Highly Active and Selective RuPd Bimetallic NPs for the Cleavage of the Diphenyl Ether C-O Bond

Article abstract of DOI:10.1021/acscatal.8b03253

The cleavage of C-O linkages of aryl ethers into aromatic platform compounds is a challenging reaction but of great importance for the sustainable future. Herein, we reported the efficient H₂-assisted C-O bond cleavage of diphenyl ether (DPE) in aqueous phase over ultrasmall RuPd bimetallic nanoparticles (NPs) supported on amine-rich silica hollow nanospheres (NH₂-SiO₂). RuPd₅/NH₂-SiO₂ with TOF of 172 h^-1 and C-O cleavage selectivity of 99% outperformed the corresponding monometallic counterparts and is among the most active solid catalysts for C-O bond cleavage of DPE. The control experiments and characterization results showed that the effective isolation of Ru sites and optimized H₂ dissociation ability mainly contributed to the enhanced catalytic performance of RuPd bimetallic NPs, in which Ru and Pd worked cooperatively with Ru sites for DPE activation and Pd sites for H₂ dissociation. The alloying of two or multiple metal atoms provides an efficient approach for designing high-performance catalysts for chemical transformations.

Full text of DOI:10.1021/acscatal.8b03253

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Article
Highly Active and Selective RuPd Bimetallic NPs
for the Cleavage of Diphenyl Ether C-O bond
Miao Guo, Juan Peng, Qihua Yang, and Can Li
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 22 Oct 2018
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Highly Active and Selective RuPd Bimetallic NPs
for the Cleavage of Diphenyl Ether C-O bond
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Miao Guo,a,b Juan Peng,a,b Qihua Yang,a* Can Lia*
aState Key Laboratory of Catalysis, iChEM, Dalian Institute of
Chemical Physics, Chinese Academy of Sciences, Dalian 116023,
People’s Republic of China
bUniversity of Chinese Academy of Sciences, Beijing 100039, Peopl
e’s Republic of China
ABSTRACT: The cleavage of C-O linkages of aryl ethers into aromatic platform compounds is
a challenge reaction but of great importance for sustainable future. Herein, we reported the
efficient H2 assisted C-O bond cleavage of diphenyl ether (DPE) in aqueous phase over
ultrasmall RuPd bimetallic NPs supported on amine-rich silica hollow nanospheres (NH2-SiO2).
RuPd5/NH2-SiO2 with TOF of 172 h-1 and C-O cleavage selectivity of 99% outperformed the
corresponding monometallic counterparts and is among the most active solid catalysts for C-O
bond cleavage of DPE. The control experiments and characterization results showed that the
effective isolation of Ru sites and optimized H2 dissociation ability mainly contributed to the
enhanced catalytic performance of RuPd bimetallic NPs, in which Ru and Pd worked
cooperatively with Ru sites for DPE activation and Pd sites for H2 dissociation. The alloying two
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or multi metal atoms provides an efficient approach for designing high performance catalysts for
chemical transformations.
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KEYWORDS: biomass, RuPd, bimetallic alloy, diphenyl ether, hydrogenolysis, hydrolysis.
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1. INTRODUCTION
Lignin composing nearly 30% of non-fossil organic carbon on Earth is an important aromatic
biopolymer.[1] Transformation of lignin and its derivatives into fuels and chemicals is of great
importance for sustainable future.[2] Up to date, cleavage of C-O linkages, accounting for two-
thirds to three-quarters of all linkages in lignin, using H2 over metal catalysts is regarded as an
effective way to transform lignin into aromatic platform compounds.[3]
OH
OH
hydrogenolysis
hydrogenation
+
+
O
C-O bond cleavage
OH
OH
hydrogenation
hydrolysis
2
2
OH
C-O bond cleavage
+
O
O
hydrogenation
Scheme 1. Summarized possible reaction pathways of H2 assisted C-O bond cleavage of
diphenyl ether in water on the basis of literature reports.
The 4-O-5 linkage (bond dissociation energy of 314 kJ mol-1) is among the strongest and
abundant C-O bonds in lignin.[1b,4] Diphenyl ether (DPE) was generally selected as the simplest
model compound of 4-O-5 linkage to investigate the C-O bond cleavage chemistry. Hydrogen
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assisted DPE cleavage over homogeneous or heterogeneous metal catalysts is very complicated.[5]
Generally, it involves competitive hydrogenolysis, (reductive) hydrolysis and hydrogenation on a
heterogeneous metal surface in water (Scheme 1).[5b,5c]
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Due to the complicated reaction pathways, selective cleavage of C-O bond of DPE is very
challenge. One of the breakthroughs was made by Sergeev et al. and they obtained 99%
selectivity to C-O bond cleavage using N-heterocyclic carbene complex for hydrogenolysis of
DPE in the presence of NaOtBu.[6] In view of the product separation, catalysts handling and
reusability, a heterogeneous catalytic process is more attractive. Solid catalysts based on Pd,[5b,7]
Pt,[8] Rh,[9] Ru,[10] Ni,[5c,11] and bimetallic catalysts[12] were developed for cleavage the C-O bond
of DPE in traditional solvents or in the supercritical CO2-H2O medium. TiN-Ni was reported to
be efficient for selective hydrogenolysis of diaryl ethers, however, only 58% selectivity to C-O
bond cleavage product was obtained using a batch reactor at 125 oC for DPE.[11d] Lercher et al.[5b]
reported reductive hydrolysis of DPE with C-O bond cleavage selectivity of 90% (88% reductive
hydrolysis selectivity and 2% hydrogenolysis selectivity) using Pd/C as a catalyst at 200 C and
40 bar H2. Though different types of solid catalysts have been reported, their selectivity and
activity towards C-O bond cleavage of DPE still need to be further improved, especially under
mild reaction conditions.
