Tuning of activity and selectivity of Ni/(Al)SBA-15 catalysts in naphthalene hydrogenation

Article abstract of DOI:10.1016/j.cattod.2019.09.018

The hydrogenation of naphthalene was performed with nickel catalysts (4 wt. % of Ni) supported on SBA-15 and Al-SBA-15 in order to evaluate the effect of the support's acidity on the activity and selectivity of the catalysts. The incorporation of aluminum into the SBA-15 support was performed during the hydrothermal synthesis of the material with the aim to reach the isomorphic substitution of Si⁴⁺ by Al³⁺ leading to the generation of Br?nsted acid sites. Two different precursors (nickel nitrate and a Ni:EDTA complex) were used for the preparation of the catalysts on each support. Catalysts were characterized by nitrogen physisorption, powder X-ray diffraction, temperature programmed reduction, temperature programmed desorption of ammonia, scanning and high-resolution transmission electron microscopy. The Al-SBA-15 support was characterized by solid state ²⁷Al MAS-NMR. The results showed that the intrinsic activity of the catalysts (TOF) increased when the Al-SBA-15 support and the Ni:EDTA complex were used in the catalyst's preparation. The catalysts prepared using the Ni:EDTA complex showed high selectivity to decalins and higher proportion of trans-decalin in the products than the catalysts prepared with nickel nitrate, which was attributed to a higher dispersion of the metallic Ni species and the larger total amount of acid sites.

Full text of DOI:10.1016/j.cattod.2019.09.018

CatalysisTodayxxx(xxxx)xxx–xxx
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Catalysis Today
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Tuning of activity and selectivity of Ni/(Al)SBA-15 catalysts in naphthalene
hydrogenation
H. Vargas-Villagrána, D. Ramírez-Suáreza, G. Ramírez-Muñoza, L.A. Calzadaa,
G. González-Garcíab, T.E. Klimovaa,⁎
a Facultad de Química, Universidad Nacional Autónoma de México (UNAM), Cd. Universitaria, Coyoacán, Ciudad de México, C.P. 04510, Mexico
b Departamento de Química, Universidad de Guanajuato, Noria Alta s/n, Guanajuato, 36050, Mexico
A R T I C L E I N F O
A B S T R A C T
Keywords:
The hydrogenation of naphthalene was performed with nickel catalysts (4 wt. % of Ni) supported on SBA-15 and
Al-SBA-15 in order to evaluate the effect of the support’s acidity on the activity and selectivity of the catalysts.
The incorporation of aluminum into the SBA-15 support was performed during the hydrothermal synthesis of the
material with the aim to reach the isomorphic substitution of Si4+ by Al3+ leading to the generation of Brönsted
acid sites. Two different precursors (nickel nitrate and a Ni:EDTA complex) were used for the preparation of the
catalysts on each support. Catalysts were characterized by nitrogen physisorption, powder X-ray diffraction,
temperature programmed reduction, temperature programmed desorption of ammonia, scanning and high-re-
solution transmission electron microscopy. The Al-SBA-15 support was characterized by solid state 27Al MAS-
NMR. The results showed that the intrinsic activity of the catalysts (TOF) increased when the Al-SBA-15 support
and the Ni:EDTA complex were used in the catalyst’s preparation. The catalysts prepared using the Ni:EDTA
complex showed high selectivity to decalins and higher proportion of trans-decalin in the products than the
catalysts prepared with nickel nitrate, which was attributed to a higher dispersion of the metallic Ni species and
the larger total amount of acid sites.
Nickel catalysts
(Al)SBA-15
Hydrogenation
Naphthalene
Activity
Selectivity
1. Introduction
petrochemical plants [4]. Particularly, for the hydrogenation of naph-
thalene, the selectivity becomes important, since the obtained products:
Nowadays, the fuel industry worldwide confronts many environ-
mental challenges and must satisfy the demands and legislations re-
stricting the emission limits of greenhouse gases. Conventionally, the
fuels’ quality is determined by the octane and cetane numbers [1]. Fuels
with a high octane number are desirable for spark-ignition engines in
order to avoid abnormal fuel combustion and also, to enable the use of
higher compression ratios resulting in better thermal efficiency. On the
other hand, fuels with good auto-ignition properties (high cetane
number) are more suitable for conventional compression-ignition en-
gines, ensuring the ignition of the heterogeneous air–fuel mixture [2].
In order to obtain a high-performance diesel fuel, the high aromatic
content should be reduced by converting the aromatic compounds into
cycloalkanes. With this aim, catalytic hydrogenation has been em-
ployed as a key process for upgrading aromatic-rich streams in the oil
refinery [3].
tetralin, cis- and trans-decalins, can be used separately as solvents
[5–7], raw materials for 6,10-Nylon production [8] and jet-fuel stabi-
lizers [9], respectively, in addition to many other possible applications.
The above makes challenging the development of novel heterogeneous
catalysts suitable for the production of desired products of naphthalene
hydrogenation.
Supported nickel catalysts have been widely studied and applied in
many industrially important catalytic processes as an alternative for the
noble metal catalysts because of their attractive cost-efficiency ratio
and their ability to activate hydrogen under mild conditions [10–12].
The performance of nickel catalysts may be significantly improved by
the support’s selection, catalyst preparation method or metal loading,
according to the treating feedstock. In our previous works [13–15], the
effects of the preparation method and metal loading in Ni/SBA-15
catalysts were studied in the hydrogenation (HYD) of naphthalene. It
was found that the activity of nickel catalysts and selectivity to trans-
decalin were enhanced when they were prepared from a Ni:EDTA
precursor solution, which was attributed to a high nickel dispersion and
Naphthalene is often used as a probe molecule, representative of
Polycyclic Aromatic Hydrocarbons (PAHs), which constitute a large
fraction in the diesel boiling point range products from oil refining and
⁎ Corresponding author.
E-mail address: klimova@unam.mx (T.E. Klimova).
https://doi.org/10.1016/j.cattod.2019.09.018
Received 21 March 2019; Received in revised form 28 August 2019; Accepted 11 September 2019
0920-5861/©2019ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:H.Vargas-Villagrán,etal.,CatalysisToday,https://doi.org/10.1016/j.cattod.2019.09.018

H. Vargas-Villagrán, et al.
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small particle size (2.5–2.7 nm) that enabled the hydrogenation of both
molecules: naphthalene and tetralin (tetrahydronaphthalene) [16–18].
In the work [19], Ni catalysts supported on SiMCM-41 and AlMCM-41
were prepared by the deposition-precipitation with urea (DPU) method.
