Ionic liquid [Bmim][AuCl4] encapsulated in ZIF-8 as precursors to synthesize N-decorated Au catalysts for selective aerobic oxidation of alcohols

Article abstract of DOI:10.1016/j.cattod.2018.11.078

Here we report a novel approach to synthesize Au NPs immobilized on N-doped carbon materials. In this strategy, an ionic liquid (IL) [Bmim][AuCl₄] was selected as the precursor for Au NPs and porous framework of ZIF-8 as the host for the IL. Raman spectroscopy, transmission electron microscopy, N₂ physical absorption and Fourier infrared spectra confirmed that the IL was successfully incorporated in the ZIF-8 pores. Followed by a thermal treatment under inert atmosphere, highly dispersed Au NPs were obtained and stabilized by nitrogen species from the carbonization of organic ligand in the metal-organic frameworks (MOFs). The [Bmim][AuCl₄]@ZIF-8-6.25percent-T materials exhibited high catalytic activity for the selective aerobic oxidation of alcohols, affording excellent yields (up to >99percent) under atmospheric air and base-free conditions. Catalytic reaction along with catalyst characterization results revealed a strong interaction between Au NPs and N species. TEM, XRD and XPS characterization results further suggested that the N species could not only prevent the Au NPs from aggregation, but also further enhance the reaction activity.

Full text of DOI:10.1016/j.cattod.2018.11.078

Accepted Manuscript
Title: Ionic liquid [Bmim][AuCl4] encapsulated in ZIF-8 as
precursors to synthesize N-decorated Au catalysts for selective
aerobic oxidation of alcohols
Authors: Fengfeng Chen, Qingwen Lu, Ting Fan, Ruiqi Fang,
Yingwei Li
PII:
S0920-5861(18)30802-2
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2018.11.078
CATTOD 11814
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
30 September 2018
22 November 2018
30 November 2018
Please cite this article as: Chen F, Lu Q, Fan T, Fang R, Li Y, Ionic liquid
[Bmim][AuCl4] encapsulated in ZIF-8 as precursors to synthesize N-decorated
Au catalysts for selective aerobic oxidation of alcohols, Catalysis Today (2018),
https://doi.org/10.1016/j.cattod.2018.11.078
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Ionic liquid [Bmim][AuCl4] encapsulated in ZIF-8 as precursors to
synthesize N-decorated Au catalysts for selective aerobic oxidation of
alcohols
Fengfeng Chen, Qingwen Lu, Ting Fan, Ruiqi Fang,* Yingwei Li*
State Key Laboratory of Pulp and Paper Engineering, School of Chemistry and Chemical
Engineering, South China University of Technology, Guangzhou 510640, China.
Corresponding author. Email: liyw@scut.edu.cn, fangrq@scut.edu.cn
Graphical Abstract
Highlights:


N-decorated Au catalysts were synthesized through a two-step protocol.
The catalysts were highly active in selective oxidation reactions of a variety of
alcohols.

The catalysts could be reused up to five times without any loss in activity.

Abstract
Here we report a novel approach to synthesize Au NPs immobilized on N-doped
carbon materials. In this strategy, an ionic liquid (IL) [Bmim][AuCl4] was selected as
the precursor for Au NPs and porous framework of ZIF-8 as the host for the IL.
Raman spectroscopy, transmission electron microscopy, N2 physical absorption and
Fourier infrared spectra confirmed that the IL was successfully incorporated in the
ZIF-8 pores. Followed by a thermal treatment under inert atmosphere, highly
dispersed Au NPs were obtained and stabilized by nitrogen species from the
carbonization of organic ligand in the metal-organic frameworks (MOFs). The
[Bmim][AuCl4]@ZIF-8-6.25%-T materials exhibited high catalytic activity for the
selective aerobic oxidation of alcohols, affording excellent yields (up to >99%) under
atmospheric air and base-free conditions. Catalytic reaction along with catalyst
characterization results revealed a strong interaction between Au NPs and N species.
TEM, XRD and XPS characterization results further suggested that the N species
could not only prevent the Au NPs from aggregation, but also further enhance the
reaction activity.
Key words：metal-organic frameworks, ionic liquid, N-doping, alcohol oxidation,
heterogeneous catalysis, Au catalysts.
1. Introduction
Selective oxidation of alcohols to carbonyl compounds is highly important in
organic synthetic chemistry [1-4]. Recently, some non-noble transitional metals such
as Co [5], Co oxides [6] and Fe [7] have been employed in order to reduce the usage

