Liquid-phase Catalytic Hydrogenation Research Articles

Development of a Novel Structured Mesh-Type Pd/γ-Al2O3/Al Catalyst on Nitrobenzene Liquid-Phase Catalytic Hydrogenation Reactions

Nitrobenzene liquid-phase catalytic hydrogenation is commonly regarded as one of the most effective technologies for aniline production. The traditional granular catalysts have the disadvantages that the reactor bed pressure drop is large and the mass transfer efficiency between gas and liquid phases is low. In this study, a novel structured mesh-type Pd/γ-Al2O3/Al catalyst was prepared by anodic oxidation and pore structures of γ-Al2O3/Al supports were constructed by acid pore-widening treatments. The results showed that acid pore-widening treatments can improve the pore size of γ-Al2O3/Al supports; the support with HNO3 pore-widening treatment exhibited the largest pore size, being enlarged from 3.7 nm to 4.6 nm. The Pd/γ-Al2O3/Al catalysts prepared with different acid pore-widening treatment supports contribute to the increased active metal Pd loading, more Pd0 content, and better dispersion of the Pd particles. The catalyst prepared with HNO3 pore-widening treatment support exhibited the largest active metal Pd loading, enlarging from 1.82% to 1.95%, the largest Pd0 content being enlarged from 52.1% to 58.5% and the smallest Pd particle size being reduced from 103 nm to 41 nm, resulting in the highest nitrobenzene conversion, increasing from 67.2% to 74.3%. Eventually, we calculated that the pressure drop of structured catalysts was 1/72 of that of granular catalysts, resulting in a better diffusion of the H2 through nitrobenzene solution to active sites on the catalyst surface and a significant increase in the catalytic activity.

SOLVENT AND SUBSTITUENT NATURE EFFECT ON THE 2-NITROAZOBENZENES HYDROGENATION SELECTIVITY ON SKELETAL NICKEL

The elucidation of the reasons and character of the influence of nature substituent and solvent composition on the kinetic features and selectivity of liquid-phase catalytic hydrogenation of compounds containing several reactive capable groups is a theoretical and practically significant task. In the presented work the process of 2-nitroazobenzene derivatives hydrogenation with various substituents was investigated in order to obtain the corresponding benzotriazoles on a skeletal nickel catalyst in a neutral azeotropic aqueous solution of 2-propanol, and in the presence of acetic acid or sodium hydroxide. According to the data obtained, the solvent composition has a decisive influence on the selectivity of this process. The transformation of azo- and nitro groups, included in the initial compound, proceeds in parallel stages, and the introduction of acidic or basic additives leads to а change in the rate ratio of these stages. Appropriately, selectivity of 2-nitroazobenzene derivatives hydrogenation changes with respect to reaction products containing a triazole ring, in particular, with respect to N-oxides of substituted benzotriazoles, and also with respect to nitrohydrazo compounds. The greatest influence of substituent nature on the hydrogenation selectivity in a solvent with any composition is manifested for the group whose transformation rate in a given medium is higher. In the presence of acid the substituent variation is more strongly reflected in the hydrogenation rate of azo group, and in the basic medium – of nitro group. It was found that the change in the transformation rate of both reactive capable groups corresponds to the Hammett constant alteration. The electron acceptor substituent has a deactivating effect on the transformation rate of nitro as well azo group. To increase the selectivity of substituted 2-nitroazobenzenes hydrogenation with respect to compounds containing a triazole ring, it is recommended to introduce a base into the aqueous solution of 2-propanol.

Nitrogen-doped carbon material NCM-T heterogeneously catalyzed liquid-phase hydrogenation of nitrobenzene to aniline

N-doped carbon materials NCM-T are prepared by in situ doping for the heterogeneous catalytic liquid-phase hydrogenation of nitrobenzene to aniline. Changes in the N species conformation and their relationship with catalytic activity are discussed.

