A theoretical study of amines adsorption in HMOR by using ONIOM2 method

The two-layered ONIOM method is used to study the interaction of amines (NH₃, MeNH₂, Me₂NH and Me₃N) with H-type mordenite (HMOR). For all the calculations, the high-layer is described by the B3LYP/6-31G(d,p) method, while the HF/3-21G method is used for the low-layer. In the adsorption complexes, proton transfer from the HMOR framework to amines is observed, and the protonated amines (R₃NH⁺) are stabilized by hydrogen bonding between the negatively charged zeolite framework and the N---H bonds. The strength of the OH---N hydrogen bonding is reflected by differences in the N---H stretching frequency of R₃NH⁺ between the adsorbed state and the gas phase. The relative order of the amine basicity on the basis of the computed adsorption energies agrees well with the experiments, but differs from those in the gas phase (proton affinity) and in solvents (pK_a).     

Heats of adsorption of CO2 on high-silicon zeolites ZSM-5 and silicalite

The heat of adsorption of C0₂on NaZSM-5 at zero occupancy is 50.0 kJ/mole. The differential heats have two linearly descending segments, corresponding to the formation of two types of adsorption complexes with one or two C0₂ molecules, on the average. The heat of adsorption on silicalite coincides with the heat of adsorption of CO₂ on the noncationic segment of the NaZSM-5 zeolite structure (28–29 kJ/mole). The adsorbate-adsorbate interaction forces are not evident on the zeolites up to 1.5 mmole/g occupancy. The isotherms for the adsorption of C0₂ on zeolite NaZSM-5 and silicalite at 303 K in the occupancy region of 0–1.5 and 0–0.5 mmole/g are completely described by VMOT equations.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2636–2638, November, 1989.     

ZSM-5 monolith of uniform mesoporous channels

A ZSM-5 monolith of uniform mesopores(meso-ZSM-5) was synthesized with the template method using carbon aerogel of uniform mesopores of great pore volume. The pore size distribution determined by N2 adsorption showed the presence of mesopores with an average pore width of 11 nm and micropores with an average pore width of 0.51 nm. Field emission scanning electron micrograph observation revealed the presence of uniform mesopores. X-ray diffraction and FT-IR provided evidence that the synthesized meso-ZSM-5 monolith has a highly crystalline ZSM-5 structure.     

An ONIOM and DFT study of water adsorption on rutile TiO2 (110) cluster

Density functional theory (DFT) calculations performed at ONIOM DFT B3LYP/6‐31G**‐MD/UFF level are employed to study molecular and dissociative water adsorption on rutile TiO₂ (110) surface represented by partially relaxed Ti₂₅O₃₇ ONIOM cluster. DFT calculations indicate that dissociative water adsorption is not favorable because of high activation barrier (23.2 kcal/mol). The adsorption energy and vibration frequency of both molecularly and dissociatively adsorbed water molecule on rutile TiO₂ (110) surface compare well with the values reported in the literature. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

H2 adsorption on Fe/ZSM-5 zeolite: a theoretical approach

The hydrogen reduction of the red-ox Fe/ZSM-5 zeolite is an essential step for catalyst preparation. In this letter, various adsorption modes of the H(2) molecules on the Fe(III)/ZSM-5 zeolite were first explored by density functional theory, with their exact configurations provided. The adsorption energies revealed that the two configurations with Fe(III) at the sextet states are the main ones, consistent with the results of inelastic neutron scattering (INS) experiments. On such basis, the vibrational and orbital analyses were made, which will be valuable for the future studies on the Fe/ZSM-5 systems.     

