The catalytic performance of Mo-impregnated HZSM-5 zeolite in CH4 aromatization: Strong influence of Mo loading and pretreatment conditions

Article abstract of DOI:10.1016/j.catcom.2017.10.008

The catalytic activity and stability of Mo/HZSM-5 solid with Mo loading higher than 10 wt% in methane aromatization are significantly improved after pretreatment with a CH₄/Ar/He (9:1:10, v/v) gas mixture. A 16 wt% Mo loading is found to provide the best catalytic performance for the aromatization of methane and better than previously reported in the literature (similar reaction conditions) over Mo-impregnated HZSM-5zeolites. After 30 h of continuous reaction at 700 °C and using a space velocity of 1500 ml g^{? 1} h^{? 1}, the 16 wt% Mo-based catalyst provides an aromatic yield of 7.8% and a CH₄ conversion of 9.5%.

Full text of DOI:10.1016/j.catcom.2017.10.008

Accepted Manuscript
The catalytic performance of Mo-impregnated HZSM-5 zeolite
in CH4 aromatization: Strong influence of Mo loading and
pretreatment conditions
Pinglian Tan
PII:
S1566-7367(17)30419-3
doi:10.1016/j.catcom.2017.10.008
CATCOM 5219
DOI:
Reference:
To appear in:
Catalysis Communications
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Revised date:
Accepted date:
12 May 2017
2 October 2017
4 October 2017
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ACCEPTED MANUSCRIPT
The catalytic performance of Mo-impregnated HZSM-5 zeolite in
CH aromatization: Strong influence of Mo loading and pretreatment
4
conditions
Pinglian Tan
Laboratory of Heterogeneous Catalysis, Anhui University of Technology, Hudong
Road 59, Maanshan City, Anhui Province 243002, People’s Republic of China
Tel.: +86 555 231 1857
Fax: +86 555 247 1263
Email: tanpinglian@sina.com
1

ACCEPTED MANUSCRIPT
Abstract: The catalytic activity and stability of Mo/HZSM-5 solid with Mo loading
higher than 10 wt% in methane aromatization are significantly improved after
pretreatment with a CH4/Ar/He (9:1:10, v/v) gas mixture. A 16 wt% Mo loading is
found to provide the best catalytic performance for the aromatization of methane and
better than previously reported in the literature (similar reaction conditions) over
o
Mo-impregnated HZSM-5 zeolites. After 30 h of continuous reaction at 700 C and
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1
-1
using a space velocity of 1500 ml g h , the 16 wt% Mo-based catalyst provides an
aromatic yield of 7.8% and a CH4 conversion of 9.5%.
Keywords: Methane aromatization; Mo/HZSM-5; Optimum Mo loading;
Pretreatment with CH4/Ar/He.
2

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1
. Introduction
Much effort has been devoted to enhancing the activity and stability of
Mo/HZSM-5 catalysts in the methane dehydroaromatization reaction since the
pioneering work of Wang et al. [1]. In various essential catalyst preparation conditions,
o
the calcination temperature of 500 C, a zeolitic Si/Al ratio between 10 and 25, and
the impregnation technique are gradually accepted as the most suitable synthetic
conditions of Mo/HZSM-5 for the above stated reaction [2,3]. However, there is a
controversy on the optimum loading of Mo, apparently as the most important
chemical composition factor for the catalytic performance of this particular Mo-based
o
catalyst prepared by the impregnation method and calcined at 500 C. It was reported
that the catalytic activity increases with Mo loading up to 2-3 wt% [4-6], 4 wt% [3],
or 6 wt% [7,8] over Mo/HZSM-5. A further increase of the Mo content resulted in a
decrease of both methane conversion and selectivity to aromatics, and an increase in
the selectivity to C2-hydrocarbons. On the other hand, for a practically acceptable
activity for the Mo/HZSM-5 catalyst in methane aromatization, the reaction
o
temperature is usually set at least 200 C higher than the optimum calcination
temperature of the catalyst applied. This temperature difference affects the influence
of pretreatment method on the catalytic performance of Mo/HZSM-5. Previous
activations of Mo/HZSM-5 solids prior to the catalytic aromatization reaction were
mostly conducted in He or Ar inert gas [1-8].
Recently, Tempelman et al. [9,10] reported that the effect of pre-reduction of
Mo/HZSM-5 in 80 vol% CH /He on the catalytic performance was more positive than
4
the pretreatment of Mo catalyst in He or air gas atmosphere. To avoid rapid release of
reaction heat due to MoO3 reduction by CH4 and an abrupt increase in the
concentration of water produced during this activation of catalyst, the present work
has developed a pretreatment method of Mo/HZSM-5 with a diluted CH4 gas mixture
(55 vol% inert gas). After the application of this new pretreatment method, the
catalytic activity and stability of Mo/HZSM-5, especially that with Mo loadings
higher than 10 wt%, were significantly enhanced. Among these catalysts, HZSM-5
zeolite loaded with 16 wt% Mo exhibited the best catalytic performance for the
methane aromatization reaction.
2
. Experimental
2.1. Catalyst preparation
HZSM-5 zeolite (Si/Al = 25, particle size distribution as shown in Fig. S1) was
supplied by Nankai University, People’s Republic of China. All the Mo/HZSM-5
catalyst samples were prepared via the wet impregnation method. A 3-g portion of
HZSM-5 zeolite was impregnated with 50 ml of aqueous solution of ammonium
3

