WO2025032125A1

WO2025032125A1 - Quality-optimized process for preparing a vanadium/phosphorous mixed oxide catalyst

Info

Publication number: WO2025032125A1
Application number: PCT/EP2024/072337
Authority: WO
Inventors: Jonglack KIM; Nicolas DUYCKAERTS; Christian Walsdorff
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2023-08-07
Filing date: 2024-08-07
Publication date: 2025-02-13
Anticipated expiration: 2026-02-07

Abstract

Quality-optimized process for preparing a vanadium/phosphorous mixed oxide catalyst A quality-optimized process for preparing a vanadium/phosphorus mixed oxide catalyst (VPO-catalyst) containing crystalline vanadyl pyrophosphate, comprising the steps: • (a) preparing a VPO catalyst precursor containing vanadyl hydrogen phosphate by reacting a mixture containing a vanadium-containing compound, preferably vanadium pentoxide, and a phosphorous- containing compound, preferably phosphoric acid or polyphosphoric acid, and optionally a promotor metal containing compound, in an alcoholic medium, • (b) pre-calcining the VPO catalyst precursor, • (c) optionally shaping the VPO catalyst precursor, • (d) calcining the VPO precursor in an oxygen containing calcination atmosphere, • (e) taking at least one sample of the calcined VPO precursor during or after step (d), • (f) thermally treating, in at least one temperature treatment step, the calcined VPO catalyst precursor in an inert gas atmosphere containing steam at a higher temperature than in the calcination step (d), wherein the final VPO catalyst is obtained, • (g) determining, via XRD, the content of crystalline vanadyl pyrophosphate of at least one sample taken in step (e), • (h) based on the result of step (g), modifying the process conditions of step (d) with regard to one or more of the process parameters selected from calcination time, calcination temperature and composition of the calcination atmosphere in order to ensure that the content of crystalline vanadyl pyrophosphate in the calcined VPO precursor is at least 45% by weight before step (f) is carried out. The VPO catalyst thus obtained is used for producing maleic anhydride from n-butane or benzene.

Description

Quality-optimized process for preparing a vanadium/phosphorous mixed oxide catalyst

Description

The invention relates to a process for preparing a vanadium/phosphorous mixed oxide catalyst (VPO catalyst), the vanadium phosphorous mixed oxide catalyst obtainable by the process and the use of the catalyst for producing maleic anhydride.

Numerous VPO catalysts, containing mixed oxides of vanadium and phosphorus, substantially in the form of vanadyl pyrophosphate, and optionally containing a promoter component, have been previously disclosed as being useful for the conversion of various hydrocarbon feed stocks to maleic anhydride. In general, such catalysts wherein the valence of the vanadium is less than +5, usually between about +4.0 and about +4.3, are particularly well suited for the production of maleic anhydride from hydrocarbons having at least four carbon atoms in a straight chain or cyclic structure, especially n-butane or benzene.

VPO catalysts are usually prepared by contacting vanadium-containing compounds, phospho- rus-containing compounds, and optionally promoter component-containing compounds under certain conditions apt to reduce pentavalent vanadium to the tetravalent state and to form the desired catalyst precursor comprising vanadyl hydrogen phosphate, optionally containing a promoter component. The catalyst precursor is recovered by separation such as filtration and typically in particulate form having a particle size ranging from a few microns to hundred microns. Then the precursor is formed into shaped bodies such as tablets or pellets by compression. Typically, a lubricant such as graphite or boron nitrate is blended into the precursor composition before compression to facilitate the tableting or pelletizing process. Finally, the shaped bodies undergo an activation procedure, which is carried out under an appropriate gas atmosphere and using a certain temperature program, to transform the catalyst precursor into the catalytically active component.

US 5,137,860 describes a three-stage activation procedure consisting of (1) an initial heat-up stage, (2) a rapid heat-up stage, and (3) a maintenance/finishing stage for VPO catalysts for maleic anhydride production by n-butane oxidation. In the initial heat-up stage, a catalyst precursor is heated in an atmosphere selected from the group consisting of air, steam, inert gas, and mixtures thereof, at any convenient heat-up rate, to a temperature not exceeding 300 °C, which is the phase transformation initiation temperature. Afterwards, the initially selected atmosphere is replaced by a molecular oxygen/steam-containing atmosphere, while maintaining the catalyst precursor at the temperature attained in the initial heat-up stage. Once the 02/steam- containing atmosphere is provided, the catalyst precursor is subjected to the rapid heat-up stage of the calcination. In the rapid heat-up stage, the initial heat-up stage temperature is increased at a programmed rate of from about 2 °C/min to about 12 °C/min. Following the rapid heat-up stage, the catalyst precursor is subjected to the maintenance/finishing stage of calcination, wherein the temperature is adjusted to from 350 to 550 °C, while maintaining the 02/steam- containing atmosphere. The adjusted temperature is then maintained, first in the molecular oxy- gen/steam-containing atmosphere and thereafter in a nonoxidizing, steam-containing atmosphere to complete the catalyst precursor-to-active catalyst transformation that yields the active catalyst.

