Efficient Cu catalyst for 5-hydroxymethylfurfural hydrogenolysis by forming Cu-O-Si bonds

Article abstract of DOI:10.1039/d0cy01032d

Selective hydrogenolysis of C-O bonds of biomass derived precursors has been identified as a promising and essential way to produce fuel additives. Supported transition metals were explored to give efficient reactivity commonly based on a bifunctionality strategy. Here, we report that covalent bonding between SiO2 and Cu features a homologous bifunctional catalyst with metallic Cu and Lewis acidic Cu cations. The catalyst gave superior reactivity for the conversion of 5-hydroxymethylfurfural into 2,5-dimethylfuran. Lewis acidic cations had more predominant roles than metallic sites for C-O hydrogenolysis by stretching and dissociating C-O bonds, whereas they remained inactive for CC bonds. The results rationalize the valence-state-sensitive catalysis for chemistry involving C-O cleavage. The covalent metal-O-Si bonding provides an alternative for developing efficient catalysts since silicates with such a feature are versatile in nature.

Full text of DOI:10.1039/d0cy01032d

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ISSN 2044-4761
PAPER
Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
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DOI: 10.1039/D0CY01032D
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Efficient Cu catalyst for 5-hydroxymethylfurfural hydrogenolysis
by forming Cu-O-Si bond
Yifeng Zhu,a Xiao Kong,b,* Bo Peng,c Luping Li,b Zhen Fangb and Yulei Zhua,d
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Selective hydrogenolysis of C-O bonds of biomass derived precursors has been identified as a promising and essential way
to produce fuel additives. Supported transition metals were explored to give efficient reactivity commonly based on
bifunctionality strategy. Here we report that covalent bonding between SiO2 and Cu features a homologous bifunctional
catalyst with metallic Cu and Lewis acidic Cu cations. The catalyst gave superior reactivity for 5-hydroxymethylfurfural to
2,5-dimethylfuran. Lewis acidic cations contributed more predominant roles than metallic sites for C-O hydrogenolysis by
stretching and dissociating C-O bonds, whereas they remained inactive for C=C bonds. The results rationalize the valence-
state-sensitive catalysis for chemistry involving C-O cleavage. The covalent metal-O-Si bonding provides an alternative for
developing efficient catalysts since silicates with such feature are versatile in nature.
catalysts.10-13 Recently, some works reported that Cu
phyllosilicate formed during the preparation of Cu/SiO2
catalyst, which can promote the hydrogenation ability.14-16
Introduction
Hydrogenolysis of C-O bond is an essential way to remove
oxygen from biomass precursors for biofuels production. For
example, C-O hydrogenolysis of 5-hydroxymethylfurfural
(HMF) can afford 2,5-dimethylfuran (DMF), which is an
alternative and sustainable fuel with high energy density (30
kJ/cm3) and octane number (RON=119).1-3 Supported
transition metals have been widely explored for C-O
hydrogenolysis based on bifunctionality strategy, including (1)
modification of metals by alloying and (2) utilization of the
support by introduction of oxygen vacancy or acid sites.4-7
Yet, the achievement of selective C-O activation by more
active, low temperature and cheap catalysts are still needed.
SiO2 is a cheap and widely used support. The inertness of its Si-
O-Si framework generally leads to the minimal interaction with
guest species, where the metal mainly interacts with SiO2 by
secondary bonds, such as Van der Vaals and electrostatic
interactions. The catalytic performance was not sufficient due
to the poorly dispersed/modulated metal species. SiO2 is also
considered to be ineffective in cleavage and hydrogenolysis of
C-O bonds, due to the absence of acidic and basic sites.8-9 The
modulation of chemical bonding of oxide and metal is a
feasible way for regulating the structures and states of
catalytically active centers, preparing effective metal
Copper phyllosilicate has lamellar structure composed of
alternate layers of SiO4 tetrahedra and discontinuous layers of
CuO6 octahedra, in which Cu-O-SiOx moieties are readily
available. Copper phyllosilicate derived Cu0/Cu2O·SiO2 catalyst
show good activity in furfural hydrogenation and a yield of 2-
methylfuran (90%) was obtained.17 Moreover, Wang et al.
found that the formation of copper phyllosilicate was more
kinetically favored over silica sol with high hydroxyl groups in
the preparation of Cu/SiO2 catalyst.18 The interaction between
Cu and Si in Cu-O-Si bond is proposed to play important role to
modulate the Cu0 and Cu+ sites, stabilize Cu sites and improve
hydrogenation ability.
