Production of p-cymene and hydrogen from a bio-renewable feedstock-1,8-cineole (eucalyptus oil)

Article abstract of DOI:10.1039/b916460j

The catalytic transformation of pure 1,8-cineole was performed in a custom-built down-flow fixed bed pyrolysis rig over various metal-doped alumina pellets controlled at temperatures between 523 K (250 °C) and 873 K (500 °C). Varying amounts of oxygen were added to the feed. Hydrophilic, hydrophobic and gaseous products were analysed separately. The hydrophilic phase was predominantly water, while the composition of the hydrophobic phase varied with catalyst type and contained mainly mixtures of both aromatic and non-aromatic C₁₀ hydrocarbons. The main gases produced were hydrogen, carbon monoxide and carbon dioxide. As the reaction temperature increased, yields of gas phase components increased for all catalysts. The palladium-doped γ-Al₂O₃ catalyst at ～250 °C showed excellent yields and selectivity for the continuous production of p-cymene together with hydrogen gas. For the best catalysts and reaction conditions, the process is very atom and carbon efficient, with all ten carbon atoms from the cineole molecule being used in the p-cymene product in an oxygen-free environment. The process uses no solvents and the high yields achieved ensure there is no waste clean-up required.

Full text of DOI:10.1039/b916460j

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www.rsc.org/greenchem | Green Chemistry
Production of p-cymene and hydrogen from a bio-renewable
feedstock–1,8-cineole (eucalyptus oil)
Benjamin A. Leita,*a,b Andrew C. Warden,a Nick Burke,b Mike S. O’Sheaa and David Trimmb
Received 10th August 2009, Accepted 17th September 2009
First published as an Advance Article on the web 23rd October 2009
DOI: 10.1039/b916460j
The catalytic transformation of pure 1,8-cineole was performed in a custom-built down-flow fixed
bed pyrolysis rig over various metal-doped alumina pellets controlled at temperatures between
523 K (250 ◦C) and 873 K (500 ◦C). Varying amounts of oxygen were added to the feed.
Hydrophilic, hydrophobic and gaseous products were analysed separately. The hydrophilic phase
was predominantly water, while the composition of the hydrophobic phase varied with catalyst
type and contained mainly mixtures of both aromatic and non-aromatic C10 hydrocarbons. The
main gases produced were hydrogen, carbon monoxide and carbon dioxide. As the reaction
temperature increased, yields of gas phase components increased for all catalysts. The
palladium-doped g-Al2O3 catalyst at ~250 ◦C showed excellent yields and selectivity for the
continuous production of p-cymene together with hydrogen gas. For the best catalysts and
reaction conditions, the process is very atom and carbon efficient, with all ten carbon atoms from
the cineole molecule being used in the p-cymene product in an oxygen-free environment. The
process uses no solvents and the high yields achieved ensure there is no waste clean-up required.
1. Introduction
The volatility of oil prices and the increasing pressure to find
sustainable, bio-derived alternatives to petrochemical feedstocks
has encouraged researchers to look into an increasingly diverse
range of biological materials for chemical precursors. 1,8-cineole
(hereafter referred to as cineole) is the main chemical component
of eucalyptus oil (~ 90%) and is obtained via steam distillation
of eucalyptus leaves1 (Fig. 1). Most of the world’s eucalyptus oil
is produced in China as a by-product of the eucalyptus timber
plantation industry. The world market for pharmaceutical and
domestic use of eucalyptus oil is about 7000 tonne each year.2
In recent years, the price has been relatively stable at between
Fig. 1 Production of 1,8-cineole.
US $4,500 and US $5,000 per tonne.2 However, the potential
production of eucalyptus oil has been estimated to be in the
millions of metric tonnes per year, globally, with estimates for
current Australian eucalyptus plantings alone reaching the level
of 800 000 tonnes per year.3 With this volume of eucalyptus oil
on the market, the prices can be expected to drop well below
current values.4
icals and chemical precursors such as menthenols, menthenes,
menthadiene and p-cymene.5
p-Cymene (Fig. 2) is of significant interest in that it is already
used in the production of p-cresol, fragrances, pharmaceu-
ticals, herbicides and fine chemicals.6 Currently, commercial
These production figures and lower costs would make cineole
a very attractive feedstock for the production of industrial
chemicals from a renewable resource. Thus, it is not surprising
that cineole has already been regarded as a potentially attractive
renewable feedstock for the production of C10 industrial chem-
aCSIRO Molecular and Health Technologies, Private Bag 10, Clayton
South VIC, 3169, Australia. E-mail: benjamin.leita@csiro.au;
Fax: +61 3 95458380; Tel: +61 3 95458130
bCSIRO Petroleum Resources, Private Bag 10, Clayton South VIC,
3169, Australia. E-mail: david.trimm@csiro.au; Fax: +61 3 95458380;
Tel: +61 2 93854340
Fig. 2 Structure of p-cymene.
