JPH09117664A - Preparation of catalyst for synthesis of unsaturated aldehyde and unsaturated carboxylic acid - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a simple method for preparing a catalyst capable of efficiently synthesizing an unsaturated aldehyde and unsaturated carboxylic acid corresponding to each of propylene, isobutylene, tertiary butyl alcohol and tertiary butyl ether through the gas-phase catalytic oxidation of these raw materials. SOLUTION: First, a mixed solution of catalytic components containing at least, molybdenum, bismuth and iron, or water slurry is dried into a substantially dense cubic particle with an average grain diameter of 1-250μm using a spray drier. Then the cubic particle-baked powder obtained by baking the dried cubic particle is mixed with water and/or an alcohol and this mixture is molded by extrusion. Further, this molding is dried and thermally treated, or only thermally treated to obtain the catalyst.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to propylene, isobutylene, tertiary butyl alcohol (hereinafter abbreviated as TBA) or methyl tertiary butyl ether (hereinafter M).
Abbreviated as TBE. The present invention relates to a process for producing a catalyst for use in synthesizing a corresponding unsaturated aldehyde and unsaturated carboxylic acid by subjecting (1) to gas phase catalytic oxidation using molecular oxygen.

[0002]

2. Description of the Related Art Conventionally, a catalyst used in the vapor phase catalytic oxidation of propylene to produce acrolein and acrylic acid, and a vapor phase catalytic oxidation of isobutylene, TBA or MTBE to produce methacrolein and methacrylic acid. Many proposals have been made for catalysts used in the above and methods for producing those catalysts. For example, JP-A-58-98143 discloses a method of adding an organic compound such as aniline, methylamine, and pentaerythrite at the time of catalyst preparation for the purpose of controlling catalyst pores in order to improve catalyst performance.
It is disclosed in JP-A-3-109946.

Since the organic compounds added during the heat treatment of the catalyst are removed, these catalysts have the advantage that the catalyst pore size can be freely controlled by changing the size of the organic compound used. However, since the sintering of the catalyst due to the combustion of the organic compound and the reduction of the catalyst due to the organic compound occur in the heat treatment stage, the heat treatment as the catalyst activation treatment becomes complicated, and the reproducibility of the catalyst production is lacking. have. Also, a method of adding starch is disclosed in JP-A-63-315.
No. 147, Japanese Patent Application Laid-Open No. 4-4048, and the like. As these examples show, it is currently desired to develop a catalyst manufacturing method that is easy, has excellent reproducibility, and can control the catalyst pore distribution freely, that is, improve catalyst performance. is there.

[0004]

DISCLOSURE OF THE INVENTION The present invention is advantageous in that the corresponding unsaturated aldehydes and unsaturated carboxylic acids are advantageously obtained by subjecting propylene, isobutylene, TBA or MTBE to vapor phase catalytic oxidation with molecular oxygen. The present invention aims to provide a simple method for producing a catalyst, which is produced.

[0005]

SUMMARY OF THE INVENTION In order to maximize the catalytic performance, the present inventors have investigated the possibility of controlling the catalyst pore distribution without adding organic substances, which has been used in the prior art. As a result of intensive studies, the inventors have found a technique capable of controlling the pore distribution of the molded body by controlling the particle size distribution and the shape of the raw material powder used in the wet shaping method of the catalyst.

That is, as the catalyst raw material powder used in the wet shaping, the average particle diameter is 1 to 250 by the spray drying method.
By forming the solid spherical particles having a diameter of μm and using the spherical sintered powder obtained by firing the dried spherical particles as a raw material to perform wet shaping, fine pores effective for the oxidation reaction are formed and excellent performance is obtained. The present invention has been completed by finding that a catalyst having the same can be produced. INDUSTRIAL APPLICABILITY The present invention is capable of expressing effective pores in a catalyst without the influence of sintering of the catalyst due to heat generation accompanying the thermal decomposition removal of the added organic material considered in the prior art, and reduction of the catalyst due to calcination of the organic material. A catalyst can be easily produced without the need for a thermal decomposition operation of organic substances.

