CA1049540A - Oxidation of alkanes to dicarboxylic acid anhydrides using promoted vanadium-phosphorus catalyst - Google Patents

Abstract

OXIDATION OF ALKANES TO DICARBOXYLIC
ACID ANHYDRIDES USING PROMOTED VANADIUM-PHOSPHORUS CATALYST
ABSTRACT OF THE DISCLOSURE

There is provided an improved process for the vapor phase oxidation of alkanes to dicarboxylic acid-anhydrides, particularly butane to maleic anhydride, which comprises using a vanadium-phosphorus-oxygen complex catalyst having a P/V atomic ratio of 0.5-2, promoted or modified with certain transition metals, preferably chromium, iron, or hafnium. Using these catalysts, the oxidation process can be carried out at lower temperatures and increased yield as compared to the unpromoted complex.

Description

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BACKGROUND OF T~IE INVENTIO~

Field of the Invention . ., This invention provides an improved catalytic process for the oxidation of alkanes to dicarboxylic acid anhydrides, particularly butane to maleic anhydride.
Description of the Prior Art The use of a vanadium-phosphorus complex catalyst to oxidize butane to maleic anhydride has been described in U.S. Patent No. 3,293,268. Such catalysts called for operat-ing temperatures greater than 500C. and, in general, yields were relatively low and not commercially attractive or feasible. Insofar as is now known, it has not been proposed to promote these catalysts with transition metals for use in ;-~
the oxidation of butane or other alkanes.
Various catalysts have been proposed for the oxida-tion of olefins, such as butene, to dicarboxylic acid anhy-drides such as maleic anhydride. It is well recognized in the art, however, that it is relatively easy to oxidi~e ole- -fins to acid anhydrides in commercially feasible yields, i.e., yields of 60 weight percent or better based upon the weight ~
o the hydrocarbon feed. Yields of 60 weight percent or ~ -better have not been noted in prior art processes for the catalytic oxidation of alkanes.
In U.S. Patent No. 3,156,705 there is described `
a process for oxidizing an olefin (butene) to a dicarboxylic " , ~
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` ~49540 ~cid anhydride (malelc anhydl~ide) using n met~1 promo~ed vana~dlum-phosphorus complex catalyst. It is tau~ht that the promoter (called a "phosphorus stabillzer" by the patentee) can be an element selected from a wide variety of elements embracins the transition metals and the rare e~rth metals. It is the discovery of the present invention that, for the oxidation of alkanes (butane), commercially reasible ylelds of anhydride are achievcd only when the promoter metal is selected fro~n a small ~roup of six metals (Cr, Fe, Hr, Zr, La, and Ce).
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SUM~RY OF TH~ INVENTIO~

This invention provides an improved process for oxidizing an alkane having from 4 to 10 carbon~atoms to the corresponding dicarboxylic acid anhydride, that comprises contacting a mixture of a molecular oxygen-containing gas (air) and an alkane having 4-10 carbon atoms, at 300-600C.,. with a catalyst that consists essentially of the complex reaction product of a vanadium oxysalt and phosphoric acid promoted with a metal selected from the group consistin~ of chromium~, ha~nium, zirconium, lanthanum,.

-and cerlum; the Rtom~c ratio Or pIIosphorus/vanadium beingbetween about O.5~and about~2; and~the atomic ratio of promoter met~l/vAnad1um beine bet~reen about O.OO5 and about O.5.

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DESCRIPTION OF THE DI~AWING

The drawlng presents curves showine; the relation-~hip between wei6ht ~ercent ma~elc an2-~dride based u~pon ~L049540 weight of butane feed and the hotspot temperature based upon typical oxldation runs using vanadium-phosphorus catalyst without promoter, Curve B, and with hafnium promoter, Curve A.
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DESCRIPTION OF SPECIFIC EMBODI~ENTS

The alkanes that are oxidized to dicarboxylic acid anhydrldes by the process of this invention are the alkanes having between 4 and 10 carbon atoms. Butane, because o~ its ready availability, is preferred. In the ~ollowing discussion and exemplification,~therefore, butane is used to demonstrate the present process for producing maleic anhydride. It is contemplated that mlxtures rich in butane can be used, such as a typical ;
butane-butene (B-B) refinery stream.