An ONIOM and DFT study of water and ammonia adsorption on anatase TiO2 (001) cluster

Density functional theory (DFT) calculations at ONIOM DFT B3LYP/ 6‐31G**‐MD/UFF level are employed to study molecular and dissociative water and ammonia adsorption on anatase TiO₂ (001) surface represented by partially relaxed Ti₂₀O₃₅ ONIOM cluster. DFT calculations indicate that water molecule is dissociated on anatase TiO₂ (001) surface by a nonactivated process with an exothermic relative energy difference of 58.12 kcal/mol. Dissociation of ammonia molecule on the same surface is energetically more favorable than molecular adsorption of ammonia (?37.17 kcal/mol vs. ?23.28 kcal/mol). The vibration frequency values also are computed for the optimized geometries of adsorbed water and ammonia molecules on anatase TiO₂ (001) surface. The computed adsorption energy and vibration frequency values are comparable with the values reported in the literature. Finally, several thermodynamical properties (ΔH°, ΔS°, and ΔG°) are calculated for temperatures corresponding to the experimental studies. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Synthesis of pyridine and picolines over modified silica-alumina and ZSM-5 catalysts

In the reaction of acetaldehyde, formaldehyde and ammononia over HZSM-5 (Si/Al-280), PbZSM-5 and WZSM-5 catalysts at 420°C, 0.5 h^–1 weight hourly space velocity, the total yields of pyridine and 3-picoline obtained were 58.2, 42.8 and 78.3 wt.% based on aldehydes, respectively. In the reaction of acetaldehyde and ammonia over typical Pb–SiO₂–Al₂O₃ (20% PbO), W–SiO₂–Al₂O₃ (10% W), Pb–Cr–SiO₂–Al₂O₃ (F) and Pb–Cu–SiO₂–Al₂O₃ (E) catalysts at 420°C, 0.5 h^–1 W.H.S.V., the yields of 2- and 4-picolines obtained were 51.1, 66.1, 80.6 and 53.7 wt.%, respectively.IICT Communication No. 3421, — dedicated to Dr. A.V. Rama Rao on his 60th birthday.     

Heats of adsorption of methanol and ethanol on high-silicon ZSM-5 zeolite

The heats of adsorption of lower alcohols on NaZSM-5 have a stepwise appearance and each step corresponds to the stoichiometric formation of adsorption complexes of Na⁺ ions with from one to four alcohol molecules. All the adsorption complexes are located at zeolite channel intersections, while the alkyl groups enter these channels. The heats of adsorption of alcohols on NaZSM in the region of the formation of adsorption complexes with cations markedly exceed the heats of adsorption on silicalite, while on the noncationic part of the NaZSM-5 structure, they are identical to the heats of adsorption on silicalite. The mean molar integral adsorption entropies of alcohols are significantly less than the entropy of the liquid. The adsorbed molecules are in a solidlike state. The isotherms for the adsorption of alcohols on NaZSM-5 are completely described by VMOT equations.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2633–2635, November, 1989.     

Infrared study of adsorption of methanol,deuterated methanol and ethylene on the surface of ZSM-5 type zeolite

The surface species resulting in exposing of the ZSM-5 zeolite at elevated temperatures to methanol, deuterated methanol or ethylene have been studied by IR method.The three-step adsorption at 150°, 300°, 420° C or one-step adsorption at 420° C have been carried out in order to prepare the samples for IR. In all cases the most prominent band appeared in the range 1495–1515 cm^?1; besides two bands at about 1470 and 1370 cm^?1 have been observed. On the basis of Greenler's results and of the shift values of the bands in our spectrum of adsorbed deuterated methanol it was supposed that the band 1495–1515 cm^?1 is due to the OCO group from the surface species. Moreover these species would involve both oxygen atoms from the surface of zeolite but not from OH groups of methanol.     

Theoretical studies on the gas-phase pyrolysis of 2-phenoxycarboxylic acids: an ONIOM approach

