Intensified continuous platform for solid-liquid systems: Zinc oxide production and photocatalytic degradation

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Instituto Tecnológico y de Estudios Superiores de Monterrey

Campus Monterrey

School of Engineering and Sciences

Intensified continuous platform for solid-liquid systems: Zinc oxide production and photocatalytic degradation

A thesis presented by

Fernando Delgado Licona

Submitted to the

School of Engineering and Sciences

in partial fulfillment of the requirements for the degree of Master of Science

In

Engineering Science

Monterrey Nuevo León, June 15^th, 2020

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Acknowledgments

I would like to express my sincere gratitude to my thesis advisors and committee members, Dr. Enrique A. López Guajardo, Dr. Alejandro Montesinos, Dr. Jaime Bonilla Ríos and Dr. Julio González García. I could not imagine having better advisors, it was a privilege working under their guidance: their support, trust and advice was always present throughout my studies. Moreover, I am grateful for the opportunity of meeting and collaborating with excellent faculty members, researchers and friends, whose lessons and example helped me experience a rich professional growth and become a better person: Dr. K.D.P. Nigam, Dr. Sara Núñez Correa, Dr. H. Alan Aguirre Gutiérrez, M.Sc.Eng. Michel Romero Flores, M.Sc.Eng. Omar Campuzano Calderón and BE Chinmay P. Tiwary, BEE Aida Taravat.

Without the financial support of Tecnologico de Monterrey and CONACyT this work would have never been possible.

Finally I would like to give my biggest thanks to my family. My parents: Victor Vicente Delgado and Aurora Licona, and my brother: Victor Alfonso Delgado Licona for their unconditional support.

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Intensified continuous platform for solid-liquid systems: Zinc oxide production and photocatalytic degradation

by

Fernando Delgado Licona

Abstract

Precise control over particle size and shape distribution (PSSD) of metal oxides enables technological advances in various domains of science and engineering. In particular, zinc oxide (ZnO) particles with narrow PSSD have been in the spotlight for their environmental applications. This thesis work presents the design and evaluation of an intensified platform that could implement: 1) the continuous-flow synthesis of tailored ZnO with narrow PSSD and 2) the continuous photodegradation of a model organic contaminant using functionalized ZnO. The platform takes advantage of the milli-fluidic coiled flow inverter (CFI) as a means to intensify mixing intensity. Key process parameters such as flow rate, reactor length, precursor concentration, and reactant molar ratios could be adjusted to obtain desired morphology (flower-like or spindle-like) in monodispersed particle sizes ranging from 280nm – 1500 nm. The interplay of hydrodynamic conditions and reactive conditions is captured in a proposed qualitative model for ZnO particle formation inside a CFI. Results show that an increase in mixing intensity and precursor molar ratio yields a homogeneous increase in particle size, due to an enhancement in mass transfer, while maintaining a narrow PSSD Polydispersity P parameter < 12.3%). At low mixing conditions (Dean number = 20) no significant change in particle size was observed for changes in molar ratio. Functionalized ZnO was shown to possess greater photodegradation capabilities in the visible light range, achieving 90%

degradation in 20 min, while a non functionalized ZnO took ~45 min. The standardized photochemical space-time yield computed for the CFI, 2.97×10-2 m³treated water/ m³

reactor·day·kW, is ~2.7 higher than a photocatalytic microreactor. Thanks to the flow inversions and the formation of secondary flow, the CFI could be positioned as an attractive alternative for continuous photochemistry and hydrothermal synthesis of monodispersed metal oxides as it is a simple, reliable and flexible platform.

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Table of Figures

Figure 2.1.1: a) Hexagonal Wurtzite structure of Zinc Oxide. b) Various crystal planes of ZnO Wurtzite structure. Adapted from Kumar et al. (2015)...17 Figure 2.1.2: Classical Nucleation mechanism. Adapted from Polte (2015)...19 Figure 2.1.3: A) Sintering process, B) Ostwald Ripening, C) Aggregation process.

Adapted from Cao (2014)...20 Figure 2.1.4: General principle of growth mechanism due to coalescence. A) Formed electrical double layer (EDL) around a nanoparticle due to the Gouy-Chapman model which consists of the inner Stern layer and the outer diffuse layer. B) Schematic of the EDL, van der Waals (VdW) and total interaction potential of two nanoparticles. C) Schematic of the generalized mechanism of nanoparticle growth due to coalescence.

Adapted from Polte (2015)...21 Figure 2.2.1 Classification of Zinc Oxide synthesis techniques, selection of challenges and key process parameters. The red highlight represents the synthesis technique used herein, as well as the challenges and key parameters of the technique...24 Figure 2.3.1: Schematic of residence time distribution from a pulse injection (top)

Velocity profile for various flow regimes inside a tube (bottom)...29 Figure 2.3.2: Schematic of Coiled Flow Inverter used in this work...36 Figure 2.3.3: CFI design parameters and corresponding cross-sectional axial velocity contours and radial velocity vectors, showing the effect of centrifugal force and flow inversions. Adapted from Klutz et al. (2015)...37 Figure 2.4.1: Lighting configurations for photoreactors. A) Batch photoreactor, B)

Continuous-flow photoreactor, C) Micro photoreactor, D) Coiled-flow inverter...42 Figure 3.1.1: Experimental setup for the continuous intensified synthesis of ZnO

particles using a CFI. The dashed arrows represent the direction of the Dean flow due to the 90° bends. B) Different CFIs for experimental set 2, they have similar geometrical parameters as the base CFI: Internal diameter 4.6 mm, Curvature Diameter 27.4 mm.. 45 Figure 3.2.1: Modular platform for the study of the Coiled Flow Inverter as photoreactor, continuous flow set up, platform dimensions and geometrical parameters of CFI...55

