Mesoporous Silicas as Versatile Supports to Tune the Palladium-Catalyzed Selective Aerobic Oxidation of Allylic Alcohols

Palladium-doped hierarchical ZSM-5 for catalytic selective oxidation of allylic and benzylic alcohols

Hierarchical zeolites have the potential to provide a breakthrough in transport limitation, which hinders pristine microporous zeolites and thus may broaden their range of applications. We have explored the use of Pd-doped hierarchical ZSM-5 zeolites for aerobic selective oxidation (selox) of cinnamyl alcohol and benzyl alcohol to their corresponding aldehydes. Hierarchical ZSM-5 with differing acidity (H-form and Na-form) were employed and compared with two microporous ZSM-5 equivalents. Characterization of the four catalysts by X-ray diffraction, nitrogen porosimetry, NH₃ temperature-programmed desorption, CO chemisorption, high-resolution scanning transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy allowed investigation of their porosity, acidity, as well as Pd active sites. The incorporation of complementary mesoporosity, within the hierarchical zeolites, enhances both active site dispersion and PdO active site generation. Likewise, alcohol conversion was also improved with the presence of secondary mesoporosity, while strong Brønsted acidity, present solely within the H-form systems, negatively impacted overall selectivity through undesirable self-etherification. Therefore, tuning support porosity and acidity alongside active site dispersion is paramount for optimal aldehyde production.

Ethanol Steam Reforming for Hydrogen Production Over Hierarchical Macroporous Mesoporous SBA-15 Supported Nickel Nanoparticles

The influence of complementary macropores, present in hierarchical macroporous mesoporous SBA-15, on the performance of supported Ni nanoparticles for ethanol steam reforming has been investigated. The increased open nature of the architecture, afforded through the incorporation of the secondary macropore network, enables superior metal dispersion. This, in turn, enhances catalytic hydrogen production performance through the generation of a greater density of active sites.

Aerobic Oxidation of Alcohols Catalyzed by in Situ Generated Gold Nanoparticles inside the Channels of Periodic Mesoporous Organosilica with Ionic Liquid Framework

In situ generated gold nanoparticles inside the nanospaces of periodic mesoporous organosilica with an imidazolium framework (Au@PMO-IL) were found to be highly active, selective, and reusable catalysts for the aerobic oxidation of activated and nonactivated alcohols under mild reaction conditions. The catalyst was characterized by nitrogen adsorption-desorption measurement, thermogravimetric analysis (TGA), transmission electron microscopy (TEM), elemental analysis (EA), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The catalyst exhibited excellent catalytic activity in the presence of either Cs₂CO₃ (35 °C) or K₂CO₃ (60 °C) as reaction bases in toluene as a reaction solvent. Under both reaction conditions, various types of alcohols (up to 35 examples) including activated benzylic, primary and secondary aliphatic, heterocyclic, and challenging cyclic aliphatic alcohols converted to the expected carbonyl compounds in good to excellent yields and selectivity. The catalyst was also recovered and reused for at least seven reaction cycles. Data from three independent leaching tests indicated that amounts of leached gold particles were negligible (<0.2 ppm). It is believed that the combination of bridged imidazolium groups and confined nanospaces of PMO-IL might be a major reason explaining the remarkable stabilization and homogeneous distribution of in situ generated gold nanoparticles, thus resulting in the highly active and recyclable catalyst system.

Palladium Intercalated into the Walls of Mesoporous Silica as Robust and Regenerable Catalysts for Hydrodeoxygenation of Phenolic Compounds

Nanostructured noble-metal catalysts traditionally suffer from sintering under high operating temperatures, leading to durability issues and process limitations. The encapsulation of nanostructured catalysts to prevent loss of activity through thermal sintering, while maintaining accessibility of active sites, remains a great challenge in the catalysis community. Here, we report a robust and regenerable palladium-based catalyst, wherein palladium particles are intercalated into the three-dimensional framework of SBA-15-type mesoporous silica. The encapsulated Pd active sites remain catalytically active as demonstrated in high-temperature/pressure phenol hydrodeoxygenation reactions. The confinement of Pd particles in the walls of SBA-15 prevents particle sintering at high temperatures. Moreover, a partially deactivated catalyst containing intercalated particles is regenerated almost completely even after several reaction cycles. In contrast, Pd particles, which are not encapsulated within the SBA-15 framework, sinter and do not recover prior activity after a regeneration procedure.

