Process of dehydration reactions

Abstract

The present invention relates to the production of acrolein, acrylic acid or methacrylic acid by dehydration reaction of renewable raw material such as glycerin or hydroxycarboxylic acids, in the presence of a novel catalyst system supported on a carrier having a bimodal structure and a high pore volume and distribution. The dehydration reactions can be carried out for longer operation duration, so that acrolein, acrylic acid or methacrylic acid can be produced at higher productivity and for longer running time.

Claims

1. A process for preparing acrolein by catalytic dehydration reaction of glycerin comprising the step of dehydrating glycerin in the presence of a supported catalyst comprising a W-containing metal oxide supported on a bipore or bimodal porous carrier, said porous carrier being made of TiO.sub.2 or a compound which is a mixture of TiO.sub.2 and at least one metal oxide selected from SiO.sub.2, Al.sub.2O.sub.3, ZrO.sub.2 or Nb.sub.2O.sub.5, wherein the W-containing metal oxide is represented by formula (I):
Aa Xb Wc Zd Oe(I) wherein A is a cation selected from the group consisting of elements of Periodic Table Groups 1 to 16, X is selected from the group consisting of P, Si, Mo and V, W is tungsten, Z is at least one element selected from the group consisting of Ti, Cr, Mn, Fe, Co, Ni, Zn, Ga, Sn, Bi, Sb, Ce, Mg, Cs and K, a, b, c and d satisfy the following ranges: 0a<9; 0b1; 0<c20; 0d20; and e is a value determined by oxidation numbers of each element and which is not 0; and wherein a ratio in the pore volume of macropores having a pore size of not smaller than 50 nm to the pore volume of mesopores having a pore size of from larger than 2 nm to smaller than 50 nm being higher than 0.5, the pore volume of said porous carrier being higher than or equal to 0.30 cm.sup.3/g, the pore volume being measured by the mercury intrusion method, and wherein a mean pore diameter of said porous carrier measured by the mercury intrusion method is between 30 nm to 100 nm.

2. The process according to claim 1, wherein the supported catalyst comprises another metal oxide, in addition to said W-containing metal oxide, of at least one metal selected from the group consisting of P, Si, Mo, and V.

3. The process according to claim 1, wherein the pore volume of said porous carrier measured by mercury intrusion method is higher than 0.30 cm.sup.3/g.

4. The process according to claim 1, wherein the mean pore diameter of said porous carrier measured by the mercury intrusion method is larger than 50 nm.

5. The process according to claim 1, wherein a salt of at least one element selected from elements belonging to Periodic Table Groups 1 to 16 is added to the catalyst other than the carrier.

6. The process according to claim 1, wherein said metal oxide represented by the formula (I) has a weight that is from 1% to 90% of the weight of said carrier.

7. A process for preparing acrylic acid from glycerin comprising a first step of catalytic dehydration of glycerin according to claim 1 to form a gaseous reaction product containing acrolein and a second step of gas phase oxidation of the gaseous reaction product containing acrolein formed by the dehydration reaction.

8. The process according to claim 7, further comprising an intermediate step of partial condensation and removal of water and heavy by-products issuing from the dehydration step.

9. The process according to claim 8, further comprising steps of (i) collecting resultant acrylic acid as a solution-containing-acrylic acid by using water or a solvent and (ii) purifying solution-containing-acrylic acid using distillation and/or crystallization.

10. A process for preparing acrylonitrile from glycerin comprising a first step of catalytic dehydration of glycerin according to claim 1 and a second step of ammoxidation of gaseous reaction product containing acrolein formed by the dehydration.

11. A process for preparing acrylic acid and methacrylic acid by catalytic dehydration reaction of hydroxycarboxylic acid, wherein the dehydration reaction of hydroxycarboxylic acid is carried out in the presence of a supported catalyst comprising a W-containing metal oxide supported on a bipore or bimodal porous carrier, said porous carrier containing at least one metal oxide selected from the group consisting of TiO.sub.2, SiO.sub.2, Al.sub.2O.sub.3, ZrO.sub.2 and Nb.sub.2O.sub.5, a ratio of the pore volume of macropores having a pore size of not smaller than 50 nm to the pore volume of mesopores having a pore size of from larger than 2 nm to smaller than 50 nm being higher than 0.5, the pore volume being measured by the mercury intrusion method, and wherein a mean pore diameter of said porous carrier measured by the mercury intrusion method is between 30 nm to 100 nm.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The Catalyst

(2) Physical properties of pores such as a pore diameter and a pore volume of the porous carrier according to this invention are determined by the mercury intrusion method at a mercury surface tension of 480 dyne/cm with a mercury contact angle of 140 by using Mercury Porosimetry (Pore Master 60-GT, Quanta Chrome Co.). Values or positions (nm) of mesopores and macropores shown in this specification are values of the maximum pore diameter of each pore obtained in a correlation graph having axis of ordinates of dV/d(log d) [cm.sup.3/g] which expresses how much mercury penetrate in the mercury intrusion method (expressed in cm.sup.3 for logarithm m for 1 g of sample) and axis of abscissas of the pore diameter. The standard test method is ASTM D 4284-83.

