Aromatization of hexane over Pt/KL catalyst: Role of intracrystalline diffusion on catalyst performance using isotope labeling

Article abstract of DOI:10.1016/j.jcat.2010.01.002

Aromatization of hexane using an equimolar mixture of C₆H₁₄ and C₆D₁₄ was investigated over 1%Pt/KL at 500 °C and 1 atm. There is hardly any kinetic isotope effect indicating that entry of hexane molecules into the wider lobes of the channels of L-zeolite through the narrow apertures leads to single-file hexane access to Pt⁰ sites. The catalyst displayed high activity and selectivity to benzene and excellent stability at the tested conditions during hexane dehydrocyclization. It is proposed that controlling the entry of hexane into the lobes of the L-zeolite containing the active sites is the reason for the exceptional Pt/KL catalytic performance for aromatization because the narrow windows of the L-zeolite channels inhibit the occurrence of bimolecular reactions that lead to coke formation and catalyst deactivation.

Full text of DOI:10.1016/j.jcat.2010.01.002

Journal of Catalysis 270 (2010) 242–248
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Aromatization of hexane over Pt/KL catalyst: Role of intracrystalline diffusion
on catalyst performance using isotope labeling
*
Khalid G. Azzam, Gary Jacobs, Wilson D. Shafer, Burtron H. Davis
Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, KY 40511, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Aromatization of hexane using an equimolar mixture of C6H14 and C6D14 was investigated over 1%Pt/KL at
500 °C and 1 atm. There is hardly any kinetic isotope effect indicating that entry of hexane molecules into
the wider lobes of the channels of L-zeolite through the narrow apertures leads to single-file hexane
access to Pt0 sites. The catalyst displayed high activity and selectivity to benzene and excellent stability
at the tested conditions during hexane dehydrocyclization. It is proposed that controlling the entry of
hexane into the lobes of the L-zeolite containing the active sites is the reason for the exceptional Pt/KL
catalytic performance for aromatization because the narrow windows of the L-zeolite channels inhibit
the occurrence of bimolecular reactions that lead to coke formation and catalyst deactivation.
Ó 2010 Elsevier Inc. All rights reserved.
Received 26 October 2009
Revised 22 December 2009
Accepted 1 January 2010
Available online 16 February 2010
Keywords:
Dehydrocyclization
Hexane-d0
Hexane-d14
Pt/KL
L-zeolite
Isotope effect
Diffusion
1. Introduction
performance of Pt/KL is due to the role of the basic zeolite in stabi-
lizing extremely small electron-rich Pt particles (<1 nm) of specific
Catalytic reforming is an important reaction used for the pro-
duction of aromatics from paraffins. The production of benzene is
important, as it is a feedstock for many industrial applications in
the production of drugs, plastics, synthetic rubber, and dyes. Com-
mercially, Pt supported on chlorinated alumina containing other
metallic components, such as Re [1–3], Sn [4–6], Ir [4,7,8], and/or
Ge [9,10], is the catalyst commonly used for catalytic reforming.
These catalysts are efficient in transforming C7 and higher paraffins
to aromatics; however, over these catalysts, the transformation of
hexane to benzene proceeds with much lower selectivity [11].
In 1980, Bernard [12] reported the exceptional activity and
selectivity of the non-acidic Pt/KL zeolite catalyst for the aromati-
zation of hexane to benzene. Since that time, several hypotheses
have been proposed in the literature [13–21] to explain the high
activity and selectivity of the Pt/KL catalyst. Tauster et al. [14]
and Derouane et al. [15] ascribed the unique feature of Pt/KL to
the geometric characteristics of the catalyst. They [14,15] proposed
that the structure of the catalyst orients the hexane molecule in the
linear channels in such a way that it favors the C1–C6 closure on the
Pt surface. Other authors [16–18] indicated that support micro-
structure does not contribute directly to the aromatization selec-
tivity over Pt/KL catalyst and rather claimed that the excellent
morphology under reaction conditions. Iglesia et al. [19,20], on the
other hand, proposed that the structure of the L-zeolite helps to
maintain the Pt particles inside the channels clean by preventing
bimolecular reactions of hexane leading to coke formation. They
[19,20] attributed the high activity and selectivity of Pt/KL to the
intrinsic properties of these clean Pt particles inside the channels.
