SG184652A1

SG184652A1 - Method for regenerating catalyst for production of methacrylic acid and process for producing methacrylic acid

Info

Publication number: SG184652A1
Application number: SG2012016952A
Authority: SG
Inventors: Shibata Junji; Shiraishi Eiichi
Original assignee: Sumitomo Chemical Co
Priority date: 2011-03-18
Filing date: 2012-03-09
Publication date: 2012-10-30
Also published as: DE102012005249A1; JP2012196608A; KR20120106634A

Abstract

METHOD FOR REGENERATING CATALYST FOR PRODUCTION OF METHACRYLIC ACID AND PROCESS FOR PRODUCING METHACRYLIC ACIDThe present invention provides a method forregenerating a catalyst for production of methacrylic acid, which can effectively recover the catalytic activity andcatalyst life of a used catalyst having been used for production of methacrylic acid and a process for producingmethacrylic acid at a good conversion and selectivity using a regenerated catalyst obtained by this method. A method for regenerating a catalyst for the production of methacrylic acid made of a heteropolyacid compoundcontaining phosphorus and molybdenum includes steps ofdrying an aqueous slurry containing a used catalyst having been used for the production of methacrylic acid, an ammonium ion, a nitrate ion and water to obtain a catalyst precursor, firstly calcining the catalyst precursor in an atmosphere of an oxidizing gas, and then secondly calciningthe product in an atmosphere of a non-oxidizing gas.

Description

SPECIFICATION

. METHOD FOR REGENERATING CATALYST FOR PRODUCTION OF

METHACRYLIC ACID AND PROCESS FOR PRODUCING METHACRYLIC ACID

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method for regenerating a catalyst for production of methacrylic acid and a process for producing methacrylic acid using a catalyst regenerated by this method.

Description of the Related Art

It is known that a catalyst for production of methacrylic acid made of a heteropolyacid compound containing phosphorus and molybdenum is degraded when the catalyst 1s used for a long time in a gas phase catalytic oxidation reaction using methacrolein or the like as a raw material, because the catalytic activity is reduced due to heat load or the like.

As a method for regenerating such degraded catalysts, for example, Japanese Patent Laid-Open No. 2008-93595 describes a method including steps of drying a mixture containing a degraded catalyst, an ammonium ion, a nitrate ion and water, and then firstly calcining the mixture in an atmosphere of air containing 2% by volume of moisture at

390°C, followed by second calcination of the mixture in a nitrogen atmosphere at 435°C.

However, the regenerated catalyst regenerated by the above-described regeneration method is not necessarily satisfactory from the viewpoint of catalyst life.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a method for regenerating a catalyst for production of methacrylic acid, which can effectively recover a catalytic activity and catalyst life of a used catalyst having been used for the production of methacrylic acid, and a process for producing methacrylic acid at a good conversion and selectivity using a regenerated catalyst obtained by this method.

As a result of intensive studies by the present inventors to solve the above-described problem, the present invention has been accomplished.

Specifically, the method for regenerating a catalyst for the production of methacrylic acid of the present invention is a method for regenerating a catalyst for production of methacrylic acid made of a heteropolyacid compound containing phosphorus and molybdenum, including steps of drying an aqueous slurry containing a used catalyst having been used for the production of methacrylic acid, an ammonium ion, a nitrate ion and water to obtain a catalyst precursor, firstly calcining the catalyst precursor in an atmosphere of an oxidizing gas containing 0.1% by volume or more and less than 2.0% by volume of moisture at 360 to 410°C, and then secondly calcining the product in an atmosphere of a non-oxidizing gas at 420 to 500°C.

In addition, the process for producing methacrylic acid of the present invention includes the steps of regenerating a catalyst for the production of methacrylic acid by the above-described method and subjecting a compound selected from methacrolein, iscobutylaldehyde, isobutane and isobutyric acid to a gas phase catalytic oxidation reaction in the presence of the regenerated catalyst.

According to the regeneration method of the present invention, a catalytic activity and catalyst life of a used catalyst having been used for the product ion of methacrylic acld can be effectively recovered, and methacrylic acid can be produced at a good conversion and selectivity over a long time using the obtained regenerated catalyst.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

<Catalyst for Production of Methacrylic Acid and Used

Catalyst>

.

