Hydroconversion of n-hexadecane on Pt/silica-alumina catalysts: Effect of metal loading and support acidity on bifunctional and hydrogenolytic activity

Article abstract of DOI:10.1016/j.apcata.2013.09.048

Bifunctional catalysts based on platinum and amorphous silica-alumina were studied in the hydroconversion of n-hexadecane. The influence of platinum loading and support acidity on activity and selectivity were assessed. The contribution of hydrogenolysis reactions on top of bifunctional hydrocracking was shown to depend not only on metal loading, but also on the effect of support acidity on the intrinsic activity of the platinum sites. The yield of cracking products, and their linear alkane fraction, increased with metal loading, while the isomerization yield was practically independent of the metal content. On a support of high Br?nsted acidity, the rate of formation of methane was proportional to the platinum surface area, indicating that demethylation occurred by metal-catalyzed hydrogenolysis. On the other hand, the methane site-time yield was one order of magnitude lower on a catalyst with negligible Br?nsted acidity. Pt-catalyzed hydrogenolysis was also investigated during selective poisoning of acid sites by co-feeding pyridine and comparing the effect of hydrogen partial pressure on reaction rates. In the presence of pyridine, total hydroconversion activity was reduced by one order of magnitude while rates to methane and linear cracking products remained relatively high. These observations indicate that acid sites do not intervene in the mechanism, but that support acidity affects the hydrogenolytic activity of platinum sites.

Full text of DOI:10.1016/j.apcata.2013.09.048

Applied Catalysis A: General 469 (2014) 328–339
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Hydroconversion of n-hexadecane on Pt/silica-alumina catalysts:
Effect of metal loading and support acidity on bifunctional and
hydrogenolytic activity
Francesco Regalia,∗, Leonarda Francesca Liottab, Anna Maria Veneziab,
Magali Boutonneta, Sven Järåsa
a KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Chemical Engineering and Technology, Teknikringen 42,
10044 Stockholm, Sweden
b Istituto per lo Studio dei Materiali Nanostrutturati, ISMN-CNR, Palermo, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Bifunctional catalysts based on platinum and amorphous silica–alumina were studied in the hydrocon-
version of n-hexadecane. The influence of platinum loading and support acidity on activity and selectivity
were assessed. The contribution of hydrogenolysis reactions on top of bifunctional hydrocracking was
shown to depend not only on metal loading, but also on the effect of support acidity on the intrinsic
activity of the platinum sites. The yield of cracking products, and their linear alkane fraction, increased
with metal loading, while the isomerization yield was practically independent of the metal content. On a
support of high Brønsted acidity, the rate of formation of methane was proportional to the platinum sur-
face area, indicating that demethylation occurred by metal-catalyzed hydrogenolysis. On the other hand,
the methane site-time yield was one order of magnitude lower on a catalyst with negligible Brønsted
acidity. Pt-catalyzed hydrogenolysis was also investigated during selective poisoning of acid sites by co-
feeding pyridine and comparing the effect of hydrogen partial pressure on reaction rates. In the presence
of pyridine, total hydroconversion activity was reduced by one order of magnitude while rates to methane
and linear cracking products remained relatively high. These observations indicate that acid sites do not
intervene in the mechanism, but that support acidity affects the hydrogenolytic activity of platinum sites.
Received 18 June 2013
Received in revised form
23 September 2013
Accepted 26 September 2013
Available online 6 October 2013
Keywords:
Bifunctional hydrocracking
Hexadecane
Hydroconversion
Hydrogenolysis
Platinum
Silica–alumina
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
rate is controlled by the acid-catalyzed isomerization and crack-
ing reactions. When this occurs, hydrocracking is defined as ideal
Hydroconversion of linear alkanes on bifunctional catalysts
has been thoroughly studied. The commonly accepted classical
bifunctional hydrocracking mechanism [1–3] states that alkanes
are converted by dehydrogenation/hydrogenation on metal sites
and carbenium ion chemistry on acid sites: The alkane molecule
is first dehydrogenated on a metal site to a corresponding alkene;
then the alkene diffuses to a Brønsted acid site where it is pro-
tonated to a carbenium ion. This carbocation undergoes skeletal
isomerization and cracking, the products of which are desorbed as
alkenes which are finally hydrogenated back to alkanes on metal
sites. At certain conditions of operating parameters and relative
strength and concentration of metal and acid sites (functional
balance), the metal-catalyzed dehydrogenation and hydrogena-
tion reactions are at quasi-equilibrium, and the overall reaction
[3,4]. Although this mechanism has gained widespread acceptance,
some details are still debated, in particular as regards the function
responsible for the activation of the alkane molecules and the role
of the diffusion of alkene intermediates to the metal sites [5,6].
Acidic supports used in combination with noble metals in hydro-
conversion processes range from chlorinated aluminas, zeolites,
amorphous silica–aluminas, to silicas or aluminas, in order of
decreasing acidity. Common noble metals such as platinum and
palladium are active, to different extents and depending on size
and morphology of the metal particles, for dehydrogenation, hydro-
genation, isomerization and hydrogenolysis [7,8]. When the metal
and acid functions are combined in bifunctional hydroconversion,
synergic effects can be observed and the resulting combination
cannot be predicted by the activities of the two functions alone
[2]. Moreover, the catalytic activity and specificity of the metal
particles are affected by their interaction with the support [9,10];
likewise, the acid function of the support is prone to be modified
by the presence of metal crystallites [11]. This situation generates
∗
Corresponding author. Tel.: +46 8 790 8243.
E-mail address: regali@kth.se (F. Regali).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.09.048

F. Regali et al. / Applied Catalysis A: General 469 (2014) 328–339
329
a high complexity in the reactivity patterns occurring on this kind
of bifunctional catalysts. The major parameter affecting product
selectivities is the metal–acid site balance, which is determined by
the strength distribution and concentration of acid sites as well as
the activity, concentration and properties of metal sites.
Sasol Germany GmbH. The supports were sieved to the desired
size (90–200 ␮m), dried at 120 ◦C for 6 h and subsequently cal-
cined in air at 500 ◦C for 2 h, increasing the temperature with
a heating rate of 2 ◦C/min. Platinum was deposited by incipi-
ent wetness impregnation, using volumes of aqueous solutions
corresponding to the total pore volume of the materials, con-
taining the appropriate amounts of tetraammineplatinum nitrate
([Pt(NH3)4](NO3)2, Sigma-Aldrich) to achieve the desired weight
loadings. After impregnation, the materials were dried at 120 ◦C
for 6 h, and then calcined in air at 500 ◦C for 4 h (heating rate
2 ◦C/min).
