A CaMnAl-hydrotalcite solid basic catalyst toward the aldol condensation reaction with a comparable level to liquid alkali catalysts

Article abstract of DOI:10.1039/c8gc00851e

In a number of heterogeneous catalysis processes, the promotion effect toward active sites is of vital importance and remains a challenge to obtain largely-improved catalytic performance. Herein, rehydrated Ca₄Mn_xAl-layered double hydroxides (denoted as re-Ca₄Mn_xAl-LDH) were prepared based on a structure memory effect of LDH precursors, which exhibited extremely high heterogeneous catalytic performance for the aldol condensation reaction, with the assistance of the promotion effect of Mn. A combination study including XRD, EXAFS, XPS, CDCl₃-FTIR and DFT calculations confirms that re-Ca₄Mn_xAl-LDH samples with Ca-O-Mn^IV structure show a highly-exposed Ca²⁺ s-orbital and strengthened coordination between Ca²⁺ with 7-fold OH^-, providing a weakened Br?nsted basic site compared with the reference sample re-Ca₄Al-LDH. The optimized re-Ca₄Mn_0.5Al-LDH catalyst exhibits prominent catalytic performance toward the condensation of isobutyraldehyde (IBD) and formaldehyde (FA) to produce hydroxypivaldehyde (HPA), with a HPA yield of 70.3%. This is significantly higher than re-Ca₄Al-LDHs (63.3%) and even comparable to the level of liquid alkali catalysts (73.2%). Studies on the structure-property correlation reveal that the weakened basic site originating from Ca-O-Mn^IV serves as a promoted active center, which accelerates the product desorption and thus largely improves HPA selectivity. This promoted re-Ca₄Mn_0.5Al-LDH catalyst can be potentially applied as a promising candidate in heterogeneous aldol condensation reactions.

Full text of DOI:10.1039/c8gc00851e

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DOI: 10.1039/C8GC00851E
Green Chemistry
ARTICLE
CaMnAl-hydrotalcite solid basic catalyst toward aldol
condensation reaction with a comparable level to liquid alkali
catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
a
a
,b
c
Weihan Bing, Huimin Wang, Lei Zheng, Deming Rao, Yusen Yang, Lirong Zheng,* Bin Wang,
,
d
,a
DOI: 10.1039/x0xx00000x
Yangdong Wang* and Min Wei*
www.rsc.org/
In a number of heterogeneous catalysis processes, promotion effect toward active sites is of vital importance
and remains a challenge to obtain largely-improved catalytic performance. Herein, rehydrated Ca Mn Al-
layered double hydroxides (denoted as re-Ca Mn Al-LDH) were prepared based on a structure memory effect
4
x
4
x
of LDH precursors, which exhibited extremely high heterogeneous catalytic performance for aldol
condensation reaction, with the assistance of promotion effect of Mn. A combination study including XRD,
IV
3 4 x
EXAFS, XPS, CDCl -FTIR and DFT calculations confirms that re-Ca Mn
Al-LDH samples with Ca−O−Mn
2
+
2+
−
structure show a highly-exposed Ca s-orbital and strengthened coordination between Ca with 7-fold OH ,
providing a weakened Brønsted basic site compared with the reference sample re-Ca Al-LDH. The optimized
re-Ca Mn0.5Al-LDH catalyst exhibits prominent catalytic performance toward the condensation of
isobutyraldehyde (IBD) and formaldehyde (FA) to produce hydroxypivaldehyde (HPA), with a HPA yield of
4
4
4
70.3%. This is significantly higher than re-Ca Al-LDHs (63.3%) and even comparable to the level of liquid alkali
catalysts (73.2%). Studies on the structure-property correlation reveal that the weakened basic site
IV
originating from Ca−O−Mn serves as a promoted active center, which accelerates the product desorption
4
and thus largely improves HPA selectivity. This promoted re-Ca Mn0.5Al-LDH catalyst can be potentially
applied as a promising candidate in heterogeneous aldol condensation reactions.
pivotal role, as their applications in many base-catalyzed industrial
processes offer opportunities to facilitate separation, avoid
1
. Introduction
7
−9
Aldol condensation is a well-known and important C−C coupling corrosion and pollution issue.
Recently, for the aldol
reaction in classic organic synthesis via reaction between aldehydes condensation of IBD with FA towards HPA, a series of perovskites
and/or ketones, which produces a variety of value-added chemicals and metal nitrides as heterogeneous base catalysts have been
with versatile commercial applications (e.g., polymers, reported. However, these solid basic catalysts suffer from a very
1
−3
pharmaceuticals, and agricultural chemicals).
Herein, the aldol sluggish development both in fundamental study and industrial
condensation of isobutyraldehyde (IBD) with formaldehyde (FA) to application, owing to great challenges in material exploration and
produce hydroxypivaldehyde (HPA) (Scheme 1), as a base-catalyzed technique limitation.
reaction with great industrial importance, has been widely applied
By virtue of the advantages of layered double hydroxides (LDHs)
in the production of polyesters, plasticizers, synthetic resin paints including versatile chemical composition and structural memory
and lubricants. Traditionally, this aldol reaction is catalysed by effect, the rehydrated CaAl-LDHs as an effective solid basic catalyst
1
0
homogeneous bases (e.g., aqueous NaOH, KOH, and triethylamine), was prepared for this aldol reaction in our previous work;
which suffers from a number of disadvantages including generation moreover, it has been verified that the 7-fold Ca−OH coordination
of waste liquid, environmental pollution and equipment structure with weak Brønsted basic site accelerates the desorption
4
−6
corrosion. Therefore, heterogeneous base catalysis plays a more of aldol condensation product, which is the key step to enhance the
selectivity toward HPA. Although rehydrated CaAl-LDHs (re-CaAl-
LDHs) as a new solid basic catalyst toward aldol condensation
reaction has been developed, its catalytic performance is not as
high as the level of liquid alkali catalysts, and a control over fine
structure of Ca−O−Al (weak Brønsted basic site structure) is highly
†
Scheme 1 Base-catalysed aldol condensation of IBD with FA towards HPA.
