Sustainable p-cymene and hydrogen from limonene

Article abstract of DOI:10.1016/j.apcata.2010.08.016

A fine chemical intermediate in a wide range of chemical processes, p-cymene, has been obtained from Limonene, solids based on a natural clay (sepiolite) modified with sodium, nickel, iron or manganese oxides and programmable focalised microwaves. The process has the added bonus of one mol of hydrogen being produced per mol of limonene converted to p-cymene.

Full text of DOI:10.1016/j.apcata.2010.08.016

Applied Catalysis A: General 387 (2010) 141–146
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Sustainable p-cymene and hydrogen from limonene
M.A. Martin-Luengoa,∗, M. Yatesb, E. Saez Rojoa,b, D. Huerta Arribasa,b, D. Aguilara,b, E. Ruiz Hitzkya
a Institute of Materials Science of Madrid, CSIC, Calle Sor Juana Ines de la Cruz, Campus UAM, Cantoblanco, 28049 Madrid, Spain
b Institute of Catálisis and Petroleochemicstry, Campus UAM, CSIC, Calle Marie Curie 2, Cantoblanco, E-28049 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 June 2010
Received in revised form 9 August 2010
Accepted 10 August 2010
Available online 17 August 2010
A fine chemical intermediate in a wide range of chemical processes, p-cymene, has been obtained from
Limonene, solids based on a natural clay (sepiolite) modified with sodium, nickel, iron or manganese
oxides and programmable focalised microwaves. The process has the added bonus of one mol of hydrogen
being produced per mol of limonene converted to p-cymene.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
p-Cymene
Limonene
Isomerisation
Dehydrogenation
Microwave
Hydrogen
Solvent less
Dry media
1. Introduction
aromatics, CFCs, etc.), as an additive in pigments, inks, adhesives,
pharmaceutical and commodity compositions [4]. New uses are
Environmental problems pose a great challenge, and are being
taken into account by ever more stringent legislation, due to the
recognition of the need for a sustainable development, particularly
in countries where water use, residues and desertic and contam-
inated soils have become a matter of the utmost concern. Efforts
from academia, industry and government are mainly based on
technological changes that improve chemical processes to avoid
negative environmental consequences for health and environment
[1]. To address this point, new renewable and sustainable chemi-
cals are now being obtained from agricultural wastes, reducing in
this way the need for non-renewable fossil resources (NRFR) [2].
This work is based on designing a clean process to transform
limonene, a renewable raw material (RRM) into p-cymene a value
added product usually prepared by alkylation of NRFR (benzene
or toluene), in a rapid, simple and economic way. Limonene, a sub-
product from citrus fruit processing, is a cyclic terpene with empiric
formula C10H16 (Fig. 1), of low toxicity (LD50 5 g kg−1 oral rat) [3],
that constitutes ca. 95% of orange peel oil. Having a six membered
ring this substance has the potential for use to obtain compounds
that are usually produced from non-renewable petroleum deriva-
tives. It is used as a biodegradable solvent (replacing mineral oils,
being sought to give added value to this subproduct, in this way
increasing the income of citrus juice industries, and also with obvi-
ous benefits to the environment and society in general.
The catalytic supports used in this study were based on sepi-
olite, a natural clay of high abundance and low cost in Madrid
[5,6], (Table 2), in order to decrease the fabrication costs and waste,
due to its negligible negative environmental impact. Sepiolite is a
hydrated magnesium silicate of the philosilicate type 2:1 with a
layer of magnesium between two layers of silica tetrahedra. The
octahedral sheet is composed mainly of Mg2+. It has substitution of
Si4+ and Mg2+ by small amounts of Al3+ and its specific surface area
(SBET) is close to 300 m2 g−1, of which 150 m2 g−1 is external (pores
with diameters >2 nm diameter) and the remainder is related to
the volume filling of micropores (<2 nm diameter). It has a density
of–SiOH groups of ca. 2.2 groups/10 nm2 originated at the edges
due to breakeage of Si–O–Si bonds. Its uses are related to its sor-
bent and colloidal properties, surface area, mechanical resistance
and thermal stability [6,7], as a support for catalysts, absorbent to
clean gaseous effluents [8,9], decolouring agent for foods [10,11],
support for insecticides [12] and more recently as a component in
nanocomposites [13].
