DE2127649B2 - METHOD FOR PRODUCING LARGE-PORED SILICA GEL BODIES - Google Patents

Description

The invention relates to the method described in more detail in the preceding claim Production of large-pored silica gel bodies with different areas of porosity and pore volumes, by ammoniacal treatment of dried silica gels in an autoclave.

Silica gel bodies and bodies that consist predominantly of silica gel will have a great deal on used in various fields, for example adsorption, chromatography and catalysis.

Of course, these bodies must have the mechanical properties required for the respective use Properties, d. H. Have abrasion resistance and compressive strength, but also certain pore properties, depending on the particular use envisaged.

For a long time, efforts have been made to develop methods that lead to the development of the Pore properties of the silica gels lead and in particular the production of substances with opposite enable the freshly prepared gels to have larger pore diameters.

Usually, for reasons of economy in the manufacture of the majority of the technical The silica gels used were based on alkali silicates, especially sodium silicates. The silica gels are generally obtained by drying highly water-containing hydrogels, which upon acidification under certain conditions the alkali silicate solutions arise and strongly with the used Acids corresponding alkali salts are contaminated and, moreover, as a result of incomplete Neutralization of these alkali silicates can have a certain residual alkalinity. Own these gels different specific surfaces and also different pore volumes, mainly from depend on the conditions of precipitation and washing of the hydrogels.

In FR-PS 14 73 240 and 14 82 867 it is stated that by firing silica gels with a defined content of alkali salts and a defined residual alkalinity at temperatures of 400 to 1200 ⁰ C gels can be obtained which have easily reproducible porosities with pores of different diameters up to several 100 nm, while the gels used as starting material, simply dried under gentle conditions, have specific surface areas in the range of several 100m ² / g,

which correspond to porosities with very fine pores.

The use of the silica gels obtained by this process is, however, in some cases because of the existing alkali compounds that are difficult to wash out and that lead to the development of pores in these gels have contributed to disadvantages. In addition, the large-pored silica gels produced in this way show at higher firing temperatures a development into crystalline forms of silica, which are used for

ίο certain areas of application are undesirable and also a noticeable reduction in the original pore volume.

Furthermore, the general effects of ammoniacal treatment are known: It leads to

r> fired silica gels to create a large-pored Develops porosity and has a rapid contraction of the freshly made silica hydrogels Result in hydrogels (DT-AS 12 66 741). Some of the described and known procedural measures

> o but do not definitely lead to the expected results; this is especially true for the treatment of freshly prepared hydrogels, the structure of which is very brittle and is in a state of rapid development. On the other hand, by the ammoniacal

2> Treatment of gels fired in the range from about 370 to 700 ^° C. does not sufficiently modify the structure of the products obtained because this structure had previously developed too much as a result of the firing (US Pat. No. 2,699,376).

It was surprising that large-pored silica gel bodies with very different porosity ranges, ie pore diameters of about 10 to about 200 nm, can be produced from washed and dried hydrogels if the treatment is carried out in the presence of an aqueous ammoniacal reacting medium in the vapor phase or in the liquid phase in the autoclave, ie under pressure the structure is not destroyed and a considerable increase in the mean pore diameter is achieved while maintaining the original pore volume if the mean pore diameter of the silica gel bodies used is at the same time above about 4 nm. In the case of silica gel bodies with an average pore diameter below 4 nm, e.g. B. 2.1 or 2.4 nm, the same ammoniacal autoclave treatment leads to an increase in the pore diameter, but at the same time to an undesirable decrease in the total porosity. The greatly changed porosity ranges achieved according to the invention are of particular importance for chromatographic use; in addition, the high purity of these silica gel bodies, which is achieved by very thorough initial washing, is often a valued advantage. The silica hydrogels must be dried so that the gels are suitable for development in the autoclave in the presence of an alkaline reacting medium; Contrary to what is known hitherto, the treatment of hydrogels, such as are obtained from silicates in an aqueous medium by the various processes, under pressure and in an ammoniacally reacting medium usually leads to their destruction. In practice, the hydrogels are usually dried at fairly low temperatures; the further development of these gently dried gels is most pronounced under the conditions according to the invention. However, the hydrogels can also be dried at temperatures in the range from 400 to 500 ^{° C. without serious disadvantage;} it just becomes

by drying at the upper limit of the specified temperature range a slightly different distribution of the get larger pores than with hydrogels that are dried at a low temperature.

