TiO2-doped Mn2O3-Na2WO4/SiO2 catalyst for oxidative coupling of methane: Solution combustion synthesis and MnTiO3-dependent low-temperature activity improvement

Article abstract of DOI:10.1016/j.apcata.2017.07.012

The Mn₂O₃-Na₂WO₄/SiO₂ catalyst is the most promising one among the enormous catalysts for the oxidative coupling of methane (OCM) but only at above 800?°C. No doubt that lowering temperature of the OCM process is at the forefront of this catalysis field. A promising low-temperature active and selective TiO₂-doped Mn₂O₃ Na₂WO₄/SiO₂ catalyst, consisting of 6?wt% TiO₂, 6?wt% Mn₂O₃, 10?wt% Na₂WO₄ and SiO₂ in balance, is developed by solution combustion synthesis (SCS) method. This catalyst is capable of converting 20% CH₄ with 70% selectivity to C₂-C₃ hydrocarbons even at 700?°C (catalyst bed temperature) and is stable for at least 250?h without deactivation sign, for a feed gas of 50% CH₄ in air using a gas hourly space velocity of 8000?mL?g_cat.^?1?h^?1. In contrast, the non-TiO₂-doped SCS catalyst is almost inactive at 700?°C whereas it can achieve reactivity (～24% CH₄ conversion and ～74% C₂-C₃ selectivity) comparable to the TiO₂-doped one at 800?°C. XRD and Raman results evidently reveal that the formation of MnTiO₃ during the OCM process appears to be important for the low-temperature OCM activity improvement by TiO₂-doping.

Full text of DOI:10.1016/j.apcata.2017.07.012

Applied Catalysis A, General 544 (2017) 77–83
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Research paper
TiO -doped Mn O -Na WO /SiO catalyst for oxidative coupling of
2
2
3
2
4
2
methane: Solution combustion synthesis and MnTiO -dependent low-
3
temperature activity improvement
Pengwei Wang , Guofeng Zhao1,⁎, Ye Liu, Yong Lu⁎
1
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai,200062,
China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The Mn
coupling of methane (OCM) but only at above 800 °C. No doubt that lowering temperature of the OCM process is
at the forefront of this catalysis field. A promising low-temperature active and selective TiO -doped Mn
Na WO /SiO catalyst, consisting of 6 wt% TiO , 6 wt% Mn , 10 wt% Na WO and SiO in balance, is de-
veloped by solution combustion synthesis (SCS) method. This catalyst is capable of converting 20% CH with
0% selectivity to C -C hydrocarbons even at 700 °C (catalyst bed temperature) and is stable for at least 250 h
without deactivation sign, for feed gas of 50% CH in air using gas hourly space velocity of
. In contrast, the non-TiO -doped SCS catalyst is almost inactive at 700 °C whereas it can
achieve reactivity (∼24% CH conversion and ∼74% C -C selectivity) comparable to the TiO -doped one at
00 °C. XRD and Raman results evidently reveal that the formation of MnTiO during the OCM process appears
to be important for the low-temperature OCM activity improvement by TiO -doping.
2 3 2 4 2
O -Na WO /SiO catalyst is the most promising one among the enormous catalysts for the oxidative
Oxidative coupling of methane
Solution combustion synthesis
Ethylene
2
2 3
O
2
4
2
2
2
O
3
2
4
2
MnTiO
Low-temperature activity
3
4
7
2
3
a
4
a
−
1
−1
8
000 mL gcat.
h
2
4
2
3
2
8
3
2
1
. Introduction
goal.
The oxidative coupling of methane (OCM) is considered to be a
promising route to directly convert methane into C -C hydrocarbon
With the ever-growing resource and world output of natural gas and
2
3
shale gas, methane conversion into the value-added chemicals and fuels
has received more and more attentions from the worldwide petro-
chemical and energy industries, in order to alleviate our strong de-
pendence on depleting oil resource [1,2]. Particularly, light olefins, the
key building blocks in modern chemical industry, need an urgent pro-
duction shift from oil to methane. Up to now, methane has been in-
dustrially converted into olefins in an indirect route, where methane is
products in the presence of molecular oxygen. It has been well indicated
from many studies that OCM is a sequential process combining the
heterogeneous-catalysis and homogeneous-coupling to achieve the
methane-to-olefins transformation [8–10]. The OCM catalysts aim to
generate methyl radicals (CH
CO, CO , and H O) over their surface [11,12]. Therefore, the most
critical performance of OCM catalyst is able to generate selective sur-
3
face oxygen radicals that act as the active sites for generating CH ·
3
·) and avoid deep oxidation (i.e., to form
2
2
firstly and forcibly broken into the syngas (i.e., CO and H
2
) through the
catalytic steam or autothermal reforming at above 700 °C, followed by
large-scaled conversion to methanol and then to olefins [3–5]. Ad-
ditionally, syngas could also be directly converted into olefins in the
presence of solid catalysts, such as the very recently reported bifunc-
[13–15]. Great attempts have been making to setup a catalyst recipe
with such properties from the simple and complex oxides of alkaline,
alkaline earth, and rare-earth elements [16–20]. Hundreds of catalysts
2 3
have been examined aiming to favor C -C selectivity and suppress
tional ZnCrO
x
-SAPO catalyst [6] and the cobalt carbide nanoprisms [7]
overoxidation, such as the representative Li/MgO [21,22] and La-based
oxides catalysts [23,24]. Li in Li/MgO catalyst acts as a structural
modifier to enhance the catalyst OCM performance [21], but suffers
from its loss during the long-term running [22]; the La-based catalysts
with outstanding selectivity under mild conditions. However, the
syngas route is negatively energy-costing and such indirect manner
causes low atom-utilization efficiency. Therefore, the direct methane
conversion into olefins is highly desirable and a large number of re-
searches have been dedicated for a couple of decays to achieving this
suffer from the relatively lower C
2 3
-C selectivity. Among these catalysts,
the Mn -Na WO /SiO is considered to be the most promising one, in
2
O
3
2
4
2
⁎
Corresponding authors.
