Base-free aerobic oxidation of 5-hydroxymethyl-furfural to 2,5-furandicarboxylic acid in water catalyzed by functionalized carbon nanotube-supported au-pd alloy nanoparticles

Article abstract of DOI:10.1021/cs5003096

The aerobic oxidation of 5-hydroxymethylfurfural (HMF), a key platform compound in cellulose transformation, into 2,5-furandicarboxylic acid (FDCA), a promising renewable alternative to petroleum-derived terephthalic acid, is one of the most attractive reactions for establishing biomass-based sustainable chemical processes. Supported Au catalysts have shown encouraging performance for this reaction, but the need of an excess amount of base additives makes the process less green and less cost-effective. Here, we report a stable and efficient carbon nanotube (CNT)-supported Au-Pd alloy catalyst for the aerobic oxidation of HMF to FDCA in water without any bases. The functionalization of CNT surfaces is crucial for FDCA formation. We have clarified that the CNT containing more carbonyl/quinone and less carboxyl groups favors FDCA formation by enhancing the adsorption of the reactant and reaction intermediates. Significant synergistic effects exist between Au and Pd in the alloy for the base-free oxidation of HMF to FDCA through three tandem steps. The present work provides understanding of the support-enhanced adsorption effect and the alloying effect for supported Au-based bimetallic catalysts, and this knowledge may help develop efficient catalysts for the aerobic oxidation of relatively complicated organic compounds with different functional groups in water.

Full text of DOI:10.1021/cs5003096

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Article
Base-Free Aerobic Oxidation of 5-Hydroxymethylfurfural to 2,5-
Furandicarboxylic Acid in Water Catalyzed by Functionalized
Carbon Nanotube-Supported Au-Pd Alloy Nanoparticles
Xiaoyue Wan, Chunmei Zhou, Jiashu Chen, Weiping Deng, Qinghong Zhang, Yanhui Yang, and Ye Wang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs5003096 • Publication Date (Web): 26 May 2014
Downloaded from http://pubs.acs.org on May 28, 2014
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Base-Free Aerobic Oxidation of 5-Hydroxymethyl-
furfural to 2,5-Furandicarboxylic Acid in Water
Catalyzed by Functionalized Carbon Nanotube-
Supported Au-Pd Alloy Nanoparticles
†
‡
†
†
,†
Xiaoyue Wan, Chunmei Zhou, Jiashu Chen, Weiping Deng, Qinghong Zhang,* Yanhui
‡
,†
Yang, and Ye Wang*
†
State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center
of Chemistry for Energy Materials, National Engineering Laboratory for Green Chemical
Productions of Alcohols, Ethers and Esters, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen 361005, China
‡
School of Chemical and Biomedical Engineering, Nanyang Technological University,
Singapore 637459, Singapore
*CORRESPONDING AUTHOR: E-mail: wangye@xmu.edu.cn or zhanqh@xmu.edu.cn;
Phone: +86-592-2186156; Fax: +86-592-2183047
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ABSTRACT: The aerobic oxidation of 5-hydroxymethylfurfural (HMF), a key platform
compound in cellulose transformation, into 2,5-furandicarboxylic acid (FDCA), a promising
renewable alternative to petroleum-derived terephthalic acid, is one of the most attractive
reactions for establishing the biomass-based sustainable chemical processes. Supported Au
catalysts have shown encouraging performance for this reaction, but the need of excess amount
of base additives makes the process less green and less cost-effective. Here, we report an
efficient and stable carbon nanotube (CNT)-supported Au-Pd alloy catalyst for the aerobic
oxidation of HMF to FDCA in water without any bases. The functionalization of CNT surfaces is
crucial for FDCA formation. We have clarified that the CNT containing more carbonyl/quinone
and less carboxyl groups favors FDCA formation by enhancing the adsorption of the reactant and
reaction intermediates. Significant synergistic effects exist between Au and Pd in the alloy for
the base-free oxidation of HMF to FDCA through three tandem steps. The present work provides
deep understanding of the support-enhanced adsorption effect and the alloying effect for
supported Au-based bimetallic catalysts, and may help develop efficient catalysts for the aerobic
oxidation of relatively complicated organic compounds with different functional groups in water.
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KEYWORDS: aerobic oxidation, 5-hydroxymethylfurfural, sustainable chemistry, carbon
nanotube, gold catalysis, alloying effect
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1
. INTRODUCTION
The depletion of fossil resources has stimulated recent research activities for the utilization of
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renewable biomass resources for the sustainable production of chemicals. An ideal route for the
chemical utilization of cellulose, which is the main component of the abundant and non-edible
lignocellulosic biomass, is to develop catalytic processes for the selective conversion of cellulose
into a platform compound under mild conditions, followed by subsequent production of high-
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valued chemicals from the platform compound.
5-Hydroxymethylfurfural (HMF) is a
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promising platform compound from cellulose or cellulose-derived carbohydrates. The aerobic
oxidation of HMF into 2,5-furandicarboxylic acid (FDCA) (Equation (1)) is one of the most
attractive reactions for establishing biomass-based sustainable chemical processes via HMF,
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since FDCA is a versatile building-block chemical. In particular, FDCA is a promising
renewable alternative to terephthalic acid, which is a petroleum-derived monomer and is widely
used for the production of key polymers such as polyethylene terephthalate (PET) and
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polybutyleneterephthalate (PBT) plastics.