(3) The catalyst is a supported catalyst on a bipore carrier or a carrier having bimodal porous structure.

(4) Namely, in the bipore carrier defined in this invention, the definition of pore size of from larger than 2 nm to smaller than 50 nm for mesopore and of not smaller than 50 nm for macropore means the value of pore diameter that the peak maximal value of each pore is located at. The mesopore is in a range of from larger than 2 nm to smaller than 50 nm and preferably in a range of 10 to 30 nm. The macropore is not smaller than 50 nm and preferably in a range of 50 to 300 nm.

(5) In the present invention, the ratio of the pore volume of macropores which are not smaller than 50 nm to the pore volume of mesopores which are within a range of from larger than 2 nm to smaller than 50 nm is higher than 0.5. This ratio is obtained by dividing a macropore volume (volume of pores having a size not smaller than 50 nm) and a mesopore volume (volume of pores having a size between from larger than 2 nm to smaller than 50 nm). In the present invention, the ratio of the macropore volume to the mesopore is higher than 0.5, preferably higher than 1.0. By selecting the above ratio, it is possible to obtain a long life catalyst for dehydration of glycerin, so that decrease in time of the conversion of glycerin and of the yield of acrolein can be prevented, in addition that acrolein can be produced at higher yield.

(6) A feature of the pore volume (the cumulative pore volume or the total pore volume) of higher than or equal to 0.30 cm.sup.3/g determined by the mercury intrusion method according to the present invention is also important. By using the porous carrier having the above pore volume preferably of higher than 0.30 cm.sup.3/g, it is possible to obtain a long life catalyst for dehydration of glycerin which permits to prevent decrease in time of the conversion of glycerin and of the yield of acrolein.

(7) Another feature of a mean pore diameter measured by the mercury intrusion method being larger than 30 nm according to in this invention is also important. The mean pore diameter is the median pore diameter or the pore diameter with assuming that the pore is cylindrical. When the cumulative pore volume is identical, if the mean pore diameter is not larger or is smaller than 30 nm, the pore volume of macropores decreases and the pore volume of mesopores increases. It was thought that the conversion of glycerin may advantageously increase when the specific surface area increase owing to increase of the pore volume of mesopores. In practice, however, decrease in catalytic activity advance rapidly because of acceleration of coking which may be caused by decrease in the efficiency of material transportation of products generated in the mesopores because of decrease of the pore volume of mesopores.

(8) Therefore, the pore volume of the mesopores also is one of important factors which extend the catalyst life in the process according to the present invention, from such view point to increase the efficiency of the material transportation and also to prevent decrease of catalytic life caused by coking in the mesopores which is thought main reaction site. However, to avoid a risk of decrease in mechanical strength caused by increase of mean pore diameter, in particular of pore diameter of the macropores, it is advantageous to select the mean pore size of 30 nm to 100 nm.

(9) In this invention it is also important that the volume-based mode diameter of the bipore carrier measured by the mercury intrusion method is larger than 50 nm. Term volume-based mode diameter means the maximum pore diameter into which mercury penetrate mostly in volume. In the present invention, the pore diameter having the maximal value of volume in the porous carrier is larger than 50 nm. For example, in case of bipore carrier having both of mesopore and macropore, when the pore volume of macropores is higher than that of mesopores, the maximum peak of macropores is located at a pore diameter of larger than 50 nm, and when the pore volume of mesopores is higher than that of macropores, the maximum peak of mesopores is located at a pore diameter of smaller than 50 nm. For a porous carrier according to this invention, the former is preferable. In fact, it is preferable that the ratio in pore volume of the macropores to the mesopores is higher than 1.0 and that the volume-based mode diameter is higher than 50 nm.

(10) According to another preferred embodiment of this invention, the specific surface of the porous carrier measured by BET method of nitrogen adsorption is 10 to 1000 m.sup.2/g, preferably 20 to 500 m.sup.2/g and more preferably 30 to 200 m.sup.2/g. Increase of the specific surface may enhance higher dispersion of supported catalyst to improve activation. However, in case of the dehydration reaction of glycerin, since the reactivity of glycerin is very high and carrier itself often possesses activity, so that the carrier itself proceed successive reactions and parallel reactions. To prevent such reactions, it is necessary to increase the amount of active species on the catalyst. Therefore, preferable specific surface is 30 to 100 m.sup.2/g.

(11) The carrier and the catalyst can have any shape without limitation such as granules, powder, pellets, rings, trilobes, quadrilobes. In case of for gas phase reaction, the catalyst can be molded into sphere, cylinder, hollow cylinder, trilobe (with or without hole), quadrilobe (with or without hole) or bars, by using a molding aid if necessary. It is also possible to shape the catalyst together with carrier and optional molding aid. By using shape such as trilobes or quadrilobes, it is possible to obtain grains of catalyst with higher size leading to reduction of pressure drop while effecting reaction. Example of a particle size of molded catalyst for sphere shape is 1 to 10 mm for a fixed bed catalyst, and a particle size of not larger than 1 mm for a fluidized bed catalyst.

