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Ind. Eng. Chem. Res. 2007, 46, 8722-8728
KINETICS, CATALYSIS, AND REACTION ENGINEERING Influence of Oxygen Addition on the Reaction of Propane Catalytic Dehydrogenation to Propylene over Modified Pt-Based Catalysts Changlin Yu, Qingjie Ge, Hengyong Xu,* and Wenzhao Li Applied Catalysis Laboratory, Dalian Institute of Chemical Physics, Graduate School of the Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China
The influences of oxygen addition to catalytic dehydrogenation system on the structure and the dehydrogenation performance of Pt-based catalysts were investigated. The results showed that a small amount of oxygen addition into the reaction system could greatly improve the catalytic performance of Pt-Sn/Ce-γ-Al2O3 catalyst although it evidently decreased the catalytic stability of Pt/γ-Al2O3, Pt-Sn/γ-Al2O3, and Pt/Ce-γ-Al2O3 catalysts. The addition of a small amount of oxygen can decrease the pressure of hydrogen, eliminate coke, and provide partial heat for catalytic dehydrogenation reaction. Hydrogen chemisorption results indicated that steam could reduce the interactions between metal and support, thus promoting the sintering of platinum particles. That adverse effect of steam on the structure on Pt-Sn/Ce-γ-Al2O3 was moderate, but it was serious on other Pt-based catalysts due to low-stability platinum particles. Both the promoting effect of Ce on the propane conversion and the inhibiting effect of Sn on the COx formation could be simultaneously achieved during the reaction over Pt-Sn/ Ce-γ-Al2O3 catalyst. At 546 °C, a 50% yield of propylene improvement was obtained due to the small oxygen addition. Introduction Propane dehydrogenation is believed to have great potential as a propylene booster in the future. Nowadays, much effort was continuously put to the improvement of PtSn catalysts for light alkane catalytic dehydrogenation.1-5 However, the propane catalytic dehydrogenation (DH) to propylene reaction is an endothermic reaction. It is a thermodynamically limited process that requires relatively high temperatures to obtain a high yield of propylene. These characteristics will certainly decrease its economy due to the huge energy consumption. Therefore, the development of another high-efficiency dehydrogenation technique is also important. Oxidative dehydrogenation (ODH) of propane, which is exothermic and a nonequilibrium limited reaction at relevant operating conditions, has been studied as an alternative. Theoretically, 100% yields of olefins are permitted by thermodynamics in ODH. But, the low selectivity is presently still detrimental to a commercial process.6 Another dehydrogenation technology proposed for obtaining olefin yields higher than equilibrium and for making the total process heat balanced or exothermic is selective hydrogen combustion (SHC).7 The primary idea is to supply heat by in situ combustion of the hydrogen and drive the equilibrium toward the product side, and the steam formed can aid in reducing coking. By physically mixing the catalytic dehydrogenation catalyst Pt-Sn/ZSM-5 with highly selective hydrogen combustion catalysts (such as Bi2O3, Bi2Mo3O12, In2Mo3O12, Sb2O4, In2O3, and WO3) or alternating zones of a Pt-Sn/ZSM-5 (DH) and the SHC catalysts, Grasselli and co-workers obtained higher than equilibrium yields of light olefins.8-9 A recent report * To whom correspondence should be addressed. E-mail: xuhy@ dicp.ac.cn. Tel.: +86(411) 84581234. Fax: +86(411) 84581234.
