Cu layered double hydroxides as catalysts for N-methylation of p-anisidine: Influence of synthesis conditions

Article abstract of DOI:10.1016/j.catcom.2019.05.004

Cu-containing layered double hydroxides were synthesized by co-precipitation method using base solution (NaOH and Na₂CO₃) with different concentration of carbonate ions. The influence of the precipitating agent concentration on formation of hydrotalcite phase was investigated. Characterization of the surface of the obtained samples calcined at 450 °C and 650 °C was performed by x-ray photoelectron spectroscopy. Catalysts based on Cu-layered double hydroxides were investigated in N-methylation of p-anisidine by methanol obtaining N-methyl-p-anisidine using autoclave reactor. Effect of precipitating agent concentration during synthesis of the Cu-layered double hydroxide on the catalyst performance was studied.

Full text of DOI:10.1016/j.catcom.2019.05.004

Catalysis Communications 127 (2019) 39–44
Contents lists available at ScienceDirect
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Short communication
Cu layered double hydroxides as catalysts for N-methylation of p-anisidine:
Influence of synthesis conditions
⁎
M.V. Bukhtiyarova , A.L. Nuzhdin, A.V. Bukhtiyarov, T.Y. Kardash, A.V. Romanenko
Boreskov Institute of Catalysis SB RAS, 630090 Novosibirsk, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords:
Cu-containing layered double hydroxides were synthesized by co-precipitation method using base solution
Layered double hydroxides
Base solution
N-Methylation
(NaOH and Na CO ) with different concentration of carbonate ions. The influence of the precipitating agent
2
3
concentration on formation of hydrotalcite phase was investigated. Characterization of the surface of the ob-
tained samples calcined at 450 °C and 650 °C was performed by x-ray photoelectron spectroscopy. Catalysts
based on Cu-layered double hydroxides were investigated in N-methylation of p-anisidine by methanol obtaining
N-methyl-p-anisidine using autoclave reactor. Effect of precipitating agent concentration during synthesis of the
Cu-layered double hydroxide on the catalyst performance was studied.
p-Anisidine conversion
1. Introduction
formate, and yield of N-alkyl-p-anisidine was 74%. However, reaction
temperature was not mentioned in [5]. Despite the fact that his catalyst
N-methyl-para-anisidine (H
3
C-NH-C
6
H
4
-OCH
3
)
can be used as
is commercial one, it is possible to decrease the price of the process by
using non-noble metals as active components for N-alkylation of
amines. K. Shimizu [6] used Ni catalysts supported on different metal
adding to gasoline due to its ability to increase the octane number. The
presence of oxygen in the N-methyl-p-anisidine molecule allows one to
increase the combustion of fuel and decrease the amount of CO and
unburned hydrocarbons in the exhaust gases. According to patent [1],
synthesis of N-methyl-p-anisidine in gas phase can be carried out by N-
alkylation of p-anisidine by methanol in the presence of Cu-containing
catalysts at the temperature range of 180–260 °C and atmospheric
pressure.
oxides, mainly θ-Al
2
O
3
2 ,
and γ-Al O in N-alkylation of aniline at 144 °C
without any additives such as base solution. High aniline conversions of
85–100% were obtained depending on alcohol and reaction time. It was
shown that it is possible to obtain conversion > 99% depending on
reaction conditions. Thus, the high conversion values can be caused by
using high alcohol concentration, high catalyst amount or long reaction
time. However, for comparison of the catalytic activity it is necessary to
Influence of the catalyst nature and the reaction conditions on
catalytic activity in N-alkylation of amines were investigated in dif-
ferent research papers [2–7]. The homogeneous catalysts such as
2 4
perform reaction at the same conditions. The Cu0.5Zn0.5Fe O catalyst
prepared by co-precipitation method was investigated in N-methylation
of aromatic amines (aniline, o-toluidine, m-toluidine, p-toluidine and p-
anisidine) at 300 °C for 10 h in fixed-bed reactor [7]. The p-anisidine
conversions and N-methyl-p-anisidine selectivities were 32–35% and
94–96%, respectively, depending on reaction time. However, conver-
sion continuously decreases with time-on-stream [7]. According to lit-
erature data, Cu-containing catalysts can be perspective for N-methy-
lation of p-anisidine to N-methyl-p-anisidine.
