Oxidative esterification of furfural by Au nanoparticles supported CMK-3 mesoporous catalysts

Article abstract of DOI:10.1016/j.apcata.2017.07.031

Furfural which is derived from the hemicellulose fraction of abundant lignocellulose has received significant attention to many researchers as its valorization yields useful products such as fine chemicals and transportation fuels. Methyl 2-furoate is one

Full text of DOI:10.1016/j.apcata.2017.07.031

Accepted Manuscript
Title: Oxidative esterification of furfural by Au nanoparticles
supported CMK-3 mesoporous catalysts
Authors: Ramakrishnan Radhakrishnan, Sivakumar
Thiripuranthagan, Arulselvan Devarajan, Sakthivel
Kumaravel, Elangovan Erusappan, Kathiravan Kannan
PII:
S0926-860X(17)30341-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2017.07.031
APCATA 16336
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
24-4-2017
5-7-2017
20-7-2017
Please cite this article as: Radhakrishnan Ramakrishnan, Thiripuranthagan
Sivakumar, Devarajan Arulselvan, Kumaravel Sakthivel, Erusappan
Elangovan, Kannan Kathiravan, Oxidative esterification of furfural by Au
nanoparticles supported CMK-3 mesoporous catalysts, Applied Catalysis A,
Generalhttp://dx.doi.org/10.1016/j.apcata.2017.07.031
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Oxidative esterification of furfural by Au nanoparticles supported CMK-3
mesoporous catalysts.
Ramakrishnan Radhakrishnan, Sivakumar Thiripuranthagan*, Arulselvan Devarajan, Sakthivel
Kumaravel, Elangovan Erusappan and Kathiravan Kannan.
Catalysis Laboratory, Department of Applied Science and Technology, A.C.Tech Campus,
Anna University, Chennai-25.
*
Corresponding author
E-mail: tssivakumar@yahoo.com; Tel.: 91-44-22359193.
Graphical abstract

HIGHLIGHTS
CMK-3 was synthesized successfully from SBA-15 and sucrose via nanocasting method.
Hydrophobic Au/CMK-3 catalyst was prepared by wet impregnation in organic medium.
Among the different metals and supports, Au and CMK-3 were found to be the best.
5
% Au/CMK-3 was found to be the best in the oxidative esterification of furfural.
Among the oxidants, O provided high conversion (99.7%) and selectivity (99.6%).
2
ABSTRACT
Furfural which is derived from the hemicellulose fraction of abundant lignocellulose has
received significant attention to many researchers as its valorization yields useful products such
as fine chemicals and transportation fuels. Methyl 2-furoate is one of the important valorized
products of furfural as it finds applications in flavor and fragrance industries. Methyl 2-furoate is
generally obtained by oxidation followed by esterification of furfural but under acidic and
corrosive conditions. Uniform rods of CMK-3 mesoporous materials were successfully
synthesized by using nanocasting method. Au nano particles were impregnated into the pores of
CMK-3 by wet impregnation method followed by hydrogen reduction at high temperature
(673 K). All the catalysts were thoroughly characterized by various analytical techniques.
Homogeneous dispersion of Au nano particles ≈ 3nm size was found over CMK-3 materials.
Qualitative and quantitative analyses of Au were made by XPS and ICP techniques. The effect of
impregnating metal ions (Ni, Pt and Pd) and supports (SBA-15, graphite, graphene, MWCNT,
activated carbon and TiO2) was studied on the catalytic conversion of furfural. Among them,
Au/CMK-3 catalyst was found to be the best. Among the Au/CMK-3 catalysts, 5% Au/CMK-3
catalyst showed remarkable conversion of furfural (99.7%) with very high selectivity towards

methyl 2-furoate (99.6%). The stability and reusability of the 5% Au/CMK-3 catalyst was found
to be very good even after five catalytic cycles.
KEYWORDS
Mesoporous Au/CMK-3, Furfural, Methyl 2-furoate, Oxidative Esterification, Nanocasting
1
. Introduction:
There is a great demand for the ordered porous materials in the area of heterogeneous
catalysis, adsorption, drug delivery and electronics [1-4]. Among the ordered materials ordered
mesoporous silica (OMS) and ordered mesoporous carbon (OMC) are of great interest and hence
the catalyst world has seen several ordered mesoporous materials synthesized by using different
soft and hard templates. The first one in the group was M41S series synthesized by Kresge et. al.
in the year of 1990 [5,6]. These mesoporous silicates have uniform pore size (2-10 nm) with long
2
range ordered nanochannels, high surface area (> 2000 m /g) and the adjustable periodicity. To
suit different applications, these OMS materials were modified either in the framework with the
introduction of hetero elements or surface modification through silanol groups. A detailed
information regarding the synthesis, characterization and applications of these various ordered
mesoporous silica (OMS) series are found in various literature reviews [7-9]. Among the OMS
materials, SBA-15 with hexagonal mesostructure synthesized by using triblock polymer has been
studied extensively due to its tunable pore size (4 – 14 nm) [10].
Similarly ordered mesoporous carbons (OMC) are another group of fascinating carbon materials.
They are highly ordered with uniform array of mesopores. Ordered mesoporous carbons (OMC)
are generally prepared using ordered mesoporous silica (OMS) as template. Ryoo et. al.
synthesized one such material using MCM-48 as template in the year of 1999 and it is referred as
CMK-1[11]. OMC materials with tunable pore size, controllable pore structure, surface area,

