Partial oxidation of ethane over K2MoO4/SiO2 and potassium promoted MoO3/SiO2 catalyst

Silica supported K₂MoO₄ and potassium-promoted MoO₃ were used as catalysts for the partial oxidation of ethane in fix-bed continuous-flow reactor at 770–823 K using N₂O as oxidant. The main products of the oxidation reaction were ethylene, acetaldehyde, CO and CO₂. Addition of various compounds of potassium to the MoO₃/SiO₂ greatly enhanced the conversion of ethane and influenced the product distribution. The highest rate and selectivity for acetaldehyde formation was found on a K₂MoO₄/SiO₂ catalyst.     

Kinetic modeling of oxidative dehydrogenation of propane with CO2 over MoOx/La2O3–Al2O3 in a fluidized bed

A 4-step kinetic model of CO₂-assisted oxidative dehydrogenation (ODH) of propane to C₂/C₃ olefins over a novel MoO_x/La₂O₃–γAl₂O₃ catalyst was developed. Kinetic experiments were conducted in a CREC Riser Simulator at various reaction temperatures (525–600 °C) and times (15–30 s). The catalyst was highly selective towards propylene at all combinations of the reaction conditions. Langmuir-Hinshelwood type kinetics were formulated considering propane ODH, uni- and bimolecular cracking of propane to produce a C₁-C₂ species. It was found that the one site type model adequately fitted the experimental data. The activation energy for the formation of propylene (67.8 kJ/mol) is much lower than that of bimolecular conversion of propane to ethane and ethylene (303 kJ/mol) as well as the direct cracking of propane to methane and ethylene (106.7 kJ/mol). The kinetic modeling revealed the positive effects of CO₂ towards enhancing the propylene selectivity over the catalyst.     

Partial oxidation of methane over silica supported molybdenum oxide catalysts

Two kinds of MoO₃/SiO₂ catalysts, MoO₃-I and MoO₃-S, were prepared by impregnation and sol-gel method, respectively. When MoO₃ loading was increased, formation of MoO₃ crystals was observed to begin at a MoO₃ loading of 8 and 16 wt% with MoO₃-I and MoO₃-S, respectively. The highest yield of formaldehyde from methane oxidation was attained also at those critical values of MoO₃ loading of 8 and 16 wt% over MoO₃-I and MoO₃-S, respectively. It is suggested that the active species for formaldehyde formation is well dispersed molybdenum oxide clusters on SiO₂ support: the optimum dispersion of the clusters affords the highest activity for formaldehyde formation.     

The molybdenum species of MoO3/SiO2 and their catalytic activities for the epoxidation of propylene with cumene hydroperoxide

The MoO₃/SiO₂ catalysts containing different surface molybdenum species were prepared by a sol–gel method, and the effects of the preparation condition and MoO₃ loading on the surface molybdenum species and property of MoO₃/SiO₂ were studied. The XRD, FT-IR, UV–vis and Raman spectroscopies were used to characterize the surface molybdenum species, and temperature-programmed desorption of NH₃ adsorbed on a catalyst was used to detect the surface acidic properties. The results show that, there were the dispersed polymolybdate, α-MoO₃, β-MoO₃, monomeric molybdenum species and silicomolybdic acid on the MoO₃/SiO₂ catalyst, and their distributions and subsistence states were affected by the preparation condition and MoO₃ loading. Different molybdenum species exhibit different catalytic activities for the epoxidation of propylene with cumene hydroperoxide. In the 15 wt% MoO₃/SiO₂ catalyst synthesized at pH 9.1 and dried appropriately, there are the small size β-MoO₃ and monomeric molybdenum species that they are mainly effective catalyst components for the epoxidation of propylene. Using this catalyst, the ～100% conversion of cumene hydroperoxide and ～100% selectivity to propylene oxide can be obtained in the tert-butyl alcohol solvent at 2.6 MPa and 80 °C for 4 h.     

