Modern Research in Catalysis, 2013, 2, 119-126 Published Online October 2013 (
N-Hexane Isomerization on Ni-Pt/Catalysts
Supported on Mordenite
Geovana S. V. Martins1, Everton R. F. dos Santos1, Meiry G. F. Rodrigues1,
Gina Pecchi2, Carlos M. N. Yoshioka3, Dilson Cardoso3
1Chemical Engineering Department, Federal University of Campina Grande,
Catalysis Laboratory (LABNOV), Campina Grande, Brazil
2Facultad de Ciencias Químicas, UniversidadConcepción, Concepción, Chile
3Chemical Engineering Department, Federal University of São Carlos,
Catalysis Laboratory (LabCat), São Carlos, Brazil
Received August 7, 2013; revised September 5, 2013; accepted September 17, 2013
Copyright © 2013 Geovana S. V. Martins et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The aim of this work was to evaluate the catalytic properties for n-hexane isomerization of bifunctional monometallic
(Ni or Pt) and bimetallic catalysts (Pt-Ni), using HMOR zeolite as support. The method used for metal dispersion in the
zeolite was competitive ion exchange using ammine complexes [Ni(NH3)6]Cl2 and [Pt(NH3)4]Cl2 as precursors. The
catalysts were characterized by X-Ray diffraction, X-Ray energy dispersion spectroscopy, temperature-programmed re-
duction and transmission electron microscopy. The n-hexane isomerization reaction using the catalysts was carried out
to evaluate the catalyst activity. The reaction was carried out in a fixed bed reactor operating at 250˚C, 1 atm, H2/C6 = 9
molar ratio. The profiles obtained from TPR suggest that, for bimetallic catalysts, the presence of platinum facilitates
the reduction of Ni2+ cations. The bimetallic catalysts presented a higher activity in the isomerization of n-hexane when
compared to the monometallic ones, as well better stability as the Pt content in the solid increases.
Keywords: N-Hexane; Isomerization; Mordenite; Nickel; Platinum
1. Introduction
Several factors determine gasoline quality. One of the
key specifications of gasoline is the octane number, which
corresponds to the fuel knocking (self-igniting) property
in internal combustion engine. High octane numbers cor-
relate to a low knocking intensity that is related to good
engine performance [1].
Usually branched paraffins have higher octane num-
bers than corresponding linear paraffins. For example,
linear hexane has an octane number equal to 25, while
2,2-dimethylbutane, an hexane isomer, has an octane
number equal to 92. For that reason, isomerization of
linear paraffins, a process in which straight-chain hydro-
carbon molecules rearrange to form branched hydrocar-
bons, is used to improve gasoline quality [2].
Commonly, users of paraffin isomerization technology
had the choice between robust zeolite based systems
[3-8]. While zeolite catalysts are characterized by their
outstanding tolerance of feedstock poison such as sulphur
and water—this is particularly true for Sud-ChemieHysopar
catalyst that operates commercially at sulphur levels ex-
ceeding 100 ppm—the chlorinated catalysts suffer from
extreme sensitivity to all kinds of feed contaminants [9].
The cases of Mordenite zeolite are employed in rela-
tively high temperature (250˚C) necessary to form the
carbocationic isomerization reactions C5/C6 that the case
of long paraffins such as n-heptane ends cracking occur-
ring faster than favoring the formation of coke and avoids
getting high fractions of branched isomers [10,11].
These catalysts are bifunctional, i.e. they consist of a
metal supported on a zeolite, and since the reaction, and
mechanism requires the dehydrogenation of the initial
alkane to form an intermediate alkene. This alkene can
then proceed through a carbocationic intermediate either
to yield the isomerized products or to undergo cracking
through a B-scission to give unwanted gaseous products
[12,13]. For this kind of bifunctional catalysts, Guisnetet
al. [14] estimated an optimum in the number of acid sites
per available platinum atom 6. If this ratio is exceeded,
the cracking reaction will be favored. Moreover, those
catalysts without a proper balance between the metallic
opyright © 2013 SciRes. MRC
and acid functions are expected to follow an alternative
mechanistic pathway involving bimolecular intermedi-
ates [12].