high pore volume, conductivity and stability are generally synthesized by nanocasting method
12, 13]. Due to these unique properties of OMC, number of environmental friendly low cost
[
energy storage and conversion devices have been developed. Apart from these applications, these
materials also find greater use in the area of catalysis and gas separations. Generally the
synthesis of OMC involves stepwise impregnation of carbon precursor (monomer, polymer, etc.)
in the pores of mesoporous silica materials to get desired silica to carbon ratio [14]. The
precursor within the pores are polymerized and carbonized in the pore structures resulting in the
formation of carbon-silica composites. The resulting composite is washed with aq.HF to remove
silica template to yield CMK-n. A large variety of OMC with different mesostructures such as
CMK-1(MCM-48, cubic Ia3d) [15], CMK-2 (SBA-1, cubic pm3n) [16], CMK-3(SBA-15, rod
type p6mm)[17-20], CMK-4 (MCM-48, Ia3d) [21], CMK-5 (SBA-15,tube type p6mm) [22],
OMC( SBA-16, cubic Im3m) [23] and CMK-8 (KIT-6, rod type) [24] were prepared by
nanocasting method using various carbon precursors such as sucrose, phenolic resins, furfural
alcohol, acenaphthene and acetonitrile [19-22]. CMK-3 has been used as a support for various
transition metals for carrying out a variety of catalytic transformations. Suzuki coupling by
Pd/CMK-3, reduction of nitrobenzene and carbonyl compounds over Ru/CMK-3, hydrogenation
of carbonyl compounds over Ni/CMK-3, oxidation of alcohols and conversion of amines to
imines over FeO /CMK-3, aqueous phase reforming of glycerol and ethylene glycol over
x
Pt/CMK-3, etc. have been reported [29-41]. Only scarce attempts such as base free oxidation of
glucose to gluconic acid and catalytic reduction of p-nitrophenol into p-aminophenol were
reported by using Au/CMK-3 [42, 43].
Herein we report the catalytic activity of Au/CMK-3 in the oxidative esterification of furfural.
Furfural is a platform chemical from which number of medicines, pesticides, paints, plastics,

commodity chemicals and solvents etc. have been prepared [44]. At present the global
production of furfural derivatives is at 947.58 kilo tons (2016) and the demand will grow at a
compound annual growth rate (CAGR) of 2.69% to reach 1,172.17 kilo tons by 2024 [45]. Hence
furfural is produced in large amounts from the biomass lignocellulose and such biomass is the
subject of interest to many researchers [46]. Several fine chemicals such as furfural alcohol, 2-
methyl furan, furoic acid, tetrahydrofuran (THF), cyclopentanol, cyclopentanone, 2-methyl
tetrahydrofuran, polytetrahydrofuran, tetrahydrofurfuryl alcohol, etc. are also derived from
furfural [47]. Recently furfural has been upgraded into gasoline and diesel fuels using efficient
catalytic technologies [48]. One of the authors converted several biomasses into green fuels and
the present authors have already reported the catalytic activities of Au-Pd/HAP-T and Au-
Ag/HAP-T bimetallic catalysts towards the oxidative esterification of furfural [49-53]. The
author performed this reaction using TBHP and air or O
present paper deals mainly with molecular O or air at different pressures. Increase in pressure of
increases the conversion and selectivity. Further the reaction was performed with O at
different pressures without using additional oxidants such as TBHP.
2
at atmospheric pressure. However the
2
O
2
2
Any valorization process will be economically and environmentally attractive only when it is
carried out under mild reaction conditions using non-corrosive reagents. Furfural can also be
valorized into methyl 2-furoate which finds application in fine chemicals, flavor and fragrance
industries [54]. As the conventional method of obtaining methyl 2-furoate uses strong oxidizing
agent such as KMnO
4
and followed by esterification using concentrated sulphuric acid in
methanol medium it causes harm to the environment. The other method of obtaining methyl 2-
-
+
furoate also involves the strong bases such as NaOH, CH
3
O Na , K
2
CO
3
, Li
2
CO
3
, CsCO etc. but
3
it is not an environmental benign method [55, 56]. Only scarce attempts have been made on the

