Platinum-tin and platinum-copper catalysts for autothermal oxidative dehydrogenation of ethane to ethylene

The addition of various metals to Pt-coated ceramic foam monoliths was examined for the autothermal oxidative dehydrogenation of ethane to ethylene at 900°C at contact times of 5 ms. The addition of Sn or Cu to Pt-monoliths enhanced both C₂H₆ conversions and C₂H₄ selectivities significantly, giving higher C₂H₄ yields. No deactivation or volatilization of the catalysts was observed. For Pt-Sn, an increase in the Sn/Pt ratio from 1/1 to 7/1 increased both the conversion and the selectivity. For Pt-Sn (Sn/Pt = 7/1) versus Pt alone the conversion increased by up to 6% and the selectivity up to 5% for an increase in optimal yield from 54.5% with Pt to 58.5% with Pt-Sn. XRD and XPS measurements showed that Pt existed in the form of PtSn and Pt₃Sn alloys. The 1/1 Pt-Cu catalyst showed comparable performance, with conversion increasing by 5% and selectivity by 3%. The addition of several other metals to Pt-monoliths decreased both C₂H₆ conversion and C₂H₄ selectivity in the order, Sn>Cu>Pt alone>Ag>Mg>Ce>Ni>La> Co. For oxidative dehydrogenation ofn-butane and isobutane, Pt-Sn and Pt-Cu also showed higher conversion than Pt.This research was partially supported by NSF under Grant CTS-9311295.     

High yields of synthesis gas by millisecond partial oxidation of higher hydrocarbons

Cyclohexane, n-hexane, and isooctane were reacted with air in a Rh-monolith reactor and converted into synthesis gas (H₂+CO) in yields exceeding 90%, with >95% conversion of fuels and 100% conversion of oxygen. The best catalyst was an 80 ppi washcoated alumina monolith containing 5 wt% Rh. There was a small effect of catalyst contact time on syngas selectivity and conversions for gas space velocities from 3×10⁵ to 3×10⁶ h^–1. Preheating the feed enhances syngas selectivities slightly, but no reactor preheat is necessary provided the fuel remains vaporized. Addition of 25 mol% toluene to isooctane also produces syngas, olefins, and methane with 90% yield, including 70% yield to syngas. Partial oxidation of gasoline–air mixtures was attempted but the catalysts were poisoned after several hours, probably by sulfur and/or metals.     

Catalytic partial oxidation of ethane to ethylene and syngas over Rh and Pt coated monoliths: Spatial profiles of temperature and composition

The catalytic partial oxidation of C₂H₆ over Pt and Rh coated monolithic supports (4.7 wt% M/α-Al₂O₃ 45 PPI) was investigated with a capillary sampling technique for a range of C₂H₆/air ratios at constant inlet flow (～8 ms contact time), with and without H₂ addition. Effluent data clearly indicate the differences in product distribution between catalysts and equilibrium. Rh effectively converts the reactant mixtures to syngas with ～80% selectivity, whereas Pt produces C₂H₄ with ～55% C-atom selectivity, while neither produces thermodynamically favored C. Spatially resolved measurements provide direct evidence of the multi-zone nature of the reactors. With Rh, complete conversion of O₂ occurs to produce mostly CO, H₂ and H₂O within the first 3 mm of catalyst, followed by a reforming zone to produce additional syngas. Pt consumes O₂ more slowly, which results in a steady increase in temperature along the reactor. Ethylene formation correlates to reactor temperatures >750 °C, regardless of C/O, in line with the onset of homogeneous reactions. Hydrogen addition tests (C₂H₆/O₂/H₂=2/1/2) clearly exhibit preferential oxidation of H₂ with O₂ over Pt, which shifts the maximum in temperature upstream while preserving a portion of the C₂H₆ for C₂H₄ production. H₂ addition modifies the concentration and temperature profiles minimally on Rh. The main differences between catalysts are the high reforming and O₂ consumption activity with Rh compared to Pt, which are likely responsible for differences in C₂H₄ yields.     

