Activity of amorphous V-AlPO4 and Co-AlPO4 in the selective synthesis of N-monoalkylated aniline via alkylation of aniline with methanol or dimethyl carbonate

Article abstract of DOI:10.1039/b208085k

The vapour phase catalytic alkylation of aniline with methanol or dimethyl carbonate (DMC) on amorphous AlPO₄ and M-AlPO₄ (M = Cu, Ni, Fe, Co and V) has been studied. V-AlPO₄ and Co-AlPO₄ showed high selectivity towards N-monomethylation of aniline. Further studies on the alkylation reactions have been carried out using AlPO₄ and M-AlPO₄ (M = V and Co) as catalysts at different temperatures and molar ratio of aniline to methanol or DMC. V-AlPO₄ is found to show 100% selectivity for N-monomethylation of aniline, irrespective of the temperature or the molar ratio of the reactants. On the other hand, Co-AlPO₄ shows 100% selectivity for N-monomethylated product only when the aniline-to-methanol (or DMC) molar ratio is 2:1. Aniline conversion towards N-methylaniline(NMA) was found to be 39%. DMC has been found to be a better methylating agent than methanol with respect to the percentage conversion of aniline to N-methylaniline.

Full text of DOI:10.1039/b208085k

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Activity of amorphous V-AlPO4 and Co-AlPO4 in the selective
synthesis of N-monoalkylated aniline via alkylation of aniline
with methanol or dimethyl carbonate
N. Nagaraju* and George Kuriakose
Department of Chemistry, St. Joseph’s College Post Graduate Center, 46 Lang ford Road,
Shanthi Nagar, Bangalore 560 027,, India
Received (in Montpellier, France) 16th August 2002, Accepted 10th December 2002
First published as an Advance Article on the web 26th February 2003
The vapour phase catalytic alkylation of aniline with methanol or dimethyl carbonate (DMC) on amorphous
AlPO4 and M-AlPO4 (M ¼ Cu, Ni, Fe, Co and V) has been studied. V-AlPO4 and Co-AlPO4 showed high
selectivity towards N-monomethylation of aniline. Further studies on the alkylation reactions have been carried
out using AlPO4 and M-AlPO4 (M ¼ V and Co) as catalysts at different temperatures and molar ratio of aniline
to methanol or DMC. V-AlPO4 is found to show 100% selectivity for N-monomethylation of aniline,
irrespective of the temperature or the molar ratio of the reactants. On the other hand, Co-AlPO4 shows 100%
selectivity for N-monomethylated product only when the aniline-to-methanol (or DMC) molar ratio is 2:1.
Aniline conversion towards N-methylaniline(NMA) was found to be 39%. DMC has been found to be a better
methylating agent than methanol with respect to the percentage conversion of aniline to N-methylaniline.
However, only 10–15% aniline conversion was observed.
Introduction
Silica-supported vanadia catalysts also were found to exhibit
good selectivity towards NMA formation but the conversion
of aniline still remained low.11 Campelo et al.14,15 reported
100% selectivity for N-alkylation (NMA + NNDMA) in this
reaction over AlPO4-based catalysts.
In contrast, we recently observed that in liquid phase alkyla-
tion of aniline with DMC over amorphous AlPO4 catalysts
incorporating cobalt or vanadium ions, biphenyl urea was
obtained as one of the major products.16 This finding indicates
that amorphous M-AlPO4 as catalyst has a much broader
scope than the crystalline counterparts. This observation
prompted us to make a systematic investigation of the effect
of the transition metal ion in vapour phase alkylation of ani-
line under different reaction conditions.
The main aims of this paper are: (i) preparation of transition
metal ion containing amorphous aluminophosphates; (ii)
determine the catalytic activity of the aluminophosphates in
the reaction between aniline and MeOH or DMC in the
vapour phase; (iii) study the effect of reaction parameters such
as catalyst bed temperature, molar ratio of the reactants and
the nature of the alkylating agent on the nature of the product.
