P-Cymenesulphonic acid: An organic acid synthesised from citrus waste

Article abstract of DOI:10.1016/j.cattod.2011.12.007

An organic acid, p-cymene-2-sulphonic acid, is synthesised from citrus waste and demonstrated to be comparable to p-toluenesulphonic acid in examples of acid catalysis. Firstly the essential oil found in citrus waste is extracted by either steam distillation or microwave irradiation. Oxidising the limonene in the citrus oil to p-cymene followed by sulphonation gives p-cymene-2-sulphonic acid.

Full text of DOI:10.1016/j.cattod.2011.12.007

Catalysis Today 190 (2012) 144–149
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Catalysis Today
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p-Cymenesulphonic acid: An organic acid synthesised from citrus waste
James H. Clark∗, Emma M. Fitzpatrick, Duncan J. Macquarrie, Lucie A. Pfaltzgraff, James Sherwood
Green Chemistry Centre of Excellence, Department of Chemistry, University of York, York YO10 5DD, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
An organic acid, p-cymene-2-sulphonic acid, is synthesised from citrus waste and demonstrated to be
comparable to p-toluenesulphonic acid in examples of acid catalysis. Firstly the essential oil found in
citrus waste is extracted by either steam distillation or microwave irradiation. Oxidising the limonene in
the citrus oil to p-cymene followed by sulphonation gives p-cymene-2-sulphonic acid.
© 2011 Elsevier B.V. All rights reserved.
Received 30 September 2011
Received in revised form 7 December 2011
Accepted 7 December 2011
Available online 4 January 2012
Keywords:
Acid catalysis
Citrus waste
p-Cymene
p-Cymenesulphonic acid
Green chemistry
Limonene
1. Introduction
Although the reaction benefits from acetone as a value-adding co-
product the transformation is no longer performed industrially.
The use of p-toluenesulphonic acid (p-TSA) is ubiquitous
throughout synthetic organic chemistry [1]. As a strong acid that
is also soluble in organic solvents, it is widely used as a Brønsted
acid catalyst for Fischer esterifications and similar reactions. His-
torically p-TSA was synthesised by the sulphonation of toluene as
an intermediate in the production of p-cresol [2]. Because this valu-
able process is based on a petrochemical substrate it is beneficial
to seek a sustainable alternative. As prices and demand for lim-
ited oil resources continue to rise worldwide, alternative renewable
feedstocks will become a necessity. The general public, followed
by the retail sector, are pushing for greener products and formula-
tions and in doing so increasing the need for bio-chemicals. Recent
developments in EU directives and future standards on bio-based
products will encourage greater use of biomass instead of finite
resources towards a point where it becomes both economically and
environmentally beneficial to do so.
The most easily accessible renewable replacement for toluene
is p-cymene, derived from naturally occurring monoterpenes. It is
a major component of the spruce turpentine obtained from the
sulphite wood pulping process [2,3]. p-Cymene can fulfil an analo-
gous role to toluene in many respects. Primarily the sulphonation
of p-cymene gives p-cymene-2-sulphonic acid (p-CSA) [3–7]. p-
Cresol can also be made from p-cymene via its hydroperoxidation.
The sulphonation of p-cymene from turpentine has been per-
formed as a means of obtaining carvacrol, analogous to the alkali
fusion of p-TSA by which p-cresol is made [3]. Sulphonation occurs
primarily at the 2-position relative to the methyl group, and this
isomer can be selectively recrystallised from the reaction mixture
(Scheme 1). Sodium p-cymenesulphonate is regarded as a good
hydrotrope [8].
An alternative route for obtaining p-cymene is the oxidation of
the (R)-(+)-limonene (henceforth referred to as limonene) present
in the peel of citrus fruits [9,10]. Although commercially signifi-
cant terpenes have been artificially synthesised since the 1950s,
limonene is marketed as a natural product sought for its aroma
and solvent properties. In terms of international trade, citrus fruits
are considered as a commodity product on a par with coffee or tea.
As the largest fruit crop in the world with over 110 million tonnes
grown every year [11], the processing industry, where up to 60% of
the whole fruit is discarded, generates citrus waste (peel, seeds,
membrane, and pulp) on a massive scale [12]. Orange process-
ing is responsible for 90% of the citrus waste created by juicing
operations [13]. The largest contributing nation is Brazil, produc-
ing 8 million tonnes of orange waste every year. The waste does not
currently have any high-value applications. Instead the majority is
disposed of or pelletised for animal feed [11,12].
