alkyl radicals which may be mentioned are methyl , ethyl , propyl , isopropyl , butyl , isobutyl and tert .- butyl radicals , preferably methyl and ethyl radicals . alkoxy radicals which may be mentioned are methoxy , ethoxy , propoxy , isopropoxy , butoxy , isobutoxy and tert .- butoxy radicals , preferably methoxy and ethoxy radicals . the starting materials for the process according to the invention are known or can be prepared analogously to processes known from the literature . preferred starting materials for the process according to the invention are n - cyclohexylidene - aniline and other wholly or partially hydrogenated diphenylamines . examples of such wholly or partially hydrogenated diphenylamines which may be mentioned are dicyclohexylamine , n - cyclohexylidene - cyclohexylamine and n - cyclohexyl - aniline ; n - cyclohexylidene - aniline may be mentioned in particular . the preferred starting materials for the process according to the invention are readily available ; n - cyclohexylidene - aniline and n - cyclohexylidene - cyclohexylamine are obtained according to known methods in very good yields from cyclohexanone and aniline or , respectively , cyclohexylamine ; n - cyclohexyl - aniline and dicyclohexylamine can also be obtained easily according to known processes , for example from the same starting materials by hydrogenating alkylation . mixtures of the said compounds are also suitable as starting materials for the process to the invention . particularly a mixture obtained by continuous operation of the process according to the invention which , after separating off the diphenylamine , contains , in addition to unreacted starting material and small amounts of other by - products , the abovementioned compounds corresponding to the starting material used , can also be used advantageously in the process according to the invention . in general , the process according to the invention is carried out in a temperature range from about 250 ° to about 400 ° c , preferably between about 280 ° and about 370 ° c and especially in the range from about 300 ° to 350 ° c . the process according to the invention is generally operated under normal pressure but it is also possible to work under reduced or elevated pressure , especially in the range from 0 . 1 to 3 . 0 bars . the selection of the dehydrogenation catalyst is particularly important for the process according to the invention . dehydrogenation catalysts used in the process according to the invention are catalysts which contain nickel and chromium and also contain manganese and / or aluminium and / or copper and / or sulphates of the alkali metals or alkaline earth metals . a group of catalysts according to the invention are nickel / chromium catalysts which also contain aluminium , copper and sulphates of the alkali metals and / or alkaline earth metals , the ratios by weight of the metals nickel , chromium , aluminium , copper and the alkali metals and / or alkaline earth metals to one another being about 40 to 60 : 6 . 8 to 17 . 5 : 1 . 5 to 8 . 0 : 0 . 05 to 1 . 0 : 0 . 5 to 7 . 0 . a preferred group of these catalysts according to the invention is known from german published pat . no . 2 , 049 , 809 . they contain nickel , chromium , aluminium , copper and alkali metal in a ratio of the metals of 40 to 60 : 6 . 8 to 17 . 1 : 1 . 6 to 8 . 0 : 0 . 05 to 1 . 0 : 0 . 65 to 3 . 4 and are prepared by adding an alkali metal compound to the catalyst base composition containing nickel , chromium , aluminium and copper , which is obtained by precipitation , subsequently drying and shaping the mixture , reducing at elevated temperature with hydrogen and , optionally , subsequently treating with co 2 under hot conditions , by a . impregnating the base composition with the appropriate amount of alkali metal sulphate as the alkali metal compound , b . shaping the impregnated and dried catalyst composition to give shaped articles with a bulk density of 0 . 8 to 1 . 6 g / ml , c . reducing the resulting catalyst mouldings at temperatures between 350 ° and 420 ° c , first with about half to about twice the equivalent amount of hydrogen calculated for reduction of the nickel compounds to metallic nickel and if appropriate of the chromium - vi compounds to compounds of trivalent chromium in the course of 1 to 4 hours and d . either , at the earliest 1 week after process stage ( c ), further reducing the catalyst mouldings with about 2 to 10 times the equivalent amount of hydrogen calculated for reduction of the nickel compounds of the catalyst to metallic nickel , in the course of 1 to 4 hours at temperatures between 350 ° and 420 ° c until the content of metallic nickel in the catalyst is about 10 to 35 percent by weight , or stabilising the catalyst mouldings after process stage ( c ), grinding and reshaping to give mouldings with a bulk density of 0 . 