The present invention relates to propylene, isobutylene, T
In the method for producing a catalyst containing at least molybdenum, bismuth and iron, which is used in the gas phase catalytic oxidation of BA or MTBE with molecular oxygen to synthesize a corresponding unsaturated aldehyde and unsaturated carboxylic acid, A mixed solution or an aqueous slurry containing the components is dried into a solid spherical particle having an average particle diameter of 1 to 250 μm by using a spray dryer and then calcined, and the obtained spherical particle calcined powder is added with water and / or alcohol. A method for producing a catalyst for synthesizing an unsaturated aldehyde and an unsaturated carboxylic acid, which comprises drying and heat treating or heat treating after extrusion molding.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, a mixed solution or an aqueous slurry containing a catalyst component is prepared, dried with a spray dryer into solid spherical particles having an average particle diameter of 1 to 250 μm, and then calcined, It is required to add water and / or alcohol to the obtained spherical particle calcined powder, extrude and mold, and dry and heat-treat or heat-treat.

In the present invention, the average particle diameter of the solid spherical particles is in the range of 1 to 250 μm, preferably 5
To 220 μm, particularly preferably 10 to 200 μm
Range. When the average particle size of the solid spherical particles is less than 1 μm, an appropriate pore size necessary for the oxidation reaction by the catalyst of the present invention cannot be obtained, and the yield of the reaction target product is remarkably reduced. On the other hand, the average particle size of the solid spherical particles is 25
When it exceeds 0 μm, the number of contact points between the spherical particles per unit volume decreases, and the mechanical strength of the catalyst molded body decreases, which is not practical.

In the spray drying method adopted in the present invention, when a mixed solution or an aqueous slurry containing a catalyst component is sprayed and released into hot air, the droplets are easily spheroidized due to surface tension. The atomizing method includes a rotating disk method and a pressure nozzle method. This spheroidized droplet is several hundred
The period of drying in hot air at ℃ is a unit of seconds, and this instantaneous drying process also has a constant rate drying period in which free water on the surface evaporates and a decreasing rate drying period in which internal water evaporates. In the constant rate drying period, the surface is instantly dried, whereas in the rate-decreasing drying period, the water inside is evaporated all at once after the constant rate drying period, resulting in hollow spherical particles. Therefore, in order to obtain the solid dense spherical particles according to the present invention, it is necessary to match the constant rate drying period and the rate-decreasing drying period,
It can be obtained by operating by selecting appropriate spraying conditions and drying temperature.

In the present invention, a pressure nozzle system is adopted as a method for producing the solid spherical particles. In the pressure type nozzle method, since the dwell time of the sprayed droplets is long,
Since the constant rate drying period and the decreasing rate drying period described above coincide with each other, a dry powder having a uniform density can be obtained.

Next, the solid spherical particles obtained by the spray drying method are fired. 200 to 50 as firing temperature
A range of 0 ° C. is appropriate. Water and / or alcohol is added to the obtained calcined powder of catalyst spherical particles, and the mixture is kneaded and then wet-molded. In kneading, in addition to water and / or alcohol, an organic binder such as gelatin, cellulose, methyl cellulose, ethyl cellulose, hydroxypropyl cellulose or the like can be added. It is also possible to add diatomaceous earth, silica sol, silica gel, bentonite, kaolin and the like as a shaping reinforcing agent, and further add glass fiber, asbestos, ceramic fiber, carbon fiber and the like.

As the shaping method, there are generally known extrusion molding, rounding, rolling granulation, carrying and the like. The extrusion molding may be performed in any shape such as a ring shape, a column shape, and a star shape.

Next, the wet shaped molded product is dried. As the drying method, it is also possible to use a method of drying in a dryer, but preferably a drying method of matching the drying rate of the inside and the outside surface of the shaped body using a humidity drying method or microwave or the like. It is valid. Further, it is also possible to carry out preliminary drying using a humidity drying method or microwaves and then main drying using a dryer or the like.

The shaped catalyst thus obtained is 300
It is heat-treated again in the range of ˜650 ° C. and used as a catalyst. The heat treatment may be performed without the drying step.

