Homogeneous gas-phase production

Information obtained during the statistical analyses was used in evaluating aerosol scavenging and aqueous-phase oxidation as the dominant pathways for introducing formate and acetate ions into precipitation. The plausibility of homogeneous gas-phase production... 

Homogeneous reactions are those in which the reactants, products, and any catalysts used form one continuous phase (gaseous or liquid). Homogeneous gas phase reactors are almost always operated continuously, whereas liquid phase reactors may be batch or continuous. Tubular (pipeline) reactors arc normally used for homogeneous gas phase reactions (e.g., in the thermal cracking of petroleum of dichloroethane lo vinyl chloride). Both tubular and stirred tank reactors are used for homogeneous liquid phase reactions. 

The catalytic conversion of NO was investigated Grst in absence of catalyst (blank). The results reported in Fig. 1 show that the homogeneous gas phase oxidation of the alkane starts at 650 K. No reduction of NO is observed in the homogeneous process, then the production of N2 can be ascribed to the catalytic reduction. The catalytic properties were determined by temperature programmed reaction (ramp 2 K min-l). The temperature was increased from 523 to 673 K and back. 

Whether or not a given PAH exists virtually entirely in the gas phase or in the particle phase, or is partitioned between them, is a critical factor in determining its physical and chemical fates in ambient air and in subsequent intra- and intermedia transport through our air/water/soil environments. This is true not only for physical processes such as wet and dry deposition but also for their chemical reactivity, lifetimes, and fates in VOC-NOx systems characteristic of polluted airsheds. For example, the homogeneous gas-phase reactions of pyrene and fluoranthene differ dramatically from the rates, mechanisms, and products of their particle-associated heterogeneous reactions (Sections E and F). 

A second concern has been indirect evidence pointing to the occurrence of chemical reactions on the walls of the chamber, followed by desorption of some of the products into the gas phase. These reactions manifest themselves by changing the observed gas-phase concentrations of one or more species in a manner that cannot be explained solely on the basis of known homogeneous gas-phase reactions. 

A systematic study to identify solid oxide catalysts for the oxidation of methane to methanol resulted in the development of a Ga203—M0O3 mixed metal oxide catalyst showing an increased methanol yield compared with the homogeneous gas-phase reaction.1080,1081 Fe-ZSM-5 after proper activation (pretreatment under vacuum at 800-900°C and activation with N20 at 250°C) shows high activity in the formation of methanol at 20°C.1082 Density functional theory studies were conducted for the reaction pathway of the methane to methanol conversion by first-row transition-metal monoxide cations (MO+).1083 These are key to the mechanistic aspects in methane hydroxylation, and CuO+ was found to be a likely excellent mediator for the reaction. A mixture of vanadate ions and pyrazine-2-carboxylic acid efficiently catalyzes the oxidation of methane with 02 and H202 to give methyl hydroperoxide and, as consecutive products, methanol and formaldehyde.1084 1085... 

Regarding the formation of by-products (acetonitrile, HCN, carbon oxides), little is known in detail. The first reaction step, formation of symmetrical allyl intermediate, is likely to be common for both main and side reactions. The work of Cathala and Germain (see below) indicates that these side reactions are complex processes that partially occur in the homogeneous gas phase. 

A more complicated reaction scheme is proposed by the authors to include the formation of the by-products acetonitrile, acetaldehyde and ethylene. However, appropriate rate coefficients cannot be given as the reactions appear to be partially homogeneous gas phase reactions, implying that factors like the reactor geometry are also involved. Regarding the oxidation mechanism, the authors assume that two hydrogen atoms are first abstracted from propene, followed by reaction with surface oxygen or NH species. 

S02 is oxidized to sulfuric acid both by homogeneous gas-phase reactions and by multiphase processes when a precursor gas is dissolved in water and then subsequently oxidized. The routes to atmospheric sulfuric acid production have been studied extensively for decades. The most important gas-phase mechanism is oxidation by OH radicals ... 

In this chapter we consider other commercially important routes to the production of olefins other than hy thermal steam cracking. Alternative processes generally involve catalytic processes rather than homogeneous gas-phase cracking. Although there have been proposals to develop catalytic processes for the production of ethylene, most of these alternative processes aim to produce propylene rather than ethylene. Some process economics of some of these routes have been compared by Houdek and Anderson and Nextant Inc. ... 

In an overwhelming majority of cases, heated polymers decompose into combustible gaseous products. The latter diffuse into the oxidative environment, mix and react with oxygen. Therefore, the self-enhancing exothermal reaction responsible for the heat generation and initiation of polymer combustion is in most cases a homogeneous gas phase reaction In order to stress this particular feature of the reactions between... 

A study has been undertaken to compare the effectiveness of molybdenum and uranium oxide and iron sodalite catalysts with the homogeneous gas-phase oxida-hon of methane [54]. Catalyst performance was evaluated in a high-pressure annular reactor and data were compared to the reactivity of the empty reactor. It was concluded that none of the catalysts gave any advantage over the homogeneous reaction. Indeed, using a catalyst only reduced the selectivity to the desired partial oxidation products. Similar conclusions have been reached for many catalysts used for the partial oxidation of methane, and therefore it is perhaps not surprising that uranium oxide catalysts are no different... 

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