FCC process for producing enhanced yields of C.sub.4 /C.sub.5 olefins

In a fluid catalytic cracking process comprising

• admixing a hydrocarbon charge stock with an FCC catalyst in the bottom section of a reactor riser at a 0.1 to 10 catalyst to oil ratio,
• passing the mixture of the hydrocarbon charge stock and the catalyst through the riser,
• thereby volatilizing the hydrocarbon charge stock and effecting cracking thereof to obtain gaseous products comprising hydrogen, C1 to C5 paraffins, olefins having 2 to 5 carbon atoms per molecule, and substantially liquid products,
• and substantially separating said gaseous products from said liquid products and solid cracking catalyst, the improvement, allowing for enhanced C.sub.4 /C.sub.5 olefin yield and decreased loss of gasoline yields, comprising
• using a catalyst formed by a method selected from:
• (i) spray drying a phosphorus-containing medium pore zeolite with a matrix to form a FCC additive having particle sizes of 40-200 microns and mechanically mixing said additive with a FCC catalyst comprising a Y-zeolite in proportions such that the weight ratio of the phosphorus-containing medium pore zeolite to Y-zeolite is 0.005 to 0.10,
• (ii) mixing both phosphorus-containing medium pore zeolite and Y-zeolite with a matrix and spray drying to form an FCC catalyst having a weight ratio of phosphorus-containing medium pore zeolite to Y-zeolite of 0.005 to 0.10.