The effect of metal–acid site ratio on hydrocrack-
ing/hydroisomerization of paraffins has been the subject of
many studies, typically using platinum on zeolite-based catalysts,
in which strong acidity is balanced by a strong hydrogenation
function. The most important findings were related to the achieve-
ment of “ideality” and its effect on observed reaction patterns and
selectivity [12–17]. Catalyst systems using platinum supported
on milder acids, such as silica–aluminas, are used for the hydro-
conversion of long chain paraffins when the desired products are
middle distillate fuels [18–21]. In such systems, metal-catalyzed
hydrogenolysis is likely to occur alongside bifunctional hydroc-
racking. This additional mechanism leads to a shift in selectivity
toward linear alkanes and to the formation of methane and ethane,
unwanted products which are not formed by purely bifunctional
hydrocracking. Ribeiro et al. studied hydroisomerization of n-
hexane on Pt/Y-zeolites and observed C1 and C2 formation on
large metal surface area catalysts [15]. Calemma et al. observed
the formation of direct cracking products on a Pt/silica-alumina
catalyst, presumably formed by the hydrogenolytic pathway, in
the hydrocracking of long chain paraffins [22]. Blomsma et al.
reported on an added hydrogenolysis contribution of platinum
on H-Beta zeolite catalysts in the hydrocracking of n-heptane
[23]; Böhringer et al. summarized the effects of non-selective
hydrogenolysis and methanolysis on the observed distribution of
cracking products from the hydrocracking of n-tetradecane [24].
However, a more detailed analysis to assess the extent of this
superimposed metal-catalyzed mechanism at varying metal–acid
site ratios has, to our best knowledge, not been published.
The purpose of this work is therefore to investigate the relation-
ship between the two catalytic functions in the hydroconversion of
n-hexadecane using a metal with high hydrogenating activity (Pt)
supported on a material of mild Brønsted acidity (silica–alumina).
This study addresses the influence of platinum loading on total
hydroconversion activity and on product distribution. Particu-
lar consideration is given to the selectivity toward methane and
the degree of isomerization of the cracking products with vary-
ing platinum loading. In order to isolate the contribution of
the monofunctional metal-catalyzed hydrogenolysis mechanism a
dual approach was used: (1) a catalyst using a support with neg-
ligible Brønsted acidity was tested; (2) selective poisoning of acid
sites by co-feeding pyridine was performed on one of the Pt/silica-
alumina catalysts. The influence of hydrogen partial pressure on the
rates to different products was studied in presence and in absence
of pyridine.
The catalysts prepared are shown in Table 1. Four different plat-
inum weight loadings were used on Siral 40: 0.06%, 0.33%, 0.6%,
1.2%. Additionally, a catalyst containing 0.33-wt% platinum was
prepared using the low acidic support Siral 1.
2.2. Catalyst characterization
Surface area and porosity were determined by nitrogen adsorp-
tion using a Micromeritics ASAP 2000 unit. Before the analysis, the
samples were degassed by evacuation at 250 ◦C for a minimum of
4 h.
Metal dispersion, crystallite size and metal surface area were
measured by hydrogen chemisorption at 35 ◦C in a Micromeritics
ASAP 2020C unit. Before each measurement 0.2–0.4 g of dried cat-
alyst powder was evacuated at 250 ◦C for 4 h, cooled to 100 ◦C and
subsequently reduced in flowing hydrogen raising the temperature
to 400 ◦C with a heating rate of 5 ◦C/min and keeping the final tem-
perature for 2 h. The sample was then evacuated and cooled to the
analysis temperature. For each sample two adsorption isotherms
were obtained in the pressure range from 50 to 500 mmHg, with
an evacuation step in between. The chemisorptive hydrogen uptake
was estimated from the difference between the two isotherms
extrapolated to zero pressure. Dispersion and average particle size
were calculated assuming a stoichiometry of adsorption of 1 hydro-
gen atom per platinum atom.
The acidic properties of the support materials and the catalysts
were determined by temperature-programed desorption of ammo-
nia (NH3-TPD) and by FTIR spectroscopy of adsorbed pyridine.
NH3-TPD experiments were carried out using a Micromeritics
Autochem 2910 apparatus equipped with a thermal conductivity
detector (TCD) and a mass quadrupole spectrometer (Thermostar,
Balzers). For each measure, a sample amount of 0.15 g was pre-
treated in helium at 500 ◦C with a heating rate of 10 ◦C/min and
holding the final temperature for 30 min. After cooling to room tem-
perature, ammonia was adsorbed by admitting a flow of 5% NH3/He
(30 ml/min) for 1 h. In order to remove all the physically adsorbed
ammonia, the sample was purged by flowing helium (100 ml/min)
at 100 ◦C for 1 h. Subsequently, after cooling to room temperature,
ammonia desorption was started by flowing helium (30 ml/min)
and heating to 500 ◦C with a rate of 10 ◦C/min. The final temperature
was held for 30 min. The NH3 desorption curves were evaluated
by QMS analysis of the NH3 fragmentation signals at mass 17, 16
and 15. A blank test at the same experimental conditions, by flow-
ing pure He, was carried out in order to exclude that H2O, CO2 or
2. Experimental
2.1. Catalyst preparation
The silica–alumina supports, Siral 40 (40-wt% SiO2, 60-wt%
Al2O3) and Siral 1 (1-wt% SiO2, 99-wt% Al2O3) were obtained from
Table 1
Physical properties of the catalysts (after calcination at 500 ◦C for 4 h).
Catalyst
Siral 40
Pt loading (wt%)
BET surface area (m2/g)
BJH adsorption pore volume (cm3/g)
BJH ads. average pore diameter (nm)
0
440
394
387
402
397
227
223
0.92
0.92
0.94
0.93
0.92
0.62
0.61
8.5
8.9
9.1
8.9
9.0
8.8
8.9
Pt0.06/Siral 40
0.06
0.33
0.60
1.20
0
Pt0.33/Siral 40
Pt0.6/Siral 40
Pt1.2/Siral 40
Siral 1
Pt0.33/Siral 1
0.33

330
F. Regali et al. / Applied Catalysis A: General 469 (2014) 328–339
Fig. 1. Simplified reaction network.
any other impurity may affect the NH3 adsorption and desorption
processes.
