Au NPs@ polystyrene resin for mild and selective aerobic oxidation of 1,4 dioxane to 1,4 dioxan-2-ol

Article abstract of DOI:10.1016/j.catcom.2016.11.016

Supported gold nanoparticles of sizes 5–8 nm have been found as highly efficient catalyst for the oxidation of 1,4 dioxane, a saturated ether, using elemental oxygen at low temperature. GC–MS analysis of the reaction mixture showed > 85% conversion of 1,4 dioxane with a TON of 1120 h^{? 1} to 1,4 dioxan-2-ol with 90% selectivity. 1,4 Dioxan-2-one was obtained as the major byproduct along with traces of acetic acid and methoxy dioxalane. The catalyst displayed excellent stability and recyclability. TEM analysis of reused catalyst indicated that there was no significant change in the size, shape and morphology of gold nanoparticles.

Full text of DOI:10.1016/j.catcom.2016.11.016

Au NPs@ polystyrene resin for mild and selective aerobic oxidation of 1,4
dioxane to 1,4 dioxan-2-ol
Anuj S Sharma, Harjinder Kaur
PII:
DOI:
Reference:
S1566-7367(16)30425-3
doi:10.1016/j.catcom.2016.11.016
CATCOM 4857
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Catalysis Communications
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Accepted date:
20 September 2016
12 November 2016
15 November 2016
Please cite this article as: Anuj S Sharma, Harjinder Kaur, Au NPs@ polystyrene resin
for mild and selective aerobic oxidation of 1,4 dioxane to 1,4 dioxan-2-ol, Catalysis Com-
munications (2016), doi:10.1016/j.catcom.2016.11.016
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Au NPs@ polystyrene resin for mild and selective aerobic oxidation of 1, 4
dioxane to 1, 4 dioxan- 2-ol
Anuj S Sharma, Harjinder Kaur*
Department of Chemistry, School of Sciences, Gujarat University, Ahmedabad, India
E- mail: hk_ss_in@yahoo.com, Fax: +91 79 26308545; Tel: +91 79 26300969
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Abstract
Supported gold nanoparticles of sizes 5-8 nm have emerged as highly efficient
catalyst for the oxidation of 1, 4 dioxane a saturated ether using elemental oxygen at low
temperature. GC-MS analysis of the reaction mixture showed > 85 % conversion of 1, 4
dioxane with a TON of 1120 h-1 to 1, 4 dioxan-2-ol with 90 % selectivity. 1, 4 dioxan-2- one
was obtained as the major byproduct along with traces of acetic acid and methoxy dioxalane.
The catalyst displayed excellent stability and recyclability. TEM analysis of reused catalyst
indicated that there was no significant change in the size, shape and morphology of gold
nanoparticles.
Keywords: resin supported AuNPs, oxidation of 1, 4 dioxane, molecular oxygen,
heterogeneous catalyst
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Introduction
Usage of nanoparticles as catalysts has revolutionized the field of chemical oxidation
in the past two decades. Especially noteworthy is the catalysis by supported gold
nanoparticles (AuNP). which has turned out to be selective, clean and with remarkable
recyclability. In fact AuNP catalysed oxidation reactions have become one of the most fertile
fields of research owing to its capability to chemisorb and dissociate molecular oxygen at low
temperatures [1-5]. Most of the initial research in this field was limited to AuNPs supported
on metal oxides such as TiO2, CeO2 and Fe2O3 for the oxidation of CO in air [6-10]. But
gradually, oxidation of a variety of substrates other than CO such as aldehyde [11],
cyclohexene [12-13], alcohols [14-15], glucose [16] etc. were also reported.
1, 4 dioxane is an important chemical that is widely used as solvent for the synthesis
of dyes, oils, paints, organic molecules, pesticides, plastics etc. It is a probable carcinogen
and is classified as hazardous compound [17]. It belongs to a class of saturated cyclic ethers
that are part of numerous natural products and associated with bioactivity [18]. Not much
work has been reported in literature about its oxidation. Owing to its probable
carcinogenicity, a few studies on its degradation by photocatalytic [19-20] and biological
processes [21-22] have been performed and reported. There are also few reports of its
heterogeneous catalytic oxidation using Fe [23], Pt [24] and Ti [25] as catalysts on porous
inorganic supports. In case of iron the activity of catalyst was studied at high temperature
leading to complete oxidation of dioxane to CO [23] and in case of Ti catalyzed oxidation,
H2O2 was used as oxidant [25].
