Process for the manufacture of maleic anhydride

Process for the manufacture of maleic anhydride which comprises contacting in the vapor phase a mixture of butane and molecular oxygen with a catalyst comprising in chemical combination phosphorus, vanadium, oxygen and a catalyst modifier comprising titanium combined with one or more elements selected from the group consisting of aluminum, cerium, cobalt and iron.

BACKGROUND OF THE INVENTION 
A. Field of the Invention 
This invention relates to a process for the manufacture of dicarboxylic 
acid anhydrides by the oxidation of hydrocarbons. More particularly, it is 
directed to a process suitable for producing maleic anhydride from 
saturated hydrocarbons in higher yields than heretofore possible. 
B. The Prior Art 
Maleic anhydride is of significant commercial interest throughout the 
world. It is used alone or in combination with other acids in the 
manufacture of alkyd and polyester resins. It is also a versatile 
intermediate for chemical synthesis. Significant quantities of maleic 
anhydride are produced each year to satisfy these needs. The prior art 
discloses a number of processes used in the conversion of organic 
feedstocks to maleic anhydride. 
Of particular interest is U.S. Pat. No. 3,293,268 which teaches a process 
of oxidizing saturated aliphatic hydrocarbons to maleic anhydride under 
controlled temperature conditions and in the presence of 
phosphorus-vanadium-oxygen catalysts. Through various improvements 
including the use of promoters such as cobalt, nickel, cadmium and 
titanium yields have been substantially increased, as exemplified in U.S. 
Pat. No. 4,111,963, U.S. Pat. No. 3,987,063, and U.S. Pat. No. 3,907,833. 
Any economically feasible method of substantially reducing the operating 
temperature or increasing the level of selectivity in the manufacture of 
maleic anhydride including alternative methods to those already known 
could be a substantial advance in the art and is an object of this 
invention. 
SUMMARY OF THE INVENTION 
These and other objects are achieved in a process for preparing maleic 
anhydride wherein a mixture of an oxygen-containing gas and a saturated 
hydrocarbon having 4 to 10 carbon atoms is contacted with a catalyst 
complex comprising phosphorus, vanadium and oxygen, the improvement 
wherein the complex also includes a catalyst modifier comprising titanium 
combined with one or more elements selected from the group consisting of 
aluminum, cerium, cobalt and iron. 
In the ordinary practice of this invention, the mixture of 
oxygen-containing gas and the hydrocarbon are reacted at about 
350.degree.-360.degree. C. The phosphorus-to-vanadium atom ratio of the 
catalyst is about 1:2 to about 2:1. At least 50 atom % of the vanadium is 
in the tetravalent state. An effective amount of the modifier can be any 
amount which reduces the required operating temperature or increases the 
selectivity in the manufacture of maleic anhydride. A typical 
titanium/vanadium atom ratio, as will be seen in the examples, is 
0.05-0.10. 
For the purposes of this invention, the term "catalytic activity" means the 
ability to convert a particular feedstock, such as butane, at a particular 
temperature to other compounds. The term "selectivity" means the ratio of 
the moles of maleic anhydride obtained to the moles of hydrocarbon 
reacted. The term "yield" means the ratio of the moles of maleic anhydride 
obtained to the moles of hydrocarbon introduced into the reaction. The 
term "space velocity" means the hourly volume of the gaseous feed 
expressed in cubic centimeters (cc) at 60.degree. F. and standard 
atmospheric pressure divided by the catalyst bulk volume expressed in 
cubic centimeters (cc), the term expressed as cc/cc/hour. 
Broadly described, the catalysts used in the process of this invention to 
convert saturated hydrocarbons to maleic anhydride are prepared by 
contacting vanadium compounds, phosphorus compounds, and the modifier 
under such conditions that a substantial amount of tetravalent vanadium is 
provided to form catalyst precursors, recovering the catalyst precursors, 
forming the catalyst precursors into structures for use in a maleic 
anhydride reactor, and calcining the structured catalyst precursors to 
form the catalysts. 
The vanadium compounds useful as a source of vanadium in the catalyst 
precursors are those known to the art to be useful for preparing catalysts 
to oxidize hydrocarbons. Suitable vanadium compounds include: vanadium 
oxides, such as vanadium pentoxide, vanadium trioxide, and the like; 
vanadium oxyhalides, such as vanadyl chloride, vanadyl trichloride, 
vanadyl dichloride, vanadyl bromide, vanadyl dibromide, vanadyl 
tribromide, and the like; vanadium salts, such as ammonium metavanadate, 
vanadium sulfate, vanadium phosphate, vanadyl formate, vanadyl oxalate and 
the like. However, vanadium pentoxide is preferred. 
