Palladium-catalyzed aerobic oxidation of 1-phenylethanol with an ionic liquid additive

Article abstract of DOI:10.1039/c1cc11643f

The aerobic oxidation of 1-phenylethanol over a carbon nanotube supported palladium catalyst was improved with an ionic liquid additive [emim][NTf ₂], showing an excellent TON of 149000 which can be maintained for 5 recycle runs.

Full text of DOI:10.1039/c1cc11643f

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COMMUNICATION
Palladium-catalyzed aerobic oxidation of 1-phenylethanol with an ionic
liquid additivew
Yuanting Chen, Linlu Bai, Chunmei Zhou, Jong-Min Lee and Yanhui Yang*
Received 22nd March 2011, Accepted 13th April 2011
DOI: 10.1039/c1cc11643f
The aerobic oxidation of 1-phenylethanol over a carbon nano-
tube supported palladium catalyst was improved with an ionic
to their unique properties, i.e., non-flammable, non-volatile,
1
thermally stable, high solvating, and non-coordinating.
2
liquid additive [emim][NTf
2
], showing an excellent TON of
Moreover, ILs are regarded as desingers’ solvents because their
physico-chemical properties can be precisely tuned by a
suitable combination of cations and anions.
1
49 000 which can be maintained for 5 recycle runs.
The selective oxidation of alcohols has been considered as one
of the most fundamental organic transformations. The corres-
ponding aldehydes or ketones are employed as high-value
intermidiates and components for the pharmaceutical,
In addition to a vast number of developments achieved using
ILs as reaction media in transition metal-catalyzed hydrogen-
1
3–15
ation, hydroformylation and coupling reactions,
several examples of catalytic alcohol oxidation in ILs.
there are
16–18
1
,2
agrochemical, and perfumery industries. From the viewpoints
of atom economy and environmental concern, developing
noble metal heterogeneous catalysts and using molecular
oxygen or air as the oxidant prevail as an attractive green
However, most of them involve homogeneous metal catalysts,
which require laborious product isolation and generate large
amounts of organic/inorganic waste. Therefore, heterogeneous
oxidation in ILs media is preferred as a greener alternative
which can also effectively solve the problems involving the
decomposition or leaching of active metal centers, poor reac-
3
technology. Numerous palladium catalysts with superior
catalytic activities have been developed since its first successful
4
application in the aerobic oxidation of secondary alcohols.
19
tants and catalyst solubility, low activities and selectivities. In
Mori et al. reported a hydroxyapatite-supported Pd catalyst
2
this communication, [emim][NTf ], a non-nucleophilic and
highly effective for the aerobic oxidation of benzylic alcohols,
5
particularly giving a TOF of 9800 h for 1-phenylethanol.
hydrophobic IL, was selected as the reaction additive for the
selective oxidation of 1-phenylethanol over a carbon nanotube-
supported Pd catalyst using molecular oxygen; the reactivity
was benchmarked against solvent-free conditions and another
representative IL [bmim]Br. The effects of IL nature and
content as well as various reaction parameters (reaction time,
À1
Li et al. showed that zeolite-supported Pd nanoparticles of
À1 6
2
.8 nm exhibited an exceptional TOF of 18 800 h . More
recently, our group has also reported the suface-functionalized
À1
TUD-1 supported Pd catalysts with a high TOF of 18 571 h
7
for benzyl alcohol oxidation.
O partial pressure, O total pressure, reaction temperature,
2 2
Ionic liquids (ILs) have risen as a focus of research interest
and made significant progress in the catalytic processes from a
green chemistry perspective since the discovery of second-
recyclibility) on the catalytic activity were examined.
Several variables have been identified to be important for
the success of 1-phenylethanol aerobic oxidation in ILs
(Table 1). In the absence of catalyst and ILs (entry 1), only
a small amount of 1-phenylethanol converts due to the
8
generation ILs in 1992. ILs are generally employed to
promote the catalytic reactions in two ways: as a supported
phase (supported ionic liquid phase catalysts, SILPCs) and as
9
medium or additive in the reaction. Making SILPCs, in which
2
0
non-catalytic oxidation. Good activity is observed over a
Pd/CNT catalyst under solvent-free conditions (entry 2),
active species are immbolized on an IL-functionalized support,
1
showing a TON of 111 000. Noteworthily, [emim][NTf ]
2
0,11
is considered as a facible and economic approach.
None-
distinctly affects the catalytic performances. The activity
increases sharply upon introducing a small amount of
theless, many studies are also centered at the use of ILs as
effective solvents for ‘‘green’’ chemical reactions. ILs can
replace the traditional volatile organic and dipolar aprotic
solvents in both laboratory and industry-scale production due
[emim][NTf ] as an additive, and reaches the maxmium with
2
1 mL of IL addition, showing an excellent TON of 149 000
(entries 3 and 4). The improvement can be attributed to the
enhanced solubility of both alcohol substrate and gaseous O
in [emim][NTf ], leading to better substrate–catalyst
contact. Further adding [emim][NTf ] results in a pronounced
2
2
a
School of Chemical and Biomedical Engineering, Nanyang
Technological University, Singapore 637459, Singapore.
