Optimizing acid-base bifunctional mesoporous catalysts for the henry reaction: effects of the surface density and site isolation of functional groups

Schiff base modified starch: A promising biosupport for palladium in Suzuki cross-coupling reactions

Starch can be used in diverse fields because it is a readily available, non-toxic polysaccharide with adaptable functionality and biodegradability. In this study, taking the aforementioned characteristics into consideration, we designed a modified starch (Starch-SB), which serves as supporting material for palladium stabilization. This new air and moisture-stable robust palladium composite [Starch-SB-Pd(II)] was characterized by FT-IR, XRD, TGA, XPS, SEM, EDX, TEM, CP/MAS ¹³C NMR, and ICP-MS analytical techniques. The catalytic studies exhibit high activity (up to 99 %) and stability in Suzuki cross-coupling reactions for this starch supported catalytic system under mild conditions (lower reaction temperature and green solvents) because of the cooperative interactions of multifunctional capturing sites on starch (Schiff base, hydroxy and amine groups) with palladium species. The experiments on reusability demonstrate that Starch-SB-Pd(II), which was prepared from functionalized starch, could be readily recycled several cycles through centrifugation. Moreover, we proposed a possibly multifunctional complex structure. This work presents an appealing and intriguing pathway for the utilization of polysaccharide as crucial support in green chemical transformations.

Cooperativity in the Aldol Condensation Using Bifunctional Mesoporous Silica-Poly(styrene) MCM-41 Organic/Inorganic Hybrid Catalysts

This work explores the efficacy of silica/organic hybrid catalysts, where the organic component is built from linear aminopolymers appended to the silica support within the support mesopores. Specifically, the role of molecular weight and polymer chain composition in amine-bearing atom transfer radical polymerization-synthesized poly(styrene-co-2-(4-vinylbenzyl)isoindoline-1,3-dione) copolymers is probed in the aldol condensation of 4-nitrobenzaldehyde and acetone. Controlled polymerization produces protected amine-containing poly(styrene) chains of controlled molecular weight and dispersity, and a grafting-to thiol-ene coupling approach followed by a phthalimide deprotection step are used to covalently tether and activate the polymer hybrid catalysts prior to the catalytic reactions. Site-normalized batch kinetics are used to assess the role of polymer molecular weight and chain composition in the cooperative catalysis. Lower-molecular-weight copolymers are demonstrated to be more active than catalysts built from only molecular organic components or from higher-molecular-weight chains. Molecular dynamics simulations are used to probe the role of polymer flexibility and morphology, whereby it is determined that higher-molecular-weight hybrid structures result in congested pores that inhibit active site cooperativity and the diffusivity of reagents, thus resulting in lower rates during the reaction.

Amine-Functionalized Nanoporous Silica Monoliths for Heterogeneous Catalysis of the Knoevenagel Condensation in Flow

Porous carrier materials functionalized with organocatalysts offer substantial advantages compared to homogeneous catalysts, e.g., easy separation of the catalyst, scalability, and an improved implementation in continuous operations. Here, we report the immobilization of (3-aminopropyl)trimethoxysilane (APTMS) onto self-prepared silica monoliths and its application as a heterogeneous catalyst in the Knoevenagel condensation between cyano ethylacetate and various aromatic aldehydes under continuous-flow conditions. The meso-macroporous silica monoliths (6-7 cm in length) were optimized to be used in flow taking advantage of their hierarchical meso- and macroporosity. The monoliths were cladded with a poly(ether ether ketone) (PEEK) tube by a refined procedure to guarantee tight connection between the carrier material and PEEK. Functionalization of the bare silica monoliths consisting of APTMS can be efficiently performed in flow in ethanol and toluene. While a large grafting gradient is obtained for toluene, the grafting in ethanol proceeds homogenously throughout the monolith, as evidenced by elemental analysis and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The silica monoliths exhibit high conversion up to 95% with concurrent low back pressures, which is of importance in flow catalysis. By connecting two monoliths, high conversions can be maintained for several flow rates. Two types of monoliths were synthesized, possessing different mesopore sizes. The monolith bearing the larger mesopore size showed an enhanced turnover frequency (TOF), while the monolith with the smaller mesopores allowed for larger quantities of the product to be synthesized, due to the higher surface area. A long-term stability test showed that the functionalized monoliths were still active after 66 h of continuous usage, while the overall yield decreased over time.

Cooperative acid–base bifunctional ordered porous solids in sequential multi-step reactions: MOF vs. mesoporous silica

Two different catalytic platforms, MOF and mesoporous silica, were compared as porous support for basic amino groups to promote sequential multi-step reactions.

Inter- and Intramolecular Cooperativity Effects in Alkanolamine-Based Acid-Base Heterogeneous Organocatalysts

Intramolecular cooperativity in heterogeneous organocatalysts is investigated using alkanolamine-functionalized silica acid-base catalysts for the aldol condensation reaction of 4-nitrobenzaldehyde and acetone. Two series of catalysts, one with and one without silanol-capping, are synthesized with varied alkyl linker lengths (two to five) connecting secondary amine and terminal hydroxyl functionalities. The reactivity of these catalysts is assessed to determine the relative potential for intermolecular (silane amine-surface silanol) vs intramolecular (amine-hydroxyl within a single silane) cooperativity, the impact of inhibitory surface-silane interactions, and the role of alkyl linker length and flexibility. For the array of catalysts tested, those with longer linker lengths generally give increased catalytic activity, although the turnover frequency trends differ between catalysts with and without surface silanol capping. Catalysts with alkyl-substituted amines lacking a terminal hydroxyl demonstrate an adverse effect of chain length, where the larger alkyl substituent on the amine provides steric hindrance depressing catalytic activity, while giving additional evidence for improved rates afforded by intramolecular cooperativity in the alkanolamine materials. The silanol-capped alkanolamine catalyst with the longest alkyl linker is found to be the most active alkanolamine catalyst due to its hydrophobized surface, which removes hypothesized silanol-alkanolamine inhibitory interactions, with the sufficient length and flexibility of its amine-hydroxyl linker allowing for favorable conformations for cooperativity. This study demonstrates the feasibility of and important factors affecting intramolecular cooperative activity in acid-base heterogeneous organocatalysis.

