Process for preparing maleic anhydride from C.sub.4 hydrocarbons

A vapor phase process for converting normal C.sub.4 hydrocarbons to maleic anhydride using novel catalyst compositions comprising as components vanadium, phosphorus, an element selected from the group of U, W, or a mixture of elements from the group Zn, Cr, U, W, Cd, Ni, B or Si.

BACKGROUND OF THE INVENTION 
The present invention relates to an improved process for the preparation of 
maleic anhydride from normal C.sub.4 hydrocarbons by the reaction of 
oxygen with the hydrocarbon in vapor phase over a particular novel 
catalyst. 
The production of dicarboxylic acid anhydride by catalytic oxidation of 
hydrocarbons is well known. The current principal route for the production 
of maleic anhydride is the catalytic oxidation of benzene. The direct 
production of maleic anhydride from the C.sub.4 hydrocarbons has been 
desirable in the past, but is now even more desirable in view of the 
particular world shortage of benzene. It can be readily appreciated that 
direct oxidation of C.sub.4 hydrocarbons would be a hydrocarbon 
conservation, since for each mol of maleic anhydride prepared from 
benzene, one mole of benzene, molecular weight 78 is consumed whereas for 
each mol of the C.sub.4 only 54 to 58 mol weight of hydrocarbon is 
consumed. The benzene process has consistently produced high conversions 
and selectivities. Although processes for the oxidation of aliphatic 
hydrocarbons are reported in the literature, there are certain defects and 
inadequacies in these processes such as short catalyst life and low yields 
of product. Furthermore, although many of the prior art methods are 
generically directed to "aliphatic" hydrocarbons, they are in all 
practical aspects directed to unsaturated aliphatic hydrocarbons. 
A more desirable process for producing maleic anhydride would be a direct 
oxidation of n-butane. There are several advantages. Principal among these 
is the greater availability of n-butane as compared to n-butenes or 
butadiene. Also n-butenes may have higher economic petrochemical 
utilization than the n-butanes, which are now, often wastefully burned as 
cheap fuel. 
In an early series of patents one of the present inventors developed a 
untique group of vanadium-phosphorus, oxidation catalysts, i.e., U.S. Pat. 
Nos. 3,156,705; 3,156,706; 3,255,211; 3,255,212; 2,255,213; 2,288,721; 
3,351,565; 3,366,648; 3,385,796 and 3,484,384. These processes and 
catalysts proved highly efficient in the oxidation of n-butenes to maleic 
anhydride. 
SUMMARY OF THE INVENTION 
It has now been discovered that vanadium-phosphorus-oxygen complex type 
catalyst modified with a particular component, Me, is excellent oxidation 
catalyst for the conversion of n-C.sub.4 hydrocarbons to maleic anhydride. 
Surprisingly, the present catalysts are excellent for the direct oxidation 
of n-butane to maleic anhydride. In addition to n-butane, n-butene, and 
butadiene can also be used as feeds. The catalyst contains only a minor 
amount of the Me component. The Me component is generally a metal or 
metalloid element. 
The precise structure of the present complex catalyst has not been 
determined, however, the complex may be represented by formula 
EQU V P.sub.a Me.sub.b O.sub.x 
wherein Me is U, W or a mixture of elements selected from the group 
consisting of Zn, Cr, U, W, Cd, Ni, B and Si, a is 0.90 to 1.3, b is 0.005 
to 0.4 This representation is not an empirical formula and has no 
significance other than representing the atom ratio of the active metal 
components of the catalysts. The x in fact has no determinate value and 
can vary widely depending on the combinations within the complex. That 
there is oxygen present is known and the O.sub.x is representative of 
this. 
In one embodiment of the present invention the catalyst complex comprises 
vanadium-phosphorus, the component, Me, and an alkali or alkaline earth 
metal, (Alk-metal) of group IA or IIA of the periodic Table of Elements.* 
FNT *Handbook of Chemistry and Physics, 51st Edition, 1970-71, The Chemical 
Rubber Company, Cleveland, Ohio, 1970, p. B-3. 
