An efficient catalytic process for converting methane into methanol could have far-reaching economic implications. Iron-containing zeolites (microporous aluminosilicate minerals) are noteworthy in this regard, having an outstanding ability to hydroxylate methane rapidly at room temperature to form methanol. Reactivity occurs at an extra-lattice active site called α-Fe(II), which is activated by nitrous oxide to form the reactive intermediate α-O; however, despite nearly three decades of research5, the nature of the active site and the factors determining its exceptional reactivity are unclear. The main difficulty is that the reactive species—α-Fe(II) and α-O—are challenging to probe spectroscopically: data from bulk techniques such as X-ray absorption spectroscopy and magnetic susceptibility are complicated by contributions from inactive ‘spectator’ iron. Here we show that a site-selective spectroscopic method regularly used in bioinorganic chemistry can overcome this problem. Magnetic circular dichroism reveals α-Fe(II) to be a mononuclear, high-spin, square planar Fe(II) site, while the reactive intermediate, α-O, is a mononuclear, high-spin Fe(IV)=O species, whose exceptional reactivity derives from a constrained coordination geometry enforced by the zeolite lattice. These findings illustrate the value of our approach to exploring active sites in heterogeneous systems. The results also suggest that using matrix constraints to activate metal sites for function—producing what is known in the context of metalloenzymes as an ‘entatic’ state6—might be a useful way to tune the activity of heterogeneous catalysts.

Nature 536, 317–321 (18 August 2016) doi:10.1038/nature19059
Received 24 March 2016 Accepted 13 June 2016 Published online 17 August 2016

http://www.nature.com/nature/journal/v536/n7616/full/nature19059.html

中文报道：甲烷转化成甲醇的高效催化过程可能带来深远的经济影响。在此方面，含铁沸石（微孔铝硅酸盐矿物质）在室温下快速羟基化甲烷形成甲醇的杰出能力引人注目。反应发生在所谓的α-Fe(II)超晶格活性位点，由一氧化二氮活化以形成反应中间体α-O；然而，尽管经过了近三十年的研究，活性位点的性质和使其呈现出高催化活性的因素目前还不清楚。主要困难是反应性物质——α-Fe(II)，α-O——很难用常规谱学检测：从大部分技术如X-射线吸收光谱和磁化率得到的数据由于不活泼的“旁观”铁物质而变得复杂难懂。本文展示了用生物无机化学经常使用的位点选择性光谱方法来克服这个难题。磁圆二色谱揭示出α-Fe(II)是一种单核、高自旋、四方平面的二价铁物质，而反应性中间体α-O，是一种单核高自旋的Fe(IV)=O物种，其高催化活性来源于由沸石晶格对铁配位几何构型所产生的约束。这些发现反映了我们所用的方法在探讨多相过程的反应活性位点中的价值。该结果还表明，使用矩阵约束以激活金属位点产生催化作用——产生具有所谓“按钮”状态的金属酶——可能是一个调控多相催化剂的活性的有效方法。（科研圈）