Nature of the acid sites in the metal triflates immobilized in SBA-15 and their role in the Friedel-Crafts acylation of naphthalene

Article abstract of DOI:10.1016/j.apcata.2009.10.021

The Zn-triflate molecules loaded (5-30 mol%) in mesoporous SBA-15 silicate exhibited considerably higher catalytic activity for liquid-phase Friedel-Crafts (FC) acylation of naphthalene with p-toluoyl chloride, as compared to the corresponding triflates o

Full text of DOI:10.1016/j.apcata.2009.10.021

Applied Catalysis A: General 372 (2010) 130–137
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Nature of the acid sites in the metal triflates immobilized in SBA-15 and their role
in the Friedel–Crafts acylation of naphthalene
1,
S. Selvakumar, Narendra M. Gupta *, A.P. Singh **
Inorganic and Catalysis Division, National Chemical Laboratory, Pune 411008, India
A R T I C L E I N F O
A B S T R A C T
Article history:
The Zn-triflate molecules loaded (5–30 mol%) in mesoporous SBA-15 silicate exhibited considerably
higher catalytic activity for liquid-phase Friedel–Crafts (FC) acylation of naphthalene with p-toluoyl
chloride, as compared to the corresponding triflates of La, Ce and Y. In situ FTIR studies revealed that the
triflate molecules occluded in the channels of SBA-15 may cause severe perturbation of surface hydroxyl
groups, without undergoing an electronic binding. The metal cations of the triflate molecules, on the
other hand, participated in direct bonding of the reactant molecules and also helped in the formation of
some Br o¨ nsted type surface acid sites. This article discusses the role of the acid sites generated on
Received 28 June 2009
Received in revised form 10 October 2009
Accepted 13 October 2009
Available online 21 October 2009
Keywords:
Acylation
occlusion of metal triflates in SBA-15, as monitored by using NH
3
-TPD and pyridine-IR spectroscopy, in
Metal triflate
Mesoporous silica
Naphthalene
the Friedel–Crafts acylation of naphthalene.
ß 2009 Elsevier B.V. All rights reserved.
1
. Introduction
[9]. There are, however, very few reports on the catalytic properties
of metal triflates supported on the ordered mesoporous silica. The
objective of the present study was to incorporate several metal
triflates in the pores of mesoporous SBA-15. The mesoporous
molecular sieve SBA-15 was chosen because of its better thermal
stability and its unique pore topology comprising of the
hexagonally arranged parallel mesopore channels [10,11]. Such
pore structures are known to serve as ideal hosts where the
mesopore channels assist in the transport of reactants or products
without much diffusional resistance, and at the same time they
provide a large surface area to enhance the number of the reaction
sites. In this paper, we describe the catalytic properties of different
Metal triflates and other triflate derivatives are known to serve
as efficient homogeneous catalysts in the Lewis acid-catalyzed
reactions for organic synthesis [1]. In addition to strong Lewis
acidity, these compounds are found to exhibit a reasonably good
tolerance towards water. A large number of metal triflate catalysts
have therefore been reported for the Friedel–Crafts alkylation and
acylation reactions. Olah et al. reported that the triflates of boron,
aluminium, and gallium serve as very efficient Friedel–Crafts
catalysts for the alkylation, isomerization, and acylation reactions
[
3
2]. Tsuchimoto et al. found that Sc(OTf) was the most efficient
catalyst among the rare earth metal triflates screened for the
benzylation of benzene with benzyl alcohol [3]. Several studies in
the last few years have reported on the heterogenization of triflic
acid, in order to facilitate the separation of the catalyst from
reaction medium and also to enhance the catalyst efficiency by
providing a higher dispersion [4–8]. Accordingly, certain silica-
supported triflates and triflic acid are found to be active and
selective catalysts for certain organic synthesis reactions, such as:
3 4 3 2
metal triflates, viz. La(OTf) , Ce(OTf) , Y(OTf) , and Zn(OTf) ,
supported on SBA-15 for the acylation of naphthalene using p-
toluoyl chloride as acylating agent to produce naphthalen-2-yl(p-
tolyl)methanone (2-acyl naphthalene). These compounds find
application as a dye intermediate and also in the photoisomeriza-
tion of cis- and trans-stilbene. In situ IR studies of pyridine
adsorption could monitor the triflate-induced acid sites respon-
sible for the FC reaction mentioned above.
rearrangement of
a-pinene oxide to campholenic aldehyde [6],
polymerization and depolymerization of cyclic ethers [5], Friedel–
2. Experimental
Crafts reactions [7,8], and alkylation of isobutane with n-butene
2.1. Materials
The precursor chemicals, i.e. Pluronic 123 (P123, average Mol.
Wt = 5800), p-toluoyl chloride (Aldrich), tetraethylorthosilicate
Merck), metal triflates (Acros Organics) and naphthalene (S.D. fine
*
Corresponding author. Tel.: +91 20 25902008.
*
* Corresponding author. Tel.: +91 20 25902497; fax: +912025902633.
(
E-mail addresses: nm.gupta@ncl.res.in (N.M. Gupta), ap.singh@ncl.res.in
(
A.P. Singh).
chemical) were of research grade and were utilized without further
purification.
1
Emeritus Scientist, CSIR.
