High heat-resistant anatase-type titanium oxide and method for producing the same

Abstract

The present invention provides highly heat-resistant anatase-type titanium oxide particles that stably maintain the highly reactive anatase-type crystalline phase in a temperature range of 700°C and have fine particles with a uniform particle size distribution. The highly heat-resistant anatase titanium oxide of the present invention comprises titanium oxide particles having an anatase crystalline phase content of 85% or more of the total crystalline phase, and a modifying layer formed on the surface of the titanium oxide particles. The modifying layer contains an organic acid having a molecular weight of 200 or less at a concentration of 1.5x10 -4 The modified polymer is an acidic solution containing 0.12 mol / L or more and 0.12 mol / L or less. The pH of the acidic solution is 0.2 to 5.

Description

Technical Field

[0001] The present invention relates to highly heat-resistant anatase-type titanium oxide and a method for producing the same. This application claims priority based on Japanese Patent Application No. 2019-224915 filed in Japan on December 12, 2019, and incorporates the content herein by reference.

Background Art

[0002] Since titanium(IV) oxide (TiO2) is a chemically stable material, it is industrially used in a wide range of fields as a white pigment and the like. The main crystal structures of titanium oxide include three types: anatase phase, brookite phase, and rutile phase. Titanium oxide having the crystal structure of the anatase phase among them (anatase-type titanium oxide) has attracted attention in applications such as dielectric raw materials, materials for solar cells, electrode material raw materials for lithium-ion batteries, and photocatalyst materials. As an example, in the application of dielectric raw materials, when synthesizing barium titanate, anatase-type titanium oxide having high reactivity with barium source raw materials is preferred.

[0003] In recent years, in order to exhibit higher functions in these applications, anatase-type titanium oxide fine particles having a high specific surface area have been demanded.

[0004] On the other hand, in the high-temperature range, sintering and grain growth of titanium oxide particles proceed. Therefore, even when titanium oxide fine particles are used as raw materials in a manufacturing process involving reaction at high temperatures, the particles grow and become coarse, so that the advantages of being titanium oxide fine particles are lost. As an example, in a manufacturing process for synthesizing barium titanate from a barium source and titanium oxide, it is necessary to react at 600°C to 800°C (Non-Patent Document 1). Therefore, not only does barium titanate become coarse due to the progress of particle grain growth, but there is also a problem that the reaction becomes non-uniform. Further, in this temperature range, anatase-type titanium oxide undergoes a phase transition to rutile-type titanium oxide having low reactivity.

[0005] In a method for producing barium titanate from a barium source and titanium oxide, as an attempt to suppress grain growth of particles in a high-temperature region, Patent Document 1 adsorbs an organic acid polymer onto barium carbonate as the barium source to enhance the heat resistance of barium carbonate and suppress sintering and grain growth.

[0006] Also, Patent Document 2 describes that by using barium tartrate as the barium source, growth of particles is suppressed and fine barium titanate particles are obtained. Thus, methods for obtaining fine barium titanate by enhancing the heat resistance of the barium source have been studied so far. On the other hand, there are few reported examples of methods for improving the heat resistance of titanium oxide particles.

[0007] In the reaction between a barium source and titanium oxide typified by the method for producing barium titanate from barium carbonate and titanium oxide, the reaction proceeds by diffusion of barium ions into titanium oxide. Therefore, if the heat resistance of titanium oxide is not improved, coarsening of barium titanate and variations in particle size distribution will occur.

[0008] As a method for improving the heat resistance of titanium oxide, a method of doping with a different element is known as in Non-Patent Document 2 and Patent Document 3. However, such a method has a problem of lowering the purity of the product.

[0009] There is also a method of modifying the surface of titanium oxide with an organic polymer, but since the polymer does not completely decompose and remains even in a high-temperature region as in Patent Document 4, there is a problem of lowering the purity of the product.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Non-Patent Document

[0011]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0012] As described above, anatase-type titanium oxide fine particles sinter, grow in grains, and undergo a phase transition in a high-temperature range. For this reason, conventional anatase-type titanium oxide fine particles have a problem that they are not suitable for use in reactions where the reaction temperature becomes high. The present invention solves this problem.

Means for Solving the Problems

[0013] The inventors deposited anatase-type titanium oxide by hydrolysis of titanium tetrachloride, and then added a predetermined amount of a low-molecular-weight organic acid having a molecular weight of 200 or less. By doing so, it was found that the low-molecular-weight organic acid modifies and stabilizes the surface of titanium oxide, so that the heat resistance becomes extremely higher than that of conventional titanium oxide. The titanium oxide produced by this method has a feature that it stably retains an anatase-type crystal phase excellent in reactivity in a temperature range of 700 ° C. and has a fine and uniform particle size distribution as compared with titanium oxide produced by a conventional method. That is, the configuration of the present invention for solving the above problems is as follows. 〔1〕Titanium oxide particles in which the content of the anatase crystal phase in all crystal phases is 85% or more, and a modification layer provided on the surface of the titanium oxide particles, wherein the modification layer contains an organic acid having a molecular weight of 200 or less at 1.5x10-4 It is modified with an acidic solution containing 0.12 mol / L or more and 1 mol / L or less, High heat-resistant anatase-type titanium oxide, characterized in that the pH of the acidic solution is 0.2 to 5. 〔2〕 The high heat-resistant anatase-type titanium oxide according to 〔1〕, wherein the organic acid is at least one organic acid selected from the group consisting of lactic acid, succinic acid, malic acid, tartaric acid, and citric acid. 〔3〕 The BET specific surface area of the titanium oxide particles is 300 to 500 m 2 / g, and the high heat-resistant anatase-type titanium oxide according to 〔1〕 or 〔2〕. 〔4〕 In the acidic solution, the molar ratio (A / T, mol / mol) of the total amount A of the organic acid to the amount T of the titanium oxide particles in terms of metallic titanium is 1.5x10 -4 The high heat-resistant anatase-type titanium oxide according to any one of 〔1〕 to 〔3〕, which is 0.12 or less and 0.12 or more. 〔5〕 The high heat-resistant anatase-type titanium oxide according to any one of 〔1〕 to 〔4〕, which satisfies all of the following (i) to (iii). (i) The content ratio of the anatase crystal phase in all the crystal phases after firing at 700 ° C for 2 hours is 80% or more. (ii) D100 calculated from the SEM image after firing at 700 ° C for 2 hours is 200 nm or less, and (iii) In the particle size calculated from the SEM image after firing at 700 ° C for 2 hours, D100 / D50 is 1.98 or less. 〔6〕 The high heat-resistant anatase-type titanium oxide according to any one of 〔1〕 to 〔5〕, wherein the titanium oxide particles have a content ratio of the anatase crystal phase in all the crystal phases of 100%. 〔7〕 A modification step of titanium oxide particles, wherein the titanium oxide particles before modification, in which the content ratio of the anatase crystal phase in all the crystal phases is 85% or more, are modified with an acidic solution containing 1.5x10 -4 mol / L or more and 0.12 mol / L or less of an organic acid having a molecular weight of 200 or less, The pH of the acidic solution is 0.2 to 5 A method for producing high heat-resistant anatase-type titanium oxide, characterized by the above. 〔8〕Before the modification step of the titanium oxide particles, it further includes a manufacturing step of titanium oxide particles for manufacturing the titanium oxide particles before modification, The manufacturing step of the titanium oxide particles is a step of depositing titanium oxide by a hydrolysis reaction of titanium tetrachloride to obtain a dispersion of titanium oxide particles, and is a method for manufacturing high heat-resistant anatase-type titanium oxide according to [7]. 〔9〕The organic acid is at least one organic acid selected from the group consisting of lactic acid, malic acid, tartaric acid, and citric acid, and is a method for manufacturing high heat-resistant anatase-type titanium oxide according to [7] or [8]. 〔10〕In the manufacturing step of the titanium oxide particles, the BET specific surface area of the titanium oxide particles is 300 to 500 m 2 / g, and is a method for manufacturing high heat-resistant anatase-type titanium oxide according to any one of [7] to [9]. 〔11〕In the modification step of the titanium oxide particles, in the acidic solution, the molar ratio (A / T, mol / mol) of the total amount A of the organic acid to the amount T in terms of metallic titanium of the titanium oxide particles before modification to be modified is 1.5x10 -4 or more and 0.12 or less, and is a method for manufacturing high heat-resistant anatase-type titanium oxide according to any one of [7] to

[10] . 〔12〕In the manufacturing step of the titanium oxide particles, sulfuric acid is added during the hydrolysis reaction of the titanium tetrachloride, and is a method for manufacturing high heat-resistant anatase-type titanium oxide according to any one of [8] to

[11] . 〔13〕It further includes a purification step of separating the high heat-resistant anatase-type titanium oxide from impurities using at least one or more selected from the group consisting of ultrafiltration membranes, reverse osmosis membranes, ion exchange resins, and electrodialysis membranes, and purifying the high heat-resistant anatase-type titanium oxide, and is a method for manufacturing high heat-resistant anatase-type titanium oxide according to any one of [7] to

[12] .