E-mail: yhyang@ntu.edu.sg; Fax: +65 6794 7553;
Tel: +65 6316 8940
w Electronic supplementary information (ESI) available: Experimental
details, supplementary catalytic results, TEM images and Raman
spectra. See DOI: 10.1039/c1cc11643f
1
9
2
decline of catalytic performances (entries 5–7), which may be
due to either the low concentration of the substrate or the
large mass-transfer resistance in the viscous alcohol–IL
mixture. The selectivities toward acetophenone show a similar
6
452 Chem. Commun., 2011, 47, 6452–6454
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a
Table 1 Variation of reaction conditions for the oxidation of 1-phenylethanol to acetophenone over a Pd/CNT catalyst
b
Select. (%)
4c
Entry
Ionic liquid
V
IL/mL
IL molar fraction (%)
Conv. (%)
TON Â 10
—
11.1
12.7
14.9
10.3
9.8
7.1
—
13.5
1.2
9.6
d
1
2
3
4
5
6
7
8
9
—
—
—
—
0.5
1
2
3
4
1
1
1
0
0
8.7
11.8
67.6
77.4
90.6
62.8
59.9
43.2
42.8
82.1
7.1
100
90.8
[emim][NTf
[emim][NTf
[emim][NTf
[emim][NTf
[emim][NTf
[emim][NTf
[emim][NTf
[emim][NTf
[emim][NTf
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
91.8
96.9
95.8
91.6
93.9
47.9
91.7
76.0
85.0
0
16.0
23.7
31.8
38.3
16.0
16.0
16.0
16.0
19.1
0
d
e
f
1
1
1
1
0
1
2
3
g
1
1
—
58.4
0
60.8
d
[bmim]Br
—
—
10.0
h
90.2
a
À1
Reaction conditions: 25 mmol of 1-phenylethanol, 5 mg of Pd/CNT, 20 mL min of O
c
2
, 160 1C, 1 h. Uncertainties: conv. Æ0.15%, select.
b
Æ0.35%, TON Æ 0.08%. Ethylbenzene is the only byproduct. TON was calculated based on DPd determined by the CO-stripping method.
d
e
f
g
h
2 2
No catalyst. [emim][NTf ] recovered after reaction. N used. Air used. Pd/CNT recovered after reaction in [bmim]Br.
À 21
In
trend along with conversions in the presence of [emim][NTf
2
],
anion, i.e., more electrophilic compared to Br .
which is higher than that under the solvent-free conditions,
accompanied by a trace amount of ethylbenzene as the only
detectable byproduct. In addition, as an environmentally
Pd-catalyzed alcohol oxidation following a dehydrogenation
mechanism, electrons are transferred through a consecutive
2
cycle from alcohol to H-acceptor (i.e., molecular O ) via Pd
2
2
benign reaction additive, [emim][NTf
2
] can be easily recovered
active species. When Pd nanoparticles are surrounded by an
electrophilic enviroment, the electron transfer is accelarated
after reaction and re-use in the next run, showing a high TON
of 135 000 (entry 9).
due to the decreased electron density on Pd, leading to an
À
The reaction time-conversion correlation was studied for
enhanced catalytic activity. The electrophilicity of [NTf ] also
2
1
-phenylethanol oxidation in [emim][NTf ] and solvent-free
2
accounts for the moderate activity in the absence of a Pd/CNT
catalyst (Table 1, entry 8). However, acetophenone and ethyl-
benzene were produced in approximately equal amounts,
suggesting that Pd/CNT can effectively suppress the hydrogeno-
lysis of 1-phenylethanol. Furthermore, in addition to the
conditions (Fig. 1(a)). Another frequently used IL [bmim]Br
was also introduced for comparison. Pd/CNT in [emim][NTf
shows superior catalytic activities to solvent-free conditions.
Bmim]Br is detrimental for the reaction, showing an extre-
mely low conversion of 3.1% (TON of 5120) after reacting for
h. The plots were fitted with first-order reaction kinetics; the
2
]
[
better solubility of O
catalytic rate in the [emim][NTf
attributed to the hydrophobicity of [emim][NTf
2
in IL, the remarkably improved
]-mediated reaction can be
], which
1
2
reaction rate constants k are 1.96 Æ 0.05, 0.0346 Æ 0.0008,
2
À1
1
.04 Æ 0.02 h for [emim][NTf
2
], [bmim]Br and solvent-free
enriches the alcohol concentration at the catalyst–liquid inter-
face where the reaction occurs and facilitates the desorption of
conditions, respectively. Considering the similarity of cations
2
3
in both ILs, the inferiority of [bmim]Br to [emim][NTf ] can be
2
hydrophobic ketone from catalyst surfaces. On the contrary,
the hydrophilic [bmim]Br is prone to adsorb on the catalyst
surfaces which are also hydrophilic after acid-pretreatment,
thus poisoning the active sites and suppressing the reaction.
