Palladium-copper Catalysts Research Articles

Reduction of nitrates dissolved in water over palladium-copper catalysts supported on a strong cationic resin

Catalysts containing 4 and 1 wt.% of palladium and copper, respectively, supported on cationic resin (DOWEX 1 × 4, gel type poly(styrene- co-divinylbenzene) with –N(CH 3) 3 +Cl − groups, particle size 100–200 mesh) were prepared and tested in the liquid-phase hydrogenation of nitrates in water. The catalyst was prepared by reduction of anionic chloro complexes of palladium(II) and copper(II) immobilized in the resin. Different reduction agents were employed, namely sodium borohydride in ethanol, or in water, and dihydrogen in either water (0.1–0.5 MPa) or methanol (0.5 MPa) solution of Na 2CO 3 (2.5%, w/w). The characterisation by means of X-ray microprobe analysis (XRMA), X-ray powder diffraction (XRPD) and transmission electron microscopy (TEM) revealed the presence of palladium metal nanoclusters and copper compounds. A cationic resin as the support was expected to facilitate the transport of the nitrate anions into the catalyst in comparison to previously employed anionic resins. This was fully proved by the generally higher specific activity of Pd-Cu catalyst supported on cationic resin. The most active and relatively stable catalyst was obtained by reduction of the immobilized metal precursors with H 2 in aqueous Na 2CO 3 at 0.5 MPa. The catalysts exhibited different stability with respect to leaching of metals under duty. Owing to the mechanical stirring of the reaction mixture, the catalytic particles were gradually ground and handling the reaction mixture with the suspended catalyst became rather problematic.

Carbon dendron nano-chains with π-extended conjugation based on end-capped N, N-dimethylamino in linear 1,4-phenylethynyl or in 1,5-naphthylethynyl subunits: fluorescence analysis

The synthesis of dendrons with the end-capped 5-( N, N-dimethylamino)naphthyl-1-ethynyl unit connected to conjugated naphthylethynyl or p-phenylethynyl chains, as the branches of the 1,3,5-substituted benzene core, have been undertaken by heterocoupling reaction between 1,3,5-triiodobenzene and the convenient end-capped 5-( N, N-dimethylamino)naphthylacetylene or 5-( N, N-dimethylamino)naphthylethynylphenylacetylene in the presence of the palladium–copper catalyst system, in excellent yields. The influence of the alternating naphthylethynyl–phenylethynyl chains on the fluorescence emission radiation, in the dendron structures, has been analyzed.

Synthesis of Melatonin Analogues Derived from Furo[2,3-b]- and [2,3-c]pyridines by Use of a Palladium-Copper Catalyst System

2,3,5-Substituted furo[2,3-b]pyridine was synthesised by palladium-catalyzed reaction of 5-bromo-2-hydroxy-3-iodopyridine and phenylacetylene with (Ph 3 P) 2 PdCl 2 , Cul in Et 3 N. A carbonylative cyclization of 5-hydroxy-2-methoxy-4-(2-phenylethynyl)pyridine with carbon monoxide in methanol with PdCl 2 , CuCl 2 under basic conditions, has been accomplished to prepare methyl 2,5-substituted furo[2,3-c]pyridine-3-carboxylate.

Selective liquid-phase semihydrogenation of functionalized acetylenes and propargylic alcohols with silica-supported bimetallic palladium-copper catalysts.

Silica-supported, bimetallic palladium-copper catalysts were prepared in solution under mild conditions by reacting lithium di(4-tolyl)cuprate with palladium acetate in the presence of silica particles. Small bimetallic palladium-copper particles were deposited on the silica surface as confirmed with TEM-EDAX and EXAFS. The new material has been applied as catalyst in the liquid-phase semihydrogenation of mono- and disubstituted alkynes and showed high selectivity toward the cis-alkenes. The influence of addition of quinoline or potassium hydroxide to the semihydrogenation reaction mixture and the effects of exposure of the catalyst to air before use have been investigated.

Effects of Cu over Pd based catalysts supported on silica or niobia

Palladium and palladium-copper catalysts supported on silica and niobia were characterized by H2 chemisorption and H2-O2 titration. Systems over silica were also analyzed by transmission electron microscopy and EXAFS. The metallic dispersion decreased from 20% to 7% when the content of Pd was increased from 0.5wt.-% to 3wt.-% in monometallic catalysts. The addition of 3 wt.-% Cu to obtain Pd-Cu catalysts caused a remarkable capacity loss of hydrogen chemisorption. TPR analysis suggested an interaction between the two metals and EXAFS characterization of the catalyst supported on silica confirmed the formation of Pd-Cu alloy. Pd/Nb2O5 catalysts showed turnover numbers higher than those obtained with the Pd/SiO2 systems in the cyclohexane dehydrogenation. However, the bimetallic catalysts showed very low turnover numbers.