WO2011023646 discloses the preparation of a VPO catalyst wherein malonic acid, known as pore-forming agent, is used for tableting of the green body. The shaped body is calcined at 390 °C under an oxidizing atmosphere (5 % O2, 45 % N2, 50 % steam), followed by thermal treatment at 425 °C under nonoxidizing atmosphere (50 % N2, 50 % steam). Example 1 a and 1 b show a BET surface area of 26 to 30 m²/g and valence of the vanadium of 4.1 to 4.2.

CN 111097467 discloses the activation of a VPO catalyst under four different calcination conditions including temperatures of 260 to 450 °C and oxygen concentrations between 16 % and 0 % oxygen in mixed gases. The catalyst precursor contains at least one promoter, for example, Zn, as disclosed in the examples. The catalyst precursor is prepared by refluxing a mixture of V2O5, isobutanol/benzyl alcohol (5:1), phosphoric acid, zinc acetylacetonate and ethylene glycol.

CN 106540728 describes the activation of a VPO catalyst precursor that comprises three activation steps and a cooling step. In the first step, the VPO catalyst precursor is heated to 200 - 220 °C under a gas flow selected from air, 1 - 2 % n-butane in air, 10 - 90 % water vapor mixed with air or 10 - 90 % water vapor mixed with inert gas. In the second activation step, the VPO catalyst precursor obtained in the first step is heated to 360 - 380 °C. In the third activation step, the temperature is increased to 400 - 500 °C under a gas flow selected from 1 - 2 % n- butane in air, 10 - 90 % water vapor mixed with air or 10 - 90 % water vapor mixed with nitrogen. In the cooling step, the temperature of the activated VPO catalyst from the third step is reduced to 290 - 310 °C under a gas flow of 1 - 2 % n-butane in air.

CN 111054409 discloses the activation of a vanadium/phosphorus mixed oxide catalyst for preparing maleic anhydride by oxidizing n-butane. The VPO catalyst precursor is heated to 380 - 480 °C, wherein the temperature in a first step is raised to the range of 120 - 270 °C under a first activation atmosphere consisting of a mixture of oxygen and nitrogen and/or air and water vapor. The temperature of 380 - 480 °C is kept while introducing a second activation atmosphere consisting of water vapor and inert gas.

JP 2021137740 discloses the activation of a phosphorus-vanadium oxide catalyst precursor. The shaped precursor is activated under a gas flow containing air and n-butane at 400 °C for 24 h.

US 2011/0257414 describes the preparation of a VPO catalyst using a belt calciner with eight heating zones at different temperatures between 140 °C and 425 °C and under different gas flows composed of air, nitrogen, nitrogen/water vapor and finally again nitrogen. It is an object of the present invention to provide a quality-optimized process for the transformation of a vanadium/phosphorus mixed oxide catalyst precursor comprising vanadyl hydrogen phosphate and optionally a promoter component into an active catalyst comprising vanadyl pyrophosphate and optionally a promoter component, suitable for the production of maleic anhydride, to provide a vanadium/phosphorus mixed oxide catalyst (VPO catalyst) giving improved yields in the production of maleic anhydrides.

The object is solved by a quality-optimized process for preparing a vanadium/phosphorus mixed oxide catalyst (VPO catalyst) containing crystalline vanadyl pyrophosphate, comprising the steps:

(a) preparing a VPO catalyst precursor containing vanadyl hydrogen phosphate by reacting a mixture containing a vanadium-containing compound, preferably vanadium pentoxide, and a phosphorous-containing compound, preferably phosphoric acid or polyphosphoric acid, and optionally a promotor metal containing compound, in an alcoholic medium,

(b) pre-calcining the VPO catalyst precursor,

(c) optionally shaping the VPO catalyst precursor,

(d) calcining the VPO precursor in an oxygen containing calcination atmosphere,

(e) taking at least one sample of the calcined VPO precursor during or after step (d),

(f) thermally treating, in at least one temperature treatment step, the calcined VPO catalyst precursor in an inert gas atmosphere containing steam at a higher temperature than in the calcination step (d), wherein the final VPO catalyst is obtained,

(g) determining, via XRD, the content of crystalline vanadyl pyrophosphate of at least one sample taken in step (e),

(h) based on the result of step (g), modifying the process conditions of step (d) with regard to one or more of the process parameters selected from calcination time, calcination temperature and composition of the calcination atmosphere in order to ensure that the content of crystalline vanadyl pyrophosphate in the calcined VPO precursor is at least 45 % by weight before step (f) is carried out.

