Insights into support wettability in tuning catalytic performance in the oxidation of aliphatic alcohols to acids

Article abstract of DOI:10.1039/c3cc43042a

A superhydrophobic catalyst was prepared by immobilizing Pt nanoparticles on superhydrophobic organic-inorganic hybrid silicas, which showed high activity and selectivity in the oxidation of aliphatic alcohols to carboxylic acids.

Full text of DOI:10.1039/c3cc43042a

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Insights into support wettability in tuning catalytic
performance in the oxidation of aliphatic alcohols
to acids†
Cite this: DOI: 10.1039/c3cc43042a
Received 24th April 2013,
Accepted 3rd June 2013
Min Wang, Feng Wang,* Jiping Ma, Chen Chen, Song Shi and Jie Xu*
DOI: 10.1039/c3cc43042a
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A
superhydrophobic catalyst was prepared by immobilizing ability can promote desorption of water and acids from the catalyst
Pt nanoparticles on superhydrophobic organic–inorganic hybrid surface; and (ii) the adhesion to less polar reactants, such as
silicas, which showed high activity and selectivity in the oxidation alcohols and aldehydes, can decrease mass transfer resistance on
of aliphatic alcohols to carboxylic acids.
active sites, increasing the activity. Several groups,6 including ours,7
have studied post-grafting or co-condensation methods to modify
Supported metal nanoparticle (MNP) catalysts constitute one of the the surface of (Ti, Ta, Co, etc.)-doped metal oxide catalysts, and their
greatly important and the most investigated catalysts in both academia applications in alkene epoxidation and hydrocarbon oxidation.
and industry.1 Substantial studies have focused on developing novel These preparation methods are experimentally simple, but inher-
synthetic methods to control metal size, morphology and anchoring ently suffer from inadequate hydrophobicity and/or that the active
methods.2 Recent discoveries have shown that catalyst supports sites are partially buried or poisoned during preparation. Unlike
could modify the electronic state of MNP and consequently influence previous studies, we herein report a novel strategy for constructing
catalysis.3 Limited to these studies, the other effects of catalyst metal nanoparticles on the robust superhydrophobic surface of
supports are greatly overlooked. For example, the wettability organic–inorganic hybrid silicas. Pt nanoparticles were localized on
(hydrophilicity–hydrophobicity) of supports can directly affect the silica through nitrogen–Pt interaction. The catalyst is highly reactive,
adsorption and desorption of the reactant and the product, elemen- selective and recyclable for oxidation of aliphatic alcohols to
tary steps in catalysis. Therefore, it is desirable to create a suitable carboxylic acids. Importantly we show that the surface wettability
microenvironment on the catalyst surface to facilitate mass transfer. of supports plays pivotal roles in tuning the catalytic performance.
Catalytic oxidation is a usual way of producing hydrocarbon
Phenyl-modified amine-bridged silica (Ph-AOS) was prepared using
oxygenates. A critical issue arises in the priority adsorption of the a water-in-oil reverse microemulsion method (Scheme S1, ESI†).8 The
polar products such as water and acids on the hydrophilic sites phenyl groups create a hydrophobic layer on the surface. The amine
(–OH, –O(H)–, –O–, defect sites, etc.) of supports by virtue of non- groups in the framework act as anchoring sites for Pt nanoparticles.
covalent bonding interaction.4 As a result, polar substance layers Transmission electron microscopy (TEM) images show that the
cover the catalyst surface, limiting reactant diffusion to active support materials with or without phenyl groups are monodispersed
sites and usually causing carbon species deposition on active nanospheres with diameter size of B90 nm (Fig. S1 and S2, ESI†).
