Formation and local structure of framework Al Lewis sites in beta zeolites.

Framework Al Lewis sites represent a substantial portion of active sites in H-BEA zeolite catalysts activated at low temperatures. We studied their nature by Al WURST-QCPMG nuclear magnetic resonance (NMR) and proposed a plausible mechanism of their formation based on periodic density functional theory calculations constrained by H MAS, Al WURST-QCPMG, and Si MAS NMR experiments and FTIR measurements. Our results show that the electron-pair acceptor of Al Lewis sites corresponds to an Al atom tricoordinated to the zeolite framework, which adsorbs a water molecule.

Dioxygen splitting at room temperature over distant binuclear transition metal centers in zeolites for direct oxidation of methane to methanol.

Here we demonstrate for the first time the splitting of dioxygen at RT over distant binuclear transition metal (M = Ni, Mn, and Co) centers stabilized in ferrierite zeolite. Cleaved dioxygen directly oxidized methane to methanol, which can be released without the aid of an effluent to the gas phase at RT.

Dioxygen dissociation over man-made system at room temperature to form the active α-oxygen for methane oxidation.

Activation of dioxygen attracts enormous attention due to its potential for utilization of methane and applications in other selective oxidation reactions. We report a cleavage of dioxygen at room temperature over distant binuclear Fe(II) species stabilized in an aluminosilicate matrix. A pair of formed distant α-oxygen species [i.

Tuning the Aluminum Distribution in Zeolites to Increase their Performance in Acid-Catalyzed Reactions.

The organization of Al atoms in the framework of Si-rich zeolites is very important and includes two classes: (i) the Al siting that determines which individual, crystallographically distinguishable framework T sites are occupied by Al atoms and (ii) the Al distribution, which describes the relation of two or more Al atoms in the framework, their distances, and the possibility of neighboring Al atoms to cooperate in the formation of active sites. The organization of Al significantly affects the catalytic properties of Si-rich, zeolite-based catalysts in acid and redox catalysis. Herein, what is known about the organization of Al in the framework of industrially very important pentasil-ring Si-rich zeolites (ZSM-5, beta zeolite, mordenite, ferrierite, MCM-22, and TNU-9), as well as the very promising SSZ-13 Si-rich zeolite with the CHA structure, is summarized.

TNU-9 Zeolite: Aluminum Distribution and Extra-Framework Sites of Divalent Cations.

The TNU-9 zeolite (TUN framework) is one of the most complex zeolites known. It represents a highly promising matrix for both acid and redox catalytic reactions. We present here a newly developed approach involving the use of Si and Al (3Q) MAS NMR spectroscopy, Co as probes monitored by UV/Vis and FTIR spectroscopy, and extensive periodic DFT calculations, including molecular dynamics, to investigating the aluminum distribution in the TUN framework and the location of aluminum pairs and divalent cations in extra-framework cationic positions.

Location of Framework Al Atoms in the Channels of ZSM-5: Effect of the (Hydrothermal) Synthesis.

(27) Al 3Q MAS NMR and UV/Vis spectroscopy with bare Co(II) ions as probes of Al pairs in the zeolite framework were employed to analyze the location of framework Al atoms in the channel system of zeolite ZSM-5. Furthermore, the effect of Na(+) ions together with tetrapropylammonium cation (TPA(+)) in the ZSM-5 synthesis gel on the location of Al in the channel system was investigated. Zeolites prepared using exclusively TPA(+) as a structure-directing agent (i.

Structure of framework aluminum Lewis sites and perturbed aluminum atoms in zeolites as determined by 27Al{1H} REDOR (3Q) MAS NMR spectroscopy and DFT/molecular mechanics.

Zeolites are highly important heterogeneous catalysts. Besides Brønsted SiOHAl acid sites, also framework AlFR Lewis acid sites are often found in their H-forms. The formation of AlFR Lewis sites in zeolites is a key issue regarding their selectivity in acid-catalyzed reactions.

Mechanism of framework oxygen exchange in Fe-zeolites: a combined DFT and mass spectrometry study.

The role of framework oxygen atoms in N(2)O decomposition [N(2)O(g)→N(2)(g) and 1/2O(2)(g)] over Fe-ferrierite is investigated employing a combined experimental (N(2)(18)O decomposition in batch experiments followed by mass spectroscopy measurements) and theoretical (density functional theory calculations) approach. The occurrence of the isotope exchange indicates that framework oxygen atoms are involved in the N(2)O decomposition catalyzed by Fe-ferrierite. Our study, using an Fe-ferrierite sample with iron exclusively present as Fe(II) cations accommodated in the cationic sites, shows that the mobility of framework oxygen atoms in the temperature range: 553 to 593 K is limited to the four framework oxygen atoms of the two AlO(4)(-) tetrahedra forming cationic sites that accomodate Fe(II).

