Catalytic performance of gallium oxide based-catalysts for the propane dehydrogenation reaction: effects of support and loading amount

Article abstract of DOI:10.1039/c6ra27204e

The different materials (ZSM-5, SBA-15, γ-Al₂O₃ and SiO₂) were used as supports for Ga₂O₃-based catalysts for the propane dehydrogenation reaction, and the effect of Ga₂O₃ content (1-9 wt%) for xGa₂O₃/SBA-15 catalysts on the catalytic activity was discussed. It is found that the supports determine the porous features, the state and dispersion of Ga species, and the acid-base properties of the corresponding catalysts. The existence of strong acid sites in catalyst can lead to more well-dispersed Ga species. The satisfied catalytic performances are obtained over 5Ga₂O₃/ZSM-5 and 5Ga₂O₃/SBA-15 catalysts. Among the Ga₂O₃-based catalysts with different supports, the 5Ga₂O₃/ZSM-5 sample exhibits the highest catalytic activity, which possesses the maximum well-dispersed gallium species and high dehydrogenation efficiency gallium ions (Ga^δ+ cations, δ < 2), and the 5Ga₂O₃/SBA-15 catalyst exhibits the highest catalytic stability. Furthermore, as for the xGa₂O₃/SBA-15 samples, the 5Ga₂O₃/SBA-15 sample exhibits the best catalytic performance. The initial propane conversion and propylene selectivity are above 32.0% and 90.0% respectively, and a final propane conversion of 17.0% is obtained after 30 h reaction. With the increase of Ga loading, the Ga species are easily agglomerated and destroy the structural integrity of the SBA-15 support, which is unfavorable to the propane dehydrogenation reaction.

Full text of DOI:10.1039/c6ra27204e

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Catalytic performance of gallium oxide based-
catalysts for the propane dehydrogenation
reaction: effects of support and loading amount
Cite this: RSC Adv., 2017, 7, 4710
*
*
Chun-Tao Shao, Wan-Zhong Lang, Xi Yan and Ya-Jun Guo
The different materials (ZSM-5, SBA-15, g-Al2O3 and SiO2) were used as supports for Ga2O3-based catalysts
for the propane dehydrogenation reaction, and the effect of Ga2O3 content (1–9 wt%) for xGa2O3/SBA-15
catalysts on the catalytic activity was discussed. It is found that the supports determine the porous features,
the state and dispersion of Ga species, and the acid–base properties of the corresponding catalysts. The
existence of strong acid sites in catalyst can lead to more well-dispersed Ga species. The satisfied
catalytic performances are obtained over 5Ga2O3/ZSM-5 and 5Ga2O3/SBA-15 catalysts. Among the
Ga2O3-based catalysts with different supports, the 5Ga2O3/ZSM-5 sample exhibits the highest catalytic
activity, which possesses the maximum well-dispersed gallium species and high dehydrogenation
efficiency gallium ions (Gad+ cations, d < 2), and the 5Ga2O3/SBA-15 catalyst exhibits the highest catalytic
stability. Furthermore, as for the xGa2O3/SBA-15 samples, the 5Ga2O3/SBA-15 sample exhibits the best
catalytic performance. The initial propane conversion and propylene selectivity are above 32.0% and
90.0% respectively, and a final propane conversion of 17.0% is obtained after 30 h reaction. With the
increase of Ga loading, the Ga species are easily agglomerated and destroy the structural integrity of the
SBA-15 support, which is unfavorable to the propane dehydrogenation reaction.
Received 23rd November 2016
Accepted 27th December 2016
DOI: 10.1039/c6ra27204e
www.rsc.org/advances
excellent catalytic efficiency as compared to conventional Cr2O3-
and V2O5-based catalysts.12–16 For instance, Nakagawa et al.14
1 Introduction
proposed that the commercial Ga2O3 had exceptionally high
activity for the dehydrogenation of ethane to ethylene, and they
found that b-Ga2O3 exhibited the highest dehydrogenation
activity among all of the polymorphs due to its abundant
surface acid sites.
Propylene is a major raw chemical for the petrochemical
industry. In recent years, more attention has been paid to the
catalytic dehydrogenation of propane to propylene because the
growing demand for propylene is beyond the production
capacity of conventional hydrocarbon steam cracking and
catalytic cracking processes.1–3 The catalytic dehydrogenation of
propane is an endothermic process, which needs a relatively
high reaction temperature to obtain a high propylene yield.
Nevertheless, under such high temperatures, the undesirable
side reactions such as hydrogenolysis, cracking and coke
deposition are inevitable.4,5
Currently, many studies are devoted to developing novel and
potential catalysts in propane dehydrogenation (PDH), such as
ceria-based catalysts,6 calcined hydrotalcite-supported plat-
inum catalysts,7 Sn/SiO2 catalysts,8 Pd-based catalysts,9 meso-
porous carbons, gallium oxide-based catalysts10,11 and so on.
Among these, gallium oxide-based catalysts have been recog-
nized as new promising alkane dehydrogenation materials due
to their unique capability to activate hydrocarbon species and
The support effects of catalysts were also considered to be an
important topic. Previous studies11,16–19 reported that the Ga2O3
species were dispersed on different supports including TiO2,
MgO, Al2O3, ZrO2, ZSM-5, HZSM-48 and MWW zeolites for
alkane dehydrogenation reaction. Many literatures veried that
acidity of support evidently inuenced the catalytic perfor-
mance of the supported catalysts. For example, Shen et al.20
reported that the stability and selectivity of Ga2O3/HZSM-5
catalyst in the propane dehydrogenation were enhanced by
increasing the Si/Al ratio of HZSM-5 support. XU et al.16 reported
that Ga2O3/TiO2, Ga2O3/Al2O3 and Ga2O3/ZrO2 showed better
activity for the dehydrogenation of propane to propylene than
Ga2O3/SiO2 and Ga2O3/MgO due to the more acid sites in
medium to strong strength. Wang et al.19 revealed that Ga/ITQ-2
exhibited higher selectivity and better stability in propane
dehydrogenation reaction as compared with those of Ga/MCM-
22, due to the higher surface area and relatively less strong acid
sites.