Surface-Initiated Gas-Phase Epoxidation of Propylene with Molecular Oxygen by Silica-Supported Molybdenum Oxide: Effects of Addition of C3H8 or NO and Reactor Design

The gas-phase epoxidation of propylene was studied over MoO _x /SiO₂ catalysts in a reaction system with a post-catalytic bed volume. In the reaction of a mixture of propylene and propane with oxygen below 578 K, propylene oxide (PO) was mainly formed from the oxidation of propylene. It was found that the oxidation reaction was very sensitive to the temperature of the post-catalytic space more than the temperature of the catalyst bed, strongly indicating that radical reactions occurring in the post-catalytic bed free space were responsible for the PO formation. The addition of NO increased propylene conversions and PO selectivity at low conversions, confirming that radical reactions were involved in the propylene reactions.     

Selective Oxidation of Ethane Over Mo–V–Al–O Oxide Catalysts: Insight to the Factors Affecting the Selectivity of Ethylene and Acetic Acid and Structure-activity Correlation Studies

Catalysts of general formula, MoVAlO _x were prepared with the initial elemental composition of 1:0.34:0.167 (Mo:V:Al) at a pH value in the range of 1–4. The elemental analysis showed that the final composition of the catalysts is pH dependant. The performance of the catalysts was tested for selective oxidation of ethane to give ethylene and acetic acid. While all of them were active for ethane oxidation with a moderate conversion, the catalyst prepared at pH 2 showed a highest activity with 23% ethane conversion and a combined selectivity of 80.6% to ethylene and acetic acid. The catalyst prepared at pH 4 was least selective to ethylene and acetic acid. Various techniques like powder XRD, SEM, Raman, UV–Vis and EPR were used to characterize the catalysts and to identify the active phases responsible for the selective oxidation of ethane. The powder XRD data showed that the catalysts prepared at pH 1 and 2 contain mainly of MoO₃ and MoV₂O₈ along with traces of Mo₄O₁₁. The amount of MoO₃ was slightly higher in the catalyst prepared at pH 1. However, the catalyst prepared at pH 3 contains mainly of MoV₂O₈ with no trace of MoO₃. The catalyst prepared at pH 4 showed V₂O₅ as the major phase along with MoVAlO₄ phase. The Raman data corroborated the XRD results. EPR and UV–Vis studies indicated the presence of traces of V⁴⁺ in pH 1 and 2 catalysts and significant amount of Mo⁵⁺ in all the catalysts. Thus, the high activity and selectivity to ethylene and acetic acid are attributed to the presence of MoV₂O₈ phase and other reduced species like Mo₄O₁₁ phase supported on MoO₃. The presence of V and Mo ions in a partially reduced form seems to play a crucial role in the selective oxidation of ethane.     

A high propylene productivity over B2O3/SiO2@honeycomb cordierite catalyst for oxidative dehydrogenation of propane

Boron-based metal-free catalysts for oxidative dehydrogenation of propane (ODHP) have drawn great attention in both academia and industry due to their impressive activity and olefin selectivity. Herein, the SiO₂ and B₂O₃ sequentially coated honeycomb cordierite catalyst is designed by a two-step wash-coat method with different B₂O₃ loadings (0.1%–10%) and calcination temperatures (600, 700, 800 °C). SiO₂ obtained by TEOS hydrolysis acts as a media layer to bridge the cordierite substrate and boron oxide via abundant SiOH groups. The well-developed straight channels of honeycomb cordierite make it possible to carry out the reactor under high gas hourly space velocity (GHSV) and the thin wash-coated B₂O₃ layer can effectively facilitate the pore diffusion on the catalyst. The prepared B₂O₃/SiO₂@HC monolithic catalyst exhibits good catalytic performance at low boron oxide loading and achieves excellent propylene selectivity (86.0%), olefin selectivity (97.6%, propylene and ethylene) and negligible CO₂ (0.1%) at 16.9% propane conversion under high GHSV of 345,600 ml·(g B₂O₃)⁻¹·h⁻¹, leading to a high propylene space time yield of 15.7 g C₃H₆·(g B₂O₃)⁻¹·h⁻¹ by suppressing the overoxidation. The obtained results strongly indicate that the boron-based monolithic catalyst can be properly fabricated to warrant the high activity and high throughput with its high gas/surface ratio and straight channels.     

