Liquid-phase Hydrogenation Reactions Research Articles

A switchable hydrogenation chemoselectivity of biomass platform compounds based on solvent regulation

Selective catalytic conversion of biomass-derived compounds to fuels and fine chemicals serves as a renewable energy pathway for the partial substitution of fossil resources, in which reaction pathway and selectivity control are key issues. Herein, we report a fully exposed Pt clusters immobilized on CoAl mixed metal oxides catalyst (denoted as Ptn/CoAl-MMOs), which exhibits prominent catalytic performance towards liquid phase hydrogenation reaction of furfural (FAL). Noteworthily, the hydrogenation chemoselectivity can be switched among four products via using four different solvents: tetrahydrofurfuryl alcohol (THFA; yield: 91.4%), furfuryl alcohol (FA; yield: 97.7%), 2-methylfuran (2-MF; yield: 92.1%) and furan (FU; yield: 90.8%) are obtained in ethanol, dioxane, isopropanol and n-hexane solvent, respectively. Experimental studies (in situ FT-IR and TPSR-Mass) combined with theoretical calculations (DFT) reveal that solvent molecules exert an essential influence on the adsorption configuration of FAL via changing the solvent-catalyst and/or substrate-catalyst interaction, which ultimately determines the hydrogenation pathway, key intermediate and final product. This work demonstrates a facile solvent-dependent product-switching strategy within one catalytic system, which opens up potential opportunities for tailoring hydrogenation selectivity in liquid-solid catalytic reactions towards biomass upgrading.

Directing Reaction Pathways on Supported Metal Catalysts with Low-Density Self-Assembled Monolayers

Controlling reactant adsorption on catalyst surfaces is crucial to reaction activity and selectivity. One method for improving selectivity is by imposing steric constraints to bias the reactant binding orientation. In this study, thiol self-assembled monolayers (SAMs) were deposited onto Pt/Al2O3 catalysts as a method for controlling activity and selectivity via steric effects. In addition to a full monolayer, a low-density SAM-coated catalyst was employed. A number of characterization techniques demonstrated the successful deposition of homogeneous low-density SAMs on the metal surface with reduced site-blocking compared to a full high-density monolayer. Reaction kinetic studies showed increased benzyl alcohol hydrodeoxygenation (HDO) selectivity for both SAM-modified catalysts. This was attributed to the inability of the reactant to adsorb on the catalyst with the aromatic ring parallel to the surface, thus preventing decarbonylation and ring hydrogenation reaction pathways. Additionally, SAM density influenced reaction activity significantly, with the low-density-modified SAM catalyst being more active than the catalyst coated with a full monolayer. Moreover, liquid-phase hydrogenation reactions were used to investigate the relationship between SAM density and reactivity for reactant molecules of various sizes. In all cases, the low-density SAM improved reaction rates relative to dense SAMs. The effect of controlling ligand density depended on the type of reaction: high ligand densities greatly diminished ring hydrogenation, while HDO was largely unaffected, suggesting a potential strategy for size-selective reaction rate and selectivity control.

Crystal-Plane-Dependent Guaiacol Hydrodeoxygenation Performance of Au on Anatase TiO2

TiO2-supported catalysts have been widely used for a range of both liquid-phase and gas-phase hydrogenation reactions. However, little is known about the effect of their different crystalline surfaces on their activity during the hydrodeoxygenation process. In this work, Au supported on anatase TiO2, mainly exposing 101 or 001 facets, was investigated for the hydrodeoxygenation (HDO) of guaiacol. At 300 °C, the strong interaction between the Au and TiO2-101 surface resulted in the facile reduction of the TiO2-101 surface with concomitant formation of oxygen vacancies, as shown by the H2-TPR and H2-TPD profiles. Meanwhile, the formation of Auδ−, as determined by CO-DRIFT spectra and in situ XPS, was found to promote the demethylation of guaiacol producing methane. However, this strong interaction was absent on the Au/TiO2-001 catalyst since TiO2-001 was relatively difficult to be reduced compared with TiO2-101. The Au on TiO2-001 just served as the active site for the dissociation of hydrogen without the formation of Auδ−. The hydrogen atoms spilled over to the surface of TiO2-001 to form a small amount of oxygen vacancies, which resulted in lower activity than that over Au/TiO2-101. The catalytic activity of the Au/TiO2 catalyst for hydrodeoxygenation will be controlled by tuning the crystal plane of the TiO2 support.

