Synthetic and natural materials with the brucite-like layers as high active catalyst for synthesis of 1-methoxy-2-propanol from methanol and propylene oxide

Article abstract of DOI:10.1016/j.molcata.2016.06.006

The aim of this study was to investigate the correlation between the basic and catalytic properties of brucite and Mg,Al-layered double hydroxides (Mg,Al-LDHs) in the reaction of propylene oxide with methanol. The basic properties of the solids were investigated by FT-IR spectroscopy using CDCl₃ as probe molecule and Hammett acidity titration with n-butylamine and benzoic acid. Both the increase in the activation temperature and the decrease in Al content favored the increase in the basicity of Mg,Al-LDH. The nature of “Mⁿ⁺[Formula presented]^2? Lewis acid-base” pairs was a key factor to control the catalytic behavior of the solids. The length of the M-O bond, the basicity of the oxygen ion and the strength of the metal ion on the surface affected the catalytic activity of the solids and their isomer selectivity in the reaction between methanol and propylene oxide to form 1-methoxy-2-propanol. It was demonstrated that brucite(150) can be applied as catalyst for at least four successive cycles without loss of activity.

Full text of DOI:10.1016/j.molcata.2016.06.006

Journal of Molecular Catalysis A: Chemical 423 (2016) 22–30
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Synthetic and natural materials with the brucite-like layers as high
active catalyst for synthesis of 1-methoxy-2-propanol from methanol
and propylene oxide
Maria N. Timofeevaa,b,∗, Aleksey E. Kapustinc, Valentina N. Panchenkoa,b
,
Eleonora O. Butenkoc, Victoria V. Krupskayad,e, Antonio Gilf,∗∗, Miguel A. Vicenteg
a Boreskov Institute of Catalysis SB RAS, Prospekt Akad. Lavrentieva 5, 630090, Novosibirsk, Russian Federation
b Novosibirsk State University, st. Pirogova 2, 630090, Novosibirsk, Russian Federation
c Azov State Technical University, Mariupol, Ukraine
d Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry RAS, pr. Staromonetny 35, 119017, Moscow, Russian Federation
e Moscow State University, Leninskie Gory 1, 119991, Moscow, Russian Federation
f
Department of Applied Chemistry, Los Acebos Building, Public University of Navarra, Campus of Arrosadia s/n, 31006 Pamplona, Spain
g Department of Inorganic Chemistry, University of Salamanca, Salamanca, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
The aim of this study was to investigate the correlation between the basic and catalytic properties of
brucite and Mg,Al-layered double hydroxides (Mg,Al-LDHs) in the reaction of propylene oxide with
methanol. The basic properties of the solids were investigated by FT-IR spectroscopy using CDCl3 as
probe molecule and Hammett acidity titration with n-butylamine and benzoic acid. Both the increase in
the activation temperature and the decrease in Al content favored the increase in the basicity of Mg,Al-
LDH. The nature of “Mn+ O2− Lewis acid-base” pairs was a key factor to control the catalytic behavior of
the solids. The length of the M-O bond, the basicity of the oxygen ion and the strength of the metal ion on
the surface affected the catalytic activity of the solids and their isomer selectivity in the reaction between
methanol and propylene oxide to form 1-methoxy-2-propanol. It was demonstrated that brucite(150)
can be applied as catalyst for at least four successive cycles without loss of activity.
Received 22 April 2016
Received in revised form 3 June 2016
Accepted 3 June 2016
Available online 5 June 2016
Keywords:
Brucite
Mg,Al-layered double hydroxides
Basicity
Propylene glycol methyl ether
Propylene oxide
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
Remarkably, the nature of the catalyst allows to control the
product distribution [2–5]. 1-methoxy-2-propanol (II) is predom-
Nowadays, propylene glycol methyl ether is widely used as
pollution-free solvent in fine chemicals synthesis in pharmaceu-
tical, chemical and cosmetic industry [1]. One of the traditional
methods for the synthesis of propylene glycol ethers is the reaction
of propylene oxide (PO) with methanol:
inantly formed in the presence of basic type catalysts (NaOH [2],
oxides [5–7], or amines [6,8]), while 2-methoxy-1-propanol (I) is
formed in the presence of acid type catalysts (BF3, H2SO4) [2,9].
Natural
hydrotalcite
(HT)
has
the
formula
[Mg6Al2(OH)16]CO3·4H2O. The hydrotalcite-type structure can be
viewed as derived from brucite (Mg(OH)2). The substitution of a
certain amount of Mg2+ by Al3+ makes the layers to have positive
charge (one charge by each substitution), which is compensated
by the carbonate anions located in the interlayer region (one
carbonate by each two substitutions). Hydration water is also
located in the interlayer region. The optimal degree of substitution
is 0.25 (one cation position from four), leading to a Mg/Al ratio
of 3, exactly the ratio observed in natural hydrotalcite, although
pure hydrotalcite phases can be obtained in the laboratory using
substitution degrees between 0.20 and 0.33 (Mg/Al ratios between
4 and 2). This structure can be adopted by combinations of a large
number of divalent (Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn, etc.) and trivalent
∗
Corresponding author at: Boreskov Institute of Catalysis SB RAS, Prospekt Akad.
Lavrentieva 5, 630090, Novosibirsk, Russian Federation.
∗∗
Corresponding author.
E-mail addresses: timofeeva@catalysis.ru, mariya-timofeeva@yandex.ru
(M.N. Timofeeva), andoni@unavarra.es (A. Gil).
http://dx.doi.org/10.1016/j.molcata.2016.06.006
1381-1169/© 2016 Elsevier B.V. All rights reserved.

