Au-Pd alloy cooperates with covalent triazine frameworks for the catalytic oxidative cleavage of β-O-4 linkages

Article abstract of DOI:10.1039/c9gc03081f

To design highly efficient catalysts for the cleavage of the C-O/C-C bond is the key task in the depolymerization of lignin. Bimetallic alloy catalysts Au-Pd-CTFs were developed to be effective in the oxidative cleavage of β-O-4 lignin model compounds with O₂. Au-Pd nanoparticles with an Au/Pd molar ratio between 1?:?1 and 1?:?1.5 showed the highest cleavage efficiency. The kinetics of the reaction process revealed that a synergistic effect between Au and Pd played a crucial role in the oxidation of C_α-OH into C_αO, which was the rate-determining step for the whole oxidative cleavage process. Further insight revealed that the cooperative effect between Au-Pd nanoparticles and the support covalent triazine frameworks (CTFs) facilitated the cleavage of the formed β-O-4 ketone compound to the corresponding aromatics. In addition, Au-Pd-CTF catalysts also showed efficiency in the oxidative transformation of the organosolv lignin. This catalytic system will provide guidance in the oxidative cleavage of β-O-4 linkages in lignin.

Full text of DOI:10.1039/c9gc03081f

Green Chemistry
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Au–Pd alloy cooperates with covalent triazine
frameworks for the catalytic oxidative cleavage
of β-O-4 linkages†
Cite this: Green Chem., 2019, 21,
707
6
a,b
a
a,b
a,b
a
a
Li Zhao, Song Shi, * Guozhi Zhu, Meng Liu, Jin Gao and Jie Xu
*
To design highly efficient catalysts for the cleavage of the C–O/C–C bond is the key task in the depoly-
merization of lignin. Bimetallic alloy catalysts Au–Pd-CTFs were developed to be effective in the oxidative
cleavage of β-O-4 lignin model compounds with O
between 1 : 1 and 1 : 1.5 showed the highest cleavage efficiency. The kinetics of the reaction process
revealed that a synergistic effect between Au and Pd played a crucial role in the oxidation of C –OH into
vO, which was the rate-determining step for the whole oxidative cleavage process. Further insight
2
. Au–Pd nanoparticles with an Au/Pd molar ratio
α
C
α
Received 2nd September 2019,
Accepted 5th November 2019
revealed that the cooperative effect between Au–Pd nanoparticles and the support covalent triazine frame-
works (CTFs) facilitated the cleavage of the formed β-O-4 ketone compound to the corresponding aro-
matics. In addition, Au–Pd-CTF catalysts also showed efficiency in the oxidative transformation of the orga-
nosolv lignin. This catalytic system will provide guidance in the oxidative cleavage of β-O-4 linkages in lignin.
DOI: 10.1039/c9gc03081f
rsc.li/greenchem
1
0
11
dominant linkage, including oxidation, reduction, solvoly-
Introduction
1
2
13
sis, pyrolysis, and catalytic cleavage assisted by an alkaline
1
4,15
16
Lignin is an important component of lignocellulosic biomass or acid,
as well as their combinations.
Recently, the catalytic oxidation method has received great
1
,2
(15–30% by weight) and widely exists in plants. As a biomass
resource, lignin is one of the few renewable sources of aro- attention because the products are oxygen-containing aro-
3
–5
6,7,17,18
matic chemicals.
The catalytic depolymerization of lignin matics with increased functionalization.
Abundant
into aromatics is a fascinating and sustainable route to research on novel catalysts for the oxidative cleavage of β-O-4
6
–9
10,19–32
produce fuels and chemicals.
Lignin primarily consists of linkages has been reported, including homogeneous
2
2,33–39
methoxylated phenylpropane structures connected through C– and heterogeneous catalysts.
