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21.
Identification of tert‐Butyl Cations in Zeolite H‐ZSM‐5: Evidence from NMR Spectroscopy and DFT Calculations
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Dr. Weili Dai Prof.Dr. Chuanming Wang Dr. Xianfeng Yi Prof. Dr. Anmin Zheng Prof. Dr. Landong Li Prof. Dr. Guangjun Wu Prof. Dr. Naijia Guan Prof. Dr. Zaiku Xie Dipl.‐Chem. Michael Dyballa Prof. Dr. Michael Hunger 《Angewandte Chemie (International ed. in English)》2015,54(30):8783-8786
Experimental evidence for the presence of tert‐butyl cations, which are important intermediates in acid‐catalyzed heterogeneous reactions, on solid acids has still not been provided to date. By combining density functional theory (DFT) calculations with 1H/13C magic‐angle‐spinning NMR spectroscopy, the tert‐butyl cation was successfully identified on zeolite H‐ZSM‐5 upon conversion of isobutene by capturing this intermediate with ammonia. 相似文献
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23.
Chao Zhang Zengtian Cheng Zaihui Fu Yachun Liu Xianfeng Yi Anmin Zheng Steven Robert Kirk Dulin Yin 《Cellulose (London, England)》2017,24(1):95-106
This article first discloses that the fluorine anion-containing ionic liquids-functionalized biochar sulfonic acids (BCSA-IL-F1–3s), which were simply synthesized by an ionic exchange of 1-trimethoxysilylpropyl-3-methylimidazolium chloride (IL-Cl) grafted on the BCSA with CF3SO3H (HF1), HBF4 (HF2), HPF6 (HF3), respectively, can efficiently catalyze cellulose hydrolysis into reducing sugars (RSs) and 5-hydroxymethyl furfural (HMF) in water under microwave irradiation. This process provides a very high catalysis efficiency (turnover numbers, 4.03–4.89) at mild temperature (80 °C) for 3 h, but also possesses an excellent repeatability. More outstandingly, they can achieve much higher HMF yields (12.70–27.94%) compared to the IL-Cl-functionalized BCSA catalyst (HMF yields are lower than 0.1%) under the same reaction conditions. This is likely because the introduction of IL-F1–3s groups can significantly improve the accessibility, acidity and thermal stability of BCSA’s SO3H sites, as supported by evidence from a solid 31P NMR spectrum and thermogravimetric analysis. It is proposed that the good selectivity for HMF perhaps originates from a co-catalysis action of the IL-F1–3s and SO3H groups on BCSA-IL-F1–3s in the further conversion of RSs to HMF. 相似文献
24.
Dr. Shing‐Jong Huang Chih‐Yi Yang Dr. Anmin Zheng Ningdong Feng Dr. Ningya Yu Pei‐Hao Wu Prof. Dr. Yu‐Chi Chang Prof. Dr. Ying‐Chih Lin Prof. Dr. Feng Deng Prof. Dr. Shang‐Bin Liu 《化学:亚洲杂志》2011,6(1):137-148
The acid and transport properties of the anhydrous Keggin‐type 12‐tungstophosphoric acid (H3PW12O40; HPW) have been studied by solid‐state 31P magic‐angle spinning NMR of absorbed trimethylphosphine oxide (TMPO) in conjunction with DFT calculations. Accordingly, 31P NMR resonances arising from various protonated complexes, such as TMPOH+ and (TMPO)2H+ adducts, could be unambiguously identified. It was found that thermal pretreatment of the sample at elevated temperatures (≥423 K) is a prerequisite for ensuring complete penetration of the TMPO guest probe molecule into HPW particles. Transport of the TMPO absorbate into the matrix of the HPW adsorbent was found to invoke a desorption/absorption process associated with the (TMPO)2H+ adducts. Consequently, three types of protonic acid sites with distinct superacid strengths, which correspond to 31P chemical shifts of 92.1, 89.4, and 87.7 ppm, were observed for HPW samples loaded with less than three molecules of TMPO per Keggin unit. Together with detailed DFT calculations, these results support the scenario that the TMPOH+ complexes are associated with protons located at three different terminal oxygen (Od) sites of the PW12O403− polyanions. Upon increasing the TMPO loading to >3.0 molecules per Keggin unit, abrupt decreases in acid strength and the corresponding structural variations were attributed to the change in secondary structure of the pseudoliquid phase of HPW in the presence of excessive guest absorbate. 相似文献
25.
