A catalytic and mechanistic study of the Friedel-Crafts benzoylation of anisole using zeolites in ionic liquids

Article abstract of DOI:10.1016/j.jcat.2004.05.034

The Friedel-Crafts benzoylation of anisole with benzoic anhydride to yield 4-methoxybenzophenone has been carried out in a range of ionic liquids using zeolite catalysts. The rates of reaction were found to be significantly higher using ionic liquids compared with organic solvents. Continuous-flow studies of successful ionic liquid systems indicate that the bulk of the catalysis is due to the formation of an acid via the ion exchange of the cation with the protons of the zeolite. The acid liberated was quantified using both titration experiments and ion-exchange experiments using sodium-exchanged zeolites.

Full text of DOI:10.1016/j.jcat.2004.05.034

Journal of Catalysis 227 (2004) 44–52
www.elsevier.com/locate/jcat
A catalytic and mechanistic study of the Friedel–Crafts benzoylation
of anisole using zeolites in ionic liquids
Christopher Hardacre a,c, Suhas P. Katdare c, David Milroy b,c, Paul Nancarrow b,
David W. Rooney b,c,∗, Jillian M. Thompson c
a
School of Chemistry, Queen’s University Belfast, Belfast, BT9 5AG, N. Ireland, UK
b
School of Chemical Engineering, Queen’s University Belfast, Belfast, BT9 5AG, N. Ireland, UK
c
QUILL Research Centre, Queen’s University Belfast, Belfast, BT9 5AG, N. Ireland, UK
Received 24 May 2004; revised 27 May 2004; accepted 28 May 2004
Available online 20 July 2004
Abstract
The Friedel–Crafts benzoylation of anisole with benzoic anhydride to yield 4-methoxybenzophenone has been carried out in a range of
ionic liquids using zeolite catalysts. The rates of reaction were found to be significantly higher using ionic liquids compared with organic
solvents. Continuous-flow studies of successful ionic liquid systems indicate that the bulk of the catalysis is due to the formation of an acid
via the ion exchange of the cation with the protons of the zeolite. The acid liberated was quantified using both titration experiments and
ion-exchange experiments using sodium-exchanged zeolites.
2004 Elsevier Inc. All rights reserved.
Keywords: Zeolite; Ionic liquids; Friedel–Crafts; Benzoylation; Anisole; Continuous flow; Ion exchange; Deactivation
1. Introduction
product and is recyclable. A number of Lewis acid cata-
lysts have been found to facilitate Friedel–Crafts acylations,
for example, metal triflate [2] and metal triflamide [3] salts
as well as solid acid catalysts such as nafion [4] and zeo-
lites [5]. For reasons including high costs of the metal salts,
low yield, particularly with unactivated aromatic substrates,
and poor recyclability the application of homogeneous cat-
alysts has remained mainly of academic interest. In con-
trast, zeolites are currently used on a large industrial scale,
for example, within the Rhodia process for the acylation of
anisole [6]. Zeolites have been shown to facilitate the acety-
lation and benzoylation of both activated aromatic substrates
such as anisole [7], veratrole [8], and 2-methoxynaphthalene
[9] and unactivated aromatics such as toluene with acid an-
hydrides [10]. In these reactions, the activity and regiose-
lectivity of the zeolite catalyst are dependent on the Si:Al
ratio, crystallite size, and pore size [10,11]. Friedel–Crafts
acylation reactions have also been carried out using zeolites
which have been cation-exchanged with iron, zinc, indium,
aluminum, and lanthanum [12] and more recently zeolites
have been used to support chlorometallate ionic liquids in
alkylation reactions [13].