To improve the selectivity of C-O bond cleavage, the hydrogenation of aromatic ring of aryl
ether should be inhibited due to the fully or partially hydrogenated dimeric compounds
((cyclohexyloxy)benzene (CHPE) and dicyclohexyl ether (DCHE) in DPE transformation) do
not easily go further C-O bond cleavage.[5c,11a] The straightforward approach to improve C-O
bond cleavage selectivity of DPE is to tune the electronic and surface structure of metal
nanoparticles (NPs) by alloying.[13] Bimetallic nanocatalysts usually exhibit improved catalytic
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performance over their monometallic counterparts because the combination of two metal atoms
could not only induce electronic and geometric effect but also generate the synergistic effect.[14]
Previously, the promotion effect of Ni-based bimetallic nanocatalysts[10d,12b,12d] have been
reported for C-O bond cleavage of aryl ether, however, the activity and selectivity is still not
very high possibly due to the intrinsic property of Ni.
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Herein, RuPd bimetallic nanocatalysts were chosen for C-O bond cleavage of DPE
considering that Ru NPs are very active in hydrogenolysis reactions and Pd NPs possess high H2
activation ability.[15] The RuPd/NH2-SiO2 exhibited remarkably enhanced activity and selectivity
in comparison with Ru/NH2-SiO2 and Pd/NH2-SiO2. The alloying effect on the catalytic
performance of RuPd/NH2-SiO2 was elucidated using RuAu/NH2-SiO2 as the control catalysts
and characterizations such as the XPS, in situ FT-IR of CO adsorption and H2-D2 exchange
experiments.
2. RESULTS AND DISCUSSION
Our group reported previously that amine groups have high binding affinity for metal cations
to afford ultrafine metal NPs.[16] Here, amine-rich silica hollow nanospheres (NH2-SiO2) were
used as supports, which were prepared via one-pot synthesis method with tetraethoxysilane
(TEOS) and (3-aminopropyl)triethoxysilane (APTES) as silane precursors. The characterization
results showed that NH2-SiO2 hollow nanospheres with particle size of 100~200 nm had the
Brunauer-Emmett-Teller (BET) surface area of 156 m2 g-1, pore size of 15 nm and N content of
3.85 mmol g-1 (Figure S1-S3 and Table S1).
RuPdx bimetallic NPs were deposited on NH2-SiO2 by the wet impregnation method using
NaBH4 as reductant (x denotes Pd/Ru molar ratio). ICP-AES analysis showed the metal contents
of RuPdx/NH2-SiO2 were almost the same as expected (Table S2). The TEM images clarified the
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uniform distribution of RuPdx NPs on NH2-SiO2 (Figure 1, Figure S4 and Figure S5). The
particle size of Ru was 2.5 nm and RuPdx had relatively smaller particle size of 2.0 to 2.2 nm,
which is possibility due to the strong interaction between the amine groups and metal cations.[17]
In the following discussion, the influence of particle size could be excluded. The High-angle
annular dark field scanning transmission electron microscopy (HAADF-STEM) image of the
representative RuPd5/NH2-SiO2 further confirmed that RuPd5 NPs (2.1 nm) are uniformly
dispersed on NH2-SiO2 (Figure 1D). The energy dispersive X-ray spectroscopy (EDS) results
also showed that the Ru and Pd are homogeneously distributed on NH2-SiO2 (Figure 1F and
1G). The lattice fringes of RuPd5/NH2-SiO2 is 0.220 nm, which is in agreement with the (111)
crystal plane of Pd. It seems that the face centered cubic packing structure of Pd is unaffected by
alloying with Ru, possibly because of large difference in reduction kinetics and big miscibility
gap between Pd and Ru.[18] EDS line scanning analysis verified that the Ru atoms were enriched
on the surface of RuPd5 NPs (Figure S7).
