The impact of the copper source on the synthesis of meso-structured CuTUD-1: A promising catalyst for phenol hydroxylation

Article abstract of DOI:10.1039/c3cy00432e

In the direct hydrothermal synthesis of copper containing mesoporous materials, various factors such as the Cu(ii) ion distribution, the coordination environment and the quantity of Cu²⁺ incorporation/grafting etc. influence catalytic activity. For these reasons, the copper source added during the synthesis plays a crucial role. In this work, three different CuTUD-1 materials with similar Si/Cu ratios (100) were synthesized with three different copper precursors (copper acetate (CuTA), copper chloride (CuTC) and copper nitrate (CuTN)). The physico-chemical properties of these CuTUD-1 materials were examined by XRD, N₂ sorption studies, ICP-OES, Diffuse Reflectance UV-Vis, FT Raman spectroscopy, TPR and XPS characterization techniques. The results revealed that the distribution of isolated Cu²⁺ species and oligonuclear CuO nanoclusters varied with different Cu sources. Copper nitrate yielded highly distributed isolated Cu²⁺ sites, whereas oligonuclear CuO was observed as the major species when copper acetate was used as the Cu source. These materials were tested as catalysts for liquid phase hydroxylation of phenol with H₂O₂. The significantly higher performance of the CuTN catalyst over that of the CuTC and CuTA catalysts was attributed to the larger amounts of active isolated Cu(ii) species connected to the TUD-1 silica lattice. Moreover, the CuTUD-1 materials showed better catalytic activity than other copper based mesoporous systems (MCM-41, MCM-48 and SBA-15). The Royal Society of Chemistry.

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Catalysis Science & Technology
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Journal Name
Cite this: DOI: 10.1039/c0xx00000x
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ARTICLE TYPE
The impact of Copper source on the synthesis of meso-structured
CuTUD-1: A promising catalyst for phenol hydroxylation
a
a
a, b
Muthusamy Poomalai Pachamuthu, Vinju Vasudevan Srinivasan, Rajamanickam Maheswari,
a
b
Kannan Shanthi * and Anand Ramanathan *
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
In a direct hydrothermal synthesis of copper containing mesoporous materials, various factors such as
2
+
Cu(II) ion distribution, its coordination environment and quantity of Cu incorporation/grafting etc.
influence their catalytic activity. For such reasons, the copper source added during the synthesis plays a
crucial role. In this work, three different CuTUD-1 with similar Si/Cu ratio (100) were synthesized with
three different copper precursors (copper acetate (CuTA), copper chloride (CuTC) and copper nitrate
1
1
2
0
5
0
(CuTN)). The physico-chemical properties of these CuTUD-1 materials were examined by XRD, N2
sorption studies, ICP-OES, Diffuse Reflectance UV-Vis, FT Raman, TPR and XPS characterization
2
+
techniques. The results revealed that the distribution of isolated Cu species and oligonuclear CuO
2
+
nanoclusters were varied with Cu source. Copper nitrate yielded highly distributed isolated Cu sites
where as oligonuclear CuO was observed as major species when copper acetate was used as Cu source.
These materials were tested as catalyst for liquid phase hydroxylation of phenol with H O . Significant
2
2
higher performance of CuTN catalyst over CuTC and CuTA catalysts was attributed to higher amounts of
active isolated Cu(II) species connected to TUD-1 silica lattice. Moreover, CuTUD-1 materials showed
better catalytic activity than the other copper based mesoporous systems (MCM-41, MCM-48 and SBA-
1
5).
Introduction
of these accessible sites might significantly enhance the activity
the catalysts. Besides, the active metal ions leaching must be
In the last two decades silica based mesoporous materials have
1
2
been paid more attention due to its important role in various
fields. Since the discovery of MCM-41, many metal ions 50 metal ions such as Cu are difficult to incorporate into the
prevented from the support. Though the high content of divalent
1
2+
2
3
3
4
4
5
0
5
0
5
containing mesoporous materials have been synthesized and
mesoporous silica, many research groups have devoted to pack
2
-5
2+
utilized for various applications particularly in catalysis.
the Cu sites as complex or direct incorporation into silica
x+
13-18
Generally, the active metal or non-metal cations (M ) have been
introduced into the SiO2 framework to make them catalytic
framework.