Friedel–Crafts acylation reactions are of great impor-
tance in the industrial manufacture of aryl ketones, and are
used extensively in the production of pharmaceuticals such
as the nonsteroidal anti-inflammatory drugs Ibuprofen and
Naproxen. Conventionally, these reactions are catalysed by
aluminum trichloride, using an acylating agent such as an
acid chloride in a volatile organic solvent [1]. Due to the
complexation of the ketone product with aluminum trichlo-
ride a stoichiometric excess must be used, which is then
destroyed in the hydrolysis step required for product isola-
tion. The large quantities of metal salt waste formed, as well
as the production of HCl, have aroused much research in the
development of new cleaner technologies, which generate
minimal waste.
A major step toward waste minimisation would be to
develop a truly catalytic process, using an acid anhydride,
where the catalyst does not form a strong complex with the
*
Corresponding author. Fax: +44 28 9038 1753.
E-mail address: d.rooney@qub.ac.uk (D.W. Rooney).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2004.05.034
2004 Elsevier Inc. All rights reserved.

C. Hardacre et al. / Journal of Catalysis 227 (2004) 44–52
45
Table 1
The chemical composition and the textural parameters of the zeolites used
Zeolite
Si:Al
ratio
BET surface area BET pore volume
2
−1
3
−1
(m g
)
(cm
g )
H-USY-29
H-USY-42
H-USY-80
H-Beta
Mordenite
H-ZSM-5
29
42
80
26
87
52
625.79
669.11
618.07
539.87
293.28
283.17
0.396
0.436
0.444
0.532
0.184
0.184
Scheme 1.
Ionic liquids represent a class of solvent, which has the
potential to reduce the use of organic solvents and lower
VOC’s. Friedel–Crafts reactions have been studied exten-
sively in ionic liquids and were one of the first synthetic reac-
tions to be carried out in this media. Acidic organochloroalu-
minate systems were shown to be very efficient catalysts for
the acylation of toluene, benzene, and chlorobenzene [14];
however, as with AlCl3 the complexation of the keto-product
to the ionic liquid necessitated its destruction in the product
isolation step. The development of water and air-stable ionic
liquids has led to a substantial increase in the number of re-
actions possible in these solvents. Here the ability of ionic
liquids to dissolve both the catalyst and the substrate can
be effectively utilised. For example, both Gmouh et al. and
Ross et al. have demonstrated that using metal triflate salts in
ionic liquids allows efficient Friedel–Crafts acylation of var-
ious aromatic substrates [15]. Furthermore, the ionic liquid
allowed extraction of substrates and subsequent recycle of
the catalyst-ionic liquid system, albeit with decreasing con-
versions.
Many of the reactions performed in ionic liquids have
used homogeneous catalysts and these have been reviewed
extensively [16]. In contrast, heterogeneous catalysts have
been much less studied. Recently, a number of investigations
using heterogeneous catalysts in ionic liquids have been
reported. Both supported and unsupported transition metal
(colloidal) catalysts have been used for a range of reactions
including the Heck reaction [17], selective hydrogenations
[18], and selective oxidations [19]. Katdare et al. have shown
that zeolites can catalyse Friedel–Crafts acylation reactions
in ionic liquids [20] and more recently microporous solids
have been used for the Prins cyclisation of aldehydes and
homoallylic alcohols [21], the alkylation of phenol [22], and
the selective oxidation of thioethers [23]. However, the cat-
alytic reaction mechanism for these reactions has not been
elucidated to date.
Anisole (99%) and benzoic anhydride (98%) were obtained
from Lancaster and used as received. Trifluoromethanesul-
phonic acid (HOTf) and bis-trifluoromethanesulphonimide
(HNTf2) were both obtained from Aldrich and used as re-
ceived.
Ionic liquids composed of either 1-alkyl-3-methylimi-
dazolium ([Cnmim]+), 1-ethyl-2,3-dimethylimidazolium
([C2dmim]+), N-butyl-N-methylpiperidinium([C4mpip]+),
N-octylpyridinium ([C8py]+), methyltrioctylammonium
([N8881]+), or trihexyltetradecylphosphonium ([P66614]+)
cations with tetrafluoroborate ([BF4]−), hexafluorophos-
phate ([PF6]−), trifluoromethanesulphonate ([OTf]−) or bis-
trifluoromethanesulphonimide ([NTf2]−) anions were syn-
thesized from the appropriate organic chloride or bromide
salt using standard literature preparative procedures [24].