The inventors have found that the yield of maleic anhydride in the oxidation of n-butane in the presence of the VPO catalyst is significantly improved if the content of crystalline vanadyl pyrophosphate of the calcined VPO catalyst precursor is at least 45 % by weight before thermal treatment in an inert gas atmosphere containing steam is carried out.

The content of crystalline vanadyl pyrophosphate in the VPO catalyst precursor is determined in step (f) by X-ray diffraction analysis. This can be carried out as follows: The shaped catalyst is ground to powder that is transferred to a standard PM MA sample holder (Bruker AXS) and flattened using a glass plate. The diffractogram of the powder sample is measured using a D8 Advance diffractometer (Bruker AXS) with fixed slits set to 0.1 ° and a linearly integrating area detector (LynxEye, Bruker AXS) in an angular range of 5°-70° 2theta with a step size of 0.02° 2theta. The quantitative analysis is performed using the software TOPAS 6 (see also TOPAS 6 User Manual, Bruker AXS GmbH, Karlsruhe, Germany, 2017), wherein the modelled phase composition is set to (VO)₂H₄P2O₉, VOPO₄, (VO)₂(P₂O₇), [VO][PO₄][H₂O]₂, (VO)(PO₄). In all phases, the lattice parameters and crystallite size are refined. The diffractogram is modelled using a 3^rd order polynomial in which intensity corrections for Lorentz and polarization effects are considered. The phase quantification is performed by standard procedures described in Klug, H. P. & Alexander, L. E. (1974). X-ray Diffraction Procedures, 2nd ed., John Wiley, New York.

Broadly described, the active VPO catalyst is prepared by reacting a vanadium-containing compound and a phosphorus-containing compound in an alcoholic medium to produce a VPO catalyst precursor, pre-calcining and optionally shaping the VPO catalyst precursor, and activating the VPO catalyst precursor by calcination in an oxygen-containing atmosphere and thermal treatment in an inert gas atmosphere to convert a substantial fraction of the precursor composition to vanadyl pyrophosphate (VO)₂P₂O₇ . The active VPO catalyst may be a material having at least 70 % by weight (VO)₂P₂O₇ based on the weight of the catalyst. In preferred embodiments, the active VPO catalyst is a material having at least 75 % by weight or even at least 80 % by weight of (VO)₂P₂O₇ based on the weight of the catalyst.

Vanadium-containing compounds in general are those containing pentavalent vanadium and include vanadium pentoxide or vanadium salts, such as ammonium metavanadate, vanadium oxytrihalides, and vanadium alkyl carboxylates. Among these compounds, vanadium pentoxide is preferred.

Phosphorus-containing compounds are preferably those that contain pentavalent phosphorus. Suitable phosphorus-containing compounds include phosphoric acid, polyphosphoric acid, phosphorus pentoxide, or phosphorus perhalides such as phosphorus pentachloride. Of these phosphorus-containing compounds, phosphoric acid and polyphosphoric acid are preferred.

Promoter elements optionally may be added as solids, suspension of solids, or solutions to the catalyst precursor slurry either prior to or after the reaction of the vanadium and phosphorus- containing compounds has taken place. Promoter compounds that may serve as sources of the promoter elements include metal halides, metal alkoxides, and metal carboxylates. Of these compounds, metal carboxylates are preferred. Suitable carboxylates for metal salts include formate, acetate, propionate, butyrate, iso-butyrate, pentanoate, hexanoate, heptanoate, octanoate, nonanoate, decanoate, and 2-ethylhexanoate. Of these carboxylates, 2-ethylhexanoate is preferred. In an embodiment, the promoter elements include Zr, Zn, Ti, Mn, Bi, Sn, Co, Ni, Mo, Nb, Or, Fe, or combinations thereof. The promoter may be less than 3.0 % by weight of the total catalyst weight.

