Side chain ethylation of toluene with ethanol over hydrotalcite-like compounds

Article abstract of DOI:10.1134/S0023158410010106

Calcined Mg, Ni, Cu, Co, and Zn hydrotalcites (CHTs) tested for their catalytic performance in the side chain ethylation of toluene with ethanol at 300, 350, 400, and 450°C. The major products were propylbenzene, β-methylstyrene, ortho-, para-, and meta-ethyltoluene. MgAl-CHTs gave side chain alkylated propylbenzene, while over Co, Ni, Cu, and Zn-CHTs, side chain as well as nuclear alkylated products and ethyltoluene were observed.

Full text of DOI:10.1134/S0023158410010106

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2010, Vol. 51, No. 1, pp. 56–62. © Pleiades Publishing, Ltd., 2010.
Published in Russian in Kinetika i Kataliz, 2010, Vol. 51, No. 1, pp. 62–68.
Side Chain Ethylation of Toluene with Ethanol
over HydrotalciteꢀLike Compounds1
R. Manivannan and A. Pandurangan
Department of Chemistry, Anna University, Chennai, India 600025
eꢀmail: manickam_mani@yahoo.co.in
Received August 30, 2008
Abstract—Calcined Mg, Ni, Cu, Co, and Zn hydrotalcites (CHTs) tested for their catalytic performance in
the side chain ethylation of toluene with ethanol at 300, 350, 400, and 450°C. The major products were proꢀ
pylbenzene, ꢀmethylstyrene, orthoꢀ, paraꢀ, and metaꢀethyltoluene. MgAlꢀCHTs gave side chain alkylated
β
propylbenzene, while over Co, Ni, Cu, and ZnꢀCHTs, side chain as well as nuclear alkylated products and
ethyltoluene were observed.
DOI: 10.1134/S0023158410010106
1
1. INTRODUCTION
various applications of hydrotalcites have been
reported elsewhere [7–10].
Alkylation of aromatics with olefins and alcohols
The basic zeolites studied for alkylation of toluene,
showed greater selectivity for nuclear alkylation than
the side chain alkylation [11]. By quantum chemical
calculation [12], the basic sites make the carbon atom
of the side chain more active than that of benzene ring;
the role of acid sites is attributed to the adsorption and
stabilization of toluene. The methyl group condenses
with the carbonyl group adsorbed on acidic sites. In
the present work, we investigate the side chain alkylaꢀ
tion of toluene with ethanol over a series of mixed
metal oxides derived from hydrotalciteꢀlike precursor.
occurs at the aromatic ring over acid catalysts and at
the side chain over basic catalysts [1]. It has been
shown that selectivity towards side chain and nuclear
alkylation depends critically on the nature of the cataꢀ
lyst and reaction environment [2, 3]. The alkylation of
toluene using methanol over alkali exchanged and also
over alkali impregnated HZSMꢀ5 were studied [4].
The active sites were calculated to be weak Brønsted
acid sites whose strength was rather reduced by alkali
cations. As a result, the nuclear alkylation decreases in
the order Li > Na > K > Rb > Cs. Huang and Ko [5]
reported the methylation of ethylbenzene over X type
zeolites that ring and side chain alkylation took place
respectively on the acid and base zeolite such as HX
and KX. The side chain alkylation and dealkylation
proceed respectively via, carbanion and carbonium
ion mechanism where as dehydrogenation occurs via,
free radical mechanism [5].