8 to 1 . 6 g / ml and then further reducing these with about 2 to 10 times the equivalent amount of hydrogen calculated for reduction of the nickel compounds of the catalyst to metallic nickel , in the course of 1 to 4 hours at temperatures between 350 ° and 420 ° c until the content of metallic nickel in the catalyst is about 10 to about 35 percent by weight . a further preferred group of these catalysts according to the invention is known from german published application no . 2 , 146 , 052 . they are prepared in a manner analogous to that used for the catalysts described above and known from german published pat . no . 2 , 049 , 809 , the difference being that the reduction is carried out until the content of metallic nickel in the catalyst is about 35 to about 45 , preferably about 42 ,% by weight . the catalysts according to the invention which contain alkaline earth metal sulphate in place of alkali metal sulphate can also be prepared in the same way . alkaline earth metals which may be mentioned are magnesium , calcium , strontium and barium , preferably magnesium . a further group of catalysts according to the invention are nickel / chromium mixed catalysts with added manganese and / or aluminium , in which the metals nickel , chromium , manganese and aluminium are present in a ratio of about 10 to 50 : about 4 to 15 : about 1 to 30 : about 0 to 5 . the preparation of these catalysts can be carried out according to methods which are in themselves known . advantageously , an aqueous solution containing nickel , chromium , manganese and optionally aluminium in the form of their watersoluble salts , for example nitrates , is used as the starting material and the mixture of the corresponding hydroxides and / or carbonates is precipitated from this solution by adding alkali metal hydroxide , bicarbonate or carbonate , or ammonia . of course , it is also possible to use a mixture of the said precipitants for precipitation . preferably , the alkali metal hydroxides , bicarbonates and carbonates used are the corresponding compounds of potassium and especially of sodium . the chromium salt used can also be a compound of 6 - valent chromium ( see , for example , handbook of catalysis , volume viii , 1st half , page 674 ( 1943 ) and houben - weyl , volume iv / 2 , page 137 et seq . ( 1955 )). the state of the art relating to the use of sodium hydroxide solution , ammonia , alkali metal carbonates and alkali metal bicarbonates is described , for example , in houben - weyl , volume iv / 2 , page 137 et seq . ( 1955 ) and handbook of catalysis , volume v ( heterogeneous catalysis ii , page 412 et seq . ( 1943 ). depending on the conditions of precipitation , that is to say on the precipitant used , the ph value of the solution and the nature of the chromium compound employed , the nickel is obtained , for example , as the hydroxide , as the basic carbonate ( see , for example , gmelin , 8th edition , part b , issue 3 , system no . 57 , ( 1966 ), page 846 to 853 ), or as ammonium nickel chromate ( see gmelin , 8th edition , part b , issue 3 , system no . 57 , page 1 , 215 to 1 , 216 ( 1966 )). a further possible variant for the preparation of the abovementioned precipitate is , for example , to precipitate a carbonate / hydroxide mixture from a solution of a nickel salt , manganese salt and optionally an aluminium salt using alkali metal carbonate solution and , after rinsing with water , to react this mixture with ammonium bichromate at elevated temperature , in a manner which is in itself known . the precipitate which is thus obtained , as the catalyst base composition , in a manner which is in itself known , for example as described above , is then washed , dried , shaped and reduced with hydrogen at elevated temperature in a manner which is in itself known . using the dehydrogenation of n - cyclohexylidene - aniline as an example , the reaction according to the invention is illustrated by the following equation : ## str3 ## in general , the process according to the invention is carried out in such a way that the starting material , optionally together with an inert gaseous diluent , is passed over the catalyst in the temperature range mentioned above and under the abovementioned pressure . gaseous diluents which may be used are , advantageously , anhydrous or substantially anhydrous gases which are inert under the reaction conditions , such as hydrogen , methane or natural gas , but especially dry nitrogen ; it is appropriate to use gases which are anhydrous or substantially anhydrous in order to prevent hydrolysis of the starting material , for example n - cyclohexylidene - aniline . it is also advantageous to work with exclusion of oxygen . the amount of the gaseous diluent or carrier gas is generally about 0 . 