Ecological synthesis of nickel–zinc–aluminium layered double hydroxides (Ni–Zn–Al LDH) in flow infrared radiated agitated tubular reactor (flow-IR-ATR)

The yield of obtaining layered double hydroxides (LDHs) remains a significant challenge that limits their practical use on a large scale. The use of flow processes is an innovative approach to solving the problem. This paper describes a method for obtaining LDH nanoparticles using an agitated tube reactor heated by infrared radiation (flow-IR-ATR). As a dedicated reactor for production of LDH nanoparticles, it is able to synthesise products at a flow rate of 1.8 dm3/h and a yield of 40 g/h. In the process, LDH NPs based on zinc–aluminium hydroxides (Zn–Al LDH) and LDH NPs modified with nickel hydroxide (Ni–Zn–Al LDH), with molar ratio of Ni to Zn 0.5, were obtained. Instrumental analyses (XRD, FTIR, SEM, DLS, BET, XPS) were used to characterise the LDH nanoparticles obtained, which showed crystallite sizes from 9 to 35 nm. The nickel-containing LDHs exhibited high photocatalytic activity. After 60 min, in the presence of UV radiation (365 nm), the photodegradation efficiency of quinoline yellow (dye concentration of 100 mg/dm3) was more than 99.9%.


Introduction
Layered double hydroxides (LDHs) belong to an unusual group of materials formed from positively charged layers stabilised by anions layered between them [1]. Majority of LDHs are described by the formula M 2+ 1 M 2+ denotes divalent metal cations, most commonly Zn, Mg, Ca, and M 3+ denotes trivalent metal cations, typically Al. Anions present between the cation layers include carbonate, nitrate, and hydroxide groups [2,3]. Both cations and anions can be substituted in the ion exchange process. The modification of LDH composition influences their sorption, mechanical, and photocatalytic properties, allowing their application potential to be extremely broad [4,5]. The methods for obtaining LDHs include co-precipitation, anion exchange, reconstitution, alcoholpolyol method, or urea method [6,7]. Among such methods, the co-precipitation method is the most widely used because of its control over particle size, structure, morphology, chemical composition, and crystallinity [8,9]. Nonetheless, their long processing times (from several hours to several days) still limit the optimization of the process, leading to large crystal sizes (micrometre range) and making the process economically unviable. Processes for obtaining LDH are based on mixing selected metal ions with hydroxide-precipitating agents. Co-precipitation, which occurs under supersaturated conditions, results in the gradual formation of LDH structures. In supersaturated reactions, a continuous change in the pH of the suspension causes a high rate of nucleation, affecting the direction of particle formation [10]. The size of LDH particles can be adjusted by regulation of the conditions under which they are obtained, either at the mixing stage, the particle nucleation stage, or at the ageing stage [11].
One of the main challenges in obtaining LDH is the need to supply energy to the system to ensure the formation of layered LDH particles. The LDH synthesis process in traditional batch systems takes several hours and requires the mixture to be maintained at a temperature of about 60-80 °C. Due to the energy intensity and the time required to obtain them, methods are sought that could reduce the cost of LDH production while maintaining their properties and layered structure.
