Gold nanoparticles supported on nanoscale amine-functionalized MIL-101(Cr) as a highly active catalyst for epoxidation of styrene

Article abstract of DOI:10.1039/c6ra24458k

In this study, well dispersed gold nanoparticles (Au Nps) were embedded on an amine-functionalized nanoscale metal-organic framework MIL-101(Cr) via a simple solution approach. The resulting Au/NH₂-MIL-101(Cr) exhibited a high isoelectric point (IEP) value with Br?nsted basic sites that played an important role in determining the size and dispersion of Au Nps onto its matrix. In addition, the uniform nanomorphology of the catalyst provides the advantage of its good dispersion in aqueous media. The synthesized Au/NH₂-MIL-101(Cr) catalyst was successfully characterized by different analytical techniques like powder X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, Brunauer-Emmett-Teller (BET) surface area analysis, Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), zeta potential analysis and atomic absorption spectroscopy (AAS). The catalytic activity of the synthesized material was explored for the epoxidation of styrene using tert-butyl hydroperoxide as oxidant. The catalyst could easily be recovered and recycled up to the fourth catalytic cycle without any significant change in its catalytic activity.

Full text of DOI:10.1039/c6ra24458k

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Gold nanoparticles supported on nanoscale amine-functionalized MIL-101(Cr) as a
highly active catalyst for epoxidation of styrene
Mrinal Saikiaa,b,Vasily Kaichevc and Lakshi Saikia*a,b
aMaterials Science and Technology Division,
CSIRꢀNorth East Institute of Science and Technology
Jorhat – 785006, Assam, India
bAcademy of Scientific and Innovative Research, Chennai, India
cSurface Science Laboratory, Boreskov Institute of Catalysis,
Lavrentieva Ave. 5,630090, Novosibirsk, Russia
*Corresponding author. Tel.: +91 376 2370 081; fax. +91 376 2370 011
*E-mail address: lakshi_saikia@yahoo.com
Abstract
In this study, well dispersed gold nanoparticles (Au Nps) were embedded on an amineꢀ
functionalized nanoscale metalꢀorganic framework MILꢀ101(Cr) via a simple solution
approach. The resulting Au/NH2ꢀMILꢀ101(Cr) exhibited high isoelctric point (IEP) value
with brønsted basic sites that played an important role in determining the size and dispersion
of Au Nps onto its matrix. In addition, the uniform nanomorphology of the catalyst provides
the advantage of its well dispersion in aqueous media. The synthesized Au/NH2ꢀMILꢀ101(Cr)
catalyst was successfully characterized by different analytical techniques like powder Xꢀray
diffraction, Xꢀray photoelectronspectroscopy, field emission scanning electron microscopy,
transmission electron microscopy, Brunauer–Emmett–Teller (BET) surface area analysis,
Fourier transform infrared spectroscopy (FTꢀIR), thermogravimetric analysis (TGA), Zeta
potential analysis and atomic absorption spectroscopy (AAS). The catalytic activity of the
synthesized material was explored for the epoxidation of styrene using tertꢀButyl
Hydroperoxide as oxidant. The catalyst could easily be recovered and recycled upto fourth
catalytic cycle without any significant change of its catalytic activity.
1. Introduction
Metal Nanoparticles (Nps) catalyzed organic chemistry transformations have been found
to be much attention in recent years1. Particularly, supported gold nanoparticles (Au Nps) has
been accepted to be a very active catalyst for a number of important organic transformations
such as selective oxidation reactions like oxidation of alcohols to aldehydes,2,3 oxidation of
Dꢀglucose,4 low temperature oxidation of CO,5 oxidation of ethylene glycol,6 epoxidation of
propene7ꢀ9 as well as asymmetrically terminal alkene.10ꢀ12 In addition, Au Nps has been

