Catalytic dehydrogenation of isobutane over a Ga2O3/ZnO interface: Reaction routes and mechanism

Article abstract of DOI:10.1039/c5cy01950h

In this work, physical mixtures of ZnO and Ga₂O₃, even with a small amount of Ga₂O₃, were found to exhibit greatly enhanced catalytic performance for isobutane dehydrogenation compared to their individual components, namely solely ZnO or Ga₂O₃. The activity test results under different packing patterns indicated that, the interface between the two component oxides played a crucial role in improving the dehydrogenation performance. Moreover, consistent with the highest dehydrogenation reactivity, the largest activation energy for isobutane desorption over ZnO-Ga₂O₃ was determined using an isobutane-TPD test. The observed synergistic effect of ZnO and Ga₂O₃ could be understood as being that, Lewis acid sites provided by Ga₂O₃ promoted the heterolytic cleavage of C-H bonds in isobutane over ZnO, thereby increasing the isobutane conversion. On the surface of ZnO-Ga₂O₃, a double-site adsorption of isobutane was further speculated through FT-IR spectra analysis, and one-step decomposition was probably the actual reaction pathway of isobutane. In addition, from understanding that catalyst deactivation was caused by highly graphitized coke deposition, the deactivated catalyst was regenerated through air combustion, and good catalyst stability was demonstrated through continuous reaction-regeneration cycles.

Full text of DOI:10.1039/c5cy01950h

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Catalytic dehydrogenation of isobutane over a
Ga2O3/ZnO interface: reaction routes and
mechanism†
Cite this: DOI: 10.1039/c5cy01950h
*
Guowei Wang, Chunyi Li and Honghong Shan
In this work, physical mixtures of ZnO and Ga2O3, even with a small amount of Ga2O3, were found to ex-
hibit greatly enhanced catalytic performance for isobutane dehydrogenation compared to their individual
components, namely solely ZnO or Ga2O3. The activity test results under different packing patterns indi-
cated that, the interface between the two component oxides played a crucial role in improving the dehy-
drogenation performance. Moreover, consistent with the highest dehydrogenation reactivity, the largest ac-
tivation energy for isobutane desorption over ZnO–Ga2O3 was determined using an isobutane-TPD test.
The observed synergistic effect of ZnO and Ga2O3 could be understood as being that, Lewis acid sites pro-
vided by Ga2O3 promoted the heterolytic cleavage of C–H bonds in isobutane over ZnO, thereby increas-
ing the isobutane conversion. On the surface of ZnO–Ga2O3, a double-site adsorption of isobutane was
further speculated through FT-IR spectra analysis, and one-step decomposition was probably the actual re-
action pathway of isobutane. In addition, from understanding that catalyst deactivation was caused by
highly graphitized coke deposition, the deactivated catalyst was regenerated through air combustion, and
good catalyst stability was demonstrated through continuous reaction–regeneration cycles.
Received 14th November 2015,
Accepted 29th November 2015
DOI: 10.1039/c5cy01950h
www.rsc.org/catalysis
Among them, the dehydrogenation performance of Mo-based
catalysts was relatively stable, but the activity needed to be
1. Introduction
With the increasing demand for light olefins, as fundamental
chemical building blocks, technologies for olefin production
have attracted great attention recently. Catalytic dehydrogena-
tion of light alkanes is an attractive approach due to the ad-
vantages of abundant feedstocks, high energy efficiencies,
and simple target product distributions.1
improved.10 As for a metal sulfide catalyst,12,13 both high ac-
tivity and selectivity were obtained, however, a demanding re-
quirement for the corrosion resistance of the equipment was
put forward.
ZnO, with n-type semiconductor character, interacts with
platinum strongly, and thus can be applied as a promoter or
support in different reactions.16 Its promoting effects occur
almost in the same way as for the addition of Sn: the forma-
tion of a PtZn alloy reduces the size of the Pt particles, and
the generation of Ptδ−–Znδ+ entities further regulates the
electronic properties of Pt.17,18 DFT calculations for the ad-
sorption of ethylene on slabs of Pt(111), Pt3ZnIJ111) and
PtZn(011) has also indicated that the addition of Zn to Pt
weakened the adsorption of CC double bond,19 conse-
quently leading to an increasing selectivity towards olefins.
