First example of high loaded polymer-stabilized nanoclusters
immobilized on hydrotalcite: effects in alkyne hydrogenation
Jules C. A. A. Roelofs* and Peter H. Berben
Engelhard De Meern b.v. Strijkviertel 67, 3454 ZG De Meern, The Netherlands.
E-mail: jules.roelofs@Engelhard.com; Fax: +31 30 666 9369; Tel: +31 30 6669306
Received (in Cambridge, UK) 16th January 2004, Accepted 19th February 2004
First published as an Advance Article on the web 16th March 2004
Poly(N-vinyl-2-pyrrolidone)-stabilized Pd-nanoclusters, for the
first time exclusively supported on the hydrotalcite lateral
surface, showed a remarkable catalytic performance in the
selective hydrogenation of 3-hexyn-1-ol, which can be ascribed
to both the influence of the protecting polymer PVP as well as
the nature of the support.
using a glass reactor equipped with baffles and 0.13 atm pressure of
H .
2
Fig. 1 shows the XRD patterns for the nanoclusters supported on
carbon (NC/C) and HT (NC/HT).† The (111) and (200) reflections
of Pd have been indicated with arrows. The obtained pattern for HT
showed the typical characteristics for this layered clay material.
The position of the (003) reflection indicated that carbonate was
7
In recent years, interest in the synthesis of colloidal metal
nanoclusters of controlled sizes and catalytic applications thereof
has emerged.1 Many preparation routes include the use of a
protecting polymer, capable of coordinating to the metal atoms and
thereby stabilizing the small metal clusters in solution. One of the
most commonly used polymers for this purpose is poly(N-vinyl-
still present in the interlayer, suggesting that the nanoclusters were
immobilized at the basal platelet surfaces. The catalysts prepared
,2
10
by Mastalir et al. showed increased basal spacings caused by Pd
particles present in the interlayer, or partly within organoclay
tactoids,12 unfavourable for catalysis. Due to the position and
intensity of interfering reflections of the basic support the Pd
signals were not visible in case of HT. However, the polymer-
protected nanoclusters were present on the platelet surface of HT,
as shown by TEM in Fig. 2. Uniform nanoparticles can be seen
throughout the image, only present on the support material (Fig.
2A). Dark field TEM shows that these Pd nanoparticles are of high
2-pyrrolidone) (PVP). Recently, the use of microwave radiation has
been applied to prepare colloidal metal nanoclusters of small sizes
3
and with narrow size distributions. This offers the possibility of
synthesizing these nanoclusters in a facile, reproducible and fast
4
way, without the commonly applied long synthesis times. Studies
2
+
on the immobilization of nanoclusters are relatively scarce and
crystallinity (Fig. 2B). This indicates that the reduction of Pd by
ethylene glycol and microwave radiation was successful, even with
the absence of stirring during radiation in our experimental setup.
From TEM analysis of the nanoclusters of uniform size an average
particle size of 4.2 ± 0.8 nm was determined. Many factors are
known to influence the final nanocluster size, i.e. pH, choice of
5
mostly limited to loadings below 1%. In recent years, a growing
interest has developed for the utilization of hydrotalcite compounds
6
(
HT) for various catalytic applications. The structure of HT
2
+
3+
consists of positively charged layers containing Mg and Al ,
octahedrally coordinated by hydroxyl groups. These octahedra
share adjacent edges to form sheets or layers. The space between
the stacked cation layers is filled with charge-compensating anions,
reducing agent, reduction time, concentrations and ratio of PVP and
Pd salt.1
3,14
Reported value sizes relevant to our procedure vary
7
3
13
with preference for carbonate having the highest affinity. Some
between 1.7 nm and 5.0 nm. When compared with TEM images
of the colloidal solution no change in size due to the immobilization
was observed. Another interesting feature is the regular distribution
of the particles on the HT support, likely due to the presence of
PVP, preventing agglomeration. PVP is removed only above
Pd-containing HTs are known, prepared by well-established
8
9
methods, i.e. co-precipitation and impregnation, usually followed
1
0
by reduction at elevated temperatures. Mastalir et al. synthesized
low-loaded organophilic Pd/HT via an inventive ion-exchange,
with interesting catalytic properties. However, this method is not
applicable for HT with interlayer carbonate and is furthermore
limited by the anion exchange capacity. In this communication, we
demonstrate that highly loaded polymer-protected Pd-nanoclusters
supported on hydrotalcite and carbon show remarkable differences
in selectivity and activity in the cis-selective hydrogenation of
-hexyn-1-ol towards the fragrance leaf alcohol.
In a typical experiment, 6 gram (27 mmol) of PVP was dissolved
in 150 gram (2.4 mol) of ethylene glycol. After obtaining a clear
solution, 7.3 gram of Na PdCl solution (10 wt% Pd) was added
and this brown solution was heated in a laboratory microwave at
50 W for 90 seconds. The colour changed to black, indicative of
350 °C, according to TPR (results not shown). As compared to
reported loadings of Pd particles on hydrotalcites,8
–10,12
a tenfold
higher loading can be achieved with the described procedure.
The hydrogenation activities of the catalysts (including a
commercial Engelhard 5% Pd/C as reference) were tested in the
hydrogenation of 3-hexyn-1-ol (Scheme 1). Besides the products
shown, further hydrogenation to hexanol and hexane is possible,
which was also observed in our experiments (Table 1). As
expected,11 the commercial Pd/C showed a high activity, but poor
selectivity. The NC/C behaved completely differently: a low
activity and a preferred formation of the cis-isomer were observed.
An almost threefold increase in activity was found with the NC/HT
catalyst as compared to the NC/C without loss of selectivity
1
1
3
2
4
7
2
+
the reduction of Pd by ethylene glycol and the applied energy
input. The hot solution was added to a 400 ml aqueous support-
containing solution. The solution was stirred overnight at room
temperature and after filtration dried at 110 °C under nitrogen
atmosphere. The Pd content was varied up to 9 wt%, which was
easily tuned by variation of the amount of support. At high loadings
not all filtrates became colourless, indicative of the presence of
nanoclusters in the filtrate. This implies that at these loadings not all
nanoclusters were immobilized on the support. However, addition
of the dried catalyst powders to known solvents for these polymer-
4
stabilized nanoclusters like methanol, ethanol and water did not
result in coloured solvents. For catalytic measurements, a known
amount of catalyst (typically 30 mg) was dispersed into 80 ml
MeOH containing 3-hexyn-1-ol (around 1.3 g, 13 mmol). The
hydrogenation reactions were performed at 29 °C and 2200 rpm
Fig. 1 (A) XRD pattern of 7% NC/C; (B) 9% NC/HT.
9
70
C h e m . C o m m u n . , 2 0 0 4 , 9 7 0 – 9 7 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
Roelofs, Jules C. A. A.
