Support Effects on the Oxidative Dehydrogenation of Ethane to

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Support Effects on the Oxidative Dehydrogenation of Ethane to Ethylene over Platinum Catalysts Rohan Gudgila† and Corey A. Leclerc*,‡,§ †

Department of Petroleum Engineering, ‡Department of Chemical Engineering, and §Department of Materials Engineering, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, United States ABSTRACT: Oxidative dehydrogenation of ethane to form ethylene was carried out over a platinum catalyst at short contact times. Alumina, zirconia, or silica reticulated foams were used as catalyst supports. The carbon selectivity to form ethylene was affected by the support material whereas the conversion of ethane was not affected to a large extent. The selectivity to form ethylene decreased from silica, to alumina, to zirconia. The temperature programmed desorption of ammonia carried out on the support materials showed that the zirconia support had a higher concentration of acid sites than either alumina or silica. After coating the supports, hydrogen chemisorption on the used catalyst showed metal dispersion was highest on silica and lowest on zirconia. The much higher selectivity on silica and alumina as compared to zirconia is explained by the lack of acid sites that catalyze the decomposition of ethylene to carbon. The higher dispersion of platinum on silica versus alumina will lead to a decrease in platinum metal costs of a real catalyst. The silica-supported catalyst achieved a yield close to that of a steam cracker without any attempt to optimize the system.

1. INTRODUCTION Light olefins are an important feedstock in chemical manufacture as intermediates to other chemicals. In 2009, over 75 million metric tons of ethylene were produced worldwide with sales in the billions.1 Industrially, ethylene is made from the steam cracking of ethane. Reactor tubes are placed in a firebox due to the endothermic nature of the reaction. A steam cracker normally achieves a conversion of ethane of 60% and a selectivity to form ethylene of 80%.2,3 Oxidative dehydrogenation (ODH) is an alternative process to steam cracking.4
6 In ODH, a small amount of oxygen is reacted with ethane to form ethylene and water: C2 H6 + 1=2O2 f C2 H4 + H2 O

ΔH ¼
104:9kJ=mol

The reaction is exothermic. Once the catalyst lights off, the reactor operates autothermally. The reactor operates at gas hourly space velocities on the order of 105
106 h
1, which translates to a residence time on the order of 1
10 ms. The millisecond residence time and exothermicity are major advantages of ODH over steam cracking. Generally, platinum-based catalysts supported on foam monoliths have shown the highest activity and best stability for ODH at millisecond contact times.7,8 Forming bimetallic catalysts of platinum and tin have further increased the activity of the catalyst with no decrease in selectivity.8 Adding hydrogen to the reactant stream can further increase the yield of hydrogen from the reactor.9 Addition of chlorine also increased activity by increasing the dispersion of the platinum catalyst.10 Chromia on alumina has shown activity similar or superior to platinum-based catalysts, but is limited in the range of feed stoichiometries and flow rates where it remains stable.11,12 Perovskites, especially those based on lanthanum and manganese, have shown excellent stability with activity that is close to that of platinum-based catalysts without expensive raw materials.13,14 r 2011 American Chemical Society

In this work, alumina, silica, and zirconia supports have been investigated for the ODH of ethane to ethylene. The catalysts have been characterized by hydrogen chemisorption and ammonia temperature-programmed desorption (NH3-TPD). Reactor results have been reported in terms of ethane conversion and ethylene selectivity as a function of ethane to oxygen feed ratio and total flow rate.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. In this study, two 5-mm monoliths (alumina, zirconia, or silica from Hi-Tech Ceramics) having 45 pores per inch (ppi) were coated with platinum at metal loadings of 2%. All monoliths had the same geometrical dimensions, such as pore diameter, independent of material. The platinum was added to the foam monoliths by impregnation with aqueous solutions of H2PtCl6. The samples were dried at room temperature, followed by calcinations at 100 °C for 1 h, 300 °C for 2 h, and 600 °C for 3 h. The actual metal loading, calculated after calcination, was within (0.2% of the 2% target. 2.2. Apparatus and Procedure. A mixture of ethane, oxygen, and nitrogen was fed at the top of reactor. Gas flows into the reactor were controlled by mass flow controllers. The total feed flow ranged from 3 to 7 standard liters per minute (slpm). The C2H6/O2 ratio was varied from 1.5 to 2.1 at a fixed nitrogen dilution of 30%. The catalyst bed was sandwiched between two inert Al2O3 monoliths which acted as radiation shields. These monoliths were sealed inside the quartz tube by silica
alumina cloth to prevent reactant bypass. The reactor was insulated by wrapping the quartz tube with high-temperature insulation. The Received: January 6, 2011 Accepted: June 6, 2011 Revised: May 25, 2011 Published: June 06, 2011 8438

