Heterogeneous hydrogenation catalyses over recyclable Pd(0) nanoparticle catalysts stabilized by PAMAM-SBA-15 organic-inorganic hybrid composites

Article abstract of DOI:10.1021/ja056424g

Pd(0) nanoparticle catalysts stabilized by Gn-PAMAM-SBA-15 (n = 1-4) organic-inorganic hybrid composites were successfully prepared. Heterogeneous hydrogenation reaction of allyl alcohol over the Pd(0)-Gn-PAMAM-SBA-15 catalysts showed their high activities with TOFs of 2185, 2266, 711, and 739 and selectivities of 79.0, 82.0, 93.4, and 91.4%, respectively. The activity over the Pd(0)-G4-PAMAM-SBA-15 catalyst was 1.5 times that over the fourth generation PAMAM encapsulated Pd(0) homogeneous catalyst. These catalysts can be easily recovered, reused multiple times, and preserved for one month in the air, maintaining high catalytic efficiencies. Copyright

Full text of DOI:10.1021/ja056424g

Published on Web 12/23/2005
Heterogeneous Hydrogenation Catalyses over Recyclable Pd(0) Nanoparticle
Catalysts Stabilized by PAMAM-SBA-15 Organic-Inorganic Hybrid
Composites
Yijun Jiang and Qiuming Gao*
State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of
Ceramics, Graduate School, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China
Received September 18, 2005; E-mail: qmgao@mail.sic.ac.cn
Noble metal nanoparticles are attractive for catalyses because
of their size effects.1-3 However, having very active surface atoms
could often lead to aggregation of the naked nanoparticles and
decreases in catalytic activity and selectivity. In particular, Pd(0)
catalysts are known to aggregate easily and form Pd(0) black,
although they realize a wide variety of useful reactions in organic
syntheses.4 Recently, many stabilizing methods have been developed
to solve the problem.5-9 Unfortunately, the strong absorption of
the stabilizers on the active sites of the nanoparticles results in losses
of the catalytic activity and selectivity. Polyamidoamine (PAMAM)
dendrimers are highly branched, well-defined, synthetic macro-
molecules available in nanometer dimensions. The characteristic
of the dendrimers is that they are soft adsorbents that permit passage
of the substrates and products of the catalytic reactions. Recently,
Crooks’ group reported that the dendrimer-encapsulated noble metal
clusters exhibited highly catalytic activity, and the dendrimers acted
as both templates and porous nanoreactors.10-12 Although, as
homogeneous catalysts, the dendrimer-encapsulated noble metal
nanoparticles show uniquely catalytic characteristics, difficulties
Figure 1. (a) The low-angle XRD patterns of G4-PAMAM-SBA-15 (1)
in separation and reuse limit the application of the materials.
Immobilization of catalysts on solid supports may result in
significant losses of catalytic activities.13 As a result, much effort
has been put into finding new catalytic systems, which effectively
combine the advantages of both heterogeneous and homogeneous
catalyses.14,15
and Pd(0)-G4-PAMAM-SBA-15 (2) with the insert of the wide-angle XRD
pattern of Pd(0)-G4-PAMAM-SBA-15. (b) HRTEM image of Pd(0)-G4-
PAMAM-SBA-15. (c) EDS spectrum of Pd(0)-G4-PAMAM-SBA-15.
Scheme 1. Preparation of SBA-15 Supported Dendrimers
During our preparation progress, two steps were followed (see
the Supporting Information (SI) for details). First, the dendrimers
(more accurate names should be hyperbranched polymers; see the
SI description) up to generation 4 (Gn, n ) 1-4) were constructed
onto the surfaces of the channels of amine-functionalized SBA-15
(Scheme 1), using modified multistep procedures, based on the
literature.16,17 A Michael-type addition reaction of the preexisting
amino groups to the methyl acrylates forms the amino propionate
esters. Subsequent amidation of the ester moieties with ethylene-
diamine completed the generation. Repetition of these two reactions
produced the desired generation of the dendrimers. The structural
characterizations of the dendrimers grafted on the surfaces of SBA-
15 were carried out by cross-polarized magic-angle spinning
(CPMAS) 13C NMR and infrared spectroscopy (Figures S1 and S2).
The decrease of the surface areas, pore volumes, and pore sizes
with the increase of generations confirmed that the dendrimers were
constructed onto the channel surfaces of SBA-15, based on the
nitrogen adsorption analyses (Figure S3 and Table S2). Overall
yields of every generation of dendrimers were also calculated by
thermogravimetric and elemental analyses (Figure S4 and Table
S3). The above results proved that the dendrimers were successfully
constructed onto the channel surfaces of SBA-15. Second, Pd(II)
ions were introduced into the dendrimers in the tunnels of SBA-
15, and they were subsequently reduced by BH4-, which resulted
Scheme 2. Formation of Nanoparticles
in the formation of dark brown powders of Pd(0)-Gn-PAMAM-
SBA-15 (Scheme 2).
