene, acenaphthene, benzene, etc. to obtain the corresponding carboxylic 
acid anhydrides. 
In use, the catalyst obtained in the above manner is packed into a tube 
having an inside diameter of 15 to 40 mm and a length of 1 to 5 meters. 
Preferably, the first-stage catalyst is packed so that it occupies 30 to 
70% of the total height of the catalyst layer from the gas inlet portion, 
and the second-stage catalyst is packed so that it occupies the remainder 
(70 to 30% from the gas outlet portion) of the total height of the 
catalyst layer. If desired, the catalyst may be packed in three or more 
layers. In this case, the P.sub.2 O.sub.5 content of the catalyst needs to 
be increased stepwise from the gas inlet portion to the gas outlet portion 
of the catalyst layer so that the aforesaid requirement of P.sub.2 O.sub.5 
content in the first-stage and second-stage catalysts is met. 
In the stacked catalyst, the other components than P.sub.2 O.sub.5 and 
their constituent porportions need not always to be the same for the 
individual layers, and can be varied as desired within the above-specified 
ranges. 
The catalytic vapor-phase oxidation of o-xylene or naphthalene to form 
phthalic anhydride using the catalyst of this invention is usually carried 
out by mixing o-xylene or naphthalene with a molecular oxygen-containing 
gas composed of 5 to 21% by volume of oxygen, 0 to 15% by volume of steam, 
0 to 3% by volume of carbon dioxide gas, 0 to 3% by volume of carbon 
monoxide and the balance being nitrogen, the concentration (to be 
abbreviated GC which stands for gas concentration) of the o-xylene or 
naphthalene being adjusted to 5 to 100 g per Nm.sup.3 of molecular 
oxygen-containing gas, and passing the gaseous mixture over the catalyst 
layer at a temperature (the temperature of the heat transfer medium; to be 
abbreviated N.T.) of 300.degree. to 400.degree. C., preferably 330.degree. 
to 380.degree. C. and a pressure of normal atmospheric pressure to 10 
atmospheres at a space velocity (to be abbreviated S.V.) of 1,000 to 6,000 
hr.sup.-1 (NTP). 
Since the catalyst of this invention can catalyze the oxidation reaction of 
o-xylene or naphthalene under the aforesaid conditions, the present 
invention has made it possible to commercially operate a process for 
production of phthalic anhydride involving recycling exhaust gases which 
although being most economical, cannot be put into practice in the 
presence of conventional catalysts. In this process, the first-stage 
catalyst and the second-stage catalyst are stacked and filled into a 
multi-tube heat-exchanger converter, and heated to a predetermined 
temperature. First, o-xylene is passed through the catalyst layer at a 
G.C. of less than 40 g/Nm.sup.3 of molecular oxygen-containing gas. At 
this time, the temperature of the starting gas is maintained at 
100.degree. to 120.degree. C. The gas which has left the converter is 
passed through a multi-tube heat exchanger and is cooled to 160.degree. C. 
The cooled gas is then conducted to a condenser adapted for recovery of 
phthalic anhydride which is filled with fin tubes, where phthalic 
anhydride is condensed. The temperature of the gas at the outlet of the 
condenser is maintained at more than the dew point of water according to 
the concentration of o-xylene. The gas which has left the condenser, 
without removing water from it, is partly recycled to the starting gas. It 
is mixed with air and again conducted to the converter together with 
o-xylene. Then, the amount of o-xylene fed is gradually increased, and 
more economically, the G.C. is increased to 80 to 90 g/Nm.sup.3 of 
molecular oxygen-containing gas. At this time, the amount of the exhaust 
gas recycled is controlled to adjust the concentration of oxygen in the 
gas at the inlet portion of the converter to not more than 12% by volume. 
