A highly efficient Ga/ZSM-5 catalyst prepared by formic acid impregnation and in situ treatment for propane aromatization

Article abstract of DOI:10.1039/c5cy00665a

A simple method for the preparation of a Ga/ZSM-5 catalyst for propane aromatization was established by formic acid impregnation and in situ treatment. The catalyst prepared by this novel method showed remarkably superior activity of propane aromatization. Under the conditions T = 540°C, P = 100 kPa, WHSV = 6000 ml g^-1·h^-1 and with a N₂/C₃H₈ molar ratio of 2, the highest propane conversion and selectivity to BTX (benzene, toluene and xylene) achieved on H-Ga/SNSA catalyst was 53.6% and 58.0%, respectively, much higher than that of the catalyst prepared using the traditional impregnation method (38.8% and 48.2%). The catalysts were characterized by nitrogen physical adsorption, ICP-AES, DRIFT, py-FTIR, NH₃-TPD, H₂-TPR, XPS and ²⁷Al MAS NMR techniques. The characterization data indicated that this facile methodology enhanced the dispersion of the Ga species and promoted the formation of highly dispersed (GaO)⁺ species, which could exchange with the acidic protons (Bronsted acid sites) of the zeolite framework, contributing to the strong Lewis acidity. The super catalytic behavior was attributed to the synergistic effect between the strong Lewis acid sites generated by the (GaO)⁺ species and the Bronsted acid sites.

Full text of DOI:10.1039/c5cy00665a

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ARTICLE TYPE
A highly efficient Ga/ZSM-5 catalyst prepared by formic acid
impregnation and in-situ treatment for propane aromatization
a,b
a
a
a
a
He Xiao, Junfeng Zhang, Xiaoxing Wang, Qingde Zhang, Hongjuan Xie,
a
a
Yizhuo Han and Yisheng Tan *
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A simple method for the preparation of a Ga/ZSMꢀ5 catalyst for propane aromatization was established
by formic acid impregnation and in-situ treatment. The catalyst prepared by this novel method showed
remarkably superior activity of propane aromatization. Under the conditions: T = 540 °C, P = 100 kPa,
WHSV = 6000 ml/(g·h) and with a N /C H molar ratio of 2, the highest propane conversion and
1
0
2
3
8
selectivity of BTX (benzene, toluene and xylene) on HꢀGa/SNSA catalyst achieved was 53.6% and 58.0%
respectively, much higher than that of the catalyst prepared using traditional impregnation method (38.8%
and 48.2%). The catalysts were characterized by nitrogen physical adsorption, ICPꢀAES, DRIFT, Pyꢀ
2
7
FTIR, NH ꢀTPD, H ꢀTPR, XPS and Al MAS NMR techniques. The characterization data indicated that
3
2
1
5
this facile methodology enhanced the dispersion of Ga species and promoted the formation of highly
dispersed (GaO) species, which could exchange acidic protons (Brønsted acid sites) of the zeolite
+
framework contributing to the strong Lewis acidity. The super catalytic behavior was attributed to the
+
synergistic effect between strong Lewis acid sites generated by the (GaO) species and the Brønsted acid
sites.
2
0
1. Introduction
with an aqueous solution of hexadecyltrimethylammonium
bromide (CTAB) and further induced a second hydrothermal
treatment resulting in a new catalyst. The activity tests reported
improved catalytic performance when converting propane to
aromatics when compared with the conventional hydrothermally
onceꢀtreated HꢀGaMFI catalyst. The authors postulated that the
addition of CTAB led to ordered mesopores in the HꢀGaMFI
zeolite, which effectively facilitated the ingress of Ga O into the
Propane (light alkane) aromatization plays an important role in
5
5
6
6
7
7
0
5
0
5
0
5
1
economic and strategic fields and has been extensively studied in
2
3, 4
5ꢀ9
recent years. In this process, Zn , Pt and Ga modified ZSMꢀ5
zeolites are currently used as the catalysts for propane
aromatization based on their superior catalytic performance over
other catalysts. Considering the deactivation of Pt/ZSMꢀ5 and the
poor stability of Zn/ZSMꢀ5 in propane aromatization—induced
2
3
3
4
4
5
0
5
0
5
2
3
micropores during the second hydrothermal treatment forming
extra framework Ga species being highly active for alkane
by rapid coke deposition and the loss of the active component
9
(
especially for Zn loaded catalysts) —research turned to focus on
18
aromatization. Kwak et al. prepared a Ga/ZSMꢀ5 catalyst for
Ga modified ZSMꢀ5 catalysts for aromatization. Research has
shown that the structure, position and state of the Ga species on
the catalyst correlates closely to the preparation method and
treatment process, which in turn has a strong effect on the
propane aromatization performance—though the understanding
of the active Ga species catalytic mechanisms still divide the
aromatization using a CVD method together with subsequent H2
treatment. The results suggested this method allowed Ga to
migrate into the zeolite channels efficiently thereby improving
19
alkane aromatization activity. Hensen et al. prepared Ga/ZSMꢀ5
by CVD of Gaꢀalkyl species including, trimethylgallium
(
Ga(CH ) ) and the dimethylgallium analogue, on a ZSMꢀ5
1
0, 11
3 3
scientific community.
A series of methods such as
zeolite host and studied the activation of the latter precursor to
reduced and oxidized species by in-situ Ga K edge XANES. It
found that the intended reductionꢀoxidation treatment to these
hydrothermal in-situ synthesis and post synthesis (ionꢀexchange,
impregnation, chemical vapour deposition [CVD] and reductionꢀ
oxidation cycles), have been developed to prepare highly efficient
Ga modified ZSMꢀ5 catalysts for propane aromatization.
