COMMUNICATION
Selective catalytic activity of ball-shaped Pd@MCM-48 nanocatalysts
Hee-Yoon Lee,*a Suyoung Ryu,a Hongkyu Kang,a Young-wook Junb and Jinwoo Cheon*b
Received (in Cambridge, UK) 2nd December 2005, Accepted 30th January 2006
First published as an Advance Article on the web 16th February 2006
DOI: 10.1039/b517082f
Remarkable selectivity is achieved in the cleavage of benzyl
ethers using ball-shaped palladium nanocatalysts, Pd@MCM-
48, in an MCM-48 matrix. The unique nanocatalysts not only
feature unprecedented complete hydrogenolysis selectivity of a
benzyl ether over hydrogenation of a double bond, but also
demonstrate selective cleavage of unsubstituted benzyl ether
over substituted benzyl ethers.
Nano-structured catalytic materials have drawn attention for their
potential durability, reactivity and selectivity in chemical processes
due to their unique structural properties.1 One interesting method
to prepare nano-structured catalysts is templated synthesis in
which active catalysts are encapsulated inside channels and pores
of a host. With the development of practical synthetic routes for
mesoporous silicate materials, extensive research effort has been
focused on their applications to catalytic reactions.2,3 Although
there have been several recent reports on the template-assisted
metallic nanocatalysts demonstrating increased reactivity,4 the
improvement of selectivity is very challenging in organic reactions.5
Recently, we described the preparation of well-defined ball-
shaped Pd nanomaterials inside a cubic phase MCM-48 matrix.6
Nanostructuring was accomplished by chemical vapor infiltration
of Pd(hfac)2 (hfac = 1,1,1,5,5,5-hexafluoro-acetylacetonate) into
the template matrix MCM-48, followed by mild thermal decom-
position under H2 gas to generate pure Pd metal inside the
template. The MCM-48 matrix has pore size of y3 nm and
Pd@MCM-48 is composed of ball shaped isolated Pd domains of
y38 nm in diameter (Fig. 1). The Pd metal surface area measured
by CO chemisorption is 12 m2 g21 Pd with a metal dispersion
of 3.3%. The observed Pd surface area of our sample is about
3–10 times lower than those obtained from one dimensional matrix
systems such as MCM-41,7 which is not surprising since those Pd
are well dispersed inside linear pore channels while ours are highly
localized ball types with Pd surface accessibility only at the end of
pore channels. Since the Pd domains of y38 nm are arranged with
an ordered array of nanoscale exposed-surface inside pore
channels, nanostructured Pd@MCM-48 may offer unexpected
catalytic activities for organic reactions.
Fig. 1 Transmission
electron
micrograph
of
Pd@MCM-48
nanocatalyst.
significant discrimination among olefins relative to Pd/SiO2 or Pd/
C (Table 1). The slower reduction rate with Pd@MCM-48 was
presumed to be due to the difference in exposed surface area of Pd
since the hydrogenation reaction rate was reported to be
independent of pore diffusion.9 However, when considering the
accessible Pd surface area of Pd/SiO2 and Pd/C (y243 m2 g21 Pd)
obtained by CO chemisorption is about 20 times higher than that
(y12 m2 g21 Pd) of Pd@MCM-48, the hydrogenation reactivity
of Pd@MCM-48 per accessible Pd surface area can be considered
far higher than that of Pd/C. Similar catalytic activity has been
observed previously in other supported Pd catalysts.7
When Pd@MCM-48 was tested as the catalyst for hydro-
genolysis of benzyl ether 1, contrary to the anticipation that the
silica support would provide an acidic environment to facilitate the
Table 1 Comparison of hydrogenation ratea
Relative rate
5% 3%
Pd/C Pd/SiO2 Pd@MCM-48
b
Substrate
First, the hydrogenation activity was compared with those of
Pd/SiO2 and Pd/C on various olefins ranging from a terminal
olefin to a trisubstituted olefin to observe for a possible
discrimination of substitution patterns of olefins by the catalyst.5d,8
The reduction rate with Pd@MCM-48 was about 1.5 to 5 times
slower than those with Pd/SiO2 or Pd/C and did not show
Methyl-3-butenoate
trans-2-Pentenyl benzoate
4-Methyl-4-pentenyl pivaloate
Cyclohex-3-enylmethyl benzoate 0.7
3,7-Dimethyloct-6-enyl benzoate 0.08 0.05
2.9
1.1
0.4
2.5
0.89
0.3
0.7
1
0.23
0.27
0.26
0.03
a
The hydrogenation reaction of a mixture of equimolar amount of
olefins and dodecanol as the standard with 10% catalyst (w/w to the
total amount of mixture of olefins) under hydrogen atmosphere
(1 atm) was monitored by gas chromatography (Column : NB-54,
30 m capillary column, conditions; temp.: gradient from 80 uC to
aCenter for Molecular Design and Synthesis, Department of Chemistry,
KAIST, Daejon, Korea. E-mail: leehy@kaist.ac.kr; Fax: 8242 869 8370;
Tel: 8242 869 2835
210 uC (10 uC min21), flow rate : 30 ml min21). The catalyst was
b
prepared by conventional wet infiltration & reduction method using
MCM-48 as the support.
bDepartment of Chemistry, Yonsei University, Seoul, Korea.
E-mail: jcheon@yonsei.ac.kr; Fax: 8223647050; Tel: 82221235631
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 1325–1327 | 1325