Amine/Hydrido Bifunctional Nanoporous Silica with Small Metal Nanoparticles Made Onsite: Efficient Dehydrogenation Catalyst

Multifunctional catalysts are of great interest in catalysis because their multiple types of catalytic or functional groups can cooperatively promote catalytic transformations better than their constituents do individually. Herein we report a new synthetic route involving the surface functionalization of nanoporous silica with a rationally designed and synthesized dihydrosilane (3-aminopropylmethylsilane) that leads to the introduction of catalytically active grafted organoamine as well as single metal atoms and ultrasmall Pd or Ag-doped Pd nanoparticles via on-site reduction of metal ions. The resulting nanomaterials serve as highly effective bifunctional dehydrogenative catalysts for generation of H₂ from formic acid.

A bio-inspired zinc finger analogue anchored in 2D hexagonal mesoporous silica for room temperature CO2activation via a hydrogenocarbonate route

A room temperature hydrogenocarbonate intermediate in CO₂activation by a carbonic anhydrase active site analog inserted in the nanopores of mesostructured porous silicas.

Nickel containing ionic liquid based ordered nanoporous organosilica: a powerful and recoverable catalyst for synthesis of polyhydroquinolines

The synthesis, characterization and catalytic application of a novel nickel containing ionic liquid based ordered mesoporous organosilica are demonstrated.

One “Click” to controlled bifunctional supported catalysts for the Cu/TEMPO-catalyzed aerobic oxidation of alcohols

A modular strategy is described for the preparation and molecular engineering of multifunctional surfaces using CuAAC chemistry and is applied to the model Cu/TEMPO-catalyzed aerobic oxidation of benzyl alcohol.

Amine-functionalized ionic liquid-based mesoporous organosilica as a highly efficient nanocatalyst for the Knoevenagel condensation

A novel aminopropyl-containing ionic liquid-based mesoporous organosilica was prepared and characterized and its catalytic performance was studied in the Knoevenagel condensation.

Surface molecular engineering in the confined space of templated porous silica

Recent developments in molecular surface engineering inside the confined space of porous materials are surveyed including a new nomenclature proposal.

Post-grafting amination of alkyl halide-functionalized silica for applications in catalysis, adsorption, and 15N NMR spectroscopy

An anhydrous synthesis of aminosilica materials from alkyl halide-functionalized mesoporous SBA-15 silica by post-grafting amination is introduced for applications in CO2 adsorption, cooperative catalysis, and (15)N solid-state NMR spectroscopy. The synthesis is demonstrated to convert terminal alkyl halide-functionalized silica materials containing Cl, Br, and I to primary alkylamines using anhydrous ammonia in a high-pressure reactor. The benefits of the post-grafting amination procedure include (i) use of anhydrous isotopically labeled ammonia, (15)NH3, to create aminosilica materials that can be investigated using (15)N solid-state NMR to elucidate potential intermediates and surface species in CO2 adsorption processes and catalysis, (ii) similar CO2 uptake in experiments extracting CO2 from dry simulated air experiments, and (iii) improved activity in acid-base bifunctional catalysis compared to traditional amine-grafted materials. The effects of the type of halide, the initial halide loading, and the total reaction time on the conversion of the halides to primary amines are explored. Physical and chemical characterizations of the materials show that the textural properties of the silica are unaffected by the reaction conditions and that quantitative conversion to primary amines is achieved even at short reaction times and high initial alkyl halide loadings. Additionally, preliminary (15)N solid-state NMR experiments indicate formation of nitrogen-containing species and demonstrate that the synthesis can be used to create materials useful for investigating surface species by NMR spectroscopy. The differences between the materials prepared via post-grafting amination vs traditional aminosilane grafting are attributed to the slightly increased spacing of the amines synthesized by amination because the alkylhalosilanes are initially better spaced on the silica surface after grafting, whereas the aminosilanes likely cluster to a greater extent when grafted on the silica surface. A slight increase in amine spacing allows for more effective amine-silanol interactions in cooperative catalysis without reducing the amine efficiency in CO2 uptake under the conditions used here.

Site isolation and coordination control of a transition metal ion by molecular surface engineering in mesoporous silica: the case of a bio-inspired copper–polyamine grafted complex

Design of a bio-inspired material, where a competition between copper ions and their triflate counterions to the grafted polyamine is evidenced.

Tunable synthesis of hierarchical mesoporous silica nanoparticles with radial wrinkle structure

We studied the formation mechanism of hierarchical mesoporous silica nanoparticles with a wrinkle structure (wrinkled silica nanoparticles, WSNs), and a method for substructure control of silica nanoparticles was proposed. We confirmed that WSNs were generated in the bicontinuous microemulsion phase of the Winsor III system. By using the phase behavior of the Winsor III system, which depends on the water-surfactant-oil mixing ratio, and by adding various cosolvents, we could precisely control the structure of silica nanoparticles from the mesoporous to the wrinkle form; furthermore, we could control the interwrinkle distance.