of noble metals. In this regard, Co nanoparticles stabilized by N-doped carbon
composite (Co/C–N) was synthesized by one-pot thermal decomposition of a
Co-containing MOF (metal-organic framework), which could catalyze the selective
oxidation of benzyl alcohols in neat water even under an atmospheric pressure of air
[5]. Zhao et al. reported a highly efficient Co-N-C catalyst for aerobic oxidation of
alcohols, which was prepared simply by pyrolysis of the commercial carbon
impregnated Co-based deep eutectic solvents (DES) [8]. However, these
transition-metal catalysts often suffered from unsatisfied catalytic performance even
in the presence of a large excess of base additives or explosive oxidizing agents [8]. In
contrary, the noble metal based catalysts such as Au [9-11], Pd [12], Pt [13], Ru [14]
and their polymetallic alloys [15] exhibited excellent activities, selectivities and
stabilities for oxidation reactions under relatively mild conditions.
Recently, Au has emerged as one of the most powerful catalysts for aerobic
oxidations owing to its high chemo-selectivity and stability under mild conditions
[16]. Extensive protocols including wetness impregnation, ions exchanging, etc. were
developed for the synthesis of Au catalysts employing different supports (e.g.,
metal-oxides, silicate, and active carbon) [9-11,17,18]. However, most of the reported
approaches suffered from aggregations of Au NPs (>5 nm), leading to unsatisfied
atom economy and catalytic performance. In the views of environment and economy,
it is highly desirable to develop facile and novel approaches for the preparation of Au
catalysts with small particle sizes.
Ionic liquids (ILs) are a class of state-of-the-art solvents and have been regarded

as one of the ideal metal resources for the synthesis of metal-carbon composites
owing to their virtually unlimited tunability, negligible vapor pressure, excellent
solubility, and high thermal stability [19-24]. On the other hand, MOFs, possessing
high surface areas, tunable porosities and ordered structures are believed to be
promising sacrificing templates for the preparation of porous carbon-based
nanoarchitectures [25-30]. Herein, we propose a novel strategy to immobilize
ultrafine metal NPs within N-doped carbon materials. Our approach relies on the
effective confinement and isolation of the ILs by the MOF pores, which could prevent
metal NPs from serious aggregation in the subsequent pyrolysis process. As a proof of
concept, we choose ZIF-8 (ZIF: zeolitic imidazolate framework) as a model host and
[Bmim][AuCl4] as the precursor to demonstrate the effectiveness of our method. After
pore filling and pyrolysis, highly-dispersed and ultrafine Au NPs have been
successfully prepared and immobilized within N-doped carbon materials. The strong
interaction between Au NPs and N species endows the nanocomposites with superior
catalytic performance in the selective oxidation of alcohols under atmospheric air and
based-free conditions.
2. Experimental
2.1. Catalyst preparation
2.1.1. Preparation of [Bmim][AuCl4]
[Bmim][AuCl4] was prepared from the neutralization of HAuCl4 and
1-methyl-3-butylimidazolium hydroxide. Briefly, an aqueous solution of
1-methyl-3-butylimidazolium hydroxide ([Bmim]OH) was first obtained from

[Bmim]Cl by using the anion-exchange resin method [31, 32], to which an equimolar
HAuCl4 was added. Then, the mixture was stirred at room temperature for 2 h.
Subsequently, the water was removed by distillation at 65 ˚C under reduced pressure.
Finally, the obtained product was dried under vacuum at 80 °C for 48 h to eliminate
traces of impurities.
2.1.2. Synthesis of ZIF-8
ZIF-8 was prepared according to the previous report [33]. Typically,
2-methylimidazole (1314 mg, 16 mmol) was dissolved in 15 mL methanol in flask A
under stirring. Zn(NO3)2·6H2O (1190 mg, 4 mmol) was dissolved in 30 mL methanol
under ultrasound for 15 min to form a clear solution in flask B. Then, the solution in
flask B was added into flask A with vigorous stirring for 1 h at room temperature. The
solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave and
heated at 120 °C for 4 h. The resulting white powder was washed with methanol for
three times and finally dried at 80 °C under vacuum overnight.
2.1.3. Synthesis of [Bmim][AuCl4]@ZIF-8-x-T
In a typical synthesis, 100 mg ZIF-8 was added into a two-necked flask (one of the
necks was sealed by a rubber stopper, and the other was connected to a high-vacuum
line). The flask was placed in an oil bath heating at 70 °C under ultrahigh vacuum (1
× 10−5 Pa) for 1 h to draw out the gas inside the ZIF-8. Then, a certain amount of
[Bmim][AuCl4] dissolved in 5 mL methanol was transferred into the flask through a
syringe. The mixture was stirred for 12 h at room temperature to fill the pores of
ZIF-8 with ILs, and then continued to agitate under ultra-high vacuum for 12 h at