A key point in improving chemical processes is decreasing
their reaction time, since this lowers energy use. In this sense
microwave irradiation has been chosen to activate the reactions,
since it allows extraordinary reaction rates to be achieved, in many
∗
Corresponding author.
E-mail address: mluengo@icmm.csic.es (M.A. Martin-Luengo).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.08.016

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M.A. Martin-Luengo et al. / Applied Catalysis A: General 387 (2010) 141–146
Table 1
Chemical analyses.
Solid
SiO2
Al2O3
MgO
Na2O
Mn2O3
Fe2O3
CaO
K2O
NiO
Sepiolite
SepNa
SepNi
SepFe
SepMn
62.5
56.8
55.4
56.5
56.8
1.2
1.3
1.3
1.3
1.4
25.2
23.9
25.1
24.1
24.8
0.1
7.3
0.1
0.1
0.1
0.5
0.5
0.5
0.5
7.4
0.6
0.7
0.7
7.2
0.9
0.4
0.3
0.3
0.3
0.4
0.3
0.2
0.2
0.2
0.3
0.0
0.0
5.7
0.0
0.0
2.2. Catalyst preparation
The iron, manganese or nickel containing sepiolites were pre-
pared so as to have similar amounts of these elements as the sodium
containing material, by mixing iron nitrate (SepFe), manganese
acetate (SepMn) or nickel nitrate (SepNi) with distilled water. The
solid–liquid mixtures were stirred in a rotary evaporator at 50 ◦C
under vacuum, until dryness.
The preparation of sodium containing sepiolite (SepNa) [19] was
carried out by mixing the sepiolite Pangel 1N NaOH solution, under
continuous stirring, introduced in a hermetically closed reactor and
kept in an oven at 100 ◦C for 16 h, cooled to room temperature
and then subsequently the product was thoroughly washed with
distilled water and dried at 100 ◦C overnight.
Fig. 1. Chemical structure of reactant, intermediates and products.
cases not comparable to those obtained by classical heating [14].
Coupling microwave irradiation with solvent less processes or with
solid supports increases the positive environmental aspect of these
systems [3,14], since it avoids the use of costly or toxic organic sol-
vents and can also avoid secondary unwanted reactions leading
to higher yields and selectivities in soft conditions [15,16]. Thus,
careful design of the processes, in many cases, can lead to conver-
sions and selectivities that cannot be achieved with conventional
heating.
All the precursors were then calcined at 400 ◦C in an air
flow of 50 mL min−1 (according to TG–DTA analyses, as explained
below), the sepiolite was also calcined at 400 ◦C for comparison
purposes.
2.3. Characterisation methods
The precursors of SepFe and SepMn, were analysed by TG–DTA
in an Stanton model STA 781 thermogravimetric analyser in an air
flow of 50 cm3 min−1 using 20–30 mg of solid, at a heating rate of
5 ◦C min−1 from room temperature to 1000 ◦C. By analysing their
thermal and weight loss behaviours, thermal stability and change in
composition, the best procedure to achieve the complete decompo-
sition of the impregnated compounds to their corresponding oxides
was determined.
The activity of the parent sepiolite was modified by adding
sodium, iron, manganese or nickel oxides, being of low price com-
pared to Au or Ag [17] or low toxicity (except nickel) compared
to other metals commonly used as catalysts for this kind of reac-
tions (i.e. Pd, Cd, Cr, etc.) (LD50 oral rat 0.3, 2, 3.4 and 0.05 g kg−1
for sodium, iron, manganese and nickel oxides, respectively
[18]).
The chemical composition of the solids after calcination was
analysed in a Thermo Jarrel Ash ICP 300 apparatus after treatment
with LiBO2 at 1000 ◦C. The amount of sodium was determined by
flame spectrophotometry.
