Catalytic cracking of n-decane over NiO–MoO3 modified Pt/ZrO2–TiO2–Al2O3 catalyst with different Al2O3 ratios

Article abstract of DOI:10.1134/S0965544117080175

A series of ZrO₂–TiO₂–Al₂O₃ composite oxides with different Al₂O₃ ratios were prepared by coprecipitation method and used as the supports of Pt/NiO–MoO₃/ZrO₂–TiO₂–Al₂O₃ catalysts. Catalytic activities for n-decane cracking over these catalysts were evaluated under high temperature and high pressure conditions. Physicochemical characteristics of as-prepared catalysts were detected by using automatic adsorption instrument, X-ray diffractometer, temperature programmed reduction, and temperature programmed desorption techniques to characterize the catalysts and supports. The results indicated that the catalyst which contained 60 wt % Al₂O₃ had the largest surface area and pore volume (152.7 m²/g and 0.39 mL/g, respectively), and also it possessed strongest medium and strong acidity as well as medium acidic density. Moreover, the catalyst with 60 wt % Al₂O₃ exhibited better cracking performances compared with the others. The gas yields over Cat3 were 1.5 and 1.2 folds higher than that obtained from thermal cracking at 650 and 700°C, respectively. In addition, the heat sinks were improved 0.27 MJ/kg and 0.25 MJ/kg at 650 and 750°C, respectively.

Full text of DOI:10.1134/S0965544117080175

ISSN 0965-5441, Petroleum Chemistry, 2017, Vol. 57, No. 8, pp. 666–672. © Pleiades Publishing, Ltd., 2017.
Catalytic Cracking of n-Decane over NiO–MoO Modified
3
1
Pt/ZrO –TiO –Al O Catalyst with Different Al O Ratios
2
2
2
d
3
2
3
b
a
a
c
a,
a
Hua Zhang , Xiongjian Li , Yi Jiao , Zhongzheng Wang , Quan Zhu *, Jianli Wang , and Xiangyuan Li
a
College of Chemical Engineering, Sichuan University, Chengdu 610065, P. R. China
b
College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, P. R. China
School of Aeronautics and Astronautics, Sichuan University, Chengdu 610065, P. R. China
c
d
Xi’an Modern Chemistry Research Institute, Xi’an 710065, Shanxi, China
*
e-mail: qzhu@scu.edu.cn
Received May 28, 2016
Abstract⎯ A series of ZrO –TiO –Al O composite oxides with different Al O ratios were prepared by co-
2
2
2
3
2
3
precipitation method and used as the supports of Pt/NiO–MoO /ZrO –TiO –Al O catalysts. Catalytic
3
2
2
2
3
activities for n-decane cracking over these catalysts were evaluated under high temperature and high pressure
conditions. Physicochemical characteristics of as-prepared catalysts were detected by using automatic
adsorption instrument, X-ray diffractometer, temperature programmed reduction, and temperature pro-
grammed desorption techniques to characterize the catalysts and supports. The results indicated that the cat-
2
alyst which contained 60 wt % Al O had the largest surface area and pore volume (152.7 m /g and 0.39 mL/g,
2
3
respectively), and also it possessed strongest medium and strong acidity as well as medium acidic density.
Moreover, the catalyst with 60 wt % Al O exhibited better cracking performances compared with the others.
2
3
The gas yields over Cat3 were 1.5 and 1.2 folds higher than that obtained from thermal cracking at 650 and
7
00°C, respectively. In addition, the heat sinks were improved 0.27 MJ/kg and 0.25 MJ/kg at 650 and 750°C,
respectively.
