Feasibility of using a rotating packed bed with blade packings to produce ZnO nanoparticles

    Feasibility of using a rotating packed bed with blade packings to produce ZnO nanoparticles Chia-Chang Lin, Chun-Jie Lin PII: DOI: Reference:

S0032-5910(17)30156-0 doi:10.1016/j.powtec.2017.02.029 PTEC 12367

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Please cite this article as: Chia-Chang Lin, Chun-Jie Lin, Feasibility of using a rotating packed bed with blade packings to produce ZnO nanoparticles, Powder Technology (2017), doi:10.1016/j.powtec.2017.02.029

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ACCEPTED MANUSCRIPT Feasibility of using a rotating packed bed with blade packings to produce ZnO nanoparticles

Chia-Chang Lin a,b,*, Chun-Jie Lina Department of Chemical and Materials Engineering, Chang Gung University, Taoyuan, Taiwan,

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a

Department of Psychiatry, Chang Gung Memorial Hospital, Linkou Branch, Taoyuan, Taiwan,

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b

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R.O.C.

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R.O.C.

*corresponding author at: Department of Chemical and Materials Engineering, Chang Gung Taoyuan,

Taiwan,

R.O.C.

Tel.:

+886

3

2118800#5760;

E-mail

address:

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University,

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[email protected] (C.-C. Lin).

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Abstract

A rotating packed bed (RPB) with blade packings was used to produce zinc oxide (ZnO)

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nanoparticles by precipitation. Precursors were firstly produced in a continuous liquid-liquid reaction of zinc chloride (ZnCl2) with sodium hydroxide (NaOH). The effects of the concentrations

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of ZnCl2 and NaOH, the flow rates of aqueous ZnCl2 and NaOH, and the rotational speed on the size of the precursors were studied. Experimental results indicate that increasing concentrations of ZnCl2 and NaOH, decreasing flow rates of aqueous ZnCl2 and NaOH, and decreasing the rotational speed reduced the size of the precursors. The smallest precursors were produced at a ZnCl2 concentration of 0.4 mol/L, an NaOH concentration of 0.8 mol/L, flow rates of aqueous ZnCl 2 and NaOH of 0.3 L/min, and a rotational speed of 600 rpm. Then, the precursors were calcined at 400°C for 1 h to generate ZnO nanoparticles with a mean size of 43 nm and a narrow size distribution. The detailed characterizations revealed that the as-produced ZnO nanoparticles were pure ZnO, which comprised a highly crystalline hexagonal wurtzite phase and exhibited a favorable optical property.

Keywords: Rotating packed bed; ZnO; Nanoparticles; Precipitation

ACCEPTED MANUSCRIPT 1. Introduction ZnO nanoparticles have received substantial interest in recent years, owing to

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their excellent optical, electrical and chemical properties. They have been proved to

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be effective for various purposes, including photocatalytic degradation [1], gas

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sensing [1], catalysis [2], solar cells [3], antibacterial applications [4], biomedical applications [5], and fluorescent applications [6]. Two classes of methods for synthesizing ZnO nanoparticles have been proposed; they are vapor-phase preparation

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methods and liquid-phase preparation methods [7]. Vapor-phase preparation [8-11]

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can easily generate high-quality ZnO nanoparticles, but it typically requires a large amount of energy and expensive equipment [7]. Liquid-phase preparation schemes

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[12-24] are favorable owing to their low energy consumption, cheap equipment, and

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high efficiency [7]. Of the liquid-phase preparation approaches, the precipitation scheme produces ZnO nanoparticles particularly easily [16, 17, 18, 21, 22, 24].

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However, conventional precipitation approaches in a batch reactor cannot yet easily produce ZnO nanoparticles on a large scale. Hence, a novel precipitation approach for

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the mass-production of ZnO nanoparticles is urgently required. Chen et al. [25] were the first to use a rotating packed bed (RPB) to produce CaCO3, Al(OH)3, and SrCO3 nanoparticles in a continuous process. In an RPB, high-gravity conditions are effectively established by rotating doughnut-shaped packings, and liquid can thus be separated into tiny droplets and thin films. Therefore, an RPB can mix liquids vigorously, providing very high and uniform supersaturation. Accordingly, it can be used to produce relatively small and fairly uniform particles by precipitation. The RPB has been widely used to produce nanoparticles, including TiO2 [26], ZnS [27], BaTiO3 [28], BaSO4 [29], Fe3O4 [30], and ZnO [31]. Recently, the authors’ laboratory group successfully used blades as packings in

