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• Controllable activity and selectivity of Pd/C hydrogenation catalysts. • Effect of the carbon support on the structure and catalytic properties of Pd NPs. • Effect of the Pd precursor on the structure and catalytic properties of Pd NPs. • Addition of a second metal and the synergistic effect in Pd-based bimetallic catalysts. Carbon-supported palladium catalysts are widely used for hydrogenation of various organic compounds in the fine chemical industry. The nanoscale geometry and electronic structure of supported Pd nanoparticles play a crucial role in providing the necessary catalytic properties. To improve catalytic activity and selectivity of Pd nanoparticles, it is possible to fine tune their intrinsic properties (e.g., size and oxidation state) by controlling the chemical transformations at different stages of catalyst preparation. Recent years have seen considerable advancement in developing new catalyst preparation techniques as well as in understanding the mechanism of active site formation. This review summarizes some of the existing approaches to regulating the catalytic properties of carbon-supported palladium by variation of the carbon support, the composition of palladium precursor and its reduction conditions, as well as the addition of a second active metal. The data presented may be useful for researchers developing efficient Pd/C catalysts for hydrogenation of polyfunctional organic compounds. [ABSTRACT FROM AUTHOR]

The paper presents the results of the synthesis and study of CuAl layered double hydroxides (LDHs) as well as their application as catalysts for the selective hydrogenation of crotonaldehyde. Phase-homogeneous LDHs were obtained by co-precipitation and mechanochemical methods, and critical parameters ensuring the formation of the target product were identified. In the case of coprecipitation, the formation of LDH is most affected by the pH of the reaction medium and the CO32−/Al3+ ratio. The optimal CO32−/Al3+ ratio is ca. 0.5–0.8 and pH 9.5–10.0. When mechanochemical synthesis is used, at 500 m·s−2 and 60 min, it is possible to obtain a single-phase CuAl LDH, whereas at higher energies, LDH is destroyed. The mechanochemical method makes it possible not only to reduce the synthesis time and the amount of alkaline wash water but also to obtain more dispersed copper particles with a higher hydrogenating activity. The conversion of 2-butenal (T = 80 °C, P = 0.5 MPa, 180 min, ethanol) for this sample was 99.9%, in contrast to 50.5% for the catalyst obtained by co-precipitation. It is important that, regardless of the conversion, both catalysts showed high selectivity (S = 90–95%) for the double bond hydrogenation. [ABSTRACT FROM AUTHOR]

Nickel catalysts with carbon-mineral supports derived from sapropel were synthesized; the effect exerted by the nature of the support (type of the initial sapropel) and active component precursor on the activity of the catalysts in the model reaction of liquid-phase nitrobenzene hydrogenation was studied. The catalysts, synthesized using the support with a smaller fraction of carbon, were more active irrespective of the precursor nature. The highest activity was observed for the catalysts synthesized from nickel nitrate and formate; nitrobenzene conversion was 65% and 51%, respectively, after 1 h of reaction. The catalysts retained high activity after six reaction cycles at 100% aniline selectivity. The presence of sulfur in the nickel precursor deteriorated the catalytic activity (convection less than 3%) due to formation of the sulfide phase. [ABSTRACT FROM AUTHOR]

Porous carbon–carbon composite materials (PCCCM) were synthesized by the alkaline dehydrochlorination of polyvinyl chloride solutions in dimethyl sulfoxide containing the modifying additives of a nanostructured component (NC): graphite oxide (GO), reduced graphite oxide (RGO) or nanoglobular carbon (NGC), with subsequent two-step thermal treatment of the obtained polyvinylene–NC composites (carbonization at 400 °C and carbon dioxide activation at 900 °C). The focus of the study was on the analysis and digital processing of transmission electron microscopy images to study local areas of carbon composite materials, as well as to determine the distances between graphene layers. TEM and low-temperature nitrogen adsorption studies revealed that the structure of the synthesized PCCCM can be considered as a porous carbon matrix in which either carbon nanoglobules (in the case of NGC) or carbon particles with the "crumpled sheet" morphology (in the case of GO or RGO used as the modifying additives) are distributed. Depending on the features of the introduced 5–7 wt.% nanostructured component, the fraction of mesopores was shown to vary from 11% to 46%, and SBET—from 791 to 1115 m2 g−1. The synthesis of PCCNC using graphite oxide and reduced graphite oxide as the modifying additives can be considered as a method for synthesizing a porous carbon material with the hierarchical structure containing both the micro- and meso/macropores. Such materials are widely applied and can serve as adsorbents, catalyst supports, elements of power storage systems, etc. [ABSTRACT FROM AUTHOR]