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Figure 4.1.1: Representative FTIR Spectra for ZnO for all experiments...57 Figure 4.1.2: Representative XRD patterns of ZnO, collection of samples from

experimental set 1, [OH^-/Zn²⁺] = 2, at different Dean numbers...58 Figure 4.1.3: ZnO morphologies at different reactive conditions from experimental set 1 at Zn²⁺ initial concentration of 31.25 mM. Spindle-like and flower-like morphology relate to [OH-/Zn2+] molar ratio...60 Figure 4.2.1: Mean particle size, standard deviation and Polydispersity P-parameter obtained from the DLS measurements for Experimental Set 1...63 Figure 4.2.2: Mean particle size, standard deviation and Polydispersity P-parameter obtained from the DLS measurements for Experimental Set 2...64 Figure 4.2.3: Mean particle size, standard deviation and Polydispersity P-parameter obtained from the DLS measurements for Experimental Set 3 compared to the

measurements from Experimental set 1...66 Figure 4.3.1: Velocity heat map on axial velocity and velocity vectors on radial velocity after a 90° bend for all flow rates...67 Figure 4.3.2: Residence Time Distribution (RTD) F-curve for the 5 turn CFI under different Dean values...69 Figure 4.3.3: Schematic representation of Zinc Oxide particle formation under different hydrodynamic and reaction conditions. A) De ≤ 20 where limiting mass transfer promotes nucleation. B) De ≥ 60 where mixing enhancement by Dean flow promotes uniform growth. Color contours represent concentration of precursors, black lines the velocity profile...71 Figure 4.4.1: Contour Graph of various synthesis parameters: Hydrodynamics (Dean number), [OH^-/Zn²⁺]molar ratio and residence time...74 Figure 4.6.1: Sedimentation at the start (t = 40 min) and at the end (t = 200) of the photocatalytic reaction, flow rate of 80 ml/min...78 Figure 4.6.2: FTIR of functionalized ZnO with APTMS...79 Figure 4.6.3: UV-Vis spectra of Flourescein with ZnO-APTMS-Au/Flourescein solutions with flourescein concentrations of 10 and 1 micro Molar...80

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Figure 4.7.1: Degradation of flourescein induced by visible light in the presence of different photocatalysts A) Normalized flourescein concentration as a function of time in the presence of ZnO-APTMS-Au B) Decay of Fluorescence emission as a function of time...81

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Index of Tables

Table 1: Summary of experiments for the intensified continuous-flow synthesis platform ...47 Table 2: Crystallite size and lattice parameters for different flow and molar ratio

conditions...59 Table 3: Distribution of Zinc complexes...61 Table 4: Xcut values for all Dean numbers tested...76

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Chapter 1 Introduction

1.1 Problem Statement and Context

Zinc oxide (ZnO) has been studied extensively in recent years due to its vast range of applications. The performance of such applications is directly dependent on the morphology and dimensions of the ZnO particle [1]. As a result, research has focused on the development of various processing methods and techniques for the precise control over particle size and shape distribution (PSSD) during the synthesis of ZnO [2–6].

There is a slight distinction on the various reported synthesis methods: 1) methods that require specialized equipment, high-energy inputs, complex process control which operate under special conditions such as vacuum; 2) methods that are considered simpler synthesis pathways, that are easy-to operate and easy-to scale-up [7]. The gap between high efficiency and high productivity, scalability and simpleness has been the topic of study for researchers in continuous flow synthesis, where wet chemical synthesis (like hydrothermal synthesis) is being intensified to meet the highest standards of particle production [8–10].

Process Intensification (PI) is defined as process improvements in terms of materials used (green/ecofriendly), often done at smaller scales (millimeter/micrometer), in a continuous manner, with higher energy efficiencies, and with overall faster, cleaner and safer operation [11]. Economic advantages (productivity and efficiency) as well as environmental issues have been a driver for process intensification in many chemical industries, like particle synthesis [12] and photochemistry [13].

Characterization of intensified synthesis processes is essential to understand particle formation and growth, hence facilitate the prediction of PSSD from selected key process parameters. Several studies have evaluated the effect of flow rate and molar ratio in intensified synthesis processes, for instance, helical reactors and coiled flow inverter (CFI) reactors [14–16]. However, a systematic study on the combined effect of reaction

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and hydrodynamic conditions inside a continuous flow system like the CFI could help achieve control over the PSSD of ZnO (or any other particle) due to its superior mixing capabilities and yet remain as a simple alternative.

The advantages of this system could be translated into new applications, such as continuous photodegradation. An exploratory study to asses the capabilities and limitations of the CFI in a continuous-flow photodegradation platform could shed light to photocatalytic performance of various catalysts alternatives and its loading.

1.2 Purpose of the Study

The purpose of this study is to evaluate different reaction conditions such as flow rate, residence time, temperature, reactant’s molar ratio and pH levels, to characterize an intensified continuous particle synthesis platform.

Additionally, to evaluate the performance of a continuous-flow photodegradation platform, using the CFI as the reactor and ZnO-based photocatalysts.

The findings of this study could provide a small step towards the democratization of particle production technology, for researchers of diverse backgrounds and for a wide range of applications of the given particle. The development of simple, inexpensive and self-contained end-to-end platforms to produce particles with a reliable morphology and size distribution, is benefited from continuous synthesis techniques, on account of their enhanced control over reaction conditions (heat and mass transfer improvement), ease of integration with detectors/sensors and fast reaction times (~minutes), as highlighted by several reviews[17–19] This could be one step into that direction. At the same time, the analysis of presented outcomes could provide insight in the field of continuous particle production, and the resulting process will reveal some of the challenges and limitations that must be overcome for scaling particle synthesis.

Even though some research efforts have been made on the synthesis of metal- oxide particles with intensified reactors [14,15,20–23], the interplay between reaction conditions and hydrodynamics as a mean to tune PSSD is worth exploring further. A

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platform with a CFI reactor component could permit a systematic study to characterize the effect of various hydrodynamic (mixing intensities) and reaction conditions, in which nucleation and growth mechanism and thus, particle shape and size, could be controlled.

Additionally, both the as-synthesized particles and the platform could serve as a basis for a photodegradation study, facilitating rapid evaluation of photocatalysts. For this particular study, the CFI was chosen based on its capabilities on heat and mass transfer enhancement.

1.3 Research Objective

Development and characterization of a milli-fluidic platform for solid-liquid systems: a continuous particle synthesis and continuous photocatalytic platform.

1.3.1 Specific Objectives

The continuous particle synthesis platform is characterized through key process parameters and its outcomes, with the following specific objectives:

1) Hydrodynamic characterization of the CFI.

2) Characterization of the as-synthesized particles (crystalline structure, Particle Size and Shape Distribution, PSSD).

3) Determine the relationship between reactive and hydrodynamic conditions:

I) Changes of molar ratio with respect on mixing intensities (vice versa).

II) Changes in PSSD under limiting mass transfer conditions (low concentration).

III) Changes in PSSD under limiting mixing conditions.

IV) Qualitative model for particle growth inside a CFI.

Exploratory research is conducted on the use of CFI as a continuous flow photoreactor with flourescein as a model pollutant and a zinc based photocatalyst. The characterization of the system would be evaluated by the following criteria:

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1) Photocatalytic performance of functionalized ZnO.

2) Capabilities and limitations of using the CFI as a continuous-flow photoreactor, through the computation of the standardized photochemical space time yield (PSTY) of the system.

1.4 Thesis Overview

The document is divided by chapters, a brief description of them is presented below:

Chapter 2 Presents the state of the art and theory. The basics of particle formation and growth are presented next to various methods of zinc oxide synthesis, their drawbacks and strengths when compared to one another. A brief introduction of continuous and batch processes would be outlined with special emphasis on the advancements on particle synthesis as an intensified continuous process. A Brief introduction to continuous photochemistry systems and how the CFI compares against other alternatives.

