Improved method for kinetic studies in microreactors using flow manipulation and noninvasive Raman spectrometry

Democratizing Microreactor Technology for Accelerated Discoveries in Chemistry and Materials Research

Microreactor technologies have emerged as versatile platforms with the potential to revolutionize chemistry and materials research, offering sustainable solutions to global challenges in environmental and health domains. This survey paper provides an in-depth review of recent advancements in microreactor technologies, focusing on their role in facilitating accelerated discoveries in chemistry and materials. Specifically, we examine the convergence of microfluidics with machine intelligence and automation, enabling the exploitation of the cyber-physical environment as a highly integrated experimentation platform for rapid scientific discovery and process development. We investigate the applicability and limitations of microreactor-enabled discovery accelerators in various chemistry and materials contexts. Despite their tremendous potential, the integration of machine intelligence and automation into microreactor-based experiments presents challenges in establishing fully integrated, automated, and intelligent systems. These challenges can hinder the broader adoption of microreactor technologies within the research community. To address this, we review emerging technologies that can help lower barriers and facilitate the implementation of microreactor-enabled discovery accelerators. Lastly, we provide our perspective on future research directions for democratizing microreactor technologies, with the aim of accelerating scientific discoveries and promoting widespread adoption of these transformative platforms.

Kinetic Aspects of Esterification and Transesterification in Microstructured Reactors

Microstructured reactors offer fast chemical engineering transfer and precise microfluidic control, enabling the determination of reactions' kinetic parameters. This review examines recent advancements in measuring microreaction kinetics. It explores kinetic modeling, reaction mechanisms, and intrinsic kinetic equations pertaining to two types of microreaction: esterification and transesterification reactions involving acids, bases, or biocatalysts. The utilization of a micro packed-bed reactor successfully achieves a harmonious combination of the micro-dispersion state and the reaction kinetic characteristics. Additionally, this review presents micro-process simulation software and explores the advanced integration of microreactors with spectroscopic analyses for reaction monitoring and data acquisition. Furthermore, it elaborates on the control principles of the micro platform. The superiority of online measurement, automation, and the digitalization of the microreaction process for kinetic measurements is highlighted, showcasing the vast prospects of artificial intelligence applications.

Dynamic Observation of the Membrane Interaction Processes of β-Lactoglobulin by Time-Resolved Vacuum-Ultraviolet Circular Dichroism

The elucidation of protein-membrane interactions is pivotal for comprehending the mechanisms underlying diverse biological phenomena and membrane-related diseases. In this investigation, vacuum-ultraviolet circular dichroism (VUVCD) spectroscopy, utilizing synchrotron radiation (SR), was employed to dynamically observe membrane interaction processes involving water-soluble proteins at the secondary-structure level. The study utilized a time-resolved (TR) T-shaped microfluidic cell, facilitating the rapid and efficient mixing of protein and membrane solutions. This system was instrumental in acquiring measurements of the time-resolved circular dichroism (TRCD) spectra of β-lactoglobulin (bLG) during its interaction with lysoDMPG micelles. The results indicate that bLG undergoes a β-α conformation change, leading to the formation of the membrane-interacting state (M-state), with structural alterations occurring in more than two steps. Global fitting analysis, employing biexponential functions with all of the TRCD spectral data sets, yielded two distinct rate constants (0.18 ± 0.01 and 0.06 ± 0.003/s) and revealed a unique spectrum corresponding to an intermediate state (I-state). Secondary-structure analysis of bLG in its native (N-, I-, and M-states) highlighted that structural changes from the N- to I-states predominantly occurred in the N- and C-terminal regions, which were prominently exposed to the membrane. Meanwhile, transitions from the I- to M-states extended into the inner barrel regions of bLG. Further examination of the physical properties of α-helical segments, such as effective charge and hydrophobicity, revealed that the N- to I- and I- to M-state transitions, which are ascribed to first- and second-rate constants, respectively, are primarily driven by electrostatic and hydrophobic interactions, respectively. These findings underscore the capability of the TR-VUVCD system as a robust tool for characterizing protein-membrane interactions at the molecular level.