Highly selective catalytic transfer hydrogenation of biomass derived furfural to furfural alcohol over Zr/SBA-15 catalysts

The effective utilization of furfural (FU) from biomass for the production of furfuryl alcohol (FOL) is becoming increasingly important due to the versatility of FOL production of valuable chemicals. This study reports a highly efficient approach for in-situ hydrothermal methods to prepare SBA-15 (SB) and x% Zr/SBA-15 (ZSB) composite catalysts with different weight percentages (x = 1, 3, and 5 wt.%). The prepared bare and most active catalysts were analyzed by XRD, FT-IR, FE-SEM, EDS, and TEM. Characterization studies showed that Zr species were present and uniformly distributed in the SB framework. All the prepared materials were used for liquid phase catalytic transfer hydrogenation (CTH) of FU with isopropanol (2-PrOH) at atmospheric N2 pressure (1 atm) under mild reaction conditions. The 3ZSB catalyst showed excellent conversion of FU (100 %) and remarkably high selectivity for FOL (97 %). To achieve the highest possible conversion of FU with high selectivity to the desired products, the experimental parameters such as reaction temperature, hydrogen donor, time, molar ratio, and catalyst weight were optimized. The reusability of the most active 3ZSB composite catalysts was also tested for up to six sequences. Moreover, no significant change in catalytic performance was observed. In this study, a green, efficient, and sustainable process for the catalytic synthesis of FOL using biomass-derived FU is presented. This process is of great importance in promoting the sustainable use of biomass resources.

Structure of 2-nitro-2’-hydroxy-5’-methylazobenzene: Theoretical and spectroscopic study

Objectives. 2-Hydroxy-nitroazobenzenes comprise reagents for the synthesis of heteroaromatic compounds, in the molecules of which the benzene and azole cycles are annulated. These reagents are widely used in the production of chemical products for various industries. In particular, 2-2’-hydroxy-5’-methylphenylbenzotriazole is used as an effective photo stabilizer for polystyrene and polyethylene. A promising method for its preparation is the liquid-phase catalytic hydrogenation of 2-nitro-2'-hydroxy-5'-methylazobenzene (2NAB). The aim of the present study was to establish the structure of 2NAB in solutions of different composition.Methods. Theoretical calculations were carried out within the framework of the density functional theory at a temperature of 298.15 K for the gas phase at B3LYP/6-311++G(d, p) and M06-2X/6-311++G(d, p) levels; for hexane, 2-propanol, toluene at B3LYP/6-311++G(d, p) level using the conductor-like polarizable continuum model. An experimental study to determine the probable isomeric structure of 2NAB in various solvents, including sodium hydroxide (NaOH) and acetic acid (CH3COOH) additives, was carried out using infrared (IR) and ultraviolet (UV) spectroscopy.Results. The most probable structure of 2NAB isomers for the gas phase and a number of solvents was determined. Experimental and theoretical IR and UV spectra were obtained. The thermodynamic characteristics of the reaction of intramolecular proton transfer from –OH to –N=N– group in the gas phase were calculated.Conclusions. A comparison of the experimental and calculated results supports the conclusion that the cis-isomer should be considered most probable for the gas phase. For the studied solutions, a trans-isomer of 2NAB with hydrogen bonds formed between the hydroxyl group hydrogen and the β-nitrogen atom of the azo group of dye molecule should be considered as the most likely structure. In the studied individual and binary solvents, prototropic equilibrium is shifted towards the azo form of the dye, while intramolecular proton transfer is possible only in aqueous diethylamine and dimethylformamide solutions with additions of NaOH.

