Catalytic hydroconversion of aryl ethers to polycyclic alkanes over Ni@MCM-41/β composite zeolite: Aromatic α-carbon activation and hydrogen transfer mechanisms

Abstract

To produce high-density liquid fuels from lignite, a novel Ni-based bifunctional MCM-41/β composite zeolite (Ni@M/β) is reported for the catalytic hydroconversion (CHC) of lignite-derived aryl ethers to high-yield polycyclic alkanes (PCAs). Ni@M/β has a three-dimensional channel-type hierarchical porous texture with β as the core and MCM-41 as the shell. Stable Brønsted/Lewis acid sites and abundant internal/inter-zeolite defects also effectively promote the CHC of aryl ethers to PCAs. Under optimal conditions (5 MPa initial H2 pressure and 160 °C), selectivities of 2-(benzyloxy)naphthalene (BON) and benzyloxybenzene derived PCAs are as high as 76.0 and 90.5 %, respectively, with dimers and trimers dominating. DFT calculation based on electrostatic potential distribution on the van der Waals surface and Mulliken charge distribution of BON and derived oxygenated monomers shows that aromatic intermediates with >CaromaticOH have multiple accessible sites and are easy to bind with cation/radical fragments. Therefore, it is speculated that PCAs are mainly derived from C-C coupling induced by cation/radical fragments after the activation of aromatic α-carbon. In addition, the activation of H2 to H+, H·, δ+H···Hδ-, and H···H is mainly attributed to the strong Ni-M/β interaction and abundant defects. The whole CHC process is triggered mainly by the adsorption of H+ released by Ni@M/β at O sites of aryl ethers, and >Caliphatic-O- bridged bond cleavage is dominant. During this period, benzyl cation/radical produced by >C-O- bond cleavage and aromatic α-carbon activation (mainly) is coupled/reformed with adjacent aromatic fragments into corresponding polymers with non-oxygen bridged bonds, which are more easily converted to PCAs. This polycrystalline composite strategy improves the accessibility of large-sized molecules to active centers, and then increases the yields of PCAs, which is attractive for the clean and efficient conversion of lignite-derived oils to high-density liquid fuels.