Keywords: ZrO –TiO –Al O composite oxides, NiO–MoO promoter, сatalytic cracking, heat sink, coke,
2
2
2
3
3
surface acidity
DOI: 10.1134/S0965544117080175
INTRODUCTION
(>750°C) should be further improved. In our previous
studies, different content Al O with larger surface
2
3
“
Thermal barrier” is a major concern associated
area and better thermal stability was introduced into
with hypersonic flight in recent years. Endothermic
hydrocarbon fuel, which can not only act as an ideal
coolant to reduce the heat load of the aircraft engine,
ZrO –TiO composite oxides, and the ZrO –TiO –
2
2
2
2
Al O composite oxides were used to the cracking of
2
3
but also can improve the propulsion efficiency, has RP-3 jet fuel [7]. It was found that there is a close cor-
drawn increasing attention [1, 2]. Over the past few relation between the cracking activity and their acidic
decades, a number of researches have been conducted properties. However, the details on acidity modulation
to obtain high chemical heat absorption, especially are rarely reported, hence it is necessary to investigate
focusing on catalytic cracking [3].
the effect of acid density and acidity on cracking activ-
ity by means of additive adding.
The catalysts for cracking reaction mainly include
noble metals, zeolites, and composite oxides. Among
It is reported that the selective catalytic reduction
them, composite oxide catalysts which exhibit more (SCR) activities of MoO –CeO catalysts can be
3
2
desirable high-temperature stability, more satisfying enhanced by Brønsted acidities provided by amor-
surface acidity, and better textural properties, have phous MoO structures, and further cobalt-molybde-
3
been widely used in fuel cracking [4, 5]. The compos-
num bimetallic catalysts were applied to Fischer-
Tropsch synthesis, ammonia decomposition and
ite oxides of ZrO –TiO in general display higher sur-
2
2
face area, better surface acidic properties, and stronger hydrodesulfurization/hydrodenitrogenation
mechanical strength than pure ZrO or TiO [6]. (HDS/HDN) reactions [8]. Cooper et al. stated that
2
2
However, the stability of ZrO –TiO as supports the addition of Mo would enhance stronger acid sites
2
2
applied to kerosene cracking at higher temperature as a consequence of the higher electron deficiency at
the interface between the Co and Mo species [9].
Besides, nickel exhibits similar basic chemical proper-
1
The article is published in the original.
666

CATALYTIC CRACKING OF n-DECANE OVER NiO–MoO3
667
ties and structure characteristics with Co element, and into the electric heated reactor, and then the reacted
Ni-doped in HDS catalysts even achieved better per- fuel was cooled through the water condenser and sep-
formances than Co-doped catalysts [10]. Moreover, arated by gas-liquid separator. In this process, the vol-
the surface acidity and reducibility can be modulated ume of gaseous product, the mass of liquid residues
when the nickel collaborates with molybdenum to and the heat sinks were measured. Finally, the gaseous
modify catalyst in Ref. [11], which is important for products were sampled and analyzed by on-line gas
cracking reactions.
chromatography (GC, GC2000III, Shanghai Institute
of Technology and Computing) equipped with TCD
and FID detectors.
In order to give a deep insight into the relationship
between the acidic property and catalytic activity, in
this work, we investigated the influence of Ni–Mo
Catalysts characterization. N adsorption/desorp-
2
promoted Pt/ZrO –TiO –Al O catalysts with vary- tion analysis was conducted under -196°C using the
2
2
2
3
ing Al O contents on the cracking of n-decane. The Quadrasorb SI Automated Surface Area analyzer
2
3
total and strong acid acidity of the catalysts were (Quantachrome Instruments, USA). The crystal
enhanced by the addition of molybdenum and nickel. structure of the samples were obtained by a power
The relationship between structure and catalytic activ- X-ray diffraction on a DX–2005 X- ray using Cu K
α
ity was also investigated. This work provided some radiation (λ = 0.15406 nm) and operating at 40 kV and
fundamental suggestions for the design of acid cata- 25 mV. The surface acidity of each catalyst was mea-
lysts and promoter screening for the catalytic cracking sured by NH temperature programmed desorption
3
of hydrocarbon under supercritical conditions.
(NH -TPR) using a TP-5076 TPD instrument. The
3
H temperature programmed reduction (H -TPR) of
2
2
samples was performed on TP-5076 instrument
equipped with a thermal conductivity detector (TCD).