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Industrial & Engineering Chemistry Research temperature at the back of the catalyst was measured using a thermocouple placed between the catalyst and the downstream radiation shield. The reactor ignition was achieved by first heating the catalyst with a Bunsen burner flame to about 220 °C. When light-off occurred a large increase in back face temperature was observed. The Bunsen burner flame was then removed and the reactor was wrapped with insulation to prevent radial heat losses. The reactor was then left to reach steady state operation and the product gases were sampled with a gastight syringe. Shutdown of the reactor was accomplished by turning off oxygen before ethane to avoid risk of explosion. The gas sample was injected into a dualcolumn Agilent 7890 gas chromatograph (GC). The GC was equipped with a HP-PLOT Q column to separate the carbon dioxide and C2 hydrocarbons in the gas sample, and a HP-PLOT molesieve column to separate the permanent gases (nitrogen, oxygen, carbon monoxide, methane, hydrogen). All products and reactants except hydrogen and water were measured by taking the area of their peak and referencing them to nitrogen. Water was measured by closing the atomic oxygen balance. Molecular hydrogen was measured by closing the atomic hydrogen balance. The elemental carbon balances were calculated and close to within (5%. Each point on the plots is an average of data from three separate experiments. 2.3. Reactor Performance. The catalysts’ suitability for the oxidative dehydrogenation of ethane was evaluated on the basis of the resulting ethane conversion and the product selectivities. The conversion of ethane is defined as the ratio of the amount of ethane which was consumed in the reactor over that which was fed to the reactor. The selectivity here is based on atomic carbon and defined as the amount of atomic carbon in a particular product divided by the total carbon in all of the products excluding ethane. The yield is defined as conversion of ethane multiplied by the selectivity of ethylene. 2.4. Catalyst Characterization. 2.4.1. NH3-TPD. NH3-TPD was used to determine the acidity of the support materials, i.e., crushed monoliths, before platinum coating. For the experiments, 10% NH3 in He was used as the preparation gas and He was used as the carrier/reference gas. Samples were degassed at 100 °C for 1 h in flowing helium to remove water vapor. The samples were then temperature programmed to 500 °C at a ramp rate of 10 °C/minute and held at that temperature for 2 h to remove strongly bound species and activate the sample. Finally, the sample was cooled to 120 °C in a stream of flowing helium. Next the sample was saturated with the ammonia at 120 °C; this temperature was used to minimize physisorption of the ammonia. The temperature-programmed desorption was performed by ramping the sample temperature at 10 °C/minute to 500 °C. It is a good rule of thumb that the end temperature during the TPD not exceed the maximum temperature used in the preparation of the sample. Exceeding the maximum preparation temperature (>1100 °C for the monolith supports) may liberate additional species from the solid unrelated to the probe molecule and cause spurious results. 2.4.2. Hydrogen Pulse Chemisorption. The active metal dispersion was determined by pulse-chemisorption using a Micromeritics Autochem II chemisorption analyzer (Micromeritics). The sample was pretreated using helium as the preparation gas and 10% H2 in Ar as the carrier/reference gas. The sample was reduced up to 900 °C before dosing. Once the amount of hydrogen absorbed was determined experimentally, the number of active sites was determined assuming that hydrogen dissociatively

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adsorbs on platinum. The metal surface area was calculated by multiplying the number of active sites times the surface area of a single active site. The dispersion was calculated from the number of exposed active sites divided by the total number of platinum atoms based on the mass of platinum added to the support. Hydrogen chemisorption was performed on the freshly calcined catalysts only. The used catalysts do not have a uniform dispersion within the monolith due to the large axial temperature gradient and oxygen concentration gradient. Due to the difficulty in breaking the monolith, it is not possible to obtain a dispersion profile as a function of axial position.

