Selective Isopropylation of Biphenyl to 4,4′-DIPB over Mordenite (MOR) Type Zeolite Obtained from a Layered Sodium Silicate Magadiite

Molybdenum carbide catalysts for water–gas shift (WGS) reaction were investigated to develop an alternate commercial LTS (Cu-Zn/Al₂O₃) catalyst for an onboard gasoline fuel processor. The catalysts were prepared by a temperature-programmed method and were characterized by N₂ physisorption, CO chemisorption, XRD and XPS. It was found that the Mo₂C catalyst showed higher activity and stability than the commercial LTS catalyst, even though both catalysts were deactivated during the thermal cycling runs. The optimum carburization temperature for preparing Mo₂C was in the range of 640–650 °C. It was found that the deactivation of the Mo₂C catalyst was caused by the transition of Mo^δ+ (IV < δ⁺ < VI, MoO_xC_y), Mo^IV and Mo₂C on the surface of the Mo₂C catalyst to Mo^VI (MoO₃) with the reaction of H₂O in the reactant. It was identified that molybdenum carbide catalyst is an attractive candidate for the alternate Cu-Zn/Al₂O₃ catalyst for automotive applications.     

Ultra‐low Sintering Temperature Microwave Dielectric Ceramics Based on Na2O‐MoO3 Binary System

The compounds in Na₂O‐MoO₃ system were prepared by the solid‐state reaction route. The phase composition, crystal structures, microstructures, and microwave dielectric properties of the compounds have been investigated. This series of compounds can be sintered well at ultra‐low temperatures of 505°C–660°C. The sintered samples exhibit good microwave dielectric properties, with the relative permittivities (ε_r) of 4.1–12.9, the Q × f values of 19900–62400 GHz, and the τ_f values of ?115 ppm/°C to ?57 ppm/°C. Among the eight compounds in this binary system, three kinds of single‐phase ceramics, namely Na₂MoO₄, Na₂Mo₂O₇ and Na₆Mo₁₁O₃₆ were formed. Furthermore, the relationship between the structure and the microwave dielectric properties in this system has been discussed. The average Na^I‐O and Mo^VI‐O bond valences have an influence on the sintering temperatures in Na₂O‐MoO₃ system. The large valence deviations of Na and Mo lead to a large temperature coefficient of resonant frequency. The X‐ray diffraction and backscattered electron image results show that Na₂MoO₄ doesn't react with Ag and Al at 660°C. Also, Na₂Mo₂O₇ has a chemical compatibility with Al at 575°C.     

A New Catalyst Modified with Nanosized Molybdenum Carbides for the Hydroisomerization of <Emphasis Type="Italic">n</Emphasis>-Alkanes and Its Catalytic Properties in the Hydroisomerization of Diesel Fractions,Part II: Preparation of Bifunctional Hydroisomerization Catalysts

The aim of this work is to develop a new hydroisomerization catalyst based on molybdenum carbides that is resistant to the influence of sulfur compounds and applicable for the synthesis of low-pour-point diesel fuels that are similar in parameters to the fuel synthesized using platinum-containing catalysts. In the first part of the work, supports with different porous structures and acidities (Beta, ZSM-5, ZSM-12, and SAPO-31) are synthesized and studied. In the second part of the work, bifunctional catalysts prepared by modifying these supports with nanosized molybdenum carbides are studied. All samples contain the same amount of Mo₂C (10% in terms of the equivalent amount of MoO₃). The catalysts are tested using a model isomerization reaction of n-decane. The catalyst based on silicoaluminophosphate ATO (SAPO-31) proves to be the most effective one. In order to optimize its composition, larger batches of samples with different Mo₂C contents (5, 7 and 10% in terms of the equivalent amount of MoO₃) are prepared. The catalytic properties are studied using the hydroisomerization of actual diesel fractions. The optimum content of Mo₂C (7 wt %) in the bifunctional catalyst is determined.     

Dry reforming of methane to synthesis gas over supported molybdenum carbide catalysts

The dry reforming of methane at elevated pressure over supported molybdenum carbide catalysts, prepared from oxide precursors using ethane TPR, has been studied. The relative stability of the catalysts is Mo₂C/Al₂O₃>Mo₂C/ZrO₂>Mo₂C/SiO₂>Mo₂C/TiO₂, and calcination of the oxide precursor for short periods was found to be beneficial to the catalyst stability. Although the support appears to play no beneficial role in the methane dry reforming reaction, the alumina-supported material was stable for long periods of time; this may be important for the production of pelletised industrial catalysts. The evidence suggests that the differences in the stabilities may be due to interaction at the precursor stage between MoO₃ and the support, while catalyst deactivation is due to oxidation of the carbide to MoO₂, which is inactive for methane dry reforming.     