1 to about 10 . 0 , preferably about 1 . 0 to about 5 . 0 and especially 1 . 5 to 2 mols per mol of the starting compound or starting compounds . in general , the process according to the invention is carried out at a catalyst load of about 0 . 05 to about 0 . 5 , preferably from about 0 . 1 to about 0 . 4 , and especially from about 0 . 2 to about 0 . 3 ( g of starting material per ml of catalyst and per hour ). the process according to the invention is generally carried out continuously , especially for economic reasons ; the particular design of the reactor is not an essential part of the invention ; this reactor can be designed in a manner which is in itself known in accordance with the state of the art , for example a reaction tube or a tubular converter with the catalyst in the tubes can be employed as the reactor . for example , it is possible to evaporate the starting material used in a dry stream of nitrogen , to heat this gaseous feed stream to a reaction temperature of , for example , 320 ° c and to pass it at this temperature through a tubular converter at a catalyst load of , for example , 0 . 1 to 0 . 4 ( g of starting material per ml of catalyst and per hour ); subsequently , the reaction product can be cooled , for example , to a temperature between 100 ° and 250 ° c , fed into a continuously operated distillation unit and separated by distillation under normal pressure or under reduced pressure of , for example 0 . 1 bar , and diphenylamine can be taken off , for example at the top , and the mixture of unreacted starting material and by - products formed in the reaction , as well as the carrier gas , can be withdrawn , if necessary after separating off the hydrogen formed , and recycled into the reaction . it can , however , also be advantageous to separate the reaction mixture , after cooling , into a liquid phase , essentially containing diphenylamine , and into a gaseous phase , which essentially contains the carrier gas and the hydrogen formed during the reaction . by cooling this stream of gas to temperatures of , for example , below 50 ° c , it is possible to separate off high - boiling by - products , like aniline , n - cyclohexyl - aniline and n - cyclohexylidene - aniline , in a liquid form and then to recycle them into the reaction , or to separate them , for example by distillation , for an appropriate utilisation of the individual compounds . the gas which leaves the separator can , for example , be recycled into the reaction as the carrier gas , as described above , or , advantageously , because of its high calorific value , can be used directly or indirectly for heating the reactor . however , working up of the reaction product in order to obtain the diphenylamine , which is obtained after separating off the carrier gas and the hydrogen formed , can also be carried out in a known manner by crystallisation from a solvent such as methanol . by - products which can be formed in the process according to the invention are , in addition to the partially hydrogenated diphenylamines formed by partial dehydrogenation of the starting material , also benzene , aniline and carbazole ; of these aniline can be employed again for the preparation of the starting material for the process according to the invention , whilst benzene and carbazole can be used as such . 