Zeolite/LDH Composites as Additives for Light Olefin Production by Catalytic Cracking of Heavy Oil Fractions.

Among the possible strategies for the enhancement of light olefin production in the catalytic cracking of heavy oils, the use of functional additives has proven to be an effective approach. Herein, we present a composite material (Z5-LDH) derived from medium-pore zeolite ZSM-5 (Z5) and layered double hydroxide (LDH)-based mixed oxides as a potential additive for catalytic cracking of heavy oils. This material shows enhanced selectivity to light olefins within the gaseous C3-C4 fraction. The Z5-LDH composite features a core-shell structure, with the zeolite forming the core, which is surrounded by an LDH shell. It exhibits reduced Brønsted acidity compared to pure ZSM-5 and surface basicity provided by the basic oxides formed upon calcination of the LDH. The catalytic performance of the composite has been compared with that of pure ZSM-5 for catalytic cracking of vacuum gasoil (VGO). When used as an additive, the composite yields a significantly higher olefin content and selectivity within the C3-C4 gas fraction than a commercial USY base catalyst alone (17.2% compared with 11.3 wt %, respectively). Furthermore, the propylene/propane and butenes/butanes ratios obtained with the Z5-LDH additive are substantially higher than those obtained with the pure ZSM-5 additive. The enhanced selectivity to C3-C4 olefins is related, on the one hand, to the lower density and acid strength of the Brønsted acid sites in Z5-LDH relative to ZSM-5, which minimizes secondary reactions such as hydrogen transfer. On the other hand, the basic mixed-metal oxide shell, derived from the LDH, inhibits the readsorption of the C3-C4 olefins produced, limiting their further conversion and promoting light olefin selectivity. The advantages of the Z5-LDH composite as a fluid catalytic cracking (FCC) additive have been demonstrated not only for conversion of conventional VGO but also in the cracking of heavier feeds, such as atmospheric residue.