Remarkable support effect of ZrO2 upon the CO2 reforming of CH4 over supported molybdenum carbide catalysts

In reforming of CH₄ with CO₂ over molybdenum carbide catalysts, the catalytic performance of unsupported hexagonal Mo₂C prepared by direct carburization of MoO₃ was considerably different from a similar composition, cubic MoC_1−x (x≈0.5), prepared through nitriding before carburization. The conversion levels over MoC_1−x were substantially higher than those over Mo₂C, although the turnover frequencies were lower. X‐ray diffraction analysis indicated that Mo₂C deactivated by conversion to MoO₂ during the reaction, but the MoC_1−x was transformed to the hexagonal Mo₂C and remained stable. The activity of Mo₂C dispersed on various supports for the CH₄–CO₂ reaction was also investigated. The performance depended strongly on the property of supports, with the ZrO₂‐supported Mo₂C catalyst exhibiting the highest activity and durability for this reaction. Moreover, deactivation of Mo₂C/ZrO₂ at ambient pressure was suppressed by decreasing the loading amount of Mo₂C. This revised version was published online in August 2006 with corrections to the Cover Date.     

Study on the reduction of commercial MoO3 with carbon black to prepare MoO2 and Mo2C nanoparticles

A simple method was developed to synthesize MoO₂ and Mo₂C nanoparticles via controlling nucleation and growth in carbothermic reduction of commercial MoO₃ with carbon black. It was found that the appropriate C/MoO₃ molar ratio for preparation of Mo₂C was 2.8, and the carbothermic reduction process followed the sequence: MoO₃ → transport phase (TP) → MoO₂ → Mo₂C. It was revealed that the most crucial issues for controlling number of produced particles of product were migration of Mo source and aid of nucleating agent, which can be achieved by using MoO₃ and carbon black as starting materials. MoO₂ nanosheets with the thickness of 12 nm and lateral size of 60 nm, as well as Mo₂C nanoparticles with particle size of 30 nm were prepared via reduction of MoO₃ with carbon black. However, MoO₂ and Mo₂C produced via reducing MoO₃ by other kinds of carbon sources (activated carbon, graphite) or gas reductants (10% CH₄/H₂, CO) had much larger particle sizes of a few micrometers, which were tens of times than those using MoO₃ and carbon black, due to the too small amount of formed nuclei. The effects of C/MoO₃ molar ratio (0.5-2.8), molybdenum sources and carbon sources on the reaction mechanisms were investigated in detail.     

Synthesis, Characterization and Catalytic Activity in the Hydrogenation of Cyclohexene with Molybdenum Carbide

Two series of molybdenum carbides are prepared from MoO₃ by temperature programmed reduction (TPR), differing in feed gas composition (20 and 40% CH₄/H₂). The heating ramp consists of two steps, one from ambient temperature to 973 K, at a rate of 10 K/min; the second from 973 K up to the final temperature (1023, 1073, 1123, 1173 K), at a rate of 0.5 K/min. The Mo₂C (hcp) phase is identified for the series prepared with 20% CH₄/H₂ at different temperatures, with surface areas between 20 and 27 m²/g. Also found is a mix of the MoC and Mo₂C (hcp) phases for the series prepared with 40% CH₄/H₂ at temperatures above 1023 K, with surface areas between 9 and 19 m²/g. Both series of catalysts reach 100% conversion of cyclohexene in 5 h or less, with those catalysts prepared with a 40% CH₄/H₂ gas mix reaching maximum conversion in the least time. Catalysts are compared to a commercial molybdenum carbide reagent as a reference.     

Synthesis and characterization of massive molybdenum carbides and supported carbide-containing catalysts Mo2C/C prepared through mechanical activation

Procedures for the synthesis of massive molybdenum carbide by the mechanical activation of a mixture of MoO₃, commercial carbon, and Zn in air and the synthesis of the supported carbide-containing catalyst Mo₂C/C by the mechanical activation of commercial carbon impregnated with a 16% aqueous solution of ammonium paramolybdate in an inert atmosphere were developed for the first time. With the use of a set of physicochemical methods, the metal contents, particle sizes, specific surface areas, and phase compositions of the mechanically activated composites were determined. The structure of the carbide-containing supported catalyst was studied by electron microscopy, and its acidic properties were studied by the temperature-programmed desorption of ammonia; catalytic tests in the model reactions of dibenzothiophene (DBT) and alkane aromatization were performed. It was found that the Mo₂C/C catalyst exhibited high activity in these reactions: the conversion of DBT at a contact time of 3–6 h was 80–85%. The conversion of n-heptane at a contact time of 2 h was 31.2%, and 100% toluene was the reaction product. An increase in the contact time to 6 h led to a decrease in the conversion of n-heptane to 1.3%, and to 47% C₆-C₇ cycloalkanes were present in the reaction products. The results of this work are indicative of the high catalytic activity of the Mo₂C/C catalyst obtained by mechanical activation.     