0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2009.10.021

S. Selvakumar et al. / Applied Catalysis A: General 372 (2010) 130–137
131
2.2. Synthesis of siliceous SBA-15
cell, fitted with water-cooled CaF
2
windows and described
previously in detail, was utilized for this purpose [12]. Self-
supporting wafers (ꢂ50 mg) of 25 mm diameter, placed in a
sample holder block, were in direct contact with a chromel–alumel
thermocouple. For acidity measurements, the samples were
heated in situ for 8–10 h at 550–575 K under vacuum
The synthesis of mesoporous SBA-15 was carried out hydro-
thermally under the autogeneous pressure of an autoclave. The
polymer surfactant P123 was used as a template and hydrochloric
acid served as a mineralizer. The following was the gel composi-
tion.
ꢁ3
(ꢂ1 ꢃ 10 Torr) and then exposed at 400 K to multiple doses of
pyridine (ꢂ1.0 mol each) for a saturation coverage. A gas mixture
m
0
:043 TEOS : 4:4 g P123 Mavg
5800 ½EO20-PO70-EO20ꢀ : 8:33 H
comprising of nitrogen + pyridine vapor was utilized for this
purpose. The gas pressure in the IR cell was monitored with the
help of a digital capacitance manometer. IR spectra were plotted at
400 K after equilibrating each sample for 15–20 min subsequent to
each pulse injection. Spectra were also recorded at room
temperature after post-exposure cooling of the sample to room
temperature, followed by pumping. A total of 300 scans were co-
¼
2
O : 0:24 HCl
Typically, 4.4 g of tri block co-polymer dispersed in 30 g of
distilled water was stirred for 1.5 h. To the resultant solution, 120 g
of 2 M HCl was added under stirring and the stirring was continued
for 2 h. Finally, 9 g of TEOS was added drop wise and the mixture
was maintained at 40 8C for 24 h with continuous stirring. The
mass was submitted to a hydrothermal treatment (100 8C, 48 h)
under static condition. The precipitate was filtered, washed with
distilled water, dried in an oven (90 8C, 12 h) and then calcined in
air (500 8C, 6 h) to remove the template completely.
ꢁ1
added for plotting of each spectrum at a resolution of 4 cm . The
numbers given in the parentheses in some of the IR spectra
represent the relative absorbance values for comparison.
The catalysts were prepared by soaking each calcined SBA-15
sample in an acetonitrile solution of a metal triflate powder,
accompanied with continuous stirring for 8–10 h. The excess of
acetonitrile was evaporated to dryness under reduced pressure,
followed by complete drying at 120 8C. The functionalized samples
are designated in the text as Zn-Tf (30)/SBA, where the number in
the parentheses represents the triflate loading in mol%.
2.5. Friedel–Crafts (FC) acylation reaction of naphthalene with p-
toluoyl chloride
Naphthalene and p-toluoyl chloride were used without further
purification. The catalyst was activated at 100 8C for 2 h and the
liquid-phase FC reaction was performed in batch mode using a
50 ml round-bottom flask fitted with a condenser. The reaction
vessel was maintained at a temperature between 80 and 160 8C by
using an oil bath. In a typical run, a mixture of naphthalene
2.3. Characterization
(
10 mmol), p-toluoyl chloride (10 mmol), nitrobenzene (5 ml) and
The powder X-ray diffraction patterns were recorded by using a
activated catalyst (0.1 g), was magnetically stirred and heated to
attain the desired reaction temperature. The product samples were
withdrawn at regular intervals of time and analyzed periodically
on a gas chromatograph (Agilent 6890N) equipped with a flame
X’Pert Pro (M/s Panalytical) diffractometer using a Ni filter and
CuK radiation and a proportional counter as detector. A divergent
a
slit of 1/88 in the primary optics and an anti-scatter slit of 1/48 in
the secondary optics were employed to measure the data in the
ionization detector and a capillary column (5 mm thick cross-
low angle region. The XRD patterns were recorded in the 2
u
range
linked methyl silicone gum, 0.2 mm ꢃ 50 m long). The main
between 0.58 and 58 at a scan rate of 18/min. N adsorption
2
product, naphthalen-2-yl(p-tolyl)methanone was separated by
1
experiments were conducted at 77 K using a NOVA 1200 (Quanta
chrome) apparatus for analyzing the surface area (BET) and pore-
size distribution in the synthesized samples. A pore-size distribu-
tion was obtained by using the BJH pore analysis applied to the
desorption branch of the nitrogen adsorption/desorption iso-
therms. FTIR spectra of solid samples were recorded in the range of
column chromatography and confirmed by GC–MS, H NMR and
1
3
1
C NMR analyse, H NMR (CDCl
3
, 200 MHz)
d
2.42 (s, 3H), 7.23–
21.71, 124.34,
1
3
8.25 (m, 11 H) ppm. C NMR (CDCl , 50 MHz)
3
d
125.7, 126.36, 127.08, 127.31, 128.33, 129.14, 129.17, 130.23,
130.56, 130.92, 144.20, 144.56, 197.75.
