Oxidative esterification of furfural over Au-Pd/HAP-T and Au-Ag/HAP-T bimetallic catalysts supported on mesoporous hydroxyapatite nanorods

Article abstract of DOI:10.1039/c6ra07614a

The conversion of furfural, a lignocellulose biomass, into fuels and value-added fine chemicals has attracted many researchers. In this study, structurally uniform and porous hydroxyapatite nanorods (HAP-T) with the gel composition 1Ca²⁺:0.66PO

Full text of DOI:10.1039/c6ra07614a

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Oxidative esterification of furfural over Au-Pd/HAP-T and Au-Ag/HAP-T
bimetallic catalysts supported on mesoporous hydroxyapatite nanorods.
Ramakrishnan
Radhakrishnan,
Kathiravan
Kannan,
Shathivel
Kumarvel
and
Sivakumar Thiripuranthagan*
ABSTRACT: The conversion of furfural, a lignocellulose biomass, into fuels and valueꢀadded
fine chemicals has attracted many researchers. In this study, structurally uniform and porous
3ꢀ
hydroxyapatite nanorods (HAPꢀT) with the gel composition of 1Ca2+: 0.66PO4 : 0.3 CTAB:
120 H2O were synthesized under hydrothermal method. The synthesized HAPꢀT was
impregnated with Au, Pd or Ag so as to obtain either monometallic (Au/HAPꢀT, Pd/HAPꢀT and
Ag/HAPꢀT) or bimetallic (Au1ꢀxPdx/HAPꢀT and Au1ꢀxAgx/HAPꢀT) catalysts. The obtained
hydroxyapatite catalysts were thoroughly characterized by using various analytical techniques
such as FTꢀIR, XRD, BET, UVꢀDRS, XPS, HRꢀSEM & HRꢀTEM. A thorough characterization
of the catalysts lead to the valid information that bimetallic Au1ꢀxPdx/HAPꢀT catalysts have coreꢀ
shell bimetallic morphology, whereas the Au1ꢀxAgx/HAPꢀT catalysts have alloy morphology. The
catalytic activities of the bare HAPꢀT, monometallic (Au/HAPꢀT, Pd/HAPꢀT and Ag/HAPꢀT)
and bimetallic (Au1ꢀxPdx/HAPꢀT and Au1ꢀxAgx/HAPꢀT) catalysts were evaluated at atmospheric
pressure towards the oxidative esterification of furfural to methyl 2ꢀfuroate using TBHP as
oxidant and methanol as solvent. Among the various synthesized catalysts, Au0.8Pd0.2/HAPꢀT
catalyst with coreꢀshell structure showed the maximum conversion of furfural (94.2%) with very
high selectivity towards methylꢀ2ꢀfuroate (99%). In addition, the efficiency of Au0.8Pd0.2/HAPꢀT
catalyst was maintained even after 5 cycles, supporting the reusability and stability.
--------------------------------------------------------------------------------------------------------------------
* Corresponding author
Catalysis Laboratory, Department of Applied Science and Technology, A.C.Tech Campus,
Anna University, Chennai-25. E-mail: tssivakumar@yahoo.com; Tel.: 91-44-22359193.

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1. Introduction
Furfural is the dehydration product of naturally abundant Cꢀ5 sugars (arabinose & xylose)
obtained from the biomass lignocellulose (hemicellulose fraction)1,2. Furfural, a natural
precursor, is a bio based alternative platform molecule for the production of everything from
medicines, pesticides, paints, plastics, commodity chemicals and solvents3ꢀ5. Currently furfural
production is about 300 KT at a price of 2000$/ton and is being projected to increase6. It is a
building block for several value added fine chemicals and fuels such as 2ꢀmethyl furan, furfuryl
alcohol, furan, cyclopentanol, cyclopentanone, tetrahydrofurfuryl alcohol, 2ꢀmethyl
tetrahydrofuran and C10ꢀC15 coupling products. Hence it is identified as one of the most
promising renewable chemicals for sustainable production of commodity chemicals and fuels.
Hence valorization of furfural is the subject of interest to many researchers in the world. One of
the authors had already reported the conversion of different biomasses into fuels7ꢀ10. The
valorization process will be attractive only when (i) it is carried under mild conditions and (ii) it
is environmentally benign. So it is very important to find a new pathway to convert furfural into
commodity chemicals. Among the different valorization processes of furfural, the synthesis of
esters particularly methyl 2ꢀfuroate find interesting applications in the fine chemical industries.
These alkyl furoates are used in flavors and fragrance industries11. Furfural to methyl 2ꢀfuroate is
typically an oxidation and esterification reaction. Conventionally methyl 2ꢀfuroate is prepared by
oxidizing furfural with strong oxidizing agent such as KMnO4 into furoic acid and then
esterification using concentrated H2SO4 in methanol medium. The usage of such corrosive acid
affects the environment badly. Hence it is desired to carry out this esterification reaction in one
step that too under milder conditions but without using any corrosive reagents. Recent literature
shows that it can be effected in mild conditions but by using bases such as NaOH, CH3ONa,