The "r _U during drying at various Ten temperatures up to the specified limits _ achieved, ebnisse differ in terms of the average pore diameter and the pore volume of the resulting dry products very little from each other. But place in the subsequent treatment in ammoniacal medium under pressure an increasing development Their organization takes place, which depends on the temperature maintained during drying; this development takes place all the faster, the higher the temperature used during drying. Above the specified temperature, the calciner then leads to a considerable decrease in the specific surface area From silica hydrogels to higher temperatures, the water loss does not proceed linearly with increasing temperature: the adsorbed water is quickly removed by drying at a lower temperature; the residual water, which corresponds to the water bound in the dry gels obtained, increases with steep This dehydration process then passes through a maximum speed which is essentially in the temperature range from 400 to 500 ^° C., which has been specified above as the limit for the drying temperatures to be maintained.

The ones used in treating the silica gel bodies Pressures can be quite low, but these bodies develop in an ammoniacal manner The higher the pressure, the faster the reacting medium. In practice there will be a pressure of some 10 bar considered sufficient; more than 50 bar can also be used without any other disadvantage than that then a more complex and therefore less economical apparatus is necessary.

The concentration of the aqueous ammoniacal medium has a very great influence on it, in the extent and the speed with which the desired results will be achieved; a concentration However, more than 100 g / l ammoniacally reacting substance in this medium is no longer effective a lot of changes in the speed of the development process.

The treatment time for the silica gel body must be long enough to keep the temperature in all Points of a given mass of this body is practically equal; in practice one is sufficient Treatment time of a few hours for technically important products.

The treatments according to the invention can be carried out in the vapor phase or in the liquid phase will; Working in the vapor phase is often preferred because of the properties in terms of There is little porosity in samples taken at various points in significant quantities are scattered; the treatment in the liquid phase, however, gives wider pore distributions, which for certain applications may be desired.

Of course, the various parameters listed must be taken into account together and the choice of the properties of the silica gel bodies which are assumed must be made with regard to this in each case the desired result occurs. So must, since the treatments according to the invention practically reduce the original pore volume of the silica gel body Change only a little, the choice of the manufacturing method of these bodies are made so that they are practical have the same pore volumes as the end products should have, while the other parameters -, then determine the different pore distributions.

The method according to the invention is not limited to bodies made of pure silica gel; Silica gel body, which also contain other oxides or hydroxides can undergo an analogous development. The presence of these other oxides does not prevent the porosity properties of the from developing Silica gel; Such bodies can especially oxides or hydroxides of magnesium, aluminum and Zirconium contains and is widely used for catalysis purposes.

In the following examples, the treatment according to the invention is carried out in an autoclave and in an ammoniacal one Medium on silica gel spheres, which are immiscible with water by coagulating gravel / acid soils in a water

_'o liquid were obtained and explained in an example of extrudates. It goes without saying that other forms of formation of silica gel bodies whose original properties are identical to same results as those of spheres or extrudates

2 "); other forms of training come for example Fragments obtained by crushing set gels or by pressing or Press obtained agglomerates in question.

Example 1

In this example it is shown that the initial drying of the silica gel bodies is quite lower Temperature should be done so that it is able to stand up in the autoclave and in ammoniacal medium

To develop large-pored porosities, not is affected.

Silica hydrogel balls obtained from sodium silicate were washed until their residual Na ₂ O content was not more than 0.02% by weight, based on the silica. These spheres were dried at a low temperature at about 100 ⁰ C and then had a mean diameter μιη of 100 to 200 and a pore volume of l, 05cm ³ / g, the mean pore diameter was about nm 12th

ν "There are three other samples of similar I lydrogelkugeln at 400, 600 and 800 ⁰ C dried and then all four samples in vapor phase for 3 hours under a pressure of 40 bar, treated in an autoclave with an aqueous solution containing 25 g / l ammonia .

The following table 1 shows the measurement data for the products obtained, namely the pore volume V, the mean pore diameter 0 m and the pore distribution.

Table 1

temperature
while drying

V 0M

(cmVg) (nm)

Pore distribution (nm)

100 1.0 85 60-120 400 1.0 70 45-110 600 1.0 41 30-70 ", 800 1.0 42 30-70

The comparison clearly shows that the mean pore diameter of the products obtained is around

the greater the lower the temperature during drying of the gel and that above 400 ° C pore enlargement is more difficult to achieve than below. In addition, the pore volume changes practically not.

