Boosting Efficiency for the Synthesis of Dimethyl Oxalate over Pd/MgAl2O4: Effect of Different Preparation Methods for MgAl2O4 Supports

High active Pd-based catalyst with low loading capacity for CO oxidative coupling with methyl nitrite (MN) to dimethyl oxalate (DMO) has aroused great interest in the large-scale production of DMO for the next hydrogenation to bring forth ethylene glycol (EG). Here, spinel composite oxide support, namely MgAl₂O₄, had been prepared through three different methods: solid combustion method (MAO-SC), hydrothermal method (MAO-HT) and co-precipitation method (MAO-CP), and further Pd nanoparticles were anchored to the surface of MgAl₂O₄ through a simple wetness impregnation method. In the CO direct esterification to DMO reaction, the Pd/MAO-SC catalyst, which used MgAl₂O₄ support prepared by solid combustion method, exhibited the highest intrinsic activity with Turnover frequency (TOF) value of 2.39 s⁻¹ and DMO weight space-time yield (956.8 g⋅kg_cat⁻¹⋅h⁻¹) with 44.1 % CO conversion and 97.8 % DMO selectivity, and showed negligible attenuation of activity during the process of continuous reaction for 100 h. The characterization results confirmed that abundant oxygen defect sites occurred in MAO-SC synthesized by combustion method not only can help to anchor the Pd nanoparticles with appropriate size and dispersion, but also can promote the interfacial electron transfer from MgAl₂O₄ support to Pd nanoparticles, thus significantly enhancing the metal-support interaction (MSI) and providing more CO bridge-adsorption sites that is favorable for the CO oxidative coupling with MN to DMO reaction.     

Selectivity Control in the Direct CO Esterification over Pd@UiO-66: The Pd Location Matters

The selectivity control of Pd nanoparticles (NPs) in the direct CO esterification with methyl nitrite toward dimethyl oxalate (DMO) or dimethyl carbonate (DMC) remains a grand challenge. Herein, Pd NPs are incorporated into isoreticular metal–organic frameworks (MOFs), namely UiO-66-X (X=-H, -NO₂, -NH₂), affording Pd@UiO-66-X, which unexpectedly exhibit high selectivity (up to 99 %) to DMC and regulated activity in the direct CO esterification. In sharp contrast, the Pd NPs supported on the MOF, yielding Pd/UiO-66, displays high selectivity (89 %) to DMO as always reported with Pd NPs. Both experimental and DFT calculation results prove that the Pd location relative to UiO-66 gives rise to discriminated microenvironment of different amounts of interface between Zr-oxo clusters and Pd NPs in Pd@UiO-66 and Pd/UiO-66, resulting in their distinctly different selectivity. This is an unprecedented finding on the production of DMC by Pd NPs, which was previously achieved by Pd(II) only, in the direct CO esterification.     

Pd(111), Pd(100)及Pd(110)表面H2和O2直接合成H2O2的密度泛函理论研究

采用周期性密度泛函理论研究了H₂和O₂在Pd(111),Pd(100)及Pd(110)表面上直接合成H₂O₂的反应机理,对反应的主要基元步骤进行了计算和分析.结果表明,Pd(111)表面对H₂O₂直接合成的催化选择性最好,表面原子密度较低的Pd(100)表面和Pd(110)表面上含有O-O键的表面物种解离严重,不利于H₂O₂的生成.H₂O₂的选择性与含有O-O键表面物种的O-O键能和表面物种的结合能有关.含有O-O键的表面物种在表面的结合能越大,越容易发生解离,不利于形成H₂O₂.     

Deactivation behaviors of hybrid Fischer–Tropsch catalysts in the production of middle distillate from synthesis gas in a dual-bed reactor

Production of middle distillate (C₁₀–C₂₀) from synthesis gas (CO + H₂) through hydrocracking of wax (>C₂₁₊) was carried out in a dual-bed reactor. Fischer–Tropsch catalyst (Co/TiO₂) was used in the first-bed reactor to produce wax from synthesis gas, and a mesoporous Pd–alumina composite catalyst (Pd–Al₂O₃) was used in the second-bed reactor to produce middle distillate through hydrocracking of wax. Both Fischer–Tropsch synthesis function of Co/TiO₂ catalyst and hydrocracking function of Pd–Al₂O₃ catalyst were deactivated during 100 h-hybrid Fischer–Tropsch synthesis reaction. It was revealed that deactivation behaviors of Co/TiO₂ and Pd–Al₂O₃ catalysts were governed by different factors. Wax accumulation and Co sintering were responsible for deactivation of Co/TiO₂ catalyst in the Fischer–Tropsch synthesis reaction. Loss of Pd dispersion and Pd surface area of Pd–Al₂O₃ catalyst was responsible for its decreased catalytic performance in the production of middle distillate through hydrocracking of wax.     