Low-angle X-ray diffraction (XRD) patterns (Figure 1a) of the
G4-PAMAM-SBA-15 and Pd(0)-G4-PAMAM-SBA-15 powders
show the peaks at 0.92, 1.57, and 1.84°, which correspond to the
(100), (110), and (200) reflections of the mesoporous SBA-15,
respectively,18 indicating that the structures of SBA-15 are well
preserved. Wide-angle XRD pattern (insert in Figure 1a) of the
Pd(0)-G4-PAMAM-SBA-15 powders shows a very weak peak at
40.2°, which is attributed to the (111) reflection of Pd(0) crystals,19
suggesting that the Pd(0) nanoparticles exist stably over the G4-
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10.1021/ja056424g CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Hydrogenation Activities and Selectivities of the
Catalysts
reaction. Generally, during the recycling and storage of the catalysts
consisting of the supported Pd(0) nanoparticles, an increase in the
size of the nanoparticles due to agglomeration and oxygenation in
the air always leads to deactivation of the catalyst.23 However, our
Pd(0) nanoparticles stabilized by the organic-inorganic hybrid
composites are stable enough to retain the activity when the catalyst
was reused several times or stored under ambient condition for one
month (Figure S10).
Pd(0)-G1-
PAMAM-
SBA-15
Pd(0)-G2-
PAMAM-
SBA-15
Pd(0)-G3-
PAMAM-
SBA-15
Pd(0)-G4-
PAMAM-
SBA-15
TOFa
2185/2150b
79.0/77.7
>99.5%
2266/2288
82.0/82.8
>99.5%
711/705
93.4/92.6
>99.5%
739/734
91.4/90.8
>99.5%
selectivityc(%)
conversion(%)
In summary, we have prepared heterogeneous Pd(0) nanoparticle
catalysts stabilized by Gn-PAMAM-SBA-15 organic-inorganic
hybrid composites. The catalysts show highly catalytic activity for
the hydrogenation of allyl alcohol. Importantly, the hydrogenation
rate and selectivity can be controlled by using different generation
catalysts. Additionally, the catalysts are stable enough to be recycled
multiple times and preserved for one month under ambient
condition, while maintaining the catalytic activities.
a The turnover frequencies (TOFs) were measured as moles hydrogenated
allyl alcohol per molar Pd per hour. b Duplicate measurements were
performed to illustrate the levels of run-to-run reproducibility. c The
selectivities to the hydrogenated products of 1-propanol.
PAMAM-SBA-15 hosts. High-resolution transmission microscopy
(HRTEM) image (Figure 1b) clearly shows that the Pd(0) particles
are quite monodisperse in the channels of SBA-15. The elemental
compositions of the nanoparticles were confirmed by the energy-
dispersive spectroscopy (EDS) and ICP analyses (Table S1).
To evaluate the catalytic characteristics of the materials, we
investigated the hydrogenation reactions of allyl alcohol over the
different generation catalysts (Figures S6, S7, and S8) under similar
conditions, which were adopted in the homogeneous hydrogenation
over the dendrimer-encapsulated Pd(0) nanoparticles.10 The turnover
frequencies (TOFs), selectivities to the 1-propanol, and conversions
for the hydrogenation of allyl alcohol are summarized in Table 1,
which suggest that all of the catalysts show highly catalytic activity
for the hydrogenation of allyl alcohol. Notably, the activity of our
Pd(0)-G4-PAMAM-SBA-15 catalyst is 1.5 times that of the fourth
generation dendrimer-encapsulated Pd(0) homogeneous catalyst.10
Probably, the existence of the inorganic carrier may reduce the
aggregation of the dendrimer-encapsulated Pd(0) nanoparticles
relative to the homogeneous dendrimer-encapsulated Pd(0) catalysts.
Generally, substrate isomerization is a common but unwanted
process in hydrogenation.20 It is desirable to minimize the isomer-
ization during the hydrogenation reaction. In our system, the
selectivity to hydrogenation product (1-propanol) was improved
when increasing the product generation. Under the same conditions,
the selectivities were 79.0, 82.0% and 93.4, 91.4%, when using
generations 1, 2 and 3, 4 catalysts, respectively. Selectivity of the
isomerization byproduct of acetone over the generation 3 catalyst
was 6.6%, which is much lower than that over the reported
heterogeneous catalyst Pd/Al2O3 (25%).20 These results indicate that
the dendrimers may not only stabilize the nanoparticles but also
modify both the environment of active sites and access to these
sites.21
Acknowledgment. This work was financially supported by the
Chinese National Science Foundation (NO. 20201013) and the
Creative Foundation (No. SCX200404) of Chinese Academy of
Sciences.
Supporting Information Available: Procedure for preparing
Pd(0)-Gn-PAMAM-SBA-15 catalysts, hydrogenation reactions, data of
catalyses, HRTEM of the G3-PAMAM-SBA-15 and Pd(0)-G3-PAMAM-
SBA-15, N2 adsorption data of the Gn-PAMAM-SBA-15, results of
the ICP-AES, TGA, EA quantitative analyses, 13C CPMAS NMR, and
FT-IR spectra of the materials (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
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