(12) Preparation of the Catalyst

(13) Source of tungsten in the W-containing metal oxide as a component to be supported on the bipore carrier is not limited specially and may be any material such as paratungstic acid or its ammonium salt, metatungstic acid or its ammonium salt, tungstic acid or its ammonium salt, tungsten oxide, tungsten chloride or organotungsten compound such as tungsten ethoxide and hexacarbonyl tungsten.

(14) The catalyst may contain as an optional component to be supported on the bipore carrier, in addition to the above W-containing metal oxide, other metal oxide of at least one metal selected from a group comprising P, Si, Mo and V. In this case, a source material of the metal oxide is preferably heteropolyacid comprising the above elements.

(15) Here, the heteropolyacid is explained briefly. Ions of tungsten and molybdenum become oxoacids in water and the resulting oxoacid is polymerized to form high molecular polyoxoacid. In this case, not only same kind of oxoacids are polymerized but also surrounding other oxoacid or oxoacids also are polymerized, resulting in formation of a polyacid consisting of more than two oxoacids, which is called heteropolyacid having a polynuclear structure. An atom that forms a center oxoacid is called as hetero-atom, while atoms forming oxoacids surrounding the center oxoacid and forming oxoacid is called as poly-atoms. The hetero-atom may be silicon, phosphorus, arsenic, sulfur, iron, cobalt, boron, aluminum, germanium, titanium, zirconium, cerium and chromium. Among them, phosphorus and silicon are preferable. The poly-atoms may be molybdenum, tungsten, vanadium, niobium and tantalum. In this invention, at least tungsten is always contained. Therefore, preferred heteropolyacid used in the glycerin dehydration is tungstophosphoric acid and tungstosilicic acid. The heteropolyacid may be a mixed coordinate type comprising phosphorus or silicon as the hetero-atom and molybdenum and tungsten as the poly-atoms and a mixed coordinate type of molybdenum and tungsten. It is known that the heteropolyacid have different structures such as Keggin type, Dawson type and Anderson type. The heteropolyacid possess generally such high molecular weight as 700 to 8,500. There is a dimmer thereof, which is included in this invention.

(16) The catalyst may contain as an optional component to be supported on the bipore carrier, in addition to the above W-containing metal oxide and optionally other metal oxide of at least one element selected from a group comprising P, Si, Mo and V, at least one cation selected from the a group comprising cations belonging to Group 1 to Group 16 of the Periodic Table of Elements.

(17) The cations belonging to Group 1 to Group 16 of the Periodic Table of Elements may be its acid salt and acid onium salts. The acid salt may be salts of sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, scandium, yttrium, lanthanide, titanium, zirconium, hafnium, chromium, manganese, rhenium, iron, ruthenium, osmium, cobalt, nickel, palladium, platinum, copper, silver, gold, zinc, gallium, thallium, germanium, tin, lead, bismuth and tellurium. The acid onium salts may be amine salt, ammonium salt, phosphonium salt and sulfonium salt. Sources of the acid salt and the acid onium salts may be nitrate, carbonate, sulfate, acetate, oxides and halide of the metals or oniums. Above-mentioned are simple examples but are not limit the scope of invention. An amount of metals or onium salt is 0.01 to 60% by weight, preferably 0.01 to 30% by weight with respect to W and optional at least one element selected from P, Si, Mo and V in addition to W.

(18) The catalyst can be prepared by any known technique such as impregnation method. As materials, nitrate, ammonium salt, hydroxide, oxide, acid of each metal element which constitutes the active components of the catalyst can be used without limitation. The catalyst supporting W-containing metal oxide also can be prepared by any known technique. In practice, an aqueous solution containing compounds which contain W and optional element selected from P, Si, Mo and V in addition to W is prepared firstly. In case of heteropolyacid also, an aqueous solution thereof is firstly prepared. Or, their aqueous solutions can be prepared after water contained in the heteropolyacid in a form of adsorptive water and crystal water is removed partially or totally under vacuum or heat-drying.

(19) The resulting aqueous solution of W-containing metal oxide is added with a carrier containing at least one metal oxide selected from a group comprising TiO.sub.2, SiO.sub.2, Al.sub.2O.sub.3, ZrO.sub.2 and Nb.sub.2O.sub.5. Optionally, an aqueous solution containing a compound or compounds containing halide, carbonate, acetate, nitrate, oxalate, phosphate, sulfate, hydroxide of metal or onium of element belonging to Group 1 to Group 16 of the Periodic Table of Elements is added. The resulting mixture is subjected to filtration or drying under reduced pressure to obtain a solid which is then calcinated finally.

(20) Addition of the salts of metal or onium of element belonging to Group 1 to Group 16 of the Periodic Table of Elements can be effected after, before or during addition of the aqueous solution containing compounds which contain W and optional element selected from P, Si, Mo and V in addition to W to porous carrier.

(21) The calcination and drying can be carried out successively for each addition of catalytic component to carrier, or can be done after all catalytic components are added.

(22) In one embodiment, the preparation of the catalyst comprises more than one cycle of contacting a carrier with a solution of the active components of the catalyst, drying and calcining the resulting solid mixture. The contacting may be performed by techniques of pore volume impregnation or excess solution impregnation. The multiple contacting procedure may be performed with different solutions at each step.

