Selective hydrogenation of substituted styrene to alkylbenzene catalyzed by Al2O3 nanoparticles

Research on Chemical Intermediates - A straightforward and suitable protocol is described for the conversion of substituted styrene to alkylbenzenes in the presence of Al2O3 nanoparticles...     

Theoretical modeling of mechanisms of phenylacetylene and styrene hydrogenation on the Pd(100) surface

Russian Chemical Bulletin - Various routes of phenylacetylene and styrene hydrogenation on the Pd(100) surface were studied by the DFT-PBE density functional method. The three-layer model of the...     

铂负载二氧化钛光催化剂在单波长下苯乙炔高选择性加氢（英文）

苯乙烯是一种非常重要的化工原料,它是工业上生产聚苯乙烯、树脂和丁苯橡胶的重要单体,而苯乙烯单体中经常含有苯乙炔杂质,影响苯乙烯的聚合性能,因此研究苯乙炔选择性加氢生成苯乙烯具有十分重要的工业意义.传统的热催化苯乙炔加氢反应会用到易燃易爆的氢气,引起操作的危险性,因此,开发具有环境友好型的加氢反应体系具有很重要的意义.光催化加氢反应利用光生电子强的还原能力,还原质子产生活性的氢物种加氢,相较于传统的热催化使用氢气作为加氢源加氢,能够在温和的条件下实现高选择性加氢.基于此,本文利用Pt/TiO2作为光催化剂,甲醇作为加氢源实现了在385 nm单波长光照下苯乙炔的高选择性加氢.首先,我们利用光沉积的方法将Pt负载在TiO2的表面,通过透射电子显微镜图像和紫外可见吸收光谱表征了负载在TiO2表面Pt的颗粒分布和光学性质.结果表明负载的Pt的颗粒大约在5 nm左右,Pt的负载改变了TiO2在可见光区的吸收性能.XPS结果显示,通过光沉积得到的Pt的价态为金属态和氧化态共存.光催化苯乙炔加氢实验表明,Pt/TiO2催化剂在室温常压条件下不仅具有高的苯乙炔光催化转化率,当光照达8 h后,苯乙炔完全转化,而且在6 h之内苯乙烯选择性保持在91.3%,具有高的苯乙烯选择性.通过对负载的Pt的含量进行了优化,筛选出当Pt的负载量为1 wt%时,苯乙炔的转化率最高.为了对比,利用传统热催化的方法氢气作为加氢源进行了苯乙炔加氢实验,结果发现,使用氢气作为加氢源时,虽然苯乙炔的转化率为100%,但产物是过加氢的产物乙苯.这主要是因为在光催化反应过程中,TiO2导带上的电子迁移至Pt颗粒上,导致Pt的电子密度增加,Pt颗粒表面高的电子密度有利于加氢中间产物苯乙烯的脱附,因此,在光催化加氢过程中不会发生过加氢反应,具有高的苯乙烯选择性.同时,扩展实验表明,Pt/TiO2光催化剂对其他类型的炔烃加氢也具有高的选择性,表明Pt/TiO2光催化炔烃加氢具有普适性.由此可见,光催化炔烃加氢未来将成为一种环境友好而高效的方法.     

Selective DNA-mediated assembly of gold nanoparticles on electroded substrates

Motivated by the technological possibilities of electronics and sensors based on gold nanoparticles (Au NPs), we investigate the selective assembly of such NPs on electrodes via DNA hybridization. Protocols are demonstrated for maximizing selectivity and coverage using 15mers as the active binding agents. Detailed studies of the dependences on time, ionic strength, and temperature are used to understand the underlying mechanisms and their limits. Under optimized conditions, coverage of Au NPs on Au electrodes patterned on silicon dioxide (SiO2) substrates was found to be approximately 25-35%. In all cases, Au NPs functionalized with non-complementary DNA show no attachment and essentially no nonspecific adsorption is observed by any Au NPs on the SiO2 surfaces of the patterned substrates. DNA-guided assembly of multilayers of NPs was also demonstrated and, as expected, found to further increase the coverage, with three deposition cycles resulting in a surface coverage of approximately 60%.     

Selective hydrogenation of aromatic compounds using modified iridium nanoparticles

Till now, Ionic liquid‐stabilized metal nanoparticles were investigated as catalytic materials, mostly in the hydrogenation of simple substrates like olefins or arenes. The adjustable hydrogenation products of aromatic compounds, including quinoline and relevant compounds, aromatic nitro compounds, aromatic ketones as well as aromatic aldehydes, are always of special interest, since they provide more choices for additional derivatization. Iridium nanoparticles (Ir NPs) were synthesized by the H₂ reduction in imidazolium ionic liquid. TEM indicated that the Ir NPs is worm‐like shape with the diameter around 12.2 nm and IR confirmed the modification of phosphine‐functionalized ionic liquids (PFILs) to the Ir NPs. With the variation of the modifier, solvent and reaction temperature, substrate like quinoline and relevant compounds, aromatic nitro compounds, aromatic ketones as well as aromatic aldehydes could be hydrogenated by Ir NPs with interesting adjustable catalytic activity and chemoselectivity. Ir NPs modified by PFILs are simple and efficient catalysts in challenging chemoselective hydrogenation of quinoline and relevant compounds, aromatic nitro compounds, aromatic ketones as well as aromatic aldehydes. The activity and chemoselectivity of the Ir NPs could be obviously impacted or adjusted by altering the modifier, solvent and reaction temperature.     

