Advances in Microﬂuidic Synthesis of Solid Catalysts

: Heterogeneous catalysis plays a central role in the chemical and energy ﬁelds, owing to the high and tunable activities of solid catalysts that are essential to achieve the favorable reaction process efﬁciency, and their ease of recycle and reuse. Numerous research efforts have been focused on the synthesis of solid catalysts towards obtaining the desired structure, property and catalytic performance. The emergence and development of microﬂuidic reactor technology provide a new and attractive platform for the controllable synthesis of solid catalysts, primarily because of its superior mixing performance and high heat/mass transfer efﬁciency. In this review, the recent research progress on the synthesis of solid catalysts based on microﬂuidic reactor technology is summarized. The ﬁrst section deals with the synthesis strategies for solid catalysts, including conventional methods in batch reactors and microﬂuidic alternatives (based on single- and two-phase ﬂow processing). Then, different kinds of solid catalysts synthesized in microﬂow are discussed, especially with regard to the catalyst type, synthetic process, structure and property, and catalytic performance. Finally, challenges in the microreactor operation and scale-up, as well as future perspectives in terms of the synthesis of more types of catalysts, catalyst performance improvement, and the combination of catalyst synthesis process and catalytic reaction in microreactors, are provided. presented photocatalytic activity in the degradation of methylene blue dye because of the signiﬁcant increase in surface defects. These results demonstrate that the change in synthetic techniques altered the surface properties (e.g., extent of oxygen vacancies and defects at the surface) and optical band gaps, and these changes in turn affected the photocatalytic properties of SnO 2 nanoparticles [132].


Introduction
Solid catalysts have attracted a tremendous amount of research interest due to their wide range of applications in science and industry [1,2]. According to statistics, more than 80-90% of industrial chemical processes use catalysts to work effectively, and the vast majority of produced chemical products and commodities involve catalytic conversions [3,4]. Moreover, the research hotspots in recent decades, such as water purification, fuel cell, solar cell, hydrogen generation and storage, biodiesel production, drug delivery and release, are inseparable from the application of catalysts [5]. Nowadays, catalysts have become the core of the entire chemistry, chemical engineering, energy and environmental industries [6].
The emergence of nanotechnology has brought new opportunities to the research and evolution of solid catalysts [7]. Micro-and nanoscale catalysts have been found to possess better catalytic properties than their macroscale counterparts [8,9]. When the size of catalytic materials decreases to micrometer or even nanometer level, its specific surface area, surface steps, wrinkles and defects increase, resulting in the increase in catalyst active sites, which can greatly improve the catalytic activity and selectivity [10,11]. Accordingly, the preparation of micro-and nanoscale catalytic materials can improve the application efficiency of catalysts to a large extent. In addition, the preparation method significantly affects the crystal structures (e.g., shape, size, composition) and texture properties (e.g., porosity, specific surface area) of catalysts, which has a great impact on the catalytic performance. Therefore, the controllable synthesis of catalysts is crucial to enhance the efficiency and sustainability of catalytic processes [12]. ondly, a chemical reaction occurs to form a supersaturated solution; thirdly, the precipitate is formed in the crystallization process including nucleation, growth and agglomeration ( Figure 1) [47]. The precipitate becomes the solid catalyst typically after filtering, washing, drying, and calcination/reduction process. When the concentration of the target product reaches supersaturation in the solution, it will be deposited by homogeneous or heterogeneous nucleation. During the precipitation process, the nucleation, growth and agglomeration of crystals have a great influence on the product particle size and morphology [48]. In order to generate non-agglomerated particles with a narrow particle size distribution, it is required that all nuclei are formed nearly simultaneously, and no secondary nucleation or particle agglomeration occurs during the subsequent crystal growth process. The particle size and its distribution and morphology of the target product depend on the relative rates of nucleation, crystal growth and agglomeration kinetics [49], which are therefore influenced by reaction conditions (temperature, concentration, solvent composition, pH, etc.) [50,51]. The morphology, size and structure have a significant impact on the physical and chemical properties of catalyst particles, which in turn affect their catalytic performance [15]. Therefore, it is of great significance to study the preparation method of catalysts to improve their catalytic activity and selectivity.

Synthesis Principle
The synthesis of (nanoscale) solid catalysts is largely based on wet chemical methods both in batch reactors and in microfluidic synthesis fields, which includes three stages: firstly, the reactant molecules are (uniformly) mixed (i.e., the micromixing process); secondly, a chemical reaction occurs to form a supersaturated solution; thirdly, the precipitate is formed in the crystallization process including nucleation, growth and agglomeration ( Figure 1) [47]. The precipitate becomes the solid catalyst typically after filtering, washing, drying, and calcination/reduction process. When the concentration of the target product reaches supersaturation in the solution, it will be deposited by homogeneous or heterogeneous nucleation. During the precipitation process, the nucleation, growth and agglomeration of crystals have a great influence on the product particle size and morphology [48]. In order to generate non-agglomerated particles with a narrow particle size distribution, it is required that all nuclei are formed nearly simultaneously, and no secondary nucleation or particle agglomeration occurs during the subsequent crystal growth process. The particle size and its distribution and morphology of the target product depend on the relative rates of nucleation, crystal growth and agglomeration kinetics [49], which are therefore influenced by reaction conditions (temperature, concentration, solvent composition, pH, etc.) [50,51]. The morphology, size and structure have a significant impact on the physical and chemical properties of catalyst particles, which in turn affect their catalytic performance [15]. Therefore, it is of great significance to study the preparation method of catalysts to improve their catalytic activity and selectivity.

Batch Processes
The commonly used methods for solid catalyst synthesis in laboratory and industry mainly include chemical precipitation method, hydrothermal method, solvothermal method, sol-gel method and thermal decomposition method, etc. Generally, these methods are operated in batch mode, and Table 1 summarizes some solid catalysts obtained by these batch processes.
The precipitation method is the most frequently used one, which uses the chemical reaction between a salt solution and a precipitant solution to form a water-insoluble salt ( Figure 2a) [52]. For example, ZnO nanoparticles were synthesized by the low temperature chemical co-precipitation between Zn(ac)2·2H2O and NaOH aqueous solution, which can be used as an effective catalyst for the degradation of Congo red dye [53]. Carbon nanotube (CNT)-Cu2O nanocomposites were successfully prepared via a facile chemical precipitation method by adding ascorbic acid solid and NaOH solution to the mixed solution of carboxylated carbon nanotubes and CuCl. The nanocomposites exhibited excel-