For the heterogeneous cat-
O and C–C bonds, which form a complex three-dimensional alysts, which are potentially superior because they are separ-
amorphous polymer with β-O-4, β-1, β-5, α-O-4, 4-O-5, and able and reusable, studies have also focused on their cleavage
other linkages, making the utilization of the material of lignin; for example, cobalt-based N-doped carbon materials
6
,9
difficult. Because of the high bond dissociation energy of and a single-atom Co catalyst performed well in the oxidative
3
6,37
the diverse structure, the core problem in depolymerizing cleavage of β-O-4 linkages,
copper–vanadium double-
lignin lies in designing highly efficient catalysts to break down layered hydrotalcites showed high activity and achieved veratric
the C–C/C–O bond of the different linkages. Among all the lin- acid as the main product in the cleavage of a β-O-4 lignin
2
2
kages, the β-O-4 structure is the most unique one because it model compound, and Pd/CeO
2
could efficiently cleave β-O-4
1
,9
accounts for approximately 50%. The designed catalysts that lignin models and depolymerize organosolv lignin to vanillin
3
9
can selectively activate and cleave the chemical bond of β-O-4 as the main product. With different catalysts, different clea-
linkages shows promise for the scissoring of lignin into aryl vage routes were put forward. Among the various catalytic
monomers. Several efficient catalytic systems have been devel- routes with the specially designed catalysts, one of the main
α α
oped to facilitate the cleavage of the C–C/C–O bond of this pre- mechanisms is that C –OH was firstly oxidized into C vO and
then followed the cleavage of the formed ketone. With the for-
mation of C vO, the BDE of the C –O bond would be substan-
tially reduced, which accounted for the predominant clea-
vage route. Inspired by this mechanism, the catalyst with the
α
β
a
40
State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy,
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,
P. R. China. E-mail: shisong@dicp.ac.cn, xujie@dicp.ac.cn
b
ability to transform C –OH would be efficient in the oxidative
α
University of Chinese Academy of Sciences, Beijing 100049, P. R. China
cleavage of β-O-4 linkages. With these encouraging results
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9gc03081f
achieved in the literature, we devote the current study to devel-
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metallic catalysts possessing different Au/Pd molar ratios were
prepared with HAuCl and PdCl as precursors through a sol-
4
2
5
9
immobilization method (Scheme 1). First, Fig. 2A shows the
XRD patterns of the samples. The materials revealed crystalline
triazine-based frameworks, and the Au–Pd-CTFs also main-
tained the crystalline structure. All the samples were separately
verified by the (100) diffraction at low angles: the (001) aro-
matic sheets with a broad peak and two additional weak peaks
5
8
assigned to the (110) and (200) diffractions. Magnified
Fig. 1 Simultaneous cleavage of β-O-4 linkages over bimetallic Au–Pd- regions of the XRD patterns of the Au–Pd nanoparticles are
CTF catalysts.
shown in Fig. 2B. For the catalysts Au-CTFs and Pd-CTFs, each
with a monometallic component, the diffraction peaks appear-
ing at 2θ = 38.1 and 39.9 were identified as Au(111) and
Pd(111) (ICDD PDF-2; Au: no. 00-002-1095 and Pd: no. 01-087-
oping efficient heterogeneous catalysts for the direct oxidative
cleavage of the β-O-4 linkages in lignin.
Bimetallic catalysts, when compared to their monometallic
counterparts, in many cases, have shown superior performance
on account of the “synergistic effects” (ligand and ensemble
4
1–44
effects).
With the pioneering studies by Hutchings and co-
workers, the Au–Pd bimetallic catalyst in a special proportion
has been demonstrated to be an efficient catalyst widely applied
4
5–52
to the heterogeneous catalytic reactions.
They have reported
that Au–Pd alloy nanoparticles could efficiently oxidize primary
4
6,47
C–H bonds in toluene and methane.
Based on their studies,
a plethora of research on Au–Pd bimetallic heterogeneous
catalytic oxidation reactions has been reported, it has shown
great ability in the transformation of different functional groups
4
2
in biomass conversion, including alcohol oxidation. For
example, the catalytic oxidation of the hydroxyl of 1,6-hexane-
Scheme 1 Process for the preparation of Au–Pd-CTF catalysts.
53,54
diol yielded 6-hydroxycaproic acid or adipic acid,
catalytic oxidation of both hydroxyl and aldehyde groups of
the
55
5
-hydroxymethylfurfural gave 2,5-furandicarboxylic acid, and
the catalytic cleavage of the C–C bond of glycerol to produce gly-
colic, glyoxylic and oxalic acids of C2 acids and formic acid of
5
6,57
C1 acid.
With its ability to convert various chemical bonds,
developing novel Au–Pd bimetallic heterogeneous catalysts may
play an important role in the oxidative cleavage of lignin.
In this paper, we reported Au–Pd nanoparticles as bi-
metallic catalysts for the effective catalytic oxidative cleavage of
β-O-4 lignin model compounds and organosolv lignin with O
Fig. 1). With the superior properties of a N-containing
2
(
periodic structure, high thermal stability and surface areas,
covalent triazine frameworks (CTFs) were applied as the support.