When the concentration of o-xylene is maintained at such a value, the gas 
at the inlet portion of the converter consists of 9 to 12% by volume of 
oxygen, 0.3 to 1.0% by volume of carbon monoxide, 1 to 4% by volume of 
carbon dioxide, 8 to 11% by volume of steam, 65 to 75% by volume of 
nitrogen and 1.7 to 1.9% by volume of o-xylene. At the exit of the 
condenser, the concentration of steam amounts to 15 to 18% by volume owing 
to the water generated at the converter. Accordingly, the temperature of 
the gas at the exit of the condenser should be maintained at a point above 
the dew point of water. 
Intermediates such as phthalide or tolualdehyde to be recycled to the 
starting gas may be advantageous to the increase of the yield of phthalic 
anhydride, and are never disadvantageous to the catalyst. Since benzoic 
acid, an over-oxidized product to be recycled, is a monocarboxylic acid, 
it is very readily decomposed in the catalyst layer, and conveniently, it 
never builds up in the condenser for phthalic anhydride. 
That part of the exhaust gas which is not recycled is sent to a catalytic 
combustion system, and after complete burning there, is released into the 
atmosphere. 
Needless to say, the catalyst in accordance with this invention is also 
applicable to other processes for producing phthalic anhydride, for 
example an ordinary oxidation process in which the exhaust gas is not 
recycled; an oxidation process in which all the exhaust gas is introduced 
into a catalytic combustion system, and after water removal, a part of the 
exhaust gas is recycled to the starting gas; and an oxidation process in 
which all the exhaust gas is sent to a washing tower for the recovery of 
maleic anhydride, and a part of the exhaust gas from the washing tower is 
recycled to the starting gas.

The following examples illustrate the process of this invention in greater 
detail. 
EXAMPLE 1 
Heated steam at 175.degree. C. was blown into an aqueous solution 
containing titanyl sulfate and sulfuric acid to form a precipitate of 
titanium hydroxide (TiO.sub.2.nH.sub.2 O). The titanium hydroxide was 
washed with water and an acid, and further with water, and calcined at 
800.degree. C. for 4 hours. The calcined product was pulverized by a jet 
stream of air to obtain porous anatase-type TiO.sub.2 having an average 
particle diameter of 0.5 micron and a BET specific surface area of 22 
m.sup.2 /g. 
To a solution of 1.8 kg of oxalic acid in 70 liters of deionized water were 
added 0.86 kg of ammonium meta-vanadate, 0.136 kg of niobium chloride, 
0.067 kg of ammonium dihydrogen phosphate, 0.01 kg of potassium hydroxide 
and 0.0556 kg of cesium sulfate, and they were fully stirred. To the 
resulting aqueous solution was added 16 kg of TiO.sub.2 produced as above, 
and they were fully emulsified for 40 minutes by an emulsifying machine to 
form a catalyst slurry. 
One hundred and fifty (150) liters of self-bonded SiC having a porosity of 
37% and a particle diameter of 5 mm as a carrier was placed in a stainless 
steel rotary oven adapted to be externally heated and having a diameter of 
2 meters and a length of 3 meters, and pre-heated to 200.degree. to 
250.degree. C. While rotating the rotary oven, the slurry prepared as 
above was sprayed onto the carrier until the catalytically active material 
was deposited at a rate of 8 g/100 cc of carrier. The resulting catalyst 
was then calcined at 550.degree. C. for 6 hours while passing air. 
The catalytically active material had the following composition by weight. 
EQU V.sub.2 O.sub.5 :TiO.sub.2 :Nb.sub.2 O.sub.5 :P.sub.2 O.sub.5 :K.sub.2 
O:Cs.sub.2 O=4:96:0.4:0.25:0.05:0.26 
The pore size distribution of the catalyst prepared as above was measured 
by a mercury penetration-type porosimeter. It was found that the total 
volume of pores having a size of 0.15 to 0.45 micron was 88% of the total 
volume of pores having a size of not more than 10 microns (this is 
abbreviated as "the volume of pores having a size of 0.15 to 0.45 micron 
was 88%"). The resulting catalyst was designated as a first-stage 
catalyst. 
