Atom efficient Friedel-Crafts acylation of toluene with propionic anhydride over solid mesoporous superacid UDCaT-5

Article abstract of DOI:10.1016/j.apcata.2012.05.031

Friedel-Crafts acylation is ubiquitous in industry and is typically carried out by using more than stoichiometric quantities of homogeneous catalysts. This creates pollution. In this work, acylation of toluene was studied in liquid phase with propionic anhydride with a variety of solid superacids to produce 4′-methylpropiophenone (4′-MPP). The solid superacids were modified versions of zirconia, namely, UDCaT-4, UDCaT-5 and UDCaT-6 developed in our laboratory; amongst which UDCaT-5 was the most active, selective and robust catalyst. The effects of various reaction parameters on the rate of reaction and selectivity were investigated to deduce the intrinsic kinetics of the reaction. The reaction is free from any external mass transfer as well as intraparticle diffusion limitations and is intrinsically kinetically controlled. The acylation conditions were: temperature 180 °C, toluene to propionic anhydride molar ratio 5:1, catalyst loading 0.06 g cm^-3, speed of agitation 1000 rpm, under autogenous pressure in a stainless steel autoclave reactor. Propionic acid generated in situ also reacts sequentially with toluene to give 4′-MPP. A conversion of 62% of priopionic anyhydride is obtained after 3 h, with 100% mono-acylated product containing 67% 4′-MPP. Water is the only co-product of the overall reaction. A suitable kinetic model was developed. The reactions were carried out without using any solvent in order to make the process cleaner and greener.

Full text of DOI:10.1016/j.apcata.2012.05.031

Applied Catalysis A: General 433–434 (2012) 265–274
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Applied Catalysis A: General
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Atom efficient Friedel–Crafts acylation of toluene with propionic anhydride over
solid mesoporous superacid UDCaT-5
Ganapati D. Yadav∗, Shashikant B. Kamble
Department of Chemical Engineering, Institute of Chemical Technology (ICT), Matunga, Mumbai 400019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Friedel–Crafts acylation is ubiquitous in industry and is typically carried out by using more than stoichio-
metric quantities of homogeneous catalysts. This creates pollution. In this work, acylation of toluene
was studied in liquid phase with propionic anhydride with a variety of solid superacids to produce
4ꢀ-methylpropiophenone (4ꢀ-MPP). The solid superacids were modified versions of zirconia, namely,
UDCaT-4, UDCaT-5 and UDCaT-6 developed in our laboratory; amongst which UDCaT-5 was the most
active, selective and robust catalyst. The effects of various reaction parameters on the rate of reaction
and selectivity were investigated to deduce the intrinsic kinetics of the reaction. The reaction is free from
any external mass transfer as well as intraparticle diffusion limitations and is intrinsically kinetically con-
trolled. The acylation conditions were: temperature 180 ◦C, toluene to propionic anhydride molar ratio
5:1, catalyst loading 0.06 g cm−3, speed of agitation 1000 rpm, under autogenous pressure in a stainless
steel autoclave reactor. Propionic acid generated in situ also reacts sequentially with toluene to give
4ꢀ-MPP. A conversion of 62% of priopionic anyhydride is obtained after 3 h, with 100% mono-acylated
product containing 67% 4ꢀ-MPP. Water is the only co-product of the overall reaction. A suitable kinetic
model was developed. The reactions were carried out without using any solvent in order to make the
process cleaner and greener.
Received 11 March 2012
Received in revised form 30 April 2012
Accepted 21 May 2012
Available online 30 May 2012
Keywords:
Friedel–Crafts acylation
Heterogeneous catalysis
Solid superacids
Kinetics
4ꢀ-Methylpropiophenone
Green chemistry
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
catalyst, AlCl3, FeCl3 or BF3 [2]. Use of these catalysts is fraught
with a numerous drawbacks and operational problems. More than
Liquid acids have been extensively used as catalysts in a vari-
ety of chemical and allied industries. Green chemistry approach
involves the replacement of corrosive and toxic liquid acids such
as HF and H2SO4 by environmentally benign heterogeneous solid
acids. The production of various pharmaceuticals, agrochemicals,
dyes and pesticides involves the synthesis of aromatic ketones
and their derivatives. The common routes for preparation of these
ketones proceed via Friedel–Crafts acylation of the concerned
aromatic hydrocarbon with derivates of carboxylic acids, which
are traditionally catalyzed by either Lewis acids such as AlCl3,
FeCl3 and BF3 or Brønsted acids such as HF or H3PO4 [1]. 4ꢀ-
Methylpropiophenone (4ꢀ-MPP) is found to have wide applications
in the area of fine chemicals, pharmaceuticals, resins, drugs, per-
fumes and specialty chemical synthesis. The classical production
of 4ꢀ-MPP is performed by Friedel–Crafts acylation process of
toluene with propionic anhydride or propionyl chloride, using sto-
ichiometric quantities of conventional homogeneous Lewis acid
stoichiometric amount of Lewis acid is required. The reaction com-
plexes formed are rather stable and their break-up to obtain the
desired product leads to the loss of the catalyst [1,3]. The corro-
sive nature of homogeneous acids results in premature ageing of
the processing equipment and associated transfer lines, which is
expensive. Use of solid acids such as shape selective zeolites in
acylation with carboxylic acids is more attractive [4].
In recent years, efforts have been directed toward the promotion
of solid acid catalysts and several synthetic procedures have been
reported. Heterogeneous solid acids are advantageous overconven-
tional homogeneous acid catalysts [5–7]. They are non-corrosive;
presenting fewer disposal problems, and their separation from
fluid phase is much easier, which allows their repeated use. They
permit the use of cheaper and non-polluting reagents, and offer
several different reactor configurations [3]. In addition, physical
and chemical properties of solid acids can be tailored and tuned
to promote reactivity and selectivity and prolonged catalyst life
[7]. Nakamura et al. [8] described the propionylation and butyryla-
tion of toluene with propionic anhydrides and butyric anhydrides
over SO4/ZrO2, SO4/SnO2, Pt-SO4/ZrO2, and Ru-SO4/ZrO2; yields of
7–9% o- and 91–93% p-isomers were 31, 26, 32, and 44% for pro-
pionylation and 46, 29, 44, and 55% for butyrylation, respectively
obtained.