To the best of our knowledge, there are no reports on the thermal oxidation of 1,
4 dioxane catalyzed by gold nanoparticles. With circumstantial, and in continuation of our
endeavours towards supported nanocatalysis, we herein report the Au NPs@resin catalyzed
selective oxidation of 1,4 dioxane to 1,4 dioxan-2-ol using molecular oxygen as green
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oxidant. We studied the effect of various reaction parameters such as temperature, nature of
the oxidant (H2O2, TBHP, and O2), catalyst quantity, reusability, reaction time etc. to
optimize conditions for the best yield of 1,4 dioxan-2-ol..
2. Experimental and Instrumentation
2.1 Materials
HAuCl4 were purchased from Sigma Aldrich. Amberlite XAD-4 resin was purchased from
Fluka Chemical Corporation. 1,4 dioxane used was of analytical grade. NaBH4 and all other
solvents were purchased from Finar chemicals. The peroxides, H2O2 (30 % in water) and
TBHP (70 % in water), were purchased from Spectrochem Fine Chemicals Ltd. All glassware
was systematically cleaned with aquaregia. Cross linked polystyrene resin Amberlite XAD-4
was impregnated with gold nanoparticles by a method reported earlier in our lab. The average
concentration of impregnated gold is 0.630 mg/g of the resin [26].
2.2 Instrumentation
GC–MS measurements were carried on Perkin Elmer USA Auto system XL; carrier gas
Helium, Column PE–5MS (condition: injection temperature 200 °C, column flow 0.7 mL /
min, carrier gas helium, oven temp varied from 70 to 280 °C at 10 °C per min. Inductively
coupled plasma atomic emission spectroscopy (ICP-AES) analysis was carried out on a HJY
Ultima-2 instrument: 1000 W, nebulizer pressure 2.96, nebulizer flow 1.29, wave length
242.795 nm. High resolution transmission electron microscopy (HRTEM) pictures were
taken using a Hitachi (H-7500) instrument attached with EDX as per protocol mentioned in
ref. 26.
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2.3 Catalytic oxidation of 1, 4 dioxane using O2
In a typical experiment, oxidation of styrene with O2 was performed in a 25 mL two-necked
round bottom flask equipped with a reflux condenser and filled with 10 mmol of 1, 4 dioxane,
and 150 mg of catalyst. The reaction mixture was heated at 80 ˚C under gentle stirring. At
atmospheric pressure, O2 gas from a balloon was bubbled into the reaction mixture at a
certain stable flow rate controlled by a mass flow controller. Care should be taken while
heating the reaction mixture as dioxane vapours can form a highly combustible mixture with
pure oxygen and explosive peroxides (MSDS 1,4 dioxane). After 24 h, the catalyst was
separated from the reaction mixture by simple filtration and the reaction mixture after proper
dilution with methanol was analyzed using GC-MS.
2.4 Catalytic oxidation of 1, 4 dioxane using H2O2 and TBHP
A 25 mL round bottom flask equipped with reflux condenser, was filled with 1, 4 dioxane
(10 mmol), 4 ml oxidant (H2O2 or TBHP), and 150 mg catalyst. The flask was placed in an
oil bath and the reaction mixture was heated at 80 ˚C with gentle stirring. After 24 h, the
catalyst was separated by filtration and the reaction mixture was extracted in ethyl acetate.
The extracted organic layer was dried with anhydrous Na2SO4 and the product was subjected
to GC-MS analysis.
3. Result and Discussion
We have earlier reported the synthesis of gold nanoparticles supported on XAD-4
resin beads (see ESI), which were found to be highly efficient, stable and reusable catalysts
[26]. In a nonfunctional resin like XAD-4, the existent micro-pores are responsible for the
stabilization of gold nanoparticles by virtue of steric hindrance afforded by polymer network
along with the electrostatic forces between the resin matrix and the nanoparticles. In the
absence of any strong surface interactions these nanoparticles are highly active. During our
studies on AuNPs catalyzed oxidation reactions we discovered that the catalyst is active
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towards oxidation of 1, 4 dioxane which was being used as solvent in the reaction and we
decided to study it in detail.
