Conversion of Methanol to Hydrocarbons: Adsorption of Adamantane on H-MCM-22 Zeolite and Its Effect on the Hydrocarbon Pool Species in Dual-Cycle Routes

Abstract

The zeolite catalyzed methanol-to-hydrocarbons (MTH) reaction can selectively obtain value-added olefin products or aromatic products. According to the dual-cycle mechanism: in the aromatic-based cycle, the aromatic hydrocarbon pool (polymethylbenzene) is formed, mainly producing ethylene, aromatics, and partial propylene; in the alkene-based cycle, the olefin hydrocarbon pool (mainly higher olefins) is formed, producing C₃₊ olefin products. However, under the influence of various factors such as the acidic properties and pore confinement effect of the zeolite, the types of hydrocarbon pool (HCP) species in the dual-cycle pathways and the corresponding dominant products will be different. Therefore, it is crucial to discern the role of aromatics/olefins, whether if it is active aromatics/olefins HCP or the inert coke precursors. At medium and low temperatures, adamantane (ADH), whose role in the dual-cycle pathway is still controversial, has been found on both CHA and MWW zeolites. In this paper, ADH was preadsorbed on the H-MCM-22 zeolite; combined with pulsed ¹²C-methanol/¹³C-methanol isotopic switching experiments and direct ¹³C-methanol isotopic pulsing experiments, it confirmed that ADH undergoes C─H bond cleavage and improves the hydrogen transfer reaction, playing a cocatalytic role in the MTH reaction; In addition, the C atoms of ADH also incorporate into ethylene, and into some aromatic HCPs, which eventually promoted the aromatic-based cycle and inhibited the olefin-based cycle. Moreover, partial carbon of ADH was incorporated into naphthalene as coke precursor in MTH reaction. It also suggested the participation of ADH in accelerating the deactivation of the catalyst. The clarified role of ADH in the dual-cycle pathway of the zeolite catalyzed MTH reaction can provide some inspiration on tuning the dual-cycle pathway and obtaining target products.