Advantages of the Invention

[0014] According to the present invention, it is possible to provide highly heat-resistant anatase-type titanium oxide particles that stably retain an anatase-type crystal phase with excellent reactivity in a temperature range of 700°C and have a fine and uniform particle size distribution.

Brief Description of the Drawings

[0015]

Figure 1

Embodiments for Carrying Out the Invention

[0016] Hereinafter, the highly heat-resistant anatase-type titanium oxide and its manufacturing method according to the embodiment of the present invention will be described. Here, "titanium oxide" refers to titanium(IV) dioxide (TiO2) unless otherwise specified. "Ti" refers to all titanium atoms constituting a compound, ion, complex, etc. unless otherwise specified. "Amount of substance of Ti" and "amount in terms of metallic titanium" are amounts in terms of titanium atoms of compounds such as titanium oxide and precursors of titanium oxide. Unit: mol or g. "Ti concentration", "amount-of-substance concentration of Ti", and "concentration in terms of Ti atoms" are concentrations of all titanium atoms constituting a compound, ion, complex, etc., and in particular, are concentrations in terms of titanium atoms of compounds such as titanium oxide and precursors of titanium oxide. Unit: mol / L (molar concentration) or mass% (mass ratio concentration).

[0017] <1. Highly Heat-Resistant Anatase-Type Titanium Oxide> The highly heat-resistant anatase-type titanium oxide (hereinafter, the "highly heat-resistant anatase-type titanium oxide" may also be simply referred to as the "highly heat-resistant titanium oxide") in the present embodiment has titanium oxide particles in which the content of the anatase crystal phase in all crystal phases is 85% or more, and a modification layer provided on the surface of the titanium oxide particles. The modification layer is obtained by modifying with an acidic solution containing an organic acid having a molecular weight of 200 or less at 1.5x10 -4 mol / L or more and 0.12 mol / L or less. The pH of the acidic solution is 0.2 to 5.

[0018] The high heat-resistant anatase-type titanium oxide is preferably titanium oxide that satisfies all of the following (i) to (iii). (i) The content rate of the anatase crystal phase in all crystal phases after firing at 700°C for 2 hours is 80% or more, (ii) D100 calculated from the SEM image of titanium oxide after firing at 700°C for 2 hours is 200 nm or less, and (iii) D100 / D50 is 1.98 or less in terms of the particle size calculated from the SEM image of titanium oxide after firing at 700°C for 2 hours.

[0019] The high heat-resistant anatase-type titanium oxide of this embodiment has a COO - M + peak observed by the FT-IR measurement method described in the examples (Fig. 1). It is presumed that a bond is formed between the surface of the titanium oxide particles and the organic acid. Also, the quantitative relationship between the COO - M + peak intensity and the organic acid bonded to the particle surface is still unknown, but a certain dependence on the concentration of the organic acid contained in the acidic solution used was observed (Fig. 1).

[0020] In the present invention, the "high heat-resistant anatase-type titanium oxide" may also be referred to as "modified layer-containing titanium oxide", "fired titanium oxide", or "modified titanium oxide". The high heat-resistant anatase-type titanium oxide of the present invention includes titanium oxide particles not containing a modified layer and a modified layer formed on the surface of these particles. The "titanium oxide particles" not containing this modified layer may also be referred to as "titanium oxide particles before modification" or "unmodified titanium oxide particles". Further, the titanium oxide obtained by subjecting the high heat-resistant anatase-type titanium oxide of the present invention to a firing treatment at 700°C (for example, firing at 700°C for 2 hours) may also be referred to as "titanium oxide after firing at 700°C" or "fired titanium oxide". In addition, in the manufacturing method of one embodiment of the present invention described later, the titanium oxide particles precipitated by hydrolysis of titanium tetrachloride are modified in an acidic dispersion before aggregation, and then purified. High heat-resistant anatase-type titanium oxide aggregated by the change in zeta potential accompanying the pH change in the process can be obtained. And after the hydrolysis of titanium tetrachloride and before the modification, the titanium oxide particles dispersed in the acidic dispersion are referred to as "titanium oxide particles precipitated by hydrolysis of titanium tetrachloride".

[0021] <1-1. Titanium Oxide Particles before Modification> The titanium oxide particles before modification that constitute the high heat-resistant anatase-type titanium oxide in this embodiment are anatase-type titanium oxide particles in which the content rate of the anatase crystal phase in the total crystal phase is 85% or more. It is preferably 90% or more, more preferably 95% or more, and still more preferably 100% in the content rate of the anatase crystal phase in the total crystal phase. When the content rate of the anatase crystal phase in the total crystal phase is 85% or more, there is an effect of improving the reactivity of titanium oxide with other substances. The evaluation method of the content rate of the anatase crystal phase in the total crystal phase will be described in detail in the examples.

[0022] The BET specific surface area of the titanium oxide particles before modification according to this embodiment is 300 m 2 / g or more and 500 m 2 / g or less, preferably 380 m 2 / g or more and 500 m 2 / g or less, more preferably 400 m 2 / g or more and 450 m 2 / g or less. When the BET specific surface area is 300 m 2 / g or more, when reacting titanium oxide with other substances, an ultra-fine particle material typified by a 0201-sized multilayer ceramic capacitor material can be obtained. When the BET specific surface area is 500 m 2 / g or less, since the crystallinity of the particles is high, abnormal grain growth of titanium oxide can be suppressed due to reaction at high temperature, and there is an effect of uniformizing the particle size distribution of the obtained material. The method for measuring the BET specific surface area will be described in detail in the examples.

[0023] Preferably, the main peak intensity width obtained by X-ray diffraction (XRD) measurement of the titanium oxide particles before modification according to the present embodiment is 1.5° or less, more preferably 1.2° or more and 1.5° or less, and still more preferably 1.3° or more and 1.45° or less. When the main peak intensity width obtained by X-ray diffraction (XRD) measurement of the titanium oxide particles is 1.5° or less, the crystallinity of the nanoparticles is high, and abnormal grain growth of titanium oxide can be suppressed because of the reaction at high temperature, and the particle size distribution of the obtained material is homogenized. X-ray diffraction (XRD) for evaluating the crystallinity of the titanium oxide particles before modification according to the present embodiment is measured as follows. X-ray source: X-ray diffraction measurement using Cu-Kα1 line Peak position: 2θ = 24.5° to 26.0°

[0024] Preferably, the titanium oxide particles before modification according to the present embodiment are nanoparticles. Here, the "nanoparticles" refer to particles with an average primary particle size of 0.5 nm to 100 nm. The average primary particle size can be calculated from the BET specific surface area by the evaluation method described below.

[0025] The average particle size of the agglomerated particles of the primary particles of titanium oxide before modification according to the present embodiment is desirably 100 nm or less, preferably 5 nm or more and 80 nm or less, and more preferably 10 nm or more and 50 nm or less. When the average particle size of the agglomerated particles is larger than 100 nm, when an organic acid is added, titanium oxide whose surface is not modified with the organic acid is generated, which is not preferable. The agglomerated particles are, for example, those in which primary particles of titanium oxide of 3 to 5 nm are agglomerated. Also, when the average particle size of the agglomerated particles of the primary particles of titanium oxide is fine particles of 5 nm or less, purification becomes extremely difficult and productivity decreases, which is not preferable. The evaluation method for the average particle size of the agglomerated particles was as follows: 1 mL of the slurry of titanium oxide after synthesis was collected, diluted 100 times, and then measured using the dynamic light scattering method (DLS).

[0026] In addition, the particle size distribution of the titanium oxide particles before modification according to the present embodiment has the following characteristics. (i) Preferably, D90 of the aggregated particle size of the primary particle aggregates measured by DLS is 100 nm or less, and more preferably 50 nm or less. (ii) Preferably, D90 / D50 of the aggregated particle size of the primary particle aggregates measured by DLS is 1.98 or less, more preferably 1.90 or less, and still more preferably 1.85 or less. When D90 / D50 exceeds 1.98, the particle size of the titanium oxide before modification is non-uniform. Even if such titanium oxide is modified with an organic acid, the particle size after firing at 700 °C for 2 h becomes non-uniform. Therefore, when a reaction is carried out in a high-temperature range using such titanium oxide as a raw material, a product having a uniform particle size distribution cannot be obtained. The method for measuring the aggregated particle size of the primary particle aggregates by the dynamic light scattering method (DLS) will be described in detail in the examples.

[0027] In addition, in the present embodiment, in the production method described below, the titanium oxide particles precipitated by hydrolysis of titanium tetrachloride are modified in an acidic dispersion before aggregation. Then, the modified particles are aggregated and purified to obtain highly heat-resistant anatase-type titanium oxide. In such an embodiment, the evaluation of the particle size, anatase crystal content, BET specific surface area, etc. of the titanium oxide particles before modification is performed as follows. First, the titanium oxide particles precipitated by hydrolysis of titanium tetrachloride are not modified with an organic acid, and a slurry containing unmodified titanium oxide particles or aggregated and purified to obtain a powder of unmodified titanium oxide particles. Then, the obtained slurry or the powder of unmodified titanium particles is evaluated by the evaluation method described in the examples. Alternatively, the modified titanium oxide (highly heat-resistant anatase-type titanium oxide) powder may be evaluated. The modified high heat-resistant anatase-type titanium oxide has substantially the same characteristics such as the particle size of the titanium oxide particles, the anatase crystal content, and the BET specific surface area as the unmodified titanium oxide particles that make up the same. That is, in the present embodiment, the modification with a predetermined amount of low-molecular-weight organic acid only modifies the particle surface, and it is considered that there is no change in at least the characteristics such as the particle size, the anatase crystal content, and the BET specific surface area of the bulk titanium oxide particles.