Role of hydroxyl on metal surface in hydrogenation reactions

The important role of surface hydroxyl in metal catalysis is attracting more attentions but rarely studied for liquid-phase hydrogenation reactions. Here we report such deliberate research with two model substrates, p-chloronitrobenzene and cinnamaldehyde. Graphene-supported iridium nanoparticles with hydroxyl on the metal surface were prepared by hydrothermal synthesis, and the surface hydroxyl can be removed by a mild pre-reduction. The catalyst with surface hydroxyl exhibited around three times higher rate for the hydrogenation of p-chloronitrobenzene than the catalyst without hydroxyl, and an opposite trend was found for the hydrogenation of cinnamaldehyde. Kinetic study revealed that although the surface hydroxyl halved the coverage of H on iridium, it increased the rate constant by a factor of eight through reducing the reaction barrier for the hydrogenation of p-chloronitrobenzene. Theoretical calculation confirmed that the surface hydroxyl can polarize the nitro group via a weak hydrogen-bond interaction and thus facilitate its hydrogenation.

Synthesis and Catalytic Activity in the Hydrogenation Reaction of Palladium-Doped Metal-Organic Frameworks Based on Oxo-Centered Zirconium Complexes

Metal-nanocluster-doped porous composite materials are attracting considerable research attention, due to their specific catalytic performance. Here we report a simple, cheap, and efficient strategy for the preparation of palladium hydrogenation catalysts based on metal-organic frameworks (MOFs). It has been shown that the synthesis of Pd/MOF results in the formation of palladium nanoparticles uniformly fixed on the surface. The composition and structure of the resulting composites were studied using elemental analysis, DSC, TGA, XRD, TEM, and IR spectroscopy. Pd nanoparticles with an average diameter of 8–12 nm were successfully confined in the UiO-type MOFs, and the obtained nanocomposites exhibited abundant porosity, high stability, and a large surface area. It has been shown that the resulting catalytic systems with high activity, selectivity, and stability reduce phenylacetylene and allyl alcohol to styrene and propanol, respectively, in liquid-phase hydrogenation reactions.

The effect of Ni particle size and carbon support on catalytic activity for glucose hydrogenation reaction

Ni nanoparticles supported on carbon black (Ni/CB) with Ni particle sizes 6.9–23.5 nm were found to exhibit a size effect, with a significant impact on catalytic performance in the glucose hydrogenation reaction. A series of Ni nanoparticles with different sizes, as determined by statistical analysis of TEM images, were obtained by varying the synthesis temperature using a sol-gel method. The highest glucose conversion and sorbitol production in glucose hydrogenation was possible with Ni/CB catalysts with Ni particles size around 17 nm obtained at 550 °C. Similarly, graphene nanoplatelets (GNP) supported Ni with different particle sizes synthesized under different temperature conditions have an identical behaviour, that is, GNP-550 exhibits the highest catalytic performance. Therefore, it can be concluded that size effect appeared when the particle size is above 10 nm, which provides a reference for the design of subsequent catalysts for liquid phase hydrogenation reactions pertinent to biomass conversion.

Catalytic Reactions at Amine-Stabilized and Ligand-Free Platinum Nanoparticles Supported on Titania during Hydrogenation of Alkenes and Aldehydes.

Ligands like amines are used in the colloidal synthesis approach to protect platinum nanoparticles (Pt NP's) from agglomeration. Normally, ligands like amines are removed by diverse pre-treatment procedures before use in heterogeneous catalysis as amines are considered as a catalyst poison. However, a possible beneficial influence of these surface modifiers on hydrogenation reactions, which is known from spectator species on metal surfaces, is often neglected. Therefore, amine-stabilized Pt nanoparticles supported by titania (P25) were used without any pre-treatment in order to elucidate a possible influence of the ligand in liquid phase hydrogenation reactions. The catalytic activity of amine-stabilized Pt nanoparticles of two different sizes was investigated in a double-walled stirring tank reactor at 69 °C to 130 °C and 1 atm hydrogen pressure. The conversion of cyclohexene to cyclohexane was determined by gas chromatography (GC) and was compared to ligand-free Pt particles. All catalysts were checked before and after reaction by transmission electron spectroscopy (TEM) and X-ray photoelectron spectroscopy (XPS) for possible changes in size, shape, and ligand shell. The hydrogenation of cyclohexene in liquid phase revealed a higher conversion for amine-stabilized Pt nanoparticles on titania than the ligand-free particles. The hydrogenation of 5-methylfurfural (5-MF) was chosen for a further test reaction, since the hydrogenation of α, β-unsaturated aldehydes is more complex and exhibits various reaction paths. However, XPS and infrared spectroscopy (IR) proved that 5-MF acts as catalyst poison at the given reaction conditions.

Hexagonal BaTiO(3-x)Hx Oxyhydride as a Water-Durable Catalyst Support for Chemoselective Hydrogenation.