Despite the different hypotheses to explain the excellent catalytic
performance of the Pt/KL, it is clear that the L-zeolite plays a signif-
icant role in enhancing both the activity and selectivity of Pt for
hexane dehydrocyclization; however, the reason for this enhance-
ment is still not completely understood.
The kH/kD normal kinetic isotopic effect (NKIE) has been used to
study the mechanism of octane dehydrocyclization over Pt/SiO2
catalyst by using a mixture of octane-d18 and octane-d0 [22–25].
In these studies, a NKIE of 3.5 0.4 was found at 482 °C, indicating
that the rate-limiting step involves breaking of the C–H bond. The
lower reactivity of C–D bond breaking relative to C–H is due to the
difference in bond energies. Similarly [26,27], the data from the
competitive conversion of octane-d18 and octane-d0 showed that
there is a NKIE for the dehydrocyclization reaction over different
catalysts (e.g., Pt/Mg(Al)O and Pt–Sn/SiO2); however, the value
(ꢀ2) was somewhat lower than that of obtained for Pt/SiO2. Never-
theless, it is obvious from these studies that the rate-determining
step is the surface reaction that involves C–H bond breaking during
alkane dehydrocyclization reactions over different catalysts.
* Corresponding author.
E-mail address: davis@caer.uky.edu (B.H. Davis).
0021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2010.01.002

K.G. Azzam et al. / Journal of Catalysis 270 (2010) 242–248
243
In a well-prepared Pt/KL catalyst, the majority of the small Pt
particles are located inside the channels of the KL zeolite [28–
30]. Therefore, under the actual catalytic conditions of hexane
dehydrocyclization, the hexane molecules likely diffuse to the Pt
active sites by Knudsen diffusion (i.e., whereby molecules fre-
quently collide with the pore wall) prior to catalytic reaction. It
is therefore possible that the entry of the hexane molecules into
the channels and intracrystalline diffusion of hexane play signifi-
cant roles in determining the selectivity of the dehydrocyclization
reaction. The aim of this research is to determine if a NKIE exists
when conducting the experiment using an equal molar ratio of
hexane-d14 and hexane-d0 as the feed.
33%H2/He for 1 h, purged with He for 15 min, and cooled to 40 °C
in 50 sccm of He flow. To probe the Pt0 clusters, the catalyst was
subjected to 50 sccm of 2%CO/He at 40 °C for 30 min. The catalyst
was then purged in 50 sccm He for 20 min. Scans were recorded
during CO adsorption and following the He purge. A 1%Pt/c-Al2O3
(Sasol Catalox 150 alumina) catalyst, prepared by incipient wet-
ness impregnation, served as the reference sample.
2.3. Activity test
Catalytic tests for hexane dehydrocyclization were carried out
in a stainless steel fixed-bed tubular reactor (0.5 in., o.d.) under
steady state conditions. Typically, the catalysts were pressed into
pellets, crushed, and sieved to yield grains of 355–600 lm diame-
2. Experimental
ter. A 250-mg catalyst sample was packed between two layers of
quartz wool. The temperature of the catalyst bed was monitored
by internal thermocouples (Fe–Cr) and maintained by a tempera-
2.1. Catalyst preparation
ture controller (
X Omega CN 3251-R). The whole system was
A sample of potassium-exchanged zeolite LTL (Süd-Chemie,
Inc.) was calcined in flowing air (100 cm3/min) at 400 °C for 4 h.
After cooling, the zeolite LTL was transferred to a glove bag filled
with inert gas, carefully mixed with platinum acetylacetonate (Alfa
Aesar), and loaded into a sublimation tube, sealed at one end. The
other end was attached to a high vacuum system, equipped with
an oil diffusion pump and capable of achieving a vacuum of
<10À6 Torr. The sublimation tube was then held at room tempera-
ture for 4 h and slowly ramped (1 °C/min) to 70, 80, 90, and then
100 °C, holding at each temperature for 1 h. Finally, the tempera-
ture was ramped to 130 °C, held for 15 min, and cooled to room
temperature. Following the chemical vapor deposition (CVD) pro-
cedure, the sample was light yellow in color. To decompose the
Pt(AcAc)2 compound, the catalyst was then calcined at 350 °C in
flowing air for 4 h. The final catalyst was light gray in color. The
loading was 1%Pt by weight. Various sublimation methods for
obtaining highly dispersed Pt/KL can be found in the literature,
e.g., [28–37].