The present invention has the general formula Mo _a Bi _b Fe _c A _d X _e Y _f Z _g Si _h O _i (wherein Mo, Bi, Fe, Si and O are molybdenum, bismuth, iron, silicon and oxygen, respectively). And A is at least 1 selected from the group consisting of cobalt and nickel.
X represents at least one element selected from the group consisting of chromium, lead, manganese, calcium, magnesium, niobium, silver, barium, tin, tantalum and zinc, and Y represents phosphorus, boron, Sulfur, selenium, tellurium,
At least one element selected from the group consisting of cerium, tungsten, antimony and titanium is shown, and Z is at least one element selected from the group consisting of lithium, sodium, potassium, rubidium, cesium and thallium. a, b, c, d, e, f, g, h and i represent the atomic ratio of each element, and when a = 12, b = 0.01
-3, c = 0.01-5, d = 1-12, e = 0-8,
f = 0 to 5, g = 0.001 to 2 and h = 0 to 20, and i is the number of oxygen atoms required to satisfy the valence of each component. It can be preferably used for a catalyst having a composition represented by).

The method for producing the catalyst powder used in the present invention does not have to be limited to a special method, and as long as there is no significant ubiquity of the components, the well-known precipitation method and oxide mixing are well known. Various methods such as a method can be used.

As the raw material of the catalyst component, a combination of oxides, sulfates, nitrates, carbonates, hydroxides, ammonium salts and halides of the respective elements can be used. For example, as a molybdenum raw material, ammonium paramolybdate, molybdenum trioxide and the like can be used.

In order to synthesize unsaturated aldehydes and unsaturated carboxylic acids using the catalyst obtained in the present invention, molecular oxygen is added to propylene, isobutylene, TBA or MTBE as a raw material and vaporized in the presence of the above catalyst. Perform phase contact oxidation. The molar ratio of propylene, isobutylene, TBA or MTBE to oxygen is preferably 1: 0.5-3. The source gas is preferably diluted with an inert gas for use. It is economical to use air as the oxygen source, but if necessary, air enriched with pure oxygen can also be used. The reaction pressure is preferably from normal pressure to several atmospheres. The reaction temperature can be selected in the range of 200 to 450 ° C, but is particularly preferably in the range of 250 to 400 ° C.

[0020]

EXAMPLES The method for producing the catalyst according to the present invention and the reaction examples using the obtained catalyst will be specifically described below. The reaction rates of the raw material olefin, TBA or MTBE, the selectivity of the unsaturated aldehyde and the unsaturated carboxylic acid to be formed in the description are defined as follows. In the description, “parts” means parts by weight. The analysis was by gas chromatography.

[0021]

(Equation 1)

[0022]

(Equation 2)

[0023]

(Equation 3)

Example 1 To 1000 parts of water, 500 parts of ammonium paramolybdate, 12.3 parts of ammonium paratungstate and 1.4 parts of potassium nitrate were added and stirred with heating (solution A). Separately, 60% nitric acid 41.
After 9 parts were added and homogenized, 103.0 parts of bismuth nitrate was added and dissolved. To this, 95.3 parts of ferric nitrate, 343.4 parts of cobalt nitrate and 7.0 parts of zinc nitrate were sequentially added, and 400 parts of water was further added and dissolved (solution B).

After adding solution B to solution A to form an aqueous slurry,
Add 20.6 parts of antimony trioxide, heat and stir, and using a spray dryer, use a "NIRO JAPAN" spray dryer (drying room SD-4.0 type, spray machine countercurrent two-fluid nozzle). The average particle size of 75.3 was obtained by adjusting the tip of the countercurrent type two-fluid nozzle of the sprayer and supplying the slurry previously adjusted so that the inlet temperature was 400 ° C and the outlet temperature was 150 ° C.
Micron-sized solid spherical particles were obtained. The obtained solid-packed spherical particles were subjected to 1 at 300 ° C. in an air atmosphere using a rotary kiln.
Heat treatment was carried out for an hour to obtain a calcined powder of the catalyst.