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etc.12. Since esterification of furfural using bases is also not a greener method, base free system
is sought by the environmentally conscious scientists.
Hydroxyapatite [Ca10(PO4)6(OH)2], a calcium phosphate based biomaterial, apart from its
biomedical and bioceramic application such as controlled drug and protein release, surface
coatings for orthopaedic surgery and dental implants, it can be a best choice for catalyst support
due to its ionꢀexchange capacity with various metal cations and anions, adsorption capacity,
acidꢀbase properties and thermal stability13ꢀ18. Not only that by tuning the Ca/P ratio one can tune
the hydroxyapatite either for acid or base catalyzed reactions.
Various catalytic transformations were reported using hydroxyapatite as support for
different transition metals19ꢀ21. Ca2+ present in the hydroxyapatite lattice can also be exchanged
3ꢀ
with cations such as Cu, Ni, Zn, Fe and Co22ꢀ26. The partial exchange of PO4 with vanadate,
tungstate and silicate are also possible19, 27, 28. Hence it is a versatile material for many catalytic
applications. Studies on oxidative esterification of furfural over various Au supported catalyst
have been reported earlier but they have certain disadvantages such as poor yield, usage of bases
or requirement of severe reaction conditions such as high oxygen pressure29ꢀ33. The addition of
second metal generally enhances the catalytic activity and stability due to the change in their
electronic and geometric properties. In this context various bimetallic catalysts have been
synthesized and their enhanced catalytic activities were reported. For example, Au/Pd bimetallic
catalysts have shown excellent catalytic properties in ethanol oxidation under alkaline conditions
vinyl acetate synthesis, benzyl alcohol oxidation and 1, 2ꢀpropanediol oxidation34ꢀ37. Similarly
Au/Ag bimetallic catalysts were reported for the various catalytic transformations such as
oxidation of benzyl alcohol, 1ꢀphenyl ethanol, TMB, methanol to methyl formate, hydrogen from
allyl alcohol, methyl acrylate, biomass to 2ꢀHMF and 2,5ꢀDMF, N2O decomposition, CꢀC
coupling, but only few studies were reported on the oxidative esterification for furfural38ꢀ46
.

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Hence it was envisaged to synthesize bimetallic catalysts supported over hydroxyapatites
(HAPꢀT) and to explore the catalytic activities towards the oxidative esterification of furfural.
Herein we report the synthesis of monometallic supported hydroxyapatites (Au/HAPꢀT,
Ag/HAPꢀT & Pd/HAPꢀT) and bimetallic supported hydroxyapatites (Au1Pd1ꢀx/HAPꢀT &
Au1ꢀxAgx/HAPꢀT, X= 0.2, 0.4, 0.6, 0.8). Detailed characterization by various instrumental
techniques and evaluation of catalytic activity towards the oxidative esterification of furfural are
also reported here. We have studied the reaction with or without the addition of oxidant and with
or without the addition of base. The results prove that the bimetallic catalysts are very active
without the bases and very selective towards the desired product namely methyl 2ꢀfuroate. Hence
it can be a good catalyst system for exploiting the renewable biomass into value added fine
chemicals.
2. Experimental
2.1 Materials
Calcium nitrate (Ca(NO3)2.4H2O, 99.9%), gold chloride trihydrate (HAuCl3.3H2O, 99.9%) and
palladium acetate (Pd(OCOCH3)2, 99.98%) were obtained from Sigma Aldrich; Silver nitrate
(AgNO3, 99%), diꢀammonium hydrogen orthophosphate ((NH4)2HPO4, 99%) and cetyl trimethyl
ammonium bromide (C19H42BrN, 99%) were purchased from SRL chemicals and sodium
borohydride (NaBH4, 98%) from Alfa Aesar was used in this study.
2.2 Synthesis of porous hydroxyapatite (HAP-T) nano rods
To the stoichiometric amount of calcium nitrate in 100 ml double distilled water, aqueous
ammonia was added to maintain pH 10.5. Then aqueous diꢀammonium hydrogen orthophosphate
was added slowly to obtain a white gel. In another vessel, cetyl trimethyl ammonium bromide
(CTAB) was dissolved in double distilled water and stirred for 1 hour. CTAB solution was
slowly added to the above calcium phosphate gel with continuous stirring. The thickened gel was

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then autoclaved at 453 K for 24 hrs. The upper transparent solution was decanted and the white
mass was filtered through the Whattman filter paper, washed with 500 ml of double distilled
water, dried at 383 K for 24 hrs, ground and finally calcined at 773 K, with the heating rate of
3ꢀ
1°C/min. This procedure was adopted to get the gel composition of 1Ca2+: 0.66PO4 : 0.3
CTAB: 120 H2O. in which Ca/P was maintained at 1.67 for stoichiometric synthesis. The
hydroxyapatite thus synthesized using the template is designated as HAPꢀT and without template
was also synthesized and designated as HAPꢀD.
2.2.1 Synthesis of monometallic Au/HAP-T, Ag/HAP-T and Pd/HAP-T catalysts. To
1000 mg of hydroxyapatite synthesized using the above procedure, known volume of aqueous
solution of HAuCl3.3H2O (0. 05 mM) was added at the constant temperature of 353 K and stirred
for 3 hrs so as to obtain 5 wt%. of Au over HAPꢀT. Then an aqueous solution of NaBH4 (25 ml,
0.01M) was rapidly added to the above solution with vigorous stirring. Bluishꢀpurple colour
dispersion formed was allowed to stir for 1 hr, centrifuged, washed with deꢀionized water and
with ethanol twice. Finally the above synthesized material was dried at 343 K for 6 hrs, calcined
at 623K under the air atmosphere for 2 hrs and completely reduced under the flow of hydrogen
(0.2LMP) at 623K for 5 hrs to obtain the Au/HAP catalyst. Similar method was also adopted for
the synthesis of Ag/HAPꢀT and Pd/HAPꢀT using AgNO3 and Pd(OAc)2 as precursors, so as to
obtain 5wt.% of metal supported over HAPꢀT.
2.2.2 Synthesis of bimetallic Au-Ag/HAP-T catalysts. To 1000 mg of hydroxyapatite
(HAPꢀT), known volume of aqueous solution of HAuCl3.3H2O
(0.05mM) and AgNO3 were
added simultaneously at the constant temperature of 353K and stirred for 6 hrs. Then an aqueous
solution of NaBH4 (25 ml, 0.01M) was rapidly added into the above solution with vigorous
stirring. The colored dispersion was allowed to stir for 1 hr, centrifuged, washed with deꢀionized
water and ethanol twice. Finally the above synthesized material was dried at 343K for 6 hrs,