XPS and TPR examinations of γ-alumina-supported Pd-Cu catalysts

Alumina-supported palladium-copper catalysts, which promote liquid-phase nitrate reduction, have been prepared according to different impregnation sequences of γ-Al 2O 3 support and characterized by X-ray photoelectron spectroscopy (XPS), X-ray induced Auger electron spectroscopy (XAES) and temperature-programmed reduction (TPR). Analysis of Pd XPS/XAES spectra reveals that palladium is present on the γ-alumina support in the metallic form. Copper is reduced at lower temperatures in the presence of palladium particles compared to the CuO/γ-Al 2O 3 sample. Due to the low copper content in catalysts ( σ Cu≈10 14 atoms cm −2), the XPS/XEAS spectral features of reduced copper species are quite different from those of bulk copper. As shown by TPR and XPS/XAES data, formation of highly dispersed Pd-Cu bimetallic clusters is suggested. It was discovered by means of TPR analysis that the catalyst preparation in which the γ-alumina support is impregnated first by copper salt, results in higher formation of the Pd-Cu alloy.

Syntheses and enantioselective recognition of chiral poly(phenyleneethynylene)s bearing bulky optically active menthyl groups

Two optically active poly(menthoxycarbonylphenyleneethynylene)s were synthesized by polycondensation using a palladium–copper catalyst system. All polymers showed Cotton effects in the absorption region of the main chain, implying that chirality is induced to the main chain. Moreover, these chiral polymers showed the enantioselective recognition ability for chiral menthol in the circular dichroism (CD) spectra.

Oxidative carbonylation of aniline over a polymer-supported palladium–copper catalyst

The polymer-supported bimetallic catalyst PVP–PdCl 2–2CuCl 2 (PVP, poly( N-vinyl-2-pyrrolidone), obtained in situ by the addition of CuCl 2 to an alcoholic solution of PVP–PdCl 2, exhibits high selectivity and activity for the oxidative carbonylation of aniline with carbon monoxide and oxygen to ethyl N-phenylcarbamate in the presence of a base (NaOAc) under atmospheric pressure. The strong synergic effect of Pd–Cu gives rise to a clear increase in the selectivity and activity.

Behavior of Palladium–Copper Catalysts for CO and NO Elimination

The behavior of a series of palladium and palladium–copper catalysts supported on ceria/alumina for the CO+NO+O2 reaction has been analyzed by a combination of electron transmission microscopy, infrared, and electron paramagnetic spectroscopies and catalytic test studies. In both systems, the catalytic behavior is dominated by the properties of the metal–ceria interface. The addition of copper to a palladium system leads to beneficial effects related to alloy formation in which both CO and NO elimination are enhanced. The chemical consequences and catalytic implications of Pd-Cu alloying are discussed.

Improving the catalytic nitrate reduction

Technologies for the nitrate removal from drinking water and waste water will be required in the near future and the catalytic nitrate reduction is one of the most promising ones. To establish a technical-scale nitrate reduction a further improvement of the catalyst is necessary and new concepts should be introduced in the process. It is shown, that palladium–tin and palladium–indium catalysts can be much more suited for an efficient nitrate reduction than palladium–copper catalysts. Furthermore, two new innovative concepts are presented — the use of in situ buffering formic acid as reductant instead of hydrogen and the application of PVAL-encapsulated catalysts with superior diffusional properties — which may contribute to solve selectivity problems.

Catalytic reduction of nitrates and nitrites in water solution on pumice-supported Pd–Cu catalysts

Two series of pumice-supported palladium and palladium–copper catalysts, prepared by impregnation with different palladium and copper precursors, were tested for the hydrogenation of aqueous nitrate and nitrite solutions. Measurements were performed in a stirred tank reactor, operating in batch conditions, in buffered water solution at atmospheric pressure and at 293 K. The activities of the catalysts were calculated in terms of nitrate and/or nitrite removal. With the monometallic Pd/pumice, the reduction of nitrite is highly selective; only 0.2% of the initial nitrite content is converted to ammonium ions. The activity in terms of turn over frequency (TOF) is higher as compared to a catalyst of Pd on silica. Addition of copper to the palladium catalyst is essential for the reduction of nitrates, although it decreases the nitrite reduction activity and increases the production of ammonium ions. Nitrate reduction appears to be structure-insensitive and a volcano-type dependence of the activity versus the overall Cu atomic weight percentage is observed for the two series of catalysts.

Palladium-Catalyzed Oxidative Cross-Coupling of 2-Phenylphenols with Alkenes

Abstract 2-Phenylphenols react with alkenes such as acrylate esters in the presence of a palladium-copper catalyst system under air to give the corresponding 6-substituted 6H-dibenzo[b,d]pyran derivatives.

Oxidative polycondensation of acetylene by iodine in the presence of a palladium-copper catalyst

A new method for oxidative coupling of mono-substituted acetylenes to corresponding di-substituted butadiynes under anaerobic conditions was applied to polycondensation of acetylene to carbyne, giving both a soluble sticky product and an insoluble black powder. The former was oligoynes that have several lengths of polyyne terminated with phenyl group and the latter resembled carbyne reported before. Electrical conductivity of the latter was an order of 10 −8 S/cm, which increased to an order of 10 −5 S/cm after k doping.