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production of cymenes is from toluene and propylene which
are obtained from crude oil. The alkylation and isomerisation
steps produce a mixture of the three different isomers of cymene,
but primarily yield the m- and p-isomers.7 Further separation
to pure p-cymene prior to the p-cresol conversion is currently
achieved using the UOP-developed “Cymex” process, where m-
and p-cymene are subject to chromatography using adsorbing
(molecular sieves) and desorbing (toluene) media.8
Subsequent production of cresols based on the oxidation
of cymenes, or cleavage of isopropyl toluenes into cresols
and acetone, are direct extensions of the phenol process from
benzene.9
iron, cobalt, chromium and palladium metals. The metal-doped
g-Al2O3 catalysts were prepared by a wet impregnation technique
using 1 M aqueous solutions of the appropriate metal salts.
In a typical procedure, 100 mL of 1 M metal nitrate solution
was poured over 70 g of g-Al2O3 pellets (Saint-Gobain NorPro,
USA) that had been heated in a vacuum oven at 90 ◦C overnight.
The mixture was stirred briefly with a spatula and left to stand at
room temperature overnight. The resultant metal-impregnated
g-Al2O3 pellets were collected, washed three times with deionised
water and dried in a vacuum oven at 90 ◦C overnight. The
coated pellets were then transferred to a crucible and calcined
◦
in air at 350 C for 12 h. As controls, undoped g-Al2O3 pellets
There have been investigations into the direct synthesis of
pure p-cymene using ZSM-5 catalysts for regioselectivity in
the alkylation of toluene10 and some further studies for pure
p-cymene production using pinenes as a feedstock.8
were subjected to the same treatment as the metal doped samples
before use, and glass beads were used as a blank reaction surface.
2.2. Catalyst characterization
These processes rely on non-renewable crude oil feedstocks
and processing. Matsuura and Waki11 studied the pyrolysis of
bio-derived cineole over calcium oxide and activated alumina,
the latter having a catalytic effect on the reaction starting at
220 ◦C, while the calcium oxide catalysed the system only
at 450 ◦C. The major liquid hydrophobic product from the
Prior to surface analysis, samples were degassed under vacuum
at 300 ◦C overnight using a VacPrep 061 Degasser. The BET
surface area was determined by N2 adsorption at 77 K using a
Micromeritics Tristar 3000. X-Ray diffraction (XRD) measure-
ments were carried out using a Phillips DW 1130 machine with
◦
˚
◦
Cu Ka (1.542 A) rad◦iation (40 kV, 25 mA) over the range 5–80
activated alumina-catalysed reactions between 250 and 350 C
2q at a scan rate of 1 min-1 with 0.1◦ step size.
was found to be dipentene, with some isomerisation due to the
rearrangement of the double bond. In contrast, Hu¨gel et al.
reacted cineole with hydrogen over pumice and silica-supported
platinum and palladium catalysts to give mixtures of cis- and
trans-p-menthanes at lower temperatures while p-cymene was
found to be produced at temperatures greater than 350 ◦C.5 Buhl
et al. investigated the conversion of a-limonene to p-cymene
over silica-supported Pd catalysts12 and later from pinenes to
p-cymene.8 A more recent study by Mart´ın-Luengo and co-
workers reported 100% batch conversion of limonene to
p-cymene using mesoporous silica–alumina supports heated by
microwave irradiation.13
In this study, we have chosen to use continuous flow fixed
bed pyrolysis to perform the catalytic transformation of cineole.
This approach is advantageous in that we employ a continuous
process that provides more scope for mass production at
either commercial scales or in smaller, decentralised plants.
In addition, the process is very atom and carbon efficient,
with all ten carbon atoms from the cineole molecule being
used in the p-cymene product without the use of any solvents
while producing no waste stream requiring costly disposal. For
the catalysts, we have used high-surface area g-Al2O3 pellets
(200 m2 g-1, Saint-Gobain, USA) initially as a slightly acidic
catalyst and then as a support where it was doped, via the
wet impregnation technique, with molybdenum, iron, cobalt,
chromium and palladium nitrate solutions.14 These catalytic
materials were subjected to vapour phase pyrolysis of cineole
using a down flow fixed bed tubular reactor at atmospheric
pressure with varying amounts of blended oxidant.