3+
Gaꢀalkyl species on the catalyst led to the formation of Ga
+
species in the form of (GaO) coordinating to the zeolite
1
2ꢀ16
Choudhary et al.
galloaluminosilicate
prepared
(HꢀGaMFI)
a
wellꢀestablished Hꢀ
framework which acted as the active sites for the aromatization of
propane. The simple and effective reductionꢀoxidation cycles
method has also been frequently adopted to prepare Ga/ZSMꢀ5
catalysts in recent years typically used for the treatment of H ꢀO
catalyst for propane
aromatization using a oneꢀstep hydrothermal synthesis method.
Their findings of the superior activity of the catalyst were
attributed to its relatively strong acidity and uniform distribution
of extra framework Gaꢀoxide species in the zeolite channels. Alꢀ
2
2
cycles to sample after the impregnation or ionꢀexchange with
6, 20ꢀ24
aqueous solutions of gallium salts. Meriaudeau et al.
studied
17
Yassir et al. mixed a hydrothermally treated HꢀGaMFI zeolite
This journal is © The Royal Society of Chemistry [year]
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in detail the effect of reductionꢀoxidation treatment to ionꢀ
exchanged Ga/ZSMꢀ5 on the dispersion and state of the Ga
species. According to the characterization data, the reductionꢀ
540 °C for 1h under static N , followed by a N flow (15 mL/min)
for 1 h to obtain the Ga/SNSA catalyst.
2
2
The precursor to the catalyst in this work, (HꢀGa/dried) was
oxidation cycles processes were shown to adjust the state of Ga 60 prepared as follows. 3 g of HZSMꢀ5 zeolite was added to 5 ml
5
0
5
0
5
0
5
0
species and thereafter enhance their diffusion into the ZSMꢀ5
zeolite channels, enhancing the dispersed Ga species (such as
Ga , GaO and to a lesser extent Ga(OH) ) on the catalyst in
substitution of framework acidic protons. In addition to the
solution of formic acid, which was composed of 0.27 g formic
acid and 0.11 g gallium nitrate. The resulting mixture was dried
at 120 °C for 8 h after mild sonicating for 1 h. Finally, the Hꢀ
Ga/SNSA catalyst was prepared by treating the HꢀGa/dried
+
+
2+
reductionꢀoxidation cycles treatment, other treatment processes 65 material in a micro fixedꢀbed reactor under the same conditions to
1
1
2
2
3
3
4
have been shown to increase the dispersion of Ga species on
that of the above Ga/SNSA. Compared with HꢀGa/SNSA, a Hꢀ
HZ/SNSA catalyst was also prepared by the same method to that
of HꢀGa/SNSA except for the omission of the gallium nitrate
precursor. To considering the factor of pH, two samples, H(half)ꢀ
2
5
Ga/ZSMꢀ5. Fricke et al.
reported improvements in Ga
dispersion as a function of pH during impregnation. The results
26
of Diakonov et al. suggested that the pH and temperature factors
of aqueous solutions greatly affect the distribution of Ga species. 70 Ga/SNSA and H(double)ꢀGa/SNSA, were also prepared by
y+
Under acid impregnation conditions, Ga(OH) precursors, such
varying the amount of formic acid to either half or double the
original formic acid volume, respectively. In addition, alternative
acids such as acetic acid, nitric acid and citric acid were used to
determine the impact of the resulting catalysts. This group of
x
+
2+
as Ga(OH)2 and Ga(OH) formed and were easily transported
into the ZSMꢀ5 zeolite channels, interacting with the acid protons
2
1
to generate active Ga species. Furthermore, steam treatment is
an alternative method to enhance the dispersion of Ga species and 75 catalysts were defined as: H(acetic)ꢀGa/SNSA, H(nitric)ꢀ
27
promote the formation of extra framework active Ga species.
Ga/SNSA and H(citric)ꢀGa/SNSA, respectively.
In this work, a novel method was adopted to prepare a highly
active Ga/ZSMꢀ5 catalyst for propane aromatization through
improved dispersion and state of the Ga species. The referred to
treatment process consists of two steps: preꢀtreatment (formic
2
.2 Catalyst characterization
The textural properties of catalyst were measured by nitrogen
physical sorption at 77K using a Tristar 3000 machine. Surface
acid impregnation) and in-situ treatment (calcination under static 80 areas were calculated by the BET method and microꢀ, mesoꢀ and
N2 atmosphere and treatment of steam generated by
decomposition of containedꢀGa species such as hydration or
hydroxylation compounds, etc. and thermal dehydration of the
zeolite catalyst) of HZSMꢀ5. The resulting catalyst was
macropore volumes by the tꢀplot method.
Diffuse reflectance infrared spectra (DRIFT) measurements
were performed using a Bruker Tensor 27 instrument with a MCT
−1
detector (64 scans, 4 cm ). Weight approximately 20 mg of
designated as HꢀGa/SNSA. The test results indicated that the Hꢀ 85 catalyst and put it in an infrared cell with KBr windows for inꢀ
Ga/SNSA exhibited high catalytic performance for propane
aromatization in comparison with catalyst prepared by
impregnation method. To understand the origin of the enhanced
catalytic performance, characterization techniques such as
situ treatments. The DRIFT spectra were recorded after treating
the catalyst at 300 °C for 1 h with argon flow.
Pyridineꢀadsorbed Fourier Transform Infrared Spectroscopy
(PyꢀFTIR) was used to determine the amount of Brønsted and
nitrogen physical adsorption, ICPꢀAES, DRIFT, PyꢀFTIR, NH ꢀ 90 Lewis acid sites in Bruker Tensor 27 equipment. Approximately
3
2
7
ꢀ1
TPD, H ꢀTPR, XPS and Al MAS NMR were used. We found
20 mg of catalyst was pressed into a regular wafer (R = 1.3 cm )
2
that this novel treatment process improved the Ga dispersion
and put in an infrared cell. The determination of the infrared
spectrum was made after sample treatment at 400 °C for 2 h
under vacuum, and this spectrum was used as background for the
+
efficiency generating ample extra framework active (GaO)
species. The superior activity of HꢀGa/SNSA was attributed to a
synergistic effect between strong Lewis acid sites generated by 95 adsorbed pyridine experiments. Pyridine was then adsorbed to a
+
ꢀ2
(
GaO) species and Brønsted acid sites.
