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 Theoretical Investigation of AcidoBasic Properties of 3D and 2D Zeolite
Tác giả hoặc Nhóm tác giả: Ho Viet Thang, Petr Nachtigall
Nơi đăng: Summer School Catalysis of Biomass, Libelice Castle, 8-11 June, Czech Republic
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; Số: xxx;Từ->đến trang: xxx;Năm: 2014
Lĩnh vực: Khoa học; Loại: Báo cáo; Thể loại: Quốc tế
TÓM TẮT
Zeolites, traditionally known as 3D crystalline solid structure, have been widely applied in heterogonous catalysis. While the small size of zeolite channels has a lot of advantages, it also prevents optimized utilization of active sites in the whole zeolite volume in catalytic reactions of larger reactants and/or products. One of the way to overcome this obstacle offers the concept of 2D layered zeolites1. The differences in properties of acidity and basicity in 3D and 2D material are investigated herein computational in order to understand the similarities and differences in catalytic properties of 2D and 3D zeolites. For acid properties, periodic models of 3D and 2D zeolites MWW, UTL, and MFI are used together with dispersion-corrected density functional theory (DFT/CC). Vibrational frequencies of Brønsted OH groups and of adsorbed CO probe are described at the ω/r correlation level that accounts for anharmonic corrections. Relative energies of Brønsted sites in the vicinity of various framework Al positions are calculated. For the energetically most stable Brønsted sites the CO adsorption complexes are investigated and their vibrational frequencies are calculated. For basic properties, acetone and pyrrole are used as probe in 3D and 2D FAU periodic model with DFT-D3 method. The relative energies of enol and enolate ion with respect to acetone are used to measure effect of sodium exchange and over–exchange. In addition, NH frequency and adsorption energies of pyrrole are also evaluated. Vibrational frequencies and adsorption enthalpies of CO calculated for corresponding 3D and 2D materials are very similar for majority of framework Al positions. However, in some cases larger decrease of OH frequency upon adsorption of CO is observed. This difference is due to the fact, that adsorption complex can easily adopt linear OH—CO geometry in 2D materials. The relative energies of enol and enolate ion of acetone and frequencies of NH are also the same in 3D and 2D FAU zeolite. It can be concluded that Brønsted acidity and basicity are not much influenced by the 3D to 2D transformation.
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ABSTRACT
Zeolites, traditionally known as 3D crystalline solid structure, have been widely applied in heterogonous catalysis. While the small size of zeolite channels has a lot of advantages, it also prevents optimized utilization of active sites in the whole zeolite volume in catalytic reactions of larger reactants and/or products. One of the way to overcome this obstacle offers the concept of 2D layered zeolites1. The differences in properties of acidity and basicity in 3D and 2D material are investigated herein computational in order to understand the similarities and differences in catalytic properties of 2D and 3D zeolites. For acid properties, periodic models of 3D and 2D zeolites MWW, UTL, and MFI are used together with dispersion-corrected density functional theory (DFT/CC). Vibrational frequencies of Brønsted OH groups and of adsorbed CO probe are described at the ω/r correlation level that accounts for anharmonic corrections. Relative energies of Brønsted sites in the vicinity of various framework Al positions are calculated. For the energetically most stable Brønsted sites the CO adsorption complexes are investigated and their vibrational frequencies are calculated. For basic properties, acetone and pyrrole are used as probe in 3D and 2D FAU periodic model with DFT-D3 method. The relative energies of enol and enolate ion with respect to acetone are used to measure effect of sodium exchange and over–exchange. In addition, NH frequency and adsorption energies of pyrrole are also evaluated. Vibrational frequencies and adsorption enthalpies of CO calculated for corresponding 3D and 2D materials are very similar for majority of framework Al positions. However, in some cases larger decrease of OH frequency upon adsorption of CO is observed. This difference is due to the fact, that adsorption complex can easily adopt linear OH—CO geometry in 2D materials. The relative energies of enol and enolate ion of acetone and frequencies of NH are also the same in 3D and 2D FAU zeolite. It can be concluded that Brønsted acidity and basicity are not much influenced by the 3D to 2D transformation.
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