ZA200602036B - Method for the production of (meth)acrolein and/or (meth)acrylic acid - Google Patents

Description

oo - PF54826

Preparation of (meth)acreolein and/or (meth)acrylic accid

The present invention rel ates to a process for prepar—ing (meth)acrolein amnd/or (meth)acrylic acid by hetesrogeneously catalyzed gass phase partial oxidat jon by charg- ing a fresh fixed catalyst bed disposed in a reactor as elevated temperatu re with a charging gas mixture whi ch, in addition to at least orm e organic precursor «compound to be partially oxidized and molecular oxygen as an oxiedant, comprises at le=ast one dilu- ent gas which behaves ssubstantially inertly under thes conditions of the he=terogeneously catalyzed gas phase parttial oxidation.

In this document, the notaation (meth)acrolein is an alobreviation of methacrolein or ac- rolein.

In this document, (meth)acrylic acid is an abbreviatio n of methacrylic acid” or acrylic acid. (Meth)acrolein and (meth )acrylic acid are reactive monomers which are suitable, for example, for preparing polymers which may find use, inter alia, as adhesives.

On the industrial scale, (meth)acrolein and (meth)acr—ylic acid are prepared predomi- nantly by heterogeneously catalyzed gas phase partial oxidation of suitab .le C3/C, pre- cursor compounds, in particular of propene and prop=ane in the case of ac rolein and acrylic acid, or of isobuterne and isobutane in the case of methacrylic acid and of methacrolein. However, a Iso suitable as starting matesrials in addition to pr-opene, pro- pane, isobutene and isobutane are other compounds containing 3 or 4 car-bon atoms, for example isobutanol, n—propanol or the methyl! ethe=r (as a precursor of = C4 precur- sor) of isobutanol. (Meth)=crylic acid can also be obtamined from (meth)acrolein.

The catalysts to be used feor such gas phase partial oxidations are normally solid state multielement oxides.

The heterogeneously cata lyzed gas phase partial oxicdation of C3/C4 precumsors to (meth)acrolein and/or (metth)acrylic acid is typically ca rried out by charging a fixed cata- lyst bed at elevated tempe rature with a charging mixtuare which, in addition to the at least one organic precurscer compound to be partially coxidized, comprises rmolecular oxygen as an oxidant.

IE PF54826

The fixed catalyst bed is norrmnally surrounded by a jacket (for e xample it may be dis- posed in the catalyst tubes of a tube bundle reactor). On this side of the jacket, the exothermic partial oxidation takes place during the residence tirme on the catalyst sur- face, and, on the other side of the jacket, a heat carrier (for exammple a salt bath) is con- ducted, in order to absorb an«d remove the heat of reaction.

In addition, the reaction partraers are generally diluted with a gaas which is substantially inert under the conditions of t he gas phase partial oxidation and, with its heat capacity, is capable of absorbing heat ©f reaction which is released addit ionally and, in most cases, is capable of simultaneously favorably influencing the explosion behavior of the charging gas mixture. In addition, it typically exerts an advantageous influence on the reaction rate. Typically, the inert diluent gases used are noncormbustible gases.

One of the most frequently ussed inert diluent gases is molecula r nitrogen which is al- ways automatically used whe nthe oxygen source for the heterogeneously catalyzed gas phase partial oxidation is air. _As a consequence of its gene=ral availability, another diluent gass which is used in many cases is steam. In many case=s, cycle gas is also used as an inert diluent gas (cf., for -example, EP-A 1180508). Cycle gas refers to the residual gas wvhich remains in the heterogeneously catalyzed g=as phase partial oxidation of the at least one organic pre- «cursor compound when the target product ((meth)acrolein and/or (meth)acrylic acid) is memoved more or less selectively (for example by absorption int o a suitable solvent) rom the product gas mixture. In general, it consists predominamtly of the inert diluent gases used for the heterogeneously catalyzed gas phase partia | oxidation, and also of & he steam typically by-produce=d in the course of the gas phase partial oxidation and of carbon oxides formed by undesired full secondary oxidation. It sometimes contains small amounts of oxygen which has not been consumed in the gas phase partial oxida- tion (residual oxygen) and/or of unconverted organic starting compounds. Typically, only a portion of the residual gas is used as cycle gas. The remaining amount of resid-

Lal gas is generally incinerate d.

Depending on the catalyst charge and reaction conditions selected, the gas phase par- t ial oxidation of the precursor compound may lead predominantly tc (meth)acrolein, or toa mixture of (meth)acrolein and (meth)acrylic acid, or predom inantly to (meth)acrylic aacid.

The reason for this is that the gas phase partial oxidation of suitable C,/C, precursor compounds to (meth)acrylic acid normally proceeds in two successive steps. The first 40 s tep leads to (meth)acrolein ard the second step to (meth)acrylic acid.

". PF54826

In general, the two steps are carried out over different catalyst charges arranged in spatial successior, and the individual catalyst ccharge is tailored to the particular reac- tion step to be catalyzed. This is then also refemred to as a multistage gas hase partial oxidation. In the fi rst stage, predominantly (metzh)acrolein is formed. The product gas mixture leaving th e first stage is subsequently, optionally after intermediate= cooling and/or supplementation of molecular oxygen (fer example in the form of air), conducted directly into the seacond stage, where (meth)acr-olein formed in the first stage is further oxidized to (methacrylic acid.

The temperature iin the particular reaction stages is normally likewise adjust ed to the optimum of the particular reaction step.

It is appropriate fr om an application point of vie w to realize the particular resaction stage in a dedicated reactor (for example in a tube bundle reactor) (cf., for exampole, EP-

A700 893 and EFP-A 700 714).

However, both reaction stages can also be carmried out in a single reactor wshich then generally has more than one temperature zone= (cf., for example, EP-A 1 106 598 and

EP-A 990 636).

However, multiele=ment oxide active compositiosns are also known which ar e capable of catalyzing more than only one step (cf., for exa mple, EP-A 962 253, EP-A 1260 495,

DE-A 10 122 027, EP-A 1 192 987 and EP-A 9 62 253). in such cases, de pending on the selected reaction conditions in a reaction stage, itis possible either to obtain substantially only (met h)acrolein, or a mixture of (meth)acrolein and (meth)acrylic acid, or substantially only (meth)acrylic acid. Nor- mally, such a reaction stage is realized in a rea ctor.

However, it will bee appreciated that a single reaction step can also be carri=ed out in a reactor which, to i mprove the target product sel ectivity, has more than one temperature zone, as recomme=nded, for example, in EP-A 8 106 598, in WO 00/53556, in

WO 00/53559, in WO 00/53557 and in WO 00/553558.

To prepare (meth Dacrolein and/or (meth)acrylic acid by a process for heterogeneously catalyzed gas phase partial oxidation, a starting reaction gas mixture compmrising at least one precursor compound to be partially oxidized, molecular oxygen ass an oxidant and at least one d iluent gas which behaves sutsstantially inertly under the czonditions of the heterogeneously catalyzed gas phase partial oxidation (the charging gaas mixture) is therefore normally conducted through a fixed catalyst bed charge at elevated tempera- 40 ture (generally a feew hundred °C, typically from 100 to 600°C). The chemical conver-

s ' PF54826 sion proceeds during the contact time on the catalyst surface and the heat of reaction is passe=d to a flowing heat exchanger in particular by indirect heat exchange.