(23) The calcination can be carried out in air or under inert gas such as nitrogen, helium and argon or under an atmosphere of a mixed gas of air and inert gas. The calcination is effected advantageously under an atmosphere of air which facilitates operations. Furnace for calcination is not limited specially and can be muffle furnace, rotary kiln, fluidized bed furnace. The calcination can be effected even in a reaction tube which is used for the glycerin dehydration reaction.

(24) Selection of the firing temperature of 400 to 900 C. is another important feature of this invention. In fact, in dehydration reactions, formation or precipitation of cokes per unit time is extremely large, so that the life of catalyst is short, resulting in that much frequent regeneration of catalyst is required. The regeneration can be carried out by a circulation of oxygen-containing gas which generates a large quantity of heat to remove cokes by burning. Sufficient burning and removal of cokes can't be expected at lower temperatures. In any way, the catalyst is exposed after all to a high temperature during reaction and regeneration stage even if the calcination of catalyst is effected at lowered temperature. Therefore, in case of a catalyst which had been fired at lower temperature, there may be a difference in performance between at an initial of reaction and after regeneration.

(25) In order to realize the same or constant performance after several reaction and regeneration cycles as at the initial reaction, and hence to facilitate operations in reaction and regeneration, it is advantageous to effect calcination at higher temperature. In case of heteropolyacid such as phosphotungstic acid and tungstosilicic acid, decomposition of their structures starts at about 350 C. and the structure of heteropolyacid will be lost when it is heated above this temperature. In case of phosphotungstic acid, it will change to oxides of phosphorus and tungsten or to complex oxide other than heteropolyacid. Use of heteropolyacid such as phosphotungstic acid as a raw material of W is suitable, but it does not show the structure of heteropolyacid because the firing temperature of 400 to 900 C. which is one of features of this invention. The firing temperature is preferably 450 to 800 C. and the firing time duration is preferably 0.5 to 10 hours.

(26) Dehydration of Glycerin

(27) The dehydration reaction of glycerin according to this invention can be carried out in gas phase or in liquid phase and the gas phase is preferable. The gas phase reaction can be carried out in a variety of reactors such as fixed bed, fluidized bed, circulating fluidized bed and moving bed. Among them, the fixed bed or the fluidized bed is preferable. Regeneration of the catalyst can be effected outside the reactor. When the catalyst is taken out of a reactor system for regeneration, the catalyst is burnt in air or in oxygen-containing gas. Regeneration of the catalyst can also be effected within the reactor, with cycles that comprise a reaction period and a regeneration period. During the regeneration period, air or an oxygen-containing gas is injected in the reactor. Preferably, two or more reactors are used in parallel so that at any time part of the reactors are used in reaction mode and part of them are used in regeneration mode.

(28) In case of liquid phase reaction, usual general reactors for liquid reactions for solid catalysts can be used. Since the difference in boiling point between glycerin (290 C.) and acrolein and acrylic acid is big, the reaction is effected preferably at relatively lower temperatures so as to distil out acrolein continuously.

(29) The reaction temperature for producing acrolein and acrylic acid by dehydration of glycerin in gas phase is effected preferably at a temperature of 200 C. to 450 C. If the temperature is lower than 200 C., the life of catalyst will be shortened due to polymerization and carbonization of glycerin and of reaction products because the boiling point of glycerin is high. On the contrary, if the temperature exceeds 450 C., the selectivity of acrolein and acrylic acid will be lowered due to increment in parallel reactions and successive reactions. Therefore, more preferable reaction temperature is 250 C. to 350 C. The catalyst regeneration is effected at a reactor temperature of 250 C. to 450 C. and preferably between 290 C. and 370 C.

(30) The pressure is not limited specially but is preferably lower than 5 atm and more preferably lower than 3 atm. Under higher pressures, evaporated glycerin will be condensed and deposition of carbon will be promoted by higher pressure so that the life of catalyst will be shortened.

(31) A feed rate of a material gas is preferably 500 to 10,000 h.sup.1 in term of the space velocity of GHSV (Gas Hourly Space Velocity, defined as the ratio between the gas flow rate in Normal Temperature and Pressure conditions and the volume of catalyst). The selectivity will be lowered if the GHSV becomes lower than 500 h.sup.1 due to successive reactions. On the contrary, if the GHSV exceeds 10,000 h.sup.1, the conversion will be lowered.

(32) The reaction temperature of the liquid phase reaction is preferably from 150 C. to 350 C. The selectivity will be reduced under highest temperatures although the conversion is improved. The reaction pressure is not limited specially but the reaction can be carried if necessary under a pressurized conditions of 3 atm to 70 atm.

(33) The material of glycerin is easily available in a form of aqueous solution of glycerin. In the invention, glycerin or glycerol may be used.

(34) The gases fed to the reactor comprise glycerol, water, oxygen and inert gases such as nitrogen, argon, CO, CO.sub.2.

(35) The water to glycerol weight ratio which is fed to the reactor is 20/1 to 1/20 and preferably 5/1 to 1/2 and most preferably 4/1 to 1/1.

