Molybdenum Catalysts Research Articles

Implication of oxidant activation on olefin epoxidation catalysed by Molybdenum catalysts with aroylhydrazonato ligands: Experimental and theoretical studies

• Dinuclear and mononuclear molybdenum(VI) complexes with aroylhydrazonato ligands were tested as catalysts for cyclooctene and oct-1-ene epoxidation. • TBHP in decane provided outstanding results in terms of cyclooctene epoxidation: TOF 2.5 min >9000 with 0.25 mol % [Mo] loading. • DFT calculations confirmed the relationship between the ligand nature and reactivity with TBHP and H 2 O 2 as oxidant agents. Dinuclear and mononuclear molybdenum(VI) complexes with aroylhydrazonato ligands were studied as catalysts for cyclooctene and oct-1-ene epoxidation. Different isomers of the OH and NH 2 functionalised aroylhydrazones led to four specific [MoO 2 L] units showing different catalytic activity. Three oxidants have been investigated: tert -butylhydroperoxide (TBHP) in water or in decane, and hydrogen peroxide (H 2 O 2 ). Catalytic processes with TBHP in decane provided outstanding results in the case of cyclooctene epoxidation: TOF 2.5 min > 9000 with 0.25 mol % [Mo] loading. DFT calculations confirmed the relationship between the ligand nature and reactivity with TBHP and H 2 O 2 as oxidant agents. In the case of TBHP as oxidant, calculations considered the solvent in which the oxidant is delivered to the olefin. A plausible mechanism with H 2 O 2 has been proposed after considering various pathways.

Alcohol Oxidation Assisted by Molybdenum Hydrazonato Catalysts Employing Hydroperoxide Oxidants

Molybdenum(VI) catalysts were obtained from methanol or acetonitrile by the reaction of [MoO2(C5H7O2)2] and isonicotinoyl- or nicotinoyl-based aroylhydrazones. Reactions in methanol resulted in the formation of the mononuclear complexes [MoO2(L1–4)(MeOH)] (1a–4a), while the ones in acetonitrile provided polynuclear complexes [MoO2(L1–4)]n (1–4). Crystals of polynuclear compound, [MoO2(L3)]n∙H2O (3∙H2O), suitable for X-ray diffraction analysis were obtained by the solvothermal procedure at 110 °C. Complexes were characterized by infrared spectroscopy (IR-ATR), nuclear magnetic resonance (NMR), elemental analysis (EA), and thermogravimetric analysis (TGA). The prepared catalysts were tested in alcohol oxidation reactions. Carveol, cyclohexanol, and butan-2-ol were investigated substrates. Because the alcohol oxidations are very challenging due to various possible pathways, the idea was to test different oxidants, H2O2, TBHP in water and decane, to optimize the researched catalytic system.

Spectroscopic and Theoretical Study on Siloxy-Based Molybdenum and Tungsten Alkylidyne Catalysts for Alkyne Metathesis

A combined spectroscopic and theoretical study on triphenyl- and dimethyl-phenyl siloxy molybdenum and tungsten alkylidyne catalysts for alkyne metathesis is reported. Using NMR, X-ray, UV–vis, and resonance Raman spectroscopy and density functional theory calculations, the influence of different ligand systems and metal centers on the geometric and electronic structure and thermochemistry of different intermediates is investigated, that is, the starting alkylidyne and the derived metallacyclobutadiene (MCBD) and metallatetrahedrane (MTd). This includes a mechanistic and kinetic study on the formation and isomerization of MCBDs and MTds. Upon changing from monodentate to tripodal siloxy ligands, higher steric strain is imposed, which modulates the relative energies of the different intermediates. Additionally, intramolecular dispersion interactions between the bound substrate and the ligand can be operative. Tungsten as the central metal leads to stronger M–C σ-bonds, which overstabilize the reaction intermediates and preclude effective turnover. Furthermore, kinetic modeling strongly suggests that MTds are off-cycle intermediates based on the high barriers for direct formation but low barriers for isomerization from MCBDs. We infer from our findings that effective catalysis can only be achieved when factors that (over)stabilize intermediates, such as strong M–C bonds or large dispersion interactions, are prevented by appropriate catalyst design.

A Bioinspired Molybdenum Catalyst for Aqueous Perchlorate Reduction.

