Hydrogenation and ring opening of aromatic and naphthenic hydrocarbons over noble metal (Ir, Pt, Rh)/Al2O3 catalysts

Article abstract of DOI:10.1007/s10562-012-0810-8

The hydrogenation and ring opening of model hydrocarbons and of naphtha was studied over commercial noble metal (Ir, Pt, Rh)/Al₂O₃ catalysts. The experiments were performed in a fixed bed reactor at temperatures between 220 and 350 °C and pressures of 1.1 and 5.0 MPa, respectively. The product distribution was determined and the cetane number was calculated. The Pt catalyst is very active for hydrogenation of aromatics but does not catalyse the ring opening of naphthenes. The Ir and Rh catalysts are active for both hydrogenation of aromatics and ring opening of naphthenes. Experiments with toluene, mxylene, propyl-benzene, and methylcyclohexane indicate that ring opening follows a selective mechanism, where the cleavage of bisecondary carbon bonds is favoured. This results in predominant formation of branched paraffins. The product distribution as well as cracking of long-chain hydrocarbons, which increase at temperatures above 260 °C, lead to an insignificant boost in the cetane number, as confirmed by experiments using real naphtha as feedstock.

Full text of DOI:10.1007/s10562-012-0810-8

Catal Lett (2012) 142:531–540
DOI 10.1007/s10562-012-0810-8
Hydrogenation and Ring Opening of Aromatic and Naphthenic
Hydrocarbons Over Noble Metal (Ir, Pt, Rh)/Al2O3 Catalysts
•
•
•
Anne Piegsa Wolfgang Korth Fehime Demir
Andreas Jess
Received: 15 February 2012 / Accepted: 19 March 2012 / Published online: 31 March 2012
ꢀ Springer Science+Business Media, LLC 2012
Abstract The hydrogenation and ring opening of model
hydrocarbons and of naphtha was studied over commercial
noble metal (Ir, Pt, Rh)/Al2O3 catalysts. The experiments
were performed in a fixed bed reactor at temperatures
between 220 and 350 ꢁC and pressures of 1.1 and 5.0 MPa,
respectively. The product distribution was determined and
the cetane number was calculated. The Pt catalyst is very
active for hydrogenation of aromatics but does not catalyse
the ring opening of naphthenes. The Ir and Rh catalysts are
active for both hydrogenation of aromatics and ring
opening of naphthenes. Experiments with toluene, m-
xylene, propyl-benzene, and methylcyclohexane indicate
that ring opening follows a selective mechanism, where the
cleavage of bisecondary carbon bonds is favoured. This
results in predominant formation of branched paraffins.
The product distribution as well as cracking of long-chain
hydrocarbons, which increase at temperatures above
260 ꢁC, lead to an insignificant boost in the cetane number,
as confirmed by experiments using real naphtha as
feedstock.
1 Introduction
In recent years, a lot of research dealing with ring opening
reactions has been undertaken. In the majority of cases, the
aim was to develop new routes for the production of high
quality transportation fuels which fulfill new, more strict
environmental regulations, e.g. a low aromatic content
[1–4]. New routes for diesel oil production are also needed,
if the demand for diesel increases in the next years. Until
the year 2030, the predicted sales volume of diesel will
double in comparison to the demand for gasoline [5]. In
this context future drive systems are also important, which
mix the most favourable characteristics of both petrol and
diesel technology, e.g. the combined combustion system
(CCS) of Volkswagen or the so-called Diesotto-technology
of Mercedes [6, 7]. Hence, the conversion of naphtha into
‘‘light’’ diesel with a preferably high cetane number (CN)
may then be an option.
This aim of a gasoline with a high CN can only be
achieved by hydrogenation of the aromatics (present in raw
naphtha) and subsequent ring opening of the naphthenes
(Fig. 1). Aromatics have a very low cetane number, e.g.
CNtoluene = -5, followed by the corresponding cycloal-
kanes, e.g. CNmethylcyclohexane = 20. Depending on the
position of the side chain iso-paraffins can have both higher
and lower cetane numbers than the corresponding naphth-
enes. For example, the CN of 2-methylhexane is 35 while the
CN of 3-methylhexane is only 17. However, linear paraffins
exhibit the highest cetane numbers (e.g. CNn-heptane = 53).
In general, the CN number of linear and branched paraffins
grows with increasing chain length, and for naphthenes and
aromatics with increasing length of side chains.
Keywords Hydrogenation ꢀ Ring opening ꢀ Naphtha ꢀ
Platinum ꢀ Iridium ꢀ Rhodium ꢀ Cetane number
A. Piegsa ꢀ W. Korth ꢀ A. Jess (&)
Department of Chemical Engineering, Faculty of Engineering
¨
Science, University of Bayreuth, Universitatsstraße 30, 95445
Bayreuth, Germany
e-mail: Jess@uni-bayreuth.de
Ring opening reactions are also interesting to produce
synthetic steam cracker feed from aromatic-rich pyrolysis
gasoline, as investigated by Weitkamp et al. [8–10].