Epoxidation of Propylene by Molecular Oxygen over Modified Ag–MoO3 Catalyst

Epoxidation of propylene to propylene oxide by molecular oxygen was studied over a modified Ag-MoO₃ catalyst. The results show that MoO₃ plays an important role in improving the efficiency of the catalyst, and a suitable content of MoO₃ is 40-50 wt%. XPS reveals that some of the silver and molybdenum in the catalyst exist as Ag⁺ and Mo^{(6 - )+}, respectively. The promotion effect of NaCl, Ce(NO₃)₃, BaCl₂ and CsNO₃ on the Ag-MoO₃ catalyst was studied. As a modifier of the Ag-MoO₃ catalyst, NaCl or Ce(NO₃)₃ are more suitable than BaCl₂ or CsNO₃ and the optimal loading of NaCl or Ce(NO₃) is about 2 wt%. Using a feedstock gas of 15.6% C₃H₆, 12.2% O₂ and balance N₂ without any addition of NO, EtCl or CO₂ at a space velocity of 4500 h^-1, 6.8% O₂ conversion and 53.1% selectivity to propylene oxide were achieved over the Ag-MoO₃-2.0% NaCl catalyst at 400 °C. At 450 °C the O₂ conversion and selectivity to propylene oxide were 11.4 and 43.6%, respectively.     

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     

Studies on probing SiO2 surface using BF3 as probe and SiO2/Al2Et3Cl3/TiCl4(PhMgCl) catalytic system for copolymerization of ethylene and propylene

Using BF₃ as probe, the surface of SiO₂ was probed. The effects of the roast temperature on the SiO₂ surface were investigated and a possible mechanism is suggested. Using SiO₂ as starting material for supports, new supported catalysts for copolymerization of ethylene and propylene were prepared and their possible structures are discussed. It was found that higher polymerization productivity, for instance, 480 kg P/mol_Ti h, can be obtained by using a SiO₂/Al₂Et₃Cl₃/TiCl₄ catalytic system and addition of PhMgCl to this catalytic system can significantly increase polymerization activity. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1583–1589, 2000     

Gas-phase epoxidation of propylene through radicals generated by silica-supported molybdenum oxide

It was found that silica-supported molybdenum oxide was high effective for the epoxidation of propylene among various silica-supported metal oxides. The post-catalytic bed volume played an important role in its formation. On a MoO_x/SiO₂ with 0.255 mmol/g-SiO₂, a propylene conversion of 17.6% and a PO selectivity of 43.6% were obtained at 5 atm, 573 K and flow rates of C₃H₆/O₂/He = 10/5/10 cm³ min⁻¹. The characterization studies indicated that crystalline MoO₃ nano-particle species was more effective for propylene epoxidation to PO than molecularly dispersed Mo oxide species. The reaction mechanism of propylene epoxidation on MoO_x/SiO₂ catalysts is hypothesized to involve gas-phase radicals generated at relatively low temperature by the dispersed molybdenum oxide species. These radicals participated in homogeneous reactions with molecular oxygen to produce propylene oxide.     

Direct Oxidation of Propylene to Propylene Oxide with Molecular Oxygen: A Review

The direct oxidation of propylene to propylene oxide (PO) using molecular oxygen has many advantages over existing chlorohydrin and hydroperoxide process, which produce side products and require complex purification schemes. Recent advances in liquid-phase and gas-phase catalytic oxidation of propylene in the presence of only molecular oxygen as oxidant and in absence of reducing agents are summarized. Liquid-phase PO processes involving soluble or insoluble Mo, W, or V catalysts have been reported which provide moderate conversions and selectivities, but these likely involve autoxidation by homogeneous chain reactions. Gas-phase PO catalysts have been mostly Ag-, Cu-, or TiO₂-based substances, although other compositions such as Au-, MoO₃-, Bi-based catalysts and photocatalysts have also been suggested as possibilities. The Ag catalysts differ from those used for ethylene oxide production in having high Ag contents and numerous additives. The additives are solid-phase alkali metals, alkaline earth metals, and halogens, with the most common substances being NaCl and CaCO₃. Nitrogen oxides in the form of gas-phase species or nitrates have also been found to be effective in enhancing PO production. Direct epoxidation by surface nitrates is a possibility. Titania catalysts supported on silicates have also been reported. These have higher PO selectivities at high conversion than silver catalysts.     