heated to 120 °C to avoid condensation of hexane. The reactor
was operated under flowing H2. Hexane-d0 and hexane-d14 (Alfa
Aesar, 99% purity) were added by infusion with a syringe pump
(Thermo Scientific, model Orion M361) through a tee-junction lo-
cated prior to the reactor. The H2/hexane ratio was kept at 6. Prior
to catalytic testing, the catalyst was reduced in H2 at 500 °C for
15 min (temperature ramp of 4 °C/min). Reaction testing was con-
ducted at a WHSV of 10 hÀ1 at atmospheric pressure and a temper-
ature of 450 and 500 °C. A purge-valve was used to automatically
send samples to the GC for analysis. The exit line from the
purge-valve is sent to a cold trap (cooled with dry ice) to collect
the products. The feed and products were analyzed using an Agi-
lent GC (7890A) which includes a HP-PLOT/Al2O3 ‘S’ deactivated
capillary column to achieve product separation. Products were de-
tected using a flame ionization detector (FID). A linear 40-min tem-
perature ramp from 35 to 180 °C provided the means for adequate
peak separation in the GC column. Due to the good separation of
hexane-d0 and hexane-d14 in the column (see Fig. 1), the conver-
sions of both reactants were calculated using the FID data. The
collected liquid products from the trap were analyzed using an Agi-
lent GC (6890)–MS (5973N) to investigate the extent of H-labeled
hydrogen incorporation in the deuterated hexane reactant and the
extent of D incorporation in the H-labeled hexane reactant.
2.2. Catalyst characterization
Brunauer–Emmett–Teller surface area (BET SA) measurements
were conducted using a Micromeritics Tri-Star system to deter-
mine whether loss of surface area occurred after loading the metal.
To degas, catalyst samples were slowly heated to 160 °C under vac-
uum (50 mTorr) and held for 14 h. Then, the sample was trans-
ferred to the adsorption unit, and nitrogen adsorption was
measured at its boiling temperature.
3. Results and discussion
The amount of hydrogen evolved from the 1%Pt/KL CVD catalyst
during TPD was 51.2 and 50.5 lmol/g, respectively, for the two
Hydrogen chemisorption by
a
temperature-programmed
reduction temperatures of 450 °C and 500 °C employed. These val-
ues correspond closely to a H/Pt ratio of 2.0 (Table 1). Virtually
identical results, obtained using the volumetric chemisorption
approach, were reported previously [28,32] and indicate that
the Pt clusters in the catalyst used in this work are extremely
well-dispersed. Although H/Pt ratios above 1.0 are common in
the literature [31,32,38–40] for Pt/KL catalysts, the CVD method
consistently yields among the highest dispersions. Not only are
desorption (TPD) method was carried out using a Zeton-Altamira
AMI-200 unit. The catalyst was slowly heated (1 °C/min) to either
450 °C or 500 °C in 30 cm3/min of a 33%H2/Ar mixture and held at
either 450 °C or 500 °C for 1 h. The catalyst was then cooled in
flowing hydrogen to 80 °C and purged with 25 cm3/min of flowing
argon to remove weakly bound hydrogen. The catalyst was then
heated in 25 cm3/min of flowing argon to either 450 °C or 500 °C
to desorb the hydrogen, and the evolved hydrogen was monitored
using a thermal conductivity detector (TCD). To calibrate the sig-
nal, ten pulses of a standard amount of hydrogen were passed into
a flowing argon stream.
Diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS) spectra were recorded using a Nicolet Nexus 870 spec-
trometer equipped with a DTGS-TEC detector. A Thermo Spectra-
Tech cell capable of high pressure/high temperature operation
and fitted with ZnSe windows served as the reaction chamber for
in situ CO adsorption measurements. Scans were taken at a resolu-
tion of 4 to give a data spacing of 1.928 cmÀ1. The catalyst
(ꢀ40 mg) was first activated at either 450 °C or 500 °C in 75 sccm
Table 1
Physical and chemical characteristics of the catalysts.
Catalyst
SBET (m2/g)
Pore volume (m3/g)
H/Pta
KLb
291
190
0.1426
0.1000
–
–
1 wt.% Pt/KLc
2d
2e
a
b
c
Measured by H2 – TPD.