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On the other hand, gold catalysis has attracted much attention in recent years.
In addition
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to the well-known examples for CO oxidation and propylene epoxidation,
supported Au
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catalysts can catalyze the aerobic oxidation of various substrates in the liquid phase.
It is
generally accepted that the Au-based catalysts are more stable and selective for the aerobic
oxidation of organic compounds in water than the conventional liquid-phase oxidation catalysts
18,25-29
2
based on Pt and Pd because the Au catalysts can offer better resistance to water and O .
However, Au may be deactivated more easily by the formed organic products or intermediates
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such as carboxylic acid in the absence of a base promoter. The alloying of another metal (e.g.,
Pd and Pt) with Au to form bimetallic alloy catalysts may combine the advantages (functions) of
different components in atomic level and may enhance the activity and stability. Such an
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alloying effect has been found in many aerobic oxidation reactions.
and co-workers have demonstrated that the supported Au-Pd alloy catalysts show excellent
performances for many oxidation reactions such as the direct synthesis of H and the selective
Many useful insights have been obtained from these
In particular, Hutchings
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oxidation of alcohols or hydrocarbons.
systems, nevertheless, more knowledge on the deep understanding of the alloying effect is still
needed.
The aerobic oxidation of HMF to FDCA requires the oxidation of both the hydroxyl and the
aldehyde groups to carboxyl groups (Equation (1)). The formation of FDCA is expected to
involve several tandem oxidation steps. Thus, the catalyst design for the aerobic oxidation of
HMF to FDCA is more complicated than that for simpler oxidation reactions such as the
oxidation of alcohols or aldehydes. Supported Au nanoparticles have shown encouraging
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catalytic performances for the aerobic oxidation of HMF to FDCA in water,
although Pt-, Pd-,
or Ru-based catalysts can also catalyze the selective oxidation of HMF to FDCA or 2,5-
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diformylfuran (DFF).
However, the stability of the supported Au catalysts in the aerobic
oxidation of HMF remains a problem. Another serious problem is the requirement of excess
amount of a base additive such as NaOH. The neutralization of the salt of carboxylic acid
product would further increase the operating cost and produce additional salt byproducts, making
the process less green and less cost-effective. The employment of methanol as a solvent instead
of water to produce dimethylfuroate may avoid the use of bases, but the oxidation of methanol
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also occurs under the reaction conditions.
The use of methanol as a solvent in the presence of
O
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may cause explosion problem in practical large-scale production.
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Thus far, only Au loaded on hydrotalcite (HT), a strong solid base, has been claimed to
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catalyze the aerobic oxidation of HMF to FDCA in water without a liquid base.
However,
extensive leaching of magnesium from HT due to the chemical interaction between the basic HT
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and the formed FDCA occurred inevitably. Therefore, the development of stable heterogeneous
catalysts for the aerobic oxidation of HMF under base-free conditions remains challenging.
Generally, the supported Au catalysts are less efficient for the oxidation of alcohol than that of
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aldehyde. Au catalysts are almost inert for the aerobic oxidation of alcohol in the absence of
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base additives or basic supports. Actually, the oxidation of the hydroxyl group in HMF has
been reported to proceed slower than that of the aldehyde group over the Au-based catalysts even
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with a base additive.
A few bimetallic catalysts such as supported Au-Cu and Au-Pd alloys
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have been reported to be more efficient and stable than the monometallic Au catalysts.
However, the nature of the alloying effect is still unclear. Furthermore, the base promoters have
still been employed in these systems.
Recently, we have attempted to develop stable heterogeneous catalysts for the base-free
aerobic oxidation of HMF to FDCA. Considering that the Au-Pd alloy can catalyze the oxidation
30,31
of alcohols very efficiently,
we have studied systematically the catalytic behaviors of the Au-
Pd nanoparticles loaded on various supports for the aerobic oxidation of HMF in water in the
absence of any bases. This article reports for the first time an efficient and recyclable carbon
nanotube (CNT)-supported Au-Pd alloy catalyst for the base-free aerobic oxidation of HMF to
FDCA. The roles of the CNT, particularly the functional groups on CNT surfaces, will be
discussed in detail. We will show how the catalyst support can play roles in the liquid-phase
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aerobic oxidation of organic molecules in water, which is a kind of very important green
chemical reactions, by enhancing the adsorption of the reactant and intermediates. We will also
demonstrate how the cooperative effect between Au and Pd in the alloy contributes to realizing
the base-free production of FDCA from HMF.
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. EXPERIMENTAL SECTION
.1. Materials. The supports employed in the work included oxides (including SiO
TiO , ZrO , CeO , and MgO with purities >99% purchased from Sinopharm or Alfa-Aesar),
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2 2 3
, Al O ,
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hydrotalcite (HT), and carbon materials. Carbon nanofibers (CNFs) were purchased from
Winchem Industrial Co. Ltd. Multiwalled CNTs with outer diameters of 20-60 nm and inner
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diameters of 3-5 nm were synthesized by a method reported previously. The CNT and CNF
were typically pretreated by 5 wt% HCl at 323 K for 2 h to remove the remaining Ni catalyst
used for the synthesis of CNTs or other possible metal impurities. Hydrotalcite (HT) and
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graphene oxide (GO) were synthesized by the procedures reported previously.
effect of functional groups on CNTs or CNFs, concentrated HNO and H were also employed
to further treat the CNTs or CNFs, which had been pretreated by 5 wt% HCl. The treatments by
HNO with concentrations of 30 and 68 wt% were carried out at 413 K under refluxing
conditions for 2 h. The treatment with 30 wt% H was performed at 338 K for 6 d. A certain
amount of 30 wt% H O (4 mL for 0.5 g CNTs or CNFs) was added into the system every 12 h.
