Intensification of heterogeneously catalyzed Suzuki-Miyaura cross-coupling reaction using ultrasound: Understanding effect of operating parameters

Palladium-catalyzed Suzuki-Miyaura cross-coupling reaction is a significant reaction for obtaining industrially important products. The current research work deals with intensification of reaction of 4-bromoanisole and phenylboronic acid catalyzed with 5 wt% Pd/C (5% by weight Pd supported on C available as commercial catalyst) using ultrasound and more importantly, without use of any additional phase transfer catalyst. Heterogeneous catalyst has been selected in the present work so as to harness the benefits of easy separation and the possible limitations of heterogeneous operation are minimized by introducing ultrasonic irradiations. The effect of operating parameters such as ultrasound power, temperature, catalyst loading and molar ratio on the progress of reaction has been investigated. It has been observed that an optimum power, temperature and catalyst loading exist for maximum benefits whereas higher molar ratio was found to be favourable for the progress of the reaction. Also, the use of ultrasound reduced the reaction time from 70 min required in conventional approach to only 35 min under conditions of frequency of 22 kHz, power dissipation of 40 W and catalyst loading as 1.5 mol% (refers to total quantum of catalyst used in the work) in ethanol-water system under ambient conditions. The work also demonstrated successful results at ten times higher volume as compared to the normally used volumes in the case of simple ultrasonic horn. Overall, the work has successfully demonstrated process intensification benefits obtained due to the use of ultrasound for heterogeneously catalyzed Suzuki-Miyaura cross-coupling reaction.     

Phosphorus-doped carbon nanoparticles supported palladium electrocatalyst for the hydrogen evolution reaction (HER) in PEM water electrolysis

Hydrogen production by PEM water electrolysis is one of the most efficient methods, due to the produced high purity of gases, high efficiency, and devoid of harmful emissions. In this study, phosphorus-doped carbon nanoparticles (P-CNPs) were synthesized by spray pyrolysis method in chemical vapor deposition (CVD). The synthesized P-CNPs were used as electron carrier support materials for the preparation of P-CNPs-supported palladium (Pd/P-CNPs) electrocatalyst and also used as the hydrogen evolution reaction (HER) electrode in PEM water electrolysis. These synthesized Pd/P-CNPs were characterized by field emission scanning electron microscope, energy-dispersive X-ray spectroscopy, X-ray diffraction, and cyclic voltammetry methods. The membrane electrode assemblies (MEAs) were fabricated using Pd/P-CNPs as a cathode catalyst for the HER and RuO₂ as the anode for oxygen evolution reaction (OER). The fabricated MEA electrochemical performances along with their corresponding yields of hydrogen production were evaluated in PEM water electrolyzer single cell assemblies at various experimental conditions. The obtained results showed that the synthesized Pd/P-CNPs observed a current density of 1 A cm^?2 at 2 V at 80 °C. Further, long-term stability tested for up to 500 h continuously and showed the reasonable stability with similar electrochemical activity compared to commercial Pt/CB. Hence, the synthesized Pd/P-CNPs could be used as the alternative to Pt-based catalysts for HER.     

Expanding cyclitol structural diversity by biocatalysis and metalocatalysis. A click chemistry approach

The palladium catalyzed cross-coupling reaction of phenyltrifluoroborate with a chemoenzymatically derived bromoazidoconduritol, combined with 1,3-dipolar cycloaddition, with a variety of alkynes is described. Fourteen new compounds were synthesized in moderate to good yields. The click chemistry reaction can be effected by using sodium ascorbate and CuSO₄ · 5H₂O as catalyst in toluene–H₂O at room temperature.     

Knoevenagel reaction in water catalyzed by amine supported on silica gel

Summary An environmentally benign and sustainable Knoevenagel reaction of aldehyde with ethyl cyanoacetate has been achieved at ambient temperature in water employing 3-aminopropylated silica gel (NAP) as a catalyst. Wide applicability of the reaction is illustrated by the results that not only arylaldehydes of both electronic characters but also aliphatic aldehydes afforded the products. The reaction condition was so mild that aldehydes having acid- or base-sensitive substituents provided substituted α-cyano-α, β-unsaturated esters. The catalyst has been efficiently recycled more than five times without any pre-treatment. Catalyst loading was successfully reduced to 0.0029 mmol% (TON = up to 9,226). This protocol was also applicable to the Knoevenagel reaction of malononitrile in good yields in water.     

