Flame-made MoO3/SiO2–Al2O3 metathesis catalysts with highly dispersed and highly active molybdate species

MoO₃/SiO₂–Al₂O₃ catalysts are prepared via flame spray pyrolysis and evaluated in the self-metathesis of propene to ethene and butene. Their specific surface area ranges between 100 and 170 m² g^?1 depending on the MoO₃ loading (1–15 wt.%, corresponding to Mo surface density between 0.3 and 6.1 Mo atoms per nm²). The catalysts were characterized by N₂-physisorption, X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectroscopy (ToF-SIMS). The silica–alumina matrix condenses first in the flame and forms non-porous spherical particles of 5–20 nm, followed by the dispersion of Mo oxide at their surface. Depending on the MoO₃ loading, different MoO_x species are stabilized: dispersed and amorphous molybdates (mono- and oligomeric) at low loadings (<5 wt.%, <1.5 Mo nm^?2) and crystalline MoO₃ species at higher loadings. Raman spectroscopy suggests the presence of monomeric species for surface densities of 0.3, 0.5 and 0.8 Mo nm^?2. The formation of MoOMo bonds is, however, clearly established by ToF-SIMS from surface densities as low as 0.5 Mo nm^?2. At 1.5 Mo nm^?2, crystallites of β-MoO₃ (2–3 nm) are detected and further increasing the loading induces the formation of bigger α- and β-MoO₃ crystals (around 20 nm). The speciation of Mo proves to have a marked impact on the metathesis activity of the catalysts. Catalysts with high Mo loading and exhibiting MoO₃ crystals are poorly active, whereas catalysts with low Mo loading (<5 wt.%) perform well in the reaction. The catalyst loaded with only 1 wt.% of MoO₃ (0.3 Mo nm^?2) is the most active, reaching turn over frequencies seven times higher than reference catalysts reported in the literature. Moreover, the specific metathesis activity is clearly inversely correlated to the degree of condensation of the molybdenum oxide phase (as evaluated by ToF-SIMS). The latter finding indicates that monomeric MoO_x species are the main active centres in the olefin metathesis.     

Oxidation of dibenzothiophene with cumene hydroperoxide on MoO3/SiO2 modified with alkaline earth metals

Catalytic oxidation of dibenzothiophene (DBT) in decalin was performed using an oil-soluble oxidant, cumene hydroperoxide (CHP), over molybdenum oxide (MoO₃) supported on silica. The effects of MoO₃ loading, reaction time and the molar ratio of CHP/DBT were investigated. At a MoO₃ loading of 15 wt%, the conversion of DBT reached 82% at 70 °C, WHSV 30 h^?1, and O/S molar ratio 3. Alkaline earth metals, such as Ca, Ba, Sr and Mg were introduced on the surface of silica, prior to the impregnation of MoO₃. The results showed that the activity in the oxidation of DBT with CHP decreased in the order: MoO₃/Ca-SiO₂ > MoO₃/Ba-SiO₂ > MoO₃/SiO₂ > MoO₃/Sr-SiO₂ > MoO₃/Mg-SiO₂. The MoO₃/Ca-SiO₂ catalysts were characterized by XRD. The DBT conversions on MoO₃/Ca-SiO₂ catalysts with various Ca/Mo ratios were studied. When the Ca/Mo ratio was 0.05, the DBT conversion was the highest (95%) at 60 °C, WHSV 30 h^?1, and O/S molar ratio 3.0.     

Sulfidability and thiophene hydrodesulfurization activity of supported NiMo carbides

Ni–Mo carbides supported on activated carbon were synthesized by carbothermal hydrogen reduction and the effect of the sulfidability on the thiophene hydrodesulfurization catalytic activity was studied. The X-ray diffraction patterns of Ni–Mo carbides showed the presence of β-Mo₂C and NiC when the atomic ratio AR = Ni/(Ni + Mo) was between 0.25 and 0.75, while for AR = 1, it only was detected metallic Ni. The X-ray photoelectron spectroscopy results showed the distribution of different surface species in the passivated catalysts: Mo^δ + (0 ≤ δ ≤ 2), Mo^4 +, Mo^6 +, Ni^δ + and Ni^2 +. After sulfiding the carbides were modified in their surface and catalytic activity.     