The contributions of the two possible routes of formation of
cracking products, i.e. bifunctional and hydrogenolytic, were esti-
mated by the simple reaction network model depicted in Fig. 1.
In this scheme, all reaction steps were considered first-order and
irreversible [22,26]. The equations describing the concentrations
profiles of the different product lumps as functions of the contact
time ꢀ (or the inverse of WHSV) are reported below.
Pyridine adsorption FTIR measurements were performed in an
ATI Mattson FTIR spectrometer. Approximately 0.20 g of sample
was pressed into a self-supporting thin wafer and placed in the FTIR
cell. The sample was pretreated in high vacuum (∼10−5 mmHg) at
450 ◦C for 1 h. The temperature was then lowered to 100 ◦C and
a background spectrum was recorded. Pyridine was adsorbed on
the sample at its ambient vapor pressure in flowing helium at
100 ◦C for 30 min. Spectra of adsorbed pyridine were recorded at
100 ◦C for 50 min; three measures were performed, after evacua-
tion for 1 h at 150, 250 and 350 ◦C, respectively. Spectral bands at
+k
)ꢀ
Cn-C16 = Cn-C160 e−(k
bif
hyd
ꢀ
ꢀ
ꢀ
−kbifCn-C160 (e−k
− e−k
)
2
bif
CMB
=
kbif − k2
Cn-C16 kbifk2e−k
Cn-C16 kbifk2(kbife−k − k2e−k
+ k3e−k
(kbif − k2)(kbif − k3)(k2 − k3)
− k3e−k
)
ꢀ
ꢀ
ꢀ
ꢀ
3
2
2
bif
bif
(kbif −0k3)(k2 − k3)
0
CDB
=
−
CCP = Cn-C16 − Cn-C16 − CMB − CDB
0
1545 cm−1 (Brønsted) and 1450 cm−1 (Lewis) were considered for
the estimation of acid site concentrations, using the molar extinc-
tion coefficients calculated by Emeis: 1.67 for the 1545 cm−1 band
and 2.22 for the 1450 cm−1 band [25].
Selectivity/conversion curves for the different product lumps
were obtained by non-linear regression of the experimental data.
First-order rate constants are given as kbif (bifunctional) and khyd
(hydrogenolysis), respectively.
The effect of H2 partial pressure was studied on a stable 1.2-
wt% Pt/Siral 40 catalyst. Experiments were performed at constant
n-hexadecane WHSV (18 h−1) and total pressure (30 bar), replacing
a part of the hydrogen feed with helium. The degree of vaporiza-
tion of n-hexadecane was considered to calculate hydrogen partial
pressure, which varied between 17.4 and 29.2 bar. The same set
of experimental points was repeated in the presence of pyridine,
mixed with n-hexadecane (0.8 wt%) in the feed vessel. The sys-
tem was run at the reference conditions until catalytic activity
reached steady state. Experimental points at reference conditions
were repeated to ensure that no further deactivation occurred
during the experiments with varying hydrogen partial pressure.
Apparent reaction orders with respect to hydrogen were calcu-
lated by linearization of the bi-logarithmic plot of reaction rates
versus inlet hydrogen pressure. Rates were approximated with
space–time yields, not considering the change in composition along
the reactor.
2.3. Catalytic testing
Hydroconversion of n-hexadecane (>99%, for synthesis, Merck
820633) was performed in a stainless steel tubular reactor, allow-
ing operation up to a pressure of 80 bar. The reaction and analysis
system has been described in detail elsewhere [26]. Briefly, the n-
hexadecane liquid feed was supplied by means of a HPLC pump, and
mixed with hydrogen prior to entering the reactor, which operated
in co-current downflow mode. The reaction products were sepa-
rated in a gas–liquid separator at room temperature. The product
gas was sent, through a downstream pressure control valve, to an
on-line gas chromatograph equipped with an Alumina PLOT col-
umn. The liquid products were sampled and analyzed off-line with
an HP-5 column.
Before each experiment, approximately 3 g of catalyst, previ-
ously sieved to a particle size of 90–200 ␮m and dried overnight,
was loaded into the reactor. The catalyst was activated in situ in
hydrogen flow (50 N ml/min/gcat) at 400 ◦C for 2 h, by raising the
temperature with a heating ramp of 5 ◦C/min. Thereafter the system
was pressurized in hydrogen and stabilized at the desired reac-
tion temperature and pressure. The conditions used were T = 310 ◦C,
P = 30 bar, H2/n-C16H34 inlet ratio = 10 mol/mol. The total conver-
sion of n-hexadecane was varied by varying the weight hourly space
velocity (WHSV) between 0.25 and 18 h−1. Under these conditions
the distribution of cracking products was practically symmetric
throughout the full conversion range, i.e. overcracking was avoided.
Conversion of n-hexadecane is defined as 1 − Fn-C16,out/Fn-C16,in
where F is the molar flow. The hydroconversion activity is reported
as the pseudo-first-order reaction rate constant for the disappear-
ance of n-hexadecane, calculated assuming plug flow behavior.
Selectivities are reported on a carbon atom basis, as moles of C
in product per mole of C of converted n-hexadecane. The products
of n-hexadecane hydroisomerization/hydrocracking were grouped
into three main lumps: mono-branched hexadecanes (MB), di-
or multi-branched hexadecanes (DB), and cracking products (CP).
3. Results
3.1. Catalyst characterization
The textural properties of the catalysts are presented in Table 1.
Addition of platinum on the Siral 40 material caused a slight
decrease of surface area (∼10%), but no significant differences
were observed for varying platinum loading. The porous struc-
ture remained practically unaffected. On Siral 1, characterized by
a smaller surface area compared to the Siral 40 (227 m2/g vs.
440 m2/g), the effect of 0.33-wt% platinum was negligible.
