SK107696A3 - Method for preparing macroporous silica gel having high-volume of pores - Google Patents

Abstract

A method of preparing macroporous silica gel with an average diameter of pores ranging from 20 to 40 nm by expanding pores of a narrow-porous silica gel can be characterised in a combined action of some of hydrous, concentrated or saturated solutions of sodium, potassium, lithium or ammonium salts of (in)organic acids in presence of an applied hydroxide cation while preserving a pH value within a range between 8 and 9. The mixture is kept at a boiling temperature for several hours depending on a required pore size. The method is carried out at atmospheric pressure using a simple apparatus. The method, when compared to those known in the art, provides considerable repeatability for preparing silica gels of expanded pores diameters whose volume is decreased only by 5 - 15 percent with respect to the source silica gel.

Description

Technical field

The invention relates to a process for preparing macroporous silica having a large pore volume.

BACKGROUND OF THE INVENTION

Porous materials are widely used in various fields of technology, e.g. how

- adsorbents for the analytical, preparative and production separation of mixtures of substances, including use as a packed column for chromatography, for the removal of undesirable impurities from food and waste water and for the controlled release of substances such as perfumes, various additives, pheromones, medicaments, etc.

function carriers, e.g. catalysts and enzymes, probes for the selective detection of analytes and various pollutants e.g. in the environment

- reinforcing plastic and rubber fillers, etc.

Of course, it is necessary to provide a material with a suitable pore size for each application.

Silica gels play an important role among the porous materials for the technology. Basically, they are amorphous substances with the general formula (SiC> 2) x (PhOjy), ie hydrated silicas with varying water content, which is physically sorbed or chemically bound and form silanol = SiOH groups [ller, RK, The-Chemistry of Silica, Wiley-Interscience, N. York 1979; Unger, KK, Porous Silica, Elsevier, Amsterdam 1979].

Silica gels are prepared by controlled polycondensation of silicic acid released from alkali silicates in a solution (waterglass) e.g. addition of acids further by polycondensation of tetraalkylsilanes e.g. by the addition of alcohol, destabilization of the silica soles, agglutination of the sol particles in an organic resin solution, which are removed by agglutination by annealing, etc.

Most of the above processes result in mesoporous silica gels having pore diameters (D) in the range of 1 to 10 and 15 nm, respectively. This pore size is not sufficient for some applications and it is necessary to prepare macroporous silica gels with pore diameters of up to 100 nm or more so that large particles can penetrate into them - e.g. polymer molecules, molecular aggregates, etc.

The simplest procedure for expanding silica gel pores is to heat them to temperatures between 600 and 1100 ° C. The mesoporous silica gel matrix is formed by a cluster of submicrometer, spherical, non-porous particles, globes, and pores are voids between globules. During heating, the globules connect, grow and the size of interglobular spaces, pores, grows. At the same time, however, the number of interglobular spaces decreases sharply, and the pore volume decreases. For this reason, the expansion of silica gel pores by simple heating is only suitable if the resulting material can have a low pore volume of less than 0.1-0.2 cm -1 g ^-1 . This situation is relatively rare.

An alternative procedure for widening the silica gel pores is to use "wet processes" which utilize the attack of water molecules on the silica gel matrix at elevated temperature. The classical procedure of expanding silica gel pores with water at elevated temperature is called hydrothermal treatment [Akshinskaya, N.V., Beznogova, V.E., Kiselev, A.V., Nikitin, Yu.S., Zh. Phys. Chim. (Moscow) 36, 1962, p. 2277]. The silica gel was heated in an autoclave for several hours in the presence of water vapor to temperatures above 150 ° C. S1O2 globules grow and interglobular pore spaces increase. In contrast to simple heating of silica gel in the absence of water, the decrease in pore volume is reduced. Depending on the temperature and time of the hydrothermal treatment, silica gels with pore diameters up to 300 nm can be prepared.

The disadvantage of the process is its difficulty in the equipment used - autoclaves and in particular the relatively low repeatability of hydrothermal treatment of silica gel - materials with bi- or multimodal pore distribution arise: in hydrothermally treated silica gel even small pores often remain because the smallest pores of the starting silica gel are not always affected.