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All catalysts are active toward C-O bond cleavage of DPE with monomers (benzene,
cyclohexane, cyclohexanol, cyclohexone and phenol) as main products and dimers, cyclohexyl
ether (CHPE) and dicyclohexyl ether (DCHE), as by products (Figure 2A and Table S3). CHPE
and DCHE were generated by hydrogenation of aromatic ring of DPE without C-O bond
cleavage. Compounds containing one C6 ring structure derived from C-O bond cleavage were
denoted as monomers. In the following discussion, the selectivity to C-O bond cleavage
designated monomers selectivity. Ru/NH2-SiO2 afforded 36% conversion with 94% selectivity to
monomers and Pd/NH2-SiO2 gave only negligible conversion (6%) and low selectivity (36%).
RuPdx/NH2-SiO2 (x = 1, 5, 9) exhibited obviously enhanced activity over Ru/NH2-SiO2 or
Pd/NH2-SiO2, showing the positive effect of alloying Ru and Pd with appropriate Pd/Ru ratio.
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The apparent activation energies of H2 assisted DPE cleavage were measured to be 49 kJ mol-1
and 43 kJ mol-1 respectively for Ru/NH2-SiO2 and RuPd5/NH2-SiO2 on the basis of the Arrhenius
plots (Figure S9), further confirming the above results. The selectivity to monomers of Ru/NH2-
SiO2 and RuPdx/NH2-SiO2 (x = 0.33, 1) is similar and varied in the range of 94-96%.
RuPdx/NH2-SiO2 (x = 5, 9) afforded the highest monomers selectivity of 99%. RuPd20/NH2-SiO2
only afforded selectivity of 87%. This suggests that the high monomers selectivity could be
obtained within a narrow range of Pd/Ru ratio.
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A
C
B
D
1
2 3 4
nm
1
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nm
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2 3
nm
E
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H
G
Ru
Pd
Figure 1. TEM images of (A) Ru/NH2-SiO2, (B) Pd/NH2-SiO2 and (C) RuPd5/NH2-SiO2. (D)
and (E) The HAADF-STEM image of RuPd5/NH2-SiO2. (F) Ru and (G) Pd STEM-EDS maps
for the panel E of RuPd5/NH2-SiO2. (H) Reconstructed overlay images of the maps shown in
panels F and G: green, Ru; red, Pd. The insets in (A), (B) and (C) show particle size distribution
and the inset in (D) for lattice fringe (scale bar 50 nm).
Due to the low activity of Pd/NH2-SiO2, the apparent TOF values (TOFRu) were calculated on
the basis of the Ru contents. As shown in Figure 2B, the TOF increased sharply as Ru contents
decreased. RuPd5/NH2-SiO2 with TOF of 172 h-1 and up to 99% selectivity to C-O bond cleavage
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is the most active and selective solid catalyst ever reported for catalytic cleavage of DPE (Table
S4). The catalytic cleavage of DPE was also performed at 80 C (Figure 2B and Table S5). At
this temperature, Pd/NH2-SiO2 showed almost no activity, while RuPdx/NH2-SiO2 could
efficiently catalyze the reaction with RuPd5/NH2-SiO2, suggesting that Ru was the active site for
DPE cleavage. The activity and selectivity of RuPdx/NH2-SiO2 at 80 C followed the same
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sequence as those at 110 C, further confirming the enhancement effect of RuPd alloy.
100
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50
B
A
TOF for RuPdx at 110 oC
TOF for RuPdx at 80 oC
TOF for RuAux at 110 oC
Conv.
for RuPdx
Conv.
for RuAux
Sel.
Sel.
80
60
40
20
0
0
0
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0
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100
Ru content (mol%)
Ru content (mol%)
D
100
50
0
C
2052
1904
e
d
2165
c
b
a
1990
2117
2200
2100
2000
1900
1800
1700
Pd
RuPd
Ru
RuAu
Au
Wavenumber (cm-1)
Figure 2. (A) Variation of the DPE conversion and monomers selectivity obtained from
RuMx/NH2-SiO2 (M = Pd or Au) catalysts as a function of Ru contents (mol%). (B) The relation
of TOF with M/Ru (M = Pd or Au) ratio in aqueous H2 assisted C-O bond cleavage of DPE over
RuMx/NH2-SiO2. (C) In situ FT-IR spectra of CO adsorption on (a) Ru/NH2-SiO2, (b)
RuPd0.33/NH2-SiO2, (c) RuPd1/NH2-SiO2, (d) RuPd5/NH2-SiO2 and (e) Pd/NH2-SiO2. (D) H2-D2
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exchange reactions results on Pd/NH2-SiO2, RuPd5/NH2-SiO2, Ru/NH2-SiO2, RuAu5/NH2-SiO2
and Au/NH2-SiO2.
