Synergistic modification of Ca/Co on Ni/HZSM-5 catalyst for pyrolysis of organic components in biomass tar

Abstract

This study used ultrasonic-assisted over-impregnation method to prepare Ni/HZSM-5 catalysts modified by Ca/Co for pyrolysis of toluene. The results showed that the additive Ca promoted the grain refinement of metallic nickel, and interacted with Ni to form the NiO-CaO solid solution, which enhanced anti-sulfidation and anti-coking properties of catalysts and improved the reforming reaction of tar. For Ni-Ca catalysts, the highest yields of H2 and CH4 were 40.2 mL/g and 69.3 mL/g, the toluene conversion was 86.2% and the liquid yield was 35.3% at 800 ℃. In order to reduce the liquid yield and further improve the gas yield, the additive Co was supported on Ni-4 wt%Ca/HZSM-5 catalyst with the loading of 0.1–2 wt%. Ni-Co solid solution alloys with small particle size and uniform distribution were formed on the surface of HZSM-5 carrier, which made the reforming reaction move to the low temperature direction and promoted the reduction of Ni species. And during the loading process, the two additive metals, Ca and Co, were combined with each other in the form of oxides, further improving the mechanical strength and reaction stability of catalysts. The Ni-4 wt%Ca-0.1 wt%Co/HZSM-5 catalyst showed the best pyrolysis performance. The liquid recovery rate was decreased by 14.6% and the yields of H2 and CH4 were increased by 17.7% and 8.9% at 800 ℃. Most importantly, with the increase of pyrolysis temperature from 700 °C to 900 °C, the yield of pyrolysis gases and toluene conversion increased significantly. Especially, at 900 °C, the yields of H2 and CH4 produced over Ni-4 wt%Ca-0.1 wt%Co catalyst were 120.3 mL/g and 107.4 mL/g, and toluene conversion reached 98.6%. Ni-4 wt%Ca-0.1 wt%Co still maintained high catalytic activity after secondary regeneration at 900 °C.