SORPTION OF ETHENE AND PROPENE ON AMORPHOUS NICKEL OXIDE-ALUMINA SILICA CATALYSTS UNDER OLIGOMERIZATION CONDITIONS

The sorption of ethene and propene on different NiO-Al2O3/SiO2 catalysts was investigated under reaction conditions by using an electronic high-pressure Sartorius microbalance coupled to a gas chromatograph. The reaction temperature was 473 K and the alkene pressure varied between 1 and 7.9 bar. The balance was operated as a differential reactor and allowed the catalyst mass and the reaction rate to be followed simultaneously. The catalyst mass increases over a long period by the interaction with the alkene. Reaction and sorption rates do not follow the number and strength of acid sites of the catalysts. The main sorption centres and catalytically active sites are coordinatively unsaturated nickel ions and high selectivities for dimers are obtained. The initial sorption rates of both alkenese are identical, from which it can be deduced that the higher reactivity of ethene is caused by the coordinative reaction mechanism. Propene leads mainly to sorbed dimers, while ethene produces higher oligomers on the catalysts. This leads to different deactivation mechanisms. The reaction rate and the sorption rate at steady state and initial conditions are directly proportional to the alkene pressure. This is consistent with an Eley-Rideal mechanism for ethene and propene oligomerization.