K. Lee, M. Choi / Journal of Catalysis 340 (2016) 66–75
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collected by centrifugation and washed with DI water until the
supernatant liquid had a pH below 9. To conduct desilication with
a surfactant, 19.6 g of cetyltrimethylammonium bromide was
added into the aforementioned KOH solution and the zeolite was
treated in a similar way. The resultant zeolites were dried at
373 K for 8 h, and calcined at 873 K for 6 h under dry air. The sam-
ples desilicated without and with CTAB were denoted as HR-KL-1
and HR-KL-2, respectively.
To support Pt catalysts, 5 g of the zeolites was suspended in
260 cm3 of 0.001 M Pt(NH3)4(NO3)2 aqueous solution and stirred
at room temperature for 24 h. The ion-exchanged zeolites were fil-
tered, washed with DI water, and dried at 373 K. The resultant
samples were calcined under a dry air flow in a plug-flow Pyrex
reactor at 593 K (ramp: 0.4 K minꢀ1) for 2 h. After calcination, the
reactor was cooled to room temperature under Ar flow. Then,
reduction was subsequently carried out under H2 flow at 573 K
(ramp: 1.2 K minꢀ1) for 2 h. After the reduction, the reactor was
cooled to room temperature under Ar flow. In order to eliminate
acid site generated during the reduction of Pt species, 5 g of
Pt/KL samples was ion-exchanged with 510 cm3 of 0.01 M KOH
aqueous solution at room temperature for 30 min. After filtration,
the final product was washed with 200 cm3 of 10ꢀ5 M KOH solu-
tion and dried at 373 K. 1 wt% Pt was also supported on a meso-
Scheme 1. Schematic representation of Pt/KL structure and preorganization of n-
hexane in the zeolite micropores during aromatization.
the C8 aromatics (o-xylene and ethylbenzene) into benzene and
toluene. It was proposed that the secondary hydrogenolysis is
caused by the retarded diffusion of bulky C8 aromatics out of the
1-dimensional microporous channel of the KL zeolite [14]. Indeed,
it was reported that the use of KL zeolite crystallites with a shorter
diffusion path length along the micropores can somewhat lower
the extent of secondary hydrogenolysis [16]. This strongly implies
that the unique 1-dimensional microporous structure of the KL
zeolite, which enables high aromatization activities in n-hexane
aromatization, can also limit the catalytic performance in larger-
alkane aromatizations (>C8) due to the hindered diffusion of bulky
alkylaromatic products. It is therefore a reasonable assumption
that the catalytic performances of a Pt/KL in larger-alkane aroma-
tizations can be significantly improved by enhancing the diffusion
of bulky aromatic products out of the catalyst structure.
porous
c-Al2O3 (97%, STREM) support by incipient wetness
impregnation of Pt(NH3)4(NO3)2 aqueous solution. The impreg-
nated sample was dried at 373 K, calcined under dry air at 573 K
for 2 h, and reduced under H2 flow at 573 K for 2 h.
2.2. Characterization
In the last decade, syntheses and catalytic applications of
hierarchical zeolites containing zeolitic microporosity as well as
secondary mesoporosity have attracted significant scientific atten-
tion [17–20], because the mesoporosity can significantly increase
molecular diffusion. The enhanced molecular diffusion within the
zeolite-based catalysts can result in significantly enhanced cat-
alytic activities [21–27], selectivities [24–27], and lifetime
[21,28–30] in various hydrocarbon conversions. In the present
work, we investigated synthesis methods to create uniformly dis-
tributed mesoporosity inside a KL zeolite (Si/Al = 3.0) via sequen-
tial dealumination and desilication process. After supporting
highly dispersed 1 wt% Pt clusters, the hierarchical Pt/KL catalysts
were used for the aromatization of various alkanes including
n-hexane, n-heptane, and n-octane in order to systematically study
the catalytic effects of mesoporosity in Pt/KL catalysts. A solely
X-ray diffraction (XRD) patterns were collected by D2-phaser
(Bruker) equipped with Cu K
a radiation (30 kV, 10 mA) and LYN-
XEYE detector. Data were recorded in the 2h range of 5–50° with
a step size 0.01° and a counting time of 2.5 s per step. N2 adsorp-
tion–desorption isotherms were collected using a BELSORP-max
volumetric analyzer at liquid nitrogen temperature (77 K). The
samples were degassed at 673 K for 4 h before the measurement.
Elemental analysis was carried out by inductively coupled plasma
optical emission spectroscopy (ICP-OES) using an Agilent ICP-OES
720 instrument. Transmission electron microscopy (TEM) images
were obtained by JEOL-TEM (200 kV) after mounting the sample
on a carbon-coated copper grid (300 mesh) using ethanol disper-
sion. High angle annular dark field-scanning transmission electron
microscopy (HAADF-STEM) investigation was carried out using a
FEI Titan cubed G2 60-300 microscope operating at 300 kV. Pt par-
ticle size distributions were determined by counting at least more
than 200 particles. H2 chemisorption was carried out using an
ASAP2020 instrument (Micromeritics). Before the experiments,
samples were degassed at 593 K under vacuum for 6 h,
re-reduced at 573 K for 1 h, and evacuated for 2 h at the same tem-
perature. H2 chemisorption was carried out at 323 K. Chemisorbed
H2 amount was calculated by extrapolation of the linear portion
(7–30 kPa) of the isotherm to zero pressure. The diffusivities of
benzene and o-xylene in the KL-supported Pt catalysts were mea-
sured using a modified thermogravimetric analysis system (TGA N-
1000 instrument, Thermo Co.) [24]. In each experiment, 5 mg of
mesoporous Pt/c-Al2O3 catalyst having a similar Pt dispersion to
those of Pt/KL catalysts was also prepared and investigated in the
catalytic reactions in order to clearly elucidate the effect of support
pore structure on aromatization activity and selectivity.
2. Experimental
2.1. Catalyst preparation
For the synthesis of micro-/mesoporous hierarchical KL zeolites,
a commercial KL zeolite (Wako Pure Chemical Industries, Ltd.) was
sequentially dealuminated and desilicated. Dealumination was
carried out in an Erlenmeyer flask equipped with a reflux con-
denser. In a typical reaction, 20 g of zeolite was suspended in
430 cm3 of 0.1 M ethylenediaminetetraacetic acid (EDTA, >99%,
Sigma–Aldrich) aqueous solution and stirred at 373 K for 8 h. The
treated zeolite was filtered, washed with hot DI water, and dried
at 373 K for 8 h. Desilication was subsequently carried out without
and with the aid of cetyltrimethylammonium bromide (CTAB,
>98%, TCI) organic surfactant. For desilication without a surfactant,
10 g of dealuminated KL was treated with 540 cm3 of 0.1 M KOH
aqueous solution at room temperature for 4 h. The product was
the sample was degassed at 573 K under N2 flow (100 cm3 minꢀ1
)
for 1 h. Aromatic vapors were introduced at 473 K by flowing N2
(100 cm3 minꢀ1) saturated with each probe molecule at 298 K.
2.3. Catalytic reactions
Catalytic n-alkane aromatizations were carried out in a stainless
steel 1/2 in. fixed-bed plug-flow reactor. As a hydrocarbon feed,
n-hexane (>96%, Junsei), n-heptane (>99%, Junsei), and n-octane
(>99%, Sigma–Aldrich) were used. Before the reaction, catalysts