All ionic liquids were dried in vacuo at 60 ◦C for more than
4 h before use and contained < 0.01 wt% water, determined
by Karl–Fischer analysis. The densities of the ionic liquid
and reagent mixtures, used to calculate concentrations, were
obtained using an Anton Parr DMA4500 densitometer.
The batch experiments were carried out in a 10-pot reac-
tor (Stemblock) at 80 ◦C with magnetic stirring at 600 rpm.
To the calcined zeolite (0.1 g), ionic liquid (2 g) and anisole
(4.6 mmol) were added. The reaction mixtures were heated
to the appropriate temperature, while stirring, before ben-
zoic anhydride (5.0 mmol) was added. Analogous proce-
dures were used for experiments using HOTf and HNTf2.
The spinning basket reactor consisted of a baffled glass
vessel (150 cm3) in which was suspended a mesh (35 mesh)
basket containing 2.2 g H-USY-29 (650 µm < particle
size < 800 µm). The basket was rotated through 60 g of
the stock solution consisting of [C2mim][NTf2] (0.15 mol),
anisole (0.46 mol), and benzoic anhydride (0.53 mol) using
an overhead stirrer at 200 rpm.
This paper reports on a study of the interaction of zeo-
lites with ionic liquids using the formation of 4-methoxy-
benzophenone by the benzoylation of anisole with benzoic
anhydride as an exemplar reaction, Scheme 1, and the mech-
anism by which the catalyst operates.
Continuous-flow experiments (Fig. 4) were carried out
using a fixed-bed reactor (4.57 mm i.d. ×76 mm) containing
∼ 0.5 g zeolite (650 µm < particle size < 800 µm) con-
nected to a pipe (1.40 mm i.d. ×1300 mm) acting as a heat
exchanger suspended in a silicone oil bath. The feedstock
made from [C2mim][NTf2] (0.23 mol), anisole (0.37 mol),
and benzoic anhydride (0.77 mol) was pumped through the
reactor bed using a BHS Labotron LDP-4 piston pump at
156 cm3 h−1 to achieve a 10 s residence time over the cata-
lyst bed.
2. Experimental
All zeolites used in the reactions were supplied by BP-
Amoco, Quest, and Chevron-Texaco and calcined in air
at 500 ◦C overnight before use. The characterisation of
the catalysts used is summarised in Table 1. The sodium-
exchanged zeolites were supplied by Catal International.

46
C. Hardacre et al. / Journal of Catalysis 227 (2004) 44–52
Fig. 1. Comparison of the percentage conversion for the benzoylation of
anisole using benzoic anhydride using H-USY-29 in a range of ionic liquids
and molecular organic solvents and in the absence of a solvent.
Fig. 2. Comparison of the percentage conversion for the benzoylation of
anisole using benzoic anhydride as a function of time using a range of zeo-
◦
lites in [C mim][NTf ] at 80 C.
2
2
For all the reactions, samples of known mass (∼ 30 mg)
were removed over time and dissolved in acetonitrile
(10 cm3) for analysis by HPLC.
For the catalyst recycle experiments, the zeolite was cal-
cined at 500 ◦C in air overnight between reactions before
recharging the reaction solution and repeating the experi-
mental procedure as described above.
Offline analysis was carried out using an Agilent HPLC
system with a diode array detector set at 224 nm. An Eclipse
XDB C8 column with an eluent of methanol/water (60/40)
at a flow rate of 1.0 cm3 min−1 was used. Yields with re-
spect to the limiting reagent, anisole, were calculated using
a calibration curve of known standards and mass balances
> 95% were obtained. Titrations were carried out using an
Orion 420A pH meter, calibrated using buffer tablets from
BDH. Atomic absorption (AAS) measurements were made
using a Perkin–Elmer AA Analyst 100 following digestion
of the zeolite in aqua regia and dilution of the ionic liquid in
water. For the aqueous extractions, titrations, and AAS dilu-
tions, double-distilled deionised water was used.