Herein, the strong covalent bonding between SiO2 and Cu
is identified, and the catalysis efficiency for C-O hydrogenolysis
is effectively boosted. This work rationalizes the valence-state-
sensitive catalysis for chemistry involving C-O cleavage. The
electrons in the hybridized orbital of Cu-O-Si bond shift from
Cu to Si-O moiety for the calcined catalyst. In contrast, because
of the changing orbital hybridization of Cu, Cu atoms show
higher affinity to electrons for the reduced catalysts. The
bonding resulted in substantial amounts of homologous Cuδ+
proximal to well-dispersed Cu0 sites on the reduced catalysts.
The Cuδ+ sites are Lewis acidic because of the high electron
affinity. The mechanistic studies revealed that Cuδ+ sites play
more predominant roles in weakening and dissociating C-O
bonds, collaborating with Cu0 sites for hydrogenolysis. As a
result, the efficiency for the hydrogenolysis of HMF to DMF
has been significantly enhanced. The covalent metal-O-Si
bonding is expected to be a facile strategy for designing
efficient metal-Lewis acid centers that does not require the
addition of promoters and increase of structural complexity.
a. State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan 030001, P. R. China.
b. College of Engineering, Nanjing Agricultural University, Nanjing 210031, P.R.
China.
c. Department of Biological Systems Engineering, Washington State University,
Richland, WA 99354-1671, USA.
d. Synfuels China Company Ltd., Beijing 101407, P. R. China.
†Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
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Journal Name
Results and discussion
View Article Online
DOI: 10.1039/D0CY01032D
Table 1 The physiochemical properties of catalysts.
SBET (m2/g)
Cu2+
dispersion
(%)b
Cu content
(wt. %)a
Cu dispersion SCu
SCu0
(mmol/g)e
SCuδ+
(mmol/g)e
Catalysts
(%)c
(mmol/g)d
256.3
305.0
391.7
481.2
484.2
81.7
CuSi-PS-5
5.0
73.8
75.6
81.7
75.6
68.2
19.3
62.7
61.5
48.4
34.8
32.2
19.9
0.49
0.97
1.48
1.61
1.91
0.77
0.1
0.39
0.75
1.04
1.10
0.97
0.32
CuSi-PS-10
CuSi-PS-20
CuSi-PS-30
CuSi-PS-40
CuSi-IM-30
10.1
19.4
29.4
37.7
24.6
0.22
0.44
0.51
0.94
0.45
a determined by ICP experiments; b calculated from XPS data of calcined samples; c calculated from XPS data of reduced samples;
d
e
determined by ICP results and Cu dispersions; determined by analysis of Cu LMM spectra and Cu dispersions of reduced
catalysts.
2.1 Cu/SiO2 catalysts with covalent Cu-O-Si features
The impregnated Cu/SiO2 was taken as a reference with a
nominal Cu loading of 30 wt. %, namely CuSi-IM-30, followed
by the similar drying and calcination procedures. Structural
characterizations revealed the presence of monoclinic CuO
nanoparticles and amorphous SiO2 for the calcined CuSi-IM-30
(Figure S3). As shown in Table 1, the formation of layered
phyllosilicate also contributes the increase of surface areas
ranging from 256.3 to 484.2 m2/g, while the CuSi-IM-30 only
has a surface area of 81.7 m2/g.
To understand the possibility of strong covalent bonding
between metals and SiO2, covalent bond classification (CBC)
gave an appropriate framework for metal oxides, for which
defined as L, Z and X types (Figure 1B).23 This depends on the
numbers of electrons (0, 1, and 2) of ligand species
contributed to the covalent bonds. During the assembly, cupric
We prepared the Cu/SiO2 composites (CuSi-PS-x, x indicates
the Cu weight loading, x= 5, 10, 20, 30, 40) by an ammonia
evaporation method (Table 1). At a base condition, the disiloxy
Si-O-Si bond would undergo nucleophilic attack by OH- anions,
producing silanol Si-OH groups and then Si-O- anions.19-21 In
the meantime, ammonia was used as the ligand to keep Cu2+
cations as the soluble species, which can be facilely removed
by thermal evaporation. The cupric ammonia complexes under
basic conditions are expected to coordinate with and assembly
to the surface anions of SiO221, and eventually form the
lamellar copper phyllosilicate by selective removal of ammonia
ligand (Figure 1A). The phyllosilicate structure was proved by
XRD results and characteristics (670 and 1034 cm-1)14 in
infrared (IR) spectra (Figure S1 and S2). This phyllosilicate is
assembled by the [CuO6] octahedron and [SiO4] tetrahedron
units through Cu-O-Si bonding with H atoms saturate the
uncoordinated sites.22
ammonia complex evolved into
a
[CuO6] octahedral
coordination with sp3d2 hybridization, for example
[Cu(NH3)x(H2O)4-x(OH)2]. If the self-polymerization to give
Cu(OH)2 is prevented, Cu atoms mainly give orbitals for O
atoms of ligands, such as [SiO4], and thus gave a L type feature
for Cu-O bonding. Similarly, the O atoms of [CuO6] donate
another lone pair for the sp3 hybrid orbitals of Si atoms and O-
Si bonds.
