Gas-solid Interaction V. Specific and Non-specific Interactions in the Adsorption of Hydrocarbons on Magnesium Oxide and other Ceramic Oxides *

The heats of adsorption on magnesium oxide of six linear chain alkanes (C4 to C 9 ) , of the corresponding 1-mono-olefins and of the cycl ic compounds cyclohexane, cyclohexene, 1.4-cyclohexadiene and benzene have been determined by gas-solid chromatography and extrapolated to zero surface coverage. The values obtained with n-alkanes are linearly dependent on the carbon atom number. By comparison with previous results for alumina and silica-gel, it is seen that the dispersion-interaction adsorption potentials for the CH 3 and CH2 -groups take very close values on the three different oxides. As for the cyc l i c molecules, a constant increment of about 1.40 kcal /mole in the heat of adsorption on M g O is observed when passing from cyc lohexane to cyclohexene and from the latter to 1.4-cyclohexadiene, in ful l agreement with the rrsults for A1 2 0 3 . The values for benzene are intermediate between those for cyclohexene and those for 1.4-cyclohexadiene. These results are interpreted in view of a specific interaction of electrostatic nature between surface hydroxyls and jr-electron systems. With linear 1-alkenes, the specific contribution becomes more apparent with increasing chain length, the difference in the heats of adsorption reaching a value of 1.50 kcal /mole for the pair 1-nonene n-nonane.