It was shown that both supports were partially destroyed during the
DPU procedure, losing the long-range hexagonal pore order of the
MCM-41 support. The activity of Ni/SiMCM-41 catalyst in naphthalene
HYD was almost twice higher than that of Ni/AlMCM-41 prepared in
the same condition that was attributed to the formation of larger Ni
particles in the latter [19]. Nevertheless, the role of the support’s
acidity in the HYD behavior of Ni catalysts was not addressed in the
above works [13–15,19]. Nowadays, it is well known that the acid sites
could promote side reactions such as alkylation, ring-opening and iso-
merization, mainly dependent on the support’s choice [20–23]. Among
the studied acidic supports, many mesostructured zeolites owning the
advantages of mesoporous materials (fast diffusion and accessibility for
bulky molecules) and microporous zeolites (strong acidity and high
hydrothermal stability) attracted attention. In both cases, the Brönsted
acidity is due to the isomorphic substitution of Si4+ in the silica matrix
by a trivalent cation such as Al3+. However, due to the amorphous
character of the silica matrix in the mesostructured MCM-41 [24–26],
SBA-15, etc. materials, their acidity is milder than that of the micro-
porous crystalline zeolites, allowing to obtain a smaller amount of side
by-products. On the other hand, the incorporation of aluminum into the
silica structure of SBA-15 is challenging due to the different hydrolysis
rates of aluminum and silicon alkoxides and the dissociation of the Al-
O-Si bond under acid hydrothermal conditions [27]. In fact, the acidity
of the amorphous silica-alumina materials strongly depends on the
preparation method used and the Si/Al ratio [28]. S. Cui et al. [29]
investigated the influence of the Si/Al molar ratio on the hydrogena-
tion, isomerization and ring opening of naphthalene over silica-alumina
supported Ni2P catalysts. They correlated the hydrogenation activity to
the small particle size of the active phase, whereas isomerization and
ring opening activities were related mainly to the amount of acid sites
depending on the Si/Al molar ratio.
Simultaneously, a second solution containing 4 g of PEO20-PPO70-PEO20
copolymer and 150 mL of aqueous HCl at pH = 1.5 was prepared. The
latter solution was transferred in a Teflon-lined autoclave, heated to
40 °C upon constant stirring. Afterwards, the first solution containing
the silicon and aluminum precursors was added dropwise to the auto-
clave, keeping this temperature for 20 h. The formed gel was aged at
110 °C for 24 h without stirring. The resultant yellowish white solid was
filtered, washed several times with deionized water, dried at room
temperature and calcined at 500 °C for 5 h in order to remove the or-
ganic template.
2.3. Preparation of catalysts
Catalysts with a nickel loading of 4 wt. % were prepared by the
conventional incipient wetness impregnation method, using aqueous
solutions of two different nickel precursor:
a nickel nitrate (Ni
(NO3)2·6H2O, Sigma Aldrich) and a [Ni(EDTA)]2− complex. A [Ni
(EDTA)]2− complex in aqueous media was prepared with a Ni:EDTA
molar ratio 1:1, using nickel nitrate as a precursor and ethylenedia-
minetetraacetic acid as a chelating agent (C10H16N2O8, Sigma Aldrich,
99.9%). The final pH of the above solution was adjusted to 9 with an
aqueous NH3 solution. After the impregnation, the catalysts were dried
at 100 °C for 12 h and calcined at 500 °C for 2 h. Hereinafter prepared
catalysts will be labeled as Ni/SBA(NN), Ni/SBA(ED), Ni/AlSBA(NN)
and Ni/AlSBA(ED) according to the support (SBA = SBA-15,
AlSBA = AlSBA-15) and nickel precursor used (NN = nickel nitrate, ED
= [Ni(EDTA)]2− complex). For comparison purposes, Ni catalysts
supported on commercial silica (Merck, grade 10180, 750 m2/g surface
area, ∼ 0.68 cm3/g pore volume and 40 Ǻ pore size) and on γ-alumina
(222 m2/g surface area, 0.55 cm3/g pore volume and 70–120 Ǻ pore
size) obtained by the calcination of boehmite Catapal B (Sasol) at
700 °C for 4 h, were prepared using a nickel nitrate precursor and the
procedure described above. These reference catalysts were labeled as
Ni/SiO2 and Ni/Al2O3, respectively.
In the present work, hydrogenation of naphthalene over Ni/AlSBA-
15 (Si/Al molar ratio of 30) catalysts was investigated and compared
over Ni/SBA-15 ones. Special emphasis was made on the effect of both,
the support’s acidity and the metal particle size on the activity and
product selectivity of the catalysts. Two different nickel precursors
(nickel nitrate and a Ni:EDTA complex) and two supports (SBA-15 silica
and AlSBA-15) were used to modify the acidity and the Ni particle size
in the catalysts.
2.4. Characterization techniques
Textural properties of supports and catalysts were determined with
a Micromeritics ASAP 2020 automatic analyzer at liquid N2 tempera-
ture (−197 °C). Prior to the experiments, the samples were degassed
(P < 10−1 Pa) for 6 h at 270 °C. N2 adsorption-desorption isotherms
were obtained and specific surface areas were calculated by the BET
method (SBET). The total pore volume (Vp) was determined by nitrogen
adsorption at a relative pressure of 0.98 and pore size distributions
were obtained from both, adsorption and desorption isotherms by the
BJH method. The reported mesopore diameters (Dads and Ddes) corre-
spond to the maxima of the pore size distributions, whereas the mi-
cropore area (Sμ) was estimated using the correlation of t-Harkins &
Jura (t-plot method).
2. Experimental
2.1. Synthesis of SBA-15
The SBA-15 support was synthesized according to the procedure
described in [30]. First, 4 g of PEO20-PPO70-PEO20 copolymer
(Mw = 5800 g∙mol−1, Sigma Aldrich) were dissolved in 120 mL of a 2 M
HCl (J. T. Baker, 36%) solution. The temperature was set to 35 °C.
Afterwards 8.5 g of tetraethyl ortosilicate (TEOS, Sigma Aldrich, 99%)
was added dropwise, keeping this temperature for 20 h. Finally, tem-
perature was raised to 80 °C and kept for 48 h without stirring in a
closed Teflon-lined autoclave. The white solid product was filtered,
dried at room temperature and calcined at 550 °C for 6 h for the organic
template removal.
