DE1073459B - Process for the production of silica as a support for catalysts - Google Patents

Description

Process for the production of silica as a support for catalysts It is known for catalytic reactions, especially for hydrogenations and dehydrogenations, to use such catalysts in which catalytically active metals on silica gels are applied in lump form. These silica gels are widely used in Manufactured in a way that you can siliceous acid hydrosols, which are obtained by adding acids Alkali silicate solution that is obtained allows it to solidify.

The silica gels thus obtained have z. B. the following physical data: surface, measured by the BET method (cf. S. Brunauer, PH Emmet and E. Teller, Am. Chem. Soc., 6Ö, p. 301 # [19381) ...... ...... 300 to 700 rn2 / g mean pore radius . .. .. .. ... 10 to 60 AU pore volume ................ . 0.5 to 1 cm3 / g Another characteristic of the silica gels obtained from the hydrosols is that they have pores of almost uniform diameter and that there are no large pores, in particular those with pore radii above 100 AEI-. Silica gel catalysts can be prepared by impregnating the lumpy gels with an aqueous solution of a metal salt and then reducing them to the metal.

It is also known that in catalytic hydrogenations or dehydrogenations in the course of operation as a result of side reactions in the pores of the support, carbon or carbon-rich compounds are deposited, as a result of which the activity of the catalyst is markedly reduced. To restore the activity, the catalyst must therefore be reactivated. This is achieved e.g. B. by burning off the carbon or the carbon-containing compounds at elevated temperature with the supply of oxygen-containing gases. Experience has shown that the last residues of carbon or the carbon-rich compounds are difficult to remove from the pores, and the more difficult the more narrow-pored the silica gel used as a carrier. The reactivation of catalysts with narrow-pore supports must therefore under more severe conditions - # d. H. using high temperatures, long reactivation times and a large excess of oxygen-containing gases - take place. However, the use of such conditions usually leads to a premature reduction in the activity and to a partial disintegration of the catalyst's grains.

It has now been found that one can use one as a support for catalysts in particular effective silica is obtained when alkali metal silicate solutions are first mixed with ammonia added and from these solutions the silica precipitates with acid.

It is advantageous to work in such a way that the precipitation of the silica begins at a pi value of about 9 and ends at a pi value of about 7 or a little below 7.

The precipitation can take place at temperatures from 20 to 100 'C. the lower the pli value at the end of the precipitation, the higher the precipitation temperature must be kept. For example, the precipitation of the 5.0 Ternperatur should be nlindestens 35 'C at a pli value at the end.

The precipitation should be carried out with intensive stirring, e.g. B. in a turbo mixer, be made. One can also work in such a way that one works with ammonia mixed flowing alkali silicate solution with the flowing precipitant in one Bringing together a nozzle or a pipe with a short mixing section with the most intimate mixing.

As a starting material for the claimed manufacture of silica Alkali silicas, in particular sodium silicate, come into consideration. For the felling mineral acids, such as. B. sulfuric acid, but also organic acids, such as z. B. vinegar saturation can be used. The easiest way to add ammonia is to add ammonia in the form of ammonia # rasser. But you can also use gaseous ammonia before the precipitation Introduce into the alkali silicate solution.

The silica obtained is a white powdery, röntgenainorpher Niedeischlag, 'filtered, elektrolytfr # i * ge ash, "etrocknet un au e-25 - d f g' is suitable molding machine molded to appropriate cylinders, spheres or other bodies.

The carriers produced according to the invention are in a known manner with aqueous solutions of metal salts, such as. B. the compounds of Vanadfn, Chromium, molybdenum, tungsten, manganese, iron, cobalt, nickel or precious metals such as platinum, Palladium and the like, soaked and then, optionally with the use of Gases, thermally treated.

The silica obtained according to the present invention is distinguished by wide pores. The pore radius is more than 100 AU and is generally in a region between 100 and 300 AU, the pore volume is between 0.9 and 1.2 cm / g.

The catalysts produced with this silica have a high activity and a long service life. They give less cause for side reactions, i. That is, they take up less carbon or carbon-containing compounds than the known silica gels and can easily be regenerated. In addition, the carbon or the carbon-containing compounds settle almost exclusively on the surface of the silica and do not penetrate into the depths of the pores as in the known gels. The regeneration can therefore take place under mild conditions, i. H. at relatively low temperatures and short reactivation times. For example, temperatures of 400 ° C. are sufficient to burn off the carbon, a reactivation time of 4 to 5 hours being sufficient. In contrast, catalysts with silica gel as a carrier require higher burn-off temperatures and longer reactivation times. In view of the ease with which the supports obtained according to the invention can be regenerated, it is not necessary to reactivate the catalysts produced therefrom in a moving stream of oxygen-containing gases. The heating can take place in still air. It is z. B. the catalyst in a layer that is a few centimeters high, spread on metal sheets. Due to the gentle keactivation in still air, no disintegration of the contact grains occurs. The used catalyst is recovered in quantitative yield as a reactivated catalyst.

