Two-Dimensional (2D) or ultrathin morphology of molecular sieve materials can significantly improve the efficiency of mass transfer and catalysis. For the synthesis of ultrathin molecular sieves, two important breakthroughs were reported in Nature by Ryoo et al (2009) and Tsapatsis et al (2017) respectively, which are extremely noticeable to scientific community. To synthesize non-layered molecular sieves in 2D ultrathin morphology, for example, CHA type, however, it is still an extreme challenge, because for those cases the 2D ultrathin molecular sieves is very unstable in nature due to the high surface energy during crystallization and growth.
Herein, we report a smart method for synthesis of CuO@SAPO-34 molecular sieves in 2D ultrathin morphology by exfoliating a kind of AlPO4 nanorolls in artificial layered structures and following a process of hydrothermal crystallization. As shown in Figure 1.
Figure 1. Schematic show of the formation process of CuO@SAPO-34 nanosheets. (a) Layered alumina phosphate nanoroll precusor with C12H25NH2 in between the layers; (b) Unfolded nanoroll induced by the insertion of TEOS and Cu(NO3)2; (c) CuO@SAPO-34 nanosheets formed after hydrothermal treatment in presence of tetraethylamonium hydroxide (TEAOH) and triethylamine (TEA).
This 2D and ultra-thin CuO @ SAPO-34 has a thickness of ~7 nm and ~ 5 cages in its thickness direction. And it is one of the thinnest molecular sieve materials up to date in published papers. CuO clusters (~ 0.7 nm) are located in the cages and increase the stability of the 2D and ultra-thin structure. At the same time, due to the 2D and ultra-thin structure, CuO clusters are mainly located in the cages of the sub-surface and can show a unique catalytic performance.
The adipic acid is one of the most important industrial chemicals for the production of nylon-66 polymer and now, over 95% of the worldwide industrial production of adipic acidis is a non-green method of nitric acid oxidation with very low overall product yields (<10%) and severe environment problems, such as corrosion and emission of vast amount of NOx. In the past decades, great efforts have been devoted to green methods for production of adipic acid. However, the direct oxidation of cyclohexane using O2 via a heterogeneous process to adipic acid remains a great challenge at the present time. Hence, we found that our catalyst of CuO@SAPO-34 nanosheets shows an excellent catalytic performance for one-pot oxidation of cyclohexane to adipic acid using O2 as oxidant (conversion: 42%; selectivity: 74%), which should be the top results of heterogeneous catalysis reported in the literature up to date. We also discovered that it may not be a radical mechanism but a new mechanism, as shown in Figure 2: (1) cyclohexane firstly converts to cyclohexanone as the main intermediate with minor cyclohexanol as by product; (2) the cyclohexanone molecules are chemically adsorbed on the partially confined CuO cluster sites and start to be oxidized to adipic acid in the following catalytic reactions.
Figure 2. Schematic show of catalytic mechanism over NS-CuO@SAPO-34.
This work was financially supported by National Science Foundation of China, and published in Angew. Chem. (On line, Dec.9, 2019), which title is “Morphology Reserved Synthesis of Discrete Nanosheets of CuO@SAPO-34 and Pore Mouth Catalysis for One-pot Oxidation of Cyclohexane” (DOI: 10.1002/anie.201911749, https://www.onlinelibrary.wiley.com/doi/10.1002/anie.201911749).
The first author is Dr. Xiangke Guo (Nanjing University), and the corresponding authors are Prof. Weiping Ding, Dr. Ming Lin (Singapore Institute of materials) and Prof. Xuefeng Guo (Nanjing University).