Hydrogen-chemisorption analyses (Table 2) showed that plat-
inum particles were well dispersed on all catalysts, with dispersion
values ranging from 120% to 61%, with dispersion decreasing as
platinum loading increased. Values of dispersion higher than 100%
are of course physically impossible, and are due to the assump-
tion of a constant hydrogen adsorption stoichiometry coefficient
equal to 1. It has been shown that adsorption stoichiometry on

F. Regali et al. / Applied Catalysis A: General 469 (2014) 328–339
331
Table 2
H2-chemisorption properties of the catalysts (after calcination at 500 ◦C for 4 h and activation in H2 at 400 ◦C for 2 h). The data are calculated from the measured chemisorption
uptake assuming spherical particles and monolayer adsorption of H2 with a stoichiometry (H/M) of 1 hydrogen atom per exposed Pt atom.
Catalyst
Pt loading (wt%)
H2 uptake at 35 ◦C (cm3 STP/g cat)
Pt dispersion (%)
Pt particle size (dp) (nm)
Pt surface area (m2/gcat
)
Siral 40
0
0
–
120a
91
89
61
–
–
Pt0.06/Siral 40
Pt0.33/Siral 40
Pt0.6/Siral 40
Pt1.2/Siral 40
Pt0.33/Siral 1
0.06
0.33
0.60
1.20
0.33
0.041
0.172
0.308
0.423
0.151
0.9
1.2
1.3
1.8
1.4
0.177
0.739
1.325
1.818
0.649
80
a
A value higher than 100% is probably due to an adsorption stoichiometry (H/M) exceeding unity on the smallest Pt particles.
highly dispersed noble metal catalysts depends on the size and
shape of the metal clusters; the hydrogen to metal (H/M) coeffi-
cient increases due to the lower coordination number of the metal
atoms in smaller particles, which causes multiple adsorption on
edges and corners [27]. For the purpose of this work a compar-
ative measure of platinum sites was needed; metal surface area
was therefore used, which is proportional to the hydrogen uptake
through the assumed stoichiometry factor and the cross sectional
area of a Pt atom. Values of dispersion and average metal particles
sizes are reported for comparison only and should not be taken as
absolute measurements.
Acidic properties of the materials have been investigated using
pyridine adsorption FTIR and NH3-TPD. In this study, Siral 40,
exhibiting the highest Brønsted acidity of the Siral series, and Siral
1, with very low Brønsted acidity, were used. Many authors have
reported before on the acidity of these materials [28–31], espe-
cially as regards the different contents of silica and alumina. Not
much work has been done about the effect of platinum addition;
moreover, there seems to be disagreement in the literature about
how Pt affects acidity. Leckel [19] observed a dramatic decrease
in the concentration of Brønsted acid sites of a Siral 40 support
after loading with 0.3-wt% of platinum, while the same amount
of platinum induced Brønsted acidity on a Siral 75 support which
showed no Brønsted acidity prior to metal addition. Larson et al.
[11] reported on the influence of Pt on ammonia adsorption proper-
ties of a silica–alumina. Higher acidity was observed at Pt contents
between 0.13 and 2.86-wt%, with a maximum increase of approxi-
mately 50% over the non-loaded support at a Pt loading of 0.53-wt%.
According to Peeters et al. [29], no significant modification was
induced by addition of 1.1-wt% Pt on Siral 40 and similar materials.
The results of acidity measurements by pyridine adsorption FTIR
are presented in Table 3. The Brønsted acidity of Siral 1 was indeed
negligible and no Brønsted acidity was induced by the incorpora-
tion of 0.33-wt% platinum on this material. The concentration of
Lewis acid sites (LAS) of all strengths was lower after the addition
of platinum.
strongest acid sites, releasing adsorbed ammonia at temperatures
above 400 ◦C, was decisively higher for the unloaded Siral 40.
Analyzing the causes behind these observed modifications in
acidity is outside the scope of this work. However, some consid-
erations can be made: Platinum crystallites supported on acidic
substrates are known to assume a lower electron density, due to
charge transfer to the support [32,33]; they can therefore accept
electron pairs, acting as Lewis acids. Kubicˇka et al. [34] observed
a similar influence on the acid properties of zeolites following
impregnation by 2-wt% Pt. They concluded that the increase in
the total measured amount of Lewis acidity, and especially of the
lower strength LAS, could be due to the interaction of the electron-
deficient Pt crystallites with pyridine. The authors ruled out the
effect of the impregnation procedure and the subsequent ther-
mal treatment on the surface structure of the solid acids, and
interpreted the changes in acid site distribution as arising from
electronic interactions between metal crystallites and the surface
atoms of the support [34]. Santi et al. [35] suggested also that charge
transfer between the metal particles and the framework oxygen
atoms was responsible for the modification of the strength of the
Brønsted sites associated with bridging hydroxyl groups.
3.2. Hydroconversion of n-hexadecane
3.2.1. Bifunctional and hydrogenolytic hydrocracking activity
The linearized form of the mass balance equation for a plug
flow reactor relates n-hexadecane conversion (ꢁ) to space veloc-
ity (WHSV) through the pseudo-first-order rate constant (k):
−ln(1 − ꢁ) = k/WHSV. The rate constants were calculated from
experimental data at different flow conditions. Table 4 shows the
mean k values and their linear fitting parameters. The values of the
intercept are close to 0 and R2 values are very close to unity, indi-
cating a high goodness of fit for the first-order reaction model. It is
directly apparent that the catalyst supported on Siral 1 is much less
On Siral 40, the incorporation of platinum caused an increase in
both Brønsted and Lewis acidity. The strongest Brønsted sites (BAS),
measured after desorption of pyridine at 350 ◦C, were removed
after the introduction of platinum, while the concentration of BAS
of lower strength increased at all loadings, peaking for the sample
containing 0.6-wt% of Pt. A similar pattern was observed for Lewis
acidity, with the difference that the medium-strength LAS were
slightly detrimentally affected by the presence of platinum. These
measurements were confirmed by the analysis of the TPD spectra of
ammonia, shown in Fig. 2 as ion current signal at mass 17. Although
ammonia desorption techniques cannot distinguish between BAS
and LAS, the same general trend of modification of the acidity of
Siral 40 by the addition of platinum was observed. The concentra-
tion of low strength acid sites (ammonia desorption temperature
<250 ◦C) increased at all Pt loadings, while medium strength acidity
(250–400 ◦C) was higher for the samples containing middle levels
of Pt and lower for the lowest (0.06-wt%) and highest (1.2-wt%)
Pt content (cfr. LAS at 250 ◦C in Table 3). The concentration of the
Fig. 2. NH3-TPD profiles of the Pt/Siral 40 catalysts.

