Changing Reaction Conditions Research Articles

Microstructure dependent microwave properties of spray deposited Tl–Ba–Ca–Cu–O films

Tl-based superconductors are found to be the potential candidate for enormous applications especially in field of microwave devices. Thick Tl-based single phase (2 2 2 3) superconducting films have been prepared successfully on CeO 2 buffered Al 2O 3 substrates by two-step technique consisting of spray pyrolysis followed by thalliation. The modifications in structure and morphology by changing reaction conditions were studied. The microstructures were found to be sensitive to synthesis conditions and in turn their dependence on surface resistance of films. The microwave absorption properties were studied in the frequency range from 1 GHz to 10 GHz. The surface resistance of Tl–Ba–Ca–Cu–O films at room temperature was measured with the help of stripe line technique and found to be 14.9 mΩ/cm 2. The effects of microstructure on microwave absorption peaks are studied and results are discussed at the length in this paper.

New Approach for Complete Reversal of Enantioselectivity Using a Single Chiral Source

Obtaining both enantiomers of a given compound in high enantiomericexcess is an attractive and important challenge, because it is oftenthe case that each enantiomer exhibits different bioactivity. Thisreview summarizes some approaches to realize complete reversal ofenantioselectivity by changing various reaction conditions. Ournew approach to achieve reversal of enantioselection in the enantioselectiveaddition of diketene to aldehydes, affording optically active 5-hydroxy-3-ketoesters in high optical yield using Schiff bases that contain anoxazoline moiety derived from L-serine,is also described. 1 Introduction 2 Reversal Based on the Nature of the Substituent 2.1 Control with Catalyst Substituent 2.2 Control with Substrate Substituent 3 Control with Central Metal 4 Reversal of Enantioselectivity by Changing Reaction Conditions 4.1 Control with Solvent 4.2 Control with Temperature 4.3 Control with Additives 4.4 Control with Other Factors 5 Recent Concepts for Reversal of Enantioselectivity 5.1 Asymmetric Addition of Readily Available Nucleophiles withAldehydes 5.2 Enantioselectivity Reversal in Asymmetric Addition of Diketenewith Aldehydes 6 Conclusions and Outlook

Synthesis of gold nanoplates in lamellar liquid crystal

Single-crystalline gold nanoplates have been synthesized in a lamellar lyotropic liquid crystal (LLC) composed of a nonionic surfactant, C 12E 4 (tetraethylene glycol monododecyl ether) and the chloroauric acid (HAuCl 4) solution. The sizes of gold nanoplates can be varied from several hundreds of nanometers to a few microns in width by changing reaction conditions. The factors which influence the product morphology, like LLC composition, capping agent and reaction temperature are studied by small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The mechanism analysis indicates that the LLC plays an important role for the consequent growth of single-crystal anisotropic plates by providing an ordered reaction medium and a limited growth space.

Facile and Highly Stereoselective One-Pot Synthesis of Either (E)- or (Z)-Nitro Alkenes

Aliphatic aldehydes were reacted with nitro alkanes in the presence of catalytic amounts of piperidine over 4 A molecular sieves. Simply by changing reaction conditions (solvent and temperature) it is possible to control the stereochemical outcome of the reactions, obtaining pure (E)- and (Z)-nitro alkenes in high to excellent yields. The role of molecular sieves on the stereochemical control seems crucial in addition to that of piperidine, especially for the synthesis of the Z isomer.

Kinetics of Oxidative Desulfurization of Sulfur Compounds in Diesel Fuel

The oxidation of diesel fuel sample was carried out using tertiary butyl hydroperoxide oxidant in the presence of two commercially available Ni-Mo catalysts. The studies were carried out by changing reaction conditions such as treat ratios (oxidant/sulfur molar ratio), WHSV, temperature, etc., to achieve product diesel with sulfur content of <20 ppm. The results of the study on diesel were compared by carrying out similar experiments on model compound, namely, dibenzothiophene dissolved in sulfur-free liquid paraffin oil. GC-PFPD instrument was used to identify and group different sulfur compounds present in diesel and the kinetics of desulfurization was studied.

High-selectivity hydrogenation of cinnamaldehyde over platinum supported on aluminosilicates

This work studies liquid-phase hydrogenation of cinnamaldehyde to cinnamylalcohol over Pt/K-10 and ion-exchanged Pt/Na-Y. The experiments show the highest selectivity of 78% for Pt/K-10 and 92% for the Pt/Na-Y. By careful analysis, characterisation and changing reaction conditions it was attempted to cover key parameters possibly responsible for the high selectivity. The parameters are described and discussed in detail.

