Recently, the National Center for Nanoscience and Technology (NCNST) research group led by Prof. Zhiyong Tang has made important progress in the catalytic conversion of unsaturated carboxylic acids under mild conditions. Through construction of graphene oxide/silver composite catalysts functionalized with amide bonds, the terminal alkynes can be converted into carboxylic acid with a high conversion efficiency and the designed catalyst shows excellent cycle stability. Related research result has been published in Matter, link to the paper: https://www.sciencedirect.com/science/article/pii/S2590238520303805 .
Carboxylation of terminal alkynes with CO2 enables not only solving the problem of carbon dioxide emissions, but also synthesizing high value-added unsaturated carboxylic acid compounds that are important chemical raw materials. For synthesis of carboxylic acid compounds, one of the typical methods is to use CO2 as an electrophile to react with a carbon nucleophile. Unfortunately, this method requires highly sensitive organometallic reagents, such as strong nucleophiles organolithium and Grignard reagent, which brings difficulties and challenges involved with large-scale production. Another strategy is to directly activate the C-H bond to make the terminal alkynes react with carbon dioxide to produce carboxylic acids. In recent years, it has been reported that copper-containing coordination polymers and composite catalysts based on silver nanoparticles exhibit good catalytic activity. However, some thorny problems still remain such as insufficient utilization of catalytic sites, harsh reaction conditions, and poor universality of reaction substrates.
The composite catalysts based on two-dimensional materials have recently received great attention. The advantages of these catalysts include full exposure of the active sites, enhanced transfer rate, and more importantly, the clear surface interface. Nevertheless, it is difficult to prepare a two-dimensional material composite catalyst with uniform and controllable morphology, specifically for graphite oxide powder of cheap price and easy availability. Tang group successfully developed a steric hindrance strategy via adding tert-butyl p-aniline molecules to react with the carboxyl groups in the graphite oxide (GO) powder during the ultrasonic process, thus allowing efficient peeling to produce ultrathin tert-GO nanosheets in large quantity (seeing Figure 1). More importantly, as-formed amide bond is conducive to capturing carbon dioxide, and the abundant hydroxyl and epoxy groups on the GO surface stably protect the silver nanoparticles. As a result, at 40℃ and ambient pressure, the synthesized Ag/tert-GO efficiently catalyzes phenylacetylene to phenylpropioic acid within 24 h, achieving a conversion rate of 97.2%, and the conversion frequency of the catalyst is up to 3.12 h -1.
The thickness of the functionalized GO nanosheets is only 4 nm. By changing the amount of tert-butyl-p-aniline added, the layer spacing of GO can be adjusted in a controllable way. Because this method is simple, green and efficient, it provides a plausible route for mass production of high-quality GO nanosheets, as well as brings numerous opportunities for development of next generation two-dimensional materials supported catalysts.
This work was supported by the National Natural Science Foundation of China and Strategic Priority Research Program of Chinese Academy of Sciences. Prof. Zhiyong Tang is the corresponding authors. Xiaofei Zhang, a postdoctoral fellow in NCNST, is the first author of this paper.
Figure: Scheme of preparing Ag/tert-GO catalysts and the carboxylation pathway of terminal acetylene with carbon dioxide