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Generally, the alloy effects may alter the geometric and electronic properties of the metal
NPs.[19] Firstly, the binding energy of Pd 3d and Ru 3p are measured using the X-ray
photoelectron spectroscopy (XPS) technique (Figure S9, Table S7). In comparison with
Ru/NH2-SiO2, the Ru 3p binding energies of RuPdx/NH2-SiO2 shifted to lower energy. The blue
shift of Pd 3d binding energies of RuPdx/NH2-SiO2 (x = 0.33, 1) was observed in comparison
with Pd/NH2-SiO2. The Ru0 content increased gradually along with the Pd content and reached a
plateau at RuPd5 and the Pd0 concentration for RuPd0.33 and RuPd1 is much lower, indicating Pd
greatly promoted the reduction of Run+.[20] The XPS data indicated the electron transfer from Pd
to Ru and the electronic state of RuPdx NPs differs from those of monometallic NPs because of
the strong interaction of Ru and Pd.[20a,21]
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The surface structure and composition of RuPdx/NH2-SiO2 which may affect their catalytic
performance[22] were investigated using in situ FT-IR of CO adsorption considering that the C-O
stretching frequency is sensitive to the change on the metal surface where CO is bonded (Figure
2C, Figure S11). For Pd/NH2-SiO2, the bridge-bonded CO on Pd surface could be observed at
1904 cm-1 and the linear-bonded CO occurred at 2052 cm-1. The CO adsorption on Ru/NH2-SiO2
is quite complicated with the bands at 2165, 2117 and 2052 cm-1 assigned to CO bonded on Run+,
the band at 1990 cm-1 derived from linear CO on Ru0 surrounded by Run+ and the weak and
broad band at 1815 cm-1 for the bridge bonded CO on Ru0.[23] With Pd content increasing, the
bands assigned to CO adsorbed on Ru NPs become less discernible accompanied by the
enhancement in the intensity of the CO bands adsorbed on Pd NPs. The bands assigned to CO
adsorbed on Ru surface disappeared completely for RuPdx/NH2-SiO2 (x ≥ 5). This is possibly
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due to the dilution and isolation of Ru by Pd with x ≥ 5. On the basis of the d-band center
theory,[24] the change in the electronic[25] and geometric[26] structure of RuPdx may modulate the
stength of the metal-adsorbate interaction and consquently tailoring their catalytic functions.
The almost linear increase in apparent TOF of RuPdx/NH2-SiO2 with x increasing indicates
that the isolated Ru sites are more active for DPE cleavage. The dilution of Ru by Pd is an
ensemble effect of alloying, which is often observed in the AuPd bimetallic system.[13b] On the
basis of the XPS results, the Pd also modifies the electronic properties of Ru. To verify the role
of Pd in RuPdx/NH2-SiO2, control samples, RuAux/NH2-SiO2 (metal loading content of 4 wt%
and x denotes the Au/Ru molar ratio, the particle size of ~2.2 nm, Figure S6), were prepared.
The EDS results show that the existence of Ru and Au on NH2-SiO2 (Figure S7). Based on the
EDS line scanning, the RuAu alloy may have a Ru surface enriched structure (Figure S8B).[27]
XPS data showed that the binding energies of Ru 3p and Au 4f of RuAux/NH2-SiO2 are
almost similar to corresponding Ru/NH2-SiO2 and Au/NH2-SiO2, showing the weak interactions
between Ru and Au (Table S8).[28] The in situ FT-IR spectra of CO adsorbed on RuAux/NH2-
SiO2 resemble that of Ru/NH2-SiO2 due to the weak adsorption of CO on Au NPs (Figure S11).
The red shift in bands assigned to CO adsorbed on Ru0 surrounded by Run+ was observed for
RuAux/NH2-SiO2 (x = 5, 9 and 20), possibly due to the weak interaction of dipole-dipole
coupling between CO molecules on metal surface.[29] The intensity of CO adsorption band
decreases as Ru content decreasing and the band disappears almost completely for RuAu5/NH2-
SiO2, suggesting that the effective isolation of Ru by Au was achieved with x  5. The above
results show that the alloying Ru with Au only induced the site isolation of Ru.[30]
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No activity was observed for Au/NH2-SiO2 in the cleavage of DPE and RuAux/NH2-SiO2
could efficiently catalyze this reaction with monomers selectivity varying in the range of 95% to
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93% (Figure 2A). The activity of RuAux/NH2-SiO2 was much lower than that of RuPdx/NH2-
SiO2. In the contrary to RuPdx/NH2-SiO2, the conversion decreases gradually as the Ru contents
decreases though the TOF of RuAux/NH2-SiO2 (x = 5, 9 and 20) increased with the Ru contents
decreasing. This suggests that the site isolation of Ru is not the main reason for the enhanced
catalytic performance of RuPdx/NH2-SiO2.
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The activation of H2 on metal surfaces was tested using H2-D2 exchange measurement
considering that the H2 activation/dissociation is one of the important element steps in
hydrogenation/hydrogenolysis reactions[31] (Figure 2D). The normalized activity followed the
order of Pd/NH2-SiO2 (100) > RuPd5/NH2-SiO2 (65) > Ru/NH2-SiO2 (2.7) > RuAu5/NH2-SiO2
(0.03) > Au/NH2-SiO2 (~0). The activity of RuPd5/NH2-SiO2 is 24 times that of Ru/NH2-SiO2.
