Gaseous phase, homogeneous reactions

Homogeneous reactions are those reactions that occur within a single phase (physical state), especially a liquid or gaseous phase. Heterogeneous reactions are those reactions that take place at least in part at the interface between two phases, such as solid and liquid or liquid and gas, etc. The discussion and problems in this chapter will address homogeneous reactions unless otherwise stated. Please note that rate means that there is a time element involved which is usually expressed in seconds (s), minutes (min), or hours (h). 

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. 

Homogeneous reactions are those in which the reactants, products, and any catalyst used form one continuous phase gaseous or liquid. 

A homogeneous catalyst exists in the same phase as the reactants. Homogeneous catalysts most often catalyze gaseous and aqueous reactions. For example, aqueous zinc chloride, ZnCh, is used to catalyze the following reaction. 

The Ruhrchemie/Rhone-Poulenc process is performed annually on a 600,000 metric ton scale (18). In this process, propylene is hydroformylated to form butyraldehyde. While the solubility of propylene in water (200 ppm) is sufficient for catalysis, the technique cannot be extended to longer-chain olefins, such as 1-octene (<3 ppm solubility) (20). Since the reaction occurs in the aqueous phase (21), the hydrophobicity of the substrate is a paramount concern. We overcame these limitations via the addition of a polar organic co-solvent coupled with subsequent phase splitting induced by dissolution of gaseous CO2. This creates the opportunity to run homogeneous reactions with extremely hydrophobic substrates in an organic/aqueous mixture with a water-soluble catalyst. After C02-induced phase separation, the catalyst-rich aqueous phase and the product-rich organic phase can be easily decanted and the aqueous catalyst recycled. 

Physically the system can be interpreted as follows. The gas A enters a channel with a velocity u. It can decompose via reversible homogeneous reaction to form two gaseous products B and C. Gas-phase species A and B can react at open sites on the surface C to form surface adsorbates A and B. These two adsorbates can react irreversibly to form a deposit and thus open two sites C. ... 

CVD is a synthesis process in which the chemical constituents react in the vapor phase near or on a heated substrate to form a solid deposit (Pierson 1999). The reactions happened in the CVD system can be divided into homogeneous gas phase reactions and heterogeneous substrate surface reactions. Normally, CVD technique is utilized to make thin films. CVD is also defined as a process whereby a thin solid film is synthesized from the gaseous phase by a chemical reaction (Hitchman and Jensen 1993). The CVD apparatus arrangement is dependant on the particular application. The apparatus is made up with three major components precursors and... 

Homogeneous catalysts are structurally well-defined complexes and because they are soluble in the reaction mix are not subject to pore diffusion limitations as are heterogeneous catalytic materials. They are almost always highly selective towards desired products. The main consideration is that the complex be stable and reactor conditions chosen such that all the gaseous reactants are adequately dissolved and mixed in the liquid phase. Homogeneous catalysts are easily characterized by standard instrumental methods for compound identification such as XRD or spectroscopy. Deactivation is associated with attack by traces of carboxylic acidic byproducts and impurities in the feed such as 02 and chlorides that attack the ligand groups. 

In a hydrogenation reaction with a soluble catalyst there are liquid and gaseous phases present. Why is the reaction called homogeneous rather than heterogeneous ... 

Photo-initiated AOPs are subdivided into VUV and UV oxidation that are operated in a homogeneous phase, and in photocatalysis (Fig. 5-15). The latter can be conducted in a homogeneous aqueous phase (photo-enhanced Fenton reaction) or in a heterogeneous aqueous or gaseous phase (titanium dioxide and certain other metal oxide catalysts). These techniques apply UV-A lamps or solar UV/VIS radiation and they are in pre-pilot or pilot status. According to Mukhetjee and Ray (1999) the development of a viable and practical reactor system for water treatment with heterogeneous photocatalysis on industrial scales has not yet been successfully achieved. This is mainly related to difficulties with the efficient distribution of electromagnetic radiation (UV/VIS) to the phase of the nominal catalyst. 

High liquid holdup and liquid-to-solid ratio in an upflow reactor. High liquid holdup will offer more liquid-phase resistance to the mass transfer of the gaseous reactant to the catalyst surface. High liquid-to-solid ratio will give more importance to the role of possible homogeneous reactions. 

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