E-mail addresses: gfzhao@chem.ecnu.edu.cn (G. Zhao), ylu@chem.ecnu.edu.cn (Y. Lu).
These authors contributed equally to this work.
1
http://dx.doi.org/10.1016/j.apcata.2017.07.012
Received 24 May 2017; Received in revised form 6 July 2017; Accepted 9 July 2017
Available online 11 July 2017
0926-860X/ © 2017 Elsevier B.V. All rights reserved.

P. Wang et al.
Applied Catalysis A, General 544 (2017) 77–83
terms of 20 ∼ 30% methane conversion with 60 ∼ 80% C
2
-C
3
se-
for the enhancement of catalyst low-temperature activity for the OCM
process.
lectivity and in particular hundreds of hours stability [25]. However,
this catalyst must be operated at above 800 °C and the slight reduction
of temperature to below 800 °C will deteriorate sharply the catalyst
activity even quench the OCM reaction [26].
2. Experimental
In order to enhance the low-temperature performance of the Mn
WO /SiO catalyst, enormous work has been carried out on catalyst
preparation and additive modification. The Mn -Na WO /SiO cat-
2
O
3
-
2.1. Materials
Na
2
4
2
2
O
3
2
4
2
All chemicals in this work were purchased from Sinopharm
Chemical Reagent Co., Ltd, China: sodium tungstate dihydrate (AR),
50 wt% manganese(II) nitrate aqueous solution (AR), tetraethyl or-
thosilicate (AR), tetrabutyl titanate (CP), titanium isopropoxide (CP),
titanium n-propoxide (CP), and 65 wt% nitric acid aqueous solution
(AR). They were all used as received.
alyst is usually prepared by the traditional methods such as incipient
wetness impregnation, mixture slurry, and sol-gel methods [27]. Re-
cently, solution combustion synthesis (SCS) strategy becomes more and
more attractive to prepare homogeneous, high-purity, and complex
nanostructured metal-oxides catalysts [28]. Compared to the traditional
methods, SCS is a one-step method by self-propagating high-tempera-
ture synthesis with the raw materials mixed in liquid phase [29–31].
For example, the typical reaction between metal nitrate and glycine for
2.2. Catalyst preparation
the preparation of nanostructured La
2
O
3
catalyst is shown by Eq. (1),
Regular Mn
WO /SiO catalyst were prepared by the reported SCS method
[35,36]. Mn(NO aqueous solution and Na WO were employed as
precursors for Mn-, Na- and W-containing components. Tetraethyl or-
thosilicate and titanate (titanium isopropoxide, titanium n-propoxide or
2 3 2 4 2 2 2 3
O -Na WO /SiO catalyst and the TiO -doped Mn O -
where ϕ represents the fuel/oxidizer ratio: ϕ of 1 indicates the stoi-
chiometric ratio that no more molecular oxygen is required; ϕ below 1
implies that molecular oxygen is generated; while ϕ above 1 means that
molecular oxygen is needed [32]. Notably, modulating synthesis para-
meters such as fuel/oxidizer ratio and the property of fuels can tune the
structure and texture of the as-synthesized catalyst [33,34]. Recently,
the SCS method has been reported to be successfully applied to syn-
thesize several promising OCM catalysts such as Sr-Al complex oxides,
Na
2
4
2
3
)
2
2
4
tetrabutyl titanate) were the precursors for SiO
while serving as fuels in the SCS process. For the SCS method, HNO
was used as oxidizer. The ϕ (i.e., fuel/oxidizer ratio) was tuned in the
range from 1 to 3 by adding appropriate amount of 65 wt% HNO
2 2
and TiO compounds
3
3
La
improved C
SiO
00 °C for the regularly-prepared counterparts to 750 °C along with
high C -C yield of ca. 25% (for a feed gas of CH :O :N = 2:1:0.63). In
addition, Ce and La can also be easily introduced into the Mn
Na WO /SiO catalyst by SCS method [35,36], which generates more
reactive oxygen to further enhance the catalyst performance.
2
O
3
, La-Sr-Al complex oxides, and Mn
2
O
3
-Na
2
WO
4
/SiO
2
[35], with
aqueous solution. In brief, the Mn, Na, and W precursors were mixed
together with the tetraethyl orthosilicate and titanate in deionized
water in a ceramic bowl, and appropriate amount of HNO according to
3
the ϕ vale was added subsequently. The mixture was then continuously
stirred at 80 °C until it was transformed into the sticky gel. Subse-
quently, the ceramic bowl with such gel was transferred into a muffle
furnace. When heating the gel to 300 °C, self-ignition combustion took
place to form brown solid product. A stainless steel screen with
2
-C yield. In particular, the SCS-prepared Mn
3
2
O
3
-Na WO
2
4
/
2
catalyst successfully reduces the reaction temperature from above
8
2
3
4
2
2
2 3
O -
2
4
2
1
40 mesh opening was used for covering the ceramic bowl to prevent
2
La(NO + 3.33ϕH N(CH )CO H + 7.5(ϕ−1)O
La + ϕ(6.67CO + 8.33H O + 1.67N ) + 3N
Besides opening up new preparation method, additive modification
3
)
3
2
2
2
2
→
the formed powders from escaping. All as-synthesized catalysts were
calcined at 500–900 °C in air for 4 h, crashed and sieved to collect
2
O
3
2
2
2
2
(1)
1
00–120 mesh fine particles for use in OCM reaction testing. The cat-
alyst was denoted as a-TiO -b-Mn -c-Na WO /SiO -ϕ-x (a, TiO
loading; b, Mn loading; c, Na WO loading; ϕ, fuel/oxidizer ratio; x,
calcination temperature).