70 °C. After cooling at room temperature for 1 h, the light yellow powders were
obtained after washing with methanol to recycle the residue ILs. The as-synthesized
materials were denoted as [Bmim][AuCl4]@ZIF-8-x, where x indicated the weight
ratio of IL to ZIF-8. To obtain Au NPs, 50 mg of the [Bmim][AuCl4]@ZIF-8-x
precursor was placed in a tubular furnace and then heated to a certain temperature
with a heating rate of 2 °C min−1 and kept for 3 h under flowing argon. The obtained
materials were denoted as [Bmim][AuCl4]@ZIF-8-x-T, where T indicated the
calcination temperatures.
2.1.4. Synthesis of Au/C catalysts
10 wt% Au/C was prepared by a traditional impregnation method. Typically, 100
mg of activated carbon was suspended in 60 mL solution of ethanol and water (v : v =
2 : 1). To which 6 mL aqueous solution of 20 mg HAuCl4·3H2O was pumped
dropwise into the flask under continuous stirring and the mixture was further stirred
﹣
for 30 min at room temperature. Then, 6 mL NaBH4 (nNaBH4/nAuCl4 = 5 : 1)
solution of alcohol and water (v : v = 2 : 1) of was quickly poured into the mixture and
stirred for another 30 min. The Au/C catalyst was collected after centrifugation,
washing with distilled water for 3 times and alcohol for 2 times, and drying under
vacuum at 80 °C for 12 h.
2.2. Catalyst characterization
The molecular structure of [Bmim][AuCl4] prepared in this work was verified
1
1
using H NMR, 13C NMR and FTIR spectra. H NMR and 13C NMR spectra were

collected on a Bruker-400 MHz spectrometer with TMS as the internal standard. FTIR
spectra were recorded on Bruker-VERTEX 33 spectrometer. The decomposition and
glass transition temperatures were determined by using a TG (SQT-600) from 25 to
900 °C at a heating rate of 10 °C/min under a continuous N2 flow of 100 mL/min.
Powder X-ray diffraction (PXRD) patterns of the samples were recorded on a Bruker
D8 ADVANCR (X’pert Power, 2.2 kW) using Cu Kα radiation (40 kV, 40 mA, λ =
1.543 Å).
The size, morphology, and elemental distribution were determined by using a
high-resolution transmission electron microscope (TEM, JEOL, JEM-2100F) with an
EDX analysis system (Bruker Xflash 5030T) at 200 kV. X-ray photoelectron
spectroscopy (XPS) with a Thermo ESCALAB 250XI multifunctional imaging
electron spectrometer was used to analyze the electronic states and surface interaction
among the elements of the samples. BET surface area and pore size measurements
were performed with N2 adsorption at ﹣196 °C on a Micromeritics ASAP 2020M
instrument. Before the analyses, the samples were degassed at 100 °C for 6 hours. The
gold contents of the samples were determined by atomic absorption spectroscopy
(AAS) by using a HITACHI Z-2300 instrument. The surface morphology of the
materials was investigated using a high-resolution scanning electron microscope
(SEM, MERLIN of ZEISS) equipped with an energy dispersive X-ray spectrometer
(EDS).
2.3. Catalytic reactions

In
a
typical run, alcohol (0.05 mmol), toluene (1 mL), and
[Bmim][AuCl4]@ZIF-8-x-T (10 mol Au% based on alcohols) were added to a 25 mL
Schlenk tube. The reaction mixture was stirred at 130 °C under atmospheric air. After
reaction, qualitative analyses of the reaction mixtures were performed on a GC-MS
spectrometer (Agilent, 7890B GC/5977A MS) equipped with a HP-5 MS capillary
column (0.25 mm × 30 m).
2.4. Recycling of catalyst
The recyclability of the [Bmim][AuCl4]@ZIF-8-6.25%-900 catalyst was tested
for the aerobic oxidation of benzyl alcohol under the same reaction conditions as
described above, except for using the recovered catalyst. Each time, the catalyst was
isolated from the reaction mixture by centrifugation at the end of catalytic reaction,
thoroughly washed with toluene for 3 times, and then reused directly as catalyst for
the next run.
3. Results and discussion
3.1. Structural characterization of [Bmim][AuCl4]@ZIF-8 composites
The preparation procedure for the [Bmim][AuCl4]@ZIF-8 composites is
illustrated in Figure 1 and Figure S1. ZIF-8, with a cavity diameter of 11.6 Å and pore
aperture diameter of 3.4 Å [34], has been used as host matrix to encapsulate Fe(acac)3
[35], Ru3(CO)12 [36], and [Bmim][Tf2N] [37] for efficient catalysis and CO2
separation. Therefore, we reasoned that [Bmim][AuCl4] ionic liquid which has the
similar size as the above-mentioned precursors could also be encapsulated in ZIF-8
for potential catalysis application (Figure S2) . We selected [Bmim][AuCl4] as

