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

In the recent study, the dehydrogenation of isobutane to isobutene was accomplished using a NiO/γ-Al2O3 catalyst, and significant improvement in the time-on-stream yield of isobutene was accomplished. During the normal catalytic dehydrogenation of alkanes, the catalyst is covered by the carbon deposition that is generated during the reaction, which drastically reduces activity with time-on-stream. Therefore, no examples of the catalytic dehydrogenation of isobutane have yet been reported. This study used either ethane or propane as a source of isobutane to examine whether the activity was improved with time-on-stream. As a result, in the dehydrogenations of both ethane and propane on a NiO/γ-Al2O3 catalyst, the catalytic activity decreased with time-on-stream when the supporting amounts of NiO was small. By contrast, when the supporting amount of NiO was large, the catalytic activity improved with time-on-stream. The results using a NiO/γ-Al2O3 catalyst with small and large NiO loadings were similar to those of isobutane dehydrogenation and it was confirmed that the dehydrogenation activity was improved with time-on-stream in the catalytic dehydrogenations of ethane, propane, and isobutane using large NiO loadings. Intermediate behavior using a moderate amount of NiO loading, which was not detected in the dehydrogenation of isobutane, was also observed, which resulted in a maximum yield of either ethylene or propylene at 2.0 or 3.25 h on-stream, respectively. We concluded that the reason the catalytic activity did not improve with time-on-stream when using a NiO/γ-Al2O3 catalyst was because the supporting amount of NiO was too small. These results show that activity with time-on-stream could also be improved in the dehydrogenations of other alkanes

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

Improvement in the dehydrogenations of ethane, propane, and isobutane over alumina-supported nickel oxides occurs together with the formation of large amounts of carbon deposits with time-on-stream, but catalyst activity is decreased with an additional increase in time-on-stream. This improvement in activity is due to metallic nickel formation that is highly dispersed over carbon nanotube-like deposits. However, the activity decreases as this highly dispersed metallic nickel is further covered with carbon deposits. We describe the use of oxygen treatment to regenerate the catalyst. Oxygen treatment to remove carbon deposits generally results in a less active catalyst due to sintering of the active species. However, we speculated that sintered nickel oxide could form carbon nanotubes in the proposed system, and that the formation of highly dispersed nickel over the nanotubes would regenerate the catalyst. To prove this hypothesis, dehydrogenations of ethane, propane, and isobutane were investigated using 18, 15, and 20 % nickel oxide supported on γ-alumina, respectively. We confirmed regeneration of the catalytic activity via oxygen treatment during subsequent dehydrogenations.アルミナ担持酸化ニッケル触媒によるエタン，プロパン，およびイソブタンの脱水素化では，通塔時間に伴う炭素析出の形成とともに，触媒活性の向上が観察されることが報告されている。この改善挙動はカーボンナノチューブ状析出物上に高分散状態で形成される金属ニッケルに起因されるが，さらに通塔時間を長くすると，カーボンナノチューブ状析出物が通常の炭素析出物によって覆われ，活性が低下する。本稿では，活性が低下したアルミナ担持酸化ニッケルを酸素処理により再生した結果について述べた。 炭素堆積物を除去するために酸素処理を用いると，活性種のシンタリングより，触媒活性成分が低分散化され，活性が低下することが一般に知られている。しかしながら，本触媒系では，酸素処理でシンタリングした低分散の酸化ニッケルが形成されたとしても，再度接触反応に用いると，そこからカーボンナノチューブが形成され，このナノチューブ上に高分散状態で金属ニッケルが形成され，良好な触媒活性が再生されることが期待される。この仮説を証明するために，エタン，プロパン，およびイソブタンの脱水素化を，γ-アルミナに酸化ニッケルを18 %，15 %，および 20 %担持した触媒を用いて検討した。その結果，これら3種類のアルカンの脱水素に対して，本稿で提案した酸素処理による触媒活性の再生が良好に行われることが明らかになった