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Al2O3 Catalysts
Accepted Manuscript Title: Synthesis Gas Production by Catalytic Partial Oxidation of Propane on Mesoporous Nanocrystalline Ni/Al2 O3 Catalysts Author: Mohammad Peymani Seyed Mehdi Alavi Mehran Rezaei PII: DOI: Reference:
S0926-860X(16)30504-X http://dx.doi.org/doi:10.1016/j.apcata.2016.10.012 APCATA 16028
To appear in:
Applied Catalysis A: General
Received date: Revised date: Accepted date:
18-8-2016 8-10-2016 12-10-2016
Please cite this article as: Mohammad Peymani, Seyed Mehdi Alavi, Mehran Rezaei, Synthesis Gas Production by Catalytic Partial Oxidation of Propane on Mesoporous Nanocrystalline Ni/Al2O3 Catalysts, Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2016.10.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis Gas Production by Catalytic Partial Oxidation of Propane on Mesoporous Nanocrystalline Ni/Al2O3 Catalysts Mohammad Peymania, Seyed Mehdi Alavib Mehran Rezaeia,c,* a
Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, P.O. Box 8731751117, Kashan, Iran
b
Chemical Engineering Department, Iran University of Science and Technology, Tehran, Iran
c
*
Institute of Nanoscience and Nanotechnology, University of Kashan, P.O. Box 8731751117, Kashan, Iran
Corresponding author: [email protected] (M. Rezaei), Fax: +98 31 5559930.
Graphical abstract
Research Highlights
Propane partial oxidation was investigated over Ni/γ-Al2O3.
C3H8 conversion had a direct relation with reaction temperature and nickel loading.
7.5 wt.% Ni/Al2O3 exhibited high stability for 12 h without any decrease in activity.
The increase in O2/C molar ratio decreased the amount of deposited carbon.
Abstract Propane partial oxidation to synthesis gas at various feed conditions was investigated over nickel catalysts supported on high surface area γ-alumina. The physicochemical characteristics of the calcined, reduced and spent samples were determined by TPR, BET, XRD, TPO and SEM analyses. The influences of Ni content, reduction and reaction temperatures, gas hourly space velocity (GHSV) and feed ratio on the catalytic properties were investigated. The activity measurements revealed that the propane conversion had a direct relation with reaction temperature and nickel loading and increased with increasing of both these factors. However, there was an optimum value for nickel content and reaction temperature in which the H2 and CO yields were maximum. The 7.5 wt.% Ni/Al2O3 exhibited high stability for 12 h without any decrease in activity. However, the selectivity declined gradually with reaction time due to carbon formation. The TPO analysis revealed that an increase in O2/C molar ratio from 0.25 to 0.75 caused a decrease in the amount of deposited carbon. Also, the amount of accumulated carbon slightly decreased with rising the reaction temperature. These results were confirmed by the SEM analysis and the filamentous carbon was observed on the catalyst surface. In addition, the increase in reduction temperature caused an increase in C3H8 conversion and H2/CO ratio. Keywords: Propane Partial Oxidation; Coke Formation; Nickel catalysts; Mesoporous; ɤalumina
1. Introduction Nowadays, synthesis gas or syngas (A mixture of H2 and CO with various ratios), is mainly used for production of chemicals such as ammonia and methanol in refineries, petrochemicals, in ferrous industries and gas-to-liquids (GTL) technology [1, 2]. Furthermore, the preferred feedstock for syngas production is methane or natural gas because of its relatively lower cost of the end product. There are some problems for natural gas as a fuel source includes difficulties in storage and transportation. Hence, light hydrocarbons, such as propane and butane, can be easily stored, distributed and transported as a source for production of syngas [3-6]. Therefore, in recent years researchers have focused on the synthesis of high active and stable catalysts for catalytic partial oxidation of light hydrocarbons. There are three possible routes for the conversion of hydrocarbons to hydrogen: steam reforming (SMR), catalytic partial oxidation (POX) and autothermal reforming (ATR). The SMR reaction is a highly endothermic reaction with a high operational cost. Nowadays, POX has received significant attention due to its availability and low-cost. Both catalytic processes suffer from the deactivation of catalysts, mainly caused by the carbon accumulation on the catalyst surface [3, 7, 8]. It is known that the metallic coordination depends on the solid structure and in oxidation reactions, which are structure sensitive, the structural characteristics of the metal oxide catalysts significantly influence the catalytic performance [9, 10]. POX reaction has been studied over Ni, Co and noble metal-based catalysts. Although the precious metals have shown high performance, but the high cost of them restricts their industrial usage [11, 12]. In contrast, the inexpensively of Ni-catalysts causes the widespread usage of nickel based catalysts in POX reactions but they are sensitive to coke formation and
sintering [7, 13]. The catalyst support can influence the catalytic performance because it can improve the active phase dispersion, decrease the inactive spinel phase formation, increase the active phase reducibility and oxygen mobility on surface [14, 15]. Alumina is one of the most popular supports and widely used in industrial catalysts due to low price and good structural properties and high surface area. In this work, the activity and selectivity of the mesoporous nanocrystalline Ni/Al2O3 catalysts with different Ni contents were studied in propane partial oxidation reaction. The effects of feed ratio, GHSV and reduction temperature on the catalytic properties of the catalysts were also studied. Furthermore, the influence of feed ratio and temperature of reaction on carbon deposition was determined.