332
F. Regali et al. / Applied Catalysis A: General 469 (2014) 328–339
Table 3
Acid properties of the catalysts (after calcination at 500 ◦C for 4 h).
Brønsted acid sites
150 ◦
Lewis acid sites
150 ◦
Catalyst
Pt loading (wt%)
C
250 ◦
C
350 ◦
C
C
250 ◦
C
350 ◦
C
(␮mol/g)
(␮mol/g)
Siral 40
0
44
54
59
67
62
0
31
38
43
39
43
0
4
0
0
0
0
0
0
114
136
146
156
127
163
133
54
43
54
56
42
98
82
12
0
0
0
0
Pt0.06/Siral 40
Pt0.33/Siral 40
Pt0.6/Siral 40
Pt1.2/Siral 40
Siral 1
0.06
0.33
0.60
1.20
0
33
17
Pt0.33/Siral 1
0.33
0
0
active than the catalysts based on Siral 40, and that the activity of
Pt/Siral 40 catalysts increases with increasing platinum loading.
Due to the sequential reaction steps involved in the isomeriza-
tion and cracking reactions, product selectivity depends strongly
on total conversion. According to the classical bifunctional mech-
anism, mono-branched feed isomers dominate at low conversion,
di- (and multi-) branched isomers start to appear at increasing con-
version, while cracking products become significant only at high
total conversion [4]. Fig. 3 shows selectivity/conversion plots for
the main product lumps on the Pt/Siral 40 catalysts. Although the
general trend of increasing selectivity to cracking products and
decreasing selectivity to mono-branched feed isomers is followed,
a marked difference is noticeable between the catalysts; the selec-
tivity to cracking products at low conversion increases consistently
with the platinum loading. The classical bifunctional mechanism
alone cannot account for these selectivity/conversion patterns. The
increasing cracking selectivity with increasing platinum content
is therefore an indication that cracking products are also directly
formed from n-hexadecane via a parallel hydrogenolytic route.
Carbon-based cracking selectivity curves are represented in
Fig. 4. The cracking selectivity of Pt/Siral 1 and a Pd/Siral 40 cat-
alyst with negligible hydrogenolytic activity [26] are reported for
Fig. 3. Selectivity/conversion plots for the Pt/Siral 40 catalysts for the product lumps: mono-branched hexadecanes (
cracking products (
). Conditions: 310 ◦C, 30 bar, H2/n-C16H34 = 10 mol/mol, WHSV = 0.75–18 h−1
); multi-branched hexadecanes (
);
.

F. Regali et al. / Applied Catalysis A: General 469 (2014) 328–339
333
Table 4
First-order rate constants and linear fitting parameters.
Catalyst
Pt loading (wt%)
ktot (h−1
)
khyd (h−1
)
R2
Intercept
Pt0.06/Siral 40
Pt0.33/Siral 40
Pt0.6/Siral 40
Pt1.2/Siral 40
Pt0.33/Siral 1
0.06
0.33
0.60
1.20
0.33
2.23 0.12
2.77 0.16
3.26 0.23
3.70 0.059
0.0682 0.0058
0.18
0.66
1.38
1.81
0.068
0.9985
0.9964
0.9991
1.0000
0.9908
0.0508
0.0548
0.0440
−0.0013
0.0116
comparison. All the Pt-containing catalysts show non-zero selec-
tivity at zero conversion; this value increases with increasing Pt
loading, ranging from ca. 8% for a Pt loading of 0.06-wt% to almost
50% for the catalyst containing 1.2-wt% of platinum. Moreover, the
Pt/Siral 1 catalyst shows almost total selectivity to cracking prod-
ucts, while the Pd/Siral 40 catalyst shows the typical pattern of
bifunctional hydrocracking. The ratio between cracking and isom-
erization selectivities at zero-conversion is a measure of the relative
importance of the two hydrocracking routes [13,22,36]. Assum-
ing that the decomposition of n-hexadecane can follow either a
bifunctional route or a “direct cracking” route, and that cracking
products at low conversion arise exclusively from direct cracking
by hydrogenolysis, the overall rate constant can be proportionally
divided in the rate constants for each route, based on their relative
selectivities extrapolated to zero-conversion. A further approxima-
tion has been made by which monofunctional hydrogenolysis gives
rise only to cracking products. Although platinum is known to be
active for alkane isomerization, its contribution to the bifunctional
route is not significant, as stated by Sinfelt [37]. This assumption is
supported by the observation that the selectivity to cracking prod-
ucts on the “monofunctional” catalyst is very close to 90% (Fig. 4).
The total (ktot), bifunctional (kbif) and hydrogenolytic (khyd) first-
order rate constants are plotted against the metal/acid site ratio in
Fig. 5. The metal/acid site ratio was calculated dividing the plat-
inum surface area (in m2/gcat) by the concentration of BAS after
desorption of pyridine at 250 ◦C (in ␮mol/gcat). Total hydroconver-
sion activity increases linearly, as does the hydrogenolytic activity,
while the bifunctional rate constant is approximately independent
of the metal/acid site ratio. This seemingly straightforward result
is in contrast to the usual pattern observed for Pt/zeolite catalysts,
where hydroconversion activity increases linearly up to the specific
metal/acid site ratio corresponding to ideal hydrocracking condi-
tions, after which a plateau is reached [12]. The explanation lies
in the fact that the increasing activity is due to monofunctional
hydrogenolysis occurring on metal sites alone, when excess metal
is present, as was originally observed by Myers and Munns [38] and
reported by Weisz [2]. Due to the lower acidity of silica–alumina
compared to zeolites, the rate of transformation of intermediate
alkenes on the tested Pt/Siral 40 catalysts is expected to be slower;
the number of platinum sites necessary to reach ideal hydrocrack-
ing conditions is therefore lower. In fact, metal sites seems to be
present in excess at a loading as low as 0.06 wt%, where the con-
tribution of Pt-catalyzed hydrogenolysis is already 8% of the total
rate.
It is possible, in principle, that the induced acidity caused by
the addition of Pt on silica–alumina, is responsible for the observed
increase in activity, as speculated by Beuther and Larson [39], who
questioned Weisz’ hypothesis of monofunctional hydrogenolysis.