M.N. Timofeeva et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 22–30
23
Table 1
Chemical composition of Mg,Al-LDHs.
Theoretical dataa
Experimental datab
Mg/(Mg + Al)
(mol/mol)
Mg/Al
(mol/mol)
Mg
Al
Mg/(Mg + Al)
(mol/mol)
Mg/Al
(mol/mol)c
(wt.%)
(mol)c
(wt.%)
(mol)c
Mg,Al-LDH(0.62)
Mg,Al-LDH(1.50)
Mg,Al-LDH(2.63)
Mg,Al-LDH(3.89)
0.52
0.72
0.81
0.86
1.08
2.57
4.26
6.14
11.0
17.4
21.0
24.7
0.453
0.716
0.864
1.016
19.8
12.9
8.86
7.05
0.734
0.478
0.328
0.261
0.38
0.60
0.72
0.80
0.62
1.50
2.63
3.89
a
Ratio of the elements during the synthesis of the Mg,Al-LDHs.
Data were determined from the chemical analyses of the Mg,Al-LDHs.
Molar amount of Mg and Al in 100 g of solid.
b
c
(Al, Ga, In, V, Cr, etc.) cations, almost with the only condition that
their size is not very different from that of Mg2+. By the presence
of two cations in their composition, this family of compounds is
also called Layered Double Hydroxides (LDHs). They are also called
Anionic Clays, as their structure resembles that of smectitic clays,
but with the layer charged positively and with compensating
anions in the interlayer [10].
rate and selectivity towards (II) in the presence of these materials.
Note that demonstrations of correlations between acid-base prop-
erties and catalytic activity of Mg,Al-LDHs are rather limited, while
application of brucite as catalyst for this reaction is absent.
2. Experimental
LDHs have been demonstrated as efficient and selective cata-
lysts for the synthesis of glycol ethers [11–15]. Thus, in the presence
of Zn,Mg,Al-catalysts derived from HT-like compounds the propy-
lene oxide conversion of PO and selectivity towards (II) were 95.9%
and 90.9%, respectively, under optimal reaction conditions (amount
of the catalysts 1.8 wt.%; molar ratio MeOH/PO 5:1; 120 ◦C and 3 h)
[11]. Zeng et al. [15] reported the preparation of Mg,Al-LDHs with
different Mg/Al molar ratios by the urea method and the investi-
gation of their catalytic properties in the synthesis of propylene
glycol methyl ether, finding the highest activity in the presence of
Mg,Al-LDHs calcinated at 500 ◦C for 7 h. Activity of activated Mg,Al-
LDHs depended on the Mg/Al molar ratio; the increase in Mg/Al
molar ratio up to 4.0 favored the improvement of the catalytic
activity (conversion of PO and selectivity towards (II)), then the
catalytic activity dropped. This trend was explained by the depen-
dence of the basicity and the crystallinity on the Mg/Al molar ratio.
In optimal reaction conditions (Mg/Al = 4.0, amount of the catalysts
0.9 wt%, molar ratio MeOH/PO 4.0, 140 ◦C, 6 h) the conversion of PO
and selectivity towards (II) were 93.2 and 97.4%, respectively.
According to Manivannan et al. [16], catalysts based on Mg,Al-
LDHs contained weakly basic OH groups, medium basic sites
formed by M O pairs and strong basic surface sites formed by the
isolated surface O2−. In our recent investigation of this reaction in
the presence of M-PILCs (M = Al, Ga and Zr) [17] and metal phos-
phate molecular sieves [18], we have suggested that the reaction
mechanism involves the interaction of reactants with active sites
“Mn+ O2− Lewis acid-base” pairs.
In the present work, two series of materials having the brucite
or hydrotalcite-type structures were investigated as catalysts for
the reaction of propylene oxide with methanol. The first series
of materials were based on natural brucite from the deposit of
Kuldur (Russian Mining Chemical Company, Moscow). The second
series of materials were based on Mg,Al-LDHs with different Mg/Al
molar ratio. As indicated before, Mg,Al-LDHs are related to brucite,
resulting from the partial substitution of Mg2+ by Al3+, the change
of chemical composition should change their acid-base properties
and, therefore, the catalytic behavior. So, one of the purposes of
this study was to demonstrate the potential application of brucite
as heterogeneous catalyst in this reaction, and other purpose was
to investigate how the changes from brucite to hydrotalcite influ-
ence on the catalytic behavior of the resulting solids. Acid-base
properties have been studied by FT-IR spectroscopy using CDCl3 as
probe molecule and Hammett acidity titration with n-butylamine
and benzoic acid. Special attention will be paid to control reaction
2.1. Materials
Propylene oxide (PO) and n-propanol were purchased from
Aldrich. Commercial methyl alcohol was used without any fur-
ther purification. Brucite came from the deposit of Kuldur (Russian
Mining Chemical Company, Moscow). Brucite was dried/calcined
at 90, 150, 250 or 400 ◦C for 2 h, and characterized and used as
catalysts without further treatments. The samples were identified
as Brucite(T), T being the temperature of drying/calcination, in ◦C.
Mg,Al-LDHs were prepared according to the method described in
[19,20]. The method involved the slowly mixing of the appropri-
ate amounts of aqueous solutions X M of Mg(NO3)2·6H2O and Y M
of Al(NO3)3·9H2O (X + Y = 1 mol/l) with a 1 M solution of NaOH and
0.1 M of Na2CO3 at 120 ◦C under vigorous stirring (the pH between
8 and 10) to yield an amorphous gel, followed by crystallization at
120 ◦C. Theoretical ratios Mg/Al of 1.08, 2.57, 4.26 and 6.14 were tar-
geted (Table 1). The precipitate was washed by water until absence
of nitrates, and then dried at 150 ◦C. The samples were identified
considering their Mg/Al ratios determined from chemical analyses,
as indicated in Table 1.