(36) The concentration of glycerin in a mixed gas which is fed in the process of the invention is 1 to 30 mol %, preferably 1 to 12 mol % and more preferably 3 to 10 mol %. Too high concentration of glycerin will result in such problems as production of glycerin ethers or undesirable reaction between the resulting acrolein and acrylic acid and material glycerin. Temperature that is necessary to evaporate glycerin is increased.

(37) The glycerin dehydration reaction of this invention is advantageously carried out in the presence of oxygen-containing gas such as oxygen or air. A concentration of oxygen is 1 to 10 mol %), preferably 2 to 7 mol %).

(38) Dehydration Reactions of Hydroxycarboxylic Acids

(39) The dehydration reactions of hydroxycarboxylic acids such as 2-hydroxypropionic acid or 3-hydroxypropionic acid and 2-hydroxyisobutyric acid or 3-hydroxyisobutyric acid can be carried out in the vapor phase or in the liquid phase in a variety of reactors such as fixed bed, fluidized bed, circulating fluidized bed and moving bed. Among them, the fixed bed or the fluidized bed are preferable.

(40) Dehydration reaction temperature is generally from 100 C. to 400 C., preferably from 200 C. to 350 C., and the pressure is about from 0.5 to 5 atm.

(41) Vapor phase reactions normally require higher temperatures than liquid phase reactions.

(42) It is possible and often desirable to include an inert gas such as nitrogen or recycle gas in the feed along with the hydroxycarboxylic acid.

(43) In the dehydration reaction of 2-hydroxypropionic acid or 3-hydroxypropionic acid to produce acrylic acid, the partial pressure is generally between 1 and 10%, preferably between 2 and 6%.

(44) In the dehydration reaction of 2-hydroxyisobutyric acid or 3-hydroxyisobutyric acid to produce methacrylic acid, the partial pressure is generally between 1 and 20%, preferably between 2 and 10%.

(45) The time for the conversion of the hydroxcarboxylic acid will vary. Reactions in the vapor phase are generally faster than those run in the liquid phase and occur within a few seconds, while reactions in the liquid phase can take from about 1 to about 6 hours. The contact time in the vapor phase is normally 0.1-15, usually 2-4 seconds.

(46) The amount of dehydration catalyst according to the invention is subject to considerable variation and is not critical.

(47) In the processes of the invention, by using the improved catalyst, the dehydration reactions can be carried out for longer operation duration, so that acrolein, acrylic acid or methacrylic acid can be produced at higher productivity and for longer running time.

(48) Owing to improvement of catalysis life, the efficiencies of catalytic reaction and of regeneration cycle of catalyst can be improved, so that complicated operations can be simplified, which is very beneficial for industrial plants. Mainly, with longer catalyst life in a fixed bed reactor, the number of reactors needed for a continuous operation is reduced since the cycle length for each reactor is longer. This contributes in a significant capital cost reduction. Longer catalyst cycle life give also more degree of freedom to optimise the regeneration parameters, since the duration of the regeneration can be extended, either to have a more complete regeneration of the catalyst, or preferably to operate the catalyst regeneration at a lower average temperature. A lower catalyst regeneration is preferable since it limits the catalyst degradation by high process temperature. It has been discovered that the bipore catalyst can be regenerated efficiently at a temperature comprised between 250 and 350 C., within the same duration than the catalyst reaction cycle.

(49) When the catalyst is used in a fluid bed, the regeneration can be done continuously using an internal or external regenerator. With the bipore catalyst of extended life it is possible to use a smaller regenerator.

(50) Finally, whatever the reactor technology, a catalyst having an extended life, requires less oxygen to be co-fed during the catalyst reaction cycle. Thereby reducing the flammability issues, especially in downstream equipments like absorption units.

(51) Manufacturing Acrylic Acid

(52) The resulting acrolein from the catalytic dehydration of glycerin is further oxidized to produce acrylic acid, according to the methods well known to the skilled.

(53) In one embodiment, the process for preparing acrylic acid from glycerin comprises an intermediate step of partial condensation and removal of water and heavy by-products issuing from the dehydration step.

(54) The said intermediate step has the aim of removing most of the water present and the heavy by-products before sending the gaseous stream comprising the acrolein and all non-condensable gases to the step for the oxidation of acrolein to give acrylic acid. This partial condensation of the water thus makes it possible to avoid damage to the catalyst of the oxidation of acrolein to give acrylic acid and to avoid, during the subsequent stages, the removal of large amounts of water, which could well be expensive and result in losses of acrylic acid. In addition, it makes it possible to remove a portion of the heavy impurities formed during the dehydration of the glycerol and to facilitate purification operations.

(55) This intermediate step is carried out on a separating unit which is a condensation plant comprising an absorption column coupled or not coupled to an evaporator, one or more heat exchangers, one or more condensers, a dephlegmator, and any item of equipment well known to a person skilled in the art which makes it possible to carry out a partial condensation of an aqueous stream.

(56) It is carried out under conditions such that the acrolein-rich stream is separated into an acrolein-rich gaseous phase and an acrolein-poor aqueous phase.

(57) From 20 to 95%, preferably from 40 to 90%, of the water present in the stream is removed in the liquid stream and the acrolein-rich phase generally comprises more than 80% and preferably more than 90% of the acrolein initially present in the stream. This result is generally obtained by lowering the temperature to a temperature of 60 C. to 120 C.

