Short communicationMicrostructured CeO2-NiO-Al2O3/Ni-foam catalyst for oxidative dehydrogenation of ethane to ethylene
Graphical abstract
Introduction
Ethylene is one of the most important commodity chemicals, going toward various high value-added products such as polyethylene, ethylene oxide, ethylene dichloride and ethylbenzene [1]. Currently, ethylene is predominantly produced from petrochemical industry by the steam cracking of naphtha and gas oil at high operation temperatures of 750–900 °C [2], which remains problematic due to its high energy demand, complex thermal management [2], [3], and the ever-increasing global depletion of crude oil. In this context, the oxidative dehydrogenation of ethane (ODE) is considered to be an appealing alternative route to produce ethylene [4], [5], [6], [7], benefiting from the following advantages: lower operation temperature in ODE than in steam cracking, coke-free process due to oxygen existence, and particularly, abundant reserves of ethane in shale gas.
Ni-based catalysts are promising candidates for ODE reaction owing to their good catalytic performance at relatively low reaction temperatures [5], [6], [7], [8]. Up to date, most efforts have been devoted to improving the catalytic performance of Ni-based catalysts by modification with various promoters (e.g., ZrO2 [5], CeO2 [6] and Nb2O5 [7]) and/or catalyst support screening [9]. For example, a high C2H4 yield of 36.3% has been reported to be achievable over a promising NiO/MgO catalyst at 600 °C [9]. Despite the above advances, their practical application still remains particularly challenging. On account of poor heat conductivity of such traditional oxide-supported NiO catalysts, the strong exothermicity of ODE reaction process will unavoidably induce “hotspots” in the reactor bed, which not only is a main cause of catalyst deactivation but is also dangerous in industrial applications. Hence, it is particularly desirable and worthwhile to render a catalyst with unique combination of excellent catalytic performance and enhanced thermal conductivity that is essential to rapidly dissipate the great release of reaction heat from such a strongly exothermic ODE process. To accomplish this goal, ceramic [6] and stainless steel cylinder foams [10] structured catalysts have been recently reported for the ODE process, while the integration of active components onto such monolithic foams involves a dissatisfactory coating process which leads to ill-controlled physic-chemical structure of the coated-catalysts as well as their poor adherence to foam-surface and binder harmful contamination. Unlike the inertness of ceramic and stainless steel foams, Ni-foam is much more chemically active, providing an opportunity for inventing a whole new non-dip-coating way to fabricate foam-structured Ni-based catalysts [11], [12], [13], [14]. For example, NiO-Al2O3 composite catalyst layer is in-situ created and firmly embedded onto the Ni-foam struts via the wet chemical etching method, exhibiting excellent catalytic activity/selectivity and stability in syngas methanation [11], [12], [13] and catalytic oxy-methane reforming [14]. Seeing the beneficial features of Ni-foam to the design of structured catalysts, a foam-structured NiPd/Ni-foam catalyst is also developed via galvanic deposition of Pd NPs on Ni-foam [15], which exhibits as-expected low pressure drop and high heat/mass transfer with excellent activity/selectivity and stability for the coalbed methane (CBM) deoxygenation via catalytic combustion of methane.
In present work, a foam-structured CeO2-NiO-Al2O3 catalyst engineered from micro- to macro-scale was developed for the ODE reaction process, which was obtained by chemically etching a Ni-foam to create NiO-Al2O3 composite layer on its strut and subsequently adding CeO2 modifier via impregnation method. This non-dip-coating approach is working effectively and efficiently to endow the highly thermal conductive Ni-foam with promising catalytic performance for the ODE reaction.
Section snippets
Experimental
A NiO-Al2O3/Ni-foam catalyst was firstly prepared by the wet chemical etching method [16]. Then, CeO2 modifier was placed onto the as-prepared NiO-Al2O3/Ni-foam samples by impregnation method with cerium nitrate aqueous solution to obtain xCeO2-NiO-Al2O3/Ni-foam catalysts (CeO2 loading: x = 3, 5, 7, or 10 wt%). For comparison, a particulate catalyst of CeO2/NiO was also prepared by impregnation method with pure NiO powder as support. The catalysts were characterized by inductively coupled
Geometry, morphology and structural features
Fig. 1 shows the geometry, morphology and structural features of our monolithic CeO2-NiO-Al2O3/Ni-foam catalysts, and Table 1 summarizes their physicochemical and textural properties. The monolithic Ni-foam support (100 pores per inch (PPI); circular chip in 8 mm diameter and 1 mm thickness) possesses 3D open cell structure with 95 vol% void volumes (Fig. S1A and S1B). As shown in Fig. 1A and Fig. S1C, after chemical etching and subsequent calcination the Ni-foam strut surface became rough and was
Conclusions
A foam-structured CeO2-NiO-Al2O3/Ni-foam catalyst has been developed via wet chemical etching of a highly thermal conductive Ni-foam and subsequent modification with CeO2 additives. This facile non-dip-coating approach is effective and efficient to combine high catalytic performance with enhanced heat/mass transfer, high permeability and unique form factor. Such catalyst is qualified for the ODE reaction with a high ethylene productivity of 425 gEthylene kgcat− 1 h− 1 for a feed gas of C2H6/O2/N2 =
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2018, Applied Catalysis A: GeneralCitation Excerpt :The above results indicated that co-modifying with CeO2 and ZrO2 indeed synergistically tamed NiO species to be more selectively active for the ODE reaction. Whereas 3CeO2-5ZrO2-NANF and 5CeO2-5ZrO2-NANF, as well as the 7CeO2-NANF catalysts reported in our previous study [46] achieved ethane conversion higher than the 1CeO2-5ZrO2-NANF at 300–450 °C, no better ethylene yield was obtainable because of their relatively poor selectivity (given that the excess of CeO2 favors the deep oxidation of ethylene at high temperature) (Figs. Fig. 33 and S4). Furthermore, the pristine activity of the CeO2-ZrO2-NANF catalysts is remarkably improved, evidenced by the higher TOFs at 350 °C ranged from 59 to 93 h−1 than that of 38 h−1 for the NANF base catalyst (Table 1).