2. Experimental 2.1. Materials
The used chemicals for preparing the catalysts were the nitrate salt of nickel (Ni(NO3)2.6H2O, Loba Chemie) and γ-Al2O3 (Sasol) as Ni active phase precursor and catalyst support, respectively. 2.2. Catalyst preparation
The catalysts were prepared by a wetness impregnation method with different nickel loadings. Prior to the impregnation, the support was crushed and sieved to obtain the particles with a size smaller than 0.25 mm. The prepared particles were stabilized by calcination at 700°C for 3h and impregnated with an aqueous solution of nickel nitrate with appropriate concentration to obtain the desired Ni loading. The slurry was agitated at room temperature for 3h and dried at 80 °C and subsequently heat treated at 600°C for 3 h. 2.3. Catalytic evaluation
All the experiments were done in a microreactor (length: 70 cm, inner diameter: 8 mm) under ambient pressure. The microreactor was loaded with a specific weight (0.1 g) of the catalyst sieved to 35-60 mesh. Prior to injection of feed, the catalysts were reduced with pure H2 at 700 °C for 3 h. The feed gas stream was consisted of C3H8 and O2 with desired molar ratio and diluted with He as carrier gas. The activity tests were carried out in the temperature range of 550-700 °C. The reactor effluent was passed through a water trap to remove the moisture and after that its composition was determined by a Younglin gas chromatograph (HID detector and Carboxen 1010 column). The values of propane conversions, H2 and CO yields and CH4, CO2, C2H4 and C2H6 selectivities were obtained using the following equations [16]:
X C3H 6 100
YH 2 100
YCO 100
FC3H 6 ,in FC3H 6 ,out FC3H 6 ,in FH 2 ,out
4( FC3H 6 ,out FC3H 6 ,in ) FCO,out
3(FC3 H 6 ,out FC3 H 6 ,in )
S n 100
(1)
(2)
(3)
Fn,out ( Ftotal,out FH 2 ,out )
(4)
Where Fin and Fout are the volumetric flow rates of the components in the feed and product gas streams, respectively. Fout was calculated with carbon balance. Where n is the component like CO, CH4, CO2, C2H4 and C2H6. 2.4. Catalyst Characterization
The crystalline phases of the fresh and reduced catalysts were identified by powder X-ray diffraction (XRD) analysis using a PANalytical X’Pert-Pro instrument. The step scans were
recorded between 2θ = 10–80°. The average crystallite size of metallic Ni was calculated by using the Scherrer equation. Nitrogen adsorption-desorption analysis of the reduced catalysts was performed at -196 °C in a gas adsorption analyzer (BELSORP mini II). A Vega@Tescan instrument was used for scanning electron microscopy (SEM) analysis. TPR (Temperature Programmed Reduction) and TPO (Temperature Programmed Oxidation) analyses of the calcined and spent catalysts were done by an automatic apparatus (Chemisorb 2750, Micromeritics). The details of TPR and TPO procedures could be found in our previous work [17].
3. Results and discussion 3.1. Structural properties of the catalysts The diffraction patterns of the support and reduced samples with different Ni loadings are shown in Fig. 1. For the catalyst with 2.5 wt.% Ni, because of the small content of nickel, the diffraction peaks assigned to the nickel and nickel oxide were not seen. Several diffraction peaks were observed by increasing of nickel content, which were located at 44.5, 51.8 and 76.5°, corresponding to (111), (200) and (220) plans of metallic Ni (JCPDS. 004-0850). As seen, in all the samples the increase in nickel content intensified the Ni diffraction peaks. The XRD results reveal that the support is in the gamma phase [18]. The observed peaks at 2θ = 37.6° and 45.79° are attributed to Al2O3 (JCPDS. 01-1303). The small peak at about 2θ = 62° is ascribed to NiAl2O4 phase (JCPDS. 71-0965). The Al2O3 and NiAl2O4 phases are overlapped at 2θ = 67° and 37°. The reduced Ni/Al2O3 catalyst at 700°C exhibited two main crystallite phases: γ-Al2O3 and NiAl2O4. γ-Al2O3 shows a pseudospinel structure and the lattice parameters of γ-Al2O3 are very close to NiAl2O4, so it is difficult to recognize these phases because the overlap between their diffraction peaks [19].
The nickel crystallite sizes calculated by Scherrer equation are presented in Table 1. The Ni crystallite sizes were increased size from 5.1 to 11.8 nm by the increase in Ni content. The textural characteristics of the catalyst carrier and reduced catalysts with various Ni contents are presented in Table 1. As shown, the catalyst carrier exhibited high BET area and with increasing the Ni content, the pore volume and surface area decreased due to blockage of the pores by Ni particles and/or destruction in mesoporous structure at high temperature. Enhancing in Ni loading does not significantly influence the pore diameter. It was suggested that proper nickel content could benefit the formation of highly dispersed NiO with high surface area [20]. The isotherms of the reduced catalysts are depicted in Fig. 2a. They can be classified as the type IV isotherm, typical for mesoporous structures. Also, for these catalysts, the H2 hysteresis loop observed at a relative pressure range (P/P0=0.6–0.9), which generally is assigned to solids made of particles crossed by channels with cylindrical shape [21, 22]. Additionally, Fig. 2b presented the pore size distributions of the samples. All the samples have a mesoporous structure with distribution of pore in the range of 2−10 nm. The pore size distribution was shifted to broader size when nickel loading increased.
3.2. Temperature-programmed reduction In order to study the redox behavior of the catalysts with different Ni loadings, H2-TPR analysis was carried out and the profiles are presented in Fig. 3. The reduction of all catalysts was started at 400°C. As can be seen, three types of peaks are seen in these patterns, which suggest the existence of three types of Ni species assigned by α, β and γ symbols, respectively. The small α peak located at 400-500 °C can be assigned to the reduction of bulk nickel oxide and weakly interacted NiO with the catalyst carrier. The β peak observed around 600 to 750°C is related to the reduction of strongly interacted NiO with Al2O3. The third
reduction peak (γ) at temperature above 800 °C is assigned to the reduction of NiAl2O4 [11, 23]. It is clearly observed that with the increase in Ni content, the reduction peaks were shifted to lower temperatures. It indicated that more reducible Ni species were formed in higher metal content. The results showed that increasing in nickel content weakened the interaction between NiO and Al2O3 and the reduction peaks moved to lower temperatures. Furthermore, the area under the reduction patterns increased with the increase of the nickel content of catalysts, due to higher hydrogen consumption on the catalyst with higher nickel content[24].
