CO2 atmosphere-enhanced methanol aromatization over the NiO-HZSM-5 catalyst

Article abstract of DOI:10.1039/c4ra06572g

The aromatization of methanol over parent HZSM-5 and the modified NiO-HZSM-5 in a fixed-bed reactor was investigated under CO₂ and N₂ flow. The as-prepared catalysts were characterized by H₂-TPR, XRD, BET, NH₃-TPD, CO₂-TPD and probe-molecule reaction. Compared with parent HZSM-5 in CO₂ or N₂ flow and with NiO-HZSM-5 in N₂ flow, NiO-HZSM-5 showed greatly improved aromatization activity and BTX (benzene, toluene and xylene) yield in CO₂ atmosphere. This is attributed to the cooperation of acid sites with the activated CO₂ (over NiO species), which not only can effectively promote dehydrogenation of alkanes to form olefin intermediates, but also can accelerate dehydrogenation in the conversion of olefin intermediates to aromatics. In particular, an optimized NiO-HZSM-5 with 2.0 wt% NiO loading showed 50.1% total aromatic yield and 35.5% BTX yield in CO₂ atmosphere. It also showed high catalytic stability resulting from the restriction of coking due to its acidity, carbonaceous elimination by the activated CO₂-reacting deposited coke, and the suppressed reduction of highly active NiO species by activated CO₂.

Full text of DOI:10.1039/c4ra06572g

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CO2 atmosphere-enhanced methanol
aromatization over the NiO-HZSM-5 catalyst
Cite this: RSC Adv., 2014, 4, 44377
*
Junhui Li, Chao Hu, Kai Tong, Hao Xiang, Zhirong Zhu and Zhonghua Hu
The aromatization of methanol over parent HZSM-5 and the modified NiO-HZSM-5 in a fixed-bed reactor
was investigated under CO2 and N2 flow. The as-prepared catalysts were characterized by H2-TPR, XRD,
BET, NH3-TPD, CO2-TPD and probe-molecule reaction. Compared with parent HZSM-5 in CO2 or N2
flow and with NiO-HZSM-5 in N2 flow, NiO-HZSM-5 showed greatly improved aromatization activity and
BTX (benzene, toluene and xylene) yield in CO2 atmosphere. This is attributed to the cooperation of acid
sites with the activated CO2 (over NiO species), which not only can effectively promote dehydrogenation
of alkanes to form olefin intermediates, but also can accelerate dehydrogenation in the conversion of
olefin intermediates to aromatics. In particular, an optimized NiO-HZSM-5 with 2.0 wt% NiO loading
showed 50.1% total aromatic yield and 35.5% BTX yield in CO2 atmosphere. It also showed high catalytic
stability resulting from the restriction of coking due to its acidity, carbonaceous elimination by the
activated CO2-reacting deposited coke, and the suppressed reduction of highly active NiO species by
activated CO2.
Received 3rd July 2014
Accepted 4th September 2014
DOI: 10.1039/c4ra06572g
www.rsc.org/advances
reported a ZnO/CuO-HZSM-5 catalyst with 69% aromatic yield.18
In addition, modication with P2O5 or Mo2C can increase the
BTX selectivity of HZSM-5;19,20 hydrothermal treatment19 and
1. Introduction
As a fundamental raw material, aromatic hydrocarbon is widely
used in pharmaceuticals, pesticides, dyes and the polymer
industry. Among the aromatic products, benzene, toluene and
xylene (BTX) are especially considered to be strategic materials.
Commercially, aromatic production is based on petrochemical
processes. However, the declining oil supply makes it crucial to
develop an alternative way to produce aromatics (especially
BTX). Although methane aromatization, especially over Mo-
containing zeolite catalysts, is a potential route for the
synthesis of aromatics,1–7 its low aromatic yield and high reac-
tion temperature greatly limit its industrial application.