15 parts of methylcellulose and 150 parts of water were added to 500 parts of this catalyst-calcined powder, and the mixture was mixed and kneaded, and a cylindrical shape having an outer diameter of 6 mm, an inner diameter of 3 mm and a length of 5 mm was formed by a piston type extruder. Was extruded. The extruded catalyst was dried at 105 ° C. for 2 hours with a dryer. The obtained dry molded catalyst was heated at 530 ° C in an air atmosphere.
Was heat-treated for 3 hours. The composition of the elements other than oxygen of the catalyst molded body thus obtained (hereinafter the same) was as follows. Mo ₁₂ W _0.2 Bi _0.9 Fe _1.0 Sb _0.6 Co _5.0 Zn
_0.1 K _0.06

The molded catalyst was filled in a stainless steel reaction tube, and a raw material mixed gas of propylene 5%, oxygen 12%, water vapor 10% and nitrogen 73% (volume%) was passed through the catalyst layer in a contact time of 3.6 seconds. And reacted at 310 ° C. As a result, the conversion of propylene was 99.1%, the selectivity of acrolein was 89.9%, and the selectivity of acrylic acid was 6.9%. In addition, as a result of observing the inside of the catalyst molded body with a scanning electron microscope, it was confirmed that the actual dense spherical particles remained spherical.

Example 2 The same procedure as in Example 1 was carried out except that the tip of the countercurrent type two-fluid nozzle of the spray dryer of the spray dryer was adjusted. Solid dense spherical particles having a size of 0.8 μm were obtained, and thereafter, firing, molding, heat treatment and reaction were performed in the same manner as in Example 1. As a result, the reaction rate of propylene was 99.3%, and the selectivity of acrolein was 9
The selectivity was 0.2% and the selectivity of acrylic acid was 6.5%. In addition, as a result of observing the inside of the catalyst molded body with a scanning electron microscope, it was confirmed that the actual close-packed spherical particles remained spherical although the actual close-packed spherical particles were partially broken.

[Embodiment 3] In Embodiment 1, the "NIR
Made by O JAPAN "spray dryer (drying room SD-1
2.5 type, sprayer countercurrent type two-fluid nozzle), the tip of the sprayer countercurrent type two-fluid nozzle is adjusted, and the operation is carried out under the same temperature conditions, and the average particle size is 195.6 μm. Dense spherical particles were obtained, and thereafter, firing, molding, heat treatment and reaction were performed in the same manner as in Example 1. As a result, the conversion of propylene was 9
The selectivity was 9.0%, the selectivity for acrolein was 90.0%, and the selectivity for acrylic acid was 7.0%. In addition, as a result of observing the inside of the catalyst molded body with a scanning electron microscope, it was confirmed that the actual dense spherical particles remained spherical.

[Comparative Example 1] [0030] The average particle size was 0 in the same manner as in Example 1 except that the tip of the countercurrent two-fluid nozzle of the spray dryer of the spray dryer was adjusted. 0.55 μm solid dense spherical particles were obtained, and thereafter, firing, molding, heat treatment and reaction were performed in the same manner as in Example 1. as a result,
The reaction rate of propylene was 99.0%, the selectivity of acrolein was 89.0%, and the selectivity of acrylic acid was 6.7%. In addition, as a result of observing the inside of the catalyst molded body with a scanning electron microscope, it was confirmed that although the actual close-packed spherical particles were somewhat broken, the actual close-packed spherical particles remained spherical.

[Comparative Example 2] In Example 1, manufactured by "Kusunoki Machinery Co., Ltd." in place of the spray dryer, 1.
187m ² double drum dryer is used, and 3.0 kg / cm ² G is used as the heat medium, the average particle size is 88.1μ.
firing in the same manner as in Example 1 except that flaky particles of m were obtained,
Molding, heat treatment and reaction were performed. As a result, the reaction rate of propylene was 98.7% and the selectivity of acrolein was 89.1.
%, And the selectivity of acrylic acid was 6.2%.

[Comparative Example 3] In Example 1, a slurry dryer RH-2 manufactured by "Okawara Seisakusho Co., Ltd." was used in place of the spray dryer, and the inlet temperature was 350 ° C and the outlet temperature was 150 ° C. And average particle size of 17.7
Firing, molding, heat treatment and reaction were performed in the same manner as in Example 1 except that flaky particles having a size of μm were obtained. As a result, the propylene reaction rate was 98.9% and the acrolein selectivity was 89.
The selectivity was 3% and the selectivity of acrylic acid was 6.3%.