Structure of 2-nitro-2’-hydroxy-5’-methylazobenzene: Theoretical and spectroscopic study

Objectives. 2-Hydroxy-nitroazobenzenes comprise reagents for the synthesis of heteroaromatic compounds, in the molecules of which the benzene and azole cycles are annulated. These reagents are widely used in the production of chemical products for various industries. In particular, 2-2’-hydroxy-5’-methylphenylbenzotriazole is used as an effective photo stabilizer for polystyrene and polyethylene. A promising method for its preparation is the liquid-phase catalytic hydrogenation of 2-nitro-2'-hydroxy-5'-methylazobenzene (2NAB). The aim of the present study was to establish the structure of 2NAB in solutions of different composition.Methods. Theoretical calculations were carried out within the framework of the density functional theory at a temperature of 298.15 K for the gas phase at B3LYP/6-311++G(d, p) and M06-2X/6-311++G(d, p) levels; for hexane, 2-propanol, toluene at B3LYP/6-311++G(d, p) level using the conductor-like polarizable continuum model. An experimental study to determine the probable isomeric structure of 2NAB in various solvents, including sodium hydroxide (NaOH) and acetic acid (CH3COOH) additives, was carried out using infrared (IR) and ultraviolet (UV) spectroscopy.Results. The most probable structure of 2NAB isomers for the gas phase and a number of solvents was determined. Experimental and theoretical IR and UV spectra were obtained. The thermodynamic characteristics of the reaction of intramolecular proton transfer from –OH to –N=N– group in the gas phase were calculated.Conclusions. A comparison of the experimental and calculated results supports the conclusion that the cis-isomer should be considered most probable for the gas phase. For the studied solutions, a trans-isomer of 2NAB with hydrogen bonds formed between the hydroxyl group hydrogen and the β-nitrogen atom of the azo group of dye molecule should be considered as the most likely structure. In the studied individual and binary solvents, prototropic equilibrium is shifted towards the azo form of the dye, while intramolecular proton transfer is possible only in aqueous diethylamine and dimethylformamide solutions with additions of NaOH.

Polyaniline coated Pt/CNT as highly stable and active catalyst for catalytic hydrogenation reduction of Cr(VI)

Liquid phase catalytic hydrogenation reduction is a feasible method to eliminate Cr(VI) in water, while supported noble metal catalysts are liable to deactivation. In this study, carbon nanotube supported Pt catalyst (Pt/CNT) coated by polyaniline (Pt/CNT@PANI) was prepared and applied in the liquid phase catalytic hydrogenation of Cr(VI). Characterization results disclose that after coating Pt/CNT is completely wrapped by PANI layers and active Pt particles are no longer accessible. Despite complete embedment of Pt particles by PANI layers, Pt/CNT@PANI remains highly active for Cr(VI) reduction in liquid phase catalytic hydrogenation. The catalytic Cr(VI) reduction on Pt/CNT@PANI can be described by a PANI oxidation-reduction mechanism, by which PANI is first oxidized by Cr(VI) to form Cr(III), and oxidized PANI is reduced by catalytic hydrogenation. The Cr(VI) reduction on Pt/CNT@PANI complies with the Langmuir-Hinshelwood model, reflecting the pivotal role of Cr(VI) adsorption. Furthermore, the catalytic activity of Pt/CNT@PANI differs with PANI layer thickness and Cr(VI) reduction is positively correlated with reaction temperature. Catalyst recycling results show that after 4 cycles Pt/CNT loses 92.4% of catalytic activity, while the initial activity of Pt/CNT@PANI slightly decreases by 11.6%, demonstrating its high catalyst stability.

Effective catalytic hydrodechlorination removal of chloroanisole odorants in water using palladium catalyst confined in zeolite Y

