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Abstract

Novel composite adsorbents Ca(NO3)2)/SiO2, LiBr/SiO2 and CaCl2/SiO2 have been intently designed for shifting the equilibrium of methanol synthesis by methanol removal from the reaction mixture. Testing of these composites in a lab-scale tubular flow reactor at the initial mixture composition CO/CO2/N2/H2 = 30.0/2.0/3.2/64.8, total pressure P = 2 MPa and temperature T = 473 K has shown their encouraging potential for this application. The former composite efficiently adsorbs methanol from the gaseous mixture under lab-scale operating conditions of methanol synthesis and demonstrates good thermal stability. It can be recommended for further testing.

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Reaction Kinetics, Mechanisms and Catalysis
Authors: Viorel Chihaia, Karl Sohlberg, Margarita Gabrovska, Rumeana Edreva-Kardjieva, Dorel Crişan, Peter Tzvetkov, Maya Shopska, and Iskra Shtereva

Abstract

The effect of nickel content on the structure and activity of co-precipitated Ni–Al layered double hydroxides (LDHs) as catalyst precursors for CO2 removal by methanation was studied by variation of the Ni2+/Al3+ molar ratio (Ni2+/Al3+ = 3.0, 1.5 and 0.5), and of the reduction and reaction temperatures as well as of the space velocities. Powder X-ray diffraction (PXRD), H2 chemisorption, and temperature programmed reduction (TPR) techniques were applied for physicochemical characterization of the samples. It was specified that the nano-scaled dimensions of the as-synthesized samples also generate nano-metrical metallic nickel particles (PXRD). The existence of readily and hardly reducible Ni2+–O species in the studied samples (TPR), affects catalytic performance. The studied catalysts hydrogenate CO2 effectively to residual concentrations of the latter in the range of 0–10 ppm at reaction temperatures from 400 to 220 °C and space velocities between 22,000 and 3000 h−1. The variation of the CO2 methanation activity with the changes of space velocities depends on the nickel content, and reduction and reaction temperatures. After reduction at 400 and 450 °C, a sample of Ni2+/Al3+ = 3.0 has demonstrated the highest conversion degree at all the reaction temperatures and space velocities, while a catalyst of Ni2+/Al3+ = 0.5 dominated in the methanation activity after reduction within 530–600 °C. The Ni2+/Al3+ = 1.5 catalyst data take intermediate position between Ni2+/Al3+ = 3.0 and Ni2+/Al3+ = 0.5 often closer to Ni2+/Al3+ = 3.0 ones. The studied Ni–Al LDH systems are found to be promising catalyst precursors for fine CO2 removal from hydrogen-rich gas streams through the methanation reaction, depending on the technological regime of catalyst activation.

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Abstract

The kinetics of the decolorization reaction of methyl violet dye (MV) with the hydroxyl ion (nucleophile) was studied in the presence of cationic micelles of cetyltrimethylammonium bromide (CTAB). The reaction follows pseudo-first order kinetics. The rate constant depends on the surfactant concentration. CTAB micelles catalyze the overall reaction between the dye carbocation and the nucleophile. Quantitative analysis of the micellar data obtained has been done by applying positive cooperativity model of enzyme catalysis. For the dye under study, the value of n (index of cooperativity) has been found to be greater than 1. The catalytic effect has been explained in terms of hydrophobic and electrostatic interactions of various species present in the reaction systems. The addition of a counterion showed inhibitory effect on the reaction.

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Reaction Kinetics, Mechanisms and Catalysis
Authors: Viorel Chihaia, Karl Sohlberg, B. Grzybowska-Świerkosz, M. Ruszel, R. Grabowski, L. Kępiński, M. A. Małecka, and J. Sobczak

Abstract

Au/MIICr2O4 (MII = Co, Mn, Fe) catalysts have been tested in the total oxidation of ethane, propane, propene at 300 °C and of CO at 35 °C, and characterized by TEM, XPS and hydrogen thermo-programmed reduction (H2TPR). The catalytic activity has been found to depend on the nature of the oxidized compound. For the CO and ethane oxidation, the activity decreases in the sequence Au/CoCr2O4 > Au/MnCr2O4 ≫ Au/FeCr2O4 and follows the sequence of the decreasing reducibility of the supports and the catalysts, estimated from H2TPR measurements. The activity decreases with the increasing Au particle size in the same order, the reactions then seem structure sensitive. No correlations between the activity and reducibility or the particle size are observed for oxidation of the C3 hydrocarbons: the sequence of the activity for propane is: Au/FeCr2O4 > Au/CoCr2O4 > Au/MnCr2O4, whereas activity in propene oxidation does not change markedly within the studied catalysts. The reaction mechanism in the oxidation of propane and propene seems then to be different from that of oxidation of CO and ethane.