A further process of spreading silica gel pores by water at elevated temperature is boiling it in aqueous salt solutions [Vespalec, R., Cigankova, M., Viska, J., J. Chromatogr. 354, 1986, p. 129]. The disadvantage of this otherwise simple and repeatable process is its limited effect: Boiling in salt solutions can only produce pores with a D of up to about 20 nm.

3 Summary of the Invention

The efficiency of the S1O2 matrix attack by water molecules can be increased by the addition of an alkali whose cations are involved in the process. However, boiling S1O2 in a dilute alkali solution does not produce the desired results: If there is an excess of hydroxyde, the silica gel dissolves and if the alkali is present in a "stoichiometric amount that effectively attacks the matrix and results in its restructuring but not sufficient to dissolve it" to about 200 nm. The desired restructuring of silica gel to form macroporous materials is achieved by the combined action of water in the presence of a stoichiometric amount of hydroxide at elevated temperature above 100 ° C - this "chemothermal process" is the essence of the invention. The elevated temperature is ensured by using saturated solutions of inorganic salts as the reaction medium or in an autoclave, at a slightly elevated pressure.

The advantages of such a preparation method are low-cost equipment and high repeatability - similar pore diameters are achieved by treating different types of starting silica gels. In contrast to the known processes, the process according to the invention makes it possible to prepare silica gel with a large pore volume, since the pore size decreases by only 5 to 15%.

The following examples illustrate the invention without limiting its scope.

DETAILED DESCRIPTION OF THE INVENTION

Example 1: Six to eight liters of distilled water are added to 1 kg of dry mesoporous silica with particles of 1 µm to 1 mm in a polyethylene 10 L vessel. The system is stirred until air is released from the silica gel. The exact amount of water added depends on the pore volume of the silica gel and must be determined experimentally. In this case it was 7 liters of water. The slurry was then transferred to a 20 L reactor and heated with stirring. After reaching a temperature of about 70 ° C, dry NaNCl 3 is gradually added until f

formation of a saturated solution. The final amount of NaNCl 3 depends on the amount of water in the system and the saturation point indicates the boiling point of the solution 118 °. Then the H2O / NaNO3 ratio is 1: 2 wt.

To the boiling slurry, 10% NaOH is added dropwise with vigorous stirring, until the pH of the mixture reaches 8-8.5. The mixture is boiled under stirring for 2 hours, then diluted with 4-6 L of distilled water heated to 40-60 ° C and slowly acidified with a 10-15% HNO 3 solution until the pH of the system reaches approximately 3. The hot suspension is filtered off and wash thoroughly with distilled water. The product is dried at 110 ° C for 2h and then calcined at 450-500 ° C for 1 hour. The material obtained has a pore diameter of 30 nm. The advantage of NaNO 3 over other salts is the reduced sensitivity to pH. With pore enlargement, the pore volume decreases by only 5-15%.

Example 2: To 1 kg of dry mesoporous silica gel was added 8 L of a 10% NH 3 OH solution and mixed well. The suspension was placed in an autoclave and heated to 150-170 ° C for 3 hours. After cooling, the suspension is transferred to a filter, washed with 1% HCl solution so that the filtrate is acidic followed by distilled water until the filtrate is neutral. The product is dried at 110 ° C for 2h and then calcined at 450-500 ° C for 1 hour. The material obtained has pore diameters of 50 nm (150 ° C) to 100 nm (170 ° C) depending on the heating temperature. The advantage of the procedure described in Example 2 is that, compared to Example 1, it is possible to obtain silica gel having a larger pore diameter, regardless of whether it is based on "soft silica gel prepared from water glass" or "hard silica gel prepared by agglutination of SiO 2 sol particles. The pore volume of the treatment decreases by only 5 to 10%. An important advantage of the procedure described in Example 2 is also that it does not contaminate the silica gel matrix with Na ⁺ or other metal ions. The purity of the resulting material is determined only by the purity of the NH 4 OH used and the quality of the autoclave material.

Claims (1)

PATENT CLAIMS Process for preparing macroporous silica gels with pore diameters of 20-100 nm by widening the pores of mesoporous silica with pore diameters of 1-15 nm, characterized in that the starting mesoporous silica is subjected to the combined action of water and a controlled amount of organic or inorganic alkali at elevated temperatures of 115-200 ° Wherein the pH of the system is up to 9 when boiling in salt solutions or up to 14 when operating under elevated pressure in the absence of salt in NH 4 OH solution.