The catalyst for production of methacrylic acid, which is regenerated by the present invention, is made of a heteropolyacid compound containing phosphorus and molybdenum, and may be made of a free heteropolyacid or a salt of a heteropolyacid. Particularly, the catalyst is preferably made of an acid salt (a partially neutralized salt) of a heteropolyacid and more preferably an acid salt of a Keggin-type heteropolyacid.

Besides phosphorus and molybdenum, the catalyst preferably contains at least one element selected from the group consisting of potassium, rubidium, cesium and thallium (hereinafter sometimes referred to as element X), and preferably contains vanadium and at least one element selected from the group consisting of copper, arsenic, antimony, boron, silver, bismuth, iron, cobalt, lanthanum and cerium (hereinafter sometimes referred to as element Y).

The composition of the heteropolyacid compound constituting the catalyst for the production of methacrylic acid (objective catalyst) is preferably, in a fresh catalyst before use, as in the following formula (i):

PaMobVcXdYeOx (i) wherein P, Mo and V represent phosphorus, molybdenum and vanadium, respectively; X represents at least one element X selected from the group consisting of potassium, rubidium, cesium and thallium; Y represents at least one element (element Y) selected from the group consisting of copper, arsenic, antimony, boron, silver, bismuth, iron, cobalt, lanthanum and cerium; O represents oxygen; a, b, c, d and e are numbers satisfying 0 <a <3, 0c <3, 0<d 5 £3, and 0 £ e £ 3, when b is 12; and x is a value determined according to the oxidation states of each element. In a case where each of X and Y is two or more elements, the total rate of two or more elements should satisfy 0 £d £3, and 0 £ e £ 3, when b 1s 12.

Specifically, the composition of the heteropolyacid compound constituting the fresh catalyst preferably has an atomic ratio of the element X to molybdenum (X : Mo) of 0.5 : 12 to 2 : 12.

The catalyst for the production of methacrylic acid as a fresh catalyst may be one produced by a conventionally known method, for example, by mixing compounds containing each element described above constituting the heteropolyacid compound (for example, an oxo acid, an oxo acid salt, an oxide, a nitrate, a carbonate, a bicarbonate, a hydroxide, or a halide of each element, and the like), molding the mixture into a desired shape, and then subjecting the mixture to calcination.

As the compound containing each element described above, examples of the compound containing phosphorus used include phosphoric acid and phosphates, examples of the compound containing molybdenum used include molybdic acid, molybdates such as ammonium molybdate, molybdenum oxide, and molybdenum chloride; examples of the compound containing vanadium used include wvanadic acid, vanadates (metavanadates) such as ammonium vanadate (ammonium metavanadate), vanadium oxide, and vanadium chloride, and examples of the compound containing the element X used include oxides such as potassium oxide, rubidium oxide and cesium oxide, nitrates such as potassium nitrate, rubidium nitrate, cesium nitrate and thallium nitrate, carbonates such as potassium carbonate, rubidium carbonate and cesium carbonate, bicarbonates such as potassium hydrogen carbonate and cesium hydrogen carbonate, hydroxides such as potassium hydroxide, rubidium hydroxide and cesium hydroxide, and halides such as potassium chloride, rubidium chloride, cesium fluoride, cesium chloride, cesium bromide and cesium iodide. In addition, examples of the compound containing the element Y used include an oxo acid, an oxo acid salt, an oxide, a nitrate, a carbonate, a hydroxide, and a halide.

When the catalyst for the production of methacrylic acid is used for the production of methacrylic acid as a fresh catalyst, the catalytic activity may be reduced due to heat load or the like.

In the production method of the present invention, a used catalyst whose catalytic activity has been reduced due to its use in the production of methacrylic acid is subjected to a regeneration treatment. Here, decomposition of active sites in the catalyst for the production of methacrylic acid can be confirmed by determining if molybdenum trioxide as a decomposition product is detected or not by performing X-ray diffraction (XRD) analysis for the catalyst.