In situ time-resolved characterization of novel Cu–MoO2 catalysts during the water–gas shift reaction

A novel and active Cu–MoO₂ catalyst was synthesized by partial reduction of a precursor CuMoO₄ mixed-metal oxide with CO or H₂ at 200–250 °C. The phase transformations of Cu–MoO₂ during H₂ reduction and the water–gas shift reaction could be followed by in situ time resolved XRD techniques. During the reduction process the diffraction pattern of the CuMoO₄ collapsed and the copper metal lines were observed on an amorphous material background that was assigned to molybdenum oxides. During the first pass of water–gas shift (WGS) reaction, diffraction lines for Cu₆Mo₅O₁₈ and MoO₂ appeared around 350 °C and Cu₆Mo₅O₁₈ was further transformed to Cu/MoO₂ at higher temperature. During subsequent passes, significant WGS catalytic activity was observed with relatively stable plateaus in product formation around 350, 400 and 500 °C. The interfacial interactions between Cu clusters and MoO₂ increased the water–gas shift catalytic activities at 350 and 400 °C.     

Preparation of Mo2C by the temperature-programmed reaction between h-MoO3 and CO

In the work, the temperature-programmed reaction (TPR) between hexagonal-shaped h-MoO₃ and high-purity CO under different heating rates was investigated in order to prepare Mo₂C. Various technologies such as TG-DTA-DTG, XRD, FESEM, FT-IR and Raman spectrum as well as the thermodynamic calculation were adopted to analyze the experimental data. The results showed that the physically adsorbed water on the sample surface, the residual ammonium and coordinated water in the internal structure of h-MoO₃ were successively released as the temperature increased, and then α-MoO₃ and Mo₄O₁₁ were formed when the temperature arrived at around 791 K. Upon further increasing the temperature, the reduction process occurred and MoO₂ will be generated. Thereafter, the carburization reaction was taken place and the subsequent reaction pathways were significantly different at lower and higher heating rates: at lower heating rates (8 and 12 K/min), the carburization process of MoO₂ to Mo₂C followed MoO₂→MoO₂+Mo₂C→Mo₂C + Mo→Mo₂C; while at higher heating rates (16 and 20 K/min), the reaction pathways followed MoO₂→MoO₂+Mo₂C→MoO₂+Mo₂C + Mo + MoO_xC_y→Mo→Mo₂C, single-phase metallic Mo can be generated. The work also discovered that the as-prepared Mo₂C always kept the same platelet-shaped morphology as that of the newly-formed MoO₂; while due to the removal of oxygen and the decrease of molar volume during the transformation process, the as-prepared Mo₂C exhibited a rougher and more porous morphological structure.     

Decomposition of metal carbides as an elementary step of carbon nanotube synthesis

The role of catalyst components in catalysts containing molybdenum, Mo/M/MgO (MNi, Co, and Fe), as well as Mo-free catalysts, M/MgO (MNi, Co, and Fe), for carbon nanotube (CNT) synthesis have been investigated by TEM, XRD, and Raman spectroscopy. CNT synthesis by the catalytic decomposition of CH₄ over M/MgO catalysts can proceed at reaction temperatures higher than the decomposition temperature of the metal carbides (Ni₃C, Co₂C, and Fe₃C), which indicates that carbon in the CNT originates from the graphitic carbon formed on the catalyst surface by the decomposition of metal carbides. For all catalysts containing Mo, thin CNT formation starts at an identical temperature of 923 K, corresponding to the decomposition temperature of MoC_1−x into Mo₂C. The significant effect of the addition of Mo is concerned with the formation of Mo₂C in a catalyst particle during CNT synthesis at high reaction temperatures. The presence of a stable Mo₂C phase leads to the formation of thin CNT with better crystallinity at high reaction temperatures. The role of Ni, Co, and Fe in the Mo/M/MgO catalysts is ascribed to the dissociation of CH₄.     