With the rapid development of the coal chemical industry,
the conversion of methanol to aromatics (MTA) over zeolite has
become a potential way of producing aromatics. The formation
of olens, olen oligomerization, cyclization and subsequent
dehydrogenation on acid sites are the key steps in this catalytic
reaction.8–10 HZSM-5 zeolite is considered to be the appropriate
catalytic material for this process, because of its adjustable
acidic properties and regular, three-dimensional, 10-ring pore
matching with BTX molecules.9 Modifying HZSM-5 with ZnO,
NiO, Ag2O, CuO, and Ga2O3 etc.,9,11–15 can improve aromatiza-
tion activity owing to the interaction-dehydrogenation16,17 of
metal oxides with acid sites. For example, Conte et al. obtained
55.1% aromatic yield over AgO/ZSM-5 catalyst.14 Zaidi et al.
mesopore structure10,21 can improve the aromatic diffusion. La
species can improve the aromatic selectivity and stability of Zn-
HZSM-5.22 However, both coke and hydrothermal deal-
umination deactivate Ag-HZSM-5,12 and the hydrolysis of GaO
bonds results in the deactivation of H-Ga-ZSM-5.23 The addition
of ZnO over CuO-HZSM-5 can reduce coke formation and retard
deactivation.18 SnO2/ZnO-modied HZSM-5 showed improved
selectivity and yield of BTX compared with Zn/HZSM-5, while its
catalytic stability was poor.24 Recently, Song et al. reported that
the introduction of n-butane to methanol decreased the rate of
coke deposition and prolonged the lifetime of Zn-HZSM-5/
HZSM-11 catalysts.25 Zheng et al. claimed that the coupling
conversion of methanol with 2,5-dimethylfuran over HZSM-5
exhibited improved BTX yield.26
On the whole, although great progress has been achieved in
methanol-to-aromatics production over modied HZSM-5, the
high added-value product (BTX) yield is still low. Besides
unsuitable acidity in the HZSM-5 channel and external acid
sites going against BTX shape-selectivity,9,10,27 two other reasons
for low BTX yield are that the alkane byproducts (especially
propane and butane) formed on acid sites cannot be effectively
converted to olen intermediates,24 and the hydrogen produced
by dehydrogenation in the conversion of olen intermediate to
aromatics cannot be immediately eliminated in situ. The
greenhouse gas CO2 has been used as a weak oxidant in the
dehydrogenation of alkanes to olens28–30 and the aromatiza-
tion of alkanes31,32 over NiO-loading catalysts; the CO2 activated
Department of Chemistry, Tongji University, 1239, Siping Road, Shanghai 200092,
China. E-mail: zhuzhirong@tongji.edu.cn; Fax: +86-21-65981097; Tel: +86-21-
65982563
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over the NiO species33 can improve the activity of these reactions thermogravimetric analysis (Netzsch STA 449C, Germany).
owing to its oxidative dehydrogenation and suppression of coke Removing coke from the used catalysts (3.0 g) was achieved
deposition.28,30 Thus, it is expected that the weak oxidizability of through calcining (in a tubular furnace with a 20 ml minꢀ1 air
CO2 could promote dehydrogenation of hydrocarbons in the ow) at 813 K for 2 h with a ramp of 3 K minꢀ1
MTA reaction over NiO-modied HZSM-5.
.
This paper investigates the inuence of CO2 atmosphere on
the performance of MTA reaction over NiO-HZSM-5. The results
2.3. Catalytic reaction
The prepared catalyst (5.0 g) was packed into a stainless steel,
tubular xed-bed reactor with 1.5 cm inner diameter and pre-
treated in high-purity nitrogen ow at 823 K for 1 h. Aer
decreasing the temperature to 783 K, methanol was pumped
into the reactor by a metering pump. Aromatization was carried
out at 1.0 hꢀ1 WHSV in 40 ml minꢀ1 CO2 or N2 ow. The effluent
liquid was analyzed by an HP-5890 gas chromatograph equip-
ped with a FID detector and a 50 m HP-FFAP capillary column.
C1–C6+ aliphatics in tail gas were analyzed by a HP-5890 gas
chromatograph equipped with a FID detector and a 50 m HP-
PLOT/Al2O3 “S” capillary column. CO, CO2 and H2 in tail gas
were analyzed by a Shimadzu GC-8A gas chromatograph
equipped with a TCD detector and a 10 m TDX01 molecular
sieve column. The probe reaction, propane aromatization, was
carried out at 783 K, 200 hꢀ1 GHSV (propane) and atmospheric
pressure in the xed-bed reactor with a mixed-gas ow (C3H8/N2
(or CO2) ¼ 1.0 (ml/ml)). The product analysis was the same as
that for methanol aromatization.
showed that CO2 activated over the NiO species can signicantly
promote methanol aromatization by means of its cooperation
with acid sites, and it can suppress coke formation and the
reduction of active NiO species. To the best of our knowledge,
this work is the rst one on the use of CO2 atmosphere for
methanol-to-aromatics reaction.
