# Hydrogenation of Adiponitrile to Hexamethylenediamine over Raney Ni and Co Catalysts
## 1 Introduction  
Nylon is a silky thermoplastic material that has been used in apparel, flooring, reinforced rubber, food packaging films, car parts, and electrical equipment parts, among other applications [1; 2; 3]. Among the different nylons, nylon 66 is an aliphatic polyamide-based nylon with high heat resistance and good mechanical properties [4]. The global market size of nylon 66 was 5.09 billion USD in 2017 and is expected to be valued at 6.7 billion USD by 2026, with an estimated compound annual growth rate of 3.1% from 2017 to 2026 [5]. In 2011, two million tons of nylon 66 was produced globally.  
Nylon 66 is synthesized via polycondensation of hexamethylenediamine (HMDA) and adipic acid in water. The HMDA is composed of a linear six-carbon linker arm with a primary amine group on each end [6; 7; 8; 9]. HMDA is produced industrially by the hydrogenation of adiponitrile (ADN).  
Ziegler-type catalysts [10], supported noble metal catalysts (e.g., Rh/Al\({}_{2}\)O\({}_{3}\)) [11], Ni-based amorphous alloy catalysts [12; 13], supported Ni catalysts [14; 15], and Ni-based bimetallic and trimetallic catalysts [16; 17; 18; 19; 20; 21] have been used for the hydrogenation of ADN to HMDA. More recently, Pd and Ag-based nanowires, synthesized by a seed-mediated growth method (to create a Pd-rich surface), were used as catalysts for the hydrogenation of ADN to HMDA with a 98% yield [22]. [PERSON] et al. used a basic ionic liquid ([bmim]OH) to suppress the formation of byproducts during the hydrogenation of ADN to HMDA over a Ni catalyst, supported on N-functionalized, multi-walled carbon nanotubes, which gave 97.8% selectivity toward HMDA and 6-aminohexanenitrile (AHN) with a 95.3% ADN conversion [23]. However, the catalytic processes previously reported have disadvantages such as complicated catalyst synthesis processes and the use of expensive metals (e.g., Pd, Rh, and Ag) and additional solvents (e.g., ionic liquid), which lead to high costs.  
Other than the catalysts mentioned above, Raney Ni and Raney Co catalysts have also been used to prepare HMDA from ADN on an industrial scale [24; 25]. They are important catalysts, routinely employed in chemical industries. Raney catalysts have higher catalytic activities at lower temperatures than conventional supported metal catalysts [26]. However, the production of HMDA by hydrogenating ADN over Raney Ni or Raney Co catalysts needs a large amount of ammonia or alkali metal hydroxide solution to impede side reactions, resulting in undesired secondary and tertiary amines [12; 27]. Despite the disadvantages of using the Raney catalysts for the hydrogenation of ADN to HMDA, they are still attractive catalysts because they are inexpensive, assuming the use of ammonia and alkali solutions can be avoided.  
Therefore, herein, an inexpensive catalytic route to HMDA via the hydrogenation of ADN without using acidic or basic solutions was investigated. Moreover, the hydrogenation of ADN over Raney Ni and Raney Co catalysts in the presence of water, ethanol, and HMDA was studied. During industrial chemical production, consideration must be given to whether a target product (e.g., HMDA) hinders the conversion of feedstock (e.g., ADN). If the target product impedes the conversion of feedstock, high yields of the target product cannot be achieved because it gradually slows the reaction rate as the reaction proceeds [28]. Thus, in this study, HMDA was added to the reactant before the hydrogenation reaction in order to investigate the effect of HMDA on ADN conversion. The effects of temperature, hydrogen pressure, reaction time, and the ADN/HMDA ratio on ADN conversion and product selectivity were also elucidated. The results of this study could aid the development of an inexpensive industrial catalytic process to produce a value-added chemical raw material used for the production of nylon.