Mo Oxide Species Research Articles

Enhanced catalytic performance for methane dehydro‐aromatization on Mo/HMCM‐49 prepared with citric acid

AbstractA simple citric acid (CA)‐ammonium molybdate co‐impregnation method was applied to prepare the Mo‐based MCM‐49 catalyst to ameliorate the dispersion of metallic Mo for methane dehydro‐aromatization reaction (MDA). The newly prepared catalyst has higher crystallinity and maintains the original morphology of zeolite, which proves that the addition of citric acid will not destroy the basic structure of the catalyst. Based on H2‐TPR, UV‐vis, NH3‐TPD and XPS characterization results, it can be corroborated that the introduction of citric acid changes the coordination state of Mo element. Brønsted acid sites can stabilize the monomeric Mo oxide species by increasing MoOx migration into the MCM‐49 zeolite pores channel after calcination, which may affect the distribution of active sites MoCx and thus improve MDA catalytic activity. The experimental results show that the Mo‐based MCM‐49 catalyst with nCA : nMo ratios of 1.0 has higher methane conversion (14.5 %) and aromatics yield (9.8 %). The combination of XPS, TG and Raman tests of the spent catalyst confirms that the newly synthesized catalysts produce less carbon deposition, thus improving the catalytic durability during the MDA reaction.

Origin of enhanced passivity of Cr–Fe–Co–Ni–Mo multi-principal element alloy surfaces

Surface analysis by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry was applied to investigate the origin of the enhanced surface passivity and resistance to a chloride-induced breakdown provided by the protective ultrathin oxide films formed on Cr–Fe–Co–Ni–Mo single-phase fcc multi-principal element alloys. A bilayer structure of the oxide films is observed with the inner barrier layer mostly constituted of Cr(III) oxide and the outer layer enriched in Cr(III) hydroxides and Mo(IV,VI) oxides. The Mo(VI) and Mo(IV) species are mainly located in the outer and inner parts of the outer layer, respectively. Anodic passivation promotes mainly the growth of the inner layer on the alloy of higher Cr bulk content and the outer layer on the alloy of higher Mo bulk content. Passivation also promotes the enrichment of Cr(III) hydroxide and Mo(IV) and Mo(VI) oxides in the outer layer. Depth distribution analysis suggests that the ultra-thin protective inner barrier contains Cr(III)-depleted heterogeneities acting as weak sites for chloride attack, which are reinforced by the Mo(IV) oxide species concentrated close to the inner barrier layer. This elemental distribution provides an explanation for the reinforcement of the resistance to localized corrosion observed on these Cr–Fe–Co–Ni–Mo alloys.

Hybrid alumina-1D titania nanotube materials as supports for NiMo catalysts with improved activity in hydrodesulfurization

γ-Alumina materials modified with different amounts of 1D titania nanotubes were synthesized, characterized and used as supports for NiMo catalysts evaluated in the hydrodesulfurization of dibenzothiophene and 4,6-dimethyldibenzothiophene. Alumina and titania components in hybrid supports exist as separate domains maintaining their characteristic mesopore structure. After thermal treatment of supports, 1D titania was partially transformed to anatase nanocrystals of <15 nm in size. The presence of 20–40 wt% of 1D titania in alumina increased the activity of the catalysts due to lower metal-support interaction, larger proportion of octahedral Mo oxide species and increased dispersion of MoS2 active phase.

Efficient and economic transesterification of waste cooking soybean oil to biodiesel catalyzed by outer surface of ZSM-22 supported different Mo catalyst