RuCl
be used for the reaction. Su and co-workers [4] used heterogeneous
system of Al -mordenite as catalyst. N-methylation was done in
fixed-bed reactor at 270 °C, 10 bars and the methanol:amine ratio of
:1. p-Anisidine was not used as a substrate in this paper. However, the
3 2 3
∗nH O-P(OBu) [2] or manganese-containing complexes [3] can
2 3
O
4
high selectivity for N,N-dimethylamine was obtained. It means that the
catalysts are not selective for N-methyl-p-anisidine which would be
desirable reaction product. Recently, it was shown [5–7] that hetero-
geneous catalysts containing transition metals such as Pd, Ni and Cu
can be used for N-alkylation of amines. Authors [5] demonstrated that
secondary amines with yields of 54–88% can be obtained by alkylation
of aniline by aliphatic or aromatic alcohols using Pd/C catalyst. p-An-
isidine was alkylated by benzyl alcohol in the presence of sodium
Layered double hydroxides (LDHs) can be used as catalysts for N-
alkylation of amines [8]. The LDHs, known as hydrotalcite-like com-
pounds,
can
(OH)
be
x+[An−
·4H
containing a positive charge due to partial substitution
represented
by
[9].
general
The
formula:
hydrotalcite
[M1−x2+M
3+
]
x/n ∙ mH
x
2
]
2
O
Mg
6
Al
2
(OH)16CO
3
2
O structure results from stacking brucite-like
layers Mg(OH)
2
⁎
Corresponding author.
E-mail address: mvb@catalysis.ru (M.V. Bukhtiyarova).
https://doi.org/10.1016/j.catcom.2019.05.004
Received 27 March 2019; Received in revised form 6 May 2019; Accepted 6 May 2019
Available online 07 May 2019
1566-7367/ © 2019 Elsevier B.V. All rights reserved.

M.V. Bukhtiyarova, et al.
Catalysis Communications 127 (2019) 39–44
The XPS measurements of the samples were performed on the
photoelectron spectrometer build by SPECS (Germany) equipped with a
of Mg2+ by Al3+. The positive charge is compensated by anions (OH–
2−
and CO
3
) which are located in the interlamellar spaces. The pure
hydrotalcite phase can be obtained when 0.2 < x < 0.33, where x is a
PHOIBOS-150 hemispherical energy analyzer and AlK
α
irradiation
3
+
2+
3+
2+
3+
ratio of M /(M +M ), resulting in the M /M ratios of 2–4 [9].
(hν = 1486.6 eV, 150 W). The binding energy (BE) scale was pre-cali-
brated using the positions of the photoelectron of Au4f7/2
(BE = 84.0 eV) and Cu2p3/2 (BE = 932.67 eV) core level peaks.
2
+
3+
These systems are perspective catalysts since Cu and Al ions can be
used as divalent and trivalent metal, respectively. The common synth-
esis method of LDH materials is co-precipitation of metal nitrates by
−9
Residual gas pressure was better than 8 × 10 . For the measurements
the samples were supported onto the double-sided conducting copper
scotch tape (Scotch 3 M©). Spectra analysis and peak fitting were
performed with XPSPeak 4.1 software. Integrated line intensities were
measured from area of the corresponding narrow regions (Al2p, C1s,
O1s, Cu2p and Na1s). The relative amount of the elements on the
sample surface and the ratio of their atomic concentrations were de-
termined from the integrated intensities of the lines corrected by their
respective atomic sensitivity factors [18].
2 3
mixture of NaOH and Na CO [10]. Works devoted to studying such
systems in N-methylation of amines are practically absent in the lit-
erature.