Citrus oil yields of up to 5% based on the dry citrus waste
mass have been obtained using supercritical carbon dioxide extrac-
tion technology, although steam distillation and its variants are
more routine methods where extraction efficiencies of over 90%
are obtainable [14,15]. Considering that 5 kg of essential oil can be
∗
Corresponding author. Tel.: +44 01904 432559; fax: +44 01904 432705.
E-mail address: james.clark@york.ac.uk (J.H. Clark).
0920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.12.007

J.H. Clark et al. / Catalysis Today 190 (2012) 144–149
145
2.2. Sulphonation of p-cymene
To p-cymene (5 mL) was slowly added 20% fuming sulphuric
acid (5 mL). The reaction was stirred at room temperature for 4 h.
After this time had elapsed stirring was stopped and water (6 mL)
carefully added to avoid the mixture becoming hot. The diluted
mixture was left to stand in a refrigerator overnight to produce a
solid, which could be recrystallised from concentrated hydrochlo-
ric acid to give p-cymene-2-sulphonic acid dihydrate, 7.29 g, 91%.
NMR (400 MHz in DMSO-d6): ıH 7.61 (1H, s), 7.08 (2H, m), 6.10 (1H,
s), 2.83 (1H, m), 2.46 (3H, s), 1.14 (6H, d)/ppm. NMR (100 MHz in
DMSO-d6): ıC 145.3, 145.1, 133.1, 131.2, 127.4, 124.7, 33.2, 24.3,
19.9/ppm. MS (−EI): m/z = 213 (M−). Melting point (Tm): 50–51 ◦C.
Scheme 1. The 2- (left) and 3- (right) isomers of p-cymenesulphonic acid. For sim-
plicity p-CSA is used in this work to refer to the former isomer only.
extracted from a tonne of oranges, then almost 92,000 tonnes of
essential oil could be obtained from Brazil’s production of oranges
alone [12]. Because limonene constitutes at least 90% of the essen-
tial oil from oranges, Brazil could potentially produce in the order
of 82,500 tonnes of limonene annually.
2.3. Steam extraction of citrus waste
Limonene is not the only useful product that can be retrieved
from citrus waste [12]. Ethanol or succinic acid can be produced by
fermentation after removal of the essential oil [15,16]. Then pectin
can be retrieved before the remaining residues are anaerobically
digested to afford methane [16]. This bio-refinery model has been
demonstrated as being economically viable [17], offering a prof-
itable and environmentally valuable alternative to current waste
disposal practices. In doing so it is possible to harness the chemi-
cal potential of waste using green chemical technologies to obtain
sought-after properties for everyday products.
The flavedo (outer peel) was separated from the albedo (inner
peel) of sixteen oranges (Navel late variety, diameter of 70–80 mm).
Using a fine grater gave 110 g of wet citrus waste. To the separated
flavedo was added 150 mL of water and distilled for 1 h. The organic
phase of the distillate was dried to give 5.71 g of the essential oil
(5.2 wt%).
2.4. Microwave assisted extraction of citrus waste
The microwave treatment of the whole orange peel (160 g)
Demand for p-cymene has made its synthesis from limonene a
viable chemical process. One burgeoning application is the synthe-
sis of terephthalate polyesters using limonene as the start material
for terephthalic acid via p-cymene [18–20]. Metal functionalised
clays have been shown to be active dehydrogenation catalysts in
the conversion of limonene to p-cymene. Microwave assisted meth-
ods give quantitative yields of p-cymene from limonene but require
almost 1.5 equivalents of an iron impregnated clay (based on iron
loading) [21]. Reducing the catalyst loading to 4 mol% resulted in
88% selectivity. More exotic catalysts have also been used, notably
palladium. This approach is hindered by the disproportionation of
limonene to give substantial quantities of p-menthane. A Pd/SiO2
catalyst is effective for this transformation only if 1-decene is used
as a hydrogen acceptor [22].
was conducted in
a Milestone ROTO SYNTH Rotative Solid
Phase Microwave Reactor with a 2.45 GHz multimode cavity. The
extraction was performed for 12.5 min with a power gradient
applied, increasing from 400 W to 1200 W (average power input
1025 W min−1) until reaching a final temperature of 200 ◦C. Liquid
products were then collected by distillation. The aqueous phase
was extracted with ethyl acetate and the combined organic frac-
tions dried to give 1.71 g of the essential oil (1.1 wt%). Analysis by
GC confirmed the similarity of the citrus oil to that extracted by
steam distillation, being predominantly limonene (≥90%).