The Education Ministry Key Laboratory of Resource Chemistry and Shanghai Key
Laboratory of Rare Earth Functional Materials, Department of Chemistry and
Chemical Engineering, Shanghai Normal University, 100 Guilin Road, Shanghai
200234, China. E-mail: wzlang@shnu.edu.cn; guoyajun@shnu.edu.cn; Fax: +86-21-
64321951; Tel: +86-21-64321951
Since gallium oxide-based catalysts were found to have good
alkane dehydrogenation properties,14,15,21 several investigations
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attempted to elucidate the reasons of high catalytic perfor- aqueous solution and stirred for 0.5 h until fully dissolved, and
mance. The previous studies reported that the gallium oxides then 6.4 g of tetraethyl orthosilicate (TEOS) was slowly dripped to
(Ga2O3, GaO+) were the most species presented on the catalyst the prepared mixture solution with strong stirring at 40 C for
ꢁ
aer calcination and were readily reduced during pretreatment 24 h to get white gel. The resulting gel was subsequently trans-
with hydrogen.16,22–24 The various gallium species (Ga2O3, GaO+ ferred into a Teon autoclave and held at 100 C for 24 h. The
ꢁ
and Ga+) as active constituents exhibited different perfor- obtained sample was ltered and washed using deionized water,
mances for light alkane dehydrogenation reaction.25 In addi- and dried in a desiccator overnight. Then, the sample was
tion, Nowak et al.26 also reported the dispersion of Ga2O3 calcined in air at 550 ꢁC for 4 h to remove the structure-directing
species varied with different supports. However, the effects of agent to obtain nal SBA-15 powder. Then, the synthesized SBA-
supports on the state of gallium species have not been fully 15 together with other three commercial materials including g-
understood, especially the causes of these changes.
Al2O3 (SBET: 180 m2 gꢀ1, 80–100 mesh), ZSM-5 (Si/Al ¼ 45–80) and
Among these support materials, alumina is extensively used SiO2 (particle size ¼ 50 nm) were used as supports. The sup-
as catalyst support due to its surface acidic properties and ported Ga2O3 catalysts were prepared by impregnating the
desirable textual properties. Nevertheless, the stability of cata- different supports with an aqueous solution of Ga(NO3)3$xH2O
lyst is still not very satisfactory.27,28 As regards ZSM-5 zeolite, it (Aldrich). Aerward, the prepared samples were aged at room
has some particular physical properties such as unique three- temperature for 4 h, and then the catalysts were dried overnight
dimensional channels, relatively large surface area and adjust- in air at 100 ꢁC, followed by calcination at 550 ꢁC for 4 h in air.
able Si/Al ratio. Especially it can hinder the formation of large The 5Ga2O3/X and xGa2O3/SBA-15 catalysts were got. Unless
hydrocarbon molecules due to the unique framework and otherwise specied, as for the 5Ga2O3/X catalysts, the loading of
channel structure, which are benecial to the catalytic stability. Ga2O3 was 5 wt%, where X represents the different support. The
Therefore, ZSM-5 is widely used in propane dehydrogenation prex x of xGa2O3/SBA-15 samples indicates the different
reaction.29,30 Besides, as advanced mesoporous materials, SBA- amounts of Ga2O3 content varying from 1 to 9 wt%.
15 and nano-silica are oen employed as supports for cata-
lysts. Generally, SBA-15 is characterized by its ordered porous
structure, high specic surface area and high thermal stability,
2.2 Catalyst characterizations
and the certain pore volume of mesoporous materials and the
2.2.1 XRD. The X-ray diffraction (XRD) patterns of different
free of acid sites can greatly resist the formation of coke.31,32 catalysts were obtained on a Bragg–Brentano diffract meter
Nano-silica also has the characteristics of large surface area, (Rigaku D/Max-2000) with monochromatic Cu Ka radiation (l ¼
chemical stability and cheapness.33 It is obvious that the cata- 1.5418 A). The XRD data were collected from 5 to 80 with a scan
lysts using different supports may show the distinctive catalytic speed of 2ꢁ minꢀ1. The X-ray tube was manipulated at 40 kV and
properties. Hence it is necessary to discuss the inuences of the 30 mA.
ꢁ
˚
different supports on the catalytic performances for Ga2O3-
2.2.2 Low temperature N2 adsorption/desorption. The
based catalysts in propane dehydrogenation. Nevertheless, textural properties of different catalysts were measured by N2
according to what I'm informed, the detailed investigations of adsorption–desorption at liquid nitrogen temperature using an
different supports on the catalytic performances for Ga2O3- automatic analyzer (NOVA 4000, Quantachrome, USA). Before
ꢁ
based catalysts are rarely reported, especially with SBA-15 adsorption, the catalysts were degassed for 4 h at 300 C. The
material as support.
specic surface areas of the samples were calculated using the
The objective of this work is to study the inuences of Brunauer–Emmett–Teller (BET) method. The pore structural
different supports on the state of gallium and the catalytic data were analyzed by the BJH (Barrett–Joyner–Halenda)
performances of Ga2O3-based catalysts for propane dehydro- method using Halsey equation for multilayer thickness.
genation, and further discuss the optimum Ga2O3 loading
2.2.3 FESEM. The morphologies of different catalysts were
content and reaction parameters of Ga2O3/SBA-15 catalysts in examined by a eld emission scanning electron microscopy
propane dehydrogenation. A series of different supports (ZSM- (FESEM, HITACHI S-4800) operated at 5.0 kV.
5, SBA-15, g-Al2O3 and SiO2) were prepared, and then loaded
2.2.4 TEM. The transmission electron microscopy (TEM)
with Ga2O3 species by impregnation method. The as-prepared images of different catalysts were analyzed using a JEM-2010
samples were characterized by several state-of-art characteriza- microscope operated at 200 kV.
tions. The structure–activity relationships of the catalysts were
analyzed and elucidated.
2.2.5 NH3-TPD. The acidity of different catalysts was
detected by temperature-programmed desorption of ammonia
(NH3-TPD). About 0.05 g of sample was placed into a quartz
reactor between two quartz wool plugs. The sample was pꢀre1-
2 Experimental
2.1 Catalyst preparation
treated at 400 ꢁC for 1 h under a dry helium ow (30 mL min
)
before being cooled to 120 ꢁC and saturated with adsorbed gas.
The mesoporous SBA-15 was prepared according to the previous A thermal conductivity detector (TCD) was carried out from 120
literature.4 The triblock copolymer P123 (EO20PO70EO20, Mn ¼ to 800 ꢁC with an increasing rate of 10 ꢁC minꢀ1 to record NH3-
5800, from Aldrich) was used as the structure-directing agent, TPD prole.
and tetraethyl orthosilicate (TEOS) was used as the silica source.