Chapter 3 The experimental section of the two continuous-flow platforms: 1) Intensified hydrothermal synthesis of ZnO using the CFI, 2) Continuous photodegradation of flourescein using functionalized ZnO with a CFI reactor.

Chapter 4 Presents how the results from experimental section fit into our understanding of the depicted processes. Discussion on the results will follow all characterization techniques, as well as the interplay between key process parameters such as mixing intensity and reactive conditions. For the photocatalytic system capabilities and limitations will be commented.

Chapter 5 A brief summary of key findings is presented, the robustness of the system is discussed and design criteria for future inquiries is envisioned.

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Chapter 2 Theory and State of the Art

2.1 Metal Oxide Particles: Zinc Oxide

Nanoparticle and micro-particle research have been studied extensively due to the wide range of technological applications. Specifically, metal oxide particles have gained interest in the pharmaceutical [24], biomedical [25], environmental [26], photovoltaic [27], optoelectronics[28] and sensor industries [29,30], among others, due to their unique physical and chemical properties [31]. These include: broad energy band (3.37 eV), large exciton binding energy (60 meV), thermal and mechanical stability at room temperature, hardness (5 GPa), low toxicity, photocatalytic activity, piezoelectric and pyroelectric properties [32]. These properties rely on parameters such as size, morphology and crystalline structure. Generally, it is possible to adjust the chemical and physical properties in any desired manner by precisely controlling one of the mentioned parameters [1]. Achieving narrow particle size and shape distribution (PSSD) not only grants control and reproducibility of the particle properties, but it could enhance the particle performance and chemical activity [26,29]. Thus, the development of various processing methods and techniques for the precise control over PSSD extend scientific and industrial domains where metal oxides could be applied. Control over the synthesis of metal oxide particles requires the understanding of the formation process.

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Among these metal oxides, zinc oxide (ZnO) plays an important role in terms of chemical and physical properties, and also in terms of morphological variety and different synthesis approaches. The vast diversity of simple shapes (rods, spindles, needles, spheres) and hierarchical structures (flower-like, star-like, sheet assemblies) make ZnO a versatile material having a broad set of applications [33]. ZnO is predominantly found in the crystalline wurtzite structure, it belongs to the hexagonal space group P6₃mc with the lattice parameters a = 3.2499 and c = 5.2066 Å. The ratio c/a = 1.602 deviates slightly from the ideal value of c/a =

√

(8/3)=1.633 Å. It contains two formula units of ZnO per unit cell [8]. It has a preferential growth direction in its c-axis, the polar Zn⁺² face [34]; under Miller index the (002) plane and under Bravais-Miller notation the [0001]

plane, the latter shown in Figure 2.1.1. Although is beyond the scope of this work to propose a detailed particle formation and crystallization mechanism inside the Coiled Flow Inverter, a brief introduction on nucleation and growth of ZnO nanostructures in wet chemical synthesis will be presented and used as a framework for this study.

Particle and nanoparticle production can be divided into two approaches: 1) “top- down” which consists of subdividing some bulk material into smaller units and 2)

“bottom-up” which consists of individual building blocks (monomers, atoms, ions,

Figure 2.1.1: a) Hexagonal Wurtzite structure of Zinc Oxide. b) Various crystal planes of ZnO Wurtzite structure. Adapted from Kumar et al. (2015)

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molecules, clusters) assembling to a larger structure. In general, “bottom-up” approaches are preferred due to the formation of uniform particles, like hydrothermal synthesis [7].

As stated above, properties of ZnO rely on parameters such as size, morphology and crystalline structure, thus for any practical application the particles must comply with the following characteristics: 1) identical size for all particles (narrow particle size distribution); 2) identical morphology (shape); 3) identical chemical composition and crystal structure; 4) Monodisperse, i.e. no agglomeration [35]. In particular, wet chemical synthesis is the most common production technique for metal oxide particles [36].

The underlying phenomena that describes the formation of ZnO particles in liquid phase routes (wet chemical synthesis) has been described by classical and non-classical nucleation and growth theories. It is important to consider the lack of fundamental knowledge, as a result of the absence of reliable experimental information about particle growth process. Some authors have even expressed such lack of understanding, by conveying the idea that synthesis of metal crystals (as well as other solid materials) has more to do with “artful skills”, than with science [1,37].

Figure 2.1.2: Classical Nucleation mechanism. Adapted from Polte (2015)

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Classical nucleation theory [CNT], essentially states that the thermodynamic system tends to minimize its Gibbs free energy (maximize entropy). Nucleation is the purely thermodynamic model which describes the process of a first order phase transition [1]. Originally it described the condensation of liquid from the vapor phase, later it was extended to other types of phase transitions, such as crystallization of solids. This thermodynamic theory was also transferred to growth processes of particles such as LaMer’s theory [38]. The fundamental characteristic of this classical approach is that nucleation whether heterogeneous (which occurs at the surface of particles) or homogeneous (which occurs spontaneously and randomly at the liquid) is a separate process preceding particle growth. In other words, there exist two distinct steps, the nucleation step followed by particle growth step without any additional nucleation.

The mechanism can be summarized as follows, as seen in Figure 2.1.2:

I. The concentration of monomers increases, and reaches a critical supersaturation level (Cm) at which homogeneous nucleation is possible but “effectively infinite”.

II. The saturation increases and reaches a level (Cₘᵢₙ) at which the energy barrier (activation energy) for nucleation can be overcome leading to a rapid self- nucleation, burst nucleation.

III. After burst nucleation, the supersaturation level lowers immediately below Cmin; growth then occurs only by diffusion of further monomers in the solution towards particle’s surface. This phenomena is known as heterogeneous nucleation or growth.

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The growth mechanism occurs mainly by diffusion as stated above. Several other mechanisms could take place simultaneously to reduce the overall surface energy such as:

aggregation, sintering or Ostwald Ripening [6], as presented in Figure 2.1.3. Aggregation is the combination of individual nanostructures into larger structures maintaining the original shape of the particles; sintering is the merging of nanostructures into a single new structure with defined solid interfaces connecting each other; Ostwald Ripening refers to a number of individual nanostructures becoming a single one, where a large particle grows at the expense of a smaller one until the latter disappears completely [35].

This happens because smaller particles have larger surface energy than bigger particles, so the system is energetically favored towards bigger particles.

Non-classical nucleation theory (NCNT) leverages from concepts in colloidal stability. DLVO theory (named after Derjaguin, Landau, Verwey and Overbeek) describes the interactions between particles in a liquid phase (colloid) by combining the effects of van der Waals and double layer forces, Figure 2.1.4B. This total interaction potential is fundamental in the explanation of particle growth processes, stating that the final particle size distribution is determined by the increase of colloidal stability [1].

