Molecular Tuning of Reactivity of Zeolite Protons in HZSM-5.

In acidic HZSM-5 zeolite, the reactivity of a methanol molecule interacting with the zeolite proton is amenable to modification via coadsorbing a stochiometric amount of an electron density donor E to form the [(E)(CH3OH)(HZ)] complex. The rate of the methanol in this complex undergoing dehydration to dimethyl ether was determined for a series of E with proton affinity (PA) ranging from 659 kJ mol-1 for C6F6 to 825 kJ mol-1 for C4H8O and was found to follow the expression: Ln(Rate) - Ln(RateN2) = ß(PA - PAN2)γ, where E = N2 is the reference and ß and γ are constants. This trend is probably due to the increased stability of the solvated proton in the [(E)(CH3OH)(HZ)] complex with increasing PA. Importantly, this is also observed in steady-state flow reactions when stoichiometric quantities of E are preadsorbed on the zeolite. As demonstrated with E being D2O, the effect on methanol reactivity diminishes when E is present in excess of the [(E)(CH3OH)(HZ)] complex. It is proposed that the methanol dehydration reaction involves [(E)(CH3OH)(CH3OH)(HZ)] as the transition state, which is supported by the isotopologue distribution of the initial dimethyl ether formed when a flow of CH3OH was passed over ZSM-5 containing one CD3OH per zeolite proton. The implication of this on the mechanism of catalytic methanol dehydration on HZSM-5 is discussed.

Supported Tetrahedral Oxo-Sn Catalyst: Single Site, Two Modes of Catalysis.

Mild calcination in ozone of a (POSS)-Sn-(POSS) complex grafted on silica generated a heterogenized catalyst that mostly retained the tetrahedral coordination of its homogeneous precursor, as evidenced by spectroscopic characterizations using EXAFS, NMR, UV-vis, and DRIFT. The Sn centers are accessible and uniform and can be quantified by stoichiometric pyridine poisoning. This Sn-catalyst is active in hydride transfer reactions as a typical solid Lewis acid. However, the Sn centers can also create Brønsted acidity with alcohol by binding the alcohol strongly as alkoxide and transferring the hydroxyl H to the neighboring Sn-O-Si bond. The resulting acidic silanol is active in epoxide ring opening and acetalization reactions.

Generating and stabilizing Co(I) in a nanocage environment.

A discrete nanocage of core-shell design, in which carboxylic acid groups were tethered to the core and silanol to the shell interior, was found to react with Co2(CO)8 to form and stabilize a Co(I)-CO species. The singular CO stretching band of this new Co species at 1958 cm(-1) and its magnetic susceptibility were consistent with Co(I) compounds. When exposed to O2, it transformed from an EPR inactive to an EPR active species indicative of oxidation of Co(I) to Co(II) with the formation of H2O2. It could be oxidized also by organoazide or water. Its residence in the nanocage interior was confirmed by size selectivity in the oxidation process and the fact that the entrapped Co species could not be accessed by an electrode.

Noncontact catalysis: Initiation of selective ethylbenzene oxidation by Au cluster-facilitated cyclooctene epoxidation.

Traditionally, a catalyst functions by direct interaction with reactants. In a new noncontact catalytic system (NCCS), an intermediate produced by one catalytic reaction serves as an intermediary to enable an independent reaction to proceed. An example is the selective oxidation of ethylbenzene, which could not occur in the presence of either solubilized Au nanoclusters or cyclooctene, but proceeded readily when both were present simultaneously. The Au-initiated selective epoxidation of cyclooctene generated cyclooctenyl peroxy and oxy radicals that served as intermediaries to initiate the ethylbenzene oxidation. This combined system effectively extended the catalytic effect of Au. The reaction mechanism was supported by reaction kinetics and spin trap experiments. NCCS enables parallel reactions to proceed without the constraints of stoichiometric relationships, offering new degrees of freedom in industrial hydrocarbon co-oxidation processes.

Striking confinement effect: AuCl4(-) binding to amines in a nanocage cavity.

Binding of AuCl(4)(-) to amine groups tethered to the interior of a 2 nm siloxane nanocage was determined in solutions containing various concentrations of acid. The mode of binding was inferred from EXAFS and UV-vis spectra to be by ligand exchange of amine for chloride, which implies that the amines remain unprotonated. Cyclic voltammetry confirmed that the Au complexes bind to the nanocage interior and established a 1:1 relationship between bound Au complex and amine groups. The results suggested a 5-7 pH unit shift in the protonation constant of the interior amines relative to free amines in solution.

Stable and solubilized active Au atom clusters for selective epoxidation of cis-cyclooctene with molecular oxygen.