The second ingredient of bifunctional catalyst is the
hydrogenation/dehydrogenation site, being Pt and Pd
very active in comparison with other transition metals.
According to Jordão et al. [16], they research with bi-
metallic catalysts Pt-Ni and Pt-Cu/HUSY in the isomeri-
zation of n-hexane, seeking a possible alternative for the
decrease of the cost of the catalyst, doing a substitution
from platinum to metals of low cost (Ni or Co). It was
verified that the bimetallic catalysts presented a great
activity when compared with the monometallic ones,
even that the pure platinum. In this same perspective, it
was analyzed that the proportions of the catalysts varied
the percentage of platinum and nickel for the isomeriza-
tion of n-hexane reaction.
Recent studies on Pt-Ni systems supported on H-USY
zeolite and Beta zeolite [15-18] showed that these pro-
vided catalytic activities were higher than those of mono-
metallic Pt catalysts.
Many studies [19-23] have been conducted using the
Pt/Mordenite catalyst in the reaction of n-hexane isom-
erization, but the literature is sparse regarding the Ni-Pt/
Mordenite catalyst.
Mordenite has an intrinsic activity for isomerization
and the initial rate is rather lowered by the presence of Pt
and hydrogen. Thus, the most important role of the metal
component is to stabilize the catalytic activity and to of-
fer higher selectivity for isomerization [24].
Mordenite consists of parallel 12-membered ring (MR)
channels (0.67 × 0.70 nm) with 8 MR side pockets (0.34
× 0.48 nm) [25]. Due to the small size of the 8 MR, for
most guest species, mordenite structure is generally con-
sidered as one-dimensional, which can induce diffusion
limitations in catalysis applications [26].
The objective of this work was to investigate the effect
of nickel on Pt/Mordenite catalyst in the reaction of
n-hexane isomerization. The catalysts were prepared by
competitive ion exchange and were characterized by X-
ray diffraction, X-Ray energy dispersion spectroscopy,
temperature-programmed reduction and transmission
electron microscopy.
2. Experimental
2.1. Catalysts Preparation
The starting material used to prepare all the catalysts was
commercial zeolite Mordenite (NH4MOR, Si/Al ratio =
10), supplied by Zeolyst International.
Monometallic Catalysts: The platinum-containing ca-
talysts were obtained by subjecting NH4MOR zeolite to a
competitive ion exchange [19] involving the cations of
the metal complex [Pt(NH3)4]2+ and 4 ions. For this,
a solution of 0.05 mol·L1 [Pt(NH3)4]2+ containing 4
(to give an 4
/[Pt(NH3)6]2+ ratio of 10) was used. To
perform the exchange, the solution was added slowly
(0.2 mL·min1, while stirring at room temperature) to a
suspension of NH4MOR that contained the volume of
water required to give a final concentration of 0.005
mol·L1 [Pt(NH3)4]2+. After a period of 70 h, the solid
was separated by filtration, washed with deionized water
and dried at 110˚C for 2 h.
For precursors containing only Ni supported on
NH4MOR, we used the same methodology described be-
fore to obtain Pt containing catalysts. However, we used
a solution containing 4
ions and [Ni(NH3)6]2+ at a
/[Ni(NH3)6]2+ molar ratio of 20 was used.
Bimetallic Catalysts: To obtain precursors of bime-
tallic catalysts containing Pt-Ni, two solutions were ini-
tially prepared: one containing the [Ni(NH3)6]Cl2 com-
plex and the other containing the [Pt(NH3)4]Cl2 complex,
which were simultaneously added to a NH4MOR zeolite
suspension in water using the same methodology used to
obtain the monometallic catalysts described above. After
stirring for 1 hour, the solid was filtered and washed with
deionized water, and dried at 110˚C for 2 hours [27].
