Conversion of glycerol to aromatic hydrocarbons over Zn-promoted HZSM-5 catalysts

• We proposed the reaction pathway for converting glycerol to aromatics over HZSM-5 catalyst.
• The acidity and hydrophobicity of HZSM-5 catalysts were the important factors for converting glycerol to aromatics.
• Compared the activity of two Zn species over HZSM-5 catalysts, Zn cations at exchanged site was more active in aromatization than ZnO.
• The aromatics yield linearly increased with increasing Zn cations at exchanged site of HZSM-5.

Catalytic conversion of glycerol to BTX aromatics has been investigated over HZSM-5 and Zn-promoted HZSM-5 catalysts. The reaction pathway of glycerol to aromatics was proposed. Glycerol was first dehydrated to three main intermediates, propenal, acetaldehyde, and acetol. The combination of oxygenate pool and olefins formed heavy aromatics in the hydrocarbon pool via oligomerization and cyclization. The disproportion and cracking of the hydrocarbon pool generated aromatics and also short alkanes. The effects of zeolite acid properties to the aromatization of glycerol were investigated over HZSM-5 with various SiO2/Al2O3 ratios i.e. 23, 30, 50, 80, and 280. In order to improve the aromatics selectivity, the dehydrogenation metal, Zn was promoted on the HZSM-5 zeolites by both aqueous phase ion-exchange (IE) and incipient wetness impregnation (IWI) methods. XPS, EXAFS, H2-TPR, NH3–TPD and TPD-IPA (temperature programmed desorption of isopropylamine) were employed to investigate the active species of Zn in aromatization of glycerol. By using aqueous phase ion-exchange method, the appearance of Zn species was only in the form of bivalent Zn cations at exchange site. Due to the exchange of these species with strong Brönsted acid site, the formation of aromatics tended to increase by suppressing the hydrogen transfer reaction and formation of light paraffins. ZnO species, formed by incipient wetness impregnation method was found to be less active for the reaction.

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