Carbon Deposition Assisting the Enhancement of Catalytic Activity with Time-on-Stream in the Dehydrogenation of Isobutane over NiO/Al₂O₃

Abstract

In the transformation reaction of alkanes in alkenes via catalytic dehydrogenation, it is generally accepted that catalytic deactivation will occur. This phenomenon causes a drastic reduction in the catalytic activity with time-on-stream. It is understood that carbon deposits generated during the reaction then covers the surface of the catalyst, leading to a drastic decrease in activity. However, in contradiction, our laboratory reported that the dehydrogenation of isobutane to isobutene on NiO/γ-Al 2 O 3 within a speci c range of NiO loading with CO 2 improved the yield of isobutene with timeon-stream. Since few such cases have been reported, in this study, isobutane was dehydrogenated with CO 2 over the NiO/α-Al 2 O 3 catalyst, with 20% NiO loading, and an improvement was again observed. To investigate the cause of the improvement, both NiO/γ-Al 2 O 3 and NiO/α-Al 2 O 3 with 20% NiO loading were examined in detail following the reaction. According to transmission electron microscopy (TEM) analysis, both catalysts were covered with a large amount of carbon deposit after the reaction; however, there was a di erence in the types. The carbon deposit on NiO/γ-Al 2 O 3 exhibited a brous nature, while that on NiO/α-Al 2 O 3 appeared to be a type of nanowire. Raman spectroscopy revealed that the carbonaceous crystal-growth properties of the two forms di ered depending on the support. In particular, a catalytically active species of metallic nickel was formed in a high degree of dispersion in and on the above two forms of carbon deposits during the reaction, and this resulted in a high catalytic activity even when the catalyst was covered with a carbon deposit.

Enhancement of the Catalytic Activity Associated with Carbon Deposition Formed on NiO/Al₂O₃ during the Dehydrogenation of Ethane and Propane

“…It has been reported that nickel oxide supported on γ-alumina is reduced to metallic nickel when using hydrogen at temperatures lower than 673 K (Heracleous et al, 2005). In addition, our laboratory also revealed that only metallic nickel was detected with XRD of NiO(20)/γ-Al 2 O 3 recovered a er the dehydrogenation of isobutane at 0.1 h on-stream (Sugiyama et al, 2022). erefore, in the present reaction systems as well, the reduction of NiO to metallic Ni is expected to proceed in the early stages of each dehydrogenation.…”

エタン，プロパンおよびイソブタンの脱水素により析出した大量の炭素に覆われたアルミナ担持酸化ニッケルの触媒再生

“…An EXSTAR6000 (Seiko Instruments Inc.) under air flow (100 mL/min) at a heating rate of 8 K/min from 298 to 1073 K was employed for thermogravimetric (TG) analysis. According to our previous studies 5), 6) , the amount of carbon deposited on the catalyst after the reaction was estimated as the weight loss obtained using TG to determine the amount of carbon deposited per 1.0 g of fresh catalyst or per 1.0 g of NiO supported on the fresh catalyst (Carbon deposition rate: CDR). A transmission electron microscope (TEM, JEM-2100F, JEOL Ltd.) was used to analyze the catalysts previously used for dehydrogenation.…”