∗
Corresponding author. Tel.: +91 22 3361 1001/1111/2222;
fax: +91 22 3361 1002/1020.
E-mail addresses: gdyadav@yahoo.com, gd.yadav@ictmumbai.edu.in
(G.D. Yadav).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.05.031

266
G.D. Yadav, S.B. Kamble / Applied Catalysis A: General 433–434 (2012) 265–274
The objective of the present work was to evaluate the solid
Nomenclature
superacid of UDCaT series of catalysts and to achieve the produc-
tion of 4ꢀ-MPP by direct propionylation of toluene with propionic
anhydride. A systematic investigation of the effects of various oper-
ating parameters is presented. A mathematical model is developed
to describe the reaction pathway and validated with experimental
results. The overall process is green and clean.
A
B
Ci
E
limiting reactant species A, propionic anhydride
excess reactant species B, toluene
concentration of species ‘i’ (mol cm−3
)
apparent activation energy (kcal mol−1
rate constant (cm6 gcat−1 mol−1 s−1
frequency factor (cm6 gcat−1 mol−1 s−1
)
)
k
)
k0
k1
k2
k3
k4
k5
k6
P1
P2
P3
P4
−ri
2. Experimental
rate constant of reaction (1) (cm6 gcat−1 mol−−11 s−1
rate constant of reaction (2) (cm6 gcat−1 mol−1 s−1
rate constant of reaction (3) (cm6 gcat−1 mol−1 s−1
rate constant of reaction (4) (cm6 gcat−1 mol−1 s−1
rate constant of reaction (5) (cm6 gcat−1 mol s−1
rate constant of reaction (6) (cm6 gcat−1 mol−1 s−1
propionic acid
)
)
)
)
)
)
2.1. Chemicals
Toluene, propionic anhydride, zirconium oxychloride, alu-
minum nitrate, ammonium persulfate and aqueous ammonia
solution were procured from Aldrich, USA. Hexadecyl amine and
chlorosulfonic acid was purchased from Spectrochem Ltd. Mum-
bai, India. Tetraethyl orthosilicate (TEOS) was procured from Fluka,
Germany. All chemicals were of analytical reagent (A.R.) grade.
These were used as received without any further purification.
2ꢀ-methylpropiophenone (2ꢀ-MPP)
3ꢀ-methylpropiophenone (3ꢀ-MPP)
4ꢀ-methylpropiophenone (4ꢀ-MPP)
rate of reaction of species ‘i’ (mol gcat−1 s−1
)
SP
selectivity
of
desired
product,
4ꢀ-
4
methylpropiophenone (4ꢀ-MPP)
2.2. Preparation of catalysts
T
t
W
w
temperature (K)
time (s)
Catalysts used in the present study namely, sulfated zirconia
(S-ZrO2), UDCaT-4, UDCaT-5, and UDCaT-6 were prepared by well
established procedures in our laboratory and the details are given
below.
water
catalyst loading (g cm−3 of liquid phase)
Subscripts
1
2
3
4
5
6
i
reaction (1)
reaction (2)
reaction (3)
reaction (4)
reaction (5)
reaction (6)
2.2.1. Preparation of sulfated zirconia (S-ZrO2)
Sulfated zirconia (S-ZrO2) was prepared by adding aqueous
ammonia solution to zirconium oxychloride solution at a pH of 10,
as detailed elsewhere [13]. The precipitate was thoroughly washed
with distilled water and made free from ammonia and chloride
ions. It was dried in an oven at 120 ◦C for 24 h. The sulfation of the
zirconia was done using 15 cm3 g−1 of 0.5 M sulfuric acid. It was
dried at 110 ◦C and calcined at 650 ◦C for 3 h.
species
P4
4ꢀ-methylpropiophenone (4ꢀ-MPP)
2.3. Preparation of UDCaT-4
The ordered hexagonal mesoporous silica (HMS) was prepared
according to our earlier work [14]. Desired quantities of zirconium
oxychloride and aluminum nitrate were dissolved in aqueous solu-
tion and added to precalcined HMS by incipient wetness technique.
After addition the solid was dried in an oven at 110 ◦C for 3 h.
The dried material was hydrolyzed by ammonia gas and washed
with deionized water until a neutral filtrate was obtained and the
absence of chlorine ion in the filtrate was detected by phenolph-
thalein indicator and silver nitrate tests. It was then dried in an
oven for 24 h at 110 ◦C. Persulfation was carried out by immersing
the above solid material in to 0.5 M aqueous solution of ammonium
persulfate for 30 min. It was dried at 110 ◦C for 24 h and calcined at
650 ◦C for 3 h to get active catalyst called UDCaT-4 with 0.6% (w/w)
of alumina [14].
Our laboratory has been pursuing the development of new
ecofriendly solid acid catalysts for several industrially relevant
reactions including process kinetics and reactor design. The activ-
ities and selectivities of different solid acid catalysts have been
successfully evaluated in our laboratory for developing green
Friedel–Crafts acylation and alkylation reactions. Alkylation of a
variety of aromatic compounds was studied over solid superacids
using different alkanols [9–12]. Among them, solid superacidic
UDCaT series catalysts have received the most attention due to
their unique structure and strong acidity. Flexibility in their acid
strength, low toxicity and fairly high thermal stability make them
excellent and versatile catalysts for a wide variety of acid catalyzed
reactions in homogeneous and heterogeneous media [13–17]. We
prepared mesoporous superacids named as UDCaT-4, UDCaT-5 and
UDCaT-6, which are modified versions of zirconia and have found
a great potential for industrially important reactions [14–16]. We
have reported, for the first time, that a sulfated zirconia, with sul-
fur content as high as 9% (w/w), was produced with preservation
of tetragonal phase by using chlorosulfonic acid as a new source
for sulfate ion. It was designated as UDCaT-5 [15]. The name of
the catalyst “UDCaT” symbolizes the authors’ research institute,
University Department of Chemical Technology (UDCT), which is
now renamed as Institute of Chemical Technology (ICT). The work
summarizes the investigation of activity of these new breed of cat-
alytic materials in atom efficient Friedel–Crafts acylation of toluene
with propionic anhydride under solventless conditions, including
reaction kinetics.
