Oxidative dehydrogenation of ethane over Pt-Sn catalysts

Studies in Surface Science and Catalysis, volume 147 X. Bao and Y. Xu (Editors) 02004 Elsevier B.V. All rights reserved.

685

Oxidative dehydrogenation of ethane over Pt-Sn catalysts r162

B. Silberova, J. Holm and A. Holmen

Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), Sem Sa~lands vei 4, Trondheim, N-7491, Norway ABSTRACT The catalytic activity of Pt and Pt-Sn monolithic catalysts has been examined for the production of ethene by short contact time oxidative dehydrogenation of ethane. The experiments were performed in a continuous flow reactor. High yields of ethene (~63%) were obtained over Pt-Sn catalyst at 850 ~ when hydrogen was added to the feed. 1

INTRODUCTION

Light hydrocarbons are important feedstocks for the production of olefins such as ethene and propene. Although large-scale steam cracking is the main process for production of olefins, several other possible routes have received considerable attention. Catalytic dehydrogenation and oxidative dehydrogenation at short contact times are examples of approaches that have been studied recently by several groups [1-5]. High yields of ethene and propene have been observed during oxidative dehydrogenation on Pt-coated foam monoliths at extremely short contact times (1 - 10 ms). It has also been shown [4] that enhancement of the ethene yield can be obtained by addition of hydrogen to the reaction mixture and by promoting the Pt catalyst by Sn. Addition of a second metal such as Sn to Pt is well known and used in other processes like catalytic reforming [6] and dehydrogenation [7]. The present study focuses on the addition of hydrogen during oxidative dehydrogenation of ethane over Pt and Pt-Sn monolithic catalysts. The experiments were performed over a large temperature range. The effect of hydrogen addition was tested by varying the H2/O2 ratio in the feed. Furthermore, Pt catalysts with various Sn loading were prepared and tested under the same reaction conditions.

686

2

EXPERIMENTAL

Cordierite ceramic monolith (2MgO*2A1203*5SiO2) with cell density of 62.2 cells/cm 2was used as support. Cylindrical pieces of the monolith (10 mm long, 15 mm diameter) were washcoated by a dispersion of Disperal P2 ( B E T - 286 mZ/g). The washcoated monoliths were dried at 120 ~ for 4 h and the washcoating step was repeated until the weight increase was 15% of the primary monolith weight. After calcination in a flow of air at 550 ~ for 4.5 h, the washcoated monoliths were impregnated with Pt(NH3)4(NO3)2 to obtain a Pt loading of about 1 wt.%. The impregnated monoliths were then dried at 80 ~ for 4 h and calcined at 550 ~ in a flow of air for 4.5 h. The Pt/monoliths were subsequently impregnated by SnClz*2H20, dried at 80 ~ and calcined in a flow of air at 100 ~ for 0.5 h and at 700 ~ for 1.5 h. Pt/monoliths with three different Sn loadings (1, 3 and 7 wt.%) were prepared and tested for oxidative dehydrogenation of ethane. The Pt-Sn monoliths were reduced in the reactor prior to the experiments in a flow of hydrogen at 700 ~ for 1.5 h. Experiments were carried out at atmospheric pressure and in the temperature range of 4 0 0 - 950 ~ in a conventional flow apparatus consisting of a quartz reactor with inner diameter of 15 mm. The Pt or Pt-Sn coated monolith was placed between two inert monolith pieces acting as radiation shields. At the reactor outlet, two water-cooled condensers were installed for removal of water. The amount of water was calculated based on the oxygen material balance and expressed as O-selectivity. The total flow rate consisting of ethane, air and argon was 2000 Nml/min. In some experiments, hydrogen was added to the reaction mixture. In these cases, the flow of argon was reduced to keep the total flow rate constant. The ethane/oxygen ratio in the feed was 2/1 and the ratio of hydrogen/oxygen was 3/1 if not otherwise mentioned. The contact time through the coated monolithic piece was estimated to 0.04 s at standard conditions. "Dry" samples of the product stream were analyzed by two on-line gas chromatographs.

