Study of W/HZSM-5-Based Catalysts for Dehydro-aromatization of CH4 in Absence of O2. I. Performance of Catalysts

With incorporation of Zn (or Mn, La, Zr ) into the W/HZSM-5 catalyst, highly active and heat-resisting W/HZSM-5-based catalysts were developed and studied. Under reaction conditions of 0.1 MPa, 1073 K, GHSV of feed-gas CH₄+10% Ar at 960 h^–1, the conversion of methane reached 18–23% in the first 2 h of reaction, and the corresponding selectivity to benzene, naphthalene, ethylene and coke was 56–48, 18, 5 and 22%, respectively. Addition of a small amount of CO₂ (2%) to the feed-gas was found to significantly enhance the conversion of methane and the selectivity of benzene, and to improve the performance of coke-resistance of the W/HZSM-5-based catalysts. Heavy deposition of carbon on the surface of the functioning catalyst was the main reason leading to deactivation of the catalyst. Reoxidation by air may regenerate the deactivated catalyst effectively. In comparison with the Mo/HZSM-5 catalyst, the promoted W/HZSM-5-based catalyst can operate under reaction temperature of 1073 K, and gain a methane conversion approximately two times as high as that of the Mo/HZSM-5 catalyst operating at 973 K. It can also operate at 973 K and have about the same methane conversion as that of the Mo/HZSM-5 catalyst at the same reaction temperature. Its main advantage is its heat-resistant performance; the high reaction temperature did not lead to loss of W component by sublimation.     

Microwave discharge-assisted NO reduction by CH4 over Co/HZSM-5 and Ni/HZSM-5 under O2 excess

Microwave discharge-assisted reduction of NO by CH₄ in the presence of excess O₂ over Co/HZSM-5 and Ni/HZSM-5 catalysts was studied. By comparing the activities of the catalysts in the microwave discharge mode with that in the conventional reaction mode, it is demonstrated that microwave discharge enhanced greatly the conversion of NO to N₂, and expanded the reaction temperature range of the catalysts. For the Co/HZSM-5 catalyst, the conversion of NO to N₂ increased by 30%, and the optimum temperature decreased by 200°C. With the Ni/HZSM-5 catalyst, the highest activity was close to 100%, and the optimum temperature decreased by 325°C. The conversion of CH₄ also increased in the microwave discharge mode over both of the catalysts.     

Interactions between pheromone traps with different strength lures for the pine beauty moth,Panolis flammea (Lepidoptera: Noctuidae)

Catches of malePanolis flammea in traps baited with lures containing 25 g of sex attractant are enhanced when in the proximity of traps baited with 125-g lures. The degree of enhancement is increased as the intertrap distance is decreased, and when the low-dose trap is upwind of the high-dose one. The patterns of alteration in trap catch suggest that moths initially attracted by one lure may divert into other traps, which may be either upwind of the original trap (overshooting), or downwind (undershooting). Overshooting can result in up to fivefold increases in catch and may provide a useful method for estimating the attractant zone of particular lure/trap combinations.     

Reduction of NO by H2 and CO on PdO-MoO3/γ-Al2O3 of low molybdena loading

The activity and selectivity in the catalytic reduction of NO by a mixture of CO and H₂ of three PdO-MoO₃/-Al₂O₃ catalysts are compared in the presence of varying amounts of oxygen at reaction temperatures from 100 to 550°C. The catalysts were prepared by different methods and contain about 2% Mo and 2% Pd. Results are compared with those for PdO/-Al₂O₃, PdO-MoO₃/-Al₂O₃ containing 2% Pd and 20% Mo, and a commercial Pt-Rh catalyst. The PdO-MoO₃/-Al₂O₃ catalysts are more active for the selective reduction of NO to N₂ and N₂O than PdO/-Al₂O₃ under slightly oxidizing conditions at temperatures from 300 to 550°C. At these reaction conditions, the fresh PdO-MoO₃/-Al₂O₃ catalysts are comparable with a commercial Pt-Rh catalyst. The improved activity of PdO-MoO₃/-Al₂O₃ relative to PdO/-Al₂O₃ is believed to be due to the interaction between Pd and Mo. The effect of O₂ on the activity and selectivity of these catalysts is different in the reduction of NO by H₂, by CO, and by a mixture of H₂ and CO. The results using the mixture of reductants cannot be inferred from the results with the single reductants.     