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calcined at 623K under the air atmosphere for 2 hrs and completely reduced under the flow of
hydrogen (0.2 LMP) at 623K for 5 hrs to obtain the AuꢀAg/HAP catalyst. By varying Au and Ag
contents, various catalysts such as Au0.8Ag0.2/HAPꢀT, Au0.6Ag0.4/HAPꢀT, Au0.4Ag0.6/HAPꢀT and
Au0.2Ag0.8/HAPꢀT were prepared.
2.2.2 Synthesis of bimetallic Au-Pd/HAP-T catalysts. To 1000 mg of hydroxyapatite,
known volume of aqueous solution of HAuCl3.3H2O (0.05mM) and Pd(OAc)2 were added at the
constant temperature of 353 K and stirred for 6 hrs. Then an aqueous solution of NaBH4 (20ml,
0.01M) was rapidly added into the above solution with vigorous stirring. The colored dispersion
was allowed to stir for 1 hr, centrifuged, washed with deꢀionized water and ethanol twice. Finally
the above synthesized material was dried at 343K for 6 hrs, calcined at 623K under the air
atmosphere for 2 hrs and completely reduced under the flow of hydrogen (0.2LMP) at 623K for
5 hrs to obtain the AuꢀPd/HAP catalyst. By varying Au and Pd content, various catalysts such as
Au0.8 Pd 0.2/HAPꢀT, Au0.6 Pd 0.4/HAPꢀT, Au0.4Pd0.6/HAPꢀT and Au0.2 Pd0.8/HAPꢀT were prepared.
2.3 Characterization
Powder Xꢀray diffraction data (PꢀXRD) were collected on a Rigaku ultima+ diffractometer using
CuꢀKα (λ=1.54056A°) radiation with a solidꢀstate detector at 40kV and 40mA. The data were
recorded in the 2θ range from 1ꢀ8° (narrow range) and 2θ range from 10ꢀ80° (wide range) with
2θ step size of 0.01° and step time of 10 s. The obtained data were analyzed using Origin 8
software for the determination of phase composition and crystallinity. The peaks were indexed
on a hexagonal unit using R3C space group. The standard JCPDF card no.09ꢀ432 was used for
comparing the hydroxyapatites. Fourier Infra Red spectra (FTꢀIR) were recorded on a Thermoꢀ
Nicolet IR 2000 spectrophotometer in wavenumber range of 4000–400 cmꢀ1. Nitrogen adsorption
and desorption isotherms studies were performed at 77 K on a BELSORP mini, BEL Japan
instrument. Prior to analysis, the samples were degassed at 473 K for 2 h under nitrogen

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atmosphere. The UV–Visible spectra of the solid samples were recorded directly using a JASCO
Vꢀ650 UVꢀVis spectrophotometer equipped with an integrated sphere for diffuse reflectance
studies. Xꢀray photoelectron spectroscopy (XPS) measurements were performed to find out the
elements present in the catalyst on an ESCA+, Omicron Nanotechnology ESCA Probe
spectrometer with monochromatized AlKα Xꢀrays (energy: 1486.6 eV) 300W with base pressure
of 5x10ꢀ5 mB. Sputterꢀcleaned Cu
(932.7 eV) and Au
(84.0 eV) core level peaks (from
4f7/2
2p3/2
foils) were used to calibrate the XPS instrument. The catalyst samples were pelletized, dried
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 highꢀresolution scans were performed for the Au4f5/2, Au4f7/2, Pd3d3/2,
Pd3d5/2, C1s, Ca2p3/2, Ca2p1/2, P2p, and O1s core levels. Quantitative determination of different
elements present in the hydroxyapatite was carried out by Inductive coupled plasma (ICP)
technique using Perkin Elmer Optima 5300 DV. 200 mg of samples were digested in Con. nitric
acid and diluted prior to the analysis. Particle morphology and structure of the hydroxyapatites
were studied by using Quanta 200 FEG scanning electron microscope (HRꢀSEM). Powdered
samples were air dispersed over the carbon tape and conductivity was enhanced by gold
sputtering. The High resolution transmission electron micrographs (HRꢀTEM) were recorded
using JEOL JEMꢀ2100JF working at the operating voltage of 200KeV. Powder samples were
dispersed in ethanol, ultrasonicated for 10 minutes prior to analysis placed over holey carbon
coated Cu grid, dried at ambient conditions and mounted for analysis.
2.4 Catalytic studies
The liquid phase oxidative esterification of furfural in the presence of methanol as solvent and
using tertꢀButyl hydroperoxide (TBHP) as an oxidant was investigated at 393K. 300ꢁl of

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furfural, 3 eq. of TBHP (70%) and 50 mg of catalyst (Au ꢀ 20 mol%) were added to 20ml of
methanol and the reaction was carried out in liquid phase. The progress of the reaction was
monitored by thin layer chromatography (TLC) using of 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. Sample aliquots
(30ꢁL) was filtered through the nylon syringe filters (0.22 microns) and analyzed using a
Shimadzu GCꢀ17A gas chromatograph equipped with a capillary column and a flame ionizing
detector (FID).
3. Results and discussion
3.1 FT-IR spectral analysis
Figure.ESI.1 shows the FTꢀIR spectra of HAPꢀT synthesized via hydrothermal method using
CTABr mediated template and is compared with the HAP synthesized via direct precipitation
method (HAPꢀD) and gold loaded (Au/HAPꢀT) apatite catalysts. The sharp and small peak
around 3570 cmꢀ1 is due to the stretching mode of non hydrogen bonded OH groups of the
hydroxyapatites. The broad band around 3431 cmꢀ1 and 1647 cmꢀ1 are due to stretching mode of
hydrogen bonded OH group and ν2 bending mode present in the adsorbed water molecules. The
1093 cmꢀ1 and 1035 cmꢀ1 band arise from triply degenerate ν3 antisymmetric PO stretching mode
3ꢀ
of PO43ꢀ , 963 cmꢀ1 peak is due to the nondegenerate PO symmetric stretching mode ν1 PO4 , 604
cmꢀ1 and 568 cmꢀ1 band arising from components of the triply degenerate ν4 OPO bending mode
3ꢀ
of PO43ꢀ and 473 cmꢀ1 is due to the doubly degenerate ν2 OPO bending mode of PO4 . The FTꢀ
IR spectra of all the hydroxyapatites look alike, indicating the structural similarity in all of them
and are in good agreement with the reports47,48. The incorporation of Au followed by reduction in
the Au/HAPꢀT also did not affect the structural features of hydroxyapatites.

