Immobilization of Ru(terpyridine)(2,6-pyridinedicarboxylate) onto MCM-41 and its catalysis in the oxidation of alcohols

Article abstract of DOI:10.1002/aoc.3484

The ruthenium complex Ru(terpyridine)(2,6-pyridinedicarboxylate) was successfully grafted onto MCM-41 using a multi-step grafting method. The immobilized ruthenium complex was characterized thoroughly using Fourier transform infrared, Raman, UV–visible diffuse reflectance and energy-dispersive X-ray spectroscopies, X-ray diffraction, N₂ adsorption, scanning electron microscopy, thermogravimetric analysis and inductively coupled plasma analysis. This immobilized ruthenium complex showed excellent performance in the oxidation of various secondary alcohols to their corresponding ketones with tert-butyl hydroperoxide as oxidant under solvent-free conditions, and had the advantages of easy recovery and good reusability.

Full text of DOI:10.1002/aoc.3484

Full paper
Received: 20 January 2016
Revised: 2 March 2016
Accepted: 8 March 2016
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3484
Immobilization of Ru(terpyridine)(2,6-
pyridinedicarboxylate) onto MCM-41 and its
catalysis in the oxidation of alcohols
Yuecheng Zhang, Xiaochen Sun, Hongyu Zhang and Jiquan Zhao*
The ruthenium complex Ru(terpyridine)(2,6-pyridinedicarboxylate) was successfully grafted onto MCM-41 using a multi-step
grafting method. The immobilized ruthenium complex was characterized thoroughly using Fourier transform infrared, Raman,
UV–visible diffuse reflectance and energy-dispersive X-ray spectroscopies, X-ray diffraction, N2 adsorption, scanning electron mi-
croscopy, thermogravimetric analysis and inductively coupled plasma analysis. This immobilized ruthenium complex showed ex-
cellent performance in the oxidation of various secondary alcohols to their corresponding ketones with tert-butyl hydroperoxide
as oxidant under solvent-free conditions, and had the advantages of easy recovery and good reusability. Copyright © 2016 John
Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publishers web site.
Keywords: Ru(terpyridine)(2,6-pyridinedicarboxylate); immobilization; oxidation; secondary alcohols; TBHP; MCM-41
(benzimidazole)(pyridinedicarboxylate) also proved to be very ef-
Introduction
fective in the oxidation of secondary alcohols to ketones utilizing
hydrogen peroxide as the oxidant.[21]
The oxidation of alcohols to their corresponding aldehydes and
ketones is of crucial importance in organic chemistry because
the obtained carbonyl compounds are widely used in the produc-
tion of bulk and fine chemicals such as perfumes, pharmaceuticals
and organic intermediates.[1–3] Classical methods to carry out this
reaction are based on stoichiometric or even over-stoichiometric
amounts of oxidants such as chromates,[4–7] manganese
dioxide[8–10] and selenium dioxide,[11] which generate large
amounts of byproducts and cause pollution to the environment.
Therefore, it is highly necessary to develop catalytic oxidation pro-
cesses for the oxidation of alcohols without using the traditional
oxidants mentioned above. From both economic and environ-
mental viewpoints, molecular oxygen is an ideal oxidant which
generates water as the only byproduct in the oxidation process.
Therefore, many catalytic systems based on stable nitroxyl radi-
cals, such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), in
combination with metals as co-catalysts have been developed.[12]
However, most of these catalytic oxidation systems are
inapplicable for the oxidation of secondary alcohols, or generally
not suitable for large-scale applications. In addition to molecular
oxygen, hydrogen peroxide and tert-butyl hydroperoxide (TBHP)
are also regarded as environmentally benign oxidants. These oxi-
dants in combination with transition metals or their complexes
have been found to be very efficient in the oxidation of secondary
alcohols to the corresponding ketones.[13–19]
Although these ruthenium complexes performed well in the re-
action, the difficult separation of the very expensive complexes
from the reaction mixture makes them impossible to recycle di-
rectly. Heterogenization of these ruthenium complexes is expected
to overcome this disadvantage. Therefore, we designed a method
to immobilize Ru(terpy)(pydic) onto MCM-41. The immobilized
ruthenium complex was evaluated in the oxidation of various alco-
hols to their carbonyl compounds. To our delight, this immobilized
complex showed excellent catalytic activity and good recyclability
in the oxidation of secondary alcohols with TBHP as oxidant under
solvent-free conditions. Herein, we report the satisfactory results.
Experimental
Materials
4-Methylbenzaldehyde, 2-acetylpyridine, 2,6-pyridinedicarboxylic
acid and TBHP (70%) were purchased from Energy Chemical, China.