Recently, gallium oxide8,20–23 has been recognized as an alter-
native catalyst active component for the dehydrogenation of
light alkanes, and the performance becomes much better in
the presence of CO2. However, the scarcity and high expense
of gallium seriously restricts the application of bulk gallium
oxide. In this work, we have found that the mixture of ZnO
and Ga2O3, even with a small amount of the latter constitu-
ent, exhibits excellent dehydrogenation activity and selectiv-
ity, substantially broadening the usage of gallium-based
catalysts.
Chromia-based and platinum-based catalysts2,3 are two
typical formulations applied commercially for the catalytic
dehydrogenation of isobutane. Both of them exhibit satisfac-
tory performance, however, the environmental concerns asso-
ciated with Cr species and the high cost of Pt-based catalysts
have limited their further worldwide applications.1 Oxidative
dehydrogenation, as an alternative route, has become a re-
search hotspot due to its exothermic property. Although re-
searchers have made great efforts in the modification of cata-
lyst formulations4–7 and the application of mild oxidants,8,9 it
is still hard to make a breakthrough regarding the control of
oxidation depth in the short term. Our research group are
also dedicated to exploring cost-effective and environment-
friendly catalyst systems and have made many attempts.10–15
State Key Laboratory of Heavy Oil Processing, China University of Petroleum,
Qingdao 266580, PR China. E-mail: chyli@upc.edu.cn, chyli_upc@126.com
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cy01950h
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This study firstly evaluated the dehydrogenation perfor-
mances of the mechanical mixtures of Ga2O3 and ZnO with
different molar ratios. Then, different packing patterns were
systematically compared to elucidate the roles of contact
tightness between the two oxides. Combining isobutane-TPD
profiles and FT-IR spectra, the adsorption strength and state
of isobutane over different samples were further investigated.
Ultimately, the reaction mechanism and deactivation origin
of the ZnO–Ga2O3 catalyst were speculated to gain a deeper
insight into the nature of the synergistic effect.
2. Experimental
2.1. Catalyst preparation
ZnO powder (99.9% purity) with a specific surface area of 4
m2 g−1 and a pore diameter of 3.5 nm, was supplied by Qing-
dao Nakasen Zinc Industry Technology Co. Ga2O3 powder
(99.9% purity) was obtained from Shandong Aluminium In-
dustry Co., and the corresponding specific surface area and
pore diameter are 5 m2 g−1 and 10.5 nm, respectively. The de-
tailed pore size distributions of these two oxides are shown
in Fig. S1.†
Fig. 2 Dehydrogenation performance of the catalysts with different
ZnO/Ga2O3 mole ratios under the same reaction conditions (560 °C, a
4 mL catalyst loading, and a 210 h−1 gas space velocity).
2.2. Catalyst characterization
The catalyst was prepared through mechanically mixing
certain proportions of the ZnO and Ga2O3 powders together
through grinding in a mortar for half an hour, and then the
uniformly mixed powder was pressed and further crushed
into 100–200 mesh particles. The as-prepared catalysts to-
gether with the 20–60 mesh quartz sands were separately
loaded into a micro-reactor (see Fig. 1), to obtain the loading
pattern (a) in Fig. 3. Later in this work, the as-obtained sam-
ples are denoted as xZnO–Ga2O3, in which x represents the
mole ratio of ZnO to Ga2O3. While for the loading patterns
(b–e) in Fig. 3, the ZnO and Ga2O3 powders were not mixed
together, but separately pressed and crushed into 100–200
mesh particles, and then fed into the reactor following a se-
ries of certain sequences.
