FIGURE 6 Proposed rection path and the reaction energy barrier
diagrams: (a) Proposed reaction path of PPE hydrogenolysis. (b) Reaction
energy barrier of PPE hydrogenolysis by Ni@N-C SAC and Ni@NC.
To explore the range of the Ni@N-C SAC catalyst, two more β-O-4 model
compounds and one α-O-4 model compound were evaluated as the substrates.
It is found that benzyl phenyl ether (α-O-4) and 2-phenylethyl phenyl
ether (β-O-4) over the Ni@N-C SAC are nearly completely converted,
providing the corresponding phenols and arenes in high yields (≥ 80%,
Table S3, entries 1-2). It is known that β-O-4 dimeric model compound
with Cγ-OH is structurally closer to the true lignin.
Gratefully, such a model compound was also completely converted (Table
S3, entry 3), affording guaiacol and 4-propylguaiacol in yields of 80%
and 53%, respectively. These data demonstrated the versatility of
Ni@N-C SAC in selective cleavage of the C-O bond in different lignin
model compounds.
The recyclability of the Ni@N-C SAC was explored by carrying out the
hydrogenolysis of PPE as listed in Figure 7a. After each run, the used
catalyst was washed with ethanol following by vacuum drying. It was
found that the conversion of PPE in Ni@N-C SAC-catalyzed hydrogenolysis
remains unchanged after three runs. Characterizations of the used
catalyst were conducted by XRD, XPS, HRTEM and aberration-corrected
HAADF-STEM. There is no Ni metal diffraction peak in XRD pattern (Figure
7b). The binding energy of Ni 2p3/2 peak (855.13 eV) of used catalyst is
almost the same as that of original Ni@N-C SAC (Figure 7c). No Ni
clusters or particles are observed on the surface of the used catalysts
(Figure 7d, e). Aberration-corrected HAADF-STEM characterizations show
that Ni atoms were still atomically dispersed with no aggregation
observed (Figure 7 f). These results demonstrate the excellent stability
of Ni@N-C SAC in the hydrogenolysis systems.