As a source of phosphorus in the catalyst precursors, useful phosphorus 
compounds are also those well known in the art useful for preparing 
catalysts to oxidize hydrocarbons. Suitable phosphorus compounds include: 
phosphorous acid, phosphoric acids, such as metaphosphoric acid, 
othophosphoric acid, triphosphoric acid, pyrophosphoric acid, and the 
like; phosphorus oxides, such as phosphorus pentoxide and the like; 
phosphorus halides, such as phosphorus oxyiodide, phosphorus 
pentachloride, phosphorus oxybromide and the like; and organophosphorus 
compounds such as ethyl phosphate, methyl phosphate and the like. However, 
phosphorous acid is preferred. 
The catalyst modifier includes titanium as well as one or more elements 
selected from the group consisting of aluminum, cerium, cobalt and iron. 
The compound or compounds must be noninterfering compounds in the sense 
that either in the ionized or neutral form it (they) does (do) not 
substantially interfere with the production of the catalyst precursor or 
catalysts or with the production of maleic anhydride. Preferably, the 
compound is a titanate of aluminum, cerium, cobalt or iron. The catalyst 
modifier is introduced into the catalyst at any stage of catalyst 
preparation prior to calcining the catalyst, but preferably it is 
introduced into the catalyst at the initial stages of precursor formation 
so as to provide an atomic ratio of titanium to vanadium of 0.05-0.01, 
preferably 0.07-0.09. 
To prepare precursors to the catalysts used in the present process, a 
pentavalent or tetravalent vanadium compound is heated with a phosphorus 
compound in an acid solution along with the catalyst modifier to dissolve 
the starting materials. A mild reducing agent is used to provide 
tetravalent vanadium and/or to maintain vanadium in the tetravalent state. 
On the other hand, an acid with reducing properties, such as hydrogen 
halide acid or oxalic acid, can serve as the acid and can provide 
tetravalent vanadium. Phosphorous acid is preferred. The acid solution 
containing the phosphorus compound and the vanadium compound are heated 
until a blue solution is obtained, indicating that a substantial amount, 
i.e., greater than 50 atom percent of the vanadium is in the tetravalent 
state. The amount of time required to dissolve the phosphorus and vanadium 
compounds and to provide a substantial amount of the vanadium in the 
tetravalent state to form the catalyst precursors varies from batch to 
batch, depending upon the compounds used as starting materials and the 
temperature at which the compounds are heated. However, as will occur to 
those skilled in the art, the solution can be analyzed to insure that most 
of the vanadium is in the tetravalent state. 
Although any number of phosphorus and vanadium compounds can be used to 
form the precursor, the atom ratio of phosphorus to vanadium in the 
precursor is important, since it controls the phosphorus-to-vanadium atom 
ratio in the final catalyst. When the precursor contains a phosphorus to 
vanadium atom ratio below about 1:2 or above about 2:1, the yield of 
maleic anhydride using the process of this invention is so low that it is 
not of commercial significance. It is preferred that the precursors have a 
phosphorus to vanadium atom ratio in the range of about 1:1 to about 
1.5:1. When the catalyst is used to convert a feed that is primarily 
butane to maleic anhydride, it is even more preferable to have a 
phosphorus to vanadium atom ratio of about 1:1 to about 1.2:1, typically 
about 1.1:1. 
After the precursors have been formed by heating the vanadium compounds, 
the phosphorus compounds and the modifier, and a substantial amount of 
vanadium has been reduced to the tetravalent state, it is necessary to 
remove most of the water in order to recover the precursor. Techniques for 
recovering the precursors from solution are well known to those skilled in 
the art. Precursors can be deposited on a carrier, such as alumina or 
titania, from solution, or excess water can be removed to provide the 
precursors. 
After the precursors are recovered from solution, they are then formed into 
structures suitable for use in a maleic anhydride reactor. Techniques for 
forming appropriate structures from the precursors for use in a fluidized 
bed reactor or in a fixed tube, heat exchanger type reactor are well known 
to those skilled in the art. For example, the precursors can be structured 
for use in a fluidized bed reactor by depositing the precursors from 
solution on a carrier, or alternatively, the dried precursors can be 
comminuted for use in a fluidized bed reactor. On the other hand, the 
precursors can be structured for use in a fixed tube reactor by prilling 
or tableting the precursors. 
