Heterogeneous Metal Oxide Catalysts Research Articles

Enhanced low-temperature behavior of selective catalytic reduction of NOx by CO on Fe-based catalyst with looping oxygen vacancy

The industrial applications of de-NOx require the high activity window of the catalysts to be shifted from high to low temperatures for energy saving. Selective catalytic reduction (SCR) of NOx by CO, as an alternative to NH3-SCR, is a promising de-NOx technology that facilitates the synergistic removal of pollutants. Herein, we report a new Fe2O3/amorphous SiO2 catalyst with high low-temperature activity for CO-SCR. A combination of experiments and density functional theory (DFT) calculations obtains direct evidence that an optimal content of oxygen vacancy enhances the low-temperature NO conversion ratio and N2 selectivity of Fe2O3-based catalyst to over 80% and maintain 100% CO conversion by promoting NN coupling via ONNO/N2O and NCO/NNCO2 intermediates. Compared with conventional defect engineering, such as doping other metals or artificially generating oxygen defects, the α-Fe2O3 (110) facet automatically enables an oxygen vacancy generation-filling cycle in the CO-SCR reaction, following Mars-van Krevelen (MvK) mechanism. This work advances a fundamental understanding of CO-SCR reaction mechanism over Fe2O3-based catalysts and provides insights for future studies to improve heterogeneous metal oxide catalysts.

Facile synthesis of M-CuFe2O4 (M= Al2O3, CeO2, La2O3, ZrO2, Mn2O3) catalysts using a one-pot method and their applications in high-temperature water gas shift reaction

Water gas shift reaction is an essential process of hydrogen production and carbon monoxide removal from syngas. In this study, the promotional effect of ZrO2, CeO2, La2O3, Al2O3, and Mn2O3 was investigated on the CO conversion and thermal stability of the copper ferrite in high-temperature water gas shift reaction (HTSR) and hydrogen purification. The powders were synthesized by a simple solid-state route and characterized by XRD, H2-TPR, SEM, FT-IR, TG-DTA, and BET analyses. Promoters (ZrO2, CeO2, La2O3, Al2O3, and Mn2O3) could affect the WGSR performance in activity and stability. In the M-CuFe2O4 catalyst, alumina acts as a texture promoter and aids in the fine dispersion of copper ferrite. The results indicated that the surface area of the Al2O3–CuFe2O4 (210 m2/g) catalyst was higher than the other samples. This catalyst presented higher CO conversion in HTSR and had higher stability at 1000 min on stream. It was found that the incorporation of different contents of alumina had a significant influence on the textural and catalytic properties of the CuFe2O4-based catalysts. The 30%Al2O3–70%CuFe2O4 catalyst exhibited the highest CO conversion of 65% at 350 °C, uniform pore size distribution, and intense interaction between copper ferrite and alumina, causing the effective stabilization of the active phase in the catalyst structure. The findings of this study represent that the solid-state method, due to its simplicity and creation of a mesoporous structure, can also be applied for the preparation of many heterogeneous metal oxide catalysts.

Regioselective Friedel-Crafts Acylation Reaction Using Single Crystalline and Ultrathin Nanosheet Assembly of Scrutinyite-SnO2.

Peculiar physicochemical properties of two-dimensional (2D) nanomaterials have attracted research interest in developing new synthetic technology and exploring their potential applications in the field of catalysis. Moreover, ultrathin metal oxide nanosheets with atomic thickness exhibit abnormal surficial properties because of the unique 2D confinement effect. In this work, we present a facile and general approach for the synthesis of single crystalline and ultrathin 2D nanosheets assembly of scrutinyite-SnO2 through a simple solvothermal method. The structural and compositional characterization using X-ray diffraction (Rietveld refinement analysis), high-resolution transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, and so on reveal that the as-synthesized 2D nanosheets are ultrathin and single crystallized in the scrutinyite-SnO2 phase with high purity. The ultrathin SnO2 nanosheets show predominant growth in the [011] direction on the main surface having a thickness of ca. 1.3 nm. The SnO2 nanosheets are further employed for the regioselective Friedel–Crafts acylation to synthesize aromatic ketones that have potential significance in chemical industry as synthetic intermediates of pharmaceuticals and fine chemicals. A series of aromatic substrates acylated over the SnO2 nanosheets have afforded the corresponding aromatic ketones with up to 92% yield under solvent-free conditions. Comprehensive catalytic investigations display the SnO2 nanosheet assembly as a better catalytic material compared to the heterogeneous metal oxide catalysts used so far in the view of its activity and reusability in solvent-free reaction conditions.

