Continuous Partial Hydrogenation Reactions by Pd@unconventional Bimodal Porous Titania Monolith Catalysts

Copper Oxide Nanoparticles over Hierarchical Silica Monoliths for Continuous-Flow Selective Alcoholysis of Styrene Oxide

A simple and reproducible approach for the synthesis of Cu-based heterogeneous catalysts, named flow chemisorption hydrolysis (flow-CH), is reported. The approach, derived from the CH method, allows size-controlled CuO nanoparticles (mean diameter 2.9 nm) to be obtained, that are highly and homogeneously dispersed into hierarchically meso-/macroporous silica monoliths. The Cu-based monolithic catalysts (CuO@SiO2-MN, 8.4 wt.% Cu) were studied in the styrene oxide ring opening reaction at 60 °C in the presence of isopropanol, under continuous flow-through conditions. A remarkable activity with a steady-state conversion of 97% for 13 h and 100% selectivity towards the corresponding β-alkoxyalcohol was observed. The performances of CuO@SiO2-MN were higher than those obtained in batch conditions with the previously reported CuO/SiO2 catalysts and with the ground CuO@SiO2-MN monolith in terms of productivity and selectivity. Moreover, a negligible Cu leaching (<0.6 wt.%) in reaction medium was observed. After 13 h CuO@SiO2-MN catalysts could be regenerated by a mild calcination (220 °C) permitting reuse.

Biocatalytic Elimination of Pharmaceutics Found in Water With Hierarchical Silica Monoliths in Continuous Flow

Pharmaceutical products (PPs) are considered as emerging micropollutans in wastewaters, river and seawaters, and sediments. The biodegradation of PPs, such as ciprofloxacin, amoxicillin, sulfamethoxazole, and tetracycline by enzymes in aqueous solution was investigated. Laccase from Trametes versicolor was immobilized on silica monoliths with hierarchical meso-/macropores. Different methods of enzyme immobilization were experienced. The most efficient process was the enzyme covalent bonding through glutaraldehyde coupling on amino-grafted silica monoliths. Silica monoliths with different macropore and mesopore diameters were studied. The best support was the monolith featuring the largest macropore diameter (20 µm) leading to the highest permeability and the lowest pressure drop and the largest mesopore diameter (20 nm) ensuring high enzyme accessibility. The optimized enzymatic reactor (150 mg) was used for the degradation of a PP mixture (20 ppm each in 30 ml) in a continuous recycling configuration at a flow rate of 1 ml/min. The PP elimination efficiency after 24 h was as high as 100% for amoxicillin, 60% for sulfamethoxazole, 55% for tetracycline, and 30% for ciprofloxacin.

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.

Greener Synthesis of Pristane by Flow Dehydrative Hydrogenation of Allylic Alcohol Using a Packed-Bed Reactor Charged by Pd/C as a Single Catalyst

Our previous work established a continuous-flow synthesis of pristane, which is a saturated branched alkane obtained from a Basking Shark. The dehydration of an allylic alcohol that is the key to a tetraene was carried out using a packed-bed reactor charged by an acid–silica catalyst (HO-SAS) and flow hydrogenation using molecular hydrogen via a Pd/C catalyst followed. The present work relies on the additional propensity of Pd/C to serve as an acid catalyst, which allows us to perform a flow synthesis of pristane from the aforementioned key allylic alcohol in the presence of molecular hydrogen using Pd/C as a single catalyst, which is applied to both dehydration and hydrogenation. The present one-column-two-reaction-flow system could eliminate the use of an acid catalyst such as HO-SAS and lead to a significant simplification of the production process.

Monolithic cellulose supported metal nanoparticles as green flow reactor with high catalytic efficiency

A highly effective, stable and reusable flow microreactor was developed by utilizing the environmentally sustainable porous monolithic cellulose based on a facile temperature induced phase separation (TIPS) method. The obtained microreator could be applied to efficiently and continuously catalysing the reduction reaction of 4-nitrophenol (an important reaction in water treatment) without any post-treatment or regeneration of catalysts. Moreover, the monolith overcame the brittleness of the crystalline cellulose and showed a good mechanical resilience, suggesting a great potential for the practical application in severe environment. Compared with previous reported Pd supported catalytic systems, this microreactor exhibited extremely high catalytic efficiency (turnover frequency, TOF = 4660 h^-1, almost 4 times higher than that of cellulose nanocrystals supported catalyst) and long-term stability. This work provided a new strategy to construct highly effective and reusable metal NPs involved catalytic system by utilizing biodegradable cellulose materials.