An alternative to the current batch processes may be flow processes, which are easier to automate and allow control of the process at each stage. Importantly, it is possible to control process parameters such as temperature, flow, amount of energy supplied, residence time of the mixture in the reactor, and dosing rate of individual components. Flow reactors provide an increased energy exchange surface area, allowing homogeneous products to be obtained throughout the reaction volume. Pascu et al. described a method to obtain pure crystalline CoNi LDH nanoplatelets with a thickness of 15 nm and a cross-sectional dimension of more than 100 nm using a tube reactor at 160 °C and 1.1 MPa for 4 h [12]. Wang et al. conducted LDH synthesis processes in a flow reactor, investigating the influence of key factors on the quality of LDH particles obtained. Changing the process temperature affected the length of Ca 2 Al-NO 3 LDH crystallites [13]. The effectiveness of scaling acquisition processes for obtaining LDH Zn 2 Al-CO 3 was confirmed by Clark et al. Reactions leading to the production of Zn 2 Al-CO 3 were carried out by the authors at laboratory, pilot, and industrial scale. All variables were maintained constant, regardless of reactor size and flow rate. The pilot-scale reactor operated at 20-30 times the laboratory scale. The plate diameter increased from 120 nm on the laboratory scale to 177 nm on the pilot scale and 165 nm on the industrial scale. However, the LDH nanoparticles retained their specific surface area regardless of the scale at which they were prepared [14].
With modified LDH, multi-step reactions are feasible and selected ions can be added further downstream in the reactor. At the same time, the safety of the process increases, as well as the fact that the cost of the product is presented as a function of time, in contrast to batch processes where the cost of production is determined by the size of the apparatus. The possibility of multistage processes was confirmed by Pascu et al. by obtaining Ca 2 Al-NO 3 LDH in a twostage reactor. In this method, ZnO was combined, due to its high photocatalytic activity, with LDH particles (Mg 2 Al-CO 3 and Ca 2 Al-NO 3 ) increasing the adsorption surface area of the final product [12].
If the LDH preparation process is carried out in a flow reaction system, the energy source and mixing method must be selected to ensure favourable conditions for the growth of LDH particles. There is no single applicable temperature value for LDH formation, and the choice of temperature depends on the method of synthesis, the ageing time, the type of metal cation, and the ratio of metal cations, among other factors. Solvothermal and hydrothermal methods use higher reaction temperatures compared to the co-precipitation method. The choice of heating method also influences the size and crystallinity of the materials [15]. In a study by Oh et al., it was confirmed that increasing the reaction temperature from 100 to 180 °C by a hydrothermal method gave LDH with a larger particle size ranging from 115 to 350 nm. The authors also showed that differences in temperature had a remarkable effect on the crystallinity of LDH [16]. The disadvantages of using a conventional energy source are the large loss factor and the small temperature rise gradient in the reaction volume. Energy sources with increased gradient include microwave radiation, hydrothermal energy, and ultrasound. Clark et al. used a flow process to obtain LDH in a hydrothermal reactor. The authors obtained ZnO combined with Mg 2 AlCO 3 LDH with a yield of 3.6 dm 3 /h [17].
The purpose of this study is to develop technology for obtaining LDH nanoparticles based on a flow process. The energy of infrared radiation combined with mixing based on shaking of the reaction system makes it possible to construct an efficient flow reactor suitable for obtaining LDH nanoparticles with a residence time of the mixture in the reactor of 30 min. Previously, the process of obtaining LDH using a shaking reactor has not been described in the literature, and there are no reports on the use of infrared radiation as an energy source in LDH synthesis processes. Modification of the structure of Zn-Al LDH with nickel hydroxide makes it possible to obtain materials with substantially higher specific surface area, which is crucial for both catalytic and photocatalytic processes of the material.

Materials
The synthesis of layered double hydroxides were carried out with use of zinc nitrate(V) (Zn(NO 3