2.2.6 H2-TPR. The temperature-programmed reduction
In brief, 4 g of P123 was added in 160 mL of 2 mol Lꢀ1 HCl (TPR) for different catalysts was measured in a programmable
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temperature system. Prior to the analysis, thꢁe sample (0.2 g) was
treated by dry argon (15 mL minꢀ1) at 500 C for 1 h and then
ꢁ
the baseline was stabilized at 25 C for 60 min. Subsequently,
the quartz reactor was heated by a mixed ow of 10% H2 in Ar
ꢁ
ꢁ
from room temperature to 950 C with a heating rate of 10 C
minꢀ1. Finally, a TCD cell was employed to monitor and record
the consumption proles of H2.
2.2.7 XPS. The X-ray photoelectron spectra (XPS) of
different catalysts were recorded on Perkin-Elmer PHI 5000C
ESCA using Al Ka radiation. All the samples were reduced in
ꢁ
a hydrogen ow at 580 C for 2.5 h. The binding energies (BE)
were calibrated using the C1s level at 284.8 eV as an internal
standard.
2.2.8 TPO. Temperature-programmed oxidation (TPO)
experiments for the spent catalysts were conducted in
a programmable temperature system. Prior to TPO analysis, the
spent catalyst (0.05 g) was purged in owing N2 (15 mL minꢀ1
)
at 500 ꢁC for 1 h. Then, the temperature was lowered to 40 ꢁC to
steady the baseline. Subsequently, the reactor was heated by
a mixed gas ow of 10% O2 in He from room temperature to
800 ꢁC at the rate of 10 ꢁC minꢀ1. Finally, a thermal conductivity
detector (TCD) cell was used to determine CO2.
2.2.9 TG. Thermogravimetric analysis was used to deter-
mine the coke content on the spent catalysts with a LCT ther-
mogravimetric analyzer (DTG-60H), which was conducted ꢀfr1om
ꢁ
ꢁ
room temperature to 800 C at a rising rate of 20 C min in
a 5% O2–95% He mixed ow (30 mL minꢀ1).
2.3 Propane dehydrogenation reaction
The propane dehydrogenation reactions were performed in
a quartz x-bed reactor (8 mm in diameter). The catalyst (0.2 g)
was placed into the center of reactor, and the feed gas including
C3H8 and Ar (C3H8/Ar molar ratio ¼ 1 : 19, WHSV ¼ 0.6 hꢀ1) was
fed to the reactor. The propane dehydrogenation reactions were
ꢁ
carried out at 620 C and atmosphere pressure. A gas chroma-
tography (GC, SP-6890) equipped with a ame ionization
detector (FID) and an AT-PLOTPORA-Q capillary column was
employed to analyze the gas compositions. The propane
conversion and propylene selectivity were calculated by
formulas as:
C3H8 conversion ¼ C3H8in ꢀ C3H8out/C3H8in
C3H6 selectivity ¼ C3H6out/(C3H8in ꢀ C3H8out
(1)
(2)
)
where C3H8in and C3H8out are the propane content in feed and
exit steams respectively; C3H6out is the propylene content in exit
steam.
3 Results and discussion
3.1 Textural properties
To evaluate the textural properties of catalysts, the low-
temperature N2-physisorption isotherms were conducted and
shown in Fig. 1(a) and (b). All the xGa2O3/SBA-15 and 5Ga2O3/
Al2O3 catalysts exhibit typical type IV adsorption isotherm with
a H1-type hysteresis loop as dened by IUPAC, implying the
Fig.
1
(a) and (b) Low temperature N2 adsorption–desorption
isotherms of xGa2O3/SBA-15, 5Ga2O3/Al2O3, 5Ga2O3/SiO2, 5Ga2O3/
ZSM-5 catalysts and (c) the corresponding pore size distributions of
xGa2O3/SBA-15 catalysts.
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presence of mesoporous structure. Moreover, the isotherms of wt%, the peak intensity of (100) diffraction plane decreases
xGa2O3/SBA-15 catalysts show comparatively sharp step with the signicantly, and the peak moves to smaller angle location,
increase of relative pressure (P/P0) as compared with that of suggesting that the introduction of overhigh Ga2O3 weakens the
5Ga2O3/Al2O3 sample. According to the previous studies,34 this degree of long-range order of SBA-15.36,37
is the characteristic of capillary condensation of nitrogen in
ordered mesopores. The low-temperature N2-physisorption
isotherm for 5Ga2O3/ZSM-5 is also illustrated in Fig. 1(a). It
3.3 FESEM and TEM
The FESEM images of the different catalysts are presented in
shows type I adsorption isotherm with a plateau at higher
Fig. 3. As can be seen, the FESEM image of 5Ga2O3/ZSM-5
relative pressure in accordance with the microporous nature of
sample reveals highly crystalline zeolites of prismatic struc-
ture. In the cases of xGa2O3/SBA-15, they all show bound worm-
like feature with a relatively uniform size of about 1 mm.
limited mesoporosity of the samples (BDDT classications31).
Furthermore, the type H3 (according to IUPAC) hysteresis loop
can be noticed for 5Ga2O3/SiO2 sample, which oen corre-
However, with the increase of Ga2O3 content from 1 wt% to 9
sponds with the slit-shaped pores.35 The textural properties of
wt%, the long-range worm-like morphology becomes chaos,
and the order degree of structure declines. It can be explained
that the increase of Ga2O3 content leads to the collapse of
molecular sieve structure.
The ordered porous structures of 5Ga2O3/ZSM-5 and xGa2O3/
SBA-15 samples were further observed by TEM. The images are
collected and showed in Fig. 4. From the TEM image of 5Ga2O3/
ZSM-5 sample, the well-ordered lattice fringes are clearly
the different catalysts are listed in Table 1. As can be seen, the
specic surface areas of all the catalysts are mainly determined
by the support materials. From Table 1, the specic surface area
(SBET) values of different Ga2O3-based catalysts are in the
following sequence: SBA-15 > ZSM-5 > Al2O3 > SiO2. As regards
xGa2O3/SBA-15 catalysts in Fig. 1(b), with the increase of
gallium oxide loading from 1 wt% to 9 wt%, the SBET and pore
volume (Vp) notably decrease, implying that gallium species
distinguished in the high-magnication image, which
might penetrate into the mesoporous channels of SBA-15 and
lead the channels to be partially blocked. Moreover, as exhibited
in Fig. 2(c), the pore size distributions of xGa2O3/SBA-15 cata-
lysts are measured by the BJH model according to the desorp-
tion branches of the isotherms, and the pore diameter (Dp)
varies little with the increase of Ga2O3 addition.