X-ray diffraction (XRD) was carried out on an X'Pert PRO
MPD diffractometer system using Cu Kα radiation at 40 kV
and 40 mA, running from 5° to 75° with a speed of 10°
min−1. Specific surface areas and pore structure properties of
the catalyst samples were determined through adsorption–de-
sorption measurements of nitrogen at liquid nitrogen tem-
perature using a Quadrasorb SI instrument. Prior to the mea-
surements, all samples were evacuated at 300 °C for 4 h at a
pressure of 1.0 × 10−3 kPa to ensure the complete removal of
adsorbed moisture. Raman spectra were obtained on a
Renishaw RM2000 Raman spectrometer with an incident la-
ser light excited at a wavelength of 532 nm. FT-IR spectra
were determined on a Bruker Tensor 27 Fourier transform in-
strument. The nature of the acid sites was probed using in-
frared spectroscopy of pyridine adsorbed on the samples.
Fig. 3 Conceptual schematic of different catalyst loading patterns: (a)
mechanically mixed 5ZnO–Ga2O3 catalyst; (b) ordered packing of
Ga2O3 (top) and ZnO (bottom); (c) ordered packing of ZnO (top) and
Ga2O3 (bottom); (d) solely ZnO and (e) Ga2O3 loaded with the same
ingredient volume.
Fig. 1 Schematic diagram of the activity test apparatus.
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To estimate the desorption activation energy of isobutane
over different catalysts, temperature-programmed desorption
of isobutane (i-C4H10-TPD) was carried out, and the gas re-
leased from the reactor was analyzed using an online Hinde
HPR20 quadrupole mass spectrometer. For each test, about
0.1 g of catalyst was loaded into a quartz tube reactor,
preheated in a flow of nitrogen (30 mL min−1) at 550 °C for
0.5 h, and then cooled down to room temperature. Adsorp-
tion was allowed to proceed for 0.5 h under an isobutane
flow (30 mL min−1). Subsequently, desorption was initiated at
different heating rates until the final temperature was
reached, with the corresponding TPD profiles being recorded.
According to Cvetanovic et al.,24 the relation between the
peak temperature (TP) and the heating rate (β) followed eqn
(1). Thus, the activation energy of isobutane desorption Ed
can be calculated through linear fitting.
The isobutane conversion and product selectivity are cal-
culated as follows:25
(2)
(3)
Here, m is the weight of the feed or product; yi‐C H is the
4
10
weight content of isobutane in the feed or product; yi‐C H is
4
8
the weight content of isobutene in the product; and the sub-
scripts F and P represent the feed and product, respectively.
2ln TP − ln β = Ed/RTP + constant
(1)
3. Results
3.1. Activity test results
The temperature-programmed surface reaction (TPSR) of
isobutane dehydrogenation over different samples was also
conducted. About 1 g of sample was loaded into a quartz tube
reactor and heated at 560 °C for 0.5 h under a nitrogen flow
(30 mL min−1), and then the temperature was cooled down to
100 °C. Subsequently, isobutane was fed into the reactor,
with the temperature increasing at a heating rate of 10 °C
min−1 to 560 °C and then was kept at 560 °C for 1 h. Mean-
while, the effluent gas was analyzed using an online Hinde
HPR20 quadrupole mass spectrometer.
TPO-MS measurement was carried out to determine the
coke deposition over the deactivated catalyst. During the test,
about 0.1 g of sample was loaded into the apparatus,
preheated in a helium flow at 200 °C for 0.5 h, and then
cooled down to 100 °C. Subsequently, the sample was
brought into contact with a mixture of 10 vol% O2/N2 (30 mL
min−1) and heated at a rate of 10 °C min−1 to the final tem-
perature. Meanwhile, the effluent gas was analyzed using a
Hinde HPR20 mass spectrometer.
3.1.1. Effect of the ZnO/Ga2O3 mole ratios. Isobutane de-
hydrogenation over solely ZnO, Ga2O3, and a series of me-
chanically mixed ZnO and Ga2O3 catalysts with different ZnO/
Ga2O3 mole ratios was conducted in a fixed bed micro-reactor
at 560 °C, and the results are displayed in Fig. 2.
