Renewable lignin-based carbon nanofiber as Ni catalyst support for depolymerization of lignin to phenols in supercritical ethanol/water
Graphical abstract
Introduction
Lignin is a heterogeneous and amorphous polymer that constitutes a large portion of the cell walls of vascular plants; it is ranked as the second most abundant biomass on earth after cellulose [1]. It is inexpensive and possesses numerous attractive properties, such as high carbon content and thermal stability, and favorable stiffness [2]. In addition to being the largest renewable source for aromatics, lignin is also rich in functional groups; thus is a potential resource for the production of renewable aromatic platform chemicals and carbon materials. With effective depolymerization, lignin can serve as a renewable feedstock for aromatic compounds [2,3]. Therefore, combining lignin-based carbon materials with lignin depolymerization to produce aromatic platform chemicals can be a new strategy for the complete use of lignin.
Hydrogenolysis of lignin (reductive method), in which H2 is used to break bonds between aromatic units in the presence of catalysts, is one of the most prevalent and efficient strategies that can be used to produce aromatic compounds from lignin; thus, it is one of the most popular and efficient techniques [4]. The process usually requires either precious metals- or nonprecious metals-based heterogeneous catalysts, and precious metals, i.e. Pt, Ru and Pd, and nonprecious metals, i.e. Ni, Fe and Mo are generally used as the active phase [5]. Kim et al. [6] have compared the effects of several carbons-supported metal catalysts in the soda lignin depolymerization. The Pt/C catalyst and ethanol has been shown to be the best combination that produces a large amount of bio-oil with the smallest amount of char. In addition to Pt/C catalyst, a number of metal-based heterogeneous catalysts supported by activated carbon materials, such as Rh/C, Ni/C, Ru/C, and other metals/C (Pd, Ir, and Mo) catalysts have been employed to produce aromatic platform chemicals by hydrogenation of lignin [7]. Nickel-based catalysts have been shown to exhibit excellent chemoselectivity for aromatic products or high activity for C–O bond cleavage, as reported by Sergeev et al. [8], Gao et al. [9], Song et al. [10] have demonstrated that Ni/C has high activity and selectivity in conversion of native lignin into monomeric phenols, propylguaiacol and propylsyringol. However, the above works mainly focus on discussing how to regulate hydrogen source, solvent type and Ni loading to achieve high lignin depolymerization efficiency. Details on the change of metal-support to avoid Ni nanoparticle aggregation, sintering and loss during lignin depolymerization processes were hardly discussed, especially when Ni nanoparticles were inserted or partially embedded (or “inlaid type”) in carbon materials. Information on the effects of the changes on lignin depolymerization efficiency, which is also interesting, is also limited.
The “inlaid type” catalyst, in which metal nanoparticles are inserted or partially embedded in mesoporous materials, is an attractive catalyst that prevents metal aggregation; it has been applied in many fields, such as catalysis, solar cells and electrode materials [[11], [12], [13]]. The inlay of Ni nanoparticles directly in carbon materials to construct the “inlaid type” catalyst and to appropriately adjust the interaction between metal and support is, therefore, a good strategy for improving the catalyst stability. To construct such a structure, it is necessary to design a suitable carbon support material that can inlay Ni during Ni/C preparation process. Lignin is a promising carbon source for the preparation of carbon materials due to its renewability and plentiful hydroxyl groups. In our recent study on lignin-based carbon material, we found that the preparation of lignin-based carbon nanofibers (LCNF) has high controllability [14]. Thus, we find that it is interesting to attempt to inlay Ni nanoparticles directly in LCNF during the preparation process of LCNF to produce the “inlaid type” catalyst.
In this work, lignin was employed as a carbon source to prepare the inlaid-type Ni/LCNF catalyst for the catalytic hydrogenolysis of lignin, with an aim to achieve the maximum utilization of lignin. We described the preparation, characterization and catalytic testing of Ni/LCNF. The effect of different Ni loadings in Ni/LCNF on the depolymerization of ethanol organosolv lignin (EOL) in the presence of hydrogen in supercritical ethanol/water were studied. We also demonstrated the recovery and reusability of the prepared catalysts. The results revealed that the catalysts had excellent stability, as was evidenced by successful reusability for three times. Finally, 2D heteronuclear single quantum coherence nuclear magnetic resonance spectroscopy (2D HSQC NMR) and gel permeation chromatography (GPC) were employed to characterize in detail the lignin residue (LR) and to track its structural changes during the reactions.
Section snippets
Materials
All commercial reagents in this study were analytical grade and used without further purification. Ni(NO3)2·6H2O (AR), ethanol, Tetrahydrofuran (THF), N,N-Dimethylformamide (DMF) and Polyethylene oxide (PEO, Mn ∼ 4,000,000) were supplied by Sinopharm Chemical Reagent Co. Ltd; n-dodecane was purchased from Sigma Aldrich. Polar wood was purchased from a local manufactory and shattered into wood sawdust (ca. 40 mesh).
Lignin extraction from poplar wood
Lignin was extracted from poplar wood by the organosolv technique using
Characterization of Ni/LCNFs catalysts
Ni/LCNF catalysts were fabricated via electrospinning from a mixture containing lignin and Ni2+ at different weight percent. As shown in Fig. 1, the scanning electron microscope (SEM) analysis showed the carbon fibers fabricated from lignin and 1%Ni had higher average diameter than those produced from lignin and 5%Ni or 10%Ni, suggesting the presence of Ni can significantly improve the spinnability of lignin. Among all fibers, the carbon fibers containing high Ni concentration (10%) had the
Conclusions
Lignin was utilized as a carbon source to prepare highly effective Ni nanoparticles inlaid in lignin-based carbon nanofibers (LCNF) catalyst that was then used for lignin depolymerization. The Ni/LCNF catalyst had excellent performance in lignin depolymerization with 91% conversion rate, yielding a maximum 7% phenols and 87% light lignin fragments. The catalyst also had high stability; it could be reused for up to three cycles without loss of catalytic reactivity. Moreover, demethoxylation and
Acknowledgments
This research was funded by the National Key Research and Development Program of China (No. 2017YFB0307900), the Opening Project of Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control (KF201803-5), the State Key Laboratory of Pulp and Paper Engineering (No. 201803), the Liaoning Providence Science and Technology Project (No. 20180550759).
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