Perchlorate (ClO4-) is a pervasive, harmful, and inert anion on both Earth and Mars. Current technologies for ClO4- reduction entail either harsh conditions or multicomponent enzymatic processes. Herein, we report a heterogeneous (L)Mo-Pd/C catalyst directly prepared from Na2MoO4, a bidentate nitrogen ligand (L), and Pd/C to reduce aqueous ClO4- into Cl- with 1 atm of H2 at room temperature. A suite of instrument characterizations and probing reactions suggest that the MoVI precursor and L at the optimal 1:1 ratio are transformed in situ into oligomeric MoIV active sites at the carbon-water interface. For each Mo site, the initial turnover frequency (TOF0) for oxygen atom transfer from ClOx- substrates reached 165 h-1. The turnover number (TON) reached 3840 after a single batch reduction of 100 mM ClO4-. This study provides a water-compatible, efficient, and robust catalyst to degrade and utilize ClO4- for water purification and space exploration.

The role of sulfur sinks and micro-structured supports on the performance of sulfur-sensitive non-PGM catalysts

The presence of traces of sulfur dioxide (i.e. 1–10 ppmV) alongside residual methane emissions induces the rapid deactivation of typical methane abatement catalysts. As a result, sulfur-induced poisoning is the main challenge faced by after-treatment technologies for residual methane abatement. The aim of this study was to synthesise a series of cobalt and molybdenum catalysts and assess their performance under realistic after-treatment conditions. The series of catalysts was characterised by BET, XRD, XPS, TPR, TGA under air flow, SEM, EDX and in situ DRIFTS. In addition, the performance of Co100 supported on an alumina hollow fibre was assessed under similar reaction conditions and characterised by SEM and EDX. It was found that the catalytic activity of the series of cobalt and molybdenum catalysts decreased in the following order: Co100 > Co80Mo20 > Co60Mo40. In addition, it was discovered that the other two catalysts (i.e. Co25Mo75 and Mo100) were inactive at 450 °C due to the lack of highly active Co3+ species. Among the active catalysts, Co100 and Co80Mo20 experienced an initial activation followed by a gradual deactivation whereas Co60Mo40 experienced a 1 h interval with stable methane conversion levels in between the activation and deactivation. The delay in poisoning of the active cobalt (II,III) oxide phase of Co60Mo40 was attributed to the sulfur sink-properties of the cobalt molybdenum oxide present in this catalyst (i.e. 44 wt%). Finally, Co100 supported on an alumina hollow fibre achieved the highest methane conversion per unit mass of catalyst and its performance was stable for a duration of 2.5 h. The high activity was attributed to the large surface area provided by the geometry of the support while the higher resistance to sulfur poisoning was credited to the sulfur sink-properties of the alumina, which delayed the poisoning of the active phase.

Preparation of a hollow HZSM-5 zeolite supported molybdenum catalyst by desilication-recrystallization for enhanced catalytic properties in propane aromatization

A hollow HZSM-5 supported molybdenum (Mo) catalyst was successfully prepared by desilication-recrystallization method of HZSM-5 in tetrapropylammonium hydroxide (TPAOH) solution. The as-synthesized catalyst exhibiting excellent performance in propane aromatization reaction was thoroughly investigated, along with the mechanism for the formation of hollow structure and mechanism for the deactivation. Results revealed that TPA ​+ ​exerted a controllable desilication and recrystallization effect. Compared with Na2CO3 treatment which led to a mesoporous structure, TPAOH treatment was able to leach out the inner Si species by OH- and then those Si species recrystallized on the external surface, resulting in the hollow structure and a silicon-rich external surface. In addition, the hollow zeolite exposing higher external surface provided more anchoring sites for Mo species, which was beneficial to the Mo related Lewis acid sites but decreased the surface Bronsted acidity. At 823 ​K and a space velocity of 1800 ​mL·g-1·h-1, the hollow Mo/HZSM-5(TPAOH) catalyst behaved a propane conversion of 73.01% and aromatics selectivity of 77.36%, exceeding those of the hierarchical Mo/HZSM-5(Na2CO3) catalyst by 5.97% and 9.96%, respectively. These features of Mo/HZSM-5(TPAOH) granted it superb diffusion ability and optimized Bronsted/Lewis acid ratio (B/L ​= ​1.3), which greatly enhanced the propane conversion and aromatic selectivity together with a preeminent catalytic stability.

