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Solvent effects on adsorption equilibrium and reaction kinetics are evaluated for hydrogenation reactions catalyzed by Pd/alumina in a series of different solvents. Three reactants – cyclohexene, benzene and benzaldehyde – and three solvents – n-heptane (HEP), methylcyclohexane (MCHA), decalin (DL) – have been investigated. Kinetic analysis of hydrogenation of cyclohexene reveals that hydrogen adsorbs on different sites from those where cyclohexene and the solvents adsorb; however, the presence of hydrogen on these separate sites affects the heats of adsorption of the hydrocarbons. When the solvent is a weakly interacting linear alkane (HEP), the rate determining step of the reaction is the first hydrogenation of adsorbed cyclohexene. This conclusion is supported by DFT calculations that show a higher enthalpy barrier for the first hydrogenation than for the second, while statistical thermodynamics analysis validates the physical significance of the entropy of adsorption parameters derived from the kinetic fitting of experimental data. By contrast, with solvents such as MCHA and DL, which interact more strongly with the metal surface and compete for active sites with the reactant and the surface intermediate, the rate limiting step seems to shift to the second hydrogenation step. One of the main conclusions of the study is that the effects observed with non-polar solvents are mostly due to competitive adsorption of the solvent, which causes a decrease in surface coverage of reactants and surface intermediates. Indeed, under identical conditions, the hydrogenation rates of C C bonds in cyclohexene (or the aromatic ring in benzene) follow the order HEP > MCHA > DL, which is consistent with the strongest solvent/metal interaction for DL and the weakest for HEP. Further, binary solvent mixtures of DL-HEP exhibit non-idealities that are clearly evident in the fitting of hydrogenation rates. It is proposed that the higher activity coefficient of liquid DL in the liquid mixture enhances its surface coverage, competing more effectively with the reactant, which causes a decrease in catalytic activity larger than that predicted assuming an ideal mixture of solvents. Likewise, large drops in benzene hydrogenation rates are observed in water-organic co-solvent mixtures due to the competitive adsorption of water on the Pd surface. Water adsorption is further enhanced by the non-ideality of the water-organic mixture. By contrast, in the hydrogenation of the C O bond of benzaldehyde, the presence of water provides a favorable alternative reaction path with lower activation barrier, resulting in higher hydrogenation activity when the reactant is a carbonyl-containing molecule, such as benzaldehyde. In this system, the H-bonded water network assists shuttling a proton from the Pd surface to the hydrophilic O atom in C O, assisted by surface charge separation. Since this H transfer pathway cannot take place with C C bonds or aromatic rings, only site inhibition occurs for these reactions.

The catalytically reactive defects act a pivotal role in enhancing the performances toward structure-sensitive reactions, such as benzene semi-hydrogenation (BSH). Herein, massive Ti3+ defects, recognized by multiple spectroscopic technics, were effectively implanted into brookite TiO2 nanosheets (BTNSs) enclosed by {1 0 1}, {2 0 1}, and {2 1 0} facets. The content of these defects, expressed by Ti3+: Ti4+ molar ratio, increased from 0.44 to 1.19 when the hydrothermal time of BTNSs prolonged from 1 to 12 h, and then decreased to 1.04 at 48 h. Benefited by such an integration of the unique microscopic architecture and atomic-scale defects, the BTNSs supported nanoparticulate Ru catalysts exhibited an increased metal size but a decreased acid amount accompanying with the extension of hydrothermal time, which synergistically determined that the Ru/BTNSs-2 catalyst with favorable Ru size and acid amount occupied the highest turnover frequency (TOF) of benzene (127.5 min−1). Meanwhile, the Ti3+ defects provided both improved net rate of cyclohexene generation and enhanced suppression of cyclohexene adsorption, resulting in a positive linearity between the initial cyclohexene selectivity (S0) and Ti3+: Ti4+ molar ratio over the Ru/BTNSs catalysts. A maximum S0 of 90.3% was achieved on the Ru/BTNSs-12 catalyst with most Ti3+ defects, outperforming the previous TiO2-supported Ru catalysts.