[0028] <1-2. Low-molecular-weight organic acid modification layer> The modification layer (hereinafter sometimes referred to as the "low-molecular-weight organic acid modification layer" or the "organic acid modification layer") that constitutes the high heat-resistant anatase-type titanium oxide in the present embodiment is obtained by modifying the surface of the titanium oxide particles before modification with an acidic solution. The acidic solution contains an organic acid having a molecular weight of 200 or less in an amount of 1.5×10 -4 mol / L or more and 0.12 mol / L or less.

[0029] The meaning of the "modification layer" of the present invention is that a part of the organic acid as a modifier or a structure derived from the organic acid may be fixed to at least a part of the surface of the titanium oxide particles, and it is not necessary to cover the entire surface. Also, at the time of filing, the applicant could not evaluate the film thickness of the modification layer with ordinary measuring means. Therefore, a modified layer formed by changing the physical properties of the surface of the titanium oxide particles by the interaction (for example, surface reaction) between the organic acid as a modifier and the surface of the titanium oxide particles may also be included as an embodiment of the present invention. The method of fixing is not particularly limited, and it is sufficient that the modification layer can be fixed without being detached from the particle surface even when processed in a later purification step. Specific examples of the method of fixing include chemical bonding, strong physical adsorption, and the like. The presence or absence of the modification layer was determined by FT-IR measurement described later.

[0030] "Low-molecular-weight organic acid" The organic acid used for forming the above-mentioned low-molecular-weight organic acid modification layer has a molecular weight of 200 or less. For example, at least one organic acid selected from the group consisting of lactic acid, succinic acid, malic acid, tartaric acid, and citric acid can be mentioned. This organic acid preferably has a molecular weight of 140 or less. For example, at least one organic acid selected from the group consisting of lactic acid, succinic acid, and malic acid can be mentioned. It is more preferable that this organic acid has a molecular weight of 100 or more and 140 or less. For example, malic acid and succinic acid can be mentioned. The reason why the organic acid with a molecular weight of 200 or less is effective is presumably that the molecular size of the organic acid is appropriate for the size of the titanium oxide nanoparticles to be modified. Since an organic acid with a molecular weight of 200 or less is used for the above-mentioned low-molecular-weight organic acid modification layer, the obtained titanium oxide has excellent heat resistance, retains the anatase type in the high-temperature range, and sintering and grain growth are suppressed.

[0031] The method for forming the organic acid modification layer is not particularly limited. For example, the acidic solution for modification may be an acidic dispersion obtained by adding an organic acid with a molecular weight of 200 or less to a dispersion of titanium oxide particles precipitated by hydrolysis of titanium tetrachloride, and adding an inorganic acid such as sulfuric acid as necessary. In that case, the titanium oxide content in the dispersion of titanium oxide particles precipitated by hydrolysis of titanium tetrachloride is preferably 0.60 mol / L or more and 1.3 mol / L or less, more preferably 0.65 mol / L or more and 1.3 mol / L or less, and still more preferably 0.70 mol / L or more and 1.1 mol / L or less. That is, first, a predetermined amount of a low-molecular-weight organic acid is added to an acidic dispersion of titanium oxide particles precipitated by hydrolysis, and a modification layer is formed on the surface of the titanium oxide particles in the dispersed state. As a result, highly heat-resistant anatase-type titanium oxide particles in the dispersed state can be obtained in the acidic dispersion. The acidic dispersion for modification has a pH of 0.2 to 5, preferably 0.5 to 3, and more preferably 0.5 to 1.5. That is, the pH of the dispersion after adding an organic acid to the dispersion of titanium oxide particles precipitated by hydrolysis of titanium tetrachloride is 0.2 to 5, preferably 0.5 to 3, and more preferably 0.5 to 1.5. When the pH is 5 or less, the isoelectric point of anatase titanium oxide is not passed, so aggregation of titanium oxide particles is suppressed, and the surface of the dispersed titanium oxide particles can be uniformly modified using an organic acid. When the pH is 0.2 or more, while the acidity is sufficiently strong, damage to the titanium oxide particles by strong acid can be suppressed.

[0032] "Modification amount of low-molecular-weight organic acid" In the high heat-resistant anatase-type titanium oxide in the present embodiment, the modification amount of the low-molecular-weight organic acid modification layer is not particularly limited, but can be adjusted according to the amount of the organic acid used as needed. For example, the addition amount of the organic acid in the dispersion containing the above organic acid and the above titanium oxide particles can be adjusted.

[0033] In one example, the amount of substance of Ti (the amount in terms of Ti atoms, also referred to as "the amount in terms of metallic titanium") T, after adding all the organic acids, the mass ratio (A / T, g / g) of the total mass A of the organic acids to the amount of substance T of Ti in the titanium oxide particles is 1.5x10 -4 It is preferably 0.12 or less and more preferably 0.005 or more and 0.05 or less. It is still more preferably 0.007 or more and 0.010 or less. By adjusting the mass ratio (A / T, g / g) of the total mass A of the organic acids to the amount of substance T of Ti in the titanium oxide particles to be in the range of 1.5x10 -4 0.12 or less, the obtained high heat-resistant anatase-type titanium oxide can exhibit predetermined heat resistance. Further, since abnormal grain growth does not occur when fired at a high temperature, the particle size of the final product obtained using this titanium oxide can be made uniform. When adding the above organic acid to the acidic dispersion of titanium oxide particles, the content of titanium oxide in the acidic dispersion is not particularly limited, but is preferably 0.6 to 1.3 mol / L.

[0034] In other examples, after adding all the organic acids, the total concentration of the organic acids is 1.5x10 -4 mol / L or more and 0.12 mol / L or less. It is more preferable that it is 1.5x10 -3 mol / L or more and 0.080 mol / L or less. It is even more preferable that it is 0.005 mol / L or more and 0.050 mol / L or less. By adjusting the total concentration of the organic acids to the range of 1.5x10 -4 mol / L or more and 0.12 mol / L or less, the resulting high heat-resistant anatase-type titanium oxide can exhibit a predetermined heat resistance. Also, since abnormal grain growth does not occur when fired at a high temperature, the particle size of the final product obtained using this titanium oxide can be made uniform. When adding the above organic acid to the acidic dispersion of titanium oxide particles, the content of titanium oxide in the acidic dispersion is not particularly limited, but it is preferably 0.6 to 1.3 mol / L.

[0035] Therefore, according to the high heat-resistant anatase-type titanium oxide of this embodiment, it is possible to provide anatase-type titanium oxide fine particles having a low rutile conversion rate at 700°C, fine particle size, and uniform particle size distribution.

[0036] <1-3. Heat resistance of high heat-resistant anatase-type titanium oxide> The high heat-resistant anatase-type titanium oxide in this embodiment is preferably a modified layer-containing titanium oxide that satisfies all of the following (i) to (iii). (i) After firing the modified layer-containing titanium oxide at 700°C for 2 hours, the content rate of the anatase crystal phase in all the crystal phases of the obtained titanium oxide is 80% or more, (ii) After firing the modified layer-containing titanium oxide at 700°C for 2 hours, D100 calculated from the SEM image analysis of the obtained titanium oxide is 200 nm or less, and (iii) After firing the modified layer-containing titanium oxide at 700°C for 2 hours, D100 / D50 is 1.98 or less in the particle size calculated from the SEM image analysis of the obtained titanium oxide.

[0037] The highly heat-resistant anatase-type titanium oxide in this embodiment is more preferably titanium oxide that satisfies all of the following (i) to (iv). (i) After calcining the modifier layer-containing titanium oxide at 700°C for 2 hours, the content rate of the anatase crystal phase in all the crystal phases obtained is 85% or more, (ii) After calcining the modifier layer-containing titanium oxide at 700°C for 2 hours, D100 calculated from the SEM image of the obtained titanium oxide is 200 nm or less, (iii) After calcining the modifier layer-containing titanium oxide at 700°C for 2 hours, D100 / D50 in the particle size calculated from the SEM image of the obtained titanium oxide is 1.98 or less, and (iv) In the obtained calcined titanium oxide, the residual carbon amount measured by XRF is 10 mass ppm or less. Preferably it is 8 mass ppm or less, and most preferably it is 6 ppm or less.

[0038] The evaluation methods for D50, D100, the content rate of the anatase crystal phase in all the crystal phases, and the residual carbon amount of the calcined titanium oxide will be described in detail in the Examples section below.