2.4. Preparation of UDCaT-5
UDCaT-5 was prepared by adding aqueous ammonia solution to
zirconium oxychloride (ZrOCl2·8H2O) solution at pH of 9–10. The
precipitated zirconium hydroxide so obtained was washed with
deionized water until a neutral filtrate was obtained. The absence
of chloride ions was detected by AgNO3 test. A material balance on
chloride ions before and after precipitation and washing shows no
retention of Cl− on the solid. Zirconium hydroxide (Zr(OH)4) was
dried in an oven for 24 h at 100 ◦C and was crushed to 100 mesh size.
Zr(OH)4 then immersedin 15 cm3 g−1 of0.5 Msolutionof chlorosul-
fonic acid and ethylene dichloride. All materials were immersed for

G.D. Yadav, S.B. Kamble / Applied Catalysis A: General 433–434 (2012) 265–274
267
5 min in the solution and then without allowing moisture absorp-
tion were kept in an oven and the heating was started slowly to
120 ◦C after about 30 min. The solids were kept in an oven at 120 ◦C
for 24 h and calcined at 650 ◦C for 3 h to get the active catalysts
UDCaT-5 [15].
2.5. Preparation of UDCaT-6
UDCaT-6 was prepared by adding an aqueous solution of 2.5 g
zirconium oxychloride to 5 g precalcined HMS by incipient wetness
technique and it was dried in an oven at 120 ◦C for 3 h. The dried
material was hydrolyzed by ammonia gas and washed with distilled
water until no chloride ions were detected which was confirmed
by the AgNO3 test. It was further dried in an oven for 2 h at 120 ◦C.
Zr(OH)4/HMS was immersed in 15 cm3 g−1 of 0.5 M chlorosulfonic
acid in ethylene dichloride. It was soaked for 5 min in the solution
and then without allowing moisture absorption, it was oven dried
to evaporate the solvent at 120 ◦C after about 30 min. The sample
was kept in an oven at 120 ◦C for further 24 h and calcined thereafter
at 650 ◦C for 3 h to get the active catalyst UDCaT-6 [16].
2.6. Catalysts characterization
Fig. 1. FTIR spectra of (a) sulfated zirconia (S-ZrO2) and (b) UDCaT-5.
The catalysts, namely, UDCaT-4, UDCaT-5, UDCaT-6 and sulfated
zirconia (S-ZrO2) were completely characterized by ammonia-
temperature programmed desorption (NH3-TPD), X-ray powder
diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area and
Fourier transform infrared (FTIR) and the details were published
by us [13–16]. Only a few salient features which are thought be
important are reported here.
intermediate and strong acidic sites present in sulfated zirconia,
UDCaT-5 also contains superacidic sites. The elemental analysis
showed the complete absence of chloride species and that 9% (w/w)
sulfate was retained on the surface of UDCaT-5 which was the high-
est then reported in the literature. The IR spectra of the S-ZrO2 show
a similar pattern as UDCaT-5 (Fig. 1) indicating the presence of the
bidentate chelating sulfate group co-ordinated to zirconia [13,15].
IR spectroscopy confirms that the chlorosulfonic acid is decom-
posed during calcination at 650 ◦C and sulfate ions are retained
on the surface of UDCaT-5 and thus the sulfur content is higher in
UDCaT-5. Powder XRD shows the crystal structure of zirconia is not
affected by a high sulfur content of 9% (w/w) in UDCaT-5. The quan-
tity of tetragonal phase of zirconia in UDCaT-5 is the same as that in
the ordinary S-ZrO2 (Fig. 2). The XRD study proved that the tetrag-
onal phase in the UDCaT-5 was preserved. The BET surface area of
UDCaT-5 (83 m2 g−1) is lower than that of S-ZrO2 (103 m2 g−1). The
2.6.1. Characterization of UDCaT-4
The XRD, BET surface area and pore size analyses provided an
explanation for the entrapment of nanoparticles of persulfated alu-
mina zirconia (PAZ) (<3.6 nm) in mesoporous of HMS. The XRD of
UDCaT-4 suggested that the structural integrity of HMS was main-
tained even after converting it into UDCaT-4. The diffractogram of
UDCaT-4 further revealed that the introduction of a small amount
of alumina (0.16%, w/w) and sulfate ion (1.17%, w/w) stabilized the
tetragonal phase of the zirconia, which is an ideal phase conducive
for superacidity in sulfated zirconia, into the pores of HMS. Fur-
thermore, the pore volume of UDCaT-4 (0.21 cm3 g−1) is much less
than that of pure HMS (0.78 cm3 g−1) indicating that large amount
of crystalline zirconia (9.01%, w/w), and alumina must be present
inside the pores of UDCaT-4. FTIR spectroscopy and energy disper-
sive X-ray (EDAX) analysis further support the assumption drawn
on introduction of sulfate ion in UDCaT-4. The sulfur Ka1 and zirco-
nium La1 distribution spectra determined by EDAX analysis show
the incorporation and homogeneous distribution of zirconia and
sulfur atoms in UDCaT-4. Temperature programmed desorption
(TPD) profiles of PAZ and UDCaT-4 show that although UDCaT-
4 possesses both weak and medium acid sites, the total number
of acid sites of UDCaT-4 (0.56 mmol g−1) are greater than those of
PAZ (0.09 mmol g−1) [14]. Scanning electron micrographs (SEM) of
UDCaT-4 revealed that similar to the morphology of HMS, UDCaT-
4 is made up of sub-micrometer sized free standing or aggregated
sphere shaped particles. SEM analysis further supports the argu-
ment that active centers of the PAZ are successfully embedded in
HMS and the structural integrity of HMS is unaltered even after it
is converted to UDCaT-4 [14].
