Hydrogenation of Aromatics in Diesel Fuels on Pt/MCM-41 Catalysts

Article abstract of DOI:10.1006/jcat.1997.1737

The hydrogenation activity of Pt supported on two mesoporous MCM-41 samples differing in their chemical composition has been studied by following the kinetics of the hydrogenation of naphthalene at 225-275°C reaction temperature and 5.0 MPa total pressure and by comparing the kinetic parameters obtained with Pt supported on a mesoporous amorphous silica-alumina (MSA) and other conventional supports, such as commercial amorphous silica-alumina (ASA), zeolite USY, γ-alumina, and silica. The two mesoporous MCM-41 and MSA materials having very high surface areas allowed for a better dispersion of the Pt particles, and they showed a superior overall hydrogenation activity as compared to the other supports. However, Pt/USY displayed the highest turnover (activity per exposed surface Pt), owing to the interaction of small Pt aggregates in the supercage of the zeolite with the strong Broensted acid sites associated to framework aluminum forming electron-deficient Pt species of known enhanced activity. Moreover, both the Al-MCM-41 and USY-based catalysts presented the highest sulfur tolerance during the hydrogenation of a naphthalene feed containing 200 ppm sulfur added as dibenzothiophene. The high metal dispersion and the interaction of the small Pt clusters with the mildly acidic sites present in Al-MCM-41 may account for its high sulfur tolerance. The superior hydrogenation activity and sulfur tolerance of Pt-MCM-41 catalyst observed in the naphthalene experiments were further confirmed during the hydrogenation of a hydrotreated light cycle oil (LCO) feed containing ca 70 wt% aromatics and 400 ppm sulfur.

Full text of DOI:10.1006/jcat.1997.1737

JOURNAL OF CATALYSIS 169, 480–489 (1997)
ARTICLE NO. CA971737
Hydrogenation of Aromatics in Diesel Fuels on Pt/MCM-41 Catalysts
A. Corma, A. Mart´ınez, ,1 and V. Martı´nez-Soria†
ꢀ
ꢀ
ꢀ
Instituto de Tecnolog´ıa Qu´ımica, UPV-CSIC, Universidad Polite´cnica de Valencia, Camino de Vera s/n, 46071 Valencia, Spain; and †Departament
d’Enginyer´ıa Qu´ımica, Universitat de Vale`ncia, Doctor Moliner 50, 46100 Burjassot (Vale`ncia), Spain
Received November 25, 1996; revised April 14, 1997; accepted April 15, 1997
shown that a reduction of the aromatics content has a pos-
itive effect on the cetane number (5, 6). Therefore, there is
considerable interest in developing new catalysts and pro-
cesses for aromatics saturation with improved activity and
stability at moderate temperatures.
The hydrogenation activity of Pt supported on two mesoporous
MCM-41 samples differing in their chemical composition has been
studied by following the kinetics of the hydrogenation of naphtha-
lene at 225–275ꢁC reaction temperature and 5.0 MPa total pres-
sure and by comparing the kinetic parameters obtained with Pt
supported on a mesoporous amorphous silica-alumina (MSA) and
other conventional supports, such as commercial amorphous silica-
alumina (ASA), zeolite USY, ꢀ -alumina, and silica. The two meso-
porous MCM-41 and MSA materials having very high surface areas
allowed for a better dispersion of the Pt particles, and they showed
a superior overall hydrogenation activity as compared to the other
supports. However, Pt/USY displayed the highest turnover (activ-
ity per exposed surface Pt), owing to the interaction of small Pt
aggregates in the supercage of the zeolite with the strong Bro¨nsted
acid sites associated to framework aluminum forming electron-
deficient Pt species of known enhanced activity. Moreover, both
the Al-MCM-41 and USY-based catalysts presented the highest
sulfur tolerance during the hydrogenation of a naphthalene feed
containing 200 ppm sulfur added as dibenzothiophene. The high
metal dispersion and the interaction of the small Pt clusters with
the mildly acidic sites present in Al-MCM-41 may account for its
high sulfur tolerance. The superior hydrogenation activity and sul-
fur tolerance of Pt-MCM-41 catalyst observed in the naphthalene
experiments were further confirmed during the hydrogenation of a
hydrotreated light cycle oil (LCO) feed containing ca 70 wt% aro-
Conventional hydrotreating catalysts containing sulfided
mixed oxides (NiMo, NiW, CoMo) can only accomplish
moderate levels of aromatics saturation under typical hy-
drotreating conditions in a single-stage operation (1–3, 7).
With these systems, increasing operation severity (tempera-
ture and H2 pressure) do not result in deep levels of aromat-
ics saturation because of thermodynamic limitations. In this
sense, noble-metal based catalysts are preferred for deep
aromatics saturation (1–4) since they can work at lower
temperatures, thusavoidingthe thermodynamicconstraints
encountered with the sulfided oxides. The main drawback of
noble-metal hydrogenation catalysts is that they are readily
poisoned by small amounts of sulfur and nitrogen organic
compounds present in the feed (8, 9), and consequently,
they are used in the second reactor of a dual-stage process,
where the feed is severely pretreated in the first reactor to
reduce the level of heteroatoms up to a few ppm. Further-
more, the sulfur and nitrogen tolerance of noble metals (Pt,
Pd) were seen to strongly improve when they are supported
on a zeolite (1–3). Thus, Pt(Pd)-USY catalysts are able to
maintain high hydrogenation activities with feeds contain-
ing several hundred ppm of sulfur under more thermody-
namically favorable conditions than those typically used in
a single-stage hydrotreating. Indeed, the nature of the noble
metal, the presence of a secondary metal as promoter, and
the type and characteristics of the support are all important
parameters affecting the hydrogenation activity and sulfur
tolerance of noble metal-based catalysts (10–15). In this re-
spect, we have recently shown (16) that NiMo-supported on
(Al)-MCM-41 material, having a hexagonal arrangement
of uniform mesopores and very high surface area (17), is
more active than conventional NiMo-alumina and NiMo-
USY catalysts for mild hydrotreating of vacuum gas oil.
