Methanol-to-olefin conversion on 3D-printed ZSM-5 monolith catalysts: Effects of metal doping, mesoporosity and acid strength

摘要

In this study, activity and selectivity in methanol-to-olefins (MTO) reaction of Cr, Cu, Ga, La, Mg, Y, and Zn-doped 3D-printed ZSM-5 zeolite monoliths were evaluated and the appropriate dopants were screened through various analysis. These 3D-printed ZSM-5 zeolite monoliths were doped with the above metals by direct addition of metal nitrate precursors into the 3D printing paste thus the metal doping procedure was integrated with the 3D printing fabrication. The effect of dopants on physical and chemical characteristics of the doped monoliths was studied through XRD, XRF, N-2, physisorption, FTIR, SEM-EDX, H-2-TPR, and NH3-TPD. The performance of these printed metal-doped zeolite monoliths in MTO reaction was systematically evaluated and compared with their non-doped counterparts at 673 K for 24 h of reaction time. It was found that doping of Cr, Mg, and Y into 3D-printed ZSM-5 zeolite monoliths leads to enhanced methanol conversion, whereas doping of Ce, Cu, Ga and Y has little or no effect on catalysts reactivity. The olefins selectivity followed the sequence Zn Mg La Y Cr Cu Ga at 10 wt% dopant contents, in line with the relative amount of moderated acid sites in the monoliths. The results indicated that among all the investigated metal dopants, 3D-printed ZSM-5 zeolite monolith doped with 10 wt% Mg exhibited the favorable effect on the light olefins production and showed ethylene and propylene selectivity of 24% and 33%, respectively with methanol conversion approaching 95% at 673 K. The increased selectivity towards ethylene and propylene was correlated to the presence of moderate Bronsted acidity which inhibited the formation of paraffins and aromatics, consistent with the NH3 -TPD results whereby moderate Bronsted acid sites were observed after Mg addition to 3D-printed ZSM-5 zeolite monoliths. The occupation of Mg dopant in the micropores, which is assigned to the cages of the ZSM-5 framework, took up the space for perspective aromatics, which are regarded to be coke precursors, and therefore reduced the coke formation.