F. Demir
Volkswagen AG, Corporate Research, Mailbox 011/17780,
38436 Wolfsburg, Germany
123

532
A. Piegsa et al.
80
70
60
50
40
30
20
10
0
high selectivity for ring opening products. The group of
Gault et al. [18] carried out experiments on the ring
opening of methylcyclopentane over platinum catalysts.
They presented a correlation between the metal dispersion
on the catalyst surface and the product distribution as well
as the reaction mechanism. It was shown that the product
distribution was non-selective on 0.2 % Pt/Al2O3 with
highly dispersed particles. The possible ring opening
products n-hexane, 2-methylpentane, and 3-methylpentane
were formed with the same probability. When using 10 %
Pt/Al2O3 with low dispersed metal particles, the ring was
only opened between bisecondary (non-substituted) carbon
atoms leading predominantly to the formation of 2-meth-
ylpentane (78 %) and not of n-hexane. Hence, Gault
proposed three different reaction mechanisms. First, the
non-selective mechanism (also known as the multiplet
mechanism). Here methylcyclohexane is supposed to
adsorb planar on the metal surface. The bisecondary and
tertiary–secondary bonds are broken with the same prob-
ability. The second mechanism proposed is called the
selective mechanism (dicarbene mechanism), where the
hydrocarbon adsorbed perpendicularly on the metal surface
via two adjacent bisecondary carbon atoms. Due to steric
hindrance the tertiary–secondary bonds are not cleaved. A
third mechanism, the ‘‘partially selective’’ mechanism,
which competes with the dicarbene mechanism, was pro-
posed to explain the product distribution, where mainly
bisecondary, but sometimes also tertiary–secondary bonds
are broken. The methyl group and a methylene group in
a-position interact with a metal site forming a metallo-
cyclobutane intermediate. This results in cleavage of
tertiary–secondary bonds [2].
n-Paraffins
3-Methyl-
Paraffins
2-Methyl-
Paraffins
Naphthenes
-10
-20
Aromatics
5
6
7
8
9
Number of carbon atoms
Fig. 1 Cetane number of different hydrocarbons versus the number
of carbon atoms
All the above mentioned cases follow the approach to
use (sometimes poorly marketable) aromatics as feedstock
for conversion into valuable intermediates/products.
Before aromatic hydrocarbons can be ring opened to
paraffins they have to be completely hydrogenated to their
corresponding naphthenes [8]. Hydrogenation of aromatics
is well described in the literature [11–14], and it is known
that group-VIII metals (Pt, Pd, Rh…) are suitable catalysts
for hydrogenations [12].
In principle, ring opening of naphthenic hydrocarbons
can be achieved catalytically by cracking on solid acidic
catalysts, hydrocracking on bifunctional catalysts or hy-
drogenolysis on noble metals.
On solid acid catalysts (e.g. zeolites), ring opening of
naphthenes is achieved by a protolytic cracking mechanism
via carbenium intermediates. Due to side reactions and
consecutive cracking reactions these catalysts are not
appropriate to yield high cetane products [2].
Other research groups showed that the size of metal
particles is less important when using iridium or rhodium.
But these metals have higher activity and selectivity to
cleave the bisecondary bonds of the C5-ring [2, 3]. Fur-
thermore it was shown by Resasco et al. [19] that the
dispersion of metal particles is less important for the
reaction mechanism taking place by converting 1,2- and
1,3-dimethylcyclohexane over different Iridium catalyst.
However, they showed the type of support influences the
ring opening mechanism strongly.
Already in 1953, Mills et al. [15] proposed a mechanism
for the hydrocracking of naphthenes over a bifunctional
catalyst based on experiments with cyclohexane, cyclohex-
ene, methylpentane, and methylpentene. The mechanism
proposes three steps [2, 15]: At first, cyclic hydrocarbons are
dehydrogenated to cyclic olefins on a metal site. Secondly,
the olefins are protonated on the acid site, which leads to
formation of carbenium ions, which are either cracked or
skeletally isomerised. The olefins formed after isomerisation
are desorbed from the acid site and finally hydrogenated on
the metal site. This mechanism and the intermediates were
confirmed by Weisz and Swegler [16].
In this work, hydrogenation and ring opening reactions
over commercial noble metal catalysts (Ir, Pt, Rh) were
studied with different model aromatic and naphthenic
hydrocarbons as well as with ‘‘real’’ naphtha. Beside the
kinetics, the impact on the cetane number was analysed.