2. EXPERIMENTAL
2.1. Preparation of Catalysts
Coꢀprecipitation at low supersatuaration and
constant pH is the method most frequently used in
the preparation of hydrotalcites [6]. They can
be represented by the general formula
n−
Ethylation of toluene is an industrially important
reaction as the polymer product, polymethylstyrene
has more commercial importance than polystyrene as
it possess high flash point, glass transition temperature
and low specific gravity. The general terms hydrotalꢀ
citeꢀlike compounds or layered double hydroxides are
widely used for the isomorphous and poly type comꢀ
pounds of hydrotalcites [6]. A broad spectrum of divaꢀ
lent and trivalent cations in different atomic composiꢀ
tions constitutes LDHs with HTꢀlike network. Interꢀ
layer anions can be varied which allows the tailoring of
desired catalytic properties with these materials [8]. A
detailed description of synthesis, characterization and
[M(II)1 M(III) (OH) ]x+[
]mH2O, where M(II) =
A x/2
⎯
x
x
2
Mg, Ni, Cu, Zn, Co and M(III) = Al, Cr, Fe, with
M(II)/M(III) atomic ratio of 3, 4, 5, 7, and 10. The
aqueous solution of metal nitrates (70–100 ml) and a
mixture of aqueous solution of NaOH (2–3 M) and
Na2CO3 (0.2–0.35 M) were mixed at room temperaꢀ
ture with vigorous stirring at the pH range 7–10. The
resulting slurry is aged at 75°C for 30 min with vigorꢀ
ous stirring. The precipitate, filtered and washed with
distilled water to eliminate the alkali metal and nitrate
ions was dried at 110°C overnight. Calcination was
carried out in the presence of air at 450°C for 12 h, and
then cooled to room temperature.
1
The article is published in the original.
56

SIDE CHAIN ETHYLATION OF TOLUENE WITH ETHANOL
57
Table 1. Chemical composition and lattice parameters of M (II) M (III) CO3 HT
Lattice parameters
(
Å
)
Surface area (m2
/g)
Catalysts
M(II)/M(III)a
a
c
uncalcined
calcinedb
Mg/Al 3.0
2.71
3.74
4.69
6.46
8.91
2.78
2.72
2.69
2.90
3.049
23.301
23.609
23.801
23.865
23.858
22.969
22.867
22.821
22.769
99.5
103.8
88.7
–
168.0
172.6
154.8
–
Mg/Al 4.0
Mg/Al 5.0
Mg/Al 7.0
Mg/Al 10.0
Ni/Al 3.0
Cu/Al 3.0
Co/Al. 3.0
Zn/Al 3.0
3.061
3.092
3.098
3.108
3.082
3.058
3.072
3.064
–
–
110.0
119.6
83.0
69.8
189.3
206.4
149.6
106.5
a
Note: ICPES result.
b
BET method at 450°C
2.2. Characterization of Catalysts
3. RESULT AND DISCUSSION
3.1. Physico Chemical Properties
The characterization of hydrotalciteꢀlike comꢀ
pounds were carried out using ICPES (Model 3410
ARL) method was adopted to estimate the ratios of
the M(II) and M(III) ions in hydrotalciteꢀlike comꢀ
pounds and BET (CarloꢀErba model 1800) surface
area measurements. Thermal behavior of HTlc was
carried out in a Perkin Elmer thermal analyzer
(TGAꢀDSC 7).
The important physico chemical characteristics of
hydrotalcites are presented in Table 1. These results
TG
DTG
(a)
2.3. Catalytic Studies
(b)
The vapour phase alkylation of toluene with ethaꢀ
nol has been carried out in a fixed bed down flow conꢀ
tinuous type quartz reactor of 8.0 mm I.D and 35 mm
length in which a porcelain disc was sintered and
sealed in the middle of the reactor to support the cataꢀ
lyst. The catalyst (1.5 g of powdered CHT, 450°C for
8 h) packed inside the reactor was supported by two
glass wool plugs, which were surrounded by ceramic
beads to fill the space. Before starting the experiment,
the catalyst was activated at 450°C for 3 h in nitrogen
flow (23 cm3/min) and then brought to the reaction
temperature. The alkylation reaction was carried out
using a mixture of toluene and alkylating agent, ethaꢀ
nol, (molar ratio 1 : 1), introduced at the top of the
reactor by means of syringe infusion pump (Electronic
Corporation of India) in the absence of carrier gas.
The liquid products are collected in cold traps, identiꢀ
fied and estimated in a Hewlett Packard 5890A gas
chromatograph with a 2ꢀm 5% bentone column using
a flame ionization detector.
(c)
(d)
(e)
60
380
Temperature, °С
700
Fig. 1. TG and DTG curves of (a) Mg/Al 3.0, (b) Mg/Al
4.0, (c) Mg/Al 5.0, (d) Mg/Al 7.0, (e) Mg/Al 10.0.
