ZnO Nanoclusters Supported on Dealuminated Zeolite β as a Novel Catalyst for Direct Dehydrogenation of Propane to Propylene

Article abstract of DOI:10.1002/cctc.201801708

Small ZnO nanoclusters supported on dealuminated β zeolite were prepared and evaluated for catalyzing direct dehydrogenation of propane to propylene (PDH), exhibiting high catalytic performance. N₂ sorption, XRD, TEM, ²⁷Al and ²⁸Si MAS NMR, IR, XRF, DR UV-vis, XPS, and NH₃-TPD techniques were employed to characterize the physicochemical properties of this novel catalyst system. It is found that the Zn species can be accommodated in the vacant T-atom sites of dealuminated β zeolite due to the reaction of aqueous zinc acetate solution with silanol groups, and thus, producing massive small ZnO nanoclusters as active phases in PDH. Additionally, dealuminated β zeolite can greatly depress side reactions attributable to the absence of strong acid sites, thereby guaranteeing high catalytic activity, propylene selectivity and stability. As a result, the optimal catalyst of 10 wt% Zn loaded on dealuminated β zeolite exhibits a high initial propane conversion of around 53 % and a superior propylene selectivity of about 93 % at a space velocity of 4000 cm³ g_cat^?1 h^?1, together with the high stability and satisfactory reusability. This study may open a new way to design and synthesize highly active PDH catalysts with high selectivity and stability.

Full text of DOI:10.1002/cctc.201801708

Accepted Article
Title: ZnO nanoclusters supported on dealuminated zeolite β as a
novel catalyst for direct dehydrogenation of propane to propylene
Authors: Chong Chen, Zhongpan Hu, Jintao Ren, Shoumin Zhang,
Zheng Wang, and Zhong-Yong Yuan
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
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the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
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To be cited as: ChemCatChem 10.1002/cctc.201801708
Link to VoR: http://dx.doi.org/10.1002/cctc.201801708
A Journal of
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ChemCatChem
10.1002/cctc.201801708
ZnO nanoclusters supported on dealuminated zeolite
β as a novel catalyst for direct dehydrogenation of
propane to propylene
[
a]
[a]
[a]
[a]
[b]
Chong Chen, Zhongpan Hu, Jintao Ren, Shoumin Zhang,* Zheng Wang, and Zhong-
Yong Yuan *[a]
[a] C. Chen, Z.P. Hu, J.T. Ren, Prof. S.M. Zhang, Prof. Z.-Y. Yuan
Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of
Chemistry, School of Materials Science and Engineering, Nankai University, Tianjin 300071 (P.R. China)
E-mail: zyyuan@nankai.edu.cn
[b] Prof. Z. Wang
State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering,
College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021 (P.R. China)
Abstract
Small ZnO nanoclusters supported on dealuminated β zeolite were prepared and evaluated
for catalyzing direct dehydrogenation of propane to propylene (PDH), exhibiting high catalytic
2
7
28
performance. N
2
sorption, XRD, TEM, Al and Si MAS NMR, IR, XRF, DR UV-vis, XPS,
and NH -TPD techniques were employed to characterize the physicochemical properties of this
3
novel catalyst system. It is found that the Zn species can be accommodated in the vacant T-
atom sites of dealuminated β zeolite due to the reaction of aqueous zinc acetate solution with
silanol groups, and thus, producing massive small ZnO nanoclusters as active phases in PDH.
1
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ChemCatChem
10.1002/cctc.201801708
Additionally, dealuminated β zeolite can greatly depress side reactions attributable to the
absence of strong acid sites, thereby guaranteeing high catalytic activity, propylene selectivity
and stability. As a result, the optimal catalyst of 10 wt% Zn loaded on dealuminated β zeolite
exhibits a high initial propane conversion of around 53% and a superior propylene selectivity
of about 93% at a space velocity of 4000 cm gcat h-1, together with the high stability and
satisfactory reusability. This study may open a new way to design and synthesize highly active
PDH catalysts with high selectivity and stability.
3
-1
1. Introduction
Propylene is a vital building block in petrochemical industry, which can synthesize
numerous high-valued chemical products such as polypropylene, acrylic acid, acrylonitrile and
so on.[1, 2, 3, 4] The traditional ways of producing propylene, including steam cracking and fluid
catalytic cracking (FCC), cannot produce enough propylene to meet with the market demand
ascribed to the depletion of petroleum. Due to the rich source of propane and the great price
gap between propane and propylene, direct dehydrogenation of propane to propylene (PDH)
has been regarded as one of the most effective ways. The traditional catalysts of PDH reaction
are Pt and CrO
based catalysts, [5, 6] while the former is costly and the latter is noxious.
x
Moreover, both of them are prone to deactivate in a very short time. Thus, the exploit of
alternative catalyst systems is of great importance.
