Studies on catalytic hydrogenation of citral by water-soluble palladium complex

The hydrogenation of citral has been studied in biphasic system using water-soluble PdCl₂(TPPTS)₂ as catalyst. The selectivity to form citronellal increased with increasing pH values of the aqueous phase. At the same pH value, the selectivity was higher when the hydrogenation was carried out in the presence of Na₂CO₃ than in the presence of NaOH. The main product was citronellal and a maximum yield of 93% had been obtained using Na₂CO₃ solution at pH 11.6. The CC bond in citronellal could be further hydrogenated to form dihydrocitronellal when the hydrogenation was carried out in distilled water at pH 6.0. The yield of dihydrocitronellal could reach 93% with prolonged reaction time to 6 h. Therefore, high yields of either citronellal or dihydrocitronellal could be obtained from citral by selecting the corresponding reaction conditions.     

Effect of transition metals (Cr, Mn, Fe, Co, Ni and Cu) on selective hydrogenation of cinnamaldehyde over Pt/CNTs catalyst

Summary The effect of transition metals (Cr, Mn, Fe, Co, Ni and Cu) on the selective hydrogenation of cinnamaldehyde (CMA) to the corresponding semi-hydrogenated product over Pt/CNTs catalyst has been studied in ethanol at 343 K under 2.0 MPa H₂ pressure. PtNi/CNTs catalyst shows good catalytic activity and selectivity of C=C bond hydrogenation, 68.4% for conversion of CMA and 97.0% for selectivity of hydrocinnamaldehyde (HCMA). PtCo/CNTs catalyst shows good catalytic activity and selectivity of C=O bond hydrogenation, 91.3% for conversion of CMA and 88.2% for selectivity of cinnamylalcohol (CMO).     

Ni-Ti Intercalated and Supported Bentonite for Selective Hydrogenation of Cinnamaldehyde

Ni-Ti intercalated bentonite catalysts (Ni-Ti-bentonite) and Ni-TiO₂ supported bentonite catalysts (Ni-TiO₂/bentonite) were prepared, and the effects of Ni-Ti supported and intercalated bentonite on the selective hydrogenation of cinnamaldehyde were investigated. Ni-Ti intercalated bentonite enhanced the Brønsted acid sites strength, decreased the acid amount and Lewis's acid sites strength, which inhibited the activation of the C=O bond and contributed to selective hydrogenation of the C=C bond. When Ni-TiO₂ was supported on bentonite, the acid amount and Lewis's acid strength of the catalyst increased, providing additional adsorption sites and increased the acetals byproducts. Due to the higher surface area, mesoporous volume, and suitable acidity, compared with Ni-TiO₂/bentonite in methanol solvent, 2 MPa, 120 °C for 1 h, Ni-Ti-bentonite exhibited a higher cinnamaldehyde (CAL) conversion of 98.8 %, as well as a higher hydrocinnamaldehyde (HCAL) selectivity of 95 %, and no acetals were found in the product.     

Selective hydrogenation of cinnamaldehyde over carbon nanotube supported pd-ru catalyst

Summary Carbon nanotube supported Pd, Ru and Pd-Ru catalysts have been prepared and tested with the hydrogenation of cinnamaldehyde as a probe reaction. It has been found that the cinnamaldehyde conversion and the selectivity towards the hydrogenation of C=O bond over Pd-Ru/PCNT catalyst could reach 56.6% and 79.1%, respectively, at 120^oC and 5.0 MPa, which is better than Pd/PCNT and Ru/PCNT catalysts under the same reaction conditions. It is assumed that the better performance of Pd-Ru/PCNT catalyst for cinnamaldehyde hydrogenation may be due to the synergic effect of Pd and Ru metals or the promoting effect of Ru metal.     