2. Experimental
2.1. Catalyst preparation
ZSM-5 zeolite, with a SiO2/Al2O3 molar ratio of 45, was
purchased from Fuxu Company of Shanghai. Firstly, it was
converted to NH4-type ZSM-5 by the conventional ion-exchange
method with aqueous NH4NO3 solution. Then, it was dried in
an oven at 303 K and subsequently calcined at 813 K for 1 h with
a ramp of 3 K minꢀ1 to obtain H-type ZSM-5. The HZSM-5 was
pressed, crushed, and sorted to get parent HZSM-5 catalysts
(particles of 12–20 mesh, denoted as Parent-Z). NiO-HZSM-5
was prepared by impregnation of parent HZSM-5 with
Ni(NO3)2$6H2O aqueous solution and calcination at 823 K for 1
h aer drying. The modied catalyst was denoted as NiO-Z.
3. Results and discussion
3.1. Physicochemical properties of the prepared catalysts
The H2-TPR results of modied HZSM-5 are shown in Fig. 1. As
can be seen, the nickel oxide mainly existed as NiO on the
2.2. Catalyst characterization
The crystal structure of the catalysts was studied by X-ray surface of these catalysts. The major peak at about 855 K was
diffraction analysis (XRD, D8ADVANCE) using Cu Ka radia- assigned to the reduction of the “free state” NiO,35 and the peak
tion. The specic surface area and pore volume were measured at about 1000 K was ascribed to the reduction of the “bound
by N2 adsorption–desorption at 77 K (NOVA 2200e, Quantach- state” NiO species, which has the strong interaction with HZSM-
rome). NH3-TPD and CO2-TPD experiments were performed on 5.36,37 With the increase of NiO content, the major peak shied
a conventional set-up equipped with a TCD (CHEMBET 3000). to a lower temperature due to the slight decrease of NiO
The samples (200 mg) were rst treated at 773 K in N2 for 1 h, dispersion.38 The gradual evolution of a high-temperature peak
cooled to the adsorption temperature (393 K for NH3-TPD, 373 K implies that much “bound state” NiO appears on the catalysts
for CO2-TPD), and then exposed to 45 ml minꢀ1 adsorption gas with large loading.
(20% NH3 or 20% CO2, N2 in balance) for 60 min. The samples
Fig. 2 shows the XRD patterns of the prepared catalysts. NiO-
then were purged with N2 at the adsorption temperature for 1 h HZSM-5 showed similar peaks to the parent HZSM-5, and no
and heated linearly at 10 K minꢀ1 to 873 K (NH3-TPD) or 773 K new diffraction peak appeared aer modication. This suggests
(CO2-TPD) in 45 ml minꢀ1 N2. NH3 or CO2 in effluent was that the structure of HZSM-5 was not changed, and no new
recorded continuously as a function of temperature. H2-TPR of phase was produced during NiO modication.
catalysts was also performed on a conventional set-up equipped
As shown in Table 1, the regular decrease of micropore
with a TCD (CHEMBET 3000). Samples (200 mg) were rst surface area, micropore volume, external surface and meso-
treated at 773 K in N2 for 1 h, then cooled to 373 K. They were porous volume for NiO-modied HZSM-5 suggests that NiO was
then shied to 10% H2–N2 and heated linearly at 10 K minꢀ1 to not only loaded on the external surface of HZSM-5, but also into
1073 K. The constraint index (CI) was determined with a xed- the channels.39
bed micro-reactor and 1/1 hexane/3-methylpetane as the
In general, loading an adequate amount of metal oxide on
mixed reactants, 1.0 hꢀ1 WHSV under 10–60% total conversion, HZSM-5 can narrow the pore diameter or pore opening and can
in helium ow at atmosphere pressure. The products were enhance its molecular shape-selectivity in catalytic reaction.40,41
analyzed using on-line gas chromatography (HP5890) with a The change in pore size was usually determined by the
50 m HP-PLOT/Al2O3 “S” capillary column and FID, and the constraint index (CI) based on the cracking reaction of probe
constraint index (CI) was calculated according to the reported molecules.27,42 As shown in Table 2, the CI values of parent
method.34 The coke on the used catalysts was analysed by HZSM-5 and NiO-modied HZSM-5 were almost equal,
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Table 2 The constraint index (CI) of the parent HZSM-5 and NiO-
modified HZSM-5
Catalyst Parent-Z 0.7NiO-Z 1.5NiO-Z 2.0NiO-Z 2.8NiO-Z 5.0NiO-Z
CI value 6.05
6.05
6.06
6.06
6.07
6.08
implying that NiO (#5.0%) modication hardly enhanced the
shape-selectivity of the HZSM-5 channel.