oxidative esterification of furfural to methyl 2-furoate without using bases [57-62]. So herein we
report the synthesis and characterization of Au/CMK-3 catalysts and the conversion of furfural to
methyl 2-furoate with and without the use of various bases and oxidants (TBHP, air and O ).
2
2
2
. Experimental section
.1 Materials
Tetraethylorthosilicate (TEOS, Merck), non-ionic triblock copolymers (P123, Poly(ethylene
glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (MW-5800); EO PO EO
Sigma Aldrich) and HCl (37%, Sigma Aldrich) were used for the synthesis of SBA-15.
Autoclavable PP bottles (Nalgene) were used for the hydrothermal preparation of SBA-15.
Glucose as a carbon source (SRL) and sulphuric acid (98%, Sigma Aldrich) as a catalyst were
used for the preparation for CMK-3. Gold chloride (AuCl .3H O, 99.9%) purchased from Sigma
n
m
n
,
3
2
Aldrich was used as a gold precursor. MWCNT (10 nm × 4.5 nm × 36 μm, Sigma Aldrich) and
activated carbon (Alfa Aesar) were used as such. Graphite flakes were purchased (Natural-325
mesh, Alfa Aesar) and used for graphene synthesis. TiO
from titanium-isopropoxide (Sigma Aldrich).
2
was synthesized by sol-gel method
2
.2 Synthesis of mesoporous SBA-15 via hydrothermal method
Mesoporous silica SBA-15 with gel composition 1 TEOS: 5.87 HCl: 194 H O: 0.017 P123 was
2
synthesized using the silicon source tetraethylorthoslicate (TEOS) and structure directing agent
Pluronic P123 in a highly acidic medium [7]. In a PP bottle, 4 g of triblock-copolymer, 130 ml of
water and 20 ml of concentrated HCl were added and stirred for 2 h for complete dissolution and
the temperature was maintained at 318 K. The silica source tetra ethylorthosilicate (TEOS) was
then added directly to the above solution with constant stirring maintaining the above

temperature for 24 h. The white suspension resulting was autoclaved at 318 K in an air oven for
2
3
4 h and cooled. The resultant white suspension was centrifuged, washed with water and dried at
93 K for 12 h. The white material formed was ground and calcined at 823 K for 6 h for the
complete removal of the templates and to obtain mesoporous SBA-15 material.
2
.3 Synthesis of mesoporous CMK-3 carbon via nanocasting method
In the typical synthesis of mesoporous CMK-3, SBA-15 was used as a hard template. In order to
ensure complete polymerization and carbonization, sucrose coatings were done twice as reported
by us [63, 64]. To 1 g of the SBA-15 material, 1.25 g of sucrose, 0.1 g of concentrated H
and 5 ml of deionised H O were added and stirred until thick slurry was formed. Then it was
dried in an air oven at 383 K for 6 h and carbonized at 433 K for another 6 h. The obtained
brown material was finely ground and used for the second coating of the sucrose precursor. To
the above brown mixture, 0.8 g of sucrose, 0.08 g of conc.H SO and 5 ml of deionised H
were added, and the same procedure was repeated. The black mass obtained was ground and
finally carbonized at 1173 K for 6 h under the flow of N . The carbonized material was then
2
SO
4
2
2
4
2
O
2
stirred with 5% HF solution for 12 h and washed with ethanol to obtain the nanoporous carbon
CMK-3.
2
.4 Synthesis of mesoporous Au/CMK-3 catalysts via high temperature H reduction
2
In the typical synthesis, 1 g of CMK-3 was dispersed in 20 ml of acetone and stirred for 5 min.
Then yellow colored HAuCl solution (0.05 mM) was added and stirred for 2 h. A transparent
3
3
+
solution obtained indicates that the Au ions were adsorbed onto the nanoporous carbon. The
resulting solution was dried at 383 K and reduced under the flow of hydrogen (30 ml/min) at

6
73 K for 3 h to get 1 % Au/CMK-3 (w/w) nanocatalyst. Similarly by varying the amount of
gold stock solution, 3%, 5% and 7 % of Au/CMK-3 (w/w) nanocatalysts were obtained.
2
.5 Synthesis of other catalysts
Graphite oxide (GO) was synthesized from graphite powder via modified Hummer’s method and
the method reported by us earlier [65]. Then it was ground, dispersed in ethanol and sonicated
for 30 min. To this, gold precursor was added and sonicated for another 15 min. For drying,
calcination and reduction the procedure described for CMK-3 was adopted. Nano titanium
dioxide (TiO ) was synthesized using sol-gel method as reported by us earlier [66]. MWCNT
2
and activated carbon were used as such without any modification. All these materials were
impregnated with 5% Au by following the same procedure given in Section 2.4.
2
.6 Characterization of catalysts
X-ray powder diffraction patterns of Au/CMK-3 were recorded using a Philips X-Pert
diffractometer equipped with a Niβ filter and Cu K radiation with λ=1.5406 A° working at the
α
operating voltage of 40 kV and the current of 30 mA. Low angle data were collected in the 2θ
range from 0.6 to 10 at a scan size of 0.01° at a scan time of 10 s. The wide angle data were
collected in the 2θ range from 10 to 80 at a scan size of 0.04°. Peak identification was performed
with Origin 8 software. The standard reference for metallic Au was obtained from the JCPDS
database. The average particle size was calculated in a direction perpendicular to the
crystallographic plane based on Scherrer formula by equation 1:
0
.9 × 휆
푋푠 =
(1)
훽 ×푐표푠휃