EXPERIMENTAL
Composite Oxides and Catalysts Preparation
ZrO –TiO –Al O composite oxides (mass ratio of
2
2
2
3
RESULTS AND DISCUSSION
ZrO : TiO = 1 : 1) with different Al2O3 mass ratios
2
2
(
0.0, 10.0, 60.0 wt %) were prepared by co-precipita-
tion method. The precipitates were dried at 120°C
and calcined at 600°C for 3 hours. Then the
catalyst powders were prepared by sequential impreg-
nation method. Firstly, the as-prepared supports were
Catalytic Activity
The distribution of gaseous product. The main com-
ponents in gaseous product are hydrogen, methane,
ethane, ethylene, propane, propylene and C4, as
shown in Table 1. As it can be seen, there are very
small variations in components content along with the
increase of Al O content in support. However, it is
co-impregnated
by
a
solution
containing
(
NH ) MO O · 4H O and Ni(NO ) · 6H O (MoO
4 6 7 24 2 3 2 2 3
content 10.0 wt % and NiO content 6.0 wt %), and
then impregnated by chloroplatinic acid (Pt content
2
3
worth noting that the amount of low-carbon olefins
ethylene and propylene) and hydrogen over catalytic
0
.50 wt %). After each impregnation step, the samples
(
were calcined for 2 hours at 500°C.
cracking is higher than thermal cracking. It is widely
accepted that unsaturated hydrocarbons and hydrogen
can provide more contribution to the heat absorption
of fuels [12]. Thus, this may be one of the reasons that
why catalytic cracking presented excellent heat sink
which will be discussed later. Besides, the formation of
a large number of small molecule hydrocarbons slows
The powders were subsequently ball-milled with
water to homogeneous slurry, and then coated on the
inner walls of stainless-steel pipes using vacuum
pump. The coated catalysts were dried overnight at
1
20°C, calcined at 500°C for 3 hours, labeled as Cat1,
Cat2 and Cat3, with Al O mass ratio of 0, 0.1 and 0.6,
2
3
respectively. The catalyst load of all catalysts is 0.2 ± down ignition delay [13].
0
.005 g/80 cm.
The gas yield and heat sink of the experimental sys-
tem are expressed as follows:
Catalytic Activity Evaluation
m1
m + m2
x =
× 100%,
(1)
(2)
The test apparatus used for cracking of n-decane
1
under supercritical condition is shown in Fig. 1. It is
composed of a fuel tank, tubular reactor, water con-
P = q Δh + P ,
in
m
loss
denser, gas-liquid separator, and an analysis system. where x is gas yield of n-decane, m and m represent
1
2
The stainless-tubes (SS304 Φ3 × 0.5 mm, 800 mm the mass of the gas and liquid residues, respectively. P
in
length) coated with the as-prepared catalysts were is the input power of the electrical heater, q the mass
m
used as reactors. The catalytic performance measure- flow rate of n-decane, Δh the heat sink of unit fuel of
ment was carried out from 600 to 750°C (fuel tempera- mass, and P the heat loss that can be minimized
loss
ture at outlet), the pressure was kept at 2.5 MPa (P = through insulation measures.
c
2
.1 MPa, T = 345°C of n-decane), and the mass flow
The gas yields and heat sinks of thermal cracking
c
of n-decane was 1.0 g/s. The n-decane was pumped and catalytic cracking at different temperatures are
PETROLEUM CHEMISTRY Vol. 57 No. 8 2017

6
68
HUA ZHANG et al.
9
10
TP3
7
2
GC
3
4
PC
8
12
TC (interval of 3 cm)
5
11
1
N2
TP1
TP2
6
13
Fig. 1. The schematic diagram of apparatus: 1, fuel tank; 2, high pressure metering pump; 3, check valve; 4, three-way valve;
5
, mass flow meter; 6, pressure and temperature control system; 7, thermocouple; 8, water condenser; 9, back pressure valve;
0, gas-liquid separator; 11, liquid receiver; 12, gas chromatograph; 13, wet gas flow meter.