[0039] <1-4. Calcined Titanium Oxide> After calcining the highly heat-resistant anatase-type titanium oxide in this embodiment at 700°C for 2 hours, the calcined titanium oxide having the following characteristics is preferable. (i) The content rate of the anatase crystal phase in the crystal phase of the titanium oxide after calcining at 700°C for 2 hours is 80% or more, (ii) D100 calculated from the SEM image of the titanium oxide after calcining at 700°C for 2 hours is 200 nm or less, (iii) In the particle size calculated from the SEM image of the titanium oxide after calcining at 700°C for 2 hours, D100 / D50 is 1.98 or less, and (iv) In the resulting titanium oxide after firing, the residual carbon content measured by XRF is 10 mass ppm or less, preferably 8 mass ppm or less, and most preferably 6 ppm or less. When the residual carbon content exceeds 10 mass ppm, impurities derived from the residual carbon are generated when titanium oxide reacts with other elements, reducing the purity of the product and causing quality deterioration.

[0040] <2. Method for Producing High Heat-Resistant Anatase-Type Titanium Oxide> The method for producing high heat-resistant anatase-type titanium oxide according to this embodiment has a step of modifying titanium oxide particles. The step of modifying titanium oxide particles is a step of modifying titanium oxide particles before modification, in which the content rate of anatase crystal phase in the total crystal phase is 85% or more, with an acidic solution. The acidic solution contains an organic acid with a molecular weight of 200 or less at 1.5x10 -4 mol / L or more and 0.12 mol / L or less. Also, the pH of the acidic solution is 0.2 to 5. The method for producing high heat-resistant anatase-type titanium oxide according to this embodiment preferably includes a step of producing the titanium oxide particles before modification and a step of modifying these titanium oxide particles. The step of producing the titanium oxide particles before modification is a step of precipitating titanium oxide by a hydrolysis reaction of titanium tetrachloride to obtain a dispersion containing titanium oxide particles. The step of modifying titanium oxide particles is a step of adding an organic acid to the dispersion obtained in the step of producing the titanium oxide particles before modification to form a modification layer on the surface of the titanium oxide particles. That is, it is preferable that the titanium oxide particles with a content rate of anatase crystal phase in the total crystal phase of 85% or more, obtained in the step of producing the titanium oxide particles before modification, are used in the next modification step in a state of being dispersed in the dispersion. Also, it is preferable that the method for producing high heat-resistant anatase-type titanium oxide according to this embodiment can produce the high heat-resistant anatase-type titanium oxide described in the aforementioned <1. High Heat-Resistant Anatase-Type Titanium Oxide>. Hereinafter, each step will be described.

[0041] <2-1. Step of Producing Titanium Oxide Particles> The manufacturing process of titanium oxide particles preferably includes a dilution step of preparing a precursor solution of titanium oxide and a step of synthesizing titanium oxide particles by a hydrolysis reaction of titanium tetrachloride. The method for synthesizing titanium oxide particles is not particularly limited as long as it can produce titanium oxide particles having a content rate of anatase crystal phase in the total crystal phase of 85% or more, preferably 95% or more, more preferably 100%. The method for synthesizing titanium oxide particles preferably produces titanium oxide particles having a BET specific surface area of 300 m 2 / g or more and 500 m 2 / g or less, more preferably 380 m 2 / g or more and 500 m 2 / g or less, still more preferably 400 m 2 / g or more and 450 m2 / g or less.

[0042] "Dilution step" In the dilution step, water is added to a titanium tetrachloride solution having a Ti atom conversion concentration (Ti concentration) of 14% by mass or more and 19% by mass or less and diluted to obtain a precursor solution of titanium oxide. It is preferably an aqueous solution. The water used here is preferably pure water. At this time, a strong acid such as hydrochloric acid may be added, but the strong acid added is preferably sulfuric acid. Also, the temperature of the aqueous solution at the time of addition may be room temperature (for example, 25°C). The addition amount of the strong acid is preferably such that the molar ratio of the amount of substance of the strong acid to the amount of substance of Ti, strong acid (mol) / Ti (mol), is 0.04 or more and 0.10 or less, more preferably 0.05 or more and 0.09 or less, and still more preferably 0.06 or more and 0.08 or less.

[0043] The Ti atom conversion concentration (Ti concentration) of the precursor aqueous solution of titanium oxide after dilution is preferably 0.60 mol / L or more and 1.3 mol / L or less, and more preferably 0.70 mol / L or more and 1.1 mol / L or less. When the Ti concentration of the titanium tetrachloride aqueous solution is 0.6 mol / L or more, titanium tetrachloride reacts with water and there is no risk of forming unstable hydroxide. As a result, it is considered that the possibility of forming nuclei of rutile-type or brookite-type titanium oxide is low. (For example, Non-Patent Document 3, Z. Wang et al, J. Colloid interface Sci. 448, 280-286 (2015)). Also, when the Ti concentration of the titanium tetrachloride aqueous solution is 1.3 mol / L or less, the number of precipitated particles is small, so the aggregation state during synthesis is relaxed and a slurry containing dispersed nanoparticles can be obtained. For example, in the examples, an aqueous solution of titanium tetrachloride with a concentration of 0.92 mol / L was used.

[0044] When a strong acid is added to the precursor aqueous solution of titanium oxide after dilution, the pH of the aqueous solution after addition is 5 or less, preferably 3 or less, and more preferably 0.5 or more and 1 or less. When the pH exceeds 5, it is near the isoelectric point of titanium oxide, and the precipitated titanium oxide aggregates. Also, when the pH becomes 1 or less, it becomes difficult to precipitate titanium oxide with low crystallinity, and the crystallinity of the obtained titanium oxide increases, so the heat resistance of titanium oxide is improved. On the other hand, when the pH becomes 0.5 or less, the precipitation rate of titanium oxide becomes slow and the productivity decreases, which is not desirable.

[0045] "Synthesis step of titanium oxide by hydrolysis reaction" The precursor solution of titanium oxide obtained in the dilution step is used to synthesize titanium oxide at a reaction temperature T [°C], and titanium oxide particles are precipitated. The reaction temperature T is preferably 60 °C or higher and lower than the boiling point of the reaction solution. For example, in the case of an aqueous solution, the reaction temperature T is preferably 60 °C or higher and 100 °C or lower, more preferably 70 °C or higher and 95 °C or lower, and even more preferably 75 °C or higher and 90 °C or lower. Further, it is preferable to determine the preferable range with respect to the Ti atom conversion concentration (also referred to as "Ti concentration", unit: mol / L) of the precursor solution of titanium oxide adjusted in the dilution step.

[0046] When the temperature of the reaction solution before this step is lower than the reaction temperature T, the reaction solution is heated. From the viewpoint of productivity, it is preferable to heat the reaction solution quickly. However, in order to suppress the precipitation of amorphous titanium oxide and improve the crystallinity, it is preferable to suppress the rapid progress of the reaction and suppress the heating rate so as to allow sufficient crystal growth. Therefore, the heating of the reaction solution until it reaches the target temperature, that is, the reaction temperature T, is preferably carried out at a heating rate of 0.05 °C / min or higher and 1.5 °C / min or lower, more preferably 0.1 °C / min or higher and 1.0 °C / min or lower, and even more preferably 0.2 °C / min or higher and 1.0 °C / min or lower.

[0047] It has been experimentally found that the reaction for generating titanium oxide from the reaction solution is an endothermic reaction. Therefore, in order to suppress the decrease in the heating rate and the decrease in temperature during heating and maintain the above temperature, it is preferable to cover the periphery of the reaction vessel with a heat insulating material or the like and then uniformly heat the reactor with a heater capable of adjusting the amount of heat applied, such as a mantle heater or a steam jacket.

[0048] In this step, after the heating is completed and the temperature of the reaction solution reaches the reaction temperature T, the holding time is preferably short in consideration of productivity. The holding time at the reaction temperature is preferably 5 hours or less, more preferably 3 hours or less, even more preferably 2 hours or less, and most preferably 0 hours. Further, in this step, it is preferable to stir the reaction solution.

[0049] <2-2. Modification Process of Titanium Oxide Particles> After obtaining the dispersion of titanium oxide particles before modification by the above hydrolysis, the titanium oxide particles may be isolated and purified to obtain the powder of titanium oxide particles before modification, and then the surface of the titanium oxide particles may be modified. That is, the surface of the titanium oxide particles may be modified with an acidic solution using the powder of the titanium oxide particles before modification. The acidic solution contains an organic acid with a molecular weight of 200 or less at 1.5x10 -4 mol / L or more and 0.12 mol / L or less. However, from the viewpoint of being able to modify the particle surface more uniformly, after obtaining titanium oxide particles by hydrolysis, it is preferable not to isolate the titanium oxide particles but to modify the surface of the titanium oxide particles. That is, it is preferable to add an organic acid with a molecular weight of 200 or less to the dispersion to adjust an acidic dispersion and modify the surface of the titanium oxide particles. The acidic dispersion contains the organic acid at 1.5x10 -4 mol / L or more and 0.12 mol / L or less. The organic acid may be added directly to the dispersion or the adjusted solution may be added.