Alumina-supported VOx-CeO2 materials are synthesized by atomic layer deposition (ALD) and evaluated in cyclohexane oxidative dehydrogenation (ODH). The order of deposition of the two metal oxides by ALD influences the size, geometry and oxidation state of the resulting surface species as indicated by Raman spectroscopy coupled with density functional theory (DFT) calculations and XPS. The individual contributions of VOx and CeO2 surface sites to the cyclohexane ODH reaction mechanism are investigated through a comparison of the catalytic activity of the supported mixed metal oxide materials, VOx/Al2O3, VOx/CeO2 and a bare CeO2 support. The highest activity is observed for CeO2, and different surface active sites are distinguished by introducing cyclohexane and cyclohexene in the reactant feed mixture. The catalytic behavior is dependent on the nature of V O S (S, Support) bonds, and small clusters of CeO2 on Al2O3 favor total oxidation, similar to exposed CeO2 sites in VOx/CeO2.

Zeolite HBEA catalyzes hydrogenation of aromatic hydrocarbons—methyl-substituted benzenes (benzene and toluene), alkenyl-substituted benzenes (styrene), and polycyclics (naphthalene)—in presence of excess H2 at high-temperatures (573–748 K) with rates that depend linearly on aromatic and H2 pressures. The observed kinetic behavior can be rationalized based on a sequence of elementary steps where the first hydrogenation step of the adsorbed benzenic intermediate is rate-determining while subsequent hydrogenation and desorption steps are quasi-equilibrated and H + is the most abundant surface species. Styrene hydrogenation exhibits the highest rates among the aromatics considered and results exclusively in ethylbenzene synthesis; in contrast, benzene/toluene and naphthalene hydrogenation results in formation of their triply-hydrogenated five-membered ring and doubly-hydrogenated ring open analogs, respectively. Based on independent studies involving co-reaction of cyclohexene and 1-methyl-1-cyclopentene with H2, we infer their facile interconversion and hydrogenation to methylcyclopentane implying that conversion of benzene to methylcyclopentane likely occurs via intervening formation of both five- and six-membered ring intermediates. Taken together, these studies demonstrate feasibility of aromatics hydrogenation and propensity of benzenic rings in these hydrocarbons to undergo ring reduction or ring opening during their activation with H2 on Bronsted acid zeolites.

The organic products of the reaction between Cr(VI)/silica and ethylene, propene, 2-pentene, 1-hexene, or cyclohexene at temperatures between 25 and 150 °C were investigated to understand the formation of chromium sites active for polymerization. A wide assortment of products were obtained, either directly or through ligand displacement or extraction, which depended on the specific olefin and the conditions. Hydrocarbon products were released under conditions relevant to commercial operation and indicated both polymerization and acid catalysis: short olefin oligomers (from polymerization), alkanes (possibly from hydride transfer), internal olefins (from double bond shift) and isoalkylbenzenes (from skeletal isomerization and alkylation of the diluent benzene). Aldehydes, ketones, and alcohols were found in polar organic extraction media, some the result of aldol condensations in the extraction medium. Surface carboxylates, evident by in situ IR spectroscopy, could be detached from the catalyst only through hydrolysis. The high threshold to desorb oxygenates implies that the chromium sites retain self-generated ligands. The nature of these ligands depends on the reductant, and, consequently, the result may be an active or an inactive site. Consistently, reduction of Cr(VI)/silica by various olefins resulted in catalysts with different polymerization behavior, and addition of external ligands to Cr(II)/silica led to a poisoned or to an active catalyst, depending on the ligand.