Continuous‐Flow Suzuki‐Miyaura and Mizoroki‐Heck Reactions under Microwave Heating Conditions

Microwave‐assisted continuous‐flow reactions have attracted significant interest from synthetic organic chemists, especially process chemists from practical points of view, due to a less complicated shift to large‐scale synthesis based on simple and continuous access to products with low energy requirements. In this personal account, we focused on the Suzuki‐Miyaura and Mizoroki‐Heck reactions, both of which are significantly important cross‐coupling reactions for the synthesis of various functional materials. Microwave power is effective for heating. Typical homogeneous palladium catalysts, such as PdCl₂(PPh₃)₂, Pd(PPh₃)₄, and Pd(OAc)₂, as well as heterogeneous palladium catalysts, such as Pd‐film, Pd/Al₂O₃, Pd/SiO₂, and Pd supported on polymers, can be used for these reactions.     

Synthesis of cyclic carbonates and dimethyl carbonate using CO<Subscript>2</Subscript> as a building block catalyzed by MOF-5/KI and MOF-5/KI/K<Subscript>2</Subscript>CO<Subscript>3</Subscript>

The synthesis of cyclic carbonates or dimethyl carbonate (DMC) using CO₂ as a building block is a very interesting topic. In this work, we found that the metalorganic framework-5 (MOF-5)/KI was an active and a selective catalytic system for the synthesis of cyclic carbonates from CO₂ and epoxides, and MOF-5/KI/K₂CO₃ was efficient for the preparation of DMC from CO₂, propylene, and methanol by a sequential route. The impacts of temperature, pressure, and reaction time length on the reactions were investigated, and the mechanism of the reactions is proposed on the basis of the experimental results.     

Direct synthesis of dimethyl carbonate from methanol and carbon dioxide using heteropolyoxometalates: the effects of cation and addenda atoms

The direct synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide over a series of Keggin-type heteropolyoxometalates has been investigated. The effects of the cations and the addenda atoms of the heteropolyoxometalate on the conversion and the product selectivities were investigated. The results showed that Co_1.5PW₁₂O₄₀ was the best catalyst of the series. The effect of reaction temperature and CO₂ pressure on the direct synthesis of DMC demonstrated that lower temperatures and higher pressures are favorable for the synthesis of DMC. Higher temperatures favor the formation of dimethoxymethane (DMM) and methyl formate (MF).     

Development of a Ni–Pd/CeZrO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst for the effective conversion of methane into hydrogen-containing gas

The effects of the Pd content (0–1 wt %) and the synthesis method (joint impregnation with Ni + Pd and Pd/Ni or Ni/Pd sequential impregnation) on the physicochemical and catalytic properties of Ni–Pd/CeZrO₂/Al₂O₃ were studied in order to develop an efficient catalyst for the conversion of methane into hydrogen-containing gas. It was shown that variation in the palladium content and a change in the method used for the introduction of an active constituent into the support matrix make it possible to regulate the redox properties of nickel cations but do not affect the size of NiO particles (14.0 ± 0.5 nm) and the phase composition of the catalyst ((γ + δ)-Al₂O₃, CeZrO₂ solid solution, and NiO). It was established that the activity of Ni–Pd catalysts in the reaction of autothermal methane reforming depends on the method of synthesis and increases in the following order: Ni + Pd < Ni/Pd < Pd/Ni. It was found that, as the Pd content of the Ni–Pd/CeZrO₂/Al₂O₃ catalyst was decreased from 1 to 0.05 wt %, the ability for self-activation, high activity, and operational stability of the catalyst under the conditions of autothermal methane reforming remained unaffected: at 850°C, the yield of hydrogen was ~70% at a methane conversion of ~100% during a 24-h reaction.     

