Die direkte katalytische Oxidation von Methan zu Methanol – eine kritische Beurteilung

Abstract

Es gibt vielzählige direkte, selektive Ansätze, um Methan partiell zu oxidieren, jedoch ist noch keiner von ihnen zur industriellen Reife gelangt. In diesem Zusammenhang ist die Oxidation von Methan zu Methanol eine schwierige, aber lohnende Aufgabe, da sie eine wirkungsvolle Methode ist, um das Abfackeln von Erdgas, das als Nebenprodukt in der Erdölindustrie anfällt, zu unterbinden, und zugleich die Möglichkeit bietet, Methan zu valorisieren. Dieser Aufsatz berücksichtigt die Synthese von Methanol und seinen Derivaten ausgehend von Methan durch homogen‐ und heterogenkatalytische Methoden. Er zeigt die starken Einschränkungen der direkten katalytischen Synthese von Methanol aus Methan auf und unterstreicht dabei die enorme Überlegenheit von Systemen, die Methanolderivate produzieren oder sich durch Besonderheiten wie die Verwendung von Mehrkomponenten‐Katalysatoren zur Methanolstabilisierung auszeichnen. Für die zukünftige Forschung erweist sich die Stabilisierung von Methanol in der homogenen und heterogenen Katalyse als unentbehrlich.

Direct Methane to Methanol: The Selectivity–Conversion Limit and Design Strategies

“…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.…”

“…15 Unfortunately, this energyintensive 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 methanol catalysis, and we propose strategies to overcome these limitations. While this work does not address the difficult design problem of methane conversion, i.e., finding a catalyst able to activate oxygen and methane and locally produce methanol, it does address the critical question of how to keep the produced methanol safe.…”

“…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. , Regarding the first problem, while methane activation barriers on transition metals are generally high (Δ G a (300 K, 1 bar) > 1.2 eV), several publications have highlighted nontransition metal catalysts able to activate methane at low temperatures or with low density functional theory (DFT)-predicted barriers. − 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. , 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 …”

“…Similarly, it was found that metal-exchanged zeolites, which had previously achieved methanol yields of ∼3% (64% CH 3 OH selectivity; 5% CH 4 conversion) in the catalytic process, could unlock higher methanol selectivities (∼98%) when used as heterogeneous protecting groups to oxidize methane to methanol stoichiometrically (TON = 1). − 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) . 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 …”

Sulfated Tin Oxide as Highly Selective Catalyst for the Chlorination of Methane to Methyl Chloride

“…One of the biggest problems is the very low selectivity to desired product with reasonably high CH 4 conversion. For example, when considering the results of direct CH 4 to CH 3 OH, which can act as a platform molecule like CH 3 Cl, the most sophisticated homogeneous and heterogeneous catalysts showed below 90% selectivity to CH 3 OH, with only 10% CH 4 conversion . Therefore, the highly selective CH 4 chlorination over STO catalyst has the potential to advance the commercialization of the direct methane transformation process.…”