We report a facile hydrothermal synthesis of nanorod-structured molybdenum-tungsten oxides as an efficient catalyst for olefin hydrogenation. The catalytic activity was demonstrated through hydrogenation of cyclohexene used as a model system. The as-prepared material was confirmed as a new phase (Mo0.6W0.4O3) via a series of characterizations, instead of a simple mixture of MoO3 and WO3. The Mo0.6W0.4O3 catalyst displays excellent catalytic activity with 100% conversion of cyclohexene in 3 h, which is much higher than that of pure MoO3 and pure WO3 synthesized with the same method. It was found that hydrogen bronze (HxMo0.6W0.4O3) was formed during the hydrogenation process, and the catalytic performance was positively correlated with the hydrogen content. The obtained nanorod-structured Mo0.6W0.4O3 without loading any noble metal exhibits much higher catalytic activity than a Pd/MoO3 catalyst. The results demonstrate that the nanorod-structured molybdenum-tungsten oxide may be a strong contender for serving as an efficient hydrogenation catalyst alternative to precious metals.

Metal Organic Frameworks (MOFs) have been explored widely to create heterogeneity in a catalytic system. Catalytic applications require the use of stable catalysts with no leaching and easy recovery. In this regard, Pt@MIL-101 is embedded in a poly-є-caprolactone (PCL) matrix by means of electrospinning to create a “catalytic carpet” which is highly efficient and can be recovered within seconds after catalysis. The obtained composite material is completely intact with a homogeneous distribution of the Pt@MIL-101 throughout the electrospun PCL fiber matrix. The catalytic carpet was examined in the hydrogenation of cyclohexene and full conversion was obtained in just 90 min. Remarkably, a very large fraction (>65%) of the total Pt-atoms participate in the reaction. Reusability tests showed that the material could be recycled for at least 4 runs without detectable Cr and Pt leaching displaying the durability of the catalytic carpets. Complete recovery is achieved with zero weight loss while fully preserving the crystalline structure.

Niobium-containing mesoporous silicates reveal superior activity and selectivity in epoxidation of alkenes using hydrogen peroxide as a green oxidant and, in contrast to mesoporous titanium silicates, catalyze epoxidation of both electron-rich and electron-deficient C C bonds. This report describes a kinetic and mechanistic investigation of epoxidation of two representative substrates, cyclooctene (CyO) and 2-methyl-1,4-naphthoquinone (MNQ), over two mesoporous niobium silicates with predominantly di(oligo)meric or isolated Nb(V) sites. The observed kinetic regularities did not depend on the state of Nb but were strictly determined by the nature of the organic substrate. The rate law established for CyO is consistent with a mechanism that involves interaction of H2O2 with Nb(V) sites to give a hydroperoxo species NbOOH and water, followed by oxygen transfer to a nucleophilic C C bond, producing an epoxide and regenerating the initial state of the catalyst. This mechanism is strongly supported by stereospecificity in epoxidation of cis-alkenes and high heterolytic pathway selectivity in the oxidation of cyclohexene. The NbOOH species is manifested by an absorption feature at 307 nm in diffuse reflectance UV–vis spectra. The addition of a base (NaOAc) causes a shift of the absorption band to 293 nm and produces a rate-retarding effect on the epoxidation reaction. Several lines of evidence, including zero reaction order in MNQ, rate-accelerating effect of base, detection of acetamide in the reaction mixture, negligible reaction rate in ethyl acetate, and recognition of weak basic sites in the niobium silicates using infrared spectroscopy of adsorbed CDCl3, all indicate that MNQ epoxidation proceeds by another mechanism that involves rate-limiting oxidation of the solvent molecule (MeCN) to generate peroxycarboximidic acid, which reacts with electron-deficient C C bonds, producing an epoxy derivative and acetamide.

