Rod-like Cu/La/O nanoparticles as a catalyst for phenol hydroxylation

Article abstract of DOI:10.1039/b510154a

Rod-like La/Cu/O nanoparticles, synthesized by a simple coprecipitation reaction with a sonication process, are found to be highly active as a catalyst for the hydroxylation of phenol. Compared to the 4-6 h, 40% yield reported in the literature, our nanop

Full text of DOI:10.1039/b510154a

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Rod-like Cu/La/O nanoparticles as a catalyst for phenol hydroxylation
Linfeng Gou and Catherine J. Murphy*
Received (in Berkeley, CA, USA) 16th July 2005, Accepted 21st September 2005
First published as an Advance Article on the web 21st October 2005
DOI: 10.1039/b510154a
Rod-like La/Cu/O nanoparticles, synthesized by a simple
coprecipitation reaction with a sonication process, are found
to be highly active as a catalyst for the hydroxylation of phenol.
Compared to the 4–6 h, 40% yield reported in the literature, our
nanoparticle catalyst demonstrates a nearly 100% conversion
within 4 h based on gas chromatography.
Phenol hydroxylation, using hydrogen peroxide, is a widely used
method of making biphenols and is a reaction of industrial
importance, as the biphenols are widely used in chemical,
27–30
pharmaceutical and food industries.
However, to date the
best conversion reported in the literature for phenol to hydroxy-
lated phenols is about 40%. As Cu(II) based complexes and oxides
31,32
are well-known catalysts for phenol hydroxylation,
we studied
Metal oxide nanoparticles are receiving more and more attention
due to their potential applications in structural materials, pigments
the catalytic performance of our rodlike La/Cu/O nanoparticles
with this reaction. (Scheme 1)
1–4
and paint/ink materials.
They are also gaining tremendous
The catalysis of phenol hydroxylation has been studied by
27,28
5
importance in catalysis. For example, CuO nanoparticles were
Maurya and Zhang et al.
and the conversion quantitated by
6–8
found to be effective catalysts for CO and NO oxidation; Cu O
2
gas chromatography. A detailed listing of reaction conditions and
results is given in Table 1. We observed a high conversion, 65% at
3 h (based on a gas chromatographic decrease in phenol
concentration and increases in dihydroxyphenol produce concen-
tration ) when La/Cu/O nanoparticles were used as catalysts. For
comparison, Xiao et al. reported a 28% conversion at 4 h when
nanoparticles were reported to catalyze the decomposition of water
9
under visible light, and Cu-based binary or ternary nanocompo-
sites were used to catalyze the fuel reforming process of short-chain
10–13
hydrocarbons.
A number of methods have been developed for the preparation
of metal oxide nanoparticles with various size and shapes. Sol–gel
and co-precipitation techniques are two traditional approaches to
3
0
2 4
Cu (OH)PO was used as catalyst; 33% phenol conversion was
14–18
make metal oxide nanoparticles.
Feldmann and coworkers
reported a polyol-mediated synthesis of a number of oxide
1
9
nanoparticles; Sugiyama et al demonstrated that microwave
cold plasma heating could be used to prepare fine metal oxide
2
0
powders; the formation of MFe
organic phase reaction was reported by Sun et al.
We attempted to make La CuO nanoparticles following Ghosh
et al.’s co-precipitation and sonication approach for ZnFe O and
2 4
O (M 5 Fe, Co, Mn) by an
21
2
4
2
4
22
MnFe
2
O
4
nanoparticles as La CuO
2
4
is also known as a catalyst
23–26
for many reactions.
However, instead of obtaining 4–5 nm
particles as they did, we observed uniform rod-like particles with
lengths of 50 ¡ 5 nm and widths of 12 ¡ 2 nm (Fig. 1).{ These
nanorods were amorphous and could not be identified as La CuO
2
4
by powder X-ray diffraction. Elemental analysis suggested that the
dried powders contained 50.89% La and 5.69% Cu by weight,
which is a y4 : 1 La : Cu mole ratio, although the initial reaction
mixture consisted of a La : Cu 2 : 1 mole ratio. Powder X-ray
diffraction data taken of powders after annealing at 650 uC were
2 4 2 3
poor, and could be indexed as a mixture of La CuO , La O , and
Fig. 1 TEM image of rod-like La/Cu/O nanoparticles.
CuO. The magnetic behavior of the nanorod samples bore no
resemblance to La CuO , and appeared as a simple paramagnet.
2
4
We found that these particles had an overall 18.78% weight loss
when they were heated to 650 uC during thermogravimetric
analysis, due to the loss of H O and the organic surfactant that
2
was also present in the initial reaction mixture.
Department of Chemistry & Biochemistry, 631 Sumter St., University of
South Carolina, Columbia, SC 29208, USA.
E-mail: murphy@mail.chem.sc.edu; Fax: +1-803-777-9521;
Tel: +1-803-777-3628
2 2
Scheme 1 Catalytic phenol hydroxylation by H O .