2. Experimental
2.1. Raw materials
The surface areas, micro- and mesopore analyses of the solids
were carried out in a Micromeritics ASAP 2010 apparatus using
adsorption-desorption of N2 at −196 ◦C, outgassing the solids
overnight at 150 ◦C to a vacuum of less than 10−2 Pa to ensure clean
dry surfaces, free from any loosely bound adsorbed species. The
specific surface areas were calculated according to the BET method,
taking the area of the nitrogen molecule as 0.162 nm2. The total
pore volume was calculated from the amount adsorbed at a rel-
Sepiolite was kindly supplied by Tolsa S.A. form Vallecas,
Madrid. The d(+)-limonene pro synthesis from Panreac with a
purity of 95% was used as the reactant since this is similar to
the purity of limonene produced as a subproduct form citrus
fruit processing. Nickel and iron nitrates and manganese acetate
used as precursor salts were from Panreac (NiNO3·6H2O 99.7%,
FeNO3·9H2O 99.5% and Mn(CH3COO)2·4H2O 99%).
Table 2
Textural characteristics of the solids.
Solid
SBET
SEXT
Vmic
Vmes
Sepiolite
Sep400
SepNa
SepNi
SepFe
SepMn
298
153
100
219
155
153
149
153
93
149
149
138
0.038
0.000
0.002
0.030
0.002
0.008
0.417
0.455
0.376
0.439
0.416
0.431
SBET = specific surface area (m2 g−1).
SEXT = external surface area (m2 g−1).
Vmes = mesopore volume (2–50 nm pore diameters, cm3 g−1).
Vt = micro + mesopore volume (cm3 g−1).
Vmic = micropore volume (0–2 nm pore diameters, cm3 g−1).
Fig. 2. Nitrogen isotherm for SepFe.

M.A. Martin-Luengo et al. / Applied Catalysis A: General 387 (2010) 141–146
143
Table 3
Conversions and selectivities under dry media conditions.
Sample
Reaction time (min)
Conversion (%)
Product distribution (%)
␣-terpinene
␥-terpinene
␣-terpinolene
p-cymene
Sepiolite
5
10
20
15
23
31
26
28
20
17
20
10
31
27
38
26
25
32
SepNa
SepNi
SepFe
SepMn
5
10
20
0
0
0
–
–
–
–
–
–
–
–
–
–
–
–
5
10
20
70
86
100
0
0
0
0
0
0
0
0
0
100
100
100
5
10
20
75
89
100
0
0
0
0
0
0
0
0
0
100
100
100
5
10
20
63
71
100
0
0
0
0
0
0
0
0
0
100
100
100
ative pressure of 0.96 on the desorption branch of the isotherms,
equivalent to the filling of all pores less than 50 nm diameter. The
pore size distributions were analysed from the desorption branch
of the isotherms using the Kelvin equation, with the thickness of
the adsorbed layer of nitrogen calculated at each relative pressure
from the Harkins–Jura equation. The pore size distributions were
obtained, applying the Barrer Joyner Halender (BJH) method. A t-
plot analysis was used to assess the presence of microporosity in
the solids, and the external surface areas, i.e. that not involved in
the micropore filling mechanism.
Transmission electron microscopy was carried out in a JEOL
model FXII electron microscope operating at 200 kV. Ethanolic sus-
pensions of the solids were deposited on carbon covered copper
grids and left to dry for analysis.
To analyse the acidity of the solids they were first thoroughly
mixed with pyridine, then introduced into a Stanton model STA 781
TG-MS apparatus and flushed with nitrogen until constant weight.
The temperature was then increased under a nitrogen flow from
room temperature to 900 ◦C at 10 ◦C min−1 and the evolution of
pyridine analysed by means of a mass spectrometer coupled to the
thermogravimetric analyser.
The catalytic reactions were carried out in Prolabo Synthewave
402 monomode programmable focalised microwave oven which
allows reproducible results when compared with conventional
microwave ovens, where heterogeneous distribution of microwave
radiation makes reproducibility difficult. Also this oven can be
programmed, by choosing time, power and temperature, with
experiments being tailored to the required parameters. Two types
of reactions were studied: dry media and solvent less conditions.
