Synergistic role of Lewis and Br?nsted acidities in Friedel-Crafts alkylation of resorcinol over gallium-zeolite beta

Article abstract of DOI:10.1016/j.catcom.2011.01.015

The role of Lewis and Br?nsted acidities in alkylation of resorcinol is demonstrated through the gallium-zeolite beta by varying the amount of Lewis and Br?nsted acid sites. The synergism of Lewis and Br?nsted acid sites takes place heterogeneously in Friedel-Crafts alkylation of resorcinol with methyl tert-butyl ether to produce 4-tert-butyl resorcinol and 4,6-di-tert-butyl resorcinol as the major and minor products, respectively.

Full text of DOI:10.1016/j.catcom.2011.01.015

Catalysis Communications 12 (2011) 822–825
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Synergistic role of Lewis and Brönsted acidities in Friedel–Crafts alkylation of
resorcinol over gallium-zeolite beta
Hadi Nur ⁎, Zainab Ramli, Jon Efendi, Aiman Najati Akmar Rahman, Sheela Chandren, Lai Sin Yuan
Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history:
The role of Lewis and Brönsted acidities in alkylation of resorcinol is demonstrated through the gallium-
zeolite beta by varying the amount of Lewis and Brönsted acid sites. The synergism of Lewis and Brönsted acid
sites takes place heterogeneously in Friedel–Crafts alkylation of resorcinol with methyl tert-butyl ether to
produce 4-tert-butyl resorcinol and 4,6-di-tert-butyl resorcinol as the major and minor products, respectively.
Received 16 November 2010
Received in revised form 19 January 2011
Accepted 21 January 2011
Available online 31 January 2011
©
2011 Elsevier B.V. All rights reserved.
Keywords:
Gallium-zeolite beta
Lewis and Brönsted acid sites
Alkylation of resorcinol
1
. Introduction
reaction is also proposed in order to show the synergistic role of Lewis
and Brönsted acidities.
Alkylation of aromatics is an important reaction in organic
chemistry as it is widely used in the synthesis of petrochemicals,
fine chemicals, perfumeries, pharmaceuticals, dyes and agrochemicals
2. Experimental
[1]. One of the important reactions is Friedel–Crafts alkylation of
resorcinol with methyl tert-butyl ether (MTBE). With an increase in
the production of methyl tert-butylether(MTBE) for use as antiknock
and octane number booster for gasoline, MTBE appears to be an
attractive source for alkylation of resorcinol [2].
Gallium containing zeolite beta was prepared by impregnation of
Ga(NO ) in water on BEA using the method described by Choudhary
3 3
et al. [8]. The resulting sample was denoted as XGa-BEA (X is the
percentage of Ga). Mesoporous alumina (MA) was prepared accord-
ing to the literature [7]. All samples were characterized by powder
X-ray diffraction (XRD) for crystallinity and phase content of the
solid materials, using a Bruker Advance D8 diffractometer with the
The use of solid acid catalysts for Friedel–Crafts alkylation reaction
such as zeolites has generated great interests and wide applications in
the industries nowadays [3]. In general, Friedel–Crafts alkylation is
carried out conventionally with the use of homogeneous Lewis and
Brönsted acid catalysts [3,4]. The disadvantages of these conventional
acid catalysts lead to the research on replacement of the homogeneous
catalyst with a heterogeneous type, predominantly zeolites [3,5]. Until
now, the role of Brönsted and Lewis acids in the alkylation reaction is
unclear as no research that systematically controlled the amount of
Lewis and Brönsted on the surface of catalyst in alkylation reaction is
carried out. Although few studies have been focused on the alkylation
of resorcinol [1,6,7], the role and mechanism of Brönsted and Lewis
acid sites have yet to be understood thoroughly. Here, we demon-
strated the effect of Brönsted and Lewis acid sites on alkylation of
resorcinol with methyl tert-butyl ether (MTBE) using zeolite beta
impregnated with gallium. One suggests that Lewis and Brönsted
acidities play a synergetic role in this reaction. The mechanism of this
Cu K
α
(λ=1.5405 Å) radiation as the diffracted monochromatic
beam at 40 kV and 40 mA. The pattern was scanned in the 2θ ranges
between 5° and 45° at a step 0.050° and step time 1 s. Infrared (IR)
spectra of the samples were collected on a Perkin Elmer Spectrum
−
1
One (FTIR) spectrometer, with a spectral resolution of 4 cm , scans
10 s, at room temperature. For acidity evaluation, the wafer of the
sample (10–12 mg) was prepared and locked in the cell equipped
with CaF
2
windows and evacuated at 400 °C under vacuum condition
for 4 h. Infrared spectra of the samples were recorded at room
−
1
temperature in the hydroxyl region of 4000–3000 cm
and
−
1
pyridine vibration region at 1700–1300 cm . Catalytic activity of
the catalysts in the alkylation of resorcinol with MTBE was performed
at 80 °C for 8 h under vigorous stirring. The reaction mixture
containing resorcinol (40 mmol) and MTBE (60 mmol) was put in a
round bottom flask equipped with a condenser. The catalyst (0.2 g)
was then added to the solution. The reaction was carried out in an oil
bath under stirring at 80 °C. The products of the reaction were
analyzed by a Hewlett-Packard 6890N gas chromatograph using an
⁎
Corresponding author. Tel.: +60 7 5536162; fax: +60 7 5536080.
E-mail address: hadi@kimia.fs.utm.my (H. Nur).
URL: http://www.hadinur.com (H. Nur).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.01.015

