JP4512689B2 - Hydroxyapatite-coated silica porous material and method for producing the same - Google Patents

Description

  The present invention relates to a catalyst carrier, a deodorizing agent, a filter aid, a light diffusion sheet, an inkjet recording sheet, a toner, a photosensitive material, a pigment, a solar cell substrate, a liquid crystal display device, a dye thermal transfer sheet, a heat resistant resin, and an ultraviolet shielding agent. Has a wide range of uses as gas detection elements and various fillers, and is a novel hydroxy material that is suitable for applications requiring biocompatibility such as cosmetics, biological tissue adsorbents, and biological tissue separation agents. The present invention relates to an apatite-coated silica porous body and a method for producing the same.

  Silicate materials are used in various applications as filter aids, adsorbents, fillers, and the like. For example, silicate-based plate crystal mica is used as various fillers and cosmetics (see Patent Document 1 and Patent Document 2), which contains a large amount of aluminum and has a purity as a silicate. In addition to being low, the specific surface area is small and the crystal structure is strong, so it is difficult to introduce functional groups necessary for hydrophobization or hydrophilization, and the amount of oil absorption is small, which limits the range of use. I can't help it.

  In addition, flaky low crystalline silica produced by hydrothermal treatment of a silica sol obtained by electrodialysis of a dilute solution of water glass is also known (see Patent Document 3). The crystalline silica that causes silicosis causes problems in environmental hygiene, and the flake-like silica is composed of lamellar scaly flakes, resulting in bending and twisting into a distorted shape. It has a drawback that it is easily formed and the specific surface area is small.

  It is also known that silica gel suitable for adsorbing and removing proteins and polyphenols in beer suspension can be obtained by treating sodium silicate with sulfuric acid to remove sodium (see Patent Document 4). ).

  However, such silica gel does not have a function as a filter aid and also exhibits excellent adsorptivity on the particle surface, but the adsorptivity inside the particle is low, and if pulverized to further increase the adsorptivity, There is a drawback that clogging occurs during solid-liquid separation.

As an improvement on the disadvantages of such silica gel, the present inventors previously described a length ratio of 5 to 50 μm, a width of 1 to 20 μm, a thickness of 0.05 to 0.5 μm, and an aspect ratio of length to thickness. 100 to 300, average pore diameter of 1 to 20 nm, total pore volume of 0.1 to 1.5 ml / g, BET specific surface area of 200 to 500 m ² / g, and in the X-ray diffraction spectrum, around 21 ° and 26 A thin plate-like silica porous material having no 2θ peak around 5 °, a fiber length of 5 to 50 μm, a fiber diameter of 0.05 to 0.5 μm, and an aspect ratio of the fiber length to the fiber diameter of 100 to 100 300, average pore diameter of 1 to 20 nm, total pore volume of 0.1 to 1.5 ml / g, BET specific surface area of 300 to 800 m ² / g, and in the X-ray diffraction spectrum, around 21 ° and 26.5 There is a 2θ peak near ° A fibrous silica porous body characterized by the absence of the porous silica was proposed (see Patent Document 5).

However, when these silica porous bodies are used as a biological tissue separating agent, adsorbent, or cosmetic base, they have a drawback that they lack biocompatibility and cannot exhibit sufficient effects.
On the other hand, by separating the silicate compound base material alternately in calcium salt aqueous solution and phosphate aqueous solution to form bone-like hydroxyapatite with orientation on the base material surface, biopolymer separating agent and cosmetic raw material It is known that a suitable hydroxyapatite complex can be obtained.

  However, the thin plate-like or fibrous silica porous body is coated with hydroxyapatite, exhibits excellent adsorption performance and separation performance, does not cause clogging during solid separation, and has improved affinity to living tissue. It has never been known to make a composite.

JP-A-5-262514 (Claims and others) JP 2000-247630 A (Claims and others) JP-A-11-29317 (Claims and others) Japanese Patent Laid-Open No. 11-138017 (claims and others) Japanese Patent Application No. 2003-418740 (Claims and others)

  The present invention has a good affinity for living tissue, has excellent adsorption performance and separation performance, and does not cause clogging as a filter medium in a solid-liquid separation process after use, or living tissue The object of the present invention is to provide a novel porous silica material suitable as a cosmetic base material that requires a high affinity.