3.2 XRD
The XRD patterns of the different catalysts are depicted in Fig. 2.
Apparently, the characteristic peaks of the corresponding
supports are found for all the samples, indicating that the
original structure of the support is not destroyed during the
catalyst preparation process. In addition, in the wide-angle
region (Fig. 2(a)), these materials do not exhibit the character-
istic peaks of Ga2O3 crystalline phases, probably due to their
small particle size and/or low concentration below XRD detec-
tion limitation. Moreover, the low-angle XRD patterns of
xGa2O3/SBA-15 samples are depicted in Fig. 2(b). As can be seen,
the unique diffraction planes of (100), (110) and (200) are
ascribed to the ordered two-dimensional hexagonal meso-
porous structures, and the diffraction peaks of mesoporous
structure vary little with the increase of Ga2O3 content from 1 to
5 wt%. As the Ga2O3 loading further increases to 7 wt% and 9
Table 1 Textural properties of the different catalysts
Samples
SBET (m2 gꢀ1
)
Vp (cm3 gꢀ1
)
Dp (nm)
5Ga2O3/ZSM-5
5Ga2O3/Al2O3
5Ga2O3/SiO2
1Ga2O3/SBA-15
3Ga2O3/SBA-15
5Ga2O3/SBA-15
7Ga2O3/SBA-15
9Ga2O3/SBA-15
327
195
137
721
661
628
623
556
0.21
0.45
0.42
1.00
0.93
0.86
0.85
0.77
—
5.06
11.5
5.47
5.46
5.48
5.46
5.46
Fig. 2 (a) Wide-angle X-ray diffraction patterns of the different cata-
lysts and (b) small-angle X-ray diffraction pattern of xGa2O3/SBA-15
catalyst.
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demonstrates the high crystallinity of ZSM-5. As for xGa2O3/ 5Ga2O3/Al2O3 catalyst, it displays three desorption peaks,
SBA-15 samples, the micrographs reveal the presence of well- a broad desorption peak (peak I) centered at ꢂ190 ꢁC with
ordered hexagonal arrays of mesoporous channels. However, a shoulder (peak II) at higher temperatures ꢂ270 ꢁC and a peak
as loading content increases to 9 wt%, less regular channels are (peak III) with a maximal temperature (TM) centered at
observed from Fig. 4(f). These TEM images are in good agree- ꢂ480 ꢁC. It is generally accepted that these three desorption
ment with the XRD, BET and SEM results. Furthermore, as peaks are ascribed to be weak, medium and strong acid sites
indicated by the arrows, the agglomerated gallium species can respectively.39 In comparison with 5Ga2O3/ZSM-5 and 5Ga2O3/
also be noticed on 7Ga2O3/SBA-15 and 9Ga2O3/SBA-15 catalysts, Al2O3 catalysts, the ammonia desorption peaks almost cannot
which implies that the dispersion of Ga species is not highly be visible over xGa2O3/SBA-15 and 5Ga2O3/SiO2 samples. This
homogeneous under high loading concentration.
phenomenon may be attributed to the inherent nature of pure
silicon support. From Table 2, though the xGa2O3/SBA-15
catalysts possess the very weak surface acidity, the acid prop-
erties still have some changes with the increase of Ga2O3
content. The 5Ga2O3/SBA-15 and 7Ga2O3/SBA-15 samples show
relatively high total acidity and weak to medium strong acid
sites as compared with those of other three samples, but the
change of the amount of strong acid sites is not obvious. The
previous studies demonstrated that undesirable side reactions
were easily initiated by the strong acid sites of catalysts.40 By
inference, the undesirable side reactions may easily occur over
5Ga2O3/ZSM-5 catalyst.
3.4 NH3-TPD
The acidity of different catalysts was probed by temperature-
programmed desorption of ammonia method, and the corre-
sponding NH3-TPD curves are displayed in Fig. 5. The semi-
quantitative analysis is achieved by deconvoluting the peaks
using Gaussian curve tting method. The calculated results
containing total acidity and acidity strength distribution are
summarized in Table 2. It is clear that 5Ga2O3/ZSM-5 catalyst
exhibits two ammonia desorption peaks. The rst peak centered
(peak I) at ꢂ220 ꢁC is attributed to the weak acid sites; while the
ꢁ
second peak centered (peak II) at ꢂ420 C is attributed to the
3.5 H2-TPR
strong acid sites.38 Furthermore, the largest strong acid sites
also can be observed over Ga2O3/ZSM-5 catalyst. From Fig. 5 and To determine the effects of different supports and loading
Table 2, it can be seen that nearly half of total acidity is ascribed amount on the reduction properties, H2-TPR technique was
to strong acid sites for 5Ga2O3/ZSM-5 catalyst. As regards the implemented, and the curves of the corresponding fresh
Fig. 3 FESEM images of (a) 5Ga2O3/ZSM-5; (b) 5Ga2O3/Al2O3; (c) 5Ga2O3/SiO2; (d) 1Ga2O3/SBA-15; (e) 3Ga2O3/SBA-15; (f) 5Ga2O3/SBA-15; (g)
7Ga2O3/SBA-15; (h) 9Ga2O3/SBA-15.