5.0×10 Pa equilibrium pressure at 40 °C. FTIR spectra were
recorded after consecutive evacuation at 150, 250, 350 and 450
°C.
2
. Experimental section
The temperatureꢀprogrammed desorption of ammonia (NH3ꢀ
00 TPD) was used to test the amount and strength of the acid sites of
2
.1 Catalyst preparation
1
HZSMꢀ5 (silica to aluminium mole ratio = 19) zeolites were
bought from Nankai University. The catalysts were prepared as
follows. Typically, 3 g of HZSMꢀ5 was added to 5 ml aqueous
solution containing 0.11 g of gallium nitrate. The resulted
asꢀprepared catalysts in TPꢀ5080 chemisorption instrument. The
45
50
55
catalyst (100 mg) was preꢀtreated at 500 °C under a flow of N
(30 ml/min) for 2 h and then cooled down to 100 °C. Then NH
2
3
was introduced into the flow system. The TPD spectra were
mixture was sonicated for 1 h at room temperature, followed by 105 recorded at a temperature rising rate of 10 °C/min from 100 °C to
drying at 120 °C for 8 h, and the obtained precursor defined as
Ga/dried. Thereafter, the Ga/dried material was calcined at 540
700 °C.
The H
ꢀTemperatureꢀprogrammed reduction (H ꢀTPR) was
2
2
°
C for 5 h under static air in a muffle oven to obtain the
carried out to study reducibility of the catalysts with
chemisorption instrument (TPꢀ5080). The catalyst (100 mg) was
Ga/calcined catalyst. Likewise, 0.15 g of the Ga/dried material
was pressed and sieved into 30ꢀ50 mesh fractions and treated in a 110 pretreated at 500 °C under a flow of N
(32 ml/min) for 1 h and
2
micro fixedꢀbed reactor for 0.5 h with a N flow (15 mL/min) to
then cooled down to 100 °C, then changed to H /N = 0.09
2
2
2
completely remove other gases and moisture. The reactor was
then heated to 540 °C at the ramp rate of 6 °C /min and kept at
mixture (35 ml/min). The temperatureꢀprogrammed reduction
was performed between 100 °C and 900 °C with a heating rate of
1
0 °C /min.
2
| Catal. Sci. Technol., [2015], [vol], 00–00
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The chemical compositions (Si, Al and Ga) of asꢀprepared
samples were determined by an inductively coupled plasmaꢀ
atomic emission spectrometer (ICPꢀAES) method using Thermo
iCAP 6300 equipment.
5
0
5
Xꢀray photoelectron spectroscopy (XPS) was used to analyze
the change of surface composition measured by AXIS ULTRA
DLD equipment. The following photoelectron lines were
recorded: Ga2p3/2, Ga2p1/2, Ga3d, Si2p Al2p, O1s, O2s and C1s.
The binding energy values were corrected for charging effect by
referring to the adventitious C1s line at 284.5 eV.
1
27
Al MAS NMR measurements were performed on Bruker
2
7
AVANCE III 600 MHz equipment. Al MAS NMR spectra were
obtained by a single pulse length of Π/6, and Al chemical shifts
were referenced to Al(NO ) .
3
3
1
The
thermal
gravimetryꢀmass
spectrum
(TGꢀMS)
measurements were performed under a flow of argon (30 ml/min)
in the temperature range from 120 to 600 °C with a heating rate of
1
0 °C /min on an Evolution and OMNI star equipment.
2
.3 Catalyst evaluation
2
2
3
0
5
0
The catalyst test for propane aromatization was carried out in a
horizontal quartz tube fixedꢀbed reactor at T = 540 °C, P = 100
kPa, WHSV = 6000 ml/(g·h) and with a N /C H molar ratio of 2.
Outlet gas was analysed after 0.5ꢀ4.5 h reaction.
The products were analysed by online gas chromatograph
(HUAAI GC 9560) equipped with a flame ionization detector
5
5
Fig. 1 Propane conversion (A) and selectivity of BTX (B) as a function of
time on stream for asꢀprepared catalysts. Reaction conditions: P = 100
kPa, T = 540 °C, WHSV = 6000 ml/(g·h) and N /C H molar ratio = 2.
2
3
8
2
3
8
shows that this simple method is highly efficient for improving
Ga activity on modified HZSMꢀ5 when compared with the
catalyst prepared by the traditional impregnation method. As
observed in Fig.1, Ga/SNSA shows a lower activity for propane
aromatization than HꢀGa/SNSA, however, the observed activity
is higher than Ga/calcined. Therefore the implication is that the
formic acid impregnation and in-situ treatment play equally
important roles in improving the catalytic performance of Hꢀ
Ga/SNSA, which principally takes effect on the Ga species.
In addition, the stability of HꢀGa/SNSA is also investigated in
comparison with that of Ga/calcined during two cycles of 900
min timeꢀonꢀstream (Fig. S1). The results (Fig. S1A) reveal that
60
(FID) with HPꢀINNOWAX capillary column, offꢀline gas
chromatograph (HUAAI GC 9560) equipped with a FID with
Al O packed column, and offline gas chromatograph (East ＆
2
3
West GC 4000A) equipped with a thermal conductivity detector
(TCD) with carbon molecular sieves packed column, respectively.
The propane conversion and product selectivity were calculated
using the following Eqs. (1) and (2).