2
2
O
3
2
4
2
2
is another aspect to promote the catalyst low-temperature performance
37–39]. As mentioned above, activating oxygen molecules into desir-
able reactive oxygen species on the catalyst surface is a critical step that
induces the methane activation to produce CH · and then C -C pro-
ducts. Freund et al. [37] reported a highly ordered CaO film modified
2
O
3
2
4
[
3
2
3
2.3. Catalyst characterization
2
+
−
with Mo , and supposed that the generated superoxide anions (O
attributes to methane activation. Trunschke et al. [38] provided the Fe
and Cu co-doped polycrystalline MgO, which can effectively activate
2
)
Catalysts were characterized by X-ray diffraction (XRD, Rigaku
Uitima IV diffractometer with Cu Kα radiation (35 kV and 25 mA);
Japan), scanning electron microscopy (SEM, Hitachi S-4800; Japan)
equipped with an energy dispersive X-Ray fluorescence spectrometer
2
−
oxygen to produce peroxy (O
ethylene formation. Therefore, a question jumping out is whether the
OCM reaction temperature is dominated by the O activation tem-
perature. If so, making efforts to lower the O activation temperature by
additive modification seems to be another key to improving the low-
temperature activity for the Mn -Na WO /SiO catalyst.
In this paper, we present a strategy to introduce TiO
-Na WO /SiO catalyst by SCS method to prepare TiO
2
) species for promoting the ethane or
(
EDX, Oxford; UK) and inductively coupled plasma atomic emission
spectrometry (ICP-AES, ICP Thermo IRIS Intrepid II XSP; USA). Specific
surface area (SSA) was determined from N adsorption isotherm at
196 °C using standard Brunauer-Emmett-Teller (BET) theory on a
2
2
2
−
2
O
3
2
4
2
Quanta chrome Autosorb–3 B instrument (USA). Raman measurements
were carried out using a Raman spectrometer (Renishaw inVia) with a
5
scanned from 800 to 2000 cm . It is equipped with a charge coupled
device (CCD) camera enabling microanalysis on a sample point.
2
into the
-doped
Mn
2
O
3
2
4
2
2
32 nm semiconductor laser as excitation and the samples were
counterpart, with the aim to enhance the low-temperature catalytic
activity and selectivity for the OCM process. The catalyst preparation
parameters (including Ti-precursors, fuel/oxidizer ratio, calcination
temperature, and active component loadings) and OCM reaction con-
−1
2.4. Reactivity tests
ditions (including CH
4 2
/O molar ratio, gas hourly space velocity
(
GHSV), and reaction temperature) were systematically investigated.
OCM reaction was performed in a fixed-bed quartz tube reactor
400 mm length and 16 mm inner diameter straight cylindrical tube)
under atmospheric pressure. The catalyst of 1.0 g was loaded in the
reactor and the catalyst bed thickness was approximately 10 mm in
each OCM reaction testing. The reactants, methane (99.99%), oxygen
Excitingly, the reaction temperature (the real catalyst bed temperature
in present work) could be reduced from > 800 °C for the regular lit-
erature Mn
through the SCS method, achieving 20% CH
selectivity to C -C . Probings of catalyst phase and surface, using XRD
and Raman, reveal that the in-situ formation of MnTiO is paramount
(
2
O
3
-Na
2
WO
4
/SiO
2
catalyst to 700 °C after TiO
2
-doping
4
conversion with 70%
2
3
(
99.999%), and nitrogen (99.99%) as dilution, were co-fed into the
reactor by three calibrated mass flow controllers. GHSV was varied in
3
78

P. Wang et al.
Applied Catalysis A, General 544 (2017) 77–83
2
Fig. 1. SEM images of typical SCS-synthesized TiO -
doped catalyst before (A) and after (B) calcining at
00 °C; (C) XRD pattern of calcined TiO -doped cat-
alyst; (D) Raman spectra: (a) the calcined TiO
4
doped catalyst, (b) Mn , (c) TiO , (d) Na WO .
8
2
2
-
2
O
3
2
2
Note of SCS preparation: titanium isopropoxide (Ti-I)
as Ti precursor, ϕ of 2, and calcination temperature
of 800 °C; 6 wt% TiO
Na WO in catalyst.
2 2 3
, 6 wt% Mn O and 10 wt%
2
4
the range of 2000–15000 mL gcat. h−1 and CH
−
1
:O
molar ratio was
collapse, being consistent with its small specific surface area (SSA) of
4
2
2
−1
investigated from 2 to 8. In our present work, reaction temperature is
the catalyst bed temperature, which was monitored by a thermo-couple
placed in the middle of the catalyst bed. The catalysts were activated by
directly undergoing the OCM reaction at 800 °C for 2 h. After that, the
catalyst bed reaction temperature was gradually reduced to 780, 760,
1.4 m g
doped Mn
tical to those of the regular Mn
them show similar small SSA (1.2-1.4 m g ) (Table 1). Fig. 1C shows
the XRD patterns of the as-calcined TiO -doped catalyst. The phases of
ɑ-cristobalite, Na WO and Mn are clearly observed, which are
critical for the regular Mn -Na WO /SiO catalyst. What to be noted
is that Mn2+ (from the Mn(NO
precursor) is fully oxidized into Mn
because of the sufficient oxygen in the calcining environment, while the
introduced Ti element is transformed into TiO . The as-calcined catalyst
was also characterized by Raman spectroscopy as well as the pure
samples of Na WO , TiO and Mn for reference. Clearly, the as-
calcined catalyst shows a very strong Mn Raman band compared to
the bands to TiO and Na WO (Fig. 1D). In combining this information
with the fact that Mn shows the weaker XRD peak than TiO and
Na WO , it is rational to infer that the Mn dominates the catalyst
surface.
(Fig. 1B, Table 1). Contents of Mn, W and Na in our TiO
-Na WO /SiO catalysts by SCS method are almost iden-
-Na WO /SiO catalyst while both of
2
-
2
O
3
2
4
2
2
O
3
2
4
−1
2
2
2
7
20, and 700 °C, to investigate their temperature-dependent OCM
2
4
2 3
O
performance. All data were collected after reaction at each temperature
for at least 0.5 h (ensure to reach steady-state). The effluent gas was
analysed with an online gas chromatograph equipped with a thermal
conductivity detector (TCD), using a 30 m DM-Plot Q capillary column
2
O
3
2
4
2
3+
3 2
)
2
(
for the separation of CO
0 m DM-Plot Msieve 5A column (for the separation of N
CH ) in parallel.
