Direct coating copper–zinc–aluminum oxalate with H-ZSM-5 to fabricate a highly efficient capsule-structured bifunctional catalyst for dimethyl ether production from syngas

Abstract

Dimethyl ether (DME) has been considered as an industrially important intermediate and clean alternative fuel. The development of a highly efficient syngas-to-DME bifunctional catalyst is of great importance but remains a challenge. In this work, we reported a facile and robust strategy for preparing a highly efficient CZA-oa@H-ZSM-5 capsule-structured bifunctional catalyst by coating an H-ZSM-5 shell on millimeter-sized copper–zinc–aluminum oxalate (CZA-oa) but not on copper–zinc–aluminum oxide (CZA-oxi) via the hydrothermal crystallization process with the subsequent calcination at 500 °C for 5 h in air. From the characterization and reaction results, we inferred that the direct use of CZA-oa as a replacement of CZA efficiently inhibited Cu leaching in the coating process, besides turning down the necessity of using a rotary oven for the good coating of the H-ZSM-5 shell on the core owing to high hydrophilic property. The developed CZA-oa@H-ZSM-5 capsule bifunctional catalyst showed 5.0 times higher turnover frequency (TOF, 1.5 min⁻¹) with 93.7% of DME selectivity as compared to CZA-oxi@H-ZSM-5 R (0.3 min⁻¹ of TOF and 92.5% of DME selectivity) prepared by a similar procedure except for the use of mixed oxide as a core to replace CZA-oa. The improvement in the intrinsic activity of the developed CZA-oa@H-ZSM-5 capsule bifunctional catalyst originated from appropriate Cu–Zn interactions. Interestingly, owing to the excellent hydrophilic property of CZA-oa, the use of the rotary oven in the coating process of the H-ZSM-5 shell can be avoided if oxalate is used to replace oxide as a core. Compared to CZA-oxi@H-ZSM-5 R (0.031 mmol m_Cu⁻² h⁻¹), CZA-oa@H-ZSM-5 showed a much higher DME formation rate (R_DME = 0.122 mmol m_Cu⁻² h⁻¹), which allowed it to be a promising catalyst for DME production from syngas. Moreover, this work also opens a new window for designing outstanding bifunctional catalysts for other applications.