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ACCEPTED MANUSCRIPT an RPB to produce nanoparticles, including CuO [32], Fe [33], and Mg(OH)2 [34]. The RPB with blade packings has been proven to exhibit a high micromixing

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efficiency [34]. Consequently, an RPB with blade packings is expected to be able to

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produce other nanoparticles. However, the production of ZnO nanoparticles in an

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RPB with blade packings has rarely been examined. In this study, the precursors of ZnO nanoparticles were produced continuously in an RPB with blade packings in which a precipitation reaction occurred. The effects of operating parameters,

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including the concentrations of the reactant and precipitant, the flow rates of the

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reactant and precipitant, and the rotational speed, on the size of the precursors were also investigated. ZnO nanoparticles were produced by calcining the smallest

2. Experimental

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precursors.

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2.1. Production of precursors and ZnO nanoparticles According to the literature [21], the production of ZnO nanoparticles by

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precipitation involves the following chemical reactions [31]. ZnCl2 is firstly reacted with NaOH to generate Zn(OH)2, as described by Eq. (1), and this Zn(OH)2 at the onset of saturation yields ZnO nuclei, according to Eqs. (2) and (3). These ZnO nuclei grow further to produce ZnO nanoparticles. Therefore, the precursors that are produced using an RPB with blade packings by precipitation with ZnCl2 and NaOH may be ZnO with a small amount of Zn(OH)2.

ZnCl2  2NaOH  Zn(OH)2  2NaCl Zn(OH)2  2OH  Zn(OH)4 Zn(OH)4

2

2

 ZnO  H2O  2OH

(1) (2) (3)

Our earlier investigation presented detailed information on the RPB with blade 2

ACCEPTED MANUSCRIPT packings [32]. Fig. 1 schematically represents the experimental procedure for producing the precursors in an RPB with blade packings. The reactant was ZnCl 2

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(Fluka, 98.0%) and the precipitant was NaOH (Mallinckrodt, 99.0%). Tank A

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contained 2 L of aqueous NaOH at a known concentration. Tank B contained 2 L of

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aqueous ZnCl2 at a known concentration. The molar ratio of ZnCl2 to NaOH was set to 1/2, which is the stoichiometric ratio of the reaction (Eq. (1)). The aqueous solutions in both tanks were kept at 25C using a temperature-controlled water bath.

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Then, the aqueous solutions from both tanks were pumped into the packed bed via

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liquid distributors at identical flow rates that were adjusted using flowmeters. In a steady state, the resulting suspension that contained the product, which was

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generated by the rapid precipitation reaction (Eqs. (1)-(3)) in the RPB with blade

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packings, was collected in tank C. The precipitates were obtained by centrifugation at 3500 rpm for 30 min, and washed several times with deionized water and ethanol.

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After they were collected, the wet precipitates were dried at 60C in air for 48 h. Finally, the white precursors were generated by milling the dry precipitates using a

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ceramic mortar. The smallest precursors were subjected to a calcination process, in which they were first maintained at 100°C for 30 min, and then heated at 10°C/min to a calcination temperature, which was retained for 1 h. Calcination yielded ZnO nanoparticles. 2.2 Characterization of precursors and ZnO nanoparticles The weight change of the precursors during calcination was detected using a thermal gravimetric analyzer (TGA, TA Instruments, Q50) in nitrogen. The crystalline structure of the precursors and ZnO nanoparticles was determined using an X-ray diffractometer (XRD, Siemens, D5005) with Cu Kα radiation and a scanning speed of 4º/min. To obtain the mean size of the precursors, the precursors were dispersed in

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ACCEPTED MANUSCRIPT water to form a dilute suspension of 100 mg/L for 30 min with ultrasonic agitation, before was analyzed using a laser diffraction particle size analyzer (Malvern,

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Zatasizer Nano ZS). The morphology of the precursors and ZnO nanoparticles was

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observed using a field-emission scanning electron microscope (FE-SEM, Hitachi,

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S5000). The shape and size of the ZnO nanoparticles were elucidated using a transmission electron microscope (TEM, Hitachi, H7500). The UV-visible absorption spectrum

of

the

ZnO

nanoparticles

was

recorded

using

a

UV-visible

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spectrophotometer (Perkin Elmer, Lambda 900). The elemental composition of the

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ZnO nanoparticles was estimated using an energy dispersive X-ray spectrometer (EDX, Oxford, INCA Energy). The surface functional groups in the ZnO

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nanoparticles were identified using a Fourier transform infrared spectroscope (FTIR,

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Bruker, TENSOR 27).

