From a homogeneous catalysis is spoken, if present in a chemical reaction, the catalyst and the reactants are in the same phase. The term is in industrial chemistry mainly its use to differentiate the heterogeneous catalysis.
Säure-/Basekatalysierte reactions such as esterification run from homogeneous catalysis. The advantages of homogeneous catalysis over heterogeneous catalysis are the milder reaction conditions such as low pressure and moderate temperatures, and often better selectivity. However, a disadvantage is the difficult separation of the catalyst from the reaction mixture, since both are in the same phase.
- 2.2.1 Redox Catalysis
- 2.2.2 Complex Catalysis
- 2.2.3 Organometallic Complex Catalysis
History of homogeneous catalysis
As first used by the people catalytic technical processes the alcohol fermentation of sugar, used by the Sumerians in Mesopotamia already 6000 BC, as well as the production of acetic acid from alcohol by means of catalytically active enzymes apply.
After these early beginnings, the discovery of a whole series of new catalytic reactions took place in the 18th and early 19th centuries. So, Antoine -Augustin Parmentier 1781, the strength of cleavage to glucose by acid catalysis. Only a year later Carl Wilhelm Scheele in 1782 discovered the acid-catalyzed esterification of alcohols and acids to form esters and shortly afterwards Joseph Priestley in 1783 Decomposition of ethanol to ethylene and water on clay.
A first process for the industrial production of a basic chemical, the lead chamber process for the manufacture of sulfuric acid has been developed by and Clement Desormes in 1806, catalyze the oxidation of sulfur dioxide in the nitrogen oxides.
As early as 1903 the French chemist Victor Henri worked in the field of enzyme catalysis. He examined the cleavage of sucrose by the enzyme sucrase into glucose and fructose. Due to the continuation of his work by the German biochemist Leonor Michaelis and Maud Menten Canadian Medical Examiner succeeded in 1913, the formulation of the Michaelis- Menten kinetics, which is valid until today cornerstone of enzyme kinetics. The potential of enzymatic catalysis for the resource-saving production of fine chemicals, pharmaceuticals, vitamins or detergents is until today, over 100 years after the discovery of the foundations, not far from exhausted. The understanding of enzyme catalysis and its stereochemistry has been extended by the work of Cornforth who was awarded the Nobel Prize in Chemistry. In addition to the development of catalytic processes for basic and intermediate products, numerous methods for the production of fine chemicals have been developed over the years.
Otto Roelen discovered in 1938 with the hydroformylation of the preparation of aldehydes from olefins, carbon monoxide and hydrogen on cobalt catalysts, which he developed to the industrial process. The hydroformylation is considered the first large-scale application of homogeneous transition metal catalysts. The original procedure Roelens has been widely developed. Be reacted with the at the Max Planck Institute developed by Karl Ziegler for Coal Research low-pressure process in which ethylene and propylene titanium / aluminum catalysts for polyolefins, the basis for the petrochemical industrial mass production of polymers was determined that ushered in the age of plastics. Ziegler was awarded together with Giulio Natta for this work with the Nobel Prize for Chemistry. At the MPI in Mülheim an der Ruhr, the fundamental work of Günther Wilke, who discovered the production of 1,5 -cyclooctadiene from 1,3-butadiene on nickel catalysts as well as the works of Wilhelm Keim on the SHOP process emerged. So the works of William S. Knowles and Ryoji Noyori "for their work on chirally catalysed hydrogenation reactions " and named after Sharpless epoxidation were awarded the Nobel Prize. Also in the 1970s discovered Richard F. Heck catalyzed by homogeneous palladium complexes cross-coupling that allows a direct olefination of aryl halides. Another Nobel Prize in the field of catalysis was awarded in 2005 for the discovery of the alkene metathesis of olefins on ruthenium catalysts to Chauvin, Grubbs and Schrock.
Molding the homogeneous catalysis
In Brönsted-Säure/-Base-Katalyse the proton H or the hydroxide ion OH - function as a catalyst. The activation of the substrate takes place by protonation / deprotonation. Typical Brönsted-Säure/-Base-katalysierte reactions are esterification, transesterification or aldol reactions.
When Lewis-Säure/-Base-Katalyse the catalysts metal ions are mostly such as the titanium cation Ti4 or Sn4 tin cation that often. In the form of organic salts, are used as alcoholates Typical reactions are approximately esterification.
