Covalently and non-covalently immobilized clusters onto nanocarbons as catalysts precursors for cinnamaldehyde selective hydrogenation

Continuous-Flow Catalysis Using Phosphine-Metal Complexes on Porous Polymers: Designing Ligands, Pores, and Reactors

Continuous-flow syntheses using immobilized catalysts can offer efficient chemical processes with easy separation and purification. Porous polymers have gained significant interests for their applications to catalytic systems in the field of organic chemistry. The porous polymers are recognized for their large surface area, high chemical stability, facile modulation of surface chemistry, and cost-effectiveness. It is crucial to immobilize transition-metal catalysts due to their difficult separation and high toxicity. Supported phosphine ligands represent a noteworthy system for the effective immobilization of metal catalysts and modulation of catalytic properties. Researchers have been actively pursuing strategies involving phosphine-metal complexes supported on porous polymers, aiming for high activities, durabilities, selectivities, and applicability to continuous-flow systems. This review provides a concise overview of phosphine-metal complexes supported on porous polymers for continuous-flow catalytic reactions. Polymer catalysts are categorized based on pore sizes, including micro-, meso-, and macroporous polymers. The characteristics of these porous polymers are explored concerning their efficiency in immobilized catalysis and continuous-flow systems.

Direct Heterogenization of the Ru-Macho Catalyst for the Chemoselective Hydrogenation of α,β-Unsaturated Carbonyl Compounds

In this study, a commercially available homogeneous pincer-type complex, Ru-Macho, was directly heterogenized via the Lewis acid-catalyzed Friedel-Crafts reaction using dichloromethane as the cross-linker to obtain a heterogeneous, pincer-type Ru porous organometallic polymer (Ru-Macho-POMP) with a high surface area. Notably, Ru-Macho-POMP was demonstrated to be an efficient heterogeneous catalyst for the chemoselective hydrogenation of α,β-unsaturated carbonyl compounds to their corresponding allylic alcohols using cinnamaldehyde as a model compound. The Ru-Macho-POMP catalyst showed a high turnover frequency (TOF = 920 h^-1) and a high turnover number (TON = 2750), with high chemoselectivity (99%) and recyclability during the selective hydrogenation of α,β-unsaturated carbonyl compounds.

Towards Atomically Precise Supported Catalysts from Monolayer-Protected Clusters: The Critical Role of the Support

Controlling the size and uniformity of metal clusters with atomic precision is essential for fine-tuning their catalytic properties, however for clusters deposited on supports, such control is challenging. Here, by combining X-ray absorption spectroscopy and density functional theory calculations, it is shown that supports play a crucial role in the evolution of monolayer-protected clusters into catalysts. Based on the acidic nature of the support, cluster-support interactions lead either to fragmentation of the cluster into isolated Au-ligand species or ligand-free metallic Au⁰ clusters. On Lewis acidic supports that bind metals strongly, the latter transformation occurs while preserving the original size of the metal cluster, as demonstrated for various Au_n sizes. These findings underline the role of the support in the design of supported catalysts and represent an important step toward the synthesis of atomically precise supported nanomaterials with tailored physico-chemical properties.

Magnetic Pt single and double core-shell structures for the catalytic selective hydrogenation of cinnmaladehyde

This study reports the catalytic preparation, characterization, and evaluation of nanoscale core-shell structures with a γ-Fe2O3 core covered by a SiO2 monoshell or by a SiO2@TiO2 multishell as a support for Pt nanoparticles (NPs) to synthesize active and operationally stable catalysts for selective liquid-phase cinnamaldehyde hydrogenation. The structures were designed with a magnetic core so they could be easily recovered from the catalytic bed by simple magnetization and with a SiO2 monoshell or a SiO2@TiO2 multishell to protect the magnetic core. At the same time, this study details the effect of the shell on the catalytic performance. Moreover, the effect of particle size on the selective production of cinnamyl alcohol was studied by preparing two families of catalysts with metal loadings of 1 wt% and 5 wt% Pt with respect to the core-shell. The particle size effect enabled the Fe2O3@SiO2-5%Pt system, with an average particle size of 5.6 nm, to reach 100 % conversion of cinnamaldehyde at 300 min of reaction, producing cinnamyl alcohol with 90 % selectivity; this result differed greatly from that of the Fe2O3@SiO2-1%Pt (dPt = 3.5 nm) system, which reached a maximum conversion at 600 min with 49 % selectivity for the product of interest. However, the Fe2O3@SiO2@TiO2-x%Pt systems showed lower levels of conversion and selectivity compared to those of the Fe2O3@SiO2-x%Pt catalysts, which is attributed to the fact that average metal particle sizes below 5.0 nm were obtained in both cases. After reduction in H2 at 773 K, the Fe2O3@SiO2@TiO2-1%Pt catalyst showed deactivation, reaching 10 % conversion at 600 min of reaction and 60 % selectivity for the product of interest. However, the reduced Fe2O3@SiO2@TiO2-5%Pt system showed 98 % conversion with 95 % selectivity for cinnamyl alcohol at 24 h of operation; the increase in selectivity is attributed to the combined effects of the increase in average particle size (~7.5 nm) and the presence of strong metal-support interaction – SMSI – effects after reduction. Finally, the most selective systems were tested for operational stability, where the Fe2O3@SiO2@-5%Pt catalyst could be reused in three consecutive operating cycles while maintaining its activity and selectivity for cinnamyl alcohol – unlike the Fe2O3@SiO2@TiO2-5%Pt reduced system, which was deactivated after the third reaction cycle due to active phase leaching.