The polar covalent Cu-O-Si bonding (Figure 1C and 1D) for
calcined CuSi-PS-x is evidenced by comparing XPS spectra of
CuSi-PS-30 with CuSi-IM-30. Cu2p features of CuSi-PS-30
significantly shifted to ~1.8 eV higher binding energy (B. E.)
than that of CuSi-IM-30. The opposite downward tendencies
(~0.8 eV) were observed for O1s and Si2p. The above cases
also fit for CuSi-PS-x samples with various Cu loadings (Figure
S4, S5, and S6). The polar Cu-O-Si bonding is rationalized by
hybrid orbital theory for which the energy of hybrid orbitals is
determined by the atomic orbitals. Four-coordinated Si
tetrahedron of phyllosilicate is hybridized in sp3 and six-
coordinated Cu octahedron is hybridized in sp3d2. Each sp3
hybrid orbital has 25% s character while sp3d2 has 17% s
character. With the increase of s character, the energy of
orbitals decreases and its affinity for electrons increases.24
Therefore, the electron density of Cu atoms is decreased with
Figure 1. (A) Assembly of CuO6 octahedron and SiO4
tetrahedron gives the lamellar copper phyllosilicate. (B) CBC
model (L refers to Lewis base bonding with the empty orbital
of metals and donating two electrons. Z is Lewis acid bonding
with the electron-rich species. X donates one electron to the
surface sites). (C) Illustrations for the polar covalent Cu-O-Si
bonding for calcined and reduced CuSi-PS-x catalysts. (D) Cu2p,
O1s and Si2p spectra of calcined CuSi-PS-30 and CuSi-IM-30. (E)
Cu2p, O1s and Si2p spectra of reduced CuSi-PS-30 and CuSi-
IM-30.
2 | J. Name., 2012, 00, 1-3
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the bonding electrons excited more difficult. The electrons
shift towards Si-O moieties, by causing the shifts of XPS
spectra. This Cu-O-Si bonding is quite stable upon drying and
calcination as evidenced by the almost identical XRD patterns
for CuSi-PS-x catalysts (Figure S7), facilitating the dispersion of
Cu species for calcined CuSi-PS-x catalysts. Quantitative
elemental analysis of surface species by XPS shows that Cu
dispersions of calcined CuSi-PS-x catalysts are greatly higher
than CuSi-IM-30 by the factors of 3.5-4.2 (Table 1). Note that
CuSi-PS-x catalysts with low Cu loadings (e.g., CuSi-PS-5) have
slightly decreased Cu dispersions. This is because the excessive
amounts of amorphous SiO2 partially confined the Cu species
into the bulk and thereby decreased the dispersion. This is
supported by the increase of reduction temperature (CuSi-PS-5)
due to the limitation of H2 diffusion by SiO2 coverage25, as
compared to the CuSi-PS-x with high loadings (Figure S8). The
TPR results also demonstrated that the Cu-O-Si bond will
benefit the Cu dispersion and make the Cu species to be
reduced more easily compared with CuSi-IM-30 catalyst.
Moreover, the covalent Cu-O-Si bonding is preserved and
evolved upon reduction of the CuSi-PS-x catalysts by H2 at 250
oC, as evidenced by the XPS results (Figure 1C and 1E). The
satellite peaks of both reduced CuSi-IM-30 and CuSi-PS-30 at
940-950 eV disappeared, indicating the reduction of Cu2+
species into Cu0 and Cuδ+. Reduced CuSi-IM-30 gave the
Cu2p3/2 and Cu2p1/2 peaks at 932.0 and 952.0 eV respectively,
revealing a typical feature of Cu0 species. This is supported by
the XRD results of reduced CuSi-IM-30 catalyst (metallic Cu
nanoparticles, ~16 nm, Figure S9). The XRD diffractions of
reduced CuSi-PS-x catalysts are rather broad, indicating the
well-dispersed surface Cu species instead of Cu crystallites.