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involved as a catalyst in a wide range of nonꢀoxidative processes such as selective
hydrogenation of NꢀO bonds,13 hydrogenation of crotonaldehyde,14 addition to multiple CꢀC
bonds, benzannulations,15 hydrochlorination of ethyne to vinyl chloride16 etc. It is important
to mention that catalytic activity of supported Au Nps is highly depends upon certain key
parameters like its size, dispersion, the nature of the support and its interaction with the
Nps.17 Different materials such as TiO2,18 SiO2,17 Al2O3,19 CuO,18 MgO,10 Fe3O4,20 ZrO2,21
CeO2,22 CNT,23 HAP,24 SBAꢀ15,25 HMS,26 MCMꢀ41,27 MSU,28 TSꢀ1,29 TSꢀ1/SiO2,30 Tiꢀ
SiO2,31 TiꢀSBAꢀ15,32 MSN,33 TiꢀMCMꢀ41,34 TUDꢀ1,35 TSG,36 Yb2O3,37 CaO37 and layered
double hydroxide38 have already been used as catalytic support for Au Nps to study their
different catalytic activities.. In this context, metalꢀorganic frameworks (MOFs) can be used
as a support for metal nanoparticles owing to its high surface area, porosity and tunable
chemical properties.39, 40 In fact, noble metal Nps have already been synthesized on MOFs 41ꢀ
43 and the combination of the porous framework structures of MOFs with metal Nps offers
more efficient catalytic activities in terms of reaction times and recyclability over other
catalysts. 44ꢀ47
Among different MOFs, MILꢀ101(Cr) has already been employed as catalytic support for
various noble metals Nps48 because of its hydrothermal stability, high surface area and
chemical inertness. In particular, MOFs at nanoscale level having uniform and wellꢀdefined
sizes and morphologies49, 50 show excellent catalytic activities due to their better dispersion in
aqueous and other solvents.51, 52 These features of MILꢀ101(Cr) are also preserved in its
amino analogue i.e. MILꢀ101ꢀNH2 in which the amino groups are introduced into the MOF
lattice either via linker modification53 or through post synthetic treatment.48 Reports are
available for MILꢀ101(Cr) supported gold nanoparticles using different synthetic protocols
like depositionꢀprecipitation method,54 double solvent method (DSM)55, 56 impregnation57 etc.
Even though few reports have been available for MOF supported Au Nps,58 yet the
deposition of Au Nps on MOF catalysts in solution is considered to be a challenging task.59, 60
Furthermore, few studies have been carried out using nanoscale MOF as catalytic support.61,
62
Therefore, applications of MOFs in heterogeneous catalysis have attracted extreme
attention in recent times.63ꢀ65
Epoxidation of styrene is an important organic transformation for the synthesis of styrene
oxide, an industrially important organic intermediate, widely employed for the production of
valuable products such as perfumes, plasticizers, sweeteners, surfactants, paints, epoxy resins
etc.66 In conventional way, epoxidation of styrene is carried out using stoichiometric amounts
of peracids67 which acts as an oxidizing agent usually display some shortcomings like

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hazardous to handle, very expensive, nonꢀselective toward epoxidation and lead to the
formation of many undesirable products. Therefore, direct epoxidation of styrene has been
found to be a promising alternative to this conventional route. Reports are available for the
epoxidation of styrene by environꢀfriendly and safer oxidizing agents such as aqueous H2O2
or urea–H2O2 adduct,66 anhydrous H2O268 or TBHP 69, 70 in presence of solid catalysts such as
cꢀAl2O3,68 Ti–SiO2,69 TSꢀ167 and TiꢀHMS70 yet the catalysts show low activity and selectivity
towards epoxides 67ꢀ70 or becomes deactivated in the presence of reaction water.68
Therefore, keep all that in mind, herein we report the synthesis of Au Nps deposited on
nanosized amine functionalized MILꢀ101(Cr) i.e. Au/NH2ꢀMILꢀ101(Cr) by employing a
solution based approach and explore its catalytic activity for the epoxidation of styrene in
presence of tꢀbutyl hydroperoxide (TBHP). To the best of our knowledge there are no reports
so far wherein NH2ꢀMILꢀ101(Cr) is employed as support for Au NPs in epoxidation of
styrene.
2. Experimental section
2.1Materials and methods
All the chemicals are of analytical grade and used as purchased without further
purification. Cr (NO3)3 .9H2O and 2ꢀaminoterephthalic acid, Gold (III) chloride hydrate,
Sodium borohydride, are purchased from SigmaꢀAldrich, USA. triꢀsodium citrate dihydrate,
tertꢀButyl hydroperoxide (TBHP) are purchased from Spectrochem Pvt. Ltd. Styrene,
C2H5OH, DMF are purchased from Merck, Germany.
The Xꢀray diffraction (XRD) patterns of the samples are performed on Rigaku, Ultima IV
X˗ray diffractometer using Cu˗Kα source (λ = 1.54 Å). Specific surface area, pore volume,
average pore diameter are measured with the Autosorb˗1 (Quantachrome, USA). X˗ray
photoelectron spectroscopy measurements are performed on an Xꢀray photoelectron
spectrometer (SPECS Surface Nano Analysis GmbH, Germany) equipped with an Xꢀray
source XRꢀ50 with a twin Al/Mg anode, a hemispherical electron energy analyzer
PHOIBOSꢀ150.The size and morphology of the nanoparticles are observed by transmission
electron microscopy (TEM) using JEOL (Model JEM˗2011) equipment. Fieldꢀemission
scanning electron microscopy (FE˗SEM, SIGMA) is applied to investigate the size and
morphology of the sample and EDS mapping was done in Oxford XMax 20 equipment. The
basic property of the support is measured through CO2ꢀTPD analysis which is carried out in
an AutoChem II 2920 V4.02 instrument. The Au content of the catalyst is determined through
Atomic
absorption
spectroscopy
(AAS)
measurement
using
PerkinElmer
(Model˗AAnalyst˗700) spectrometer. TGA analysis of the material has been carried out in a

