The Role of Nanostructural Chemistry in the Design of Solid Catalysts

We report here the results of molecular dynamics and quantum chemical calculations on porous catalysts and supported metal catalysts to bring out the catalytic role played by nanostructures. We present three case studies where the computer simulation techniques have revealed the important structural aspects involved in the catalyst design. The role of exchanged metal cations in zeolite A for the molecular sieving of nitrogen and oxygen, controlled pore opening of hydrated VPI-5 by careful removal of water and the behavior of palladium metal particles supported over MgO are explained

Application of hard-soft acid-base principle to study bronsted acid sites in zeolite clusters: a quantum chemical study

The effect of isomorphous substitution of Si by Al, B, and Ga in the tetrahedral sites of a zeolite framework on the properties of Bronsted acid sites is investigated by computational techniques. We used the density functional theory (DFT) based acidity descriptor, "local softness”, and a new acidity descriptor, "relative electrophilicity"to predict the acidity sequence of the zeolite clusters. The finite difference approximation is used to calculate local softness. The relative electrophilicity is found to be a suitable descriptor of acidity of zeolite clusters. The trend of acidity derived from local softness values depends on several factors such as basis sets, geometry of zeolite clusters, and different population analyses used to calculate them. The acidity trend derived from the relative electrophilicity follows the experimental trend, and it is not affected by the choice of basis sets or cluster models

Quantum chemical calculations on the stability of different conformations of silicate building block structures in relevance to zeolite synthesis

The relative influence of various geometric parameters such as bond lengths, bond angles and dihedral angles on the energetics of silicate structures is studied. The influence of dihedral angles, in particular, on the preferred conformation and energy of silicate building blocks is highlighted. The possibility of formation of ring structures and the diameters of the rings will depend on these Si-O-Si-O and O-Si-O-Si dihedral angles. Different conformations were obtained for a dimer cluster model by varying the two types of dihedral angles and their relative stability was studied using quantum chemical calculations. The study of variation of the dihedral angles on the stability of silicate structures is further extended to larger cluster models. The importance of dihedral angles is discussed in the context of formation of silicate building blocks during the synthesis of zeolites. The combinations of Si-O-Si-O and O-Si-O-Si dihedral angles, such as 0° and 60°, 120° and 300° etc., leading to staggered adjacent SiO4 groups are energetically preferred. The energetic preference of the staggered structures over eclipsed structures is an order of magnitude higher for pentamers cluster models than for dimers cluster models

Molecular modeling studies on zeolite catalysts for shape-selective electrophilic substitution: I acylation of 2-methoxynaphthalene

The diffusion characteristics of acylated 2-methoxynaphthalene inside large pore zeolites were investigated. The interaction of the three isomers with fully siliceous zeolite lattices was studied by energy-minimization calculations. The favorable adsorption sites and the orientation of the acylated products of 2-methoxynaphthalene were analyzed in detail. Three large pore zeolites having 12 m channels were selected: (i) mordenite with an elliptical 1-d channel, (ii) zeolite-L with a circular channel and 2-d cages, and (iii) zeolite-β with circular 3-d channel systems. It was observed that the shape selectivity properties of the zeolites could be profitably used to produce 2-acyl-6-methoxynaphthalene. In the case of mordenite, the diffusion of all three isomers is facile, whereas in the case of zeolite-L, the diffusion of 1-acyl-7-methoxynaphthalene is more facile than 2-acyl-6-methoxynaphthalene and 1-acyl-2-methoxynaphthalene. In the case of zeolite-β, the energy barrier for the diffusion of 2-acyl-6-methoxynaphthalene is significantly smaller than those of the other two isomers. Thus zeolite-β is predicted to be a suitable catalyst for the shape-selective acylation of 2-methoxynaphthalene to 2-acyl-6-methoxynaphthalene

Molecular modeling studies on zeolite catalysts for shape-selective electrophilic substitution: formation of xylenes

Energy minimization methodology has been used to study the interaction of isomers of xylene with ZSM-5, mordenite and MCM-22 zeolites. Adsorption sites and the mechanism of diffusion of the reactant molecules in the alkylation of toluene and xylene isomers inside these zeolites were studied. When molecules diffuse through 10-member and 12-member ring channels, they pass through favorable and unfavorable adsorption sites. ZSM-5 shows significant selectivity for p-xylene over the o- and m-xylenes. In large pore mordenite, the difference between favorable and unfavorable adsorption sites is not significant for xylene isomers. In the case of MCM-22, there are significant energy barriers, and the diffusivity is not high in spite of the presence of large cages. Results of this study emphasize the importance of the shape and the size of molecules as well as the pore dimensions and architecture of zeolites in effectively controlling molecular diffusion characteristics. The diffusion characteristics of the molecules inside a zeolite are sensitive to its pore architecture. The results offer the reasons based on molecular level interactions, for the experimentally observed shape selectivities in the above three zeolites

The local hard-soft acid-base principle: a critical study

The recently proposed local hard-soft acid-base principle characterizes the reactive centers of two systems on the basis of equal Fukui functions or/and equal local softnesses. We make a quantum chemical study of this principle using ab initio and density functional calculations in cases where the global softnesses of the reacting systems are different. We consider reactions of a dimer cluster model of faujasite X-type zeolite with the probe molecules CO, NH3, and H2O

Influence of cation exchange on M-Pt-ETS-10 molecular sieve: correlation between ab initio results, catalytic activity, and physicochemical investigations

The electronic property of Pt supported on cation-exchanged Pt-M-ETS-10 [where M = Li, Na, K, Rb, Cs, Mg(OH), Ca(OH), Sr(OH), and Ba(OH) ions] depends on the location of Pt and the nature of the exchanged metal ion. Electronic changes on the Pt cluster are highly influenced when it is near TiO6 rather than SiO4 units. The benzene selectivity in the transformation of n-hexane over Pt-M-ETS-10 molecular sieves is found to correlate with the basicity of the exchanged cations and the average electron density of Pt. The relationship between electron density on Pt and the amount of CO2 adsorbed (from TPD) and the frequency of the ν3 band of CO2 adsorbed in FTIR is established. We report here the results of ab initio Hartree-Fock calculations on model clusters representing the Pt (active) site, molecular sieve with cations, Pt-M-molecular sieve, benzene, and H2S adsorbed over Pt-molecular sieve. Generally, the charge density on Pt decreases in the order Cs > Rb > K > Ba(OH) > Na > Sr(OH) > Ca(OH) > Li > Mg(OH) and indicates that the electron transfer is from the support to the Pt. Catalytic activity of Pt-M-ETS-10 corroborates well and explains the experimentally observed higher activity as compared to commercial Pt-Al2O3 catalyst. The larger benzene yield in the case of Pt supported over basic zeolites could be attributed to the ease of desorption of benzene, and it is supported by a decreasing binding energy of benzene from Li to Cs and Mg(OH) to Ba(OH) in Pt-M-ETS-10. The electron density on Pt decreases drastically in the presence of sulfur, in the order Cs < Rb < K < Na < Li and Ba(OH) < Sr(OH) < Ca(OH) < Mg(OH)