showed that over a Pt/Na-[Fe] ZSM5 catalyst (small Pt clusters within Na-[Fe]ZSM5-protected channels) by staged oxygen introduction for selective dehydrogenation, near-equilibrium alkane conversions were obtained.10 Our recent research showed that zinc-doped platinum catalysts have high selectivity to propylene and ceria promoted platinumtin catalysts show high stability in propane dehydrogenation.1-2 To further improve the yield of propylene over those catalysts, we used the SHC technology to the propane catalytic dehydrogenation over platinum catalysts. In this paper, the effects of a small amount of oxygen addition into the reaction of propane catalytic dehydrogenation to propylene over modified Pt/Al catalysts were investigated. Experimental Section Catalysts Preparation. A series of alumina supported Pt catalysts were prepared by an impregnation method as described in the literature.1,2 A commercial γ-Al2O3 (SBET: g250 m2 g-1), previously 40-60 mesh (average particles size 0.54 mm) sieved and calcined in air at 550 °C for 4 h, was used as support. Pt/γ-Al2O3 catalyst was prepared by impregnating γ-Al2O3 with excess aqueous solution of the Pt precursor (H2PtCl6). After impregnation, the catalyst was dried at 110 °C for 10 h and then calcined at 500 °C in air for 4 h. For Pt-Sn/γ-Al2O3 and Pt/Ce-γ-Al2O3 catalysts, Sn or Ce was first deposited by impregnating with SnCl2 ethanol solution or Ce(NO3)3 aqueous solution, followed by drying and calcination, and finally, the Pt component was added from aqueous Pt solution in a similar way. In the case of ceria promoted Pt-Sn/γ-Al2O3 catalyst, Sn and Ce were first deposited by coimpregnating SnCl2 and Ce(NO3)3 ethanol solution, then dried, and calcined. Finally, the Pt component was added as described previously. In all cases, the loadings of Pt, Sn, and Ce were 0.3, 0.9, and 2.2 wt %,
10.1021/ie0704952 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/09/2007
Ind. Eng. Chem. Res., Vol. 46, No. 25, 2007 8723 Table 1. Effects of Gas Medium on the Catalytic Performance of Different Catalysts in Propane Dehydrogenationa selectivity (mol %) conversion (mol %) catalyst Pt/Al Pt-Sn/Al Pt/Ce-Al Pt-Sn/Ce-Al
b
C1-2
COx
yield of C3H6 (mol %)
C3H6
gas medium
startb
endc
start
end
start
end
start
end
H2/He H2/O2/He H2/He H2/O2/He H2/He H2/O2/He H2/He H2/O2/He
29.0 33.8 33.2 34.3 35.9 46.5 33.9 44.8
20.3 9.0 25.0 18.5 23.8 26.3 33.3 38.7
13.7 24.7 11.7 14.2 29.8 25.8 9.1 5.1
10.6 13.5 5.8 4.1 13.4 22.0 3.4 1.8
0 6.0 0 1.8 0.2 23.6 0.1 5.6
0 7.1 0 1.9 0 25 0 5.2
86.3 69.3 88.3 84.0 70.0 50.6 90.8 89.3
89.4 79.4 94.2 94.0 86.6 53.0 96.6 93.0
start 25.0 23.4 29.3 28.8 25.1 23.5 30.8 40.0
end 18.1 7.1 23.5 17.4 20.6 13.8 32.1 36.0
a Reaction conditions: T ) 576 °C; total GHSV ) 5000 h-1, H /C H /He ) 1:1:3; total GHSV ) 5400 h-1, H /O /C H /He ) 1:0.4:1:3 (molar ratio). 2 3 8 2 2 3 8 Start, reaction time 5 min. c End, 155 min.
respectively. The catalysts were labeled as follows: Pt/Al for platinum supported on alumina, Pt-Sn/Al for platinum-tin supported on alumina, and Pt-Sn/Ce-Al for platinum-tincerium supported on alumina. Catalytic Test. The reactions were studied in a fixed bed quartz microreactor with an inner diameter of 5 mm. The reactions were carried out at atmospheric pressure. The catalyst loading was 0.3 g for all experiments. All catalysts were previously reduced under flowing pure H2 (12.6 mL/min) at 576 °C for 2.5 h. The reactor was placed in an electrical furnace and the temperature controlled using external and internal thermocouples. The reactant mixture with varying compositions of propane, oxygen, hydrogen, and helium was premixed and fed to the reactor using electronic mass flow controllers (Bronkhurst). The product gas was analyzed for hydrocarbons and for gases (O2, CO, CO2) by Shimadzu GC14-C equipped with a TCD (Carbosieve SII packed column) and a FID (Porapak-Q packed column). From the composition data, the fractional conversion of propane was calculated. For the convenience of calculation in propane conversion, carbon contained in coke is not considered. The products’ (hydrocarbon and carbon oxides) selectivity is defined as moles of product per mole of converted propane. Water and hydrogen in the exit gas was not analyzed by the GC. Catalyst Characterization. (1) Coke Quantitative Analysis. The amount of coke over reacted catalysts was measured by O2-pulse technology. The