  The inventors of the present invention perform hydrothermal reaction of a predetermined proportion of a silicic acid raw material and a lime raw material in water or an aqueous alkali hydroxide solution to prepare a calcium silicate-containing slurry, and then add acid to produce a by-product. When a part of calcium oxide is dissolved and removed to form a porous body, and phosphoric acid or a phosphoric acid generating substance is further added and reacted with the remaining calcium oxide, hydroxyapatite is generated, and the surface of the porous silica body is The present inventors have found that it is coated with hydroxyapatite and have come to make the present invention based on this finding.

That is, the present invention has a length of 0.3 to 50 μm, a width of 0.1 to 20 μm, a thickness of 0.05 to 1.5 μm, an aspect ratio of length to thickness of 5 to 300, an average pore diameter of 1 to 30 nm, It has a pore volume of 0.1 to 1.5 ml / g, a BET specific surface area of 50 to 800 m ² / g, and in the X-ray diffraction spectrum, there are 2θ peaks near 26 ° and 32 ° inherent to hydroxyapatite. When the hydroxyapatite-coated silica porous material and the silicic acid raw material powder and the lime raw material powder having an average particle diameter of 20 μm or less are converted into SiO ₂ and CaO, respectively, the molar ratio CaO / SiO ₂ is 0.00. After mixing at a ratio of 6 to 5.0 and causing a hydrothermal reaction in the presence of water or an aqueous alkali hydroxide solution to prepare a calcium silicate-containing aqueous slurry, an acidic substance is added to the slurry. A part of calcium oxide is dissolved and removed to form a porous body, and then phosphoric acid or a phosphoric acid generating substance is further added to react with the remaining calcium oxide to form hydroxyapatite to form the surface of the porous body. The present invention provides a method for producing a hydroxyapatite-coated silica porous material, which is coated with hydroxyapatite.

The silica porous body serving as the base of the hydroxyapatite-coated silica porous body of the present invention is not particularly limited, but a thin plate-like or fibrous silica porous body having specific physical properties is used. This preferred thin plate-like silica porous body has a length of 5 to 50 μm, preferably 10 to 30 μm, a width of 1 to 20 μm, preferably 5 to 10 μm, and a thickness of 0.05 to 0.5 μm, preferably 0.1. The aspect ratio of length to thickness is 5 to 100, preferably 10 to 50. Further, it has pores having an average pore diameter of 1 to 20 nm, preferably 3 to 10 nm, and the total pore volume is 0.1 to 1.5 ml / g, preferably 0.3 to 1.0 ml / g. , and the and a BET specific surface area of 50 to 300 m ² / g, preferably it has a 100 to 250 m ² / g.

Further, a preferred fibrous silica porous body has a fiber length of 5 to 50 μm, preferably 10 to 30 μm, a fiber diameter of 0.05 to 0.5 μm, preferably 0.1 to 0.3 μm, and a fiber length. The aspect ratio of the fiber diameter is in the range of 100 to 300, preferably 70 to 250. Further, it has pores having an average pore diameter of 1 to 20 nm, preferably 3 to 10 nm, and the total pore volume is 0.1 to 1.5 ml / g, preferably 0.3 to 1.0 ml / g. And a BET specific surface area of 300 to 800 m ² / g, preferably 450 to 550 m ² / g.

  These silica porous materials are new materials not yet described in the literature, and X-ray diffraction measured by a conventional thin plate-like silica porous material obtained using a silicic acid raw material having a relatively large particle diameter using CuKα rays. In the spectrum, it is characterized by showing prominent peaks around 21 ° and 26.5 °, while not showing any peaks.

This thin plate-like silica porous body has a molar ratio CaO / SiO ₂ of 0.6 to 5.0 when, for example, a silicic acid raw material and a lime raw material having an average particle diameter of 20 μm or less are converted into SiO ₂ and CaO, respectively. The mixture is mixed at a ratio, and hydrothermal reaction is carried out in the presence of water or an aqueous alkali hydroxide solution to prepare a thin plate-like calcium silicate-containing aqueous slurry. Then, an acidic substance is introduced into this, and calcium oxide therein Is gradually dissolved and removed to form a thin plate-like silica porous body, and the fibrous silica porous body converts, for example, a silicic acid raw material and a lime raw material having an average particle diameter of 20 μm or less into SiO ₂ and CaO, respectively. weight ratio to a molar ratio CaO / SiO ₂ when the is 0.6 to 5.0, in the presence of water or an aqueous alkali hydroxide solution to perform the hydrothermal reaction, first lamellar calcium silicate containing After forming an aqueous slurry and continuing the hydrothermal reaction to transfer to an aqueous slurry containing fibrous calcium silicate, an acidic substance is introduced into this, and the calcium oxide therein is gradually dissolved and removed. It is manufactured by forming a fibrous silica porous body.

