Flow Chemistry – Applications

Ferenc Darvas, Volker Hessel, György Dorman, Ferenc Darvas, Volker Hessel, György Dorman

• Publisher: De Gruyter
• Year: 2014
• ISBN:
  9783110392609
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Chemical & Biochemical Engineering
• Index: Technology & Engineering

" Flow Chemistry fills the gap in graduate education by covering chemistry and reaction principles along with current practice, including examples of relevant commercial reaction, separation, automation, and analytical equipment. The Editors of Flow Chemistry are commended for having taken the initiative to bring together experts from the field to provide a comprehensive treatment of fundamental and practical considerations underlying flow chemistry. It promises to become a useful study text and as well as reference for the graduate students and practitioners of flow chemistry." Professor Klavs Jensen Massachusetts Institute of Technology, USA
Broader theoretical insight in driving a chemical reaction automatically opens the window towards new technologies particularly to flow chemistry. This emerging concept promotes the transformation of present day's organic processes into a more rapid continuous set of synthesis operations, more compatible with the envisioned sustainable world. These two volumes Fundamentals and Applications provide both the theoretical foundation as well as the practical aspects.

Part I: Catalysis and activation

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Clemens Brechtelsbauer and King Kuok (Mimi) Hii

1 Catalysis in flow

1.1 Introduction

The practice of catalysis on the industrial scale is closely associated with flow. Indeed, the most extensive and important industrial processes, namely, the Haber−Borsch process, water-gas shift reaction and methanol production, are all conducted under continuous flow (CF) over (heterogeneous) catalysts.

1.1.1 Flow versus batch chemistry

Catalytic flow chemistry embraces some of the most important principles of Green Chemistry and Engineering. The basic function of a catalyst is to lower the activation energy, and therefore the overall energy consumption, of a given reaction, while side-products can be eliminated or reduced by designing more atom-economical and selective processes. Concurrently, engineering tools can be applied to shift an unfavorable thermodynamic equilibrium towards product formation via the operation of Le Chatelier’s principle (as is the case with the three industrial processes mentioned above).

Traditionally, fine and pharmaceutical industrial sectors have largely favored the use of batch reactors, due to the long sequences of unit operations required for the assembly of complex molecules, their low production volume, and last but by no means least, the inherent similarity between a chemist’s round bottom flask and an engineer’s stirred tank reactor. In recent years, however, there is increasing recognition of the potential value of flow processes [1], particularly in combination with catalytic reactions [2], as a way of mitigating the high E-factors associated with the production of complex molecules [3]. Even more importantly, the use of flow reactors enables highly reactive intermediates and gases to be handled safely, thus enabling otherwise hazardous reactions to be performed at scale with no risks to the operator.

In this subchapter, the application of immobilized organo- and metal catalysts in combination with flow chemistry will be discussed.

1.1.2 Development of catalytic reactions and flow for organic synthesis

For the development of catalytic flow processes, it is almost always preferable to work with multiphase reactions, where the catalyst is immobilized to allow it to be easily separated and/or recovered from the product. There are many different ways of keeping catalysts separated from the reaction mixture and/or the product stream, which have been described in recent reviews [4-6]. As organic reactants are almost always dissolved in a solvent, the immobilization of catalyst invariably generates biphasic solid-liquid or liquid-liquid, or sometimes, gas-liquid-solid tri-phasic reaction mixtures. In turn, catalytic activity can be limited by mass transport efficiency or through catalyst deactivation. Almost all pharmaceutically active substances are multifunctional molecules, and it is not uncommon that the reaction partners themselves can poison the catalyst. Furthermore, catalyst leaching is also a significant issue, particularly for the manufacturing of pharmaceutical products where the amount of impurities in the final product (including residual metal) is subject to strict quality control. Catalyst leaching is dependent on several factors, including the nature of the support, the choice of solvent, reaction temperature and the nature of the reactant/products.

Fixed-bed (or packed-bed) reactor: Usually a tubular reactor in which the heterogeneous catalyst is fixed in a packed-bed between filters. The packed-bed can consist solely of the catalyst, or can be diluted through the use of inert material of similar or larger particle size. This can be used to, for example, dissipate heat, or reduce the amount of pump energy that is required to generate the flow, that is, lower back-pressure.