Fig. 3 shows the NH3-TPD results of the prepared catalysts.
The peaks below 573 K were assigned to the desorption of NH3
adsorbed on weak acid sites, and the peaks above 573 K corre-
sponded to strong acid sites.43,44 The desorption peaks (espe-
cially high-temperature peaks) shied to a lower temperature,
and their intensity weakened aer modication. This suggests
that NiO species covered the partial acid sites of HZSM-5 during
modication and resulted in the lower acidic density of modi-
ed HZSM-5.45 Moreover, strong acid sites were preferentially
neutralized. As a result, 5.0NiO-Z showed the lowest acidic
density and acidic strength, owing to its highest loading.
Fig. 4 shows the CO2-TPD results of the prepared catalysts.
The two peaks at about 435 K and 485 K were ascribed to the
desorption of weakly adsorbed CO2, and another peak at about
640 K corresponded to strongly adsorbed CO2.46,47 The low-
temperature desorption peaks shied to a higher temperature
aer NiO modication. Moreover, the intensity of the high-
temperature desorption peak increased with the increase of
NiO loading, although it decreased beyond 2.0 wt%. The above
changes indicate that the appropriate amount of NiO loading
can obviously enhance the CO2 adsorptivity of HZSM-5 thanks
to the basic NiO species, which serves as adsorbing sites for CO2
Fig. 1 H2-TPR results of nickel oxide-modified HZSM-5.
Fig. 2 XRD patterns of parent HZSM-5 and NiO-modified HZSM-5.
Table 1 Specific surface areas and pore volumes of parent HZSM-5
and NiO-modified HZSM-5
Surface area (m2 gꢀ1
)
Pore volume (ml gꢀ1
)
Catalyst
Total (BET) External Internal Mesopore Micropore
Parent-Z 338.3
0.7NiO-Z 334.1
1.5NiO-Z 329.5
2.0NiO-Z 325.4
2.8NiO-Z 319.7
5.0NiO-Z 302.6
58.8
56.4
53.9
52.1
49.8
42.5
279.5
277.7
275.6
273.3
269.9
260.1
0.061
0.059
0.058
0.056
0.054
0.049
0.120
0.119
0.117
0.115
0.112
0.105
Fig.
HZSM-5.
3 NH3-TPD results of parent HZSM-5 and NiO-modified
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Comparatively, those were only about 32% and 18% over parent-
Z, respectively. Moreover, although the interaction14 of NiO with
acid sites slightly improved aromatization over NiO-Z in N2
atmosphere, the product yield was still relatively low. These
results strongly suggest that the integration of CO2 atmosphere
and NiO loading signicantly enhanced the aromatization
activity and BTX yield in the MTA reaction over HZSM-5.
Ding et al. pointed out that NiO species is highly active for
the coupled dehydrogenation of isobutane with CO2 over NiOx/
AC (nickel supported on active carbon) catalyst.30 This coupled
reaction is described as follows:
(CH3)3CH ¼ (CH3)2CCH2 + H2
CO2 + H2 ¼ CO + H2O
(1)
(2)
The reverse water-gas shi reaction of CO2 (activated by NiO
species) with the hydrogen from isobutane dehydrogenation
enhanced the isobutene yield to form CO. A similar result was
also obtained over NiO/Al2O3 catalyst.28 Owing to the dehydro-
genation of hydrocarbon in aromatization, H2 can be produced
in the MTA reaction. As listed in Table 4, the H2 yield (below
0.16%) over NiO-Z in CO2 atmosphere was much lower than
those (above 1.0%) on parent-Z in the two atmospheres and over
NiO-Z under N2 atmosphere, while the CO yield was much
higher (above 2.10%). This proves that the dehydrogenation of
hydrocarbon in the aromatization over NiO-HZSM-5 under CO2
atmosphere was coupled with the reverse water-gas shi reac-
tion by CO2 activated over the NiO species. Besides, the small
amount of CO (below 0.28%) over parent-Z under the two
atmospheres and over NiO-Z under N2 atmosphere was
produced by the decomposition of methanol without acidic
adsorption,48 and the water-gas reaction49 of the deposited
carbon with the water formed by methanol dehydration.