The B. E. of Cu2p of reduced CuSi-PS-30 is 0.9 eV higher than
reduced CuSi-IM-30, which is because that Cu species are
composed of Cu0 and substantial amount of Cuδ+ for reduced
CuSi-PS-30. In compared to reduced CuSi-IM-30, the O 1s and
Si 2p both shifted to higher B. E. for CuSi-PS-30, indicating the
electron density for O and Si atoms decreased. The shifts of
XPS peaks are also observed for reduced CuSi-PS-x catalysts
(Figure S10, S11, and S12). The phenomena can also be
interpreted by the hybridization of orbitals. Cuprous cation
was taken as a simplified and stable model for the reduced
CuSi-PS-x, since Chen et al. have observed the formation of
slight intermediate Cu2O when the copper phyllosilicate is
reduced14. The cuprous cation is hybridized in sp model in
oxide, i.e. Cu2O. The sp hybrid orbitals have 50% s characters,
indicating the more affinity for electrons than that of sp3 of Si
atoms. Therefore, the electrons shifted from Si and O atoms
towards Cu atoms for reduced catalysts, which gave the
different electronic features from the calcined catalysts. The
generation of substantial amount of Cuδ+ species is related to
the chemical bonding between Cu and SiO2. We here
successfully observed the covalent bonding between Cu and
an irreducible oxide i.e. SiO2, leading to the greatly modified
electronic and structural properties of the as-prepared
catalysts and reduced catalysts.
View Article Online
DOI: 10.1039/D0CY01032D
Figure 2. High-resolution TEM images of the reduced CuSi-IM-
30 (A, B) and CuSi-PS-30 (C, D) catalysts.
Figure 3. (A) Deconvolution of Cu XAES spectra of reduced
catalysts reveals the diverse surface Cu species with both
metallic and cationic features. (B) Proportions of surface Cu0
and Cuδ+ species as the functions of catalyst compositions and
methods.
The dispersion of Cu species was greatly enhanced for the
reduced CuSi-PS-x catalysts by the Cu-O-Si bonding, which is in
consistent with improvement of Cu dispersion for calcined
catalysts (Table 1). Quantitative analysis by XPS showed the
high surface Cu dispersion and concentrations for reduced
CuSi-PS-x, which detects all the surface Cu species with
different valence states. The reduced CuSi-PS-x catalysts gave
the 1.6-3.2 times of surface Cu dispersions in compared with
the CuSi-IM-30. Considering the different Cu loadings of CuSi-
PS-x catalysts, reduced CuSi-PS-30 showed the highest surface
Cu concentration among the catalysts. In consistent with the
XPS results, the broad peaks of XRD results and high-resolution
TEM results confirmed the well dispersed Cu species on the
reduced CuSi-PS-x catalysts (Figure S9 and Figure 2). It was
also found that the Cu dispersion determined by quantitative
XPS is linearly correlated with N2O titration (Figure S13).
Because N2O titration cannot differentiate the Cu0 and Cuδ+
species, we employed quantitative XPS to determine the
concentrations of Cu species with different valence states.
To determine the surface contents of Cu species with
different valence states, we turned to Cu X-ray Auger electron
spectra (XAES, Figure 3A). The metallic Cu0 and cationic Cuδ+
2.2 Cu-O-Si bonding introduces the well-dispersed metallic Cu0 and
Lewis acidic Cuδ+ sites
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The vibration at 1450 cm-1 increased upon calcining CuSi-
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PS-30, whereas it decreased in intensity DuOpIo: n10f.1u0r3t9h/eDr0CreYd01u0c3in2Dg
the catalyst. It has been revealed from XRD results that both
the dried and calcined CuSi-PS-30 are composed of copper
phyllosilicate. The increase of 1450 cm-1 band caused by
calcination is attributed to the generation of coordinatively
unsaturated (CUS) Cu2+ species, since the calcination can
partially remove the surface OH groups and create more CUS
Cu2+ species27. XPS results have shown the reduction
eliminated Cu2+ species by generating highly dispersed Cu0-
Cuδ+ on SiO2. The reduction-induced decrease of 1450 cm-1
band is attributed to the elimination of Cu2+ species.