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3.2 Powder x-ray diffraction
The small and wide angle xꢀray powder diffraction patterns of bare hydroxyapatite (HAPꢀT) are
shown in Fig. 1 & 2
Figure.1 & Figure.2
The broad peak appearing from 2θ = 2° to 4° indicates the mesoporous nature of
synthesized hydroxyapatite. However the broadness of the peak suggests that they have irregular
mesopores. The high angle XRD shows intense and well resolved peaks confirming the nature
of crystalline hydroxyapatite. The xꢀray diffraction patterns were identified and indexed in
comparison with the standard JCPDSꢀICDD values49. The standards with card nos. 04ꢀ932
(Ca10(PO4)6(OH)2), 09ꢀ0169 (βꢀCa3(PO4)9), 04ꢀ0777(CaO) and 04ꢀ0733(Ca(OH)2) were used to
check whether these compounds were present in the hydroxyapatites. On comparison, it was
found that none of the above was found and this indicates the phase purity of hydroxyapatites.
The average crystallite size (XS, nm) in a direction perpendicular to the crystallographic plane
based on Scherrer formula was calculated by using the equation:
0.9 × λ
Xꢀ =
(βꢁꢂꢃ × cosθ)
Where, λ – is the wavelength of the monochromatic xꢀray beam (CuꢀKα1, 1.54056A°);
βhkl – broadening of the hkl diffraction peak measured at half of its maximum intensity (radians)
and θ – diffraction angle50.
The unit cell lattice parameters were indexed using a hexagonal unit cell lattice and
determined using the following formula and were found to be a = b = 9.432A°, c = 6.881A°.
ꢄ
ꢅ
ꢄꢆ
1
dꢄ
4 h + hk + k
lꢄ
cꢄ
=
+
3
aꢄ

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These values are found to be in good agreement with our reported literature51. Using the
obtained lattice parameters values of a and c, the volume of the unit cell was calculated and was
found to be 530.14 cm3:
V = aꢄc × sin 60°
Figure.3 shows the wide angle xꢀray powder diffraction patterns of Au/HAPꢀT, Pd/HAPꢀ
T and Au1ꢀxPdx/HAPꢀT catalysts(x=0.2ꢀ0.8)
Figure.3
The appearance of diffraction peaks at 2θ= 39.68(111), 46.48(200), 68.64(220) confirm
the presence of Pd and at 2θ=38.24(111), 44.32(200), 64.4(220) confirm the presence of Au. The
peaks are indexed in the FCC lattice and compared with the standard JCPDS cards 41ꢀ1043(Pd)
and 65ꢀ2870(Au) respectively. Using the Scherrer equation the average particle size was
calculated and they were found to be 10.9 nm for Au/HAPꢀT, 9.7 nm for Au0.8Pd0.2/HAPꢀT, 9.3
nm for Au0.6Pd0.4/HAPꢀT, 9.5 nm for Au0.4Pd0.6/HAPꢀT, 9.8 nm for Au0.2Pd0.8/HAPꢀT and 13.7
nm for Pd/HAPꢀT. It was observed that the bimetallic shows smaller particles than the
monometallic hydroxyapatites as reported52,53. The average particle sizes of metal particles
calculated from the XRD are given in Table.1.
Table -1
A well separable peak for the metal species indicates that the nano metallic particles in
either mono or bimetallic are well dispersed over the porous hydroxyapatite. The peak intensity
of Au in the hydroxyapatites of AuxPdxꢀ1/HAPꢀT (x=0.4, 0.6 & 0.8) was found to be almost the
same, whereas the peak intensity of Pd increases with concentration. Such possibility will occur
only when there is a coreꢀshell formation with Au in the core and Pd as the shell. Such formation
of core –shell is justified from the changes in the interplanar spacing and lattice parameter. As

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per the Vegard’s law, the formation of ordered bimetallic species can be confirmed from the
linearity of the plot of molar ratio Vs dꢀspacing (Figure.ESI2). The corresponding dꢀspacing and
lattice parameter of both mono and bimetallics of Au and Pd are given in Table 2. Since interꢀ
planar spacing “d” is proportional to lattice parameter “a”, any effect would change the
interatomic distance which in turn changes the geometric and electronic properties also.
Table.2
The linearity reveals that both the metals are in zero oxidation state, the existence of
oxides of Au and Pd are ruled out. The absence of peaks at 2θ = 33.216(002), 34.352(101) and
42.854(110) indicates the absence of PdO species(JCDFS card no. 88ꢀ2434) and this further
confirms that both the Au and Pd were completely involved in the formation of bimetallics. The
fact that (002) plane corresponding to cꢀaxis of the hydroxyapatite remains unaffected confirms
that both the crystallinity and stability are retained even after the impregnation of metals and
reduction under H2 atmosphere.
Figure.4 shows the wide angle xꢀray powder diffraction patterns of Au/HAPꢀT, Ag/HAPꢀ
T and Au1ꢀxAgx/HAPꢀT catalysts(x=0.2ꢀ0.8).
Figure.4
On comparison of these powder diffraction patterns with standard JCPDS of Au(04ꢀ
0784) and Ag(04ꢀ0783), it was found that the peaks match at 2θ= 38.04°(111), 44.2°(200) and
64.72°(220) and are indexed in the face centered cubic (fcc) lattice. The powder diffraction
patterns further confirm the absence of peaks of the oxides of Au and Ag in the synthesized
hydroxyapatites. Since the d values of Au (d111=0.2355nm) and Ag (d111=0.2359nm) are close to
each other, the powder diffraction patterns of both mono and bimetallics of Au and Ag