Millisecond catalytic reforming of monoaromatics over noble metals

The millisecond autothermal reforming of benzene, toluene, ethylbenzene, cumene, and styrene were independently studied over five noble metal‐based catalysts: Pt, Rh, Rh/γ‐Al₂O₃, Rh–Ce, and Rh–Ce/γ‐Al₂O₃, as a function of carbon‐to‐oxygen feed ratio. The Rh–Ce/γ‐Al₂O₃ catalyst exhibited the highest feedstock conversion as well as selectivities to both synthesis gas and hydrocarbon products (lowest selectivities to H₂O and CO₂). Experimental results demonstrate a high stability of aromatic rings within the reactor system. Benzene and toluene seem to react primarily heterogeneously, producing only syngas and combustion products. Ethylbenzene and cumene behaved similarly, with higher conversions than benzene and toluene, and high product selectivity to styrene, likely due to homogeneous reactions involving their alkyl groups. Styrene exhibited low conversions over Rh–Ce/γ‐Al₂O₃, emphasizing the stability of styrene in the reactor system. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Preferential CO Oxidation in the Presence of H2, H2O and CO2 at Short Contact-times

High selectivities and conversions in the preferential oxidation of CO in the presence of large quantities of H₂, H₂O and CO₂ are demonstrated on noble metal catalysts at millisecond contact times (~10–15 ms) for temperatures between 150 and 500 °C. With a simulated water-gas shift product stream containing 0.5% CO and varying amounts of H₂, H₂O and CO₂, we are able to achieve ~90% CO conversions on a Ru catalyst at temperatures of ~300 °C using a stoichiometric amount of O₂ (0.25%). Experiments with and without O₂ and with varying H₂O reveal that significant water-gas shift occurs on Pt and Pt-ceria catalysts at temperatures between 250 and 400 °C, while significant CH₄ is formed on Ru and Rh catalysts at temperatures greater than 250 and 350 °C, respectively. The presence of H₂O blocks H₂ adsorption and allows preferential CO oxidation at higher temperatures where rates are high. We propose that a multistage preferential oxidation reactor using these catalysts can be used to bring down CO content from 5000 ppm at the reactor entrance to less than 100 ppm at very short contact-times.     

Synthesis gas formation by direct oxidation of methane over Rh monoliths

The production of H₂ and CO by catalytic partial oxidation of CH₄ in air or O₂ at atmospheric pressure has been examined over Rh-coated monoliths at residence times between 10^–4 and 10^–2 s and compared to previously reported results for Pt-coated monoliths. Using O₂, selectivities for H₂ ( ) as high as 90% and CO selectivities (S _CO) of 96% can be obtained with Rh catalysts. With room temperature feeds using air, Rh catalysts give of about 70% compared to only about 40% for Pt catalysts. The optimal selectivities for either Pt or Rh can be improved by increasing the adiabatic reaction temperature by preheating the reactant gases or using O₂ instead of air. The superiority of Rh over Pt for H₂ generation can be explained by a methane pyrolysis surface reaction mechanism of oxidation at high temperatures on these noble metals. Because of the higher activation energy for OH formation on Rh (20 kcal/mol) than on Pt (2.5 kcal/mol), H adatoms are more likely to combine and desorb as H₂ than on Pt, on which the O+ H OH reaction is much faster.This research was partially supported by DOE under Grant No. DE-FG02-88ER13878-AO2.     

Simulating cyclohexane millisecond oxidation: Coupled chemistry and fluid dynamics

Cyclohexane partial oxidation over a 40-mesh Pt–10% Rh single-gauze catalyst can produce ∼85% selectivity to oxygenates and olefins at 25% cyclohexane conversion and 100% oxygen conversion, with cyclohexene and 5-hexenal as the dominant products. A detailed 2-D model of the reactor is solved using density-functional theory (with 35 reactions among 25 species) and computational fluid dynamics. Rapid quenching in the wake of the wires allows highly nonequilibrium species to be preserved. The simulations show that the competition between cyclohexyl and cyclohexylperoxy radicals is crucial in determining product selectivities. At high temperatures and low pressures, the cyclohexyl radical is favored, leading to high selectivities to cyclohexene. At lower temperatures or high pressures, cyclohexylperoxy radicals are favored, allowing the formation of parent oxygenates to dominate. Numerical simulations suggest ways to tune reactor operation for desired product distributions and allow the investigation of dangerous or costly operating conditions, such as high pressure.     