It is apparent that both ZnO and Ga2O3 exhibited moder-
ate dehydrogenation activity, with isobutane conversions of
45.1 and 43.1 wt%, respectively. Since a cracking reaction oc-
curred to some extent, the selectivity to isobutene was much
lower for Ga2O3. Surprisingly, a remarkably improved dehy-
drogenation performance was observed when ZnO and Ga2O3
were mixed together. With the increase of the ZnO content,
the selectivity to isobutene was firstly elevated, and then it
decreased once the ZnO/Ga2O3 ratio exceeded 12.0. With a
ZnO/Ga2O3 ratio of 5.0, the isobutane conversion was up to
65.4 wt%, which was about 20 wt% higher than for ZnO and
Ga2O3, consequently yielding an isobutene yield higher than
55 wt%. That is, the mixture of ZnO and Ga2O3 exhibited a
superior performance in isobutane dehydrogenation than
solely ZnO or Ga2O3, and a synergistic effect has been
established in between. The isobutane conversion over a ZnO
and Ga2O3 mixture is even better than the commercial level
of 45–55%.26
3.1.2. Effect of catalyst loading patterns in the fixed-bed
reactor. To further confirm the synergy between the two com-
ponent oxides of the mixed catalyst, the five different catalyst
loading patterns shown in Fig. 3 were evaluated for isobutane
dehydrogenation. Here, the catalyst with a ZnO/Ga2O3 mole
ratio of 5.0 (denoted as 5ZnO–Ga2O3) was loaded, and isobu-
tane was fed from the top of the fixed-bed.
Among the five different loading patterns, the
5ZnO–Ga2O3 catalyst, a mechanical mixture of ZnO and
Ga2O3, exhibited the best dehydrogenation performance with
an isobutene yield of up to 55.1 wt% (see Table 1). In con-
trast, the ordered packing patterns of ZnO and Ga2O3, no
2.3. Catalytic activity test
The feedstock used in the activity test was composed of 99.98
wt% isobutane and 0.02 wt% propane. Isobutane dehydroge-
nation was conducted in a fixed bed micro-reactor (18 mm
outer diameter and 14 mm inner diameter) at 560 °C under
atmospheric pressure with a 4 mL catalyst loading. The sche-
matic diagram of the experimental apparatus is shown in
Fig. 1. Prior to the reaction, the catalyst was degassed at the
reaction temperature in a nitrogen flow to remove adsorbed
oxygen and water from the catalyst surface.
During the test, isobutane was fed into the reactor at a
fixed flow rate of 2 mL min−1 with a nitrogen flow of 12 mL
min−1. The composition of the effluent gaseous products was
analyzed using a Bruker 450 Gas Chromatograph equipped
with a FID detector to determine the composition of hydro-
carbons and two TCD detectors to analyze the content of hy-
drogen, carbon monoxide and carbon dioxide.
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Table 1 Reaction results of the different catalyst loading patternsa
Catalyst loading pattern
Case (a)
Case (b)
Case (c)
Case (d)
Case (e)
Isobutane conversion, wt%
Selectivity to isobutene, wt%
Isobutene yield, wt%
65.4
84.3
55.1
54.6
80.3
43.8
53.0
82.9
43.9
40.3
82.1
33.1
22.5
71.8
16.2
a
Reaction conditions: 560 °C for cases (a–e); a 4 mL catalyst loading, and a 210 h−1 gas space velocity for cases (a–c); a 3.02 mL catalyst
loading, and a 278 h−1 gas space velocity for case (d); a 0.98 mL catalyst loading, and a 857 h−1 gas space velocity for case (e).
matter whether the ZnO layer was at the top or the bottom,
presented an inferior conversion level, demonstrating that a
close contact between the two components was essential for
an ideal catalytic performance. In other words, what the de-
hydrogenation reaction needed was the simultaneously com-
bined impact of Ga2O3 and ZnO, rather than a successive ef-
fect of the two oxides, which directly verified the synergy
between them.
occurred preferentially, and the highest selectivity to isobutene
was yielded for the 5ZnO–Ga2O3 catalyst (Fig. 2). Moreover,
5ZnO–Ga2O3 exhibited the lowest temperature for producing
isobutene, which indicated that the mixture possessed the
highest ability to initiate the dehydrogenation reaction, thereby
leading to the best dehydrogenation activity and performance
(Fig. 2).