Batch Processes
The commonly used methods for solid catalyst synthesis in laboratory and industry mainly include chemical precipitation method, hydrothermal method, solvothermal method, sol-gel method and thermal decomposition method, etc. Generally, these methods are operated in batch mode, and Table 1 summarizes some solid catalysts obtained by these batch processes.
The precipitation method is the most frequently used one, which uses the chemical reaction between a salt solution and a precipitant solution to form a water-insoluble salt ( Figure 2a) [52]. For example, ZnO nanoparticles were synthesized by the low temperature chemical co-precipitation between Zn(ac) 2 ·2H 2 O and NaOH aqueous solution, which can be used as an effective catalyst for the degradation of Congo red dye [53]. Carbon nanotube (CNT)-Cu 2 O nanocomposites were successfully prepared via a facile chemical precipitation method by adding ascorbic acid solid and NaOH solution to the mixed solution of carboxylated carbon nanotubes and CuCl. The nanocomposites exhibited excellent catalytic performance in the reduction of p-nitrophenol [54]. As a catalyst for NO oxidation, MnO x -CeO 2 could be obtained by the co-precipitation of the mixture of Ce(NO 3 ) 3 ·6H 2 O and Mn(CH 3 COO) 2 ·4H 2 O with (NH 4 ) 2 CO 3 solution [55], as summarized in Table 1.
In the hydrothermal method ( Figure 2b) [56], a precipitant or chelating agent is added to the salt aqueous solution to prepare the precursor. Then, the mother liquor containing the precursor is poured into a closed vessel. Under the reaction conditions of high temperature and high pressure in a closed system, the insoluble substances are dissolved and recrystallized by using air in the kettle or the added specific oxidant. Many inorganic functional materials, metal oxide and composite oxide nanomaterials have been successfully prepared by this method (Table 1). For instance, Ni-Ce-ZrO δ catalysts were obtained via a one-pot hydrothermal method by mixing the Ni/Ce/Zr metal salt mixture solution and urea solution and keeping at 105 • C for 50 h. The catalyst exhibited a good catalytic activity and CH 4 selectivity for CO 2 methanation [57]. Mn-Ce oxides, which can be used as catalysts for the oxidation of toluene, were successfully synthesized via the combination of redox-precipitation and hydrothermal approach (i.e., by mixing KMnO 4 [59]. The solvothermal method utilizes the principle similar to the hydrothermal method to prepare materials that cannot grow in aqueous solution or are susceptible to oxidation, hydrolysis or water sensitivity ( Figure 2b). Typical solvents used are alcohols, benzene, acids, and amines. As exemplified in Table 1, Co 3 O 4 /Co(OH) 2 hybrids were synthesized via a facile solvothermal method by heating the mixture of Co(NO 3 ) 2 ·6H 2 O, sodium oleate, water, ethanol, n-hexane, and KOH at 200 • C for 12 h. The hybrids exhibited superior activity for photocatalytic water oxidation [60]. Near spherical-shaped TiO 2 nanoparticles with uniform size distribution were synthesized by the solvothermal method using titanium tetraisopropoxide as precursor, ethanol as solvent, and H 2 SO 4 as hydrolyzing agent. The obtained TiO 2 nanoparticles can be used as a photocatalyst for the degradation of methyl orange and methylene blue dyes [61]. The monodisperse PtCu dodecahedral nanoframes, as a promising catalyst in hydrogen evolution reaction, were obtained by a simple solvothermal method, where Pt(acac) 2 and CuCl 2 were used as precursors, cetyltrimethylammonium chloride as structure-director and dispersing agent, oleylamine as reductant, allantoin as co-reductant and co-structure directing agent [62].
The sol-gel method uses metal alkoxides or inorganic compounds (e.g., nitrates, chlorides, acetates) as raw materials to carry out a series of hydrolysis and condensation reactions to form a stable sol (Figure 2c) [63]. The sol undergoes aging, colloidal particle aggregation, or polymerization to form a gel. Then, the gel is dried, sintered and solidified to obtain metal oxides or metals. As summarized in Table 1, when the solutions of Cu(NO 3 ) 2 ·3H 2 O and CNTs (both in ethanol) were separately added to the mixture of tetrabutyl-orthotitanate, benzyl alcohol, ethanol and water, Cu doped TiO 2 -10 wt.% CNT nanocomposite powder was successfully prepared by the sol-gel method. The nanocomposite can be used as a photocatalyst for the degradation of methylene blue [64]. The mixture of carbon spheres, MoCl 5 , and diphenyl diselenide was ultrasonically dissolved in excessive ethanol to synthesize C@MoSe 2 core-shell nanocomposites by a simple sol-gel approach, and the nanocomposite can be used as an electrocatalyst for the hydrogen evolution reaction [65]. The mesoporous Ni-M/Al 2 O 3 (M: Fe, Co, Zr, La and Cu) nanoparticles were prepared by a one-pot sol-gel method using Ni(NO 3 ) 2 ·6H 2 O, Al(NO 3 ) 3 ·9H 2 O and M(NO 3 ) x ·yH 2 O as metal precursors, ethanol and propylene oxide as the solvent and gelation agent, respectively. The nanoparticles were shown as a promising catalyst in carbon dioxide methanation reaction [66].
In addition, the thermal decomposition method features the decomposition of the precursors (e.g., metal hydroxides, metal salts; usually obtained in a previous step from precipitation or crystallization in solution in batch) at a certain temperature to obtain metals or metal oxides (e.g., in the calcination stage) as catalysts. The obtained product generally maintains the morphology of the precursors. Co 3 O 4 hexagonal nanoplates were successfully fabricated through the simple thermal decomposition of β-Co(OH) 2 nanoplates (previously obtained in the precipitation process by adding CoCl 2 and NaOH aqueous solutions into water), and exhibited a high catalytic activity in the reduction of p-nitrophenol [67]. Mesoporous CeO 2 nanocrystals could be obtained by thermal decomposition of Ce-MOF (previously prepared via one-step hydrothermal synthesis), which had a superior catalyst activity for benzene combustion [68]. The magnetic porous Mn/Fe 3 O 4 cubes were also synthesized by thermal decomposition of Fe-Mn-oxalate precipitant (previously obtained by adding FeSO 4 ·7H 2 O and MnSO 4 ·H 2 O to the oxalate solution), and this binary oxide showed a good catalytic activity and stability during the degradation of bisphenol A (Table 1) [69].
precursors (e.g., metal hydroxides, metal salts; usually obtained in a previous step from precipitation or crystallization in solution in batch) at a certain temperature to obtain metals or metal oxides (e.g., in the calcination stage) as catalysts. The obtained product generally maintains the morphology of the precursors. Co3O4 hexagonal nanoplates were successfully fabricated through the simple thermal decomposition of β-Co(OH)2 nanoplates (previously obtained in the precipitation process by adding CoCl2 and NaOH aqueous solutions into water), and exhibited a high catalytic activity in the reduction of p-nitrophenol [67]. Mesoporous CeO2 nanocrystals could be obtained by thermal decomposition of Ce-MOF (previously prepared via one-step hydrothermal synthesis), which had a superior catalyst activity for benzene combustion [68]. The magnetic porous Mn/Fe3O4 cubes were also synthesized by thermal decomposition of Fe-Mn-oxalate precipitant (previously obtained by adding FeSO4·7H2O and MnSO4·H2O to the oxalate solution), and this binary oxide showed a good catalytic activity and stability during the degradation of bisphenol A (Table 1) [69].   Although the above wet chemical methods performed in batch stirred tanks have the advantages of simple operation and low technical requirements, their inherent limitations tend to hinder their wide applications in both academia and industry. The conventional batch synthesis processes usually require bulky instruments or expensive autoclaves and complex stepwise operations, associated with non-optimized or low efficiency in the consumption of reagent, power and labor, and require long synthesis time. Moreover, because of its low heat transfer efficiency, for strongly exothermic reactions, the heat may not be dissipated in time especially when large amount of reagents are used in batch. This may cause an instantaneous increase in temperature and pressure, resulting in potential safety concerns. In addition, the low mixing rate and heat/mass transfer efficiency of batch reactors render temperature and concentration gradients during the synthesis process. Thus, the (nano)particle formation process cannot be precisely controlled, resulting in a large particle size, wide particle size distribution, inhomogeneous morphology and poor batch-to-batch repeatability.