With the aim of developing efficient heterogeneous catalysts for production of clean biodiesel through the transesterification process, a novel Mo catalyst supported on the outer surface of ZSM-22 (xMo/ZSM-22) was developed and synthesized successfully with the wet impregnation method. The characterization results showed that the Mo(VI) oxide species were highly dispersed on the surface of catalysts under the Mo-loading not higher than 5%, and the aggregation of the MoO3 nanoparticles were detected above the Mo-loading of 5%. Meanwhile, the maximum acid intensity was also exhibited in the 5Mo/ZSM-22 catalyst. As a result, the catalytic performance of Mo/ZSM-22 catalysts in the transesterification of waste cooking soybean oil (WCSO) was significantly improved compared with the untreated ZSM-22 catalyst. Especially, the 5Mo/ZSM-22 showed the good activity (XWCSO of 59.29%, SBiodiesel of 99.86%), which was mainly due to the well dispersed Mo species and the larger acidity. Additionally, the 5Mo/ZSM-22 catalyst exhibits good stability and the high biodiesel selectivity, and the fuel properties of the as-synthesized biodiesel meet ASTM D6751 standards. This work provides a useful synthesis strategy of transesterification catalysts for the preparation of biodiesel.

The influence of lanthanum (La) promoter on Mo/Al2O3 for the catalytic synthesis of methyl mercaptan (CH3SH) from COS/H2/H2S

The cooperative transition of sulfur-containing pollutants of COS/H2S/H2 to the chemical feedstock of methyl mercaptan (CH3SH) catalyzed by Mo-based catalysts has good application prospects. However, the role of rare earth elements as promoters in this reaction system has not been investigated currently. In this study, the effects of the rare earth of lanthanum (La) on the catalytic performance of Mo-based catalysts were investigated by XRD, Raman, TEM, H2-TPR, and XPS characterizations. The results showed that the addition of La species led to the formation of larger particle size and lower sulfidation degree of Mo species, but the higher, rather than lower, catalytic activity was obtained. This unusual phenomenon is confirmed to be due to the generation of La2(MoO4)3 crystals covered with surface MoS2 layers caused by the interaction of Mo oxide and La species at the sulfidation atmosphere. The presence of La2(MoO4)3 changes the electronic and chemical environment of MoS2 in the catalyst, which lead a positive effect on the catalytic activity, outweighing the negative effect caused by the larger particle size of Mo species and the reduced sulfidation.

Sulfated TiO2 supported molybdenum-based catalysts for transesterification of Jatropha seed oil: Effect of molybdenum species and acidity properties

A series of sulfated TiO2 supported Mo (xMo/1.5ST) catalysts were synthesized successfully and evaluated for the transesterification of Jatropha seed oil (JO) to biodiesel. Extensive characterization exhibits that the well dispersed Mo oxide species, with Mo6+ and Mo5+ oxidation states, were formed on the surface of the sulfated TiO2. The results indicate the presence of many sulfates species leads to the reduction of Mo6+ species to Mo5+ species. Furthermore, the catalytic activity of the xMo/1.5ST catalysts gradually increases with the increase of Mo-loading, although the sulfates content and the total acid intensity decrease. The transesterification process was fully investigated, and the 10Mo/1.5ST shows the highest yield of biodiesel of up to 97.42%. Additionally, the possible mechanism for the JO transesterification over Mo/1.5ST was described, and the fuel properties of the as-synthesized biodiesel conformed to the current international standards.

Computational Insights into Active Site Formation during Alkene Metathesis over a MoOx/SiO2 Catalyst: The Role of Surface Silanols

A MoOx/SiO2 system is an effective catalyst for alkene metathesis; however, the mechanism of the transformation of the surface metal oxide species into active alkylidene sites is not well recognized. In this work, comprehensive density functional theory studies of the initiation mechanisms for alkene metathesis on the MoOx/SiO2 catalyst have been performed. It is shown that surface silanol groups interacting with Mo species and constituting Brønsted acid sites can play a key role in reduction of the dioxo Mo(VI) species to the mono-oxo Mo(IV) species by alkene, through Mo(VI) alkoxy species, and in subsequent formation of the Mo(VI) alkylidene species. An alternative activation pathway avoiding the reduction step is also possible. The proposed mechanisms of silanol-assisted reduction/initiation with propene are predicted to be more kinetically and thermodynamically accessible than the often assumed pseudo-Wittig mechanism. The silanol-assisted activation of the mono-oxo Mo(IV) species by propene is kinetically preferred over non-silanol–assisted initiation mechanisms, that is, 1,2-hydrogen shift mechanism, allyl mechanism, and oxidative coupling mechanism involving molybdacyclopentane species. The reactivity of the Mo sites is significantly affected by their geometry and the local structure of silica. Our results suggest that only a small fraction of the Mo oxide species with a suitable geometry and neighborhood can be effectively activated by alkenes.

