catalysts
Article
Dehydrogenative Oxidation of Alcohols Catalyzed
by Highly Dispersed Ruthenium Incorporated
Titanium Oxide
Youngyong Kim 1 , Seokhoon Ahn 2 , Jun Yeon Hwang 2 , Doo-Hyun Ko 3, * and Ki-Young Kwon 1, *
1
2
3
*
Department of Chemistry, Gyeongsang National University and RINS, Jinju 660-701, Korea;
[email protected]
Institute of Advanced Composite Materials, Korea Institute of Science and Technology, Chudong-ro 92,
Bongdong-eup, Wanju-gun, Jeonbuk 565-950, Korea; [email protected] (S.A.);
[email protected] (J.Y.H.)
Department of Applied Chemistry, Kyung Hee University, 1 Seochun-Dong, Giheung-Gu, Yongin-Si,
Gyeonggi-Do 446-701, Korea
Correspondence: [email protected] (D.-H.K.); [email protected] (K.-Y.K.);
Tel.: +82-31-201-3844 (D.-H.K.); +82-55-772-1493 (K.-Y.K.)
Academic Editors: Albert Demonceau, Ileana Dragutan and Valerian Dragutan
Received: 30 October 2016; Accepted: 21 December 2016; Published: 28 December 2016
Abstract: Ruthenium incorporated titanium oxides (Rux TiO2 ) were prepared by a one-step
hydrothermal method using Ti(SO4 )2 and RuCl3 as the precursor of Ti and Ru, respectively. X-ray
diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM),
Energy-dispersive X-ray spectroscopy (EDS) mapping, and BET were applied for the analyses of
catalysts. Ruthenium atoms are well dispersed in the anatase phase of TiO2 and the crystallite size
of Rux TiO2 (≈17 nm) is smaller than that of pure TiO2 (≈45 nm). In particular, we found that our
homemade pure TiO2 exhibits a strong Lewis acid property. Therefore, the cooperation of ruthenium
atoms playing a role in the hydride elimination and the Lewis acid site of TiO2 can efficiently transfer
primary alcohols into corresponding aldehydes in an oxidant-free condition.
Keywords: ruthenium catalyst; titanium oxide; dehydrogenative oxidation
1. Introduction
The oxidation of alcohols into their carbonyl compounds by a cost effective and environmentally
benign means is important in synthetic chemistry. Particularly, many researchers have made
concentrated efforts on the development of catalytic oxidation reactions using molecular oxygen
(O2 ) as an oxidant instead of using stoichiometric amounts of expensive and harmful oxidizing
reagents [1–3]. Moreover, there have been lots of studies seeking the novel oxidation catalytic
systems of alcohols in which alcohol oxidation is accomplished by a dehydrogenation pathway under
oxidant-free conditions. In homogeneous catalytic systems, iridium complexes, osmium, or ruthenium
pincer-type complexes, and Grubbs’ catalyst were commonly applied for the dehydrogenative
oxidation [4–7]. Recently, nickel complexes were also reported [8]. In heterogeneous dehydrogenative
catalytic systems, mainly noble metals catalysts were reported: alumina-supported silver cluster,
hydrotalcite supported silver nanoparticles, ceria supported gold nanoparticles, alumina-supported Pt
nanoparticles, alumina-supported Re nanoparticles [9–13]. Particularly, high reactivity was reported
for the oxidation of aliphatic alcohols in anaerobic condition [12]. While aerobic oxidation generates
water as byproduct, anaerobic conditions produce hydrogen gas. Therefore, the dehydrogenative
oxidation prevents the deactivation of catalysts by water and is safely applied to substrates containing
water sensitive functional groups.
Catalysts 2017, 7, 7; doi:10.3390/catal7010007
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Nanocrystalline TiO2 has been intensively investigated in a wide range research fields due to its
practical applications, such as photocatalysts, solar cells, materials for water or air purification, and so
on [14–17]. It is the chemical, physical, and photo-stability of TiO2 that all applications mentioned
above make possible. Therefore, besides the catalytic properties of TiO2 , it can be a suitable material for
supporting catalytic active centers in heterogeneous catalysts. Particularly, TiO2 supported ruthenium
catalysts were utilized for diverse oxidation reactions including aerobic oxidation [18,19], catalytic
wet air oxidation of various chemicals [20–22], and electro-oxidation of alcohols [23]. However, to the
best of our knowledge, ruthenium incorporated TiO2 has not been applied to the oxidation of alcohols
under oxidant-free conditions. Here we report the dehydrogenative oxidation of benzylic, allylic,
and aliphatic alcohols into their corresponding aldehydes using ruthenium incorporated TiO2 under
oxidant-free conditions. We found that benzylic and allylic alcohols exhibited enhanced catalytic
activity compared to aliphatic alcohol. Particularly, our homemade pure TiO2 is a strong Lewis
acid catalyst enough to form the Lewis acid-catalyzed Friedel-Craft alkylation products in a benzyl
alcohol reaction. Therefore, the dehydrogenative oxidation pathway is accomplished by the synergistic
catalysis of the Lewis acidic TiO2 surface and the highly dispersed ruthenium species acting as the
hydride elimination of α-carbon.
2. Results and Discussion
2.1. Catalyst Characterization
Figure 1 shows the crystal phases of ruthenium incorporated titanium oxides (Rux TiO2 ,
x represents the molar ratio of Ru with respect to Ti) obtained by hydrolysis of Ti(SO4 )2 in aqueous
solution with the different amount of RuCl3 . The pure TiO2 and Rux TiO2 represent the anatase phase
of TiO2 (JCPDS data file No. 00-21-1272). It was reported that TiO2 prepared by a hydrothermal
method using Ti(SO4 )2 as a precursor exhibited the anatase phase [24]. The reflection peaks originated
from RuO2 are not presented for all the samples obtained from a hydrothermal condition (160 ◦ C).
After annealing the Ru0.07 TiO2 sample (A-Ru0.07 TiO2 ) at 700 ◦ C for six hours, we find the appearance of
Bragg’s reflection of RuO2 (JCPDS file No. 00-040-1290) and phase transformation of TiO2 from anatase
to rutile. This result suggests that ruthenium atoms are randomly incorporated into the crystallite
of TiO2 before annealing. However, the phase segregation of TiO2 (rutile) and RuO2 takes place at
an elevated temperature. The crystallite sizes of samples in Table 1 was calculated from X-ray line
broadening by the Scherrer equation (D = 0.89λ/(βcos θ)), where D, λ, β, and θ represent the average
crystal size, the Cu Kα wavelength (0.15406 nm), the full-width at half-maximum, and the diffraction
angle, respectively. The calculated size of the pure TiO2 is 26.4 nm which is bigger than that of Rux TiO2
(13–15 nm). The surface area of the pure TiO2 obtained by the BET method is approximately two-times
smaller than that of Rux TiO2 (Table 1).
Table 1. The physical properties of the samples.
Samples
Pure TiO2
Ru0.01 TiO2
Ru0.03 TiO2
Ru0.05 TiO2
Ru0.07 TiO2
Size (nm)
Surface Area
Ru Content
(µmol/g)
9
24
37
51
XRD
TEM
(m2 /g)
26.4
13.2
13.3
13.4
14.7
45.7 ± 5.3
16.7 ± 0.9
16.9 ± 1.0
17.0 ± 1.4
17.3 ± 1.1
48
104
111
125
126
Catalysts 2017, 7, 7
Catalysts 2017, 7, 7 Catalysts 2017, 7, 7 3 of 11
3 of 11 3 of 11 Figure 1. XRD patterns of synthesized Ru
TiO22. . .
Figure 1. XRD patterns of synthesized Ru
Figure
1. XRD patterns of synthesized RuxxxTiO
TiO
2
The elemental surface analyses of samples were done by X‐ray photoelectron spectroscopy (XPS). The elemental surface analyses of samples were done by X‐ray photoelectron spectroscopy (XPS). The elemental surface analyses of samples were done by X-ray photoelectron spectroscopy (XPS).