65
ꢇꢈꢉꢊꢋꢉꢌꢍꢎ,ꢏꢍꢐꢈꢉꢊꢋꢉꢌꢍꢎ,ꢋꢑꢒ
ꢓ
Xꢀꢁꢂꢀꢃꢄꢅ
ꢆ
ꢔ 100%
(1)
ꢈ
ꢉꢊꢋꢉꢌꢍꢎ,ꢏꢍ
ꢈ
ꢏ,ꢋꢑꢒ
70 HꢀGa/SNSA and Ga/calcined both show gradual deactivation
behavior under the conditions of large space velocity as a
function of time on stream, but HꢀGa/SNSA prepared using our
proposed novel method shows much better catalytic performance
than Ga/calcined prepared using conventional impregnation
75 method always during the whole test period. On the other hand,
from the test results of regenerated catalysts in Fig. S1B, it can be
observed that activities of HꢀGa/SNSA and Ga/calcined can be
recovered substantially through a thermoꢀtreatment to remove
coke deposition at 540 °C in air. Meanwhile, stability patterns of
Sꢕ ꢆꢖ∑
ꢔ
100%
(2)
ꢈꢏ,ꢋꢑꢒ
3
5
where Xpropane is the conversion of propane, S is the selectivity of
i
target product i (i = BTX, CH , etc), N
the numbers of moles of propane in the inlet and outlet gas phases,
respectively, and Ni,out is the total number of moles of product i.
and Npropane,out are
propane,in
4
3
. Results and discussion
4
0
3.1 Catalytic activity
Fig. 1A and Fig. 1B show the propane conversion and selectivity 80 HꢀGa/SNSA and Ga/calcined are similar to those before
of BTX (benzene, toluene and xylene) over asꢀprepared catalysts
respectively. The data clearly shows propane conversion and
selectivity of BTX over Ga/calcined and HꢀGa/SNSA are
superior to that over HZSMꢀ5 and HꢀHZ/SNSA respectively,
regeneration as a function of time on stream. Furthermore, the
propane conversion and selectivity of BTX over HꢀGa/SNSA are
still higher than those on Ga/calcined for the whole reaction time.
The results of the durability and regeneration tests suggest that
4
5
suggesting the introduction of Ga to ZSMꢀ5 (treated or untreated) 85 the catalyst crystals are reasonably stable under the conditions of
improves the catalytic performance. At the same time, it is
noticeable that the catalytic activity of HꢀHZ/SNSA is lower for
propane aromatization than that of HZSMꢀ5, while that of Hꢀ
Ga/SNSA is higher than Ga/calcined and HZSMꢀ5. The catalytic
data indicates that this novel treatment process exclusively takes
effect on the Ga species rather than the ZSMꢀ5 zeolite as the
reason for improved catalytic behavior. Furthermore, Fig. 1 also
the aromatization reaction.
.2 Catalyst characterization
3.2.1 Catalyst composition and textural properties
3
5
0
21, 28ꢀ31
Previous researches
show that the XPS technique is
90
certainly effective for charactering the situation on the surface of
catalysts. Chemical states and the quantity of the elements that
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are present within the top 1ꢀ12 nm of the sample surface can be
obtained by this technique. The bulk and surface compositions of
the asꢀprepared catalysts are listed in Table. 1. As observed,
comparison between ICPꢀAES and XPS data for Ga modified
HZSMꢀ5 zeolites shows surface of these catalysts are enriched in
aluminum according to the smaller Si/Al ratios measured by XPS
than the bulk Si/Al ratios determined by ICPꢀAES. After preꢀ
treatment to HZSMꢀ5 (impregnated with the bear gallium nitrate
aqueous solution with and without formic acid, respectively), the
surface aluminium content of the resulting Ga/dried and Hꢀ
Ga/dried samples are higher than that of untreated HZSMꢀ5
according to the surface Si/Al ratios, surface Al mol%, Al /Al
5
0
5
0
5
0
5
1
1
2
2
3
3
Td
Oh
4
0
ratios and Al % (Td: fourꢀcoordinate framework Al and Oh:
Oh
Fig. 2 DRIFT spectra in the OH stretching region of asꢀprepared ZSMꢀ5
zeolites. (a) HZSMꢀ5; (b) HꢀHZ/SNSA (c) Ga/calcined; (d) Ga/SNSA; (e)
HꢀGa/SNSA.
2
7
octahedral extra framework Al as shown in Fig. S2). The
spectra indicate that the framework of the precursor suffers from
dealumination, especially for the sample impregnated by both
gallium nitrate and formic acid, the dealumination is very severe.
As for the samples after in-situ treatment such as Ga/SNSA and
HꢀGa/SNSA, their surface aluminium contents are higher than
that of Ga/dried and HꢀGa/dried, respectively. The results suggest
that a second dealumination of the framework occurs during the
in-situ treatment. Compared with that of Ga/dried, a lower
surface aluminium content of Ga/calcined indicates that a portion
of the surface aluminium species of Ga/calcined reꢀinsert into the
intracrystalline region. The HꢀGa/SNSA zeolite undergoes the
the quantity of elements can be distinguished obviously by the
results of XPS at the same test conditions based on the same kind
of materials. Furthermore, the galliumꢀtoꢀframework aluminium
ratio (Ga/AlFM ) of the Ga modified zeolites, measured by ICPꢀ
AES (Ga) and NMR spectroscopy (AlFM ), can be used to judge
the dispersion of the Ga species, because it’s positively related to
4
5
5
6
6
5
0
5
0
5
32
the dispersion of the Ga species. The ratios decrease in the
following order for the three catalysts: HꢀGa/SNSA (0.160) >
Ga/SNSA (0.154) > Ga/calcined (0.150). It is inferred that the Hꢀ
Ga/SNSA catalyst holds a higher degree of Ga dispersion over
that of the other catalysts suggesting that this simple method has
promise for catalytic applications.