H. Nur et al. / Catalysis Communications 12 (2011) 822–825
823
3
740
Ultra-1 column and a Hewlett-Packard GC-MSD instrument using a
HP5 column.
(
(
(
e)
d)
c)
3
. Results and discussion
Impregnation of gallium on zeolite beta was done at different wt.%
of Ga which varies from 3 to 25 wt.%. The XRD patterns for all XGa-BEA
X=3–25 wt.% Ga) samples (Fig. 1) showed that the structure of
zeolite beta is maintained even at high Ga loading without the
formation of Ga crystal phases. X-ray diffractograms of the
(
2 3
O
impregnated gallium on zeolite beta showed no evidence of any
crystalline Ga containing compounds suggesting that the Ga species is
either in the amorphous form or was highly dispersed as very small
particles on the zeolite support.
3
608
Fig. 2 shows the FTIR spectra of the gallium-zeolite beta samples
after evacuation at 400 °C for 4 h under vacuum. In the region of
−1
hydroxyl groups, two bands were observed at 3608 and 3740 cm
.
The former had been assigned to acidic bridging OH from the Si–O
(
(
b)
a)
(
H)–Al band while the latter is attributed to the defect sites and
−1
terminal Si–OH [9]. Bands at 3740 cm
can be clearly seen in all
samples. However, the band assigned to the bridging hydroxyl group,
−1
at 3608 cm , can be seen with different intensities of peak area in all
samples. As shown in Fig. 2, it is clearly observed that the peak area at
−1
3
608 cm decrease with an increase in the amount Ga impregnated
on the surface of zeolite beta. This suggests that the gallium atoms
have interacted with the Brönsted acid of zeolite beta. Mesoporous
alumina (MA) was also used for comparison. The absence of peak at
−1
4000
3800
3600
3400
3200
3000
3
608 cm in the FTIR spectrum of MA suggests that this sample did
Wavenumber / cm-1
not have the acidic bridging hydroxyl groups (see Fig. 2a).
The acidity of the XGa-BEA and MA catalysts was further
monitored by FTIR using pyridine as a probe molecule. The purpose
of this experiment is to determine the amount and type of acid in the
catalyst. Fig. 3 shows the IR spectra of samples after the adsorption of
Fig. 2. The FTIR spectra of hydroxyl group of (a) MA, (b) 25%Ga-BEA, (c) 10%Ga-BEA, (d)
8%Ga-BEA and (e) 3%Ga-BEA.
−
1
pyridine. The bands at 1450 cm
in the spectra are attributed to
adsorbed pyridine bound coordinative with Lewis acid sites [10]. The
1540
o
1450
2
θ = 22.3 (302)
o
2
θ = 7.4 (001)
(e)
25%Ga-BEA
(
(
d)
c)
1
0%Ga-BEA
8
%Ga-BEA
3%Ga-BEA
(b)
Ga O
2
3
(a)
1560 1540 1520 1500 1480 1460 1440 1420 1400
5
10
20
30
40
Wavenumber / cm-1
o
2θ/
Fig. 3. The FTIR spectra of pyridine absorbed on (a) MA, (b) 25%Ga-BEA, (c) 10%Ga-BEA,
Fig. 1. X-ray diffractograms of gallium-zeolite beta and Ga
2
O
3.
(d) 8%Ga-BEA and (e) 3%Ga-BEA after desorption of pyridine at 150 °C for 1 h.