Homogeneous catalysis on the gas-phase dehydration reaction of tertiary alcohols by hydrogen bromide. Density functional theory calculation

The gas-phase thermal dehydration mechanism of tert-butanol, 2-methyl-2-butanol, 2-methyl-2-pentanol and 2,3-dimethyl-2-butanol by homogeneous catalysis of hydrogen bromide was examined by density functional theory calculations with the hybrid functionals: M062X, CAMB3LYP and WB97XD. Reasonable agreements were found between theoretical and experimental enthalpy values at the WB97XD/6-311++G(d,p) level. The dehydration mechanism of tert-butanol with and without catalysis was evaluated in order to examine the catalyst effect on the mechanism. The elimination reaction without catalysis involves a four-membered transition state (TS), while the reaction with catalysis involves a six-membered TS. The mechanism without catalysis has enthalpy activation over 150 kJ mol^–1 greater than the catalysed reaction. In all these reactions, the elongation of the C–O bond is significant in the TS. The un-catalysed reaction is controlled by breaking of C–O bond, and it was found to be more synchronous (Sy ≈ 0.91) than the hydrogen bromide catalysed reactions (Sy ≈ 0.75–0.78); the latter reactions are dominated by the three reaction coordinates associated with water formation. No significant effect on the enthalpies of activation was observed when the size of the alkyl chain was increased.     

Green preparation of Pd nanoparticles on SBA-15 via supercritical fluid deposition and application on Suzuki–Miyaura cross-coupling reaction

A well-dispersed green Pd/SBA-15 catalyst with an average size of 13.7 nm and 492.6 m²/g BET surface area is prepared via supercritical fluid deposition method with a new bipyridyl precursor that enables reduction at mild conditions at 80 °C and 17.2 MPa. The catalytic performance of Pd/SBA-15 prepared using scCO₂ with hydrogen reduction was assessed for Suzuki–Miyaura coupling reaction of bromobenzene and phenylboronic acid that was chosen as a model coupling reaction. The catalyst was tested in six different solutions and in three organic and inorganic bases during reactions. In general, the effect of bases is investigated when solvents are held constant and K₂CO₃ appears to have the best results in the activity studies used. For each of the 3 bases used, the highest catalytic activity was reached as the result of the solvent system being ethanol/water (1:1). The highest catalytic conversion was obtained in the ethanol-K₂CO₃ solvent-base pair. The catalyst synthesized in this study exhibited high activities and TON value was found as 160.8 at room temperature.     

固-液-气三相环境下非均相苯加氢反应的原位核磁共振研究

本文使用原位核磁共振（NMR）方法成功实现了对固-液-气三相环境下加氢反应的在线监测.在反应过程中,无需分离产物.以Pd纳米颗粒粉末及不同晶面形貌的Pd催化剂作为研究对象,我们研究了反应压强、反应时间,以及催化剂表面性质对加氢反应的影响.结果表明,压强和反应时间都可以影响加氢反应产物的产率,使用暴露不同晶面的Pd共催化剂时,苯加氢产物不同.该原位NMR方法为研究固-液-气三相催化环境下的加氢反应机理提供了可能.     

