Recyclable hydrotalcite catalysts for alcohol imination via acceptorless dehydrogenation

Article abstract of DOI:10.1039/c5gc00312a

Here we report that hydrotalcite-like materials (HTs) are active heterogeneous catalysts for alcohol imination, which proceeds through acceptorless alcohol dehydrogenation. The catalytic activity of a series of Mg-Al hydrotalcites doped with Fe³⁺, Zn²⁺, Ni²⁺, Cr³⁺ and Cu²⁺ is dependent on their composition, and Fe : Mg : Al HT yields up to 92% imine under mild conditions. Impregnation of Fe : Mg : Al HT with Pd⁰ resulted in an enhancement of activity for acceptorless dehydrogenation, but a decrease in the isolated yield of imine in a loading-dependent manner. This is attributed to the Pd loading-dependent retention of imine and aldehyde on the catalysts. The substrate scope for alcohol imination and recyclability of the catalysts is discussed.

Full text of DOI:10.1039/c5gc00312a

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Recyclable hydrotalcite catalysts for alcohol
imination via acceptorless dehydrogenation†
Cite this: Green Chem., 2015, 17,
2271
John Bain, Philip Cho and Adelina Voutchkova-Kostal*
Here we report that hydrotalcite-like materials (HTs) are active heterogeneous catalysts for alcohol imina-
tion, which proceeds through acceptorless alcohol dehydrogenation. The catalytic activity of a series of
Mg–Al hydrotalcites doped with Fe3+, Zn2+, Ni2+, Cr3+ and Cu2+ is dependent on their composition, and
Fe : Mg : Al HT yields up to 92% imine under mild conditions. Impregnation of Fe : Mg : Al HT with Pd0
resulted in an enhancement of activity for acceptorless dehydrogenation, but a decrease in the isolated
yield of imine in a loading-dependent manner. This is attributed to the Pd loading-dependent retention of
imine and aldehyde on the catalysts. The substrate scope for alcohol imination and recyclability of the
catalysts is discussed.
Received 9th February 2015,
Accepted 4th March 2015
DOI: 10.1039/c5gc00312a
www.rsc.org/greenchem
Hydrotalcites (HTs) are layered double hydroxides (LDHs)24
Introduction
3+
with the general formula [M2+
M )x/n·mH2O,
x(OH)2]x+(An−
1−x
Imines are highly versatile intermediates for the stereoselective where A− represents anions (e.g. CO32−, OH−, Cl−, NO32−, SO4
synthesis of amines and N-heterocycles.1 Synthetic routes to etc.) and M2+ and M3+ are compatible alkali earth and tran-
imines that utilize readily available starting materials, such as sition metal ions (e.g. Fe3+, Zn2+, Ni2+, Cr3+, Cu2+, Co2+ etc.).
alcohols, are typically more atom-economical than convention- Although variation in the composition does not impact the
al routes requiring carbonyl compounds. Alcohol imination brucite-like structure,24 it does affect the acid/base,25–28
reactions can proceed through alcohol oxidation to the corres- redox,28 and catalytic properties of these materials.25,29–33 Par-
ponding ketone or aldehyde, followed by condensation with ticle size and morphology can also be varied by modifying the
the amine. Amination catalysts typically proceed through synthetic protocol.34–37 In addition, the LDH structure itself
alcohol dehydrogenation, amine condensation and imine can be semi-reversibly transformed to that of a mixed metal
hydrogenation (Scheme 1a).2–10 Alcohol imination can proceed oxide (MMO) by calcination at ∼450 °C.38,39 MMOs outperform
through a similar route lacking the hydrogenation step the corresponding LDHs in a number of catalytic applications,
(Scheme 1b). Although a number of highly active catalysts have as they are significantly more basic and resistant to sintering.38
been reported for alcohol amination,4,8,10–18 catalytic systems Rehydration of MMOs causes incomplete reversion to a more
that yield the imine are rare and offer the advantageous possi- basic LDH phase via a “memory effect”.33,40 Thus, by control-
2−
bility of further imine functionalization.17,19–23
ling composition, synthetic protocol and post-synthetic treat-
ment a palette of synthetic HTs can be accessed with
comparable structures, but substantial differences in catalytic
activities. This allows optimization of the catalytic activity by
varying the HT composition.38
HTs are increasingly used as heterogeneous catalysts
for a number of base-assisted solution-phase reactions that
operate under mild conditions. Examples include aldol
condensations,41–43 Claisen–Schmidt condensations,44 acyla-
tion of phenols, amines and thiols,45 epoxidations,46 trans-
esterifications,47 and N-arylation of amines.48 HTs have also
been reported as catalysts for alcohol oxidation that takes
place under mild conditions: Sakthivel et al. have recently
reported a Co/Al hydrotalcite system;49 and Choudary50 and
Takehira51 have reported a Ni/Mg/Al hydrotalcite system for
alcohol oxidation using molecular oxygen. However, acceptorless
Scheme 1 Catalytic alcohol (a) amination and (b) imination via
dehydrogenation.
Department of Chemistry, The George Washington University, Washington, DC, USA.
E-mail: avoutchkova@email.gwu.edu; Fax: +202-994-5873; Tel: +202-994-6121
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5gc00312a
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alcohol dehydrogenation is a more attractive approach due to
the higher atom economy and generation of hydrogen gas. The
catalytic activity of Mg–Al HTs for acceptorless alcohol de-
hydrogenation is only reported under high temperature and
pressure conditions (533 K, 100 kPa) in the gas phase.52
Alcohol imination using HTs has not been reported to the best
of our knowledge, but there is one example of alcohol amin-
ation using a Cu/Al hydrotalcite, which likely proceeds through
alcohol dehydrogenation.53
Table 1 Characterization data of hydrotalcites (HTs) 1–6
% Elemental
PXRD crystallographic
composition (FAAS) parametersb
BET S.A.