In the fine chemical, pharmaceutical, drug and dye industries
the synthesis of organic intermediates is a high activity
research area. Friedel–Crafts (FC) alkylation is one of the
well-established reactions in ganic synthesis. It is a well-known
fact that Lewis and Brønsted acids catalyse FC reactions.
Lewis acids such as AlCl3 , TiCl4 and BF3 have been exten-
sively used as catalysts in alkylation reactions. A survey of
the literature on heterogeneous catalysis research in the area
of alkylation reactions clearly indicated that several attempts
have been made to discover the utility of solid acids such as
oxides, mixed oxides, clays and zeolites as catalysts in alkyla-
tion reactions.1–9 Such studies have established that solid acids
can successfully replace the traditional and hazardous Lewis
acid catalysts in acid-catalysed organic synthesis reactions
and at the same time these catalysts can be suitably modified
to increase the selectivity towards a particular product.
Alkylation of aniline is an important reaction in organic
synthesis. This reaction form products such as N-alkylated
and/or C-alkylated anilines that are used as intermediates
or additives in dyes, synthetic rubber, herbicides and phar-
maceuticals. N-Alkylated anilines are synthesised via alkyla-
tion of anilines. Different alcohols and dimethyl cabonate
are used as alkylating agents under different reaction
conditions.10,11
Experimental
Several reports have appeared on the alkylation of aniline
using methanol or DMC as alkylating agents over various
solid acid-base catalysts. It is established that formation of
N-and/or C-alkylated anilines depends not only on the reac-
tion conditions but also on the catalysts used. Woo et al.,12
using metallosilicates of the pentasil family, suggested that
weak to moderate and strong acid sites are responsible for
N-alkylated and C-alkylated products, respectively. As for
as aniline alkylation with DMC is concerned Sue and Bartho-
meuf13 reported 100% selectivity towards N-mono- and
dialkylation (NMA/NNDMA) in the vapour phase reaction
of aniline over KX and KY zeolites as basic catalysts.
Catalyst preparation and characterisation
Amorphous aluminophosphates and M-aluminophosphates
(M ¼ V, Fe, Co, Ni and Cu) were prepared by the precipita-
tion method.17 In a typical preparation aluminium nitrate,
metal acetate and phosphoric acid were dissolved in 500 cm3
of water in a 0.95:0.05:1 molar ratio of aluminium, metal
and phosphorous. The solution was heated to 80 ꢀC. To this
hot solution 28% aqueous ammonia was added dropwise until
the pH of the solution was 6–7.5. The precipitate, after wash-
ing with deionised water, was initially dried overnight at
120 ꢀC and finally calcined at 550 ꢀC.
DOI: 10.1039/b208085k
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765
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All the catalysts were analysed for their chemical composi-
tion following standard wet analytical and spectroscopic meth-
ods.18 X-Ray diffraction (XRD) patterns of the calcined
powders were recorded on a Philips PW 1349/30 diffract-
ometer. BET surface area of the samples were measured using
a NOVA-1000 (ver: 3.7). Total surface acidity was estimated
by the n-butylamine back-titration method using dry benzene
as the solvent19,20 as well as by TPD of ammonia. Thermal
stability of a few catalysts was investigated by recording their
TGA and DTA patterns.
method. However, the acidity values determined by TPD indi-
cate an increase in the surface acidity of M-AlPO4 with respect
to plain AlPO4 . It is found from the TPD studies that all the
adsorbed ammonia was desorbed when the catalysts were
heated in the temperature range of 250–300 ꢀC. This indicates
that transition metal ion incorporation through co-precipita-
tion with AlPO4 enhances the density of moderate to weak
acid sites. Strong acid sites of the catalysts are known to retain
adsorbed ammonia up to 550 ꢀC and above. Analysis of cata-
lysts for metal ion content showed that the later was present at
about 5%. The absence of any peaks in the XRD pattern of the
catalyst samples indicates that they are all amorphous. Ther-
mal analysis (TGA) results indicate that there is continuous
loss in weight up to 250 ꢀC, mainly due to the removal of water
from the samples. Between 250 and 1000 ꢀC there is no weight
loss and no peaks in the DTA curves. This indicates that the
materials are thermally stable.