3.1 Oxidation of 1, 4 dioxane
A systematic investigation of the catalytic activity of the AuNPs@ XAD-4 toward the
oxidation of 1, 4 dioxane was carried out in presence of molecular O2 at ambient pressure,
H2O2 and tert-butyl hydroperoxide (TBHP). GC-MS analysis of the reaction mixture
indicated a high conversion of 1, 4 dioxane (85 %) in molecular oxygen as compared to other
oxidants. (Scheme 1). Conversion of 1, 4 dioxane in the presence of different oxidants was
carried out and the results are shown in Table 1. It was also observed that 1, 4 dioxan-2-ol
(90 %) was the major product while 1, 4 dioxan-2-one was obtained as major side product
along with trace amount of acetic acid and methoxy dioxalane (Entry 3). The products as
identified by GC-MS showed retention time of 2.10, 3.40 and 5.20 min respectively for 1, 4
dioxane, 1, 4 dioxane 2-ol and 1, 4 dioxan-2-one (see ESI). 1, 4 dioxan-2-ol was identified by
its characteristic m/z peaks at 104, 103, 102, 73, 58, 44, 42 (see ESI) whereas, 1, 4 dioxan-2-
one was identified by its characteristic m/z peaks at 103, 102, 73, 58, 44, 42 (see ESI).
Oxidation of C-H bond of 1, 4 dioxane, a saturated ether is an indication of the highly active
nature of AuNPs supported on the resin.
Scheme 1. Reaction scheme for the oxidation of 1, 4 Dioxane by Au NPs@ Resin.
Table 1. Effect of oxidant on oxidation of 1, 4 dioxane by Au NPs@ Resin.
Selectivity
Oxidant
Conversion (%)
1, 4 dioxan-2-ol
1, 4 dioxan-2-one
Other
39
10
TBHP*
25
40
14
35
47
55
*
H2O2
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O2
85
90
07
03
Reaction conditions: 1, 4 dioxane (10 mmol), catalyst (150 mg), 24 hour; 80 ° C * H2O2 or
TBHP (4 mL) conversion and selectivity determined by GC-MS analysis.
3.3 Effect of Temperature
Effect of temperature on conversion of the reactant was studied at temperatures ranging from
60 to 100 ˚C and it was observed that at 60 ˚C the conversion of 1, 4 dioxane was 36 % with
56 % selectivity for 1, 4 dioxan-2-ol. As we increased the temperature from 60 ˚C to 80 ˚C,
the conversion increased from 36 to 85 %. At this temperature maximum selectivity for 1, 4
dioxan-2-ol (90 %) was observed. On further raising the temperature from 80 to 100 ˚C, it
was observed that conversion of 1, 4 dioxane increased from 85 to 92 % but selectivity for 1,
4 dioxan-2-ol decreased to 40 % due to its over oxidation (Figure 1). Therefore, 80 ˚C was
selected as the optimum temperature and the effect of other important parameters such as
catalyst concentration and reaction time were studied at this temperature.
Figure 1. Effect of Temperature on 1, 4 dioxane oxidation. (Reaction conditions: 1, 4 dioxane
(10 mmol), Catalyst (150 mg), 24 h, 1 atm O2).
3.4 Effect of reaction time
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To ascertain the conversion of 1, 4 dioxane and selectivity of 1, 4 dioxane 2-ol, reaction
mixture was analyzed at intervals of 4 h by GC-MS. It was observed that after 4 h, the
conversion of 1, 4 dioxan-2-ol was only 28 % with 70 % selectivity for 1, 4 dioxan-2-ol as
shown in the Figure 2. As the time increased from 4 h to 24 h, conversion of 1, 4 dioxane
increased from 28 to 85 %. Further increase in the reaction time up to 28 h increased the
conversion of 1,4 dioxane from 85 to 90 % but selectivity decreased to 65 %.
Figure 2. Effect of reaction time on 1, 4 dioxane oxidation. (Reaction conditions: 1, 4 dioxane
(10 mmol), Catalyst (150 mg), 80 ˚C, 1 atm O2).