In the above preparation of the catalyst slurry, the amount of ammonium 
dihydrogen phosphate was changed to 0.134 kg, and otherwise, the same 
procedure was repeated. There was obtained a catalyst in which the 
catalytically active material had the following composition by weight. 
EQU V.sub.2 O.sub.5 :TiO.sub.2 :Nb.sub.2 O.sub.5 :P.sub.2 O.sub.5 :K.sub.2 
O:Cs.sub.2 O=4:96:0.4:0.5:0.05:0.26 
The volume of pores having a size of 0.15 to 0.45 micron was 86%. The 
resulting catalyst was designated as a second-stage catalyst. 
First, the second-stage catalyst was packed to a height of 1.25 meters into 
a multi-tube heat exchanger converter consisting of 250 iron tubes having 
an inside diameter of 20 mm and a height of 3 meters whose inside surface 
was rust-proofed and treated with phosphoric acid. Then, the first-stage 
catalyst was packed into it to a height of 1.25 meters so that it was 
placed on top of the second-stage catalyst of a height of 1.25 meters. A 
molten salt as a heat transfer medium was circulated through the converter 
to maintain the temperature at 370.degree. C. 
A gaseous mixture of o-xylene and air preheated to 120.degree. C. was 
introduced into the converter from its upper portion at a space velocity 
of 2,500 hr.sup.-1 (NTP), and the concentration of the o-xylene was 
maintained at 40 g/Nm.sup.3 of air. Then, an exhaust gas circulating 
blower was operated, and when the concentration of oxygen in the starting 
gas at the inlet of the converter reached 10%, the amount of the o-xylene 
fed was gradually increased, and finally to 83 g/Nm.sup.3 of molecular 
oxygen-containing gas. At this time, the amount of the exhaust gas 
recycled was controlled with an increase in the amount of o-xylene fed so 
as to maintain the concentration of oxygen in the gas at the inlet of the 
converter at 10% by volume. 
The gas which left the converter was cooled to 160.degree. C. in a heat 
exchanger, and introduced into a switch condenser to crystallize phthalic 
anhydride. The exhaust gas left the condenser while maintaining the 
temperature of the outlet of the condenser at 77.degree. C., and further 
passed through a conduit kept at 120.degree. to 130.degree. C. to mix 58% 
of it with air. The mixture was then introduced into the converter. The 
remainder of the exhaust gas (42%) was conducted to a catalytic combustion 
system, and after complete combustion, was released into the atmosphere. 
Under these conditions, the concentration of steam in the gas at the inlet 
of the converter reached about 9%. In a long-term operation over about a 
year, the reaction results shown in Table 1 were obtained. 
TABLE 1 
______________________________________ 
Yield of 
phthalic 
Reaction 
N.T. S.V. G.C. anhydride 
.DELTA.T.sub.1 (*) 
.DELTA.T.sub.2 (**) 
time (.degree.C.) 
(hr.sup.-1) 
(g/Nm.sup.3) 
(wt. %) (.degree.C.) 
(.degree.C.) 
______________________________________ 
Initial 
370 2500 83 113.6 68 25 
stage 
months 370 2500 83 113.8 65 27 
6 
months 372 2500 83 113.1 67 24 
12 
months 375 2500 83 112.7 64 29 
______________________________________ 
(*) .DELTA.T.sub.1 = .DELTA.T with the firststage catalyst (the same 
definition applies to the following tables) 
(**) .DELTA.T.sub.2 = .DELTA.T with the secondstage catalyst (the same 
definition applies to the following tables) 
EXAMPLE 2 
The titanium hydroxide obtained in Example 1 was calcined at 750.degree. C. 
for 4 hours, and treated in the same way as in Example 1 to form porous 
anatase-type TiO.sub.2 having an average particle diameter of 0.45 micron 
and a BET specific surface area of 28 m.sup.2 /g. By operating similarly 
to Example I, catalysts having the following compositions were prepared by 
using the resulting anatase-type TiO.sub.2 and a molded carrier composed 
of 2% by weight of alumina, 92% by weight of silicon carbide and the 
remainder being silica and having a porosity of 42% and a diameter of 5 
mm. 