Synthesis of LDH nanoparticles
In the current study, a flow process was designed to obtain Zn-Al LDH and Ni-Zn-Al LDH nanoparticles. The addition of nickel ions was aimed at increasing the specific surface area and improving the photocatalytic activity of the material. For this purpose, first time, a stirred tube reactor (ATR) was constructed using an alternative method of stirring the reaction stream based on shaking. The ATR reactor consists of a reactor tube connected to a shaking system. Inside the tube, a glass rod is located, which, when the reactor is set in motion, disrupts the fluid flow inside. The inner rod also prevents contaminants from settling on the reactor walls as a result of shaking and improves the turbulence of the system [18]. Two substrate streams are introduced into the tube. The ATR reactor enables the preparation of dispersed suspensions of nanoparticles, especially with gel consistency. In addition, the reaction system was modified by using infrared radiation as an external energy source, which allowed heating the suspension inside the reactor to a temperature of more than 50 °C. A schematic of the system is shown in Fig. 1.
To obtain a suspension of double layered hydroxides, initial solutions of a mixture of metal salts (Zn(NO 3 ) 2 , Al(NO 3 ) 3 , NiCl 2 with a molar ratio of Zn 2+ :Ni 2+ :Al 3+ equal to 0.5:0.5:1 for synthesis of flow-Ni-Zn-Al LDH NPs or Zn(NO 3 ) 2 , Al(NO 3 ) 3 with a molar ratio of Zn 2+ :Al 3+ equal to 1:1 for synthesis of flow-Zn-Al LDH) and a mixture of NaOH with Na 2 CO 3 (molar ratio of NaOH:Na 2 CO 3 equal to 1:0.5) were prepared. The two solution streams were connected by a T-junction to a tubular reactor. The flow rate of metal salts and alkaline reagents was 10 and 20 cm 3 / min, respectively. The residence time of the entire solution was 30 min. Two 1200 W IR lamps were selected as the energy source. After obtaining a suspension of LDH nanoparticles, the material was filtered, washed, and dried at 50 °C, overnight.
To compare the flow-Zn-Al LDH and flow-Ni-Zn-Al LDH NPs, in parallel, LDH nanoparticles in a batch process were obtained. IR radiation as an energy source was also used in the batch method. For this purpose, 10 g Zn(NO 3 ) 2 and 15 g Al(NO 3 ) 3 or 4.82 g Ni(NO 3 ) 2 , 5 g Zn(NO 3 ) 2 , and 15 g Al(NO 3 ) 3 were dissolved in 200 cm 3 deionized water, followed by the addition of 30 cm 3 of 5 g NaOH and 4 g Na 2 CO 3 solution. The solution was mixed in 200 rpm for 30 min in the presence of IR radiation with 1200 W, placed 30 cm above the sample. The finished products (Zn-Al LDH or Ni-Zn-Al LDH) were filtered, washed, and dried in 50 °C overnight.

XRD
The crystallographic properties of LDH nanoparticles were analysed by XRD method (Philips X'Pert camera with PW 1752/00 CuKα monochromator (λ = 1.540598 Å). X-ray diffractograms were measured in the 2θ 10-70° range with a step of 0.02° (measurement every 3 s). The obtained XRD diffractograms were analysed by the Rietveld method, using Match! Software with FullProf. The elemental cell parameters and the phase composition of the materials were defined. In addition, based on Scherrer's equation, the size of the crystallites was determined: where d cr is the mean size of the crystallite, K a dimensionless shape factor (K = 0.90), the X-ray wavelength, the line broadening at half the maximum intensity, and the Bragg angle.

SEM and SEM-EDS
Shape and size of LDH nanoparticles were determined by scanning electron microscopy (SEM). The analysis was performed on a Quanta 3D FEG apparatus, with an accelerating voltage of 30 kV and a resolving capacity of 1.2 nm. Mapping of constituent elements was carried out by energydispersive X-ray spectroscopy (EDX).

DLS analysis
Using DLS analysis (Zeta Sizer Malvern, ZS-90 and Brookhaven, ZetaPALS), an average hydrodynamic diameter of LDH nanoparticles suspended in deionized water was determined. The LDH materials were compared in terms of the nature of the surface of the resulting materials by determining their point zero charge (PZC). The zero-point value was tested by preparing 50 ml each of H 2 O solutions with the addition of 0.1 M NaOH/HCl to establish a pH in the range of 2 to 12. To the solutions, 25 mg of LDH nanoparticles was added, and the whole was stirred for 24 h (200 rpm). After this period, the zeta potential for each suspension was determined. The line of intersection of the pH-dependent potential values defines the point of zero charge of the material.

UV-Vis analysis
The energy gap was determined as the absorption, which corresponds to the energy difference between the upper part of the valence band and the lower part of the conduction band. The value of the energy gap was determined using the UV-Vis method [19], using the following equation: where h is the Planck's constant h = 6.626⋅10 −34 J • s, c speed of light c = 3.0⋅10 8 m/s, and is the cut of wavelength-absorption edge.

