Flower-like Au/Ni-Al hydrotalcite with hierarchical pore structure as a multifunctional catalyst for catalytic oxidation of alcohol

Article abstract of DOI:10.1039/c5cy00160a

Flower-like hierarchical Au/NiAl-LDH catalysts were synthesized for selective oxidation of alcohols. The abundant hydrogen vacancies at the edge of the flowers as nucleation centers contributed to the uniform dispersion of Au NPs. The confinement effect of the hierarchical pores promoted 60% higher activity than the common Au/NiAl-LDH nanoparticle catalyst in the oxidation of benzyl alcohol by heightening the effective collisions between substrates and active sites. The evolution process of the hierarchical pores in the support was further proposed. Moreover, the reaction mechanism of the cooperation among Bronsted base sites, Ni^III coordinatively unsaturated metal sites and isolated gold cations was concretely proved. In the oxidation of other typical alcoholic substrates, the flower-like catalyst showed higher activity than the common nanoparticle one except for linear alcohols, which could be attributed to the shape selectivity of straight macropores.

Full text of DOI:10.1039/c5cy00160a

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Flower-like Au/Ni-Al Hydrotalcite with
Hierarchical Pore Structure as a Multifunctional
Catalyst for Catalytic Oxidation of Alcohol
Cite this: DOI: 10.1039/x0xx00000x
Y. Y. Du, Q. Jin, J. T. Feng*, N. Zhang, Y. F. He, D. Q. Li*
Received 00th January 2012,
Accepted 00th January 2012
Flower-like hierarchical Au/NiAl-LDH catalysts were synthesized for selective oxidation of alcohols.
Abundant hydrogen vacancies at the edge of flowers as the nucleation center made contributions for
the uniform dispersion of AuNPs. The confinement effect of the hierarchical pores promoted 60%
higher activity than the common Au/NiAl-LDH nanoparticle catalyst in the oxidation of benzyl alcohol
by heightening the effective collisions between substrates and active sites. The evolution process of
hierarchical pores in support was further proposed. Moreover, the reaction mechanism of the
cooperation among Brønsted base sites, NiIII coordinatively unsaturated metal sites and isolated gold
cations was concretely proved. In the oxidation of other typical alcoholic substrates, the flower-like
catalyst kept higher activity than the common nanoparticle one except for linear alcohols, which could
be attributed to the shape selectivity of straight macro-pores.
DOI: 10.1039/x0xx00000x
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of the high price and less reserve of noble metals, the
alternation of one metal with nonꢀnoble one is more potential in
the application of oxidized catalysts.
Introduction
Liquid phase oxidation of alcohols is of great importance and
preferred for the environmentꢀfriendly synthesis of aldehydes
without generation of greenhouse gas CO2. In the mechanism of
alcoholic oxidation, βꢀH fracture is generally accepted as the
determined step. It has been widely considered that the addition
of alkali can efficiently enhance the catalytic activity by
accelerating this step. However, it is not an environmentꢀ
friendly way due to the difficulties in separation. Solid base
supports which possess abundant hydroxyls are therefore
desirable in the baseꢀfree oxidation of alcohols in view of green
and sustainable chemistry. Layered double hydroxide (LDH),
known as a family of synthetic anionic clays, are a class of twoꢀ
Gold nanoparticles (AuNPs) demonstrate high selectivity in
heterogeneous selective liquid oxidations5, 6 but lower activity
compared with other noble metallic nanoparticles especially
PdNPs7ꢀ10. Recently, there has been a growing interest in the
design and synthesis of efficient Auꢀbased catalysts synergized
by reducible supports with transition metals such as Cr2, Mn11
and Ni12ꢀ14. Liu et al.2 reported the application of CrMgAlꢀLDH
supported Au catalyst and revealed the strong AuꢀCr synergy
which was related to a Cr3+
↔
Cr6+ redox cycle at the
Au/CrMgAlꢀLDH interface, where O2 activation took place
accompanied with electron transfer from CrMgAlꢀLDH to Au.
However, Auꢀbased catalysts supported by LDHs with other
reducible metals are rarely reported. Comparing with other
reducible metals, the application of Niꢀbased support with
hypotoxicity and efficiency is more significantly in accordance
with the main tendency of green chemistry.
The property of easy aggregation for nanomaterials in liquid
reactions is an undeniable fact. Preparing microꢀlevel
hierarchical materials is considered as an effective way to solve
this problem. This kind of materials can inherit many properties
from building units such as high surface area and a large
amount of active sites and generate some new characteristics
such as specific internal porosity, plentiful coordinatively
unsaturated metal sites (CUS) and outstanding separation and
recirculation ability. Yu et al.15 fabricated hollow spherical
dimensional brucite Mg(OH)2ꢀlike layered inorganic materials.
