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.