The selective epoxidation of olefins with hydrogen peroxide (H2O2) over transition metal substituted zeolites is less environmentally impactful than epoxidation schemes that use chlorinated or organic oxidants. The structure and reactivity of reactive intermediates derived from H2O2 and the mechanism for olefin epoxidation on such materials are debated. Here, cyclohexene oxide formation and H2O2 decomposition rates (measured as functions of reactant and product concentrations) and in situ infrared (IR) and UV-vis spectroscopy are used to probe the intervening elementary steps for cyclohexene (C6H10) epoxidation and the identity of the reactive intermediates on a Nb-β catalyst. IR and UV-vis spectra acquired in situ show that the reactive intermediates are predominantly superoxide species (NbIV-(O2)−, observed also by X-ray photoelectron spectroscopy), which form by the irreversible activation of H2O2 over Nb centers. Similar M-(O2)∗ (M = Ti or Ta) intermediates were previously assumed to form via reversible processes; however, in situ IR and UV-vis measurements directly show that NbIV-(O2)− forms irreversibly in both H2O and acetonitrile. Activation enthalpies (ΔH‡) for C6H10 epoxidation are 27 kJ mol−1 higher than for H2O2 decomposition, while activation entropies (ΔS‡) for epoxidation are 56 J mol−1 K−1 lower than for H2O2. These comparisons show that the selectivities for epoxidation, via primary reaction pathways, increase with increasing reaction temperatures. Collectively, these results provide a self-consistent mechanism for C6H10 epoxidation that is also in agreement with previously published data. These findings will aid the rational design and study of alternative metal oxide catalysts for olefin oxidation reactions.

Intercalated composite materials were prepared with CaAl-layered double hydroxide as the host and Mn(II)-amino acid (l-cysteine, l-histidine and l-tyrosine) complex anions as the guest. Two methods (intercalation of the ligand first followed by constructing the complex; preforming the complex first, then introducing it among the layers of the host) and optimization of the auxiliary conditions were performed to arrive at composites having the complex exclusively among the layers. The obtained substances were structurally characterized by powder X-ray diffractometry, mid IR spectroscopy in diffuse reflectance mode and with ATR or photoacoustic detections, and scanning electron microscopy. The structural features of the intercalant (coordination number, coordination sites) were elucidated by classical chemical and energy dispersive X-ray analyses, EPR, X-ray absorption and far IR spectroscopies. Structural models are also given. The catalytic activities, selectivities and recycling abilities of the substances were studied in the oxidation reactions of cyclohexene with peracetic acid and in situ formed iodosylbenzene as oxidants and allylic alcohol with peracetic acid, in the liquid phase. The intercalated substances proved to be efficient and highly selective (with peracetic acid: outstanding epoxide, with iodosylbenzene superior diol selectivities) catalysts with very good recycling abilities. (Less)

Titanosilicate zeolite beads with hierarchical porosity and 0.2-0.5 mm diameter (HPB-TS-1) have been synthesized from a titanosilicate solution, employing a porous anion-exchange resin as shape- and structure-directing template. The characterization results showed the existence of crystalline TS-1 nanoparticles and of a network of connected large meso/macropores in the interior of the beads. These bead materials are active and selective heterogeneous catalysts for two classes of industrially relevant oxidation reactions: the hydroxylation of phenol and the epoxidation of alkenes. In both cases, a green oxidant such as hydrogen peroxide was utilized. HPB-TS-1 beads displayed high activity in the hydroxylation of phenol, due to their crystalline TS-1 structure with isolated tetrahedral Ti species. In the epoxidation of cyclohexene, HPB-TS-1 gave conversion comparable to that over a benchmark catalyst such as Ti-MCM-41, but more than double yield and selectivity of the target product, cyclohexene oxide. HPB-TS-1 was also active and selective in the epoxidation of other bulky alkenes, i.e., cis-cyclooctene and trans, trans, cis-1,5,9-cyclododecatriene. The catalytic performance of HPB-TS-1 stems from the presence of crystalline TS-1 nanodomains, which are accessible through a network of mesoimacropores within the beads. Notably, the bead format of these catalysts causes their spontaneous settling upon cessation of the agitation, thus enabling their straightforward separation from the reaction mixture. The HPB-TS-1 catalysts could be efficiently recycled in the hydroxylation of phenol with aqueous H2O2 via a calcination approach. (C) 2015 Elsevier Inc. All rights reserved.