[0050] "pH of the Dispersion" The acidic solution containing the organic acid or the acidic dispersion obtained by adding an organic acid to the dispersion precipitated by hydrolysis has a pH of 0.2 to 5, preferably 0.5 to 3, and more preferably 0.5 to 1.5. At this time, when the pH is 5 or less, the isoelectric point of anatase titanium oxide is not passed, so aggregation of titanium oxide particles is suppressed, and the organic acid can be uniformly modified on the surface of the titanium oxide. Also, when the pH is 0.2 or more, while the acidity is sufficiently strong, damage to the titanium oxide particles by strong acid can be suppressed.

[0051] "Type of Organic Acid" The organic acid to be added is a low-molecular-weight organic acid with a molecular weight of 200 or less. When an organic acid polymer is added, since the polymer chain is long for the precipitated titanium oxide particles, the surface of the titanium oxide cannot be uniformly modified, and titanium oxide with high heat resistance cannot be obtained.

[0052] As the low-molecular-weight organic acid with a molecular weight of 200 or less, carboxylic acids such as succinic acid are preferred, and α-hydroxycarboxylic acids are more preferred. For example, at least one organic acid selected from the group consisting of lactic acid, succinic acid, malic acid, tartaric acid, and citric acid can be mentioned. This organic acid preferably has a molecular weight of 140 or less. For example, at least one organic acid selected from the group consisting of lactic acid, succinic acid, and malic acid can be mentioned. This organic acid more preferably has a molecular weight of 100 or more and 140 or less. For example, malic acid and succinic acid can be mentioned. The reason why the organic acid with a molecular weight of 200 or less is effective is presumably because the molecular size of the organic acid is appropriate for the size of the titanium oxide nanoparticles to be modified. Since the modified layer modified with the above low-molecular-weight organic acid uses an organic acid with a molecular weight of 200 or less, the surface of titanium oxide can be uniformly modified. In addition, the obtained titanium oxide has excellent heat resistance, and sintering and grain growth in the high-temperature range are suppressed.

[0053] "Addition amount of low-molecular-weight organic acid" In the modification step, the addition amount of the organic acid can be adjusted so that an acidic solution containing 1.5x10 -4 mol / L or more and 0.12 mol / L or less of the organic acid with a molecular weight of 200 or less is obtained. In one example, after adding all the organic acids, the mass ratio (A / T, g / g) of the total mass A of the organic acids to the amount of substance T of Ti in the titanium oxide particles is 1.5x10 -4 It is preferably 14 or more and 0.12 or less, more preferably 0.005 or more and 0.05 or less. It is even more preferably 0.007 or more and 0.010 or less. The mass ratio of the total mass A of the organic acids to the amount of substance T of Ti in the titanium oxide particles is 1.5x10 -4By adjusting as described above, the surface of the precipitated titanium oxide can be uniformly modified with an organic acid, and high heat resistance can be imparted to the titanium oxide. Further, when the mass ratio of the total concentration A of the organic acid to the amount of substance T of Ti of the titanium oxide particles is 0.12 or less, it is possible to suppress the dehydration condensation of the excess organic acid that could not completely modify the titanium oxide surface as it is heated. As a result, it is possible to suppress the aggregation of titanium oxide particles and the formation of contact points. Therefore, this contact point does not become a starting point for sintering, and the particle size distribution when fired at 700 °C does not become non-uniform. When adding the above organic acid to the acidic dispersion of titanium oxide particles, the content of titanium oxide in the acidic dispersion is not particularly limited, but is preferably 0.6 to 1.3 mol / L.

[0054] In other examples, after adding all the organic acids, the total concentration A of the organic acids is preferably 1.5x10 -4 mol / L or more and 0.12 mol / L or less. More preferably, it is 1.5x10 -3 mol / L or more and 0.080 mol / L or less, and even more preferably 0.005 mol / L or more and 0.050 mol / L or less. By adjusting to 1.5x10 -4 mol / L or more, the surface of the precipitated titanium oxide can be uniformly modified with an organic acid, and high heat resistance can be imparted to the titanium oxide. On the other hand, when the total concentration A of the organic acid is 0.12 mol / L or less, it is possible to suppress the dehydration condensation of the excess organic acid that could not completely modify the titanium oxide surface as it is heated. As a result, it is possible to suppress the aggregation of titanium oxide particles and the formation of contact points. Therefore, this contact point does not become a starting point for sintering, and the particle size distribution when fired at 700 °C does not become non-uniform. When adding the above organic acid to the acidic dispersion of titanium oxide particles, the content of titanium oxide in the acidic dispersion is not particularly limited, but is preferably 0.6 to 1.3 mol / L.

[0055] For example, in the dispersion of titanium oxide particles precipitated by hydrolysis of titanium tetrachloride, the titanium oxide content is 0.85 mol / L or more and 0.95 mol / L or less, and the organic acid is α-hydroxycarboxylic acid. In that case, the amount of the organic acid to be added is preferably such that the molar ratio of the amount of substance (mol) of the α-hydroxycarboxylic acid added to the amount of substance (mol) of Ti of titanium oxide in the aqueous dispersion is from 0.00020 to 0.12. More preferably, it is added such that the ratio is from 0.00070 to 0.010, and most preferably, it is added such that the ratio is from 0.0050 to 0.0080. If it is 0.00020 or more, the surface of the precipitated titanium oxide can be uniformly modified with the organic acid, and high heat resistance can be imparted to the titanium oxide. On the other hand, when it is 0.12 or less, it is possible to suppress the dehydration condensation of the excess organic acid that has not completely modified the surface of the titanium oxide as it is heated. As a result, it is possible to suppress the aggregation of titanium oxide particles and the formation of contact points. Therefore, this contact point does not become a starting point for sintering, and the particle size distribution when fired at 700 °C does not become non-uniform.

[0056] Also, according to the investigations by the inventors, it has been found that even if the same low-molecular organic acid as in the present invention is added at a stage before the precipitation of titanium oxide, for example, before the start of the hydrolysis reaction of titanium tetrachloride or during the reaction, the heat resistance is not sufficiently improved (for example, Comparative Example 7 described later). This is considered to be because the surface of the titanium oxide is not effectively modified. The reason for the ineffective modification is that, in the process of the precipitated titanium oxide forming aggregates, the organic acid is taken into the aggregates, and thus it is presumed that the dispersion after the hydrolysis of titanium tetrachloride does not contain the organic acid at the required concentration.

[0057] <2-3. Neutralization step> After the modification step of the titanium oxide particles, if necessary, a basic substance, for example, an aqueous ammonia solution, may be further added to the acidic dispersion containing the low molecular weight organic acid to adjust the pH to 7 to 9, preferably 7.5 to 8.5. For example, an aqueous ammonia solution with an ammonia concentration of 25% by mass may be added to adjust the pH of the acidic dispersion to 8.0. By setting the pH to 7 or higher, the chloride ions derived from the raw materials adsorbed on the surface of the titanium oxide particles are released, enabling easy removal by water washing. If the pH is set to 9 or higher, unreacted Ti components precipitate as poorly crystalline titanium hydroxide, which is not preferable as it reduces the heat resistance of the product.

[0058] <2-4. Purification Process of Titanium Oxide> The method for producing highly heat-resistant anatase-type titanium oxide according to this embodiment may include a purification step of removing impurities from the highly heat-resistant anatase-type titanium oxide (modified titanium oxide) obtained in the above modification step of the titanium oxide particles, as required for the application. For example, in this step, in order to improve the purity of titanium oxide, impurities such as chlorine (Cl), sulfur (S), and carbon (C) contained in the dispersion (slurry) of titanium oxide are removed. The purification method is not particularly limited, but at least one or two or more selected from the group consisting of ultrafiltration membranes, reverse osmosis membranes, ion exchange resins, and electrodialysis membranes may be used. That is, the method for producing highly heat-resistant anatase-type titanium oxide according to this embodiment may further include a purification step of separating the highly heat-resistant anatase-type titanium oxide from the impurities and purifying the highly heat-resistant anatase-type titanium oxide to a high purity by using at least one or more selected from the group consisting of ultrafiltration membranes, reverse osmosis membranes, ion exchange resins, and electrodialysis membranes. As the impurity concentration after purification, for example, in the case of using BaTiO3 as a raw material, the total concentration of impurities containing chlorine (Cl) and sulfur (S) is preferably 100 ppm (mass / mass) or less, more preferably 50 ppm or less, and still more preferably 10 ppm or less. When the total concentration of impurities of chlorine (Cl) and sulfur (S) (in terms of the amount of chlorine atoms and sulfur atoms) is 100 ppm (mass / mass) or less with respect to the mass of titanium oxide after purification, it is possible to suppress the occurrence of a decrease in the dielectric constant when used as a raw material for BaTiO3.

[0059] The purified titanium oxide may be crushed as necessary. The crushing method is not particularly limited, and examples include methods using a mortar, a ball mill, etc.

Examples

[0060] Hereinafter, examples of the present invention will be described, but the present invention is not limited to these examples.

