Understanding the Solvent Effect on the Catalytic Oxidation of 1,4‐Butanediol in Methanol over Au/TiO2 Catalyst: NMR Diffusion and Relaxation Studies

The effect of water on the catalytic oxidation of 1,4‐butanediol in methanol over Au/TiO₂ has been investigated by catalytic reaction studies and NMR diffusion and relaxation studies. The addition of water to the dry catalytic system led to a decrease of both conversion and selectivity towards dimethyl succinate. Pulsed‐field gradient (PFG)‐NMR spectroscopy was used to assess the effect of water addition on the effective self‐diffusivity of the reactant within the catalyst. NMR relaxation studies were also carried out to probe the strength of surface interaction of the reactant in the absence and presence of water. PFG‐NMR studies revealed that the addition of water to the initial system, although increasing the dilution of the system, leads to a significant decrease of effective diffusion rate of the reactant within the catalyst. From T₁ and T₂ relaxation measurements it was possible to infer the strength of surface interaction of the reactant with the catalyst surface. The addition of water was found to inhibit the adsorption of the reactant over the catalyst surface, with the T₁/T₂ ratio of 1,4‐butanediol decreasing significantly when water was added. The results overall suggest that both the decrease of diffusion rate and adsorption strength of the reactant within the catalyst, due to water addition, limits the access of reactant molecules to the catalytic sites, which results in a decrease of reaction rate and conversion.     

不同温度焙烧的Au-Pd/SiO2催化剂上甲醇选择性氧化制备甲酸甲酯

以溶胶固定法制备了Au-Pd/SiO₂催化剂,考察了催化剂焙烧温度对甲醇选择氧化制甲酸甲酯反应性能的影响。在200~500℃,400℃焙烧的Au-Pd/SiO₂具有最好的低温催化性能,在室温下就有活性,反应温度为100℃时甲醇转化率为25.3%,甲酸甲酯的选择性为100%。采用BET、XRD、UV-vis DRS、XPS、TEM和DRIFTS技术对催化剂进行表征,结果表明,催化剂中活性组分Au和Pd的高分散性,合适的Au和Pd粒径,Au-Pd合金的形成以及Au和Pd之间的强相互作用力,有利于甲醇氧化为甲酸甲酯反应的进行。初步推测出了甲醇在Au-Pd/SiO₂上氧化为甲酸甲酯的反应机理,甲醇在Au-Pd/SiO₂催化剂上是通过甲氧基中间体得到甲酸甲酯的。     

Dispersion of TiO2 on high‐surface‐area mesoporous silica: functionalization with tungstophosphoric acid and application in solvent‐free,aerobic oxidation of n‐hexadecane

Imidazole type ionic liquid, 1‐hexadecyl‐3‐methylimidazolium chloride, was used to template the synthesis of high‐surface‐area mesoporous silica under acidic conditions and crystalline titanium dioxide (TiO₂) nanoparticles of anatase phase were inserted utilizing a solvent evaporation‐induced method. The surface area of more than 700 m² g^?1 was obtained after TiO₂ impregnation. Further, the polyoxometalate, 12‐tungstophosphoric acid (PW₁₂) was dispersed on the surface of TiO₂ to form PW₁₂–TiO₂–silica hybrid catalytic materials. The catalytic activity of this hybrid material was tested for solvent‐free, aerobic oxidation of n‐hexadecane. The experimental investigation shows that PW₁₂–TiO₂ nanocrystals did not block the pore channels and gave good conversion. Copyright © 2012 John Wiley & Sons, Ltd.     

Detection of Polyols by HPLC Coupled with an On‐Line Photochemical Oxidation Reactor and Chemiluminescence Detection

In this paper we propose a new post‐column detection method for polyols containing 1,2‐diol, 1,3‐diol, and saccharides. The polyols are oxidized in a photochemical reactor to yield oxalate with subsequent chemiluminescence detection using [Ru(III)(bpy)₃]³⁺. A mixing solution of eluate and oxidizing reagent is delivered to a reaction coil, which is then irradiated with ultraviolet light to promote the oxidation reaction. The detection limits for 1,2‐ethanediol (ethylene glycol) and 1,3‐propanediol were 38 pmol and 23 pmol, respectively.     