precursor because the cations contain imidazolium and the anions contain Au species
to form Au NPs, and the highly stable ZIF-8 with a high carbon content and large
surface area is a good candidate to prepare porous carbons directly from carbonization
[38]. After pyrolysis, the imidazolium motif can form nitrogen doped carbon which
may increase the oxidation stability significantly [39]. On the other hand, the formed
Au NPs or Au SACs have demonstrated excellent catalytic performances in a variety
of heterogeneous transformations [40-42].
Fig. 1 (a) Schematic illustration of the incorporation of [Bmim][AuCl4] within ZIF-8.
(b) Experimental setup used in this work to prepare [Bmim][AuCl4]@ZIF-8.
To ensure an efficient incorporation of ILs into the nanocages of ZIF-8, the
ZIF-8 was degassed thoroughly under high vacuum to completely remove any gas
residual in the MOF pores [43, 44]. Then, [Bmim][AuCl4] dissolved in CH3OH was
added to impregnate the MOF through a syringe (Figure S3). The resultant mixture
was cooled and collected by centrifugation. As shown in Figure S3c, the methanol
solution in the upper layer was clear, indicating that the ILs has been incorporated in

the ZIF-8 pores.
As can be seen from Figure 2a, the powder X-ray diffraction (PXRD) patterns
matched well with those of the parent ZIF-8 even after the incorporation of 50%
[Bmim][AuCl4]. Raman spectra provided evidence that the successful confinement of
[Bmim][AuCl4] into ZIF-8 was achieved (Figure 2b). Two vibration bands at 335 cm-1
and 371 cm-1 were assigned to Au-Cl bands in [Bmim][AuCl4]@ZIF-8-50%.
Compared with bulk IL, blue shifts of the Au-Cl bands were observed, which could be
attributed to a strong interaction of Cl atoms of [AuCl4]− with the metal nodes of
ZIF-8 [37]. In addition, we also collected the Raman spectra of IL coated on the outer
surface of ZIF-8 (i.e., [Bmim][AuCl4]/ZIF-8), no significant shifts of the Au-Cl bands
were observed.
A new characteristic peak appeared at 837 cm-1 in the Fourier transform infrared
(FTIR) spectra (Figure 2c), which can be assigned to the incorporated IL. However,
the stretching vibration peak of [Bmim]+ was not clearly observed in Figure 2c,
because it has the same functional group as ZIF-8. Then, we conducted the FTIR
difference spectra of ZIF-8 and [Bmim][AuCl4]@ZIF-8-50% in the range of 500 to

Fig. 2 Confirming the structure properties of [Bmim][AuCl4]@ZIF-8-50%. (a)
PXRD patterns, (b) Raman spectra, (c) FT-IR spectra, (d) FTIR difference spectra of
ZIF-8 and [Bmim][AuCl4]@ZIF-8-50% in the range of 500 to 1800 cm-1, (e) EDX
patterns of [Bmim][AuCl4]@ZIF-8-50%, (f) TG curves, (g) N2 adsorption isotherms,
(h) pore size distributions.