2.2. Instrumental measurements
The porous structure of the materials was studied from nitrogen
(Air Liquide, 99.999%) adsorption at −196 ◦C using a static volu-
metric apparatus (Micromeritics ASAP 2010 adsorption analyser).
All samples (0.2 g) were degassed for 24 h at 200 ◦C at a pressure
lower than 0.133 Pa. The specific surface area (SBET) was calculated
from the adsorption data over the relative pressure range between
0.05 and 0.20. The total pore volume (Vp) was calculated from the
amount of nitrogen adsorbed at a relative pressure of 0.99. The
micropore volume (V␮p) and the external specific surface area (Sext
were calculated using the t-plot method, and the pore diameter
applying the BJH method.
)
The chemical analyses of the solids were carried out by means
of inductively coupled plasma–atomic emission spectrometry (ICP-
AES).
The X-ray diffraction patterns from random samples were col-
lected on ɑ X-ray diffractometer (Ultima-IV, Rigaku), acquired as
part of the Moscow State University Development Program, using
Cu-K␣ radiation (␭ = 1.54056 Å), rapid detector D/Tex-Ultra, scan
range 3.6–65◦ 2␪, scan speed 5◦/min, step 0.02◦ 2␪, maximum
intensity ∼25000 counts. Quantative mineral analysis has been
done by Rietveld full-pattern fitting method [21].

24
M.N. Timofeeva et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 22–30
Table 2
Textural properties of brucite calcinated at several temperatures.
SBET (m2/g)
Sext (m2/g)
Vp (cm3/g)
V
(cm3/g)
␮p
Brucite(90)
Brucite(150)
Brucite(250)
Brucite(400)
10 (C = 82)a
10 (C = 103)a
15 (C = 170)a
271 (C = 38)a
9
7
11
37
0.050
0.049
0.054
0.215
<0.001
<0.001
0.002
0.113
a
C is the BET constant related to the heat of adsorption.
Table 3
Structural and textural properties of Mg,Al-LDHs with several Mg/Al atomic ratios.
Structural properties
Textural properties
a (Å)
c (Å)
d003 (Å)
h (Å)
SBET (m2/g)
Vp (cm3/g)
Mg,Al-LDH(0.62)
Mg,Al-LDH(1.50)
Mg,Al-LDH(2.63)
Mg,Al-LDH(3.89)
3.038
3.036
3.045
3.058
22.62
22.67
22.81
23.40
7.54
7.56
7.60
7.80
2.74
2.76
2.80
3.00
250
200
192
180
0.5
0.2
0.2
0.2
d003–basal spacing; a and c – lattice parameters; h – distance between layers.
The DR-UVvis spectra were recorded on a UV-2501 PC Shimadzu
3. Results and discussion
spectrometer with an IRS-250A accessory in the 190–900 nm range
with a resolution of 2 nm; BaSO4 was used as standard reference.
The basicity of Mg,Al-LDHs was determined by the method of
indicator titration with 0.01 M benzoic acid in benzene. Mg,Al-
LDHs were calcinated at 150 ◦C for 1 h. Calcined barium oxide
was used as a comparison standard. Bromothymol blue (pKa = 7.2),
dinitroaniline (pKa = 15.0), 4-chloro-2-nitroaniline (pKa = 17.2), 4-
chloroaniline (pKa = 26.5) are used as indicators. The concentration
of acid sites was determined by the method of indicator titration
with butylamine in benzene.
3.1. Physicochemical and textural properties of the catalysts
3.1.1. Characterization of brucite
Chemical composition of natural brucite was (wt.%): 38.3% Mg,
0.03% Al and 0.005% Na. Its X-ray diffractogram (Fig. S1, Support-
ing information) pointed out that it mainly consisted on brucite
(89.5%), identified by the diffraction peaks at 4.77, 2.72, 2.37, 1.79,
1.57 and 1.49 Å. Moreover, it contained impurities of dolomite
(2.4%, diffraction peak at 2.89 Å), calcite (1.4%, diffraction peak
at 3.03 Å), magnesite (0.8%, diffraction peaks at 2.72 and 2.11 Å),
lizardite (1.5%, diffraction peaks at 7.32, 4.60, 3.65, 2.62, 2.50, 2.34
and 2.15 Å), and traces of smectite (diffraction peaks at 13.6, 6.5,
4.45 and 2.19 Å). Heating of brucite at 90–400 ◦C led to its dehy-
dration, and only at 400 ◦C the phase change from brucite to MgO
– periclase was completed, this phase being characterized by the
diffraction peaks at 2.11 and 1.50 Å. The mineralogical impuri-
ties remained practically unchanged, as carbonates and silicates
decompose at higher temperatures.
Basicity of brucite was studied by IR spectroscopy using CDCl3
as probe molecule. The analysis method was described in [22]. The
strength of the base sites was estimated using the following equa-
tion:
logꢀC-D = 0.0066PA − 4.36(1)
IR spectra of brucite activated at several temperatures in the
region of hydroxyl stretching vibration are shown in Fig. S2A (Sup-
porting information). The spectrum of natural brucite showed an
intense band at 3697 cm−1 that can be attributed to hydroxyl
stretching vibration [23,24]. Moreover, the bands at 3649 and
3625 cm−1 can be attributed to the stretching vibration of Mg OH
groups (Table S1, Supporting information). The intensity of all these
bands slightly decreased after calcination of sample at 90 ◦C for 2 h,
while calcination at higher temperature led to its sharp decrease.