2
, CH
4
, C
2
H
4 2
, C H
6 3
, C H
6
, and C
3
H
8
) and a
6
2
, O
2
, CO, and
2
4
2
2 3
O
4
2 3
O
2
2
4
2
O
3
2
3
. Results and discussion
.1. Catalyst characteristics
Fig. 1 and Table 1 show the characteristics of catalysts in terms of
2
4
2 3
O
3
2 2 3 2 4 2
3.2. Catalytic OCM performance of TiO -doped Mn O -Na WO /SiO
surface morphology, chemical composition, and textural and structural
properties. Fig. 1A shows the SEM image of the typical as-synthesized
catalyst
2 2 3 2 4 2
TiO -doped Mn O -Na WO /SiO catalyst by SCS method. It can be
seen that the catalyst surface is rough and porous, which is the char-
acteristic of sample prepared by SCS method [28]. After calcining at
3
.2.1. Effects of Ti precursors, ϕ, and calcination temperature
As well known, the SCS preparation parameters including Ti pre-
cursors, ϕ, and calcination temperature, always play critical roles in
tuning the catalyst performance [28]. The effect of Ti-precursors was
studied on the OCM performance of our SCS catalysts, by employing the
common isopropoxide (denoted as Ti-I), titanium n-propoxide (denoted
as Ti-II), and tetrabutyl titanate (denoted as Ti-III). By setting TiO
Mn , and Na WO loadings as 6, 6, and 10 wt%, ϕ as 2, and calci-
nation temperature as 800 °C, three catalysts were obtained by SCS
method and denoted as TiO -I-Mn -Na WO /SiO , TiO -II-Mn
Na WO /SiO , and TiO -III-Mn -Na WO /SiO . These catalysts were
tested in the OCM reaction at 700 °C for feed gas of
CH :O :N
= 5:1:4, using a total GHSV of 8000 mL gcat. h−1, with
the results as shown in Fig. 2A. Prior to testing experiments at 700 °C,
800 °C, the rough surface become smooth associated with serious pore
Table 1
SSA and real contents of Mn, W, Na, and Ti of the calcined catalysts.
2
,
2
a
2
O
3
2
4
Catalyst
SSA (m /g) Pore Size (nm) Content (wt%)
Mn
W
Na Ti
O
2 3
O -
2
2
3
2
4
2
2
2
4
2
2
2
O
3
2
4
2
TiO
2
-Mn
2
O
3
eNa
WO
2
WO
/SiO
4
/SiO
2
1.6
1.4
24.8
23.2
4.0 5.2 1.1 3.5
3.9 4.9 1.0 0.0
a
Mn
2
3
O eNa
2
4
2
−
1
4
2
2
a
All contents were determined by AES-ICP measurements.
79

P. Wang et al.
Applied Catalysis A, General 544 (2017) 77–83
Fig. 2. Effects of (A) Ti precursor, (B) fuel/oxidizer
ratio, and (C) calcination temperature on the cata-
lytic OCM performance of the Ti-doped Mn
Na WO /SiO catalyst. Reaction conditions: 1.0 g
catalyst, 700 °C, GHSV of 8000 mL gcat.
/N molar ratio of 5/1/4, atmospheric pressure.
2 3
O -
2
4
2
−1
−1, CH
/
4
h
O
2
2
Note of catalyst activation: by undergoing OCM re-
action at 800 °C for 2 h.
the catalysts were in situ activated by undergoing the OCM reaction at
from 4 to 20 wt% with a constant Na
conversion and C -C selectivity against the loading of TiO
2
are shown in Fig. 3A. At 700 °C, clearly, the CH conversion and C -C
2
WO
4
loading of 10 wt%. CH
4
3
3
8
00 °C for 2 h. Clearly, the TiO
2
-I-Mn
2
O
3
-Na
2
WO
4
/SiO
-II-Mn
. Over the TiO
conversion of 20% is ob-
selectivity of 70%. Therefore, titanium iso-
2
catalyst
2
3
2
plus Mn O
2
achieves higher activity and selectivity compared to the TiO
Na
Mn
2
2
O
3
-
4
2
WO
4
/SiO
2
and TiO
/SiO catalyst, a high CH
-C
2
-III-Mn
2
O
3
-Na
2
WO
4
/SiO
2
2
-I-
selectivity are promoted from 15 to 20% and 55–70%, along with the
increase in the loading of TiO plus Mn from 4 to 12 wt%. Never-
theless, further increasing the loading of TiO plus Mn to 20 wt%
does not yield better CH conversion as well as C -C selectivity. Sub-
sequently, effect of Na loading (from 5 to 20 wt%) was in-
vestigated on the TiO -I-Mn -Na WO /SiO catalyst performance. To
plus Mn was fixed at 12 wt%.
conversion and C -C selectivity are slightly
increased from 18 to 20% and 68–70% with increasing the Na WO
loading from 5 to 10 wt%, and then plateau with further increasing the
Na WO loading up to 20 wt%.
Based on the above systematic investigation and analysis, the cat-
alyst preparation conditions can be optimized as following: titanium
isopropoxide (Ti-1) as Ti precursor, ϕ of 2, TiO plus Mn loading of
12 wt% with Mn/Ti molar ratio of 1:1 (i.e., 6 wt% Mn and 6 wt%
TiO ), Na WO loading of 10 wt%, and calcination temperature of
800 °C. This promising catalyst is denoted as 6-TiO -I-6-Mn -10-
Na WO /SiO -2-800.
2
O
3
-Na
2
WO
4
2
4
2
2 3
O
tained with a high C
2
3
2
2 3
O
propoxide (Ti-I) is selected to prepare the OCM catalysts by SCS method
for follow-up studies.
4
2
3
2
WO
4
In addition, ϕ is another important parameter for the SCS catalysts.