To study the
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O
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2
O
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2
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The powdery CNTs or CNFs after each treatment were recovered by filtration, followed by
thorough washing with deionized water and drying at 373 K. In the case of H treatment, the
2
O
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treated CNTs or CNFs underwent further drying at 423 K for 6 h in vacuum to remove the
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2 2
remaining H O . The specific surface areas and the structure information of the supports were
summarized in Table S1 in the Supporting Information.
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Colloidal Au-Pd nanoparticles stabilized with polyvinyl alcohol (PVA) were synthesized by
addition of 1.0 wt% PVA (Aldrich, MW = 10000) aqueous solution into a mixed solution of
PdCl and HAuCl . The ratio of PVA to (Au + Pd) was controlled at 1.2/1. A freshly prepared
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4
NaBH
4
solution (0.1 M) was added into the mixture, forming a dark-brown sol containing the
4
colloidal Au-Pd nanoparticles. The molar ratio of NaBH to (Au + Pd) was regulated to be 5/1.
The supported Au-Pd bimetallic nanoparticles were prepared by immobilization of the PVA-
stabilized colloidal Au-Pd nanoparticles onto different supports. Typically, the powdery support
was added into the dark-brown sol with a proper pH under vigorous stirring. When the CNT was
used as support, the pH of the sol was adjusted to 7.0 by using diluted HCl aqueous solution.
After 2 h, the solid catalyst was recovered by filtration and washed repeatedly with hot distilled
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water (368 K) to remove the PVA stabilizer. The resultant was dried at 393 K overnight. The
supported monometallic catalysts were prepared with the same procedure. The expected loading
of metal nanoparticles was typically 1.0 wt% unless otherwise stated.
2
.2. Characterizations. Inductively coupled plasma mass spectrometry (ICP-MS)
measurements were performed with an Agilent ICP-MS 4500 instrument to measure the actual
loadings of Au and Pd in each sample. Specific surface areas of the supports or catalysts were
measured by N physisorption with Micromeritics Tristar II 3020. Laser Raman spectroscopic
2
measurements were performed on a Renishaw 1000R Raman system. A green laser at 532 nm
-1
was used as the excitation source and the spectra were recorded in the range of 100-2000 cm .
Transmission electron microscopy (TEM) measurements were performed on a Philips Analytical
FEI Tecnai 20 electron microscopy operated at an acceleration voltage of 200 kV. Samples for
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TEM measurements were suspended in ethanol and dispersed ultrasonically. Drops of
suspensions were applied on a copper grid coated with carbon. X-ray photoelectron spectroscopy
(XPS) measurements were performed on a Quantum 2000 Scanning ESCA Microprobe (Physical
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Electronics) using Al-K radiation (1846.6 eV) as the X-ray source. Temperature-programmed
desorption (TPD) was carried out using a Micromeritics AutoChem II 2920 instrument.
Typically, the sample was loaded and pretreated in a quartz reactor with high-purity Ar
(99.999%) at 473 K for 2 h, followed by cooling down to 373 K. Then, the temperature was
-1
raised to 1293 K at a rate of 5 K min . The desorbed CO and CO were detected with a mass
2
spectrometer (ThermoStar GSD 301 T2) by monitoring the signals with m/e = 28 and m/e = 44,
respectively. The adsorption amounts of the reactant or possible reaction intermediates on
various supports or the CNTs pretreated by different methods were measured by addition of the
support (0.10 g) into an aqueous solution containing a certain amount of the reactant (HMF) or
possible reaction intermediates. The suspension was stirred for 12 h and was allowed to rest for
another 12 h at room temperature. The amount of the reactant or possible intermediates
remaining in the supernate was measured to determine the adsorption amount.
2
.3. Catalytic Reaction. The aerobic oxidation of HMF was performed in a batch-type
Teflon-lined stainless-steel autoclave (70 mL). Typically, the powdery catalyst and HMF were
added into the reactor pre-charged with H O (20 mL). After the purge and the introduction of O
2
2
with a certain pressure (typically 0.5 MPa), and the reactor was placed in an oil-bath. Once the
system reached the desired temperature (typically 373 K), the reaction was initiated by vigorous
stirring with a magnet stirrer. The stirring speed was typically fixed at 500 rpm. After a fixed
time, the reaction was quickly terminated by cooling the reactor to room temperature. The liquid
products were analyzed by high-performance liquid chromatography (Shimazu LC-20A)
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equipped with an refractive index (RI) detector and a Shodex SUGARSH-1011 column (8 mm ×
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300 mm) using a dilute H SO aqueous solution as a mobile phase. The conversion of HMF and
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the selectivity of each product were defined as the molar percentage of HMF converted and the
molar percentage of each product in HMF converted, respectively. Both the conversion and
selectivity were calculated on a carbon basis.