The distribution of xylene isomers was at the thermody-
namic equilibrium value in the MTA reaction over all the
prepared catalysts, and the total aromatic yield was similar over
these catalysts in N2 atmosphere. This again proves that NiO
modication had no enhancement on the shape selectivity of
HZSM-5 channel and indicates that the changes in product
distribution for modied HZSM-5 are unrelated to porous
shape-selectivity. Because the pore diameter of HZSM-5 is near
the molecular size of xylene,9 enhanced shape-selectivity of the
channel can restrict formation or diffusion of alkylbenzenes
with relatively large molecular size, resulting in the increase of
para-selectivity of xylene27,39–41 and decrease of total aromatic
selectivity.27 Signicantly, hydrocarbon yield was similar for the
prepared catalysts in the two atmospheres. The selectivity of
total aliphatics, total alkanes and total olens over NiO-Z in CO2
atmosphere was much lower than that under N2 atmosphere,
while total aromatic selectivity was much higher. Olens are the
intermediates of aromatization. Increasing olen intermediates
in situ24 and accelerating the dehydrogenation in the subse-
quent conversion of olen intermediates to aromatics50 are two
effective ways to enhance the aromatic yield. Moreover, the
conversion of olen to aromatics occurs more easily than
dehydrogenation of alkanes in this reaction.24 Thus, it can be
Fig.
HZSM-5.
4 CO2-TPD results of parent HZSM-5 and NiO-modified
molecules.33 As a result, 2.0NiO-Z showed the strongest
adsorptivity. Besides, the adsorptivity of 2.8NiO-Z and 5.0NiO-Z
were slightly weaker than that of 2.0NiO-Z; this may result from
their relatively small pore volumes and low NiO dispersion.
3.2. Methanol aromatization over the prepared catalysts in
N2 and CO2 atmospheres
The yields of total aromatics and BTX in the MTA reaction over
the prepared catalysts in N2 and CO2 atmospheres are shown in
Table 3. As can be seen, total aromatic yield and BTX yield
increased with NiO amounts below 2.0 wt%, especially in CO2
atmosphere. However, excessive loading was detrimental to
aromatization owing to the great weakening of acidity for 2.8NiO-
Z and 5.0NiO-Z. 50.1% total aromatic yield and 35.5% BTX yield
were obtained over 2.0NiO-Z catalyst in CO2 atmosphere.
Table 3 The total aromatics yield, BTX selectivity and BTX yield over
the prepared catalysts in N2 and CO2 atmospheresa
Total aromatic
yield (%)
BTX selectivity
in aromatics (%)
BTX yield (%)
Catalysts
N2
CO2
N2
CO2
N2
CO2
Parent-Z
0.7NiO-Z
1.5NiO-Z
2.0NiO-Z
2.8NiO-Z
5.0NiO-Z
32.0
32.8
34.1
35.0
33.1
30.5
32.2
36.6
42.1
50.1
44.2
39.7
56.6
60.1
58.7
67.4
71.3
69.2
57.1
68.3
67.0
70.9
65.6
65.7
18.1
19.7
20.0
23.6
23.6
21.1
18.4
25.0
28.2
35.5
29.0
26.1
a
MTA reaction was carried out at 783 K and 1.0 hꢀ1 WHSV in a
40 ml minꢀ1 CO2 or N2 ow. Product samples were tested at 1.0 h.