The 27Al Magic Angle Spinning nuclear magnetic resonance (27Al
MAS-NMR) spectra was obtained using a Varian/Agilent 600 spectro-
meter with rotors of ZrO2 (diameter = 4.2 mm), at 22 °C using a source
frequency of 78.2 MHz.
The XRD measurements of all samples were performed on a Siemens
D5000 diffractometer, using CuKα radiation (λ = 1.5406 Å) and a go-
niometer speed of 1°(2θ) per minute. The diffraction patterns were re-
corded from 3° to 80°(2θ).
Temperature programmed reduction (TPR) and temperature pro-
grammed desorption of ammonia (NH3-TPD) experiments were carried
out on a Micromeritics AutoChem II 2920 automatic analyzer equipped
with a thermal conductivity detector. In the TPR experiments, about
50 mg of catalyst were placed in a quartz reactor in a tubular furnace,
where the samples were first pretreated in situ at 400 °C for 2 h under air
flow and then cooled in an Ar stream. Then, the reduction step was
performed from room temperature to 1000 °C with a heating rate of
2.2. Synthesis of Al-SBA-15
The AlSBA-15 support was synthesized by the hydrothermal method
following the procedure reported in [31]. First, 8.5 g of TEOS and the
calculated amount of aluminum isopropoxide (Al(i-PrO)3, Aldrich,
99%) to obtain a desired molar ratio Si/Al = 30, were added to 10 mL
of aqueous HCl at pH = 1.5 upon constant magnetic stirring for 4 h.
2

H. Vargas-Villagrán, et al.
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10 °C/min under a stream of an H2/Ar mixture (10:90 mol:mol, 50 mL/
min flow). The consumption of H2 at different temperatures was re-
corded. In the TPD experiments, first 50 mg of a calcined catalyst were
pretreated in situ at 500 °C for 30 min in a helium flow to remove water
and other contaminants. Afterwards, the samples were cooled to 120 °C
and exposed to a NH3/He mixture stream (10:90 mol:mol, 20 mL/min
flow) for 30 min. The desorption of ammonia was performed in a He
stream (50 mL/min flow) from 120 °C to 500 °C with a heating rate of
10 °C/min, keeping the final temperature until the trace reached the
baseline.
(5)) for the kinetic analysis.
k
1
k
2
Naphthalene (NP)→ Tetralin (TR)→ Decalins(DECS)
(5)
Catalytic activity was determined by measuring naphthalene con-
centrations at different reaction times. The conversion of naphthalene
(XNP) was calculated as shown in Eq. (6), where CNP0 is the initial
naphthalene concentration (mol∙L−1) in the reaction mixture and CNP is
the concentration (mol∙L−1) of naphthalene at different reaction times
(t, h). The reaction rate constants (k1 and k2) were determined as-
suming that the naphthalene (NP) and tetralin (TR) hydrogenation re-
actions are of the pseudo-first order (Eq. (5)) due to the excess of H2.
The rate constants k1 and k2 were calculated using a curve-fitting
method for Eqs. (7) and (8), respectively. Finally, k1 and k2 were nor-
malized (k1N and k2N) by the volume (V, L) of the reaction mixture per
The chemical composition of the catalysts was determined by a
SEM-EDX analysis, using a JEOL 5900 L V microscope with OXFORD
ISIS equipment.
The high-resolution transmission electron microscopy (HRTEM)
characterization of reduced catalysts was performed using a JEOL 2010
microscope (resolving power 1.9 Å). The solids were ultrasonically
dispersed in heptane and the suspension was collected on carbon coated
grids. The dispersion of Ni in reduced catalysts was determined based
on the average particle size resulting from the measurements of the size
of at least 300 Ni particles observed in different HRTEM images of the
same sample. The dispersion of Ni (DNi) was calculated using Eqs. (1)
and (2), where dNi at. is the atomic diameter of nickel (2.48 Ǻ) and dNi is
the average particle size of metallic Ni in the catalyst [32,33]. Then, the
number of Ni surface sites (Ni atoms accessible for interaction with
reactant molecules) was calculated with Eqs. (3) and (4), where Ni(T) is
the total number of nickel atoms on the catalyst, Ni(S) is the number of
surface atoms, WNi is the mass fraction of Ni on the catalyst, and MNi is
the molar mass of Ni. The number of Ni surface sites (Ni(S)) was used to
calculate the turnover frequency (TOF) of the catalysts in the hydro-
genation of naphthalene.
weight of the catalyst (mcat, g) and amount of Ni in the catalysts (mNi
,
g
Ni/gcat) as shown in Eq. (9).
CNP − CNP
0
XNP
=
x 100 %
CNP
(6)
(7)
0
CNP = CNP ∙e−k t
1
0
CNP ∙k1
0
1
2
CTR
kxN
=
[e−k t − e−k t
]
k2 − k1
(8)
(9)
(V)(kx)
=
, where x = 1, 2
(mcat)(mNi)
3. Results and discussion
3.1. Supports’ and catalysts’ characterization
5.01∙dNi at.
DNi
DNi
=
=
, when dNi > 6 nm
dNi
(1)
The chemical composition of the AlSBA-15 support and prepared
nickel catalysts was determined by the SEM-EDX method (Table 1). The
aluminum content in the AlSBA-15 support was 3.6 wt. % (Si/Al molar
ratio of 31), which is consistent with the theoretically-expected Si/Al
molar ratio of 30. In addition, the real Ni loadings in the catalysts were
close to the theoretically-expected ones (4 wt. %).
0.81
3.32 ∙ dNi at.
⎛
⎞
⎟
⎜
, when 1 nm< dNi < 6 nm
dNi
(2)
(3)
⎝
⎠
Ni(S) = DNi ∙Ni(T)
The 27Al MAS-NMR spectrum of the AlSBA-15 support presented
two signals (Fig. 2). The first higher-intensity signal located at 55 ppm
was assigned to the tetrahedrally-coordinated Al3+ species (AlO4
structural units, Al(Td)). These aluminum species were incorporated
into the silica structure isomorphically substituting Si4+ cations and
they were responsible for the generation of the Brönsted acid sites [34].
The second, lower intensity signal at 0 ppm was attributed to the oc-
tahedrally-coordinated Al3+ species (AlO6 structural units, Al(Oh))
corresponding to extra-framework aluminum oxide that is not a part of
the SBA-15 structure [34–36]. The integration of the areas of the Al(Td)
and Al(Oh) peaks resulted in the Al(Td):Al(Oh) ratio = 1.6:1, indicating
that the majority of the added aluminum (∼60%) was incorporated
into the SBA-15 framework structure.
