Microbial or Photochemical Conversion of Methane to Methanol

Certain bacteria, known as methanotrophs, can obtain all the energy and carbon they need for life from methane [187]. The key step for their utilization of methane is its selective conversion to methanol using oxygen. In subsequent biological processes, methanol is further oxidized to formaldehyde, which in turn can be either incorporated into biomass or oxidized to CO2, thereby providing the energy needed by the bacteria. Nature's catalyst for the conversion of methane to methanol is an enzyme called methane monooxygenase (MMO), which can operate in aqueous solution at ambient temperature and pressure [206]. In such a system composed of a number of imbricated proteins, the reactivity is controlled by molecular recognition and the regulation characteristics of the enzyme, allowing a virtually complete selectivity to methanol. The conversion is achieved by reductive activation of O2 with a reductant known as NADH.

Two varieties of MMO systems have been found in methanotrophic organisms. The first, which contains copper, is a membrane-bound enzyme which has proven difficult to isolate and eluded full characterization. Most attention has thus been focused on the soluble, iron-containing MMO, especially from the species Methy-losinus trichosporium and Methylococcus capsulatus. The active site in this enzyme which is directly responsible for methane oxidation contains a pair of iron atoms. Given the ability of MMO to activate methane at room temperature and ambient pressure, much research as been devoted in trying to reproduce such activity for the large-scale production of methanol. Our understanding of the reaction mechanisms taking place in MMO has greatly improved over the years, and simpler catalyst systems which try to model the behavior of MMO have been developed and tested. However, because of their complexity, the direct use of MMO enzymes has proven difficult to apply for the practical production of methanol. Besides MMO systems and their ability to oxidize specifically the simplest alkane, methane, enzymes of the cytochrome P-450 family, with a less complicated structure than MMO, have been found to catalyze the oxidation of many types of hydrocarbons to alcohols [207]. At the heart of the cytochrome P-450 is a porphyrin system containing at its center a metal atom, most commonly iron, catalyzing the oxidation reaction. Although to date no P-450 system has been shown capable of oxidizing methane, significant advances have been made in that direction through the genetic engineering of enzymes based on P-450, specifically designed for high activity towards small alkanes. In attempting to approach Nature's ability to activate methane under mild conditions, methanol has recently also been obtained by reacting methane with oxygen and H2O2 in water at temperatures below 75 °C, using a vanadium-based catalyst [208]. The finding of a practical, highly efficient and selective catalyst based on Nature's example is, however, still a long term goal. It must also be pointed out that MMO is by far not a perfect system. When O2 is used as the oxidant, it requires the presence of a reductant, NADH. Alternatively, a reduced form of O2, for example H2O2, is generally used in model experiments. In practical applications however, this would most probably imply the use of hydrogen as a reducing agent, making the process less attractive. Ideally, methane should be oxidized by only O2 to form methanol.

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