Even under these strictly “dry” conditions no reaction oc-
curred. Furthermore, using a water-immiscible ionic liquid,
[C8py][BF4], no activity was observed, implying that the
anion has a strong effect on the activity of the catalyst. It
should be noted that the lack of activity found in the water-
miscible ionic liquids, which are more difficult to obtain
halide free, is not due to chloride impurities remaining af-
ter preparation. Addition of 100 ppm Cl− to [C2mim][NTf2]
had little effect on the reaction rate. Although benzoylation
was found to proceed in [C4mim][PF6], the zeolite partly
dissolved in the reaction mixture, indicating the presence of
HF, itself an active catalyst for the reaction. The hydrolysis
of [PF6]−-based ionic liquids to yield HF is well known [25]
and therefore no further study was carried out on this system.
The effect of the cation was investigated using [NTf2]−-
based ionic liquids. The reaction rates were found to be sim-
ilar in [Cnmim][NTf2] (n = 2, 4, 6, 10), [C2dmim][NTf2],
[C8py][NTf2], and [C4mpip][NTf2]; however, the reaction
proceeded only slowly in [N8881][NTf2] and no reaction was
observed in [P66614][NTf2].
Based on these observations [C2mim][NTf2] was used as
the solvent with a range of zeolites of varying Si:Al ratios,
surface areas, and pore size. With the exception of amor-
phous silica, the sodium forms of ZSM-5 and zeolite Y, all
the zeolites tested facilitated the reaction (Fig. 2). It can be
observed that in the case of the H-USY zeolites the conver-
sion appears to be dependent on the Si:Al ratio: i.e., as the
ratio increased the conversion decreased. To determine the
effect of internal diffusion and external mass transfer, par-
ticles of H-USY-29 of various sizes ranging from less than
106 µm to greater than 500 µm were used in [C2mim][NTf2]
at different stirring speeds from 300 to 1200 rpm. However,
neither aggregate size nor stirring speed had any effect on
the rate of reaction.
3. Results and discussion
Fig. 1 compares the variation of conversion with time in
a standard batch reactor for a range of [NTf2]−-based ionic
liquids with conventional solvents using H-USY-29, for the
benzoylation of anisole with benzoic anhydride. Clearly,
higher activity is observed in the ionic liquids compared
with either the molecular solvents or in the absence of a sol-
vent. In each reaction the para selectivity was > 94%. Not
all the ionic liquids screened showed this variation. No re-
action was observed in the water-miscible [C4mim][BF4],
[C6mim]Cl, or [C4mim][OTf] ionic liquids. Therefore to test
whether the poor reactivity was due to water adsorption from
the atmosphere, reactions were also performed in a dry box.

C. Hardacre et al. / Journal of Catalysis 227 (2004) 44–52
47
It can be observed from Figs. 1 and 2 that many reactions
show a similar kinetic profile regardless of the ionic liquid or
zeolite used. For each combination, the reaction rate is ini-
tially high but quickly decreases to a much lower rate. This
profile is similar to that reported for other zeolite-catalysed
acylation reactions. For example, Rohan et al. [26] observed
a fast reaction on the fresh catalyst which was then followed
by rapid deactivation. The deactivation was attributed to the
adsorption of product in the mesopores of the zeolites and
the subsequent polyacetylation to form di- and tri-acetylated
products which blocked the micropores of the zeolite. Using
an excess of anisole reduced this deactivation by reducing
the adsorption of product on the surface of the zeolite. This
not only increased the number of catalyst sites available but
also reduced the formation of polyacetylated products. Der-
ouane et al. [27] also found a similar kinetic profile for the
acetylation of anisole using H-Beta with excess substrate as
the solvent. In addition, they reported that, during the ex-
periments, the zeolite changed colour from white to orange,
which was attributed to the adsorption of product onto the
surface of the zeolite. Similar visual changes were observed
in the present study in the absence of solvent and using
organic solvents; i.e., the zeolite became orange in colour
while the liquid phase remained colourless. Contrastingly,
when the reaction was carried out in ionic liquids, the cat-
alyst barely changed in colour while the ionic liquid phase
became dark orange.