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TA SDT˗Q600 instrument under N2 atmosphere. Gasꢀ Chromatography (GC) analysis of the
reaction is carried out in a gas chromatography (Thermo Scientific, TRACE 1300) equipped
with a flame ionization detector (FID) and a Thermo Scientific (Polyethelene glycol, 30 m ×
0.32 mm × 0.25 ꢁm) capillary column. Isolectric point (IEP) value of the support are
analyzed by carry out zeta potential in a Zetasizer (Model No. nano ZS), MPTꢀ2 instrument
using auto titration method. FT˗IR spectra (4000˗400 cmꢀ1) are recorded on KBr discs in a
Perkin˗Elmer system 2000 FT˗IR spectrophotometer.
2.2 Synthesis of NH2-MIL-101(Cr)
NH2ꢀMILꢀ101(Cr) was synthesized according to the reported procedure by Chen et al. 53
A
mixture of Cr(NO3)3. 9H2O (2 mmol), 2ꢀaminoterephthalic acid (2 mmol) and sodium
hydroxide (5 mmol) were dispersed and then stirring in 15 ml of deꢀionized water for 5
minutes. The resulting solution was heated in a 100 ml Teflon˗lined autoclave at 150°C for
12 h. After cooling to room temperature, the green precipitate was collected by
centrifugation, washed it with DMF to remove the unreacted 2ꢀaminoterephthalic acid and
then further purified by solvothermal treatment in ethanol at 100°C for 24 h. Finally, the
product was dried at 80°C in air.
2.3 Synthesis of Au/NH2-MIL-101(Cr)
The activated NH2˗MIL˗101(Cr) (0.3 g)was dispersed in 50 mL of HAuCl4 (2.5×10ꢀ4M)
aqueous solution at a pH ~ 3 for 10 minutes. The mixture was then stirred in a 100 ml round
bottom flask for another 12 h followed by centrifugation and washed several times with deꢀ
ꢀ
+
ionized water. The resulting AuCl4 NH3 ꢀMILꢀ101(Cr) was then dispersed in 20 mL of deꢀ
ionized water with the addition of triꢀsodium citrate dihydrate (0.0036 g) and NaBH4 (0.0056
g dissolved in 1.5 mL ice cold water) subsequently. After the addition of NaBH4, the solution
was continuously stirred at room temperature for another 6 h to yield Au/NH2ꢀMILꢀ101(Cr).
It was observed that the colour of the resulting solid was changed from light green colour to
dark green indicating the formation of Au Nps. The solid is then separated through
centrifugation, repeatedly washed with deꢀionized water and then dried under vacuum. The
Au content (0.3 wt %) in the synthesized material was calculated from AAS technique.
2.4 Catalytic activity for epoxidation of styrene
A liquid reaction mixture containing styrene (1 mmol), TBHP (1.5 mmol) and toluene (5
mL) was stirred under reflux condition at 110°C for a period of 8 h in presence of Au/NH2ꢀ
MILꢀ101(Cr) (0.025 mol%). After the reaction, the reaction mixture was cooled to room
temperature and the solid catalyst was separated through filtration. The products were
analyzed by a Thermo Scientific, TRACE 1300 gas chromatograph equipped with a Thermo