A survey spectrum verify verify the the presence presence of of titanium, titanium, oxygen, oxygen, ruthenium, ruthenium, and and carbon carbon contaminant contaminant A survey spectrum A survey spectrum verify the presence of titanium, oxygen, ruthenium, and carbon contaminant
without other species (Figure S1). The Ru 3d XPS spectra of Ru
0.07
TiO
2
is shown in Figure 2. It It is is without other species (Figure S1). The Ru 3d XPS spectra of Ru0.07TiO2 is shown in Figure 2. without other species (Figure S1). The Ru 3d XPS spectra of Ru0.07 TiO2 is shown in Figure 2. It is
mainly composed of two parts which corresponds to the Ru 3d5/2
5/2 (280.7 eV) and Ru 3d
(280.7 eV) and Ru 3d3/2
3/2 (284.6 eV) (284.6 eV) mainly composed of two parts which corresponds to the Ru 3d
mainly composed of two parts which corresponds to the Ru 3d5/2 (280.7 eV) and Ru 3d3/2 (284.6 eV)
band originated from spin‐orbital coupling. Ru
0.07
TiO
2
was synthesized under highly acidic aqueous band originated from spin‐orbital coupling. Ru0.07TiO2 was synthesized under highly acidic aqueous band originated from spin-orbital coupling. Ru0.07 TiO2 was synthesized under highly acidic aqueous
conditions because because no no base base was was added added in in the the hydrolysis hydrolysis of of Ti(SO
Ti(SO44))22. . Therefore, Therefore, we we attributed attributed the the conditions conditions because no base was added in the hydrolysis of Ti(SO4 )2 . Therefore, we attributed
binding energy of 3d
5/2
(280.7 eV) to the Ru species with +4 oxidation state considering the above binding energy of 3d5/2 (280.7 eV) to the Ru species with +4 oxidation state considering the above the binding energy of 3d5/2 (280.7 eV) to the Ru species with +4 oxidation state considering the
mentioned synthetic condition and previous literature data [25–27]. It was known that the Ru 3d3/2
3/2 mentioned synthetic condition and previous literature data [25–27]. It was known that the Ru 3d
above mentioned synthetic condition and previous literature data [25–27]. It was known that the Ru
band overlaps with carbon 1s band and therefore, the Ru 3d5/2
5/2 band is used for quantitative analysis band is used for quantitative analysis band overlaps with carbon 1s band and therefore, the Ru 3d
3d3/2 band overlaps with carbon 1s band and therefore, the Ru 3d5/2 band is used for quantitative
[23,28]. The amount of Ru in Ru
0.07
TiO
2
is 0.58 atomic percent and the ratio of Ru to Ti is 0.026 which [23,28]. The amount of Ru in Ru0.07TiO2 is 0.58 atomic percent and the ratio of Ru to Ti is 0.026 which analysis [23,28]. The amount of Ru in Ru0.07 TiO2 is 0.58 atomic percent and the ratio of Ru to Ti is 0.026
is smaller than the added molar ratio (0.07) (In quantitative analysis, the relative sensitivity factor is smaller than the added molar ratio (0.07) (In quantitative analysis, the relative sensitivity factor which is smaller than the added molar ratio (0.07) (In quantitative analysis, the relative sensitivity
(RSF) of Ti 2p and Ru 3d5/2
5/2 is 2.077 and 2.705, respectively.). The XPS peak intensities of 3d Ru of other is 2.077 and 2.705, respectively.). The XPS peak intensities of 3d Ru of other (RSF) of Ti 2p and Ru 3d
factor (RSF) of Ti 2p and Ru 3d5/2 is 2.077 and 2.705, respectively.). The XPS peak intensities of 3d Ru
Ru
x
HAP are too weak to analyze quantitatively. RuxHAP are too weak to analyze quantitatively. of other Rux HAP are too weak to analyze quantitatively.
Figure 2. Ru (3d) X‐ray photoelectron spectroscopy (XPS) spectrum of the Ru
0.07TiO
TiO222. . .
Figure
2. Ru (3d) X-ray photoelectron spectroscopy (XPS) spectrum of the Ru0.07
Figure 2. Ru (3d) X‐ray photoelectron spectroscopy (XPS) spectrum of the Ru
0.07
The morphology of samples was characterized by transmission electron microscopy (Figure 3). The morphology of samples was characterized by transmission electron microscopy (Figure 3). The
morphology of samples was characterized by transmission electron microscopy (Figure 3).
The pure TiO
is similar to corn‐shape (Figure 3a). The average size of pure TiO
is 45.7 ± 5.3 nm, The pure TiO
The pure TiO222 is similar to corn‐shape (Figure 3a). The average size of pure TiO
is similar to corn-shape (Figure 3a). The average size of pure TiO222 is 45.7 ± 5.3 nm, is 45.7 ± 5.3 nm,
standard deviation. The shape of Ru
xTiO
TiO
2 is similar to pure TiO
is similar to pure TiO
2, however, the average size is smaller, , however, the average size is smaller, standard deviation. The shape of Ru
x
2
2
standard deviation. The shape of Rux TiO2 is similar to pure TiO2 , however, the average size is
approximately 16–27 16–27 nm nm (Figure (Figure 3b,e). 3b,e). This This result result agrees agrees with with the the crystallite crystallite size size calculated calculated by by approximately Scherrer’s equation based on the full width at half maximum (FWHM) of the (101) facet of anatase. It Scherrer’s equation based on the full width at half maximum (FWHM) of the (101) facet of anatase. It Catalysts 2017, 7, 7
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smaller, approximately 16–27 nm (Figure 3b,e). This result agrees with the crystallite size calculated by
Catalysts 2017, 7, 7 4 of 11 Scherrer’s
equation based on the full width at half maximum (FWHM) of the (101) facet of
anatase.
+
2+
3+
It was reported that the addition of metal ions—such as+ Na2+ , Zn , 3+and Fe —influence the crystal
was reported that the addition of metal ions—such as Na , Zn , and Fe —influence the crystal phase phase resulting in a decrease of the crystal size of TiO2 [29–32]. We believe that incorporation of
resulting in a decrease of the crystal size of TiO2 [29–32]. We believe that incorporation of Ru in the Ru inTiO
the
TiO2 lattice
lattice deformation
which
mightthe inhibit
thegrowth crystals
of TiO2 .
2 lattice causes causes
lattice deformation which might inhibit crystals of growth
TiO2. After ◦
Afterannealing the Ru
annealing the Ru
0.072TiO
2 at 700 C, several crystallites aggregate with each other to be the size
0.07TiO
at 700 °C, several crystallites aggregate with each other to be the size of a few of a few
hundred
nanometers
(Figure 3f and Figure S2) with rutile phase. Particularly, dark spots
hundred nanometers (Figure 3f and Figure S2) with rutile phase. Particularly, dark spots (red circle in (red circle
in Figure 3f) are embedded in the crystallite. These dark spots seem
to be RuO2 crystallites
Figure 3f) are embedded in the crystallite. These dark spots seem to be RuO
2 crystallites observed in XRD data. observed
in XRD data.
(a) (b)
(c) (d)
(e) (f)
Figure 3. TEM image of RuxTiO2, (a) TiO2; (b) Ru0.01TiO2; (c) Ru0.03TiO2; (d) Ru0.05TiO2; (e) Ru0.07TiO2; Figure 3. TEM image of Rux TiO2 , (a) TiO2 ; (b) Ru0.01 TiO2 ; (c) Ru0.03 TiO2 ; (d) Ru0.05 TiO2 ; (e) Ru0.07 TiO2 ;
and (f) A‐Ru0.07TiO2. Red circles: RuO2. and (f) A-Ru0.07 TiO2 . Red circles: RuO2 .
The elemental mapping of Ru0.07TiO2 was investigated by energy dispersive X‐ray spectroscopy (Figure 4). Individual mapping of TiO
Ti, O, and investigated
Ru is shown by
Figures 4b–d, respectively. 4e The elemental
mapping
of Ru0.07
energy
dispersive
X-rayFigure spectroscopy
2 was
represents the elemental mapping with Ti, O, and Ru overlaid. We found that Ru is homogeneously (Figure 4). Individual mapping of Ti, O, and Ru is shown Figure 4b–d, respectively. Figure 4e represents
dispersed within TiO
2. This result is in good accordance with the previous XRD data of samples in the elemental
mapping with
Ti, O, and Ru overlaid. We found that Ru is homogeneously dispersed
which the reflection peak from RuO2 does not observed before annealing Ru0.07TiO2. In addition, high within TiO2 . This result is in good accordance with the previous XRD data of samples in which the
reflection peak from RuO2 does not observed before annealing Ru0.07 TiO2 . In addition, high resolution
Catalysts 2017, 7, 7
Catalysts 2017, 7, 7 5 of 11
5 of 11 resolution TEM image also showed no aggregation of Ru species (Figure 4f). Therefore, combining XRD TEM
image also showed no aggregation of Ru species (Figure 4f). Therefore, combining XRD and TEM
and TEM data, we conclude that ruthenium species are highly dispersed in TiO
2 in monomeric form. data, we conclude that ruthenium species are highly dispersed in TiO2 in monomeric
form.
(a) (b)
(c) (d)
(e) (f)
Figure 4. TEM image and Energy‐dispersive X‐ray spectroscopy (EDS) element mapping of Ru0.07TiO2. Figure 4. TEM image and Energy-dispersive X-ray spectroscopy (EDS) element mapping of Ru0.07 TiO2 .
(a) TEM image of Ru0.07TiO2; (b) Ti mapping; (c) O mapping; (d) Ru mapping; (e) overlapping of Ti, (a) TEM image of Ru0.07 TiO2 ; (b) Ti mapping; (c) O mapping; (d) Ru mapping; (e) overlapping of Ti, O,
O, and Ru; (f) high resolution TEM image of Ru0.07TiO2 (insert: interlayer spacing of d(101) = 0.35 nm). and Ru; (f) high resolution TEM image of Ru0.07 TiO2 (insert: interlayer spacing of d(101) = 0.35 nm).