Figure 2.1.3: A) Sintering process, B) Ostwald Ripening, C) Aggregation process. Adapted from Cao (2014)

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A schematic of the generalized mechanism (Figure 2.1.4C) can be summarized as follows:

I. Reduction rate is high, the number of particles increases.

II. Reduction continues but at a much slower rate, the formed particles have weak stabilization and undergo coalescence process leading to a decrease in the number of particles, forming dimers and small clusters.

III. In the last step, the remaining metal particles grow into the existing ones increasing in size until colloid stability is reached and the final particle size is achieved. The particles will encounter the aggregation barrier (peak of the black line in Figure 2.1.4B), this imposes an activation energy two particles have to overcome when they collide, this barrier decreases the probability of coalescence.

Figure 2.1.4: General principle of growth mechanism due to coalescence. A) Formed electrical double layer (EDL) around a nanoparticle due to the Gouy-Chapman model which consists of the

inner Stern layer and the outer diffuse layer. B) Schematic of the EDL, van der Waals (VdW) and total interaction potential of two nanoparticles. C) Schematic of the generalized mechanism of

nanoparticle growth due to coalescence. Adapted from Polte (2015)

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This process can be thought as seeded growth, where the “seeds” are stable coalesced particles (steps I and II) which further growth depends on the stability of the colloid and its “deactivation energy” (an energy barrier at which aggregation or coalescence stops), whereas the CNT deals with an activation energy to form the first thermodynamically stable particles. In this sense the colloidal stability and not the thermodynamic stability determine the minimum particle size. Precursor addition rate and precursor addition order plays a major role in particle growth since it directly affects colloidal stability [1,39].

Particularly for metal oxide systems, like ZnO, both mechanisms have been reported: classical and non classical crystallization pathways [33,40,41]. The factors that determine which one prevails are numerous (concentration, mixing, precursors, capping agents, aging time, etc.) and even when synthesis parameters are adjusted very subtly it is challenging to make the distinction [33]. Achievements in experimental time resolved in- situ measurements have shed light into these two different nucleation and growth theories, but much work is still needed [41,43–46].

Hydrothermal processing consists of hydrolysis and condensation of precursors.

In the case of ZnO hydrolysis and condensation reactions are both multiple-step processes, occurring sequentially and in parallel. Condensation reactions result in the formation of nanoscale clusters of metal oxides or hydroxides, as stated above, these clusters will coalesce to form “seeds” for seeded growth (NCT) or as fully formed nuclei for subsequent growth by diffusion (CNT). The Equations (2.1) – (2.3) exemplify a typical hydrothermal pathway for the formation of Zinc Oxide with NaOH and Zinc Nitrate Hexahydrate [46].

Zn(NO₃)₂⋅6 H₂O+2 NaOH ←→Zn(OH )₂+2 NaNO₃+6 H₂O (2.1) Zn(OH)2+2OH^-←→[Zn(OH )4]^{2 -} (2.2) [Zn(OH)4]^{2 -}←→ZnO+2OH^-+H2O (2.3) Some species are directly related to nucleation of ZnO crystals, while others act as intermediate species or as growth units, it has been reported for ZnO synthesis that the

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Zn(OH)42- specie is responsible for the nucleation of ZnO crystals [15]. Therefore, it is important to obtain the species distribution in order to understand the outcomes of the synthesis procedure [6]. One method to achieve this is from the Specific Ion Theory method [6,47], which is a modification of the Debye-Hückel theory that allows for the prediction of activity coefficients for ions in non-dilute solutions (high concentrations 1 – 5 molality) [48]. The range of species distribution is obtained by providing the reaction conditions such as temperature, initial concentration and pH. For this reason such parameters are essential to control growth and morphology of the ZnO particle.

Nonetheless, particle shape and size control will not only depend on the above-mentioned key parameters, but also on the ones specific to the technique and synthesis method used, which have their particular set of additional key parameters. To understand the decision on the wet chemical route, and the use of continuous hydrothermal synthesis for ZnO a brief introduction to other methods of synthesis will be presented.

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2.2 Synthesis Techniques and Methods

Many synthesis techniques and methods have been developed to precisely control particle size and shape distribution (PSSD). These various methods can be classified according to the phase at which the particles are synthesized: gas, liquid and solid; as well as the different routes to produce the particles: physical, chemical and biological [7].

Figure 2.2.1, summarizes some alternatives to synthesize ZnO particles, in addition to common challenges and particular key parameters to achieve narrow PSSD.

Physical routes are commonly classified as top-down approaches. For instance, laser ablation, is the process of removing with laser portions of zinc metal (bulk material) in a solution. Some characteristic key process parameters include ablation time, fluence and wavelength of the laser. Fine tuning these could achieve narrow PSSD [49]. Another example is thermal evaporation, which consists of vaporizing some bulk material (Zinc metal) at elevated temperatures under certain atmospheric compositions. Although top- down approaches may seem technically simple (single step), they require specialized equipment, high energy inputs and highly controlled environments (pressure, gas

Figure 2.2.1 Classification of Zinc Oxide synthesis techniques, selection of challenges and key process parameters. The red highlight represents the synthesis technique used herein, as well as

the challenges and key parameters of the technique.

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composition and complex process control) making them difficult to scale-up and less attractive for industrial-scale production [30].

Gas, liquid (wet) and solid chemical methods are by definition bottom-up approaches and are expected to produce structures with less defects and homogeneous chemical composition. Among all, wet chemical synthesis methods are most commonly used because they are easy-to-operate (non complex equipment and easy-to-handle reactants), have low operation cost (low energy input) and are easy-to scale-up, some are environmentally friendly [51,52]. Some wet chemical synthesis methods are sol-gel, microemulsion and hydrothermal.

Sol-gel technique involves the preparation of a colloidal solution, a solid dispersed in a liquid (sol) that acts as a precursor which is then transformed to a gel and solid material. This type of chemical technique involves hydrolization reactions, condensation reactions and polymerization reactions. Typical precursors are metal alkoxides (Zn(OR)x) or corresponding chlorides, in an aqueous or organic medium (alcohol is often used). Some key parameters include temperature, concentration of species in the solvent, molar ratio of the precursors, solvent used and any additives such as surfactants or stabilizing agents [28].

Microemulsion techniques involve a thermodynamically stablized dispersion of two immiscible liquids – usually water and hydrocarbon, but several different additives (surfactants) can be added. This type of technique, confines the reaction to just take place inside one of the two phases, gaining control over the precipitated structures, this could be seen as nanoreactors [9,53] since the radii of the micelles is typically smaller than 100nm. Key parameters to ensure narrow particle size and morphology distribution are concentration, reactants, additives (surfactants).

