Hexamethylene diamine purification

Pure xylan is not employed in industry. but crude xylan or pentosans are of industrial importance. Xylan has been proposed as a textile size but is not employed as yet for this purpose.130 Perhaps the largest use of pentosans is in their conversion to furfural, which has many applications and serves as the source of other furan derivatives. At the present time, large quantities of furfural are used in the extractive purification of petroleum products, and recently a large plant has been constructed to convert furfural by a series of reactions to adipic acid and hexamethylene-diamine, basic ingredients in the synthesis of nylon. In commercial furfural manufacture, rough ground corn cobs are subjected to steam distillation in the presence of hydrochloric acid. As mentioned above, direct preferential hydrolysis of the pentosan in cobs or other pentosan-bearing products could be used for the commercial manufacture of D-xylose. 

Nylon-6,6 is made commercially from adipic acid and hexamethylene diamine with the intermediate formation of a salt (see below). Isolation of this salt provides a means of further purification to the stringent levels required for polymerization to high molecular weight. 

The hydrogenation reaction takes place in the presence of a Raney nickel catalyst at 120-150°C (250-300°F) and 5000 psig. Nylon properties are sensitive to impurities in the feedstock so the reaction and purification steps for both adiponitrile and hexamethylene diamine are carried out to minimize by-products and impurities in the HMDA product. The hydrogenation of adiponitrile is carried out in a large quantity of ammonia which prevents the formation of hexamethyleneimine by-product. Ammonia also acts as a heat transfer fluid and helps maintain the reaction temperature. The conversion of adiponitrile is nearly 100% and the selectivity to HMDA is 97% giving an overall yield of HMDA from adiponitrile of 96.7%. 

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