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3. Results and Discussion 3.1. Size of precursors

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3.1.1. Effect of concentrations of ZnCl2 and NaOH The effect of the concentrations of ZnCl2 and NaOH on the size of the precursors was examined with the other operating parameters fixed. The flow rates of aqueous ZnCl2 and NaOH were fixed at 0.5 L/min and the rotational speed was set at 1800 rpm. As indicated in Table 1, increasing the concentrations of ZnCl2 and NaOH reduced the mean size of the precursors. A reduction in the size of the precursors with increasing concentrations of ZnCl2 and NaOH has also been observed in the production of ZnO nanoparticles in the RPB with structured packings [31]. The precipitation process is normally conducted at a high reactant concentration, which favors a high reaction rate and therefore a high degree of supersaturation,

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ACCEPTED MANUSCRIPT producing instantaneously a large amount of small primary particles [35]. Furthermore, as proposed in our earlier investigation [34], the RPB with blade

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packings supported a high micromixing efficiency. Therefore, a much higher degree

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of supersaturation can be established at higher concentrations of ZnCl2 and NaOH.

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The nucleation of many crystals dominated the production of the precursors because the crystal growth was limited by the short residence time in the RPB with blade packings, which can be less than 1 s. Hence, uniform and tiny precursors were

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obtained at high concentrations of ZnCl2 and NaOH. A ZnCl2 concentration of 0.4

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mol/L and an NaOH concentration of 0.8 mol/L yielded the smallest precursors. The morphology of the precursors depended strongly on the concentrations of

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ZnCl2 and NaOH, as displayed in Fig. 2. The precursors with the flower-like shape

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were produced at a ZnCl2 concentration of 0.05 mol/L and an NaOH concentration of 0.1 mol/L, as displayed in Fig. 2(a). An increase in the concentrations of ZnCl 2 and

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NaOH (0.1 and 0.2 mol/L) also yielded flower-like structure, as displayed in Fig. 2(b). A transition from flower-like to quasi-ellipsoidal precursors was observed at ZnCl2

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and NaOH concentrations of 0.2 and 0.4 mol/L, respectively, as displayed in Fig. 2(c). Smaller and rounder precursors were obtained at a ZnCl2 concentration of 0.4 mol/L and an NaOH concentration of 0.8 mol/L, as displayed in Fig. 2(d). 3.1.2. Effect of flow rates and rotational speed Table 2 presents the effects of the flow rates of aqueous ZnCl2 and NaOH and rotational speed on the mean size of the precursors. The concentrations of ZnCl2 and NaOH were 0.4 mol/L and 0.8 mol/L, respectively. According to Table 2, the mean size of the precursors increased with the flow rates and rotational speed. This trend is the opposite of that obtained for the production of ZnO nanoparticles in an RPB with structured packings [31], in which the size of the precursors decreased as the flow

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ACCEPTED MANUSCRIPT rates and rotational speed were increased. As proposed in our earlier investigation [34], the micromixing efficiency increased with the flow rates and rotational speed in

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the RPB with blade packings. Thus, smaller particles were expected to be formed at

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higher flow rates and rotational speeds. However, the micromixing time in an RPB

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with blade packings may exceed the time required to induce the nucleation of the precursors. The difference between the micromixing and nucleation time may be larger at higher rotational speeds. Consequently, flow rates of aqueous ZnCl2 and

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3.2. Properties of ZnO nanoparticles

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NaOH of 0.3 L/min and a rotational speed of 600 rpm yielded the smallest precursors.