Highly Active Carbene Ruthenium Catalyst for Metathesis of 1‐Hexene

AbstractA new carbene ruthenium complex, 1,3‐bis(2,6‐dimethylphenyl)‐4,5‐dihydroimidazol‐2‐ylidene)(PPh3)Cl2‐Ru=CHPh, was synthesized and used as catalyst for the metathesis of 1‐hexene. The resulting complex exhibited very high catalytic activity whose TOF is up to 6680 h−1. However, at the same time significant olefin isomerization was observed and could be surpressed by changing reaction conditions, such as temperature, time, alkene/Ru molar ratio and solvent.

Hetero-metal Mn–Cu and Mn–Ni clusters with tridentate Schiff-base ligands

Novel hetero-metal di-, tetra-, and octanuclear Mn–Cu clusters of [Mn IIICu II(5-Br-sap) 2Cl(EtOH)] ( 1), [Mn IIICu II(5-Cl-sap) 2Cl(MeOH)] ( 2), [ Mn 2 III Cu 2 II ( 5 -Br-sap ) 4 ( H 2 O ) 2 ] ( ClO 4 ) 2 ( 3(ClO 4) 2), [ Mn III Cu 3 II ( 5 -Br-sae ) 4 ( H 2 O ) 2 ] ( PF 6 ) · 2 Et 2 O ( 4(PF 6) · 2Et 2O), [ Mn 2 III Mn 2 II Cu 4 II ( 3 -OMe-sap ) 4 ( μ 4 -O ) 2 ( μ 2 -OMe ) 2 Cl 2 ] Cl 2 · 2 MeOH ( 5Cl 2 · 2MeOH), as well as tetranuclear Mn–Ni clusters of [ Mn 2 III Ni 2 II ( sap ) 2 ( sal ) 2 ( μ 3 -OMe ) 2 ( OAc ) 2 ] ( 6), [ Mn 2 III Ni 2 II ( sap ) 2 ( sal ) 2 ( μ 3 -OMe ) 2 ( NO 3 ) 2 ( MeOH ) 2 ] ( 7) were prepared by simple one-pot reactions of multidentate Schiff base ligands with metal sources. All clusters were selectively obtained by changing reaction conditions, employment of suitable co-ligands, and slightly modified ligands. We report here syntheses, structure and magnetic properties of a series of hetero-metal Mn–Cu and Mn–Ni clusters.

W(CO) 5(L)-catalyzed 6- endo-selective cyclization and formal Cope rearrangement of allenyl silyl enol ethers

On treatment of 5-siloxy-1,2,5-trienes with a catalytic amount of W(CO) 6 under photoirradiation, two types of synthetically useful compounds, that is, 6- endo-cyclized products or formal Cope rearrangement products, are obtained selectively via the same intermediates simply by changing reaction conditions. In these reactions, electrophilic activation of the allene moiety is effectively achieved by coordination of W(CO) 5, allowing intramolecular attack by neutral carbon nucleophiles in a 6- endo manner.

Mass spectrometric identification and characterization of antimony complexes with ribose-containing biomolecules and an RNA oligomer

Mass spectrometric techniques have been used to study the interaction of inorganic Sb(V) with biomolecules containing a ribose or deoxyribose moiety. Electrospray (ES) mass spectra of reaction mixtures containing inorganic Sb(V) and one of several biomolecules (adenosine, cytidine, guanosine, uridine, adenosine-5'-monophosphate, adenosine-3',5'-cyclic monophosphate, ribose, or 2'-deoxyadenosine) afforded high-mass antimony-containing ions corresponding to Sb(V)-biomolecule complexes of stoichiometry 1:1, 1:2, or 1:3. The complexes were characterized by collision-induced dissociation (CID) tandem mass spectrometry (MS) using ion-trap multistage MS. The CID results revealed that Sb(V) binds to the ribose or deoxyribose moiety. Structures are proposed for the Sb-biomolecule complexes. Analysis of the reaction mixtures by reversed-phase chromatography coupled on-line to either inductively coupled plasma (ICP) MS or ES-MS showed that in solution Sb(V) forms complexes with all the analyzed biomolecules with vicinal cis hydroxyl groups. Evidence (from size-exclusion chromatography ICP-MS and direct infusion ES-MS) of complexation of Sb(V) with an RNA oligomer, but not with a DNA oligomer, supports the suggestion that the presence of vicinal cis hydroxyl groups is critical for complexation to occur. This is the first direct evidence of complexation of Sb(V) with RNA. Results obtained by studying the effect of changing reaction conditions, i.e. pH, reaction time, and Sb/biomolecule molar ratio, on the extent of Sb-biomolecule formation suggest the reaction may be of physiological importance. Selected reaction monitoring (SRM) and precursor-ion-scanning tandem MS were investigated to determine their potential to detect trace levels of the Sb-biomolecule complexes in biological samples. Application of SRM MS-MS in combination with high-performance liquid chromatography enabled successful detection of an Sb-adenosine complex that had been spiked into a complex biological matrix (liver homogenate).