Oxygen atom transfer catalysis by dioxidomolybdenum(VI) complexes of pyridyl aminophenolate ligands

A series of new cationic dioxidomolybdenum(VI) complexes [MoO2(Ln)]PF6 (2–5) with the tripodal tetradentate pyridyl aminophenolate ligands HL2-HL5 have been synthesized and characterized. Ligands HL2-HL4 carry substituents in the 4-position of the phenolate ring, viz. Cl, Br and NO2, respectively, whereas the ligand HL5, N-(2-hydroxy-3,5-di-tert-butylbenzyl)-N,N-bis(2-pyridylmethyl)amine, is a derivative of 3,5-di-tert-butylsalicylaldehyde. X-ray crystal structures of complexes 2, 3 and 5 reveal that they have a distorted octahedral geometry with the bonding parameters around the metal centres being practically similar. Stoichiometric oxygen atom transfer (OAT) properties of 5 with PPh3 were investigated using UV–Vis, 31P NMR and mass spectrometry. In CH2Cl2 solution, a dimeric Mo(V) complex [(µ-O){MoO(L5)}2](PF6)26 was formed while in methanol solution an air-sensitive Mo(IV) complex [MoO(OCH3)(L5)] 7 was obtained. The solid-state structure of the µ-oxo bridged dimer 6 was determined by X-ray diffraction. Complex 7 is unstable under ambient conditions and capable of reducing DMSO, thus showing reactivity analogous to that of DMSO reductases. Similarly, the OAT reactions of complexes 2–4 also resulted in the formation of dimeric Mo(V) and unsaturated monomeric Mo(IV) complexes that are analogous to complexes 6 and 7. Catalytic OAT at 25 °C could also be observed, using complexes 1–5 as catalysts for oxidation of PPh3 in deuterated dimethylsulfoxide (DMSO‑d6), which functioned both as a solvent and oxidant. All complexes were also tested as catalysts for sulfoxidation of methyl-p-tolylsulfide and epoxidation of various alkene substrates with tert-butyl hydroperoxide (TBHP) as an oxidant. Complex 1 did not exhibit any sulfoxidation activity under the conditions used, while 2–5 catalyzed the sulfoxidation of methyl-p-tolylsulfide. Only complexes 2 and 3, with ligands containing halide substituents, exhibited good to moderate activity for epoxidation of all alkene substrates studied, and, in general, good activity for all molybdenum(VI) catalysts was only exhibited when cis-cyclooctene was used as a substrate. No complex catalysed epoxidation of cis-cyclooctene when an aqueous solution of H2O2 was used as potential oxidant.

Insights into the Structure-Activity Relationship in Aerobic Alcohol Oxidation over a Metal-Organic-Framework-Supported Molybdenum(VI) Catalyst.

The understanding of structure-activity relationships at the atomic level has played a profound role in heterogeneous catalysis, providing valuable insights into designing suitable heterogeneous catalysts. However, uncovering the detailed roles of how such active species' structures affect their catalytic performance remains a challenge owing to the lack of direct structural information on a specific active species. Herein, we deposited molybdenum(VI), an active species in oxidation reactions, on the Zr6 node of a mesoporous zirconium-based metal-organic framework (MOF) NU-1200, using solvothermal deposition in MOFs (SIM). Due to the high crystallinity of the NU-1200 support, the precise structure of the resulting molybdenum catalyst, Mo-NU-1200, was characterized through single-crystal X-ray diffraction (SCXRD). Two distinct anchoring modes of the molybdenum species were observed: one mode (Mo1), displaying an octahedral geometry, coordinated to the node through one terminal oxygen atom and the other mode (Mo2) coordinated to two adjacent Zr6 node oxygen atoms in a tetrahedral geometry. To investigate the role of base in the catalytic activity of these Mo centers, we assessed the activity of Mo-NU-1200 for the aerobic oxidation of 4-methoxybenzyl alcohol as a model reaction. The results revealed that Mo-NU-1200 exhibited remarkably higher catalytic reactivity under base-free conditions, while the presence of base inhibited the catalytic reactivity of this species. SCXRD studies revealed that the molybdenum binding motifs (structures of the supported metal on the Zr6 node in the MOF) changed over the course of the reactions. Following the oxidation without base, both pristine coordination modes (Mo1 and Mo2) evolved into a new coordination mode (Mo3), in which the molybdenum atom coordinated to two adjacent oxygen atoms from the Zr6 node in an octahedral geometry, while in the presence of base, the pristine Mo1 coordination mode evolved entirely into the pristine Mo2. This study demonstrates the direct observation of an active species' structural evolution from metal installation to subsequent catalytic reaction. As a result, these subtle structural changes in catalyst binding motifs led to distinct differences in catalytic activities, providing a compelling strategy for elucidating structure-activity relationships.