The significantly enhanced H2 activation ability of RuPd5/NH2-SiO2 may be due to the electronic
effect of RuPd alloy and superior hydrogen dissociation capabilities of Pd.[15c, 32] On the contrary,
RuAu5/NH2-SiO2 exhibit inferior catalytic performance due to the low activation ability of the
Au NPs.[33] On the basis of kinetic studies, a 0.64-order dependence of H2 and the large KIE
effect (KH/KD = 3.1) was observed for RuPd5/NH2-SiO2 (Figure S12), suggesting the
dissociation adsorption of H2 may be involved in the rate-limiting step in the hydrogenation and
hydrogenolysis reactions.[11a,34] In correlation of the H2 activation ability and catalytic
performance, it could be seen that RuPd5/NH2-SiO2 with moderate H2 activation ability showed
the highest activity and selectivity. Thus, the high catalytic performance of RuPd5/NH2-SiO2
could be possibility attributed to the combined effect of optimized H2 activation ability and site
isolation of Ru.
With RuPd5/NH2-SiO2 as a model catalyst, the product selectivity as a function of reaction
time was investigated (Table 1). The DPE conversion increased steadily with reaction time and
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approached 99% in 60 min. Benzene, cyclohexanol and cyclohexanone together with a small
amount of phenol were detected in initial 3 min. No phenol was detected after 15 min because of
its high hydrogenation activity on RuPd5/NH2-SiO2. The selectivity to cyclohexane and
cyclohexanol increased while the selectivity to benzene and cyclohexanone decreased as the
reaction time was prolonged. This indicated that benzene, phenol and cyclohexanone were
hydrogenated into saturated products, cyclohexane and cyclohexanol. Only 1% CHPE was
detected throughout the whole reaction process, suggesting that CHPE is not the intermediate for
C-O bond cleavage of DPE. This was further confirmed by poor activity and selectivity of
RuPd5/NH2-SiO2 in the cleavage of CHPE (26% Conv. with 31% Sel.). Furthermore, DCHE
cannot be converted on RuPd5/NH2-SiO2 under current reaction conditions. This could exclude
the reaction pathway via hydrogenolysis/hydrolysis of DCHE.[5c,11a] It can be concluded that the
C-O bond of DPE is directly cleaved on RuPd5/NH2-SiO2.
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Table 1. The C-O bond cleavage of DPE over RuPd5/NH2-SiO2 as a function of reaction time.a
Product Sel. (%) b
T
Conv. (%)
Sel. (%) c
(min)
CH Bz
CHOH
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CHO
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2
PhOH
CHPE DCHE
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0
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~ 0
~1
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0
0
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71
89
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99
> 99
99
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99
69
1
99
0
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0
1
99
aReaction conditions: 110 oC, 0.22 mmol DPE, 0.022 mmol metals, 3 mL H2O, 10 bar H2. b CH
(cyclohexane), Bz (benzene), CHOH (cyclohexanol), CHO (cyclohexanone), PhOH (phenol),
DCHE (dicyclohexyl ether), CHPE ((cyclohexyloxy)benzene). cMonomers selectivity.
The hydrogenolysis of DPE is the main reaction pathway for RuPd5/NH2-SiO2 and Ru/NH2-
SiO2 at the initial time (88% vs. 68%, similar conversion of ~15%) (Figure 3A-B). With the
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reaction time prolonging, the selectivity to hydrogenolysis products decreased accompanied with
the selectivity to hydrolysis products increasing for the two catalysts. Finally, the products ratio
of hydrogenolysis to hydrolysis on RuPd5/NH2-SiO2 and Ru/NH2-SiO2 reached to 3:2 and 2.4:2,
respectively. The higher hydrogenolysis ratio on RuPd5/NH2-SiO2 may be related to its higher H2
dissociation ability and lower aromatic ring hydrogenation ability (Table S11). No conversion
was obtained on RuPd5/NH2-SiO2 in the N2 atmosphere, showing the important role of H2 in the
catalysis. Only hydrogenolysis (65%) and hydrogenation (35%) products was obtained on
RuPd5/NH2-SiO2 using isopropanol as solvent (Table S6). The hydrogenolysis selectivity in
water and in isopropanol on RuPd5/NH2-SiO2 was similar, indicating that the water does not
involve in the hydorgenolysis reaction pathway.
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0
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hydrogenolysis
hydrolysis
hydrogenation
B
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hydrogenolysis
hydrolysis
hydrogenation
Conv.
Conv./%
0
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0
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T (min)
T (min)
C
Hydrogenolysis
Hydrogenation
H
H
Ru
Pd
Ru
Pd
C-O bond
directcleavage
H
Ru
Pd
Hydrogenation
Hydrolysis
H
H
Ru
Pd
Ru
Pd
Figure 3. Reaction profiles of C-O bond cleavage of DPE with the reaction time catalyzed by (A)
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RuPd5/NH2-SiO2 and (B) Ru/NH2-SiO2. Reaction conditions: 110 C, 0.22 mmol DPE, 0.022
mmol metals, 3 mL H2O, 10 bar H2. Selctivity to hydrogenolysis = 2 ╳ (CH + Bz), selectivity to
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hydrolysis = (PhOH + CHO + CHOH) – (CH + Bz). Abbreviation see Table 1. (C) Proposed
reaction pathways for the DPE cleavage over RuPd5/NH2-SiO2 in water.