Another advantage of the silica according to the invention is that they respect to water - both in the liquid and in the Dampfpliase - is stable. The moldings made from the silica are mechanically and chemically resistant to heat . In other words, in the presence of water there is no grain disintegration, and the conversion of the amorphous phase into the crystalline phase only begins at temperatures above 700 ° C. In contrast, silica gels tend to disintegrate very easily in the presence of water, and the transition from the amorphous phase to the crystalline modification of the silica takes place at temperatures which are 200 to 300 ° C lower. The formation of the crystalline modification of the silica is, however, accompanied by a loss of surface area and porosity, i. H. especially the valuable carrier properties.

It was already known to add sodium silicate with hexamethylenetetramine mix and add this mixture to acid. A sol is primarily obtained, which is later converted into a gel. ' In contrast, after the Operation of the present invention converts the acid into a mixture of sodium silicate and Animoniak initiated, whereby a gel is formed immediately. The Silicic acid is characterized by a particularly large pore radius and is suitable excellently as a carrier for the production of active catalysts with a long service life.

It was also known to carry out the precipitation of the silica with ammonium chloride, ammonium nitrate or pyridine hydrochloride. This procedure also gives a silica with a considerably smaller pore radius. Example A stirred tank is charged with 136 parts by weight of technical water glass (with about 25 percent by weight SiO.). The water glass is diluted with 200 parts by volume of water. 28 parts by weight of 25 percent strength by weight ammonia water are added to this solution.

With vigorous stirring, sulfuric acid with a density of 1.125 (17.66 percent strength by weight sulfuric acid) is allowed to run into the sodium silicate-ammonia solution until a pi value of 6 to 6.5 is reached. The temperature should be about 20 ° C at the beginning of the precipitation; it rises by about 5 ° C. during the precipitation.

The mixture is left to stir for 2 hours. During the subsequent stirring, the p.1 value of the solution increases somewhat. It is set back to pr = 6 to 6.5 by adding sulfuric acid.

The precipitated silica is filtered, freed from sodium sulfate by washing with water and then shaped into tablets, strands, spheres or similar bodies on suitable molding machines. The briquettes are heated to about 600 ° C. to form the surface and to solidify.

The calcined grains have a bulk density of 500 g / l, a pore volume of 0.9 to 1 cm3 / g; the mean pore radius is 150 AU.

1 kg of these grains is impregnated with the same amount of palladium nitrate solution containing 0.8 g Pd / 1 in a rotating drum at 70 to 80 "C. The palladium-coated silicic acid moldings are reduced at about 150 ° C. in a stream of hydrogen.

The catalyst can e.g. B. can be used for the hydrogenation of crotonaldehyde to n-butyraldehyde, the procedure being as follows: Via 301 of the palladium-silica catalyst described above, 0.6 kg of 95 percent by weight crotonaldehyde together with 8 Nir0 hydrogen at a temperature of 130 to 140 per hour ' C headed. The yield of n-butyraldehyde is 95 percent by weight. The catalytic converter is removed after 300 days of operation. The carbon content is 4.6 percent by weight.

For reactivation, the used catalyst is placed in a layer from a height of 30 to 40 minutes on tray plates and heated for 5 hours at 400 ° C. in still air. After this time, the impurities are completely removed. After the reactivating treatment, the contact shows no disintegration and no reduction in mechanical strength or its hydrogenation activity.

If, however, a palladiniertes silica gel with the same Pd-content and a mean pore radius of 40 AE under the gieichen conditions as - spent catalyst for the hydrogenation of crotonaldehyde Vera, so after an operating time of 300 days, the carbon content of the catalyst is 12.4 percent by weight. The reactivation to remove the carbon or the carbon-rich substances in this case requires a temperature of 500 ° C., which must be maintained for 15 hours. After reactivation, the decomposition of the catalyst originally used is 23 percent by weight.

Claims (2)

PATENT PROCESS: 1. Process for the production of silica as a support for catalysts, characterized in that ammonia is first added to alkali silicate solutions and the silica is precipitated from these solutions by adding acid. 2. The method according to claim 1, characterized in that the precipitation of the silica begins at a pi value of about 9 and ends at a p, i value of about 7. References considered: British Patent No. 561750.