Chloroanisoles is a class of odorous pollutants commonly identified in drinking water. In the present study, we confined noble metal palladium (Pd) in the micropores of zeolite Y (ie-Pd@Y) using an ion exchange method, and applied it for the catalytic hydrodechlorination removal of chloroanisoles (represented by 2,4,6-trichloroanisole/TCA) in water. Pd supported on zeolite Y surface (im-Pd/Y, prepared by conventional impregnation method) was used as the benchmarking catalyst. The characterization results revealed that ie-Pd@Y had smaller Pd particle size and higher Pdn+/Pd0 ratio than im-Pd/Y. The catalytic hydrodechlorination of TCA followed a concerted dechlorination pathway and the Langmuir-Hinshelwood model. The ie-Pd@Y catalysts with different Pd loadings exhibit excellent catalytic activities with more than 95% of TCA removed within 30 min, which is far superior to the im-Pd/Y catalysts (27–70%). Moreover, due to the confinement effect of zeolite Y, ie-Pd@Y displayed enhanced catalytic stability as compared with im-Pd/Y. The initial activity of ie-Pd@Y was more than 20 times higher than that of im-Pd/Y after five reaction cycles. Additionally, with the assistance of sieving effect, ie-Pd@Y displayed much stronger capability against the interference from dissolved organic matter than im-Pd/Y. The present results demonstrate that the confined catalysts ie-Pd@Y can be applied in liquid phase catalytic hydrogenation to effectively eliminate halogenated odorants in waters.

Highly Active Large Au Clusters and Even More Active Ag Nanoparticles Supported on Ceria-Zirconia: Impact of Particle Size and Potassium Ion Loading on Activity in Catalytic Transfer Hydrogenation

Although heterogeneous monometallic gold catalysts are commonly more active when the gold particles are smaller, this study shows that the reverse is true in the case of liquid phase catalytic transfer hydrogenation of acetophenone with 2-pentanol. Higher catalytic activity of larger gold particles, i.e., over 30 nm in diameter, than of smaller particles of average 4 nm in size was observed. Moreover, this effect was contradictory to that observed for supported monometallic silver catalysts in which the interaction with the support and hence particle size was shown to cause drastic changes in the activity in this reaction, with the large particles being completely inactive and tiny ones being the most active system studied. In this reaction, the ceria-zirconia solid solutions were used as the supports for the catalysts and both zirconium doped ceria, as well as cerium doped zirconia carriers were tested. The supports themselves exhibited little activity in this reaction. It was shown that the activity of the supports and catalysts depends on the Ce/Zr ratio and potassium content. Both types of catalysts showed excellent selectivity to 1-phenylethanol and conversion of acetophenone, although it was noted that a high loading of potassium carbonate in the gold catalysts propelled undesired reactions, thereby reducing the selectivity to 1-phenylethanol.

Mechanism of transfer hydrogenation of carbonyl compounds by zirconium and hafnium-containing metal-organic frameworks

Liquid phase catalytic hydrogenation is essential to produce platform chemicals from biomass-derived carbonyl compounds. Carbonyl compounds can be upgraded to corresponding alcohols by catalytic transfer hydrogenation using hydrogen-donor solvents and mild reaction conditions. The challenge in transfer hydrogenation is the development of selective, active, and reusable catalysts. Here we show the chemical pathway of transfer hydrogenation of benzaldehyde by Hf- and Zr-containing MOF-808 and UiO-66 catalysts. MOF-808(Hf) was the most selective catalyst with 95% selectivity to benzyl alcohol at 99% conversion. Furthermore, the quantum calculations revealed that the transfer hydrogenation by MOF-808(Hf) proceeded by Meerwein–Ponndorf–Verley (MPV) reduction, which resulted in high selectivity and conversion. These findings of the effects of metal and acid sites of these MOFs enable maximizing the selectivity for transfer hydrogenation. Moreover, understanding these effects provide opportunities for these MOFs in other biomass conversion reactions.

Polyphenylene sulfide as an efficient solid-phase ligand for improved selective alkyne hydrogenation

Liquid phase catalytic selective hydrogenation of alkynes to alkenes is a widely used reaction in the industry. Pairing nanocatalysts with soluble ligands or developing advanced catalytic nanostructures are two general protocols to realize preferable hydrogenation performance. However, both protocols are not cost-effective, and it is still challenging to achieve high selectivity and high activity with conventional hydrogenation nanocatalysts. In this work, commercial available, low-cost, and reactant-inert polyphenylene sulfide (PPS) is used as a new solid-phase ligand for palladium nanocatalysts in selective alkyne hydrogenation. Hydrogenation experiments have been carried out on different terminal and internal alkyne substrates, to investigate the universal effects of PPS on hydrogenation. As a result, over-hydrogenation and/or isomerization could be effectively suppressed upon the addition of solid PPS powders. The beneficial effects can be easily tuned by varying the PPS dosage. Importantly, both solid supported nanocatalysts and PPS can be simply separated via sedimentation and washing for subsequent experiments. A recycling experiment with PPS/Pd@CaCO3 has demonstrated the good cyclability (ten times of usage without much changes in effectiveness) of the catalyst system. X-ray photoelectron spectroscopy results indicate that the weak binding of PPS as a solid-phase ligand to nanocatalysts may contribute to the improved selective hydrogenation performance.