Hydrothermal synthesis refers to the production of particles via chemical reactions in a sealed and heated solution above ambient temperature and pressure, usually ranging from 60°C to 280°C and reaction times from 2 to 24h [6,50,54,55]. In addition to

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these, molar ratio, concentration, surfactants or stabilizers, provide control over particle size and shape.

Nonetheless all these methods have some inherent complications, 1) the use of additional stabilizers such as surfactants and ligands may require extra steps of purification after the synthesis, 2) a wide residence time distribution (RTD) could be expected from the use of conventional reactors such as stirred tanks, leading to concentration and temperature gradients, 3) most of these processes operate in batch, consequently the processes require constant human intervention, furthermore, when trying scale up they may run to elevated energy requirements or wider RTD.

The development of technologies which could reduce the above-mentioned drawbacks, have been reported in the literature in recent years, this is known as process intensification (PI). It is defined as process improvements in terms of materials used (green/ecofriendly), often at smaller scales, with higher energy efficiencies, and with overall faster, cleaner and safer operation [11]. Improving a process in any of these areas could translate in reduced operational costs. Some of these approaches have been reported, such as continuous-flow reactor technology [8–10,26,56], as a means to intensify the production of ZnO particles. For this particular work the synthesis route is highlighted in red in Figure 2.2.1, alongside the corresponding challenges it attenuates and a selection of key processes parameters such as flow rate, precursor concentration, [OH^-/Zn²⁺] ratio and reactor length. In essence a continuous milli-fluidic wet-chemical (hydrothermal synthesis) process will be followed without the use of additional stabilizers.

2.3 Continuous-flow reactors

Continuous-flow micro/milli-reactor technology is preferred due to their high surface-to-volume ratios resulting in an enhancement of heat and mass transfer coefficients and therefore, a complete control over the two steps process of particle formation (nucleation and particle growth) which require low and uniform gradients of both, concentration and temperature throughout the reactor [56]. Particularly,

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homogeneous particle growth in continuous hydrothermal reactors, could be expected from a quick, short and synchronous nucleation (Burst nucleation from Classical Nucleation Theory, CNT) followed by growth under homogeneous chemical conditions.

This can be expected from vigorous mixing or from fast heat transfer (if nucleation is activated by either mixing or reaction temperature) [10]. Higher mixing intensity leads to a homogenization of the reactants concentration and temperature control, and thus, homogenization of the produced particles size [15].

Several studies have explored the use of micro and milli-reactors for the continuous hydrothermal synthesis of ZnO particles [10,16,57–59]. For instance, He et al.

[58] showed an apparent control over the size of ZnO nanoparticles (rods) by tuning the flow rate in a coiled polytetrafluoroethylene capillary tube micro-reactor: internal diameter (di ) was 0.5 mm with a total reactor length (L) of 2 m. Variation of key process parameters such as temperature (80°C, 100°C and 120°C) and flow rate (196.3, 39.26, 19.63 μL/min) presented the following effects: Increasing the temperature led to bigger particles (faster growth), increased flow rate led to a reduction in mean particle size of ZnO (size ranged from 8 to 50 nm). The experiment used EDA (ethylenediamine) as a surfactant to foment the heterogeneous growth of ZnO and CTAB (cetyltrimethylammonium bromide) for surface passivation of the reactor inner walls and as a stabilizer.

Li et al. [10] reported an apparent increment in aspect ratio in flower-like structures as both the [OH^-/Zn²⁺] molar ratio and initial concentration increased. Unlike He et al. [58], the experimental arrangement introduced a second immiscible phase into the reactor (di = 1 mm, L = 1 m) allowing the formation of slugs that act as a “moving batch micro-reactors”. The ranges of concentration used were 0.02 – 0.05M for Zn²⁺ and 0.2 – 1.0M for OH^-with mean particle diameters of 0.428 – 3.523μm. Size and shape was controlled by changing concentration conditions on both precursors. Overall, narrow PSSD were obtained due to the two phase configuration, since the formation of slugs greatly improves the homogenization of reactant concentration and causes a significant

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enhancement in heat and mass transfer. Hence, reducing concentration and temperature gradients.

Both continuous approaches, micro-fluidic hydrothermal synthesis and segmented-flow hydrothermal synthesis, exemplified by the above mentioned experiments, present a viable way for the production of ZnO. Nonetheless, the obtained results from He et al. [58] may not take into account that the reacting system could be controlled by the dispersion of the components within the micro-reactor. Since the micro- coiled tube used may hydrodynamically behave as a straight-tube under laminar flow, the system may present low mixing intensities at which the formed particles and precursors could experience non homogeneous chemical conditions. Without the use of stabilizers or surfactants the products may present a broad PSD. This is greatly improved upon by Li et al. [10] however, the introduction of two immiscible liquids may require further purification steps through the synthesis, which could be avoided by modifying the hydrodynamic behavior using devices with the same mixing intensity achieved in slug flow, yet without recurring to two-phase flow.

Higher mixing intensity could be accomplished in part via a narrowing of the residence time distribution of a given system. A narrow residence time distribution ensures all chemical species experience the same time inside the reactor, hence the uniform chemical environment is neither axially nor radially dispersed.

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2.3.1 RTD in continuous flow reactors

To prevent particles or chemical species experience different Residence Time Distribution (RTD) a proper control over the hydrodynamics of the system is required to attain a near-plug flow behavior, which may lead to a narrow particle size distribution (PSD).

Residence time in continuous flow reactors can be idealized as a plug-flow reactor (PFR), where there is no axial or radial dispersion, in other words, all the material leaving the reactor have been inside it for exactly the same amount of time. Ideal PFRs have a perfectly narrow residence time distribution, a single-valued residence time [60]

As can be seen from Figure 2.3.1, residence time depends on the average flow velocity profile. Different flow regimes: laminar, transitional (not shown) and turbulent, are related to different velocity profiles. The Reynolds number is defined as the ratio between inertial forces and viscous forces (2.4), it can be used to predict the flow regime

Figure 2.3.1: Schematic of residence time distribution from a pulse injection (top) Velocity profile for various flow regimes inside a tube (bottom)

Concentration

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of a liquid under different fluid flow conditions, in laminar flow viscous forces dominate over inertial forces, and vice versa for turbulent flow regime [61].