A disadvantage of a heterogeneously catalyzed gas phase partial oxicdation carried out in this way is that the heat of reaction has &o be removed on the one haand at a suffi- cient raate to prevent overheating of the sysstem. On the other hand, thes heat removal must not be too fast, since the reaction othmerwise in some cases ceases. Conversely, the rea ction, especially at the beginning, h as to develop heat to a suff icient extent in order tc commence at all. This balance is complicated by the reactant concentration not beimg constant while passing through t he catalyst charge, but rath er decreasing.

In the exit region of the reaction gas mixtuwe from the fixed catalyst be=d, this has the effect ©f reducing the reaction rate and the= associated evolution of he -at, while, in the entrance region of the reaction gas mixture into the catalyst charge, tte high reactant concertration accelerates the exothermic evolution of heat.

The above-described state of affairs is additionally complicated by a faresh fixed catalyst bed no t having steady-state activity behav ior, but rather passing throLugh what is known as a conditioning phase.

In orde=r to prevent excessive, in some casses uncontrolled, localized eevolution of heat when bringing a fresh catalyst charge on stream, WO 02/098827 recommends chang- ing the composition of the charging gas m ixture with time in such a waay that a charging gas mixture having a very low content of tbe organic compound to be partially oxidized (typica Hy from 0 to < 0.5% by volume) is initially used for at least one hour. Subse- quently, the reactant content in the chargirg gas mixture is increased in stages. At the same time as the reactant concentration im the charging gas mixture i ncreases, the reactart ratio is varied. Finally, a charging gas mixture having a substzantially constant compo sition is conducted over the fixed catalyst bed.

As soomn as the charging gas mixture of the organic compound to be oartially oxidized compri ses, the hourly space velocity of ch arging gas mixture on the c atalyst charge is kept constant.

However, the procedure of WO 02/098827 described is disadvantage-ous in that, when it is performed over several operating hours, there is no prevailing subostantially stable chargirg gas mixture composition. This is disadvantageous in that the charging gas mixturez, depending on its composition, car have explosive and nonexplosive states (cf.

DE-A ¥0232482). Frequent changes in its composition should therefo re be avoided.

4 7 a © PF54826

Furthermore, it is snot advantageous to carry out the startup of the gas phase partial oxidation under fu 1lioad (final hourly space veloc ity of charging gas mixture on the fixed catalyst bed), since a high hourly space velsocity on the fixed catamlyst bed causes a short average re=sidence time in the fixed catalyst bed. However, a stort residence 5 time curtails the peeriod available for reaction at the catalyst.

A need exists to s ubstantially remedy the disadvantages of the proced ure of the ac- knowledged prior -art.

We have found thaat this need is fulfilled by a process for preparing (me=th)acrolein and/or (meth)acry lic acid by heterogeneously cat-alyzed gas phase partial oxidation by charging a fresh flixed catalyst bed disposed in a reactor at elevated te -mperature with a charging gas mixt ure which, in addition to at leasst one organic precursor compound to be partially oxidize=d and molecular oxygen as arm oxidant, comprises aut least one dilu- ent gas which belaves substantially inertly undemr the conditions of the heterogeneously catalyzed gas phase partial oxidation, which cormmprises carrying out th e process, after the composition o f the charging gas mixture has been established, at substantially con- stant conversion cf the organic precursor compo und and at substantia ily constant composition of the charging gas mixture, initially over a startup period of from 3 days to 10 days at a low Hourly space velocity and subsequently at a higher hourly space ve- locity, of the charging gas mixture on the catalysft charge. in this context, the= hourly space velocity of charging gas mixture on a —fixed catalyst bed of a reaction stagee refers to the amount of charg ing gas mixture in litemrs at STP (=1(STP); the vol ume in liters which would be ta_ken up under standar—d conditions, i.e. at 25°C and 1 bar—, by the amount of charging ga s mixture in question) which is con- ducted through ore liter of fixed catalyst bed per- hour (upstream and cdownstream beds of pure inert mate=rial are not counted as part of the fixed catalyst bed; in contrast, ho- mogeneous mixtures of shaped inert material bo=dies and shaped catemlyst bodies are counted as being part of the fixed catalyst bed).

The advantage of™ the process according to the irvention over the priomr art process is based on the fact that it curtails excessive heat evolution not by reduc ing the reactant content of the chaarging gas mixture, but rather, xt full reactant content=, by reducing the hourly space velocity of charging gas mixture on the fixed catalyst bec.

The conversion o -f the organic precursor compound (based on single Dass of the charg- ing gas mixture trrough the fixed catalyst bed) is set substantially con:=stantly to the de- sired target conversion. In this context, substantially constantly meanss that the maxi- 40 mum deviation of ~ the arithmetic average converssion over time is not more than +10%,

AMEND ED SHER

- PF5-4326 preferably not more than +5% (the basis is the arithmetic average con version over time ).

Equally, in the present context, “at substantially constant composition of the charging gas mixture” means that the maximum cleviation of the proportion by volume of one of the components (molecular oxygen, organic precursor compound and inert diluent gas) of th e charging gas mixture from the particular arithmetic average proportion by volume over time of the particular component of the charging gas mixture is nt more than + 10% , preferably not more than + 5% (thes basis is the particular arithmestic average pro- porti on by volume over time of the particular component of the chargirmg gas mixture).

The composition of the charging gas mixture, and also the temperatures of the fixed catalyst bed, for the process according t© the invention can in principles be established by the procedure described in WO 02/0938827. However, the period re «quired for this purp ose is normally distinctly below one hour. However, it can also be effected by add- ing, lin a line leading through a static mixer, to the reactor containing trae fixed catalyst bed =charge, initially only inert gas (inclucling steam) (optionally with a content of from 2 to 4%4% by volume of oxygen), then the at least one organic precursor compound and finall y the oxygen source (normally air). “The fixed catalyst bed is brought to the tem- perature at which the low hourly space velocity is required as early as during the inert gas Feed by means of the heat carrier, in order to achieve the target conversion on sin- gle peass through the catalyst charge.

A lov=v hourly space velocity of charging gas mixture on the fixed cataly st bed at sub- stantially constant composition of the charging gas mixture is equivalertto a low hourly spac e velocity of reactant on the fixed catalyst bed.

Whemn the hourly space velocity of chargi ng gas mixture on the fixed catalyst bed is incre ased in the later course of the process according to the invention, this reduces the averaage residence time of the reactants in the catalyst charge. In order to achieve a substantially constant conversion at a sh ort residence time of the at least one organic precursor compound, it is therefore norm ally necessary to increase thes temperature of the h eat carrier used for the indirect heat exchange.

Inthe process according to the invention, low hourly space velocity of charging gas mixtu re on the fixed catalyst bed means that the low hourly space velocity is typically from ~40 to 80%, preferably from 50 to 70%, of the higher desired (final) hourly space velocity for which the reactor including its catalyst charge is designed.