Ag/Au Mixed Sites Promote Oxidative Coupling of Methanol on the Alloy Surface

Nanoporous gold, a dilute alloy of Ag in Au, activates molecular oxygen and promotes the oxygen‐assisted catalytic coupling of methanol. Because this trace amount of Ag inherent to nanoporous gold has been proposed as the source of oxygen activation, a thin film Ag/Au alloy surface was studied as a model system for probing the origin of this reactivity. Thin alloy layers of Ag_xAu_1?x, with 0.15≤x≤0.40, were examined for dioxygen activation and methanol self‐coupling. These alloy surfaces recombine atomic oxygen at different temperatures depending on the alloy composition. Total conversion of methanol to selective oxidation products, that is, formaldehyde and methyl formate, was achieved at low initial oxygen coverage and at low temperature. Reaction channels for methyl formate formation occurred on both Au and Au/Ag mixed sites with a ratio, as was predicted from the local 2‐dimensional composition.     

Controlling the Selectivity of the Surface Plasmon Resonance Mediated Oxidation of p‐Aminothiophenol on Au Nanoparticles by Charge Transfer from UV‐excited TiO2

Although catalytic processes mediated by surface plasmon resonance (SPR) excitation have emerged as a new frontier in catalysis, the selectivity of these processes remains poorly understood. Here, the selectivity of the SPR‐mediated oxidation of p‐aminothiophenol (PATP) employing Au NPs as catalysts was controlled by the choice of catalysts (Au or TiO₂‐Au NPs) and by the modulation of the charge transfer from UV‐excited TiO₂ to Au. When Au NPs were employed as catalyst, the SPR‐mediated oxidation of PATP yielded p,p‐dimercaptobenzene (DMAB). When TiO₂‐Au NPs were employed as catalysts under both UV illumination and SPR excitation, p‐nitrophenol (PNTP) was formed from PATP in a single step. Interestingly, PNTP molecules were further reduced to DMAB after the UV illumination was removed. Our data show that control over charge‐transfer processes may play an important role to tune activity, product formation, and selectivity in SPR‐mediated catalytic processes.     

Fe3O4@L‐arginine magnetic nanoparticles: A novel and magnetically retrievable catalyst for the synthesis of 1′3‐diaryl‐2N‐azaphenalene

The catalytic activity of l ‐arginine‐coated nano‐Fe₃O₄ particles (Fe₃O₄@l ‐arginine) proves they are a novel magnetic catalyst without the use of heat and reflux for the synthesis of 1,3‐diaryl‐2‐N‐azaphenalene derivatives and n‐acyl‐1,3‐diaryl‐2‐N‐azaphenylene derivatives in a one‐pot pseudo‐five‐component condensation reaction of compounds of 2,7‐naphthalene diol, aldehydes, and ammonia derivatives (ammonium acetate or ammonium hydrogen phosphate) and solvent (water and alcohol) with high yield and short reaction times, economical, and simple workup. The structure and magnetic properties of the obtained nanoparticles were characterized via Fourier transform infrared spectroscopy (IR) and field emission scanning electron microscopy (FE‐SEM). The results demonstrated that the average size of the synthesized magnetite nanoparticles is about 21 nm. In addition, the heterogeneous catalyst can be easily recovered magnetically and can be reused for further runs without significant loss of its catalytic activity.     

Gold(Core)–Lead(Shell) Nanoparticle‐Loaded Titanium(IV) Oxide Prepared by Underpotential Photodeposition: Plasmonic Water Oxidation

Underpotential photodeposition of Pb yields an ultrathin shell layer on the Au(111) surface of Au nanoparticle(NP)‐loaded TiO₂ (Au/TiO₂) with heteroepitaxial nanojunctions. The localized surface plasmon resonance of Au/TiO₂ undergoes no damping with the Pb‐shell formation, and the Pb shell offers resistance to aerobic oxidation. Mesoporous films comprising the Au(core)–Pb(shell) NP‐loaded TiO₂ and unmodified Au/TiO₂ were formed on fluorine‐doped tin oxide (FTO) electrode. Using them as the photoanode, photoelectrochemical cells were fabricated, and the photocurrent was measured under illumination of simulated sunlight. The photocurrent for water splitting is dramatically enhanced by the Pb‐shell formation. The photoelectrochemical measurements of the hot‐electron lifetime and density functional theory calculations for model clusters indicate that the Pb‐shell effect originates from the charge separation enhancement.     