Various ONIOM combinations-ONIOM(HF/6-31G*: PM3), ONIOM(B3LYP/6-31G*: PM3), ONIOM(MP2/6-31G*: PM3), and ONIOM(MP2/6-31G*: HF/3-21G)--were applied to investigate thermal decomposition mechanisms of four 2-phenoxycarboxylic acids (2-phenoxyacetic acid, 2-phenoxypropionic acid, 2-phenoxybutyric acid, and 2-phenoxyisobutyric acid) in the gas phase. All the transition states and intermediates of the reaction paths were optimized. The reaction pathway of four reactants yielding the phenol, CO, and the corresponding carbonyl compound was characterized on the potential energy surface and found to proceed stepwise. The first step corresponds to the elimination of phenol and the formation of alpha-lactone intermediate through a five-membered ring transition state, and the second step is the cycloreversion process of alpha-lactone intermediate to form CO and the corresponding carbonyl compound. The reaction pathway of latter three compounds to produce the carboxylic acid and phenol via a four-membered cyclic transition structure was also examined theoretically. Comparison with experiment indicates that the activation parameters for the fist reaction channel are accurately predicted at the ONIOM(MP2/6-31G*: HF/3-21G) level of theory.     

A study of binary adsorption of aromatics in completely siliceous zeolite ZSM-5 by FT-Raman spectroscopy

In the present work, binary adsorption of p-xylene and toluene in completely siliceous zeolite ZSM-5 was examined by FT-Raman spectroscopy in combination with powder X-ray diffraction. The results indicate that at total loadings of < or = 4 molecules per unit cell (u.c.) both p-xylene and toluene molecules prefer to reside at the intersections of straight and zigzag channels. The structure of the sorbate/sorbent complexes likely belongs to orthorhombic. At high loadings of 7 and 8 molecules/u.c., the framework of the zeolite transforms to another orthorhombic phase. In this high-loaded phase, toluene molecules are distributed evenly in both the channel intersection and the midsection of straight channel. Although p-xylene has access to both the channel intersection and the zigzag channel, it does show a slight preference for the channel intersection. The adsorptive behavior of these aromatics at intermediate total loadings of 5 and 6 molecules/u.c. is more complicated because the zeolitic framework is a mixture of two different orthorhombic phases.     

不同硅铝比ZSM-5的合成及其吸附脱除柴油中苯胺和吡啶的性能

合成了一系列不同硅铝比的ZSM-5分子筛,采用XRD、FT-IR、ICP、SEM、NH_3-TPD和N_2吸附-脱附等方法对其进行了表征,研究了不同硅铝比ZSM-5分子筛对模拟柴油中苯胺和吡啶的吸附脱除性能。结果表明,所合成的ZSM-5分子筛均具有典型MFI结构;与合成原料混合物中的硅铝比相比,实际硅铝比稍有降低。ZSM-5分子筛的酸量随硅铝比的增加而降低,硅铝比较小的ZSM-5(1)和ZSM-5(2)的吸附脱除苯胺或吡啶的效果明显优于其他样品,并且所有样品吸附脱除吡啶的效果均优于苯胺。ZSM-5(2)上苯胺和吡啶的吸附等温线符合Langmuir-Freundlich混合模型。     

Synthesis, characterization, and adsorption properties of nanocrystalline ZSM-5

Nanocrystalline ZSM-5 with a Si/Al ratio of 20 was synthesized using clear solutions and a hydrothermal synthesis procedure. The resulting ZSM-5 materials were characterized by powder X-ray diffraction, scanning electron microscopy (SEM), nitrogen adsorption isotherms, solid-state nuclear magnetic resonance, and toluene adsorption. A commercial ZSM-5 sample was similarly characterized for comparison with the synthesized materials. The particle sizes of the synthesized ZSM-5 samples were calculated using the measured external surface areas and were determined to be 15 and 60 nm. SEM images indicated that the ZSM-5 samples consist of agglomerated and possibly intergrown particles. Toluene adsorption measurements showed that the ZSM-5 sample with a particle size of 15 nm adsorbed approximately 50% more toluene than the other ZSM-5 samples, most likely due to the adsorption of toluene on the external surface. For the toluene adsorbed on the internal zeolite surface, approximately one toluene molecule was adsorbed per channel intersection for each of the ZSM-5 samples.     