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necessary for a further improvement. In the past decades, many Synthesis of Ca Mn Al-LDH precursor: Ca Al-LDH and Ca Mn Al-
4
x
4
4
x
elements, such as lanthanum, cerium, zirconium and manganese
have been investigated as promoters to enhance catalytic
performance toward aldol condensation reactions; among these
promoters, manganese as an effective promoter has attracted
considerable attention, due to its versatile valence state and
electronic structure. On the other hand, based on our previous
understanding, if the active basic site could be tuned via changing
the electronic structure of Ca−O−Al, a further promoꢀon of catalyꢀc
performance would be achieved. Therefore, partial substitution of
LDH precursors with varying Mn/Al molar ratios were synthesized
using a coprecipitation method as follows. In a typical procedure,
0
.04 mol of Ca(NO
Mn(NO aqueous solution with a given Mn : Al molar ratio (0, 0.3,
.5, 0.8, 1.0 and 1.5, respectively) were dissolved in deionized and
3 2 2 3 3 2
) •4H O, 0.01 mol of Al(NO ) •9H O and
3 2
)
0
decarbonated water (100 mL), which was then added
simultaneously into a three-neck round-bottom flask with NaOH
aqueous solution (100 mL, 1.1 M) in N
2
atomosphere. The
Al by another metal element with special M−O bonding (e.g., precipitate was stirred vigorously at pH 12.6−13.2 at room
IV
Mn −O) would modify the fine structure of basic site and therefore temperature for 6 h, followed by separation, washing thoroughly
make a further enhancement of catalytic performance toward aldol and drying at 80 °C for 24 h.
condensation reaction.
4 x
Synthesis of re-Ca Mn Al-LDH: The rehydrated LDH catalysts were
Inspired by the above idea, herein, on the basis of rehydrated
CaAl-LDHs, Mn element was further introduced to prepare
rehydrated CaMnAl-LDHs solid base catalyst, which was used in
aldol condensation of isobutyraldehyde (IBD) with formaldehyde
prepared in a two-step procedure. Firstly, 0.5 g of the as-
synthesized LDH precursor was heated at 500 °C for 4 h in air
−
1
atomosphere at a heating rate of 2 °C min . Subsequently, the
calcination sample was cooled down to room temperature and then
dispersed in a NaOH solution with stirring for 2 h. We chose an
appropriate pH value of 13.40 for the preparation of rehydrated
LDHs samples with a high purity. The obtained product was filtered,
washed thoroughly with water and ethanol, finally dried at 80 °C for
(
FA) to produce hydroxypivalaldehyde (HPA). Among them, the
optimized re-Ca Mn0.5Al-LDH sample shows an extremely high
catalytic performance with a HPA yield of 70.3%, significantly higher
than re-Ca Al-LDHs (63.3%) and even comparable to the level of
4
4
liquid alkali catalysts (73.2%). A combination study including XRD,
EXAFS, and XPS confirms that low Mn-containing samples (Mn/Al
1
4 x
2 h, labeled as re-Ca Mn Al-LDH (x=0, 0.3, 0.5, 0.8, 1.0 and 1.5).
IV
molar ratio: 0.3, 0.5) involve predominant Mn species; the
2
.3 Catalytic reaction
The aldol condensation reaction was carried out using a batch-
type reactor (cylindrical glass tube), avoiding exposing the catalyst
to air. Typically, the freshly prepared re-Ca Mn Al-LDH catalyst (0.2
IV
2+
resulting Ca−O−Mn structure with a highly-exposed Ca s-orbital
−
and strengthened interaction with 7-fold OH , provides a weakened
Brønsted basic site revealed by deuterated chloroform−FTIR
4
x
spectra. In contrast, high Mn-containing samples (Mn/Al molar ratio
g) was dispersed into a mixture of IBD (0.55 mol), FA (0.80 mol, 37%
aqueous solution), dioxane (0.2 g, as internal standard) and CTAB
III
from 0.8 to 1.5) possess abundant Ca−O
v
−Mn structure (O
v
denotes oxygen vacancy) with a decreased interaction between
(
0.1 g) in Ar atomosphere. Herein CTAB is used as a phase transfer
2
+
−
Ca and 7-fold OH , giving rise to medium Brønsted basic site. By
establishing the correlation between catalytic performance and
relative concentration of Mn species, it is found that the weakened
catalyst, which can facilitate the migration of IBD from oil phase
into water phase to accelerate reaction, so as to obtain a high
conversion and inhibit byproducts in oil-water two-phase reaction
system. After vacuuming and inletting argon gas, the solution was
magnetically stirred in oil bath at 70 °C. The power and rotating rate
of magnetic stirrer were also studied to reduce the influence of
mass transfer, and an optimized stirring condition (15 W, 600 r/min)
was applied in this work. The reactant conversion and product yield
were monitored by GC-2014 Shimadzu gas chromatograph (GC)
equipped with a flame ionization detector (FID) and an Rtx-5
capillary column (0.25 mm in diameter, 30 m in length). The
products were identified by gas chromatography−mass
spectrometry (GC−MS) equipped with the same column as GC.
IV
basic site originating from Ca−O−Mn facilitates the product
desorption and inhibits further polycondensation, accounting for
4
the significantly enhanced HPA yield over re-Ca Mn0.5Al-LDH. DFT
IV
calculations reveal that Ca−O−Mn structure gives a lattice
expansion and a shortened 7-fold Ca−O bond, which induces a
weakened basic site and a markedly decreased desorption energy
of HPA (0.179 eV) relative to pristine Ca−O−Al structure (0.557 eV).
This work demonstrates
a
successful paradigm for the
enhancement of LDHs-based solid base catalyst by introducing
IV
promoter Mn , and provides an in-depth understanding on the
IV
promotion mechanism of Ca−O−Mn .
2
.4 Characterization
X-ray diffraction (XRD) patterns were recorded on a Shimadzu
2
. Experiment Section
XRD-6000 diffractometer, using Cu Kα radiation (λ = 0.1542 nm) at
0 kV, 30 mA. X-ray photoelectron spectra (XPS) were recorded on
a Thermo VG Escalab 250 X-ray photoelectron spectrometer at a
4
2.1 Materials
3 2 2 3 3 2
Chemical reagents, including Ca(NO ) •4H O, Al(NO ) •9H O,
−9
pressure of ∼2 × 10 Pa with Al Kα X-rays as the excitation source.