First-stage catalyst 
EQU V.sub.2 O.sub.5 :TiO.sub.2 :Nb.sub.2 O.sub.5 :P.sub.2 O.sub.5 :Rb.sub.2 
O=15:85:0.5:0.35:0.40 (by weight) 
Second-stage catalyst 
EQU V.sub.2 O.sub.5 :TiO.sub.2 :Nb.sub.2 O.sub.5 :P.sub.2 O.sub.5 :Tl.sub.2 
O=8:92:0.5:1.0:0.8 (by weight) 
The volume of pores having a size of 0.15 to 0.45 micron was 83% in the 
first-stage catalyst, and 86% in the second-stage catalyst. 
In a stainless steel tube having an inside diameter of 20 mm and a height 
of 3 meters, the first-stage catalyst was packed to a height of 0.8 meter, 
and the second-stage catalyst, to a height of 1.7 meters. A synthetic gas 
composed of 10% by volume of oxygen, 12% by volume of steam and 78% by 
volume of nitrogen was mixed with 80 g/Nm.sup.3 of synthetic gas of 
o-xylene, and the gaseous mixture was passed throught the catalyst layers. 
The results obtained are shown in Table 2. 
TABLE 2 
______________________________________ 
Yield of 
phthalic 
Reaction 
N.T. S.V. G.C. anhydride 
.DELTA.T.sub.1 
.DELTA.T.sub.2 
time (.degree.C.) 
(hr.sup.-1) 
(g/Nm.sup.3) 
(wt. %) (.degree.C.) 
(.degree.C.) 
______________________________________ 
Initial 
373 3000 80 112.8 71 21 
stage 
months 375 3000 80 112.5 68 24 
6 
months 378 3000 80 112.4 70 20 
______________________________________ 
EXAMPLE 3 
The titanium hydroxide obtained in Example 1 was calcined at 850.degree. C. 
for 6 hours, and treated by the same procedure as in Example 1 to afford 
porous anatase-type TiO.sub.2 having a BET specific surface area of 17 
m.sup.2 /g. Catalysts having the following compositions were prepared 
similarly to Example 1 by using the TiO.sub.2 and a spherical powder of 
self-bonded SiC having a porosity of 35% as a carrier. 
First-stage catalyst 
EQU V.sub.2 O.sub.5 :TiO.sub.2 :Nb.sub.2 O.sub.5 :P.sub.2 O.sub.5 
:Cs.sub.2)=2:98:0.4:0.2:0.3 (by weight) 
Second-stage catalyst 
EQU V.sub.2 O.sub.5 :TiO.sub.2 :Nb.sub.2 O.sub.5 :P.sub.2 O.sub.5 :Cs.sub.2 
O=2:98:0.4:0.6:0.3 (by weight) 
The volume of pores having a size of 0.15 to 0.45 microns was 80% in the 
first-stage catalyst, and 84% in the second-stage catalyst. 
Into a stainless steel tube having an inside diameter of 20 mm and a height 
of 5 meters, the first-stage catalyst was packed to a height of 1.8 
meters, and the second-stage catalyst, to a height of 1.2 meters. A 
synthetic gas composed of 11% by volume of oxygen, 10% by volume of steam 
and 79% by volume of nitrogen was mixed with 85 g/Nm.sup.3 of synthetic 
gas of o-xylene. The gaseous mixture was passed through the catalyst layer 
to react it. The results obtained are shown in Table 3. 
TABLE 3 
______________________________________ 
Yield of 
phthalic 
Reaction 
N.T. S.V. G.C. anhydride 
.DELTA.T.sub.1 
.DELTA.T.sub.2 
time (.degree.C.) 
(hr.sup.-1) 
(g/Nm.sup.3) 
(wt. %) (.degree.C.) 
(.degree.C.) 