Combustion of methane over supported palladium-copper bimetallic catalysts

Monometallic and bimetallic catalysts based on palladium and copper deposited on a spinel carrier have been investigated in the catalytic combustion of methane. Great differences were found in catalytic activity, according to the sequence Pd/MgAl2O4>CuO−Pd/MgAl2O4>Pd−CuO/MgAl2O4>CuO/MgAl2O4. They were explained by changes in surface composition of the catalysts. In the case of bimetallic catalysts the metallic surface is preferentially enriched in copper, which acts as a diluting agent for the Pd atom ensembles. As a consequence, the adsorption of reactants is limited and the catalysts so obtained behave like copper slightly doped with palladium.

Surface characterization of palladium-copper bimetallic catalysts by FTIR spectroscopy and test reactions

Different bi-metallic palladium-copper catalysts have been prepared according to different sequences of impregnation of a commercial alumina. CO adsorption followed by FTIR spectroscopy was performed in order to identify the nature of the superficial sites. Two test reactions were also used to characterize the nature of the active surface site: gas phase toluene hydrogenation was selected in order to show the hydrogenating activity of metallic palladium, whereas the reaction of decomposition of ethanol, which exhibits 100% selectivity in the formation of ethanal on pure copper catalysts, was also studied on this series of catalysts. The results obtained by both methods are complementary. By comparison with the behaviour of pure palladium or copper based catalysts, they allow to propose hypotheses on the surface composition: separated phases, alloys or a phase covered by the other one.

Development of catalysts for a selective nitrate and nitrite removal from drinking water

The increasing nitrate concentration of ground water has stimulated intensive research on the denitrification of drinking water. At present nitrate is removed either by physicochemical or biological methods. The application of catalysts in the purification of drinking water was not examined. We found that nitrate and nitrite can be reduced with hydrogen on noble metal catalysts. Products of the reaction are gaseous nitrogen and dissolved ammonia, which is undesired in drinking water. Here we describe the development of a supported palladium catalyst, which reduces nitrite to nitrogen with a selectivity of 99.9 %. After activation with a second metal, e.g. copper, this catalyst shows nitrate removal activity, but also a higher formation of ammonia. The addition of palladium catalysts to the palladium-copper catalysts and hydrogen limitation reduce the ammonia formation of the nitrate reduction. After complete removal of 100 mg l nitrate, less than 0.5 mg l ammonia were formed, so that the application of the catalysts in the purification of drinking water seems possible.

Novel Synthesis of Functionalized Benzo[b]furans and Homologues by Use of a Palladium-Copper Catalyst System

Novel Synthesis of Functionalized Benzo[<i>b</i>]furans and Homologues by Use of a Palladium-Copper Catalyst System

Phase structure of carbon supported palladium-copper catalysts

The phase structure of 7%Pd-3%Cu on activated carbon catalysts was studied by potentiodynamic polarization and XRD methods. Unalloyed Pd, Pd-rich disordered alloy, unalloyed Cu and Cu 3Pd phases and adsorbed Cu have been detected, depending on the preparation methods. Activity in hydrogenation reactions was attributed to the Pd-rich disordered alloy phase.

Investigation of the possibility of intermediate formation of allyl alcohol in the process of oxidative acetoxylation of propylene on a palladium-copper zeolite catalyst

1. The formation of allyl acetate in reactions of oxidative acetoxylation of propylene by labeled acetic acid (AcA) and esterification of labeled acetic acid by allyl alcohol on a Pd, Cu-zeolite catalyst occur with complete conservation of the labeled oxygen of the original AcA in the reaction product. 2. A reaction scheme is proposed for the oxidative acetoxylation of propylene, providing for the formation of allyl alcohol as an intermediate compound, present in a chemisorbed state in the form of a complex with a Pd atom.

Differential thermal analysis of PdCu zeolites: Catalysts of oxidation of lower olefins into carbonyl compounds

1. Type A, X, and Y initial zeolites and mordenite and palladium-copper catalysts for oxidation of olefins into carbonyl compounds prepared from them by ion exchange and impregnation methods were investigated by differential thermal analysis (DTA). 2. The thermal stability of the aluminosilicate framework of the starting and PdCu forms increased with an increase in the SiO2/Al2O3 ratio. For type Y zeolites and the impregnation catalysts based on them, the thermal stability increased with an increase in the size of the alkali metal cations contained in the zeolite. 3. Based on the DTA data, the processes which take place in activation and regeneration of PdCu catalysts were interpreted. Activation of the starting PdCu zeolites in air was accompanied by oxidation of the NH3 added to the catalyst during preparation. Both desorption of low-boiling products of the reaction and oxidation of difficult-to-desorb substances take place in conditions of oxidative regeneration of the PdCu samples after catalysis.