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heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O, Sigma-Aldrich) containing the
required amount of molybdenum (x wt% loading) at room temperature for 24 h. The
o
resulting material was dried in a Binder oven at 120 C and then calcined in a
o
Nabertherm furnace at 500 C in static air for 6 h. The freshly prepared Mo/HZSM-5
samples were then pressed, crushed and sieved to 20-40 mesh for catalytic evaluation.
In the following Sections, a Mo/HZSM-5 catalyst with different Mo loading is
denoted as M-x (M=Mo/HZSM-5; x=Mo loading value).
2.2. Catalyst evaluation
Catalytic tests were carried out in a continuous fixed-bed flow quartz reactor (i.d.=
o
7
mm) system at 700 C and atmospheric pressure. The gases used in this work were
all UHP grade. Typically, 0.5-g of catalyst sample was first heated in the reactor to
o
-1
7
5
00 oC at the ramp rate of ~22 C min under a gas mixture containing 45 vol% CH4,
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1
vol% Ar and 50 vol% He at the flow rate of 25 ml min in order to reduce the
catalyst and remove adsorbed/produced water. When the temperature reached 700 C,
o
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1
another gas mixture consisting of 90 vol% CH4 and 10 vol% Ar (12.5 ml min ) was
introduced into the reactor in place of the pretreatment gas mixture and the catalyst
test was then started. For comparison, the effects of typical pretreatment methods (Fig.
S2) reported in the literature [7,8,11-13] on the catalytic performance of Mo/HZSM-5
were also investigated. All reactants and products were analyzed using two Agilent
5
820 online gas chromatographs equipped with flame ionization and thermal
conductivity detectors, where Ar in CH4 was used as an internal standard. To prevent
product condensation the reactor outlet gas lines and gas sampling valves were kept
o
above 160 C. CH4 and higher hydrocarbon products (e.g. C2H4, C2H6, C6H6, C7H8,
and C10H8) were separated by a Porapak-P packed column and then analyzed using
the FID detector. H2, Ar, CO, CH4, and CO2 were separated by an activated carbon
packed column and then analyzed using the TCD detector. CH4 conversion and
selectivities/yields of carbon-containing products were calculated on a carbon number
basis.
2.3. Catalyst characterization
Nitrogen physical adsorption measurements of the parent and Mo-modified
o
HZSM-5 zeolites at -196 C were carried out on a Micromeritics automatic surface
area and pore size analyzer (TriStar II 3020). All the samples were outgassed under
o
vacuum at 350 C for 5 h prior to analysis. The micropores volume was determined
according to the t-plot method.
The Mo content was determined by ICP-AES (Optima 7300DV). The various
solutions for analysis were prepared by digesting ca. 0.25 g of the dried sample (120
o
C for 6 h) in a 10-ml concentrated nitric acid solution, followed by adding 15 ml
o
concentrated HF solution at 0 C and then some deionized water up to 50 ml. The
spent sample was treated at 600 C for 2 h under air for coke removal before digesting.
o
Mo content analysis (Table 1) showed that most of Mo loaded was remained on the
o
fresh Mo/HZSM-5 catalyst after calcination at 500 C in air for 6 h. After treatment of
o
the 16 wt% Mo-impregnated HZSM-5 zeolite with CH4/Ar/He (9:1:10, v/v) at 700 C
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and space velocity of 1500 ml g h for 3 min, there was a slight decrease in the Mo
loading of the catalyst, ca. from 14.2 to 13.6 wt%.
3
. Results and discussion
This work first investigated the effects of typical pretreatment methods developed
in the present work and those reported in the literature [7,8,11-13] on the catalytic
performance of 6 wt% Mo/HZSM-5, the most widely studied catalyst for methane
aromatization. As shown in Figs. 1a and S3a, although activation of the catalyst at 600
o
C under inert helium before reaction could enhance the activity of Mo/HZSM-5, this
Mo-based catalyst exhibits better catalytic activity and stability after pretreatment
with a reductive gas than an inert one. Furthermore, compared with H2 gas
o
pretreatmentat 350 C for 6 h, pretreatment with CH4/Ar/He (9:1:10, v/v) is a very
simple but highly efficient method for improving the catalytic performance of the
Mo/HZSM-5 solid. On the other hand, with increasing Mo loading from 6 to 16 wt%,
differences in the effects of various pretreatment methods on the catalytic
performance of Mo/HZSM-5 become obvious (Figs. 1b and S3b).
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Activities and aromatic yields at 700 C and a space velocity of 1500 ml g h over
the 2-28 wt% Mo/HZSM-5 zeolites after pretreatment in CH4/Ar/He (9:1:10, v/v)
mixture are given in Figs. 2 and S4, and Table 1. In all cases, benzene is found to be
the major aromatic product, accompanied by other heavier aromatic molecules that
include mainly naphthalene and a small amount of toluene; C2-hydrocarbon products
include ethylene and ethane (yield ratio of C2H4/C2H6 > 1). After reaction for 10 h, a
methane conversion of 5.5%, an aromatic yield of 3.2% and a C2-hydrocarbon yield
of 0.8% are observed over the 2 wt% Mo loaded catalyst. The ratio of aromatic yield
to methane conversion (Yaromatics/CCH4) is 0.58.
Methane conversion, aromatic yield, and Yaromatics/CCH4 increase with increasing Mo
loading and reach maximum values of 11.4%, 9.8%, and 0.86, respectively, at the Mo
loading of 16 wt%. Meanwhile, C2-hydrocarbon yield slightly decreases to 0.5%. By
further increasing the Mo loading, a decrease of methane conversion and aromatics
yield and a slight increase of selectivity to aromatics and C -hydrocarbon yield are
2
obtained. When the Mo loading reaches 28 wt%, the Mo/HZSM-5 gives an aromatic
yield of 8.4% and a methane conversion of 9.6% (Yaromatics/CCH4 = 0.87), both values
being higher than those obtained over the 6 wt% Mo-based catalyst. It is interesting to
note that HZSM-5 loaded with a Mo content of 50 wt% is still active for the methane
aromatization reaction under the same pretreatment and reaction conditions reported
for the other Mo loadings; an aromatic yield of 5.7% at a methane conversion of 12.4%
is obtained after 2 h of reaction.
On the other hand, with increasing Mo loading the evolution of catalytic stability is
similar to that observed for the catalytic activity of the Mo/HZSM-5 sample in the
dehydro-aromatization of methane, although the selectivity to coke decreases with
increasing Mo loading from 2 to 28 wt%. Of those catalysts investigated, the 16 wt%
Mo-impregnated HZSM-5 zeolite exhibits the most stable catalytic performance.
o
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After 30 h of continuous reaction at 700 C and a space velocity of 1500 ml g h , an
5