The ring opening of naphthenic hydrocarbons can also
occur on noble metal catalysts without acidic sites (hy-
drogenolysis). Galperin et al. [17] inhibited the activity of
the acid sites of a bifunctional platinum catalyst by addi-
tion of potassium. They showed that the catalyst acted then
like a monofunctional metallic catalyst, which enabled a
2 Experimental
All experiments were carried out in a continuously operated
fixed bed reactor (Fig. 2), designed to operate at
123

Hydrogenation and Ring Opening of Aromatic
533
Fig. 2 Flow diagram of the
experimental plant (left) and
cross section of the fixed bed
reactor (right)
1
1
2
3
4
Thermal element
Catalyst bed
E-1
Educt tank
V-5/6
V-7/8
V-9
Stop valve
E-2
Gas bottle N2
Gas bottle H2
Educt pump
Three-way valve
Needle valve
Vaporizer
Inert bed (glass beads)
Frit
E-3
P-1
R-1
V-1/2
V-3/4
Check valve
R-2
Fixed bed reactor
Sampling
Mass flow controller
R-3
temperatures up to 450 ꢁC and pressures up to 6.0 MPa. The
set-up is subdivided into three units, the vaporizer (45 mm
inner diameter, 460 mm length), the fixed bed reactor
(15 mm inner diameter, 360 mm length), and the sampling/
analysis. Each part is separately heated and temperature
controlled. The lab-scale plant is supplied with hydrogen and
nitrogen via two mass flow controllers [5 to 100 l/h (STP)].
Nitrogen is needed to check on leak tightness and to flush the
set-up with inert gas after experimental usage. A high pres-
sure pump feeds the liquid hydrocarbon, and the combined
stream of the hydrocarbon and of hydrogen enters the
vaporizer where the hydrocarbon evaporates. Afterwards the
gas mixture streams into the reactor containing the catalyst
fixed bed. The catalyst bed is placed in the middle of the
reactor to achieve almost isothermal conditions. Hence, the
reactor is half-filled with glass beads (inert bed), on which
the catalyst particles are placed. Running the experiments
without catalyst cracking or conversion of the hydrocarbons
was not observed. Thus a blind activity of the reactor can be
excluded. The reactor is equipped with a metal frit (90 lm)
to avoid discharge and deposit of glass beads or catalyst
material into the pipework. Downstream the reactor, the
pressure is reduced to standard pressure by a needle valve.
Both gaseous and liquid samples are analysed using a gas
chromatograph (Perkin Elmer, CLARUS 500) equipped
with a PONA-column (100 m in length, 0.25 mm inner
diameter, 0.5 mm film thickness) and a Flame Ionization
Detector. The gas chromatographic (GC) raw data are
interpreted by using ARNEL DHA Software.
weighting coefficient, whereby only hydrocarbons with 6
or more carbon atoms were taken into account.
The catalysts used are commercially available. The
platinum catalyst (0.3 wt% Pt on Al2O3, HDMAX800, Su¨d
Chemie) is of cylindrical shape (4.5 mm length and
diameter). The iridium catalyst (1 % Ir on Al2O3, Alfa
Aesar) is spherical with a core-shell structure (0.5 mm
shell thickness). The Rhodium catalyst (0.5 % Rh on
Al2O3, Alfa Aesar) is cylindrical and also has a core-shell
structure (0.5 mm shell thickness). Further properties of the
catalysts are listed in Table 1. Before usage the catalysts
were reduced for 2 h in H2 at 250 ꢁC.
Toluene (C99.7 % purity), ethylbenzene (99 % purity),
propylbenzene (98 % purity), cyclohexane (99.9 % purity),
and methylcyclohexane (99 % purity) were obtained from
Sigma Aldrich. m-Xylene (C99 %) was purchased from
Merck. All hydrocarbons were used without further puri-
fication. Naphtha (boiling range 30–230 ꢁC) was provided
by the MiRO Refinery (Karlsruhe, Germany). Due to the
sulphur content of 600 ppm, the naphtha was desulfurized
(hydrotreated) before usage to avoid catalyst poisoning
(350 ꢁC; H2-pressure: 5.0 MPa; catalyst: NiMo/Al2O3, Su¨d
Chemie). After desulfurization the S-content was less than
10 ppm. H2 (purity grade 3.0) and N2 (purity grade 5.0)
were purchased from Riessner Gase.
3 Results and Discussion
The cetane number was calculated based on the product
analysis simply with the individual mass content as
At first, hydrogenation and ring opening reactions were
studied with model substances. The results with iridium,
123

534
A. Piegsa et al.
Table 1 Physical properties of the catalysts used in this work
Catalyst
HDMax 800 (Su¨d Chemie)
Ir/Al2O3 (Alfa Aesar)
Rh/Al2O3 (Alfa Aesar)
Composition
Apparent density
Surface area (BET)
Porosity
0.3 wt% Pt, Al2O3
1,260 kg/cm3
185 m2/g
1.0 wt% Ir, Al2O3
1,100 kg/m3
140 m2/g
0.5 wt% Rh, Al2O3
1,650 kg/m3
100 m2/g
68 %
72 %
60 %
Geometry
Cylindrical; length
and diameter: 4.5 mm
Spherical; diameter: 5 mm,
iridium shell: 0.5 mm
Cylindrical; length and diameter:
4 mm, Rh shell: 0.5 mm
1
rhodium and platinum as catalysts are shown in Sect. 3.1.