The mesoporous aluminosilicate MCM-41 has also been
shown to be a suitable support for preparing noble metal-
based catalysts. Thus, Pt and/or Pd containing Al-MCM-41
was already claimed as a catalyst for the low temperature
matics and 400 ppm sulfur.
ꢂc 1997 Academic Press
INTRODUCTION
As a consequence of the stringent environmental regu-
lations directed to lower hazard emissions from vehicle ex-
hausts, together with the growing demand for high quality
diesel fuels, hydrotreating processes are playing an impor-
tant role in the modern refinery strategies. New diesel fuel
specifications will include a reduction of sulfur to 0.05 wt%
maximum and of aromatics content, while the cetane num-
ber will be set to a minimum value of 40 (1–4). Although
the role of aromatics in particulate emissions of diesel fu-
els has not been at present clearly established, it has been
1 To whom correspondence should be addressed. E-mail: amart@
itq.upv.es.
480
0021-9517/97 $25.00
c
Copyright ꢂ 1997 by Academic Press
All rights of reproduction in any form reserved.

HYDROGENATION OF AROMATICS
481
hydrogenation of benzene (18) and for the hydrogenation (150 cm3/min) at 540ꢁC for 1 h, and, finally, calcined in air
of aromatics in diesel and kerosene feeds (19). On the (150 cm3/min) at the same temperature for 6 h.
other hand, Reddy and Song found that the use of alu-
For comparison purposes, the following materials were
minum isopropoxide as the Al source during the synthe- also used as supports: an amorphous mesoporous silica-
sis of Al-MCM-41 led to samples presenting a higher Al alumina (MSA, Si/Al = 100) supplied by Enichem and ex-
incorporation, better crystallinity, and higher catalytic ac- hibiting a narrow pore size distribution (23); a commercially
tivity than those prepared from pseudo-bohemite (20). available amorphous silica-alumina (ASA) (Crossfield,
Moreover, it wasalso shown that Pt supported on Al-MCM- 25 wt% alumina); a USY zeolite (CBV760, PQ Corp.,
41 prepared from aluminum isopropoxide is highly active a0 = 2.426 nm); ꢀ -Al2O3 (Merck); and SiO2 (BASF).
for aromatics hydrogenation (21). The same authors have
All these supports were impregnated with the required
also reported that Pt and Pd supported on Al-MCM-41 are amount of hexachloroplatinic acid in a 0.2 N HCl aqueous
more active for the hydrogenation of naphthalene at low solution (3 ml/g solid) and the solvent evaporated in a ro-
temperaturesascompared to alumina and titania supported tatory evaporator, dried at 100ꢁC and, finally, calcined at
catalysts (22).
500ꢁC for 3 h. The nominal Pt content in all catalysts was
In this work we have further investigated the catalytic 0.5 wt% and 1 wt% for the hydrogenation of naphthalene
performance of Pt-supported MCM-41 catalysts for the hy- and LCO, respectively.
drogenation of aromatics. For this purpose, we firstly stud-
X-ray diffraction (Philips PW 1830, CuKꢁ radiation)
ied the hydrogenation ofnaphthalene asa modelcompound was used to ascertain the structure and pore size of the
in a batch reactor at 225–275ꢁC reaction temperature and mesoporous MCM-41 materials. The textural properties of
5.0 MPa total pressure on two MCM-41 catalysts differing the different supports were evaluated from the nitrogen
in their chemical composition. The results obtained with adsorption–desorption isotherms at 77 K in a ASAP-2000
Pt/MCM-41 were then compared with those obtained using apparatus (Micromeritics). Infrared spectroscopy (Nicolet
different supports, i.e., a mesoporous silica-alumina (MSA) 710 FTIR equipment) with adsorption–desorption of pyri-
synthesized by Eni (23), a commercial amorphous silica- dine was used to measure the acidity of MCM-41 (sample
alumina (ASA, 25 wt% alumina), an ultrastable Y zeolite MC-2), MSA, ASA, and USY catalysts. The details of the
(USY), ꢀ -Al2O3, and SiO2, all of them having the same Pt experimental conditions used for the IR-pyridine experi-
content. The sulfur tolerance of these catalysts was studied ments can be found in Ref. (25). Chemisorption of hydro-
by adding 200 ppm(w) of dibenzothiophene (DBT) to the gen was performed at 20ꢁC using the pulse flow method in
naphthalene feed. In the second part of the work, we have a Chemisorb 2700 (Micromeritics) apparatus. Before the
carried out the hydrogenation of a light cycle oil (LCO) chemisorption measurements the catalysts were reduced in
feedstock containing ca 70 wt% aromatics and 1600 ppm H2 at 450ꢁC for 2 h, followed by cleaning of the surface at
sulfur previously hydrotreated to reduce the sulfur level to the same temperature for 1 h in N2 flow.
