S. A. Yashnik

Cu and Fe-containing ZSM-5 zeolites as catalysts for wet peroxide oxidation of organic contaminants: reaction kinetics

AbstractnThe peroxide oxidation of model substrates (formic acid and phenol) was studied in the presence of copper- and iron-containing catalysts (0.5xa0% Cu–ZSM-5-30 and 0.65xa0% Fe–ZSM-5-30). The aim was to develop optimal kinetic models for describing the kinetics of peroxide oxidation. The real kinetics of phenol and formic acid oxidation in the presence of these catalysts at varied reaction parameters (concentrations and temperature) was studied. The copper-containing catalysts were more active to formic acid oxidation than the iron-containing catalyst over all the temperature range studied. The rate of destruction of pollutants decreases with a decrease in the H2O2 concentration and the catalyst weight. The observed rate dependences on the initial substrate concentration appeared to be different for the substrate used. With formic acid, an increase of initial concentration leads to a slight increase in the reaction rate. In the case of phenol peroxide oxidation, the negative order with respect to the substrate concentration was observed. This may be explained by strong inhibition of the reaction rates by phenol and intermediates (hydroquinone, catechol, etc.) of its oxidation. The mathematical modeling of the kinetics was performed for various types of kinetic equations that correspond to different hypotheses on the kinetic reaction scheme. The selected kinetic models based on logical kinetic schemes allowed describing the peroxide oxidation of model substrates at an appropriate accuracy.

Effect of support modification on the physicochemical properties of a NiPd/Al2O3 catalyst for the autothermal reforming of methane

For the development of effective catalysts for the autothermal reforming of methane, the NiPd catalysts were synthesized based on modified aluminum oxide and their physicochemical properties were studied using X-ray diffraction analysis, low-temperature nitrogen adsorption, transmission electron microscopy, and temperature-programmed reduction with hydrogen. It was found that the variation of modifying components (CeO2, ZrO2, La2O3, Ce0.5Zr0.5O2, and La2O3/Ce0.5Zr0.5O2) and their concentrations (10–30 wt %) makes it possible to regulate the particle size of NiO, the composition of a Ni-containing phase (NiO, La2NiO4, NiAl2O4, or Ni-La-Al-O) and the redox properties of nickel ions. It was shown that the average particle size of NiO increased from 6.7 to 17.5 nm in the following order of supports: La2O3/Al2O3 < La2O3/Ce0.5Zr0.5O2/Al2O3 < Al2O3 < Ce0.5Zr0.5O2/Al2O3 < CeO2/Al2O3 < ZrO2/Al2O3. On the introduction of the modifying oxides CeO2 and ZrO2 into aluminum oxide, the fraction of nickel in the composition of NiAl2O4 decreased and, therefore, the fraction of difficult-to-reduce Ni2+ decreased. The addition of La2O3 and La2O3/Ce0.5Zr0.5O2 strengthened the interaction of nickel cations with the support up to the formation of Ni-La-Al-O and La2NiO4 phases and increased the fraction of difficult-to-reduce Ni2+ ions. The resulting NiPd catalysts are promising in the catalysis of the autothermal reforming of methane.

Low-temperature activation of nitrogen oxide on Cu-ZSM-5 catalysts

The adsorption and activation of NO molecules on Cu-ZSM-5 catalysts with different Cu/Al and Si/Al ratios (from 0.05 to 1.4 and from 17 to 45, respectively) subjected to different pretreatment was studied by ultraviolet-visible diffuse reflectance (UV-Vis DR). It was found that the amount of chemisorbed NO and the catalyst activity in NO decomposition increased with an increase in the Cu/Al ratio to 0.35–0.40. The intensity of absorption bands at 18400 and 25600 cm−1 in the UV-Vis DR spectra increased symbatically. It was hypothesized that the adsorption of NO occurs at Cu+ ions localized in chain copper oxide structures with the formation of mono- and dinitrosyl Cu(I) complexes, and this process is accompanied by the Cu2+...Cu+ intervalence transfer band in the region of 18400 cm−1. The low-temperature activation of NO occurs through the conversion of the dinitrosyl Cu(I) complex into the π-radical anion (N2O2)− stabilized at the Cu2+ ion of the chain structure, [Cu2+-cis-(N2O2)−], by electron transfer from the Cu+ ion to the cis dimer (NO)2. This complex corresponds to the L → M charge transfer band in the region of 25600 cm−1. The subsequent destruction of the complex [Cu2+-cis-(N2O2)−] at temperatures of 150–300°C leads to the release of N2O and the formation of the complex [Cu2+O−], which further participates in the formation of the nitrite-nitrate complexes [Cu2+(NO2)−], [Cu2+(NO)(NO2)−], and [Cu2+(NO3)−] and NO decomposition products.