A new Ti-incorporated MCM-36 material (shortly termed Si/Ti-MCM-36) with bimodal pore structures was prepared through sequential steps. The hexadecyltrimethylammonium swollen MCM-22 precursor was pillared with titanosilicate formed via hydrolysis of an interlayer mixture of tetraethylorthosilicate (TEOS) and tetrabutylorthotitanate (TBOT). Hydrolyzation of TEOS and TBOT was a key step for intercalation efficiency. The X-ray diffraction (XRD) patterns and N 2 sorption isotherm data confirmed the intercalation efficiency and the retention of basal layer MWW structure. A slightly ordered stacking of MWW sheets along c -direction was observed, generating mesopores of ca. 2 nm in diameter. Ti(IV) was well dispersed in the matrix of titanosilicate pillars, which expand the MWW layers apart. The Ti coordination environment examined by DR UV–Vis and Ti K -edge X-ray absorption spectroscopies was mainly in tetrahedral coordination. Post-treatment of the Si/Ti-MCM-36 materials with acid resulted in a new Ti-incorporated MCM-36 with good catalytic activities in cyclohexene epoxidation.

Impacts of water, methanol, and hexadecane solvents on the individual steps of phenol hydrodeoxygenation are investigated over Pd/C and HZSM-5 catalyst components at 473 Kin presence of H-2. Hydrodeoxygenation of phenol to cyclohexane includes four individual steps of phenol hydrogenation to cyclohexanone on Pd/C, cyclohexanone hydrogenation to cyclohexanol on Pd/C, cyclohexanol dehydration to cyclohexene on HZSM-5, and cyclohexene hydrogenation to cyclohexane on Pd/C. Individual phenol and cyclohexanone hydrogenation rates are much lower in methanol and hexadecane than in water, while rates of cyclohexanol dehydration and cyclohexene hydrogenation are similar in three solvents. The slow rate in methanol is due to the strong solvation of reactants and the adsorption of methanol on Pd, as well as to the reaction between methanol and the cyclohexanone intermediate. The low solubility of phenol and strong interaction of hexadecane with Pd lead to the slow rate in hexadecane. The apparent activation energies for hydrogenation follow the order E-a phenol > E-a cyclonexanone > E-a cyclohexene, and the sequences of individual reaction rates are reverse in three solvents. The dehydration rates (1.1-1.8 x 10(3) mol mol(BAS)(-1) h(-1))and apparent activation energies (115-124 kJ mol(-1)) are comparable in three solvents. In situ liquid-phase IR spectroscopy shows the rates consistent with kinetics derived from chromatographic evidence in the aqueous phase and verifies that hydrogenation of phenol and cyclohexanone follows reaction orders of 1.0 and 0.55 over Pd/C, respectively. Conversion of cyclohexanol with HZSM-5 shows first-order dependence in approaching the dehydration-hydration equilibrium in the aqueous phase. Published by Elsevier Inc.

Phenol and substituted phenols are hydroalkylated and hydrodeoxygenated to bi-cycloalkanes in a tandem reaction over Pd nanoclusters supported on a large-pore molecular sieve HBEA at 473-523 K using water as solvent. The HBEA-supported Pd catalyst (metal-acid ratio: 1:22 mol/mol) optimally balances the competing rates of metal catalyzed hydrogenation as well as of solid acid-catalyzed dehydration and carbon-carbon coupling to combine hydrodeoxygenation and dimerization of phenol derivatives to C-12-C-18 bicycloalkanes in a single reaction sequence. A detailed kinetic study of the elementary reactions of (substituted) phenol and their potential products (cyclohexanol, cyclohexanone, and cyclohexene) demonstrates that phenol selectively reacts with the in situ generated cyclohexanol or cyclohexene on Bronsted acid sites. The acid-catalyzed alkylation of phenol with alcohol intermediates and alcohol dehydration are parallel reactions, which are subtly influenced by the competing hydrogenation reactions as well as by the presence of water as solvent. IR spectroscopy of adsorbed species and preliminary molecular modeling indicate that phenol and cyclohexanol enrichment in the large pores of zeolite HBEA is critical for the high activity and hydroalkylation selectivity. (C) 2012 Elsevier Inc. All rights reserved.