In situ Diazonium Salt Formation and Photochemical Aryl-Aryl Coupling in Continuous Flow Monitored by Inline NMR Spectroscopy

A novel class of diazonium salts is introduced for the photochemical aryl-aryl coupling to produce (substituted) biphenyls. As common diazonium tetrafluoroborate salts fail, soluble and safe aryl diazonium trifluoroacetates are applied. In this mild synthesis route no catalysts are required to generate an aryl-radical by irradiation with UV-A light (365 nm). This reactive species undergoes direct C-H arylation at an arene, forming the product in reasonable reaction times. With the implementation of a continuous flow setup in a capillary photoreactor 13 different biphenyl derivatives are successfully synthesized. By integrating an inline 19F-NMR benchtop spectrometer, samples are reliably quantified as the fluorine-substituents act as a probe. Here, real-time NMR spectroscopy is a perfect tool to monitor the continuously operated system, which produces fine chemicals of industrial relevance even in a multigram scale.

Enabling High Throughput Kinetic Experimentation by Using Flow as a Differential Kinetic Technique

Kinetic data is most commonly collected through the generation of time-series data under either batch or flow conditions. Existing methods to generate kinetic data in flow collect integral data (concentration over time) only. Here, we report a method for the rapid and direct collection of differential kinetic data (direct measurement of rate) in flow by performing a series of instantaneous rate measurements on sequential small-scale reactions. This technique decouples the time required to generate a full kinetic profile from the time required for a reaction to reach completion, enabling high throughput kinetic experimentation. In addition, comparison of kinetic profiles constructed at different residence times allows the robustness, or stability, of homogeneously catalysed reactions to be interrogated. This approach makes use of a segmented flow platform which was shown to quantitatively reproduce batch kinetic data. The proline mediated aldol reaction was chosen as a model reaction to perform a high throughput kinetic screen of 216 kinetic profiles in 90 hours, one every 25 minutes, which would have taken an estimated continuous 3500 hours in batch, an almost 40-fold increase in experimental throughput matched by a corresponding reduction in material consumption.

Visualizing Chain Growth of Polytelluoxane via Polymerization Induced Emission

Visualizing polymer chain growth is always a hot topic for tailoring structure-function properties in polymer chemistry. However, current characterization methods are limited in their ability to differentiate the degree of polymerization in real-time without isolating the samples from the reaction vessel, let alone to detect insoluble polymers. Herein, a reliable relationship is established between polymer chain growth and fluorescence properties through polymerization induced emission. (TPE-C2)2 -Te is used to realize in situ oxidative polymerization, leading to the aggregation of fluorophores. The relationship between polymerization degree of growing polytelluoxane (PTeO) and fluorescence intensity is constructed, enabling real-time monitoring of the polymerization reaction. More importantly, this novel method can be further applied to the observation of the polymerization process for growing insoluble polymer via surface polymerization. Therefore, the development of visualization technology will open a new avenue for visualizing polymer chain growth in real-time, regardless of polymer solubility.

Parallel multi-droplet platform for reaction kinetics and optimization

We present an automated droplet reactor platform possessing parallel reactor channels and a scheduling algorithm that orchestrates all of the parallel hardware operations and ensures droplet integrity as well as overall efficiency. We design and incorporate all of the necessary hardware and software to enable the platform to be used to study both thermal and photochemical reactions. We incorporate a Bayesian optimization algorithm into the control software to enable reaction optimization over both categorical and continuous variables. We demonstrate the capabilities of both the preliminary single-channel and parallelized versions of the platform using a series of model thermal and photochemical reactions. We conduct a series of reaction optimization campaigns and demonstrate rapid acquisition of the data necessary to determine reaction kinetics. The platform is flexible in terms of use case: it can be used either to investigate reaction kinetics or to perform reaction optimization over a wide range of chemical domains.

Automated optimization under dynamic flow conditions

The combination of feedback optimization with dynamic operations leads to enhanced data-rich experimentation in flow.

