Enhanced Catalytic Performance of Hierarchical Mesoporous Fe-Modified ZSM-5 for Light Olefin Synthesis via CO and CO₂ Hydrogenation

Introduction

The catalytic conversion of carbon oxides into value-added hydrocarbons offers a pathway for greenhouse gas utilization and sustainable fuel production. ZSM-5 zeolites are widely used due to their thermal stability and tunable acidity, though microporous diffusional limitations reduce efficiency. The introduction of mesoporosity with iron incorporation enhances active site accessibility and olefin selectivity.

Methods

Catalytic performance was evaluated in a fixed-bed flow reactor employing CO/H₂ and CO₂/H₂ feeds across 298-723 K. Product streams were analyzed by gas chromatography for quantitative determination of C₁-C₄ hydrocarbons. Two systems were compared: conventional Na-ZSM-5/Fe₃O₄ and hierarchically structured Fe-modified ZSM-5 (MO-30) prepared via alkaline desilication and Fe ion exchange.

Results

Both catalysts were active toward hydrocarbon formation, with productivity increasing with temperature. Na-ZSM-5/Fe₃O₄ achieved high CO conversion (88% at 523 K, stabilizing at ~92% above 573 K), yet exhibited pronounced methane formation - the rate at 573 K (W ≈ 2.87×10^-4 mol·g^-1·h^-1) exceeded that of Fe-ZSM-5 by three orders of magnitude, confirming the dominant methanation pathway of the conventional composite. The mesoporous Fe-ZSM-5 demonstrated superior olefin selectivity: C₂–C₄ selectivity reached 71.2% at 523 K, peaked at 87.5% at 598 K, and remained at 68.5% at 723 K, with methane selectivity suppressed to 12.5-29.7%. CO conversion rose from 44.6% at 523 K to 90.4% at 723 K, while CO₂ conversion reached 97.8% at 723 K, demonstrating applicability across both feed compositions. Enhanced olefin selectivity is attributed to improved intracrystalline diffusion via the mesopore network and greater accessibility of iron active sites.

Conclusions

Hierarchically porous Fe-modified ZSM-5 demonstrates superior selectivity toward light olefins compared to Na-ZSM-5/Fe₃O₄, with methane formation suppressed by nearly three orders of magnitude. The combination of alkaline treatment and iron functionalization positions these materials as promising catalysts for selective CO and CO₂ hydrogenation toward valuable C₂-C₄ olefinic feedstocks.