Extend X-ray absorption fine structure spectroscopy (EXAFS) at the
Mn(NO
formaldehyde
3
)
2
(50% aqueous solution), NaOH, isobutyraldehyde (IBD),
(FA, 37% aqueous solution), dioxane,
hydroxypivaldehyde (HPA) and cetyl trimethyl ammonium bromide Mn and Ca K-edge was performed at the beamline 1W1B and 4B7A
CTAB) were purchased and used without further purification. of the Beijing Synchrotron Radiation Facility (BSRF), Institute of High
Deionized and decarbonated water was used in all the experimental Energy Physics (IHEP), Chinese Academy of Sciences (CAS). For the
(
processes.
Mn EXAFS, the typical energy of the storage ring was 2.5 GeV with a
maximum current of 250 mA. The typical energy of the storage ring
of Ca EXAFS was 2.2 GeV with a maximum current of 100 mA. The Si
2.2 Preparation of catalysts
2
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−
(
111) double crystal monochromator was used. The IFFEFIT 1.2.11 the reconstruction of layered structure with OH as interlayer
date analysis package (Athena, Artemis, Atoms, and FEFF6) was anion. Moreover, some weak peaks are observed in XRD patterns of
used for the date analysis.
Ca
to the presence of a small amount of CaCO
phase (Mn or MnO ). Correspondingly, the cell parameters of re-
Ca Mn Al-LDH (listed in Table 1) are calculated based on (003) and
(110) reflection as c = 22.80 Å and a = 3.32 Å, respectively, which
are in accordance with the characteristics of OH intercalated
Ca Mn Al-LDH. SEM images of these rehydrated LDH samples (Fig.
S2) show plate-like microcrystals with lateral diameter
distribution of 1.0 2.0 μm. Nitrogen adsorption–desorption
isotherms of re-Ca Mn Al-LDH samples display comparable specific
4
Mn
x
Al-LDH precursors and re-Ca
4
Mn
x
Al-LDH samples, attributed
3
and manganese oxide
Deuterated chloroform was used as a basicity probe molecule
for the measurement of FTIR transmission spectroscopy, which was
carried out in a Bruker Equinox 55 spectrometer, between 4000 and
3
O
4
2
4
x
−1
−1
∼
∼
4
00 cm with a resolution of 4 cm after 600 scans per spectrum.
The nature of the basic site was characterized by the FTIR
spectroscopy of adsorbed deuterated chloroform (CDCl -IR). About
8 mg of sample was pressed into a wafer with a diameter of 1.3
−
4
x
3
a
1
−
2
cm, installed in an IR cell with CaF windows, treated at 150 °C for
4
x
0
.5 h, followed by an evacuation treatment at this temperature.
surface, average pore size and total pore volume (Table 1).
Moreover, the molar ratios of Ca/Mn/Al of Ca Mn Al-LDH and re-
Subsequently, the sample was cooled to 50 °C for recording the
background spectrum. With the introduction of deuterated
chloroform into the cell for 1 h, another evacuation was performed
to remove physisorbed deuterated chloroform; afterwards, the
4
x
Ca Mn Al-LDH samples determined by inductively coupled plasma–
4
x
atomic emission spectrometer (ICP−AES) are also summarized in
Table 1 and Table S1, which are close to the nominal values. The
aluminium content in re-Ca Mn Al-LDH samples does not show
−1
spectrum was recorded with a step of 4 cm and an accumulation
of 32 sans. The strength of base site involved in the adsorption of
4
x
obvious change after rehydration with NaOH solution, indicating
rather less Al leaching.
CDCl was estimated based on the following equation:
3
ꢀꢁꢂꢃꢄꢅꢆ ꢇ 0.0066ꢈꢉꢊ ꢋ ꢊ4.36
(1)
−1
where Δν is the shift of νCD (cm ), and PA is the proton affinity (kJ
CD
−
1
mol ).
2
.5 Computational methods
The density functional theory calculations were performed with
the Vienna Ab-initio Simulation Package (VASP) in slab models. The
model of CaMnAl-OH-LDH was constructed according to the crystal
data of these LDHs reported previously. The space group of LDHs
was r3m, with unit cell parameters α = β = 90°, and γ = 120°. The
other three unit cell parameters a, b and c, are referred to the
powder X-ray diffraction data of these LDHs. The supercell of bulk
CaMnAl-LDH is set to be 3×3×1 in the a-, b-, and c-direction with the
accustomed molar ratio of Ca : Mn : Al = 4 : 1 : 1. Three hydroxyl
ions were placed into the interlayer gallery of bulk Ca Mn Al -LDH
4
1
1
to keep the charge neutral. The exchange-correlation potential was
described by the Perdew-Burke-Ernzerhof (PBE) in generalized
gradient approach (GGA). The structure optimizations were based
Fig. 1 XRD patterns of (a) re-Ca
4
Al-LDH, (b) re-Ca
Mn0.8Al-LDH, (e) re-Ca Mn1.0Al-LDH and (f) re-
Mn1.5Al-LDH, respectively.
4
Mn0.3Al-LDH, (c) re-
Ca
Ca
4
Mn0.5Al-LDH, (d) re-Ca
4
4
−4
on the following points: (1) an energy tolerance of 1.0×10 eV per
atom; (2) a maximum force tolerance of 0.2 eV; (3) the k-points
were set to be 3×3×1 in calculating the electronic structure.