This complex may be represented by the configuration 
EQU V P.sub.a Me.sub.b Alk.sub.c O.sub.x 
wherein Me, a, b and x as described above and Alk is a metal selected from 
the group of elements of Groups IA and IIA of the Periodic Table of 
Elements, and c is 0.001 to 0.1. Particular Group IA and IIA elements for 
the present invention are Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr and Ba, even 
more preferably Li, Na, Mg and Ba. 
DETAILED DESCRIPTION OF THE INVENTION 
The catalyst may be prepared in a number of ways. The catalyst may be 
prepared by dissolving the vanadium, phosphorus, Me and Group IA and IIA 
components (referred to herein as alk metals) in a common solvent, such as 
hot hydrochloric acid and thereafter depositing the solution onto a 
carrier. The catalyst may also be prepared by precipitating the metal 
compounds, either with or without a carrier, from a colloidal dispersion 
of the ingredients in an inert liquid. In some instances the catalyst may 
be deposited as molten metal compounds onto a carrier; however, care must 
be taken not to vaporize off any of the ingredients such as phosphorus. 
The catalyst may also be prepared by heating and mixing anhydrous forms of 
phosphorus acids with vanadium compounds, Me compounds, and the alk - 
metal compound. The catalysts may be used as either fluid bed or fixed bed 
catalysts. In any of the methods of preparation heat may be applied to 
accelerate the formation of the complex. Although some methods of catalyst 
preparation are preferred, any method may be used which results in the 
formation of the catalyst complex containing the specified ratios of 
vanadium, Me elements, phosphorus and alk metal. 
One method to obtain catalysts which produce high yields of maleic 
anhydride upon oxidation of C.sub.4 hydrocarbons is whereby the catalyst 
complex is formed in solution and deposited as a solution onto the 
carrier. According to one solution method, the vanadium is present in 
solution with an average valence of less than plus 5 in the finally formed 
complex in solution. Preferably the vanadium has an average valency of 
less than plus 5 at the time the solution of catalyst complex is deposited 
onto the carrier, if a carrier is used. The reduced vanadium with a 
valence of less than 5 may be obtained either by initially using a 
vanadium compound wherein the vanadium has a valence of less than 5 such 
as vanadyl chloride, or by initially using a vanadium compound with a 
valence of plus 5 such as V.sub.2 O.sub.5 and thereafter reducing to the 
lower valence with, for example, hydrochloric acid during the catalyst 
preparation to form the vanadium oxysalt, vanadyl chloride, in situ. The 
vanadium compound may be dissolved in a reducing solvent, such as 
hydrochloric acid, which solvent functions not only to form a solvent for 
the reaction, but also to reduce the valence of the vanadium compound to a 
valence of less than 5. For example, a vanadium compound, a zinc compound, 
phosphorus compound and alk metal compound may be dissolved in any order 
in a suitable reducing solvent and the formation of the complex allowed to 
take place. Preferably, the vanadium compound is first dissolved in the 
solvent and thereafter the phosphorus, Me and alk metal compounds are 
added. The reaction to form the complex may be accelerated by the 
application of heat. The deep blue color of the solution shows the 
vanadium has an average valence of less than 5. The complex formed is 
then, without a precipitation step, deposited as a solution onto a carrier 
and dried. In this procedure, the vanadium has an average valence of less 
than plus 5, such as about plus 4, at the time it is deposited onto the 
carrier. Generally, the average valence of the vanadium will be between 
about plus 2.5 and 4.6 at the time of deposition onto the carrier. 
When the above described solution method is employed, reducing agents for 
the vanadium may be either organic or inorganic. Acids such as 
hydrochloric, hydroiodic, hydrobromic, acetic, oxalic, malic, citric, 
formic and mixtures thereof such as a mixture of hydrochloric and oxalic 
may be used. Sulphur dioxide may be used. Less desirably, sulfuric and 
hydrofluoric acids may be employed. Other reducing agents which may be 
employed, but which have not given as desirable catalysts are organic 
aldehydes such as formaldehyde and acetaldehyde; alcohols such as 
pentaerythritol, diacetone alcohol and diethanol amine. Additional 
reducing agents are such as hydroxyl amines, hydrazine, and nitric oxide. 
