Controllable synthesis of gallium-containing MFI zeolite for bifunctionally catalyzing methanol aromatization

The synergistic interplay of hierarchy, crystal size, and Ga-promotion in the methanol-to-aromatics process over ZSM-5 zeolites

“…Therefore, the consumption and oligomerization of shorter olefin appear to be more influenced by particle size, leading directly to alkylated aromatics via subsequent cyclization/aromatization, bypassing the need for further alkylation. As recently reported by multiple research groups independently, 67,73–78 besides the effects brought by the acid distribution characteristics, Ga-promoter could induce methanol dehydrogenation and disproportionation step to yield formaldehyde, which is considered a pivotal intermediate in the MTA process. 30,81 This intermediate could readily participate in hydrogen transfer, Prins reaction with dienes, and eventually promotes the catalyst deactivation.…”

“…S5†). 72,73 This observation indicates that these materials exhibit a similar distribution of Ga species, with the majority entering the zeolite framework and existing as tetrahedrally coordinated Ga species after calcination. Notably, the content of surface Ga 2 O 3 species correlates with crystal size (and/or HF), as reflected by the higher proportion of surface Ga 2 O 3 (Ga( i )) species in Ga/ZSM-5 (h-coffin), which has the largest crystal size (∼4.6 μm).…”

“…When Ga was introduced via the impregnation method, it preferentially interacted with the Brønsted acid sites within the zeolite pores, replacing some of the hydroxyl H + at these sites. 36,45,72,73 Herein, upon calcination, a portion of the Ga 2+ typically transforms into neutral and ionic Ga species in the forms of oxides and hydroxides. Here, similar XPS signals for Ga were observed on all our Ga-based zeolites: Ga 2p 3/2 (Fig.…”

“…S5,† 1116–1122 eV). 72,73–75 These signals can be deconvoluted to fit three species: Ga 2 O 3 species (Ga( i )) on the zeolite surface, with a peak at ∼1117.6 eV; tetrahedrally coordinated Ga( ii ) species in the zeolite framework, with a peak at ∼1118.6 eV; and extra-framework Ga( iii ) species interacting with the zeolite framework, with a peak at ∼1120.4 eV (see Fig. S5†).…”

“…The introduction of Ga species and the resulting modification of acid site distribution within Ga/ZSM-5 zeolites can influence their overall catalytic performance. For instance, these changes can enhance the formation of certain intermediates (such as formaldehyde), reduce hydrogen transfer, and facilitate the oligomerization and cyclization/aromatization of short-chain olefins, 73–78 thereby consuming short-chain olefins and increasing the desired aromatics selectivity.…”