Ni/HZSM-5 catalysts for hydrodeoxygenation of polycarbonate plastic wastes into cycloalkanes for sustainable aviation fuels

Abstract

Hydrodeoxygenation (HDO) is one of the effective methods for upcycling polycarbonate (PC) plastic waste, but the development of low-cost, high-performance HDO catalysts still faces great challenges. In this work, we report that a bifunctional Ni/HZSM-5 catalyst can directly upgrade PC plastic wastes into sustainable aviation fuel ranged cycloalkanes in high yields (99.3 %) with 81.2 % yield of C15 dicycloalkane under mild reaction conditions (190 °C, 4 MPa H2). Controlled reactions of PC and probe molecules demonstrated the conversion of PC into C15 dicycloalkanes involves PC depolymerization by direct cleavage of C-O bonds adjacent to benzene ring and ester group, forming C15 aromatics and C15 monophenols at the metal sites, followed by hydrogenation of C15 aromatics rings on metallic Ni sites and HDO of C15 monophenols on metal-acid dual sites parallel to generate C15 dicycloalkane. The C-C cracking mainly occurred on the tertiary carbon adjacent to the benzene ring on acidic sites of HZSM-5, thus forming C6∼C9 monocyclic alkanes. The realization of this process can be attributed to the cooperation between metal sites and acid centers on Ni/HZSM-5. Besides, the metal-acid balance (MAB) in Ni/HZSM-5 significantly affected the HDO activity and product distribution. 20Ni/HZSM-5(200) with suitable MAB showed good HDO activity and excellent selectivity to C15 dicycloalkane. In addition, the obtained catalyst was also capable of converting common real PC plastics wastes (CD disk, DVD disk and PC sheet) into cycloalkanes with the yield up to 98 % and exhibited excellent stability. This approach to construct inexpensive bifunctional catalysts highlights an efficient and reliable route for HDO of PC to sustainable aviation fuel, providing a new avenue towards a circular economy.