Numerous novel catalyst systems such as Zn, Sn, Co, Ga and carbon based catalysts have
been reported for catalyzing PDH, [3, 7, 8, 9, 10, 11, 12, 13] which are void of shortcomings typical of
commercial CrO
x
and Pt based catalysts. Among them, Zn based materials are found to be one
2
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ChemCatChem
10.1002/cctc.201801708
of the most promising catalysts due to the low price, environmental-friendly-ness and high
catalytic performance in PDH. [10, 14, 15] Several studies have verified that isolated Zn and
2+
+
[16, 17]
Zn(OH) species are the catalytically active sites in PDH,
put attention on the catalytic activity of small ZnO clusters or nanoparticles. It is reported that
ZnO supported on Al could serve as Lewis acid sites (LAS), which can efficiently catalyze
while very few investigations
2
O
3
PDH. [14] In accordance with previous reports, small ZnO nanoclusters (SN ZnO) are capable
of activating C-H bond, [18, 19] suggesting its potentially catalytic ability for PDH. However,
active ZnO nanoclusters are very small and unstable, especially under harsh operation
conditions. [20] Fortunately, zeolites can provide well-ordered pore structure, as well as steady
and antacid conditions for dispersing and anchoring ZnO nanoclusters. In addition, the
microporous zeolites are able to tailor the size of ZnO nanoclusters. Therefore, zeolite is one of
the best suitable hosts for the preparation of SN ZnO. For instance, Wang et al. [21] prepared
sub-nanometric ZnO clusters in the micropores of zeolite L by using the incipient wetness
impregnation method. Li’s group [22] encapsulated ZnO clusters inside the channel of zeolites
including HZSM-5 and HY. However, all the reports indicated that only a small quantity of ZnO
clusters could be introduced into the zeolite frameworks, and most of Zn species would be
aggregated on the external surface of zeolite into bulk ZnO. So, the exploitation of new methods
to obtain more SN ZnO in zeolite framework is very necessary.
β zeolites are BEA-type featuring three-dimensional structure with 12-ring channel
systems, and have larger pores than those of ZSM-5 zeolites. [23] The acidity of BEA-type β
zeolites can be easily adjusted by eliminating lattice Al atoms from the framework.[24, 25, 26, 27,
2
8]
Moreover, the left vacant T-atom sites associated with silanol groups in β zeolites after
3
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ChemCatChem
10.1002/cctc.201801708
dealumination could accommodate and anchor metal species, [29, 30, 31] attributable to the
reaction between metal species and silanol groups.[32, 33] It has been reported the exchanged
2
+
+
[16, 34, 35]
Zn ions or Zn(OH) species in Al-containing zeolites are active for catalyzing PDH.
The issue of this kind of catalysts is that a large proportion of Zn species is prone to form
macrocrystalline ZnO on the external zeolite surface, and thus SN ZnO with catalytic potential
are hard to be obtained. [16] Although the dealuminated β zeolite is free of framework Al atoms,
the vacant T-atom sites associated with silanol groups can disperse and stabilize the introduced
metal species.[28, 36, 37] When the loading amount is low enough, metal heteroatoms can even be
entirely incorporated into vacant T-atom sites to form mononuclear active sites. [38, 39] Therefore,
it can be speculated that the dealuminated β zeolite as support might obtain highly-dispersed
and small-sized ZnO species.
Herein, the SN ZnO catalysts supported on dealuminated β zeolite were prepared via two-
step post-synthesis method involving dealumination and conventional impregnation procedures.
It is revealed that the SN ZnO supported on dealuminated β exhibited high catalytic activity,
selectivity and stability for PDH. The systematic characterizations and a comparison with raw
Hβ support indicate the important role of SN ZnO located in the dealuminated β zeolite. Also,
the function of silanol groups in dispersing and forming SN ZnO was clarified. Further, the
reaction pathway of PDH over this catalyst was proposed.
2
. Experimental section
2
.1. Catalyst preparation
Hβ zeolite with SiO
2
/Al
2
O molar ratio of 60 was provided by Tianjin Shenneng Sci-Tech
3
-1
Co., Ltd. The Hβ zeolite was treated with concentrated nitric acid solution (13 mol L ) at 100
4
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ChemCatChem
10.1002/cctc.201801708
oC for 10 h to get rid of the framework Al atoms, followed by washing with distilled water for
o
several times, and then drying at 100 C overnight. The obtained product was named as DeAlβ.
Afterward, 1 g of DeAlβ zeolite was added into the zinc acetate solution with desired Zn content
o
and stirred at room temperature for 12 h. The mixture was dried at 80 C overnight, and
o
o
-1
subsequently calcined in air at 600 C for 6 h with a heating rate of 10 C min . The as-prepared
samples were denoted as Znβ-x, where the x refers to the weight percent of Zn content (x = 3,
7
, 10, 20). For comparison purpose, ZnHβ with 10 wt% of Zn content, using the raw Hβ zeolite
as support, was synthesized by the same method to that of Znβ-x catalysts.
2
.2. Catalyst characterization
Powder X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku
D/MAX-2500 (Mini FlexII) using Cu-Kα radiation. Transmission electron microscopy (TEM)
was performed on a JEOL JSM-2800ꢀ microscope at 200ꢀkV. Fourier transform infrared (FT-IR)
spectra were measured on a Bruker Tensor 37 with the technique of KBr pellet, and the spectrum
-1
range was from 4000 to 400 cm . Diffuse reflectance infrared Fourier transform (DRIFT)
spectra of samples were carried out a Bruker Tensor 27 spectrometer with 128 scans at a
−1
resolution of 2 cm . Diffuse reflectance UV-vis (DR UV-vis) spectra were recorded on a
Shimadzu UV2450, and BaSO was used as an reference.