Calcined at 400 °C.
Calcined at 350 °C.
Reduced in H2 at 450 °C.
Reduced in H2 at 500 °C.
d
e

244
K.G. Azzam et al. / Journal of Catalysis 270 (2010) 242–248
Fig. 1. FID gas chromatogram showing the efficiency of separating the hexane-H14 and hexane-D14 using the HP-PLOT/Al2O3 ‘S’ deactivated capillary column, (A) before
reaction and (B) after reaction.
the particles generally smaller, but since they are not clustered to
form a large particle, they exhibit greater spatial separations with-
in the zeolite pores [28].
CVD versus the 1%Pt/
of bands for 1%Pt/KL CVD are positioned at wave numbers much
lower than the linear (CO) band observed for the typical 1%Pt/c-
c-Al2O3 reference compound. The majority
m
One method to verify the likelihood that the Pt0 clusters are
mainly within the zeolite channels is to use CO as a probe and
follow the evolution of Pt carbonyl species by Fourier transform
infrared (FTIR) spectroscopy. These carbonyl species are formed
by the disruption of only highly dispersed Pt clusters by CO; they
are stabilized by a ligand effect, proposed to be due to the inter-
action between electron-rich zeolite O atoms, located inside the
zeolite channels, and the PtX(CO)Y species [41,42]. That is, the ba-
sicity of the support is responsible for the ligand effect, in a sim-
ilar manner that Longoni–Chini Pt-carbonyls are stabilized in
solution by, for example, tertiary phosphines [43]. In agreement
with this perspective, Pt particles in KL zeolite have been ob-
served to be completely disrupted by in situ EXAFS [44]. Another
view is that there is an electrostatic interaction [45], and Lane
et al. [46] suggested this occurs between the potassium cation
and linearly bound CO; in another perspective, an electronic ef-
fect due to electron transfer from the zeolite to small Pt particles
has been postulated [40]. Fig. 2 (left, A and B) shows the evolu-
tion of Pt carbonyl species for 30 min, while Fig. 2 (right, C and
D) shows a comparison after a 20-min purge in He of 1%Pt/KL
Al2O3 catalyst. Furthermore, the low-wave number bands appear
at discrete positions (e.g., for the catalyst reduced at 500 °C prior
to CO adsorption, the bands are positioned at 2049, 2035, 2030,
2014, 2006, with a series of shoulders from 2000 to 1915 cmÀ1),
suggesting that they likely correspond to PtX(CO)Y species stabi-
lized by the L-zeolite support.
Although the DRIFTS results of adsorbed CO indicate that a high
fraction of Pt resides inside the channels (Pt carbonyl bands rang-
ing from 2049 cmÀ1 to 1915 cmÀ1), there is also a shoulder present
at 2067 cmÀ1 for CO adsorbed on larger Pt metal crystallites. These
particles are likely large enough to block the narrow window of the
L-zeolite and therefore completely cut off reactant access to a frac-
tion of the channels. The DRIFTS spectra were fitted using a least
squares fitting procedure with Gaussian peaks. Two Gaussian
peaks were used to define the
m(CO) of linearly bound CO on larger
Pt crystallites (2067 cmÀ1 +), and five Gaussian peaks were used to
define the m(CO) bands of PtX(CO)Y species stabilized by the KL zeo-
lite (<2067 cmÀ1), as depicted in Fig. 2E and F. Approximately 20%
of the linear stretching bands correspond to those assigned to CO
adsorbed on larger Pt crystallites. This is consistent with a decrease

K.G. Azzam et al. / Journal of Catalysis 270 (2010) 242–248
245
Fig. 2. (Left) CO adsorption over 1%Pt/KL using 50 ccm of 2%CO/He at 40 °C after reduction at (A) 450 °C or (B) 500 °C in 75 ccm of 33%H2/He and cooling in 50 ccm He. (Right)
Corresponding spectra (C and D) after a 20 min 50 ccm He purge for (solid line) 1%Pt/KL and (dashed line) 1%Pt/ -Al2O3. Note the height of the (CO) band for Pt/alumina was
multiplied by 0.65 (450 °C data) and 0.45 (500 °C data) to normalize. Gaussian fitting of (CO) for linearly bound CO (dashed, two components) and (CO) in Pt-carbonyls
(dash-dotted, five components) for (E) 450 °C and (F) 500 °C H2-reduced catalysts following CO adsorption at 40 °C and He purge.