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hydroxyapatites look alike. The closeness of the interplanar spacing “d” and the lattice parameter
“a” make it difficult to distinguish the peaks not only for the (111) plane but also with the other
planes such as (200) and (220). The powder diffraction patterns of mono and bimetallics shows
sharp peaks indicating high crystallinity. The particle size of Au/HAPꢀT (10.9 nm) is the least in
the series whereas it is the highest for the Ag/HAPꢀT (32.7 nm). The increase in Ag content in
bimetallic hydroxyapatites increases the particle size54,55. The average particle sizes calculated
from the XRD are given in Table.1.
3.3 UV-Visible diffuse reflectance studies (UV-DRS)
Figure.5 shows the UVꢀVisible diffuse reflectance spectra of monometallic (Au/HAPꢀT &
Pd/HAPꢀT) and bimetallic (Au1ꢀxPdx/HAPꢀT) hydroxyapatites. Among the hydroxyapatites, the
monometallic Au/HAPꢀT shows surface plasmonic resonance(SPR) band around 520 nm due to
the transition of e¯ from occupied 5d to vacant 6sp, which corresponds to the band gap of
2.4 eV56,57. Such SPR transition is not seen in Pd/HAPꢀT. With increase in the Pd concentration
in the bimetallics, the intensity of the surface plasmonic resonance band decreases. The dramatic
decrease in the intensity of this band occurred even when a small amount of Pd (x = 0.2) was
incorporated. The interaction between Au and Pd in the bimetallic allows a large change in the
electronic properties as reported58,59. The absence of SPR bands at 308 nm and 407 nm indicates
the absence of other ionic species such as Au3+ and Pd2+ respectively60. In any bimetallic
formation, either a coreꢀshell structure or alloy structure is possible. The fact that the intensity of
the surface plasmonic resonance (SPR) decreases with the increase in the concentration of Pd
suggest the formation of coreꢀshell structure of Au/PdꢀHAPꢀT. When a small amount of Pd is
present in bimetallics, it forms a thin nonꢀhomogeneous shell over Au core. When the Pd
concentration increases, the bimetallic hydroxyapatites show decrease in the intensity of SPR
band around 520 nm and get completely vanished beyond Pd = 0.4. This is mainly due to the

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thick coverage of Pd over Au. Such reduction of SPR bands when any of the group 10 metals is
added to Au was also reported earlier60,61. Such large change in the electronic properties was also
reported62.Yong D et. al. reported the reorientation of the AuꢀPd as coreꢀshell structures when
the bimetallics was heated to 523K leading to the formation of alloy63. However the bimetallic
hydroxyapatites synthesized by us did not reorient and was intact even after the reduction at
623K for 5hrs.
Figure.5
Figure.6 shows the UVꢀVisible diffuse reflectance spectra of monometallic (Au/HAPꢀT
& Ag/HAPꢀT) and bimetallic (Au1ꢀxAgx/HAPꢀT) hydroxyapatites.
Figure.6
The UVꢀDRS spectra of Au/HAPꢀT and Ag/HAPꢀT are different from that of the Auꢀ
Pd/HAPꢀT system. In the UVꢀDRS spectra of Au and Ag system, the SPR bands occur at two
different wavelengths at 526nm and 470nm for the Au/HAPꢀT and Ag/HAPꢀT respectively64. In
this bimetallic, the increase in Ag wt. % has direct impact on the shifting of the surface plasmon
resonance (SPR) bands; 511.6nm for Au0.8Ag0.2/pꢀHAP, 500.2nm for Au0.6Ag0.4/pꢀHAP,
489.5nm for Au0.4Ag0.6/pꢀHAP and 476.2nm for Au0.2Ag0.8/pꢀHAP65. The formation of alloy
structure66ꢀ68 or coreꢀshell structure69ꢀ71 can be derived by the shift in the SPR bands. The
absence of two separate SPR bands indicate that the bimetallic is not a physical mixture of Auꢀ
Ag system and also rules out the Aucore/Agshell or Agcore/Aushell structure72ꢀ74. The formation of
single SPR band and its gradual blue shifting and broadening with increase in Ag content in the
bimetallic suggest the complete interaction between Au and Ag forming an alloy at all the
concentrations (Figure.ESI3). The linearity in the plot between % mole fractions of Ag to λmax

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of SPR bands further confirms the formation of alloy structure (Figure.ESI4) for AuꢀAg
bimetallic.
3.4 BET Nitrogen Adsorption-Desorption Measurements
To check whether the synthesized hydroxyapatite is mesoporous in nature, nitrogen adsorption
and desorption isotherms were constructed at liquid N2 temperature in the relative partial
pressure of P/P0=0.05 to 1.0. Figure.7 shows the BET adsorption isotherm and (b) pore size
distribution of HAPꢀT. This figure shows that the hydroxyapatite exhibits a type IV isotherm
with steep H1 hysteresis loop occurring in the partial pressure between 0.6 and 0.9 confirming
the presence of mesopores with open ends in the hydroxyapatites. The BET surface area and the
pore volume of the hydroxyapatite were also calculated and they were found to be 44 m2/gm and
0.2485 cm3/gm. The BJH pore size distribution of the HAPꢀT sample is shown in Figure.8.
Figure.7 & 8
The synthesized hydroxyapatite has large population of the pores with diameter of 2.4
nm, 3.1 nm and 5.4 nm. The HRꢀTEM micrograph (discussed later) also confirms pores in the
above mentioned ranges. Hydroxyapatites with such different pore diameters have also been
reported75ꢀ77
.
3.5 X-ray photoelectron spectra
Xꢀray photoelectron spectra (XPS) were recorded for AuꢀPdꢀHAP and Au/Agꢀ HAP catalysts to
understand their structural and electronic properties.
Figure.9
Figure.9 illustrates the XPS survey spectra of Au1ꢀHAP. The measured XPS core levels
of Ca(Ca2p3/2ꢀ346.6 eV, Ca2p1/2ꢀ350.1eV), P (P2pꢀ 132.6eV) and O (O 1s) were in good agreement