  When dissolving and removing calcium oxide by the above acidic substance, first, a porous silica is formed using an acidic substance in an amount that dissolves and removes a part of calcium oxide, and then phosphoric acid or a phosphoric acid generating substance. Is added to react with the remaining calcium oxide to produce hydroxyapatite, which covers the surface of the porous body, so that the hydroxyapatite-coated silica porous body of the present invention is obtained.

  Examples of the phosphoric acid-generating substance include trialkali metal phosphates such as trisodium phosphate and tripotassium phosphate, dialkali metal phosphates such as disodium hydrogen phosphate and dipotassium hydrogen phosphate. Alkali metal dihydrogen phosphates such as sodium dihydrogen phosphate, potassium dihydrogen phosphate and the corresponding ammonium salts are used.

  In the production of the lamellar silica porous body, fibrous silica porous bodies and secondary particle aggregates are by-produced, but when the hydrothermal reaction is carried out in the presence of lamellar calcium silicate seed crystals, The production of these by-products is suppressed, and a thin plate-like silica porous body can be selectively obtained, which is preferable.

  As this seed crystal, from a reaction mixture obtained by hydrothermal reaction of silicate raw material powder and lime raw material powder in water or an aqueous alkali hydroxide solution in advance, fibrous calcium silicate and secondary particle aggregates Excluding such impurities, those recovered as pure lamellar calcium silicate crystals are used.

  This seed crystal is added at a ratio of 0.01 to 10% by mass based on the total mass of the silicic acid raw material and the lime raw material. As this seed crystal, those having a major axis of 5 to 50 μm, a thickness of 0.05 to 0.5 μm, and an aspect ratio of the major axis to the thickness of 100 to 300 are usually used. The size of the seed crystal is appropriately selected. Thereby, the length of the obtained sheet-like calcium silicate can be adjusted in the range of 1 to 30 μm.

In the case where no seed crystal was added, it was possible to produce thin plate-like calcium silicate by appropriately selecting the hydrothermal reaction conditions when the raw material CaO / SiO ₂ molar ratio was 1.5. However, mixing of spherical secondary particle aggregates could not be avoided, and in order to avoid this mixing, the CaO / SiO ₂ molar ratio had to be 1.9 or more.
However, in the method of the present invention, pure lamellar calcium silicate can be stably produced even when the CaO / SiO ₂ molar ratio is 1.2 or less by adding this seed crystal.

  Further, in order to suppress the formation of the secondary particle aggregates, water or an alkaline aqueous solution that is 10 times or more the total mass of the silicic acid raw material and the lime raw material must be used. Even if it is about double, lamellar calcium silicate can be produced without forming secondary particle aggregates.

Furthermore, when seed crystals are not added, even if the CaO / SiO ₂ molar ratio is selected to the optimum 2.0, spherical secondary particle aggregates and fibrous calcium silicate are produced due to slight differences in the hydrothermal reaction temperature. Although it was unavoidable that the quality became unstable, the addition of seed crystals can suppress the mixing of products with different shapes, and obtain a thin plate-like calcium silicate of stable quality. Can do.

  The silicic acid raw material used in the above method is not particularly limited as long as it is usually used as a silicic acid raw material. Examples of such silicic acid raw materials include silica-containing substances such as quartz, silica sand, amorphous silicic acid, white carbon, sodium feldspar, potash feldspar, glass, porcelain stone, shirasu, fly ash, slag, and pearlite. Can be mentioned. These may be used alone or in combination of two or more.

  These silicate raw materials need to have an average particle diameter of 20 μm or less, but especially dispersibility in the slurry, hydrothermal reactivity, economy, etc., especially the dispersed calcium silicate plate and fiber In view of the above, an average particle diameter of 0.01 to 20 μm, preferably 0.1 to 18 μm is selected.

  Next, as a lime raw material used in combination with this silicic acid raw material, powders such as quick lime (calcium oxide) and slaked lime (calcium hydroxide) used as a normal lime raw material can be used. These may be used alone or in combination of two or more. The viscosity of the lime raw material is preferably one having the same average particle diameter as that of the silicic acid raw material, but other sized particle may be used.