For TGA, the precursors that were produced at a ZnCl2 concentration of 0.4

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mol/L, an NaOH concentration of 0.8 mol/L, a rotational speed of 600 rpm, and flow

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rates of aqueous ZnCl2 and NaOH of 0.3 L/min, were heated from 30C to 700C at a rate of 10C/min. Fig. 3 presents the weight of the precursors as a percentage of the

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initial weight versus temperature. The weight percentages at 400C, 450C, and 550C were 97.3%, 97.1%, and 96.8%, respectively. These weight percentages were

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much lower than the stoichiometric weight ratio of 82%, which was obtained using Eq. (4). This dramatic difference confirms that the precursors that were produced at a ZnCl2 concentration of 0.4 mol/L, an NaOH concentration of 0.8 mol/L, a rotational speed of 600 rpm, and flow rates of aqueous ZnCl2 and NaOH of 0.3 L/min, comprised ZnO with a small amount of Zn(OH)2. Thus, a temperature of over 400C allows the transformation of the precursors into highly pure ZnO nanoparticles. Therefore, the calcination temperatures were set to 400C, 450C, and 550C.

Zn(OH)2  ZnO  H2O

(4)

The crystalline structures of the ZnO nanoparticles that were produced by calcination at various temperatures were characterized using XRD, as shown in Fig. 4. 6

ACCEPTED MANUSCRIPT At the three calcination temperatures, all of the reflection peaks were assigned to diffraction from the (100), (002), (101), (102), (110), (103), (200), (112), and (201)

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planes of ZnO with a hexagonal wurtzite phase (JCPDS 36-1451), so all peaks from

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the as-produced ZnO nanoparticles at the three calcination temperatures were

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consistent with pure ZnO. Moreover, the intensity of (101) peak from the ZnO nanoparticles increased with the calcination temperature, suggesting that the crystallinity of the ZnO nanoparticles that were produced at a calcination temperature

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of 550C exceeded those of the ZnO nanoparticles that were obtained using

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calcination temperatures of 400C and 450C.

A parameter “degree of orientation” [36] of the (002) plane is defined as (5)

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I002 I100  I002  I101

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c ( 002) 

where I100, I002, and I101 are the intensities of (100), (002), and (101) peaks,

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respectively. Similarly, c(100) and c(101) represent the degrees of orientation of the (100) and (101) planes, respectively. Table 3 indicates the effect the calcination

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temperature on the degree of orientation. As evidenced by Table 3, all c values were almost independent of the calcination temperature. Since the c(100) value was the largest at all calcination temperatures, the orientation of the crystallites was mostly directed along the (101) plane at all calcination temperatures. Table 4 shows the effect of calcination temperature on the mean size of the ZnO nanoparticles, estimated from the SEM and TEM images that are displayed in Figs. 5 and 6. The mean size of the ZnO nanoparticles increased with the calcination temperature. This behavior may be attributed to the fact that increasing the calcination temperature increased the extent of agglomeration of the ZnO nanoparticles and thereby reduced their surface area [37]. However, the morphology of the ZnO

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ACCEPTED MANUSCRIPT nanoparticles was found to be almost independent of the calcination temperature, as displayed in Fig. 5. Additionally, the ZnO nanoparticles at all calcination

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temperatures exhibited a narrow size distribution, as displayed in Fig. 6.

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Fig. 7 presents typical UV-visible spectra of the ZnO nanoparticles that were

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produced using different calcination temperatures. A shift of the sharp peak from 370 nm to 377 nm with increasing calcination temperature is observed, perhaps because crystal defects in the form of oxygen vacancies were formed at higher calcination

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temperatures [37]. These oxygen vacancies may be neutral (VOX), singly charged

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(VO+), or doubly charged (VO++) [37, 38]. This shift in the sharp peak to the higher wavelengths with increasing calcination temperature reduced the band gap of the ZnO

1242  max (nm)

(6)

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E g (eV ) 

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nanoparticles, consistent with Eq. (6) [39].