KINETICS AND CATALYSIS Vol. 51
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58
MANIVANNAN, PANDURANGAN
0.2 to 0.3. Instead of Mg2+ various M(II) ions (M =
Co, Ni, Cu, and Zn) were used, keeping Al3+ constant.
3.1.1. Thermal analytical studies. The thermoꢀ
gravimetry (TG) and its differential curves (DTG) for
the Mg/Al synthesized samples are shown in the
Fig. 1. Two consecutive weight losses are recorded as
expected for HTlcs with vaporizable interlayer anion
[13]. The first weight loss in the temperature range
(120–250°C) is attributed to the removal of physꢀ
isorbed interlayer water molecules. The second endotꢀ
hermic weight loss occurring in the temperature range
(250–450°C) is ascribed to the removal of structural
water (from layer hydroxyl groups) and of CO2 (from
the interlayer carbonate anions) leading to the
destruction of the layered structure [14]. This temperꢀ
ature range changes very little with the Mg/Al atomic
e
d
c
b
а
ratio, both transformation temperatures
T1 and T2
decrease with increase in Mg/Al atomic ratio. This can
be attributed to the decrease in electrostatic attraction
between the layer and interlayer owing to the lowering
of electropositive nature of the brucite layer. The therꢀ
mograms agree with the thermal results of various
authors and characteristic thermal transitions are
retained even up to atomic ratio variations [15, 16].
The DSC of these samples substantiates the TG
60
260
Temperature, °С
460
Fig. 2. DSC curves of
5.0; d, Mg/Al 7.0; e, Mg/Al 10.0.
a, Mg/Al 3.0;
b, Mg/Al 4.0;
c, Mg/Al
enable one to know the exact ‘x’ values. The appropriꢀ
ate lattice parameters required for a perfect HTlc forꢀ results giving two endotherms in the same temperature
mation were generally achieved by MgAlꢀHT for
range (Fig. 2). Although the DSC transformation temꢀ
peratures 1 and 2 are slightly higher compared to
x
=
T
T
that of TG transformation temperatures, the observed
trend with respect to Mg/Al atomic ratio is similar in
these experiments.
The thermal decompositions of M(II) AlꢀHT were
studied by TG and DSC experiments. The TG and
DSC curves of these compounds are given in Figs. 3
and 4, respectively. The transformation temperatures
Table 2. Decomposition of cyclohexanol over catalyst derived
from CHTs and conventional oxides
Product selectivity, wt %
Cyclohexanol,
Catalysts
conversion,
wt %
cycloꢀ
cyclohexene
T1 and T2 in TG and DSC correlated well with each
hexanone
other. The thermograms of other compounds except
CuAlꢀHTlc exhibit two stages of weight loss correꢀ
sponding to dehydroxylation and decarboxylation.
NiAlꢀHT showed considerable thermal stability (up to
300°C) while Cu, Co, and Zn containing samples
exhibited poor thermal stability (less than 250°C).
MgAlꢀHT exhibited a high thermal stability in comꢀ
parison with other hydrotalcites. These observations
would suggest that the amount of structural water assoꢀ
ciated with Mg is high in comparison with other
hydrotalcites.
3.1.2. BET surface area measurement. Thermal
decomposition of hydrotalcite precursors led to the
formation of high surface area. Also creation of large
surface area is an important characteristic need for
catalysis. Surface areas of CHTs were significantly
higher than in uncalcined HT samples [17]. The
increase in surface area during calcination at 450°C
can be attributed to the formation of mesopores due to
expulsion of CO2 and H2O from the hydrotalcite preꢀ
cursors [18]. Mg/Alꢀ4.0 CHT showed a maximum
observed surface area. Due to variation in the radii of
M (II) ions and M (III) ions substitution in the MgAl–
Mg /Al 3.0
Mg /Al 4.0
Mg /Al 5.0
Mg/Al 7.0
Mg/Al 10.0
Co/Alꢀ3.0
Ni/Alꢀ3.0
Cu/Alꢀ3.0
Zn/Alꢀ3.0
MgO
37.6
36.1
34.0
28.2
21.4
39.3
50.4
52.2
36.5
8.6
14.3
13.8
10.9
6.3
85.7
86.2
89.1
93.7
98.7
65.9
7.2
1.3
34.1
92.8
95.6
82.1
1.0
4.4
17.9
99.0
–
Al2O3
39.5
100
*
MgO–Al2O3
43.8
77.6
22.4
Note: Temperature: 300°C; Flow rate: 2 ml/h; Catalyst weight: 1.5 g;
*Physical mixture of MgO and Al O .