ꢁ
1
4
000–400 cm to monitor the nature of the triflate bonding with
3. Results
the host matrix. Thin sample wafers (ca. 20 mol% of catalyst mixed
with KBr powder) were used for this purpose after proper drying
and the spectra were recorded in transmission mode using a
Shimadzu model-8201 spectrophotometer. Thermo gravimetric
analyses (TGA and DTA) were performed using a Diamond TG/DTA
instrument at a heating rate of 10 8C/min under airflow. Experi-
ments on the saturation adsorption of ammonia and its subsequent
3.1. X-ray diffraction (XRD)
Fig. 1 shows the representative XRD patterns of calcined SBA-
15, without (curve (a)) and after functionalization with 30 mol% of
different metal triflates (curves (b–e)). The triflate containing
samples exhibited a XRD pattern similar to that of SBA-15, where
three well-resolved diffraction peaks due to {1 0 0}, {1 1 0}, and
{2 0 0} reflections were observed. These reflections correspond to
the two dimensional hexagonal p6mm symmetry, a characteristic
of well defined SBA-15 mesoporous material. The XRD patterns in
Fig. 1(b–e) thus confirm the integrity of the characteristic
hexagonal structure of SBA-15 in the triflate loaded samples. A
progressive decrease observed in the intensity of the XRD lines,
however, reveals a partial loss of long-range order of SBA-15 due to
the incorporation of triflate molecules. The values of the unit cell
3
temperature-programmed desorption (TPD-NH ) were performed
on a Micromeritics Autochem 2910 instrument. About 0.1 g of a
fresh sample was placed in a U-shaped, flow-through, quartz micro
reactor for each experiment. The catalyst was activated at 220 8C
for 2 h under helium flow (30 ml/min) and then cooled to 100 8C
before being exposed to ammonia. Each sample was flushed again
in He for 1 h to remove any physisorbed ammonia, and the
desorption profile was then recorded by increasing the sample
temperature from 100 to 900 8C at a ramp rate of 10 8C/min.
0
parameter (a ) of SBA-15 and of the corresponding triflate-
2.4. Pyridine-IR studies
modified samples are given in Table 1.
The nature of the acid sites in different samples was monitored
3.2. Nitrogen sorption studies
by recording the transmission-mode IR spectra of adsorbed
pyridine vapor. A Thermo Nicolet (model Nexus 870) FTIR
equipped with a high-pressure high-temperature stainless steel
The functionalized SBA-15 samples displayed type-IV iso-
therms with H1 hysteresis, a characteristic feature of the highly

1
32
S. Selvakumar et al. / Applied Catalysis A: General 372 (2010) 130–137
Fig. 2. Ammonia TPD profiles of (a) pure SBA-15, (b) Zn-Tr (30)/SBA, (c) Ce-Tr (30)/
SBA, (d) La-Tr (30)/SBA and (e) Y-Tr (30)/SBA.
Fig. 1. Low angle powder-XRD patterns of metal triflate loaded SBA-15. (b) Zn-Tr
30)/SBA, (c) Y-Tr (30)/SBA, (d) La-Tr (30)/SBA and (e) Ce-Tr (30)/SBA. Curve (a) is a
(
comparative XRD pattern of pure SBA-15.
moisture (figure not shown), it is imperative that the triflate
molecules are immobilized firmly in the channels of SBA-15 and
are thermally stable at the temperatures up to 400 8C.
ordered mesoporous materials. The BET surface area (SBET) and
total pore volume (Vtotal) of SBA-15 was found to decrease
progressively with increasing loading of metal triflate, as shown in
the data given in Table 1. Thus, in a representative case, the loading
of 30 mol% zinc triflate in SBA-15 resulted in the decrease of
surface area from 684.3 to 550.3 m g . This trend may be
attributed to the ingress of triflate molecules in the pore structure
and also to the partial collapse of host structure, as revealed by the
XRD results in Fig. 1.
3.4. Acidity measurement
Curves (b–e) in Fig. 2 exhibit the temperature-programmed
desorption (TPD) profiles of ammonia, recorded on SBA-15
samples containing different metal triflates at a uniform loading
of 30 mol%. The pristine SBA-15 showed negligibly small adsorp-
tion of ammonia and the comparative TPD profile is shown in curve
2
ꢁ1
(
a). The intensity of the TPD profiles in curves (b–e) was found to
3
.3. Thermal analysis
vary progressively with triflate loading. The density of acid sites in
different catalyst samples, as computed from the area under each
TPD profile, is included in Table 1. As shown in Table 1, the density
of the acid sites is considerably higher in zinc triflate loaded SBA-
15, as compared to the samples containing rare earth triflates.
Furthermore, the TPD results in Fig. 2 display the presence of at
least three distinct ammonia adsorption sites. The high tempera-
ture of the desorption peaks (>325 8C) in this figure is indicative of
the strong acidic character of the synthesized catalysts. These
results also reveal that the acid strength distribution may depend
strongly on the cation associated with a triflate molecule. For
The thermal stability of the catalysts was checked by thermo
gravimetric analysis in air atmosphere from 30 to 1000 8C with a
temperature ramp of 10 8C/min. That pure zinc triflate decomposes
at ꢂ540 8C, but its decomposition in the case of SBA-supported
samples occurs at a lower temperature (ꢂ435 8C). A similar
lowering of TG peak temperature was observed in the case of SBA-
1
5 samples containing other metal triflates. This may be ascribed
to the reaction of highly dispersed Zn(OTf) with the surface
hydroxyl groups on SBA-15 at elevated temperature to produce
ZnF and ZnSO [13]. A similar observation has been reported in the
2
2
4
instance, the Zn-triflate containing sample exhibited a distinct NH
3
earlier studies on the triflic acid supported over mesoporous
materials [4,6]. The –OH groups on silica are found to promote such
thermal reactions of triflic acid [14]. Since no measurable weight
loss is observed at the temperatures lower than 400 8C, besides a
small peak at about 100 8C due to a small amount of adsorbed
TPD peak at a higher temperature of ꢂ450 8C (curve (b)), as
compared to the samples containing similar loading of rare earth
metal triflates (Fig. 2, curves (c–e)). These results conform with the
pyridine-IR results described below, where the presence of distinct
Lewis acid sites in Zn-Tr–SBA is demonstrated.