Example 4 To 400 parts of water was added 60% nitric acid 42
After adding 1 part to make a uniform solution, 68.7 parts of bismuth nitrate were added and dissolved. To this, 240.2 parts of nickel nitrate and 24.1 parts of antimony trioxide were sequentially added and dissolved and dispersed. 165 parts of 28% ammonia water was added to this mixed solution to obtain a white precipitate and a blue solution. This was heated and stirred, and most of the water was evaporated. The obtained slurry-like substance is added to 120
After drying at ℃ for 16 hours, heat treatment at 750 ℃ for 2 hours,
Finely ground. 1000 parts of water to 500 parts of ammonium paramolybdate and ammonium paratungstate 12.
3 parts and 20.7 parts of cesium nitrate were added, and the mixture was heated and stirred (Liquid A).

Separately, ferric nitrate 190.7 was added to 700 parts of water.
Parts, cobalt nitrate 240.4 parts and magnesium nitrate 6
0.5 part was sequentially added and dissolved (solution B). After adding liquid B to liquid A to form a slurry, 425.5 parts of 20% silica sol and the fine powder of the bismuth-nickel-antimony compound described above are added and stirred by heating, and a "NIRO JAPAN" spray dryer (drying room SD -4.0 type, sprayer countercurrent two-fluid nozzle) is used to adjust the tip of the sprayer countercurrent two-fluid nozzle, and the inlet temperature is 400 ° C and the outlet temperature is 150 ° C. The average particle size of 5
Solid spherical particles of 1.3 μm were obtained. The solid spherical particles were heat-treated in a rotary kiln in an air atmosphere at 300 ° C. for 1 hour to obtain a calcined powder of a catalyst. 15 parts of hydroxypropyl cellulose was added to 500 parts of this catalyst baked powder.
Part and 150 parts of water are added, mixed and kneaded, and an outer diameter of 6 mm, an inner diameter of 3 mm, and a length of 5 by a piston type extruder.
Extrusion molding was carried out to mm. The extruded catalyst was dried at 105 ° C. for 2 hours with a dryer. The obtained dry-molded catalyst was heat-treated at 530 ° C. for 3 hours in an air atmosphere. The composition of elements of the obtained catalyst molded body was as follows. Mo ₁₂ W _0.2 Bi _0.6 Fe _2.0 Sb _0.7 Ni _3.5 Co
_3.5 Mg _1.0 Cs _0.45 Si _6.0

The molded catalyst was filled in a stainless steel reaction tube, and a raw material mixed gas of isobutylene 5%, oxygen 12%, water vapor 10% and nitrogen 73% (volume%) was contacted for 3.6 hours.
It passed through the catalyst layer for 2 seconds and reacted at 350 ° C. As a result, the conversion of isobutylene was 97.9%, the selectivity of methacrolein was 89.9%, and the selectivity of methacrylic acid was 4.0%. In addition, as a result of observing the inside of the catalyst molded body with a scanning electron microscope, it was confirmed that the actual dense spherical particles remained spherical.

[Embodiment 5] The embodiment was carried out under the same temperature conditions as in Embodiment 4 except that the tip of the countercurrent type two-fluid nozzle of the sprayer of the spray dryer was adjusted. Real dense spherical particles of 0.2 μm were obtained, and thereafter, firing, molding, heat treatment and reaction were performed in the same manner as in Example 4. As a result, the conversion of isobutylene was 98.0%, the selectivity of methacrolein was 90.0%, and the selectivity of methacrylic acid was 3.8%. In addition, as a result of observing the inside of the catalyst molded body with a scanning electron microscope, it was confirmed that the actual close-packed spherical particles remained spherical although the actual close-packed spherical particles were partially broken.

[Embodiment 6] In Embodiment 4, the "NIR
Made by O JAPAN "spray dryer (drying room SD-1
2.5 type, sprayer countercurrent two-fluid nozzle) was used and the average particle diameter was obtained under the same temperature conditions as in Example 4 except that the tip of the sprayer countercurrent two-fluid nozzle was adjusted. 168.5μ
m to obtain solid dense spherical particles, which are then fired in the same manner as in Example 4,
Molding, heat treatment and reaction were performed. As a result, the reaction rate of isobutylene was 97.8% and the selectivity of methacrolein was 90.
The selectivity was 1% and methacrylic acid was 3.7%. Also,
As a result of observing the inside of the catalyst molded body with a scanning electron microscope, it was confirmed that the actual dense spherical particles remained spherical.