Where, λ – is the wavelength of the monochromatic X-ray beam (Cu-Kα1, 1.54056 A°); βhkl
broadening of the hkl diffraction peak measured at half of its maximum intensity (radians) and θ
diffraction angle [67].
–
–
Nitrogen adsorption - desorption isotherms were constructed at 77 K using a constant volume
adsorption apparatus (Micrometrics ASAP 2020) for the determination of surface area and other
texture characteristics. Prior to the analysis, CMK-3 samples were degassed and pretreated at
5
83 K for 6 h. The specific surface area was calculated from the Brunauer –Emmett – Teller
BET) equation from the adsorption data in the relative partial pressure from 0.05 to 0.3. The
total pore volume was derived from the BET plot by calculating the amount of N at a partial
(
2
pressure of 0.95. The pore size distribution (PSD) was calculated from the desorption branch of
the isotherm using Barrett – Joyner − Halenda (BJH) model.
Catalyst morphology and structure of Au/CMK-3 were studied using Quanta 200 FEG scanning
electron microscope (HR-SEM) and the composition analysis was performed using energy-
dispersive X-ray spectroscopy (EDS). Prior to the analysis, powdered samples were dried at
3
73 K for 6 h, air dispersed over the carbon tape and mounted for analysis. The high resolution
transmission electron micrographs (HR-TEM) were recorded using JEOL JEM-2100JF working
at the operating voltage of 200 keV. Powder samples were dispersed in absolute ethanol,
ultrasonicated for 10 minutes, placed over holey carbon coated Cu grid, dried at ambient
conditions and mounted for the analysis.
Quantitative determination of gold was carried out by inductively coupled plasma (ICP)
technique using Perkin Elmer Optima 5300 DV. 200 mg of samples were digested in mixture of
concentrated acid for 6 h, diluted and filtered through syringe filter prior to the analysis.

XPS measurements were carried out to find out the elements present in the mesoporous catalysts
on an ESCA+, Omicron Nanotechnology ESCA Probe spectrometer with monochromatized
-5
AlK
α
X-rays (energy: 1486.6 eV) 300W with base pressure of 5x10 mB. Sputter-cleaned Cu
2
p3/2 (932.7 eV) and Au 4f7/2 (84.0 eV) core level peaks (from foils) were used to calibrate the
XPS instrument. The catalyst samples were pelletized, dried at 398 K under vacuum and
mounted in the sample port. Spectra in the required binding energy range were collected for 8
times (except survey scan) and an average spectrum was taken. The binding energy (B.E) scale
was referenced by setting the C (1s) binding energy (B.E) of adventitious carbon to 284.6 eV.
XPS high-resolution scans were performed for the Au 4f5/2, Au 4f7/2 and C1s.
2
.7 Catalytic experiments
The liquid phase oxidative esterification of furfural in the methanol medium using different
oxidants such as tert-butyl hydroperoxide (TBHP) was investigated using a batch reactor of 150
ml capacity made up of glass at 393 K as reported by us [53]. 0.3ml of furfural, 3 eq. of TBHP
(70%) and 50 mg of catalyst were added to 20 ml of methanol and the reaction was performed in
liquid phase. Thin layer chromatography (TLC) was used to study the progress of the reaction
using petroleum ether: ethyl acetate ( 9:1 ) as mobile phase. After the reaction, catalyst was
separated by centrifuging and then treated with chloroform water mixture in a separating funnel
for the removal of excess of methanol. The golden yellow layer was separated and dried over
anhydrous sodium sulphate. Air or O were fed into the high pressure reactor made up of
2
stainless steel with 100 ml capacity [Parker autoclaves engineers]. After 3 h of reaction, the
reactor was allowed to cool in cold water and the sample aliquots (30 μL) was centrifuged,
filtered through nylon syringe filters (0.22 microns) and analyzed using gas chromatograph

(
Shimadzu GC-17A) equipped with a ZB-FFAP capillary column and a flame ionization detector
FID).
(
3
3
. Results and discussion
.1 Powder X-ray diffraction
The low angle x-ray powder diffraction patterns of SBA-15 and CMK-3 were recorded to
determine the structural order. Fig.S1 shows the low angle x-ray powder diffraction of
mesoporous silicious SBA-15 and its carbon replica phase CMK-3. The texture parameters are
given in Table.1.
Figure.S1
Both SBA-15 and CMK-3 show strong reflections in the low angle indicating the uniform
pore structures. The powder diffraction patterns of silicious SBA-15 show sharp peak at 2θ =
0
.96 (100) and two weak reflections at the angles of 2θ = 1.62 (110) and 1.84 (200). Similarly
CMK-3 obtained from the replica of SBA-15 also shows a sharp and well intense peak at 2θ =
.03(100) and the weak peaks at 1.77 (110), 2.06 (200) [68-70]. SBA-15 material possesses 2D
1
hexagonal mesoporous structure with long range order connecting micropores and small
mesopores [71]. As the replica CMK-3 was obtained from the pore filling of SBA-15, uniform
sized hexagonal carbon rods with interlinked carbon bridges were formed. This is indexed in the
hexagonal lattice with the crystallographic space group of p6mm [72].
The XRD peaks of CMK-3 were found to be broader, less intense and observed at slight
by higher 2θ angle than SBA-15. The periodic distance decreases when CMK-3 shrinks and
distorts at high temperature treatments and hence there is slight broadening and slight shift in the