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where Eg is the band gap and max is the wavelength of the sharp peak. The band gaps of the ZnO nanoparticles that were produced using calcination temperatures of 400C,

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450C, and 550C were 3.36 eV, 3.35 eV, 3.29 eV, respectively. All as-produced ZnO nanoparticles exhibited a single and well-defined absorption peak, which was related to the characteristic peak for the hexagonal wurtzite phase of pure ZnO, as suggested in the literature [37, 40]. A single absorption peak from the as-produced ZnO nanoparticles further indicates that these ZnO nanoparticles exhibited favorable optical properties [37, 40]. Furthermore, the absorbance at the sharp peak declined as the calcination temperature was increased. The highest absorbance was obtained at a calcination temperature of 400C. As presented in Fig. 7, a very strong absorption that was centered around 370 nm reveals that the ZnO nanoparticles that were produced using a calcination temperature of

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ACCEPTED MANUSCRIPT 400C had a narrow size distribution [17,41]. As shown in Fig. 4, the XRD patterns included no other reflection peak

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associated with any impurity, up to the limit of XRD detection, further revealing that

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the ZnO nanoparticles that were produced using a calcination temperature of 400C

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were pure ZnO without any significant impurity. This finding was further verified by EDX, as shown in Fig. 8. The obtained EDX spectrum included only zinc and oxygen peaks and no other peak associated with any impurity. ZnO nanoparticles that were

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produced using a calcination temperature of 400C revealed a composition of 49.6%

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Zn and 50.4% O, confirming the purity of the ZnO, which comprised zinc and oxygen only.

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Fig. 9 displays the FT-IR spectrum of the ZnO nanoparticles that were produced

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at a calcination temperature of 400C. The spectrum includes numerous well-defined peaks at 420 cm-1, 1630 cm-1, and 3449 cm-1. The well-defined peak at 420 cm-1 arose

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from the Zn-O bond and so verified the formation of ZnO [42]. A broad peak at 3449 cm-1 probably arose from the stretching vibration of the O-H bond, while the peak at

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1630 cm-1 was associated with the bending vibration of the O-H bond [37,40]. These peaks were all associated with the adsorbed water in the ZnO nanoparticles. The yield of ZnO nanoparticles by precipitation in the RPB with blade packings was approximately 80%. The rate of production of ZnO nanoparticles was estimated using the following equation, under the production conditions [ZnCl2]=0.4 mol/L, [NaOH]=0.8 mol/L, flow rate=0.3 L/min, and yield=80%. Rate of production (kg ZnO/day) = flow rate (L/min)×60 (min/h)×24 (h/day)×[Zn2+] (mol/L) ×yield (mol of ZnO/mol of Zn2+)×0.0814 (kg of ZnO/mol of ZnO)

(7)

Accordingly, the rate of production of ZnO nanoparticles by precipitation in the RPB 9

ACCEPTED MANUSCRIPT with blade packings was 11.3 kg/day.

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4. Conclusions

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In this study, precursors were produced by continuously feeding aqueous ZnCl2

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and aqueous NaOH into an RPB with blade packings. The size of the precursors was found to be related to the concentrations of ZnCl2 and NaOH, the flow rates of aqueous ZnCl2 and NaOH, and the rotational speed. The size of the precursors

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decreased as the concentrations of ZnCl2 and NaOH were increased, but increased

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with the flow rates of aqueous ZnCl2 and NaOH and the rotational speed. ZnO nanoparticles were produced by calcining the precursors that were themselves

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produced at a ZnCl2 concentration of 0.4 mol/L, an NaOH concentration of 0.8 mol/L,

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flow rates of aqueous ZnCl2 and NaOH of 0.3 L/min, and a rotational speed of 600 rpm, at 400C for 1 hr. ZnO nanoparticles that were produced under these conditions

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had a mean size of 43 nm with a narrow size distribution. According to detailed characterizations, as-produced ZnO nanoparticles were highly crystalline with the

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hexagonal wurtzite phase and exhibited a favorable optical property. The rate of production of ZnO nanoparticles was 11.3 kg/day. Accordingly, precipitation in an RPB with blade packings is effective in producing ZnO nanoparticles on a large scale.

Acknowledgements The authors are thankful for the financial support of the Chang Gung Memorial Hospital (CMRPD 290081, CMRPD 290082) and the Ministry of Science and Technology of the Republic of China, Taiwan (MOST 102-2221-E-182-001-MY3).

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[39] R. Kumar, G. Kumar, M.S. Akhtar, A. Umar, Sonophotocatalytic degradation of methyl orange using ZnO nano-aggregates, J. Alloys Compd. 629 (2015)

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167-172.