4
N
2
adsorption-desorption isotherms were recorded on a Quantachrome NOVA 2000e
o
sorption analyzer at 77 K, and all the samples were degassed at 200 C over 7 h before the
measurement. The ammonia temperature-programmed desorption (NH -TPD) profiles were
3
obtained from a Quantachrome CHEMBET-3000 analyzer, consisting of a U-shaped tubular
micro-reactor which was placed inside an electrical furnace. Typically, 100 mg of the sample
5
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ChemCatChem
10.1002/cctc.201801708
o
o
was pretreated with He at 600 C for 1 h, then the sample was cooled to 80 C. NH adsorption
3
o
was performed by switching the He flow to NH
3
gas and then keeping at 80 C for 30 min.
Afterward, the reactor having loaded sample was purged with He to remove the adsorbed NH
3
o
o
-1
from 80 to 700 C at a heating rate of 10 C min .
X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo
Scientific ESCALAB 250Xi spectrometer equipped with a monochromatic Al-Kα X-ray source
(
1486.6ꢀeV). The metal elements were detected using X-ray fluorescence (XRF, Rigaku
Supermini). Solid-state magic angle spinning (MAS) nuclear magnetic resonance (NMR)
experiments were performed on a Bruker Advance III 400WB spectrometer at resonance
2
7
29
frequencies of 104.3 MHz for Al and 79.5 MHz for Si NMR.
2
.3. Catalytic activity testing
The catalytic tests of PDH over the as-prepared catalysts were performed in a quartzose
fixed-bed microreactor (inner diameter, 6 mm) at atmospheric pressure. 0.3 g of catalyst
particles were hold in isothermal zone of the microreactor with two quartz wool plugs. A
thermocouple fixed at the isothermal zone was used to measure the operation temperature. Then,
the effluent products were transferred into an on-line gas chromatograph (SP-6800A6), which
was equipped with a flame ionization detector (FID). The following reaction conditions were
o
settled in the catalytic test: reaction temperature, 600 C; the total flow rate of gas reactant, 20
3
-1
cm min (5 vol% propane and a balance of nitrogen). The propane conversion (Cpropane),
propylene selectivity (Spropylene), propylene yield (Ypropylene) and carbon balance were calculated
as follows:
moles of propane reacted
Cpropane
=
×100%
moles of propane fed
6
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ChemCatChem
10.1002/cctc.201801708
moles of propylene formed
Spropylene
Ypropylene
=
=
×100%
×100%
moles of propane reacted
moles of propylene formed
moles of propane fed
moles of carbon in products
Carbon balance =
×100%
moles of carbonꢀinꢀpropane fed
ꢀ
ꢀ
3. Results and discussion
3
.1. Preparation of SN ZnO supported on dealuminated β zeolite
2
2.52
(
a)
(b)
Znβ-10
DeAlβ
2
2.58
ZnHβ-10
2
2.54
Znβ-10
ZnO
Hβ
JCPDS No.36-1451
5
10 15 20 25 30 35 40 45 50 55 60
5
10 15 20 25 30 35 40 45 50 55 60
2
Theta ( degree)
2 Theta (degree)
Figure 1. XRD patterns of (a) selected samples, (b) Znβ-10, ZnHβ-10 catalysts and standard
ZnO.
The XRD patterns of Hβ zeolite, DeAlβ, Znβ-10, ZnHβ-10 and standard ZnO are shown
in Figure 1. For Hβ zeolite, the sharp diffraction peaks indicate the well-crystallized structure.
Noticeably, the structure of Hβ zeolite remains unchanged after the dealumination and
subsequent impregnation with zinc acetate solution, though unavoidably the crystallinity
slightly reduces because of the elimination of framework Al atoms and the incorporation of Zn
o
species. The position of the main diffraction peak at 2θ around 22.6 , corresponding to d302
7
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ChemCatChem
10.1002/cctc.201801708
spacing, has slightly shifted within different samples. The d302 value of the fabricated materials
obtained from the corresponding 2θ value (Table 1), decreases from 3.9417 Å (Hβ, 2θ = 22.54o)
o
to 3.9346 Å (DeAlβ, 2θ = 22.58 ), indicating the contraction of BEA matrix after the
dealumination procedure. Then, the d302 spacing increases to 3.9448 Å (Znβ-10, 2θ = 22.52o),
evidencing the expansion of the framework of DeAlβ zeolite caused by the incorporation of Zn
species.[30] With respect to Znβ catalysts with different Zn loading amounts (Figure S1), in
o
o
o
addition to the diffraction peaks of β zeolite, three new peaks at 2θ = 31.78 , 34.44 and 36.25
attributed to hexagonal phase ZnO (JCPD No. 35-1451) can be seen when the Zn loading is up
to 20 wt%, which indicates the growth of Zn oxides with the increase of the Zn loading amount.
The diffraction lines corresponding to hexagonal phase ZnO are not observed in Znβ-10, in
contrast, which can be easily identified in ZnHβ-10 (Figure 1b). Although the Zn loading
amount is identical (10 wt%), more macrocrystalline ZnO formed in ZnHβ-10 catalyst while
more amorphous or well-dispersed ZnO species are present in Znβ-10 based on the
corresponding XRD patterns.