The SBA-15 and AlSBA-15 supports exhibited a type IV nitrogen
adsorption-desorption isotherm (Fig. 3A and A’, respectively) corre-
sponding to mesoporous solid materials. The H1 hysteresis loop of the
SBA-15 material is associated to the presence of uniform cylindrical
pore channels. On the other hand, the AlSBA-15 support presented the
H2 hysteresis loop due to either pore-blocking/percolation in a narrow
range of pore necks or to cavitation-induced evaporation. This kind of
hysteresis has also been observed for silica gel or mesostructured ma-
terials such as SBA-16 or KIT-5 [37]. The difference in the shape of the
hysteresis loop between SBA-15 and AlSBA-15 materials can be due to
the different conditions of the synthesis of these supports (see experi-
mental part) and to the incorporation of aluminum into the SBA-15
structure leading to its distortion. All Ni catalysts preserved the char-
acteristic isotherm features of the corresponding supports (Fig. 3).
Nevertheless, a decrease in the specific textural characteristics (SBET, Sμ
WNi
MNi
Ni(T)
=
(4)
2.5. Catalytic activity testing
Prior to the catalytic tests, the catalysts were reduced ex-situ in a U-
shaped glass flow reactor at 400 °C for 4 h, under a stream of H2/Ar
(30:70 mol:mol) at atmospheric pressure. The hydrogenation of naph-
thalene was performed in a 300 mL stainless steel batch reactor (Parr),
with a 0.06 M solution of naphthalene (Aldrich, 99%) in hexadecane
(Aldrich, 99.5%). In each activity test, 50 mL of naphthalene solution
and 100 mg of reduced Ni catalyst were used. The reactions were per-
formed at 300 °C and 7.3 MPa of total H2 pressure for 6 h. Small aliquots
of the reaction mixture were taken every 15 min for the first hour and
then hourly. The course of the reaction was followed by analyzing the
taken aliquots in an Agilent 6890 gas chromatograph equipped with a
flame ionization detector and a non-polar methyl siloxane capillary
column HP-1 (50 m x 0.32 mm inner diameter and 0.52 μm film
thickness). Products' identification was performed in an Agilent 7890A
GC system equipped with a 5975C MS detector. The reaction scheme of
naphthalene hydrogenation is presented in Fig. 1. According to this
mechanism, naphthalene is firstly hydrogenated to tetralin (inter-
mediate product), which can suffer further hydrogenation to cis- and
trans- isomers of decalin as final reaction products. Upon the reaction
conditions used in the present study (large excess of hydrogen), the
equilibrium between naphthalene and tetralin is shifted towards the
product. Based on this, we propose a simplified reaction scheme (Eq.
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Fig. 1. Scheme of the naphthalene hydrogenation.
Table 1
Al-containing support. Fig. 3B and B’ show the pore diameter dis-
tributions of SBA-15 and AlSBA-15 supports and their respective cata-
lysts. A monomodal and uniform distribution of pore sizes was observed
for the SBA-15 materials and Ni/SBA catalysts, while for the AlSBA-15
material and Ni catalysts, broader pore size distributions were obtained,
with the pore size maxima located at 9.3 nm (adsorption) and 7 nm
(desorption). This pore broadening is consistent with the higher tem-
perature used for the synthesis of AlSBA-15 material [38,39] and
moreover, to the increasing amount of isopropanol present in the re-
action mixture, which is formed by the hydrolysis of the aluminum
precursor that is inserted into the Pluronic P123 micelles, increasing
their size [38,40]. Additionally, pore size distributions obtained from
the desorption branches of the isotherms for the AlSBA-15 support and
catalysts (Fig. 3C’) were also broader than those of the corresponding
materials of the SBA-15 series. In spite of the above-described differ-
ences in the textural properties, all prepared Ni catalysts had attractive
textural characteristics, being smaller the BET surface areas and total
pore volumes in the catalysts prepared using a [Ni(EDTA)]2− complex.
In this case, the impregnation solutions used in the catalysts’ prepara-
tion had a basic pH value (9) that could have provoked the partial
dissolution of SBA-15 and AlSBA-15 materials in the basic media
leading to a decrease in the textural properties.
Chemical compositiona and textural characteristicsb of the prepared supports
and catalysts.
Sample
Ni loading
(wt. %)
SBET
Sμ
Vp
Dads (nm) Ddes (nm)
(m2/g) (m2/
g)
(cm3/
g)
SBA-15
–
861
737
523
570
534
433
129
121
51
17
14
1.12
0.94
0.85
0.92
0.86
0.80
7.8
7.6
7.4
9.3
9.3
9.3
6.1
5.6
6.3
7.0
6.8
6.8
Ni/SBA(NN)
Ni/SBA(ED)
AlSBA-15
Ni/AlSBA(NN) 3.9
Ni/AlSBA(ED) 4.0
4.1
4.2
–
0.7
0.7
0.6
0.8
40
a
Determined by SEM-EDX. The theoretical Ni loading in the catalysts was
4 wt. %.
b
SBET, BET surface area; Sμ, micropore area; Vp, total pore volume; Dads and
Ddes, pore diameters determined from adsorption and desorption branches of N2
adsorption-desorption isotherm, respectively.
The diffraction patterns of the SBA-15 and AlSBA-15 supports and
prepared Ni catalysts in their oxide form are shown in Fig. 4A. All of the
XRD patterns exhibited a broad signal between 15 and 30° (2θ) char-
acteristic of the amorphous silica. In addition to the above signal, four
reflections were observed at 37.2, 43.3, 62.9 and 75.4°(2θ) in the dif-
fraction patterns of the Ni/SBA(NN) and Ni/AlSBA(NN) catalysts evi-
dencing the presence of the crystalline NiO phase (JCPDS-ICDD card
01-071-1179). For the Ni/SBA(ED) and Ni/AlSBA(ED) catalysts, no
reflections of any crystalline phase were detected. These results show
that the nickel precursor used in the catalyst’s preparation has an im-
portant effect on the dispersion of the supported NiO species. The use of
a [Ni(EDTA)]2− complex resulted in a better dispersion of the deposited
Ni species on both SBA-15 and AlSBA-15 supports. It seems that the
effect of the precursor on the dispersion of NiO is much stronger than
the effect of the incorporation of aluminum to the SBA-15 material.
TPR profiles of the calcined Ni catalysts are shown in Fig. 5. The
reduction of NiO takes place in a single step (NiO + H2 → Ni + H2O).