Note that bands at 3763 and 3798 cm−1 can be revealed in the spec-
trum of brucite, which can be assigned to Al OH groups due to the
presence of a small amount of Al in brucite (0.3 wt.%). Moreover,
bands from carbonate groups in the region of 3200–3700 cm−1 are
also observed in the spectrum (Fig. S2A, Table S1, Supporting infor-
where ꢀC-D is the shift, in cm−1, of C D vibration and PA is the pro-
ton affinity. IR spectra were recorded on a Shimadzu FTIR–8300 S
spectrometer in the range between 400 and 6000 cm−1 with a res-
olution of 4 cm−1
.
2.3. Catalytic test
Brucite and LDHs were tested in the synthesis of 1-methoxy-
2-propanol from methanol and propylene oxide (PO) at 120 and
140 ◦C. PO (99.5%) was purchased from Acros Organic. Commercial
methanol was used without any further purification. The reac-
tion was carried out in a stainless steel autoclave reactor with an
inner volume of 25 cm3. The standard procedure was as follows:
7.4 mmol PO, MeOH (MeOH/PO = 8 mol/mol) and catalyst (3 wt.%)
were introduced into the autoclave. After running at 120 or 140 ◦C
for 1 or 7.5 h under magnetic stirring, the reactor was cooled down
to room temperature. The products were analyzed by a gas chro-
matograph (Agilent 7820) with a flame ionization detector on a
HP-5 capillary column, after separation by centrifugation of the cat-
alysts. n-Propanol was used as the internal standard to calculate the
amount of the products.
Table 4
Acid-base properties of Mg,Al-LDHs with several Mg/Al atomic ratios.
Acid-base properties (mmol/g)
BS
BAS
BS
ꢁ
BAS
Mg,Al-LDH(0.62)
Mg,Al-LDH(1.50)
Mg,Al-LDH(2.63)
Mg,Al-LDH(3.89)
0.41
0.32
0.21
0.06
0.73
0.54
0.63
0.85
1.8
1.7
3.0
0.694
0.730
0.746
0.755
14.2

M.N. Timofeeva et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 22–30
25
Fig. 1. Correlation between optical basicity ꢁ and molar ratios of Mg/(Mg + Al) in Mg,Al-LDHs (A) and molar ratio of basic sites/acidic sites in Mg,Al-LDHs (B).
90
75
60
45
90
75
60
45
A
B
20
10
0
20
10
0
880
890
PA, kJ/mol
900
910
100
200
300
400
o
Temperature of activation, C
Fig. 2. Effect of temperature activation of brucite on conversion of PO and selectivity towards (II) (Experimental conditions: 4.1 wt% of catalyst, MeOH/PO = 8 mol/mol, 120 ◦C,
1 h).
Fig. 3. (A) Correlations between molar ratio of basic sites/acidic sites and conversion of PO and molar ratio of (II)/(I) for 7.5 h; (B) Correlations between optical basicity (ꢁ)
and conversion of PO and molar ratio of (II)/(I) for 7.5 h; (C) Correlations between strength of basic sites and conversion of PO and molar ratio of (II)/(I) for 7.5 h.
of brucite [23]. Bands at 798, 1018 and 1080 cm−1 may be attributed
to the presence of traces of smectite. Bands at 853, 881, 1429 and
1489 cm−1 were assigned to the presence of calcite, dolomite or
magnesite [23]. The decrease in intensity of these bands with cal-
cination of sample suggested the beginning of the decomposition
of the carbonates.
mation). The intensity of these bands decreases with increasing the
temperature of calcination of brucite.
The low frequency bands in the IR spectra of brucite activated
at several temperatures are shown in Fig. S2B (Supporting infor-
mation). Bands were observed close to 964 cm−1 (high-frequency
Mg
Mg
O
O
H angle-bending mode), 1018 cm−1 (normal modes for
H interactions), at similar positions that in the IR spectrum

26
M.N. Timofeeva et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 22–30
Table 5
Catalytic behavior of brucite and Mg,Al-LDHs in the reaction of propylene oxide with methanol.
Catalyst
Experimental conditions
Catalytic test
Catalyst (wt.%)
МeOН/PO (mol/mol)
T (◦C)
time (h)
Conversion of
PO (%)
Selectivity (%)
(I)
(II)
Other
Brucite(90)
Brucite(150)
Brucite(250)
Brucite(400)
4.1
2.2
4.1
2.2
4.1
2.2
4.1
2.2
2.2
2.2
2.2
2.2
2.2
2.2
8
12
8
12
8
12
8
12
12
12
12
12
12
12
120
120
120
120
120
120
120
120
140
120
120
120
120
140
1
7.5
1
7.5
1
7.5
1
7.5
7.5
7.5
7.5
7.5
7.5
7.5
13
30
20
36
19
35
20
36
42
7
12.5
16.8
9.1
8.5
8.2
8.0
8.1
7.9
9.2
36.4
38.0
21.1
14.2
11.6
72.0
70.3
86.6
87.3
87.5
86.9
87.8
87.9
87.7
53.5
52.0
72.1
84.6
88.2
15.5
12.9
4.3
4.2
4.3
5.1
4.1
4.2
3.1
10.1
10.0
6.8
1.2
0.2
Brucite(150)
Mg,Al-LDH(0.62)a
Mg,Al-LDH(1.50)a
Mg,Al-LDH(2.63)a
Mg,Al-LDH(3.89)a
Mg,Al-LDH(3.89)a
8
10
15
28
a
Calcinated at 400 ◦C.
