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Catálisis heterogénea. Materiales catalíticos.
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Tema 9
Materiales catalíticos
Materiales cataliticos
• Redox catalyst– Hydrogenation of alkenes, aromatics, and other compounds with double bonds– Hydrogenation of CO and CO2 to methane– Ammonia synthesis– Synthesis of hydrocarbons and alcohols from synthesis gas– Oxidation of hydrocarbons, SO2, NH3, etc.– Dehydrogenation of organic compounds– Decomposition of formic acid– Polymerization of hydrocarbons
Materiales catalíticos
• Acid‐Base catalyst
– Hydrolysis– Hydration and dehydration– Polymerization and polycondensation– Cracking reactions– Alkylation– Isomerization
Ej O2
Ej H2
Metales
Configuración electrónica está directamente relacionada con la actividad catalíticaTambien existen efectos geometricos (Tema 8)
Metales
1) Metals are used as catalysts for hydrogenation, isomerization, and oxidation.2) For reactions involving hydrogen (alone or in combination with hydrocarbons),the following activity series holds:Ru, Rh, Pd, Os, Ir, Pt>Fe, Co, Ni,>Ta, W, Cr≈Cu3) Pd is an excellent catalyst that is often active and selective. Pd enables selectivehydrogenation of double bonds to be carried out in the presence of other functionalgroups.4) Activities sometimes correlate with the percentage d character of the metallicbonding, but there are many exceptions.5) Activities sometimes correlate with the lattice parameters of the metal.6) The following metals are particularly stable towards oxygen and sulfur:Rh Pd AgIr Pt Pd7) The activity of metals decreases in the order:(W‐Mo)>Rh>Ni>Co>Fe
Reglas empíricas…
Metales
Metales
Catalizadores bimetálicosHidrogenación acetato de etilo a etanol
– Isolation of Rh atoms by Sn atoms at the catalyst surface– The chemsorption ability of the bimetallic particles for CO and H2 is drastically reduced– IR: Rh/Sn‐SiO2 shows only terminal carbonyl groups at 2000 cm–1– Ea: Rh/SiO2 75.2 kJ/mol; Rh/Sn‐SiO2 46 kJ/mol– Electronic effect of Sn increases the electron density of Rh
Catalizadores bimetalicos
Semiconductoresn‐type p‐type
Quimisorcion en semiconductores
Semiconductores
1) Formation of an electron bond between the starting material to be oxidized (e. g., a hydrocarbon) and the catalyst; chemisorption of the starting material.2) Chemisorption of oxygen.3) Transfer of electrons from the molecule to be oxidized (the donor) to the acceptor (O2) by the catalyst.4) Interaction between the resulting ion, radical, or radical ion of the starting materialand the oxygen ion with formation of an intermediate (or the oxidation product).5) Possible rearrangement of the intermediate.6) Desorption of the oxidation product.
Reacciones de oxidación
Semiconductores
Semiconductores
Semiconductores
Aislantes: Cat acidos y básicos
Acidez
In comparison with Al2O3, Lewis centers are not so readily formed on the surfaceof SiO2 since the OH groups are very strongly bound, so that Brønsted acidity predominates, albeit in a weak form, comparable to acetic acid.
Aluminosilicatoszeolitas
Catalizadores ácidos
Zeolitas selectividad forma