Gasification of Radical Coke with Steam and Steam–Hydrogen Mixtures over Manganese–Chromium Oxides

Abstract

The coke gasification performance of synthesized Mn−Cr−O catalysts, as well as MnO x , Cr 2 O 3 , and α-Al 2 O 3 , is evaluated in the presence of steam and steam−hydrogen mixtures. Radical coke is deposited in situ onto the catalysts via ethylene pyrolysis at an elevated temperature. Raman spectroscopy analysis reveals that the coke deposited on the various catalysts is similar in nature across all catalysts. Rates of coke gasification are determined from thermogravimetric analysis (TGA) using different gas/vapor feeds including inert/steam and steam/hydrogen mixtures. Addition of hydrogen to a steam feed results in a considerable reduction in reaction rates, suggesting that hydrogen inhibits the production of surface oxygen that is needed for coke gasification. Active Mn 3+ species on the surface of Mn-rich spinel (Mn 1.5 Cr 1.5 O 4 ) and MnO catalysts are hypothesized to be responsible for the significantly higher reaction rates and lower apparent activation barriers for coke gasification under mixed steam−hydrogen conditions.

Enhanced Coke Gasification Activity of the Mn1.5Cr1.5O4 Spinel Catalyst during Coking in Ethylene–Steam Mixtures

“…The Mn–Cr–O spinel oxides were prepared using a sol–gel method, and the oxides were acquired from the same batch of chemicals as studied in our previous work. , The Mn/Cr ratio in the spinel catalysts was controlled by changing the Mn/Cr ratio in the synthesis gel; MnCr 2 O 4 and Mn 1.5 Cr 1.5 O 4 catalysts were selected for evaluation, along with single oxides of manganese­(II) oxide (MnO, Alfa Aesar, 99.99%) and chromium­(III) oxide (Cr 2 O 3 , Sigma-Aldrich, 99.9%) obtained from commercial sources.…”

“…Detailed characterization of the fresh Mn–Cr–O spinel oxide catalysts was performed and described in our previous work. , The crystal structure of the fresh and coked catalysts was characterized using powder X-ray diffraction (XRD, PANalytical X’Pert Pro). XRD patterns were obtained at room temperature using Cu Kα radiation.…”

“…The first-order Raman spectra were collected using an Ar + ion laser (λ = 488 nm) operated at 5 mW source intensity for 2 s. A small amount (10 mg) of the coked catalyst was placed onto a glass slide, and the focused laser was then exposed onto at least three different areas for each sample. The Raman spectra were deconvoluted using OriginPro software, with constrained peak analyzer settings for peak fitting …”

“…The decoking of the catalysts was performed by a temperature-programmed gasification (TPG) procedure under steam–nitrogen flow. A steam generator was used to controllably evaporate 2.2 g/h of liquid water, which was diluted with 100 mL/min of nitrogen to generate a mixture of 33% steam in nitrogen . The coked catalysts were placed inside the TGA on the alumina crucible and heated up to 950 °C (10 °C/min) in the steam–nitrogen mixture to remove the coke before proceeding to the next coking cycle.…”

“…Under the conditions during pretreatment and steam cracking processes, Mn–Cr spinel oxides can form with varying Mn/Cr ratios: MnCr 2 O 4 , Mn 0.5 Cr 2.5 O 4 (which is likely a combination of MnCr 2 O 4 and Cr 2 O 3 ), and Mn 1.5 Cr 1.5 O 4 . − In our previous work, we synthesized and characterized these catalysts, as well as analyzed their reaction kinetics for ethylene steam reforming . We also studied these oxides as catalysts for gasifying radical coke formed in situ during ethylene pyrolysis . The results demonstrated that Mn-rich spinel oxide (Mn 1.5 Cr 1.5 O 4 ) is significantly more active for coke gasification under steam and steam–hydrogen mixtures, likely due to the presence of active Mn 3+ species in the spinel structure, compared to the other compositions.…”