O2-pulse experiment is a special technology used for quantitative coke analysis. The pulse experiments were carried out at 800 °C by injecting pulses of pure O2 (99.99%) to the coke deposited catalysts (0.03 g), which were maintained under flowing pure Ar between two successive pulses. The CO2 generated was continuously monitored with a TCD cell and recorded. Pulses of pure O2 stopped until deposited carbon entirely converted to CO2. Then the amount of generated CO2 was converted to the amount of deposited coke on the catalyst. (2) Pulse Chemisorption of Hydrogen. The ability of fresh and reacted catalysts to adsorb hydrogen was determined by pulse chemisorption of hydrogen experiments on a conventional setup. Before pulse chemisorption experiments, fresh samples (0.2 g) were reduced under flowing pure H2 (12.6 mL/min) at 576 °C for 2.5 h, then purged in Ar at 550 °C for 2 h, and successively cooled to 330, 200, and 0 °C in flowing pure Ar. Then, samples were saturated by a hydrogen pulse at each temperature. The procedure of pulse chemisorption of hydrogen for reacted catalysts and fresh samples treated in a steam atmosphere is the same as described previously. The pulse size was 0.32 mL of 5% (v/v) H2 in Ar mixture, and the time between pulses was 3 min. The total amount of H2 uptake
(volume at room temperature) equals the sum of H2 uptake at different temperatures. Results and Discussion Catalytic Reaction. (1) Effects of Oxygen Introduction. A blank experiment test showed that there were no significant homogeneous thermal pyrolysis reactions at temperatures below 610 °C in the empty reactor using a gas mixture with or without oxygen. At 576 °C, the conversions of propane were about 0.5 and 1%, respectively, in H2/He and H2/O2/He gas medium. The possibility of mass-transfer limitation on experimental conditions was excluded. The effects of oxygen introduction on the performance of platinum catalysts were examined first. The results are shown in Table 1. It can be seen that the presence of oxygen changes the catalytic behavior. Due to oxygen addition, a slight increase in propane initial conversion is observed over Pt/Al, Pt-Sn/Al catalysts and a big increase in initial conversion appears over Pt/Ce-Al catalyst. These catalysts all show poorer stability in oxygen atmosphere than in H2/He gas medium. In H2/He gas medium, the final conversions of propane over Pt/Al, Pt-Sn/ Al, and Pt/Ce-Al catalysts are 20.3, 25.0, and 23.8%, respectively. However, in H2/O2/He gas medium, the final conversions are only held to 9.0, 18.5, and 26.3%, respectively, over each corresponding catalyst. As for Pt-S n/Ce-Al catalyst, a remarkable increase in both initial and final conversions is obtained. The presence of oxygen largely promotes the initial conversion of propane from 33.9 to 44.8% and final conversion from 33.3 to 38.7%. Ceria produces a notable increase in activity and a stabilization effect with respect to Pt-Sn/Al2O3 catalyst in oxygen atmospheres. The introduction of oxygen leads to a decrease in the selectivity of propylene, which strongly depends on the catalysts. Over Pt/Al catalyst, the initial selectivity toward propylene decreases from 86.3 to 69.3% with oxygen present and the selectivity to COx is about 6-7%. At the same time, the C1-2 selectivity is much higher than that in H2/He atmosphere. Over Pt-Sn/Al catalyst, oxygen does not bring about a large number of COx molecules. It shows the lowest COx selectivity (1.81.9%). Over Pt/Ce-Al catalyst, oxygen presence causes a sharp drop in propylene selectivity (from 70 to 50%). A large number of COx molecules are produced. The selectivity of COx increases to as high as 23-25%. Only a small decrease in propylene selectivity is shown over Pt-Sn/Ce-Al catalyst in oxygen atmosphere. Obviously, the presence of tin can evidently reduce the formation of COx with respect to Pt/Ce-Al catalyst. A big improvement in the yield of propylene is achieved over PtSn/Ce-Al catalyst due to a great increase in propane conversion and a small decrease in propylene selectivity.
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Figure 1. Effects of the feed composition on propane dehydrogenation over Pt-Sn/Ce-Al catalyst. Reaction conditions: total GHSV ) 5400 h-1; T ) 576 °C. 9, H2/O2/C3H8/He ) 1:0.2:1:3.2; b, H2/O2/C3H8/He ) 1:0.4:1:3; 2, H2/O2/C3H8/He ) 1:0.6:1:2.8; 1, H2/O2/C3H8/He ) 0:0.4:1:4.