sites. If the surface wettability can be tuned according to the
The material composition was analyzed using Fourier transform
character of reaction, better activity and selectivity can be infrared (FT-IR), and 29Si magic angle spinning nuclear magnetic
anticipated. However, the preparation of hydrophobic catalysts resonance (29Si MAS NMR) spectroscopy and 13C cross-polarization
with robustness and reusability still remains a challenging task. MAS NMR (13C CP/MAS NMR) spectroscopy (Fig. 1, Fig. S3 and S4,
We were inspired by recent advances in superhydrophobic ESI†). In the FT-IR spectra, both samples show C–H vibration bands
materials with the water contact angle (WCA) Z 1501.5 We thought of the bridging aliphatic carbon chain at 2930, 2883, 1460 and
that superhydrophobic materials can potentially be used as hetero- 1414 cmÀ1, Si–C vibration at 1119 cmÀ1 and the Si–O–Si band at
geneous catalysts once suitable catalytic active sites are created on 1032 cmÀ1.9 TheQCH stretching aromatic vibrations (weak peaks at
them because of the reasons that (i) their strong water repellent 3030–3080 cmÀ1) and mono-substituted benzene out-of-plane bend-
ing vibration (739 and 698 cmÀ1) in the spectrum of Ph-AOS confirm
the successful incorporation of phenyl groups.7c The 29Si MAS NMR
Dalian National Laboratory for Clean Energy, State Key Laboratory of Catalysis,
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023,
spectrum shows characteristic peaks of the organosiloxane network
P. R. China. E-mail: wangfeng@dicp.ac.cn, xujie@dicp.ac.cn;
[Tm = RSi(OSi)m(OH)3Àm, m = 1–3] (Fig. 1). The peaks at around À60
Fax: +86-411-84379762; Tel: +86-411-84379762
and À70 ppm are assigned to [T3, CSi(OSi)3] and [T2, CSi(OSi)2(OH)]
† Electronic supplementary information (ESI) available: Experimental details,
characterization data and supplementary materials. See DOI: 10.1039/c3cc43042a silicon connected with the bridging groups, respectively.9 The peak
c
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Fig. 2 The dispersion of (a–c) Pt/AOS and (d–f) Pt/Ph-AOS in water (lower phase)
and 1-butanol (a and d), butanal (b and e) and toluene (c and f) mixture (upper phase).
Fig. 1 29Si MAS-NMR spectra of (a) AOS and (b) Ph-AOS. R1 and R2 indicate the
bridging group and the phenyl group, respectively.
at À78 ppm in the Ph-AOS spectrum is attributed to the
[T3, CSi(OSi)3] silicon connected to phenyl groups. In the 13C
CP/MAS NMR spectrum of Ph-AOS, the peaks at 100–150 ppm and
below 100 ppm are ascribed to the aromatic carbon in phenyl groups
and the aliphatic carbon, respectively (Fig. S4, ESI†). Thermogravi-
metric analysis (TGA) and temperature programmed oxidation-mass
spectroscopy (TPO-MS) indicate that both samples are thermally
stable up to 285 1C, after which point the bridging organic groups
start to decompose (Fig. S5 and S6, ESI†). Another weight loss at
500 1C in Ph-AOS is caused by the decomposition of phenyl groups.7b
All these characterizations confirm that the prepared silica has the
expected structure. The presence of phenyl groups greatly changed
the surface hydrophilicity–hydrophobicity. Water contact angle
measurement indicates that the amine-bridged silica without phenyl
groups (AOS) is hydrophilic with a WCA of 201 (Fig. S1a, ESI,† the
inseted image), and Ph-AOS shows superhydrophobicity with WCA
up to 1501 (Fig. S1b, ESI,† the inseted image).
We employed the as-synthesized silica as a support for Pt nano-
particles. The Pt nanoparticles with diameter size of 2.48 and 2.51 nm
for Pt/AOS and Pt/Ph-AOS, respectively, were supported via the N–Pt
interaction (Fig. S7 and S8, ESI†). Pt nanoparticles are difficult to be
supported on silica without amine groups as anchoring sites (Fig. S9
and S10, ESI†). The Pt content of Pt/AOS and Pt/Ph-AOS was 2.02 wt%
and 1.94 wt%, respectively, as analyzed using inductively coupled
plasma (ICP). The X-ray photoelectron spectrum (XPS) of Ph-AOS
shows N1s (399.3 eV) and C1s (285.0 eV) peaks, indicating that
organic groups are present on the material surface (Fig. S11, ESI†).
