<![CDATA[Biogas is a renewable methane-containing feedstock that can be upgraded to methanol, linking waste treatment with the production of a transportable liquid chemical. Conventional methanol manufacture relies on reforming, syngas conditioning, elevated temperature, and pressure. Methanotrophic bacteria and methane monooxygenase (MMO) provide a different route: methane can be hydroxylated selectively under comparatively mild conditions. The difficulty is that biological methane-to-methanol conversion is still limited by the low solubility of methane and oxygen, rapid methanol oxidation by methanol dehydrogenase (MDH), redox and cofactor constraints, loss of biocatalyst activity, gas impurities, and modest volumetric productivity. This review examines recent progress through three closely connected engineering themes: methanotrophic biocatalysis, immobilization of cells or enzymes, and reactor intensification. Studies using chitosan, lignocellulosic carriers, alginate/silica matrices, banana leaves, and magnetic-particle-containing polymers show that immobilization can improve reuse and cumulative methanol production. At reactor scale, trickle beds, inverse membrane systems, airlift reactors, pressurized stirred tanks, fine-bubble spargers, silicone-oil mass-transfer vectors, and emerging nanobubble approaches have been explored to address gas-transfer limitations. Molecular developments, including engineered mini-sMMO and photo-biocatalytic activation of MMO, also offer routes that reduce dependence on native methanotrophic metabolism and costly NADH regeneration. On this basis, an integrated process is proposed in which anaerobic digestion, selective biogas conditioning, intensified gas delivery, stable biocatalysis, controlled MDH activity, and methanol recovery are designed as a single process. The main challenge is no longer demonstrating methane hydroxylation, but maintaining useful methanol productivity with realistic biogas under scalable mass-transfer conditions]]>
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