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aromatic yield of 7.8% at a methane conversion of 9.5% is still able to be obtained
(Fig. 2d). In addition, the induction time of the Mo/HZSM-5 becomes slightly longer
at higher Mo loadings.
The above results are different from those reported in the literature [3-8], in which
Mo/HZSM-5 with Mo loading higher than 10 wt% exhibits poorer or much poorer
catalytic performance for methane aromatization than the Mo/HZSM-5 catalyst with
Mo loadings of 6 wt% or lower (Fig. S5). For the HZSM-5 zeolite, the adsorption of
perfluorotributyl amine showed that most of its Brønsted acid sites (>96%) were
located inside the zeolitic channels [7]. After impregnation of the zeolite with
ammonium heptamolybdate tetrahydrate aqueous solution, Wang et al. [14] reported
o
that even after the Mo-impregnated HZSM-5 zeolite was treated at 400 C, zeolitic
acidity was not modified, and relatively little Mo was present in the channels of
o
ZSM-5. It is well known [6-8,14] that calcination temperature of 500 C could drive
in part of Mo species loaded into the zeolitic channels, where the cationic Mo-oxo
complexes could replace the protons and anchor on the internal surface of the zeolite.
This Mo diffusion is enhanced with increasing Mo loading from 6 to 28 wt%, as
reflected in the reduction of the specific volume of micropores [Fig. S6]. Since those
Mo species dispersed inside the zeolite pores are reported to be the precursors of
active phases responsible for the activation of methane [15,16], this can reasonably
explain the enhanced catalytic activity of Mo/HZSM-5 with suitably increasing Mo
loading. On the other hand, it is reported that methane is activated on Mo sites via
heterolytic splitting of C-H bonds assisted by the Brønsted acid sites of HZSM-5, and
synergy between the Mo2C clusters and the acid sites plays an important role in this
activation [4]. Excessive Mo loading, such as 28 wt%, leads to the formation of
inadequate synergistic sites between active Mo and Brønsted acid sites for methane
activation. Thus, the observed reduction in the catalytic activity of Mo/HZSM-5
appears reasonable. Moreover, insufficient acidic sites due to Mo overload would
promote dehydrogenation of reaction intermediates to carbonaceous materials, which
would deposit on the active sites, thus causing deactivation of Mo/HZSM-5. As a
result, the 16 wt% Mo loading is found to be the optimum one for the catalytic
performance of Mo/HZSM-5 in CH transformation to aromatic hydrocarbons at the
4
examined experimental conditions.
4
. Conclusions
Mo/HZSM-5 with Mo loading larger than 10 wt% shows significantly enhanced
catalytic activity and stability in the non-oxidative conversion of methane to aromatic
hydrocarbons after pretreatment with a diluted CH4 gas mixture (55 vol% Ar/He).
Among these Mo loadings, 16 wt% is the most suitable for the catalytic performance
of Mo-impregnated HZSM-5 zeolites in this aromatization reaction. After 30 h of
o
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continuous reaction at 700 C and 1500 ml g h , this 16 wt% Mo-based catalyst can
give rise to an aromatic yield of 7.8% at a CH4 conversion of 9.5%. Hitherto, the most
widely studied catalysts for methane aromatization are those based on Mo/HZSM-5
with Mo loading of 6 wt% or lower. This work presents novel results about the
6