Previously we demonstrated [11] that the Cu2+/Al3+ molar ratio in
CuAl-LDHs has influence on the catalytic activity in N-methylation of p-
anisidine. It was shown that the best catalytic performance is obtained
2
+
3+
over the CuAl sample with the Cu /Al molar ratio of 1 calcined at
50 °C. This paper is devoted to the investigation of effect of the base
4
concentration used for the synthesis of LDH material on the phase
composition and Cu surface state. Besides, the catalytic activity of the
mixed metal oxides on the base of CuAl-LDH in N-methylation of p-
anisidine by methanol in “one-pot” synthesis was studied.
2.3. Catalytic test
The catalytic properties of the samples were investigated in auto-
clave reactor Tinyclave Steel (Buchiglasuster, Switzerland) with volume
of 15 ml. The reaction mixture contained p-anisidine (0.1 M), methanol
and n-decane as internal standard in o-xylene (10 ml). The catalyst mass
2. Experimental part
2.1. Synthesis of CuAl-layered double hydroxide
2
was 27 mg. Reaction was carried out at 180–200 °C and H pressure of 4
bars for 5 h. The liquid samples were characterized by gas chromato-
graph (Agilent) with FID detector. The gas analysis was done by gas
chromatograph Chromos-1000. The conversion of p-anisidine was de-
termined by internal standard (n-decane) method.
The layered double hydroxides with the Cu2+/Al3+ molar ratio of
:1 were synthesized by co-precipitation method at constant
1
pH 9.0 ± 0.1 and temperature of 70 °C by the base solution of NaOH
and Na CO . There are several opinions about base concentrations
which should be used during synthesis of layered double hydroxides. S.
2
3
3. Results and discussion
2
−
3+
Miyata et al. [12] used base solution with the [CO
.7 closed to stoichiometric value while most papers [13,14] are
dedicated to usage of the higher concentration of carbonate-ions:
3
]/[Al ] ratio of
0
3.1. Catalyst characterization
2−
3+
[
CO
3
]/[Al ] = 2.0. The concentration of carbonate-ions should be
For the samples calcined at 450 °C the concentrations of the main
components (Cu and Al) obtained by chemical analysis are present in
Table 1. These values are lower than the theoretical values. It can be
sufficient for the formation of pure carbonate form of hydrotalcite
phase [15]. The authors of the work have chosen the following con-
2−
3+
–
centration of carbonate ions: [CO
3
]/[Al ] = 0.5, 0.86 and 1.0.
caused by the incomplete removal of the interlayer anions such as OH
and CO32 during heat treatment of the samples at 450 °C. The Cu:Al
ratios were calculated based on these concentrations. The calculated
Cu:Al ratios are practically the same as the theoretical values indicating
that the degree of precipitation of the two species were very similar. It
means that the concentrations of the base solution are sufficient for the
precipitation of the layered double hydroxides.
–
Materials were denoted as CuAl-LDH-1, CuAl-LDH-2, CuAl-LDH-3, re-
spectively. Hydroxide concentration for all samples was
−
[
OH ] = 1.6([Al3+] + [Cu2+]), that is sufficient for the formation of
the layered double hydroxide [16]. The aging was performed at 70 °C
for 4 h. After aging the precipitate was filtered and washed with hot
water (5 l). The precipitate was dried at 110 °C for 14 h and calcined at
4
50 °C for 4 h. The samples were denoted as CuAl-x (where is x = 1, 2
Chemical analysis data were confirmed by thermal analysis data.