Interestingly, the bands in 1600-1630 cm-1 for CuSi-PS-30
greatly elevated in intensity after reduction, which was
reported to be sensitive to the nature and strength of Lewis
acids.28 After the reduction, the valence states of Cu species
were altered greatly whereas the concentration of surface acid
was not changed greatly (Figure S14). Since SiO2, metallic Cu0
Figure 4. (A) NH3-TPD of reduced CuSi-PS-x and CuSi-IM-30. (B)
Pyridine-adsorbed IR spectra of dried, calcined, reduced CuSi-
PS-30 and calcined, reduced CuSi-IM-30. (C) Schematic
representation of nature of Lewis acids over reduced CuSi-PS-x
catalysts. (D) Correlations between surface concentrations of
Cuδ+ species and acidity measured by XPS and NH3-TPD
respectively.
and even terrace kinks or edges on Cu0 (species possibly exist
29
on reduced CuSi-PS-x) have very slight acidity,26,
the
surface Cuδ+ species are responsible for surface Lewis acids
(Figure 4C). The results suggested the formation of Lewis acid
on CuSi-PS-x by Cu-O-Si bonding. The vibrations of CuSi-IM-30
slightly red-shifted, indicating the decrease of Lewis acid
strength or the presence of OH groups hydrogen-bonding with
pyridine.26 This is in accordance with weak NH3 desorption
peak of reduced CuSi-IM-30. The reduction also induced a
slight decrease of 1450 cm-1 band due to the elimination of
Cu2+ species with forming Cu0 sites on SiO2. The peak at 1610
cm-1 was very weak over reduced CuSi-IM-30, indicating the
importance of Cu-O-Si bond on the formation of large amounts
of Cuδ+ sites. According to the above results, the acid
concentration of reduced catalysts can be correlated with the
surface Cuδ+ concentrations (Figure 4D).
species located at ~917.8 eV and ~915.8 eV at Cu XAES spectra
respectively, which were quantitatively differentiated by
deconvolution of the peaks. The proportions of surface Cuδ+
species of reduced CuSi-PS-x decreased gradually from 78.9%
to 50.5% with the increase of Cu contents from 5 wt. % to 40
wt. % (Figure 3B). In contrast, reduced CuSi-IM-30 gave the
minimal proportion of Cuδ+ species. The existence of large
amount of Cuδ+ sites is related to the interfacial Cu-O-Si
bonding that altering the Cu properties to cationic, as
illustrated by the structural evolutions observed in XPS analysis.
The heterogeneous distributions of surface species (e.g., Cu0
and Cuδ+) would alter the surface and adsorption properties,
which may therefore influence the catalysis performance.
Based on the measured Cu dispersions and distributions of Cu
species of reduced catalysts, the surface concentrations of Cu0
and Cuδ+ species were determined (Table 1).
2.3 Structure-performance relationship for C-O hydrogenolysis
The performance for HMF hydrogenolysis to DMF was shown
in Figure 5A. The high temperatures (e.g., 220 oC) were usually
needed for DMF synthesis over Cu-based catalysts to facilitate
o
C-O cleavage.30-32 We here chose 180 C to investigate the
efficiency of CuSi-PS-x catalysts. The main products from HMF
conversions under H2 atmosphere can be categorized into 4
types: hydrogenated-, semi-hydrogenolysis-, hydrogenolysis-
and decarbonylation-related compounds (Figure 5B). The over-
hydrogenation of C=C bonds is not observed for all the
catalysts. High yields of target DMF can be achieved over CuSi-
PS-x catalysts within 2 h, in particular over CuSi-PS-30 (93.4%).
In contrast, CuSi-IM-30 only gave a trace amount of DMF (0.2%)
and 51.7% hydrogenated product 2,5-dihydroxymethylfuran,
indicating the insufficiency for C-O hydrogenolysis. Moreover,
the product distributions are varied with the Cu loadings for
CuSi-PS-x catalysts. The increase of Cu loadings promotes the
efficiency for C-O hydrogenolysis and the production of DMF.
