Chemical vapor deposition of trimethylaluminium on dealuminated faujasite zeolite

Chemical vapor deposition of trimethylaluminum (TMA) was explored as an approach for the preparation of model faujasite-type catalysts containing extraframework aluminum. The decomposition of the grafted organoaluminum species was investigated in hydrogen and oxygen atmosphere. The process of grafting Al-containing species and the associated changes of the zeolite hydroxyl groups were followed by in situ FTIR spectroscopy. The state of intrazeolite Al atoms, the changes in zeolite structure and acidity caused by the CVD procedure as well as by subsequent treatment were analyzed in detail by 1H, 29Si and 27Al MAS NMR, COads IR, H/D exchange of acidic hydroxyl groups with perdeuterobenzene and propane cracking. Reaction of an extraframework aluminum-free high-silica faujasite zeolite with TMA leads to nearly complete substitution of the bridging hydroxyl groups with Al species. The reaction, however, does not produce uniform homogeneously distributed species. Because of the high reactivity of TMA, the zeolite lattice is partially decomposed resulting in its partial dealumination and formation of stable Si-CH3 moieties. The exact conditions of post-CVD treatment influence strongly the chemical and catalytic properties of the zeolites. The strongest increase of the propane conversion rate was observed when grafted TMA species were decomposed in H2 at high temperature. Such zeolite displays much higher activity per Brønsted acid site in propane cracking than a commercial ultrastabilized Y zeolite. It is proposed that the activity enhancement is related to strong polarization of a fraction of the zeolite Brønsted acid sites by Lewis acid sites formed by the hydrogenolysis of grafted TMA complexes

Lightweight hydrogen-storage material Mg0.65Sc0.35D2 studied with 2H and 2H–{45Sc} MAS NMR exchange spectroscopy

Using double-quantum 2H MAS NMR with 45Sc recoupling and Bloch–Siegert compensated 2H–{45Sc} TRAPDOR we have identified the overlapping NMR signals of deuterium with and without scandium neighbors in Mg0.65Sc0.35D2, a candidate lightweight material for hydrogen storage. At room temperature we also observe a third type of mobile deuterium. Deuterium mobility among the three NMR-distinct sites has been investigated by means of one-and two-dimensional exchange spectroscopy (Exsy). Complete deuterium exchange within 0.1 s is observed, which indicates that the three NMR-distinct sites are close together in the crystal lattice. The weak temperature- and MAS-rate dependences observed in Exsy are indicative for a combination of chemical exchange and spin diffusion

Coherent Cross-Polarization Theory for a Spin-½ Coupled to a General Object

Zero-order average-Hamiltonian theory is used to extend the product-operator description of coherent spin–spin cross-polarization to the case of a spin-½ coupled to a general object, like a molecular rotor or a quantum oscillator. The object, which is not necessarily in a Boltzmann equilibrium state, is assumed to have no interaction with the lattice and no internal relaxation capacity. The Bloch–Wangsness–Redfield (BWR) theory for incoherent processes like spin–lattice relaxation does not apply for such an isolated spin– object pair. Nevertheless spectral density at the Larmor frequency, of key importance in BWR theory, also plays a central role in object-induced spin polarization. Spectral density in our theory is represented by quantum operators J2 and J1. If J2 and J1 do not commute, the spin– object coupling may cause spin polarization in an initially saturated spin system. This represents a coherent mechanism for spin cooling, which in specific cases may lead to enhanced spin polarization above the thermal equilibrium value. A master equation is derived for general spin– object crosspolarization, and applied to the case of a spin pair inside a uniaxial rotor, and a spin coupled to a microelectronic LC circuit

Brownian motion in a deformable medium

Diffusion in a deformable medium shows a remarkable transition at the onset of concerted particle-medium motions. This is derived for Brownian motion in a periodic potential coupled to a broad phonon spectrum, and obsd. for rotation of org. mols. inside zeolite cavities. Interactions between the particle and local overdamped phonons in the potential wells cause residence-time dependent lattice deformations, which, in our model, decrease the energy barriers. The model explains the non-Arrhenius type of rotational motion of pentane inside zeolite ZK-5 obsd. with 1H-13C NMR relaxometry