This effect is taken into account by relating hydroconversion activ-
ity to the ratio between the two types of active sites. Anyhow, no
correlation could be observed between catalytic activity and the
measured Brønsted acidity, whereas a linear correlation was found
with metal surface area. This shows that the effect of change in
acidity can be neglected in comparison to the added activity due to
metal-catalyzed hydrogenolysis.
The total hydroconversion activity on Pt0.33/Siral 1 is much
lower than that on the Pt0.33/Siral 40 catalyst. The two catalysts
have the same nominal loading of Pt and similar metal surface area.
The only property that is greatly different is the Brønsted acidity,
responsible for the bifunctional hydrocracking mechanism. If the
Fig. 5. Hydroconversion activity vs. metal/acid site ratio for the Pt/Siral 40 catalysts.
Conditions: 310 ◦C, 30 bar, H2/n-C16H34 = 10 mol/mol, WHSV = 0.75–18 h−1. The val-
ues of ktot (ꢁ), kbif (ꢀ) and khyd (ꢂ) are mean values of the apparent first-order rate
constants calculated at different WHSV. Platinum surface area and BAS concentra-
tion after desorption of pyridine at 250 ◦C were used in calculating the metal/acid
site ratios.
Fig. 4. Cracking selectivity/conversion plots. Pt0.06/S40 (᭹), Pt0.33/S40 (ꢀ),
Pt0.6/S40 (ꢁ), Pt1.2/S40
(
), Pt0.33/S1 (♦). Conditions: 310 ◦C, 30 bar, H2/n-
C16H34 = 10 mol/mol, WHSV = 0.75–18 h−1. Data obtained on a purely bifunctional
0.33-wt% Pd on Siral 40 (ꢀ) are reported for comparison [26].

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F. Regali et al. / Applied Catalysis A: General 469 (2014) 328–339
Fig. 6. Cracking product distribution on: (A) Pt0.06/Siral 40, n-C16H34 conversion = 14.2%; (B) Pt0.06/Siral 40, n-C16H34 conversion = 86.7%; (C) Pt1.2/Siral 40, n-C16H34
conversion = 18.9%; (D) Pt1.2/Siral 40, n-C16H34 conversion = 91.5%; (E) Pt1.2/Siral 40 during addition of pyridine, n-C16H34 conversion = 6.0%; (F) Pt 0.33/Siral 1, n-C16H34
conversion = 6.5%.

F. Regali et al. / Applied Catalysis A: General 469 (2014) 328–339
335
Fig. 8. Space–time yield of methane as a function of n-hexadecane conversion for
Pt0.06/S40 (᭹), Pt0.33/S40 (ꢀ), Pt0.6/S40 (ꢁ), Pt1.2/S40 ( ), Pt0.33/S1 (♦). Condi-
tions: 310 ◦C, 30 bar, H2/n-C16H34 = 10 mol/mol, WHSV = 0.75–18 h−1
.
[40]. The rate of formation of methane can therefore be used
for discriminating between the two hydrocracking routes. The
space–time yield (STY, defined as millimoles produced per unit
mass of catalyst per unit time) of methane is plotted versus total
conversion in Fig. 8. A strong dependence both on Pt content and
conversion can be observed: methane yield increases with increas-
ing loading and decreases with increasing total conversion.
The dependence on conversion of the methane STY can be com-
pared with that of mono-branched hexadecanes (Fig. 9), which are
primary products generated from the bifunctional route. The iso-
mer yield decreases linearly with total conversion, but shows no
dependence on Pt loading. This is further indication that isomeri-
zation products are formed by acid-catalyzed reactions whose rate
is not limited by the reaction steps occurring on the metal sites.
The effect of conversion on methane yield can be explained by
two concurring factors: At high conversion the contribution of the
bifunctional mechanism increases, increasing the concentration of
isomerized species, which are prone to undergo acid-catalyzed
Fig. 7. Iso-to-normal paraffin ratio in the cracking products as
a function
of n-hexadecane conversion for Pt0.06/S40 (᭹), Pt0.33/S40 (ꢀ), Pt0.6/S40 (ꢁ),
Pt1.2/S40 ( ), Pt0.33/S1 (♦). Conditions: 310 ◦C, 30 bar, H2/n-C16H34 = 10 mol/mol,
WHSV = 0.75–18 h−1. The lines are eye guides only.
speculations about the added hydrogenolytic activity of the Pt sites
are correct, we expect that the total activity of the Pt0.33/Siral 1 cat-
alyst would be similar to the hydrogenolytic activity on Pt0.33/Siral
40, i.e. similar specific rates on platinum sites. The rate values
reported in Table 4 show that this is not the case: The rate constant
for hydrogenolysis on the non-acidic catalyst (Pt0.33/Siral 1) is one
order of magnitude lower. Consequently, it could be argued that
either the hydrogenolytic activity on Pt0.33/Siral 40 is not only due
to the platinum sites, or that their monofunctional catalytic activity
depends on the support.
3.2.2. Distribution of cracking products
Distributions of cracking products obtained on two of the
Pt/Siral 40 catalysts at high and low total conversion are shown
in Fig. 6. Symmetric carbon-number distributions were typically
observed, with a slight contribution of secondary cracking at
high conversions; the typical ratios between moles of crack-
ing products and moles of cracked n-hexadecane were within 2
and 2.1. Remarkable differences were observed in the content of
branched isomers. The ratio between branched and normal paraf-
fins (iso/normal ratio) increased with increasing total conversion
and with decreasing Pt loading. The dependence on conversion
is expected in a bifunctional hydrocracking scheme, where the
increasing population of multi-branched isomers promotes the
occurrence of Type A ␤-scission, which gives rise to two branched
fragments [40]. The iso/normal ratio is also dependent on the Pt
loading, decreasing markedly with increasing amount of platinum
on the catalyst (Fig. 7). At high total conversion (ca. 85%) a differ-
ence larger than factor four is observed between the Pt0.06/Siral
40 and the Pt0.6/Siral 40 (1.76 vs. 0.37). This follows naturally from
the hypothesis of the increasing importance of the hydrogenolysis
mechanism, which produces only linear fragments.