Under dry media, 200 mg of solid were thoroughly mixed with
0.057 cm3 of limonene and introduced in the reactor, being the
microwave oven programmed as designed. In the solvent less reac-
tions a maximum temperature of 165 ◦C was chosen to transform
limonene, based on the results obtained under dry media condi-
tions. In this case 5 cm3 of limonene and a maximum of 500 mg of
solid were usedwitharefluxattachmentincludedinthe microwave
oven. At the end of the reactions, the system was allowed to cool
and the solid eluted with 2 cm3 ethanol (dry media) or 0.057 cm3
of liquid were dissolved in 2 cm3 of ethanol (liquid) to extract the
reactants and products. The liquid was then filtered and analysed
in a Hewlett Packard 5890 Series II chromatograph, coupled to a
Hewlett Packard Series 5971 mass spectrometer with a 25 m cap-
illary column of methyl silicone, (50–170 ◦C at a heating rate of
6 ◦C min−1).
3. Results
3.1. Catalyst characterisation
After preparation, the solid precursors of SepNi, SepFe and
SepMn were analysed by TG–DTA in order to assess the best condi-
tions to achieve the thermal decomposition of the precursor salts,
leading to the corresponding impregnated oxides. The thermo-
grams obtained (not shown) indicate a total weight loss of ca. 20%
due to decomposition of the precursor salt anions, divided in ca. 8%
in the interval 20–120 ◦C (loss of adsorbed water), 5% between 120
and 250 ◦C (loss of zeolitic water, also found in the parent sepio-
lite) and decomposition of the anions at ca. 400 ◦C for nickel or iron
nitrate and at ca. 300 ◦C for manganese acetate, with weight losses
of ca. 8%. In accordance with these findings all the precursor solids
were calcined at 400 ◦C for 4 h in an air flow of 50 mL min−1. FTIR
analyses, were in agreement with the disappearance of the anions.
The compositions of the catalysts are included in Table 1, the
main differences between the samples being due to the introduc-
tion of sodium or nickel, iron and manganese oxides, as expected.
XRD analyses showed that the structure of sepiolite is main-
tained after impregnation and calcination to produce the supported
oxides, as shown by the (1 1 0) reflection at 7.42◦ (2ꢀ), but did not
show peaks corresponding to the oxides, due to the high dispersion
of their particles, in agreement with the TEM results showing sizes
of ca. 3–4 nm (Fig. 3), whose XRD peaks would have very low inten-
sities and would thus be occluded inside the background noise of
the XRD patterns [6].
The nitrogen adsorption isotherms for the parent sepiolite gave
a mixed type I/II form, due to the presence of both micropores
(0–2 nm) and mesopores (2–50 nm) that extend into the macrop-
ore region (>50 nm). The textural characteristics of the solids are
included in Table 2. The original sepiolite had a surface area of
ca. 300 m2 g−1 as expected. On calcination the surface areas of the
solids decreased to ca. 150 m2 g−1 mainly due to the rotation and
folding of the clay structure which causes the collapse of the micro-
porous channels, giving type II adsorption branches, as shown in
Fig. 2 and to only 100 m2 g−1 for SepNa (due also to blocking of
the pores with sodium entities) [6,19]. For all samples the hystere-
sis loops were of type H3, typical for solids with slit-shaped pores,
commonly found with clay materials [7,20,21]. The pore size distri-
butions of the solids show the presence of mesopores with maxima
at ca. 30 nm, extending into the macropore region at pore diam-
eters >50 nm. The external surface areas corresponding to pores

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M.A. Martin-Luengo et al. / Applied Catalysis A: General 387 (2010) 141–146
Fig. 4. Measurement of acidity with pyridine desorption.
solids present no observable alteration of the sepiolite micro-
tubules (0.2–2 ␮m in length) and the oxide particles have sizes
of 3–4 nm. More homogeneous distributions of the oxide parti-
cles were observed for SepNi and SepFe compared to SepMn, the
latter displaying agglomerates of particles. Homogeneous particle
sizes have been claimed to be a positive effect for the reactivity
in this kind of reactions [22]. However, there are researchers that
claim different effects of the oxide particle sizes on the reactiv-
ity and selectivity for dehydrogenations, for example White et al.
found that changes in the particle size did not alter the overall
activity, although larger particles gave lower selectivities for C6
products than smaller ones in the oxidative dehydrogenation over
bismuth oxide (475–625 ◦C) [23], Chen et al. claimed the same effect
on oxidative dehydrogenation on zirconia-supported molybdenum
oxide catalysts [24] and Frank et al. found that propene selectivity
in the oxidative dehydrogenation of propane decreased with an
increase in the size of the catalyst particles [25].
