Monolithic Metal Foam-structured NiO-Based Catalysts For Oxidative Dehydrogenation Of Ethane To Ethylene

The conversion of light alkanes to high value-added olefin is an important catalytic reaction.Among them,the oxidative dehydrogenation of ethane to ethylene（ODE）,which is a low-cost,green and non-oil route,has been arousing great interests.However,traditional particulate catalysts as their poor thermal conductivity are detrimental to rapid dissipation of reaction heat released in this strongly exothermic ODE reaction,which causes severe hotspots in the catalyst bed,and thereby leads to the deep oxidation of ethane/ethylene and deactivation of catalyst sintering.Recently,the monolithic metal substrate has been attracting great interests in heterogeneous catalysis due to its intensified mass/heat transfer.It is noteworthy that the metallic Ni-foam not only is chemically active,but also offers Ni element to form the ODE reaction-needed component of NiO,thereby giving an opportunity to tailor catalysts for this strongly exothermic ODE reactions.However,the catalytic functionalization of this monolithic metal substrate still remains a challenge due to their low surface area.Therefore,a series of NiO-based structured catalysts integrated the excellent ODE performance and enhanced heat/mass transfer was developed by the novel catalytic functionalization strategies,such as wet etching,hydrothermal method and solvothermal method.The main contents are summarized as follows:（1）Preparation of NiO-based structured catalysts via the chemical etching method and post-modification with CeO₂ and/or ZrO₂ and its catalytic performance for ODE reactionA series of NiO-based structured catalysts for the ODE reaction was prepared by chemical etching and wet impregnation method.The detailed results are as follows:（i）NiO-Al₂O₃/Ni-foam catalyst prepared by chemical etching method exhibits the high activity,but its selectivity to ethylene is unacceptably low due to the excess of non-selective oxygen species in surface.（ii）Doping of the NiO-Al₂O₃/Ni-foam using CeO₂dramatically increases activity and selectivity,but the excess of CeO₂ is favorable for the deep oxidation.The insight of modification is that CeO₂ additive apparently increases the NiO reducibility but reduces the amount of non-selective oxygen species,thereby improving the ODE performance.（iii）Doping of the NiO-Al₂O₃/Ni-foam using ZrO₂ delivers a remarkable improvement in the ethylene selectivity but seriously deteriorates the activity.The insight of modification is that ZrO₂ additive remarkably weakens the NiO reducibility and reduces the amounts of non-selective oxygen species.（iiii）Co-doping of the NiO-Al₂O₃/Ni-foam using CeO₂ and ZrO₂ not only markedly promote the selectivity to ethylene but also keep the high level of activity.For example,C₂H₆ conversion of 40.3%and C₂H₄ selectivity of 60.6%are achieved with a stability of at least 14 h for a feed gas of C₂H₆/O₂/N₂=1/1/8 at 500 ^oC and a gas hourly space velocity（GHSV）of 18000 L kg^-1 h^-1 over the 1CeO₂-5ZrO₂-NiO-Al₂O₃/Ni-foam（CeO₂:1 wt%,ZrO₂:5 wt%）catalyst.In nature,co-doping with CeO₂ and ZrO₂ synergistically tamed the NiO reducibility and the amounts of non-selective oxygen species.Moreover,the temperature-rising（i.e.,the temperature difference between catalyst bed and reactor wall）in our Ni-foam-structured catalyst bed is much lower than that in particulate counterpart bed due to the enhanced heat transfer of Ni-foam-structured design.