Active-oxygen species on non-reducible rare-earth-oxide-based catalysts in oxidative coupling of methane

From supplementary in situ Raman spectroscopic studies of active-oxygen species on non-reducible rare-earth-oxide-based catalysts in the oxidative coupling of methane (OCM) and structural adaptability considerations, further support has been obtained for our proposal that there may be an active and elusive precursor (of O₂ ^– and O₂ ^2– adspecies), most probably O₃ ^2– formed from reversible redox coupling of an O₂ adspecies at an anionic vacancy with a neighboring O^2– in the surface lattice. This active precursor may initiate H abstraction from CH₄ and be itself converted to OH^–+O₂ ^–, or it may abstract an electron from the oxide lattice and be converted to O₂ ^2–+O^–. The prospect of developing this type of OCM catalysts is discussed.     

Electrochemical behaviour of viologen-cyclodextrin inclusion complexes. The case of non-alkyl group substituted viologen

The electrochemical behavior of non-alkyl substituted viologen, 4,4-dibenzyl bipyridinium (BzV), 4,4-dicyanophenyl bipyridinium (CyV) and -,-,-cyclodextrin (, , -CD) was studied using cyclic voltammetry and a spectroelectrochemical method. It was found that BzV and Fe(CN) ₆ ^4– formed a charge-transfer (CT) complex with a ratio of 21 and the colour of the solution faded with the addition of an electrolyte. This behaviour is the same as in then-heptyl viologen and ferrocyanide system [1]. BzV, -CD and -CD formed an inclusion complex only in the reduced state, whilst BzV and -CD formed an inclusion complex in both the oxidized and the reduced state. An EC scheme in which a chemical reaction follows an electrochemical reaction was considered to predominate in the BzV and -, -CD systems, while a CE scheme in which a chemical reaction preceded an electrochemical reaction predominated in the BzV and -CD system. On the other hand, CyV was found to form an inclusion complex with -, -, -CD in both the oxidized and the reduced states. therefore a CE scheme was considered to predominate in the CyV--, -, -CD systems.     

Catalytic Decomposition of Hydrazine over Supported Molybdenum Nitride Catalysts in a Monopropellant Thruster

The catalytic decomposition of hydrazine over a series of MoN_x/-Al₂O₃ catalysts with different Mo loadings was investigated in a monopropellant thruster (10 N). When the Mo loading is equal to or higher than the monolayer coverage of MoO₃ on -Al₂O₃, the catalytic performance of the supported molybdenum nitride catalyst is close to that of the conventionally used Ir/-Al₂O₃ catalyst. The MoN_x/-Al₂O₃ catalyst with a loading of about 23 wt% Mo (1.5 monolayers) shows the highest activity for hydrazine decomposition. There is an activation process for the MoN_x/-Al₂O₃ catalysts at the early stage of hydrazine decomposition, which is probably due to the reduction of the oxide layer formed in the passivation procedure.     

Interaction of methane with surfaces of silica,aluminas and HZSM-5 zeolite. A comparative FT-IR study

Infrared investigations on the interaction of methane with silica, aluminas (, and ) and HZSM-5 zeolite have been carried out. At low temperature (173 K), methane adsorption was observed over these oxides and HZSM-5 zeolite. Our findings featured that the infrared inactive ₁ band (2917 cm^–1) of a gaseous methane molecule became active and shifted to lower frequencies (2900 and 2890 cm^–1) when it adsorbed on the surfaces of these adsorbents. Our results also demonstrate that hydroxyl groups played a very important role in methane adsorption over the acidic oxides and the HZSM-5 zeolite. When interaction between the hydroxyl groups and methane took place, the band shift of the hydroxyl groups varied with different oxides. The strength of the interaction decreased according to the following sequence, Si-OH-Al>Al-OH>Si-OH, which is in accordance with the order of their acidities. At higher temperatures, methane interacted quite differently with various oxides and HZSM-5 zeolite. It has been observed that the hydroxyl groups of silica, -alumina and HZSM-5 zeolite could exchange with CD₄ at temperatures higher than 773K, while those on -alumina could exchange at a temperature as low as 573 K. Another interesting observation was the formation of formate species over Al₂O₃ (both and ) at temperatures higher than 473 K. The formate species would decompose to CO₂, or produce carbonate at much higher temperatures. Formation of formate species was not observed over silica and HZSM-5 under similar conditions, -Al₂O₃ did not adsorb or react with methane in any case.     