3.5 Effect of catalyst concentration
The presence of catalyst was necessary for the progress of the reaction as in its
absence only 3 % conversion of 1, 4 dioxane was observed. We changed the amount of
catalyst from 110 to 190 mg while keeping the other parameters constant (Figure 3). With the
increase in quantity of catalyst, we observed that the conversion of styrene increased from 45
to 85 %. Thereafter, it remained constant. However, maximum selectivity for 1, 4 dioxan-2-ol
(90 %) was obtained with 150 mg of catalyst (0.045 mol % of Au). This corresponds to a
turnover number of 1120 h-1which is reasonably good for a heterogeneous reaction.
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Figure 3. Effect of catalyst quantity on 1, 4 dioxane oxidation. (Reaction condition: 1, 4
dioxane (10 mmol), 80 ˚C, 24 h, 1 atm O2.)
3.6 Recyclability of the catalyst and mechanism
Recyclability of the catalyst is one of the main advantages of a heterogeneous catalyst and an
important parameter that is required for the evaluation of catalytic performance. The
reusability of catalyst was evaluated by performing five consecutive oxidation reactions using
the same catalyst. After each reaction the catalyst was separated from reaction mixture by
filtration and frequently washed with hot DI water and ethanol to remove any sorbed organic
products. After cleaning and drying, it was ready for the next batch. As shown in Figure 4,
the conversion of styrene and 1, 4 dioxane remained almost constant during five consecutive
cycles, which indicated the remarkable stability of the catalyst. After separation of catalyst
from the reaction mixture, the filtrate was analyzed by ICP-AES for the presence of leached
gold species. Reports indicate absence of measurable gold species in the filtrate.
Transmission electron microscopy is a useful tool to study the size of nanoparticles.
TEM analysis of used catalyst is of considerable importance as catalytic activity of gold
nanoparticles is highly size dependent. TEM pictures of the catalyst taken after the fifth cycle
(Figure 5) clearly indicated that the majority of the particles were still less than 5 nm in size.
No big agglomerations of nanoparticles were observed (See ESI). Therefore, we can finally
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conclude that the AuNPs present in the polystyrene resin matrix are highly stable and absence
of any leached AuNPs in the reaction mixture indicated that the reaction is enormously
heterogeneous.
Figure 4. Recyclability of the catalyst and post reaction TEM of used catalyst
Numerous experimental and DFT studies have indicated that the surface electron
density of the AuNPs is highly favorable for the adsorption of molecular oxygen and to
-
-
produce active oxygen species (·O2 ) [27-28]. This active superoxo-like form (·O2 ) can
abstract hydrogen from co-adsorbed reactants [29] which is 1, 4 dioxane in the present case to
produce intermediate 2-hydroperoxy-1, 4-dioxane. The formed 2-hydroperoxy-1, 4-dioxane
then undergoes decomposition to produce 1, 4-dioxan-2-ol and 1, 4-dioxan-2-one (Figure 5).
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Figure 5. Schematic representation of the mechanism for aerobic oxidation of 1, 4 dioxane
over Au NPs @ XAD-4 resin.
4.
Conclusions
So far, oxidation of 1, 4 dioxane has not been studied much, especially, with an aim to get
useful oxidation products. We are reporting for the first time gold nanoparticles catalyzed
oxidation of 1, 4 dioxane. In view of above results, we can say that gold nanoparticles
supported on polystyrene resin are highly active for liquid phase oxidation using molecular
oxygen at low temperature with high TON. Though, pure oxygen in contact with organic
liquids is potentially hazardous due to combustible nature of the reaction mixture [30], the
advantages of oxygen as oxidant on environment are too many to be ignored. This study
opens up a new area of aerobic C-H oxidation of saturated ethers by gold nanoparticles. A
further work on oxidation of similar compounds is under progress.
Acknowledgments
Authors are grateful to Nanomission DST New Delhi for the financial support to carry out
this work and to SICART Vallabh Vidyanagar, Gujarat for GC-MS, MNIT-Jaipur for HR-
TEM and EDX analysis, PRL Ahmedabad for ICP-AES results.
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Graphical abstract
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Highlights
 Au NPs@ polystyrene resin successfully used for aerobic oxidation of 1, 4 dioxane.
 High conversion (85 %) of 1,4 dioxane to 1, 4 dioxan-2-ol with 90 % selectivity.
 Green protocol with molecular oxygen as oxidant at low temperature.
 A recyclable and reusable catalyst.
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