Cat. Ma Mg
Al
Ma
a (Å)
c (Å)
L (nm) (m2 g−1
)
HT1 28.3 9.28
HT2 Zn 20.9 6.95 29.0
HT3 Cu 21.9 8.54 13.2
—
—
3.001 23.348 16.9
3.005 23.186 10.0
2.992 22.986 15.0
3.082 23.145 20.0
63
128
30
73
64
5
HT4 Fe
HT5 Ni
HT6 Cr
29.4 3.05 11.4
25.0 10.2 14.0c 2.998 23.346 12.9
25.2 3.36 10.1c 3.020 23.067 11.9
In addition to the interest in HTs as heterogeneous cata-
lysts, they are also increasingly used as “inert” supports for
other catalysts. Examples include supported Rh,54 V,55 and
Ru56 oxides and nanoparticles (n.p.) of Ag, Pd,57 Au,58 Cu,59
Ru,60 Rh,61 and Ni.62 Nanoparticles of Ag, Pd, and Cu sup-
ported on Mg–Al HTs are reported as active catalysts for accep-
torless alcohol dehydrogenation under mild conditions.
Although these reports note that the HT support is critical for
catalytic activity,59,60,63–66 they do not attribute inherent cataly-
tic activity for dehydrogenation to the HT itself.51,60,67,68
While exploring the catalytic activity of HT-supported
Pd n.p. for alcohol imination, we observed that some HTs
exhibit inherent catalytic activity for alcohol dehydrogena-
tion and imination. Here we report the optimization of a
modified hydrotalcite that can act as an efficient, cheap and
recyclable heterogeneous catalyst for alcohol imination
under mild conditions. We show that supported Pd0 can
enhance activity for dehydrogenation, but actually retard
activity for imination.
a The third metal added to the Mg/Al HT. b a, the average cation–cation
distance; c, three times the distance from the centre of one brucite-like
layer to the next layer; L, the average crystallite size (lower bound,
calculated using Scherrer’s formula).69 c Values obtained by EDX.
Results and discussion
Six modified Mg–Al hydrotalcites (HTs) were synthesized with
the addition of the first row transition metal ions. The first
row metals were selected due to their relative abundance in
comparison with the second and third row transition metals,
and the compatibility of their ionic radii for incorporation into
Fig. 1 PXRD patterns of (a) HT4, (b) 0.02% Pd/HT4, (c) 1% Pd/HT4, and
(d) 5% Pd/HT4.
the HT lattice. The metal ions selected (Cr3+, Fe3+, Ni2+, Cu2+ times the distance from the centre of one brucite-like layer to
and Zn2+) provide a range of acid–base properties, hydridic/ the next, respectively) were all within 0.015 Å and 0.92 Å
protic behaviour and redox potentials. HTs 1–6 were syn- ranges respectively, suggesting only minute differences in
thesized by co-precipitation and aging of the corresponding the lattice structures of the HTs. The variation in compo-
transition metal nitrate salt with Mg(NO3)2·6H2O and Al sition is reflected in an evident variability in the BET surface
(NO3)3·9H2O. Table
1
shows the elemental composition, areas of HTs 1–6 (5–128 m2 g−1). This is due to small differ-
powder X-ray diffraction (PXRD) parameters and BET surface ences in the synthetic protocols for the respective HTs, differ-
areas of the resulting HTs 1–6. The PXRD and BET surface ences in the rates of co-precipitation and porosity of the
area parameters are consistent with literature values (ESI resulting HT. The BET surface area of the HT6, which con-
Table S1†).
tains 10.1% Cr3+, is reproducible but anomalously low due to
Powder X-ray diffraction (PXRD) patterns confirmed the very low porosity relative to the other HTs. PXRD and FT-IR
phase, phase transition, crystallinity and lattice parameters characterization of HT6 was consistent with HT as the
of the HTs, as well as Pd-impregnated HTs (Pd/HTs) dis- only phase.
cussed in a later part of the manuscript. Exemplary PXRD
HTs 1–6 were also characterized by FT-IR. Fig. 2 shows
patterns are shown in Fig. 1. The PXRD patterns of all six exemplary FT-IR spectra of HT4 and the series of Pd/HT4 with
HTs (shown in ESI Fig. S1†) were consistent with the pure HT varying loadings of Pd, while spectra for HTs 1–6 are shown in
phase, characterized by sharp symmetric reflections (003 and ESI Fig. S2.† Carbonate anions were detected in all HTs and
006) and broad asymmetric reflections.24 The lattice para- Pd/HTs, as indicated by the vCvO at 1350–1370 cm−1. Broad
meters a and c (average cation–cation distance and three bands arising from bending vibrations of interlayer water are
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Fig. 2 FT-IR spectra of (a) HT4, (b) 0.02% Pd/HT4, (c) 1% Pd/HT4, and
(d) 5% Pd/HT4.
observed between ∼1400 and 1700 cm−1. Since the symmetry
of the interlayer water molecules is affected by composition,
some HTs show additional broad bands in that region. Nitrate
anions were not detected (no sharp peak at 1376 cm−1),70 indi-
cating that nitrate salts of the starting materials had been
washed out of the lattice.
Fig. 3 Percent yields of benzaldehyde and 4-methoxy-N-(phenyl-
methylene)benzenamine from reactions in Schemes 2 and 3. Alcohol
dehydrogenation reaction conditions: 0.24 mmol of benzyl alcohol,
0.100 g of catalyst, 3 mL toluene, 110 °C, 48 h. Yields were determined
from GC-FID with an internal standard (trimethoxybenzene). Alcohol
iminations were carried out under identical conditions, with the addition
of 0.29 mmol p-anisidine.