The reaction (a) is carried out in an alcoholic reaction medium. In general, the alcoholic reaction medium contains primary or secondary alcohols which can be oxidized to the corresponding aldehydes or ketones. Examples are methanol, ethanol, 1 -propanol, 2-propanol (iso-propanol), 2- methyl-1 -propanol (isobutyl alcohol), 3-methyl-2-butanol, 2, 2-dimethyl-1 -propanol, ethylene glycol and benzyl alcohol. The alcoholic medium has reducing properties. A preferred alcoholic reaction medium contains isobutyl alcohol or mixtures of isobutyl alcohol and benzyl alcohol. The reaction (a) between the vanadium and phosphorus-containing compounds may be carried out at any suitable temperature. In an embodiment, the reaction is carried out by reflux in a stirred tank reactor at a temperature within a range of 80 to 130 °C, preferably 90 to 120 °C, under ambient or elevated pressure and at a phosphorus/vanadium atom ratio (PN) of 1 .00 to 1.15.

During the course of carrying out the reaction, the VPO catalyst precursor forms and precipitates from the precursor slurry as a finely divided precipitate that may also contain the optional promoter elements. The VPO catalyst precursor may be recovered after cooling to below 50 °C by conventional techniques well known to those skilled in the art, including filtration, centrifugation, and decantation.

The VPO catalyst precursor may then be dried at a temperature of fer example 150 to 200 °C, and then subjected to (b) pre-calcination at a temperature preferably in the range of 250 to 350 °C. Pre-calcination is usually carried out in air. It may be one object of such a pre-calcination to reduce the content of organic carbon, for example relatively tightly bound alcoholate residues not removed in previous separation or drying steps.

The VPO catalyst precursor is converted to an active VPO catalyst by one or more (d) calcination treatments under an oxygen containing calcination atmosphere and one or more (f) thermal treatments or it may be first in a shaping step (c) be compressed in a press or die to produce shaped bodies (“green” shaped bodies I “green” tablets) and then subjected to calcination and thermal treatment. The VPO catalyst precursor may be compressed into any desired shape or form, such as a cylinder, hollow cylinder, ring, sphere or other preferred shapes, for example described in WO2021239483 A1 and DE102010052126 A1. Binding and/or lubricating agents may be added, if desired, at amounts ranging from 2 to 6 % by weight based on the total weight of the VPO catalyst precursor and may include starch, calcium stearate, stearic acid and graphite. In a preferred embodiment, VPO precursor material in powder form is first subjected to a pre-compacting step in order to obtain a material with a more uniform particle (agglomerate) size distribution better suited for a tableting process. Such a pre-compacting step may include a compression step, followed by a crushing and sieving step, where fines and bigger agglomerates may be returned to the compression step and a middle fraction of relatively defined and narrow size distribution is provided for a shaping process such as tableting. In a preferred embodiment some binding or lubricating agents or part of the total amount of lubricating or binding agents are already added and mixed with the VPO catalyst precursor material prior to pre-compacting.

In general, the at least one calcination step (d) is carried at a temperature of from 300 °C to 450 °C, preferably at a temperature of from 360 °C to 450 °C, in an oxygen containing atmosphere. Calcination times are in general from 20 to 80 min, preferably from 35 to 60 min. Calcina- tion temperature includes a calcination temperature profile. Calcination temperature and/or calcination time may be systematically varied, based on the results of steps (e) and (g), until a crystallinity of the VPO precursor of at least 45% by weight after calcination step (d) is obtained.

The oxygen containing atmosphere can contain inert gases such as nitrogen and steam. In general, the oxygen containing atmosphere contains nitrogen and steam. Preferably, the oxygen containing calcination atmosphere in at least one calcination step (d) contains from 2 to 5 vol.- %, specifically from 2 to 4 vol.-% oxygen. In general, the oxygen containing calcination atmosphere in at least one calcination step (d) contains steam, preferably from 25 to 75 vol.-%, specifically from 40 to 60 vol.-% steam. Calcination atmosphere may be systematically varied, based on the results of steps (e) and (g), until a crystallinity of the VPO precursor of at least 45 % by weight after calcination step (d) is obtained.

It is essential for the present invention that the content of crystalline vanadyl pyrophosphate of the calcined VPO catalyst precursor is at least 45 % by weight, preferably at least 50 % by weight after the calcination step (d) and before the temperature treatment step (f) is carried out.