Highly Active Supported Pt Nanocatalysts Synthesized by Alcohol Reduction towards Hydrogenation of Cinnamaldehyde: Synergy of Metal Valence and Hydroxyl Groups

The hydrogenation of α,β‐unsaturated aldehydes to allylic alcohols or saturated aldehydes provides a typical example to study the catalytic effect on structure‐sensitive reactions. In this work, supported platinum nanocatalysts over hydrotalcite were synthesized by an alcohol reduction method. The Pt catalyst prepared by the reduction with a polyol (ethylene glycol) outperforms those prepared with ethanol and methanol in the hydrogenation of cinnamaldehyde. The selectivity towards the C=O bond is the highest over the former, although its mean size of Pt particles is the smallest. The hydroxyl groups on hydrotalcite could act as an internally accessible promoter to enhance the selectivity towards the C=O bond. The optimal Pt catalyst showed a high activity with an initial turnover frequency (TOF) of 2.314 s^?1. This work unveils the synergic effect of metal valence and in situ promoter on the chemoselective hydrogenation, which could open up a new direction in designing hydrogenation catalysts.     

Palladium‐Catalyzed Hydrolytic Cleavage of Aromatic C−O Bonds

Metallic palladium surfaces are highly selective in promoting the reductive hydrolysis of aromatic ethers in aqueous phase at relatively mild temperatures and pressures of H₂. At quantitative conversions, the selectivity to hydrolysis products of PhOR ethers was observed to range from 50 % (R=Ph) to greater than 90 % (R=n ‐C₄H₉, cyclohexyl, and PhCH₂CH₂). By analysis of the evolution of products with and without incorporation of H₂¹⁸O, the pathway was concluded to be initiated by palladium metal catalyzed partial hydrogenation of the phenyl group to an enol ether. Water then rapidly adds to the enol ether to form a hemiacetal, which then undergoes elimination to cyclohexanone and phenol/alkanol products. A remarkable feature of the reaction is that the stronger Ph−O bond is cleaved rather than the weaker aliphatic O−R bond.     

Synthesis and application of palladium stearates as precursors for the preparation of palladium nanoparticles

This report successfully demonstrates the synthesis and application of palladium stearates. It was found that the branching of the carboxylate anion of metal precursors could influence the size and shape of palladium nanoparticles (PdNPs). Worm‐like nanowires were formed when using the branched isomer palladium isostearate (PdISt₂), while triangular nanoparticles were produced in a majority when using the normal form: palladium stearate (PdSt₂). Furthermore, when applying CO₂ to the system, both types of PdNPs transformed into more spherical shapes with smaller sizes. The formation of carbamates from the amine stabilizer with CO₂ could prevent the further growth and aggregation of PdNPs. The PdNPs were tested as catalysts for the hydrogenation of styrene, and higher catalytic activities were achieved with PdNPs that were prepared with the assistance of CO₂. Copyright © 2012 John Wiley & Sons, Ltd.     

Reaction of carbon dioxide with a bimetallic octadienylbridged palladium complex

Reaction of equimolar amounts of μ-1–3-η:6–8-η-octadienatobis(1,1,1,5,5,5-hexafluoroacetylacetonatopalladium) and triisopropylphosphine gives a bimetallic octadienyl-bridged complex, in which one palladium atom is η¹ -bound to the terminal carbon of the octadienyl chain. Insertion of CO₂ into this PdC bond gives a carboxylate complex; acidic decomposition and hydrogenation of the carboxylate complex gives pelargonic acid. The results are discussed in relation to the mechanism of the palladium-catalyzed reaction between butadiene and carbon dioxide.     

Baker's yeast reduction of α-methyleneketones

The bioreduction of α-methyleneketones, R¹C(O)C(CH₂)R² (R¹=Me, Et, Pr, iso-Bu, Ph, CH₂CH₂Ph; R²=Cl, Me, Et, n-Pr, iso-Pr, n-Bu, n-C₆H₁₃, Ph, CH₂Ph), was mediated by baker's yeast (Saccharomyces cerevisiae) to obtain the corresponding α-methylketones. The R¹ and R² groups had a significant influence on the rate and enantioselectivity of the reductions. The rate of CC bond reduction was higher than that of CO bond reduction. Only α-methyleneketones having R¹=Me yielded α-methylketones in high enantioselectivity with e.e.s of 88–99%.     