Table 1
Physico-chemical properties of metal triflate-grafted SBA-15 catalysts.
Sample
d
1 0 0 ( A˚ )
a
0
( A˚ )a
Total acidity (mmol NH
3
/g)
b
Pore diameter (A)
˚
Pore volume (cm g
3
ꢁ1
)
S
2 ꢁ1
BET (m g )
Calcined SBA-15
Zn-Tr (10)/SBA
Zn-Tr (30)/SBA
Ce-Tr (30)/SBA
Y-Tr (30)/SBA
La-Tr (30)/SBA
93.85
109.5
115.2
116.4
116.4
116.4
116.4
0.01
2.85
9.13
6.96
6.20
7.01
84.3
81.0
75.0
–
0.94
0.90
0.74
–
684.3
628.3
550.3
575.3
525.6
518.6
100.31
101.31
101.31
101.31
101.31
–
–
–
–
a
Unit cell parameter (a
0
) = d1 0 0 ꢃ 2/H3.
method.
b
Measured by using TPD-NH
3

S. Selvakumar et al. / Applied Catalysis A: General 372 (2010) 130–137
133
Table 2
ꢁ
1
Assignment of IR band frequency (cm ) of zinc triflate, without and after grafting
in SBA-15.
ꢁ1 a
)
IR band frequency (cm
Assignment
Zinc triflate
Zn-Tr (30)/SBA
5
5
6
7
20
76
45
68
519
578
650
–
d
d
d
d
n
n
n
n
n
as (SO
3
)
as (CF
3
)
s
s
3
(SO )
(CF
(SO
3
) +
n
s
(CS)
1
1
1
1
1
033
180
245
270
346
1035
1182
–
s
3
)
3
as (CF )
s
3
(CF )
1270
–
as (SO
as (SO
3
)
)
3
Notations:
n, bond stretch; d, group angle deformation; s, symmetric stretch; as,
asymmetric stretch.
a
Assignments as per the references [17,18].
3
3
.5. IR spectroscopy results
.5.1. FTIR Spectra of triflate loaded SBA-15
The mid-IR region spectrum of a representative Zn-Tr (30)/SBA
catalyst, recorded after compensating for the framework vibra-
tional bands of pure SBA-15 in this region with pure zinc triflate, is
given in Table 2. The comparison of the IR spectra reveals clearly
that the frequency of different vibrational bands of zinc triflate
molecules remained mostly unchanged on immobilization in
mesoporous SBA-15. Significant changes are, however, noticeable
in the relative intensity of different stretching and deformation
ꢁ1
Fig. 3. The relative intensity (absorbance) of 3740 cm band as a function of: curve
a) loading of zinc triflate in SBA-15, (b) pyridine adsorption over SBA-15, and (c)
(
pyridine adsorption over Zn-Tr (10)/SBA.
the intensity increasing systematically with the increase in the
amount of adsorbed pyridine. A similar trend has been reported
earlier for the adsorption of pyridine over MCM-41 [15]. Fig. 4
displays the IR spectra of Zn-Tr (10)/SBA samples for the dosing of
3 3
mode vibrational bands arising due to SO and CF groups of the
triflate ions. These results are indicative of certain perturbations
experienced by the occluded triflate molecules, as discussed later
in the text.
0–7
the 3480 cm
mmol of pyridine vapor. We observed here that the growth of
ꢁ1
band matches well with the decrease in the
ꢁ1
intensity of the 3740 cm band. The intensity of this new band at
ꢂ3480 cmꢁ was however quite low in the case of the pyridine
adsorption over triflate-free SBA-15 and it was found to merge
1
3
.5.2. Effect of triflate and pyridine loading on hydroxyl region bands
of SBA-15
The IR spectrum of calcined SBA-15, recorded at 400 K after
evacuation of the sample at 525 K for 8 h, exhibited a prominent
ꢁ1
with the broad IR band in 3700–3600 cm region. These results
indicate that the perturbation of –OH groups in SBA-15 is more
severe on occlusion of triflate molecules alone as compared to
when pyridine vapor was adsorbed. The data presented in Fig. 3(c)
ꢁ1
sharp band at 3740 cm
absorption band in the 3650–3450 cm
reported, the 3740 cm band arises due to the non-acidic terminal
silanol stretching vibrations while the lower frequency absorption
band is assigned to the hydrogen-bonded internal SiOH groups in
mesoporous silicates [15].