3-Aminopropyltriethoxysilane, N-bromsuccinimide (NBS), α,α′-
azoisobutyroniltrile (AIBN) and H2O2 (30%) were obtained from
Tianjin Guangfu Fine Chemical Research Institute. Dichloro(p-
cymene)ruthenium(II) dimer ([Ru(p-cymene)Cl2]2) was purchased
from Xi’an Kaili Chemical Company. MCM-41 was purchased from
In 2007, Beller and co-workers[20] applied Ru(terpyridine)(2,6-
pyridinedicarboxylate) – Ru(terpy)(pydic) – to the oxidation of
secondary alcohols to their corresponding ketones using hydrogen
peroxide as oxidant. This complex showed excellent performance
in the reaction in the absence of co-catalysts or organic solvents.
Later, a similar ruthenium complex known as ruthenium bis
*
Correspondence to: Jiquan Zhao, School of Chemical Engineering and
Technology, Hebei University of Technology, Tianjin 300130, PR China. E-mail:
zhaojq@hebut.edu.cn
School of Chemical Engineering and Technology, Hebei University of Technology,
Tianjin 300130, PR China
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.

Y. Zhang et al.
Preparation of MCM-41-NH-Terpy (C)
the Catalyst Plant of Nankai University, China. The secondary alco-
hols were obtained from Alfa Aesar China (Tianjin) Co. Ltd. Solvents
were purified by standard methods and dried if necessary.
In a typical process, B (7.5g) and 2 (4.3g, 0.01 mol) were dispersed
in anhydrous toluene (120 ml) under a nitrogen atmosphere. The
mixture was heated and refluxed for 10h. Then the solid product
was filtered, washed with anhydrous tetrahydrofuran and dried at
90°C for 5 h to afford the terpy-functionalized MCM-41 (C).
Characterization
1H NMR spectra were recorded with tetramethylsilane as internal
standard using a Bruker AC-P 400 type NMR spectrometer. Fourier
transform infrared (FT-IR) spectra were recorded in the range
400–4000 cmꢀ1 with a Bruker Vector 22 type instrument using
KBr pellets. UV–visible diffuse reflectance spectra (UV-vis DRS) were
obtained in the range 200–800 nm with a Varian Cary 300 UV–
visible spectrophotometer. Powder X-ray diffraction (XRD) patterns
were recorded using a Bruker X-ray diffractometer with D8 FOCUS
lynx eye detector in the range of 2θ between 1° and 10° operating
with a Cu anode at 40 kV and 40 mA. Analyses of surface area and
Preparation of immobilized ruthenium complex MCM-41-Ru (D)
To a mixture of C (8.0 g) and [Ru(p-cymene)Cl2]2 (1.8 g, 0.003mol) in
methanol (90 ml) was added a solution of disodium pyridine-2,6-
dicarboxylate (1.3 g, 0.006 mol) in methanol–water (2:1, 80 ml)
under a nitrogen atmosphere. The mixture was heated to reflux
for 5 h. The solid product was filtered, washed with methanol and
water and dried at 90 °C for 5 h to afford D.
Typical procedure for oxidation of alcohols
pore structure were carried out using
a
Micromeritics
ASAP2020M + C automated gas-sorption system, employing nitro-
gen as the adsorbate, after pretreatment of the sample at 120 °C
for 10 h under reduced pressure. Scanning electron microscopy
(SEM) and energy-dispersive X-ray spectroscopy (EDX) were carried
out with a Nova Nano SEM450 instrument and an Octane Pro Det.
The ruthenium content of the immobilized complex was analyzed
using an X7 series inductively coupled plasma (ICP) mass spectrom-
eter. Thermogravimetric analysis (TGA) was performed with an
SDT/Q600 thermogravimetric analyzer at a temperature ranging
from 20 to 800 °C with a temperature ramp of 10 °Cminꢀ1 under
an air atmosphere. Raman spectra were measured at room temper-
ature using an inVia Reflex visible Raman spectrometer in backscat-
tering geometry equipped with excitation argon ion lasers working
at 532 nm.
A 5 ml two-necked round-bottom flask equipped with a magnetic
stirrer was charged in succession with 0.02 g of D and 2 mmol of al-
cohol. The mixture was stirred and heated to 70 °C, and then
2.6 mmol of TBHP was added slowly at this temperature for 3 h.
The progress of the reaction was monitored using GC with a PEG-
20M column. The product was purified by column chromatography
over silica gel (n-hexane–ethyl acetate).