1
7
second dealumination perhaps because of the presence of steam
generated by selfꢀdecomposition of surface Ga (and other)
species—inferred from Fig.S3. In addition, it can be seen that
surface Si/Al ratios of the catalysts are lower than that of its bulk.
It is probable that the partial surface Ga species could transport
into the zeolite intracrystalline region. Surface Si/Ga ratios of Hꢀ
Ga/SNSA and Ga/SNSA are higher than those of HꢀGa/dried and
Ga/dried respectively, which indicates that the in-situ treatment
step is in favour of promoting the surface Ga into the
intracrystalline region/channels of zeolites. However, the surface
Si/Ga ratio of Ga/calcined is almost equal to that of Ga/dried.
This implies that calcination under air greatly hinders the
insertion of Ga into the intracrystalline region/channels of zeolite.
To some extent, distinctions among the catalysts with regard to
Table. 2 reports the textural properties of the asꢀprepared
catalysts. One can observe that the micropore surface area and
micropore volumes for the Ga modified ZSMꢀ5 zeolites
uniformly decrease in comparison with that for pure ZSMꢀ5.
Understandably, the decrease is because of pore blockage resulted
3
2
from the dealumination and incorporation of Ga. In addition, no
significant changes are observed in the surface area and pore
volume in the mesoꢀ and macropores regions. Based on the
results of catalyst testing, it is plausible that the textual properties
of the catalysts do not correlate with their catalytic activity. This
32
is consistent with the results of Rodrigues et al.
Table 1 Bulk and surface compositions of asꢀprepared ZSMꢀ5 zeolites.
b
c
c
Si/Al ratio
Al mol%
Al /Al ratio
Al %
Si/Ga ratio
Surface
Td
Oh
Oh
a
b
a
b
Sample
Bulk
19.1
18.9
18.8
18.8
19.1
Surface
Surface
1.53
1.62
1.69
1.74
Bulk
/
150
152
152
149
149
HZSMꢀ5
Ga/calcined
Ga/dried
Ga/SNSA
HꢀGa/dried
20.9
17.2
16.9
16.4
14.7
10.9
18.3
15.7
14.6
10.8
11.1
8.51
5.18
5.99
6.39
8.42
8.24
10.5
/
21.2
20.1
35.1
11.5
18.9
1.92
3.67
HꢀGa/SNSA 19.1
a
b
c
27
determined by ICPꢀAES method; determined by XPS; determined by Al MAS NMR..
Table 2 Physical parameters of different asꢀprepared ZSMꢀ5 zeolites
a
a
b
b
S
S
Sext
(m /g)
95.8
99.1
99.1
99.2
V
V
V
ext
BET
micro
total
micro
2
2
2
2
2
2
Sample
(m /g)
420
400
397
367
b
(m /g)
324
301
298
(m /g)
0.185
0.180
0.178
0.167
(m /g)
0.129
0.121
0.119
0.108
(m /g)
0.056
0.059
0.059
0.059
HZSMꢀ5
Ga/calcined
Ga/SNSA
HꢀGa/SNSA
a
267
0
70
determined by tꢀmethod; determined by Volume adsorbed at p/p =0.97.
4
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DOI: 10.1039/C5CY00665A
1
2
2
3
5
0
5
0
3.2.2 DRIFT and Pyridine-adsorption FTIR.
The infrared spectra in the OH stretching region for the catalysts
ꢀ
1
are shown in Fig. 2. In this region, the peak at 3601 cm is
ascribed to the stretching vibration of Si(OH)Al bridged bonds,
2
5
which are associated with the zeolite Brønsted acid sites. By
comparing the Ga/calcined material with HZSMꢀ5, a reduction in
the intensity of this peak can be observed with Ga incorporation
analogous to that of HꢀGa/SNSA compared with HꢀHZ/SNSA.
The spectra suggest that the number of Brønsted acid sites
decreases because of the exchange of acidic protons (Brønsted
33
acid sites) with Ga species. Another important feature is that the
ꢀ
1
intensity of the peak centred close to 3650 cm , which is usually
2
5
associated with extraꢀframework aluminium OH species, is
enhanced with incorporation of Ga, indicating that the
dealumination has occurred as previously described, which has
been verified from the results of solidꢀstate NMR and XPS in
ꢀ
1
Table. 1. In addition, the intensity of the peak ~3671 cm also
increases, implying that the extraꢀframework gallium OH species
are formed with quantitative incorporation of Ga, accordingly.
The FTIR spectra of the pyridine adsorption on the catalysts
and the quantified results of different acid sites are shown in Fig.
3
4
4
5
5
6
6
5
0
5
0
5
0
5
−1
3
. The 1540 and 1450 cm bands are attributed to pyridine
adsorption on Brønsted acid sites and Lewis acid sites,
−
1
respectively. As for the 1490 cm band, it is believed to be a coꢀ
contribution of pyridine adsorption on the Lewis sites as well as
3
2
on Brønsted acid sites (as shown in Fig. 3A). In this work, the
ꢀ
1
peaks ~1450 and 1540 cm are used to quantify separately the
Lewis and Brønsted acid sites. From Fig. 3B, it is clearly
observed that the amount of Brønsted sites declines as a function
of increasing evacuation treatment temperature for all samples.