in addition to a weak and broad
ꢁ1
region. As is well
ꢁ1
The intensity of both these n(OH) vibrational bands was found
to decrease progressively when the increasing amounts of a
particular metal triflate were loaded into SBA-15. No measurable
shift was observed in the frequency of these n(OH) bands in all the
samples, irrespective of the nature or the amount of triflate
introduced. Plot (a) in Fig. 3 shows the progressive variation in the
ꢁ1
intensity of 3740 cm
n(OH) band as a function of zinc triflate
loading. A decrease by a factor of ꢂ70% in the intensity of
ꢁ1
3
740 cm band is noticeable in Fig. 3(a) at the saturation loading
of zinc triflate. A much smaller (ꢂ25%) decrease in the intensity of
this (OH) band was observed for adsorption of pyridine vapor in
n
SBA-15, even after saturation coverage. These results are presented
in Fig. 3(b). Again, no shift was observed in the frequency of the
n(OH) bands. Interesting results were observed when the pyridine
vapor was dosed over triflate loaded SBA-15 under the identical
condition. The representative results on the relative intensity of
ꢁ1
3
740 cm band for different loadings of pyridine in Zn-Tr (10)/
SBA samples are shown in curve (c) of Fig. 3. We observe in this
case that, even though the intensity of (OH) band has attained a
n
saturation after the loading of triflate alone (cf. Fig. 3(a)), a further
decrease in the intensity took place as a result of subsequent
pyridine loading (Fig. 3(c)). At the same time, the adsorption of
Fig. 4. The relative intensity of O–H stretching bands of Zn-Tr (10)/SBA on exposure
to different amounts of pyridine. Amount of pyridine in mmol: Curve (a) nil, (b) 1, (c)
3 and (d) 7. The numbers given in the parentheses represent the relative absorbance
values.
pyridine over Zn-Tr/SBA gave rise to a strong band at ꢂ3480 cmꢁ1
,

1
34
S. Selvakumar et al. / Applied Catalysis A: General 372 (2010) 130–137
further reveal that the mode of pyridine binding in the pores of
SBA-15 is influenced by the presence of the triflate molecules.
These aspects are discussed later in more detail.
the individual bands was found to increase progressively with the
increasing amount of pyridine adsorbed.
Furthermore, the frequency and the intensity of various IR
bands shown in [Fig. 5(A), (e)] were found to vary considerably
when different metal triflates were dispersed in SBA-15. Curves (b–
e) in Fig. 5B exhibit IR spectra of SBA-15 samples, loaded with
30 mol% of different metal triflates and exposed uniformly to
3.5.3. IR bands of adsorbed pyridine
Exposure of SBA-15 to pyridine vapor gave rise to two
ꢁ
1
symmetrical broad IR bands at 1596 and 1446 cm . Curve (c)
in Fig. 5(A) exhibits a typical IR spectrum of calcined SBA-15
5
m
mol pyridine. Curve (a) is again a comparative IR spectrum of
sample, recorded at 400 K after an exposure to 5
For comparison, curves (a and b) in Fig. 5(A) show the IR bands of
vapor and liquid-phase pyridine in this region. Whereas spectrum
mmol pyridine.
pyridine adsorbed over pure SBA-15. It is important to notice that
ꢁ1
instead of a pair of weak bands at 1596 and 1446 cm in Fig. 5(B)
ꢁ1
(b–d), a new pair of IR bands is observed at 1614 and 1454 cm in
the case of pyridine adsorption over zinc triflate containing SBA-
(a) was recorded by introducing pyridine vapor + nitrogen in the
ꢁ1
gas cell without including a catalyst sample, spectrum (b) was
plotted by placing a drop of pyridine in between the two KBr
pellets.
15, Fig. 5(B) (e). Moreover, the intensity of the 1546 cm band is
found to be much smaller in the IR spectrum in Fig. 5(B) (e) as
compared to that in Fig. 5(B) (b–d). A weak and overlapping IR band
ꢁ
1
ꢁ1
The intensity of both the IR bands at 1596 and 1446 cm in
Fig. 5(A), (c) followed a linear relationship with the increase in the
amount of pyridine adsorbed, and no saturation was observed. A
plot showing typical loading dependent variation in the intensity
at ꢂ1618 cm is also noticeable (figure not shown). The IR bands
shown in Fig. 5(B) were found to be fairly stable when the samples
exposed to pyridine vapor at 425 K were cooled to room
temperature followed by 15 min pumping.
ꢁ1
of 1596 cm band for pure SBA-15 showed progressive growth of
the pyridine-IR band (figure not shown), which is in contrast to the
3.6. Catalytic reactions
ꢁ1
decrease observed in the intensity of 3740 cm –OH band, which
showed a saturation effect even at 4 mol pyridine loading
Fig. 3(b)]. This indicates clearly that the decrease in the intensity
m
3.6.1. Effect of triflate loading and reaction temperature
[
The host material SBA-15, with no triflate loading, showed no
measurable catalytic activity for acylation of naphthalene under
the reaction conditions of this study. In the case of triflate-
functionalized SBA-15, the conversion of naphthalene was found to
increase progressively with the increase in the triflate loading.