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with the reported literature78,79. The Ca/P ratio calculated from XPS lie in the range of 1.55ꢀ1.59
which is almost close to both the Ca/P=1.63 obtained from the ICP and theoretical Ca/P= 1.67.
The determined Ca/P ratio rules out the formation of other calcium phosphate species such as αꢀ
TCP, βꢀTCP, ACP(Ca/P=1.50), OCP(Ca/P=1.3) and DCPD(Ca/P=1.0). Hence XPS study further
confirms the phase formation and stability of HAPꢀT besides FTꢀIR and XRD studies79, 80
.
Figure.10 shows the XPS spectra of the Au, Ag and Pd core levels of Au/HAPꢀT,
Ag/HAPꢀT and Pd/HAPꢀT. The binding energy of Au (Au4f7/2 – 83.37eV & Au4f5/2 – 87.03eV),
Ag (Ag3d5/2 – 367.4 eV & Ag3d3/2 – 373.3eV) and Pd (Pd3d3/2 – 334.9 eV & Pd3d5/2 – 340.3eV) are
in agreement with the literature values81.
Figure.10
The XPS spectra of Au1Pd1ꢀx/HAPꢀT catalysts are shown in Figure.11. From the Fig. the
binding energies and surface concentration of Au and Pd were calculated and given in Table.3.
Figure.11
Table.3
The Table.3 also contains the theoretical and ICP values of metals content and their
surface concentration. The bimetallics are present in the surface in significant quantities. In the
coreꢀshell arrangement, there are possibilities for the formations of either AucoreꢀPdshell or
PdcoreꢀAushell. Due to the higher reduction potential, electronegativity and electron affinity of
Au3+, Au gets reduced first forming core when compared to Pd. In this AuꢀPd system, Au gains
sp eꢀ from Pd, whereas Pd gains d eꢀ from Au and hence the stabilization of AuꢀPd coreꢀshell
arrangement is mainly due to the net charge transfer82. The gain of dꢀcharacter coupled with
better dispersion of Pd over Au, makes it a better catalytic system83. The intensity of the Au
peaks at 83.37eV and 87.03eV decrease with increase in the concentration Pd and vanish
completely, while the Pd peaks at 334.9eV and 340.3eV increases with increasing Pd content.

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The complete disappearance of the peaks could be explained only when the formation of
bimetallic takes place through coreꢀshell arrangement. At higher concentration of Pd in
bimetallics, the envelope of Pd layer over gold takes place. This increase in the thickness of Pd
over Au suppresses the existence of peaks for Au. This study again confirms the AucoreꢀPdshell
bimetallics. Similar synthesis of coreꢀshell structures of AuꢀPd bimetallics have been
reported 84ꢀ86. Figure.12 illustrates the XPS spectra containing the core levels of Au 4f, Ag 3d of
Au1Ag1ꢀx/HAPꢀT catalysts. The alloy formation between AuꢀAg is further supported by the XPS
analysis. The surface concentration of Au and Ag over the hydroxyapatite (HAPꢀT) was
calculated from the intensity of the peak obtained at respective binding energies and the values
are given in Table.4.
Figure.12
Table.4
A reasonable amount of Au and Ag is present on the surface of the hydroxyapatite. On
increasing the concentration of Ag from 0.2 to 1, i.e. from Au0.8Ag0.2/HAP to Ag1/HAP the
intensity of the Au peaks at Au4f7/2ꢀ83.57eV and Au4f5/2ꢀ87.22eV decreased and the intensity of
the Ag peak at Ag3d5/2ꢀ367.2 eV and Ag3d1/2ꢀ372.9 eV increased. In the Au/Pd system, the
intensity of the Au core peaks vanished, but in Au/Ag system, even at Au0.2Ag0.8/HAP, the Au
core peaks are not vanished. The presence of peaks even at high loading of Ag suggests the
formation of alloy structure rather than coreꢀshell system. Previous reports also support the
Au/Ag alloy structure formation87.
3.6 Electron microscope studies of hydroxyapatite
The surface topography and morphology of the HAPꢀT synthesized are depicted in HRꢀ
SEM and HRꢀTEM images (Figure.13).

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Figure.13
The nano rods ≈ 250nm x 40nm (length x breadth) are seen in the HRꢀSEM micrographs
(Figure.13a). All the nano rods were found to have similar size. The HRꢀTEM micrographs of
these nano rods (Figure.13b & 13c) confirm the hexagonal shape with the size of 207nm x 38nm.
The pores ≈ 6nm were also clearly seen on the hydroxyapatites nano rods (Figure.13d). Further
magnification shows the lattice fringes with d002=0.347nm and indicates the preferential growth
along the cꢀaxis (Figure.13e). This is in agreement with the XRD pattern. Hydroxyapatites with
similar dꢀspacing were also reported in the literatures88. The SAED patterns (Figure.13f)
corresponding to the strong reflections at d002 = 0.346nm, d211 = 0.282nm, d210 = 0.312nm, d112
=
0.278nm and d213 = 0.185nm reveal the polycrystalline nature of the hydroxyapatites. This
HRꢀTEM result coincides with the powder diffraction results discussed earlier.
The HRꢀTEM images of the monometallic Au/HAPꢀT, Pd/HAPꢀT and Ag/HAPꢀT are
shown in Figures.14 (aꢀc). In all the micrographs the presence of corresponding metal particles
was seen. Among the metal nano particles, Au particles are the smallest, and Ag particles are the
biggest. The lattice fringes of the 002 plane corresponding to monometallic hydroxyapatites are
shown in Figure 14(dꢀf). The dꢀvalues are also in clear agreement with the values obtained from
the powder diffraction patterns.
Figure.14
The presence of lattice plane at d111(0.235nm) and d200(0.203nm) confirms the presence
of Au, the lattice planes at d111(0.225nm) confirms the presence of monometallic Pd and the
lattice planes at d111(0.237nm) confirms the presence of Ag as shown in Figure14(dꢀf). The HRꢀ
TEM images of the most active bimetallic hydroxyapatite of Au0.8Pd0.2/HAPꢀT with different
magnifications are shown in Figure.15(aꢀc).