3
. RESULTS AND DISCUSSION
.1. Structure and Unique Catalytic Behavior of Au-Pd/CNT for the Base-Free Aerobic
3
Oxidation of HMF into FDCA. The actual loading amounts of (Au + Pd) and molar ratios of
Au/Pd in the Au-Pd catalysts loaded on different supports, which have been measured by the
ICP-MS, are displayed in Table 1. The results show that almost all of the Au-Pd nanoparticles in
the colloids can be immobilized onto the support by the colloidal immobilization method and the
Au/Pd molar ratios in these catalysts are also close to 1/1, which has been used in the preparation
2
stage. The specific surface areas of these catalysts measured by the N physisorption were
similar to those of the corresponding supports (Supporting Information Table S2).
The TEM measurements for these catalysts clarified that the mean sizes of Au-Pd (Au/Pd = 1/1)
nanoparticles loaded on different supports were similar, being 2.4-2.7 nm except for the Au-
2
Pd/SiO , which possessed a slightly larger mean size of Au-Pd particles (Table 1, see Supporting
Information Figure S1 for TEM images of the catalysts). Figure 1 shows a representative TEM
image for the Au-Pd/CNT catalyst. We have performed the line-scan energy-dispersive X-ray
spectroscopy (EDS) analyses for several Au-Pd nanoparticles located on CNT. The EDS
analyses reveal that Au and Pd elements are uniformly distributed in all these nanoparticles (see
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Figures 1b and 1c for two Au-Pd particles with sizes of ~4.0 and ~2.3 nm, respectively). This
suggests that the nanoparticles over this catalyst are composed of Au-Pd alloy.
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Our XPS studies showed that the binding energy of Au 4f7/2 in the Au-Pd/CNT catalyst was
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4.0 eV, which was ~0.4 eV lower than that in the Au/CNT (84.4 eV), whereas the binding
energy of Pd 3d5/2 in the bimetallic catalyst was 336 eV, higher than that in the Pd/CNT catalyst
335.6 eV) (Figure 2). These shifts of binding energies of Au 4f7/2 and Pd 3d5/2 further confirm
(
the formation of alloy nanoparticles between Au and Pd, and may suggest the electron transfer
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from Pd to Au.
Catalytic studies demonstrated that the support played crucial roles in the aerobic oxidation of
HMF over the supported Au-Pd catalysts (Table 1). The combination of the catalytic
performances with the specific surface areas (Supporting Information Table S2) clearly suggests
that the surface area is not a key factor determining the catalytic behaviors. The employment of
metal oxides except for MgO as the supports led to the formations of 2,5-diformylfuran (DFF)
and 5-formyl-2-furancarboxylic acid (FFCA) but almost no FDCA irrespective of the specific
surface areas. When MgO, a strong solid base, was used as the support, HMF was completely
converted with a FDCA selectivity of 99%. However, the Au-Pd/MgO catalyst was unstable
during the reaction and could not be used recyclably due to the dissolution of MgO into the
reaction solution containing FDCA (Figure 3a). We confirmed that the Au-Pd/HT catalyst, which
also showed better FDCA yield, underwent similar deactivation during the repeated uses (Figure
3b).
The Au-Pd nanoparticles loaded on the carbon materials showed higher HMF conversions and
FDCA selectivities than those loaded on metal oxides except for MgO or HT (Table 1). In
particular, the CNT was an excellent support for the aerobic oxidation of HMF to FDCA in water;
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the HMF conversion and FDCA selectivity over the Au-Pd/CNT catalyst reached 100% and 94%,
respectively. It should be noted that Prati and co-workers recently reported that Au-Pd
nanoparticles loaded on an activated carbon support could also catalyze the aerobic oxidation of
HMF to FDCA with a high efficiency; an FDCA yield of >99% could be obtained for the
conversion of HMF with a concentration of 0.15 M under relatively mild conditions (333 K, 4
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h). However, the presence of a base additive, i.e., NaOH, is indispensable for the conversion of
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HMF under such mild conditions. We have demonstrated that CNT is a unique support for the
aerobic oxidation of HMF to FDCA in the absence of a base additive. Nevertheless, it should be
mentioned that relatively higher temperatures and longer reaction times are required to obtain a
full conversion of HMF under base-free conditions. We have also performed the aerobic
oxidation of HMF with a higher concentration (0.15 M), and an FDCA yield of 91% has been
obtained over the Au-Pd/CNT catalyst after 18 h of reaction at 373 K (Table 1). Moreover, as
compared to the Au-Pd/MgO and Au-Pd/HT, the Au-Pd/CNT was much more stable during the
recycling uses (Figure 4). Although the selectivity of FDCA decreased slightly in the initial three
recycles, both the HMF conversion and FDCA selectivity were sustained in the further recycling
uses. Thus, the Au-Pd/CNT is a highly efficient and recyclable catalyst for the conversion of
HMF to FDCA under base-free conditions.
