3D-printed zeolite monoliths with hierarchical porosity for selective methanol to light olefin reaction
Herein, we report the rapid synthesis of customized zeolite monoliths with various compositions and hierarchical porosity (macro–meso–micro) using a 3D printing technique. Moreover, several 3D-printed monoliths were synthesized from HZSM-5 and HZSM-5/silica, and SAPO-34 crystals were grown on the as-synthesized 3D-printed ZSM-5 monoliths via a secondary growth method. The 3D-printed zeolite monoliths exhibited hierarchical porosity with pore sizes ranging from 1.5 nm to 1 μm. Characterization results suggested that transformation of zeolite paste into the monolith structure enhanced the mesopore volume and moderated the acidity of the structures. Further incorporation of amorphous silica into the HZSM-5 monoliths reduced the acid site density. The obtained monoliths were evaluated in the methanol to olefin (MTO) reaction and found to exhibit higher stability than their powder counterparts. The selectivity to light olefins was significantly increased as a result of modification in the acidity and porosity of the monolith catalysts, which in turn mitigated the hydrogen transfer, hence suppressing the formation of paraffins and aromatics. The SAPO-34 coating tended to increase the ethylene selectivity due to its intrinsic framework structure. The analysis of spent zeolite monoliths by TGA-DTA indicated that the amount of polyaromatic species formed during the reaction was much lower than that on the powder analogues, due to their diluted acid site density and decreased Brønsted acid sites, as proven by NH3-TPD profiles and py-FTIR spectra. Furthermore, 29Si MAS NMR results confirmed slight dealumination of 3D-printed monoliths after the MTO reaction.