2.5. Complete synthesis of p-CSA from the essential oil of oranges
The steam extracted citrus oil of sixteen oranges was added
dropwise to a mixture of 10 wt% Pd/C (0.434 g, 1 mol%) and K-10
montmorillonite clay (0.815 g), pre-heated to 140 ◦C, and stirred for
an hour once addition of the citrus oil was complete. The reaction
was allowed to cool and 100 mL of water added. The solution was
distilled, and the organic phase of the distillate dried to give crude
p-cymene (4.43 g, 88% yield based on limonene content of citrus
oil, 70% selectivity). To this colourless liquid was carefully added
3.5 mL of 20% fuming sulphuric acid. The reaction was stirred at
room temperature for 4 h. After this time had elapsed stirring was
stopped and water (4.2 mL) carefully added to avoid the mixture
becoming hot. At this stage the p-menthane co-product caused by
limonene disproportionation can be decanted. The diluted mixture
was left to stand in a refrigerator overnight to solidify. The solid was
retrieved by filtration to give p-cymene-2-sulphonic acid dihydrate
(27% yield based on the p-cymene content of the reaction distillate,
16% total yield based on the limonene content of the citrus oil, and
27 wt% based on the mass of citrus oil).
2. Experimental
All reactions were conducted under an ambient atmosphere and
without any purification of reagents prior to use. The Hammett
acidity functions were determined using a Jasco V-550 UV/visible
spectrophotometer. Thermal decomposition temperatures were
obtained using a PL Thermal Sciences system (STA 625). Character-
isation of reaction products was consistent with authentic samples
where available.
2.1. Synthesis of p-cymene
A mixture of 10 wt% Pd/C (0.788 g, 1 mol%) and K-10 montmo-
rillonite clay (1.48 g) was heated to 140 ◦C, and limonene (12 mL,
74.1 mmol) slowly added dropwise. The mixture was stirred at
140 ◦C for 1 h. The reaction then was cooled to room temperature
and water (100 mL) added prior to steam distillation. The organic
phase of the distillate was dried to give p-cymene as a colourless
liquid (8.87 g, 89% yield, 71% selectivity as determined by GC–MS
and 1H NMR spectroscopy). NMR (400 MHz in CDCl3): ıH 7.22 (4H,
m), 2.98 (1H, m), 2.43 (3H, s), 1.35 (6H, d)/ppm. NMR (100 MHz in
CDCl3): ıC 146.1, 135.3, 129.2, 126.5, 33.9, 24.3, 21.1/ppm. MS (+EI):
m/z = 135 (M+).
2.6. Esterification kinetic experiments
To a solution of benzyl alcohol (0.541 g, 5.0 mmol) and the acid
catalyst (p-TSA or p-CSA, 0.05 mmol) in the chosen solvent (5 mL)
stirred at 50 ◦C was added acetic acid (0.330 g, 5.5 mmol). Aliquots
of the reaction mixture were removed at convenient intervals and
diluted with deuterated chloroform to allow the reaction progress
to be monitored by 1H NMR spectroscopy by the same method

146
J.H. Clark et al. / Catalysis Today 190 (2012) 144–149
described by Welton and co-workers [23]. Comparison of the ben-
zylic signals of benzyl alcohol (e.g. 4.73 ppm where p-cymene was
the reaction solvent) and benzyl acetate (e.g. 5.14 ppm where p-
cymene was the reaction solvent) could be used to determine the
extent of acetylation using the integrated form of the second order
rate equation as follows:
ꢀ
ꢁ
1
[A]0([B]0 − [P]t)
[B]0([A]0 − [P]t)
k2t =
· ln
Scheme 2. Two step synthesis of p-CSA from limonene.
[B]0 − [A]o
Key: k2, rate constant (dm3 mol−1 s−1); t, time (s); [A]0, initial con-
centration of reactant A (mol dm−3); [B]0, initial concentration of
reactant B (mol dm−3); [P]t, concentration of product P at time t
(mol dm−3).
