High performance nanosheet-like silicoaluminophosphate molecular sieves: synthesis, 3D EDT structural analysis and MTO catalytic studies

Abstract

Nanosheet-like silicoaluminophosphate (SAPO) molecular sieves SAPO-34 (CHA-type) and SAPO-18 (AEI-type) with different silicon contents were synthesized under hydrothermal conditions by using tetraethylammonium hydroxide as the template. Three-dimensional electron diffraction tomography (3D EDT) technique was applied for ab initio structure solutions. Electron microscopy observations confirmed the existence of defects, i.e., intergrowth of CHA- and AEI-type frameworks caused by the different stacking manners of double 6-ring layers, but the layers were highly coherent along c direction. Compositions, acidities, chemical environments, and texture properties of all the samples were characterized by ICP, EDS, NH₃-TPD, MAS NMR, and N₂ adsorption–desorption measurements. The catalytic performances of methanol-to-olefin (MTO) reactions over nano SAPO catalysts with different silicon contents were systematically studied. All of these catalysts showed excellent catalytic activity, among which SAPO-34 showed superior catalytic performance compared to SAPO-18. DFT calculations were utilized to study the different catalytic performance of CHA and AEI. Significantly to date, SAPO-34 with the lowest silicon content exhibited the longest catalyst lifetime and the lowest coking rate in the MTO reaction than any of the reported catalysts, tested under similar conditions. The straight 8-ring pore channel along the c direction provided the optimum diffusion pathway as well as the shortest diffusion length for reactant and generated olefins, thus significantly reduced the coking rate. This work demonstrates that a 3D EDT approach combined with TEM and EDS analysis from a single nanocrystal can provide a clear crystal structure, crystal orientation and compositional information of nanocrystals, which are useful for the better understanding of the catalytic performance of nanosized crystalline catalysts.