Modifying Zeolite ZSM-5 to Increase the Yield of Light Olefins in Cracking Feedstocks of Petroleum and Vegetable Origin

Abstract

The effect modifying the HZSM-5 zeolite contained in a bizeolite catalyst has on the conversion of hydrotreated vacuum gas oil, vegetable oil, and a vacuum gas oil–vegetable oil mixture under conditions of catalytic cracking is investigated. It is found that modification lowers both the specific surface area and the volume of the meso- and micropores of HZSM-5 zeolite; the higher the phosphorus content, the greater the reduction in the main characteristics of the pore structure of the zeolite. A drop in the total acidity of P/HZSM-5 and a quantitative redistribution of weak and medium-strength acid sites are also observed. Catalytic tests of the zeolites in the cracking process show that phosphorus modification helps increase the total yield of propane–propylene and butane–butylene fractions with high olefin contents. Alkaline treatment of HZSM-5 zeolite with a high SiO₂/Al₂O₃ ratio facilitates the extraction of silicon and increases the specific surface area of mesopores considerably. In addition, weakening of the strong acid sites of the zeolite and/or a change in the accessibility of these sites due to the partial removal of silicon is observed.