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Scientific (Polyethelene glycol, 30 m × 0.32 mm × 0.25 ꢁm) capillary column and a flame
ionization detector (FID).
3. Results and discussion
The synthesis of Au Nps on the amine functionalized MILꢀ101(Cr) was carried out using
HAuCl4 as the gold precursor in presence of NaBH4 that acts as a reducing agent. Trisodium
citrate was used as a stabilizer which provides the optimum citrate concentration necessary
for the stabilization of the Nps. In addition, the MOF matrix, acting as a support plays its role
by enhancing the distribution of HAuCl4 over its surfaces. The amine functionality of NH2ꢀ
MILꢀ101(Cr) facilitate the absorption of [AuCl4]ꢀ ions from the solution phase to the support
through an electrostatic interaction between NH3+ and [AuCl4]ꢀ that can be well demonstrated
with UVꢀvis spectroscopy (Fig S1, ESI). The peak of [AuCl4]ꢀ ions at around 307 nm was
completely disappeared as well as its yellow (Fig S2, ESI) colour after the addition of the
support (Fig S1, ESI), conforming the absorption of [AuCl4]ꢀ on to the support. In addition,
we have calculated the isoelectric point (IEP) value of the sample by determining zeta
potential as a function of pH using autotitration method (Fig S3, ESI). The IEP value of the
support was found to be 8.43, that confirms the positive surface charge of NH2ꢀMILꢀ101(Cr)
which facilitate the attraction of negatively charged species of [Au(OH)nCl4ꢀn]ꢀ (n=1ꢀ4) from
solution. Furthermore, the Brønsted basic sites of the support are advantageous for the
formation and stabilization of small gold particles.71 We have measured the basic properties
of NH2ꢀMILꢀ101(Cr) by employing CO2ꢀTPD analysis (Fig S4, ESI). From the analysis the
desorption amount of CO2 is found to be 0.1324 mmol gꢀ1. Therefore both the IEP value of
the support as well as the brønsted basic sites plays a major role in the distribution of gold
nanoparticles on NH2ꢀMILꢀ101(Cr). The successful incorporation of Au Nps after the
reduction of [AuCl4]ꢀ to Au(0) on to the framework of NH2ꢀMILꢀ101(Cr) was verified by
employing different characterization techniques such as PXRD, N2ꢀphysisorption, FEꢀSEM,
HRTEM, XPS, TGA, FTꢀIR analysis.
The PXRD pattern of the synthesized Au/NH2ꢀMILꢀ101(Cr) (Fig 1.(red)) remained nearly
unchanged in comparison to the parent NH2ꢀMILꢀ101(Cr) (Fig 1.(blue)).The expected (111),
(200), (220), (311) and (222) reflexes of face centred cubic (fcc) structure (JCPDS card no.
04ꢀ0784) of metallic72 Au were found in the diffractogram (red colour) in addition to the
other characteristic peaks of the parent MOF. The broad Bragg reflection73 clearly indicate
the typical nanoscale Au/NH2ꢀMILꢀ101(Cr), confirming by the FEꢀSEM analysis (Fig S5,
ESI) where it was observed that the morphology of the support remain unaltered after loading
of the Au Nps on to its matrix. FTꢀIR spectra (Fig S6, ESI) display double peaks at 3463 and

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3397 cmꢀ1 which could be attributed to the asymmetrical and symmetrical stretching of the
amine moieties.74, 75 Two other characteristics bands of amino group were observed at 1624
cmꢀ1 and 1338 cmꢀ1 that could be ascribed as the N–H bending (scissoring) vibration and CꢀN
stretching absorption respectively. It was observed that after incorporation of the Au
nanoparticles (Fig S6 (red), ESI), the chemical functionality of parent MOF remains
unchanged that clearly confirmed the robustness of the support. The EDS (Fig 2 (a)) and
mapping analysis (Fig 2 (b)) showed the distribution of Au Nps along with the different
elements present in the material which was further confirmed by the XPS analysis (Fig S7,
ESI) showing peaks of binding energies at 83.8 eV and 87.4 eV assigned to 4f7/2 and 4f5/2 of
Au (0).The specific surface area and pore volumes of synthesized materials were evaluated
by the N2 physisorption analysis at 77 K (Fig 3) The BET surface area of NH2ꢀMILꢀ101(Cr)
and Au/NH2ꢀMILꢀ101(Cr) were determined to be 1502 m2gꢀ1 and 1055 m2gꢀ1 with pore
volumes 1.674 cm3gꢀ1 and 0.594 cm3gꢀ1 respectively. The BJH plot (Fig S8, ESI) further
confirms the uniform pore distribution of the support after incorporation of Au Nps on it. The
considerable decrease in the surface area and pore volume of Au/NH2ꢀMILꢀ101(Cr) clearly
indicates that the cavities of NH2ꢀMILꢀ101(Cr) were occupied either by the highly dispersed
Au Nps or potential blockage of the pores by the Nps.
The morphological evaluation of Au Nps in the Au/NH2ꢀMILꢀ101(Cr) was investigated
through TEM analysis (Fig 4). From the images it was observed that the synthesized Au NPs
were highly dispersed on the MOF matrix with a narrow size distribution between 2–10 nm
(Fig 4(e)) which may be attributed to the presence of amine functionality attached to the
framework structure that stabilizes the ion pair (NH3)+AuCl4 as well as the effect of trisodium
citrate, acting as stabilizer for the Nps formed upon treatment of NaBH4. HRꢀTEM image of
a single Au Np (Fig 4 (d)) shows lattice fringes with fringe spacing about 0.23 nm further
confirms the fcc (111) structure of Au (0) crystals. Thermal stability of the synthesized
Au/NH2ꢀMILꢀ101(Cr) material (Fig S9, ESI) was examined through thermogravimetric
analysis (TGA). Two major weight loss step has been obtained, the first step in the range 30ꢀ
100° C (calculated loss 8%) is due to the loss of adsorbed water molecules and the second
weight loss step in the range 300ꢀ600° C (Calculated loss 34% ) may ascribed to the
framework decomposition as shown by the corresponding derivative curve.
The catalytic activity of Au/NH2ꢀMILꢀ101(Cr) was tested for the epoxidation of styrene in
presence of TBHP (Schemeꢀ1). The effect of various key parameters like catalyst dosage,
temperature, solvent was also examined in order to get the optimized reaction condition. The
effect of different mol% of the catalyst was summarized in Table 1. With neat NH2ꢀMILꢀ