In agreement with the findings of Rohan et al. [26] and
Derouane et al. [27], the use of a 10-fold molar excess of
anisole increased the reaction rate. In addition, when using
ionic liquids, a 10-fold excess of benzoic anhydride also re-
sulted in a similar increase in rate, indicating that in the case
of ionic liquids adsorption of the product and further reac-
tion may not be the main source of deactivation.
To study the deactivation a spinning basket reactor was
initially used. This was run in batch mode for the first
30 min of operation, before being changed to continuous
mode by pumping the reagents in and out at a flow rate of
0.5 cm3 min−1 for a further 260 min. Fig. 3 shows the change
in product concentration with respect to time when the sys-
tem was run as a CSTR. Clearly, the reaction did not achieve
steady state in the time studied; however, as product was still
being formed over the duration of the experiment, as shown
by comparison with the calculated dilution curve (i.e., the
expected outlet concentration due to dilution of the bulk liq-
uid by the incoming feed), the catalyst was still active. It was
also observed that the catalyst deactivation was reversible
Fig. 3. Concentration of 4-methoxybenzophenone at the outlet of the spin-
ning basket reactor as a function of time on stream using freshly calcined
◦
and recycled H-USY-29 in [C mim][NTf ] at 80 C.
2
2
and similar activity was found both for the fresh catalyst
and after recycle, provided the catalyst had been calcined
at 500 ◦C between reactions. It should be noted that recycles
performed without precalcination of the catalyst showed lit-
tle reaction.
The reaction was also run as a continuous plug flow sys-
tem over a fixed bed of zeolite, shown schematically in
Fig. 4. A comparison of product concentration at the outlet
with respect to time on stream is shown in Fig. 5 for reac-
tions performed in [C2mim][NTf2] and 1,2-dichloroethane
(DCE). As expected from the batch experiments, a lower ini-
tial conversion was observed when the reaction was carried
out in DCE than in [C2mim][NTf2]. In DCE the conversion
remained constant with respect to time, indicating contin-
ued active sites on the surface; however, in the ionic liquid,
a rapid decrease in conversion was observed with time on
stream. A similar profile was observed when the ionic liquid
reaction was carried out at 150 ◦C; as expected a higher level
of activity was observed when compared to 90 ◦C. The de-
pendence of the ionic liquid concentration present in the feed
was also tested using the plug-flow reactor. Here it was ob-
served that at high [C2mim][NTf2] concentrations (ranging
from 835 to 13.6 mM), the previously observed conversion
profile was obtained indicating that the concentration of the
ionic liquid had little effect. However, at lower concentra-
tions of [C2mim][NTf2] (1.420 mM) a much lower initial ac-
Fig. 4. Schematic diagram of the fixed-bed reactor.