[Comparative Example 4] The average particle diameter was 0 in Example 4 except that the tip of the countercurrent type two-fluid nozzle of the spray dryer of the spray dryer was adjusted. 0.43 μm dense solid spherical particles were obtained, and thereafter, firing, molding, heat treatment and reaction were performed in the same manner as in Example 4. as a result,
The conversion of isobutylene was 98.2%, the selectivity of methacrolein was 89.2%, and the selectivity of methacrylic acid was 3.4%. In addition, as a result of observing the inside of the catalyst molded body with a scanning electron microscope, it was confirmed that although the actual close-packed spherical particles were somewhat broken, the actual close-packed spherical particles remained spherical.

[Comparative Example 5] In Example 4, manufactured by "Kusunoki Machinery Co., Ltd." in place of the spray dryer, 1.
187 m ² double drum dryer is used, and 3.0 kg / cm ² G is used as the heat medium, the average particle size is 92.3 μ.
Firing in the same manner as in Example 4 except that flaky particles of m were obtained,
Molding, heat treatment and reaction were performed. As a result, the conversion of isobutylene was 97.2% and the selectivity of methacrolein was 89.
The selectivity was 0% and methacrylic acid was 3.1%.

[Comparative Example 6] In Example 4, a slurry dryer RH-2 manufactured by "Okawara Seisakusho Co., Ltd." was used in place of the spray dryer, and supplied at an inlet temperature of 350 ° C and an outlet temperature of 150 ° C. And average particle size 22.6
Firing, molding, heat treatment and reaction were carried out in the same manner as in Example 4 except that flaky particles of μm were obtained. As a result, the reaction rate of isobutylene was 97.3% and the selectivity of methacrolein was 8
The selectivity was 9.4% and the selectivity of methacrylic acid was 3.3%.

Example 7 The reaction was carried out in the same manner as in Example 4, except that the raw material was changed to TBA using the catalyst of Example 4. As a result, the reaction rate of TBA was 100%, the selectivity of isobutylene was 1.9%, and the selectivity of methacrolein was 87.
The selectivity was 5% and methacrylic acid was 2.6%.

Example 8 Using the catalyst of Example 4, the raw material was changed to MTBE and the reaction was carried out in the same manner as in Example 4. As a result, MTBE reaction rate 100%, isobutylene selectivity 1.8%, methacrolein selectivity 8
The selectivity was 7.6% and the selectivity for methacrylic acid was 2.7%.

[0043]

INDUSTRIAL APPLICABILITY According to the present invention, a mixed solution or slurry containing a catalyst component is sprayed with an average particle diameter of 1 to 2
The pores effective for the oxidation reaction can be expressed by utilizing the voids between the solid spherical particles of 50 μm. That is, by molding the spherical particles while maintaining them, they have connecting pores and an appropriate pore diameter can be obtained. Therefore, the use of the catalyst of the present invention improves the yield of the oxidation reaction. Further, regarding the addition method of the pore-forming agent as a conventional method, while problems such as complicated heat treatment for removal of various pore-forming agent occurs, for the pore expression by the present invention In the method of utilizing the voids between the actual dense spherical particles having an average particle diameter of 1 to 250 μm, it is not necessary to remove the pore developing agent and the like, and a molded catalyst product can be obtained by a simple means with good reproducibility.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical indication location C07C 45/37 C07C 45/37 47/22 9049-4H 47/22 A 9049-4H J 57/045 2115-4H 57/045 57/055 2115-4H 57/055 Z // C07B 61/00 300 C07B 61/00 300

Claims (1)

[Claims] 1. When synthesizing a corresponding unsaturated aldehyde or unsaturated carboxylic acid by subjecting propylene, isobutylene, tertiary butyl alcohol or methyl tertiary butyl ether to gas phase catalytic oxidation using molecular oxygen. In the method for producing a catalyst containing at least molybdenum, bismuth and iron used, a mixed solution containing a catalyst component or an aqueous slurry is used with a spray dryer to have an average particle size of 1 to 25.
Unsaturation characterized by drying and heat-treating the obtained spherical-particle fired powder after drying into 0 μm solid dense spherical particles and adding the water and / or alcohol Process for producing catalyst for synthesis of aldehyde and unsaturated carboxylic acid.