2
3
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SIDE CHAIN ETHYLATION OF TOLUENE WITH ETHANOL
59
TG
DTG
a
b
(a)
(b)
(c)
c
(d)
d
60
220
380
540 700
Temperature, °С
60
260
Temperature, °С
460
Fig. 4. DSC curves of a, Cu/Al 3.0; b, Ni/Al 3.0; c, Co/Al
3.0; and d, Zn/Al 3.0.
Fig. 3. TG and DTG curves of (a) Cu/Al 3.0, (b) Ni/Al 3.0,
(c) Co/Al 3.0 and (d) Zn/Al 3.0.
Except Mg/Alꢀ3.0 CHT, all other CHTs showed
more than 90% selectivity of cyclohexanone indicatꢀ
ing that these compounds possesses predominantly
basic sites. The higher selectivity of cyclohexene in the
case of Mg/Alꢀ3.0 CHT compared to other CHTs
indicates the higher acidity of this catalyst which can
be attributed to the dissolution of more amount of Al3+
in the MgO lattice introducing Lewis acidity. From
these results it can be concluded that MgAlꢀCHT posꢀ
sesses predominantly basic sites whose basicity is
slightly less than that of MgO but higher than that of
pure Al2O3. Jianyi Shen et al. [19] also observed similar
observation from microcalorimetric determination of
heat of adsorption study. They found that MgꢀAl
mixed metal oxides obtained from hydrotalcites do not
possess acid sites and base sites with differential heats
higher than 120 and 150 kJ/mol respectively. The
HT precursors there will be a lowering of surface area
of CHTs (Table 1).
3.1.3. Acid–base properties. Catalytic conversion
of cyclohexanol is often used as a model reaction to
evaluate acidity, basicity and catalytic activity of the
catalysts. It has been reported that the determination
of acidꢀbase properties of a catalyst by test reactions is
superior to other conventional methods. This test
reaction has been applied in order to evaluate the acidꢀ
base properties of some samples of hydrotalciteꢀlike
materials. The results of conversion of cyclohexanol
over a series of MgAlꢀCHT are shown in Table 2. The
catalytic activity of the catalyst is also monitored
through the percentage extent of formation of cycloꢀ
hexanone and cyclohexene. As expected, MgO
showed exclusive selectivity for cyclohexanone indiꢀ
cating the presence of strong basic sites while Al2O3
showed 100% selectivity for cyclohexene. The physiꢀ
cally mixed MgO–Al2O3 products possess approxiꢀ
mately proportional amounts of cyclohexene and
cyclohexanone which bears a close relation to the proꢀ
portion of MgO–Al2O3 (~3.5 : 1). It has been observed
that when Mg(II)–Al(III)–HTs were tested for cycloꢀ
hexanol model reaction, although they possess both
acidic and basic sites, ketones are formed predomiꢀ
nantly than alkenes. This suggests that basicity domiꢀ
nates over the acidity in Mg(II)–Al(III)–HTs.
γ
ꢀAl2O3 possesses acid sites with differential heat from
60 to 160 kJ/mol, and MgO has base sites with differꢀ
ential heats as high as 170 kJ/mol. The results of
decomposition of cyclohexanol over Co, Ni, Zn, and
Cu containing HTs at 300°C are shown in the Table 2.
CuAlꢀCHT and NiAlꢀCHT are most selective for
cyclohexene (around 90%). Many authors have
claimed that the selectivity to cyclohexene in the
decomposition of cyclohexanol can be taken as a meaꢀ
sure of acidic strength of catalyst [20–22]. Taking into
account the selectivity to cyclohexene, the activity of
KINETICS AND CATALYSIS Vol. 51
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60
Toluene conversion, wt %
MANIVANNAN, PANDURANGAN
Yield, mol %
20
80
60
40
Mg/Alꢀ3
Mg/Alꢀ4
Mg/Alꢀ5
Mg/Alꢀ7
Mg/Alꢀ10
Mg/Alꢀ3
Mg/Alꢀ4
Mg/Alꢀ5
Mg/Alꢀ7
Mg/Alꢀ10
10
0
20
0
250
300
350
400
450
500
300
350
400 450
Temperature, °С
80
60
40
Fig. 6. Ethylation of toluene: the effect of temperature on
Co/Alꢀ3
Ni/Alꢀ3
Cu/Alꢀ3
Zn/Alꢀ3
the yield of
β
ꢀmethylstyrene.
