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Monolithic reactor concepts are currently intensively discussed in the literature. For the oxidative coupling of methane relying on a balance of surface and gas phase reactions, such concepts have previously been claimed to have beneficial effects with respect to obtainable C 2 yields. In order to verify the superior performance in the case of a foam catalyst, ceria and samaria foams were fabricated by a direct foaming process. In both cases, mechanically stable, homogeneous open-cell foams were obtained as revealed by 3D magnetic resonance imaging, Hg-porosimetry, and scanning electron microscopy. As a characteristic feature of the introduced foaming methodology the process resulted in bimodal pore size distributions, ensuring low pressure drops on the one hand and sufficiently large surface areas on the other hand. Oxidative coupling of methane was carried out over the samaria foams. It was possible to obtain C 2 yields that were indeed higher than those obtained with the samaria powder, in contrast to honeycomb monoliths previously studied in the literature.

Rare earth oxides (REOs), particularly the sesquioxides, such as Sm 2 O 3 and La 2 O 3 , have been investigated as promising catalysts in the oxidative coupling of methane (OCM). Much less attention has been paid to the reducible REOs because they are expected to give oxidation products, such as CO and CO 2 (CO x ), rather than the desirable ethane and ethylene (C 2+ ). Because Li addition can improve the performance of Sm 2 O 3 in the OCM reaction and Li/MgO is commonly used as a reference OCM catalyst, the effects of lithium addition to a reducible oxide, TbO x , were investigated in detail in this study and compared with a Sm 2 O 3 catalyst, which is the best single component OCM catalyst. Because of the well-documented volatility of lithium under OCM conditions, particularly for the Li/MgO system, the stability of lithiumdoped samaria and terbia catalysts was examined as a function of preparation methods in this study. As expected, terbia supported on nanoparticle magnesia (n-MgO) is not a very active or selective OCM catalyst, and most of the observed selectivity toward C 2+ products is likely due to the n-MgO support. In contrast, Li-doped TbO x /n-MgO prepared using a coimpregnation method yields a highly active and selective catalyst. The Li-TbO x /n-MgO catalyst yields the same methane conversion as pure Sm 2 O 3 , and has a higher C 2+ selectivity than the Li-Sm 2 O 3 /n-MgO catalyst. The stability of the Li-TbO x /n-MgO catalyst is also higher than that of the Li-Sm 2 O 3 /n-MgO catalyst, and the loss of activity for the lithium-doped terbia catalyst appears to be the same as for the undoped Sm 2 O 3 /n-MgO catalyst (and undoped TbO x /n-MgO). The characterization data indicate stronger interactions between Li and TbO x than between Li and Sm 2 O 3 , which may explain the higher stability of the Li-TbO x /n-MgO catalysts. There are also indications that Li enters the TbO x lattice and reduces TbO 1.81 , to Tb 2 O 3 during reaction, which can explain the higher C 2+ selectivity compared with undoped TbO x /n-MgO. Furthermore, the Li-TbO x /n-MgO catalyst in this study is active at lower temperatures (600−700 °C) than typically used in the OCM (around 800 °C). Therefore, the Li-TbO x / n-MgO catalysts have potential to be very effective OCM catalysts, even though undoped TbO x /n-MgO catalysts are more selective toward CO x than C 2+ products.

A new sol-gel synthesis route for aluminasamaria mixed aero-and xerogel catalysts based on the socalled epoxide addition method and the use of these systems as catalysts for the oxidative coupling of methane (OCM) is reported. As precursors simple chloride or nitrate salts can be used. The mesoporous materials are X-ray amorphous even after calcination to 800°C and show an intimate mixing of Al and Sm on the nanoscale. In the case of the xerogels derived from chlorides, C 2 yields comparable to pure samaria can be achieved under OCM reaction conditions with 100 % O 2 conversion. Even at lower O 2 conversions the activity of the xerogel is competitive with a pure samaria reference catalyst taking the lower samaria content of 20 % into account. Accordingly, the approach is suitable to reduce the costs associated with the rare earth oxide. In addition to the preparation of aerogel and xerogel particles, the presented synthesis also allows the fabrication of xerogel films which can be coated on a suitable (monolithic) support. First results of such films are presented. Graphical AbstractKeywords Oxidative methane coupling Á Sol-gel chemistry Á Rare earth oxide catalyst Electronic supplementary material The online version of this article (