2.2. Catalytic Tests 2.2. Catalytic Tests
Catalytic properties were tested by the oxidation reaction of alcohols (benzyl alcohol, cinnamyl Catalytic properties were tested by the oxidation reaction of alcohols (benzyl alcohol, cinnamyl
alcohol, and 1‐octanol) under oxidant‐free condition (Table 2). The entries 1–10 showed the oxidative alcohol,
andactivities 1-octanol)
condition
2). The
entries
1–10 showed
oxidative
catalytic of under
benzyl oxidant-free
alcohol. Compared to (Table
the short reaction period (30 min) the
under O2 catalytic
activities
of
benzyl
alcohol.
Compared
to
the
short
reaction
period
(30
min)
under
atmosphere (Table 3), reaction time was adjusted to 3 h in N2 environment. The conversion was less O2
atmosphere
(Table 3), reaction time was adjusted to 3 h in N2 environment. The conversion was less
than 2% in the absence of catalyst or in the presence of Degussa, P25 (entries 1 and 2). However, the than 2% in the absence of catalyst or in the presence of Degussa, P25 (entries 1 and 2). However,
Catalysts 2017, 7, 7
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the conversion of benzyl alcohol with homemade pure TiO2 (entry 3) was 49% which is high
compared
to the commercial Degussa P25 (Degussa, Frankfurt, Germany) (entry 2). 6 of 11 Importantly,
Catalysts 2017, 7, 7 6 of 11 Catalysts 2017, 7, 7 Catalysts 2017, 7, 7 6 of 11 Catalysts 2017, 7, 7 6 of 11 the
conversion
into benzaldehyde is only 2%. Instead, o, m, p-benzyltoluenes (the molar
ratio of
Catalysts 2017, 7, 7 6 of 11 Catalysts 2017, 7, 7 6 of 11 ortho/meta/para
= 43:6:51) and dibenzyl ether were
as the major products. These o, m,
conversion of benzyl alcohol with homemade pure TiO
2produced
(entry 3) was 49% which is high compared conversion of benzyl alcohol with homemade pure TiO
2 (entry 3) was 49% which is high compared conversion of benzyl alcohol with homemade pure TiO
2 (entry 3) was 49% which is high compared conversion of benzyl alcohol with homemade pure TiO
2 (entry 3) was 49% which is high compared conversion of benzyl alcohol with homemade pure TiO
2 (entry 3) was 49% which is high compared conversion of benzyl alcohol with homemade pure TiO
2 (entry 3) was 49% which is high compared to the commercial Degussa P25 (Degussa, Frankfurt, Germany) (entry 2). Importantly, the conversion to the commercial Degussa P25 (Degussa, Frankfurt, Germany) (entry 2). Importantly, the conversion p-benzyltoluenes
were
produced
by
the
acid
catalyzed
Friedel-Crafts alkylation of toluene (solvent)
to the commercial Degussa P25 (Degussa, Frankfurt, Germany) (entry 2). Importantly, the conversion to the commercial Degussa P25 (Degussa, Frankfurt, Germany) (entry 2). Importantly, the conversion to the commercial Degussa P25 (Degussa, Frankfurt, Germany) (entry 2). Importantly, the conversion to the commercial Degussa P25 (Degussa, Frankfurt, Germany) (entry 2). Importantly, the conversion into benzaldehyde is only 2%. Instead, o, m, p‐benzyltoluenes (the molar ratio of ortho/meta/para = into benzaldehyde is only 2%. Instead, o, m, p‐benzyltoluenes (the molar ratio of ortho/meta/para = with
benzyl alcohol (Figure S3). Dibenzyl ether is produced by the homo-coupling of benzyl alcohol.
into benzaldehyde is only 2%. Instead, o, m, p‐benzyltoluenes (the molar ratio of ortho/meta/para = into benzaldehyde is only 2%. Instead, o, m, p‐benzyltoluenes (the molar ratio of ortho/meta/para = into benzaldehyde is only 2%. Instead, o, m, p‐benzyltoluenes (the molar ratio of ortho/meta/para = into benzaldehyde is only 2%. Instead, o, m, p‐benzyltoluenes (the molar ratio of ortho/meta/para = 43:6:51) and dibenzyl ether were produced as the major products. These o, m, p‐benzyltoluenes were 43:6:51) and dibenzyl ether were produced as the major products. These o, m, p‐benzyltoluenes were Both
reactions are proceeded by the same intermediate that is benzylic carbocation as indicated in
43:6:51) and dibenzyl ether were produced as the major products. These o, m, p‐benzyltoluenes were 43:6:51) and dibenzyl ether were produced as the major products. These o, m, p‐benzyltoluenes were 43:6:51) and dibenzyl ether were produced as the major products. These o, m, p‐benzyltoluenes were 43:6:51) and dibenzyl ether were produced as the major products. These o, m, p‐benzyltoluenes were produced acid catalyzed Friedel‐Crafts alkylation toluene (solvent) with benzyl alcohol produced by by the the acid catalyzed Friedel‐Crafts alkylation of of toluene (solvent) with benzyl alcohol produced the acid catalyzed Friedel‐Crafts alkylation toluene (solvent) with benzyl alcohol It was
the
Scheme
1.by Therefore,
the surface
of homemade
pure
TiO
as a Lewis
acid
catalyst.
produced by the acid catalyzed Friedel‐Crafts alkylation of of toluene (solvent) with benzyl alcohol 2 acts
produced acid catalyzed Friedel‐Crafts alkylation toluene (solvent) with benzyl alcohol produced by by the the acid catalyzed Friedel‐Crafts alkylation of of toluene (solvent) with benzyl alcohol (Figure S3). Dibenzyl ether is produced by the homo‐coupling of benzyl alcohol. Both reactions are (Figure S3). Dibenzyl ether is produced by the homo‐coupling of benzyl alcohol. Both reactions are (Figure S3). Dibenzyl ether is produced by the homo‐coupling of benzyl alcohol. Both reactions are (Figure S3). Dibenzyl ether is produced by the homo‐coupling of benzyl alcohol. Both reactions are reported
that
TiO
and
sulfated
TiO
are
applied
to
Friedel-Crafts
acylation
or
Beckman
rearrangement
2
2
(Figure S3). Dibenzyl ether is produced by the homo‐coupling of benzyl alcohol. Both reactions are (Figure S3). Dibenzyl ether is produced by the homo‐coupling of benzyl alcohol. Both reactions are proceeded same intermediate that benzylic carbocation indicated Scheme proceeded by by the the same intermediate that is is benzylic carbocation as as indicated in in the the Scheme 1. 1. proceeded the same intermediate that is benzylic carbocation indicated the Scheme 1. major
proceeded by by the same intermediate that is acid
benzylic carbocation as as indicated in in the Scheme 1. inTherefore, the surface of homemade pure TiO
which
these
solid
catalysts
act as Lewis
catalysts
[33,34].
In
the case
Ru
the
x TiO
2 , 1. proceeded the same intermediate that is benzylic carbocation indicated in the Scheme proceeded by by the same intermediate that is 2 acts as a Lewis acid catalyst. It was reported that TiO
benzylic carbocation as as indicated in of
the Scheme Therefore, the surface of homemade pure TiO
2 acts as a Lewis acid catalyst. It was reported that TiO
2 2 1. Therefore, the surface of homemade pure TiO
2 acts as a Lewis acid catalyst. It was reported that TiO
Therefore, the surface of homemade pure TiO
2 acts as a Lewis acid catalyst. It was reported that TiO
2 2 product
is benzaldehyde,
however, still approximately
7% of benzyl toluene and benzyl ether
are
Therefore, the surface of homemade pure TiO
2 acts as a Lewis acid catalyst. It was reported that TiO
Therefore, the surface of homemade pure TiO
2 acts as a Lewis acid catalyst. It was reported that TiO
2 2 and sulfated TiO
2 are applied to Friedel‐Crafts acylation or Beckman rearrangement in which these and sulfated TiO
2 are applied to Friedel‐Crafts acylation or Beckman rearrangement in which these and sulfated TiO
2 are applied to Friedel‐Crafts acylation or Beckman rearrangement in which these and sulfated TiO
2 are applied to Friedel‐Crafts acylation or Beckman rearrangement in which these and sulfated TiO
2
are applied to Friedel‐Crafts acylation or Beckman rearrangement in which these remained
(entries
4–9).
As
the
Ru
content
increases,
the
catalytic
activity
increases
(entries
4–7
in
and sulfated TiO
2
are applied to Friedel‐Crafts acylation or Beckman rearrangement in which these solid catalysts act as Lewis acid catalysts [33,34]. In the case of Ru
x
TiO
2
, the major product is solid catalysts act as Lewis acid catalysts [33,34]. In the case of RuxTiO2, the major product is solid catalysts Lewis acid catalysts [33,34]. case xTiO
2, the major product solid catalysts act act as as Lewis acid catalysts [33,34]. In In the the case of of RuRu
xTiO
2, the major product is is solid catalysts act as Lewis acid catalysts [33,34]. In the case of Ru
x
TiO
2
, the major product is solid catalysts act as Lewis acid catalysts [33,34]. In the case of Ru
x
TiO
2
, the major product is benzaldehyde, however, still approximately 7% of benzyl toluene and benzyl ether are remained Table
2).