Calcination can be done in a batch or continuous process. Both steps (d) and (f) can be done in one or separate calciners. Both steps can be done separately isolating the intermediate product or in a single operation subjecting the product of step (d) directly to step (f) conditions. In a continuous process care needs to be taken in order to separate different gas-atmospheres such as in step (d) and (f). Continuous calcination can be done in a single belt-calciner or a series of belt-calciners where varying gas-atmosphere compositions of step (d) and (f) can be sufficiently separated in successive compartments. In a preferred embodiment, two separate belt-calciners or belt calcination steps are used for steps (d) and (f). In a batch process, conditions of step (d) and (f) can be applied directly after each other in the same calciner. Calcination is preferably done close to ambient pressure.

In a commercial process calcination (d) and thermal treatment (f) may be preferably carried out directly after another in the same piece of equipment or in a series of pieces of equipment.

In one embodiment a batch process can be applied. For example, a tray-oven can be used that is subjected to a temperature program combined with a program of varying atmospheres without removing or manipulating the catalyst precursor body until calcination (d) and thermal treatment (f) have been completed.

In another embodiment a continuous process may be applied for calcination (d) and thermal treatment (f). For example, a belt-oven (belt-calciner) or a series of consecutive belt-ovens can be applied. In a preferred option, a program of varying temperature and gas atmospheres is enforced onto the catalyst precursor bodies when the catalyst precursor bodies are moved through consecutive domains of the belt-oven with varying temperature or atmospheric conditions. In a preferred embodiment temperature is measured and controlled at representative positions within a tray-or belt-oven. In another preferred embodiment characteristic parameters referring to the atmospheric composition, such as the oxygen or water content or their respective partial pressure, inside the oven are monitored.

In a preferred embodiment, taking samples according to step (e) of the present invention and doing XRD measurements according to step (g) is carried following a more or less random or preferably a more systematic pattern, for example after production of a certain amount of VPO catalyst material or after passing a certain amount of time. In a more preferred embodiment, sampling and XRD analytics are especially carried out when operation conditions have been changed, for example when certain pieces of equipment or instrumentation have been replaced, throughput rates or filling-modes or charging of catalyst precursor shapes have been changed. When the process is operated under relatively stable conditions, sampling and analytics frequency may be reduced.

According to the invention, based on the result of step (g), the process conditions of step (d) with regard to one or more of the process parameters selected from calcination time, calcination temperature and composition of the calcination atmosphere are modified in order to ensure that the content of crystalline vanadyl pyrophosphate in the calcined VPO precursor is at least 45% by weight before step (f) is carried out. I. e., when XRD results of sampled material reveal a deviation of crystal composition from the target range, operation parameters of calcination step (d) such as temperature profiles, residence times and/or composition of the calcination atmosphere are adjusted for future operation.

In one embodiment, at least the calcination time is modified, based on the result of step (g). In another embodiment, at least the calcination temperature, including a temperature profile, is modified, based on the result of step (g). In a further embodiment, at least the composition of the calcination atmosphere is modified, based on the result of step (g).

In general, the at least one temperature treatment step (f) is carried at a temperature of from 400 to 500 °C, preferably at a temperature of from 420 to 480 °C in an inert gas atmosphere containing steam. In general, the inert gas atmosphere can contain nitrogen and steam as inert gases. In general, the inert gas atmosphere in the at least one temperature treatment step (h) contains from 25 to 75 vol.-%, specifically from 40 to 60 vol.-% steam.

The inert, nonoxidizing, steam-containing atmosphere needs not necessarily be completely free of molecular oxygen. However, such atmosphere preferably is substantially free of molecular oxygen. Accordingly, molecular oxygen may be present in an amount which is not effective to cause further oxidation of the vanadium beyond the desired average oxidation state of about +4.0 to about +4.3, more particularly, not beyond the maximum desired average oxidation state of about +4.3. In general, molecular oxygen may be present in amounts which do not exceed about 0.5 mol % of the nonoxidizing, steam-containing atmosphere. In general, the content of crystalline vanadyl pyrophosphate in the final VPO catalyst is at least 70 % by weight, preferably at least 75 % by weight. The average valence of vanadium in the final VPO catalyst is in general from +3.8 to +4.5, preferably from +4.0 to +4.3. The average valence of vanadium in the final VPO catalyst is analyzed by a double-titration method, whereby vanadium of the dissolved catalyst in acidic solution (mixture of sulfuric acid and phosphoric acid solution) is oxidized to +5 by a potassium permanganate solution and the titration solution is afterwards reduced by an ammonium iron(ll) sulfate solution.

The invention also relates to the vanadium/phosphorus mixed oxide catalyst perse, obtainable by the process according to the invention.