Calcination: After the precursors were prepared, they
were submitted to calcinations. This was done to remove
the ligands coordinated to the metal and decompose the
NH4+ cations presents in the NH4MOR zeolite, thus,
forming Bronsted acid sites (HMOR). The samples were
heated at 10˚C·min1 rate from room temperature to
200˚C, under N2 flow (100 cm3·min1·gcat1) and the
sample remained at this temperature for 1 h. Then, the
samples were subjected to syntheticair flow (100
cm3·min1·gcat1) and the temperature was increased up
to 500˚C at 2˚C·min1. With the samples remaining under
the air flow, the 500˚C temperature was kept constant for
2 h, in order to complete the calcinations process.
X-Ray Energy Dispersion Spectroscopy (EDX):
Elemental analysis was determined through energy dis-
persive X-Ray spectrophotometry, in a Shimadzu EDX-
700 instrument.
X-Ray Diffraction (XRD): The powder method has
been used, whereby the samples were sieved in an ABNT
no 200 (0.074 mm) sieve and then placed in an aluminum
sample door for the X-Ray diffraction, using a Shimadzu
XRD 6000 equipment. Operational details of the tech-
nique have been set as follows: Copper Kα radiation at
40 KV/30 mA, with a goniometer velocity of 2˚/min and
a step of 0.02˚ in the range of 2θ scanning from 2˚ to 45˚.
The average diameter of the sample crystallites was de-
termined by the Scherrer equation.
Analysis by Temperature Programmed Reduction
(TPR): Calcinated samples were characterized by TPR
Copyright © 2013 SciRes. MRC
(Micromeritics-ChemiSorb 2705), under a mixed flow of
H2-N2 (5% H2 30 mL·min1). Approximately, 150 mg of
the sample were heated at a rate of 10˚C·min1 in the
range of 20˚C - 1000˚C. Before beginning data acquisi-
tion, the samples were subjected to a pre-treatment proc-
ess that consisted of heating from room temperature up to
200˚C (at 10˚C·min1) and keeping this temperature for 1
h under N2 flow (30 mL·min1).
Transmission Electron Microscopy (TEM): The
analyses were performed on JEOL equipment Model
JEM-1200 EX II Instrument with the technique of em-
bedding in Araldite resin and then cut with Sorvall MT
5000 ultra micron.
2.2. Catalytic Tests
Before carrying out the catalytic experiments, 100 mg of
the calcined precursor samples were reduced “in situ”,
using the same conditions as Jordão et al. [27]: tempera-
ture of 500˚C for 6 hours under 55 mL·min1 hydrogen
flow, at STP and 2˚C·min1 heating rate. The catalysts
stability and activity were measured during the period of
3 hours of reaction using a fixed bed microreactor. The
reaction was n-hexane isomerization at 250˚C and 1 atm
pressure. Hydrogen and n-hexane were fed to the reactor
at 55 mL·min1 and 2 mL·h1, respectively, giving a mo-
lar feeding ratio of 9:1 hydrogen/n-hexane. The reaction
products were analyzed online using a LM1 capillary
column (50 m and 0.25 mm i.d.), coupled to a gas chro-
matograph (VARIAN STAR 3400) equipped with a flame
ionization detector.
3. Results and Discussion
The results of the elemental analyses obtained for mono
and bimetallic catalysts are presented in Table 1. Accor-
ding to the data, it is possible to verify that the NH4MOR
zeolite showed high percentage of silicon oxide (SiO2).
After a competitive ion exchange the analyses performed
by EDX revealed that a 100 wt% Ni (nickel monometal-
lic catalyst), 100 wt% Pt (platinum monometallic cata-
lyst), 60Pt40Ni/HMOR (bimetallic catalyst) and 40Pt60Ni/
HMOR (bimetallic catalyst) were effectively incorpo-
rated in the Mordenite (MOR) structure.
Table 1. Chemical composition of NH4MOR and catalysts.
Sample SiO2
(%) Impurities (%)
NH4MOR 90.2 8.6 - - 0.23
100Ni/HMOR 89.0 8.5 100 - 0.82
60Pt40Ni/HMOR 87.3 8.4 23.4 76.4 0.88
40Pt60Ni/HMOR 87.3 8.4 76.9 23.1 0.94
100Pt/HMOR 85.5 8.2 - 100 0.74
X-Ray diffraction pattern of the NH4MOR zeolite, mo-
nometallic catalyst and bimetallic catalyst are presented
in Figure 1.
