Promotion of cracking catalyst octane yield performance

In an improved process for increasing gasoline octane number in catalytic cracking of a hydrocarbon chargestock in the presence of a cracking catalyst comprising an active component and a matrix and under cracking conditions, the improvement which comprises addition to said catalyst of a shape selective additive promoter, characterized by a particle size of less than 5 microns made up of crystals in which atoms constituting at least those of silicon or germanium are tetrahydrally coordinated to oxygen atoms to provide structures of definite configuration having an X-ray diffraction pattern substantially as shown in Table 1, said additive promoter not being intimately combined with a substantial amount of other solid prior to contact thereof with said cracking catalyst and wherein the concentration of said additive promoter is between about 0.01 and about 1.0 weight percent based on the total quantity of cracking catalyst.