Secondly, ‘‘real’’ naphtha was converted over the Pt and Ir
catalysts (Sect. 3.2).
Equilibrium
conversion
Toluene
0,8
0,6
0,4
0,2
0
Ethylbenzene
Propylbenzene
m-Xylene
3.1 Experiments with Model Substances
3.1.1 Hydrogenation of Aromatics Over Pt/Al2O3
Toluene
Experimental
Ethylbenzene
Propylbenzene
m-Xylene
It is well known that platinum catalyses the hydrogenation
of aromatics. Several research groups also claim that it is
possible to ring open naphthenes over platinum catalysts
[3, 17]. However, the results on the conversion of different
aromatics over the platinum catalyst used here clearly
indicate that ring opening is not possible. All aromatics are
completely converted to the corresponding naphthenes, but
a consecutive ring opening was not detected, and even at
the highest temperature of 420 ꢁC and long residence times
(90 g s/cm3) only the corresponding naphthenes were
found as products. Even the direct usage of naphthenes
as feedstock did not lead to a ring opening over the
Pt-catalyst.
200
300
400
500
Temperature [°C]
Fig. 3 Hydrogenation of different aromatics over Pt/Al2O3
(1.1 MPa; hydrogen/naromatic = 15; modified residence time:
0.28 g s/cm3)
n
conversion, the temperature was raised successively from
220 to 320 ꢁC. At temperatures lower than 240 ꢁC, the
conversion of cyclohexane is less than 10 % and the
selectivity to n-hexane is higher than 90 % (Fig. 4, left).
With raising temperatures the conversion of cyclohexane
increases while the selectivity to n-hexane decreases
mainly via further cracking to short chain hydrocarbons. At
320 ꢁC, nearly complete cyclohexane conversion (97 %) is
reached, but the remaining selectivity of n-hexane is only
2 %. The main products are then methane, ethane, and
propane (subsequently referred to as C1 to C3) with a
selectivity of 94 %. In the medium temperature range
between 250 and 280 ꢁC, linear and branched butane and
pentane as well as 2- and 3-methylpentane are also formed
as intermediates (in Fig. 4 referred to as C4 to C6, except
n-hexane), which are further cracked at T [ 280 ꢁC. These
results show that the ring opening of cyclohexane only
leads dominantly to n-hexane at a low degree of conver-
sion, i.e. here at temperatures below 250 ꢁC. At higher
temperatures the paraffinic hydrocarbons are cracked over
Ir/Al2O3 and the formation of C1 to C3 hydrocarbons
increases drastically.
At a high modified residence time (=ratio of total mass
of catalyst to the volumetric flow rate at reaction condi-
tions) of e.g. 2 g s/cm3, the conversion of the model
aromatics toluene, ethylbenzene, propylbenzene, and
m-xylene to the corresponding naphthenes is almost com-
plete even at 200 ꢁC. Fig. 3 depicts the conversion versus
temperature (Fig. 3) at a very short residence time of
0.28 g s/cm3 in a temperature range from 200 to 420 ꢁC.
At about 300 ꢁC the reaction shifts from the regime of
kinetic control to the regime of control by thermodynam-
ics, i.e. the equilibrium limits the conversion, which then
decreases with increasing temperature.
3.1.2 Ring Opening of Cyclohexane
and Methylcyclohexane Over Ir/Al2O3 and Rh/Al2O3
In contrast to the Pt catalyst, ring opening takes place, if
the Ir catalyst is used. In Fig. 4 (right), the conversion of
cyclohexane over Ir/Al2O3 and the selectivities of the dif-
ferent products are depicted. In order to increase the
The appearance of small amounts 2- and 3-methylpen-
tane raises the question about the reaction mechanism
123

Hydrogenation and Ring Opening of Aromatic
535
Fig. 4 Conversion of
cyclohexane (left) and
methylcyclohexane (right) over
Ir/Al2O3 and the product
selectivities (1.1 MPa; nhydrogen
nhydrocarbon = 15; modified
residence time: 2 g s/cm3)
1
0.8
0.6
0.4
0.2
0
1
0.8
0.6
0.4
0.2
0
Methylcyclohexane
n-Hexane
C1 to C3
/
3-Methylhexane
C1 to C3
Cyclohexane
2-Methylhexane
C to C6
4
C to C6
4
(except n-Hexane)
n-Heptane
200
250
300
350
200
250
300
350
Temperature [°C]
Temperature [°C]
1
0.8
0.6
0.4
0.2
0
occurring on the iridium catalyst. It is known that bifunc-
tional catalysts with metal and acid sites contract the
cyclohexane ring to form methylcyclopentane, which is
subsequently ring opened to n-hexane but also 2- and
3-methylpentane [20]. Experiments with methylcyclopen-
tane as feedstock over the iridium catalyst were carried out
to get an indication about the reaction mechanism and the
catalysts’ acidity. The reaction conditions were equal to the
ones listed in Fig. 4. The conversion of methylcyclopen-
tane resulted in the formation of 2- and 3-methylpentane in
a ratio of 2:1 but no n-hexane. This result shows that ring
opening of cyclohexane over the iridium catalyst is not
achieved by ring contraction but by direct opening on the
metal site. Thus, acid sites on the iridium catalyst can be
excluded.