Kinetics and mechanism of uncatalysed and ruthenium(III) catalysed oxidation of allyl alcohol by diperiodatoargentate(III) in aqueous alkaline medium

The oxidation of allyl alcohol by diperiodatoargentate(III) (DPA) is carried out both in the absence and presence of ruthenium(III) catalyst in alkaline medium at 298 K and a constant ionic strength of 1.1 mol dm^?3 was studied spectrophotometrically. The oxidation products in both the cases were acrolein and Ag(I), identified by spectral studies. The stoichiometry is same in both the cases, that is, [AA]/[DPA] = 1:1. The reaction shows first order in [DPA] and has less than unit order dependence each in both [AA] and [Alkali] and retarding effect of [IO] in both the catalysed and uncatalysed cases. The order in [Ru(III)] is unity. The active species of DPA is understood to be as monoperiodatoargentate(III) (MPA) in both the cases. The uncatalysed reaction in alkaline medium has been shown to proceed via a MPA–allyl alcohol complex, which decomposes in a rate determining step to give the products. In catalysed reaction, it has been shown to proceed via a Ru(III)‐allyl alcohol complex, which further reacts with one molecule of MPA in a rate determining step to give the products. The reaction constants involved in the different steps of the mechanisms were calculated for both reactions. The catalytic constant (K_c) was also calculated for catalysed reaction at different temperatures. The activation parameters with respect to slow step of the mechanisms were computed and discussed for both the cases. The thermodynamic quantities were also determined for both reactions. Copyright © 2007 John Wiley & Sons, Ltd.     

Preparation of metallic Pd nanoparticles using supercritical CO<Subscript>2</Subscript> deposition: An efficient catalyst for Suzuki cross-coupling reaction

Ligand-free palladium nanoparticles supported on multi-walled carbon nanotubes (Pd/MWCNT) were prepared by the supercritical carbon dioxide (scCO₂) deposition method using a novel scCO₂-soluble Pd organometallic complex as a precursor. The precursor with the perfluoroalkyl chain group was synthesized and identified by microanalytic methods. The deposition was carried out at the temperature of 363.15 K and pressure of 27.6 MPa CO₂. The prepared metallic nanoparticles were obtained with an average size of 2 nm. Pd/MWCNT was utilized as a heterogeneous catalyst in Suzuki cross-coupling reaction. The nanocatalyst was found very effective in Suzuki reaction and it could also be recovered easily from the reaction media and reused over several cycles without significant loss of catalytic activity under mild conditions.

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Graphical Abstract Pd/MWCNT was prepared by the scCO2 deposition method using a new synthesized perfluroalkylated vic-dioxime Pd complex as the precursor. The prepared nanoparticle was very effective as catalyst and reusable for Suzuki cross coupling reaction under mild conditions.

     

The Suzuki homocoupling reaction under high-intensity ultrasound

The Pd-catalysed Suzuki homocoupling of boronic acids was successfully carried out in water under high-intensity ultrasound (HIU). Heterogeneous catalysis with Pd/C, facilitating work up and purification, could be used without adding phosphine ligands. Reaction rates and yields were strongly influenced by the oxidant employed; excellent results were obtained using either molecular oxygen or 3-bromo-4-hydroxycoumarin. 3-Arylation of the latter with the Suzuki procedure had failed, exclusively affording the homocoupling products, symmetric biaryls. Besides offering a number of operational advantages, the use of HIU broadens the field of application for the Suzuki reaction.     

Enhanced electro-catalytic degradation of chloroorganic compounds in the presence of ultrasound

The application of finely divided (black) Pd and Pd-Fe powder in the sono-electro-catalytic reduction of chlorophenoxy herbicides (2,4-D) and chlorophenols (2,4-DCP) in aqueous solutions allows for effective destruction of toxic chlorinated aromatic compounds. At 20 degrees C complete conversion of these compounds is observed within 10 min. On bimetallic Pd/Fe catalyst, intermediates due to the oxidation reaction are detected in addition to the products of dechlorination. The bimetallic catalyst appears to be energetically and economically superior to the Pd. In both cases, the reaction times were considerably shortened in comparison with traditional electro-catalytic processes.     

Arylboronic acids as versatile coupling partners in fast microwave promoted oxidative Heck chemistry

The useful and selective reactivity of arylboronic acids makes them favourite building blocks for many modern organic chemistry applications like the metal-mediated formation of C-C, C-O, C-N, and C-S bonds. This report describes oxidative Heck coupling reactions of arylboronic acids and olefins, which were conveniently and rapidly (5-30 min) carried out under air with temperature-controlled microwave heating. Different reaction conditions were investigated with regard to both microwave heating capability and chemical productivity. Copper(II) acetate was identified as a microwave compatible reoxidant of Pd(0). The scope and limitations of this high-speed chemistry protocol with diverse olefins and organoboronic acids are discussed.     

