JP4974757B2 - Method for producing fine particle-supported porous body - Google Patents

Description

  The present invention relates to a method for producing a fine particle-supporting porous body in which fine particles such as a catalyst are uniformly supported in a porous body such as an inorganic fiber molded body.

  Conventionally, catalysts and adsorbents are often used by being supported on a porous body. As one method of supporting functional fine particles such as catalyst and adsorbent on a porous material, a porous material such as an inorganic fiber molded product is impregnated with a dispersion of fine particles having a function of catalyst or adsorption, and then heated. There is a method of supporting by drying.

  However, in such a conventional impregnation method, surface migration occurs during drying, so that a uniform distribution of fine particles cannot be obtained in the porous body. That is, when the porous material is impregnated with the fine particle dispersion and then directly dried by hot air drying or the like, evaporation of water starts from the surface of the porous material. And if the water | moisture content of a porous body surface begins to reduce, a water | moisture content will move from the inside of a porous body to the surface side in the state of water, will evaporate on the surface, and finally the whole will dry. At this time, since the fine particles also move to the surface side of the porous body together with water, the fine particles are concentrated on the surface.

  In such a non-uniform distribution state, even if the function of the fine particles is obtained, the large surface area of the fine particles cannot be effectively used, and thus the activity cannot be sufficiently extracted. In addition, the porous body is used as a carrier such as a catalyst because the specific surface area is large. If the fine particles are not uniformly dispersed on a large surface inside the porous body, the merit of the porous body cannot be utilized. Therefore, in order to obtain sufficient performance of the functional fine particles in such a non-uniform distribution state, it is possible to carry the functional fine particles excessively, but in this case, there is a problem that the cost of the product becomes high There is.

  As a method for preventing the above-described surface migration of fine particles, a gelation method is known in which fine particles and water are confined in a gel so that the fine particles cannot move and only water is evaporated. For example, in JP-A-01-242737, a solution prepared by hydrolyzing an alkoxide such as Si or Al and adding a gelling agent thereto to be gelled in a short time is used for forming a fiber molded body. It is described that it is used as a binder (fine particles).

JP 04-059675 A prepares a slurry in which ceramic fibers and / or ceramic whiskers are dispersed in a solution of a gelling agent, and heats or cools the slurry to cause gelation and solidification. A method for producing an inorganic fibrous porous material is described in which the obtained gel body is dried and then fired. As the gelling agent, agar, gelatin, basic salts such as (NH ₄ ) ₂ CO _{3 and} the like are known.

  However, in order to gel the fine particles, the gelling agent is added, or the fine particles themselves are limited to those that can be gelled. In addition, when a gelling agent is added, since the normal gelling agent is an organic substance, it is necessary to finally calcinate and remove the organic substance in order to prevent the fine particles from being covered with the organic substance and losing its activity. . However, since most of the functional fine particles are deactivated by firing (there is an upper limit on the firing temperature), it is desirable to avoid the firing step. Even when the gelling agent is an inorganic material, the surface of the fine particles is covered with an inorganic gel, so that the performance cannot often be exhibited.

  In addition, when using an inorganic fiber molded object as a porous body, it is also possible to hold | maintain microparticles | fine-particles in the molded object simultaneously with shaping | molding of a fiber molded object. For example, an inorganic fiber molded article carrying fine particles can be obtained by mixing and dispersing fine particles together with inorganic fibers and a binder in water, forming the mixture by a technique such as suction molding, and drying or firing. However, since this method requires a binder for formation, if fine particles are mixed together with the binder during molding, the fine particles are covered with the binder, and the function of the fine particles is lost.

Japanese Patent Laid-Open No. 01-242737 JP 04-059675 A

  In view of the above-described conventional circumstances, the present invention uses a simple impregnation method in which fine particles are impregnated into a porous body such as a fiber molded body when producing a porous body having fine particles supported in internal voids. It is an object of the present invention to provide a method for producing a fine particle-carrying porous body that can carry fine particles uniformly in the inside thereof and can improve the utilization efficiency of functional fine particles.