______________________________________ 
Initial 
370 2700 85 113.3 78 32 
stage 
months 370 2700 85 113.4 75 36 
6 
months 372 2700 85 113.4 76 33 
______________________________________ 
EXAMPLES 4 AND 5 
Into a stainless steel tube having an inside diameter of 27 mm and a height 
of 3 meters, the first-stage catalyst obtained in Example 1 was packed to 
a height of 1.5 meters and the second-stage catalyst obtained in Example 
1, to a height of 1.5 meters. Using air as an oxidizer, o-xylene was 
oxidized under the conditions shown in Table 4 using the catalyst layers 
obtained. The results are shown in Table 4. 
TABLE 4 
__________________________________________________________________________ 
Yield of 
phthalic 
Reaction N.T. 
S.V. 
G.C. anhydride 
.DELTA.T.sub.1 
.DELTA.T.sub.2 
time (.degree.C.) 
(hr.sup.-1) 
(g/Nm.sup.3) 
(wt. %) 
(.degree.C.) 
(.degree.C.) 
__________________________________________________________________________ 
Initial 
360 
3000 
40 116.8 45 18 
stage 
Example 
3 months 
360 
3000 
40 116.4 44 18 
4 6 months 
360 
3000 
40 116.5 44 19 
Initial 
365 
2700 
60 114.1 68 28 
stage 
Example 
3 months 
365 
2700 
60 113.8 66 28 
5 6 months 
365 
2700 
60 113.6 65 30 
__________________________________________________________________________ 
COMATIVE EXAMPLE 1 
Ammonium titanium sulfate [(NH.sub.4).sub.2 SO.sub.4.TiOSO.sub.4.H.sub.2 O] 
was heat-treated at 900.degree. C. for about 3 hours in accordance with 
the disclosure of Example 1 of the specification of Japanese Patent 
Publication No. 4538/77, and pulverized by a jet stream of air to afford 
finely divided anatase-type TiO.sub.2 having a primary particle diameter 
of 0.25 micron and a specific surface area of 15 m.sup.2 /g. Using a 
self-bonded SiC carrier having a particle diameter of 6 mm and a porosity 
of 35%, catalysts having the following compositions were prepared in the 
same way as in Example 1. 
EQU V.sub.2 O.sub.5 :TiO.sub.2 :Nb.sub.2 O.sub.5 :P.sub.2 O.sub.5 :K.sub.2 
O:Na.sub.2 O=2:98:0.25:1.02:0.15:0.1 (by weight) (A) 
EQU V.sub.2 O.sub.5 :TiO.sub.2 :Nb.sub.2 O.sub.5 :P.sub.2 O.sub.5 :K.sub.2 
O:Na.sub.2 O=2:98:0.25:1.3:0.15:0.1 (by weight) (B) 
Into a stainless steel tube having an inside diameter of 20 mm and a height 
of 3 meters, the catalyst (A) was packed to a height of 1.25 m at the gas 
inlet portion, and the catalyst (B), to a height of 1.25 m at the gas 
outlet portion. Two such converters were provided. Oxidation of o-xylene 
was performed for a long period of time under the same loading conditions 
except that in one converter the content of steam in the inlet gas was 
adjusted to zero, and in the other converter, the content of steam in the 
inlet gas was adjusted to 10% by volume. The results are shown in Table 5. 
TABLE 5 
______________________________________ 
Yield of 
phthalic 
Re- anhy- 
Composition of 
action N.T. S.V. G.C. dride 
the inlet gas 
time (.degree.C.) 