Low-temperature nitrogen sorption (BET analysis)
The pore size and distribution, as well as the specific surface area of the materials, were determined by low-temperature nitrogen sorption (Gemini VI apparatus from Micromeritics, USA). The multi-point BET specific surface areas of the samples were determined by the N2 physical sorption method at 77 K using a Quantachrome Autosorb Station apparatus. Prior to the measurements, the samples were pretreated at 400 °C in helium.

FTIR analysis
The nature of the surface of LDH nanoparticles was investigated using Fourier transform infrared spectroscopy with attenuated total reflection (FTIR-ATR). IR spectra were recorded using a Nicolet 6700 spectrometer. Spectra were collected in the range of 400-4000 cm −1 , with a step of 2 nm (64 scans).

XPS analysis
The qualitative composition of the flow-Ni-Zn-Al LDH final material was determined by XPS X-ray photoelectron spectroscopy method. The analysis was performed on a PHI Quantum 2000 instrument, Physical Electronics, Inc: Photoelectron excitation source: a VG Scienta SAX 100 X-ray tube with an aluminium anode, equipped with a VG Scienta XM 780 monochromator, emitting radiation with an Al Kα characteristic line and an energy of 1486.7 eV.

Photocatalytic studies
The photocatalytic properties of LDH nanoparticles, i.e. Zn-Al LDH and Ni-Zn-Al LDH obtained by batch and flow processes, were studied in the photodegradation of quinoline yellow. For this purpose, 50 mg of photocatalyst was mixed with 200 cm 3 of 100 mg/dm 3 dye solution. After 30 min of stirring in the darkness (to achieve sorption equilibrium of the dye), the mixture was exposed to UV light. A 365-nm lamp was placed 20 cm above the sample (intensity value was 5.3 mW/cm 2 ). At successive intervals, 10 cm 3 of the suspension was taken, filtered with a syringe, and the filtrate was subjected to spectrophotometric analysis. The absorbance of the degraded dye was measured by UV-Vis spectroscopy (Rayleigh UV-1800 spectrophotometer) at a wavelength of 412 nm, for which the maximum absorbance of the quinoline yellow solution is reached. The efficiency of the process was presented as the ratio of the removed dye to its initial concentration, expressed as a percentage: where E is the efficiency of quinoline yellow decomposition, C 0 is the initial concentration of the dye, and C t is the concentration of the dye after time t.