The synergistic effect between Au and Pd and the cooperation of
Au–Pd nanoparticles with support CTFs facilitated the cleavage
α α
of lignin. Here, C –OH was first oxidized to C vO with Au–Pd
nanoparticles. Then, Au–Pd cooperated with CTFs to promote
the cleavage of the C–C and C–O bonds in the formed ketones to
yield the corresponding aromatic compounds.
Results and discussion
Characterization studies of bimetallic catalysts
Fig. 2 (A) XRD patterns and (B) Partial regions of XRD patterns for (a)
CTFs, (b) Au-CTFs, (c) Au
Pd -CTFs, (f) Au –Pd1.5-CTFs, (g) Au
–Pd -CTFs, and ( j) Pd-CTFs.
1
–Pd0.25-CTFs, (d) Au
1
–Pd0.5-CTFs, (e) Au
1
–
CTFs were prepared with 1,4-dicyanobenzene monomers using
1
1
1
–Pd -CTFs, (h) Au
2
1
–Pd -CTFs, (i)
4
5
8
the ionothermal method and a range of Au–Pd-CTF bi- Au
1
8
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Fig. 3 Dependence of the lattice parameter for Au–Pd-CTFs on the
relative amount of Au.
0
643), respectively. In the spectra of the bimetallic Au–Pd-CTF
Fig. 4 (A) HAADF-STEM image of Au
1
–Pd1.5-CTFs and the corres-
catalysts, the peaks appeared between Au(111) and Pd(111), ponding elemental mapping images, (B) an HRTEM image of Au –Pd1.5-
1
and with decreasing Au amounts in Au–Pd-CTFs, the diffrac- CTFs and (C) a line-scan EDS analysis across one nanoparticle.
tion peaks gradually shifted to higher angles. The lattice para-
meters of the Au–Pd nanoparticles estimated from the XRD
diffraction patterns of the (111) phase, as a function of the Au
amount determined by ICP-OES, are plotted in Fig. 3. For the
monometallic Pd-CTF and Au-CTF samples, the lattice con-
stants were 3.91 and 4.09, respectively. Increasing the content
particles were homogeneously dispersed in alloy form; no
core–shell form or other structural incoherency was exhibited
by the present Au–Pd nanoparticles.
1
To attain the electronic state of the supported Au –Pd1.5-
of Au in the bimetallic Au–Pd nanoparticles led to an increase
in the lattice parameter, and a linear relationship between
their composition and the lattice parameter was obtained.
These results indicated that the two different metals in the
CTF alloy catalyst, XPS measurements were carried out. Fig. 5A
shows the Au 4f spectra of Au-CTFs and Au –Pd1.5-CTFs. The
1
peak for Au 4f7/2 showed a negative shift from 83.7 to 83.5 eV
along with an 87.4 to 87.2 eV shift in the peak for Au 4f5/2
4
9,60
nanoparticles were forming homogeneous alloys.
going from Au-CTFs to Au –Pd1.5-CTFs, which corresponded to
1
The morphologies of the Au–Pd-CTF samples were charac-
terized by TEM, and the size distribution histograms of the
Au–Pd nanoparticles were measured (Fig. S1†). Except for the
monometallic Au-CTFs, which exhibited a small fraction with
large diameters, the samples all presented well-dispersed
nanoparticles of approximately 4.3 nm. No clear particle size
differences among the several Au–Pd-CTF alloy catalysts were
observed, which also verified that the catalytic activity differ-
ences in the following reactions were not caused by the par-
the Au(0) species. Similarly, for the Pd 3d spectra in Fig. 5B,
the 3d3/2 peak for the Pd(0) species also showed a negative
shift with 340.4 eV for Pd-CTFs to 340.2 eV for Au –Pd1.5-CTFs;
1
at the same time, a negative shift from 335.2 to 335.0 eV for
the peak of Pd 3d5/2 was shown, which had an overlap with the
Au 4d5/2. The lower binding energies for both Pd 3d and Au 4f
ticle size. In addition, a representative sample of Au –Pd1.5-
1
CTFs was further studied. The HAADF-STEM and elemental
mapping images are shown in Fig. 4A. Apparently, the support
of C and N and the supported bimetallic Au and Pd were all
dispersed evenly and overlapped with each other. The typical
lattice spacing of the (111) plane from the HR-TEM image was
found to be 0.230 nm (Fig. 4B), which was right between
Au(111) (0.236 nm) and Pd(111) (0.226 nm). This result was
strong evidence for the formation of the bimetallic alloy. In
addition, the line-scan EDS analysis showed that Au and Pd
elements were uniformly distributed in the nanoparticles
(Fig. 4C). These results were in agreement with the XRD
1
Fig. 5 (A) Au 4f and (B) Pd 3d spectral region of Au –Pd1.5-CTFs com-
results and further confirmed that the bimetallic Au–Pd nano- pared with that of Au-CTFs and Pd-CTFs.