Quantum chemical analysis of mechanisms of phenylacetylene and styrene hydrogenation to ethylbenzene on the Pd{111} surface

The mechanisms of the hydrogenation of phenylacetylene and styrene to ethylbenzene on the Pd{111} surface, which are secondary reactions of the selective hydrogenation of phenylacetylene, were studied by the DFT-PBE method. The position of the Ph group of the styrene molecule with respect to the Pd surface is shown to exert a noticeable effect on the mechanism of the process. If the Ph group is adsorbed on the Pd surface, then the addition of the first H atom to the terminal C atom of the styrene molecule is most probable. If Ph does not contact the surface, then the most substituted C atom of the styrene molecule is first hydrogenated. On the whole, the interaction of the Ph group with the Pd{111} surface results in stable adsorption structures, the hydrogenation of which on the Pd{111} surface is strongly hindered. The Gibbs activation energy (ΔG^≠₂₉₈) is 22.9 and 27.1 kcal mol^–1 in the most probable reaction routes of styrene hydrogenation to ethylbenzene and direct hydrogenation of phenylacetylene to ethylbenzene, respectively.     

Selective hydrogenation of phenylacetylene on Ni and Ni-Pd catalysts modified with heteropoly compounds of the Keggin type

It is established that unmodified Ni catalysts and Ni catalysts modified with Mo- and W-heteropoly compounds (HPC) of the Keggin type (6 wt %) along with catalyst containing 6% K₄SiW₁₂O₄₀/Al₂O₃ appear to be active in the reaction of phenylacetylene (PA) hydrogenation. At low temperatures (100?C150°C), the selectivity of the process strongly depends on the nature of the modifier or second active metal (Pd). It is demonstrated that in the presence of 6% Ni-0.015% Pd/Al₂O₃ modified by HPC K₄SiMo₆W₆O₄₀, the conversion of PA at 100°C was 87% at a styrene: ethylbenzene ratio of 1: 1. The acidity of HPC is found to influence the side reactions of alkylation and condensation. Transmission electron microscopy demonstrates that Ni in modified HPC 6% Ni/Al₂O₃ is present in the form of the particles below 2 nm in size, and these particles of Ni become larger when affected by the reaction medium during PA hydrogenation.     

Acetylene hydrogenation on SiO2 supported gold nanoparticles

Acetylene hydrogenation has been investigated on 1.8 wt.% Au(I)/SiO₂ and 1.9wt.% Au(II)/SiO₂ catalysts prepared by fixation of Au sol to SiO₂ (Aerosil 200). The mean particle size measured by TEM is 3.7 and 6.1 nm, respectively. For the sake of comparison a 2.1 wt.% Au/TiO₂ sample was prepared by deposition-precipitation (DP) technique (mean particle size of Au is 3.3 nm). Transformation of acetylene was measured at 5 K/min ramp rate with gas mixtures containing the reactants at H₂/C₂H₂=2 and 70 ratios. The C₂H₂ content of the gas mixture was 0.11% (0.11 kPa C₂H₂). The activity sequence at 423 K was: Au/TiO₂>Au(I)/SiO₂≫Au(II)/SiO₂. Both the partial pressure of hydrogen and the temperature significantly affect the activity (acetylene conversion) and ethylene selectivity. Above 500–550 K over-hydrogenation (ethane formation) and hydrogenolysis (methane formation) decrease the ethylene selectivity. Faster deactivation and larger amount of deposit was observed on Au/TiO₂ than on Au(I)/SiO₂. A reaction scheme is proposed suggesting formation of sigma bonded intermediates as sp carbon hybridises to sp² and sp³.     