XRD analysis
Four materials, i.e. Zn-Al LDH and Ni-Zn-AL LDH obtained in a flow reactor and Zn-Al LDH and Ni-Zn-Al LDH obtained by a batch method, were examined by XRD analysis. In all spectra, the presence of peaks at 11.6° (003), 23.3° (006), 34.5° (102),  [20,21]. Table 1 shows the basic dimensions of crystallites (d cr ), including elemental cell dimensions (a, c, d) as well as the relative degree of crystallinity (I/I (LDH) ) of the materials studied. The sizes of crystallites determined by the XRD method for materials obtained in the flow reactor take on larger values than those obtained in the batch system. The differences in size are related to the reduced crystallinity of the particles. In this study, conventional water bath heating was substituted for infrared heating. The use of an IR lamp results in an increased heating gradient, while the temperatures obtained with this method are similar to those of conventional heating. The XRD analysis revealed materials with lower crystallinity (Table 1), but all characteristic peaks corresponding to the LDH phase are present (Fig. 2). The formation of additional phases was also not observed, which indicates the possibility of using IR as an alternative for the preparation of LDH nanoparticles. The crystal structure of the materials obtained in the flow-through process with infrared heating applied differs slightly from those obtained in batch processes. The lower crystallinity of the materials is probably a result of the reduced process time of 30 min, compared to the batch process of 4 h. The parameter d takes on approximately 5% lower values, which is indicative of smaller distances between the planes. In contrast, the particle size is also reduced, making it possible to obtain nanoparticles of smaller size. Minor contamination with the ZnO phase, which is formed in all LDH materials, was also observed. Flow processes for the preparation of LDH were described by Clark et al. The authors compared the preparation of Zn-Al-CO 3 -LDH materials obtained by continuous synthesis processes at laboratory, pilot, and industrial scale and were investigated for a range of properties. Platelet size determined from TEM images showed that the average platelet size in LDHs obtained from the pilot process was 177 nm, while in LDHs obtained from the batch process, it was 120 nm. Surface area values were quite comparable, averaging 58 m 2 /g, although the highest value of 64 m 2 /g was obtained at the industrial scale. The authors concluded that the LDH size distribution and surface area changes of the samples are associated with an increase in scale [14]. These results are consistent with the data obtained in the paper.
Analysis of SEM and SEM-EDS LDH nanoparticles SEM microphotographs of the obtained LDH nanoparticles in the batch system heated with infrared radiation and the LDH in the flow system heated with infrared radiation are shown in Fig. 3. Depending on the method used, nanoplatelets ranging from 100 to 272 nm in length and 13.5 to 36 nm in thickness were obtained. SEM-EDS analysis confirmed a uniform distribution of Zn, Ni, Al, O, and C. Ma et al. obtained γ-Fe 2 O 3 /Cd 2+ -Ni 2+ -Fe 3+ LDHs in a batch process consisting of homogeneous hexagonal platelets with an average transverse size of 6-8 μm and an approximate thickness of 50-100 nm [22]. The application of the flow process resulted in the formation of smaller nanoparticles of approximately 100 nm in size with reduced crystallinity of the samples, which is consistent with the XRD results. The addition of nickel ions further influences the reduction in size of LDH nanoparticles regardless of the modification of the method.  Table 2. The particle size was determined and compared using DLS, SEM, and XRD methods.
The size values of LDH crystallites from XRD analysis and their thickness determined by SEM are similar. The higher particle size values obtained by DLS method in comparison with the width of LDH nanoparticles result from the inclusion of hydration layer (Fig. 4A). The addition of nickel in the structure of the materials results in a reduction of the LDH particle size. Table 2 summarises the band gap (BG) energy values of the LDH nanoparticles obtained, which enable to compare the materials and verify their photocatalytic properties. The energy gap values for LDHs obtained by the batch process were 3.29 eV for both Zn-Al LDH and Ni0.5-Zn-Al LDH (Fig. 4B). These values were comparable to those obtained for ZnO [23]. In contrast, the materials obtained by the flow process are characterized by lower BG values. These changes may be due to a different process of particle formation and increased incorporation of nickel and zinc into LDH structures. Comparing zinc-aluminiumbased LDH materials with LDH containing nickel, zinc, and aluminium, it can be observed that there is no clear effect of nickel addition on the energy gap value. According to this, it can be concluded that the presence of nickel hydroxide in the structure favourably increases the active surface area of the photocatalyst [24].   The BET analysis LDH nanoparticles obtained in a flow-through ATR reactor modified with nickel were compared with zincbased material. The presence of nickel in the composition caused an increase in the surface area by more than 150%, i.e. from a value of 42.03 m 2 /g for pure flow-Zn-Al LDH to 66.94 m 2 /g for flow-Ni-Zn-Al LDH nanoparticles ( Table 3). The presence of nickel increased the pore width of the material (Fig. 5). The P/Po plot has the form of type II hysteresis, which corresponds to the formation of interlaminar pores. The material with nickel addition is characterised by an increased presence of nanopores with diameters below 1 nm and a small number of micropores with sizes of 5-50 nm.

