Scaling up of continuous-flow, microwave-assisted, organic reactions by varying the size of Pd-functionalized catalytic monoliths

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.

Microflow chemistry and its electrification for sustainable chemical manufacturing

Sustainability is vital in solving global societal problems. Still, it requires a holistic view by considering renewable energy and carbon sources, recycling waste streams, environmentally friendly resource extraction and handling, and green manufacturing. Flow chemistry at the microscale can enable continuous sustainable manufacturing by opening up new operating windows, precise residence time control, enhanced mixing and transport, improved yield and productivity, and inherent safety. Furthermore, integrating microfluidic systems with alternative energy sources, such as microwaves and plasmas, offers tremendous promise for electrifying and intensifying modular and distributed chemical processing. This review provides an overview of microflow chemistry, electrification, their integration toward sustainable manufacturing, and their application to biomass upgrade (a select number of other processes are also touched upon). Finally, we identify critical areas for future research, such as matching technology to the scale of the application, techno-economic analysis, and life cycle assessment.

Development of Facile and Simple Processes for the Heterogeneous Pd-Catalyzed Ligand-Free Continuous-Flow Suzuki–Miyaura Coupling

The Suzuki–Miyaura coupling reaction is one of the most widely utilized C–C bond forming methods to create (hetero)biaryl scaffolds. The continuous-flow reaction using heterogeneous catalyst-packed cartridges is a practical and efficient synthetic method to replace batch-type reactions. A continuous-flow ligand-free Suzuki–Miyaura coupling reaction of (hetero)aryl iodides, bromides, and chlorides with (hetero)aryl boronic acids was developed using cartridges packed with spherical resin (tertiary amine-based chelate resin: WA30)-supported palladium catalysts (7% Pd/WA30). The void space in the cartridge caused by the spherical catalyst structures enables the smooth flow of a homogeneously dissolved reaction solution that consists of a mixture of organic and aqueous solvents and is delivered by the use of a single syringe pump. Clogging or serious backpressure was not observed.

Suzuki–Miyaura Coupling Using Monolithic Pd Reactors and Scaling-Up by Series Connection of the Reactors

The space integration of the lithiation of aryl halides, the borylation of aryllithiums, and Suzuki–Miyaura coupling using a Pd catalyst supported by a polymer monolith flow reactor without using an intentionally added base was achieved. To scale up the process, a series connection of the monolith Pd reactor was examined. To suppress the increase in the pressure drop caused by the series connection, a monolith reactor having larger pore sizes was developed by varying the temperature of the monolith preparation. The monolithic Pd reactor having larger pore sizes enabled Suzuki–Miyaura coupling at a higher flow rate because of a lower pressure drop and, therefore, an increase in productivity. The present study indicates that series connection of the reactors with a higher flow rate serves as a good method for increasing the productivity without decreasing the yields.

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.

Continuous-Flow Suzuki-Miyaura and Mizoroki-Heck Reactions under Microwave Heating Conditions

Microwave-assisted continuous-flow reactions have attracted significant interest from synthetic organic chemists, especially process chemists from practical points of view, due to a less complicated shift to large-scale synthesis based on simple and continuous access to products with low energy requirements. In this personal account, we focused on the Suzuki-Miyaura and Mizoroki-Heck reactions, both of which are significantly important cross-coupling reactions for the synthesis of various functional materials. Microwave power is effective for heating. Typical homogeneous palladium catalysts, such as PdCl₂ (PPh₃ )₂ , Pd(PPh₃ )₄ , and Pd(OAc)₂ , as well as heterogeneous palladium catalysts, such as Pd-film, Pd/Al₂ O₃ , Pd/SiO₂ , and Pd supported on polymers, can be used for these reactions.

The Hitchhiker's Guide to Flow Chemistry ∥

Flow chemistry involves the use of channels or tubing to conduct a reaction in a continuous stream rather than in a flask. Flow equipment provides chemists with unique control over reaction parameters enhancing reactivity or in some cases enabling new reactions. This relatively young technology has received a remarkable amount of attention in the past decade with many reports on what can be done in flow. Until recently, however, the question, "Should we do this in flow?" has merely been an afterthought. This review introduces readers to the basic principles and fundamentals of flow chemistry and critically discusses recent flow chemistry accounts.

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 hydrogenations using novel catalytic static mixers inside a tubular reactor

Continuous flow reactor for the hydrogenation of organic substrates using novel catalytic static mixers.

Integration of borylation of aryllithiums and Suzuki–Miyaura coupling using monolithic Pd catalyst

Integration of the preparation of arylboronic esters and Suzuki–Miyaura coupling using monolithic Pd catalyst was successfully achieved.

