Revealing Active Sites and Reaction Pathways in Methane Non-Oxidative Coupling over Iron-Containing Zeolites

Non-oxidative coupling of methane is a promising route to obtain ethylene directly from natural gas. We synthesized siliceous [Fe]zeolites with MFI and CHA topologies and found that they display high selectivity (>90 % for MFI and >99 % for CHA) to ethylene and ethane among gas-phase products. Deactivated [Fe]zeolites can be regenerated by burning coke in air. In situ X-ray absorption spectroscopy demonstrates that the isolated Fe³⁺ centers in zeolite framework of fresh catalysts are reduced during the reaction to the active sites, including Fe²⁺ species and Fe (oxy)carbides dispersed in zeolite pores. Photoelectron photoion coincidence spectroscopy results show that methyl radicals are the reaction intermediates formed upon methane activation. Ethane is formed by methyl radical coupling, followed by its dehydrogenation to ethylene. Based on the observation of intermediates including allene, vinylacetylene, 1,3-butadiene, 2-butyne, and cyclopentadiene over [Fe]MFI, a reaction network is proposed leading to polyaromatic species. Such reaction intermediates are not observed over the small-pore [Fe]CHA, where ethylene and ethane are the only gas-phase products.     

甲烷/甲醇光催化转化研究进展

在能源需求不断上涨及石油供应日益紧张的背景下,开展对煤、天然气或生物质等非油基资源(CO、CO_2、CH_3OH、CH_4等)的高效利用显得尤为重要。C_1小分子(CO、CO_2、CH_3OH、CH_4等)经催化转化可得到燃料及多种化学品,一直受到学术界及工业界的广泛关注。甲烷/甲醇作为重要的C_1平台分子,其催化转化在C_1化学中占据重要地位。为了提高目标产物的选择性,需要有效地控制甲烷/甲醇中C―H键的活化。传统热催化作为甲烷/甲醇最常见的转化方法发展已久,但仍然面临着反应条件苛刻、能耗大、产率和选择性低等问题。光催化反应通过引入光能弥补反应中吉布斯自由能的上升,同时具有反应条件温和、操作简单、能耗低等特点,从而为甲烷/甲醇转化提供了新的途径。通过调节光的波长、强度以及催化剂的氧化能力可以实现甲烷/甲醇的选择性转化,减少副产物的生成。此外,光催化能够选择性活化甲醇的C―H键而非O―H键,从而实现甲醇的C―C偶联反应。本文主要围绕甲烷/甲醇的重整、氧化和偶联反应,总结近年来的光催化转化进展,并对进一步提高光催化性能做了展望。     

Non-oxidative Methane Coupling over Silica versus Silica-Supported Iron(II) Single Sites

Non-oxidative CH₄ coupling is promoted by silica with incorporated iron sites, but the role of these sites and their speciation under reaction conditions are poorly understood. Here, silica-supported iron(II) single sites, prepared via surface organometallic chemistry and stable at 1020 °C in vacuum, are shown to rapidly initiate CH₄ coupling at 1000 °C, leading to 15–22 % hydrocarbons selectivity at 3–4 % conversion. During this process, iron reduces and forms carburized iron(0) nanoparticles. This reactivity contrasts with what is observed for (iron-free) partially dehydroxylated silica, that readily converts methane, albeit with low hydrocarbon selectivity and after an induction period. This study supports that iron sites facilitate faster initiation of radical reactions and tame the surface reactivity.     

Dinuclear Gold-Catalyzed para-Selective C−H Arylation of Undirected Arenes by Noncovalent Interactions

The regioselectivity of C−H functionalization is commonly achieved by directing groups, electronic factors, or steric hindrance, which facilitate the identification of reaction sites. However, such strategies are less effective for reactants such as simple monofluoroarenes due to their relatively low reactivity and the modest steric demands of the fluorine atom. Herein, we present an undirected gold-catalyzed para-C−H arylation of a wide array of monofluoroarenes using air-stable aryl silanes and germanes at room temperature. A high para-regioselectivity (up to 98 : 2) can be realized by utilizing a dinuclear dppm(AuOTs)₂ (dppm=bis(diphenylphosphino)methane) as the catalyst and hexafluorobenzene as the solvent. This provides a general and practical protocol for the concise construction of structurally diverse para-arylated monofluoroarenes through C−H activation manner. It features excellent functional group tolerance and a broad substrate scope (>80 examples). Besides, this strategy is also robust for other simple monosubstituted arenes and heteroarenes. Our mechanistic studies and theoretical calculations suggest that para-C−H selectivity arises from highly electrophilic and structurally flexible dinuclear Ar−Au(III)−Au(I) species, coupled with noncovalent interaction induced by hexafluorobenzene.     

Na-W-Mn/SiO_2催化剂活化甲烷的研究 Ⅰ.活性中心的结构

制备了不同 Na、W、Mn组分的 Na- W- Mn/ Si O2 催化剂 ,并进行了甲烷氧化偶联反应催化性能的评价和XPS、XRD、L RS表征 .研究结果表明 ,Na不但有强的表面富集能力 ,而且可以使 Mn向表面发生迁移 .首次发现 Na2 W2 O7也是活化甲烷的活性相 .活化甲烷的活性中心是 Na- O- Mn和 Na- O- W结构单元 ,且 Na- O-Mn主要存在于催化剂的表面 ,Na- O- W则分布在距催化剂表面较深的部位     

甲烷氧化偶联反应稀土催化剂表面活性氧种的研究

有关甲烷氧化偶联（OCM）中活性氧种的研究已有许多报道[1~4]．本文利用XPS、O2－TPD、XRD等方法研究了高活性RE2O3体系催化剂上不同表面氧种对偶联反应的影响，并进行了氧种归属，探讨了对偶联反应起关键作用的活性氧种及作用机理．1实验部分1．1催化剂制备及活性评价将高温焙烧后的混合稀土氧化物粉末（Y2O3为主要成分）、MgO及P2O5按一定摩尔比混合研磨调匀、烘干，于950℃焙烧后压环粉碎，取0．38～0．75mm颗粒备用．活性评价在常压固定床流动体系中进行，采用内径为4．5mm的石英反应管，催化剂重量0．8g，反应温度780℃，空速…     

Organic Additive-derived Films on Cu Electrodes Promote Electrochemical CO2 Reduction to C2+ Products Under Strongly Acidic Conditions

Electrochemical CO₂ reduction (CO₂R) at low pH is desired for high CO₂ utilization; the competing hydrogen evolution reaction (HER) remains a challenge. High alkali cation concentration at a high operating current density has recently been used to promote electrochemical CO₂R at low pH. Herein we report an alternative approach to selective CO₂R (>70 % Faradaic efficiency for C₂₊ products, FE_C2+) at low pH (pH 2; H₃PO₄/KH₂PO₄) and low potassium concentration ([K⁺]=0.1 M) using organic film-modified polycrystalline copper (Modified-Cu). Such an electrode effectively mitigates HER due to attenuated proton transport. Modified-Cu still achieves high FE_C2+ (45 % with Cu foil /55 % with Cu GDE) under 1.0 M H₃PO₄ (pH≈1) at low [K⁺] (0.1 M), even at low operating current, conditions where HER can otherwise dominate.