Advantages of Microfluidic Reactor Technology
For all the above-mentioned methods of solid catalyst synthesis, a good mixing between reagents in the solution and a fine control over other reaction parameters (e.g., concentration, temperature and reaction time) are important in determining the obtained catalyst structure, property and performance. Thus, there is an incentive to switch from batch to continuous flow processing in microreactors for process improvement. Numerous researches have reported the advantages of microfluidic reactor technology over conventional batch operation for the synthesis of nanoparticles using wet chemical methods [76,77]. When the precipitation process is conducted in a miniaturized microreactor, it is a facile, versatile and reproducible method due to the continuous operation mode, superior mixing efficiency and well-controlled residence time [78]. The precipitation process in microreactors can effectively differentiate the nucleation and growth processes, as well as mitigate the problem of nanoparticle agglomeration often encountered in a stirred tank, so that nanoparticles obtained in microreactors usually have a smaller particle size and more uniform particle size distribution than in batch [14]. It takes typically seconds to tens of minutes to synthesize solid catalyst nanoparticles by hydrothermal or solvothermal method in the microreactor, while it usually takes hours to days in a batch reactor [40,79]. Similarly, the synthesis of catalyst nanoparticles by the sol-gel method in a microreactor usually requires a short reaction time (e.g., 1 min), and the obtained nanoparticles have a small particle size, narrow and controllable particle size distribution [12,80].
From the discussion above, we can reasonably conclude that microfluidic reactor technology offers a series of advantages for the synthesis of solid catalysts. Firstly, the superior micromixing therein provides a (close to) uniform reaction environment for the precipitation process. Secondly, the efficient heat/mass transfer in microreactors renders a (nearly) uniform temperature/concentration distribution [81]. In addition, a fine control over the residence time is attainable in microreactors, which is conducive to the preparation of nanomaterials with a narrow particle size distribution and high repeatability between different flow operation sequences [82][83][84][85]. Moreover, nucleation, growth and agglomeration processes can be effectively controlled by accurately adjusting parameters during reactions in microflow. This unique characteristic enables microreactors to regulate the size, shape, and structure properties of solid catalysts. Finally, it is relatively easy to scale up compared with batch reactors. The scale-up of microreactors is primarily based on the numbering-up concept, that is, the superposition of microchannels. During this scale-up process, the synthesis parameters optimized previously in a single-channel microreactor are kept, so that the transport/reaction characteristics across a multitude of microchannels are identical in principle, which is beneficial to the fast, reliable and modular production of solid catalysts on a large scale [46].

Classification of Microfluidic Methods
According to the flow pattern manipulation during solid catalyst synthesis, microfluidic methods can be divided mainly into two types, namely single-phase flow (continuous flow, Figure 3a) and two-phase flow methods [84]. The two-phase flow method can be subdivided into gas-liquid/liquid-liquid segmented flow ( Figure 3b) and liquid-liquid droplet flow (Figure 3c) methods. The length of droplets (if present) in segmented flow is larger than the microchannel diameter and there is usually a film present between the droplet and microchannel wall, while the droplet size is smaller than the microchannel diameter in the droplet flow mode and droplets are surrounded by the continuous liquid phase. In both biphasic flow modes, reactions can occur either in the dispersed phase ( Figure 3bI,cI) or in the continuous phase (Figure 3bII,cII). In general, for reactions that are prone to blockage, reagents are usually confined in the dispersed phase, and the generated solid particles are prevented from contacting or interacting with the microchannel, thereby clogging can be prevented. Both single-and two-phase flow methods are characterized by low Reynolds numbers (and thus laminar flow conditions) due to small microchannel sizes, but the mixing behavior of reagents is largely different. The mixing of reagents in microreactors under single-phase flow operation is relatively slow and primarily driven by molecular interdiffusion, whereas reagents under two-phase flow operation can be more efficiently mixed (e.g., on sub-millisecond timescales) through the strong advection generated in droplets [86].
production of solid catalysts on a large scale [46].

Classification of Microfluidic Methods
According to the flow pattern manipulation during solid catalyst synthesis, fluidic methods can be divided mainly into two types, namely single-phase flow ( uous flow, Figure 3a) and two-phase flow methods [84]. The two-phase flow meth be subdivided into gas-liquid/liquid-liquid segmented flow ( Figure 3b) and liquid droplet flow (Figure 3c) methods. The length of droplets (if present) in segmented larger than the microchannel diameter and there is usually a film present betwe droplet and microchannel wall, while the droplet size is smaller than the microc diameter in the droplet flow mode and droplets are surrounded by the continuous phase. In both biphasic flow modes, reactions can occur either in the dispersed phas ure 3bI,cI) or in the continuous phase (Figure 3bII,cII). In general, for reactions t prone to blockage, reagents are usually confined in the dispersed phase, and the gen solid particles are prevented from contacting or interacting with the microch thereby clogging can be prevented. Both single-and two-phase flow methods are c terized by low Reynolds numbers (and thus laminar flow conditions) due to small channel sizes, but the mixing behavior of reagents is largely different. The mixing gents in microreactors under single-phase flow operation is relatively slow and pri driven by molecular interdiffusion, whereas reagents under two-phase flow operati be more efficiently mixed (e.g., on sub-millisecond timescales) through the strong tion generated in droplets [86].

