Effects of temperature pretreatment on propane cracking over H-SSZ-13 zeolites

Article abstract of DOI:10.1039/c4cy00731j

The effect of thermal treatment of SSZ-13 on catalytic activity has been investigated by using monomolecular propane conversion as a probe reaction. Two samples with Si/Al ratios of 12 and 6 were prepared and characterized by solid-state nuclear magnetic resonance (NMR), X-ray powder diffraction (XRD), N₂ adsorption, and temperature-programmed desorption (TPD). Samples were pretreated at different temperatures: mild activation at 773 K (treatment 1) and high-temperature activation at 823, 873, 973, and 1073 K (treatment 2). The catalysts showed similar catalytic propane reaction rates within the experimental error. On the other hand, after treatment 1 the catalysts showed higher cracking selectivity than dehydrogenation selectivity by a factor of 2, while the selectivity towards dehydrogenation was enhanced for the catalysts treated at high temperatures (cracking-to-dehydrogenation ratio of ～1). The change in selectivity suggests that the generation of different active sites - other than Bronsted acid sites (BAS), typically considered the active sites for hydrocarbon conversion. The SSZ-13-6 treated at 823 and 873 K showed different selectivity, without a notable change in reactivity and activation energies compared to the acid catalyst after treatment 1. After high-temperature treatments, SSZ-13-12 showed gradually decreasing activation energies and increasing selectivity toward dehydrogenation with increasing pretreatment temperature (from 823 to 1073 K). These results lead us to the hypothesis that different types of new active sites are generated in SSZ-13-12 and SSZ-13-6 samples.

Full text of DOI:10.1039/c4cy00731j

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Effects of temperature pretreatment on propane
cracking over H-SSZ-13 zeolites
Cite this: Catal. Sci. Technol., 2015,
5, 264
J. H. Yun and R. F. Lobo*
The effect of thermal treatment of SSZ-13 on catalytic activity has been investigated by using monomolec-
ular propane conversion as a probe reaction. Two samples with Si/Al ratios of 12 and 6 were prepared and
characterized by solid-state nuclear magnetic resonance (NMR), X-ray powder diffraction (XRD), N
2
adsorption, and temperature-programmed desorption (TPD). Samples were pretreated at different temper-
atures: mild activation at 773 K (treatment 1) and high-temperature activation at 823, 873, 973, and 1073 K
(treatment 2). The catalysts showed similar catalytic propane reaction rates within the experimental error.
On the other hand, after treatment 1 the catalysts showed higher cracking selectivity than dehydrogenation
selectivity by a factor of 2, while the selectivity towards dehydrogenation was enhanced for the catalysts
treated at high temperatures (cracking-to-dehydrogenation ratio of ~1). The change in selectivity suggests
that the generation of different active sites—other than Brønsted acid sites (BAS), typically considered the
active sites for hydrocarbon conversion. The SSZ-13-6 treated at 823 and 873 K showed different selectivity,
without a notable change in reactivity and activation energies compared to the acid catalyst after treatment 1.
After high-temperature treatments, SSZ-13-12 showed gradually decreasing activation energies and
increasing selectivity toward dehydrogenation with increasing pretreatment temperature (from 823 to
Received 4th June 2014,
Accepted 11th August 2014
DOI: 10.1039/c4cy00731j
1
073 K). These results lead us to the hypothesis that different types of new active sites are generated in
www.rsc.org/catalysis
SSZ-13-12 and SSZ-13-6 samples.
Under a dry inert atmosphere, there are two mechanisms
by which BAS dehydroxylation can proceed. In the heterolytic
mechanism, dehydroxylation proceeds through the dehydra-
Introduction
Understanding the effects of post-synthesis zeolite treatments,
including high temperature, is important for improving and
modifying the catalytic properties
tion of BAS, leading to the formation of water and Lewis acid
1
–11
12,13,19
of zeolites in general.
sites (LAS), as illustrated in Scheme 1.
In the homolytic
High-temperature treatment of zeolites, normally above
mechanism (Scheme 2), the hydrogen atoms of the acid sites
are lost as H˙, that is hydrogen atoms, leading to the forma-
tion of an oxidized species on the zeolite surface. Nash et al.
observed the production of hydrogen during dehydroxylation
processes corroborating that dehydroxylation could proceed
through a homolytic mechanism in high-silica zeolites. The
electron-deficient sites formed by dehydrogenation of BAS
8
73 K, leads to the thermal decomposition of Brønsted acid
1
2,13
sites (BAS), Si–O(H)–Al, by a process called dehydroxylation.