Ordered meso- and macroporous perovskite oxide catalysts for emerging applications

Hierarchically ordered perovskite materials which have potential applications in chemistry, energy and materials science.

Tunable Ag@SiO2 core–shell nanocomposites for broad spectrum antibacterial applications

Silica encapsulated silver nanoparticle core–shell nanocomposites of tunable dimensions were synthesised via a one-pot reverse microemulsion route to achieve controlled release of Ag⁺ ions for broad spectrum antibacterial application.

Synthesis and characterization of bifunctional lipophilic and basic mesoporous organosilica supported palladium nanoparticles as an efficient and ecofriendly nanocomposite in aqueous Heck reaction

In the present study, a highly ordered mesoporous organosilica nanocomposite having modified pore channels with both lipophilic and basic units, SBA-R/Im-NH₂, was synthesized through surfactant-templated sol–gel methodology and post modification process.

Novel access to carbonyl and acetylated compounds: the role of the tetra-n-butylammonium bromide/sodium nitrite catalyst

A novel aerobic oxidation of alcohols without the use of any oxidants was developed.

Rice husk derived nanosilica supported Cu(ii) complex: an efficient heterogeneous catalyst for oxidation of alcohols using TBHP

We have developed a heterogenized catalyst of Cu(ii) complex immobilized onto nanosilica obtained from rice husk (RH), through Schiff-base condensation between 3-aminopropyltriethoxysilane (APTES) functionalized nanosilica and acetamide.

Selective Palladium-Catalysed Aerobic Oxidation of Alcohols

Palladium has a significant track record as a catalyst for a range of oxidation reactions and it has been explored for the selective oxidation of alcohols for many years. This chapter focuses on the two main types of aerobic Pd catalysts: heterogeneous and ligand-modulated systems. In the case of heterogeneous systems, the mechanistic understanding of these systems and the use of in situ and operando techniques to obtain this knowledge are discussed. The current state-of-the-art is also summarized in terms of catalytic performance and substrate scope for heterogeneous Pd-based catalysts. In terms of ligand-modulated systems, leading examples of molecular Pd(ii) catalysts which undergo direct O2 coupled turnover are highlighted. The catalyst performance for such catalysts is exemplified and mechanistic understanding for these molecular systems is discussed.

Dual application of Pd nanoparticles supported on mesoporous silica SBA-15 and MSU-2: supported catalysts for C–C coupling reactions and cytotoxic agents against human cancer cell lines

Pd nanoparticles have been supported onto SBA-15 and MSU-2 and present interesting catalytic and cytotoxic properties.

Heterogeneous Pd catalysts supported on silica matrices

Features, advantages and limitations associated with palladium catalysts deposited over various siliceous supports in different types of practically useful organic transformations are reviewed.

Application of the polyacrylonitrile fiber as a support for the green heterogeneous base catalyst and supported phase-transfer catalyst

Highly efficient and recyclable polyethylene polyamine functionalized polyacrylonitrile fiber as the heterogeneous base catalyst and supported phase-transfer catalyst.

Tunable Pt nanocatalysts for the aerobic selox of cinnamyl alcohol

The selective aerobic oxidation of cinnamyl alcohol over Pt nanoparticles has been tuned via the use of mesoporous silica supports to control their dispersion and oxidation state. High area two-dimensional SBA-15, and three-dimensional, interconnected KIT-6 silica significantly enhance Pt dispersion, and thus surface PtO2 concentration, over that achievable via commercial low surface area silica. Selective oxidation activity scales with Pt dispersion in the order KIT-6 ≥ SBA-15 > SiO2, evidencing surface PtO2 as the active site for cinnamyl alcohol selox to cinnamaldehyde. Kinetic mapping has quantified key reaction pathways, and the importance of high O2 partial pressures for cinnamaldehyde production.