Catalytic activity of hydrotalcites for alcohol dehydrogenation
and imination
product under the screening conditions. The observed relative
catalytic activity of the HTs for imination is as follows:
The catalytic activity of HTs 1–6 towards alcohol dehydrogena-
tion (Scheme 2) and imination (Scheme 3) of benzyl alcohol in
the absence of a base is summarized in Fig. 3.
Fe : Mg : Al ðHT4Þ > Ni : Mg : Al ðHT5Þ > Mg : Al ðHT1Þ
> Cu : Mg : Al ðHT3Þ > Zn : Mg : Al ðHT2Þ
ꢀ Cr : Mg : Al ðHT6Þ
The yields of aldehyde resulting from alcohol dehydrogena-
tion are lower than the yields of imine, which implies that the
amine improves dehydrogenation yield by drawing the equili-
brium towards products via condensation with the aldehyde. A
general correlation between the activities for both reactions
was observed, consistent with imination occurring via de-
hydrogenation. The molar ratio of H2 to benzaldehyde was
within experimental error of being stoichiometric (89 : 100) in
the dehydrogenation of benzyl alcohol, thus indicating that H2
is generated quantitatively during the reaction. A clear differ-
ence in catalytic activity was observed among the six HTs. In
contrast to the negligible yield of the imine afforded by HT6
(Cr : Mg : Al), HT4 (Fe : Mg : Al) afforded 85% imination
Optimization of solvents for imination reactions increased
the yield afforded by HT4 to 92% when xylenes were used as
solvents. The reaction proceeds without the need for an exter-
nal base, and addition of K2CO3 did not improve the yield.
Although the aniline present is itself a homogeneous base,
results from alcohol dehydrogenation experiments confirm
that no external base is necessary for dehydrogenation. In con-
trast, homogeneous catalysts for similar processes do not gen-
erally operate efficiently without an external base.17
Given the similar lattice structures of the six HTs, these
differences in activity are likely due to the respective acid–base
properties, redox properties, and catalytic surface areas.
Although differences in surface areas cannot account for the
variation in catalytic activity, the lack of activity for HT6 is
likely associated with its anomalously low surface area and the
lack of porosity that is apparent from the gas adsorption iso-
therms. The differences in acid–base and redox properties are
currently under investigation, and likely have a predominant
influence on catalytic activity. Preliminary microcalorimetry
results76 that probe the heats of adsorption of CO2 showed
HT4 to have intermediate basicity among the series, which
implies that there is likely an optimal basicity associated with
the highest catalytic activity. It should also be noted that
trends from microcalorimetric studies are impacted by the
surface area, volume of pore or diameter of pore. The latter
three are not consistent across HTs 1–6, as indicated in
Scheme 2 Catalyst screening for alcohol dehydrogenation.
Scheme 3 Catalyst screening for alcohol imination.
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Table 2 Percent yields of (a) 4-methoxy-N-(phenylmethylene)benzen-
amine and (b) benzaldehyde from imination or dehydrogenation of
benzyl alcohol facilitated by HT4 with different thermal treatments
(HT4: freshly prepared; HT4calc: calcined at 450 °C and HT4reh: calcined
at 450 °C and then rehydrated)
Table 3 Characterization data of Pd/HT4 catalysts (Pd/HT4): Pd com-
position (determined from atomic absorption spectroscopy) and PXRD
crystallographic parameters (a, c and L)
PXRD crystallographic parametersa
Catalyst
% Pd (AAS)
a (Å)
c (Å)
L (nm)
HT4
HT4calc
HT4reh
Dehydrogenation (Scheme 2)a
Imination (Scheme 3)a
23
92
37
77
8
38
0.02% Pd/HT4
1% Pd/HT4
5% Pd/HT4
0.020
1.050
4.875
3.097
3.052
3.055
23.640
22.711
23.226
16.9
12.6
12.0
a Conditions as described in Fig. 3.
a a, the average cation–cation distance; c, three times the distance from
the center of one brucite-like layer to the next layer; L, the average
crystallite size (lower bound, calculated using Scherrer’s formula).69
Table 1, so further research efforts are focused on optimizing
the synthetic protocols so as to make surface areas consistent.
This will allow us to explicitly probe the effect of basicity.
Since the basicity of HTs can be enhanced semi-reversibly
by calcination38,39 and by calcination–rehydration,33,40 we also
probed the catalytic activity after calcination of HT4 at 450 °C
and after calcination–rehydration. This treatment afforded
HT4calc and HT4reh respectively. Under the optimized con-
ditions both were less active for imination than the HT4,
although HT4calc showed higher dehydrogenation activity
(Table 2).
We initially hypothesized that the enhanced basicity of
HT4calc and HT4reh would translate into higher activity for
dehydrogenation, as the basic sites are implicated in deproto-
nation of the alcohol and β-hydride elimination. Indeed we
observe a higher dehydrogenation yield with HT4calc. However,
HT4calc and HT4reh both afford lower imination yields than
HT4. To test whether this decrease in yield is due to the
adsorption of aldehyde and/or imine to the HT, we monitored
the concentration of free aldehyde and imine respectively
when each was reacted with HT4, HT4calc and HT4reh respect-
ively. We found that HT4calc retains 53% and 41% after 24 h of
reaction. Under the same conditions HT4reh retains 28% of the
imine and 35% of the aldehyde respectively. Given that HT4
retains smaller fractions of aldehyde and imine, the lower
yields of aldehyde (for HT4reh) and imine (for HT4calc and
HT4reh) are likely due to the adsorption of the products on the
catalyst. Extensive washing of the catalyst did not release
additional products.
are shown in Table 3. PXRD patterns showed no detectable Pd
phase and only minor variations from the crystallographic
parameters of HT4 (Fig. 1). No detectable change in the HT4
interlayer spacing by PXRD is consistent with a highly dis-
persed Pd phase.72,73 The FT-IR spectra of Pd/HT4 show broad-
ening of the O–H (vO–H at 3000–3600 cm−1) bands, suggesting
a greater degree of hydrogen bonding in the surface and/or
interlayer water. Small shifts in the carbonate band (vCvO at
∼1400 cm−1) and stretches related to interlayer water at
1560–1700 cm−1 likely result from the changes in the sym-
metry of interlayer water molecules.