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Table 1 The physical properties of the samples
a
b
c
d
e
e
S
BET
D
(nm)
pore
V
pore
d
(nm)
mean
Au content
(wt%)
Pd content
(wt%)
Au/Pd molar
ratio
2
(cm g−1)
3
e
Entry
Sample
(m g−1)
1
2
3
4
5
6
7
8
9
CTFs
Au-CTFs
967
877
888
890
885
889
866
888
888
864
1.23
1.23
1.23
1.23
1.23
1.23
1.23
1.23
1.23
1.23
0.47
0.43
0.43
0.44
0.43
0.43
0.42
0.43
0.43
0.43
—
—
—
—
—
1 : 0
4.0
4.2
4.0
4.2
4.3
4.5
4.2
4.2
4.1
0.92
0.80
0.66
0.58
0.49
0.43
0.28
0.17
—
Au
Au
Au
Au
Au
Au
Au
1
1
1
1
1
1
1
–Pd0.25-CTFs
–Pd0.5-CTFs
–Pd
0.15
0.19
0.33
0.43
0.51
0.64
0.77
0.91
1 : 0.34
1 : 0.54
1 : 1.05
1 : 1.63
1 : 2.17
1 : 4.18
1 : 8.11
0 : 1
1
-CTFs
–Pd1.5-CTFs
–Pd
–Pd
–Pd
2
-CTFs
4
-CTFs
8
-CTFs
10
Pd-CTFs
a
b
c
d
Calculated with the multipoint BET equation. Calculated with the NLDFT model. Calculated at P/P
0
= 0.98. Au
x
Pd
y
nanoparticles were calcu-
e
lated from TEM. Determined by ICP-OES.
6
1–64
revealed that the net charge was flowing into Au and Pd,
and the electronic states of Pd and Au would be modified by
each other in the alloyed form. In addition, the peaks at 337.4
5
0
and 342.6 eV were attributed to Pd(II), compared with mono-
metallic Pd-CTFs, the proportion of Pd(II) in the bimetallic
alloy Au –Pd1.5-CTFs decreased, further indicating that the
1
electron flowed into Pd. Combined with the above results of
the electron shifts of Au 4f and Pd 3d, and with the rich
N-containing of CTFs, there possibly is interaction between
CTFs and nanoparticles. Such an electron interaction had the
potential to promote the catalytic oxidative cleavage of lignin.
The specific composition contents of Au and Pd were
measured by ICP-OES and are shown in Table 1. The actual
amount of total metal was approximately 0.9 wt%, and no
evident deviation occurred. Furthermore, the corresponding
Au/Pd molar ratio was close to the theoretical value.
The specific surface area and porosity data for Au–Pd-CTFs
were determined and calculated by N2 physical adsorption
measurements (Fig. S2 and S3†) and are shown in Table 1.
Compared with the CTF support, which had a specific area of
Fig. 6 Catalytic conversion of β-O-4 model compound 1 over Au–Pd-
CTFs with different Au/Pd molar ratios. Reaction conditions: Substrate,
0
3 2
.2 mmol; catalyst, 40 mg; CH OH, 5 mL; O , 0.5 MPa; 160 °C; 4 h.
2
−1
9
67 m
g , the supported alloy Au–Pd-CTF catalysts (with
different Au/Pd molar ratios) all had approximately the same
high specific surface area, which was only slightly lower than sisted of phenol 2, phenylglyoxal 3, methyl benzoate 4, benzoic
that of CTFs. In addition, the samples had identical pore dia- acid 5, methyl benzoylformate 6 and a minute amount of
meters and high total volumes. These characteristics were ben- 2-phenoxyacetophenone 7. With the adjustment of the Au/Pd
eficial for the following catalytic reactions.
