Al2O3 catalyst

Al2O3 catalyst

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights rese...

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S.Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

1835

Oxidative Dehydrogenation of Ethane to Ethylene over NiO/AI203 Catalyst Xinjie Zhang, Gang Yu, Yunqing Gong, De'en Jiang and Youchang Xie* Institute of Physical Chemistry, Peking University, Beijing 100871, P.R.China Oxidative dehydrogenation of ethane to ethylene was studied over NiO/AI203 catalysts. The effects of various A1203 supports, NiO loading, calcination temperature of the supports and of the catalysts, as well as some dopants were investigated. It was found that P205 is the best dopant. An optimum NiO-P2Os/AI203 catalyst can give ethane conversion of 58.3% and selectivity to ethylene of 72.0% with yield to ethylene of 42.0% at reaction temperature of 450~ 1. INTRODUCTION The thermal pyrolysis of ethane to ethylene has been one of the most important industrial processes for a long time, but it is costly in energy consumption and investment, because it is an endothermic process carried out at high temperature and special alloy reactor is used. The catalytic oxidative dehydrogenation of ethane (ODE) to ethylene has received more and more attention since 70's, because it is an exothermic reaction and can be carried out at low temperature with high conversion and selectivity if a good catalyst is found. For the last two decades, a variety of catalysts such as MoVNbSbCaOx[1-5], Li/MgO[6-8], La/CaOx[9,10], have been examined for ODE. Among them, MoVNbSbCaOx catalyst developed by Union Carbide Corporation is the only type of catalyst that can be operated at low temperature with good conversion and selectivity. Recently, Martin et al.[ 11 ] reported another new type of low temperature catalyst, NiO/SiO2, for ODE. Li et al.[12] improved this catalyst by using A1203 as support, but the highest yield to ethylene over their catalysts is only about 25.3%, much lower than the yield of 50% given by Union Carbide catalyst. The present study is to look for a better NiO/A1203 catalyst by screening various A1203 supports and studying the effects of NiO loading, calcination temperature of the supports and of the catalysts, as well as some additives on the catalytic properties of the catalysts. It was found that a good catalyst can be obtained if suitable amount of NiO is supported on a suitable A1203, and the catalyst is calcined at a suitable temperature with P205 as a promoter. 2. EXPERIMENTAL

2.1. Catalyst preparation In general, the catalysts were prepared by the impregnation of A1203 powders in aqueous solution of Ni(NO3)E'6H20, followed by evaporation to dryness at 140~ ovemight. The resulting solids were then calcined at 450~ in air for 5 hours unless stated otherwise. If an additive was used, it was dissolved in the Ni(NOa)2"6H20 aqueous solution for impregnation. * Corresponding author. E-mail" [email protected]

1836 The catalysts were crushed and sieved to 40-80 mesh for the evaluation of catalytic property.

2.2. Catalytic property measurement The catalytic tests were carried out with a fixed-bed flow glass microreactor, using 0.5g catalyst and operating under atmospheric pressure. The reaction temperature ranged from 350~ to 450~ The feed consisted of 10(vol)% ethane, 10(vol)% 02 and 80(vol)% N2 with a space velocity of 338 ml C2H6/g cat. h (GHSV at STP). The products were analyzed on an on-line gas chromatography with a 4m Porapak Q column and a hydrogen flame ionization detector. A methanator fitted with Ni catalyst was equipped to the GC for the analysis of carbon monoxide and carbon dioxide. 2.3. Catalyst characterization X-ray diffraction analysis was performed with a BD-86 X-ray diffractometer using Cu Kct radiation at 40 kV and 20 mA. The BET surface areas of the samples were measured by using nitrogen adsorption at 77K. H2-TPR was carried out by using 5 vol% H2 in Ar as reducing gas with a flow rate of 20 ml/min and a heating rate of 10~ 3. RESULTS AND DISCUSSION 3.1. Effect of various Ai203 supports Several A1203 supports made from different precursor aluminum salts were used to prepare NiO/AI203 catalysts. Since these supports have different surface areas, we prepared these catalysts with NiO loading on the base of the surface area of the supports. The catalysts, C1 to C4, have NiO loading of 0.24g/100m 2 surface of A1203. Their catalytic properties are reported in Table 1. Table 1 Catal ic ro erties of the catal sts usin different A1203 su _ys

Cat.*

.......................... .3__5..0.............................................

S% C% .............................. Y%

C2H4 CO2

.40.0

orts

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.................................................

S% C% .............................. Y%

C2H4 CO2

.4_50 .........................

S% C% .............................. Y%

C2H4 CO2

C1 12.0 79.9 20.1 9.6 31.8 75.2 24.8 23.9 57.2 67.1 32.9 38.4 C2 9.2 75.2 24.8 6.9 26.6 71.6 28.4 19.0 52.2 67.6 32.4 35.3 C3 14.7 53.4 46.6 7.8 36.8 52.0 48.0 1 9 . 1 52.7 53.4 46.6 24.1 C4 10.4 76.9 23.1 8.0 29.2 69.6 30.4 20.3 55.6 65.3 34.7 36.3 C%: ethane conversion, S%: selectivity, Y%: yield to ethylene * The A1203 supports used in C1, C2, C3 and C4 were prepared by hydrolysis of aluminum alkoxide, precipitation of aluminum sulphate by ammonia solution, aluminum nitrate by ammonia solution and sodium aluminate by aluminum sulphate, respectively. .............................................

