Adams' catalyst

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.

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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 Adams.^[1] The procedure involves first preparing a platinum-nitrate which is then heated to expel nitrogen oxides.^[2]

H₂PtCl₆ + 6 NaNO₃ → Pt(NO₃)₄ + 6 NaCl _(aq) + 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 followed by ammonia.

Uses

Adams' catalyst is used for many applications. It has shown to be valuable for hydrogenation and hydrogenolysis, dehydrogenation, and oxidation reactions. 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. 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.

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:

"...[S]everal of the problems I assigned my students involved catalytic reduction. For this purpose we were using as a catalyst platinum black 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."^[3]

Safety

Little precaution is necessary with the oxide but, after exposure to H₂, the resulting platinum black can be pyrophoric. Therefore, it should not be allowed to dry and all exposure to oxygen should be minimized.

References

1. ^ Voorhees, V.; Adams, R. "The Use of the Oxides of Platinum for the Catalytic Reduction of Organic Compounds". J. Amer. Chem. Soc., (1922) 44, 1397-1405. doi:10.1021/ja01427a021
2. ^ Adams, Roger (1928). "Platinum Catalyst for Reductions". Organic Syntheses Ann. 8 (4): 92.
3. ^ Hunt, LB (October 1962). "The Story of Adams' Catalyst: Platinum Oxide in Catalytic Reductions". Platinum Metals Rev. 6 (4): 150-2.