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101(Cr), the reaction was not proceeded, ensuring the catalytic role of Au Nps on the MOF
support to carry out the chemical transformation. With increase in the mol% of the catalyst,
both the conversion of the reaction and selectivity to styrene oxide was increased and attained
maximum conversion (88.2%) and selectivity (74.7%) (Table 1, entry 3) with 0.025 mol% of
the catalyst which could be attributed to the increase in the number of catalytically active
centres with increase in catalyst dosage. However, beyond 0.025 mol%, conversion of the
reaction decreased abruptly (44.1%) with slightly increase in the selectivity (78.2%) (Table 1,
entry 4). Hence, 0.025 mol% was taken as the optimized catalyst dosage for this epoxidation
reaction and all further studies were performed using this amount of catalyst.
The effect of different solvent system in the styrene epoxidation was examined by
employing solvents having different polarities (Table 2).The highest conversion was obtained
in DMF (96.7%) (Table 2, entry 2) with a very low styrene oxide selectivity (5.6%) (Table 2,
entry 2). In case of acetone, although a highest selectivity towards styrene oxide was obtained
(85.3%) (Table 2, entry 3), yet the conversion of the reaction was found to be low (7.6%)
(Table 2, entry 3). A moderate reaction conversion and styrene oxide selectivity was
observed in ethanol, acetonitrile and THF solvent system (Table 2, entry 5, 1, 7) while DCM
displayed lowest reaction conversion (5.1%) (Table 2, entry 4) with medium styrene oxide
selectivity (58.3%) (Table 2, entry 4). In comparison to other solvents, toluene showed better
reaction conversion (88.2%) (Table 2, entry 6) as well as selectivity for styrene oxide
(74.7%) (Table 2, entry 6). Under solvent free condition (Table 2, entry 8) no formation of
styrene oxide was obtained although the conversion of the reaction was around 97.8% with
95.5 % selectivity towards the side products. Hence, toluene was found to be the most
suitable solvent for this reaction and all further studies were performed in toluene.
The effect of different oxidizing agents like H2O2, air and TBHP was also explored for the
epoxidation of styrene using our catalytic system (Table 2). With H2O2 the reaction was not
proceeded while with air the reaction conversion was found to be 7.2% with 100% styrene
oxide selectivity. Using TBHP as oxidizing agent, maximum conversion of 88.2% was
observed with 74.7% selectivity towards styrene oxide and therefore, TBHP was found to be
an appropriate oxidizing agent for the mentioned chemical reaction.
The reaction temperature had a pronounced effect on styrene epoxidation as listed in Table
3. At room temperature the reaction was not proceeded as indicated by reaction conversion
<1% (Table 3, entry 1). Increase in reaction temperature resulted in the increase in conversion
(Table 3, entry 2, 3, 4, 5) and slightly increased in selectivity at high temperature (Table 3,
entry 3, 4, 5). Temperature above 110°C was found to be detrimental for the reaction (Table

