Flow Chemistry – Applications

Ferenc Darvas, György Dormán, Volker Hessel, Steven V. Ley, Ferenc Darvas, György Dormán, Volker Hessel, Steven V. Ley

• Verlag: De Gruyter
• Jahr: 2021
• ISBN:
  9783110693768
• Auflage: 2
• Thema: Technik & Maschinenbau
• Thema: Chemie- & Biochemietechnik

The fully up-dated edition of the two-volume work covers both the theoretical foundation as well as the practical aspects. A strong insight in driving a chemical reaction is crucial for a deeper understanding of new potential technologies. New procedures for warranty of safety and green principles are discussed.

Vol. 1: Fundamentals.

1 Photochemical transformations in continuous-flow reactors

Gabriele Laudadio

Timothy Noël

1.1 Introduction

The use of photons to overcome kinetic and thermodynamic barriers has provided diverse opportunities to access novel and unique synthetic pathways to organic chemists for the construction of organic molecules [1]. In the past decade, photocatalysis has become a vibrant research field with many researchers from both academia and industry implementing this mode of molecule activation into their scientific programs [2]. Despite the rapid progress in terms of synthetic chemistry, some technological issues were encountered. Most of these issues have been associated with the Bouguer–Lambert–Beer law (vide infra), which dictates that photons will be absorbed as they travel through the reaction medium. This means that the light intensity will rapidly diminish with increasing reactor diameters. Hence, photochemistry is perceived as inherently not scalable. However, this statement can be regarded as false when using continuous-flow reactors with small internal dimensions, for example, micro- or millireactor technology [3]. In such reactors, reactants can be continuously introduced into the narrow channels (i.e., <1 mm to several millimeter inner diameter) that are homogeneously irradiated [4]. As a consequence of the high photon flux, reduced reaction times are observed, which allow keeping the selectivity of the selected transformations high [5].

In this chapter, we will provide an up-to-date insight into continuous-flow photochemistry and establish guidelines on how to recognize and solve potential issues associated with this highly valuable activation mode.

1.2 Photochemical versus thermochemical activation of molecules

Classical thermochemical pathways utilize elevated temperatures to increase the reaction rate, as shown by the Arrhenius equation (Equation (1.1); Figure 1.1). The higher the reaction temperature, the more the molecules that have the minimal required energy to transform into products. However, in many cases, the temperature needed for reaction would be so high that the molecules would first decompose. Hence, additional reagents for activation, such as catalysts, acids/bases, or reductants/oxidants, are often required:

(1.1)$k=A^{\left(-\frac{E_{\mathrm{a}}}{RT}\right)}$

where k is the rate constant [unit depends on the order of the reaction], T is the absolute temperature [K], A is the pre-exponential factor, which is a constant for each chemical reaction, E_a is the activation energy for the reaction [J⋅mol⁻¹], and R is the universal gas constant [8.314 J⋅K⁻¹⋅mol⁻¹].

Alternatively, the selective absorption of photons allows production of complex organic molecules that cannot be easily constructed using thermochemical pathways (Figure 1.1). Notable examples are [2 + 2] cycloadditions yielding strained cyclobutanes in a single step [6] and Norrish-type photoreactions that allow homolytic cleavage of C–C bonds [7]. Notably, the kinetics of such photochemical transformations is strongly dependent on the photon flux as follows:

(1.2)$k=\alpha \cdot I^{\beta }$

where k is the rate constant, α is a constant depending on the type of photochemistry, I is the light intensity, and β is a constant depending on the photon flux.For lower light intensities (<around 200–250 W⋅m^–2), β is 1.0 [8]. This means that the rate constant increases linearly with increasing photon fluxes (Figure 1.2); such a situation allows one to tune the reaction rate whilst keeping the reaction temperature constant. In other words, the reaction is purely governed by the flux of photons. Consequently, the vast majority of photochemical reactions can be easily quenched by simply switching off the light, which is important in terms of process safety. Furthermore, it can be easily understood that the higher the photocatalyst concentration, the longer the linear regime.

For intermediate light intensities, β is around 0.5 (i.e., square root behavior), while for higher light intensities, β becomes 0 and, consequently, the reaction rate becomes independent of the light intensity. In the latter situation, all catalyst molecules are permanently active, and not all photons are being absorbed. Such a situation is to be avoided because of the increased energy losses and higher costs. However, at such high photon fluxes, higher catalyst loadings can be used, allowing further reduction in reaction times and thus increasing productivity.

Figure 1.1: Thermochemical (A) versus photochemical activation (B).

Figure 1.2: The average reaction rate in function of the photon flux. (A) Linear regime with β is 1.0 can be observed at lower light intensities. The reaction is photon limited in the whole reactor. (B) For intermediate light intensities, β is around 0.5 and kinetic limitations appear in parts of the reactor. (C) For high photon fluxes, β becomes 0 and, consequently, the reaction rate becomes independent of the light intensity. Kinetic limitations are observed in the entire reactor.

(For further details on this issue, please see Volume 1, Chapter 1, Title: Fundamentals of Flow Chemistry)

1.3 Important considerations when performing photochemistry in microreactors

The importance of light in photochemical transformations is exemplified by several important laws [9]. The first law of photochemistry (also called the Grotthüs-Draper law) details that, in order to be effective, light needs to be absorbed by the reacting system. Since photocatalysis is chromoselective, it is also important to match the emission wavelength of the light source with the absorption characteristics of the photochemical transformation. Typically, an LED with a narrow emission wavelength is selected to cover the absorption maximum of the photocatalyst, which is the wavelength range at which the substance shows maximum absorbance. The matching of light source and photochemical needs is also important to avoid undesired byproduct formation due to the absorption of other wavelengths by the starting material or product (Figure 1.3) [10]. Ideally, only the photocatalyst or the targeted functional group absorbs, while the other components in the reaction mixture are transparent to the irradiation. In other words, light quality is more important than light quantity for photochemical transformations: a light source, which is high in intensity but has a broad spectral emission (e.g., CFLs or medium/high-pressure mercury lamps have multiple emission bands; also the sun has a broad spectral distribution, see Figure 1.3), is less energy-effective and less desired compared to monochromatic LEDs. Consequently, it is important to characterize the purchased light sources, as essentially there are no two light sources that are identical (even LEDs differ slightly in color and intensity) [11]. This can be done with so-called integrating spheres that enable the precise determination of the luminous flux and ensure complete integration of all wavelengths present in the light source.

The first law of photochemistry is also important with regard to tuning the lamp positioning to the reactor; it should be noted that the light decreases with distance from the light source (so-called inverse-square law of light, Figure 1.4). To avoid lost irradiation and to maximize radiation transfer to the reactor, the distance between the light source and the reactor needs to be optimized, as shown in Fig...

Inhaltsverzeichnis

• Title Page
• Copyright
• Contents
• About the editors
• 1 Photochemical transformations in continuous-flow reactors
• 2 Electrochemical processes in flow
• 3 Continuous flow methods for synthesis of functional materials
• 4 Polymer synthesis in continuous flow
• 5 Flow chemistry for nanotechnology
• 6 From green chemistry principles to sustainable flow chemistry
• 7 Flow chemistry in fine chemical production
• 8 Scale-up of flow chemistry system
• 9 Exothermic advanced manufacturing techniques in reactor engineering: 3D printing applications in flow chemistry
• 10 Continuous-flow biocatalysis with enzymes and cells
• 11 Outlook, future directions, and emerging applications
• Answers to the study questions
• Index