The IR absorption band of the OH groups of the BAS is
−
1
19
detected in the range of 3600–3660 cm in the IR spectra of
3
,14
dehydrated samples,
and the concentration of the BAS can
be estimated based on the relative intensity of this signal. It
has been shown that samples treated at high temperatures
have new chemical properties, including the ability to extract
electrons from adsorbed species. For example, xanthene,
iminostilbene, fluorene, 2,5-dimethylhexa-2,4-diene, biphenyl,
1
5–18
and naphthalene
form radical cations upon adsorption
on pretreated zeolites. It is also important to understand the
properties of dehydroxylated zeolites because high tempera-
tures are employed in important industrial processes, such as
in the regenerators of a fluidized catalytic cracking (FCC) unit
or in the catalytic converters of automobiles.
Scheme 1 Dehydroxylation of Brønsted acid sites by dehydration.
Center for Catalytic Science and Technology, Department of Chemical &
Biomolecular Engineering, University of Delaware, Newark, DE 19716, USA.
E-mail: lobo@udel.edu
Scheme 2 Dehydroxylation of Brønsted acid sites by dehydrogenation.
264 | Catal. Sci. Technol., 2015, 5, 264–273
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3
2–37
(
Scheme 2) can have redox properties that may impact the
(MTO) process due to its unique shape selectivity.
adsorption capacity of SSZ-13 at ambient temperature and
pressure also makes it as a promising adsorbent for CO
The
1
9
catalytic properties of the sample. More detailed investiga-
tion of the sites generated after dehydroxylation has been
2
1
6
38
difficult because these sites are EPR-silent and do not
appear to have any well-defined UV/Vis or IR signature. We
have shown that the sites that are generated after dehydroxy-
lation have the ability to extract a single electron from a
neutral organic molecule, such as naphthalene, forming
capture. Copper-exchanged SSZ-13 is a promising catalyst
for selective catalytic reduction of NOx with ammonia.
Cu-SSZ-13 has high activity for NH -SCR and shows high
3
3
9,40
nitrogen formation selectivity.
We report the catalytic reaction rates and selectivity over
SSZ-13 with different Si/Al ratios (6 and 12) after thermal
treatments for the monomolecular propane reaction. The
samples treated at different temperatures were characterized
4
radical cations. In addition, long-lived electron–hole pairs
are formed by migration of electrons from the zeolite frame-
work to the radical cations. This observation suggests that the
formation of new sites could lead to catalytic activation of
adsorbed species by a redox mechanism instead of the typical
protolytic mechanism.
The activation of small alkanes has been used frequently
as a model reaction because the cleavage of the C–C and C–H
bonds in the hydrocarbon is a problem of both scientific and
technological interest. Alkane activation on acid sites in zeo-
lites can proceed through bimolecular and monomolecular
pathways. The bimolecular pathway involves hydride transfer
between an alkane and an adsorbed carbenium ion when the
concentration of surface species is high. In contrast, the
monomolecular pathway (protolytic mechanism), involving
2
7
by XRD, N
2
adsorption isotherm, TPD, and
Al and
2
9
Si solid-state NMR. The product distribution and kinetic
analysis indicate that different types of active sites are gener-
ated by thermal treatments. The catalytic properties of the
newly generated sites are also different for the samples with
different Si/Al ratios. In the case of SSZ-13-6, the selectivity
has changed dramatically with just 50 K of change in the
treatment temperature from 773 K to 823 K, which is the
“normal” calcination temperature for acid zeolites prior to
reaction tests. The activation energy does not change despite
this drastic selectivity change. In the case of SSZ-13-12,
the selectivity and activation energy changed gradually
with the treatment temperature, indicating the generation of
different active sites after thermal treatments. Further work
is still necessary to establish the molecular structure of these
new sites.
2
0–28
the formation of alkanium-like ions,
is kinetically
dominant at high temperatures (~773 K), low alkane partial
pressure, and low conversion (<2%). The propane reaction
rates and selectivity over H-ZSM-5 before and after dehydroxy-
3
lation were investigated, showing comparable catalytic rates
for propane activation but different selectivity and different
activation energies. In the monomolecular propane reaction,
Experimental section
Synthesis of SSZ-13
3
the reaction proceeds only through two channels, i.e., crack-
ing and dehydrogenation. While acid catalysts have higher
propane cracking selectivity than dehydrogenation selectivity
by a factor of about three, the catalysts show similar cracking
SSZ-13 samples were synthesized using N,N,N-trimethyl-1-
adamantanammonium ion (TMAda ) as the structure-
directing agent (SDA). Two different methods were used to
prepare the SSZ-13 samples with different Si/Al ratios. SSZ-13
with a low Si/Al ratio (SSZ-13-6, Si/Al ~6) was synthesized
+
3
and dehydrogenation rates after dehydroxylation. Reduced
−
1
apparent activation energies for cracking (from 184 kJ mol
−
1
−1
38,41
to 144 kJ mol ) and dehydrogenation (from 187 kJ mol to
27 kJ mol ) are also observed. These changes indicate that
using the procedure reported by Zones.
