Conversion of syngas toward aromatics over hybrid Fe-based Fischer-Tropsch catalysts and HZSM-5 zeolites

Abstract

Direct conversion of syngas (CO + H2) toward aromatics has been demonstrated over a hybrid catalyst system composed of Fe-based FTS catalyst and HZSM-5 zeolite. Over the catalyst system, the aromatics were highly selective in the liquid hydrocarbon products with a content greater than 95%, and 70–90% of the aromatics were concentered on the toluene, xylene and 3C-branched alkylbenzenes. In this study, we systematically investigated the effects of reaction conditions (temperature, pressure, space velocity and H2/CO ratio) and catalyst combination (Si/Al ratio of zeolite, weight ratio of HZSM-5 to Fe-based catalyst, metal modification of HZSM-5, and kind of Fe-based catalyst) on the CO conversion, selectivity to aromatics, aromatic distribution, and catalyst deactivation, combining with some characterization technologies (XRD, BET, NH3-TPD, H2-TPR, TG and 27Al NMR). The obtained results show that HZSM-5 zeolite was the crucial component for aromatic formation. Excessive loading of the zeolite in bed would greatly suppress the carburization of reduced Fe into Fe carbides as well as CO dissociation on the Fe carbides because of the increasingly unfavorable direct contact between the acidic HZSM-5 and Fe-based catalyst, resulting in a remarkably lower CO conversion. HZSM-5 zeolite having high Brønsted acidity favored the formation of aromatics with a high content in liquid phase. However, as HZSM-5 had originally lower Brønsted acidity or the Brønsted acidity decreased during the aromatic synthesis due to more coke deposition, the selectivity to hydrocarbons in liquid phase gradually shifted to iso-paraffins rather than aromatics. Reactions operated over FeMn-HZSM-5 catalyst system at lower temperature or higher pressure led to an easier deactivation of HZSM-5 mainly from coke deposition. Compared with the FeMn-HZSM-5, FeK-HZSM-5 deactivated seriously at same conditions probably due to heavier and olefinic primary hydrocarbons over the FeK catalyst. In addition, 27Al NMR showed that extraction of framework Al by in situ formed H2O and CO2 could be ignorable for the HZSM-5 deactivation. More detailed results and discussion from abundant experimental work on clarifying their effects on the aromatic synthesis can be found in the article.