1 1 1
molar ratio, Au –Pd -CTF and Au –Pd1.5-CTF catalysts pre-
sented the highest and comparative substrate conversions of
6% and cleavage selectivity of 91%. These results illustrated
that the monometallic Au-CTFs and Pd-CTFs were not active
Catalytic oxidative cleavage of β-O-4 lignin models over Au–Pd-
CTF catalysts
9
To study the C–C/C–O bond cleavage regularity of lignin, a towards this reaction under the present conditions; however,
representative simple β-O-4 model compound, 2-phenoxy-1- the bimetallic alloy catalysts dramatically enhanced the conver-
3
9
phenylethanol 1, was used to evaluate the catalytic perform- sion of the substrate, verifying a clear synergistic effect
ance of Au–Pd-CTFs with different Au/Pd molar ratios (Fig. 6 between Au and Pd that promoted the reaction successfully.
and Table S1†). For no catalyst or the support CTFs only, there For the physical mixture of Au-CTFs and Pd-CTFs, with the
was hardly any activity towards the conversion of the substrate amounts of Au and Pd equal to those of Au
1
–Pd1.5-CTFs, the
(
less than 5%). Similarly, the monometallic supported catalysts system showed little enhancement reaction, demonstrating the
Au-CTFs and Pd-CTFs also showed no apparent catalytic per- molecular-scale synergy effect exhibited by the alloy form. In
formance. However, the bimetallic alloy Au–Pd-CTF catalysts addition, when the reaction was conducted in nitrogen, only a
exhibited an enhanced activity towards the cleavage of the slight conversion of the substrate followed, demonstrating the
β-O-4 lignin model compound 1. The products mainly con- necessity of oxygen for the reaction.
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Table 2 Catalytic cleavage of β-O-4 compounds with methoxy or
a
1
γ-OH groups over Au –Pd1.5-CTFs
Product distribution (%)
Substrate
Conv. (%)
2a
4a
5a
6a
2b
2c
>
99
26
48
15
11
—
—
9
7
7
50
34
30
9
5
8
—
—
>
>
99
99
47
10
—
29
33
67
63
—
—
—
—
—
—
4
4
8
9
9
8
25
70
—
—
—
5
a
Reaction conditions: Substrate, 0.2 mmol; Au
OH, 5 mL; 180 °C; 8 h; O , 0.5 MPa. The product distribution was
calculated via the area normalization method by GC. 16 h. 20 mol%
1
–Pd1.5-CTFs, 40 mg;
CH
3
2
b
c
d
2 3
K CO was added, 16 h. 24 h.
Methoxy as well as γ-OH groups largely exist in lignin,
which may have effects on the conversion of β-O-4 linkages.
Herein, the optimal catalyst Au –Pd1.5-CTFs was tested on the Fig. 7 Partial 2D HSQC NMR spectra of (A) organosolv birch lignin and
1
(
B) after the reaction. The color-coded cross-peaks were assigned to the
corresponding linkages. Reaction conditions: Organosolv birch lignin,
.1 g; Au –Pd1.5-CTFs, 40 mg; CH OH, 10 mL; O , 0.5 MPa; 180 °C; 16 h.
conversion of several β-O-4 compounds with methoxy or γ-OH
Table 2). When R was methoxy group substituted, β-O-4 alco-
(
2
0
1
3
2
hols almost completely converted to acids and esters with a
low amount of phenols, and even partially with quinone.
Similarly, the β-O-4 alcohol compound containing the γ-OH
group was also totally converted. However, when R1 was aromatic monomer products were quantitatively determined
methoxy group substituted, a small amount of base K CO
was after the cleavage reaction (approximately 5.2 wt% of the total
2
3
added to promote the cleavage of alcohols with a longer reac- monomers, Fig. S4†). In detail, the identified monomers were
tion time, which was to improve the C –OH oxidation, or the esters, quinone derived from the oxidized phenol and a small
α
products mainly focused on the ethers 2c. In summary, Au
1
–
amount of aldehydes, i.e., vanillin, syringaldehyde, methyl
Pd1.5-CTFs had the ability to cleave β-O-4 linkages with 4-hydroxybenzoate, methyl syringate, methyl vanillate and 2,6-
different groups.
dimethoxy-1,4-benzoquinon.
Catalytic oxidative depolymerization of organosolv lignin
Cleavage process of the β-O-4 compound over Au -Pd1.5-CTFs
1
With the successful oxidative cleavage of the β-O-4 lignin For the oxidative cleavage of the β-O-4 model compound, the
models, the applicability of Au –Pd1.5-CTFs to the oxidative substrate may undergo C –OH oxidation to a ketone, dehydra-
depolymerization of birch lignin extracted from a dioxane- tion to enol ether intermediates or other radical process.
