Effects of Pre-Treatment of a Silica-Supported Gallium Oxide Catalyst with H2 on Its Catalytic Performance for Dehydrogenation of Propane

The pre-treatment of a silica-supported gallium oxide catalyst with H₂ at 823 K increased the yield of aromatics and the selectivity to aromatics in the dehydrogenation of propane over the catalyst at 823 K. Gallium oxide in the catalyst was partially reduced with H₂ at 823 K. NH₃ desorption and DRIFTS studies on the gallium oxide catalyst suggest that the dehydrogenation of propane over a silica-supported gallium oxide catalyst would proceed in the following way: (1) the dehydrogenation of propane to produce propene would occur on Ga sites including Ga^δ+–H sites and (2) the aromatization of propene to aromatics on Ga–O–H acid sites.     

Effect of temperature on the product selectivity and aromatics distribution in aromatization of propane over H-GaAlMFI zeolite

The influence of temperature on the product selectivity, aromatics distribution, p-X/o-X and p-X/m-X product ratios, aromatization/cracking and aromatization/dehydrogenation ratios have been exhaustively investigated on H-GaAlMFI at different iso-conversions of propane. These studies show a profound influence of temperature on the product selectivity/distribution of the propane aromatization reaction. The aromatization activity of the H-GaAlMFI zeolite decreases at higher temperatures relative to the cracking and dehydrogenation activities. The benzene selectivity increases whereas the toluene and C₈ selectivity decreases with increasing reaction temperature. The product selectivity and the formation of xylene isomers are kinetically controlled.     

Structure and function of metal cations in light alkane reactions catalyzed by modified H-ZSM5

The rate of propane dehydrocyclodimerization to form C₆ aromatics is limited by a sequence of irreversible dehydrogenation reactions leading to propene, higher alkenes, dienes, trienes, and aromatics. Quasi-equilibrated acid—catalyzed cracking, oligometization, and cyclization reactions of alkene intermediates occur in sequence with these dehydrogenation reactions. Each dehydrogenation reaction is in turn limited by the rate of elementary steps that dispose of H-atoms formed in C-H bond activation steps. The rate of C-H bond activation, recombinative hydrogen desorption, and propane chemical conversion have been measured from the rates of isotopic redistribution and chemical conversion during reactions of C₃H₈/C₃/D₈and D₂/C₃/H₈ mixtures on H-ZSM5, Ga/H-ZSM5, and Zn/H-ZSM5. Isotopic studies show that C-H activation steps are fast during steady-state propane dehydrocyclodimerization on H-ZSM5, Ga/H-ZSM5, and Zn/H-ZSM5.

Ga and Zn species increase the rates of propane chemical conversion, recombinative hydrogen desorption, and deuterium incorporation from D₂into reaction products. Disposal of hydrogen formed in C-H bond activation steps occurs by transfer of H-atoms to unsaturated species to form alkanes or to Ga and Zn species, which catalyze the recombinative desorption of H-atoms to form dihydrogen (H₂). The sequential release of several H-atoms during a propane dehydrocyclodimerization turnover limits the rate and selectivity of this reaction on H-ZSM5.

In-situ X-ray absorption studies suggest that Ga and Zn species reside at cation exchange sites as monomeric cations and that recombinative desorption involve reduction—oxidation cycles of such cations during each dehydrocyclodimerization turnover. These monomeric species form directly during exchange of Zn ions from solution onto H-ZSM5. Ga³⁺species, however, do not exchange directly from solution onto H-ZSM5, but instead form extrazeolitic Ga₂O₃ crystals. Ion exchange occurs during subsequent contact with propane or hydrogen at 700-800 K via vapor phase exchange of volatile Ga¹⁺ species.     