Polymer Molecular Weight Determination via Fluorescence Lifetime

Control of polymer molecular weight is critical for tailoring structure-function properties; however, traditional molecular weight characterization techniques have limited ability to determine the molecular weight of polymers in real time without sample removal from the reaction mixture, with spatial resolution, and of insoluble polymers. In this work, a fluorescence lifetime imaging microscopy (FLIM) method was developed that overcomes these limitations. The method is demonstrated with polynorbornene and polydicyclopentadiene, polymers derived from ruthenium-catalyzed ring-opening metathesis polymerization (ROMP). The polymer M_w, ranging from 35 to 570 kg/mol as determined by gel-permeation chromatography, was quantitatively correlated with the fluorescence lifetime. The revealed correlation then enabled time-resolved measurement of M_w during an ongoing ROMP reaction, requiring only 1 s per measurement (of a 45 μm × 45 μm polymer sample area), and provided spatial resolution, resulting in simultaneous characterization of polymer morphology. To provide the fluorescence signal, the initial reaction solutions contained a very low doping of a reactive norbornene monomer labeled with fluorescent boron dipyrromethene (BODIPY), such that 1 in every 10⁷ monomers contained a fluorophore. The resulting FLIM visualization method enables the rapid determination of the molecular weights of growing polymers without removal from the reaction mixture and regardless of polymer solubility.

Reactor performance estimation in microscale flow calorimeter for rapid characterization of exothermic reactions

Abstract

Continuous flow calorimeters are a promising tool in process development and safety engineering, especially for flow chemistry applications to characterize the heat release and kinetic parameters of rapid chemical reactions. In this study, the digital accompaniment of an isoperibolic flow calorimeter for characterization of exothermic reactions is presented. To support experimental planning and evaluation, computational fluid dynamic simulations are carried out for single-phase flow in the microreactor. The residence time distribution is obtained and used for estimation of conversion and temperature profiles along the microreactor channel. This leads to an integration of CFD simulations into the calorimeter’s software-guided workflow reducing the experimental effort regarding the determination of thermokinetic data. The approach is tested for a highly exothermic test reaction, which provides further hints for future investigations.

Article highlights

• Estimation of conversion and temperature profiles within a microscale calorimeter

• Combination of CFD simulations and reactor performance estimation

• Approach was tested for highly oxidation of sodium thiosulfate

• Estimated conversion and temperature profiles are in good agreement with experimental data

Graphical abstract

Review of Optical Thermometry Techniques for Flows at the Microscale towards Their Applicability to Gas Microflows

Thermometry techniques have been widely developed during the last decades to analyze thermal properties of various fluid flows. Following the increasing interest for microfluidic applications, most of these techniques have been adapted to the microscale and some new experimental approaches have emerged. In the last years, the need for a detailed experimental analysis of gaseous microflows has drastically grown due to a variety of exciting new applications. Unfortunately, thermometry is not yet well developed for analyzing gas flows at the microscale. Thus, the present review aims at analyzing the main currently available thermometry techniques adapted to microflows. Following a rapid presentation and classification of these techniques, the review is focused on optical techniques, which are the most suited for application at microscale. Their presentation is followed by a discussion about their applicability to gas microflows, especially in confined conditions, and the current challenges to be overcome are presented. A special place is dedicated to Raman and molecular tagging thermometry techniques due to their high potential and low intrusiveness.

Measuring Kinetics in Flow Using Isoperibolic Flow Calorimetry

Continuous flow calorimeters are a promising tool in process development and safety engineering, particularly for flow chemistry applications. An isoperibolic flow calorimeter is presented for the characterization of exothermic reactions. The calorimeter is adapted to commercially available plate microreactors made of glass and uses Seebeck elements to quantify the heat of reaction. For automation of calibration procedures and calorimetric measurements, the device is connected to a lab automation system. Reaction enthalpy of exothermic reactions is determined via an energy balance of the entire calorimeter. Characterization of reaction kinetics is carried out via a local balancing of the individual Seebeck elements without changing the experimental setup, while using the previous measurements and additional ones at higher flow rates. The calorimeter and the associated measurement procedures were tested with the oxidation of sodium thiosulfate using hydrogen peroxide. Reaction enthalpy was determined to be 594.3 ± 0.7 kJ mol−1, which is within the range of literature values.