The catalytic activity of the Pd/HT4 as it relates to the Pd
loading is illustrated in Fig. 4 and Table 4.
The aldehyde yields isolated from alcohol dehydrogenation
(Scheme 2) were dependent on the Pd0 loading, with the
highest yield (85%) obtained with the 1% Pd/HT4 catalyst after
2 h (0.039 mol% Pd vs. alcohol), dropping to 65% when Pd0
loading on HT4 was increased to ∼5% (0.191 mol% Pd). The
trend is illustrated in Fig. 4. The lower yields with 5% Pd/HT4
are likely due to the agglomeration of Pd, as confirmed by
TEM. In contrast, in the 1% Pd/HT4 catalyst no distinct nano-
particles were observed by TEM, suggesting a more disperse
Pd phase. The fact that Pd impregnation enhances
the aldehyde yield suggests that HT4-supported Pd0 is more
Catalytic activity of Pd0 on Fe–Mg–Al hydrotalcite for alcohol
dehydrogenation and imination
Given that HT4 was the most active HT catalyst for both dehy-
drogenation and imination, we expected that immobilization
of small quantities of Pd(0) on this support would boost the
catalytic activity. This is consistent with literature reports that
document alcohol dehydrogenation facilitated by immobilized
Pd on Mg/Al HT.60,63,71 We thus synthesized and characterized
three Pd/HT4 catalysts with different Pd loadings (Table 3).
The presence of Pd was confirmed by AAS and TEM/EDX. TEM
images did not show distinct Pd nanoparticles, suggesting that
the Pd phase is highly dispersed (ESI Fig. S3†). The PXRD para-
Fig. 4 Percent yields of (a) imine from Scheme 3 and (b) benzaldehyde
from Scheme 2 versus % Pd loading on HT4. Reaction conditions:
0.24 mmol of benzylalcohol, mol% Pd as shown in Table 4, 3 mL
toluene, 110 °C, 24 h. Yields were determined from GC-FID using tri-
methoxybenzene as an internal standard. Error bars designate estimated
meters and the elemental composition of palladium on HT4 standard error arising from measurement error and multiple runs.
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fully established, as the active sites on these catalysts have not
yet been identified or quantified. That is to say, we cannot
assume that the Pd species are the only active catalyst, as our
data clearly show the HT4 itself is catalytically active for
alcohol dehydrogenation. The investigation of the mechanism
and active sites of these catalysts is the topic of ongoing
studies, which include an in-depth study of the electronic
support interactions between Pd and HT4.39,74
Table 4 Percent yields of benzaldehyde and 4-methoxy-N-(phenyl-
methylene)benzenamine from reactions in Schemes 2 and 3
%Yield
mol%
Pda
Imine,
24 hb
Aldehyde,
2 hb
Aldehyde,
24 hb
Catalyst
HT4
0
85
82
73
29
15
26
85
65
23
53
67
7
0.02% Pd/HT4
1% Pd/HT4
5% Pd/HT4
0.001
0.039
0.191
Substrate scope for imination
a mol% Pd relative to benzyl alcohol. b Conditions as described in
Fig. 3.
The alcohol substrate scope for imination using HT4 under
optimized reaction conditions is shown in Table 5. Benzylic
alcohols generally showed good to excellent yields. The reac-
tion is favoured by electron-donating substituents of the
benzene ring, such as methoxy (entries 1 and 3: 97% and
89%) and disfavoured by strongly electron-withdrawing nitro
groups, which reduce the yield to 75% (entry 5). Benzyl
alcohol itself also gave excellent yields of imine (92%, entry
2). Interestingly, a low yield was obtained with 4-(hydroxy-
methyl)phenol (15%, entry 7) despite the electron-donating
–OH group. Phenol could be deprotonated by the basic cata-
lyst to form the corresponding phenoxide, which could com-
petitively inhibit activation of the benzylic alcohol on the
HT4. The resulting phenoxide might also intercalate and
become immobilized in the HT interlayers, resulting in
decreased product yield. The extension of the benzylic alcohol
by one methylene group to phenethyl alcohol reduces the
yield to 32%, indicating that this mild catalytic system is
selective for activated alcohols. Primary and secondary ali-
phatic alcohols show no reaction under these conditions
(entries 8 and 9). It is notable that under the conditions
reported by Uemura et al., Pd/HT systems do not show a
similar selectivity for alcohol dehydrogenation,67,68,75 and are
reactive for aliphatic alcohols.
The amine substrate scope using optimized reaction con-
ditions and HT4 is shown in Table 6. The yields observed with
anilines were strongly influenced by the ring substituents. Ani-
lines with electron donating groups, such as methoxy (entry 1),
afforded high yields, while those with strongly electron-with-
drawing nitro groups resulted in lower isolated yields (entry 2).