400 ppm. This study was performed in a continuous fixed-
bed reaction system at 300–350ꢁC and 5.0 MPa.
Reaction System and Procedure
The kinetic experiments for naphthalene hydrogenation
were performed in a 0.5 L batch stirred reactor (Autoclave
EXPERIMENTAL
Engineers, Inc.) equipped with inlet and outlet valves for
adding or removing gases, and a liquid sample valve at-
Preparation and Characterization of Catalysts
A pure silica and a Al-containing (Si/Al = 16) MCM-41 tached to a dip tube extending to the bottom of the reac-
samples (denoted as MC-1 and MC-2, respectively), were tor in order to sample products at different reaction times.
synthesized from Aerosil as silica source and Al2O3 4H2O The system includes a reactant loader for good zero-time
as the alumina source (in the case of MC-2), trimethylam- determination. In this way, 1 g of prereduced catalyst is
monium hydroxide (TMAOH), and hexadecyltrimethy- introduced into the reactor and heated up to the desired
lammonium (CTMA) bromide as the surfactant template, reaction temperature in hydrogen atmosphere. Then, 100 g
following the procedure reported in Ref. (24). Thus, gels of of a 10 wt% solution of naphthalene (Merck, >98% purity)
the following molar composition:
dissolved in n-decane (Arcross, >99.5% purity) are added
from the loader to the reactor, and the system is pressurized
with H2 at the operating pressure. For the determination of
sulfur tolerance of Pt-supported catalysts, 200 ppm(w) of
sulfur as dibenzothiophene (DBT) were added to the naph-
MC-1) 0.15 CTMA: 24.3 H2O: 0.26 TMAOH: SiO2
MC-2) 0.40 CTMA: 24.3 H2O: 0.26 TMAOH: 0.036
Al2O3: SiO2
were crystallized in teflon-lined stainless steel autoclaves thalene solution. The hydrogenation experiments were car-
at 135ꢁC in static conditions, and then the solids were ried out at 225–275ꢁC reaction temperature and 5.0 MPa
washed, filtered, dried at 60ꢁC, calcined in nitrogen flow total pressure. The catalysts were previously reduced at

´ ´
CORMA, MARTINEZ, AND MARTINEZ-SORIA
482
450ꢁC for 1 h in H2 flow (250 cm3 minꢃ1) at atmospheric
pressure in a fixed bed system, and immediately trans-
ferred to the batch reactor for the kinetic measurements.
In order to determine the possible influence of external
mass transfer limitations we performed preliminary hy-
drogenation experiments at 275ꢁC and 5.0 MPa in which
we varied the stirring speed from 200 to 2000 rpm while
keeping constant the rest of parameters. It was seen that
the pseudo-first-order kinetic rate constant, first, increased
from 200 to 500 rpm and, then, hardly changed in the range
of 500–2000 rpm. According to these results, we selected
an agitation rate of 1000 rpm for the kinetic experiments.
Similarly, we did not observe significant changes in the re-
action rate when using catalyst particles of 0.1–0.25 mm or
0.59–0.84 mm diameter, so we used the smaller particles to
be sure that in our experiments the reaction was not con-
trolled by intraparticle mass resistance. The mass balance in
all experiments was above 98% of the initial reaction mass.
The products were analyzed by GC (Varian 3400) equipped
with a FID detector and a capillary column (Supelco SPB-1,
30-m length).
Hydrogenation of a hydrotreated LCO was conducted in
a fixed-bed stainless steel tubular reactor at 300 and 350ꢁC
reaction temperature, 5.0 MPa total pressure, WHSV =
4 hꢃ1, and a H2/feed ratio of 1000 stp. Under these condi-
tions some deactivation of the catalysts was observed dur-
ing the first 3–5 h onstream. The catalytic data reported
for these flow reactor studies correspond to reaction times
above 6 h onstream, for which a stationary state behavior
was achieved. The total aromatics content and aromatics
distribution in the feed and hydrogenated products were
determined by gas chromatography in a GC Varian 3400
apparatus equipped with a FID and a capillary column
(Petrocol DH-100, fused silica, 100-m length). Identifica-
tion of the main aromatic components in the LCO feed
and the hydrogenated products was carried out by mass
spectrometry in a GC-MS (Varian 3400) equipped with the
same kind of capillary column described above in order
to obtain the same retention times for each component
in the two GC analysis. All the catalysts were previously
reduced “in situ” at 400ꢁC for 2 h at atmospheric pres-
sure in hydrogen flow. Hydrotreatment of the original LCO
was carried out with a commercial CoMo/alumina catalyst
(TK-550, Haldor–Topsoe) at 400ꢁC, 3.0 MPa, WHSV =
0.3 hꢃ1, and H2/feed ratio of 1000 stp in order to reduce
the sulfur content.
FIG. 1. XRD patterns of calcined MCM-41 samples: (a) MC-1;
(b) MC-2; and (c) Pt/MC-2.
tern corresponding to an hexagonal arrangement of pores
is observed for the siliceous MC-1 sample (Fig. 1a) as
compared to the aluminosilicate MC-2 sample (Fig. 1b).
The pore size calculated from the position of the (100)
XRD peak is slightly higher for the Al-containing MC-2
˚
sample (42.6 A) as compared to the pure silica MC-1 sam-
˚
ple (37.3 A). Moreover, impregnation of Pt did not change
appreciably the pore size of MCM-41 (Fig. 1c).