4 - aminodiphenyl is formed in the process according to the invention in the order of magnitude of 1 ppm . the technical advance of the process according to the invention may be summarized as follows . the catalysts according to the invention exhibit a high mechanical stability and a long life . the degree of conversion and the selectivity during the life time of the catalysts according to the invention are also high . it is true that these do decrease somewhat with regard to the desired product but the formation of by - products which cannot be re - used as starting materials is virtually independent of time in the process according to the invention , whilst the formation of by - products which can be used again as starting material increases . since these by - products can be recycled into the desired reaction and so are not lost to the reaction , the advantage of the long life of the catalyst outweighs the slight reduction in its activity and selectivity . accordingly , a selectivity of the catalyst of more than 90 % can be assumed for virtually its entire life . furthermore , there are virtually no corrosion problems , such as are known from the customary preparation of diphenylamine from aniline with acid catalysts . therefore , commercially available steels , for example normal c steel , can be used as the material for the equipment used for carrying out the process according to the invention , especially for the reactor . a catalyst base composition containing 38 . 2 % by weight of nickel , 8 . 9 % by weight of chromium , 3 . 2 % by weight of aluminium and 0 . 2 % by weight of copper was prepared according to the instructions of dt - as ( german published specification ) no . 2 , 049 , 809 ; 12 , 870 parts by weight of this composition were stirred with a solution of 337 . 5 parts by weight of potassium sulphate in 11 , 700 parts by weight of water and the mixture was dried and ground . the catalyst composition thus obtained was mixed with 1 . 5 % of its weight of graphite and shaped under a moderate moulding pressure to give cylinders 5 mm in diameter and 5 mm in height . these cylinders were treated for 2 . 5 hours at about 390 ° c with 100 parts by volume of hydrogen per part by weight of catalyst and per hour . after cooling to room temperature , the cylinders were treated with ammonia and then with carbon dioxide , until the exothermic effects which arises had subsided . they were then ground and , together with 3 % of their weight of graphite , shaped again under a higher moulding pressure to give cylinders of the abovementioned size . the moulded catalyst was finally treated for 2 . 5 hours at about 390 ° c with 725 parts by volume of hydrogen per part by weight of catalyst and per hour and then heat - treated for 30 hours at about 100 ° c in a stream of carbon dioxide . the catalyst thus obtained has a bulk density of 1 . 2 g per ml and contains 26 . 6 % by weight of metallic nickel for a total nickel content of 51 . 6 % by weight . the amounts of starting material and the volumes of inert gas indicated in the following examples were fed into a reaction tube with a diameter of 17 mm and a length of about 500 mm , the upper part of which is used as an evaporating zone and the lower part of which is filled with 30 ml of the catalyst obtained according to example 1 . the reaction mixture leaves the reaction tube at the lower end and , after cooling and gas / liquid phase separation , is worked up in a known manner by distillation or crystallisation or is analysed by gas chromatography . the amounts and volumes indicated in each case were measured at room temperature , the volumes were measured by means of rotameter and the feed rates are given in grams per hour or , respectively , in 1 per hour . 3 . 0 g of n - cyclohexylidene - aniline and 1 . 0 l of nitrogen were metered into the reactor described in example 2 ; the catalyst temperature was kept at about 320 ° c . after a start - up period of a few days , a reaction product which consists to the extent of 89 . 6 % by weight of diphenylamine is obtained , the relative degree of conversion being 99 %. the product also contains 2 . 1 % by weight of benzene , 2 . 5 % by weight of aniline , 4 . 6 % by weight of n - cyclohexyl - aniline and 0 . 3 % by weight of carbazole as well as 0 . 9 % by weight of other impurities ; the 4 - aminodiphenyl content in the crude product is 1 ppm . the yield of diphenylamine thus corresponds to about 95 % of theory , based on the amount of aniline which was employed for the virtually quantitative conversion to n - cyclohexylidene - aniline . after a running period of 5 , 000 hours , the reaction product contained 85 % by weight of diphenylamine , whilst the amounts of the by - products benzene , aniline and carbazole were virtually unchanged . on the other hand , the concentration of n - cyclohexyl - aniline , which can be used again , and of unreacted n - cyclohexylidene - aniline had risen to about 10 % by weight . the concentration of 4 - aminodiphenyl in the reaction product had not changed . a . 