NH4MOR showed peaks at 2
= 22.24˚; 25.68˚; 26.24˚
and 27.62˚, a typical spectrum for the mordenite structure
[28]. X-Ray diffraction profiles did not change signifi-
cantly after competitive ion exchange with Ni, Pt and
Ni-Pt (Figures 1(a)-(e)). It was found catalyst 60Ni40Pt/
HMOR the presence of a peak at 2θ = 39.74˚ can be at-
tributed to segregation of particles of PtO2.
The average crystallite sizes are presented in Table 2.
It is important to note that after calcination and ion
exchange, there was a small decrease in the average size
of the crystallites. These changes are significant and can
be attributed to the sizes of the radii of the embedded
elements: nickel, platinum and nickel + platinum. The
values of the radii of nickel and platinum are 1.25 Å and
1.39 Å, respectively, and are smaller in comparison with
the ammonium ions.
Figure 2 shows the profile of the Temperature Pro-
grammed Reduction (TPR) obtained for 100Ni/HMOR,
40Ni60Pt/HMOR, 60Ni40Pt/HMOR and 100Pt/HMOR
0 1020304050
Figure 1. X-Ray diffraction patterns of zeolite (a) NH4MOR,
of mono and bimetallic catalysts (b) 100Ni/HMOR (c) 40Ni-
60Pt/HMOR (d) 60Ni40Pt/HMOR and (e) 100Pt/HMOR.
Table 2. Results of crystallites average size.
Sample Average size of crystallites (nm)
NH4MOR 45.9
100Ni/HMOR 42.3
60Pt40Ni/HMOR 37.5
40Pt60Ni/HMOR 45.2
100Pt/HMOR 43.4
Copyright © 2013 SciRes. MRC
Copyright © 2013 SciRes. MRC
0200 400 600 8001000
H2 Consumption (u. a.)
Temperature ()
100Ni / HMOR
260˚C 626˚C
0200 400 600 800
Temperature ()
H2 Consumption (u. a.)
40Ni60Pt/ HMOR
(a) (b)
0200 400 600 800
Temperature ()
H2 Consumption (u. a.)
Temperature ()
H2 Consumption (u. a.)
60Ni40Pt/ HMOR
(c) (d)
Figure 2. TPR of (00Pt/HMOR.
is impor
a) 100Ni/HMOR; (b) 40Ni60Pt/HMOR; (c) 60Ni40Pt/HMOR; (d) 1
tant to note that the reduction profiles shown (125˚C, 301˚C, 480˚C and 572˚C). This fact can be atIt
Figure 2(a) exhibited three peaks reduction at 260˚C,
626˚C and 820˚C. The first peak, at 260˚C, can be related
to a few particles of nickel (NiO) that lie freely in large
channels (12 rings) of mordenite. According to Cardona
et al. [29] the reduction of nickel oxide (NiO) at low
temperature is believed to interact weakly with mordenite.
The second reduction peak, at 626˚C, equivalent to re-
ducing the Ni2+ that are in positions exchange channel 12
rings, and the third peak at 820˚C can be attributed to the
presence of Ni2+ which is strongly interacting in lateral
channels (8 MR) of the mordenite. Because of the great
mobility of nickel, ions migration for the small mordenite
channels (8 rings) may occur during heating, thereby in-
creasing the reduction temperature of these species.
The profile of temperature-programmed reduction
catalyst for 40Ni60Pt/HMOR shown in Figure 2(b) ex-
hibited three peaks (135˚C, 280˚C and 460˚C) and the
catalyst 60Ni40Pt/HMOR proved four reduction peaks
tributed to the increase in platinum content in the bime-
tallic catalysts that are totally reducing the nickel cations
at 460˚C.
According to Jao et al. [30] as the content of platinum
increases, greater the possibility of interaction of Pt-Ni,
explaining that the presence of the peak at 460˚C can be
attributed to the reduction of particulate nickel and plati-
num bimetallic catalyst 40Ni60Pt/HMOR, so it can be
seen that nickel is forming “cluster” with platinum.