We present heavily H--doped BaTiO(3-x)Hx (x ≈ 1) as an efficient and water-durable catalyst support for Pd nanoparticles applicable to liquid-phase hydrogenation reactions. The BaTiO(3-x)Hx oxyhydride with a hexagonal crystal structure (P63/mmc) was synthesized by the direct reaction of BaH2 and TiO2 at 800 °C under a stream of hydrogen, and the estimated chemical composition was BaTiO2.01H0.96. Density functional theory calculations and magnetic measurements indicated that such heavy H- doping results in a metallic nature with delocalized electrons and a low work function. The potential of BaTiO(3-x)Hx as a catalyst support was examined for the selective hydrogenation of unsaturated C-C bonds by Pd nanoparticles deposited on BaTiO(3-x)Hx. We found that the turnover frequency for phenylacetylene hydrogenation per total amount of Pd in Pd/BaTiO(3-x)Hx was the highest among the supported Pd catalysts reported to date. The strong electronic charge transfer between Pd and the support, as confirmed by X-ray photoelectron spectroscopy measurements, can be attributed to be responsible for such high catalytic activity. The combination of the BaTiO(3-x)Hx support and Pd nanoparticles provides for the selective hydrogenation of unsaturated C-C bonds and highlights the validity of catalyst design that integrates H- in support materials.

On the role of bismuth as modifier in AuPdBi catalysts: Effects on liquid-phase oxidation and hydrogenation reactions

There is much about the action of bismuth within heterogeneous catalysis that still require a deeper understanding. We observed that, when Bi was added to AuPd bimetallic nanoparticles (NPs) supported on activated carbon, Bi affected the activity and significantly alters the selectivity in two model liquid phase reactions, namely the oxidation of cinnamyl alcohol and the hydrogenation of cinnamaldehyde. A combination of transmission electron microscopy and X-ray absorption spectroscopy provided a detailed characterization of trimetallic AuPdBi systems. We propose that the introduction of bismuth on AuPd NPs results in a partial blockage of most active sites, limiting the occurrence of consecutive reactions.

Hydrogenation of Ethylbenzene Over Ru/γ-Al₂O₃ Catalyst in Trickle-Bed Reactor.

The objective of this study is to evaluate the catalytic performance of pellet-type Ru/γ-Al₂O₃ as a catalyst during liquid-phase hydrogenation of the aromatic hydrocarbon. The Ru/γ-Al₂O₃ catalyst was prepared using a wet impregnation method. After adding a binder to Ru/γ-Al₂O₃, a pellet-type catalyst was obtained through an extrusion method. Nanoporous structures are well developed in the pellet-type Ru/γ-Al₂O₃ catalyst. The average pore sizes of the Ru/γ-Al₂O₃ catalysts were approximately 10 nm. The catalytic performance of the pellet-type Ru/γ-Al₂O₃ catalyst during ethylbenzene hydrogenation was evaluated in a trickle-bed reactor. When the ruthenium loading increased from 1 to 5 wt%, the number of active sites effective for the hydrogenation of ethylbenzene increased proportionally. In order to maximize the conversion of ethylbenzene to ethylcyclohexane, it was necessary to maintain a liquid phase hydrogenation reaction in the trickle bed reactor. In this regards, the reaction temperature should be lower than 90 °C. The conversion of ethylbenzene to ethylcyclohexane on the Ru(5 wt%)/γ-Al₂O₃ catalyst was highest, which is ascribed to the largest number of active sites of the catalyst.

Structure and Energy Profile of the Skeletal Nickel Surface According to the Small-Angle X-Ray Diffraction and Adsorption Calorimetry Data

A systematic study of the surface of Raney nickel-based pyrophoric catalysts by small-angle X-ray diffraction analysis was carried out for the first time. The parameters of the crystallites constituting the active catalyst surface were determined. A correlation between the surface structure and the activity of skeletal nickel for the liquid-phase hydrogenation reactions was demonstrated. The effect of treatment of Ni–Al alloy with sodium hydroxide and hydrogen peroxide on the catalyst particle size and on the course of the hydrogenation reaction was revealed and substantiated. The X-ray diffraction data on the catalyst surface structure and the adsorption calorimetry data were correlated with the skeletal nickel activity parameters.