20 g of sodium silicate (Na O Si, Sigma-Aldrich) and 0.64 g of
2 3
In this procedure,
−
1
3
1
the new sites catalyze the hydrocarbon conversion through a
different reaction mechanism; a redox mechanism is a poten-
sodium hydroxide (NaOH, Fisher Scientific) were mixed with
48 g of deionized water at room temperature for 15 min.
tial path for the reaction. Deuterium-labeled propane (C
3
D
8
)
Next, 2 g of NH -Y zeolite (Zeolyst CBV-100) was added to the
4
was used to evaluate the redox mechanism by measuring the
solution and stirred for 30 min. This was followed by the
addition of 8.4 g of TMAdaOH (SACHEM Inc., 25% aqueous
solution) to the solution and mixing for another 30 min in
a covered container. The hydrothermal conversion of the
synthesis solutions was carried out in Teflon-lined Parr auto-
claves at a temperature of 413 K under rotation for 6 days.
The zeolite samples were separated from the solution
using vacuum filtration, washed using deionized water, and
dried at room temperature. The as-synthesized zeolites were
relative reaction rates of cracking and dehydrogenation
3
(
r
C H8/rC D ) before and after dehydroxylation. The relative
3
3
8
reaction rate of cracking is not affected by the deuterium
label, while that of dehydrogenation is increased from 1.13 to
3
3
.5. This change in the relative reaction rate of dehydrogena-
2
3
tion is consistent with radical cation chemistry. The complex-
ity of the ZSM-5 zeolite catalysts (12 distinctive tetrahedral sites)
precludes complete understanding of the structure of the active
sites. The presence of 12 distinctive T-sites results in different
−
1
calcined in air at 823 K with a heating rate of 2 K min for
2
9
+
turnover rates for the monomolecular propane conversion.
8 h to remove the occluded TMAda .
In this report, SSZ-13 (CHA) was used to activate propane
because it has only one topologically distinct tetrahedron site
The synthesis method of SSZ-13 with a high Si/Al ratio
(SSZ-13-12, Si/Al = ~12) was also based on the reports written
by Zones and by Eilertsen et al. 13 g of tetraethyl ortho-
3
0,31
42
43
and only four non-equivalent oxygen atoms
in the structure.
SSZ-13 is a promising solid catalyst for the methanol-to-olefin
silicate (TEOS, Sigma-Aldrich) and 26.4 g of TMAdaOH were
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mixed with 2.7 g of water at room temperature. The solution
was stirred for 2 h and then 0.7 g of aluminum ethoxide
(
2 5 3
Al(C H O) , Sigma-Aldrich) was added and mixed for addi-
tional 1 h. The resulting solution was then hydrothermally
treated in Teflon-lined Parr autoclaves at 413 K under static
conditions for 6 days. Sample filtration, washing, drying, and
calcination were identical to those of the SSZ-13-6 sample.
Analytical
X-ray powder diffraction (XRD) patterns were collected with a
Philips X'Pert X-ray diffractometer using CuKα radiation.
EDX spectral analysis integrated into a JEOL JSM-7400F scan-
ning electron microscope (SEM) was used to determine the
composition of the samples. The micropore volumes of the
Fig. 1 Temperature protocols for two treatments: 1) treatment 1 for
the acid catalyst and 2) treatment 2 for the dehydroxylated catalyst.
2
samples were determined using N adsorption isotherms
measured by using a Micromeritics ASAP 2020 instrument.
The samples were degassed at 623 K for 1 day using an
in-house-built silica tube. The adsorption isotherms were
obtained for the acid form of the samples and the dehydroxy-
lated samples.
furnace (C5232, Hoskins Mfg. Co.) and the temperature was
managed using a temperature controller (NC 74000, Omega
Engineering). Treatment protocols for treatments 1 and 2 are
illustrated in Fig. 1. The zeolites were first heated at 673 K for
Ammonia temperature-programmed desorption (TPD)
was measured by using a catalyst characterization system
2
h to remove water in the samples. To obtain acid catalysts,
the samples were then heated at 773 K for additional 3 h
treatment 1, solid black line). The other thermally treated
(Altamira Instruments, AMI-200iρ). About 30 mg of the sample
(
was loaded in a quartz U-tube reactor and the first TPD
experiment was carried out by heating the sample to 773 K
catalysts were prepared by heating the samples once again
at 823, 873, 973 (dashed grey lines), or 1073 K for 1.5 h
−
1
at a ramping rate of 10 K min . The acid form of the sam-
ple was obtained after this treatment. The ammonium form
of the sample was restored by exposing to NH3 flow for
(
1073 K – treatment 2, solid grey line). After treatments, the
reaction temperatures varied from 733 K to 803 K. The flow
rate of the reactant gas, which contains 5 mole% of propane
3
0 min at 373 K, and then the TCD signal was recorded up
(
2
Matheson Tri-Gas, research grade) diluted in N , was fixed
to 800 K. For the thermally treated catalyst, the sample was
first heated to 1073 K (and 873 K for SSZ-13-6) at a ramp rate
to 80 sccm. The conversion was kept low under 2% for the
monomolecular propane reaction. The product distribution
was collected by using a gas chromatography instrument
−
1
of 10 K min , cooled down, exposed to NH
3
flow for 30 min
at 373 K, and then the TCD signal was recorded up to 800 K.