diffraction angles. Such deformations in the periodic structures are frequently observed in the
synthesis of mesoporous carbon. The d100 for SBA-15 and CMK-3 were found to be 9.2 nm and
8
1
.6 nm. The corresponding unit cell parameters calculated using the formula a
0
=2d100/√3 were
0.6 nm and 9.9 nm respectively. The unit cell parameter (a ) of CMK-3 decreases in
0
comparison with SBA-15 indicates the structural shrinkage due to nanocasting. These calculated
d spacing and unit cell parameters are in good agreement with the reported literature [73, 74].
Fig.1 depicts the low angle x-ray powder diffraction patterns of 1%, 3%, 5% and 7%
Au/CMK-3 catalysts. All the catalysts show sharp, intense peak around 2θ = 1.03 (100) and two
less intense peaks at 2θ = 1.77 (110) and 2θ = 2.05 (200). The introduction of Au into the
mesopores of the CMK-3 generally increase the scattering effect which further increase the
phase cancellation between scattering from the walls and pores. Due to this reason, the increase
in Au content from 1% to 7%, further decreased the peak intensity [75, 76]. The XRD patterns of
Au impregnated CMK-3 catalysts reveal that the structural periodicity is not lost despite the
introduction of Au and higher calcination/reduction temperature employed during the synthesis.
Figure.1
Fig.2 presents the wide angle x-ray diffraction patterns of Au/CMK-3 with different Au
(1 %, 3 %, 5 % and 7 %) contents and compared with the JCPDS pattern of pure metallic Au.
Figure.2
The effect of dispersion and crystalline nature Au metal nano particles over CMK-3 is
better observed from the wide angle diffraction pattern. Only 5% Au/CMK-3 and 7 % Au/CMK-
catalysts show weak characteristic peak for Au (FCC lattice) at 2θ = 38.2(111) indicating the
formation of Au nano clusters outside the mesochannels of CMK-3 [77]. However Au (1% &
3

3
%)/ CMK-3 catalysts, the peaks corresponding to Au at 2θ = 38.24(111), 44.32(200), 64.4(220)
are not observed due to the fact that Au nano particles are dispersed within the mesochannels.
However when the concentration of Au increased to 5% or 7%, less intense diffraction peaks
were observed owing to the dispersion of Au nano particles on the surface. Similar such reports
have been reported for Pt/CMK-3 [78]. The average particle size was calculated to be 2.3 nm and
2
.5 nm for 5% and 7% respectively. Further texture evidence of Au nano structures over CMK-3
is detailed with N adsorption studies.
2
3
.2 Nitrogen sorption analysis
The texture characteristics of porous materials through N adsorption at low temperature (77 K)
2
provide information about the microporosity and mesoporosity. Adsorption – desorption
isotherm exhibit a hysteresis loop provides information about the shape and size of the
mesopores [79]. The N adsorption - desorption isotherm and the pore size distribution of
2
SBA-15 and CMK-3 are shown in Figs. S2 and S3.
Figure. S2
Figure. S3
Both SBA-15 and CMK-3 materials exhibit typical type IV isotherm with H hysteresis
1
loop (0.6 - 0.8) which is characteristic of capillary condensation in the mesochannels according
to the IUPAC classification. A sharp increase at lower P/P (0.45) indicates that the pores are
o
expected to be narrow [80, 81]. Surface area, pore volume, pore diameter and carbon wall
thickness calculated for SBA-15, CMK-3 and Au/CMK-3 (1%, 3%, 5% and 7 %) are
summarized in Table.1.

Table.1
It is evident from the texture properties that CMK-3 derived from the SBA-15 as template
2
2
shows the highest surface area of 1096 m /g followed by SBA-15 of 740 m /g. These values are
in good agreement with literature reports [82]. This difference may be due to the bulk density
difference between carbon (1.8 ml/g) and silica (2.2 ml/g) [83]. The decreased pore diameter and
pore volume of CMK-3 when compared to SBA-15 may be due to the increased microporosity in
the nano channels. The pore size distribution (PSD) of SBA-15 and CMK-3 calculated from the
BJH model were 7.69 nm and 4.56 nm respectively and the peak was found to be sharp which is
indicative of uniform regular pores. The N adsorption - desorption isotherms and the pore size
2
distribution (PSD) of Au/CMK-3 with different Au contents (1%, 3%, 5% and 7 %) are
illustrated in Fig.S4 and S5 respectively.
Figure. S4
Figure. S5
All the Au/CMK-3 catalysts show similar type IV isotherm with H hysteresis loop which
1
2
2
is typical for mesoporous materials. Surface area (SBET) decreased from 1039 m /g to 880 m /g
as the content of Au increased from 1% to 7 %. The pore volume and pore diameter also
3
gradually decreased with increase in Au content. The pore volume decreased from 0.97 cm /g to
3
0
.81 cm /g, the pore diameter decreased from 4.10 nm to 3.80 nm and the wall thickness
increased from 5.99 nm to 6.09 nm. In general, type, size and shape of the metal, preparation
methods and the amount of loading are the important factors which are responsible for either
decrease or increase in the texture properties [84-89].