Pd/WO<Subscript>3</Subscript>/C nanocomposite with APTMS-functionalized tungsten oxide nanosheet for formic acid electrooxidation enhancement

A Pd/WO₃/C nanocomposite with 3-aminopropyltrimethoxysilane (APTMS)-functionalized tungsten oxide nanosheets (Pd/WO₃/C-APTMS) was synthesized and applied as the efficient anode catalyst for direct formic acid fuel cells (DFAFCs). The mechanism for synthesizing the nanocomposite is as follows: initially, [PdCl₄]^2? was assembled onto the tungsten oxide nanosheets modified with APTMS. Following this, Pd nanoparticles were reduced via traditional impregnation reduction of [PdCl₄]^2? with NaBH₄. The transmission electron microscope (TEM) images revealed that the Pd nanoparticles were uniformly dispersed on WO₃ nanosheets and were approximately 2.7 nm in size. The electrochemical test results showed that enhanced electrocatalytic activity for the formic acid oxidation reaction (FAOR) was obtained on the Pd/WO₃/C catalyst compared with Pd/C. The higher electrocatalytic activity might be attributed to the uniform distribution of Pd with smaller particles. Furthermore, it is likely that the improvement in catalytic stability for the Pd/WO₃/C catalyst is due to the hydrogen spillover effect of WO₃ particles. These results indicate that this novel Pd/WO₃/C-APTMS nanocomposite exhibits promising potential for use as an anode electrocatalyst in DFAFCs.     

Making precious metals cheap: A sonoelectrochemical – Hydrodynamic cavitation method to recycle platinum group metals from spent automotive catalysts

Platinum group metals, such as Pd and Pt, found in three-way catalyst converters were recycled in a two-step method: hydrodynamic cavitation followed by sonoelectrochemical dissolution. High shear forces were obtained by using a convergent nozzle with a throat diameter of 0.2 mm, feeded by a plunger pump at a pressure of 60 MPa. Cavitating submerged jets acted locally on the water dispersed waste catalyst. As-obtained samples were analyzed by scanning electron microscopy and transmission electron microscopy. Electron microscopy on the initial sample showed that round shaped Pd and Pt nanoparticles were randomly distributed on the Al₂O₃ matrix. Cavitated samples show two zones in which Pt and Pd were partially and completely separated from the cordierite. The hydrodynamic cavitation separates the Pd and Pt from the cordierite leading to an apparent increase in Pd and Pt concentrations of 9% and 34% respectively. Conventional electrochemistry showed a dissolution of 20% in 1 h. To further accelerate the dissolution, a sonotrode operating at 20 kHz and 75 W was placed inside an electrochemical cell in order to increase the mass transport and obtain high dissolution rates. Indeed, the results showed that 40% of the available Pd and Pt can be recycled in just 1 h. In the absence of hydrodynamic cavitation and using conventional electrochemistry less than 10% of the available Pt and Pd is recovered in 1 h. The cost analysis showed that Pd and Pt can be recovered at less than 10 EUR per g which is 5 times smaller than their current market price.     

Sonocatalyzed facile and mild one pot synthesis of gem-dichloroaziridine derivatives under alkaline conditions

In this research, rapid and efficient preparation of 2,2-dichloro-1,3-diarylaziridines through the reaction of Schiff base compounds with dichlorocarbene yielded in situ from chloroform and sodium hydroxide without any phase transfer catalyst under ultrasonic irradiation is described. The advantages of this reaction are very short reaction times, excellent product yields, simplicity of the method and high purity of products.     