3.3. Catalytic performance 3.3.1. Effect of reaction temperature and Ni content The influences of temperature and nickel content on the activity of catalysts in propane partial oxidation reaction were studied and the results are presented in Fig. 4a. The propane conversions enhanced when the temperature increased from 550 to 700 °C due to increasing in the rate of reaction with the increase of temperature. The results showed that C3H8 conversion increased with the enhancement of Ni content from 2.5 to 10 wt.% due to increase of available nickel to reactants. With increasing in reaction temperature the product ratio (H2/CO ratio) for all samples except of the catalyst with 2.5% Ni decreased due to occurrence of the reverse water gas shift reaction (RWGS) [25] as shown in Fig. 4b. Also, The 7.5 Ni/Al2O3 catalyst exhibited maximum H2/CO ratio at various reaction temperatures. The yields of H2 and CO of the catalysts are shown in Fig. 4c and 4d, respectively. The H2 and CO selectivities improved with the increase of the reaction temperature up to 600°C and the further increase in temperature declined the H2 and CO yields due to formation of byproducts. Also, with increasing in Ni loading from 2.5 to 5 wt.%, H2 and CO selectivities increased and then decreased at nickel loadings higher than 5 wt.%.
The by-product selectivities are shown in Fig. 5. The selectivity to methane is between 0.5 to 17% at different temperatures as shown in Fig. 5a. The minimum selectivity to methane was observed at 600°C in catalysts with various Ni loadings. The results in Fig. 5b showed the decrease in CO2 selectivity with increasing in the reaction temperature, due to reverse water gas shift and CO2 consumption [26]. As shown in Fig. 5c and 5d, the by-product formation occurred at 650°C. The optimal temperature for propane partial oxidation is the temperature with maximum hydrogen yield and the lowest byproducts production. The results showed the maximum hydrogen production was occurred at a temperature of 650 °C for propane [4]. The textural properties of the spent catalysts with various nickel contents are reported in Table 1. It is clearly observed that the BET area and pore volume decreased after reaction due to thermal sintering of catalyst particles at high temperature and also the carbon formation. The accumulated carbon on the catalyst surface can block the pores and reduce the BET area.
3.3.2. Effect of GHSV Fig. 6 presents the influence of GHSV on the catalytic activity and selectivity of 7.5% Ni/Al2O3 catalyst for partial oxidation at 600 °C and C3H8:O2 = 1 : 1.5. The graphs show that with increasing in GHSV from 15000 to 60000 mL/(gcat·h), the C3H8 conversion increased from 53 to 70% and further increase in GHSV does not have a significant effect on propane conversion. It may be due to increasing in the amount of heat released from the bed of catalyst by enhancing the space velocity. It is noted that the catalyst bed temperature was controlled with inserted thermocouple in bottom of the catalyst bed. However, the enhancement of the temperature of the inner surface of the catalyst particles was inevitable. In fact, enhancing of the space velocity indirectly leads to increase the temperature of the catalyst particles [27]. The same trend was also seen for the selectivities of H2 and CO.
3.3.3. Effect of feed ratio Fig. 7 shows the influence of the molar feed ratio on the activity and selectivity of the catalysts. It is clear that increasing in oxygen content of feed caused to consuming propane due to occurring of both partial and complete oxidation. It is known that the total combustion is more favorable than the partial oxidation at higher O2/C ratios, and at these conditions the high quantities of CO2 and H2O were formed. The CO2 selectivities increased significantly while the CH4 selectivities were nearly the same. The higher O2/C molar ratios caused a decrease of the H2 selectivity. Due to the low content of CH4, hydrogen might be reacted by the excess O2 , which led to higher H2O selectivity (not shown in the Fig. 7) [26]. 3.3.4. Catalytic stability The stability of C3H8 conversion, H2 yield, CO, C2H4 and C3H6 selectivities for 7.5% Ni/Al2O3 catalyst during 12 h time on stream at 700 °C are shown in Fig. 8. The catalyst possessed high stability without any decline in propane conversion. It is seen that with increasing time, the H2 yield and CO selectivities were decreased due to the formation of byproducts such as ethylene and propylene. 3.3.5. Effect of reduction temperature Table 2 presented the textural characteristics of the 7.5% Ni/Al2O3 catalyst with different reduction temperatures. It can be seen that increasing in reduction temperature caused a decline in the BET area. However, it did not have a significant influence on the pore volume and pore size. The nitrogen adsorption/desorption isotherms of the reduced catalysts at different temperatures are shown in Fig. 9a. As shown, IV type isotherms with H2 shape hysteresis loop were observed in all catalysts. The pore size distributions of these catalysts are shown in Fig. 9b. The effects of reduction temperature on the catalytic performance of the 7.5%Ni/Al2O3 catalyst were investigated and the results are shown in Fig. 10a and 10b. As expected, the
increase in reaction temperature leads to increase in propane conversion. The increase in reduction temperature slightly increased the propane conversion due to more reduced Ni on the catalyst surface. The increase in reduction temperature also enhanced the product ratio (H2/CO molar ratio) in all temperatures, Fig. 10b. 3.4. TPO analysis The TPO analyses of the 7.5% Ni/Al2O3 catalysts after reaction with various O2/C ratios are presented in Fig. 11. The results indicated that the amount of accumulated carbon decreased with the increase of the O2/C ratio. No detectable carbon was observed at O2/C ratio of 0.75. The low amount of accumulated carbon could be related to high dispersion of nickel in high surface area of alumina and the excess of oxygen content respect to the stoichiometric value can increase oxygen species in surface and therefore reduce the deposited carbon. The highest content of carbon was observed at O2/C=0.25, due to the highest area under the TPO profile. The elementary chemical potential calculations in a free energy minimization technique showed that, the potential of carbon formation was increased with decreasing the O2/C ratio [13, 28]. The carbon deposition over the 7.5% Ni/Al2O3 at various reaction temperatures at O2/C=0.25 was studied and the TPO curves are presented in Fig. 12. It is clearly seen that an increase in temperature of reaction leads to decrease in the intensity and the area of the TPO curves. It means lower amounts of carbon formation at higher reaction temperature, due to exothermic nature of CO disproportionation, which is thermodynamically unfavorable at high reaction temperature [28-30]. 3.5. SEM analysis SEM images of the spent 7.5% Ni/Al2O3 catalysts evaluated under different O2/C ratios are presented in Fig. 13. It is clear that the amount of whisker carbon increased with the decrease
of O2/C ratio. As shown in Fig. 13, no observable carbon was seen at O2/C=0.75. This confirms the results of TPO analysis, which showed lower content of carbon deposition at higher O2/C ratio. Fig. 14 shows the SEM images of the 7.5% Ni/Al2O3 evaluated under various reaction temperatures. As shown, the content of whisker carbon decreased with the increase of reaction temperature.