The kinetics of hydrodesulphurisation of thiophene over a nickel-molybdenum catalyst

The kinetics of vapour phase hydrogenolysis of thiophene over a series of unsupported and γ-alumina supported nickel-molybdenum catalysts were studied at atmospheric pressure using a fixed bed integral microreactor. A kinetic expression describing the effects of temperature and space velocity was experimentally verified. The hydrogenolysis of thiophene was found to be 0.8th order with respect to thiophene and 0.8Sth order with respect to hydrogen. The activation energy of the reaction over nickel-molybdenum (NiO:MoO₃ weight ratio 30:70) catalyst was found to be 19.23 kJ mol⁻¹ in the temperature range 250–350°C, while above this temperature the reaction was affected by adsorbed thiophenic sulphur. X-ray and infrared analyses of the catalysts indicated that MoO₃ (Mo⁶⁺, a main component), which after pretreatment forms MoS₂ or MoS₂+MoO₂, requires the incorporation of a specific concentration of nickel (Ni:Mo ca 0.5) in the form of α-NiMoO₄ for maximum hydrodesulphurisation activity.     

Study on Reduction of MoO2 Powders with CO to Produce Mo2C

Molybdenum hemicarbide (Mo₂C), which is widely applied in steel and metal ceramics as well as catalysts, was successfully synthesized by using a simple method of reducing MoO₂ powders by CO. It was found that the final reduction product was all Mo₂C under both isothermal and nonisothermal conditions. However, the reduction mechanisms were significantly different at lower and higher temperatures: at lower temperatures (1070 to 1342 K), reduction of MoO₂ to Mo₂C followed a one‐step reaction (simultaneous reduction and carburization), while at higher temperatures (1423 to 1485 K), MoO₂ was first reduced to metallic Mo, and then Mo was carburized to Mo₂C. Mo₂C particles obtained at higher temperatures contained micrometer‐sized surface features which formed during the MoO₂ to Mo reduction step.     

Carbide and Nitride Supported Methanol Steam Reforming Catalysts: Parallel Synthesis and High Throughput Screening

A series of Mo₂C and Mo₂N supported catalysts have been synthesized using a parallel synthesis and high throughput screening approach. The high surface area Mo₂C and Mo₂N supports were prepared using temperature programmed reaction methods. Metals including Co, Cu, Fe, Ni, Pd, Pt, Ru, and Sn were impregnated onto these supports using a synthesis system. Methanol steam reforming (MSR) activities and selectivities for these materials were evaluated using a high throughput-screening reactor. The support type, metal type and concentration, and metal precursor type influenced the activity and selectivity patterns. Of more than 400 materials that were synthesized and evaluated, the Pt/Mo₂N, Pt–Ni/Mo₂N, Pt–Fe/Mo₂N, and Pd–Fe/Mo₂C catalysts possessed the highest activities. Some of these formulations were more active than a commercial Cu/Zn/Al₂O₃ catalyst, however, the CO₂ selectivities were typically lower. At similar conversions, materials that were highly active were not selective while the less active materials were very selective. Many of the highly active catalysts included noble metals while the highly selective catalysts included base metals.     

Laser Raman spectroscopy (LRS) and time differential perturbed angular correlation (TDPAC) study of surface species on Mo/SiO2 and Mo,Na/SiO2. Their role in the partial oxidation of methane

Silica-supported molybdenum (1.6 and 5.0 wt%) and molybdenum (5 wt%)-sodium (0.4 wt%) catalysts have been characterized by laser Raman spectroscopy (LRS), time differential perturbed angular correlation (TDPAC), temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The presence of different molybdenum species was correlated with activity and selectivity to formaldehyde during the methane partial oxidation reaction. The main species identified on the Mo(5.0 wt%) /SiO₂ surface were MoO₃ and monomeric species with a single Mo=O terminal bond. The pre-impregnation of the silica support with sodium strongly diminishes the Mo=O concentration due to the formation of Na₂Mo₂O₇ species and tetrahedral monomers with a high degree of symmetry. As a result of these modifications, both methane conversion and formaldehyde formation are strongly inhibited. The combination of LRS and TDPAC techniques resulted in a powerful tool for the identification and quantification of the molybdenum species present on the surface of a silica support. This revised version was published online in July 2006 with corrections to the Cover Date.     

Selectivities in methanol oxidation over silica supported molybdena

Selectivities in methanol oxidation over silica supported molybdenum oxide catalysts were investigated in relation with the phase distribution. The supported catalysts were prepared by impregnation with ammonium heptamolybdate. In addition to crystalline MoO₃, Mo containing cluster species of 1–2 nm size were observed by STEM even from a used catalyst with 13% catalyst loading. The percentage of Mo present as crystalline MoO₃ increases with the catalyst loading. An ESCA study indicates that part of surface Mo in the supported catalysts is reduced to Mo⁵⁺. The dimethyl ether selectivity increases with the catalyst loading and its formation occurs over the crystalline MoO₃ phase. The Selectivities to CO and methyl formate are greatly enhanced because of the presence of support, and are relatively independent of the catalyst loading and phase distribution. The dependence and independence of the Selectivities of different byproducts on the loading make the silica supported catalysts with high catalyst loadings less selective for the partial oxidation of methanol to formaldehyde.     