(
GC, Shimadzu 2014). A thermal conductivity detector (TCD)
2
7
29
Al and Si NMR experiments were recorded using a
and a flame ionization detector (FID) were used to detect
hydrogen and hydrocarbons, respectively.
Bruker AVIII 500 solid-state NMR spectrometer operating at a
1
Larmor frequency of 500.138 MHz for H and 99.362 MHz for
2
9
Si. A 4 mm HX MAS probe was used for all measurements.
All spectra were collected at a MAS frequency of 10 kHz,
Results and discussion
Effect of treatment on the structure of the sample
2
9
controlled to within ±2Hz using a Bruker MAS controller. Si
single-pulse and cross polarization (CP) experiments were
performed on each sample. For Si single-pulse MAS experi-
XRD and N adsorption analyses. The XRD patterns of the
2
9
2
samples with two different Si/Al ratios indicated that the
calcined SSZ-13 samples (treatment 1) have high crystallinity
ments, a 90 degree pulse with a width of 4.3 μs was used and
1
29
the recycle delay was 30 s. For H- Si CP MAS experiment,
(
Fig. 2) and are free of any crystalline or amorphous impuri-
1
H 90° pulse duration was 2.5 μs, a linear amplitude ramp
ties. The full width at half maximum (FWHM) values of the
1
(
80~100%) on H was used with a contact time of 4.5 μs, and
(
−210) and (−311) reflections (2θ = 20.6° and 30.7°, respec-
2
7
the recycle delay was 5 s. For Al MAS with and without high-
power proton decoupling, a pulse with a width of 1.35 μs was
used and the recycle delay was 1 s.
tively) were 0.21° and 0.26° for SSZ-13-12 after treatment 1.
The FWHM of the (−210) reflection did not change, while the
FWHM of the (−311) reflection increased by approximately
1
1% (from 0.26° to 0.29°) after treatment 2. However, the
Thermal treatment of zeolite and catalysis
SSZ-13-12 sample was still highly crystalline after treat-
ment 2 because structural damage and an amorphous phase
(background), other than the slightly broadened XRD peaks,
were not observed. On the contrary, the XRD pattern of the
SSZ-13-6 sample after treatment 2 showed reduced peak
intensities and reveals the presence of an (minor) amorphous
The samples were treated in a quartz plug flow reactor
(
ID = 5 mm). To support the sample in the center of the
reactor, quartz wool was placed near the bottom of the reactor
tube and quartz chips were located between the quartz wool
and the sample. The reactor was heated using a cylindrical
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The micropore volumes of H-SSZ-13-12 and H-SSZ-13-6
3
−1
3
−1
after treatment 1 were 0.25 cm g and 0.27 cm g , respec-
tively, showing good agreement with the expected values
4
5
based on the chabazite structure and previous reports.
Table 1 shows that the volume of micropores in SSZ-13-12
decreased by 5% after treatment 2; there was no substantial
structural damage even if the decomposition of BAS led to a
minor reduction in the volume of the micropores. After
heating the SSZ-13-6 sample to 873 K, the microporous
volume did not decrease, but after heating the SSZ-13-6
samples to 973 K and 1073 K (treatment 2), there was a
significant reduction in the microporous volume, which is
evidence of structural damage. XRD analysis and N adsorp-
2
tion analysis show then that the sample with a Si/Al ratio of 6
(
SSZ-13-6) is less thermally stable and reveal some collapse of
the crystalline structure at temperatures over 973 K. Conse-
quently, the maximal treatment temperatures were 873 K for
SSZ-13-6 and 1073 K for SSZ-13-12.