Table 1. The XRD parameters and textural properties of Hβ, DeAlβ and Znβ-x samples
2
-1 a
3
-1 b
Sample
Hβ
2θ (degree)
d
302 (Å)
S
BET (m g )
472
V
total (cm g )
22.54
3.9417
0.380
DeAlβ
Znβ-3
Znβ-7
Znβ-10
Znβ-20
22.58
3.9346
471
0.374
-
-
-
468
0.365
-
22.52
-
455
0.359
3.9448
445
0.339
-
389
0.297
8
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ChemCatChem
10.1002/cctc.201801708
a
b
S
BET is the specific BET surface area; Vtotal is the total pore volume determined at P/P = 0.98.
0
The textural properties of β zeolite before and after dealumination, and Zn species
introduction were studied by N
2
sorption method (Figure S2 and Table 1). It is notable that Hβ,
2
DeAlβ, Znβ-3, Znβ-7 and Znβ-10 have similar BET surface areas (445-480 m /g) and total pore
3
volumes (0.339-0.380 cm /g), revealing the well preserved pore structure upon the two-step
post-synthesis procedure. When the Zn loading is up to 20%, the specific surface area and total
pore volume are significantly reduced, ascribing to the formation of bulk ZnO which can block
access to partial inner micropores. [19, 40]
5
4
-110
112
DeAlβ
31
1
-
(
a)
(b)
-
115
-
103
Znβ-10
DeAlβ
Hβ
-
106
Hβ
-100
-50
0
50
100
150
200
-140 -130 -120 -110 -100
-90
-80

27 Al / ppm
29 Si / ppm
2
7
28
Figure 2. (a) Al and (b) Si MAS NMR spectra of the selected samples.
2
7
The Al MAS NMR spectroscopy was employed to verify the elimination of Al atoms
after dealumination procedure. In Figure 2a, the characteristic peak at 54 ppm corresponding to
tetrahedrally coordinated Al species progressively decreases, indicating almost all lattice Al
atoms have been eliminated from the zeolite framework. [41] In addition, no peak at 0 ppm
related to extraframework octahedral Al can be observed, further suggesting the negligible
content of Al atoms after dealumination. Accordingly, the generation of silanols occurs during
9
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ChemCatChem
10.1002/cctc.201801708
the dealumination process. [28, 36] The Si MAS NMR spectra of the selected samples are shown
29
in Figure 2b. The spectrum of raw Hβ zeolite shows the peaks centered at -103, -106 and -110
ppm, corresponding to the framework Si(OSi)
3
(OH), Si(OSi)
3
(OAl) and Si(OSi) ,
4
respectively.[42] Upon the dealumination, the peak at -103 ppm has no obvious change while the
peak at -106 ppm almost disappears, further evidencing the removal of Al atoms from the
framework. The introduction of Zn species into DeAlβ zeolite causes the weakening of the peak
at -103 ppm, indicating the interaction between Zn species and SiOH in these Si(OSi)
species. [25] The broad peak at -110 ppm in Hβ splits into two sharp peaks centered at -110 and
112 ppm in DeAlβ, ascribing to the generation of two types of surroundings of Si atoms in the
framework Si(OSi)
species. [42] Then, the introduced Zn in DeAlβ can modify the silicon
environment in Si(OSi) , and thus, leading to the change of the two peaks at -110 and -112 ppm.
The new emerging peak at -115 ppm in DeAlβ is another characteristic signal of Si(OSi)
species at different crystallographic sites.[36, 43] The incorporation of Zn species doesn’t
influence its intensity, because that the Zn species cannot interact with this kind of Si(OSi)
3
(OH)
-
4
4
4
4
species due to the absence of hydroxyl. [43] This result shows that the concentrated nitric acid
can remove lattice Al from the zeolite framework, and the supported Zn species have interaction
with the silanol groups in DeAlβ zeolite.
The structural properties of Hβ, DeAlβ, Znβ-10 and ZnHβ-10 catalysts were also
characterized by IR. The FT-IR spectra of Hβ and DeAlβ samples exhibit the similar bands
(
Figure S3a), indicating the dealumination procedure did not disrupt the characteristic structure
of zeolite. The existence of a mass of silanol groups in DeAlβ is proved by the characteristic
-1
band at 968 cm , as shown in Figure S3b, which is attributable to the stretching vibration of
10
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ChemCatChem
10.1002/cctc.201801708
Si-O corresponding to uncoupled SiO
4
tetrahedral with a hydroxyl group. [39, 44] The intensity
of this band is very weak in Hβ, while it is more intense in DeAlβ, revealing the presence of T-
atom sites associated with silanol groups which are generated upon the dealumination of Hβ
zeolite. [45] This band nearly disappears after the Zn species introduction, indicating the
consumption of silanol groups which is caused by the reaction between Zn species and the
hydroxyls of silanol groups. Further, Figure S4 displays the DRIFT spectra in the hydroxyl
stretching vibration region of Hβ, DeAlβ and Znβ-10. The bare Hβ zeolite shows two
-1
characteristic bands at 3740 and 3600 cm , related with isolated terminal silanol (Si(OH) and
bridging hydroxyl groups (Si(OH)Al) respectively.[46] After the dealumination procedure, the
-1
3
600 cm band corresponding to Al species disappears due to the removal of framework Al
-1
atoms. Meanwhile, a new and broad band at 3460 cm can be detected, indicating the formation
of vacant T sites in the framework. This band is derived from the formation of silanol groups
with hydrogen-bonded SiOH groups in vacant T sites. After loading with Zn species, the band
-1
at 3740 cm becomes weak, indicating that Zn species bind to terminal oxygen atoms and even
replace the silanol groups to some extent. Moreover, an intensive reduction of the 3460 cm-1
band is observed, revealing the Zn species are mainly boned to the silanol groups in vacant T
sites. [47]
11
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ChemCatChem
10.1002/cctc.201801708
4
(b)
Hβ
DeAl
Znβ-10
Znβ-3
(
a)
0.016 mmolNH3/g
β
0
4
ZnHβ-10
0.023 mmolNH3/g
Znβ-7
0
4
0.032 mmolNH3/g
Znβ-10
0
4
0.015 mmolNH3/g Znβ-20
0
1
00
200
300
400
500
600
0
100 200 300 400o 500 600
o
Temperature
(
C
)
Temperature
( C)
Figure 3. NH
3
-TPD curves of (a) the selected samples and (b) Znβ-x catalysts with different
desorption amount shown inside).
