Activities of γ-Al2O3-Supported Metal Oxide Catalysts in Propane Oxidative Dehydrogenation

The activities of metal oxide catalysts in propane oxidative dehydrogenation to propene have been studied. The catalysts are M/-Al₂O₃ (where M is an oxide of Cr, Mn, Zr, Ni, Ba, Y, Dy, Tb, Yb, Ce, Tm, Ho or Pr). Both transition metal oxides (TMO) and rare-earth metal oxides (REO) are found to catalyze the reaction at 350-450 °C, 1 atm and a feed rate of 75 cm³/min of a mixture of C₃H₈, O₂ and He in a molar ratio of 4:1:10. Among the catalysts, Cr-Al-O is found to exhibit the best performance. The selectivity to propene is 41.1% at 350 °C while it is 54.1% at 450 °C. Dy-Al-O has the highest C₃H₆ selectivity among the REO. At 450 °C, the other catalysts show C₃H₆ selectivity ranging from 16.2 to 37.7%. In general TMO show higher C₃H₆ selectivity than REO, which, however, show higher C₂H₄ selectivity. An attempt is made to correlate propane conversion and selectivity to C₃H₆ with metal-oxygen bond strength in the catalysts. For the TMO a linear correlation is found between the standard aqueous reduction potential of the metal cation of the respective catalyst and its selectivity to propane at 11% conversion. No such correlation has been found in the case of REO. Analyses of the product distributions suggest that for TMO propane activation the redox mechanism seems to prevail while the REO activate it by adsorbed oxygen.     

Numerical simulation of particle/monolithic two-stage catalyst bed reactor with beds-interspace distributed dioxygen feeding for oxidative coupling of methane

A three-dimensional geometry model of the particle/monolithic two-stage reactor with beds-interspace distributed dioxygen feeding for oxidative coupling of methane (OCM) was set up. The improved Stansch kinetic model adapting different operating temperatures was established to calculate the OCM reactor performance using computational fluid dynamics (CFD) and FLUENT software. The results showed that the calculated values matched well with the experimental values of the conversion of CH₄ and the selectivity of products (C₂H₆, C₂H₄, CO₂, CO) in the OCM reactor. The distributed dioxygen feeding with the percentage of 5–20% based oxygen flow rate of top inlet promoted the OCM reaction in monolithic catalyst bed and led to the conversion of CH₄ and the selectivity and yield of C₂ (C₂H₆ and C₂H₄) increase obviously. The distributed dioxygen feeding was 15%, the conversion of CH₄, the selectivity and the yield of C₂ reached 34.1%, 68.2% and 23.3%, respectively.     

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.     

Characterization of Ti/Si binary oxides prepared by the sol-gel method and their photocatalytic properties: The hydrogenation and hydrogenolysis of CH3CCH with H2O

Titanium-silicon (Ti/Si) binary oxides having different Ti content were prepared by the sol-gel method and utilized as photocatalysts for the hydrogenation and hydrogenolysis of CH₂CCH with H₂O. The photocatalytic reactivity and selectivity of these catalysts were investigated as a function of the Ti content and it was found that the hydrogenolysis reaction (C₂H₆ formation) was predominant in regions of low Ti content, while the hydrogenation reaction (C₃H₆ formation) proceeded in regions of high Ti content. The in situ photoluminescence, diffuse reflectance absorption, FT-IR, XAFS (XANES and EXAFS), and XPS spectroscopic investigations of these Ti/Si binary oxides indicated that the titanium oxide species are highly dispersed in the SiO₂ matrices and exist in a tetrahedral coordination exhibiting a characteristic photoluminescence spectrum. The charge transfer excited state of the tetrahedrally coordinated titanium oxide species plays a significant role in the efficient photoreaction with a high selectivity for the hydrogenolysis of CH₃CCH to produces mainly C₂H₆ and CH₄, while the catalysts involving the aggregated octahedrally coordinated titanium oxide species show a high selectivity for the hydrogenation of CH₃CCH to produce C₃H₆, being similar to reactions of the powdered TiO₂ catalysts. The good parallel relationship between the yield of the photoluminescence and the specific photocatalytic reactivity of the Ti/Si binary oxides as a function of the Ti content clearly indicates that the high photocatalytic reactivity of the Ti/Si binary oxides having low Ti content is associated with the high reactivity of the charge transfer excited state of the isolated titanium oxide species in tetrahedral coordination, [Ti³⁺-O⁻]*.     