E PF54826

In other worcs, if the reactor and the fixed catalyst bed are designed for a firal hourly space velocifty of, for example, 150 | (STP) of propene/l of fixed catalyst bed «h (the propene con-tent in the charging gas mixture of a propene partial oxidation tco acrolein and/or acrylics acid is typically from 4 to 12% by volume), the inventive 3- to —10-day startup is typeically carried out at an hourly space velocity of 100 | (STP) of p ropenel/l ¢ h. However, the aforementioned startup could also be carried out at appropriate hourly space velocities of from 80 to 120 | (STP) of propene/l «= h.

If a final houmrly space velocity of from 180 to 1 90 | (STP) of propene/l « his i ntended, the inventive 3- to 10-day startup is typically carried out at an hourly space velocity of 120 | (STP) of propenell = h. However, the aforementioned startup could alseo be carried out at approporiate hourly space velocities of from 100 to 140 | (STP) of prop enell « h. in general, the desired final hourly space velocity of organic precursor compoLmnd is at values of > 801 (STP) « h, usually at values of = 1001 (STP)/I + h or of > 12&0 | (STP) + 15h. Final hourly space velocities of 600 | (STP)/1 + h or in many cases 300 | (STP) +h are generallyw not exceeded.

After the startup period, the hourly space velocity can be increased to the dessired final hourly space= velocity sharply, continuously or stepwise.

The advanta- ge of the process according to the invention is that on completicon of the startup phasee of from 3 to 10 days, frequently from 4 to 9 or from Sto 8 dayss, it enables the process tEo be continued at higher hourly space velocity with comparativealy in- creased targ- et product selectivity and at the same time comparatively lower heat car- rier temperat-ure. The basis for comparison in this case is a shortened or ab=sent startup phase at low er hourly space velocity. The process also enables minimum hotspot tem- peratures (th e term for the highest temperature within the fixed catalyst bed flowed through by reaction gas mixture).

Useful fixed Eoed catalysts for the process according to the invention for preparing (meth)acrole in, in particular for preparing acrolein from propene, are all of trmose whose active composition is at least one multimetal oxide containing Mo, Bi and Fe=. They are to be referred to here as fixed bed catalysts 1.

In other words, the fixed bed catalysts 1 used may in principle be any of those disclosed in tthe documents DE-C 3338380, DE-A 19902562, EP-A 15565,

DE-C 23807635, EP-A 807465, EP-A 279374, [DE-A 3300044, EP-A 575897, US-

A 4438217, [DE-A 19855913, WO 98/24746, D E-A 19746210 (those of the g eneral formula Il), JFP-A 91/294239, EP-A 293224 and EP-A 700714. This is true in particular 40 of the exemp lary embodiments in these documents, among which particular preference

’ PF54826 is given to those of EP-A 15565, EP-A 575897, DE-A 197465210 and DE-A 198555913.

In this context, particular emphas is is given to a catalyst according to example 1c of

EP-A 15565 and also to a catalyst to be prepared in a corre-sponding manner but whose active composition, has thee composition Mo 1,Nig sZN=Fe2BitPo.00s5K0.060x 1 0SiO,.

Emphasis is further given to the example having the serial mo. 3 of DE-A 198559 13 (stoichiometry: Mos,CorFe3BiosK.0sSi160x) as an unsupported hollow cylinder catalyst of geometry 5 mm x 3 mm x 2 mam (external diameter x height x internal diamete=r) and also to the multimetal oxide Il unsupported catalyst accordirg to example 1 of

DE-A 19746210. The multimetal oxide catalysts of US-A 44 38217 should also bes mentioned. The latter is particula rly true when these hollow cylinders have a gecometry of 5mm x2 mm x 2mm, or 5 mrnx 3mm x 2mm, or6mmix 3 mmx 3mm, or 7mm x 3 mm x 4 mm (each external diameter x height x internal diameter).

A multitude of the multimetal oxicle active compositions suit able for fixed bed catalysts 1 can be encompassed the gene ral formula

Mo12BiaF 8X X2uX2X On 0 where the variables are defined &s follows: X'= nickel and/or cobalt,

X2 = thallium, an alkali metal and/or an alkaline earth metal x2 = zinc, phosphorus, arsenic, tooron, antimony, tin, cerium, lead and/or tungsten,

X* = silicon, aluminum, titanium and/or zirconium, a=05to05, b = 0.01 to 5, preferably 2 to 4, ¢ = 0 to 10, preferably 3 to 10, d = 0 to 2, preferably 0.02 to 2, e = 0 to 8, preferably 0 to 5, f=0to 10 and n = a number which is determine=d by the valency and frequaency of the elements other than oxygen in I.

They are obtainable in a manner known per se (see, for example, DE-A 4023239) and are customarily shaped in bulk to give spheres, rings or cyl®nders or used in the form of coated catalysts, i.e. preshaped inert support bodies coatecd with the active composition. However, it will be appreciated that they may -also be used in powcder form as fixed bed catalyst 1. It will be appreciated that the fixed bed catalyst 1 used nay also be the multimetal oxide catalyst ACS-4 from Nippon Shhokubai comprising E3i, Mo 40 and Fe in accordance with the in vention.

PF54826

Ld

In principle, active compositions suitable for fixed bed catalysts 1, in gparticular those of the general formula |, can be prepared in a simple manner by generaating a very intimate, preferably finely divided dry mixture having a composition ceorresponding to its stoichiometrsy from suitable sources of its eleemental constituents and calcining it at temperaturess of from 350 to 650°C. The calcination may be effected either under inert gas or under an oxidative atmosphere, for esxample air (mixture of inert gas and oxygen), or Linder a reducing atmosphere (for example a mixture of imert gas, NH, CO and/or H,). The calcination time may be fron a few minutes to a few hours and customarily falls with temperature. Useful sources of the elemental ceonstituents of the multimetal o=xide active compositions | inclucle those compounds whisch are already oxides and/car those compounds which can oe converted to oxides by heating, at least in the preserce of oxygen.

In addition to the oxides, other useful starting compounds are in particular halides, nitrates, formates, oxalates, citrates, acetatems, carbonates, amine co mplexes, ammonium ssalts and/or hydroxides (compounds such as NH,OH, (N H,)2CO3, NH4NO3,

NH,CHO,, C=H;COOH, NH,CH;3CO, and/or ammonium oxalate whichm decompose and/or fall apart on subsequent calcining at the latest to give compounds released in gaseous forrm may additionally be incorporated in the intimate dry mi_xture).

The intimate mixing of the starting compounds for preparing multimeital oxide active compositions | may be effected in dry or wet form. Where it is effecte=d in dry form, the starting comgoounds are advantageously use=d as fine powders and smubjected to calcination a—fter mixing and optionally compacting. However, prefere nce is given to effecting the intimate mixing in wet form. Cusstomarily, the starting co mpounds are mixed with e ach other in the form of an aqueous solution and/or susfoension.

Particularly irmtimate dry mixtures are obtained in the mixing process =described when the starting raaterials are exclusively sources of the elemental constituents present in dissolved form. The solvent used is preferably water. The aqueous ceomposition obtained is then dried, and the drying procedure is preferably effecte=d by spray drying the aqueous mixture at exit temperatures of from 100 to 150°C.