optimisation purposes, our initial studies were performed on pure
limonene (Sigma–Aldrich®) for the transformation to p-cymene
is key in this synthesis. Despite a catalogue of successful exam-
ples in the literature, the reaction is not uncomplicated as the
sequential isomerisation-dehydrogenation of limonene is prone
to interference by unwanted side reactions. The initial isomerisa-
tion of limonene to ␣-terpinene in situ can be acid catalysed but
the use of strong acids was observed to lead to the decomposi-
tion of the substrate. The isomerisation of limonene to terpinolene
is slow, while the irreversible isomerisation of this intermediate
to ␣-terpinene is relatively rapid [24]. Under acidic conditions at
elevated temperatures, ␣-terpinene is expected to be in equilib-
rium with the other terpinene isomers, although only ␣-terpinene
can be directly dehydrogenated to give p-cymene. Therefore it is
desirable to suppress the reactions of the unwanted isomers and
intercept the isomerisation of limonene at the stage where dehy-
drogenation to p-cymene can occur. Heating a slurry of limonene
and K-10 montmorillonite clay to 100 ◦C for 1 h was again unsat-
isfactory, as was the use of clay supported metal salts. However
reducing the loading of K-10 montmorillonite clay to 1 mol% (based
on the number of acid sites [25]) and applied in conjunction with
10 wt% Pd/C improved selectivity from 28% to 54%. Increasing the
palladium load beyond 1 mol% had minimal benefit, but increasing
the reaction temperature from 100 ◦C to 140 ◦C gave 71% selectiv-
ity. The predominant co-product of the reaction was p-menthane,
produced in the disproportionation of limonene [22]. Attempts to
use hydrogen acceptors (1-decene, levulinic acid) actually inhib-
ited the reaction. Diluting the reaction using p-cymene as a solvent
did not improve the selectivity of the reaction either.
Reactions were typically allowed to proceed beyond 50% con-
version to guarantee accuracy in the calculation of rate constants.
Predicted conversions were also calculated using the integrated
second order rate equation. Spectra were recorded with a Bruker
400 MHz spectrometer, and chemical shifts calibrated against the
residual solvent signal. No calibration of the NMR proton signal
integrals was found to be necessary.
2.7. Synthesis of ethyl levulinate
To a solution of levulinic acid (0.581 g, 5.0 mmol) in the chosen
solvent (5 mL) was added either a Brønsted acid (p-TSA or p-CSA,
0.05 mmol) or a Lewis acid (In(OTf)3, InCl3, or FeCl3, 0.25 mmol)
followed by ethanol (0.230 g, 5.0 mmol). The reaction mixture was
stirred at 50 ◦C under the ambient atmosphere for 20 h. After this
time, the reaction was cooled and potassium carbonate added. Fil-
tration of the reactions performed in either toluene or 2-MeTHF
gave a filtrate that could be concentrated in vacuo to give the desired
product, ethyl levulinate, in yields of up to 73% of the theoretical
yield. Reactions conducted in p-cymene were similarly filtered and
purified by column chromatography (hexane:ethyl acetate) to give
ethyl levulinate (up to 76% yield).
2.8. Synthesis of 4-bromochalcone
A
mixture of acetophenone (0.25 mL, 2.03 mmol), 4-
Concerned by the rarity and market price of palladium, other
metals were screened as potential dehydrogenation catalysts.
Heating ␣-terpinene in combination with different metal salts at
140 ◦C always resulted in some p-cymene being formed (Table 1).
The reaction between ␣-terpinene and various palladium species
gave a comparable yield of p-cymene after 1 h to that obtained
from the action of K-10 montmorillonite clay and 10 wt% Pd/C
on limonene. Again the reaction principally proceeds via a dis-
proportionation mechanism to give p-menthane as a co-product
(Scheme 3). These results suggest that the acidic clay mediated iso-
merisation of limonene to ␣-terpinene is quite satisfactory and it
is the action of the metal catalyst that is responsible for the less
than complete selectivity towards p-cymene. It was decided to pro-
ceed using Pd/C in combination with K-10 montmorillonite clay as
bromobenzaldehyde (0.562 g, 3.04 mmol, and the acid catalyst
(p-TSA or p-CSA, 0.10 mmol) was stirred at 120 ◦C under the
ambient atmosphere for 24 h. The reaction was then allowed to
cool, giving rise to fine needles of a white solid and an amorphous
orange solid. The latter could be removed by recrystallisation with
ethanol to give 4-bromochalcone (73% yield).
2.9. Synthesis of 2-(4-nitrophenyl)-1,3-dioxolane
A suspension of 4-nitrobenzaldehyde (1.511 g, 10.00 mmol) and
the acid catalyst (p-TSA or p-CSA, 0.200 mmol) in cyclohexane was
heated to reflux in Dean–Stark apparatus, at which point the mix-
ture became homogeneous. 1,2-Ethanediol (0.621 g, 10.00 mmol)
was then added and the reaction stirred at reflux under the ambi-
ent atmosphere for 5 h. The reaction was the left to cool, allowing
the product, 2-(4-nitrophenyl)-1,3-dioxolane, to precipitate and be
isolated by filtration (92% yield).