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3, entry 6) as the conversion of the styrene was decreased with slight increase in styrene
oxide selectivity, perhaps due to the instability of the BuO./.OH radical species at high
temperature. At 110°C the reaction conversion and selectivity to styrene oxide were
maximum (Table 3, entry 5), establishing it as the optimum reaction temperature for the
chemical transformation.
The effect of reaction time in the epoxidation of styrene was also studied and the results
are summarized in Table S1, ESI. After 4h of the reaction, the styrene conversion was found
to be 55.9% with 84.4% selectivity to styrene oxide (Table S1, entry 1, ESI). Extension of
reaction time from 4h to 8h led to a significant increase in reaction conversion with slight
decrease in selectivity (Table S1, entry 2, ESI). However, further increase in reaction time
from 8h to 12h (Table S1, entry 3, ESI) resulted in the increase of conversion with a very low
selectivity towards styrene oxide may be attributed to the sequential oxidation of styrene
oxide to benzaldehyde and/or phenyl acetaldehyde as the reaction proceeds.61
The influence of the amount of TBHP in the styrene epoxidation was observed and the
results were summarized in Table 4. Without TBHP, the reaction was not move forward with
a very low conversion of 1.1%. With the increase of amount of TBHP (Table 4, entry 1, 2,
3), the conversion increased continually from 50.9% (Table 4, entry 1) to 88.2 % (Table 4,
entry 3) whereas the selectivity to styrene oxide decreased from 87.9% (Table 4, entry 1) to
74.7% (Table 4, entry 3). However, further increased in the TBHP amount (Table 4, entry 4)
resulted low conversion and selectivity. All these observations clearly confirmed the requisite
TBHP amount, i.e 1.5 mmol for carried out the epoxidation reaction.
In order to assess the efficiency of the present catalytic system, different Au based
catalytic system was screened based on the reactivity and selectivity to epoxides (Table S2,
ESI) under similar reaction conditions. The present Au/NH2ꢀMILꢀ101(Cr) exhibited excellent
catalytic activity as compared to the previously reported catalysts available in the literature.
The key advantages of our protocol are low catalyst dosage, facile recovery and reusability,
High TOF etc. Furthermore, the high surface area of the parent MOF, it provides not only a
better dispersion of the Au Nps but also controls its size which seemed to be a critical role in
enhancing its catalytic activity.
The heterogeneous nature of catalyst was evaluated by removing the catalyst from the
reaction medium after 4h of the reaction and then allowed the reaction to run for another 4h.
No further increase in the styrene conversion was observed after the removal of the catalyst.
The atomic absorption spectroscopic (AAS) analysis revealed the absence of the Au in the
supernatant solution of the reaction mixture, confirming that the Au Nps were not leached out

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from the NH2ꢀMILꢀ101(Cr) matrix during the reaction condition, establishing the true
heterogeneous nature of the catalyst.
A plausible reaction mechanism has been proposed for the epoxidation reaction catalysed
by Au/NH2ꢀMILꢀ101(Cr) in presence of TBHP. In fact the reaction was proceeded through
the formation of a hydroperoxy species ((I) Scheme 2) which undergo dehydration and
rearrangement to form peroxyformate intermediate ((II) Scheme 2). Styrene is then activated
on the surface of Au Nps via its nucleophilic C=C group and further reacts with OꢀO group of
peroxyformate intermediate through a sideꢀbyꢀside interaction to yield species (III) (Scheme
2). The final product of the reaction, i.e. styrene oxide is formed through a metalloepoxy
intermediate by the transfer of oxygen to the olefinic bond of styrene. The formation of
benzaldehyde may be attributed to the breaking of the CꢀC bond of the species (III) whereas
phenylacetaldehyde is formed through isomerisation of styrene oxide.76
The reusability of the catalyst is an important feature of heterogeneous catalyst that makes
it economically preferable over homogeneous catalyst. After completion of the reaction the
catalyst was separated through filtration, washed it with acetone for several times and then
dried in an oven at 70°C for 5h. The catalyst was then further employed for the next catalytic
runs under identical reaction condition. It was observed that the catalyst showed effective
catalytic performance upto fourth catalytic cycles without any significant loss of its catalytic
activity (Fig 5) which was in good agreement with the results of PXRD, FEꢀSEM, TEM, FTꢀ
IR analysis of the recovered catalyst. PXRD analysis (Fig 1(black)) of the recovered catalyst
displayed the conservation of its crystallinity to that of the fresh one. The nanoscale
morphology of the support i,e. NH2ꢀMILꢀ101(Cr) was well maintained in the recovered
catalyst as indicated by the FEꢀSEM analysis (Fig S5 (c), ESI). The EDSꢀmapping analysis of
the recovered catalyst (Fig S10, ESI) further clarified the presence of Au Nps on the support
after the catalytic runs. The dispersion of the Au Nps in the recovered catalyst (Fig S11, ESI)
was well preserved during the chemical transformation as demonstrated by TEM analysis.
The BET surface area of the recovered catalyst was found to be 947 m2gꢀ1 (Fig 3,(blue))
which was slightly lower than that of the fresh catalyst may be owing to the blockage of
pores of the support by the reactant molecules. From the FTꢀIR analysis (Fig S6, (blue), ESI)
it was observed that the chemical functionality of parent MOF remained unchanged after the
catalytic cycles, confirming the robustness of the support. Finally all the results clearly
signified the true heterogeneous nature of the synthesized Au/NH2ꢀMILꢀ101(Cr) material as
well as the robustness of the host matrix.