[0061] (Example 1) To an aqueous solution of titanium tetrachloride with a Ti concentration of 15% by mass (59% by mass of titanium tetrachloride), dilute sulfuric acid with a mass of 48% was added so that the ratio of the amount of substance of sulfuric acid to the amount of substance of Ti was 0.080. Pure water was added thereto and diluted so that the molar concentration of Ti was 0.92 mol / L. 300 ml of this aqueous solution was heated to 85°C at a heating rate of 0.2°C / min using an external heater while stirring at 300 rpm using a magnetic stirrer. After the liquid temperature reached 85°C, the heating was immediately stopped and left standing for 24 h, and then allowed to cool to room temperature. After cooling, citric acid monohydrate was added so that the molar ratio of citric acid to the amount of substance of Ti was 0.00070 (mass ratio: 0.0017, citric acid concentration in the dispersion: 0.00065 mol / L), and the pH of the acidic solution after addition was adjusted to 1.0. Stirring was carried out at 300 rpm for 1 h using a magnetic stirrer. After stirring, an aqueous ammonia solution with an ammonia concentration of 25% by mass was added to adjust the pH to 8.0. Subsequently, the obtained slurry was filtered through an ultrafiltration membrane, and the obtained titanium oxide was washed with ion-exchanged water. The washed titanium oxide was placed in an oven and dried at 150 °C. Using the FT-IR measurement method described below, the powder of the obtained high heat-resistant anatase-type titanium oxide was measured, and the COO - M + peak was confirmed. The results are shown in Fig. 1. Using the evaluation method described below, the obtained titanium oxide was evaluated, and the results are shown in Table 1. Also, it was fired at 700 °C by the method described below, and the fired titanium oxide obtained was evaluated, and the results are shown in Table 2.

[0062] (Example 2) It was produced and evaluated in the same manner as in Example 1 except that citric acid monohydrate was added so that the molar ratio of citric acid to the amount of substance of Ti was 0.0070. Using the FT-IR measurement method described below, the powder of the obtained high heat-resistant anatase-type titanium oxide was measured, and the COO - M + peak was confirmed. The results are shown in Fig. 1.

[0063] (Example 3) It was produced and evaluated in the same manner as in Example 1 except that lactic acid was added instead of citric acid monohydrate so that the molar ratio of lactic acid to the amount of substance of Ti was 0.00070.

[0064] (Example 4) It was produced and evaluated in the same manner as in Example 1 except that lactic acid was added instead of citric acid monohydrate so that the molar ratio of lactic acid to the amount of substance of Ti was 0.0073.

[0065] (Example 5) It was produced and evaluated in the same manner as in Example 1 except that malic acid was added instead of citric acid monohydrate so that the molar ratio of malic acid to the amount of substance of Ti was 0.00070.

[0066] (Example 6) It was produced and evaluated in the same manner as in Example 1, except that malic acid was added instead of citric acid monohydrate so that the molar ratio of the amount of malic acid to the amount of substance of Ti was 0.0070.

[0067] (Example 7) It was produced and evaluated in the same manner as in Example 1, except that tartaric acid was added instead of citric acid monohydrate so that the molar ratio of the amount of tartaric acid to the amount of substance of Ti was 0.00070.

[0068] (Example 8) It was produced and evaluated in the same manner as in Example 1, except that tartaric acid was added instead of citric acid monohydrate so that the molar ratio of the amount of tartaric acid to the amount of substance of Ti was 0.0070.

[0069] (Example 9) It was produced and evaluated in the same manner as in Example 1, except that tartaric acid was added instead of citric acid monohydrate so that the molar ratio of the amount of tartaric acid to the amount of substance of Ti was 0.070.

[0070] (Example 10) It was produced and evaluated in the same manner as in Example 1, except that succinic acid was added instead of citric acid monohydrate so that the molar ratio of the amount of succinic acid to the amount of substance of Ti was 0.0070.

[0071] (Comparative Example 1) To an aqueous titanium tetrachloride solution with a Ti concentration of 15% by mass (59% by mass of titanium tetrachloride), 48% by mass of dilute sulfuric acid was added so that the ratio of the amount of sulfuric acid to the amount of substance of Ti was 0.080. Pure water was added thereto and diluted so that the molar concentration of Ti became 0.92 mol / L. 300 ml of this aqueous solution was heated to 85°C at a heating rate of 0.2°C / min using an external heater while stirring at 300 rpm using a magnetic stirrer. After the liquid temperature reached 85°C, the heating was immediately stopped and allowed to stand for 24 h, and then cooled to room temperature. After cooling, an aqueous ammonia solution with an ammonia concentration of 25% by mass was added to adjust the pH to 8.0. Thereafter, the obtained slurry was filtered through an ultrafiltration membrane, and the obtained titanium oxide was washed with ion-exchanged water. The washed titanium oxide was placed in an oven and dried at 150 °C. The obtained titanium oxide was evaluated by the evaluation method described below, and the results are shown in Table 1. Also, it was fired at 700 °C by the method described below, and the obtained fired titanium oxide was evaluated, and the results are shown in Table 2.

[0072] (Comparative Example 2) Titanium oxide was obtained based on the method described in Example 1 of Patent Document 5 (Japanese Patent No. 5021106). Specifically, 690 mL of ion-exchanged water was placed in a reaction tank equipped with a comb-shaped stirrer and preheated to 95 °C. Stirring was carried out at 300 rpm, and while stirring and maintaining the temperature at 95 °C, 50 g of an aqueous titanium tetrachloride solution at 20 °C (Ti concentration: 18% by mass) was added dropwise over 30 seconds, and the mixture was stirred in the reaction tank and the temperature was maintained at 95 °C for 4 min. The reaction tank was cooled to 50 °C in an ice bath in less than 1 min. Hydrochloric acid generated by the reaction was removed using an electrodialysis membrane. The washed titanium oxide was placed in an oven and dried at 100 °C. The obtained titanium oxide was evaluated by the evaluation method described below, and the results are shown in Table 1. Also, it was fired at 700 °C by the method described below, and the obtained fired titanium oxide was evaluated, and the results are shown in Table 2.

[0073] (Comparative Example 3) Titanium oxide was obtained based on the method described in Example 1 of Patent Document 6 (International Publication No. 2016 / 002755). Specifically, while maintaining an aqueous titanium tetrachloride solution at 100 g / L in terms of TiO2 (Ti concentration: 1.25 mol / L) at 25 °C, citric acid monohydrate was added and stirred for 30 min. The pH was 0.4. The addition amount of the citric acid monohydrate was 3% by mass in terms of anhydrous citric acid with respect to the mass of titanium tetrachloride contained in the aqueous solution converted to titanium oxide. The obtained aqueous solution was used as a precursor aqueous solution. Next, the temperature of the aqueous solution was raised using an external heater, and it was stirred at 92 °C for 30 min. Then, the resulting solution was cooled to 70 °C, and the pH was adjusted to 6.5 with aqueous ammonia (ammonia concentration 25% by mass). Then, the resulting slurry was cooled to 25 °C and filtered through an ultrafiltration membrane, and the recovered titanium oxide was washed with ion-exchanged water. The washed titanium oxide was placed in an oven and dried at 60 °C. The obtained titanium oxide was evaluated by the evaluation method described below, and the results are shown in Table 1. Also, it was fired at 700 °C by the method described below, and the obtained fired titanium oxide was evaluated, and the results are shown in Table 2.

[0074] (Comparative Example 4) It was produced and evaluated in the same manner as in Example 1 except that kaocel 2000 (molecular weight 2000) was added instead of citric acid monohydrate so that the mass ratio of the amount of kaocel 2000 to the amount of substance of Ti was 0.012.

[0075] (Comparative Example 5) It was produced and evaluated in the same manner as in Example 1 except that polyacrylic acid (molecular weight 25000) was added instead of citric acid monohydrate so that the mass ratio of the amount of polyacrylic acid to the amount of substance of Ti was 0.011.

[0076] (Comparative Example 6) To an aqueous solution of titanium tetrachloride with a Ti concentration of 15% by mass (59% by mass of titanium tetrachloride), 48% by mass of dilute sulfuric acid was added so that the ratio of the amount of substance of sulfuric acid to the amount of substance of Ti was 0.080. Pure water was added thereto and diluted so that the molar concentration of Ti became 0.92 mol / L. 300 ml of this aqueous solution was heated to 85 °C at a heating rate of 0.2 °C / min using an external heater while stirring at 300 rpm using a magnetic stirrer. After the liquid temperature reached 85 °C, the heating was immediately stopped and it was allowed to stand for 24 h and then cooled to room temperature. After cooling, an aqueous ammonia solution with an ammonia concentration of 25% by mass was added to adjust the pH to 8.0. After adjusting the pH, citric acid monohydrate was added so that the molar ratio of the amount of citric acid to the amount of Ti was 0.0070, and the mixture was stirred at 300 rpm for 1 h using a magnetic stirrer. Thereafter, the obtained slurry was filtered through an ultrafiltration membrane, and the obtained titanium oxide was washed with ion-exchanged water. The washed titanium oxide was placed in an oven and dried at 150 °C. The obtained titanium oxide was evaluated by the evaluation method described below, and the results are shown in Table 1. Further, it was calcined at 700 °C by the method described below, and the obtained calcined titanium oxide was evaluated, and the results are shown in Table 2.