In the regeneration method of the present invention, a used catalyst having been used for production of methacrylic acid, an ammonium ion, a nitrate ion and water are mixed to obtain an aqueous slurry. Examples of a source material compound of an ammonium ion include ammonia, and ammonium salts such as ammonium nitrate, ammonium carbonate and ammonium acetate, and preferably ammonia and ammonium nitrate. Examples of the source material compound of a nitrate ion include nitric acid, and nitrates such as ammonium nitrate, and preferably nitric acid and ammonium nitrate.

In addition, when an ammonium salt or a nitrate is used for a compound added as a dissipated component described below, these ammonium salt and nitrate are also the ammonium ion and nitrate ion.

When the catalyst for the production of methacrylic acid is used in production of methacrylic acid, part of the

: constituents of the catalyst such as phosphorus and molybdenum may be dissipated. In such a case, preferably, kinds of the dissipated constituents are analyzed by a fluorescent X-ray analysis or an inductively-coupled plasma (ICP) emission spectrometry, and the dissipated amount is calculated from the obtained result, and then the dissipated elements are added upon preparing the above- described aqueous slurry. )

As the compounds to be added as the dissipated elements, one kind or two or more kinds may be properly selected from the compounds including each element that can be used for the production of the above-described fresh catalyst.

When the element X is present in the above-described aqueous slurry, it is preferred that the atomic ratio of the element X to molybdenum contained in the aqueous slurry (X : Mo) is adjusted to 0.5 : 12 to 2 : 12 and preferably to 1.0 : 12 to 1.8 : 12.

Specifically, adjustment of the atomic ratio may be conducted by adding at least one of compounds containing the element X (element X-containing compound) and a molybdenum compound.

The catalyst composition (kinds and amounts of constituents) of the used catalyst before being subjected to regeneration is previously analyzed by a fluorescent X-

ray analysis, an ICP emission spectrometry or the like, and a mixing amount of the element X-containing compound, the molybdenum compound and the like may be determined based on the catalyst composition of the used catalyst, so as to have an atomic ratio of the element X to molybdenum (X :

Mo) after adding the element X-containing compound and/or the molybdenum compound in the above-described range.

Usually, the element X~containing compound is added considering the amount of molybdenum in the used catalyst, and when molybdenum is dissipated to disappear due to heat load or the like by being used in the production of methacrylic acid for a long time, the composition of the used catalyst may be at the X : Mo ratio in the aqueous slurry described above, depending on the reduction amount.

In that case, it is also possible to add neither the element X-containing compound nor the molybdenum compound.

As the molybdenum compound or the element X-containing compound to be mixed in the preparation of the aqueous slurry, one kind or two or more kinds may be properly selected from compounds containing molybdenum and the element X-containing compounds that can be used in the production of the above-described fresh catalyst.

In addition, when the agueous slurry is prepared, a compound containing catalyst constituents other than molybdenum and element X can be added as necessary, based on the catalyst composition of the used catalyst. As the compound containing catalyst constituents other than molybdenum and element X, one kind or two or more kinds may be properly selected from compounds including each element : that can be used in the production of the above-described fresh catalyst.

The preparation method of the aqueous slurry is not particularly limited, and for example, the used catalyst may be suspended in water, followed by addition of a source material compound of ammonium ion and a source material compound of nitrate ion, or the above-described used catalyst may be suspended in an aqueous solution containing an ammonium ion and a nitrate ion.

As the ratio of the nitrate ion to the ammonium ion contained in the aqueous slurry, an amount of the ammonium ion is preferably 1.3 mol or less and more preferably 0.5 to 1.3 mol, based on 1 mol of the nitrate ion. When the amount of ammonium ion is out of the above-described range, the catalytic activity may not be sufficiently recovered.

When a compound containing an ammonium ion and a nitrate ion is added to the aqueous slurry as the dissipated elements, the amounts are adjusted to the above-described range, also considering these ammonium ion and nitrate ion.

As water used in the preparation of the aqueous slurry, ion-exchange water 1s usually used. The amount of water used is usually 1 to 20 parts by weight based on 1 part by weight of molybdenum (total of molybdenum contained in the used catalyst and molybdenum contained in the molybdenum compound added) in the aqueous slurry.

When aqueous slurry is prepared, the used catalyst may be directly subjected to mixing or may be previously heat- treated as a pretreatment.