Nitric acid and similar oxidizing acids which would oxidize the vanadium 
from a valence of 4 to 5 during the preparation of the catalyst should be 
avoided. Generally, the reducing agents form oxysalts of vanadium. For 
example, if V.sub.2 O.sub.5 is dissolved in hydrochloric or oxalic acid, 
the corresponding vanadium oxysalts are produced. These vanadium oxysalts 
should have as the salt forming anion an anion which is more volatile than 
the phosphate anion. 
According to this method, the time at which the Me and alk metal compounds 
are incorporated into the solution is not critical so long as it is in 
solution before the catalyst complex is coated onto the carrier. The Me 
and alk metal compounds may be added after the vanadium compound and the 
phosphorous compound have been reacted or may be added either before, at 
the same time or after either the vanadium or phosphorus compound has been 
added. 
Any vanadium, Me, phosphorus and alk metal compounds may be used as 
starting materials which when the compounds are combined and heated to 
dryness in air at a temperature of, for example, 300.degree. - 350.degree. 
C. will leave as a deposit a catalyst complex having relative proportions 
within the described ranges. In the solution methods, preferred are 
vanadium, Me, phosphorus and alk metal compounds which are essentially 
completely soluble in boiling aqueous hydrochloric acid at 760 mm. of 
mercury, containing 37 percent by weight hydrochloric acid. Generally, 
phosphorus compounds are used which have as the cation an ion which is 
more volatile than the phosphate anion, for example, H.sub.3 PO.sub.4. 
Also, generally any vanadium, or Me compound which has an anion which is 
either the phosphate ion or an ion which is more volatile than the 
phosphate anion, for example, vanadyl chloride or zinc chloride, nickel 
chloride or the like may be used. 
In this method, the catalyst complex containing vanadium, Me, phosphorus 
and Group I or IIA elements may be formed by simply causing the 
combination of each of the ingredient components in a solution or 
dispersion. Heat may be applied to accelerate the formation of the complex 
and one method of forming the complex is by causing the ingredients to 
react under reflux conditions at atmospheric pressure. Under reflux 
conditions this solution reaction generally takes about one to two hours. 
Although the catalysts prepared by this method may be separately formed and 
used as pellets, it may be more economical and practical to deposit this 
material on a carrier such as aluminum oxide, silica or niobium oxide. 
Before the carrier is combined with the catalyst the solution of catalyst 
is preferably concentrated to a solution which contains from about 30 to 
80 percent volatiles and better results have been obtained when there is 
from about 50 to 70 percent volatiles by weight. The carrier may be added 
to the catalyst solution or the catalyst solution may be poured onto the 
carrier. Less desirably, the Alundum or other carrier may be present 
during the whole course of reactions to provide the desired 
vanadium-oxygen phosphorus-alkali metal complex. After the catalyst 
complex has been coated onto the carrier, the vanadium may be converted to 
a more active form by heating in the presence of an oxidizing gas. 
Another example of the catalyst preparation is to mix with heating at a 
temperature of about 100.degree. to 600.degree. C. an anhydrous phosphoric 
acid such as ortho-phosphoric acid, pyrophosphoric acid, triphosphoric 
acid or metaphosphoric acid with a vanadium compound such as vanadium 
pentoxide or ammonium metavanadate, a Me compound such as uranyl acetate 
and an alkali such as potassium chloride. After the exothermic reaction 
between the ingredients the catalyst may be used. The reaction mixture may 
be formed onto carriers or shaped into forms such as pellets prior to the 
reaction to form the catalyst. 
Another example of the preparation of the catalyst complex is to dissolve 
the Me and alk metal compounds and a vanadium compound such as ammonium 
metavanadate or vanadium pentoxide in an aqueous solution of phosphoric 
acid. After the components have been dissolved the solution is heated 
until precipitation occurs. The precipitant can then be dried and used as 
a catalyst, or a carrier may be combined with the liquid phase either 
before or after the precipitation. 