c
m
m
m
in the BET surface area and pore volume by a fraction of ꢀ1/3 (see
Table 1) observed after Pt addition and calcination relative to KL
zeolite alone. Nevertheless, DRIFTS results demonstrate that the
majority of Pt is small enough, and in sufficient contact with the
KL zeolite channels, as to be converted to KL support-stabilized
bands). As mentioned previously, adequate basicity is a require-
ment in the formation of Pt carbonyl species and can only be pro-
vided in this case by the KL zeolite. From the hydrogen
chemisorption and DRIFTS results, one can conclude that the
majority of Pt clusters are ultra-well-dispersed and located within
the channels of the L-zeolite.
Pt carbonyl species upon CO adsorption (ꢀ80% of the linear
m(CO)

246
K.G. Azzam et al. / Journal of Catalysis 270 (2010) 242–248
Table 2
Dehydrocyclization of equimolar mixture of hexane-d0 and hexane-d14 over 1%Pt/KL
catalyst at 500 °C, 1 atm, WHSV = 10 hÀ1, H2/hexane ratio of 6.
Time (h)
Conversion (%)
Conversion-h14/conversion-d14
d0
d14
0.25
1.75
2.50
4.00
5.50
43
37
38
43
45
38
30
29
37
38
1.13
1.23
1.31
1.16
1.18
Average
St. Dev.
41.2
3.5
34.4
4.5
1.2
0.1
Fig. 4. Deuterium distribution in the recovered hexane (equimolar feed of hexane-
d14/hexane-d0) after the dehydrocyclization reaction over 1%Pt/KL at 300 °C, 1 atm,
WHSV = 10 hÀ1, H2/hexane ratio of 6.
Fig. 3. Deuterium distribution in the recovered hexane (equimolar feed of hexane-
d14/hexane-d0) after the dehydrocyclization reaction over 1%Pt/KL at 450 °C, 1 atm,
WHSV = 10 hÀ1, H2/hexane ratio of 6.
Table 2 shows the relative conversions of the equimolar C6H14
and C6D14 over the 1%Pt/KL versus time on-stream at 500 °C and
atmospheric pressure. Over the tested period (5.5 h), the catalyst
showed good stability with average conversion of 41% and 34%
for C6H14 and C6D14, respectively, and an average calculated NKIE
(kH/kD) of 1.2 0.1. The carbon balance was good in all analyses
with a value of 1 0.07. The results indicate that there is hardly
any NKIE detected for the conversion of an equimolar mixture of
C6H14 and C6D14 on the tested catalyst (1%Pt/KL). It is well estab-
lished in the literature [22–25,47,48] that (i) the adsorption of a
mixture of alkanes–alkanes or alkanes–cycloalkanes is competitive
and irreversible during the dehydrocyclization reaction over Pt cat-
alysts at temperatures at or above ca. 480 °C and 1 atm and (ii) a
reaction that exhibits a significant decrease in conversion when
switching from a C–H label to the C–D label in the reactant in-
volves breaking of that C–H bond in the rate-limiting step due to
the NKIE arising from the difference in bond energy [23,49,50].
Irreversible adsorption has been shown to be temperature depen-
dent [22,47]. Thus, at 300 °C, adsorption is not rate limiting so that
the cycloalkane is converted much more rapidly than the alkane
[17,43]. Furthermore, it was observed that there was more H–D
exchange in the unconverted reactant following exposure to the
catalyst at the low-temperature condition [22,47,48]. In this study,
H–D exchange in the unconverted hexane when the reaction was
conducted at 450 °C (Fig. 3) or at 300 °C (Fig. 4) was more
pronounced compared with the 500 °C condition (Fig. 5). The
occurrence of H–D exchange at 300 °C or at 450 °C and the lack
of exchange at 500 °C indicate a change from reversible adsorption
Fig. 5. Deuterium distribution in the recovered hexane (equimolar feed of hexane-
d
14/hexane-d0) after the dehydrocyclization reaction over 1%Pt/KL at 500 °C, 1 atm,
WHSV = 10 hÀ1, H2/hexane ratio of 6.
at lower temperature to irreversible adsorption at 500 °C, the typ-
ical temperature for aromatization over Pt/KL.