; ~ The promoted catalysts utilizable herein are 15~ prepared by refluxing a reaction mixture of vanadium -~
oxlde3 phosphoric acid, a hydrogen~halide (usually hydro-chloric acid), and a speci~ied promoter metal compound.
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Alternatively, the promoter metal compound~can be added at a later st&ge o~ the catalyst preparation. For example, ~ ~ -the promoter metal compound can be added just prior to -catalyst pelletization. The proportions~of reactants are selected to a~ord an atomic ratio of phosphorus/-anadi~n o~ between about 0.5 and~about 2 and an atomic ratio o~ promoter metal/vanadium of between about 0.0025 .
and about 1, preferably between about 0.005 and about 0.5. ~ -.~ ' , ' :

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The optimum atomic ratio of promoter metal/V willdepend upon the particular metal selected. In general, the best ratio can be found from the specific working examples, infra, or can be readily determined with a minimum of experi-ments. For example, a ratio of 0.1 cerium is less effective as compared to a ratio of 0.013, but in the case of hafnium a ratio of 0.088 is better than 0.032. With iron, a middle ratio appears more effective.
The reaction mixture is heated at reflux temperature for between about 0.5 hour and about 24 hours, during which time the solution changes color, usually from brown to dark blue. Then, the reaction mixture is concentrated and eva-porated to dryness. The catalyst is prepared by grinding the resultant solid material to about 20-60 mesh size and pelletiz- ~`
ing, for example, to 1/8" x 5/32" cylindrical pellets.
Optionally, a binder, such as stearic acid, can be added hefore pelletizing. Alternatively, the reaction mixture con-centrate can be used to impregnate a suitable carrier, such as alumina or "Alundum", silica, silicon carbide, silica-alumina, or zirconia, to produce a supported catalyst suitable for use in a fixed or fluidized bed reactor. As a further alternative, the dried catalyst (unsupported) can be ground to produce a powdered catalyst for use in a fluidized bed reactor.
In practice, the vanadium salt is added as vanadium oxychloride, whlch is formed by reaction in situ of vanadium * Trade mark for a line of fused alumina refractory and abrasive products.