O2 Activation with a Sterically Encumbered, Oxygen-Deficient Polyoxovanadate-Alkoxide Cluster.

The isolation of the oxygen-deficient, polyoxovanadate-alkoxide (POV-alkoxide) cluster, [nBu4N][V6O6(OMe)12(MeCN)], and its subsequent reactivity with oxygen (O2), has demonstrated the utility of these assemblies as molecular models for heterogeneous metal oxide catalysts. However, the mechanism through which this cluster activates and reduces O2 to generate the oxygenated species is poorly understood. Currently it is speculated that this POV-alkoxide mediates the four-electron O-O bond cleavage through an O2 bridged dimeric intermediate, a mechanism which is not viable for O2 reduction at solid-state metal oxide surfaces. Here, we report the successful activation and reduction of O2 by the calix-functionalized POV-alkoxide cluster, [nBu4N][(calix)V6O6(OMe)8](MeCN)] (calix = 4-tert-butylcalix[4]arene). The steric hindrance imparted to the open vanadium site by the calix motif eliminates the possibility of cooperative, bimolecular O2 activation, allowing for a comparison of the reactivity of this system with that of the nonfunctionalized POV-alkoxide described previously. Rigorous characterization of the calix-substituted assembly, enabled by its newfound solubility in organic solvent, reveals that the incorporation of the tetradentate aryloxide ligand into the POV-alkoxide scaffold perturbs the electronic communication between the site-differentiated vanadium(III) ion and the cluster core. Collectively, our results provide insight into the physiochemical factors that are important during the O2 reduction reaction at oxygen-deficient sites in reduced POV-alkoxide clusters.

Mesoporous ZrO2 Nanopowder Catalysts for the Synthesis of 5-Hydroxymethylfurfural

Balancing the large surface area and high crystallinity of heterogeneous metal oxide catalysts is quite tricky. They are usually needed together to get both a large amount of active catalytic sites...

Aerobic Oxidations of Light Alkanes over Solid Metal Oxide Catalysts.

Heterogeneous metal oxide catalysts are widely studied for the aerobic oxidations of C1-C4 alkanes to form olefins and oxygenates. In this review, we outline the properties of supported metal oxides, mixed-metal oxides, and zeolites and detail their most common applications as catalysts for partial oxidations of light alkanes. By doing this we establish similarities between different classes of metal oxides and identify common themes in reaction mechanisms and research strategies for catalyst improvement. For example, almost all partial alkane oxidations, regardless of the metal oxide, follow Mars-van Krevelen reaction kinetics, which utilize lattice oxygen atoms to reoxidize the reduced metal centers while the gaseous O2 reactant replenishes these lattice oxygen vacancies. Many of the most-promising metal oxide catalysts include V5+ surface species as a necessary constituent to convert the alkane. Transformations involving sequential oxidation steps (i.e., propane to acrylic acid) require specific reaction sites for each oxidation step and benefit from site isolation provided by spectator species. These themes, and others, are discussed in the text.

Distinguishing Homogeneous from Heterogeneous Water Oxidation Catalysis when Beginning with Polyoxometalates

Polyoxometalates (POMs) have been proposed to be excellent homogeneous water oxidation catalysts (WOCs) due to their oxidative stability and activity. However, recent literature indicates that even these relatively robust compounds can be transformed into heterogeneous, metal-oxide WOCs under the oxidizing reaction conditions needed to drive O2 evolution. This review covers the experimental methodology for distinguishing homogeneous and heterogeneous WOCs; it then addresses the “what is the true catalyst?” problem for POMs used as precatalysts in the oxidation of water to O2. These results are also compared to the broader WOC literature. The primary findings in this review are the following: (1) Multiple, complementary experiments are needed to determine the true catalyst, including determination of catalyst stability, speciation, and kinetics under operating conditions. (2) Controls with hypothetical heterogeneous metal-oxide catalysts are required to determine their kinetic competence in the reaction an...

Transesterification kinetics of Camelina sativa oil on metal oxide catalysts under conventional and microwave heating conditions

The reaction kinetics of transesterification of Camelina sativa oil using metal oxide catalysts under the conventional heating and the microwave-heating conditions were investigated. The transesterification reaction rates and the fatty acid methyl ester (FAME) conversion rates were determined using heterogeneous metal oxide catalysts, i.e., BaO, CaO, MgO and SrO and two different heating methods. It was observed during the kinetic studies in this work that the BaO and SrO catalysts generated higher FAME yields than the CaO and MgO catalysts. A comparison between the conventional heating and the microwave-assisted transesterification processes showed that the reaction rate constants obtained in the microwave-assisted transesterification process are of two orders of magnitude higher than those obtained with the conventional heating method. Therefore, it can be well concluded that the heating method and solid catalysts play vital roles in improving the reaction kinetics and optimizing the reactors for biomass conversion to biodiesel.