Example 2

This example describes a 3 hour autoclave treatment in the vapor phase under a pressure of 10 bar, in an aqueous ammonia solution 5 g / l. The treatment was carried out with silica gel balls with a diameter of 40 to 350 μίτι, which by Drying hydrogel spheres had been obtained and a pore volume of 0.7 cmVg and one had an average pore diameter of 6 nm. The balls were dried at different temperatures been; the results are summarized in Table 2 below.

Table 2

temperature
while drying

V 0M

(emtyg) (nm)

Pore distribution
(nm)

150 0.7 44 30-70 400 0.7 40 25-65 600 0.7 36 20-60 800 0.7 35 20-60

NHs-Gehah
the solution

Pore volume V

(cmVg)

Pore distribution

(nm) (nm)

0.06 1.06 13th 8-35 0.25 1.00 36 20-70 0.50 1.05 50 30-80 1.0 1.02 52 30-80 2.5 1.05 60 35-90 5 1.00 70 40-100 10 1.05 80 40-110 25th 1.02 82 40-110 60 1.01 80 40-110 95 1.01 84 45-120

Example 4

In this example, ammonia was replaced by other substances that react with ammonia, such as ethylamine and ethylenediamine, which were used in a concentration of 50 g / l and 100 g / l. The same, dried at 10O ⁰ C balls as in Example 1 and 3 were treated; The duration of the experiment was 3 hours and the pressure was 40 bar. The results are summarized in Table 4 below.

Table 4

Amine content of
aqueous solution

This example again shows that the gels dried at low temperatures more easily have large pores Develop porosity.

Example 3

In Giesem example the effect of increasing ammonia concentration in the aqueous autoclave solution is examined. The experiments were carried out with the same Kieselsäuregelkugeln as in Example 1 which had been dried at 100 ⁰ C. Various samples of these spheres were treated in the autoclave for 3 hours under a pressure of 10 bar with different NH3 concentrations. The results are summarized in Table 3 below.

Table 3

These results clearly show that above a certain NHj content of the aqueous solution a An increase in the concentration only has a slight effect and that a concentration of 100 g / l can be seen as an upper limit, which there is no further advantage to exceeding.

(cmVg)

(nm)

Pore distribution

(nm)

Ethylamine 50 1.09 70 40-140

Ethylenediamine 100 1.07 75 40-200

The results show an analogy with the results obtained with ammonia.

Example 5

This example describes autoclaving in the liquid phase in an ammoniacal medium -'i at two different concentrations: 0.25 g / 1 and 25 g / l NHj.

The same balls were used as in the

Examples 1, 3 and 4, which had been dried at 100 ⁰ C. The one after 3 hours of treatment under one

«Ι pressure of 40 bar obtained results are in the summarized in Table 5 below:

Table 5 V
(cmVg) 0M
(nm) Pores
distribution
(nm) NH3 content of the
'"> Autoclave solution
(g / i) 0.98
0.80 21
72 10-30
35-170 0.25
25th

This example shows that the development of large pores is also possible in the liquid phase.

Example 6

In this example, extrusions obtained were mm with a diameter of 3 mm and height of 5 treated silica hydrogel of microspheres, which were dried at 12O ⁰ C and a pore volume of 1 cmVg and an average pore diameter of 12.5 nm possessed. Two samples were treated: the first in the vapor phase and the second in the liquid phase, each for 3 hours at 40 bar and with an NH ₃ content of the aqueous solution of 50 g / l.

The results are summarized in the following table 6:

Table 6

Treatment in

(cnWg)

0M

(nm)

Pore distribution

(nm)

Vapor phase 1.00 103 80-120

liquid phase 1.01 35 10-80

This example shows that the development of the body in the liquid phase, although analogous to the development in Vapor phase, leads to different mean pore diameters and a different pore distribution.

Claims (1)

Claim: Process for the production of large-pored silica gel bodies with different areas of porosity and pore volumes, which may contain catalytic amounts of other oxides or hydroxides, in particular for catalysis and chromatography, by ammoniacal treatment of silica hydrogels which have been dried ^{at temperatures of a maximum of 500 ° C. thereby} characterized in that the ammonia treatment is carried out in an autoclave and dried silica gel bodies are used which have an average pore diameter of over 4 nm.