The above results show that propane’s initial conversion is higher with oxygen present in the feed than the case without oxygen. Oxygen presence can increase the conversion of propane, which is especially distinct over ceria promoted catalysts. The data of COx selectivity indicate that tin can exhibit good ability to restrain the production of COx. (2) Effects of the Feed Composition. Experiments were performed at varying feed composition by changing oxygen concentration in the range of 3.7-11% with a total GHSV of 5400 h-1 and a constant propane GHSV of 1000 h-1. The measured conversions of propane and selectivities of propylene and COx at 576 °C are reported in Figure 1. Figure 1a indicates that, with fixed hydrogen feed, propane’s initial conversion progressively increases from 42 to 48% with increasing oxygen concentration. The product distribution, represented in Figure 1b and Figure 1c, is also significantly affected by oxygen concentration. According to the balance of mass, the selectively of water is obtained by the assumed equation, SH2O ) 100% - X‚SCOx because only two products containing oxygen are produced (H2O and COx) and oxygen conversion is almost 100% in all different oxygen concentrations. When the ratio of O2/H2 is below 0.6, >100 - 5x% of oxygen is selectively converted to water and COx selectivity lowers 5%. Propylene selectivity can be kept almost constant at nearly g85%.The ratio of H2/O2 justly corresponds to the stoichiometry of the H2-O2 reaction for water formation. The results suggest that as long as the O2 is fed to the reactor at a less than stoichiometric amount of the reaction 1, H2 + 0.5O2 ) H2O (1), the O2 mainly reacts with hydrogen to form water. When the ratio of O2/H2 is above 0.6, CO and CO2 are formed in increasing amounts. Almost the same conversions of propane are observed when the ratio of H2/O2 is 1:0.2 and 1:0.4, respectively. If oxygen is only involved in the combustion of hydrogen, the more oxygen is introduced, the more hydrogen should be consumed, and the higher increase of propane conversion is expected. However, in a low concentration of oxygen, a high propane conversion could still be maintained, which indicates that steam formed may participate in the reaction of propane dehydrogenation. The adsorbed steam may produce hydroxyl groups, which participate in the propane dehydrogenation reaction by H-elimination of propane. Such a conclusion could also be proved by the initial propane conversion data, which are higher than the equilibrium conversion of propane catalytic dehydrogenation. Figure 1a also shows that hydrogen presence in the feed is crucial for maintaining the stability of the catalyst. Hydrogen presence may inhibit the oxidation of platinum particles caused by oxygen or steam. Furthermore, oxygen or steam adsorption over the platinum surface could reduce the interactions between metal and support,
thus promoting the sintering of platinum particles. But the presence of hydrogen may reduce this adverse effect of oxygen or steam. More detail discussion about the effect of steam on the reaction mechanism and structure of Pt-based catalysts will be made later. (3) Effects of Temperature. Figure 2 shows the effects of temperature on typical developments of the activity and selectivity in propane dehydrogenation over the Pt-Sn/Ce-Al catalyst with respect to time. At 516 °C, the performance of the Pt-Sn/Ce-Al catalyst in different gas mixtures is shown in Figure 2a,b. Propane conversion is much higher with oxygen present in the feed than that in the gas mixtures without oxygen. The conversion of propane is only ∼15% in H2/He gas medium. In an oxygen atmosphere, it increases to 26%. Only a slight decrease in the selectivity to propylene is found in an oxygen atmosphere. The selectivity to COx is initial 0.8%. Obviously, at this temperature, oxygen existence dose not lead to an obviously extra loss in propylene selectivity. The mean yield of propylene in the case of O2 presence is initial 24%, which is much higher than that (14%) in gas medium. At 546 °C, in Figure 2c,d, a similar picture is given. The conversion of propane increases from about 25 to 36% and approaches the equilibrium value (initial 36%) due to oxygen presence. The COx selectivity shows a slight increase. The mean yield of propylene (34%) is also much higher than that in H2/He gas medium (initial 23%).When the temperature increases to 606 °C, propane shows high initial conversion, but the catalyst deactivates quickly compared to a no-oxygen atmosphere. At the same time, the selectivity toward propylene suffers great loss. The selectivity of COx reaches to 2-6%. No obvious oxygen-promoting effect is observed at this elevated temperature. The elevated temperature must aggravate the adverse effect of oxygen or steam on platinum active sites, thus speeding up the deactivation rate of catalyst. (4) Effects of Contact Time. The effects of contact time on the performance of catalyst were investigated by varying the packed volume of catalysts while keeping the flow rate constant and the feed composition at H2/O2/C3H8/He )1:0.4:1:3 mole ratios. Contact time (τ) ranges between 0.22 and 0.67 s. The results are shown in Figure 3. Figure 3a shows that propane conversion increases rapidly with increasing contact time from 0.22 to 0.43 s; with further increase of τ (from 0.43 to 0.67 s), conversion of propane shows no obvious change. As for the product distribution, indicated in Figure 3b, propylene’s initial selectivity decreases by nearly 6% with the increase of τ. COx initial selectivity increases moderately from 0.7 to 1.2% at increasing contact time. With the contact time increase, the hydrogenolysis phenomenon also becomes more serious.