The BET surface area of the final VPO catalyst is in general 20 to 40 m²/g, preferably 22 to 35 m²/g.

The pore volume of the final VPO catalyst is in general 0.3 to 0.4 ml/g, preferably 0.31 to 0.36 ml/g.

The pore volume of the calcined VPO catalyst precursor is, before the temperature treatment step (h) is carried out, in general 0.260 to 0.320 ml/g, preferably 0.280 to 0.320 ml/g, more preferably 0.285 to 0.320 ml/g, in particular 0.290 to 0.315 ml/g.

The invention also relates to the use of the vanadium phosphorus mixed oxide-catalyst for producing maleic anhydride from C4-hydrocarbons or benzene.

In general, the reaction to convert C4-hydrocarbons to maleic anhydride using the catalysts transformed from the catalyst precursors in accordance with the process of the present invention requires only contacting the C4-hydrocarbons admixed with a molecular oxygen-containing gas (including pure molecular oxygen), such as air or molecular oxygen-enriched air, with the catalyst at elevated temperatures. In addition to the C4-hydrocarbons and molecular oxygen, other gases, such as nitrogen or steam, may be present or added to the reactant feed stream. Typically, the C4-hydrocarbons are admixed with the molecular oxygen-containing gas, preferably air, at a concentration of from about 1 mol % to about 10 mol % hydrocarbon and contacted with the catalyst at a gas hourly space velocity (GHSV) of from 100 hr¹ up to 5 000 hr¹ and at a temperature of from 300 to 600 °C, preferably from 1 000 hr¹ to 3 000 hr¹ and from 325 to 500 °C to produce maleic anhydride. The catalyst can be also used in a recycle process for oxidation of C4-hydrocarton to maleic anhydride, whereby more or less pure oxygen is applied as an oxidant instead of air as described in, for example, US7345167B2.

The invention is further illustrated by the following examples. Examples

Catalyst Production

Powder synthesis:

The vanadium hydrogen phosphate hemihydrate (= vanadium hydrogen phosphate) powder was synthesized by refluxing of a mixture of vanadium pentoxide (ca. 690 kg), polyphosphoric acid 105% (ca. 800 kg) and iso-butanol (ca. 4600 kg) at 110 °C for 14 h in a stirred tank reactor. After being cooled down to RT, the material formed was filtered and dried under vacuum (90 mbara) at 170 °C. The drying was over when a vacuum of 25 mbara was reached.

Pre-calcination:

The dried powder was pre-calcined in a rotary kiln to reduce the residual carbon content (< 4 wt%), main content of which is due to residual iso-butanol in more or less loosely or chemically bound state, at 250 - 350 °C under air flow. The atmosphere in or from the rotary kiln may contain various combustible components such as iso-butanol, iso-butene or carbon monoxide and care should be taken to avoid an explosive composition. An explosive atmosphere composition may be avoided for example by dilution of combustible components using a sufficient supply of air feed or by limiting oxygen content to a level below the limiting oxygen concentration (LOC) using an inert gas such as nitrogen for dilution. It may be beneficial to use a recycle gas stream in order to save inert gas consumption.

Granulation and tableting:

The precursor powder after pre-calcination was granulated by compaction of dried powder, which is mixed with 1 % by weight of graphite, into a particle size of 400 - 1500 pm. The catalyst precursor granules had a density of 700 - 750 g/l. Fine particles remaining after granulation were sieved and recycled.

A shaped catalyst precursor (green body) was then obtained by tableting the catalyst precursor granule mixed with 2 - 3 % by weight of graphite into 6.5x5x3.7 mm hollow cylinders with tablet bulk density of 1 .40 - 1 .6 kg/l, preferably 1 .45 - 1 .55 kg/l . The tablet bulk density is defined as the mass of tablet per total volume occupied by material, internal pore, and inter-particle void. The tablet bulk density is measured by mercury porosimetry at low pressure, for example, 0.2 MPa.

Calcination and thermal treatment of the green body:

Calcination and thermal treatment of the shaped green body was carried out batchwise by two steps using a calciner 1 .) under an oxidizing atmosphere and 2.) under a non-oxidizing atmosphere. The calciner was comprised of a tube reactor of 1 .1 L that was electrically heated. During the first calcination step, the calciner was heated to an inner temperature of 330 - 375 °C (2 °C/ min) and kept for 40 - 50 min under a gas flow of 1200 NL/h, comprising of 200 NL/h air, 400 NL/h N2 and 600 NL/h steam. The steam was introduced after the inner temperature of the oven reached 200 °C. Afterwards, the oven was cooled down to 150 °C, wherein the flow of steam and air were removed at 200 and 150 °C, respectively, and kept for 2 h under 600 NL/h N2 before it was further cooled to RT.