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Combining the above results, the possible reaction pathway on RuPd5/NH2-SiO2 can be
proposed as follows (Figure 3C). Firstly, H2 is activated on Pd sites to generate active H species.
Meanwhile, DPE is chemisorbed on Ru active sites via strong π interactions between one
aromatic ring and d-states of the metal (possibly the most stable adsorption mode).[15b,35]
Subsequently, the active H species spilled over to Ru sites to attack C-O bond,[36] consequently
resulting in the direct C-O bond scission. The produced monomers transferred to metal surface to
be hydrogenated for the formation saturated cyclohexane and cyclohexanol.
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The organosolv lignin extracted from beech sawdust was used to investigate the catalytic
performance of RuPd alloy. The reaction was performed under H2 atmosphere (30 bar) at 150 oC
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for 10 h. 2D C-1H heteronuclear single quantum correlation (HSQC) NMR experiments were
conducted to monitor the cleavage of the characteristic interconnecting bonds within lignin
(Figure S13). After reaction, the signals which can be ascribed to the phenylcoumaran structures
B (Bα, Bβ and Bγ) and the resinol structures C (Cα, Cβ and Cγ) significantly reduced, showing
RuPd5/NH2-SiO2 could be able to degrade the lignin at mild conditions. However, most of the β-
O-4 linkages A- and A’-signals still present, suggesting the β-O-4 bonds cannot be easily cleaved
under this conditions.
The recycle stability of RuPd5/NH2-SiO2 was tested in the cleavage of DPE (Figure S14). The
result revealed that RuPd5/NH2-SiO2 could be stably reusable for at least 5 times. After 5 cycles,
RuPd5/NH2-SiO2 could still afforded 94% conversion. TEM results shows that no aggregation of
metal NPs could be observed. Furthermore, no traces of leached Ru or Pd species were detected
in the reaction filtrate, demonstrating its good recyclability stability under current conditions.
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In summary, the RuPd bimetallic NPs (2.1 nm) uniformly dispersed on amine-rich silica
hollow nanospheres (NH2-SiO2) could efficiently catalyze the H2 assisted C-O bond cleavage of
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diphenyl ether in H2O even at temperature as low as 80 C. The obvious volcano relationship
between conversion and Ru content suggested the synergistic effect of Ru and Pd. The TOF of
RuPd NPs increased remarkably as the Ru contents decreased, indicating that the isolated Ru
sites were more active than Ru crystallites. The studies show that the unique catalytic
performance of RuPd bimetallic NPs was strongly related with the optimized H2 activation
ability and the site isolation of Ru resulting from the alloying effect. The excellent catalytic
performance and good recyclability make the RuPd5/SiO2-NH2 catalyst attractive for both
fundamental research and practical applications.
4. EXPERIMENT SECTION
Chemicals and agents. All materials were of analytical grade and used as received without
any further purication. Diphenyl ether was purchased from J&K Chemicals. Phenol and decane
were obtained from TCI Chemicals (Shanghai). Fluorocarbon surfactant (FC4) was bought from
YickVic Chemicals (Hong Kong). Cyclohexyloxybenzene were purchered from Fluorochem
(UK). Oxydicyclohexane were synthesized according to the previous method.[5c] (3-
Aminepropyl)triethoxysilane (APTES), tetraethoxysilane (TEOS), cetyltrimethylammonium
bromide (CTAB) and other reagents were purchased from Shanghai Chemical Reagent, Inc. of
the Chinese Medicine Group.
Preparation of amine-rich mesoporous silica hollow nanospheres (NH2-SiO2). Step1.
Tetraethoxysilane (TEOS) (0.25 mL) was added into 23 mL of aqueous solution containing 0.05
g of CTAB and 0.18 mL of NaOH solution (2.0 M) under slow stirring. Then, 0.2 mL of ethyl
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acetate was successively added to the solution, and the synthetic medium was kept at 70 °C for
30 min without stirring. Step 2. 60 mL of aqueous solution (40 mL) with ethanol (20 mL),
CTAB (0.15 g), and NH3·H2O (0.5 mL, 25 wt%) was added to the above mixture. After cooling
down to 38 °C, the reaction mixture was kept at this temperature with stirring for 30 min, then
TEOS (0.25 mL) was added under stirring. The mixture was stirred for 1 h followed by the
addition of 1.5 mL of aqueous solution with FC4 (0.02 g), CTAB (0.04 g), and NH3·H2O (0.1
mL, 25 wt%). Step 3. Then, 1.33 mL of ethanol solution with APTES (0.33 mL) was added
under vigorous stirring. After stirring for 1 h, the reaction mixture was stirred at 80 °C for 10 h
with the stopper open to evaporate the solvent. The powder product was collected by filtration
and dried at room temperature. To remove the surfactant, the as-synthesized materials (1 g) was
dispersed in 120 mL of ethanol (95%) with ammonium nitrate (80 mg) and sirred at 60 °C for 20
min. This process was repeated for three times.