For 100 Pt/HMOR catalyst, three reduction peaks lo-
cated at 225˚C, 478˚C and 680˚C respectively, were ob-
served. The first reduction peak at 225˚C can be attrib-
ut 2+
ed to the presence of PtO2 and some ions Pt and Pt4+,
which is not formed, and oxides which are in free chan-
nels, but which are exchanged on the surface.
The second reduction peak at 478˚C and the third re-
duction peak at 680˚C can be corresponded to the pres-
ence of Pt2+ ions, which are strongly interacting with the
channels (8 MR) of the mordenite, according Jimenez et
be noted that the addition of platinum alters
creasing reduction
e Ni2+ cations present in the zeo-
well distributed along the catalyst grain.
The catalyst 100 Ni/
ce the temperature at which the reaction is
platinum assists in the reduction of
. [31].
Comparing the reduction profiles of monometallic and
bimetallic catalysts, we observed a shift of peak reduc-
tion of cations to lower temperatures.
It may
gnificantly reducing the profiles, indicating that bi-
metallic catalysts, for the presence of platinum is facili-
tating the reduction of cations Ni2+, de
mperature of cations.
Yoshioka et al. [15] observed that this behavior occurs
because initially the platinum is reduced causing metal
sites that dissociate hydrogen molecules into atomic hy-
drogen, which reduce th
e mordenite.
Representative TEM images of the catalysts after re-
duction are shown in Figure 3. The metal particles found
in Figures 3(a)-(d) present diameters in the range of 8 - 18
nm that are very
Figure 4 shows the activities in the isomerization of
n-hexane during 3 hours of Pt-Ni catalysts supported on
HMOR zeolite with different Pt/Ni ratios and a total
metal content of 180 μmol/gcat.
Figure 4 shows the results of the activities for the
mono-and bimetallic catalysts.
MOR showed lower performance than the other cata-
lysts (40Ni60Pt/HMOR, 60Ni40Pt/HMOR and 100Pt/
This is due to the difficulty that the nickel particles
have to redu
rformed (250˚C) as was observed in the reduction pro-
files shown in Figure 2(a). It is noticed that few nickel
particles are reduced, and this implies that there are few
metal sites formed.
There is an initial increase in activity with increasing
platinum content as
ckel cations in the bimetallic catalysts, in such a way
that these catalysts have higher activities. This fact can
be explained with the dispersion of metals in the zeolite.
The formation of metallic platinum is influenced by
the presence of nickel, resulting in a smaller average par-
le size. That is, Ni-Pt/HMOR bimetallic catalysts have
metal particles with smaller diameter when compared to
monometallic Ni/HMOR or Pt/HMOR. Therefore, the
15 nm
100Ni (a)
40Ni60Pt (b)
12.5 nm
8 nm
(a) (b)
60Ni40Pt (c)
14 nm 12.5 nm
100Pt (d)
18 nm
(c) (d)
Figure 3. Micrographs of samples: (a) 100Ni/HMOR; (b) i/HMOR; (c) 40Pt60Ni/HMOR; and (d) 100Pt/HOR. 60Pt40N M
Copyright © 2013 SciRes. MRC
bimetallic catalysts have higher
ening of 6.5 to 7 Å (12 ring mem-
metal dispersion for the
nction, thus presenting a greater activity in the isom-
erization of n-hexane.
The structure of mordenite shows a system formed by
channels with large op
rs) connected by parallel channels of small dimensions
from 2.7 to 5.7 Å (8 rings members). In view of the di-
mensions of organic molecules is important to note that
the diffusion of n-hexane, whose diameter has dimen-
sions of 4.3 Å and its isomers larger number octane (3-
methyl-pentane, 2,2-dimethyl-butane and 2,3-dimethyl-
butane) have diameters of 5 Å, 6.2 Å and 5.6 Å diffusion
occurs only in the large channels of mordenite 12 mem-
bers. However, because of its one-dimensional porous
system, mordenite is susceptible to deactivation due to
blocking of its channels by deposition of coke formed
during the course of the reaction.