4
EXAFS spectroscopy was performed to investigate the detailed
structure (Fig. 2), and the normalized Mn K-edge XANES spectra for
these five re-Ca Mn Al-LDH samples and reference samples (MnO,
Mn and MnO ) are shown in Fig. 2A. Pure MnO has the lowest
absorption edge, followed by Mn , and MnO displays the highest
absorption edge. For all these re-Ca Mn Al-LDH samples, the edge
Al-LDH solid basic catalysts with various position is located between Mn and MnO , indicating the
Mn/Al ratios (x=0, 0.3, 0.5, 0.8, 1.0, 1.5) were prepared via a facile coexistence of trivalent manganese (Mn ) and tetravalent
4
x
3
. Results and discussion
2
O
3
2
2
O
3
2
3
.1 Structural and morphological characterizations
4
x
The rehydrated Ca
4
Mn
x
2
O
3
2
III
IV
11−14
coprecipitation method, followed by a calcination and a rehydration manganese (Mn ) species,
process in an aqueous NaOH. XRD patterns of Ca Mn
Al-LDH absorption spectroscopy (Fig. S3). Notably, the absorption edge of
precursors (Fig. S1A) and the calcination samples Ca Mn
Al-MMO re-Ca Mn Al-LDH samples shifts toward low photon energy with the
Fig. S1B) show a series of reflections indexed to a characteristic increment of Mn content, indicating reduced relative
which is also confirmed by UV−vis
1
5
4
x
4
x
4
x
(
a
3
−
IV
feature of layered NO -containing LDH phase and CaO-periclase concentration of Mn (Fig. 2A and inset). Moreover, as shown in
phase, respectively. As shown in Fig. 1, the rehydration samples fourier-transform EXAFS spectra at Mn K-edge (Fig. 2B), Mn−O
(
denoted as re-Ca
4
Mn
x
Al-LDH) display characteristic peaks of an distance in the first shell of re-Ca
and MnO
to NO -containing LDH precursor (11.5° vs. 10.5°). This indicates coordination number of Mn−O bond decreases obviously with the
Mn
4
Al-LDH samples is 1.48 Å, the
x
LDH phase, with the (003) reflection shifting to a higher 2θ relative same to that of Mn
O
3
. It should be noted that the
2
2
3
−
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2
+
enhancement of Mn/Al ratio, which suggests the existence of low- state and average electron density of Ca in re-Ca
4 x
Mn Al-LDH
coordinated Mn−O structure such as oxygen vacancies (i.e., samples. As the Mn/Al ratio increases from x=0 to x=0.5, the
III
2+
Ca−O
For the five re-Ca
Al-LDH, Ca K-edge XANES spectra (Fig. 2C) the FT k
EXAFS spectra (Fig. 2D) are also studied. Fig. 2C shows a weak pre- in re-Ca
edge feature at 4043 eV corresponding to transitions from 1s to Ca Al-LDH (
d state, which indicates the distorted 7-fold Ca −OH with a low octahedron. This lattice expansion induces an enhanced exposure
−Mn ).
average electron density of Ca increases and reaches the maximal
Al-LDH samples and reference sample re- value in re-Ca Mn0.5Al-LDH sample; meanwhile, as shown in fourier-
-weighted transform EXAFS spectra at Ca K-edge (Fig. 2D), the Ca−O distance
Mn0.5Al-LDH ( 1.90 Å) is obviously longer than that in re-
1.82 Å), indicating a lattice expansion of Ca−O−Mn
v
4
Mn
x
4
Ca
4
3
4
∼
IV
∼
4
∼
2
+
3
2
+
2+
structural symmetry existing in Ca-containing LDHs. With the of Ca s-orbital and thus strengthens the interaction between Ca
−
−
increase of Mn/Al molar ratio, the photon energy of central peak and the 7-fold OH (OH group as an electron donor), which
2
+
(
white line) decreases firstly and then increases, and the minimal increases the average electron density of Ca . In addition, from
photon energy (4052.0 eV) presents in the sample of re-Ca
Mn0.5Al- x=0.5 to x=1.5, Ca−O distance becomes shorter with a reduced
LDH (Fig. 2C). Depeyrot et al. has reported a positive correlation coordination number (Fig. 2D), which indicates the existence of
4
1
6
III
between mean oxidation state of metal cation and photon energy.
Ca−O −Mn structure, in accordance with the results of Mn K-edge
v
Herein, we used the similar method to estimate mean oxidation EXAFS spectroscopy.
4 x
Table 1. Physicochemical Properties of re-Ca Mn Al-LDH samples
Ca/Mn/Al molar
BET surface area
Pore diameter
(nm)
Total pore
b
c
Sample
a
2
−1
3
−1
c (Å)
a (Å)
ratio
(m g )
volume(cm g )
3
3
3
3
3
3
.94 : 0.00 : 1
.92 : 0.26 : 1
.98 : 0.51 : 1
.91 : 0.87 : 1
.91 : 1.11 : 1
.99 : 1.64 : 1
re-Ca
4
Al-LDH
28.2
34.3
26.5
33.3
22.1
29.6
7.4
7.9
0.139
0.191
0.220
0.225
0.318
22.98
22.90
22.82
22.70
22.78
22.82
3.32
3.33
3.32
3.32
3.31
3.31
re-Ca
re-Ca
re-Ca
re-Ca
re-Ca
4
4
4
4
4
Mn0.3Al-LDH
Mn0.5Al-LDH
Mn0.8Al-LDH
Mn1.0Al-LDH
Mn1.5Al-LDH
10.3
9.5
12.3
11.6
0.426
a
b
Ca/Mn/Al molar ratio was measured by inductively coupled plasma–atomic emission spectroscopy (ICP–AES). Lattice parameter a was determined by the
c
formula a = 2d110, which is related to the average cation–cation distance in the brucite-like sheets. Lattice parameter c was determined by the formula c =
3
d003, which is indicative of interlayer distance.
Fig. 2 (A) Normalized XANES spectra and (B) Fourier-transform EXAFS spectra at Mn K-edge for re-Ca
Ca Mn1.0Al-LDH, re-Ca Mn1.5Al-LDH, MnO, Mn and MnO , respectively; inset: the enlarged view of absorption edge selected from (A). (C) Normalized
XANES spectra and (D) Fourier-transform EXAFS spectra at Ca K-edge for re-Ca Al-LDH, re-Ca Mn0.3Al-LDH, re-Ca Mn0.5Al-LDH, re-Ca Mn0.8Al-LDH, re-
Ca Mn1.0Al-LDH and re-Ca Mn1.5Al-LDH, respectively.