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~ ~4~5~0 pentoxide with hydrochloric acid. Alternatively, other oxyhalide salts of vanadium can be used, suitably prepared by reacting vanadium pentoxide with another acid such as hydrofluoric, hydrobromic or hydroiodic. The phosphoric acid used generally will have a strength of between about 25 per cent and about 100 per cent. The promotor metal compound can be any compound of the promotor metal, such as nitrate, chloride, -acetate, oxide, carbonate and the like. The promoter metals -utiliæable in the catalysts contemplated herein are chromium, iron, hafnium, zirconium, lanthanum, and cerium. Iron and `
hafnium are particularly preferred.
In some of the earlier work in the process of this invention, the catalyst was "conditioned" in the reactor by passing a hydrocarbon-air mixture through the catalyst bed prior to running the oxidation reaction. Such conditioning is, however, not necessary to obtain catalyst efficiency. , In this regard, note Examples 10 and 11. In practice, anhy-dride product can be obtained upon commencing the flow of oxid-ation feed through the reactor. ~
The oxidation of n-butane (or other alkane) to maleic ! .
anhydride (or other anhydride) is carried out using air or other molecular oxygen-containing gases, such as mixtures of carbon dioxide and oxygen or mixtures of nitrogen or steam with air or oxygen. Air is preferred. The oxidation reaction is carried out at temperatures of 300-600C., prefer- `
ably 400-550C. The feed concentration will be 0.5-6 volume percent butane in the oxygen-containing gas -~
and preferably 1-5 volume percent. The contact time will ''``~ ' , 10~9S40 vary between about 0.08-3 seconds, preferably about 0.16-1.6 seconds for fixed bed operation. Contact times of up to 10 seconds can be used in the case of a fluidized bed opera-tion. Thus contact time, depending upon the type of opera-tion will be about 0.08-10 seconds. Although the reaction ;
can be carried out at 0.5-20 atmospheres pressure, it is pre-ferably carried out at substantially atmospheric pressure.
The reaction can be carried out in any suitable reactor for effecting vapor phase oxidation reactions. Suit-ably and preferably, a fixed catalyst bed can be employed.
The reaction can be carried out, however, by using smaller catalyst particles in a fluidized reactor bed. In most of the following examples, except as noted, there was used a fixed-bed reactor consisting of 14" x 3/4" i.d. stainless steel tube equipped with a 1/4" o.d. axial thermowell for tempera-ture measurement. a 12" portion of the reactor was encased in a brass block. Temperatures were measured at the hottest point in the catalyst bed and in the brass block. Heat was supplied to the reactor through tubular electrical heaters.
In the examples and tables, "~ MA'I indicates maleic anhydride yield expressed as weight per cent based -upon the weight of butane feed and was determined by -titration. The flow rates of air and butane were measured `
at room temperature and pressure. Temperatures are designated as "hotspot" for the temperature measured at !~:
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- 10~54~) the hottest pOint ln the axlal therrnowell or "Jacket~' for the temperature of the brasa block. Unless otherwise specified, all temperatures are hotspot temperatures.

A vanadium-phosphorus catalyst having a P~V ratio of 1.06:1 (atomic ratio) was prepared as follows: 258 grams of V205 was added to a solution of 393 grams of 85 per cent H3P04 and 2 liters of concentrated (37~ by wt.) HCl. The mixture was re~luxed for two hours during whic~ time the solution changed color from brown to dark blue. The solution was concentrated until viscous and - .
; ~ ~ dried under vacuum at 110C. for 17 hours. ~ ~
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The resulting blue-green solid was ground to :~
~ 20-60 mesh. To 80 g. of this solid, 10 per ;~
cent by weight stearic acid was added as binder, and the material was formed into 1/8"
x 5/32" cylindrical pellets.
The catalyst was charged to the reactor ~ at room temperature and butane and alr were passed over the catalyst at 20~ml. per minute and 2000 ml. per minute, respectlvely. The reactor was heated at 490C. for about 16 hours. ~
A mixturé of 20 ml. per minute butane and - 2000 ml. per minute air was then passed over : , ~` 8 : -~ ~ , :
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the catalyst at 490C. Malelc anhydride product was determlned by scrubbing the exit gases through water followed by tltration of an ~liquot of the aqueous solution. In successive one hour sampling periods, MA yields of 46 and 44 per cent were obtained. Some ~urther examples of MA
yields obtalned at other flow rates are glven in Table 1.
., ., .:, ~ EXAMPLE 2 A catalyst having a P/V~Hf ratio (atomic) of 1.25/1/0.030 was prepared as follows: 258 grams of V205 was added to a solution of 393 g ~ ~ of a5 per cent H3P04 and 2 liters concentrated ~ (37%~by wt.) HCl. The mixture was refluxed for 2 hours during which time the V205 dissolved~
and the~solution changed from brown to blue-green.
, 29.1 grams o~ HfC14 was added and the solution was stirred for one more hour. The resulting

2~ solution was concentrated,~dried under vacuum at ;~ ~ 110C., ground~to pass 40 mesh, and pelletized . to 1/8" x 5/32" using 5 per cent stearic acid~
as binder.
70 ml. of the catalyst was charged to the ~25 reactor and heated overnight at 490C~ with a flow of 10 ml. per minute butane and 1000 ml.
per minute air passing over it. -:
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~ . A mlxture o~ 10 ml. per minute butane :: and 1000 ml. per m~nute air was then passed :~
o~er the catalyst at 490C. and the maleic anhydride yield was determined as in Example 1. In two successive one-hour sampling periods : - ~ the maleic anhydride yields were 69 and 73 per cent based on butane feed. ~
, Further comparisons between this catalyst and the unmodi~ied V/P catalyst of Example 1 are summarized in Table I.