It has been well known that one of the important
2
+
factor that determines the dispersion of Cu active site which are
active. However these direct synthesis methods are limited only 55 in turn influenced in the direct synthesis method by structural
x+
for loading small amounts of M ion modifications (typically
Si/M ratio of about 30). A higher loading of metal ion
modifications leads to structural collapse or formation of higher
nature of copper precursors, coordinative environment, rate of
hydrolysis and solubility. Hence, the selection of copper source is
of prime concern in the copper based mesoporous material
synthesis.
amount of crystalline metal oxides (MO ) in ordered mesoporous
x
6
materials. In the series of mesoporous materials family, recently
60
Phenol hydroxylation by hydrogen peroxide (H O ) is a
2
2
added TUD-1 material have been studied as excellent support for
typical environmental friendly catalytic reaction. The obtained
products catechol and hydroquinone (diphenols) have been
widely used as antioxidants, photographic chemicals,
polymerization inhibitors, pesticides and flavoring agents.
7
various catalytically active species. Though TUD-1 possessed
disordered pores, advantages such as easy synthesis,
interconnected narrow pore channels, tunable pore size,
hydrothermal stability etc., have been succeeding its applications 65 Though various types of catalysts have been studied for this
7
as a heterogeneous catalyst.
reaction, copper based catalyst have been fascinating due its
1
9-22
Copper salts or copper complexes have been employed as an
active catalyst for oxidative transformation of organic
precise OH radical activating and phenol coordinating nature.
2
3,
Very recently copper containing mesoporous molecular sieves,
biopolymer based material (Cu (II) alginate), metal-organic
8
24
25
compounds. Heterogenizing such active catalytic sites by various
9
-11
26
methods have been explored.
Since the active sites are the 70 framework were shown to exhibit good catalytic activity for this
main factor which determines the catalyst activity, a large number
reaction. Zhang et al. reported that in mesoporous Cu/MCM-41
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the Cu species and phenol hydroxylation activities are varied with
the preparation methods.
In our previous report we have synthesized CuTUD-1 with
different Si/Cu ratios (100 to 10) by using triethanolamine (TEA)
spectrometer using BaSO as reference in the range of 200 – 800
4
2
2
nm. Raman spectroscopic measurements were carried out in the
-
1
range of 200 to 800 cm using a 1064 nm excitation source on
Bruker FT-Raman System 1000R. The X-ray photoelectron
5
as a bi-functional organic molecule (mesopore structure directing 60 spectroscopy (XPS) spectra were carried out by using an
2
7
and complexing of metal ions). It is well known that the Cu (II)
Omicron ESCA probe spectrometer with Al Kα X-rays (hv =
1486.6 eV) radiation. Electron spin resonance (ESR) spectra were
recorded on a JEOL (JES FA-300) spectrometer at room
temperature operating in the X-band region. Temperature
2
8
triethanolamine complexes are stable and highly active. In this
work, we have investigated the effect of three different copper
sources namely, copper nitrate, copper chloride and copper
1
0
acetate in the synthesis of CuTUD-1 (Si/Cu = 100). Thus 65 programmed reduction (TPR) measurements were carried out in a
prepared materials are characterized by XRD, N2 sorption
studies, ICP-OES, DRS UV-Vis, EPR, FT Raman, SEM HRTEM
Quantachrome ChemBET TPR/TPD chemisorptions analyzer.
For the sample analysis 5 vol % H in Ar gas mixture was used.
2
and H -TPR studies. The catalysts activity comparisons and
reaction condition optimizations are also carried out in detail.
High resolution transmission electron microscopy (HRTEM) was
carried out on a JEOL 3010 instrument with a UHR pole piece
operated at an accelerating voltage of 300 kV. The sample
morphologies were examined using SEM-EDAX imaging FEI
Quanta FEG 200 high resolution scanning electron microscope.
The elemental composition of the CuTUD-1 catalysts before and
after phenol hydroxylation was analyzed using ICP-OES Perkin
2
70
1
5
Materials and methods
Materials
The chemicals used in this study include tetraethyl orthosilicate
(
TEOS, 99%, Sigma Aldrich), Copper nitrate (Cu(NO ) .3H O, 75 Elmer Optima 5300 DV elemental analysis instrument.