The temperature for heat treatment carried out as a pretreatment of the used catalyst is not particularly limited, and is preferably 350 to 600°C. The heat treatment time is not particularly limited, and is usually 0.1 to 24 hours and preferably 0.5 to 10 hours. In addition, the heat treatment carried out as a pretreatment of the used catalyst may be carried out in an atmosphere of an oxidizing gas such as an oxygen-containing gas, or may be carried out in an atmosphere of a non-oxidizing gas such as nitrogen.

Also, when the used catalyst subjected to the preparation of aqueous slurry is a molded product, the used catalyst may be directly used or can be previously subjected to a crushing treatment by a conventionally known method as necessary. However, when the molded product contains fibers and the like that develop strength of the catalyst, strength degradation is concerned once the fibers and the like are cut. Therefore, when the used catalyst is crushed, it 1s preferred not to cut the fibers and the like.

When the used catalyst subjected to the preparation of the aqueous slurry is subjected to both the crushing treatment and the heat treatment carried out as a pretreatment, an order of both the treatments is not particularly limited. Usually, the heat treatment is carried out after carrying out the crushing treatment.

The above-described aqueous slurry is dried to obtain a catalyst precursor. As a drying method, methods usually used in this field, for example, evaporation to dryness, spray drying, drum drying, flash drying and the like can be adopted. In addition, the drying conditions are not particularly limited so long as they are appropriately set so that -a water content in the aqueous slurry is sufficiently reduced, and the temperature 1s usually less than 300°C.

The aqueous slurry is preferably heat-treated at 100°C or more prior to drying, to mature, from the viewpoint of recovery of the catalytic activity. The heat treatment temperature is preferably 200°C or less and more preferably 150°C or less. The heat treatment can be usually carried out in a closed vessel. The heat treatment time is usually 0.1 hour or more, preferably 2 hours or more, and more preferably 2 to 10 hours. When the heat treatment time is shorter than 0.1 hour, a sufficient activity recovery effect is not likely to be obtained. On the other hand, the heat treatment time is preferably 10 hours or less from the viewpoint of the productivity.

In addition, the obtained dried product (catalyst precursor) may be directly calcined, or is preferably molded into the form of a ring, a pellet, a sphere, a cylinder or the like, by tablet compression, extrusion molding or the like. In this case, a molding aid such as ceramic fibers, glass fibers and also ammonium nitrate may be used as necessary, in order to enhance the strength.

Particularly, ammonium nitrate functions as a pore-forming agent besides a molding aid.

When the catalyst precursor is molded as described above, it is preferred that the temperature-humidity conditioning is carried out by exposing the obtained molded product specifically to an atmosphere of 40 to 100°C and a relative humidity of 10 to 60% for about 0.5 to 10 hours, and then calcination is carried out, for further effectively recovering the catalytic activity. The conditioning may be carried out, for example, in a container having controlled temperature and humidity, or by blowing a gas having controlled temperature and humidity to the molded product. Also, as an atmosphere gas in the conditioning, the air is usually used while an inert gas such as nitrogen may be used.

The above-described dried product (catalyst precursor) is directly calcined, or molded, then subjected to the temperature~-humidity conditioning, followed by calcination, whereby a regenerated catalyst can be obtained. In the calcination, first calcination is carried out in an atmosphere of an oxidizing gas containing 0.1% by volume or more and less than 2.0% by volume of moisture at 360 to 410°C, and then second calcination is carried out in an atmosphere of a non-oxidizing gas at 420 to 500°C. Such a two-step calcination process can more effectively recover the catalytic activity.

The moisture content in an atmosphere in the first calcination is 0.1% by volume or more and less than 2.0% by volume, preferably 0.3 to 1.8% by volume, and more preferably 0.6 to 1.6% by velume. When the moisture content is less than 0.1% by volume, part of the catalyst ’ may be decomposed to MoO; and the like, and the activity of the regenerated catalyst may not be sufficiently recovered.

When the moisture content is 2.0% by volume or more, the life of the regenerated catalyst may not be sufficiently recovered.

The oxidizing gas used in the first calcination is a gas containing an oxidizing substance, and examples thereof include an oxygen-containing gas. The concentration of oxygen in the oxygen-containing gas is usually about 1 to

30% by volume. As a source of oxygen, the air or pure oxygen is usually used, and the oxygen source is diluted with an inert gas as necessary. The oxidizing gas is particularly preferably the air.