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TABLE I

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Air Butane Wt.% MA per pa~s ml./mln. ml~/min. Temp-C Example 1 Ex~le 2 2000 20 410 ~ 51 ~` 510 _ 44 : 525 56 - ::
530 _ 27 : 575 32 _ - ~ 600 24 2000 40 ~ ~ 410 - ~ 41 : 430 - : 49 ; .
450 - : 63 : : 470 . - ~: 64 490 39 : 61 : 530 - ~ 52 ~ ~:

` ~ . 34 ~:It will be apparent from the data in the Table that .
greater yields o~ maleic anhydride are obtained at less : ~evere conditions when using the ha~nium-promotèd ca~alyst :~
of Example 2. The difference between promoted and un- ~.
promoted catalyst becomes more apparent upon reference to the drawing. The curves in the drawing show the relation~
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ship between temperature and yield of:maleic anhydride based upon the data in Table I UfiIng an air flow rate of 2000 ml. per minute and a bùtane flow rate of 20 ml. per ::
10` minute. Curve A shows this relationship using the hafnium~
promoted catalyst of Example 2. .Curve B shows this ~ ;
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., ~ILat49S40 r -relationship when using the unpromoted catalyst of Example lo It will be noted (Curve A) that significantly higher yields of MA are obtained at substantially lower tempera-tures when the promoted catalyst is used.

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- _ A catalyst having a P~V/Hf atomic ratio oP 1.21/1/0.058 (about twice as much hafnium as the catalyst in Example 2) was prepared as follows: a mixture of 97 g V205 and 1.5 liter concentrated ~Cl (37% by wt.) was refluxed for ; . .
~our hours. 147.3 g 85% H3P04 was then;added and the solution was refluxed for 16 hours more. 21.8 g HfC14 was then added and re~luxing was continued for one hour. The solution was ~ -then treated as described in Examples I and 2 -.:
to produce the finished catalyst. ~ ~
70 ml. of the catalyst was charged to the ~ ~ -fixed-bed reactor already described and heated ~
for one day at 462C. under a flow of 2000 ml./min. -~ Bir and 10 ml./min. butene ~or about two hours~
and then the butene was replaced with butane.
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After fifteen days operation with varying .
conditions of temperature and air and butane flows, butane and air were passed over the catalyst at 40 ml./minute and 2000 ml./minute, respectively, at a reactor temperature of 490C.
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; me maleic anhydride yield under these conditions was 73 per cent. The V~P/Hf catalyst of Example 2 _12- ~

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~49540 .
produced 58-64 per cent MA under these condltions. me V/P catalyst of Example 1 produced 39 per cent MA under these conditions, The general procedure of Example 3 was , repeated. Seventy ml. of a catalyst having an atomlc ratio of phosphorus/vanadium/zirconium of 1.2/1/0.148 was used (zirconlum added as ~ zirconyl acetate). At a reactor temperature of 509C. and butane and air flows of 30 ml.
per min. and 1000 ml. per min., respectively~
the yield of maleic anhydride was 71%. At a ' reactor temperatu.re of 499C. and butane and air :, flows of 45 ml. per min. and l500 ml.,per min., , respectively, the yield of maleic anhydride was 64%.
-, , EXAMPLE 5 ~ ~ ~
, ~he general procedure of Example 3 was repeated. Seventy ml, of a catalyst having an , atomic ratio of phosphorus/vanadium/cerium of 1.2/1/0.013 was used (cerium added as ceric ammonium nitrate). At a reactor temperature oi 490C. and butane nd,air flows of 19 ml.
-per min. and 1000 ml. per min., respectively, ,25 tbe maleic anhydride yield was~,81%. At a re-actor temperature of 520C. and butane and air ~, flows oi 70 ml. per min. and 3500 ml. per min., ' , 13 , .