3 2 2
Merck), Copper chloride (CuCl .2H O, Sd fine), Copper acetate
(Cu(CH COO) .H O, SRL), Triethanolamine (TEA, SRL),
3 2 2
2
2
CuTUD-1 catalysed Phenol hydroxylation
2
0
Tetraethyl ammonium hydroxide (TEAOH, Sigma Aldrich,
5%), Cetyltrimethyl ammonium bromide (CTAB, Merck),
Pluronics P123 (MW:5800, Aldrich), Hydrochloric acid (HCl,
The hydroxylation of phenol over copper containing catalyst was
carried out in a two neck round-bottom flask equipped with water
condenser, septum cap and magnetic stirrer. Prior to the runs, the
3
SRL), Fumed silica (SiO , Sigma Aldrich), Sodium hydroxide 80 catalysts were activated (pretreated) for 2 h in air at 300 °C. In a
2
2
5
(NaOH, Merck).
typical run, 50 mg of CuTUD-1 catalyst was taken in the flask.
Then, 5 mmol of phenol in 10 mL of water were added to the
flask. The reaction mixture was equilibrated to desired
temperature in an oil bath. Finally, H O oxidant (5 mmol) was
Catalyst synthesis
CuTUD-1 with Si/Cu ratio of 100 was prepared from three
2
2
different copper sources by sol gel method according to the 85 added drop wise to the reaction mixture with continuous stirring
2
7
procedure with a composition of 1SiO : 0.01 CuO: 0.5
(~750 RPM) which is considered as the start of the reaction. 0.5
ml of liquid samples were then withdrawn at different time
intervals and the products were identified and quantified by a
Shimadzu high performance liquid chromatography (HPLC)
2
3
3
4
4
0
5
0
5
TEAOH: 1 TEA: 11 H O. In a typical synthesis, 10 g of TEOS
and 2 g of deionised water were mixed and stirred for 30 minutes.
Calculated amounts of copper precursor (copper nitrate or copper
2
chloride or copper acetate) dissolved in 1 g water added to the 90 equipped with an UV detector (254 nm) and using a C18 column
above stirring mixture. Then 7 g of TEA was added and stirred
for 1 h. Finally, 10 g of TEAOH was added drop wise to the
above mixture and continued stirring for another 3h. The
obtained gel was aged at 30 °C and dried at 98 °C for 24 h each.
(Phenomenex Luna 5μ C18(2) 100Å). The HPLC grade
acetonitrile and water (1:1) mixture were used as a mobile phase
(flow rate of 1ml/min, 0.5 μl of sample injection at 40 °C
temperature). By using standard compounds the reactant and
The wet solid kept 180 °C in Teflon coated autoclave for 8 h. 95 products were identified and quantified (see supplementary
The organic moieties are removed by the calcination at 600 °C
for 10 h. The final grey colored CuTUD-1 solid samples are
hereafter mentioned as CuTN (Nitrate source), CuTC (Chloride
source) and CuTA (Acetate source) respectively. Other Si/Cu
ratios of 50, 25 and 10 also prepared using copper nitrate for
comparative studies.
information). The conversion of H O (%) was calculated by
2 2
iodometry titration method.
Results and discussion
Mechanism of different copper species formation
Cu-MCM-41 and Cu-MCM-48 with Si/Cu ratio 50 were also 100 During the synthesis of CuTUD-1 catalyst, the formation and
synthesized as per literature procedure using copper nitrate as a
distribution of Cu species (isolated/grafted and oxide) followed
an entirely different mechanism compared to other Cu-containing
ordered mesoporous silicates. The general synthesis of metal
containing TUD-1 utilizes triethanolamine (TEA) as a mesopore
05 structure directing agent as well as a chelating agent for both the
silicon and the metal ion species. Considerably, this ‘atrane route’
2
9, 30
copper source.
In addition, 2 wt % of Cu/SBA-15 and
Cu/SiO2 were prepared by incipient wetness impregnation
method.
1
5
0
Catalyst characterization
n+
X-ray powder diffraction was performed on a Bruker D8 using
Cu Kα radiation (λ = 1.54 Å) operating at 30 kV and 15 mV. N2-
physisorption isotherms and specific surface areas were measured
yielded a significantly higher amounts of active metal ion (M )
7
species in the silicate network. In the synthesis gel of CuTUD-1,
2
+
31
triethanolamine chelates with copper ions (Cu tea , log β = 4.3)
on a Quantachrome porosimeter (Quantosorb SI). Diffuse 110 that are then interacted with silatrane molecules (Scheme 1).