Selective Amplification of CO Bond Hydrogenation on Pt/TiO2: Catalytic Reaction and Sum‐Frequency Generation Vibrational Spectroscopy Studies of Crotonaldehyde Hydrogenation

The hydrogenation of crotonaldehyde in the presence of supported platinum nanoparticles was used to determine how the interaction between the metal particles and their support can control catalytic performance. Using gas‐phase catalytic reaction studies and in situ sum‐frequency generation vibrational spectroscopy (SFG) to study Pt/TiO₂ and Pt/SiO₂ catalysts, a unique reaction pathway was identified for Pt/TiO₂, which selectively produces alcohol products. The catalytic and spectroscopic data obtained for the Pt/SiO₂ catalyst shows that SiO₂ has no active role in this reaction. SFG spectra obtained for the Pt/TiO₂ catalyst indicate the presence of a crotyl‐oxy surface intermediate. By adsorption through the aldehyde oxygen atom to an O‐vacancy site on the TiO₂ surface, the C?O bond of crotonaldehyde is activated, by charge transfer, for hydrogenation. This intermediate reacts with spillover H provided by the Pt to produce crotyl alcohol.     

Solution pH: A Selectivity Switch in Aqueous Organometallic Catalysis—Hydrogenation of Unsaturated Aldehydes Catalyzed by Sulfonatophenylphosphane–Ru Complexes

The equilibribrium distribution of water-soluble ruthenium hydrides [HRuCl(tppms)₃] and [H₂Ru(tppms)₄] (tppms = (3-sulfonatophenyl)diphenylphosphane) in the reaction with H₂ is governed by the pH value. As a consequence, the selectivity in the hydrogenation of cinnamaldehyde for reaction at C=C or C=O can be completely inverted by changing the pH value (see drawing below).     

Selective Hydrogenation of α,β-Unsaturated Aldehydes Catalyzed by Amine-Capped Platinum-Cobalt Nanocrystals

More Greasy, More Selective: Amine-capped Pt(3) Co nanocatalysts were synthesized and used for the hydrogenation of cinnamaldehyde (CAL). Capping the catalysts with amines that contain long carbon chains results in an ordered surface "array", in which high selectivity towards C?O hydrogenation can be achieved because the C?C bond in CAL does not interact with the surface. The longer the carbon chains in the amine, the higher the selectivity.     

ZrO_2负载Pt催化巴豆醛选择性加氢

以高比表面积ZrO2为载体,采用浸渍法制备了负载型Pt催化剂,应用于常压下气相巴豆醛加氢反应,考察了Pt负载量和H2还原温度等对巴豆醛选择性加氢性能的影响.实验结果表明,Pt负载量(质量分数)为3%的3Pt/ZrO2催化剂经500℃还原后,具有较高的巴豆醛选择性加氢性能:巴豆醛转化率为27%,巴豆醇的选择性为55%.X射线粉末衍射(XRD)分析,CO化学吸附,NH3程序升温脱附(NH3-TPD)表征结果表明Pt/ZrO2催化剂上Lewis强酸中心和适宜的Pt颗粒(约为8nm)有利于巴豆醛选择性加氢生成巴豆醇.     