The appearance of Pt4d and Pt4f peaks indicate that Pt particles are
localized on the surface. The in situ FT-IR spectrum of CO adsorption
on Pt/Ph-AOS shows a strong band at 2040 cmÀ1 and a weak band at
1825 cmÀ1, which are attributed to the linearly bonded CO on Pt
atoms and the bridged adsorption of CO on neighbouring Pt atoms,
respectively (Fig. S12, ESI†). Compared to the adsorption of CO on
metallic neutral Pt (2090–2100 cmÀ1), a large red shift observed on
Pt/Ph-AOS indicates d–p* electron donation from Pt to CO, indicating
that Pt is negatively charged.10 This further proves that nitrogen
donates electrons (p–d) to Pt particles. In a 1-butanol (or butanal–
toluene) and water mixed solvent, Pt/AOS is dispersed in the lower
water phase, and Pt/Ph-AOS is selectively suspended in the upper
1-butanol, butanal and toluene phase (Fig. 2), indicating the hydro-
phobic nature of Pt/Ph-AOS. Alcohols and aldehydes tend to adsorb
onto Pt/Ph-AOS, and water onto Pt/AOS.
Fig. 3 Catalytic activities of Pt/AOS and Pt/Ph-AOS in the oxidation of 1-butanol.
Reaction conditions: 0.2 mmol 1-butanol, 0.05 mmol mesitylene, 10 mg catalyst,
2 mL toluene, 1.0 MPa oxygen, 24 h.
most important and challenging reactions in organic synthesis.11 At
80 1C, the reaction over Pt/AOS was sluggish and only 19% conver-
sion of 1-butanol was obtained. The conversion increased with an
increase in temperature and reached 80% at 120 1C. For the same
reaction over Pt/Ph-AOS, the activity was greatly enhanced. The
conversion reached 80% at 80 1C which was nearly four times of
that obtained over Pt/AOS. A further increase in temperature to
90 1C completely converted 1-butanol. The main product over
Pt/AOS was butanal at 80 1C. Notably the selectivity of butanic acid
gradually increased with an increase in temperature. In comparison,
the reaction over Pt/Ph-AOS produced butanic acid as the main
product, independent of the reaction temperature investigated.
We adjusted the content of phenyl group to obtain silica with
differential hydrophobicity, as illustrated by WCA measurement.
The WCA increased with an increase in phenyl group contents
from 0, 1.3, 1.9, to 2.5 mmol gÀ1 (Fig. S13, ESI†).12 As expected,
the activity and selectivity of the butanic acid increased with
an increase in WCA (Fig. 4). This demonstrates that the surface
hydrophobicity is favorable for oxidation of 1-butanol to
butanic acid.
Fig. 4 The effect of hydrophobicity of the support on catalysis. Reaction condi-
We then test their catalytic performance in the selective oxida-
tions: 0.2 mmol 1-butanol, 0.05 mmol mesitylene, 10 mg catalyst with different
tion of aliphatic alcohols to acids (Fig. 3). The reaction is one of the hydrophobicity, 2 mL toluene, 1.0 MPa oxygen, 90 1C, 24 h.
c
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solution through centrifugation. After washing with ethanol and
drying, it could be repeatedly used at least three times without
apparent activity loss (Fig. S15, ESI†). The ICP mass analysis revealed
that the Pt content in the reaction solution was only 1.6 ppm,
indicating that the catalyst is heterogeneous. The TEM image of the
used catalyst shows that the morphology of Pt nanoparticles remains
the same as the fresh catalyst, suggesting that Pt particles are stable
on catalysts (Fig. S16, ESI†). This high stability is ascribed to a strong
binding function of amine groups on the hydrophobic surface.