Dehydrogenation of propane over a silica-supported vanadium oxide catalyst

The dehydrogenation of propane over a silica-supported vanadium oxide catalyst was investigated at 823 K under atmospheric pressure in the presence/absence of CO₂. The yield of propene and the selectivity to propene were higher in the dehydrogenation in the presence of CO₂ than those in the dehydrogenation in the absence of CO₂. On the other hand, the yield of aromatics and the selectivity to aromatics were much higher in the dehydrogenation in the absence of CO₂ than those in the dehydrogenation in the presence of CO₂. TPR measurements, NH₃ desorption studies and in-situ UV–vis studies on the catalyst were also performed to elucidate the effects of CO₂ on the behavior of the vanadium oxide in the catalyst during the dehydrogenation of propane.     

Differences between ZSM-5 and ZSM-11 zeolite catalysts in 1-hexene aromatization and isomerization

ZSM-5 and ZSM-11 zeolites with high crystallinity are synthesized and tested in the aromatization and isomerization reactions of 1-hexene at 370 °C in a continuous flow fixed bed. The results indicate that ZSM-5 and ZSM-11 zeolites possess similar acid site amount and strength, and most of the acid sites belong to Brønsted acid. When the ZSM-5 and ZSM-11 zeolites were used as catalysts, the aromatics selectivity over ZSM-11 catalyst was higher than that over ZSM-5 catalyst in contrast to i-paraffins selectivity, maybe attributed to that the C₇ and C₈ aromatics have an easier exit from the ZSM-11 zeolite. Moreover, the decrease of particle size can present superior aromatics selectivity and less i-paraffins selectivity in the aromatization and isomerization of 1-hexene over the ZSM-11 catalyst.     

Selective conversion of propane to propene by the catalytic oxidative dehydrogenation over cobalt and magnesium molybdates

Catalytic performances of various metal molybdates were tested in the oxidative dehydrogenation of propane to propene with molecular oxygen under an atmospheric pressure. Most of the molybdates tested promoted the selective oxidative conversion of propane to propene and among them cobalt and magnesium molybdates were found highest in the activity and selectivity. It was also found that their catalytic activities were highly sensitive to the catalyst composition, and it turned out that Co_0.95MoO _x and Mg_0.95MoO _x catalysts which have slightly excess molybdenum showed the highest activity in the oxidative dehydrogenation of propane. Under the optimized reaction conditions, higher reaction temperatures and lower partial pressures of oxygen, these catalysts gave 60% selectivity to propene at 20% conversion of propane. Since the molybdates having the surface enriched with molybdenum oxide tended to show high activity for the propane oxidation, surface molybdenum oxide clusters supported on metal molybdate matrix seem to be the active sites for the selective oxidative dehydrogenation of propane.     

Aromatization of n-Butane Over Supported Mo2C Catalysts

Similarly to the case of methane, ethane and propane, Mo₂C deposited on ZSM-5 significantly enhanced the aromatization of n-butane observed on ZSM-5 (SiO₂/Al₂O₃ ratio of 80) alone. The catalytic performance of Mo₂C/ZSM-5 sensitively depended on its preparation and pretreatment. The selectivity of aromatics measured for pure ZSM-5 increased from 11-13% to 28-34% at the conversion level of 60-65%. The formation of aromatics was also observed over Mo₂C/SiO₂.     

Effect of shock-wave loading on bismuth oxide-containing piezo- and ferroelectrics

The effect of shock-wave loading on bismuth and germanium oxides, their mechanical mixtures, and the compounds Bi₁₂GeO₂₀, Bi₄Ge₃O₁₂, and Bi₂GeO₅ in the form of single- and polycrystals was studied by methods of physicochemical analysis. It was established that the distribution of the elements Bi, Ge, and Fe is nonuniform because of displacement of layers from the surface into the bulk. In bismuth and germanium oxides after loading, the effect of admixtures (iron oxide and oxygen) and distortion of the crystal lattices were revealed. Under shock-wave loading, the starting components in a mixture with composition of 61 react to form the metastable Bi₂GeO₅ phase. A comparison of differential thermal analysis data of specimens on heating and cooling with metastable state diagrams makes it possible to distinguish between the effects of stuctural distortions and of admixtures on the properties of the materials after loading.Scientific-Research Physico-Technical Institute, Krasnoyarsk 660036. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 4, pp. 84–88, July–August, 1995.     