2
2
O
3
2
4
2
Its effect on the TiO
2
-I-Mn
2
O
3
-Na
2
WO
4
/SiO
2
catalyst was investigated
address this issue, the loading of TiO
As seen in Fig. 3B, the CH
2
2 3
O
in a range from 1 to 3 (i.e., ϕ = 1.0, 1.5, 2.0, 2.5, and 3.0, respectively)
4
2
3
with the results as shown in Fig. 2B. At 700 °C and a GHSV of 8000
2
4
−1
−1
mL gcat.
h
4 2 3
, CH conversion and C -C selectivity are improved
from 14 to 20% and from 65 to 70% along with the increase of ϕ from
2
4
1
3
.0 to 2.0, and then reaches a plateau with further increasing ϕ up to
.0. Accordingly, the optimal value of ϕ is determined as 2.0.
Moreover, the effect of calcination temperature was also studied on
2
2 3
O
the OCM performance of the TiO
fixing ϕ to 2.0. As shown in Fig. 2C, in the calcination temperature
range studied from 500 to 900 °C, a volcano-shaped CH conversion
evolution is observed with a maxima of 20% at a calcination tem-
perature of 800 °C; while C -C selectivity is increased from 66 to 70%
2
-I-Mn
2
O
3
-Na
2
WO
4
/SiO
2
catalyst by
2 3
O
2
2
4
4
2
2 3
O
2
4
2
2
3
with calcination temperature from 500 to 800 °C but shows no altera-
tion with further increasing the calcination temperature to 900 °C.
Clearly, the optimal calcination temperature is found to be 800 °C for
achieving a good SCS catalyst using titanium isopropoxide (Ti-I) as
precursor at a ϕ of 2.0.
3
.2.3. Effects of reaction variables
Fig. 4 shows the effect of reaction variables on the OCM perfor-
mance over the 6-TiO -I-6-Mn -10-Na WO /SiO -2-800 catalyst.
Clearly, the CH conversion and C -C selectivity are dependent on the
CH /O molar ratio, GHSV, and catalyst bed temperature. The CH
molar ratio is very critical to the CH
selectivity to C -C products. As shown in Fig. 4A, use of low CH
2
2
O
3
2
4
2
4
2
3
4
2
4
/O
2
2
4
conversion and especially the
3
.2.2. Effects of TiO
Effects of TiO , Mn
eNa WO /SiO
2
, Mn
2
O
3
, and Na
, and Na
2
WO
4
loadings
loadings on the TiO
2
3
4
/O
2
2
O
3
2
WO
4
2
-I-
molar ratio (i.e., high oxygen concentration in catalyst bed) results in a
reduction of selectivity due to the facilitation of deep oxidation,
whereas the high CH
worsens the CH conversion other than the selectivity. Such observation
is in accordance with the reported results [40]. At 700 °C and a GHSV of
8000 mL gcat.
from 26.5 to 15.8% while the C
76.5% with the increase in CH
volcano-shaped yield to C -C
ratio and a C -C yield maxima of 14% appears at the CH
ratio of 5. In this case, the CH
high C -C selectivity of 70%.
Fig. 4B shows the GHSV dependent CH
lectivity for a feed gas with CH /O molar ratio of 5 at 700 °C. With the
increase in the GHSV from 2000 to 8000 mL gcat.
Mn
2
O
3
2
4
2
catalyst performance were systematically in-
−
1
−1
vestigated at a GHSV of 8000 mL gcat.
CH :O :N = 5:1:4. What to be first noted is that Mn and Ti species
could be combined and transformed into MnTiO in the real reaction
h
for a feed gas of
4 2
/O molar ratio (i.e., low oxygen concentration)
4
2
2
4
3
−1
−1
stream, which is paramount for the improvement of low-temperature
OCM activity of catalysts (detailed results and discussion given in the
posterior sections of 3.4 and 3.5). The Mn:Ti ratios of 1:1.5, 1:2, 1.5:1
and 2:1 were also tested (Fig. S2). The results show that the catalytic
performance with these Mn:Ti ratios is similar to that with Mn:Ti ratio
of 1:1 (Fig. 3A), indicating that the catalyst performance is mainly
h
4
, the CH conversion is monotonously decreased
2
-C
/O
3
selectivity is inversely from 52.5 to
molar ratio from 2 to 8 (Fig. 4A). A
4
2
2
3
is observed against the CH
4
/O
2
molar
2
molar
2
3
4
/O
4
conversion of 20% is achieved with a
determined by MnTiO
catalysts with Mn:Ti ratios of 1:1.5 and 1:2) or Mn
with Mn:Ti ratios of 1.5:1 and 2:1). Therefore, the loading of TiO
Mn at a fixed Ti/Mn molar ratio of 1:1 was initially investigated
3
rather than the excessively-formed TiO
(for the catalysts
plus
2
(for the
2
3
2
O
3
4 2 3
conversion and C -C se-
2
4
2
−1
−1
2
O
3
h
, clearly, the
Fig. 3. Effects of the loadings of (A) TiO
Na WO
Mn -Na
.0 g catalyst, 700 °C, GHSV of 8000 mL gcat.
/N molar ratio of 5/1/4, atmospheric pressure. Note of
catalyst activation: see that in Fig. 2.
2
plus Mn
2
O
3
and (B)
-I-
-2-800 catalyst. Reaction conditions:
2
4
on the catalytic OCM performance of the TiO
WO /SiO
2
2
O
3
2
4
2
−1
−1
1
h
4
, CH /
O
2
2
80

P. Wang et al.
Applied Catalysis A, General 544 (2017) 77–83
Fig. 4. CH
4
conversion and C
2
-C
3
selectivity for the
2
ratio at GHSV of
OCM reaction against (A) CH
4
/O
−1
−1
8
000 mL gcat.
CH /O /N = 5/1/4, using 6-TiO
Na WO /SiO -2-800 catalyst. Reaction temperature
i.e., catalyst bed temperature) dependent CH con-
version and C -C selectivity for the OCM reaction
for feed gas of CH /O /N = 5/1/4 at GHSV of 8000
using (C) 6-TiO -I-6-Mn -10-
-2-800 and (D) 6-Mn -10-Na WO
-2-800 catalysts. Note of catalyst activation: see
that in Fig. 2.
h
and (B) GHSV for feed gas of
4
2
2
2
-I-6-Mn -10-
2 3
O
2
4
2
(
4
2
3
4
2
2
−
1
−1
mL gcat.