Table 1
Dehydrogenation of ␣-terpinene (analysed by 1H NMR spectroscopy).
Catalyst
Loading
Conversion
Selectivity
3. Results and discussion
10 wt% Pd/C
PdCl2
Pd(OAc)2
PtCl2
5%
5%
5%
5%
5%
5%
5%
5%
0%
100%
100%
91%
61%
55%
52%
52%
51%
41%
82%
72%
70%
66%
31%
68%
26%
46%
49%
3.1. Synthesis of p-CSA
FeCl3
The conversion of limonene to p-CSA consists of three dis-
tinct stages (Scheme 2). Firstly the isolation of the essential oil
from citrus waste, followed by the aromatisation of limonene
to p-cymene, and finally sulphonation to give the acid. For
Ni(OAc)2·4H2O
CuCl2
ZnCl2
Blank

J.H. Clark et al. / Catalysis Today 190 (2012) 144–149
147
Table 3
Selected physical properties of p-TSA and p-CSA.
Property
p-TSA
p-CSA
Tm (◦C)
106–107
206
1.22
50–51
203
1.26
Td (◦C)
H0 (80 mmol dm−3
)
3.2. Characterisation of p-CSA
Scheme 3. Disproportionation of ␣-terpinene.
Prior to its application as an acid catalyst it was fitting to deter-
mine the acidity of p-CSA. The Hammett acidity function (H0) is an
equivalent measurement to pH for strong acids [28]. As a method
of calculating the ability of strong acids to protonate a weak base
by UV–visible spectroscopy, it is now routinely used as part of
the characterisation of novel sulphonic acids [29]. As expected
from electronic arguments, the acid strength of alkyl functionalised
arenesulphonic acids is inversely proportional to the degree of sub-
stitution present, and so p-CSA is a slightly weaker acid than p-TSA.
However the difference is less than that between p-TSA and ben-
zenesulphonic acid (H0 = 1.12 at 80 mmol dm−3).
One benefit of using p-TSA as a catalyst in synthesis is that it can
be easily washed out of an organic phase with water. Biphasic sys-
tems of ethyl acetate (20 mL) and water (20 mL) containing either
0.05 mmol of p-TSA or p-CSA were prepared to represent a post-
reaction work-up. Neither organic layer contained any sulphonic
acid in the organic layer after mixing as determined by 1H NMR
spectroscopy. It is noticeable that the system containing p-CSA is
slower to separate after agitation, but addition of sufficient water
allowed the complete removal of the acid from the organic phase.
The thermal stability of both sulphonic acids is also comparable.
These observations are summarised in Table 3.
co-catalysts to aid any potential recycling of the palladium. Steam
distillation can be used to retrieve the p-cymene prior to sulphona-
tion although unfortunately p-menthane is also observed in the
distillate.
In the optimisation of the final sulphonation stage, the reac-
tion of p-cymene (Sigma–Aldrich®) with concentrated sulphuric
acid was found to be unsatisfactory; after stirring with 2 equiva-
lents of concentrated sulphuric acid at 100 ◦C for 4 h a yield of only
30% was obtained upon cooling and subsequent solidification of
the reaction mixture. The use of 20% fuming sulphuric acid at room
temperature increased the yield significantly [4]. The crude prod-
uct could be isolated from the reaction by the addition of water,
which induces the solidification of the product [3,7]. Recrystallisa-
tion from concentrated hydrochloric acid results in a white solid,
previously described in the literature as the dihydrate of p-CSA in
yields exceeding 90% of the theoretical maximum [4,5].
Once the individual stages of the synthesis had been demon-
strated, the complete synthesis of p-CSA from citrus waste was
explored. Two methods for the isolation of citrus oil were
attempted. Steam distillation of orange peel afforded the essential
oil in yields equating to a little over 1 g of essential oil for every three
fruits. The alternative process was based on microwave assisted oil
extraction [26]. Processing of food waste with microwave irradia-
tion may prove useful for creating further valuable products within
a bio-refinery as previously demonstrated for wheat straw [27].
However the procedure resulted in lower than anticipated yields
of citrus oil and significant colourisation. Furthermore yields of p-
cymene were lower when the citrus oil was obtained by microwave
assisted extraction, most probably due to trace impurities interfer-
ing with the mode of catalysis (Table 2).