Fig. 3. Transmission electron micrographs (a: Sep, b: SepMn and c: SepFe). Scale
bar: a: 40 nm, b and c: 50 nm.
of diameters >2 nm determined form t-plot analyses of the corre-
sponding nitrogen isotherms were similar for the Sep, SepFe and
SepMn samples.
Transmission electron microscopy was used to obtain informa-
tion about the morphology and particle size of the oxide containing
solids. Selected electron micrographs are presented in Fig. 3. The
Fig. 5. Temperature variation with reaction time. a. Dry media. b. Liquid phase.

M.A. Martin-Luengo et al. / Applied Catalysis A: General 387 (2010) 141–146
145
Fig. 6. Variation of conversions and selectivities with reaction time.
The amount and strength of acid sites, determined by the TPD
of pyridine, are shown in Fig. 4. From these results it was observed
that pyridine desorbs from sepiolite in the temperature range of
200–260 ◦C, while temperatures in excess of 400 ◦C were required
to clean the surfaces of the oxide doped sepiolites. The increase
in the amount and strength of acid sites by addition of oxides to
these supports is mainly related to the presence of Mex+ ions [26]
(Lewis acidity) and to surface OH groups (Brönsted acidity) [27–30]
and followed the order: SepFe > SepNi > SepMn > Sep. As expected
no pyridine adsorption was found for the sodium containing sample
(SepNa). Furthermore, due to the strong relationship between OH-
and water, the presence of Brönsted acidity is usually difficult to
demonstrate and relate to catalytic activities [30].
for the solids studied 100% selectivity to p-cymene was found after
only 5 min of reaction. The increase of activity, compared to the par-
ent sepiolite, can be related to the metal oxide contents, since they
are microwave-adsorbing centres, but not to the final temperature
attained, since they did not follow the same trend (Fig. 5a).
3.2.2. Liquid phase reaction
Only the solids that presented activity under dry media con-
ditions were tested in the liquid phase reactions, designed to
allow higher liquid to solid ratios. Chosen sets of conditions were
repeated three times and the differences found were less than
3% in conversions and selectivities. It may be observed (Fig. 6)
that the selectivity to p-cymene increased with longer reaction
times and higher temperatures, in accordance with the forma-
tion of this product being an endothermic reaction. For SepFe,
SepNi and SepMn, 100% conversion was reached after 20 min of
reaction with high selectivities to p-cymene (88, 82 and 77%,
respectively). The temperatures reached by the systems were in
the order Sep > SepNi > SepMn > SepFe (Fig. 5b). Although similar
reactivities and selectivities are found for these oxide containing
solids, it should not be forgotten the higher toxicity of SepNi, com-
pared to SepMn and SepFe, which makes these last two easier to
dispose of after use. The higher microporosity of the SepNi cat-
alyst appears to have little effect on the overall reactivity. This
suggests that the active sites on the more accessible external area,
3.2. Transformation of limonene to p-cymene
3.2.1. Dry media conditions
The results for dry media conditions are presented in Table 3. The
SepNa material was chosen for comparison purposes, but due to its
lack of acid sites and since it showed no activity it has been omitted
from the table. A bibliographic search indicated that for basic sites
to react in isomerisation and dehydrogenation of terpenes, temper-
atures close to 400 ◦C are required [31]. Since with mw irradiation
the temperatures were not higher than 210 ◦C, the inactivity of this
material was to be expected. From Table 3 it may also be seen that

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M.A. Martin-Luengo et al. / Applied Catalysis A: General 387 (2010) 141–146
which are similar for all three samples, are where the reaction takes
place.
