No abstract

In its element: Zn(2+) at the M7 site of MFI-type zeolites activates H(2), via ZnH and OH species, and leads to Zn(0) species. The Zn(0) species returns to its original state, a Zn(2+) ion, upon evacuation of the zeolite at 873 K (see picture). The formation of the Zn(0) species is supported by DFT calculations.

Although a terminal oxyl species bound to certain metal ions is believed to be the intermediate for various oxidation reactions, such as O-O bond generation in photosystem II (PSII), such systems have not been characterized. Herein, we report a stable Zn -oxyl species induced by an MFI-type zeolite lattice and its reversible reactivity with O at room temperature. Its intriguing characteristics were confirmed by in situ spectroscopic studies in combination with quantum-chemical calculations, namely analyses of the vibronic Franck-Condon progressions and the ESR signal features of both Zn -oxyl and Zn -ozonide species formed during this reversible process. Molecular orbital analyses revealed that the reversible reaction between a Zn -oxyl species and an O molecule proceeds via a radical O-O coupling-decoupling mechanism; the unpaired electron of the oxyl species plays a pivotal role in the O-O bond generation process.

We investigate how nanospaces surrounded by a 10-membered ring of ZSM-5 zeolite affect the reaction intermediates formed during dioxygen activation by enclosed dicopper cations. Two types of dioxygen intermediates are considered: one is an O(2)...Cu(2) complex, where dioxygen binds to the two Cu cations, and the other is a bis(mu-oxo)dicopper complex converted from an O(2)...Cu(2) complex by the cleavage of the O-O bond. We employ large-scale density functional theory (DFT) calculations with the B3LYP functional to examine the energetics of the two dioxygen intermediates inside a 10-membered ring of ZSM-5 with double Si --> Al substitutions at variable locations. The properties of the O(2)...Cu(2) complexes, such as the dioxygen bridging modes and dioxygen activation, are strongly affected by the locations of the two Al atoms within the 10-membered ring. In particular, the O(2)...Cu(2) complexes have either end-on or side-on bridging modes depending on the substituted Al positions. On the other hand, the steric hindrances of a ZSM-5 cavity play crucial roles in determining the properties of the bis(mu-oxo)dicopper complexes containing a diamond Cu(2)O(2) core. By restricting its Cu(2)O(2) core to a 10-membered ring of ZSM-5 in which the two Al atoms are second-nearest neighbors, each Cu cation is tetrahedral four-coordinate. On the other hand, the Cu cations have almost square planar coordination inside a ZSM-5 where the Al atoms are fourth-nearest neighbors. The different Cu coordination environments are responsible for the different levels of stability; the planar diamond Cu(2)O(2) core is 30.7 kcal/mol more stable relative to the tetrahedral case. Since the ZSM-5 nanospaces directly influence the stability of the bis(mu-oxo)dicopper complexes by changing the Cu coordination environments, zeolite confinement effects on the bis(mu-oxo)dicopper complexes are more noticeable than those in the O(2)...Cu(2) cases. The DFT findings are important in terms of catalytic functions, because the spatial constraint from the ZSM-5 should significantly contribute to the stability of the reaction intermediates formed during the dioxygen activation.

We propose theoretically that the reactivity of O2-bound Cu-ZSM-5 toward methane is enhanced by the presence of one water molecule near a dinuclear copper site inside a 10-membered ring of the zeolite cavity. The current study employed density functional theory (DFT) calculations with the B3LYP functional to elucidate reaction intermediates during dioxygen activation by Cu-ZSM-5 in the presence of one water molecule attached to a dicopper site. The initial event is the formation of a hydroperoxo species bridged by the dicopper site via an H atom transfer from an attached water to the bound dioxygen. After the formation of the intermediate, the hydroperoxo O–O bond is completely cleaved to form radical oxygen containing intermediates, such as a Cu–O–Cu species bound by two OH groups (HO–Cu–O–Cu–OH), as well as a copper oxyl group containing intermediate (HO–Cu–OH–CuO). The radical oxygen containing intermediates can cleave a methane C–H bond in a homolytic fashion. Examining the barrier for the C–H bond activation obtained from DFT calculations, we found that the two types of intermediates have the power to more effectively cleave methane C–H bonds than the Cu–O–Cu intermediate that has been proposed to be formed in the absence of a water molecule. The current DFT findings propose that O2-bound Cu-ZSM-5 in the presence of one water molecule is a potential candidate for catalysts desired for methane to methanol conversion under mild conditions. Recently, techniques for controlling the number of water molecules near the active site of a ZSM-5 zeolite have been developed, and therefore the DFT findings should stimulate experimental efforts for constructing catalysts for direct methane hydroxylation.