Results and discussion
Preparation of immobilized ruthenium complex D
The preparation route to D is shown in Scheme 1. First we synthe-
sized 4′-(4-bromomethylphenyl)-[2,2′:6′,2′′]terpyridine as described
1
in the literature.[22] The H NMR spectrum of the compound con-
forms to that in the literature.[22] After obtaining this compound,
we began to immobilize the ruthenium complex onto MCM-41
using a multi-step grafting method. B was synthesized from
MCM-41 and 3-aminopropyltriethoxysilane using a method previ-
ously reported by us.[23] In this process, the accessible silanol
groups condensed with the triethoxysiloxane groups smoothly,
which led to the attachment of a reactive terminal primary amine
group to the surface of MCM-41.
In the process for preparing C, the amino group of B underwent a
substitution reaction on the bromomethyl group of 2 smoothly to
afford terpy-functionalized MCM-41 C. Using the same procedure
for preparing Ru(terpy)(pydic) as in the literature,[24] the supported
terpy ligand in combination with sodium 2,6-pyridinedicarboxylate
coordinated with [Ru(p-cymene)Cl2]2 to give the immobilized ru-
thenium complex D.
The immobilized complex MCM-41-Ru was washed with metha-
nol and water thoroughly to ensure the removal of substances
absorbed on the outer surface of the support. The ruthenium con-
tent of the immobilized complex was determined using ICP analysis
to be 1.93 wt%. Due to the washing procedure removing all
absorbed substances from the MCM-41 surface, it is reasonable that
the ruthenium content is from the complex attached onto MCM-41.
Catalyst preparation
Synthesis of 4′-(p-methylphenyl)-2,2′:6′,2″-terpyridine (1)
To a solution of 4-methylbenzaldehyde (19.4g, 0.16 mol) and 2-
acetylpyridine (37.8g, 0.32 mol) in ethanol (300 ml) were added
NaOH (12.2 g, 0.3 mol) and NH4OH (460 ml, 25%) successively. The
mixture was heated to 34°C for 24 h, and then cooled to 20°C.
The off-white solid was collected by filtration and washed with
ice-cold ethanol (60 ml). Recrystallization of the off-white solid from
ethanol afforded white crystalline solid 1. The characterization of 1
is shown in Fig. S1.
Synthesis of 4′-(4-bromomethylphenyl)-[2,2′:6′,2′′]terpyridine (2)
A mixture of 1 (19.4 g, 0.06 mol), NBS (12.8g, 0.07 mol) and AIBN
(0.8 g, 0.005 mol) in dry CCl4 (280 ml) was refluxed for 9 h. The warm
reaction mixture was filtered to remove the succinimide byproduct,
and the solvent was evaporated under vacuum. The crude product
was recrystallized from ethanol–acetone (2:1) to afford 2 as a pale-
yellow solid. The characterization of 2 is shown in Fig. S2.
Amino group-functionalized MCM-41
3-Aminopropyltriethoxysilane (2.6ml, 0.01 mol) was slowly added
to anhydrous toluene (300 ml) containing MCM-41 (8.0g) under
constant stirring and a nitrogen atmosphere at room temperature.
The mixture was heated to reflux for 24 h. Then the solid product
was filtered, washed with anhydrous ethanol and dried at 90 °C
for 5 h to afford the amino group-functionalized material MCM-
41-NH2 (B).
Characterization
FT-IR spectroscopy
FT-IR spectra of the samples are shown in Fig. 1. In all spectra the
intense absorption band at 1081 cmꢀ1 with a shoulder around
1200 cmꢀ1 and those at 746 and 459 cmꢀ1 are attributed to the
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Synthesis of MCM-41-Ru and its catalysis in oxidation of alcohols
2928 cmꢀ1, which indicates that 3-aminopropyl group is attached
successfully onto the wall of MCM-41. Compared with that of B,
the spectrum of MCM-41-NH-Terpy (C) shows two additional bands
at 1587 and 1486 cmꢀ1, attributed to the stretching vibration of
C¼N of the pyridine ring,[29] which indicates that the 4′-(p-
methylenephenyl)-2,2′:6′,2″-terpyridine is successfully grafted onto
the wall of MCM-41 through an iminepropyl chain. In the spectrum
of MCM-41-Ru (D), a weak band at 614 cm-1 attributed to Ru–O
vibration[30] is observed, indicating the formation of the
immobilized complex. The used sample showed almost same spec-
trum compared with the fresh one, confirming that MCM-41-Ru is
stable in a catalytic run.
Raman studies
Raman analysis was carried out to further confirm the formation of
the immobilized complex. As shown in Fig. 2, the spectrum of
MCM-41-Ru almost coincides with that of the homogeneous com-
plex Ru(terpyridine)(2,6-pyridinedicarboxylate). In the spectra, the
bands at 1607 and 1477 cmꢀ1 are attributed to the stretching vibra-
tion of C¼N from disodium pyridine-2,6-dicarboxylate,[29] and the
bands at 1528 and 1356 cmꢀ1 arise from the stretching vibration
of C¼N of terpyridine. The characteristic band at 644 cmꢀ1 attrib-
uted to Ru–O vibration,[30] and the weak band at 458 cmꢀ1 attrib-
uted to Ru–N vibration[30] are clearly observed in both spectra,
indicating that the immobilized complex is successfully formed.