Molecular simulation of adsorption and diffusion of chlorinated alkenes in ZSM-5 zeolites

In this work, adsorption and diffusion of trichloroethylene (TCE) and tetrachloroethylene (PCE) in ZSM-5-type zeolites were studied using molecular simulation methods. Grand canonical Monte Carlo technique was to calculate adsorption isotherms and heats of vaporization of TCE and PCE in zeolite. The results demonstrated that the Pnma-P2(1)2(1)2(1) symmetry transition of the zeolite framework has no significant effect on the TCE adsorption capacity of the silicalite, but it causes an increase of the PCE adsorption capacity. Simulations using a silicalite framework with Pnma symmetry showed that the adsorption capacity of the silicalite was limited to five molecules per unit cell. However, when a framework with P2(1)2(1)2(1) symmetry was used in the simulations, the capacity reached to eight molecules per unit cell, which is the actual adsorption capacity. To calculate intracrystalline diffusion coefficients of these compounds, molecular dynamics simulations were performed at different temperatures and loadings. The results show that the zeolite symmetry has a significant impact on diffusion coefficients of the sorbate molecules.     

A computational study on N2 adsorption in Cu-ZSM-5

The present computational study investigates the adsorption of N(2) by Cu-ZSM-5, with particular regard to the interaction with pairs of Cu(+) ions, employing simple cluster models in the calculations. It shows that several interaction patterns between N(2) and couples of Cu(+) sites are possible within the Cu-ZSM-5 structure. In particular, when pairs of Cu(+) ions are located at opposite sides of ten-membered rings, in the region where linear and sinusoidal channels intersect each other, a quasi-linear Cu-N-N-Cu adsorption occurs. Although lattice restraints cause small deviations from linearity, such interaction turned out to be more favourable than other adsorption patterns within the Cu-ZSM-5 structure. The linearity of the Cu-N-N-Cu fragment and the relatively low concentration of the related sites cause a low extinction coefficient for the N-N IR stretching mode, which is usually detected with very low intensity or not detected at all. The results of the present calculations may explain the experimental evidence for a nearly IR-silent fraction of nitrogen strongly adsorbed in the Cu-ZSM-5 catalyst which, as shown in a previous work, is linearly related to the number of active sites for NO decomposition.     

Theoretical study of modes of adsorption of water dimer on H-ZSM-5 and H-Faujasite zeolites

Modes of adsorption of water dimer on H-ZSM-5 and H-Faujasite (H-FAU) zeolites have been investigated by a quantum embedded cluster approach, using the hybrid B3LYP density functional theory. The results indicate that there are two possible adsorption pathways, namely the stepwise process where only one water binds strongly to the (-O)3-Al-O(H) tetrahedral unit while the other weakly binds to the zeolite framework and the concerted process where both water molecules form a large ring of hydrogen-bonding network with the Br?nsted proton and an oxygen framework. With inclusion of the effects of the Madelung potential from the extended zeolite framework, for adsorption on H-ZSM-5 zeolite, both the neutral and ion-pair complexes exist with adsorption energies of -15.13 and -14.73 kcal/mol, respectively. For adsorption on the H-FAU, only the ion-pair complex exists with the adsorption energy of -14.63 kcal/mol. Our results indicate that adsorption properties depend not only on the acidity of the Br?nsted acidic site but also on the topology of the zeolite framework, such as on the spatial confinement effects which lead to very different adsorption structures for the ion-pair complexes in H-ZSM-5 and H-FAU, even though their adsorption energies are quite similar. Our calculated vibrational spectra for these ion-pair complexes support previous experimental IR interpretations.     

Theoretical study for quercetin/β-cyclodextrin complexes: quantum chemical calculations based on the PM3 and ONIOM2 method

The inclusion interaction between quercetin and β-cyclodextrin (β-CD) binding site has been investigated, based on PM3 and ONIOM2 methods. The obtained results clearly indicate that the orientation in which the B ring of the guest molecule located near the secondary hydroxyls of the β-CD cavity is preferred in the binding energy. Moreover, Analyses regarding the complex structures suggest that one hydrogen bond between 7-hydroxy group (OH) of quercetin and 6-OH of β-CD is formed. This hydrogen bond interaction plays an important role in the bound quercetin/β-CD complex.