For
example,
Ru
TiO
exhibits
98%
conversion
with
93%
selectivity
within
3
h
(entry
7).
benzaldehyde, however, still approximately 7% of benzyl toluene and benzyl ether are remained 0.07
2
benzaldehyde, however, still approximately benzyl toluene and benzyl ether remained benzaldehyde, however, still approximately 7% 7% of of benzyl toluene and benzyl ether are are remained benzaldehyde, however, still approximately of benzyl toluene and benzyl ether remained benzaldehyde, however, still approximately 7% 7% of benzyl toluene and benzyl ether are are remained (entries 4–9). As the Ru content increases, the catalytic activity increases (entries 4–7 in Table 2). For In(entries 4–9). As the Ru content increases, the catalytic activity increases (entries 4–7 in Table 2). For addition,
Ru
TiO
exhibited
the
catalytic
activity
to
be
retained
without
significant
loss for up
0.07
2
(entries 4–9). As the Ru content increases, the catalytic activity increases (entries 4–7 in Table 2). For (entries 4–9). As the Ru content increases, the catalytic activity increases (entries 4–7 in Table 2). For (entries 4–9). As the Ru content increases, the catalytic activity increases (entries 4–7 in Table 2). For (entries 4–9). As the Ru content increases, the catalytic activity increases (entries 4–7 in Table 2). For ◦ C, the catalytic activity
example, Ru
0.07TiO
2 exhibits 98% conversion with 93% selectivity within 3 h (entry 7). In addition, 0.07TiO
2 exhibits 98% conversion with 93% selectivity within 3 h (entry 7). In addition, toexample, Ru
three
cycles
(entries
7–9).
However,
after
annealing
Ru
TiO
at
700
0.07
2
example, Ru
0.07TiO
2 exhibits 98% conversion with 93% selectivity within 3 h (entry 7). In addition, example, Ru
0.07TiO
2 exhibits 98% conversion with 93% selectivity within 3 h (entry 7). In addition, example, Ru
0.07TiO
2 exhibits 98% conversion with 93% selectivity within 3 h (entry 7). In addition, example, Ru
0.07TiO
2 exhibits 98% conversion with 93% selectivity within 3 h (entry 7). In addition, Ru
0.07TiO
2 exhibited the catalytic activity to be retained without significant loss for up to three cycles Ru
0.07TiO
2 exhibited the catalytic activity to be retained without significant loss for up to three cycles decreased
(entry 10), and the Lewis acid catalyzed products (o, m, p-benzyltoluenes and
0.07TiO
2 exhibited the catalytic activity to be retained without significant loss for up to three cycles RuRu
0.07TiO
2dramatically
exhibited the catalytic activity to be retained without significant loss for up to three cycles Ru
0.07TiO
2 exhibited the catalytic activity to be retained without significant loss for up to three cycles Ru(entries 0.07TiO
2 exhibited the catalytic activity to be retained without significant loss for up to three cycles 7–9). However, after annealing 0.07TiO
2 at 700 catalytic activity decreased (entries 7–9). However, after annealing RuRu
0.07TiO
2 at 700 °C, °C, the the catalytic activity decreased (entries 7–9). However, after annealing 0.07TiO
22
at 700 °C, catalytic activity decreased dibenzyl
ether)
were
not after produced
by A-Ru
TiO
in
the
same
reaction
conditions.
Based on the
(entries 7–9). However, annealing RuRu
0.07
TiO
2 at 700 °C, the the catalytic activity decreased 0.07
(entries 7–9). However, after annealing 0.07TiO
2 at 700 catalytic activity decreased (entries 7–9). However, after annealing RuRu
0.07TiO
2 at 700 °C, °C, the the catalytic activity decreased dramatically (entry 10), and the Lewis acid catalyzed products (o, m, p‐benzyltoluenes and dibenzyl dramatically (entry 10), and the Lewis acid catalyzed products (o, m, p‐benzyltoluenes and dibenzyl dramatically (entry 10), and the Lewis acid catalyzed products (o, m, p‐benzyltoluenes and dibenzyl dramatically (entry 10), and the Lewis acid catalyzed products (o, m, p‐benzyltoluenes and dibenzyl previous
mechanistic
studies
using
the
single
crystal
of
TiO
(typically
rutile
(110)
surface)
in vacuum
2 conditions. dramatically (entry 10), and the Lewis acid catalyzed products (o, m, p‐benzyltoluenes and dibenzyl dramatically (entry 10), and the Lewis acid catalyzed products (o, m, p‐benzyltoluenes and dibenzyl ether) were not produced A‐Ru
0.07TiO
2 in same reaction conditions. Based previous ether) were not produced by by A‐Ru
0.07TiO
2 in the the same reaction Based on on the the previous ether) were not produced by A‐Ru
0.07TiO
2 in the same reaction conditions. Based on the previous ether) were not produced by A‐Ru
0.07
TiO
2
in the same reaction conditions. Based on the previous conditions,
the
reactivity
of
alcohol
on
isof strongly
dependent
on
the
surface
defects
of TiO2 ,
2of the ether) were not produced by A‐Ru
0.07
TiO
2 in same reaction conditions. Based the previous ether) were not produced by A‐Ru
0.07TiO
2crystal TiO
in the same reaction conditions. Based on on the previous mechanistic studies using the single TiO
2 (typically rutile (110) surface) in vacuum mechanistic studies using the single crystal TiO
2 (typically rutile (110) surface) in vacuum mechanistic studies using the single crystal of TiO
2 (typically rutile (110) surface) in vacuum of Ru
mechanistic studies using the single crystal of TiO
2 (typically rutile (110) surface) in vacuum such
as
oxygen
vacant
sites
[35–37].
Besides
the
reduction
of
surface
area
and
the
aggregation
mechanistic studies using the single crystal of TiO
2
(typically rutile (110) surface) in vacuum mechanistic studies using the single crystal of TiO
2
(typically rutile (110) surface) in vacuum conditions, the reactivity of alcohol on TiO
2 is strongly dependent on the surface defects of TiO
2, such conditions, the reactivity of alcohol on TiO
2 is strongly dependent on the surface defects of TiO
2, such conditions, the reactivity of alcohol on TiO
2 is strongly dependent on the surface defects of TiO
2, such conditions, the reactivity of alcohol on TiO
2 is strongly dependent on the surface defects of TiO
2, such ◦ C might
conditions, the reactivity of alcohol on TiO
2 is strongly dependent on the surface defects of TiO
2, such into
RuO2 during annealing process, the 2decrease
of the surface defects after annealing at2, such 700
conditions, the reactivity of alcohol on TiO
is strongly dependent on the surface defects of TiO
as oxygen vacant sites [35–37]. Besides the reduction of surface area and the aggregation of Ru into as oxygen vacant sites [35–37]. Besides the reduction of surface area and the aggregation of Ru into as oxygen vacant sites [35–37]. Besides the reduction of surface area and the aggregation of Ru into as oxygen vacant sites [35–37]. Besides the reduction of surface area and the aggregation of Ru into as oxygen vacant sites [35–37]. Besides the reduction of surface area and the aggregation of Ru into as oxygen vacant sites [35–37]. Besides the reduction of surface area and the aggregation of Ru into RuO
2the
during annealing process, the decrease of the surface defects after annealing at 700 °C might result
low reactivity of A-Ru0.07 TiO2 .
RuO
2in
during annealing process, the decrease of the surface defects after annealing at 700 °C might RuO2 during annealing process, the decrease of the surface defects after annealing at 700 °C might RuO2 during annealing process, the decrease of the surface defects after annealing at 700 °C might RuO
2 during annealing process, the decrease of the surface defects after annealing at 700 °C might RuO
2 during annealing process, the decrease of the surface defects after annealing at 700 °C might result in the low reactivity of A‐Ru
0.07TiO
result in the low reactivity of A‐Ru
0.07TiO
2. 2. result in the low reactivity of A‐Ru
0.07TiO
result in the low reactivity of A‐Ru
0.07TiO
2. 2. Table
of
oxidant-free conditions
conditions 11..