The effect of the temperature on the textural properties is
also clear and confirms the results found with other techniques
(Table 2). Up to a temperature of 250 ◦C, no appreciable changes
are observed in the specific surface area and pore volume, obtaining
values of 15 m2/g and 0.054 cm3/g. However, these values increase
significantly after treatment at 400 ◦C, obtaining 270 m2/g and
0.215 cm3/g. The thermal treatment produces a phase change from
brucite to MgO-periclase that is confirmed by the type of adsorption
isotherm, from type II–III to type I–II (see Fig. S3, Supporting infor-
mation). The phase change is also confirmed from the modification
of the C constant, related to the interaction adsorbate-adsorbent.
The increase of C from 82 to 170 could be related to the dehy-
dration process. The phase change can affect strongly this type of
interaction, decreasing C up to 38.
ratio, so being related to the charge of the layers and consequently
to the amount of compensating interlayer carbonate anions, the
increase in the Al content led to the decrease in the basal spac-
ing between the layers d003 and the lattice parameters, a and c.
The decrease in the basal spacing implied a parallel decrease of
the distance between layers, h, when increasing the Al content
in the solids (Table 3). This trend can be explained by the higher
electrostatic attraction between the positive hydroxide layers and
negative interlayers when increasing the Al content [27,28]. The
distance between the layers was calculated using the following
equation:
h = d003 − 4.8(2)
where 4.8 Å is the thickness of the brucite-like layer.
3.1.2. Characterization of Mg,Al-LDHs
Moreover, the experimental data pointed out that the variation
of the Al content affected both the crystallinity and the lattice strain
of Mg,Al-LDHs. Thus, according to the intensity and width of (0 0 3)
reflection, the increase in the Al content led to the decrease in the
crystallinity of samples and, moreover, to the decrease in the unit
cell dimension. First of all, this phenomenon can be explained by
the difference in size of metal ions, i.e. the size of Al3+ (0.0535 nm)
is much smaller than Mg2+ (0.072 nm). This difference can lead to
the strong interaction of trivalent cations with OH groups com-
pared to divalent cations, i.e Al3+ cations in the brucite-like layers
of Mg,Al-LDHs attracted the OH groups more strongly than Mg2+
cations, what favored the changing of the average a lattice param-
eter (Table 3). This effect has been demonstrated for Mg,Al-LDHs
[29,30] and Mg,Fe-LDHs [31].
The textural properties of Mg,Al-LDHs also depended on the
Al content. As can be seen from Table 3 specific surface area and
pore volume rose with increase in Al content in Mg,Al-LDH. This
may be due to the presence in these samples of alumina, with high
specific surface area, and not detected in the diffractograms by its
amorphous nature.
The disorder in the interlayer spacing of Mg,Al-LDHs has been
also followed from IR spectra (Fig. S5, Supporting information). The
spectra clearly showed the relatively broad carbonate modes at
670 cm−1 (ꢀ4), 880 cm−1 (ꢀ2), 1050–1069 cm−1 (ꢀ1), and that the
relatively broad carbonate ꢀ3 mode at around 1380 cm−1 split into
two bands 1368 and 1398 cm−1 [25,32,33]. The mode ꢀ1 of car-
bonate is not active in IR, so its activation indicates that it has low
symmetry, as also suggested by the splitting of ꢀ3 mode, due to
the location of this anion in the interlayer. The shift of ꢀ1 mode
from 1069 cm−1 in the spectrum of Mg,Al-LDH(0.61) to 1050 cm−1
The X-ray powder diffraction patterns of Mg,Al-LDHs are shown
in Fig. S4 (Supporting information). All diffractograms were typ-
ical for solids having the hydrotalcite structure (JCPDS-14-191).
Although nitrates were the source of the cations, as sodium car-
bonate was used in the synthesis, it was expected that carbonate
was the interlayer anion, due to the strong preference of this
anion for the interlayer region. The characteristic reflections were
observed in the low angle region (2␪ < 25◦), the middle angle region
(2␪ = 30◦ − 50◦) and the high angle region (2␪ = 55◦ − 65◦), assigned
to sharp and intense basal 00 l reflections of 0 0 3 and 0 0 6 planes,
broad 0kl reflections of 0 1 2 and 0 1 5 planes, and sharp hk0 and
hkl reflections of 1 1 0 and 1 1 3 planes, respectively [16,26–28].
Very crystalline solids are obtained in the cases of Mg,Al-LDH(3.89)
and, especially, Mg,Al-LDH(2.63), and less crystalline in the cases of
Mg,Al-LDH(0.62) and Mg,Al-LDH(1.50), which agrees with the fact
that the first two solids show Mg/Al ratios in the range allowed for
the formation of hydrotalcite structure while the last two solids are
out of this range (as hydrotalcite formally derives from brucite, low
amounts of Al do not allow the formation of pure hydrotalcite, while
high Al amounts provoke repulsion between Al-containing octahe-
dra, distorting the structure). In fact, for solids Mg,Al-LDH(0.62)
and Mg,Al-LDH(1.50), although low-crystalline hydrotalcite is the
only phase observed, their chemical composition indicates that
an excess of amorphous alumina may also exist. The observed
reflections can be indexed in a three-layer 3R polytype with rhom-
bohedral symmetry (space group R3m). The values of the lattice
parameters, a and c, calculated from 00 l reflections of 1 1 0 and 0 0 3
planes (c = 3d003 and a = 2d110) are shown in Table 3. The basal spac-
ings varied between 7.54 and 7.80 Å, with the experimental Mg/Al

M.N. Timofeeva et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 22–30
27
in the spectrum of Mg,Al-LDH(0.86) pointed out to the decrease of
the trivalent cation in the brucite layer [34].