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respectively~ the maleic anhydride yield was 52%. ~ -- EXhMPLE 6 ~ :
~ The general procedure o~ Example 3 was repeated. Seventy ml. of a catalyst having an atomic ratio of phosphorusjvanadium/iron of 1.2/1/0.03 was used (iron added as iron - (III) oxlde). At a reactor temperature of 508C. and butane and air flows of 41 ml. per ~ -min. and 2000 ml. per min., respectively, the yield of maleic anhydride was 69~. At i~re- -actor temperature of 538C. (jacket temperature was 460C.~ and butane and air flows of 62.5 ~ and 2000 ml. per min.~ respectl~ely, the 15 ~ ~ - maleic anhydride yield was 73%

: ~ : : - , A catalyst having a phosphorus/vanadium atomic ratlo of 1.2/1 was prepared according to the general procedure of Example 3. After ~20 the catalyst was dried, 0.2 weight percent iron .. .
(a8 iron oxide) was added and the catalyst was ~
formed into pellets.~ Over 70 ml. of the~catalyst at a temperature of 490C. was passed 10 ml/min.
. - . ' butane and 1000 ml./min. air. The malelc anhydride yield~was 73%. At a reactor temperature~of 510C.
and butane and air flow rates of 40 ml./min. and 2000 ml./min. respectively, the maleic anhydride yleld Was 56%.

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It is apparent from this Example and previous examples that the promotor may be added with benefit at different stages of the catalyst preparation.

The catalyst of Example 2 was ground to a powder (approximately 100 mesh), and 70 ml.
was placed in a fluid bed type of reactor com-prising a glass tube about 18" long and having an inside diameter of about 1". A mixture of 30 ml./min. butane and 1200 ml./min.air was passed upward through the reactor with the bed temperature at 480 to 500C. The yield of maleic anhydride was 53%.

Four catalysts were prepared in an iden-tical fashion using the method described in Examples 1-3. The catalysts were deposited on a silica-alumina support. Vanadium pentoxide and phosphoric acid were used in quantities such that the catalyst contained 11 per cent V2O5 and 10 per cent P2O5 by weight. One catalyst contained only V and P;
another contained l per cent Ce(added as Ce~NO3]3) ;
in addition to V and P; another contained 0.9% Sm (added as Sm[OAc]3) in addition to V and P; the fourth contained 0.8 per cent Hf (added as HfC14) in addition to V and P.

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The catalysts were evaluated as described above. The u~nodified V/P
~atalyst produced an average MA yield of 34 per cent from n-butane. The cerium .
modified V/P catalyst gave an average MA
yield of 46 per cent. The samarium and hafnium modified catalysts gave average MA
- yields of 49 per cent and 47 per cent, respectively, from n-butane. -~ EXAMPLE 10 The cataly2t described in Example 2 was ~ ~ .
charged to the reactor. m en, the reactor was rapidly hèated to 500C. (jacket temperature3 . . .
while 1000 ml./min. air was passed through the catalyst bed. A mixture o~ 20 ml./min. bùtane ~ `
~15 ~ and ~000 ml./min. air was then passed in contact~
with the catalyst at a bed (hotspot) temperature of 490C. and MA yield determined as in Example ~ -In two successi~e~hourly sampling periods ; ~ the MA yieIds wera 63~ and 66~ based on weight ~ 20 of butane feed. mis run demonstrates that ; ~ prior conditioning of the catalyst is not~
necessary and is not critical for~good catalyst~
` performance.