Role of glutamate 64 in the activation of the prodrug 5-fluorocytosine by yeast cytosine deaminase.

Yeast cytosine deaminase (yCD) catalyzes the hydrolytic deamination of cytosine to uracil as well as the deamination of the prodrug 5-fluorocytosine (5FC) to the anticancer drug 5-fluorouracil. In this study, the role of Glu64 in the activation of the prodrug 5FC was investigated by site-directed mutagenesis, biochemical, nuclear magnetic resonance (NMR), and computational studies. Steady-state kinetics studies showed that the mutation of Glu64 causes a dramatic decrease in k(cat) and a dramatic increase in K(m), indicating Glu64 is important for both binding and catalysis in the activation of 5FC.

Aluminium siting in the ZSM-5 framework by combination of high resolution 27Al NMR and DFT/MM calculations.

The Al siting in the ZSM-5 zeolite was investigated by (27)Al 3Q MAS NMR spectroscopy and QM/MM calculations. It was found that the occupation of the framework T-sites by Al and the concentration of Al in these T-sites are neither random nor controlled by a simple rule. They both depend on the conditions of the zeolite synthesis.

Permanganate oxidation of alkenes. Substituent and solvent effects. Difficulties with MP2 calculations.

The permanganate oxidation of alkenes has been studied both experimentally and computationally. Transition state structures were located for the reaction of permanganate ion with a variety of monosubstituted alkenes at the B3LYP/6-311++G** level. Although the calculated activation energy for the reaction with ethene was reasonable, the calculated effect of substituents, based on the energies of the reactants, was much larger than that experimentally found.

A molecular dynamics exploration of the catalytic mechanism of yeast cytosine deaminase.

Yeast cytosine deaminase (yCD), a zinc metalloenzyme of significant biomedical interest, is investigated by a series of molecular dynamics simulations in its free form and complexed with its reactant (cytosine), product (uracil), several reaction intermediates, and an intermediate analogue. Quantum chemical calculations, used to construct a model for the catalytic Zn ion with its ligands (two cysteines, a histidine, and one water) show, by comparison with crystal structure data, that the cysteines are deprotonated and the histidine is monoprotonated. The simulations suggest that Glu64 plays a critical role in the catalysis by yCD.

CuNO2 and Cu(+)NO2 Revisited: A Comparative ab Initio and DFT Study.

We have reinvestigated CuNO2 and Cu(+)NO2 at ab initio as well as at pure and hybrid DFT levels of approximation employing large ANO basis sets. The systems were fully optimized using the CCSD(T), QCISD(T), BPW91, PBE, PBE0, and B3LYP methods. Several stationary points (minima and transition structures) were found on the related potential energy surfaces (PES).

Catalytic mechanism of yeast cytosine deaminase: an ONIOM computational study.

The complete path for the deamination reaction catalyzed by yeast cytosine deaminase (yCD), a zinc metalloenzyme of significant biomedical interest, has been investigated using the ONIOM method. Cytosine deamination proceeds via a sequential mechanism involving the protonation of N(3), the nucleophilic attack of C(4) by the zinc-coordinated hydroxide, and the cleavage of the C(4)-N(4) bond. The last step is the rate determining step for the generation of the zinc bound uracil.

A comprehensive investigation of the chemistry and basicity of a parent amidoruthenium complex.

trans-(DMPE)(2)Ru(H)(NH(2)) (1) dehydrogenates cyclohexadiene and 9,10-dihydroanthracene to yield benzene (or anthracene), (DMPE)(2)Ru(H)(2), and ammonia. Addition of fluorene to 1 results in the formation of the ion pair [trans-(DMPE)(2)Ru(H)(NH(3))(+)][A(-)] (A(-) = fluorenide, 4a). Complex 1 also reacts with weak acids A-H (A-H = phenylacetylene, 1,2-propadiene, phenylacetonitrile, 4-(alpha,alpha,alpha-trifluoromethyl)phenylacetonitrile, cyclobutanone, phenol, p-cresol, aniline) to form ammonia and trans-(DMPE)(2)Ru(H)(A) (7, 8, 9a, 9b, 10, 11b, 11c, 12, respectively).