T. Yagyu et al. / Inorganica Chimica Acta 412 (2014) 114–120
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example, monomeric ruthenium species having one hydroxy and
two aqua ligands grafted upon the HT surface acted as a multifunc-
tional catalyst for direct -alkylation of nitriles with primary alco-
a
hols [20]. This grafting method is simpler than previous ones using
the solid or polymer supported transition metal complexes [21].
More recently, a unique vanadium complex grafted on the surface
of the HT (V/HT) acted as a solid catalyst for highly efficient dehy-
dration of various amides into the corresponding nitriles [22].
These unique properties of HT and HT-like compounds, based
on their multifunctionarity including the cation and anion ex-
change ability, surface tunable basicity, and metal adsorption
capacity [19], will provide an attractive method for novel design
of heterogeneous catalyst. For example, they were known to pro-
mote aldol and Knoevenagel condensations [23]. Since the basic
characters are important for promoting these reactions, transeste-
rification is also expected to accelerate by HT [6,24]. In addition,
the lanthanum ion accelerated the transesterification as mentioned
above [7–17]. Here, we describe that lanthanum species grafted on
the surface of hydrotalcite (La/HT) can act as a solid catalyst for
transesterification. The design concept of La/HT catalyst is shown
in Fig. 1; (i) immobilization of lanthanum ion to the surface of
HT through the formation of La–O–M (M = Al, or Mg) bond; (ii) Le-
wis acid moiety for activation of ester through the coordination of
carbonyl group, (iii) Lewis base moiety for activation of nucleo-
phile, and (iv) surface hydroxyl function for formation of hydro-
philic field. These integrated functionalities will enable the high
performance catalytic activity.
Fig. 2. XRD pattern of HT and HT-like compounds; HT (A), HT-Ba (B), HT-Ni (C), HT-
Sc (D), HT-Fe (E), HT-In (F), and HT-La (G). Asterisks in HT-La (G) correspond to the
XRD pattern of La2(CO3)3.
somewhat higher than that of original HT (entry 6). In the La(III)
compound prepared by the co-precipitation method in aqueous
alkaline media, moderate activity (10% yield) was observed among
the ion-exchanged HT-like compound here tested (entry 7). The
yield of the transesterification products increased with increasing
the reaction time. On the other hand, simple La(III) compounds
did not show any activity for the transesterification (entries 8
and 9). The combined system of lanthanum ion and HT was impor-
tant for demonstrating catalytic activity. Accordingly, we carried
out the improved preparation method for La/HT catalyst.
2. Results and discussion
2.1. Preparation and characterization of HT and HT-like compounds
To investigate the catalytic activity for transesterification, some
HT and HT-like compounds were prepared using co-precipitation
method of inorganic salts in aqueous alkaline media. The ratio of
Mg/Al in the original HT was 3:1. In the preparation of HT-like
compounds, divalent or trivalent metal ions were partially ex-
changed for magnesium or aluminum ions of the brucite layer of
HT, respectively. In the structure of the HT-like compounds ana-
lyzed by XRD method (Fig. 2), the brucite sheet of these com-
pounds was similar to that of the original HT. We consider that
the slight difference in the XRD pattern depends on difference in
the crystal lattice of the original HT. The catalytic activities of these
HT-like compounds for transesterication of n-butyl decanoate in
methanol solution were shown in Table 1. The activity of Mg–
Ba–Al (2.7:0.3:1.0) compound containing the larger ionic radii of
Ba compared with Mg ion was lower than that of original HT (entry
2). Other ion-exchanged HT-like compounds containing similar io-
nic radii to Mg(II) and Al(III) showed similar activity compared
with the case of the original HT (entries 4 and 5). The activity of
the HT-like compound containing slightly larger indium ion was
2.2. Improvement of La/HT catalyst preparation
Since the activity of the lanthanum compound shown in Table 1
was not sufficient for the transesterication, we prepared new lan-
thanum catalyst combined with HT. According to the design con-
cept shown in Fig. 1, La(III) ion was immobilized on the surface
of HT. Complexation of La(III) with HT was carried out under vari-
ous aqueous conditions. Typical preparation method for La/HT was
as follows: The La(III) ion (La(NO3)3ꢀ6H2O 104 mg (0.25 mmol))
was dissolved in 100 mL of deionized water, to which HT (1 g)
was added, and appropriate amount of 1 M NaOH solution was
added to this reaction mixture to adjust the pH condition. The sus-
pension was stirred for appropriate maturing time at 60 °C and
cooled. The resulting precipitate was filtered and washed with
deionized water until the filtrate became neutral. The obtained
La/HT as a white powder was dried at 60 °C overnight.
As shown in Table 2, the catalytic activities of some lanthanum
compounds immobilized on the HT surface (La/HT) were higher
than that of the ion-exchanged one in the brucite layer for same
the reaction time (Table 1 entry 7 versus Table 2 entries 6, 8, and
11). Although there have been some reports on the hydrotalcite-
catalyze transesterication [6,24], the catalytic performance of this
La/HT was obviously superior to that of simple HT previously re-
ported. This activity of La/HTs formed through the complexation
of La(III) ion and HT as a macroligand might be derived from the
coordination sphere around La ion, which is discussed in the fol-
lowing section. Among the La/HT catalysts, those prepared at pH
7 and 9 showed slight low activities (Table 2, entries 1–4). The
highest catalytic activity of La/HT was obtained using that pre-
pared at pH 10, and the yield of the transesterication products
using La/HT catalyst was 54% (entry 6). La/HT catalyst was 54%
Activation of nucleophile
Formation of hydrophilic field
Activation of ester
HO
OH2
OH
M
La
OH
M
OH
M
O
O
OH
M
M
M
Coordination of La(III)
2-
CO3
surface
HT
M = Al or Mg
Fig. 1. The design concept of lanthanum species grafted on the surface of
hydrotalcite (La/HT).