Liquid phase catalytic transfer hydrogenation of ethyl levulinate to γ-valerolactone over ZrO2/SBA-15

γ-Valerolactone (GVL) is an important bio-derived platform molecule whose atom- and energy efficient, and scalable, catalytic synthesis is highly desirable.

Efficient Reduction of Selenite to Elemental Selenium by Liquid-Phase Catalytic Hydrogenation Using a Highly Stable Multiwalled Carbon Nanotube-Supported Pt Catalyst Coated by N-Doped Carbon.

A stable catalyst, Pt/carbon nanotube (CNT) coated with N-doped carbon (Pt/CNT@CN), was designed to reduce selenite (Se(IV)) in water to elemental selenium by liquid-phase catalytic hydrogenation. Commercial Pt/C, pristine Pt/CNT, and carbon-coated Pt/CNT (Pt/CNT@C) were used for benchmarking. The Pt particles in Pt/CNT@CN were completely embedded beneath the coatings to minimize leaching and were not easily accessible to Se(IV). However, Schottky-Mott-type metal-carbon junctions that activate H2 were formed on the coated catalyst, facilitating effective reduction of Se(IV). The initial activity of Pt/CNT@CN (900.5 mg L-1 gcat-1 h-1) was two times higher than that of commercial Pt/C (448.6 mg L-1 gcat-1 h-1). The commercial Pt/C and uncoated Pt/CNT lost their initial activities during reuse and were almost inactive after 10 cycles due to significant Pt leaching (>90%) during the reaction and acid-washing regeneration processes. Pt/CNT@CN maintained 33% of the initial activity after the first cycle and stabilized over the following 9 cycles due to effective protection of Pt particles by carbon coatings. After 10 cycles, the activity of Pt/CNT@CN was over 20 times higher than that of Pt/C and uncoated Pt/CNT. Overall, catalytic hydrogenation using carbon-coated-supported Pt catalysts is an effective and promising approach to remove Se(IV) in water.

The effect of metal precursor on copper phase dispersion and nanoparticle formation for the catalytic transformations of furfural

The formation of copper-based catalysts ranging from nanoparticles to isolated and dimeric Cu species supported on nanophased alumina is reported and utilised for the catalytic liquid-phase hydrogenation of furfural. The materials were synthesised via wet impregnation using various copper precursors (nitrate, acetate and sulphate) at two different loadings. A high Cu loading (5.0 wt.%) led to the formation of well-defined nanoparticles, while a lower loading (1.0 wt.%) generated a highly dispersed phase consisting mostly of atomic and dimeric Cu species dispersed on Al2O3. The catalytic reaction was found to be structure sensitive, promoting decarbonylation reactions with low Cu loading. Copper sulphate derived catalysts were found to severely decrease furfuryl alcohol selectivity from 94.6% to 0.8%, promoting the formation of side reactions. The sulphur-free catalysts represent a greener and more sustainable alternative to the toxic catalysts currently used in industry, operating at milder conditions of 50 °C and 1.5 bar H2.