Ind. Eng. Chem. Res., Vol. 46, No. 25, 2007 8725
Figure 2. Effect of the temperatures on propane dehydrogenation over Pt-Sn/Ce-Al catalyst. Reaction conditions: Total GHSV ) 5000 h-1, 9, H2/ C3H8/He ) 1:1:3; Total GHSV ) 5400 h-1, b, H2/O2/C3H8/He ) 1:0.4:1:3; 2, COx. Dashed line represents equilibrium conversion of propane in catalytic dehydrogenation in the composition of H2/C3H8/He ) 1:1:3.
(5) Stability Examination. Based on the optimization of the feed composition, reaction temperature, and contact time, a stability test of propane dehydrogenation was carried out. The results are presented in Figure 4. With time extending, the catalyst exhibits a slight decrease in propane conversion and the selectivity to propylene shows moderate increase. The selectivity of COx does not surpass 2% during the whole reaction course in mixtures with oxygen. No obvious deactivation rate is observed in oxygen atmosphere at this temperature. The mean yield of propylene is initial 33% at g95% selectivity in H2/O2/ He mixtures as compared to 22 and g97% selectivity in H2/He mixtures. Considering the strong endothermic character of the propane dehydrogenation reaction eq 1, and exothermic reaction of
C3H8(g) ) C3H6(g) + H2(g) - 124 kJ/mol
(1)
hydrogen combustion (SHC) according to eq 2,
H2(g) +0.5O2(g) ) H2O(g) + 242 kJ/mol
(2)
an almost heat balance will be obtained in the total process if
the process operation is adiabatic. Additional benefits in this process are that the consumption of hydrogen will drive the equilibrium toward the product side, and the steam formed may aid in reducing coking. Catalyst Characterization. (1) Coke Analysis. Coke is one of the important factors that influence the performance of Pt catalyst in the dehydrogenation reaction. Coke over reacted catalysts in different atmospheres was quantitatively analyzed by a special O2-pulse technology. Table 2 shows the results of measurements. In H2/He gas medium, a large amount of coke was detected over these reacted catalysts. A very high amount of coke over Pt/Al and Pt/Ce-Al catalysts was detected. In the catalytic dehydrogenation reaction system, there are two side reactions that compete with dehydrogenation: hydrogenolysis and coke formation. Side reactions are significant over a monoPt catalyst due to the presence of large platinum ensembles. Large platinum ensembles are needed for the reactions of hydrogenolysis and coke.11,12 As regards promoted Pt catalyst, the surface properties of Pt are changed. The surface of platinum metal is diluted, giving small ensembles of Pt, with different electronic properties. This modifications lead to a more stable
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Figure 5. Effects of steam on propane dehydrogenation over Pt-Sn/2.2CeAl catalyst. Reaction conditions: T ) 576 °C, total GHSV ) 5400 h-1. Table 3. Total Hydrogen Chemisorption of the Fresh and Reacted Catalysts in Different Atmospheres (mL of H2/g of Pt)a H2/He gas medium
Figure 3. Effects of contact time on the catalytic performance of Pt-Sn/ 2.2Ce-Al catalyst. Reaction conditions: T ) 546 °C, H2/O2/C3H8/He ) 1:0.4:1:3. Contact time: 9 0.22, b 0.43, and 2 0.67 s.