Non-isothermal temperature-programmed treatment of the CuAl-LDH-2
sample in the temperature range of 25–700 °C is accompanied by endo-
effects on DTA curves (Fig. 1). Thermal analysis was done only for one
sample due to the similarity in chemical composition of all samples. The
mass loss for the sample is 38.2 wt%. The first step at 60–200 °C is re-
lated to removal of physical adsorbed and interlayer water. The second
step at 200–300 °C can be attributed to the dehydroxylation of brucite-
like layers and decarbonization of interlayer carbonate ions that leads
to the collapse of the layer structure [19]. It can be concluded that
processes of dehydroxylation and decarbonization occur at tempera-
tures lower than 500 °C. The further sample heating promotes slightly
mass loss around 5.2 wt%. This step can be attributed to the decom-
position of strongly bound high-temperature carbonate [20,21]. A
and 3). After drying the CuAl-LDH-2 sample was also calcined at 650 °C
for 4 h (CuAl-2-650).
2.2. Characterization of the catalysts
Chemical analysis of the samples was done by atomic-absorption
method [17].
X-ray patterns were obtained by Bruker D8 (CuKα-radiation,
θ = 5–70° with step of 0.05°, the accumulation time is 3 s). The mul-
tichannel LynxEye detector was used for the signal detection. Phase
analysis was done by comparison of interlayer distances d and in-
tensities I of corresponding reflexes with theoretical values from ICDD
2
i
i
PDF-2 database.
Table 1
Chemical composition of the Cu-layered double hydroxides calcined at 450 °C.
Sample
Cu concentration, wt%
Theoretical
Al concentration, wt%
Cu:Al molar ratio
Theoretical
1:1
Calculated
Theoretical
20.4
Calculated
Calculated
CuAl-1
CuAl-2
CuAl-3
48.9
40.3
40.7
40.2
17.5
17.2
17.6
0.97
1.00
0.98
40

M.V. Bukhtiyarova, et al.
Catalysis Communications 127 (2019) 39–44
Fig. 1. Thermal analysis of air-dried CuAl-LDH-2 sample.
higher decomposition temperature of this carbonate species is an in-
dicator for strong interaction across interface and grain boundaries
which were formed during the first decomposition step. Carbonate ions
can be present as monodentate or bidentate ligand in interlayer of
hydrotalcite phase. Monodentate carbonate decomposes at lower tem-
perature whereas bidentate carbonate is stable at higher temperatures
[
22]. Thus, the CuAl-LDH-1, CuAl-LDH-2, CuAl-LDH-3 samples were
calcined at 450 °C due to the fact that dehydroxilation and dec-
arbonization occur at temperature lower that 500 °C. The CuAl-LDH-2
sample was calcined at 650 °C to investigate influence of the calcination
temperature on catalytic performance.
Thermal analysis data were confirmed by XRD analysis. XRD was
done for the CuAl-LDH-2 sample dried at 110 °C (Fig. 2a), calcined at
450 and 650 °C (Fig. 2b) and for the CuAl-1 and CuAl-3 samples (not
presented). XRD pattern of the CuAl-LDH-2 sample dried at 110 °C
shows the characteristic peaks corresponding to the (00l) crystal-
lographic planes. This provides a clear evidence for the formation of the
Fig. 2. XRD of (a) CuAl-LDH-2 sample, (b) CuAl-2 and CuAl-2-650 samples.
typical for Cu2+ state in CuO) for samples calcined at 450 and 650°С
means the partial decomposition of copper hydroxide and formation of
CuO. XP spectra of the CuAl-1, CuAl-2 and CuAl-3 samples (not pre-
sented here) are practically the same: the binding energy is in the range
1
hydrotalcite phase [21]. The 2H trigonal prismatic structure of two-
layered polytype is formed [23]. Calcination of the CuAl-LDH-2 sample
at 450°С leads to destruction of the hydrotalcite phase and formation of
oxide phase. XRD pattern (Fig. 2b) of the CuAl-2 sample shows the
presence of broad peak at ~36° which can correspond to low-crystal-
lized CuO that is in agreement with data of other research group
2
+
of 935.4–935.5 eV. It means that all samples contain copper in Cu
state. The changes obtained in Auger spectra basically prove the sug-
gestion concerning the CuO formation on the samples surface during
calcination. For the samples dried at 110 °C there is a peak at 915.3 eV
[
21,24]. The same phase is obtained for the CuAl-1 and CuAl-3 samples.