Solvent free synthesis of chalcone and flavanone over zinc oxide supported metal oxide catalysts

Liquid phase Claisen–Schmidt condensation between 2′-hydroxyacetophenone and benzaldehyde to form 2′-hydroxychalcone, followed by intramolecular cyclisation to form flavanone was carried out over zinc oxide supported metal oxide catalysts under solvent free condition. The reaction was carried out over ZnO supported MgO, BaO, K₂O and Na₂O catalysts with 0.2 g of each catalyst at 140 °C for 3 h. Magnesium oxide impregnated zinc oxide was observed to offer higher conversion of 2′-hydroxyacetophenone than other catalysts. Further MgO impregnated with various other supports such as HZSM-5, Al₂O₃ and SiO₂ were also used for the reaction to assess the suitability of the support. The order of activity of the support is ZnO > SiO₂ > Al₂O₃ > HZSM-5. Various weight percentage of MgO was loaded on ZnO to optimize maximum efficiency of the catalyst system. The impregnation of MgO (wt%) in ZnO was optimized for better conversion of 2′-hydroxyacetophenone. The effect of temperature and catalyst loading was studied for the reaction.     

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.     

Microwave dielectric properties of Pb2MoO5 ceramic with ultra-low sintering temperature

Ultra-low firing microwave dielectric ceramic Pb₂MoO₅ with monoclinic structure was prepared via a conventional solid state reaction method. The sintering temperature ranged from 530 °C to 650 °C. The relative densities of the ceramic samples were about 97% when the sintering temperature was greater than 570 °C. The best microwave dielectric properties were obtained in the ceramic sintered at 610 °C for 2 h with a permittivity ∼19.1, a Q × f value about 21,960 GHz (at 7.461 GHz) and a temperature coefficient value of −60 ppm/°C. From the X-ray diffraction, backscattered electron image results of the co-fired samples with 30 wt% silver and aluminum additive, the Pb₂MoO₅ ceramics were found not to react with Ag and Al at 610 °C for 4 h. The microwave dielectric properties and ultra-low sintering temperature of Pb₂MoO₅ ceramic make it a promising candidate for low temperature co-fired ceramic applications.     

Identification of the coke accumulation and deactivation sites of Mo2C/HZSM-5 catalyst in CH4 dehydroaromatization

Identifying the preferentially coking sites of Mo₂C/HZSM-5 catalyst in the CH₄ dehydroaromatization at non-oxidative conditions was attempted by employing a physically separable Mo₂C/α-Al₂O₃ + HZSM-5 mixture instead of Mo₂C/HZSM-5 as catalyst. Photographic observation on the spent Mo₂C/α-Al₂O₃ and HZSM-5 components separated from the deactivated mixture and their thermogravimetric analysis clearly revealed that coke accumulation occurred predominantly on the HZSM-5. Then, the comparative activity tests with the physical blends of the deactivated Mo₂C/α-Al₂O₃ + HZSM-5 mixture sample with fresh Mo₂C/α-Al₂O₃ or HZSM-5 further confirmed that the coked HZSM-5 component in the deactivated mixture definitely deactivated while that un-coked Mo₂C constituent remained highly active.     

Synthesis of β-SiAlON powder by carbothermal reduction–nitridation of zeolites with different compositions

β-SiAlON was synthesized from select zeolite Y compositions with different Si/Al ratios by carbothermal reduction–nitridation (CRN), and the correlation between the starting compositions and products was investigated. The carbon content in all of the zeolite samples was fixed at 1.2 times the required stoichiometric value. Zeolite–carbon mixtures were placed in a carbon boat and fired in a furnace at 1300 °C for 0 min, and 1450 °C for 0, 120 min in a N₂ flow of 0.5 l/min. The main phase in each of the samples fired at 1450 °C for 120 min was determined from XRD results as β-SiAlON. It was also found that the ratio of β-SiAlON to minor phases such as α-Si₃N₄ and Si₂N₂O is typically higher in samples prepared from zeolites rather than from silica–alumina mixtures of the same compositions. This indicates that zeolites are ideal raw materials for the CRN synthesis of high purity β-SiAlONs by CRN with various z values.     

