Fundamental reaction kinetics of high-pressure reductive amination of polyalkylene glycol
Introduction
Polyetheramine (PEA) has been employed in various application fields. Epoxy coating is the representative area where PEA is utilized as a cross-linking agent for connecting epoxy resins [1]. In polyurea synthesis, its reaction with isocyanate provides urea bonding [2], [3]. In addition, PEA can be employed as one of gas hydrate inhibitors [4], [5], [6]. PEA is normally synthesized from polyalkylene glycol (PAG) having an alcohol functional group through reductive amination by introducing ammonia and hydrogen. The terminal alcohol group is dehydrogenated to a ketone (aldehyde) intermediate and then the ketone (aldehyde) is aminated to form an imine intermediate, releasing water generated during the amination. Finally, PEA is synthesized via hydrogenation of imine (Scheme 1) [7], [8], [9]. PEA has been prepared via one-pot synthesis, and the dehydrogenation of alcohol (the 1st step) is known to be the rate-limiting step [10], [11], [12]. Because PEA is non-volatile with a vapor pressure of around 1.24x10-3 bar at 235 °C, it is quite difficult to separate PEA from the unreacted product mixture. The PEA reaction thus should take place as much as possible to reach a high target yield. In this respect, it is crucial to determine dominant kinetic factors influencing the activity toward PEA and selectivity toward the primary amine.
The reductive amination of alcohol is called a hydrogen-borrowing reaction or hydrogen auto-transfer reaction [13], [14] because hydrogen formed from dehydrogenation of alcohol is utilized again for the hydrogenation step from imine to amine. The hydrogen-borrowing reaction is known to be atom-economical and environmentally friendly since it does not require additional reductants for the hydrogenation step or generate any side products other than water [13], [14], [15].
However, when the reductive amination is carried out in the presence of a catalyst, excess hydrogen is required in order to avoid the formation of metal nitride. In our previous studies, as the formation of inactive metal nitride sites is a reversible reaction, providing sufficient hydrogen was able to reduce the catalyst to the active metal state from the inactive metal nitride and the increase in hydrogen amount brought the improved catalytic activity toward PEA [9], [16]. Additional hydrogen also plays an important role in inhibiting the formation of carbonaceous deposits on the catalyst [17].
Considering the rate determining step of alcohol dehydrogenation, the amount of NH3 is not expected to be a main factor influencing activity toward the amine. Previous studies showed that the amination step was a zero order reaction and did not dominate the overall reaction rate [18], [19], [20]. Increasing the amount of NH3 did not lead to an increase in the amine yield from n-octanol [19] and dodecanol [20]. However, it is noteworthy that PEA synthesis was carried out at high pressure of around 150 bar or in some cases, above 200 bar, owing to the introduction of an excess amount of NH3 [21], [22]. Most of reductive amination reactions were performed at reaction pressure lower than that of the PEA synthesis. For example, the amination reaction of n-octanol was implemented at about 1 bar to give octylamines [19], dodecylamine was prepared at 4 bar of NH3 and 2 bar of H2 from dodecanol [20], and isopropanol was aminated to isopropyl amine at around 20 bar [23]. This study will make the first attempt to elucidate why the PEA reaction should be carried out under high pressure conditions with excess NH3, indicating that the conditions in the reductive amination of PAG existed as the liquid phase is quite different with that of small molecular alcohol in the previous studies.
In addition, a proper kinetic model will be set up based on the most influential factors affecting the activity toward PEA, the NH3 amount and temperature. The kinetic parameters such as the rate constant and activation energy for the PEA reaction will subsequently be determined. To the best of our knowledge, it is the first time to obtain kinetic factors for the synthesis of PEA from PAG at high reaction pressure conditions. Obtaining these kinetic parameters at such high pressure above 150 bar offers great benefit to develop large-scale processes.
In the reductive amination, some side products such as secondary and tertiary amines are generated as well as a primary amine as the target material [19], [24] and the existence of these secondary and tertiary amines in the final product may adversely affect subsequent reactions for the primary amine. It is thus important to determine the factors that affect selectivity toward the primary amine in order to obtain high purity of primary PEA. In this present study, factors influencing the selectivity and activity such as the NH3 amount, reaction pressure, and the existence of hydrogen will be investigated because it is quite difficult to separate the primary amine from the product mixture due to its low vapor pressure, as mentioned above. In addition, another route by which the secondary amine can be generated from the primary amine as a starting material will be investigated to shed light on the factors affecting the formation of secondary amine.
We employed Fourier transform infrared spectroscopy (FT-IR) to analyze the relative amount of primary amine or imine in the product as well as secondary amine. By analyzing the total amine or primary amine yields in terms of reaction temperature, amount of NH3, and presence of H2O, kinetic modelling of the PEA reaction was established with the Langmuir-Hinshelwood model. The effect of hydrogen on the reverse reaction of PEA into secondary or tertiary amines was further investigated by calculating the amine values. These efforts to find factors improving the activity toward PEA and selectivity toward the primary amine can provide insight into developing a competitive synthesis method for PEA.
Section snippets
Preparation of catalysts
Y2O3-inserted Co-Pd/molecular sieve 13X (MS 13X) catalysts were prepared by a precipitation method, referring to our previous studies [9], [16]. First, 44.035 g of Co(NO3)2∙6H2O (ACROS), 0.525 g of Y(NO3)3∙4H2O (Sigma-Aldrich), and 0.056 g of Pd(NO3)2∙2H2O (KOJIMA) were each placed in a beaker, followed by the introduction of 200 mL, 10 mL, and 10 mL, respectively, of deionized water (DIW) to dissolve each precursor. These precursor solutions were introduced to a glass-reactor equipped with an
Reactor setup and P-T curve measurement of NH3
PEA reaction was carried out at high pressure and temperature in a batch reactor by introducing NH3 and H2. The reaction system was composed of feed introduction, reactor and post-treatment section (Fig. 1). For the feed introduction section, anhydrous NH3 in a cylinder was first filled in the NH3 vessel whose NH3 level measurement is available for a correct amount of NH3 injected. The temperature of NH3 vessel was kept to −20 °C by a chiller. After opening the valve between the NH3 vessel and
Conclusions
The activity and selectivity for the PEA reaction were investigated with the variation of numerous reaction parameters such as reaction temperature, NH3 amount, reaction pressure, and H2O amount. Though prior studies reported that the amount of NH3 had no influence on the conversion, an increase in the NH3 amount brought improved activity toward PEA, which resulted from the enhanced solubility of NH3 to the liquid phase of PPG. Increasing the NH3 amount also led to improved selectivity toward
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was supported by the Global Ph.D. Fellowship Program (NRF-2019H1A2A1076070) and KC30 (1711100593) funded by National Research Foundation of Korea.
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