Ammonia TPD experiment. The thermal conductivity
detector (TCD) signal profiles of ammonia desorption from
the samples are illustrated in Fig. 3. After treatment 2, the
amount of ammonia desorbed clearly decreases on SSZ-13-12
(
Fig. 3(a)); a large fraction of the Brønsted acid sites involved
Fig. 2 XRD patterns of (a) H-SSZ-13-12 and (b) H-SSZ-13-6: calcined
vs. thermally treated ones.
phase in the background, while the XRD pattern of the
SSZ-13-6 treated at 873 K was still highly crystalline. The
FWHM of the (−210) reflection (2θ = 20.7°) for SSZ-13-6
increased stepwise, i.e., from 0.18° to 0.19°, from 0.19° to
0
.20°, and from 0.20° to 0.24° as the treatment temperature
increased from 773 K (treatment 1) to 873 K, from 873 K to
73 K, and from 973 K to 1073 K (treatment 2), respectively.
9
The FWHM of the (−311) reflection (2θ = 20.7°) for SSZ-13-6
also increased from 0.26 to 0.29, from 0.29 to 0.31, and from
0
.31 to 0.33. The effective particle size of the crystal, calcu-
4
4
lated using the Scherrer equation with the FWHM of the
(
−210) reflection for SSZ-13-6, decreased from 46.9 to 44.4,
from 44.4 to 42.2, and from 42.2 to 35.1 nm as the treatment
temperatures increased from 773 K (treatment 1) to 873 K, to
9
73 K, and to 1073 K (treatment 2).
3
−1
Table 1 Micropore volumes (cm g )
Treatment protocol
H-SSZ-13-12
H-SSZ-13-6
Treatment 1
0.257
—
—
0.275
0.274
0.192
0.178
Treated at 873 K
Treated at 973 K
Treatment 2
Fig. 3 Ammonia TPD of (a) SSZ-13-12 after treatments 1 (black) and
2 (grey) and (b) SSZ-13-6 after treatment 1 (black), after treated at
873 K (grey), and after treatment 2 (light grey).
0.240
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in ammonia adsorption was destroyed by heating to 1073 K.
In addition, the position of the peak maximum in the signal
profile shifted to a lower temperature from 741 K to 705 K.
The temperature of the peak maximum is affected by the
number of acid sites in the zeolite, the change in the zeolite
structure, the carrier gas flow rate, or the amount of the sam-
band at around 60 ppm is assigned to the Al atoms in
tetrahedral coordination, and the additional band around
0 ppm is normally assigned to the extraframework Al in
4
7–52
octahedral coordination.
Fig. 4 shows that the NH -form
4
of SSZ-13-12 has predominantly tetrahedral Al sites. Extra-
framework octahedral Al sites were generated in the SSZ-13-12
sample after treatment 1, but this fraction decreased after treat-
ment 2. A broad band was detected between 50 and 40 ppm in
SSZ-13-12 after treatment 2. This broad band is related to the
extraframework pentacoordinated Al sites. The extraframework
4
6
ple. Thus, the reduction in the peak maximum temperature
reflects a smaller number of acid sites after treatment 2.
It has also been reported that ZSM-5 after thermal dehydroxy-
lation at 1093 K exhibited much lower TCD trace than ZSM-5
3
49,52
treated at 823 K, similarly indicating a reduction in the
Al sites can be related to the dealumination of the samples.
number of BAS during dehydroxylation and the formation
of non- or weakly interacting sites with ammonia. The sites
produced in the SSZ-13-12 sample by treatment 2 are then
only mildly acidic and do not strongly adsorb ammonia. For
SSZ-13-6, the TCD signal of the sample treated at 873 K did not
significantly change after treatment at 873 K (Fig. 3(b)). The
amount of ammonia desorbed decreased after treatment 2 due
to dehydroxylation or structural damage and the position of
the peak maximum cannot be determined.
The SSZ-13-6 sample also showed the peak at 60 ppm in the
NH -form, and a peak at around 0 ppm was generated after
4
thermal treatments. The peak at ~60 ppm becomes quite broad
and the fraction of the extraframework octahedral Al sites
increases after treatment 2.
2
9
Si MAS NMR spectra of the NH -form and thermally
4
treated samples are depicted in Fig. 5. The three different
resonances are assigned to the different Si(nAl) environ-
ments. The resonances at around −110 ppm, −104 ppm, and
−98 ppm are related to Si4, Si3Al1, and Si2Al2 sites, respec-
2
7
29
Solid-state Al and Si MAS NMR analysis. The local
4
7,48,53
structure of aluminum species in zeolites can be determined
tively.
The Si/Al compositions of the crystalline frame-
2
7
using Al MAS NMR measurements (Fig. 4). The resonance
work can be estimated from the relationship:
2
7
29
Fig. 4
Al NMR of a) SSZ-13-12 samples: NH
4
-form, treated at 773 K
Fig. 5
Si NMR of a) SSZ-13-12 samples: NH
4
-form, treated at 773 K
and 1073 K and b) SSZ-13-6 samples: NH
and 1073 K.
