Selective oxidation of methane to methanol over Cu-exchanged zeolites

<sec><p indent="0mm">Methane, as the primary component of natural gas, is abundant in nature and considered a promising clean energy source. Efficient methods for methane utilization are crucial to achieve sustainable energy development. Currently, methane is typically converted into methanol based on steam reforming, a process that requires high temperature and pressure, leading to significant energy consumption, complex equipment, and economic and environmental challenges. Therefore, there is an increasing interest in developing direct methane conversion technologies that operate under milder conditions with high efficiency and product selectivity. Methane monooxygenase (MMO), a natural enzyme, catalyzes the oxidation of methane to methanol under mild conditions, offering a valuable model for the design of biomimetic catalysts. Inspired by the function of MMO, copper-exchanged zeolites have emerged as promising candidates for methane oxidation, owing to their tunable structures and the presence of highly active copper species. Copper-exchanged zeolites are believed to have significant potential for catalyzing methane selective oxidation to methanol. However, these zeolites face challenges in achieving both high selectivity and conversion in the methane-to-methanol oxidation process. This review focuses on the progress of copper-exchanged zeolites for the selective oxidation of methane to methanol. </sec><sec> Firstly, it discusses the rational design of methane oxidation catalysts, including precious metal catalysts, single-atom catalysts, and iron-exchanged zeolite catalysts. This section provides a detailed analysis of their respective advantages and optimization strategies. Special emphasis is placed on copper-exchanged zeolite catalysts, covering the role of different oxidants and the diversity of copper species structures. </sec><sec> Next, the review discusses the catalytic approaches in methane oxidation and their respective advantages. Chemical looping, by separating the oxidation and reduction steps, effectively reduces side reactions, ensuring high selectivity for methanol production. However, its complexity, particularly in terms of frequent gas switching, heating and cooling cycles, and catalyst reactivation, limits its large-scale industrial application. Future research should focus on enhancing the material’s reactivity and optimizing the cycle time to improve process efficiency. In contrast, continuous catalytic oxidation offers a simpler operation and allows for the continuous conversion of methane at lower oxygen concentrations without the need for frequent changes in reaction conditions, leading to a higher methanol production rate. However, this method faces challenges related to the balance between catalytic activity and catalyst stability, especially in maintaining the efficient Cu²⁺↔Cu⁺ cycle and preventing the over-oxidation of methanol. </sec><sec> The final section provides a detailed review of the reaction mechanisms of diverse copper species in selective methane oxidation reactions. Monocopper, binuclear and trinuclear copper species exhibit different levels of activity and stability during the oxidation process. Typically, binuclear and trinuclear copper species, due to their multinuclear structures, show higher catalytic activity and thermal stability. In contrast, monocopper species, although exhibiting relatively lower activity, can still participate in the reaction under specific confined environments through unique redox cycling mechanisms, demonstrating a certain degree of catalytic activity. Therefore, a deeper understanding and differentiation of the activity and stability of these copper species is crucial for designing more efficient and selective catalysts. </sec><sec> Copper-exchanged zeolites enable selective methane oxidation under mild conditions due to the synergistic effects of their unique pore structure and active sites. In these catalysts, copper typically exists in the forms of Cu⁺ and Cu²⁺, which can undergo redox cycles to facilitate the selective oxidation of methane to methanol. The geometry of the zeolite pore structure not only influences the dispersion of copper species but also dictates the diffusion and transformation pathways of reactants and intermediates. By optimizing copper concentration and its distribution within the zeolite framework, the activity and methanol selectivity of copper-exchanged zeolite catalysts can be significantly enhanced. </sec>