9 . 0 g of n - cyclohexylidene - aniline and 8 . 5 l of nitrogen were metered into the reactor described in example 2 , the catalyst temperature being 320 ° c . after a reaction time of 94 hours , the reaction product contained 92 . 8 % by weight of diphenylamine , 0 . 3 % by weight of n - cyclohexylidene - aniline , 0 . 3 % by weight of n - cyclohexyl - aniline , 1 . 4 % by weight of benzene and 5 . 1 % by weight of aniline . b . after a running period of 286 hours , the feed mixture was reduced to 6 . 0 g of n - cyclohexylidene - aniline per hour and 5 . 5 l of nitrogen per hour . after 958 hours , the reaction product consisted of 91 . 1 % by weight of diphenylamine , 3 . 0 % by weight of n - cyclohexylidene - aniline , 1 . 0 % by weight of n - cyclohexyl - aniline , 0 . 7 % by weight of benzene and 4 . 2 % by weight of aniline . c . after the catalyst had been in operation for 1 , 507 hours , the feed mixture was changed again , the amount of nitrogen being reduced to 4 . 2 l per hour . after a total running period of 1 , 800 hours , the reaction mixture had the following composition : 91 . 2 % by weight of diphenylamine , 0 . 2 % by weight of n - cyclohexylidene - aniline , 1 . 2 % weight of n - cyclohexyl - aniline , 1 . 5 % by weight of benzene and 5 . 2 % by weight of aniline , as well as 0 . 7 % by weight of several other substances . 3 . 0 g of n - cyclohexylidene - aniline and 5 . 0 l of nitrogen were metered at the temperature indicated in table i which follows , into the reactor described in example 2 . the composition of the reaction product is also given in this table . table 1__________________________________________________________________________reaction product (% by weight ) temperature diphenyl - n - cyclohexylidene - n - cyclohexyl - remainder ( several (° c ) amine aniline aniline aniline carbazole substances ) __________________________________________________________________________280 69 22 5 2 1 1300 88 5 2 4 1 -- 320 92 1 2 4 1 -- 350 89 0 . 2 -- 4 5 1 . 8370 86 0 . 2 -- 5 7 1 . 8390 83 0 . 1 -- 4 12 0 . 9__________________________________________________________________________ the amount of n - cyclohexylidene - aniline indicated in table ii which follows and 5 . 0 l of nitrogen are fed into the reactor described in example 2 , the reaction temperature being 325 ° c . the composition of the reaction product obtained in each case is also given in table ii . table ii______________________________________reaction product (% by weight ) n - cyclohexyl - di - n - cyclo - car - remainderdene - aniline phenyl - hexyl - ani - ba - ( several ( g / h ) amine aniline line zole substances ) ______________________________________5 . 1 93 -- 4 2 -- 5 . 7 94 -- 3 -- 1 . 07 . 2 92 1 3 0 . 1 0 . 98 . 4 88 3 4 -- 1 . 09 . 9 85 6 4 -- 1 . 0______________________________________ 6 . 6 g of n - cyclohexylidene - aniline and the amount of nitrogen indicated in table iii which follows were charged to the reactor described in example 2 , the reaction temperature being 320 ° c . the composition of the reaction product obtained in each case can also be seen in table iii which follows . table iii______________________________________reaction product (% by weight ) remain - n - cyclo - dernitro - di - hexyl - n - cyclo - car - ( sever - gen phenyl - idene - hexyl - ani - ba - al sub -( l / h amine aniline aniline line zole stances______________________________________5 91 0 . 1 1 6 1 0 . 92 90 0 . 1 3 6 0 . 9 -- 0 85 0 . 1 3 9 2 . 9 0______________________________________ 6 . 0 g of n - cyclohexylidene - aniline and 2 . 0 l of hydrogen are metered into the reactor described in example 2 , the catalyst temperature being 350 ° c . under these conditions the catalyst has a life time of more than 6 , 000 hours . table iv which follows gives the relative degree of conversion and the main constituents of the resulting reaction mixture as a function of the running time of the catalyst . table iv______________________________________reaction product (% by weight ) relativerunning degree of di - n - cyclo - n - cyclo - time conversion phenyl - hexylidene - hexyl - ani -( hours ) (%) amine aniline aniline line______________________________________3 , 000 98 . 5 80 1 . 3 1 . 1 8 . 54 , 000 98 . 0 80 0 . 9 4 . 4 8 . 25 , 000 95 . 0 76 3 . 7 7 . 0 7 . 46 , 000 88 . 0 73 6 . 4 6 . 5 9 . 2______________________________________ 3 . 0 g of n - cyclohexylidene - cyclohexylamine and 1 . 