GB731819A - Improvements in or relating to the production of hexamethylene diamine and salts thereof - Google Patents

Abstract

Hexamethylenediamine obtained by catalytically hydrogenating adiponitrile and then removing at least a portion of the 1,2-diaminocyclohexane produced during the hydrogenation so as to recover hexamethylenediamine containing less than 500 parts per million of 1,2-diaminocyclohexane (see Groups III and IV (b)) is stated to be well adapted for use in preparing superior polyamides of improved colour stability and strength. The polyamides may be obtained either by direct polymerization with a dibasic carboxylic acid such as adipic acid or by first forming a salt with the dibasic acid and then polymerizing in a second step. The salts may be prepared by the water strike technique, thus the purified hexamethylene-diamine may be reacted with an equivalent amount of adipic acid in an aqueous medium to give a salt of improved colour stability which may be converted into a synthetic linear condensation polyamide capable of being formed into strong colourless fibres. Specifications 474,999, 490,922, and 536,940, [Group V], are referred to.ALSO:In the production of hexamethylenediamine by catalytically hydrogenating adiponitrile at least a portion of the 1,2-diaminocyclohexane produced during the hydrogenation is removed from the hexamethylene diamine and a hexamethylenediamine product containing less than 500 parts per million of 1,2-diaminocyclohexane is recovered. The hydrogenation may be effected using hydrogenation catalysts such as nickel, cobalt, copper, zinc, platinum, palladium, and ruthenium, either in the form of the free metals or in the form of compounds such as the oxides or salts, if desired in conjunction with known promoters and/or supports. The process may be carried out in the presence of ammonia and may be operated in the liquid or vapour phase and the catalyst may be employed in fluidized form. Cobalt catalysts and particularly catalysts comprising reduced cobalt oxide are preferred. The adiponitrile used may be obtained by the reaction of adipic acid with ammonia, by reacting dichlorobutane with hydrogen cyanide, or by the hydrogenation of dicyanobutene. Removal of the 1,2-diamino cyclohexane from the hexamethylenediamine may be effected in several ways, e.g. by fractional crystallization of the anhydrous hexamethylenediamine product or by a chemical separation, e.g. by forming the 1,2-diaminocyclohexane into a complex by addition of hydroxyacetic acid or by titrating with picric acid to a pH of 7 when the hexamethylenediamine is removed as an insoluble picrate salt and then further titrating to a pH of about 3 when the di-salt of 1,2-diaminocyclohexane with picric acid precipitates. The latter may be treated with a base to decompose the salt and recover the diamine. The preferred method of removing the 1,2-diaminocyclohexane is by a carefully controlled fractional distillation of the hexamethylene diamine the distillation being preferably carried out in the presence of a small amount of water which may be introduced along with the crude hexamethylenediamine, or additional water or steam may be added directly to the column or columns in which the 1,2-diaminocyclohexane is removed (see Group III). The 1,2-diaminocyclohexane which is removed as an overhead stream during the distillation procedure can be recovered as the substantially pure compound by further distillation. It is stated that the purified hexamethylenediamine containing less than 500 parts per million of 1,2-diaminocyclohexane is well adapted for use in preparing superior polyamides, either by direct polymerization with a dibasic carboxylic acid, such as adipic acid, or by formation of a salt with the dibasic acid followed by polymerization in a second step. The salts may be prepared by the water strike technique, thus the purified hexamethylenediamine may be reacted with an equivalent amount of adipic acid in an aqueous medium to give a salt of good colour stability. Specifications 474,999, 490,922 and 536,940, [Group V], are referred to.ALSO:<PICT:0731819/III/1> In the production of hexamethylenediamine by catalytically hydrogenating adiponitrile (see Group IV (b)) at least a portion of the 1,2-diaminocyclohexane produced in the hydrogenation is removed from the product and a hexamethylenediamine product containing less than 500 parts per million of 1,2-diaminocyclohexane is recovered. The preferred method of removing the 1,2-diaminocyclohexane is by a carefully controlled fractional distillation which is preferably carried out in the presence of a small amount of water which may be introduced along with the crude hexamethylenediamine, or additional water or steam, may be added directly to the column or columns in which the 1,2-diaminocyclohexane is removed. In the distillation system shown in the drawing the crude hexamethylenediamine together with added water is introduced via inlet line 2 into column 1 which is generally a packed column operated at about atmospheric pressure and serves to remove most of the water overhead through pipe 3 together with hexamethyleneimine. The tails from this column pass via line 4 to column 5 which is operated at reduced pressure, e.g. 50 to 400 mm. mercury, and the bulk of the 1,2-diaminocyclohexane is taken overhead via line 6 together with any pentamethylenediamine that may be present, some water, and some hexamethylenediamine. The tails are passed via line 7 to column 11 whilst the heads are passed via line 6 into the heads concentrator column 8 from which the 1,2-diaminocyclohexane is removed overhead. The hexamethylenediamine recovered in the tails may be recycled to column 1 via inlet line 2 or to column 5 via line 4 or may be added directly to the final column 11 wherein the purified diamine is separated from high boilers and recovered overhead through line 12. If desired the system can be modified by elimination of column 8 in which case the effluent from line 6 would be removed from the system. Furthermore the entire system can be incorporated into a single column with feeds and outlets located at the proper levels. Preferred conditions are given for operating the four-column system so as to recover a hexamethylenediamine product containing about 200 parts per million of 1,2-diaminocyclohexane and it is stated that by increasing the reflux ratio in column 8 from 20 : 1 to 35 : 1 the impurity content is reduced to about 100 parts per million. The purified hexamethylenediamine is stated to be well adapted for use in preparing superior polyamides either by direct polymerization with a dicarboxylic acid such as adipic acid or by formation of a salt with the dibasic acid followed by polymerization in a second step. Specifications 474,999, [Group IV], 490,922 and 536,940, [Group V], are referred to.