4 4 4
Mn0.3Al-LDH, re-Ca Mn0.5Al-LDH, re-Ca Mn0.8Al-LDH, re-
4
4
2
O
3
2
4
4
4
4
4
4
4
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XPS spectra of Mn2p and O1s for these re-Ca
ARTICLE
IV
4
x
Mn Al-LDH dominant Ca−O−Mn structure induces a strengthened 7-fold
samples were investigated, so as to further quantify Mn species and Ca−OH coordinaꢀon and therefore a low electron density of
explore the electronic structure of O species. Fig. 3A shows the hydroxyl oxygen species; in contrast, for samples with Mn/Al molar
III
spin–orbit splitting of Mn2p1/2 and Mn2p3/2 signals for re-Ca
4
x
Mn Al- ratio from 0.5 to 1.5, the predominant Ca−O
v
−Mn structure leads
LDH samples. The broad, asymmetric Mn2p3/2 peak can be to a weakened 7-fold Ca−OH coordinaꢀon with a high electron
deconvoluted into two symmetry peaks cantered at 642.15 and density of hydroxyl oxygen species, in accordance with Ca XANES
∼
III
IV
6
43.45 eV, which are assigned to Mn and Mn species, spectra. In addition, the deconvoluted peak due to Ca−O−Mn lattice
1
7−19
IV
III
respectively.
With the increment of Mn content, the Mn /Mn
Mn Al-LDH samples estimated from the peak binding energy as the Mn/Al molar ratio increases from x=0 to
intensities decreases significantly (Fig. 3A and Table 2), which is x=1.5, which could be attributed to the electron transfer from Mn
oxygen species becomes augmented and shifts toward lower
molar ratio in re-Ca
4
x
III
IV
IV
indicative of the gradual transformation from Ca−O−Mn to (with a higher electron density relative to Mn ) to adjacent lattice
III
v 4 x
Ca−O −Mn structure. Moreover, Mn3s spectra of re-Ca Mn Al-LDH oxygen.
samples were collected to determine the average oxidation state of
Mn cations (Fig. S4). As has been reported, the Mn average
oxidation state (AOS) can be determined through the exchange
3
4 x
.2 Surface basic properties of re-Ca Mn Al-LDH samples
splitting of Mn3s spectra (ꢀE ) according to the linear equation:
3s
2
0−21
AOS = 8.956 − 1.13 (
without observed Mn3s peak, AOS of Mn species decreases in the
following sequence: re-Ca Mn0.5Al-LDH (4.75) re-Ca Mn0.8Al-LDH
ꢀ
E ).
Therefore, except re-Ca Mn0.3Al-LDH
3
s
4
>
4
4
(
4.68) > re-Ca Mn1.0Al-LDH (4.57) > re-Ca Mn Al-LDH (4.50). This
4 4 1.5
IV
indicates that the relative concentration of Mn decreases along
with the increment of Mn content, in accordance with the results of
Mn K-edge XANES spectroscopy.
Fig. 4 FTIR transmission spectra of (a) re-Ca
c) re-Ca Mn0.5Al-LDH, (d) re-Ca Mn0.8Al-LDH, (e) re-Ca
re-Ca Mn1.5Al-LDH recorded in 2170−2300 cm after CDCl
at 25 °C.
4
Al-LDH, (b) re-Ca
Mn1.0Al-LDH and (f)
chemiadsorption
4
Mn0.3Al-LDH,
(
4
4
4
−1
4
3
Fourier-Transform infrared spectroscopy (FTIR), with deuterated
chloroform (CDCl , a weak acid) as probe molecule, was employed
Fig. 3 (A) XPS Mn2p of (a) re-Ca
Ca Mn0.8Al-LDH, (d) re-Ca Mn1.0Al-LDH and (e) re-Ca
O1s of (a) re-Ca Al-LDH, (b) re-Ca Mn0.3Al-LDH, (c) re-Ca
Ca Mn0.8Al-LDH, (e) re-Ca Mn1.0Al-LDH and (f) re-Ca Mn1.5Al-LDH.
4
Mn0.3Al-LDH, (b) re-Ca
Mn1.5Al-LDH. (B) XPS
Mn0.5Al-LDH, (d) re-
4
Mn0.5Al-LDH, (c) re-
3
4
4
4
to study basic properties of solid materials under mild
22−23
4
4
4
conditions.
4 x
For these re-Ca Mn Al-LDH samples, a broad FTIR
−1
4
4
4
band located in the range 2170−2300 cm is obtained and
deconvoluted to four peaks via a Gaussian peak fitting method (Fig.
Fig. 3B shows a broad O1s spectrum within 528.8−535.2 eV,
which are deconvoluted to three peaks by Gaussian peak fitting
−1
4
). The two peaks at relatively high wavenumber (
∼
2275 cm and
2264 cm ) are assigned to C−D stretching vibration of CDCl
gas
red-shift
−1
∼
3
method for these re-Ca Mn Al-LDH samples. The two peaks at
4
x
2
4−25
and physically adsorbed CDCl
3
,
respectively.
A
∼
532.6 eV and ∼531.6 eV are assigned to surface hydroxyl in 7-fold
Ca−OH coordination and Ca−O−Al lattice oxygen species in brucite-
−
1
magnitude relative to the CDCl
3
gas peak (∼2275 cm ) and the
resulting proton affinities (calculated by eq. 1) are related to the
like layers of edge-sharing Ca(OH)6 octahedron; whilst the peak
26−27
Brønsted basic strength.
