Korean J. Chem. Eng., 34(10), 2610-2618 (2017) pISSN: 0256-1115 DOI: 10.1007/s11814-017-0164-4 eISSN: 1975-7220 INVITED REVIEW PAPER INVITED REVIEW PAPER Reductive amination of ethanol to ethylamines over Ni/Al2O3 catalysts Jung-Hyun Park*,‡, Eunpyo Hong**,‡, Sang Hee An**, Dong-Hee Lim***, and Chae-Ho Shin**,† *Greenhouse Gas Resources Research Group, Korea Research Institute of Chemical Technology, Daejeon 34114, Korea **Department of Chemical Engineering, Chungbuk National University, Chungbuk 28644, Korea ***Department of Environmental Engineering, Chungbuk National University, Chungbuk 28644, Korea (Received 12 April 2017 • accepted 12 June 2017) − Abstract Ni(x)/Al2O3 (x=wt%) catalysts with Ni loadings of 5-25 wt% were prepared via a wet impregnation method γ on an -Al2O3 support and subsequently applied in the reductive amination of ethanol to ethylamines. Among the vari- ous catalysts prepared, Ni(10)/Al2O3 exhibited the highest metal dispersion and the smallest Ni particle size, resulting in the highest catalytic performance. To reveal the effects of reaction parameters, a reductive amination process was per- formed by varying the reaction temperature (T), weight hourly space velocity (WHSV), and NH3 and H2 partial pres- sures in the reactions. In addition, on/off experiments for NH3 and H2 were also carried out. In the absence of NH3 in the reactant stream, the ethanol conversion and selectivities towards the different ethylamine products were signifi- cantly reduced, while the selectivity to ethylene was dominant due to the dehydration of ethanol. In contrast, in the absence of H2, the selectivity to acetonitrile significantly increased due to dehydrogenation of the imine intermediate. Although a small amount of catalyst deactivation was observed in the conversion of ethanol up to 10 h on stream due to the formation of nickel nitride, the Ni(10)/Al2O3 catalyst exhibited stable catalytic performance over 90 h under the o −1 optimized reaction conditions (i.e., T=190 C, WHSV=0.9 h , and EtOH/NH3/H2 molar ratio=1/1/6). Keywords: Ni/Al2O3 Catalyst, Ethanol, Reductive Amination, Ethylamines, Nickel Nitride INTRODUCTION by consecutive reactions with EtOH. As hydrogenation/dehydro- genation processes are involved in the preparation of EAs via this Ethylamines (EAs) have been widely used as intermediates to mechanism, a catalyst surface bearing metallic functionalities must produce fine chemicals, such as herbicides, pharmaceuticals, tex- be employed. In this context, the dispersion and reducibility of the tile chemicals, rubbers, and corrosion inhibitors [1-3]. Due to the metal species are key factors to determine catalytic performances increasing demand for amine-containing compounds in the chem- in the reductive amination reaction [1,9,13,14]. ical industry, a variety of methods for their syntheses have been The noble metal catalysts containing Pt, Pd, and Rh have been proposed, including the hydrogenation of nitrile compounds [4,5], applied in reductive amination reactions due to their excellent hy- the reductive amination of ethanol (EtOH) and acetaldehyde [1- drogenation and dehydrogenation capabilities [15-18]. However, 3,6], and the hydroamination of olefins [7]. Among the various their application has been restricted due to high costs, and thus the approaches, reductive amination has been considered a promising development of alternative catalysts has received growing atten- route for the preparation of EAs due to the environmentally fri- tion. From this perspective, Ni- and Co-containing catalysts have endly nature of the process [3,8]. In addition, the synthesis of EAs been widely studied for application in reductive amination reac- via other reaction pathways usually requires harsh reaction condi- tions because of their suitable activities and selectivities, in addition tions, including high temperatures and pressures, while the amina- to their wide availability [1,2,9,11-13,19]. Sewell et al. [2] investi- tion reaction can instead proceed under relatively mild conditions gated the reductive amination of alcohols over Ni- or Co-catalysts [1]. supported on SiO2. Although a higher EtOH conversion was ob- As shown in Fig. 1, the reductive amination of EtOH progresses tained for the Co-based catalyst compared to that of the Ni-based through successive stages, beginning with the dehydrogenation of catalyst, selectivity towards the formation of different amine spe- EtOH to acetaldehyde [9-12]. An imine intermediate is then ob- cies varied depending on the type of metal. More specifically, the tained by condensation and reaction of the carbonyl compound Co-based catalyst favored the production of the primary amine, with NH3. As amine compounds can be produced as either pri- while the Ni-based catalyst was more selective towards the sec- mary, secondary, or tertiary amines, monoethylamine (MEA) can ondary amine. In addition, the deactivation