Product distribution in the aromatization of dilute ethene over H-GaAlMFI zeolite: effect of space velocity

Influence of space velocity on the aromatization of dilute ethene (5 mol% in N₂) over H-GaAlMFI zeolite catalyst, having high acidity (0.46 mmol g⁻¹, measured in terms of the pyridine chemisorbed at 400 °C) and high concentration of non-framework Ga-oxide species (0.32 mmol g⁻¹), at atmospheric pressure covering a wide temperature range (300–500 °C) has been thoroughly investigated. The selectivity of aromatics, propene, propane and C₄ hydrocarbons and alkane/aromatics and H₂/aromatics mole ratios are strongly influenced by the space velocity. The results indicate that the aromatization involves H₂ transfer reactions predominantly at the lower temperatures and/or higher space velocities whereas dehydrogenation reactions become predominant at higher temperatures and/or lower space velocities. The distribution of aromatics and C₈-aromatic isomers depends strongly upon the amount (i.e. yield) of aromatics and C₈-aromatics, respectively, formed in the process. The primary aromatics produced in the process are found to be mainly p- and o-xylenes. The aromatics distribution is, however, controlled by the aromatics inter-transformation (viz. isomerization, alkylation/dealkylation and disproportionation) reactions. The p-xylene/m-xylene ratio is decreased as expected, but the p-xylene/o-xylene ratio is increased with increasing both the space velocity and temperature. The increase of p-xylene/o-xylene ratio is found to be unusual, much above the equilibrium value.     

Influence of Indium Content on the Properties of Pt–Re/Al2O3 Naphtha Reforming Catalysts

The influence of indium on the properties of Pt–Re/Al₂O₃ catalysts used in naphtha reforming is studied. The addition of indium to the Pt–Re/Al₂O₃ catalyst produces a big decrease of acidity. It also produces an inhibition of the metal function, i.e., dehydrogenation and hydrogenolysis activity. The reaction of n-C₅ isomerization shows that indium addition decreases the total activity of the Pt–Re catalyst but increases the selectivity to the i-C₅ isomers. The selectivity to low cost light gases (C₁–C₃) is particularly decreased. The reaction of n-C₇ reforming showed that addition of indium increases the stability of the catalyst and the selectivity to aromatics, and decreases the production of light gases.     

Propane Reactions over Faujasite Structure Zeolites Type-X and USY: Effect of Zeolite Silica over Alumina Ratio, Strength of Acidity and Kind of Exchanged Metal Ion

In this work we prepared various zeolite-X and USY samples partially exchanged with copper, iron and platinum. These samples were characterized by XRD, Chemical Analysis, SEM-EDS, N₂-adsorption–desorption, ammonia-TPD, and tested as catalysts in high temperature (400 and 550 °C) propane transformation. The obtained results revealed the strong effect of Si/Al ratio in faujasite zeolite structure, the number and strength of acid sites and of the presence of different metal ions in countered ion sites, on the catalytic activity and selectivity of zeolite-X and USY. The highest propane dehydrogenation activity was achieved with the platinum-exchanged X zeolite (∼11.2% propylene yield, ∼31% selectivity). On the contrary USY zeolites showed high cracking capability and relatively low dehydrogenation activity excepting the platinum-exchanged sample which yielded notably high aromatization products.     

Nano-scale uniform distribution of Ge/Cu3Ge phase and its electrochemical performance for lithium-ion batteries