Catalytic activity studies
The catalytic activity of all the samples in alkylation of aniline
with methanol or DMC was investigated in the vapour phase.
Vapour phase reactions were conducted in a continuous down-
flow fixed-bed glass reactor under ambient pressure. For each
reaction 500 mg of freshly calcined catalyst was used. The
effect of change in the reaction parameters such as the catalyst
bed temperature, molar ratio of the reactants or duration of
the reaction on the product distribution was investigated.
The reaction products were analysed on a gas chromato-
graph with FID detector using a 2 m SS column packed with
10% Apeozonal and 10% KOH on chromosorb. The products
were identified using authentic samples of the expected pro-
ducts. The conversions and selectivity were calculated with
respect to aniline using the following expressions:21
Catalytic studies
All the catalytic activity studies were conducted in the vapour
phase. GC analysis of the products from these studies revealed
that the reaction between aniline and methanol or DMC in the
presence of AlPO4 and M-AlPO4 (M ¼ V, Fe, Co, Ni, and Cu)
catalysts resulted only in N-alkylated anilines such as
N-methylaniline (NMA) and N,N-dimethylaniline (NNDMA).
The reaction product analysis did not indicate the presence
of any C-alkylated compounds of aniline, such as toludines.
Formation of only N-alkylated anilines further confirms
the TPD results that the catalysts used are associated with
only weak acid sites. Alkylation of aniline is a sequential reac-
tion and follows the order aniline ! NMA ! NNDMA !
toludines. Weak acidity and low reaction temperature favours
NMA and NNDMA formation while for the formation of
toludines strong acid sites are required.11
XAðmol %Þ ¼ ½1À ðmole of aniline in the product
per mole of aniline in the feedފ  100
Sðmol %Þ ¼ ½weight of the aromatic product
per mole of aniline reactedŠ  100
The quantitative results of the vapour phase alkylation of
aniline with methanol (1:1 molar ratio) in the presence of
AlPO4 and M-AlPO4 as catalysts is presented in Table 2 An
analysis of the data given in this table indicates that the aniline
conversion activity of the catalysts is in the following order:
Results and discussion
Catalyst properties
The catalysts used and their physico-chemical properties such
as total surface acidity, determined by n-butylamine titration
and TPD of NH3 methods, BET surface area, percentage of
metal and the colour of the as-synthesised samples are listed
in Table 1. The total surface acidity and BET surface area
are quite in agreement with what is reported in the literature
for similar solid acids.17 Both the n-butylamine back-titration
method and TPD of NH3 give the total surface acidity of
solids. The acidity values obtained by these methods are not
exactly the same due to the difference in the nature of the inter-
action of these basic molecules and the surface acid sites.