The physicochemical and textural properties of the dif-
ferent supports are given in Table 1. The two MCM-41
samples show, together with the mesoporous silica-alumina
(MSA), the highest surface area. The total pore volume is
TABLE 1
Chemical Composition and Textural Properties of MCM-41
Materials and Different Supports Determined by Nitrogen
Adsorption–Desorption Experiments
BET surface area
(m2 gꢃ1
Pore volume
(cm3 gꢃ1
Bulk
SiO2/Al2O3
ratio
)
)
APDa
˚
Support
Total Micropore Total Micropore
(A)
MC-1
Pt/MC-1
MC-2
Pt/MC-2
MSA
ASA
USY
ꢀ -Al2O3
SiO2
∞
∞
32
32
100
3
56b
100
0
1046
989
834
694
750
268
551
122
137
—
—
—
—
—
21
362
—
6
0.91
0.82
1.03
0.73
—
0.31
0.41
0.18
0.74
—
—
—
—
—
0.01
0.18
—
27.7
ndc
30.9
nd
nd
46.0
21.0
nd
RESULTS AND DISCUSSION
Characterization of Catalysts
0
nd
The X-ray diffractograms of the calcined MCM-41 sam-
ples (Fig. 1) show the typical low angle reflection at about
22 = 2ꢁ characteristic of this material (17). In agreement
with previous works (26, 27) a more well-defined XRD pat-
a Average pore diameter as measured from Ar adsorption–desorption
isotherms at 87 K and using the Horvath–Kawazoe equation (45).
b
˚
Framework Si/Al ratio = 62.2 (a0 = 24.26 A).
c nd = not determined.

HYDROGENATION OF AROMATICS
483
TABLE 2
TABLE 3
Results of H2 Chemisorption Experiments on the Different Pt-
Acidity of the Different Al-Containing Supports Used as Mea-
sured by IR Spectroscopy with Adsorption of Pyridine and Desorp- Supported Catalysts,a after Reduction with Hydrogen at 450ꢁC
tion at Different Temperatures
for 2 h
Acidity (ꢂmol pyridine/g catalyst)a
Hirr/Pt
(% )
Area
Catalyst
(m2 Pt/g cat)
d (A)
˚
Bro¨nsted
250ꢁC
Lewis
250ꢁC
Pt/MC-1
Pt/MC-2
Pt/MSA
Pt/ASA
Pt/USY
Pt/ꢀ -Al2O3
Pt/SiO2
82
68
85
19
13
77
6
1.12
0.94
1.18
0.26
0.18
1.06
0.08
12.4
14.9
11.9
53.8
77.2
13.2
175.0
Support
150ꢁC
350ꢁC
150ꢁC
350ꢁC
MC-2
MSA
ASA
USY
19
17
22
—
6
5
10
37
0
0
2
58
53
60
—
39
42
38
20
22
28
22
13
15
a Calculated using the extinction coefficients given in Ref. (46).
a Nominal Pt content = 0.5 wt% .
are observed for Pt/ASA and Pt/USY catalysts. In the case
of Pt/USY, the low Pt dispersion might be attributed to a
bimodal Pt distribution, with very small Pt clusters located
inside the zeolite cavities and large Pt aggregates predomi-
nanting on the external surface. This aspect will be further
discussed along the work. The lowest hydrogen uptake was
observed for Pt supported on silica (Hirr/Pt = 0.06). Proba-
bly, the low interaction of the metal with the silica surface
favors sintering of the metal at the reduction temperature
used, producing the formation of larger metal aggregates
and lowering the number of exposed surface metal atoms.
also higher for the MCM-41 samples. Contrary to MCM-41
and MSA supports, the USY zeolite presents most of its
pores in the micropore range, although some mesoporos-
ity, probably created during the hydrothermal treatments,
is also observed in the zeolite. As shown in Table 1, both
the BET surface area and pore volume of MCM-41 slightly
decrease after the impregnation of Pt, the decrease being
more significant in the case of the Al-containing Pt/MC-2
catalyst. Anyway, the surface area and pore volume of the
Pt-containing MCM-41 samples are still very high, which,
besides the XRD data, indicate that most of the MCM-41
structure was preserved after the impregnation step.
The acidity results measured by IR-pyridine at differ-
ent desorption temperatures are given in Table 2. USY ze-
olite presents the higher Bro¨nsted acidity, both in terms
of acid site density and acid strength. On the other hand,
the Al-containing MCM-41 (sample MC-2), the MSA, and
the commercial amorphous silica-alumina (ASA) show a
similar Bro¨nsted acidity, with most of the sites being of
weak-medium strength. Indeed, no Bro¨nsted acid sites are
practically observed on these supports at 350ꢁC desorption
temperature.
The hydrogen chemisorption results obtained for the
Pt/supported catalysts (0.5 wt% nominal Pt content) are
shown in Table 3. It is seen there that the amount of hy-
drogen irreversibly adsorbed, Hirr/Pt, after reduction with
hydrogen at 450ꢁC for 2 h, is higher for the mesoporous
MCM-41 and MSA supports, suggesting a fairly high Pt
dispersion as a consequence of the high surface area of
these materials. Accordingly, a higher Pt dispersion is also
observed for the siliceous MCM-41 sample (Pt/MC-1) as
compared the Al-containing Pt/MC-2 catalyst, which may
be ascribed to the significantly higher surface area of the
former sample (Table 1). A high dispersion (77% ) is also
observed for Pt/ꢀ -Al2O3, probably due to a strong inter-
action of the metal with the alumina surface, preventing
sintering of the Pt particles during the thermal treatments.