0 l of nitrogen are fed into the reactor described in example 2 , the catalyst temperature being 320 ° c . after about 100 hours , the reaction product contains 58 . 0 % by weight of diphenylamine , 34 . 0 % by weight of n - cyclohexyl - aniline and 1 . 0 % by weight of n - cyclohexylidene - aniline as well as 7 . 0 % by weight of other by - products . the reactor described in example 2 is charged at a catalyst temperature of 320 ° c with 2 . 0 l of nitrogen and 3 . 0 g of a mixture having the composition : 33 . 4 % by weight of n - cyclohexylidene - aniline , 37 . 9 % by weight of n - cyclohexyl - aniline , 3 . 8 % by weight of diphenylamine and 24 . 9 % by weight of aniline , such as can be obtained as the intermediate runnings when a crude diphenylamine is worked up by distillation . n - cyclohexylidene - aniline and n - cyclohexyl - aniline are dehydrogenated on the catalyst , with relative degree of conversion of 98 . 5 and 99 %, to give a reaction product of the following composition : 47 . 5 % by weight of diphenylamine , 46 . 1 % by weight of aniline , 3 . 2 % by weight of benzene , 0 . 7 % by weight of n - cyclohexylidene - aniline , 0 . 4 % by weight of n - cyclohexyl - aniline and 0 . 5 % by weight of carbazole ; the amount of further impurities was 1 . 6 % by weight . in each case 3 . 0 g of the substances mentioned in table v which follows , together with 1 . 0 l of nitrogen , are metered into the reactor described in example 2 ; the catalyst temperature is kept at about 320 ° c . after a reaction time of about 200 hours , the reaction product was in each case analysed by gas chromatography ; its composition is listed in table v which follows , &# 34 ; remainder &# 34 ; indicating the sum of all the by - products , that is to say both those which can be used again as starting material for the desired product and those which cannot be used again . table v__________________________________________________________________________ reaction mixture obtained unreacted starting remainderno . starting substance reaction product ( designation ) % substance (%) (%) __________________________________________________________________________1 n - cyclohexylidene - 2 - methyl - 2 - methyl - diphenylamine 93 . 0 3 . 0 4 . 0 aniline2 n - cyclohexylidene - 3 - methyl - 3 - methyl - diphenylamine 90 . 0 aniline3 n - cyclohexylidene - 4 - methyl - 4 - methyl - diphenylamine 85 . 0 7 . 5 7 . 5 aniline4 n - cyclohexylidene - 2 , 3 - 2 , 3 - dimethyl - diphenylamine 90 . 0 1 . 4 8 . 6 dimethylaniline5 n - cyclohexylidene - 3 - methoxy - 4 - methoxy - diphenalamine 85 . 3 1 . 7 13 . 0 aniline6 75 . 0 9 . 4 15 . 67 n - cyclohexylidene - 2 - ethoxy - 2 - ethoxy - diphenylamine 69 . 0 12 . 7 18 . 3 aniline8 n - cyclohexylidene - 4 - ethoxy - 4 - ethoxy - diphenylamine 64 . 7 11 . 3 24 . 0 aniline9 n -( 3 - methyl - cyclohexylidene )- 3 - methyl - diphenylamine 76 . 0 12 . 0 12 . 0 aniline10 n -( 3 - methyl - cyclohexylidene )- 3 , 3 &# 39 ;- dimethyl - diphenylamine 30 . 8 58 . 2 11 . 0 3 - methylaniline11 n -( 3 - methyl - cyclohexylidene )- 3 , 4 &# 39 ;- dimethyl - diphenylamine 55 . 3 28 . 2 17 . 0 4 - methylaniline__________________________________________________________________________ a 12 % strength by weight aqueous sodium carbonate solution is added to an aqueous solution of 2 . 7 g of manganese sulphate and 12 . 0 g of nickel sulphate in 250 ml of water at about 80 ° c until the ph value reaches 8 . 2 . after the solution has cooled , the precipitate which has deposited is separated off , washed with water until sulphate - free and suspended in 120 ml of water ; this suspension is treated at about 85 ° c with a solution of 35 . 0 g of ammonium dichromate in 120 ml of water and mixture is stirred for 2 hours . after the suspension has cooled to room temperature , the precipitate is again filtered off , dried at 120 ° c and pressed to give cylinders 5 mm in diameter and 5 mm in height . the catalyst thus obtained contains 41 . 0 % by weight of nickel , 11 . 5 % by weight of chromium and 1 . 5 % by weight of manganese , calculated as the metal in each case . before use , the catalyst is treated with hydrogen for 2 hours at about 250 ° c . 30 ml of the catalyst obtained according to example 3 are filled into the reaction tube described in example 2 and the reactor is charged , in the manner described in example 2 , with , per hour , 3 . 0 g of n - cyclohexylidene - aniline and 1 . 