An exploratory study of diesel soot oxidation with NO2 and O2 on supported metal oxide catalysts

A number of supported metal oxide catalysts were screened for their catalytic performance for the oxidation of carbon black (CB; a model diesel soot) using NO₂ as the main oxidant. It was found that contact between the carbon and catalyst was a key factor in determining the rate of oxidation by NO₂. Oxides with low melting points, such as Re₂O₇, MoO₃ and V₂O₅ showed higher activities than did Fe₃O₄ and Co₃O₄. The activities of MoO₃ and V₂O₅ on various supporting materials were also examined. MoO₃/SiO₂ was the most active catalyst among the supported MoO₃ examined, whereas, V₂O₅/MCM-41 showed the highest activity among the supported V₂O₅. Different performances of the supported MoO₃ catalysts were explained by the interaction of MoO₃ with the supports: a strong MoO₃/support interaction may result in a poor mobility of MoO₃ and a poor activity for oxidation of carbon by NO₂. The high activity of V₂O₅/MCM-41 was associated with its catalysis of the oxidation of SO₂ by NO₂ to form SO₃, which substantially promotes oxidation of carbon by NO₂. Addition of transition metal oxides or sulfates to supported MoO₃ and V₂O₅ was also investigated. Combining MoO₃ or V₂O₅ with CuO on SiO₂, adding VOSO₄ to MoO₃/SiO₂ or MoO₃/Al₂O₃ and adding TiOSO₄ or CuSO₄ to V₂O₅/Al₂O₃ improved the catalytic performance.     

Copolymerization of ethylene–propylene using high‐activity bi‐supported Ziegler–Natta TiCl4 catalyst

Heterogeneous Ziegler–Natta TiCl₄ catalyst using MgCl₂ and SiO₂ as supports was prepared under controlled conditions. Mg(OEt)₂ was used as a starting material and was expected to convert to active MgCl₂ during catalyst preparation. Due to the high surface area and good morphological control, SiO₂ was chosen as well. Slurry copolymerization of ethylene and propylene (EPM) was carried out in dry n‐heptane by using the catalyst system SiO₂/MgCl₂/TiCl₄/EB/TiBA or TEA/MPT/H₂ at temperatures of 40–70°C, different molar ratios of alkyl aluminum : MPT : Ti, hydrogen concentrations, and relative and total monomers pressure. Titanium content of the catalyst was 2.96% and surface area of the catalyst was 78 m²/g. Triisobutyl aluminum (TiBA) and triethyl aluminum (TEA) were used as cocatalysts, while ethyl benzoate (EB) and methyl p‐toluate (MPT) were used as internal and external donors, respectively. H₂ was used as a chain‐transfer agent. Good‐quality ethylene propylene rubber (EPR) of rubber was obtained at the ratio of [TiBA] : [MPT] : [Ti] = 320 : 16 : 1 and polymerization temperature was 60°C. When TiBA was used as a cocatalyst, a higher and more rubberlike copolymer was obtained. For both of the cocatalysts, an optimum ratio of Al/Ti was obtained relative to the catalyst productivity. Ethylene content of the copolymer obtained increased with increasing TiBA concentration, while inverse results were obtained by using TEA. Addition of H₂ increased the reactivity of the catalyst. The highest product was obtained when 150 mL H₂/L solvent was used. Increasing temperature from 40 to 70°C decreased the productivity of the catalyst, while irregular behavior was observed on ethylene content. Relative pressure of P_P/P_E = 1.4 : 1 and total pressure of 1 atm was the best condition for the copolymerization. Polymers with ethylene contents of 25–84% were obtained. Increasing ethylene content of EPR decreased T_g of the polymer obtained to a limiting value. Viscosity‐average molecular weight (M_v) decreased with increasing temperature and TiBA and H₂ concentration. However, increasing the polymerization time increased the M_v. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2597–2605, 2004     

The Promoting Effect of Tellurium on K2MoO4/SiO2 Catalyst for Methanethiol Synthesis from High H2S-Containing Syngas

Tellurium used as promoter was investigated in the preparation of K₂MoO₄/SiO₂ catalyst for methanethiol synthesis from high H₂S-containing syngas. The experimental results showed that the addition of Te to K₂MoO₄/SiO₂ improved the activity of catalysts and the selectivity of methanethiol. ESR results reveal that the addition of Te decreases the content of “oxo-Mo⁵⁺” species and increase that of the “oxysulfo-Mo⁵⁺” species, simultaneously increase the low valence states of sulfur species. XPS results reveal that the addition of Te to K₂MoO₄/SiO₂ increase the amount of low valence states of molybdenum and sulfur species, which are related to the formation of methanethiol.     

