Adams' catalyst
Encyclopedia
Adams' catalyst, also known as platinum dioxide, is usually represented as platinum
(IV) oxide
hydrate, PtO2-H2O. It is a catalyst for hydrogenation
and hydrogenolysis
in organic synthesis
. This dark brown powder is commercially available. The oxide itself is not an active catalyst, but it becomes active after exposure to hydrogen whereupon it converts to platinum black
, which is responsible for reactions.
. The procedure involves first preparing a platinum nitrate which is then heated to expel nitrogen oxides.
The resulting brown cake is washed with water to free it from nitrates. The catalyst can either be used as is or dried and stored in a desiccator for later use. Platinum can be recovered from spent catalyst by conversion to ammonium chloroplatinate using aqua regia
followed by ammonia
.
and hydrogenolysis, dehydrogenation
, and oxidation reactions. During the reaction, platinum metal (platinum black
) is formed which has been cited to be the active catalyst. Hydrogenation
occurs with syn stereochemstry when used on an alkyne resulting in a cis-alkene. Some of the most important transformations include the hydrogenation
of nitro compounds to amines and ketones to alcohols. Although, reductions of alkenes can be preformed with Adam's catalyst in the presence of nitro groups without reducing the nitro group. When reducing nitro compounds to amines, platinum catalysts are preferred over palladium catalysts to minimize hydrogenolysis. The catalyst is also used for the hydrogenolysis of phenyl phosphate esters, a reaction that does not occur with palladium catalysts. The pH of the solvent significantly affects the reaction course, and reactions of the catalyst are often enhanced by conducting the reduction in neat acetic acid, or solutions of acetic acid in other solvents.
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
(IV) oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
hydrate, PtO2-H2O. It is a catalyst for hydrogenation
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
and hydrogenolysis
Hydrogenolysis
Hydrogenolysis is a chemical reaction whereby a carbon–carbon or carbon–heteroatom single bond is cleaved or undergoes "lysis" by hydrogen. The heteroatom may vary, but it usually is oxygen, nitrogen, or sulfur. A related reaction is hydrogenation, where hydrogen is added to the molecule, without...
in organic synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
. This dark brown powder is commercially available. The oxide itself is not an active catalyst, but it becomes active after exposure to hydrogen whereupon it converts to platinum black
Platinum black
Platinum black is a fine powder of platinum with good catalytic properties. The name of platinum black is due to its black color....
, which is responsible for reactions.
Preparation
Adams' catalyst is prepared from chloroplatinic acid H2PtCl6 or ammonium chloroplatinate, (NH4)2PtCl6, by fusion with sodium nitrate. The first published preparation was reported by V. Voorhees and Roger AdamsRoger Adams
Roger Adams was an American organic chemist. He is best-known for the eponymous Adams' catalyst, and his work did much to determine the composition of naturally occurring substances such as complex vegetable oils and plant alkaloids...
. The procedure involves first preparing a platinum nitrate which is then heated to expel nitrogen oxides.
-
- H2PtCl6 + 6 NaNO3 → Pt(NO3)4 + 6 NaCl (aq) + 2 HNO3
-
- Pt(NO3)4 → PtO2 + 4 NO2 + O2
The resulting brown cake is washed with water to free it from nitrates. The catalyst can either be used as is or dried and stored in a desiccator for later use. Platinum can be recovered from spent catalyst by conversion to ammonium chloroplatinate using aqua regia
Aqua regia
Aqua regia or aqua regis is a highly corrosive mixture of acids, fuming yellow or red solution, also called nitro-hydrochloric acid. The mixture is formed by freshly mixing concentrated nitric acid and hydrochloric acid, usually in a volume ratio of 1:3, respectively...
followed by ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
.
Uses
Adams' catalyst is used for many applications. It has shown to be valuable for hydrogenationHydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
and hydrogenolysis, dehydrogenation
Dehydrogenation
Dehydrogenation is a chemical reaction that involves the elimination of hydrogen . It is the reverse process of hydrogenation. Dehydrogenation reactions may be either large scale industrial processes or smaller scale laboratory procedures....
, and oxidation reactions. During the reaction, platinum metal (platinum black
Platinum black
Platinum black is a fine powder of platinum with good catalytic properties. The name of platinum black is due to its black color....
) is formed which has been cited to be the active catalyst. Hydrogenation
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
occurs with syn stereochemstry when used on an alkyne resulting in a cis-alkene. Some of the most important transformations include the hydrogenation
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
of nitro compounds to amines and ketones to alcohols. Although, reductions of alkenes can be preformed with Adam's catalyst in the presence of nitro groups without reducing the nitro group. When reducing nitro compounds to amines, platinum catalysts are preferred over palladium catalysts to minimize hydrogenolysis. The catalyst is also used for the hydrogenolysis of phenyl phosphate esters, a reaction that does not occur with palladium catalysts. The pH of the solvent significantly affects the reaction course, and reactions of the catalyst are often enhanced by conducting the reduction in neat acetic acid, or solutions of acetic acid in other solvents.
-
- PtO2 + 4 H2 → Pt + 2 H2O
Development
Prior to the development of Adams' Catalyst, organic reductions were carried out using colloidal palladium, colloidal platinum or platinum black. The colloidal catalysts were more active but posed difficulties in isolating reaction products. This led to more widespread use of platinum black. In Adams' own words:"...Several of the problems I assigned my students involved catalytic reduction. For this purpose we were using as a catalyst platinum blackPlatinum blackPlatinum black is a fine powder of platinum with good catalytic properties. The name of platinum black is due to its black color....
made by the generally accepted best method known at the time. The students had much trouble with the catalyst they obtained in that frequently it proved to be inactive even though prepared by the same detailed procedure which resulted occasionally in an active product. I therefore initiated a research to find conditions for preparing this catalyst with uniform activity."