Development of an efficient method for preparation of 1,3,5-trihydroxyisocyanuric acid (THICA) and its use as aerobic oxidation catalyst

1,3,5-Trihydroxyisocyanuric acid (THICA), which serves as an efficient radical-producing catalyst from hydrocarbons, was successfully prepared by two methods. The reaction of O-benzylhydroxyamine with phenyl chloroformate gave formbenzyloxycarbamic acid phenyl ester of which subsequent treatment with dimethylaminopyridine (DMAP) produced 1,3,5-tribenzyloxyisocyanurate leading to THICA by hydrogenation with H₂ on Pd/C. The other method involved the direct synthesis of 1,3,5-tribenzyloxyisocyanurate from O-benzylhydroxyamine and diphenyl carbonate. The aerobic oxidation of p-methylanisole catalyzed using THICA as a key catalyst afforded p-anisic acid in almost quantitative yield (>99%).     

Metal-Oxo Electronic Tuning via In Situ CO Decoration for Promoting Methane Conversion to Oxygenates over Single-Atom Catalysts

Direct methane conversion (DMC) to oxygenates at low temperature is of great value but remains challenging due to the high energy barrier for C−H bond activation. Here, we report that in situ decoration of Pd₁-ZSM-5 single atom catalyst (SAC) by CO molecules significantly promoted the DMC reaction, giving the highest turnover frequency of 207 h⁻¹ ever reported at room temperature and ~100 % oxygenates selectivity with H₂O₂ as oxidant. Combined characterizations and DFT calculations illustrate that the C-atom of CO prefers to coordinate with Pd₁, which donates electrons to the Pd₁−O active center (L−Pd₁−O, L=CO) generated by H₂O₂ oxidation. The correspondingly improved electron density over Pd−O pair renders a favorable heterolytic dissociation of C−H bond with low energy barrier of 0.48 eV. Applying CO decoration strategy to M₁-ZSM-5 (M=Pd, Rh, Ru, Fe) enables improvement of oxygenates productivity by 3.2–11.3 times, highlighting the generalizability of this method in tuning metal-oxo electronic structure of SACs for efficient DMC process.     

One-pot synthesis of a rose-like Pd–Fe3O4 nanocatalyst for Sonogashira coupling reactions

A one-pot synthesis of rose-like Pd–Fe₃O₄ nanocomposites via the controlled thermal decomposition of Fe(CO)₅ and reduction of Pd(OAc)₂ is reported. This rose-like Pd–Fe₃O₄ composite structure has a high surface area owing to the individual Pd–Fe₃O₄ nanosheets, which imparted a high catalytic activity for Sonogashira coupling reactions. Moreover, the catalyst also demonstrated magnetic recyclability.     

A new palladium complex supported on magnetic nanoparticles and applied as an catalyst in amination of aryl halides,Heck and Suzuki reactions

A simple, efficient and less expensive protocol for the phosphine-free C–C coupling reactions and synthesis of anilines in the presence of 2-aminobenzamide complex of palladium supported on Fe₃O₄ magnetic nanoparticles (Pd(0)-ABA-Fe₃O₄) has been reported. The Suzuki reaction was carried out in water or PEG using phenylboronic acid (PhB(OH)₂) or sodium tetraphenyl borate (NaBPh₄). Pd(0)-ABA-Fe₃O₄ has been found promising for Heck reaction of butyl acrylate, styrene or acrylonitrile with aryl halides (including Cl, Br and I). Also, Pd(0)-ABA-Fe₃O₄ has been found as efficient catalyst for the amination of aryl halides using aqueous ammonia. The products have been obtained in short reaction times and high yields. The catalyst was easily separated using an external magnet from the reaction mixture and reused for several runs without significant loss of its catalytic efficiency or palladium leaching. The leaching of catalyst has been examined by hot filtration and ICP-OES technique. The nanomagnetical catalyst was characterized by FTIR, TGA, XRD, VSM, TEM, SEM, EDS, DLS and ICP-OES techniques.     

Active control of methanol carbonylation selectivity over Au/carbon anode by electrochemical potential

Electrochemical oxidative carbonylation of methanol was studied over Au supported carbon anode in CO. The major carbonylation products were dimethyl oxalate (DMO) and dimethyl carbonate (DMC). The minor oxidation products were dimethoxy methane (DMM) and methyl formate (MF) from methanol and CO(2). Influences of various reaction conditions were studied on carbonylation activities and selectivities. The selectivities to DMO and DMC can be controlled by the electrochemical potential. Electrocatalysis of Au/carbon anode was studied by cyclic voltammetry (CV), stoichiometric reactions among Au(3+), methanol, and CO, and UV-vis spectra. The Au/carbon anode was characterized by XRD, SEM, and BE images before and after the carbonylation. These experimental facts strongly suggest that transition of oxidation states of Au affects changing of the carbonylation selectivities to DMO and DMC. Au(0) is the active species for the selective DMO formation by direct electrochemical carbonylation at low potentials (<+1.2 V (Ag/AgCl)). On the other hand, Au(3+) is the active spices for the selective DMC formation by indirect electrochemical carbonylation through Au(3+)/Au(+) redox at high potentials (>+1.3 V).     