A germanium/copper germanide/carbon (Ge/Cu₃Ge/C) composite is prepared by the Pechini method using GeO₂, CuSO₄, and citric acid/ethylene glycol as the Ge, Cu and carbon sources, respectively. The microstructure and electrochemical properties of this nano-composite are compared to those of germanium/carbon (Ge/C) that is prepared without the copper precursor. In the latter composite, irregular-shaped submicron-sized (0.4-0.5 μm) Ge particles are dispersed inside a carbon matrix. In the former, however, sphere-shaped nano-sized (<100 nm) Ge particles are produced, inside of which a Cu₃Ge phase is uniformly dispersed in nano-scale. The pure Ge component in both electrodes is lithiated up to Li₁₅Ge₄ after a lithiation down to 0.0 V at a current density of 100 mA g⁻¹. The Cu₃Ge-containing electrode gives a superior cycle and rate performance to that of the Cu₃Ge-free counterpart. This difference has been ascribed to the favorable roles provided by the nano-sized Cu₃Ge phase that is intimately contacted with the nano-sized Ge particles within a sphere. The electrochemical dilatometry study reveals that the electrode swelling is much smaller for the Cu₃Ge-containing electrode, illustrating that the electrochemically inactive Cu₃Ge phase serves as a buffer against a massive volume change encountered in the Ge component.     

C3N Non-metallic Catalyst for Propane Dehydrogenation: A Density Functional Theory Study

In this work, density functional theory is used to study the mechanism of propane dehydrogenation over non-metallic C₃N catalyst. The structure, electrostatic potential and density of state of C₃N are introduced, as well as the adsorption of reactants on catalyst is studied. The propane dehydrogenation reaction is divided into the first dehydrogenation and the second dehydrogenation (deep dehydrogenation). We explore the possible dehydrogenation pathways in two-step dehydrogenation. The rate control step of the first dehydrogenation is the removal of methylene hydrogen atom from propane, and its energy barrier is 47.79 kcal/mol, which reflected the catalytic activity of the catalyst. The rate control step of deep dehydrogenation is the process of removing the first hydrogen atom of the product propylene to produce the by-product. The energy barrier is 72.80 kcal/mol, which is much larger than that of the first step of dehydrogenation, reflecting the excellent selectivity of the catalyst.

Graphic Abstract

     

Simulation of Propane Dehydrogenation to Propylene in a Radial‐Flow Reactor over Pt‐Sn/Al2O3 as the Catalyst

Catalytic paraffin dehydrogenation for manufacturing olefins is considered to be one of the most significant production routes in the petrochemical industries. A reactor kinetic model for the dehydrogenation of propane to propylene in a radial‐flow reactor over Pt‐Sn/Al₂O₃ as the catalyst was investigated here. The model showed that the catalyst activity was highly time dependent. In addition, the component concentrations and the temperature varied along the reactor radius owing to the occurring endothermic reaction. Moreover, a similar trend was noticed for the propane conversion as for the propylene selectivity, with both of them decreasing over the time period studied. Furthermore, a reversal of this trend was also revealed when the feed temperature was enhanced or when argon was added into the feed as an inert gas.     

Pure Hydrogen and Propylene Coproduction in Catalytic Membrane Reactor-Assisted Propane Dehydrogenation

Propane dehydrogenation on a commercial Pt-Sn/Al₂O₃ catalyst in a Pd-Ag membrane reactor is considered. A mathematical model is developed to evaluate the performance of the catalytic membrane reactor for the process of propane dehydrogenation. Design and operating conditions are systematically evaluated for key performance metrics such as propane conversion, propylene selectivity, hydrogen selectivity, and hydrogen recovery under different operating conditions. The results confirm that the high performance of the membrane reactor is related to the continuous removal of hydrogen from the reaction zone to shift the reaction equilibrium towards the formation of more propylene and hydrogen.     

The effect of intermetallic hydrogen acceptor on catalytic aromatization of propane

The effect of the hydride-forming intermetallic compound Zr₂Fe on the aromatization of propane over high-silica zeolites of CVM type (Russian equivalent of ZSM-5) modified by Zn, Ga or Pt cations has been investigated. Aromatics yield and selectivity of aromatization are shown to increase essentially as a result of releasing hydrogen elimination by the intermetallic acceptor. The effect of hydrogen acceptor on propane conversion and product distribution appeared to be different depending on the composition of the catalyst used. Possible changes in the reaction mechanism in hydrogen removal conditions are discussed.     