In the general formula of [M2+
3+
M
x(OH)2]x+(Anꢀ)x/n·mH2O,
1ꢀx
M2+ locates on the layers substituted by M3+ partially with
regular hydroxyl arrays, Anꢀ acts as balancing anions in the
interlayer space with water molecules. As a consequence of the
tailored structureꢀdesign, LDHs and their calcined products
mixed metal oxides (MMO) with tunable acidic and basic
properties can provide extremely potential opportunities for
designing and innovating novel catalytic supports in the baseꢀ
free oxidation of alcohols.1ꢀ4 Very recently, we reported the
research on the novel heterogeneous catalyst which combined
acid–base bifunctional material of MgAlꢀMMO with bimetallic
AuPd nanoparticles for solventꢀfree oxidation of benzyl alcohol
(BA) and exhibited dramatic performance.3 However, in view
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CeO2 supported Au catalyst which showed high activity and a
long lifetime in the catalytic reduction of ꢀnitrophenol because
p
of the nanosized interconnected chambers of the hierarchical
support resulting in the large specific surface area and narrow
mesopore distribution. Zhang et al.4 assembled MgAlꢀLDH
nanosheets onto the magnetic Fe3O4 cores for the preparation of
the Au/LDH/Fe3O4 catalyst. The outer honeycombꢀlike
microstructure of the obtained coreꢀshell Fe3O4@LDH
nanospheres provided abundant accessible edges and junction
sites of LDH crystals. Besides, perpendicularly interlaced
MgAlꢀLDH nanocrystals facilitated the immobilization of
AuNPs along with avoiding the possible aggregation. However,
it is rarely reported the catalyst which simultaneously contains
the superiorities of hierarchical materials and the advantages of
rich hydroxy in LDHs, as well as the strong synergy between
reducible metal species and Au in the oxidation of alcohols.
Therefore, we designed and fabricated flowerꢀlike NiAlꢀLDH
microparticles as the multifunctional support for Au catalyst.
The catalytic performance of asꢀfabricated flowerꢀlike
Au/NiAlꢀLDH catalyst was evaluated through various alcoholic
oxidations with molecular O2 and BA oxidation was chosen as
the probe reaction to investigate the structureꢀperformance
relationships. By comparing with the common nanoparticle
catalyst, the structural effect of flowerꢀlike catalysts in the
catalytic performance was revealed. Furthermore, the
synergistic effect between Ni and Au was also evaluated by
identifying the oxidation states and the redox process. The
rational reaction mechanism and route were further clarified.
Fig. 2 XRD patterns (A) of NiAl-LDH-P-36 (a), NiAl-LDH-F-36 (b), NiAl-LDH-F-24 (c)
and NiAl-LDH-F-12 (d) precursors and partial enlarged views (B) from 16.5o to
20.5o.
XRD analysis in Fig. 2A(a) and (b) for NiAlꢀLDHꢀPꢀ36 and
NiAlꢀLDHꢀFꢀ36, respectively.17
HRTEM images of AuNPs dispersed on NiAlꢀLDHꢀPꢀ36 and
NiAlꢀLDHꢀFꢀ36 supports are shown in Fig. 3(a), (b), (d) and
(e) in which lowꢀmagnification images are on the left and highꢀ
magnification ones are in the middle. From lowꢀmagnification
images, the AuNPs distribute more evenly on the surface of the
NiAlꢀLDHꢀFꢀ36 (Fig. 3(d)) than on NiAlꢀLDHꢀPꢀ36 (Fig.
3(a)). It is worth noticing that, in Au/NiAlꢀLDHꢀFꢀ36 catalyst,
AuNPs are more inclined to load near the edge of the “flowers
petals” with the distance range of around 150 nm from the
edges where locate abundant CUS as “dangling orbital” with
high densities leading to larger cohesive energy than that on the
top site for the preferential deposition of AuNPs.18 The
corresponding particle size distributions by measuring more
than 200 particles from different regions are also shown in Fig.
3. The AuNPs size distributions and average sizes (Au/NiAlꢀ
LDHꢀPꢀ36 2.44 nm and Au/NiAlꢀLDHꢀFꢀ36 2.45 nm) are both
similar in Au/NiAlꢀLDHꢀPꢀ36 and Au/NiAlꢀLDHꢀFꢀ36.
Furthermore, there is no visible difference in the morphology of
metal nanoparticles.
Results and discussion
Characterization and catalytic activities of various catalysts
The SEM images of NiAlꢀLDHꢀPꢀ36 and NiAlꢀLDHꢀFꢀ36 are
shown in Fig. 1(a) and (b). In NiAlꢀLDHꢀPꢀ36, shapeless
aggregations are observed. Correspondingly, the flowerꢀlike
structure with outer trumpetꢀlike pores is obviously
demonstrated in NiAlꢀLDHꢀFꢀ36. The wellꢀdefined layered
structure characteristic of pure hydrotalcite was confirmed by
The catalytic performances of asꢀsynthesized Au/NiAlꢀLDHꢀ
Pꢀ36 and Au/NiAlꢀLDHꢀFꢀ36 catalysts were tested in the
oxidation of BA. The catalytic data are listed in Table 1.
Benzaldehyde (BD) was the predominant product (the
selectivity
≥99%) in all cases. The turnover frequency (TOF)
and BD yield of the flowerꢀlike Au/NiAlꢀLDHꢀFꢀ36 catalyst
both perform nearly 60% higher than Au/NiAlꢀLDHꢀPꢀ36. In
order to investigate the factors for the increase of activity, ICP
and MIP were carried out. From the composition analysis,
Ni/Al molar ratios as well as the Au loading of Au/NiAlꢀLDHꢀ
Pꢀ36 and Au/NiAlꢀLDHꢀFꢀ36 are very close to the feeding ratio
(3.0:1.0 and 1.0 wt%). Therefore, it can be surmised that the
enhancement of activity in Au/NiAlꢀLDHꢀFꢀ36 is mainly due to
the structural effect of supports. In NiAlꢀLDHꢀPꢀ36, pores are
formed mainly by the aggregation of NiAlꢀLDH nanoparticles,
Fig. 1 SEM images of NiAl-LDH-P-36 (a), NiAl-LDH-F-36 (b), NiAl-LDH-F-24 (c) and
NiAl-LDH-F-12 (d).