3.2.3. Methane selectivity on acid and non-acid catalysts
Fig. 9. Space–time yield of mono-branched hexadecanes as
a function of
Methane can only be formed through the hydrogenolytic
pathway. The high instability of primary carbenium ions makes
methane formation through Type D ␤-scission very unfavorable
n-hexadecane conversion for Pt0.06/S40 (᭹), Pt0.33/S40 (ꢀ), Pt0.6/S40 (ꢁ),
Pt1.2/S40 ( ), Pt0.33/S1 (♦). Conditions: 310 ◦C, 30 bar, H2/n-C16H34 = 10 mol/mol,
WHSV = 0.75–18 h−1
.

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F. Regali et al. / Applied Catalysis A: General 469 (2014) 328–339
Fig. 11. Activity drop during the co-feeding of pyridine on Pt1.2/S40. Conditions:
310 ◦C, 30 bar, H2/n-C16H34 = 10 mol/mol, WHSV = 0.75–18 h−1
.
et al. singled out the role of hydrogen coverage, which decreased
with increasing acidity of the support; neopentane adsorbed more
strongly at lower hydrogen coverage [42]. Another study pro-
posed that the role of Brønsted acid sites was to form protonated
palladium species which were highly active in the conversion of
neopentane [43].
In the light of our results, it seems that direct hydrogenolysis
of n-alkanes on Pt sites might be affected by interactions between
the metal particles and the support in a similar manner as that
observed for neopentane hydrogenolysis. To gather more proof that
this behavior is not due to mechanistic involvement of Brønsted
sites, but rather to intrinsic catalytic properties of the metal parti-
cles, selective poisoning of acid sites on a Pt/Siral 40 catalyst was
performed.
Fig. 10. Specific methane yield per Pt surface area as a function of n-hexadecane
conversion. Pt0.06/S40 (᭹), Pt0.33/S40 (ꢀ), Pt0.6/S40 (ꢁ), Pt1.2/S40 ( ), Pt0.33/S1
(♦). Conditions: 310 ◦C, 30 bar, H2/n-C16H34 = 10 mol/mol, WHSV = 0.75–18 h−1
.
cracking by the fast modes of ␤-scission [40] and are therefore less
readily demethylated on the metal sites. The higher conversion will
also increase the concentration of olefinic intermediates, which will
compete for adsorption on metal sites. The methane STY is very low
on Pt0.33/Siral 1, despite its metal surface area being comparable
with that of the Pt0.33/Siral 40 catalyst.
The methane yield can be recalculated as a site-time yield,
defined as millimoles produced per unit Pt surface area per unit
time. The site-time yields for the tested catalysts are shown in
Fig. 10. If methane is produced only by hydrogenolysis occurring
exclusively by metal catalysis, no variations of the specific yield
on the different catalysts should be observed, except for possi-
ble effects of metal crystallite size. Site-time yields fall indeed in
a narrow range on the Pt/Siral 40 catalysts, except for the slightly
higher values observed for Pt1.2/Siral 40. This deviation, particu-
larly apparent at higher conversion, can be ascribed to secondary
demethylation, occurring to some extent on the catalyst with high-
est Pt loading, as can be seen by the higher fraction of methane in
the cracking products distribution (Fig. 6D). A major deviation is
the much lower methane site-time yield on the non-acid catalyst,
with values below 0.05 mmol/h/m2, which is lower by a factor five
compared to the Siral 40-based catalysts.
This observation either implies that Brønsted acid sites partici-
pate in the hydrogenolytic mechanism, or that Pt sites supported on
the Siral 40 material are intrinsically more active for hydrogenol-
ysis. Since particle size and dispersion are quite similar for the
Pt0.33/Siral 1 and the Pt0.33/Siral 40 catalysts, the reason for
the different activity have to be found in other properties of the
metal crystallites. It is generally known that metal-support inter-
actions affect the activity of metal catalysts in hydrogenation and
hydrogenolysis reactions, but the underlying reasons are debated.
The increased activity for benzene hydrogenation of Pt supported
on acid supports was ascribed to adsorption of reaction interme-
diates in the metal–acid interfacial region, which then reacted
with spilt-over hydrogen from Pt sites [41]; Lercher and cowork-
ers explained the higher turnover frequency of Pt/ASA catalysts
for neopentane hydrogenolysis with the increasing electronega-
tivity of the support, increasing the electron deficiency of Pt sites,
causing a stronger adsorption of the reactant [33]. Koningsberger
3.2.4. Effect of hydrogen pressure and selective poisoning of acid
sites
When the effects of metal–acid interactions on hydrogenoly-
sis reactions are studied, care must be taken to avoid introducing
bifunctional reactions which might influence alkane hydroconver-
sion in the presence of acidity. Neopentane is generally used as it
cannot form alkene intermediates, necessary for the bifunctional
hydrocracking mechanism [42]. For the investigation of similar
effects on the hydrogenolysis of linear alkanes, a way of inhibi-
ting bifunctional activity is to selectively poison the acid sites with
a base such as pyridine [44,45]. This approach allows for the qual-
itative study of monofunctional, metal-catalyzed reactions in the
presence of an acid support.
Pyridine was therefore co-fed with n-hexadecane on a 1.2-wt%
Pt/Siral 40 catalyst with stable activity. The effect of pyridine was
immediately noticeable: After one hour of co-feeding, no branched
hydrocarbons could be detected in the gaseous products. The total
catalytic activity dropped considerably and stabilized after a few
hours (Fig. 11). After that, experiments at varying hydrogen partial
pressures were performed.
Pyridine decomposed into n-pentane and ammonia on the
Pt1.2/Siral 40 catalyst. The conversion of pyridine, calculated by the
added amount of n-pentane observed on top of the typical crack-
ing products distribution, was approximately 50%. Decomposition
of pyridine occurs most probably on Pt sites, through piperidine
and 1-pentylamine, by hydrogenation of double bonds followed by
hydrodenitrogenation [46]. This is, of course, a drawback for the
determination of the absolute hydrogenolytic activity of the metal
sites. Nevertheless, a qualitative analysis can still be of interest. The

F. Regali et al. / Applied Catalysis A: General 469 (2014) 328–339
337
Fig. 12. Space–time yield of mono-branched (A) and multi-branched hexadecanes (B) as a function of hydrogen partial pressure before (ꢂ) and during (ꢁ) the addition of
pyridine. Catalyst: Pt1.2/S40. Conditions: 310 ◦C, 30 bar, H2/n-C16H34 = 6–10 mol/mol, WHSV = 18 h−1
.
fact that space–time yields of branched hexadecanes (Fig. 12) are
reduced by one order of magnitude, while yields to methane and
normal alkanes (Fig. 13) are much less affected, shows that pyridine
blocks the hydroisomerization/cracking function of the acid sites.
