Preparation of Active MnO2 from KMnO4 under Basic Conditions (Attenburrow).
A solution of MnSO4.4H2O (110 g) in H2O (1.5 L) and a solution of NaOH (40%; 1.17 L) were added simultaneously during 1 h to a hot stirred solution of KMnO4 (960 g) in H2O (6 L). MnO2 precipitated soon after as a fine brown solid. Stirring was continued for an additional hour and the solid was then collected with a centrifuge and washed with water until the washings were colorless. The solid was dried in an oven at 100-120 °C and ground to a fine powder (960 g) before use.
Preparation of Active MnO2 from KMnO4 under Acidic Conditions.
Active MnO2 was made by mixing hot solutions of MnSO4 and KMnO4, maintaining a slight excess of the latter for several hours, washing the product thoroughly with water and drying at 110-120 °C. Its activity was unchanged after storage for many months, but it was deactivated by H2O, MeOH, thiols, or excessive heat (500 °C). MnO2 was less active when prepared in the presence of alkali and ineffective when precipitated from hot solutions containing a large excess of MnSO4.
Preparation of Highly Active MnO2 from KMnO4.
A solution of MnCl2.4H2O (200 g) in H2O (2 L) at 70 °C was gradually added during 10 min, with stirring, to a solution of KMnO4 (160 g) in H2O (2 L) at 60 °C in a hood. A vigorous reaction ensued with evolution of chlorine; the suspension was stirred for 2 h and kept overnight at rt. The precipitate was filtered off, washed thoroughly with H2O (4 L) until pH 6.5-7 and the washing gave a negligible chloride test. The filter cake was then dried at 120-130 °C for 18 h; this gave a chocolate-brown, amorphous powder; yield 195-200 g. Alternatively, the wet cake was mixed with benzene (1.2 L) and H2O was removed by azeotropic distillation giving a chocolate-brown, amorphous powder; yield 195 g. The last procedure gave a slightly less active material.
Preparation of Active MnO2 by Pyrolysis of MnCO3.
Powdered MnCO3 was spread in a one-inch thick layer in a Pyrex glass and heated at 220-280 °C for about 18 h in an oven in which air circulated by convection. The initially tan powder turned darker at about 180 °C, and black when maintained at over 220 °C. No attempt was made to determine lower temperature or time limits, nor the upper limit of temperature. The MnO2 prepared as above was stirred with about 1 L of a solution made up of 15% HNO3 in H2O. The slurry was filtered with suction, the solid was washed on the Buchner funnel with distilled water until the washes were about pH 5, and finally was dried at 220-250 °C. The caked, black solid was readily crushed to a powder which retained its oxidizing ability even after having been stored for several months in a loosely stoppered container.
Preparation of g-MnO2.
To a solution of MnSO4 (151 g) in H2O (2.87 L) at 60 °C was added, with stirring, a solution of KMnO4 (105 g) in H2O (2 L), and the suspension was stirred at 60 °C for 1 h, filtered, and the precipitate washed with water until free of sulfate ions. The precipitate was dried to constant weight at 60 °C; yield 120 g (dark-brown, amorphous powder).
Preparation of Active MnO2 on Silica Gel.13n
KMnO4 (3.79 g) was dissolved in water (60 mL) at rt. Chromatographic grade silica gel (Merck, 70-230 mesh, 60 g) was added with stirring, and the flask connected to a rotary evaporator to strip off the water at 60 °C. The purple solid was ground to fine powder and then added with vigorous stirring to a solution of MnSO4.H2O (9.3 g) in H2O (100 mL). The resulting brown precipitate was filtered with water until no more MnII ion could be detected in the wash water by adding ammonia. After being dried at 100 °C for 2 h, each gram of this supported reagent contained 0.83 mmol of MnO2.
As shown above, a wide range of products of various activities are called active MnO2 and the results described in the literature are sometimes difficult to reproduce since the nature of the MnO2 which was used is not always well defined. Now, the commercial materials give reproducible results but they are not always convenient to perform all the oxidation reactions described in the literature. In addition, their origin (method of preparation) is not often indicated and comparison with the active MnO2 described in the literature is sometimes difficult. For all these reasons, the use of activated MnO2 has been somewhat restricted in spite of the efficiency and selectivity of its reactions since only an empirical approach and a careful examination of the literature allow selection of the suitable activity of MnO2 and the optimum reaction conditions for a defined substrate.
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