Probably, it can be caused by incomplete removal of interlayer anions.
Increase of calcination temperature of the CuAl-2 sample to 650 °C re-
sults in higher degree of crystallization of the CuO phase (Fig. 2b). More
pronounced peaks appear at ~35.5, 39 and 48° which are characteristic
peaks of tenorite CuO phase. The obtained results reveal that hydro-
which is attributed to Cu2+ in copper hydroxide Cu(OH)
that is the
2
evidence of the formation of layered double hydroxide. Increase of
calcination temperature results in shifting the peak position to the
higher binding energy. For the CuAl-2-650 sample the binding energy is
916.8 eV, which is characteristic for tenorite phase. However, the pre-
sence of Cu2+ in hydroxide structure cannot be excluded. The samples
differ in the Cu:Al atomic ratio on the surface (Table 2).
2
+
talcite phase gradually transforms to tenorite phase. As a result Cu
ions which are present in hydroxide structure turn into Cu2+ ions which
are in oxide structure. This is confirmed by XPS data.
Increase of the calcination temperature of the CuAl-LDH-2 sample
results in decrease of the Cu:Al surface ratio. It can be caused by the
The surface copper state in the CuAl-LDH-2 sample dried at 110 °C,
calcined at 450 and 650°С was investigated by XPS. The lines corre-
sponded to Al, C, O, and Cu are present in survey photoelectron spectra
measured for the samples. Other elements were not obtained within the
sensitivity of the XPS. The measured Cu2p XP spectra and CuLMM
Auger spectra are shown on Fig. 3. Analysis of Cu2p lines (shape and
position) measured for the sample dried at 110 °C shown that copper is
presented only in one state with binding energy (BE) value near
decomposition of Cu(OH)
2
in a structure of layered double hydroxide
with simultaneous formation of CuO particles on the sample surface.
3.2. Catalytic properties
Catalytic properties of the CuAl-1, CuAl-2, CuAl-2-650 and CuAl-3
samples were investigated in N-methylation of p-anisidine in autoclave
reactor using o-xylene as solvent.
9
[
35.5 eV which corresponds to the Cu in Cu2+ state in Cu(OH)
18,25]. The shift of BE to lower values ~934.9 eV (which is more
2
41

M.V. Bukhtiyarova, et al.
Catalysis Communications 127 (2019) 39–44
Fig. 3. XP and Auger spectra of the CuAl-LDH-2, CuAl-2 and CuAl-2-650 samples.
Table 2
4 2
simultaneous side reactions with formation of CH , CO , and CO. As it is
Atomic Cu:Al ratio on the sample surface and catalytic behavior of the samples
depending on the calcination temperature and the concentration of carbonate-
ions.
known [27,28], several reactions involving methanol (methanol steam
reforming and methanol decomposition) can occur over Cu-containing
catalysts. The steam reforming takes place due to the water formation
during N-methylation of p-anisidine. Water will react with methanol to
2
−
]/[Al3+
Sample
Treatment
[CO
3
]
Cu:Al Conversion of p-
temperature, °C
anisidine, %
produce CO
2 2 2
and H . Besides, interaction of CO with H promotes
formation of methane in trace amounts [27].
CuAl-LDH-2 110
0.86
0.5
0.86
1.0
0.69
0.58
0.62
0.74
0.57
–
CuAl-1
CuAl-2
CuAl-3
CuAl-2-650
450
650
59
48
37
77
CH
3
OH + H
2
O → CO
2
+ 3H
2
CH
3
OH → CO + 2H
2
0.86
CO + 3H
2
→ CH
4
+ H
2
O
Reaction conditions: p-anisidine (0,10 M), о-xylene (10 ml), 1000 rpm, τ = 5 h.
The formation of CH
4
, CO
2
, and CO is confirmed by the chromato-
graph analysis of gas phase obtained during N-methylation of p-anisi-
dine.