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Preparation of RuPdx/NH2-SiO2 via the impregnation method. Typically, 400 mg of NH2-
SiO2 was dispersed in 2 mL of deionized water in a centrifuge tube under ultrasound. Then
desired amount of Na2PdCl4 and RuCl3 aqueous solution (0.01 g mL-1) was added into the tube
successively. After ultrasound treatment for 10 min, a freshly prepared NaBH4 aqueous solution
(7.5 mg mL-1) was added dropwise. The brownish red colour transformed into dark gray. After
ultrasound treatment for another 30 min, the mixture was filtered and the black powder product
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was washed with deionized water and EtOH for several times. After drying in an oven at 60 C
overnight, RuPdx/NH2-SiO2 was obtained where x refers to molar ratio of Pd per Ru.
RuAux/NH2-SiO2 bimetallic NPs were prepared in a similar method with RuPd5/NH2-SiO2
except that HAuCl4 was used.
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The reaction of diphenyl ether. In a typical experiment, a desired amount of solid catalyst
(0.022 mmol of metals) was added in an ampoule tube, followed by the addition of diphenyl
ether (0.22 mmol) (S/C=10) and 3 mL of water. The ampoule tube was loaded into a stainless
steel autoclave (300 mL) with a thermocouple probed detector. After purging with hydrogen for
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six times, the final pressure was adjusted to 10 bar and the reactor was heated to 110 C with
vigorous stirring. After reaction, the reactor was quenched to ambient temperature using cooling
water, and the organic products were extracted using ethyl acetate (5 mL). The water phase and
oil phase were respectively analyzed by an Agilent 7890B GC equipped with an Agilent J&W
GC HP-INNOWax capillary column (30 m × 0.32 mm × 0.5 μm). Conversion and selectivity
were determined using n-decane and EtOH as the internal standard respectively for oil and water
phase and defined as eqs 1-5. The monomers refer to benzene, cyclohexane, phenol,
cyclohexanone and cyclohexanol. The dimers refer to (cyclohexyloxy)benzene and dicyclohexyl
ether. The carbon balance for all the reactions was > 90%.
6 *(The amount of monomers/mmol) 12 *(The amount of dimers/mmol)
12 *(The initial amount of diphenyl ether/mmol)
Conversion(%) 
*100
(1)
(2)
6 *(The amount of benzene and cyclohexane/mmol)
Selectivity to hydrogenolysis(%)  2 *
*100
6 *(The amount of monomers/mmol) 12 *(The amount of dimers/mmol)
12 *(The amount of dimers/mmol)
(3)
(4)
Selectivity to hydrogenation(%) 
*100
6 *(The amount of monomers/mmol) 12 *(The amount of dimers/mmol)
Selectivity to hydrolysis(%) 
6 *(The amount of phenol,cyclohexanone and cyclohexol/mmol) 6 *(The amount of benzene and cyclohexane/mmol)
6 *(The amount of monomers/mmol) 12 *(The amount of dimers/mmol)
*100
(5)
Selectivity to the C - O bond(%)  (Selectivity to hydrogenolysis/%) (Selectivity to hydrolysis/%)
Characterization
Transmission electron microscopy (TEM) was performed on a HITACHI 7700 at an
acceleration voltage of 100 kV. Scanning transmission electron microscopy (STEM) was
undertaken on a HITACHI S-5500 scanning electron microscope operating at an acceleration
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voltage of 1-20 kV. The EDS mapping were obtained using a JEOL F200. 2D 13C-1H
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heteronuclear single quantum correlation (HSQC) NMR spectra were recorded at 25 C using a
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Bruker Avance III HD 700 MHz spectrometer. 60 mg sample was dispersed in 0.5 mL of
DMSO-d6. The central DMSO solvent peak was used as an internal chemical shift reference
point (δC/δH 39.52/2.49).[37] The N2 adsorption-desorption experiments were performed at 77 K
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using a Micromeritrics ASAP 2020. Samples were degassed at 120 C for 6 hours prior to the
measurements. FT-IR spectra were collected with a Nicolet Nexus 470 IR spectrometer (KBr
pellets were prepared) in the range of 400-4000 cm-1. The thermogravimetric analysis (TGA)
was performed using a NETZSCH STA 449F3 analyzer from 30 oC to 900 oC at a heating rate of
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10 C ∙min-1 under air atmosphere. The metal content was determined by PLASAMSPEC-II
inductively coupled plasma atomic emission spectrometry (ICP-AES). X-ray photoelectron
spectroscopy (XPS) was recorded on VG ESCALAB MK2 apparatus using Al Kα (hλ = 1486.6
eV) as the excitation light source. The peaks in the spectra were fitted by using the shareware
program XPS-PEAK with Gaussian-Lorentzian peak shapes and a Shirley background. The
binding energies are corrected with reference to the C 1s line at 284.6 eV.