The introduction of Ga to materials such as Ga/calcined and Hꢀ
Ga/SNSA leads to a modest decline in the number of Brønsted
acid sites. Yet the decline in Brønsted acid sites is greatly
deteriorated when those intermediate precursors were
impregnated with formic acid followed by in-situ treatment. The
data agrees with the previously shown infrared results in the OH
stretching region. Fig. 3C shows pyridine adsorbed on Lewis acid
sites. The observation is that Lewis acid sites of HZSMꢀ5 and Hꢀ
HZ/SNSA are almost completely removed after evacuation at 250
°
C. However, as for other samples, especially HꢀGa/SNSA,
pyridine absorption is still present even at temperatures as high as
50 °C, indicating that stronger Lewis acid sites than originally
4
present in the zeolite appear as a result of Ga incorporation via
formic acid impregnation and in-situ treatment. Already
32,34ꢀ37
published work
studying Zn and Ga modified zeolites, also
concluded that the Brønsted acid sites were replaced by strong
Lewis acid sites after Zn or Ga incorporation, and the strong
Lewis acid sites associated with Ga modified ZSMꢀ5 zeolites
were related to highly dispersed Ga species. According to
previous test results, the Ga/SNSA and HꢀGa/SNSA materials
have good catalytic activity. However, these two catalysts show
the lowest ratio of Brønsted acid site/Lewis acid site as shown in
Fig. 3D. Therefore it is feasible that tuning the ratio of Brønsted
acid site/Lewis acid site has a significant effect on catalytic
activity of propane aromatization, that is, the synergies between
Fig. 3 The FTIR spectra of the pyridine adsorption on the catalysts and
5
0
the quantified results of different acid sites. (A) Spectrum of HZSMꢀ5
after evacuation at 450 °C; (B) Brønsted acid site concentration of
samples at different temperatures; (C) Lewis acid site concentration of
samples at different temperatures; (D) Brønsted acid site
concentration/Lewis acid site concentration ratios of samples at different
temperatures.
1
70 Brønsted acid sites and Lewis acid sites have a significant and
direct impact on propane aromatization.
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DOI: 10.1039/C5CY00665A
60
3
.2.3 NH -TPD.
3
The distribution of acidity on asꢀprepared catalysts was measured
by NH ꢀTPD and the results shown in Fig. 4. All catalysts show
3
5
0
5
two desorption peaks of NH centred at 150–340 °C and 360–590
3
°C, which are associated with weak acid sites and strong acid
sites, respectively. The desorption peaks of the Ga modified
samples at 150–340 °C show reduced intensity than that of ZSMꢀ
5
. However, the intensity of the desorption peak of HꢀHZ/SNSA
1
at 360–590 °C dramatically declines, which implies that the sites
associated with strong acidity are no longer present. This mainly
results from zeolite dealumination via formic acid impregnation.
The desorption peaks of Ga/calcined ascribed to the strong acid
sites slightly decreases, however, the peak position shifts to
higher temperatures (~500 °C), indicating that the acidic strength
can be enhanced as a function of Ga incorporation. As for
Ga/SNSA and HꢀGa/SNSA, the peak area at 360–590 °C is
almost identical with both materials displaying a higher degree of
reduction than that of the other three catalysts. It is plausible that
the in-situ treatment to (Hꢀ)Ga/dried precursor leads to a decline
in the numbers of acid sites because of the exchange of acidic
protons (Brønsted acid sites) with ionic gallium species entering
into zeolite intracrystalline region/ channels.
1
Fig.4 NH
3
ꢀTPD profiles of different asꢀprepared ZSMꢀ5 zeolites. (a)
HZSMꢀ5; (b) HꢀHZ/SNSA (c) Ga/calcined; (d) Ga/SNSA; (e) Hꢀ
Ga/SNSA.
20
3
.2.4 H -TPR.
2
25
H ꢀTPR profiles of the asꢀprepared samples are displayed in Fig.
2
5
. Pure Ga O exhibits two reduction peaks, centred close to 507
2 3
°
C and 771 °C (in Fig. 5), which are ascribed to small Ga O
2 3
2
1
particles and segregated bulk Ga O , respectively. Upon Ga
2
3
introduction to the ZSMꢀ5 zeolite, the reduction becomes difficult,
and as such the two reduction peaks of the Ga modified ZSMꢀ5
catalysts translate to higher temperatures (centred ~528ꢀ533 °C
and 810ꢀ839 °C) as observed in curves b, c and d, Fig. 5. The
principal reason in the increase of the reduction temperature is
related to the Ga O species being highly dispersed on the ZSMꢀ5
30
35
40
2
3
zeolite host. Besides, an additional, and new reduction peak
centred at ~648ꢀ671 °C for the Ga modified ZSMꢀ5 catalysts is
65
1
8, 23, 24, 38
present. Previous literature
has reported the strong
Fig. 5 H ꢀTPR profiles of different samples. (a) Ga O ; (b)Ga/calcined;
2
2
3
interaction between highly dispersed Ga species and ZSMꢀ5
(c)Ga/SNSA;(d) HꢀGa/ SNSA
+
+
zeolite produced (GaO) (III) species, while the (GaO) (III)
+
species could be further reduced to Ga (I). The peak associated
with the reduction from (GaO) (III) to Ga (I) corresponds to the
peak at 648ꢀ671 °C. Through fitting of the TPR profiles and
+
+
+
calculating their peak areas, the data suggests the (GaO) content
of HꢀGa/SNSA is higher than that of the Ga/calcined and
Ga/SNSA (Table S1).
45
3
.2.5 XPS.