where ICP is the so-called ionic-covalent parameter. Early [17],
we have suggested to use the ICP parameter for the quantitative
description of the acid-base properties of Al-, Zr- and Ga-containing
pillared clays. ICP parameter takes into account both the type (ionic
to covalent) of Me O bond and the extent (through polarizability)
of the negative charge borne by oxygen, and is calculated by the
following equation [38,39]:
3.1.3. Basicity of brucite and Mg,Al-LDHs
The change of the basicity of brucite as a function of the
activation temperature and of Mg,Al-LDHs depending on their com-
position was investigated by IR spectroscopy using CDCl3 as probe
molecule. The spectra of CDCl3 adsorbed on brucite activated at
90, 150, 250 and 400 ◦C are shown in Fig. S6A (Supporting infor-
mation). For brucite activated at 90 ◦C, three bands at 2211, 2237
and 2252 cm−1 can be disclosed in the spectrum after deconvo-
lution. At the same time, bands at 2210–2212, 2230–2232 and
2245–2247 cm−1 were observed in the spectra of the solids heated
at 150, 250 and 400 ◦C. The positions of the bands in these spec-
tra are given in Table S2 (Supporting information). All these
bands can be assigned to the interaction between CDCl3 and basic
sites. According to Paukshtis et al. [35], bands in the region of
2210–2220 cm−1 are the result of the interaction of CDCl3 with an
oxygen atom through a proton (Cl3C-D → O), while bands in the
region of 2230–2260 cm−1 are due to the interaction of CDCl3 with
OH groups. This assertion is in agreement with our experimental
data. The position of the bands at 2210–2212 cm−1 did not change
with increasing of temperature activation of brucite, while bands at
ICP = logP − 1.38 × ꢂ + 2.07(4)
where ␹ is the electronegativity (in Pauling-type units [40]) and P
is the polarizing power of the cation (P = e/r2, e – formal charge, r
– Shannon ionic radius). Thus, the bulk parameter optical basicity
ꢁ may be the most appropriate parameter for characterization of
Mg,Al-LDHs, because this parameter takes into account both the
type (ionic to covalent) of Me O bond and the extent (through
polarizability) of the negative charge borne by oxygen, i.e. the actual
coordination of each ion.
The values of optical basicity for Mg,Al-LDHs were estimated by
the following equation [38]:
ꢁ = XMgꢁMg + XAlꢁAl(5)
where XAl,XMg, ꢁAl and ꢁMg are the equivalent fractions of met-
als and optical basicity of Mg and Al, respectively. The main results
are shown in Table 4. The linear correlation between atomic ratio
of Mg/Al and ꢁ is clearly observed in Fig. 1A. Optical basicity rose
when increasing the Mg/Al atomic ratio, i.e. with the decreasing
of Al content. This trend was in agreement with the data obtained
by IR spectroscopy using CDCl3 as probe molecule. Moreover, this
correlation was also in agreement with the data determined by the
method of indicator titration in benzene (Table 4, Fig. 1B), which
indicated that Mg,Al-LDHs possessed both Brønsted acid sites (BAS)
and basic sites (BS). The decrease in Al content in Mg,Al-LDHs
favored the simultaneous decrease in the amount of BAS and the
increase in BS.
2252 and 2237 cm−1 shifted to 2245–2247 and 2230–2232 cm−1
,
respectively (Fig. S6, Table S2, Supporting information). Unfor-
tunately, the nature of bands at 2210–2212 cm−1 was not clear.
Bands in the regions of 2240–2260 cm−1 (2252 and 2245 cm−1
)
and 2220–2240 cm−1 were likely to arise from interaction between
CDCl3 with basic sites on the brucite layer. The strength of these
basic sites was estimated from the shift of ꢀC-D using Eq. (1) [35].
The values for the proton affinity (PA) are presented in Table S2
(Supporting information). It was deduced that the increasing of
the activation temperature from 90 to 150 ◦C was accompanied by
the increase of the strength of two basic sites of brucite (PA) from
843 and 886 kJ/mol to 867 and 900 kJ/mol, respectively. The further
increasing of activation temperature did not affect the strength of
the basic sites.
3.2. Catalytic properties of brucite and LDHs
IR spectra of CDCl3 adsorbed on Mg,Al-LDHs activated at 400 ◦C
are shown in Fig. S6B (Supporting information). After deconvolu-
tion, three bands at 2210–2212, 2220–2235 and 2247–2250 cm−1
can also be disclosed in all the spectra, their positions are listed in
Table S2 (Supporting information). Only the position of the band
in the region of 2220–2235 cm−1 depended on the Mg/Al atomic
ratio, shifting from 2235 to 2220 cm−1 when Mg/Al increased from
0.61 to 4.00. At the same time, the increase in Mg/Al from 0.61 to
4.00 led to a parallel increase of the strength of the basic sites from
884 to 915 kJ/mol (Table S2, Supporting information). According
to Manivannan et al. [16], Mg,Al-LDHs possess three type of sur-
face basic sites, namely the sites of weak ( OH groups), medium
(M O pairs), and strong (O2− anions) basicity. The surface concen-
tration of weak and medium strength basic sites increased when
decreasing the Al content. Therefore, the shift of the band in the
region of 2220–2235 cm−1 can point out the change of the nature
of OH groups and M O pairs in Mg,Al-LDHs with the variation of
Al content.