(hr.sup.-1) 
(g/Nm.sup.3) 
(wt. %) 
______________________________________ 
O.sub.2 
10% Initial 375 2500 85 114.8 
H.sub.2 O 
0 stage 
3 
o-Xylene 
85g/Nm.sup.3 
months 376 2500 85 114.4 
6 
N.sub.2 
balance months 378 2500 85 114.1 
O.sub.2 
10% Initial 375 2500 85 114.9 
H.sub.2 O 
10% stage 
3 
o-Xylene 
85g/Nm.sup.3 
months 381 2500 85 109.1 
6 
N.sub.2 
balance months 387 2500 85 107.2 
______________________________________ 
COMATIVE EXAMPLES 2 AND 3 
Catalysts were prepared in the same way as in Examples 1 and 4 of the 
specification of Japanese Patent Publication No. 4538/77 except that a 
self-bonded SiC powder having a particle diameter of 5 mm and a porosity 
of 35% was used. Into a stainless steel tube having an inside diameter of 
20 mm and a height of 3 meters, the first stage catalyst and the 
second-stage catalyst were packed to a height of 1.25 meters respectively. 
A gas composed of 10% by volume of oxygen, 10% by volume of steam and 80% 
by volume of nitrogen was mixed with 83 g/Nm.sup.3 of o-xylene, and the 
gaseous mixture was passed through the catalyst layer to react o-xylene. 
The results obtained are shown in Table 6. 
TABLE 6 
______________________________________ 
Yield of 
phthalic 
G.C. anhy- 
Reaction N.T. S.V. (g/ dride .DELTA.T.sub.1 
.DELTA.T.sub.2 
time (.degree.C.) 
(hr.sup.-1) 
Nm.sup.3) 
(wt. %) 
(.degree.C.) 
(.degree.C.) 
______________________________________ 
Com- Initial 375 2500 83 113.7 67 21 
para- stage 
tive 3 
Ex- months 381 2500 83 110.3 53 32 
ample 
2 (*) 6 
months 386 2500 83 108.1 38 43 
Com- Initial 385 2500 83 113.3 64 19 
para- stage 
tive 3 
Ex- months 393 2500 83 109.3 47 34 
ample 6 
3(**) months 401 2500 83 107.3 32 51 
______________________________________ 
(*) The catalysts in accordance with Example 1 of Japanese Patent 
Publication No. 4538/77. 
(**) The catalysts in accordance with Example 4 of Japanese Patent 
Publication No. 4538/77. 
EXAMPLE 6 
The following two catalysts were prepared in accordance with Example 1 
using the TiO.sub.2 obtained in Example 1. The carrier used was 
self-bonded SiC having a particle diameter of 5 mm and a porosity of 35%. 
______________________________________ 
Volume (%) of pores 
having a size of 
0.15-0.45 micron 
______________________________________ 
A: V.sub.2 O.sub.5 :TiO.sub.2 :Nb.sub.2 O.sub.5 :Rb.sub.2 O:P.sub.2 
O.sub.5 85 
4 : 98 : 0.6 : 0.35 : 0.3 
B: V.sub.2 O.sub.5 :TiO.sub.2 :Nb.sub.2 O.sub.5 :Rb.sub.2 O:P.sub.2 
O.sub.5 88 
2 : 98 : 0.6 : 0.35 : 1.1 
______________________________________ 
In a multi-tube heat exchanger converter consisting of twenty stainless 
steel tubes having an inside diameter of 20 mm and a height of 3.5 meters, 
the catalyst B was first packed to a height of 1.5 meters, and on top of 
it, the catalyst A was stacked to a height of 1.5 meters. A molten salt as 
a heat transfer medium was circulated through the converter to maintain 
the temperature at 365.degree. C. 
A gaseous mixture preheated to 120.degree. C. of o-xylene and air was 
introduced into the converter from its upper portion at a space velocity 
of 2,000 hr.sup.-1 (NTP). First, the concentration of o-xylene was 
maintained at 40 g/Nm.sup.3 of air. Then, an exhaust gas circulating 
blower was operated, and when the concentration of oxygen in the starting 
gas reached 11% by volume, the concentration of o-xylene was increased 
gradually, and finally to 100 g/Nm.sup.3 of molecular oxygen-containing 
gas. The amount of the exhaust gas recycled was controlled with an 
increase of in the amount of o-xylene fed so that the concentration of 
oxygen in the starting gas was maintained at 11% by volume. 