The catalyst intermediate, the green body after the calcination procedure, was subjected to a thermal treatment step under non-oxidizing atmosphere. Inner temperature of the oven was heated to 450 °C (2 °C/ min) and kept for 6 h under a gas flow of 1200 NL/h, comprising of 600 NL/h N2 and 600 NL/h steam. The steam was introduced after the inner temperature of the oven reached 200 °C. Afterwards, the oven was cooled down to RT °C, wherein the flow of steam was removed at 200 °C.

The conditions of the calcination step and the thermal treatment step are summarized in Table 1.

Table 1 Condition of 1^st calcination and 2^nd high-temperature treatment (HTT) for catalyst activation

The BET surface area was determined by nitrogen physisorption as follows: A Shaped body was ground in a mortar to a size of approx. 3-6 mm. The samples are pre-dried in an open, heat-resistant sample vessel for 1 h in the drying cabinet at 120°C. After thermal treatment and evacuation of a sample, the adsorption of gaseous nitrogen is measured at the surface of a sample at the boiling temperature of nitrogen. The surface area of shaped body was obtained using the adsorbed volume of nitrogen and Brunauer-Emmet-Teller (BET) analysis model.

The pore volume was determined by mercury porosimetry as follows: Pore volume and pore distribution of shaped body (catalyst intermediate and catalyst) were measured by mercury po- rosimeter. The volume of mercury intrusion at different pressure up to ca. 415 MPa was recorded by MicroActive AutoPore V 9600. The pore radius and pore volume were calculated from the pressure and volume using Washburn’s equation. Prior to the measurement, samples are pre-dried at least 2 hours at 120 °C under atmospheric pressure. Afterwards, shaped body were used in their original form.

Determination of the crystalline vanadyl pyrophosphate content by XRD before and after the thermal treatment step was carried out as follows: A shaped catalyst was ground to powder that was transferred to a standard PM MA sample holder (Bruker AXS) and flattened using a glass plate. The diffractogram of the powder sample was measured using a D8 Advance diffractometer (Bruker AXS) with fixed slits set to 0.1 ° and a linearly integrating area detector (LynxEye, Bruker AXS) in an angular range of 5°-70° 2theta with a step size of 0.02° 2theta. The quantitative analysis was performed using the software TOPAS 6, wherein the modelled phase composition was set to (VO)₂H₄P2O₉, VOPO₄, (VO)₂(P₂O₇), [VO][PO₄][H₂O]₂, (VO)(PO₄). In all phases, the lattice parameters and crystallite size were refined. The diffractogram was modelled using a 3^rd order Polynomial in which intensity corrections for Lorentz and polarization effects were considered. The phase quantification was performed by standard procedures described in Klug, H. P. & Alexander, L. E. (1974). X-ray Diffraction Procedures, 2nd ed., John Wiley, New York.

Oxidation state of V (V_ox) was determined by double titration, whereby vanadium of the dissolved catalyst in acidic solution (mixture of sulfuric acid and phosphoric acid solution) is oxidized to +5 by a potassium permanganate solution and the titration solution is afterwards reduced by an ammonium iron(ll) sulfate solution.

The results are summarized in Table 2 and Figure 1 .

Catalyst Performance Test

Performance of prepared catalyst was carried out by using a shell-and-tube reactor with inner diameter of 22.3 mm and length of 6.5 m. A multi-thermocouple having 20 temperature measuring points was located in a protective tube with an external diameter of 6 mm within the reactor tube. The reactor was heated by a salt melt used as heat transfer medium. The upper 0.2 m of the reactor remained unfilled and was followed by preheating zone of 0.3 m, wherein shaped steatite bodies were filled as inert material. The preheating zone was followed by a catalyst bed that amounted to 2173 ml. The reaction gas mixture of 3600 - 4400 NL/h (corresponding to GHSV 1700 - 2000 h’¹), comprising of 1 .0 - 2.0 vol.% n-butane (purity of 99.5%), 0 - 2.3 ppm triethyl phosphate (TEP), 3 vol.% H₂O and air, flowed from the top downward through the reactor (single pass). Gaseous product was taken off downstream of the reactor unit and passed to gas chromatographic on-line analysis. In the exhaust gas, CO₂, CO, acetic acid and acrylic acid were analyzed as by-products. During the performance test, n-butane loading (NL_n-butane/Lreactor/h) was increased step by step from 17 - 40 NL_n-butane/Lcataiystbed/h. The yield of maleic anhydride shown in this patent application is back calculated yield (BCY), which was obtained at:

GHSV = 1750 IT¹ , n-butane.in ~ 1 -85 VOl.-%,

Vn2O,in = 3 vol.-%,

VrEP n = 2.25 vol. -ppm, conversion of n-butane = 80%, wherein V_k,in - vol.-% of molecule k in the inlet gas mixture.