4
-form, treated at 773 K, 873 K,
and 1073 K and b) SSZ-13-6 samples: NH
873 K, and 1073 K.
4
-form, treated at 773 K,
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P
P
Si/Al = ISi(nAl)/0.25 nISi(nAl)
(1)
where n = 0 ~ 4 and ISi(nAl) is the area of the peak for the Si
environment. The estimated Si/Al ratios of SSZ-13-12 samples
were 13.1, 12.1, and 12.2 for the NH -form of the sample, the
4
sample after treatment 1, and the sample after treatment 2,
respectively (Table 2). The Si/Al ratios show little reduction
with increments in treatment temperature, but the ratios are
in a reasonable range of batch composition for the synthesis
procedure. The Si/Al ratios of the SSZ-13-6 sample after treat-
ment 1 and after treated at 873 K were all 6.4, almost same
as the ratio of NH
of the SSZ-13-6 sample after treatment 2 increased to 10.9
Table 2), indicating that the framework Al atoms were lost
after the treatment. From Al MAS NMR measurements, the
increase in the Si/Al ratio of the sample after treatment 2 can
be related to the generation of the extraframework octahedral
Al sites. At the same time, the increase in the Si/Al ratio could
be related to the structural damage in the framework of the
SSZ-13-6 sample because of low thermal stability.
4
-form SSZ-13-6. However, the Si/Al ratio
(
Fig. 6 Reaction rates for cracking and dehydrogenation of propane at
reaction temperature of 773 K over SSZ-13-12 and SSZ-13-6 treated at
different pretreatment temperatures.
2
7
The total reaction rates over the SSZ-13-12 samples treated
at 823, 873, 973, and 1073 K were similar to the reaction rates
observed over the SSZ-13-12 after treatment 1 (773 K), despite
of a slight increase in the reaction rates over the sample
Effect of treatment temperature on the monomolecular
propane reaction
3,4
treated at 823 K. Earlier reports and the TPD experiment
reported above (Fig. 3) have found that the number of BAS
decreases by dehydroxylation. It is notable that reaction rates
of SSZ-13-12 after thermal treatments are similar to the rates
of the samples treated at low temperature. This implies that
new active sites, different from BAS, can be generated and
can catalyze propane conversion. The cracking and dehydro-
genation selectivity showed noticeable changes even though
the total reaction rates over SSZ-13-12 samples treated at
different temperatures barely changed. For the SSZ-13-12
samples, dehydrogenation selectivity increases step by step as
the treatment temperature increases, and the reaction rates
of the dehydrogenation channel become identical to the reac-
Reaction rates and selectivity. The SSZ-13-12 samples
were treated at five different temperatures: 773 K (treatment 1)
and 823 K, 873 K, 973 K, and 1073 K (treatment 2). After being
treated at temperatures above 873 K, the samples showed very
high initial reaction rates of propane, an early period of deacti-
vation, and then stabilization. In contrast, the reaction rates
were stable for the catalyst treated at 773 K only. For the
samples treated at higher temperatures, the reaction rates were
recorded after the rates became stable. Conversions under 2%
indicate that the reactions were performed under differential
conditions; this is also verified by the stoichiometric ratio of
methane to ethylene for the cracking channel and of hydrogen
to propylene for the dehydrogenation channel. Fig. 6 shows
the reaction rates of the cracking and the dehydrogenation
channels at the reaction temperature of 773 K over SSZ-13-12
and SSZ-13-6 samples treated at different temperatures.
The acid form of SSZ-13-12 (after treatment 1) showed higher
−
1
tion rates of the cracking channel (0.32 mol s per gram
7
catalyst × 10 ) at 973 K. It can be assumed that the propor-
tion of sites different from BAS increases stepwise with
increasing treatment temperature and that the mechanism of
the reaction changed starting from the temperature of 973 K.
SSZ-13-6 samples were treated at 773 (treatment 1), 823,
and 873 K. The temperatures of 973 and 1073 K were not
employed because of the samples' low thermal stability as
discussed above. The acid form of SSZ-13-6 (after treatment 1)
−
1
reaction rates for the cracking channel (0.40 mol s per gram
7
catalyst × 10 ) than those for the dehydrogenation channel
−
1
7
(
0.17 mol s per gram catalyst × 10 ) and a cracking-to-
−
1
dehydrogenation ratio of ~2.39 (Table 3).