[0077] (Comparative Example 7) To an aqueous titanium tetrachloride solution with a Ti concentration of 15% by mass (59% by mass of titanium tetrachloride), 48% by mass dilute sulfuric acid was added so that the ratio of the amount of sulfuric acid to the amount of Ti was 0.080. Pure water and citric acid monohydrate were added thereto so that the mass ratio of the amount of citric acid to the amount of Ti was 0.017, and it was diluted so that the molar concentration of Ti was 0.92 mol / L. While stirring 300 ml of this aqueous solution at 300 rpm using a magnetic stirrer, the temperature was raised to 85 °C at a rate of 0.2 °C / min using an external heater. At this time, the concentration of citric acid was less than 0.00015 mol / L. After the liquid temperature reached 85 °C, the temperature increase was immediately stopped, and it was allowed to stand for 24 h and then cooled to room temperature. After cooling, an aqueous ammonia solution with an ammonia concentration of 25% by mass was added to adjust the pH to 8.0. Thereafter, the obtained slurry was filtered through an ultrafiltration membrane, and the obtained titanium oxide was washed with ion-exchanged water. The washed titanium oxide was placed in an oven and dried at 150 °C. The obtained titanium oxide was evaluated by the evaluation method described below, and the results are shown in Table 1. Further, it was calcined at 700 °C by the method described below, and the obtained calcined titanium oxide was evaluated, and the results are shown in Table 2.

[0078] (Comparative Example 8) Malic acid was added instead of citric acid monohydrate so that the molar ratio of the amount of malic acid to the amount of Ti in terms of the amount of substance was 0.14, and the product was produced and evaluated in the same manner as in Example 1 except for this.

[0079] (Comparative Example 9) Malic acid was added instead of citric acid monohydrate so that the molar ratio of the amount of malic acid to the amount of Ti (in terms of the amount of metallic titanium) in terms of the amount of substance was 0.00014, and the product was produced and evaluated in the same manner as in Example 1 except for this.

[0080] [Evaluation Method] The XRD, BET, and DLC evaluation results of the "titanium oxide particles before modification" shown in Table 1 are the evaluation results of titanium oxide particles obtained by a method without only the modification step in the above Examples and Comparative Examples. The XRD, BET, and SEM evaluation results of the modified titanium oxide (before firing) obtained in the above Examples and Comparative Examples are the same as those of the titanium oxide particles before modification obtained by a method without only the modification step. Regarding the evaluation of the residual carbon amount of the modified titanium oxide (before firing) obtained in the above Examples and Comparative Examples, and the titanium oxide after firing them in the following firing experiment, the evaluation was performed by the following XRF measurement. The results are shown in Table 1. The following evaluations were performed on the titanium oxide obtained by firing the modified titanium oxide obtained in each of the above Examples and Comparative Examples in the following firing experiment. The results are shown in Table 2.

[0081] [Firing Test] The following firing test was performed on the titanium oxide obtained in each of the above Examples and Comparative Examples. First, 2 g of the obtained titanium oxide powder was placed in an alumina crucible, and the temperature was raised from 25°C to 700°C in 2 hours in an air atmosphere in an electric furnace, and it was left in a 700°C environment for 2 hours. Then, the alumina crucible containing the titanium oxide powder was taken out of the electric furnace and allowed to cool at room temperature (25°C).

[0082] [Measurement of Content Ratio of Each Crystal Phase] For each titanium oxide before and after the firing test, X-ray diffraction measurement was performed as follows, and the ratios of the anatase, rutile, and brookite crystal phases contained in the crystal phase of titanium oxide were calculated. The powder X-ray analysis measurement was performed using X’pert PRO manufactured by PANalytical, and X-ray diffraction measurement was performed under the following conditions. The measurement conditions were as follows: using the Cu-Kα1 line of a copper target, tube voltage 45 kV, tube current 40 mA, measurement range 2θ = 20 to 35°, sampling width 0.0167°, and scanning speed 0.0192° / s.

[0083] In the measurement, the background with only the glass cell was measured, and the diffraction pattern of the sample was corrected by subtracting the diffraction intensity of the background from the diffraction intensity measured with the sample containing titanium oxide and the glass cell. Let the diffraction intensity at 2θ of the sample containing titanium oxide and the glass cell be I S (2θ), and the diffraction intensity at 2θ of only the glass cell be I B (2θ). Then, the diffraction intensity of titanium oxide after background correction is I(2θ) = I S (2θ) - I B (2θ).

[0084] Regarding the content ratio of the anatase crystal phase in the total crystal phase of titanium oxide obtained in each of the above Examples and Comparative Examples (the anatase content ratio or anatase ratio of titanium oxide before modification, or the anatase residual ratio of titanium oxide after the firing treatment), I a 、I r 、and I b were calculated from the following equations, respectively. Anatase content ratio [%] = {I a / (I a + I b + I r )} x 100% Anatase residual ratio [%] = {I a / (I a + I b + I r )} x 100% In the equations, I a: Intensity of the peak corresponding to the anatase crystal phase (2θ = 24.5 - 26.0°); I r : Intensity of the peak corresponding to the rutile crystal phase (2θ = 26.6 - 28.1°); I b : Intensity of the peak corresponding to the brookite crystal phase (2θ = 30.8 - 32.3°).

[0085] <Measurement of BET specific surface area> For the titanium oxide obtained in each of the above Examples and Comparative Examples, and the titanium oxide after the firing test, the specific surface area (BET specific surface area (m 2 / g)) was measured using QUADRASORV evo manufactured by Quantachrome Corporation.

[0086] <Measurement of average primary particle size> The average primary particle size of titanium oxide is converted from the BET specific surface area. Using the specific surface area S (m 2 / g) obtained by the above method, the average primary particle size was calculated from the following formula. Average primary particle size = 6000 / (S × ρ) Here, ρ represents the density of titanium oxide (g / cm 3 ). Since the titanium oxide of the present invention has anatase as the main component, ρ = 4.0 was used.

[0087] 〈Evaluation of particle size distribution by dynamic light scattering method〉 The method for evaluating the average particle size of the aggregated particles of the titanium oxide particles before modification was as follows: 1 mL of the titanium oxide slurry after synthesis in Comparative Example 1 was collected, diluted 100-fold, and then measured using the dynamic light scattering method (Dynamic Light Scattering, DLS). Shown in Table 1. Measuring instrument: Otsuka Electronics Co., Ltd. ELSZ - 2000 Measurement method: 1 ml of the titanium oxide slurry was diluted to 100 ml with pure water, placed in a 2.5 ml disposable cell, and measurement was performed.

[0088] 〈Evaluation of particle size distribution by image analysis〉 For the evaluation samples, the titanium oxide powders obtained in each of the above Examples and Comparative Examples were used as powders fired at 700 °C for 2 hours in the above firing test. 0.1 g of the powder was placed on a carbon tape laid on a sample stage No. 3 manufactured by Hitachi High-Tech Fielding Co., Ltd., and an image at 100,000 times magnification was taken using a scanning electron microscope. The scanning electron microscope was an S-5500 type field emission scanning electron microscope manufactured by Hitachi High-Technologies Corporation. The taken image was analyzed using image analysis software Mac-View ver 4.0 manufactured by Mountech Co., Ltd. The particle size of 500 particles per visual field was approximated as a perfect circle to obtain the diameter, and the arithmetic mean was calculated to calculate the particle size distribution.

[0089] <FT-IR Measurement> For the FT-IR measurement, a Fourier transform infrared spectrophotometer, FT / IR-M-500 (manufactured by JASCO Corporation), was used. The measurement conditions were as follows: using the ATR method under a nitrogen atmosphere, with a resolution of 4 cm -1 , and the number of accumulations was 1024 times. For the peak waveform separation in the infrared absorption spectrum at wavenumbers from 1900 cm -1 to 1200 cm -1 , curve fitting was performed using the spectrum analysis program attached to the control software of the above FT / IR-M-500 so that the sum of squared residuals was 0.01 or less. The measurement was performed using the dried powder of the titanium oxide powder obtained in each of the Examples and Comparative Examples. The difference spectra between the spectra of Example 1 and 2 and the spectrum of Comparative Example 1 obtained were obtained.

[0090] <XRF Measurement (Evaluation of Residual Carbon Content)> The fluorescence X-ray analysis (XRF) measurement was performed under the following measurement conditions. Using a multi-element simultaneous fluorescence X-ray analyzer Simultxix14 manufactured by Rigaku, 5 g of the powder was formed in the sample holder attached to the fluorescence X-ray analyzer, and the measurement was performed under the conditions of 50 kV and 50 mA.

[0091]

Table 1

[0092] Notes in Table 1: *1: *1 is the addition ratio calculated using the amount of organic acid added before precipitating titanium oxide by hydrolysis. It is not the amount of organic acid contained in the acidic dispersion obtained after precipitating titanium oxide by hydrolysis. *2: Could not be determined because it was significantly aggregated. *3: Before firing: Titanium oxide after modification, After firing: Titanium oxide after firing at 700 °C.

[0093]

Table 2

[0094] <Discussion> Compared with Comparative Example 1, Examples 1 to 10 have a higher proportion of the anatase phase remaining after firing at 700 °C for 2 hours. Also, they have a more uniform particle size distribution compared to Comparative Example 1. From these facts, it is suggested that by modifying the surface of titanium oxide with an organic acid, the surface of titanium oxide was stabilized and high heat resistance was exhibited.