Temperature: constant (240 °C)
Residence time: varied (2 to 8 g s/cm )
3
Trend lines
Temperature: varied (220 to 320 °C)
Residence time: constant (2 g s/cm )
3
0
0.2
0.4
0.6
0.8
1
Conversion [-]
Fig. 5 Conversion of Cyclohexane over Ir/Al2O3 at constant tem-
perature and at constant residence time (pressure: 1.1 MPa; nhydrogen
ncylohexane = 15)
/
To exclude temperature effects, the conversion of
cyclohexane was also increased at a constant temperature
(240 ꢁC) by raising modified residence time successively
from 2 to 8 g s/cm3 (Fig. 5). For comparison, the result of
the experiments discussed before at a constant residence
time (2 g s/cm3) but with varied temperature are also
shown. At a conversion of about 30 %, in both cases the
selectivity of n-hexane is about 80 %. Fig. 5 indicates that
the selectivity to n-hexane is improved, if the conversion is
enhanced by a higher residence time and not by increasing
the temperature. For example at a conversion of 80 %, the
n-hexane selectivity is still 61 % at 240 ꢁC (s = 8 g s/
cm3) compared to only 16 % at 280 ꢁC (2 g s/cm3).
A similar picture is obtained with methylcyclohexane as
feedstock (Fig. 4, right), which was used to obtain infor-
mations on ring opening of 6-ring naphthenes with side
chains. At temperatures below 260 ꢁC and a corresponding
low conversion of methylcyclohexane, the main products
are 2-methylhexane (selectivity: 34 % at 240 ꢁC) and
3-methylhexane (52 %) while C1 to C3 (5 %) and C4 to C6
hydrocarbons (HCs) (7 %) are formed to a very low extent.
The selectivity to n-heptane is low (2 %) and remains
below 3 % throughout the entire temperature range inves-
tigated. At higher temperatures ([240 ꢁC), the conversion
increases, and the selectivities to 2- and 3-methylhexane
strongly decrease. The selectivity of C4 to C6 HCs passes a
maximum at 290 ꢁC (36 %) and then also drops. Hence,
cracking of all these intermediates finally leads to C1 to C3
HCs (selectivity: 90 % at 340 ꢁC).
This results show that the ring opening of cyclohexane
and methylcyclohexane to the corresponding paraffins with
the same carbon number is only possible at low tempera-
tures. The selectivity decreases with increasing conversion,
but to a lower extent, if the residence time and not the
temperature is increased to achieve a higher conversion
without loosing product quality (Fig. 5).
The experiments with methylcyclohexane also give a
hint about the mechanism responsible for the product dis-
tribution of ring opened naphthenes with a side chain.
Depending on where the ring is cleaved, three products are
possible to be formed: 2-methylhexane, 3-methylhexane
and n-heptane. If the mechanism is non-selective the three
products would be formed with the same probability. As
the product spectrum shows an 8:11:1 distribution of
2-methylhexane, 3-methylhexane, and n-heptane (Fig. 5) it
is obvious that the mechanism is selective. This implies
123

536
A. Piegsa et al.
that mainly bisecondary bonds are cleaved according to the
dicarbene mechanism. But the appearance of small
amounts of n-heptane in the product spectrum shows that
also tertiary-secondary bonds are broken. This leads to the
conclusion that at least to a minor extent also the partially
selective mechanism proceeds resulting in the formation of
n-heptane via metallocyclobutane intermediates.
Based on the results shown in Fig. 4 the reaction rates
-1
cat
(rm in mol kg s-1) of the conversion of cyclohexane and
methylcyclohexane were calculated.
The solution of the power law reaction rate approach
(with s as modified residence time = ratio of total mass of
catalyst to the volumetric flow rate at reaction conditions)
dci
ꢁ
¼ rm ¼ kðTÞcmi cn
ð1Þ
In summary, it can be stated that the ring opening of
naphthenes with a side chain over the iridium catalyst
predominantly forms branched paraffins, which is a draw-
back regarding the Cetane number (Fig. 1). Conversion of
methylcyclohexane (CN = 20) mainly leads to 2-methyl-
hexane (CN = 35) and 3-methylhexane (CN = 17). Only
5 % of n-heptane (CN = 53) is formed. Hence, the average
CN only increases from 20 to 26 (Fig. 6).