These electronic effects of ring substitution mirror the trends
observed in the alcohol substrate series and are reflective of
the more nucleophilic anilines being more reactive. The yields
obtained with primary aliphatic amines were reasonably good
(43–57%, entries 3–5). The results are not reflective of a direct
trend with amine basicity, likely because more basic amines
adsorb on the HT more strongly and are not available to con-
dense with the aldehydes.
catalytically active for dehydrogenation than the support (HT4)
alone. Interestingly, the aldehyde yields at 2 h of reaction were
higher than those at 24 h (Table 4). The decrease was
especially noticeable for reactions with 5% Pd/HT4, where the
aldehyde yield decreased from 65% (2 h) to 7% (24 h). Since
the recovered unreacted alcohol can account for mass balance,
we hypothesized that this was due to aldehyde being adsorbed
to the catalyst. Thus we quantitated the fraction of free alde-
hyde in control reactions, where the alcohol was replaced by
aldehyde under identical reaction conditions. We found that
while the loss of aldehyde with 0.02% Pd/HT4 is negligible,
with 1% Pd/HT4 we lose up to 30% of the aldehyde in 2 h,
while with the 5% Pd/HT4 that number increases to 44%, and
these quantities increase with time. These results suggest that
the lower yield of aldehyde after 24 h vs. after 2 h could be
attributed to adsorption of the aldehyde to Pd/HT4. The quan-
tity of aldehyde bound is dependent on the loading of Pd on
the HT (and the mol% Pd).
The yield of imine from the reaction in Scheme 3
decreases almost linearly with increasing Pd loading (Table 4
and Fig. 4). Thus, HT4 is the most active catalyst (85% yield),
but just 0.02% Pd loading on the catalyst (0.001 mol% Pd)
causes a decrease in the isolated yield to 82%. We proposed
that this trend is due to the retention of aldehyde, amine
and/or imine by the catalyst, and this retention is dependent
on the Pd loading. Free aniline was observed in the reactions
and is accounted for by mass balance, and a Pd loading-
dependent retention of aldehyde has already been esta-
blished in the previously described experiment. The amount
of imine retained was almost equal for HT4 and 1% Pd/HT4
(22–26%) but higher for 5% Pd/HT4 (43%). Strong binding of
Pd to imines is not surprising given the homogeneous cataly-
sis literature. Despite the fact that these results are consistent
with the hypothesis, we have not excluded the possibility that
there may be more complex reasons that drive the difference
in imine yields obtained from Pd/HT4 catalysts and HT4,
such as the effect of immobilized Pd on the number and
strength of acid–base sites on the HT4. Microcalorimetric
studies are underway to investigate the effect of Pd on the acid–
base properties of HTs.
Catalyst recycling
The ability to reuse the heterogeneous catalyst and identify
compositional and morphological changes post-catalysis is
critical for synthetic applications. Since in a few instances we
observed conversions that were greater than yields, it was
prudent to characterize the used catalysts, ensure that the
organic material is not retained and check whether activity is
Although we typically report TON and TOF when describing
a new catalyst, in this case these metrics cannot be meaning-
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Table 5 Alcohol substrate scope for HT4-catalyzed alcohol imination
Yield
(%)
Conversion
(%)
Entry
1a
Alcohol substrate
Product
97
99
2a
92
89
97
3a
100
4a
84
75
98
93
5a
6a
32
15
90
54
7a,b
8b
9b
—
—
NR
NR
94
73
a Reaction conditions: 0.24 mmol alcohol, 0.60 mmol p-anisidine, 100 mg catalyst, 3 mL xylenes, 110 °C, 24 h. b 200 mg catalyst.
retained in multiple use cycles. Indeed, TGA of the used and shows new reflections at 8.23° and 31.5° (2θ), which could not
unwashed catalyst (HT4) from the reaction in entry 1 in be readily attributed to an identifiable phase, but disappeared
Table 5 showed a new mass loss at 185 °C. We find that after aqueous K2CO3 wash. It is notable that the PXRD patterns
aqueous potassium carbonate provides the necessary abun- of the unwashed catalyst do not show any shift of the 003
dance of carbonate ions to reform the active catalyst quickly reflection, which indicates that there is no change in interlayer
during the wash and remove adsorbed substrates. The refor- spacing that would likely result from intercalation of sub-
mation to a LDH phase was confirmed by FT-IR and PXRD strates or products. The FT-IR spectra were consistent with the
(Fig. 6), while the loss of residual organics was confirmed by PXRD data – the unwashed catalyst shows several new IR
TGA. The PXRD pattern of the used catalyst prior to wash stretches in the hydroxyl and carbonyl regions that are elimi-
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Table 6 Amine substrate scope for HT4-catalyzed alcohol imination
Yield
(%)
Entry
1a
Amine substrate
Product
97
2a
20
3a
4a
43
57
5b
57
Fig. 6 Powder X-ray diffraction patterns (above) and FT-IR spectra
(below) of (a) HT4 washed with K2CO3 (aq.) after an imination reaction,
(b) HT4 – used and unwashed and (c) fresh HT4.
a Reaction conditions: 0.24 mmol benzyl alcohol, 0.29 mmol amine,
100 mg catalyst, 3 mL xylenes, 110 °C, 24 h. b 0.6 mmol amine.
(Fig. 5). In contrast, the unwashed catalyst shows loss of cataly-
tic activity quickly over 3 cycles (Fig. 5).
Conclusions
We developed an active hydrotalcite-like (HT) heterogeneous
catalyst for alcohol imination proceeding through acceptorless
alcohol dehydrogenation. After screening HTs with different
compositions (up to 15% of Fe3+, Zn2+, Ni2+, Cr3+ or Cu2+), we
identified Fe : Mg : Al HT (HT4) as the most active for both
dehydrogenation and imination. The yields of imine generally
exceeded those of aldehydes, suggesting that the amine drives
the equilibrium forward by condensation. Calcination slightly
increased the activity for dehydrogenation but significantly
lowered the activity for imination. Thus, we opted to enhance
the catalytic activity of non-calcined HT4 by impregnating with
Pd0 at 0.02%, ∼1% and ∼5% loading. The dehydrogenation
activity of the catalysts was enhanced by Pd(0) immobilization,
with 1% Pd/HT4 affording the highest yield of aldehyde.