3. RESULTS 3.1. Feed Stoichiometry. The major products obtained for ethane oxidation were C2H4, CH4, CO, CO2, and H2. Contrary to previous reports,8
10,12,13 significant amounts of acetylene were not detected over the range of operating conditions investigated. Figure 1 below illustrates the effect of a varying C2H6/O2 feed ratio on the (a) back face temperature, (b) ethane conversion, (c) ethylene selectivity, (d) yield of ethylene, and (e) carbon oxide selectivities. For these graphs the gas hourly space velocity was kept constant at 160 000 h
1. 3.1.1. Back-Face Temperature. For all of the catalysts, the back-face temperatures decrease continuously in response to increases of the feed ratio. The back-face temperature of zirconia was highest and decreased from 1015 to 975 °C as the feed ratio increased. The back-face temperature of silica decreased from 980 to 905 °C. The alumina support experienced the lowest back-face temperature which decreased from 950 to 850 °C. 3.1.2. Ethane Conversion. Pt
SiO2 demonstrates the highest conversion of 76% for the C2H6/O2 ratio of 1.5. For other ratios of 1.7
2.1, Pt
ZrO2 shows almost 5% higher conversion compared to the other two catalysts. The ethane conversion for all the three catalysts decreased continuously with increases in C2H6/O2 feed ratios. The Pt
ZrO2 catalyst’s ethane conversion dropped from 74% to 45%, the Pt
SiO2 catalyst’s ethane conversion declined from 76% to 40%, and for Pt
Al2O3, it decreased from 73% to 32% as the C2H6/O2 feed ratio increased from 1.5 to 2.1. 3.1.3. Ethylene Selectivity and Yield. The highest selectivity was obtained for the Pt
SiO2 catalyst which was ∼63% for the C2H6/O2 ratio of 1.7. The selectivity for Pt
SiO2 was greater than that for Pt
Al2O3 and Pt
ZrO2 for all the C2H6/O2 ratios. There is a decrease of almost 20% in selectivity of Pt
ZrO2 compared to Pt
SiO2. The selectivity for the ratios 1.7 and 1.9 showed a slight maximum compared to the ratios 1.5 and 2.1 for all three catalysts. Also Pt
SiO2 has better yield compared to the other two catalysts. The highest yield obtained was ∼46% which is ∼5% better than Pt
Al2O3 (41%). Pt
ZrO2 gave poor yield; it went down from ∼27% to ∼17%. 3.1.4. CO and CO2 Selectivities. Correspondingly, the carbon monoxide selectivity shows a decrease as a result of an increase in feed ratio. The carbon monoxide selectivity observed with the Pt
SiO2 catalyst ranges from 26% to 20%, while for the Pt
ZrO2 catalyst it ranges from 56% to 51% and for Pt
Al2O3 it is from 33% to 15%. Conversely, the carbon dioxide selectivity remained constant over the range of feed ratios for all catalysts. It was below 7% for Pt
ZrO2 catalyst for all C2H6/O2 ratios. 3.2. Flow Rate Experiments. Experiments were carried out in which the feed flow rate was varied from 3 to 7 slpm. The 8439

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Figure 1. (a) Back face temperature, (b) ethane conversion, (c) ethylene selectivity, (d) ethylene yield, and (e) carbon monoxide (dashed)/carbon dioxide (solid) selectivity for varying ethane to oxygen ratios for platinum catalysts supported on alumina ((), silica (9), and zirconia (2).