4. Conclusions The Ni/Al2O3 catalysts with various nickel loadings were prepared by a wetness impregnation method. The results showed that these catalysts possessed mesoporous structure with a distribution of pores in the range of 2−10 nm. The synthesized catalysts with various Ni contents (2.5-10 wt %) were evaluated in partial oxidation of propane. The catalytic results showed that increasing in nickel loading increased the catalytic activity but H2 and CO selectivities have an optimum depending on the Ni content and reaction temperature. The optimal temperature for partial oxidation of propane was 650°C. These catalysts exhibited high stability for 12 h without any decline in conversion but H2 and CO selectivities decreased during time on stream. Increasing in reduction temperature from 500°C to 700°C led to increase the catalyst activity and selectivities. TPO results showed that the decreasing the feed ratio and reaction temperature led to increasing the content of accumulated on the catalyst surface. SEM analyses showed the formation of whisker type carbon over the catalyst surface.
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Figure Captions
Fig. 1. XRD patterns of the carrier and reduced catalysts. Fig. 2. (a) N2 adsorption-desorption isotherms and (b) Pore size distributions of the reduced samples. Fig. 3. H2-TPR profiles of the calcined catalysts. Fig. 4. (a) C3H8 conversion , (b) H2/CO ratio, (c) H2 yield and (d) CO yield, O2/C=0.5, GHSV= 60,000 (ml/h·gcat),C3H8=10%. Fig. 5. (a) CH4 selectivity , (b) CO2 selectivity, (c) C2H4 selectivity and (d) C2H6 selectivity, O2/C=0.5, GHSV= 60,000 (ml/h·gcat), C3H8=10%. Fig. 6. Effect of GHSV on the activity and selectivity of 7.5%Ni/Al2O3,O2/C=0.5,T=600 °C. Fig. 7. Effect of O2/C ratio on the activity and selectivity of 7.5%Ni/Al2O3, GHSV=60000 mL/(gcat.·h),T=600 °C. Fig. 8. Stability of C3H8 conversion, H2 yield and CO, C2H4 and C3H6 selectivities of 7.5%Ni/Al2O3, GHSV=60000 mL/(gcat.h),T=700 °C,O2/C=0.5. Fig. 9. (a) N2 adsorption-desorption isotherms and (b) Pore size distributions of 7.5% Ni/ Al2O3 reduced at different temperatures. Fig. 10. (a) C3H8 conversion and (b) H2/CO ratio of 7.5% Ni/Al2O3 catalyst , O2/C=0.5, GHSV= 60,000 (ml/h·gcat). Fig. 11. TPO profiles of 7.5% Ni/Al2O3 evaluated under different O2/C ratios (T=600°C,GHSV=60000 mL/(gcat·h)). Fig. 12. Effect of reaction temperature on the TPO profile of 7.5% Ni/Al2O3 (O2/C=0.25, GHSV=60000 mL/(gcat·h), ).
Fig. 13. SEM images of 7.5%Ni/Al2O3 catalyst at (a) O2/C=0.75, (b) O2/C=0.5 and (c) O2/C=0.25. Fig. 14. SEM images of 7.5%Ni/Al2O3 catalyst at (a) T=500°C, (b) T=600°C and (c) T=700°C.