The silica raw material and the lime raw material are desirably used in such a ratio that the CaO / SiO ₂ molar ratio is in the range of 0.6 to 5.0, preferably 1.0 to 4.0. Calcium silicate can be obtained even at a CaO / SiO ₂ molar ratio of 0.6 or less, or 5.0 or more, but a large amount of unreacted silica remains and must be removed, which is not economical. A particularly preferable range is 1.8 or more, particularly around 2.0.

  The hydrothermal reaction is performed by dispersing the above-described silicic acid raw material and lime raw material in water or an alkali hydroxide aqueous solution at a predetermined ratio. As the alkali hydroxide aqueous solution at this time, for example, a solution prepared by dissolving an alkali metal hydroxide such as lithium hydroxide, sodium hydroxide, or potassium hydroxide in water is used. These alkali hydroxides may be used alone or as a mixture of two or more.

  In this case, the concentration of the alkaline aqueous solution is preferably 0.01 to 1.0 mol. When the concentration of the aqueous alkali solution is less than 0.01 mol, the effect of adding an alkali for changing the crystal form of the generated calcium silicate or promoting the hydrothermal reaction is not sufficiently exhibited. Moreover, even if it exceeds 1.0 mol, the improvement of an alkali addition effect is not recognized.

  On the other hand, the concentration of water or alkali hydroxide aqueous solution slurry containing silica raw material and lime raw material is not particularly limited, but considering hydrothermal reactivity and volume efficiency, the total amount of silicic acid raw material and lime raw material A slurry containing an aqueous solvent at a ratio of 2 to 30 times mass is preferable.

  The hydrothermal reaction is carried out in a temperature range of 100 to 250 ° C., for example, in an autoclave. This hydrothermal reaction proceeds under an autogenous pressure, but the reaction may be carried out by appropriately applying pressure as necessary. Moreover, in order to accelerate | stimulate reaction rate during reaction, you may stir as needed.

  If the hydrothermal reaction temperature is less than 100 ° C., the reaction rate is too slow, requiring a long time, and is not practical. If the reaction temperature exceeds 250 ° C., the self-generated pressure becomes too high, which is economically disadvantageous in terms of equipment. The reaction time depends on the slurry concentration, the type and particle size of the raw material, the reaction temperature, and the like, and cannot be determined generally, but usually about 0.5 to 24 hours is sufficient.

  Next, after carrying out a hydrothermal reaction to form a porous silica material, an acidic substance is introduced into the reaction mixture, and a part of the by-produced calcium oxide is dissolved and removed.

  Examples of the acidic substance used here include inorganic acids such as hydrochloric acid, sulfuric acid, nitric acid, and carbonic acid, and organic acids such as formic acid, oxalic acid, acetic acid, propionic acid, maleic acid, lactic acid, and acidic cation exchangers. . The inorganic acid may be in the form of a salt such as ammonium nitrate.

  As the inorganic acid, hydrochloric acid, nitric acid or a mixture thereof is preferable. However, since these have a high degree of ionization and the pH is rapidly lowered, a dilute acid aqueous solution is gradually added so that the pH does not fall abruptly. . If it does in this way, calcium oxide will be removed, without changing the form of calcium silicate. Depending on the crystallinity of calcium silicate, calcium oxide can be more efficiently removed by heating the calcium silicate slurry in the range of room temperature to 100 ° C.

  On the other hand, in the case of acetic acid, carbonic acid, etc. with low ionization degree, even if calcium silicate is directly treated with a high concentration acid, the removal of calcium oxide gradually proceeds and the form of calcium silicate is maintained. A porous silica material is obtained. Further, when the calcium silicate slurry is heated in the range of room temperature to 100 ° C., the degree of ionization increases and the removal of calcium oxide is promoted.

  The calcium silicate treated with carbonic acid can be obtained by dissolving and removing calcium carbonate with hydrochloric acid or the like because calcium carbonate that is sparingly soluble in the thin plate-like silica porous material and water is obtained. The time required for this acid treatment depends on the crystallinity of calcium silicate, the type of acid used, the concentration, the treatment conditions, etc., but is usually in the range of 1 to 120 minutes.

  As an acid used for removing calcium oxide from calcium silicate, an organic acid can also be used. As such an organic acid, formic acid, acetic acid, oxalic acid, propionic acid, maleic acid, lactic acid, an acidic cation exchanger and the like are preferable.

  As this acid treatment, a method in which carbon dioxide gas is blown into a calcium silicate slurry obtained by a hydrothermal reaction is advantageous in that calcium oxide can be gradually dissolved and removed.