Kinetics of Catalytic Thermal Pretreatment (Catalytic Thermolysis) of Distillery Wastewater and Bio-Digester Effluent of Alcohol Production Plant at Atmospheric Pressure

This paper presents the kinetics of catalytic thermolysis of the sugarcane molasses based alcohol distillery wastewater coming out from distillation unit (distillery wastewater or DWW) and the biodigester (biodigester effluent or BDE) having high biochemical and chemical oxygen demand (BOD and COD). The effluents were thermally pretreated at 80-100°C and atmospheric pressure in the presence of various catalysts. The homogeneous copper sulphate and the heterogeneous metal oxide catalysts, such as CuO, MnO2-CeO2 and ZnO were used for the catalytic thermolysis. The CuO catalyst was found to be best amongst all the catalysts, giving 47% COD reduction, 68% colour reduction for DWW and 61% COD reduction, 78% colour reduction for BDE at 100°C using 4 kg m-3 catalyst loading in 12h. The experimental data on the catalytic thermolysis exhibit two clearly distinct COD reduction phases: the first phase being fast followed by the second phase which is a slower. Both the phases of the catalytic thermolysis are found to follow first order COD reduction kinetics. Catalytic thermolysis with 3 kg m-3 CuO at 100°C and atmospheric pressure shows about 1.6 and 1.5 times higher COD reductions in comparison to that without using a catalyst for the DWW and BDE, respectively.

Microwave-Assisted Catalytic Transesterification of Camelina Sativa Oil

Catalytic conversion of Camelina Sativa oil to biodiesel through both conventional heating and microwave radiation was investigated. Three different types of catalysts: homogeneous catalysts (NaOH and KOH), heterogeneous metal oxide catalysts (BaO and SrO), and sol−gel derived catalysts (BaCl2/AA and SrCl2/AA) were evaluated for their efficacy in biodiesel production. The following conditions were obtained for the catalysts based on the maximum biodiesel yield: potassium hydroxide/methanol to oil ratio of 1:9, catalyst concentration of 1% (w/w), and reaction time of 60 s; sodium hydroxide/methanol to oil ratio of 1:9, catalyst concentration of 0.5 wt %, and reaction time of 60 s; barium oxide/methanol to oil ratio of 1:9, catalyst concentration of 1.5% (w/w), and reaction time of 4 min; strontium oxide/methanol to oil ratio of 1:9, catalyst concentration of 2 wt %, and reaction time of 4 min. In the case of sol−gel derived catalysts, different catalyst loading rates in the range of 1−10 mmol/g were evaluated. Low biodiesel yields of 10−25% on the sol−gel derived catalysts were observed. On the basis of energy consumptions in the transesterification processes with both conventional heating and microwave-heating methods evaluated in this study, it was estimated that the microwave-heating method consumes less than 10% of the energy to achieve the same yield as the conventional heating method. The fuel properties of camelina biodiesel produced were compared with those of the regular diesel and found to be conforming to the American Society for Testing and Materials (ASTM) standards.

Biodiesel Production from Jatropha Curcas, Waste Cooking, and Camelina Sativa Oils

Process parameter evaluation and catalyst performance study was conducted for biodiesel production using jatropha curcas, waste cooking, and camelina sativa oils. Conversion of triglycerides to methyl esters involves esterification and/or transesterification, depending on the nature of the feedstock. A two-step transesterification process (acid esterification followed by alkali transesterification) was employed to produce biodiesel from high free fatty acids (FFA) in jatropha curcas and waste cooking oils, and a single-step transesterifcation process (alkali transesterifcation) was used for camelina sativa oil conversion. Catalyst selection is vital in transesterification process because it determines biodiesel yield and cost. Transesterification of jatropha curcas and waste cooking oil was optimized by using H2SO4 and ferric sulfate catalysts in the acid esterification step and a KOH catalyst in the alkali transesterification, respectively. Heterogeneous metal oxide catalysts including BaO, SrO, MgO, and CaO were used for the transesterification of camelina sativa oil. Jatropha curcas oil was observed to have yields in the range of 90−95%, with a maximum of 95% obtained at the following process conditions: acid esterification, methanol to oil molar ratio of 6:1, 0.5% of H2SO4, and 40 ± 5 °C; alkali tranesterification, methanol to oil molar ratio of 9:1, 2% of KOH, and 60 °C. Waste cooking oil yielded biodiesel in the range of 85−96%, with a maximum of 96% observed at the following optimized process conditions: 2 h reaction at 100 °C, methanol to oil molar ratio of 9:1, and 2% of a ferric sulfate catalyst. Comparative experiments on camelina sativa oil conversion showed that the most effective catalyst was BaO that showed >80% yield of camelina to biodiesel. Fuel properties of biodiesel produced from the three different feedstocks were determined and compared with the ASTM standards for biodiesel and petroleum diesel.

