Ruthenium-incorporated hydroxyapatites for the oxidation of alcohols and amines using molecular oxygen as an oxidant

Full text of DOI:10.1002/bkcs.10006

Communication
DOI: 10.1002/bkcs.10006
BULLETIN OF THE
D. Kim et al.
KOREAN CHEMICAL SOCIETY
Ruthenium-Incorporated Hydroxyapatites for the Oxidation of Alcohols
and Amines Using Molecular Oxygen as an Oxidant
Daehyun Kim,† Youngyeong Kim,† Kyungmun Jung,† Myong Yong Choi,† Minjun Park,‡
§,
†,
Byung Yang Lee,‡, Tae Hyun Kim, and Ki-Young Kwon
*
*
*
†Department of Chemistry, RINS, Gyeongsang National University, Jinju 660-701, South Korea.
*E-mail: kykwon@gnu.ac.kr
‡Department of Mechanical Engineering, Korea University, Seoungbuk-gu, Seoul 136-713, South Korea.
*E-mail: blee@korea.ac.kr
§Department of Chemistry, Soonchunhyang University, Asan 336-745, South Korea. *E-mail: thkim@sch.ac.kr
Received August 29, 2014, Accepted September 1, 2014, Published online December 19, 2014
Keywords: Hydroxyapatite, Aerobic oxidation, Ruthenium, Supported catalyst
Oxidation of alcohols and amines into the corresponding car-
bonyl and nitrile compounds in an environmentally benign
and cost-effective wayhas been animportanttopicinsynthetic
chemistry. Particularly, as a replacement for the treatment of
stoichiometric amounts of expensive and toxic oxidizing
reagents, recently there have been intensive efforts on the
development of novel oxidation reactions by using molecular
oxygen as an oxidant under mild conditions.1–3
Figure 2 presents the morphological changes of calcium
phosphates as a function of the NaOH concentration.
Figure 2(a) shows the morphology of dicalcium phosphate
in which the crystallites exhibit an elongated rod shape with
a few micrometers in size. At higher NaOH concentrations
(Figure 2(b)–(g)), nanocrystalline HAPs present as elongated
hexagonal rods. Particularly, as the NaOH concentration
increases, the aspect ratio seems to decrease. Figure 2(h) shows
the transmission electron microscopy (TEM) image of HAP-
4M NaOH after ion exchange under 50 mM RuCl3 solution
(RuHAP (4 M)). Ruthenium clusters (< ~2 nm) are dispersed
evenly on the surface of HAP after the ion-exchange reaction.
Hydroxyapatite (HAP, Ca10(PO4)6(OH)2)4 is known to be a
major inorganic constituent of mammalian bone and teeth. In
particular, it is reported that various kinds of transition metals
having catalytic activities can be readily incorporated into
hydroxyapatite by a cation exchange reaction. In particular,
the Kaneda group reported that HAPs containing Pd and
Ru can be applied for various reactions.5,6 Recently, we
reported the aerobic oxidation of alcohols using ruthenium-
incorporated HAP (RuHAP) in which the HAP used as cata-
lytic support was (1) a single crystal prepared by molten salt
synthesis, (2) an amorphous HAP, or (3) a calcium-deficient
HAP.7 In this study, we have synthesized seven calcium phos-
phates that were prepared under seven different NaOH con-
centrations. We have utilized these calcium phosphates as
catalytic supports for ruthenium, and applied RuHAP for
the oxidation of alcohols and amines using O2 as oxidant.
We found that RuHAPs prepared under NaOH concentration
ranging from 2 to 5 M exhibited high catalytic property.
The synthesis of calcium phosphates was reported previ-
ously.7,8 RuHAPwaspreparedbysoakingeachHAP(0.5 g)into
20 mM of RuCl3 solution (20 mL). The solution was filtered and
washed three times using 100 mL of deionized (DI) water, fol-
lowed by drying overnight in an oven at 70 oC temperature.
