Oxidative coupling of methane
The oxidative coupling of methane (OCM) is a type of chemical reaction discovered in the 1980s for the direct conversion of natural gas, primarily consisting of methane, into value-added chemicals. Direct conversion of methane into other useful products is one of the most challenging subjects to be studied in heterogeneous catalysis. Methane activation is difficult because of its thermodynamic stability with a noble gas-like electronic configuration. The strong tetrahedral C–H bonds (435 kJ/mol) offer no functional group, magnetic moments, or polar distributions to undergo chemical attack. This makes methane less reactive than nearly all its conversion products, and limits efficient utilization of natural gas, the world’s most abundant petrochemical resource.
The principal product of OCM is ethylene, the world’s largest commodity chemical and the fundamental building block of the chemical industry. While the ability to convert methane to ethylene is highly attractive from an economic point of view, this is a major scientific challenge. Thirty years of research failed to produce a commercial OCM catalyst, preventing this promising process from advancing beyond experimental stage.
Ethylene is the world’s most valuable commodity chemical with an estimated value of $160 billion/year. As a fundamental building block of chemical industry and the world’s largest chemical intermediate (annual demand estimated at 120–140 million ton), ethylene derivatives are found in products as diverse as food packaging, eyeglasses, cars, medical devices, lubricants, engine coolants and liquid crystal displays. While indispensable to our daily lives, ethylene production by steam cracking consumes more energy than any other chemical process, uses valuable oil fractions, such as naphtha, and is the largest contributor to GHG emissions in the chemical industry.
- 2CH4 + O2 → C2H4 + 2H2O
The reaction is exothermic (∆H = -67kcals/mole) and occurs at high temperatures (750–950 ˚C). In the reaction, methane (CH4) is activated heterogeneously on the catalyst surface, forming methyl free radicals, which then couple in the gas phase to form ethane (C2H6). The ethane subsequently undergoes dehydrogenation to form ethylene (C2H4). The yield of the desired C2 products is reduced by non-selective reactions of methyl radicals with the surface and oxygen in the gas phase.
The economic promise of OCM has attracted significant industrial interest. In the 1980s and 1990s multiple research efforts were pursued by the world’s leading academic investigators and petrochemical companies. Hundreds of catalysts have been tested, and several promising ones identified and extensively studied. Following the initial excitement, it eventually became obvious that the reaction did not proceed with the selectivities that were required for an economically viable operation. Instead of converting into ethylene, the majority of methane was being non-selectively oxidized to carbon dioxide. This lack of chemoselectivity rendered the process economically unattractive.
From further mechanistic studies, it became apparent that the lack of selectivity was related to the poor properties of conventional catalysts. Specifically, known catalysts have poor C-H activation properties, resulting in the need for high reaction temperatures (750 ˚C and 950 ˚C) to activate the C-H bond. This high reaction temperature leads to a secondary gas-phase reaction mechanism pathway, whereby the desired reaction of methyl radical coupling to C2 products (leading to ethylene) strongly competes with COx side reactions. This undesired and non-selective competing pathway negatively impacts selectivity.
The high temperature of the conventional catalysts is not only detrimental to the product selectivity, but also presents a challenge for the reaction engineering. Among the process engineering challenges are the requirements for expensive metallurgy, lack of industry experience with operating high temperature catalytic processes, and the potential need for new reactor design to manage heat transfer efficiently.
Due to lack of selective and robust catalysts that function in an acceptable temperature regime, none of the OCM efforts have advanced beyond exploratory stage. It has even been postulated that there exists an inherent limit to OCM selectivity, which is fundamental and cannot be overcome, with the apparent conclusion that “expecting substantial improvements in the OCM performance might not be wise”.
Eventually, the inability to discover a selective catalyst led to a gradual loss of interest in OCM. Beginning in the mid-1990s, research activity in this area began to decline significantly, as evidenced by the decreasing number of patents filed and peer-reviewed publications.
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