Moreover, the variation in the acidity of AlPO4 by the incor-
poration of transition metal ions cannot be clearly explained
with respect to the values obtained by the n-butylamine
Fe-AlPO4 > Ni-AlPO4 > Cu-AlPO4 >
Co-AlPO4 > V-AlPO4 > AlPO4
A comparison of the catalytic activity of the various cata-
lysts as concerns aniline conversion and their surface acidity
(Table 1) indicates that there is no simple correlation between
these two quantities. However, it is interesting to note that the
products from the alkylation reactions carried out in the pre-
sence of Co-or V-containing AlPO catalysts showed the for-
4
mation of only NMA and NNDMA. On the other hand,
Fe-, Ni- and Cu-containing samples also resulted in other
Table 2 Percentage conversion in the vapour phase alkylation of
aniline with methanol in the presence of M-AIPO4 catalystsa
Table 1 The catalysts used in aniline alkylation and their physico-
chemical properties
Selectivity
Total
surface
aciditya /
mmol gÀ1
BET
surface
area/
% Other
products
Catalysts
% Conversion
% NMA
% NNDMA
Catalyst
m2 gÀ1 % Metal colour
No. Catalyst
AlPO4
14
20
51
41
48
43
35.7
100
42.8
–
21.4
–
V-AlPO4
Fe-AlPO4
Co-AlPO4
Ni-AlPO4
Cu-AlPO4
1
2
3
4
5
6
AlPO4
V-AlPO4
Fe-AIPO4
0.41 (0.22) 172
0.39 (0.72) 160
0.38 (0.87) 161
–
White
45.0
43.0
41.6
48.8
29.0
46.3
33.0
27.9
25.4
–
4.9
4.7
4.75
4.8
4.9
Pale yellow
Light brown
Violet blue
Pale yellow
Light blue
25.0
23.2
Co-AIPO4 0.46 (1.10) 159
Ni-AIPO4 0.32 (0.89) 171
Cu-AIPO4 0.34 (0.91) 168
a
Conditions: 1:1 molar ratio of the reactants; reaction temperature
250 ꢀC; flow rate 5 ml hÀ1
a
.
Ammonia TPD acidity values are given within parenthesis.
766
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Table 3 Catalytic activity of Co-AIPO4 in the vapour phase alkylation of anline under different reaction conditions
Selectivity
Selectivity
T/ꢀC
Aniline: CH3OH
% Conversion
% NMA
% NNDMA
Aniline: DMC
% Conversion
% NMA
% NNDMA
200
1:5
1:1
2:1
1:5
1:1
2:1
1:5
1:1
2:1
29
22
14
53
54
30
32
36
19
34.5
59.0
100
65.5
40.9
–
1:5
1:1
2:1
1:5
1:1
2:1
1:5
1:1
2:1
60
62
30
43
44
32
30
36
23
41.6
51.6
100
58.3
48.3
–
250
300
47.1
55.5
100
52.8
44.4
–
34.8
54.5
100
65.1
45.4
–
43.75
58.3
100
56.25
41.6
–
46.6
63.8
100
53.3
36.1
–
Table 4 Catalytic activity of V-AlPO4 in the vapour phase alkylation of aniline under different reaction conditions
Selectivity
Selectivity
% NMA
T/ꢀC
Aniline: CH3OH
% Conversion
% NMA
% NNDMA
Aniline: DMC
% Conversion
% NNDMA
200
1:5
1:1
2:1
1:5
1:1
2:1
1:5
1:1
2:1
8
30
18
14
32
20
12
17
16
100
100
100
100
100
100
100
100
100
–
–
–
–
–
–
–
–
–
1:5
1:1
2:1
1:5
1:1
2:1
1:5
1:1
2:1
14
36
20
17
39
25
16
18
16
100
100
100
100
100
100
100
100
100
–
–
–
–
–
–
–
–
–
250
300
products (not identified) besides NMA and NNDMA. This
observation indicates that the former sets of catalysts are
associated with only weak acid sites that are active in
N-alkylation of aniline.
Effect of catalyst bed temperature. Aniline methylation using
methanol or DMC was carried out at three different tempera-
tures, 200, 250 and 300 ꢀC. At 250 ꢀC both Co-AlPO4 and V-
AlPO4 showed the highest catalytic activity for the formation
of NMA. A change in the ratio of aniline to methanol or
DMC did not show any change in the temperature at which
these catalysts exhibited their maximum activity. These obser-
vations indicate that 250 ꢀC is the optimum reaction tempera-
ture for the reaction between aniline and methanol or DMC to
yield NMA in the presence of Co-AlPO4 and V-AlPO4 as cat-
alysts. At higher temperatures aniline would probably undergo
oxidation and also decompose, resulting in coke formation.
This may be the reason for the decrease in the catalytic activity
for the formation of NMA at higher temperatures.