Correlation of the Catalytic Activity for Oxidation Taking Place on Various TiO2 Surfaces with Surface OH Groups and Surface Oxygen Vacancies

Three catalytic oxidation reactions have been studied: The ultraviolet (UV) light induced photocatalytic decomposition of the synthetic dye sulforhodamine B (SRB) in the presence of TiO₂ nanostructures in water, together with two reactions employing Au/TiO₂ nanostructure catalysts, namely, CO oxidation in air and the decomposition of formaldehyde under visible light irradiation. Four kinds of TiO₂ nanotubes and nanorods with different phases and compositions were prepared for this study, and gold nanoparticle (Au‐NP) catalysts were supported on some of these TiO₂ nanostructures (to form Au/TiO₂ catalysts). FTIR emission spectroscopy (IES) measurements provided evidence that the order of the surface OH regeneration ability of the four types of TiO₂ nanostructures studied gave the same trend as the catalytic activities of the TiO₂ nanostructures or their respective Au/TiO₂ catalysts for the three oxidation reactions. Both IES and X‐ray photoelectron spectroscopy (XPS) proved that anatase TiO₂ had the strongest OH regeneration ability among the four types of TiO₂ phases or compositions. Based on these results, a model for the surface OH group generation, absorption, and activation of molecular oxygen has been proposed: The oxygen vacancies at the bridging O^2? sites on TiO₂ surfaces dissociatively absorb water molecules to form OH groups that facilitate adsorption and activation of O₂ molecules in nearby oxygen vacancies by lowering the absorption energy of molecular O₂. A new mechanism for the photocatalytic formaldehyde decomposition with the Au/TiO₂ catalysts is also proposed, based on the photocatalytic activity of the Au‐NPs under visible light. The Au‐NPs absorb the light owing to the surface plasmon resonance effect and mediate the electron transfers that the reaction needs.     

Carbon Monoxide Oxidation by Polyoxometalate‐Supported Gold Nanoparticulate Catalysts: Activity,Stability, and Temperature‐ Dependent Activation Properties

Nanoparticulate gold supported on a Keggin‐type polyoxometalate (POM), Cs₄[α‐SiW₁₂O₄₀]⋅n H₂O, was prepared by the sol immobilization method. The size of the gold nanoparticles (NPs) was approximately 2 nm, which was almost the same as the size of the gold colloid precursor. Deposition of gold NPs smaller than 2 nm onto POM (Au/POM) was essential for a high catalytic activity for CO oxidation. The temperature for 50 % CO conversion was −67 °C. The catalyst showed extremely high stability for at least one month at 0 °C with full conversion. The catalytic activity and the reaction mechanism drastically changed at temperatures higher than 40 °C, showing a unique behavior called a U‐shaped curve. It was revealed by IR measurement that Au^δ+ was a CO adsorption site and that adsorbed water promoted CO oxidation for the Au/POM catalyst. This is the first report on CO oxidation utilizing Au/POMs catalysts, and there is a potential for expansion to various gas‐phase reactions.     

Carbon Monoxide Oxidation by Polyoxometalate‐Supported Gold Nanoparticulate Catalysts: Activity,Stability, and Temperature‐ Dependent Activation Properties

Nanoparticulate gold supported on a Keggin‐type polyoxometalate (POM), Cs₄[α‐SiW₁₂O₄₀]?n H₂O, was prepared by the sol immobilization method. The size of the gold nanoparticles (NPs) was approximately 2 nm, which was almost the same as the size of the gold colloid precursor. Deposition of gold NPs smaller than 2 nm onto POM (Au/POM) was essential for a high catalytic activity for CO oxidation. The temperature for 50 % CO conversion was ?67 °C. The catalyst showed extremely high stability for at least one month at 0 °C with full conversion. The catalytic activity and the reaction mechanism drastically changed at temperatures higher than 40 °C, showing a unique behavior called a U‐shaped curve. It was revealed by IR measurement that Au^δ+ was a CO adsorption site and that adsorbed water promoted CO oxidation for the Au/POM catalyst. This is the first report on CO oxidation utilizing Au/POMs catalysts, and there is a potential for expansion to various gas‐phase reactions.     