(58) The process for preparing acrylic acid according to the invention further comprises the steps of collecting the resultant acrylic acid as a solution by using water or a solvent, and then of purifying the resultant solution containing acrylic acid by using for example a distillation step for removing low- and high-boiling point materials and/or a crystallization step for purifying acrylic acid by crystallizing it.

(59) The acrylic acid thus obtained can be used to produce for example polyacrylic acids or salts as water-soluble polymers or water-absorbent resins, by known methods.

(60) Now, the present invention will be explained in detail with referring illustrative examples but this invention should not be limited to those described in following examples. In the following Examples and Comparative Examples, % means mole %.

EXPERIMENTAL SECTION

Preparation of Catalysts

Example 1

(61) Catalyst of PW/TiO.sub.2 for dehydration reaction of glycerin was prepared as following: 13.2 g of phosphotungstic acid (Nippon Inorganic Color & Chemical Co., Ltd.) were dissolved in 100 ml of pure water to obtain an aqueous solution of phosphotungstic acid. Separately, 100 g of Anatase TiO.sub.2 pellets (Saint-Gobain ST 31119: diameters of 3.2 mmlength of 5 mm, 48.2 m.sup.2/g, 0.36 cm.sup.3/g, a pore volume ratio=1.4) was placed on a porcelain dish onto which the above aqueous solution of phosphotungstic acid was added. After an assembly was left for 2 hours, the aqueous solution of phosphotungstic acid was dried-up at 120 C. for 10 hours and then calcinated in an atmosphere of air at 500 C. for 3 hours.

Example 2

(62) Example 1 was repeated, except Anatase TiO.sub.2 pellet (Saint-Gobain ST 31119: diameters of 3.2 mmlength of 5 mm, 29.2 m.sup.2/g, 0.30 cm.sup.3/g, the pore volume ratio=1.5) was used as TiO.sub.2.

Example 3

(63) Example 1 was repeated, except Anatase TiO.sub.2 pellet (Saint-Gobain ST 31119: diameters of 3.2 mmlength of 5 mm, 35.6 m.sup.2/g, 0.33 cm.sup.3/g, the pore volume ratio=1.1) was used as TiO.sub.2.

(64) The cumulative pore volume, the ratio of pore volume and the mean pore diameter of Examples 2 and 3 are within a range of this invention.

Example 4

(65) Example 1 was repeated, except Anatase TiO.sub.2 pellet (Saint-Gobain ST 31119: diameters of 3.2 mmlength of 5 mm, 39.9 m.sup.2/g, 0.36 cm.sup.3/g, the pore volume ratio=1.3) was used as TiO.sub.2.

Example 5

(66) Example 1 was repeated, except Anatase TiO.sub.2 pellet (Saint-Gobain ST 31119: diameters of 3.2 mmlength of 5 mm, 45.6 m.sup.2/g, 0.36 cm.sup.3/g, the pore volume ratio=1.4) was used as TiO.sub.2.

Example 6

(67) Catalyst of W/TiO.sub.2 for dehydration reaction of glycerin was prepared as following: 12.2 g of ammonium metatungstate (Nippon Inorganic Color & Chemical Co., Ltd.) were dissolved in 100 ml of pure water to obtain an aqueous solution of ammonium metatungstate. Separately, 100 g of Anatase TiO.sub.2 pellets (Saint-Gobain ST 31119: diameters of 3.2 mmlength of 5 mm, 48.2 m.sup.2/g, 0.36 cm.sup.3/g, a pore volume ratio=1.4) was placed on a porcelain dish onto which the above aqueous solution of was added. After an assembly was left for 2 hours, the aqueous solution of ammonium metatungustate was dried up at 120 C. for 10 hours and then calcinated in an atmosphere of air at 500 C. for 3 hours.

Example 7

(68) Example 1 was repeated, except Anatase TiO.sub.2 ring (Saint-Gobain ST 31119: diameters of 4.5 mmlength of 5 mm, 54 m.sup.2/g, 0.32 cm.sup.3/g, the pore volume ratio=0.5) was used as TiO.sub.2.

Example 8

(69) Example 1 was repeated, except Anatase TiO.sub.2 trilobes (Saint-Gobain ST 31119: diameters of 4.5 mmlength of 5 mm, 53 m.sup.2/g, 0.40 cm.sup.3/g, the pore volume ratio=0.9) was used as TiO.sub.2.

Example 9

(70) Example 1 was repeated, except Anatase TiO.sub.2 quadrilobes (Saint-Gobain ST 31119: diameters of 4.5 mmlength of 5 mm, 54 m.sup.2/g, 0.38 cm.sup.3/g, the pore volume ratio=0.9) was used as TiO.sub.2.

Comparative Example 1

(71) In Comparative Example 1, a sample which is outside the scope of invention was prepared and evaluated.

(72) Example 1 was repeated, except Anatase TiO.sub.2 pellet (Sakai Chemistry CS-300 S: diameter of 3.0 mm, 75 m.sup.2/g, 0.40 cm.sup.3/g, the pore volume ratio=0.4) was used as TiO.sub.2.