The citrus oil obtained by steam distillation was dried and
reacted with K-10 montmorillonite clay and 10 wt% Pd/C at 100 ◦C
for 1 h, after which the mixture was cooled then steam distilled to
give the crude p-cymene. The extent of aromatisation was simi-
lar to the optimisation studies using pure limonene. Unfortunately
the sulphonation of the crude p-cymene distillate was much poorer
than the analogous procedure on pure p-cymene. In both cir-
cumstances the quantity of 20% fuming sulphuric acid added was
proportional to the amount of p-cymene present in the substrate,
and the reaction stirred at room temperature for 4 h. Despite this
the yield of p-CSA dropped to 27%. Nevertheless the product was
identical to p-CSA made from pure p-cymene. 1H NMR and 13C NMR
spectroscopy techniques were used to establish the regioselectiv-
ity of the sulphonation on the isolated product. As expected only
the 2-isomer was present after recrystallisation.
3.3. Application of p-CSA
Chosen from the most common uses of organic acids, ester-
ification, aldol condensation, and acetal forming reactions were
conducted to demonstrate the similar performances of p-TSA and
p-CSA in the catalysis of these procedures. Firstly the kinetics of
a model esterification were explored in a variety of different sol-
vents (Scheme 4). The choice of solvent was found to have an
influence on the catalysis of the reaction. In agreement with previ-
ous studies, solvents with low hydrogen bond accepting abilities,
as measured on the Kamlet–Abbout–Taft ␤ scale, accentuate the
kinetic benefit provided by the catalyst to the esterification process
[23]. Values of the ␤ scale were calculated using a comparison of the
UV–visible spectra of 4-nitroaniline and N,N-diethyl-4-nitroaniline
in accordance with literature precedent [30]. Good examples of
appropriate solvents are toluene and p-cymene, the precursors to
the sulphonic acids evaluated in this work. Solvents that are able to
accept hydrogen bonds are likely to interact with acid catalysts and
hinder protonation of the reactants as required in the rate deter-
mining step. This effect can be represented by a linear free energy
relationship (Fig. 1).
Table 2
Dependency of p-cymene production efficiency on limonene purity.
Source of limonene
Yield
Selectivity
Purchased (Sigma–Aldrich®
Citrus waste (steam distillation)
Citrus waste (microwave extraction)
)
89%
88%
64%
71%
70%
79%
Scheme 4. Model esterification: Acetylation of benzyl alcohol to yield benzyl
acetate.

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J.H. Clark et al. / Catalysis Today 190 (2012) 144–149
Fig. 2. Isolated yields of ethyl levulinate arising from different conditions. Reactions
were performed overnight at 50 ◦C.
Fig. 1. Rates of esterification catalysed by p-TSA (diamonds/dashed line) and p-CSA
(squares/solid line) in the following solvents (left to right): chloroform, toluene,
p-cymene, acetonitrile, butanone, and 2-methyltetrahydrofuran (2-MeTHF).
Table 4
Calculated acetylation yields.
Solvent
Calculated conversion after 6 h
p-CSA catalysed
p-TSA catalysed
p-Cymene
Acetonitrile
2-MeTHF
73%
30%
10%
71%
33%
12%
Scheme 6. Reaction B: Chalcone synthesis via the condensation of an aldehyde and
a ketone to give 4-bromochalcone.
in p-cymene, toluene and 2-MeTHF using either p-TSA (1 mol%), p-
CSA (1 mol%), In(OTf)3 (5 mol%), InCl3 (5 mol%), or FeCl3 (5 mol%)
as catalysts. The performances of the catalysts were fairly simi-
lar, although the Lewis acids had to be used in higher loadings to
compete with the sulphonic acids. The comparable yields suggest
that the equilibrium position of the reaction was being approached,
which is not unreasonable given that equimolar quantities of the
reactants were used. Both the Brønsted acid catalysts were effective
(Fig. 2).
Furthermore, the synthesis of 4-bromochalcone via an aldol
condensation (Reaction B, Scheme 6) and the acetal protection of
4-nitrobenzaldehyde with 1,2-ethanediol (Reaction C, Scheme 7)
were performed as further demonstrations of acid catalysis by p-
CSA. Both these types of procedure have been used previously as
demonstrations of novel sulphonic acid catalysis and so were fit-
ting in this context also [31,32]. Reaction B was performed without
an auxiliary solvent, and the product recrystallised from ethanol.
After refluxing in cyclohexane, the acetal product of Reaction C
precipitated from the reaction mixture upon cooling.