5
5
reflectance (UV Vis) spectra were recorded using Bruker Tensor
These formed complexes are very strong enough to incorporate
2
|
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2
+
Cu ions into the silica matrix during condensation/thermal
treatment (See Figure S1). Besides, varies types of Cu
3
1
2+
2+
complexes
Cu tea (OH)
2
such as Cutea , Cu(OH) (H O)tea
and
2
2
2
+
accounts for the formation of isolated Cu and
2
2
5
CuO species after hydrothermal treatment and calcination
process. Since TUD-1 synthesis was carried out in basic
conditions the formation of binuclear complex can also be
3
2
expected. Additionally, it is believed that mononuclear Cu-
complex (scheme 1 a) are the sources of isolated Cu species
whereas unbound surface Cu-complex (scheme 1 b) and
binuclear complexes (scheme 1 c) mainly leads to crystalline
CuO species formations.
1
0
Figure 1. (A) Low angle and (B) wide angle XRD patterns of CuTA,
Silatrane
40
CuTC and CuTN samples.
O
N
Cu-tea
Si
The N physisorption isotherms of CuTA, CuTC and CuTN
2
O
O
showed a Type IV isotherm with H hysteresis (Figure 2a), which
OH
O
2
confirms the disordered and interconnected mesoporous nature of
a
(
)
N
O
Si
45 the material. The physical characteristics derived from nitrogen
sorption such as BET surface area, BJH pore size and pore
volume are tabulated in Table1. The CuTUD-1 samples
Cu
O
O
OH
O
2
-1
Si-tea,
possessed surface area in the range of 431–589 m g with a pore
3
-1
SiO
volume in the range of 0.71–0.79 cm g . However the pore
50 diameter distribution (Figure 2b) determined by using BJH
method is quite narrow. The CuTA samples showed the higher
pore size with broader distributions than the CuTC and CuTN.
The ICP-OES analysis (Table1) of CuTUD-1 samples revealed
that Si/Cu atomic ratio is nearly the same as that in the synthesis
2
b
(
c
)
( )
H O
2
N
O
O
OH
O
H
O
Si-tea,
O
Cu
Cu
OH
S
iO
2
N
Cu OH
O
N
H O
5
5
gel implying that all copper ions were incorporated in the final
solid. Additionally, disordered worm-hole like arrangement of
mesopores was further evidenced from HR-TEM image for
CuTN sample (Supplementary information S2). Further, the SEM
images of the CuTUD-1 samples (Supplementary information S3)
displayed uneven size and shape of individual silica particles and
the corresponding Energy Dispersive X-ray analysis (EDAX)
revealed the atomic percentage of Cu to be in the range of 1-1.2
%.
OH
O
Si-tea,
SiO
2
Scheme 1 Some possible Cu-complexes with silatrane formed in
Cu/TUD-1 synthesis, a) Cutea complex connected with silatrane layer, b)
individual Cu complex on the surface; c) binuclear Cu-complex
60
1
5
Catalyst Characterization
The low angle XRD patterns of CuTA, CuTC and CuTN
materials are shown in Figure 1A. The CuT materials displayed a
low intense and broad peak in the low angle which confirms the
meso-structured and disordered porous nature of these
materials. The intensity of this peak varied with different copper
source suggesting that the disordered regular pore channel arrays
were further influenced by the added source. In addition, all the
three CuTUD-1 samples, showed a broad peak between 2θ value
of 10 and 30° in the wide angle XRD due to the amorphous
nature of mesoporous silica Figure 1B. The presence of
crystalline CuO in all CuT materials even at very low Cu loading
2
2
3
3
0
5
0
5
2
7
(Si/Cu = 100) was evidenced from a low intense reflections at
35.5 and 38.6° (002 and 111 respectively) suggesting the
2
+
presence of either incorporated/grafted Cu ions into TUD-1
silica matrix or CuO species inside the pore channels. Due to low
signal to noise ratio, the average crystallite size of CuO was
estimated (using the Debye-Scherer method) after smoothening
the signals. For CuTA sample, the average CuO crystal size was
estimated to be ~6 nm whereas for CuTN and CuTC it was
calculated to be ~2 - 4 nm.
65
2
Figure 2. (A) N adsorption–desorption isotherms and (B) BJH pore size
distributions of CuTA, CuTC and CuTN samples.