Automated Flow and Real-Time Analytics Approach for Screening Functional Group Tolerance in Heterogeneous Catalytic Reactions

Heterogeneous hydrogenation reactions are widely used in synthesis, and performing them using continuous flow technologies addresses many of the safety, scalability and sustainability issues. However, one of the main potential...

Modern advancements in continuous-flow aided kinetic analysis

Although kinetic analysis has traditionally been conducted in a batch vessel, continuous-flow aided kinetic analysis continues to swell in popularity.

Discovery of unexpectedly complex reaction pathways for the Knorr pyrazole synthesis via transient flow

Kinetic data for the Knorr pyrazole synthesis were acquired using transient flow methods, including a novel reactant stoichiometry ramping method, providing new insights into the mechanism, including autocatalysis and an unexpected intermediate.

Autonomous model-based experimental design for rapid reaction development

To automate and democratize model-based experimental design for flow chemistry applications, we report the development of open-source software, Optipus. Reaction models are built in an iterative and automated fashion, for rapid reaction development.

A continuous flow investigation of sulfonyl chloride synthesis using N-chloroamides: optimization, kinetics and mechanism

We develop a continuous flow protocol for the synthesis of sulfonyl chlorides from disulfides and thiols, using 1,3-dichloro-5,5-dimethylhydantoin (DCH) as a dual-function reagent for oxidative chlorination.

Quantification and Prediction of Imine Formation Kinetics in Aqueous Solution by Microfluidic NMR Spectroscopy

Quantitatively predicting the reactivity of dynamic covalent reaction is essential to understand and rationally design complex structures and reaction networks. Herein, the reactivity of aldehydes and amines in various rapid imine formation in aqueous solution by microfluidic NMR spectroscopy was quantified. Investigation of reaction kinetics allowed to quantify the forward rate constants k₊ by an empirical equation, of which three independent parameters were introduced as reactivity parameters of aldehydes (S_E , E) and amines (N). Furthermore, these reactivity parameters were successfully used to predict the unknown forward rate constants of imine formation. Finally, two competitive reaction networks were rationally designed based on the proposed reactivity parameters. Our work has demonstrated the capability of microfluidic NMR spectroscopy in quantifying the kinetics of label-free chemical reactions, especially rapid reactions that are complete in minutes.

A comprehensive review of flow chemistry techniques tailored to the flavours and fragrances industries

Due to their intrinsic physical properties, which includes being able to perform as volatile liquids at room and biological temperatures, fragrance ingredients/intermediates make ideal candidates for continuous-flow manufacturing. This review highlights the potential crossover between a multibillion dollar industry and the flourishing sub-field of flow chemistry evolving within the discipline of organic synthesis. This is illustrated through selected examples of industrially important transformations specific to the fragrances and flavours industry and by highlighting the advantages of conducting these transformations by using a flow approach. This review is designed to be a compendium of techniques and apparatus already published in the chemical and engineering literature which would constitute a known solution or inspiration for commonly encountered procedures in the manufacture of fragrance and flavour chemicals.

Microfluidic synthesis of pyrrolidin-2-ones via photoinduced organocatalyzed cyclization of styrene, α-bromoalkyl esters and primary amines

A green and efficient reaction route for the synthesis of pyrrolidin-2-ones via photoinduced organocatalyzed three-component cyclization of styrene, tertiary α-bromoalkyl esters and primary amines in a microchannel reactor under visible light conditions has been developed. Moreover, the overall process can be carried out under mild conditions without using a metal. In addition, a reasonable reaction mechanism was proposed based on control experiments.

Design of dynamic trajectories for efficient and data-rich exploration of flow reaction design spaces

Sinusoidal variations of operative parameters in flow chemistry allows the fast exploration of chemical design spaces through inline measurements of an objective function.