We examined the state of sodium electrochemically inserted in HC prepared at 700-2000 C using solid state Na magic angle spinning (MAS) NMR and multiple quantum (MQ) MAS NMR. The 23 Na MAS NMR spectra of Na-inserted HC samples showed signals only in the range between +30 and À60 ppm. Each observed spectrum was ascribed to combinations of Na + ions from the electrolyte, reversible ionic Na components, irreversible Na components assigned to solid electrolyte interphase (SEI) or nonextractable sodium ions in HC, and decomposed Na compounds such as Na 2 CO 3 . No quasi-metallic sodium component was observed to be dissimilar to the case of Li inserted in HC. MQMAS NMR implies that heat treatment of HC higher than 1600 C decreases defect sites in the carbon structure. To elucidate the difference in cluster formation between Na and Li in HC, the condensation mechanism and stability of Na and Li atoms on a carbon layer were also studied using DFT calculation. Na 3 triangle clusters standing perpendicular to the carbon surface were obtained as a stable structure of Na, whereas Li 2 linear and Li 4 square clusters, all with Li atoms being attached directly to the surface, were estimated by optimization. Models of Na and Li storage in HC, based on the calculated cluster structures were proposed, which elucidate why the adequate heat treatment temperature of HC for high-capacity sodium storage is higher than the temperature for lithium storage. † Electronic supplementary information (ESI) available: XRD patterns of HC samples, wide range 23 Na NMR spectra, Na NMR spectra of some inorganic sodium compounds and NaPF 6 /PC solutions, charge/discharge curves of reassembled cells, and DFT optimizations of an alkali atom (Li or Na) set at the center of C 150 H 30 . See

In this work, we used both experimental and density functional theory (DFT) calculation methods to examine the mechanism of CH 4 activation taking place on the Zn 2+ ion exchanged MFI-type zeolite (ZnMFI). The heterolytic dissociation of CH 4 on ZnMFI around 300 K was observed experimentally, causing the appearance of IR bands at 3615, 2930, and 2892 cm −1 . The first band can be assigned to the OH stretching vibration associated with the formation of the Brønsted acid site and the latter to the C−H stretching modes ascribable to the −[ZnCH 3 ] + species. Combining the IR spectroscopy with a DFT calculation, it is apparent that the heterolytic C−H bond dissociation of CH 4 has an activation energy of 15 kJ mol −1 and takes place on a monomeric Zn 2+ at the M7S2 site. The M7S2 site has a specific Al arrangement in MFI and exhibits a pronounced reactivity for the H−H bond cleavage of H 2 , even at room temperature. In addition, to our knowledge, we are the first to succeed in explaining the dissociation process of CH 4 by applying natural bond orbital (NBO) and interaction localized orbital (ILO) analyses to the present system; the donation interaction from the CH 4 −σ(C−H) orbital to the Zn−4s orbital triggers the cleavage of the C−H bond of CH 4 under mild conditions.

Four possible isomers of the Ti2C80 metallofullerene are discussed in detail at the B3LYP DFT level of theory: two isomers in Ti2@C80 formula with two Ti atoms encapsulated inside a C80 cage and the other two in Ti2C2@C78 formula with a Ti2C2 cluster involved inside a C78 cage. In the encaged Ti2C2 cluster, there are end-on and side-on C2 bridging modes into the two Ti atoms. The optimized end-on cluster has a linear Ti-C-C-Ti array, whereas the side-on cluster has a butterfly-like structure where the two Ti atoms and the C2 unit do not lie in a plane. DFT calculations show that the Ti2C2@C78 molecule with the end-on Ti2C2 cluster is energetically most favorable in the four isomers. Stabilities of the Ti2C80 molecules are essentially dominated by Ti binding sites inside fullerene cages. The Ti atoms bind over the hexagon rings in preference to a junction between hexagon and pentagon rings. In the Ti2C2@C78 molecules, orbital interactions between the Ti2C2 cluster and the outer cage play a significant role in determining the C2 bridging modes into the dititanium center and their relative stabilities.