Table 1 Structural properties of the carrier and reduced catalysts. Sample
BET surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
Crystallite size ( nm)
Reduced
Spent
Reduced
Spent
Reduced
Spent
Al2O3
191.53
-
0.502
-
10.50
-
-
2.5Ni/Al2O3
169.82
119.42
0.513
0.290
12.09
9.38
-
5Ni/Al2O3
160.35
92.39
0.484
0.187
12.07
8.09
5.1
7.5Ni/Al2O3
157.26
75.57
0.474
0.156
12.04
8.27
9.1
10Ni/Al2O3
148.60
65.77
0.451
0.133
12.15
8.10
11.8
Table 2 Textural properties of 7.5% Ni/Al2O3 catalyst at different reduction temperatures. Reduction temperature (°C)
BET surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
500
159.12
0.474
11.91
600
158.53
0.474
700
157.26
0.474
11.96 12.04
Figure 1
Figure 2a
Figure 2b
Figure 3
Figure 4a
Figure 4b
Figure 4c
Figure 4d
Figure 5a
Figure 5b
Figure 5c
Figure 5d
Figure 6
Figure 7
Figure 8
Figure 9a
Figure 9b
Figure 10a
Figure 10b
Figure 11
Figure 12
Figure 13
Figure 14
S0926-860X(16)30504-X http://dx.doi.org/doi:10.1016/j.apcata.2016.10.012 APCATA 16028
To appear in:
Applied Catalysis A: General
Received date: Revised date: Accepted date:
18-8-2016 8-10-2016 12-10-2016
Please cite this article as: Mohammad Peymani, Seyed Mehdi Alavi, Mehran Rezaei, Synthesis Gas Production by Catalytic Partial Oxidation of Propane on Mesoporous Nanocrystalline Ni/Al2O3 Catalysts, Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2016.10.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis Gas Production by Catalytic Partial Oxidation of Propane on Mesoporous Nanocrystalline Ni/Al2O3 Catalysts Mohammad Peymania, Seyed Mehdi Alavib Mehran Rezaeia,c,* a
Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, P.O. Box 8731751117, Kashan, Iran
b
Chemical Engineering Department, Iran University of Science and Technology, Tehran, Iran
c
*
Institute of Nanoscience and Nanotechnology, University of Kashan, P.O. Box 8731751117, Kashan, Iran
Corresponding author: [email protected] (M. Rezaei), Fax: +98 31 5559930.
Graphical abstract
Research Highlights
Propane partial oxidation was investigated over Ni/γ-Al2O3.
C3H8 conversion had a direct relation with reaction temperature and nickel loading.
7.5 wt.% Ni/Al2O3 exhibited high stability for 12 h without any decrease in activity.
The increase in O2/C molar ratio decreased the amount of deposited carbon.
Abstract Propane partial oxidation to synthesis gas at various feed conditions was investigated over nickel catalysts supported on high surface area γ-alumina. The physicochemical characteristics of the calcined, reduced and spent samples were determined by TPR, BET, XRD, TPO and SEM analyses. The influences of Ni content, reduction and reaction temperatures, gas hourly space velocity (GHSV) and feed ratio on the catalytic properties were investigated. The activity measurements revealed that the propane conversion had a direct relation with reaction temperature and nickel loading and increased with increasing of both these factors. However, there was an optimum value for nickel content and reaction temperature in which the H2 and CO yields were maximum. The 7.5 wt.% Ni/Al2O3 exhibited high stability for 12 h without any decrease in activity. However, the selectivity declined gradually with reaction time due to carbon formation. The TPO analysis revealed that an increase in O2/C molar ratio from 0.25 to 0.75 caused a decrease in the amount of deposited carbon. Also, the amount of accumulated carbon slightly decreased with rising the reaction temperature. These results were confirmed by the SEM analysis and the filamentous carbon was observed on the catalyst surface. In addition, the increase in reduction temperature caused an increase in C3H8 conversion and H2/CO ratio. Keywords: Propane Partial Oxidation; Coke Formation; Nickel catalysts; Mesoporous; ɤalumina
1. Introduction Nowadays, synthesis gas or syngas (A mixture of H2 and CO with various ratios), is mainly used for production of chemicals such as ammonia and methanol in refineries, petrochemicals, in ferrous industries and gas-to-liquids (GTL) technology [1, 2]. Furthermore, the preferred feedstock for syngas production is methane or natural gas because of its relatively lower cost of the end product. There are some problems for natural gas as a fuel source includes difficulties in storage and transportation. Hence, light hydrocarbons, such as propane and butane, can be easily stored, distributed and transported as a source for production of syngas [3-6]. Therefore, in recent years researchers have focused on the synthesis of high active and stable catalysts for catalytic partial oxidation of light hydrocarbons. There are three possible routes for the conversion of hydrocarbons to hydrogen: steam reforming (SMR), catalytic partial oxidation (POX) and autothermal reforming (ATR). The SMR reaction is a highly endothermic reaction with a high operational cost. Nowadays, POX has received significant attention due to its availability and low-cost. Both catalytic processes suffer from the deactivation of catalysts, mainly caused by the carbon accumulation on the catalyst surface [3, 7, 8]. It is known that the metallic coordination depends on the solid structure and in oxidation reactions, which are structure sensitive, the structural characteristics of the metal oxide catalysts significantly influence the catalytic performance [9, 10]. POX reaction has been studied over Ni, Co and noble metal-based catalysts. Although the precious metals have shown high performance, but the high cost of them restricts their industrial usage [11, 12]. In contrast, the inexpensively of Ni-catalysts causes the widespread usage of nickel based catalysts in POX reactions but they are sensitive to coke formation and
sintering [7, 13]. The catalyst support can influence the catalytic performance because it can improve the active phase dispersion, decrease the inactive spinel phase formation, increase the active phase reducibility and oxygen mobility on surface [14, 15]. Alumina is one of the most popular supports and widely used in industrial catalysts due to low price and good structural properties and high surface area. In this work, the activity and selectivity of the mesoporous nanocrystalline Ni/Al2O3 catalysts with different Ni contents were studied in propane partial oxidation reaction. The effects of feed ratio, GHSV and reduction temperature on the catalytic properties of the catalysts were also studied. Furthermore, the influence of feed ratio and temperature of reaction on carbon deposition was determined.