Other approach for the estimation of the change of the nature
of OH groups and M O pairs in Mg,Al-LDHs with the variation of
Al content is based on the “optical basicity”. According to Duffi and
Ingram [36], the optical basicity characterizes the electron donor
power of the catalyst lattice oxygen and depends on the cation(s)
valence and coordination. Lebouteiller and Courtine [37] demon-
strated that optical basicity can be calculated for any oxide or
complex. Generally, the optical basicity (ꢁ) can be calculated using
the following Eq. (3):
The catalytic properties of brucite and the anionic clays were
studied in the synthesis of propylene glycol methyl ethers from
methanol and propylene oxide (PO). Special attention was focused
on the relationships between chemical composition, basic proper-
ties and catalytic performance of the materials. The heterogeneous
character of the reaction in the presence of brucite and Mg,Al-LDHs
was confirmed in special experiments. Thus, brucite(150) catalyst
was filtered off after 1 h of the reaction at 120 ◦C, 8 mol/mol of
MeOH/PO, 4.1 wt.% of catalyst, where the conversion of PO was
about 20%. Then, the filtrate without the catalyst was further stirred
at 120 ◦C for 3 h in autoclave. After removing the catalyst, the
change of conversion of PO (21%) was negligible.
The catalytic behavior of brucite was investigated at 4.1 wt.% of
catalyst, 120 ◦C and using a MeOH/PO molar ratio of 8; the results
obtained are summarized on Table 2. The heating temperature
of the catalysts affected the conversion of PO and the selectivity
towards 1-methoxy-2-propanol (II). The increase in the heating
temperature from 90 to 150 ◦C led to a clear increase both in the
conversion of PO (from 13 to 20%) and the selectivity towards (II)
(from 72.0 to 87.7%), respectively (Table 2, Fig. 2). The heating of
brucite at higher temperatures did not change these values. These
changes in the catalytic behavior were parallel to the effect of the
activation temperature on the basicity of brucite (Fig. 2, Table S2,
Supporting information). Similar trends were observed at 2.2 wt.%
of catalyst, 120 ◦C and MeOH/PO molar ratio of 12 (Table 2).
The catalytic behaviour of Mg,Al-LDHs in this reaction was also
performed at 2.2 wt.% of catalyst, 120 ◦C and using a MeOH/PO
molar ratio of 12. The amount of Al in the Mg,Al-LDH clearly affected
ICP = aꢁ + b(3)

28
M.N. Timofeeva et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 22–30
Table 6
Catalytic activity of different catalysts in the reaction of propylene oxide with methanol.
Experimental conditions
Conversion of
(PO) (%)
Selectivity of (II)
(%)
TOF
Ref.
MeOH/PO
(mol/mol)
Catalyst
(wt.%)
T (◦C)
Time (h)
Brucitea
4
4
4
4
5
5
5
5
1.8
1.8
0.9
1.2
1.8
1.8
120
120
120
140
120
120
120
120
3
3
8
6
3
3
5
5
68.1
61.4
82.1
92.4
88.7
95.9
37.3
71.1
93.8
94.6
98.1
96.5
85.7
90.9
85.1
89.2
12.6
11.3
11.4
12.8
16.4
17.6
–
Our data
Our data
[11]
[11]
[15]
[15]
[7]
[7]
Mg,Al-LDH(3.89)a
Mg,Al-LDH (Mg/Al = 4/1)a
Mg,Al-LDH (Mg/Al = 4/1)a
Mg,Al-LDH (Mg/Al = 3/1)a
Zn,Mg,Al-LDH (Zn/Mg/Al = 0.03/3/1)b
CaO
Not determine
Not determine
MgO
–
a
Calcinated at 500 ◦C.
b
Calcinated at 450 ◦C.
the conversion of PO and the selectivity to the isomers (Table 2). The
decrease in Al content led to the increase in the selectivity towards
(II); thus, the increase in the Mg/Al atomic ratio from 0.61 to 4.00
rose the selectivity towards (II) from 53.5 to 84.6%. The highest
selectivity towards (I) (36.4%) for Mg,Al-LDH(0.61) catalyst can be
explained by the large amount of BAS sites in this solid (Table 4,
Fig. 3A). Our previous investigations on the catalytic properties of
M-PILCs (M–Al, Ga and Zr) [17] and metal phosphate molecular
sieves [18] allowed to suggest that the strength of the basic sites
determined the isomer selectivity towards (II). This trend is now
also observed for LDH catalysts, the increase in the strength of basic
sites of Mg,Al-LDHs, i.e increase in ꢁ and PA, led to the rise in the
selectivity towards (II) and the conversion of PO (Fig. 3B,C).
The amount of Al in Mg,Al-LDH also conditioned the conversion
of PO (Table 5). The increase in the Mg/Al atomic ratio from 0.61
to 4.00 ran parallel to the rise in the conversion of PO from 7 to
15%, which can be related to the increase in the amount of basic
sites, as the ratio BS/BAS increased from 1.8 to 14.2 (Fig. 3A). At the
same time, the increase in the strength of the basic sites of Mg,Al-
LDHs, i.e. increase in ꢁ and PA, led to the rise in the selectivity
towards (II) and the conversion of PO (Fig. 3B,C). In our previous
investigations, it was found that the strength of the basic sites also
affected the reaction, but in the opposite direction; the increase in
the strength of basic sites led to a decrease in the reaction rate both
in the presence of M-PILCs (M–Al, Ga and Zr) [17] or of metal phos-
phate molecular sieves [18]. This observation can be explained in
terms of the reaction mechanism, assuming that the active sites are
“Mn+ O2− Lewis acid-base” pairs. The adsorption of MeOH on the
active sites caused the formation of methoxide species via the dis-
sociation of MeOH into methoxide ion and proton on Lewis-acidic
and basic sites, respectively.
length of the M O bond, were very important for the activation of
the reactants and the following rearrangement of the intermedi-
ates, because the basicity of Mg,Al-LDHs can be characterized by
the extent of negative charge on the oxygen atom. Comparing with
XRD data (Table 3), it was found that the conversion of PO rose from
7 to 15% with the increasing of the values of the lattice parameters, a
and c, from 3.038 to 3.058 Å and from 22.62 to 23.40 Å, respectively.