824
H. Nur et al. / Catalysis Communications 12 (2011) 822–825
absorption bands at 1540 cm− correspond to pyridine interacting
with Brönsted acid sites [10]. The presence of these bands for all
samples indicates that all samples contain both Lewis and Brönsted
acid sites. For calculation of the pyridine concentrations adsorbed on
Brönsted site and Lewis site, the values of integrated molar extinction
coefficients chosen are 1.67 and 2.22 cm/mmol, respectively [12]. As
shown in Fig. 3, it is observed that the number of Lewis acid sites of
XGa-BEA increases with increasing Ga loading. On the contrary, the
number of Brönsted acid decreases with increasing Ga loading. When
the acidity of XGa-BEA is compared with MA, as expected, only the
Lewis acid is observed in the MA sample (Fig. 3a).
1
The calculated amount of Brönsted and Lewis acid sites after
thermodesorption of pyridine for samples with different wt.% of Ga
loadings and the effects of Brönsted acid in gallium-zeolite beta to
the conversion of resorcinol are shown in Fig. 4. An interesting effect
of Lewis and Brönsted acids to the alkylation of resorcinol is
demonstrated in this figure. The maximum conversion of resorcinol
to 4-tert-butyl resorcinol and 4,6-di tert-butyl resorcinol by using
10%Ga-BEA is observed when the amount of Lewis and Brönsted acid
is one to one ratio. This includes the amount of Brönsted and Lewis
acids of MA and sulfuric acid which are also considered in the
calculation.
From Fig. 3, it was also proposed that the presence of Brönsted
acid is more dominant than Lewis acid at a low amount of Ga loading
Table 1 summarized the catalytic alkylation of resorcinol over
XGa-BEA, MA and sulfuric acid. All Ga impregnated samples were
(
ca. 3 wt.% Ga). However, the amount of Lewis acid is significantly
found active as a catalyst in the Friedel–Crafts alkylation of
higher than Brönsted acid when the amount of Ga is 25 wt.% Ga. The
increase in Lewis acid sites in the samples is suggested to originate
resorcinol, giving 4-tert-butyl resorcinol and 4, 6-di-tert-butyl
resorcinol as major and minor products, respectively. Interestingly,
as shown in Fig. 3 and Table 1, MA and sulphuric acid which
possess 100% of Lewis and Brönsted acids, respectively, did not
provide significant activities in the alkylation of resorcinol with
MTBE while 10%Ga-BEA possessing both Brönsted and Lewis acids
showed high catalytic activity. These results indicated the possibility
of synergism between Brönsted and Lewis acids in the Friedel–
Crafts alkylation of resorcinol.
from the agglomeration of Ga
11].
2 3
O particles, creating Lewis acid sites
[
1
1
1
40
20
00
In order to confirm the importance of synergism between Lewis
and Brönsted sites, we compared the activity of MA with MA in a
solution containing H
that the catalytic activity of the reaction system containing MA and
liquid H SO (entry 7 in Table 1) is similar to those observed for the
SO homogeneous catalyst (entry 6 in Table 1). Although H SO has
2 4
SO (see entries 1 and 7 in Table 1). It shows
2
4
H
2
4
2
4
5
0
0
a stronger acid strength than those of zeolite beta, HZSM-5 and HY
zeolite [13,14], the yield of products from the Friedel–Crafts alkylation
of resorcinol using H SO as a catalyst is lower than that of XGa-BEA
2 4
indicating the occurrence of the synergism between Brönsted and
Lewis acids in Friedel–Crafts alkylation of resorcinol. Based on these
results, one suggests that the presence of only Brönsted acid in the
liquid form, even though with a high acid strength, is not suitable for
catalyzing the alkylation of resorcinol with MTBE. In our previous
0
5
10
15
20
25
%
Ga loading
1
1
1
40
20
00
paper, H
alumina (6%H
2
SO
4
was heterogenized on the surface of mesoporous
SO /MA) [7]. The catalytic activity of 6%H SO /MA is
2
4
2
4
5
0
0
Table 1
Catalytic alkylation of resorcinol to 4-tert butyl resorcinol and 4,6-di tert-butyl
resorcinol
a
.
0
5
10
%
15
20
25
Entry Catalysts
Conversion/% Product Selectivity/%
yield/
Ratio of
Lewis acid
Ga loading
4-tert
4,6-di
mmol
to Brönsted
butyl
tert-butyl
acid b
resorcinol resorcinol
80
60
40
20
0
1
0%Ga-BEA
1
2
3
4
5
6
MA
0
0
0
97.4
95.8
100
100
96.0
0
0
0.5
0.7
1.0
3%Ga-BEA
8%Ga-BEA
10%Ga-BEA
38.0
54.4
59.1
32.2
6.5
15.6
21.7
23.6
12.9
2.6
2.6
4.2
0
0
4.0
8
%Ga-BEA
25%Ga-BEA
1.5
3
%Ga-BEA
c
H
2
SO
4
contain only
Brönsted acid
1.0
d
e
7
8
MA + H
6%H SO
2
SO
4
6.0
/MA 11.0
2.4
4.4
96.0
80.0
4.0
20.0
2
4
1.5
2
5%Ga-BEA
a
All reactions were carried out at 80 °C for 8 h with resorcinol (40 mmol), MTBE
(
60 mmol) and catalyst (0.2 g) with vigorous stirring.
H
2
SO
4
MA
b
The ratio of Lewis acid to Brönsted acid is calculated by using the peak area of peaks
−1
−1
at wavenumber of 1540 cm
respectively (see Fig. 3). For calculation of the pyridine concentrations adsorbed on
and 1450 cm
for Brönsted and Lewis acids,
0
25
50
75
100
Amount of Lewisacidper total amounf of
Brönsted and Lewis acids / %
Brönsted site and Lewis site, the values of integrated molar extinction coefficients
chosen are 1.67 and 2.22 cm/mmol, respectively [12].
c
The amount of H
2
SO
The MA in a solution containing H
entries 1 and 6, respectively.
4
is 25 μmol.
d
Fig. 4. Calculated amount of Brönsted and Lewis acid sites after thermodesorption of
pyridine for samples with different wt.% of Ga loadings and the effects of Lewis and
Brönsted acids in gallium-zeolite base to the conversion of resorcinol.
2 4 2 4
SO . The amount of MA and H SO is similar as
e
The catalyst is the same as those reported in our previous publication [7].