The deconvoluted FTIR spectra of all
∼
530.6 eV is attributed to the Ca−O−Mn lattice oxygen species.
these samples display two bathochromic peaks with a maximum in
With the increase of Mn/Al molar ratio from 0.3 to 0.5, the peak of
oxygen species in 7-fold Ca−OH coordination shifts to higher
−
1
−1
the region 2251
identified as C−D stretching vibraꢀon of CDCl
basic site and on medium basic site, respectively.
the integral intensity of absorption bands of CDCl
x
medium basic sites, we compared re-Ca Mn Al-LDH samples in this
−
2257 cm and 2214
−
2224 cm , which are
adsorbed on weak
binding energy (532.7 eV and 532.8 eV) relative to re-Ca Al-LDH
3
4
2
7−29
According to
on weak and
reference (532.6 eV), and then shifts to lower binding energy (532.2
eV) with the Mn/Al molar ratio from 0.5 to 1.5. The results indicate
that for the low Mn-containing samples (Mn/Al molar: 0.3, 0.5), the
3
4
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journal name
IV
work with other types of LDHs, which were listed in Table S2. The Ca−O−Mn structure (as confirmed by EXAFS and XPS) in samples
results show for previously reported LDH samples, both weak basic with a low content of Mn (Mn/Al molar: 0.3 and 0.5) induces a high
2
+
site (proton affinity: 851
proton affinity: 919 931 kJ/mol) are observed. For re-Mg
samples, FTIR band at 2214 cm assigned to medium basic site and a weakened Brønsted basic site. In the case of samples with a
−867 kJ/mol) and medium basic site s-orbital exposure of Ca and a strong interaction with the 7-fold
−
(
−
4
Al-LDH OH , resulting in a decreased electron density of hydroxyl oxygen
−1
(
proton affinity: ~925 kJ/mol) is dominant, due to the presence of high content of Mn (Mn/Al molar: 0.8, 1.0 and 1.5), the absorption
−1
4 x
surface/interlayer hydroxyl group. In the case of re-Ca Mn Al-LDH band corresponding to weak basic site shifts to 2252 cm (proton
−1
−1
samples however, FTIR band at 2256 cm corresponding to weak affinity of 867 kJ mol ) with an increased strength of weak base
III
basic site (proton affinity: ~854 kJ/mol) is predominant, resulting site, which indicates that the formation of Ca−O
from the seven-fold Ca−OH coordination. Based on our previous increased electron density of hydroxyl oxygen in 7-fold OH and a
−Mn leads to an
v
−
1
0
work, we identified weak basic site with an FTIR band of adsorbed rather strengthened Brønsted basic site. Meanwhile, the
−
1
−1
CDCl
3
at ∼2255 cm and a proton affinity of ∼855 kJ mol , and normalized peak area of medium basic site increases significantly.
−1
III
III
medium basic site with band at
∼
2214 cm and a proton affinity of This indicates that Mn and resulting Ca−O −Mn structure in high
v
−
1
∼
925 kJ mol . Our previous work has also versified the weak basic Mn-containing samples reduce the concentration of 7-fold Ca−OH
site and medium basic site in re-Ca Al-LDH material are derived coordination and facilitate the formation of physically-adsorbed
4
from the active 7-fold Ca−OH coordinaꢀon and physically-adsorbed surface hydroxyl, leading to less weak basic sites and more medium
1
0
IV
surface hydroxyl group, respectively. As shown in Fig. 4, for re- basic sites. Therefore, a controlled formation of Ca−O−Mn via
Ca Al-LDH and the low Mn-containing samples (Mn/Al molar: 0.3, changing the content of promoter Mn induces a high s-orbital
4
2
+
−
0
.5), it is observed that the weak basic site is dominant with almost exposure of Ca and a strong interaction with the 7-fold OH ,
−1
constant intensity. A red shift from 2255 to 2257 cm is observed resulting in a decreased electron density of hydroxyl oxygen and a
with the increase of Mn/Al molar ratio from 0 to 0.5, which weakened Brønsted basic site; in contrast, the formation of
III
indicates a further weakened weak basic site from 7-fold Ca−OH Ca−O −Mn leads to an opposite effect and a rather strengthened
v
coordination structure (with proton affinity from 858 to 851 kJ Brønsted basic site.
−
1
IV
mol ), as a result of the influence of Ca−O−Mn structure.
4 x 4
Fig. 5 (A) IBD conversion, HPA selectivity and HPA yield over re-Ca Mn Al-LDH catalysts. (B) Catalytic behaviour of re-Ca Mn0.5Al-LDH for 6 consecutive cycles.
Reaction conditions: IBD (0.55 mmol), FA (0.80 mmol, 37% aqueous solution), cetyl trimethyl ammonium bromide (CTAB 0.1 g), dioxane (0.2 g), catalyst (0.2
g), 70 °C, 8 h, Ar atomosphere.
a
Table 2. Catalytic performances of various samples toward aldol condensation of isobutyraldehyde with formaldehyde
III
IV
c
c
d
Mn /Mn
molar ratios
B
W
B
M
IBD Conversion
(%)
HPA Selectivity
(%)
HPA
Yield (%)
Formation Rate
Sample
b
1
1
1
−
1
−
−
−
(Area g )
44.65
(Area g )
10.26
9.87
(mmol g h )
57.1
re-Ca
4
Al-LDH
/
81.4
83.2
85.9
80.5
82.8
77.7
77.8
77.3
81.9
70.8
63.1
49.3
63.3
64.3
70.3
57.1
52.2
38.3
re-Ca
re-Ca
re-Ca
re-Ca
re-Ca
4
4
4
4
4
Mn0.3Al-LDH
/
44.72
69.5
Mn0.5Al-LDH
Mn0.8Al-LDH
Mn1.0Al-LDH
Mn1.5Al-LDH
0.407
0.588
0.688
0.789
46.18
9.56
98.0
50.67
48.00
64.33
150.00
49.5
24.82
34.3
15.67
21.0
a
Reaction conditions: isobutyraldehyde (0.55 mol), formaldehyde (0.80 mol), cetyl trimethyl ammonium bromide (0.1 g), dioxane (0.2 g), catalyst (0.2 g), 70
b
III
IV
III
IV
°
C, 8 h, Ar atomosphere. Mn /Mn is calculated by the normalized XPS peak area assigned to Mn and Mn , respectively, according to each integral area
c
divided by sample quantity. B
W
and B
M
are calculated based on integral intensity of absorption bands of CDCl
3
on weak and medium basic sites divided by
d
sample quantity, respectively. Formation rate of hydroxypivaldehyde (HPA) is calculated on the basis of tangent slope of the yield-reaction time plot at the
2
% yield of HPA.