Efficient removal and recovery of copper by liquid phase catalytic hydrogenation using highly active and stable carbon-coated Pt catalyst supported on carbon nanotube

In this work, we coated carbon nanotubes (CNT) supported Pt catalyst by conductive carbon layers (labelled as Pt/CNT@C) and the catalyst was further functionalized by surface oxidation (denoted as Pt/CNT@Oxi-C). The textural properties of the catalysts were extensively characterized and liquid phase catalytic hydrogenation reduction of Cu2+ was conducted. Results showed that Pt particles of Pt/CNT@C and Pt/CNT@Oxi-C were completely embedded beneath carbon overcoatings. Furthermore, contrary to Pt/CNT no CO chemisorption was observed on both Pt/CNT@C and Pt/CNT@Oxi-C, indicative of the absence of exposed Pt particles in carbon-coated Pt/CNT. Effective Cu2+ reduction and metallic Cu deposition by catalytic hydrogenation were achieved on catalyst surface. Surface oxidation of Pt/CNT@C resulted in increased surface wetting and functionality content, leading to noticeable enhancement in catalytic activity for Cu2+ reduction. Additionally, Cu2+ reduction on Pt/CNT@Oxi-C proceeded through the Langmuir-Hinshelwood model, suggesting that the reduction of Cu2+ adsorbed on catalyst surface was the rate-determining step. Carbonization of overcoatings exhibited a volcano-type relationship between carbonization temperature and catalytic activity of Pt/CNT@C for Cu2+ reduction. As for catalyst reuse, Pt/CNT lost 92 % of initial activity after five consecutive reaction cycles, whereas Pt/CNT@Oxi-C maintained a high catalytic activity without remarkable deactivation.

Complete dehalogenation of bromochloroacetic acid by liquid phase catalytic hydrogenation over Pd/CeO2 catalysts

Bromochloroacetic acid is classified as one of the typical disinfection byproducts (DBPs). In this work, supported palladium catalysts on different supports (CeO2, Al2O3, SiO2 and activated carbon (AC)) (labelled as Pd/support) were synthesized via the deposition-precipitation method (D-P method) and their activities for the complete dehalogenation of bromochloroacetic acid by liquid phase catalytic hydrogenation were evaluated. Comprehensive characterizations of the catalysts were conducted by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), point of zero charge (PZC), X-ray photoelectron spectroscopy (XPS) and CO chemisorption. Results indicated that the PZCs of the supports varied with each other. The stronger Pd-support interaction and higher Pd dispersion of Pd/CeO2 and Pd/Al2O3 than those of Pd/AC and Pd/SiO2 were confirmed by X-ray photoelectron spectroscopy and CO chemisorption. Pd/CeO2 had a higher ratio of positively charged Pd to metallic Pd (Pdn+/Pd0) than Pd/Al2O3 and Pd/AC due to a stronger metal-support interaction. Accordingly, a negligible bromochloroacetic acid conversion was observed on Pd/SiO2, whereas bromochloroacetic acid was found to be readily decomposed on Pd/CeO2, Pd/Al2O3 and Pd/AC. However, the dechlorination reaction could not further proceed on Pd/Al2O3 and Pd/AC catalysts after the bromine functionality was removed from bromochloroacetic acid. A complete dehalogenation of bromochloroacetic acid occurred only on Pd/CeO2. Furthermore, the dechlorination rate constants of monochloroacetic acid and bromochloroacetic acid over Pd(1.40)/CeO2 were 0.018 and 0.031 min−1 respectively, confirming an induced synergistic effect due to the existence of bromine atoms. It was worth noting that a stepwise-concerted pathway was verified during the liquid phase catalytic hydrodehalogenation of bromochloroacetic acid.