3.2. Ethylation of Toluene
Ethylation of toluene with ethanol was carried out
over Mg/Al, Ni/Al, Cu/Al, Co/Al, and Zn/Al calꢀ
cined hydrotalcites from 300–450°C in steps of 50°C
with toluene to ethanol feed ratio 1 : 1. The nuclear
and side chain alkylated products of toluene (propylꢀ
20
0
benzene, ꢀmethylstyrene, o, p, and mꢀethyltoluene),
β
250
300
350
400
450 500
Temperature, °С
and also the products of the side reactions of ethanol
(ethane and diethyl ether) were formed.
Fig. 5. Ethylation of toluene: the effect of temperature on
Conversion of toluene increases with increase in
temperature from 300 to 400°C as shown in Fig. 5.
Yield of side chain alkylated products namely propylꢀ
toluene conversion.
benzene and ꢀmethylstyrene (Table 3) is exclusive
β
the samples could be ordered as Cu > Ni > Zn > Co
indicating that the acidity also decreases in the same
order.
over MgAl–CHTs, while substitution of Mg2+ by other
divalent metal ions shows higher nuclear alkylated
product namely ethyltoluene (Table 3). Among the
MgAl–CHT series, it is to be noted that Mg/Al 3.0
CHT is more active than other mole ratios and gives
more of propylbenzene. Divalent substitution gives
lesser yield of propylbenzene than MgAlꢀCHT. Forꢀ
mation of ethyltoluene by nuclear alkylation is higher
over the series of divalent metal substitution.
Table 3. Ethylation of toluene: the effect of CHTs on product
distribution
Product yield, mole %
Toluene
Catalysts conversion,
propyl βꢀmethyl
benzene styrene
ethyl
toluene
wt %
3.3. Effect of Temperature
Mg/Alꢀ3.0
Mg/Alꢀ4.0
Mg/Alꢀ5.0
Mg/Alꢀ7.0
Mg/Alꢀ10.0
Co/Alꢀ3.0
Ni/Alꢀ3.0
Cu/Alꢀ3.0
Zn/Alꢀ3.0
31.8
31.2
25.6
19.7
15.2
22.8
27.5
29.3
17.5
19.2
18.5
13.8
10.8
6.7
7.5
9.3
9.1
7.9
8.2
–
3.6
2.9
–
The effect of temperature on product selectivity
over MgAl–CHT resulted in higher selectivity to
methylstyrene at low temperatures in Fig. 6. As the
temperature increases, the selectivity to ꢀmethylstyꢀ
β
ꢀ
β
–
rene decreased with concomitant increase in selectivꢀ
ity for propylbenzene (Fig. 7) and ethyltoluene
(Fig. 8). The selectivity for propylbenzene and ethylꢀ
toluene showed maximum at 400°C. On the other
hand, substitution of Mg2+ by other divalent cations
resulted in predominant nuclear alkylation even at low
–
12.8
11.0
12.2
13.4
9.9
15.4
11.6
3.1
–
–
temperatures. ꢀmethylstyrene, an intermediate for
β
the formation of propylbenzene, is noticed over
MgAl–CHTs. Moreover, substitution of Mg2+ by other
divalent metal ions does not favour the selectivity of
–
Note: Temperature: 300°C; Toluene : Ethanol = 1 : 1; Catalyst
weight: 1.5 g; Flow rate: 2 ml/h.