Single-Phase Flow Method
The reagents involved in the single-phase flow method are all miscible (e.g., in commonly used solvents such as water, ethanol and ethylene glycol), and the process of synthesizing solid catalysts is to inject different reagents into microchannels to mix and react. Single-phase flow microfluidic systems for (catalytic) nanomaterial synthesis are considered to be very promising, because this method has the advantages of simple implementation, high flux, and easy control compared with two-phase flow method. However, there are two principal drawbacks that limit the application scope of single-phase flow method. One is that the products deposited on the microreactor wall may cause fouling or even blockage of the microchannel, posing difficulties for long-term operation. The other is that there is a wide distribution of residence time within the microchannel associated with the laminar parabolic velocity profile (Figure 3a), often resulting in a significant dispersion in the particle size and properties of the synthesized solid catalysts.

Two-Phase Flow Method
The two-phase flow method usually uses an inert gas or liquid that does not react or is immiscible with the reagent phase to divide the reagent phase into continuous slugs (i.e., when the wall is wetted by the liquid) or discrete droplets, which can greatly improve the mixing efficiency in the microchannel due to the recirculation of the liquid [87]. This method overcomes the main drawbacks of single-phase flow method, namely, the significant elimination of velocity dispersion and reduction of reactor contamination (due to the confinement of reactions in droplets) [27]. Moreover, the two-phase flow method has the advantages of a large phase boundary area, low axial backmixing degree, and narrow residence time distribution. Therefore, many studies have been focused on the controlled synthesis of nanoparticles using segmented flow and droplet flow methods [88,89].

Examples
Microfluidic reactor technology has been extensively used for the synthesis of various catalysts such as metals, metal oxides, and porous materials (zeolite, silica, MOFs). Most of the reports only deal with catalyst synthesis by microfluidic single-or two-phase flow method, and the performance study of catalysts (as listed in Tables 2 and 3). In addition, some studies adopted different methods to prepare catalysts, and compared the performance of catalysts obtained by different methods and the influence of preparation method on the catalyst properties in detail. In this section, some examples are provided for solid catalysts synthesized by microfluidic methods in recent years. The effects of different synthesis methods on the structural properties and catalytic performance of catalysts are also discussed.

Metal Catalyst
Metal nanoparticles are widely used in various technological fields, such as catalysis, fine chemicals, petrochemicals, pharmaceuticals, electronics, sensing, fuel cells, and environmental protection due to their excellent physical and chemical properties [90,91]. The size, shape, and structure of metal nanoparticles greatly influence their catalytic, optical, electronic and biomedical properties [92]. Researchers have explored a variety of synthetic strategies for the preparation of metal catalysts with a narrow particle size distribution [93].

Monometallic Catalyst
Among all metals, noble metals (e.g., gold, silver, palladium and platinum) are the most commonly used catalyst materials because of the easy adsorption of reactants on their surfaces, high catalyst activity and selectivity [94]. In the past two decades, the synthesis of gold nanoparticles by microfluidic reactor technology has become a major research hotspot and various gold nanoparticles with controlled sizes and morphologies have been successfully obtained.
As summarized in Table 2, ultrasmall Au nanoparticles with an average diameter of 1.0 nm and narrow size distribution were synthesized in a continuous single-phase flow microfluidic setup using HAuCl 4 ·3H 2 O as precursor, NaBH 4 as reducing agent and PVP as stabilizer [76]. In comparison, the average diameter of Au nanoparticles produced in batch increased to 1.9 nm and the particle size distribution was also slightly broader. Depositing the obtained Au nanoparticles on TiO 2 could be used to prepare the catalyst for the oxidation of CO. Au nanoflowers with tunable sizes were synthesized by a rapid continuous single-phase microfluidic approach within 3 min using HAuCl 4 ·4H 2 O as precursor and HEPES aqueous solution as reducing agent and size regulator ( Table 2) [95]. The as-prepared Au nanoflowers exhibited a high catalytic activity for the reduction of 4-nitrophenol. A gas-liquid segmented flow microfluidic reactor was used to synthesize Au nanoparticles ( Table 3) with diameters of 3-25 nm at room temperature (using HAuCl 4 as precursor and CO as gaseous reductant) [96]. The size and hydrophilicity of Au nanoparticles could be tuned by using different capping agents (Figure 4). most commonly used catalyst materials because of the easy adsorption of reactants o their surfaces, high catalyst activity and selectivity [94]. In the past two decades, the syn thesis of gold nanoparticles by microfluidic reactor technology has become a major re search hotspot and various gold nanoparticles with controlled sizes and morphologie have been successfully obtained.
As summarized in Table 2, ultrasmall Au nanoparticles with an average diameter o 1.0 nm and narrow size distribution were synthesized in a continuous single-phase flow microfluidic setup using HAuCl4·3H2O as precursor, NaBH4 as reducing agent and PV as stabilizer [76]. In comparison, the average diameter of Au nanoparticles produced i batch increased to 1.9 nm and the particle size distribution was also slightly broader. De positing the obtained Au nanoparticles on TiO2 could be used to prepare the catalyst fo the oxidation of CO. Au nanoflowers with tunable sizes were synthesized by a rapid con tinuous single-phase microfluidic approach within 3 min using HAuCl4·4H2O as precu sor and HEPES aqueous solution as reducing agent and size regulator ( Table 2) [95]. Th as-prepared Au nanoflowers exhibited a high catalytic activity for the reduction of 4-n trophenol. A gas-liquid segmented flow microfluidic reactor was used to synthesize A nanoparticles ( Table 3) with diameters of 3-25 nm at room temperature (using HAuCl4 a precursor and CO as gaseous reductant) [96]. The size and hydrophilicity of Au nanopa ticles could be tuned by using different capping agents (Figure 4).  The synthesis of silver nanoparticles has also attracted great research interest because of their special chemical and optical properties [97]. Silver nanoshells with an average shell thickness of 11.6 ± 5.3 nm were successfully synthesized in a single-phase flow central collision-type microreactor using silver-ammonia complexes as precursor, AuNP-SiO 2 as core particles and formaldehyde as reducing agent (Table 2) [98]. Compared with the batch synthesis process, the microfluidic synthesis process was time-saving and the obtained shell structures were homogeneous due to the intensive mixing and large surface to volume ratio of the microreactor. Spherical silver nanoparticles with a mean diameter of 7.6 ± 1.8 nm were successfully obtained in a droplet flow microfluidic device using AgNO 3 as precursor, NaBH 4 as reducing agent, EDTA as complexant and PVP as dispersant (Table 3) [99]. The spherical-shaped Ag nanoparticles with an average size of 4 nm and Au nanoparticles with an average size of 14-17 nm could be obtained in a segmented flow capillary microreactor using AgNO 3 and AuHCl 4 ·3H 2 O as the respective metal precursors, silicon oil as continuous phase, NaBH 4 and sodium citrate as reducing agent [100].
Much effort has been taken on the preparation of palladium and platinum nanoparticles with a controllable size and shape due to their wide application in the field of catalysis [101,102]. Pd nanoparticles, which can be used as an electrochemical catalyst, were synthesized in a flow microreactor (under single-phase flow) using PdCl 2 as precursor, NaBH 4 as reducing agent, and cetyltrimethylammonium bromide as stabilizing agent, and the size of the nanoparticles could be controlled within 5-200 nm by changing the solution flow rate [32]. Ultra-small Pt nanoparticles as well as alloyed nanoparticles including PtPd, PtRu and PtRh (Table 2), which can be used for the catalytic oxidation of volatile organic compounds, were prepared in microreactors (under single-phase flow) using H 2 PtCl 6 (K 2 PdClO 4 , RuCl 3 , RhCl 3 ) as precursor and tetrakis(hydroxymethyl)phosphonium chloride as reducing and stabilizing agent [36]. Compared with the traditional batch process for the synthesis of Pt nanoparticles, the microfluidic method had a short residence time, high yield and high productivity. Pd nanorods with a diameter of about 4 nm and an aspect ratio of up to 6 were prepared by a continuous segmented flow microfluidic method using air as the segmentation gas and Na 2 PdCl 4 mixed in the mixture of water, KBr, ethylene glycol, and polyvinyl pyrrolidone as precursor (Table 3) [103]. This method only required 2 min to synthesize Pd nanorods, while it usually took hours in batch systems. Moreover, the obtained Pd nanorods showed an excellent activity for the catalytic hydrogenation of styrene.