2.6.2. Characterization of UDCaT-5
UDCaT-5 was fully characterized by FTIR, XRD, BET surface
area, NH3-TPD, elemental analysis and the details have been pub-
lished by us [15]. Ammonia-TPD was used to determine the acid
strength of UDCaT-5. Ammonia-TPD analysis shows that apart from
Fig. 2. XRD pattern of (a) sulfated zirconia (S-ZrO2) and (b) UDCaT-5.

268
G.D. Yadav, S.B. Kamble / Applied Catalysis A: General 433–434 (2012) 265–274
70
60
50
40
30
20
10
0
surface area of S-ZrO2 gradually increases at low sulfate contents
up to 4% (w/w) (119 m2 g−1) but it decreases abruptly at a maxi-
mum sulfate content of 5.64% (71 m2 g−1) due to the migration of
sulfate ions to the bulk phase of zirconia. The surface area of UDCaT-
5 decreased abruptly at the maximum sulfur content. It was due to
the migration of sulfate ions from bulk phase to zirconia matrix.
Thus maximum sulfur present on surface of the catalyst decreases
its surface area. Thus, UDCaT-5 is superacidic in nature due to the
presence of very high sulfur content present on the zirconia matrix
with preservation of tetragonal phase of zirconia [15].
2.6.3. Characterization of UDCaT-6
FTIR spectroscopy and EDAX analyses support the introduc-
tion and retention of sulfate ion in UDCaT-6. XRD, BET surface
area and pore size analysis provided an explanation for entrap-
ment of nanoparticles of zirconia in mesoporous of HMS. The XRD
of UDCaT-6 suggested that the structural integrity of HMS was
retained even after converting it into UDCaT-6. Furthermore, the
pore volume of UDCaT-6 (0.7 cm3 g−1) is much less than that of pure
HMS (1.2 cm3 g−1) indicating that large amount of crystalline nano-
particles of zirconia must be present inside the pores of UDCaT-6.
The sulfur Ka1 and zirconium La1 distribution spectra determined
by EDAX analysis shows the incorporation and homogeneous dis-
tribution of zirconia and sulfur atoms in UDCaT-6. The SEM of
UDCaT-6 revealed that similar to the morphology of HMS, UDCaT-
6 is made up of sub-micrometer sized free standing or aggregated
sphere shaped particle and that active centers of zirconia are suc-
cessfully embedded in HMS and the structural integrity of HMS is
unaltered even after it is converted to UDCaT-6 [16].
0
20
40
60
80
100 120
Time (min)
140
160
180
200
Fig. 3. Effect of different catalysts on conversion of propionic anhydride. Reac-
tion conditions: speed of agitation = 1000 rpm, catalyst loading = 0.06 g cm−3, mole
ratio of toluene:propionic anhydride = 5:1, temperature = 180 ◦C, total reaction vol-
ume = 50 cm3, autogenous pressure: (ꢀ) UDCaT-5, (ꢁ) UDCaT-4, (᭹) UDCaT-6, (ꢂ)
sulfated zirconia (S-ZrO2).
3. Results and discussions
3.1. Efficacies of various catalysts
The effects of various process parameters on conversion, rates
of reaction and product distribution were studied systemati-
cally. Various parallel reactions were take place over UDCaT-5 as
shown in Scheme 1. Toluene (B) and propionic anhydride (A) are
chemisorbed on the acidic sites. This is a complex reaction network
with parallel and consecutive reactions of toluene with propionic
anhydride and propionic acid; which was in operation during the
reaction process as shown in reactions (1)–(6) of Scheme 1.
Various solid superacids synthesized in our laboratory, namely,
UDCaT-4, UDCaT-5, UDCaT-6 and sulfated zirconia were screened
to assess their efficacy in this reaction. A 0.06 g cm−3 loading
of the particular catalyst based on the volume of the reac-
tion mixture was employed at 180 ◦C at a speed of agitation of
1000 rpm. It was found that UDCaT-5 showed higher conversion
of the limiting reactant, propionic anhydride (62%) as compared
to the other catalysts with selectivity (67%) towards the desired
product, 4ꢀ-methylpropiophenone (4ꢀ-MPP) during 3 h of reaction
and the order of activity was: UDCaT-5 (most active) > UDCaT-
4 > UDCaT-6 > sulfated zirconia (S-ZrO2) (least active) (Fig. 3). All
other catalysts contain fewer acidic sites as compared to UDCaT-5
and thus the results are in this order. The purpose of using sev-
eral different solid acid catalysts was to study the effect of nature,
strength and distribution of acidity, pore size distribution and sta-
bility of the catalyst on conversion and selectivity in a complex
network of reactions involving acylation and dehydration. Since the
substrate is bulky and involves generation of water, it is essential
that sulfated zirconia which gets deactivated is modified. Besides,
our group was the first one to report the highest superacidity in
solid superacids. This has been already published by us [15]. The
catalyst properties, selectivity of 4ꢀ-MPP and final conversion of
propionic anhydride are given in Table 1, which clearly indicates
that UDCaT-5 is the most active and selective catalyst in terms of
conversion and selectivity.
2.7. Reaction procedure
The reactions were carried out in a 100 cm3 capacity Parr auto-
clave reactor with an internal diameter of 5 cm, equipped with a
four bladed pitched turbine impeller. The temperature was main-
tained at 1 ◦C of the desired value with the help of an in-built
proportional integral differential (PID) controller. Specific quanti-
ties of desired reactants and catalyst were charged into the reactor
and the temperature was raised to the desired value. Then, an ini-
tial sample was withdrawn and agitation started. Further samples
were withdrawn at periodic time intervals up to 3 h to monitor
the reaction. All catalysts were dried in an oven at 120 ◦C for 1 h
before use. In a typical reaction, 0.376 mol toluene was reacted with
0.0752 mol propionic anhydride (PA) (5:1 mole ratio of toluene to
propionic anhydride) with 3 g of catalyst; this makes the catalyst
loading as 0.06 g cm−3 of liquid phase. The total volume of the reac-
tion mixture was 50 cm3. The reaction was carried out at 180 ◦C at
a speed of agitation of 1000 rpm under autogenous pressure. The
reaction was carried out without any solvent.