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Figure.15
The HRꢀTEM micrographs reveal the porous nature of the hydroxyapatite (white dots)
and metal particles (black dots). The uniform distribution of the bimetallics over HAPꢀT was
clearly seen in the Figure 15d. The existence of bimetallic particles in Au0.8Pd0.2/HAPꢀT can be
easily understood from their corresponding d values as shown in Figures.15e. As the
concentration of the Pd in Au0.8Pd0.2/HAPꢀT is low a thorough coverage of Pd is not seen
(d111=0.225nm for Pd & d111=0.235nm for Au), whereas in the other bimetallic of
Au0.6Pd0.4/HAPꢀT a better coverage of Pd over Au is seen Figure.15f. The higher coverage of Pd
over Au is due to the higher concentration of Pd in the bimetallic. Hence HRꢀTEM studies
further confirm the formation of AuꢀPd coreꢀshell besides UVꢀDRS, XRD and XPS
observations.
3.7 Catalytic studies of mono and bimetallic hydroxyapatites
Catalytic oxidation esterification of furfural was carried out in a batch reactor using methanol as
solvent, TBHP as an oxidant over mono and bimetallic catalysts. Two different products namely
methyl 2ꢀfuroate and 2ꢀfuraldehydeꢀdimethyl acetal were formed . No alkyl peroxides were
formed.
3.7.1 Effect of Reaction Temperature. The effect of reaction temperature on the
conversion of furfural and the selectivity towards methyl 2ꢀfuroate was first studied in the
temperature range of 333K to 413K over Au/HAPꢀT and the results are given in Table.5
Table.5
The Au/HAPꢀT catalyst showed the highest conversion at 413K and highest selectivity
towards methyl 2ꢀfuroate at 393K. Since large amount of the undesired products namely 2ꢀ

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furaldehydeꢀdimethyl acetal is formed at 413K, temperature of 393K was optimized for further
studies. Table.6 contains the results obtained in the oxidative esterification of furfural over the
Au1Pd1ꢀx/HAPꢀT bimetallic catalysts at the optimized reaction temperature of 393K. Between the
two monometallic Au/HAPꢀT and PdꢀHAPꢀT catalysts, the latter shows higher conversion of
furfural (86.6%) than Au/HAPꢀT (76%). Among the bimetallic hydroxyapatites,
Au0.8Pd0.2/HAPꢀT showed the highest conversion of 94.2% of furfural and the highest selectivity
(99%) towards methyl 2ꢀfuroate. As far as selectivity towards methyl 2ꢀfuroate is concerned, a
contrast behavior is shown by the monometallic hydroxyapatites, i.e Au/HAPꢀT showed higher
selectivity of (94.2%) than Pd/HAPꢀT (2.3%).
Table.6
The results suggest that the introduction of Pd into Au/HAPꢀT increases the conversion,
but decreases the selectivity towards methyl 2ꢀfuroate. However a significant increase in both
conversion and selectivity was seen when small amount of Pd was introduced in to Au/HAPꢀT.
The Au0.8Pd0.2/HAPꢀT catalyst shows the highest conversion of 94.2% and almost 99.0%
selectivity towards the desired product of methyl 2ꢀfuroate. Hence this study reveals the
significance of the interaction of low levels of Pd in Au0.8Pd0.2/HAPꢀT. As far as the bimetallics
are concerned, the formation of the other products namely 4ꢀfuraldehydeꢀdimethylacetal was also
found to increase with increase in the Pd content. Pd/HAPꢀT although show significant
conversion, the selectivity towards methyl 2ꢀfuroate was too low (2.3%). In the Pdꢀrich
bimetallic, a coreꢀshell arrangement is completely formed. The Au clusters are not exposed to
reactant molecules and hence results in poor conversion and selectivity. Similarly among the Auꢀ
Ag bimetallics (Table.7), Au0.8Ag0.2/HAPꢀT was found to convert 89.6% of furfural with

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selectivity as high as 99% towards methyl 2ꢀfuroate, whereas the Ag/HAPꢀT showed poor
selectivity (25%) to methyl 2ꢀfuroate.
Table.7
3.7.2 Effect of addition of base in the oxidative esterification of furfural. In the
present study TBHP has been used for the oxidative esterification of furfural. In order to check
the effect of other oxidants in the formation of ester, air or oxygen was bubbled into the reactor
and the products were analyzed by gas chromatograph. The conversion and selectivity towards
the two different products namely methyl 2ꢀfuroate and 4ꢀfuraldehydeꢀdimethylꢀacetal are given
in Table.8.
Table.8
It was found that the most active catalyst namely Au0.8Pd0.2/HAPꢀT did not convert
furfural in air medium and only converted 5.8% in O2 atmosphere. In general oxidative
esterification reactions are carried out in the presence of base such as NaOH, NaOCH3, K2CO3 .
Hence in order to check the influence of the base on the conversion and selectivity, 5ml of 10%
Aq. solution of NaOH/NaOCH3/K2CO3 was added and the reactions were performed in O2
atmosphere under the optimized conditions and the results are given in Table.8. It was found that
the addition of base has tremendous improvement on the conversion and selectivity. Among the
bases, NaOCH3 was found to have the maximum conversion of furfural (78.1%) with the highest
selectivity of (96%) towards methylꢀ2ꢀfuroate. Since the bases are corrosive in nature, another
oxidant namely TBHP has been used and the reactions were carried out with and without
oxygen. Replacement of base with TBHP yielded remarkable results both in conversion and
selectivity. The usage of O2 in addition to TBHP in the reaction over the most active catalyst has