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which is obviously different from that in NiAlꢀLDHꢀFꢀ36
circled by “flower petals”. In order to investigate the structural
transformation in the growth process of flower, we thus
synthesized flowerꢀlike catalysts with reduced hydrothermal
duration of 24 h and 12 h for researching the structureꢀ
performance relationships.
Structural effect of support on catalytic performance
The AuNPs morphology of Au/NiAlꢀLDHꢀFꢀ24 and Au/NiAlꢀ
LDHꢀFꢀ12 are shown in Fig. 3(g), (h), (j) and (k). Similar with
Au/NiAlꢀLDHꢀFꢀ36 catalyst, AuNPs in Au/NiAlꢀLDHꢀFꢀ24
and Au/NiAlꢀLDHꢀFꢀ12 with ~1 wt% loadings also locate at
the edge of “flower petals” with the average size of 2.99 nm
and 2.24 nm, respectively. In the test of BA oxidation,
interestingly, the trend of activities is proportional to the
hydrothermal duration. Therefore, we speculate that the activity
could be related to the structural evolution of the pores in
supports.
In Fig. 1(c) and (d), trumpetꢀlike microstructure of NiAlꢀ
LDHꢀFꢀ24 and NiAlꢀLDHꢀFꢀ12 can also be obviously
observed. However, with the extension of hydrothermal
duration, slight morphological distinguishes appear especially
in the size of “flower petals” and the depth of pores.
Significantly, an individual structure of solid “coreꢀpetal” can
be recognized in NiAlꢀLDHꢀFꢀ12, which is different from
NiAlꢀLDHꢀFꢀ24 and NiAlꢀLDHꢀFꢀ36 with no obvious solid
cores. Furthermore, the pores circled by “petals” become
deeper followed by the extension of hydrothermal duration. For
accurately investigating the evolution process and extent of
“flower petals” and pores in hydrothermal procedure, XRD,
ICP and MIP analysis were carried out.
Fig. 3 HRTEM images and AuNPs size distributions of Au/NiAl-LDH-P-36 (a-c),
Au/NiAl-LDH-F-36 (d-f), Au/NiAl-LDH-F-24 (g-i) and Au/NiAl-LDH-F-12 (j-l). The
low-magnification images (a, d, g and j) are on the left. The high-magnification
images (b, e, h and k) are in the middle. The AuNPs size distributions (c, f, I and l)
are on the right.
Table 1 Aerobic oxidation of benzyl alcohol over various Au/NiAlꢀLDH catalysts.a
Catalyst
S (m2/g)b
42
Ni/Al molar ratioc
2.99:1.0
[Au] (wt%)c
1.00
dAu (nm)
2.45
2.44
2.24
2.99
ꢀ
2.46
2.44
4.58h
3.48h
Yield (%)d
TOF (hꢀ1)e
267
423
286
199
ꢀ
Au/NiAlꢀLDHꢀPꢀ36
Au/NiAlꢀLDHꢀFꢀ36
Au/NiAlꢀLDHꢀFꢀ24
Au/NiAlꢀLDHꢀFꢀ12
NiAlꢀLDHꢀFꢀ36
Au/MgAlꢀLDHꢀPꢀ36
Au/NiAlꢀLDHꢀFꢀ36f
Au/NiAlꢀLDHꢀPꢀ36g
Au/NiAlꢀLDHꢀFꢀ36g
34
53
36
25
2
13
13
30
51
70
64
61
70
12
70
42
70
3.07:1.0
3.31:1.0
1.32:1.0
0.99
0.98
1.00
ꢀ
3.07:1.0
3.00:1.0i
3.07:1.0
0.99
103
104
235
409
0.99
2.99:1.0
3.07:1.0
0.94h
0.99h
a Reaction conditions: benzyl alcohol (1 mmol), catalysts (0.05 g), toluene (10 mL), O2 bubbling (20 mL minꢀ1), 100 oC, 0.5 h.
b Determined by MIP.
c Determined by ICP.
d Yields of BD were determined by GCꢀFID.
e Turnover frequency based on BD yield and gold loading for 0.5 h, and given in molBD moltotal Auꢀ1 hꢀ1.
f Without preꢀtreated by calcining in air.
g Oxidation results for the 3rd run.
h Corresponding data after 3rd run.
i Mg/Al molar ratio.