Converting Cyclohexanone to Liquid Fuel-Grade Products: A Characterization and Comparison Study of Hydrotreating Molybdenum Catalysts

Developing biomass-based strategies for liquid bio-fuels production is promising for the reduction of the aftereffects of fossil fuels. The conversion of lignin-derived intermediates such as cyclohexanone is currently of a great deal of interest. The current study aims to evaluate the molybdenum-based hydrotreating catalysts for the conversion of cyclohexanone in the presence of hydrogen. Catalytic experiments at 400 °C, 15 bar, and a range of WHSV were developed. The experiments reveal that catalyst type and reaction WHSV affect the cyclohexanone conversion, product distribution, deoxygenation efficiency, total hydrocarbon production capacity, and heating value of the product blend. The main products include C6 cyclic and aromatic hydrocarbons and oxygenates. Cyclohexane, cyclohexene, benzene, cyclohexanol, and phenol are major products. Small quantities of methylcyclopentane and bicyclic hydrocarbons and oxygenates are also reported in some cases. Increasing WHSV reduced the cyclohexanone conversion. Cyclohexanone conversions up to 89% were observed at the lowest WHSVs over a cobalt-molybdenum sample. The highest hydrocarbon production capacity (99.53%) was managed at WHSV = 5.240 gcyclohexanone/gcat h over a cobalt-molybdenum sample, while the highest deoxygenation efficiency, i.e. 81.29% degree of deoxygenation and 5.35 C/O ratio enhancement were achieved at WHSV = 0.262 gcyclohexanone/gcat h by a nickel-molybdenum sample. The heating values would be enhanced by up to 22.7% when cyclohexanone is converted over the utilized catalysts. The larger heating value (44.90 MJ/kg, 22.7% enhancement) was obtained over a nickel-molybdenum catalyst, which is comparable to the energy density of the conventional fuels. The results reveal that the catalysts are efficient in the conversion of cyclohexanone to liquid bio-fuels.

Interference from HONO in the measurement of ambient air NO2 via photolytic conversion and quantification of NO

The reference method to quantify mixing ratios of the criteria air pollutant nitrogen dioxide (NO2) is NO-O3 chemiluminescence (CL), in which mixing ratios of nitric oxide (NO) are measured by sampling ambient air directly, and mixing ratios of NOx (= sum of NO and NO2) are measured by converting NO2 to NO using, for example, heated molybdenum catalyst or, more selectively, photolytic conversion (P-CL). In this work, the nitrous acid (HONO) interference in the measurement of NO2 by P-CL was investigated. Results with two photolytic NO2 converters are presented. The first used radiation centered at 395 nm, a wavelength region commonly utilized in P-CL. The second used light at 415 nm, where the overlap with the HONO absorption spectrum and hence its photolysis rate are less. Mixing ratios of NO2, NOx and HONO entering and exiting the converters were quantified by Thermal Dissociation Cavity Ring-down Spectroscopy (TD-CRDS). Both converters exhibited high NO2 conversion efficiency (CFNO2; > 90%) and partial conversion of HONO. Plots of CF against flow rate were consistent with photolysis frequencies of 4.2 s-1 and 2.9 s-1 for NO2 and 0.25 s-1 and 0.10 s−1 for HONO at 395 nm and 415 nm, respectively. CFHONO was larger than predicted from the overlap of the emission and HONO absorption spectra. The results imply that measurements of NO2 by P-CL marginally but systematically overestimate true NO2 concentrations, and that this interference should be considered in environments with high HONO:NO2 ratios such as the marine boundary layer or in biomass burning plumes.

