Browse

You are looking at 91 - 100 of 392 items for :

  • Chemistry and Chemical Engineering x
  • Architecture and Architectonics x
  • All content x
Clear All
Reaction Kinetics, Mechanisms and Catalysis
Authors: Viorel Chihaia, Karl Sohlberg, Rodica Zăvoianu, Anca Cruceanu, Octavian Dumitru Pavel, Emilian Angelescu, Ana Paula Soares Dias, and Ruxandra Bîrjega

Abstract

This contribution presents several aspects concerning the production and characterization of hydrotalcite-like compounds containing Mo-species in the interlayer region (Mo-HT), the corresponding derived mixed oxides (Mo-CHT) and their catalytic activity in the demercaptanization of gasoline contaminated by tert-butanethiol. Mo-HT samples were obtained using two different molybdenum sources, e.g. Na2MoO4 or (NH4)6Mo7O24 and two preparation procedures: (i) ionic exchange and (ii) coprecipitation at pH 10 under high supersaturation. The derived mixed oxides were obtained by calcination at 450 °C during 24 h. The solids have been characterized by chemical analysis, TG/DTA, XRD, SEM–EDX, FTIR, DR-UV–Vis and Raman spectroscopy as well as determination of base sites. The specific area of the solids was determined using the BET method. The best catalysts were found to be those containing higher amounts of Mo species with tetrahedral coordination obtained by calcination of the Mo-HT precursors prepared at pH 10, either by ionic exchange or by co-precipitation using Na2MoO4 as a molybdenum source. Meanwhile, the catalysts containing mainly octahedrally coordinated Mo species (obtained from Mo-HT prepared at pH 10 using (NH4)6Mo7O24·6H2O as molybdenum source) were characterized by a higher concentration of molybdenum species with octahedral coordination, lower number of base sites, smaller surface area, and a poor catalytic activity.

Restricted access

Abstract

In this study, a series of imidazolium-based ionic liquids (ILs) having carboxylic acid moieties were synthesized and used as new homogeneous catalysts to synthesize cyclic carbonates from CO2 and epoxides. Even in the absence of any co-catalyst and organic solvent, carboxylic-acid-functionalized ILs showed better catalytic activity in the coupling reaction of CO2 and styrene oxide for the production of styrene carbonate than did hydroxyl-functionalized ILs and conventional ILs without any functional moieties. A detailed investigation was carried out on a variety of factors that affected the reactivity, such as the alkyl chain length and the molecular composition of IL molecules including the halide ions. The effect of various reaction parameters such as reaction time, temperature, CO2 pressure and catalyst amount was also investigated in detail. The mechanism underlying the enhanced rate of the cycloaddition reaction in the presence of carboxylic-acid-functionalized ILs was proposed.

Restricted access

Abstract

An immobilized layer-by-layer TiO2/Chitosan (CS) biopolymer onto glass plate has been fabricated, optimized and applied for the removal of 10 mg L−1 phenol and its intermediates under a 45-W fluorescent lamp with a UV leakage irradiance of 4.4 Wm−2. A simple dip-coating technique using the dip-coating solution containing TiO2, epoxidized natural rubber (ENR50) and phenol–formaldehyde resin (PF) was used to immobilize TiO2 onto the CS sub-layer. However, the CS sub-layer did not improve the adsorption of phenol while photodegradation of ENR50 and PF binders acted as macro-pores forming agent within the surface of the TiO2 layer. The degradation kinetics of phenol fitted the pseudo-first order rate model and the main intermediates were qualitatively and quantitatively identified by HPLC analysis to be maleic acid, fumaric acid, hydroquinone and catechol. These intermediates and end-products of the degraded phenol by both TiO2 single layer and layer-by-layer TiO2/CS systems were identical. The total removal of phenol and its intermediates can be achieved within 120 min of irradiation by using layer-by-layer TiO2/CS system, while the same results can be obtained within 360 min of irradiation by using TiO2 single layer.

Restricted access

Abstract

SnO2 catalysts were prepared with the precipitation method by varying the pH and SnCl4 concentration. For comparison, another two SnO2 samples were also synthesized with sol–gel and combustion methods. All of the catalysts were characterized by N2-BET, XRD and H2-TPR. It was found that the CO oxidation activity of the precipitation samples increases with an increase in pH, while the CH4 oxidation activity seems unchanged. The change of the SnCl4 concentration has little influence on the activity for both CO and CH4 oxidation. The sol–gel sample has relatively higher CO but similar CH4 oxidation activity, due to its higher surface area and finer particle sizes. On the contrary, the combustion sample displays the lowest activity for both reactions, due to its extra low surface area and inert oxygen species.