FTIR analysis of LDH nanoparticles
FTIR-ATR analysis confirmed the presence of bonds assigned to groups specific for LDH (Fig. 6). The broad absorption band with a maximum at 3420 cm −1 originates from the stretching vibrations of -OH interlayer water and hydroxide molecules. The high-intensity absorption peaks at 1360 and 779 cm −1 derive from symmetric stretching vibrations and out-of-plane deformation vibrations of CO 3 2− , indicating the presence of interstitial carbonate ions [25]. The higher band intensity at 1360 cm −1 suggests an ordered arrangement of CO 3 2− ions between layers of LDH obtained by batch processes (batch LDH). The weak absorptions in the low-frequency region of 550 and 460 cm −1 are attributed to the stretching bonds of Me-O [22]. Absorption peaks in the 780-645 cm −1 range are associated with vibrations of the Me-O-Me as well as O-Me-O bonds [26].

The XPS analysis
The broad spectrum obtained by XPS analysis (Fig. 7) confirmed the existence of the elements Zn, Al, Ni, C, O, Cl. Figures show the deconvolution of the main peaks (Ni2p, Al2p, Zn2p, C1s, and O1s) of the high-resolution XPS spectrum. The analysis confirmed the formation of Ni(OH) 2 , Zn(OH) 2 , and Al(OH) 3 [27]. The formation of additional forms of metal carbonates, mainly ZnCO 3 , was also confirmed. Ni2p, Zn2p, and Al2p indicate that the oxidation state of these elements is mainly based on ions [28].
The results show that the valence state of zinc on the flow-Ni-Zn-Al LDH surface is + 2 (Lu et al. 2017). The peaks located at 1021.91, 1022.62, and 1023.74 eV correspond to the forms of zinc, respectively: Zn-OH, Zn-CO 3 , and to a small extent Zn-Cl [29] ( Table 4). The O1s spectrum shows signals from all oxygen species on the surface. XPS analysis shows that Ni 2+ and Al 3+ elements are also present [30].  The adsorption and photocatalytic properties of LDH nanoparticles Based on photodegradation studies of quinoline yellow, it was found that the addition of nickel to the LDH structure improves their photocatalytic properties using both batch and flow processes. Zn-Al-LDH materials after 30 min of sorption reached the dye removal efficiency of 40-75%, which was related to the developed surface of the materials. Additionally, the neutral surface character of LDH (PZC of 6.95) improved the adhesion of the natural dye to the catalyst surface. After 60 min of the process conducted in the presence of UV light, degradation rates of 50% and 75% for Zn-Al-LDH materials and 91 and over 99.9% for   materials with nickel addition were achieved (Fig. 8). The high photocatalytic activity of LDH with Ni addition might be due to, among others, the formation of smaller particles, as shown by DLS analysis, and the reduction of the anionic character of the catalyst surface [45].
Based on quinoline yellow photodegradation studies, it was found that the addition of nickel to the LDH structure improves their properties, respectively, improving the dye sorption efficiency of batch-LDH materials to 1%, while for materials obtained in the flow-LDH system (flow-LDH) the difference was already 43%. After 60 min of the process carried out in the presence of UV light, degradation rates of 50% and 75% were obtained for Zn-Al-LDH materials in the flow and periodic systems, respectively, and 91 and over 99.9% for materials with nickel addition (Fig. 8). These changes are due to an increase in the specific surface area of the photocatalysts, which increases 1.5 times for the flow-formed materials after the introduction of nickel. Zn-Al-LDH materials reached a dye removal efficiency of 40-75% after 30 min of sorption. In addition, the addition of nickel reduced the anionic character of the catalyst surface. Thus, the modified material showed a neutral LDH surface character (PZC of 6.95), which improved the adhesion of the natural dye to the catalyst surface (Razzaq et al. 2020). The validity of using nickelmodified LDH materials was also confirmed by Dang et al. They successfully obtained three-dimensional LDH structures based on nickel and aluminium modified with additives, mainly iron ions. The materials succeeded in fluoride ion removal and methyl orange photodegradation processes. The authors confirmed the existence of oxygen vacancies and the formation of heterojunctions in the material, improving its photocatalytic activity [46].