Selective hydrogenation of amides using Rh/Mo catalysts

Rh/Mo catalysts formed in situ from Rh₆(CO)₁₆ and Mo(CO)₆ are effective for the liquid phase hydrogenation of CyCONH₂ to CyCH₂NH₂ in up to 87% selectivity, without the requirement for ammonia to inhibit secondary amine formation. Use of in situ HP-FTIR spectroscopy has shown that decomposition of metal carbonyl precursors occurs during an extended induction period, with the generation of recyclable, heterogeneous, bimetallic catalysts. Variations in Mo:Rh content have revealed significant synergistic effects on catalysis, with optimum performance at values of ca. 0.6, and substantially reduced selectivities at ?1. Good amide conversions are noted within the reaction condition regimes 50–100 bar H₂ and 130–160 °C. Ex situ characterization of the catalysts, using XRD, XPS and EDX-STEM, has provided evidence for intimately mixed (ca. 2–4 nm) particles that contain metallic Rh and reduced Mo oxides, together with MoO₃. Silica-supported Rh/Mo analogues, although active, perform poorly at <150 °C and deactivate during recycle.     

Improvements in acidity for TiO2 and SnO2 via impregnation with MoO3 for the esterification of fatty acids

The impregnation of TiO₂ or SnO₂ with MoO₃ forms solid materials exhibiting high acidity (MoO₃/TiO₂ and MoO₃/SnO₂) that might be applied as catalysts for the esterification of fatty acids. TiO₂ and SnO₂ oxide matrixes were prepared using a metal-chitosan complex method, and the acidity of these materials was modified via impregnation with MoO₃. These catalytic systems were characterized by FTIR, N₂ adsorption/desorption isotherms, thermogravimetric analysis, and NH₃ temperature programmed desorption (NH₃-TPD) and tested in the esterification of fatty acids in the presence of methanol. The esterification reactions were carried out at three different temperatures (120 °C, 140 °C and 160 °C) with a 400:100 molar ratio of alcohol:fatty acid and 1% (mass) catalyst loading. Both TiO₂ and SnO₂ only exhibited catalytic activity after their acidity was improved via impregnation with MoO₃, and yields of 80% and 90%, respectively, were achieved at 6 h and 160 ºC.     

Effect of the activation process on thiophene hydrodesulfurization activity of activated carbon-supported bimetallic carbides

The effect of passivation and presulfidation after carbiding of activated carbon-supported Fe–Mo, Co–Mo and Ni–Mo catalysts on their thiophene HDS activity was evaluated. Catalytic precursors were prepared by co-impregnation of the support with solutions of ammonium heptamolybdate and the promotor nitrates or sulfates. Carbiding was achieved by means of the carbothermal method, employing pure H₂ as reductant and the support as the carbon source. Carbided samples were submitted to one out of three types of procedures before HDS tests: (a) passivation at room temperature followed by presulfiding; (b) presulfiding (no passivation); and (c) neither passivation nor sulfiding before HDS. Samples of passivated catalysts prepared from the sulfates of Fe, Co or Ni contained variable amounts of sulfur, as shown by XPS and elemental analysis, while XRD showed only metals and mixed Fe₃Mo₃C or η-M₆Mo₆C₂ (MCo, or Ni) phases. The nitrate-derived catalysts only presented β-Mo₂C and metals (XRD). Sulfur containing catalysts showed high initial activities although deactivate strongly during the first 40 min on the reaction stream, while the unsulfided nitrate-derived samples showed a more stable behavior and lower activities during the 2–3 h of testing. In general, samples submitted to passivation followed by presulfiding showed the higher steady state activities and those neither passivated nor sulfided were the less active. The results show the benefits of a passivating treatment on these carbon-supported catalysts, and point out to the importance of sulfided surface phases in HDS on carbides of transition metal catalysts.     

Molybdenum doped carbon aerogels with catalytic potential

Mo-doped carbon aerogels were obtained in the polycondensation reaction of aqueous resorcinol and formaldehyde by adding Mo-salt at two different stages of the synthesis: (i) to the initial sol; (ii) by incipient wetting impregnation of the supercritically dried polymer gel. Molybdenum added during the polymerization yielded a more compact gel structure with practically no mesoporosity. With post-impregnation, by contrast, mesopores of diameter 3–15 nm were generated. Carbonization appreciably enhanced the microporous character of both samples, but in the mesopore range their pore size distribution was conserved. The Mo-content of the samples was also different: Mo was lost during the solvent exchange before the supercritical drying (i.e., the Mo failed to bind chemically to the polymer matrix). The residual Mo congregated into 25–60 nm bulk clusters of α-Mo₂C. In the other carbon aerogel, finely dispersed α-Mo₂C and η-Mo₃C₂ crystals formed, of size 8–20 nm. On the surface of both carbons the Mo formed oxides. In the model test reaction (acetic acid hydroconversion) the catalytic activity of both carbon aerogels was enhanced by molybdenum. The more open pore structure, higher concentration and finer Mo distribution, as well as its chemical form, may all be responsible for the greater conversion and higher value products obtained with the post-impregnated sample.     