Correlation Between the Characteristics and Catalytic Performance of Ni–V–O Catalysts in Oxidative Dehydrogenation of Propane

Ni–V–O series catalysts for the oxidative dehydrogenation (ODH) of propane were prepared and characterized by BET, XRD, H₂-TPR, O₂-TPD-MS and electrical conductivity. At 425°C a C₃H₆ selectivity of 49.9% was observed on Ni_0.9V_0.1O _Y at a C₃H₈ conversion of 19.4%, and the obtained selectivity is almost two times higher than that over NiO at the roughly same conversion of C₃H₈. The mobile oxygen species created by the interaction of NiO and V₂O₅ has been found in the composite catalysts by O₂-TPD-MS and electrical conductivity studies, which seems to be responsible for the enhanced selectivity of the propane oxidative dehydrogenation.     

The effect of alkali promoters in oxidative coupling of methane with Mn-oxide catalysts

The catalytic oxidalive coupling of metnane to ethylene and ethane with manganese oxide catalysts promoted with alkali metal and alkali metallic-chloride has been studied at atmospheric pressure in a fixed bed flow reactor. The main studies of reaction were carried out over maganese oxide catalysts promoted with sodium chloride and the structure and surface morphology of these catalysts was characterized by an X-ray diffraction and a scanning electron microscope. The powdered MnO₂ was changed into Mn₂O₃, and MnO₂ containing alkali metallic-chlorides was not changed to new ternary oxides but changed into Mn₃O₄ and/or Mn₂O₃ at higher calcination temperature(above 780°C). The optimum content of NaCl promoted was 10–20wt%, an in over 10wt%, the conversion and the selectivity were kept constant. The main factor on deactivation of catalysts was the loss of thepromoter(NaCl). The addition of alkali metal salts to manganese oxide catalyst has enhanced C₂(C₂H₄ + C₂H₆) selectivity due to neutralizing acid sites more than the electronic factor. It was confirmed that chlorine in alkali metallicchloride has enhanced the formation of C₂H₄, resulting in a good C₂-yield (up to 25.7%).     

Slurry-phase CO2 hydrogenation to hydrocarbons over a precipitated Fe-Cu-Al/K catalyst: Investigation of reaction conditions

The hydrogenation of CO₂ to hydrocarbons over a precipitated Fe-Cu-Al/K catalyst was studied in a slurry reactor for the first time. Reducibility of the catalyst and effect of reaction variables (temperature, pressure and H₂/CO₂ ratio of the feed gas) on the catalytic reaction performance were investigated. The reaction results indicated that the Fe-Cu-Al/K catalyst showed a good CO₂ hydrogenation performance at a relatively low temperature (533 K). With the increase of reaction temperature CO₂ conversion and olefin to paraffin (O/P) ratio in C₂-C₄ hydrocarbons as well as the selectivity to C₂-C₄ fraction increased, while CO and CH₄ selectivity showed a reverse trend. With the increase in reaction pressure, CO₂ conversion and the selectivity to hydrocarbons increased, while the CO selectivity and O/P ratio of C₂-C₄ hydrocarbons decreased. The investigation of H2/CO₂ ratio revealed that CO₂ conversion and CH₄ selectivity increased while CO selectivity and O/P ratio of C₂-C₄ decreased with increasing H₂/CO₂ ratio.     

Promotion effect of K2O and MnO additives on the selective production of light alkenes via syngas over Fe/silicalite-2 catalysts

The addition of K₂O and MnO promoters enhances catalyst activity and selectivity to light alkenes during CO hydrogenation over silicate-2 (Si-2) supported Fe catalysts. The results of CO hydrogenation and CO-TPD, CO/H₂-TPSR, C₂H₄/H₂-TPSR and C₂H₄/H₂ pulse reaction over Fe/Si-2 catalysts with and without promoters clearly show that the MnO promoter mainly prohibits the hydrogenation of C₂H₄ and C₃H₆. Therefore, it enhances the selectivity to C₂H₄ and C₃H₄ products. Meanwhile further incorporating the K₂O additive into the FeMn/ Si-2 catalyst leads to a remarkable increase in both the capacity and strength of the strong CO adspecies. These produce much more [C_ad] via their dissociation and disproportionation at higher temperatures. This results in an increase in the CO conversion and the selectivity to light olefins. Moreover, the K₂O additive modifies the hydrogenating reactivity of [C_ad] and suppresses the disproportionation of C₂H₄ that occurs as a side-reaction. Both K₂O and MnO promoters play key roles for enhancing the selective production of light alkenes from CO hydrogenation over Fe/Si-2 catalyst.     

