1. Introduction
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Currently, there exists no industrial process capable of directly converting methane to methanol. While many processes have been explored, none to date has proven cost-effective. A consequence of the paucity of catalysts for the direct oxidation of methane to methanol is the annual flaring of 140 billion cubic meters of natural gas at remote oil drilling locations around the world, accounting for 1% of global CO2 emissions with no associated energy gains.(1) Two distinct problems are often cited as being responsible for the lack of catalysts available for such a process: the large barriers associated with activating the nonpolar and highly symmetric methane molecule and the higher relative reactivity of the desired products.(2,3) Regarding the first problem, while methane activation barriers on transition metals are generally high (ΔGa(300 K, 1 bar) > 1.2 eV),(4) several publications have highlighted nontransition metal catalysts able to activate methane at low temperatures or with low density functional theory (DFT)-predicted barriers.(5−8) However, solutions to the second problem, that of product reactivity, have proven more elusive. Even if methanol can be locally produced by a catalyst at low temperatures, it is difficult to stop its CH bonds, which have a 0.4 eV lower bond dissociation energy (BDE) than those in methane, from being further oxidized.(3,9) Indeed, an example of a continuous process able to simultaneously achieve both high methane conversion and high methanol selectivity has yet to be established, pointing to a robust selectivity–conversion trade-off.(10)
In light of this challenge, many efforts have shifted focus from catalytic to stepwise processes, in which reactant consumption and product collection are decoupled. These systems bypass the aforementioned selectivity–conversion trade-off by producing a protected methanol derivative that is less prone to further oxidation compared to free methanol. Examples in homogeneous catalysis are often quasi-catalytic, i.e., turnover number (TON) > 1, and proceed through the use of small-molecule protecting groups. For example, Periana et al. oxidized methane to a stable methyl bisulfate product that could later be hydrolyzed to yield methanol and sulfuric acid.(11,12) However, these systems are limited by expensive oxidants and the cost of recycling protecting groups. Similarly, it was found that metal-exchanged zeolites, which had previously achieved methanol yields of ∼3% (64% CH3OH selectivity; 5% CH4 conversion) in the catalytic process,(13) could unlock higher methanol selectivities (∼98%) when used as heterogeneous protecting groups to oxidize methane to methanol stoichiometrically (TON = 1).(14−18) Such processes typically involve three steps: zeolite activation at high temperatures (∼450 °C), stoichiometric methane oxidation at lower temperatures (∼150 °C), and methanol recovery by flowing water (∼150 °C).(15) Unfortunately, this energy-intensive temperature cycling in combination with the expensive oxidizing agents required to reactivate the catalyst and low methanol yields per cycle tend to limit the practical application of these approaches.(10)
Herein, we aim to understand the limitations of direct methane to
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