Magnetic Fe₂O₃⁻SiO₂⁻MeO₂⁻Pt (Me = Ti, Sn, Ce) as Catalysts for the Selective Hydrogenation of Cinnamaldehyde. Effect of the Nature of the Metal Oxide

The type of metal oxide affects the activity and selectivity of Fe₂O₃–SiO₂–MeO₂–Pt (Me = Ti, Sn, Ce) catalysts on the hydrogenation of cinnamaldehyde. The double shell structure design is thought to protect the magnetic Fe₂O₃ cores, and also act as a platform for depositing a second shell of TiO₂, SnO₂ or CeO₂ metal oxide. To obtain a homogeneous metallic dispersion, the incorporation of 5 wt % of Pt was carried out over Fe₂O₃–SiO₂–MeO₂ (Me = Ti, Sn, Ce) structures modified with (3-aminopropyl)triethoxysilane by successive impregnation-reduction cycles. The full characterization by HR-TEM, STEM-EDX, XRD, N₂ adsorption isotherm at −196 °C, TPR-H₂ and VSM of the catalysts indicates that homogeneous core-shell structures with controlled nano-sized magnetic cores, multi-shells and metallic Pt were obtained. The nature of the metal oxide affects the Pt nanoparticle sizes where the mean Pt diameter is in the order: –TiO₂–Pt > –SnO₂–Pt > –CeO₂–Pt. Among the catalysts studied, –CeO₂–Pt had the best catalytic performance, reaching the maximum of conversion at 240 min. of reaction without producing hydrocinnamaldehyde (HCAL). It also showed a plot volcano type for the production of cinnamic alcohol (COL), with 3-phenyl-1-propanol (HCOL) as a main product. The –SnO₂–Pt catalyst showed a poor catalytic performance attributable to the Pt clusters’ occlusion in the irregular surface of the –SnO₂. Finally, the –TiO₂–Pt catalyst showed a continuous production of COL with a 100% conversion and 65% selectivity at 600 min of reaction.

Size Regulation and Stability Enhancement of Pt Nanoparticle Catalyst via Polypyrrole Functionalization of Carbon-Nanotube-Supported Pt Tetranuclear Complex

A novel multiwall carbon nanotube (MWCNT) and polypyrrole (PPy) composite was found to be useful for preparing durable Pt nanoparticle catalysts of highly regulated sizes. A new pyrene-functionalized Pt₄ complex was attached to the MWCNT surface which was functionalized with PPy matrix to yield Pt₄ complex/PPy/MWCNT composites without decomposition of the Pt₄ complex units. The attached Pt₄ complexes in the composite were transformed into Pt⁰ nanoparticles with sizes of 1.0-1.3 nm at a Pt loading range of 2 to 4 wt %. The Pt nanoparticles in the composites were found to be active and durable catalysts for the N-alkylation of aniline with benzyl alcohol. In particular, the Pt nanoparticles with PPy matrix exhibited high catalyst durability in up to four repetitions of the catalyst recycling experiment compared with nonsize-regulated Pt nanoparticles prepared without PPy matrix. These results demonstrate that the PPy matrix act to regulate the size of Pt nanoparticles, and the PPy matrix also offers stability for repeated usage for Pt nanoparticle catalysis.

Efficient Pt–FeOx/TiO2@SBA-15 catalysts for selective hydrogenation of cinnamaldehyde to cinnamyl alcohol

Pt–FeO_x/TiO₂@SBA-15 serves as an efficient and recyclable catalyst for liquid-phase selective hydrogenation of cinnamaldehyde to cinnamyl alcohol under mild conditions.

Effect of Carbon Supported Pt Catalysts on Selective Hydrogenation of Cinnamaldehyde

Selective hydrogenation of cinnamaldehyde (CAL) to cinnamyl alcohol (COL) is of both fundamental and industrial interest. It is of great significance to evaluate the possible differences between different supports arising from metal dispersion and electronic effects, in terms of activity and selectivity. Herein, Pt catalysts on different carbon supports including carbon nanotubes (CNTs) and reduced graphene oxides (RGO) were developed by a simple wet impregnation method. The resultant catalysts were well characterized by XRD, Raman, N2physisorption, TEM, and XPS analysis. Applied in the hydrogenation of cinnamaldehyde, 3.5 wt% Pt/CNT shows much higher selectivity towards cinnamyl alcohol (62%) than 3.5 wt% Pt/RGO@SiO2(48%). The enhanced activity can be ascribed to the high graphitization degree of CNTs and high density of dispersed Pt electron cloud.

pH controlled adsorption of water-soluble ruthenium clusters onto carbon nanotubes and nanofiber surfaces

Nanocarbon supported catalysts were prepared from water-soluble molecular clusters by pH controlled impregnations in order to probe the clusters/surface interactions and to maximize them.