β
ꢀmethylstyrene. On the other hand, substitution of
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SIDE CHAIN ETHYLATION OF TOLUENE WITH ETHANOL
61
Yield, mol %
60
Yield, mol %
20
Mg/Alꢀ3
Mg/Alꢀ3
Mg/Alꢀ4
Mg/Alꢀ5
Mg/Alꢀ7
Mg/Alꢀ4
Mg/Alꢀ5
Mg/Alꢀ7
Mg/Alꢀ10
40
20
10
Mg/Alꢀ10
0
0
300
350
400
450
300
350
400
450
30
60
40
20
0
Co/Alꢀ3
Ni/Alꢀ3
Cu/Alꢀ3
Zn/Alꢀ3
Co/Alꢀ3
Ni/Alꢀ3
Cu/Alꢀ3
Zn/Alꢀ3
20
10
0
300
350
400 450
Temperature, °С
300
350
400 450
Temperature, °С
Fig. 7. Ethylation of toluene: the effect of temperature on
the yield of propyl benzene.
Fig. 8. Ethylation of toluene: the effect of temperature on
the yield of ethyl toluene.
Table 4. Effect of temperature on isomer distribution of ethyl
toluene
Mg2+ in the HT framework influences the nuclear
alkylation to give ethyltoluene predominantly.
Product yield, mole %
In Table 3, the toluene conversion and products
selectivity are compared at constant temperature of
300°C. It can be seen that MgAlꢀCHT with Mg/Al 3.0
is most selective catalysts for both nuclear and side
chain alkylated products, while other divalent ions
metal substituted catalysts show selective nuclear alkyꢀ
lation to give ethyltoluene. The selectivity for ethyltolꢀ
uene decreases in the order ZnAl > NiAl > CuAl >
CoAl > MgAl. The isomer distribution in the alkylaꢀ
tion of toluene with ethanol is given in the Table 4.
MgAlꢀCHTs show higher oꢀethyltoluene, while other
divalent substitutions give pꢀethyltoluene in excess at
low temperature. It is clear that with increase in temꢀ
perature (i.e., at 350 and 400°C) mꢀethyltoluene
increases with corresponding decrease in pꢀ and oꢀisoꢀ
mer, but there is no significant change in oꢀ ethyltoluꢀ
ene in MgAl–CHT.
Temperaꢀ
Catalysts
oꢀethyl
toluene
mꢀethyl
toluene
pꢀethyl
toluene
ture, C
°
Mg/Alꢀ3.0
Co/Alꢀ3.0
Ni/Alꢀ3.0
Cu/Alꢀ3.0
Zn/Alꢀ3.0
300
350
400
450
300
350
400
450
300
350
400
450
300
350
400
450
300
350
400
450
2.8
3.6
5.2
4.5
3.6
4.4
4.0
3.0
4.5
4.2
3.8
3.3
4.8
4.6
4.2
3.6
1.9
2.2
2.0
1.2
–
0.8
3.2
6.7
2.0
1.0
2.6
2.2
1.5
4.0
8.4
7.2
3.0
4.2
13.6
11.7
2.6
3.5
11.6
12.8
–
4.8
10.2
15.6
8.4
7.9
10.8
17.4
13.1
4.2
10.5
14.3
13.6
1.2
4. CONCLUSIONS
From this investigation, it is concluded that the
combined participation of acidic and basic sites of the
hydrotalciteꢀlike compounds are found to be crucial
for both side chain and ring alkylation of toluene with
ethanol. Among these calcined hydrotalcites, MgAlꢀ
CHTs gave propylbenzene by side chain alkylation;
while over Cu, Ni, Co, and ZnꢀCHTs gave side chain
1.2
13.4
13.9
5.4
17.3
7.0
Note: Toluene : Ethanol = 1 : 1; Catalyst weight: 1.5 g; Flow rate:
2 ml/h.
KINETICS AND CATALYSIS Vol. 51
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62
MANIVANNAN, PANDURANGAN
11. Yashima, T., Aamhad, H., Yamasaki, K., Katsuta, M.,
as well as nuclear alkylated product such as ethyl toluꢀ
ene. Among studied hydrotalcites, Mg/AlꢀCHTs are
better for side chain ethylation of toluene.
and Hara, N., J. Catal., 1970, vol. 16, p. 273.
12. Itoh, H., Miyamoto, A., and Murakami, Y., J. Catal.
1980, vol. 64, p. 284.
,
13. Kloprogge, J.T. and Frost, R.L., Appl. Catal., A, 1999,
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