(73) Comparative Example 1 is outside the present invention because the ratio in pore volume of the macropores to the mesopores is 0.4 which is outside a requirement of higher than 0.5 of this invention. In fact, its mean pore diameter is as small as 23.2 nm due to larger pore volume of the mesopores, and is outside a requirement of larger than 30 nm of this invention.

Comparative Example 2

(74) Example 1 was repeated, except Anatase TiO.sub.2 pellet (Sakai Chemistry CS-300 S: diameter of 3.5 mm, 52 m.sup.2/g, 0.32 cm.sup.3/g, the pore volume ratio=0.3) was used as TiO.sub.2.

(75) Catalysts of Comparative Examples 1 and 2 have same or higher pore volume and specific surface as Examples 1, 4 and 5.

Comparative Example 3

(76) Example 1 was repeated, except Anatase TiO.sub.2 pellet (Saint-Gobain ST 31119: diameters of 3.2 mmlength of 5 mm, 42.4 m.sup.2/g, 0.26 cm.sup.3/g, a pore volume ratio=1.6) was used as TiO.sub.2.

(77) This catalyst of Comparative Example 3 has similar specific surface, mean pore diameter and pore volume as Examples 1 and 5 and satisfy required their ranges in physical properties, but the pore volume of 0.26 cm.sup.3/g is outside the claimed range of higher than 0.30 cm.sup.3/g of this invention.

Comparative Example 4

(78) Example 6 was repeated, except Anatase TiO.sub.2 pellet (Saint-Gobain ST 31119: diameters of 3.2 mmlength of 5 mm, 42.4 m.sup.2/g, 0.26 cm.sup.3/g, a pore volume ratio=1.6) was used as TiO.sub.2.

(79) By comparing Comparative Example 4 and Example 6, difference in physical properties of TiO.sub.2 when the carrier supports a metal oxide consisting of tungsten oxide alone can be evaluated in comparison to other W-containing metal oxides. In fact, the carrier of Comparative Example 4 has same specific surface, mean pore diameter and the ratio in pore volumes as Example 6 and satisfy required their ranges in physical properties, but the pore volume of 0.26 cm.sup.3/g is outside the claimed range of higher than 0.30 cm.sup.3/g of this invention, as is the case of PW/TiO.sub.2 in Comparative Example 3.

(80) The characteristics of these catalysts are summarized in Table 1

(81) TABLE-US-00001 TABLE 1 Total Ratio of pore Mean Pore diameter Pore diameter volume- pore volumes of pore of mesopore at of macropore at based mode specific volume macropore/ diameter peak maximum peak maximum diameter surfaca area Catalyst (cm.sup.3/g) mesopore (nm) (nm) (nm) (nm) (m.sup.2/g) Example 1 0.36 1.4 37.5 18.5 130 130 48.2 Example 2 0.30 1.5 47.9 22.2 149 149 29.2 Example 3 0.33 1.1 36.6 20.7 136 136 35.6 Example 4 0.36 1.3 39.7 19.9 160 160 39.9 Example 5 0.36 1.4 37.6 18.4 159 159 45.6 Example 6 0.36 1.4 37.5 18.5 130 130 48.2 Example 7 0.32 0.5 22.6 19.9 83 20 54 Example 8 0.40 0.9 44.1 20.5 134 134 53 Example 9 0.38 0.9 34.1 19.9 127 127 54 Comparative 0.40 0.4 23.2 16.6 452 16.6 75.0 Example 1 Comparative 0.32 0.3 23.9 18.9 140 18.9 52.0 Example 2 Comparative 0.26 1.6 33.9 14.8 155 155 42.4 Example 3 Comparative 0.26 1.6 33.9 14.8 155 155 42.4 Example 4
Reactivity of Catalysts

(82) The catalysts were subjected to evaluation test in a pass-through type fixed bed reactor operated at ambient pressure. 30 cm.sup.3 of catalyst was packed in a SUS reaction tube (20 mm diameter). An aqueous solution of 50 wt % of glycerin was pre-heated in a vaporizer heated at 330 C. and the resulting gasified glycerin was fed directly to the catalytic bed together with air at a flow rate of 23.4 h/hr. The reactor containing the catalyst was heated at 290 C. The feed stream have following composition:glycerin:oxygen:nitrogen:water=4.7 mol %:2.8 mol %:68.5 mol %:24.0 mol %. GHSV was 2,020 h.sup.1.

(83) Products were collected in a condenser as condensates and the quantitative determination of all components was carried out by a gas chromatograph (Agilent 7890A, DB-WAX column). Factor of each product was corrected by the gas chromatograph to obtain absolute contents of glycerin fed, glycerin remained and products for determining the conversion of raw material (glycerin conversion), the selectivity of targeted substance (acrolein selectivity) the yield of objective substance (acrolein yield) were calculated by following equations:
The conversion of material (%)=(a mole number of material reacted/a mole number of material fed)*100
The selectivity of objective substance (%)=(a mole number of objective substance obtained/a mole number of material reacted)*100
The yield of objective substance (%)=(a mole number of objective substance obtained/a mole number of material fed)*100

(84) The quantitative analysis was carried out in every several hours and the glycerin conversion and the acrolein yield were compared between a reaction time of 19 hrs and a reaction time of 43 hours.