In situ FT-IR of CO adsorption were collected on a Bruker EQUINOX 55 infrared
spectrometer with a DTGS detector. Prior to CO chemisorption, as-prepared samples were
pretreated at 200 oC under flowing H2 atmosphere (20 mL min-1) for 1 h, followed by evacuation
at room temperature for 1 h, then cooled down to room temperature. After pretreatment, a
background spectrum was collected from samples and subtracted automatically from subsequent
spectra. CO adsorption experiments were carried out sequentially on a single sample. Gas-phase
CO spectra were collected at the same pressure and subtracted from the corresponding sample
spectra.[15d]
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H2-D2 exchange reactions were carried out in a flow quartz reactor at 22 oC.[38] The formation
rate of HD was measured by mass signal intensity (ion current). Before the test, the catalysts
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were heated in H2 (10 mL min-1) at 200 C for 20 min. After the sample was cooled at room
temperature, D2 (10 mL min-1) mixed with H2 was passed through the sample. The gas hourly
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space velocity (GHSV) is 1.16×108 mL·h-1·gmetal-1. Under these conditions, the H2-D2 exchange
conversions were always kept below 15% for calculation of turnover frequency (TOF).[38b]
Products (HD, H2, and D2) were analyzed with an online mass spectrometer (GAM200,
InProcess Instruments). The mass/charge ratio (m/z) values used are 2 for H2, 4 for D2, and 3 for
HD. The background HD exchanges from the corresponding support were deducted from the
results.
Characterization of NH2-SiO2.
The results of TEM and HRSEM characterizations show that NH2-SiO2 is composed of mono-
dispersed nanospheres with particle size in the range of 100~200 nm and has a rough surface
formed with connected nanoparticles with size of 10-25 nm (Figure S1). A broken nanosphere
chosen intentionally clearly confirmed the hollow nanostructures (Figure S1B). NH2-SiO2 has
the BET surface area of 156 m2 g-1 with pore size distribution centered at 15 nm derived mainly
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from inter void space of the connected nanoparticles (Table S1). The solid-state C CP/MAS
NMR spectrum displays the chemical shifts at 8.8 ppm, 20.8 ppm and 40.9 ppm, which could be
assigned to C1, C2 and C3 of -C1H2C2H2C3H2NH2, suggesting the successful incorporation of
amine groups (Figure S2A). The solid-state 29Si NMR spectrum exhibits both T sites (-69.9 ppm
for T3) and Q sites (-101.7 ppm for Q3 and -113.8 ppm for Q4) with T/(Q+T) ratio of about 22.4
%, showing the integration of APTES in NH2-SiO2 hollow nanospheres (Figure S2B). FT-IR
spectrum of NH2-SiO2 clearly show the vibration peaks assigned to propyl groups at 2937 cm-1
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(υC-H ) and 1385 cm-1 (δC-H) and amine group at 3441 cm-1 (υN-H) and 1558 cm-1 (δN-H) (Figure
S3A). On the basis of the TG analysis (Figure S3B), the weight loss of 22.9 wt% in the range of
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250 to 750 C could be assigned to the content of amine groups, which corresponds to amine
amount of 3.95 mmol g-1. The C, H, N elemental analysis affords N content of 5.4 wt%. The
calculated amine amount is 3.85 mmol g-1, which is consistent with the TG analysis result. The
above results confirm the formation of porous silica hollow nanospheres rich in amine group via
one-pot synthesis method.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the xxx.
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TEM and STEM images, C CP/MAS NMR spectrum, Si MAS NMR spectrum, FT-IR
spectrum, TG analysis results and physical parameters of NH2-SiO2, TEM images of
RuPd0.33/NH2-SiO2, RuPd1/NH2-SiO2, (C) RuPd9/NH2-SiO2 and (D) RuPd20/NH2-SiO2,
EDS results of RuAu5/NH2-SiO2, arrhenius plots of Ru/NH2-SiO2 and RuPd5/NH2-SiO2,
XPS results of RuPdx/NH2-SiO2 and RuAux/NH2-SiO2, in situ FT-IR spectra of CO
adsorption results on RuAux/NH2-SiO2, RuPd9/NH2-SiO2 and RuPd20/NH2-SiO2, the results
of H2-D2 exchange reactions and the reusability results.
AUTHOR INFORMATION
Corresponding Author
yangqh@dicp.ac.cn;
canli@dicp.ac.cn;
Author Contributions
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M. G. did all the experiment and wrote the manuscript, J. P. designed and synthesized the
material, and Q. Y. and C. L. made the research plan, supervised the research, and organized the
paper.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was financially supported by the National Key R&D Program of China,
2017YFB0702800, the Natural Science Foundation of China (No. 21733009, 21232008,
21621063) and the Strategic Priority Research Program of the Chinese Academy of Sciences
Grant No. XDB17020200.
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