result in the low reactivity of A‐Ru
0.07TiO
result in the low reactivity of A‐Ru
0.07TiO
2. Table 2.
2. Catalytic
Catalytic properties
properties
of2. alcohol
alcohol oxidation
oxidation under
under oxidant-free
1. 1. Table 2. Catalytic properties of alcohol oxidation under oxidant‐free conditions Table 2. Catalytic properties of alcohol oxidation under oxidant‐free conditions 1. 1. Table 2. Catalytic properties of alcohol oxidation under oxidant‐free conditions Table 2. Catalytic properties of alcohol oxidation under oxidant‐free conditions 1. 1. Table 2. Catalytic properties of alcohol oxidation under oxidant‐free conditions Table 2. Catalytic properties of alcohol oxidation under oxidant‐free conditions Entry Catalysts
Catalysts
Substrates
Products
to to
Aldehyde
(%)
(%) (%)
Entry
Substrates
Products Conversion
Conversion(%)
(%) Yield
Yield
Aldehyde
(%) Selectivity
Selectivity
(%) (%) Selectivity (%)
Entry Catalysts Catalysts Substrates Substrates Products
Products Conversion
Conversion
Yield to Aldehyde
Selectivity (%)
Entry (%)(%)Yield to Aldehyde
1 Entry None
None
0(%)
- (%) - Catalysts Substrates Substrates Products
Products Conversion
Conversion
(%)Yield to Aldehyde
Yield to Aldehyde
(%) Selectivity (%)
Selectivity (%)
Entry Catalysts 1 Entry 0
1 None Substrates 1 Entry None 0 0 (%)(%)Yield to Aldehyde
‐ ‐ ‐ ‐ Catalysts Substrates Products
Products Conversion
Conversion
Yield to Aldehyde
Selectivity (%)
Catalysts (%) (%) Selectivity (%)
None P25
60 60
1 1 Degussa,
None 0 0 2 2
‐ ‐ 1 1
‐ ‐ 2 2 2 P25
2 Degussa, P25 2 1 60 1 Degussa, P25 None 0 ‐ ‐ 1 Degussa,
None 0 2 ‐ 1 ‐ 60 Degussa, P25 2 2 pure
Degussa, P25 2 2 4949
1 1 1 1
60 60 pure
TiO
2
2 2
3 3 3 TiO
2 pure TiO
49 1 1 2 60 pure TiO
2 2 2 2 Degussa, P25 Degussa, P25 2 2 3 2 49 60 pure TiO
22 2 49 49 1 1 3434
2 2 3 3 pure TiO
Ru
0.01
TiO
4141
86 86
4 4 4 Ru0.01
TiO
0.01
Ru
0.01
TiO
22 22 41 34 86 Ru
2 TiO
49 1 2 3 pure TiO
49 41 1 34 2 86 3 4 pure TiO
0.01TiO
RuTiO
0.01
2 2 41 41 34 34 7272
86 86 93 93
4 4 Ru0.03
Ru
7979
5 5 5 Ru0.03
0.03
RuTiO
0.03
TiO
222 22 79 72 93 RuTiO
2 TiO
0.01
41 34 86 4 0.01
41 79 34 72 86 93 4 5 0.03TiO
79 72 5 Ru0.03
RuTiO
2 2 79 72 93 93 5 6 6 6 Ru0.05
0.05
RuTiO
Ru
0.05
8888
93 93
0.05
222 22 88 82 93 93 RuTiO
0.03
79 72 8282
5 2 TiO
0.03
TiO
79 88 72 82 5 6 0.05TiO
RuTiO
2 2 88 88 82 82 93 93 6 6 Ru0.05
7 7 7 Ru0.07
>99
0.07
RuTiO
0.07
TiO
222 22 >99 92 93 93 RuTiO
0.05
88 82 9292
6 Ru
0.07
>99
93 93
2 TiO
0.05
88 >99 82 92 6 7 0.07TiO
RuTiO
2 2 >99 >99 92 92 93 93 7 7 Ru0.07
8 8 8 7TiO
99
92
93
Reuse 7 99 92 93 Reuse 7 99 92 93 0.07
TiO
>99 7 Reuse
Ru
0.07
2 2 >99 7 8 Ru
Reuse
7
99
92
Reuse 7 8 8 Reuse 7 99 99 92 92 93 93 93
Reuse 8 9 9 9 8Reuse 7 98
Reuse 8 98 90 92 8 Reuse
99 92 9090
93 92 92
8 9 Reuse 7 99 98 92 90 93 92 Reuse
8
98
Reuse 8 9 9 Reuse 8 98 98 90 90 92 92 0.07
2 2
2 10 A‐Ru
0.07
TiO
2 2 2 98 2 90 99 10 10 TiO
2
99
9 A‐Ru
Reuse 8 92 9 A-Ru
Reuse 8 98 90 92 99 0.07
2 TiO
1010 0.07
0.07TiO
10 A-Ru
A‐Ru
0.07TiO
TiO
22 2 2 2 2
2 2 2
99 99 99
A‐Ru
11 None 0 ‐ ‐ 11 None 0 ‐ 2 ‐ 99 0.07TiO
2 10 A‐Ru
A‐Ru
0.07
TiO
2 2 2 2 99 10 11 1111 11 None
0
None None
- None 0 0 0
‐ ‐ ‐ ‐ Degussa, P25 12 12 3 3 99 11 Degussa, P25 None 0 ‐ ‐ None 0 3 ‐ 3 ‐ 99 12 1211 P25
12 Degussa,
Degussa, P25 12 Degussa,
Degussa, P25 3 3 3 3
3 3 3 3
99 99 99 99
P25
pure TiO
2 2 44 43 98 13 13 pure TiO
12 Degussa, P25 Degussa, P25 3 3 99 3 44 3 43 99 98 13 1312 purepure TiO
TiO
pure TiO
44 44 43 43 4343
98 98 98 98
13 13 2 22 2 TiO
4444
0.01TiO
Ru
0.01
TiO
22 22 78 78 >99 14 Ru
44 43 98 13 pure
pure TiO
44 78 43 78 98 >99 13 14 pure TiO
14 1414 14 Ru0.01
>99
0.01
RuTiO
0.01
2 2 78 78 78 78 7878
>99 >99 RuTiO
2 TiO
7878
0.03
RuTiO
0.03
TiO
222 22 85 84 >99 >99 >99
15 15 Ru0.01
0.01TiO
78 78 14 Ru
0.01
78 85 78 84 15 14 Ru0.03
>99
0.03
TiO
RuTiO
0.03
2 2 85 85 85
84 84 84
>99 >99 15 15 RuTiO
2 TiO
0.05
2
96 94 98 16 Ru
1516 Ru
0.03
TiO
2
85
84
>99
0.05
2
96 94 98 Ru
0.03 2
2
15 0.03TiO
85 85 84 84 >99 >99 0.05
TiO
RuTiO
16 15 Ru0.05
98
0.05
2 2 96 96 96
94 94 94
98 98 16 16 RuTiO
2
0.07
RuTiO
0.07
222 22 >99 98 99 Ru0.05
0.05TiO
96 94 94
98 98
16 Ru
1617 96
0.05
TiO
96 >99 94 98 98 99 16 17 0.07
RuTiO
17 17 17 Ru0.07
>99
99
0.07
2 2 >99 >99 98 98 98
99 99 RuTiO
2 TiO
18 None 0 ‐ ‐ 18 None 0 ‐ ‐ 0.07
TiO
2
>99 98 99 17 Ru
0.07TiO
TiO22 >99 >99
98 99 Ru0.07
1717 Ru
98
99
None 18 18 None 0 0 ‐ ‐ ‐ ‐ 18 1819 Degussa, P25 Degussa, P25 3 2 93 18 None
None 0 0
‐ ‐ 18 19 None 0 3 ‐ 2 ‐ 93 None
- Degussa, P25 19 19 Degussa, P25 3 3 0
2 2 93 93 20 pure TiO
19 20 Degussa,
P25 2 2 4 4 98 pure TiO
19 Degussa,
Degussa, P25 3 3 3
2 2 2
93 93 93
Degussa, P25 3 4 2 4 93 98 1919 P25
pure TiO
2 2 4 4 4 4 98 98 20 20 pure TiO
Ru
20 21 pureRu
TiO
4
98
0.01
TiO
22 22 12 12 >99 4 4
4 98 20 pure TiO
4 12 4 12 98 >99 20 21 pure TiO
20.01TiO
0.01TiO
Ru
2021 21 pure
TiO
TiO
2 2 12 12 4
12 12 4
>99 >99 98
Ru0.01
0.03
TiO
2
23 22 97 RuTiO
0.03
2
23 22 97 RuTiO
21 22 Ru0.01
12
12
>99
0.01
2
12 12 >99 21 0.01
2
12 12 >99 21 22 2
0.03
TiO
2
23 22 97 22 Ru
0.03TiO
TiO22 23 12
22 97 22 Ru0.01
>99 97
0.05
RuTiO
0.05
22 22 23 23 22 22 1222
>99 RuTiO
22 2123 Ru Ru
23
0.03
97 22 0.03
97 >99 22 23 2 TiO
0.05TiO
RuTiO
0.05
22 2 23 23 22 22 22
>99 >99 97
23 23 0.03
Ru0.03
Ru
2323
0.07
TiO
22 24 23 98 RuTiO
0.07
TiO
2
24 23 98 RuTiO
23 2224 Ru0.05
22
>99
0.05
23 22 >99 23 0.05
2
23 22 >99 23 24 2
0.07TiO
RuTiO
0.07
2 2 24 24 23 23 22
98 98 24 24 Ru
Ru0.05
TiO
2324
amount: mmol, 0.07
2 substrate 23 time: 24 Ru
mmol, 24 123
Ru0.07
TiO
233 h, 3 Conversion 98
1 Reaction 0.07
TiO
22 substrate 24 24 23 98 98 is >99
24 Ru
2 TiO
conditions: 0.04 catalyst amount: 5 mg, reaction time: h, Conversion is Reaction conditions: amount: 0.04 catalyst amount: 5 mg, reaction 1 Reaction conditions: substrate amount: 0.04 mmol, catalyst amount: 5 mg, reaction time: 3 h, Conversion is 1 Reaction conditions: 5 mg, reaction time: 3 h, Conversion is 98
Ru
0.07TiO2 substrate amount: 0.04 mmol, catalyst amount: 24amount: 23time: 1 Reaction 1 Reaction 124
conditions: substrate amount: 0.04 mmol, 5 mg, reaction 3 Conversion h, (Figure is is
conditions: substrate amount: 0.04 mmol, amount: 5 mg, time: 3 time:
h, is S4). calculated by an internal standard method. Yield to catalyst aldehyde is calculated by calibration curves calculated by an internal standard method. Yield to catalyst aldehyde is calculated by calibration curves S4). Reaction
conditions:
substrate
amount:
0.04
mmol,
catalyst
amount:
5reaction mg,
reaction
3Conversion h,(Figure Conversion
calculated internal standard method. Yield to aldehyde is calculated calibration curves (Figure calculated by by an an internal standard method. Yield to aldehyde is calculated by by calibration curves (Figure S4). S4). 1 Reaction
conditions:
substrate
amount:
0.04
mmol,
catalyst
amount:
5by mg,
reaction
time:
3 (Figure h, Conversion
calculated
byby anan internal
standard
method.