nitrogen dilution was held constant at 30% and the C2H6/O2 was held constant at 1.5. Figure 2 shows the results for the reactor runs. In general, the conversion of ethane increased whereas the ethylene selectivity stayed constant. In terms of supports, the trend that silica is slightly better than alumina which is much better than zirconia for ethylene selectivity continued. 3.3. Surface Acidity. Figure 3 shows the NH3-TPD profiles for the three different support materials. The alumina and silica supports show no significant peaks. Zirconia exhibits a large peak centered at 550 °C. This peak corresponds to strong binding to acidic sites. 3.4. Metal Dispersion and Particle Size. Table 1 lists metal dispersion, metallic surface area, and active particle diameter on the three catalysts. Pt
SiO2 showed better metal dispersion and

had a smaller active particle diameter. The metal dispersion was ∼50% higher compared to Pt
Al2O3.

4. DISCUSSION 4.1. Reactor Performance. All catalysts exhibit decreasing conversion and temperature as the C2H6/O2 ratio increases, in agreement with previous results.9 The selectivity to form ethylene was not as sensitive to changes in feed ratio. As the flow rate increased, the conversion increased and the temperature increased approaching a plateau at the intermediate flow rate. These changes are much smaller as compared to the effects of the C2H6/O2 ratio. The ethylene selectivity was not sensitive to the change in the flow rate. 8440

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Figure 2. 9a) Ethane conversion, (b) ethylene selectivity, (c) carbon monoxide (dashed)/carbon dioxide (solid) selectivity, and (d) back-face temperature for dilution experiments. Ethane to oxygen ratio of 1.5 is used for platinum catalysts supported on alumina ((), silica (9), and zirconia (2).

Figure 3. NH3-TPD spectrum of monolith supports.

The silica-supported catalyst achieves higher ethylene selectivity and yield than the other two catalysts. Zirconia achieves the highest ethane conversion at times, but the greatly reduced ethylene selectivity leads to a low yield. The alumina-supported catalyst achieves conversions, selectivities, and yields that are similar to silica, but less favorable to ethylene production.

At the base case of 5 slpm inlet flow, C2H6/O2 = 1.5, and 30% N2 dilution, the silica-supported catalyst achieves a yield that is 5% higher (46% vs 41%) than the alumina-supported catalyst. This translates into a greater than 10% increase in the yield of ethylene from the reactor. For a multibillion-dollar industry, this type of increase would have enormous financial implications. 8441

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Table 1. Pulse Chemisorption Results on All Three Catalysts Pt
SiO2

Pt
Al2O3

Pt
ZrO2

metal dispersion (%)

4.01

2.83

1.53

metallic surface area (m2/g metal)

9.92

6.97

3.72

28.18

39.98

75.15

active particle diameter (nm)

Additionally, the 46% yield achieved by the silica-supported catalysts is similar to the 48% yield achieved by one pass in a steam cracker without any attempt at optimization by adding hydrogen to the feed, mixing platinum with tin, or adding a recycle loop. Optimization should boost the yield of the silicasupported catalyst over that of ethylene. Bell et al.15 have shown that for a similar reactor system analyzed by gas chromatography, conversions and selectivities have an uncertainty of only 0.5%. The small changes in reactor performance as measured by gas chromatography in these experiments are in fact statistically significant. 4.2. Effect of Support Acidity. A relationship between the support acidity and the selectivity to form ethylene exists based on the experimental results. The more acidic zirconia support exhibits a lower selectivity than the silica and alumina which show little to no surface acidity in the NH3-TPD results. In contrast, the conversion of ethane shows little relationship to the surface acidity. At low C2H6/O2 ratios, there seems to be no effect of acidity on conversion, whereas at higher ratios there is a slight increase in the conversion of ethane. These phenomena are explained by the ability of acid sites to decompose ethylene to carbon.16 The conversion and selectivity are likely identical on the platinum surface, but ethylene that is formed decomposes at acid sites on the support. Because the zirconia support has more acid sites, more ethylene decomposes to carbon leading to a lower ethylene selectivity. The oxidative conditions present in the catalyst lead to the burn-off of carbon to carbon monoxide and carbon dioxide which have higher selecitivites on the zirconia-supported catalysts as compared to those supported on alumina or silica. None of the catalysts deactivate over a period of ∼10 h and no visible coke is formed on the surface of the catalysts. 4.3. Effect of Catalyst Dispersion. All supports have low dispersions of platinum on the surface. However, the approximately 50% increase in platinum dispersion on silica versus alumina is a significant benefit. With the high cost of noble metals, optimizing the dispersion will be of prime importance. Without making attempts to optimize the dispersion and operating at temperatures where sintering is occurring, the silica support will require much less platinum to obtain the same number of active sites. Characterization after use in the reactor would be ideal to confirm these results, but was not performed here due to the expected gradient in particle size axially through the monolith. There appears to be no link between ethylene selectivity and catalyst dispersion as was observed by Nakamura et al.17 This is likely due to the much larger particle size in this work as compared to their work, which rules out platinum size effects and structure sensitivity on the product selectivity. Additionally, Hickman and Schmidt18,19 have shown that the millisecond contact time reactors are mass transfer limited catalytic processes indicating that the amount of metal on the surface is not important to achieve high conversion. Therefore, the more highly dispersed platinum on silica should not achieve a higher