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Abstract

In the current paper, the metal organic coordination polymer Zn4O(OH)2(BDC)2(H2O)2.7 (Zn-MOCP) with high thermal and chemical stability was synthesized by a direct mixing method at room temperature. Then the catalyst Ni@Zn-MOCP (7.5 wt% Ni) was successfully prepared via a wet impregnation strategy employing Ni(acac)2 (acac = acetylacetonate) as the precursor. The hydrogenation of crotonaldehyde was utilized as the probe reaction to explore its catalytic activity. The samples were characterized by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), N2 adsorption–desorption measurements, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). PXRD patterns of Ni@Zn-MOCP showed good coincidence with that of Zn-MOCP, and the pore texture of Zn-MOCP was still maintained after impregnation. Most of Ni(acac)2 over Zn-MOCP were reduced to Ni0 after reduction based on XPS analysis. In terms of the turnover of frequency (TOF) of crotonaldehyde, Ni@Zn-MOCP (53.6 h−1) exhibited much higher activity than the industrial catalyst Ni/SiO2 (29.5 h−1). Furthermore, the reusability of the catalyst Ni@Zn-MOCP over the hydrogenation for crotonaldehyde was tested.

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Abstract

The influence of Si/V ratio of mesoporous V-SBA-15 on the structure and catalytic behavior has been investigated for the vapor phase oxidation of diphenylmethane (DPM) to benzophenone (BP) using CO2 free air as oxidant under atmospheric pressure. The reaction conditions were optimized for a maximum conversion of DPM and selectivity for BP on V-SBA-15 (25). These catalysts were synthesized with different Si/V ratios of 10, 25, 50, 75 and 100 by the direct hydrothermal method under mild acidic conditions and characterized by using X-ray diffraction, ICP-OES, EDAX, DRUV–Vis, photoluminescence and transmission electron microscopy techniques. It was found that a large amount of vanadium is incorporated mainly in isolated tetrahedral environment with a terminal V=O bond. Under the optimized conditions, the activity of all the catalysts were compared and found that V-SBA-15 with Si/V = 25 ratio, exhibited the maximum activity. The difference in the catalytic activity is related to their physico-chemical properties.

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Abstract

Platinum–germanium and platinum–tin catalysts supported on silica, containing different amounts of Sn and Ge, were synthesized using the surface organometallic chemistry on metals technique. The catalysts were characterized using transmission electron microscopy, X-ray photoelectron spectra and X-ray absorption near edge structure and extended X-ray absorption fine structure; and were tested in the liquid-phase selective hydrogenation of furfural. The atomic ratio between the two metals resulted the key factor towards the optimization of the activity and selectivity of the bimetallic catalysts. The bimetallic catalysts were more active than the parent Pt/SiO2 catalyst in the hydrogenation of furfural. These results can be accounted for by considering a new type of active site having an architecture which would favor the hydrogenation of the C=O group is created, allowing an increased activity towards obtaining furfuryl alcohol. All the studied systems allowed to obtain furfuryl alcohol with high selectivity.

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Abstract

Direct catalytic decomposition of N2O in inert and oxidative atmosphere was investigated over two series of binary oxide catalysts containing CuO coupled with Ga2O3 or SnO2 dispersed over a silica-alumina support (CuGa/SA and CuSn/SA series catalysts). The activity for N2O decomposition in the absence of oxygen was between 0.1 and 0.2 μmol gcat s−1 in the 500–600 °C interval over the CuSn/SA series catalysts with conversion between 80 and 90%. Lower activities with larger differences were observed within the CuGa/SA series catalysts. The presence of oxygen in the feed (O2/N2O ratio up to 20:1) did not affect the activity of the CuGa/SA series while a light decrease of activity could be noticed on the CuSn/SA series. The effect of methane reductant in the selective N2O reduction (N2O-SCR) was tested. On the CuGa/SA series samples, the best reaction selectivity was observed.

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Abstract

A kinetic model of cobalt-based Fischer–Tropsch synthesis was developed through the detailed kinetic study of the reaction mechanism. Experimental evidence and previously reported theoretical analyses were used to suggest the mechanism and derive reaction rates for the formation of hydrocarbon products by applying the equilibrium constants of the adsorbents and the quasi steady state assumption to intermediate species on the surface of the catalyst. The comparison between experimental data and simulated results with kinetic parameters validated the effectiveness of the developed model. Further analysis showed that temperature and H2/CO ratio significantly influenced the entire distribution of hydrocarbon products. The effects of operating conditions were also predicted in accordance with previous work, thus demonstrating that the developed model can contribute to a better understanding of the kinetic mechanism of FT synthesis.

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Abstract

Monocomponent and bicomponent iron and/or titanium modified MCM-41 materials prepared by direct synthesis at room temperature were investigated using X-ray diffraction (XRD), nitrogen physisorption, UV–Vis diffuse reflectance, FT-IR and Mössbauer spectroscopy. Their catalytic behavior was studied in cylohexanol conversion. Materials with high surface area and well-ordered pore structure were obtained. All modified mesoporous silicas possess high activity in cyclohexanol conversion. Bicomponent FeTiMCM-41 catalyst pretreated in hydrogen at 723 K presented the highest activity and selectivity to cyclohexanone.

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