In the various methods of preparation any vanadium, Me, phosphorus and alk 
metal compounds may be used as starting materials which when the compounds 
are combined and heated to dryness in air at a temperature of, for 
example, 300.degree. - 350.degree. C. will leave as a deposit a catalyst 
complex having relative proportions within the above described ranges. 
In another method a solution of the vanadium component is prepared by 
adding a portion of the reducing agent, such as oxalic acid and 
isopropanol solution to be employed, to a solution of water and phosphoric 
acid and heating this mixture to a temperature generally of around 
50.degree. -80.degree. C. A vanadium compound such as V.sub.2 O.sub.5 is 
added incrementally to this heated mixture with stirring. The blue 
solution which indicates vanadium of average valency less than 5, is 
maintained by added increments of the remaining oxalic acid - isopropanol 
solution. After concentration of this solution, solutions of alkali and 
alkaline earth metals and the Me components are added to vanadium solution 
and this resultant solution concentrated to a paste-like consistency, 
which may be coated on a carrier or mixed with a carrier, heated at 
moderate temperatures, i.e., 250.degree. - 500.degree. C. for a few 
minutes to several hours and prepared in pellets or chips. 
As the source of phosphorus, various phosphorus compounds may be used, such 
as metaphosphoric acid, triphosphoric acid, pyrophosphoric acid, 
ortho-phosphoric acid, phosphorus pentoxide, phosphorus oxyiodide, ethyl 
phosphate, methyl phosphate, amine phosphate, phosphorus pentachloride, 
phosphorus trichloride, phosphorus oxybromide and the like. 
Suitable vanadium compounds useful as starting materials are compounds such 
as vanadium pentoxide, ammonium metavanadate, vanadium trioxide, vanadyl 
chloride, vanadyl dichloride, vanadyl trichloride, vanadium sulfate, 
vanadium phosphate, vanadium tribromide, vanadyl formate, vanadyl oxalate, 
metavanadic acid, pyrovanadic acid, and the like. Mixtures of the various 
vanadium, Me, and phosphorus compounds may be used as starting materials 
to form the described catalyst complex. 
The Me component is also suitably introduced by employing the various 
compounds thereof such as the acetates, carbonates, chlorides, bromides, 
oxides, hydroxides, nitrates, chromates, chromites, tellurates, sulfides, 
phosphates and the like. These compounds are entirely conventional and 
those of ordinary skill in the art know these materials and can readily 
determine suitable compounds to prepare the catalyst, with little, if any, 
experimentation. A few illustrative compounds are uranyl acetate, uranyl 
sulfate, zinc chloride, zinc oxalate, tungstic acid, tungsten dioxide, 
nickel chloride, chromium sulfate, chromium trioxide, chromium chloride, 
cadmium chloride, boric acid, SiO.sub.2 (30 percent colloidal solution -- 
ammonia stabilized) and similar compounds. 
The alk-metal may suitably be introduced as compounds such as alkali and 
alkaline earth metal salts with examples being lithium acetate, lithium 
bromide, lithium carbonate, lithium chloride, lithium hydroxide, lithium 
iodide, lithium oxide, lithium sulfate, lithium orthophosphate, lithium 
metavanadate, potassium sulfate, potassium chloride, potassium hydroxide, 
sodium chloride, sodium hydroxide, rubidium nitrate, cesium chloride, 
beryllium nitrate, beryllium sulfate, magnesium sulfate, magnesium 
bromide, magnesium carbonate, calcium carbonate, calcium chromite, 
strontium chloride, strontium chromate, barium acetate, barium chlorate, 
radium carbonate and the like. Mixtures of two or more alk metal compounds 
may be used, such as a mixture of lithium hydroxide and sodium chloride or 
a mixture of lithium chloride and potassium chloride. Preferred alk-metal 
elements are lithium, sodium and potassium, and mixtures thereof, with 
lithium being particularly preferred. When the above described solution 
method of catalyst preparation is employed, the alkali metal compound will 
suitably be an alkali metal compound which either has a phosphate anion as 
the anion, that is a compound such as lithium phosphate, or a compound 
which has an anion which is more volatile than the phosphate anion. 
The function of the Group IA element is not completely understood that 
superior results are obtained when the catalyst contains these elements. 