New advances in using Raman spectroscopy for the characterization of catalysts and catalytic reactions

Gaining insight into the mode of operation of heterogeneous catalysts is of great scientific and economic interest. Raman spectroscopy has proven its potential as a powerful vibrational spectroscopic technique for a fundamental and molecular-level characterization of catalysts and catalytic reactions. Raman spectra provide important insight into reaction mechanisms by revealing specific information on the catalysts' (defect) structure in the bulk and at the surface, as well as the presence of adsorbates and reaction intermediates. Modern Raman instrumentation based on single-stage spectrometers allows high throughput and versatility in design of in situ/operando cells to study working catalysts. This review highlights major advances in the use of Raman spectroscopy for the characterization of heterogeneous catalysts made during the past decade, including the development of new methods and potential directions of research for applying Raman spectroscopy to working catalysts. The main focus will be on gas-solid catalytic reactions, but (photo)catalytic reactions in the liquid phase will be touched on if it appears appropriate. The discussion begins with the main instrumentation now available for applying vibrational Raman spectroscopy to catalysis research, including in situ/operando cells for studying gas-solid catalytic processes. The focus then moves to the different types of information available from Raman spectra in the bulk and on the surface of solid catalysts, including adsorbates and surface depositions, as well as the use of theoretical calculations to facilitate band assignments and to describe (resonance) Raman effects. This is followed by a presentation of major developments in enhancing the Raman signal of heterogeneous catalysts by use of UV resonance Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS), and shell-isolated nanoparticle surface-enhanced Raman spectroscopy (SHINERS). The application of time-resolved Raman studies to structural and kinetic characterization is then discussed. Finally, recent developments in spatially resolved Raman analysis of catalysts and catalytic processes are presented, including the use of coherent anti-Stokes Raman spectroscopy (CARS) and tip-enhanced Raman spectroscopy (TERS). The review concludes with an outlook on potential future developments and applications of Raman spectroscopy in heterogeneous catalysis.

Reaction Cycling for Kinetic Analysis in Flow

A reactor capable of efficiently collecting kinetic data in flow is presented. Conversion over time data is obtained by cycling a discrete reaction slug back and forth between two residence coils, with analysis performed each time the solution is passed between the two. In contrast to a traditional steady-state continuous flow system, which requires upward of 5× the total reaction time to obtain reaction progress data, this design achieves much higher efficiency by collecting all data during a single reaction. In combination with minimal material consumption (reactions performed in 300 μL slugs), this represents an improvement in efficiency for typical kinetic experimentation in batch as well. Application to kinetic analysis of a wide variety of transformations (acylation, S_NAr, silylation, solvolysis, Pd catalyzed C-S cross-coupling and cycloadditions) is demonstrated, highlighting both the versatility of the reactor and the benefits of performing kinetic analysis as a routine part of reaction optimization/development. Extension to the monitoring of multiple reactions simultaneously is also realized by operating the reactor with multiple reaction slugs at the same time.

The Medicinal Chemistry in the Era of Machines and Automation: Recent Advances in Continuous Flow Technology

Medicinal chemistry plays a fundamental and underlying role in chemical biology, pharmacology, and medicine to discover safe and efficacious drugs. Small molecule medicinal chemistry relies on iterative learning cycles composed of compound design, synthesis, testing, and data analysis to provide new chemical probes and lead compounds for novel and druggable targets. Using traditional approaches, the time from hypothesis to obtaining the results can be protracted, thus limiting the number of compounds that can be advanced into clinical studies. This challenge can be tackled with the recourse of enabling technologies that are showing great potential in improving the drug discovery process. In this Perspective, we highlight recent developments toward innovative medicinal chemistry strategies based on continuous flow systems coupled with automation and bioassays. After a discussion of the aims and concepts, we describe equipment and representative examples of automated flow systems and end-to-end prototypes realized to expedite medicinal chemistry discovery cycles.

Multivariate analysis of inline benchtop NMR data enables rapid optimization of a complex nitration in flow

Multivariate analysis is applied to inline benchtop NMR data for a complex nitration in flow. This rapid quantification enables reaction optimization using advanced techniques in flow, such as design of experiments and dynamic experimentation.

