Synergetic effect of HZSM-5 crystals and Na/Fe-Zn for direct syngas-to-aromatic transformation

초록

The selective synthesis of aromatics, specifically benzene, toluene, ethylbenzene, and xylenes (BTEX), from syngas was studied by integrating Fischer-Tropsch synthesis with subsequent aromatization over composite catalysts. The composite catalysts consisted of sodium-promoted zinc ferrite (Na/Fe-Zn) and HZSM-5 zeolites with controlled crystallite sizes (10-90 nm). This reaction involved converting syngas to olefins at Fe-carbide sites in Na/Fe-Zn, followed by the transformation of olefins into aromatics at the acidic sites of HZSM-5. Simply adding HZSM-5 to Na/Fe-Zn approximately doubled CO conversion, confirming its synergistic effect on reaction activity by preventing the oxidation of Fe carbides. The highest catalytic efficiency was achieved using Na/Fe-Zn physically mixed with HZSM-5 (30 nm crystallite size), which exhibited 36.3 % selectivity toward aromatics and 17.8 % selectivity toward BTEX within the hydrocarbon product. Improved H2O transfer suppressed CO2 formation via the water-gas shift reaction (WGS), while Na migration in Na/Fe-Zn reduced coke deposition on the Fe surface and the optimal crystallinity of HZSM-5 minimized the accumulation of carbonaceous species within the micropores. The aromatic selectivity was strongly influenced by the zeolite acidity, while the reaction stability was improved by optimizing the HZSM-5 crystallite size. The composite catalyst (PM-30) exhibits a strong synergistic effect that promotes the selective conversion of syngas to aromatics through the Fischer-Tropsch synthesis (FTS) route.

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