Ultrasound irradiation promoted efficient solvent-free lipase-catalyzed production of mono- and diacylglycerols from olive oil

This work reports the enzymatic production of mono- and diacylglycerols under the influence of ultrasound irradiation, in a solvent-free system, with and without the presence of surfactants at a constant temperature of 65°C, glycerol to oil molar ratio of 2:1 and a commercial immobilized lipase (Novozym 435) as catalyst. For this purpose, two operation modes were adopted: the use of a sonotrode (ultrasonic probe), without agitation, varying reaction time, irradiation amplitude (25-45% of the total power) and type of surfactant, and a mechanically stirred reactor (600 rpm) under ultrasound irradiation in a water bath, testing different surfactants. Results show that very satisfactory MAG and DAG yields, above 50 wt.%, can be obtained without the use of surfactant, at mild irradiation power supply (～130 W), with no important enzyme activity losses verified, in a relatively short reaction time (2h), and low enzyme content (7.5 wt.%). Also, reaction kinetic results show that contents of MAG+DAG as high as ～65 wt.% can be achieved at longer times (6h), indicating a promising route for producing MAG and DAG using ultrasound irradiation.     

Ultrasound-assisted reaction of 2,3,4,6-tetra-O-acetyl-alpha-D-glucopyranosyl bromide with potassium salt of curcumin under PTC conditions

Reaction of 2,3,4,6-tetra-O-acetyl-alpha-D-glucopyranosyl bromide with potassium salt of curcumin [bis-1,7-(3'-methoxy-4'-hydroxy)phenyl-5-hydroxy-1,4,6-heptatrien-3-one] under either thermal or high pressure conditions affect the labile substrate, curcumin, thus resulting in drastic reduction in the yields of the glucosides. This drawback could be effectively overcome by carrying out the biphasic reaction in the presence of a phase transfer catalyst under the effect of ultrasound. The reaction under the sonochemical conditions was faster and resulted in the increased yield of the glucoside products. The reaction was investigated in detail with a view to optimizing the yield of the glucosides. The detailed study clearly indicated the important role of the nature and quantity of the phase transfer catalyst employed in the reaction. Also, the selectivity with respect to the formation of mono- or di-beta-glucosides under both mono- and biphasic reaction conditions was clearly discernable. The study establishes a simple synthetic protocol for the glucoside derivatives of curcumin in high yields and selectivity using ultrasonic waves.     

Pd@C2N单原子催化剂用于HCOOH分解制氢的第一性原理研究

用于HCOOH分解制氢的贵金属催化剂存在着反应选择性差和资源稀缺等问题. 单原子催化可以有效改善催化剂性能并降低成本,C2N作为一种新型二维材料可为过渡金属原子提供良好的结合位点. 本文利用密度泛函理论,设计了Pd@C2N 单原子催化剂,研究了HCOOH在Pd@C2N表面上的吸附和分解制氢反应机理. 结果表明,HCOOH可在Pd原子顶位吸附,并在C2N表面N原子的协同作用下通过甲酸盐路径生成氢气. 通过比较,Pd@C2N具有比Pd (111)表面更好的催化反应活性与选择性.     

葡萄糖催化氧化反应用Pd-Co/C催化剂的XPS研究

活性炭为载体，制备了Pd/C、Pd-Co/C催化剂，并进行了活性评价和XPS表征。在Pd-Co／C催化剂上反应3h后，葡萄糖的转化率达92％，选择性为94％，较Pd/C催化剂有显著提高。Co的添加及焙烧过程改变了Pd/C催化剂的表面组成及结构，提高了贵金属Pd在催化剂表面的分散程度，有利于Pd的还原，使Pd-Co/C催化剂表现出良好的催化活性和选择性。     

Microwave-assisted Pd(0)-catalyzed amination of aryl halides on solid support

In this study, the microwave-assisted Pd(0)-catalyzed coupling of aryl halides to Rink resins is compared to a conventional heating protocol. While yields and purities of the coupling products were comparable under both conditions, microwave heating allows a significant acceleration of the reaction. The coupling of electron-deficient aryl halides proceeded within 10-15 minutes, while under conventional conditions 20 h of heating were necessary for complete conversion.