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Table 4 The detailed results of MTA reaction over parent HZSM-5 and NiO-HZSM-5 in N2 and CO2 atmospheresa
Catalyst/atmosphere Parent-Z/N2 Parent-Z/CO2 1.5NiO-Z/N2 1.5NiO-Z/CO2 2.0NiO-Z/N2 2.0NiO-Z/CO2 5.0NiO-Z/N2 5.0NiO-Z/CO2
Hydrocarbon yield (%)
97.8
97.5
98.6
97.6
98.2
97.0
98.0
97.2
Hydrocarbon distribution (%)
CH4
C2H6
C2H4
C3H8
C3H6
i-C4H10
n-C4H10
t-2-C4H8
1-C4H8
i-C4H8
c-2-C4H8
C5
C6+
B
T
EB
3.79
1.63
1.62
25.72
2.49
15.74
6.49
0.41
0.29
0.83
0.29
7.21
0.80
1.81
4.73
0.50
2.80
6.29
2.88
7.80
4.15
1.73
67.31
60.58
6.73
32.69
18.51
5.11
4.48
3.41
4.18
7.05
5.92
5.03
2.13
1.24
28.41
2.29
13.86
6.14
0.39
0.22
0.77
0.25
5.76
0.43
1.25
5.53
0.43
2.89
6.36
2.84
7.62
4.87
1.21
67.00
61.32
5.68
33.00
18.87
3.12
3.14
17.36
6.15
14.22
5.08
0.91
0.59
2.01
0.69
7.17
0.49
1.01
6.44
0.62
3.15
6.71
2.97
6.64
5.97
1.08
65.41
50.13
15.28
34.59
20.28
3.81
2.04
18.36
3.68
12.50
4.68
0.51
0.23
0.87
0.29
6.04
0.50
2.53
10.49
0.63
3.97
8.26
3.64
7.52
5.03
1.01
56.92
48.31
8.61
43.08
28.89
4.51
2.34
18.06
4.28
13.37
4.97
0.71
0.46
1.33
0.50
8.95
0.73
1.98
7.64
0.59
3.47
7.57
3.37
5.79
4.30
0.90
64.39
53.07
11.32
35.61
24.03
5.01
1.55
12.54
2.93
8.92
3.26
0.46
0.27
0.92
0.32
4.85
0.22
3.84
12.47
0.87
4.89
10.69
4.71
7.71
5.38
1.14
48.30
41.09
7.21
51.70
36.6
2.98
3.02
17.02
6.02
13.68
4.93
1.02
0.60
2.05
0.80
9.97
0.91
1.79
6.80
0.58
3.14
6.78
3.02
5.58
2.79
0.60
68.92
52.88
16.04
31.08
21.53
3.81
2.34
15.86
4.95
11.63
4.37
0.55
0.29
1.26
0.48
8.21
0.43
2.01
8.54
0.66
3.92
8.56
3.82
8.09
4.2
p-X
m-X
o-X
C9
C10
Other aromatics
Total aliphatics
Total alkanes
Total olens
Total aromatics
BTX
0.99
59.21
47.65
11.56
40.79
26.85
Non-hydrocarbon yield (%)
H2
CO
CO2
1.11
0.15
0.09
1.03
0.14
1.01
0.18
0.11
0.15
2.10
1.26
0.20
0.15
0.06
2.80
1.41
0.27
0.16
0.05
2.61
a
C5: C5 aliphatics; C6+: C6+ aliphatics; B: benzene; T: toluene; EB: ethylbenzene; p-X: para-xylene; m-X: meta-xylene; o-X: ortho-xylene; C9: C9
aromatics; C10: C10 aromatics. MTA reaction was carried out at 783 K and 1.0 hꢀ1 WHSV in a 40 ml minꢀ1 CO2 or N2 ow. Product samples
were tested at 1.0 h; conversion of methanol was 100% over these prepared catalysts.
conrmed that the coupled reaction not only can effectively the coupled reaction on the conversion of aliphatics to BTX and
promote dehydrogenation of alkanes to form olen intermedi- the weaker acidity of NiO-Z, which can suppress the deep
ates, but also can accelerate dehydrogenation in the conversion reaction of BTX.51 Owing to its strongest CO2 adsorptivity, the
of olen intermediates to aromatics over NiO-HZSM-5 in CO2 highest total aromatic yield and BTX yield were obtained over
atmosphere. Of course, the diffusion of hydrocarbon species is 2.0NiO-Z with appropriate acidity.