It was established that the surface functionality of nanoglobular carbon (NGC) can be effectively altered by treatment at temperatures of 573 – 1173 K in an inert atmosphere, without affecting the structure and morphology of the material as a whole. The destruction and loss of surface oxygen groups occurs as a result of this treatment, which is accompanied by a decrease in the concentration of paramagnetic centers. At a temperature of 1173 K, a restructuring and “smoothing” of the carbon surface apparently takes place, which is expressed by annealing of defects (sources of EPR signal). It was found that changes in the surface functionality of NGC affect the reducibility of supported palladium precursor and the formation of palladium nanoparticles, without causing changes in palladium dispersion state. The study of the obtained Pd/NGC catalysts in the practically important hydrogenation of 4-nitrobenzoic acid ethyl ester and furfural showed that thermal pre-treatment of the support affects the catalytic performance in these reactions. It is important that varying temperature of such pre-treatment over a fairly wide range, which has a significant impact on the functionality of the support surface, leads to only relatively small changes in the activity and selectivity of the resulting catalysts. In this regard, thermal pre-treatment of carbon support should be considered as an approach to fine tune the performance of carbon-supported palladium catalysts.Graphical Abstract: It was established that the surface functionality of nanoglobular carbon (NGC) can be effectively altered by treatment at temperatures of 573 – 1173 K in an inert atmosphere, without affecting the structure and morphology of the material as a whole. The destruction and loss of surface oxygen groups occurs as a result of this treatment, which is accompanied by a decrease in the concentration of paramagnetic centers. At a temperature of 1173 K, a restructuring and “smoothing” of the carbon surface apparently takes place, which is expressed by annealing of defects (sources of EPR signal). It was found that changes in the surface functionality of NGC affect the reducibility of supported palladium precursor and the formation of palladium nanoparticles, without causing changes in palladium dispersion state. The study of the obtained Pd/NGC catalysts in the practically important hydrogenation of 4-nitrobenzoic acid ethyl ester and furfural showed that thermal pre-treatment of the support affects the catalytic performance in these reactions. It is important that varying temperature of such pre-treatment over a fairly wide range, which has a significant impact on the functionality of the support surface, leads to only relatively small changes in the activity and selectivity of the resulting catalysts. In this regard, thermal pre-treatment of carbon support should be considered as an approach to fine tune the performance of carbon-supported palladium catalysts. [ABSTRACT FROM AUTHOR]

Changes in the structural and textural properties of NiAl-layered double hydroxides (LDHs) (with 2–4 molar ratios of metals) and state of nickel that occur in different steps of the synthesis of nickel catalysts were studied using XRD, thermal analysis, TPR, low-temperature nitrogen adsorption, XANES, EXAFS, and electron microscopy methods. Relations between nickel content, catalyst reduction conditions, state of nickel, and its catalytic properties were revealed. It was shown that the use of NiAl LDH as the catalyst precursor even at a high content of active metal allows for the obtaining of the dispersed particles of supported nickel that are active in the aqueous-phase hydrogenation of furfural. The catalyst activity and conversion of furfural were found to increase with elevation of the catalyst reduction temperature and the corresponding growth of the fraction of reduced nickel. However, a lower reduction temperature (500 °C) makes it possible to form smaller nickel particles with the size of 4–6 nm, and a high Ni content (Ni:Al = 4) can be used to obtain the active Ni@NiAlOx catalyst. Under mild reaction conditions (90 °C, 2.0 MPa), the furfural conversion reached 93%, and furfuryl alcohol was formed with the selectivity of 70%. Under more severe reaction conditions (150 °C, 3.0 MPa), complete conversion of furfural was achieved, and cyclopentanol and tetrahydrofurfuryl alcohol were the main hydrogenation products. [ABSTRACT FROM AUTHOR]