Zn loading amounts (the corresponding NH
3
The acidic properties of β zeolite before and after acid treatment and the Zn incorporation
were evaluated by the mean of NH -TPD technique. As shown in Figure 3a, the raw Hβ zeolite
shows two major NH desorption peaks centered at about 230 and 450 ℃, revealing the
existence of weak and strong acid sites, respectively. [48] The NH
desorption peaks disappear
3
3
3
after the removal of framework Al atoms, indicating that both types of acid sites have been
eliminated. After the introduction of Zn species, the broad peak appearing at the range of 200
to 450 ℃ can be ascribed to the generation of LAS, which is linked to the anchoring of Zn
species to hydroxyls in zeolite framework. [22, 49, 50] Compared with that of Znβ-10, the NH
3
desorption peak intensity of ZnHβ-10 is much stronger, due to the remained strong acid sites.
Concretely, a quantitative analysis on the LAS density of Znβ catalysts with different Zn
loadings, can be determined from the NH
3
desorption amounts calculated from the NH -TPD
3
curves in Figure 3b. The amounts of Lewis acid sites decrease as the order of Znβ-10 (0.032
mmolNH3/g) > Znβ-7 (0.023 mmolNH3/g) > Znβ-3 (0.016 mmolNH3/g) > Znβ-20 (0.015
12
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ChemCatChem
10.1002/cctc.201801708
mmolNH3/g), indicating that the most LAS can be obtained in Znβ-10 catalyst.
Figure 4. (a, b) TEM images of ZnHβ-10, and (c-e) TEM images and (f) the corresponding
element mapping images of Znβ-10.
In order to identify the distributions of Zn species in Znβ-10 and ZnHβ-10 catalysts, TEM
measurement was carried out. From the TEM images of ZnHβ-10 shown in Figure 4a-b, distinct
ZnO nanoparticles can be easily observed in ZnHβ-10, and the size of the ZnO particles is up
to tens nanometers. As for Znβ-10, no ZnO particles can be seen in Figure 4c, while a small
1
3
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ChemCatChem
10.1002/cctc.201801708
number of ZnO nanoparticles with uniform particle sizes smaller than 5 nm can be discerned in
the high magnification images (Figure 4d). Although Zn species are dispersed well in both
DeAlβ and Hβ supports, the surface ZnO particles in DeAlβ are obviously smaller than that in
Hβ. As shown in Figure S5, the particle sizes of surface ZnO in Znβ-10 and ZnHβ-10 catalysts
are mainly distributed in 2.9 ± 1.5 and 7.3 ± 4.0 nm, respectively. Although partial large ZnO
particles are inevitably formed on the surface, it still can be used to demonstrate that the DeAlβ
can favor the dispersion of Zn species. The low-resolution TEM image of Znβ-10 in Figure 4e
exhibits the homogeneous contrast, indicating the uniform distribution of Zn species in Znβ-10.
Besides, the homogeneous distribution of Si, O and Zn elements can be observed on the
corresponding element mapping images (Figure 4f). Especially, the Zn image confirms the
homogeneous dispersion of Zn species throughout the whole zeolite. Additionally, very weak
Al signal can be observed in Figure 4f, indicating that most of Al species have be wiped off.
Therefore, after acid treatment, β zeolite becomes a positive support for the dispersion and
formation of SN ZnO.
3
68
(
a)
(b) 195
Pure ZnO
3
68
Znβ-3
Znβ-7
Znβ-10
Znβ-20
2
10
Hβ
1
95
ZnHβ-10
Znβ-10
2
00 250 300 350 400 450 500
200
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Figure 5. DR UV-vis spectra of (a) the selected samples, and (b) Znβ catalysts with different
Zn loading amounts.
14
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ChemCatChem
10.1002/cctc.201801708
The DR UV-vis spectroscopy is an effective method of distinguishing the chemical
environment of Zn species on zeolite support. So, the DR UV-vis analysis was performed and
the spectra are presented in Figure 5. The spectra of Hβ and pure ZnO were regarded as
references. The absorption band of 368 nm corresponds to the band gap width of
2
-
2+
macrocrystalline ZnO is resulted from the charge transfer of ligand-to-metal from O to Zn .