UV-vis DRS
Figure 3 shows the UV-vis DRS spectra following the synthesis pro-
cedure step by step. The spectra of MCM-41 (A) and MCM-41-NH2
(B) exhibit no absorption bands above 300 nm. In the spectrum of
MCM-41-NH-Terpy (C), band maxima at 214 and 290 nm are de-
Scheme 1. Route for the synthesis of immobilized ruthenium complex.
rived from the π → π* transitions of phenyl and C¼N groups[31]
;
and the band around 578 nm is from n → π* of C¼N in the conjuga-
tion system. In the spectrum of MCM-41-Ru (D), band at 510 nm is
attributed to the d–d transition between Ru and ligands.[32] The re-
sult of UV-vis DRS is consistent with that of FT-IR spectra.
XRD analysis
The XRD pattern of MCM-41 (Fig. 4) exhibits three peaks in the 2θ
range of 2–8° corresponding to the d100, d110 and d200 reflections
of a regular hexagonal array of pores.[33] The patterns of MCM-41-
NH2 (B) and MCM-41-NH-Terpy (C) have no significant change
Figure 1. FT-IR spectra of MCM-41 (A), MCM-41-NH2 (B), MCM-41-NH-Terpy
(C), MCM-41-Ru (D), and used MCM-41-Ru (E).
Si–O–Si stretching and deformation vibrations, and the shoulder in
particular is attributed to the porous nature of the MCM-41.[25,26]
The band at 960 cmꢀ1 in the spectrum of MCM-41 (A) is assigned
to the bending vibration of Si–O–H,[27] and its intensity decreases
with the modification of the wall of MCM-41 by triethoxysiloxane.
The absorption bands at 3422 and 1632 cmꢀ1 correspond to
H–O–H bending vibrations of physisorbed water.[28] In the spec-
trum of MCM-41-NH2 (B), the characteristic bands attributed to
the stretching vibration of –CH2– can be seen around 2973 and
Figure 2. Raman spectra of MCM-41-Ru (a) and Ru(terpyridine)(2,6-
pyridinedicarboxylate) (b).
Appl. Organometal. Chem. (2016)
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Y. Zhang et al.
Table 1. BET surface area, pore volume and pore size of MCM-41,
MCM-41-NH2, MCM-41-NH-Terpy and MCM-41-Rua
Entry
Sample
SBET
Pore volume
Pore diameter
(nm)
(m2 gꢀ1
)
(cm3 gꢀ1
)
1
2
3
MCM-41
1006.4
781.6
281.7
0.8
0.6
0.2
3.5
3.2
3.0
MCM-41-NH2
MCM-41-NH-
Terpy
4
MCM-41-Ru
33.3
0.1
11.8
aPore size calculated using the BJH method.
SEM analysis
The SEM images of MCM-41 (A), MCM-41-NH2 (B), MCM-41-NH-
Terpy (C), MCM-41-Ru (D) and used MCM-41-Ru (E) are displayed
in Fig. 5. The images reveal that all samples consist of many
banana-like particles with particle size of about 800 × 300 nm, and
the functionalized samples retain nearly the same morphology as
that of the original MCM-41. These images indicate that the MCM-
41 structure does not change largely during the ruthenium com-
plex immobilization process. The same morphology of the used
sample with that of the fresh MCM-41-Ru confirms that MCM-41-
Ru is stable in a catalytic run. Meanwhile, EDX analysis was carried
out and the results clearly illustrate the existence of Ru in the cata-
lyst surface (Fig. 5(F)).
Figure 3. UV-vis DRS spectra of MCM-41 (A), MCM-41-NH2 (B), MCM-41-NH-
Terpy (C) and MCM-41-Ru (D).
Thermal analysis
Figure 6 shows the TGA and DSC plots of MCM-41-Ru. The initial en-
dothermic first stage occurs in the range 25 to 70 °C, with a weight
loss of 5%, attributed to desorption of physisorbed water and other
small molecules. The second stage lies between 300 and 450 °C,
which corresponds to the pyrolytic endothermic degradation of
the immobilized complex. A weight loss of 30% takes place in this
stage. The results reveal that the immobilized Ru complex as a cat-
alyst is stable during a catalytic run.
Figure 4. XRD analysis of MCM-41 (A), MCM-41-NH2 (B), MCM-41-NH-Terpy
(C) and MCM-41-Ru (D).