The results further supported the categorized reaction routes
for DMF synthesis via hydrogenolysis mechanism. Turnover
frequencies (TOF) of DMF over different catalysts were
calculated in Figure S15. The CuSi-PS-x catalysts showed much
Temperature-programmed desorption of ammonia (NH3-
TPD) was used to probe the surface acid sites of reduced
catalysts (Figure 4A). All the catalysts exhibited the desorption
peaks in 150-400 oC which correspond to weak acid sites. CuSi-
PS-x catalysts showed higher acid concentrations than CuSi-
IM-30. The acid concentration of CuSi-PS-x varied with Cu
loadings.
To elucidate the nature of surface acid, we performed
pyridine-adsorbed IR experiments of the as-prepared, calcined
and reduced CuSi-PS-30, and calcined and reduced CuSi-IM-30
(Figure 4B). The vibration at 1540 cm-1 was not observed for all
the samples, indicating the absence of Brønsted acid sites. The
vibration (~1600 cm-1) of pyridine interacting with OH groups
via hydrogen bonding was also not observed for CuSi-PS-30.
The vibrations at ~1450, 1490 and 1600-1620 cm-1 were
observed for CuSi-PS-30, attributing to vibrations of pyridine
coordinating with Lewis acid.26
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DOI: 10.1039/D0CY01032D
Figure 5. (A) Catalytic performance of CuSi-PS-x and CuSi-IM-
o
30 catalysts for HMF hydrogenolysis at 180 C and 15 bar.
(conditions: HMF 0.75 g, catalyst 0.1 g, 1,4-dioxane solvent 35
g, 2 h) (B) Reaction pathways for HMF conversions under H2
include C=O hydrogenation (type I), hydrogenolysis (type II and
III), decarbonylation (type IV), and C=C hydrogenation
(undetected in our case). (C) Comparison of TOFs normalized
by the surface Cu0 sites, Cuδ+ sites and overall Cu0-Cuδ+ sites
and the relationship with surface Cuδ+ concentrations.
higher TOF values than CuSi-IM-30 catalyst, demonstrating the
superior hydrogenolysis ability of CuSi-PS-x catalysts for C-O
bond. Moreover, CuSi-PS-30 catalyst exhibited the highest TOF
for Cu0, indicating the promotion effect of Cu+ sites. The
catalytic performance of CuSi-PS was compared with other
reported Cu-based catalysts in Table S1. It can be found the
alloying with Co or Ni is a commonly used strategy to lower the
reaction temperature and to obtain high DMF yield. Here we
created an easy way to realize the efficient conversion of HMF
to DMF.
Figure 6. (A) TOFs normalized by the surface Cu0 and Cuδ+ sites
as the function of TOFs for overall Cu (Cu0+Cuδ+) sites (the dash
lines refer to the related linear fitting functions). (B) IR spectra
of gas-phase furfural and furfural-adsorbed IR spectra of
reduced CuSi-PS-30.
TOF of DMF, normalized by the surface Cu0 sites, Cuδ+
sites and overall Cu sites (including both Cu0 and Cuδ+) were
summarized (Figure 5C), based on the catalytic results at DMF
yield lower than 20%. Although the TOF values cannot be
calculated with the same accuracy due to the indirect way for
measuring the concentrations of active sites (e.g., Cu0 and
Cuδ+), the difference of apparent TOF values is quite large and
the trends are meaningful.33 For the overall Cu sites, the TOF
follows a volcano-type trend by firstly increasing and then
decreasing with the Cuδ+ proportion, demonstrating the
coorporation of the Cu0 and Lewis acidic Cuδ+ sites. TOF for
Cuδ+ and Cu0 sites followed the similar trends to that of overall
Cu sites while exhibited the higher values. Moreover, TOF
based on the surface Cu0 sites increased dramatically at the
topmost point, indicating that another surface center (i.e. Cuδ+)
may contribute more to C-O hydrogenolysis under the
condition. The highest intrinsic reactivity is achieved when the
proportions of Cuδ+ and Cu0 are 68% and 32% respectively. Dai
group34 and Gong group35 have also experimentally observed
that the performance for ester hydrogenation to ethylene
glycol or ethanol is related to the ratios of Cu0/Cuδ+ species.
Both TOFs on Cu0 and Cuδ+ sites correlated linearly with
that of overall Cu sites (Figure 6A), confirming that Cu0 and
Cuδ+ sites serve as the dual active sites for C-O hydrogenolysis.
