Recombination Reactions and Diffusive Properties of Diatomic Molecules in Two Different Microporous Structures:  Silicalite and ZK4

Demontis, Pierfranco; Suffritti, and Giuseppe B.; Tilocca, Antonio

doi:10.1021/jp991449l

Cited by 9 publications

(5 citation statements)

References 60 publications

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“…We then proceeded with the study of a dissociation-recombination reaction in silicalite. [4][5][6] Such kind of processes, exploiting many of the specific properties of zeolites, result particularly suitable as a starting point to obtain comparative information on the catalytic activity of different zeolitic hosts. Radical processes are usually not activated, thus a statistically meaningful number of reactive trajectories may be easily generated and examined by dissociating a stable molecule in different initial conditions and following its subsequent dynamics.…”

Section: Computer Simulationsmentioning

confidence: 99%

Molecular dynamics simulation of an activated transfer reaction in zeolites

Demontis

Suffritti

Tilocca

1999

The Journal of Chemical Physics

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The activated transfer of a light particle between two heavier species in the micropores of silicalite and ZK4 zeolites has been studied through molecular dynamics ͑MD͒ simulations. A three-body potential controls the exchange of the light particle between the heavier ones; an effective barrier of a few k B T separates the two stable regions corresponding to symmetric ''reactant'' and ''product'' species. Harmonic forces always retain the reactants at favorable distances so that in principle only the energetic requirement must be fulfilled for the transfer to occur. The rate constant for the process ͑obtained from a correlation analysis of equilibrium MD trajectories͒ decreases by more than one order of magnitude when the barrier height is increased from 2k B T to 5k B T following an Arrhenius-type behavior. The transfer rates are always lower in ZK4. When the reaction is studied in a liquid solvent the calculated rate constants are closer to those obtained in silicalite. Since with this model the diffusive approach of the reactants is almost irrelevant on the reactive dynamics, only the different ability of each environment to transfer the appropriate energy amount to the reactants and then promote the barrier passage could be invoked to explain the observed behavior. We found that structural, rather than energetic, effects are mainly involved on this point. The lower efficiency of ZK4 seems to arise from the frequent trapping of the reactive complex in the narrow ZK4 windows in which the transfer is forbidden and from the weaker interaction of the reactive complex with the host framework compared to silicalite.

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Section: Computer Simulationsmentioning

confidence: 99%

Molecular dynamics simulation of an activated transfer reaction in zeolites

Demontis

Suffritti

Tilocca

1999

The Journal of Chemical Physics

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“…The size of the micropore or cavity plays an important role in the selectivity process. The widespread and diverse uses of zeolites are as catalysts and molecular sieves in the chemical industry and as ion exchangers, in particular as absorbents in detergents. − Diffusion phenomena, which are the basis of those applications, lie in the adsorption and the transportation processes. , Interest in the water−zeolite interaction arises from the fact that water plays strong and essential roles for both absorption and catalytic properties of zeolite, , as it is known that all natural zeolites are hydrated. In addition, water molecules facilitate the exchange of the charge-compensating cations, which are essential for the industrial catalysts.…”

Section: Introductionmentioning

confidence: 99%

Understanding the Movement, Encapsulation, and Energy Barrier of Water Molecule Diffusion into and in Silicalites Using Ab Initio Calculations

2001

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Quantum chemical calculations at the Hartree-Fock and MP2 levels have been performed to investigate water−silicalite interactions as well as the energy barrier and water orientations during diffusion into and in silicalite. Experimental geometries of water and silicalite have been used and kept constant throughout. The silicalite crystal structure has been represented by three fragments consisting of 20, 52, and 64 heavy atoms (oxygen and silicon atoms). Calculations have been performed using extended 6-31G and 6-31G* basis sets with BSSE (basis set superposition error) corrections. The results indicate obviously how the water molecule moves and turns via diffusion through the center of the silicalite pore in order to find the optimal route. The energy barriers for the water molecule to enter the pore and to diffuse from one channel to another have been clearly examined. The most stable binding site inside the pore is to be encapsulated in the intersection channel. It was also found that a water molecule enters and leaves the pores by pointing its dipole vector toward the center of the cavity.

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“…Compelling evidence exists that zeolite pore walls act as an efficient third body for halide radicals produced by photodissociation of I 2 , Cl 2 , and Br 2 in the zeolite pores. 22,23,52,53 However, Xu et al found no evidence for zeolites acting as a third body in the coupling of methyl radicals. 54 If collisions with the walls of the zeolite pores can push the rate of the H + OH + M reaction into the high pressure limit (as justified in the ESIw), then in the 200-400 1C temperature range, the pseudo-second-order rate constant for H + OH + M is larger than the rate constant for H + NO 2 by a factor of B1.7.…”

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confidence: 99%

“…The zeolite walls could potentially act as a third body/collision partner in association reactions and unimolecular decomposition reactions. For example, it has been shown that zeolite pore walls can act as a third body in radical recombination reactions, 22,23 and it has also been speculated that aerosols may act as a third body in the atmosphere. 24 Such effects could alter reaction mechanisms in the zeolite relative to the gas phase at low total pressures.…”

Section: Introductionmentioning

confidence: 99%

TPD of nitric acid from BaNa–Y: evidence that a nanoscale environment can alter a reaction mechanism

Savara

Danon

Sachtler

et al. 2009

Phys. Chem. Chem. Phys.

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The mechanism of temperature-programmed desorption (TPD) of nitric acid chemisorbed on BaNa-Y was studied over the temperature range from 200 to 400 degrees C, in the presence and absence of CO. Nitric acid dissociates to form H(+) and NO(3)(-) when chemisorbed on BaNa-Y. The results of these experiments are consistent with H(+) and NO(3)(-) either reacting directly to produce OH and NO(2) or recombining to produce HNO(3), which is desorbed and rapidly decomposes within the zeolite pores to OH and NO(2). The kinetics and stoichiometry suggest that the hydroxyl radicals produced react with CO and NO(2) to form CO(2) + H and NO + HO(2), respectively. The H atoms thus formed react with OH in preference to NO(2), a change in mechanism consistent with literature rate constants and the expectation that the zeolite pore walls act as a third body for the reaction of H with OH. Finally, OH may react with NO(2) to form HO(2), which can undergo further reactions to form O(2), H(2)O, and/or H(2). No reaction between CO and NO(3) or CO and surface-bound NO(3)(-) was observed.

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Recombination Reactions and Diffusive Properties of Diatomic Molecules in Two Different Microporous Structures: Silicalite and ZK4

Cited by 9 publications

References 60 publications

Molecular dynamics simulation of an activated transfer reaction in zeolites

Molecular dynamics simulation of an activated transfer reaction in zeolites

Understanding the Movement, Encapsulation, and Energy Barrier of Water Molecule Diffusion into and in Silicalites Using Ab Initio Calculations

TPD of nitric acid from BaNa–Y: evidence that a nanoscale environment can alter a reaction mechanism

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