|
The heats of adsorption on magnesium oxide of six linear chain alkanes (C4 to C9) , of the corresponding 1-mono-olefins and of the cyclic 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 CH3- and CH2-groups take very close values on the three different oxides. As for the cyclic molecules, a constant increment of about 1.40 kcal/mole in the heat of adsorption on MgO is observed when passing from cyclohexane to cyclohexene and from the latter to 1.4-cyclohexadiene, in full agreement with the rrsults for Al2O3. 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 π-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.
|