Selective catalytic reduction of NO by ammonia using mesoporous Fe-containing HZSM-5 and HZSM-12 zeolite catalysts: An option for automotive applications

Mesoporous and conventional Fe-containing ZSM-5 and ZSM-12 catalysts (0.5–8 wt% Fe) were prepared using a simple impregnation method and tested in the selective catalytic reduction (SCR) of NO with NH₃. It was found that for both Fe/HZSM-5 and Fe/HZSM-12 catalysts with similar Fe contents, the activity of the mesoporous samples in NO SCR with NH₃ is significantly higher than for conventional samples. Such a difference in the activity is probably related with the better diffusion of reactants and products in the mesopores and better dispersion of the iron particles in the mesoporous zeolite as was confirmed by SEM analysis. Moreover, the maximum activity for the mesoporous zeolites is found at higher Fe concentrations than for the conventional zeolites. This also illustrates that the mesoporous zeolites allow a better dispersion of the metal component than the conventional zeolites. Finally, the influence of different pretreatment conditions on the catalytic activity was studied and interestingly, it was found that it is possible to increase the SCR performance significantly by preactivation of the catalysts in a 1% NH₃/N₂ mixture at 500 °C for 5 h. After preactivation, the activity of mesoporous 6 wt% Fe/HZSM-5 and 6 wt% Fe/HZSM-12 catalyst is comparable with that of traditional 3 wt% V₂O₅/TiO₂ catalyst used as a reference at temperatures below 400 °C and even more active at higher temperatures.     

An efficient Ce-doped MoO3 catalyst and its photo-thermal catalytic synergetic degradation performance for dye pollutant

Herein, an efficient Ce-doped MoO₃ catalyst is prepared by an impregnation method. Under visible light irradiation (≥ 420 nm) or light-off, Ce(5)-doped MoO₃ shows a degradation activity of methylene blue (MB) dye 10 times higher than MoO₃ due to the doped Ce. It is proposed that the light-off degradation is predominated by a Mo/Ce redox cycle, instead of a photocatalytic reaction; while the light-on degradation is via a synergetic effect of both photocatalytic oxidation and Mo/Ce redox cycle. This study offers a new concept for environmental cleaning from cost-saving and energy-saving viewpoints.     

CO removal from reformed fuel over Cu/ZnO/Al2O3 catalysts prepared by impregnation and coprecipitation methods

A composition of Cu/ZnO/Al₂O₃ catalysts prepared by the impregnation method was optimized for water gas shift reaction (WGSR) coupled with CO oxidation in the reformed gas. The optimum composition of the impregnated catalyst for high WGSR activity was 5 wt.% Cu/5 wt.% ZnO/Al₂O₃. The optimum loading amounts of Cu and ZnO in the impregnated catalyst were smaller than those in the coprecipitated catalyst. Its catalytic activity above 200 °C was comparable to that of the conventional coprecipitated Cu/ZnO/Al₂O₃ catalyst. However, the activity of the impregnated Cu/ZnO/Al₂O₃ catalysts was significantly lowered at 150 °C, whereas no deactivation was observed for the coprecipitated catalyst at the same temperature. It was found that deactivation occurred over impregnated catalysts with H₂O and/or O₂ in the reaction gas; it prevented CO adsorption on the surface.     

Optimized vacuum/pressure sol impregnation processing of wood for the synthesis of porous,biomorphic SiC ceramics

Pine (Pinus silvestris) wood with shaped sample dimensions of 20 mm × 20 mm × 5 mm (axial) was selected as the raw material. Samples were dried and, for a half of the samples, resin extraction from the sample was applied. SiO₂ sol was prepared, and samples were impregnated under different vacuum/pressure conditions. Relative impregnation efficiency was calculated for impregnated samples and varied from 95 up to 105% of the theoretical value for different samples and impregnation conditions. Impregnation and drying procedures were repeated up to three times to increase the SiO₂ amount introduced in the sample. Impregnated samples were pyrolyzed at 500 °C under oxygen free atmosphere with the subsequent high temperature treatment at 1600 °C in an Ar atmosphere. Biomorphic SiC ceramics and its precursors were investigated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). An experimental result shows that the optimized vacuum/pressure impregnation technique is highly effective for the introduction of SiO₂ in the wood.     