1
shown in Fig. 2a,b, respectively. It is obviously that the monitored through the differential pressure between
gas yields and heat sinks over catalysts are higher than the inlet and outlet of the tubular reactor [14]. As
that produced from thermal cracking at the same shown in Fig. 3, the differential pressure of the thermal
experimental condition. Among the catalysts, Cat3 cracking is generally lower than those over the cata-
shows stable and preferable catalytic activity in the lysts during the whole experimental run. It is well
whole experimental temperature range. The gas prod- known that the number of acid sites and the distribu-
ucts produced over Cat3 are 1.5 and 1.3 folds higher tion of them are important factors for cracking activity
than thermal cracking at 650 and 700°C, respectively. also for catalyst coking deactivation. High acid site
And the heat sinks increase by 0.27 MJ/kg and density and strong acid sites may result in long carbe-
0
.25 MJ/kg correspondingly. At lower temperature nium ion residence time on the surface, and then the
(
<650°C), the catalytic activity over Cat2 is the high- carbenium ions would undergo bimolecular reactions
est from the point of gas yield and heat sink. However, more readily, leading to serious carbon deposition.
the experiment by Cat2 was forced to stop above Therefore, with the introduction of catalysts, the
6
50°C because of the serious coke blocking, which amount of carbon increased in different degree [18].
might be due to the too strong acidic sites of the cata- Especially Cat2 exhibit highest coke formation among
lyst. Also, Cat1 cannot be used at high temperature all experiments, it soared to 0.6 MPa even around
and the reason is possibly the same [5]. Above all, it is 675°C, which is the reason why the experiment over
indicated that the catalyst with 60 wt % Al O has not Cat2 was forced to stop and confirmed by catalytic
2
3
only excellent thermal stability but also more positive activity. Besides, among all the catalysts, Cat3 per-
inhibiting ability of coking formation.
forms the perfect anti-coking capacity.
Coking inhibition. To investigate the coking growth
Textural properties of catalysts. Table 2 summarizes
on the monolithic catalysts, the flow resistance was the specific surface area, pore volume, and average
Table 1. The distribution of gas product from thermal and catalytic cracking of n-decane at 700°C
Mol fraction, %
T, °C
Samples
H2
CH4
C H
C H
C H
C H
6
C4
2
6
2
4
3
8
3
Thermal
4.2
5.7
5.2
5.4
18.2
20.2
19.3
18.5
16.9
15.9
15.5
14.8
34.6
35.6
35.3
35.2
4.4
3.2
3.2
3.1
16.1
16.6
16.3
16.5
4.9
3.9
5.2
8.4
700
Cat1
Cat2
Cat3
PETROLEUM CHEMISTRY
Vol. 57
No. 8
2017

CATALYTIC CRACKING OF n-DECANE OVER NiO–MoO3
а) (b)
669
(
Thermal
Cat1
Thermal
Cat1
7
5
3.6
3.2
2.8
2.4
2.0
1.6
Cat2
Cat2
Cat3
Cat3
60
45
30
15
0
6
00 625 650 675 700 725 750
600
625
650
675
T, °C
700
725
750
T, °C
Fig. 2. The catalytic activity of thermal cracking and catalytic cracking: (a) gas yield (b) heat sink.
pore diameter of the as-prepared catalysts. As can be centers, and reduction of aggregation of active sites.
observed, the specific surface areas and pore volumes
N adsorption data show that the ZrO –TiO –Al O
2 2 2 2 3
samples present typical type IV adsorption isotherms
in conjunction with a H2-shaped hysteresis loop (as
per the IUPAC classification), indicating that there
are both slit and bottle-type pore shapes. Besides, the
pore sizes of samples calculated by BJH method are
increase obviously with the increasing of Al O con-
2
3
tent, with maximized surface area Al O content of
2
3
2
6
0 wt % (152.7 m /g and 0.39 mL/g). A large surface
area is beneficial to the catalytic activity due to the
uniform disperse of active sites, small size of active less than 7.5 nm, and both slit or bottle-type pore
shapes and mesoporous pore size distribution are ben-
eficial to high space velocity reactions [15].