Effect of pressure on three catalytic partial oxidation reactions at millisecond contact times

The effects of pressure on reactant conversion and product selectivities in three catalytic oxidation systems have been examined at pressures between 1 and > 5 atm. Reaction was sustained autothermally near adiabatic operating conditions at temperatures of 1000°C with residence times over the noble metal catalysts between 10^–4 and 10^–2 s. The three systems investigated were (1) HCN synthesis over Pt-10% Rh gauze catalysts, (2) methane oxidation to synthesis gas (CO and H₂) over rhodium-coated monoliths, and (3) ethane conversion to ethylene over platinum-coated monoliths. We find that selectivities in all three reactions do not change dramatically with approximately a five-fold increase in pressure. This strongly suggests that free radical homogeneous chain reactions are not significant in these processes and that they can be operated reliably above atmospheric pressure. For the synthesis of HCN over Pt-10% Rh gauzes, the selectivity to HCN can be maintained above 0.75 at pressures up to 5.5 atm. Selectivities to synthesis gas (CO and H₂) from a methane-air mixture over a Rh-coated foam monolith at pressures up to 5.5 atm were maintained above 0.90. Over a Pt-coated foam monolith, the selectivity to ethylene from ethane-air and ethane-O₂ mixtures was independent of pressure up to 6.5 atm and conversion rose slightly although it was necessary to maintain constant velocity and residence time over the catalyst to avoid carbon formation.This research was supported by DOE under Grant No. DE-FG02-88ER13878.     

Partial oxidation of methane and the effect of sulfur on catalytic activity and selectivity

Partial oxidation of methane into syngas was conducted over fresh and sulfided catalysts at a temperature range of 450–750 °C. The temperature dependence of conversion, H₂/CO ratio, and the CO₂ concentration were measured for both fresh and sulfided catalysts. Regardless of metal type, metal loading, support type, and the methods of preparation it appears that all the fresh catalysts were very active and conversions of higher than 70% with H₂/CO ratio of about 2 were observed at 750 °C. Pulse sulfidation appears to be reversible for some of the catalysts but not for all. Under pulse sulfidation conditions, the Rh(0.5%)/Al₂O₃ and NiMg₂O_x-1100 °C (solid solution) catalysts were fully regenerated after reduction with hydrogen. Rh catalyst showed the best overall activity, less carbon deposition, both fresh and when it was exposed to pulses of H₂S. Sulfidation under steady-state conditions, flowing H₂S/Ar mixture over the catalysts, significantly reduce catalyst activity. The catalysts were characterized before and after reaction with H₂S using temperature-programmed oxidation (TPO) and reduction (TPR), X-ray diffraction, and XPS.     

Modification of the catalytic properties of supported noble metal catalysts by carrier doping

The influence of altervalent cation doping of TiO₂ carriers on the chemisorptive and catalytic properties of supported Pt and Rh crystallites has been investigated. It was observed that doping of the carrier with higher valence cations leads to suppression of the H₂ and CO chemisorption capacity of Pt catalysts, while their activity in hydrogenation and oxidation reactions is significantly reduced. The opposite effects were observed in the case of Rh catalysts supported on higher valence doped TiO₂. These catalysts were found to possess higher activity in CO and CO₂ hydrogenation, in aromatics hydrogenation and in CO and C₂H₄ oxidation. Their stability characteristics were also found to be superior to those of the undoped Rh/TiO₂ catalyst. These effects are believed to originate from an electronic type interaction at the metal-support interface, induced by doping, which results in electron transfer from the support to the metal crystallites.     

Direct synthesis of hydrogen peroxide from H2 and O2 and in situ oxidation using zeolite-supported catalysts

Four microporous materials, zeolites HZSM-5, Y, Beta and TS-1, were used as the supports to prepare supported gold catalysts using impregnation or deposition precipitation. The gold catalysts were tested in the direct synthesis of hydrogen peroxide from H₂ and O₂ and for CO oxidation. The effect on the catalytic activity of different metal (e.g., Pd, Pt, Cu, Ag, Rh or Ru) on the synthesis of hydrogen peroxide was also tested. Organic substrates, such as cyclohexane or cyclooctene, were introduced to investigate the possibility of in situ H₂O₂ oxidation with these catalysts.     

Enhanced Production of C2–C4 Olefins Directly from Synthesis Gas

An attempt made for the selective production of C₂–C₄ olefins directly from the synthesis gas (CO + H₂) has led to the development of a dual catalyst system having a Fischer–Tropsch (K/Fe–Cu/AlOx) catalyst and cracking (H-ZSM-5) catalyst operate in consecutive dual reactors. The flow rate (space velocity) and H₂/CO molar ratio of the feed have been optimized for achieving higher CO conversions and olefin selectivities. The selectivity to C₂–C₄ olefins is further enhanced by optimizing the reaction temperature in the second reactor (cracking), where the product exhibited 51% selectivity to C₂–C₄ hydrocarbons rich in olefins (77%) with a stable time-on-stream performance in a studied period of 100 h.     