The characterization of XPS is effective for chemical states of the
2
1,
elements on the surface of the zeolites. Already published work
2
5, 39, 40
has reported that XPS technique is used to characterize the
50
states of Ga in nonꢀframework or extraꢀframework positions on
the surface of the catalysts. Fig.6 shows Ga 2p3/2 XPS spectra of
the asꢀprepared catalysts. It can be observed that the Ga 2p3/2
binding energy value (BEs) for Ga/calcined (1119.1 eV) is higher
than that for Ga O (1117.9 eV), indicating that a strong
2
3
55
interaction between the ionized Ga species and ZSMꢀ5 zeolite,
while the Ga 2p3/2 BEs for the HꢀGa/SNSA (1118.7 eV) is lower
Fig. 6 Ga 2p3/2 XPS spectra of different samples. (a) Ga O ; (b)
2
3
than that for Ga/calcined. This decrease of BEs for Ga/SNSA is 70 Ga/calcined; (c) Ga/SNSA; (d) HꢀGa/SNSA.
related to the strong covalent bonding character between the
surface highly dispersed Ga species (GaO) and the framework of
21
+
the ZSMꢀ5 zeolite according to Izabela Nowak et al. The XPS
6
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DOI: 10.1039/C5CY00665A
data suggests that this novel treatment method is favourable for
+
the formation of (GaO) species. It has been proved that highly
+
dispersed (GaO) species which exchange with acidic protons
(
Brønsted acid sites) of the zeolite framework presented as Siꢀ
+
5
O((GaO) )ꢀAl in extraꢀframework positions on the surface of the
3
2
catalysts, contribute to strong Lewis acidity , concordant with
the PyꢀFTIR results that there are greater numbers of strong
Lewis acid sites on the HꢀGa/SNSA material.
3
.3 Discussion
1, 25
1
1
2
2
3
3
4
4
5
5
0
5
0
5
0
5
0
5
0
5
According to previous literature
and the results reported
herein, the dispersion and state of the Ga species within the
catalyst strongly dictated their activity to propane aromatization.
3
+
The addition of trivalent Ga
ions into ZSMꢀ5 using
conventional impregnation methods is quite difficult because of
the highly positive electrostatic charge and the bulky size of
Ga(H O) ] aquacomplexes, and thus Ga ions tend to reside
2 6
predominantly on the external surface of zeolite as single
extracrystalline Ga O . Hence, prepared catalysts by this
methodology display suboptimal activity. However, in the case of
3+
41
3+
[
Fig. 7 Propane conversion and selectivity of BTX on asꢀprepared ZSMꢀ5
6
0
zeolites. Reaction conditions: P = 100 kPa, T = 540 °C, WHSV = 6000
2
3
2 3 8
ml/(g·h), N /C H molar ratio = 2, time on stream is 0.5h.(a) Ga/calcined;
+
(b)HꢀGa/SNSA;(c)H(half)ꢀGa/SNSA;(d)H(double)ꢀa/SNSA;(e)H(acetic)ꢀ
our prepared HꢀGa/SNSA catalyst, the Ga component (GaO) )
Ga/SNSA; (f) H(nitric)ꢀGa/SNSA;(g) H(citric)ꢀGa/SNSA.
was found to be well dispersed within the zeolite intracrystalline
region/channels via formic acid impregnation and in-situ
treatment method is remarkably generic to allow preparation of
highly active catalysts for propane aromatization. As reported,
the Ga(OH)x (x = 1, 2; y = 3 − x) species can be produced in an
26
y+
y+
acid environment. In our experiment, an abundance of Ga(OH)x
species are accordingly formed on catalyst surface because of the
presence of formic acid. The formed precursor species were
prone to move into the zeolite channels and interact with
framework protons (Si(OH)Al) of the ZSMꢀ5. Finally, the linkage
between Ga and ZSMꢀ5 is favourable for subsequent treatment
procedures to prepare highly dispersed and active Ga modified
catalysts. In addition, it is reported that the pH value of the
solution during impregnation affects the dispersion of Ga
species. Therefore, the quantity and nature of the acid used
certainly leads to different Ga dispersion. The samples prepared 70 process. Here, zeolite texture is probably affected during
by treatment with different acids and their amount, for example,
with formic acid (such as H(half)ꢀGa/SNSA and H(double)ꢀ
Ga/SNSA), differ in their catalytic activities (shown in Fig. 7).
The catalytic activities of H(half)ꢀGa/SNSA and H(double)ꢀ
65
Scheme 1 Probable mechanistic route of HꢀGa/SNSA
strength to formic acid, H(acetic)ꢀGa/SNSA shows slight change
of activity. As for H(citric)ꢀGa/SNSA, the residues of citric acid
can be only removed off through subsequent calcination
2
5
44
calcination, thereby the activity of H(citric)ꢀGa/SNSA decreases.
y+
In the case of in-situ treatment, the above mentioned Ga(OH)
x
+
species decomposed to (GaO) under a static N atmosphere at
high temperature as Eq. (3). At the same time, a significant
2
32
Ga/SNSA are lower than that of HꢀGa/SNSA but remain higher 75 quantity of steam was generated and maintained within the
than that of Ga/calcined. It suggests that an optimum acidity to
the solution is required to optimize the catalyst properties,
thereby leading to differences in activity. As for the catalysts
impregnated by acetic acid, nitric acid and citric acid,
improvements in their respective activities are significant
together with their selectivity toward BTX and are similar to Hꢀ
Ga/SNSA. The results indicate that the acidity of the solution
plays a crucial role the observed activity, as each catalyst results
in differing degrees of Ga dispersion. On the other hand, one
should not neglect that the acid treatment has a modest effect on
the zeolite structure/morphology such as pore size, dealumination
and acidity, etc. Although the increase of amount of formic acid
is in favour for the formation of Ga(OH) precursors, such as
reactor. The
decomposition of the samples was monitored by TGꢀMS (Fig.S3).
Z−Ga(OH)2+ → Z−GaO + H O
+
(3)
2
−
80
(Z represents an ionꢀexchange site at the zeolite framework)
45, 46
Ausavasukhi and Sooknoi
Ga O of Ga/HZSMꢀ5 (prepared by conventional impregnation
2 3
reported that part of the bulk
2
5
methods) could be reꢀdispersed under the current steaming
conditions, thereby forming GaO(OH), which can easily interact
with protons from Brønsted acid sites and further convert to extra
85
+
y+
framework (GaO) species (as show in Eq.(4)) and Eq. (5)).
x
+
2+
Ga(OH)2 and Ga(OH) , the dealumanition and damage to the
ZSMꢀ5 framework are remarkably severe as well as the increased
Ga O + H O → 2GaO(OH)
(4)
(5)
2
3
2
4
2, 43
+
+
acid strength of nitrate acid solution.