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In Fig. 2A(c) and (d), there are phase separation phenomenon beginning, Al3+ ions are rapidly precipitated because of the
in NiAlꢀLDHꢀFꢀ24 and NiAlꢀLDHꢀFꢀ12 at around 2θ 18.8o hydrolysis of urea, leading to the formation of spherical
arising from a few Al(OH)3 (PDF No. 74ꢀ1119). In the partial Al(OH)3 cores as observed in SEM and XRD. Meanwhile, Ni2+
enlarged views (Fig. 2B), the peak intensity of Al(OH)3 phase is complexed by Fꢀ. With the slow slight release of Ni2+ ions
drops gradually with the extension of synthesized duration until and further formation of NiAlꢀLDH crystal in 12 h, a dramatic
disappears at 36 h. Additionally, the actual ratio of Ni/Al in change in morphology occurres. Tiny NiAlꢀLDH “petals” grow
NiAlꢀLDHꢀFꢀ12 (1.32:1.0) is evidently lower than the out on the surface of Al(OH)3 spheres by in situ growth along
theoretical value (3.0:1.0). However, it approaches 3.0:1.0 after with the appearance of macroꢀpores as the initial morphology
36 h.
of flowers in Fig. 1(d). Subsequently, the release of massive
Pore distributions were investigated by MIP and BET as Ni2+ ions accompanies with the dissolution of Al(OH)3 by
shown in Fig. 4 and Fig. S1, respectively. In the support of alkaline condition, which results in not only the extension of
NiAlꢀLDHꢀPꢀ36, all the pores are in the range of mesoꢀscale NiAlꢀLDH platelets but also the shrink of Al(OH)3 spheres as
(below 50 nm). The NiAlꢀLDHꢀFꢀ36 support also has abundant mentioned above in SEM analysis. At this time, the acquired Fꢀ
mesoꢀpores and the pore distribution in mesoꢀscale is almost ions from the dissociation of complexes adsorb on the {001}
identical with NiAlꢀLDHꢀPꢀ36 as shown in Fig. S1(A). From facets and perform as the capping agent, which suppresses the
the BET analysis, the structures of the mesoꢀpores in NiAlꢀ growth of LDH along cꢀaxis leading to the extension of flower
LDHꢀPꢀ36 and NiAlꢀLDHꢀFꢀ36 are similar as shown in Fig. “petals”. This speculation is similar with that in the literature.22
S1(B), which means the adsorption processes in mesoꢀpores for The gradual increase of surface area accompanied with the
substrates have no obvious differences in these two catalysts.19 extension of hydrothermal duration in flowerꢀlike NiAlꢀLDH
Besides, the NiAlꢀLDHꢀFꢀ36 support possesses a number of support is the powerful testimony for the growth of LDH
macroꢀpores in the range of 170 nm~1 ꢁm, which can be platelets and the deepening of pores. Significantly, much higher
regarded as abundant microꢀspaces separated by “flower petals” Ni/Al actual molar ratio of 3.31:1.0 determined by ICP analysis
and increase not only the accessibility of active sites but also compared to theoretical value in NiAlꢀLDHꢀFꢀ24 suggests that
the chance for effective collisions between substrates and active the dissolution rate of Al(OH)3 exerts faster than the releasing
sites.20, 21 Therefore, it can be conjectured that the confinement rate of Ni2+ ions. When the hydrothermal time reaches 36 h,
effect of hierarchical pores is the key factor for the almost all Ni2+ and Al3+ enter into the layers of NiAlꢀLDH,
enhancement of activity in Au/NiAlꢀLDHꢀFꢀ36 for BA which has been confirmed by XRD and ICP. No characteristic
oxidation. Significantly, in Fig. 4(c) and (d), the macoꢀpores in diffraction peaks of Al(OH)3 can be observed by XRD and the
NiAlꢀLDHꢀFꢀ24 and NiAlꢀLDHꢀFꢀ12 exhibit similar bimodal Ni/Al molar ratio approaches to the theoretical value. In the
distributions with the same peak positions around 210 nm and analysis of pore structure, the decrease of pore size along with
520 nm. However, in NiAlꢀLDHꢀFꢀ36, the peak at 520 nm the increase of surface area beyond 24 h is caused by the
tends to shift to nearly 380 nm as shown in Fig. 4(b), which growth of new “petals” among old ones, which separates more
could be attributed to the growth of abundant new “petals” microspaces and therefore leads to an increase in activity.
among the old ones from 24 h to 36 h leading to the separation
Synergistic effect of Au and Ni
of the original pores.
Based on above results, the growth process of asꢀsynthesized
flowerꢀlike NiAlꢀLDH is proposed as the scheme 1. At
To investigate more information on the reaction mechanism,
some contrastive catalysts were test by BA oxidation as shown
in Table 1. Considering with the influence of support
composition, MgAlꢀLDH supported Au catalyst was
synthesized similarly with Au/NiAlꢀLDHꢀPꢀ36 catalyst (Fig. S2
Fig. 4 Pore size distributions of NiAl-LDH-P-36 (a), NiAl-LDH-F-36 (b), NiAl-LDH-F-
Scheme 1 Schematic representation for the growth process of flower-like NiAl-
LDH.
24 (c) and NiAl-LDH-F-12 (d).
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and S3). In comparison with Au/NiAlꢀLDHꢀPꢀ36, 60% lower
activity is performed in Au/MgAlꢀLDHꢀPꢀ36, which confirms
the support is of importance in respect to the origin of the
catalyst activity. However, the pristine NiAlꢀLDH is almost
inactive in our case. Hence, the benefit of utilizing the NiAlꢀ
LDH as a support can be attributed to the synergistic effect
between Ni and Au. In general, pretreating process can also
affect the performance of catalysts. In the test of nonꢀpretreated
Au/NiAlꢀLDHꢀFꢀ36, 75% lower activity than the pretreated one
implies the importance of oxidized states in this hybrid system
for the enhancement of activity after calcination. Concentrating
above, the synergy between Au and Ni was further researched
by surveying the oxidized states and verifying the main reaction
center in this hybrid system.