Catalytic properties of immobilized catalyst
compared with that of MCM-41, which indicates that the MCM-41
framework structure remains after the multi-step grafting pro-
cesses. A slight shift of the characteristic reflection to a higher angle
and the obvious decrease of the intensity of the peak representing
the d100 reflection are observed after the formation of the
immobilized complex MCM-41-Ru (D), which may be ascribed to
the partial structure collapse of parent MCM-41.[34]
The catalytic performance of MCM-41-Ru was evaluated for the
oxidation of 1-phenylethanol as a model substrate with various
oxidants. Molecular oxygen is the best oxidant from both economic
and environmental viewpoints; therefore, we first tested molecular
oxygen as oxidant in the oxidation of 1-phenylethanol. Almost no
oxidation reaction takes place (Table 2, entry 1). Then hydrogen
peroxide was tried, again due to its good performance in the
oxidation of secondary alcohols catalyzed by Ru(terpy)(pydic).[20]
Unexpectedly, very low conversion is obtained when 30% aqueous
H2O2 is used as oxidant, and extending the reaction time has no sig-
nificant effect on the reaction (Table 2, entries 2 and 4), which is
very different from the same reaction catalyzed by homogeneous
Ru(terpyridine)(2,6-pyridinedicarboxylate). It is difficult to tell what
causes the large difference between the free and immobilized com-
plex. Finally, we utilized TBHP as oxidant to carry out the reaction,
and a delightful result was obtained. As evident from Table 2, a con-
version of 99% with a selectivity of 99% toward acetophenone is
obtained after 2.2h at 70 °C (Table 2, entry 3).
Nitrogen adsorption
The BET surface area, pore size and pore volume of MCM-41, MCM-
41 functionalized with each of amino and terpy, and the
immobilized ruthenium complex are presented in Table 1. The
surface area and pore volume of the samples decrease with the at-
tachment of aminopropyl and terpy successively, and the forma-
tion of the immobilized complex. On the contrary, the pore
diameter increases. The cooperative coordination of the supported
terpy ligand and sodium 2,6-pyridinedicarboxylate with [Ru(p-
cymene)Cl2]2 to form the immobilized complex leads to a sharp de-
crease of surface area and pore volume. The surface area and pore
Control experiments were performed to evaluate the necessity of
the immobilized complex in the oxidation of 1-phenylethanol with
TBHP as oxidant. As evident from Table 2, the use of the
volume of MCM-41-Ru are only 33.3m2 gꢀ1 and 0.1 cm3 gꢀ1
,
respectively.
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Synthesis of MCM-41-Ru and its catalysis in oxidation of alcohols
Figure 6. TGA and DSC curves of the thermal degradation of MCM-41-Ru.
Table 2. Selection of oxidant and necessity of the immobilized com-
plex with 1-phenylethanol as model substratea
Entry Oxidant
Catalyst
Time Conv. Select. TON TOF
(h)
(%)b
(%)b
(hꢀ1
)
1
2
3
4
5
6
7
8
O2
MCM-41-Ru
MCM-41-Ru
MCM-41-Ru
MCM-41-Ru
—
2.2
2.2
2.2
5.5
2.2
2.2
2.2
2.2
2.0
22.2
>99
23.2
28.4
29.0
18.6
31.3
>99
10.5
4.8
H2O2
TBHP
H2O2
TBHP
TBHP
TBHP
TBHP
>99 116.8 53.1
>99 526.3 239.2
>99 122.1 22.2
>99
—
—
MCM-41
>99 244.7 48.9
>99 182.6 36.5
>99 185.3 37.0
MCM-41-NH2
MCM-41-NH-
Terpy
aReaction conditions: 1-phenylethanol (2 mmol), catalyst (0.02 g,
0.0038 mmol), oxidant (2.6 mmol), reaction temperature 70 °C.
bDetermined by GC.
immobilized ruthenium complex is essential. Only 28.4% of 1-
phenylethanol is converted to acetophenone in 2.2 h when no
catalyst is added (Table 2, entry 5). When MCM-41, MCM-41-NH2
and MCM-41-NH-Terpy instead of the immobilized ruthenium com-
plex are used as catalysts respectively, the conversion of 1-
phenylethanol is 29.0, 18.6 and 31.3% in 2.2 h (Table 2, entries 6–8).