Almost no conversion of HMF was observed over CNT alone. DFF and FFCA were the major
products over the catalyst with a lower loading of Au-Pd nanoparticles (Table 2). The HMF
conversion and FDCA selectivity increased with the loading of Au-Pd nanoparticles on CNTs,
both reaching >90% as the Au-Pd loading became ≥0.67 wt%. These observations confirm that
the Au-Pd alloy nanoparticle is essential for the aerobic oxidation of HMF to FDCA. The Au-
Pd/CNT catalyst could also catalyze the oxidation of HMF with air; HMF conversion and FDCA
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selectivity reached 100% and 96%, respectively, after a reaction under an air pressure of 1.0 MPa
and 373 K for 12 h. These results further demonstrate that the Au-Pd/CNT is a promising
catalyst for the base-free aerobic oxidation of HMF to FDCA in water.
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.2. Support-Enhanced Adsorption Effect for the Au-Pd/CNT-Catalyzed Aerobic
Oxidation of HMF into FDCA. The results in Table 1 clearly show that the support plays a key
role in the aerobic oxidation of HMF to FDCA. Generally, a catalyst support may offer better
dispersions of active sites and modify the electronic or geometric state of the active metal
66,67
nanoclusters, consequently influencing the catalytic behavior.
Another important effect of the
support, which is usually overlooked, is the enhancement of the adsorption of the reactant and
the reaction intermediates. This effect becomes particularly vital for the heterogeneous catalytic
conversion of organic molecules in water medium. The preferential adsorption of water and polar
products such as acids on the hydrophilic sites of supports may hamper the access of the organic
reactants or reaction intermediates to the active sites and thus limit the catalytic performance and
the catalyst stability. The enhancement of the hydrophobicity of catalyst surfaces is a useful
68-71
strategy to increase the catalyst performance for such kind of reactions.
Thus far, the organic
modification of inorganic supports is typically employed for the preparation of hydrophobic
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catalysts,
but the recyclability of these catalysts is still problematic. Moreover, few studies
have been devoted to finely tuning the organic functional groups on catalyst surfaces.
The relatively better catalytic performances of carbon material-supported Au-Pd catalysts
(Table 1) allow us to speculate that the organic feature of the support may contribute to the
adsorption of the organic reactant (HMF) and possible intermediates (DFF and FFCA) from
2
water solution. In particular, we speculate that the sp -bonded carbon networks on CNT or GO
surfaces may favor the interaction with the aromatic furan ring in the reactant or intermediates,
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and thus facilitate the reaction. To confirm this point, we have measured the adsorption amounts
of HMF and the possible reaction intermediates, i.e., DFF and FDCA, on different supports. The
combination of the adsorption results for different supports (Supporting Information Table S3)
with the surface areas of these supports (Supporting Information Table S1) suggests that the
surface area may influence the adsorption but is not the determining factor. The carbon materials,
in particular CNT, showed higher adsorption amounts of these compounds (especially DFF and
FFCA) than the metal oxides except for MgO (Supporting Information Table S3). MgO
exhibited an adsorption amount of FFCA comparable to CNT. It is easy to understand that MgO
can adsorb a larger amount of FFCA because of the acid-base interaction. The combination of
the results in Table 1 and Supporting Information Table S3 allows us to propose that the
difference in the adsorption ability of support may largely affect the catalytic behavior of the
supported Au-Pd catalysts.
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We have also performed co-adsorption studies for three combinations, i.e., (HMF + DFF),
(HMF + FFCA), and (HMF + FDCA), over CNTs. Both HMF and the possible reaction
intermediate (DFF or FFCA) could be adsorbed simultaneously on CNTs, and the adsorption
amount of either DFF or FFCA was higher than that of HMF under the conditions used for the
co-adsorption (Supporting Information Table S4), suggesting that CNTs exhibited better
adsorption capacity for these possible reaction intermediates. On the other hand, the adsorption
amount of FDCA was significantly lower than that of HMF. In other words, the product, FDCA,
did not adsorb strongly on CNT surfaces. This would favor both the conversion of HMF and the
selectivity of FDCA. We further clarified that the co-adsorption of the possible intermediate
(
particularly FFCA) or product (FDCA) decreased the adsorption amount of HMF, but such a
decrease was not very serious (Supporting Information Table S4).
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Many studies have pointed out that the oxygen-containing functional groups (including phenol,
carbonyl/quinone, anhydride, ester, lactone, and carboxyl groups) on the surfaces of carbon
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materials, particularly CNTs, play significant roles in the catalysis of carbon-based materials.
Rass et al. once reported that the oxygenated functional groups such as carboxyl groups on
activated carbon support were detrimental to the Pt-catalyzed aerobic oxidation of HMF to
82
FDCA in the presence of a base additive. The pretreatment of CNTs with different methods can
create different types of functional groups. It is expected that the tuning of functional groups on
CNT surfaces may regulate the adsorption amount of HMF or the possible intermediates and thus
affect the catalytic behavior.
We have functionalized CNT surfaces by HNO
pretreatment by 5 wt% HCl aqueous solution. Since different surface oxygen-containing groups
can be decomposed to CO and/or CO at different temperatures, the temperature-programmed
desorption (TPD) is a powerful technique to quantify the concentration of functional groups on
3 2 2
and H O in addition to the standard
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2
carbon materials. From the CO and CO desorption peaks in TPD profiles for the Au-Pd/CNT
catalysts with CNTs pretreated by different methods (Supporting Information Figure S2), we
have estimated the relative concentrations of various functional groups, and the results are
summarized in Table 3. It should be noted that the relative concentration of the strongest peak
for each functional group among the four differently pretreated CNT samples has been
normalized to 100%. For the catalyst with CNT pretreated by 5 wt% HCl (standard pretreatment),
the relative concentrations of phenol and carbonyl/quinone groups were higher, while those of
carboxyl, anhydride, ester, and lactone groups were lower.
