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Hualan Zhou et al. / Chinese Journal of Catalysis 38 (2017) 529–536
lysts are deficient in terms of poor stability and lack of durabil‐
ity during recovery [1,10,11]. Thus, the development of novel
supports to improve the stability of these catalysts is not only
highly desirable but also timely.
H‐SUZ‐4 or H‐ZSM‐5 with an aqueous mixture of H2PtCl6 and
SnCl4 (H2PtCl6 = 5 mg/mL, SnCl4 = 5.85 mg/mL) and with
aqueous NaCl (0.5 mol/L). The impregnated samples were
dried at 110 °C for 4 h, calcined at 520 °C for 4 h, and then
dechlorinated in air containing water vapor at 530 °C for 4 h.
In recent years, zeolites such as SUZ‐4 and ZSM‐5 have been
found to be good supports because of their high surface areas,
good thermal stabilities, large pore volumes and tunable acidity
[12–14]. SUZ‐4 is a new type of synthetic zeolite patented by
the British Petroleum Company in 1992. In the three dimen‐
sional topological structure of this material, straight
ten‐membered channels intersect with two eight‐membered
channels at an angle of approximately 74°, which is similar to
the structure of ZSM‐5 zeolite [15,16]. SUZ‐4‐supported cata‐
lysts have been widely used in many processes, such as the
conversion of n‐hexane [16], the synthesis of dimethyl ether
from methanol [17], and the elimination of nitrogen oxides
[18]. However, there have not yet been any reports concerning
the application of SUZ‐4‐supported catalysts to propane dehy‐
drogenation, while ZSM‐5‐supported catalysts have attracted
significant attention in this regard [13,19]. In contrast to
γ‐Al2O3, the three‐dimensional microporous ZSM‐5 zeolite has
a well‐defined, ten‐membered, ring‐crossed channel system
that prevents the formation of large hydrocarbon molecules,
thus improving the catalyst’s stability [13]. Recently, Zhou’s
group [20] investigated propane dehydrogenation over
ZSM‐5‐supported Pt‐Sn catalysts and found that the propylene
selectivity was significantly improved by introducing promot‐
ers to neutralize the support acidity. The addition of hydrogen
to the reaction system effectively inhibited the cracking of pro‐
pane to C1 and C2 products and also reduced carbon deposi‐
tion on the catalyst surface, thus improving both the dehydro‐
genation selectivity and catalytic stability [21]. Despite this, the
ZSM‐5‐based catalysts were still easily deactivated by carbon
deposition under the chosen reaction conditions. To resolve
this issue, our own group developed the SUZ‐4‐supported cat‐
alyst PtSnNa/SUZ‐4, which afforded a 20% propylene yield.
Although propylene can be obtained in similar 18%–23%
yields by increasing the Sn loading when using a PtSn/ZSM‐5
catalyst [22], our catalyst has the advantage of being more ro‐
bust and undergoing very little deactivation due to carbon
deposition.
2.2. Catalyst characterization
The powder X‐ray diffraction (XRD) patterns of all samples
were obtained with a Philips X’pert pro diffractometer using Cu
Kα radiation at 40 kV and 40 mA, from 5° to 50°. Surface areas
were calculated by the BET method based on N2 adsorption
isotherms recorded at the temperature of liquid nitrogen using
a Micromeritics ASAP2010 analyzer. The samples were
degassed at 300 °C and 0.133 Pa prior to analysis, after which
isotherms were acquired at −196 °C. NH3‐TPD profiles of the
specimens were obtained in a flow‐type fixed‐bed reactor at
ambient pressure. The catalysts were pre‐treated at 500 °C for
2 h under an Ar flow. The NH3 adsorption temperature was 100
°C, and the temperature was raised at a rate of 10 °C/min. The
desorbed NH3 was detected by a gas chromatograph (GC)
equipped with a thermal conductivity detector (TCD).
H2 chemisorption on the supported PtSnNa catalysts was
assessed both before and after the propane dehydrogenation
reaction according to a previously described procedure [25].
Each of the catalysts was reduced in a H2 flow at 500 °C for 2 h
and then out‐gassed in an Ar flow at 540 °C for 2 h before H2
chemisorption measurements.
The amount of carbonaceous material deposited on each
catalyst during the propane dehydrogenation reaction was
measured using thermo‐gravimetric (TG) analysis (STA
449C‐Thermal star 300 TA‐MS apparatus). Catalyst samples
(each approximately 0.02 g) obtained after a 10‐h reaction
were heated from room temperature to 900 °C in O2 (at 25
mL/min) at a heating rate of 10 °C/min, and the amounts of
coke on the specimens was calculated from the resulting TG
curves.
Temperature‐programmed oxidation (TPO) was deter‐
mined with the same apparatus as used for the H2
chemisorption experiments. An approximately 0.1‐g sample
was placed in a quartz reactor and then heated to 800 °C in a
mixture of O2 (at 3.0 mL/min) and Ar (at 30 mL/min) at a
heating rate of 10 °C/min. Temperature‐programmed
reduction (TPR) was performed using the same apparatus em‐
ployed during the TPO assessments. In these trials, approxi‐
mately 0.1‐g samples were placed in a quartz reactor and sub‐
sequently heated in a flow of 5% H2‐95% Ar (at 20 mL/min) at
a heating rate of 10 °C/min.
2. Experimental
2.1. Catalyst preparation
An SUZ‐4 zeolite with the molar ratio SiO2/Al2O3 = 21 and a
ZSM‐5 zeolite with the molar ratio SiO2/Al2O3 = 20 were
prepared by methods previously described in the literature
[23,24]. In each case, the resulting solid phase was filtered,
washed with distilled water several times, dried at 110 °C for
12 h and then calcined at 550 °C for 4 h. This was followed by
Purposely poisoned catalysts (containing 0.02% S) were
prepared by impregnating the reduced PtSnNa/SUZ‐4 or
PtSnNa/ZSM‐5 catalysts with an ethanol solution of
dibenzothiophene (0.2 mg/mL), followed by flushing with
nitrogen for 1 to 2 h to evaporate residual ethanol and drying at
110 °C for 4 h.
All catalysts were reduced in H2 at 500 °C for 2 h before
catalytic evaluation. The propane dehydrogenation reaction
was carried out in a quartz tubular micro‐reactor under
+
NH4 exchange in aqueous NH4Cl (1 mol/L). H‐SUZ‐4 and
H‐ZSM‐5 were obtained by calcining the ammonium forms of
SUZ‐4 and ZSM‐5 at 550 °C for 4 h. PtSnNa catalysts supported
on either the SUZ‐4 or the ZSM‐5 zeolite (Pt = 0.5%, Sn = 2.0%,
Na = 1.0%) were prepared by sequentially impregnating the