0 l of nitrogen , the catalyst temperature being kept at about 320 ° c . in the start - up period of a few days , the reaction product contains , on average , 82 % by weight of diphenylamine . subsequently , the diphenylamine content of the reaction product rises to 84 . 5 % by weight . the reaction product also contains : 2 . 1 % by weight of benzene , 9 . 4 % by weight of aniline , 2 . 9 % by weight of n - cyclohexyl - aniline , 1 . 0 % by weight of carbazole and 2 ppm of 4 - aminodiphenyl . a 12 % strength by weight aqueous sodium carbonate solution is added to a solution of 250 . 0 g of nickel sulphate , 350 . 0 g of manganese sulphate and 140 . 0 g of aluminium sulphate in 1 , 000 ml of water at about 80 ° c until the ph value reaches 8 . 2 . after cooling the precipitate which is deposited is filtered off , washed with water until sulphate - free and suspended in 1 , 500 ml of water ; the suspension is treated at about 85 ° c , whilst stirring , with an aqueous solution of 100 . 0 g of ammonium dichromate . after stirring for about 2 hours at this temperature , the precipitate is filtered off , dried and pressed to give cylinders 5 mm in height and 5 mm in diameter . the catalyst thus obtained contains 11 . 8 % by weight of nickel , 27 . 4 % by weight of manganese , 5 . 8 % by weight of chromium and 4 . 1 % by weight of aluminium , in each case calculated as the metal . before use , the catalyst is treated with hydrogen for 2 hours at about 390 ° c . 30 ml of the catalyst obtained according to example 5 are filled into the reaction tube described in example 2 . the reactor is charged , in the manner described in example 2 , with 3 . 0 g of n - cyclohexylidene - aniline and 1 . 0 l of nitrogen the catalyst temperature being maintained at about 320 ° c . the reaction product contains 90 . 3 % by weight of diphenylamine , 8 . 0 % by weight of aniline , 2 . 4 % by weight of n - cyclohexylidene - aniline , 0 . 7 % by weight of benzene , 1 . 1 % by weight of carbazole and 2 ppm of 4 - aminodiphenyl . example 7 . 1 ( comparison example according to german published application no . 2 , 331 , 878 ) 30 ml of a al 2 o 3 catalyst containing 14 % by weight of nickel , which has been prepared in a known manner by impregnating aluminium oxide pellets with the calculated amount of an ammoniacal solution of nickel formate , subsequently drying and treating for one hour with 30 l of hydrogen at 200 ° c and for 1 hour with 30 l of hydrogen at 380 ° c , are filled into the reaction tube described in example 2 . 12 . 9 g of n - cyclohexylidene - aniline , 83 . 0 l of nitrogen and 13 . 8 l of hydrogen are metered in , per hour , in the manner described in example 2 , the catalyst temperature being 370 ° c . the molar ratio of n - cyclohexylidene - aniline to nitrogen to hydrogen is 1 . 0 : 46 . 4 : 7 . 7 and the residence time on the catalyst is 0 . 5 seconds . in the first 70 hours , a reaction mixture consisting of : 86 . 6 % by weight of n - cyclohexylidene - aniline , 5 . 2 % by weight of diphenylamine , 4 . 5 % by weight of n - cyclohexyl - aniline , 2 . 6 % by weight of aniline , 0 . 5 % by weight of carbazole and 0 . 4 % by weight of cyclohexanone , is obtained . in the subsequent 18 hours , a reaction mixture of the following composition : 91 . 9 % by weight of n - cyclohexylidene - aniline , 2 . 0 % by weight of diphenylamine , 2 . 5 % by weight of n - cyclohexyl - aniline , 2 . 2 % by weight of aniline , 1 . 0 % by weight of carbazole and 0 . 4 % by weight of cyclohexanone , is obtained . 3 . 0 of n - cyclohexylidene - aniline and 1 . 0 l of nitrogen are passed over the catalyst mentioned in example 7 . 1 in the reaction tube described in example 2 , the catalyst temperature being about 320 ° c . in this case the molar ratio of n - cyclohexylidene - aniline to nitrogen is 1 : 2 . 6 . in the first 44 hours , a reaction mixture which contains 69 . 0 % by weight of n - cyclohexylidene - aniline , 4 . 0 % by weight of diphenylamine , 1 . 8 % by weight of n - cyclohexyl - aniline , 16 . 6 % by weight of aniline , 0 . 1 % by weight of carbazole , 4 . 1 % by weight of cyclohexanone and 2 . 7 % by weight of benzene , is obtained . in the subsequent 72 hours , a reaction mixture which contains 78 . 5 % by weight of n - cyclohexylidene - aniline , 2 . 9 % by weight of diphenylamine , 4 . 3 % by weight of n - cyclohexyl - aniline , 8 . 4 % by weight of aniline , 0 . 1 % by weight of carbazole , 3 . 4 % by weight of cyclohexanone and 0 . 3 % by weight of benzene , is obtained .