Effect of the nature of sulfur compounds on their reactivity in the oxidative desulfurization of hydrocarbon fuels with oxygen over a modified CuZnAlO catalyst

The reactivity of thiophene, dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT), which are the representatives of the main classes of sulfur compounds that are the constituents of diesel fractions, was studied in the course of their oxidative desulfurization with oxygen on a CuO/ZnO/Al2O3 catalyst modified with boron and molybdenum additives. At T ≥ 375°C, the reactivity increased in the order thiophene < DBT < 4,6-DMDBT. The degree of sulfur removal in the form of SO2 from hydrocarbon fuel, which was simulated by a solution of 4,6-DMDBT in toluene, was 80%. Under the assumption of a first order reaction with respect to sulfur compound and oxygen, the apparent activation energies of the test processes were calculated. An attempt was made to reveal the role of the adsorption of sulfur compounds in the overall process of oxidative desulfurization with the use of X-ray diffraction analysis, X-ray photoelectron spectroscopy, and differential thermal and thermogravimetric analysis with the massspectrometric monitoring of gas phase composition.

Structure and properties of Pd–Mn hexaaluminate catalysts modified with platinum for the high-temperature oxidation of methane

The effect of Pt additives on the catalytic characteristics of a Pd-containing catalyst based on manganese hexaaluminate was studied. It was found that the bimetallic PtPd-containing catalysts based on MnLaAl11O19 with the Pt/Pd atomic ratio smaller than 0.25 exhibited a comparable or somewhat smaller activity in the methane oxidation, but their stability at elevated temperatures and gas flow rates was higher than that of the Pd-based catalyst. The state of the active constituent of the resulting catalysts was investigated. Main correlations between the state of the active component and the catalytic activity were revealed.

Cu-Zn-Al-O catalysts for the oxidative desulfurization of dibenzothiophene, a typical sulfur-containing compound of the diesel fraction

The possibility of oxidative desulfurization of dibenzothiophene dissolved in toluene as a model organosulfur compound of diesel fuel in the presence of catalytic CuZnAl compositions was studied. It was shown that the CuZnAl catalysts at 350–450°C ensure sulfur removal from the model fuel to the extent of up to 40–45%. Part of the sulfur is removed as SO2, and some sulfur is absorbed on the catalyst surface.

Ethylene production by the oxidative condensation of methane in the presence of MnMW/SiO 2 catalysts (M = Na, K, and Rb)

The samples of MnMW/SiO2 (M = Na, K, and Rb) were synthesized using various synthesis methods under varied heat treatment conditions and their physicochemical properties and activity in the reaction of the oxidative condensation of methane (OCM) were studied for the development of an effective catalyst for the resource-saving process of natural gas conversion into ethylene. It was found that the preparation method exerts an effect on the textural characteristics of the samples and the reducing properties of the cations of manganese and tungsten. It was determined that the composition of a W-containing phase depends on the alkali metal, and a ratio between the polymorphous modifications of SiO2 is controlled by the method of synthesis and the conditions of catalyst heat treatment. It was established that the yield of C2 hydrocarbons in the OCM reaction increased with the use of incipient wetness impregnation instead of the method of mixing with a suspension for catalyst preparation and with an increase in the catalyst heat treatment temperature from 700 to 1000°C. The optimum composition of the catalyst and the condition of its synthesis were found: 2Mn0.8Na3W/SiO2 obtained by the impregnation method and calcined at 1000°C ensured the yield of target products of ~20% with a CH4 conversion of ~35% at a reaction temperature of 850°C.