Figure 1 shows the X-ray diffraction (XRD) data of calcium
phosphates prepared under different NaOH concentrations (0,
2, 4, 5, 10, 15, and 20 M). We find that monetite (CaHPO4,
dicalcium phosphate anhydrous, DCP) is synthesized at 0 M
NaOH concentration (JCPDS card number 70-1425). With
NaOH concentration >2 M, HAPs are synthesized without
the presence of other crystal phases. In general, monetite is
prepared under acidic conditions.9
Figure1. XRDpatterns ofcalciumphosphates synthesizedunderdif-
ferent NaOH concentrations.
Bull. Korean Chem. Soc. 2015, Vol. 36, 1–2
© 2014 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
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BULLETIN OF THE
Communication
KOREAN CHEMICAL SOCIETY
Table 1. Oxidation of benzyl alcohol, 1-phenyl ethanol, and benzyl
amine catalyzed by RuHAP using O2.
Time
(min)
Conversion
(%)
Catalyst
Substrate
Product
RuHAP(0 M)
RuHAP(2 M)
RuHAP(4 M)
RuHAP(5 M)
RuHAP(10 M)
RuHAP(15 M)
RuHAP(20 M)
RuHAP(0 M)
RuHAP(2 M)
RuHAP(4 M)
RuHAP(5 M)
RuHAP(10 M)
RuHAP(15 M)
RuHAP(20 M)
RuHAP(0 M)
RuHAP(2 M)
RuHAP(4 M)
RuHAP(5 M)
RuHAP(10 M)
RuHAP(15 M)
RuHAP(20 M)
30
30
30
30
30
17
91
88
85
53
70
43
40
90
98
49
65
83
72
8
30
30
120
120
120
120
120
120
120
60
60
60
60
60
64
54
46
42
43
49
60
60
Reaction conditions: catalyst 50 mg, benzyl alcohol (50 mM), 1-phenyl
ethanol (50 mM), benzyl amine (10 mM) in 2 mL toluene at 105 oC.
Acknowledgments. B.Y.L. acknowledges the support
from the Korea Ministry of Environment through
“The Environmental Health Action Program” (grant
number: ARQ201303173002), and “The Conversing Tech-
nology Program” (grant numbers: 2013001650001,
ARQ201403075001). K.Y.K acknowledges that this research
was supported by 2014 Korean-style SGER through the
National Research Foundation of Korea(NRF) funded by
the Ministry of Education(NRF-2014R1A1A2A16055892).
Figure 2. (a–g) TEM images of calcium phosphate prepared by dif-
ferentNaOHconcentrations. (a)0 M, (b)2 M, (c)4 M, (d)5 M, (e)10
M, (f ) 15 M, and (g) 20 M. (h) TEM image of RuHAP (4 M).
Table 1 shows the catalytic property of our solid catalysts for
oxidation of alcohols and amines into the corresponding car-
bonyl and nitrile compounds using O2 (under 1 atm) as
oxidant. We find thatRuHAPs prepared under NaOH concentra-
tion ranging from 2 to 5 M exhibit higher catalytic ability for the
oxidation of all three compounds (benzyl amine, 1-phenyl etha-
nol, and benzalhehyde). As shown in Figure 2, the sizes of HAP
crystallites prepared under NaOH concentration between 2 and
5 M are small compared to those prepared under other concen-
trations. Therefore, surface area might be playing an important
role in the catalytic activity. We plan to investigate the relation
between the catalytic property and the structure of HAPs.
In summary, we prepared calcium phosphates under seven
different NaOH concentrations. When these HAPs were tested
as catalytic supports of Ru for alcohol and amine oxidation,
we found that RuHAPs prepared under NaOH concentration
ranging from 2 to 5 M exhibited high catalytic activity.
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Bull. Korean Chem. Soc. 2015, Vol. 36, 1–2
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