Thus, length 0.3-50 μm, width 0.1-20 μm, thickness 0.05-1.5 μm, length-to-thickness aspect ratio 5-300, average pore diameter 1-30 nm, total fineness It has a pore volume of 0.1 to 1.5 ml / g, a BET specific surface area of 50 to 800 m ² / g, and in the X-ray diffraction spectrum, there are 2θ peaks in the vicinity of 26 ° and 32 ° inherent to hydroxyapatite. A hydroxyapatite-coated silica porous material can be obtained.

  ADVANTAGE OF THE INVENTION According to this invention, the hydroxyapatite covering silica porous body which is a novel material used suitably in the field | area where biocompatibility is requested | required, such as cosmetics, the biological tissue adsorption agent, the biological tissue separation agent, is provided.

  The general description of the means for solving the problems of the present invention has been described above. Next, the best mode for carrying out the present invention will be described by way of examples. In addition, the physical property in each case is calculated | required with the following method.

(1) Average particle size and particle size distribution of silica raw material Using a laser diffraction / scattering type particle size distribution measuring device, the particle size was determined on a volume basis, and the average particle size (median diameter) and particle size distribution were determined.

(2) Dimensional measurement Using a scanning electron microscope (SEM), about 30 dimensional measurements were made to obtain an average dimension.

(3) BET specific surface area, total pore volume and average pore diameter Using a BET specific surface area measuring device, a specific surface area based on a multipoint method in which nitrogen gas is adsorbed on a sample sufficiently heated and degassed at 250 ° C., total pores Volume and average pore diameter were determined.

(4) Silicic acid content The silicic acid content was measured using fluorescent X-rays.

(5) Transmittance (Darcy)
Using a cylindrical pressure filter with a filtration area of 9.6 cm ² , a cake layer of about 3 cm was formed on the filter plate, and then 200 ml of water was poured so as not to break the cake layer, and 0.5 kg / cm ^The pressure was increased at ² , and the permeability (Darky) was determined from the amount of filtrate collected.

(6) Adsorption rate of cytochrome C 100 ml of a 500 μg / ml cytochrome C aqueous solution adjusted to pH 4 was sampled, 0.3 g of the sample was put into this, and infiltrated with a constant temperature incubator at 30 ° C. for 1 hour in the afternoon 5 C filter paper And filtered. Absorbance (wavelength 410 nm) of the remaining amount of cytochrome C in the filtrate was measured using a spectrophotometer, and the adsorption rate was determined from the difference from the initial concentration.

(7) Oil absorption amount According to JIS K5101, 1 g of sample was used.

Reference example (seed crystal preparation method)
Amorphous silicic acid raw material and quicklime raw material pulverized to an average particle size of 0.5 μm are mixed so that the CaO / SiO ₂ molar ratio is 2.0, and the mass ratio is 20 times the total amount of the raw material. A 0.1 mol NaOH aqueous solution was added and stirred to prepare a slurry.
Next, this slurry was put in an autoclave and subjected to a hydrothermal reaction at 200 ° C. for 4 hours with stirring to obtain a calcium silicate slurry. After confirming that the obtained calcium silicate slurry is lamellar calcium silicate by SEM observation, the slurry was washed and filtered, and water of 20 times mass by mass ratio was added to the total amount of raw materials, and a ball mill was used. The seed crystals were obtained by wet pulverization for 48 hours, washing, filtering and drying at 150 ° C.

Amorphous silicic acid raw material crushed to an average particle size of 17.2 μm and quicklime raw material are mixed so that the CaO / SiO ₂ molar ratio is 2.0, and the mass ratio is 10 times the total amount of the raw material. A 0.2 mol NaOH aqueous solution was added and stirred to prepare a slurry. Next, this slurry was put in an autoclave and subjected to a hydrothermal reaction at 200 ° C. for 8 hours with stirring to obtain a calcium silicate slurry. The slurry was cooled to 70 ° C. to neutralize 0.2 mol NaOH aqueous solution and to dissolve and remove 70% by mass of calcium oxide in calcium silicate (concentration 99.7%). ) And stirring for 60 minutes to form a porous silica, and then adding a sufficient amount of diammonium hydrogen phosphate to hydroxyapatite the remaining 30% by mass of calcium oxide, The mixture was reacted for 60 minutes with stirring and then dried at 120 ° C. to obtain a hydroxyapatite-coated thin plate-like silica porous material.
This product had 2θ peaks around 26 ° and 32 ° in the X-ray diffraction spectrum. The physical properties of this product are shown in Table 1.