2. Experimental 2.1. Materials
The used chemicals for preparing the catalysts were the nitrate salt of nickel (Ni(NO3)2.6H2O, Loba Chemie) and γ-Al2O3 (Sasol) as Ni active phase precursor and catalyst support, respectively. 2.2. Catalyst preparation
The catalysts were prepared by a wetness impregnation method with different nickel loadings. Prior to the impregnation, the support was crushed and sieved to obtain the particles with a size smaller than 0.25 mm. The prepared particles were stabilized by calcination at 700°C for 3h and impregnated with an aqueous solution of nickel nitrate with appropriate concentration to obtain the desired Ni loading. The slurry was agitated at room temperature for 3h and dried at 80 °C and subsequently heat treated at 600°C for 3 h. 2.3. Catalytic evaluation
All the experiments were done in a microreactor (length: 70 cm, inner diameter: 8 mm) under ambient pressure. The microreactor was loaded with a specific weight (0.1 g) of the catalyst sieved to 35-60 mesh. Prior to injection of feed, the catalysts were reduced with pure H2 at 700 °C for 3 h. The feed gas stream was consisted of C3H8 and O2 with desired molar ratio and diluted with He as carrier gas. The activity tests were carried out in the temperature range of 550-700 °C. The reactor effluent was passed through a water trap to remove the moisture and after that its composition was determined by a Younglin gas chromatograph (HID detector and Carboxen 1010 column). The values of propane conversions, H2 and CO yields and CH4, CO2, C2H4 and C2H6 selectivities were obtained using the following equations [16]:
X C3H 6 100
YH 2 100
YCO 100
FC3H 6 ,in FC3H 6 ,out FC3H 6 ,in FH 2 ,out
4( FC3H 6 ,out FC3H 6 ,in ) FCO,out
3(FC3 H 6 ,out FC3 H 6 ,in )
S n 100
(1)
(2)
(3)
Fn,out ( Ftotal,out FH 2 ,out )
(4)
Where Fin and Fout are the volumetric flow rates of the components in the feed and product gas streams, respectively. Fout was calculated with carbon balance. Where n is the component like CO, CH4, CO2, C2H4 and C2H6. 2.4. Catalyst Characterization
The crystalline phases of the fresh and reduced catalysts were identified by powder X-ray diffraction (XRD) analysis using a PANalytical X’Pert-Pro instrument. The step scans were
recorded between 2θ = 10–80°. The average crystallite size of metallic Ni was calculated by using the Scherrer equation. Nitrogen adsorption-desorption analysis of the reduced catalysts was performed at -196 °C in a gas adsorption analyzer (BELSORP mini II). A Vega@Tescan instrument was used for scanning electron microscopy (SEM) analysis. TPR (Temperature Programmed Reduction) and TPO (Temperature Programmed Oxidation) analyses of the calcined and spent catalysts were done by an automatic apparatus (Chemisorb 2750, Micromeritics). The details of TPR and TPO procedures could be found in our previous work [17].
3. Results and discussion 3.1. Structural properties of the catalysts The diffraction patterns of the support and reduced samples with different Ni loadings are shown in Fig. 1. For the catalyst with 2.5 wt.% Ni, because of the small content of nickel, the diffraction peaks assigned to the nickel and nickel oxide were not seen. Several diffraction peaks were observed by increasing of nickel content, which were located at 44.5, 51.8 and 76.5°, corresponding to (111), (200) and (220) plans of metallic Ni (JCPDS. 004-0850). As seen, in all the samples the increase in nickel content intensified the Ni diffraction peaks. The XRD results reveal that the support is in the gamma phase [18]. The observed peaks at 2θ = 37.6° and 45.79° are attributed to Al2O3 (JCPDS. 01-1303). The small peak at about 2θ = 62° is ascribed to NiAl2O4 phase (JCPDS. 71-0965). The Al2O3 and NiAl2O4 phases are overlapped at 2θ = 67° and 37°. The reduced Ni/Al2O3 catalyst at 700°C exhibited two main crystallite phases: γ-Al2O3 and NiAl2O4. γ-Al2O3 shows a pseudospinel structure and the lattice parameters of γ-Al2O3 are very close to NiAl2O4, so it is difficult to recognize these phases because the overlap between their diffraction peaks [19].
The nickel crystallite sizes calculated by Scherrer equation are presented in Table 1. The Ni crystallite sizes were increased size from 5.1 to 11.8 nm by the increase in Ni content. The textural characteristics of the catalyst carrier and reduced catalysts with various Ni contents are presented in Table 1. As shown, the catalyst carrier exhibited high BET area and with increasing the Ni content, the pore volume and surface area decreased due to blockage of the pores by Ni particles and/or destruction in mesoporous structure at high temperature. Enhancing in Ni loading does not significantly influence the pore diameter. It was suggested that proper nickel content could benefit the formation of highly dispersed NiO with high surface area [20]. The isotherms of the reduced catalysts are depicted in Fig. 2a. They can be classified as the type IV isotherm, typical for mesoporous structures. Also, for these catalysts, the H2 hysteresis loop observed at a relative pressure range (P/P0=0.6–0.9), which generally is assigned to solids made of particles crossed by channels with cylindrical shape [21, 22]. Additionally, Fig. 2b presented the pore size distributions of the samples. All the samples have a mesoporous structure with distribution of pore in the range of 2−10 nm. The pore size distribution was shifted to broader size when nickel loading increased.
3.2. Temperature-programmed reduction In order to study the redox behavior of the catalysts with different Ni loadings, H2-TPR analysis was carried out and the profiles are presented in Fig. 3. The reduction of all catalysts was started at 400°C. As can be seen, three types of peaks are seen in these patterns, which suggest the existence of three types of Ni species assigned by α, β and γ symbols, respectively. The small α peak located at 400-500 °C can be assigned to the reduction of bulk nickel oxide and weakly interacted NiO with the catalyst carrier. The β peak observed around 600 to 750°C is related to the reduction of strongly interacted NiO with Al2O3. The third
reduction peak (γ) at temperature above 800 °C is assigned to the reduction of NiAl2O4 [11, 23]. It is clearly observed that with the increase in Ni content, the reduction peaks were shifted to lower temperatures. It indicated that more reducible Ni species were formed in higher metal content. The results showed that increasing in nickel content weakened the interaction between NiO and Al2O3 and the reduction peaks moved to lower temperatures. Furthermore, the area under the reduction patterns increased with the increase of the nickel content of catalysts, due to higher hydrogen consumption on the catalyst with higher nickel content[24].