The back calculated yield of maleic anhydride was obtained by subtracting the amount of carbon of by-products from the amount of carbon of the reacted n-butane as Eq. (1).

wherein V_{k in} = vol.-% of molecule k in inlet gas; V_{k out} = vol.-% of molecule k in exhaust gas.

The results are summarized in Table 2 and in Figure 1.

Table 2

^a sample number in parentheses used for analysis of catalyst intermediate obtained after 1^st calcination ^b back-calculated yield of maleic anhydride at 80 % of n-butane conversion. Reaction conditions: GHSV of 1750 h^-1, n-butane of 1.85 vol.-% in feed gas (butane load of 32.4 l/l/h), TEP of 2.25 ppm.

Claims

Patent claims

1 . A quality-optimized process for preparing a vanadium/phosphorus mixed oxide catalyst (VPO catalyst) containing crystalline vanadyl pyrophosphate, comprising the steps:

(b) pre-calcining the VPO catalyst precursor,

(c) optionally shaping the VPO catalyst precursor,

(d) calcining the VPO precursor in an oxygen containing calcination atmosphere,

(h) based on the result of step (g), modifying the process conditions of step (d) with regard to one or more of the process parameters selected from calcination time, calcination temperature and composition of the calcination atmosphere in order to ensure that the content of crystalline vanadyl pyrophosphate in the calcined VPO precursor is at least 45% by weight before step (f) is carried out.

2. The process of claim 1 , wherein the calcination step (d) is carried out at a temperature of from 300 to 450 °C.

3. The process of claim 2, wherein the calcination step (d) is carried out at a temperature of from 360 to 450 °C.

4. The process of any one of claims 1 to 3, wherein the oxygen containing calcination atmosphere in the calcination step (d) contains from 2 to 5 vol.-% oxygen.

5. The process of any one of claims 1 to 4, wherein the oxygen containing calcination atmosphere in the calcination step (d) contains from 25 to 75 vol.-% steam.

6. The process of any one of claims 1 to 5, wherein the at least one temperature treatment step (f) is carried out at a temperature of from 400 to 500 °C.

7. The process of claim 6, wherein the at least one temperature treatment step (f) is carried out at a temperature of from 420 to 480 °C.

8. The process of any one of claims 1 to 7, wherein the inert gas atmosphere in the at least one temperature treatment step (f) contains from 25 to 75 vol.-% steam.

9. The process of any one of claims 1 to 8, wherein the pre-calcination step (b) is carried out at a temperature of from 250 to 350 °C.

10. The process of any one of claims 1 to 9, wherein the pre-calcination step (b) is carried out in air.

11 . The process of any one of claims 1 to 10, wherein the phosphorous containing acid in step (a) is phosphoric acid or polyphosphoric acid.

12. The process of any one of claims 1 to 11 , wherein the alcoholic medium in step (a) contains isobutyl alcohol or a mixture of isobutyl alcohol and benzyl alcohol.

13. The vanadium phosphorus mixed oxide-catalyst, obtainable by the process according to any one of claims 1 to 12.

14. The vanadium phosphorus mixed oxide-catalyst of claim 13, wherein the content of crystalline vanadyl pyrophosphate in the final VPO catalyst is at least 70 %, preferably at least 75 %.

15. The vanadium phosphorus mixed oxide-catalyst of claim 13 or 14, wherein the average valence of vanadium in the final VPO catalyst is from +4.0 to +4.3.

16. The vanadium phosphorus mixed oxide-catalyst of any one of claims 13 to 15, wherein the BET surface area of the final VPO catalyst is 20 to 40 m²/g.

17. The vanadium phosphorus mixed oxide-catalyst of any one of claims 13 to 16, wherein the pore volume of the final VPO catalyst is 0.3 to 0.4 ml/g.

18. The use of the vanadium phosphorus mixed oxide-catalyst according to any one of claims 13 to 17 for producing maleic anhydride from C4-hydrocarbons or benzene.