Controlling
the selectivity in single-step conversion of syngas
to single aromatic hydrocarbon to enhance CO utilization is a big
challenge. By adapting the reaction coupling methodology, which allows
the precise control of C–C coupling reaction, we obtained a
high selectivity of ∼70% of a single product, tetramethylbenzene
(TeMB), in hydrocarbons, at total CO conversion of 37%. This was enabled
by the reaction of H2-deficient syngas over a composite
catalyst of physically mixed nanosized ZnCr2O4 and H-ZSM-5. The H-ZSM-5 employed in this work appeared as a coffin
shape with short straight channels [010] along the b-axis that exhibit low molecular-diffusion resistance, resulting
in high selectivity of aromatics, particularly TeMB. Due to selective
methanol formation and enhanced molecular diffusion, we observed an
aromatic vacancy created inside H-ZSM-5 pores, which boosts the transformation
of olefins into aromatics, thus making the aromatic cycle dominant
in a dual-cycle mechanism and giving a high yield of aromatics and
TeMB. Furthermore, no catalyst deactivation was observed within 600
h of reaction time using H2-deficient syngas. Therefore,
by rejecting the need for extra H2 addition into the syngas-to-aromatics
(STA) reaction system, direct conversion of H2-deficient
syngas derived from coal/biomass into TeMB makes an attractive industrial
process.
Selectivity control in the single-step conversion of syngas to a single aromatic product is a big challenge. Here, we report an aldol-aromatic mechanism composed of aldol, phenolic, and aromatic cycles, that gave high selectivity >70% of a single product, tetramethylbenzene (TeMB) in hydrocarbons, at a reaction temperature as low as 275 °C. We evidently found the existence of oxygenated-aromatic compounds in the carbon pool, which remained active throughout the reaction and acted as key intermediates for the formation of the aromatics. The physical contact of ZnCr 2 O 4 with H-ZSM-5 exhibited a strong coupling effect that promoted surface diffusion of C 1 oxygenates (i.e., formaldehyde and methanol) from ZnCr 2 O 4 into H-ZSM-5 and transformed into aromatics via an aldol-aromatic reaction pathway, thus overcoming the most difficult step for first carbon−carbon bond formation. In addition, ZnCr 2 O 4 promoted the aromatics desorption by lowering the desorption activation energy and prevented the oversaturation of carbon pool species. Furthermore, it was found that a combination of thermodynamic equilibrium, surface methylation, and static repulsion are the key factors for giving high selectivity of TeMB in both carbon pool and final aromatics. This aldol-aromatic mechanism will open an efficient reaction pathway to upscale the process for selective aromatic synthesis in high yield from syngas.
We herein report a composite catalyst containing partially reducible and highly active manganese oxide and nano-size H-ZSM-5 with short b-axis, prepared for the direct conversion of syngas into aromatics.
The
surplus production of glycerol as a by-product of biodiesel
has become a serious problem for the sustainability of the biodiesel
industry. Glycerol dehydration over MFI zeolites offers a green route
to acrolein production. However, conventional MFI zeolites suffer
diffusion limitation, which inhibits accessibility to active sites
and causes quick deactivation of catalysts because of severe coke
deposition. In this study, self-pillared MFI nanosheet catalysts were
prepared with different Si/Al ratios to catalyze glycerol dehydration.
The nanosheets with interlayered meso-space possessing wide a,c-planes of ∼2 nm thickness in
the b-axis provide a shortened diffusion pathway
to easily access the active acid sites. In contrast to conventional
MFI zeolites, the nanosheet morphology significantly enhanced the
catalyst selectivity, activity, and stability; gave a high acrolein
selectivity (92%) at a higher WHSV of 4 h–1; and
remained stable after up to 100 h of the reaction of 5% glycerol in
water.
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