3
.4 X-ray photoelectron spectroscopy (XPS)
X-ray photoelectron spectral analysis were further performed on CMK-3, 3% Au/CMK-3, 5%
Au/CMK-3 and 7% Au/CMK-3 and the corresponding XPS survey spectra are shown in Fig. S6.
Figure. S6
The survey spectrum of CMK-3 shows core levels of C1s (284.7 eV) and O1s (529.5 eV)
after the charge correction. Au peaks at 83.37 eV and 97.03 eV are noticed due to the core levels
of Au 4f7/2 and Au 4f5/2. These values are in good agreement with the reported ones [90, 91].
Fig. 3 shows the XPS core level spectra of 1%, 3%, 5% and 7% Au/CMK-3.
Figure.3
Among the Au loaded catalysts, only 5% Au/CMK-3 and 7% Au/CMK-3 catalysts show
corresponding core level peaks for Au at 83.37 eV (Au 4f7/2) and 87.03 eV (Au 4f5/2). 1%
Au/CMK-3 and 3% Au/CMK-3 did not show any core level peaks for Au due to the presence of
Au particles within the pores making inaccessible for the analysis.
Table.1 shows the Au metal content values determined by various instrumental
techniques such as ICP, EDS and XPS. The Au metal concentration determined by EDS and
XPS in 5% and 7% Au/CMK-3 catalysts were found to be lower when compared with the values
obtained from the ICP results. This clearly shows that most of the Au particles are retained in the
pores. The Au diffraction peaks found only in 5% Au/CMK-3 and 7% Au/CMK-3 catalysts in
their XRD analysis further support this observation.

3
.5 High resolution scanning electron microscope (HR-SEM)
The morphological structure of SBA-15 and CMK-3 was investigated by high resolution
scanning electron microscope (HR-SEM) in Fig.4 (a-d). Low magnified HR-SEM images of
SBA-15 show uniform and linear rod like morphology arranged together to form bundles as
shown in Fig.4 (a). Similarly CMK-3 obtained from SBA-15 also shows similar lengthy rod like
morphology with similar dimensions of 20 micron x 400 nm as shown in Fig.4 (b). These images
clearly show the successful replication process of SBA-15. In the high magnification image of
CMK-3 it is noticed that the width of the bundle decreased (Figs.4 (b) and 4 (d)). This may be
due to the shrinkage occurred during the high temperature treatment. The obtained micrographs
of SBA-15 and CMK-3 are in good agreement with the previous literature reports [92].
Figure.4
On high magnification, the HR-SEM of SBA-15 shows rods of width ≈ 380 nm whereas
the CMK-3 rods show rods of ≈ 333 nm but importantly without much irregular carbon deposits.
This study reveals that just enough quantity of sucrose has been used in the replication process. It
is evident that the individual rods of CMK-3 are arranged in an array forming long bundles of
structure. The HR-SEM of the representative samples of Au loaded CMK-3 are shown in Fig.5
(
a-b). Gold particles are not seen in the HR-SEM but clearly seen in the HR-TEM (discussed
later). The highly magnified micrographs of Au/CMK-3 with 5% and 7% Au show similar
surface structure indicating that Au has been incorporated into the channels of CMK-3 matrix as
shown in Fig. 5 (c).
Figure.5

However during the impregnation of gold chloride over CMK-3, the yellow colour of the
precursor becomes transparent indicating that the gold ions are adsorbed completely by CMK-3.
The effect of strong capillary force within the pore channels with high specific surface area and
3+
high adsorption capacity of CMK-3 makes the precursor Au to be highly adsorbed into the
channels during the impregnation process. This material which on reduction under H
2
atmosphere yields highly dispersed Au nano structures. Even at high loading of Au at 7 % the
gold nano structures are too small to be identified by HR-SEM. However the EDS analysis of
5
% Au/CMK-3 shows a very small peak at the binding energy of 2.120 KeV as shown in
Fig.5 (d). Similar weak peak was also obtained in the XRD patterns at 2θ = 38.2 (111). However
the presence of the Au nano particles is better seen from the HR-TEM micrographs.
3
.6 High resolution transmission electron microscope (HR-TEM)
The morphology of the silica template SBA-15 and the carbon replica CMK-3 support were also
examined by HR-TEM technique and the micrographs are shown in Fig.S7 (a & b).
Figure.S7
The periodic, structural and texture properties of both SBA-15 and the carbon replica
CMK-3 are similar. These images suggest that even after the replication process and high
temperature reduction the structure of CMK-3 remains intact. The sizes of SBA-15 and CMK-3
rods measured from the HR-TEM analysis were found to be 360 nm and 300 nm respectively.
These are in agreement with the HR-SEM values.
The structure and extent of Au dispersion over CMK-3 are also examined by HR-TEM
technique. The HR-TEM micrographs of 5% and 7 % Au/CMK-3 catalysts are shown in Fig. 6
(
a-f). Low magnification images of Au loaded CMK-3 show comparable structure and no gold

particles are seen in these images (Fig. 6 (a) & (b)). However, high magnification images of 5%
Au/CMK-3 and 7% Au/CMK-3 catalysts show Au nano particles dispersed nicely without any
agglomeration as shown Fig. 6 (c) & (d).
Figure.6
6
0 particles were chosen from HR-TEM micrograph of 5% Au/CMK-3 to find out the
average particle size distribution and they were found to be in the narrow range of approximately
- 4 nm with maximum particles around 2.5 nm (Fig.6e). The determined values are in good
1
agreement with the average particle size calculated using Scherrer equations from the high angle
XRD diffraction pattern. The lattice fringe was determined to be d = 0.2380 nm for the
corresponding metallic Au.
3
.7 Catalytic studies on oxidative esterification of furfural
3
.7.1 Evaluation of catalytic activities of various supported catalysts: The oxidative
esterification of furfural to methyl 2-furoate( as shown in Scheme.1) was carried out in a batch
reactor using methanol as solvent, TBHP as an oxidant over various Au supported catalysts such
as Au/CMK-3, Pd/CMK-3, Pt/CMK-3 and Ni/CMK-3, Au/SBA-15, Au/graphite, Au/graphene,
Au/MWCNT, Au/activated carbon, Au/TiO and the results are given in Table.2.
2
Table.2
Among the Au supported catalysts Au/TiO
2
showed maximum conversion of furfural
showed poor
(
93.9%) followed by Pd/CMK-3, Au/CMK-3, Au/graphite etc. But Au/TiO
2
selectivity towards the desired product of methyl 2-furoate. Although Ni/CMK-3 shows 100%
selectivity towards methyl 2-furoate, it is able to convert only 29.1% of furfural. As far as