Yield
toaldehyde aldehyde
calculated
by
calibration
curves
(Figure
calculated internal standard method. Yield to is is
calculated by calibration curves calculated by an internal standard method. Yield to aldehyde is calculated calibration curves (Figure S4). S4). S4).is
Selectivity is calculated by the peak area of aldehyde divided by the total peak area of product. Selectivity is calculated by the peak area of aldehyde divided by the total peak area of product. Selectivity is calculated by the peak area of aldehyde divided by the total peak area of product. Selectivity is calculated by the peak area of aldehyde divided by the total peak area of product. Selectivity
calculated
the peak
area of aldehyde
divided by
total peak
of product.
Selectivity is calculated by the peak area of aldehyde divided by the total peak area of product. calculated
byisan
internal by
standard
method.
Yield to aldehyde
is the
calculated
byarea
calibration
curves (Figure S4).
Selectivity is calculated by the peak area of aldehyde divided by the total peak area of product. Table 3. Catalytic properties of alcohol oxidation using O
2 as an oxidant. Table 3. Catalytic properties of alcohol oxidation using O
2 as an oxidant. Selectivity is calculated
by the peak area of aldehyde divided by the total peak
area of product.
Table 3. Catalytic properties of alcohol oxidation using O
2 as an oxidant. Table 3. Catalytic properties of alcohol oxidation using O
2 as an oxidant. Table 3. Catalytic properties of alcohol oxidation using O
2 as an oxidant. Table 3. Catalytic properties of alcohol oxidation using O
2 as an oxidant. Table 3. Catalytic properties of alcohol oxidation using O2 as an oxidant.
23
24
1
Ru0.05TiO2
Ru0.07TiO2
23
24
22
23
>99
98
Reaction conditions: substrate amount: 0.04 mmol, catalyst amount: 5 mg, reaction time: 3 h, Conversion is
calculated by an internal standard method. Yield to aldehyde is calculated by calibration curves (Figure S4).
Selectivity is calculated by the peak area of aldehyde divided by the total peak area of product.
Catalysts 2017, 7, 7
Catalysts 2017, 7, 7 Catalysts 2017, 7, 7 Catalysts 2017, 7, 7 Table
3. Catalytic
of alcohol
alcohol oxidation
oxidation using
using O
O2 as
an oxidant.
Table 3.
Catalytic properties
properties of
2 as an oxidant.
7 of 11
7 of 11 7 of 11 7 of 11 Entry EntryCatalysts
Catalysts Substrates
Substrates
1
2
3
4
5
6
7
Products Conversion
Conversion
Selectivity
(%)
Products
(%) (%)
Selectivity
(%)
Entry Catalysts Substrates
Products
Conversion
Selectivity (%) Entry (%)(%)
Entry Catalysts Catalysts Substrates
Substrates Products
Products Conversion
Conversion
(%) Selectivity (%) Selectivity (%) 1
None
0
None 1 1 None 0 0 ‐ ‐ ‐ 1 None
None 0 0
2
Degussa, P25
3
>99
Degussa,
P25
Degussa, P25 >99 2 2 Degussa, P25 3 3 >99 2 Degussa, P25 3 3
>99 >99
3
pure TiO2
3
>99
TiO
>99 pure TiO
3 3 >99 3 3 pure TiO
3 3
>99 >99
3 pure
pure TiO
2 2 22 4
Ru0.01TiO2
38
>99
TiO
38
>99
0.01
TiO
2
38 >99 0.01
TiO
2
38 >99 4 4 RuRu
0.01
TiO
2
38 >99 4 Ru0.01
Ru
2
5
Ru0.03TiO2
77
>99
TiO
0.03
>99 0.03
TiO
2 22 77 77 >99 5 5 RuRu
0.03
TiO
77 77
>99 >99
5 Ru0.03
Ru
2TiO
6
Ru0.05TiO2
94
>99
TiO
0.05
>99 0.05
TiO
2 22 94 94 >99 6 6 RuRu
0.05TiO
TiO
94 94
>99 >99
6 Ru0.05
Ru
7
Ru0.072TiO2
>99
>99
0.07
>99 >99 0.07
TiO
2 22 >99 >99 7 7 RuRu
0.07
TiO
>99 >99
>99 >99
7 Ru0.07
Ru
TiO
2TiO
Scheme
1.
Lewis
acid
catalyzed
an electrophilic
aromatic
substitution
and
Scheme Lewis acid (TiO
22 surface) catalyzed electrophilic aromatic substitution and homo‐
Scheme 1. 1. Lewis acid (TiO
2 (TiO
surface) catalyzed an an electrophilic aromatic substitution and homo‐
Scheme 1. Lewis acid (TiO
surface) catalyzed an electrophilic aromatic substitution and homo‐
2 surface)
1
homo-coupling
of benzyl alcohol.
coupling of benzyl alcohol. coupling of benzyl alcohol. coupling of benzyl alcohol. xxTiO
successfully applied oxidation allylic alcohol (cinnamyl alcohol) into RuRu
xTiO
2 2can be be successfully applied oxidation of of allylic alcohol (cinnamyl alcohol) into Ru
TiO
2 can can be successfully applied oxidation of allylic alcohol (cinnamyl alcohol) into Ru
x TiO2 can be successfully applied oxidation of allylic alcohol (cinnamyl alcohol) into
corresponding a α,β‐unsaturated carbonyl compound (cinnamaldehyde) with high selectivity (>98%). corresponding a α,β‐unsaturated carbonyl compound (cinnamaldehyde) with high selectivity (>98%). corresponding a α,β‐unsaturated carbonyl compound (cinnamaldehyde) with high selectivity (>98%). corresponding
a α,β-unsaturated carbonyl compound (cinnamaldehyde) with high selectivity (>98%).
In the case of cinnamyl alcohol, the by‐products from the electrophilic aromatic substitution or acid In the case of cinnamyl alcohol, the by‐products from the electrophilic aromatic substitution or acid In the case of cinnamyl alcohol, the by‐products from the electrophilic aromatic substitution or acid In the case of cinnamyl alcohol, the by-products from the electrophilic aromatic substitution or acid
catalyzed homo‐coupling were not produced (entries 14–17 in Table 2). Similar benzyl alcohol catalyzed homo‐coupling were not produced (entries 14–17 in in
Table 2). Similar to to benzyl alcohol catalyzed homo‐coupling were not produced (entries 14–17 in Table 2). Similar to benzyl alcohol catalyzed
homo-coupling
were
not
produced
(entries
14–17
Table
2).
Similar
to
benzyl
alcohol
oxidation, homemade pure TiO
22 showed the high conversion compared to Degussa, P25 (entries 12 oxidation, homemade pure TiO
2 showed the high conversion compared to Degussa, P25 (entries 12 oxidation, homemade pure TiO
showed the high conversion compared to Degussa, P25 (entries 12 oxidation,
homemade pure TiO
2 showed the high conversion compared to Degussa, P25 (entries 12
and 13 in Table 2). In addition, pure TiO
22 exhibits high selectivity transformation into the and 13 in Table 2). In addition, TiO
2 exhibits high selectivity in in transformation into the and 13 in Table 2). In addition, pure TiO
exhibits high selectivity in transformation into the and 13 in Table 2). In addition,pure pure
TiO
2 exhibits high selectivity in transformation into the
cinnamaldehyde (entry Table 2). We want point out that the conversion alcohol into cinnamaldehyde (entry 13 in in Table We want to to point out that the conversion of of alcohol into cinnamaldehyde (entry 13 in Table 2). We want to point out that the conversion of alcohol into cinnamaldehyde
(entry
1313 in
Table
2).2). We
want
to point
out
that
the
conversion
of alcohol
into
aldehyde
aldehyde in our catalytic system is not a common phenomenon because the dehydration of various aldehyde in our catalytic system is not a common phenomenon because the dehydration of various aldehyde in our catalytic system is not a common phenomenon because the dehydration of various in our catalytic system is not a common phenomenon because the dehydration of various alcohols
alcohols into alkenes is a more general reaction on TiO
22 surface instead of dehydrogenation reaction alcohols into alkenes is a more general reaction on TiO
2 surface instead of dehydrogenation reaction alcohols into alkenes is a more general reaction on TiO
surface instead of dehydrogenation reaction into
alkenes is a more general reaction on TiO2 surface
instead of dehydrogenation reaction [35,38].