conversion of ethane because it has a higher concentration of active sites.

5. CONCLUSIONS Silica-supported platinum catalysts show superior ethylene yield as compared to alumina and zirconia supports. At a C2H6/ O2 feed ratio of 1.5, feed flow rate of 5 slpm, and nitrogen dilution of 30%, the silica-supported catalyst achieved a yield of 46%, the alumina-supported catalyst achieved a yield of 41%, and the zirconia-supported catalysts achieved a yield of 26%. In general, all catalysts had similar ethane conversions, so the increased yield was due primarily to increases in ethylene selectivity. The large increase in ethylene on silica-supported catalysts is due to the absence of acid sites which decompose ethylene to carbon, which is in turn oxidized to carbon oxides on the zirconia supports. Because the process is mass transfer limited, increases in platinum dispersion on silica versus alumina do not lead to an increase in ethane conversion, but will lead to lower platinum costs for the silica-supported catalyst. This study shows that one must be careful to choose support materials for ODH reactors that lack acid sites, which degrade ethylene selectivity. The 5% increase in yield achieved by switching from alumina to silica translates into millions of dollars based on the size of the ethylene industry. The yield achieved by the silica-supported catalysts is within 2% of that achieved by a steam cracker in one pass without the benefit of optimization by mixing platinum with tin, chlorine addition to the catalyst, or hydrogen addition to the feed, which have all been shown to increase the yield of ethylene, but at a residence time that is 2
3 orders of magnitude lower that a steam cracker and without the addition of external heating. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: (575) 835-5293. Fax: (575) 835-5210. E-mail: leclerc@ nmt.edu.

’ ACKNOWLEDGMENT We thank the Department of Energy for funding through the Experimental Program to Stimulate Competitive Research (EPSCoR) award DE-FG02-08ER46530. ’ REFERENCES (1) Output Declines In U.S., Europe. Chem. Eng. News 2010. 88 (27), 54. (2) Cavani, F.; Ballarini, N.; Cericola, A. Oxidative dehydrogenation of ethane and propane: How far from commercial implementation? Catal Today 2007, 127, 113. (3) Kung, H. H.; Kung, M. C. Oxidative dehydrogenation of alkanes over vanadium-magnesium-oxides. Appl. Catal., A 1997, 157, 105. (4) Cavani, F.; Trifiro, F. The oxidative dehydrogenation of ethane and propane as an alternative way for the production of light olefins. Catal. Today 1995, 24, 307. (5) Kung, H. H. Oxidative dehydrogenation of light (C2 to C4) alkanes. Adv. Catal. 1994, 40, 1. (6) Huff, M.; Schmidt, L. D. Ethylene formation by oxidative dehydrogenation of ethane over monoliths at very short contact times. J. Phys. Chem. 1993, 97, 11815. (7) Flick, D. W.; Huff, M. C. Oxidative dehydrogenation of ethane over a Pt-coated monolith versus Pt-loaded pellets: Surface area and thermal effects. J. Catal. 1998, 178, 315. 8442

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