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

Abstract

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.

DMAP Immobilized on Porous Silica Particles and Monoliths for the Esterification of Phenylethanol in Continuous Flow

“…PEEK was fit tightly onto the rods by inserting it into a PTFE shrinking tube and heating it just below the melting point of PEEK. The softened PEEK envelope is pressed against the silica due to the shrinking tube and a tight connection between cladding and material is created [35,36] …”

“…The softened PEEK envelope is pressed against the silica due to the shrinking tube and a tight connection between cladding and material is created. [35,36] A copper-catalyzed cycloaddition was used for immobilization of a DMAP derivative on silica (Scheme 2). First, (3azidopropyl)trimethoxysilane (Az-PTMS) was grafted onto the material (Scheme 2 a).…”

“…We used a process adapted from Chamieh et al in which a PEEK tube is used as cladding. [35,36] By further optimizing this technique, we achieved control over all steps, encompassing sol-gel synthesis of the monoliths, functionalization, and finally generation of cladded, functionalized monoliths. Since we can control and analyze the properties of the reactor material from cradle to grave, we were thus able to modify the reactors along our needs with respect to the porosity and catalyst loading.…”

“…We used a process adapted from Chamieh et al. in which a PEEK tube is used as cladding [35,36] . By further optimizing this technique, we achieved control over all steps, encompassing sol‐gel synthesis of the monoliths, functionalization, and finally generation of cladded, functionalized monoliths.…”

Mass Transfer in Hierarchical Silica Monoliths Loaded With Pt in the Continuous-Flow Liquid-Phase Hydrogenation of p-Nitrophenol

“…Owing to the extensively studied surface chemistry and versatility in synthesis steps, HSM supports can be functionalized via in situ as well as post-synthesis treatment with a variety of moieties and applied in numerous catalytic applications. Recent studies show successful implementation of silica monolithic catalysts as flow reactors in various chemical and biochemical transformations (El Kadib et al, 2009;Sachse et al, 2010;Linares et al, 2012;Sachse et al, 2012;Sachse et al, 2013;Szymańska et al, 2013;Hakat et al, 2016;Ciemięga et al, 2017;Haas et al, 2017;Liguori et al, 2017;Pélisson et al, 2017;Mrowiec-Białoń et al, 2018;Russell et al, 2020;Yamada et al, 2020;Ahmad et al, 2021;Turke et al, 2021).…”