1800 cm-1 [45]. As shown in Figure 2d, a few peaks at 1589, 1406, 1316, 1157, 740,
674 cm-1 appeared in the difference spectra of [Bmim][AuCl4]@ZIF-8 and ZIF-8.
Compared with the bulk IL, shifts in band positions were noticed, which can be
explained by the strong interaction of N atom of [Bmim]+ with the metal nodes of
ZIF-8 [37].
The Energy-dispersive X-ray spectroscopy (EDX) confirmed the presence of Au
and Cl elements in [Bmim][AuCl4]@ZIF-8-50% (Figure 2e). Thermogravimetric
analysis (TGA) further confirmed the strong interactions between the IL and ZIF-8, as
the IL’s decomposition temperature was significantly enhanced. As shown in Figure
2f, the TGA data showed the bulk [Bmim][AuCl4] started to decompose at 224 °C,
whereas the [Bmim][AuCl4]@ZIF-8-50% decomposed at a higher temperature of
258 °C. A second weight loss was detected at about 530 °C, which could be attributed
to the decomposition of ZIF-8.
N2 adsorption and desorption isotherms of ZIF-8 and [Bmim][AuCl4]@ZIF-8-50%
were measured at ﹣ 196 °C. As shown in Figure 2g, both ZIF-8 and
[Bmim][AuCl4]@ZIF-8-50% displayed a type-I isotherm, characteristics of
microporous materials. Owing to the space-occupying of ILs in the ZIF-8 pores, the
N2 adsorption amount decreased significantly, whereas the porous nature and high
surface
areas
were
still
preserved
after
IL incorporation.
The
Brunauner-Emmett-Teller (BET) surface areas decreased from 1399 m2 g-1 for ZIF-8
to 739 m2 g-1 for [Bmim][AuCl4]@ZIF-8-50% (Table S1), together with a decrease in
the pore volume from 0.67 mL g-1 to 0.35 mL g-1. The pore size distribution also

indicated that the pore volume of ZIF-8 was remarkably reduced after the
incorporation of ILs (Figure 2h), suggesting that ILs were encapsulated inside the
pores of ZIF-8 rather than on the external surface.
The morphology and microstructure of the [Bmim][AuCl4]@ZIF-8-50% and
ZIF-8 were investigated by scanning electron microscope (SEM), transmission
electron microscopy (TEM) and high-angle annular dark-field scanning transmission
electron microscopy (HAADF-STEM). The typical SEM and TEM images showed
spheric morphology with smooth surface, which is consistent with the XRD results
(Figure S4, Figure 2a). It can be seen that the NPs were evenly distributed in ZIF-8
(Figure S4f), probably due to a slight aggregation of [Bmim][AuCl4] owing to the
high loading (i.e., 50%). The TEM and HAADF-STEM images of a representative
ultrathin slice from [Bmim][AuCl4]@ZIF-8-50% further confirmed that the ILs were
successfully confined and evenly distributed in the ZIF-8 pores (Figure 3).
Subsequently, various [Bmim][AuCl4]@ZIF-8 composites with different contents
of IL were prepared and the photographs of the as-prepared materials were shown in
Figure S5. The PXRD patterns of the IL-incorporated samples (Figure S6) showed no

Fig. 3 (a-d) TEM and HAADF-STEM images of an ultrathin cut from
[Bmim][AuCl4]@ZIF-8-50%, and (e-h) the corresponding elemental mappings of Au,
Zn, N and Au+Zn+N.
obvious crystallinity loss for IL@ZIF-8 after incorporating [Bmim][AuCl4],
consistent with the SEM images (Figure S7). SEM images showed that at relatively
low loadings of ILs, the obtained IL@ZIF-8 composites basically maintained their
original rhombic dodecahedron shape. N2 physical adsorption measurements indicated
that the BET surface area of pristine ZIF-8 decreased from 1399 to 1159, 1263 and
1358 m2 g-1 when the loading amount of the IL was 12.5, 6.25, 3.13 wt% (Table S1
and Figure S8), respectively.
3.2. Structural characterization of [Bmim][AuCl4]@ZIF-8-x-T
The obtained [Bmim][AuCl4]@ZIF-8 was pyrolyzed at high temperatures to
yield nitrogen doped porous carbon stabilized Au NPs. TGA of
[Bmim][AuCl4]@ZIF-8 indicated that the ZIF-8 structure began to decompose when
the temperature was increased to ca. 530 °C under nitrogen (Figure 2f). Therefore, the
calcination temperatures for carbonization of [Bmim][AuCl4]@ZIF-8 were varied
from 700 to 1000 °C under flowing argon.
The crystal structures of the [Bmim][AuCl4]@ZIF-8-6.25%-T (T = 700, 800, 900,
1000) were identified by XRD analysis (Figure S9). All these
[Bmim][AuCl4]@ZIF-8-6.25%-T materials exhibited weak and broad peaks at around
2θ = 38.2º, 44.4º, 64.6º and 77.5º indexed to the (111), (200), (220) and (311)
crystalline planes of metallic Au (JCPDS file 04-0784), respectively. Furthermore,