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catalytic aromatization of methane over Mo/HZSM-5 zeolite impregnated with high
Mo loadings (e.g. ≥10 wt%).
Acknowledgements
The author acknowledges financial support from the Anhui University of
Technology.
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[
[
[
8
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[
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6
Captions to Figures
Figure 1. The effect of pretreatment method on the catalytic activity of Mo/HZSM-5
o
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in methane aromatization at 700 C and 1500 ml g h : (a, top) Mo/HZSM-5 catalyst
with Mo loading of 6 wt%, pretreatment methods 1-5; (b, bottom) Mo/HZSM-5
catalysts with Mo loadings of 6, 10 and 16 wt%, pretreatment methods 3 and 5.
Note: the activity data were recorded after 10 h of reaction; Methods 1, 2, and 3:
o
o
o
pretreatment in helium at 500 C, 600 C and 700 C, respectively, for 0.5 h; Method 4:
o
pretreatment in hydrogen at 350 C for 6 h; Method 5: pretreatment in CH /Ar/He
4
(9:1:10, volume).
o
-1 -1
Figure 2. Catalytic dehydroaromatization of methane at 700 C and 1500 ml g h
over Mo/HZSM-5 catalyst: (a), (b) and (c) report catalytic performance (CH4
conversion and various hydrocarbon yields) of the Mo/HZSM-5 with various Mo
loadings (2-28 wt%) after a 10-h test; (d) catalytic performance of Mo/HZSM-5 with
a Mo loading of 16 wt%.
Note: A CH4 conversion of 47.1% and traces of C2-hydrocarbon products were
observed on the M-28 catalyst after 0.5 h on stream.
8