[
51, 52]
The broad and intense absorption band at 368 nm of pure ZnO validates it well. This band
in ZnHβ-10 is much stronger compared with that in Znβ-10 (Figure 5a), suggesting the high
content of macrocrystalline ZnO in ZnHβ-10. The bands at 195 nm can be assigned to the charge
2
-
transfer transitions of Zn species with lattice O in zeolite framework, thus which are regarded
as the distinguishing proof of Zn species in or bonded to the framework of zeolite. [35, 53] In
contrast to ZnHβ-10, the intensity of band at 195 nm in Znβ-10 is much stronger, indicating
2
+
[51]
that there are more ZnO clusters or Zn bonding to the framework of DeAlβ zeolite.
The
DR UV-vis spectra of Znβ with different Zn loadings are also displayed in Figure 5b. With the
increase of Zn loading amount, the band located at 368 nm becomes more intensive because
crystalline ZnO are growing on the external surface. Although it has the weakest band of 368
nm in Znβ-3, the band at 195 nm corresponding to the incorporated Zn species shows much
lower intensity than that of Znβ-10 due to the low Zn loading amount. When the Zn loading
amount exceeds 10 wt%, the band of 195 nm becomes weaker again, attributable to more Zn
species forming into bulk ZnO rather than bonded ZnO clusters. According to the intensity of
band at 195 nm, it can be anticipated that there are much more bonded Zn species in the zeolite
framework of Znβ-10 than that of Znβ-3, Znβ-7 and Znβ-20.
15
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ChemCatChem
10.1002/cctc.201801708
9
85.9
(
a)
1021.3
Znβ-10
ZnHβ-10
Pure ZnO
(b)
Znβ-10
ZnHβ-10
9
85.6
1
044.6
1
021.6
1
044.9
1
020
1030
1040
1050
980
982
984
986
988
990
Binding energy (eV)
Kinetic energy (eV)
Figure 6. (a) XPS Zn 2p spectra of pure ZnO, ZnHβ-10 and Znβ-10 samples, (b) Zn LMM core
level spectra of ZnHβ-10 and Znβ-10 catalysts.
The XPS analysis was used to analyze the electronic states of Zn species in the selected
samples, and the results are showed in Figure 6. The Zn 2p binding energies of 1021.6 and
1
044.9 eV in Znβ-10 and ZnHβ-10 samples, are obviously higher than that of pure ZnO. Due
to the lower electronegativity of Zn compared to Si atom, the introduced Zn species are more
positively charged in Zn-O-Si bands than in Zn-O-Zn bands, leading to the chemical shift
towards higher location. [48, 54] However, the Zn 2p spectra cannot differentiate the Zn2+
chemical environment within Znβ-10 and ZnHβ-10 samples. Fortunately, the existent states of
the Zn species can be discerned by the Auger Zn LMM spectra, as shown in Figure 6b, which
are more sensitive to the difference in the Zn chemical environment in comparison with the Zn
2
p spectra. The difference of the kinetic energy peak locations can be attributed to the Zn2+
species in different electronic environment. [55] Compared with that of ZnHβ-10, the lower
kinetic energy of 985.6 eV of Znβ-10 catalyst suggests that there are more Zn species have
interaction with lattice O atoms in zeolite framework.
16
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ChemCatChem
10.1002/cctc.201801708
3
.2. Catalytic performance
8
6
4
2
0
0
0
0
0
100
(
a)
(b)
ZnHβ-10
Znβ-10
8
6
4
2
0
0
0
0
0
ZnHβ-10
Znβ-10
0
1
2
3
4
5
6
0
1
2
3
4
5
6
Time on steam (h)
Time on steam (h)
Figure 7. (a) Propane conversions and (b) propylene selectivities as a function of reaction time
over ZnHβ-10 and Znβ-10 catalysts.
The duration of PDH test was fixed to 6 h since that the deactivation rate and tendency of
propane conversions and propylene selectivities could be observed over all the samples. The
calculated values of carbon balance are about 95% for Znβ-10 catalyst, as shown in Figure S6,
verifying the correctness of the analytical determinations examined for the test. The catalytic
performance over all the prepared catalysts are given in Table 2. The catalytic performance of
ZnHβ-10 is unsatisfactory with a low propane conversion less than 20%, as shown in Figure 7a.
In addition, the propylene selectivity of ZnHβ-10 is very low, only 7.6% at the beginning of
reaction, verifying the occurrence of serious side reactions. However, the Znβ-10 shows an
o
initial propane conversion of about 53.3% at 600 C, which is much higher than that of ZnHβ-
1
0. Noticeably, the propylene selectivity of Znβ-10 is up to 92.9% at the beginning of reaction
and stabilizes at around 97% after 2.5 h on stream. The activity results obviously indicate that
the dealumination treatment render β zeolite become a very positive support of Zn catalyst in
17
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ChemCatChem
10.1002/cctc.201801708
PDH reaction.