If the surface Cu0 and Lewis acidic Cuδ+ sites contribute equally
to HMF hydrogenolysis, the reaction is valence-state-
insensitive. It means that TOFs based on either Cu0 or Cuδ+
sites would correlate with the TOFs on total surface Cu sites
with the similar slopes. However, the slope of fitting functions
for Cu0 is ~2 times that for Cuδ+ sites. The results clearly show
that Cuδ+ sites contributed more predominant roles for the
reaction.
We expect that the Lewis acidic Cuδ+ sites promote the
adsorption and activation of C-O bonds. Wang et al.36 reported
that Cuδ+ is capable of adsorption of methanol and presumed
to form a Cuδ+-OCH3 configuration. Dai37 and Ma38 groups have
also observed that the reactivity for ester hydrogenation to
ethylene glycol or ethanol is related to the Cu0/Cuδ+ ratio.
Whether the C-O bond is stretched and activated by Cuδ+
species is not indicated by the experiments. Thus, in-situ
diffuse reflectance IR of furfural adsorption was performed
(Figure 6B). Due to the high boiling point of HMF, we used
furfural to study the selective adsorption and activation of C=O
bonds on reduced CuSi-PS-30. The gas-phase furfural has the
vibrations of C=O and C=C bonds at 1720 and 1470 cm-1
respectively. The downward shift of C=O vibration to 1670 cm-1
was observed for the chemisorbed furfural, whereas the
frequency of C=C vibration was slightly altered. It is reported
that C=O bond is polarized by interacting with the Lewis acidic
solvents via the lone pairs of O atoms.39 Along with the
redshift of C=O vibration, the occupancy of π(C=O) increases
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because of the electron transfer from adjacent π(C=C). A more Kα X-ray source (1486.6 eV). For the XPS analysis of reduced
View Article Online
stable resonance C+-O- configuration with increased single- catalysts, the samples were in-situ pre-reduced with H2 flow at
DOI: 10.1039/D0CY01032D
o
bond character was formed, leading to the partially charged C 250 C. All the spectra were calibrated by the C1s spectrum.
and O atoms. The C=O bond is stretched and bond length is The electron flood gun was employed for charge correction.
increased for ~0.73% at an IR band shift of 40-50 cm-1.39 Thus, XRD patterns were analyzed by an X-ray diffractometer
the redshift of C=O vibration can be ascribed to the withdrawal (MiniFlex II, Rigaku) with the Cu Kα radiation (40 kV) and a rate
of electron density from C=O bonds by the surface Lewis acidic of 4 degree per minute. The XRD patterns of reduced catalysts
Cuδ+ sites.40 In contrast, Lewis acids was inactive for C=C were recorded after the samples were reduced ex-situ in H2
o
hydrogenation, as the vibration shift of C=C bond and the ring- flow at 250 C and sealed in inert medium. TPR and N2O
saturated products were not observed. The above results titration experiments were performed on a Tianjin XQ TP-5080
highlight the covalent Cu-O-Si bonding drives the cooperation instrument equipped with a TCD detector. The IR experiments
of metallic Cu and Lewis acidic Cuδ+ sites, giving the high were performed on a Nicolet Nexus 470 spectrometer in the
efficiency for C-O hydrogenolysis. The mechanistic insights of range of 400-4000 cm-1 with the resolution of 4 cm-1. The
metal-Lewis acid catalysis for the heterogenous Cu/SiO2 pyridine adsorbed IR spectra were recorded by a Bruker
catalyst rationlized the previous observations on the valence- VERTEX70 spectrometer equipped with a deuterium triglycine
state-sensitivity for both hydrogenation and hydrogenolysis sulphate detector. TPR experiments were performed on a
reactions (e.g., Cu34-35 and Co41 catalysts).
Tianjin XQ TP-5080 instrument equipped with a TCD detector.
NH3-TPD experiments were performed on a Micromeritics
ASAP 2920 instrument with an online quadrupole mass
spectrometer as the detector. The m/z=17 fragment of NH3
would be interfered by OH fragment of H2O. Thus, the m/z=16
fragment with an 80% intensity of m/z=17 fragment can be
drawn as the characteristic of NH3, which was barely altered by
H2O molecules.