Also, the best results were obtained by using the equi-molar
mixture of the reactants, i.e. naphthalene + p-toluoyl chloride (p-
TC). The conversion of naphthalene was found to decrease
drastically for smaller amounts of p-TC taken for a reaction. In
the representative experiments conducted at 140 8C over zinc
triflate loaded SBA-15 and using an equi-molar mixture of
naphthalene + p-toluoyl chloride (p-TC), the overall conversion
of naphthalene was found to increase progressively from ꢂ35 to
55% for the increase in the triflate loading from 5 to 30 mol%. The 2-
acyl naphthalene was the major reaction product (ꢂ92% yield), the
minor product being 1-acyl naphthalene. However, similar product
selectivity was obtained by using the catalyst samples containing
different loadings of a particular triflate.
of
n(OH) bands, as seen in Fig. 3(b), may not arise due to any direct
bonding of pyridine molecules to hydroxy groups.
Several new vibrational bands were detected when a triflate
containing SBA-15 was exposed to pyridine vapor. Curve (d) in
Fig. 5(A) presents the IR spectrum of Zn-Tr (5)/SBA sample,
recorded on exposure to 5
three prominent vibrational bands at 1614, 1491 and 1454 cm
in addition to some low intensity IR bands with the frequency
maxima at 1640, 1578 and 1547 cm . The intensity of these bands
was found to increase considerably for the samples containing
higher amounts of zinc triflate. For instance, curve (e) in Fig. 5(A)
mmol pyridine. We find in this figure
ꢁ1
,
ꢁ
1
shows the IR spectrum of Zn-Tr (10)/SBA on exposure to 5
mmol
pyridine. The numbers given in the parentheses in this figure
represent the comparative absorbance values of different IR bands.
The intensity values of these IR bands, however, reached saturation
for the triflate loadings of 20 mol% and above. At the same time, in
case of each of the zinc triflate containing samples, the intensity of
Fig. 5. (A) Comparative IR Spectra of pyridine (5
mmol) adsorbed at 125 8C over (c) pure SBA-15, (d) Zn-Tr (5)/SBA and (e) Zn-Tr (10)/SBA catalysts. Curves (a and b) show the
characteristic IR spectra of vapor and liquid states of pyridine, respectively. (B) Comparative FTIR spectra of pyridine (5
Tr (30)/SBA, (c) Ce-Tr (30)/SBA, (d) Y-Tr (30)/SBA, and (e) Zn-Tr (30)/SBA.
mmol) adsorbed at 125 8C over (a) pure SBA-15, (b) La-

S. Selvakumar et al. / Applied Catalysis A: General 372 (2010) 130–137
135
The increase in the reaction temperature gave rise to higher
conversions and also in the increase in the rate of FC reaction of
naphthalene with p-toluoyl chloride. Taking Zn-Tr (30)/SBA-15 as a
model catalyst again, the effect of the rise in reaction temperature
on the time-dependent conversion of naphthalene is depicted in
Fig. 6. These results show that the equilibrium conversion of
naphthalene increases from 12.2 to 53.6 wt% with the increase in
reaction temperature from 80 to 160 8C. At the same time, the rate
of the reaction also increased considerably with the increase in
reaction temperature. Thus, while an equilibrium conversion was
achieved in ꢂ24 h at 80 8C (curve (a)), the reaction was complete in
about 3 h time at the temperatures above 120 8C (curves (d and e)).
However, the selectivity for 2-acyl naphthalene remained nearly
unchanged with the increase in reaction temperature.
3.6.2. Acylation of naphthalene over SBA-15 loaded with different
metal triflates
The comparative catalytic activity values of different metal
triflates (Ce, La, Y and Zn, loaded at 30 mol% on SBA-15) for FC
reaction of naphthalene with p-toluoyl chloride are shown in Fig. 7.
The results reported in this figure are for the reaction conducted at
1
40 8C over a period of ca. 9 h, using the reactants in an equi-molar
ratio. As mentioned above, the main reaction product was
naphthalen-2-yl(p-tolyl)methanone. Also, the product distribution
was almost independent of the catalyst used. As seen in Fig. 7, the
Zn-Tr (30)/SBA catalyst showed much higher catalytic activity as
compared to the catalysts containing rare earth triflates, even
though the selectivity for 2-acyl naphthalene production was
almost similar (94–98%). In order to demonstrate a relationship
between the activity and the acidic strength, the number of acid
sites in different catalysts (TPD results) is plotted in Fig. 7. We may
add that all the triflate based catalysts showed remarkable stability
and reproducibility in catalytic activity when utilized for 5–6
consecutive test runs, the activity loss in the process being ca. 5%.
Fig. 7. Comparative catalytic activity for the naphthalene acylation reaction carried
out using different metal triflate loaded (30 mol%) SBA-15 catalysts. Reaction
parameters: catalyst amount 0.1 g; naphthalene = 10 mmol; p-TC = 10 mmol;
nitrobenzene = 5 ml; reaction temperature = 140 8C; reaction time = 9 h.
