CO2 atmosphere-enhanced methanol aromatization over the NiO-HZSM-5 catalyst

Abstract

The aromatization of methanol over parent HZSM-5 and the modified NiO-HZSM-5 in a fixed-bed reactor was investigated under CO₂ and N₂ flow. The as-prepared catalysts were characterized by H₂-TPR, XRD, BET, NH₃-TPD, CO₂-TPD and probe-molecule reaction. Compared with parent HZSM-5 in CO₂ or N₂ flow and with NiO-HZSM-5 in N₂ flow, NiO-HZSM-5 showed greatly improved aromatization activity and BTX (benzene, toluene and xylene) yield in CO₂ atmosphere. This is attributed to the cooperation of acid sites with the activated CO₂ (over NiO species), which not only can effectively promote dehydrogenation of alkanes to form olefin intermediates, but also can accelerate dehydrogenation in the conversion of olefin intermediates to aromatics. In particular, an optimized NiO-HZSM-5 with 2.0 wt% NiO loading showed 50.1% total aromatic yield and 35.5% BTX yield in CO₂ atmosphere. It also showed high catalytic stability resulting from the restriction of coking due to its acidity, carbonaceous elimination by the activated CO₂-reacting deposited coke, and the suppressed reduction of highly active NiO species by activated CO₂.