Development of a Ni–Pd/CeZrO 2 /Al 2 O 3 catalyst for the effective conversion of methane into hydrogen-containing gas

The effects of the Pd content (0–1 wt %) and the synthesis method (joint impregnation with Ni + Pd and Pd/Ni or Ni/Pd sequential impregnation) on the physicochemical and catalytic properties of Ni–Pd/CeZrO2/Al2O3 were studied in order to develop an efficient catalyst for the conversion of methane into hydrogen-containing gas. It was shown that variation in the palladium content and a change in the method used for the introduction of an active constituent into the support matrix make it possible to regulate the redox properties of nickel cations but do not affect the size of NiO particles (14.0 ± 0.5 nm) and the phase composition of the catalyst ((γ + δ)-Al2O3, CeZrO2 solid solution, and NiO). It was established that the activity of Ni–Pd catalysts in the reaction of autothermal methane reforming depends on the method of synthesis and increases in the following order: Ni + Pd < Ni/Pd < Pd/Ni. It was found that, as the Pd content of the Ni–Pd/CeZrO2/Al2O3 catalyst was decreased from 1 to 0.05 wt %, the ability for self-activation, high activity, and operational stability of the catalyst under the conditions of autothermal methane reforming remained unaffected: at 850°C, the yield of hydrogen was ~70% at a methane conversion of ~100% during a 24-h reaction.

Zeolite ZSM-5 containing copper ions: The effect of the copper salt anion and NH4OH/Cu2+ ratio on the state of the copper ions and on the reactivity of the zeolite in DeNOx

Isotherms of copper cation sorption by H-ZSM-5 zeolite from aqueous and aqueous ammonia solutions of copper acetate, chloride, nitrate, and sulfate are considered in terms of Langmuir’s monomolecular adsorption model. Using UV-Vis diffuse reflectance spectroscopy, IR spectroscopy, and temperatureprogrammed reduction with hydrogen and carbon monoxide, it has been demonstrated that the electronic state of the copper ions is determined by the ion exchange and heat treatment conditions. The state of the copper ions has an effect on the redox properties and reactivity of the Cu-ZSM-5 catalysts in the selective catalytic reduction (SCR) of NO with propane and in N2O decomposition. The amount of Cu2+ that is sorbed by zeolite H-ZSM-5 from aqueous solution and is stabilized as isolated Cu2+ cations in cationexchange sites of the zeolite depends largely on the copper salt anion. The quantity of Cu(II) cations sorbed from aqueous solutions of copper salts of strong acids is smaller than the quantity of the same cations sorbed from the copper acetate solution. When copper chloride or sulfate is used, the zeolite is modified by the chloride or sulfate anion. Because of the presence of these anions, the redox properties and nitrogen oxides removal (DeNOx) efficiency of the Cu-ZSM-5 catalysts prepared using the copper salts of strong acids are worse than the same characteristics of the sample prepared using the copper acetate solution. The addition of ammonia to the aqueous solutions of copper salts diminishes the copper salt anion effect on the amount of Cu(II) sorbed from these solutions and hampers the nonspecific sorption of anions on the zeolite surface. As a consequence, the redox and DeNOx properties of Cu-ZSM-5 depend considerably on the NH4OH/Cu2+ ratio in the solution used in ion exchange. The aqueous ammonia solutions of the copper salts with NH4OH/Cu2+ = 6–10 stabilize, in the Cu-ZSM-5 structure, Cu2+ ions bonded with extraframework oxygen, which are more active in DeNOx than isolated Cu2+ ions (which form at NH4OH/Cu2+ = 30) or nanosized CuO particles (which form at NH4OH/Cu2+ = 3).

Formation of Active Structures in Monolith Copper–Manganese Oxide Catalysts for Air-Heating Devices

Impregnation catalysts based on CuO, MnOx, and CuO–MnOx with different Cu/Mn ratios supported on ceramic monoliths of alumina and silica are studied by BET, mercury porosimetry, X-ray diffraction analysis, transmission and scanning electron microscopy, temperature-programmed reduction with H2, diffuse reflectance electron spectroscopy, and differential dissolution. It is found that, in the butane oxidation reaction, CuO–MnOx catalysts exert a synergistic effect, which is attributed to the formation of highly defective phases of complex oxides of the nonstoichiometric spinel type with a large number of interparticle boundaries in the near-surface layers of the support.