R e=ρ_sol⋅v⋅di

µ_sol =inertial forces

viscous forces (2.4)

Most continuous micro and milli-reactors operate under Reynolds numbers below 2100 which corresponds to a laminar flow regime. The contrast between laminar flow and plug flow velocity profile leads to a difference in residence time, as observed in Figure 2.3.1. Laminar flow residence time has a distribution rather than a definite value like the plug-flow. The distribution could be narrowed by reaching turbulent-flow-like profiles (or even plug-flow), nevertheless, for milli/micro-fluidic devices it is difficult to reach such high Reynolds numbers (due to high pressures, >10 bar [62]) so other techniques must be used [63]. Mixing improvements on devices of this length scale (millimeters and micrometers) can be done via two routes: Active mixing, achieved by an outside energy input, to promote mixing such as acoustic energy, electric potential, impellers, or any other moving part that is not part of the system [64]. The other route, passive mixing, concerns all mixing that takes place by the alteration of the structure or geometry of the fluid channel. The energy of the flow is used to reduce the diffusion length scale of the fluid and simultaneously increases contact interfaces, hence, enhancing mixing. This phenomena is known as chaotic advection. Passive mixing strategies include: introduction of obstacles to the fluid, collision of jets, creation of thin lamellae, creation of eddy-based flow patterns, among others [63,64].

It is important to understand which phenomena influence RTD and how to characterize it. For any given flow inside a channel transport by diffusion is taking place as long as a concentration gradient exists. In an ideal plug-flow reactor, it is expected that no radial diffusion is taking place, since concentration and the velocity profile is strictly identical along the cross section. Nonetheless, non-ideal continuous tubular reactors could present axial dispersion, that is, convective diffusion (dispersion) allows particles or molecules diffuse ahead or behind the molar average velocity [61]. The model that

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captures deviations from ideal plug-flow, in terms of axial dispersion, have been studied and derived for straight tubes [65,66], this model, the advection-dispersion model (ADM), assumes that concentration is radially uniform, and is represented by Equation (2.5):

∂C

∂t =−v ∂C

∂ Z+D_ax∂²C

∂ Z² (2.5)

Where C is the tracer concentration that could be changing through time and axial coordinate Z. The average axial velocity of the fluid is given by v and the axial dispersion coefficient, Dax, is assumed to be constant and independent of both the axial position and the tracer concentration [67].

To characterize the ADM under different conditions the dimensionless number Bodenstein (Bo) could be adopted. The use of dimensionless numbers in engineering gives us insight into characteristic phenomena or properties of any given system. These in turn can be used to parameterize or simplify (group different phenomena) the behavior of the system as have been explained for the previously presented dimensionless number (Re). To obtain a given dimensional number, the model should be scaled so that all dimensions are eliminated. This could be done by introducing of certain dimensionless ratios. For instance, shown in Equation (2.6) is the dimensionless time t*, the ratio of time and mean residence time τ, Equation (2.7) scales the axial position Z with the reactor length L, and Equation (2.8) shows the ratio of the concentration (C) to the initial concentration (Co)

t^∗=t τ , τ=L

v (2.6)

Z^∗=z

L (2.7)

C^∗=C

C_o (2.8)

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Introducing these ratios Equations (2.6 – 2.8) to the ADM Equation (2.5) would lead to the creation of the dimensionless number Bo.

∂C^∗

∂t^∗ =−∂C^∗

∂ Z^∗ + 1 Bo

∂²C^∗

∂ Z^∗2 (2.9)

Bodenstein (Bo) is defined as the ratio between convective mass transport, advection (bulk movement of the fluid), and the mass transport through axial dispersion Equation (2.10). It can also be interpreted as the combined contributions of the axial dispersion time, tD,axial, and the radial dispersion time tD,radial [68]. Ideal PFR would have an infinitely high Bo, systems with Bo > 100 have small deviations from the ideal PFR, however, Bo higher than 1000 is enough to consider it plug-flow in any practical application [69].

Bo=v⋅L D_ax= L²v

D_mL+192D_mL

d_i²v =t_{D, axial}

τ +192 τ

t_{D ,radial} (2.10)

There are two main axial dispersion models for the coefficient Dax inside a tube with circular cross section, mainly it would depend on how much the system behaves as a plug-flow reactor. Having said this, the Dax could be computed either by Equation (2.11) if Bo < 100 and by Equation (2.12) if Bo > 100. Dm stands for the molecular diffusivity of the tracer [67]. The right side of Equation (2.10) is obtained by plugin Equation (2.11) into the definition of Bodenstein. It may be clear now, that when a tubular reactor behaves as an ideal plug-flow, no axial dispersion is present, as the term for axial dispersion time is neglected.

D_ax=Dm+ v²d_i²

192 D_m (2.11)

D_ax= v²di2

192 D_m (2.12)

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The residence time distribution function E(t) Equation (2.13) measures the effects of the axially-dispersed flow model from Aris and Taylor. An axially-dispersed flow (also known as Taylor dispersion) would tend to broaden the residence time distribution of a concentration pulse or step injection, a deviation from the single-valued residence time of the plug-flow in Figure 2.3.1. It is possible to find the fraction of molecules in the exit stream that have spent any range of time between t and t+dt in the reactor by integrating E(t), similar to a probability density function (PDF):

E(t )= C(t )

∫

0

∞

C(t )dt (2.13)

The cumulative distribution function F(t) Equation (2.14) is defined as the integral of the residence time distribution function (E(t)). It represents the fraction of molecules exiting the reactor that have spent a time t or less in the reactor. When the input of a tracer is an ideal step, then the expected output is an F-curve, when the input of a tracer is a Dirac delta function (pulse input), then the expected output is an E-curve, which integral is the F-curve.

F(t)=

∫

0

∞

E(t)dt (2.14)

When assuming small axial dispersion (Bo > 100) an expected dimensionless residence time distribution E(t*) can be obtained after a pulse injection, shown in Equation (2.15). This can be then integrated to obtain a dimensionless F-curve, for plug- flow systems [70]. Similarly, it could be obtained analytically when assuming a material flowing after another through the same tube (step injection), Equation (2.16).

E(t^∗)=1

2

√

^Bo^π ^exp

^[

^−(1−t⁴^∗⁾^2Bo

^]

^(2.15)

F(t^∗)=C C₀=1

2

{

^erf

^[ √

^Bo⁴ ^⋅(t^∗⁻¹⁾

^]

^{+erf (}

√

^Bo⁴ ⁾

}

^(2.16)

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Where t* is the dimensionless time, C and Co correspond to the concentration through time and the initial concentration respectively. Near-to-ideal plug flow reactors can be modeled using these equations, each reactor would have a characteristic RTD, F- curve and Bodenstein number, such that comparisons between them are possible.

To compare the narrowness of a given set of RTDs, the Relative Width, Equation (2.17), is a simple criteria that measures how fast a reactor outputs the total content of material inside it.