The multime®al oxide compositions suitable i n accordance with the in vention as fixed bed catalysts 1, in particular those of the gereral formula |, may be u sed for the process according to the invention either in powder form or shaped irto certain catalyst geometries, i n which case the shaping may Ioe effected before or aftesrthe final calcination. FFor example, unsupported catalysts may be prepared fro m the powder form of the aective composition or its uncalcin ed and/or partially calcin ed precursor 40 composition Boy compacting to the desired ca talyst geometry (for exarmple by tableting,

E PF54826 extruding or pressing to gi ve strands), in the course of which assistants, for example syraphite or stearic acid as lubricants and/or shaping assistants and reinforcing agesnts =such as microfibers of glass, asbestos, silicon carbide or potassium titanate, may «optionally be added. Examples of useful unsupported catalayst geometries include solid agylinders or hollow cylindesrs having an external diameter amd a length of from 2 tc —10 mm. In the case of the hollow cylinders, a wall thicknesss of from 1to 3 mm is =ppropriate. It will be appr eciated that the unsupported catalyst may also have =spherical geometry, in whi ch case the sphere diameter maw/ be from 2 to 10 mm.

Mt will be appreciated that the pulverulent active compositior or its pulverulent precursor composition which has as yet only been partially calcined, if at all, may also be shaaped

Eoy applying to preshaped inert catalyst supports. The coatirg of the support bodie=s for oreparing the coated catal ysts is generally performed in a suitable rotatable vesse-|, as disclosed, for example, in DE-A 2909671, EP-A 293850 or EP-A 714700. To coat the ssupport bodies, the pulverwulent composition to be applied iss appropriately moisten ed and dried again after appli cation, for example by means of Ehot air. The layer thickress

Of the pulverulent composi tion applied to the support bodies is appropriately selected within the range from 10 to 1000 mm, preferably within the mange from 50 to 500 nm =nd more preferably withirs the range from 150 to 250 mm.

Useful support materials nay be customary porous or nonp» orous aluminum oxides, ssilicon dioxide, thorium dio xide, zirconium dioxide, silicon carbide or silicates such as rmagnesium or aluminum s ilicate. The support bodies may have a regular or irregu lar shape, although preference is given to regularly shaped support bodies having distinct ssurface roughness, for example spheres or hollow cylinderss. The use of substantially ronporous, spherical supp orts having surface roughness ard made of steatite whose diameter is from 1 to 8 mm, preferably from 4 to 5 mm, is suitable. However, usefuml ss upport bodies are also cylinders whose length is from 2 to 10 mm and whose extemal diameter is from 4 to 10 mr. In the case of rings suitable ass support bodies according too the invention, the wall th ickness is further customarily frorm 1 to 4 mm. Annular s: upport bodies to be used “with preference according to the invention have a lengti— of fr-om 3 to 6 mm, an external diameter of from 4 to 8 mm and a wall thickness of frorm 1 to 2 mm. In particular, rings of geometry 7 mm x 3 mm x 4 rm (external diameter = le=ngth x internal diameter) are also suitable as support bodies according to the irvention. It will be appreciated that the fineness of the catal ytically active oxide ceompositions to be applied to the surface of the support bod y is adapted to the des-ired cwoating thickness (cf. EP-A 714 700).

V*ultimetal oxide active conmpositions to be used according to the invention which awe 40 agppropriate as fixed bed catalysts 1 are also compositions off the general formula Il

PF54826 [Ya YZ Oude Yoe Ya Y 5.8 Y7Y%,00]q { 1) where the variables are defined as follows:

Y' = bismuth, tellurium, antimony, tin and/or copper,

YZ = molybdenum and/or tungsten,

Y> = an alkali metal, thallium and/or samarium,

Y#+ = an alkaline earth metal, nickel, cob alt, copper, manganese, zinc, tir, cadmium amd/or mercury,

Y* = iron, chromium, cerium and/or vanadium,

Y& = phosphorus, arsenic, boron and/or antimony,

Y7 = arare earth metal, titanium, zirconium, niobium, tantalum, rhenium, ruthenium, rheodium, silver, gold, aluminum, gallium, indium, silicon, germanium, leaad, thorium amad/or uranium, a’ =0.01t08, b’ =0.1to 30, c¢ =0to4, dd =0to20, e' =0to 20, f=0t%o6, g’ =0to 15, h’ =810 16, x, y =numbers which are determined by the valency and frequency of the elements otter than oxygen in Il and p, q= numbers whose p/q ratio is from O.1 to 10, co ntaining three-dimensional regions of the chemical composition Y',Y2 LO, which are de-limited from their local environment owing to their different compositio nto the local en vironment and whose largest diameter (longest line passing through tlhe center of the region and connecting two points on the surface (interface) of the region )is from 1 nm to 100 um, frequently from 10 nm to 500 nm or from 1 pm to 50 or 25 pm.

Particularly advantageous multimetal oxide compositions ll according to the invention are those in which Y' is bismuth.

Armong these, preference is given in turm to those which correspond to tie general for mula ill 40 [Big Z2-O ly [22122%-Z 4 Fee 22-284 Z Oy) (IN)

E PF548R26 where the variants are defined as fol lows:

Z2 = molybdenum and/or tungsten, Z%= nickel and/or cobalt,

Z* = thallium, an alkali metal and/or &n alkaline earth metal,

Z® = phosphorus, arsenic, boron, antimony, tin, cerium and/or lead,

Z% = silicon, aluminum, titanium and/ or zirconium,

Z’ = ceopper, silver and/or gold, a”"=0 .1to1, b"=0 .2t02, c"=3 1010, d”=0 .02to 2, e”=0 .01 to 5, preferably 0.1 to 3, f'=0 105, g"=0 to10, h"=0 to1, x”, y" =numbers which are determin ed by the valency and frequency of the elements other ®&han oxygen in lll, p”.q” =numbers whose p”/q” ratio iss from 0.1 to 5, preferablyw from 0.5 to 2, and veary particular preference is giveen to those compositions Il where Z%- = (tungssten),- and Z%;, = (molybdenum )r.

Itis fu rther advantageous when at le ast 25 mol% (preferably at least 50 mol% and more goreferably at least 100 mol%) Of the entire [Y's Y2,Ox]p( CBia-Z%-Ox lpr) proportion of the multimetal oxide compositions Il (multimetal oxide comgoositions Ill) suitable according to the invention as fixed bed catalysts 1 in the multmmetal oxide compositions

Il (mul timetal oxide compositions Ill) suitable according to the invention is in the form of three-edimensional regions of the che=mical composition YY 04(Bia-Z%-Ox] which are delimited from their local environmerit owing to their different echemical composition from their local environment and whose largest diameter is in the range from 1 nm to 100 prm.

With re=gard to the shaping, the same applies to multimetal ox ide composition Il catalyssts as was said for multimetal Oxide composition | catalysts.

The preparation of multimetal oxide composition Ii active com positions is described, for example, in EP-A 575897 and also im DE-A 19855913.

E PF54826

It is appropriate from an application point of view to carry out the heterogeneously cata- lyzed gas phase partial oxidation of an organic precursor compound to (meth)acrolein in a tube bundle reactor charged with the fixed bed catalysts 1 as described, for exam- ple, in EP-A 700714.