H2
ds
leads to the following equation for the reaction rate
constant k(T)
1ꢁm
ð1 ꢁ XÞ
ðm ꢁ 1Þc
ꢁ 1
kðTÞ ¼
ð2Þ
ðmꢁ1Þ
cnH s
2
i;in
X is the conversion of the hydrocarbon, ci,in the initial
hydrocarbon concentration, and cH2 the constant hydrogen
concentration (high surplus).
1
1
In the area of low conversions (T \ 260 ꢁC), the rate
constant k(T) is in agreement with Arrhenius’ law:
E
A
RT
kðTÞ ¼ k0eꢁ
ð3Þ
2
2
It was proven by respective calculations, that internal
and external mass transfer limitations are not present in this
temperature range. The values for the activation energy EA,
the pre-exponential factor k0, and the reaction orders m and
n are listed in Table 2. The reaction orders for hydrogen
and for the hydrocarbon were experimentally determined
by varying separately the respective feed concentration
(Table 2). The Arrhenius plot is shown in Fig. 7. Due to
different reaction orders of hydrogen, and thus different
units of k, Fig. 7 depicts the reaction rate (and not k) versus
the reciprocal temperature in order to compare the activity
of the Ir catalyst for conversion (ring opening) of
cyclohexane and methylcyclohexane. The activation
energy is similar in both cases (around 200 kJ/mol,
Table 2). Hence, the rates differ by a constant factor,
whereby the activity to convert cyclohexane is (at
1.1 MPa) by a factor of three higher. Interestingly
(although the reason is still an open question), the rate of
ring opening of methylcyclohexane is slightly enhanced
with increasing hydrogen concentration (rMCH * c0H.25),
whereas for cyclohexane this is reverse and the rate
decreases (rCH * 1/cH2).
3
3
Methylcyclohexane (CN = 20)
+ H2
Selectivity:
0.05
n-Heptane (CN = 53)
2-Methylhexane (CN = 35)
3-Methylhexane (CN = 17)
1
0.4
2
3
0.55
average CN = 26
Fig. 6 Product distribution after the cleavage of bisecondary (2 and
3) and tertiary–secondary (1) bonds of the methylcyclohexane ring
over Ir/Al2O3
The ring opening of cyclohexane was also studied over a
rhodium catalyst (0.5 % Rh/Al2O3, cylindrical shape,
Table 2 Kinetic data of cyclohexane and methylcyclohexane conversion (ring opening) over Ir/Al2O3 (1.1 MPa; nhydrogen/nhydrocarbon = 15;
modified residence time: 2 g s/cm3)
EA in kJ
k0
Reaction order H2 (m)
Reaction order hydrocarbon (n)
Cyclohexane
204
197
1.7 9 1019 mol1.5/m1.5 kg s
4.8 9 1014 m3/kg s
-1
0.5
0.5
Methylcyclohexane
0.5
123

Hydrogenation and Ring Opening of Aromatic
537
1
0.8
0.6
0.4
0.2
0
250
°
C
200 °C
a
0.1
0.01
Experimental data
Calculated model
Toluene
0.001
Cyclohexane
0.0001
0.00001
0.000001
Methylcyclohexane
Methylcyclohexane
3-Methylhexane
2-Methylhexane
C
4 to C6
0.0018
0.0019
0.002
0.0021
0.0022
C1 to C3
Reciprocal temperature [1/K]
n-Heptane
Fig. 7 Arrhenius plot of cyclohexane and methylcyclohexane ring
opening over Ir/Al2O3 (200–260 ꢁC, 1.1 MPa; nhydrogen/nhydrocar-
bon
200
220
240
260
280
300
= 15; modified residence time: 2 g s/cm3)
1
0.8
0.6
0.4
0.2
0
b
Propylbenzene
1
Propylcyclohexane
C1to C3
Selectivity
n-hexane
(trend line)
0.8
0.6
0.4
0.2
0
Conversion
cyclohexane
(trend line)
4-Ethylhept.
4-Methyloct.
C7/ C8
Iridium
Rhodium
C to C6
4
200
220
240
260
280
300
200
220
240
260
280
300
Temperature [°C]
Fig. 8 Conversion of cyclohexane over 0.5% Rh/Al2O3 and 1.0 % Ir/
Al2O3 and the selectivity of n-hexane (5 MPa; nhydrogen/ncylohex-
1
0.8
0.6
0.4
0.2
0
c
= 15, modified residence time: 30 g s/cm3 (Rh) and 15 g s/cm3
ane
m-Xylene
(Ir); note that the residence time related to the mass of the noble metal
is constant
1,3-Dimethylcyclohexane
4.5 mm in height and diameter, purchased from Alfa Ae-
sar). Neither the conversion of cyclohexane nor the selec-
tivity of n-hexane is influenced by the type of noble metal
(Fig. 8), if the residence time related to the mass of the
noble metal is kept constant. Thus, it can be assumed that
the results shown here, which were mainly performed over
iridium, could also be achieved by using a similar rhodium
based catalyst.