The reduction profile of the Ni/SBA(NN) catalyst showed a broad in-
terval of hydrogen consumption (between 300 and 500 °C) with a
maximum at 382 °C (Fig. 5A). It can be seen that this broad peak is
formed by multiple overlapped signals that indicates the presence of
nickel species of different characteristics. Therefore, it can be concluded
that the Ni/SBA(NN) catalyst should have some nickel oxide species
located on the external surface of the SBA-15 support (detected by
Fig. 2. 27Al MAS-NMR spectrum of the AlSBA-15 support (Si/Al molar
ratio = 30).
and VP) was observed after the incorporation of nickel to the supports
(Table 1) associated to the increase of the materials’ density. The BET
surface area of the AlSBA-15 support was ∼34% lower than of the SBA-
15. Also, the amount of micropores was significantly smaller in the
AlSBA-15 than in the SBA-15 analog. This can be attributed again to the
difference in the preparation conditions used for the synthesis of these
materials and to the presence of extra-framework alumina species in the
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Fig. 3. (A) N2 adsorption-desorption isotherms of the SBA-15 support and SBA-15-supported Ni catalysts. (B and C) Pore size distributions from the adsorption and
desorption isotherm branches, respectively. (A’, B’, C’) The same for the AlSBA-15 support and the AlSBA-15-supported Ni catalysts.
XRD), which can be easily reduced, and other NiO species, more diffi-
cult to be reduced, that are in stronger interaction with the silica sup-
port or are located inside the mesopores of the SBA-15 support [41,42].
The TPR profile of the Ni/SBA(ED) catalyst was symmetric, with the
maximum at increased temperature (456 °C), indicating a stronger
metal-support interaction than in the Ni/SBA(NN) catalyst. On the
other hand, the reduction temperatures of the main reduction signal of
the AlSBA-15-supported nickel catalysts (Fig. 5B) were similar to those
of the SBA-15-supported ones. A similar increase in the reduction
temperature (from 385 to 450 °C) was observed when the nickel pre-
cursor was changed from nickel nitrate to a [Ni(EDTA)]2− complex. In
addition, the reduction of nickel species in the AlSBA-15-supported
SEM-EDX. It was observed for the SBA-15-supported catalysts, the Ni/
SBA(ED) had a lower degree of reduction than the Ni/SBA(NN) sample.
In line with the positions of the reduction peaks described above, this
can be attributed to a stronger metal-support interaction in the first
catalyst that makes more difficult the complete reduction of the sup-
ported NiO species to the metallic Ni. Similar trends were also observed
for the AlSBA-15-supported catalysts, where the Ni/AlSBA(ED) catalyst
showed
a lower reducibility than its Ni/AlSBA(NN) counterpart.
Nevertheless, the difference in the values of α was much smaller for the
AlSBA-15 series of the catalysts than for the SBA-15-supported samples.
This points out that on the Al-containing support, the nature of the
nickel precursor used has a small effect on the degree of reduction of
the deposited Ni species, probably, because of a strong metal-support
interaction. By the same reason, the proportions of the reduced Ni
species in the Ni/AlSBA catalysts (α, Table 2) were smaller than in the
corresponding Ni/SBA ones.
catalysts was also prolonged at
a higher temperature interval
(500–700 °C) that can be associated to NiO species in a stronger inter-
action with the framework and extra-framework aluminum species of
the AlSBA-15 support. Table 2 shows the degrees of reduction of the
catalysts (α), determined taking as a reference the theoretical hydrogen
consumption of each catalyst based on its real Ni content estimated by
The ammonia TPD profiles of the supports and calcined catalysts are
displayed in Fig. 6 and the results of the quantification of the amount of
5

H. Vargas-Villagrán, et al.
CatalysisTodayxxx(xxxx)xxx–xxx
Fig. 4. Powder X-ray diffraction patterns of (A)
supports and nickel catalysts in their oxide
state and (B) reduced catalysts. ▲ NiO (JCPDS
card 01-071-1179), ★ Ni (JCPDS card 01-071-
4655).
acid sites are presented in Table 3. In this table, it can be observed that
the incorporation of aluminum to the SBA-15 material resulted in a
more than five times increase in the total amount of acid sites compared
to the SBA-15 silica support. For the Ni catalysts, also the samples
supported on the AlSBA-15 exhibited significantly higher total acidity
than the corresponding Ni/SBA catalysts. Regarding the nickel pre-
cursor used, the catalysts prepared from a [Ni(EDTA)]2- complex
showed higher surface acidity than their analogs prepared from nickel
nitrate, which can be correlated with the dispersion of the deposited
NiO species in the catalysts (XRD results, Fig. 4). It is worth mentioning
that the Ni/AlSBA(ED) catalyst showed the highest amount of total acid
sites among all catalysts prepared in the present work. In order to se-
parate the contributions of the support and the supported Ni oxide
species to the acidity of the catalysts, specific acidity was calculated.
This acidity represents an increase in the amount of acid sites nor-
malized per m2 of the catalyst surface due to the incorporation of nickel
oxide species in the catalysts. This specific acidity was larger on the Al-
containing SBA-15 support, as well as when a [Ni(EDTA)]2− complex
was used in the preparation.
3.2. Characterization of the active phase in reduced catalysts
After the catalysts’ reduction at 400 °C for 4 h under a H2/Ar stream,
the active phase was characterized by XRD and HRTEM. The XRD
patterns of the reduced catalysts are shown in Fig. 4B. Three catalysts,
Ni/SBA(NN), Ni/AlSBA(NN) and Ni/AlSBA(ED), presented three re-
flections at 44.6, 52.0 and 76.3° (2θ), attributed to metallic Ni (JCPDS-
ICDD card 01-071-4655). In addition, the catalysts prepared using a
nickel nitrate solution, Ni/SBA(NN) and Ni/AlSBA(NN), also had three
low-intensity reflections of the remaining NiO phase. The size of Ni
crystallites in the reduced catalysts, estimated using the Scherrer
equation, increased from Ni/AlSBA(ED) (7.2 nm) to Ni/SBA(NN)
(11.0 nm) and Ni/AlSBA(NN) (12.1 nm) (Table 4). It is worth men-
tioning that, according to the XRD results for the catalysts in oxide state
(Fig. 4A), the size of the NiO crystallites in the calcined catalysts also
increased in the same order; namely, no NiO crystals were detected in
the Ni/AlSBA(ED) sample (size below 5 nm), while for the Ni/SBA(NN)
and the Ni/AlSBA(NN) catalysts, the size of the NiO crystallites was
13.0 nm and 16.8 nm, respectively (Table 4). Larger NiO and Ni crys-
tallites were formed on the AlSBA-15 support compared to those on the
Fig. 5. Reduction profiles of the calcined Ni catalysts supported on (A) SBA-15 and (B) AlSBA-15.