The effect of MCM-41-Ru loading on the reaction was studied at
40°C under the same conditions, and the results are shown in
Table 3. When MCM-41-Ru loading is increased from 0.095 to
0.24 mol%, the time required to finish the reaction decreases from
12.5 to 5.0h (Table 3, entries 1–4). Further increasing MCM-41-Ru
loading does not increase reaction rate, but decreases it. This can
be attributed to the direct decomposition of TBHP catalyzed by
the over-loaded immobilized ruthenium complex.[35,36]
To obtain the optimal reaction conditions, the reaction tempera-
ture was examined. The results are listed in Table 4. It can be seen
that the time to finish the reaction shortens with an increase of tem-
perature from 40 to 70°C, and the time to finish the reaction is only
2.2 h at 70 °C (Table 4, entries 1–4). An unfavorable result was ob-
tained with a further increase of the reaction temperature. For
instance, 1-phenylethanol is not completely converted to
acetophenone when the temperature is 80°C (Table 4, entry 5). This
Figure 5. SEM images of MCM-41 (A), MCM-41-NH2 (B), MCM-41-NH-Terpy
(C), MCM-41-Ru (D) and used MCM-41-Ru (E). EDX analysis of MCM-41-Ru (F).
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
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Y. Zhang et al.
Table 3. Effect of MCM-41-Ru loading on oxidation of 1-
From the above experimental results we determined the optimal
reaction conditions which are: 1-phenylethanol, 2 mmol; catalyst,
0.02 g (0.0038mmol); TBHP, 2.6mmol; reaction temperature, 70 °C.
Under the optimal reaction conditions, 1-phenylethanol is almost
quantitatively converted to acetophenone in 2.2h in the absence
of solvent.
phenylethanola
Entry
MCM-41-Ru
(mol%)
Time
(h)
Conv.
(%)b
Select.
(%)b
TON
TOF
(hꢀ1
)
1
2
3
4
5
0.095
0.14
0.19
0.24
0.28
12.5
8.0
5.5
5.0
5.5
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
1052.6
714.3
526.3
416.7
357.1
84.2
89.2
95.7
83.3
64.9
Having determined the optimal conditions, the oxidation of
various alcohols to their corresponding carbonyl compounds was
explored under the optimal reaction conditions to evaluate the ver-
satility of this novel catalytic system. As evident from Table 6, all sec-
ondary benzylic alcohols, including those bearing both electron-
withdrawing and electron-donating groups, are selectively con-
verted to their corresponding aromatic ketones in excellent yields
(Table 6, entries 1–12). However, the time required to finish the re-
action is much different. It seems that the catalytic system is more
efficient for a substrate with an electron-withdrawing substituent
(Table 6, entries 2–7). The position of the substituent on the ben-
zene ring has obvious effects on the reactivity of secondary ben-
zylic alcohols. A substrate with an o-substituent shows poor
reactivity compared to that with an m- or p-substituent due to
the steric hindrance (Table 6, entries 2–4, 7–9). Compared with their
p- and m-isomers, high TBHP loading is required to get high yields
in the cases of o-methyl-1-phenylethanol and o-chloro-1-
phenylethanol as substrates (Table 6, entries 4 and 9). Overall, the
reaction rate is determined by a combination of the electronic
and steric effects. The immobilized ruthenium complex also shows
high activity in oxidation of secondary aliphatic alcohols and
cyclohexanol at high TBHP loading (Table 6, entries 13–15). These
alcohols are generally difficult to oxidize using molecular oxygen
catalyzed by copper/TEMPO-based catalysts.[37–40] However, the
catalytic oxidation system does not work well with menthol due
to large steric hindrance present in the structure (Table 6, entry 16).
The immobilized ruthenium complex was also applied to the ox-
idation of primary benzylic alcohols with TBHP as oxidant to see
whether it was possible to obtain selectivity towards either the al-
dehyde or the carboxylic acid. Unfortunately, the selectivity is not
controlled; only carboxylic acids as main products are obtained in
all the cases (Table 6, entries 17–19). However, primary heterocyclic
alcohols including 2-pyridylmethanol and 5-hydroxymethylfurfural
are converted to their corresponding aldehydes in moderate yield
at 40 °C (Table 6, entries 20 and 21); no deep oxidation products
are observed in these cases. This may be due to the coordination
of the heterocyclic atom to the Ru center, which decreased the ac-
tivity of the immobilized ruthenium complex.
aReaction conditions: 1-phenylethanol (2 mmol), oxidant (6 mmol),
reaction temperature 40 °C.
bDetermined by GC.
Table 4. Effect of reaction temperature on oxidation of 1-
phenylethanola
Entry Temperature Time Conv. Select. (%)b TON TOF (hꢀ1
)
(°C)
(h)
(%)b
1
2
3
4
5
40
50
60
70
80
25.0
88.3
>99
>99
>99
>99
>99
464.7
526.3
526.3
526.3
482.1
18.6
42.1
12.5 >99
7.5 >99
2.2 >99
70.2
239.2
56.7
8.5
91.6
aReaction conditions: 1-phenylethanol (2 mmol), catalyst (0.02 g,
0.0038 mmol), oxidant (2.6 mmol).
bDetermined by GC.
result is attributed to the decomposition of TBHP at higher temper-
ature. Therefore, 70 °C was chosen as the reaction temperature in
subsequent experiments.