Amorphous silicic acid raw material and quick lime raw material pulverized to an average particle size of 0.5 μm are mixed so that the CaO / SiO ₂ molar ratio is 1.5, and a reference example in terms of mass ratio with respect to the total amount of raw material A slurry was prepared by adding 0.1 wt% of the seed crystal obtained in the above and 10 times water and stirring.
Next, this slurry was put in an autoclave and subjected to a hydrothermal reaction at 200 ° C. for 2 hours with stirring to obtain a calcium silicate slurry.
Next, the calcium silicate slurry is cooled to 70 ° C., and high-concentration acetic acid (concentration 99.7%) necessary for removing 70% by mass of calcium oxide in the calcium silicate is added and stirred for 10 minutes. did. After washing and filtering this, water is added, and a sufficient amount of diammonium hydrogen phosphate is added to make the remaining calcium oxide hydroxyapatite. The mixture is stirred at 70 ° C. for 60 minutes, then washed and filtered at 120 ° C. By carrying out the drying treatment, a hydroxyapatite-coated thin plate-like silica porous body was obtained.
This product had 2θ peaks around 26 ° and 32 ° in the X-ray diffraction spectrum. The physical properties of this product are shown in Table 1.

  The hydroxyapatite-coated silica porous material of the present invention includes a catalyst carrier, a deodorizing agent, a filter aid, a light diffusion sheet, an inkjet recording sheet, a toner, a photosensitive material, a pigment, a solar cell substrate, a liquid crystal display element, a dye thermal transfer sheet, It is useful as a heat-resistant resin, an ultraviolet shielding agent, a gas detection element, various fillers, and the like.

Claims (10)

Length 0.3-50 μm, width 0.1-20 μm, thickness 0.05-1.5 μm, length-to-thickness aspect ratio 5-300, average pore diameter 1-30 nm, total pore volume 0.1 It has a BET specific surface area of 50 to 800 m ² / g and has 2θ peaks in the vicinity of 26 ° and 32 ° inherent to hydroxyapatite in the X-ray diffraction spectrum. Hydroxyapatite-coated silica porous material. Silica raw material powder and lime raw material powder having an average particle size of 20 μm or less are mixed at a ratio of molar ratio CaO / SiO ₂ of 0.6 to 5.0 when converted to SiO ₂ and CaO, respectively, A hydrothermal reaction is performed in the presence of an aqueous alkali hydroxide solution to prepare an aqueous slurry containing calcium silicate, and then an acidic substance is added to dissolve and remove part of the calcium oxide in the slurry to form a porous body. Then, further adding phosphoric acid or a phosphoric acid generating substance, reacting with the remaining calcium oxide to form hydroxyapatite, and covering the surface of the porous body with hydroxyapatite, Production method.   The method for producing a hydroxyapatite-coated silica porous material according to claim 2, wherein the thin plate-like silica porous material is separated and recovered from the reaction mixture, dried, and further heat-treated at 300 to 1400 ° C.   The silicic acid raw material is at least one silicic acid-containing substance selected from quartz, silica sand, amorphous silicic acid, white carbon, feldspar, porcelain stone, glass, shirasu, fly ash, slag and perlite. Item 4. A method for producing a hydroxyapatite-coated silica porous material according to Item 2 or 3.   The method for producing a hydroxyapatite-coated silica porous body according to claim 2, 3 or 4, wherein the lime raw material is quick lime, slaked lime or a mixture thereof.   The method for producing a porous hydroxyapatite-coated silica according to any one of claims 2 to 5, wherein the acidic substance is an aqueous solution or a diluted aqueous solution of an inorganic acid.   The method for producing a porous hydroxyapatite-coated silica according to claim 6, wherein the inorganic acid is at least one selected from hydrochloric acid, sulfuric acid, nitric acid and carbonic acid.   The method for producing a porous hydroxyapatite-coated silica material according to any one of claims 2 to 5, wherein the acidic substance is an organic acid aqueous solution.   The method for producing a porous hydroxyapatite-coated silica according to claim 8, wherein the organic acid is formic acid, acetic acid, oxalic acid, propionic acid, maleic acid, lactic acid or an acidic cation exchanger. The method for producing a porous hydroxyapatite-coated silica material according to any one of claims 2 to 5, wherein the acidic substance is gaseous carbon dioxide.