48
C. Hardacre et al. / Journal of Catalysis 227 (2004) 44–52
Fig. 6. Variation of the anisole conversion with respect to time for out-
let fractions of [C mim][NTf ] collected after contact with H-USY-29 at
Fig. 5. Continuous-flow study using the fixed-bed reactor containing
◦
2
2
H-USY-29 at 90 C in [C mim][NTf ] and 1,2-dichloroethane and at
2
2
◦
80 C.
◦
150 C in [C mim][NTf ].
2
2
To prove that ion exchange had occurred, acid–base titra-
tions and exchange experiments with Na zeolites were car-
ried out in an attempt to quantify the acid liberated. Fig. 7
shows a series of titration curves from HNTf2 in water;
the aqueous extract of HNTf2 dissolved in [C2mim][NTf2],
the aqueous extract of [C2mim][NTf2] contacted with
H-USY-29, and the aqueous extract of [C2dmim][NTf2] con-
tacted with H-USY-29 at 80 ◦C using NaOH (2.3 mM) as
the titrant. The reference titration of HNTf2 in water showed
a typical strong acid-strong base titration curve; however,
when [C2mim][NTf2] was pumped through a fixed bed of
H-USY-29 at 80 ◦C and the [C2mim][NTf2] washed with
water the resulting titration curve was ill-defined, with two
points of inflection. In these cases the resulting data were fit-
ted using cubic spline interpolation and the points of inflec-
tion found by finding the maximum of the first derivatives.
A similar titration curve was obtained following the aqueous
extraction of 1.55 mM HNTf2 dissolved in [C2mim][NTf2]
with water, showing that the second inflection point cor-
responds to the neutralisation of HNTf2. The first titration
point is due to the dissolution of [C2mim][NTf2] in the
aqueous phase, albeit only a relatively small amount due
to its low water miscibility. Washing [C2mim][NTf2] with
D2O and analysing the solution with 1H NMR confirmed the
presence of the [C2mim]+ cation. It is well known that the
C(2) proton is acidic, as shown by the disappearance of the
1H NMR signal on the addition of base, and therefore the
presence of the [C2mim]+ cation results in a diprotic acid
titration curve [29]. To confirm this, HNTf2 was dissolved
in [C2dmim][NTf2], where the C(2) position is protected
by a methyl group, extracted into water, and titrated against
sodium hydroxide. In this case the titration curve was well
defined with a single inflection point. From the titration data
it can be calculated that 76.6 mg of HNTf2 is produced per
gram of zeolite used. This corresponds to a concentration
Scheme 2.
tivity was observed which decreased at a significantly slower
rate.
This type of rapid deactivation indicates a change in the
zeolite catalyst to facilitate a homogeneous catalytic mech-
anism. No gross changes were observed on the zeolite, for
example, neither XRD of the catalyst nor ICP analysis of
the ionic liquid solution following the reactions showed any
significant structural breakdown of the catalyst or removal
of framework aluminum. This is consistent with the good
recyclability of the zeolite, albeit after calcinations, and indi-
cated that the reaction was taking place via an ion-exchange
mechanism whereby the ionic liquid cation exchanges with
the acidic proton on the zeolite releasing H+ into the ionic
liquid, as shown in Scheme 2. Similar effects have been
observed with water whereby strong Brønsted acids are
formed following the hydrolysis of the surface Lewis acid
sites [28].
In a further experiment different fractions of pure ionic
liquid feed were collected (3-min intervals) from the out-
let stream of the plug-flow reactor operating at 80 ◦C
and 40 cm3 h−1. These fractions were then used in stan-
dard batch reactions without any additional catalyst present
(Fig. 6). The activity of each subsequent fraction decreased,
although the shape of the conversion profile remained the
same. It should be noted that analogous experiments per-
formed using samples of the zeolite removed from the fixed
bed after washing with ionic liquid showed no residual activ-
ity. These results from the continuous experiments support
the hypothesis that ion exchange is occurring and the result-
ing acid is the active catalytic species.

C. Hardacre et al. / Journal of Catalysis 227 (2004) 44–52
49
◦
Fig. 7. Titration of (a) HNTf in water, (b) aqueous extract from [C mim][NTf ] contacted with H-USY-29 at 80 C, (c) aqueous extract of HNTf from
2
2
2
2
[C mim][NTf ], and (d) aqueous extract of HNTf from [C dmim][NTf ] with 2.3 mM NaOH.