2.8. Analytical methods
Clear liquid samples were withdrawn at regular time inter-
vals by reducing the speed of agitation momentarily to zero
and allowing the catalyst to settle at the bottom of the reac-
tor. Analysis of the samples or compounds were performed by
Gas Chromatograph (Chemito, India: Model 8610 GC) equipped
with a 10% SE-30 (liquid stationary phase) stainless steel column
(3.175 mm diameter × 4 m length) with flame ionization detec-
tor (FID). Products were isolated and confirmed through gas
chromatography–mass spectrometry (GC–MS) and their physical
properties and retention times were recorded and compared with
authentic samples. Calibrations were done with authentic samples
for quantification of data. The conversions were based on the dis-
appearance of propionic anhydride, the limiting reactant in the
reaction mixture.
Propionic acid, which is formed during the reaction course
(Scheme 1) reacts further with toluene to produce 4ꢀ-MPP and
generates water. This was also verified by doing an independent
experiment with toluene (excess) and propionic acid (limiting) to

G.D. Yadav, S.B. Kamble / Applied Catalysis A: General 433–434 (2012) 265–274
269
Scheme 1. Friedel–Crafts acylation of toluene with propionic anhydride over UDCaT-5. Where, A = propionic anhydride, B = toluene,
P
1 = propionic acid, P2 = 2ꢀ-
methylpropiophenone (2ꢀ-MPP), P3 = 3ꢀ-methylpropiophenone (3ꢀ-MPP), P4 = 4ꢀ-methylpropiophenone (4ꢀ-MPP), W = water.

270
G.D. Yadav, S.B. Kamble / Applied Catalysis A: General 433–434 (2012) 265–274
Table 1
Properties of catalysts and conversion of propionic anhydride.a
Catalyst
Source
Average pore
diameter (A)
Surface area
Acidity by
NH3-TPD
(mmol g−1
Selectivity
of 4ꢀ-MPP
after 3 h (%)
Conversion of
propionic
anhydride after
3 h (%)
(m2 g−1
)
˚
)
UDCaT-4
UDCaT-5
UDCaT-6
Sulfated
zirconia
(S-ZrO2)
This work
This work
This work
This work
30
40
32
40
233
83
877
100
0.562
0.584
0.508
0.433
52
67
56
43
55
62
51
38
a
Reaction conditions: speed of agitation = 1000 rpm, catalyst loading = 0.06 g cm−3, mole ratio of toluene:propionic anhydride = 5:1, temperature = 180 ◦C, total reaction
volume = 50 cm3, autogenous pressure.
observe that the reaction indeed took place. Novel mesoporous
solid superacid catalyst UDCaT-5 showed the highest activity com-
paring to other catalysts even in the presence of water as a
co-product of this reaction. Hence further experiments were con-
ducted with UDCaT-5. The concentration profile of reactants and
the products at 180 ◦C are shown in Fig. 4. Concentration of pro-
pionic anhydride decreases gradually, whereas the concentration
of 4ꢀ-MPP increases sharply due to the consumption of toluene
and propionic anhydride during the reaction. Even at this condi-
tion, the concentration of 2ꢀ-methylpropiophenone (2ꢀ-MPP) and
3ꢀ-methylpropiophenone (3ꢀ-MPP) increases slightly up to certain
limits. The scenario also shows that during reaction, the concen-
tration of propionic acid increases suddenly up to 20 min and then
drops gradually toward zero. This supports the above discussion of
formation and consumption of propionic acid during the reaction.
Indeed, carboxylic acids are good acylating agents with strong solid
acids [4].
(1). On the contrary because of the superacidity of UDCaT-5 used
here, a second reaction with propionic acid takes place (reaction
(4)) for 4ꢀ-methylpropiophenone. Toluene (MW = 92.14 g mol−1
)
reacts with priopionic anhydride (MW = 130.14 g mol−1) to produce
4ꢀ-methylpropiophenone (MW = 148.2 g mol−1) and propionic acid
(MW = 74.08 g mol−1) which further reacts to give the product
and water. Thus, in this case, 2 mol of toluene reacts with
1 mol of priopionic anhydride giving 2 mol of product and 1
mole of water (Scheme 1). In other words, in this reaction,
{(2 × 148.2)/(2 × 92.14 + 130.14)} × 100 = 94.26% atoms are incor-
porated in the monoacylated product. All three products 2ꢀ, 3ꢀ and
4ꢀ-methylpropiophenone are useful intermediates and can be sep-
arated. Thus, in this process atom efficiency is achieved. Indeed,
because of the superacidic nature of the catalyst; waste is reduced.
The other green parameters are given in Table 2 [18–21].
3.3. Effect of speed of agitation
3.2. Green metrics of the process
The effect of speed of agitation was studied in the range of
800–1200 rpm at a catalyst loading 0.06 g cm−3 at 180 ◦C (Fig. 5).
The mole ratio of toluene to propionic anhydride was kept 5:1.
There was no significant change in the rate and conversion patterns,
which was indicative of the absence of any resistance to exter-
nal mass transfer of propionic anhydride to the external surface
of the catalysts. However, all further reactions were carried out at
a speed of 1000 rpm. We have given the theoretical development
in some of our earlier work [9]. Theoretical calculations were also
done to establish that there was absence of external mass transfer
The green metrics of this process are presented in Table 2. Typ-
ically in acylation reactions; carboxylic acid anhydrides are used,
which lead to the formation of ketone and corresponding carboxylic
acid. Thus, the carboxylic acid has to be recovered. Depending on
the type of anhydride, the carboxylic acid loss is significant. In the
current case, the expected product of reaction is given by reaction
1.6
1.4
1.2
1
70
60
50
40
30
20
10
0
0.8
0.6
0.4
0.2
0
0
20
40
60
80
100 120 140
160 180 200
Time (min)
0
20
40
60
80
100 120
140
160
180
200
Time (min)
Fig. 4. Concentration profile of various products in acylation of toluene with
propionic anhydride. Reaction conditions: catalyst = UDCaT-5, speed of agita-
tion = 1000 rpm, catalyst loading = 0.06 g cm−3
, mole ratio of toluene:propionic
Fig. 5. Effect of speed of agitation on conversion of propionic anhydride.