Tunability of Propane Conversion over Alumina Supported Pt and Rh Catalysts

Propane conversion over alumina supported Pt and Rh (1 wt% metals loading) was examined under fuel rich conditions (C₃H₈:O₂:He = 1:2.25:9) over the temperature range 450–650 °C. Morphological characteristics of the catalyst materials were varied by calcining at selected temperatures between 500 and 1,200 °C. X-ray diffraction and BET analysis showed the treatment generated catalyts metals with particle sizes in the range of <10 to >500 nm, and support surface areas in the range of 20–240 m²/g. Remarkably, both Rh and Pt yielded product compositions close to equilibrium values (with high H₂ and CO selectivity, complete oxygen conversion and almost complete propane conversion) so long as the metal particle size was sufficiently low, ≲10–15 nm. In cases where the particle size was large, primarily complete oxidation rather than partial oxidation products were observed, along with unreacted C₃H₈, indicative of a direct oxidation pathway in which gas-phase CO and H₂ are not present as intermediate species. It is proposed that the high resistance of Rh to coarsening is largely responsible for the observation of a higher selectivity of this material for syngas products when prepared by procedures similar to those for Pt. Overall, the tunability of the product composition obtained over Rh and Pt via processing steps has direct significance for the incorporation of such catalyts into the anodes of solid oxide fuel cells.     

Catalytic reduction of N2O with CH4 and C3H6 over Ag–Rh/Al2O3 bimetallic catalyst in the presence of oxygen

A study of nitrous oxide (N₂O) reduction with methane (CH₄) and propene (C₃H₆) in the presence of oxygen (5%) over Ag/Al₂O₃, Rh/Al₂O₃ and Ag–Rh/Al₂O₃ catalysts, with Ag and Rh loadings of 5 wt% and 0.05 wt% respectively, has been performed. From the results, it was observed that the Ag–Rh bimetallic catalyst was the most active for both nitrous oxide removal (more than 95%) and hydrocarbon oxidation. This high activity seems to be connected with a synergistic effect between Ag and Rh. The findings from X‐ray diffraction and X‐ray photoelectron spectroscopy studies showed also, that there were no strong interactions (eg alloying) between Ag and Rh. Copyright © 2005 Society of Chemical Industry     

Hydrogenation of carbon monoxide to higher alcohols over ordered mesoporous carbon nanoparticle-supported Rh-based catalysts

Two-dimensional hexagonally ordered mesoporous carbon nanoparticles (MCNs) were synthesized using the templating synthesis method. MCNs were introduced as supports for the catalytic thermochemical conversion of syngas to higher alcohols. The catalytic test of the promoted Rh/MCNs was performed using a fixed bed reactor. The catalytic results reveal that the nano-sized MCN-supported catalysts exhibited higher C₂₊ alcohol production with a high ethanol selectivity compared with the micro-sized ordered mesoporous carbon-supported catalysts. The promoted Rh/CMK-5-MCN with a hollow framework configuration exhibited a superior space–time yield of the total C₂₊ alcohols compared with the promoted Rh/CMK-3-MCN with a rod carbon framework. It indicates that the promoted Rh/MCNs exhibited different catalytic activities and selectivities of higher alcohols, which is attributed to the Rh particle size and the reactant accessibility to active sites through the morphological effects of the MCNs.     

Catalytic partial oxidation of methane to synthesis gas over γ-Al2O2-supported rhodium catalysts

The partial oxidation of methane was studied over -Al₂O₃-supported catalysts for Rh loadings between 0.01 and 5.0 wt%. It was found that the activity and selectivities for loadings between 0.5 and 5.0 wt% are almost the same. As an example, detailed information is presented for the 1.0 wt% Rh/-Al₂O₃, which provides at 750°C (furnace temperature) an activity of 82% and selectivities of 96% to CO and 98% to H₂, at a gas hourly space velocity (GHSV) of 720000 ml g^–1 h^–1. Its activity remained stable during our experiment which lasted 120 h. Possible explanations for this high stability are proposed based on TPR and XRD experiments. Pulse reactions with small pulses of CH₄ and CH₄/O₂ (2/1) were performed over the reduced and unreduced Rh catalysts to probe the mechanistic aspects of the reaction. The partial oxidation of methane to syngas was found to be initiated by metallic rhodium sites, since the CO selectivity increased with increasing number of such sites.     