Fine chemical syntheses under flow using SiliaCat catalysts

After reviewing selected recent applications from diverse laboratories, we show how mesoporous organosilica-entrapped catalysts of the SiliaCat series have been successfully used for clean manufacturing of APIs in a number of different reactions carried out under laminar flow in microreactors.

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.

One-step synthesis of pyridines and dihydropyridines in a continuous flow microwave reactor

The Bohlmann-Rahtz pyridine synthesis and the Hantzsch dihydropyridine synthesis can be carried out in a microwave flow reactor or using a conductive heating flow platform for the continuous processing of material. In the Bohlmann-Rahtz reaction, the use of a Brønsted acid catalyst allows Michael addition and cyclodehydration to be carried out in a single step without isolation of intermediates to give the corresponding trisubstituted pyridine as a single regioisomer in good yield. Furthermore, 3-substituted propargyl aldehydes undergo Hantzsch dihydropyridine synthesis in preference to Bohlmann-Rahtz reaction in a very high yielding process that is readily transferred to continuous flow processing.

Efficient continuous-flow synthesis of novel 1,2,3-triazole-substituted β-aminocyclohexanecarboxylic acid derivatives with gram-scale production

The preparation of novel multi-substituted 1,2,3-triazole-modified β-aminocyclohexanecarboxylic acid derivatives in a simple and efficient continuous-flow procedure is reported. The 1,3-dipolar cycloaddition reactions were performed with copper powder as a readily accessible Cu(I) source. Initially, high reaction rates were achieved under high-pressure/high-temperature conditions. Subsequently, the reaction temperature was lowered to room temperature by the joint use of both basic and acidic additives to improve the safety of the synthesis, as azides were to be handled as unstable reactants. Scale-up experiments were also performed, which led to the achievement of gram-scale production in a safe and straightforward way. The obtained 1,2,3-triazole-substituted β-aminocyclohexanecarboxylates can be regarded as interesting precursors for drugs with possible biological effects.

Development and Evaluation of a Raman Flow Cell for Monitoring Continuous Flow Reactions

We show how in-line Raman spectroscopy can be used to monitor both reactant and product concentrations for a heterogeneously catalysed Suzuki cross reaction operating in continuous flow. The flow system consisted of an HPLC pump to drive a homogeneous mixture of the reactants (4-bromobenzonitrile, phenylboronic acid, and potassium carbonate) through an oven heated (80°C) palladium catalyst immobilised on a silica monolith. A custom built PTFE in-line flow cell with a quartz window enabled the coupling of an Ocean Optics Raman spectrometer probe to monitor both the reactants and product (4-cyanobiphenyl). Calibration was based on obtaining multivariate spectral data in the range 1530 cm–1 and 1640 cm–1 and using partial least-squares regression (PLSR) to obtain a calibration model which was validated using gas chromatography–mass spectrometry (GCMS) analysis. In-line Raman monitoring of the reactant and product concentrations enable (i) determination of reaction kinetic information such as the empirical rate law and associated rate constant and (ii) optimisation of either the product conversion (61 % at 0.02 mL min–1 generating 17 g h–1) or product yield (14 % at 0.24 mL min–1 generating 53 g h–1).

Selective hydrogenation over Pd nanoparticles supported on a pore-flow-through silica monolith microreactor with hierarchical porosity

Well-dispersed Pd nanoparticles have been synthesized inside the mesoporosity of a silica monolith featuring hierarchical porosity of homogeneous interconnected macropores (4 microns) and mesopores (11 nm). These monoliths have been implemented as microreactors for selective hydrogenation reactions. Conversion and selectivity can be tuned by adjusting the flow rates of hydrogen and substrates. In the selective hydrogenation of cyclooctadiene, a conversion of 95% and a selectivity of 90% in the monohydrogenated product, constant over a period of 70 h, have been reached. These figures correspond to a productivity of 4.2 mmol s(-1) g(-1)(MonoSil) (or 0.32 mol s(-1) g(-1)(Pd)). In the stereoselective hydrogenation of 3-hexyn-1-ol a constant conversion of 85% was observed, with however moderate selectivity into the cis isomer, over a test period of 7 h. These results open the route to the synthesis of important chemicals and intermediates via safe and green processes.

Continuous-flow hydration-condensation reaction: Synthesis of α,β-unsaturated ketones from alkynes and aldehydes by using a heterogeneous solid acid catalyst

A simple, practical and efficient continuous-flow hydration-condensation protocol was developed for the synthesis of α,β-unsaturated ketones starting from alkynes and aldehydes by employing a heterogeneous catalyst in a flow microwave. The procedure presents a straightforward and convenient access to valuable differently substituted chalcones and can be applied on multigram scale.