In other words, in the simplest manner, the fixed bed catalyst 1 to be used is disposed in the metal tubes of a tube bundie reactor and a heating medismum (one-zone method), generally a salt melt, is conducted around the metal tubes. Sal—t melt and reaction gas mixture can be conducted in a simple cocurrent or countercurrent flow. However, the salt melt (the heating medium) can also be conducted, viewed over the reactor, ina meandering manner around the tube bundles, in such a way thm at only viewed over the entire reactor does a cocurrent or countercurrent to the flow dir—ection of the reaction gas mixture exist. The flow rate of the heating medium (heat exchange medium) is typically such that the temperature rise (caused by the exotheramicity of the reaction) of the heat exchange medium from the entry point into the reactomr to the exit point out of the reactor is from > 0 to 10°C, frequently from > 2 to 8°C, oftemn from > 3 to 6°C. The entrance temperature of the heat exchange medium into the tu be bundle reactor, es- pecially in the case of the conversion of propene to acrolein, is generally from 310 to 360°C, frequently from 320 to 340°C.

Suitable heat exchange media are in particular fluid heating media. itis particularly favorable to use melts of salts such as potassium nitrate, potasssium nitrite, sodium nitrite and/or sodium nitrate, or of low-melting metals such as s odium, mercury and also alloys of different metals.

Itis appropriate to feed the charging gas mixture to the charge of fixed bed catalyst 1 preheated to the desired reaction temperature.

Especially in the case of the desired high (for example > 1601 ( STP)/l + h, but generally <600 | (STP)/I « h) final hourly space velocities of the at least ore organic precursor compound to be partially oxidized (for example of propene) on ®&he charge of fixed bed catalyst 1, it is appropriate to carry out the process according to the invention in a two- zone tube bundle reactor. A preferred variant of a two-zone tub-e bundle reactor which can be used in accordance with the invention is disclosed by DEE-C 2830765. However, ~the two-zone tube bundle reactors disclosed in DE-C 2513405, US-A 3147084, DE-

A 2201528, EP-A 383224 and DE—A 2903218 are also suitables.

Jn other words, in the simplest manner, the fixed bed catalyst 1 to be used is disposed 40 ®Enthe metal tubes of a tube bundie reactor and two substantially spatially separated

PF54826 heating media, generally sait melts, are conducted around the metal tubes. The tube section over which the particular salt bath extends represent sa reaction zone. Preferaa- bly, for example, a salt bath A flows around that section of thme tubes (the reaction zone

A) in which, for example, the oxidative conversion of propenee (on single pass) pro- ceeds until a conversion in the range from 40 to 80 mol% is —achieved, and a salt bath B flows around, for example, the section of the tubes (the reaction zone B) in which, for example, the subsequent Oxidative conversion of propene (on single pass) proceeds until a conversion value of generally at least 90 mol% is achieved (if required, further reaction zones which are k ept at individual temperatures mamy follow the reaction zone-5s

A,B).

Within the particular temperature zone, the salt bath can in principle be conducted as in the one-zone method. The temperature of the salt bath B is normally at least 5°C above the temperature of the salt bath A.

Otherwise, the two-zone hi gh load method can be carried out, for example, as de- scribed in DE-A 19948523 or as described in DE-A 199482488.

The example of the gas ph ase partial oxidation of propene teo acrolein will now be use d by way of example to provide more details. The other gas phase partial oxidations ac— cording to the invention of organic precursor compounds cam be carried out in a similar manner.

Accordingly, the process a ccording to the invention is suitabmle for propene hourly space velocities on the fixed bed catalyst charge 1 of > 70 | (STP)/M + h, > 1301 (STP)/I * h, > 1801 (STP) « h, > 2401 (STP)/I +h, 2300 (STP)/I + h, butt normally < 600 | (STP)/| = h.

The inert gas to be used for the charging gas mixture may ¢ onsist of > 20% by volume, or > 30% by volume, or > 40% by volume, or > 50% by volur me, or > 60% by volume, or > 70% by volume, or > 80% by volume, or > 90% by volurme, or > 95% by volume, of molecular nitrogen.

However, at propene hourly space velocities on the fixed be d catalyst charge 1 of above 250 | (STP)! « h, the use is recommended for the process according to the in- vention of inert (inert diluert gases here are intended generally to refer to those of which less than 5%, prefer ably less than 2%, is converted om single pass through the particular fixed bed catalyst charge) diluent gases such as p=ropane, ethane, methane , pentane, butane, CO,, CO, steam and/or noble gases for thes charging gas mixture.

PF54826

With increasing propoene hourly space velocity, the two-zone method described, as already mentioned, i s preferred over the one-zone snethod described.

The working pressure in the process according to tlhe invention in the prope=ene partial oxidation may be either below atmospheric pressur-e (for example down to 0.5 bar) or above atmospheric pressure. Typically, the working pressure will be at valmues of from 1 to 5 bar, frequently f rom 1.5 to 3.5 bar. Normally, the reaction pressure in fhe propene partial oxidation will not exceed 100 bar.

The molar 0,:CsHg ratio in the charging gas mixtures is normally = 1. Typically, this ratio will be at values < 3.. Frequently, the molar O:C3sHe ratio in the charging g=s mixture wilbe=1.5and £2 .0.

Useful sources for tke molecular oxygen required &are either air or, for exarmple, air de- pleted in molecular mitrogen (for example = 90% by volume of O,, £ 10% k=y volume of

Na).

The propene fractior in the charging gas mixture nay be, for example, at wvalues of from 4 to 15% by vo lume, frequently from 5 to 12%. by volume or from 5 tom 8% by vol- ume (based in each case on the total volume).

Frequently, the proc-ess according to the invention ~will be carried out at a yoropene : oxygen : inert gases: (including steam) volume ratio in the starting reaction gas mixture (charging gas mixtur—e) of 1:(1.0 to 3.0):(5 to 25), preferably 1:(1.5 to 2.3):( 10 to 15).

However, the chargi ng gas mixture compositions of DE-A 10313209 can also be em- ployed in accordance with the invention.

To prepare acrylic a-cid from acrolein, useful [lacuna] for the process according to the invention are all thosse whose active composition is at least one muitimetal oxide con- taining Mo and V. They are to be referred to here as fixed bed catalysts 2.

Such suitable fixed toed catalysts 2 can be taken, for example, from US-A 3775 474,

US-A 3 954 855, USS-A 3 893 951 and US-A 4 339 355. Also particularly suitable are the multimetal oxide compositions of EP-A 427 5083, DE-A 2 909 671, DE —

C 31 51 805, DE-ASs 2 626 887, DE—A 43 02 991, EEP-A 700 893, EP-A 7 14 700 and

DE-A 19 73 6105 foer fixed bed catalysts 2. Particular preference is given i n this context to the exemplary emmbodiments of EP-A 714 700, and also of DE-A 1973 6105.

A multitude of the mmultimetal oxide active composit ions suitable as fixed be=d catalysts 40 2 can be encompasssed by the general formula IV

PF54826

MO 15V a X XZ XXX 5X On (IV) in which the variables are defined as follows:

X'= W, Nb, Ta, Cr and/or Ce,

X?= Cu, Ni, Co, Fe, Mn and/or Zn,

X*= Sb and/or Bi,

X*= one or more alkali metals,

M40 X°= one or more alkaline earth metals, x8 = Si, Al, Ti and/or Zr, a= 1to6, b= 02to4, c = 05t018, d= 0t!040, e = 0to2, f= O0to4, g = 0to40and n = a number which is determined by the valency and frequency off the elements other than oxygen in IV.