XPS were examined on the fresh, pretreated and recycled
Au/NiAlꢀLDHꢀFꢀ36 catalysts and NiAlꢀLDHꢀFꢀ36 support as
shown in Fig. 5A and B, respectively and the relative fractions
of the species in Ni 2p3/2 and Au 4f7/2 are listed in Table S1. In
all cases, the FWHM was constrained to be equal in each
sample for all deconvoluted peaks from photoemission spectra
of the same species using an 80%/20% Gaussian/Lorentzian
sum and a Shirley background. The curves of Au 4f electron
were fitted well to double groups of peaks, corresponding to
4f7/2 and 4f2/5, respectively, with the same peak area ratio (4:3)
in per pair of corresponding peaks. By comparison, it is worth
mentioning that a wide band is obviously obtained at 852.85 eV
in the fresh pure LDH which is attributable to unsaturated Ni2+ꢀ
O lattice species13, however, it disappears after loading AuNPs.
Therefore, we speculate that Au atoms locate on the support
surface through the AuꢀOꢀNi model, which indicates that the
hydrogen vacancies as a kind of CUS on the surface of NiAlꢀ
LDHꢀFꢀ36 are the priority nucleation center for Au
nanoparticles. It can be considered as the above mentioned
“dangling orbital” with high energy at the edges of the “flower
petals”.18 After loading, surface hydrogen vacancies are almost
completely covered by AuNPs or eliminated by filling in
hydrogen species from the decomposition of overweight
NaBH4, which leads to the disappearance of the characteristic
peak. Furthermore, the BE value of the Ni 2p3/2 electron in
fresh Au/NiAlꢀLDHꢀFꢀ36 is slightly higher than that in the
fresh support, indicating that there is a slight electron transfer
from Ni species to AuNPs because of the higher
electronegativity of Au.
Fig. 5 XPS spectra in Ni 2p or Au 4f regions on the flower-like catalyst (A) and
corresponding support (B) surface: the fresh sample (a), the pretreated sample
(b), the pretreated sample experienced the oxidation of benzyl alcohol at
reaction conditions (c).
Meanwhile, the amount of Au+ species decrease from 70.37%
to 54.94%, while the amount of Au0 species remained about the
same. Therefore, we speculate that during calcination, Au+ꢀNi2+
hybrid species are oxidized to Au3+ꢀNi3+ ones in which the Au3+
species can be stabilized on the LDH surface by Ni3+ and
oxygenꢀdeficient sites.25 In addition, after reaction, the peak of
Au3+ species almost disappears, which means Au3+ꢀNi3+ hybrid
species could be reduced by alcohols to Au+ꢀNi2+ ones during
the reaction.
Through the XPS data, more details are proved in the
mechanism of BA oxidation. After pretreatment, the work
function of catalyst reveals upward trend due to the formation
of Ni3+ and Au3+, resulting in the increase of electron transfer
29
from benzyl alcohol to the catalyst.28,
In benzyl alcohol
molecules, alcoholic hydroxyl groups as the origins of
transferred electron undergo coordination with catalyst to form
an unsteady alkoxide intermediate for further elimination of βꢀ
→ →
H. Hence, the transformation of Ni2+ Ni3+ and Au+ Au3+ can
promote the determined step of alcoholic oxidations. Moreover,
the isolated Au3+ cations make a great contribution to the
reaction between active O* adspecies with hydride. Au2O3 is
composed of a network of identical AuO4 units in which the Au
ion breaks two AuꢀO bonds and binds to one oxygen atom of
the O2 adsorbed in the oxygen vacancy at metalꢀsupport
perimeter interface.30ꢀ33 The hydride spilled over from support
could react with this active oxygen adspecies.34 On these bases,
we propose that the Au3+ꢀNi3+ hybrid site is the reaction center
possessing dramatically higher activity, which has been proved
by the rising yield form 13% to 53% after pretreatment.
Furthermore, the metabolic oxidation state of Ni species in
the pristine NiAlꢀLDHꢀFꢀ36 support was also inspected in the
whole reaction recycle as shown in Fig. 5B. It is worth noticing
that Ni3+ species cannot be reduced to Ni2+ in the oxidation
reactions, indicating that, without Au species, active O*
adspecies stored at O vacancies on the surface of support could
not join into the reaction.
In Fig. 5A, the peaks of Ni2+ꢀOH, Ni3+ꢀOH in Ni 2p3/2
binding energy (BE) are distinguished in these samples, as well
as Au0, Au+ and Au3+ in Au 4f7/2 12, 13, 23ꢀ25 Significantly, the BE
.
value of metallic Au is 82.9 eV, and remarkably lower than
84.0 eV, which is mainly because of the electron donation from
supports.2, 26, 27 Besides, the recycle process of Ni2+ Ni3+ in the
↔
whole reaction cycle is in accordance with the recent reports.12,
23
However, we find that the valent cycle of Au species is
different from that reported in other AuꢀNi system which is
23
often considered as the process of Au0
↔
Au+.12,
In asꢀ
synthesized fresh catalyst, Au species present two different
types assigned to Au0 and cationic Au+. However, experiencing
pretreatment, a new peak caused by Au3+ species is observed.