2.3. Catalytic activity measurements
The vapour phase catalytic conversion of cineole was performed
using an electrically heated tubular down-flow reactor (13.5 mm
internal diameter, 300 mm length) with the catalyst held as a
fixed bed at atmospheric pressure. A K-type thermocouple was
used to monitor the temperature of the bed. All thermocouples,
furnaces, heating bands and mass flow controllers (MFC)
were controlled and data were logged using specially designed
software.15
The liquid product was collected at 40 ◦C in a stainless steel
trap. The gaseous products were sent through a second trap at
0 ◦C to an online gas chromatograph (GC).
In a typical variable temperature experiment, 3 g of catalyst
was loaded into a stainless steel mesh basket, which was placed
inside the tubular reactor. The furnace was set to an initial
temperature of 250 ◦C and the temperature was allowed to
equilibrate for one hour. Cineole was injected upstream of the
pre-heater at a rate of 0.5 mL min-1 with an ISCO 500D syringe
pump. The carrier gas was a blended mixture of the amount of
oxygen required in argon containing a 5.1% helium internal
standard; this was fed at a constant rate of 150 mL min-1.
Once at equilibrium, gas samples were taken and liquid products
collected. The furnace temperature was then raised by 50 ◦C and
the procedure was repeated until the final reaction temperature
of 500 ◦C was reached.
2.4. Analysis of liquid products
The liquid product obtained for the majority of the samples
consisted of an oily, hydrophobic phase and an aqueous
phase. The analysis of liquid products was performed using
a GCMS fitted with an auto sampler. For analysis, 10 mL of
the hydrophobic phase was dissolved in 1.5 mL of acetonitrile
(Aldrich) that had been doped with 0.1% mesitylene (Aldrich)
2. Experimental
2.1. Catalyst preparation
High surface area g-Al2O3 pellets (200 m2g-1) were used as a
slightly acidic catalyst and as a solid support for molybdenum,
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Scheme 1 The catalytic transformation of cineole over metal-doped g-Al2O3 catalysts.
Table 1 Surface area and pore volume of fresh (as prepared) metal
doped g-Al2O3 catalysts
as an internal standard. Chromatographic standards of 1,8-
cineole, p-cymene and dipentene were run using the same sample
preparation method. Major products were also characterised
Surface
Pore
Average pore
width/A
1
by H and 13C NMR. The yield of p-cymene was defined as
˚
Catalyst
area/m2 g-1
volume/cm3 g-1
the percentage of p-cymene in the whole hydrophobic phase.
Selectivity for p-cymene is defined as the percentage of p-cymene
in the non-cineole fraction of the hydrophobic phase. Unless
otherwise noted, the non-cineole fraction of the hydrophobic
phase was comprised of the products shown in Scheme 1. The
hydrophilic phase was found to be mainly water.
g-Al2O3
247
172
239
210
216
243
0.76
0.51
0.71
0.66
0.69
0.81
12.9
13.1
12.8
12.7
12.7
12.4
Mo–g-Al2O3
Cr–g-Al2O3
Co–g-Al2O3
Fe–g-Al2O3
Pd–g-Al2O3
2.5. Analysis of gaseous products
Analysis of the reagent and product gases was performed with an
online Shimadzu GC 8A fitted with a Valco sampling valve with
a 1 mL sample loop. The GC was fitted with a 12 m HAYESEP
Q column and a thermal conductivity detector (TCD). The GC
was calibrated with a calibration gas mixture containing known
concentrations of hydrogen (5.02%), helium (4.92%), carbon
monoxide (4.97%) and carbon dioxide (10.20%) (BOC) before
each run. Argon was used as the reference gas for the TCD in the
GC analyses, allowing for maximum sensitivity to hydrogen. An
internal standard gas mixture containing 5.1% helium in argon
was used as the carrier gas for the liquid cineole feedstock.
Online gas analysis was conducted at the time of collection of
liquid samples. Results for the gas phase analysis were accurate
to within 3%.
Fig. 3 The XRD patterns of the metal-doped g-Al2O3 catalysts.
technique has shown a minimal amount of surface area loss with
the exception of the Mo-doped catalyst.
3. Results and discussion
3.1 Catalyst characterization
3.2 Catalytic activity
The XRD patterns of the metal doped g-Al2O3 catalysts
prepared by the wet impregnation technique are shown in Fig. 3.
The pure g-Al2O3 showed some crystalline nature while the
molybdenum-doped catalyst clearly shows a more crystalline
nature with intense peaks at 2q = 12.9, 23.4, 25.8 and 27.4◦
corresponding to a-MoO3 in the orthorhombic phase.16 The
other metal-doped g-Al2O3 samples display XRD patterns
associated with the support material. There is some evidence
of added crystallinity in all of the metal-doped samples.