[0095] When XRD measurement was performed on Comparative Example 2 after firing at 700 °C for 2 hours, no remaining anatase phase was confirmed. In contrast, Examples 1 to 10 show a high anatase residual rate even after firing at 700 °C for 2 hours. Also, Comparative Example 2 shows coarsening of grain growth when fired at 700 °C for 2 hours. Compared with this, since D100 is 155 nm or less and D100 / D50 is 2.00 or less for Examples 1 to 10 even after firing at 700 °C for 2 hours, it is suggested that grain growth in the high-temperature range is suppressed compared to existing titanium oxide.

[0096] Ammonia was added in the step of precipitating titanium oxide in Comparative Example 2. At this time, it is presumed that the pH temporarily rises and the zeta potential changes, causing the dispersed titanium oxide to aggregate. When an organic acid is added to such aggregated titanium oxide, it is presumed that the organic acid cannot be evenly modified on the primary particles of titanium oxide, so high heat resistance as in Examples 1 to 10 cannot be exhibited.

[0097] When Comparative Example 3 was subjected to XRD measurement after firing at 700°C for 2 hours, no remaining anatase phase was confirmed. In contrast, Examples 1 to 10 exhibit a high anatase remaining rate even after firing at 700°C for 2 hours. Also, in Comparative Example 3, grain growth becomes coarse when fired at 700°C for 2 hours. In comparison, since D100 is 155 nm or less even after firing Examples 1 to 10 at 700°C for 2 hours, it is suggested that grain growth in the high-temperature range is suppressed compared to existing titanium oxides.

[0098] In Comparative Example 3, synthesis is carried out at a low Ti concentration in the step of depositing titanium oxide. For this reason, the supersaturation during synthesis is low, and titanium oxide particles with low crystallinity are deposited. Therefore, the heat resistance is lower compared to Comparative Example 1.

[0099] From this, it is inferred that the high heat resistance of Examples 1 to 10 not only results from the effect of modifying the surface of titanium oxide with an organic acid, but also from synthesizing highly crystalline titanium oxide by appropriately controlling the Ti concentration during precipitation.

[0100] Although Comparative Examples 4 and 5 add an organic acid in almost the same mass ratio as in Example 2, their heat resistance is lower compared to Example 2. This is presumably because in Comparative Examples 4 and 5, polymers are used, so the size of the organic matter is large with respect to the titanium oxide particles to be modified, and the surface of the titanium oxide cannot be efficiently modified.

[0101] Although Comparative Example 6 adds the same amount of citric acid as in Example 2, its heat resistance is low. This is presumably because Example 2 adds citric acid to the titanium oxide particles precipitated in an acidic solution with pH 1 to modify the surface, while Comparative Example 6 adds citric acid in a basic solution with pH 8.

[0102] Although Comparative Example 7 added the same amount of citric acid as in Example 2, its heat resistance was low. This is presumably because in Example 2, citric acid was added to the precipitated titanium oxide particles to modify the surface, while in Comparative Example 7, citric acid was added to the titanium tetrachloride aqueous solution which is a titanium oxide precursor. When citric acid is added to the precursor aqueous solution before titanium oxide precipitates, citric acid preferentially modifies the surface of the titanium oxide precipitated in the initial stage of the reaction. Therefore, it is presumed that the modification state of the titanium oxide surface by citric acid is different between the titanium oxide particles precipitated in the initial stage of the reaction and those precipitated in the later stage of the reaction. Thus, when an organic acid is added to the precursor aqueous solution before titanium oxide particles precipitate as in Comparative Example 7, it is presumed that titanium oxide with high heat resistance as in the examples cannot be obtained.

[0103] Although Comparative Example 8 added the malic acid added in Examples 5 and 6, high heat-resistant titanium oxide was not obtained. This is presumably because in the case of Comparative Example 8, the amount of the organic acid added is small relative to the amount of the precipitated titanium oxide. When the amount of the organic acid added is significantly small relative to the precipitated titanium oxide, it is presumed that titanium oxide whose surface is not modified with the organic acid is formed. For this reason, it is presumed that high heat resistance as in the examples was not exhibited in Comparative Example 8.

[0104] Although Comparative Example 9 added the malic acid added in Examples 5 and 6, high heat-resistant titanium oxide was not obtained. This is presumably because in the case of Comparative Example 9, the amount of the organic acid added is excessive relative to the amount of the precipitated titanium oxide. When the amount of the organic acid added is significantly large relative to the precipitated titanium oxide, it is presumed that excessive organic acids cause dehydration condensation during drying, crosslink the space between titanium oxide particles, aggregate them, and create an environment in which titanium oxides are likely to sinter. For this reason, it is presumed that high heat resistance as in the examples was not exhibited in Comparative Example 9.

Claims

1. Titanium oxide particles in which the content rate of anatase crystal phase in the fully crystalline phase is 85% or more, a modification layer provided on the surface of the titanium oxide particles, and The modification layer is modified with an acidic solution containing an organic acid with a molecular weight of 200 or less in an amount of 1.5x10 －４ mol / L or more and 0.12 mol / L or less, the organic acid is at least one organic acid selected from the group consisting of lactic acid, succinic acid, malic acid, tartaric acid, and citric acid, wherein the pH of the acidic solution is 0.2 to 1.5 characterized in that it is a highly heat-resistant anatase-type titanium oxide.

2. The modification layer is modified with an acidic solution containing an organic acid having a molecular weight of 200 or less in an amount of 1.5x10 －４ mol / L or more and 0.05 mol / L or less The highly heat-resistant anatase-type titanium oxide according to Claim 1.

3. The BET specific surface area of the titanium oxide particles is 300 to 500 m ２ / g The highly heat-resistant anatase-type titanium oxide according to any one of Claims 1 or 2.

4. In the acidic solution, the molar ratio (A / T, mol / mol) of the total amount A of the organic acid to the amount T of the titanium oxide particles in terms of metallic titanium is 1.5×10 －４ or more and 0.12 or less The highly heat-resistant anatase-type titanium oxide according to any one of Claims 1 to 3.

5. The highly heat-resistant anatase-type titanium oxide according to any one of Claims 1 to 4, satisfying all of the following (i) to (iii): (i) The content rate of the anatase crystal phase in the fully crystalline phase after firing at 700°C for 2 hours is 80% or more, (ii) D100 calculated from the SEM image of titanium oxide after firing at 700°C for 2 hours is 200 nm or less, and (iii) D100 / D50 is 1.98 or less in terms of the particle size calculated from the SEM image of titanium oxide after firing at 700°C for 2 hours.

6. The highly heat-resistant anatase-type titanium oxide according to any one of Claims 1 to 5, wherein the titanium oxide particles are titanium oxide particles in which the content rate of the anatase crystal phase in the fully crystalline phase is 100%.

7. Titanium oxide particles before modification, in which the content of anatase crystal phase in the fully crystalline phase is 85% or more, are modified with an acidic solution containing an organic acid having a molecular weight of 200 or less in an amount of 1.5x10 －４ mol / L or more and 0.12 mol / L or less, and the modification step of the titanium oxide particles is provided. The organic acid is at least one organic acid selected from the group consisting of lactic acid, succinic acid, malic acid, tartaric acid, and citric acid, wherein the pH of the acidic solution is 0.2 to 1.5 characterized in that it is a method for producing a highly heat-resistant anatase-type titanium oxide.

8. Before the modification step of the titanium oxide particles, it further includes a manufacturing step of the titanium oxide particles for manufacturing the titanium oxide particles before modification, wherein the manufacturing step of the titanium oxide particles is a step of precipitating titanium oxide by a hydrolysis reaction of titanium tetrachloride to obtain a dispersion of titanium oxide particles characterized in that it is a method for producing the highly heat-resistant anatase-type titanium oxide according to Claim 7.

9. In the manufacturing step of the titanium oxide particles, sulfuric acid is added during the hydrolysis reaction of the titanium tetrachloride characterized in that it is a method for producing the highly heat-resistant anatase-type titanium oxide according to Claim 8.

10. In the process for producing the titanium oxide particles, the BET specific surface area of the titanium oxide particles is 300 to 500 m ２ / g. The method for producing highly heat-resistant anatase-type titanium oxide according to claim 8 or claim 9.

11. In the step of modifying the titanium oxide particles, in the acidic solution, the molar ratio (A / T, mol / mol) of the total amount A of the organic acid to the amount T in terms of metallic titanium of the titanium oxide particles before modification to be modified is 1.5x10 －４ or more and 0.12 or less A method for producing highly heat-resistant anatase-type titanium oxide according to any one of claims 7 to 10.

12. Further comprising a purification step of separating the highly heat-resistant anatase-type titanium oxide from impurities using at least one selected from the group consisting of ultrafiltration membranes, reverse osmosis membranes, ion exchange resins, and electrodialysis membranes, and purifying the highly heat-resistant anatase-type titanium oxide A method for producing highly heat-resistant anatase-type titanium oxide according to any one of claims 7 to 11.

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