Abstract: The mechanism of the propylene oxidation by Pd(NOn)Cl2−m(CH3CN)2 complexes (n=2,3; m=0,1,2) in chloroform solutions has been studied by 1H NMR and IR spectroscopy. The main reaction products are acetone and 2-nitropropylene, with their ratio depending on the equilibrium existing in the reaction solutions between palladium complexes containing NOn ligands bonded to a palladium atom via either an oxygen or a nitrogen atom. Reactivities of the oxygen bonded nitrato and nitrito complexes are significantly higher than that of the nitrogen bonded nitro complex. Various new organopalladium intermediates have been observed and monitored in situ. A reversible insertion of the coordinated propylene into the Pd–O or Pd–N bonds results in nitrato-, nitrito- and nitropalladation intermediates, which then decompose via a β-hydrogen elimination. Two isomers of the nitritopalladation intermediate have been detected, i.e., a palladium metallacycle and an open ring complex, with the latter being much more reactive towards the β-hydrogen elimination than the former. The decomposition of the nitrato- and nitritopalladation intermediates results in the organometallic precursor of acetone, i.e., an acetonylpalladium complex, and then in acetone itself. On the other hand, the nitropalladation intermediate originates 2-nitropropylene. In the presence of dioxygen, which re-oxidizes the nitrosyl groups, the acetone formation becomes a catalytic reaction with respect to both palladium and nitrogen. [Copyright &y& Elsevier]

• Purposeful change of palladium species localization in LDH structure. • Effect of precursor localization on sizes and morphology of supported Pd. • Effect of Pd precursor fixing on palladium activity in aqueous-phase FAL hydrogenation. • Predominant route of FAL conversion through furan ring hydrogenation on Pd/MgAl-LDH. Pd(II) chloride complexes were anchored using magnesium-aluminum layered double hydroxides (LDHs) with interlayer anions (CO 3 2– and OH–), which possess different exchange properties, and MgAl mixed oxide during its rehydration. It was shown that the catalysts of the same chemical composition with different size, morphology and electronic state of supported palladium particles can be synthesized by varying the localization of Pd precursor. The properties of Pd/MgAlOx catalysts were studied in aqueous-phase hydrogenation of furfural. Anchoring of the Pd precursor in the interlayer space of LDHs is accompanied by the formation of non-isometric agglomerated palladium particles which contain less oxidized metal and show a higher activity toward hydrogenation of furfural. Magnesium-aluminum oxides in Pd/MgAlO x catalysts are rehydrated in the aqueous-phase reaction to yield the activated MgAl-LDH species as a support, which promotes the furfural conversion via hydrogenation of the furan cycle. [ABSTRACT FROM AUTHOR]

• Mechanism of Pd/C-catalyzed hydrogenation of furfural in H 2 O was established. • Deuterium labeling is one of the techniques used to study the reaction mechanism. • Aqueous-phase furfural hydrogenation is realized through four pathways. • Mechanism includes reductive and acid-catalyzed furan ring opening reactions. Catalytic hydrogenation of furfural under hydrothermal conditions is of great importance and has recently attracted huge attention as an approach to obtaining a wide range of valuable chemicals from a renewable source. However, the data concerning the reaction mechanism of aqueous-phase furfural hydrogenation are very scarce, and it is still not clear which reactions lead to various carbonyl compounds, such as cyclopentanone, 4-oxopentanal, 5-hydroxy-2-pentanone. Herein, we present the results of an experimental study of the mechanism of reactions occurring in the furfural hydrogenation under hydrothermal conditions over the 1% Pd/CNTs catalyst. The following approaches have been used to study the reaction mechanism: hydrogenation of key intermediates formed upon the furfural conversion, and isotopic labeling experiments using D 2 O as a tracer in hydrogenation of furfural, which provides direct evidence of the occurring reactions. It was found that the furfural hydrogenation process is realized through four pathways including various reductive and acid-catalyzed furan ring opening reactions. The methods and approaches used in the present work, as well as the results obtained, can be useful in studying the mechanism of aqueous-phase hydrogenation of a wide range of oxygen-containing substrates in the presence of catalysts of different compositions. [ABSTRACT FROM AUTHOR]