1
α
2
0,36,67
6
5
based organosolv process was carried out. 2D HSQC NMR In this report, no inhibition effect occurred with the radical
spectra were obtained to detect the variations in the prominent scavenger 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) or
6
6
linkages (Fig. 7). Successfully, the signals for β-O-4 linkage A butylated hydroxytoluene (BHT) added to the system
and cyclic α-O-4 linkage B disappeared; besides, the β–β (Table S2†), therefore excluding the radical process.
linkage C also being converted, indicating that an effective Additionally, during the cleavage process, the C
conversion of organosolv lignin was achieved over the Au product ketone 7 was detected, and the reaction time profile
Pd1.5-CTF bimetallic alloy catalyst. The yields of the identified over Au –Pd1.5-CTFs is shown in Fig. S5.† Furthermore, the
α
–OH oxidation
1
–
1
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apparent rate constants (kapp) for the cleavage of substrate 1
and ketone 7 under the same conditions were determined as
−
3
−1
−3
k
Ketone(H) = 33.8 × 10 min and kAlcohol(H) = 2.63 × 10
−
1
min , respectively (Fig. S6†). This result revealed that the clea-
vage rate of ketone 7 was 12.9 times that of the alcohol sub-
strate 1 (eqn (1)). The large reaction rate discrepancy indicated
a substantially faster transformation of ketone 7 than that of
alcohol 1, meaning that once ketone 7 was formed, it was con-
verted. This observation was consistent with the results in
Fig. S5,† in which ketone 7 always exhibited a low content.
Doubtlessly, when ketone 7 was used as the substrate, it could
be totally converted (eqn (2)). All these conclusions confirmed
Scheme 2 The proposed cleavage pathways for 2-phenoxy-1-
phenylethanol.
that the C
ate. In addition, for this oxidation step, a deuterium-labeling
experiment of C –H of substrate 1 was carried out. The kapp
value of C
-deuterium-labeled 2-phenoxy-1-phenylethanol over glyoxal was also detected (Fig. S8†), which might have been
α
–OH oxidation product ketone 7 was the intermedi-
α
α
−3
−1
Au –Pd1.5-CTFs was kAlcohol(D) = 1.38 × 10 min , showing a formed through the cleavage of a C–O bond yielding phenol. A
1
first-order kinetic isotope effect (kAlcohol(H)/kAlcohol(D) = 1.9). total conversion of phenyl formate and a 92% conversion of
Therefore, the activation of the C
α
–H bond was the rate-deter- phenylglyoxal was observed (eqn (4) and (5)), suggesting that
mining step for the oxidation step (eqn (3) and Fig. S6†). these two compounds were possible intermediates during the
Compared with the control experiment, which had no conver- second cleavage step. In addition, the deuterium-labeling
sion of the substrate, Au
Cα–H bond activation, and in turn, facilitated the cleavage.
Then, for the following cleavage process of the formed k′Ketone(D) = 3.7) (eqn (6) and Fig. S9†), suggesting that for the
ketone 7, a route similar to that of our previous report was second cleavage process the activation of the C –H bond was
–Pd1.5-CTFs may have
–Pd1.5-CTFs would also
1
–Pd1.5-CTFs might have promoted the experiment of C
β
–H on ketone 7 was also studied. Similarly,
the results showed a primary kinetic isotope effect (k′Ketone(H)
/
β
6
8
shown. First, the peroxide generated through the activation the rate-determining step. Because Au
of C
–H was determined, as proved by titration (Fig. S7†). facilitated the activation of C–H, Au
Additionally, a small amount of phenyl formate was detected
1
β
1
have promoted the conversion of this step (discussed below).
In summary, the β-O-4 alcohol model compound was first oxi-
dized to a ketone, which was a slow step, and then, the formed
β-O-4 ketone quickly cleaved to the corresponding products.
ð1Þ
1
During the process, Au –Pd1.5-CTFs may have the ability to
promote the C–H bond activation.
As a summary of the foregoing discussion, a possible clea-
vage mechanism is proposed in Scheme 2. First, 2-phenoxy-1-
phenylethanol was oxidized to 2-phenoxyacetophenone, which
was then converted to the corresponding peroxide. Next, along
with the cleavage of the formed peroxide, a C–C bond cleavage
happened to form phenyl formate and benzoic acid; at the
same time, the cleavage of a C–O bond occurred to produce
phenylglyoxal and phenol. Then, phenyl formate was further
converted to phenol and phenylglyoxal to benzoic acid, methyl
ð2Þ
ð3Þ
ð4Þ
benzoate and methyl benzoylformate over Au –Pd1.5-CTFs.