Acknowledgements
The characterisation by pyridine desorption (Fig. 4) indi-
cated that the amount of acid sites followed the order:
SepFe > SepNi > SepMn > Sep and no acid sites were found for
SepNa. Previous work has shown that the acid sites of a series of
silica–aluminas could dehydrogenate limonene towards p-cymene,
with higher activities found for the solids that contained more acid
sites [3], in agreement with the results found here, where the order
of activity (and selectivity to p-cymene) was also the order of acidity
of the solids, as found with pyridine desorption.
When SepFe was reacted using conventional heating (Fig. 6e),
much lower activities and selectivities to p-cymene were found in
agreement with literature data [32,33]. We believe the short reac-
tion times required using dielectric heating were responsible for
the higher conversions and selectivities found, due to the presence
of paramagnetic absorbing centres, thus avoiding unwanted prod-
ucts, that are formed during the longer reaction times necessary
with conventional heating.
This work has been supported by the CICYT (Spain; Projects:
BTE2003-05757-C02-02, MAT2006-03356, MAT2009-09960) and
the Comunidad de Madrid (Spain; Project S-0505/MAT/000227).
The assistance of Mr. T. Garcia Somolinos from the Adsorption Unit
of the ICMM, CSIC for the textural analyses and provision of sepiolite
Pangel from Tolsa SA are acknowledged.
References
[1] J. Clark, Green Chem. 1 (1999) G1.
[2] J.H. Clark, V. Budarin, F.E.I. Deswarte, J.J.E. Hardy, F.M. Kerton, A.J. Hunt, R. Luque,
D.J. Macquarrie, K. Milkowski, A. Rodriguez, O. Samuel, S.J. Tavener, R.J. White,
A.J. Wilson, Green Chem. 10 (2006) 853–860.
[3] 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.
[4] M. Virot, V. Tomao, C. Ginies, F. Visinoni, F. Chemat, J. Chromatogr. A 1196–1197
(2008) 147.
[5] (a) E. Ruiz-Hitzky, B. Casal, J. Catal. 92 (1985) 291;
(b) J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas, React. Solid 3 (1987) 263.
[6] M.A. Martin, J.A. Pajares, L. Gonzalez Tejuca, J. Colloid Interface Sci. 107 (1985)
540.
[7] J. Blanco, A.L. Petre, M. Yates, M.P. Martín, J.A. Martín, M.A. Martín-Luengo, Appl.
Catal. B: Environ. 73 (2007) 128.
Although similar conversions and selectivities to those found
in this study have recently been reported for the transformation
of limonene, toxic catalysts (i.e. Cr) or hydrogen pressures, greater
than atmospheric and the use of toxic and expensive palladium
catalysts were required [34,35].
[8] M. Yates, J. Blanco, M.A. Martin-Luengo, M.P. Martin, Micropor. Mesopor. Mater.
65 (2003) 219.
[9] A. Brasseur, A. Gambin, J. Laudet, B. Marien, J.P. Pirard, Chemosphere 56 (2004)
745.
Regarding the mechanism of reaction, according to the work
of Catrinescu [33], limonene can react in the pores of acid solids
in two main ways: isomerise to terpinenes and terpinolenes, with
further dehydrogenation of these, or disproportionate to p-cymene
and menthenes. The presence of isomerisation compounds (ter-
pinenes and terpinolenes) in the products would support the
first mechanism, while the presence of menthenes or menthanes
would support the second mechanism. Since only terpinenes, ter-
pinolenes and p-cymene were found, the results agree with the
first mechanism (isomerisation and dehydrogenation, but not dis-
proportionation).
[10] U.H. Ibrahim, B. Erdogan, Appl. Clay Sci. 12 (1998) 419.
[11] E. Sabah, M. C¸ inar, M.S. C¸ elik, Food Chem. 100 (2007) 1661.
[12] B. Casal, J. Merino, E. Ruiz-Hitzky, L. Lami, A. Alvarez, L. Cuadra, Green Chem. 4
(1999) 199.
[13] R. Kun, P. Aranda, M.A. Martín-Luengo, S. Letaïef, I. Dékány, E. Ruiz-Hitzky,
Chem. Mater. 20 (2008) 84.