As has been indicated hereinbefore, U.S. Patent~
No. 3,156~705 teaches that olefins can be oxidized to maleic anhydride in ~ood yields using ~irtually all , .. , . ., :...... . . .. ..... . ... . .

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transition metals and rare earth promoters ~or the V/P
catalyst. The following example sets forth the results of a serles of runs with various promoters, which shows that in the oxidation of alkanes (butane) only certain promoters are effective to produce MA from butane in commercially feasible ylelds (60 wt.% or better).
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., EXAMPLE 11 ~
A number of series of runs were made to ,. : : ~ .
oxidize butane to MA, using V/P catalysts having different promoter metals in each ..
series. In each series~of runs, temper~ture was varied from run to run using in all runs ,. . ~ :
~ ~ a feed of 2 volume per cent butane in air and ... .
a ~eed rate space velocity of ~.54 lb. moles/-` ~t.3/hr- After a temperature series was run : . . . ~ . , for each promoter, the optimum temperature for optimum ~A yield was determined. The pertinent data of promoter metal, promoterfV atomic~ratio, temperature, and yield per pass are set forth in Table II. The temperatures shown are~jacket ~` ~ temperatures corresponding to ~verage bed tempera-ture, unless otherwise indicated in Table II. The .::
Table is divided as to co~ercially feasible , . . . .
yields in the first portion and to yields not ~
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commercially feasible in the second (lower) portion. ~
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T E II

Atomic Ra~io Wt. ~ MA
Promoter Promoter/V Temp.,C.Per Pass Cr 0.031 4~ 6 Fe 0.031 470 7 Hf 0.0~ 415 73 Zr 0.072 460 66 Ce 0.013 L~50 64 Fe o.o56 455 64 ~f 0.032 L~90* 64 La 0.03 4~0 63 Fe 0.014 530-~ 61 Mn 0.03 500 59 Zn 0O03 512 5~9 Ru 0 01 L~20 5~
Mo 0 03 460 56 Sn 0.03 49~ 54 l Ti 0.03 4~0 54 Sb 0.03 512 54 Th 0~03 490 52 Pr 0.03 465 52 None __ 470 52 W 0.03 4~0 51 Ce 0.10 450 49 Sm 0.03 460 49 Ag 0.03 4~5 49 Nb 0~03 510 47 Ni 0.03 535 46 T1 0.03 495 44 U 0.03 502 42 Cu-Li 0.024-0.072 4~0 41 Co 0.03 5O 39 , .
;` * Hottest tempe~ature in bed.
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Although the present invention has been descrlbed with prefer~ed embodiments, it is to be - understodd th~t modifications and variations may be resorted to, without departing from the spirit and scope of this invention~ as those skilled in the art will readily unders-tand. Such modifications and variatlons are considered to be within the purview : and scope of the appended claims.

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Claims (7)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An improved process for oxidizing an alkane having from 4 to 10 carbon atoms to the corresponding dicarboxylic acid anhydride, that comprises contacting a mixture of a molecular oxygen-containing gas and an alkane having 4-10 carbon atoms, at a temperature of about 300°-600°C., with a catalyst that consists essentially of the complex reaction product of a vanadium oxysalt and phosphoric acid promoted with a metal selected from the group consisting of chromium, hafnium, zirconium, lanthanum, and cerium; the atomic ratio of phosphorus-vanadium being between about 0.5 and about 2; and the atomic ratio of promoter metal/vanadium being between about 0.0025 and about one.

2. The process of claim 1, wherein said alkane is butane.

3. m e process of claim 1, wherein said alkane is butane, said oxygen-containing gas is air, said temperature is about 400-550°C., and said atomic ratio of promoter metal/
vanadium is between about 0.005 and about 0.5.

4. The process of claim 3, wherein said promoter metal is chromium.

5. The process of claim 3, wherein said promoter metal is hafnium.

6. The process of claim 3, wherein said promoter metal is zirconium.

7. The process of claim 3, wherein said promoter metal is cerium.