Adsorption of parent nitrosamine on the nanocrystaline M-ZSM-5 zeolite: A density functional study (original) (raw)
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Density Functional Theory Study of NO Adsorbed in A-Zeolite
The Journal of Physical Chemistry B, 2005
Density functional theory was employed to investigate the adsorption site and hyperfine interactions of nitric oxide adsorbed in Na-LTA (previous name NaA) zeolite. Three different cluster models of increasing complexity were used to represent the zeolite network: (1) a six-membered ring terminated by hydrogen atoms with one sodium ion above the ring, (2) as model 1 with the addition of three sodium ions located at the centers of three imagined four-membered rings adjacent to the six-membered ring, and (3) as model 2 with the addition of the three four-membered rings adjacent to the six-membered ring. Calculations on the largest system (model 3) showed very good agreement with measured electronic Zeeman interaction couplings, 14 N hyperfine coupling tensors, and 23 Na hyperfine and nuclear quadruple coupling tensors of the S ) 1 / 2 Na + ‚‚‚N-O adsorption complex when the position of the sodium ion was relaxed. The optimized geometry of the complex agreed nicely with that estimated experimentally, except for the Na-N distance, where the present results indicate that the distance deduced from previous ENDOR experiments may be underestimated by as much as 0.5 Å.
The Journal of Physical Chemistry C, 2013
Adsorption and activation of ethene, ethyne and formaldehyde by copper and silver sites in ZSM-5 are reinspected by quantitative assessment of the donation and backdonation processes between the three molecules and either models comprising bare cations (M +) or the cations embedded in zeolite framework (M(I)). ETS-NOCV analysis (decoupling the deformation density upon substrate bonding into independent components) reveals two predominant channels for electron transfer between adsorbed molecule and a cation or a cationic site, namely π*-backdonation from metal d orbitals to π* antibonding orbital of a substrate and σ-donation from π bonding or lone pair orbitals of the substrate to the cation. Alternative fragmentation of a complex modeling the cation embedded in zeolitic framework, treated as three-part entity, allows for extracting predominant electron transfer channels from framework oxygens to the cation with bound substrate, namely opposing σ-donation and supporting π*-backdonation. Critical analysis of charge flows between various parts of a complex system shows that effective activity of the cationic site comes from two contradictory processes and must be viewed as resulting from the framework effect on the sensitive balance between opposing electron transfer channels. Juxtaposition of density transfer channels between the substrate with CC and CO multiple bonds and either a bare metal cation or the cation embedded in a zeolite framework shows that σ-donation prevails over π*-backdonation for free cations while for zeolitic sites their order is reversed. Quantitative analysis reveals that for title systems activation decrease due to the reduction of σ-donation becomes outweighed by the increase in π *-backdonation for Cu(I) but not for Ag(I). Thus zeolitic framework, regarded as an electron reservoir, may either support (Cu +) or impair (Ag +) electronic processes underlying catalytic activation of multiple bonds.
Adsorption of carbon monoxide in H-ZSM-5 and Li-ZSM-5 zeolites: an embedded ab initio cluster study
Journal of Molecular …, 2000
The absorption of carbon monoxide with H-ZSM-5 and metal-substituted Li-ZSM-5 zeolites has been investigated by Ž. using both cluster and embedded cluster approaches at the HFr6-31G d,p level of theory. For the H-ZSM-5 zeolite, the binding energy of CO on a 3T quantum cluster is predicted to be 2.25 kcalrmol for the C-bound complex. The O-bound complex was found to be less stable by about 0.84 kcalrmol. Inclusion of the Madelung potential was found to increase the acidity of the Brønsted acidic site and the CO-binding energy to 4.95 kcalrmol, consequently, it leads better agreement with experimental observation. Similar results were also obtained for the Li-ZSM-5rCO complex. The Madelung potential field from the zeolite framework was found to reverse the order of relative stability of C-bound and O-bound adducts in comparison to the Li q-CO system.
Computer simulation of zeolite structure and reactivity using embedded cluster methods
Faraday Discussions, 1997
The use of bare cluster models to understand the nature of zeoliteÈsubstrate interactions may be improved to take account of the environment of the acid site. We consider two models for introducing the electrostatic BrÔnsted e †ects of the zeolite lattice. The Ðrst involves generating a specialised correction potential by Ðtting a non-periodic array of ca. 60 point charges to the di †erence between the bare cluster and periodic potentials. The second starts by Ðtting a periodic array of atomic charges to the potential of the inÐnite lattice and then builds up a classical cluster of ca. 2000 atoms into which the QM cluster is embedded. Such embedded cluster calculations, employing a T3 cluster, with electron correlation at the density functional theory level, are described, to model the interaction of water at a acid site. BrÔnsted Structures of the waterÈzeolite complex, and associated vibrational frequencies and 1H NMR shifts are calculated and compared with calculations of bare clusters of varying size and with experimental data. We then describe a mixed quantum mechanicalÈmolecular mechanical (QMÈMM) model derived by combining charges from the second model with a standard aluminosilicate force Ðeld. We report preliminary results on the e †ect of embedding on the energetics of a prototypical hydrocarbon cracking reaction ; the methyl-shift reaction of a propenium ion coordinated to the acid site.