6
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ARTICLE
(
70.3%). In addition, the formation rate of product (Fig. S5) follows
3
.3 Catalytic performance evaluations
a similar change tendency, and re-Ca Mn0.5Al-LDH exhibits the
4
−
1 −1
maximal value (98 mmol g h ). Meanwhile, some byproducts
from Cannizzaro reaction or deep condensation are detected by
HPLC-MS, including major neopentyl glycol, minor 3-hydroxy-2,2,4-
trimethyl-pentanal and polymerization products. Our previous
work has shown that weak Brønsted basic site (7-fold Ca−OH
coordination) in rehydrated CaAl-LDHs serves as active centre to
catalyse the aldol condensation, which accelerates the product
desorption and thus promotes the HPA selectivity; therefore,
tuning the strength and/or concentration of weak Brønsted base
structure would play a key role in improving the selectivity toward
aldol condensation product. Herein, to study the promotion effect
The aldol condensation of isobutyraldehyde (IBD) with
formaldehyde (FA) to produce hydroxypivaldehyde (HPA, an
intermediate in the production of neopentyl glycol) is a base-
catalysed reaction with great industrial importance, which requires
3
0
efficient heterogeneous catalysts for a “green” synthetic route.
In this work, the aldol condensation of IBD with FA was performed
over these rehydrated LDH solid basic catalysts, and the catalytic
evaluation data are shown in Fig. 5A and Table 2. With the
increase of Mn/Al molar ratio, the equilibrium conversion
maintains at ∼80%; while the selectivity increases markedly at first
and then decreases, and the maximal value (81.9%) is obtained
of promoter Mn on catalytic performance of re-Ca Mn Al-LDH
over re-Ca Mn0.5Al-LDH catalyst with a maximal yield of 70.3%.
4
x
4
samples, the initial formation rate of HPA vs. relative
The results show that re-Ca Mn Al-LDH catalysts exhibit largely
4
x
IV
IV
III
IV
concentration of Mn species (Mn /(Mn +Mn )) or relative
concentration of weak basic site (B /(B +B )) (based on the
enhanced performance (HPA yield: 49−82%) in comparison with
the Ca Mn Al-LDH precursors (HPA yield: 28−32%) or Ca Mn Al-
W
W
M
4
x
4
x
normalized XPS or FTIR peak area listed in Table 2) for these re-
Ca Mn Al-LDH catalysts are displayed in Fig. 6B and 6C,
MMOs (HPA yield: 20−37%) (Table S3). A comparison study
between re-Ca Mn0.5Al-LDH and previously reported catalysts
4
x
4
respectively. A linear correlation between initial formation rate
toward the synthesis of HPA is performed, and the results show
the largest HPA yield demonstrated in this work (Table S4). Most
IV
and the relative concentration of Mn is observed (Fig. 6B), which
IV
IV
manifests that Mn (Ca−O−Mn structure) serving as promoter
improves the catalytic performance in aldol condensation reaction,
importantly, the catalytic behaviour of re-Ca Mn0.5Al-LDH in this
4
work, exceeds conventional solid base catalysts (33.4%) and is
rather close to liquid alkali catalysts (73.2%) (see Table S3 for a
III
III
other than the Mn site (Ca−O −Mn structure). On the other
v
hand, as shown in Fig. 6C, for low Mn-containing samples, the
relative concentration of BW remains unchanged but initial
formation rate increases significantly; however, in the case of the
systematic comparison). When the re-Ca Mn0.5Al-LDH catalyst was
4
reused for 6 consecutive cycles, both the conversion and yield
decreased by ∼5% after the firstly 4 consecutive cycles, and then
maintained at a stable level within 5−6 cycles (Fig. 5B).
high Mn-containing samples, both the relative concentration of BW
and initial formation rate decreases markedly with the increment
In order to investigate the promotion effect of Mn on
rehydrated CaAl-hydrotalcite catalysts, the yield of HPA vs.
reaction time and initial formation rate at 2% IBD conversion that
reflects the intrinsic catalytic property, are shown in Fig. 6A. For all
these samples, HPA yield increases gradually along with reaction
time and reaches the equilibrium maximum at 8 h. The largest
IV
IV
of Mn content. This demonstrates that Mn (Ca−O−Mn ) in low
Mn-containing samples further decreases the strength of weak
basic site, which accelerates the product desorption and largely
promotes the HPA selectivity. For the high Mn-containing samples,
III
III
Mn (Ca−O −Mn ) would decrease the concentration of 7-fold
v
Ca−OH coordinaꢀon and thus reduce the content of acꢀve site
equilibrium yield is present in the sample of re-Ca Mn0.5Al-LDH
4
(
weak basic sites), resulting in a low initial formation rate. The
Fig. 6 (A) Profiles of HPA yield vs. reaction time over re-Ca
4
Mn
IV
x
Al-LDH catalysts. (B) Formation rate of HPA as a function of the normalized FTIR peak area
assigned to relative concentration of Mn (Mn /(Mn +Mn )). (C) Formation rate of HPA as a function of the normalized FTIR peak area assigned to weak
basic site (B /(B +B )) over (a) re-Ca Al-LDH, (b) re-Ca Mn0.3Al-LDH, (c) re-Ca Mn0.5Al-LDH, (d) re-Ca Mn0.8Al-LDH, (e) re-Ca Mn1.0Al-LDH, and (f) re-
Ca Mn1.5Al-LDH, respectively.