The oxidative treatment of CNTs by concentrated HNO increased the concentrations of
3
carboxyl, anhydride, ester, and lactone groups, and decreased those of phenol and
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2 2
carbonyl/quinone groups. On the other hand, the treatment by H O increased not only the
concentrations of anhydride, ester, and lactone groups but also those of phenol and
carbonyl/quinone groups. However, the concentration of carboxyl group was not significantly
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altered. Our XPS studies also suggest that the CNT pretreated by concentrated HNO
higher fraction of carboxyl groups, whereas the fraction of carbonyl/quinone groups becomes
higher over the CNT pretreated by H (Supporting Information Figure S3 and Table S5).
As compared to the Au-Pd/CNT (Au/Pd = 1/1) catalyst with CNT pretreated by 5 wt% HCl,
the catalysts with CNTs further treated by HNO showed lower HMF conversions and FDCA
selectivities (Figure 5). The conversion of HMF decreased to 20% and no FDCA was formed
when 68 wt% HNO was used for the treatment of CNTs. On the other hand, the treatment of
CNTs by H afforded a more efficient catalyst, and the FDCA yield reached 95% even after
0 h of reaction at 373 K under base-free conditions.
As confirmed by TEM measurements, there were no significant differences in the mean sizes
of Au-Pd alloy nanoparticles over the catalysts with CNTs pretreated differently (Supporting
Information Figure S4). N physisorption measurements showed that the surface areas and pore
volumes for the catalysts with CNTs pretreated by different methods were in the ranges of
3
possesses
2 2
O
3
3
2 2
O
1
2
2
-1
2
-1
1
05~116 m g and 0.36-0.39 cm g , respectively (Supporting Information Table S6). This
indicated that the pretreatment by concentrated HNO or H did not lead to significant changes
in the porous structure of CNT. We further performed Raman spectroscopic measurements. The
3
2 2
O
-1
ratios of the integrated intensities of the D-band at 1340-1380 cm and the G-band at 1570-1600
-1
cm , which could be used to characterize the defects in the graphitic structure of carbon
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materials, were in the range of 1.12-1.19 for the catalysts with CNTs pretreated by different
methods (Supporting Information Figure S5 and Table S6). This further confirms that the CNT
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structure does not undergo significant changes after the pretreatment used in this work. Thus, we
can conclude that the type of functional groups on CNTs plays pivotal roles in determining the
catalytic performance of the supported Au-Pd catalyst. The correlation of the catalytic
performance (Figure 5) with the relative concentrations of different types of functional groups on
CNTs (Table 3 and Supporting Information Table S5) suggests that the carboxyl group is
unfavorable for HMF conversion and FDCA formation, whereas the carbonyl/quinone and/or
phenol group enhances FDCA formation.
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To gain more insights into the effects of the functional groups on carbon materials on the
catalytic behaviors, we have further investigated the catalysts with CNFs pretreated by different
methods. In addition to that pretreated by 5 wt% HCl (standard pretreatment), the CNFs
pretreated by 68 wt% HNO and 30 wt% H O were also used as the catalyst supports for the
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Au-Pd nanoparticles (loading amount = 1.0 wt%, Au/Pd = 1/1). The catalytic studies for these
samples in the aerobic oxidation of HMF at 373 K showed that the catalyst with the CNF
3
pretreated by 68 wt% HNO , on which a higher density of carboxyl groups could be generated,
exhibited much lower conversion of HMF and lower selectivity to FDCA than the catalyst with
the CNF pretreated by 5 wt% HCl (Supporting Information Table S7). On the other hand, over
2 2
the Au-Pd/CNF catalyst with the CNF pretreated by H O , on which a higher density of
carbonyl/quinone and phenol groups could be generated, significantly higher HMF conversion
and FDCA selectivity were obtained (Supporting Information Table S7). Thus, the carboxyl
groups on CNFs play a negative role in the catalytic reaction, while the carbonyl/quinone and/or
phenol groups favor the conversion of HMF to FDCA. These results are in good agreement with
those obtained for the Au-Pd/CNT catalysts with CNTs pretreated by different methods and
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further confirm the important roles of the functional groups on the surfaces of carbon materials
in the aerobic oxidation of HMF under base-free conditions.
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We have measured the adsorption amounts of HMF and the two possible intermediates (DFF
and FFCA) on the CNTs pretreated by different methods. As compared to the CNT pretreated by
5
wt% HCl, the CNTs further treated by 30 wt% and 68 wt% HNO exhibited lower adsorption
3
amounts of both HMF and the possible intermediates (Table 4). On the other hand, the
adsorption of HMF and the possible intermediates was enhanced on the CNT pretreated by H
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Thus, it becomes clear that the carbonyl/quinone and/or phenol groups enhance the adsorption of
HMF and the possible intermediates onto the CNT surfaces, whereas the carboxyl groups are
disadvantageous to the adsorption of these molecules. These results further suggest that the
adsorption ability of the support for HMF and the possible reaction intermediates contributes
significantly to the catalysis for the aerobic oxidation of HMF to FDCA in water.
