Formation of products responsible for motor and research octane of gasolines produced by cracking The implication of framework Si/Al ratio and operation variables

An alkane in the range of gasoline fraction (n-heptane) has been used as a reactant to study the influence of zeolite Y catalyst and process variables (i.e., framework Si Al ratio and procedure of dealumination, time on stream, and contact time) on the formation of products responsible for motor and research octane of gasoline during cracking, namely branched, aromatics, and olefins. It is found that the branched isomers in the C5 and C6 fractions appear as primary products and are partly produced by disproportionation, since the ratio of iso to normal compounds is above equilibrium. The ratio of branched to linear products strongly decreases when the number of carbon atoms of the product fraction increases. It goes through a maximum for samples with unit cell size in the range 24.30–24.40 Å. The selectivity to aromatics is strongly dependent on total conversion and increases with time on stream, while dealumination decreases the selectivity to aromatics. The saturation of olefins, via hydrogen transfer, decreases with increasing dealumination, increasing temperature and decreasing time on stream. These observations are explained not only on the basis of site density and strength but also on the basis of the adsorption characteristics of the samples. Finally, methane and ethane can be formed by a protolytic cracking of branched products. Part of these two gases and most of the ethylene are formed by a radical type of cracking, in which extraframework aluminium plays an important role.