C7
(except 2,4-DMCH)
2,4-Dimethylhexane
(main RO product)
C1to C3
C4to C6
280 300
200
220
240
260
3.1.3 Conversion of Aromatics Over Ir/Al2O3
Temperature [°C]
Fig. 9 Conversion of toluene (a), propylbenzene (b) and m-xylene
(c) over Ir/Al2O3 and product selectivities (1.1 MPa; nhydrogen
nhydrocarbon = 50, modified residence time: 2 g s/cm3)
The conversion of toluene, propylbenzene, and m-xylene
over Ir/Al2O3 was studied at temperature from 220 to
280 ꢁC. In all three cases the hydrogenation to the corre-
sponding naphthenes is very fast, and almost complete
conversion of the aromatics is reached even at the lowest
temperature of 220 ꢁC (Fig. 9).
/
In case of toluene (Fig. 9a), the primary product at
220 ꢁC is methylcyclohexane, which is subsequently ring
opened mainly to 2- and 3-methylhexane. As expected in
123

538
A. Piegsa et al.
respect to initial fast hydrogenation of toluene, the distri-
bution of ring opening products is equal to the results of the
conversion of methylcyclohexane. Again, cracking reac-
tions at temperatures higher than 240 ꢁC predominantly
lead to formation of C1 to C6 hydrocarbons.
that the 1,3-DMCH is opened predominantly in bisecondary
positions.
Figure 10 summarizes the reactions and the product
selectivities obtained in with the different aromatic
hydrocarbons with Ir/Al2O3 as catalyst. In the best case, i.e.
if cracking is negligible, the increase of the cetane number
is relatively high for benzene (from -10 to 42), toluene
(-5 to 26), and m-xylene (1 to 29), but rather low for
propylbenzene (15 to 16).
The conversion of propylbenzene (Fig. 9b) leads ini-
tially to the formation of propylcyclohexane, which is ring
opened subsequently. The expected products n-nonane,
4-methyloctane, and 4-ethylheptane are formed in a ratio of
1:10:20. This fits in the theory that ring opening over
iridium follows the dicarbene mechanism, where mainly
the bisecondary bonds are cleaved leading to the formation
of branched hydrocarbons. At 240 ꢁC, the formation of C7/
C8 and C1 to C3 hydrocarbons escalates. Propylcyclohex-
ane is split into ethane and methylcyclohexane, which is
subsequently ring opened to 2- and 3-methylhexane.
Unfortunately, the yield of C9 ring opening products is
therefore \15 %. To avoid side chain cleavage of the
naphthenes temperatures below 240 ꢁC are necessary.
In Fig. 9c, the conversion of m-xylene over Ir/Al2O3 is
depicted. The hydrogenation to 1,3-dimethylcyclohexane
(1,3-DMCH) is very fast, but compared to methyl- and pro-
pylcyclohexane the ring opening of 1,3-DMCH to paraffins is
slower. Even at 280 ꢁC 1,3-DMCH is still the main product
with a selectivity of 55 %. The distribution of the three pos-
sible ring opening products 4-methylheptane, 2-methylhep-
tane, and 2,4-dimethylhexane is 1:4:30. As expected, the
main ring opening product is 2,4-dimethylhexane, showing
3.2 Hydrogenation and Ring Opening of Aromatic
and Naphthenic Compounds of Naphtha
The composition of the naphtha used here before and after
conversion over platinum and iridium catalysts at temper-
atures between 250 and 290 ꢁC and the resulting cetane
numbers are listed in Table 3. For hydrogenation over Pt/
Al2O3 the cetane number raised only from 32 to 34. This
marginal increase is due to the composition of the feed.
The used naphtha already contains almost 60 % paraffins,
which are the target product, and 32 % naphthenes, which
are not ring opened over the platinum catalyst. Only the
aromatics, which are present to a small extent (9 %), are
nearly completely converted to their corresponding naph-
thenes. Thus, the cetane number of low aromatic but high
paraffinic and naphthenic feed stocks cannot be signifi-
cantly changed by using a catalyst, which is only active for
aromatic hydrogenation.