Front Cover Picture: Recent Developments in Z‐Selective Olefin Metathesis Reactions by Molybdenum, Tungsten, Ruthenium, and Vanadium Catalysts (8/2021)

The front cover picture, designed by K. Nomura and K. M. Dawood illustrates the basic mechanism for Z-selective (cis-specific) olefin metathesis by molybdenum, ruthenium and vanadium catalysts. The review includes reports for transition metal catalyzed Z-selective olefin metathesis reactions (EROCM, CM, ROMP, and RCM) applied for organic synthesis and polymer synthesis. The review also includes basic background in the catalyst design, ligand modifications, which play an essential role for the success. Details can be found in the review by Kamal M. Dawood and Kotohiro Nomura (K. M. Dawood, K. Nomura, Adv. Synth. Catal. 2021, 363, 1970–1997; DOI: 10.1002/adsc.202001117).

New heptacoordinate tungsten(II) complexes with α-diimine ligands in the catalytic oxidation of multifunctional olefins

New tungsten(II) and molybdenum(II) heptacoordinate complexes [MX2(CO)3(LY)] (MXLy: M = W, Mo; X = Br, I; LY = C5H4NCY = N(CH2)2CH3 with Y = H (L1), Me (L2), Ph (L3)) were synthesized and characterized by spectroscopic techniques and elemental analysis. The two tungsten complexes WXL1 (X = Br, I) were also structurally characterized by single crystal X-ray diffraction. The metal coordination environment is in both a distorted capped octahedron. The complexes with L1 and L2 ligands were grafted in MCM-41, after functionalization of the ligands with a Si(OEt)3 group. The new materials were characterized by elemental analysis, N2 adsorption isotherms, 29Si MAS and 13C MAS NMR. The tungsten(II) complexes and materials were the first examples of this type reported. All complexes and materials were tested as homogeneous and heterogeneous catalysts in the oxidation of multifunctional olefins (cis-hex-3-en-1-ol, trans-hex-3-en-1-ol, geraniol, S-limonene, and 1-octene), with tert-butyl hydroperoxide (TBHP) as oxidant. The molybdenum(II) catalyst precursors are in general very active, reaching 99% conversion and 100% selectivity in the epoxidation of trans-hex-3-en-1-ol. Their performance is comparable with that of the [Mo(η3-C3H5)X(CO)2(LY)] complexes, but it increases with immobilization. On the other hand, most of the W(II) complexes display an activity similar or inferior to that of the Mo(II) analogues and it decreases after they are supported in MCM-41. DFT calculations show that tungsten complexes and iodide ligands are more easily oxidized from M(II) to M(VI) than molybdenum ones, while the energies of relevant species in the catalytic cycle are very similar for all complexes, making the theoretical rationalization of experimental catalytic data difficult.

Recent Developments in Z‐Selective Olefin Metathesis Reactions by Molybdenum, Tungsten, Ruthenium, and Vanadium Catalysts

AbstractThis review article summarizes reports for Z‐selective olefin metathesis reactions by molybdenum, tungsten, ruthenium, and vanadium catalysts. It is clearly demonstrated that the ligand modifications play essential roles in achieving the stereospecific olefin metathesis reactions. In particular, Mo‐ and W‐alkylidene based complex catalysts having pyrrolide‐aryloxide [or bis(aryloxide)] were highly effective in the efficient enantioselective ring opening/cross‐metathesis (EROCM), cross metathesis (CM), homodimerization, ring‐opening metathesis polymerization (ROMP), and in ring closing metathesis (RCM) with high Z‐ selectivities (∼>98%). Ruthenium‐carbene catalysts having cycloadamantyl, catechothiolate, phenolate and thiophenolate ligands display pronounced Z‐selectivity (>98%) in efficient CM, ROCM, AROCM, RCM, ROMP reactions. Halogenated (arylimido)‐alkoxo vanadium‐alkylidenes were thermally robust and effective for efficient highly cis‐specific ROMP of cyclic olefins to afford end‐functionalized ring‐opened polymers.magnified image

Extraction Methods and Their Influence on Yield When Extracting Thermo-Vacuum-Modified Chestnut Wood