[14] S. Letaïef, B. Casal, P. Aranda, M.A. Martín-Luengo, E. Ruiz-Hitzky, Appl. Clay Sci.
22 (2003) 263.
[15] B. Toukoniitty, J.P. Mikkola, D.Yu. Murzin, T. Salmi, Appl. Catal. A: Gen. 279
(2005) 1.
[16] R.S. Varma, Green Chem. 1 (1999) 43.
[17] R.S. Varma, R.K. Saini, R. Dahiya, Tetrahedron Lett. 38 (1997) 7823.
[18] Materials safety data sheets.
[19] R. Salvador, B. Casal, M. Yates, M.A. Martín-Luengo, E. Ruiz-Hitzky, Appl. Clay
Sci. 22 (2002) 103.
[20] F. Rouquerol, J. Rouquerol, K.S.W. Sing, Adsorption by Powders and Porous
Solids: Principles, Methodology and Applications, 1999.
[21] Y. Watanabe, K. Banno, M. Sugiura, Appl. Clay Sci. 16 (2000) 59.
[22] C. Zhao, W. Gan, X. Fan, Z. Cai, P.J. Dyson, Y. Kou, J. Catal. 254 (2008) 244.
[23] M.G. White, J.W. Hightower, J. Catal. 82 (1983) 185.
[24] K. Chen, S. Xie, E. Iglesia, A.T. Bell, J. Catal. 189 (2000) 421.
[25] B. Frank, A. Dinse, O. Ovsitser, E.V. Kondratenko, R. Schomäcker, Appl. Catal. A:
Gen. 323 (2007) 66.
From an environmental point of view, it is important to note
that this method does not need hydrogen [34] and every mol of
limonene that produces p-cymene leads to the production of a mol
of hydrogen [36].
4. Conclusions
The transformation of limonene, an inexpensive subproduct
from citrus fruit processing, to p-cymene, a fine chemical interme-
diate, by means of a cheaper, simpler and lower toxicity method
than the catalytic dehydrogenation presently used in industry, has
been demonstrated. In this reaction for each mol of limonene con-
verted to p-cymene a mol of hydrogen is produced.
[26] L. Chmielarz, P. Ku
s´trowski, R. Dziembaj, P. Cool, E.F. Vansant, Appl. Catal. B:
Environ. 62 (2006) 369.
[27] T. Zaki, J. Colloid Interface Sci. 284 (2005) 606.
[28] V. Sanchez Escribano, J.M. Gallardo Amores, E. Finocchio, M. Daturi, G. Busca, J.
Mater. Chem. 5 (1995) 1943.
[29] E. Finocchio, G. Busca, Catal. Today 70 (2001) 213.
[30] V. Sanchez Escribano, J.M. Gallardo Amores, F. Finocchio, M. Daturi, G. Busca, J.
Mater. Chem. 5 (1995) 1943.
Under microwave irradiation the reaction is activated by the
microwave-adsorbing paramagnetic centres. Solids with both acid
and paramagnetic sites lead to increased reaction rates and selec-
tivities, compared to conventional heating. SepFe and SepNi had
more highly dispersed oxide particles and greater acidity than
SepMn, (although the latter had stronger basic sites), and showed
also higher activities and selectivities towards p-cymene.
[31] M. Albeck, Ch. Rav-Acha, E. Gil-Av, O. Schachter, J. Catal. 22 (1971) 219.
[32] C.G. Ramankutty, S. Sugunan, B. Thomas, J. Mol. Catal. A 187 (2002) 105.
[33] C. Catrinescu, C. Fernandes, P. Castilho, C. Breen, Appl. Catal. A: Gen. 311 (2006)
172.
[34] C. Zhao, W. Gan, X. Fan, Z. Cai, P.J. Dyson, Y. Kou, J. Catal 254 (2008) 244.
[35] F. Al-Wadaani, E.F. Kozhevnikova, I.V. Kozhevnikov, Appl. Catal. A: Gen. 363
(2009) 153.
[36] B.A. Leita, A.C. Warden, N. Burke, M.S. O’Shea, D. Trimm, Green Chem. 12 (2010)
70.