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Host–Guest Interactions and Their Catalytic Consequences in MTO Conversion on Zeolites Studied by ss-NMR Spectroscopy

Release time:2017-8-18      Source:admin      Reads:802




   Methanol conversion over zeolites with different topologies (H-ZSM-5, H-SSZ-13, and H-MOR) was studied using solid-state NMR spectroscopy and gas chromatrography mass spectroscopy (GCMS). Host-guest interactions between active hydrocarbon pool (HP) species and zeolite framework (Brønsted acid/base site) were observed,and the supramolecular reaction centers (SMCs) generated by the interactions were unambiguously identified by 13C-27Al double-resonance NMR. The internuclear spatial interaction/proximity between the 13C nuclei (associated with HP species) and the 27Al nuclei (associated with Brønsted acid/base site) was analyzed and compared over the three zeolites. The product shape selectivity of zeolites greatly influences the catalytic performance that can be linked to the nature of HP species and the host-guest interactions. Closer spatial proximity and stronger interaction between methylbenzenes (MBs) and Brønsted acid/base sites were observed over H-SSZ-13 and H-MOR zeolites, which facilitate aromatic-based reaction routes and rationalize the higher selectivity to ethene on the two catalysts. This leads to rapid deactivation at high temperature due to coke deposition on the active sites caused by the evolution of active MBs. For H-ZSM-5, the lesser amount of retained MBs and their weaker interactions with the active sites at high temperature cause the aromatic-based reactions to be insignificant and lead to the prevalence of alkene-based and carbocation-involved reactions, which are responsible for the high resistance of H-ZSM-5 to deactivation. In contrast, at lower temperature, the aromatic-based reaction route is favored with the MBs-composed SMCs prevailing. The distribution of the carbonaceous species in deactivated catalysts was revealed by host-guest interactions.