very important. On the one hand, the alkanes formed from
The main alkane product was propane over the prepared
methanol and the intermediates (for example, formed by catalysts in MTA reaction. Therefore, as the probe reaction,
cyclization of olens) in the conversion of olens to aromatics propane aromatization was investigated over parent-Z and
on acid sites must be desorbed and diffuse to react with CO2 2.0NiO-Z in the two atmospheres to evaluate the effect of CO2
activated over the NiO species. On the other hand, the species atmosphere. The detailed results are listed in Table 5. As can be
formed by their dehydrogenation through reacting with acti- seen, reaction results were very similar over parent-Z in N2 and
vated CO2 must diffuse to be adsorbed on acid sites for the CO2 atmospheres. Although 2.0NiO-Z showed weaker acidity
subsequent reaction. Under the cooperation of acid sites with than parent-Z, it exhibited slightly higher propane conversion
the CO2 activated over NiO species, the conversion of aliphatics (41.54%) and aromatic selectivity (31.76%) in N2 atmosphere,
to aromatics was promoted, and aromatization activity was owing to the interaction-dehydrogenation of NiO species with
enhanced in the MTA reaction. In addition, as listed in Tables 3 acid sites.14 Signicantly, much higher propane conversion
and 4, NiO-Z in CO2 atmosphere showed much higher BTX (62.73%) and aromatic selectivity (49.44%) were obtained on
selectivity than parent-Z. This is due to the promoting effect of 2.0NiO-Z in CO2 atmosphere. There are two ways of propane
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Table 5 The results of propane aromatization over parent HZSM-5 and 2.0% NiO-HZSM-5 in N2 and CO2 atmospheresa
Catalysts
Parent-Z/N2
35.71
Parent-Z/CO2
36.30
2.0NiO-Z/N2
41.54
2.0NiO-Z/CO2
62.73
Propane conversion (%)
Product distribution (%)
CH4
C2H4
C2H6
C3H6
C4+ aliphatics
Benzene
Toluene
Xylene
EB + C9
16.32
14.82
12.26
11.88
14.24
9.17
13.34
7.45
0.52
17.50
13.62
12.68
12.15
14.13
8.86
13.15
7.22
0.69
14.77
13.88
11.76
12.23
15.60
9.86
14.23
6.97
0.70
8.64
9.80
8.58
9.09
14.45
13.92
22.83
11.43
1.26
Aromatic selectivity (%)
30.48
29.92
31.76
49.44
H2 yield (%)
CO yield (%)
1.67
0.10
1.71
0.12
1.90
0.15
0.14
2.85
a
EB: ethylbenzene; C9: C9 aromatics; propane aromatization was carried out at 783 K, 200 hꢀ1 GHSV (propane) and atmospheric pressure on the
xed-bed reactor with a mixed gas ow (C3H8/N2 (or CO2) ¼ 1.0 (ml mlꢀ1)); product samples were tested at 1.0 h.
decomposition on acid sites in this reaction: decomposing to
form propylene through the hydrogen abstraction reaction of
carbonium ion, and cracking to form ethylene and methane.52
Aromatics are formed from olens.50 As listed in Table 5, the H2
yield (0.14%) over 2.0NiO-Z in CO2 atmosphere was much lower
than those (above 1.66%) on parent-Z in the two atmospheres
and over NiO-Z under N2 atmosphere, but CO yield was much
higher. Moreover, although the highest propane conversion was
obtained over 2.0NiO-Z in CO2 atmosphere, methane and olen
selectivity were lowest. These imply that the CO2 activated over
NiO species not only can promote the dehydrogenation of
propane to produce more propylene intermediate, but also can
accelerate the dehydrogenation of the intermediates in the
conversion of olens to aromatics, enhancing aromatic yield by
means of its cooperation with acid sites on this catalyst.
Furthermore, this result powerfully proves the promoting effect
of the coupled reaction on methanol aromatization over NiO-
HZSM-5 under CO2 atmosphere. To clearly explain this
process, a conceptual diagram is shown in Fig. 5.
3.3. Catalytic stability of 2.0% NiO-HZSM-5 in the MTA
reaction under CO2 atmosphere
Fig. 6 shows the comparison of the catalytic stability of parent-Z
and 2.0NiO-Z under N2 and CO2 atmospheres. As can be seen,
Fig. 6 The time on-stream total aromatics yield (a) and BTX selectivity
(in aromatics) (b) in the MTA reaction over parent-Z and 2.0NiO-Z
under N2 and CO2 atmospheres.
Fig. 5 Schematic of the promoting effect of CO2 atmosphere on
methanol aromatization over NiO-HZSM-5.