Transesterification of Camelina Sativa Oil Using Heterogeneous Metal Oxide Catalysts

Camelina sativa oil provides a reliable solution for biodiesel production by providing a sustainable, low-input biofuel feedstock option in some climates. In the present study, optimization of the transesterification of camelina sativa oil using different heterogeneous metal oxide catalysts, i.e., BaO, SrO, MgO, and CaO, was evaluated. The results of comparative experiments showed that the most effective catalyst was 1 wt % BaO, which showed an >80% yield of camelina to biodiesel conversion in 3 h at 100 °C. The relative order of the effectiveness of the catalysts was BaO > SrO > CaO > MgO. These catalysts were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), and Brunauer−Emmett−Teller (BET) to obtain their physical and chemical properties. The important variables affecting the methyl ester yield during the transesterification reaction are the molar ratio of alcohol/oil, catalyst amount, and reaction temperature. The fuel properties of biodiesel produced were compared to American Society for Testing and Materials (ASTM) standards for biodiesel.

Reaction Kinetics of Soybean Oil Transesterification Using Heterogeneous Metal Oxide Catalysts

AbstractHomogeneous acid or base catalysts dissolve fully in the glycerol layer and partially in the fatty acid methyl ester (biodiesel) layer in the triglyceride transesterification process. Heterogeneous (solid) catalysts, on the other hand, can prevent catalyst contamination making product separation much simpler. In the present work, the transesterification kinetics of five different solid catalysts with soybean oil is presented. It is found that heterogeneous catalysts require much higher temperatures and pressures to achieve acceptable conversion levels compared to homogeneous catalysts. Subsequent to preliminary investigations, transesterifications were conducted for selected high performance solid catalysts, i.e., MgO, CaO, BaO, PbO, and MnO2 in a high pressure reactor up to a temperature of 215 °C. The yield of the fatty acid methyl esters and the kinetics (rate constant and order) of the reaction are estimated and are compared for each catalyst.

Development of active oxide catalysts for the direct oxidation of methane to formaldehyde

Formaldehyde is currently produced from methane by a three-step process involving H 2/CO synthesis gas and methanol as intermediates, and development of a single-step process would have great economic incentive for producing this commodity chemical. A historical perspective is presented here in regard to the research carried out with heterogeneous metal oxide catalysts in attempts to achieve selective oxidative conversion of methane to formaldehyde. The concepts employed, both chemical and engineering, are described, and these include dual redox promoters and double-bed catalysts. More recent work in this laboratory has found V 2O 5/SiO 2 catalysts to be very active partial oxidation catalysts. The space-time yield of and selectivity toward formaldehyde are improved by the presence of steam in the methane/air reactant mixture, and an attractive feature of the product mixture is the low quantity of carbon dioxide produced. Space-time yields of >1.2 kg CH 2O/kg catalyst per h have been achieved.

Comparative energy barriers for hydrogen activation by homogeneous and heterogeneous metal oxide catalysts

Triethylene glycol solutions of alkali and alkaline earth metal hydroxide complexes are well-defined soluble oxide water-gas shift catalysts which equilibrate the reaction of carbon monoxide and water to yield hydrogen and carbon dioxide at temperatures ranging from 150 ° to 250 °C and carbon monoxide pressures of 1 to 300 atm. Significantly, catalysis proceeds cleanly, even in the complete absence of a metal center in the soluble oxide system. Thus, the rate of hydroxide ion catalyzed hydrogen evolution is highest in the presence of a noncoordinating organic cation: BuΔN+>Cs+>Na+>H+>Ca+2. Furthermore, the activation energy for the homogeneous sodium hydroxide catalyst in triethylene glycol solution, 26±1 kcal, is comparable to that exhibited by a commercially used heterogeneous iron oxide catalyst, 27±0.2 kcal. The alkali metal hydroxide system may be modified for metal cocatalysis. Thus, lead (II) oxide dissolves in the triethylene glycol solutions to yield a new species which exhibits a207Pb NMR resonance shifted 3350 ppm downfield from lead perchlorate. The activity of this lead modified system is improved by three orders of magnitude. Yet, the activation energy is unchanged, 26±1 kcal, suggesting that entropic factors may be important in these homogeneous metal oxide hydrogen evolution/activation systems.