Redox oxidative cracking of n-hexane with Fe-substituted barium hexaaluminates as redox catalysts

Abstract

Light olefins such as ethylene and propylene are critical building blocks in the chemical industry and are currently produced mainly from steam cracking of naphtha. However, the highly endothermic nature of steam generation and the cracking reactions make this process highly energy- and CO₂-intensive. Coke formation in the steam cracker tubes represents another obstacle. To address these challenges, we proposed a chemical looping approach to convert naphtha into olefins via a redox oxidative cracking (ROC) process. In the present work, Fe-substituted barium hexaaluminates (BaFe_xAl_12−xO₁₉) were investigated as oxygen carriers, or redox catalysts, to convert n-hexane to olefins via ROC. The cyclic redox scheme facilitated by the Fe-substituted hexaaluminates allows autothermal operation and higher olefin yields relative to the endothermic steam-cracking process. While base BaFe_xAl_12−xO₁₉ (x = 1, 2, 3, 4, 6) oxides showed high CO_x yields (8.4–55.2%) in ROC of n-hexane, 20 wt% Na₂WO₄ promotion of BaFe₆Al₆O₁₉ (20-NaW/BaFe6) significantly inhibited CO_x formation (0.6–1.2% CO_x yield) while oxidizing all the H₂ produced during cracking within a temperature range of 600–700 °C and GHSV of 9000 h⁻¹. Benefiting from the donation of selective lattice oxygen, 20-NaW/BaFe6 more than doubled the olefin yield when compared to that from thermal cracking (26.0% vs. 12.8%). Moreover, decreasing the GHSV from 9000 h⁻¹ to 2250 h⁻¹ resulted in 8.5% increase in n-hexane conversion on an absolute basis, while maintaining nearly the same olefin selectivity. Long-term stability of the 20-NaW/BaFe6 oxygen carrier was also demonstrated within 30 cycles at 700 °C and 2250 h⁻¹, achieving an olefin yield in the range of 31.3–32.4% and low CO_x yield of 0.6–0.7%. XPS analysis of cycled BaFe6 revealed a shift towards near-surface Ba enrichment upon redox cycling. In comparison, both as-prepared and cycled 20-NaW/BaFe6 showed suppression of near surface Ba content, indicating that Na₂WO₄ inhibited the migration of Ba into the near-surface region during n-hexane ROC and effectively suppressed non-selective oxidation reactions.