Crystal Phase-dependent Activity of MnGaO_x Observed in Direct Syngas to Light Olefins

Two notable metal oxide structures, spinel-type oxide and solid solution-type oxide, are widely used in Oxide-Zeolite (OXZEO) bifunctional catalysts for CO/CO₂ hydrogenation reactions.

Identifying the crystallographic structure sensitivity of catalysts in chemical reactions is helpful to the rational design of catalysts. However, direct and convincing study to correlate the crystal structure of oxide to its catalytic performance is still lacking.

Recently, a joint research team led by Prof. BAO Xinhe, Prof. PAN Xiulian, Assoc. Prof. JIAO Feng and Prof. XIAO Jianping from the Dalian Institute of Chemical Physics(DICP) of the Chinese Academy of Sciences (CAS) has observed the strongly crystal phase-dependent activity of MnGaO_x in direct syngas conversion.

This study was published in Angewandte Chemie International Edition on April 18.

"The main challenge to observing the crystal structure of oxide is the lack of a well-defined material-synthesis method to obtain metal oxides with the same element components but different crystallographic structures," said JIAO.

In this study, the researchers found co-precipitation method and hydrothermal method could synthesize bimetallic oxides, which were composed of amorphous MnO₂ and Ga₂O₃ with hexagonal close-packed (HCP) or Face centered-cubic (FCC) crystal phase, respectively. More interestingly, they found that the HCP oxide remained unchanged as HCP MnO-Ga₂O₃ solid solution oxides after reduction under H₂ or CO, while the FCC solid solution oxide transformed into FCC spinel structure, where reduced Mn²⁺ took the A-site of AB₂O₄ spinel structure.

Crystal-phase-dependent of syngas conversion activity of MnGaO_x-SAPO-18 (Image by BAI Bing and JIAO Feng)

They obtained 40% CO conversion, 81% light olefins selectivity, and 0.17 g·gcat^-1·h^-1 space-time yield of light olefins with the combination of FCC MnGaO_x-Spinel and SAPO-18. In comparison, they obtained a much inferior activity with solid solution MnGaO_x with a similar chemical composition.

They further proved that the superior activity of MnGaO_x-Spinel was attributed to its higher reducibility and the presence of coordinatively unsaturated Ga³⁺ site, which facilitated the dissociation of the C-O bond via a more efficient ketene-acetate pathway to light olefins.

"Our findings may further optimize metal oxides for OXZEO syngas conversion,"said Prof. PAN.