CO‐Assisted Direct Methane Conversion into C 1 and C 2 Oxygenates over ZSM‐5 Supported Transition and Platinum Group Metal Catalysts Using Oxygen as an Oxidant

Metal-Oxo Electronic Tuning via In Situ CO Decoration for Promoting Methane Conversion to Oxygenates over Single-Atom Catalysts

Direct methane conversion (DMC) to oxygenates at low temperature is of great value but remains challenging due to the high energy barrier for C-H bond activation. Here, we report that in situ decoration of Pd1-ZSM-5 single atom catalyst (SAC) by CO molecules significantly promoted the DMC reaction, giving the highest turnover frequency of 207 h-1 ever reported at room temperature and ~100 % oxygenates selectivity with H2O2 as oxidant. Combined characterizations and DFT calculations illustrate that the C-atom of CO prefers to coordinate with Pd1, which donates electrons to the Pd1-O active center (L-Pd1-O, L=CO) generated by H2O2 oxidation. The correspondingly improved electron density over Pd-O pair renders a favorable heterolytic dissociation of C-H bond with low energy barrier of 0.48 eV. Applying CO decoration strategy to M1-ZSM-5 (M=Pd, Rh, Ru, Fe) enables improvement of oxygenates productivity by 3.2-11.3 times, highlighting the generalizability of this method in tuning metal-oxo electronic structure of SACs for efficient DMC process.

Light-driven flow synthesis of acetic acid from methane with chemical looping

Oxidative carbonylation of methane is an appealing approach to the synthesis of acetic acid but is limited by the demand for additional reagents. Here, we report a direct synthesis of CH₃COOH solely from CH₄ via photochemical conversion without additional reagents. This is made possible through the construction of the PdO/Pd-WO₃ heterointerface nanocomposite containing active sites for CH₄ activation and C-C coupling. In situ characterizations reveal that CH₄ is dissociated into methyl groups on Pd sites while oxygen from PdO is the responsible for carbonyl formation. The cascade reaction between the methyl and carbonyl groups generates an acetyl precursor which is subsequently converted to CH₃COOH. Remarkably, a production rate of 1.5 mmol g_Pd^-1 h^-1 and selectivity of 91.6% toward CH₃COOH is achieved in a photochemical flow reactor. This work provides insights into intermediate control via material design, and opens an avenue to conversion of CH₄ to oxygenates.

Selective Formation of Acetic Acid and Methanol by Direct Methane Oxidation Using Rhodium Single-Atom Catalysts

Atomically dispersed catalysts such as single-atom catalysts have been shown to be effective in selectively oxidizing methane, promising a direct synthetic route to value-added oxygenates such as acetic acid or methanol. However, an important challenge of this approach has been that the loading of active sites by single-atom catalysts is low, leading to a low overall yield of the products. Here, we report an approach that can address this issue. It utilizes a metal-organic framework built with porphyrin as the linker, which provides high concentrations of binding sites to support atomically dispersed rhodium. It is shown that up to 5 wt% rhodium loading can be achieved with excellent dispersity. When used for acetic acid synthesis by methane oxidation, a new benchmark performance of 23.62 mmol·g_cat^-1·h^-1 was measured. Furthermore, the catalyst exhibits a unique sensitivity to light, producing acetic acid (under illumination, up to 66.4% selectivity) or methanol (in the dark, up to 65.0% selectivity) under otherwise identical reaction conditions.

Methane Oxidation to Methanol

The direct transformation of methane to methanol remains a significant challenge for operation at a larger scale. Central to this challenge is the low reactivity of methane at conditions that can facilitate product recovery. This review discusses the issue through examination of several promising routes to methanol and an evaluation of performance targets that are required to develop the process at scale. We explore the methods currently used, the emergence of active heterogeneous catalysts and their design and reaction mechanisms and provide a critical perspective on future operation. Initial experiments are discussed where identification of gas phase radical chemistry limited further development by this approach. Subsequently, a new class of catalytic materials based on natural systems such as iron or copper containing zeolites were explored at milder conditions. The key issues of these technologies are low methane conversion and often significant overoxidation of products. Despite this, interest remains high in this reaction and the wider appeal of an effective route to key products from C-H activation, particularly with the need to transition to net carbon zero with new routes from renewable methane sources is exciting.

Selective Methane Oxidation to Acetic Acid Using Molecular Oxygen over a Mono-Copper Hydroxyl Catalyst

Acetic acid is an industrially important chemical, produced mainly via carbonylation of methanol using precious metal-based homogeneous catalysts. As a low-cost feedstock, methane is commercially transformed to acetic acid via a multistep process involving energy-intensive methane steam reforming, methanol synthesis, and, subsequently, methanol carbonylation. Here, we report a direct single-step conversion of methane to acetic acid using molecular oxygen (O₂) as the oxidant under mild conditions over a mono-copper hydroxyl site confined in a porous cerium metal-organic framework (MOF), Ce-UiO-Cu(OH). The Ce-UiO MOF-supported single-site copper hydroxyl catalyst gave exceptionally high acetic acid productivity of 335 mmolg_cat^-1 in 96% selectivity with a Cu TON up to 400 at 115 °C in water. Our spectroscopic and theoretical studies and controlled experiments reveal that the conversion of methane to acetic acid occurs via oxidative carbonylation, where methane is first activated at the copper hydroxyl site via σ-bond metathesis to afford Cu-methyl species, followed by carbonylation with in situ-generated carbon monoxide and subsequent hydrolysis by water. This work may guide the rational design of heterogeneous abundant metal catalysts for the activation and conversion of methane to acetic acid and other valuable chemicals under mild and environmentally friendly reaction conditions.

Highly Efficient CO-Assisted Conversion of Methane to Acetic Acid over Rh-Encapsulated MFI Zeolite Prepared Using RhCl3 Molten Salt

Direct catalytic conversion of methane to value-added chemicals with a high reaction rate and selectivity under mild conditions remains a great challenge. Rh-supported zeolite is one of the promising heterogeneous...