The ability of Au catalysts to effect the challenging task of utilizing molecular oxygen for the selective epoxidation of cyclooctene is fascinating. Although supported nanometre-size Au particles are poorly active, here we show that solubilized atomic Au clusters, present in ng ml-1 concentrations and stabilized by ligands derived from the oxidized hydrocarbon products, are active. They can be formed from various Au sources. They generate initiators and propagators to trigger the onset of the auto-oxidation reaction with an apparent turnover frequency of 440 s-1, and continue to generate additional initiators throughout the auto-oxidation cycle without direct participation in the cycle. Spectroscopic characterization suggests that 7-8 atom clusters are effective catalytically. Extension of work based on these understandings leads to the demonstration that these Au clusters are also effective in selective oxidation of cyclohexene, and that solubilized Pt clusters are also capable of generating initiators for cyclooctene epoxidation.

Erratum: Stable and solubilized active Au atom clusters for selective epoxidation of cis-cyclooctene with molecular oxygen.

This corrects the article DOI: 10.1038/ncomms14881.

In situ transient FTIR and XANES studies of the evolution of surface species in CO oxidation on Au/TiO2.

The adsorption of CO and its reaction with oxygen were investigated using a combination of in situ Fourier transform infrared spectroscopy, step response measurements in a microreactor, (18)O isotopic labeling, and X-ray absorption near edge structure spectroscopy. An as-prepared sample in which Au is present as a surface oxyhydroxy complex does not adsorb CO. On an activated sample in which only metallic Au is detected, 0.18 +/- 0.03 mol CO/(mol Au) are adsorbed on Au at -60 degrees C, which shows an IR band at 2090 cm(-1). When oxygen is present in the gas phase, this species reacts with a turnover rate of 1.4 +/- 0.2 mol CO(mol Au min)(-1), which is close to the steady-state turnover rate. In contrast, there is a very small quantity of adsorbed oxygen on Au. A small IR peak at 1242 cm(-1) appears when an activated sample is exposed to CO. It reacts rapidly with oxygen and is shifted to 1236 cm(-1) if (18)O is used. It is assigned to the possible intermediate hydroxycarbonyl.

Activation of Au/TiO2 catalyst for CO oxidation.

Changes in a Au/TiO(2) catalyst during the activation process from an as-prepared state, consisting of supported AuO(x)(OH)(4-2x)(-) species, were monitored with X-ray absorption spectroscopy and FTIR spectroscopy, complemented with XPS, microcalorimetry, and TEM characterization. When the catalyst was activated with H(2) pulses at 298 K, there was an induction period when little changes were detected. This was followed by a period of increasing rate of reduction of Au(3+) to Au(0), before the reduction rate decreased until the sample was fully reduced. A similar trend in the activation process was observed if CO pulses at 273 K or a steady flow of CO at about 240 K was used to activate the sample. With both activation procedures, the CO oxidation activity of the catalyst at 195 K increased with the degree of reduction up to 70% reduction, and decreased slightly beyond 80% reduction. The results were consistent with metallic Au being necessary for catalytic activity.

Rational synthesis of asymmetric bicyclic siloxane.

A rational and versatile method to synthesize bicyclosiloxane of design structures is presented. The method is used to synthesize a new, asymmetric bicyclo[7.5.3]octasiloxane and other bicyclosiloxanes.

Stepwise synthesis of siloxane chains.

Siloxane chains of designated lengths can be synthesized with high yields by reacting tris(tert-butoxy)silanol alternately with dichlorosilane and silanediol.

Organosilicon platforms: bridging homogeneous, heterogeneous, and bioinspired catalysis.

Organosilicon compounds, in the form of cubic metallasiloxanes, cage-like silsesquioxanes, macromolecular nanocages, and flexible structures such as dendrimers and linear metallsiloxanes, have found useful applications as catalysts, ligands for metal complexes, and catalyst supports. Illustrative examples of these are presented. The well-defined structures of these compounds make them particularly suitable as molecular analogues of zeolites or silica-supported catalysts. A unique feature of many of these compounds is the presence of flexible siloxane bonds, which accommodate large fluctuations in the framework geometry, reminiscent of the adaptability of enzymes to conformational changes, and distinguish siloxane containing materials from carbon based synthetic materials. New preparative pathways and the use of the versatile silyl ester as a protection group have greatly expanded synthetic possibilities, pointing to the possibility of assembling these structures to form multifunctional catalytic structures. Some nanocage structures, with functionalities organized in close proximity, exhibit nanoconfinement effects.

Free-standing nitrogen-doped graphene paper as electrodes for high-performance lithium/dissolved polysulfide batteries.