Longer useful catalyst life has been observed when the IA element is 
present, probably due, at least in part, to the partially stabilizing 
effect of the alkali on phosphorus, and Me compounds. 
The atomic ratio of the total atoms of Group IA elements to vanadium should 
be about 0.003 to 0.08 atom of alkali per atom of vanadium. The preferred 
amount of alkali is about 0.01 to 0.04 atom per atom of vanadium. 
A catalyst support, if used, provides not only the required surface for the 
catalyst, but gives physical strength and stablity to the catalyst 
material. The carrier or support normally has a low surface area, as 
usually measured, from about 0.110 to about 5 square meters per gram. A 
desirable form of carrier is one which has a dense non-absorbing center 
and a rough enough surface to aid in retaining the catalyst adhered 
thereto during handling and under reaction conditions. The carrier may 
vary in size but generally is from about 21/2 mesh to about 10 mesh in the 
Tyler Standard screen size. Alundum particles as large as 1/4 inch are 
satisfactory. Carriers much smaller than 10 to 12 mesh normally cause an 
undesirable pressure drop in the reactor, unless the catalysts are being 
used in a fluid bed apparatus. Very useful carriers are Alundum and 
silicon carbide or Carborundum. Any of the Alundums or other inert alumina 
carriers or low surface may be used. Likewise, a variety of silicon 
carbides may be employed. Silica gel may be used. 
Other materials which can serve as carriers are Nb.sub.2 O.sub.5, WO.sub.3, 
Sb.sub.2 O.sub.3 and mixtures of these and other supports. The support 
material is not necessarily inert, that is, the particular support may 
cause an increase in the catalyst efficiency by its chemical or physical 
nature or both. 
The amount of the catalyst complex on the carrier is usually in the range 
of about 15 to about 95 weight percent of the total weight of complex plus 
carrier and preferably in the range of 50 to 90 weight percent and more 
preferably at least about 60 weight percent on the carrier. The amount of 
the catalyst complex deposited on the carrier should be enough to 
substantially coat the surface of the carrier and this normally is 
obtained with the ranges set forth above. With more absorbent carriers, 
larger amounts of material will be required to obtain essentially complete 
coverage of the carrier. In a fixed bed process the final particle size of 
the catalyst particles which are coated on a carrier will also preferably 
be about 21/2 to about 10 mesh size. The carriers may be of a variety of 
shapes, the preferred shape of the carriers is in the shape of cylinders 
or spheres. Although more economical use of the catalyst on a carrier in 
fixed beds is obtained, as has been mentioned, the catalyst may be 
employed in fluid bed systems. Of course, the particle size of the 
catalyst used in fluidized beds is quite small, usually varying from about 
10 to about 150 microns, and in such systems the catalyst normally will 
not be provided with a carrier but will be formed into the particle size 
after drying from solution. 
Inert diluents may be present in the catalyst, but the combined weight of 
the active ingredients, e.g., vanadium, oxygen, phosphorus, Me, and alk 
metal should preferably consist essentially of at least about 50 weight 
percent of the composition which is coated on the carrier, if any, and 
preferably these components are at least about 75 weight percent of the 
composition coated on the carrier, and more preferably at least about 95 
weight percent. 
The oxidation of the n-C.sub.4 hydrocarbon to maleic anhydride may be 
accomplished by contacting, e.g., n-butane, in low concentrations in 
oxygen with the described catalyst. Air is entirely satisfactory as a 
source of oxygen, but synthetic mixtures of oxygen and diluent gases, such 
as nitrogen, also may be employed. Air enriched with oxygen may be 
employed. 
The gaseous feed stream to the oxidation reactors normally will contain air 
and about 0.5 to about 2.5 mol percent hydrocarbons such as n-butane. 