T, °C
675
XRD results. The XRD patterns are presented in
600
625
650
700
725
750
Fig. 4. The complete ZrTiO and anatase TiO crystal
4
2
0
.6
.5
phase are formed in Cat1 (ZrO -TiO ), while Cat2 and
2 2
Cat3 exist as basic amorphous structures. No crystal
0
phases were observed after the addition of Al O , and
2
3
3+
the possible explanation is that Al prevents the
Cat2
0.4
0.3
0.2
4
+
4+
agglomeration of Zr and Ti during the co-precipi-
tation process. It is worth noting that the crystalliza-
tion of supports will seriously affect the surface area
and acidity, which is consistent with the results of BET
Cat1
Cat3
Thermal
40
and NH -TPD test. Besides, no diffraction peaks of
3
0
.1
PtO, NiO, and MoO are observed in Cat2 and Cat3,
3
which might due to the low loading of Pt and high
disperse of oxides on the surface of the catalysts [16].
The high degree of disperse could promote the cata-
0
0
10
20
Time, min
30
50 lytic activity, which agrees well with the experimental
results. XRD results show that the addition of Al O
2
3
could effectively shift the crystallization temperature
of ZrO and ZrTiO crystal phases, which reduces the
Fig. 3. The pressure drop of thermal and catalytic cracking.
2
4
PETROLEUM CHEMISTRY Vol. 57 No. 8 2017

6
70
HUA ZHANG et al.
Table 2. The textural performance of catalysts
2
Samples
Surface are, m /g
Pore volume, mL/g
Average pore diameter, nm
Cat1
Cat2
Cat3
83.5
135.2
152.7
0.29
0.34
0.39
7.1
5.7
4.5
Table 3. Acidic sites concentration (mmol/g) of the prepared catalysts
Catalysts
Weak acid sites
Medium acid sites
Strong acid sites
Total acid sites
Cat1
Cat2
Cat3
0.295
0.667
0.790
0.161
0.120
0.173
0.376
0.329
0.431
0.832
1.016
1.394
degradation of surface area and acidity and thus through rapid coking. In contrast, Cat3 possesses
improves the catalytic activity.
moderate total and strong acid sites densities. Table 4
summarizes the percentages of various acid sites
account for total acidic amount, and we see that the
acidic property of Cat3 is in the middle zone among all
the catalysts. In conclusion, the desirable amount and
NH -TPD results. The NH -TPD plots obtained
3
3
from different samples are presented in Fig. 5.
Absorbed NH continuously stripped from the sur-
3
faces of catalysts, indicating that the distribution of the concentration of strong acid centers of Cat3 result in
surface acidic sites is homogeneous and continuous, its better catalytic performances.
and there are many types of surface acidity. All the pat-
H -TPR results. Figure 6 gives out the H -TPR
terns show the weak acid desorption peak (100–
2
2
3
5
8
50°C) and medium acid desorption peak (350– plots of prepared catalysts. The results reveal that there
00°C) as well as strong acid desorption peak (500– are several oxide species in the catalysts, and the sup-
00°C) [17]. Tables 3 and 4 present the acidic distribu- port composition has a significant impact on the for-
tions and density of these catalysts, respectively. It is mation of them. Among all the plots, there is a broad
reported that the catalytic cracking of hydrocarbon is and weak peak at low temperature (~242°C), which
likely to proceed via classical carbenium ions formed can be attributed to the reduction of free NiO and Pt
on acid sites of the catalysts [3]. When the fraction of on the catalytic surface [15]. In relative high tempera-
C–C bond tends to proceed at strongly acid sites, the ture region (>400°C), a large reduction peak is
hydrogen transfer reaction competing with C–C bond observed around 610°C with regard to Cat1, indicating
rupture can carry out at different acid centers (weak, the existence of MoO and superficial NiMoO spe-
3 4
medium and strong acid). What is more, the hydrogen cies [19]. For Cat2 and Cat3, there are two obvious
transfer reaction is one of the important reactions of
carbon deposition. Besides, report [18] stated that
high acid site density and strength may result in long
carbenium ion residence time on the surface, then the
carbenium ions will undergo bimolecular reactions
more readily, and hence leads to serious carbon depo-
sition. Thus, numerous strong acid sites is benefical to
cracking of hydrocarbon, but high density of strong
acid centers would lead to rapid coking. One can read-
ily see that such opposite effect results in the supposi-
tion that coke formation and cracking activiy reach an
optimum combination at some intermediate density
and amount of strong acid sites. As can be seen from
Table 3, the total amount of acid sites increases obvi-
ously with the increased level of Al O , in which Cat3
ZrTiO4
TiO2
Cat1
Cat2
2
3
Cat3
70
has the largest acidic amount (1.394 mmol/g), which
is favorable to cracking. Table 3 gives densities of dif-
ferent acid centers in each catalyst. Here we can see
that Cat1 has the largest concentration of total and
10
20
30
40
50
60
80
2
θ, deg
2
2
strong acid sites, 9.96 μmol/m and 4.5 μmol/m
respectively. This might be the reason why Cat1 goes
Fig. 4. XRD profiles of different catalysts.