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Figure 1
1
2
0
8
6
4
2
0
C2-hydrocarbon yield
Aromatic yield
6
wt% Mo/HZSM-5
1
CH4conversion
(a)
Method 1 Method 2 Method 3 Method 4 Method 5
Pretreatment methods
1
4
C2-hydrocarbon yield
Pretreatment method5
Aromatic yield
CH4conversion
1
2
0
8
6
4
2
0
(b)
1
Pretreatment method3
6
wt%
10wt%
16wt%
6 wt%
10wt%
16wt%
Mo/HZSM-5 with various Mo target loadings
9

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Figure 2
16
14
12
10
8
6
4
12
10
8
6
4
2
0
M-2
M-6
M-2
M-6
M-10
M-16
M-20
M-28
M-10
M-16
M-20
M-28
(b)
(a)
0
1
2
3
4
5
6
7
8
9
10
0
1
2
3
4
5
6
7
8
9
10
Time on stream (h)
Time on stream (h)
0
0
0
0
0
0
0
0
.9
.8
.7
.6
.5
.4
.3
.2
16
16
14
12
10
8
Aromatics
C2
M-2
M-6
1
1
1
4
2
0
(d)
(
c)
C6H6
M-10
M-16
M-20
M-28
C7H8
C10H8
8
6
4
2
0
6
4
2
0
0
1
2
3
4
5
6
7
8
9
10
0
3
6
9
12
15
18
21
24
27
30
Time on stream (h)
Time on stream (h)
1
0

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Table 1 Catalytic activity of Mo/HZSM-5 materials with different Mo loadings in
o
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methane aromatization at 700 C and 1500 ml g h
Nominal
Analyzed
Methane
conversion
(mol%)
5.5
8.9
10.1
11.2
11.4
10.8
10.3
Selectivity (%)
Mo loading Mo loading
b
b
c
d
(wt%)
(wt%)
1.9
C2
C6H6
44.1
62.1
67.4
70.8
71.9
74.2
74.2
73.8
C7H8
3.2
3.9
3.9
3.6
3.7
3.9
4.0
4.3
C10H8 Coke
2
6
1
1
1
2
2
14.1
7.1
5.3
4.5
4.3
4.7
5.3
5.9
10.8
11.7
12.1
11.2
10.5
8.1
27.8
15.2
11.3
9.9
9.6
9.1
5.4
8.9
0
2
6
0
4
8
10.7
14.2
17.7
21.1
24.4
8.5
8.0
2
a
9.6
9.0
7.0
Data were recorded after 10 h of reaction time
Mo content of fresh Mo/HZSM-5 catalysts
SC2 = SC2H4 + SC2H6
b
c
d
SCoke = 100% – (S + S
+ SC7H8 + S10H8)
C2
C6H6
1
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Graphical abstract
1
1
1
4
2
0
8
6
4
2
0
16
16
14
12
10
8
C2-hydrocarbon yield
Aromatic yield
CH4conversion
Activity data recorded after
C for 10 h
Aromatics
C2
reaction at 700
o
14
16 wt% Mo/HZSM-5
C6H6
C7H8
C10H8
1
1
2
0
8
6
4
2
0
6
Reaction at 700
oC and 1500ml g-1
h
-1
4
2
0
2
wt% 6wt% 10w t% 12wt% 16wt% 20wt% 24w t% 28wt%
Mo/HZSM-5 with various Mo target loadings
0
3
6
9
12
15
18
21
24
27
30
Time on stream (h)
1
2

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Highlights



The catalytic performance of >10 wt% Mo/HZSM-5 was significantly
improved
16 wt% Mo-doped HZSM-5 shows the best behavior in catalytic aromatization
of CH4
Prereduction with 45 vol% CH4/Ar/He is a highly efficient pretreatment
method
1
3