In order to achieve the above object, a method for producing a fine particle-supporting porous material provided by the present invention is a method for producing a porous material in which fine particles are supported in internal voids, in which functional fine particles are dispersed in water. It is characterized by impregnating a liquid with a porous body, rapidly cooling the porous body to freeze water in the voids, and then heating and drying the frozen porous body at 80 to 300 ° C. in a hot air drying furnace. To do.

In the production method of the particulate support porous body according to the present invention, it is preferable that the functional particle in the water to control the temperature of the fine particle dispersion is dispersed in 5 ° C. or less, the porous body of the inorganic fiber molded It is preferable that it is a body .

  According to the present invention, while using a simple impregnation method, the surface migration of fine particles, which has been a problem in the past, can be eliminated, and a fine particle-supporting porous body in which fine particles are uniformly supported inside the porous body can be obtained. . In addition, since there is no need to add a binder or gelling agent as in the past and no baking process is required, the surface of the fine particles is not covered with other components, and the use of fine particles is combined with uniform dispersion. It becomes possible to increase efficiency and to exhibit sufficient functions.

  Further, when the water frozen in the porous body is dried, it may be transferred to a normal hot-air drying furnace or the like and heated and dried, and it is not necessary to keep it in a vacuum state and dry it as in the so-called freeze-drying. Therefore, there is no need to provide a pump or vacuum chamber for evacuating, and no equipment or power costs are required. Therefore, the manufacturing cost can be reduced in addition to not using the above-mentioned binder or gelling agent. it can.

  In the method of the present invention, first, a porous material is impregnated with fine particles dispersed in water by an impregnation method. When this porous body impregnated with water and fine particles is frozen, only water becomes ice, and the fine particles are expelled between ice crystals and agglomerate to become secondary particles and become coarse. Since the coarse secondary particles aggregated inside the porous body during freezing in this way remain secondary particles after thawing and do not return to the primary particles, they are supported without being able to move inside the porous body during subsequent heating and drying. The

  That is, when the frozen porous body is heated, the ice inside the porous body melts, but the aggregated secondary particles do not return to the primary particles. Therefore, the water inside the porous body evaporates from the surface, and the water moves from the inside to the surface side (surface migration). However, the aggregated secondary particles cannot move because they are coarse. Retained. As a result, the concentration distribution of fine particles is eliminated between the surface and the inside of the porous body, and the fine particles can be uniformly supported. Needless to say, even in the state of agglomerated secondary particles, the particle surface is much easier to use (the effective surface area is wide) than in the state of being concentrated on the surface.

  The method of the present invention will be described more specifically. First, a fine particle dispersion for impregnation is prepared by adding the fine particles to water and sufficiently stirring and mixing. The fine particles to be used are not limited in kind, crystal structure, surface state, etc. For example, various functional fine particles such as a catalyst such as titanium oxide and a noble metal, and an adsorbent such as zeolite can be used.

  The size of the fine particles to be used needs to be large enough to enter the inside of the porous body together with water, and is generally about one-sixth or less of the size (diameter) of the pores of the porous body. It is preferable. If the fine particles are too large, they are filtered on the surface of the porous body during impregnation and cannot be impregnated inside the porous body. Even if the raw material fine particles form secondary particles, the secondary particles in the raw material fine particles may have any size as long as they become primary particles when dispersed in water and can be impregnated into the porous body.

  In addition, the fine particles that immediately settle are settled before freezing in the impregnated porous body, and a concentration distribution is likely to occur in the porous body, so that it is difficult to use for impregnation. In the case of such fine particles, it is necessary to add a dispersant, a viscosity modifier or the like to prevent sedimentation. However, in that case, a baking step at a low temperature is required to remove the added drug, so care must be taken not to lose the activity particularly when functional fine particles are used. Moreover, a quick freezer, liquid nitrogen, etc. can also be utilized so that microparticles can aggregate before sedimentation.