Investigation of MoOx/Al2O3 under Cyclic Operation for Oxidative and Non-Oxidative Dehydrogenation of Propane

A MoOx/Al2O3 catalyst was synthesised and tested for oxidative (ODP) and non-oxidative (DP) dehydrogenation of propane in a reaction cycle of ODP followed by DP and a second ODP run. Characterisation results show that the fresh catalyst contains highly dispersed Mo oxide species in the +6 oxidation state with tetrahedral coordination as [MoVIO4]2− moieties. In situ X-ray Absorption Spectroscopy (XAS) shows that [MoVIO4]2− is present during the first ODP run of the reaction cycle and is reduced to MoIVO2 in the following DP run. The reduced species are partly re-oxidised in the subsequent second ODP run of the reaction cycle. The partly re-oxidised species exhibit oxidation and coordination states that are lower than 6 but higher than 4 and are referred to as MoxOy. These species significantly improved propene formation (relatively 27% higher) in the second ODP run at similar propane conversion activity. Accordingly, the initial tetrahedral [MoVIO4]2− present during the first ODP run of the reaction cycle is active for propane conversion; however, it is unselective for propene. The reduced MoIVO2 species are relatively less active and selective for DP. It is suggested that the MoxOy species generated by the reaction cycle are active and selective for ODP. The vibrational spectroscopic data indicate that the retained surface species are amorphous carbon deposits with a higher proportion of aromatic/olefinic like species.

Molecular Characteristics of Light Cycle Oil Hydrodesulfurization over Silica-Alumina-Supported NiMo Catalysts.

A detailed understanding of the catalytic upgrading of light cycle oil (LCO) is important to achieve effective deep hydrodesulfurization (HDS) when LCO is mixed with straight run gas oil in the diesel pool. Herein, HDS of polyaromatic-rich LCO was studied at the molecular level over three NiMo catalysts on silica–alumina supports, which were synthesized on the pilot scale using different silica/alumina mixing procedures. Gas chromatography with atomic emission detection and two-dimensional gas chromatography with time-of-flight mass spectrometry were used to evaluate the HDS performance through determining the feed and product compositions, respectively, at the molecular level. Furthermore, the textural properties of the catalysts were evaluated using Raman spectroscopy, transmission electron microscopy, and the temperature-programmed desorption of NH3. The performance of the best catalyst was attributed to its higher content of octahedrally coordinated Mo oxide species, a lower number of layered stacks, and the more acidic sites on the surface. In addition, the hydrotreating reactivity of various family groups in LCO over the catalyst was investigated.

Box-behnken design to enhance the corrosion resistance of plasma sprayed Fe-based amorphous coating

Fe48Cr15Mo14C15B6Y2 (at. %) amorphous coatings under various spraying parameters were deposited on Q235 steel by atmospheric plasma spraying. Passive film thickness was estimated by power-law formula proposed by Hirschorn to investigate the corrosion properties of passive film on the coating surface. Box-behnken design (BBD) in combination with response surface methodology (RSM) was utilized to gain the thickest passive film (about 1.60 nm) in 3.5 wt% NaCl solution. XPS results indicated that passive film formed on the optimal coating comprised of Fe, Cr and Mo oxide species and certain amount of bounded water. Mott-Schottky curve further confirmed an enhanced corrosion resistance of the optimal coating due to a low carrier density under the magnitude of 1018 cm−3. The surface morphology after 20 days immersion suggested that no corrosion products were observed on the optimal coating surface and the passive film retained its stability to protect the metal substrate effectively.