In the current article, the structures and properties of intermediates during the hydride transfer for the prior coordination of the carbonyl oxygen of keto oxime ether at B(2) of oxazaborolidine are discussed. All the structures are optimized completely by means of the Hartree–Fock (HF) and the density functional methods at the HF/6‐31G(d) and Becke's three‐parameter exchange functional and the gradient‐corrected functional of Lee, Yang, and Paar (B3LYP)/6‐31G(d) levels. The hydride transfer from BH3 to the carbonyl carbon in oxazaborolidine‐borane‐keto oxime ether adduct results in the formation of the adduct 4a* with a seven‐membered ring. This adduct has four stable structures. Another hydride of BH2 transfers to the oxime carbon in 4a* , leading to the adduct 5a* , which has also four stable structures. Among all the structures of 5a* , the most stable structure can generate (1S, 2R)‐cis amino alcohol, which is in agreement with that obtained in the experiment. This enantioselective reduction may go through the process in which oxazaborolidine‐borane‐keto oxime ether adduct is directly transformed into the adduct 4a* with a seven‐membered ring. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 93: 307–316, 2003 相似文献
26.
Xu J Zheng A Yang J Su Y Wang J Zeng D Zhang M Ye C Deng F 《The journal of physical chemistry. B》2006,110(22):10662-10671
The acidity of mesoporous MoO(x)/ZrO2 and WO(x)/ZrO2 materials was studied in detail by multinuclear solid-state NMR techniques as well as DFT quantum chemical calculations. The 1H MAS NMR experiments clearly revealed the presence of two different types of strong Br?nsted acid sites on both MoO(x)/ZrO2 and WO(x)/ZrO2 mesoporous materials, which were able to prontonate adsorbed pyrine-d5 (resulting in 1H NMR signals at chemical shifts in the range 16-19 ppm) as well as adsorbed trimethylphosphine (giving rise to 31P NMR signal at ca. 0 ppm). The 13C NMR of adsorbed 2-(13)C-acetone indicated that the average Br?nsted acid strength of the two mesoporous materials was stronger than that of zeolite HZSM-5 but still weaker than that of 100% H2SO4, which was in good agreement with theoretical predictions. The quantum chemical calculations revealed the detailed structures of the two distinct types of Br?nsted acid sites formed on the mesoporous MoO(x)/ZrO2 and WO(x)/ZrO2. The existence of both monomer and oligomer Mo (or W) species containing a Mo-OH-Zr (or W-OH-Zr) bridging OH group was confirmed with the former having an acid strength close to zeolite HZSM-5, with the latter having an acid strength similar to sulfated zirconia. On the basis of our NMR experimental and theoretical calculation results, a possible mechanism was proposed for the formation of acid sites on these mesoporous materials. 相似文献
27.
A molecular design was performed for the caged molecule (CH)8: the replacement of CH groups by N atoms to increase the content of N as well as reduce the content of H. A series of caged molecules were obtained: (CH)xN(8-x) (0 < or = n < or = 8). The studied aspects are as follows: (i) molecular geometries and electronic structures, (ii) the analysis of the electronic structure using natural bond orbital (NBO) and atoms in molecules (AIM), and (iii) some physicochemical properties of studied molecules, such as the dipole moments, IR vibrational spectra, NMR chemical shifts, heats of formation, and relative specific impulses, were provided. Our studies show that these molecules should be a kind of potential and novel energetic material. Our work provides some useful information for the experimental study of these molecules. The effect of the substitution of N atoms for CH groups on the properties of this kind of caged molecule is presented. 相似文献
28.