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Table 1. Physical properties of CuTA, CuTC and CuTN catalyst
a
b
c
d
Catalyst
Si/Cu (molar ratio)
S
BET
V
p, BJH
d
P, BJH
Synthesis
Calcined
gel
a
m²/g
cc/g
Å
CuTA
CuTC
CuTN
100
100
100
92
112
104
431
589
531
0.79
0.71
0.73
81
41
48
a
b
c
Elemental analysis by ICP-OES; SBET = specific surface area. VP,BJH
=
d
total pore volume at P/Po = 0.98; dP,BJH= BJH adsorption pore diameter.
Three absorption peaks at 235, 310 and 350-800 nm were
noticed in the diffuse reflectance UV–Vis spectra of the CuT
samples in the 200–800 nm region (Figure 3). The absorption
5
0
5
0
2
-
2+
peaks at ca. 235 and 310 nm are assigned to O →Cu ligand to
metal charge transfer (LMCT) and oligomers or aggregated CuO
2
7, 33
clusters respectively.
In addition, very broad peak at above
25
1
1
2
400 nm is assigned to pseudo-octohedral ligand oxygen
Figure 4. FT Raman spectra of CuTA, CuTC and CuTN samples.
1
7, 18, 23, 24, 33
environment of copper.
Among the three samples,
CuTA showed an intense absorption peak at 310 nm, which
The FT Raman spectra of the CuTUD-1 samples are shown in
1
8
-1
indicate of presence of oligonuclear CuO clusters and also an
Figure 4. The Raman peaks at 296, 345 and 620 cm are due to
2
+
34
indirect evidence for lesser incorporation of Cu into the silica
A
g
and B
g
Raman active modes of CuO. Even at this low Cu
matrix. Furthermore, for all the CuTUD-1 samples a relative 30 loading (Si/Cu = 100), all samples displayed CuO Raman active
percentage of Cu species were estimated from the respective
absorption peak areas. From these results, the amounts of isolated
Cu2+ speices were estimated to be approximately 65 %, 42 %
and 30 % respectively for CuTN, CuTC and CuTA samples.
Notably, ~36% of oligonuclear and or aggregated Cu species 35 species which are not incorporated in the silica matrix might be
were found in the CuTA sample.
modes. This observation is in consistent with wide angle XRD
and DRS UV-Vis analysis. CuTA showed this peak with high
intensity compared to the other two CuT samples evidencing
higher population of nanocrystalline CuO suggesting that Cu
δ+
δ-
δ+
forming linear oligonuclear [Cu ···O ···Cu ]
clusters
n
(extraframework species). In particular an unbounded copper
which readily converts to oxides are in a higher proportion when
acetate source was used. Wang et al. has studied the influence of
different copper sources, anion and cation on CuO dispersion and
mesoporosity of SBA-15 and concluded that CuO dispersion
4
4
5
5
0
5
0
5
-
-
-
favored the following anionic order: NO <Cl < CH COO
3
3
3
5
(lyotropic series).
The ESR technique was employed to study the environment of
copper ion coordination and the room temperature ESR spectrum
of CuTA, CuTC and CuTN samples are presented in Figure 5.
The signal intensity increased in the following order:
CuTA<CuTC<CuTN. The principal g values calculated using the
formula, g = hν/Bβ are consistent with the reported distorted
3
6
octahedral environment of Cu(II) ion. Bulk CuO is ESR silent,
whereas both dispersed CuO and isolated Cu(II) sites are ESR
1
5
active and the signal intensities were affected by the presence of
1
3
Figure 3. Diffuse reflectance UV-Vis spectra of CuTA, CuTC and CuTN
these neighboring species. The lower signal intensity observed
for CuTA sample indicate the presence of a higher amount of
agglomerated CuO species. On the other hand, a highly dispersed
CuO or Cu(II) sites have been shown to be present in CuTN
sample.
samples.
4
|
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Catalysis Science & Technology
present in CuTN followed by CuTC and CuTA, whereas a higher
concentration of oligonuclear and/or bulk CuO is noticed in
CuTA followed by CuTC and CuTN.
35
Figure 7. (A) XPS Survey spectra and (B) Cu 2p core level binding
energy spectra of CuTN sample.
Figure 5. ESR spectra of CuTA, CuTC and CuTN samples measured at
room temperature.