Exceptionally thermal-stable Al2O3/TiO2 nanofibers by depressing surface-initiated grain growth as new supports for anti-sintering Pt nanoparticles

Electrospun ceramic nanofibers can be ideal supports for catalyst, but commonly restricted by the rapid grain growth during fabrication and thus lose their superior functions. In this work, we reported grain growth suppressed nanofibers made of a blend of TiO2 and Al2O3 with an average crystal size of 22.2 nm and 47.1 wt% of anatase on 900 °C. Al2O3 preferred to locate at the grain boundaries of TiO2 nanograins, significantly hindering the coalescence among the neighboring TiO2 nanograins. More intriguingly, Al2O3 spontaneously migrated to the surface and thus naturally formed a particle-on-fiber morphology. When serving as support, Al2O3/TiO2 nanofibers offered promoted adhesion for Pt nanoparticles, by taking advantages of the well-preserved grain boundaries and anatase phase. Moreover, the dual-oxide construction built kinetic bottleneck to prevent Pt nanoparticles from moving across the support to attach to or merge with each other. Therefore, Pt nanoparticles can be stabilized against sintering up to 500 °C with an ultra-close neighboring distance of 4.56 nm. This sinter-resistant catalyst exhibited high activities toward both liquid-phase hydrogenation and gas-phase oxidation reactions (i.e. soot oxidation) at high temperatures. Such a robust and thermally stable catalyst can be further used for catalytic soot oxidation, giving great prospects for emission control.

Effect of the distribution and dispersion of palladium nanoparticles on the reducibility and performance of Pd/Al2O3 catalyst in liquid-phase hydrogenation of olefins

A series of Pd/γ-Al2O3 catalysts were prepared by wet impregnation method at different pH and time. The effect of metal distribution and dispersion on the catalyst reducibility in liquid-phase hydrogenation of olefins was studied. The samples were characterized by BET, XRD, ICP, FESEM, CO-chemisorption and H2-TPR techniques. Liquid-phase hydrogenation reaction was carried out in a three-phase trickle bed reactor to evaluate the performance of the catalyst samples. The best catalyst performance was attributed to a combination of its desirable characteristics, that is, high reducibility at lower temperatures, high Pd dispersion and an egg-shell Pd distribution which increase the accessibility of the reactants to Pd active sites in this diffusion-limited reaction.

Magnetically induced synthesis of mesoporous amorphous CoB nanochains for efficient selective hydrogenation of cinnamaldehyde to cinnamyl alcohol

Amorphous metal-boron alloys have attracted extensive interest in catalytic reactions because a large number of randomly oriented bonds constitute a glass-like structure with abundant defects and unsaturated surface atoms which are catalytically active. However, the synthesis of non-precious amorphous metal-boron with a multilevel structure remains a challenging task. Here, ultra-long mesoporous cobalt-boron (CoB) amorphous alloy nanochains are firstly synthesized by using the soft-templating approach with the assistance of a magnetic field. Owing to the high surface area and the unique behavior of Co within the Co-B alloy, mesoporous CoB nanochains show superior activity and selectivity in liquid-phase cinnamaldehyde (CMA) hydrogenation in cinnamyl alcohol (CMO). This work opens an avenue for the exploration of mesoporous amorphous metal alloys towards liquid-phase selective hydrogenation reaction.

Engineering of Yolk/Core-Shell Structured Nanoreactors for Thermal Hydrogenations.

Heterogeneous hydrogenation reactions are of great importance for chemical upgrading and synthesis, but still face the challenges of controlling selectivity and long-term stability. To improve the catalytic performance, many hydrogenation reactions utilize special yolk/core-shell nanoreactors (YCSNs) with unique architectures and advantageous properties. This work presents the developmental and technological challenges in the preparation of YCSNs that are potentially useful for hydrogenation reactions, and provides a summary of the properties of these materials. The work also addresses the scientific challenges in applications of these YCSNs in various gas and liquid-phase hydrogenation reactions. The catalyst structures, catalytic performance, structure-performance relationships, reaction mechanisms, and unsolved problems are discussed too. Also, a brief outlook and opportunities for future research in this field are presented. This work on the advancements in YCSNs might inspire the creation of new materials with desired structures for achieving maximal hydrogenation performances.

Tandem Catalysis of Ammonia Borane Dehydrogenation and Phenylacetylene Hydrogenation Catalyzed by CeO2 Nanotube/Pd@MIL-53(Al).

Heterogeneously catalyzed, selective hydrogenation in the liquid phase is widely used in industry for the synthesis of chemicals. However, it can be a challenge to prevent active nanoparticles (e.g., palladium) from aggregation/leaching and meanwhile achieve high conversion as well as selectivity, especially under mild conditions. To address these issues, a CeO2 nanotube/Pd@MIL-53(Al) sandwich-structured catalyst has been prepared in which the MIL-53(Al) porous shell can efficiently stabilize the palladium nanoparticles. When this catalyst was used in a tandem catalytic reaction involving the dehydrogenation of ammonia borane and the hydrogenation of phenylacetylene, remarkably, the hydrogen released from the dehydrogenation of ammonia borane boosted the catalytic process, with 100 % conversion of phenylacetylene and a selectivity of 96.2 % for styrene, even at room temperature and atmospheric pressure, within 1 min. This work therefore provides an alternative strategy for balancing the conversion and selectivity of liquid-phase hydrogenation reactions.

