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

Universal, Time-Cost-Effective, and Inside-Out Strategy for Synthesis of Plant-Based Flow Catalysis Microreactors

The utilization of plant-based flow catalytic microreactors has been increasingly gaining traction in the fields of water treatment, energy generation, and biotechnological science due to their inherent channel structures, renewable properties, and environmentally friendly nature. The conventional outside-in strategy for synthesizing plant-based monolithic microreactors typically entails prolonged hydrothermal modification, extensive chemical usage, or energy-intensive equipment. The present study presents a universal inside-out strategy for the rapid synthesis of monolithic catalytic microreactors derived from plant materials. This approach enables the direct formation of catalytic metal nanoparticles within specific plant microchannels through regioselective deposition, resulting in reduced chemical usage and an accelerated process. Moreover, this method effectively minimizes the required catalyst dosage. In this process, the plant monolith's aligned, narrow, and accessible channels provided a higher contact area, shorter diffusion path, and abundant oxygen-containing functional groups for rapid transformation of metal salt precursors into catalytic metal nanoparticles with excellent dispersion. The inside-out strategy can be extended to various plant-based monoliths and diverse metal/metal oxide/MOF materials within the plant monolith, thereby offering a facile, time- and cost-effective universal approach for skillfully designing plant-based flow microreactors for a wide range of applications.

Time-of-Flight Secondary Ion Mass Spectrometry Revealing the Organocatalyst Distribution in Functionalized Silica Monoliths

Hierarchically porous monolithic silica shows promise as a carrier material for immobilized organocatalysts. Conventional analysis usually includes physisorption, infrared spectroscopy and elemental analysis, among others, to elucidate the pore space and degree of functionalization of the material. However, these methods do not yield information about the spatial distribution of the organic species inside the monolithic reactor. In this work, time-of-flight secondary ion mass spectrometry has been applied to characterize the surface of organically functionalized silica monoliths. Cross sections of a silica monolith functionalized with 4-dimethylaminopyridine were analyzed and the results were compared with physisorption and elemental analysis experiments of the same material. This way, insight into the radial distribution of the catalyst could be achieved, which might assist in interpreting the performance of such reactors in heterogeneous flow catalysis.

A Continuous-Flow Microreactor for Knoevenagel Condensation of Ethyl Cyanoacetate with Benzaldehyde: The Effect of Grafted Amino Groups on Catalytic Activity

Ethyl α-cyanocinnamate was synthesized in the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate in flow monolithic microreactor of 0.63 cm³ volume. The catalytically active core was made of silica monolith modified with various amine group precursors. Structural properties of the support, surface density of NH_x groups, and catalytic activity were investigated. It was found that the poly- or di-amine groups attached to the silica surface appeared to be more effective than the aminopropyl groups. Microreactors grafted with diamine functional groups, accompanied by hydrophobic methyl groups, showed the highest activity and stability. It was proved that the decisive role on the activity of catalysts was exerted by the presence of primary amines in diamine chain. The reaction conditions were optimized and it was found that almost full substrate conversion could be achieved in 6 min at 50 °C in the microreactor with low concentration of diamine groups equal to 0.33 mmol g^-1 .

Selective Pd recovery from acidic leachates by 3-mercaptopropylphosphonic acid grafted TiO2: does surface coverage correlate to performance?

Modification of metal oxides with organophosphonic acids (PAs) provides the ability to control and tailor the surface properties. The metal oxide phosphonic acid bond (M-O-P) is known to be stable under harsh conditions, making PAs a promising candidate for the recovery of metals from complex acidic leachates. The thiol functional group is an excellent regenerable scavenging group for these applications. However, the research on organophosphonic acid grafting with thiol groups is very limited. In this study, four different metal sorbent materials were designed with different thiol surface coverages. An aqueous-based grafting of 3-mercaptopropylphosphonic acid (3MPPA) on mesoporous TiO₂ was employed. Surface grafted thiol groups could be obtained in the range from 0.9 to 1.9 groups per nm². The different obtained surface properties were studied and correlated to the Pd adsorption performance. High Pd/S adsorption efficiencies were achieved, indicating the presence of readily available sorption sites. A large difference in their selectivity towards Pd removal from a spend automotive catalyst leachate was observed due to the co-adsorption of Fe on the titania support. The highest surface coverage showed the highest selectivity (K _d: 530 mL g^-1) and adsorption capacity (Q _max: 0.32 mmol g^-1) towards Pd, while strongly reducing the co-adsorption of Fe on remaining TiO₂ sites.

Silicone Nanofilament Coatings as Flexible Catalyst Supports for a Knoevenagel Condensation Reaction in Batch and Flow Systems

In this work, silicone nanofilament (SNF) coatings were prepared via a droplet-assisted growth and shaping (DAGS) approach, where the preparation of the coatings is allowed under ambient conditions. The application of SNF coatings as catalyst supports for amino moieties from (3-aminopropyl)triethoxysilane (APTES) was investigated. With the optimized coating conditions identified, the Brunauer-Emmett-Teller surface areas of a bare glass filter substrate and bare glass beads after the coating have increased by 5-fold and 16-fold, respectively. The SNF-coated filters were readily functionalized with amino groups via a liquid-phase deposition process, and their catalytic activities for a Knoevenagel reaction were evaluated using a batch reactor and a packed bed reactor. In both reactors, the as-prepared filters demonstrated superior catalytic performance over the functionalized filters without SNF coatings. Notably, the unique flexibility of the SNF coatings allowed the facile preparation of a packed bed reactor and a scalable catalytic system. It is expected that the packed bed system established in this study will support the development and the use of various SNF-supported organocatalysts and catalytic materials.

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

Sol-gel-based silica monoliths with hierarchical mesopores/macropores are promising catalyst support and flow reactors. Here, we report the successful preparation of cylindrically shaped Pt-loaded silica monoliths (length: 2 cm, diameter: 0.5 cm) with a variable mean macropore width of 1, 6, 10, or 27 μm at a fixed mean mesopore width of 17 nm. The Pt-loaded monolithic catalysts were housed in a robust cladding made of borosilicate glass for use as a flow reactor. The monolithic reactors exhibit a permeability as high as 2 μm2 with a pressure drop below 9 bars over a flow rate range of 2–20 cm3 min−1 (solvent: water). The aqueous-phase hydrogenation of p-nitrophenol to p-aminophenol with NaBH4 as a reducing agent was used as a test reaction to study the influence of mass transfer on catalytic activity in continuous flow. No influence of flow rate on conversion at a fixed contact time of 2.6 s was observed for monolithic catalysts with mean macropore widths of 1, 10, or 27 µm. As opposed to earlier studies conducted at much lower flow velocities, this strongly indicates the absence of external mass-transfer limitations or stagnant layer formation in the macropores of the monolithic catalysts.

N-Doped carbon as a solid base catalyst for continuous flow Knoevenagel condensation

A stability survey of a basic N-doped carbon catalyst in a continuous flow process together with a study of process parameters affecting the batch system and their correlation to the flow protocol is described.