Catalytic activity of CuTUD-1 in Phenol hydroxylation
In the CuTUD-1 catalyzed phenol hydroxylation (Scheme 2)
catechol (CC), hydroquinone (HQ) and benzoquinone (BQ) are
identified as major products. The reaction parameters such as
time, temperature, solvent and mole ratios of phenol and H O
40
2
2
are optimized in the presence of CuTN catalyst. The catalytic
efficiencies of CuTA, CuTC and other copper containing ordered
mesoporous catalysts are then compared under this optimized
condition.
45
OH
OH
O
OH
OH
CuTUD-1
+
H O
+
+
2
2
H O, 60°C
2
OH
O
Phenol
enzo u none
Catachol(CC) Hydroquinone(HQ)
B
q
i
(BQ)
5
Figure 6. H
2
-TPR profiles of CuTA, CuTC and CuTN samples
Scheme 2. Phenol hydroxylation over CuTUD-1 catalyst and possible
reaction products
H -TPR profiles of all CuTUD-1 samples (Figure 6) showed
two peaks in the temperature range of 100 – 320 °C. The low
temperature reduction peak observed around 160 °C can be
2
Influence of reaction temperature over CuTN catalyst was
carried out from 30 °C to 80 °C and the results are shown in
Figure 8. A maximum catalytic activities were observed at a
reaction time of 6h, 4h, 2h and 1h for the reaction temperatures of
5
5
6
6
7
0
5
0
5
0
attributed to the one step reduction of isolated CuO clusters to
x
0
2+
+ 37
1
1
2
2
3
0
5
0
5
0
Cu and the partial reduction of isolated Cu ions to Cu . The
second and broad high temperature peak observed between 220 –
3
0, 40, 60 and 80 °C respectively. In addition, the catalytic
conversion of H O over CuTN and/or decomposition of H O
2
50 °C is associated with the reduction of highly dispersed CuO
2
2
2
2
with reaction temperature also influences the conversion of
phenol. For instance, at 60 °C, for a H O conversion of 69%
nanoparticles and/or oligomeric CuO clusters. The variation in
the intensity of these peaks in CuTA and CuTC suggest the
presence of predominantly CuO clusters with variable sizes in
2
2
phenol conversion was observed to be 54% whereas at 80 °C, the
phenol conversion was only 62% even though H2O2 conversion
was 100%. This observation clearly signifies the effect of
1
8
these samples.
To probe further, XPS was assessed to understand the Cu ions
oxidation state and coordination. XPS spectrum of CuTN (Figure
and deconvoluted figure in supplement information S4) showed
temperature on the H O decomposition and the activity of
2
2
catalyst. As expected, increasing the reaction temperature, an
increase in the phenol conversion was noticed whereas a
maximum catechol selectivity of about 85% was achieved at 60
7
characteristic spin-orbit split of Cu 2p3/2 and Cu 2p1/2 with
binding energy value of 935.7 eV and 955.8 eV respectively.
Asymmetric satellite peak approximately at 932.9 eV was also
noticed. These results are well in consistent with Cu (II)
°C. At a higher temperature (80 °C), visible tar like products were
detected in the reaction which is due to the deep oxidation of
benzoquinone and their yield was estimated to be about 5% by
1
9, 23, 24
containing SBA-16 and other copper based materials.
1
7, 21
weight
. Similar observations were also reported for metallic
Further the asymmetric peak region could be divided into two
peaks, the lower B.E peak at 932.9 eV is assigned to dispersed
3
8
Cu nanoparticles catalyzed phenol hydroxylation . Considering
the phenol conversion and catechol selectivity, 60 °C was
selected as optimum temperature for the phenol hydroxylation
over this catalyst.
2
+ 23
CuO and higher energy region due to silica connected to Cu .
These observations are in line with DRS UV-Vis and EPR
studies. From the foregoing characterization results, it can be
summarized that a higher amount of isolated Cu (II) species is
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Table 2. Effect of solvent on phenol hydroxylation over CuTN catalyst
Solvent
Dielectric
constant
80
24.55
21
X
Phenol (%)
Selectivity (%)
a
CC
85
60
50
79
HQ
8
26
42
11
BQ
7
14
8
Water
Ethanol
Acetone
54
15
10
37
Acetonitrile
37.5
10
30
Reaction conditions: 5.0 mmol phenol, 5.0 mmol H
CuTN,10 ml solvent, 60 °C.
