Water-Controlled Coking Dynamics during High-Pressure Methanol-to-Olefins Reaction over SAPO-34
Chengwei Zhang, Xinqiang Wu, Yanan Zhang, Li Wang, Yan Jin, Mingbin Gao, Mao Ye, Yingxu Wei, Zhongmin Liu
Abstract
Water, as a co-feed and decoking agent for catalyst regeneration, is increasingly recognized as a crucial component in methanol to olefins (MTO) catalysis over zeolites. In this study, water-controlled coking dynamics and improved diffusion efficiency have been revealed in a high-pressure MTO reaction over the SAPO-34 zeolite catalyst. Through gas chromatograph–mass spectrometry (GC-MS), matrix-assisted laser desorption/ionization Fourier-transform ion cyclotron resonance mass spectrometry (MALDI FT-ICR MS), and ultraviolet–visible spectroscopy (UV–vis), the kinetic behavior of water-delayed coking has been confirmed mainly in two aspects: suppressing the aging of active hydrocarbon pool species (HCPs, e.g., phenyl, naphthyl species) to form polyaromatic hydrocarbons (PAHs) within the CHA cages and hindering the cross-linking of PAHs between CHA cages. For the deactivated SAPO-34 catalyst, the restoration of methanol conversion from 5% to 40% upon switching from methanol to water–methanol co-feed and from 5% to 100% after high-pressure steam treatment further confirms the in situ coke decomposition capability of high-pressure water under the real MTO reaction conditions. Moreover, structured illumination microscopy (SIM) offers a direct visualization of the retained organic species and their spatiotemporal distribution within individual SAPO-34 crystals under the influence of water, thereby providing visual evidence for water-delayed coking dynamics and the improved diffusion process. Thus, the mechanistic insights into water-controlled coking and diffusion dynamics unveiled in this study provide a crucial theoretical foundation for the application of water-related techniques in the MTO industry.