1
Effect of Au–Pd nanoparticles
As we stated above, for the oxidative cleavage of the β-O-4 com-
pound, the activation of the C –H bond was the rate-determin-
α
ð5Þ
ð6Þ
ing step, followed by the benzylic alcohol being oxidized to a
ketone. The tremendous catalytic activity difference among
Au–Pd-CTFs with different Au/Pd molar ratios was attributed
to the different catalytic performances with respect to the
alcohol oxidation. Here, a model reaction of the catalytic oxi-
dation of α-phenethyl alcohol to acetophenone was conducted
(Fig. S8†) and was probably formed through the C–C bond to study the effect of the Au/Pd molar ratio (Fig. 8). The results
cleavage along with the cleavage of the formed peroxide, pro- showed that the law of oxidative activity was the same as that
ducing benzoic acid at the same time. Meanwhile, phenyl- of the former. In other words, monometallic Au-CTFs and Pd-
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Fig. 8 Catalytic oxidation of α-phenethyl alcohol over Au–Pd-CTFs
with different Au/Pd molar ratios. Reaction conditions: Substrate,
Fig. 9 The kapp values of ketone compound 7 over the catalysts.
0
.2 mmol; catalyst, 40 mg; CH
3
OH, 5 mL; O
2
, 0.5 MPa; 120 °C; 2 h.
Reaction conditions: Substrate, 0.2 mmol; catalyst, 40 mg; CH
3
OH,
5
mL; O , 0.5 MPa; 120 °C.
2
CTFs presented nearly no activity under the present con-
ditions, whereas the synergistic effect of Au–Pd nanoparticles suggesting that Au–Pd-CTFs could accelerate the C–H acti-
promoted the oxidation process. The catalysts with optimal vation. In summary, Au–Pd nanoparticles played a crucial role
Au/Pd molar ratios, i.e., Au
exhibited the highest catalytic performances. This phenom- into ketones, and the cleavage of the formed ketones was then
1 1 1
–Pd -CTFs and Au
–Pd1.5-CTFs, in the oxidation of the benzylic alcohol of β-O-4 lignin models
4
9,50,69–71
enon has also been disclosed in other reports.
In facilitated over Au–Pd nanoparticles cooperating with CTFs.
brief, the effect of Au–Pd nanoparticles involved the oxidation
of C–OH to CvO, which was favored through the promotion of
C–H bond activation.
Effect of the catalyst support
Except for CTFs, other supports including SiO , ZrO , Al O
2 3
2
2
As for the cleavage of ketone 7, the cooperative effect and TiO were used as catalysts or catalyst supports with Au –
2
1
between Au–Pd nanoparticles, with different Au/Pd molar Pd1.5 nanoparticles for the oxidative cleavage of β-O-4 com-
ratios, and support CTFs was studied under related mild con- pound 1 (Fig. 10). As seen, all supports showed nearly no cata-
ditions (Table S3†). Compared with CTFs, which had a sub- lytic activity, as was the same with CTFs. For the supports
strate conversion of 54%, and regardless of whether monome- loaded with Au –Pd bimetallic nanoparticles, a full conver-
1
1.5
tallic Au-CTF and Pd-CTF or bimetallic alloy Au –Pd0.5-CTF, sion of the substrate was exhibited; however, the selectivity of
1
1 1 1 1 2
α
Au –Pd -CTF, Au –Pd1.5-CTF and Au –Pd -CTF catalysts were the cleavage product was quite low, and the C –OH oxidized
used, these supported catalysts all presented enhanced cata- product 7 was the main focus. We inferred that supports SiO2,
lytic properties. Surprisingly, for whichever monometallic or ZrO , Al O and TiO2 had no catalytic activity towards the
2
2 3
bimetallic catalyst implemented, the catalytic activity was β-O-4 lignin model compound. When the Au –Pd bimetallic
1
1.5
always with an approximately 70% substrate conversion. To nanoparticles were supported on these supports, the catalytic
further confirm the phenomenon, the apparent rate constant oxidation activity was enhanced to mainly generate the oxi-
(kapp) was measured among CTFs, Au-CTFs, Au –Pd1.5-CTFs dized product ketone 7. Nevertheless, the cleavage perform-
1
and Pd-CTFs (Fig. 9 and S10†), which possibly had different ance was poor compared with that of Au –Pd1.5-CTFs, which
1
catalytic activities. The results showed that for the cleavage of demonstrated a substrate conversion of 96% with a selectivity
−
3
−1
ketone 7, CTFs had a kapp value of 6.7 × 10 min , while Au- of the cleavage product of 91%. Therefore, a cooperative effect
–Pd1.5-CTFs and Pd-CTFs had kapp of 9.2 × 10− , 9.3 × between Au –Pd
3
CTFs, Au
1
1.5
nanoparticles and the CTF support to
1
−
3
−3
−1
1
0
, and 9.2 × 10 min , respectively. This observation indi- promote the oxidative cleavage of a substrate was observed.