[35,38]. Unfortunately, the kinetics of primary aliphatic alcohol (1‐octanol) were not as efficient as [35,38]. Unfortunately, the kinetics of primary aliphatic alcohol (1‐octanol) were not as efficient as [35,38]. Unfortunately, the kinetics of primary aliphatic alcohol (1‐octanol) were not as efficient as Unfortunately,
the kinetics of primary aliphatic alcohol (1-octanol) were not as efficient as that of
that of benzyl alcohol and cinnamyl alcohol in the given conditions (entries 19–24 in Table 2). In the that of benzyl alcohol and cinnamyl alcohol in the given conditions (entries 19–24 in Table 2). In the that of benzyl alcohol and cinnamyl alcohol in the given conditions (entries 19–24 in Table 2). In the benzyl
alcohol and cinnamyl alcohol in the given conditions (entries 19–24 in Table 2). In the case
case of the oxidation of secondary alcohol (1‐phenyl ethanol), the major product is styrene instead of case of the oxidation of secondary alcohol (1‐phenyl ethanol), the major product is styrene instead of case of the oxidation of secondary alcohol (1‐phenyl ethanol), the major product is styrene instead of of
the oxidation of secondary alcohol (1-phenyl ethanol), the major product is styrene instead of
acetophenone, which indicates that secondary benzylic alcohols are prone to elimination under the acetophenone, which indicates that secondary benzylic alcohols are prone to elimination under the acetophenone, which indicates that secondary benzylic alcohols are prone to elimination under the acetophenone,
which indicates that secondary benzylic alcohols are prone to elimination under the
same experimental conditions (Table S1). same experimental conditions (Table S1). same experimental conditions (Table S1). same experimental conditions (Table S1).
In our initial experiment set‐up, the role of TiO
22 is catalytic supports to incorporate ruthenium In our initial experiment set‐up, the role of TiO
2 is catalytic supports to incorporate ruthenium In our initial experiment set‐up, the role of TiO
is catalytic supports to incorporate ruthenium In
our initial experiment set-up, the role of TiO
2 is catalytic supports to incorporate ruthenium
atoms that are catalytic active centers. However, our homemade TiO
22 itself significantly involves in atoms that are catalytic active centers. However, our homemade TiO
2 itself significantly involves in atoms that are catalytic active centers. However, our homemade TiO
itself significantly involves in atoms that are catalytic active centers. However, our homemade TiO
2 itself significantly involves in
the reaction (entries 3 and 13 in Table 2). For the conversion of alcohol into aldehyde, two hydrogen the reaction (entries 3 and 13 in Table 2). For the conversion of alcohol into aldehyde, two hydrogen the reaction (entries 3 and 13 in Table 2). For the conversion of alcohol into aldehyde, two hydrogen the reaction (entries 3 and 13 in Table 2). For the conversion of alcohol into aldehyde, two hydrogen
atoms (one is attached to oxygen, the other is attached to α‐carbon) are detached. A previous high atoms (one is attached to oxygen, the other is attached to α‐carbon) are detached. A previous high atoms (one is attached to oxygen, the other is attached to α‐carbon) are detached. A previous high atoms
(one is attached to oxygen, the other is attached to α-carbon) are detached. A previous
resolution STM study verified that adsorption process alcohol on TiO
22 surface induces the resolution STM study verified that adsorption process of of alcohol on TiO
2 surface induces the resolution STM study verified that adsorption process of alcohol on TiO
surface induces the high resolution
STM
study
verified
that
adsorption
process
of
alcohol
on
TiO
2 surface induces the
detachment of the hydroxyl hydrogen in which the regular surface Ti sites of TiO
22 enable alcohols to detachment of the hydroxyl hydrogen in which the regular surface Ti sites of TiO
2 enable alcohols to detachment of the hydroxyl hydrogen in which the regular surface Ti sites of TiO
enable alcohols to detachment of the hydroxyl hydrogen in which the regular surface Ti sites of TiO2 enable alcohols
adsorbed molecularly and dissociatively (deprotonated alkoxide) temperature [37]. be be adsorbed molecularly and dissociatively (deprotonated alkoxide) at at room temperature [37]. be adsorbed molecularly and dissociatively (deprotonated alkoxide) at room temperature [37]. to
be
adsorbed
molecularly
and
dissociatively
(deprotonated
alkoxide)
atroom room
temperature
[37].
Particularly, our homemade highly acidic surface of pure TiO
22 facilitates alcohol adsorption on the Particularly, our homemade highly acidic surface of pure TiO
2 facilitates alcohol adsorption on the Particularly, our homemade highly acidic surface of pure TiO
facilitates alcohol adsorption on the Particularly, our homemade highly acidic surface of pure TiO2 facilitates alcohol adsorption on the
surface to form deprotonated alkoxide species. The hydrogen elimination of the α‐carbon seems to surface to form deprotonated alkoxide species. The hydrogen elimination of the α‐carbon seems to surface to form deprotonated alkoxide species. The hydrogen elimination of the α‐carbon seems to surface
to form deprotonated alkoxide species. The hydrogen elimination of the α-carbon seems to
take place on exclusively ruthenium species (Figure 5). This interpretation is supported by a dramatic take place on exclusively ruthenium species (Figure 5). This interpretation is supported by a dramatic take place on exclusively ruthenium species (Figure 5). This interpretation is supported by a dramatic take place on exclusively ruthenium species (Figure 5). This interpretation is supported by a dramatic
change of major products from o, m, p‐benzyltoluenes to benzaldehyde when pure TiO
22 is replaced change of major products from o, m, p‐benzyltoluenes to benzaldehyde when pure TiO
2 is replaced change of major products from o, m, p‐benzyltoluenes to benzaldehyde when pure TiO
is replaced change
of major products from o, m, p-benzyltoluenes to benzaldehyde when pure TiO
2 is replaced
TiO
22 (entries Table Similarly, Yamaguchi proposed that alcohol RuRu
0.010.01
TiO
2 (entries 3 3 and 4 4 in Table 2). 2). Similarly, Yamaguchi et et al. al. proposed that in in an an alcohol Ru
0.01
TiO
(entries 3 and and 4 in in Table 2). Similarly, Yamaguchi et al. proposed that in an alcohol Ru
0.01 TiO2 (entries 3 and 4 in Table 2). Similarly, Yamaguchi et al. proposed that in an alcohol
oxidation reaction on Ru
xxTiO
22 using molecular oxygen as oxidant, the reaction was accomplished by oxidation reaction on Ru
xTiO
2 using molecular oxygen as oxidant, the reaction was accomplished by oxidation reaction on Ru
TiO
using molecular oxygen as oxidant, the reaction was accomplished by oxidation reaction on Rux TiO2 using molecular oxygen as oxidant, the reaction was accomplished by
the ruthenium alcoholate formation followed by hydride elimination and the rate determining step the ruthenium alcoholate formation followed by hydride elimination and the rate determining step the ruthenium alcoholate formation followed by hydride elimination and the rate determining step is the hydride abstraction step [18]. is the hydride abstraction step [18]. is the hydride abstraction step [18]. Catalysts 2017, 7, 7
8 of 11
the ruthenium alcoholate formation followed by hydride elimination and the rate determining step is
Catalysts 2017, 7, 7 8 of 11 the
hydride abstraction step [18].
Run+
H2
Ti
O
Ti
O
Ti
O
Ti
OH
1. dissociative
adsorption
4. hydrogen
removal
H
H
Ti
O
O
Ti
H
O
H H
H O
Run+
O
O
O
Ti
Ti
Ti
Ti
Run+
O
Ti
Ti
hydrogen
abstration
H
O
H
Ti
O
Ti
O
H
2. surface
diffusion
Run+
O
Ti
Ti
Figure
5. The proposed mechanism for the dehydration of benzyl alcohol by RuxxTiO
Figure 5. The proposed mechanism for the dehydration of benzyl alcohol by Ru
TiO22. .
The turnover
turnover frequency
frequency (TOF) based on for benzyl oxidation using 0.01TiO2 The
(TOF)
based
on Ru
forRu benzyl
alcohol alcohol oxidation
using Ru0.01
TiO2Ru
amounts
−1 where the total amount of ruthenium obtained from Inductively coupled plasma −
1
amounts to 130 h
to 130 h where the total amount of ruthenium obtained from Inductively coupled plasma mass
mass spectrometry (ICP) data are used for TOF calculation. In comparison with other heterogeneous spectrometry
(ICP) data are used for TOF calculation. In comparison with other heterogeneous
−1 [7], Ag/Al2O3 (>2 h−
−1
catalytic systems in oxidant-free
oxidant‐free conditions
conditions (Au/CeO
(Au/CeO2,, (TOF
(TOF == 125
125 catalytic systems in
h−h1 )) [7],
Ag/Al2 O3 (>2 h 1) ) [9], [9],
2
−1
−1
−1
1
Ru/AlO(OH) (7 h
3 (8.3 h −
) [41]), our value is higher Ru/AlO(OH)
(7 h−1) [39], Cu/hydrotalcite (1.5 h
) [39], Cu/hydrotalcite (1.5 h−1) [40], Cu/Al
) [40], Cu/Al2O
2 O3 (8.3 h ) [41]), our value is higher
−1
except for the silver nanoparticles on the hydrotalcite system (590 h
except
for the silver nanoparticles on the hydrotalcite system (590 h−1) [11]. ) [11].
