Facile synthesis of polystyrene/gold composite particles as a highly active and reusable catalyst for aerobic oxidation of benzyl alcohol in water

Article abstract of DOI:10.1039/c4ra01522c

PS/Au composite particles have been synthesized facilely based on a thermodynamic effect. More significantly, the PS/Au composite particles can catalyze the aerobic oxidation of benzyl alcohol remarkably under mild conditions (1 atm, air as oxidant, K

Full text of DOI:10.1039/c4ra01522c

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Facile synthesis of polystyrene/gold composite
particles as a highly active and reusable catalyst for
aerobic oxidation of benzyl alcohol in water†
Cite this: RSC Adv., 2014, 4, 24769
Received 21st February 2014
Accepted 28th May 2014
*
Yunxing Li, Yan Gao, Cheng Yang, Shengsheng Sha, Jiefu Hao and Yan Wu
DOI: 10.1039/c4ra01522c
www.rsc.org/advances
PS/Au composite particles have been synthesized facilely based on a presented a synthetic strategy, in which reductants were
thermodynamic effect. More significantly, the PS/Au composite attached onto the surface of PS microspheres by a series of
particles can catalyze the aerobic oxidation of benzyl alcohol reactions and subsequently metal ions were reduced in situ to
remarkably under mild conditions (1 atm, air as oxidant, K2CO3, 30 ꢀC, form polymer-supported AuNPs.19 On the other hand, there has
in water) and be reused several times without significant loss of been tremendous interest in the development of liquid-phase
activity.
aerobic oxidation of alcohols with supported heterogeneous
catalyst, and signicant progress has been achieved. Neverthe-
less, environmentally undesirable solvents, high temperature,
or high atmospheric pressure are used in many cases.17,19–21,23,24
Therefore, in order to meet the requirement of green chemistry
and sustainable development, there is still substantially
necessary for exploring the supported heterogeneous catalyst
that can be synthesized facilely, recovered effectively, and used
directly under mild condition.
Recently, we reported a facile and controllable method to
coat the polystyrene (PS) microspheres with AuNPs based on a
thermodynamic effect.25–27 Scheme 1 shows the synthetic
method employed there. The PS microspheres synthesized by
conventional dispersion polymerization were collected and
dispersed in water by centrifugation. During this process, the
PVP adsorbed on the surface of PS microspheres was reduced,
although the PVP could not be removed completely. Then, the
hydrophobic PS microspheres became metastable in water
owing to the lack of protection of enough PVP. Aer mixing
them with the hydrophilic AuNPs in water, the latter played the
role of solid stabilizer spontaneously for the PS microspheres
and distributed uniformly on the surface of PS microspheres to
reduce the total interfacial energy of the colloidal system.
The selective oxidation of alcohols to the corresponding
carbonyl compounds is one of the most important organic
reactions in the synthesis of ne chemicals.1–4 Traditionally,
these oxidations are performed using stoichiometric amounts
of inorganic oxidants, such as chromate or permanganate, but
these reagents are expensive and toxic, and produce a large
amount of waste.5 Recently, noble metal nanoparticles have
been found to be extremely catalytically active for aerobic
oxidation of alcohols.6–13 Unfortunately, it is oen difficult to
use these nanoparticles directly owing to their strong tendency
to aggregate in the course of the catalytic reaction.9,14 In addi-
tion, one problem to solve in using noble metal nanoparticles as
catalysts is the difficulty in their simple and effective recovery
for reuse. For these reasons, noble metal nanoparticles are oen
stabilized by a solid support and used as supported heteroge-
neous catalyst.12–22 However, conventional preparation methods
of these catalysts are usually time-consuming and cumbersome
owing to surface pretreatments of supports or noble metal
nanoparticles, which are used to promote the deposition of
noble metal nanoparticles on the supports. For example, Rossi's
group synthesized Fe3O4@SiO2 supported gold nanoparticles
(AuNPs) based on the adsorption of AuClÀ ions by silica-coated
magnetic nanoparticles, previously functionalized with –NH2
groups, followed by metal reduction.21 Pan and co-workers
The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School
of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China.
E-mail: yunxingli@jiangnan.edu.cn; Fax: +86-510-8591-7763; Tel: +86-510-8591-
7090
† Electronic supplementary information (ESI) available: Experimental section. See
DOI: 10.1039/c4ra01522c
Scheme 1 Schematic representation of our method for synthesizing
the PS/Au composite particles.
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The advantages of this unique synthesis are obvious. The EDX spectrum of the composite particles (Fig. 2, le), the signal
complicated surface pretreatments of particulate substrates of Au element is present evidently. Moreover, it can be seen
were not necessary at all. Moreover, the resultant PS/Au from the XRD pattern that there are four peaks, which corre-
composite particles had stable structure during the repeated spond to (111), (200), (220), and (311) lattice planes for the
centrifugations and ultrasonic treatments in water. Another Bragg reection of Au, indicating the existence of AuNPs in the
convincing phenomenon about the good stability was the fact composite particles (Fig. 2, right).
that the coverage density of AuNPs on the PS microspheres did
Having demonstrated the successful synthesis of PS/Au
not decrease, when adding some naked PS microspheres into composite particles, we turned to the examination of catalytic