Restricted access

Abstract

In order to examine the capability of AlPO4 to serve as a catalytic support, we analyzed naphthalene hydrogenation over Pd/AlPO4 catalyst in the presence of CO. This process is an effective storage strategy in the organic hydride method that uses low grade hydrogen. Pd/AlPO4 demonstrated high activity in naphthalene hydrogenation similar to Pd/SiO2–Al2O3, and retained the activity in much higher extent in the presence of CO as compared to Pd/Al2O3. The existence of acidic sites on the surface of AlPO4 was confirmed by ammonia adsorption. FT–IR analysis of adsorbed CO after high temperature evacuation revealed that CO desorbed more easily on Pd surface when supported on acidic materials. Detailed analysis of the IR spectra suggested that acidic support decreased the electronic density of Pd and weakened the adsorption bond. Because CO retardation decreased on acidic supports, Pd catalysts demonstrated high activity in the presence of CO. We found that AlPO4 was an effective acidic support for Pd catalyzed hydrogenation.

Restricted access
Reaction Kinetics, Mechanisms and Catalysis
Authors: Viorel Chihaia, Karl Sohlberg, M. Scurtu, S. Mihaiu, M. Caldararu, and M. Zaharescu

Abstract

Tin and cerium based oxide ceramics, due to their peculiar properties, are good candidates for using as sensors, solid electrolytes in fuel cells, and catalysts. In the present work, Sn–Ce–O powders with the composition of interest for catalysis applications were obtained by solid state reactions and by thermal decomposition of the different tin and cerium precursors. The structural characterization of the resulted samples was performed by X-ray diffraction. Morphological characteristics were evaluated from X-ray microstructural parameters and BET surface areas measurements. Surface evolution of the studied powders was investigated by electrical measurements in various atmospheres in the 25–400 °C temperature range. The samples studied behave as a n-type semiconductor. The catalytic activity for CO oxidation was measured in gas flow between 25 and 400 °C. The conversion degree depends on the preparation method of the samples. Magnetic susceptibility measurements at room temperature of the samples before and after catalytic test indicated a paramagnetic behavior. Higher values of magnetic susceptibility of the samples after catalytic test compared to initial ones could indicate the influence of CeO2 on the catalytic activity.

Restricted access
Reaction Kinetics, Mechanisms and Catalysis
Authors: Shakeel Ahmed, Faizur Rahman, Adnan M. J. Al-Amer, Eid M. Al-Mutairi, Uwais Baduruthamal, and Khurshid Alam

Abstract

A series of metal incorporated M-MCM-41 (M = Ce, Mo, and/or V) mesoporous materials were synthesized by the hydrothermal method. The synthesized mesoporous materials were characterized by the XRD, BET, TPR and EPR techniques. The extent of mesoporous structural ordering was evident from the XRD pattern of M-MCM-41. The catalytic properties of the metal containing MCM-41 catalysts were evaluated for oxidative dehydrogenation of propane and n-butane in a fixed-bed micro-reactor. The results showed higher activity in case of V-MCM-41 catalyst as compared to that of Mo-MCM-41 catalysts over a temperature range of 500–600 °C at atmospheric pressure. Cerium containing MCM-41 catalyst showed high selectivity (78%) for butadiene at moderate conversion of about 4%. The major products include ethylene, propylene and butanes. Propane conversion of about 20% with corresponding propylene selectivity of around 30% was obtained at 575 °C over V-MCM-41 catalyst. A small amount of H2 besides COx was also produced during catalytic runs under the conditions of catalyst performance evaluation.

Restricted access

Abstract

The semi-batch slurry polymerization of propylene using a heterogeneous multi-site type Ziegler–Natta catalytic system was studied. A simple kinetic model including initiation, propagation, spontaneous chain transfer, chain transfer to hydrogen, chain transfer to monomer and chain transfer to cocatalyst, and spontaneous deactivation was developed to predict instantaneous rates of polymerization and average molecular weights of final products. Estimation of kinetic parameters was performed using online measurements of polymerization rate and end of batch measurements of average molecular weights. The multivariable nonlinear optimization problem was solved using the Nelder–Mead simplex method for three different site types at three levels of temperatures. The model predicts that the propagation reaction has a lower activation energy than chain transfer reactions which leads to a decrease of molecular weight at elevated temperatures. The deactivation reaction has a higher activation energy than the propagation reaction, which results in decreasing the final rate of polymerization at higher temperatures.

Restricted access
Reaction Kinetics, Mechanisms and Catalysis
Authors: F. J. Lona-Ramírez, R. Herrera-Muñoz, V. Rico-Ramírez, F. Louvier Hernández, G. Luna-Bárcenas, and G. González-Alatorre

Abstract

Although the product resulting from the nitrosation of 1,1,3-trimethylurea is a carcinogenic agent, there have been no kinetic studies reported on such a reaction. This work determines the kinetic parameters and the corresponding reaction mechanism.

Restricted access