Bimetallic Catalyst
By combining two metals, the properties of the resulting nanoparticles may differ from those of pure metals due to the interaction between metal particles. The bimetallic particles often form alloys or core-shell structures and usually exhibit an excellent catalytic performance because of the synergistic effect of metal atoms [104]. For example, Au@Ag core-shell nanoparticles were synthesized (using HAuCl 4 and sodium citrate for the synthesis of Au core, and AgNO 3 and ascorbic acid for the preparation of Ag shell) in a single-phase microreactor ( Figure 5a) and a conventional batch reactor, respectively [105]. Compared with the batch results, the spherical Au@Ag core-shell nanoparticles synthesized in the microreactor exhibited a more uniform shell size (Figure 5b,c) due to the faster mixing of reagents and better control over the reaction time. Moreover, the microreactor could be used to produce various uniform core-shell nanoparticles, such as Au@Pd (Figure 5d), Au@Au, and Au@Au with an interior nanogap. Hybrid Au-Pd dumbbell-like nanostructures, which involved the synthesis of Pd nanorods using Na 2 PdCl 4 as precursor and ethylene glycol as reducing agent and solvent, as well as the growth of Au on Pd nanorods using HAuCl 4 as precursor and ethylene glycol as reducing agent, were successfully synthesized ( Table 2) [106]. In more detail, Pd nanorods were synthesized in a sequential-addition microfluidic reactor by segmented flow method, and the growth of Au was conducted in an ultrasonic integrated microfluidic reactor by single-phase flow method. Core@shell Au@Pd nanostructures with controlled shape and dimensions have been obtained in a continuous flow droplet microreactor, including the synthesis of Au core (using HAuCl 4 ·3H 2 O as precursor, NaBH 4 or sodium citrate as reducing agent) and the growth of Pd shell (using H 2 PdCl 4 as precursor, L-ascorbic acid as reducing agent, and silicone oil as the carrier medium) ( Table 3) [107]. The droplet-based microreactor further enabled the inline analysis and modification of process conditions compared with the batch method.
Powders 2022, 2, FOR PEER REVIEW 12 been obtained in a continuous flow droplet microreactor, including the synthesis of Au core (using HAuCl4·3H2O as precursor, NaBH4 or sodium citrate as reducing agent) and the growth of Pd shell (using H2PdCl4 as precursor, L-ascorbic acid as reducing agent, and silicone oil as the carrier medium) ( Table 3) [107]. The droplet-based microreactor further enabled the inline analysis and modification of process conditions compared with the batch method.

Supported Metal Catalyst
Metal nanoparticles used for catalytic reactions usually have a small particle size and high surface free energy, making them easy to agglomerate and difficult to recover and recycle. To overcome these shortcomings, they are usually anchored on solid supports (e.g., carbon materials, metal oxides). The specific surface area of solid supports is usually large, which greatly increases the dispersibility and stability of metal nanoparticles. More-