Kinetics of Naphthalene Hydrogenation
on Pt-Supported Catalysts
The general reaction scheme proposed for the hydro-
genation of naphthalene is presented in Fig. 2. The hydro-
genation occurs in a sequential manner, with the rate of
tetraline hydrogenation being of an order of magnitude
less than that of naphthalene hydrogenation (29). In our
case, tetraline was the major product under the relatively
mild operating conditions used in this work (P = 5.0 MPa,
225ꢁC ꢄ T ꢄ 275ꢁC). Only at high conversions the hydro-
genation of tetraline into cis- and trans-decaline occurred
at a significant extent. At the highest temperature studied
(275ꢁC) and at high conversions a small amount (less than
0.1 wt% ) of cracking and isomerization products, such as
FIG. 2. General reaction scheme for the hydrogenation of naphtha-
However, relatively low metal dispersions (less than 20% ) lene.

´ ´
CORMA, MARTINEZ, AND MARTINEZ-SORIA
484
methylindanes, alkylbenzenes, and alkylcyclohexanes were
also detected in the reaction products. The conversion-time
curves obtained at different reaction temperatures on the
two Pt/MCM-41 catalysts are shown in Figs. 3a–b. As an ex-
ample, the selectivity to tetraline and decalines obtained at
the three temperatures studied is also presented in Fig. 4 as
a function of naphthalene conversion for the Pt/MC-1 sam-
ple. It can be seen that the selectivity pattern of the catalyst
does not depend on the reaction temperature, at least in
the range studied here, but it depends only on naphtha-
lene conversion. Furthermore, Fig. 4 also shows that the
hydrogenation of the second ring only occurs to a signif-
icant extent at very high conversions, clearly showing the
secondary character of decaline, in agreement with the re-
action scheme presented in Fig. 2. Similar trends were also
observed for noble metal Pt and Pd catalysts supported on
alumina and titania during the low temperature (200ꢁC)
hydrogenation of naphthalene (28).
FIG. 4. Selectivity to tetraline (filled symbols) and decalines (open
symbols) as a function of naphthalene conversion obtained on Pt/MC-1
catalyst at different reaction temperatures: 225ꢁC (᭿); 250ꢁC (᭡); and
275ꢁC (᭹).
According to the reaction scheme in Fig. 2, the hydro-
genation of naphthalene to tetraline is a reversible reac-
tion, while the dehydrogenation rates of decalines are neg-
ligible (29). However, in the range of temperatures used
in this work the equilibrium concentration of naphthalene
is negligible (2, 30) and the formation of tetraline can be
considered as an irreversible process. Taking this into ac-
count, and in order to determine the kinetic rate constant
for the process, and to be able to compare the activity of the
different catalysts on the bases of this parameter, we have
considered that the system can follow a first-order kinetic
equation with respect to the hydrocarbon reactant, which
after integration results in the equation
ꢃln(1 ꢃ X) = k1t.
When this equation was fitted with the experimental re-
sults obtained for the different catalysts, Figs. 5 and 6 show
that the above assumption is adequate, in agreement with
previous published results (2, 12, 28, 31, 32).
As shown in Fig. 5, the catalyst based on the pure sil-
ica MCM-41 (sample Pt/MC-1) is more active in the whole
range of temperatures than the Al-containing Pt/MC-2
sample. The higher surface area and better Pt dispersion
obtained with Pt/MC-1 (Tables 1 and 3) can be responsible
for its higher hydrogenation activity.
The pseudo-first-order rate constants for the hydrogena-
tion of naphthalene, k1, obtained at the three temperatures
studied are given in Table 4. The corresponding apparent
activation energies calculated from the Arrhenius plot of
the rate constants (Fig. 8) are also included in Table 4. At
the lower temperature, Pt/USY and Pt/MC-1 catalysts show
the highest hydrogenation activity, while Pt/SiO2 is the less
active catalyst. However, at 275ꢁC the mesoporous Pt/MSA
and Pt/MC-1 catalysts show their superior hydrogenation
activity. When the activity of the different catalysts is ana-
lyzed, one would be tempted to correlate, in a first approx-
imation, the hydrogenation activity with the metal disper-
sion, or what is equivalent, with the metallic area calculated
from the H2 chemisorption measurements. Thus, when the
pseudo-first-order rate constant obtained at 225ꢁC reaction
FIG. 3. Kinetics of naphthalene hydrogenation on Pt/MC-1 (a) and
Pt/MC-2 (b) catalysts at different temperatures: 225ꢁC (᭿); 250ꢁC (᭹);
and 275ꢁC (᭡).