The development of mild and efficient process for the selective oxidation of organic compounds with molecular oxygen can be one of the key technologies for synthesizing oxygenates. Here, a visible light-driven metal chloride to catalyze the selective oxidation of cyclohexane was carried out at ambient temperature under a pure O2 atmosphere. Among the metal salts examined, only a few metal chlorides, with easily changeable valence, such as CuCl2·2H2O, VOCl3, and FeCl3·6H2O, were found to be active to this photo-oxidation reaction in acetonitrile or acetone, providing cyclohexanol, cyclohexanone, chlorocyclohexane, and cyclohexene as main products. This is likely because the weak coordination of these metal chlorides with solvent molecules plays key roles in absorbing visible light and realizing photoredox cycle, as supported by UV–Vis spectrum and cyclic voltammetry measurements. Among these active metal chlorides, CuCl2·2H2O showed a higher conversion and better selectivity for cyclohexanol and cyclohexanone (the oxygenated products) than the other two metal chlorides, and its activity and selectivity for chlorocyclohexane were significantly improved in the case of adding concentrated HCl, because HCl promotes the photocatalytic cycling, as supported by UV–Vis spectra. Notably, a high turnover frequency (TOF, 7.4 h−1) and an excellent selectivity for the oxygenated products (93%) were achieved upon a low concentration of CuCl2·2H2O (0.002 mol L−1), 0.1 ml of concentrated HCl and 2 atm of O2 pressure. Based on these findings, a free radical mechanism for the present photocatalysis system was proposed.

We report on our systematic investigation of Au nanoparticles highly dispersed in the mesopores of (s)-(–)-2-pyrrolidinone-5-carboxylic acid (Py)-modified SBA-15 (Au/SBA-15-Py) by using a series of modern techniques. 13C NMR and IR spectroscopies indicate that Py species are successfully grafted on the surface of mesopores in SBA-15; XRD patterns and N2 adsorption isotherms show that the sample mesostructures are well preserved; TEM images clearly confirm the uniform Au nanoparticles in the mesopores; and XPS suggests an interaction between Au nanoparticles with Py species. Interestingly, Au/SBA-15-Py catalysts always exhibit superior catalytic properties in the oxidation of cyclohexene and styrene by molecular oxygen at atmospheric pressure, compared with the pyrrolidone-free SBA-15 supported Au catalyst (Au/SBA-15-N). This phenomenon is reasonably related to the interaction between Au nanoparticles with Py species, which is consistent with results analyzed from our density-functional theory (DFT) calculations.

The current work utilizes the adsorption of hydrogen and the hydrogenation of cyclohexene as probe reactions to bridge the materials gap between single crystal and polycrystalline Ni/Pt bimetallic surfaces. Previous studies on Ni-modified Pt(1 1 1) have identified the formation of a Pt–Ni–Pt(1 1 1) subsurface structure that binds atomic hydrogen and cyclohexene more weakly, leading to a low-temperature hydrogenation pathway not present on either of the parent metal surfaces. In the current work, the Pt–Ni–Pt subsurface structure is prepared by depositing Ni on a polycrystalline Pt foil substrate. The adsorption of hydrogen and hydrogenation of cyclohexene are investigated using surface science experiments and density functional theory (DFT) calculations. The polycrystalline Pt–Ni–Pt subsurface structure shows general similarities to Pt–Ni–Pt(1 1 1) in the low-temperature hydrogenation pathway, demonstrating the validity of using single crystals to predict activity trends for more complex surfaces.

The goal of this study is the preparation of new heterogeneous catalysts to be used in the oxidation of organic molecules under mild conditions. The metals in zeolite (M–Y) were prepared by the ion exchange method . The in situ encapsulation of selected transition metal complexes as a guest, namely cobalt(II), nickel(II), copper(II), and zinc(II), with 1-(2-pyridylazo)-2-naphthol (PAN) ligand in supercages of Y zeolite (host) was accomplished by the flexible ligand method . The coordination geometry of neat and Y-encapsulated metal complexes has been determined by molecular simulations. The resulting catalysts were fully characterized by different techniques (FTIR, SEM, and chemical analysis), and the results indicated that complexes were encapsulated in supercages of Y zeolite. Catalytic studies were performed in liquid phase for cyclohexene and cyclohexanol oxidations, by using tert -butylhydroperoxide as the oxidizing agent at 40 and 60 °C, respectively. All prepared catalysts exhibited catalytic activity for the oxidation reactions.