Independent Tailoring of Macropore and Mesopore Space in TiO2 Monoliths

TiO₂ monoliths were synthesized by a partially hindered sol-gel process. Various synthesis parameters like precursor concentrations and gelation temperature were varied to investigate changes in the macroporosity (being in the range of micrometers) and to determine influences on the macropore formation mechanism. Ionic liquids (ILs) were used as templates to vary the mesopore size independently from the macropore size. Depending on the synthesis parameters, TiO₂ monoliths with exclusive mesoporosity or with hierarchical meso-/macropore structure were received, and the range of macropores can be shifted between 100 nm and 10 μm without influencing the mesopore diameter. Pore volumes up to 880 mm³/g were achieved, as determined by mercury intrusion porosimetry. The mesopores' diameter can be adjusted between 6 and 25 nm by adding different amounts of IL, and surface areas up to 260 m²/g and mesopore volumes of 0.5 cm³/g were obtained, based on N₂-physisorption measurements. The monoliths were cladded by polymer, allowing for studying the flow-through properties depending on the macropore size. This precise control for tailored macropores enables the design of optimized TiO₂ monoliths with respect to the desired application requirements.

Tailored macroporous hydrogel–nanoparticle nanocomposites for monolithic flow-through catalytic reactors

Highly porous poly(N-isopropylacrylamide) PNIPAam hydrogel monoliths with tunable microstructures and comprising gold, silver or palladium nanoparticles, display significant catalytic activity when used in flow-through microreactors.

Low-Temperature Continuous-Flow Dehydration of Xylose Over Water-Tolerant Niobia-Titania Heterogeneous Catalysts

The sustainable conversion of vegetable biomass-derived feeds to useful chemicals requires innovative routes meeting environmental and economical criteria. The approach herein pursued is the synthesis of water-tolerant, unconventional solid acid monolithic catalysts based on a mixed niobia-titania skeleton building up a hierarchical open-cell network of meso- and macropores, and tailored for use under continuous-flow conditions. The materials were characterized by spectroscopic, microscopy, and diffraction techniques, showing a reproducible isotropic structure and an increasing Lewis/Brønsted acid sites ratio with increasing Nb content. The catalytic dehydration reaction of xylose to furfural was investigated as a representative application. The efficiency of the catalyst was found to be dramatically affected by the niobia content in the titania lattice. The presence of as low as 2 wt % niobium resulted in the highest furfural yield at 140 °C under continuous-flow conditions, by using H₂ O/γ-valerolactone as a safe monophasic solvent system. The interception of a transient 2,5-anhydroxylose species suggested the dehydration process occurs via a cyclic intermediates mechanism. The catalytic activity and the formation of the anhydro intermediate were related to the Lewis acid sites (LAS)/Brønsted acid sites (BAS) ratio and indicated a significant contribution of xylose-xylulose isomerization. No significant catalyst deactivation was observed over 4 days usage.

Cucurbit[7]uril-based high-performance catalytic microreactors

Catalytic microreactors manufactured using microfluidic devices have received significant research interest in recent years. However, little attention has been paid to immobilising metallic nanoparticles (NPs) onto microchannel walls for high efficiency catalytic reactions. We demonstrate a facile preparation of cucurbit[7]uril-based catalytic microreactors, where metallic NPs are immobilised onto microchannels via supramolecular complexation with methyl viologen@cucurbit[7]uril (CB[7]). These microreactors exhibit a remarkable catalytic activity owing to the substantially high surface area to volume ratio of the microchannels and metallic NPs. Superior to most conventional heterogeneous catalytic reactions, separation post reaction and complicated recycling steps of the catalysts are not required. Moreover, CB[7] can complex a variety of metallic NPs to its portal, providing a multifunctional high-performance in situ catalytic platform.

Engineering Porous Polymer Hollow Fiber Microfluidic Reactors for Sustainable C-H Functionalization

Highly hydrophilic and solvent-stable porous polyamide-imide (PAI) hollow fibers were created by cross-linking of bare PAI hollow fibers with 3-aminopropyl trimethoxysilane (APS). The APS-grafted PAI hollow fibers were then functionalized with salicylic aldehyde for binding catalytically active Pd(II) ions through a covalent postmodification method. The catalytic activity of the composite hollow fiber microfluidic reactors (Pd(II) immobilized APS-grafted PAI hollow fibers) was tested via heterogeneous Heck coupling reaction of aryl halides under both batch and continuous-flow reactions in polar aprotic solvents at high temperature (120 °C) and low operating pressure. X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma (ICP) analyses of the starting and recycled composite hollow fibers indicated that the fibers contain very similar loadings of Pd(II), implying no degree of catalyst leaching from the hollow fibers during reaction. The composite hollow fiber microfluidic reactors showed long-term stability and strong control over the leaching of Pd species.