To gain further evidence for the role of the surface functional groups, we have studied the
influence of the modification of the Au-Pd/Al
and quinone, which may function as model organic molecules of different functional groups. The
Au-Pd/Al catalyst without modification showed a HMF conversion of 34%, and the major
products were DFF and FFCA. The modification by benzoic acid decreased the HMF conversion
and FDCA yield (Table 5). On the other hand, the modification of the Au-Pd/Al with quinone
2 3
O (Au/Pd = 1/1) catalyst by benzoic acid, phenol,
2
O
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significantly accelerated both HMF conversion and FDCA selectivity. The FDCA yield was also
improved to some extent by the modification with phenol. These observations agree well with
the results obtained for the Au-Pd/CNT catalysts containing different functional groups on CNT
surfaces. Furthermore, the result in Table 5 may suggest that the quinone groups play more
positive effects on FDCA formation than the phenol groups on CNT-supported catalysts.
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Therefore, we have clearly demonstrated the importance of the adsorption of the reactant and
possible reaction intermediates by using the functionalized CNT supports. We expect that the
effective control of the support-enhanced adsorption effect may finely tune the reaction rates in
different elementary steps and may be further utilized for the design of efficient heterogeneous
catalysts for the aerobic oxidation of organic molecules in water.
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.3. Alloying Effect for the Au-Pd/CNT-Catalyzed Aerobic Oxidation of HMF into
FDCA. To gain insights into the alloying effect, we have investigated the CNT-supported
bimetallic catalysts with different Au/Pd ratios as well as monometallic catalysts. The ICP-MS
measurements for these catalysts confirm that the loading amounts of Au and/or Pd and the
molar ratios of Au/Pd in the catalysts are close to those used for preparation (Table 6). Our TEM
measurements reveal that the bimetallic or monometallic nanoparticles are homogeneously
dispersed on CNTs over these catalysts (Supporting Information Figure S6). The mean sizes of
nanoparticles over these catalysts are almost the same (2.5-2.7 nm). Table 6 shows that the CNT-
supported monometallic Au or Pd catalyst is less active and selective for the oxidation of HMF
to FDCA than the supported alloy catalysts. It is noteworthy that 5-hydroxymethyl-2-
furancarboxylic acid (HMFCA), the product from the oxidation of the aldehyde group in HMF,
was formed over the Au/CNT catalysts, whereas this product was not observed over the other
catalysts in Table 6. Byproducts from ring-opening and degradation were also formed with
higher selectivities over the Au/CNT catalyst under our reaction conditions. Significant
synergistic effects exist between Au and Pd in the alloys on enhancing the HMF conversion and
FDCA selectivity. The catalyst with an Au/Pd molar ratio of 1/1 exhibits the best performance
for the conversion of HMF to FDCA.
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The time-course analysis for the conversion of HMF can provide information on the reaction
paths and reaction mechanism. The result for the Au-Pd/CNT (Au/Pd = 1/1) catalyst in Figure 6
shows that the selectivity to DFF decreases quickly with reaction time in the initial 4 h and that
to FFCA begins to decrease as the reaction time exceeds 4 h, while the selectivity to FDCA
increases monotonically with reaction time. These observations clearly indicate a tandem
pathway for the conversion of HMF to FDCA via DFF and FFCA (Scheme 1a).
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We also investigated the time courses for the conversion of HMF over the CNT-supported
monometallic catalysts to gain deeper insights into the alloying effect on the reaction mechanism.
The result for the Pd/CNT catalyst (Supporting Information Figure S7) suggests the same tandem
reaction pathway with that for the Au-Pd/CNT catalyst (Scheme 1a). However, the Au/CNT
behaved differently during the oxidation of HMF. HMFCA was formed with considerable
selectivities at the initial reaction stage over the Au/CNT catalyst (Figure 7), indicating that
HMFCA was a major primary product together with DFF (Scheme 1b). The ring-opening and
degradation products (including gluconic acid, glucuronic acid, succinic acid, glycolic acid, and
formic acid) were also formed over the Au/CNT catalyst, and the selectivity of these byproducts
increased at the expense of the HMFCA selectivity at longer reaction times (Figure 7).
We have further performed the conversion of HMFCA over the Au/CNT catalyst and clarified
that the ring-opening and degradation products stem from HMFCA (Supporting Information
Figure S8). The ring-opening of HMFCA occurs first and then the degradation takes place over
the Au/CNT catalyst under base-free conditions, leading to the very low FDCA selectivity (Table
6
and Figure 7). On the other hand, HMFCA was not formed over the Pd/CNT and the Au-
Pd/CNT catalysts.
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We further performed the aerobic oxidations of DFF and FFCA, two major intermediates for
FDCA formation, over the Au/CNT, Pd/CNT, and Au-Pd/CNT (Au/Pd = 1/1) catalysts. The
conversion of DFF produced FFCA and FDCA, while FDCA was the only product from FFCA
over these catalysts (Supporting Information Figures S9 and S10), further confirming the
reaction pathways in Scheme 1.