The distribution of cracking products on a Pt/Siral 40 catalyst
during addition of pyridine (Fig. 6E) can be compared with that
obtained on the non-acidic Pt/Siral 1 catalyst (Fig. 6F). In both case
the isomer content is very low and the only difference is in the
relative amounts of C1 and C15, which are higher on the non-
acidic catalyst. This implies that the rate of terminal vs. internal
bond hydrogenolysis is relatively lower on the pyridine-poisoned
Pt1.2/Siral 40 catalyst.
The results of the study on the effect of the partial pressure of
hydrogen are reported in a comparative way. The same conditions
of total pressure, temperature and flow rates were used in absence
and presence of pyridine. Considering the large excess of hydro-
gen, its partial pressure can be assumed constant throughout the
reactor. Moreover, n-hexadecane conversion levels were quite low
(<20%); the use of inlet hydrogen pressure and of space–time yields
instead of rates is therefore justified.
It is known that, for ideal hydrocracking conditions, hydrogen
partial pressure changes the equilibrium of the alkane dehy-
drogenation reaction. At constant alkane concentration, lower
hydrogen pressures cause a proportional increase of the steady
Fig. 13. Space–time yield of methane (A) and n-octane (B) as a function of hydrogen partial pressure before (ꢂ) and during (о) the addition of pyridine. Catalyst: Pt1.2/S40.
Conditions: 310 ◦C, 30 bar, H2/n-C16H34 = 6–10 mol/mol, WHSV = 18 h−1
.

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F. Regali et al. / Applied Catalysis A: General 469 (2014) 328–339
silica–alumina with Brønsted and Lewis acidity, compared to plat-
inum sites supported on a silica–alumina of comparable Lewis
acidity but negligible Brønsted acidity.
4. Conclusions
A series of catalysts consisting of platinum on silica–aluminas
were tested in the hydroconversion of n-hexadecane. The effect
of platinum loading on the acid properties of the catalysts and
on their catalytic activity and selectivity was investigated. Total
hydroconversion activity showed a linear increase with platinum
loading.
The bifunctional hydrocracking pathway showed constant
reaction rates at platinum loadings ranging between 0.06 and
1.2 wt%, and the observed increase in activity could be related
to a hydrogenolytic pathway yielding normal alkanes, including
methane and ethane. The yield of methane, as well as the yield
of linear hydrocarbons at low n-hexadecane conversion could
be linearly related to platinum surface area on catalysts with
similar acidic properties and platinum dispersion, showing that
hydrogenolysis is purely metal catalyzed.
Addition of platinum was found to impact on the acidic prop-
erties of the catalysts, increasing the concentration of weak
Brønsted and Lewis acid sites. Conversely, support acidity strongly
influenced the catalytic properties of platinum crystallites. High
hydrogenolytic activity was observed on catalysts containing
Brønsted acidity. A catalyst with similar metal dispersion, but
using a support of negligible Brønsted acidity, presented rates of
hydrogenolysis one order of magnitude lower.
Selective poisoning of acid sites of a Pt/silica-alumina catalyst
was performed. Bifunctional activity was successfully suppressed,
while hydrogenolysis rates were less affected by the presence of
pyridine. This indicates that the role of acidity in the hydrogenol-
ysis reactions is related to changes induced on the platinum sites,
probably by charge transfer from the metal particle to the frame-
work atoms of the support, and not to the involvement of acid sites
in the mechanism.
Fig. 14. Bi-logarithmic plot of the overall reaction rate as a function of hydrogen par-
tial pressure before (ꢂ) and during (о) the addition of pyridine. Catalyst: Pt1.2/S40.
Conditions: 310 ◦C, 30 bar, H2/n-C16H34 = 6–10 mol/mol, WHSV = 18 h−1
.
state concentration of the olefins formed on the metal sites, which
subsequently react on the acid sites. Its influence on the overall rate
is thermodynamic rather than kinetic. According to this scheme the
apparent reaction order for hydrogen under ideal hydrocracking
conditions is expected to be −1, as indeed reported in the liter-
ature [21,22]. In Fig. 14 the bi-logarithmic plot of the total rate
against hydrogen partial pressure is shown. The reaction order for
hydrogen was calculated by linear regression, and resulted to be
−0.42 on the non-poisoned catalyst. This discrepancy with litera-
ture data is probably due to the high contribution of hydrogenolysis
on the total rate. In the presence of pyridine, the overall reaction
rate seems to be unaffected by hydrogen pressure (apparent reac-
tion order = 0.05). As can be seen in Fig. 12, the isomerization rate
is affected in a similar way: negative order at reference conditions
(reaction order = −1.0) and little or no influence of hydrogen pres-
sure in the presence of pyridine. These facts are further indications
that acid-catalyzed reactions are indeed inhibited by pyridine.
Fig. 13A and B shows the dependence on hydrogen pressure
of the rates of formation of methane and n-octane, respectively.
It can be noted that the absolute values of space–time yields are
of the same order of magnitude with and without addition of
pyridine, indication that hydrogenolytic reactions are much less
affected by the presence of pyridine compared to the acid-catalyzed
reactions. At reference conditions, the effect of hydrogen partial
pressure is positive for both the rate to methane and the rate to
other cracking products. The latter shows first-order dependence
while a less marked positive influence is observed on methane
formation (apparent reaction order: 0.35). In the presence of pyri-
dine, hydrogen pressure has a positive effect on the space–time
yield to metal-catalyzed hydrogenolysis products, while the rate
to methane shows a negative order in hydrogen.
Acknowledgments
Atte Aho, Process Chemistry Centre, Åbo Akademi University is
kindly acknowledged for the pyridine adsorption measurements.
The authors wish to thank Rodrigo Suárez París and Matteo Lualdi
for fruitful discussion and Christina Hörnell for improving the
English language of this paper. The Swedish Energy Agency is
acknowledged for financial support and Sasol Germany GmbH for
providing the silica–alumina materials.
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