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BA and the adsorbing sites. In NiAlꢀLDHꢀFꢀ36 support, it is
known that BA adsorbs on the Brønsted base sites29, 35 in two
modes, on Ni2+ꢀOH (1370 cmꢀ1) and Ni3+ꢀOH (1380 cmꢀ1) sites.
However, in Au/NiAlꢀLDHꢀFꢀ36, only a tailing peak with
significant blue shift appears in this range with a maximum at
1390 cmꢀ1, suggesting that Au cations further promotes the
electron transfer from alcoholic hydroxyl groups to catalysts. In
other words, all the Brønsted base sites, NiIII CUS and Au
cations in the system of Au/NiAlꢀLDHꢀFꢀ36 make the
contributions to the βꢀH eliminate for enhancing the activity. In
addition, it is worth noticing that in the highꢀfrequency region,
no signals are present between 1650 cmꢀ1 and 1750 cmꢀ1 in
NiAlꢀLDHꢀFꢀ36 support. However, the signals of carbonyl
bands in Au/NiAlꢀLDHꢀFꢀ36 centered at 1702 and 1720 cmꢀ1
clearly appear and belong to BD. The appearance of these
bands implies that the production of BD strongly adsorbs on the
catalyst, which reconfirms that without the assistance of
AuNPs, active O* adspecies could not join into the reaction.
Fig. 6 DRIFT spectra of benzyl alcohol dosed Au/NiAl-LDH-F-36 catalyst (a) and
NiAl-LDH-F-36 support (b) at 100 oC.
Although a number of researches have proposed possible
mechanisms in the system of Au/LDH,3, 29, 35 to the best of our
knowledge, the concrete evidences are still not given. DRIFT
spectroscopic studies were performed on Au/NiAlꢀLDHꢀFꢀ36
catalyst and NiAlꢀLDHꢀFꢀ36 support dosed with small
quantities of BA. The spectra are shown in Fig. 6 and the
detailed band information are shown in Table S2. For both
samples, bands associating with aromatic C=C and CꢀH modes
(1454 cmꢀ1, 1496 cmꢀ1, 1584 cmꢀ1, 1596 cmꢀ1, 1598 cmꢀ1 and
1606 cmꢀ1) are observed. In the lowꢀfrequency region, all bands
centered from 1360 cmꢀ1 to 1400 cmꢀ1 indicate the perturbation
of the adsorbed hydroxyl groups of BA.36, 37 However, obvious
differences in the peak position and quantity are demonstrated
in both samples, indicating the different interactions between
Mechanistic investigation
Based on our experimental results, we proposed a possible
mechanism with Au3+ꢀNi3+ꢀOH as a hybrid active site (Scheme
2). In the pretreatment process, Ni2+ and Au+ are oxidized to
Ni3+ and Au3+, respectively. Meanwhile, oxygen molecules are
adsorbed and activated to active O* species which are further
stored in the O vacancies at metalꢀsupport perimeter interface.
At the beginning of reaction, BA molecules attack basic NiꢀOH
sites to give an alkoxide intermediate. Subsequently, the
intermediate undergoes coordination to afford unsteady metalꢀ
alcoholateꢀspecies, which is considered as the rateꢀdetermining
step and promoted by both NiIII CUS and Au cations, leading to
the elimination of βꢀH. The final step is the rapid oxidation of
Scheme 2 The possible reaction pathway of the oxidation for benzyl alcohol over Au/NiAl-LDH catalyst.
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5CY00160A
common nanoparticle NiAlꢀLDHꢀPꢀ36 support in the 3rd run
comparing with the 1st. However the Au/NiAlꢀLDHꢀFꢀ36
，
catalyst with flowerꢀlike support only shows around 4%
deactivation after the 3rd run. The HRTEM images of the used
Au catalysts demonstrate that the aggregation from 2.45 nm to
4.58 nm is the key reason for the activity loss in Au/NiAlꢀLDHꢀ
Pꢀ36 (Fig. S5). In contrast, the aggregation of AuNPs in
Au/NiAlꢀLDHꢀFꢀ36 is rarely conspicuous. However, the
aggregation of AuNPs cannot explain the obvious decrease of
selectivity and tiny increase of activity in the 3rd run, thereby
ICP was taken for the catalysts after recycle. The results
suggest that the leaching of AuNPs from support to solution
becomes the reason for the side reactions (Table 1).38
Thereupon, the remarkable recyclability of the flowerꢀlike
catalyst with almost full reusability has been highlighted by
contrast, resulting from the perpendicularly interlaced
morphology of “flower petals” which can facilitate the
immobilization of nanoꢀmetallic particles for avoiding the
possible aggregation and leaching.4 In summary, the
hierarchical Au/NiAlꢀLDH catalyst possesses not only higher
activity but also preferable stability during the oxidation of
benzyl alcohol.