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The rates of HMF, DFF, and FFCA conversions over the Au/CNT, Pd/CNT, and Au-Pd/CNT
(
Au/Pd = 1/1) catalysts have been evaluated using the results obtained for the oxidation of these
substrates at the initial reaction stage. As summarized in Table 7, the addition of Pd to Au/CNT
to form Au-Pd alloy significantly accelerates the oxidation of the hydroxyl group of HMF,
forming DFF (Scheme 1, Step 1), and retards the path for HMFCA formation (Scheme 1, Step
1
’). The incorporation of Pd to the supported Au catalyst also enhanced the conversions of DFF
and FFCA. In particular, the oxidation of FFCA (Scheme 1, Step 3) was significantly accelerated
and the rate of FFCA conversion increased ~220 times owing to the presence of Pd. It is
noteworthy that the Au/CNT catalyst is almost inactive for the conversion of FFCA under our
base-free conditions. This is quite different from the result obtained for the supported Au
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catalysts in the presence of a base, where the conversion of FFCA is a fast step.
The base, i.e.,
−
OH , has been reported to accelerate the conversion of FFCA to FDCA via a hemiacetal
42-47
intermediate.
Thus, we have clearly demonstrated that the use of the supported Au-Pd alloy instead of
monometallic Au catalyst is a useful strategy to avoid the formation of HMFCA, which leads to
the ring-opening and degradation reactions under base-free conditions, and to direct the
formation of FDCA via DFF and FFCA. Furthermore, the incorporation of Pd enhances the
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conversion of FFCA to FDCA, which is a quite difficult elementary step under base-free
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On the other hand, the reaction pathways were the same for the Pd/CNT and Au-Pd/CNT
catalysts (Scheme 1a). However, as compared to the Pd/CNT, the supported alloy catalyst
exhibited higher activities for all the three tandem reaction steps (Table 7). In particular, the rate
for the oxidation of HMF to DFF over the supported alloy catalyst was ~8 times higher than that
over the supported Pd catalyst. This means that the embedding of Au into Pd can also
significantly accelerate the oxidation of the hydroxyl group in HMF.
4. CONCLUSIONS
We have reported for the first time that CNT-supported Au-Pd alloy nanoparticles are highly
efficient and recyclable heterogeneous catalysts for the aerobic oxidation of HMF to FDCA in
water under base-free conditions. It has been demonstrated that the support-enhanced adsorption
effect and the alloying effect mainly contribute to the realization of the base-free oxidation of
HMF to FDCA. Enhanced adsorption of HMF and the intermediates (i.e., DFF and FFCA) from
water onto the functionalized CNT surfaces contributes significantly to the aerobic oxidation of
HMF to FDCA. We have clarified that the carbonyl/quinone and phenol (especially the former)
groups on CNT surfaces facilitate the adsorption of HMF and the intermediates, and thus
accelerate the formation of FDCA. On the other hand, the carboxyl groups on CNTs are
unfavorable for the adsorption of HMF and reaction intermediates, and thus, retard the reaction.
In addition to the support-enhanced adsorption effect, significant synergistic effects also exist
between Au and Pd in the alloy for the oxidation of HMF to FDCA in three tandem steps via
DFF and FFCA. The CNT-supported Au monometallic catalyst preferentially catalyzes the
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oxidation of the aldehyde group in HMF, forming HMFCA, which mainly undergoes ring-
opening and degradation reactions to byproducts under base-free conditions. The use of the
supported Au-Pd alloy nanoparticles changes the main route from HMFCA formation to DFF
formation by accelerating the oxidation of the hydroxyl group in HMF. DFF is then oxidized into
FDCA via FFCA. The incorporation of Pd further enhances the oxidation of FFCA to FDCA, a
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difficult elementary step over the supported monometallic Au catalyst under base-free conditions.
On the other hand, although the Pd/CNT can also catalyze the oxidation of HMF to FDCA via
DFF and FFCA, the Au-Pd/CNT catalyst exhibits higher rates in the three tandem elementary
steps. In particular, the rate of the oxidation of the hydroxyl group in HMF is significantly
accelerated by the presence of Au. We believe that these insights into the support-enhanced
adsorption effect and the alloying effect can be applied to other oxidative transformations, in
particular those involving tandem oxidation of organic compounds with different functional
groups in water.
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ASSOCIATED CONTENT
Supporting Information
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Specific surface areas and structural information of supports, Specific surface areas and pore
volumes of supported catalysts, TEM micrographs for Au-Pd nanoparticles loaded on different
supports, Adsorption amounts of HMF, DFF, and FDCA on supports, Co-adsorption of HMF
and possible reaction intermediates or products, TPD profiles, XPS spectra, TEM micrographs,
2
N physisorption results, and Raman spectra for Au-Pd/CNT catalysts with CNTs pretreated by
different methods, Catalytic performances of Au-Pd/CNF catalysts with CNFs pretreated by
different methods, TEM micrographs for CNT-supported bimetallic catalysts with different
Au/Pd ratios, Time course for the conversion of HMF over Pd/CNT, Time course for the
conversion of HMFCA over Au/CNT, Time courses for the conversions of DFF and FFCA over
Au/CNT, Pd/CNT, and Au-Pd/CNT.
AUTHOR INFORMATION
Corresponding Author
*
E-mail: wangye@xmu.edu.cn; zhangqh@xmu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by the Natural Science Foundation of China (Nos. 21173172,
21103143, 21033006, and 21161130522), the National Basic Research Program of China (No.
2013CB933100), the Research Fund for the Doctoral Program of Higher Education
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(
2013012113001), and the Program for Innovative Research Team in Chinese Universities (No.
IRT1036).
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