Modification of reaction intermediates formed from steam reforming of acetic acid with HZSM-5 or Mg-Al-LDH in upper-bed influences property of coke in lower-bed Ni/KIT-6

Abstract

Modification of structures of precursors of coke is a method tackling coking in steam reforming. In this study, HZSM-5 or Mg-Al-LDH, with abundance of acidic sites or alkaline sites, were employed as upper-bed catalyst for tailoring structures of the intermediates derived from acetic acid before being reformed over Ni/KIT-6 in the lower bed. In-situ IR characterization of steam reforming indicated that aromatization of the reaction intermediates was promoted with HZSM-5, which formed more coke than with Ni/KIT-6 alone. However, the coke had a highly aromatic nature and other related features like higher thermal stability, crystallinity and hydrophobicity, which did not significantly influence the catalytic stability. Instead, the lower-bed Ni/KIT-6 showed a higher activity due to the modification of intermediates by HZSM-5, resulting in a higher hydrogen yield. In comparison, alkaline sites on Mg-Al-LDH catalyzed ketonization reactions and further condensation of ketones, forming abundant aliphatics bearing olefinic CC. Mg-Al-LDH presence formed less coke over Ni/KIT-6, but the highly polymeric coke, featuring with lower thermal stability, low C/H ratio and high hydrophilicity, led to fast deactivation of the Ni/KIT-6 in lower bed. However, the hydrogen yield was still improved by modifying the intermediates with upper-bed Mg-Al-LDH.