Automated generation of photochemical reaction data by transient flow experiments coupled with online HPLC analysis

Competing homo- and crossdimerization reactions between coumarin and 1-methyl-2-quinolinone are investigated by transient continuous-flow experiments combined with online HPLC, enabling the generation and acquisition of large reaction data sets.

Characterization of reaction enthalpy and kinetics in a microscale flow platform

We report an isothermal flow calorimeter for characterization of reaction enthalpy and kinetics.

Model-based design of transient flow experiments for the identification of kinetic parameters

Rapid and precise estimation of kinetic parameters is facilitated by transient flow experiments designed using model-based design of experiments.

Kinetics Studies on a Multicomponent Knoevenagel-Michael Domino Reaction by an Automated Flow Reactor

The optimization of complex chemical reaction systems is often a troublesome and time‐consuming process. The application of modern technologies, including automated reactors and analytics, opens the avenue for generating large data sets on chemical reaction processes in a short period of time. In this work, an automated flow reactor is used to present detailed kinetics and mechanistic studies about an amine‐catalyzed Knoevenagel−Michael domino reaction to yield tetrahydrochromene derivatives. High‐performance monoliths as catalyst supports and online coupled HPLC analysis allow for time‐efficient data generation. We show that the two‐step multicomponent domino reaction does not follow the kinetics of consecutive reaction steps proceeding independently from each other. Instead, the starting materials of both individual reactions compete for the active sites on the heterogeneous catalyst, which lowers the rate constants of both steps. This knowledge was used to implement a more efficient experimental setup which increased the turnover numbers of the catalyst, without adjusting common reaction parameters like temperature, reaction time, and concentrations.

Laboratory of the future: a modular flow platform with multiple integrated PAT tools for multistep reactions

The coupling of a modular microreactor platform, real-time inline analysis by IR and NMR, and online UPLC, leads to efficient optimization of a multistep organolithium transformation to a given product without the need for human intervention.

An autonomous microreactor platform for the rapid identification of kinetic models

Rapid estimation of kinetic parameters with high precision is facilitated by automation combined with online Model-Based Design of Experiments.

Multidimensional dynamic experiments for data-rich process development of reactions in flow

The use of multidimensional dynamic flow experiments for reaction profiling and generation of an empirical surface response model for a Knoevenagel condensation reaction is described.

Model-based scale-up and reactor design for solvent-free synthesis of an ionic liquid in a millistructured flow reactor

Based on kinetic investigations in microreactors, a millistructured plate reactor for a solvent-free ionic liquid synthesis was designed by combining several scale-up concepts to maintain thermal stability.

Efficient kinetic experiments in continuous flow microreactors

Transient temperature and flowrates in continuous flow reaction systems allows for the rapid generation of kinetic data.

Design and construction of an open source-based photometer and its applications in flow chemistry

An inexpensive and easy to build photometer using a movable measuring cell for flow chemistry applications was designed with temporal resolution down to 1 ms.

Rapid multistep kinetic model generation from transient flow data

S_NAr reaction profiles were generated using an automated reactor, collected in less than 3 hours, and allowed accurate estimation of kinetic parameters.

Today, the generation of kinetic models is still seen as a resource intensive and specialised activity. We report an efficient method of generating reaction profiles from transient flows using a state-of-the-art continuous-flow platform. Experimental data for multistep aromatic nucleophilic substitution reactions are collected from an automated linear gradient flow ramp with online HPLC at the reactor outlet. Using this approach, we generated 16 profiles, at 3 different inlet concentrations and 4 temperatures, in less than 3 hours run time. The kinetic parameters, 4 rate constants and 4 activation energies were fitted with less than 4% uncertainty. We derived an expression for the error in the observed rate constants due to dispersion and showed that such error is 5% or lower. The large range of operational conditions prevented the need to isolate individual reaction steps. Our approach enables early identification of the sensitivity of product quality to parameter changes and early use of unit operation models to identify optimal process-equipment combinations in silico, greatly reducing scale up risks.