Usually, the first calcination is carried out in the stream of the oxidizing gas. The temperature in the first calcination is 360 to 410°C and preferably 380 to 400°C.

The non-oxidizing gas used in the second calcination is a gas containing substantially no oxidizing substance such as oxygen. Examples thereof include inert gases such as nitrogen, carbon dioxide, helium, and argon. Also, the non-oxidizing gas may contain moisture as necessary.

However, the concentration of moisture is usually 10% by volume or less. In particular, nitrogen is preferable as a non-oxidizing gas. Usually, the second calcination is carried out in the stream of the non-oxidizing gas. The temperature in the second calcination is 420 to 500°C and preferably 420 to 450°C.

Prior to the calcination, the catalyst precursor is preferably heat-treated (pre-calcined) in an atmosphere of an oxidizing gas or a non-oxidizing gas at a temperature of about 180 to 300°C. <Regenerated Catalyst>

The thus-obtained regenerated catalyst is made of a heteropolyacid compound, and may be made of a free heteropolyacid or a salt of a heteropolyacid. Particularly, the catalyst is preferably made of an acid salt of a heteropolyacid and more preferably an acid salt of a

Keggin-type heteropolyacid. Also, more preferably, the structure of the Keggin-type heteropolyacid salt is formed upon the heat treatment (pre-calcination). The regenerated catalyst has a preferred composition (the above-described formula (i)) same as the above-described fresh catalyst, and when the element X is contained, an atomic ratio of the element X to molybdenum (X : Mo) in the composition of the heteropolyacid compound constituting the regenerated catalyst is preferably 0.5 : 12 to 2 : 12.

The method for regenerating methacrylic acid of the present invention is to regenerate a used catalyst having been used for the production of methacrylic acid. The regeneration method of the present invention can also be carried out to regenerate, for example, a fresh catalyst unused for production of methacrylic acid such as a loss powder generated in the process of producing a fresh catalyst and a fresh catalyst which does not have desired performance, and good effect can also be obtained in that case similarly to a case of regenerating a used catalyst.

The regenerated catalyst is a catalyst whose catalytic activity has been effectively recovered, and methacrylic acid can be produced with a good conversion and selectivity by subjecting the raw material compound such as methacrolein to a gas phase catalytic oxidation reaction in the presence of this regenerated catalyst. <Process for Producing Methacrylic Acid>

Methacrylic acid is usually produced by charging a catalyst (including the regenerated catalyst according to the present invention) in a fixed-bed multitubular reactor and supplying a raw material compound selected from methacrolein, isobutyl aldehyde, isobutane and isobutyric acid and a raw material gas containing oxygen thereto, and a reaction system such as a fluidized bed or a moving bed can be adopted. As an oxygen source, an air is usually used. As a component besides the raw material compound and oxygen, the raw material gas may contain nitrogen, carbon dioxide, carbon monoxide, water wvapor, and the like.

For example, when methacrolein is used as a raw material, the reaction is carried out usually under conditions of the concentration of methacrolein in the raw material gas of 1 to 10% by volume, the concentration of water vapor of 1 to 30% by volume, the molar ratio of oxygen to methacrolein of 1 to 5, the space velocity of 500 to 5000 h™ (on the normal state basis), the reaction temperature of 250 to 350°C, and the reaction pressure of 0.1 to 0.3 MPa. The raw material methacrolein does not necessarily have to be a purified product with a high

: purity. For example, a methacrolein-containing reaction product gas obtained by a gas phase catalytic oxidization reaction of isobutylene or t-butyl alcohol can be used.

Also, when isobutane is used as a raw material, the reaction is carried out usually under conditions of the concentration of isobutane in the raw material gas of 1 to 85% by volume, the water vapor concentration of 3 to 30% by volume, the molar ratio of oxygen to isobutane of 0.05 to 4, the space velocity of 400 to 5000 h™' (on the normal state basis), the reaction temperature of 250 to 400°C, and the reaction pressure of 0.1 to 1 MPa. When isobutyric acid or isobutylaldehyde 1s used as a raw material, almost the same reaction conditions as those employed when methacrolein is used as the raw material are usually adopted.

