CH385175A - Process for the production of catalytically active coatings on reaction spaces - Google Patents

Description

       

  
 



  Process for the production of catalytically active coatings on reaction spaces
It is known for the hydrogen cyanide synthesis from ammonia and methane at temperatures of z. B. 900 to 1400 elongated reaction chambers, in particular tubes, to use, the walls of which are flushed by the gases are provided with catalysts. The catalysts used were mostly platinum metals, which could usefully be alloyed or mixed with other metals or with oxides such as A1203. The metallic and / or oxidic coatings adhering firmly to the walls were formed in that solutions containing metal compounds, e.g. B. of Al or Cu or Pt or Pt metals by themselves or in mixtures or alloys, e.g. B.



  Pt-Ru, Pt-Os, contain e.g. B. were filled into the tubes in a horizontal position, after which the tubes are then heated with rotation and escape of the evaporated solvent through axial openings guided through the pipe closures, gas outlet pipes or the like.



   In this way, intermediate, i.e. H. obtained as an intermediate stage dried metal salt coatings, wherein in order to achieve the desired thickness of e.g. B. to achieve 20 mg of platinum metal per cm2 of active area, this operation is repeated several times. The coatings produced in this way were then annealed by applying them with or without the passage of non-oxidizing gases, e.g. B. without purging or with air admission or with passage of air or the like, or with the exclusion of air or with purging, for example with H2, NH3, N2.



   It has now been shown that in order to obtain very active and permanent catalysts, one has to form and activate the known coatings of aluminum oxide and platinum metals in a certain way.



   The invention consists in a process for the production of catalytically active coatings on reaction chambers for the hydrogen cyanide synthesis from ammonia and methane at temperatures of 900 to 1400 C in which the walls are provided with coatings of platinum metals and aluminum oxide by the walls with the aqueous solutions of metal salts are impregnated, the water evaporates, the wall dried, heated in a non-oxidizing gas stream and then cooled, which is characterized in that the platinum metal salts and aluminum salts containing coatings produced by removing the water and drying at temperatures up to 8000 C are first passed through heated by reducing gases, then cooled and this operation repeated,

   until enough platinum metals and Al2O3 have been applied, and that the catalyst layer obtained in this way is then activated by heating to the reaction temperature in the presence of the reaction gases, initially only passing ammonia through and then slowly increasing the methane addition in such a way that the methane content is increased to avoid soot formation Residual gas freed from NH5 and HCN never exceeds 1% and that a molar ratio of NH3 to CH4 of 1 to 0.8 is reached after 4 days at the earliest.



   In this way, hard, permanent coatings made of porous A1203 and platinum metals, in which the latter are finely distributed in the A1203, can be obtained. By embedding the platinum metals in the aluminum oxide layer, the evaporation of the metals during operation is largely suppressed and the activity of the catalyst is thereby considerably extended. Furthermore, the aforementioned type of activation normally avoids the formation of soot and thus primarily an undesirable carburization of the platinum metal transitions. It has been found that soot formation can lead to the formation of alloys (e.g. PtAl2) in addition to the formation of AlN through reduction of the A1203 to metallic aluminum. It could z.

   For example, it can be found that, with more soot, metal alloys form in the form of small spheres, which migrate into the lower parts of the reaction tube or collect in the lower end of the tube. In particular, it was found that, in the case of sooty catalysts, there can be a disadvantageous decrease in activity during operation due to migration and aging of the finely divided platinum metals.



   The soot is mainly formed by the decomposition of any higher hydrocarbons in the methane. The inclusion of soot in the catalytic coating must expediently be avoided when the catalyst is activated, which is achieved according to the invention by increasing the addition of methane so slowly, while avoiding soot formation, that the methane content in the residual gas freed from NH3 and HCN Never exceeds 1%. Should some soot still form, which can be seen from the increase in pressure in the pipe, the activation must be interrupted and the soot deposit formed in the upper part of the insert pipe can be removed by treating with air at about 900 "and then the activation continued.



   example
The inner wall of a pipe made of sillimanite about 40 mm in diameter and 1500 mm in length was impregnated with a hydrochloric acid solution containing aluminum in an amount of 4.25% calculated as A1203 and 4.0% platinum metals (Pt8Ru12). For this purpose, the horizontally lying pipe is charged with the solution and then slowly rotated around the pipe axis or an axis parallel to it under weak heating. The heating takes place evenly, for example with heating gas by a series of small flames or electrically. The evaporation of the solution is accelerated by passing some air through it. After the solution has evaporated up to 1500 while air is passed through, a dry coating of the salts mentioned remains.

   The tube with the coating obtained in this way is then heated in an annealing furnace to 800 "while passing a little ammonia through it and then allowed to cool again
Operation of impregnation and heating is repeated 5 times.



   In this way a coating is obtained which consists of a porous aluminum oxide layer in which the platinum metals are finely divided and which is completely free of chlorine.



   The coating is now activated by heating the tube with the catalyst layer in a molybdenum furnace to an average temperature of 1200.



  At the same time, a stream of 400 liters of ammonia is passed through the pipe and then methane is added, the methane addition being increased so slowly that the methane content in the residual gas freed from NH3 and HCN never exceeds 1%. Within 8 days the methane content could be increased so that the molar ratio of ammonia to methane reached about 1 to 0.7. The activation in the activation furnace is now complete and the reaction tube is ready for installation in the reaction furnace.



   It is expedient to use two concentric tubes extending into one another to carry out the reaction, the reaction gases being passed through the annular space between the two tubes. In this case, the outer side of the inner tube is also covered with the catalyst layer and activated.


    

Claims (1)

PATENT CLAIM Process for the production of catalytically active coatings on reaction spaces for hydrogen cyanide synthesis from ammonia and methane at temperatures of 90001400 "C, at which the walls are provided with coatings of platinum metals and aluminum oxide by impregnating the walls with the aqueous solutions of metal salts, the water evaporated, the wall is dried, heated in a non-oxidizing gas stream and then cooled, characterized in that the platinum metal salts and coatings containing aluminum salts produced by removing the water and drying are first heated at temperatures up to 8000 C while passing reducing gases and then cooled and repeat this operation until enough platinum metals and A1203 are applied, and that the catalyst layer obtained in this way is then activated by heating to the reaction temperature in the presence of the reaction gases, initially only passing ammonia through and then slowly increasing the methane addition in such a way that the methane content in the residual gas freed from NH3 and HCN is 1 Ó never exceeds and that a molar ratio of NH3 to CH4 of 1 to 0.8 is reached after 4 days at the earliest.