Furthermore, Fig. 4 demonstrates that the partial pressure of
isobutene decreased in the sequence 5ZnO–Ga2O3 > ZnO >
Ga2O3, corresponding well to the order of the isobutene yield
(Fig. 2). At the same time, it is worth noting that both the shape
and initial temperature of the hydrogen flow were consistent
with those of isobutene, which indicated simultaneous forma-
tion due to the catalytic dehydrogenation of isobutane. The sub-
sequent decrease of the isobutene and hydrogen partial
As for the separately loaded patterns, both a higher con-
version and isobutene selectivity were obtained for ZnO than
for Ga2O3, which was consistent with the results shown in
Fig. 2. Moreover, attributed to additional loading of Ga2O3
and ZnO, the packing patterns (b) and (c) achieved a higher
isobutane conversion compared with cases (e) and (d), re-
spectively. If quartz sand was loaded between the two oxides
in patterns (b) and (c), lower conversions of isobutane and
selectivity to isobutene were obtained, indicating that the
interface between the two component oxides played an im-
portant role in improving the dehydrogenation performance.
Furthermore, two additional control samples of 5ZnO–Ga2O3
were prepared and evaluated for isobutane dehydrogenation.
Sample (a) was fabricated through separately pressing and
grinding ZnO and Ga2O3 into 20–60 mesh particles, and then
mixing together. In contrast, sample (b) was fabricated through
mixing ZnO and Ga2O3 powder together firstly, and then press-
ing and crushing the mixture into 20–60 mesh particles. In this
situation, the influence of particle size was excluded. Based on
the activity test results (Table S1†), namely that sample (b) with
a closer contact between the ZnO and Ga2O3 exhibited higher
dehydrogenation activity, the synergistic effect between the two
individual oxides was concluded, which further confirmed the
significance of the interface between the two oxides in achieving
excellent dehydrogenation performance.
3.1.3. Temperature-programmed surface reaction results.
The reactivities of ZnO, Ga2O3, and their mixture were further
examined through temperature-programmed surface reaction
(TPSR) tests. For the TPSR profiles shown in Fig. 4, the tem-
peratures corresponding to the occurrence of isobutene and
methane represent the reactivity for the dehydrogenation and
cracking reactions, respectively. That is, a lower starting tem-
perature indicates a much easier reaction occurrence. Mean-
while, the partial pressure of each product can be regarded
as a reflection of its yield.
As illustrated in Fig. 4, the starting temperature of methane
generation increased in the order Ga2O3 < ZnO < 5ZnO–Ga2O3,
which indicated that the cracking reaction was much harder
over the mixed catalyst. Therefore, the dehydrogenation reaction
Fig. 4 TPSR profiles of isobutane over different catalysts: (a) Ga2O3;
(b) ZnO; (c) 5ZnO–Ga2O3.
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pressures indicated a fast catalyst deactivation, possibly attrib-
uted to coke deposition and covered active sites. With regard to
the three samples, the following generation of methane after
isobutene suggested that methane was probably a secondary
product from isobutene cracking.
3.2. Structure and properties of the catalysts
3.2.1. Phase characterization. To determine the phase
change of the two mixed oxides under the reaction condi-
tions, the mechanically mixed 5ZnO–Ga2O3 sample before
and after the reaction, as well as 5ZnO–Ga2O3 calcined at the
reaction temperature of 560 °C for 2 h, were all characterized
using XRD, with the patterns shown in Fig. 5.
As displayed in Fig. 5, the fresh mixture exhibited the char-
acteristic peaks of both ZnO and Ga2O3, indicative of the na-
ture of physical mixing. For the calcined sample, the intensity
of the original peaks increased to some extent, and no new
peaks emerged, which suggested that chemical interaction be-
tween ZnO and Ga2O3 hardly occurred during the calcination
process. After the dehydrogenation reaction, the intensities of
the characteristic peaks decreased slightly due to coke deposi-
tion, and also no phase change or reduction of ZnO and
Ga2O3 were detected. In conclusion, the active phase of the
mixed 5ZnO–Ga2O3 catalyst for dehydrogenation is the oxides
themselves rather than the their reduced states. The fact that
isobutane conversion reached the maximum at the initial
stage of the reaction (Fig. S2†) also confirmed this point.