Na WO
SiO
h
,
2
2 3
O
2
4
/SiO
2
2
O
3
2
4
/
2
CH
selectivity to C
With further increasing the GHSV up to 15000 mL gcat.
conversion shows a big reduction to 12.8% and the selectivity to C
is improved further but slowly to 74.8%. Similar observation is con-
sistent with the literature report [41].
4
conversion is decreased slightly from 24.8 to 20% whereas the
4 2
whole CH /O ratio investigated.
2
-C is inversely increased from 55 to 70% dramatically.
3
−
1
−1
h
, the CH
-C
4
3.3. Insight into the TiO
2
-doping chemistry in low-temperature OCM
2
3
catalysis: XRD and Raman studies
To allow the above evidenced low-temperature activity improve-
ment effect by TiO -doping to be optimized and exploited more ra-
tionally in the future work, it is imperative to explore the TiO -doping
Fig. 4C shows the CH
4
conversion and C
2
-C
:O
, showing that high temperature is favourable to
improve not only the CH conversion but also the C -C selectivity. At a
high temperature of 800 °C, a high CH conversion of 25% is achievable
with a high C -C selectivity to 74%. Most notably, the CH conversion
and C -C selectivity at this level (i.e. ∼20% conversion and ∼70%
selectivity) is well sustained after decreasing the catalyst bed tem-
perature to 700 °C. Even at 680 °C, an interesting CH conversion of
4% can still be achieved with a fair C -C selectivity of 55%. For
comparison, the non-TiO -doped Mn -6-Na WO -10/SiO (6 wt%
Mn and 10 wt% Na WO and without TiO -doping) catalyst was
3
selectivity against the
:N = 5:1:4 at a GHSV
2
2
reaction temperature using a feed gas of CH
4
2
−
1
−1
2
of 8000 mL gcat.
h
catalysis of the OCM reaction. To accomplish this goal, XRD and Raman
techniques were employed to track the bulk and surface evolution be-
4
2
3
4
haviors of the 6-Mn
2 3 2 4 2
O -10-Na WO /SiO catalysts before and after
2
3
4
TiO -doping. The used catalysts were particularly characterized and
2
2
3
analyzed because of the inevitable reconstruction of the surface even
bulk phase.
4
The freshly-synthesized, calcined, and used samples of the 6-Mn
2 3
O -
1
2
3
1
0-Na WO /SiO and 6-TiO -I-6-Mn -10-Na WO /SiO catalysts
2
4
2
2
2
O
3
2
4
2
2
2
O
3
2
4
2
were characterized by XRD, and their patterns are shown in Fig. 5. For
the freshly-synthesized samples (patterns a and b), no any characteristic
diffraction peak of Ti-, Mn-, Na-, and/or W- containing compounds is
observable but a dispersing peak for amorphous SiO
indicating the high dispersion of these species. After calcination at
2
O
3
2
4
2
also prepared by SCS method [35,36] under the optimum preparation
conditions, and was examined in the OCM reaction under the identical
reaction conditions. As shown in Fig. 4D, not surprisingly, this reference
2
at 2 theta of 23.3°,
catalyst achieves a high CH
lectivity of 72% at 800 °C, being comparable to those of our TiO
SCS catalyst (Fig. 4C). However, at 700 °C it almost loses the activity
less than 6% CH conversion) and achieves a poor C -C selectivity of
less than 30% (Fig. 4D). These distinct results solidly evidence that
TiO -doping remarkably enhances the low-temperature OCM activity/
selectivity of the Mn -Na WO /SiO . In addition, the OCM product
distributions (21.3% C , 43.5% C , 3.3% C , and 2% C ) for
the 6-TiO -I-6-Mn -10-Na WO /SiO -2-800 catalyst at 700 °C using a
feed gas of CH :O = 5:1:4 and a GHSV of 8000 mL gcat. are
4
conversion of 24% with a nice C
2
-C
3
se-
800 °C in air for 4 h (patterns c and d), amorphous SiO
2
is transformed
2
-doped
Rutile
(
4
2
3
Ƶ
*
ͩ
MnTiO
Na WO
a-Crist.
Mn
Quartz
Tridymite
MnWO
3
*
ƽ
ƹ
Ƶ
Ƶ
2
ƽ
ͩ
ƽ
ƾ
ƽ
Ƶ
ƻ
ƶƶ
ƶƶ
ͩ
2
4
f
2
O
3
2
4
2
2
H
6
2
H
4
3
H
8
3
H
6
e
O
3
2
4
2
ƾ
ƹ
2 3
O
2
2
d
c
−1
−1
4
2
:N
2
h
shown in Fig. S1, and the effects of reaction conditions and catalyst
preparation parameters on the product distributions are also in-
vestigated with the results as shown in Table S1 and Table S2. Clearly,
the product distribution is sensitive to the reaction conditions (espe-
cially the reaction temperature and GHSV) but not to the catalyst pre-
paration parameters studied. With the increase in either the reaction
temperature or GHSV, ethylene formation is favourable thereby leading
to an increase in the ratio of ethylene to ethane. Tuning the CH /O
4 2
ratio makes the product distributions changed a little, whereas the ratio
of ethylene to ethane remains almost constant at ∼2 at 700 °C in the
b
a
ƻ
ƶ
4
1
5
20
25
30
35
40
Fig. 5. XRD patterns of fresh as-synthesized samples without (a) and with (b) TiO
doping, catalysts after calcining at 800 °C without (c) and with (d) TiO -doping, used
catalysts without (e) and with (f) TiO -doping. Reaction conditions: 1.0 g catalyst, 700 °C,
/O /N molar ratio of 5/1/4, atmospheric pressure.
Note of SCS preparation: see that in Fig. 1. Note of catalyst activation: see that in Fig. 2.
