Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Phosphorus or compound containing same
Reexamination Certificate
2001-06-01
2002-06-18
Bell, Mark L. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Phosphorus or compound containing same
C502S210000, C502S211000, C502S212000, C502S213000, C502S306000, C502S311000, C502S312000, C502S313000, C502S314000, C502S315000, C502S316000
Reexamination Certificate
active
06407030
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
Applicants claim priority under 35 U.S.C. §119 of German Application No. 198 40 224.4 filed Sep. 3, 1998. Applicants also claim priority under 35 U.S.C. §120 of PCT/EP99/06274 filed Aug. 26, 1999. The international application under PCT article 21(2) was not published in English.
BACKGROUND OF THE INVENTION
1) Field of the Invention
The invention relates to a process for preparing catalysts for the partial gas-phase oxidation of saturated and/or unsaturated C
4
-hydrocarbons to form maleic anhydride (MA).
2) Background Art
The preparation of maleic anhydride from C
4
-hydrocarbons has been known for more than 20 years. It is carried out using catalysts based on vanadyl phosphates (vanadyl pyrophosphate, vanadium-phosphorous oxides). These vanadyl phosphates are prepared via a catalyst precursor which can be prepared in aqueous or organic medium. The precursor is then, in a second step before or after shaping, converted either in the reactor (in situ) or externally into the actual catalytic reactive substance.
The reaction of the C
4
-hydrocarbon over the catalyst is carried out in various types of reactor. Reactors used are fixed-bed, fluidized-bed and also riser reactors; in the case of fixed-bed reactors, use is made exclusively of catalysts based on vanadyl phosphates in the form of unsupported catalysts. EP-B 72381 and WO-A 96/25230 disclose the use of coated catalysts too.
The precursor can be prepared either in an aqueous medium or in an organic medium. In industry, preference is given to carrying out the preparation in an organic solvent. Here, isobutanol (2-methylpropan-1-ol) has proven useful as solvent or reaction medium. Compared with the catalysts prepared in an organic medium, those prepared in an aqueous medium have significantly lower activity and selectivity. A cause of the difference is the lower specific surface area of the catalysts prepared by the aqueous route. Furthermore, the use of an organic solvent brings process engineering advantages, namely the precursor formed is easier to separate from the reaction mixture since the product is insoluble in the solvent. The solvent has to meet a number of prerequisites. Thus, for example, it must not react with the phosphorous compounds required for formation of the vanadyl phosphate. In addition, its boiling point has to be within a range in which the formation of the desired vanadyl phosphates is observed. As is generally known, the reaction temperature strongly influences the rate of catalyst formation. Since the reaction is usually carried out in the boiling solvent, the boiling point of the solvent plays an important role. The solvent also has a strong influence on the morphology of the reaction product and sometimes even on the phases formed (Doi, T.; Miyake, T.; Applied Catal. A. General 164 (1997), 141-148).
The vanadium-containing starting compound used is generally vanadium pentoxide. Phosphoric acid generally serves as phosphorous compound. Since the vanadium is present in the oxidation state IV in the target compound, a reduction by means of a suitable reducing agent is necessary. Many compounds have been described for this purpose. Thus, use is made of, for example, gaseous HCl, oxalic acid, benzyl alcohol, isobutanol, hydrazine and also many other reducing agents. The nature of the reducing agent plays a minor role in the reaction.
U.S. Pat. No. 4,132,670 describes a process for preparing vanadyl phosphate catalysts. In this process, vanadium pentoxide is reacted in an alcohol, preferably isobutanol, to give a vanadium(IV) compound. The reaction of vanadium(V) to vanadium(IV) is effected by the solvent used. Subsequently, the vanadium phosphate precursor is prepared using concentrated H
3
PO
4
. The water content of the reaction mixture in the preparation of the precursor is here described as an important parameter and a low water content in the reaction mixture is recommended.
U.S. Pat. No. 4,382,876 discloses a process for preparing catalysts in which a vanadium(V) compound and isobutanol as solvent are placed in a reaction vessel and H
3
PO
3
as reducing agent is added in relatively small amounts. The reduction of the vanadium(V) compound to vanadium(IV) in that invention occurs by means of a combined reduction by the tertiary alcohol and the phosphorous acid. The water formed in this process together with the solvent are distilled off batchwise a number of times. This procedure has the disadvantage that further water is continuously formed in the reaction mixture and this is only distilled off after a certain time. Thus, a continually low water content is not possible, which leads to a poorer performance of the future catalyst.
WO-A 95/29006 likewise discloses a process for preparing vanadyl phosphate catalysts. Here too, a vanadium(V) compound is reacted with phosphoric acid in isobutanol. In. this process, the water content is controlled by addition of highly concentrated phosphoric acid to the reaction mixture in order to absorb the water of reaction formed. The total water content of the reaction mixture is here determined by the phosphorous compounds used (85% H
3
PO
4
, 100% H
3
PO
4
, 106% H
3
PO
4
or polyphosphoric acids) and by the water formed in the reaction (by the reduction of vanadium(V) to vanadium(IV) compounds). Major-disadvantages of this process are the high costs and the difficulty of handling concentrated phosphoric acid and polyphosphoric acid.
The reaction of the starting substances to form the desired vanadyl phosphate can in principle be carried out in various ways:
1. reduction of the vanadium(V) compound to vanadium(IV), subsequent reaction with the phosphorous compound to give vanadyl(IV) phosphate,
2. reaction of the vanadium(V) compound with the phosphorous compound to form the corresponding vanadium(V) phosphate and subsequent reduction to vanadyl(IV) phosphate, or
3. parallel reduction and reaction with the phosphorous compound to give vanadyl(IV) phosphate.
After the reaction is complete, the suspended or dissolved vanadyl phosphate is separated from the reaction mixture by suitable methods, for example filtration or evaporation, dried and converted into the active catalyst in a further step, namely calcination or activation. Shaping for use in fixed-bed reactors is carried out either before or after calcination, for example by tableting or by extrusion. A further possible way of shaping the catalyst is the production of coated catalysts as are also used in other processes, for example the synthesis of phthalic anhydride from o-xylene or naphthalene.
The calcination of the precursor, i.e. the conversion into the actual catalyst, can be carried out either externally or in the reactor (in situ) in which the conversion of the C
4
-hydrocarbons into maleic anhydride is carried out.
U.S. Pat. No. 4,382,876 discloses a process in which the precursor obtained in the first step is converted into the actual catalyst in a second step in the reactor itself. Here, the precursor is activated by heating at a rate of less than 1° C./min in order to avoid hot spots in the catalyst. This in-situ calcination is carried out in the presence of C
4
-hydrocarbon and air and the precursor is heated to a temperature of from 450° C. to 510° C. After this temperature has been held for from about 12 to 72 hours, the calcined material is slowly cooled again.
A major disadvantage of this process is that the final activity of the catalyst is reached only after some hundreds of hours.
Apart from the calcination processes described, external activations are also known. WO-A 95/29006 describes a process for calcination in which the precursor is reacted in an atmosphere of air, water vapour and inert gas. Here, the precursor is slowly heated at a heating rate of less than 1° C./min in a number of steps to a temperature in the range from 350° C. to 550° C. This temperature is held for from 2 to 8 hours, giving a mean oxidation state of vanadium of less than +4.5. U.S. Pat. No. 5,185,455 discloses a similar process for external calcination. Here, the t
Bosch Richard
Eberle Hans-Jürgen
Groke Dirk
Lotz Joachim
Bell Mark L.
Collard & Roe P.C.
Consortium fur elektrochemische Industrie GmbH
Hailey Patricia L.
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