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Fig. 4 TEM micrographs of (a) 5Ga2O3/ZSM-5; (b) 1Ga2O3/SBA-15; (c) 3Ga2O3/SBA-15; (d) 5Ga2O3/SBA-15; (e) 7Ga2O3/SBA-15; (f) 9Ga2O3/SBA-15.
catalysts are illustrated in Fig. 6(a). The semi-quantitative H2 Table 4, the ratio of well-dispersed Ga in 5Ga2O3/ZSM-5 to
consumption is summarized in Table 3, which is obtained by 5Ga2O3/Al2O3 is approximately equal to that of the strong acid
tting the H2-TPR curves using the Gaussian deconvolution peak area in the corresponding samples. This similar
method. It can be seen that all the catalysts present two phenomenon was also mentioned by Ausavasukhi et al.25 In
ꢁ
reduction consumption peaks at about 600 ꢁC (peak I) and addition, a small peak at about 690 C can be noticed, which
900 ꢁC (peak II). As reported previously,26 the peak I is ascribed should be attributed to the reduction of OH groups or Al of
to the reduction of well-dispersed Ga species, such as micro alumina support.41 On the other hand, Fig. 6(b) shows the TPR
Ga2O3 particles and/or GaO+ species interacting with the curves of xGa2O3/SBA-15 catalysts. It can be seen that all the
support, and the peak II can be assigned to bulk Ga2O3 particles samples also display similar two peaks, a small shoulder peak at
and the loosely supported part of Ga2O3, respectively. In the lower temperature and intensive one at very high temperature
case of 5Ga2O3/ZSM-5, the strongest reduction peak I and the even over 950 ꢁC. More specically, the hydrogen consumption
weakest reduction peak II can be observed, implying that the areas of peak II over xGa2O3/SBA-15 catalysts increase as the
incorporated Ga species of 5Ga2O3/ZSM-5 catalyst are mostly Ga2O3 content increases. The 5Ga2O3/SBA-15 catalyst exhibits
retained as well-dispersed Ga species. The analysis shows that the strongest signal value among all the catalysts, which indi-
compared with 5Ga2O3/ZSM-5 catalyst, a weaker reduction peak cates that the 5Ga2O3/SBA-15 sample possesses the largest
I and slightly stronger reduction peak II can be observed over amount of well-dispersed Ga species. Furthermore, for alꢁl the
the 5Ga2O3/Al2O3 catalyst, suggesting that the amount of well- xGa2O3/SBA-15 catalysts, the peak intensity at about 600 C is
dispersed Ga species on 5Ga2O3/Al2O3 catalyst is less than much weaker than those of 5Ga2O3/ZSM-5 and 5Ga2O3/Al2O3
that of 5Ga2O3/ZSM-5 catalyst. This phenomenon should be catalysts. This phenomenon can be explained that the low
highly related to the acidic properties of catalysts. As shown in acidity of xGa2O3/SBA-15 catalysts is adverse to the dispersion of
Ga species. As for the TPR curve of 5Ga2O3/SiO2 sample, two
weak reduction peaks for Ga species can be found, which means
that the SiO2 support is hard to stabilize small Ga2O3 particles
or GaO+ species. It is very unfavorable to the reaction. The
similar results were also mentioned in previous reports.42,43
Table 2 The semi-quantitative results of NH3-TPD measurements
Peak area
fraction (%)
TM (ꢁC)
I
Total area
(a.u.)
Catalysts
II
III
I + II
III
5Ga2O3/ZSM-5
5Ga2O3/Al2O3
5Ga2O3/SiO2
1Ga2O3/SBA-15
3Ga2O3/SBA-15
5Ga2O3/SBA-15
7Ga2O3/SBA-15
9Ga2O3/SBA-15
220
190
218
198
206
200
180
179
—
420
480
495
502
513
460
455
486
399
403
53.4
67.3
34.5
15.8
13.4
27.8
23.2
18.3
46.6
32.7
65.5
84.2
86.6
72.2
76.8
81.7
270
314
320
316
301
318
301
69.2
51.4
48.6
60.8
53.2
49.3
Fig. 5 NH3-TPD profiles of the different catalysts.
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Table
3
The semi-quantitative H2-TPR results for the different
catalysts
Total area
Peak I area
Peak II area
Catalysts
(a.u.) (ꢃ104)
(a.u.) (ꢃ104)
(a.u.) (ꢃ104)
5Ga2O3/ZSM-5
5Ga2O3/SBA-15
5Ga2O3/Al2O3
5Ga2O3/SiO2
1.60
1.34
1.41
0.05
1.38
0.05
0.99
0.01
0.22
1.29
0.42
0.04
catalyst increases with the increase of Ga2O3 content from 1
to 9 wt%. Moreover, it can be seen from Table 5 that the Ga3d
banding energy of the gallium species in the Ga2O3/SiO2 is
reduced by 0.5 as compared with those in the other three
samples. It is remarkable that the Ga3d banding energy of the
gallium species in pure b-Ga2O3 is the same as that in 5Ga2O3/
SiO2 catalyst,44 indicating that using ZSM-5, Al2O3 and SBA-15
supports can strengthen the interaction between Ga species
and support. The similar conclusion was also reported by Xu
et al.18 These results imply that the different supports can play
a signicant role in determining the chemical state of Ga
species.
3.7 Catalytic performances
3.7.1 Effect of different supports. The catalysts with
different supports were subjected to the propane dehydroge-
nation reaction for 30 h. The propane conversion and propylene
selectivity are displayed in Fig. 8. As can be seen, the initial
conversions of propane for different catalysts decrease in the
following order: 5Ga2O3/ZSM-5 > 5Ga2O3/Al2O3 > 5Ga2O3/SBA-15
> 5Ga2O3/SiO2. The 5Ga2O3/ZSM-5 catalyst shows the highest
initial propane conversion (78.1%), which is much higher than
those of the other samples. However, the worst stability of
5Ga2O3/ZSM-5 catalyst is found among all the samples. The
nal propane conversion is only 11.5%. As regards the 5Ga2O3/
SBA-15 sample, although the initial conversion of propane is
29.7%, it exhibits the highest catalytic stability. The nal
propane conversion still attains 21.4% aer 30 h of propane
dehydrogenation reaction. When the Al2O3 is chosen as the
support, the relatively high initial propane conversion (46.0%)
over 5Ga2O3/Al2O3 can be obtained, but the quick deactivation
of this sample is also observed, and the nal conversion value is
only 5.3%. For the 5Ga2O3/SiO2 catalyst, a low and stable
propane conversion (around 5.5%) can be observed. The TOF
values of different catalysts at 20 h are calculated and included
Fig. 6 H2-TPR profiles of the different catalysts.