In summary, we have reported a novel superhydrophobic
catalyst by supporting Pt nanoparticles on superhydrophobic
supports. This study makes us believe that careful tuning of the
support wettability to meet reaction requirements will open a
new avenue for heterogeneous catalysts.
Scheme 1 The process of 1-butanol oxidation over Pt/Ph-AOS.
We attribute the excellent catalytic activity of Pt/Ph-AOS to its
surface hydrophobicity (Scheme 1). The surface phenyl groups create
a hydrophobic microenvironment and the supported Pt nano-
particles function as active phases. The hydrophobic index (HI)
was 3.5 times that of Pt/AOS.13 This indicates that Pt/Ph-AOS is
much more hydrophobic than Pt/AOS. Less polar organic substrates,
such as 1-butanol and butanal, can be enriched on the surface of
Pt/Ph-AOS, which enable the Pt nanoparticles to have better acces-
sibility to organic molecules, thus resulting in higher conversion.
And, the intermediate of butanal can be more easily adsorbed and
oxidized over Pt/Ph-AOS than over Pt/AOS to butanic acid. As a result,
the reactions over two catalysts have different major products. Water
is obtained as the inevitable product during the oxidation reaction.
The highly polar acid product, together with water, could be expelled
from the hydrophobic surface by a reactant and a solvent, avoiding
further oxidation and increasing selectivity. In comparison, water
and acids prefer to adsorb on the hydrophilic surface of Pt/AOS,
which is disadvantageous to the adsorption on the substrate.
The reaction over Pt/Ph-AOS was little affected by adding water
as solvent. As seen in Fig. S14 (ESI†), the conversion of 1-butanol
decreased dramatically from 60% without adding water to 25%
when 5% water was added over Pt/AOS. In contrast, the activity of
Pt/Ph-AOS slightly decreased and high conversion of 1-butanol
was still maintained. These results reinforce the importance of
the superhydrophobic surface of Pt/Ph-AOS.
This work was supported by the National Natural Science
Foundation of China (Project 21073184, 21273231, 21233008,
21103175), and One Hundred Person Project of the Chinese
Academy of Sciences.
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Using Pt/Ph-AOS as a catalyst, selective oxidation of other aliphatic
alcohols was carried out (Table 1). Ph-AOS is not active in this reaction
(entry 1). The catalyst is chemoselective and preferentially oxidizes
primary hydroxyl over secondary hydroxyl groups. Aliphatic primary
alcohols were converted into acids with high activity (entries 2–8).
Secondary alcohols such as 2-octanol and cyclohexanol exhibited low
activity (entries 9 and 10), which may be ascribed to the steric
hindrance. The catalyst can be simply separated from the reaction
Table 1 Oxidation of various aliphatic alcohols with Pt/Ph-AOSa
Entry
Substrate
Conversion [%]
Selectivityb [%]
1c
2
3
4
5
6
7
8
9
1-Butanol
1-Butanol
Isobutyl alcohol
1-Pentanol
Isoamyl alcohol
1-Hexanol
2-Phenylethanol
1-Octanol
2-Octanol
—
98
82
96
89
89
69
85
3
—
95
95
92
89
95
79
93
>99
>99
10
Cyclohexanol
32
12 The surface areas of phenyl groups were calculated based on
that the phenyl groups were completely introduced into materials
during the preparation process and on the surface of the silica.
a
Reaction conditions: 0.2 mmol alcohol, 0.05 mmol mesitylene, 10 mg
b
Pt/Ph-AOS, 2 mL toluene, 1.0 MPa oxygen, 100 1C, 24 h. The main
products were the corresponding acids or ketones for secondary alcohols. 13 The hydrophobic index (HI) is defined as the ratio of the adsorbed
c
Others are aldehyde and esters. Ph-AOS was used as catalyst.
quantity of benzene vapor to that of water vapor. HI = Vbenzene/VWater.
c
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