To a good approximation both p-TSA and p-CSA perform equally
well as acid catalysts in this esterification at a 1 mol% loading, as
shown by the following equations derived from Fig. 1:
p-CSA catalysis : ln(k) = −8.38 − 6.68ˇ, R2 = 0.981
p-TSA catalysis : ln(k) = −8.67 − 5.72ˇ, R2 = 0.988
However there is an indication that p-TSA is superior to p-CSA in
high polarity solvents, while p-CSA marginally enhances reaction
rates in low polarity solvents. Given the slighter superior acidity of
p-TSA, the former observation is expected. The latter observation is
most probably due to differences in the solubility of the catalysts,
or more precisely the stability of their conjugate bases in solution
during protonation of the reactive substrate. To alleviate any con-
cern that in certain solvent systems p-CSA would be a significantly
poorer catalyst than p-TSA, we have used the observed rate con-
stants of this model reaction to calculate the expected conversion
to the ester product after 6 h under the conditions explained in
Section 2. There is a maximum of only 3% difference between con-
versions in any solvent system, three of which are shown in Table 4.
Regardless, this reaction and similar carbonyl additions proceed
much more efficiently in solvents without a strong tendency to
accept hydrogen bonds, and in these instances p-CSA accelerates
the reaction beyond the rates obtained by p-TSA catalysis.
A comparison of Reaction A, Reaction B, and Reaction C shows
that p-TSA fairs no better than p-CSA in its role as an acid catalyst
(Table 5). Reaction B and Reaction C appear to be less sensitive to the
choice of catalyst than Reaction A. After performing each reaction
in triplicate no difference in the mean yield of either reaction is
Another esterification (Reaction A) was attempted to confirm
the similarity of the two acid catalysts and their activity com-
pared to Lewis acids (Scheme 5). Ethyl levulinate was synthesised
Scheme 7. Reaction C: Acetal protection using ethylene diol to give 2-(4-
nitrophenyl)-1,3-dioxolane.
Scheme 5. Reaction A: The Fischer esterification of levulinic acid.

J.H. Clark et al. / Catalysis Today 190 (2012) 144–149
149
Table 5
References
Isolated product yields from acid catalysed condensation chemistries (esterification,
aldol condensation, and acetal protection).
[1] As of 30/11/11 the Scifinder® database cites 1,148,722 instances in which p-TSA
or its monohydrate salt have been applied as a catalyst. By comparison sulfuric
acid has been recorded as a catalyst in 251,064 different reactions.
[2] W. Gerhartz (Ed.), Ullmann’s Encyclopedia of Industrial Chemistry, 5th edition,
VCH, Weinheim, 1985.
[3] A.W. Hixson, R.H. McKee, J. Ind. Eng. Chem. 10 (1918) 982.
[4] R.J.W. Le Fèvre, J. Chem. Soc. (1934) 1501.
[5] P. Spica, Ber. Dtsch. Chem. Ges. 14 (1881) 652.
Reaction
Yield
p-CSA catalysed
p-TSA catalysed
A (p-cymene)
A (toluene)
A (2-MeTHF)
B
C
71%
65%
11%
73%
92%
69%
67%
10%
73%
92%
[6] M. Phillips, J. Am. Chem. Soc. 46 (1924) 686.
[7] A.W. Schorger, J. Ind. Eng. Chem. 10 (1918) 258.
[8] P.C. Ho, C.-H. Ho, K.A. Kraus, J. Chem. Eng. Data 24 (1979) 115.
[9] M.A. Martín-Luengo, M. Yates, M.J. Martínez Domingo, B. Casal, M. Iglesias, M.
Esteban, E. Ruiz-Hitzky, Appl. Catal. B: Environ. 81 (2008) 218.
[10] M.A. Martín-Luengo, M. Yates, E. Saez-Rojo, D. Huerta-Arribas, D. Aguilar, E.
Ruiz-Hitzky, Appl. Catal. A: Gen. 387 (2010) 141.
observed, unlike Reaction A in which up to 2% variation in yield
is observed. Indeed within the context of a synthetic procedure
no major allowances have to be made or additional considerations
accounted for if replacing p-TSA with p-CSA.
[11] V. Ferreira-Leitão, L.M.F. Gottschalk, M.A. Ferrara, A.L. Nepomuceno, H.B.C.
Molinari, E.P.S. Bon, Waste Biomass Valor. 1 (2010) 65.