Activity and stability of laser-photoreduced supported molybdenum oxide metathesis catalysts

Photoreduction of MoO₃/SiO₂ and MoO₃/SiO₂·Al₂O₃ catalysts in CO with a laser beam at 308 nm resulted, after addition of cyclopropane to form the active sites, in a high activity for alkene metathesis. Both catalyst systems are active for normal alkenes, e.g. propene, and for functionalized alkenes, e.g. methyl oleate, already at room temperature.     

Selective oxidation of propane over PMoV?M/α‐Al2O3 (M: Co,Fe, or Ni)

Alumina supported phosphovanodomolybdic acid and alumina supported phosphovanodomolybdic acid‐transition metal ions (M: Fe³⁺, Co²⁺, or Ni²⁺) were prepared by impregnation. The thermal decomposition, in situ at 400°C, of supported catalysts showed the formation of V₂O₅, P₂O₅, MoO₃ and MoO₃, CoMoO₄, (Mo_0.3V_0.7)₂O₅ phases, on the alumina surface, in the presence of H₄PMo₁₁VO₄₀/α‐Al₂O₃ and H₄PMo₁₁VO₄₀? Co/α‐Al₂O₃, respectively. The catalytic activity of alumina‐supported catalysts was evaluated in the reaction of propane oxidation at 380 and 400°C. The addition of transition metal increases the conversion and changes the reaction products distribution. The reaction conditions (temperature and propane/oxygen ratio) have also modified the behaviour of the studied catalysts.     

Selective oxidation of methane to formaldehyde and C2 hydrocarbons over double layered Sr/La2O3 and MoO3/SiO2 catalyst bed

A double layered catalyst bed of Sr/La₂O₃ followed by MoO₃/SiO₂ has been used to produce C₂ hydrocarbons and formaldehyde from a CH₄/air mixture with a formaldehyde space time yield of 187 g (kg cat)^–1 h^–1, which is significantly higher than those yields obtained with single bed catalysts or with mechanically mixed catalyst bed at ambient pressure and 630 ° C.     

Contrasting the Behaviour of MoO3 and MoO2 for the Oxidation of Methanol

The oxidation of methanol has been measured on MoO₃ and MoO₂. The properties of these two materials are interchangeable, depending upon the conditions in which the reaction is run. MoO₃ produces high yields of formaldehyde, but MoO₂ does not, due to the importance of the Mo⁶⁺ state for the selective reaction. However, if the MoO₃ material is run in anaerobic conditions it behaves in a very similar way to MoO₂, due to the presence of Mo⁴⁺ in the surface layers. In complement to this MoO₂ converts to high yield behaviour when run in aerobic conditions, due to the conversion of the material to Mo⁶⁺ at the surface, and, ultimately to MoO₃ in the bulk. In TPD experiments MoO₃ yields formaldehyde, whereas MoO₂ yields CO. In both materials oxygen transport within the lattice becomes appreciable above 300 °C, and the reaction proceeds via the Mars-van Krevelen mechanism.     

Demonstration of a packed bed membrane reactor for the oxidative dehydrogenation of propane

An experimental demonstration of the oxidative dehydrogenation of propane (ODHP) in a lab-scale packed bed membrane reactor has been performed. Experiments were carried out with both premixed and distributed oxygen feed over a Ga₂O₃/MoO₃ catalyst and compared, and the influence of the gas composition, flow rate and the extent of dilution was investigated. The experimental results were found to compare very well with detailed reactor simulations. The results revealed that, in comparison with conventional reactor concepts for the ODHP (fixed bed with premixed reactants feed), a significantly higher propylene yield can be achieved at higher propane conversions in a packed bed membrane reactor.