After the precursors have been formed into the structures in which they 
will be used in the maleic anhydride reactor, they can be calcined in an 
oxygen-containing atmosphere, such as air, at temperatures of from about 
350.degree. C. to about 600.degree. C. for at least about 2 hours to 
convert the precursors to the catalysts for use in the present process. 
If more than about 90 atom percent of the vanadium is oxidized to 
pentavalent vanadium, usually caused by calcining in air at too high a 
temperature, the selectivity of the catalyst and the yield of maleic 
anhydride decrease markedly. On the other hand, oxidation of less than 
about 20 atom percent of the vanadium during air calcination does not seem 
to be more beneficial than calcination in an inert atmosphere. 
After the precursors have been calcined to form the catalysts of this 
process, the catalysts can be used to convert a saturated hydrocarbon to 
maleic anhydride. However, the initial yield of maleic anhydride may be 
low; and if this is the case, the catalysts can be conditioned, as will 
occur to those skilled in the art by passing low concentrations of a 
saturated hydrocarbon in air through the catalyst for a period of time 
before production operations begin. It has been discovered, however, that 
a unique feature of this invention is that with the preferred bimetallic 
or multimetallic content of the catalyst the initial yield of maleic 
anhydride will not be lower, so that no conditioning is in fact required. 
The catalysts of the present process are useful in a variety of reactors to 
convert saturated hydrocarbons to maleic anhydride. Both fluidized bed 
reactors and fixed tube heat exchanger type reactors are satisfactory and 
details of the operation of such reactors are well known to those skilled 
in the art. The reaction to convert saturated hydrocarbons to maleic 
anhydride requires only contacting the saturated hydrocarbon admixed with 
a free-oxygen containing gas, such as air or oxygen-enriched air, with the 
catalysts at elevated temperatures. The saturated hydrocarbons are 
contacted with the catalysts at a concentration of about 1.5 to about 10 
volume percent saturated hydrocarbons at a space velocity of about 100 to 
4,000 cc/cc/hour to provide maleic anhydride yields of greater than 40% at 
temperatures between about 350.degree. C. and 600.degree. C. 
The catalysts of the present process are particularly useful in fixed tube 
heat exchanger type reactors. The tubes of such reactors can vary in 
diameter from about 1/4 inch to about 1.5 inches and the length can vary 
from about 6 inches to about 10 or more feet. It is desirable to have the 
surfaces of the reactors at relatively constant temperature, and some 
medium to conduct heat from the reactors is necessary to aid temperature 
control. Such media can be Woods metal, molten sulfur, mercury, molten 
lead and the like or eutectic salt baths. A metal block reactor whereby 
the metals surrounding the tube act as a temperature regulating body can 
also be used. The reactor or reaction can be iron, stainless steel, carbon 
steel, glass and the like. 
Maleic anhydride prepared by using the process of this invention can be 
recovered by any number of means well known to those skilled in the art. 
For example, the maleic anhydride can be recovered by direct condensation 
or by absorption in suitable media with subsequent separation and 
purification of the anhydride. 
The pressure in the reactor is not generally critical. Therefore, the 
reaction can be run at atmospheric, superatmospheric and subatmospheric 
pressures, although superatmospheric pressures are usually employed. 
A large number of saturated hydrocarbons having from 4 to 10 carbon atoms 
can be converted to maleic anhydride using the process of the present 
invention. It is only necessary that hydrocarbons contain not less than 4 
carbon atoms in a straight chain. As an example, the preferred saturated 
hydrocarbon is butane, but isobutane which does not contain 4 carbon atoms 
in a straight chain is not satisfactory for conversion to maleic 
anhydride, although its presence is not harmful. In addition to butane, 
other saturated hydrocarbons within the scope of this invention include 
the pentanes, the hexanes, the heptanes, the octanes, the nonanes, the 
decanes, or mixtures of any of these with or without butane. In addition 
to the above compounds, cyclic compound such as cyclopentane or 
cyclohexane are satisfactory feed materials for conversion to maleic 
anhydride. Also, the feedstock can be technical grade hydrocarbons 
containing up to about 25 weight percent of olefinically unsaturated 
hydrocarbons or other hydrocarbon fractions. 
The principal product from the oxidation of the above feed materials is 
maleic anhydride. It should be noted that small amounts of citraconic 
anhydride may also be produced when the feedstock is a saturated 
hydrocarbon containing more than 4 carbon atoms.