The TBHP loading was optimized at 70 °C keeping the other pa-
rameters constant. Increasing the TBHP loading improves the reac-
tion effectively. The results are listed in Table 5, from which it can be
learned that the oxidation reaction could not finish at TBHP loading
lower than or equal to 1:1.2. When the molar ratio of 1-
phenylethanol to TBHP reaches 1:1.3, 1-phenylethanol is almost
quantitatively converted to acetophenone in 2.2 h. Then the time
to finish the reaction shortens with further increasing TBHP loading.
Over-loading of TBHP will lead to unnecessary waste, thus the suit-
able molar ratio of 1-phenylethanol to TBHP is 1:1.3.
Recycling of catalyst
A recycle test was carried out under the optimal conditions deter-
mined as described above using 1-phenylethanol as a model
substrate. After reaction, the catalyst was filtered, washed with
methylene chloride, dried at 80 °C for 2 h and subjected directly
to the next run. The results are summarized in Table 7. Comparing
the results between the second and the first runs, the conversion
decreases about 1.8%. With an increase of the number of recycles,
the conversion of 1-phenylethanol decreases more and more
(Table 7, entries 1–6). In the seventh run the conversion of
1-phenylethanol decreases to 75.6% (Table 7, entry 7), which is a re-
duction in activity of about 24.4% compared with that in the first
catalytic run. To elucidate the reason, the ruthenium content of
the catalyst after the seventh run was analyzed using ICP and is
found to be 1.49%. The ruthenium loss is about 22.8% compared
with that of the fresh catalyst, which agrees well with the activity
Table 5. Effect of TBHP loading on oxidation reaction of 1-
phenylethanola
Entry Substrate:TBHP Time Conv. Select.
TON
TOF (hꢀ1
)
(h)
(%)b
(%)b
1
2
3
4
5
6
1:1.0
1:1.1
1:1.2
1:1.3
1:1.4
1:1.5
5.0
5.0
5.0
2.2
1.8
1.5
78.8
88.6
95.0
>99
>99
>99
>99
>99
>99
414.7
466.3
500.0
526.3
526.3
526.3
82.9
93.3
100.0
239.2
292.4
350.9
>99
>99
>99
aReaction conditions: 1-phenylethanol (2 mmol), catalyst (0.02 g,
0.0038 mmol), temperature 70 °C.
bDetermined by GC.
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Appl. Organometal. Chem. (2016)

Synthesis of MCM-41-Ru and its catalysis in oxidation of alcohols
Table 6. Oxidation of various alcohols with TBHP catalyzed by MCM-41-Ru
Entry
Substrate
1-Phenylethanol
Product
Acetophenone
Time (h)
Conv. (%)a
Yield (%)b
Select. (%)a
TON
TOF (hꢀ1
)
1
2c
2.2
6.0
8.0
5.5
3.2
4.0
3.6
5.0
8.0
6.0
4.3
3.3
8.0
13.0
9.0
5.5
7.0
9.8
8.2
16.0
7.0
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
94.2
>99
37.3
>99
>99
>99
58.0
74.0
88.1
92.8
86.2
85.3
86.3
91.5
88.0
87.8
90.0
85.3
89.6
94.4
90.1
85.3
89.1
33.9
78.0
75.0
73.3
41.5
52.9
>99
>99
>99
>99
>99
>99
>99
>99
97.6
>99
>99
>99
>99
>99
>99
>99
85.3
83.3
80.9
98.2
98.6
526.3
526.3
526.3
526.3
526.3
526.3
526.3
526.3
526.3
526.3
526.3
526.3
526.3
495.8
526.3
196.3
526.3
526.3
526.3
305.2
389.4
239.2
39.8
65.8
95.7
164.4
131.5
146.2
105.3
65.8
87.7
122.4
159.5
65.8
38.1
58.5
35.7
75.2
90.7
64.2
19.0
55.6
1-(4-Methylphenyl)ethanol
1-(3-Methylphenyl)ethanol
1-(2-Methylphenyl)ethanol
1-(4-Fluorophenyl)ethanol
1-(4-Bromophenyl)ethanol
1-(4-Chlorophenyl)ethanol
1-(3-Chlorophenyl)ethanol
1-(2-Chlorophenyl)ethanol
1-Phenyl-1-propanol
1-Indanol
4-Methylacetophenone
3-Methylacetophenone
2-Methylacetophenone
4-Fluoroacetophenone
4-Bromoacetophenone
4-Chloroacetophenone
3-Chloroacetophenone
2-Chloroacetophenone
1-Phenyl-1-propanone
1-Indanone
3
4e
5
6
7
8
9d
10f
11f
12
13e
14g
15e
16
17h
18e
19h
20i
21i
Benzhydrol
Benzophenone
Cyclohexanol
Cyclohexanone
2-Octanol
2-Octanone
1-Cyclohexyl-1-propanol
D,L-Menthol
Cyclohexyl ethyl ketone
Menthone
Benzyl alcohol
Benzoic acid
2-Chlorobenzenemethanol
4-Chlorobenzenemethanol
2-Pyridinemethanol
2-Thiophenemethanol
2-Chlorobenzoic acid
4-Chlorobenzoic acid
2-Pyridinecarboxaldehyde
2-Thenaldehyde
Reaction conditions: 1-phenylethanol (2 mmol), catalyst (0.02 g, 0.0038 mmol), oxidant (2.6 mmol), temperature 70 °C.