Figure 5 shows the selectivity of mono-branched pro-
ducts as a function global conversi
050100 150 200
Activity/(m mol g-1cat-1h-1)
Figure 4. The activity of catalysts with 180 μmol/gcat dur-
ing 3-h reaction.
0510 15 20 25 30
Conversion (%)
Figure 5. Selectivity for mono-branched products as a func-
tion of global conversion.
es obtained for the mono-and
When comparing the valu
bimetallic catalysts it is clear that the selectivity was al
ways above 65%, except for the catalyst only containing
nickel (100Ni/HMOR) which proved to be less selective
isomerization thus confirming its weak dehydrogenating
power capacity and low adsorption of molecules of n-
hexane. However, it appears that increasing the platinum
content bimetallic catalysts for the conversion of the re-
actant increases and the selectivity to isomerization. Stu-
dies by Jordão [32] showed that the isomerization selec-
tivity is low (~60%) and this fact gives great ability of
nickel to promote the formation of cracking products
stemmed from the hydrogenolysis reactions of this metal.
Figures 6 and 7 illustrate the relationship between the
mono- and di-branched isomers as a function of conver-
sion for mono e bimetallic catalysts.
Figure 6 shows that the catalysts exhibited a high
0510 15 20 25 30
1,6 ______________________ ______ ______ ______ _____
2mC5/3mC5 (molar)
Conversion (%)
Figure 6. Ratio between the mono-branched isomers of n-
hexane as a function of the conversion for mono and bi-
metallic catalysts.
0510 15 20 25 30
2,2-dmC4/2,3-dmC4 (molar)
Figure 7. Ratio between the di-branched isomers of n-hex-
ane as a function of the conversion for mono and bimetallic
Copyright © 2013 SciRes. MRC
value of the ratio of mono-branched isomers (2-mC5/
3-mC5) and this ratio was almost constant for all catalysts,
the balance being above (1.5) only 60Ni40Pt/HMOR
catalyst showed less than the equilibrium value which
can be attributed to segregation of particles of this cata-
lyst PtO2 observed by XRD analysis.
These results indicate that there is a higher selectivity
to the formation of 2-mC5 the formation of 3-mC5, which
makes them the most promising catalysts for isomeriza-
tion, since the 2-methyl-pentane has a higher octane
number (RON = 75) than 3-mC5 (RON = 73).
It is noted in Figure 7 that the molar ratio of branched
bi-products (2,2-dmC4/2,3-dmC4) for all the catalysts was
considerably below the equilibrium value (2, 5). Th
behavior can be explained by the higher stability o t
-dmC requires two successive branches th
0) is 2,3-dimethyl-
elho Nacional de Desenvolvimento Ci-
entifico e Tecnológico (CNPq, Brazil) and Petrobras for
rt to this research.
tertiary carbocation, compared to the secondary, promot-
ing the formation of the product 2,3-dmC4. The forma-
tion of 2,34
protonation of cyclopropane in the acidic sites. This
branch corresponds to the limiting step of the bifunc-
tional mechanism of 2,3-dmC4 succeed in that methyl
pentane, simultaneously with the formation of 2,2-dmC4.
Thus, this is advantageous because the isomer which has
a higher octane number (RON = 10
tane, while 2,2-dimethyl-butane presented RON equal
to 92, which is somewhat lower when compared to 2,3-
4. Conclusions
The profiles obtained from TPR suggest that, for bime-
tallic catalysts, the presence of platinum facilitates the
reduction of Ni2+ cations.
The mono and bimetallic catalysts were more selective
for the formation of isomers with high octane index (2-
mC5 dmC4 and 2.3) which are products of interest in the
petroleum industry.
The bimetallic catalysts presented a higher activity in
the isomerization of n-hexane when compared to the mo-
nometallic ones, as well better stability as the Pt content
in the solid increases.
5. Acknowledgements
The authors would like to make special acknowledge-
ments to the Cons
their financial suppo
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