PETROLEUM CHEMISTRY Vol. 57 No. 8
2017

CATALYTIC CRACKING OF n-DECANE OVER NiO–MoO3
671
Cat3
Cat2
Cat3
Cat2
Cat1
Cat1
1
00 200 300 400 500 600 700 800 900
100
200
300
400
500
600
700
800
T, °C
T, °C
Fig. 5. NH -TPD patterns for different catalysts.
Fig. 6. The H -TPR profiles of as-prepared catalysts.
2
3
reduction peaks at the range of 400–850°C. The peak interaction between Mo and Ni may result in less
centered at 515°C belongs to superficial NiMoO spe- NiMoO species, in other words, the excess strong
4
4
cies and MoO mentioned in Cat1. The other around acid sites will decrease, which is unanimous with the
3
7
50°C is caused by Al (MoO ) species and NiAl O
results of NH -TPD.
3
2
4 3
2
4
spinel or NiAl O -like aluminate species [10]. The
2
4
slight distinction among each catalyst in this region
may be caused by two possible reasons. One is related
to the formation of new species. When Al O was
CONCLUSIONS
2
3
In conclusion, in NiO-MoO modified catalysts,
3
doped into ZrO –TiO support, it can interact with
2
2
different content of Al O brings about the formation
2
3
MoO and NiO then resulted in the formation of Mo–
3
of different species on the surface of catalysts, and the
surface acid sites are tuned for this reason. The Cat3
O–Al bond and NiAl O spinel as well as NiAl O -like
2
4
2
4
species. These species are more difficult to be reduced
2
has the highest surface area (152.7 m /g), pore volume
than NiMoO species, thus the high temperature peak
4
(0.39 mL/g), and moderate concentration of strong
splitting into two obvious peaks [20]. Another reason is acid (0.431 mmol/g, account for 31% of the total). All
the amount of new species derived from the interac- these factors facilitate the catalytic cracking reactions,
tion between Al O and MoO /NiO. With the and hence lead to a higher gas yield and heat sink.
2
3
3
Therefore, surface loading with Cat3 is a potential cat-
alyst pattern in active cooling of hypersonic flights.
increases of Al O content, more NiAl O or NiAl O -
2
3
2
4
2
4
like species will be formed, and the reducibility of
NiAl O spinel is better than that MoAl O species.
2
4
2
4
Therefore, the reduction peak area of Cat3 around
50°C is larger than Cat2, and the maximum tempera-
ture moved to 736°C. Above all, the H -TPR results
ACKNOWLEDGMENTS
7
2
The research was supported by the National Natu-
ral Science Foundation of China (91441132) and the
illustrate that the addition of Al O reduces the syn-
2
3
ergy between Mo and Ni, and improves the reduction Program for New Century Excellent Talents in Uni-
performance of catalyst [21], eventually enhances the versity of Ministry of Education of China (NCET-13-
reducibility of the catalysts. Moreover, the diminished 0398).
2
Table 4. Acidic sites density (μmol/m ) of the prepared catalysts
Catalysts
Cat1
Weak acid sites
Medium acid sites
Strong acid sites
Total acid sites
3.53
4.93
5.17
1.93
0.89
1.13
4.5
1.69
2.82
9.96
7.51
9.13
Cat2
Cat3
PETROLEUM CHEMISTRY Vol. 57 No. 8 2017

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HUA ZHANG et al.
11. W. Cheng, Y. M. Li, R. J. Wang, and J. Y. Zhang,
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PETROLEUM CHEMISTRY
Vol. 57
No. 8
2017