  Next, the porous body is immersed in the fine particle dispersion, and the porous body is impregnated with fine particles together with water. The porous body to be used is not particularly limited as long as fine particles can enter the inside through voids. For example, inorganic fiber molded bodies, ceramic porous fired bodies, diatomaceous earth fired grains, porous refractory bricks, etc. Can be used. In addition, the aperture of the porous body is important in relation to the impregnation of fine particles as described above, but by controlling the fiber diameter and density in the inorganic fiber molded body, by using a pore-imparting agent in the porous fired body, etc. In any case, the aperture can be adjusted.

  In particular, the inorganic fiber molded body is preferable because it can carry a large amount of fine particles on the inner fiber surface and can carry out sufficient performance if functional fine particles are supported. In the production of an inorganic fiber molded body, an organic and inorganic binder can be added and fired at a high temperature to obtain a high-strength inorganic fiber molded body. However, the organic binder disappears during this high-temperature firing. In addition, the functional fine particles impregnated later into the inorganic fiber molded body are not covered with a binder and are not exposed to high temperatures, so that the crystal structure changes or the specific surface area decreases. Therefore, its activity is not lost.

  The porous body impregnated with fine particles together with water is rapidly cooled to freeze the water in the voids. During the period from impregnation to freezing, the fine particle dispersion (impregnating liquid) impregnated in the porous body is held by the capillary force due to the gap between the porous bodies. Therefore, it is preferable that the impregnated porous body is immediately transferred to a refrigeration apparatus and rapidly cooled to shorten the time during which gravity sedimentation is possible. Further, since the viscosity of water increases as the temperature decreases, the temperature of the water in which the fine particles are dispersed is preferably as low as possible. In particular, if the temperature is controlled to 5 ° C. or less, gravity sedimentation can be suppressed.

  Thereafter, the frozen porous body is heat-dried to remove moisture from the porous body, thereby obtaining a porous body carrying fine particles therein. Various heat drying ovens or heat drying apparatuses can be used for heat drying of the frozen porous body, and among them, the use of a hot air drying furnace is preferable. In addition, if the heating temperature is 80 ° C. or higher, efficient drying is possible and it can be used up to about 300 ° C., but if the temperature is up to about 100 ° C., the activity of the functional fine particles is lost. Particularly preferred.

By adding 10 kg of anatase-type titanium oxide (TiO ₂ : average diameter 0.5 μm, specific surface area 100 m ² / g) as functional fine particles to 90 kg of water and sufficiently stirring and mixing using a uniaxial stirrer, A fine particle dispersion for impregnation was prepared.

On the other hand, the compounding tank containing the concentration 20 wt% of colloidal silica 25kg of added water 5000 kg (5 m ^3), after the addition of alumina-silica fibers 50kg to prepare a slurry, was added a coagulant, by using a mold suction And then molded. This molded body was further dehydrated and fired at 1200 ° C. to obtain an inorganic fiber molded body having a density of 250 kg / m ³ .

  The inorganic fiber molded body was immersed in the fine particle dispersion, and the inorganic fiber molded body was impregnated with a fine particle dispersion containing water and titanium oxide fine particles. Immediately after the impregnation, the inorganic fiber molded body was put in a freezer and kept at −20 ° C. for 24 hours to be frozen. The frozen inorganic fiber molded body was transferred to a hot air drying furnace and dried with hot air at 120 ° C. for 24 hours.

  The obtained fine particle-supported porous body supported 40% of anatase-type titanium oxide in the inorganic fiber molded body, and the anatase-type titanium oxide particles formed a porous aggregate of about 10 to 300 μm. This fine particle-supported porous body was useful as a filter supporting an oxidation catalyst.

Claims (3)

A method for producing a porous body in which fine particles are supported in internal voids, wherein the porous body is impregnated with a fine particle dispersion in which functional fine particles are dispersed in water, and the porous body is rapidly cooled to drain water in the voids. A method for producing a fine particle-supporting porous body, comprising: freezing and then drying the frozen porous body in a hot air drying furnace at 80 to 300 ° C. The method for producing a fine particle-supporting porous body according to claim 1, wherein the temperature of the fine particle dispersion in which the functional fine particles are dispersed in water is controlled to 5 ° C or lower. The method for producing a fine particle-supporting porous body according to claim 1, wherein the porous body is an inorganic fiber molded body .