The BET surface areas for all as-prepared catalysts and
g-Al2O3 are given in Table 1. The surface area of the g-Al2O3
was the highest while all other catalysts were lower in surface
area. Metal doping of the g-Al2O3 using the wet impregnation
Products of the pyrolysis process were separated into three
phases: hydrophobic and hydrophilic liquid, and gaseous
(Scheme 1). Table 2 shows the compositions of the hydrophobic
phases produced during the variable temperature experiments
using the five metal-doped catalysts and the undoped g-Al2O3
catalyst. Gaseous phase composition is shown in Fig. 7. In all
experiments, the only detectable hydrophilic phase component
was water, while the hydrophobic phase generally contained a
mixture of both aromatic and non-aromatic C10 hydrocarbons
with dipentene and p-cymene as the major products. The
hydrophilic liquids were analysed by NMR and GCMS. The only
gases detected were hydrogen, carbon monoxide and carbon
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dioxide. The gas phase composition was dependent on gas feed
composition and reaction temperature.
Fig. 4 shows p-cymene yield as a function of catalyst
bed temperature for all catalysts. Cineole remained relatively
unchanged for all experiments using the glass beads. The amount
of oxygen blended into the carrier gas had only a mild effect on
the hydrophobic phase products. A moderate level of introduced
oxygen (7.3%) generally increased the yields of p-cymene, while
higher levels (14.6%) decreased p-cymene yields and increased
the formation of CO and CO2.
Fig. 5 Selectivity for p-cymene versus bed temperature with 7.3%
oxygen.
Catalysts such as the undoped alumina and the chromium-
based catalyst displayed very high selectivities for p-cymene
(96 and 100%, respectively) at very low bed temperatures
(~220 ◦C) but had poor overall conversion of cineole (24 and
9%, respectively). The molybdenum catalyst displayed both poor
selectivity and overall production of p-cymene, whereas the iron
and cobalt catalysts produced no p-cymene under any of the
reaction conditions tested.
Fig. 4 Yield of p-cymene versus bed temperature with 7.3% oxygen.
There was a general trend towards decreasing selectivity with
increasing bed temperatures greater than ~250 ◦C for most
of the catalysts, with the notable exception of the palladium
catalyst, which maintained very high selectivity for p-cymene
in a zero-oxygen environment. Conversion increased from 37%
at 214 ◦C to nearly complete cineole consumption (94%) at
385 ◦C. Addition of oxygen at moderate levels increased the
selectivity to p-cymene to 99–100% at low bed temperatures
while exhibiting excellent conversion of cineole (>90%).
Interestingly, the undoped alumina, molybdenum, chromium
and palladium catalysts produced no dipentene in any of the
runs, yet selectivity for dipentene was very high under mild
conditions for the iron and cobalt catalysts. Conversion of
cineole was poor for cobalt (<10%) but slightly better for iron,
reaching a maximum of 58% at 250 ◦C, with 97% selectivity. As
was the case for p-cymene, selectivity towards dipentene dropped
off past ~250 ◦C for both the iron and cobalt catalysts. The
addition of a moderate level of oxygen during runs with the iron
catalyst dramatically improved the conversion level of cineole
from 13 to 81% at low bed temperatures However, additional
oxygen past 7.3% reduced cineole conversion to 5%. Selectivity
for dipentene was only marginally affected by the varying oxygen
levels. The best result for the cobalt catalyst was 74% cineole
conversion with 89% selectivity for dipentene at 267 ◦C with no
oxygen.
Experiments performed with the undoped g-Al2O3 pellets
showed some formation of p-cymene with yields of 47, 60
and 50%, for 0, 7.3 and 14.6% oxygen, respectively at around
400 ◦C. The selectivity of the undoped g-Al2O3 pellets at this
temperature was approximately 70% for p-cymene. The major
product formed for both the iron- and cobalt-doped g-Al2O3
catalysts was dipentene, with no apparent production of
p-cymene.
The chromium-doped g-Al2O3 catalyst catalysed the forma-
tion of p-cymene with a maximum yield of 55% using 14.6%
oxygen at ~400 ◦C. At this bed temperature, the conversion
of cineole ranged from 67–87% depending on the amount of
oxygen used. The selectivity of this catalyst to p-cymene was
initially quite high (>95%) but with very little conversion of the
cineole (9–15%). At 400 ◦C, selectivity was again reasonable,
being approximately 70% (Fig. 5).