2
-
2
2
−1
−1
GHSV of 8000 mL gcat.
h
, CH
4
2
2
81

P. Wang et al.
Applied Catalysis A, General 544 (2017) 77–83
into ɑ-cristobalite, tridymite and/or some quartz whereas phases of
at 800 °C for 2 h and then at 700 °C for another 0.5 h, a MnTiO
riched surface is observed with some Na WO , TiO and α-cristobalite
but no any Mn is detected (Fig. 7B, spectrum a). On the used 6-
Mn -10-Na WO /SiO catalyst, the surface is dominated by MnWO
with detectable α-cristobalite and Na WO (Fig. 7B, spectrum b). When
directly running at 700 °C, both catalysts with and without TiO -doping
3
-en-
Na
MnWO
than for the 6-TiO
and d), suggesting that the MnWO
2
WO
4
, Mn
phase are stronger for the 6-Mn
-I-6-Mn -10-Na WO
formation is more preferable in the
2
O
3
and MnWO
4
appear; interestingly, XRD peaks of
-10-Na WO /SiO catalyst
/SiO catalyst (patterns c
2
4
2
4
2
O
3
2
4
2
2 3
O
2
2
O
3
2
4
2
2
O
3
2
4
2
4
4
2
4
former than in the latter during the calcination treatment at 800 °C.
After reaction (at 800 °C for 2 h and subsequently at 700 °C for another
2
show comparable but poor activity and selectivity, and meanwhile, si-
milar Raman spectra are observed on their surfaces, which are very
0
.5 h), no appearance of new compound phase is observable on the
used 6-Mn -10-Na WO /SiO catalyst whereas the MnWO phase
peaks become stronger compared to the calcined one (patterns c and e);
in contrast, on the used 6-TiO -I-6-Mn -10-Na WO /SiO catalyst, a
new phase formation of MnTiO compound is evidently detectable by
XRD while the peak intensity of MnWO is in turn weakened in com-
parison with the used 6-Mn -10-Na WO /SiO catalyst (patterns e
and f). Regarding the Na WO , its role in improving the selectivity and
promoting the transformation from amorphous SiO to ɑ-cristobalite in
2
O
3
2
4
2
4
close to the pure Mn
reaction at 800 °C for 2 h and then at 700 °C for 0.5 h, a MnTiO
riched surface appears on the 6-TiO -I-6-Mn -10-Na WO /SiO
alyst (Fig. 7C), associated with a sharp increase of CH
6% to 20% and C -C selectivity 28% to 70% (Fig. 6B). The above
Raman-indicated MnTiO -enriched surface formation in correspon-
dence to the high CH conversion and high C -C selectivity solidly
confirms again that MnTiO governs the low-temperature OCM activity
for our TiO -doped Mn -10-Na WO /SiO catalysts.
2 3
O sample (Fig. 7C). After undergoing the OCM
3
-en-
cat-
2
2
O
3
2
4
2
2
2
O
3
2
4
2
3
4
conversion from
4
2
3
2
O
3
2
4
2
3
2
4
4
2
3
2
3
the OCM reaction has been widely confirmed and accepted in a large
2
2
O
3
2
4
2
number of studies [25,26]. In combining this information with the fact
that the TiO
activity (e.g., at or even below 700 °C), it is rational to infer that the
formation of MnTiO is paramount for the dramatic improvement of the
2
-doped catalyst achieved satisfying low-temperature OCM
3
.4. Stability
3
Stability is a very critical practical consideration for a hetero-
catalyst low-temperature activity.
−1 −1
geneous catalyst. At 700 °C and a total GHSV of 8000 mL gcat.
shown in Fig. 8A, the 6-TiO -I-6-Mn -10-Na WO /SiO
stable for at least 250 h without deactivation sign for a feed gas of
CH :O :N = 5:1:4; the CH conversion remains at 19–21% throughout
entire 250 h testing while C -C selectivity is retained at 68–72% with
an ethylene/ethane ratio of 2. After 250 h testing, the used catalyst was
also characterized by XRD and Raman, with the results as shown in
Fig. 8B,C. Clearly, the bulk phase and surface compositions almost
maintain unchanged even after 250 testing, compared to those at the
beginning (Fig. 5, XRD pattern a; Fig. 7B, Raman spectrum a). Most
h
, as
To further verify the promotion effect of MnTiO
3
on the low-tem-
plus Mn loading
formation) from 4 to 20 wt
, after undergoing the OCM reaction as in the Section of 3.2.2, were
2
2
O
3
2
4
2
catalyst is
perature activity, the catalysts with increased TiO
Ti/Mn ratio of 1:1 to correspond to MnTiO
2
2 3
O
(
%
3
4
2
2
4
2
3
also characterized by the XRD. The XRD patterns are shown in Fig. 6A
as well as the part of 2 theta from 30 to 40° magnified in Fig. 6B.
Clearly, formation of MnTiO
3
compound is observed on all these used
catalysts and its peak intensity is monotonously increased with the in-
crease in the TiO
2
plus Mn
2
O
3
loading from 4 to 20 wt%. As show in
Fig. 3A, at 700 °C the CH
4
conversion and C -C selectivity are re-
2
3
3 3
notably, the bulk phase of formed MnTiO is sustained with a MnTiO -
enriched surface throughout the 250 h testing (Fig. 8B,C), which is
critical to assure the promising activity/selectivity maintenance.
spectively promoted from 15 to 20% and 55–70% along with the in-
crease in the TiO plus Mn loading from 4 to 12 wt%, and then
illustrate a plateau with further increasing it up to 20 wt%. No doubt
that the MnTiO formation is essential to improve the catalyst low-
2
2 3
O
3
4
. Conclusions
temperature activity but in a certain amount range.
