Solvent Polarity-Induced Pore Selectivity in H-ZSM-5 Catalysis

Abstract

Molecular-sized micropores of ZSM-5 zeolite catalysts provide spatial restrictions around catalytic sites that allow for shape-selective catalysis. However, the fact that ZSM-5 has two main pore systems with different geometries is relatively unexploited as a potential source of additional shape selectivity. Here, we use confocal laser-scanning microscopy to show that by changing the polarity of the solvent, the acid-catalyzed furfuryl alcohol oligomerization can be directed to selectively occur within either of two locations in the microporous network. This finding is confirmed for H-ZSM-5 particles with different Si/Al ratios and indicates a general trend for shape-selective catalytic reactions.

Maximizing sinusoidal channels of HZSM-5 for high shape-selectivity to p-xylene

“…We observed the edge of each crystal, regardless of the crystal lying with its (010) or (100) facet, are less illuminated. This has been proved by Roeffaers et al 32 to be a result of the preferential occupation of oligomers in straight channel when water was used as solvent. The straight channels run parallel to the direction of polarized excitation light and thus the oligomer fluorophores in straight channels showed the strongest fluorescence emission.…”

“…Detailed orientation of the channels of this highly intergrowth HZSM-5_T could also be characterized by laser confocal fluorescence microscopy, which was demonstrated clearly by the pioneer works of Weckhuysen et al 42–44 and Roeffaers et al 32,45,46 They used this technology to study the structure of zeolite and diffusion behaviour of various molecules in zeolite channels. Here, with furfuryl alcohol as the probe molecule, which emits strong fluorescence when catalysed by Brönsted acid sites, the internal structure of the twinned HZSM-5_T can be determined.…”

“…The straight channels run parallel to the direction of polarized excitation light and thus the oligomer fluorophores in straight channels showed the strongest fluorescence emission. The existence of the areas without illumination is because that the straight channel runs perpendicular to the polarized excitation light and sinusoidal channels lacks fluorophores when water was used as solvent 32 . The depth scan also confirmed the hourglass shape in the interior of the HZSM-5_T crystal determined by TEM in Fig.…”

“…Therefore, straight channels are less efficient for xylene isomers shape/size sieving and are possibly one of the main reasons for the generation of undesired m -xylene and o -xylene in the methanol–toluene alkylation reaction. However, the co-existence of the two different pore systems makes the shape-selective capability of HZSM-5 hard to be fully exploited 32 .…”

Highly selective conversion of methanol to propylene: design of an MFI zeolite with selective blockage of (010) surfaces

“…[2][3][4] By structure modification, the selectivity of specific products can be selectively suppressed or increased. [5][6][7][8][9][10][11][12][13] Methanol-to-olefin/propylene (MTO/MTP) process has attracted much attention from both industrial field and academic field, and it has been regarded as one of the central processes of methanol economy 14 and modern coal chemical industry 15 . In China, more than 12,000,000 t per year of olefin production ability from methanol has been built up in 2017, which has become one of the largest coal-to-chemical activities ever in human history.…”