3.6 XPS
To further elucidate the chemical state of Ga species, XPS
analyses were measured. The XPS spectra and semi-quantitative
results of various catalysts are shown in Fig. 7 and Table 5
respectively. As can be seen in Fig. 7, 5Ga2O3/ZSM-5 and
5Ga2O3/Al2O3 catalysts present three peaks at ꢂ25 eV (peak I),
ꢂ21 eV (peak II) and ꢂ19.8 eV (peak III) similar to the previous
literatures.16,44,45 The high binding energy (ꢂ25 eV) is ascribed to
O2s band; the binding energy (ꢂ21 eV) is assigned to Ga3d
band, and the low energy peak at ꢂ19.8 eV is attributed to Gad+
species (d < 2).45 From Table 5, the percentages of Gad+ species (d
< 2) for 5Ga2O3/ZSM-5 and 5Ga2O3/Al2O3 catalysts are about
62% and 11% respectively, implying that partial Ga2O3 is
reduced on these two supports during the catalyst preparation
process. The similar viewpoint was also reported in the previous
literature.46 As for xGa2O3/SBA-15 and 5Ga2O3/SiO2 catalysts,
there are two deconvoluted peaks (peak I and peak II). The
fraction of gallium in the oxidized state for xGa2O3/SBA-15
Table 4 The relationship between the reduction properties and the
strong acid sites on the different supports
5Ga2O3/ZSM-5
1.38
5Ga2O3/Al2O3
0.99
Area ratio
1.39
H2-TPR peak I area
(a.u.)
NH3-TPD peak III area
(a.u.)
185.93
131.78
1.41
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Fig. 7 Ga3d XPS spectra of the different catalysts.
in Table 6. It is remarkable that the TOF values show the similar three kinds of active centers in the fresh catalysts: well-
variation regularity to that of propane conversion over the cor- dispersed Ga species, GaO+ and bulk Ga2O3.25,47 The well-
responding catalysts. The highest TOF value can be found over dispersed Ga species and GaO+ usually have higher dehydro-
5Ga2O3/SBA-15 catalyst, indicating that it displays the high genation activity as compared with bulk Ga2O3.25 It can be seen
dehydrogenation efficiency of Ga species.
from the H2-TPR curves in Fig. 6 that the initial catalytic activ-
From Fig. 8, it can be seen that the propylene selectivity ities of different catalysts are in good agreement with the
exhibits an apparent rise of for 5Ga2O3/ZSM-5 catalyst in the amount of well-dispersed Ga species of the corresponding
initial several hours and then gradually stabilizes at around catalysts. Furthermore, the TEM images also certify the exis-
89% aer 8 h reaction. The 5Ga2O3/Al2O3 catalyst presents the tence of bulk Ga2O3 in 7Ga2O3/SBA-15 and 9Ga2O3/SBA-15
highest initial propylene selectivity, and slightly decreases from samples. Secondly, the researchers reported that the reduced
95.8% to 85.1% aer 30 h reaction. The propylene selectivity for gallium ions (Gad+ cations, d < 2) were considered to be a reason
5Ga2O3/SBA-15 catalyst attains about 92.0% and varies little in for the high activity, and monovalent Ga was regarded as the
the whole 30 h reaction period. As for the 5Ga2O3/SiO2 sample, second most active species.25,47,48 As shown in Fig. 7 and Table 5,
a relatively low propylene selectivity of around 86% is observed. the content of the reduced gallium species on 5Ga2O3/ZSM-5
The variation of the catalytic performances can be explained catalyst and 5Ga2O3/Al2O3 catalyst are 62% and 11% respec-
as following. In general, for Ga2O3-supported catalysts, there are tively. Nevertheless, the reduced gallium species cannot be
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Table 5 The semi-quantitative XPS results for the different catalysts
exist the relatively high selectivities to methane and ethane
(Fig. 9), implying the occurrence of hydrogenolysis reaction, and
the following increased selectivity should be due to the cover of
acid sites by coke deposits. For the 5Ga2O3/Al2O3 catalyst, the
NH3-TPD results (Fig. 5) show that the strong acid sites are rela-
tively suitable, and thus the undesired side reactions can be
suppressed. The 5Ga2O3/SBA-15 and 5Ga2O3/SiO2 samples
possess the relatively high propylene selectivity values among all
the samples, which are consistent with the extremely weak acidity.
3.7.2 Effect of Ga2O3 content. The catalytic performances
over xGa2O3/SBA-15 catalysts with different Ga2O3 content are
Ga3d binding
energy (eV)
Catalysts
Ga3+ (%)
Gad+a (%)
5Ga2O3/ZSM-5
xGa2O3/SBA-15
5Ga2O3/Al2O3
5Ga2O3/SiO2
21.0/19.8
21.0
21.0/19.8
20.5
38
100
89
62
0
11
0
100
a
d < 2.
found over the other two pure silica supported catalysts. In this illustrated in Fig. 10. It can be observed that 5Ga2O3/SBA-15
sense, this is another reasonable explanation for the much sample shows the highest conversion, selectivity and catalytic
better catalytic performance of 5Ga2O3/ZSM-5 than the other stability, which has the initial propane conversion and nal one
three catalysts. Thirdly, as shown in Table 5, the weak interac- of 31.7% and 30.3%. However, the higher or lower Ga2O3
tion between Ga species and SiO2 support is found for 5Ga2O3/ loading amount both leads to a decline of initial propane
SiO2 sample, but the other three catalysts exhibit the strong conversion. 1Ga2O3/SBA-15 sample exhibits the lowest catalytic
interactions between Ga species and the supports. This may be activity among all the catalysts. The TOF values of xGa2O3/SBA-
the reason why 5Ga2O3/SiO2 catalyst possesses the lowest cata- 15 are also shown in Table 6. It can be seen that 1Ga2O3/SBA-15
lytic activity. Finally, according to the textural parameters in sample has the highest TOF value, which should be ascribed to
Fig. 1 and Table 1, the relatively high surface area and unique the high Ga species dispersion under the minimum Ga2O3
structure of ZSM-5 support may be responsible for the relatively loading. As for the other samples, the TOF values rst increase,
high catalytic activity of 5Ga2O3/ZSM-5 catalyst.
and then decrease with the increase of the amount of gallium
As for the changes of propylene selectivity, it is well-known that oxide, and the 5Ga2O3/SBA-15 sample possesses the second
the strong acid sites generally promote the deep dehydrogenation highest TOF value. This phenomenon can be probably attrib-
due to isomerization and coking reactions on the surfaces of uted to the formation of bulk Ga2O3, which leads to the decline
catalysts. Aromatization of propylene would also take place on the of dehydrogenation efficiency of gallium oxide.