Effect of oxygen mobility in the lattice of Au/TiO<Subscript>2</Subscript> on formaldehyde oxidation

Two Au catalysts supported on TiO₂ were prepared by impregnation method followed by sodium borohydride reduction or calcination in air (Au/TiO₂-R and Au/TiO₂-C, respectively). The 1 wt % Au/TiO₂-R sample was found to be highly efficient for the oxidation of low concentrated formaldehyde at room temperature. A HCHO conversion of 98.5% was achieved with this catalyst, whereas the Au/TiO₂-C sample showed almost no activity under the same conditions. Highly dispersed metallic Au nanoparticles with small size (∼3.5 nm) were identified in the 1 wt % Au/TiO₂-R catalyst. A significant negative shift of Au4f peak in XPS spectra with respect to bulk metallic Au was observed for the 1 wt % Au/TiO₂-R but no similar phenomena was found for the heat-treated catalyst. More Au nanoparticles and higher content of surface active oxygen were identified on the surface of the Au/TiO₂-R in comparison with the Au/TiO₂-C, suggesting that the Au/TiO₂-R catalyst can enhance the amount of active sites and species involved in for HCHO oxidation. The reduction treatment by sodium borohydride promotes the formation of dispersed metallic Au nanoparticles with small size because it facilitates the electron transfer and increases the content of surface Au nanoparticles and activated oxygen. All these factors are responsible for a high activity of this catalyst in the oxidation of HCHO.     

Enzymatic Synthesis and Polymerization of Cyclic Trimethylene Carbonate Monomer with/without Methyl Substituent

The direct enzymatic synthesis of a cyclic trimethylene carbonate (1,3‐dioxane‐2‐one) monomer with/without a methyl substituent was carried out using dimethyl or diethyl carbonate and 1,3‐diol with the objective of producing aliphatic poly(trimethylene carbonate), a typical biodegradable synthetic plastic. The lipase‐catalyzed condensation of dimethyl or diethyl carbonate with aliphatic 1,3‐diols using immobilized Candida antarctica lipase (lipase CA) in an organic solvent at 70 °C afforded the corresponding methyl‐substituted and unsubstituted cyclic trimethylene carbonates. The cyclic trimethylene carbonates obtained by the reaction of dimethyl or diethyl carbonates with 1,3‐propanediol and 2‐methyl‐1,3‐propanediol were polymerized by lipase to produce the corresponding polycarbonates.

Total TMC yield as a function of the reaction time.     

A Novel Non-phosgene Process for the Synthesis of Methyl N-Phenyl Carbamate from Methanol and Phenylurea： Effect of Solvent and Catalyst

A novel environmentally benign process for the synthesis of methyl N-phenyl carbamate (MPC) from methanol and phenylurea was studied. Effect of solvent and catalyst on the reaction behavior was investigated. The IR spectra of methanol and phenylurea dissolved in different solvents were also recorded. Compared with use of methanol as both a reactant and a solvent, phenylurea conversion and selectivity to MPC increased by using toluene, benzene or anisole as a solvent, while phenylurea conversion decreased slightly by using n-octane as a solvent. The phenylurea conversion declined nearly 50% when dimethyl sulfoxide (DMSO) was used as a reaction media, and MPC selectivity decreased as well. The catalytic reaction tests showed that a basic catalyst enhanced the selectivity to MPC while an acidic catalyst promoted the formation of methyl carbamate and aniline. Moderate degree of basicity showed the best catalytic performance in the cases studied.     