In situ carburization of metallic molybdenum during catalytic reactions of carbon-containing gases

Supported molybdenum clusters were prepared by sublimation of Mo(CO)₆ onto dehy-droxylated alumina followed by decomposition in flowing dihydrogen at 970 K. These alumina-supported molybdenum clusters were found by XAFS to transform into Mo₂C if heated in a 20% methane/H₂ mixture at 950 K. For the hydrogenolysis ofn-butane at 510 K and CO-H₂ reactions at 570 K, both at atmospheric pressure, molybdenum and carburized molybdenum showed similar, but different for each reaction, turnover rates. The product distribution was the same for each reaction on Mo and Mo₂C. In both reactions, in situ XAFS data for fresh and used catalysts indicated that Mo clusters progressively transformed into Mo₂C under the reaction conditions     

Mixed Alcohol Synthesis from Syngas on K–Co–Mo/C Catalyst Prepared by a Sol–Gel Method

A kind of K–Co–Mo/C catalyst with homogenous components distribution and small particle size was prepared by sol–gel method with citric acid as complexant. Its structure and catalytic performance for mixed alcohol synthesis were investigated. By heat treating the dried gel in argon, the decomposition of citric acid resulted in the formations of amorphous carbon and low-valence MoO₂ species. The incorporation of potassium and cobalt increased significantly the alcohol synthesis activity, especially improved the C₂₊OH selectivity. The optimal atomic composition of catalyst was: 0.10 K: 0.50 Co: 1.0 Mo. Comparing with the similar catalysts reported in literatures, the sol–gel derived K–Co–Mo catalyst showed better performance, especially much higher C₂₊OH selectivity for mixed alcohol synthesis. A 150 h reaction test indicated that the catalyst had good stability during the entire experimental period. It suggested that the homogenous components distribution and small particle size enhanced the synergistic effect of promoters and created more active species, leading to a high catalytic performance. The formation of MoO₂ species was also favorable to improve the catalytic activity because the low-valence Mo⁴⁺ was known to be more active in CO hydrogenation.     

Study on reduction reaction of MoO2 powder with NH3

Reaction mechanism between MoO₂ powder and NH₃ in the temperature range of 1028‐1273 K has been investigated. The results show that reduction products are strongly dependent on the reaction temperature employed. It is found that molybdenum nitride (Mo₂N) can be successfully synthesized when the temperature is below 1078 K, with the morphologies keep the same as that of the raw material MoO₂. Although the temperature is above 1078 K, metal Mo powder will be turned up; at the even higher temperatures, the thermodynamically stable phase will be metal Mo. In addition, MoO_xN_1?x as an intermediate product is formed during the reduction processes.     

Cupric Oxide-Molybdenum Oxide Phase Diagram in Air and in Oxygen

Phase diagrams of the CuO-MoO₃ system were determined in oxygen, in air, and under oxygen pressure. The first two diagrams are quite similar, with temperatures up to 40° higher occurring in oxygen than in air. Stable compounds obtained by low-temperature sintering were CuMoO₄ and CUZMoO₅, which melted incongruently at 835° and 880° in oxygen and at 812° and 840°C in air, respectively. The MoO₃-CuMoO₄ eutectic occurs near 30 molyo CuO at 705° in oxygen and at 700° C in air. Above temperatures as low as 865° in oxygen and 840° C in air, oxygen is lost with the production of cuprous compounds. This phenomenon probably accounts for all of the inconsistencies in previous phase diagrams. The dominant cuprous compound in the central region at high temperatures is Cu₆Mo₄O₁₅, melting congruently at 880° and 895°C in air and oxygen, respectively. On cooling in oxygen-containing atmospheres, this compound changes to cupric Cu₃Mo₂O₉. Unit cell dimensions and an indexed powder pattern are presented for this compound. Experiments under an oxygen pressure of 2 to 3 atm indicate that the cupric compounds in the simple CuO-MoO₄, system without reduction are CuMoO₄, Cu₃Mo₂O₉, and Cu₂MoO₅, melting incongruently at 850°, 910°, and 940°C, respectively. In all atmospheres Cu₃Mo₂O₉ is metastable with respect to CuMoO₄ and Cu₂MoO₅ below the incongruent melting point of CuMoO₄.