Brønsted acid sites of supports. All these factors will lower
From Fig. 10, 5Ga2O3/SBA-15 catalyst presents the highest
propylene selectivity.49 Moreover, not only the surface acidic propylene selectivity among all the samples. 1Ga2O3/SBA-15 and
properties of support but also the active components can give rise 9Ga2O3/SBA-15 catalysts show the relatively lower propylene
to the variation of propylene selectivity. The previous study has selectivities. Also, it is found that the propylene selectivity has
proved that the dihydrogen molecule is dissociatively adsorbed on the similar variation trend to propane conversion with regard to
gallium oxide on the catalyst surface, which may cause the the Ga2O3 content for xGa2O3/SBA-15 catalysts.
occurrence of hydrogenolysis reaction.50 With regards to 5Ga2O3/
The catalytic performances over the synthesized catalysts
ZSM-5 sample, the lowest selectivity in the initial hours must be with different loadings can be explained as following. According
related to the strong acidity sites and well-dispersed Ga species,
which results in more side reactions (Fig. 9). Meantime, there also
Table 6 Amounts of coke on the spent different supported and
xGa2O3/SBA-15 catalysts for propane dehydrogenation after 30 h and
2 h reaction respectively, and TOF values of the different catalystsa
Samples
Coke amountb (%)
TOF (sꢀ1) (ꢃ10ꢀ2
)
5Ga2O3/ZSM-5c
5Ga2O3/SBA-15c
12.6
4.55
2.86
0.96
3.97
5.85
3.11
6.32
6.52
0.82
1.81
2.91
1.16
0.80
6.92
3.72
4.20
2.08
1.64
—
c
5Ga2O3/Al2O3
c
5Ga2O3/SiO2
1Ga2O3/SBA-15d
3Ga2O3/SBA-15d
5Ga2O3/SBA-15d
7Ga2O3/SBA-15d
9Ga2O3/SBA-15d
SBA-15
a
Note: the coke on the SBA-15 and the Ga2O3 catalysts supported on
of propane
dehydrogenation reaction; the coke on the xGa2O3/SBA-15 was
measured aer of propane dehydrogenation reaction.
Experimental value calculated from thermogravimetric (TG) analysis.
various supports was measured aer 30
h
2
h
b
Fig. 8 Propane conversion and propylene selectivity of the different
catalysts as a function of time (reaction conditions: T ¼ 620 ꢁC;
C3H8 : Ar (molar ratio) ¼ 1 : 19; WHSV ¼ 0.6 hꢀ1; mcat. ¼ 0.2 g).
c
d
C3H8 turn frequency (TOF) in the 20
frequency (TOF) in the 2 h reaction.
h reaction. C3H8 turn
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Fig. 11 The effect of reaction temperature on the performance of
5Ga2O3/SBA-15 catalyst in propane dehydrogenation.
Fig. 9 Selectivities to alkanes (alkenes) over the different catalysts in
the initial 30 min reaction: (reaction conditions: T ¼ 620 ꢁC; C3H8 : Ar
(molar ratio) ¼ 1 : 19; WHSV ¼ 0.6 hꢀ1; mcat. ¼ 0.2 g).
begin to enrich in the form of large Ga2O3 clusters rather than
well-dispersed species. Ausavasukhi et al.43 also reported
a similar conclusion.
to XRD, BET, SEM and TEM results, the high Ga2O3 content can
block the pores of SBA-15, destroy the structural integrity of
SBA-15 and lead to the formation of bulk Ga species on the
support. The H2-TPR curves in Fig. 6(b) also verify that more
well-dispersed Ga species exist on 5Ga2O3/SBA-15 sample,
which is in coincidence with the best catalytic performance.
Furthermore, as shown in Table 2, the catalytic performances of
xGa2O3/SBA-15 catalysts are closely related to the variation of
the total acid of the catalysts with respect to Ga2O3 content. It is
notable that the strong acid sites of xGa2O3/SBA-15 catalysts
almost vary little with the increase of Ga2O3 content, and all the
catalysts have the similar amounts of well-dispersed Ga species,
implying that the inuence of the amount of well-dispersed Ga
species is mostly related to the acidity rather than the loading
content of Ga2O3. When Ga content increases, the Ga species
3.7.3 Effect of reaction temperature. Fig. 11 shows the
effect of reaction temperature on the performance of 5Ga2O3/
SBA-15 catalyst in propane dehydrogenation. The initial
propane conversions of 21.7%, 28.3%, 33.8% and 45.0% are
obtained at 580, 600, 620 and 640 ꢁC respectively, and they
decrease to 16.1%, 20.4%, 26.6% and 36.0% aer 20 h reaction.
The initial propylene selectivities are 94.5%, 93.5%, 92.8% and
90% at 580, 600, 620 and 640 ꢁC respectively. The decreased
selectivity with the increase of temperature can be ascribed to
the side reactions like thermal crackingꢁetc. Though the highest
propane conversion is achieved at 640 C at the expense of the
lowest selectivity, it is not a desirable reaction parameter.
Compared the results at 580 ꢁC with others, the propylene
selectivity evidently decreases at other three temperatures.
However, the initial propane conversion is only 16.1% when the
reaction is carried out at 580 ꢁC. In general, the reaction
Fig. 10 The effect of Ga2O3 content of the catalysts on the catalytic
performance in propane dehydrogenation (reaction conditions: T ¼
620 ꢁC; C3H8 : Ar (molar ratio) ¼ 1 : 19; WHSV ¼ 0.6 hꢀ1; mcat. ¼ 0.2 g). Fig. 12 TPO profiles of the spent catalysts.
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should be responsible for the largest amount of coke deposi-
tion. When SBA-15 material is used as the support, the second
most coke amount of 5Ga2O3/SBA-15 which has the low acidity
may owe to the high temperature, Ga species and largest SBET
value. From Table 6, a small amount of coke can be found on
pure SBA-15 support, implying that the propane thermal
cracking reactions are inevitable during the long-term reaction
at such a high reaction temperature (620ꢁ). Furthermore, the
Ga2O3-based catalysts are known to be effective catalysts for
promoting the selective conversion of light alkanes to
aromatics. Many authors agree that the aromatization reaction
occurs via a bifunctional mechanism. Not only strong acid sites
but also Ga species can enhance the dehydrogenation steps,
including the dehydrogenation of alkane, higher olens, and
cycloolens. Meriaudeau illustrated that the dihydrogen mole-
cule was dissociatively adsorbed on gallium oxide. These might
result in producing a large amount of coke precursors during
the process of reaction. Moreover, the mesoporous character
and high SBET of SBA-15 make the support have a certain pore
volume, which can contain the coke easily. On the contrary,
with respect to 5Ga2O3/g-Al2O3 catalyst, without high SBET value,
the coke deposition quickly covers the active sites, and results in
an evident decrease in the catalyst activity. When SiO2 material
is used as the support, the lowest amount of coke on the
5Ga2O3/SiO2 can be found, which is well coincident with the
dreadful catalytic activity.