It was observed that the catalyst which was reduced directly in re-
2
action media (4 bar of H ) at 200 °C reveals the same activity as the
sample which was reduced prior to reaction in hydrogen atmosphere at
3
00 °C (Table 3, lines 3, 4). p-Anisidine conversion reaches ~32% in
2+
0
both cases. Thus, it can be assumed that reduction of Cu ions to Cu
occurs in autoclave at reaction conditions: 200 °C and 4 bar and in H
2
atmosphere. Thus, it is not necessary to reduce the catalyst prior to the
reaction.
Increase of methanol concentration from 0.15 to 0.65 M (Table 3,
lines 4–6) promotes raise of p-anisidine conversion from 33 to 99.8%
while selectivity to N-methyl-p-anisidine remains practically the same
At the beginning, the search of optimal conditions for comparison of
catalytic activity was done. For this purpose the CuAl-2 catalyst was
used. F. Santoro and co-workers [26] showed that N-alkylation of ani-
line by different alcohols can be performed using reduced form of the
(
99.9% vs. 97.5%). Influence of the catalyst amount on the catalytic
activity was also investigated. Decrease of catalyst amount leads to
decline in p-anisidine conversion from 99.8 to 34% (Table 3, lines 6–8).
Using higher methanol concentration of 0.9 M promotes increase of p-
anisidine conversion to 48% (Table 3, line 9). At the same time the
selectivity to N-methyl-p-anisidine is 99.0–99.8%. It can be concluded
that the catalyst amount and methanol concentration should be high
enough to obtain high p-anisidine conversion. It was established that
optimum hydrogen pressure for N-methylation of p-anisidine is 4 bars.
Pressure decrease to 2 bars, probably, leads to incomplete reduction of
2 3
Cu/Al O catalyst. Therefore, N-methylation of p-anisidine was per-
formed over the catalyst reduced in hydrogen atmosphere at 300 °C.
Results of liquid phase synthesis of N-methyl-p-anisidine are presented
in Table 3.
Using hydrogen atmosphere of 4 bars for N-methylation allows one
to observe consumption of p-anisidine (Table 3, line 1), although its
conversion is only 6% at 180 °C. Temperature increase from 180 to
2
(
00 °C provides significant increase in p-anisidine conversion to 34%
Table 3, lines 1, 2). Rate of methanol conversion during reaction is
much higher than rate of p-anisidine N-methylation. It can be related to
2+
Cu in the catalyst that promotes decrease in catalytic activity from 48
to 31% (Table 3, lines 9, 10). Pressure increase to 6 bars results in
42

M.V. Bukhtiyarova, et al.
Catalysis Communications 127 (2019) 39–44
Table 3
Liquid-phase synthesis of N-methyl-p-anisidine over CuAl-2 catalyst.
№
[CH
3
OH], М
Substrate:Cu
P, bar
T, °C
Conversion MeOH, %
Conversion, %
Selectivity, %
1a
0.10
0.10
0.15
0.15
0.40
0.65
0.65
0.65
0.90
0.90
0.90
1.33
1.33
1.33
1.33
1.33
1.33
2.66
5
4 (H
4 (H
4 (H
4 (H
4 (H
4 (H
4 (H
4 (H
4 (H
2 (H
6 (H
180
200
200
200
200
200
200
200
200
200
200
n.d.