Transesterification of rice bran oil to biodiesel using mesoporous NaBeta zeolite-supported molybdenum catalyst: Experimental and kinetic studies

Mesoporous NaBeta zeolite was synthesized successfully in the presence of a mesoscale copolymer (RCC) template, and a series of mesoporous NaBeta-supported molybdenum with varying Mo loadings (xMo/NaBeta) catalysts were prepared carefully for biodiesel production via the transesterification of rice bran oil. The resultant catalyst exhibits high activity and excellent stability in this transesterification reaction, attributing to the formation of highly dispersed Mo active species and the well interaction between the Mo oxide species and support. The physical properties of the as-produced biodiesel accorded with the current international biodiesel standards. Furthermore, the intrinsic kinetics of the transesterification of rice bran oil with methanol over 7Mo/NaBeta catalyst was also studied in the absence of mass transfer. The corresponding kinetic equation was established by correlating the data of initial rate and reactant concentration, and the apparent activation energy was calculated to be 59.05 kJ mol−1.

Production of green diesel from karanja oil (Pongamia pinnata) using mesoporous NiMo-alumina composite catalysts

NiMo-alumina catalysts with a small quantity of Mo showed ordered mesoporous structure. For 4.3 mmol total metals content, mesoporous structure, however, became disorder for 1.7 mmol and higher Mo content. The C18 alkane was the leading product among hydrocarbons (C15–C22) formed during hydrodeoxygenation of karanja oil. The catalytically active NiMo complex and Mo oxide species with lower oxidation states were substantial in NiMo catalysts with the modest quantity of both Mo and Ni and relatively small for the other two extremes. The catalytic activity and selectivity to C18 alkane were thus enhanced with the rise in Mo content up to 3.4 mmol. The catalytic activity was also improved with growing total metals content and temperature. The optimum catalyst (0.9 mmol Ni and 3.4 mmol Mo) showed the complete conversion of oxygenates with 13 wt% < C18, 75 wt% C18, and 12 wt% > C18 alkanes at 340 °C and 4 h reaction.

Effect of sulfiding conditions on the hydrodesulfurization performance of the ex-situ presulfided CoMoS/γ-Al2O3 catalysts

The influence of sulfiding conditions in the ex-situ presulfidation process on the morphology and performances of CoMoS/γ-Al2O3 catalysts was studied in details for hydrodesulfurization (HDS) of dibenzothiophene. Surface and bulk characterization using XRD, HRTEM, XPS, TPR and FT-IR confirmed that the sulfiding agent mixture of (NH4)2S and elemental S effectively react with the supported Mo oxide to form intermediate thiosalts from mononuclear ammonium thiomolybdates to polynuclear ammonium thiomolybdates, namely the formed thiosalts from (NH4)2MoS4 to (NH4)2Mo2S12 and (NH4)2Mo3S13,which weaken the strong interaction between the γ-Al2O3 support and Mo oxide, resulting in maximum sulfidation of Mo oxide species. Mo species tend to agglomerate as the number of Mo nuclear increases, which further induces significant changes in the morphology of MoS2 particles. Such observation was further correlated with HDS activity and selectivity. The CoMoS/γ-Al2O3 catalyst with (NH4)2Mo2S12 as intermediate exhibits suitable slab lengths and stacking numbers of MoS2 slabs with optimal compromisebetween the Mo dispersion and ratios of edge sites to rim sites, and show the highest HDS activity. Therefore, a tailor-made catalyst with excellent HDS performance could be developed by adjusting the sulfiding conditions through ex-situ presulfidation method.

Modeling the interaction of molybdenum species adsorbed on a pyrolytic graphite platform and correlations with XPS spectra at different ETAAS stages

Adsorptions of molybdenum species and interactions on a model pyrolytic graphite platform (PGP) sites were calculated employing quantum methods (density functional theory (DFT) and parametric method number 6 (PM6)). The aim of this work is to propose, at the molecular level, the structure of different chemisorbed species on the PGP surface used in the electrothermal atomic absorption spectroscopy (ETAAS) process for a Mo analyte. The carbon surface was modeled by a seven-ring polycyclic aromatic system (coronene). Molybdenum species (Mo, Mo2, MoO, MoO2, MoO3, and MoO4) adsorbed on a coronene surface were studied on different adsorption sites. Calculations of these species showed that chemisorption strengths are stronger on dehydrogenated and decarbonized edge sites than on sites located in the flat central region of the model graphite. It was confirmed that Mo-oxygenated species interactions are very strong at the edge sites and therefore the reduction of these species must occur before the atomization (evaporation of Mo species) takes place, in agreement with experimental results. Comparisons between experimental XPS spectra for ETAAS process (drying, ashing, and atomization) and calculated adsorption energies of Mo species allow an interpretation of which species are present on the PGP at each stage. The presence of very strong bonded molybdenum carbide species on decarbonized sites may explain the memory effects. Experimentally observed migration of Mo oxide species to the edges of the PGP is also corroborated by their small chemisorption energies on central sites of the PGP model.