It is noted that, prior to reacting at 700 °C for 0.5 h, these catalysts
2 3
Low-temperature active, selective and stable Ti-doped Mn O -
all undergo OCM reaction at 800 °C for 2 h and therefore the MnTiO
compounds are already formed. If directly reacting at 700 °C, the 6-
3
Na
the OCM reaction. The most promising catalyst, with 6 wt% TiO
Mn and 10 wt%Na WO , is obtained using titanium isopropoxide
Ti-I) as Ti precursor, ϕ of 2 and calcination temperature of 800 °C. This
catalyst, activated by undergoing the OCM at 800 °C for 2 h, achieves a
high CH conversion of ∼20% with a high C -C selectivity of ∼70%
and is stable for at least 250 h at 700 °C and GHSV of 8000
2
WO
4
/SiO
2
catalyst has been developed through the SCS method for
2
, 6 wt
TiO
TiO
with a C
2
-I-6-Mn
-doped one (Fig. 4D), delivers a very low CH
-C selectivity of only 28% because no any MnTiO
is formed (Fig. 6B). After elevating this TiO -doped catalyst bed tem-
perature to 800 °C and running the OCM reaction for 2 h, not surpris-
ingly, the formation of MnTiO compound is clearly observed (Fig. 6B).
Accompanied with the formation of MnTiO compound, interestingly,
this TiO -doped catalyst become highly active and selective at low
temperature of 700 °C, achieving a high CH conversion of 20% with a
high C eC selectivity of 70% (Fig. 6B).
The Raman spectroscopy was also employed to monitor the catalyst
surface. The Raman spectra for the pure MnTiO , Mn and MnWO
compounds were recorded as reference, which are displayed in Fig. 7A.
On the used 6-TiO -I-6-Mn -10-Na WO /SiO catalyst after reaction
2
O
3
-10-Na
2
WO
4
/SiO
2
catalyst, quite similar to the non-
conversion (6%)
compound
%
(
2
O
3
2
4
2
4
2
3
3
2
4
2
3
3
−
1
−1
mL gcat.
h
for a feed gas of CH
Raman results consistently reveal that bulk MnTiO
during the OCM process in association with a derived MnTiO
4
:O
2
:N
2
= 5:1:4. Both XRD and
phase is formed
-enriched
3
3
2
3
4
catalyst surface, which appears to be important for the dramatic im-
provement of catalyst low-temperature activity for the OCM reaction.
2
3
3
2
O
3
4
Acknowledgments
2
2
O
3
2
4
2
This work has been funded by the NSF of China (21473057,
Fig. 6. (A) XRD patterns of the TiO
Na WO /SiO -2-800 catalyst with various loadings
of TiO plus Mn after undergoing the OCM re-
action at 800 °C for 2 h and then at 700 °C for 0.5 h.
B) XRD patterns, CH conversion (CCH4) and C -C
selectivity (SC2-C3 for the 6-TiO -I-6-Mn -10-
Na WO /SiO -2-800 catalysts after undergoing the
2 2 3
-I-Mn O -10-
2
4
2
2
2 3
O
(
4
2
3
)
2
2 3
O
2
4
2
OCM reaction first directly at 700 °C for 0.5 h (top)
and subsequently at 800 °C for 2 h then at 700 °C for
another 0.5 h (bottom). Reaction conditions: 1.0 g
−1
−1
catalyst, GHSV of 8000 mL gcat.
h
4 2 2
, CH /O /N
molar ratio of 5/1/4, atmospheric pressure.
82

P. Wang et al.
Applied Catalysis A, General 544 (2017) 77–83
Fig. 7. Raman spectra of the various samples. (A)
TiO
MnTiO
MnWO
2
Ƶ
ƽ
D-Cristobalite
B
C
Pure Mn
2
O
3
, MnWO
-10-Na WO
WO /SiO -2-800 (b) catalysts after under-
4
, and MnTiO
3 2
, (B) used 6-TiO -I-
ͩ
ƶ
3
Na
2
WO
4
Close to Mn O
2 3
6
1
-Mn
0-Na
2
O
3
2
4
/SiO -2-800 (a) and 6-Mn
2
2 3
O -
4
ͩ
2
4
2
2
Mn O3
o
7
00 C
going the OCM reaction at 800 °C for 2 h then at
700 °C for another 0.5 h. (C) 6-TiO -I-6-Mn O -10-
ͩ
ͩ
ƽ
ͩ
ͩ
2
2 3
ͩ
Ƶ
ͩ
ͩ
a
b
MnWO4
Ƶ
Ƶ
Close to MnTiO3
Na WO /SiO -2-800 catalyst after undergoing the
2
4
2
ƶ
OCM reaction first directly at 700 °C for 0.5 h (top)
and subsequently at 800 °C for 2 h then at 700 °C for
another 0.5 h (bottom). Reaction conditions: 1.0 g
Ƶ
ƶ
ƶ ƶ
MnTiO3
ƶ
ƶ
ƶ
ƶ
ƶ
ƽ
o
o
8
00 Cĺ700 C
−
1
−1
catalyst, GHSV of 8000 mL gcat.
h
4 2 2
, CH /O /N
3
00 600 900 1200
300
600
900
300
600
900
molar ratio of 5/1/4, atmospheric pressure.
4 2 3
Fig. 8. (A) CH conversion and C -C selectivity for
8
6
4
2
0
ͩ
B
C
ͩ
ƽ
MnTiO3
the OCM reaction against the time on stream at
700 °C using the 6-TiO -I-6-Mn O -10-Na WO /
ͩ
ƾ
MnTiO3
Na WO4
2
2
2
3
2
4
2
Mn O3
0
0
0
C
2-
C
selectivity
SiO -2-800 catalyst. (B) XRD pattern and (C) Raman
TiO2
2
3
ƽ
Na WO4
spectrum of the catalyst after 250 h OCM reaction
testing. Note of catalyst activation: see that in Fig. 2.
2
ͩ
ͩ
ƽ
Reaction conditions: catalyst of 1.0 g, GHSV of 8000
CH conversion
4
ͩ
−1 −1
ƽ
ͩ
mL gcat.
h
4 2 2
, CH /O /N molar ratio of 5/1/4,
ƾ
ͩ
atmospheric pressure.
0
50 100 150 200 250
15 20 25 30 35 40
300 600 900 1200
)
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