About 1.0 to about 1.5 mol percent of the n-C.sub.4 hydrocarbon are 
satisfactory for optimum yield of product for the process of this 
invention. Although higher concentrations may be employed, explosive 
hazards may be encountered. Lower concentrations of C.sub.4, less than 
about 1 percent, of course, will reduce the total yields obtained at 
equivalent flow rates and thus are not normally economically employed. The 
flow rate of the gaseous stream through the reactor may be varied within 
rather wide limits but a preferred range of operations is at the rate of 
about 50 to 300grams of C.sub.4 per liter of catalyst per hour and more 
preferably about 100 to about 250 grams of C.sub.4 per liter of catalyst 
per hour. Residence times of the gas stream will normally be less than 
about 4 seconds, more preferably less than about one second, and down to a 
rate where less efficient operations are obtained. The flow rates and 
residence times are calculated at standard conditions of 760 mm. of 
mercury and at 25.degree. C. A preferred feed for the catalyst of the 
present invention for conversion to maleic anhydride is a n-C.sub.4 
hydrocarbon comprising a predominant amount of n-butane and more 
preferably at least 90 mol percent n-butane. 
A variety of reactors will be found to be useful and multiple tube heat 
exchanger type reactors are quite satisfactory. The tubes of such reactors 
may vary in diameter from about 1/4 to about 3 inches, and the length may 
be varied from about 3 to about 10 or more feet. The oxidation reaction is 
an exothermic reaction and, therefore, relatively close control of the 
reaction temperature should be maintained. It is desirable to have the 
surface of the reactors at a relatively constant temperature and some 
medium to conduct heat from the reactors is necessary to aid temperature 
control. Such media may be Woods metal, molten sulfur, mercury, molten 
lead, and the like, but it has been found that eutectic salt baths are 
completely satisfactory. One such salt bath is a sodium nitrate-sodium 
nitrite-potassium nitrate eutectic constant temperature mixture. An 
additional method of temperature control is to use a metal block reactor 
whereby the metal surrounding the tube acts as a temperature regulating 
body. As will be recognized by the man skilled in the art, the heat 
exchange medium may be kept at the proper temperature by heat exchangers 
and the like. The reactor or reaction tubes may be iron, stainless steel, 
carbon-steel, nickel, glass tubes such as Vycor and the like. Both 
carbon-steel and nickel tubes have excellent long life under the 
conditions of the reactions described herein. Normally, the reactors 
contain a preheat zone of an inert material such as 1/4 inch Alundum 
pellets, inert ceramic balls, nickel balls or chips and the like, present 
at about one-half to one-tenth the volume of the active catalyst present. 
The temperature of reaction may be varied within some limits, but normally 
the reaction should be conducted at temperatures within a rather critical 
range. The oxidation reaction is exothermic and once reaction is underway, 
the main purpose of the salt bath or other media is to conduct heat away 
from the walls of the reactor and control the reaction. Better operations 
are normally obtained when the reaction temperature employed is no greater 
than about 100.degree. C. above the salt bath temperature. The temperature 
in the reactor, of course, will also depend to some extent upon the size 
of the reactor and the C.sub.4 concentration. Under usual operating 
conditions, in compliance with the preferred procedure of this invention, 
the temperature in the center of the reactor, measured by themocouple, is 
about 375.degree. to about 550.degree. C. The range of temperature 
preferably employed in the reactor, measured as above, should be from 
about 400.degree. to about 515.degree. C. and the best results are 
ordinarily obtained at temperatures from about 420.degree. to about 
470.degree. C. Described another way, in terms of salt bath reactors with 
carbon steel reactor tubes about 1.0 inch in diameter, the salt bath 
temperature will usually be controlled between about 350.degree. to about 
550.degree. C. Under normal conditions, the temperature in the reactor 
ordinarily should not be allowed to go above about 470.degree. C. for 
extended lengths of time because of decreased yields and possible 
deactivation of the novel catalyst of this invention. 
The reaction may be conducted at atmospheric, super-atmospheric or below 
atmospheric pressure. The exit pressure will be at least slightly higher 
than the ambient pressure to insure a positive flow from the reaction. The 
pressure of the inert gases may be sufficiently high to overcome the 
pressure drop through the reactor. 
In one utilization of the present catalyst compositions, the oxidation is 
carried out at 15 to 100 psig, preferably about 20 to 50 psig and more 
preferably about 25 to 40 psig, which is disclosed and claimed in a 
commonly assigned patent application of Bruno J. Barone and Stone D. 