compared with metallic Au, both [Bmim][AuCl4]@ZIF-8-6.25%-900 and
[Bmim][AuCl4]@ZIF-8-6.25%-1000 showed a slight shift in the characteristic (111)
peak, with a value of 0.7° and 0.5°, respectively. This result suggested a strong
interaction between Au NPs and N species, which may be beneficial for the
preparation of highly dispersed Au NPs on N-doped carbon materials. To further
confirm such a strong interaction in [Bmim][AuCl4]@ZIF-8-6.25%-T, TEM and
HAADF-STEM were performed on [Bmim][AuCl4]@ZIF-8-6.25%-T (Figure 4).
From the TEM and HAADF-STEM images (Figure 4a-4d), it can be seen that Au NPs
was highly dispersed on the N-doped carbon in [Bmim][AuCl4]@ZIF-8-6.25%-T, of
which the average size was 2.98 nm with a narrow size distribution (insert in Figure
4a). The corresponding high-resolution TEM (HRTEM) image (Figure 4e) showed
clear lattice fringes with the interplane distances of 0.252 nm and 0.292 nm, larger
than that of metallic Au NPs, which might result from the interactions between N and
Fig. 4 Characterization of [Bmim][AuCl4]@ZIF-8-6.25%-900. (a, c) TEM, (b, d)
HAADF-STEM images, and (e) HRTEM image of an individual Au nanoparticle. (f-i)
The corresponding elemental mappings of C, N, Au and Au+N. (j) The elemental
line-scan profiles of a selected Au nanoparticle. Insert in (a) is the size distribution
histogram of Au particles.

Au species [46]. The corresponding STEM-EDX elemental mapping images (Figure
4f-4i) indicated that all the C, N and Au were homogeneously dispersed in
[Bmim][AuCl4]@ZIF-8-6.25%-900. Notably, the EDS line-scanning profile (Figure
4j) across a single Au NP revealed similar distribution of Au and N elements, further
confirmed the interaction between Au and N in the as-synthesized materials.
In order to investigate the influence of IL’s loading on the size of Au NPs, we
prepared [Bmim][AuCl4]@ZIF-8 with different IL loadings in mass and then
pyrolyzed them at 900 °C, obtaining a series of [Bmim][AuCl4]@ZIF-8-x-900
materials. As shown in Figure 5, we can see that uniform Au NPs were evenly
distributed on these N-doped carbon materials. The hollow morphology of
[Bmim][AuCl4]@ZIF-8-x-900 was related to the generation of HCl during the
decomposition of [Bmim][AuCl4]@ZIF-8 [47, 48]. Besides, with the increment of IL
loadings, the Au ions derived from IL tend to aggregate during calcination. The mean

Fig.
5
Characterization
of
[Bmim][AuCl4]@ZIF-8-12.5%-900,
[Bmim][AuCl4]@ZIF-8-6.25%-900 and [Bmim][AuCl4]@ZIF-8-3.13%-900. (a, b, e,
f, i, j) TEM, and (c, g, k) HAADF-STEM images. (d, h, l) The size distribution
histogram of Au particles.
Au
NPs
sizes
of
[Bmim][AuCl4]@ZIF-8-3.13%-900
and
[Bmim][AuCl4]@ZIF-8-6.25%-900 is 2.91 and 2.98 nm, respectively. However, the
average particle size of the [Bmim][AuCl4]@ZIF-8-12.5%-900 reached 3.69 nm,
indicating the significant aggregation of Au NPs. Therefore, 6.25 wt% is the most
favorable IL loading to obtain desirable [Bmim][AuCl4]@ZIF-8-6.25%-900 with both
high dispersity and small particle size. As expected, the BET surface areas decreased
with an increase in the IL loadings (Figure S10, Table S1 and S2). To gain further

Fig. 6 (a, b) Au 4f and N 1s spectra for [Bmim][AuCl4]@ZIF-8-6.25%-T (T = 700,
800, 900, 1000) (c, d) Au 4f and N 1s spectra for [Bmim][AuCl4]@ZIF-8-12.5%-900,
[Bmim][AuCl4]@ZIF-8-6.25%-900 and [Bmim][AuCl4]@ZIF-8-3.13%-900.
different temperatures. As shown in Figure 6a, the Au 4f spectra of
[Bmim][AuCl4]@ZIF-8-6.25%-T demonstrated that Au element in these samples
mainly existed in the form of Au0 and Au3+ (Table S3) [49]. Moreover, the binding
energies
of
[Bmim][AuCl4]@ZIF-8-6.25%-900
and
[Bmim][AuCl4]@ZIF-8-6.25%-1000 exhibited about 1.5 eV shift as compared with
[Bmim][AuCl4]@ZIF-8-6.25%-700 for the Au3+ 4f5/2 band. The decreased binding
energy
of
Au3+
4f5/2
for
[Bmim][AuCl4]@ZIF-8-6.25%-900
and
[Bmim][AuCl4]@ZIF-8-6.25%-1000 suggested the electron transfer from N to Au. At
the same time, four peaks in the N 1s spectra located at 398.5, 398.96, 400.96 and
403.9 eV (Figure 6b and Table S4) were attributed to pyridinic-N, Au-Nx, pyrrolic-N
and
ammonium-N
in
[Bmim][AuCl4]@ZIF-8-6.25%-900
and
[Bmim][AuCl4]@ZIF-8-6.25%-1000, respectively [50]. Furthermore, XPS
characterization was also performed on the different pyrolysis products at 900 °C with
different IL mass loadings, as shown in Figure 6c and Figure 6d. It can be seen that
the [Bmim][AuCl4]@ZIF-8-6.25%-900 showed the lowest binding energy for Au3+
4f5/2 and N1s in Au-Nx. This could be attributed to the strongest interaction between
Au and N. Such a coordination interaction was advantageous in preventing a serious
aggregation of Au in the course of MOF pyrolysis, which is consistent with the
average size of Au NPs as presented in Figure 5.