Dehydration of 2‐methylbutanal to isoprene using aluminium phosphate catalysts

The synthesis of isoprene from the dehydration of 2methylbutanal is described using aluminium phosphates as catalysts. Two samples of aluminium phosphate are studied prepared from the reaction of phosphoric acid with aluminium chloride or sulphate. The chloride route gives a mixed cristobalite/tridymite AlPO₄ and this is shown to be more active than a catalyst containing only the tridymite form of AlPO₄ formed from the sulphate route. The AlPO₄ catalysts are also shown to be active catalysts for the synthesis of isoprene from 3methylbutan2one, which is the major byproduct formed from the reaction of 2methylbutanal. The AlPO₄ catalysts are deactivated due to the deposition of coke in addition to loss of phosphorus from the surface. Catalytic activity can be totally restored by a simple calcination procedure at 800°C.     

Reduction of NO by CO on PdO-MoO3/γ-Al2O3 of low molybdena loading

The catalytic activity for the reduction of NO by CO of five PdO-MoO₃/-Al₂O₃ catalysts is compared in the presence of varying amounts of oxygen at reaction temperatures from 25 to 550 °C. The samples were prepared by different methods and contain about 2% of Mo and 2% Pd. Results are compared with the activities and selectivities of PdO/ -A1₂O₃ and PdO-MoO₃/-Al₂O₃ containing 2% Pd and 2% Pd + 20% Mo, respectively. All catalysts showed appreciable activity at temperatures between 300 and 550 °C and at stoichiometric ratios,R, of the oxidizing to reducing gases of 0.1 <R < 1.1. The activity of three PdO-MoO₃/ -A1₂O₃ catalysts with low concentrations of Mo and Pd was found to be significantly higher than the activity of PdO/-Al₂O₃ at 1.1 <R < 1.3 and at temperatures between 300 and 500 °C. The improved activity is ascribed to the interaction of the active metals.     

Reduction of NO by H2 on PdO-MoO3/γ-Al2O3 of low molybdena loading

The catalytic activity and selectivity of three PdO-MoO₃/-Al₂O₃ catalysts containing about 2% Pd and 2% Mo were studied for the reduction of NO by h₂ in the presence of varying amounts of oxygen at temperatures from 50 to 550 °C. The results are compared with those for PdO/-Al₂O₃, PdO-MoO₃/-Al₂O₃ containing 2% Pd and 20% Mo, and a commercial Pt-Rh catalyst. In the absence of oxygen, the conversion of NO to N₂ and N₂O is higher on the three catalysts than it is on PdO/-Al₂O₃ at 500 and 550 °C. In the presence of oxygen, the yields of N₂ and N₂O are generally lower on two of the PdO-MoO₃/-Al₂O₃ catalysts than on PdO/-Al₂O₃.     

Lean NOx catalysis over alumina‐supported catalysts

aluminasupported catalysts show promise as lean NO_x catalysts. The role of alumina in influencing the structural and chemical properties of the active phase supported on it is discussed using some effective aluminabased lean NO_x catalysts. These include Ag/Al₂O₃, CoO_x/Al₂O₃ and SnO₂/Al₂O₃. Alumina plays an important role in stabilizing Ag in the oxidic phase and cobalt in the 2+ oxidation state. For SnO₂/Al₂O₃, alumina increases the SnO₂ surface area. On both Ag/Al₂O₃ and SnO₂/Al₂O₃, alumina also participates actively in the NO_x reduction reaction. An active organic intermediate is formed on Ag or Sn oxide which reacts with NO_x subsequently on alumina to form N₂.     

Catalytic oxidation of methanol to methyl formate over silver – a new purpose of a traditional catalysis system

Catalytic reaction was performed in the unregarded temperature region over silver catalysts with long catalytic lifetime for the conversion of methanol to methyl formate. O-saturated or O-saturated silver catalysts were studied individually to identify the roles of O, O in the oxidative esterification of methanol over an unsupported polycrystalline silver catalyst. A synergic process is proposed based on the coexistence of -oxygen species and -oxygen species on the surface of polycrystalline silver at about 573 K.     