HYDROGENATION OF AROMATICS
485
TABLE 4
Pseudo-First-Order Kinetic Rate Constants and Activation En-
ergies for the Hydrogenation of Naphthalene (k1) Obtained on the
Different Pt-Supported Catalysts
k1 (hꢃ1
)
Eact
(KJ/mol)
Catalyst
225ꢁC
250ꢁC
275ꢁC
Pt/MC-1
Pt/MC-2
Pt/MSA
Pt/ASA
Pt/USY
Pt/ꢀ -alumina
Pt/SiO2
0.114
0.082
0.071
0.035
0.138
0.065
0.013
0.556
0.221
0.440
0.127
0.352
0.245
0.062
0.911
0.710
1.069
0.589
0.554
0.659
0.129
95.2
97.9
116.9
128.6
63.5
105.4
104.8
very high activity for the hydrogenation of naphthalene on
a Pt/HY catalyst having very large (ca 170 nm) metal parti-
cle size has been recently reported by Smittz et al. (33). The
high activity of the zeolite-based catalyst, in comparison
with the other supports, might be explained, considering
the formation of a bidisperse metallic phase, with most of
the reaction occurring on small Pt clusters of ca 1-nm size
fitting in the supercages of the Y zeolite (33). Then, the large
Pt aggregates on the external surface of the zeolite crystals
could explain the low H2 uptake obtained for this catalyst
(Table 3) (34, 35). Such small Pt clusters would be greatly
influenced by the zeolite lattice, giving rise to the formation
ofelectron-deficient Pt speciesbyelectron transfer from the
metal to the zeolite acid centers, and would exhibit an en-
hanced catalytic activity towards hydrogenation reactions,
as is well reported in the literature (36).
FIG. 5. Pseudo-first-order kinetic plots for naphthalene hydrogena-
tion on Pt/MC-1 (a) and Pt/MC-2 (b) catalysts at 225ꢁC (᭿); 250ꢁC (᭹);
and 275ꢁC (᭡) reaction temperature.
temperature (Table 4) is plotted against the area of the
metallic phase (Table 3), a fairly good linear correlation is
obtained for all catalysts except for Pt/USY, which displays
an anomalous high hydrogenation activity (Fig. 7) despite
its relatively low metallic area (and Pt dispersion). Indeed, a
The apparent activation energies reported in Table 4 fall
in the range of 95–115 KJ/mol for most of the catalysts
studied, which is in close agreement with previously pub-
lished data (37) for the hydrogenation of the first ring of
naphthalene on metal sulfide catalysts. This value does
FIG. 7. Pseudo-first-order kinetic rate constant (at 225ꢁC) for the hy-
FIG. 6. Pseudo-first-order kinetic plots for naphthalene hydrogena- drogenation of naphthalene, k1, as a function of the metallic area of the
tion on different Pt-supported catalysts at 275ꢁC reaction temperature.
different Pt-supported catalysts.

´ ´
CORMA, MARTINEZ, AND MARTINEZ-SORIA
486
TABLE 5
Turnover Frequencies (TOF) for Hydrogenation of Naphthalene
on Different Pt-Supported Catalysts
TOF (sꢃ1
250ꢁC
)
Catalyst
225ꢁC
275ꢁC
Pt/MC-1
Pt/MC-2
Pt/MSA
Pt/ASA
Pt/USY
Pt/ꢀ -alumina
Pt/SiO2
1.17
1.02
0.70
1.56
8.98
0.70
1.80
5.70
2.73
4.38
5.63
22.89
2.66
8.75
9.38
8.83
10.63
26.25
36.09
7.27
18.20
MCM-41 and MSA materials show TOF similar to that of
ꢀ -alumina, which as shown in Table 3 were the supports
presenting a higher hydrogen uptake, and presumably, a
higher metal-support interfacial area. Moreover, results of
Table 5 suggest that the acidity of the support cannot be the
only parameter to explain the specific hydrogenation activ-
ity of the catalysts, since both MCM-41 based catalysts, one
of them having no Bro¨nsted acid sites (MC-1), show sim-
ilar turnover frequencies for naphthalene hydrogenation.
Therefore, other factors related to the dispersion and state
of the metal and the location of metal particles, which will
depend on the pore geometry of the support, might also
contribute to the observed specific hydrogenation rates.
The rate constants for the hydrogenation of tetraline
into cis- and trans-decaline, k2 + k3, calculated assuming an
irreversible pseudo-first-order consecutive reaction, and
the product selectivity defined as the ratio k1/(k2 + k3),
obtained at 275ꢁC reaction temperature are presented in
Table 6. The hydrogenation rates for the second ring of
naphthalene are, in agreement with previous results (29),
one order of magnitude lower than the hydrogenation
rates of naphthalene to tetraline. At this temperature,
the pseudo-first-order rate constants for tetraline hy-
drogenation obtained on the different catalysts follow
FIG. 8. Arrhenius plots for hydrogenation of naphthalene on the two
Pt/MCM-41 (a) and the rest of catalysts (b) studied.
not vary much with the nature of the support, except for
Pt/USY, which shows a relatively low apparent activation
energy of ca 65 KJ/mol (Table 4). This is probably due to
a stronger adsorption of the aromatic compound on the
Bro¨nsted acid sites of the zeolite.
The turnover frequency (TOF) of the different Pt-
supported catalysts, calculated from the pseudo-first-order
rate constants given in Table 4 and the number of exposed
surface metal atoms derived from the H2 chemisorption
data in Table 3, are given in Table 5. It can be seen there
that the activity per exposed surface platinum clearly de-
pends on the nature of the support. Higher TOF are ob-
tained for the most acidic Pt/USY catalyst, especially at
lower reaction temperatures. Increased turnover frequen-
cies displayed by more acidic supports have also been ob-
served during the hydrogenation of benzene and toluene
on different Pt-supported systems (10, 11, 38, 39). This fact
has been explained considering an additional contribution
to the overall hydrogenation rate of spilled-over hydrogen
from the metal to the aromatic ring adsorbed on the acid
sites of the support. On the other hand, the mesoporous
TABLE 6
Rate of Hydrogenation of Tetraline to cis- and trans-decaline
(k2 + k3), and Relative Hydrogenation Rates for the First and Sec-
ond Rings of Naphthalene, k1/(k2 + k3), Obtained at 275ꢁC on
Pt-Supported Catalysts
Catalyst
(k2 + k3) ꢅ 102 (hꢃ1
)
k1/(k2 + k3) ratio
Pt/MC-1
Pt/MC-2
Pt/MSA
Pt/ASA
Pt/USY
Pt/ꢀ -alumina
Pt/SiO2
2.6
2.1
4.9
1.9
1.9
2.1
0.6
35
34
22
31
29
31
18

HYDROGENATION OF AROMATICS
487
MSA, and ASA supports having similar acidity (Table 2)
present quite different sulfur tolerances. Moreover, the Al-
free Pt/MC-1 catalyst does not possess Bro¨nsted acidity
but it shows a relatively high resistance to sulfur poison-
ing. These results suggest again that not only the intrinsic
hydrogenation activity but also the sulfur resistance of Pt-
based catalysts is a complex function of the metal-support
properties (14).