A novel heterogeneous catalyst has been developed by immobilizing Ti(IV)–Schiff base complex in Zn–Al/LDH by an ion-exchange method. Powder X-ray diffraction, Fourier transform infrared spectroscopy, diffuse reflectance UV–Visible spectroscopy, N 2 -adsorption and thermal studies confirm the successful intercalation of the Ti-complex within the LDH structure. The supported catalyst shows an excellent catalytic activity in cyclohexene epoxidation reaction. In this paper, we report an environmentally benign reaction pathway in a solvent-free condition, taking H 2 O 2 as oxidant. As high as 95% conversion of cyclohexene took place at 70 °C in 6 h. The heterogeneous catalyst can be recovered easily and reused multiple times without significant loss in catalytic activity and selectivity, which is a better green alternative for practical applications.

Supported Ta(V) oxide catalysts were prepared by grafting calixarene–Ta(V) and TaX 5 complexes on SiO 2 at coverages less than 0.25 Ta nm −2 . Thermogravimetric and elemental analyses and UV–visible spectroscopy indicate that catalysts consist of isolated, 1:1 Ta:ligand surface sites. Catalysts obtained by this one-pot procedure were studied in cyclohexene and cyclooctene epoxidation with H 2 O 2 . In sharp contrast with bare oxides, calixarene-containing catalysts had initial cyclohexene direct epoxidation turnover rates of 3.9 ± 0.1 × 10 −2 s −1 unaffected by surface density, demonstrating single-site character. Calixarene-containing catalysts were up to 3× more active than the corresponding TaCl 5 -based catalysts at high surface densities and were also up to 95% selective to direct (non-radical) cyclohexene epoxidation versus 5 -based catalysts or only ∼20% selectivity for grafted calixarene–Ti(IV) catalysts. Capping silanols with octanol reduced epoxide hydrolysis from >50% to 2 O 2 and demonstrate that a surface ligand on an otherwise traditionally prepared supported oxide can selectively direct oxidation down mechanistically distinct pathways.

Tailor-made and size-controlled ruthenium nanoparticles, RuNPs, of three distinct sizes between 1 and 3 nm are generated from the decomposition of (η 4 -1,5-cyclooctadiene)(η 6 -1,3,5-cyclooctatriene)ruthenium(0) [Ru(COD)(COT)], under H 2 in 1-butyl-3-methylimidazolium bis(trifluoromethanesulphonyl)imide, C 1 C 4 ImNTf 2 , by simply varying experimental conditions. Catalytic hydrogenation of 1,3-cyclohexadiene, CYD, and cyclohexene, CYE, in C 1 C 4 ImNTf 2 , has been used as a probe for the relationship between size and catalytic performance (activity and selectivity) of RuNPs. To allow comparison between different reactions, all catalytic reaction mixtures were diligently prepared in order that the parameters such as substrate/catalyst and substrate/ionic liquid ratio, and therefore, viscosity and mass transport factors remained constant. It was found that the catalytic activity increases with the NP size, while high selectivity is only observed with the smaller NPs. In addition, the studied RuNPs exhibit a high level of recyclability with neither loss of activity nor significant agglomeration.