3 3.1

RESULTS AND DISCUSSION

Effect of H~ addition and Pt-Sn catalysts Oxidative dehydrogenation of ethane was performed over Pt and Pt-Sn monolithic catalysts without and with addition of hydrogen. At lower temperatures the lowest conversions of ethane were obtained using Pt-Sn catalyst with addition of hydrogen. However, at high temperatures the conversion of ethane was the same for all the experiments performed (Fig. 1A). Reactor temperatures measured inside the reactor were higher with addition of hydrogen to the ethane/oxygen mixture during the experiments over the Pt catalyst and even more using the Pt-Sn catalyst (Fig. 1A). If less ethane is converted but the reactor temperatures are increased when hydrogen is added, other exothermic reactions

687 must occur. Looking at the production of water during the experiments over the Pt catalyst (Fig. 1B), a large amount of water was formed when hydrogen was present in the feed stream. Using the Pt-Sn catalyst, even more significant amounts of water was produced (Fig. 1B). It is obvious that oxygen reacts with hydrogen leading to the formation of large amounts of water. Using Pt catalyst, the conversion of oxygen was already complete at 600 ~ when hydrogen was added to the feed. Without addition of hydrogen, not all oxygen was converted at the same temperature. Higher temperatures (< 850 ~ were necessary to achieve total conversion of oxygen over the Pt-Sn catalyst. As shown on Fig. 2A, the addition of hydrogen obviously contributed to higher selectivities of ethene and lower selectivities of carbon oxides for the Pt catalyst (Figs. 2B and 2C). The selectivities of ethene were even more enhanced by adding Sn to the Pt catalyst (Fig. 2A). This effect can be assigned firstly to the reaction of oxygen with hydrogen, which is preferred to the reaction of oxygen with ethane and secondly to high activity of the Pt-Sn catalyst for dehydrogenation of light alkanes [7]. A high activity of Pt-Sn catalysts for this type of reaction is ascribed to the fact that Sn inhibits the formation of highly dehydrogenated surface species and Sn also decreases the size of surface Pt ensembles [8]. The Pt-Sn catalyst also showed a significant effect on the formation of carbon monoxide (Fig. 2B) and carbon dioxide (Fig. 2C), which were lowered in the whole temperature range studied. Methane is also one of the components formed during oxidative dehydrogenation of ethane (Fig. 2D). 100

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C2H6 conversion [%]

Fig. 1. Oxidative dehydrogenation of ethane over Pt and Pt-lwt.%Sn catalysts with and without addition of H2. Feed [Nml/min]: C2H6(308), air (733), H2 (0 - 462) and Ar (959 - 497). Total flow rate: 2000 Nml/min. A: Conversion of ethane and reactor temperature as a function of the furnace temperature. B: O-selectivity of water as a function of the conversion of ethane.

688 100

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Fig. 2. Effect of addition of H2 on Pt and Pt-1wt.%Sn monolithic catalysts on the selectivity of ethene (A), carbon monoxide (B), carbon dioxide (C) and methane (D). Feed [Nml/min]: C2H6 (308), air (733), H2 (0 - 462) and Ar (959 - 497). Total flow rate" 2000 Nml/min. Higher formation of methane was obtained over the Pt catalyst with hydrogen addition compared to the one without hydrogen addition at low furnace temperature. The addition of hydrogen might thus contribute to the formation of methane via CHy species, which are formed at low furnace temperatures on the Pt catalysts [9-11]. Using the Pt-Sn catalyst, the formation of methane was even higher compared to the Pt catalyst, which is probably due to higher activity of the Pt-Sn catalyst for hydrogenolysis of light alkanes [12]. Higher hydrocarbons such as C3, C4 and C5 components were also formed during oxidative dehydrogenation. The addition of hydrogen to the ethane/oxygen mixture enhanced slightly the yield of C3 hydrocarbons (from 0.47% to 0.50%) but a more significant effect was observed by addition of Sn to the Pt catalyst (the yield of C3 hydrocarbons increased to 0.83%). The highest yields of C4 and C5