also showed higher reaction rates for cracking (0.58 mol s per
7
−1
gram catalyst × 10 ) than for dehydrogenation (0.22 mol s per
7
gram catalyst × 10 ), resulting in a cracking-to-dehydrogenation
29
Table 2 Normalized relative peak area from the Si MAS NMR of SSZ-13
ratio of ~2.59 (Table 3). The SSZ-13-6 treated at 823 exhibited a
samples
Treatment
NH -form
Treatment 1
Treatment 2
NH -form
4
Treatment 1
Si4
Si3Al1
Si2Al2
Si/Al ratio
Table 3 Cracking-to-dehydrogenation ratios at reaction temperature of
SSZ-13-12
SSZ-13-6
4
0.73
0.68
0.69
0.42
0.44
0.44
0.66
0.24
0.31
0.30
0.52
0.49
0.49
0.32
0.03
0.01
0.02
0.07
0.07
0.06
0.02
13.1
12.1
12.2
6.2
6.4
6.4
773 K for SSZ-13-12 and SSZ-13-6 treated at different temperatures
Treatment
temp., K
773 (treatment 1) 823 873 973 1073 (treatment 2)
1.88 1.35 1.00 1.03
Treated at 873 K
Treatment 2
SSZ-13-12 2.39
SSZ-13-6 2.59
10.9
0.99 0.98
—
—
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little higher total reaction rate, and the SSZ-13-6 treated at 873 K
showed similar total reaction rates to that over the SSZ-13-6
after treatment 1 (773 K). The SSZ-13-6 samples showed
very low reaction rates after treatment at 973 K and 1073 K
for both samples even if the treatment temperatures for
SSZ-13-12 and SSZ-13-6 are different.
Reaction mechanism. The reaction rates and selectivity
patterns suggest that the reaction mechanism on the
pretreated SSZ-13 samples is different from the typical
protolytic mechanism on the acid catalysts (treatment 1). An
identical trend of selectivity change in the propane reaction
over H-ZSM-5, before and after dehydroxylation, has been
(
data not shown), as expected given that their structure was
damaged by the treatment (see above).
The SSZ-13-6 samples showed drastic changes in selectivity
with a small change in treatment temperatures. After a
pretreatment temperature of 823 K, the cracking-to-
dehydrogenation ratio decreased from 2.59 to 0.99 with just a
3
reported. The formation of redox sites and Lewis acid
sites has a role in the reactivity and selectivity of the zeolite
3
5
0 degree increase in the treatment temperature (Table 3). This
samples and these sites have the ability to extract electrons
4
drastic change in selectivity without apparent decomposition
of BAS (as shown by the ammonia TPD experiments) cannot
be explained at this time.
from neutral organic materials, such as naphthalene.
A kinetic analysis of the data has been carried out by
using following relationships:
In Fig. 7, the reaction rates of cracking and dehydrogena-
tion over SSZ-13-12 and SSZ-13-6 were compared for pre-
treatment temperatures of 773 K and for the respective
highest treatment temperatures (1073 K for SSZ-13-12 and
K P
1
C3H8
r  k3
(2)
(3)
1
 K P  KP
 KP
1
C3H8
4
C3H6 4 C2H4
8
73 K for SSZ-13-6). The cracking rates decreased slightly
r = k K P
= kappPC3H8
H
8
3
1
C
3
after thermal treatment for both samples, but dehydrogena-
tion rates were significantly increased. Note that the change
in selectivity after complete dehydroxylation is similar
Under the conditions for monomolecular propane reac-
tion, such as low partial pressure, low conversion, and high
reaction temperatures, the rate expression can be simplified
to a first-order rate equation (eqn (3)). The measured activa-
tion energies are reported in Table 4. For H-SSZ-13-12 after
−
1
treatment 1, the activation energy for cracking is 170 kJ mol ,
−
1
a little lower than that for dehydrogenation (174 kJ mol ).
H-SSZ-13-6 after treatment 1 also exhibited a higher activation
−
1
energy for dehydrogenation (208 kJ mol ) than that for crack-
−
1
ing (179 kJ mol ). A higher activation barrier is expected for
dehydrogenation than for cracking because of the relative
stabilities of the protonated C–H and C–C bonds in the
3
0
propane molecule. The difference in the activation energy
Ea,meas for cracking < Ea,meas for dehydrogenation) for the
(
acid catalyst was consistent with previous reports. At the same
time, the activation energy values showed good agreement
3
,23
with the experimental values for ZSM-5 samples
and
the values determined from the simulation of the propane
5
3,54
reaction over the acidic chabazite.
The activation energies for reactions with SSZ-13-12
samples show a stepwise decrease as the treatment tempera-
−
1
−1
tures increased (146 kJ mol and 114 kJ mol , for cracking
and dehydrogenation, respectively), which is consistent with
Table 4 Measured activation energies for SSZ-13-12 and SSZ-13-6
treated at different temperatures
−1
Measured activation energy, Ea,meas (kJ mol
SSZ-13-12 SSZ-13-6
Cracking Dehydrogenation Cracking Dehydrogenation
)
Treatment
temp., K
773
170
174
179
208
(treatment 1)
8
8
973
1073
(treatment 2)
23
73
170
165
114
146
161
158
84
178
179
206
198
Fig. 7 Log scale reaction rates for cracking and dehydrogenation of
propane over a) SSZ-13-12 after treatments 1 and 2 and b) SSZ-13-6
after treatment 1 and after treated at 873 K.