conversion obtained by using Zn-Tr (30)/SBA, as compared to the
rareearthtriflatecontainingsamples, isreflectedintheconsiderably
higheracidityofthiscatalyst (Table 1). Onthe other hand, the almost
identical activity of the three rare earth-based catalysts is in
consonance with the number of the acid sites present in these
samples. The higher activity of zinc containing samples can be
explained by visualizing the mode in which triflate molecules are
bonded in the channels of the SBA host. The IR spectra assignments
in Table 2 help us in establishing the binding states of a grafted
4
. Discussion
The results presented in Fig. 7 exhibit a direct relationship
between the naphthalene conversion and the density of the acid
sites in a particular catalyst sample. Thus, the almost 5-times higher
triflate molecule. As is well known, the non-coordinated triflate ion
ꢁ
(i.e. CF
3
SO
3
) has a point-symmetry C
as(SO ), with E-symmetry is doubly degenerate,
while the A1 symmetric mode, (SO ), is non-degenerate [16]. Both
3
n
, where the antisymmetric
stretching mode,
n
3
n
s
3
these modes are IR active. The coordination of triflate ion causes the
lowering of the symmetry and this causes the splitting of the
ꢁ
antisymmetricSO
3
stretching mode intotwo components [17]. The
extent of this splitting will depend upon the physical state of the
ꢁ1
molecules. The overlappingIRbandsat1270and 1340 cm arethus
assigned to the antisymmetric SO stretching, following the
assignments reported in the previous studies [16–18]. Based on
3
1
8
the studies conducted on O-labeled triflates [18], the strong IR
ꢁ1
bands appearing at 1245 and 1180 cm in Table 2 for pure zinc
triflate are assigned to the symmetric and antisymmetric stretching
3
modes of (CF ) groups. The corresponding asymmetric deformation
ꢁ1
band appears at 576 cm . No appreciable shift in these frequencies
is observed when the zinc triflate molecules are grafted on to SBA-
15A (Table 2). No significant electronic interaction is therefore
envisaged between the guest molecules and the surface of the host
silicate. At the same time the relative intensities of the IR bands due
3 3
to (SO ) and (CF ) groups undergo considerable changes. Thus, the
comparison of spectra of pure zinc triflate and of zinc triflate loaded
SBA-15 reveals a drastic decrease in the intensity of the IR bands,
bothsymmetricandantisymmetricmodes, relatedmainly tothe CF
3
groups. These observations point to a significant perturbation of
triflatemoleculesasaresultoftheirocclusioninthechannelsofSBA-
Fig. 6. Conversion of naphthalene over Zn-Tr (30)/SBA catalyst as a function of time
15. The natureof thisperturbationbecomes clear whenwe take onto
and at different reaction temperatures: curve (a) 80, (b) 100, (c) 120, (d) 140 and (e)
account the changes occurring in the stretching vibrations of the
hydroxyl groups.
1
60 8C. Reaction parameters: catalyst amount = 0.1 g; naphthalene = 10 mmol; p-
TC = 10 mmol; nitrobenzene = 5 ml.

1
36
S. Selvakumar et al. / Applied Catalysis A: General 372 (2010) 130–137
ꢁ
1
Fig. 3(a) shows that the intensity of the 3740 cm band, arising
nature of the acid sites in a catalyst sample may depend
considerably on the cation associated with a metal triflate, as
can be seen clearly in Fig. 5(B). A pair of strong IR bands observed at
1614 and 1454 cm in the case of pyridine adsorption over zinc
triflate loaded SBA-15 [Fig. 5(B), (e)] arises due to well reported 8a
duetotheO–HstretchingoftheisolatedsilanolsinSBA-15,decreases
progressively with the increasing loading of the zinc triflate. No shift
was, however, observed in the frequency of this band as a result of
triflate loading (figure not shown). This behavior has been attributed
earliertotheinteractionoftriflatemoleculeswithsurfaceOHgroups;
ꢁ1
n(C–C) and 19b
n(C–C) vibrations of pyridine adsorbed at the Lewis
3 3
both the (SO ) and the (CF ) groups are shown to participate
acid (designated as L1) sites [15,25]. Another pair of bands
ꢁ
1
individually in such interactions [19–22]. Various possible binding
observed at 1640 and 1546 cm in this figure is known to arise
due to the vibrations of pyridine molecules bound at bridge-
ꢁ
modes of triflate ion (CF
SO
3 3
) at the surface silanols, viz.
ꢁ
1
monodentate, bidentate, tridentate and tetradentate, have also been
postulated in an earlier publication [4]. We may, however, point out
that an electronic interaction between the triflate ions with the
surfacesitesofSBA-15islikelytochangethefrequencyofthebonded
guestmolecules,ashasbeendiscussedinourearlierpublication[23].
bonded Br o¨ nsted (B) sites. The band appearing at 1491 cm is
attributed to the adsorption of pyridine at both the Lewis and
Br o¨ nsted acid sites. In the case of SBA-15 samples containing the
triflates of Y, La and Ce, instead of a pair of bands at 1614 and
ꢁ
ꢁ
1
1454 cm
we observe two low intensity bands at 1597 and
1
Similarly, the
n(OH) frequency is expected to increase because of the
1447 cm [Fig. 5(B), (b–d)], similar to the IR bands observed for
pure SBA-15 [Fig. 5(B), (a)]. As discussed in our earlier paper in
detail [25], these bands are attributable to weak Lewis acid sites
(designated as L2), a characteristic feature of the siliceous
mesoporous silicates having a weak acid character. The higher
increased electron density along the hydroxyl group. Therefore, the
spectroscopyresultsofourstudycannotbeascribedtoanyelectronic
interactionbetweenthetriflateionsandthesurfacehydroxylgroups.