3.2.2. Raman spectra and TPO-MS profiles. Raman spectra
of ZnO, Ga2O3, as well as 5ZnO–Ga2O3 before and after reac-
tion are shown in Fig. 6. The bands at 378, 437, 1084 and
1149 cm−1 detected for ZnO represent the bulk phase.27,28 As
for the spectrum of Ga2O3, characteristic peaks of β-Ga2O3
were observed. The sharp band at 200 cm−1 is attributed to
the bending modes of O–Ga–O. The peaks at 319 and 348
cm−1 are assigned to the bending modes of the GaO2 unit.
Fig. 6 Raman spectra of the different catalysts.
The symmetric stretching modes of the GaO2 unit give rise to
the bands at 418 and 476 cm−1. Moreover, the bands at 630,
656 and 769 cm−1 correspond to the bending and stretching
of the GaO4 unit.29 According to Rao et al.,30 peaks at 144
and 169 cm−1 are also characteristic of bulk Ga2O3. Consis-
tent with the XRD patterns (Fig. 5), the spectrum of fresh
5ZnO–Ga2O3 was a combination of that of ZnO and Ga2O3.
With regard to the deactivated 5ZnO–Ga2O3 catalyst, the
intensity of the characteristic peaks corresponding to ZnO and
Ga2O3 decreased obviously. Meanwhile, two additional broad
peaks emerged at around 1336 and 1600 cm−1 ascribed to the
C–H bending and CC stretching in aromatic and alkyl
hydrocarbons,31–33 which was the origin of the catalyst deacti-
vation. These two bands were further deconvoluted into four
individual peaks, including R1, D, R2 and G, and the peak
intensity ratio of D to G represented the degree of graphitiza-
tion of the carbon materials.34 The as-determined ID/IG value of
the deactivated 5ZnO–Ga2O3 catalyst was 0.57, indicating that
the graphitization degree was relatively high, and the depos-
ited coke species contained a low content of hydrogen.33
Further TPO-MS measurements were carried out to deter-
mine the amount of coke deposited on the spent 5ZnO–Ga2O3
catalyst after 1 h of reaction. The TPO-MS profiles in Fig. 7 indi-
cate that the coke was burnt to form CO2 to a much larger ex-
tent, while a low intensity of H2O was also detected. This result
suggested that the deposited coke contained a high content of
carbon but a low amount of hydrogen, which is consistent with
the Raman spectra results (Fig. 6). Moreover, the amount of
coke speculated from the peak area of CO2 and H2O was about
0.1 wt% on the catalyst surface. Although the amount is rela-
tively low, the catalyst deactivated quickly due to the insuffi-
cient surface area.
3.2.3. Desorption behavior of the different catalysts. The
adsorption–desorption behavior of the catalysts exert significant
influence on their catalytic performances. For example, the com-
petition between propene dehydrogenation and its desorption
Fig. 5 XRD patterns of the different 5ZnO–Ga2O3 catalysts.
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At the same heating rate, the peak temperature of isobutane de-
sorption increased in the order, Ga2O3 < ZnO < 5ZnO–Ga2O3,
which revealed that it was hardest for the isobutane molecule to
desorb from the surface of the mixed catalyst, thus contributing
to an enhanced probability for isobutane conversion.
The activation energy also increased in the same se-
quence, Ga2O3 < ZnO < 5ZnO–Ga2O3. A lower activation en-
ergy of desorption means that the binding between the cata-
lyst surface and isobutane molecules is less intense and the
reactant desorbs easily, consequently reducing the probability
of the reaction. Therefore, the mixed catalyst with the biggest
desorption energy, presented a higher isobutane conversion
than both solely ZnO or Ga2O3 (Fig. 2).