The relative stabilities of nonisomers are investigated. Twenty-two species of nitrogen cage molecules N(2n) (N6 (D(3h)), N8 (Oh), N10 (D(5h)), N12 (D(6h)), N12 (D(3d)), N16 (D(4d)), N18 (D(3h)), N20 (Ih), N24 (D(3d)), N24 (D(4h)), N24 (D(6d)), N30 (D(3h)), N30 (D(5h)), N32 (D(4d)), N36 (D(3d)), N40 (D(4h)), N42 (D(3h)), N48 (D(4d)), N48 (D(3d)), N54 (D(3h)), N56 (D(4h)), and N60 (D(3d))), which are divided into four sets, have been studied in detail. The geometries and varieties of energies are examined extensively, and NBO analysis and AIM analysis are applied to investigate the bonding properties of the cage molecules. The introducing of the concept of "layer" can well assist in explaining why one nonisomer molecule is more stable than another one. The results show that the lengths of bonds, on both sides of which are five-membered rings (referred to as pentagons), are the shortest and the orbital energies are the lowest. The nonlocalized electron numbers of orbitals, on at least one side of which is a triangle, are the greatest. Pentagons play a major role in the stability of a cage molecule, and the three-membered rings (referred to as triangles) play the second one. The layers in nitrogen cage molecules also contribute to the relative stabilities. 相似文献
29.
We concern the sublinear Schrödinger-Poisson equations \(\left\{ \begin{gathered}- \Delta u + \lambda V\left( x \right)u + \phi u = f\left( {x,u} \right)in{\mathbb{R}^3} \hfill \\- \Delta \phi = {u^2}in{\mathbb{R}^3} \hfill \\ \end{gathered} \right.\) where λ > 0 is a parameter, V ∈ C(R3,[0,+∞)), f ∈ C(R3×R,R) and V-1(0) has nonempty interior. We establish the existence of solution and explore the concentration of solutions on the set V-1(0) as λ → ∞ as well. Our results improve and extend some related works. 相似文献
30.
Jun Xu Victor V. Terskikh Yueying Chu Anmin Zheng Yining Huang 《Magnetic resonance in chemistry : MRC》2020,58(11):1082-1090
Metal–organic frameworks (MOFs) are a class of important porous materials with many current and potential applications. Their applications almost always involve the interaction between host framework and guest species. Therefore, understanding of host–guest interaction in MOF systems is fundamentally important. Solid-state NMR spectroscopy is an excellent technique for investigating host–guest interaction as it provides information complementary to that obtained from X-ray diffraction. In this work, using MOF α-Mg3(HCOO)6 as an example, we demonstrated that 13C chemical shift tensor of organic linker can be utilized to probe the host–guest interaction in MOFs. Obtaining 13C chemical shift tensor components (δ11, δ22, and δ33, where δ11 ≥ δ22 ≥ δ33) in this MOF is particularly challenging as there are six coordinatively equivalent but crystallographically non-equivalent carbons in the unit cell with very similar local coordination environment. Two-dimensional magic-angle-turning experiments were employed to measure the 13C chemical shift tensors of each individual crystallographically non-equivalent carbon in three microporous α-Mg3(HCOO)6 samples with different guest species. The results indicate that the δ22 component (with its direction approximately being co-planar with the formate anion and perpendicular to the C−H bond) is more sensitive to the adsorbate molecules inside the MOF channel due to the weak C−H···O hydrogen bonding or the ring current effect of benzene. The 13C isotropic chemical shift, on the other hand, seems much less sensitive to the subtle changes in the local environment around formate linker induced by adsorption. The approach described in this study may be used in future studies on host–guest interaction within MOFs. 相似文献