Hydrothermal Synthesis of AlV-Rich Nonporous Amorphous Silica-Alumina for Stable Cracking of Plastics Waste into Light Olefins

Fast coking and limited selectivity toward light olefins are two drawbacks of the zeolitic upcycling of polyolefin-based plastic waste. The Al^V–rich amorphous silica-alumina (ASA) catalysts with the strong Brønsted acidic sites (BAS) might be an alternative to the zeolites; however, the existing preparations are costly and complex. A hydrothermal preparation of Al^V-rich (37.90–43.46%) nonporous ASA catalysts is presented, which is achieved by collecting the synthesis gel of an FAU-type structure during its induction period. The resultant ASA catalysts demonstrated (very) strong BAS, depending on the Al^V/Al^IV ratio, and a high density of Lewis acidic sites (LAS), which are stronger than the industrially relevant zeolites. The ASA catalysts examined in the cracking of various polyolefin-based plastics resulted in excellent polymer degradation activity, increased light olefins production, high-quality heavy aromatics-free liquid oils, and up to zilch coking, which are clearly superior to those obtained using a crystalline zeolite. Despite having (very) strong BAS, the exceptional light olefins production and suppressed coke formation by the ASA catalysts are possibly due to the synergy between a high density of LAS and free mass transfer, which favors the monomolecular over the bimolecular cracking mechanism.