Hydrogenation of Light Cycle Oil (LCO) Feed
LCO is a by-product of fluid catalytic cracking (FCC)
units which has poor blending properties for diesel fuel ow-
ing to its very high aromatics content (about 70 wt% ) and
high levels of S and N compounds. However, due to the in-
creasing demand for high quality diesel fuel, upgrading of
the LCO fraction would be necessary if LCO is to be used
as a diesel blending component.
Here we have studied the hydrogenation performance
of Pt supported on the mesoporous MCM-41 and amor-
phous silica-alumina supports, as well as of the Pt/USY
catalyst, using a LCO feed obtained by distillation of a
FCC liquid product. Due to the high sulfur content of
LCO (usually 1–2 wt% ), noble metal catalysts have to be
used in a dual-stage hydrogenation process, where the feed
has been previously hydrotreated to reduce the sulfur con-
tent. Therefore, we have hydrotreated the LCO feed using
a commercial CoMo/alumina hydrotreating catalyst (TK-
550 from Haldor–Topsoe) before performing the hydro-
genation experiments with the different Pt-based catalysts.
The characteristics of the untreated and hydrotreated LCO
(LCO-HT) are given in Table 7. It can be seen that the
FIG. 9. Relative hydrogenation activity at 275ꢁC of Pt-supported
catalysts after addition of 200 ppm sulfur to the naphthalene feed.
the same trend previously found for the hydrogenation of
the first ring of naphthalene (Table 4), i.e., MSA > MC-1 >
MC-2 = ꢀ -alumina > ASA = USY > SiO2. On the other
hand, Pt supported on the mesoporous MSA and on silica
give a higher selectivity towards the hydrogenation of the
second ring of naphthalene, as can be seen from the ratio
between the first, k1, and second hydrogenations, K2 + K3.
Pt/MCM-41 catalysts show similar selectivity pattern to
Pt supported on amorphous silica-alumina, USY, and
ꢀ -alumina.
Sulfur Resistance of Pt-Supported Catalysts
Since sulfur tolerance is an important aspect of noble
metal hydrogenation catalysts, we have studied here the
activity of the different Pt-supported catalysts for hydro-
genation of naphthalene in the presence of 200 ppm sul-
fur, added as dibenzothiophene (DBT) to the naphthalene
solution. The relative decrease of the pseudo-first-order
rate constants after the addition of sulfur at 275ꢁC reac-
tion temperature is shown in Fig. 9. It can be seen that the
Al-containing Pt/MCM-41 (sample Pt/MC-2) and Pt/USY
are the most stable catalysts towards poisoning by sulfur
under the reaction conditions used here.
TABLE 7
Properties of the LCO Feed before and after Hydrotreating
with a Commercial CoMo/Alumina Catalyst
LCO
LCO-HT
Sulfur content (ppm)
Nitrogen (ppm)
1600
643
0.9174
400
480
—
The high sulfur tolerance of zeolite-based Pt catalysts is
believed to arise from the formation of electron-deficient Pt
particles, Ptn+, upon interaction of the reduced metal with
the Bro¨nsted acid sites of the zeolite, which in turn lowers
Density at 15ꢁC (g cmꢃ3
)
Distillation curve:
IBP
5%
64.3
57.8
146.9
177.1
227.8
253.5
288.5
336.7
361.1
451.5
172.9
205.8
244.4
270.8
306.0
349.4
369.9
467.2
–
10%
30%
50%
70%
90%
95%
FBP
the strength of the S Pt bond (35, 36, 40–43). Taking into
account the low metal dispersion in our Pt/USY catalyst
(Table 3), the high thioresistance obtained for this system
might also be attributed to the formation of small Pt clus-
ters in the supercages of the zeolite that, as discussed above,
could be responsible for its high intrinsic activity. More-
over, the higher sulfur resistance of Pt/MC-2 with respect to
Pt/MC-1 may also be ascribed to the presence of Bro¨nsted
acid sites in the Al-containing MCM-41 sample (MC-2).
