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The process contrasts with homogeneous catalysis where the reagents, products and catalyst exist in the same phase. Phase distinguishes between not only solid , liquid , and gas components, but also immiscible mixtures (e.g., oil and water ), or anywhere an interface is present.
In contrast, heterogeneous catalysis describes processes where the catalysts and substrate are in distinct phases, typically solid and gas, respectively. [1] The term is used almost exclusively to describe solutions and implies catalysis by organometallic compounds. Homogeneous catalysis is an established technology that continues to evolve.
A second class of Ziegler–Natta catalysts are soluble in the reaction medium. Traditionally such homogeneous catalysts were derived from metallocenes, but the structures of active catalysts have been significantly broadened to include nitrogen-based ligands. A post-metallocene catalyst developed at Dow Chemical. [8]
Aspects are described in an early review: "using a non-protic Lewis acid, aluminium tribromide, were able to demonstrate the truly catalytic effect of titanium by treating dinitrogen with a mixture of titanium tetrachloride, metallic aluminium, and aluminium tribromide at 50 °C, either in the absence or in the presence of a solvent, e.g. benzene.
In chemistry, a catalyst support or carrier is a material, usually a solid with a high surface area, to which a catalyst is affixed. [1] The activity of heterogeneous catalysts is mainly promoted by atoms present at the accessible surface of the material. Consequently, great effort is made to maximize the specific surface area of a catalyst ...
Conversely, if the reaction rate is fast compared to the rate of diffusion, a poisoned shell will form on the exterior layers of the catalyst, a situation known as "pore-mouth" poisoning, and the rate of catalytic reaction may become limited by the rate of diffusion through the inactive shell. [4]
The heterogeneous process ultimately failed due to catalyst inactivation and was replaced by the water-based homogeneous system for which a pilot plant was operational in 1958. Problems with the aggressive catalyst solution were solved by adopting titanium (newly available for industrial use) as construction material for reactors and pumps ...
Oxidation catalysis is conducted by both heterogeneous catalysis and homogeneous catalysis. In the heterogeneous processes, gaseous substrate and oxygen (or air) are passed over solid catalysts. Typical catalysts are platinum, and redox-active oxides of iron, vanadium, and molybdenum.