Hydrogenation of Adiponitrile to Hexamethylenediamine Over Raney Ni and Co Catalysts
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applied sciences Article Hydrogenation of Adiponitrile to Hexamethylenediamine over Raney Ni and Co Catalysts Younghyun Lee 1, Sung Woo Lee 2, Hyung Ju Kim 2, Yong Tae Kim 2 , Kun-Yi Andrew Lin 3 and Jechan Lee 1,4,* 1 Department of Environmental Engineering, Ajou University, 206 Worldcup-ro, Suwon 16499, Korea; [email protected] 2 C1 Gas & Carbon Convergent Research Center, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Daejeon 34114, Korea; [email protected] (S.W.L.); [email protected] (H.J.K.); [email protected] (Y.T.K.) 3 Department of Environmental Engineering & Innovation and Development Center of Sustainable Agriculture & Research Center of Sustainable Energy and Nanotechnology, National Chung Hsing University, 250 Kuo-Kuang Road, Taichung 402, Taiwan; [email protected] 4 Department of Energy Systems Research, Ajou University, 206 Worldcup-ro, Suwon 16499, Korea * Correspondence: [email protected] Received: 13 October 2020; Accepted: 23 October 2020; Published: 26 October 2020 Abstract: Hexamethylenediamine (HMDA), a chemical for producing nylon, was produced on Raney Ni and Raney Co catalysts via the hydrogenation of adiponitrile (ADN). HMDA was hydrogenated from ADN via 6-aminohexanenitrile (AHN). For the two catalysts, the effects of five different reaction parameters (reaction temperature, H2 pressure, catalyst loading, and ADN/HMDA ratio in the reactant) on the hydrogenation of ADN were investigated. Similar general trends demonstrating the dependence of ADN hydrogenation on the reaction conditions for both catalysts were observed: higher temperature (60–80 ◦C) and H2 pressure, as well as lower ADN/catalyst and ADN/HMDA ratios, led to higher HMDA yields. A further increase in temperature from 80 to 100 ◦C increased the HMDA yield from 90.5 to 100% for the Raney Ni catalyst, but did not affect the HMDA yield (85~87%) for the Raney Co catalyst. A 100% HMDA yield (the highest yield reported to date) was also achieved via ADN hydrogenation over the Raney Ni catalyst, with a high HMDA content in the reactant (e.g., ADN/HMDA volumetric ratio of 0.06). No sign of metal leaching into the product solution was found, meaning that the Raney Ni and Raney Co catalysts were stable during ADN hydrogenation. Keywords: adiponitrile; 6-aminocapronitrile; nylon; hydrogenation; hexamethylenediamine; polymer industry 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 [1–3]. 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 [6–9]. HMDA is produced industrially by the hydrogenation of adiponitrile (ADN). Appl. Sci. 2020, 10, 7506; doi:10.3390/app10217506 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 7506 2 of 9 Ziegler-type catalysts [10], supported noble metal catalysts (e.g., Rh/Al2O3)[11], Ni-based amorphous alloy catalysts [12,13], supported Ni catalysts [14,15], and Ni-based bimetallic and trimetallic catalysts [16–21] 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]. Lv 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. 2. Experimental Section 2.1. Materials The Raney Ni catalyst (product number: 510033; Raney® 2400), ADN (purity: 99%; product number: D77001), and HMDA (purity: 98%; product number: H11696) were purchased from Sigma-Aldrich (St. Louis, MO, USA). The Raney Co catalyst (Raney® 2724) was provided by W. R. Grace and Company (Columbia, MD, USA). Ethanol (purity: 99.5%; product number: 054-00461) was purchased from FUJIFILM Wako Pure Chemical (Osaka, Japan). Other chemicals (AHN, azacycloheptene, and azepane) used for gas chromatography (GC) calibrations were purchased from Sigma-Aldrich (St. Louis, MO, USA). Hydrogen gas (ultra-high purity (UHP)) was purchased from DK Gas (Hwaseong, Korea). All chemicals were used as received. A Millipore SAS Direct-Q3 UV system was used to produce the ultrapure water (18.2 MW cm at 25 ◦C) used for all experiments. 2.2. Catalyst Characterization Physisorption measurements of the Raney Ni and Raney Co catalysts were carried out with a Micromeritics ASAP 2020 system (Micromeritics Instrument Co., Norcross, GA, USA), using N2 as analysis adsorptive to assess the adsorption–desorption isotherm at 196 C. Before the N − ◦ 2 Appl. Sci. 2020, 10, 7506 3 of 9 physisorption measurement, degassing of the samples was performed at 90 ◦C for 30 min, and then at 150 ◦C under a vacuum. The surface area of each catalyst was calculated using the Brunauer–Emmett–Teller (BET) method. The total pore volume and average pore diameter of each catalyst were calculated using the Barrett–Joyner–Halenda (BJH) desorption method. Chemisorption measurements of the Raney Ni and Raney Co catalysts were carried out with a Micromeritics ASAP 2010 system (Micromeritics Instrument Co., Norcross, GA, USA), using H2 as analysis gas. Before the H2 chemisorption measurement, pretreatment of the sample was conducted by flowing H2 at 300 ◦C for 2 h. After pretreatment, the sample was evacuated at 35 ◦C for 2 h under a vacuum. The first isotherm was recorded at 35 ◦C by adding doses of H2 until reaching the equilibrium pressure of 560 mmHg. The dosed H2 was then evacuated, and again, H2 was re-introduced to determine the amount of weakly adsorbed hydrogen (i.e., reversible H2 uptake). The amount of strongly adsorbed hydrogen (i.e., irreversible H2 uptake) was determined by subtracting the second isotherm from the first one. 2.3. Hydrogenation Reaction Batch reactions of ADN hydrogenation were carried out in a 100 mL high-pressure batch reactor (M-Series reactor system, Rexo Engineering Inc., Seoul, Korea) equipped with a magnetic drive agitator. A thermocouple was also equipped on the reactor to measure the actual temperature of the reaction. For an experiment, water, ethanol, ADN, HMDA (total volume of 56 mL), and a catalyst (Raney Ni or Raney Co) with a desired ratio of ADN/HMDA and ADN/catalyst were loaded into the reactor. Considering that the Raney Ni and Co catalysts are highly pyrophoric, great care was taken to prevent exposure to air when loading them into the reactor. Thus, we loaded the catalyst into the reactor according to the following procedure: (1) Thermogravimetric analysis of the Raney Ni and Raney Co catalysts was conducted in order to determine their water content (note that the catalysts were delivered as a slurry in water); (2) water (mass of water required for an experiment the mass of − water contained in a catalyst slurry (i.e., catalyst + water) required for the experiment) was placed on a weighing dish and then weighed; (3) the catalyst slurry was taken on the water-loaded weighing dish and then weighed; and (4) the water with the catalyst slurry was poured together into the reactor.