Sorption and diffusion of methanol and water in PVDF‐g‐PSSA and Nafion® 117 polymer electrolyte membranes

Sorption and diffusion properties of poly(vinylidene fluoride)‐graft‐poly(styrene sulfonic acid) (PVDF‐g‐PSSA) and Nafion® 117 polymer electrolyte membranes were studied in water/methanol mixtures. The two types of membranes were found to have different sorption properties. The Nafion 117 membrane was found to have a maximum in‐solvent uptake around 0.4 to 0.6 mole fraction of methanol, while the PVDF‐g‐PSSA membranes took up less solvent with increasing methanol concentration. The proton NMR spectra were recorded for membranes immersed in deuterated water/methanol mixtures. The spectra showed that the hydroxyl protons inside the membrane exhibit resonance lines different from the resonance lines of hydroxyl protons in the external solvent. The spectral features of the lines of these internal hydroxyl groups in the membranes were different in the Nafion membrane compared with the PVDF‐g‐PSSA membranes. Diffusion measurements with the pulsed field gradient NMR (PFG‐NMR) method showed that the diffusion coefficient of the internal hydroxyl groups in the solvent immersed Nafion membrane mirrors the changes in the diffusion coefficients of hydroxyl and methyl protons in the external solvent. For the PVDF‐g‐PSSA membranes, a decrease in the diffusion coefficient of the internal hydroxyl protons was seen with increasing methanol concentration. These results indicate that the morphology and chemical structure of the membranes have an effect on their solvent sorption and diffusion characteristics. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3277–3284, 2000     

Avoiding Self‐Poisoning: A Key Feature for the High Activity of Au/Mg(OH)2 Catalysts in Continuous Low‐Temperature CO Oxidation

Au/Mg(OH)₂ catalysts have been reported to be far more active in the catalytic low‐temperature CO oxidation (below 0 °C) than the thoroughly investigated Au/TiO₂ catalysts. Based on kinetic and in situ infrared spectroscopy (DRIFTS) measurements, we demonstrate that the comparatively weak interaction of Au/Mg(OH)₂ with CO₂ formed during the low‐temperature reaction is the main reason for the superior catalyst performance. This feature enables rapid product desorption and hence continuous CO oxidation at temperatures well below 0 °C. At these temperatures, Au/TiO₂ also catalyzes CO₂ formation, but does not allow for CO₂ desorption, which results in self‐poisoning. At higher temperatures (above 0 °C), however, CO₂ formation is rate‐limiting, which results in a much higher activity for Au/TiO₂ under these reaction conditions.     

Uncovering Key Structural Features of an Enantioselective Peptide‐Catalyzed Acylation Utilizing Advanced NMR Techniques

We report on a detailed NMR spectroscopic study of the catalyst‐substrate interaction of a highly enantioselective oligopeptide catalyst that is used for the kinetic resolution of trans‐cycloalkane‐1,2‐diols via monoacylation. The extraordinary selectivity has been rationalized by molecular dynamics as well as density functional theory (DFT) computations. Herein we describe the conformational analysis of the organocatalyst studied by a combination of nuclear Overhauser effect (NOE) and residual dipolar coupling (RDC)‐based methods that resulted in an ensemble of four final conformers. To corroborate the proposed mechanism, we also investigated the catalyst in mixtures with both trans‐cyclohexane‐1,2‐diol enantiomers separately, using advanced NMR methods such as T₁ relaxation time and diffusion‐ordered spectroscopy (DOSY) measurements to probe molecular aggregation. We determined intramolecular distance changes within the catalyst after diol addition from quantitative NOE data. Finally, we developed a pure shift EASY ROESY experiment using PSYCHE homodecoupling to directly observe intermolecular NOE contacts between the trans‐1,2‐diol and the cyclohexyl moiety of the catalyst hidden by spectral overlap in conventional spectra. All experimental NMR data support the results proposed by earlier computations including the proposed key role of dispersion interaction.     