Cooley Ser. No. 558,737 filed Mar. 15, 1975. 
Operating under pressure as described above, the temperature in the center 
of the reactor, measured by themocouple is about 375.degree. to about 
550.degree. C, with the preferred temperature range for operating 
according to the present invention being 430.degree. to 480.degree. C. and 
the best results are ordinarily obtained at temperatures from about 
430.degree. to about 455.degree. C. Described another way, in terms of 
salt bath reactors with carbon steel reactor tubes about 1.0 inch in 
diameter, the salt bath temperature will usually be controlled between 
about 325.degree. to about 455.degree. C. Under these conditions, the 
temperature in the reactor ordinarily should not be allowed to go above 
about 410.degree. C for extended lengths of time because of decreased 
yields and possible deactivation of the novel catalyst of this invention. 
The maleic anhydride may be recovered by a number of ways well known to 
those skilled in the art. For example, the recovery may be by adsorption 
in suitable media, with subsequent separation and purification of the 
maleic anhydride.

EXAMPLES 
In the following examples, the reactor comprised a 4-tube cylindrical brass 
block (8 inches O.D. .times. 18 inches) reactor made of alloy 360. The 
block was heated by two 2500 watt (220 volt) cartridge heaters controlled 
by means of a 25 amp. thermoelectric proportional controller with 
automatic reset. Prior to its insulation, the block was tightly wound with 
a coil of 3/8 inch copper tubing. This external coil was connected to a 
manifold containing water and air inlets for cooling of the reactor block. 
The reactors were made of a 304 stainless steel tube, 1.315 inches O.D. 
and 1.049 inches I.D., 231/2 inches long, containing a centered 1/8inch 
O.D. stainless steel thermocouple well. The lower end of the reactor was 
packed with a 1 inch bed of 3 mm pyrex beads. The next 12 inches of the 
reactor were packed with catalyst (1/8inch .times. 1/8inch pellets or 6-12 
mesh granules) followed by about a 10 inches bed of 3 mm pyrex beads. The 
gas streams are separately metered into a common line entering the top of 
the reactor. The reaction vapors are passed through two 2000 ml gas 
scrubbing bottles containing 800 ml of water. The vapors from the 
scrubbers then go through a wet test meter and are vented. The inlet gases 
are sampled before entering the reactor and after the water scrubbers. The 
feed is normally 0.5 to 1.8 mol percent C.sub.4, e.g., n-butane, in air, 
adjusted to maintain a desired temperature. In addition, operating 
temperature can be further controlled by dilution of the air with an inert 
gas. 
The inlet gases and water scrubbed outlet gases were analyzed by gas 
chromatography using the peak area method. Butane, carbon dioxide and any 
olefins or diolefins present in the gas streams were determined using a 
1/4 inch column with a 5 feet foresection, containing 13 weight percent 
vacuum pump oil on 35/80 mesh chromosorb, followed by a 40 feet sectiion 
containing 26 weight percent of a 70-30 volume ratio of propylene 
carbonate to 2,4-dimethylsulfolane on 35/80 mesh chromosorb. The analysis 
was conducted at room temperature with hydrogen as the carrier gas (100 
ml/minute). Carbon monoxide was analyzed on a 1/4inch column with a 3 feet 
foresection of activated carbon followed by a 6 feet section of 40/50 mesh 
5A molecular sieves. This analysis was run at 35.degree. C with helium as 
the carrier gas (20 psi). 
The water scrub solutions were combined and diluted to 3000 ml in a 
volumetric flask. An aliquot of this solution was titrated with 0.1 N 
sodium hydroxide solution to determine maleic acid (first end point) and 
weak acids in solution and titrated to determine the carbonyls, using 
hydroxylamine hydrochloride. 
