The Effects of the Crystalline Phase of Zirconia on C–O Activation and C–C Coupling in Converting Syngas into Aromatics

Zirconia has recently been used as an efficient catalyst in the conversion of syngas. The crystalline phases of ZrO2 in ZrO2/HZSM-5 bi-functional catalysts have important effects on C–O activation and C–C coupling in converting syngas into aromatics and been investigated in this work. Monoclinic ZrO2 (m-ZrO2) and tetragonal ZrO2 (t-ZrO2) were synthesized by hydrothermal and chemical precipitation methods, respectively. The results of in situ diffuse reflection infrared Fourier transform spectroscopy (DRIFTs) revealed that there were more active hydroxyl groups existing on the surface of m-ZrO2, and CO temperature programmed desorption (CO-TPD) results indicated that the CO adsorption capacity of m-ZrO2 was higher than that of t-ZrO2, which can facilitate the C–O activation of m-ZrO2 for syngas conversion compared to that of t-ZrO2. And the CO conversion on the m-ZrO2 catalyst was about 50% more than that on the t-ZrO2 catalyst. 31P and 13C magic angle spinning nuclear magnetic resonance (MAS NMR) analysis revealed a higher acid and base density of m-ZrO2 than that of t-ZrO2, which enhanced the C–C coupling. The selectivity to CH4 on the m-ZrO2 catalyst was about 1/5 of that on the t-ZrO2 catalyst in syngas conversion. The selectivity to C2+ hydrocarbons over m-ZrO2 or t-ZrO2 as well as the proximity of the ZrO2 sample and HZSM-5 greatly affected the further aromatization in converting syngas into aromatics.