of these catalysts by be synthesized via the hydrogenation of the imine intermediate, the formation of metal nitrides through interactions between NH3 while diethylamine (DEA) and triethylamine (TEA) are formed and the active metal species has also been well documented [13, 20,21]. †To whom correspondence should be addressed. We herein report the preparation of Ni/Al2O3 catalysts contain- E-mail: [email protected] ing different Ni loadings for application in the reductive amination ‡These authors contributed equally to this work. of EtOH to EAs. In addition, the relationship between the metal- Copyright by The Korean Institute of Chemical Engineers. lic surface area and the Ni loading was examined, and the catalytic 2610 Reductive amination of ethanol to ethylamines over Ni/Al2O3 catalysts 2611 Fig. 1. Possible reaction pathways for the reductive amination of ethanol. performance of the different catalysts were carefully investigated. in the relative pressure range between 0.05-0.2. The total pore vol- As reaction parameters have been shown to significantly influ- umes were obtained from the volumes of N2 adsorbed at P/P0= ence alcohol conversion and selectivity towards different nitrogen- 0.995, and the average pore sizes were calculated from the desorp- containing compounds [1,2,9], we also examined the influence of tion branch using the Barrett-Joyner-Hallenda (BJH) equation. reaction temperature (T), weight hourly space velocity (WHSV), The metal dispersions, particle sizes, and metallic surface areas and the partial pressures of NH3 and H2. Finally, to further under- of the different Ni(x)/Al2O3 catalysts were determined by H2 stand the effects of the key reactants, on/off experiments for NH3 chemisorption (Micromeritics ASAP2020C). Prior to analysis, the o and H2 were also carried out. samples (0.3g) were reduced at 600 C for 3h under a flow of H2. Each sample was then cooled to 100oC under reduced pressure, EXPERIMENTAL and the chemisorption isotherm was obtained from the difference between the first and second adsorption isotherms. The first iso- 1. Catalyst Preparation therm involved both quantities of chemisorption and physisorp- Ni/Al2O3 catalysts with Ni loading ranging from 5 to 25 wt% tion, while second contained only the physisorbed quantity. In ad- were prepared via a wet impregnation method with a nickel nitrate dition, the metal dispersion was calculated for each sample assum- hexahydrate solution (Ni(NO3)2·6H2O, >98%, Samchun) and com- ing an H/Ni stoichiometry of 1.0, while the reduction degree of γ 2 μ mercial -Al2O3 (SBET=194 m /g, particle size=100-180 m, Proca- each Ni(x)/Al2O3 catalyst was determined using the O2 titration talyse) as the Ni metal precursor and the support, respectively. The method. The O2 titration method was subsequently conducted on precursor and the support in the desired ratio was stirred at room an identical apparatus, following the H2 chemisorption. The sam- o temperature for 1 h, prior to evaporation of the solvent at 70 C ple used in the H2 chemisorption experiment was evacuated at using a rotary evaporator (Eylar N1000). The impregnated sample 300 oC for 1 h, and the reduction degree was calculated in combi- o was then dried at 100 C for 12 h, and subsequently calcined at nation with the following equation: [the amount of O2 consump- o → 500 C for 2 h under a flow of air. The prepared Ni/Al2O3 catalysts tion (mmol O2; 2Ni+O2 2NiO]/[the theoretical amount of H2 were denoted as Ni(x)/Al2O3, where x (x=5, 10, 15, 20, and 25) consumption with the assumption of fully reduced nickel oxides → 0 represents the Ni loading in weight percent (wt%). (mmol H2; NiO+H2 Ni +H2O)]×100)] [22], where the amount o 2. Characterization of O2 consumption was determined at 300 C by plotting the amount X-ray diffraction (XRD) patterns were obtained using a Rigaku of consumed O2 as a function of the O2 partial pressure, and back diffractometer (Ultima IV) equipped with a Cu-Kα X-ray source extrapolating to zero. operating at 40 kV and 40 mA. The crystalline phases were identi- H2-temperature programmed reduction (H2-TPR) was performed fied by matching the patterns to the JCPDS powder diffraction files, using a quadrupole mass spectrometer (QMS, Balzers QMS 200) and the particle sizes of NiO and Ni were calculated using the Scher- as the detector. Each sample (0.1g) was loaded in a quartz reactor rer equation. The specific surface area, total pore volume, and aver- (I.D. 12 mm) and pretreated at 400 oC for 1h under a flow of Ar 3 o 3 age pore size of the Ni(x)/Al2O3 catalysts were then determined by (50 cm /min). After cooling to 50 C, a flow of 5% H2/Ar (50 cm / − o N2-sorption isotherms at 196 C on a Micromeritics ASAP2020 min) was then introduced into the catalyst bed, and the tempera- o o apparatus. Prior to N2-sorption analysis, the samples were degassed ture was increased to 800 C with a heating rate of 10 C/min.