Partial oxidation of methane to synthesis gas over Rh/α-Al2O3 at high temperatures

The partial oxidation of methane to synthesis gas has been studied in a continuous flow reactor using a Rh/α-Al₂O₃ catalyst under conditions as close as possible to those industrially relevant: pressures up to 800 kPa and temperatures higher than 1274 K in order to avoid the formation of carbon and to obtain high equilibrium selectivities to CO and H₂. Intrinsic kinetic data were obtained when the feed was diluted with helium. Gas-phase reactions were found to occur at 500 kPa when the feed was not diluted. A reaction network has been derived from experimental results in which oxygen conversions range from 0 to 1. CO₂, C₂H₆ and H₂O are the primary products. C₂H₄ is formed by oxidative dehydrogenation of C₂H₆. CO and H₂ are formed by reforming of CH₄ by CO₂ and H₂O; an additional direct route to CO and H₂ at low oxygen conversions cannot be excluded. The catalyst appears to be present in two states, the transition being at an oxygen conversion of 0.4 under the conditions used. The support probably enhances oxidation reactions by reverse spillover of oxygen or hydroxyl species onto rhodium. The support as such behaves similarly to the catalyst at low oxygen conversions, but shows no reforming activity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Oxydehydrogenation of propane over Mg-V-Sb-oxide catalysts. I. Reaction network

Magnesium vanadates have been shown by various groups to be active oxydehydrogenation catalysts for the conversion of light paraffins to the corresponding olefins. The olefins produced have significant commercial value in petroleum and petrochemical industry. Recently, we reported that doping of the magnesium vanadates with antimony, antimony-phosphorus, or boron, produces catalysts with significantly better selectivities to olefins than those of the parent undoped catalysts. Among these, the composition Mg₄V₂SbO_x was selected for further study of propane oxydehydrogenation and its reaction network. At 500°C and atmospheric pressure, the selectivity to propylene decreases monotonically from 75% to 5% as propane conversion is increased from 2% to 68%. An analysis of the reaction network reveals, that propylene is the only useful first formed product, that CO_x is produced largely by sequential oxidation of the in situ formed propylene, but also to a lesser extent direct from propane by a deep oxidation route. The presence of two parallel pathways for CO_x formation is of interest, as it suggests that partial and deep oxidations may occur at different surface sites or involve different forms of reactive oxygens. Both of these might be amenable to electronic modification by substitution or doping to achieve higher propylene selectivities and yields at higher propane conversions, or their catalytic behavior might be advantageously alterable through site isolation of the paraffin activation centers.     

Disk-shaped packed bed micro-reactor for butane-to-syngas processing

A novel disk-shaped packed bed micro-reactor containing Rh/ceria/zirconia nanoparticles is investigated with respect to catalytic butane-to-syngas processing at moderate temperatures of 550 °C. The main goal of this study is the development of an efficient butane processor which can be integrated into a micro solid oxide fuel cell system due to its small size, easily packaged geometry in layered microdevices, high compactness, low pressure drop, and low reaction temperature. It is shown that Rh/ceria/zirconia has an excellent long-term stability and achieves very high C₄H₁₀ conversion and syngas selectivity, considering the relatively low operating temperature. The yields of H₂ and CO can be increased up to 71% and 57%, respectively, by optimizing operational parameters such as the C/O ratio and the total inlet flow rate. The introduced disk-shaped packed bed reactor shows significant advantages in catalytic behavior, at a 6.5 times lower pressure drop compared to an equivalent tubular packed bed reactor. This increased catalytic performance is pursued extensively by investigating possible reaction pathways in three regions of the radial-flow reactor, leading to the significant discovery of a threefold pathway of syngas production on a single catalyst. To this end, it is shown that the excellent selectivities to H₂ and CO for high flow rates are due to the combination of partial oxidation, steam reforming, and dry reforming of C₄H₁₀, indicating one direct and two indirect reaction paths.     

NO reduction by C3H6 in excess oxygen over fresh and sulfated Pt‐ and Rh‐based catalysts

The selective catalytic reduction (SCR) of NO with C₃H₆ was studied over three noble‐metal‐based catalysts: 2% Pt/γ‐Al₂O₃, 2% Rh/γ‐Al₂O₃ and 1.5% Rh/TiO₂(4% WO₃). The SO₂ effect on the catalyst activity was examined using sulfated samples of the above catalysts and SO₂‐containing feeds. Temperature‐programmed desorption and oxidation studies were carried out to examine the adsorption characteristics of NO and C₃H₆, respectively, in the absence or the presence of SO₂. The adsorption data were linked to variations in the NO reduction rates over fresh and sulfated samples. Modification of the support surface as a result of the SO₂ presence affects the NO and propene sorption characteristics, the NO oxidation and the propene consumption rates. This revised version was published online in August 2006 with corrections to the Cover Date.