Conversion of light paraffins for preparing small olefins over ZSM-5 zeolites

The conversion of C₃-C₉ paraffins to small olefins over ZSM-5 zeolite is investigated. The small olefins are primary products and are usually converted into other more stable secondary products such as aromatics on the ZSM-5 zeolites. Thermally treated HZSM-5, K/HZSM-5 and Ba/HZSM-5 catalysts were developed and favourable oxidative conditions were introduced for the conversion process to maximize selective conversion of light paraffins to small olefins at the relatively low temperature of 873 K. The role of K and Ba is to minimize bimolecular hydrogen transfer reactions and enhance the dehydrogenation activity of the catalysts. Meanwhile, the oxygen in the gas phase is effective to improve the olefin selectivity and yield. C₂-C₄ olefin selectivities of 70.4 and 66.8% have been obtained for propane andn-hexane feed-stocks, respectively, at a temperature of 873 K.     

Selective oxidation of ethane and propane over sulfated zirconia‐supported nickel oxide catalysts

The oxidative dehydrogenations of ethane and propane were investigated over a series of zirconia and nickel‐oxide supported on zirconia catalysts. It was found that zirconia, sulfated zirconia as well as NiO‐based zirconia catalysts showed high catalytic activities for oxidative dehydrogenation of ethane and propane. However, conversion and selectivity differed depending on the nature of the catalysts. Zirconia, sulfated zirconia (SZ) and their supported NiO catalysts showed high ethane conversions but lesser selectivities to olefins while NiO/Li₂ZrO₃ exhibited high selectivities to ethylene and propylene. Addition of an LiCl promoter in the NiO/SZ catalyst increased the catalytic activity and olefin selectivity, thus resulting in a higher olefin yield. In the oxidative dehydrogenations of ethane and propane NiO–LiCl/SZ exhibited 79% ethylene selectivity at 93% ethane conversion at 650 °C and 52% selectivity to propylene at 20% propane conversion at 600 °C, respectively. Characterization showed that the physico‐chemical properties of the catalysts determine the catalytic activity and selectivity. © 2001 Society of Chemical Industry     

Initial activity/selectivity of H-gallosilicate (MFI) in propane aromatization: influence of H+ exchange and thermal/hydrothermal pretreatments

Initial activity/selectivity of H-gallosilicate (MFI) zeolite with different degrees of H⁺ exchange and pretreated under different thermal and hydrothermal conditions in propane aromatization (at 500C) has been determined using a pulse microreactor connected to GC. It is found to be strongly influenced by the degree of H⁺ exchange, calcination temperature and hydrothermal treatment at different temperatures and concentrations of steam. There exists a close relationship between the acidity (measured in terms of pyridine chemisorbed at 400 C) of the gallosilicate and its initial propane conversion and aromatization activity. Presence of strong acidic sites (attributed to FW Ga) at high concentration is essential for the well dispersed non-FW Ga oxide species to be active for dehydrogenation in the propane aromatization over the zeolite.     

Influence of the different dechlorination time on catalytic performances of PtSnNa/ZSM-5 catalyst for propane dehydrogenation

Catalytic dehydrogenation of propane has recently received considerable attention because of the increasing demand for propene. Among several catalysts, PtSnNa/ZSM-5 catalyst is one of the most suitable ones. In this study, PtSnNa/ZSM-5 catalysts with different content of chlorine were prepared by changing the time of catalyst dechlorination. The obtained catalysts were characterized by X-ray fluorescence (XRF), XRD, nitrogen adsorption, ²⁷Al MAS NMR, NH₃-TPD, H₂ chemisorption and TPR. It was found that with the increase of treatment time, more framework aluminum atoms were removed from tetrahedral positions, leading to the loss of Sn species and the decrease of catalyst acidity. Meantime, the porous properties and the interactions between Pt and Sn of the catalysts changed remarkably, which was disadvantageous to the reaction. Compared with the dechlorinated catalysts, the fresh sample with suitable content of chlorine exhibited the best reaction activity and stability. The average yield of propene was about 30.4% over 45 h for the reaction of propane dehydrogenation at 590 °C. Finally, a model was proposed for the influence of dechlorinated treatment on catalytic properties of PtSnNa/ZSM-5 catalyst for propane dehydrogenation.