However, the imination activity was reduced in a loading-
dependent manner. Our experiments suggest that the primary
reason for this was the Pd loading-dependent retention of
Fig. 5 Percent yields of 4-methoxy-N-(phenylmethylene)benzenamine
from repeated cycles of the alcohol imination reaction between benzyl
alcohol and p-anisidine catalyzed by HT4.
nated after the aqueous K2CO3 wash. The PXRD patterns and aldehyde and imine on the catalysts. Substrate scope investi-
FT-IR spectra of the washed catalyst closely resemble those of gations revealed that the catalytic system is selective towards
the fresh catalyst.
activated alcohols and aromatic amines. Recyclability experi-
Given these results, it was thus not surprising that as long ments showed that the catalytic activity for imination can be
as an aqueous K2CO3 wash is performed, the catalyst retains retained for at least three cycles when HT4 is washed in
its activity for at least 3 cycles with minimal loss in activity aqueous K2CO3.
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equiv.) and 3 mL of degassed solvent were added. The reaction
mixture was heated to 110 °C for 24 hours in a Heidolph
Radleys Carousel 12 Plus Reaction Station. The reaction was fil-
Experimental
Synthesis of HTs
Magnesium–aluminium hydrotalcites containing 0–15% Fe3+, tered and the spent catalyst was washed with toluene (3 ×
Zn2+, Ni2+, Cr3+, and Cu2+ were synthesized by aqueous co- 5 mL) and K2CO3 (aq.) (3 × 5 mL). The filtrate was dried in
precipitation of M2+ and M3+ nitrate salts under basic conditions vacuo, and the product was isolated by column chromato-
(pH 8–10) in the presence of sodium carbonate (see Scheme 4 graphy. The product was characterized by GC-MS and 1H NMR.
and ESI† for details).24 The precipitates were filtered and
washed copiously with deionized water until no more nitrate
was present in the filtrate. The resulting hydrotalcites were
dried at 110 °C for 12 hours, and a fraction of each sample was
Acknowledgements
calcined at 450 °C for 24 h.
The authors would like to thank both Dr Chris Cahill and Dr
Michael Wagner for the use of PXRD and BET measurements.
The authors would also like to thank Dr Anastas Popratilof
and the Center for Microscopy and Image Analysis (CMIA) for
providing training and access to the TEM.
Pd0 supported on hydrotalcites
A series of Pd0/HT4 catalysts were prepared by wet precipi-
tation–reduction. In a typical reaction, 2.00 g of calcined HT
was suspended in deionized water (50 mL). To this suspension
a known quantity of PdCl2 was added, followed by the addition
of 1 mL of a 1 M NaOH solution. The solution was allowed to
stir at room temperature for 12 hours, after which the solution
was removed via vacuum filtration, and the solid was washed
with deionized water (3 × 25 mL) and dried for 12 hours at
110 °C. The solid was then resuspended in deionized water
(50 mL) and reduced by dropwise addition of a NaBH4 solution
(0.01 g, 3 equivalents dissolved in 25 mL distilled water). The
suspension was allowed to stir at room temperature for
2 hours, and then the solid was filtered and washed with
deionized water (3 × 15 mL) and dried for 12 hours at 100 °C.
Catalyst characterization is described in the ESI.†
Notes and references
1 S. I. Murahashi, Angew. Chem., Int. Ed. Engl., 1995, 34,
2443–2465.
2 M. A. Berliner, S. P. A. Dubant, T. Makowski, K. Ng,
B. Sitter, C. Wager and Y. S. Zhang, Org. Process Res. Dev.,
2011, 15, 1052–1062.
3 J. F. Moulder, W. F. Stickel, P. E. Sobol and K. D. Bomben,
Handbook of X-ray Photoelectron Spectroscopy, Physical-
Electronics Inc., Eden Prairie, MN, 1995.
4 R. Kawahara, K. Fujita and R. Yamaguchi, Adv. Synth.
Catal., 2011, 353, 1161–1168.
5 R. Yamaguchi, S. Kawagoe, C. Asai and K. I. Fujita, Org.
Lett., 2008, 10, 181–184.
6 K. Fujita, Z. Z. Li, N. Ozeki and R. Yamaguchi, Tetrahedron
Lett., 2003, 44, 2687–2690.
7 M. Benitez, E. Mas-Marza, J. A. Mata and E. Peris, Chem. –
Eur. J., 2011, 17, 10453–10461.
8 G. E. Dobereiner and R. H. Crabtree, Chem. Rev., 2010, 110,
681–703.
9 K. Fujita, Z. Li, N. Ozeki and R. Yamaguchi, Tetrahedron
Lett., 2003, 44.
10 D. Pingen, C. Muller and D. Vogt, Angew. Chem., Int. Ed.,
2010, 49, 8130–8133.
11 B. Gnanaprakasam, J. Zhang and D. Milstein, Angew.
Chem., Int. Ed., 2010, 49, 1468–1471.