Free-standing N-doped graphene papers (NGP), generated by pyrolysis of polydiallyldimethylammonium chloride, were successfully used as binder-free electrodes for the state-of-the-art Li/polysulfide-catholyte batteries. They exhibited high specific capacities of approximately 1000 mA h g(-1) (based on S) after 100 cycles and coulombic efficiencies great than 98%, significantly better than undoped graphene paper (GP). These NGP were characterized with XRD, X-ray photoelectron spectroscopy, thermogravimetric analysis, AFM, electron microscopy, and Raman and impedance spectroscopy before and after cycling. Spectroscopic evidence suggested stronger binding of sulfide to NGP relative to GP, and modelling results from DFT calculation, substantiated with experimental data, indicated that pyrrolic and pyridinic N atoms interacted more strongly with Li polysulfides than quaternary N atoms. Thus, more favorable partition of polysulfides between the electrode and the electrolyte and the corresponding effect on the morphology of the passivation layer were the causes of the beneficial effect of N doping.

Tetrahedral Sn-silsesquioxane: synthesis, characterization and catalysis.

A tetrahedral stannasilsesquioxane complex was synthesized as a racemic mixture using Sn(O(i)Pr)4 and silsesquioxanediol, and its structure was confirmed with X-ray crystallography, NMR, and EXAFS. The complex was a Lewis acid, and both anti and syn-binding with Lewis bases were possible with the formation of octahedral Sn complexes. It was also a Lewis acid catalyst active for epoxide ring opening and hydride transfer.

Spherosilicates with peripheral malonic acid and vinyl end groups.

Two novel spherosilicates comprised of an octahedral Si8O12 core, [Si8O12]-(OSiMe2CH2CH2CH2CH(COOH)2)8 and [Si8O12]-(OSiMe2CH2CH2CH2CH(COOSi(CH=CH2)3)2)8, were synthesized from [Si8O12]-(OSiMe2H)8. These new structures have high densities of peripheral functional groups, and the second structure also possesses silyl ester bonds that are easily cleavable under mild conditions. These functionalities enable these structures to be modified further and to have many potential applications. We demonstrated one by cross-linking the vinyl-terminated spherosilicate to form nanospheres with a narrow size distribution and utilizing the hydrophilic interior to accommodate a Pd salt in a toluene solution.

Metal nanoparticle catalysts decorated with metal oxide clusters.

Au nanoparticles decorated with mononuclear Ti-oxo units dispersed in silica clusters were formed by activating Au nanoparticles (~2 nm) stabilized with Ti- and amine-functionalized siloxane oligomers. These Au nanoparticles were active catalysts for selective oxidation of propane to acetone, and the activity increased with increasing Ti density.

Photothermal-assisted fabrication of iron fluoride-graphene composite paper cathodes for high-energy lithium-ion batteries.

A facile route that combines co-assembly and photothermal reduction was developed to synthesize free-standing, flexible FeF(3)-graphene papers. The papers contain well-dispersed FeF(3) nanoparticles and open diffusion channels in a porous, electrically conducting network of graphene sheets, and demonstrate promising applications as cathodes in high-energy density Li-ion batteries.

Coking- and sintering-resistant palladium catalysts achieved through atomic layer deposition.

We showed that alumina (Al(2)O(3)) overcoating of supported metal nanoparticles (NPs) effectively reduced deactivation by coking and sintering in high-temperature applications of heterogeneous catalysts. We overcoated palladium NPs with 45 layers of alumina through an atomic layer deposition (ALD) process that alternated exposures of the catalysts to trimethylaluminum and water at 200°C. When these catalysts were used for 1 hour in oxidative dehydrogenation of ethane to ethylene at 650°C, they were found by thermogravimetric analysis to contain less than 6% of the coke formed on the uncoated catalysts. Scanning transmission electron microscopy showed no visible morphology changes after reaction at 675°C for 28 hours. The yield of ethylene was improved on all ALD Al(2)O(3) overcoated Pd catalysts.

Flexible holey graphene paper electrodes with enhanced rate capability for energy storage applications.

The unique combination of high surface area, high electrical conductivity and robust mechanical integrity has attracted great interest in the use of graphene sheets for future electronics applications. Their potential applications for high-power energy storage devices, however, are restricted by the accessible volume, which may be only a fraction of the physical volume, a consequence of the compact geometry of the stack and the ion mobility. Here we demonstrated that remarkably enhanced power delivery can be realized in graphene papers for the use in Li-ion batteries by controlled generation of in-plane porosity via a mechanical cavitation-chemical oxidation approach. These flexible, holey graphene papers, created via facile microscopic engineering, possess abundant ion binding sites, enhanced ion diffusion kinetics, and excellent high-rate lithium-ion storage capabilities, and are suitable for high-performance energy storage devices.

Isotope labelling study of CO oxidation-assisted epoxidation of propene. Implications for oxygen activation on Au catalysts.

(18)O isotope labelling studies of the CO oxidation-assisted epoxidation of propene, catalyzed by a mixture of Au/TiO(2) and TS-1, using a methanol-H(2)O solvent showed the O in the epoxide was exclusively from O(2) and not H(2)O or methanol.