However, the acidity of the support cannot by itself ex-
plain the results presented in Fig. 9, since the Al-MCM-41,
Aromatics content (wt% ):
Mono-
Di-
20.7
36.3
13.6
70.6
24.0
32.4
11.6
68.0
Tri+
Total

´
´
488
CORMA, MARTINEZ, AND MARTINEZ-SORIA
TABLE 8
Results of the Hydrogenation of a Hydrotreated LCO Feed (LCO-HT) at 300 and 350ꢁC Reaction
Temperature on the Different Pt-Supported Catalysts (1 wt% Pt)
Pt/MC-2
Pt/MSA
Pt/ASA
Pt/USY
300ꢁC
350ꢁC
300ꢁC
350ꢁC
300ꢁC
350ꢁC
300ꢁC
350ꢁC
Total aromatics reduction (% )
40.3
46.2
32.1
45.4
10.9
22.8
23.4
35.7
Aromatics distribution (wt% ):
Mono-
Di-
Tri+
Total
19.8
16.3
4.5
20.4
13.5
2.7
20.9
20.5
4.8
21.4
11.7
4.0
23.1
28.0
9.5
22.3
22.9
7.3
22.4
22.9
6.8
32.7
10.4
0.6
40.6
36.6
46.2
37.1
60.6
52.5
52.1
43.7
sulfur content was reduced from 1600 to 400 ppm and that, sites present in the zeolite induced on the small Pt clusters
besides sulfur removal, a partial hydrocracking also oc- inside the zeolite cavities. Then, the electron-donating ni-
curred during the hydrotreatment. Moreover, only a slight trogen compounds present in the hydrotreated LCO feed
reduction in total aromatics content from 70.6% to 68.0% may strongly adsorb on the electron-deficient Pt species
was achieved during the feed pretreatment, with the dis- in Pt/USY, reducing their hydrogenation activity. On the
tribution of aromatics shifted towards the monoaromatics other hand, the nitrogen compounds may also interact with
fraction (from 20.7% to 24.0% ) as a consequence of the the strong Bro¨nsted acid sites, decreasing the acidity of the
easier hydrogenation of polyaromatics, as compared to the zeolite (44), and consequently reducing the number of the
more refractory monoaromatics.
highly active electron-deficient Pt particles in the catalyst.
The results of the hydrogenation of the hydrotreated
LCO feed on the different Pt-catalysts at two reaction tem-
peratures are presented in Table 8. It can be observed that
the Al-containing Pt/MCM-41 catalyst (Pt/MC-2) produces
CONCLUSIONS
It has been shown that MCM-41 materials having very
the highest aromatics reduction of LCO at the two tem- high surface area and a regular arrangement of uniform
peratures studied, and especially at the lowest temperature mesopores are excellent supports for preparing highly
(300ꢁC), the difference between Pt/MCM-41 and the rest of disperse Pt-supported catalysts. These catalysts showed,
catalysts is more evident. The catalyst based on the meso- together with the mesoporous amorphous silica-alumina
porous amorphous silica-alumina (MSA) also presents a (MSA), a superior activity for the hydrogenation of
relatively high aromatics reduction in LCO, compared to naphthalene than other conventional Pt-containing sup-
Pt supported on a conventional silica-alumina (ASA) and ports, such as commercial amorphous silica-alumina, sil-
a USY zeolite. No significant hydrocracking of the LCO ica, ꢀ -alumina, and zeolite USY at moderate temperatures.
feed was observed at the two temperatures studied for the However, the turnover number (activity per surface ex-
catalysts investigated.
posed Pt) was seen to be higher for the zeolite-based cata-
The results obtained using the LCO feed confirmed the lyst, despite its lower metal dispersion. This is explained
higher hydrogenation activities observed for the MCM-41 by an important contribution of small Pt clusters localized
and MSA mesoporous materials during the hydrogenation in the supercages of the zeolite, which are subjected to a
of naphthalene in the absence of sulfur compounds. How- higher interaction with the strong Bro¨nsted acid sites of the
ever, it appears that the mesoporous MSA material per- zeolite to form electron-deficient Pt particles of enhanced
forms much better and the USY based catalyst performs hydrogenation activity and improved sulfur tolerance.
much poorer with the LCO feed, in comparison with the
Indeed, when a naphthalene feed containing of 200 ppm
experiments of hydrogenation of naphthalene in the pres- sulfur added as dibenzothiophene (DBT) was used, both
ence of 200 ppm sulfur. This fact may be associated with the Al-containing MCM-41 and USY supports showed the
the relatively high concentration of nitrogen compounds higher sulfur resistance among the catalysts studied. The
still present in the hydrotreated LCO feed (Table 7). Ni- high metal dispersion and the interaction of the small Pt
˚
trogen compounds are, besides sulfur, well-known poisons clusters (10–15 A mean particle size) with the Bro¨nsted acid
of noble metal hydrogenation catalysts (2, 3). Previously, it sites of mild acidity present in Al-MCM-41 may account for
has been shown that the high specific hydrogenation rate the high sulfur tolerance of this catalyst.
of the Pt/USY sample (Table 5) could be associated to the
Furthermore, Pt/(Al)MCM-41 also presented the highest
formation of electron-deficient Pt species promoted by the activity for hydrogenation of aromatics in a hydrotreated
electron withdrawing effect that the strong Bro¨nsted acid light cycle oil (LCO) feedstock containing 400 ppm sulfur,

HYDROGENATION OF AROMATICS
489
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ACKNOWLEDGMENTS
Financial support by the Direccio´n General de Investigacio´n Cientı´fica
y Te´cnica of Spain (Project MAT 94-0166) is gratefully acknowledged.
The authors also thank the Centro de Investigacio´n of CEPSA (Madrid,
Spain) and Eniricerche (Milan, Italy) for the analytical support given. V.M.
acknowledges financial support from the Generalitat Valenciana.
26. Corma, A., Grande, M. S., Gonzalez-Alfaro, V., and Orchille´s, A. V.,
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