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4. Conclusion
The work demonstrated the Au NPs embedded on an amineꢀfunctionalized nanoscale
metalꢀorganic framework by a facile solution approach. The resultant Au/NH2ꢀMILꢀ101(Cr)
exhibited excellent catalytic activity for the epoxidation of styrene in the presence of aqueous
TBHP. Low catalyst dose, high turnover frequency (TOF) and reusability make this method
superior to some of the reported catalytic methods available in the literature. The better
catalytic activity may be owing to the high surface area of the parent MOF with high
isolectric point value (IEP) and the presence of brønsted basic sites together provides a better
dispersion of the catalytically active Au nanoparticles that results a better catalytic activity of
the catalyst.. Furthermore, the catalyst is easily separable and recyclable up to forth catalytic
cycles without much change in its catalytic activity and selectivity. The present work also
demonstrated that the chemical functionalization can significantly improves the catalytic
activity of MOF materials which can be extended to other metal NPs for implementing
important organic transformations.
Acknowledgement
The authors are grateful to the Director of CSIRꢀNorth East Institute of Science and
Technology, Jorhat, Assam, India for his kind permission to publish the work. We are
thankful to Dr P. Sengupta, Head of Materials Sciences and Technology Division for support,
and non network project MLPꢀ6000 (WPꢀ1) and network project (CSCꢀ0408) project for
financial assistance. M. S. acknowledges DST New Delhi for an INSPIRE fellowship.
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Captions
1. Fig 1.PXRD pattern of NH2ꢀMILꢀ101(Cr) (blue), Fresh Au/NH2ꢀMILꢀ101(Cr) (red) and
recovered Au/NH2ꢀMILꢀ101(Cr) (black)
2. Fig 2. a) EDS analysis of Au/NH2ꢀMILꢀ101(Cr), b) mapping of different elements of
Au/NH2ꢀMILꢀ101(Cr)
3. Fig 3. N2 adsorption isotherm of NH2˗MIL˗101(Cr) (red) and Au/NH2˗MIL˗101(Cr)
(black)
4. Fig 4. (aꢀc) TEM images of Au/NH2ꢀMILꢀ101(Cr) (aꢀc), (d) HRTEM image of a Au (0)
NP showing the lattice fringes and (e) size distribution of Au Nps
5. Fig 5. Catalyst recycling test for Au/NH2ꢀMILꢀ101(Cr)
6. Table 1. Effect of catalyst dosage
7. Table 2. Effect of solvent
8. Table 3. Effect of temperature
9. Table 4: Effect of amount of TBHP
10. Scheme 1. Epoxidation of styrene using Au/NH2ꢀMILꢀ101(Cr) as catalyst.
11. Scheme 2. Plausible reaction mechanism for the epoxidation of styrene in the presence of
Au/NH2ꢀMILꢀ101(Cr)

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20
40
60
80
2 Theta (degree)
Fig 1 PXRD pattern of NH2ꢀMILꢀ101(Cr)(blue), Fresh Au/NH2ꢀMILꢀ101(Cr)(red) and
recovered Au/NH2ꢀMILꢀ101(Cr) (black)

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a)
b)
Au
C
N
O
Cr
Fig 2 a) EDS analysis of Au/NH2ꢀMILꢀ101(Cr), b) mapping of different elements of Au/NH2ꢀ
MILꢀ101(Cr)

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1200
1000
800
600
400
200
0
(a)
(b)
(c)
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
Fig 3 N2 adsorption isotherm of NH2˗MIL˗101(Cr) (red) and Au/NH2˗MIL˗101(Cr) (black) and
recovered Au/NH2˗MIL˗101(Cr) (blue)

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(b)
(a)
(c)
(d)
30
25
20
15
10
5
(e)
0
0
2
4
6
8
10
Diameter (nm)
Fig 4 (aꢀc) TEM images of Au/NH2ꢀMILꢀ101(Cr) (aꢀc), (d) HRTEM image of a Au (0) NP
showing the lattice fringes and (e) size distribution of Au Nps