Alcohol dehydrogenation reactions
In a typical procedure, a 20 mL reaction tube was charged with
0.100 g of catalyst (or other amounts, as indicated). It was then
evacuated and placed under a nitrogen atmosphere. Sub-
sequently, the alcohol (0.24 mmol, 1 equiv.) and 3 mL of
degassed solvent were added to the reaction tube. The reaction
mixture was heated to 110 °C for 24 hours in a Heidolph
Radleys Carousel 12 Plus Reaction Station. The reaction was fil-
tered and the spent catalyst was washed with toluene (3 ×
5 mL) and K2CO3 (aq.) (3 × 5 mL). The filtrate was dried in
vacuo, and the product was isolated by column chromato-
graphy using 80 : 20 hexanes–ethyl acetate. The isolated
product was characterized by GC-MS, GC-FID and 1H NMR.
12 M. Hamid, C. L. Allen, G. W. Lamb, A. C. Maxwell,
H. C. Maytum, A. J. A. Watson and J. M. J. Williams, J. Am.
Chem. Soc., 2009, 131, 1766–1774.
13 N. Andrushko, V. Andrushko, P. Roose, K. Moonen and
A. Borner, ChemCatChem, 2010, 2, 640–643.
14 A. P. da Costa, M. Viciano, M. Sanau, S. Merino, J. Tejeda,
E. Peris and B. Royo, Organometallics, 2008, 27, 1305–1309.
15 D. Gnanamgari, E. L. O. Sauer, N. D. Schley, C. Butler,
C. D. Incarvito and R. H. Crabtree, Organometallics, 2009,
28, 321–325.
Alcohol imination reactions
In a typical procedure, a 20 mL reaction tube was charged with
0.100 g of catalyst (or other amounts, as indicated), then evacu-
ated and placed under a nitrogen atmosphere. Subsequently
the alcohol (0.24 mmol, 1 equiv.), amine (0.29 mmol, 1.2
16 K. I. Fujita, Y. Enoki and R. Yamaguchi, Tetrahedron, 2008,
64, 1943–1954.
Scheme 4 Co-precipitation synthesis of hydrotalcites.
2278 | Green Chem., 2015, 17, 2271–2280
This journal is © The Royal Society of Chemistry 2015

View Article Online
Green Chemistry
Paper
17 Y. Z. Qiang Kang, Green Chem., 2012, 14, 1016–1019.
18 G. Guillena, D. J. Ramon and M. Yus, Chem. Rev., 2010,
110, 1611–1641.
19 J. Xu, R. Zhuang, L. Bao, G. Tang and Y. Zhao, Green Chem.,
2012, 14, 2384–2387.
20 E. L. Zhang, H. W. Tian, S. D. Xu, X. C. Yu and Q. Xu, Org.
Lett., 2013, 15, 2704–2707.
21 G. Q. Zhang and S. K. Hanson, Org. Lett., 2013, 15, 650–
653.
22 Y. Nakajima, Y. Okamoto, Y. H. Chang and F. Ozawa, Orga-
nometallics, 2013, 32, 2918–2925.
23 S. Musa, S. Fronton, L. Vaccaro and D. Gelman, Organo-
metallics, 2013, 32, 3069–3073.
24 F. Cavani, F. Trifiro and A. Vaccari, Catal. Today, 1991, 11,
173–301.
25 D. Meloni, M. F. Sini, M. G. Cutrufello, R. Monaci, E. Rombi
and I. Ferino, J. Therm. Anal. Calorim., 2012, 108, 783–791.
26 V. R. L. Constantino and T. J. Pinnavaia, Inorg. Chem., 1995,
34, 883–892.
46 O. D. Pavel, B. Cojocaru, E. Angelescu and V. I. Parvulescu,
Appl. Catal., A, 2011, 403, 83–90.
47 D. G. Cantrell, L. J. Gillie, A. F. Lee and K. Wilson, Appl.
Catal., A, 2005, 287, 183–190.
48 V. H. Jadhav, D. K. Dumbre, V. B. Phapale, H. B. Borate and
R. D. Wakharkar, Catal. Commun., 2007, 8, 65–68.
49 T. Baskaran, R. Kumaravel, J. Christopher and A. Sakthivel,
RSC Adv., 2014, 4, 11188–11196.
50 B. M. Choudary, M. L. Kantam, A. Rahman, C. V. Reddy
and K. K. Rao, Angew. Chem., Int. Ed., 2001, 40, 763–766.
51 K. Takehira and T. Shishido, Catal. Surv. Asia, 2007, 11,
1–30.
52 V. K. Diez, C. R. Apesteguia and J. I. Di Cosimo, J. Catal.,
2003, 215, 220–233.
53 P. R. Likhar, R. Arundhathi, M. L. Kantam and
P. S. Prathima, Eur. J. Org. Chem., 2009, 5383–5389.
54 K. Motokura, N. Hashimoto, T. Hara, T. Mitsudome,
T. Mizugaki, K. Jitsukawa and K. Kaneda, Green Chem.,
2011, 13, 2416–2422.
27 J. Kostal, A. Voutchkova-Kostal, B. Weeks, J. B. Zimmerman 55 S. Sueoka, T. Mitsudome, T. Mizugaki, K. Jitsukawa and
and P. T. Anastas, Chem. Res. Toxicol., 2012, 25, 2780–2787.
28 A. Vaccari, Appl. Clay Sci., 1999, 14, 161–198.
29 J. S. Valente, F. Figueras, M. Gravelle, P. Kumbhar, J. López
and J. P. Besse, J. Catal., 2000, 189, 370.
K. Kaneda, Chem. Commun., 2010, 46, 8243–8245.
56 K. Motokura, D. Nishimura, K. Mori, T. Mizugaki,
K. Ebitani and K. Kaneda, J. Am. Chem. Soc., 2004, 126,
5662–5663.
30 S. Velu and C. S. Swamy, Appl. Catal., A, 1997, 162, 81–91.
31 L. Dussault, J. C. Dupin, E. Dumitriu, A. Auroux and
C. Guimon, Thermochim. Acta, 2005, 434, 93–99.
57 B. M. Choudary, S. Madhi, N. S. Chowdari, M. L. Kantam
and B. Sreedhar, J. Am. Chem. Soc., 2002, 124, 14127–14136.