Relationships between the structure of a zeolite and its adsorption properties
Surface Science, 1998
The adsorptmn of N 2 has been stu&ed m different Na-LSX Zeohtes, using an embedded cluster approach. Embedded catmn-molecule systems modeling sates II and sites III of Ideal LSX zeolltes (Si/A1 = 1 ) have been treated quantum chemically, using a method based on density functmnal theory. Two experlmental structures have been compared with two artificially modafied structures The adsorptaon strength of the caUomc sites is correlated with the values of the electrac field and electric field gradaent in the supercages © 1998 Elsevxer Science B V
Journal of Molecular Graphics and Modelling, 2006
The adsorption properties of pyridine on H-ZSM-5 zeolites have been investigated by cluster calculations with the ONIOM scheme and with an embedded-ONIOM scheme. The active site has been modeled with cluster sizes of up to 46 tetrahedra. Two different types of pyridine adsorption complexes on the zeolite models are found. If Zeolite is modeled by a small 3T quantum cluster, the adsorption energy of the hydrogen-bonded pyridine complex (Py-Hz), is found to be À18.5 kcal/mol. When a larger cluster or the ONIOM models are employed, the optimized geometries show the formation of pyrdinium cation [PyH + ] bound as an ion-pair complex [PyH + ][Z À ]. The calculated energy of formation for this ion-pair complex is À36.8 kcal/mol in the ONIOM (B3LYP/6-31G(d,p):UFF) model. Both values are considerably lower than the experimentally estimated heat of adsorption of pyridine in ZSM-5 zeolite of À47.6 kcal/mol. Inclusion of the electrostatic effects of the zeolite crystal lattice via the embedded ONIOM model increases the adsorption energy to À44.4 kcal/mol. Performing the quantum-chemical treatment at the MP2/6-31G(d,p) level instead of the B3LYP/6-31G(d,p) leads to a slightly lower adsorption energy to À45.9 kcal/mol. These data suggest that the embedded ONIOM scheme provides an accurate method of studying the interaction of small organic molecules with zeolites.
Modeling of Adsorption Properties of Zeolites: Correlation with the Structure
The Journal of Physical Chemistry B, 1997
The adsorption of N 2 and CO in Na X-zeolites has been studied for different framework structures and extraframework cation distributions. To this aim, the cation-molecule system modeling one site has been embedded in a set of external point charges which simulate the zeolite environment of the site and has been treated quantum chemically, using a method based on density functional theory. This procedure has been applied to the 64 cationic sites accessible for adsorption in a crystal unit cell of an ideal X-zeolite with a Si/Al ratio equal to 1. These calculations have shown that only a few cations are favorable for initial adsorption and that those cations are always of type III(III′). Their efficiency depends both on the framework geometry and on their location in the supercages. The analysis of the quantum chemical results in terms of a classical description involving electrostatic and induction interaction energies with the framework has led to the conclusion that the direction of the electric field vector created by the zeolite in the supercages is an important factor determining the zeolite adsorption properties.
The Journal of Physical Chemistry B, 1999
We present an embedded cluster approach for modeling interactions in zeolites and an application of this model to the study of NH 3 and NH 4 + adsorption in chabazite. This model utilizes the SCREEP (surface charge representation of the electrostatic embedding potential) formalism to include an accurate description of the Madelung potential in quantum mechanical calculations. The model is validated by comparison with previous cluster, embedded cluster, and periodic calculations on this system. The importance of including the Madelung potential and geometry relaxation in zeolite calculations is addressed. After considering the effects of electron correlation, basis set superposition error, and the zero-point energy, the model yields a heat of adsorption of-170 kJ/mol for NH 4 + in chabazite, in good agreement with experimental TPD data.
Hydrogen bond vs proton transfer in HZSM5 zeolite. A theoretical study
The journal of physical chemistry. B, 2005
The interaction of a large set of bases covering a wide range of the basicity scale with HZSM5 medium-size zeolites has been investigated through the use of two model clusters, namely 5T and 7T:63T. The 5T cluster has been treated fully ab initio at the B3LYP level, whereas the 63T cluster has been treated with the ONIOM2 scheme using the B3LYP:MNDO combination for geometry optimizations and B3LYP:HF/3-21G for adsorption energies. The optimized geometries of the different hydrogen bond (HB) and ion pair (IP) complexes obtained with both models are rather similar. However, there are significant dissimilarities as far as the adsorption energies are concerned, in particular when dealing with IP clusters whose intrinsic stability is largely underestimated when the simpler 5T model is used. 5T clusters could be used to obtain reasonable estimates of adsorption energies provided these are scaled by a factor of 1.1 for HB complexes and 1.4 for IP complexes. The zeolite cavity favors the pr...