3.2.4. IR Spectra of isobutane adsorbed on the mixed cata-
lyst. To further investigate the adsorption state of isobutane
over the mixed catalyst, the gaseous isobutane and isobutane
chemisorbed on the 5ZnO–Ga2O3 catalyst surface were char-
acterized using FT-IR, with the spectra shown in Fig. 8. In
Fig. 8a, the bands at 2960 and 2892 cm−1 were attributed to
the methyl antisymmetric and symmetric C–H stretching
modes, while the bands at around 1479 and 1176 cm−1 were
respectively assigned to the methyl in-plane HCH deforma-
tion and C–C stretching modes.40,41 In addition, the charac-
teristic bands observed at 1380 and 1333 cm−1 were due to
the vibration modes of tert-butyl.
Fig. 7 TPO-MS profiles of the spent 5ZnO–Ga2O3 catalyst after 1 h of
reaction.
essentially controls the selectivity for propane dehydrogenation
on a Pt catalyst.35,36 The reported TPD measurements and DFT
calculations both indicated that the addition of Sn into Pt weak-
ened propene adsorption, and consequently increases the olefin
selectivity.37 According to Yang et al.,38 with an appropriate ad-
sorption energy of the reactant, the interaction between the reac-
tant and catalyst surface was appropriately mediated, and the
ideal coverage of free sites and the adsorbed species facilitated
the adsorption of reactants on the catalyst surface and the subse-
quent desorption of the products.
After isobutane was adsorbed onto the mixed catalyst
(Fig. 8b), the bands at 2960 and 2892 cm−1 were shifted to 2966
and 2894 cm−1, with a new peak at 2953 cm−1 branching out
The desorption behaviors of Ga2O3, ZnO, and 5ZnO–Ga2O3
were investigated to understand their different dehydrogenation
performances. To estimate the desorption energy of isobutane,
the TPD method24,39 proposed by Cvetanovic et al. was adopted.
Based on the desorption peak temperature (Tp) of the TPD pro-
files collected under different heating rates (β), the activation
energy for desorption (Ed) was calculated through linear fitting
according to eqn (1), in which the influences of diffusion and
the re-adsorption of reactants were ignored (Fig. S3†). The linear
heating rates and the corresponding peak temperatures, as well
as the as-determined isobutane desorption activation energies
over Ga2O3, ZnO, and 5ZnO–Ga2O3 are summarized in Table 2.
Table 2 Summary of the linear heating rate, peak temperature and esti-
mated desorption energy over different catalysts
Ga2O3
β (K min−1
)
5
10
38
15
49
20
55
Tp (°C)
25
32
Ed (kJ mol−1
)
)
)
ZnO
β (K min−1
)
3.3
35
42
5
39
10
51
15
62
Tp (°C)
Ed (kJ mol−1
5ZnO–Ga2O3
β (K min−1
)
3.3
46
54
5
50
10
60
15
69
Fig. 8 FT-IR spectra of (a) gaseous isobutane and (b) the isobutane
outgassed from the 5ZnO–Ga2O3 catalyst at different time periods.
Tp (°C)
Ed (kJ mol−1
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from the former band and a remarkably reduced intensity of the
latter one. At the same time, the vibration modes of tert-butyl
also shifted to higher wavenumbers at ∼1384 and 1337 cm−1.
On the contrary, the two bands at 1479 and 1176 cm−1 experi-
enced a red shift to 1476 and 1153 cm−1, respectively. Compared
with the IR spectrum of gaseous isobutane (Fig. 8a), the above
variations observed for the C–H stretching mode of –CH3, the
C–C stretching mode, and the vibration mode of tert-butyl
(Fig. 8b) jointly indicate a possible double-site adsorption mode
for isobutane over the mixed 5ZnO–Ga2O3 catalyst.
Notably, two additional bands centered at 1635 and 1458
cm−1, associated with the CC stretching mode and the CH
deformation mode of olefin species appeared,40,41 as shown
in Fig. 8b. This observation suggested that the isobutane
molecules might react with the catalyst at very low tempera-
tures of ∼50 °C, thus generating abundant olefin species as
detected through IR. With the extension of the outgassing
time, the intensity of the bands assigned to isobutane de-
creased remarkably. However, no decreasing trend was ob-
served for the bands of olefin species, which indicated that
the olefin species interacted with the catalyst surface more
strongly than isobutane molecules.
given the fact that Ga2O3 itself is active for isobutane dehy-
drogenation, the performance of 5ZnO–Al2O3 is worse than
that of 5ZnO–Ga2O3.