58 W. H. Fang, Q. H. Zhang, J. Chen, W. P. Deng and Y. Wang,
Chem. Commun., 2010, 46, 1547–1549.
32 S. Casenave, H. Martinez, C. Guimon, A. Auroux, V. Hulea,
A. Cordoneanu and E. Dumitriu, Thermochim. Acta, 2001, 59 T. Mitsudome, Y. Mikami, K. Ebata, T. Mizugaki,
379, 85–93.
K. Jitsukawa and K. Kaneda, Chem. Commun., 2008, 4804–
4806.
60 T. Mitsudome, Y. Mikami, H. Funai, T. Mizugaki,
K. Jitsukawa and K. Kaneda, Angew. Chem., Int. Ed., 2008,
47, 138–141.
33 J. S. Valente, F. Figueras, M. Gravelle, P. Kumbhar, J. Lopez
and J. P. Besse, J. Catal., 2000, 189, 370–381.
34 M. R. Othman, Z. Helwani, Martunus and W. J. N. Fernando,
Appl. Organomet. Chem., 2009, 23, 335–346.
35 S. K. Yun and T. J. Pinnavaia, Chem. Mater., 1995, 7, 348– 61 K. Kaneda, T. Mitsudome, T. Mizugaki and K. Jitsukawa,
354. Molecules, 2010, 15, 8988–9007.
36 S. K. Sharma, P. K. Kushwaha, V. K. Srivastava, S. D. Bhatt 62 K. Takehira, T. Shishido, P. Wang, T. Kosaka and K. Takaki,
and R. V. Jasra, Ind. Eng. Chem. Res., 2007, 46, 4856–4865. Phys. Chem. Chem. Phys., 2003, 5, 3801–3810.
37 D. G. Evans and D. A. Xue, Chem. Commun., 2006, 485–496. 63 J. Chen, Q. H. Zhang, W. H. Fang, Y. Wang and H. L. Wan,
38 G. Fan, F. Li, D. G. Evans and X. Duan, Chem. Soc. Rev.,
2014, 43, 7040–7060.
39 S. Nishimura, A. Takagaki and K. Ebitani, Green Chem.,
2013, 15, 2026–2042.
40 J. Perez-Ramirez, S. Abello and N. M. van der Pers, Chem. –
Eur. J., 2007, 13, 870–878.
41 K. K. Rao, M. Gravelle, J. S. Valente and F. Figueras,
J. Catal., 1998, 173, 115–121.
Chinese J. Catal., 2010, 31, 1061–1070.
64 W. H. Fang, J. S. Chen, Q. H. Zhang, W. P. Deng and
Y. Wang, Chem. – Eur. J., 2011, 17, 1247–1256.
65 R. K. Marella, C. K. P. Neeli, S. R. R. Kamaraju and
D. R. Burri, Catal. Sci. Technol., 2012, 2, 1833–1838.
66 R. J. Shi, F. Wang, X. L. Mu, N. Ta, Y. Li, X. M. Huang and
W. J. Shen, Chin. J. Catal., 2010, 31, 626–630.
67 N. Kakiuchi, Y. Maeda, T. Nishimura and S. Uemura, J. Org.
Chem., 2001, 66, 6620–6625.
42 D. P. Debecker, E. M. Gaigneaux and G. Busca, Chem. –
Eur. J., 2009, 15, 3920–3935.
43 S. Abello, F. Medina, D. Tichit, J. Perez-Ramirez,
68 T. Nishimura, N. Kakiuchi, M. Inoue and S. Uemura, Chem.
Commun., 2000, 1245–1246.
J. C. Groen, J. E. Sueiras, P. Salagre and Y. Cesteros, Chem. 69 R. Jenkins and R. L. Snyder, Introduction to X-ray Powder
– Eur. J., 2005, 11, 728–739. Diffractometry, John Wiley & Sons Inc., 1996.
44 M. J. Climent, A. Corma, S. Iborra and A. Velty, J. Catal., 70 S. Velu, V. Ramkumar, A. Narayanan and C. S. Swamy,
2004, 221, 474–482. J. Mater. Sci., 1997, 32, 957–964.
45 A. R. Massah, R. J. Kalabashi, M. Toghiani, B. Hojati and 71 T. Chen, F. Z. Zhang and Y. Zhu, Catal. Lett., 2013, 143,
M. Adibnejad, E-J. Chem., 2012, 9, 2501–2508.
206–218.
This journal is © The Royal Society of Chemistry 2015
Green Chem., 2015, 17, 2271–2280 | 2279

View Article Online
Paper
Green Chemistry
72 J. T. Feng, X. Y. Ma, Y. F. He, D. G. Evans and D. Q. Li, Appl. 75 N. Kakiuchi, T. Nishimura, M. Inoue and S. Uemura, Bull.
Catal., A, 2012, 413, 10–20. Chem. Soc. Jpn., 2001, 74, 165–172.
73 S. Narayanan and K. Krishna, Appl. Catal., A, 1998, 174, 76 M. Granollers, R. Brown, N. An, P. Yaseneva,
221–229.
A. P. Voutchkova-Kostal and A. Lapkin, Microcalorimetric
studies of doped hydrotalcite-like materials: effect of syn-
thetic conditions. Manuscript in preparation.
74 D. E. Ramaker, J. de Graaf, J. A. R. van Veen and
D. C. Koningsberger, J. Catal., 2001, 203, 7–17.
2280 | Green Chem., 2015, 17, 2271–2280
This journal is © The Royal Society of Chemistry 2015