H. Nur et al. / Catalysis Communications 12 (2011) 822–825
825
In order to clarify the effect of the pore structure of zeolite beta in
Friedel–Crafts alkylation of resorcinol, a comparison has been made
with mesoporous alumina loaded with sulphuric acid (entry 8 in
Table 1) [7]. Although the amount of Brönsted and Lewis acid sites of
6
%H
2
SO
4
/MA is similar to those of 25%Ga-BEA, the catalytic activity of
SO /MA (see entries 5 and 8 in
2
5%Ga-BEA is higher than that of 6%H
2
4
Table 1). This suggests that the pore structure of zeolite beta plays a
role in the catalytic reaction.
Based on the results obtained, the mechanism of alkylation of
resorcinol with MTBE is proposed (see Fig. 5). MTBE and resorcinol
preferentially adsorbed on Brönsted and Lewis acid sites, respectively.
The activation of MTBE is initiated by hydrogen bonding between
Brönsted hydroxyl proton and oxygen of MTBE. It leads to the
releasing of carbocation by MTBE. The formation of carbocation from
MTBE is necessary in order to initiate the alkylation. The carbocation
can only form if the MTBE is adsorbed on Brönsted acid sites. On the
Lewis site, high electronegativity of oxygen of resorcinol coordinated
weakly with Lewis acid site. The proposed mechanism clearly shows
that both Lewis and Brönsted acidities play a synergetic role in this
reaction.
Acknowledgements
We gratefully acknowledge the funding from The Ministry of
Science Technology and Innovation Malaysia (MOSTI), under Science
Fund grant no. 03-01-06-SF0755 and The Ministry of Higher
Education Malaysia (MOHE), under Fundamental Research Grant
Scheme.
References
[
[
[
1] S. Narayanan, K.V.V.S.B.S.R. Murthy, Appl. Catal., A 213 (2001) 273.
2] K.G. Chandra, M.M. Sharma, Catal. Lett. 19 (1993) 309.
3] G. Sartori, R. Maggi, Advances in Friedel-Crafts Acylation Reactions: Catalytic and
Green Processes, CRC Press, 2009.
[
[
4] N.E. Poh, H. Nur, M.N.M. Muhid, H. Hamdan, Catal. Today 114 (2006) 257.
5] J.M. Thomas, W.J. Thomas, Principles and Practice of Heterogeneous Catalysis,
Wiley-VCH, 1997.
[
6] B.M. Devassy, S.B. Halligudi, S.P. Elangovan, S. Ernst, M. Hartmann, F. Lefebvrec,
J. Mol. Catal. A: Chem. 221 (2004) 113.
[
[
7] S. Chandren, Z. Ramli, H. Nur, Int. J. Chem. Reactor Eng. 8 (2010) A40.
8] V.R. Choudhary, S.K. Jana, N.S. Patil, S.K. Bhargava, Microporous Mesoporous
Mater. 57 (2003) 21.
[
9] M. Niwa, S. Nishikawa, N. Katada, Microporous Mesoporous Mater. 82 (2005)
105–112.
[
[
10] A. Corma, Chem. Rev. 95 (1995) 559.
11] K. Okumura, K. Nishigaki, M. Niwa, Microporous Mesoporous Mater. 44–45
Fig. 5. Proposed mechanism of the alkylation of resorcinol with MTBE.
(2001) 509.
[
[
12] C.A. Emeis, J. Catal. 141 (1993) 347.
13] S.G. Hegde, R. Kumar, R.N. Bhat, P. Ratnasamy, Zeolites 9 (1989) 2317.
much higher than that of H
2
4
SO . This also implies that the synergism
[14] J.F. Haw, Phys. Chem. Chem. Phys. 4 (2002) 5437.
between Lewis and Brönsted acid sites take place heterogeneously on
the surface of solid catalysts in the Friedel–Crafts alkylation of
resorcinol.