Highly Efficient and Recyclable Nitrogen-Doped Mesoporous Carbon-Supported Ru Catalyst for the Reductive Amination of Levulinic Acid/Esters to Pyrrolidones

Abstract

A highly effective transformation from renewable biomass and its derivatives into high-value-added chemicals and biofuels is important and challenging. Herein, a N-doped mesoporous carbon (NMC)-supported Ru catalyst developed by an incipient wetness impregnation method was reported for the reductive amination of levulinic acid (LA)/esters with amines to Nsubstituted 5-methyl-2-pyrrolidinones. A complete conversion of LA with a 99.8% yield of N-hexyl-5-methyl-2-pyrrolidinone (HMP) was achieved over 0.05 mol % Ru/NMC under mild conditions of 120 °C and 1.5 MPa H 2 for 4 h, giving a high turnover frequency (3667 h −1 ). Furthermore, Ru/NMC exhibited superior stability and recyclability (reusable 21 cycles without loss of activity). The excellent performance of the Ru/NMC catalyst was mainly attributed to the introduction of N species to the mesoporous carbon, which not only improved the anchoring and the dispersion of Ru nanoparticles on the catalyst surface but also provided moderate basic sites and transferred electrons from NMC to Ru nanoparticles, as revealed by detailed characterizations. Additionally, the developed Ru/NMC catalytic system was applicable to LA/ esters and various amines to afford the corresponding N-substituted 5-methyl-2-pyrrolidinones with excellent yields (80−99%).

CoNi Alloy Nanoparticles Confined in N-Doped Porous Carbon as an Efficient and Versatile Catalyst for Reductive Amination of Levulinic Acid/Esters to N-Substituted Pyrrolidones

“…Finally, M2 was hydrogenated by NiCo@NC to form the BMP, which is the rate-determining step. This reaction mechanism is consistent with that of our previous report of the reductive amination of LA with n -hexylamine over Ru/NMC …”

“…It is worth noting that there are two obvious peaks in the CO 2 -TPD profile at around 92 and 567 °C, corresponding to weak and strong basic sites, respectively, which originate from N-containing groups in CoNi@NC, such as CN heterocycles and the pyridinic nitrogen-oxide. Our previous study found that the basic sites in the carbon support are conducive to the formation of negatively charged metals . Furthermore, it is reported that the N-derived base sites on Co@NC could play a similar role to base additives, which can not only inhibit the formation of polyamines or prevent the accumulation of products on the catalyst surface but also effectively facilitate the hydrogenation and reductive amination …”

“…Our previous study found that the basic sites in the carbon support are conducive to the formation of negatively charged metals. 56 Furthermore, it is reported that the N-derived base sites on Co@NC could play a similar role to base additives, which can not only inhibit the formation of polyamines or prevent the accumulation of products on the catalyst surface but also effectively facilitate the hydrogenation and reductive amination. 57 3.2.…”

“…The excellent performance of CoNi@NC was mainly attributed to the formation of more metal active sites after pyrolysis at 900 °C, the uniformly dispersed CoNi alloy, and the synergistic effect between Ni and Co. 57−60 Meanwhile, the basic property of NC was also beneficial for the reductive amination of LA, as we have confirmed before. 56 The effect of reaction temperature and hydrogen pressure on the reductive amination of LA with benzylamine to BMP was investigated over CoNi@NC. As shown in Figure 9a, even at a lower temperature (100 °C), LA was well converted (89% conversion) after 6 h, offering a 55% yield of BMP accompanied by 33% of M1 and 1% of M2, which indicates the high activity of CoNi@NC.…”

“…This reaction mechanism is consistent with that of our previous report of the reductive amination of LA with n-hexylamine over Ru/NMC. 56 3.3. Catalyst Stability.…”