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no effect on the conversion and selectivity. This study clearly indicates that such oxidative
esterification can be successfully carried out in base free and environmentally benign conditions.
3.7.3 Effect of recyclability of Au0.8Pd0.2/n-HAP on the oxidative esterification of
furfural. The best catalyst namely Au0.8Pd0.2/HAPꢀT was recycled for 4 more times to check
efficiency of the catalyst to convert furfural into methyl 2ꢀfuroate and the results are shown in
Figure.16. It clearly shows that there is only ≈ 4% difference in the conversion at the end of the
5th cycle. However there is no difference in the selectivity towards methyl 2ꢀfuroate. It remained
around 99%. This study confirms the stability of the Au0.8Pd0.2/HAPꢀT catalyst.
Figure.16
4. Conclusion
In summary, we succeed in the hydrothermal synthesis of mesoporous hydroxyapatite nanorods
(HAPꢀT) using CTAB as template. Mono and bimetallics of Au, Ag and Pd were impregnated
and reduced simultaneously over hydroxyapatite nanorods (HAPꢀT) with better dispersion of the
metal nanoparticles. Among the bimetallics, AuꢀPd adopted core ꢀ shell morphology while the
AuꢀAg system showed alloy morphology over hydroxyapatite nanorods. The FTꢀIR, XRD and
XPS analyses supports the formation and stability of the hydroxyapatites. AucoreꢀPdshell and
AualloyꢀAgalloy morphology over HAPꢀT were confirmed by UVꢀDRS, HRꢀTEM and XPS
analyses. Mesoporosity of the synthesized HAPꢀT was confirmed by BET measurements. The
optimized reaction temperature for the oxidative esterification of furfural was found to be 393K.
Introduction of small amounts of either Pd or Ag in AuꢀHAPꢀT increased the conversion of
furfural. Among the synthesized hydroxyapatite catalysts Au0.8Pd0.2/HAPꢀT showed 94.2%

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furfural conversion and 99% methyl 2ꢀfuroate selectivity. The replacement of corrosive bases by
TBHP in the oxidative esterification of furfural yielded significant results and hence this study
proves that such reactions can be performed in a mild and an environmentally benign conditions.
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FIGURE CAPTIONS
Fig.1. Small angle xꢀray powder diffraction pattern of hydroxyapatite (HAPꢀT) catalyst.
Fig.2.Wide angle xꢀray powder diffraction pattern of hydroxyapatite (HAPꢀT) catalyst.
Fig.3. Wide angle xꢀray powder diffraction pattern of (a) Au/HAPꢀT, (b) Au0.8Pd0.2/HAPꢀT
(c) Au0.6Pd0.4/HAPꢀT, (d) Au0.4Pd0.6/HAPꢀT, (e) Au0.2Pd0.8/HAPꢀT and (f) Pd/HAPꢀT.
(HꢀHydroxyapatite, Aꢀ Gold, PꢀPalladium).
Fig.4. Wide angle xꢀray powder diffraction pattern of (a) Au0.8Ag0.2/HAPꢀT,
(b)
Au0.6Ag0.4/HAPꢀT, (c) Au0.4Ag0.6/HAPꢀT, (d) Au0.2Ag0.8/HAPꢀT and (e) Ag/HAPꢀT.
(HꢀHydroxyapatite, Aꢀ Gold & Silver).
Fig.5. UVꢀDRS spectra of (a) Au/HAPꢀT, (b) Au0.8Pd0.2/HAPꢀT, (c) Au0.6Pd0.4/HAPꢀT, (d)
Au0.4Pd0.6/HAPꢀT, (e) Au0.2Pd0.8/HAPꢀT and (f) Pd/HAPꢀT.
Fig.6. UVꢀDRS spectra of (a) HAPꢀT, (b)Au/HAPꢀT, (c) Au0.8Ag0.2/HAPꢀT,
(d) Au0.6Ag0.4/HAPꢀT, (e) Au0.4Ag0.6/HAPꢀT, (f) Au0.2Ag0.8/HAPꢀT and (g) Ag/HAPꢀT.
Fig.7. Nitrogen adsorption –desorption isotherm of bare HAPꢀT.
Fig.8. Pore size distribution of bare HAPꢀT calculated from the desorption branch using BJH
algorithm.
Fig.9. Overall XPS survey spectra of AuꢀHAPꢀT catalyst.
Fig.10. XPS spectra of Au4f (Au4f7/2 & Au4f5/2), Pd3d (Pd3d3/2 & Pd3d5/2) & (Ag3d3/2
&
Ag3d5/2) core levels of the catalysts (a) Au/HAPꢀT (b) Ag/HAPꢀT and (c) Pd/HAPꢀT.

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Fig.11. XPS spectra in the of Au(Au4f7/2 & Au4f5/2) and Ag(Ag3/2 & Ag3d5/2) core levels for the
catalysts (a) Au0.8Ag0.2/HAPꢀT, (b) Au0.6Ag0.4/HAPꢀT, (c) Au0.8Ag0.2/HAPꢀT and
(d) Au0.8Ag0.2/HAPꢀT.
Fig.12. XPS spectra of Au4f (Au4f7/2 & Au4f5/2) and Pd3d (Pd3d3/2 & Pd3d5/2) core levels of the
bimetallic catalysts (a) Au0.8Pd0.2/HAPꢀT, (b) Au0.6Pd0.4/HAPꢀT, (c) Au0.4Pd0.6/HAPꢀT and
(d) Au0.2Pd0.8/HAPꢀT.
Fig.13. (a) HRꢀSEM images of bare HAPꢀT at Low magnification (b) High magnification and its
HRꢀTEM images (c) 200nm magnification (d) 20nm magnification (e) 5nm magnification and its
(f) SAED pattern.
Fig.14. HRꢀTEM images of Au/HAPꢀT (a, d), Ag/HAPꢀT (b, e) and Pd/HAPꢀT (c, f) catalysts
with magnification at 20nm and 5nm.
Fig.15. HRꢀTEM images of Au0.8Pd0.2/HAPꢀT (aꢀe) and Au0.6Pd0.4/HAPꢀT (f) catalysts.
Fig.16. Effect of recyclability of Au0.8Pd0.2/HAPꢀT on conversion and selectivity in the oxidative
esterification of furfural..