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Table 1 Percent yield of phenol hydroxylation under different reaction conditions
Catalyst
2 2
Amount of phenol/g Amount of H O (30% ) Solvent/mL Temp./uC Highest phenol conv. (%) [1,2]/[1,4]/[1,3] (%)
a
La/Cu/O NPs
2.0
4.7
2.0
5.0 mL
5.4 g
4.5 mL
MeCN (8)
MeCN (2)
80
80
70
y100
40
39
67/33/0
63/18/16
y90/10/0
b
[
CuCl
Cu(hybe)]-Y
+ SiW12
c
2
H
2
O (12)
a
b
c
This work. From reference 27. From reference 28. [1,2], [1,4] and [1,3] represent the relative amounts of the isomers of dihydroxyphenol
formed.
in 40 mL DI water and mixed with 60 mL 0.1 M cetyltrimethylammonium
bromide (CTAB) solution. CTAB is a surfactant that we have previously
3
7
2
found useful to control the particle size of Cu O nanoparticles. The entire
solution was heated to 70 uC, and an aqueous 3.0 M NaOH solution was
added dropwise to adjust the pH to y12. The temperature was maintained
at 70 uC for one hour, while the blue precipitate turned dark. The
precipitate was collected by centrifugation at 5500 rpm for 10 min, washed
with DI water to remove salts and excess surfactant, and were re-
centrifuged in the same manner, then dried in an oven at 60 uC overnight.
A 0.2 g sample of the dried powder was then transferred to a solution
which contained 9.5 mL toluene and 0.5 mL n-octylamine and was
sonicated in a bath sonicator for 4 h. The precipitate was collected by the
same centrifugation, washing and drying process described above. For the
catalyst reaction, 25 mg of the dried powder was redispersed in 1.0 mL DI
water and injected to the reaction solution.
The catalyst reaction was carried out in a 50 mL flask fitted with a water-
cooled condenser and magnetic stirrer. In a typical reaction, 2.0 g phenol
was dissolved in 8 mL acetonitrile, and 25 mg of solid was then added while
the reaction mixture was heated at 80 uC with continuous stirring.
Fig. 2 Catalytic phenol hydroxylation by H
injected in a pattern of 2.0 mL at 0 min, 1.0 mL at 30 min, and 2.0 mL at
0 min, solvent: MeCN; squares: H was injected all at once at 0 min,
solvent: MeCN; triangles: H was injected in a pattern of 2.0 mL at
min, 1.0 mL at 30 min, and 2.0 mL at 60 min, solvent was DI water.
2 2 2 2
O . Diamonds: H O was
Subsequently, 5 mL of a 30% H
mixture in a pattern of 2.0 mL at 0 min, 1.0 mL at 30 min, and 2.0 mL at
0 min. At certain time intervals during the reaction, 0.3 mL of the reaction
mixture was withdrawn and mixed with 0.2 mL 1.0 M HCl solution and
.0 mL methylene chloride. After phase separation, 0.6 mL of the organic
2 2
O solution was added into the reaction
6
2 2
O
6
2 2
O
0
2
Conversion was based on gas chromatography, which included all
isomers.
phase was diluted with 3.0 mL methylene chloride, and the products were
then separated and analyzed by gas chromatography (Shimadzu GC-17A,
with helium as the carrier gas). The products were also characterized by a
H NMR (Varian Mercury 400 MHz) to determine the different isomers
with deuterated methylene chloride as the solvent at 25 uC.
1
2
9
achieved by Villa et al; and 40% conversion was achieved in 4–6 h
as reported in ref. 27 and 28. Furthermore, we found the yield of
hydroxylated phenols could be further improved by altering the
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Materials, 1st edn, Wiley-VCH, Weinheim, 1998.
2 2
way of adding H O . A nearly 100% conversion was achieved
2
K. J. Klabunde, Nanoscale Materials in Chemistry, Wiley -VCH,
Weinheim, 2001.
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2
2
1
.0 mL at 30 min and 2.0 mL at 60 min instead of adding the H
2
O
2
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solution into the reaction mixture all at once (Fig. 2). We did this
because both lanthanum and copper oxides are also able to
4
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2
.
catalyze the decomposition of H
2
O
Our results on how the
5
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2
2
27
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2
although our process has much higher yield and produces none of
the 1,3 dihydroxyphenol isomer. Peroxide added without the
nanorod catalyst produced less than 1% of the product.
7
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27
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deionized (DI) water was used in Zhang’s report. We found the
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hydroxylation reaction, compared to only 48% when water was
used as the solvent (Fig. 2).
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{ Experimental details: The rod-like La/Cu/O nanoparticles were prepared
as follows: 2.971 g LaCl ?7H O and 0.5378 g CuCl ?2H O were dissolved
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