• MgAl-layered hydroxides were synthesized on the surface of γ-Al 2 O 3 using its Al atoms. • Modification of alumina changed the mechanism of [PtCl 6 ]2− anchoring. • Adsorption occurs with localization of 2–4 nm platinum particles in the MgAl-phase. • Catalyst with supported LDH provides a higher stability in propane dehydrogenation. Layered magnesium-aluminum hydroxides of hydrotalcite-like structure were synthesized on the surface of γ-Al 2 O 3. The synthesis was performed via the interaction of aluminum and magnesium hydroxides and carbonate ions in the pores of alumina support. Novelty of the approach was that the synthesis involved the boehmite phase AlO(OH) that formed directly on the surface of γ-Al 2 O 3 under hydrothermal conditions. This approach provides fixation of the forming layered phase on the surface of initial support and retains textural parameters of the support. The formation of a layered structure on the γ-Al 2 O 3 surface was confirmed by XRD analysis and electron microscopy studies. Investigation of the H 2 [PtCl 6 ] anchoring on γ-Al 2 O 3 with supported magnesium-aluminum hydroxide by UV–vis DRS, 195Pt NMR and HRTEM showed that platinum was adsorbed on the surface as chloride forms of the complexes with a low degree of hydrolysis even at a low metal content. Catalytic properties of the synthesized supported platinum catalysts were examined in a model reaction of propane dehydrogenation. In comparison with the alumina-platinum catalyst, the modified sample showed a better stability at a somewhat higher yield of propylene because the formation of heavy products was suppressed. [ABSTRACT FROM AUTHOR]

Carbon nanofibers (CNFs) have been rapidly synthesized on the surface of low-cost commercial chlorinated polymers (chlorinated polyvinyl chloride, polyvinyl chloride), which contains iron compounds, under the action of a high power ion beam at room temperature. Organic (Fe(C 5 H 5 ) 2 ) and inorganic (FeCl 3 ⋅6H 2 O и Fe(NO 3 ) 3 ·9H 2 O) iron compounds were used as catalytic additions. SEM, TEM and Raman spectroscopy were used to investigate the morphologies and structure of irradiated polymers. CNFs had a most probable diameter in the range 50–90 nm and a maximum length of up to 10 μm. The maximum CNFs growth rate was estimated as 160 μm/μs. A possible growth mechanism for CNFs is discussed. [ABSTRACT FROM AUTHOR]

The palladium catalysts prepared using carbon nanoglobules (CNGs) as the support and containing no more than 2 wt% Pd were shown to be highly effective for the liquid-phase hydrogenation of ethyl 4-nitrobenzoate under mild conditions. Regardless of the kind of CNGs and the catalyst preparation method, the most of the Pd/CNGs catalysts allow achieving a complete conversion of substrate giving benzocaine in a 100% yield. The Pd/CNGs catalyst is easily recycled many times without loss of its catalytic performance. The results obtained may be useful for the development of effective Pd/C catalysts for chemoselective hydrogenation of various nitroarenes. [ABSTRACT FROM AUTHOR]

Abstract: A facile direct deposition approach for the preparation of recyclable Pd/C catalysts simply by stirring a solution of tris(dibenzylideneacetone)dipalladium(0) with a suitable carbon material was evaluated. An extraordinarily rapid catalyst preparation procedure (<5 min) under mild conditions and its excellent performance in cross‐coupling and hydrogenation reactions were demonstrated. The key point for catalyst design is the direct deposition of Pd0 centers onto the highly accessible surface area and the avoidance of ill‐defined PdII/Pd0 states. [ABSTRACT FROM AUTHOR]

