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Local structure elucidation and reaction mechanism of light naphtha aromatization over Ga embedded H-ZSM-5 zeolite: Combined DFT and experimental study
(2020-10-15)
Wongnongwa, Yutthana
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Kidkhunthod, Pinit
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Sukkha, Usa
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Pengpanich, Sitthiphong
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Thavornprasert, Kaew arpha
Local structures and mechanisms for n-pentane aromatization on Ga embedded H-ZSM-5 zeolite (Ga/ZSM-5) were elucidated using Synchrotron-based X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations to understand the role of Ga/ZSM-5 zeolite in aromatics synthesis. XAS data suggests that Ga ligates with four oxygen or four hydrogen atoms. Catalytic tests results suggest that conversion by Ga/ZSM-5 catalyst cannot occur via C6–C8 non-aromatic intermediates, while the availability of Ga metal sites promotes the aromatization of C2 and C3 species. Therefore, conversion of n-pentane to benzene or toluene comprises four steps, i) cracking, ii) GaH<inf>2</inf> activation, iii) cyclization, and iv) dehydrogenation. Our model predicts the key intermediate in n-pentane aromatization on Ga/ZSM-5 zeolite to be a five-membered Ga-C4 ring structure. The ring undergoes expansion to form a seven-membered Ga-C6 ring. Moreover, we discuss thermodynamics and kinetic results for the benzene and toluene formation pathways. Our results provide new finding for the role of Ga/ZSM-5 zeolites in n-pentane aromatization processes.
2
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Nitric oxide oxidation on warped nanographene (C80H30): a DFT study
(2019-01-01)
Roongcharoen, Thantip
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Kungwan, Nawee
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Daengngern, Rathawat
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Sattayanon, Chanchai
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Namuangruk, Supawadee
The possible use of the recently synthesized warped nanographene C<inf>80</inf>H<inf>30</inf> for NO oxidation by O<inf>2</inf> molecule has been investigated using density functional theory. The reaction starts with the adsorption and dissociation of O<inf>2</inf> molecule on the central pentagon of C<inf>80</inf>H<inf>30</inf> with the activation energies of 24.2–26.6 kcal/mol depending on the active sites. Then, the dissociated O atoms readily oxidize NO to NO<inf>2</inf> twice. The first NO oxidation occurs with barrierless, while the second NO oxidation requires a small energy barrier of 16.0 kcal/mol. The low activation energy barrier pathway indicates high catalytic activity of this nanographene for NO oxidation. Charge analysis reveals that such high catalytic activity of nanographene is attributed to the charge transfer from the saddle-shaped C<inf>80</inf>H<inf>30</inf> to the dissociated O atoms which makes it reactive to NO molecule. Desorption of NO<inf>2</inf> product, which is the rate-limiting step of NO oxidation in some catalysts, is easily occurred in this nanographene (less than 2 kcal/mol), indicating the prevention of catalyst poisoning. This study suggests that C<inf>80</inf>H<inf>30</inf> nanographene is a promising catalyst for NO removal in ambient condition.
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A spectroscopic study of indigo dye in aqueous solution: A combined experimental and TD-DFT study
(2018-12-01)
Jiwalak, Naparat
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Daengngern, Rathawat
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Rungrotmongkol, Thanyada
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Jungsuttiwong, Siriporn
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Namuangruk, Supawadee
This study reports UV–Visible spectra and electronic structures of indigo (IG) in aqueous solution using a combination of experimental and theoretical methods. In the visible region, the experimental absorption spectrum of the solution showed a broad peak with the longest wavelength of maximal absorption (λ<inf>max</inf>) value at 708 nm. For the theoretical method, a trans-IG monomer and a trans-IG bound with two water molecules (IG.2W) were optimized in the ground state using the B3LYP and B3LYP-D3 calculations with the 6-31 + G(d,p) basis set and the SCRF-CPCM model for taking solvent effect into account was also applied. Sequentially, the UV–Visible spectra and λ<inf>max</inf> of the optimized trans-IG and IG.2W models in the implicit water were simulated by the time-dependent density functional theory (TD-DFT) calculations. The TD-DFT methods including BLYP, B3LYP, PBE0, CAM-B3LYP, M06-2X, ωB97XD, LC-BLYP, and LC-ωPBE functionals without and with the D3 correction and the 6-31 + G(d,p) basis set were selected. The results pointed out that BLYP and BLYP-D3 were the best methods because they could reproduce the experimental λ<inf>max</inf> value of IG in aqueous solution. The predicted λ<inf>max</inf> values of IG.2W were almost equal to 708 nm (the experimental data), indicating that IG.2W could be responsible for optical properties of IG.
3
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Heteroatom effect on photophysical properties of 2-(2′-hydroxyphenyl)benzimidazole and its derivatives as fluorescent dyes: A TD-DFT study
(2017-08-01)
Manojai, Natthaporn
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Daengngern, Rathawat
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Kerdpol, Khanittha
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Ngaojampa, Chanisorn
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Kungwan, Nawee
The effects of hetero nitrogen substitution on the photophysical properties and excited-state intramolecular proton transfer (ESIPT) of HBX derivatives (HBI, HBO and HBT where X=NH, O and S, respectively) were investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations at B3LYP/6-311+G(d,p) level, which is the suitable method of choice validated from various density functionals. Geometries and absorption spectra of all derivatives as well as emission spectra of selected derivatives were calculated at the same level of theory. The hetero nitrogen substitution of HBX resulted in the red shift emission spectra, which implies that the lone pair of electron in the substituted nitrogen has the effect on the π-conjugated system. In addition, results of frontier molecular orbital analysis show that vertical S<inf>0</inf>→S<inf>1</inf> transition of these molecules corresponds essentially to the excitation from HOMO (π) to LUMO (π*). The potential energy curves (PECs) of selected derivatives were used to investigate the occurrence of ESIPT and the chance was found in this order: HBT>HBI>HBO. The hetero nitrogen substituted on 1,4 and 3,4 position of HBX derivatives could easily facilitate the ESIPT process, because the nitrogen substitution makes O–H bond of selected HBX derivatives weaker than those of other derivatives and the hydrogen bond in N–H becomes stronger in the excited state, resulting in lower PT barrier. The obtained information of the electronic structure, the photophysical property and the chance of ESIPT of hetero nitrogen substituted HBX molecules is useful for molecular design of fluorescent molecular probes.

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