NiMo catalysts supported on Al, Nb, Ti or Zr-containing MCM-41 for dibenzothiophene hydrodesulfurization

xMCM-41 materials (x = Al, Nb, Ti or Zr) with nominal x loading of 0.5 mmol/g were prepared by chemical grafting of Al2O3, Nb2O5, TiO2 and ZrO2 precursors and used as supports for NiMo/xMCM-41 catalysts. Small-angle XRD characterization of supports showed typical patterns of solids with hexagonal arrangement of mesopores. After the deposition of Ni and Mo oxide species on the supports a decrease in their long-range pore arrangement was observed. Nevertheless, the prepared NiMo catalysts possessed high specific surface areas and pore volumes. Powder XRD patterns showed the formation of the MoO3 crystalline phase in the NiMo/MCM-41 reference. This phase was not detected in the catalysts supported on xMCM-41 materials. An increase in the dispersion of MoO3 species with the incorporation of alumina, niobia, titania and zirconia in the MCM-41 support was due to the stronger interaction of Mo and Ni oxide species with xMCM-41 materials (H2-TPR and UV–vis DRS). Surface modification of the MCM-41 with different metal oxides increased the dibenzothiophene conversion and pseudo first-order rate constants. This effect was more evident for the NiMo/TiMCM-41 catalyst. Selectivity of the catalysts (product distributions) was also affected by the incorporation of Al-, Zr-, Ti- and Nb-oxide species in the support.

Effect of Si/Al2 ratios in Mo/H-MCM-22 on methane dehydroaromatization

A series of Mo(5)/H-MCM-22 catalysts with various Si/Al2 ratios were applied to methane dehydroaromatization reaction. According to the NH3 TPD, XPS, TPO, UV Raman and STEM-EDS analysis, it is clearly verified that Brønsted acid site, determined by the Si/Al2 ratios, stabilizes monomeric Mo oxide species, by facilitating to migrate Mo oxides into the microchannel of zeolite, so that it can influence the distribution of catalytically active Mo oxide species. Combined TPO and UV Raman analysis of post-reaction catalysts obviously confirmed that HT (high temperature) type coke is primarily composed of polyaromatics such as naphthalene and anthracene, which brings about the deactivation of the catalyst during MDA reaction. The formation of HT type coke is mainly affected by the amount of Mo oxides present on the external surface of zeolite, which indicates that the major role of zeolite microchannel is to provide a shape-selective environment during the conversion of methane to benzene. To sum up, an increment in Brønsted acid site enhances the dispersion of Mo oxides, thus leading to the improved methane conversion rate and benzene formation rate while suppressing the formation of HT type coke concomitantly.

Solvothermal Synthesis, Gas‐Sensing Properties, and Solar Cell‐Aided Investigation of TiO2–MoOx Nanocrystals

AbstractTitania anatase nanocrystals were prepared by sol‐gel/solvothermal synthesis in oleic acid at 250 °C, and modified by co‐reaction with Mo chloroalkoxide, aimed at investigating the effects on gas‐sensing properties induced by tailored nanocrystals surface modification with ultra‐thin layers of MoOx species. For the lowest Mo concentration, only anatase nanocrystals were obtained, surface modified by a disordered ultra‐thin layer of mainly octahedral MoVI oxide species. For larger Mo concentrations, early MoO2 phase segregation occurred. Upon heat treatment up to 500 °C, the sample with the lowest Mo concentration did not feature any Mo oxide phase segregation, and the surface Mo layer was converted to dense octahedral MoVI oxide. At larger Mo concentrations all segregated MoO2 was converted to MoO3. The two different materials typologies, depending on the Mo concentration, were used for processing gas‐sensing devices and tested toward acetone and carbon monoxide, which gave a greatly enhanced response, for all Mo concentrations, to acetone (two orders of magnitude) and carbon monoxide with respect to pure TiO2. For the lowest Mo concentration, dye‐sensitized solar cells were also prepared to investigate the influence of anatase surface modification on the electrical transport properties, which showed that the charge transport mainly occurred in the ultra‐thin MoOx surface layer.