Because of near acid
GaO(OH) + H (Brønsted sites) → GaO + H O
z 2
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DOI: 10.1039/C5CY00665A
y+
3+
In our experiment, Ga(OH)x or [Ga(H O) ] species, within
catalyst prepared using traditional impregnation method (38.8%
and 48.2%) when tested after 0.5 h onꢀstream. Furthermore, the
2
6
the formic acid impregnated samples, under the experimental
conditions, also should transform into extracrystalline Ga O3 60 activity of HꢀGa/SNSA was shown to be constantly higher than
2
during the air drying at 120 °C for 8 h. Here, the extracrystalline
that of Ga/calcined in an evaluation period of 4.5 h.
+
5
0
5
0
5
0
5
0
5
0
Ga O can be further converted into (GaO) species under a steam
Characterization results such as BET, XPS, H ꢀTPR, etc.
2
3
2
environment. Based on the above, the novel formic acid
impregnation and in-situ treatment method favors promoting the
suggested that formic acid impregnation and in-situ treatment
both made the ZSMꢀ5 zeolites exposed to dealumination.
surface Ga into the intracrystalline region forming highly 65 Simultaneously, this novel treatment process enhanced the
+
dispersed active (GaO) species.
dispersion of gallium species and promoted the formation of
+
1
1
2
2
3
3
4
4
5
According to above, a probable mechanistic route for the Hꢀ
Ga/SNSA catalyst can be deduced as scheme 1. Briefly speaking,
highly dispersed (GaO) species in the zeolite intracrystalline
region through decomposition of intermediate species, such as
y+
y+
in the process of formic acid impregnation, the Ga(OH)x species
Ga(OH)x and the interaction between Ga O and water (steam).
2
3
+
are formed under the acidic environment. Subsequently, a portion 70 (GaO) species were shown to exchange acidic protons (Brønsted
y+
of the Ga(OH)x
species are inevitably converted to
acid sites) of the zeolite framework and contributed to the strong
Lewis acidity, which was favourable for propane
dehydrogenation as well as the contribution of the Brønsted acid
extracrystalline Ga O via drying, while the remainder moves into
2
3
the ZSMꢀ5 intracrystalline region, owing to their small size, and
interacts with the framework of the ZSMꢀ5 zeolites. When the
resulting sample is subjected to the in-situ treatment, the 75 strong Lewis acid sites generated by the (GaO) species and the
sites to the whole aromatization processes. It is apparent that the
+
y+
undecomposed Ga(OH)
species are further converted to
Brønsted acid sites have a synergistic effect on propane
aromatization.
x
+
(GaO) , accompanied by the byꢀproduct steam, wherein, the
extracrystalline Ga O also easily reacts with the steam, finally
leading to the formation of highly active (GaO) species in
abundance.
2
3
+
Acknowledgements
This research was supported by Chinese Academy of Sciences
Knowledge Innovation Project KGCX2ꢀYWꢀ318ꢀ1 from Chinese
Academy of Sciences.
Propane aromatization contains a series of processes: olefin
formation through dehydrogenation of propane to propene,
oligomerization of olefins, subsequent cracking (olefin
interconversion), formation of new oligomers through olefin
8
0
Notes and references
alkylation followed by
dehydrogenation, and cyclization and aromatization. Among
them, the dehydrogenation of propane is one of most critical
a
hydrogen transfer and/or
4
7
a.
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry,
Chinese Academy of Sciences, Taiyuan 030001, China;
b
1
85 University of Chinese Academy of Sciences, Beijing 100049, China.
process of propane aromatization. Generally, it is thought that
†
Eꢀmail: tan@sxicc.ac.cn. Tel:+86ꢀ351ꢀ4044287. Fax:+86ꢀ351ꢀ4044287.
the acidity of the metalꢀmodified zeolite catalyst is quite an
important and relevant factor affecting catalytic activity in
Supplementary text; Fig.S1ꢀS3; Table S1. See DOI: 10.1039/b000000x/
heterogeneous catalysis reactions. In our present catalytic system,
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+
the highly dispersed (GaO) species generate strong Lewis acid 90 2. M. S. Scurrell, Appl. Catal., 1988, 41, 89ꢀ98.
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4
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6
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.
.
.
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95
+
7
8
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.
B. S. Kwak, W. M. H. Sachtler and W. O. Haag, J. Catal. , 1994,
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1
49, 465ꢀ473.
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1
1
1
00 9. G. Krishnamurthy, A. Bhan and W. N. Delgass, J. Catal. , 2010, 271,
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1
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At this stage of the research, it could not be easily established
+
as to how exactly the (GaO) species present on the catalyst are
+
converted to the reduced state species (Z−Ga ) during
05
10
1
7, 22, 25, 48ꢀ50
aromatization as other literatures
however, on at least
12. V. R. Choudhary, A. K. Kinage, C. Sivadinarayana, P. Devadas, S.
+
one count, the (GaO) species are quite a useful intermediate for
propane aromatization. The work is still onꢀgoing.
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1
1
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55
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1
and selectivity of BTX on the HꢀGa/SNSA catalyst achieved
3.6% and 58.0% respectively—significantly higher than that of
5
8
|
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Catal. Sci. Technol., [2015], [vol], 00–00 | 9

Catalysis Science & Technology
Page 10 of 10
View Article Online
DOI: 10.1039/C5CY00665A
A highly efficient Ga/ZSM-5 catalyst prepared by formic acid
impregnation and in-situ treatment shows remarkably superior
activity for propane aromatization.