Examples

Hereinafter, examples of the present invention are shown, but the present invention is not limited to these examples.

Here, nitrogen used in each example is substantially free of moisture.

The composition analysis of the catalyst and evaluation of the catalyst performance in the examples were performed as follows. <Activity Test of Catalyst>

A glass microreactor having an inner diameter of 16 mm was filled with 9 g of a catalyst, and a raw material gas (composition: 4% by volume of methacrolein, 12% by volume of molecular oxygen, 17% by volume of water vapor, and 67% by volume of nitrogen) prepared by mixing methacrolein, air, steam and nitrogen was fed to the microreactor at a space velocity of 670 h™!. The furnace temperature (the ‘temperature of a furnace used for heating the microreactor) was raised to 355°C, then the temperature was maintained for 1 hour. The furnace temperature was then lowered to 280°C. Thereafter, the reaction was continued at the temperature for 1 hour. After 1 hour from the start of the reaction (after setting the furnace temperature to 280°C), an exit gas (a gas after reaction) was sampled and analyzed by gas chromatography, and the conversion of methacrolein (%), the selectivity of methacrylic acid (%) and the yield (3) were obtained by the following equations.

The conversion, selectivity and yield are defined as follows.

Conversion (%) = Number of Moles of Methacrolein

Reacted/Number of Moles of Methacrolein Fed x 100

Selectivity (%) = Number of Moles of Methacrylic Acid

Generated/Number of Moles of Methacrolein Reacted x 100

Yield (%) = [Conversion (%) x Selectivity (%)]1/100 <Life Test of Catalyst>

A glass microreactor having an inner diameter of 16 mm was filled with 4.5 g of a catalyst, and a raw material gas (composition: 4% by volume of methacrolein, 12% by volume of molecular oxygen, 17% by volume of water vapor, and 67% by volume of nitrogen) prepared by mixing methacrolein, air, steam and nitrogen was fed to the microreactor at a space velocity of 1,340 hr? and reacted at a furnace temperature of 320°C for 50 days or more. During the reaction, the conversion of methacrolein was determined every 7 to 14 days. The results were plotted with the reaction time on an abscissa and the conversion on an ordinate. The slope of the line was determined by the least-square method, and then the decreasing rate of the conversion (%/day) was calculated. <Detection of Molybdenum Trioxide by X-Ray Diffraction

Measurement>

An X-ray diffraction measurement was performed by a powder method using MiniFlex manufactured by Rigaku

Corporation as an X-ray diffraction measurement apparatus, and the presence of a diffraction line in a value d of 3.24 to 3.26 derived from molybdenum trioxide (MoOs3) was observed. <Reference Example 1> (Preparation of Fresh Catalyst)

In 224 kg of ion-exchange water heated to 40°C, 38.2 kg of cesium nitrate [CsNO3], 27.4 kg of 75% by weight orthophosphoric acid and 25.2 kg of 70% by weight nitric acid were dissolved, to prepare Solution A.

Separately, 297 kg of ammonium molybdate tetrahydrate [ (NHg) 6M07024-4H,0] was dissolved in 330 kg of ion-exchange water heated to 40°C, followed by suspending 8.19 kg of ammonium metavanadate [NH,VO3] therein, to prepare Solution

B.

Solutions A and B were adjusted to 40°C, and Solution

A was added dropwise to Solution B while stirring, then the mixture was further stirred for 5.8 hours at 120°C in a closed vessel. Subsequently, 10.2 kg of antimony trioxide [Sb203] and 10.2 kg of copper nitrate trihydrate [Cu(NO3)2-3H,0] was suspended in 23 kg of ion-exchange water and added thereto. Thereafter, the mixture was stirred at 120°C for 5 hours in the closed vessel.