aDetermined by GC.
bIsolated yield.
cOxidant: 2.8 mmol.
dOxidant: 7.6 mmol.
eOxidant: 5.8 mmol.
fOxidant: 3.2 mmol.
gOxidant: 5.2 mmol.
hOxidant: 4.6 mmol.
iOxidant: 6.0 mmol, temperature 40 °C.
Table 7. Recycle test of MCM-41-Ru for oxidation of 1-phenylethanola
Anyway, this immobilized ruthenium catalyst can be recycled four
times without a large reduction of its activity.
Entry
Run number
Conversion (%)b
Selectivity (%)b
1
2
3
4
5
6
7
1
2
3
4
5
6
7
>99
97.2
95.1
91.0
84.1
78.6
75.6
>99
>99
>99
>99
>99
>99
>99
Conclusions
The ruthenium complex Ru(terpyridine)(2,6-pyridinedicarboxylate)
was successfully grafted onto MCM-41 using a multi-step grafting
method. This immobilized ruthenium showed excellent perfor-
mance in the oxidation of various secondary alcohols to the
corresponding ketones with TBHP as an oxidant under solvent-free
conditions. In addition, the immobilized complex also showed ad-
vantages of easy recovery and good recyclability. ICP analysis indi-
cated that the slow deactivation of the immobilized ruthenium
complex is mainly due to the leaching of ruthenium during the cat-
alytic runs.
aReaction conditions: 1-phenylethanol (40 mmol), catalyst (0.4 g,
0.076 mmol), oxidant (52 mmol), temperature 70 °C, reaction time
2.2 h.
bDetermined by GC.
Acknowledgments
reduction of the immobilized ruthenium complex. These results in-
dicate that the deactivation of the immobilized ruthenium complex
is mainly due to the leaching of ruthenium during the catalytic runs.
The authors are grateful for financial support from the National
Natural Science Foundation of China (no. 21276061) and the Natu-
ral Science Foundation of Hebei Province, China (no. B2013202158).
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

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Supporting Information
Additional supporting information may be found in the online ver-
sion of this article at the publisher’s web site.
Figure S1. 1H NMR of 4′-(p-Methylphenyl)-2,2′:6′,2″-terpyridine
Figure S2. 1H NMR of 4′-(4-Bromomethylphenyl)-[2,2′:6′,2′′]
terpyridine
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Figure S7. 1H NMR of 4-Fluoroacetophenone (CDCl3) (Table 6, entry
5)
Figure S8. 1H NMR of 4-Bromoacetophenone (CDCl3) (Table 6, entry
6)
1
Figure S9. H NMR of 4-Chloroacetophenone ((CD3)2SO) (Table 6,
entry 7)
Figure S10. 1H NMR of 3-Chloroacetophenone (CDCl3) (Table 6, en-
try 8)
Figure S11. 1H NMR of 2-Chloroacetophenone (CDCl3) (Table 6, en-
try 9)
Figure S12. 1H NMR of 1-Phenyl-1-propanone (CDCl3) (Table 6, en-
try 10)
Figure S13. 1H NMR of 1-Indanone (CDCl3) (Table 6, entry 11)
Figure S14. 1H NMR of benzophenone (CDCl3) (Table 6, entry 12)
1
Figure S15. H NMR of cyclohexanone (CDCl3) (Table 6, entry 13)
Figure S16. 1H NMR of 2-Octanone (CDCl3) (Table 6, entry 14)
Figure S17. 1H NMR of cyclohexyl ethyl ketone (CDCl3) (Table 6, en-
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Figure S19. 1H NMR of benzoic acid (CDCl3) (Table 6, entry 17)
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entry 19)
Figure S22. 1H NMR of 2-Pyridinecarboxaldehyde (CDCl3) (Table 6,
entry 20)
Figure S23. 1H NMR of 2-Thenaldehyde (CDCl3) (Table 6, entry 21)
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