Electrochemical behaviour of indium ions in molten equimolar CaCl2–NaCl mixture at 550 ∘C

The stability of indium chloride and oxide as well as the electrochemical behaviour of indium ions have been studied in the equimolar CaCl2–NaCl melt at 550 C by X-ray diffraction (XRD) and different electrochemical techniques, using molybdenum and tungsten wires as working electrodes. Voltammetric and chronopotentiometric studies showed signals attributed to the presence of three oxidation states of indium, i.e. 0, i and iii. The standard potential of the redox couples, as well as the solubility products of indium oxides have been determined, showing that In(iii) ions are completely reduced to monovalent indium by the indium metal according to the reaction: In () + 2 In 3 In () and that In₂O is a strong oxide donor according to the reaction: In₂O(s) 2 In() + O^2- These results have allowed the construction of E-pO2– equilibrium diagrams summarising the properties of In–O compounds. The electrodeposition of indium was uncomplicated at Mo and W electrodes. Very good adherence of liquid indium to the electrode materials was observed, with the formation of Na–In alloys at highly reducing potentials, and there was no evidence of indium dissolution into the melt. Moreover, the voltammograms corresponding to the electrochemical In(iii)/In(i) exchange were well defined. The two electrochemical steps were found to be quasi-reversible, and the values of the kinetic parameters, ko and , for both reactions, as well as the diffusion coefficients, DIn(III) and DIn(I) were calculated.     

The effect of the duration of n-butane/air pretreatment on the morphology and reactivity of (VO)2P2O7 catalysts

Three vanadium pyrophosphate catalysts have been prepared by calcining vanadium hydrogen phosphate hemihydrate (VOHPO₄0.5H₂O, prepared in an organic medium) for different lengths of time (40, 100 and 132 h) in a n-butane (0.75%)/air mixture at 473 K. The catalysts were designated VPO40, VPO100 and VPO132. Increasing the duration of reaction with n-butane/air mixture led to an increase in the total surface area from 21.3 m²g^–1 (VPO40) to 24.9 m²g^–1(VPO100) and to 27.0 m²g^–1(VPO132). It also led to the complete removal of the VOPO₄ phase from catalysts VPO100 and VPO132, this VOPO₄ phase having seen as a minor component of catalyst VPO40. Scanning electron microscopy showed that longer periods of pretreatment in the n-butane/air mixture produced catalysts with increasing amounts of a characteristic rosette-type of agglomerate. Temperature-programmed reduction with H₂ resulted in the removal of 11 monolayers equivalent of oxygen from all three of these catalysts at a peak maximum temperature of 1000 K with the development of a second reduction peak at 1100 K which increases with increasing time of n-butane/air pretreatment. The morphology produced by extended pretreatment in the n-butane/air mixture at 673 K is therefore predisposed to reaction with H₂ (and probably with n-butane). Apparently paradoxically, increasing the duration of n-butane/air pretreatment results in catalysts which on temperature-programmed desorption desorb less oxygen.     

Study of the Supported K2MoO4 Catalyst for Methanethiol Synthesis by One Step from High H2S-Containing Syngas

ESR and XPS are used to study the Mo-based catalysts MoO₃/K₂CO₃/SiO₂ and K₂MoO₄/SiO₂ prepared with two kinds of precursors, (NH₄)₆Mo₇O₂₄4H₂O and K₂MoO₄. The catalytic properties of the catalysts for methanethiol synthesis from high H₂S-containing syngas are explored. The activity assay shows that the two catalysts have much the same activity for the reaction. By the ESR characterization of both functioning catalysts, the resonant signals of oxo-Mo(V) (g=1.93), thio-Mo(V) (g=1.98) and S (g=2.01 or 2.04) can be detected. In the catalyst MoO₃/SiO₂ modified with K₂CO₃, as increasing amounts of K₂CO₃ are added, the content of oxo-Mo(V) increases, but thio-Mo(V) decreases. The XPS characterization indicates that Mo has mixed valence states of Mo⁴⁺, Mo⁵⁺ and Mo⁶⁺, and that S includes three kinds of species: S^2– (161.5 eV), [S–S]^2– (162.5 eV) and S⁶⁺ (168.5 eV). Adding K₂CO₃ promoter to the catalysts, the Mo species of high valence state is easily sulphided and reduced to Mo₂S and oxo-M(V), and the derivation of [S–S]^2– and S^2– species from S is promoted simultaneously. The methanethiol synthesis is favored if the mole ratio of (Mo⁶⁺ + Mo⁵⁺)/Mo⁴⁺ 0.8 and S^2–/[S–S]^2– is kept at a value of about 1.     