Supported Metal Catalyst
Metal nanoparticles used for catalytic reactions usually have a small particle size and high surface free energy, making them easy to agglomerate and difficult to recover and recycle. To overcome these shortcomings, they are usually anchored on solid supports (e.g., carbon materials, metal oxides). The specific surface area of solid supports is usually large, which greatly increases the dispersibility and stability of metal nanoparticles. Moreover, there may be interactions between particles and supports, which will further enhance the catalyst performance [108,109]. Therefore, the design and application of supported metal catalysts is an important method to improve and ensure its catalytic activity and selectivity.
Highly dispersed PtFeCu/C ternary nanocatalysts (Table 2), which showed an excellent catalytic performance for methanol oxidation, were synthesized by single-phase flow method in a microfluidic setup with four parallel channels [110]. The aqueous solution of FeCl 3 ·6H 2 O, CuCl 2 ·2H 2 O, and H 2 PtCl 6 ·6H 2 O containing suspended carbon black was used as metal precursor solution, and NH 3 BH 3 as reducing agent. The high flow rate and multiple channels increased the throughput obviously. M/TiO 2 (M = Pd, Pt or Au) nanoparticles were continuously synthesized by a stabilizer-free method based on a segmented flow microfluidic system using Pd(NO 3 ) 2 , H 2 PtCl 6 or HAuCl 4 mixed with TiO 2 aqueous suspension as precursor, NaBH 4 with NaOH as reducing agent, and n-octane as continuous phase (Table 3) [77]. Compared with M/TiO 2 nanoparticles prepared by the batch method, the obtained nanoparticles in microflow had a smaller noble metal particle size (<5 nm), better dispersity, and thus showed a higher catalytic activity in the hydrolysis of ammonia borane. Ultrafine Ag nanoparticles with a controllable particle size (1.5-5.6 nm) and narrow particle size distribution were uniformly deposited on rGO by segmented flow processing in the microreactor (Figure 6a,c,d), using AgNO 3 as precursor, trisodium citrate and NaBH 4 as reducing agent, sodium dodecyl sulfate (SDS) as stabilizing agent and octane as continuous phase [111]. The average particle size of Ag nanoparticles in Ag-rGO composites was smaller than those synthesized in batch (Figure 6b).
Powders 2022, 2, FOR PEER REVIEW 13 over, there may be interactions between particles and supports, which will further enhance the catalyst performance [108,109]. Therefore, the design and application of supported metal catalysts is an important method to improve and ensure its catalytic activity and selectivity. Highly dispersed PtFeCu/C ternary nanocatalysts (Table 2), which showed an excellent catalytic performance for methanol oxidation, were synthesized by single-phase flow method in a microfluidic setup with four parallel channels [110]. The aqueous solution of FeCl3·6H2O, CuCl2·2H2O, and H2PtCl6·6H2O containing suspended carbon black was used as metal precursor solution, and NH3BH3 as reducing agent. The high flow rate and multiple channels increased the throughput obviously. M/TiO2 (M = Pd, Pt or Au) nanoparticles were continuously synthesized by a stabilizer-free method based on a segmented flow microfluidic system using Pd(NO3)2, H2PtCl6 or HAuCl4 mixed with TiO2 aqueous suspension as precursor, NaBH4 with NaOH as reducing agent, and n-octane as continuous phase (Table 3) [77]. Compared with M/TiO2 nanoparticles prepared by the batch method, the obtained nanoparticles in microflow had a smaller noble metal particle size (<5 nm), better dispersity, and thus showed a higher catalytic activity in the hydrolysis of ammonia borane. Ultrafine Ag nanoparticles with a controllable particle size (1.5-5.6 nm) and narrow particle size distribution were uniformly deposited on rGO by segmented flow processing in the microreactor (Figure 6a,c,d), using AgNO3 as precursor, trisodium citrate and NaBH4 as reducing agent, sodium dodecyl sulfate (SDS) as stabilizing agent and octane as continuous phase [111]. The average particle size of Ag nanoparticles in Ag-rGO composites was smaller than those synthesized in batch (Figure 6b).

Metal Oxide Catalyst
Although noble metal catalysts have an excellent catalytic performance, the shortage of resources and high price limit their application scale. Transition metal oxides (e.g., TiO 2 , ZnO, CeO 2 , CuO, Fe 2 O 3 ) are inexpensive and abundant. They have excellent redox properties and can be used as catalysts for many reactions, though their catalytic activity is usually lower than that of noble metal catalysts [15,128]. For example, phosphated TiO 2 catalysts were prepared by a continuous sol-gel method in a microreactor (under single-phase flow), which is an eco-friendly catalyst for the selective synthesis of 5-hydroxymethylfurfural (a biobased platform chemical) [12]. Because of the great flexibility and precise control of chemical process, the yield and space-time yield of phosphated TiO 2 obtained in the microreactor process were higher than that of the conventional batch synthesis process and the phosphated TiO 2 exhibited a remarkably higher glucose conversion and slightly improved selectivity during the catalytic process compared with that obtained in the batch reactor. Benefiting from the high microscopic mixing rate of the membrane dispersion microreactor, the mesoporous γ-Al 2 O 3 nanofibers were synthesized by a simple, template-free and singlephase microfluidic method using NaAlO 2 and Al(NO 3 ) 3 ·9H 2 O as raw materials. And the obtained γ-Al 2 O 3 nanofibers showed considerable pore volumes and uniform pore size distributions compared with those prepared by traditional batch stirring methods [129]. The morphology-controlled synthesis of ZnO nanostructures (spindles, sheets and spheres) was reported via the precipitation of Zn(NO 3 ) 2 and NaOH by a facile surfactant-free method in microreactors (under single-phase flow) [130]. These ZnO nanostructures were tested as photocatalysts for the degradation of dye and exhibited an excellent catalytic efficiency.
CeO 2 nanoparticles were synthesized through the precipitation of Ce(NO 3 ) 3 ·6H 2 O and NH 3 ·H 2 O using a single-phase flow microreactor and a batch stirred tank reactor under the same experimental conditions [39]. CeO 2 nanoparticles prepared in the microreactor were nearly 2 times smaller in size, had a better dispersion and narrower particle size distribution than those obtained in batch (Figure 7). When used as catalyst for the mineralization of acid orange 7, the degradation efficiency approached 84.1% for CeO 2 nanoparticles synthesized in the microreactor within 0.5 h, which was~40.6% higher than those prepared in batch. CeO 2 nanorods and nanocubes were synthesized by hydrothermal method using Ce(NO 3 ) 3 ·6H 2 O and NaOH as reagents in a self-made single-phase flow microfluidic system [131]. CeO 2 nanorods were transformed to nanocubes by introducing PVP as surfactant under hydrothermal conditions. For comparison, CeO 2 nanoparticles were prepared by precipitation method in a stirred tank reactor. The preparation of CeO 2 nanostructures by hydrothermal method took only 8 min in the microreactor, while several days were needed in the conventional batch system. The catalytic activity for the decomposition of hydrogen peroxide followed the sequence: CeO 2 nanocubes > CeO 2 nanorods > CeO 2 nanoparticles, which was explained by the microfluidic reaction system that generated nanostructures with a smaller particle size, thereby obtaining larger specific surface areas and better catalytic performance.
In addition, SnO 2 nanoparticles were synthesized in a single-phase flow microreactor and a batch reactor using SnCl 4 ·5H 2 O and NaOH as raw materials to compare its structure and photocatalytic performance [132]. SnO 2 nanoparticles obtained in batch were characterized by agglomerated structures composed of extremely tiny nanoparticles with an average size of around 2-3 nm. In contrast, those synthesized in the microreactor had less agglomeration and consisted of extremely small SnO 2 nanoparticles with an average size of 3-5 nm. The SnO 2 nanoparticles prepared in the microreactor presented photocatalytic activity in the degradation of methylene blue dye because of the significant increase in surface defects. These results demonstrate that the change in synthetic techniques altered the surface properties (e.g., extent of oxygen vacancies and defects at the surface) and optical band gaps, and these changes in turn affected the photocatalytic properties of SnO 2 nanoparticles [132].