,

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Table 1 shows that the catalyst C 1, which was made with an A1203 support prepared by hydrolysis of aluminum alkoxide, is the best one. It gives good ethane conversion and good selectivity to ethylene for the whole reaction temperature, and the highest yield is obtained at 450~ Therefore, this A1203 support was used to prepare the catalysts in the following experiments.

1837 3.2. Effect of NiO loading The effect of NiO loading on the catalytic with the A1203 support calcined at 900~ for 5 Fig. 1. In Fig. 1, the NiO loading is counted on the base of 100m 2 surface of the A1203 supports. As the NiO loading increases from 0.03 to 0.21g NiO/100m 2 surface of A1203, the ethane conversion increases, and then the ethane conversion decreases slightly when the NiO loading exceeds 0.2 lg NiO/100m 2 surface of A1203. In Fig. 1, the selectivity to ethylene decreases gradually as the NiO loading increases. The highest yield to ethylene is obtained over the catalysts with NiO loading of 0.21-0.27g NiO/100m 2 surface of A1203.

property was tested over a series of catalysts hours. Their catalytic properties are shown in 0

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0.4

NiO/AI20 3 (g/100m 2 surface)

Fig. 1. Ethane conversion(o), selectivity (o) and yield to ethylene(,,) as a function of NiO loading. Reaction temperature: 450~

3.3. Effect of calcination temperature of the supports A1203 was calcined at 550~ 800~ 900~ 1000~ 1100~ and 1200~ for 5 hours to obtain supports with different surface areas and surface properties. The surface areas of the supports decrease as the calcination temperature increases, as is shown in Table 2. Table 2 Surface areas of A1203 calcined at different temperatures ........................ .Sup.p_o_a_.s_ ............................. . % . 0 . ........... ._s__8._0.0........... _ _s..9__0_9_......... _.S..l__0._0_0_ .......... __S_l_l_0__0_ .......... . % 0 0 _ .....

Calcination temp_.erature.(~

550

According to the results of 3.2, we chose 0.24g NiO/100m 2 surface of A1203 as a suitable NiO loading to prepare the catalysts with the A1203 supports obtained at different calcination temperatures. The ethane conversion, selectivity and yield to ethylene over the catalysts at the reaction temperature of 450~ are shown in Fig. 2. Fig. 2 shows that the calcination temperature of the supports has significant effect on the catalytic properties of the catalysts. Though the amount of NiO on the catalysts decreases as the calcination temperature of their A1203 supports raises, the ethane conversion and yield to ethylene

800

o-e,

900

1100

1200

80 ~/0 ~0-.-.-----0~ " / 0 - - - - - - - _ 0

0

60

o___________o

N

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~

20

o

1000

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~ o k ,

~

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500 600 700 800 900 1000"1100 1200 1300 Calcination temperature of the support(~

Fig. 2. Ethane conversion (o), selectivity (o) and yield (m) to ethylene as a function of calcination temperature of supports. Reaction temperature" 450~

1838 increase slightly rather than decrease over these catalysts as the calcination temperature of supports increases from 550~ to 900~ Fig. 2 also shows that as the calcination temperature of the supports increases from 550~ to 1100~ the selectivity to ethylene over the respective catalysts increases. When the calcination temperature raises to 1200~ the support converts to t~-A1203, and the selectivity to ethylene decreases abruptly. The data in Table 1 and Fig. 2 reveal that the best catalyst can be obtained by using support prepared by hydrolysis of aluminum alkoxide and calcined at 900~ 3.4. Effect of calcination temperature of the catalysts

A series of catalysts with various NiO loading were prepared with the A1203 supports calcined at 900~ for 5 hours, and the catalysts were calcined at different temperatures. The catalysts are designated as NiOxTy, where x represents NiO loading (g/100m 2 surface of A1203), and y represents the calcination temperature (~ of the catalysts. The catalytic properties of the NiO0.24Ty catalysts are shown in Table 3. Table 3 Effe ct 0fca!cinatir0 n temperature 0fthecatalysts on the catal~ic ~pr0Pe~ies

...........

.......... _T(!.c)_ ............................ 3_5.0 ......................................... _40.0......................................... _45_0.....................

S% S% C% .......................... Y% C% .......................... Y% C% C2H4 CO2 C2H4 CO2 NiO0.24T400 23.4 66.9 33.1 15.7 44.7 66.1 33.9 29.5 . NiO0.24T500 15.2 73.8 26.2 11.2 39.7 69.7 30.3 27.7 59.5 NiO0.24T600 4.8 81.0 19.0 3.9 14.9 78.0 22.0 11.6 33.4 NiO0.24T700 . . . . 4.0 83.0 17.0 3.3 11.0 C%: ethane conversion, S%: selectivity, Y%: yield to ethylene Cat.