（2）Endogenous growth of NiC₂O₄ nanorods on Ni-foam via the hydrothermal method and subsequent modification with Nb₂O₅ and its catalytic performance for ODE reactionWe established a facile and efficient strategy for endogenous growth of NiC₂O₄nanorods onto Ni-foam via the hydrothermal oxidation reaction between Ni-foam and oxalic acid（H₂C₂O₄+Ni+2H₂O=NiC₂O₄·2H₂O+H₂）to prepare the NiC₂O₄/Ni-foam material,of which NiC₂O₄ loading（NiO loading）can be easily and finely tuned by controlling the preparation conditions.Then such NiC₂O₄/Ni-foam material was modification with Nb₂O₅ precursor（ammonium niobium oxalate）via wet impregnation method and followed by calcination to form the Nb₂O₅-NiO/Ni-foam catalyst.What to be noted is that the physicochemical characteristics of NiO（i.e.,NiO reducibility）and the desorption properties of oxygen species（i.e.,the amounts of non-selective oxygen species）can be systematically modulated by the preparation conditions,such as the support（i.e.NiC₂O₄/Ni-foam and NiO/Ni-foam）,Nb₂O₅ loading and calcination temperature.The optimal catalyst exhibits C₂H₆ conversion of 59.5%and C₂H₄selectivity of 68.2%for a feed of C₂H₆/O₂/N₂=1/1/8 at 410 ^oC and a high GHSV of9000 L kg^-1 h^-1,and also the good stability of 207 h at 375 ^oC.Moreover,the NiO-Nb₂O₅/Ni-foam catalyst shows the better mass/heat transfer and the catalytic property than the corresponding powdered catalyst.（3）In-situ growth of nickel terephthalate（Ni-Tp）nanosheets on Ni-foam via the hydrothermal method and subsequent modification with Nb₂O₅ and its catalytic performance for ODE reactionNi-Tp nanosheets were in-situ grown onto Ni-foam via the solvothermal method to form the Ni-Tp/Ni-foam material.Then,the Nb₂O₅ precursor（ammonium niobium oxalate）was loaded on such material via wet impregnation method and followed by calcination to form the Nb₂O₅-NiO/Ni-foam-NS catalysts.The optimal catalyst exhibits C₂H₆ conversion of 58.4%and C₂H₄ selectivity of 75.4%at 425 ^oC for a feed of C₂H₆/O₂/N₂=1/1/8 and a high GHSV of 9000 L kg^-1 h^-1,and also the promising stability of 72 h at 375 ^oC.This excellent ODE performance is attributed to the strong Nb₂O₅-NiO interaction over the 5Nb₂O₅-NiO/Ni-foam-NS catalyst,which not only reduces the NiO size（or increased SSA）and NiO reducibility,but also makes the non-selective oxygen species almost disappear.In addition,such optimal catalyst shows the good moisture resistance under the 29 vol%water vapor content.（4）The mechanism of NiO-precursor morphology dependent ODE performance over the Nb₂O₅-NiO/Ni-foam catalystLast but not least,the nature of NiO-precursor morphology dependent ODE performance for the Nb₂O₅-NiO/Ni-foam catalyst was investigated.Firstly,three kinds of NiO-precursors layer with different morphologies were in situ grown onto Ni-foam via different methods:clump-like Ni（OH）₂ layer by ammonia evaporation method,rod-like NiC₂O₄ layer by hydrothermal method,and nanosheet-like Ni-Tp by solvothermal method.Subsequently,above-obtained Ni（OH）₂/Ni-foam,NiC₂O₄/Ni-foam and Ni-Tp/Ni-foam materials were modification with Nb₂O₅ precursor via wet impregnation method and followed by calcination to form the Ni-foam-structured Nb₂O₅-NiO catalysts with different morphologies.The characterization and experimental results indicate the NiO precursor morphology（or thickness）-dependent NiO-Nb₂O₅interaction,and thus the ODE performance（especially the selectivity）.Inspired by this exciting findings,we designed the Nb₂O₅-NiO/Ni-foam catalyst with ultrathin nano-sheet morphology（²0 nm）,which possesses C₂H₆ conversion of 60.0%and C₂H₄selectivity of 80.0%at 425 ^oC for a feed of C₂H₆/O₂/N₂=1/1/8 and a high GHSV of9000 L kg^-1 h^-1,and achieves the favorable stability of 240 h at 400 ^oC.In addition,such advanced catalyst delivers the best ODE performance than other NiO-based structured catalysts and NiO-based powdered catalysts in the literature.More remarkably,this work provides an interesting clue to tailor high-performance ODE catalyst via morphology modulation strategy.