Catalysis Letters 68 (2000) 191–196 191 New efficient catalysts for the oxidative coupling of methane Alejandra Palermo, Juan Pedro Holgado Vazquez ∗ and Richard M. Lambert ∗∗ Chemistry Department, Cambridge University, Cambridge CB2 1EW, UK E-mail: rml1@cam.ac.uk Received 27 June 2000; accepted 30 June 2000 During calcination of OCM catalyst precursors, Li...Cs spectacularly lower the amorphous silica → α-cristobalite phase transition temperature, shown here to be a critically important requirement for production of effective catalysts. Incorporation of W switches on OCM activity and newly discovered K/W and Rb/W formulations exhibit unsurpassed ethylene selectivity at high methane conversion. Addition of Mn significantly improves the performance of the former. An alkali-stabilised tungsten oxo species is thought to be the OCM active site. Keywords: oxidative coupling of methane, OCM, silica phase transition, alkalis, ethylene selectivity 1. Introduction The oxidative coupling of methane (OCM) to higher hy- drocarbons in order to generate either chemical feedstocks or fuels continues to attract considerable interest. In the last three years alone over 200 papers have appeared in which a range of catalysts and reactor strategies have been explored. In a recent paper Pak et al. [1] provide a concise review of recycle reactor and dual reactor strategies. They also describe their own encouraging results achieved with an OCM/H-ZSM-5 dual bed reactor with which they achieved 100% methane conversion with good selectivity towards C 4+ products. The OCM catalyst they employed was an efficient trimetallic Mn/Na 2 WO 4 /SiO 2 formulation – which belongs to the class of materials that is the subject of the present paper. Whatever the implementation of OCM, se- lectivity towards ethylene (as opposed to ethylene + ethane) is of paramount importance. Leaving aside the issue of long term stability, ethylene selectivity at high methane conver- sion provides the most important single figure of merit for an OCM catalyst. Here, among other things, we report on a new bimetallic formulation and a new trimetallic for- mulation which, on the above basis, outperform all other catalysts described thus far. In 1992 Fang et al. [2] identified a very promising OCM catalyst with the formulation 1.9 wt% Mn, 5% Na 2 WO 4 supported on SiO 2 . In a subsequent paper [3] they pro- posed tetrahedral WO 4 surface as the OCM active site, with manganese oxide enhancing the exchange between gaseous and lattice oxygen. The model was then elaborated to in- clude a redox mechanism involving lattice oxygen ions and the W 6+ /W 5+ couple [4] The same catalyst was studied by Lunsford et al. [5] who took a somewhat different view as to the roles of the various chemical components in the full ∗ Instituto de Ciencia de Materiales de Sevilla, CSIC, Am´ erico Vespucio S/N, Isla de la Cartuja, 41092 Seville, Spain. ∗∗ To whom correspondence should be addressed. trimetallic system. They suggested that a Na–O–Mn species is responsible for the activation of methane, and from a later kinetic study they concluded [6] that the catalyst was unreactive towards methyl radicals even at 790 ◦ C. It was inferred that this property could be responsible for its good performance as an OCM catalyst. We too investigated this Na/W/Mn OCM catalyst by means of XRD, XPS/XAES, TPR analysis and microreactor testing [7]. We also studied similar formulations without one or more of Na, Mn and W. A clear correlation emerged between catalyst perfor- mance and support structure in the final calcined mate- rial. Amorphous silica gave active but very unselective catalysts. Crystalline SiO 2 (α-cristobalite) generated active and highly selective catalysts – especially with respect to the formation of ethylene. We showed that the presence of Na was essential for the anomalous low temperature silica → cristobalite support phase transition to occur, and that the occurrence of this process is critically important in the genesis of high performance catalysts. The strategy of progressively “taking apart” the trimetallic system also enabled us to demonstrate that the presence of Mn is not crucial for obtaining high ethylene yields, and the struc- tural, catalytic and spectroscopic results indicated that Na plays a dual role as both structural and chemical promoter. Given the key role played by Na in this catalytic system, it is of interest to investigate the consequences of replacing Na with other alkalis or alkaline earths. Here we report the structural and reactive properties of a series of OCM catalysts incorporating Li, K, Rb, Cs, Mg, or Ca/WO 4 sup- ported on silica and (in certain cases) in the presence of Mn. It is found that K- and Rb-based catalysts are best of all. The K/WO 4 catalysts exceed the performance of the corresponding Na system while the Rb analogue is even better. Addition of Mn somewhat improves the K-based catalyst but degrades the Rb-based system.  J.C. Baltzer AG, Science Publishers