Metal complex catalysis
In the redox catalysis metal complexes act as a catalyst, which catalyzes the exchange of electrons between an oxidation and a reducing agent. The catalyst activates the substrate by electron transfer.
In the complex catalytic metal complexes act as catalysts. The activation of the substrate takes place by coordinative interactions.
Organometallic Complex Catalysis
In the homogeneous catalysis of transition metals are often used. Here, the central metal atom of the ligand is complexed, whereby the catalyst can have a significant influence on selectivity and conversion of a reaction.
The elementary steps of homogeneous catalysis are:
In addition to the rearrangement and the insertion of the oxidative addition and reductive elimination are the two main elementary steps of homogeneous transition metal catalysis. In the oxidative addition of a metal takes two previously covalently interconnected groups in its coordination sphere. Thus its oxidation number is increased by two units. The output of this complex must have two free coordination sites.
The reductive elimination is the reverse of the oxidative addition. In this case, two ligands ( A and B) are removed as AB molecule from the coordination sphere and decreases the oxidation number of the central metal atom to two.
The homogenous transition metal complex is to achieve a better separation in some processes heterogenized through the use of special, generally water-soluble ligand, and transferred to a water phase.
In a process variant of the hydroformylation, the Ruhrchemie Rhone-Poulenc process, the removal of the catalyst by means of a rhodium complex Triphenylphosphantrisulfonat (TPPTS ) is reached. Sulfonated by the ligand of the catalyst remains in the aqueous phase, while the product of the process, the n-butanal produced from propene, carbon monoxide and hydrogen is an organic phase.
1966 succeeded Nozaki, through the use of a chiral copper complex, the first asymmetric link of styrene and ethyl diazoacetate to give cis -and trans- Cyclopropancarboxylaten in low ee. In the following period, the influence of chiral ligands has been extensively studied on the stereochemical control of catalytic reactions. In 2001, Barry Sharpless, Ryoji Noyori and William S. Knowles were honored for their research in the field of asymmetric catalysis with the Nobel Prize.
In organocatalysis small organic molecules act as a catalyst. The substrate activation occurs mostly through formation of an intermediate molecule. Both organic and inorganic substances can be obtained. A well-known example is the anthraquinone process, wherein hydrogen peroxide is produced.
In enzyme catalysis enzyme complex structure act as catalysts. The substrate activation mechanisms are diverse, ranging from protonation reactions to redox catalysis, such as metalloenzymes.
Catalysis by small inorganic molecules
Small inorganic molecules can act as a catalyst. In the lead chamber process, one of the first large-scale applications of homogeneous catalysis, nitrous oxide acts as a catalyst. The OMEGA process as carbon dioxide acts selectively on the formation of ethylene glycol. In the benzoin addition, the addition of aromatic aldehydes under catalysis of cyanide to aromatic α - hydroxy ketones takes place.
The selection of solvent will depend on the polarity of the substrates. Polar substrates are typically implemented in alcohols or water, nonpolar substrates in alkanes or simple aromatics, such as benzene or toluene. Also ethers, for example, tetrahydrofuran are used.
The solvent may act in addition to its solvent properties as ligand and occupy catalytically active sites in the complex.
A homogeneous catalytic process is the large scale hydroformylation ( oxo synthesis, Roelen Reaction) of olefins to aldehydes with carbon monoxide and hydrogen, such as of propylene to n / iso -butanal. Here, a rhodium - triphenylphosphine complex for the selective conversion of propene to n- butanal.
With the Reppe reaction can be produced from acetylene, carbon monoxide and water with nickel carbonyl catalysts acrylic acid. Accordingly, can be synthesized from ethene propionic acid.
With Lewis acids catalyzed alkylation reactions and allow the synthesis of carboxylic acids from olefins, carbon monoxide and water or alcohols.
The maximum - Wacker process is prepared from ethylene and air or pure oxygen with soluble palladium complex salts acetaldehyde.
In the hydrocyanation, the addition of HCN to an alkene is catalyzed by a nickel complex. An important synthesis is the hydrocyanation of 1,3 -butadiene to adiponitrile (DuPont).
By the Ziegler - Natta process ethylene is polymerized in a large scale to polyethylene. Accordingly, dienes can oligomerize, eg 1,3-butadiene with nickel complexes cyclododeca -1 ,5,9 -triene or to cyclo -1 ,5-diene.