TYPICAL CATALYST PREATION 
A catalyst having the representative formula VP.sub.1.5 Zn.sub.0.19 O.sub.x 
for oxidation of n-butane to maleic anhydride was prepared as follows: 
145.44g of vanadium pentoxide (V.sub.2 O.sub.5) was added to 1500 
milliliters of 37 percent hydrochloric acid. The mixture was refluxed 
slowly and after the initial reaction 212.14g of 85 percent phosphoric 
acid (H.sub.3 PO.sub.4) was added and the mixture refluxed for about 13 
hours. After a blue solution was obtained, showing that the vanadium had 
an average valence of less than plus five, 41.43g of ZnCl.sub.2 was added 
to the solution and the mixture again refluxed. The resulting deep blue 
solution was concentrated to a paste like consistency and dried in an oven 
at 130.degree. C. The dried catalyst was chipped, calcined at 350.degree. 
C for 2 hours, ground to 60 mesh and tableted. 
Other catalyst shown below were prepared in the same manner with additional 
or different component compounds being added when the ZnCl.sub.2 was added 
and in the appropriate amounts to give the atom ratios shown. 
The results of the testing of the catalyst are set forth in the Table 
below: 
TABLE 
__________________________________________________________________________ 
Head Mole % 
Catalyst Composition 
Temp., .degree.C 
Pressure 
n-C.sub.4 
GHSV Mole % M.A. M.A. Output 
Example 
Atom Ratio Block 
Hot Spot 
psig Feed V/V/H 
Conv. 
Select. 
Yield 
g/lit.cat/hr. 
__________________________________________________________________________ 
1 VP.sub.1.12 O.sub.x 
415 451 0 0.930 
847 87 53 46 15.3 
380 450 20 1.272 
850 82 52 43 22.9 
2 VP.sub.1.15 Zn.sub.0.19 Si.sub.0.167 O.sub.x 
410 467 0 0.947 
1115 76 62 47 21.8 
388 456 35 1.104 
1346 80 73 58 38.0 
3 VP.sub.1.17 Zn.sub.0.19 Cr.sub.0.01 B.sub.0.01 O.sub.x 
390 464 0 1.031 
1218 80 60 48 26.6 
350 465 30 1.321 
1342 85 63 54 41.6 
4 VP.sub.1.15 U.sub.0.25 B.sub.0.01 O.sub.x 
360 460 0 1.161 
1347 81 50 41 27.8 
325 453 20 1.253 
1348 79 54 43 31.6 
5 VP.sub.1.1 W.sub.0.06 O.sub.x 
405 466 0 1.038 
582 79 54 42 11.3 
380 467 30 0.787 
1217 85 58 49 20.7 
6 VP.sub.1.2 W.sub.0.06 O.sub.x 
410 477 0 1.205 
757 83 56 47 18.7 
370 460 35 1.464 
838 84 64 53 28.6 
7 VP.sub.1.1 U.sub.0.25 O.sub.x 
360 460 0 1.034 
1346 83 43 36 21.8 
8 VP.sub.1.15 W.sub.0.06 Zn.sub.0.1 O.sub.x 
405 461 0 0.781 
705 82 54 44 10.7 
375 455 25 0.841 
1116 78 60 47 19.2 
9 VP.sub.1.15 Ni.sub.0.19 Cr.sub.0.005 Li.sub.0.01 O.sub.x 
390 457 0 1.040 
1116 82 59 49 24.8 
340 464 30 1.159 
1340 84 61 51 34.9 
10 VP.sub.1.15 U.sub.0.25 Cr.sub.0.005 Li.sub.0.01 O.sub.x 
360 459 0 1.136 
1346 80 55 44 29.5 
335 458 30 0.797 
7860 85 54 46 42.9 
11 VP.sub.1.175 Cd.sub.0.19 Cr.sub.0.005 Li.sub.0.001 O.sub.x 
410 469 0 0.979 
892 82 54 45 17.0 
375 468 30 1.237 
1348 86 61 53 38.5 
12 VP.sub.1.2 W.sub.0.06 Cr.sub.0.005 Li.sub.0.01 O.sub.x 
410 461 0 0.760 
1117 83 57 47 17.5 
393 463 30 1.336 
1209 83 61 51 35.9 
13 VP.sub.1.15 Ni.sub.0.19 W.sub.0.03 B.sub.0.005 O.sub.x 
390 460 0 0.805 
1339 84 55 46 21.9 
345 468 20 1.189 
1347 86 57 49 34.4 
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