Trinuclear Pd3O2 Intermediate in Aerobic Oxidation Catalysis

The activation of O₂ is a key step in selective catalytic aerobic oxidation reactions mediated by transition metals. The bridging trinuclear palladium species, [(LPd^II)₃(μ³‐O)₂]²⁺ (L=2,9‐dimethylphenanthroline), was identified during the [LPd(OAc)]₂(OTf)₂‐catalyzed aerobic oxidation of 1,2‐propanediol. Independent synthesis, structural characterization, and catalytic studies of the trinuclear compound show that it is a product of oxygen activation by reduced palladium species and is a competent intermediate in the catalytic aerobic oxidation of alcohols. The formation and catalytic activity of the trinuclear Pd₃O₂ species illuminates a multinuclear pathway for aerobic oxidation reactions catalyzed by Pd complexes.     

Selective hydrogenation of citral over Au-based bimetallic catalysts in supercritical carbon dioxide

Selective hydrogenation of citral was investigated over Au-based bimetallic catalysts in the environmentally benign supercritical carbon dioxide (scCO₂) medium. The catalytic performances were different in citral hydrogenation when Pd or Ru was mixed (physically and chemically) with Au. Compared with the corresponding monometallic catalyst, the total conversion and the selectivity to citronellal (CAL) were significantly enhanced over TiO₂ supported Pd and Au bimetallic catalysts (physically and chemically mixed); however, the conversion and selectivity did not change when Ru was physically mixed with Au catalyst compared to the monometallic Ru/TiO₂, and the chemically mixed Ru-Au/TiO₂ catalyst did not show any activity. The effect of CO₂ pressure on the conversion of citral and product selectivity was significantly different over the Au/TiO₂, Pd-Au/TiO₂, and Pd/TiO₂ catalysts. It was assumed to be ascribed to the difference in the interactions between Au, Pd nanoparticles and CO₂ under different CO₂ pressures.     

Volumetric,Viscometric, and Refractive Index Behaviour of Glycine in Aqueous Diols at Different Temperatures

Densities, ρ, viscosities, η, and refractive indices, n_D, of glycine (Gly) (0.1 — 0.5 M) in aqueous 1,2‐ethanediol (1,2‐EtD), 1,2‐propanediol (1,2‐PrD), and 1,3‐butanediol (1,3‐BuD) (30% v/v) were measured at 298, 303, 308, and 313 K. Experimental values of ρ and η were used to calculate partial molar volumes, ?⁰_v, partial molar volumes of transfer of Gly from water to aqueous diol solutions, ?⁰_v(tr), Falkenhagen and Jones ‐Dole coefficients, A and B, respectively, free energies of activation of viscous flow, Δμ⁰*₁ and Δμ⁰*₂, per mole of solvent and solute, respectively, enthalpies, ΔH* and entropies, ΔS* of activation of viscous flow. Large positive values of ?⁰_v, and an increasing value of S_v*, for all the three mixtures at each temperature suggest the presence of strong solute‐solvent interaction, and this interaction decreases as the size of alkyl moiety increases from 1,2‐EtD to 1,3‐BuD. Positive ?⁰_v(tr) values tend to decrease with increasing the number of CH₂ group, thereby indicating that the electrostriction effect in diols follows the sequence; 1,2‐EtD > 1,2‐PrD > 1,3‐BuD. Small A values, with large values of B, are indicative of weak solute‐solute and strong solute‐solvent interactions that operate in the present systems, and that the magnitudes of B are in the sequence: 1,2‐EtD > 1,2‐PrD > 1,3‐BuD and, thus, the sequence represents the strength of interaction between Gly and diol molecules. Moreover, positive SB/ST values suggest the structure‐breaking nature of Gly in diol + water mixtures. The observed values of Δμ⁰*₂ fall in the sequence: 1,2‐EtD > 1,2‐PrD > 1,3‐BuD which, like ?⁰_v and S_v*, reinforce that Gly‐diol interaction decreases with subsequent addition of CH₂ group in diols. The trends in the variation of ΔH* and ΔS* with Gly concentration also reveal the presence of significant solute‐solvent interaction in all three systems. An almost linear increase in RD with an increasing amount of Gly reveals that Gly tends to increase the polarizability of the aqueous‐diol molecules under study. The variation of all these parameters with concentration of Gly and with temperature suggests the presence of strong solute‐solvent interaction, which decreases as the size of alkyl moiety in diols increases from 1,2‐EtD to 1,3‐BuD.