Preferred embodiments among the active multimetal oxides IV are thmose which are encompassed by the following definitions of the variables of the genezral formula IV:

X'= W, Nb, and/or Cr,

X?= Cu, Ni, Co, and/or Fe,

X3= Sb,

X*= Na and/or K,

X5= Ca, Sr and/or Ba,

X®= Is, Al and/or Ti, a= 15t05, b= 05to2, c= 05t03, d= 0to2 e= 0to0.2 f= 0to1and n = a number which is determined by the valency and frequency of the elements other than oxygen in IV.

PF $4826

Ho=wever, very particularly preferr ed multimetal oxides IV a re those of the general fom- mula Vv

MoVaY lu Y2 Yo YS,0, (V) wheere

Y'== W and/or Nb,

Y? = Cu and/or Ni,

Y® = Ca and/or Sr,

Y® = Siandlor Al a'= 2to4, b= 1to1.5, c= 1to3, f= 0to05 g'= 0to8and n' = a number which is determired by the valency and frequency of the elements other than oxygen in V.

The multimetal oxide active comps ositions (IV) suitable acccording to the invention are= obt ainable in a manner known per se, for example as disclcosed in DE-A 4335973 or in

EP—A 714700.

In principle, multimetal oxide active compositions suitable &according to the invention for fixe=d bed catalysts 2, in particular those of the general formula IV, can be prepared im a simaple manner by generating a veary intimate, preferably fin ely divided dry mixture hawing a composition correspondi ng to its stoichiometry fro m suitable sources of its elemental constituents and calcining it at temperatures of from 350 to 600°C. The calcination may be effected either under inert gas or under an oxidative atmosphere, for example air (mixture of inert gas and oxygen), or else urder a reducing atmosphezre (for- example mixtures of inert gas and reducing gases such as Hz, NH, CO, methan e ancd/or acrolein, or the reducing gases alone). The calcinatison time may be from a fexw mimmutes to a few hours and custormarily falls with temperature. Useful sources of the elermental constituents of the mult imetal oxide active compositions IV include those compounds which are already oxicles and/or those compou nds which can be convertzed to cmxides by heating, at least in thes presence of oxygen.

The= intimate mixing of the starting compounds for preparingg multimetal oxide active compositions IV may be effected im dry or wet form. Where itis effected in dry form, t he 40 starting compounds are appropriately used as fine powders and subjected to

PF54826 calcination after mixing and optionally compacting. H owever, preference is given to effecting the intimates mixing in wet form.

Customarily, the starting compounds are mixed with each other in the form off an aqueous solution ame dior suspension. Particularly intimate dry mixtures are ototained in the mixing process described when the starting materials are exclusively sou rces of the elemental constituerits present in dissolved form. Thee solvent used is prefera: bly water.

The aqueous compcsition obtained is then dried, ancd the drying procedure iss preferably effected toy spray drying the aqueous mixture at exit temperatures of from 100 to 150°C.

The muitimetal oxides compositions suitable in accorcdance with the invention as fixed bed catalysts 2, in psarticular those of the general formmula IV, may be used fo rthe process according teo the invention either in powder form or shaped into certain catalyst geometries, in which case the shaping may be effected before or after the firal calcination. For exa ample, unsupported catalysts may be prepared from the p owder form of the active composition or its uncaicined prec ursor composition by cormpacting to the desired catalyst geometry (for example by tab leting, extruding or presssing to give strands), in the cour—se of which assistants, for exam ple graphite or stearic acid as lubricants and/or sh aping assistants and reinforcing agents such as microfibe=rs of glass, asbestos, sili=con carbide or potassium titanate, may optionally be add=ed.

Examples of useful unsupported catalyst geometries include solid cylinders or hollow cylinders having an external diameter and a length of from 2 to 10 mm. In the case of the hollow cylinders=, a wall thickness of from 1 to 3 rnm is appropriate. It will be appreciated that thee unsupported catalyst may also have spherical geometry=, in which case the sphere diammeter may be from 2 to 10 mm.

It will be appreciate «d that the pulverulent active com position or its pulverulermt precursor composition which Fas not yet been calcined may altso be shaped by applyirag to preshaped inert catalyst supports. The coating of thes support bodies for prepoaring the coated catalysts is generally performed in a suitable rotatable vessel, as disclosed, for example, in DE-A 2 909671, EP-A 293859 or EP-A 7714700.

To coat the support bodies, the pulverulent composi tion to be applied is appwopriately moistened and drieed again after application, for exarmple by means of hot air-. The layer thickness of the pul-verulent composition applied to t he support bodies is appropriately selected within the mrange from 10 to 1 000 pm, preferably within the range freom 50 to 500 ym and more pereferably within the range from 1 50 to 250 um.

PF54826

Useful support ma terials may be customary porous or nonporous aluminum oxides , silicon dioxide, thorium dioxide, zirconium dioxide, silicon carbide or silicates such =s magnesium or alumminum silicate. The support bodies may have a regular or irregular shape, although pmreference is given to regularly shaped support bodies having dist inct surface roughnesss, for example spheres or hollow cylinders. The use of substantia 1ly nonporous, spheri cal supports having surface rough ness and made of steatite who-se diameter is from 1 to 8 mm, preferably from 4 to 5 mm, is suitable. However, usefull support bodies are also cylinders whose length is from 2 to 10 mm and whose exte=rnal diameter is from 4 to 10 mm. In the case of rings suitable as support bodies accorclling to the invention, tke wall thickness is further customarily from 1 to 4 mm. Annular support bodies to ‘be used with preference according to the invention have a lengtha of from 3 to 6 mm, ar external diameter of from 4 to 8 rnm and a wall thickness of frorm 1 to 2 mm. In particular, rings of geometry 7 mm x 3 mim x 4 mm (external diameter =< length x internal d:iameter) are also suitable as support bodies according to the invention. It will bea appreciated that the fineness of the catalytically active oxide compositions to bez applied to the surface of the suport body is adapted to the dessired coating thickness (cf. EP-A 714 700).

Favorable multimestal oxide active compositions to be used in accordance with the #in- vention as fixed bead catalysts 2 are also compositions of the general formula VI, [DIE] (VI) in which the varialoles are defined as follows:

D = MouValZ'wZiZ3Z 2% 20x,

E = Z",,Cu H:Op,

Z' = W,Nb_ Ta, Crand/or Ce,

Z?> = Cu, Ni_ Co, Fe, Mn and/or Zn, z* = SbandiorBi,

Z* = Li Na, K, Rb, Csand/orH

Z° = Mg, Ca, Srandior Ba,

Z° = Si Al TTiandlor Zr,

Z' = Mo, W,V,Nbandlor Ta, a” = 1to 8, b” = 0.2 to 5, c’ = Oto 23 , d" = Oto 50 , 40 ee" = 0to 2,

‘ PF54826 f = 0to 5, g = 0 to 550, h" = 4 to 30, i" = 0 to 20 and x"y’ = numbmers which are determined by time valency and frequenc y of the ele- mentss other than oxygen in VI and

P.q = numbers other than zero whose p/q ratio is from 160:1 to 1:71, and which are obtainable by separately preforming a multimetal oxide composition E

Z’2CupHyOy (E) in finely divided #form (starting composition 1) ard subsequently incorpcarating the pre- formed solid stamting composition 1 into an aqueous solution, an aqueo us suspension orinto a finely divided dry mixture of sources off the elements Mo, V, Z' , Z%, 2°, Zz, 2° 7% which contaimns the aforementioned elements in the stoichiometry D

MoV Z 22022022 25% 20 (D) (starting compossition 2) in the desired p:q ratio, drying, where appropriate, the resulting aqueous mixtures, and calcining the resulting dry precursor compositiorm, before or after its drying to the desired catalyst geometry, at temperatures of from 250m to 600°C.