Furthermore, it is noticed that Co-AlPO4 , with the higher
TPD acidity, exhibited a lower selectivity for NMA because
NNDMA was also formed. On the other hand, V-AlPO4 , with
a lower acidity, exhibited 100% selectivity towards NMA. This
is in accordance with the expected trend that catalysts with
low acidity exhibit higher selectivity towards NMA formation.
With respect to other catalysts containing Fe, Ni and Cu with
acid sites whose concentration is lower than in cobalt-contain-
ing AlPO4 , the latter, however, showed poor selectivity to
NMA. These observations clearly indicate that it is not the
total surface acidity that is responsible for the catalytic activity
of the solid acids but the presence of acid sites with a suitable
acid strength and their concentration which is important.
Probably among all the metal aluminophosphates only Co-
and V-AlPO4 possess acid sites of medium strength that would
bring about selective N-methylation of aniline because it is
known that the solid acids with higher surface acid strength
show greater activity towards the formation of C-alkylated
anilines.17
Effect of the molar ratio of the reactants and the methylating
agent. Aniline alkylation reactions were conducted using three
Variation of catalytic parameters
Further experiments were conducted only on V-AlPO4 and
Co-AlPO4 to determine the effect of reaction temperature,
molar ratio of the reactants and the nature of the methylating
agent on the percentage conversion and selectivity towards N-
methylated products of aniline. The results of the experiments
are presented in Table 3 and 4. In Table 3 is given the catalytic
activity of Co-AlPO4 in the alkylation of aniline with methanol
or DMC, carried out at three different reaction temperatures
and using three different molar ratios of aniline and alkylating
agents. In Table 4 the results of similar studies carried out
using V-AIPO4 are presented.
Fig. 1 The effect of temperature and molar ratio of reactants on the
vapour phase alkylation of aniline with DMC over V-AIPO4 .
New J. Chem., 2003, 27, 765–768
767

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apparent deactivation of the catalysts at the selected reaction
temperature.
Conclusion
Amorphous aluminophosphates modified by transition metal
ions have been found to be good catalysts for the vapour phase
synthesis of N-alkylated anilines, using MeOH or DMC as the
alkylation agents. V-AlPO4has been found to show 100% selec-
tivity for the formation of NMA. No simple correlation
between the selectivity and the total surface acidity has been
observed. However, none of the catalysts showed any activity
for C-alkylation of aniline, indicating that they possess only
acid sites with moderate acid strength.
Acknowledgements
Fig. 2 The effect of temperature and molar ratio of reactants on the
vapour phase alkylation of aniline with DMC over Co-AIPO4 .
The authors are grateful to the Department of Science and
Technology (DST), New Delhi, India, for financial support,
to IISc, Banglore, India, for providing library facilities and
to Dr. A. B. Halgari and Dr. J. Das (IPCL, Baroda, India)
for useful scientific discussions.
different molar ratios (1:5, 1:1, 2:1) of reactants, using aniline
and MeOH or DMC as the methylating agent, at three differ-
ent catalyst bed temperatures. Five hundred milligrams of
either Co-AlPO4 or V-AlPO4 catalyst was used in each reac-
tion. It is observed from the results given in Table 4 that V-
AlPO4 showed 100% selectivity for the formation of NMA,
irrespective of the molar ratio of the reactants, alkylating
agents and the reaction temperature. On the other hand, Co-
AlPO4 (Table 3) showed a similar selectivity only when the
molar ratio was 2:1, that is in the presence of a lower concen-
tration of methylating agent in the reaction mixture. In the
case of Co-AlPO4 , due to the higher acid site concentration,
when an excess of methylating agent is present further alkyla-
tion of the NMA formed would probably take place, resulting
in a poor selectivity for NMA. On the other hand, V-AlPO4 ,
with acid sites probably with suitable strength, brings about
100% selectivity for NMA. The results of the experiments with
respect to the % of NMA formed at different molar ratios of
the reactants and the reaction temperature are represented in
the form of bar diagrams in Fig. 1 and 2.
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New J. Chem., 2003, 27, 765–768