2 2
O ,50 mg of
a
Dielectric Constant. (From the source - macro.lsu.edu. Retrieved on
013-01-26).
2
3
4
4
5
0
5
Table
3
shows the effect of reaction time for phenol
hydroxylation over CuTA, CuTC and CuTN catalysts. Though a
linear increase in H O conversion was observed, no significant
2
2
increase in phenol conversion was noticed after 1h of reaction
over CuTA and CuTC catalysts. Besides these catalysts
performed significantly lower (about 12-20% in the phenol
conversion) compared to CuTN catalyst. Furthermore, relatively
a linear increase in phenol conversion was noticed over CuTN
catalyst. This could be attributed to presence of a higher number
of active Cu sites present in CuTN catalyst compared to CuTA
and CuTC catalysts. Further, 100% conversion of H O was
noticed for all the catalysts at the end of 4h runs. It clearly reveals
the catalytic efficiency of the catalysts. Prolonging reaction time
over 4h leads to formation of a number of unidentified products
(including tar) in all these catalysts.
Figure 8. The effect of reaction temperature on phenol hydroxylation
over CuTN catalyst.
The molar ratio of H O2 to phenol was varied from 0.5 to 2
2
5
and the results are displayed in Figure 9. Near 95% H O
2
2
efficiency was noticed when H O /Phenol ratio was 0.5. Two
2
2
fold and fourfold increase in the H O mole ratio did increase
2
2
2
2
phenol conversion by approximately 10% only. A drastic
decrease in CC selectivity with concomitant increase in HQ and
BQ selectivity were observed at H O /Phenol ratio of 2. This
1
0
2
2
could be due to over oxidation of formed products and formation
other unidentified products (mainly acetic and oxalic acids).
3
9
Therefore, a mole ratio of H O /Phenol ≤ 1 is favorable for 50 Table 3. Phenol hydroxylation reaction over CuTUD-1 catalyst prepared
2
2
H O efficiency and CC selectivity.
with different copper source.
2
2
Catalyst
Time
X
Phenol (%)
X
H2O2 (%)
Selectivity (%)
(h)
CC
HQ
BQ
CuTA
CuTC
CuTN
1
2
4
1
2
4
1
2
4
35
34
36
37
37
43
42
54
56
38
49
100
60
74
100
46
69
100
72
65
58
68
60
64
78
85
71
13
15
18
16
17
16
11
8
15
20
24
16
23
20
11
7
14
15
Reaction conditions: 5.0 mmol phenol, 5.0 mmol H
CuTUD-1,10 ml water, 60 °C.
2 2
O ,50 mg of
55
We also varied the amount of Cu in CuTN (Si/Cu = 100 to
0) for phenol hydroxylation and the results are tabulated in
1
5
27
1
Figure 9. Effect of Phenol:H
2
O
2
mole ratio on phenol hydroxylation over
Table 4. Phenol conversion increased with Cu loading up to
Si/Cu ratio of 20, however, a further increase in Cu loading
(Si/Cu = 10), the activity was found similar to CuTN (Si/Cu =
CuTN catalyst.
The effect of solvents has been shown to play a major role in
the phenol hydroxylation and product selectivity.
1
7, 33, 40
Influence 60 100). This could be attributed to higher amounts of bulk CuO
2
0
of polar protic and aprotic solvents for the CuTN catalyzed
phenol hydroxylation is presented in Table 2. Among various
solvents studied, phenol conversion follows the order: water >
acetonitrile > ethanol > acetone. From the dielectric constant
(crystalline size ~10 nm) that can block the pores or active sites
thus impeding the contact between the substrate/oxidant to the
active sites. However, finely dispersed CuO on MCM-48 and
4
2, 43
SBA-16 catalyst showed good hydroxylation activity.
Our
values of different solvents it can be inferred that the polarity of 65 studies on CuO dispersed SBA-15 and SiO systems also showed
2
2
5
solvents influences the conversion of phenol. Moreover, the water
has been employed as a preferred solvent for the phenol
hydroxylation reactions probably due to the ease of H O
hydroxylation activity with high BQ selectivity (see Table 4,
entry 7 and 8). Nevertheless, CuTUD-1 catalyst displayed higher
catalytic activity and selectivity than other Cu containing
2
2
4
1
activation over active sites thus enhancing the reaction rate.