Porous Catalytic Material
Porous materials have attracted an extensive scientific research interest in the field of catalysis due to their uniform, abundant, and tunable porous structures, high specific surface area, large adsorption capacity, and excellent physicochemical properties. The most common porous materials that can be used as catalysts include among others zeolites, silica, and MOFs [133].

Silica
Silica is a kind of porous material and its synthesis has received a great deal of research attention due to its wide application in many fields, especially as catalysts and catalyst supports. Over the past decade, microfluidic reactor technology has been introduced to overcome some of the challenges faced in the synthesis of silica, such as

Porous Catalytic Material
Porous materials have attracted an extensive scientific research interest in the field of catalysis due to their uniform, abundant, and tunable porous structures, high specific surface area, large adsorption capacity, and excellent physicochemical properties. The most common porous materials that can be used as catalysts include among others zeolites, silica, and MOFs [133].

Silica
Silica is a kind of porous material and its synthesis has received a great deal of research attention due to its wide application in many fields, especially as catalysts and catalyst supports. Over the past decade, microfluidic reactor technology has been introduced to overcome some of the challenges faced in the synthesis of silica, such as precise regulation of its morphology, particle size, and porosity [134]. For instance, silica spheres (diame-ter: 300-400 nm), core-shell spheres with the mesoporous silica shell (up to ca. 60 nm) and the dense silica core (SiO 2 @mSiO 2 ) as well as gold-encapsulated spheres (Au@SiO 2 ) were efficiently produced in a microreactor under gas-liquid segmented flow (Table 2; Figure 8a) [125]. Typically, silica spheres were prepared using ammonia as catalyst in the ethanol-water mixture and TEOS in ethanol as precursor. For the silica shell deposition, the mixture of TEOS and OTMS in ethanol was employed as the second precursor. Au@SiO 2 particles were synthesized by further adding the aqueous gold colloids in the mixture of ammonia and ethanol. The segmentation of flow was found crucial to obtain the desirable monodispersity of silica spheres or core-shell particles, whereas a significant shell thickness was present under single-phase laminar flow operation.
Hollow silica nanoparticles with an average diameter of ∼30 nm were prepared using tetraethylorthosilicate as precursor, poly (acrylic acid) as template by a micromixer (under single-phase flow) [135]. Compared with the batch reactor, the fast mixing efficiency in the microreactor dramatically reduced the aggregation of poly (acrylic acid) template, which was beneficial to obtain silica nanoparticles with a smaller diameter and narrower size distribution. Silica bearing different surface silanol groups was synthesized using sodium silicate aqueous solution and CO 2 as raw materials in a segmented flow microreactor and conventional batch reactor under similar reaction conditions [136]. The obtained silica samples were employed as supports for Pd-based catalysts for benzyl alcohol oxidation. Compared with conventional batch method, silica prepared in the microreactor had a higher specific surface area, more uniform distribution of surface silanol groups and better catalytic effect due to the high mass transfer efficiency of the microreactor.

Zeolite
Zeolite is a kind of crystalline aluminosilicate with regular and uniform pores that has a wide range of applications, especially in the field of catalysis, due to its nanoporosity and acidity [137]. The preparation process of zeolite generally consists of two steps; firstly the mixing of silica and alumina solutions to prepare the hydrogel, and then the hydrothermal crystallization process of the hydrogel. Zeolites with adjustable sizes were prepared in a segmented flow-based microreactor by the one-step hydrothermal synthesis, using silica and alumina solutions as dispersed phase and liquid paraffin as continuous phase (Figure 8b) [126]. The microreactor system had the advantages of no clogging as well as saving time and cost for zeolite synthesis, because of the in situ mixing of silica and alumina solutions (Table 3).
Porous zeolite A nanocrystals with a uniform particle size were efficiently synthesized by a hydrothermal method conducted in a droplet flow-based microreactor, using fluorocarbon oil as continuous phase and the gel solution as dispersed phase (precursor), that is, obtained by the aging of an alumina precursor solution and tetraethylorthosilicate [40]. The aqueous droplets were surrounded by oil medium, preventing the contact of the formed nanocrystals with the microchannel wall, thereby preventing channel clogging. Moreover, it took only tens of minutes to synthesize nanocrystals in the microreactor, compared with hours to days in batch. The obtained zeolite A exhibited a superior catalytic performance and an improved stability in the synthesis of alkyl borate.

MOFs
MOFs, consisting of metal clusters and organic bridging ligands, are a promising class of porous-crystalline materials that can be used in many fields such as catalysis, adsorption, sensing and separation [138]. These materials usually have plenty of pores and high specific surface areas, different transition metals or functional groups, which are very suitable as catalysts. The continuous synthesis of MOFs by means of microreactors has become a research hotspot in recent years due to the ultra-high mixing efficiency and precise control of reaction parameters therein [42].
For example, MIL-88B type MOFs with average sizes of 90-900 nm and narrow particle size distributions were prepared under liquid-liquid segmented flow processing through microreactors, using FeCl 3 ·6H 2 O solution and three different ligands solution (H 2 BDC (benzene-1,4-dicarboxylic acid), NH 2 -H 2 BDC, Br-H 2 BDC) as continuous phases and silicone oil as discrete droplets (Figure 8c) [139]. MOFs could be obtained at a residence time as short as 20 s in the microreactor and this method can effectively reduce microreactor clogging by preventing the obtained particles from interacting with the microchannel walls. The synthesis of uniform HKUST-1 microcubes, composed of Cu(II) dimers and 1,3,5-benzenetricarboxylate, was also reported in droplet flow-based microreactors, using silicon oil as continuous phase and the DMSO solution of the precursor of Cu(NO 3 ) 2 and 1,3,5-benzenetricarboxylate containing uniform polystyrene nanoparticles as dispersed phase [140].
very suitable as catalysts. The continuous synthesis of MOFs by means of microreactors has become a research hotspot in recent years due to the ultra-high mixing efficiency and precise control of reaction parameters therein [42].
For example, MIL-88B type MOFs with average sizes of 90-900 nm and narrow particle size distributions were prepared under liquid-liquid segmented flow processing through microreactors, using FeCl3·6H2O solution and three different ligands solution (H2BDC (benzene-1,4-dicarboxylic acid), NH2-H2BDC, Br-H2BDC) as continuous phases and silicone oil as discrete droplets (Figure 8c) [139]. MOFs could be obtained at a residence time as short as 20 s in the microreactor and this method can effectively reduce microreactor clogging by preventing the obtained particles from interacting with the microchannel walls. The synthesis of uniform HKUST-1 microcubes, composed of Cu(II) dimers and 1,3,5-benzenetricarboxylate, was also reported in droplet flow-based microreactors, using silicon oil as continuous phase and the DMSO solution of the precursor of Cu(NO3)2 and 1,3,5-benzenetricarboxylate containing uniform polystyrene nanoparticles as dispersed phase [140].