Figure 4. Stability test of Pt-Sn/Ce-Al catalyst in propane dehydrogenation in different gas media. Reaction conditions: T ) 546 °C. Total GHSV ) 5000 h-1, H2/C3H8/He ) 1:1:3; total GHSV ) 5400 h-1, H2/O2/C3H8/ He ) 1:0.4:1:3 (molar ratio). Table 2. Amount of Coke on the Reacted Catalysts (mg of C/g of Cat)a reaction gas medium catalyst
H2/He
H2/O2/He
Pt/Al Pt-Sn/Al Pt/Ce-Al Pt-Sn/Ce-Al
5.8 2.4 4.6 2.0
4.0 0.7 0 0
a Reaction conditions: T ) 576 °C; total GHSV ) 5000 h-1, H /C H / 2 3 8 He ) 1:1:3; total GHSV ) 5400 h-1, H2/O2/C3H8/He ) 1:0.4:1:3 (molar ratio). Reaction time: 155 min.
catalyst. Our previous research has shown that addition of Ce to Pt/Al catalyst could not decrease the amount of coke,1 which is related to the high acidity over Pt/Ce-Al catalyst. But, addition of Sn to Pt/Al catalyst could moderately reduce the
H2/O2/He gas medium
catalyst
fresh FTH
To
RTH/ FTH (%)
To
RTH/ FTH (%)
Pt/Al Pt-Sn/Al Pt/Ce-Al Pt-Sn/Ce-Al
48 62 49 55
24 40 26 48
50 64 53 87
19 50 39 52
39 80 79 94
a FTH, total hydrogen chemisorption of fresh catalyst; RTH, total hydrogen chemisorption of reacted catalyst. Reaction conditions: T ) 576 °C; reaction time 155 mim. Total GHSV ) 5000 h-1, H2/C3H8/He ) 1:1:3; total GHSV ) 5400 h-1, H2/O2/C3H8/He ) 1:0.4:1:3 (molar ratio).
reaction of coke. The high amount of coke over these two catalysts leads to the poor stability of these catalysts. In H2/ O2/He gas medium, the situation of coke changes. Over the Pt/ Al catalyst, a large amount of coke is also found. The reasons could be that the rate of coke production is much faster than the rate of coke elimination. Coke rapidly blocks platinum particles, resulting in the quick activity loss of this catalyst. However, over ceria promoted catalysts, we did not detect any coke in oxygen atmospheres. The high active oxygen species in ceria could participate in the elimination of coke, which was verified by previous temperature-programmed oxidation experiments.1 Another possible reason for the low amount of coke in H2/O2/He atmospheres may be that the steam formed over promoted catalysts could aid in reducing coke. From the coke analysis, it could be learned that the main reason for the deactivation of Pt-Sn/Al and Pt/Ce-Al catalysts in an oxygen atmosphere must not be the coke formation. (2) Measurements of Hydrogen Chemisorption. The sintering of platinum particles and the destruction of the interactions between platinum, promoter, and support are the other important factors for the deactivation of promoted-Pt catalysts. Usually, in an oxygen or a steam atmosphere, the sintering of platinum particles is more serious. A pulse of hydrogen chemisorption technique was used to estimate the sintering of platinum particles and the structure change of platinum catalysts. The results of measurements are shown in Table 3. Here, the total amount of H2 uptake of the sample at low and high temperatures is the sum of H2 uptake at 330, 200, and 0 °C.13 According to the literature,14,15 the amount of chemisorption hydrogen at low temperature (0 °C) can be used to characterize the size of the Pt particles, and at high temperatures, chemisorption of hydrogen must be related to the interactions between platinum and promoter or support. As shown in Table 3, the amount of
Ind. Eng. Chem. Res., Vol. 46, No. 25, 2007 8727 Table 4. Effects of Steam Treatment on Hydrogen Chemisorption of Catalyst (mL H2/g Pt)a reduced
treated with H2O
catalyst
total
0 °C
high temp
total
0 °C
high temp
Pt/Al Pt-Sn/Al Pt/Ce-Al Pt-Sn/Ce-Al
48 62 49 55
20 25 22 26
28 42 27 29
30 37 34 48
10 16 18 25
20 21 16 23
a Steam-treated conditions: catalysts after reduction were treated with 7%H2O/He for 60 min at 520 °C.