Fig. 7 Reusability of Au/NiAl-LDH-P-36 (red) and Au/NiAl-LDH-F-36 (black).
the hydride by active O*. Along with the consumption of active
O* adatoms, the Au3+ꢀNi3+ hybrid site is reduced to the Au+ꢀ
Ni2+. With the desorption of BD and water molecules, the
catalytic cycle is thereby completed. According to this reaction
mechanism, both hydroxyl arrays and the synergy between Ni
and Au play as the key factors for the enhancement of
activity.18
Recyclability of various catalysts
Various catalytic aerobic oxidation of alcohols over AuNPs
supported on flower-like NiAl-LDH
The reusability of the catalysts was investigated for three
successive reactions as shown in Fig. 7. There is a considerable
decrease of activity (nearly 12%) over the Au catalyst with
To obtain more information about hierarchical catalysts, the
Table 2 Aerobic oxidation of various alcohols using Au/NiAlꢀLDH.
Entry
1a
Substrate
Main Product[c]
Catalyst
Au/NiAlꢀLDHꢀPꢀ36
Au/NiAlꢀLDHꢀFꢀ36
Temperature (oC)
TOF (hꢀ1)
Ref.
this work
this work
100
100
45
45
2a
Crotonic acid
Crotonyl alcohol
3a
4a
Au/NiAlꢀLDHꢀPꢀ36
Au/NiAlꢀLDHꢀFꢀ36
100
100
123
126
this work
this work
1ꢀoctoic acid
1ꢀoctanol
5a
6a
Au/NiAlꢀLDHꢀPꢀ36
Au/NiAlꢀLDHꢀFꢀ36
100
100
148
239
this work
this work
Cinnamyl aldehyde
4ꢀMethyl benzaldehyde
Cyclohexanone
Cinnamic alcohol
7a
8a
Au/NiAlꢀLDHꢀPꢀ36
Au/NiAlꢀLDHꢀFꢀ36
100
100
259
347
this work
this work
4ꢀMethylbenzyl alcohol
Cyclohexanol
9a
Au/NiAlꢀLDHꢀPꢀ36
Au/NiAlꢀLDHꢀFꢀ36
100
100
101
225
this work
this work
10a
11a
12a
Au/NiAlꢀLDHꢀPꢀ36
Au/NiAlꢀLDHꢀFꢀ36
100
100
309
445
this work
this work
Benzophenone
Benzaldehyde
Benzhydrol
13b
14b
15
16
17
18
19
Au/NiAlꢀLDHꢀPꢀ36
Au/NiAlꢀLDHꢀFꢀ36
Au/MgAlꢀMMO
Au/SBAꢀ16
140
140
140
140
140
140
160
8023
10026
~4500
1937
6420
7830
this work
this work
3
39
40
40
41
AuPd/TiO2
AuPd/C
Au/TiO2
Benzyl alcohol
12400
a The reaction conditions are the same as those in Table 1.
b Reaction conditions: benzyl alcohol (1 mL), catalysts (10 mg), O2 (0.1 MPa), 140 oC, 0.5 h, 1000 rpm, 50 mL glass stirred reactors.
c Determined by GCꢀMS.
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oxidation of various typical alcohols was evaluated in toluene benzyl alcohol was proved by DRIFT analysis and the rational
as shown in Table 2. We note that Au/NiAlꢀLDHꢀFꢀ36 can reaction route was therefore clarified. Moreover, the oxidations
oxidize a wide range of alcohols to the corresponding carbonyl of other alcohols in various types were further measured and
compounds with higher activity than Au/NiAlꢀLDHꢀPꢀ36 the asꢀfabricated flowerꢀlike catalyst also performed more
except for crotonyl alcohol and 1ꢀoctanol which are both linear effective than the common nanoparticle one in the most
alcohols. The similar TOFs in each group suggest that the oxidations except for in linear alcohols, which is attributed to
effective collision between linear alcohols and active sites the shape selectivity of straight macroꢀpores by affecting the
cannot be enhanced in the straight macroꢀpores. It is worth effective collision between active sites and substrates. We
noticing that the effective collision can also be enhanced in the believe that the synthesized hierarchical multifunctional
benzhydrol oxidation of which the molecule size is beyond 1 catalyst in this work could offer a novel strategy for designing
nm, although the increase amount of TOF is relatively lower high performance catalysts for green, hypotoxicity and efficient
than others. It is mainly attributed to the steric constraint for the oxidations.
bulky molecule structure with bigger size.
Distinctively, considering that toluene is a byproduct in BA
oxidation, the solventꢀfree oxidation of BA was thus tested. The
TOF of Au/NiAlꢀLDHꢀFꢀ36 is 25% higher than Au/NiAlꢀLDHꢀ
Pꢀ36, which results from the special hierarchitecture as
mentioned above. Moreover, the TOF (10025 hꢀ1) of Au/NiAlꢀ
LDHꢀFꢀ36 is much higher than previously reports such as
~4500 hꢀ1 of Au/MgAlꢀMMO3, 1937 hꢀ1 of Au/SBAꢀ1639, and
even higher than 6420 hꢀ1 of AuPd/TiO2 and 7830 hꢀ1 of
AuPd/C which are both regarded as two types of the most
efficient catalysts for oxidation at present40. Significantly, the
TOF of Au/NiAlꢀLDHꢀFꢀ36 at 140 oC is comparable with
Experimental
Preparation of supported Au/LDH catalysts
Synthesis of LDH support
The hierarchical NiAlꢀLDHs with different [Ni2+]/[Al3+] molar
ratio were synthesized by a urea decomposition method with
the assistance of NH4F as morphology control agent similar as
that ascribed by Wei et al16. In brief, the Ni(NO3)2·6H2O (0.006
M), Al(NO3)3·9H2O (0.002 M), urea (0.04 M) and NH4F (0.016
M) were dissolved in deionized water (100 mL) to give a
transparent solution. The resulting solution was aged in sealedꢀ
o
12400 hꢀ1 of the Au/TiO2 catalyst at 160 C41. All superiorities
are brought from the combination of hierarchitecture, AuꢀNi
synergy and hydroxyl arrays.