689 hydrocarbons (1.94% and 0.06%, respectively) were obtained over Pt-Sn catalysts with addition of hydrogen. 3.2

Effect of Sn l o a d i n g

The addition of l wt.%Sn to the Pt catalyst and with addition of hydrogen showed a significant effect on the selectivity of ethene. However, further addition of Sn to the Pt catalyst did not reveal any large enhancement in the selectivity of ethene. The highest yield of ethene (~ 63%) was obtained over the Pt-7wt.%Sn catalyst at 850 ~ which is only a few percent higher value compared to using Pt-Sn catalysts with Sn loadings of l wt.% and 3wt.%. The yields of ethene obtained are higher than the ones reported by Henning and Schmidt (45-57% at 920-925~ [4] or Buyevskaya et al. (34-46% at 855-867 ~ [5]. The selectivities of carbon monoxide were the same for all three different catalysts at high temperatures. Even though the selectivity of carbon monoxide was highest using the Pt-3wt.%Sn at low temperatures, no obvious effect of the Sn loading on the carbon monoxide production can be deduced. The selectivities of methane and carbon dioxide were hardly influenced by higher Sn loading on the Pt catalyst. 3.3

H2 o u t p u t

The consumption and the production of hydrogen are demonstrated by H2/O2 ratios as a function temperature in Fig. 3A. It is obvious from Fig. 3A that the amount of hydrogen in the product stream depends very much on the temperature using both Pt and Pt-Sn catalysts. 5,0 =

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Moles of H2_inlO2_in

Fig. 3. Comparison between amount of moles of H2 in the feed and in the product stream related to the amount of O2 in the feed. Total flow rate: 2000 Nml/min. A) Effect of the temperature. Feed [Nml/min]: C2H6 (308), air (733), H2 (462) and Ar (497). B) Effect of the Hz_in/Oz_in. Feed [Nml/min]: C2H6(308), air (733), H2 (154-462) and Ar (805-497).

690 The H2_out/O2_in ratio increases with increasing temperature. In the case of the Pt catalyst, a temperature of 550 ~ is high enough to obtain H2_out/Oz_in ratio equal to H2_in/Oz_in, while a temperature of 850 ~ is required to reach the same point using the Pt-lwt.%Sn catalyst. The experiments with various Hz_in/Oz_in ratios in the feed were performed at 850 ~ Fig. 3B shows that higher amount of H2 moles is coming out compared to what was fed for both the Pt and the Pt-Sn catalyst. To obtain an even higher amount of hydrogen in the product gas, the Hz_in/Oz_in ratio can be reduced. This is clearly shown by the increasing difference between Hz_in/Oz_in and Hz_out/Oz_in ratios with decreasing Hz_in/O2_in ratios (Fig. 3B). 4 CONCLUSIONS The addition of hydrogen contributed to a lower production of carbon oxides and to a higher production of ethene. The ethene selectivity was substantially increased also by addition of Sn to the Pt monolithic catalyst. The Pt-Sn catalyst also showed a significant effect on the formation of carbon monoxide and on the selectivity of carbon dioxide, which were lowered remarkably. Further addition of Sn above l wt.% to the Pt monolithic catalysts did not affect the selectivities of products to any large degree. The selectivity of methane was enhanced by the addition of hydrogen. It has been shown that the amount of hydrogen in the product stream depends strongly on the temperature and on the H2/O2 ratio in the feed. ACKNOWLEDGEMENT The financial support from the Norwegian Research Council and the Norwegian University of Science and Technology in Trondheim is greatly acknowledged. REFERENCES

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