室温光驱动甲烷活化

如何在较温和的条件下将甲烷转化为其它更有价值的有机衍生物,如醇、芳烃、长链烷烃和烯烃等,长期以来是催化、化学及化工领域的热点课题和难点课题之一。为了提高甲烷的转化效率,过去几十年里,研究人员不断开发新的催化剂和新的反应路径。与传统高温热催化方法相比,如果能利用自然界中丰富的太阳能驱动甲烷转化,将同时满足能源和环保两方面的要求,是各种新型非常规策略中比较令人期待的一种。本文从光催化材料的组成、结构及催化路线、催化机制等方面进行总结,对当前室温光活化甲烷分子的研究现状加以论述。

Photo-Induced Activation of Methane at Room Temperature

Methane, the most abundant constituent of natural gas, is a potential substitute for the dwindling petroleum resources for the chemical industry as a carbon-based feedstock. Over the last two decades, global research endeavors have focused on the development of more efficient and selective catalysts for the conversion of ubiquitous but inert methane. In addition, the transportation of gaseous methane in pipelines is unavoidably accompanied by leakage, and methane is recognized as a potent greenhouse gas (20 times more powerful than carbon dioxide per molecule). Thus, the conversion of methane into heavier derivatives is also of crucial environmental concern. Unfortunately, there is still a lack of economical and practical routes for methane conversion. Currently, the major route for methane conversion is the steam reforming of methane into synthetic gases, which is a multistep and energy-consuming route. Another option is to use photoenergy to drive the conversion of methane, which has significant advantages such as the capacity to minimize coking by running at room temperature. A promising approach to photocatalytic methane conversion is the photo-powered direct coupling or oxidation of methane to form ethane, methanol and hydrogen. The ethane or methanol produced can, in turn, be converted into ethene or liquid fuels through metathesis or dehydrogenation, respectively. Furthermore, the direct dehydrogenation of methane is the best way to produce clean H₂ energy from fossil fuels since methane has the highest H/C ratio among hydrocarbons. However, the methane conversion efficiency of previously reported photocatalysts is low. Furthermore, the wavelength of light used in previously reported photocatalytic systems usually needs to be less than 270 nm, which is beyond the range of the solar spectrum (wavelength λ > 290 nm) reaching the Earth's surface. To achieve substantial yield and selectivity, and to exploit solar energy effectively, the development of photocatalytic systems with distinctly higher activity, higher selectivity, and lower photon energy threshold is desired. Over the past decades, many efforts have been made to activate the strong C―H bond in methane by light at room temperature. Based on the current state of research on photocatalytic methane conversion, we have focused our review on the following aspects: non-oxidative coupling of methane, dehydroaromatization of methane, and total and partial oxidation of methane. Finally, we summarize the difference between photocatalysis and thermal catalysis in the methane conversion reaction.