Aqueous−Organic Phase-Transfer of Highly Stable Gold, Silver, and Platinum Nanoparticles and New Route for Fabrication of Gold Nanofilms at the Oil/Water Interface and on Solid Supports

A simple but effective aqueous-organic phase-transfer method for gold, silver, and platinum nanoparticles was developed on the basis of the decrease of the PVP's solubility in water with the temperature increase. The present method is superior in the transfer efficiency of highly stable nanoparticles to the common phase-transfer methods. The gold, silver, and platinum nanoparticles transferred to the 1-butanol phase dispersed well, especially silver and platinum particles almost kept the previous particle size. Electrochemical synthesis of gold nanoparticles in an oil-water system was achieved by controlling the reaction temperature at 80 degrees C, which provides great conveniences for collecting metal particles at the oil/water interface and especially for fabricating dense metal nanoparticle films. A technique to fabricate gold nanofilms on solid supports was also established. The shapes and sizes of gold nanoparticles as the building blocks may be controllable through changing reaction conditions.

Structure and oxidation state of gold on different supports under various CO oxidation conditions

The structure of gold clusters of different sizes supported on various metal oxides (Al 2O 3, TiO 2, SiO 2) exposed to different CO oxidation conditions was investigated in situ using X-ray absorption spectroscopy at the Au L 3 edge. In all catalysts, the only phase detected during catalysis was Au 0. In the most active sample with small gold particles (Au/Al 2O 3), variation in the electronic structure of the gold clusters with changing reaction conditions was observed by XANES spectroscopy and ascribed to the adsorption of CO on the metallic gold clusters. FEFF8 calculations proved that the changes in the XANES signature of Au/Al 2O 3 can be explained by backdonation of d-electrons into the π ∗ orbitals of CO. For Au/Al 2O 3, the presence of Au O backscattering in the EXAFS suggested weak cluster–support interactions. For Au/SiO 2 and Au/TiO 2, the structure of the gold clusters remained unchanged throughout all experiments.

Morphologies and microstructures of tree-like carbon produced at different reaction conditions in a CVD process

Centimeter-size multi-branched tree-like carbon structures have been generated by the catalytic chemical vapor deposition of toluene using ferrocene as the catalyst precursor and investigated by means of SEM, TEM, and EDX. It is found that a temperature of 1000–1200 °C and a carrier gas flow rate of 1000–2500 ml/min are necessary for the generation of the carbon trees. Their morphologies and microstructures change greatly with the changing reaction conditions. The fractal dimensions of the trees are calculated to quantitatively investigate the influence of different reaction temperatures on the morphologies.

Synthesis and flocculability of sodium alginate grafted with acrylamide

Graft copolymers of sodium alginate (SA) with acrylamide (AM) were synthesized using a ceric ion initiated solution polymerization technique. The acrylamide conversions were studied by changing reaction conditions, such as temperature, molecular weight of sodium alginate and reaction time. The flocculation performance of the graft copolymer (SAG) was investigated in kaolin suspension and also in dyeing waste water. It was found that SAG is more efficient in flocculation behavior as compared to polyacrylamide and SA in kaolin suspension, and in removal capacities for COD Cr and colority in dyeing wastewater.

Hydrothermal Soft Chemical Synthesis and Particle Morphology Control of BaTiO3in Surfactant Solutions

Barium titanate (BaTiO3) particles with book‐like and spherical morphology were prepared by using a hydrothermal soft chemical process in the presence of a cationic surfactant. A layered titanate of H1.07Ti1.73O4with a lepidocrocite‐like structure and plate‐like particle morphology was used as the precursor. The layered titanate was hydrothermally treated in a Ba(OH)2–(HTMA‐OH) (n‐hexadecyltrimethylammonium hydroxide) solution or a Ba(OH)2–(HTMA‐Br) (n‐hexadecyltrimethylammonium bromide) solution in a temperature range of 80°–250°C to prepare BaTiO3. The intercalation reaction of HTMA+with the layered titanate promotes the structural transformation reaction from the layered titanates to BaTiO3, while it inhibits the structural transformation reaction to anatase under the hydrothermal conditions. The particle morphology of BaTiO3prepared by this method dramatically changes with changing reaction conditions. HTMA+plays an important role in changing particle morphology in the hydrothermal soft chemical process.