Role of chlorine in improving selectivity in the oxidative coupling of methane to ethylene

Samarium, magnesium and manganese oxide and alkali-promoted oxide catalysts have been prepared and tested for the oxidative coupling of methane. The results show that alkali-promoted oxides inhibit total oxidation and have a higher selectivity for the formation of C₂ products than the undoped metal oxides. These catalysts have been promoted by injecting pulses of gaseous chlorinated compounds (dichloromethane and chloroform) during the reaction. It has been found that these chlorinated compounds markedly increase the selectivity for the formation of C₂ products for all the MnO₂-based catalysts and for lithium-doped MgO and Sm₂O₃ catalysts. The effect is greatest in MnO₂-based catalysts. When dichloromethane is added to a pure, unpromoted MnO₂ catalyst the selectivity for the formation of carbon dioxide decreases from 82.6% to 4.1% and the selectivity for the formation of C₂H₄ increases from virtually zero to 56.3%. The highest C₂ selectivity observed after promotion of pure MnO₂ by dichloromethane is about 93%. Promotion of these pure oxide catalysts by gaseous chlorinated compounds provides an alternative to alkali promotion as a method of inhibiting total oxidation and of increasing ethylene production.     

Alkene selectivity enhancement in the oxidation of propane on calcium-based catalysts

The oxidation of propane has been investigated in the presence and absence of tetrachloromethane (TCM) on calcium hydroxyapatite (CaHAp), Ca₃(PO₄)₂, CaSO₄ and CaO at 723 K. In the absence of TCM, the conversion of C₃H₈ on CaHAp was 7.7–9.2% during 6 h on-stream while that on Ca₃(PO₄)₂, CaSO₄ and CaO was 0.6, 0 and 0.2–0.4%, respectively. The principal products on all catalysts in the absence of TCM were CO and CO₂ with small selectivities to C₃H₆ and C₂H₄ (both 5–6%) observed on CaHAp. Upon addition of TCM, the selectivity to C₃H₆ on all catalysts and the conversion of C₃H₈ on CaSO₄ increased while, with increasing time-on-stream, the changes in the conversion and selectivity were dependent upon the nature of the catalysts. XPS and XRD analyses provide evidence for the presence of chlorine in the surface and/or bulk of three of the catalysts, suggesting that chlorinated species on the solids play a role in the selectivity enhancement, but the absence of chlorine from the sulphate demonstrates the dissimilarities of the catalysts in their abilities to sorb and decompose TCM. This revised version was published online in July 2006 with corrections to the Cover Date.     

Regeneration behaviors of Fe/Si‐2 and Fe–Mn/Si‐2 catalysts for C2H6 dehydrogenation with CO2 to C2H4

The catalytic performance of Fe/Si‐2 and Fe–Mn/Si‐2 catalysts for conversion of C₂H₆ with CO₂ to C₂H₄ was examined in a continuous‐flow and fixed‐bed reactor. The results show that the Fe–Mn/Si‐2 catalyst exhibits much better reaction activity and selectivity to C₂H₄ than those of the Fe/Si‐2 catalyst. Furthermore, the coking–decoking behaviors of these catalysts were studied through TG. The catalytic performances of the catalysts after regeneration for conversion of C₂H₆ or dilute C₂H₆ in FCC off‐gas with CO₂ to C₂H₄ were also examined. The results show that both activity and selectivity of the Fe–Mn/Si‐2 catalyst after regeneration reached the same level as those of the fresh catalyst, whereas it is difficult for the Fe/Si‐2 catalyst to refresh its reaction behavior after regeneration. This revised version was published online in July 2006 with corrections to the Cover Date.     

Methane conversion to higher hydrocarbons in a dielectric-barrier discharge reactor with Pt/γ-Al2O3 catalyst

Plasma catalytic reactions were applied to the conversion of methane to C₂, C₃ or higher hydrocarbons in a dielectric-barrier discharge (DBD) reactor at atmospheric pressure. Methane conversion was increased with the increase of Pt loading on γ-Al₂O₃. The highest C₂H₆ selectivity was 50.3% when 3 wt% Pt/γ-Al₂O₃ catalyst was calcined at 573 K. Methane conversion was increased with the increase of the catalyst weight in DBD reactor. The major products were C₂H₆ and C₃H₈, which were independent of catalyst weight in the presence of catalyst.     

Selective N-propargylation of imidazole under microwave irradiation using some magnesium oxides as catalysts

The alkylation of imidazole (C₃H₄N₂) with propargyl bromide (C₃H₃Br) using some magnesium oxides as catalysts under microwave irradiation was carried out with a high activity and selectivity. It has been found that it is possible to prepare efficiently N-alkyl derivatives of heterocycles under microwave irradiation in absence of solvent. Under these experimental conditions N-alkylated heterocycles can be obtained with higher selectivities than when using other basic media.     