Selective oxidative dehydrogenation of ethane over MoO3/V2O5–Al2O3 catalysts: Heteropolymolybdate as a precursor for MoO3

Oxidative dehydrogenation of ethane to ethylene was investigated over a series of MoO₃ added V₂O₅–Al₂O₃ catalysts. The catalysts were characterized by BET, XRD, Laser-Raman and FT-IR spectroscopies and TPR technique. Catalytic tests were carried out in a fixed bed stainless steel reactor in the temperature range from 450 to 600 °C. Results revealed that the loading of molybdophosphoric acid (MPA) and the method of preparation had a clear influence on the catalytic performance. Among all, 10 wt.% MPA/V₂O₅–Al₂O₃ solid was found to possess superior activity and selectivity (X-C₂H₆ ~ 35% and S-C₂H₄ ~ 65%). Formation of Mo–V mixed oxide phases on Al₂O₃ appeared to be responsible for this improved performance. This best catalyst also exhibited good long-term stability over a period of ca. 36 h.     

MoO3/reduced graphene oxide composites as anode material for sodium ion batteries

MoO₃/reduced graphene oxide (MoO₃/RGO) composites were successfully prepared via a facile one-step hydrothermal method, and evaluated as anode materials for sodium ion batteries (SIBs). The crystal structures, morphologies and electrochemical properties of the as-prepared samples were characterized by X-ray diffraction, field-emission scanning electron microscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge/discharge tests, respectively. The results show that the introduction of RGO can enhance the electrochemical performances of MoO₃/RGO composites. MoO₃/RGO composite with 6 wt% RGO delivers the highest reversible capacity of ~208 mA h g⁻¹ at 50 mA g⁻¹ after 50 cycles with good cycling stability and excellent rate performance for SIBs. The excellent sodium storage performance of MoO₃/RGO should be attributed to the synergistic effect between MoO₃ and RGO, which offers the increased electrical conductivity, the facilitated electron transfer ability and the buffering of volume expansion.     

Controllable synthesis of α-MoC1-x and β-Mo2C nanowires for highly selective CO2 reduction to CO

Transition metal carbides are attractive catalysts because of their similar properties to precious metals. Here, we report the controllable synthesis of α-MoC_1-x and β-Mo₂C nanowires as highly active and selective catalysts for CO₂ reduction to CO (CO₂ + H₂ → CO + H₂O, reverse water-gas shift reaction, RWGS). CO₂ conversion of > 60% together with nearly 100% CO selectivity was achieved at 600 °C, H₂:CO₂ molar ratio of 4:1, and space velocity of 36,000 mL g^− 1 h^− 1. A formate decomposition mechanism for the RWGS reaction was proposed based on the in-situ DRIFTS results.     

Study on SO42−/ZrO2-MoO3 in the integrative transformation of cottonseed oil deodorizing distillate

With the integrative transformation of non α-tocopherols, glycerides, free fatty acids, and methyl alcohol in cottonseed oil deodorizing distillate as the target reaction, we prepared highly catalytic SO₄^2?/ZrO₂-MoO₃ solid acid catalyst by precipitation–wet impregnation. The optimal conditions for catalyst preparation were then determined by varying sulfuric acid concentration, MoO₃ loading factor, calcination temperature, and calcination time. The structure of SO₄^2?/ZrO₂-MoO₃ solid acid catalyst was then examined by X-ray diffraction (XRD), Brunauer–Emmett–Teller measurements, scanning electron microscopy, and other methods. Results show that the MoO₃ loading factor (percentage weight ratio of MoO₃ to ZrO₂), impregnation concentration of sulfuric acid, and calcination temperature were the most important factors that influenced catalytic activity. The optimal conditions for catalyst preparation were an MoO₃ loading factor of 20%, a sulfuric acid impregnation concentration of 0.75 mol/L, a calcination temperature of 550 °C, and a calcination time of 6 h. The obtained catalyst exhibited the highest catalytic activity under these conditions.