Probing the Role of a Non‐Thermal Plasma (NTP) in the Hybrid NTP Catalytic Oxidation of Methane

Three recurring hypotheses are often used to explain the effect of non‐thermal plasmas (NTPs) on NTP catalytic hybrid reactions; namely, modification or heating of the catalyst or creation of new reaction pathways by plasma‐produced species. NTP‐assisted methane (CH₄) oxidation over Pd/Al₂O₃ was investigated by direct monitoring of the X‐ray absorption fine structure of the catalyst, coupled with end‐of‐pipe mass spectrometry. This in situ study revealed that the catalyst did not undergo any significant structural changes under NTP conditions. However, the NTP did lead to an increase in the temperature of the Pd nanoparticles; although this temperature rise was insufficient to activate the thermal CH₄ oxidation reaction. The contribution of a lower activation barrier alternative reaction pathway involving the formation of CH₃(g) from electron impact reactions is proposed.     

Magnetically recyclable palladium nanoparticles (Fe3O4‐Pd) for oxidative coupling between amides and olefins at room temperature

A convenient method for the synthesis of magnetically recyclable palladium nanoparticles (Fe₃O₄‐Pd) is described. The catalytic application of the Fe₃O₄‐Pd nanoparticles was explored for the first time in oxidative coupling between amides and olefins. p‐Toluenesulfonic acid plays a significant role in the oxidative amidation reaction. The reaction proceeds at room temperature, resulting in (Z)‐enamides under ambient air in the absence of co‐catalyst and ligand. The superparamagnetic nature of Fe₃O₄‐Pd facilitates easy, quantitative recovery of the catalyst from a reaction mixture, and it can be reused for up to three consecutive cycles with a slight decrease in catalytic activity.     

Fe3O4‐SAHPG‐Pd0 nanoparticles: A ligand‐free and low Pd loading quasiheterogeneous catalyst active for mild Suzuki–Miyaura coupling and C?H activation of pyrimidine cores

This paper reports a green magnetic quasiheterogeneous efficient palladium catalyst in which Pd⁰ nanoparticles have been immobilized in self‐assembled hyperbranched polyglycidole (SAHPG)‐coated magnetic Fe₃O₄ nanoparticles (Fe₃O₄‐SAHPG‐Pd⁰). This catalyst has been used for effective ligandless Pd catalyzed Suzuki–Miyaura coupling reactions of different aryl halides with substituted boronic acids at room temperature and in aqueous media. Herein, SAHPG is used as support; it also acts as a reducing agent and stabilizer to promote the transformation of Pd^II to Pd⁰ nanoparticles. Also, this environmental friendly quasiheterogeneous catalyst is employed for the first time in the synthesis of new pyrimido[4,5‐b]indoles via oxidative addition/C? H activation reactions on the pyrimidine rings, which were obtained with higher yield and faster than when Pd(OAc)₂ was used as the catalyst. Interestingly, the above‐mentioned catalyst could be recovered in a facile manner from the reaction mixture by applying an external magnet device and recycled several times with no significant decrease in the catalytic activity.     

Preparation and characterization of palladium immobilized on silica‐coated Fe3O4 and its catalytic performance for Suzuki reaction under microwave irradiation

