Effects of Li Doping on MgO-Supported Sm₂O₃ and TbO_x Catalysts in the Oxidative Coupling of Methane

Abstract

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.

Characterization of Mn–Na2WO4/SiO2 and Mn–Na2WO4/MgO catalysts for the oxidative coupling of methane

“…Compared to the SiO 2 -supported catalysts, the surface area of the Mn-Na 2 WO 4 /n-MgO is higher. In fact, the surface area of the n-MgOsupported Mn-Na 2 WO 4 catalyst (12.3 m 2 /g) is higher than our previously reported Li-TbO x /nMgO catalyst (5.6 m 2 /g) [29]. While a high surface area normally is a desirable property of heterogeneous catalysts, this is not necessarily true in OCM, since a high surface area can induce unwanted side reactions on the surface, with a concomitant reduction in selectivity.…”

Sol–Gel Preparation of Samaria Catalysts for the Oxidative Coupling of Methane

“…While Mn-Na 2 WO 4 /SiO 2 is considered to be the state-of-the-art OCM catalyst, it is a very complex system, and the nature of the active sites is still under debate [7]. Li/MgO is receiving less attention in recent literature as it is associated with rapid deactivation due to Li removal during reaction [8,9]. Other catalysts commonly used for the OCM reaction are doped and undoped rare earth oxides (REOs) [7].…”

Synthesis and characterization of Sm(OH)3 and Sm2O3 nanoroll sticks

“…Wang et al used high-k Sm 2 O 3 thin film as a pH-sensing layer [5]. Application to catalyst has also been of interest [6][7][8][9][10]. Elkins et al demonstrated that Sm 2 O 3 could be a promising catalyst for the oxidative coupling of methane [8].…”

“…Application to catalyst has also been of interest [6][7][8][9][10]. Elkins et al demonstrated that Sm 2 O 3 could be a promising catalyst for the oxidative coupling of methane [8]. Wang et al reported a potential application to photocatalytic dye degradation [9,10].…”