Preference is given to the muitimetal oxide corpositions VI in which th e preformed solid starting commposition 1 is incorporated intc> an aqueous starting co mposition 2 at a temperature of << 70°C. A detailed description Of the preparation of mul-timetal oxide composition VI czatalysts is contained, for example, in EP—-A 668104, D E-A 19736105 and DE-A 1952_8646.

With regard to tte shaping, the same applies to multimetal oxide composition VI cata- lysts as was sai=d for the multimetal oxide composition IV catalysts.

Appropriately from an application point of view, the inventive heterogeneously cata- lyzed gas phase partial oxidation of acrolein to acrylic acid will be carrieed out in a tube bundle reactor charged with the fixed bed cata lysts 2, as described, fom example, in

EP-A 700893.

In other words, &n the simplest manner, the fixeaed bed catalyst 2 to be u sed is disposed in the metal tube=s of a tube bundle reactor anc¥ a heating medium (one=-zone method), 40 generally a salt melt, is conducted around the mmetal tubes. Salt melt ard reaction gas

PF54826 mixture can be conducted in simple cocurrent or countercurrent. However, the heatin g medium (the salt melt) can also be conducted, viewed «over the reactor, in a meander—- ing manner around the tube bundles, in such a way thaat only when viewed over the entire reactor does a cocurrent or countercurrent to thes flow direction of the reaction gas mixture exist. Th e flow rate of the heating medium (heat exchange medium) is typically such that thes temperature rise (caused by the exothermicity of the reaction) of the heat exchange nmaedium from the entry point into thee reactor to the exit point from the reactor is from 0 to 10°C, frequently from 2 to 8°C, often from 3 to 6°C. The entry temperature of the hesat exchange medium into the tubee bundie reactor is generally from 230 to 300°C, frequently from 245 to 285°C or fro m 245 to 265°C. Suitable heat exchange media are the same fluid heating media as already described for the inverm- tive heterogeneously catalyzed gas phase partial oxidation of an organic precursor compound to (meth)aacrolein.

The charging gas mi xture is appropriately fed to the charge of fixed bed catalyst 2 pr-e- heated to the desirecd reaction temperature.

Especially in the cas-e of desired high (for example > 1 40 1 (STP) * h, but generally < 600 | (STP) = h) fi nal hourly space velocities of acrolein on the charge of fixed becd catalyst 2, it is appropriate to carry out the process, according to the invention in a tv-vo- zone tube bundle reaactor. A preferred variant of a two—zone tube bundle reactor which can be used in acco rdance with the invention for this purpose is disclosed by DE—

C 2830765. Howeve=r, the two-zone tube bundle react«ors disclosed in DE-C 251340»5,

US—A 3147084, DE—A 2201528, EP—A 383224 and D E-A 2903582 are also suitable.

In other words, in a ssimple manner, the fixed bed cata. lyst 2 to be used in accordanc e with the invention is disposed in the metal tubes of a t ube bundle reactor, and two stib- stantially spatially sesparated heating media, generally salt melts, are conducted around the metal tubes. The tube section over which the parti cular salt bath extends repre- sents areaction zorme.

Preferably, for exammmple, a salt bath C flows around these sections of the tubes (the reaction zone C) in which the oxidative conversion of acrolein (on single pass) pro- ceeds until a conversion value in the range from 55 to» 85 mol% is achieved, and a s=alt bath D flows around the section of the tubes (the reaction zone D) in which the subsse- quent oxidative conversion of acrolein (on single pass=) proceeds until a conversion value of generally at: least 90 mol% is achieved (if req uired, further reaction zones which are kept at individual temperatures may follow the reaction zones C,D to be e m- ployed in accordanc=e with the invention).

E | PF54826

Within the particular temperature zone, the salitbath can in principle be cconducted as in the one-zone method. The temperature of the salt bath D is normally at least 5t0 10°C above the temperature of the salt bath C.

Otherwise, the twwo-zone high-load method cam be carried out as descriloed, for exam- ple, in DE-A 19948523 or as described in DE-_A 19948248.

Accordingly, the process according to the invention is suitable for acrole=in hourly space velocities on the fixed bed catalyst charge 2 of >701(STP)I<h, 21301 (STP)/I+h, 2 1801 (STP) +h, 22401 (STP) +h, = 300! (S-TP)! +h, but normally < 6«00 | (STP)/l « h.

The inert gas to be used for the charging gas mixture may consist of = 20% by volume, or > 30% by volume, or > 40% by volume, or = 50% by volume, or > 60% by volume, or > 70% by volum e, or = 80% by volume, or > 9 0% by volume, or 2 95% boy volume, of molecular nitrog en.

If the gas phase= partial oxidation of acrolein is the second reaction stag e of a two-stage gas phase partial oxidation of propene to acry~lic acid, the inert diluent g as will fre- quently consist ©f from 5 to 20% by weight of H,O (is formed in the first reaction stage) and of from 70 t © 90% by volume of Na.

However, at acrolein hourly space velocities on the fixed bed catalyst 2 of above 250 (STP) + h, it is Tecommended for the processs according to the inventiom to use inert diluent gases such as propane, ethane, methane, butane, pentane, CO», CO, steam and/or noble ga ses. It will be appreciated that: these gases can also be used even at relatively low acsrolein hourly space velocities —

The working pressure in the gas phase partia | oxidation of acrolein mays be either below atmospheric pressure (for example up to 0.5 bar) or above atmospheric pressure.

Typically, the weorking pressure in the gas phaase partial oxidation of acr-olein will be at values of from 1% to 5 bar, frequently from 1 to 3 bar.

Normally, the rezaction pressure in the acroleimn partial oxidation will not exceed 100 bar.

The molar O,:accrolein ratio in the charging ges mixture of the fixed bed catalyst charge 2 tube will norma ally be > 1. Typically, this ratio will be at values of < 3. According to the invention, the maolar O,:acrolein ratio in the aforementioned charging gaas mixture will be from 1 to 2 or from 1 to 1.5. In many casess, the process according to the invention in the case of acrolein partial oxidation will be- performed at an

PF54826 acrolein:oxygen:ssteam:inert gas volume ratio (| (STP™)) in the charging gas mixtzure of 1:(1 to 3) : (0 to 20) : (3 to 30), preferably of 1:(1 to 3-): (0.510 10) : (7 to 10).

The acrolein fraction in the charging gas mixture ma=y be, for example, at value=s of from 3to 15% by volu me, frequently from 4 to 10% by vol ume or from 5 to 8% by volume (based in each case on the total volume).