3.3. Catalytic performance 3.3.1. Effect of reaction temperature and Ni content The influences of temperature and nickel content on the activity of catalysts in propane partial oxidation reaction were studied and the results are presented in Fig. 4a. The propane conversions enhanced when the temperature increased from 550 to 700 °C due to increasing in the rate of reaction with the increase of temperature. The results showed that C3H8 conversion increased with the enhancement of Ni content from 2.5 to 10 wt.% due to increase of available nickel to reactants. With increasing in reaction temperature the product ratio (H2/CO ratio) for all samples except of the catalyst with 2.5% Ni decreased due to occurrence of the reverse water gas shift reaction (RWGS) [25] as shown in Fig. 4b. Also, The 7.5 Ni/Al2O3 catalyst exhibited maximum H2/CO ratio at various reaction temperatures. The yields of H2 and CO of the catalysts are shown in Fig. 4c and 4d, respectively. The H2 and CO selectivities improved with the increase of the reaction temperature up to 600°C and the further increase in temperature declined the H2 and CO yields due to formation of byproducts. Also, with increasing in Ni loading from 2.5 to 5 wt.%, H2 and CO selectivities increased and then decreased at nickel loadings higher than 5 wt.%.
The by-product selectivities are shown in Fig. 5. The selectivity to methane is between 0.5 to 17% at different temperatures as shown in Fig. 5a. The minimum selectivity to methane was observed at 600°C in catalysts with various Ni loadings. The results in Fig. 5b showed the decrease in CO2 selectivity with increasing in the reaction temperature, due to reverse water gas shift and CO2 consumption [26]. As shown in Fig. 5c and 5d, the by-product formation occurred at 650°C. The optimal temperature for propane partial oxidation is the temperature with maximum hydrogen yield and the lowest byproducts production. The results showed the maximum hydrogen production was occurred at a temperature of 650 °C for propane [4]. The textural properties of the spent catalysts with various nickel contents are reported in Table 1. It is clearly observed that the BET area and pore volume decreased after reaction due to thermal sintering of catalyst particles at high temperature and also the carbon formation. The accumulated carbon on the catalyst surface can block the pores and reduce the BET area.
3.3.2. Effect of GHSV Fig. 6 presents the influence of GHSV on the catalytic activity and selectivity of 7.5% Ni/Al2O3 catalyst for partial oxidation at 600 °C and C3H8:O2 = 1 : 1.5. The graphs show that with increasing in GHSV from 15000 to 60000 mL/(gcat·h), the C3H8 conversion increased from 53 to 70% and further increase in GHSV does not have a significant effect on propane conversion. It may be due to increasing in the amount of heat released from the bed of catalyst by enhancing the space velocity. It is noted that the catalyst bed temperature was controlled with inserted thermocouple in bottom of the catalyst bed. However, the enhancement of the temperature of the inner surface of the catalyst particles was inevitable. In fact, enhancing of the space velocity indirectly leads to increase the temperature of the catalyst particles [27]. The same trend was also seen for the selectivities of H2 and CO.
3.3.3. Effect of feed ratio Fig. 7 shows the influence of the molar feed ratio on the activity and selectivity of the catalysts. It is clear that increasing in oxygen content of feed caused to consuming propane due to occurring of both partial and complete oxidation. It is known that the total combustion is more favorable than the partial oxidation at higher O2/C ratios, and at these conditions the high quantities of CO2 and H2O were formed. The CO2 selectivities increased significantly while the CH4 selectivities were nearly the same. The higher O2/C molar ratios caused a decrease of the H2 selectivity. Due to the low content of CH4, hydrogen might be reacted by the excess O2 , which led to higher H2O selectivity (not shown in the Fig. 7) [26]. 3.3.4. Catalytic stability The stability of C3H8 conversion, H2 yield, CO, C2H4 and C3H6 selectivities for 7.5% Ni/Al2O3 catalyst during 12 h time on stream at 700 °C are shown in Fig. 8. The catalyst possessed high stability without any decline in propane conversion. It is seen that with increasing time, the H2 yield and CO selectivities were decreased due to the formation of byproducts such as ethylene and propylene. 3.3.5. Effect of reduction temperature Table 2 presented the textural characteristics of the 7.5% Ni/Al2O3 catalyst with different reduction temperatures. It can be seen that increasing in reduction temperature caused a decline in the BET area. However, it did not have a significant influence on the pore volume and pore size. The nitrogen adsorption/desorption isotherms of the reduced catalysts at different temperatures are shown in Fig. 9a. As shown, IV type isotherms with H2 shape hysteresis loop were observed in all catalysts. The pore size distributions of these catalysts are shown in Fig. 9b. The effects of reduction temperature on the catalytic performance of the 7.5%Ni/Al2O3 catalyst were investigated and the results are shown in Fig. 10a and 10b. As expected, the
increase in reaction temperature leads to increase in propane conversion. The increase in reduction temperature slightly increased the propane conversion due to more reduced Ni on the catalyst surface. The increase in reduction temperature also enhanced the product ratio (H2/CO molar ratio) in all temperatures, Fig. 10b. 3.4. TPO analysis The TPO analyses of the 7.5% Ni/Al2O3 catalysts after reaction with various O2/C ratios are presented in Fig. 11. The results indicated that the amount of accumulated carbon decreased with the increase of the O2/C ratio. No detectable carbon was observed at O2/C ratio of 0.75. The low amount of accumulated carbon could be related to high dispersion of nickel in high surface area of alumina and the excess of oxygen content respect to the stoichiometric value can increase oxygen species in surface and therefore reduce the deposited carbon. The highest content of carbon was observed at O2/C=0.25, due to the highest area under the TPO profile. The elementary chemical potential calculations in a free energy minimization technique showed that, the potential of carbon formation was increased with decreasing the O2/C ratio [13, 28]. The carbon deposition over the 7.5% Ni/Al2O3 at various reaction temperatures at O2/C=0.25 was studied and the TPO curves are presented in Fig. 12. It is clearly seen that an increase in temperature of reaction leads to decrease in the intensity and the area of the TPO curves. It means lower amounts of carbon formation at higher reaction temperature, due to exothermic nature of CO disproportionation, which is thermodynamically unfavorable at high reaction temperature [28-30]. 3.5. SEM analysis SEM images of the spent 7.5% Ni/Al2O3 catalysts evaluated under different O2/C ratios are presented in Fig. 13. It is clear that the amount of whisker carbon increased with the decrease
of O2/C ratio. As shown in Fig. 13, no observable carbon was seen at O2/C=0.75. This confirms the results of TPO analysis, which showed lower content of carbon deposition at higher O2/C ratio. Fig. 14 shows the SEM images of the 7.5% Ni/Al2O3 evaluated under various reaction temperatures. As shown, the content of whisker carbon decreased with the increase of reaction temperature.