Mass transfer: The movement of chemical species from one physical phase (e.g., a gas) into a different phase (e.g., a liquid or a solid) which is governed by diffusion and convection. The rate at which mass transfer occurs has a direct impact on the rate of catalytic reactions, as only molecules that have been transported to the active centers of the catalyst can react.

Most commercially-available catalysts consist of metallic particles supported on inorganic supports (e.g., alumina, silica, titania, ceria, and zeolites), or incarcerated within organic matrixes (e.g., polyurea, PVP). While these catalysts are highly effective for certain processes (such as hydrogenation reactions), they are not generally sufficiently active or selective enough for organic synthesis. In contrast, catalysts based on discreet organometallic complexes, organocatalysts, or enzymes can offer exquisite (stereo-)selectivity, but will require some form of immobilization, most commonly by attachment onto an insoluble solid support [7], or be retained in a distinct liquid phase, for example an ionic liquid [8].

The ultimate design of the flow reactor is very much determined by the nature of the catalyst and their support, particularly physical properties such as catalyst loading, density, pore and particle sizes. When a fixed-bed reactor is deployed, a compromise often has to be struck between residence time for maximum conversion, mass transport and unacceptable back-pressure; certain polymeric supports, for example polystyrene, swell upon exposure to organic solvents. The size and configuration of a flow reactor are also determined by catalyst activity and stability, which will determine the optimal residence time (rate and selectivity), and heat management (heat of reaction).

The unique physical properties of ionic liquids (IL’s) and supercritical CO₂ (sc-CO ₂) have been widely exploited in the development of continuous flow processes involving organometallic [9] and bio-catalysts [10]. Ionic liquids may be used to dissolve the catalyst, generating a solution which will remain immiscible with the organic solvent during the reaction [11]. To circumvent the mass-transport problem associated with the mixing between two liquid phases, a thin film of the catalytic solution can be dispersed onto a solid support with a high surface area, such as silica, for deployment in a fixed-bed reactor. The method is known as Supported Ionic-Liquid Phase (SILP) catalysis. On the other hand, sc-CO₂ has been substituted for organic solvents in several flow systems. This is particularly useful as the physical property of CO₂ can be varied between a gas and a liquid at different pressures. Therefore, leaching is less problematic with such systems, facilitating recovery and reuse of the catalyst. However, solubility of polar molecules in sc-CO₂ is a major limitation against its wider use in organic chemistry. The potential for CO₂ to react with nucleophilic reagents, for example amines, is another major consideration.

1.2 Reactor types, catalytic reactions and productivity

The productivity of a catalytic flow process is largely dependent upon catalytic activity, defined by the electronic and steric nature of the catalyst as well as the reactant(s). Application of an appropriate flow reactor enhances mass transport (e.g., mixing, adsorption /desorption of substrates onto a catalyst surface), heat management (e.g., effective dissipation of a reaction exotherm) and precise control of residence time (flow rate). For multiphasic systems, the design and implementation of the flow reactor is equally important as the nature of the catalyst. In this subchapter, the turnover frequency (TOF), as mmol(product)/mmol(cat)/h, will be adopted as the indication of productivity. However, it has to be noted that these numbers are often extrapolated and may not correspond to catalyst longevity, that is, the extent of catalyst deactivation and leaching, which can only be revealed by time-on-stream studies. These are not always performed in the laboratory, but are essential before a process can be scaled up into production.

Turnover frequency: A measurement of how often the catalytic cycle occurs per unit of time and, therefore, a quantitative description of the activity and effectiveness of a catalyst. Although strictly speaking associated with the number of catalytically active sites, in practice it gets more conveniently quantified by amount of product generated per amount of catalyst and unit of time.

Catalyst lo...

Table of contents

• Also of Interest
• Title Page
• Copyright Page
• Preface
• Table of Contents
• About the editors
• Contributing authors
• Abbreviations
• Part I: Catalysis and activation
• Part II: Cutting-edge applications in advanced and functional materials
• Part III: Additional features of the Flow Process: in-line analytics, safety and green principles
• Answers to the study questions
• Index