The mixture thus obtained was spray-dried with a spray dryer. To 100 parts by weight of the dried powder, 4 parts by weight of ceramic fibers, 13 parts by weight of ammonium nitrate and 9.7 parts by weight of ion-exchange water were added, and the mixture was kneaded and extrusion-molded into cylinders each having a diameter of 5 mm and a height of 6 mm. The molded product was dried at a temperature of 90°C and a relative humidity of 30% for 3 hours, then heat- treated (pre-calcined) at 220°C for 22 hours in an air stream and then at 250°C for 1 hour in an air stream, and thereafter heated to 435°C in a nitrogen stream and kept at the same temperature for 3 hours. Further, the product was cooled to 300°C in a nitrogen stream, and then nitrogen was changed to air containing 3.5% by volume of moisture. The product was heated to 390°C in the air stream and kept at the same temperature for 3 hours. Thereafter, the product was cooled to 70°C in an air stream, and a fresh catalyst was taken out.

This fresh catalyst was made of an acid salt of a

Keggin-type heteropolyacid containing phosphorus, molybdenum, vanadium, antimony, copper and cesium at an atomic ratio of 1.5, 12, 0.50, 0.5, 0.3 and 1.4 respectively. Here, molybdenum trioxide was not detected.

The results of the activity test and life test of this fresh catalyst are shown in Table 1. <Reference Example 2> (Preparation of Used Catalyst and Activity Test

Thereof)

The fresh catalyst obtained in Reference Example 1 was subjected to a catalytic gas phase oxidation reaction of methacrolein for a long time, to obtain a used catalyst.

The metal elements contained in this used catalyst were phosphorus, molybdenum, vanadium, antimony, copper and cesium at an atomic ratio of 1.3, 9.9, 0.49, 0.5, 0.3 and

1.4 respectively. Here, molybdenum trioxide was detected.

The activity test was performed for this used catalyst.

The result is shown in Table 1. <Example 1> (Preparation of Regenerated Catalyst)

To 7.32 kg of ion-exchange water was added 4.20 kg of the used catalyst obtained in Reference Example 2, and the mixture was stirred. The kinds and amounts of the deficient components (dissipated components) of the used catalyst for the fresh catalyst obtained in Reference

Example 1 were calculated by a fluorescent X-ray analysis.

In order to compensate the deficient components, 0.65 kg of molybdenum trioxide [MoOs], 0.06 kg of 75% by weight orthophosphoric acid, and 0.004 kg of ammonium metavanadate [NH,VO3] were added thereto.

Next, a solution prepared by adding 1.44 kg of ammonium nitrate [NH4NO3] to 1.08 kg of ion-exchange water was added thereto, and the mixture was heated to 70°C and kept at the same temperature for 1 hour. Thereafter, 0.26 kg of 25% by weight aqueous ammonia was added. The mixture was kept at 70°C for 1 hour and then stirred for 5 hours at 120°C in a closed vessel. The amount of the ammonium ion in the aqueous slurry A was 1.2 mol based on 1 mol of the nitrate ion. Also, the metal elements contained in the aqueous slurry A were phosphorus, molybdenum, vanadium,

antimony, copper and cesium at an atomic ratio of 1.5, 12, 0.50, 0.5, 0.3 and 1.4 respectively, and the atomic ratio of cesium to molybdenum was 1.4 : 12.

The aqueous slurry A was spray-dried at 120°C using a spray dryer. To the resulting dried product, 9 parts by weight of ammonium nitrate and 8 parts by weight of ion- exchange water were added based on 100 parts by weight of the dried product, and the mixture was kneaded and extrusion-molded into cylinders each having a diameter of 5 mm and a height of 6 mm. The molded product was dried at a temperature of 90°C and a relative humidity of 30% for 3 hours and thereafter heat-treated (pre-calcined) at 220°C for 22 hours and then at 250°C for 1 hour in an air stream, to obtain a pre-calcined catalyst precursor made of a

Keggin-type heteropolyacid salt.

Subsequently, the catalyst precursor was heated to 390°C in a stream of a mixed gas of alr and steam (the content of water is 1.4% by volume) and kept at the same temperature for 4 hours, to carry out the first calcination.

Thereafter, air was changed to nitrogen, and the catalyst precursor was heated to 435°C in a nitrogen stream and kept at the same temperature for 4 hours, to carry out the second calcination. Thereafter, the resulting product was cooled to 70°C in a nitrogen stream, and then regenerated catalyst (1) was recovered.