Reaction conditions: catalyst = UDCaT-5, catalyst loading = 0.06 g cm−3, mole ratio
of toluene:propionic anhydride = 5:1, temperature = 180 ◦C, total reaction vol-
ume = 50 cm3, autogenous pressure: (᭹) 800 rpm, (ꢀ) 1000 rpm, (ꢁ) 1200 rpm.
anhydride = 5:1, temperature = 180 ◦C, total reaction volume = 50 cm3, autogenous
pressure: (ꢀ) propionic anhydride, (ꢂ) 4ꢀ-MPP, (ꢁ) 3ꢀ-MPP, (×) 2ꢀ-MPP, (᭹) propionic
acid.

G.D. Yadav, S.B. Kamble / Applied Catalysis A: General 433–434 (2012) 265–274
271
Table 2
Green metrics for the current work.
MW of product
MW of all reactants used in reaction
Atom economy for monoacylation
× 100
× 100
94.26%
ꢀ
ꢁ
Final mol of limiting reactant
Initial mol of limiting reactant
Conversion of propionic anhydride (limiting reactant)
1 −
62%
Final mol of 4ꢀ-methylpropiophenone
Total mol of all products
Selectivity of 4ꢀ-methylpropiophenone
Yield of 4ꢀ-methylpropiophenone
× 100
× 100
× 100
67%
Final mol of 4ꢀ-methylpropiophenone
Initial mol of propionic anhydride
41.54%
MW of product
MW of all reactants used in reaction
Atom economy of 4ꢀ-methylpropiophenone
Reaction mass efficiency (RME)
63.15%
20.77%
100%
Mass of isolated products
Total mass of reactants used in reaction
× 100
× 100
Mass of carbon in monoacylated products
Total mass of carbon in key reactants
Carbon efficiency of monoacylated products
Mass of carbon product
Total mass of carbon in key reactants
Carbon efficiency of 4ꢀ-methylpropiophenone
E factor (monoacylation)
× 100
67%
Mass of waste
Mass of product
0.06
NB: All ketones are useful products and can be separated. The selectivity to 4ꢀ-methylpropiophenone is considered. There was only monoacylation and water is the only
co-product of the overall reaction.
resistance. It confirms that the mass transfer rates were much
higher than rates of reaction and hence speed of agitation had no
influence on reaction rate.
directly proportional to the catalyst loading based on the entire
liquid phase volume. This indicates that, as the catalyst load-
ing increased, the conversion of propionic anhydride increases,
which is due to proportional increase in the number of active
sites of catalyst. There was only a little higher conversion of
propionic anhydride in case of 0.07 g cm−3 catalyst loading than
that of 0.06 g cm−3 (Fig. 6); even in this case the rate of reac-
tion was almost constant between 0.06 g cm−3 and 0.07 g cm−3
catalyst loading (Fig. 7). This suggests that the number of sites
available were more than required and hence all further reactions
were carried out with 0.06 g cm−3 catalyst loading as used in the
3.4. Effect of catalyst loading
The effect of catalyst loading was studied over range of
0.01–0.07 g cm−3 (Fig. 6). The initial rates of reaction, in the
absence of external mass transfer resistance, are plotted against
catalyst loading in Fig. 7. It demonstrates that the rates are
70
60
50
40
30
20
10
0
3.5
3
y = 45.858x
R2 = 0.9944
2.5
2
1.5
1
0.5
0
0
20
40
60
80
100
120
140
160
180
200
0
0.01
0.02
0.03
0.04
0.05
-3
0.06
0.07
0.08
Time (min)
Catalyst loading (g cm )
Fig. 6. Effect of catalyst loading on conversion of propionic anhydride. Reac-
tion conditions: catalyst = UDCaT-5, speed of agitation = 1000 rpm, mole ratio
of toluene:propionic anhydride = 5:1, temperature = 180 ◦C, total reaction vol-
Fig. 7. Plot of initial rate of reaction as a function of catalyst loading in liquid
phase. Reaction conditions: catalyst = UDCaT-5, speed of agitation = 1000 rpm, mole
ratio of toluene:propionic anhydride = 5:1, temperature = 180 ◦C, total reaction vol-
ume = 50 cm3, autogenous pressure.
ume = 50 cm3, autogenous pressure: (ꢂ) 0.01 g cm−3
,
(ꢁ) 0.02 g cm−3
, (ꢁ)
0.04 g cm−3, (×) 0.05 g cm−3, (ꢀ) 0.06 g cm−3, (᭹) 0.07 g cm−3
.

272
G.D. Yadav, S.B. Kamble / Applied Catalysis A: General 433–434 (2012) 265–274
70
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
0
20
40
60
80
100
120
140
160
180
200
0
20
40
60
80
100
120
140
160
180
200
Time (min)
Time (min)
Fig. 9. Effect of temperature on conversion of propionic anhydride. Reaction
conditions: catalyst = UDCaT-5, speed of agitation = 1000 rpm, catalyst load-
ing = 0.06 g cm−3, mole ratio of toluene:propionic anhydride = 5:1, total reaction
volume = 50 cm3, autogenous pressure: (ꢂ) 160 ◦C, (᭹) 170 ◦C, (ꢀ) 180 ◦C, (ꢁ) 190 ◦C.
Fig. 8. Effect of mole ratio of toluene:propionic anhydride on conversion of
propionic anhydride. Reaction conditions: catalyst = UDCaT-5, speed of agita-
tion = 1000 rpm, catalyst loading = 0.06 g cm−3, temperature = 180 ◦C, total reaction
volume = 50 cm3, autogenous pressure: (ꢂ) 1:1, (×) 3:1, (᭹) 4:1, (ꢀ) 5:1, (ꢁ) 6:1.
3.7. Effect of temperature
standard reaction. At this loading, the intraparticle diffusion resis-
tance sets in.
After the external mass transfer and intra-particle diffusion lim-
itations were eliminated, the effect of temperature on conversion
of propionic anhydride was studied under otherwise similar con-
ditions at 160, 170, 180 and 190 ◦C, respectively. It was observed
that the conversion of propionic anhydride increased with tem-
perature (Fig. 9). This would suggest an intrinsically kinetically
controlled mechanism which shows significant increase in the con-
version profile with temperature. With an increase in temperature,
both the rate of reaction as well as selectivity for 4ꢀ-MPP increased.