84
90
88
90
85
76
68
54
73
66
6
100
2
)
2
)
2
)
2
)
2
)
2
)
2
)
2
)
2
)
2
)
2
)
a
2
3
16
32
33
69
99.8
84
34
48
31
40
99.9
99.9
99.9
99.8
97.5
99.5
99.8
99.7
99.9
99.8
a
4
5
6
7
8
9
5
5
5
1
1
0
1
Reaction conditions: p-anisidine (0.10 M), о-xylene (10 ml), 1000 rpm, τ = 5 h.
a
Catalysts were reduced in hydrogen flow at 300 °C in fixed-bed reactor for 2 h. Obtained sample was cooled down in hydrogen flow and placed in a Schlenk flask
under nitrogen atmosphere.
decline of p-anisidine conversion to 40% (Table 3, line 11) that is
caused by shifting reaction equilibrium to methanol on the step of
methanol dehydrogenation to formaldehyde [29]. Based on the ob-
tained results and previous papers [5,6,29], the plausible mechanism
was proposed. The reaction begins with dehydrogenation of methanol
by Cu sites to formaldehyde, which further reacts with p-anisidine to
give corresponding imine. Finally, N-methyl-p-anisidine is obtained by
hydrogenation of imine by surface hydrogen atoms of Cu hydride spe-
cies. Taking into account XPS data, Cu species are responsible for the
dehydrogenation and hydrogenation transfer steps.
Thus, it was shown that CuAl-2 sample can provide high selectivity
of N-methyl-p-anisidine performing reaction in liquid phase with an
4. Conclusions
The catalytic properties of the samples on the base of Cu-layered
double hydroxides in N-methylation of p-anisidine were investigated.
Effect of the reaction conditions was also investigated. The CuAl-2
catalyst was chosen as a reference catalyst. It was shown that increase
in reaction temperature promotes raising p-anisidine conversion. The
pressure change leads to conversion decrease that can be caused by side
reactions of methanol. Using lower amount of the catalyst does not
improve catalytic activity. The optimum reaction conditions for per-
forming N-methylation of p-anisidine are following: 200 °C, 4 bars, the
substrate:Cu ratio of 5.
0
excess of methanol in H
2
atmosphere. The optimal reaction conditions
Varying synthesis conditions of the CuAl-LDH sample is possible
decision to obtain more active catalyst for N-methylation of p-anisidine.
Different sodium carbonate concentration in the base solution was used
for the synthesis of LDHs. It was shown that p-anisidine conversion over
Cu catalysts on the base of LDH increases in the same order as carbo-
nate concentration decreases. Thus, lower concentration promotes
better catalyst performance in N-methylation of p-anisidine. Besides,
higher calcination temperature of CuAl-2 results in formation of well-
crystallized tenorite phase CuO that facilitates higher p-anisidine con-
version. CuAl-layered double hydroxides can be used as the catalysts in
the N-methylation of other aniline derivatives.
were obtained: temperature of 200 °C, pressure of 4 bars, methanol
concentration of 0.9 M, p-anisidine concentration of 0.1 M, substrate:Cu
ratio of 5 despite the fact that p-anisidine conversion of ~100% was
achieved at other reaction conditions (Table 3, line 6). However, it is
incorrect to compare the catalytic activity at conversion of 100%. Thus,
the optimal conditions were used for the next experiments.
The effect of base solution concentration used for the synthesis of
Cu-LDHs on catalytic properties in N-methylation of p-anisidine was
investigated. Four samples were taken for this purpose: CuAl-1, CuAl-2,
CuAl-3 and CuAl-2-650. The effect of calcination temperature of the
CuAl-2 sample on the catalyst performance was also studied in the
mentioned reaction. The p-anisidine conversion increases from 37 to
Acknowledgments
2−
3+
59% with decrease of the [CO
3
]/[Al ] ratio from 0.5 to 1.0
(
Table 2). The lower the Cu:Al surface ratio, the higher p-anisidine
The work was conducted within the framework of budget projects
No. 0303-2016-0004 and АААА-А17-117041710083-5 for Boreskov
Institute of Catalysis.
conversion. As it was mentioned before the decrease in the molar Cu:Al
surface ratio can be caused by formation of the CuO particle on the
sample surface. However, increase in the calcination temperature of the
CuAl-2 sample to 650 °C leads to rise in the p-anisidine conversion from
References
48 to 77% while the molar Cu:Al surface ratio is the same as that for the
CuAl-1 sample calcined at 450 °C (Table 2).
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