6
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Journal Name, [year], [vol], 00–00
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Page 7 of 8
Catalysis Science & Technology
Table 5. Leaching studies of CuTA, CuTC and CuTN catalysts by hot-
mesoporous and non mesoporous catalysts which is mainly due to
filtration experiment
the 3D interconnected mesoporous structure and presence of
highly dispersed of Cu active sites. Moreover, the isolated copper
active sites are mainly present in the internal surface of the pore
a
Catalyst
Cu leaching
%)
XH2O2 (%)
(
30 min.
4 h
42
70
58
CuTA
CuTC
CuTN
12
20
8
29
53
30
5
walls (grafted) that activates H O and phenol easily (Scheme 3).
2 2
It is proposed that reaction follows the Haber Weis mechanism
1
7, 24,
similar to those reported for Cu based materials (Scheme 3).
3
8, 44
2 2
Reaction conditions: 5.0 mmol phenol, 5.0 mmol H O , 50 mg of
catalyst, 10 ml water, 60 °C. Reaction mixture hot-filtered after 30 min of
run.
35
Table 4. Comparison of Catalytic activity of Cu based ordered and
1
0
disordered mesoporous silicates for phenol hydroxylation
a
Calculated from ICP-OES method.
Catalyst (Si/Cu)
X
Phenol (%)
Selectivity (%)
CC
85
72
70
63
60
74
46
35
HQ
8
BQ
7
CuTN (104)
CuTN (53)
CuTN (21)
CuTN (10)
CuMCM-41(50)
CuMCM-48 (50)
54
69
72
59
41
46
22
16
Conclusions
14
16
20
28
26
19
26
14
14
17
12
10
35
39
We have shown that use of different copper source have profound
40 effect on the mesoporous structure of TUD-1 as seen from low
angle XRD and N2 sorption studies. Two types of Cu species
were observed in these materials, namely isolated Cu and
oligonuclear CuO species whose concentration depend clearly on
2
% Cu/SBA-15
% Cu/SiO
2+
2
2
Reaction conditions: 5.0 mmol phenol, 5.0 mmol H
catalyst, 10 ml water, 60 °C.
2
O
2
, 50 mg of
the Cu source. The use of Copper nitrate as source predominantly
2+
4
5
5
5
0
5
yielded isolated Cu
species with lesser amount of
nanocrystalline CuO species. On the other hand copper acetate
produced higher amounts of oligonuclear and/or bulk CuO
species in TUD-1. Phenol hydroxylation over CuTN catalyst
showed a better catalytic performance compared to CuTA and
2
+
CuTC catalyst due to the presence of more isolated Cu active
sites. Also lower amount of Cu leaching (8 %) and the hot-
filtration studies confirm the catalyst heterogeneous nature.
Comparing the catalytic performance of CuTUD-1 with other
supported systems, the phenol conversion and selectivity of
catechol are noticeably higher, which was accounted to higher
2
+
amounts of framework Cu species, dispersed CuO, and 3D
dimensional porous nature of material.
ACKNOWLEDGMENT
The author M.P is thankful to DST-FIST, UGC for instrument
facility and financial support. The authors are thankful to Prof. B.
Viswanathan, NCCR, IIT-Madras for providing laboratory and
characterization facilities to carry out this work.
60
Scheme 3. Proposed reaction pathway for phenol hydroxylation over
1
5
CuTUD-1 catalyst
Notes and references
In general, leaching of active sites are inevitable in oxidation
catalysis (solvolysis of M-O-Si bonds) owing to the use of strong
a
Department of Chemistry, Anna University, Chennai 600025, India,
6
7
5
0
Tel.: +91-9840146642; E-Mail: kshanthiramesh@yahoo.com (K.S)
1
2
b
oxidants (e.g. H O ). To examine the Cu leaching from
Center for Environmentally Beneficial Catalysis (CEBC), The University
2
2
of Kansas, Lawrence, KS 66047, USA, Tel.: +1-785-864-1631; Fax: +1-
7
CuTUD-1 samples, end of the reaction hot-filtered samples were
analysed by ICP-OES method. A Cu atomic weight loss of about
85-864-6051; E-mail: anand@ku.edu
2
2
3
0
5
0
1
2, 20 and 8 % respectively were observed for CuTA, CuTC and
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DOI: 10.1039/C3CY00432E
Catalysis Science & Technology
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