(85) Results are shown in Table 2.

(86) TABLE-US-00002 TABLE 2 Reaction time 19 hr Reaction time 43 hr Conversion Conversion of glycerin, Yield of of glycerin, Yield of Catalyst % acrolein, % % acrolein, % Example 1 99.6 77.0 95.6 73.6 Example 2 97.9 76.7 88.4 67.8 Example 3 98.2 75.0 89.4 67.0 Example 4 99.5 77.4 93.2 70.9 Example 5 99.7 76.9 94.8 72.1 Example 6 99.6 74.3 93.6 68.3 Example 7 99.6 74.2 93.6 71.6 Example 8 99.4 74.5 94.7 69.4 Example 9 98.8 72.8 91.4 65.2 Comparative 99.3 73.5 88.2 64.8 Example 1 Comparative 96.8 70.4 78.0 56.2 Example 2 Comparative 96.3 76.0 83.1 64.8 Example 3 Comparative 95.4 73.4 78.9 58.9 Example 4
Catalyst of example 7 was also tested in the following conditions.

(87) The feed stream I was composed of: glycerin:oxygen:nitrogen:water=6.0 mol %:2.0 mol %:61.3 mol %:30.7 mol %. GHSV was 2,020 h.sup.1. Acrolein yield after 19 hr on stream is 73.5%.

(88) The cumulative pore volume, the ratio of pore volume and the mean pore diameter of Examples 2 and 3 are within a range of this invention. Results reveal that these catalysts show high glycerin conversion and high acrolein yield and reduced decrease of the glycerin conversion and of the acrolein yield, in spite of relatively lower specific surface area to Comparative Examples.

(89) Catalysts of Comparative Examples 1 and 2 have same or higher pore volume and specific surface as Examples 1, 4 and 5, but the catalytic activity decrease much rapidly (comparison in the glycerin conversion and the acrolein yield between 19 hr and 43 hr). Results reveal that the life of catalyst is influenced by the ratio of pore volumes and the mean pore diameter.

(90) With regards to comparative example 3, the result reveals that deterioration in time of the glycerin conversion and of the acrolein yield is influenced also by the cumulative pore volume in addition to the ratio of specific surface, mean pore diameter and the ratio of pore volumes.

(91) By comparing Comparative Example 4 and Example 6, the result reveals that deterioration in time of the glycerin conversion and of the acrolein yield is influenced also by the ranges of physical properties when a porous carrier supports a metal oxide consisting of tungsten oxide alone.

(92) With regards to Examples 7-9, the result reveals that the shape of the carrier/catalyst such as rings, trilobes or quadrilobes leads to high conversion and yield maintained for a long time.

(93) From these results of Examples and Comparative Examples, following conclusions are obtained: (1) When a catalyst comprising a W-containing metal oxide and optionally other metal oxide containing at least one element selected from P, Si, Mo and V supported on a bipore carrier containing at least one metal oxide selected from a group comprising TiO.sub.2, SiO.sub.2, Al.sub.2O.sub.3, ZrO.sub.2 and Nb.sub.2O.sub.5, a ratio of the pore volume of macropores having a pore size of not smaller than 50 nm to the pore volume of mesopores having a pore size of from larger than 2 nm to smaller than 50 nm being higher than 0.5, there is no substantial difference in glycerin conversion and in acrolein yield after the reaction time of 19 hours and 43 hours, so that the life of catalyst is improved. (2) In case of known catalysis in which a carrier is a porous support like bipore and has a higher pore volume than 0.3 cm.sup.3/g, no decrease in time of glycerin conversion and of acrolein yield is realized so that long life of catalyst can be expected. Still more, there is such risk that mechanical strength is lost simply when the pore volume is increased. Advantage of the higher pore volume and hence longer catalyst life can be obtained only when all requirement of the ratio in pore volume of macropores to the mesopores, and hence the mean pore diameter and the volume-based mode diameter according to the present invention are satisfied, resulting in that the life of catalyst is improved. (3) In other words, decrease in time of catalytic activity or the life of catalyst can be improved without spoiling the glycerin conversion and the acrolein yields, comparing to TiO.sub.2 having physical properties outside the present invention, when a catalyst comprising a W-containing metal oxide and optionally other metal oxide containing at least one element selected from P, Si, Mo and V supported on a bipore carrier containing at least one metal oxide selected from a group comprising TiO.sub.2, SiO.sub.2, Al.sub.2O.sub.3, ZrO.sub.2 and Nb.sub.2O.sub.5, in particular bipore TiO.sub.2 carrier having a bimodal pore structure, a ratio of the pore volume of macropores having a pore size of not smaller than 50 nm to the pore volume of mesopores having a pore size of from larger than 2 nm to smaller than 50 nm being higher than 0.5, in particular higher than 1.0, a mean pore diameter of larger than 30 nm and the volume-based mode diameter of larger than 50 nm, and the pore volume of said porous carrier being higher than or equal to 0.30 cm.sup.3/g.