The heterogeneously catalyzed gas phase partial oxidation of methacrolein to methacrylic acid can be carried out in a similar manrer to that of acrolein to ac=rylic acid. However, the catalysts used are preferably tho se of EP-A 668103. The remaining reaction conditioms are likewise advantageously established in accordance wit h EP-

A 668103.

For the gas phasse partial oxidation of propane to acmrylic acid or of isobutane to methacrylic acid, the muitimetal oxide catalysts used will advantageously be those rec- ommended, for esxample, by the documents DE-A 100248584, DE-A 10029338. DE-

A 10033121, DE -A 10261186, DE-A 10254278, DE—A 10034825, EP-A 962253, EP-

A 1260495, DE-AA 10122027, EP-A 1192987 and DE=-A 10254279.

The reaction corditions can likewise be selected in &ccordance with these documents.

The reactor used will typically be a one-zone reactom.

Finally, it is emplhasized that, in a two-stage heteroggeneously catalyzed gas pase par- tial oxidation for preparing (meth)acrylic acid (for example from propene to acr-olein (1 stage) and then acrolein to acrylic acid (2™ stage)), in which the product gas rnixture of the first stage, opotionally after cooling and metering in of air as an oxygen souwce, is conducted into the second stage, application of the gorocess according to the irvention to the first stage is automatically accompanied by application of the process aeccording to the invention to the second stage.

It is also emphassized that, in the process according to the invention, at the tramnsition from the low howarly space velocity to the higher housrly space velocity, it may bee appro- priate to slightly reduce the cycle gas fraction in the charging gas mixture (the reactant fraction in the chwarging gas mixture then rises slightly). The aforementioned is appro- priate when the design of the cycle gas compressor limits the maximum comp ressible amount of gas. Finally it is emphasized that the process according to the invertion can also be employe d on freshly regenerated (for example according to EP-A 169449,

EP-A 614872, EFP-A 339119, DE-A 10249797 or DE -A 10350822) catalyst beds dis- 40 posed in reactors.

P*F54826

Example and Comparative Examples aa) Experimental arrangenment

A reaction tube (V2A steel; external diameter 30 mr, wall thickness 2 mm, inter- nal diameter 26 mm, le=ngth: 350 cm and a thermal tube (external diameter 4 mm), centered in the middle of the reaction tube, to accommodate a ther— moelement which can Ibe used to determine the temperature in the reactiorm tube over its entire length) vwas freshly charged from top to bottom as follows:

Section 1: length 8@0 cm

Steatite rings of geometry 7 mm x 7 mmm x 4 mm (external di=ame- ter x length x internal diameter) as a preliminary bed.

Section 2: length 1 0 cm

Catalyst charge of a homogeneous rixture of 30% by weigh tof steatite rings of geometry 5 mm x 3 rmm x 2 mm (external diameter x length x internal diameter) and 70% by weight of unsupported catalyst from section 3.

Section 3: length 1 70 cm

Catalyst: charge of annular (6 mm x 3mm x2 mm = external di- ameter = length x internal diameter) unsupported catalyst ac—cord- ing to Example 1 of DE-A 10046957 (stoichiometry: [Biz2W20- ge 2WO3]o. 5 [M012C0s sFe2.04Si1.59K0.080 1).

The reaction tube is heated by means of a salt batlh pumped in countercurrent. b) Experimental procedumre

The experimental arra ngement described, in each case freshly prepared, v-vas in each case charged co ntinuously with a charging gaas mixture (mixture of ai r, polymer-grade propylesne and cycle gas) of the cormposition 5.4% by volume of propene, 10.5% by volume of oxygen, 1.2% by volume of CO, 81.3% by volume of N, and 40 1.6% by volume of HO,

E PF $4826 and the hourly space velocity and the thermostatting of the reaction tube were varied over time. The reactio n tube was thermostatted in stuch a way that the propene conversion C (Mol%a) on single pass of the chargimg gas mixture through the reaction tube continuous ly was about 95.0 mol%.

The tables which follow show the product of value selectivities SP (mol%) (sum of the selectivity of acrolein forrmation and the selectivity of acrylic acid formation) achieved and also the maxirmum temperatures Tmax measu red along the reaction tube in °C, as a function of tie hourly space velocity on the= fixed catalyst bed (expressed as propene loading in | (STP) h) and the salt bath temperature Ts (°C). The desired final hourly, space velocity was 150 | (STE2)/l « h. The results re- ported always relate to the e=nd of the particular operating foeriod.

Exxample

Operating period Propene loading | C Ss” LE Tmax rn imo [ia 5) [5

Comparative Example

Operating period Propene loading | C Ss" Ts T max a =r re pO

A comparison of example and corparative example shows that ‘when the fresh catalyst charge is immediately brought on stream under the desired final hourly space velocity, th is results in a catalyst charge which requires significantly higher salt bath tempera- tu res for the same conversion. Thee higher maximum temperaturess additionally cause premature aging of the catalyst chaarge.

US Provisional Patent Application No. 60/49814, filed on August 14, 2003, is in <corporated into the present application by literature reference.

« 0, PF54826

With regard to the abovementioned teachings, numerous altterations and deviations from the present invention are possible. it is therefore possible to assume that the invention, within the scope of the appended claims, can be poerformed in a different way from that specifically described herein. “Comprises/comprising” when used in this specification is tzmken to specify the pres- ence of stated features, integers, steps or components but cdoes not preclude the pres- ence or addition of one or more other features, integers, ste=ps or components or groups thereof.

AMENDED SHEET

Claims (6)

« V. : PF548265 We clair;

1. A porocess for preparing (meth)acrolein aryd/or (meth)acrylic acid by hete=rogene- ou sly catalyzed gas phase partial oxidation by charging a fresh fixed catalyst bed dissposed in a reactor at elevated temperature with a charging gas mixtu re which, in addition to at least one organic precursor compound to be partially ox idized and molecular oxygen as an oxidant, comprises at least one diluent gas which be haves substantially inertly under the casnditions of the heterogeneous®y cata- lyz=ed gas phase partial oxidation, which comprises carrying out the process, after thea composition of the charging gas mixture has been established, at sumbstan- tia lly constant conversion of the organic precursor compound and at sutostantially co nstant composition of the charging gas mixture, initially over a startups period of froem 3 days to 10 days at a low hourly sp ace velocity and subsequently ata hicgher hourly space velocity of the chargi ng gas mixture on the catalyst charge.

2. A process as claimed in claim 1, wherein the low hourly space velocity sduring the staartup period is from 40 to 80% of the desired higher final hourly space velocity.

3. A process as claimed in claim 2, whereire the desired final hourly space velocity wilith charging gas mixture, expressed as the final hourly space velocity of organic precursor compound, is > 80 | (STP)/1 = hs.

4. A process as claimed in any one claims 1 to 3, which is for heterogenecously catalyzed gas phase partial oxidation of propene to acrolein and/or acrylic acid.

5. A process according to the invention for preparing (meth)acrolein and/or (rmmeth)acrylic acid, substantially as hereimbefore described or exemplified.

6. A process for preparing (meth)acrolein a nd/or (meth)acrylic acid includang any new and inventive integer or combinatior of integers, substantially as h erein de- scribed. AMENDED SHEET