increased after elongating the reaction time, which allows the
intermediate products to be further converted to phenols.
One of the challenging tasks in converting lignosulfonate is
the sulfur poisoning which occurs in most catalysts. In our
study, sulfur is catalytically removed from reaction system as
H2S gas. The formation of H2S was detected by Pb(Ac)2
solution, which instantly changed dark upon adsorbing
evolved H2S gas, forming PbS precipitation. We did not
observe the PbS precipitation when water was the solvent,
instead of EG, or under N2 atmosphere, instead of H2,
indicating that the reduction of sulfonate requires the presence
of EG and H2. We checked the used catalyst by X-ray
diffraction spectrometry (XRD). The typical diffraction of
the Ni(0) metal phase (111, 200, 220) disappeared as shown
in Fig. S29w. Alternatively, a mixture of nickel sulfides (NiSx)
was generated. To confirm the process, we treated the Ni/AC
with lignosulfonate under H2-free conditions and obtained
nickel sulfides, the XRD pattern of which takes after that of
the used catalyst (see Fig. S30w). The as-prepared nickel
sulfides were reduced to Ni(0) metal and H2S gas was detected
under the reaction conditions, inferring that a similar process
takes place in the hydrogenolysis of lignosulfonate. However,
the Ni(0) metal is highly dispersed on activated carbon, which
is blind to detection by XRD, suggesting that the Ni(0)
crystallites are fragmented into smaller particles during the
reaction.10
be reduced under reaction conditions (Fig. S33–S34, ESIw). In
comparison, the used Ni/AC catalyst was still in the original
loose form, and was reducible (Fig. S32, ESIw).
Nickel(0) metal sites may have three key functions in the
reaction: (i) as active sites for hydrogenolysis of C–O–C
bonds; (ii) as active sites for hydrogenolysis of C–OH bonds
at side chains to alkane chains; (iii) as reservoirs and convertors
for sulfonate to H2S. Through a complete catalytic cycle, Ni(0)
metal sites stay in dynamic states. Initially, the dissociation of
H2 on nickel sites forms surface active H* species. Aryl–alkyl
bonds and alcoholic bonds of lignosulfonate get hydrogenated
to phenols. The decomposition of C–S bonds on nickel sites
generates metastable nickel sulfides.11 The combination of active
H* species with nickel sulfides produces H2S, regenerating Ni(0)
sites for the next cycle.
This work was supported by the National Natural Science
Foundation of China (21073184), and the One Hundred
Person Project of the Chinese Academy of Sciences.
Notes and references
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One question remains: whether nickel sulfides are active for
the cleavage of a C–O–C bond? Table 2 shows the catalytic
activities of Ni/AC and NiS/AC in the hydrogenolysis of lignin
model compounds, dibenzyl ether and phenylbenzyl ether. The
Ni/AC achieves 499% and 62% conversions of dibenzyl ether
and phenylbenzyl ether, respectively (entries 1–2, Table 2).
Major products are toluene and phenol. In comparison, the
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atmosphere (such as in H2) during reaction to stabilize the
Ni(0) state. The precious metal catalyst, Pd/AC, is too active
for the hydrogenolysis reaction. No H2S gas was evolved when
the Pd/AC catalyst was used (Fig. S31w). After reaction,
the Pd/AC catalyst severely congregated to hard particles
(Fig. S32w), a sign of heavy sulfur poisoning, and could not
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Table 2 Hydrogenolysis of lignin model compounda
Selectivity (%)
Toluene Phenol
Entry Substrate
Cat. Conv. (%)
10 The fragmentation of Ni(0) particles may be due to sulfur diffu-
sion. It was observed when carbon is inserted into a nickel lattice to
form carbon nanotubes. See literature: S. Helveg, C. Lopez-Cartes,
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catalyst is known. See references: (a) E. Wenkert and T. W.
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281–284.
1
2
3
4
dibenzyl ether Ni
phenylbenzyl ether Ni
dibenzyl ether NiS n.d.
phenylbenzyl ether NiS n.d.
499
62
499
42
—
—
—
58
—
—
a
Reaction conditions: model compound (50 mg), catalyst (50 mg),
ethanol (10 mL), H2 (2 MPa), 393 K, 2 h. The conversion and product
selectivity was determined by an internal standard method. n.d., not
determined.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 7019–7021 7021