[40] R. Kumar, G. Kumar, A. Umar, ZnO nano-mushrooms for photocatalytic

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degradation of methyl orange, Matt. Lett. 97 (2013) 100-103. [41] H.J. Zhai, W.H. Wu, F. Lu, H.S. Wang, C. Wang, Effects of ammonia and cetyltrimethylammonium bromide (CTAB) on morphologies of ZnO nano- and micromaterials under solvothermal process, Mater. Chem. Phys. 112 (2008) 1024-1028. [42] A. Becheri, M. Dürr, P.L Nostro, P. Baglioni, Synthesis and characterization of zinc oxide nanoparticles: application to textiles as UV-absorbers, J. Nanopart. Res. 10 (2008) 679-689.

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ACCEPTED MANUSCRIPT Figure Captions Experimental procedure for producing ZnO nanoparticles.

Fig. 2.

FE-SEM images of precursors obtained at (a) [ZnCl2]=0.05 mol/L,

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Fig. 1.

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[NaOH]=0.1 mol/L, (b) [ZnCl2]=0.1 mol/L, [NaOH]=0.2 mol/L, (c)

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[ZnCl2]=0.2 mol/L, [NaOH]=0.4 mol/L, and (d) [ZnCl2]=0.4 mol/L, [NaOH]=0.8 mol/L. TGA of precursors.

Fig. 4.

XRD patterns of ZnO nanoparticles obtained at calcination temperatures of

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Fig. 3.

Fig. 5.

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(a) 400C, (b) 450C, and (c) 550C.

FE-SEM images of ZnO nanoparticles obtained at calcination temperatures

TEM images of ZnO nanoparticles obtained at calcination temperatures of

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Fig. 6.

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of (a) 400C, (b) 450C, and (c) 550C.

(a) 400C, (b) 450C, and (c) 550C. UV-visible absorption spectra of ZnO nanoparticles obtained at calcination

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Fig. 7.

temperatures of (a) 400C, (b) 450C, and (c) 550C.

Fig. 9.

EDX spectrum of ZnO nanoparticles obtained at 400C.

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Fig. 8.

FT-IR spectrum of ZnO nanoparticles obtained at 400C.

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ACCEPTED MANUSCRIPT Table Legends Table 1. Effect of concentrations of ZnCl2 and NaOH on mean size of precursors.

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Table 2. Mean size of precursors at various flow rates and rotational speeds.

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Table 3. Effect of calcination temperature on c.

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Table 4. Effect of calcination temperature on mean size of ZnO nanoparticles.

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Fig. 1

blade packings

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blade packings

Motor

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Tank C

Flowmeter

Valve

Tank A

Flowmeter

Valve

Tank B Pump

Pump

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(b)

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D

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(a)

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Fig. 2

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(c)

(d)

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Fig. 3

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Fig. 4

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(a)

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(b)

(c)

ACCEPTED MANUSCRIPT Fig. 6

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(a)

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(b)

(c)

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Fig. 7

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Fig. 8

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Fig. 9

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[NaOH]

Mean size

(mol/L)

(nm)

0.1

533

0.2

486

0.2

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0.4

165

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Table 1

[ZnCl2]

0.4

0.8

65

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(mol/L)

0.1

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0.05

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Rotational speed (rpm)

Flow rate

600

0.3

600

NU

27

0.5

40

0.7

55

0.3

30

0.5

47

0.7

69

1400

0.3

38

1400

0.5

69

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Table 2

1400

0.7

76

1800

0.3

56

1800

0.5

65

1800

0.7

77

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(L/min)

600

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1000

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1000

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1000

Mean size (nm)

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Table 3

Calcination temperature (C)

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550

c(002)

c(101)

0.2835

0.2446

0.4719

0.2828

0.2481

0.4691

0.2866

0.2294

0.4840

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450

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400

c(100)

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Table 4

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450

TE

400

AC

550

SEM mean size (nm)

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Calcination temperature (C)

TEM mean size (nm)

39

43

46

50

71

67

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Graphical abstract

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Highlights

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 RPB with blade packings can produce ZnO nanoparticles on a large scale.

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 ZnO nanoparticles exhibit a good optical property.

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 ZnO nanoparticles have a mean size of 43 nm.