This regenerated catalyst (1) was made of an acid salt of a Keggin-type heteropolyacid containing phosphorus, molybdenum, vanadium, antimony, copper and cesium at an atomic ratio of 1.5, 12, 0.50, 0.5, 0.3 and 1.4 | respectively. Here, molybdenum trioxide was not detected.

The results of the activity test and life test of this regenerated catalyst (1) are shown in Table 1. <Example 2> (Preparation of Regenerated Catalyst)

The same procedures as in Example 1 were carried out except that the catalyst precursor was heated to 390°C in a stream of a mixed gas of air and steam (the content of water is 0.5% by volume), in place of the stream of a mixed gas of air and steam (the content of water is 1.4% by volume) in the first calcination in Example 1, to obtain regenerated catalyst (2). Here, molybdenum trioxide was not detected.

The results of the activity test and life test of this regenerated catalyst (2) are shown in Table 1. <Comparative Example 1> (Preparation of Regenerated Catalyst)

The same procedures as in Example 1 were carried out except that the catalyst precursor was heated to 390°C in a stream of a mixed gas of air and steam (the content of water is 2.8% by volume), in place of the stream of a mixed gas of air and steam (the content of water is 1.4% by volume) in the first calcination in Example 1, to obtain regenerated catalyst (Cl). Here, molybdenum trioxide was not detected.

The results of the activity test and life test of this regenerated catalyst (Cl) are shown in Table 1. <Comparative Example 2> (Preparation of Regenerated Catalyst)

The same procedures as in Example 1 were carried out except that the catalyst precursor was heated to 390°C in an alr stream (the content of water is 0% by volume), in place of the stream of a mixed gas of air and steam (the content of water is 1.4% by volume) in the first calcination in Example 1, to obtain regenerated catalyst (C2). Here, molybdenum trioxide was detected.

The result of the activity test of this regenerated catalyst (C2) is shown in Table 1.

In the following Table 1, examples are described in ascending order of the content of water in the mixed gas of air and steam in the first calcination.

oO j= 0S wl ~

Hl GE 5

Lx ~ | ~ | o~ | ~ ) > >

PP

0 7

Q

BD

< 5 -— 0)

Hoo — ~N ~N <

Qo mM eo] [0)} aN a

O

0

Tee ee o | = | «

J 0-0 gloeo =

AZ

— oc 8S

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Claims

WHAT IS CLAIMED IS:

1. A method for regenerating a catalyst for production of methacrylic acid comprising a heteropolyacid compound containing phosphorus and molybdenum, comprising the steps of: drying an aqueous slurry containing a used catalyst having been used for production of methacrylic acid, an ammonium ion, a nitrate ion and water to obtain a catalyst precursor; : firstly calcining the catalyst precursor in an atmosphere of an oxidizing gas containing 0.1% by volume or more and less than 2.0% by volume of moisture at 360 to 410°C; and then secondly calcining the product in an atmosphere of a non-oxidizing gas at 420 to 500°C.

2. The method according to claim 1, wherein the heteropolyacid compound contains at least one element X selected from the group consisting of potassium, rubidium, cesium and thallium and has an atomic ratio of the element X to molybdenum contained in the aqueous slurry (X : Mo) of

0.5 : 12 to 2 : 12.

3. The method according to claim 1 or 2, wherein the amount of the ammonium ion contained in the aqueous slurry is 1.3 mol or less based on 1 mol of the nitrate ion.

4. The method according to any one of claims 1 to 3, wherein the aqueous slurry is heat-treated at 100°C or more and then dried to produce the catalyst precursor.

5. The method according to any one of claims 1 to 4, wherein the catalyst precursor is exposed to an atmosphere having a relative humidity of 10 to 60% at 40 to 100°C for

0.5 to 10 hours and then subjected to the first calcination.

6. The method according to any one of claims 1 to 5, wherein the heteropolyacid compound further comprises vanadium, and at least one element selected from the group consisting of copper, arsenic, antimony, boron, silver, bismuth, iron, cobalt, lanthanum and cerium.

7. A process for producing methacrylic acid, comprising the steps of: regenerating a catalyst for production of methacrylic acid by the method as defined in any one of claims 1 to 6; and subjecting a compound selected from methacrolein, isobutylaldehyde, isobutane and isobutyric acid to a gas phase catalytic oxidation reaction in the presence of the regenerated catalyst.