アミン類を主剤とする防食剤の研究 (第42報)

インヒビターと接触還元反応の促進剤

抄録

From our studies on organic corrosion inhibitors, it has been proposed that water molecule was adsorbed on metallic surface by the donation of electron pairs of its oxygen atom and organic corrosion inhibitors could be adsorbed by displacing the water, as shown in followings,
M: O-H-H RNH₂→M: O-H-H…NH₂R→M
+O-H-H…NH₂R→M: NH₂R
+O-H-H…NH₂R
This theory was applied to Ni-catalysts, often used for hydrogenations. Ni is stored in water or in alcohol as usual. The surface of Ni in water should be adsorbed by water molecule and one in alcohol by its molecule, as illustrated in next formulae,
Ni: O-H-H(I) and Ni: O-H-C₂H₅ (II)
Hydrogenation of acetone to isopropylalcohol with these two types of catalysts was carried out, and with I the reaction proceeded at room temperature, while with II it could not at less than 50°C. For these, it was concluded that acetone couldbe reduced by its displacing ability with the adsorbed water.
Ni: O-H-H…OC-CH₃-CH₃→Ni: OC-CH₃-CH₃
H₂→Ni+HOCH-CH₃-CH₃
But ethyl radical in II could block the approach of acetone to metal and it could not be adsorbed. Then it should be needed to pull off alcohol molecule from the surface by heating to be adsorbed with acetone. Immediately after acetone was adsorbed the hydrogenation reaction should take place.
When isooctene was reduced to isooctane with these catalysts, heating was necessary. But with II the reaction was more promptly carried out than with I, as the alcohol could be more easily desorbed than the water.
As the active site on metallic surface was adsorbed by the water, desorption of the water will make the active site open for the reactants. Strong electron-donating substances, such as acetone or triethylamine, will promote hydrogenation reaction by pulling the water apart from the active site, as following manners,
Ni: O-H-HCH₃COCH₃→Ni+O-H-H…OC-CH₃-CH₃,
Ni: O-H-H(C₂H₅)₃N→Ni+O-H-H…N(C₂H₅)₃
These promoters have been tried in the isooctenehydrogenation with water-dipped Ni, and have found that they, only in the limited amounts (7×10^-5mol/g Ni), could lower the reaction temperature, and that the addition in the amounts increased the reaction rate at the same temperature. This showed that the active site of Ni should be best open for isooctene when electron-donating substances were just equimolecular with the adsorbed water.