Improvements in the yield and solubility of chestnut wood extractives, by using different extraction methods and molybdenum catalysts as support, have rarely been reported in literature. Many studies focus on the different parts of trees, except for the chemical characteristics of the remaining extractives achieved from thermally modified (THM) chestnut (Castanea sativa Mill) wood. This research seeks to better understand the effects of extraction techniques and catalysts on the yield and solubility of extractives. GC-MS analysis of the chloroform soluble and insoluble fractions was also used. Accelerated Solvent Extraction (ASE) 110 °C, Soxhlet, and autoclave extraction techniques were used to obtain extractives from untreated and thermally modified (THM) chestnut wood (170 °C for 3 h). Ethanol/H2O, ethanol/toluene, and water were the solvents used for each technique. A polyoxometalate compound (H3PMo12O40) and MoO3 supported on silica were used as catalysts. The THM induced a change in the wood’s surface color (ΔE = 21.5) and an increase in mass loss (5.9%), while the equilibrium moisture content (EMC) was reduced by 17.4% compared to the control wood. The yields of the extractives and their solubility were always higher in THM and mainly used ASE as the technique. GC-MS analysis of the extractives, without catalyst support, showed different results for each extraction technique and type of wood (untreated and THM). Ultimately, the amount of extractive compound dissolved in each solvent will differ, and the choice of extraction technique will depend on the intended final application of the extracted chemical product.

Influence of defect density on the gas sensing properties of multi-layered graphene grown by chemical vapor deposition

Chemical vapor deposition (CVD) has been demonstrated as a highly promising technique for the production of graphene on large scale and enabling tunability of the intrinsic defects of the films during the synthesis.In this work, we report on the correlation between the density of defects (DoD) and the kinetics of interaction of multi-layered graphene (MLG) with nitrogen dioxide (NO2) used as a target gas. We grow MLG on a pre-patterned molybdenum (Mo) catalyst layer, tailoring the DoD while growing MLG at temperatures from 850 °C to 980 °C. Analysing the Raman spectra, we show the lowering of the DoD as well as a quality dependence of MLG as a function of the growth temperature. The chemi-resistors based on MLG grown at different temperatures unambiguously highlight that, both during the exposure and the subsequent purge phase, the more defective the MLG, the more intense the NO2’s molecules interaction with MLG. Our results significantly mark a step forward in tuning the sensing properties of MLG without the need of any post-processing of the material after synthesis.

Effect of nickel cobalt co-catalyst on catalytic activity of molybdenumnaphthenatefor the hydroprocessing of coal tar pitch in suspension bed

Five dispersed molybdenum, nickel and cobalt oil soluble homogeneous catalysts were synthesized. The hydrogenation of coal tar pitch was under the conditions of 370°C, 10MPa hydrogen pressure, and 4 h reaction time in an autoclave reactor. The effects of molybdenum naphthenate addition, molybdenum nickel and molybdenum cobalt bimetallic ratios on hydrogenation were investigated. The catalytic hydrogenation effect was evaluated by the liquid yield. A variety of analytical methods, such as elemental analysis, ICP-MS, TEM, XPS, and four component separation, were used to explore the optimal catalytic system for the slurry bed hydrogenation of coal tar pitch. The results show that the optimal catalytic system is molybdenum naphthenate and nickel naphthenate ratio of 1:1 at catalyst amount of 2×103. Under optimal conditions, the liquid yield is 85.3%, the residue yield is 10.6%, and the gas yield is 4.1%.

Synthesis layered material of MnO2 and TiO2 as the support of molybdenum catalyst for bio-crude oil upgrading process

Layered materials are attractive for many applications because of their unique structure consisting of stacked sheets that can be controlled and modified. In this work, various synthetic layered materials (SLMs) i.e., Na-MnO2, H-MnO2, and Cs-TiO2 have been successfully prepared as the support for molybdenum (Mo)-based catalyst and applied for bio-crude oil hydrodeoxygenation (HDO) process. A commercial Mo/Al2O3 catalysts will also be utilized as a benchmark. The physical and chemical properties of the catalysts were characterized by using an X-ray diffractometer, X-ray fluorescence, surface area analyzer, NH3-temperature-programmed desorption, and scanning electron microscopy. The characterization results showed that the interlayer ion exchange of Na+ ions with H+ ions was successful for MnO2 support. The impregnation of the MoO3 into SLMs increased the surface area, pore-volume, average pore size, and total acidity. The HDO of bio-crude oil using MoO3/Na-MnO2 catalyst produced a higher HHV value of 38.75 MJ/Kg and a DOD value of 82.75% compared to other prepared catalysts or even the MoO3 commercial catalyst.