Comparative Study on the Mechanisms of Partial Oxidation of Methane to Syngas Over Rhodium Supported on SiO2 and γ-Al2O3

A comparative study on the mechanisms of the partial oxidation of methane (POM) to syngas over SiO₂- and -Al₂O₃-supported Rh catalysts was carried out using in situ time-resolved FTIR spectroscopy to follow the primary products of POM reaction over the catalysts. Experiments of catalytic performance evaluation and temperature-programmed reduction (TPR) characterization of the catalysts, as well as the in situ FTIR spectroscopic study using CO to probe the oxidation state of Rh species over the catalysts were performed. It was found that the direct oxidation of CH₄ to syngas is the main pathway of the POM reaction over Rh/SiO₂ catalysts, while the combustion--reforming mechanism is the dominant pathway of syngas formation over Rh/-Al₂O₃ catalysts. The results of TPR characterization indicate that Rh supported on -Al₂O₃ is more difficult to reduce than Rh supported on SiO₂. The IR experiments of CO adsorption over Rh/SiO₂ and Rh/-Al₂O₃ after the POM reaction reveal that the surface of the Rh/-Al₂O₃ catalyst contains more partially oxidized rhodium (Rh⁺) species as compared to the Rh/SiO₂ catalyst. These results suggest that the significant difference in the mechanisms of the POM reaction over Rh/SiO₂ and Rh/-Al₂O₃ catalysts can be related to the difference in the surface concentration of O^2- species over the catalysts under the reaction conditions mainly due to the difference in oxygen affinity of the Rh species on the two supports.     

Temperature programmed desorption studies on hydrodesulfurization catalysts

The temperature programmed-desorption (TPD) of butane, butene, butadiene and thiophene over a series of Co-Mo/-Al₂O₃ catalysts with varying Co to Mo ratio has been investigated. The TPD of butane, butene and butadiene over catalysts containing no Co showed a single desorption profile while incorporation of Co created an additional site without significantly affecting desorption from the original site. The TPD of thiophene over a series of catalysts with varying Co content showed identical desorption temperature as well as heat of desorption. It was concluded that thiophene was adsorbed on the Mo-S component of the catalyst and was unaffected by the presence of Co.     

Thiophene adsorption studies on clean and hydrogen-preadsorbed MoS2(100) surface

It is known from temperature-programmed desorption studies that the binding energy of thiophene over Mo/-Al₂O₃ and Co-Mo/-Al₂O₃, hydrodesulfurization catalysts, is lower in the presence of hydrogen. The adsorption of thiophene on clean and hydrogen-adsorbed MoS₂ was modelled using extended Hückel tight binding band structure calculations. In the ¹ adsorption configuration the calculations show a lower binding energy for adsorption on the hydrogen-preadsorbed surface similar to that observed experimentally. The lowering is due to an increased occupancy of the Mo density of states in the presence of hydrogen.     

Shape-selective synthesis of 4,4′-dimethylbiphenyl. 1. Methylation of 4-methylbiphenyl over modified zeolite catalysts

Methylation of 4-methylbiphenyl with methanol was carried out using zeolites HY, HM, and HZSM-5 as catalysts under fixed-bed down-flow conditions. HY and HM display no shape selectivity, but HZSM-5 shows moderate selectivity to the target product, 4,4-dimethylbiphenyl (4,4-DMBP) under proper reaction conditions. Modification of the surface of HZSM-5 zeolite can improve the selectivity to 4,4-DMBP, especially the simultaneous modification of external and internal surface with inorganic P compound. The selectivity to 4,4-DMBP can be increased to as high as 65% over the HZSM-5 modified with a small amount of ammonium hydrogen phosphate.