NiMo catalysts supported on Mn-Al2O3 for dibenzothiophene hydrodesulfurization application

Modification of the traditional Al2O3 support through addition of manganese to Al2O3 mixed Mn-Al oxides was herein envisaged to obtain highly active NiMo catalysts for hydrodesulfurization application. The effect of adding manganese was determined considering different Mn-Al2O3 supports synthetized using a sol–gel approach. The manganese-containing supports were furthermore impregnated with Ni(NO3)2+(NH4)6Mo7O24 aqueous solutions at pH=9 and characterized at their oxide state using UV–vis diffuse reflectance and Raman spectroscopies after drying and calcination steps. NiMo/Mn-Al2O3 catalysts were also characterized at the sulfide state mainly by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Finally, the sulfide catalysts were evaluated in the hydrodesulfurization of dibenzothiophene.Results show that the oxidation state of manganese species directly influences the nature of the Mo oxide species and their interaction with the Al2O3 support. At low Mn content (up to 0.5mol% Mn as MnO), Mn2+ species leads to weaker interaction with the support and a higher intrinsic activity of the NiMoS species. However, these promoted sites are also formed in a lower amount than without adding Mn to the support. At too high manganese content (≥2mol% Mn as MnO), Mn3+ species are formed and react with Ni to form a spinel phase decreasing the proportion of promoted phase to be formed after sulfidation. The highest activity is therefore observed at an intermediate Mn content of 1mol% for which a higher intrinsic activity resulting from weaker support interaction and higher sulfidation rate combine together to achieve highly active NiMo HDS catalysts.

Ammoxidation of ethylene to acetonitrile over vanadium and molybdenum supported zeolite catalysts prepared by solid-state ion exchange

This work reports on the investigation of the influence of the parent zeolite topology (MFI, MOR and USY) on the ammoxidation of ethylene to acetonitrile over vanadium and molybdenum oxide supported zeolite catalysts. The physico-chemical properties were investigated by several characterization techniques such as XRD, N2-adsorption, 27Al MAS NMR, TEM, XPS, DR UV–vis, Raman and DRIFT spectroscopies and H2-TPR. From the catalytic results, V and Mo oxide species in USY and MOR zeolites led to less active catalysts when compared to MFI structure. These results suggest that the catalytic performances depend strongly on the zeolite structure and thus, the size of the formed metal oxide particles. The catalytic activity and selectivity are controlled by the porous structure and the chemical state of V and Mo species. The extent of dispersion and reducibility of supported M-Ox (MV or Mo) species are governed by the chemical identity of the support as detected by TPR analysis and optical absorption spectroscopy. The good catalytic performance of MFI-type zeolite might be related to the high dispersion of metal oxide species and to the internal pore space which permits an effective accessibility of the reactants.

A MoVI grafted Metal Organic Framework: Synthesis, characterization and catalytic investigations

We present the post-modification of a gallium based Metal Organic Framework, COMOC-4, with a Mo-complex. The resulting Mo@COMOC-4 was characterized by means of N2 sorption, XRPD, DRIFT, TGA, XRF, XPS and TEM analysis. The results demonstrate that even at high Mo-complex loadings on the framework, no aggregation or any Mo or Mo oxide species are formed. Moreover, the Mo@COMOC-4 was evaluated as a catalyst in the epoxidation of cyclohexene, cyclooctene and cyclododecene employing TBHP in decane as oxidant. The post-modified COMOC-4 exhibits a very high selectivity toward the epoxide (up to 100%). Regenerability and stability tests have been carried out demonstrating that the catalyst can be recycled without leaching of Mo or loss of crystallinity.