Surprisingly, H–D exchange at 450 °C occurred to a greater ex-
tent than at 300 °C, and the H–D exchange in the unreacted hexane
at 300 °C is far from equilibrium. It was reported by Shi and Davis
[22], based on their observation of H–D exchange in octane over Pt/
SiO2 catalyst using an octane/methylcyclohexane mixture, that the
rate of isotope exchange and the depth of exchange increased as
the temperature decreased. Our observation on H–D exchange at
300 and 450 °C over the Pt/KL catalyst indicate that this is not
the case for the microporous Pt/KL catalyst. It is well established
that in a well-prepared Pt/KL catalyst, as confirmed by our hydro-
gen chemisorption and DRIFTS results, the active sites reside inside
the channels of the L-zeolite [16,28,29,51]. Considering the orders
of magnitudes in pore diameter of the nanoporous materials, this
means that Pt sites in microporous (0.7–1.1 nm) Pt/KL will exhibit
less open contact relative to the mesoporous catalysts like Pt/SiO2
(ꢀ27 nm for PQ CS-2133 silica). Thus, since the hexane molecules
must enter and diffuse through the channels of the L-zeolite to
reach the Pt active sites in order for H–D exchange to occur, a
higher fraction of hexane molecules will not access the Pt sites if

K.G. Azzam et al. / Journal of Catalysis 270 (2010) 242–248
247
diffusion of hexane is slow. Therefore, if at lower temperatures the
diffusion of hexane molecules is the rate-limiting step of H–D ex-
change, then at higher temperature (450 °C) the diffusion of the
hexane molecules through the channels of the L-zeolite will be fas-
ter than at 300 °C and hence more H–D exchange can occur, in
agreement with our observations. Another possibility that could
explain the finding that a higher rate of exchange occurs at
450 °C is due to the faster kinetics of the surface reaction. However,
in previous studies by Shi and Davis [22], the exchange achieved
equilibrium over Pt/SiO2 catalyst even at 300 °C; thus, the previous
explanation regarding diffusion control appears to be valid.
The lack of a NKIE at 500 °C (see Table 2) reveals that the chan-
nels of the L-zeolite play a significant role in controlling the entry
of hexane molecules to the active surface sites; these are located
on the Pt0 crystallites, which reside inside the lobes of the uni-axial
channels. In contrast to the non-microporous catalysts (e.g., Pt/
SiO2) in which irreversible reactant adsorption is reported to be
the rate-determining step based on the lack of H/D exchange in
the recovered reactants and significant NKIE [22–26], the entry
into the pore opening and intracrystalline diffusion of hexane
seems to be the slowest step in the dehydrocyclization reaction
over Pt/KL. If this was not the case, a more pronounced NKIE should
be observed. Indeed, the adsorption of hexane-d0 or hexane-d14 is
also irreversible in our case based on the lack of H/D exchange of
the recovered reactants at 500 °C shown in Fig. 5. It is obvious from
Fig. 5 that there is very little H incorporated into hexane-d14, and
this also applies to hexane-h14 where there is little D incorporation
observed. This means that almost all the reactants that are ad-
sorbed on Pt sites complete the reaction to final products [23].
However, based on the fact that there is no NKIE when using the
equimolar C6H14 and C6D14 mixture over Pt/KL, the entry of hexane
into the pore channel and diffusion of the hexane molecules to Pt
sites within the channels of the Pt/KL catalyst seem to determine
the selectivity for, or absence of, H–D exchange in the reactants.
Moreover, the L-zeolite is reported to have uni-axial pores with a
narrow 0.7-nm aperture leading to cavities of about 1.1 nm in
diameter [52], and the average diameter of hexane molecule is
about 0.454 nm [53]. Comparing the diameter of the aperture of
pores and the diameter of hexane molecules, one can expect that
hexane enters and diffuses on a single-file basis (i.e., one-by-
one). Therefore, based on our hexane isotope distribution data
and the size characteristics of hexane molecules versus the size
of the narrow windows of the L-zeolite channels, we conclude that
hexane dehydrocyclization over Pt/KL is controlled by the proba-
bility of reactant entry into the wider lobes of the L-zeolite chan-
nels via the narrow apertures. Since the concentration of C6H14
and C6D14 is equal, the probability of entry into each narrow pore
opening is essentially identical and thus equal to unity, and this is
nearly the ratio that is obtained experimentally. One cannot rule
out a slight steric effect due to the presence of the larger D atoms.