Rw=t_{0.5 %}^∗

t^∗_99.5% (2.17)

Were the t*0.5% and t*99.5% represents the dimensionless time at which the 0.5% and 99.5% of the maximum concentration is obtained. A value of Rw = 1 is the narrowest possible distribution, everything comes out at the same time, as can be seen in the plug flow pattern in Figure 2.3.1.

Insight from these measurements, Rw, Bo, F(t*), allow comparison between different reactor designs and different flow conditions based on RTD. As introduced earlier, passive mixing (for instance, changes in reactor geometry) could introduce chaotic advection, significantly improving RTD when compared to a straight tube. Coiled geometries have been proven effective in heat transfer equipment [71], not only because of their compactness when compared to a straight tube, but because of the presence of secondary flow (Dean flow, which will be covered in the Section 2.3.2) along the tube.

The secondary flow patterns reduce the axial dispersion of the system by enhancing radial mixing, narrowing the RTD and improving heat and mass transfer coefficients.

Helical reactors have been also explored as a synthesis platform, due to the formation of secondary flow, which, as discussed above improve mixing intensity and narrowing of the RTD. Choi et al. [15] studied the effect of flow rate in the assembly of ZnO nanocrystals in a helical reactor (di = 1.22 mm, L = 1.3). He found that increasing

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the flow rate promoted the aggregation and assembly of mesoporous structures, but did not study the effect on growth or particle size distribution. Wu et al. [23] investigated the synthesis of silver nanoparticles in a helical reactor (di = 0.762 mm, L = {0.6, 0.95, 1.3 m}) without the use of surfactants or capping agents (single phase). In this experiment, increasing the flow rate led to a decrease in particle size, as well as a narrowing in PSD;

while increasing residence time (by increasing reactor length) led to an increase in particle size.

These studies show favorable results in terms of PSD, they support the idea of using different geometrical reactor configurations to promote the necessary hydrodynamic conditions in order to remove the need of surfactants or stabilizing agents.

A simple modification of the helical geometry could further improve mixing intensities and promote higher chaotic advection. A complete flow inversion, made possible by simply bending the helical tube, enables the change of direction of these secondary flows, further narrowing the RTD [72]. This type of coiled geometry is called Coiled Flow Inverter (CFI) elucidating the inversion of velocity profiles at each bend.

2.3.2 Coiled Flow Inverter (CFI)

Figure 2.3.2: Schematic of Coiled Flow Inverter used in this work.

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Coiled geometries such as the Coiled Flow Inverter (CFI, Figure 2.3.2) [73–75] is a passive mixing device which has been shown as a promising crystallizer technology due to the narrower RTD and enhanced heat and mass transfer coefficient obtained at certain flow conditions [76–78]. This enhancement is related to both, flow inversions at the 90° bends and to an increase in radial mixing (Dean flow), diminishing the effects of axial dispersion. As shown in Figure 2.3.1 inside a straight tube under laminar flow, a Poiseuille flow pattern will form. Instead, inside a coiled geometry, the characteristic Poiseuille profile will be deformed, shifting the point of maximum velocity from the center of the tube towards the concave wall of the channel, this is due the centrifugal force experienced by the fluid in the curved pipe. As a result, a velocity gradient between the now-shifted maximum velocity region and the walls (since velocity near the wall is low) is created. This velocity gradient successively causes an increase in pressure, so the local velocity near the wall is not sufficient to balance the pressure gradient. Driven by the pressure difference, fluid recirculates in the form of vortices directed from the center of the channel towards the outer channel wall. These vortices are called Dean vortices [56]. Moreover, inside a CFI, the flow inversions causes the changes in direction (90°) of both the now-shifted region of maximum velocity and the Dean vortices, as can be seen in Figure 2.3.3.

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As it can be seen from Figure 2.3.3 the pressure and velocity gradient imbalance caused by Dean instability results in Dean vortices, as shown by the black arrows. The black arrows represent the velocity vector along the transverse plane of the pipe.

Furthermore, 90° direction changes of the secondary flow and the deformed Poiseuille pattern will further narrow the RTD, in Figure 2.3.2 three bends are shown.

The intensity of these vortices is captured by the dimensionless number by the same name, the Dean number. It encompasses the ratio of inertial and centripetal forces to viscous forces (2.18).

Figure 2.3.3: CFI design parameters and corresponding cross-sectional axial velocity contours and radial velocity vectors, showing the effect of centrifugal force and flow

inversions. Adapted from Klutz et al. (2015)

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De=

√

⁽¹²Inertial forces⋅centripetal forces)

viscous forces =R e

√

⁽^λ¹^c⁾ ^(2.18)

Where the value λc (2.19) represents the curvature ratio between the outer curvature (Dc) and the internal diameter (di).

λ_c=D_c

d_i (2.19)

The higher the intensity of these vortices the better radial mixing effect. This enables the fluid components to experience narrower residence time distribution, since the portion of time any particle remains in a given axial velocity region would now be affected by the secondary flow. Generally, under laminar flow, radial transport is dominated by Brownian motion (diffusion), but as Dean vortices appear and increase their strength, the fluid components could follow those radial streamlines, flattening the characteristic Poiseuille curve and resembling a plug-flow profile. A coiled reactor, with a high enough Dean number would reduce the axial dispersion, since secondary flow promotes an even distribution of the material over the cross section [80].

From Equation (2.18) it is straight forward that to increase the Dean number, hence the secondary vortices, it is needed to either increase the Reynolds number or decrease λc, in other words, decrease the ratio of the curvature diameter (Dc) and the internal diameter (di). Nonetheless, other dimensionless numbers, such as the Torsion parameter (T, 2.20) and the modified Torsion parameter (T*, 2.21), have been reported to aid in reducing the RTD.

T= p

2⋅π⋅r_c⋅R e (2.20)

T^∗=π⋅rc⋅R e

p (2.21)

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Both of these numbers take into account the pitch distance (p), the length of one turn within the helical coil and the Reynolds number. They describe how different the geometry is compared to a straight tube, we can imagine a straight tube with infinite pitch. Stephan Klutz et al. recommend a T* > 500, this would result in the narrowest RTD curve [70]. Curvature ratio λc effect is less significant when operating at low Reynolds numbers (< 1) but becomes quite significant at higher Reynolds numbers (>10), overall λc

< 10 is recommended [81].

2.3.3 Continuous flow synthesis in CFI

Several studies have used the micro-CFI as a medium of tuning the particle size and distribution of silver-palladium, silver and gold nanoparticles [14,22,82]. However, those studies were focused more on controlling the particle size by changing the reaction conditions such as ratio of the reactants, while using multiple CFIs for seed preparation and growth, rather than studying the effect of hydrodynamics (increasing the mixing intensity by increasing the flow rate) on the different stages of particle synthesis [14,22].