chemisorption hydrogen over the reacted catalysts is closely related to the reaction atmospheres. Two factors mainly influence the ability of reacted catalysts to adsorb hydrogen. One is coke deposition, and the other is the sintering of platinum particles. In H2/He gas medium, coke deposited over platinum particles must be the main reason for the decrease in the amount of H2 uptake. The high amount of coke over Pt/Al and Pt/CeAl catalysts could be responsible for the great decrease in total H2 uptake. The smallest decrease in H2 uptake over Pt-Sn/ Ce-Al could be related to the lowest coke and the highest stability of platinum particles over this sample, as reported in the literature.1 However, in H2/O2 gas medium, it is not the case. We almost did not detect any coke over the reacted catalysts with the exception of Pt/Al catalyst. Thus, the agglomeration of platinum particles and the change of interactions between platinum, promoter, and support may be the main reasons leading to the decrease of H2 uptake. After reaction in H2/O2 gas medium, among all catalysts, Pt-Sn/Ce-Al shows the smallest decrease in total H2 uptake, which suggests that platinum particles over this catalyst may be the most stable. Previous studies,16,17 have shown that oxygen or steam adsorption over platinum surface can distinctly reduce the interactions between metal and support, thus promoting the sintering of platinum particles. Steam could also destroy platinum-tin clusters supported by γ-Al2O3.18 However, the adverse effects of oxygen or steam adsorption over the platinum surface are different for different platinum catalysts. The effects of steam atmosphere on the structure of Pt catalysts were investigated. The fresh catalysts after reduction were treated with 7% H2O/He for 60 min at 520 °C, and then the same hydrogen chemisorption measurements were carried out. The results of experiments are shown in Table 4. Steam treatment causes a sharp decrease in low-temperature hydrogen uptake over Pt/Al and Pt-Sn/Al catalysts, which indicates the sintering of platinum particles becomes serious in a steam atmosphere. The smallest decrease in low-temperature hydrogen uptake is observed over Pt-Sn/Ce-Al catalyst. As for the hightemperature hydrogen uptake, Pt-Sn/Ce-Al catalyst also shows the smallest changes. The hydrogen chemisorption measurements show that the adverse effects of steam on the structure of Pt-Sn/Ce-Al catalyst are lowest among the catalysts. Strong interactions between the Pt-Sn-Ce three components indicated in the literature,1 may minimize these adverse effects of steam.
The platinum particles have high stability over Pt-Sn/Ce-Al catalyst even the steam atmosphere. Reaction Mechanism Discussion. The studies have shown that, under certain conditions, feeding a small quantity of oxygen to the catalytic dehydrogenation system can remarkably promote the yield of propylene and not obviously at the expense of propylene selectivity. The results are dependent on catalyst. Ceria promoted Pt-Sn/Al catalyst exhibits an excellent performance in oxygen atmosphere. The best performance can first be ascribed to the unique role of ceria to platinum catalyst. Ceria has a notable stabilizing effect on Pt-Sn/Al catalyst in an oxygen atmosphere. The oxygen or steam produces an adverse effect on active sites of platinum metal, which causes the fast deactivation in oxygen atmosphere. At elevated temperature, this effect becomes more distinct. The hydrogen chemisorption measurements show that the presence of ceria could greatly decrease this adverse effect and promote the stability of catalyst in an oxygen atmosphere. Second, in an oxygen atmosphere, Sn also plays an important role in promoting propylene selectivity. The selectivity toward COx increases on the order Pt-Sn/Al < Pt-Sn/Ce-Al < Pt/Al < Pt/Ce-Al. Sn shows an excellent property by reducing the generation of COx as described in the literature.19 In this literature, a possible reason was proposed that Sn had the role of favoring hydrogen adsorption and activating hydrogen reaction with adsorbed oxygen. Oxygen can directly react with hydrogen, producing H2O. The pressure of hydrogen is decreased, the thermodynamic equilibrium is broken through, and the reaction is driven toward the product side according to the Le Chatelier,s principle. The steam formed could eliminate coke and further promote the performance of Pt-Sn/Ce-Al catalyst. A possible reaction mechanism for propane dehydrogenation over platinum was put forward by Biloen.20 He proposed that the rate-determining step of propane catalytic dehydrogenation over platinum catalyst is the β H-elimination:
The increase in the rate of β H-elimination will promote the whole dehydrogenation reaction. Over Pt-In/corundum catalysts, a faster dehydrogenation rate in a steam atmosphere than in a hydrogen atmosphere was observed by Kogan.21 They proposed that the hydroxyl groups adsorbed on the Pt-In bimetallic clusters reacted with β H of propane, thus increasing the rate of dehydrogenation. According to the literature,22-24 hydroxyl groups can be produced over the surface of CeO2 treated with hydrogen in 400 °C. A possible reaction mechanism is suggested as follows.