Epoxidation of propylene by air over modified silver catalyst

Epoxidation of C₃H₆ to C₃H₆O by air was studied over a silver catalyst modified with alkali or alkaline earth chloride salts. The catalyst preparation factors and the operational conditions could affect obviously the catalytic epoxidation property of the silver catalyst. It was shown that, as a promoter of the silver catalyst, NaCl or BaCl₂ is more suitable than LiCl or NH₄Cl. The loading of NaCl should be controlled at about 3.8 wt%. Using a feed gas of 10% C₃H₆/air at a space velocity of 1.75×10⁴ h⁻¹, 18.6% C₃H₆ conversion and 33.4% selectivity to C₃H₆O were obtained at 350°C. Using a feed gas of 5% C₃H₆/air at a space velocity of 2.4×10⁴ h⁻¹, 54.0% C₃H₆ conversion and 26.3% selectivity to C₃H₆O were obtained at 390°C. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of synthesis gas composition on the Fischer–Tropsch synthesis over Co/γ-Al2O3 and Co–Re/γ-Al2O3 catalysts

The Fischer–Tropsch synthesis over Co/γ-Al₂O₃ and Co–Re/γ-Al₂O₃ was investigated in a fixed-bed reactor at 20 bar and 483 K using feed gases with molar H₂/CO ratios of 2.1, 1.5 and 1.0 simulating synthesis gas derived from biomass. With lower H₂/CO ratios in the feed, the CO conversion and the CH₄ selectivity decreased, while the C₅₊ selectivity and olefin/paraffin ratio for C₂–C₄ increased slightly. The water–gas shift activity was low for both catalysts, resulting in high molar usage ratios of H₂/CO (close to 2.0), even at the lower inlet ratios (i.e. 1.5 and 1.0). For both catalysts, the drop in the production rate of hydrocarbons when shifting from an inlet ratio of 2.1 to 1.5 was significant mainly because the H₂/CO usage ratio did not follow the change in the inlet ratio. The hydrocarbon selectivities were rather similar for inlet H₂/CO ratios of 2.1 and 1.5, while significantly deviating from those for an inlet ratio of 1.0. With the studied catalysts, it is possible to utilize the advantages of an inlet ratio of 1.0 (higher selectivity to C₅₊, lower selectivity to CH₄, no water–gas shifting of the bio-syngas needed prior to the FT reactor) if a low syngas conversion is accepted.     

Propane Oxidative Dehydrogenation over Metal Pyrophosphates Catalysts

Metal pyrophosphates (M–P₂O₇, where M is V, Zr, Cr, Mg, Mn, Ni or Ce) have been found to catalyze the oxidative dehydrogenation of propane to propene. The reaction was conducted at 1 atm, 450–550°C and feed flowrate of 75 cm³/min (20 cm³/min propane, 5 cm³/min oxygen and the balance is helium). All catalysts showed increase in degrees of conversion and decrease in olefins selectivity with increase in reaction temperature. At 550°C, MnP₂O₇ exhibited the highest activity (40.7% conversion) and total olefins (C₃H₆ and C₂H₄) yield (29.3%). The other catalysts, indicated by their respective metals, may be ranked (based on olefins yield) as V (16.9%) < Cr (17.5%) < Ce (25.1%) < Zr (26.2%) < Ni (26.8%) < Mg (27.9%). The reactivity of the lattice oxygen was estimated from energy of formation of the corresponding metal oxides. Correlation between the selectivity to propene and the standard energy of formation was attempted. Although there was no clear correlation, the result suggested that the lattice oxygen play a key role in the selectivity-determining step.     

Effect of Vanadium Promotion on Activated Carbon-Supported Cobalt Catalysts in Fischer–Tropsch Synthesis

The effect of vanadium promotion on activated carbon (AC)-supported cobalt catalysts in Fischer–Tropsch synthesis has been studied by means of XRD, TPR, CO-TPD, H₂-TPSR of chemisorbed CO and F-T reaction. It was found that the CO conversion could be significantly increased from 38.9 to 87.4% when 4 wt.% V was added into Co/AC catalyst. Small amount of vanadium promoter could improve the selectivity toward C₁₀–C₂₀ fraction and suppress the formation of light hydrocarbon. The results of CO-TPD and H₂-TPSR of adsorbed CO showed that the addition of vanadium increased the concentration of surface-active carbon species by enhancing CO dissociation and further improved the selectivity of long chain hydrocarbons. However, excess of vanadium increased methane selectivity and decreased C₅⁺ selectivity.     