By means of density functional theory calculations using
the cluster modeling approach, the researchers47,48 studied the
mechanism of ethane dehydrogenation over Zn/ZSM-5 and Ga/
ZSM-5 zeolite, and one-step decomposition was proven to be
both kinetically and thermodynamically favorable compared
with β-H transfer. Accordingly, the possible reaction pathways
of isobutane on the surface of the mixed ZnO–Ga2O3 catalyst
are inferred in Scheme 1. On the surface of ZnO, heterolytic
C–H bond cleavage occurs initially, and then a cyclic transition
state is formed, corresponding well with the double-site ad-
sorption of isobutane speculated from the IR spectra. After
that, the yielded isobutene and hydrogen molecules are re-
leased simultaneously to the gaseous phase rather than only
the desorption of the olefin, since the activation energy of the
latter case is 33 kJ mol−1 higher than that of the former one.48
Meanwhile, the Lewis acid sites provided by Ga2O3 effectively
promote the heterolytic cleavage of C–H bonds in isobutane,
and thus increase the conversion of isobutane.
4.2. Deactivation and regeneration of the ZnO–Ga2O3 mixed
catalyst
4. Discussion
4.1. Promoting mechanism of the ZnO–Ga2O3 mixture
According to the above activity test results and Raman spectra,
the deactivation of the mixed catalyst is primarily attributed to
coke deposition on the catalyst surface, which contained a low
content of hydrogen and exhibited a high graphitization degree.
Therefore, the deactivated catalyst was regenerated through
recalcination in an air flow at 700 °C for 0.5 h to compose one
reaction–regeneration cycle. Afterwards, the regenerated catalyst
was purged with nitrogen and reacted with isobutane for an-
other 0.5 h. As displayed in Fig. 9, during five reaction–regenera-
tion cycles, the initial conversion of isobutane and selectivity to
isobutene of each cycle remained relatively constant, and a sta-
ble isobutene yield of around 55 wt% was obtained, fully dem-
onstrating a good stability of the catalyst.
As for the aromatization reaction of light alkanes, a synergis-
tic effect between metal sites and acid sites has been
obtained for Zn or Ga-promoted HZSM-5 catalysts.42–46 The
acid sites stabilize the adsorption of alkane molecules and
activate C–H bonds, and the metal sites facilitate the transfer
and elimination of H atoms. However, with regard to the
mixed oxides studied here, there are no zeolites to supply the
acid sites. Therefore, to guarantee the ability to adsorb isobu-
tane and activate C–H bonds, the acid properties should be
provided by the oxides themselves.
According to the results of pyridine-IR tests (Fig. S4†),
both ZnO and Ga2O3 presented an inconspicuous band near
1540 cm−1, which was indicative of pyridine coordinated to
the Brønsted acid sites. As for the Lewis acid sites, a signifi-
cantly larger characterization band at around 1450 cm−1 was
observed for Ga2O3 compared to that of ZnO, further
resulting in a moderate Lewis acid amount of ZnO–Ga2O3. In
a study of light alkane activation over Zn–ZSM-5, Pidko and
Santen reported that the increased Lewis acidity facilitated
heterolytic C–H bond cleavage.47 Therefore, it was inferred
here that the introduced Ga2O3 acted as a promoter to supply
Lewis acid sites that enhance C–H bond cleavage, thus lead-
ing to an improved reactivity of the ZnO–Ga2O3 mixture com-
pared with solely ZnO or Ga2O3 (Fig. 2). To verify this point, a
mixture of ZnO and Al2O3 (providing the Lewis acid) with a
mole ratio of 5.0 was prepared, and Table S1† lists its dehy-
drogenation performance. With the introduction of Al2O3, an
isobutane conversion of up to 50.5 wt%, much larger than
that over ZnO and Al2O3, was obtained, directly confirming
the important promoting effect of a Lewis acid. However,
Scheme 1 Possible reaction pathway of isobutane over a ZnO–Ga2O3
mixture.
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Acknowledgements
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