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Conversion (%)
Selectivity (%)
100
80
60
40
20
0
Fresh
Cycle 1
Cycle 2
Cycle 3
Cycle 4
No. of Cycle
Fig 5 Catalyst recycling test for Au/NH2ꢀMILꢀ101(Cr)
Table 1 Effect of catalyst dosagea
Entry
Catalyst
Dosage
(mol%)
0.008
0.016
0.025
Conversion
(%)
Selectivity (%)
Styrene oxide Benzaldehyde Phenylacetaldehyde
Other
products
19.8
24.4
24.9
1
2
3
4
59.7
84.0
88.2
44.1
35.5
40.3
74.7
78.2
44.6
33.0
0.0
0.0
2.1
0.4
0.6
0.038
0.0
21.2
aReaction condition: Styrene (1 mmol), TBHP ( 1.5 mmol), toluene (5 mL), time (8 h), Temperature
(110° C), GC conversion and selectivity.

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Table 2 Effect of solventsa
Entryb
Solvents
Conversion
(%)
Selectivity (%)
Styrene oxide Benzaldehyde Phenylacetaldehyde
Other
products
10.1
87.8
0.0
0.2
53.8
24.9
24.0
1
2
3
4
5
6
7
8
Acetonitrile
DMF
Acetone
DCM
Ethanol
Toluene
THF
29.4
96.7
7.6
78.5
5.6
0.08
6.5
14.6
0.0
0.0
0.0
5.7
4.4
11.2
0.0
0.0
41.3
3.4
0.4
85.3
58.3
42.7
74.7
70.2
0.0
5.1
47.1
88.2
14.5
97.8
0.0
0.0
Solvent free
95.5
aReaction condition: Styrene (1 mmol), TBHP ( 1.5 mmol), solvent (5 mL), time (8 h), Au/NH2ꢀMILꢀ
101(Cr) (0.025 mol%), Reflux condition, GC conversion and selectivity
b H2O2 = No reaction, bAir = Conversion (7.2%), Selectivity (100%), bTBHP = Conversion (88.2%),
Selectivity (74.7%)
Table 3 Effect of temperaturea
Entry Temperature Conversion
Selectivity (%)
(° C)
(%)
Styrene oxide Benzaldehyde Phenylacetaldehyde
Other
products
81.6
31.6
16.1
25.0
24.9
23.3
1
2
3
4
5
6
RT
50
70
90
110
120
<1
1.3
30.9
53.3
88.2
66.5
18.3
32.4
73.8
73.6
74.7
76.6
0.0
34.5
0.0
0.0
0.0
0.0
1.2
10.0
1.2
0.4
0.0
0.0
aReaction condition: Styrene (1 mmol), TBHP ( 1.5 mmol), toluene (5 mL), time (8 h), Au/NH2ꢀMILꢀ
101(Cr) (0.025 mol%), GC conversion and selectivity
Table 4 Effect of amount of TBHPa
Entry Amount of Conversionb
Selectivity (%)
Styrene oxide Benzaldehyde Phenylacetaldehyde
TBHPc
(%)
Other
products
11.06
14.5
24.9
22.1
(mmol)
0.5
1
1.5
3
1
2
3
4
50.9
61.2
88.2
48.4
87.9
84.0
74.7
18.2
0.0
0.0
0.0
0.97
1.3
0.35
10.0
49.5
aReaction condition: Styrene (1 mmol), Au/NH2ꢀMILꢀ101(Cr) (0.025 mol%), toluene (5 mL), time (8 h),
Temperature (110° C), bGC conversion and selectivity
c Without TBHP, conversion= 1.1%

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Scheme 1 Epoxidation of styrene using Au/NH2ꢀMILꢀ101(Cr) as catalyst
Scheme 2 Plausible reaction mechanism for the epoxidation of styrene in the presence of
Au/NH2ꢀMILꢀ101(Cr)

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Gold nanoparticles supported on nanoscale amine-functionalized MIL-101(Cr) as a
highly active catalyst for epoxidation of styrene
Gold nanoparticles can effectively be incorporated on to the matrix of an
amine˗functionalized MIL˗101(Cr) by a facile solution approach. The resulting
nanocatalyst, Au/NH2˗MIL˗101(Cr) can act an as efficient heterogeneous catalyst for
epoxidation of styrene in presence of tertꢀButyl hydroperoxide
.
O
TBHP
Au/NH2-MIL-101(Cr)