Table 5. Catalytic Performance Comparison of Different Catalysts in Propane Dehydrogenation in Steam Atmosphere selectivity (mol %) conversion (mol %)
C1-2
COx
yield of C3H6 (mol %)
C3H6
catalyst
startb
endc
start
end
start
end
start
end
Pt/Al Pt-Sn/Al Pt-Sn/2.2Ce-Al
43.0 41.4 46.2
14.0 31.2 44.4
30.0 33.0 13.5
12.0 10.6 4.9
11.0 4.0 6.8
13.0 2.0 3.1
56.0 62.0 80.0
65.0 87.0 92.0
a
start 24.0 25.0 36.4
end 12.3 27.0 40.5
Reaction conditions: T ) 576 °C, total GHSV ) 5400h-1, H2/H2O/C3H8/He ) 1:3:2:5 (molar ratio). b Start, reaction time 5 min. c End, 155 min.
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Over the Pt-Sn/Ce-Al catalyst, hydrogen molecules first split to active hydrogen atoms (5). Then, hydrogen atoms transform from the surface of Pt to ceria support. The spillover hydrogen atoms adsorb on anionic sites of the support, thus yielding hydroxyl groups and releasing free electrons to the support (6).25-27
H2(g) + 2Pts f 2Pts-H
(5)
2Pts-H + O2- f Pts + OH- + e-
(6)
The hydroxyl groups could take part in the elimination of β H and further promote the rate of dehydrogenation:
The steam formed may also promote the activity of the catalyst. The effects of steam on propane dehydrogenation over Pt-Sn/ 2.2Ce-Al catalyst were investigated by replacing partial inertia helium with steam. The test results are shown in Figure 5.The presence of steam results in an increase in propane conversion, from 33 to 36%, which suggests that steam can promote the activity of Pt-Sn/2.2Ce-Al catalyst. The catalytic performance of different platinum catalysts in a propane dehydrogenationin steam atmosphere was also investigated. It can be seen from Table 5 that, for Pt/Al and Pt-Sn/Al catalysts, the steam presence could lead to an obvious decrease in the stability of catalysts. Pt-Sn/2.2Ce-Al catalyst shows the highest activity and stability in the steam atmosphere. Conclusions Over ceria promoted Pt-Sn/Al catalyst, the presence of a small quantity of oxygen can remarkably promote the yield of propylene. To maintain the high selectivity of propylene and restrain COx generation, the amounts of oxygen in the feed must be kept less than the amount necessary for the stoichiometric reaction with the available hydrogen. Oxygen could directly react with hydrogen, producing H2O, and the reaction is driven to the product side according to Le Chatelier,s principle. Steam formed can eliminate coke and further promote the activity of Pt-Sn/Ce-Al catalyst. OH groups may be produced over PtSn/Ce-Al catalyst in an oxygen atmosphere. The OH groups may participate in H-elimination of adsorbed propane and greatly increase the rate of dehydrogenation. Literature Cited (1) Yu, C. L.; Ge, Q. J.; Xu, H. Y.; Li,W. Z. Effects of Ce Addition on the Pt-Sn/γ-Al2O3 Catalyst for Propane Dehydrogenation to Propylene. Appl. Catal. A 2006, 315, 58-67. (2) Yu, C. L.; Xu, H. Y.; Ge, Q. J.; Li, W. Z. Properties of the Metallic Phase of Zinc-Doped Platinum Catalysts for Propane Dehydrogenation. J. Mol. Catal. A 2007, 266, 80-87. (3) Zhang, Y. W.; Zhou, Y. M.; Qiu, A. D.; Wang, Y.; Xu, Y.; Wu, P. C. Effect of Alumina Binder on Catalytic Performance of PtSnNa/ZSM-5 Catalyst for Propane Dehydrogenation. Ind. Eng. Chem. Res. 2006, 45, 2213-2219. (4) Akporiaye D.; Jensen, S. F.; Olsbye, U. A. Novel, Highly Efficient Catalyst for Propane Dehydrogenation. Ind. Eng. Chem. Res. 2001, 40, 4741-4748. (5) Rombi, E.; Cutrufello, M. G.; Solinas, V.; Rosso, S. D.; Ferraris,G.; Pistone, A. Effects of Potassium Addition on the Acidity and Reducibility of Chromia/Alumina Dehydrogenation Catalysts. Appl. Catal. A 2003, 251, 255.
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ReceiVed for reView April 6, 2007 ReVised manuscript receiVed June 27, 2007 Accepted August 30, 2007 IE070495