The kinetics of the degradation of quinoline yellow were determined using the Langmuir-Hinshelwood equation, which in linear form takes the form of a pseudo-first-order equation: where k is the photodegradation rate constant [min −1 ]. Kinetic data were analysed, by fitting the logarithm of C 0 /C against the reaction time using experimental curves. The results are shown in Table 5. For QY degradation, the rate constant was 0.01018, 0.0185, and 0.0068 min −1 for pure LDH and 0.0562, 0.0546, and 0.0273 min −1 for Ni-modified LDH. The process rate constants after LDH modification with Ni are more than 10 times higher, indicating a significant improvement in reaction rates. Abderrazek et al. investigated how the calcination process affects the structure of LDH Zn-Al materials and their photocatalytic properties towards MB decomposition. For materials without calcination, the decomposition rate ranged from 0.036 to 0.0519 min −1 , confirming agreement with the results obtained [47]. Based on the studies and literature sources, LDH composites have been shown to be promising adsorbents and photocatalysts for dye removal due to their simple synthesis, low cost, and high compound removal capacity. Nevertheless, one of the main problems, and an essential technological challenge, of using nanoparticles extensively is the transition from the laboratory scale to the industrial scale. Importantly, the increase in scale should not adversely affect the reproducibility of the manufactured nanoparticles and their quality. Flow methods greatly improve the producibility of precipitation methods that have been known to produce polydisperse samples. Mahin et al. presented a continuous, scalable, and reproducible synthesis of PEG-functionalized Fe 3 O 4 nanoparticles. Nanoparticles were obtained using a modular flow system. The obtained nanoparticles proved to be biocompatible and showed reactivity comparable to other commercial iron oxide-based MRI contrast agents. Also, cost analysis, which is one of the key parameters in the development of a method, showed the commercial viability of the synthetic process (80% lower operating and labour costs, compared to the similar batch process) [48].
Further challenges include regulating process control and developing reliable technologies for obtaining a range of nanoparticles, including LDH. Research results in recent years show that flow synthesis methods are able to overcome the challenges of increasing the scale of synthesis of complex nanomaterials, which are successfully finding applications beyond the laboratory scale [49]. Liang et al. presented the controlled synthesis of layered double hydroxide composed of aluminium and cobalt using a newly developed high-temperature, high-pressure flow hydrothermal reactor, which enables better control of precursor saturation, thereby affecting the morphology and size of the nanostructure [50].
An ongoing challenge is the development of methods for separation and drying of the resulting nanoparticles. Although more and more literature is describing successive methods for the flow synthesis of LDH NPs, very limited research has focused on the separation of nanoparticles prepared in this  way from the reaction suspension. After the precipitation of LDH nanoparticles, it is necessary to wash, filtrate, and dry the materials, and these processes are, by standard approaches, batch processes. This suggests the need for further research and development of solutions that can be applied on a large scale over time. Flegler et al. presented a flow LDH synthesis involving a continuous precipitation process using static stirrers. They found that the combination of continuous LDH precipitation in static mixers followed by downstream separation and rinsing in a semi-continuous tube centrifuge in a single pass was ideal for obtaining nanosized LDH and providing steady-state conditions and immediate "quenching" of the precipitation result. This solution has helped not only to improve the precipitability of the entire process but also has had a beneficial effect on process control, since precipitation parameters can be varied at will during synthesis [51].

Conclusion
This paper presents processes for the synthesis of Zn-Al LDH and Ni-Zn-Al LDH nanoparticles in flow infrared radiated agitated tubular reactor (flow-IR-ATR). The materials after modification with nickel hydroxide exhibited high photocatalytic activity and high sorption capacity towards quinoline yellow. An innovative approach combining excellent mixing in the flow by using a shaking process with the delivery of highly efficient IR energy made it possible to reduce the formation time of LDH nanoparticles from a few hours to 30 min. The materials exhibited increased particle size while achieving lower crystallinity. QY photocatalysis studies allowed the determination of QY removal efficiency over 99%. These results indicate the applicability of Ni-Zn-Al LDHs as photocatalysts in organic compound removal processes.