Continuous-flow processes for the catalytic partial hydrogenation reaction of alkynes

The catalytic partial hydrogenation of substituted alkynes to alkenes is a process of high importance in the manufacture of several market chemicals. The present paper shortly reviews the heterogeneous catalytic systems engineered for this reaction under continuous flow and in the liquid phase. The main contributions appeared in the literature from 1997 up to August 2016 are discussed in terms of reactor design. A comparison with batch and industrial processes is provided whenever possible.

Grafted Polymethylhydrosiloxane on Hierarchically Porous Silica Monoliths: A New Path to Monolith-Supported Palladium Nanoparticles for Continuous Flow Catalysis Applications

Polymethylhydrosiloxane has been grafted on the surface of a hierarchically porous silica monolith using a facile catalytic reaction between Si-H and silanol to anchor the polymer. This easy methodology leads to the functionalization of the surface of a silica monolith, where a large amount of free Si-H bonds remain available for reducing metal ions in solution. Palladium nanoparticles of 15 nm have been synthesized homogeneously inside the mesopores of the monolith without any stabilizers, using a flow of a solution containing Pd²⁺. This monolith was used as column-type fixed bed catalyst for continuous flow hydrogenation of styrene and selective hydrogenation of 3-hexyn-1-ol, in each case without a significant decrease of the catalytic activity after several hours or days. Conversion, selectivity, and stereoselectivity of the alkyne hydrogenation can be tuned by flow rates of hydrogen and the substrate solution, leading to high productivity (1.57 mol g(Pd)^-1 h^-1) of the corresponding cis-alkene.

Advances in monoliths and related porous materials for microfluidics

In recent years, the use of monolithic porous polymers has seen significant growth. These materials present a highly useful support for various analytical and biochemical applications. Since their introduction, various approaches have been introduced to produce monoliths in a broad range of materials. Simple preparation has enabled their easy implementation in microchannels, extending the range of applications where microfluidics can be successfully utilized. This review summarizes progress regarding monoliths and related porous materials in the field of microfluidics between 2010 and 2015. Recent developments in monolith preparation, solid-phase extraction, separations, and catalysis are critically discussed. Finally, a brief overview of the use of these porous materials for analysis of subcellular and larger structures is given.

Structuring hybrid palladium nanoparticles in metallic monolithic reactors for continuous-flow three-phase alkyne hydrogenation

Ligand-modified palladium nanoparticles are stabilised in the microchannels of an alumina-coated metallic monolith and advantageously used in the continuous-flow three-phase semi-hydrogenation of acetylenic compounds.

Nanocatalysis in Flow

Nanocatalysis in flow is catalysis by metallic nanoparticles (NPs; 1-50 nm) performed in microstructured reactors. These catalytic processes make use of the enhanced catalytic activity and selectivity of NPs and fulfill the requirements of green chemistry. Anchoring catalytically active metal NPs within a microfluidic reactor enhances the reagent/catalyst interaction, while avoiding diffusion limitations experienced in classical approaches. Different strategies for supporting NPs are reviewed herein, namely, packed-bed reactors, monolithic flow-through reactors, wall catalysts, and a selection of novel approaches (NPs embedded on nanotubes, nanowires, catalytic membranes, and magnetic NPs). Through a number of catalytic reactions, such as hydrogenations, oxidations, and cross-coupling reactions, the advantages and possible drawbacks of each approach are illustrated.

Mesoporous materials for clean energy technologies

Alternative energy technologies are greatly hindered by significant limitations in materials science. From low activity to poor stability, and from mineral scarcity to high cost, the current materials are not able to cope with the significant challenges of clean energy technologies. However, recent advances in the preparation of nanomaterials, porous solids, and nanostructured solids are providing hope in the race for a better, cleaner energy production. The present contribution critically reviews the development and role of mesoporosity in a wide range of technologies, as this provides for critical improvements in accessibility, the dispersion of the active phase and a higher surface area. Relevant examples of the development of mesoporosity by a wide range of techniques are provided, including the preparation of hierarchical structures with pore systems in different scale ranges. Mesoporosity plays a significant role in catalysis, especially in the most challenging processes where bulky molecules, like those obtained from biomass or highly unreactive species, such as CO2 should be transformed into most valuable products. Furthermore, mesoporous materials also play a significant role as electrodes in fuel and solar cells and in thermoelectric devices, technologies which are benefiting from improved accessibility and a better dispersion of materials with controlled porosity.

Continuous-flow hydrogenation of olefins and nitrobenzenes catalyzed by platinum nanoparticles dispersed in an amphiphilic polymer

Continuous-flow hydrogenation of olefins and nitrobenzenes with ARP-Pt.

Continuous flow synthesis of Rh and Pd nanoparticles onto ion-exchange borate monoliths: application to selective catalytic hydrogenation of unsaturated carbonyl compounds under flow conditions

Rh and Pd NPs, generated within an unconventional borate monolith, performed highly in continuous flow hydrogenation of unsaturated carbonyl compounds.