Moreover, Baber et al.[82], focused on the hydrodynamics by the combination of static mixers (coaxial flow reactor and micro-CFI) to obtain nanoparticles with a controlled PSD. This combination of reactors resulted in an increase in particle size while achieving a narrow PSD with an increase in flow rate. However, the focus of this study was in the characterization and capability of nanoparticle production mainly of the coaxial flow reactor. The hydrodynamics of these intensified equipment plays a fundamental role on control of shape and size of the as-produced particles since the reactants could experience the same chemical environment.

Even though research efforts have been made on the synthesis of metal-oxide particles with enhanced hydrodynamics equipment, the interplay between reaction conditions (concentration and molar ratio) and hydrodynamics as a mean to control shape and size of the particles needs further characterization. That is, a systematic study is needed to comprehend the effect of various hydrodynamic (mixing intensities) and reaction conditions in which nucleation and growth mechanism and thus, particle shape and size, could be easily controlled. Therefore, the present research explores the use of a

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CFI (at different flow conditions and OH^- /Zn²⁺ratios) as a mean to carry out the intensified hydrothermal synthesis of ZnO with control over particle shape and size in the simplest and fastest way possible.

2.4 Zinc oxide in a continuous-flow photochemistry system

Having discussed the CFI as a continuous synthesis platform for the production of highly controlled zinc oxide particles (over shape and size), an exploratory application study involving them is presented below. Insight from both components, the CFI and ZnO particles could lead to improvements in other engineering domains. As discussed in the first couple of paragraphs of this chapter, Zinc Oxide has been in the spotlight for a multitude of applications that arise from its morphological diversity. Zinc Oxide properties can further be tuned with techniques such as functionalization [30].

Meanwhile, the CFI has positioned as an attractive alternative for continuous flow applications, improving mixing, hence, reaching high mass and heat transfer coefficients.

Therefore, the CFI could be useful in other solid-liquid continuous flow applications.

One of such applications that could benefit not only from ZnO properties, but from the CFI as a continuous flow platform, is photochemistry [83].

From the particle perspective, Zinc Oxide has been recognized as a competitive photocatalyst to be used in photodegradation of organic pollutants due to its low production cost (up to 75% compared to TiO2 and Al2O3), ecofriendly, low toxicity and overall efficiency in the absorption across the solar spectrum, which could be further tuned to the visible region [84]. Following sustainability efforts, it is sought to tune the photocatalytic activity of ZnO to the visible wavelengths. The sun is the ultimate energy source, irradiating nearly 3×10²⁴ J per year, and around 40% of its energy is in the visible range (400 – 700 nm) [85]

From the reactive system perspective, an analogous trend to the continuous manufacturing of particles is seen: advancements in process intensification are taking photoreactor systems move from batch systems, that present large timescales (hours to days) with significant heat, mass and photon transfer inefficiencies [86] into miniaturized

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(reducing the characteristic length scale) and continuous technologies, such as milli/micro-reactors which greatly enhance photon absorption efficiencies [87]. In spite of this, inherent limitations are faced by these types of systems, some have already been discussed, mainly the Taylor dispersion and rate of mass transfer in laminar flow regimes can limit the productivity of the system. Additional limitations include the transport of solid particles inside the system, the amount of photocatalyst per unit volume the system can handle without incurring into clogging by sedimentation, fouling or limiting depth of light penetration (due to absorption and scattering). Lastly, light irradiation uniformity could result problematic when dealing with complex 3D shapes such as the CFI [83].

It has already been discussed that chaotic advection could diminish mass transfer limitations. Clogging could happen when sedimenting particles block the channel preventing flow. There are several techniques to diminish clogging, such as carrying reactions in two immiscible liquids, adding another phase (Gas-Liquid-Solid), increasing the velocity of the fluid, increasing chaotic flow patters, or completely abandon the use of suspended particles (slurry reactors) and instead use immobilized thin-film catalysts [83].

Light depth penetration has been typically improved by reducing the length scale of the reactor down to centimeters, or even millimeters. Several strategies for a uniform light irradiation have been proposed, from flexible LED strips [83,88] and simple planar surfaces that ensure complete light irradiation [83,89], to optical fibers guiding the light to specific region of the reactor [83,90] or simply taking advantage of the sun irradiation [67,75].

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Using the CFI as a platform for the intensification of continuous photochemistry (which no one has to date [78]) shows alignment with some of the most promising trends found in literature [83]. Having the particles suspended assures higher number of active sites per unit volume of the reaction medium [92] when compared to immobilized catalysts. The length scale (di ~ 5 mm) allows for high throughput and high productivity when compared to microreactors which often operate in the order of μL/min [93], as well as reducing the need for deep light penetration. There is evidence coiled geometries that present Dean vortices enhance particle fluidization, thus reducing the possibility of clogging by sedimentation [20,21], as well as improve homogenization of the particle concentration in the radial direction. This could in turn allow for higher catalyst loadings, without hampering mass and photon transfer, hence the residence time necessary to achieve relevant efficiencies and productivities improves. Hydrodynamic advantages are

Figure 2.4.1: Lighting configurations for photoreactors. A) Batch photoreactor, B) Continuous- flow photoreactor, C) Micro photoreactor, D) Coiled-flow inverter

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seldom considered in the design of these systems, as a result, at least one of the following parameters will be adjusted to impractical levels, concentration, flow rates, reactor length and light intensity, for example, low concentrations due to sub-optimal mixing in laminar flow regime, would require longer reactor lengths or higher intensity bulbs, Figure 2.4.1.

An evident major obstacle still persists: light uniformity in a complex 3D geometry. For the CFI in particular, this has been studied by analyzing different multiaxial coplanar irradiation configurations with light-source-to-reactor distances less than 5 cm, and comparing it to single-source irradiation (light source distance between 1 – 5 cm) jointly with a reflective surfaces, where the latter seems to improve light uniformity across the 3D geometry [78].

Intensified continuous platform for solid-liquid systems: Zinc oxide production and photocatalytic degradation

Instituto Tecnológico y de Estudios Superiores de Monterrey

School of Engineering and Sciences

Intensified continuous platform for solid-liquid systems: Zinc oxide production and photocatalytic degradation

Fernando Delgado Licona

Acknowledgments

Abstract

Contents

Table of Figures

Index of Tables

Chapter 1 Introduction

1.1 Problem Statement and Context

1.2 Purpose of the Study

1.3 Research Objective

1.4 Thesis Overview

Chapter 2 Theory and State of the Art

2.1 Metal Oxide Particles: Zinc Oxide

√

2.2 Synthesis Techniques and Methods

2.3 Continuous-flow reactors

∫

∫

√

[

]

{

[ √

]

√

}

√

√

2.4 Zinc oxide in a continuous-flow photochemistry system

^[

^]

^[ √

^]