[12] J.A.S. Lopez, Q. Li, I.P. Thompson, Crit. Rev. Biotechnol. 30 (2010) 63.
[13] W. Widmer, W. Zhou, K. Grohmann, Bioresour. Technol. 101 (2010) 5242.
[14] B. Mira, M. Blasco, A. Berna, S. Subirats, J. Supercrit. Fluids 14 (1999) 95.
[15] Q. Li, J.A. Silas, I.P. Thompson, Appl. Mircobiol. Biotechnol. 88 (2010) 671.
[16] M. Pourbafrani, G. Forgács, I.S. Horváth, C. Niklasson, M.J. Taherzadeh, Biore-
sour. Technol. 101 (2010) 4246.
4. Conclusion
The sulphonic acid derivative of p-cymene has been synthe-
sised from limonene for the first time, and demonstrated as
being equal to p-TSA in applications as an organic acid cata-
lyst. The synthesis of p-CSA does not require organic solvents
or rely on any other petrochemical auxiliaries. Although p-CSA
has the undeniable benefit of being the product of a sustainable
and cheap feedstock, it is not commercially available whereas
p-TSA is supplied by many chemical manufacturers. Although a
matter of inconvenience at present, if further chemistry is devel-
oped promoting the use of p-CSA the market pull may result in
a commercial supply being available in the future. The necessity
for sustainable chemical products will only increase with time,
and as such p-CSA is an ideal candidate to fulfil this role in acid
catalysis.
[17] M. Lohrasbi, M. Pourbafrani, C. Niklasson, M.J. Taherzadeh, Bioresour. Technol.
101 (2010) 7382.
[18] P.W.D. Mitchell, D.E. Sasser, European Patent 0 522 839 A2 (1993).
[19] M. Colonna, C. Berti, M. Fiorini, E. Binassi, M. Mazzacurati, M. Vannini, S.
Karanam, Green Chem. 13 (2011) 2543.
[20] C. Berti, E. Binassi, M. Colonna, M. Fiorini, G. Kannan, S. Karanam, M. Mazzacu-
rati, I. Odeh, M. Vannini, International Patent 078328 A2 (2010).
[21] M.A. Martin-Luengo, M. Yates, E.S. Rojo, D.H. Arribas, D. Aguilar, E.R. Hitzky,
Appl. Catal. A: Gen. 387 (2010) 141.
[22] P. Lesage, J.P. Candy, C. Hirigoyen, F. Humblot, J.M. Basset, J. Mol. Catal. A: Chem.
112 (1996) 431.
[23] T.P. Wells, J.P. Hallett, C.K. Williams, T. Welton, J. Org. Chem. 73 (2008) 5585.
[24] R.E. Kirk, D.F. Othmer, M. Grayson, D. Eckroth (Eds.), Kirk–Othmer Encyclopedia
of Chemical Technology, 3rd edition, John Wiley and Sons, New York, 1984.
[25] V.L.C. Gonc¸ alves, B.P. Pinto, J.C. Silva, C.J.A. Mota, Catal. Today 133–135 (2008)
673.
[26] A. Farhat, A.-S. Fabiano-Tixier, M. El Maataoui, J.-F. Maingonnat, M. Romdhane,
F. Chemat, Food Chem. 125 (2011) 255.
[27] V.L. Budarin, P.S. Shuttleworth, J.R. Dodson, A.J. Hunt, B. Lanigan, R. Marriott,
K.J. Milkowski, A.J. Wilson, S.W. Breeden, J. Fan, E.H.K. Sin, J.H. Clark, Energy
Environ. Sci. 4 (2011) 471.
Acknowledgements
[28] L.P. Hammett, A.J. Deyrup, J. Am. Chem. Soc. 54 (1932) 2721.
[29] D.-J. Tao, Y.-T. Wu, Z. Zhou, J. Geng, X.-B. Hu, Z.-B. Zhang, Ind. Eng. Chem. Res.
50 (2011) 1989.
[30] M.J. Kamlet, R.W. Taft, J. Am. Chem. Soc. 98 (1976) 377.
[31] Q. Xu, Z. Yang, D. Yin, F. Zhang, Catal. Commun. 9 (2008) 1579.
[32] X. Liang, S. Gao, G. Gong, Y. Wang, J.-G. Yang, Catal. Lett. 124 (2008) 352.
The authors thank Mr. Paul Elliott for determining the thermal
decomposition points of the sulphonic acids, Mr. Andrew Marriott
for performing the mass spectrometry of p-CSA, and Dr. Thomas
Farmer for proposing the concept of this article.