Fig. 10 Reaction paths of
aromatic hydrogenation and
ring opening over Ir/Al2O3 (and
Rh/Al2O3)
123

Hydrogenation and Ring Opening of Aromatic
539
Table 3 Conversion of naphtha over platinum and iridium catalysts at different temperatures (5 MPa; nhydrogen/nnaphtha = 15; modified resi-
dence time: 90 g s/cm3)
Catalyst
Fractions in wt%
CN
n-Paraffins
i-Paraffins
C5-naphthenes
C6-naphthenes
Aromatics
Feed (Naphtha)
250 ꢁC
270 ꢁC
290 ꢁC
250 ꢁC
25.4
25.8
25.7
25.9
25.8
24.7
15.1
33.8
32.6
32.4
32.5
48.2
64.9
79.4
13.8
14.5
14.6
14.5
7.5
18.2
26.2
26.4
26.2
18.2
7.0
8.8
0.8
0.8
0.9
0.3
0.2
0.3
32
34
34
34
32
31
28
Pt/Al2O3
Ir/Al2O3
270 ꢁC
280 ꢁC
3.2
2.3
2.9
Table 4 Carbon number distribution of naphtha before and after conversion over Ir/Al2O3
Number of carbon atoms
Naphtha
250 ꢁC
270 ꢁC
280 ꢁC
C1/C2
0.0
0.6
4.4
8.2
C3–C5
C6–C8
C9–C11
9.4
14.8
23.9
27.6
69.1 (76.4)
21.4 (23.6)
76.2 (90)
8.5 (10)
68.2 (95)
3.6 (5)
59.2 (92.2)
5.0 (7.8)
Bold hydrocarbons used to calculate the cetane number, values in brackets indicate the content within the C6 to C11 fraction, which is used to
calculate the CN
To hydrogenate all aromatics and additionally ring open
the resulting naphthenes (and the naphthenes already
present in the feed), the naphtha was converted over Ir/
Al2O3. Unfortunately, the CN remains constant (250 ꢁC) or
even decreases (280 ꢁC) from 32 to 28 (Table 3). (Note
that the CN was calculated for the fraction ‘‘C6 and higher’’
only.) This negative effect on the cetane number can be
attributed to three reasons:
(naphtha) and the product after conversion over the Ir
catalyst. The feed consists of 69 % C6 to C8 compared to
21 % C9 to C11 HCs, i.e. 24 % within the C6 to C11 fraction
used to calculate the CN. With conversion over Ir/Al2O3
and rising temperatures the composition of the product is
shifted to short-chained hydrocarbons. At 250 ꢁC the
fraction of C6 to C8 (within the C6 to C11 fraction)
increases to 90 % and is even higher for 270 and 280 ꢁC.
Hence, long chained hydrocarbons like C9 to C11, which
have a fraction of more than 20 % in the feed, are almost
completely cracked to shorter ones over the iridium cata-
lyst, which decreases the cetane number (Fig. 1).
Firstly, the reaction mechanism leads to an unfavourable
product distribution. Even at the lowest temperature of
250 ꢁC the aromatics are completely converted (content
\0.3 %) and the content of 5-ring-naphthenes drops from
14 to 8 %. The fraction of 6-ring-naphthenes remains
constant (18 %), but one has to consider that this fraction
also contains the hydrogenated aromatics, which are not
completely converted to paraffins at 250 ꢁC. The fraction
of n-paraffins remains unchanged while the fraction of i-
paraffins increases from 33 to 48 %. As discussed in Sect.
3.1.2, the conversion of naphthenes over the Ir catalyst
leads predominantly to a cleavage of bisecondary bonds,
and hence mainly branched paraffins (with low CN) are
formed. With rising temperature and conversion this effect
is intensified. At 280 ꢁC the residual content of 5- and
6-ring-naphthenes is low (B3 %) and iso-paraffins are
dominant (79 %).
A drawback is also, that the fraction of C3 and C5
hydrocarbons increases from 9 % in the feed to 15 % at
250 ꢁC and at 280 ꢁC even to 28 %. C1 and C2 hydrocar-
bons (methane and ethane), which are very undesirable, are
also formed at temperatures above about 270 ꢁC.
Thirdly, the naphtha used here is already rich in paraffines
(59 %, see Table 3). Thus, the conversion of the aromatics
(only 9 %) and of the naphthenes (about 32 %) does not lead
to an improvement of the CN compared to the expected case
of a highly aromatic/naphthenic naphtha as feedstock.
4 Summary
The second reason that leads to a low cetane number is
the cracking of hydrocarbons taking place over Ir/Al2O3.
Table 4 shows the carbon number distribution of the feed
The conversion of aromatics (toluene, propylbenzene,
m-xylene) over Ir/Al2O3 and Rh/Al2O3 reveals that it is
123

540
A. Piegsa et al.
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cetane number (CN) is high for benzene (from -10 to 42),
toluene (-5 to 26), and m-xylene (1 to 29), but rather low for
propylbenzene (15 to 16). Nevertheless, these values are still
low compared to commercial diesel oils with CN of around
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CN) and - with the exception of benzene only to a small
extent n-paraffines (high CN, selectivity \ 5 %) are formed.
If real naphtha is used as feedstock, the increase of CN may
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