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an evident decline of activity of parent-Z and 2.0NiO-Z under N2
atmosphere was observed, with an obvious increase of BTX
selectivity. However, 2.0NiO-Z showed the highest catalytic
stability under CO2 atmosphere; there was hardly any decline
for total aromatic yield in 10 h reaction. To understand the
difference in catalytic stability, the coke on the used catalysts
was analysed by TG, and the framework and porous structure of
the used catalysts were studied by BET and XRD aer removing
coke by calcination. Then, the valence-state of Ni species on the
used 2.0NiO-Z in N2 and CO2 atmospheres was also determined
by H2-TPR.
In the MTA reaction, 1,3,5-trimethylbenzene is the smallest-
sized coke molecule in the HZSM-5 channel, and its boiling
point is 438 K. Therefore, it can be conrmed that the loss of
coke begins at 438 K in TG analysis.39 As can be seen in Fig. 7,
the mass loss at the temperature range of 438–773 K was
ascribed to soluble coke (large organic molecules), while that at
773–1023 K corresponded to the insoluble coke (with high mole
ratio of C/H) formed from soluble coke.53,39 The coke amounts
on the used parent-Z in N2 and CO2 atmospheres were up to
5.63% and 5.70% respectively. Moreover, the results of XRD
(shown in Fig. 8) and BET (listed in Table 6) for the used cata-
lysts aer removing coke prove that the framework and porous
structure of these catalysts were not destroyed during the 10 h
reaction. These facts indicate that serious coke deposition
resulted in the obvious decline of parent-Z activity, owing to the
high density of strong acid sites, which is in favor of deep
reaction of aromatics and coke formation.27,51,54,55 With coke
covering more and more strong acid sites, the rates of coke
deposition and activity decline decreased gradually (as can be
seen in Fig. 6).
Fig.
8
XRD patterns of the used parent-Z and 2.0NiO-Z after
removing coke.
Table 6 Specific surface areas and pore volumes of the used parent-Z
and 2.0NiO-Z after removing coke
Surface area (m2 gꢀ1
)
Pore volume (ml gꢀ1
)
Catalyst
Total (BET) External Internal Mesopore Micropore
N2-parent-Z
336.1
58.1
58.3
51.7
51.9
278.0
278.4
272.6
273.0
0.060
0.061
0.056
0.056
0.118
0.119
0.114
0.115
CO2-parent-Z 336.7
N2-2.0NiO-Z 324.3
CO2-2.0NiO-Z 324.9
Although the coke on the used 2.0NiO-Z in N2 atmosphere
was less than that on the used parent-Z owing to the relative
weak acidity (of 2.0NiO-Z), which can prevent the strong deep
reaction of aromatics and to a certain extent reduce coke
formation,27,51,54 it is still up to 5.33%. Nevertheless, that on
the used 2.0NiO-Z in CO2 atmosphere was much less (only
3.85%). Besides, as shown in Table 7, about 48.5% NiO on
2.0NiO-Z was reduced to Ni in the 10 h reaction under N2
atmosphere, while 87.7% NiO was retained in CO2 atmo-
sphere. The above facts imply that CO2 activated by NiO
species not only can restrict the carbonaceous deposition by
reaction with partial adjacent coke,28,30 but also can effectively
suppress the reduction of highly active NiO species in the
reaction.28,30 These effects of activated CO2 and the appropriate
Table 7 The H2 consumption in H2-TPR and the Ni/NiO molar ratio
for the fresh 2.0NiO-Z and used 2.0NiO-Z in the MTA reactions for
10 h
H2 consumption
Ni/NiO molar
ratio
Catalyst
(mmol gꢀ1
)
Fresh
0.33
0.29
0.17
0
0.14
0.94
Used in CO2 atmosphere
Used in N2 atmosphere
Fig. 7 TG curves of the used parent-Z and 2.0NiO-Z in aromatization
for 10 h under N2 and CO2 atmospheres.
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acidity of 2.0NiO-Z ensure the high catalytic stability of this 12 T. Tian, W. Z. Qian, X. P. Tang, S. Yu and F. Wei, Acta Phys.-
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Conversion of methanol to aromatics was investigated over
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Acknowledgements
This work was nancially supported by Shanghai Key Basic
Research (Grant no. 11JC1412500) and National Natural Science 24 Y. B. Xin, P. Y. Qi, X. P. Duan, H. Q. Lin and Y. Z. Yuan, Catal.
Foundation of China (Grant no. 51174277).
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25 C. Song, K. F. Liu, D. Z. Zhang, S. L. Liu, X. J. Li, S. J. Xie and
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