IV
IV
III
W
W
M
4
4
4
4
4
4
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results demonstrate that the electronic effect imposed by Mn
journal name
IV
p-orbitals of O atom decreases to three main peaks with a slight
IV
(
Ca−O−Mn ) modifies the 7-fold Ca−OH coordinaꢀon, which overlap with Ca atom, accounting for the weakened interaction
2+
−
2+
−
causes a strengthened Ca /OH interaction and a decreased between Ca and the 7-fold OH , in agreement with the results of
strength of weak basic site, as confirmed by EXAFS, XPS and FTIR. Ca K-edge EXAFS. In addition, based on catalytic evaluations in this
This tuned weak basic site promotes HPA desorption and inhibits work, we found that the equilibrium conversion of all catalysts
its deep condensation, which is responsible for the largely maintains a high level at ∼80%, but re-Ca Mn0.5Al-LDH sample with
4
enhanced HPA selectivity in the sample of re-Ca Mn0.5Al-LDH.
decreased strength of weak basic site exhibits the largest HPA
selectivity and initial formation rate. The aldol condensation of IBD
and FA to produce HPA is a classical one-step C–C coupling
reaction, and deep condensation of HPA is the main side reaction;
therefore, timely desorption of HPA from active site is the
selectivity-determining step. A weak basic site is prone to release
4
+
and transfer H to deprotonated HPA, which facilitates the
desorption of HPA. To further confirm this conclusion, the
IV
desorption behaviour of HPA from Ca−O−Mn , Ca−O−Al or
III
Ca−O −Mn was studied (Fig. 7D). The result reveals HPA
v
IV
desorption energy from Ca−O−Mn is 0.179 eV, significantly lower
III
than that from Ca−O−Al (0.557 eV) and Ca−O −Mn (1.976 eV),
v
which is responsible for the largely enhanced HPA yield over the
re-Ca Mn0.5Al-LDH catalyst.
4
4
. Conclusions
In summary, rehydrated CaMnAl-layered double hydroxides
IV
III
Fig. 7 Schematic view of (A) optimized Ca−O−Mn and (B) Ca−O
v
−Mn
(
LDHs) as a heterogeneous basic catalyst were prepared based on
based on DFT calculations. The hydroxyl in 7-fold Ca−OH coordination for
the memory effect of LDH precursors. The resulting re-Ca Mn0.5Al-
4
IV
III
Ca−O−Mn system moves inward to Ca atom, and that for Ca−O
v
−Mn
LDH exhibits a largely enhanced catalytic performance toward
aldol condensation of isobutyraldehyde with formaldehyde (HPA
yield of 70.3%), rather close to the level of liquid alkali catalysts
system moves outward. (C) Local density of states (LDOS) of Ca and O atom
in 7-fold Ca−OH coordination and (D) adsorption energies of HPA molecule
IV
III
in the system of: (a) Ca−O−Mn , (b) Ca−O−Al and (c) Ca−O
v
−Mn ,
(
73.2%). A combination study including XPS, EXAFS and deuterated
respectively.
chloroform−FTIR spectra verifies that re-Ca Mn0.5Al-LDH contains a
4
2+
To further understand the promotion effect of Mn on
rehydrated CaAl-hydrotalcite catalysts, density functional theory
highly-exposed Ca s-orbital and strengthened coordination
2+
−
between Ca and 7-fold OH , providing a weakened Brønsted
(
DFT) calculations were performed. As shown in Fig. 7A and 7B, the
basic site compared with the reference sample re-Ca Al-LDH.
4
IV
III
structural models of Ca−O−Mn and Ca−O −Mn structure are
v
Studies on the structure-property correlation based on
experimental investigation and DFT calculation reveal that the
built according to their XRD patterns, including a targeted Ca atom,
Mn atom and several circumjacent O atoms in Ca−O−M octahedra
IV
decreased strength of weak Brønsted basic site modified by Mn
IV
and 7-fold Ca−OH coordination. In the optimized Ca−O−Mn
further accelerates the product desorption, accounting for the
largely promoted HPA selectivity. This work provides a facile and
cost effective approach for the preparation of LDHs-based solid
basic catalysts with high performance, which can be used as a
promising candidate in green catalysis of aldol condensation
reactions.
IV
model (Fig. 7A), an obvious lattice expansion of Ca−O−Mn
IV
octahedron with a Ca−O−Mn bond length of 3.184 Å is found,
larger than that of Ca−O−Al (3.134 Å). Otherwise, a shorter Ca−O
bond length (2.251 Å) for 7-fold Ca−OH coordination is also
observed, compared with Ca−O−Al structure (2.361 Å). The results
confirm that the lattice expansion induces an enhanced exposure
2+
2+
of Ca s-orbital and thus strengthens the interaction between Ca
−
and the 7-fold OH , in accordance with Ca K-edge EXAFS results. In Acknowledgment
the case of Ca−O −Mn system however (Fig. 7B), we found a
III
v
This work was supported by the National Key Research and
longer Ca−O bond length (2.483 Å) in 7-fold Ca−OH, which is
Development Programme (Grant No. 2017YFA0206804), the
National Natural Science Foundation of China (NSFC) and the
Fundamental Research Funds for the Central Universities
2+
attributed to the lower exposure of Ca s-orbital and a weaker
2+
−
Ca /7-fold OH interaction. The local density of states (Fig. 7C)
was calculated to understand the interaction between Ca atom
(
buctylkxj01).
IV
and O atom in 7-fold Ca−OH. For Ca−O−Mn , the four main peaks
of p-orbitals associated with O atom show a strong hybridization
with the p and d orbitals of Ca atom, indicating a strong Ca–O
III
Notes and references
interaction. For Ca−O−Al and Ca−O −Mn , the hybridization from
v
8
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C8GC00851E
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Green Chemistry
Page 10 of 11
View Article Online
DOI: 10.1039/C8GC00851E
CaMnAl-hydrotalcite solid basic catalyst toward aldol condensation
reaction with a comparable level to liquid alkali catalysts
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Weihan Bing, Huimin Wang, Lei Zheng, Deming Rao, Yusen Yang, Lirong Zheng,*
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Bin Wang, Yangdong Wang,* Min Wei*
1
State Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation
Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology,
Beijing 100029, P. R. China
2
Institute of High Energy Physics, the Chinese Academy of Sciences, Beijing 100049, P. R.
China
3
Beijing Research Institute of Chemical Industry, Sinopec Group, Beijing 100013, P. R. China
4
SINOPEC Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, P. R.
China
Graphical Abstract

Page 11 of 11
Green Chemistry
View Article Online
DOI: 10.1039/C8GC00851E
A CaMnAl-hydrotalcite solid basic catalsyt was prepared based on the memory effect of LDHs,
which exhibited extremely high catalytic performance toward aldol condensation reaction,
comparable to level of liquid alkali catalysts.