Table 2. Activity test results of PDH over as-prepared catalysts.a
Catalysts
ZnHβ-10
Znβ-20
Znβ-10
Znβ-7
Con. (%)b
6.5 (10.7)
36.1 (20.4)
53.3 (37.9)
46.3 (31.1)
37.9 (23.6)
Sel. (%)c
7.6 (80.1)
94.4 (96.4)
92.9 (96.9)
95.0 (97.4)
95.6 (97.7)
Yie. (%)d
0.5 (8.6)
Dea. (%)e
-
34.1 (19.7)
49.5 (36.7)
44.0 (30.3)
36.2 (23.1)
42.2
25.9
31.1
36.2
Znβ-3
a
b
e
The value outside and inside the parentheses are the data obtained at first and 6 h, respectively.
c
d
Conversion of propane. Selectivity of propylene. Yield of propylene.
Deactivation rate of propylene yield after 6 h on stream.
6
0
5
0
5
0
5
0
5
0
5
0
100
(
a)5
(b) 99
Znβ-3
Znβ-7
Znβ-10
Znβ-20
5
4
4
3
3
2
2
1
1
98
97
96
95
94
93
92
91
90
Znβ-3
Znβ-7
Znβ-10
Znβ-20
0
1
2
3
4
5
6
0
1
2
3
4
5
6
Time on steam (h)
Time on steam (h)
Figure 8. (a) Propane conversions and (b) propylene selectivities as a function of reaction time
over the Znβ-x catalysts with different Zn loading amounts.
18
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ChemCatChem
10.1002/cctc.201801708
The activity tests of PDH over Znβ catalysts with different Zn loading amounts were
carried out, and the propane conversions and propylene selectivities achieved over these
catalysts are presented in Figure 8. As discussed above, the Znβ catalysts show very high
selectivity to propylene. The effect of Zn loading in Znβ catalysts on the propane conversion is
o
significant, especially after the activity tests were performed at 600 C for 6 h (Figure 8a). At
the beginning of the reaction, with the increasing Zn loading on DeAlβ from 3 to 10 wt%, the
propane conversion progressively increases from 37.9% to 53.3% and the propylene selectivity
slightly decreases from 95.6% to 92.9%. However, it is difficult to determine the TOF
parameters due to the high propane conversions. When the Zn loading is up to 20%, the propane
conversion has an evident reduction, ascribed to the formed bulk ZnO which is inactive and can
cover active sites. Overall, the optimal Znβ-10 shows superior catalytic performance with the
initial propane conversion as high as 53.3% at a high propylene selectivity of 92.9%. After 6 h
on stream, the propane conversion reduces to 37.9%, while the propylene selectivity keeps
steady at about 97%. Noticeably, such an ideal catalytic performance of Znβ catalyst in PDH is
superior to that of a large majority of previously reported catalyst systems (Table S1).
1
00
fresh
8
6
4
2
0
0
0
0
0
after the first regeneration
after the second regeneration
0
1
2
3
4
5
6
Time on steam (h)
Figure 9. Propylene yields during different reaction-regeneration cycles as a function of
19
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ChemCatChem
10.1002/cctc.201801708
reaction time over Znβ-10 catalyst.
In order to evaluate the reusability of this catalyst, the regeneration treatment of Znβ-10
o
catalyst after 6 h of reaction at 600 C was performed, and the result is shown in Figure 9. The
propylene yield on the fresh catalyst decreases from 51.2% to 31.1% after 6 h on stream. Then,
o
the air was introduced at 600 C for 2 h to remove carbon species deposited on the catalyst. The
propylene yield decreases to 38.0%, lower than that of the fresh catalyst, indicating the original
activity of the catalyst could not be fully recovered. The recovery of catalytic activity after the
first regeneration can be ascribed to the removal of formed coke, and the decrease of it is may
be caused by the loss of some active sites. After the second regeneration procedure, the catalytic
performance is almost restored. Anyway, as to what causes the trivial loss of activity during 6
h on stream, it still needs further investigation. Generally, the Znβ-10 catalyst possesses good
reusability, indicating its great potential of application in commercial process of PDH.
3
.3. Discussion
Scheme 1. Possible reaction pathway of propane dehydrogenation over Znβ catalyst.
2
0
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ChemCatChem
10.1002/cctc.201801708
According to the Si/Al atom ratio (37.3) of raw Hβ zeolite determined by XRF (Table S2),
2
+
the approaching saturation of framework Zn in DeAlβ zeolite is supposed to be at Si/Zn ratio
of around 37.3, [34, 56] thus excess Zn loadings must form ZnO clusters covering the framework
2
+
2+
Zn . If only the isolated Zn are active sites, when the Si/Zn ratio is less than 37.3, the catalytic
activity would decrease due to the formation of ZnO clusters. However, the decrease of Si/Zn
ratio of 5.32 in Znβ-3 to 2.9 in Znβ-10 (Table S2) greatly improves the catalytic performance,
suggesting the formed ZnO clusters are also catalytically active sites for PDH. Referencing to
previous reports, [8, 15] a simple reaction mechanism is proposed and drawn in Scheme 1. It can
be speculated that propane dissociative adsorption takes placing on the constrained SN ZnO
surface. [40] The isopropyl group and dissociated H atom are bonded to Zn and the adjacent O
atom, respectively. Then, propylene is formed through the β-H cleavage of the adsorbed
2+
isopropyl species. Finally, the active sites are recovered after the H molecular is released, and
2
the reaction pathway is completed. Figure 10 illustrates the possible formation procedure of
ZnHβ-10 and Znβ-10 catalysts. Directly loading Zn species on Hβ zeolite can cause a part of
Zn species form bulk ZnO, which are inactive for PDH. [16] Upon the dealumination procedure,
almost all framework Al atoms of Hβ zeolite have been wiped off, and massive vacant T-atom
sites associated with silanol groups are created. After loading with Zn species, ZnO particles
can be well accommodated into vacant T-atom sites in the DeAlβ framework, and thus,
producing into a large amount of SN ZnO with the catalytic capacity for PDH in Znβ catalyst.