o
Teflon autoclave at 110 C for 36 h. The precipitations were
washed until pH reached 7 with water and dried at 60 oC
overnight. The asꢀsynthesized NiAlꢀLDH was denoted as NiAlꢀ
LDHꢀFꢀ36. Considering the effects brought from synthesized
duration for LDH structures, hydrothermal time was shortened
to 24 h and 12 h which were called NiAlꢀLDHꢀFꢀ24 and NiAlꢀ
LDHꢀFꢀ12, respectively. The other processes were taken like
ones in preparing NiAlꢀLDHꢀFꢀ36. The nanoparticle NiAlꢀ
LDH was obtained similarly as NiAlꢀLDHꢀFꢀ36 without NH4F
with the name NiAlꢀLDHꢀPꢀ36. MgAlꢀLDHꢀPꢀ36 was
synthesized analogously as NiAlꢀLDHꢀPꢀ36 with the only
replacement of Ni(NO3)2·6H2O to Mg(NO3)2·6H2O in the same
proportion.
Conclusions
In this work, a oneꢀstep hydrothermal method was utilized for
the synthesis of flowerꢀlike NiAlꢀLDH with outer trumpetꢀlike
hierarchical pore structure. XPS results indicated that abundant
hydrogen vacancies at the edge of “flower petals” provided the
nucleation centers for AuNPs in the process of solꢀ
immobilization, resulting in the uniform dispersion of AuNPs.
As comparison, common particle Au/NiAlꢀLDH catalyst was
also synthesized. The catalytic performance of flowerꢀlike
Au/NiAlꢀLDH catalyst and the contrastive one were tested in
the selective oxidation of benzyl alcohol. By contrast, the
activity of flowerꢀlike catalyst was dramatically enhanced by
60% due to confinement effect of the hierarchical pores which
increase the effective collisions between substrates and active
sites. Besides, the influence of pretreatment was also
investigated. After pretreatment in air, Au+ꢀNi2+ hybrid sites at
interface perimeter were partially oxidized to Au3+ꢀNi3+ as
more active reaction centers, which led to further increase in
activity. The cooperation of the Brønsted base sites, NiIII CUS,
and isolated gold cations in the system for the βꢀH eliminate of
Synthesis of Au/LDH catalyst
All the supported Au/LDH catalysts were prepared by the solꢀ
immobilization method. An aqueous solution of hydrogen
tetrachloroaurate (III) trihydrate (HAuCl4·3H2O, AR) with the
desired concentration was stirred with magnetic force which
was named as solution A. The required amount of a PVA
solution (1 wt.%) was added into the solution A (PVA/Au=2
wt./wt.), then a freshly prepared solution of NaBH4 (0.1 M) was
dumped quickly (NaBH4/Au=5 mol/mol) to form a darkꢀbrown
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DOI: 10.1039/C5CY00160A
sol. After the sol generation with 30 min, the reducible AuNPs standard technique. Mesitylene was used as external standard
was immobilized by adding support of which requirement was for GC analysis. In all cases, the carbon balances were
calculated so as to have a total final metal loading of 1% wt. 100±5%.
After 1 h the slurry was filtered, the catalyst washed thoroughly
o
with acetone and deionized water followed by drying at 80 C
overnight.
Acknowledgements
This work was supported by National Natural Science
Foundation, the 973 Project (2011CBA00506), Beijing Natural
Science Foundation (2132032), the Fundamental Research
Funds for the Central Universities (YS1406) and the Beijing
Engineering Center for Hierarchical Catalysts.
Catalyst characterization
The morphology and structure of the samples were examined
using a Zeiss Supra 55 scanning electron microscope (SEM).
Xꢀray diffraction (XRD) patterns were performed by a
Shimadzu XRDꢀ6000 diffractometer using Cu Kα source (λ=
0.154 nm) in the 2θ range from 3° to 70° and a scan step of 10°
minꢀ1.The lattice fringes of the catalysts were characterized
using a JEOL JEMꢀ2100 highꢀresolution transmission electron
microscope (HRTEM). Information on the pore structure of asꢀ
synthesized samples was obtained by mercury intrusion
porosimetry (MIP) using a poreꢀsize analyzer (PoreMasterGT
60, Quantachrome Inc.), which measures the amount of
mercury penetration as a function of the applied pressure. The
BrunauerꢀEmmettꢀTeller (BET) method is based on the
adsorption isotherm. The BarrettꢀJoynerꢀHalenda (BJH) method
was used to calculate the pore volume and the pore size
distribution. Chemical analyses were obtained with inductively
coupled plasma emission spectroscopy (ICPꢀAES; a Shimadzu
ICPSꢀ75000).
Notes and references
*
State Key Laboratory of Chemical Resource Engineering, Beijing
University of Chemical Technology, Box 98, 15 Bei San Huan East
Road, Beijing 100029, China.
Eꢀmail: fengjt@mail.buct.edu.cn, lidq@mail.buct.edu.cn
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
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