The gas which left the converter was cooled to 160.degree. C. in a heat 
exchanger, and introduced into a switch condenser to crystallize phthalic 
anhydride. At this time, about 33% of the total phthalic anhydride formed 
was recovered in the liquid state. The exhaust gas was withdrawn from the 
condenser while maintaining the temperature of the outlet of the condenser 
at 78.degree. C., and was conducted to a tower for recovering maleic 
anhydride. About 35% of the exhaust gas containing steam saturated at a 
tower top temperature of 35.degree. C. was recycled to the starting gas. 
The remainder was released into the atmosphere through the 
complete-combustion system. Under these conditions, the concentration of 
steam in the starting gas was about 3%. In a long-term operation over 
about 6 months, the reaction results shown in Table 7 were obtained. 
TABLE 7 
______________________________________ 
Yield of 
phthalic 
Reaction 
N.T. S.V. G.C. anhydride 
.DELTA.T.sub.1 
.DELTA.T.sub.2 
time (.degree.C.) 
(hr.sup.-1) 
(g/Nm.sup.3) 
(wt. %) (.degree.C.) 
(.degree.C.) 
______________________________________ 
Initial 
365 2000 100 111.9 65 35 
stage 
months 367 2000 100 111.8 63 37 
6 
months 370 2000 100 111.5 60 40 
______________________________________ 
EXAMPLE 7 
The same hydrous titanium oxide as obtained in Example 1 was calcined at 
700.degree. C. for 5 hours in a current of air, and pulverized by a jet of 
air stream to obtain porous TiO.sub.2 having a particle diameter of 0.45 
micron and a specific surface area of 33 m.sup.2 /g. 
The following two catalysts were prepared in accordance with Example 1 
using a self-bonded SiC carrier having a particle diameter of 5 mm and a 
porosity of 35%. 
______________________________________ 
Volume of pores 
having a size 
of 0.15-0.45 
microns 
______________________________________ 
A: V.sub.2 O.sub.5 :TiO.sub.2 :Nb.sub.2 O.sub.5 :Rb.sub.2 O:P.sub.2 
O.sub.5 80 
15 : 85 : 1.0 : 0.28 : 0.35 
B: V.sub.2 O.sub.5 :TiO.sub.2 :Nb.sub.2 O.sub.5 :Rb.sub.2 O:P.sub.2 
O.sub.5 82 
15 : 85 : 1.0 : 0.28 : 1.0 
______________________________________ 
Into the same converter as in Example 6, the catalyst B was first packed to 
a height of 1 meter, and then the catalyst A was stacked to a height of 
1.5 meters. The temperature of the heat transfer medium was maintained at 
360.degree. C. A gaseous mixture of naphthalene and air preheated to 
140.degree. C. was introduced into the converter from its upper portion at 
a space velocity of 3,000 hr.sup.-1. The concentration of naphthalene was 
first maintained at 40 g/Nm.sup.3 of air, and then by the same operation 
as in Example 6, the concentration of a naphthalene was finally increased 
to 60 g/Nm.sup.3 of molecular oxygen-containing gas. 
The tower top temperature of a tower for recovery of maleic anhydride and 
quinone was maintained at 35.degree. C. The ratio of the exhaust gas which 
was recycled was 66% (the concentration of oxygen in the gas at the inlet 
portion of the converter was 11% by volume), and the concentration of 
steam in the gas at the inlet of the reactor was about 4%. 
The results of the reaction are shown in Table 8. 
TABLE 8 
______________________________________ 
Yield of 
phthalic 
Reaction 
N.T. S.V. G.C. anhydride 
.DELTA.T.sub.1 
.DELTA.T.sub.2 
time (.degree.C.) 
(hr.sup.-1) 
(g/Nm.sup.3) 
(wt. %) (.degree.C.) 
(.degree.C.) 
______________________________________ 
Initial 
360 3000 60 103.2 60 32 
stage 
months 364 3000 60 102.9 56 35 
4 
months 366 3000 60 102.7 54 36 
6 
months 368 3000 60 102.6 53 36 
______________________________________