3.3. Catalytic performances
The catalytic properties of [Bmim][AuCl4]@ZIF-8-x-900 for selective oxidation
of alcohols were investigated. The reactions were performed under base-free
conditions using atmospheric pressure of air as the green oxidant. In the absence of
any catalyst, the reaction hardly occurred under the investigated condition (Table 1,
entry 1). For comparison, the catalytic activities of ZIF-8-900 and Au/C were also
examined, both of which showed a low yield of benzaldehyde (entries 2 and 9). A
series of [Bmim][AuCl4]@ZIF-8-x-T materials were then tested as catalysts in the
aerobic oxidation of benzyl alcohol. As is shown in Table 1,
[Bmim][AuCl4]@ZIF-8-6.25%-900 was the most active one for benzyl alcohol
oxidation with benzaldehyde as the predominant product (Table 1 entry 5). The
[Bmim][AuCl4]@ZIF-8-6.25%-700 and [Bmim][AuCl4]@ZIF-8-6.25%-800 gave a
very low yield of benzaldehyde possibly due to the absence of interaction between Au
Table 1. Oxidation of benzyl alcohol catalyzed by various catalysts.a
Entry
Catalyst
Au-Nx
Conversion
Yieldc (%)
content (%)b
-
(%)
<1
1
2d
3
-
<1
26
4
ZIF-8-900
-
26
4
[Bmim][AuCl4]@ZIF-8-6.25%-700
[Bmim][AuCl4]@ZIF-8-6.25%-800
[Bmim][AuCl4]@ZIF-8-6.25%-900
[Bmim][AuCl4]@ZIF-8-6.25%-1000
[Bmim][AuCl4]@ZIF-8-12.5%-900
[Bmim][AuCl4]@ZIF-8-3.13%-900
Au/C
0
4
0
12
99
46
79
98
7
12
99
46
79
98
7
5
3.11
0.59
2.70
4.07
-
6
7
8
9
a Reaction conditions: benzyl alcohol (0.05 mmol), catalyst (10 mol% Au based on

benzyl alcohol), toluene (1.0 mL), air, 4 h.
b Measured by XPS.
c Conversion and yield was determined by GC-MS.
d Catalyst: 10 mg.
NPs and N species in these two materials, as determined by XPS. The lower activity
of
[Bmim][AuCl4]@ZIF-8-6.25%-1000
as
compared
to
[Bmim][AuCl4]@ZIF-8-6.25%-900 might be due to the bigger Au NPs obtained at
1000 °C and the lower percentage of Au-Nx species in the total N atoms of
[Bmim][AuCl4]@ZIF-8-6.25%-1000 (Au-Nx
=
0.59%) as compared to
[Bmim][AuCl4]@ZIF-8-6.25%-900 (Au-Nx = 3.11%). Based on the catalysis and
characterization results, we proposed that the heat treatment at 900 ºC was preferable
for the formation of Au-Nx species, which were crucial and would significantly
enhance the catalytic performance in benzyl alcohol oxidation. Interestingly, the Au/C
catalyst showed a very low yield of benzaldehyde due to the absence of Au-Nx
species, which further evidenced the significance of Au-Nx species. Besides, the large
amount of uniform distributed small Au NPs and high BET surface areas also
contributed to the high benzaldehyde yield.
Moreover, when [Bmim][AuCl4]@ZIF-8-6.25%-900 was removed by a hot
filtration when the reaction was stopped at 1 h (ca. 54% conversion), the oxidation
reaction of alcohol almost no longer proceeded. AAS analysis of the reaction solution
revealed that the Au content in the solution was below the detection limit, implying no
significant leaching of Au NPs occurred during the oxidation reaction.