The effect of the nature of alkaline agent and pH value of hydrolysis of Pd(II) chloride complex into palladium polynuclear hydroxo complexes, as precursor of active component in Pd/C catalyst, on the dispersion of palladium particles supported on carbon material Sibunit was revealed. It was shown that an increase in the particle size of supported palladium enhances the specific catalytic activity in hydrogenation of 2,4,6-trinitrobenzoic acid and changes the direction of the transformation (increases the fraction of non-aromatic intermediate, cyclohexane-1,3,5-trione trioxime, in the products). The results obtained are essential for optimizing the particle size of supported palladium in selective hydrogenation of polyfunctional organic compounds. [ABSTRACT FROM AUTHOR]

Bimetallic Pd-Ru/C catalysts prepared with different carbon supports were shown to be much more active in the liquid-phase hydrogenation of benzaldehyde in comparison with their monometallic analogs. In the case of systems based on nanoglobular carbon, the synergistic effect manifested itself as an increased selectivity for benzyl alcohol (86–90%) at a complete conversion of benzaldehyde, whereas the catalysts containing carbon nanotubes were highly active in hydrogenolysis of the C O bonds, which caused an increased yield of toluene (up to 56%). The synergistic effect between palladium and ruthenium in the reaction under study can be related to the changes in the electronic and dispersion state of the supported metals upon interaction with each other and formation of Pd-Ru alloy. In terms of the electronic factor, the enhanced catalytic performance of bimetallic systems can be a result of facilitation of electrophilic activation of the C O bond in benzaldehyde owing to an increased fraction of the electron-deficient palladium species. The nature of carbon support was shown to affect the electronic state of supported metals and thus catalytic properties of bimetallic Pd-Ru/C systems. [ABSTRACT FROM AUTHOR]

Results of the catalytic hydrogenation of 2,4,6-trinitrobenzoic acid (TNBA) in the presence of 1% Pd/C catalyst with analysis of the reaction products at the steps corresponding to the consumption of 1, 3, 5, 6, 8 and 9 mols of hydrogen per mol of TNBA are presented. Numerous reaction intermediates were identified using the 1 H and 13 C NMR spectroscopy; the reaction mixture composition was assessed quantitatively at different steps of the hydrogenation. The data obtained were used to propose a scheme of TNBA transformations over the Pd/C catalyst in the aqueous-phase reaction conditions. The data on the component composition at different steps of TNBA hydrogenation are important to understand the process chemistry, to explore the possibility of selective formation of different types of intermediate nitroamino compounds, and to synthesize high-performance catalysts for the hydrogenation of aromatic polynitro compounds. [ABSTRACT FROM AUTHOR]

The study considered the formation and catalytic properties of the active sites in Pd/C and Ru/C catalysts supported on carbon nanotubes (CNT) and carbon black (CB). The nature of carbon support was shown to affect the dispersion of supported metal and its catalytic properties in the aqueous-phase hydrogenation of furfural. The 1.5% Pd/CB catalyst demonstrated a high selectivity (99%) for furfuryl alcohol during hydrogenation of furfural at a temperature of 50 °C and hydrogen pressure of 0.5 MPa. The 1.5% Pd/CNT catalyst was not active under mild conditions of the reaction (50 °C, 0.5 MPa), but had a greater tendency to reduce the furan ring under severe conditions (90 °C, 0.5 or 2.0 MPa) as demonstrated by an increased yield of tetrahydrofurfuryl alcohol. The ruthenium samples showed a low activity in aqueous-phase hydrogenation of furfural irrespective of the support nature and reaction conditions, possibly due to irreversible adsorption of water on the active sites. [ABSTRACT FROM AUTHOR]

The Pearson IV function was used to fit the asymmetric solid-state 27 Al NMR spectra of alumina based catalysts. A high convergence (correlation coefficient is no less than 0.997) between experimental and simulated spectra was achieved. The decomposition of the 27 Al NMR spectra of zinc/aluminum mixed oxides with different Zn/Al molar ratio revealed an increased fraction (6–9%) of pentacoordinated aluminum atoms in these oxides as compared to γ-Al 2 O 3 . As the Zn/Al ratio is raised, the fraction of [AlO 6 ] octahedral units decreases, while the fraction of [AlO 4 ] tetrahedra increases. [ABSTRACT FROM AUTHOR]