There is no significant difference in the final conversion of propi-
onic anhydride between 180 ◦C (62%) and 190 ◦C (65%), and also the
selectivity for 4ꢀ-MPP was 67% at 180 ◦C and 69% at 190 ◦C after 3 h of
the reaction course. Therefore, 180 ◦C is the optimum temperature
for this reaction.
3.5. Proof of absence of intraparticle resistance
The average particle size of UDCaT-5 was found to be in the
range of 40–50 ␮m. The catalyst is amorphous in nature; so it was
not possible to study the effect of catalyst particle size on the
rate of reaction. The average particle diameter of UDCaT-5 used
in the reactions was 0.0045 cm. Therefore, a theoretical calcula-
tion was done based on the Weisz-Prater criterion [22,23] to assess
the influence of intraparticle diffusion resistance. According to the
Weisz-Prater criterion, the value of {−robsꢀpRp2/De[As]} has to be
much less than unity for the reaction to be intrinsically kineti-
cally controlled; which represents the ratio of the intrinsic reaction
rate to the intraparticle diffusion rate. This can be evaluated from
the observed rate of reaction (−robs = 4.86 × 10−6 mol gcat−1 s−1),
density of catalyst particle (ꢀp = 1.255 g cm−3), the particle
radius (Rp = 2.25 × 10−3 cm), the effective diffusivity of the
limiting reactant (De = 1.549 × 10−4 cm2 s−1), and the concentra-
tion of the reactant at the external surface of the particle
([As] = 1.51 × 10−3 mol cm−3). The calculated value of 1.32 × 10−4 is
less than unity, which further reveals that the absence of intraparti-
cle diffusion resistance at the reaction conditions. A further proof of
the absence of the intraparticle diffusion resistance was obtained
through the study of the effect of temperature as discussed later.
3.8. Reusability of catalyst
The stability of the active species in solution has been of con-
cern for solid acids, since UDCaT-5 showed the best results, the
reusability studies were performed by employing them in a model
reaction. The model reaction was carried out by using 3 g of cat-
alyst and the experiments were properly scaled up. The catalyst
reusability was studied three times, including the use of fresh cata-
lyst. After each run the catalyst was filtered, and then refluxed with
50 cm3 methanol for 1 h in order to remove any adsorbed material
from catalyst surface within pores. The catalyst was then dried at
120 ◦C for 3 h and weighed before using in the next batch. The actual
amount of catalyst used in the next batch, was almost 5% less than
the previous batch. There was some attrition of catalyst particle
during agitation. In a typical batch reaction, there were inevitably
losses of particles during filtration due to attrition. Although the
catalyst was washed after filtration to remove all adsorbed reac-
tants and products, there was still a possibility of retention of small
amount of adsorbed reactants and products species which might
cause the blockage of active sites of the catalyst and loss in yield
on successive runs. These are apparent factors for the loss in activ-
ity, which have been taken into account because no fresh catalyst
was added into the reaction mixture to maintain the same catalyst
loading during reusability tests. But this catalyst could be recovered
3.6. Effect of mole ratio
The effect of toluene to propionic anhydride mole ratio was
studied at 1:1 to 6:1 by keeping the catalyst loading constant. The
conversion of propionic anhydride was found to increase with an
increase in concentration of toluene (Fig. 8). Beyond a mole ratio
5:1 there was no significant difference between the conversion lev-
els of propionic anhydride. Therefore, all further reactions having
been studied by using a toluene to propionic anhydride mole ratio
of 5:1.

G.D. Yadav, S.B. Kamble / Applied Catalysis A: General 433–434 (2012) 265–274
273
0.0
70
60
50
40
30
20
10
0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
2.14E-03
2.18E-03
2.22E-03
2.26E-03
1/T (K -1
2.30E-03
2.34E-03
2.38E-03
)
Fig. 11. Arrhenius plots. (ꢀ) reaction (1), (ꢁ) reaction (2), (᭹) reaction (3), (ꢂ) reac-
tion (4), (ꢁ) reaction (5), (×) reaction (6).
0
20
40
60
80
100
Time (min)
120
140
160
180
200
Fig. 10. Reusability of catalyst. Reaction conditions: catalyst = UDCaT-5, speed of
agitation = 1000 rpm, catalyst loading = 0.06 g cm−3, mole ratio of toluene:propionic
anhydride = 5:1, temperature = 180 ◦C, total reaction volume = 50 cm3, autogenous
pressure: (ꢀ) fresh catalyst, (᭹) first reuse, (ꢂ) second reuse.
where, A = propionic anhydride, B = toluene, P1 = propionic
acid,
P2 = 2ꢀ-methylpropiophenone
(2ꢀ-MPP),
P3 = 3ꢀ-
methylpropiophenone (3ꢀ-MPP), P4 = 4ꢀ-methylpropiophenone
(4ꢀ-MPP), W = water, and k1, k2, k3, k4, k5, k6 are the rate constants
of respective reactions.
and subsequently reused several times in a solventless system. It
showed no significant loss of activity after three successive runs. It
was observed that there was only a marginal decrease in conver-
sion (Fig. 10) and there was no effect on selectivity of the products.
When make-up quantity of the catalyst was added, almost similar
conversions were found to suggest that the catalyst was stable and
reusable.
The net rate of consumption of propionic anhydride (A) by three
parallel reactions (Eqs. (1)–(3)) is given by:
dCA
−rA = −
= k1wCACB + k2wCACB + k3wCACB
dt
= (k1 + k2 + k3)wCACB
(7)
3.9. Reaction kinetics
The rate of reaction of propionic anhydride (−rA) is in
mol cm−3 s−1, the concentrations in mol cm−3, the second order
rate constant in cm6 gcat−1 mol−1 s−1 and catalyst loading (w)
in g cm−3. Similarly, the net rate of consumption of toluene
(−rB) and the net rate of formations of propionic acid (rP1 ),
The above results were used to build a kinetic model. The
reaction involves two organic phase reactants, limiting reactant
A (propionic anhydride), excess reactant B (toluene); the desired
product P4 (4ꢀ-methylpropiophenone) and co-product P1 (propi-
onic acid). Since A and B are liquid phase reactants, they need to
diffuse to the interior surface of the catalyst.