the dispersion of PS/Au composite particles.28,29 This is a activity of these particles toward the aerobic oxidation of benzyl
noticeable advantage when these composite particles are used alcohol, which is oen used as a model alcohol to test the
as supported heterogeneous catalyst, because the good struc- activity of catalyst.1,2 Inductively coupled plasma (ICP) analyses
tural stability allows them to retain high activity for reuse.
showed that 1.16, 0.65, and 0.51 wt% Au had been incorporated
In this work, the size of AuNPs supported on the PS micro- into the composite particles, respectively, as the size of AuNPs is
spheres was decreased to less than 10 nm, which is a critical size about 9.5, 6.7, and 3.5 nm. A series of reaction conditions was
for a dramatic change in their catalytic activity,21,30,31 and then investigated by varying the size of supported AuNPs, atmo-
the synthesized PS/Au composite particles as supported sphere, reaction time and temperature, and nature of base. The
heterogeneous catalysts were applied to the aerobic oxidation of corresponding yields of benzoic acid formed under these
benzyl alcohol in water under different reaction conditions.
conditions are listed in Table 1. It is evident that the supported
Fig. 1a displays the high-resolution transmission electron AuNPs with lower size exhibited higher catalytic activity.
microscopy (HR-TEM) image of the as-prepared AuNPs. However, the yield of benzoic acid exceeded 90% in all cases,
Inspection of this image reveals the AuNPs have well-dened
morphology and a narrow size distribution. Statistical diameter
measurement indicates that the average size of the AuNPs is
about 3.5 nm (insert in Fig. 1). In Fig. 1b, the dense dark spots
can be observed clearly, indicating that the PS microspheres are
decorated with the AuNPs successfully. In addition, by selecting
different molar ratio of HAuCl4 and NaBH4, two different-sized
AuNPs (i.e., 9.5 nm and 6.7 nm) were obtained and subse-
quently deposited on the surface of PS microspheres, as shown
in Fig. 1c and d. In fact, the formation of PS/Au composite
particles can be further conrmed by the energy-disperse X-ray
Fig. 2 EDX spectrum (left) and XRD pattern (right) of PS/Au composite
(EDX) spectroscopy and X-ray diffraction (XRD). As displayed in
particles (supported AuNPs, 3.5 nm).
Fig. 1 HR-TEM images of AuNPs with a size of 3.5 nm (a) and PS/Au composite particles with different size of supported AuNPs: 3.5 nm (b), 6.7
nm (c), 9.5 nm (d). The insert in part (a) shows the corresponding histogram of the size distribution of AuNPs.
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Table 1 Aerobic oxidation of benzyl alcohol using PS/Au composite
particles as catalyst under different reaction conditionsa
run (Table 2). In contrast, AuNPs stabilized by linear polymer
PVP could not be reused owing to their negative aggregation
during the course of catalytic reaction.8,9,32 Therefore, taking
practical application into account, the PS/Au composite parti-
cles reported in this work is hopeful to offer one kind of reus-
able catalyst for aerobic oxidation of alcohols.
Entry
Sizeb [nm]
Time [h]
Base
Yieldc [%]
1
2
3
4
5
6
7
9.5
6.7
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
12
12
12
6
3
2
1
3
3
3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
KOH
94
97
99
97
90
72
58
99
99
99
5
Conclusions
In summary, ultrane AuNPs (<10 nm) were attached onto the
surface of PS microspheres facilely based on a thermodynamic
effect. The synthesized PS/Au composite particles exhibited
high catalytic activity for the aerobic oxidation of benzyl alcohol
8d
9e
10
11f
24
None
ꢀ
under mild condition (1 atm, air as oxidant, K2CO3, 30 C, in
a
Reaction condition: benzyl alcohol (0.1 mmol), Au catalyst (3 Â 10À3
water). Moreover, the PS/Au composite particles can be recov-
ered simply and effectively because of large size of support and
reused several times without signicant loss of catalytic activity.
Hence, combining with facile synthesis and convenient
recovery, it can be expected that the PS/Au composite particles
show great potential for practical application in catalytic
oxidation of alcohols.
mmol), base (0.3 mmol), water (10 mL), air as oxidant, 1 atm, 30 ꢀC.
b
d
c
Size of supported AuNPs.
Estimated from GC analysis.
e
f
Temperature is 50 ꢀC. Oxygen as oxidant. Production is only
benzaldehyde.
when the supported AuNPs with different size were used. At the
given condition, the yield of benzoic acid increased with
increasing the reaction time. As the supported AuNPs with the
lowest size were used, the yield of benzoic acid was higher than
90% in 3 hours under air at 30 ꢀC. If under otherwise identical
conditions air was replaced by oxygen, the yield of benzoic acid
reached 99%. Alternatively, when the reaction time was
extended to 12 hours under air at 30 ꢀC, we were pleased to nd
that the oxidation of benzyl alcohol also gave benzoic acid in
almost quantitative yield (99%). This is a signicant result
because the use of air and water rather than oxygen and organic
solvents under 1 atm is an ideal green process for practical
application.17,20 In addition, it is apparent that the addition of
base could improve the catalytic activity of PS/Au composite
particles dramatically, and oppositely, the oxidation reaction
almost did not proceed at all in the absence of base. The yield of
benzoic acid increased to 99% at 30 ꢀC aer 3 h as the KOH was
added, which is comparable to other known catalysts.9,13
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (no. 21204030) and MOE & SAFEA for the
111 Project (B13025).
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