Experimental
Catalyst preparation
An ammonia evaporation method was employed to
demonstrate the chemical bonding of metal cations and SiO2,
for which selective removal of ammonia was adopt by
Catalytic tests and analytic methods
o
hydrothermal evaporation at 80 C. Typically, the ammonia
The hydrogenolysis of HMF was performed on a 100 mL
stainless tank reactor. For a typical procedure, the tank was
fed with the reduced catalysts, HMF and 1,4-dioxane solvent,
then it was sealed and purged by pure H2 for 5 times. A pre-
reduction of catalysts at 250 oC and H2 atmosphere was
performed. The reactor was then filled with H2 and heated to
the reaction temperature within 20 minutes. After the
measurements, the tank was quenched in ice-water and the
products in both liquid and gas phase were analyzed by GC-MS
and GC instrument. The determination of liquid products was
performed on a GC-MS equipment and the quantitative
analysis of liquid products was conducted on a GC instrument
equipped with DB-WAX capillary column and FID detector. The
gas products was also gathered and analyzed by the GC
instrument. Carbon balances were calculated for the catalysts,
which was in the range of 100%±5%. Therefore, the product
selectivity and HMF conversions were determined by
calibrated area normalization.
solution (25-28 wt. %) was added to and mixed with the
aqueous solutions of Cu nitrate with preset contents at room
temperature. The ratio of NH3/Cu was controlled to be 6, 1.5
2+
times than the coordination number of Cu(NH3)4 complex.
Thereafter, the SiO2 sol gel was added to the above solution
under vigorous stirring. The mixture was stirred for further 12
hours at room temperature by forming a dark-blue sol gel,
which serves as the precursor for ammonia evaporation. The
blue sol gel was heated to 80 oC to undergo the evaporation of
ammonia until the pH value of the suspension decreased to
approximate 6-7. Then, the precipitate was filtered, washed,
o
dried overnight and annealed at 450 C. The samples were
denoted as CuSi-PS-x catalysts, of which x indicates the weight
loading of Cu species over the catalysts. The actual Cu loadings
were shown in Table S1. For the catalytic tests, the catalysts
o
were pre-reduced at 250 C under H2 flow. The impregnated
CuSi-IM-30 catalyst with a Cu loading of 30 wt. % was prepared
as a reference using copper nitrate and SiO2 sol gel as the
starting materials. The SiO2 was synthesized from the drying of
o
SiO2 sol gel at 80 C, which was crushed to be a fine powder.
Conclusions
The solution of copper nitrate was then impregnated onto the
SiO2 powder. The received material was further dried and The covalent-bonding chemistry and structural evolution upon
calcined at 450 oC.
reduction of copper and irreducible SiO2 have been explored.
An enhanced catalysis by synergistic metallic Cu and Lewis
acidic Cuδ+ sites was demonstrated as an atomic-scale
sequence of covalent Cu-O-Si bonding. The Cu-O-Si bonding of
calcined catalysts was evolved upon reduction by giving
substantial amounts of surface metallic Cu and Lewis acidic
Cuδ+ sites, facilitating the efficient C-O hydrogenolysis
reactivity at moderate temperature, for example
hydrogenolysis of HMF to DMF. The high DMF yield of ~93.4%
Characterizations
The BET surface area was tested via N2 physical adsorption at -
196 oC by a Micromeritics ASAP 2420 instrument. Before tests,
the samples were degassed at 90 oC for 1 h and 350 oC for 8 h.
The elemental compositions were measured by inductively
coupled plasma-optical emission spectroscopy (ICP, Perkin
Elmer Optima 2100DV). XPS analysis was conducted on an AXIS
ULTRA DLD spectrometer equipped with a monochromatic Al
6 | J. Name., 2012, 00, 1-3
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9. Robinson, A. M.; Hensley, J. E.; Medlin, J. W., Bifunctional
was received at 180 C and 15 bar H2. A relationship between
surface Cu valence states and intrinsic reactivity was
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and Lewis acidic Cuδ+ sites. Moreover, Lewis acidic Cuδ+ sites
contributed more predominant roles than Cu0 sites for C-O
hydrogenolysis. In-situ adsorption of furfural model revealed
that the Cuδ+ sites stretch and dissociatively activate the C-O
bonds while they remain inactive for C=C bonds, thus
facilitating selective hydrogenolysis of C-O bonds. The work
not only rationalizes the valence-state-sensitive catalysis for
reactions involving C-O cleavage, but opens up an alternative
for designing efficient catalysts by covalent metal-O-Si
bonding.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Natural Science Foundation in
Jiangsu Province (BK20170707), National Natural Science
Foundation of China (21802073, 21878161), and Major State
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DOI: 10.1039/D0CY01032D
Strong covalent bonding (Cu-O-Si) modulates the Cu status and boosts the C-O
hydrogenolysis.