首页|Acidic property of YNU-5 zeolite influenced by its unique micropore system
Acidic property of YNU-5 zeolite influenced by its unique micropore system
扫码查看
点击上方二维码区域，可以放大扫码查看
原文链接
NSTL
Elsevier
YNU-5 zeolite has the YFI-type framework with a 12-12-8 ring system (2-dimensional 12-oxygen membered ring pores 3-dimensionally connected by twin 8-ring channels), and isolated 8-ring channels separated from the system by a thin (mono-atomic silicate) wall. In this study, the acidic property of YNU-5 zeolite was analyzed mainly by means of the ammonia IRMS-TPD (infrared/mass spectroscopy temperature-programmed desorption) method. In addition, the accessibility of acid sites was evaluated through adsorption of pyridine. The number of Br:misted acid sites approximately agreed with the number of Al atoms, and the amount of Lewis acid sites was negligible, indicating that most acid sites have the nature of bridging Si(OH)Al group in the YFI framework. Most of the Br:misted acidic OH groups on the YNU-5 zeolites (including the dealuminated samples) reacted with pyridine vapor at 343 K to form pyridinium cations, indicating that the Br:misted acid sites are highly accessible through the 12-12-8-ring system. Enthalpy of ammonia desorption from the Br:misted acid site, which can be an index of acid strength or reactivity of the acid site with a basic reactant, was in the following order: FAU < MOR (12-ring) approximate to *BEA < MFI < MWW approximate to YFI < MOR (8-ring), indicating the markedly large ammonia desorption enthalpy on the YNU-5 zeolite compared to the other 12-ring zeolites. The DFT calculations suggest that the Br:misted acidic protons with especially high ammonia desorption enthalpy are located in the isolated 8-ring but accessible from the 12-12-8-ring system side. In addition, dealumination at a high temperature under the reflux conditions with nitric acid resulted in the preferential removal of Br:misted acid sites with low ammonia desorption enthalpy, probably in the 12-12-8-ring system. The presence of reactive Br:misted acid sites in the isolated 8-ring, accessibility to them from the 12-12-8-ring system, and preferential removal of the Br:misted acid sites from the 12-12-8-ring system are reasonably consistent with the previously reported catalytic properties for dimethyl ether-to-olefin reaction.
NSTL
Elsevier