Ontheotherhand, theresultsreportedhereareakintothefindingsof
our previous studies on the occlusion of small molecules, such as
methanol, benzene and cyclohexane, in the channels of micro- and
mesoporous silicates [23,24]. Based on the detailed discussion
ꢁ
1
intensity of the 1546 cm
band in curves (b–d) of Fig. 5(B)
indicates that the samples loaded with the triflates of La, Ce and Y
exhibit much higher Br o¨ nsted acidity and at the same time a lower
Lewis acidity, as compared to the zinc triflate containing sample.
The ratio B/L (L1 or L2) follows a trend La (9.5) > Y (4.7) > Ce
(3.0) = Zn (0.3), and this matches well with the catalytic activity
results of Fig. 7.
ꢁ
1
presented in these references, the removal of 3740 cm band on
triflate or pyridine loading can be attributed to the physical
perturbation or the displacement of the –OH groups. This may
facilitate a weak van der Waals type interaction between the two
displaced hydroxyl groups, leading to the progressive removal of
The important role played by the metal cation in an occluded
triflate molecule becomes apparent when we compare the IR bands
in spectra (c–e) in Fig. 5(A) with the corresponding spectra of
pyridine in its vapor (curve (a)) and liquid (curve (b)) forms. In the
case of pyridine adsorption over pure SBA-15 [Fig. 5(A), (c)], the
two observed bands are quite broad and at the same time they lack
any splitting of the bands, a characteristic feature of the IR
spectrum of pyridine vapor [cf. Fig. 5(A), (a)]. Further, the full width
at half maximum (FWHM) of the IR bands seen in Fig. 5(A), (c) is
ꢂ15 cmꢁ , and this lies in between the FWHM values for IR bands
ꢁ
1
3
740 cm bandandthedevelopmentofanewlowerfrequencyband
ꢁ
1
at ꢂ3485 cm (Fig. 4). This is shown graphically in Fig. 8. This mode
of triflate occlusion in SBA-15 would thus enable a metal cation to
serve as a distinct Lewis acid site for the activation of a reactant
molecule, as shown in Fig. 8. The reaction mechanism for the
acylationofnaphthalenecanthusbeexpressedbytworeactionsteps.
The first step consists of an interaction of toluoyl chloride with the
metal triflate to form an acyl cation-zinc triflate activated complex.
This transient complex may promote the formation of some highly
active species, e.g. acylium ion. In the second step the acylium ion
mayreactwiththe naphthalene, givingrisetotheformationof 2-acyl
naphthalene and eventual regeneration of the catalyst.
1
ꢁ
1
ꢁ1
of vapor (ꢂ28 cm ) and liquid (ꢂ4 cm ) phases of pyridine. This
indicates that the pyridine molecules entrapped in SBA-15 may
exist in a non-equilibrium state, in between the vapor and the
liquid form. In this adsorbed state, the pyridine molecules are
likely to retain their rotational motion without any strong bonding
at the SBA sites. On the other hand, a considerable shift is observed
IR results in Fig. 5(A) confirm that the acid density in a catalyst
was directly related to the loading of a metal triflate. Moreover, the
in the frequencies of n8a and n19b C–C bands for the adsorption of
pyridine molecules in Zn-Tr/SBA samples [Fig. 5(A), (d and e)]. In
addition, significant narrowing of the peaks (FWHM ꢂ6 cmꢁ ) is
noticeable in spectra (d) and (e) of Fig. 5(A), as compared to the IR
bands of pyridine vapor [Fig. 5(A), (a)]. This represents an immobile
state of pyridine molecules having no rotational motion. It can thus
be concluded that the triflate metal cations may participate in the
direct binding of a guest molecule, a situation similar to that
presented schematically in Fig. 8. Further, the systematic increase
in the intensity of pyridine bands as a function of increasing triflate
loading in SBA-15 [Fig. 5(A), (d and e)] and the dependence of Lewis
acidity in the functionalized silicate samples on the nature of the
occluded metal cation [Fig. 5(B)] also serve as evidence for the
direct participation of the triflate cation in binding of the pyridine
molecules. Logically, other reactant molecules are likely to
undergo similar binding at the cation sites, as depicted in Fig. 8.
The binding of the pyridine molecules at triflate metal cations may
eventually lead to further perturbation of the surface hydroxyl
groups in the host matrix, thus giving rise to some synergistic
1
changes in the intensity of n(OH) band, as shown in Fig. 3(c). We
surmise that the perturbed –OH groups may give rise to certain
bridge bonded hydroxyl species with the participation of a triflate
metal cation, and these species may be associated with a broad IR
Fig. 8. Schematic presentation of the perturbation of –OH groups caused by the
confinement of a metal triflate molecule in a silicate channel, and the postulated
binding mode of a reactant molecule.
ꢁ
1
band observed at 3485 cm
in Fig. 4. These bridge bonded

S. Selvakumar et al. / Applied Catalysis A: General 372 (2010) 130–137
137
hydroxyl groups may in turn serve as new Br o¨ nsted acid sites, the
presence of which is seen clearly in IR spectra in Fig. 5(A), (d and e).
The very small intensity of 3485 cm band in case of pyridine
adsorption over triflate-free SBA-15, as mentioned above, and the
near absence of the Br o¨ nsted acid sites in Fig. 5(A), (c) are in
conformity with our ideas.
discussion. SK thanks the Council of Scientific and Industrial
Research (CSIR), New Delhi, for a fellowship and NMG thanks the
CSIR for an Emeritus Scientist research grant.
ꢁ
1
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