4. Conclusions The Ni/Al2O3 catalysts with various nickel loadings were prepared by a wetness impregnation method. The results showed that these catalysts possessed mesoporous structure with a distribution of pores in the range of 2−10 nm. The synthesized catalysts with various Ni contents (2.5-10 wt %) were evaluated in partial oxidation of propane. The catalytic results showed that increasing in nickel loading increased the catalytic activity but H2 and CO selectivities have an optimum depending on the Ni content and reaction temperature. The optimal temperature for partial oxidation of propane was 650°C. These catalysts exhibited high stability for 12 h without any decline in conversion but H2 and CO selectivities decreased during time on stream. Increasing in reduction temperature from 500°C to 700°C led to increase the catalyst activity and selectivities. TPO results showed that the decreasing the feed ratio and reaction temperature led to increasing the content of accumulated on the catalyst surface. SEM analyses showed the formation of whisker type carbon over the catalyst surface.
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Figure Captions
Fig. 1. XRD patterns of the carrier and reduced catalysts. Fig. 2. (a) N2 adsorption-desorption isotherms and (b) Pore size distributions of the reduced samples. Fig. 3. H2-TPR profiles of the calcined catalysts. Fig. 4. (a) C3H8 conversion , (b) H2/CO ratio, (c) H2 yield and (d) CO yield, O2/C=0.5, GHSV= 60,000 (ml/h·gcat),C3H8=10%. Fig. 5. (a) CH4 selectivity , (b) CO2 selectivity, (c) C2H4 selectivity and (d) C2H6 selectivity, O2/C=0.5, GHSV= 60,000 (ml/h·gcat), C3H8=10%. Fig. 6. Effect of GHSV on the activity and selectivity of 7.5%Ni/Al2O3,O2/C=0.5,T=600 °C. Fig. 7. Effect of O2/C ratio on the activity and selectivity of 7.5%Ni/Al2O3, GHSV=60000 mL/(gcat.·h),T=600 °C. Fig. 8. Stability of C3H8 conversion, H2 yield and CO, C2H4 and C3H6 selectivities of 7.5%Ni/Al2O3, GHSV=60000 mL/(gcat.h),T=700 °C,O2/C=0.5. Fig. 9. (a) N2 adsorption-desorption isotherms and (b) Pore size distributions of 7.5% Ni/ Al2O3 reduced at different temperatures. Fig. 10. (a) C3H8 conversion and (b) H2/CO ratio of 7.5% Ni/Al2O3 catalyst , O2/C=0.5, GHSV= 60,000 (ml/h·gcat). Fig. 11. TPO profiles of 7.5% Ni/Al2O3 evaluated under different O2/C ratios (T=600°C,GHSV=60000 mL/(gcat·h)). Fig. 12. Effect of reaction temperature on the TPO profile of 7.5% Ni/Al2O3 (O2/C=0.25, GHSV=60000 mL/(gcat·h), ).
Fig. 13. SEM images of 7.5%Ni/Al2O3 catalyst at (a) O2/C=0.75, (b) O2/C=0.5 and (c) O2/C=0.25. Fig. 14. SEM images of 7.5%Ni/Al2O3 catalyst at (a) T=500°C, (b) T=600°C and (c) T=700°C.
Table 1 Structural properties of the carrier and reduced catalysts. Sample
BET surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
Crystallite size ( nm)
Reduced
Spent
Reduced
Spent
Reduced
Spent
Al2O3
191.53
-
0.502
-
10.50
-
-
2.5Ni/Al2O3
169.82
119.42
0.513
0.290
12.09
9.38
-
5Ni/Al2O3
160.35
92.39
0.484
0.187
12.07
8.09
5.1
7.5Ni/Al2O3
157.26
75.57
0.474
0.156
12.04
8.27
9.1
10Ni/Al2O3
148.60
65.77
0.451
0.133
12.15
8.10
11.8
Table 2 Textural properties of 7.5% Ni/Al2O3 catalyst at different reduction temperatures. Reduction temperature (°C)
BET surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
500
159.12
0.474
11.91
600
158.53
0.474
700
157.26
0.474
11.96 12.04
Figure 1
Figure 2a
Figure 2b
Figure 3
Figure 4a
Figure 4b
Figure 4c
Figure 4d
Figure 5a
Figure 5b
Figure 5c
Figure 5d
Figure 6
Figure 7
Figure 8
Figure 9a
Figure 9b
Figure 10a
Figure 10b
Figure 11
Figure 12
Figure 13
Figure 14