Modified zeolite Y catalyst composition

A catalyst composition, comprising (a) 10 to 90% by weight of a modified zeolite Y (II) formed from a modified zeolite Y (I) and (b) 10 to 90% by weight of a binder (III) formed from a binder material containing an aluminum compound, in a degree of mixing of not less than 0.5, the catalyst composition having a clear absorption peak in an IR spectrum of a wavelength of 3602.+-.5 cm.sup.-1, the catalyst composition having a pore size distribution which has a maximum value of 5 to 50 nm and a maximum value of 50 to 1000 nm, the catalyst composition having a total pore volume of not less than 0.3 ml/g, containing not less than 25% by volume of pores of 5 to 50 nm in a pore size based on a total volume of pores of a pore size of 5 to 1000 nm, containing not less than 10% by volume of pores of 50 to 1000 nm in a pore size based on a total volume of pores of a pore size of 5 to 1000 nm, the catalyst composition being prepared by mixing a first slurry (bS) of the binder material and a second slurry (zS) of the modified zeolite Y (I), the modified zeolite Y (I) being prepared by modifying an unmodified zeolite Y having a SiO.sub.2 /Al.sub.3 O.sub.3 molar ratio of not less than 3.5, by (i) steaming and by a wet acid treatment, or (ii) by steaming, a wet acid treatment and treatment with an aqueous metal salt solution which contains at least one salt of at least one metal selected from the group consisting of metals of Group VIA and Group VIII of the Periodic Table and has a pH of 2.0 to 12.0, or (iii) by steaming and by treatment with an aqueous metal salt solution which contains at least one salt of at least one metal selected from the group consisting of metals of Group VIA and Group VIII of the Periodic Table and has a pH of lower than 1.5, the modified zeolite Y (I) having a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of not less than 7, having no clear absorption peak in an IR spectrum of a wavelength of 3602.+-.5 cm.sup.-1, containing not more than 14% by weight of four-coordinate Al in a zeolite crystal lattice, as measured by a solid NMR method, having a relative crystallinity measured by X-ray powder diffraction analysis of not less than 40% relative to the unmodified zeolite Y and containing not less than 20% by volume of pores of 3 to 30 nm in a pore size based on a total pore volume, the modified zeolite Y (II) containing four-coordinate Al in a zeolite crystal lattice in an amount which is increased by not less than 0.1% by weight as compared with the content of the four-coordinate Al in the zeolite crystal lattice of the modified zeolite Y (I), having a relative crystallinity measured by X-ray powder diffraction analysis of not less than 35% relative to the unmodified zeolite and having a U.D. value which is increased by not less than 0.001 nm as compared with the U.D. value of the modified zeolite Y (I).