Environmentally Compatible Hybrid‐Type Polyurethane Foams Containing Saccharide and Lignin Components

AbstractSemi‐rigid polyurethane (PU) foams were prepared using lignin‐molasses‐ poly(ethylene glycol) polyols. Two kinds of lignin, kraft lignin (KL) and sodium lignosulfonate (LS), were used. Both lignin and molasses polyols were mixed with various ratios and were reacted with poly(phenylene methylene) polyisocyanate (MDI) in the presence of silicone surfactant and di‐n‐butyltin dilaurate. A small amount of water was used as a foaming agent. The apparent density of PU foams increased with increasing lignin content. The compression strength and elastic modulus linearly increase with increasing apparent density, suggesting that mechanical properties are controllable by changing reaction conditions. The PU foams were amorphous and glass transition was detected by differential scanning calorimetry. The glass transition temperature (Tg ) maintained an almost constant value, regardless of the mixing ratio. This indicates that both the phenolic group of lignin and the glucopyranose ring of molasses act as rigid components in PU crosslinking network structures, and both groups contribute to the main chain motion to the same extent. By thermogravimetry (TG), it was confirmed that PU foams are thermally stable up to around 300 °C. By differential scanning calorimetry, Tg was observed at temperatures from 80 to 120 °C.

Morphologies of nanostructured bismuth sulfide prepared by different synthesis routes

Crystalline and poorly crystalline bismuth sulfide with different sizes were synthesized using hydrothermal, solvothermal and liquid-phase method at low temperature (110–180°C) through changing reaction conditions. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED). The results indicated that solvent, reaction temperature and time were key factors for purity, size, and morphology of the resulting products. A reasonable mechanism was proposed for these processes. For the established precursors, we put forward the optimum synthesis conditions for preparing nanoscale bismuth sulfide.

In Situ Synthesis of Noble Metal Nanoparticles in Ultrathin TiO2−Gel Films by a Combination of Ion-Exchange and Reduction Processes

We have explored in situ synthesis of noble metal nanoparticles in ultrathin TiO2−gel films, in view of the increasing importance of such nanoparticle-containing thin films. The two-step procedure consists of ion exchange in the ultrathin matrix and subsequent reduction of metal ions to nanoparticles by low-temperature H2 plasma, by aqueous NaBH4, and by UV irradiation. Smooth and transparent ultrathin TiO2 films containing noble metal (Ag, Au, Pd, Pt) particles of 2−10 nm were readily obtained by this approach. The size and distribution of metal nanoparticles could be controlled by changing reaction conditions. The film thickness was precisely controlled by the cycle of film assembly. The low-temperature H2 plasma treatment, first used for preparation of noble metal nanoparticles, showed interesting advantages over chemical and photoinduced reductions in fabrication of nanoparticle-containing inorganic thin films.

Outstanding Mechanistic Questions in Heterogeneous Catalysis

This Feature Article summarizes key results from our studies on the kinetics of chemical reactions on solid surfaces as they relate to heterogeneous catalysis. Emphasis is placed on the identification of specific issues of relevance to the design of catalytic processes, particularly the need for highly selective catalytic reactions. From the kinetic point of view, issues of selectivity translate into controlling relative activation energies within a small fraction of their absolute value. In the case of catalytic reforming of hydrocarbons, the nature of the final products is determined by the regioselectivity of an early dehydrogenation from surface alkyl intermediates (from the α, β, or γ position), a step greatly affected by the electronic structure of the metal used as catalyst. In the case of partial oxidation reactions, both the geometry of the local ensemble of surface atoms in the catalytic site and the presence of surface species such as hydroxide groups play determining roles in defining selectivity. A third factor is adsorption geometry, which often varies with changing reaction conditions: variations in concentration of the modifier in chiral catalysis, for example, may lead to dramatic changes in the enantioselectivity of hydrogenation processes. High pressures can induce new types of bonding between adsorbates and surfaces as well, opening new mechanistic routes for catalytic reactions: the hydrogenation of olefins, where π bonding occurs on top of strongly bonded carbonaceous deposits, falls into this category. Finally, the inhomogeneous distribution of adsorbates within the surface can be manifested in the formation of local islands, and those can promote specific catalytic reactions; witness, for instance, the effect of islanding on reactivity in the catalytic oxidation of carbon monoxide and during the catalytic reduction of nitrogen oxide.

A new catalytic enantioselective reducing reagent system from (−)-α,α-diphenylpyrrolidinemethanol and 9-borabicyclo[3.3.1]nonane, especially effective for hindered and substituted aralkylketones

New catalytic enantioselective reduction systems were prepared from aminoalcohols and dialkylboranes, for the enantioselective reductions of prochiral aromatic ketones. Among these, the system prepared from (−)-α,α-diphenylpyrrolidinemethanol with 9-borabicyclo[3.3.1]nonane proved especially promising for such reductions. This complex catalyzes the reduction of prochiral aralkyl ketones to the corresponding alcohols with BH 3–THF, with enantioselectivities 82–99.2%. Also, this catalyst is particularly effective for the more hindered and substituted aralkyl ketones. Various modifications in this new catalytic reduction system, such as changing reaction conditions, reducing agent and dialkylborane, were also examined.