S% ......................... Y%

C2H4 CO2

.

. . 64.5 35.5 38.4 70.7 29.3 23.6 81.3 18.7 8.9

In Table 3, as the calcination temperature of the catalysts increases, the ethane conversion decreases for all the reaction temperature, while the selectivity to ethylene increases. The other catalysts demonstrate similar trend as the calcination temperature of the catalysts increases. The catalysts were characterized by XRD and H2-TPR to investigate the change of the catalytic property. XRD patterns of NiO0.12Ty and NiO0.30Ty are shown in Figs. 3 and 4.

9 NiO

10

20

30

40

50

60

70

2 Theta/~ Fig. 3. XRD patterns of NiO0.12Ty.

10

20

9

30

9

40 50 60 70 2 Theta/~ Fig. 4. XRD patterns of NiO0.30Ty.

1839 The XRD pattems of NiO0.12Ty catalysts in Fig. 3 have no obvious difference with that of the A1203 support, and there is no evident crystalline NiO on these catalysts. The result indicates that NiO is highly dispersed on the NiO0.12Ty catalysts. In Fig. 4, for the catalysts (NiO0.30Ty) with higher NiO loading, it is observed that there is crystalline NiO on the catalysts. Fig. 4 also shows that almost all the crystalline NiO on the NiO0.30Ty catalysts does not change greatly as the calcination temperatures of the catalysts increase. According to the results of Xie et al. tl31, not all NiO is crystalline NiO on the catalysts if the NiO loading exceeds 0.12g NiO/100m 2 surface of A1203, so there exists both highly dispersed NiO and crystalline NiO on the NiO0.24Ty catalysts. Fig. 5 shows the H2-TPR curves of the NiO0.30Ty catalysts. There are two kinds of reduction peaks. The peaks at low temperature ( 300~176 ) are due to the reduction of crystalline NiO, and the peaks at high 571 temperature ( 400~176 ) are due to the ~NiO0.30T70( reduction of highly dispersed NiO[13]. Though 547 both the low temperature peaks and the high .~~NiO0.30T60( temperature peaks shift to higher temperature as the calcination temperature of the catalysts ----~---~NiO0.30T50( increases, the high temperature peaks shift more than the low temperature peaks. The TPR peaks ~ N i O 0 .i3 0 T!4 0 ( shift to higher temperature maybe ascribes to that 100 200 300 400 500 600 700 800 900 some highly dispersed NiO has formed spinel NiA1204 with A1203 owing to the calcination at Temperature (~ higher temperature. This is consistent with the Fig. 5. H2-TPR curves of NiO0.30Ty. activity decline of the catalysts calcined at higher temperature. *

3.5. Effect of doping oxides Several oxides such as WO3, MOO3, B203, P205, were used to dope NiO/A1203 catalysts, and their effects on the catalytic properties were tested. The data show that P205 is the best dopant. The catalytic property of a NiO-P2Os/A1203 catalyst (0.24g NiO/100m 2 surface of A1203 and Ni:P=I 0:1 (mol)) is shown in Fig. 6, which shows that the dopant P205 can make the selectivity to ethylene increase about 5%, while the ethane conversion changes a little. The P205 doped catalyst gives ethane conversion of 58.3% and selectivity to ethylene of 72.0% with yield to ethylene of 42.0% at 450~

9

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~ 60 --o-- Sel. overNiO/AI203 --ZX--Conv. overNiO-P~Os/Ai20 ~ ~ / . ~ _ o --V-- Sel. overNiO-P2Os/Al20.a/~~" 40 =~ 20 O w 340

,

i

,

360

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380

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i

400

,

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420

,

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440

,

460

Reaction temperature (~ Fig. 6. Ethane conversion and selectivity to ethylene as a function of reaction temperature over the NiO/A1203 catalysts with or without P205 dopant.

1840 4. CONCLUSION This work investigated oxidative dehydrogenation of ethane to ethylene over NiO/AI203 catalysts. The effects of various A1203 supports, NiO loading, calcination temperature of the supports and of the catalysts, as well as some dopants were studied. The A1203 prepared by the hydrolysis of aluminum alkoxide and calcined at 900~ is an appropriate support. The appropriate NiO loading on the support is 0.21-~0.27g NiO/100m 2 surface of A1203. The appropriate calcination temperature of the catalyst is 450-500~ P205 is a good promoter for the NiO/A1203 catalyst. It can increase selectivity to ethylene, while ethane conversion does not decrease. For an optimum NiO-P2Os/AI203 catalyst, ethane conversion is 58.3% with a selectivity to ethylene of 72.0%, i.e., yield to ethylene of 42.0% is obtained at the reaction temperature of 450~ The NiO-P2Os/A1203 catalyst is a better low temperature catalyst for ODE. ACKNOWLEDGEMENT The authors acknowledge the National Science Foundation of China for financial support (Project No: 29733080 ). REFERENCES

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