Fig. 7 Influence of catalyst dosage (a), and reaction temperature (b) on the aerobic
oxidation of alcohols over [Bmim][AuCl4]@ZIF-8-6.25%-900.
With benzyl alcohol as the model substrate, the effect of various solvents on the
catalytic performances was investigated. Toluene was shown to be the best one among
the solvents investigated (Table S5, entry 4). The catalyst dosages and reaction
temperatures were further optimized. As shown in Figure 7a, when the catalyst dosage
increased from 5 mol% to 10 mol% (based on alcohols), the yield of benzaldehyde
was rapidly enhanced from 22% to 99% after 4 h of reaction. However, further
increasing the catalyst dosage to 15 mol%, the reaction rate was almost unchanged
and no obvious increase in the yield of benzaldehyde was observed; and further
prolonging the reaction time, no by-product was detected in the reaction. As shown in
Figure 7b, the yield of benzaldehyde increased rapidly with an increase in the reaction
temperature from 100 °C to 130 °C.
Furthermore, the substrate scope of selective alcohol oxidation over the
[Bmim][AuCl4]@ZIF-8-6.25%-900 material was also investigated under atmospheric
pressure of air and base-free conditions (Table 2). It can be seen that various benzyl
alcohols bearing electron-donating or electron withdrawing groups could approach the

corresponding substituted benzaldehydes in excellent yields by simply prolonging the
reaction time.
Finally, the reusability of the [Bmim][AuCl4]@ZIF-8-6.25%-900 catalyst in the
aerobic oxidation of benzyle alcohol was investigated. The catalyst could be separated
from the reaction mixture by centrifugation. It can be seen from Figure S11 that
[Bmim][AuCl4]@ZIF-8-6.25%-900 can be recovered and reused for at least six runs
in the subsequent reaction without significant decreases in catalytic activity after each
run, indicating the catalyst was stable under the investigated conditions.
Table 2. Selective oxidation of various alcohols catalyzed by
[Bmim][AuCl4]@ZIF-8-6.25%-900a
Entry
Substrate
Product
Time (h)
Con (%)
99
Sel (%)
>99
99
1
2
3
4
5
6
4
12
12
36
12
24
96
98
99
20
99
99
99
98
99

7
36
96
24
36
36
48
72
96
72
36
36
96
25
93
97
99
93
99
33
80
95
100
99
8
99
9
99
10
11
12
13
14
15
16
17
>99
99
>99
99
99
99
95
99
a Reaction conditions: alcohols (0.05 mmol), Catalyst ([Bmim][AuCl4]@ZIF-8-6.25%-900: 10
mol% Au based on benzyl alcohol), toluene (1 mL), air, 130 ºC, reflux. Conversion and
selectivity were determined by GC-MS.
4. Conclusion
In conclusion, we have developed a novel, highly efficient, and reusable

[Bmim][AuCl4]@ZIF-8-6.25%-900 for selective aerobic oxidation of alcohols. The
[Bmim][AuCl4]@ZIF-8-x-T composites are synthesized by one-step thermolysis of
[Bmim][AuCl4]@ZIF-8, in which [Bmim][AuCl4] is isolated in the pores of ZIF-8.
During the pyrolysis process, the carbonization of organic ligands in the MOF may
prevent Au NPs from aggregation, and the presence of N species is beneficial for the
improvement of metal dispersion of the yielded metal-carbon materials. The
as-prepared [Bmim][AuCl4]@ZIF-8-6.25%-900 materials are highly active and
selective in the aerobic oxidation of alcohols under atmospheric pressure of air and
base-free conditions. The excellent catalytic performances are suggested to be related
to the high dispersion of Au NPs and the strong interaction between Au NPs and N
species. Our study not only opens up a new avenue for the design of high-dispersion
NPs immobilized in N-doped carbon materials with high loadings and small sizes, but
also directly demonstrates the efficient catalysis of Au and N synergistic effect for
selective aerobic oxidation of alcohols.
Acknowledgements
This work was supported by the National Natural Science Foundation of China
(21436005, 21576095, 21825802), the Fundamental Research Funds for the Central
Universities (2017PY004), the Science and Technology Program of Guangzhou
(201804020009), the State Key Laboratory of Pulp and Paper Engineering
(2017ZD04, 2018TS03), and the Natural Science Foundation of Guangdong Province
(2016A050502004, 2017A030312005).

Competing financial interests
The authors declare no competing financial interests.
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