The molybdenum-doped g-Al2O3 catalyst also produced
p-cymene as the major product with a maximum yield of 56%
at 0 and 7.3% oxygen and ~390 ◦C. At lower temperatures, the
selectivity of this catalyst was quite low (<20%) but reached
66% at 378 ◦C. p-Cymene was the major product produced
with high selectivity for the palladium-doped g-Al2O3 catalyst.
This catalyst gave excellent conversion efficiencies to p-cymene
at much lower bed temperatures compared to the other catalysts
tested. The experiments with this catalyst containing some
oxygen in the carrier gas showed almost complete conversion of
the cineole while conversion efficiencies to p-cymene were 95%
at a reaction bed temperature of approximately 250 ◦C. The
selectivity of this catalyst for p-cymene was also very high, being
>98% at this bed temperature.
3.3 Analysis of gas products
The yields of hydrogen for the different catalysts with 7.3%
oxygen in the carrier gas are shown in Fig. 6.
The only gas product observed during oxygen-free experi-
ments was hydrogen. Across all catalysts tested, increasing the
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Fig. 7 Gaseous products formed with the palladium doped g-Al2O3
catalyst.
Fig. 6 Yield of hydrogen for experiments with 7.3% oxygen.
amount of oxygen in the carrier gas increased the production of
CO and CO2.
3.4 Reaction mechanism
The effect of oxygen in the carrier gas stream on the
composition of the gas phase produced by the palladium doped
g-Al2O3 catalyst is shown in Fig. 7. When oxygen was used in
the reaction, the amount of hydrogen produced decreased, while
the amount of CO2 increased. This effect was more pronounced
at higher temperatures. The CO production was negligible for
all the palladium-doped g-Al2O3 catalyst experiments. Almost
total conversion of oxygen was observed during experiments
containing oxygen.
Based on the observed products, a possible reaction mechanism
for the conversion of cineole into p-cymene over g-Al2O3 sup-
ported catalysts is shown in Scheme 2. The reaction consists of a
dehydration step followed by a dehydrogenation step. According
to the thermophysical and theoretical study undertaken by Leal
et al.,17 the weakest bond in the cineole molecule is the oxygen
to carbon bond with only one methyl group attached. This bond
is broken first, and the dehydration of cineole takes place very
rapidly, producing a mixture of doubly unsaturated isomers.
Scheme 2 The proposed reaction sequence for the catalytic transformation of cineole into p-cymene.
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In the second step, dehydrogenation and rearrangement of the
double bonds occurs to produce p-cymene.
perform the dehydration step, the palladium-doped g-Al2O3
pellets showed the best activity, yielding >90% p-cymene while
producing large amounts of hydrogen in the exit gas at a bed
temperature of ~250 ◦C. Future work is focusing on studying the
palladium catalyst system in more detail including: optimizing
the reaction conditions to maximize both conversion of cineole
and selectivity to p-cymene, and studying catalyst lifetime.
The undoped g-Al2O3 catalyst was active for both the
dehydration and dehydrogenation reactions. The dehydration
reaction yielded a mixture of non-aromatic intermediates. This
was followed up by the dehydrogenation of these intermediates
into the fully aromatic p-cymene (Scheme 2). Similar products
were also observed for the chromium- and molybdenum-doped
g-Al2O3 catalyst. For the cobalt- and iron-doped g-Al2O3
catalyst, there was no observation of p-cymene in the products
indicating that these catalysts are not able to affect the dehy-
drogenation step. The major products for both the cobalt- and
iron-doped g-Al2O3 catalysts suggest that these catalysts were
only capable of performing the dehydration of the cineole while
also giving some selectivity towards dipentene. They were not
able to perform the dehydrogenation step.
For the palladium-doped g-Al2O3 catalyst, a very high
selectivity towards the p-cymene was observed. This catalyst
is capable of performing both the dehydration of the cineole
followed by a rapid dehydrogenation into p-cymene. Due to
the absence of dehydration products observed when using the
palladium-doped g-Al2O3 catalyst, the palladium plays a vital
role in the dehydrogenation step. This is not surprising as
palladium is a known hydrogenation/dehydrogenation catalyst.
The high selectivity of the dehydrogenation step to p-cymene
over the Pd catalyst suggests this step proceeds at a greater rate
than the cineole dehydration step.
Acknowledgements
The authors wish to thank the CSIRO OCE PDF funding
scheme for financial support for this project.
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4. Conclusions
This study has shown that a continuous synthetic route exists for
the conversion of cineole to valuable products. The high temper-
ature catalytic transformation of the bio-renewable feedstock,
1,8-cineole, over a number of metal-doped g-Al2O3 catalysts has
shown p-cymene to be the major hydrophobic liquid product
and hydrogen the major gaseous product. From an industrial
perspective, the separation of the liquid products for runs with
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