We investigated the activity, epoxide selectivity, H 2 O 2 efficiency, and recyclability of heterogeneous alkene epoxidation catalysts prepared by encapsulation of tripodal Ti silsesquioxane complexes in polydimethylsiloxane (PDMS) membrane. We found that [Ti(NMe 2 ){( c -C 6 H 11 ) 7 Si 7 O 12 }]/PDMS ( 3 ) and [Ti(NMe 2 ){( i -C 4 H 9 ) 7 Si 7 O 12 }]/PDMS ( 4 ) displayed high activity, epoxide selectivity (⩾97%), and H 2 O 2 efficiency (⩾97%) in cyclohexene- and 1-octene epoxidation with aqueous H 2 O 2 . Moreover, these catalysts were highly recyclable. The high H 2 O 2 efficiency can be attributed to the uniformly non-polar environment provided about the Ti silsesquioxane complex in 3 and 4 by the PDMS membrane, which presumably results in low local water concentrations and higher [alkene]:[H 2 O 2 ] ratios at the Ti center than in the bulk reaction medium; both of these effects favor the epoxide selectivity and minimal leaching of titanium. Our study of the effect of solvent on catalyst activity revealed that acetonitrile is to be preferred as solvent, since methanol inhibits the epoxidation at long reaction times.

Hexagonal mesoporous solids were synthesized from solutions containing TS-1 seeds. The products were characterized by XRD, nitrogen and argon physisorption, TEM, TG/DTA of template decomposition (also after extraction of the mesopore template), UV–Vis and IR spectroscopy, and XANES at the TiK edge. Their catalytic activities were assessed for cyclohexene epoxidation in hydrophilic and hydrophobic environment (CH3OH/water, with H2O2 oxidant, and decane, with tert-butyl hydro-peroxide oxidant, respectively) and for n-hexene epoxidation in hydrophilic environment. The mesopore system was clearly documented by XRD, physisorption measurements, and TEM, whereas evidence for micropores by physisorption proved elusive. However, the micropore template was detected in the solids by TG/DTA even after extraction of the mesopore template, and among the Ti sites, which were confirmed to be tetrahedrally coordinated by UV–Vis and XANES, a clear majority was able to coordinate two water molecules. It was concluded that the pore walls had been built up from nanoparticulate TS-1 precursors resulting in walls of ca. 1.5 nm thickness, which resemble rather the exterior layers of a TS-1 crystallite than its (hydrophobic) interior. In cyclohexene epoxidation, the micro-mesophases were by 1–2 orders of magnitude more active than TS-1 and outperformed also Ti-MCM-41, at similar selectivity in hydrophobic medium. With 1-hexene in hydrophilic medium, however, the micro-mesophases failed completely whereas TS-1 exhibited high activity.

The [Ru 3 O(H 3 CCO 2 ) 6 (py) 2 (L)]PF 6 clusters, where L = methanol or dimethyl sulfoxide, can be activated by peroxide or oxygen donor species, such as tert -butyl hydroperoxide (TBHP) or iodosylbenzene (PhIO), respectively, generating reactive intermediates of the type [Ru IV,IV,III 3 O] + . In this way, they catalyse the oxidation of cyclohexane or cyclohexene by TBHP and PhIO, via oxygen atom transfer, rather than by the alternative oxygen radical mechanism characteristic of this type of complexes. In addition to their ability to perform efficient olefin epoxydation catalysis, these clusters also promote the cleavage of the C H bond in hydrocarbons, resembling the oxidation catalysis by metal porphyrins.

Uniform particles of Ni 3 Ge intermetallic compound (IMC) were prepared inside the pores of MCM-41 mesoporous silica with various mesopore diameters. XRD and TEM revealed that Ni–Ge/MCM-41 thus prepared contained mainly fine metallic particles smaller than the pore size. IR spectra of adsorbed CO suggest the formation of a Ni–Ge IMC, probably Ni 3 Ge, and the electron transfer from Ni to Ge. For H 2 –D 2 equilibration, Ni 3 Ge/MCM-41 had higher intrinsic activity than Ni/MCM-41 because of the high activity of Ni atoms at the corner and edge sites. Among MCM-41 and SiO 2 supports, those with smaller mesopores gave higher activity. The similar effect of pore size also was observed for the hydrogenation of cyclohexene. The difference in activity is discussed in terms of the confinement effect inside mesopores. For the hydrogenation of acetylene, Ni 3 Ge gave higher ethylene selectivity than Ni because of the geometric and electronic effects through the formation of IMC.