cated that CTFs loaded with Au, Pd or Au –Pd1.5 all expressed
1
In addition, due to the ketone compound 7 being the
an enhanced catalytic activity compared with CTFs, and primary oxidation product and acting as the intermediate
judging from kapp, we could see that the facilitating effect during the catalytic cleavage of the β-O-4 compound 1, the
β
2 2 3 2 2
made no difference. The activation of the C –H bond was the catalytic activities of the support CTFs, SiO , Al O , TiO , ZrO ,
rate-determining step for the oxidative cleavage of ketone 7 XC-72R and C N accompanied with the supported catalysts
3
4
(eqn (6)), and Au–Pd-CTFs expressed a higher reaction rate, were also tested, and the results are shown in Fig. 11. Only
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1
Fig. 12 Recycling test of Au –Pd1.5-CTFs in the catalytic oxidative clea-
vage of β-O-4 compound 1.
Fig. 10 Catalytic conversion of β-O-4 compound 1 over the catalysts.
Reaction conditions: Substrate, 0.2 mmol; catalyst, 40 mg; CH
3
OH,
5
mL; O , 0.5 MPa; 160 °C; 4 h.
2
Table 3 Recycle of Au
compound 1 over 5 cycles
1
–Pd1.5-CTFs in the catalytic cleavage of β-O-4
a
TOF (h−
)
1
Run
Time (h)
Conv. (%)
1
2
3
4
5
2
2
2
4
6
53
49
50
52
51
19.4
19.0
18.8
10.0
6.5
a
Data extrapolated from the kinetic studies. TOF numbers were calcu-
lated on the basis of the total loading of metals.
pound further cleaved over Au –Pd1.5-CTFs. A cooperative
1
effect between Au–Pd nanoparticles and CTFs was necessary
for the oxidative cleavage of the β-O-4 lignin models.
Recycling experiments
The recycling test of Au –Pd -CTFs for the oxidative cleavage
1
1.5
7
2,73
of β-O-4 compound 1 is shown in Fig. 12.
As seen from the
repeated experimental study, the catalyst retained high cata-
Fig. 11 Catalytic oxidative cleavage of ketone compound 7 over the lytic activity in the first three cycles, and an obvious activity
catalysts. Reaction conditions: Substrate, 0.2 mmol; catalyst, 40 mg;
decrease gradually occurred during the fourth and fifth runs.
3 2
CH OH, 5 mL; O , 0.5 MPa; 140 °C; 2 h.
Meanwhile, initial TOFs extrapolated from the kinetic studies
decreased from 19.4 to 6.5 h− (Table 3). Through the TEM
1
characterization of the Au –Pd1.5-CTF catalyst reused five times
1
CTFs had a high reactivity towards the oxidative cleavage of
ketone compound 7, with a conversion of 84% and a cleavage
selectivity of 85% under the present conditions. Whether the
(
Fig. S1†), an increase in the Au–Pd particle size from the
initial 4.3 nm to 7.1 nm was observed, which might be one
reason for the catalytic performance decline.
support was SiO
2
, Al
2
O
3
2 2 3 4
, TiO , ZrO , XC-72R or C N unmodi-
fied or with the Au –Pd
bimetallic catalyst, all expressed
1
1.5
rather low values for the conversion and cleavage selectivity.
Conclusions
Combined with the above results, we can confirm that Au –
1
Pd1.5 nanoparticles had the ability to oxidize C –OH of β-O-4 The CTF supported Au–Pd nanoparticles, with a special Au/Pd
α
compound 1 to ketone 7, and then, the formed ketone com- molar ratio, were developed to have high efficiency in the cata-
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lytic oxidative cleavage of β-O-4 lignin models and applied to
the catalytic conversion of β-O-4 linkages in organosolv lignin.
Compared with monometallic Au-CTFs and Pd-CTFs, which
had no catalytic activities towards the conversion of β-O-4
lignin models, bimetallic Au–Pd nanoparticles supported on
CTFs with an Au/Pd molar ratio between 1 : 1 and 1 : 1.5 pro-
ceeded efficiently with the highest catalytic performances. The
synergistic effect between Au and Pd and the cooperation of
Au–Pd nanoparticles with support CTFs facilitated the cleavage
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There are no conflicts to declare.
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This work was supported by the National Natural Science
Foundation of China (Grant No. 21790331 and 21603218) and
the Strategic Priority Research Program of the Chinese
Academy of Sciences (XDA21030400 and XDB17020300). This
work was dedicated to the 70th anniversary of the Dalian
Institute of Chemical Physics.
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