2
2
2
2
2
of 9.46 mM of acid in a typical reaction mixture, 10.68 mg
[C2mim][NTf2] consumed, i.e., only ∼ 0.5% ionic liquid
loss corresponding to approximately 49% of the acid sites
on the zeolite exchanged by [C2mim]+.
Table 2
Extent of cation exchange from sodium-exchange zeolites with 1,3-di-
alkylimidazolium, tetraalkylammonium, and tetraalkylphosphonium ionic
liquids
a
H-USY-5
H-USY-29
H-ZSM-5
To further quantify the amount of ion exchange and to
probe why no reaction occurred in either [OTf]−- or [BF4]−-
based ionic liquids, the extent of ion exchange from a se-
ries of Na-exchanged zeolites with a range of ionic liquids
was determined. In each case, the zeolites were stirred in
the ionic liquid at 80 ◦C, the zeolite and ionic liquid were
separated by filtration, and the sodium content of both the
ionic liquid and zeolite was determined by AAS; the re-
sults are summarised in Table 2. From the data it is clear
that the anion does not affect the ion exchange signifi-
cantly, similar sodium ion exchange was observed for equiv-
alent [NTf2]−-, [OTf]−-, and [BF4]−-based ionic liquids. It
should be noted that while the anion has little effect on the
extent of exchange, there are marked differences in the re-
activity of the resulting catalytic species. Conversely, the
[C mim][NTf ]
42 4%
31 3%
42 1%
19 1%
34 4%
48 1%
47 1%
42 2%
41 1%
4
2
[C mim][BF ]
4
4
[C mim][OTf]
4
[N
[P
][NTf ]
0
0
1%
2%
7
0
1%
3%
8
0
2%
3%
8881
2
][NTf ]
66614
a
2
Si:Al = 52.
low or no activity for the Friedel–Crafts reaction using the
zeolite catalysts. Since these cations do not readily facili-
tate the exchange, little acid is formed and therefore an in-
significant reaction occurs when using these ionic liquids.
The lack of exchange with the tetraalkylammonium and
tetraalkylphosphonium cations is probably due to the shape
and size of the cations compared with the imidazolium, pyri-
dinium, and piperidinium ions. The tetraalkylammonium
and tetraalkylphosphonium cations are much more bulky
than the corresponding heterocyclic ring systems studied
which may prevent them from entering the pores of the ze-
olites. In addition, the charge distribution in these cations is
cation does effect the exchange as seen by the level of
+
exchange observed with the bulkier cations [N8881
]
and
[P66614]+. The latter explains why the tetraalkylammonium
and tetraalkylphosphonium-based ionic liquids show very

50
C. Hardacre et al. / Journal of Catalysis 227 (2004) 44–52
Fig. 9. Effect of triflic acid catalyst concentration on the initial rate of reac-
tion. Initial rate is given as the initial rate of consumption of anisole.
Fig. 8. Comparison of the percentage conversion as a function of time
observed with H-USY-29 with that from the calculated equivalent concen-
◦
tration of HNTf using [C mim][NTf ] at 80 C.
2
2
2
ton in the triflate ionic liquid, is known to be a highly active
Friedel–Crafts catalyst [30]. However, despite the concen-
tration of acid generated in situ being greater in the [OTf]−-
based ionic liquid than in the [NTf2]−-based ionic liquid, no
reaction occurred. To assess the relative activity of HNTf2
and HOTf as Friedel–Crafts catalysts in the corresponding
ionic liquid, reactions were performed as a function of the
acid concentration. Fig. 9 shows the initial rate of change in
anisole concentration as a function of triflic acid concentra-
tion (determined from the differential of the best fit third-
order polynomial). Although there is a linear correlation
between the concentration and initial rate data, the regres-
sion line does not intersect with the origin and shows that a
threshold acid concentration is required before activity is ob-
served. This type of threshold is typical of reactions which
undergo catalyst poisoning and the cause is currently under
further investigation. Comparison of the threshold concen-
tration (42.4 mM) with the concentration estimated from the
ion exchange with the H-USY-29 zeolite (116 mg HOTf g−1
zeolite which is equivalent to 26.8 mM in a typical reaction
mixture) shows that there is an insufficient amount of acid
generated using 0.1 g of zeolite in 2 g of triflate ionic liq-
uid to facilitate reaction. In the analogous experiments using
HNTf2 no threshold value for concentration was observed.