Fig. 6 shows the product selectivity of Pt/KL to benzene versus
time on-stream at 500 °C and 1 atm. Over a period of a 5.5 h of test-
ing, the selectivity to benzene product is constant at about 70%. It
is obvious from the conversion and selectivity data that the cata-
lyst has superior activity, stability, and selectivity to benzene when
Pt/KL catalyst is prepared by the CVD procedure, in agreement with
the previous findings of Mielczarski et al. [16], Jacobs et al. [28,29],
and Dossi et al. [54]. The authors found that using CVD methods for
catalyst preparation resulted in better catalytic performance for Pt/
KL than when prepared by ion exchange (IE) or incipient wetness
impregnation (IWI) methods. This is because the CVD method re-
sults in a majority of ultra-tiny Pt clusters present inside the L-zeo-
lite channels rather than external to the zeolite or in close
proximity to each other at the pore mouth (i.e., resulting in signif-
icant agglomeration and noticeable initial deactivation trend). This
also agrees with our hydrogen chemisorption and DRIFTS results
Fig. 6. Benzene selectivity vs. time on-stream obtained from the dehydrocycliza-
tion of equimolar mixture of hexane-d0 and hexane-d14 over 1%Pt/KL catalyst at
500 °C, 1 atm, WHSV = 10 hÀ1, H2/hexane ratio of 6.
which show that the majority of Pt clusters are well-dispersed
and located within the channels of the L-zeolite. The location of
highly dispersed Pt clusters inside the channels is believed to
prevent Pt sintering (i.e., the narrow windows provide a kinetic
barrier to Pt mobility) and suppress carbon deposition (i.e., by
inhibiting bimolecular reaction pathways that lead to coking
[19,20,28,29,55]).
The observed results in this study, including the lack of NKIE
at 500 °C when using an equimolar of C6H14 and C6D14 over Pt/
KL CVD catalyst, the enhanced catalyst stability relative to other
preparation methods, and the excellent dehydrocyclization activ-
ity and selectivity to benzene are consistent with the hypothesis
of Iglesia et al. [19,20] that an important role of L-zeolite is to
help maintain the Pt particles inside the channels clean by pre-
venting bimolecular reaction pathways from occurring [19,20].
We propose based on the kinetic isotopic effect study that the
dehydrocyclization reaction over Pt/KL catalyst is controlled by
the entry of hexane molecules into the wider lobes of the L-zeo-
lite where the Pt0 active sites reside. That is, in contrast to non-
microporous-supported Pt catalysts, the tiny windows of the L-
zeolite channels allow hexane to pass through one-by-one to
the larger lobes containing the Pt clusters. This in turn hinders
the bimolecular reactions that lead to coke formation and thus
prevents catalyst deactivation.
4. Conclusions
Hydrogen chemisorption and DRIFTS revealed that the majority
of Pt clusters are ultra-tiny and well-dispersed within the channels
of L-zeolite when the catalyst was prepared using the CVD method.
There is hardly any kinetic isotope effect for the conversion of an
equimolar mixture of C6H14 and C6D14 on 1%Pt/KL catalyst at
500 °C and 1 atm, indicating that the dehydrocyclization reaction
of hexane over Pt/KL is controlled by hexane entry into the wider
lobes of the L-zeolite channels where the Pt clusters reside. In con-
trast with non-microporous-supported Pt catalysts, the tiny win-
dows of the L-zeolite channels allow hexane to pass through
one-by-one to the larger lobes containing the Pt clusters. This con-
trolled diffusion of hexane inside the channels seems to be the rea-
son for the high stability and selectivity of the catalyst, most likely
because it inhibits the bimolecular reactions that lead to coke for-
mation and catalyst deactivation, in agreement with the earlier
hypothesis of Iglesia.

248
K.G. Azzam et al. / Journal of Catalysis 270 (2010) 242–248
[30] E.W. Berg, F.R. Hartlage, J. Anal. Chim. Acta 33 (1965) 173.
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