Reexamination on transition-metal substituted MFI zeolites for catalytic conversion of methanol into light olefins

Zeolites with MFI structure were hydrothermally synthesized according to silicon-to-heteroatom ratios around 56 and were characterized by using XRD, SEM, nitrogen adsorption–desorption, NH3-TPD, ICP-AES, and particle size analyses. For methanol-to-olefin (MTO) reaction, the catalytic performance of these zeolites was found to correlate with the transition metal combinations and acid properties. Among the MFI-metallosilicate catalysts, aluminous H-ZSM-5 revealed the highest methanol conversion but relatively lower selectivity towards light olefins. For the introduction of Fe or Ga species, the isostructural incorporation of iron ions into silicate and aluminosilicate frameworks led to considerable increases in light olefin selectivity, whereas the presence of Ga or Ga–Al in MFI frameworks promoted the formation of C5+ and aromatic fractions. In contrast to the H-ZSM-5 (H-AlMFI), H-FeAlMFI zeolite showed the features of selectively converting methanol into light olefins highly favorable in propylene; H-GaAlMFI zeolite facilitated the heavier fractions rich in aromatics. H-FeGaAlMFI zeolite presented the highest MTO reaction activity among the MFI-metallosilicates except for H-ZSM-5, and showed the light olefin selectivity that is almost equivalent to that of H-FeAlMFI. The H-FeAlMFI for the MTO reaction revealed an enhanced active stability compared with the referred H-ZSM-5. With the iron species introduced in ZSM-5 structure a framework site has catalytic advantages over an ion-exchange site.