Molybdenum and nickel-molybdenum nitride catalysts supported on MgO-Al2O3 for the dry reforming of methane

Molybdenum and nickel-molybdenum nitride catalysts supported on MgO-Al2O3 were synthesized and tested for methane dry reforming at temperatures of 760−840 °C. Two in-situ nitridation procedures involving heating the catalyst in NH3/N2 or H2/N2 gas at a space velocity of 44 s−1 and 5 °C/min heating rate, were assessed and compared with a conventional reduction procedure. The activity of the bimetallic nitride was much higher than that of molybdenum nitride, with a slightly better performance with H2/N2-treated catalysts. The nitrides showed deactivation and poor stability at atmospheric pressure. The nitride phase was shown to transition into an oxide/carbide phase during reaction. Furthermore, hydrogen/nitrogen treatment caused partial reduction to occur, which explains the activity improvement in bimetallic Ni-Mo nitrides. Whisker carbon was significantly decreased although carbon formation was observed. The basic support improves coking resistance by enhancing CO2 adsorption. Initial results indicate a unique activation site for both methane and CO2 through a Mars-Van Krevelen mechanism, with a phase transformation followed by a carbide-oxide redox cycle mechanism taking place.

N-Heterocyclic carbene-nitrogen molybdenum catalysts for utilization of CO2

Three new N-heterocyclic carbene-nitrogen molybdenum complex was synthesized, and its catalytic activity was evaluated in the cycloaddition of epoxides with CO2. The molybdenum complex combined with tetrabutyl ammonium iodide (TBAI) resulted in a catalytic system for efficient conversion of a wide range of terminal and internal epoxides under 80 °C and 5–7 bar pressure for CO2. The cooperative catalysis mechanism between molybdenum complex and TBAI was elucidated, in which molybdenum complex was used as Lewis acid, and TBAI was employed as nucleophilic reagent. In addition, the NHC-Mo catalytic system was also successfully applied for the direct carboxylation of terminal alkynes with CO2.

Synthesis and characteristics of aluminate spinel ceramic pigments based on spent aluminium-cobalt-molybdenum catalyst

PurposeThe purpose of this paper is to conduct the directed synthesis of blue-colour aluminate spinel pigments based on spent aluminium–cobalt–molybdenum (ACM) catalyst and to study peculiar features of mineral formation processes and changes in their colour indices depending on composition and firing temperature.Design/methodology/approachAluminate spinel ceramic pigments with specified colour indices were synthesised by directed formation of their mineral composition and identification of the most rational technological parameters of production. Mineral composition of synthesised pigments was evaluated by X-ray phase analysis. The colour indices of pigments and pigment-containing glaze coatings were studied on the comparator colour. The chemical resistance of pigments was determined by loss of their weight loss after boiling in 1 N hydrochloric acid solution and 1 N sodium hydroxide solution.FindingsPeculiar features of formation of the mineral composition of aluminate spinel pigments based on the ACM catalyst were studied. The expediency of using magnesium and zinc oxides in their composition has been proved. It is found that for the formation of stable pigments of intense blue colour, a concentration of cobalt (II) oxide in the amount of 0.5 mol is sufficient, which is 23.1 Wt.%. The colour of such pigments is determined by the ratios of cobalt, magnesium and zinc aluminates, which form a spinel solid solution.Practical implicationsThe use of developed aluminate spinel pigments provides obtaining of high-quality glass coatings of blue colour, in particular, for ceramics.Originality/valueAluminium oxide in the spent ACM catalyst is predominantly in the active form (of amorphous aluminium hydroxide and y-Al2O3). This is a prerequisite for the use of this waste material as a complete substitute for chemically pure Al2O3 in the technology of aluminate spinel pigments and reduction of their firing temperature. Besides, spent ACM catalyst already contains 5 Wt.% of expensive cobalt (II) oxide in the form of stable colour-bearing phase CoAl2O4.