Similarities and differences between aromatic-based and olefin-based cycles in H-SAPO-34 and H-SSZ-13 for methanol-to-olefins conversion: insights from energetic span model†
Zeolite catalyzed methanol-to-olefins (MTO) conversion proceeds through a hydrocarbon pool mechanism involving a series of elementary steps. The nature of the active hydrocarbon pool species is yet to be made clear in different zeolites. In this work, both aromatic-based and olefin-based cycles in H-SAPO-34 and H-SSZ-13 were systematically investigated using periodic DFT calculations with a van der Waals (vdW) interaction corrected XC functional. Combining static adsorption energies and interconversion thermodynamics, we theoretically proved that 1,2,4,5-tetramethylbenzene (1,2,4,5-TMB) is the primary component of methylbenzenes in CHA-structured zeolites. The energetic span model was employed to compare the kinetics of both cycles in which 1,2,4,5-TMB and 2,3-dimethyl-2-butene (iso-C6) were taken as hydrocarbon pool species. Both cycles follow a similar sequence of elementary steps. We demonstrate that the iso-C6-based cycle is kinetically facile for the MTO conversion in H-SAPO-34 and H-SSZ-13. The rate-determining transition states are identified as the propagation of ethyl side chain in the 1,2,4,5-TMB-based cycle and the cracking of alkyl chain in the iso-C6-based cycle. Our results show that the reactivity of 1,2,4,5-TMB increases from H-SAPO-34 to H-SSZ-13. The stabilities of carbenium ions, important intermediates in the olefin-based cycle, increase with their size and zeolite acidity. These theoretical insights from the energetic span model enable us to highlight the importance of the olefin-based cycle in MTO conversion and understand the dependence of the reaction mechanism on zeolite frameworks.