Modeling the oxidative coupling of methane using artificial neural network and optimizing of its operational conditions using genetic algorithm

The effect of some operating conditions such as temperature, gas hourly space velocity (GHSV), CH₄/O₂ ratio and diluents gas (mol% N₂) on ethylene production by oxidative coupling of methane (OCM) in a fixed bed reactor at atmospheric pressure was studied over Mn/Na₂WO₄/SiO₂ catalyst. Based on the properties of neural networks, an artificial neural network was used for model developing from experimental data. To prevent network complexity and effective data input to the network, principal component analysis method was used and the number of output parameters was reduced from 4 to 2. A feed-forward back-propagation network was used for simulating the relations between process operating conditions and those aspects of catalytic performance including conversion of methane, C₂ products selectivity, C2 yielding and C₂H₄/C₂H₆ ratio. Levenberg-Marquardt method is presented to train the network. For the first output, an optimum network with 4-9-1 topology and for the second output, an optimum network with 4-6-1 topology was prepared. After simulating the process as well as using ANNs, the operating conditions were optimized and a genetic algorithm based on maximum yield of C₂ was used. The average error in comparing the experimental and simulated values for methane conversion, C₂ products selectivity, yield of C₂ and C₂H₄/C₂H₆ ratio, was estimated as 2.73%, 10.66%, 5.48% and 10.28%, respectively.     

Modelling and optimization of catalytic-dielectric barrier discharge plasma reactor for methane and carbon dioxide conversion using hybrid artificial neural network—genetic algorithm technique

A hybrid artificial neural network-genetic algorithm (ANN-GA) was developed to model, simulate and optimize the catalytic-dielectric barrier discharge plasma reactor. Effects of CH₄/CO₂ feed ratio, total feed flow rate, discharge voltage and reactor wall temperature on the performance of the reactor was investigated by the ANN-based model simulation. Pareto optimal solutions and the corresponding optimal operating parameter range based on multi-objectives can be suggested for two cases, i.e., simultaneous maximization of CH₄ conversion and C₂₊ selectivity (Case 1), and H₂ selectivity and H₂/CO ratio (Case 2). It can be concluded that the hybrid catalytic-dielectric barrier discharge plasma reactor is potential for co-generation of synthesis gas and higher hydrocarbons from methane and carbon dioxide and performed better than the conventional fixed-bed reactor with respect to CH₄ conversion, C₂₊ yield and H₂ selectivity.     

Halogenated La1.6Sr.04CuO4 catalysts active for ethane selective oxidation to ethene

The catalytic performances and characterization of the catalysts La_1.6Sr_0.4CuO_3.852, La_1.6Sr_0.4CuO_3.857F_0.143, and La_1.6Sr_0.4 CuO_3.856Cl_0.126 have been investigated for the oxidative dehydrogenation of ethane (ODE) to ethene. X‐ray diffraction results indicated that the three catalysts have a single‐phase tetragonal K₂NiF₄-type structure. The incorporation of fluoride or chloride ions in the La_1.6Sr_0.4CuO_4-δ lattice can significantly enhance C₂H₆ conversion and C₂H₄ selectivity. We observed 83.2% C₂H₆ conversion, 76.7% C₂H₄ selectivity, and 63.8% C₂H₄ yield over La_1.6Sr_0.4CuO_3.857F_0.143> and 79.6% C₂H₆ conversion, 74.6% C₂H₄ selectivity, and 59.4% C₂H₄ yield over La_1.6Sr_0.4CuO_3.856Cl_0.126 under the reaction conditions of C₂H₆/O₂/N₂ molar ratio 2/1/3.7, temperature 660°C, and space velocity 6000 ml h^-1 g^-1. With the rise in space velocity, C₂H₆ conversion decreased, whereas C₂H₄ selectivity increased. Life studies showed that the two catalysts were durable within 60 h of on‐stream ODE reaction. Based on the results of X‐ray photoelectron spectroscopy, O₂ temperature-programmed desorption, and C₂H₆ and C₂H₆/O₂/N₂ (2/1/3.7 molar ratio) pulse studies, we conclude that (i) the inclusion of halide ions in the La_1.6Sr_0.4CuO_4δ lattice could promote lattice oxygen mobility, and (ii) the O^- species accommodated in oxygen vacancies and desorbed below 600°C favor ethane complete oxidation whereas the lattice oxygen species desorbed in the 600–700°C range are active for ethane selective oxidation to ethene. By regulating the oxygen vacancy density and Cu³/Cu ratio in the K₂NiF₄-type halo-oxide catalyst, one can generate a durable catalyst with good performance for the ODE reaction. This revised version was published online in July 2006 with corrections to the Cover Date.