Preparation of Pd/C catalysts via deposition of palladium hydroxide onto Sibunit carbon and their application to partial hydrogenation of rapeseed oil

Pd/Sibunit catalysts were prepared by deposition of palladium hydroxide onto the support surface in an alkaline medium. It was found that the palladium distribution throughout the catalyst grain, and the dispersion of Pd particles depend on (i) the order of the addition of H₂PdCl₄ and Na₂CO₃ to carbon suspension, (ii) Na₂CO₃ to H₂PdCl₄ ratio, and (iii) aging time of the mixture H₂PdCl₄ + Na₂CO₃ before its addition to the carbon. The catalysts were tested in the hydrogenation of cyclohexene and rapeseed oil under static conditions. The yield of trans-isomers as products of partial hydrogenation of rapeseed oil was found to decrease with decreasing the Pd particle size in the catalysts, as well as with increasing the Pd concentration on the periphery of the support grains.     

温控聚乙二醇两相体系中纳米钯催化肉桂醛选择性加氢反应

将具有“高温混溶、室温分相”功能的聚乙二醇4000(PEG₄₀₀₀)与甲苯-正庚烷组成的两相体系用于纳米钯催化的肉桂醛选择性加氢反应中.在优化的反应条件下,肉桂醛转化率和氢化肉桂醛选择性分别为99%和98%.钯纳米催化剂经简单分相即可与产物分离,且循环使用8次,其活性和选择性基本保持不变.     

Reduction of olefins,nitroarenes and Schiff base compounds by a polymer-supported [2-(2′-pyridyl)benzimidazole]palladium complex

2-(2′-Pyridyl)benzimidazole (PBIMH) was functionalized onto chloromethylated polystyrene beads crosslinked with 6.5 % divinylbenzene, and this solid support was then reacted with Na₂PdCl₄ in methanol. The functionalized beads were then activated using sodium borohydride. The resultant polymer-supported [2-(2′-pyridyl)benzimidazole]palladium complex (PSDVB–PBIM–PdCl₂) and its activated form were characterized by various physicochemical techniques. XPS studies confirmed the +2 oxidation state of palladium in the supported complex. The activated complex was found to catalyse the hydrogenation of various organic substrates including olefins, nitro and Schiff base compounds. Kinetic measurements for the hydrogenation of cyclopentene, cyclohexene and cyclooctene were carried out by varying temperature, catalyst and substrate concentration. The energy and entropy of activation were evaluated from the kinetic data. The catalyst showed an excellent recycling efficiency over six cycles without leaching of metal from the polymer support, whereas the unsupported complex was unstable as metal leached out into the solution during the first run.     

Enhancing Metal–Support Interactions by Molybdenum Carbide: An Efficient Strategy toward the Chemoselective Hydrogenation of α,β‐Unsaturated Aldehydes

Metal–support interactions are desired to optimize the catalytic turnover on metals. Herein, the enhanced interactions by using a Mo₂C nanowires support were utilized to modify the charge density of an Ir surface, accomplishing the selective hydrogenation of α,β‐unsaturated aldehydes on negatively charged Ir^δ? species. The combined experimental and theoretical investigations showed that the Ir^δ? species derive from the higher work function of Ir (vs. Mo₂C) and the consequently electron transfer. In crotonaldehyde hydrogenation, Ir/Mo₂C delivered a crotyl alcohol selectivity as high as 80 %, outperforming those of counterparts (<30 %) on silica. Moreover, such electronic metal–support interactions were also confirmed for Pt and Au, as compared with which, Ir/Mo₂C was highlighted by its higher selectivity as well as the better activity. Additionally, the efficacy for various substrates further verified our Ir/Mo₂C system to be competitive for chemoselective hydrogenation.     

Influence of electronic state and dispersion of platinum particles on the conversion and selectivity of hydrogenation of an α,β-unsaturated aldehyde in supercritical carbon dioxide

The selective hydrogenation of cinnamaldehyde (CAL) was investigated using silica supported platinum catalysts in supercritical carbon dioxide. Selectivity to cinnamyl alcohol (COL) is enhanced as Pt⁰/Pt²⁺ ratio increases; namely, zero-valent metallic surface is beneficial to the formation of COL compared with less reduced surface. The influence of Pt⁰/Pt²⁺ ratio is more significant on the selectivity than on the total conversion. For the catalyst with small Pt⁰/Pt²⁺ value, the selectivity also depends on the degree of platinum dispersion. The selectivity to COL is higher for higher degree of platinum dispersion. The CO₂ pressure did not affect the conversion and selectivity so much.     