INTERRELATION BETWEEN CATALYTIC REDUCTION RATE AND SOLUBILITY OF NITROBENZOIC ACID ISOMERS

The solubility of nitrobenzoic acid isomers is determined from the concentration of solution in equilibrium with the solid phase at a temperature of 303 K in an azeotropic aqueous solution of 2-propanol, including additives of acetic acid and sodium hydroxide, with an error of no more than 5%. It is shown that the lowest solubility in all these media is observed for para-nitrobenzoic acid, and the maximum for the meta-isomer. The addition of acid or base to the liquid phase assists an increase in solubility for all isomers of nitrobenzoic acid. The most significant changes in solubility occur in the presence of sodium hydroxide in the water-alcohol solvent. The shift of the maxima in the absorption spectra was compared with the change in the solubility value. It has been established that with increasing solubility, the rate of liquid phase catalytic hydrogenation of nitrobenzoic acid isomers on the skeletal nickel catalyst decreases. А linear correlation between the solubility of reagents and the rate of their reduction is revealed. The observed interrelation for para- and meta-isomers in neutral and acidic aqueous-alcoholic media is mostly caused by the influence of solubility on the adsorption ability of the hydrogenated compound on the surface of the nickel catalyst. Deviations from this dependence in the presence of sodium hydroxide can be explained by ionization of the of nitrobenzoic acid isomers molecules and by redistribution of the forms of hydrogen adsorbed on the surface of the nickel catalyst. The reason of the distinction in a similar correlation for isomers with a substituent in the ortho-position relative to the nitrogroup is the possibility for forming an intramolecular hydrogen bond in all used media.

Hydrogenation of Furfural to Furfuryl Alcohol in the Presence of Ru-Containing Catalysts Based on New Zeolite-Like Materials

Catalytic liquid-phase hydrogenation of furfural to commercially important furfuryl alcohol was studied in the presence of Ru-containing catalysts based on Sibunit and zeo-type materials, such as nickel phosphate molecular sieves (VSB-5 and Fe-VSB-5) and zirconium-containing metal–organic frameworks (UiO-66(Zr)-R, R = H and NH2), at 30 and 50°C and 5–25 atm of hydrogen. The yield of furfuryl alcohol was found to depend on the nature of the support and increases in the following order: Ru/UiO-66(Zr)-NH2 < Ru/UiO-66(Zr) < Ru/Fe-VSB-5 < Ru/VSB-5. The effect of hydrogen pressure and temperature on the selectivity toward furfuryl alcohol was investigated.

Highly effective bromate reduction by liquid phase catalytic hydrogenation over Pd catalysts supported on core-shell structured magnetites: Impact of shell properties

Liquid phase catalytic reduction of bromate with supported noble metals as the catalysts is a promising method to remove bromate in water. Magnetic supports provide a feasible way to recover catalysts whose surface properties also strongly influence the catalytic efficiency. In this study, Pd nanoparticles supported on core-shell structured magnetites with varied shells (e.g., carbon, SiO2, polypyrrole, polyaniline, polydopamine and chitosan) were prepared and catalytic reduction of bromate on the catalysts was investigated. The results showed that in comparison with other catalysts Pd/(Fe3O4@polyaniline) exhibited a higher catalytic efficiency due to its higher point of zero charge and surface hydrophilicity. In parallel, bromate reduction on Pd/(Fe3O4@polyaniline) followed the Langmuir-Hinshelwood model, confirming the crucial role of bromate adsorption. At pH 5.6 and a catalyst dosage of 0.05 g/L, 0.4 mM bromate could be completely reduced into bromide within 120 min. Furthermore, the magnetic catalysts could be effectively separated and recovered under an external magnetic field within 3 min. The results of catalyst reuse showed that after five consecutive catalytic reduction cycles Pd/(Fe3O4@polyaniline) retained 87% of its fresh catalyst activity. The present findings indicate that Pd/(Fe3O4@polyaniline) with polyaniline as the shell is a highly active, stable and recyclable catalyst for liquid phase catalytic hydrogenation of pollutants in water.

Pt embedded in carbon rods of N-doped CMK-3 as a highly active and stable catalyst for catalytic hydrogenation reduction of bromate.

A novel Pt-based catalyst with fine and homogeneous Pt particles embedded in carbon rods of N-doped CMK-3 was fabricated by a two-step infiltration method using SBA-15 as the template. Due to its fine particle size, N-containing functionality and effective embedment of Pt particles in carbon rods, the catalyst exhibited superior catalytic activity and stability in the liquid phase catalytic hydrogenation of bromate in water.