Supported palladium catalyst (Pd/Fe₃O₄@SiO₂) was easily prepared by supporting PdCl₂ on silica‐coated magnetic nanoparticles Fe₃O₄ in ethylene glycol. The as‐prepared sample was characterized by infrared spectroscopy (IR), X‐ray diffraction (XRD) and X‐ray photoelectron spectrometer (XPS). The formation of active specie Pd(0) was confirmed by XRD and XPS, and the Pd loading for the fresh and recovered catalyst was determined by atomic absorption spectroscopy (AAS). Pd/Fe₃O₄@SiO₂ was employed for the synthesis of biphenyl derivatives via Suzuki reaction. In terms of the yield of biphenyl, the supported catalyst displayed nearly equal catalytic performance to that of homologous PdCl₂ under microwave irradiation for 30 min but higher than that obtained by traditional heating method for 12 h. The catalytic performance of Pd/Fe₃O₄@SiO₂ for Suzuki reactions involving various aryl halides and arylboronic acids were also examined. Impressive yield of biphenyl at 68.2% was obtained even in the presence of unreactive aryl chlorides. Pd/Fe₃O₄@SiO₂ was recovered by a permanent magnet and directly reused in the next run, and no obvious deactivation was observed for up to 6 times. Copyright © 2016 John Wiley & Sons, Ltd.     

Palladium immobilized on amidoxime‐functionalized magnetic Fe3O4 nanoparticles: a highly stable and efficient magnetically recoverable nanocatalyst for sonogashira coupling reaction

We describe the synthesis of a novel Fe₃O₄/amidoxime (AO)/Pd nanocatalyst by grafting of AO groups on Fe₃O₄ nanoparticles and subsequent deposition of Pd nanoparticles. Prior to grafting of AO, the 2‐cyanoethyl‐functionalized Fe₃O₄ nanoparticles prepared through combining 2‐cyanoethyltriethoxysilane and Fe₃O₄ were treated with hydroxylamine. The AO‐grafted Fe₃O₄ nanoparticles were then used as a platform for the deposition of Pd nanoparticles. The catalyst was characterized using Fourier transform infrared spectroscopy, X‐ray diffraction, scanning and transmission electron microscopies, vibrating sample magnetometry, wavelength‐ and energy‐dispersive X‐ray spectroscopies and inductively coupled plasma analysis. Fe₃O₄/AO/Pd is novel phosphine‐free recyclable heterogeneous catalyst for Sonogashira reactions. Interestingly, the novel catalyst could be recovered in a facile manner from the reaction mixture by applying an external magnet device and recycled seven times without any significant loss in activity. Copyright © 2015 John Wiley & Sons, Ltd.     

Carbonylation of nitrobenzene in methanol with palladium bidentate phosphane complexes: an unexpectedly complex network of catalytic reactions, centred around a Pd-imido intermediate

The reactivity of palladium complexes of bidentate diaryl phosphane ligands (P₂) was studied in the reaction of nitrobenzene with CO in methanol. Careful analysis of the reaction mixtures revealed that, besides the frequently reported reduction products of nitrobenzene [methyl phenyl carbamate (MPC), N,N′‐diphenylurea (DPU), aniline, azobenzene (Azo) and azoxybenzene (Azoxy)], large quantities of oxidation products of methanol were co‐produced (dimethyl carbonate (DMC), dimethyl oxalate (DMO), methyl formate (MF), H₂O, and CO). From these observations, it is concluded that several catalytic processes operate simultaneously, and are coupled via common catalytic intermediates. Starting from a P₂Pd⁰ compound formed in situ, oxidation to a palladium imido compound P₂Pd^II?NPh, can be achieved by de‐oxygenation of nitrobenzene 1) with two molecules of CO, 2) with two molecules of CO and the acidic protons of two methanol molecules, or 3) with all four hydrogen atoms of one methanol molecule. Reduction of P₂Pd^II?NPh to P₂Pd⁰ makes the overall process catalytic, while at the same time forming Azo(xy), MPC, DPU and aniline. It is proposed that the Pd–imido species is the central key intermediate that can link together all reduction products of nitrobenzene and all oxidation products of methanol in one unified mechanistic scheme. The relative occurrence of the various catalytic processes is shown to be dependent on the characteristics of the catalysts, as imposed by the ligand structure.     

Active control of catalysis and product selectivity by a fuel cell system

A new concept to control catalysis and catalytic reaction through partial oxidation of alkenes with O₂ is described. Oxidation of alkenes was studied by alkene/Pd-anode/H₃PO₄-electrolyte/cathode/O₂ fuel cell (FC). An idea based on electrocatalysis and electrochemical reactions to control reaction rates and product selectivity was proposed and proven through the oxidation of propylene, Wacker and π-allyl oxidation. The oxidation rate and the product selectivity to the Wacker and the π-allyl oxidations could be controlled by changing electrode potentials. We could active control oxidation states of Pd on the anode, Pd(II) or Pd(0), during the oxidation from outer circuit. The oxidation states of Pd on the anode decided the product selectivity.