Glucose dehydration into HMF by Lewis acidic metal chlorides : development of a hetero-geneous MeCIx/ionic liquid/SBA-15 catalyst

The efficient utilization of cellulosic biomass is limited by the selective conversion of fructose, the dominant sugar in cellulose, to 5-Hydroxymethylfurfural (HMF). HMF is a platform chemical for the production ofuseful intermediates. It can be formed in homogeneous metal chloride/ionic liquid systems. In industrial scale processes a cheaper solvent for glucose delivery is required and heterogeneous catalysts have clear advantages. Therefore we developed a heterogeneous catalyst by successfully grafting the ionic liquid to the surface of ordered mesoporous SBA-15 and doping this catalyst with metal chlorides. The catalysts were characterized by XRD, N2 physisorption, TEM, 29Si CP-MAS NMR. Besides, EXAFS measurements were performed for metal chloride ionic liquid system and the heterogeneous catalyst. The Cr/IL/SBA- 15 catalyst showed 46% glucose conversion and 22% HMF yield at 150 °C after 3 h of reaction. Although the HMF yields are somewhat lower than the homogeneous case, the system is promising, especially when the replacement of the ionic liquid solvent with water is considered

Glucose dehydration into HMF by Lewis acidic metal chlorides : development of a hetero-geneous MeCIx/ionic liquid/SBA-15 catalyst

The efficient utilization of cellulosic biomass is limited by the selective conversion of fructose, the dominant sugar in cellulose, to 5-Hydroxymethylfurfural (HMF). HMF is a platform chemical for the production ofuseful intermediates. It can be formed in homogeneous metal chloride/ionic liquid systems. In industrial scale processes a cheaper solvent for glucose delivery is required and heterogeneous catalysts have clear advantages. Therefore we developed a heterogeneous catalyst by successfully grafting the ionic liquid to the surface of ordered mesoporous SBA-15 and doping this catalyst with metal chlorides. The catalysts were characterized by XRD, N2 physisorption, TEM, 29Si CP-MAS NMR. Besides, EXAFS measurements were performed for metal chloride ionic liquid system and the heterogeneous catalyst. The Cr/IL/SBA- 15 catalyst showed 46% glucose conversion and 22% HMF yield at 150 °C after 3 h of reaction. Although the HMF yields are somewhat lower than the homogeneous case, the system is promising, especially when the replacement of the ionic liquid solvent with water is considered

A Comparative Study on Gels and Clathrates of Syndiotactic Polystyrene; Solvent Mobility in Polymer-Solvent Compounds

Gels of syndiotactic polystyrene with different solvents have been compared to clathrates. Wide-angle X-ray diffraction results using toluene (good solvent for sPS) and decalin (relatively poor solvent for sPS) show that the structure of the crystalline junctions of the gels is similar to that of the clathrate d-phase. A difference can be found in the width of the (010) reflection, which is, relative to the width of the (10) reflection, much broader for the gel than for the clathrate. This is caused by the difference in mechanism involved for crystal formation in gels and clathrates. Experiments performed on quenched samples of syndiotactic polystyrene with the monomer benzyl methacrylate show that also for this gel the structure of the crystalline part is similar to that of the clathrate phase. This means that solvent is present in both the crystalline and the amorphous part of the gel. By solid-state NMR studies, a clear difference in mobility of solvent molecules in the crystalline and amorphous parts of the gel has been observe

Organo-bridged silsesquioxane titanates for heterogeneous catalytic epoxidation with aqueous hydrogen peroxide