Deactivation of Supported Nickel-Based Hydrogenation Catalysts with Sulfide Ions

Kinetics of the liquid-phase hydrogenation of a multiple carbon bond is studied in an aqueous medium on supported nickel catalysts at different hydrogen pressures in the system under conditions of partly controlled deactivation of the active surface sites with sulfide ions. The pattern of deactivation of the active surface sites of Ni/SiO2 catalysts containing different amounts of the active metal on the surface with sulfide ions in water is determined. The resistance of the studied catalysts to deactivation during the reduction of diethyl maleate (DM) and propen-2-ol-1 is determined experimentally. It is shown that the catalyst is more resistant to deactivation during the hydrogenation of propen-2-ol-1; this finding is attributed to the steric factor. It is found that the hydrogen pressure in the system does not affect the deactivation pattern. Excessive pressure slightly alters the deactivation resistance of the catalyst during the hydrogenation of propen-2-ol-1. It is shown experimentally that the catalytic properties of nickel in liquid-phase hydrogenation reactions can be controlled by introducing small amounts of a catalytic poison into the system at high hydrogen pressures.

Study of the Adsorption Activity of Skeletal Nickel with Respect to Reactive Hydrogen by Simultaneous Thermal Analysis Mass Spectrometry

A method for the determination of the total amounts and physical parameters of adsorbed individual hydrogen species on the surface and in the bulk of metal catalysts of liquid-phase hydrogenation reactions by simultaneous thermal analysis mass spectrometry was proposed. The adsorptions of reactive hydrogen on a skeletal nickel catalyst in an aqueous medium were estimated. The concentrations of metal-hydrogen complexes with different binding energies on the surface of the catalyst in the liquid phase were determined by a direct experimental method.

Phase‐Dependent Stability and Substrate‐Induced Deactivation by Strong Metal‐Support Interaction of Ru/TiO2 Catalysts for the Hydrogenation of Levulinic Acid

AbstractThe choice of support type has a profound influence on catalyst performance in liquid phase hydrogenation reactions, including the catalytic hydrogenation of biomass‐derived levulinic acid (LA) to γ‐valerolactone (GVL). Catalytic performance, including stability, of three Ru/TiO2 catalysts, having a similar mean Ru metal nanoparticle size but supported on three types of TiO2, namely P25, rutile and anatase, is evaluated by multiple reuse under batch reactor conditions. T3he catalysts’ physicochemical properties before and after recycling are characterized by XRD, STEM, TGA and FT‐IR after CO stepwise adsorption. The results show that the deactivation seen for (mixed) anatase‐supported catalysts in dioxane can be attributed to strong metal‐support interaction (SMSI) rather than coke formation or metal sintering, with the rutile‐based catalyst being more resistant against such support reduction. Notably, SMSI formation under the applied, relatively mild conditions only occurs in the presence of organic acids, such as LA or valeric acid.

Efficient and Selective Ni/Al2O3–C Catalyst Derived from Metal–Organic Frameworks for the Hydrogenation of Furfural to Furfuryl Alcohol

Currently, precious metal catalysts or toxic copper-chromium catalysts are widely used in the catalytic hydrogenation of furfural. In this study, the selective hydrogenation of furfural to furfuryl alcohol was performed over alumina-carbon matrix embedded Ni nanoparticles catalyst. The Ni-loaded metal organic framework precursors were synthesized by reverse microemulsion method (water in oil), then the Ni/Al2O3–C catalysts were obtained by pyrolysis under a nitrogen atmosphere. Various characterizations including XRD, XPS, SEM, TEM, TPR, TPD and TG were carried out to reveal the microstructure and chemical composition of the prepared samples. The results showed that the hexagonal platelet structure and the smaller nickel nanoparticle size(~ 8 nm) benefits from the alumina–carbon matrix support, which helped the uniform dispersion and prevented it from accumulation at high temperatures. In addition, the catalyst performance was greatly affected by the calcination conditions and metal loading. At optimized operating conditions, 98.7% conversion of furfural and 92% yield of furfuryl alcohol could be achieved over Ni0.15/Al2O3–C catalyst in the liquid phase hydrogenation reaction with a good reusability.