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Số người truy cập: 106,055,593

 Computational Investigation of the Lewis Acidity in Three- and Corresponding Two-Dimensional Zeolites: UTL vs. IPC-1P
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Tác giả hoặc Nhóm tác giả: Ho Viet Thang, Miroslav Rubes, Ota Bludsky, and Petr Nachtigall
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Nơi đăng: The Journal of Physical Chemistry A; Số: 118;Từ->đến trang: 7526-7534;Năm: 2014
Lĩnh vực: Khoa học; Loại: Bài báo khoa học; Thể loại: Quốc tế
TÓM TẮT
The adsorption and catalytic properties of three-dimensional zeolite UTL were investigated computationally along with properties of its two-dimensional analog IPC-1P that can be obtained from UTL by a removal of D4R units. Adsorption properties and Lewis acidity of extra-framework Li+ sites were investigated for both two- and three- dimensional forms of UTL using the carbon monoxide as a probe molecule. The CO adsorption enthalpies, calculated with various dispersion-corrected DFT methods, including DFT/CC, DFT-D2, and vdW-DF2, and the CO stretching frequencies obtained with the νCO/rCO correlation method, are compared for corresponding Li+ sites in 3D and 2D UTL zeolite. For majority of framework Al positions the Li+ cation is preferably located in one of the channel wall sites and such sites remains unchanged upon the 3D → 2D UTL transformation; consequently, the adsorption enthalpies become only slightly smaller in 2D UTL (less than 3 kJ mol-1) due to the missing part of dispersion interactions and νCO becomes also only up to 5 cm-1 smaller in 2D UTL due to the missing repulsion with framework oxygen atoms from the opposite site of the zeolite channel (effect from the top). However, when Li+ is located in the intersection site in 3D UTL (about 20% probability) its coordination with the framework is significantly increased in 2D UTL and that is accompanied by significant decrease of both νCO (about 20 cm-1) and adsorption enthalpy (about 20 kJ mol-1). Since the intersection sites in 3D UTL are the most active adsorption and catalytic Lewis sites, the results reported herein suggest that the 3D → 2D transformation of UTL zeolite is connected with partial decrease of zeolite activity in processes driven by Lewis acid sites.
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ABSTRACT
The adsorption and catalytic properties of three-dimensional zeolite UTL were investigated computationally along with properties of its two-dimensional analog IPC-1P that can be obtained from UTL by a removal of D4R units. Adsorption properties and Lewis acidity of extra-framework Li+ sites were investigated for both two- and three- dimensional forms of UTL using the carbon monoxide as a probe molecule. The CO adsorption enthalpies, calculated with various dispersion-corrected DFT methods, including DFT/CC, DFT-D2, and vdW-DF2, and the CO stretching frequencies obtained with the νCO/rCO correlation method, are compared for corresponding Li+ sites in 3D and 2D UTL zeolite. For majority of framework Al positions the Li+ cation is preferably located in one of the channel wall sites and such sites remains unchanged upon the 3D → 2D UTL transformation; consequently, the adsorption enthalpies become only slightly smaller in 2D UTL (less than 3 kJ mol-1) due to the missing part of dispersion interactions and νCO becomes also only up to 5 cm-1 smaller in 2D UTL due to the missing repulsion with framework oxygen atoms from the opposite site of the zeolite channel (effect from the top). However, when Li+ is located in the intersection site in 3D UTL (about 20% probability) its coordination with the framework is significantly increased in 2D UTL and that is accompanied by significant decrease of both νCO (about 20 cm-1) and adsorption enthalpy (about 20 kJ mol-1). Since the intersection sites in 3D UTL are the most active adsorption and catalytic Lewis sites, the results reported herein suggest that the 3D → 2D transformation of UTL zeolite is connected with partial decrease of zeolite activity in processes driven by Lewis acid sites.
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