Zn4Si2O7(OH)2H2O盐修饰的纳米Ru催化剂催化苯选择加氢制环己烯

利用沉淀法制备了纳米Ru催化剂,在ZnSO₄存在下考察了Na₂SiO₃·9H₂O和二乙醇胺作反应修饰剂对Ru催化剂催化苯选择加氢制环己烯性能的影响,并用X-射线衍射(XRD)、X-射线荧光光谱(XRF)和透射电镜-能量散射谱(TEM-EDS)等物理化学手段对加氢前后Ru催化剂进行了表征。结果表明,在水溶液中Na₂SiO₃与ZnSO₄可以反应生成Zn₄Si₂O₇(OH)₂H₂O盐、H₂SO₄和Na₂SO₄,化学吸附在Ru催化剂表面上的Zn₄Si₂O₇(OH)₂H₂O盐起着提高Ru催化剂环己烯选择性的关键作用。Na₂SiO₃·9H₂O量的增加,生成的Zn₄Si₂O₇(OH)₂H₂O盐逐渐增加,Ru催化剂的活性降低,环己烯选择性逐渐升高。向反应体系中加入二乙醇胺,它可以中和Na₂SiO₃与ZnSO₄反应生成的硫酸,使化学平衡向生成更多的Zn₄Si₂O₇(OH)₂H₂O盐的方向移动,导致Ru催化剂环己烯选择性增加。当Ru催化剂与ZnSO₄·7H₂O、Na₂SiO₃·9H₂O和二乙醇胺、分散剂ZrO₂的质量比为1.0:24.6:0.4:0.2:5.0时,2 g Ru催化剂上苯转化73%时环己烯选择性和收率分别为75%和55%,而且该催化剂体系具有良好的重复使用性和稳定性。     

Zn4Si2O7(OH)2H2O盐修饰的纳米Ru催化剂催化苯选择加氢制环己烯

利用沉淀法制备了纳米Ru催化剂, 在ZnSO₄存在下考察了Na₂SiO₃·9H₂O和二乙醇胺作反应修饰剂对Ru催化剂催化苯选择加氢制环己烯性能的影响, 并用X-射线衍射(XRD)、X-射线荧光光谱(XRF)和透射电镜-能量散射谱(TEM-EDS)等物理化学手段对加氢前后Ru催化剂进行了表征。结果表明, 在水溶液中Na₂SiO₃与ZnSO₄可以反应生成Zn₄Si₂O₇(OH)₂H₂O盐、H₂SO₄和Na₂SO₄, 化学吸附在Ru催化剂表面上的Zn₄Si₂O₇(OH)₂H₂O盐起着提高Ru催化剂环己烯选择性的关键作用。Na₂SiO₃·9H₂O量的增加, 生成的Zn₄Si₂O₇(OH)₂H₂O盐逐渐增加, Ru催化剂的活性降低, 环己烯选择性逐渐升高。向反应体系中加入二乙醇胺, 它可以中和Na₂SiO₃与ZnSO₄反应生成的硫酸, 使化学平衡向生成更多的Zn₄Si₂O₇(OH)₂H₂O盐的方向移动, 导致Ru催化剂环己烯选择性增加。当Ru催化剂与ZnSO₄·7H₂O、Na₂SiO₃·9H₂O和二乙醇胺、分散剂ZrO₂的质量比为1.0:24.6:0.4:0.2:5.0时, 2 g Ru催化剂上苯转化73%时环己烯选择性和收率分别为75%和55%, 而且该催化剂体系具有良好的重复使用性和稳定性。