6

H. Vargas-Villagrán, et al.
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Table 2
showed the lowest Ni particle size among all the catalysts.
The average particle size of metallic Ni determined based on the
HRTEM results was also used for the calculation of Ni dispersion (DNi
Hydrogen consumptions and degree of reduction (α) of the prepared nickel
catalysts.
,
Table 4) in the reduced catalysts. It was observed that the highest
dispersion of Ni was obtained in the Ni/SBA(ED) catalyst (38.2%) fol-
lowed by the Ni/AlSBA(ED) catalyst (20.0%). On the other hand, the
catalysts prepared using a nickel nitrate solution, Ni/AlSBA(NN) and
Ni/SBA(NN), presented similar and relatively low Ni dispersions
(10.1% and 10.5%, respectively). The above results point out to the
clear effect of the nickel precursor used on the dispersion of metallic Ni
on the SBA-15 and AlSBA-15 supports, namely, the use of a [Ni
(EDTA)]2− complex led to a high dispersion, while a nickel nitrate to a
low one.
Catalyst
H2 consumption (mmol/g)
Experimental
αa
Theoretical
Ni/SBA(NN)
Ni/SBA(ED)
Ni/AlSBA(NN)
Ni/AlSBA(ED)
0.581
0.494
0.484
0.479
0.699
0.716
0.664
0.682
0.84
0.69
0.73
0.70
a
α, degree of reduction was determined as a ratio of hydrogen consumed by
each catalyst to the theoretical hydrogen consumption calculated based on the
experimentally determined Ni loading.
SBA-15 silica, which can be attributed to the smaller surface area of the
AlSBA-15 support (570 m2/g) than that of the SBA-15 (861 m2/g,
Table 1). According to the above results, the incorporation of aluminum
to the SBA-15 support did not improve the dispersion of the NiO and Ni
phases. Finally, it should be mentioned that the Ni/SBA(ED) catalyst
was the only one in which the presence of any crystalline phase of
nickel (NiO or Ni) was not clearly detected by XRD (Fig. 4A and B)
evidencing the best dispersion of the deposited nickel species in both,
oxide and reduced states.
HRTEM images of the reduced Ni catalysts are presented in Fig. 7.
Large Ni particles with a size varying from 5 to 25 nm were observed on
the images of the Ni/SBA(NN) catalyst. In contrast, for the Ni/SBA(ED)
catalyst, only the presence of very small Ni particles (from 1 to 6 nm)
was detected, that is in line with the results obtained for this catalyst by
XRD, as described above. Regarding the catalysts supported on the Al-
containing support, Ni/AlSBA(NN) showed Ni particles with a size be-
tween 5 and 25 nm, while the Ni/AlSBA(ED) had smaller particles
(from 3 to 12 nm size). More detailed information on the dispersion of
metallic Ni in the catalysts was obtained from the particle size dis-
tributions also shown in Fig. 7. These distributions were calculated by
measuring the size of at least 300 Ni particles on different images of the
same catalyst. The average particle size of metallic Ni in the reduced
catalysts was also estimated from these distributions giving the results
shown in Table 4. According to these results, the average particle size of
reduced Ni particles increased in the following order: Ni/SBA(ED) <
Ni/AlSBA(ED) < Ni/SBA(NN) < Ni/AlSBA(NN). This result is in a good
agreement with the particle sizes calculated from the XRD patterns of
the reduced catalysts (Table 4). In both cases, the Ni/SBA(ED) catalyst
3.3. Catalytic performance in the hydrogenation of naphthalene
Catalytic performance of the prepared Ni catalysts was evaluated in
the hydrogenation (HYD) of naphthalene (NP). Table 5 shows the ki-
netic parameters (k1, k2, k1N and k2N) obtained based on the simplified
reaction scheme (Eq. (5)) and assuming that both hydrogenation re-
actions (HYD of naphthalene and HYD of tetraline) are of the pseudo-
first order, since hydrogen was taken in a large excess. According to
this, k1 and k1N constants correspond to naphthalene hydrogenation,
whereas k2 and k2N represent hydrogenation of tetralin. It can be ob-
served in Table 5 that the catalytic activity of the Ni/SBA(NN) and Ni/
AlSBA(NN) catalysts prepared in the present work was higher than that
of the reference Ni/SiO2 and especially the Ni/Al2O3 samples prepared
using nickel nitrate as a precursor. It is interesting to note that the k1
and k1N values obtained for the Ni/SiO2 reference were only slightly
lower than those of the Ni/SBA(NN), which can be attributed to the
high surface area of the commercial silica (750 m2/g) used as a support
for the reference catalyst. The catalysts prepared using a [Ni(EDTA)]2−
complex, Ni/SBA(ED) and Ni/AlSBA(ED), resulted to be more active in
the hydrogenation of both, naphthalene and tetralin (k1 and k2 values,
respectively) than the catalysts prepared from a nickel nitrate pre-
cursor, Ni/SBA(NN) and Ni/AlSBA(NN). A similar trend was also ob-
served for the specific activity of the catalysts represented by the k1N
and k2N constants normalized per gram of Ni and per volume of the
reaction mixture (Table 5). Comparison of k1N values points out that the
rate of hydrogenation of naphthalene increased in the following order
Ni/Al2O3 < Ni/SiO2 < Ni/SBA(NN) < Ni/AlSBA(NN) < Ni/
SBA(ED) < Ni/AlSBA(ED), whereas the order of activity of the catalysts
Fig. 6. Ammonia TPD profiles of the supports and the calcined Ni catalysts supported on (A) SBA-15 and (B) AlSBA-15.
7

H. Vargas-Villagrán, et al.
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Table 3
Acidic properties of the supported nickel catalysts.
Sample
Amount of acid sites (μmol NH3/gcat
)
Specific aciditya(μmol NH3/m2)
Weak (120–200 °C)
Medium (200–400 °C)
Strong (400–500 °C)
Total
SBA-15
5.0
41.4
56.7
157.6
208.2
293.5
379.9
12.1
33.1
36.9
29.7
46.7
59.0
58.5
–
Ni/SBA(NN)
Ni/SBA(ED)
AlSBA-15
Ni/AlSBA(NN)
Ni/AlSBA(ED)
10.4
60.8
107.8
140.1
146.0
100.2
255.3
345.7
480.3
584.9
0.06
0.38
–
0.25
0.55
a
Calculated as the difference between the total acidity of the catalyst and the total acidity of the corresponding support, normalized per SBET of the catalyst.
parameter values (both k1 and k2) were obtained for the catalysts with
smaller metallic Ni particles, Ni/SBA(ED) and Ni/AlSBA(ED).