Miscellaneous Catalysts
Other kinds of particles (e.g., semiconductors, quantum dots and metal salts) as potential catalyst candidates have also been prepared in microreactors [141]. For example, Ag 2 S/ZnS core-shell quantum dots (QDs) with a different ZnS shell thickness could be prepared in a droplet microreactor, by interfacial reaction between Zn-oleylamine precursor (in the continuous oil phase) and Ag 2 S QDs (in the dispersed aqueous phase) [142]. Spherical Ag 3 PO 4 nanoparticles with a narrow size distribution were successfully synthesized in a T-shaped microreactor (via single-phase flow method) using silver nitrate and disodium hydrogen phosphate as raw materials [118]. The photocatalytic efficiency of these Ag 3 PO 4 nanoparticles for the degradation of Rhodamine B dye could reach >97% within only 15 min.

Conclusions and Perspectives
The development of microfluidic reactor technology has opened a new and attractive platform for the controlled synthesis of solid catalysts, including among others monometallic and bimetallic catalysts, supported metal catalysts, metal oxides and porous catalytic materials. Compared with solid catalyst synthesis using wet chemical methods (e.g., precipitation, hydrothermal/solvothermal, sol-gel methods as well as thermal decomposition methods) performed in traditional batch reactors, continuous processing in microreactors based on the single-phase or two-phase flow mode could achieve a much better micromixing and heat/mass transfer efficiency. This allowed a (close to) uniform reaction environment, and a fine control over reaction parameters (e.g., concentration, temperature and reaction time), and thus the preparation of solid catalysts with a smaller particle size, more uniform particle size distribution and better morphology control in a significantly shorter reaction time (e.g., typically minutes or even seconds regarding the formation of nanoparticles or nanocrystals) in microreactors than in batch. As a result, solid catalysts prepared by microfluidic approach often exhibited a better catalytic performance in the target applications than their counterparts obtained via batch protocols.
Although a great research progress has been made in the microfludic synthesis of solid catalysts in recent years, there are still considerable scopes for optimization in terms of the synthesis process, catalyst quality, functionality and yield. To advance this technology, some aspects need to be addressed carefully. Firstly, the fouling or even clogging of microchannels may happen by solid products formed in the reaction process because of the agglomeration of particles or the deposition of particles on the microchannel walls due to the gravity or particle-wall interaction. Such phenomena can occur especially during the single-phase flow mode and long-term operation, which might be mitigated via the introduction of strong cavitation effects by coupling ultrasonication with microreactors, the employment of mechanical vibration in continuous flow, the modification of the wettability of microchannel surface (e.g., to be superhydrophobic or superoleophobic), or the use of the two-phase flow method to confine the reaction in the dispersed phase that is not in contact with the wall [143,144]. Secondly, a better synthesis optimization and control can be assisted via coupling microreactors with some online analysis and measurement systems, which enables real-time monitoring and accurate analysis of the as-synthesized particles and immediate adjustment of synthesis conditions to obtain the desired products [19,145]. Thirdly, the majority of current research remains still on the laboratory scale. Effective scale-up strategies for large-scale fabrication of solid catalysts in microreactors with a multitude of microchannels need to be researched further, especially those addressing well the local particle-fluid hydrodynamics [46]. The up-to-date scale-up strategies include the internal numbering-up method (i.e., via arranging a number of identical microchannels in parallel within one microreactor module), the external numbering-up method (i.e., via providing an individual supply system to each microreactor), and the sizing up method (i.e., selective dimension enlarging) [146]. The numbering-up (both internal and external) method may require hundreds to thousands of reaction units to meet high throughput of industrial production. The question of how to achieve a uniform fluid distribution in each microchannel in the microreactor system realized by the internal numbering-up method represents a somewhat significant challenge, especially when it comes to multiphase fluid flow, let alone to the presence of particles [147,148]. In the external numbering-up method, each microreactor requires its own supply system (e.g., pump, buffer tank) and auxiliary equipment (e.g., mixing unit, pressure detector unit, temperature controller unit), resulting in excessive installation and operating costs. The size (e.g., length or diameter) of the microchannel can be increased to some extent to achieve a higher throughput while maintaining the mixing and transfer characteristics at the microscale (the sizing up method). Though this size increase can greatly reduce the occurrence of clogging, the hydrodynamic and transport properties might change dramatically if the channel size becomes too large. Thus, the throughput might be only increased to a somewhat limited extent by this method. To date, a practical and feasible approach for the scale-up of microreactors to satisfy the industrial demand is to combine the strategies of internal numbering-up, external numbering-up, and sizing up, followed by the optimization of the microreactor structures. This approach would facilitate a fast and reliable reproduction of laboratory achievements on the industrial scale [146][147][148][149].
The future research interests are envisaged in the following areas. It is necessary to synthesize more catalytic materials that have not been investigated in microreactors so far, in order to explore the full potential of microfluidic synthesis approach. There exists a lot of room for improvement regarding the activity, selectivity and stability of solid catalysts obtained in microreactors, which necessitates an in-depth fundamental study. Last but not least, the integration of solid catalyst synthesis in microreactors and its further incorporation in microreactors (e.g., in the form of nanofluids, packed beds or catalyst slurries [43,46]) for more sustainable and efficient chemical synthesis represents an emerging area to develop.
To summarize, although microfluidic reactor technology is still in its development phase and faces some challenges, it has become an indispensable tool in the synthesis of solid catalysts. With the continuous and rapid development of this technology, these challenges will be gradually resolved, holding promises for breakthroughs in solid catalyst synthesis. In the near future, microfluidic reactor technology may open new research frontiers and provide unparalleled opportunities for the production of widely used catalysts that not only enable the precise tuning of catalyst particle size, morphology and microstructure, but also catalytic activity, selectivity and other desired functions to fully fulfil the target reaction performance requirements.

Conflicts of Interest:
The authors declare no conflict of interest.