If the hypothesis outlined above is correct then it should
be possible to generate a known, controlled concentration
of HNTf2 in situ in an organic solvent by ion exchange
using a source of [NTf2]− with a zeolite. Fig. 10 shows
a comparison of the benzoylation kinetics of H-USY-29 in
[C2mim][NTf2] and H-USY-29 in DCE in the presence of
LiNTf2. Similar rates of reaction were observed in each case
whereas no reaction was observed in the absence of H-USY-
29. This indicates that a similar mechanism is occurring in
both media, where the acidic proton on the zeolite exchanges
with the cation, either lithium or [C2mim]+, producing the
active HNTf2.
highly localized on the central nitrogen/phosphorus and, due
to the surrounding alkyl chains, this prevents a strong inter-
action between the positive charge on the cation and the sur-
face sites. This may be compared with the more delocalised
charge on the imidazolium-, pyridinium-, and piperidinium-
based ionic liquids where the charge is easily accessible
from either face of the heterocyclic ring. This exchange is
also found to be temperature dependent. At higher temper-
atures the Na-exchange experiments do show an increase
in the percentage of sodium exchanged, 7% exchange of
sodium from USY-29 in [N8881][NTf2] at 80 ◦C was found
compared with 64% exchange at 150 ◦C. Interestingly, the
Na-exchange results indicate that the amount of acid formed
from USY-29 in [C4mim][NTf2] was 83.7 mg g−1 zeolite
which is equivalent to 10.34 mM in a typical reaction. This
value correlates well with the values obtained from the titra-
tion experiments and was further confirmed by comparing
the reaction profiles generated from 0.1 g of H-USY-29 and
a 9.71 mM acid concentration (Fig. 8). In agreement with
the activity data, the close correlation shown between the
[C2mim][NTf2] titration results and the sodium-exchange
results from [C4mim][NTf2] also indicates the weak depen-
dence on alkyl chain length for [Cnmim]+-based ionic liq-
uids.
The fact that not all the acid sites are exchanged by the
ionic liquid despite the large excess of cation present indi-
cates that the activity is dependent on the accessibility of
the exchangeable sites as well as the number of sites. This
is consistent with the results from the continuous-plug-flow
study whereby increasing the temperature increased the ac-
tivity due to increased availability of the exchangeable sur-
face sites and no further increase in activity was observed
above a certain concentration of ionic liquid.
The sodium ion exchange results do not explain the lack
of activity in [OTf]−- and [BF4]−-based ionic liquids. Triflic
acid (HOTf), formed from the exchange of the zeolite pro-

C. Hardacre et al. / Journal of Catalysis 227 (2004) 44–52
51
acids are generated in these cases to facilitate the reaction.
Whereas [P66614][NTf2] does not support sodium exchange
and [N8881][NTf2] will exchange at low levels and do not
therefore facilitate the reaction. The amount of acid gener-
ated at the reaction temperature (80 ◦C) has been quantified
using titration and sodium-exchange experiments with both
methods giving comparable results which correlate well with
kinetic data.
Acknowledgments
The authors thank BP, Chevron-Texaco, and Quest for
samples of zeolite, and QUILL, DEL in Northern Ire-
land, LINK, and the EPSRC under Grants GR/N02085 and
GR/R42061 for funding this project.
Fig. 10. Comparison of anisole conversion as a function of time in the
absence of catalyst, addition of LiNTf in [C mim][NTf ], H-USY-29
2
2
2
in DCE, H-USY-29 in [C mim][NTf ], HNTf in [C mim][NTf ], and
2
2
2
2
2
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