21
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ChemCatChem
10.1002/cctc.201801708
Figure 10. The illustration of the preparation procedure for ZnHβ and Znβ catalysts.
So, the dealumination of Hβ zeolite makes a good condition for forming and anchoring
SN ZnO. As shown in Figure 1a and Figure 2b, the Zn species are successfully incorporated
2
8
into the DeAlβ zeolite framework. Moreover, the IR and Si MAS NMR results demonstrate
that ZnO clusters are in or bonded the supercages of T-atom sites where there have been acidic
sites, proving silanol groups really play an important role in obtaining SN ZnO. There are still
a fraction of large ZnO particles can be observed on the DeAlβ surface (Figure 4), but the EDS
mapping clearly demonstrates the Zn species in Znβ-10 are dispersed well throughout the whole
support. Moreover, the TEM image of Znβ-3 catalyst shows no ZnO particles, as shown in
Figure S7, indicating that the Zn is completely located in the zeolite framework when the Zn
loading amount is low. The new emerging NH desorption peak in Znβ catalysts (Figure 3)
3
confirms that the incorporated SN ZnO possess the character of LAS, which are active for
catalyzing PDH. Further, the DR UV-vis and XPS results confirm that the highly dispersed SN
ZnO bonded to the DeAlβ framework are derived from the interaction between Zn species and
silanol groups. This work indicates that dealuminating β zeolite is an attractive method to
generate silanol groups which can be employed to interact with Zn complex, and thus, a lot of
2
2
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10.1002/cctc.201801708
active SN ZnO can be obtained in DeAlβ.
The catalytic performance of Zn supported on raw Hβ and DeAlβ supports are evaluated
in PDH (Figure 7), and the influence of Zn loading amounts on the catalytic activity is exhibited
in Figure 8. The optimal catalyst of Znβ-10 exhibits high propane conversion and great
propylene selectivity compared with the ZnHβ-10 and other Znβ catalysts. The initial propane
conversion of Znβ catalysts with different Zn loadings has perfect linear relationship to the
corresponding NH desorption amounts (Figure S8), which is in consistence with the LAS
3
density. Noticeably, the LAS are derived from the bonded SN ZnO, it thereby can be concluded
that the catalytic activity of Znβ is dependent on the content of bonded SN ZnO. As for the Zn-
containing Hβ, a small amount of bonded SN ZnO can be created, while most of Zn species
exist on the zeolite surface in the form of macrocrystalline ZnO (Figure 1b). Excess Zn loading
or using raw Hβ as support lead to weak catalytic performance ascribed to the formation of
inactive bulk ZnO. [16] Additionally, the dealuminated β zeolite is almost free of strong acid
sites, effectively reducing side reactions. Thus, the propylene selectivity of Znβ-10 catalyst can
be greatly enhanced. With massive active SN ZnO and free of strong acid sites, Zn supported
on dealuminated β zeolite exhibits excellent catalytic performance in the propylene production.
Moreover, the optimal Znβ-10 catalyst possesses great reusability (Figure 9), indicating its great
potential in industrial application.
4
. Conclusions
The dealuminated β zeolite, possessing a great deal of vacant T-atom sites associated with
silanol groups, can be used as an ideal support for obtaining well-dispersed SN ZnO by the
host-guest interaction. On the whole, the created vacant T-atom sites are in favor of constraining
23
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ChemCatChem
10.1002/cctc.201801708
SN ZnO in the zeolite framework, and thus reducing the possibility of inactive bulk ZnO formed
on the external surface of zeolite. The highly dispersed SN ZnO in or bonded to the zeolite
framework as LAS are catalytically active sites in PDH. In addition, the absence of strong acid
sites in the dealuminated β zeolite after removing Al atoms can reduce side reactions to a great
extent. So the Zn catalyst supported on DeAlβ zeolite, with a large number of catalytically
active SN ZnO and free of strong acid sites, showing high catalytic activity and excellent
propylene selectivity in PDH. Strikingly, the catalytic capacity of SN ZnO on the dealuminated
zeolite in PDH is the first time to be reported, and thus, opening a wide avenue for the design
of Zn based dehydrogenation catalysts.
Acknowledgements
This work was supported by the National Natural Science Foundation of China
(
21421001, 21573115), the 111 project (B12015), the Fundamental Research Funds for
the Central Universities (63185015), and the Foundation of State Key Laboratory of
High-efficiency Utilization of Coal and Green Chemical Engineering (2017-K13).
Conflicts of interest
There are no conflicts of interest to declare.
Keywords:active sites; dealuminated zeolite β; propane dehydrogenation; propylene; ZnO.
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Table of Contents entry
Small ZnO nanoclusters supported on dealuminated β zeolite can be obtained via a two-step
post-synthesis method, showing much better catalytic performance for direct dehydrogenation
of propane to propylene than that on raw Hβ zeolite.
2
7
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