114
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the change in selectivity. For the SSZ-13-12 sample after
treatment 2, the activation energy for cracking was greater
than that for dehydrogenation. The differences in activation
energy indicate that the different intermediates are generated
during the propane conversion over the SSZ-13-12 sample
after treatment 2 compared to the sample after treatment 1.
The NMR and ammonium TPD experiments indicate that the
selectivity change in SSZ-13-12 is related to decomposition of
BAS. In a previous report, different activation energies were
also observed for propane conversion over ZSM-5 before and
propylene or the ethylene radical cations. Note that the activa-
tion energy at the treatment temperature of 973 K is exception-
ally lower than the values observed at other temperatures. It is
possible that 1) some new sites, which have the different ability
of the catalytic reaction than redox sites or LAS, were generated
at 973 K but disappeared at 1073 K, and 2) some specific
portion of the newly generated sites may affect the high dehy-
drogenation selectivity and the kinetics of the reaction.
The activation energies of SSZ-13-6 treated at 823 K were
indistinguishable from those of SSZ-13-6 after treatment
1 (the acid catalyst). The activation energy of the dehydroge-
3
after thermal treatments. It was suggested that redox sites
−
1
−1
are generated in ZSM-5 after dehydroxylation, the sites that
nation reaction decreased by ~10 kJ mol (208 kJ mol
3
−1
can extract an electron from neutral adsorbed molecules.
for SSZ-13-6 after treatment 1 to 198 kJ mol for SSZ-13-6
treated at 873 K), while the values of the cracking reaction
did not change. At this time, the molecular origin of
this change in selectivity is not clear. It may be that only a
small number of new dehydrogenation sites are formed given
that the ammonia TPD traces are nearly unchanged and
that the change in selectivity reflects the contributions
of these sites.
Here, the activation energy for the reaction with SSZ-13-12
was significantly lower after treatment 2, as in the case of
ZSM-5. We conclude that new sites must be generated by the
thermal treatment and their effect increases as the treatment
temperature increases because more BAS are decomposed.
Consequently, the properties of the thermally treated catalysts
lead to greater selectivity for the dehydrogenation channel in
propane conversion.
The proposed redox catalytic cycle over the dehydroxylated
SSZ-13-12 sample is depicted in Scheme 3. An electron is
extracted from the propane molecule, resulting in the forma-
tion of a propane radical cation. The propane radical cation
then dissociates into a propylene radical cation with hydro-
gen or an ethylene radical cation with methane. The reaction
cycle is completed after the electron is given back to the
Conclusions
The structure–activity relationship of the SSZ-13 samples
after thermal treatments is summarized in Table 5. The low
thermal stability of SSZ-13-6 limited the pretreatment temper-
ature to a maximum of 873 K. Therefore, thermal decomposi-
tion of BAS was not observed in the SSZ-13-6 sample at given
treatment temperatures (823 and 873 K) even if the hetero-
lytic mechanism is expected to be predominant due to
locations of BAS being close to each other. Instead, structural
damage was observed for the samples after treated over
8
73 K. The reaction rates of the propane conversion
were similar for the acid catalyst and the thermally treated
catalysts. However, SSZ-13-6 treated at 873 K showed higher
selectivity in the dehydrogenation but the identical kinetic
parameters. The observed results indicate that the selectivity
change is not a result of generation of LAS or redox sites, and
the change in the cell volume after thermal treatment may
lead to the confinement effect, promoting C–H cleavage.
Different measured activation energies were observed for the
SSZ-13-12 sample treated at 1073 K despite of higher selectivity
in the dehydrogenation channel. The decreased activation
energies and the change in selectivity suggest the generation
of different active sites, other than BAS, in SSZ-13-12.
Scheme 3 Redox catalytic cycle for propane conversion over zeolite
after high-temperature treatment.
Table 5 Summary of the effect of treatment temperatures on SSZ-13 samples
SSZ-13-12
SSZ-13-6
Structure change with
increase in treatment
temperature
<973 K
≥973 K
Partial dehydroxylation
Complete dehydroxylation
<973 K
≥973 K
Not changed
Structural damage
Selectivity change
Activation energy change
Assumed generated sites
Gradually (773 to 973 K)
Decrease gradually
Redox sites
Drastically (773 to 823 K)
Not changed
New sites & structural
damage (≥973 K)
(
electron-deficient sites)
Lewis acid sites
&
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After thermal treatments, LAS and the redox sites can be
generated and could play an important role in the catalytic
activity of the SSZ-13-12 sample. The observations in this
work confirm the existence of different active sites other than
BAS and their role in increasing the catalytic activity for
hydrocarbon conversion.
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