Au/CMK-3 is concerned, it gives the best conversion of furfural (77.7 %) and selectivity towards
methyl 2-furoate (96.8%). The superior activity of CMK-3 support over other supports is evident
from the above results. This may be due to the very good dispersion of smaller Au particles onto
the pore channels of CMK-3. Further, the oxidative esterification does not produce any alkyl
peroxides.
3
3
.7.2 Optimization of reaction conditions
.7.2.1 Optimization of Au content: The effect of Au content on the catalytic oxidative
esterification of furfural was studied by varying the Au content from 1 % to 7 % at the same
reaction conditions and the results are given in Table.3.
Table.3
Although there is not much difference in % selectivity of methyl 2-furoate, a significant
difference in % conversion was observed over 1, 3, 5 % Au/CMK-3 catalysts. However on
increasing the Au content further to 7% there was no remarkable improvement in both the %
conversion and % selectivity. Higher loading of 7% Au impregnation over CMK-3 induces
agglomeration and may result in a slight loss of catalytic activity. The HR-TEM images of these
two catalysts also suggest the higher agglomeration of Au on 7% Au/CMK-3 than on 5%
Au/CMK-3. Hence 5% Au/CMK-3 has been chosen for further optimization studies.
3
.7.2.2 Effect of reaction temperature on the catalytic activity: The effect of reaction
temperature on the conversion of furfural and the selectivity towards methyl 2-furoate was
investigated in the temperature range of 333 K to 413 K over 5% Au /CMK-3 catalyst under the
same reaction conditions and the results obtained after 3 h are given in Table.4.

Table.4
The results clearly show that the reaction temperature plays a definite role in the conversion of
furfural as only 23.5% of furfural was converted at 333 K and it increases as high as 77.7% with
increase in temperature (393 K). Increasing reaction temperature also decreases the selectivity
towards methyl 2-furoate due to the formation of 4-furaldehyde-dimethyl-acetal. Hence 393 K
was found to be the optimum temperature. The effect of time on stream on the catalytic activity
was also studied for 5 h at the reaction temperature of 393 K and the results are given in Table.5.
Table.5
Table.5 indicates that conversion increased with time and reached maximum of 85.1 % at the 5th
hr. However the selectivity to methyl 2-furoate decreased with time on stream. Hence reaction
time has been optimized at 3 h.
3
.7.2.3 Effect of various bases on the catalytic activity:
In general such oxidative esterification of furfural is carried out by using various bases
such as NaOH, NaOCH , K CO , Li CO and CsCO3. Hence in order to check the influence of
the base on the conversion and selectivity, 5 ml of 10% aqueous solution of bases was added and
the reaction was performed in O atmosphere under the optimized conditions but without TBHP
3
2
3
2
3
2
and the results are given in Table.6.
Table.6
It was found that bases also convert furfural with reasonable conversion and selectivity towards
methyl 2-furoate. Among the various bases, the order of catalytic reactivity was found to be

CsCO
3
> Li
2
CO
3
> NaOCH
3
> K
2
CO > NaOH. As the bases are corrosive in nature and not
3
environmentally benign they are to be avoided for such conversions.
3
.7.2.4 Effect of oxidants on the catalytic activity: In the present study different
oxidants such as tert-butyl hydroperoxide, air and oxygen have been used for the oxidative
esterification of furfural. The reaction was performed under the optimized reaction conditions
over 5% Au/CMK-3 and the results obtained are given in Table.7.
Table.7
Air at ambient conditions does not convert furfural and the conversion increased with increasing
pressure of air and oxygen. However reactions carried out under the optimized reaction
temperature in air and oxygen at high pressure (5-15 bar) shows increased conversion and
selectivity. With the increase in the operating pressure from 0 to 15 bar with air and oxygen, the
conversion of furfural and selectivity to methyl 2-furoate increases. The reaction carried out in
presence of oxygen at 15 bar shows higher conversion of furfural (99.7%) and higher selectivity
(
99.6%) than air atm. The selectivity towards the undesired product namely 4-furaldehyde
dimethyl- acetal was found to be in the range of 3.2 % to 9.9%, however when air or oxygen
alone was used the selectivity towards this undesired product was found to be less than 1. Hence
this study clearly indicates that for both selectivity and economic points of view, the oxidative
esterification of furfural can be done effectively by using only oxidants such as air and oxygen.
3
.7.2.5 Effect of reusability of 5% Au/CMK-3 catalyst:

In order to investigate the reusability of the 5% Au/CMK-3 catalyst, the oxidative esterification
of furfural was carried out successively for another five times at 393 K in O atmosphere (15
2
bar). The catalysts after 3 h of reaction time was gently washed with water and methanol and
reused. The efficiency was investigated for five catalytic cycles and the results are given in
Table.8.
Table.8
The conversion and selectivity results reveal that 5% Au/CMK-3 catalyst is a sturdy catalyst and
can be reused without much loss of catalytic activity. Au/CMK-3 prepared by Qi et. al. (2015)
showed loss of catalytic activity from 92% to 70% in the base free oxidation of glucose to
gluconic acid after 4 catalytic cycles [42]. The authors have impregnated Au over CMK-3 in the
aqueous medium followed by sodium borohydride reduction. Since the CMK-3 material is
highly hydrophobic in nature, the CMK-3 will prefer the Au particle to occupy the surface and
not into the pores and hence there was a loss in the catalytic activity due to leaching. Moreover
the particle will have bigger size. However our method of synthesis of Au/CMK-3 involves the
deposition of Au in organic medium followed by hydrogen reduction. This method has the
advantage of keeping the Au particles inside the pores of CMK-3 and hence the Au particles are
not leached during the reaction. Similar observations were made by Ryoo. et. al. (2001) over Pt
clusters over CMK-3[92]. The particle inside the pores has the pore size distribution in the range
of 1-3 nm. Therefore the catalytic activity is retained even after five cycles. This study also
clearly indicates the importance of method of preparation of catalysts. Figure.7 shows the HR-
TEM micrographs and the average particle size distribution of the spent catalyst used for five
catalytic cycles. The intact nature of the Au particles with no aggregation was noticed.

Figure.7
In order to check the effect of oxidants such as TBHP and oxygen on metal leaching, catalysts
after five catalytic cycles were subjected to ICP analysis to know the Au concentration. Prior to
analysis the catalysts were washed with distilled water, methanol, dried at 393 K and digested in
a mixture of concentrated acids. The Au content in the spent catalyst (5% Au/CMK-3) catalyzed
in the O atmosphere was found to 4.82% which is much closer to the theoretical value of 5%.
2
But when the Au content in the spent catalyst (5% Au/CMK-3) catalyzed along with TBHP was
found to be 4.32% showing much loss due to leaching in the reaction. Thus it is concluded that
the hydrophobicity keeps the Au metal particles onto the pores of CMK-3 helping in well
dispersion of Au particles in the size range of 1-3 nm without agglomeration.
Moreover, the usage of TBHP as an oxidant introduces new functional groups such as –OH,
–
COOH etc, on the surface of CMK-3. Fig.8 shows the FT-IR spectra of the spent catalyst (5%
Au/CMK-3) catalyzed along with TBHP at two different reaction temperatures (393 K & 413 K)
-1
-1
showed OH stretching peak at 3438 cm , the C=O stretching peaks at 1720 and 1563 cm , the
-1
C-O-C bending vibration at 1233 cm . Slightly higher leaching of Au particles was noticed in
the spent catalyst used along with TBHP. This is because of creation of the above mentioned
functional groups, opening up a thin layer of surface leading to slight leaching of the Au
particles. The presence of functional groups is also responsible for the formation of undesired
products namely 4-furaldehyde dimethyl-acetal.
Figure.8
4
. Conclusions

CMK-3 was successfully synthesized using a hard template SBA-15, and sucrose, a
carbon source, using nanocasting method. Au was impregnated over different supports such as
SBA-15, graphite, graphene, MWCNT, activated carbon and TiO
were compared. Among the catalysts 5% Au/CMK-3 was found to be the best catalyst in terms
of conversion (99.7%) and selectivity (99.6 %) under the O pressure of 15 bar. Various
2
and their catalytic activities
2
Au/CMK-3 catalysts with different Au contents were prepared using wet impregnation method
followed by hydrogen reduction. The XRD and BET analysis confirm the mesostructure of the
catalyst while the HR-SEM and HR-TEM micrographs confirm the rod like morphology for both
CMK-3 and Au/CMK-3 catalysts. Fine dispersion of Au particles over CMK-3 was evidenced by
HR-TEM. The study of effect of metal ion on the catalytic activity shows that Au catalysts are
the best catalysts in terms of furfural conversion and selectivity towards methyl 2-furoate.
Similarly, the study on the effect of support on the catalytic activity shows that CMK-3 is the
best support. Optimization studies revealed that the reaction temperature of 393 K and the
reaction time of 3 h yield maximum conversion of furfural and selectivity towards methyl 2-
furoate.The study on effect of oxidants on the catalytic activity indicated that among the different
oxidants namely TBHP, O
2
and air, O was found to be the best oxidant as it offers very high
2
catalytic activity. Unlike TBHP the oxidative esterification of furfural in presence of oxygen
does not create any unwanted functional groups on the surface of the catalysts and hence
improves the selectivity. Among the Au/CMK-3 catalysts, 5% Au/CMK-3 was found to be the
most active catalyst as it converted 99.7% of furfural with methyl 2-furoate selectivity as high as
9
9.6%. The recyclability of 5% Au/CMK-3 proved that the catalyst did not show much loss in
the catalytic activity even after five catalytic cycles.
Acknowledgement
The authors acknowledge the funding received from DRDO-LSRB and CPRI, INDIA.

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