Nichakorn Pornnongsan

The reverse water–gas shift (RWGS) reaction is a promising route for converting CO₂ into CO, an important feedstock for synthetic fuels and chemicals. In this study, Zn–Co zeolitic imidazolate frameworks (ZIFs), including ZIF-8, ZIF-67, ZIF-67-8, and the core–shell structures ZIF-8@67 and ZIF-67@8, were synthesized as catalyst precursors and thermally activated prior to catalytic testing. Transmission electron microscopy and elemental mapping confirmed the formation of well-defined core–shell architectures, while synchrotron X-ray diffraction verified the characteristic sodalite-type framework. Thermogravimetric analysis revealed substantial framework decomposition during activation at 700 °C. In situ time-resolved X-ray absorption spectroscopy (TR-XAS) showed that Zn remained predominantly in the Zn²⁺ state throughout heating, whereas Co²⁺ underwent progressive reduction to metallic Co⁰ at temperatures approaching 600 °C. Ex situ X-ray absorption spectroscopy confirmed the presence of Zn²⁺ species and metallic cobalt after activation. Catalytic testing of the activated ZIF-derived materials showed that Co-containing catalysts exhibited significantly higher RWGS activity than Zn-only ZIF-8-derived catalyst. Among the investigated samples, ZIF-67@8_C-500 achieved the highest performance, producing 2.50 μmol CO (equivalent to 50 μmol g⁻¹ catalyst) at 600 °C with a H₂/CO₂ ratio of 2:1. The strong dependence of activity on ZIF-67 core loading indicates that metallic cobalt generated from the Co-rich core plays a dominant role in CO₂ conversion. Thermal activation transformed the highly porous ZIF precursors into metallic Co-containing carbonaceous catalysts. The resulting structural evolution, rather than retention of the original porous MOF framework, governed the catalytic performance in the RWGS reaction.

Zeolite NaY synthesized from a typical procedure could suffer from agglomeration. Adding non-refluxed cogon grass (NG) to the synthesis gel could produce NaY with good dispersion and uniform crystal size. Small molecules produced from cogon grass in alkaline conditions could prevent agglomeration. The obtained zeolite (Y-NG) has a crystallinity and surface area comparable to the synthesis without grass (Y-WG). Y-NG demonstrated similar paraquat adsorption capacity to Y-WG at high initial concentrations. On the other hand, the zeolite from the addition of acid-refluxed grass (Y-RG) has the lowest crystallinity, smallest surface area, and poorest paraquat adsorption capacity. The effect of grass amount on the zeolite structure was studied. One gram of cogon grass was the optimum amount to add to the synthesis gel.