3.5 Catalytic Hydrogenolysis of Lignin
The high catalytic activity of Ni@N-C SAC toward the hydrogenolysis of lignin model compounds encouraged us to explore the application of Ni@N-C SAC in real lignin decomposition. In this work, birch organosolv lignin was employed as the substrate in the reaction. The yield of the aromatic monomers of Ni@N-C SAC is 31.2% with propylguaiacol and propysyringol as the main products (Table S4), which is about twice higher than those of Ni@NC (14.9%) and Raney nickel catalyst (12.3%), indicating Ni@N-C SAC is an efficient catalyst for lignin depolymerization. Figure 8 shows the 2D HSQC spectra of birch organosolv lignin and the corresponding oily products. The signals in the 2D HSQC spectra are labeled and summarized in Table S5. It is found that almost all the signals of A (β-O-4), B (β-5) and C (β-β) in lignin oil are disappeared after catalyzed reaction (Figure 8a vs Figure 8c) compared with that in lignin. In detail, the relative content of β-O-4 linkages in birch organosolv lignin was 42/100Ar, while the signals of Aαand Aβ corresponding to benzylic alcohols and secondary alkyl protons of β-O-4 linkages almost completely disappeared, implying most β-O-4 linkages have been cleaved after reaction. Moreover, the signals of B (β-5) and C (β-β) in oily products are no longer observed, indicating that the stubborn C-C bonds are also cleaved after reaction. It is therefore interesting that Ni@N-C SAC shows high activity not only in cleaving C-O bonds, but also in cracking C-C bonds in lignin. It is also found that G and S units are observed with the amounts of 29% and 71% in fresh lignin. After reaction, the corresponding amounts were similarly 31% and 69%, respectively, suggesting that most of the aromatic units are not destroyed after reaction. As a comparison, Ni@NC exhibits lower depolymerization efficiency according to Figure 8b, and Figure 8e. Of which, the signals of A (β-O-4), B (β-5) and C (β-β) in lignin oil are still calculated 21%, 2% and 4%, respectively, and the corresponding amounts of the primary units (S and G) are also similarly to that in original lignin. The above results further confirm the higher catalytic activity of Ni@N-C SAC than Ni@NC in lignin depolymerization. GPC analysis gives another evidence of the excellent activity of Ni@N-C SAC. The GPC spectrum of birch organosolv lignin (Figure S10 and Table S6) shows a board molecular weight distribution with an average M W of 4150 g/mol. After reaction, the average molecular weight of the two lignin oil samples catalyzed by Ni@N-C SAC and Ni@NC are 590 g/mol and 1870 g/mol, respectively, which are much lower than that of birch organosolv lignin.