Particularly, this effect was stronger for the hydrogenation of tetralin
(k2N, Table 5), which is structure-sensitive from the adsorption point of
view [43]. However, the intrinsic activity of nickel catalysts (TOF va-
lues, Table 5) was improved by increasing the support's acidity by
aluminum incorporation. The above results are reasonable from the
point of view of the literature reports, where it was noted that the
electron-deficient surface of metallic Ni enhances the adsorption of H2
(beneficial for the intrinsic hydrogenation activity), which is favored
with acidic supports [44,45].
Table 4
Average particle size and dispersion of nickel in the prepared catalysts.
c
d
Catalyst
NiO particle
sizea (nm)
Ni particle size (nm)
DNi (%) Ni(S)
(mmolNi
gcat
/
XRDa
HRTEMb
)
Ni/SBA(NN)
Ni/SBA(ED)
Ni/AlSBA(NN) 16.8
Ni/AlSBA(ED) n. d.
13.0
11.0
n. d.
12.1
7.2
11.8
2.7
12.3
6.2
10.5
38.2
10.1
20.0
0.073
0.273
0.067
0.137
n. d.e
a
Determined by the Scherrer equation from the main reflection peaks of NiO
and metallic Ni in each catalyst.
Table 6 shows the composition of reaction products obtained with
each catalyst at 50 and 95% of naphthalene (NP) conversion. It can be
observed that the catalysts prepared using a nickel nitrate solution, Ni/
SBA(NN) and Ni/AlSBA(NN), showed a high proportion of tetralin in
the products (above 90%) even at a high naphthalene conversion
(95%). The Ni/AlSBA(NN) catalyst was 100% selective to tetralin at
50% of NP conversion and 99.3% selective at 95% of NP conversion.
This can be attributed to a large average size of the reduced Ni particles
in these catalysts (11.8 nm in Ni/SBA(NN) and 12.3 nm in Ni/AlS-
BA(NN), Table 4). On the other side, the catalysts prepared using a [Ni
(EDTA)]2− complex, having smaller Ni particles (2.7–6.2 nm, Table 4)
and higher acidity, resulted in the formation of considerable amounts of
cis- and trans-decalins at both naphthalene conversions (50 and 95%).
The Ni/SBA(ED) catalyst showed the highest selectivity to decalins
among all prepared samples. This result is in line with previous report
[43], where it was shown that tetralin hydrogenation is structure-sen-
sitive to very fine nickel particles [43]. In addition, the Ni/SBA(ED)
catalyst showed an increase in the ratio of trans-decalin to cis-decalin
with the increase of naphthalene conversion (from 1.24 at 50% of NP
conversion to 2.27 at 95% conversion) that indicates the ability of this
catalyst to cis-/trans- isomerization of decalins. The isomerization of
decalins on this catalyst became more notorious at high NP conversions
(above 80%), when concentrations of both naphthalene and tetralin
were small (Fig. 8). Other catalyst, Ni/AlSBA(ED), also selective to
decalins, showed only a small increase in the trans-decalin/cis-decalin
ratio (from 1.37 to 1.43) with increasing NP conversion from 50 to
95%. In this case, the cis-/trans- isomerization of decalins was not ac-
celerated even at high NP conversion (Fig. 8). It can be observed that
the isomerization of decalins does not follow the trends of the changes
in the acidity of the catalysts, since the Ni/SBA(ED) catalyst with the
highest isomerization activity was less acidic than the Ni/AlSBA(ED)
and Ni/AlSBA(NN) counterparts (Table 3). This points out to the bi-
functional character of the cis-/trans- isomerization of decalin, which is
a saturated bicyclic molecule. This molecule cannot be activated di-
rectly on the Brönsted acidic site of the catalyst because of very low
basicity, similar to that of the other saturated hydrocarbons (paraffins).
Therefore, the most probable mechanism is through the participation of
two types of active sites: the metal and the acid ones. At the first step,
dehydrogenation of cis-decalin occurs at a metal active site leading to
the formation of a double bond, which can be easily protonated at a
Brönsted acid site with the subsequent cis-/trans-isomerization of the
b
Determined from the measurement of the size of at least 300 Ni particles
observed on different HRTEM images of the same sample.
c
DNi, nickel dispersion calculated based on the average size of Ni particles
determined from HRTEM.
d
Ni(S), nickel atoms located on the surface of Ni particles (Ni atoms acces-
sible for interaction with reactant molecules).
e
n. d., crystalline phases were not detected by XRD.
in the hydrogenation of tetralin was different: Ni/Al2O3
SiO2 < < Ni/AlSBA(NN) < Ni/SBA(NN) < Ni/AlSBA(ED) < Ni/
SBA(ED). For the hydrogenation of both molecules, the catalysts pre-
pared using a [Ni(EDTA)]2− complex were more active than those
prepared from a nickel nitrate solution.
= Ni/
The turnover frequency (TOF) values were also calculated for HYD
of naphthalene with the tested catalysts based on the nickel dispersion
determined from the HRTEM characterization of the reduced catalysts.
The TOF values represent the intrinsic activity of the prepared catalysts
normalized per one Ni atom located on the catalyst’s surface and ac-
cessible for the interaction with naphthalene molecules. It was observed
that the TOF values increased in the following order: Ni/SBA(ED) < Ni/
SBA(NN) < Ni/AlSBA(NN) < Ni/AlSBA(ED), which differs from the
trends observed for the rate constants. The Ni/SBA(ED) catalyst had the
lowest TOF value of 150 h−1, while the Ni/AlSBA(ED) catalyst showed
the highest TOF value (320 h−1). This result points out that in general,
the dispersion of metallic Ni species (depending on the nickel precursor
used, Table 4) and the acidity of the catalysts (Table 3) contribute to the
TOF values. It can be mentioned that the catalysts prepared in the
present work have two types of acidity: Brönsted acidity that can be
attributed to the support used and Lewis acidity generated by the de-
position of the nickel species during the catalyst’s preparation. An in-
crease in the support's acidity and the presence of Brönsted acid sites
due to the incorporation of Al atoms into the SBA-15 support resulted in
higher TOF values of the Ni catalysts in hydrogenation of naphthalene.