Further investigations on the effects of various reaction
addressed by the different nucleophilicities of two anions.
À
[
NTf2] is considered as a non-nucleophilic and innocent
parameters will be focused on using [emim][NTf
additive. N and air were employed as alternatives of O
Table 1, entries 10 and 11), showing that the catalytic activity
increases with a higher O partial pressure. When using N , the
2
] as an
2
2
(
2
2
low alcohol conversion corresponds to the dehydrogenation
mechanism where b-H elimination occurs upon dissociative
adsorption of the substrate on active sites even in the absence
of O but suffers a fast catalyst deactivation due to product
2
2
,22
deposition on the Pd surface.
The considerably low aceto-
phonone selectivity is contributed by the C–O bond hydrogeno-
lysis, affording the formation of ethylbenezne due to the
abundant hydride species covered on the Pd surface and
2
4
2
consequent catalyst deactivation. Therefore, sufficient O is
essential to maintain the reaction in the kinetic region where
the dehydrogenation but not the mass-transport is the rate-
limitting step and to continuously consume surface Pd–H
Fig. 1 Comparison of the effects of reaction parameters for the
oxidation of 1-phenylethanol over a Pd/CNT catalyst. (a) Time-
2
conversion plots for reaction; (b) effect of O pressures; (c) effect of
reaction temperatures; (d) recyclability of Pd/CNT. Run conditions:
see footnote of Table 1.
2
species. Elevating the O total pressure from the atmosphere
pressure to 3 atm leads to a drastically increased activity due
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to the enhanced solubility of O
2
(Fig. 1(b)). Nonetheless, the
alcohol substrate. Considering the presence of F (Fig. 2(e)),
Pdd+ which is ca. 28% of Pd surfaces in Pd/CNT used in
activity reaches a plateau when further increasing the O
pressure, implying that the reaction is of zero order in O2.
2
1
[emim][NTf ] is attributed to Pd–F bonds, indicating the
2
À
Meantime, it is also speculated that abundant PdO phases
may be formed concomitantly in the presence of excess O2,
resulting in the loss of metallic Pd active sites and con-
interaction of [NTf ] and Pd active sites during the
2
2
6
reaction.
Surprisingly, no Pd signal is detectable in
Pd/CNT used in [bmim]Br while the existence of Br has been
verified (Fig. 2(f)). TEM observation shows Pd nanoparticles
for this particular spent catalyst (Fig. S2(d), ESIw) and no Pd
leaching occurs during the reaction. We suggest that Pd was
covered by a thick layer of [bmim]Br. Two control experiments
were carried out to verify this speculation (Table 1, entries 12
and 13). [Bmim]Br alone shows no activity in the oxidation.
After the catalyst in [bmim]Br was recovered by washing with
acetone and re-used in a new reaction without any additive,
comparable activity was observed with the solvent-free
condition (entry 2). Therefore, when employed as an additive,
hydrophilic [bmim]Br quickly adsorbs and accumulates on
catalyst surfaces, resulting in catalyst deactivation. This can be
2
4
sequently surpressed dehydrogenation step.
The effect of reaction temperature was also examined
Fig. 1(c)). The acetophenone selectivity is well maintained
(
above 95% in the studied temperature range. The activity is
hardly detectable at low reaction temperatures, whereas shows
a sharp step increase above 100 1C. Despite the increased O
solubility, the rate of alcohol dehydrogenation decreases
2
2
remarkably at low temperatures. The high activation energy
4
À1
barrier (71.9 kJ mol ) to initiate the reaction implies the
7
absence of mass-transfer limitation in this catlaytic system.
Other than recycling the [emim][NTf ] additive (Table 1,
2
entry 9), the recyclability of a Pd/CNT catalyst was also
investigated (Fig. 1(d)). There is only a slight deterioration for
both catalytic activity and acetophenone selectivity during the
repeated use of catalyst for 5 cycles, which can be contributed
by the partial loss of catalysts during the recovery. Therefore,
D G
also confirmed by the decreased I /I ratio in the Raman
spectrum, which implies the diminished CNT defects due to
[bmim]Br coverage (Fig. S3, ESIw).
[
2
emim][NTf ] not only promotes the catalytic performance but
Notes and references
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verified that no observable leaching of the Pd catalyst occurs.
Negligible conversion is observed when the liquid filtrate is used
as catalyst for further reaction. It is suggested that metal is more
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454 Chem. Commun., 2011, 47, 6452–6454
This journal is c The Royal Society of Chemistry 2011