Highlights: [•] Increase the fraction of bridging OH groups and concentration of Lewis acid sites. [•] Preferential formation of outer-sphere complexes. [•] Changes in the dispersion and electronic state of supported Pt. [•] Effect of hydrothermal treatment of the support on catalytic properties of Pt/Al2O3. [ABSTRACT FROM AUTHOR]

Abstract: A method for in situ reduction of the sulfur content in catalytic cracking gasoline is reported. A comparison of thermal and catalytic transformations of sulfide and thiophene sulfur is presented. Main transformations of various organosulfur compounds are demonstrated. The effect of [H]-donor activity of hydrocarbons on conversion of thiophene sulfur into hydrogen sulfide is revealed. The impact of acid–base properties of the additives to the cracking catalysts on sulfur content reduction in the liquid products is examined in detail (TPD-NH3, 27Al NMR, CO2 adsorption). The choice of optimal qualitative composition of a modifying additive for reducing the sulfur content in catalytic cracking gasoline is substantiated. [Copyright &y& Elsevier]

Abstract: The state of palladium in Pd/SO4 2−–ZrO2 (Pd/SZ) isomerization catalyst was investigated by the temperature-programmed reduction (TPR), chemisorption technique, infrared spectroscopy of adsorbed carbon monoxide (FTIRS), X-ray photoelectron spectroscopy (XPS), diffuse-reflectance UV–vis spectroscopy (UV–vis DRS), and benzene hydrogenation as a test reaction. It has been stated that reduction temperature has a great impact on the metal function of Pd-promoted sulfated zirconia catalyst. Metal centers are formed at about 30–70°C and characterized by high palladium dispersion and activity in benzene hydrogenation. At temperatures above 200°C, intensive sulfate decomposition occurs and products of sulfate reduction poison the metal function of the catalyst. According to XPS and FTIRS study, palladium particles in the poisoned samples are only partly oxidized, but the main part is presented by metallic phase without large amount of PdS. Reduction of Pd-containing catalyst at 150°C (instead of 250°C) leads to higher conversion and 2,2-dimethylbutane yield in acid-catalyzed reaction of n-hexane isomerization. Higher isomerization activity in this case is provided by prevention of active sulfate species decomposition due to the capability of palladium metallic particles formation at low reduction temperatures. [ABSTRACT FROM AUTHOR]

Ni(Mg)Al-layered double hydroxides (LDH) were synthesized by coprecipitation. Structural properties of LDH and mixed oxides Ni(Mg)AlO x , obtained by LDH calcination, were studied by XRD and SEM. Textural properties of Ni(Mg)AlO x were studied by the nitrogen adsorption-desorption method. It was found that the formation of the single-phase LDH was occurred for any nickel content in the samples. The mixed oxides have a high specific surface area, the value of which decreases with increasing nickel content in the samples. Ni(Mg)AlO x are characterized by high activity in furfural hydrogenation. Selectivity of the formation of different reaction products depends on the nickel content in the catalysts. The full furfural conversion with the furfuryl alcohol selectivity no more than 65% was observed for the catalyst with a low nickel content. At the same time an increase in selectivity for furfuryl alcohol up to 84% occurs for the NiAl-catalyst with a furfural conversion not exceeding 43%. [Display omitted] • Synthesis of single-phase Ni(Mg)Al-layered hydroxides by coprecipitation. • Structural and textural properties of the Ni(Mg)Al-layered hydroxides and their calcined forms. • High activity of the Ni(Mg)AlO x catalysts in the reaction of aqua-phase furfural hydrogenation. • Selectivity of products formation can vary by changing the nickel amount in the Ni(Mg)AlO x catalysts. [ABSTRACT FROM AUTHOR]

Review: 79 refs. [ABSTRACT FROM AUTHOR]