Fig. 13 TG profiles of the spent 5Ga2O3/SBA-15 catalysts.
temperature at 600–620 ꢁC should be the ideal selection to
balance the conversion and selectivity of propane dehydroge-
nation reaction.
As mentioned above, in this work, coke is one main reason to
catalyst deactivation.51,52 The strong acidity can promote the
coke deposition on the catalyst surface.53 The total coke
amounts of the spent catalysts were analyzed by TPO and TG
measurements. It can be seen from Fig. 12 that all the TPO
curves of the spent catalysts present one major peak,54 which is
attributed to coke combustion on the catalysts. The peak for
5Ga2O3/SiO2 sample presents a relatively lower temperature
than the other three catalysts, meaning that the deposited coke
is easily combusted. Coke quantitative analysis (Table 6) is in
good agreement with TPO results, and the order of coke amount
is as follows: 5Ga2O3/ZSM-5 > 5Ga2O3/SBA-15 > 5Ga2O3/Al2O3 >
5Ga2O3/SiO2. The spent 5Ga2O3/ZSM-5 catalyst has the largest
coke amount (12.6%), which coincides with the propylene
selectivity in Fig. 8. In this case, as discussed before (Table 2),
the 5Ga2O3/ZSM-5 possesses the quite strong acidity. This
As for xGa2O3/SBA-15 catalysts, the total coke amounts of the
spent catalysts are also illustrated in Table 6. The 5Ga2O3/SBA-
15 and 9Ga2O3/SBA-15 catalysts possess the lowest and highest
coke content among all the samples respectively, which are in
accord with the catalytic performance. Meanwhile, the different
reaction time (2 h and 30 h) on the nature of coke components
are also discussed. As can be seen from Fig. 13, the weight loss
ꢁ
between 0 and 300 C can be considered as the loss of surface
water and water adsorption phase, and the weight loss between
300 and 600 ꢁC is attributed to the coke combustion on the
catalyst surface. It should be noted that the weight loss trend of
Table 7 Comparison of the catalytic performances of different catalysts for propane dehydrogenation reaction
WHSV (hꢀ1
)
Xpropane (%)
Spropylene (%)
Ref.
ab
ab
Catalysts
Ga2O3 content (wt%)
T (ꢁC)
Ga2O3/MCM-22
Ga2O3/ITQ-2
Ga2O3/HZSM-48
Ga2O3/ZSM-5
Ga2O3/TiO2
Ga2O3/Al2O3
Ga2O3/ZrO2
Ga2O3/SiO2
Ga2O3/MgO
Ga2O3/Al2O3
Ga2O3/ZSM-5
Ga2O3/SiO2
5
5
5
5
5
5
5
5
5
5
5
5
5
600
600
600
600
600
600
600
600
600
620
620
620
620
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
57.8(20.6)c
30.0(19.3)c
40.0(36.3)c
76.3(25.0)c
23.0(3.0)d
33.0(18.0)d
39.0(5.0)d
7.2(6.5)d
37.5(64.1)c
72.0(78.4)c
53.8(54.5)c
—
85.0(—)d
92.0(—)d
74.0(—)d
92.0(—)d
34.0(—)d
95.2(93.1)c
60.2(86.1)c
90.1(91.1)c
92.5(91.6)c
19
19
11
18
16
16
16
16
5.3(4.0)d
16
46.0(15.6)c
78.1(28.8)c
6.7(5.1)c
This work
This work
This work
This work
Ga2O3/SBA-15
28.5(22.1)c
a
b
Note: the given data of Xpropane and Spropylene are estimated according to the given curves in the literatures. The values outside and inside the
c
parenthesis are the data obtained in the initial and terminal times, respectively. The propane conversion (%) and corresponding propylene
selectivity (%) aer reaction for 8 h. The propane conversion (%) and corresponding propylene selectivity (%) aer reaction for 3 h.
d
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different samples in the range of 200–600 ꢁC are markedly
different, suggesting that the weight loss of these two samples
are caused by the combustion of different coke species. There-
fore, it can be concluded that the reaction time may play an
important effect on the graphitization degree of coke.
Table 7 displays some reports about the catalytic perfor-
mances of Ga2O3-based catalysts for propane dehydrogenation
reaction. The initial and nal propane conversions and
propylene selectivities are collected. As can be seen from Table
7, the supports of catalyst have great inuence on the catalytic
performance, especially the propylene selectivity. The previous
presented the low propylene selectivity and catalytic stability.
However, the 5Ga2O3/SBA-15 catalyst in this study displays the
excellent catalytic stability accompanied with the relatively high
conversion and selectivity, and the catalytic stability is evidently
higher than those of the catalysts reported in literatures.
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Among the catalysts with different supports, the 5Ga2O3/
ZSM-5 catalyst shows the best catalytic activity and the lowest
catalytic stability, possesses the maximum gallium ions (Gad+
cations, d < 2), well-dispersed Ga species and strong acid sites.
The 5Ga2O3/SBA-15 catalyst exhibits the highest catalytic
stability, the highest SBET value and the lowest acidity. The
propane conversion only declines 11% aer reaction for 30 h. As
for xGa2O3/SBA-15 catalysts, the agglomeration degree of Ga
species gradually increased as the increase of Ga2O3 loading
amount. The 5Ga2O3/SBA-15 sample exhibits the highest cata-
lytic performance, possesses the most well-dispersed Ga species
and the best structure accompany with the relatively high
loading amount, the nal propane conversion and propylene
selectivity still attain above 17.5% and 92.0% aer 30 h propane
ꢁ
dehydrogenation reaction at 620 C.
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The research is supported by Science and Technology
Commission of Shanghai Municipality (14520502900) and
Internation Joint Laboratory on Resource Chemistry (IJLRC).
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