Various parallel as well as consecutive reactions take place
over UDCaT-5 as shown in Scheme 1. Toluene (B) and propi-
onic anhydride (A) are chemisorbed on the acidic sites. This is a
complex reaction network with parallel reaction of toluene with
propionic anhydride and consecutive reactions of toluene with
propionic acid (which was produced during the reaction pro-
cess) take place as shown in reactions (1)–(6) of Scheme 1. The
Langmuir–Hinshelwood–Hougen–Watson (LHHW) type of kinetic
model will result into a Power Law model; if the adsorption of all
species is weak [24]. The different reaction steps involved in this
process are according to the Power Law model, which was found
to hold for acid catalyzed reactions. For a fixed catalyst loading (w),
the rates of reaction or formation can be established. The following
reaction steps occurred over UDCaT-5:
2ꢀ-methylpropiophenone (rP ), 3ꢀ-methylpropiophenone (rP3 ), 4ꢀ-
2
methylpropiophenone (rP ) and water (rW) are as follows:
4
dCB
−rB = −
= (k1 + k2 + k3)wCACB + (k4 + k5 + k6)wCBCP
(8)
(9)
1
dt
dCP
4
3
2
1
rP
rP
rP
rP
=
=
=
=
=
= k1wCACB + k4wCBCP
4
3
2
1
1
dt
dCP
dt
= k2wCACB + k5wCBCP
(10)
(11)
(12)
(13)
1
dCP
dt
= k3wCACB + k6wCBCP
1
dCP
dt
= (k1 + k2 + k3)wCACB − (k4 + k5 + k6)wCBCP
1
dCW
dt
rW
= (k4 + k5 + k6)wCBCP
1
k
From the knowledge of concentration profiles (Fig. 4), the indi-
1
A + B−→ P4 + P1
(1)
(2)
(3)
(4)
(5)
(6)
vidual reaction rate constants could be calculated. Mathcad or
Polymath package was used to extract the rate constants (Table 3).
The values of rate constant or initial rate at different tempera-
tures were used to estimate the frequency factor (k0) and apparent
activation energy (E) of each reaction by making Arrhenius plots
(Fig. 11) and are tabulated in Table 3. The values of rate constant and
activation energy of all six reactions suggest that reaction 1 is faster
as compared to other reactions and the rates of all six reactions
are in the following order: reaction (1) (fast reaction) > reaction
(3) > reaction (4) > reaction (2) > reaction (6) > reaction (5) (slow
k
2
A + B−→ P3 + P1
k
3
A + B−→ P2 + P1
k
4
P1 + B−→ P4 + W
k
5
P1 + B−→ P3 + W
k
6
P1 + B−→ P2 + W

274
G.D. Yadav, S.B. Kamble / Applied Catalysis A: General 433–434 (2012) 265–274
Table 3
Kinetic parameters of various reactions.
Reaction number
Reactiona
Rate
Frequency
Activation
energy, E
constant, k at 180 ◦C
(cm6 gcat−1 mol−1 s−1
factor, k0
)
(cm6 gcat−1 mol−1 s−1
)
(kcal mol−1
)
k
1
1
2
3
4
5
A + B−→ P4 + P1
0.386
0.148
0.219
0.165
0.084
0.118
2.87 × 102
1.21 × 104
8.93 × 102
3.03 × 103
2.53 × 106
4.95 × 105
5.97
10.15
7.43
k
2
A + B−→ P3 + P1
k
3
A + B−→ P2 + P1
k
4
P1 + B−→ P4 + W
P1 + B−→ P3 + W
P1 + B−→ P2 + W
8.75
k
5
15.53
13.72
k
6
6
a
Where, A = propionic anhydride, B = toluene, P1 = propionic acid, P2 = 2ꢀ-methylpropiophenone (2ꢀ-MPP), P3 = 3ꢀ-methylpropiophenone (3ꢀ-MPP), P4 = 4ꢀ-
methylpropiophenone (4ꢀ-MPP), W = water, and k1, k2, k3, k4, k5, k6 are the rate constants of respective reactions.
reaction). This also indicates that acylation of toluene with propi-
onic anhydride is the faster reaction than acylation of toluene with
propionic acid (Table 3). The values of activation energy also sup-
ported the fact that the overall rate of reaction is not influenced by
either external mass transfer or intraparticle diffusion resistance
and it is an intrinsically kinetically controlled reaction on active
sites of the catalyst.
Professorships. He also thanks Department of Science and Technol-
ogy, Government of India for J.C. Bose National Fellowship. S.B.K.
acknowledges receipt of Senior Research Fellowship (SRF) from
University Grants Commission (UGC), New Delhi.
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Friedel–Crafts acylation of toluene with propionic anhydride
to synthesise 4ꢀ-methylpropiophenone was studied at 180 ◦C over
variety of ecofriendly solid acid catalysts such as UDCaT-4, UDCaT-
5, UDCaT-6 and sulfated zirconia. UDCaT-5 was found to be the
most active, selective, stable and reusable catalyst. The effect of
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product distribution over UDCaT-5 were discussed systematically.
The rate of reaction increased with temperature, catalyst loading
and toluene to propionic anhydride mole ratio. A kinetic model for
the reaction mechanism was successfully developed and it follows
Power Law model, whereby toluene and propionic anhydride are
weakly adsorbed on the surface of the catalyst particle. The co-
product propionic acid generated in situ also reacts with toluene.
Water is the only co-product of the overall reaction. The values
of activation energy also supported the fact that the overall rate
of reaction is not influenced by either external mass transfer or
intraparticle diffusion resistance and it is an intrinsically kinetically
controlled reaction on active sites of the catalysts. The reaction is
completely solvent free; hence leads to minimizing the waste and
hazards, which could be advantageous as a green and clean method
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G.D.Y. acknowledges financial support from Darbari Seth
Endowment and R.T. Mody Endowment for the Chair of