Organo-bridged silsesquioxane titanates for heterogeneous catalytic epoxidation with aqueous hydrogen peroxide were synthesized through the acid-catalyzed hydrolysis and co-condensation of organotrialkoxysilane monomers and a,¿-bis(trialkoxysilyl) alkane cross-linkers in ethanol–water solution, with subsequent titanium grafting. These hybrid catalysts show high activity and selectivity in the epoxidation of cyclooctene with both aqueous hydrogen peroxide and tert-butyl hydroperoxide (TBHP) as oxidants, although they are nonporous. The catalysts can be reused at least six times with no loss of activity with TBHP as the oxidant and at least four times (although with gradual activity loss) with aqueous H2O2 as the oxidant. Furthermore, the catalytic activities of these hybrid titanates relate to the nature and the composition of the starting materials used. The epoxidation activity of the catalysts using aqueous H2O2 as the oxidant seems to increase with increasing carbon length in the starting organosilane cross-linkers. UV–vis DRS and XPS measurements indicate that tetrahedrally coordinated Ti species are the active centers

Influence of nickel and silicon addition on the deuterium siting and mobility in fcc Mg-Ti hydride studied with 2H MAS NMR.

Fluorite-structured Mg–Ti hydrides are interesting for hydrogen storage applications because of their high gravimetric hydrogen storage capacity, and improved (de)hydrogenation kinetics compared to MgH2. In the present study we have investigated the potential catalytic effect of Ni and Si as third element on the siting and mobility of electrochemically loaded deuterium in ball-milled Mg0.63Ti0.27Ni0.10 and Mg0.63Ti0.27Si0.10 alloys. Magic angle spinning (MAS) 2H NMR reveals that Ni and Si induce new types of deuterium sites in addition to the Mg-rich and Ti-rich sites already present in Mg0.65Ti0.35D1.2. 2D exchange NMR spectroscopy shows a substantial deuterium exchange between the various types of sites, which reflects their close interconnectivity in the crystal structure. Furthermore, the time scale and temperature dependence of the deuterium mobility have been quantified by 1D exchange NMR. The obtained effective residence times for deuterium atoms in the Mg-rich and Ti-rich nanodomains in Mg0.65Ti0.35D1.2, Mg0.63Ti0.27Ni0.10D1.3, and Mg0.63Ti0.27Si0.10D1.1 at 300 K are 0.4, 0.3, and 0.8 s, respectively, and the respective apparent activation energies 17, 21, and 27 kJ mol–1. The addition of Ni promotes deuterium mobility inside Mg–Ti hydrides, which is in agreement with the observed catalytic effect of Ni on the electrochemical (de)hydrogenation of these materials

Heterogeneous distribution of entanglements in the polymer melt and its influence on crystallization

Recently we demonstrated that it is possible to obtain single chains forming single crystals, where chains are adjacently re-entrant. It is feasible to melt these crystals, either by simple consecutive detachment of chain stems from the crystalline substrate or by cluster melting, where several chain stems are involved. The consecutive detachment of chain stems occurs at the melting point predicted from the Gibbs-Thomson equation, whereas the cluster melting takes place at much higher temperatures. Melting by consecutive detachment of chain stems from the crystal substrate and their diffusion in the melt ultimately results in a new melt state having a heterogeneous distribution of physical entanglements. Because of differences in the transverse relaxation times of the chains in the entangled and disentangled domains, solid-state NMR is the technique used to follow differences in the molecular mobility of the two domains. In this paper, with the help of solid-state NMR, we follow the mechanism involved in the development of the heterogeneous melt state. Observations are that the entangled and disentangled domains are maintained at higher temperatures resulting into a thermodynamically nonequilibrium melt state. On cooling, the heterogeneous melt influence of entanglements on the initial stages of crystallization is followed. It is found that the disentangled chains segments crystallize faster than the entangled chains, which is suggestive for the homogeneous nucleation to occur faster than the heterogeneous nucleation. Rheological studies are performed to follow the influence of disentangled domains on crystallization. With the increasing number of entanglements per unit chain, the time required for the onset of crystallization increases. In the sample from the same batch having lesser number of entanglements per unit chain, the crystallization time can be reduced by a decade