On the other hand, the effect of employing a [Ni(EDTA)]2− precursor
instead of nickel nitrate was not as clear: a slight decrease of the TOF
value was observed for the Ni/SBA(ED) catalyst compared to the Ni/
SBA(NN), while the Ni/AlSBA(ED) catalyst showed a notorious increase
in the TOF value in comparison with Ni/AlSBA(NN).
In the case of the catalytic activity represented by the reaction rate
constants, it seems that the hydrogenation of naphthalene, as well as of
tetralin, is related mostly to nickel particle size. Thus, higher kinetic
8

H. Vargas-Villagrán, et al.
CatalysisTodayxxx(xxxx)xxx–xxx
Fig. 7. HRTEM micrographs and particle size distributions of the reduced nickel catalysts.
Table 5
Activity of the prepared Ni catalysts in the hydrogenation of naphthalene and tetralin.
Catalyst
Pseudo-first order rate constants
TOFa (mmolNP mmolNi(S)−1⋅ h−1
)
k1 (h−1
)
k2 (h−1
)
k1N (L⋅gNi−1⋅ h−1
)
k2N (L⋅gNi−1⋅ h−1
)
Ni/SBA(NN)
Ni/SBA(ED)
Ni/AlSBA(NN)
Ni/AlSBA(ED)
Ni/SiO2
0.41
1.36
0.48
1.46
0.38
0.19
0.20
22.02
0.06
3.31
0.0
5.0
16.2
6.2
18.3
4.7
2.4
2.4
262.1
0.8
41.4
0.0
0.0
168
150
215
320
n. d.b
n. d.
Ni/Al2O3
0.0
a
TOF (turnover frequency) was calculated for naphthalene (NP) hydrogenation by multiplying the initial reaction rate by the volume of the reaction mixture and
then dividing among the amount of accessible surface Ni sites (Ni(S)) in the catalyst used.
b
n. d., not determined.
obtained carbocation and deprotonation. Finally, hydrogenation of the
obtained isomer takes place again at the metal site. This supposition
considering participation of the metal active sites at the beginning of
the reaction can be confirmed by the fact that the cis-/trans-iso-
merization of decalins is related to the size of the metallic Ni particles in
the catalysts. Thus, the isomerization occurs quickly at the Ni/SBA(ED)
catalyst with very small Ni particles (average size 2.7 nm, 38.2% of Ni
dispersion, Table 4), it becomes much slower at the Ni/AlSBA(ED)
sample (average size of Ni particles 6.2 nm, 20.0% of Ni dispersion) and
no isomerization was detected on the Ni/SBA(NN) and Ni/AlSBA(NN)
catalysts (Fig. 8) with large Ni particles (11.8–12.3 nm, about 10% Ni
dispersion). In addition, the above results are in line with the previous
9

H. Vargas-Villagrán, et al.
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Table 6
4. Conclusions
Product yields and selectivity to decalins obtained with the prepared Ni catalyst
at different naphthalene (NP) conversions.
In the present work, two series of nickel catalysts supported on SBA-
15 and AlSBA-15 were prepared using two different precursors (nickel
nitrate and a [Ni(EDTA)]2− complex). The changes in the support’s
composition and the nickel(II) compound used resulted in catalysts
with different acidity and dispersion of the metallic Ni particles. The
evaluation of the prepared catalysts in the hydrogenation of naphtha-
lene showed that the catalysts presented significant differences in the
activity and selectivity of the products. It was found that the use of
nickel nitrate as a precursor resulted in catalysts with relatively large
reduced Ni particles (11.8–12.3 nm average size), which were highly
selective towards the formation of tetralin. On the other hand, the
catalysts prepared using a [Ni(EDTA)]2− complex were more active
than those prepared from a nickel nitrate and showed high selectivity to
the formation of cis- and trans-decalins. In addition, the Ni/SBA(ED)
catalyst resulted to be the only one among all prepared samples with
high activity for the cis-/trans-isomerization of decalins, which was
attributed to the acidic characteristics of this catalyst and the presence
of very small (2.7 nm average size) metallic Ni particles. Therefore, the
principal contribution of the present study is that it was shown that the
correct selection of the Ni(II) precursor and the SBA-15 support, with or
without Al, opens a wide field of possibilities for the preparation of
supported Ni catalysts with improved activity and tunable selectivity to
the desired products.
Catalyst
Yields of the productsa (%)
Selectivity to decalins
b
TR trans-DEC
cis-DEC SDECS (%) trans-DEC/cis-
DEC ratio
At 50 % of NP conversion
Ni/SBA(NN)
Ni/SBA(ED)
Ni/AlSBA(NN) 100.0
Ni/AlSBA(ED) 44.2
90.4
19.2
4.0
44.8
0.0
5.6
36.0
0.0
9.6
80.8
0.0
0.71
1.24
–
32.2
23.5
55.8
1.37
At 95 % of NP conversion
Ni/SBA(NN)c
Ni/SBA(ED)
–
–
–
–
–
0.9
68.8
0.3
53.8
30.3
0.4
37.7
99.1
0.7
91.5
2.27
0.75
1.43
Ni/AlSBA(NN) 99.3
Ni/AlSBA(ED) 8.5
a
TR, tetralin; cis-DEC, cis-decalin; trans-DEC, trans-decalin.
SDECS, selectivity to decalins calculated as a sum of cis- and trans-decalins
b
in the products.
c
Maximum naphthalene conversion reached with this catalyst at 6 h reac-
tion time was 56%.
study [46], where the isomerization of the C6 naphthenic rings of
decalins was studied with the bifunctional Pt/USY catalyst. It was ob-
served that the isomerization rate and the formation of ring opening
products increased in the presence of Pt in the Pt/USY catalyst, as
compared to the monofunctional USY sample, and with increasing the
proximity between the Pt and acid sites. According to this report and
our results, it can be concluded that the efficient catalyst for the cis-/
trans-isomerization of decalins should be bifunctional with very small
Ni particles and Brönsted acid sites.
Acknowledgements
Financial support by Consejo Nacional de Ciencia y Tecnología
(CONACyT, México - Ph. D. scholarship to H. Vargas CVU421070) and
DGAPA-UNAM, México (PAPIIT project IN-115218 and postdoctoral
grant to L. Calzada) is gratefully acknowledged. The authors thank I.
Fig. 8. Reaction product compositions obtained in the hydrogenation of naphthalene.
10

H. Vargas-Villagrán, et al.
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Puente Lee and C. Salcedo Luna for technical assistance with electron
microscopy and powder XRD, respectively.
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