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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]

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]

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]

• 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]

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]

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]