Hydrometallatrane-catalyzed regioselective hydrosilylation of styrene and phenylacetylene

Russian Chemical Bulletin -     

Selective hydrogenation by Pd nanoparticles embedded in polyelectrolyte multilayers

Alternating adsorption of poly(acrylic acid) and a polyethylenimine-Pd(II) complex on alumina and subsequent reduction of Pd(II) by NaBH4 yield catalytic Pd nanoparticles embedded in multilayer polyelectrolyte films. The polyelectrolytes limit aggregation of the particles and impart catalytic selectivity in the hydrogenation of alpha-substituted unsaturated alcohols by restricting access to catalytic sites. Hydrogenation of allyl alcohol by encapsulated Pd(0) nanoparticles can occur as much as 24-fold faster than hydrogenation of 3-methyl-1-penten-3-ol. Additionally, the nanoparticle/polyelectrolyte system suppresses unwanted substrate isomerization, when compared to a commercial palladium catalyst. Selective diffusion through poly(acrylic acid)/polyethlyenimine membranes suggests that hydrogenation selectivities are due to different rates of diffusion to nanoparticle catalysts. First-order kinetics are also consistent with a diffusion-limited mechanism. Further exploitation of the versatility of polyelectrolyte films should increase selectivity in hydrogenation as well as other reactions.     

Selective hydrogenation of styrene/1-octene mixtures over a monolithic Pd catalyst

Mixtures of styrene and 1-octene in toluene were hydrogenated over a monolithic Pd catalyst in the three-phase regime. Styrene was preferably hydrogenated to ethylbenzene, while a major part of the initial quantity of 1-octene was isomerized to internal olefins. Formation of alkylcyclohexanes was negligible. Observed rates of styrene hydrogenation were high compared to the values listed in literature.     

Selective enhancement of nucleases by polyvalent DNA-functionalized gold nanoparticles

We demonstrate that polyvalent DNA-functionalized gold nanoparticles (DNA-Au NPs) selectively enhance ribonuclease H (RNase H) activity while inhibiting most biologically relevant nucleases. This combination of properties is particularly interesting in the context of gene regulation, since high RNase H activity results in rapid mRNA degradation and general nuclease inhibition results in high biological stability. We have investigated the mechanism of selective RNase H activation and found that the high DNA density of DNA-Au NPs is responsible for this unusual behavior. This work adds to our understanding of polyvalent DNA-Au NPs as gene regulation agents and suggests a new model for selectively controlling protein-nanoparticle interactions.     

Selective enhancement of resonance light-scattering of gold nanoparticles by glycogen

Simple and sensitive methods for the determination of glycogen are desirable. This study was to establish the determination of glycogen based on the enhancement of the light-scattering intensity of gold nanoparticles (AuNPs) at nanogram per milliliter levels. Glycogen enhancement was independent of temperature and was highest at pH 7. The enhancement was linear from 10 to 800 ngmL(-1) with a 2 ngmL(-1) detection limit (3S/N). The standard deviation of the slope for 5 consecutive calibration runs was within 5%. Analysis of three artificial samples with varied glycogen concentrations had 98-102% recovery. Selected amino acids, glucose, lactose, BSA and trichloroacetic acid all showed minor effects on glycogen enhancement demonstrating the potential for glycogen determination in complex samples.     

Kinetics and mechanism of phenylacetylene hydrogenation under hydrogen pressure

Kinetic and potentiometric curves of phenylacetylene hydrogenation on a 5% Pt/C catalyst in ethanol under hydrogen pressure have been studied. On the basis of chromatographic data a possible scheme of the process is suggested.

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. 5% Pt/C . .

     

Selective co-aggregation of gold nanoparticles functionalised with complementary hydrogen-bonding groups

Molecular recognition between two species of Au55 clusters bearing complementary hydrogen-bonding groups provides a facile route to accomplish solution-based assembly of two-component nanoparticle aggregates.     

Selective partial hydrogenation of hydroxy aromatic derivatives with palladium nanoparticles supported on hydrophilic carbon

Selective hydrogenation of phenol to cyclohexanol in the aqueous phase was achieved using a new catalytic system based on palladium particles supported on hydrophilic carbon prepared by one-pot hydrothermal carbonisation.     

Selective hydrogenation of monosubstituted alkenes by Pd nanoparticles embedded in polyelectrolyte films

Pd nanoparticles embedded in multilayer polyelectrolyte films can be easily prepared through layer-by-layer adsorption of poly(acrylic acid) (PAA) and poly(ethyleneimine)-Pd2+ (PEI-Pd(II)) complexes followed by reduction of Pd(II) with NaBH4. Transmission electron microscopy confirms the formation of Pd particles with diameters of 1-3 nm. Remarkably, [PAA/PEI-Pd(0)]3PAA films catalyze the hydrogenation of monosubstituted alkenes with turnover frequencies that are as much as 100-fold higher than turnover frequencies for hydrogenation of multiply substituted double bonds. Selectivities in the hydrogenation of monosubstituted over multisubstituted double bonds are higher than those of Wilkinson's catalyst. Moreover, the turnover frequency for the hydrogenation of allyl alcohol did not change when the catalyst was recycled three times. Intramolecular selectivity for the hydrogenation of monosubstituted alkenes also occurs when substrate molecules contain both mono and multiply substituted double bonds. The combination of the encapsulating polyelectrolyte film and small nanoparticles apparently results in hindered access of multiply substituted double bonds to catalytic sites.