It may be remarked that the importance of the length of the M-O
bond can also be revealed in the methoxycarbonylation of ketones
with dimethyl carbonate in the presence of oxides of Ca, Mg, La and
Zr [7]. In spite of its strong basic sites, the activity of CaO (radius
of Ca2+ is 0.100 nm) was lower than that of MgO (radius of Mg2+ is
0.072 nm). In our case the difference in size of metal ions (radius
of Al3+ and Mg2+ are 0.0535 and 0.072 nm, respectively) can also
affect the reaction rate.
The comparison between the isomer selectivity and activity
of brucite and Mg,Al-LDHs is especially interesting. As observed
in Table 5, the activity of Mg,Al-LDHs was lower than that
of brucite, while selectivities towards (II) were close. For bet-
ter comparison, the reaction was carried out using Brucite(150)
and Mg,Al-LDH(3.89) catalysts, and under the same experimen-
tal conditions, temperature of reaction 140 ◦C, 4.1 wt.% of catalyst,
MeOH/PO molar ratio of 12 and time of reaction 7.5 h, finding con-
versions of PO of 42 and 28%, respectively. Higher conversion of
brucite (150) (36%) compared with Mg,Al-LDH(3.89) (15%) was also
observed at 2.2 wt.% of catalyst and 120 ◦C. This can be explained
by the difference of the nature of “Mn+ O2− Lewis acid-base” pairs;
the strength of medium and strong basic sites of Mg,Al-LDH(3.89)
is higher (915 and 928 kJ/mol) than that of Brucite(150) (900 and
927 kJ/mol) (Table S2, Supporting information). Secondly, it can be
assumed that the higher activity of Brucite(150) compared with
Mg,Al-LDH(3.89) may be related to the difference in the surface
basicities.
Therefore, a necessary condition for high reaction rates and
selectivity for (II) was the presence of “Mn+ O2− Lewis acid-base”
pairs with the optimal combination of the length of the M O bond,
the basicity of the oxygen ion and the strength of the metal ion on
the surface of the solid.
Moreover, the interaction of PO with the active sites can also
lead to the dissociative activation of PO:
3.3. Stability and efficiency of brucite and Mg,Al-LDH
The increase in the strength of the basic sites should favor the
activation of PO and methanol. However, a high basicity should
lead to a strong stabilization of (A) and (B) species on the surface
of the solid and, therefore, lead to a decrease in the reaction rate.
However, our experimental data were not in agreement with these
assumptions. So, it can be proposed that not only the basicity of
the oxygen ion and the strength of the metal ion, as well as the
One of the important properties for application of catalysts is
their stability in the reaction mixture. The stability of brucite(150)
was investigated, and it was found that this sample can be used
repeatedly without significant loss of catalytic activity during at
least 4 catalytic cycles (Fig. 4). After each catalytic test, brucite(150)
catalyst was separated from the reaction mixture by simple filtra-
tion, washed with methanol, dried in air and activated at 150 ◦C for
2 h.

M.N. Timofeeva et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 22–30
29
mer selectivity to (II). In LDHs, the decrease in Al content led to the
increase in the selectivity towards (II) and the conversion of PO. The
nature of “Mn+ O2− Lewis acid-base” pairs was proposed to be a key
factor to control the catalytic behavior of these solids. The length
of the M-O bond, the basicity of the oxygen ion and the strength
of the metal ion on the surface affected the catalytic activity of the
solids and their isomer selectivity in the reaction studied.
Brucite(150) can be used repeatedly without significant loss of
catalytic activity during at least four catalytic cycles. The com-
parison of the efficiencies (TOF) of brucite and Mg,Al-LDH(3.89)
activated at 500 ◦C with Mg,Al-LDH(Mg/Al = 4/1) reported in liter-
ature points that their TOF values are close.
95
90
85
80
30
15
0
Acknowledgments
This work was supported by the thank the support from
the Spanish Ministry of Economy and Competitiveness (MINECO)
and the European Regional Development Fund (FEDER) (project
MAT2013-47811-C2-R).
I
II
Cycle
III
IV
Fig. 4. Brucite(150) recycling in the reaction of propylene oxide with methanol.
Experimental conditions: 2.2 wt% of catalyst, MeOH/PO = 12 mol/mol, 120 ◦C, 7.5 h.
Amount of reactants was corrected based on reaction conditions.
Appendix A. Supplementary data
The efficiencies of brucite and Mg,Al-LDH(3.89) were compared
to those of Mg,Al-LDH, MgO and CaO catalysts reported in the liter-
ature [7,11,15]. Because experimental conditions are different, the
comparison was made by estimation of their efficiency by means
of the TOF numbers:
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molcata.2016.06.
006.
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ꢁ
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Time×W
TOF =
,
mol/ h × g where Wcatalyst is the amount
(
)
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