3. Materials and Methods 3.
Materials and Methods
3.1.
Synthesis of TiO2 and Ruthenium‐Incorporated TiO
and Ruthenium-Incorporated TiO2 (Ru
(RuxxTiO
TiO22) )
3.1. Synthesis of TiO
Ti(SO4)22 and RuCl
and RuCl3 nH
2O were purchased from Kanto and Sigma‐Aldrich, respectively. TiO
and 3 nH
2 O were purchased from Kanto and Sigma-Aldrich, respectively. 2TiO
2
and
Ru2x were synthesized by a hydrothermal method using Ti(SO
TiO2 were synthesized by a hydrothermal method using4)2Ti(SO
)
and
RuCl
nH
O
as
the
RuxTiO
and RuCl
3
nH
2
O as the precursor 4 2
3
2
precursor
of Ti and Ru, respectively. 15 mL of 24%
Ti(SO4 )2 (Ti: 20 mmol) was added dropwise into the
of Ti and Ru, respectively. 15 mL of 24% Ti(SO
4)2 (Ti: 20 mmol) was added dropwise into the 60 mL 60
of aqueous
solution
containing
the different
amount
of RuCl
0.2,
0.6,1, 1,1.4 1.4mmol) mmol) under under
of mL
aqueous solution containing the different amount of RuCl
3 (Ru: 0, 0,
0.2, 0.6, 3 (Ru:
vigorous
stirring.
The
samples
are
labeled
Ru
TiO
,
in
which
x
represents
the
mole
ratio
of
ruthenium
vigorous stirring. The samples are labeled RuxxTiO22, in which x represents the mole ratio of ruthenium with
respect to titanium. The solution was transferred to a Teflon-lined autoclave reactor and treated
with respect to titanium. The solution was transferred to a Teflon‐lined autoclave reactor and treated at
160 ◦ C for 6 h. After the reaction vessel was cooled down to room temperature, the samples
at 160 °C for 6 h. After the reaction vessel was cooled down to room temperature, the samples were were
separated
by centrifugation
and purified
by washing
with distilled
water
three
times.
Finally,
separated by centrifugation and purified by washing with distilled water three times. Finally, the the
precipitates were obtained by drying under vacuum at room temperature.
A-Ru
precipitates were obtained by drying under vacuum at room temperature. A‐Ru
0.07TiO
2 was prepared 0.07 TiO2 was
◦
prepared
by annealing
the 2Ru
by annealing the Ru
0.07TiO
at 700 °C for 6 h. 0.07 TiO2 at 700 C for 6 h.
3.2.
Characterization of Catalysts
3.2. Characterization of Catalysts X-ray
diffraction (XRD) patterns were obtained using a Bruker D8 Advance X-ray diffractometer
X‐ray diffraction (XRD) patterns were obtained using a Bruker D8 Advance X‐ray diffractometer with
Cu
Kα
Diffraction patterns patterns were were taken taken over over the the 2θ 2θ
with Cu Kα radiation
radiation (Bruker,
(Bruker, Karlsruhe,
Karlsruhe, Germany).
Germany). Diffraction ◦
◦
range
20 –75 . The XPS results were obtained on a PHI 5000 Versaprobe instrument (ULVAC-PHI Inc.,
range 20°–75°. The XPS results were obtained on a PHI 5000 Versaprobe instrument (ULVAC‐PHI Chigasaki,
Japan) in which monochromated Al Kα radiation (hν = 1486.6 eV) is illuminated as the light
Inc., Chigasaki, Japan) in which monochromated Al Kα radiation (hν = 1486.6 eV) is illuminated as source.
The
X-ray anode was run at 25 W and the voltage was maintained at 15 kV. The pass energy
the light source. The X‐ray anode was run at 25 W and the voltage was maintained at 15 kV. The pass was
fixed at 23.5 eV to ensure sufficient resolution to determine peak positions accurately. The binding
energy was fixed at 23.5 eV to ensure sufficient resolution to determine peak positions accurately. energies
were calibrated by using the Au 4f7/2 signal at 84.0
eV. The prepared sample morphology
The binding energies were calibrated by using the Au 4f
7/2 signal at 84.0 eV. The prepared sample was
characterized
by
a
TITAN
G2
60-300
(FEI,
Hillsboro,
OR,
USA)
transmission
microscopy
morphology was characterized by a TITAN G2 60‐300 (FEI, Hillsboro, OR, electron
USA) transmission (TEM)
equipped
with(TEM) CETCOR
corrector
andCETCOR 4-silicon corrector drift detector
energy
X-ray
electron microscopy equipped with and system
4‐silicon drift dispersive
detector system energy dispersive X‐ray spectroscopy (Super‐X EDS, Bruker) operating at an accelerating voltage of 80 kV. The quantitative EDS area mapping was carried out using a scanning electron probe (Bruker) with a size of 1 nm over the sample. The surface area of samples was measured by Brunauer‐Emmett‐
Catalysts 2017, 7, 7
9 of 11
spectroscopy (Super-X EDS, Bruker) operating at an accelerating voltage of 80 kV. The quantitative
EDS area mapping was carried out using a scanning electron probe (Bruker) with a size of 1 nm over
the sample. The surface area of samples was measured by Brunauer-Emmett-Teller (BET) method. N2
adsorption-desorption isotherms of samples at liquid nitrogen temperature (77 K) were obtained using
Belsorp Mini II (BEL Japan, Osaka, Japan).
3.3. Catalytic Tests
2 mL of alcohols (benzyl alcohol, 1-octanol, cynnamyl alcohol, 20 mM) dissolved in toluene was
added to 16 mL test tubes containing 5 mg of catalysts. To ensure oxygen-free conditions, nitrogen
gas was flowed for 20 min in the reaction mixture for degassing, followed by connection of an N2
balloon that was connected to a syringe needle, which was inserted into the septum covering the test
tube. The reaction mixture was heated 105 ◦ C for 3 h. After cooling the reaction mixture to room
temperature, the solid catalysts were separated from solution phase by centrifugation. The solution
phase was analyzed by gas chromatography (GC, aglient 7890A, Agilent Technologies, Santa Clara,
CA, USA) with a flame ionization detector (FID). Dodecane was used as an internal standard for
the calculation of conversion. The catalytic property of the commercial Degussa P25 was also tested
for comparison.
4. Conclusions
In summary, we prepared the highly dispersed ruthenium incorporated TiO2 by a one-pot
hydrothermal method. Rux TiO2 can transfer the benzylic, allylic, and alkyl alcohols into corresponding
carbonyl compounds in oxidant-free conditions. In our catalytic system, particularly, TiO2 acts as not
only the catalytic support but also a Lewis acid catalytic site which improves reactivity by facilitating
the deprotonated adsorption of alcohols on the catalysts. Therefore, the synergistic effect of hydride
elimination by ruthenium and the Lewis acid site on TiO2 can efficiently transfer primary alcohols into
corresponding aldehydes in oxidant-free conditions.
Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/7/1/7/s1,
Figure S1: The survey XPS spectrum of Ru0.07 TiO2 , Figure S2: TEM image of A-Ru0.07 /TiO2 . After annealing,
crystallites aggregate each other to form large crystallites without particular shape. (a) TEM image; (b) aggregation
of crystallites, Figure S3: (Top) Conversion data from analysis of gas chromatography (GC, Agilent 7890A, Agilent
Technologies, Santa Clara, CA, USA). Dodecane was used as internal standard for the calculation of conversion
(middle); the main products by pure TiO2 were o, m, p-benzyltoluenes which are produced by Friedel-Crafts
alkylation of toluene (solvent) with benzyl alcohol. In addition dibenzyl ether is also produced by the acid
catalyzed homo-coupling of benzyl alcohol (bottom), Figure S4: The calibration curves of alcohols and aldehydes.
The flame ionization detector was used. (a) Benzyl alcohol and benzaldehyde; (b) 1-octanol and 1-octanal;
(c) cinnamyl alcohol and cinnamaldehyde, Table S1: Catalytic properties of 1-phenyl ethanol oxidation under
oxidant-free condition.
Acknowledgments: Ki-Young Kwon acknowledges that this research was supported by the Creative Human
Resource Training Project for Regional Innovation through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education (NRF-2015H1C1A1035812).
Author Contributions: Ki-Young Kwon and Doo-Hyun Ko conceived and designed the experiments;
Youngyong Kim performed syntheses of catalysts and the evaluation of catalytic properties; Seokhoon Ahn
and Jun Yeon Hwang contributed TEM experiments; Ki-Young Kwon and Youngyong Kim wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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