Impinging Jet Micromixer for Flow Synthesis of Nanocrystalline MgO

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Impinging Jet Micromixer for Flow Synthesis of Nanocrystalline MgO: Role of Mixing/Impingement Zone D. V. Ravi Kumar,† B. L. V. Prasad,*,† and A. A. Kulkarni*,‡ †

Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune − 411 008, India Chemical Engineering and Process Development Division, CSIR-National Chemical Laboratory, Pune − 411 008, India

‡

S Supporting Information *

ABSTRACT: Continuous flow synthesis of nanomaterials via sol-gel process using microreactors has not received much attention. For the synthesis of gels where the reaction time is of the order of few seconds to few tens of seconds, microchannel reactors are an excellent processing option. However since a ‘gel’ does not ‘flow’ easily, making it in a microchannel usually clogs the microchannel. We envisaged that by using an impinging jet micromixer, where the two reactants impinge and collide to form a mixing zone outside the micromixer, this problem could be circumvented. Here we report a successful implementation of continuous flow synthesis of metal oxides formed by the rapid sol-gel process taking the nanocrystalline MgO (NC-MgO) as a specific example. Furthermore, we clearly demonstrate that the nature of the mixing zone formed by the impinging jets governs the surface area of the product. Specific flow rate and impingement angle are identified that yield high surface area materials.

1. INTRODUCTION Continuous flow synthesis using microreactors/miniaturized devices is now a known and accepted approach for process intensification for the synthesis of organic compounds1 and inorganic and polymeric materials.2,3An ample number of examples can be found in the literature where microreactors have been used for the synthesis of several nanomaterials (metals, metal oxides, quantum dots, polymeric nanoparticles, etc.) and their combinations.2,4 Although this offers a greater control over features such as size distribution, homogeneous composition, and shape control of materials than the conventional batch synthesis approach, these are still in nascent stages with respect to their applicability to complex systems. In this context though, a plethora of reactor designs have been used to prepare almost all types of nanomaterials; the methods of synthesis have largely been restricted to those that result in the formation of nanoparticle dispersions in solvents rather than precipitation/gels. Materials accessed via sol-gel processes are generally considered not suitable for continuous flow synthesis, because a ‘gel’ by definition does not ‘flow’. On the other hand, confined impinging jet reactors (CIJs) are used successfully for very fast reactions such as precipitation5 where mixing is considered an important phenomenon in nanoparticle synthesis.6 CIJs are well-studied experimentally and theoretically to understand and characterize the mixing and are known to have advantages over the conventional stirred tank reactors.7,8 Also, a few examples of nanoparticle synthesis using CIJs are known.5,9,10 However, CIJs are unfavorable to sol-gel processes because of the same reason of channel blocking as mixing does not occur ina “wallfree environment”.11 With this background, we envisaged that if the mixing can be made to happen in a free space we could realize the preparation of a nanocrystalline metal oxide via solgel processes in a continuous flow manner. Rapid mixing by impingement of two thin liquid sheets in free space was initially studied by Demyanovich and co-workers.12,13 The nature of © 2013 American Chemical Society

mixing zones (i.e., mixing sheet formed by the impingement of two liquid jets) was further studied by Li et al.,14 and the atomization pattern produced by the liquid jets was investigated by Jung et al.15 Mixing in impinging liquid jets depends upon the thickness of the mixing sheet, pressure drop, and viscosity of the fluids. The inelastic nature of the collision results in high energy dissipation making the flow turbulent; hence, effective mixing can be expected. Relatively efficient mixing can be achieved for Reynolds number, Re > 1000. In this contribution we show the adoption of a free impingement jet micromixer16 (IJM) concept for the synthesis of nanocrystalline MgO (NC-MgO) noting that it can be easily generalized for any other material whose synthesis is accomplished by sol−gel processes. NC-MgO is an effective catalyst for many organic reactions17 and has been established as “destructive adsorbent” for toxic materials.18 The surface area of the commercially available NC-MgO is ≥250 m2/g and is prepared by sol-gel process.19 Klabunde and co-workers showed that adding toluene as a spectator solvent during the gelation alters the gelation kinetics and increases the quality of the product in terms of surface area. This process involves the hydrolysis of Mg(OCH3)2 in the presence of methanol and toluene as the solvent mixture and forms a rigid gel very rapidly (gelation time < 30 s). The rigid gel was subjected to supercritical drying to obtain the high surface area aerogels.20,21 The typical synthesis method based on the sol−gel process involves mixing equal volumes of 0.4 M Mg(OCH3)2 and 0.8 M H2O solution in methanol−toluene mixture (with the toluene/ methanol volume ratio 1.60 after mixing). Within a few seconds the resultant mixture of two solutions turned to rigid gel. The gel was allowed to dry for 12 days under ambient conditions, Received: Revised: Accepted: Published: 17376

June 29, 2013 September 11, 2013 October 31, 2013 October 31, 2013 dx.doi.org/10.1021/ie402012x | Ind. Eng. Chem. Res. 2013, 52, 17376−17382

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Figure 1. (A) Design of the impinging jet micromixer. (1 and 2: Inlets for reactants, 3: support plates, 4: support tension springs, 5: screw for adjusting angle of the impinging sections, 6: metallic blocks having microscopic bore, 7: impingement zone.) (B) Photograph of the microjet/ micromixer without the spring attachment, (C) Schematic representation of the experimental setup. (D) Schematic of the mixing zone. The different parameters used to analyze the mixing zone are marked.

vacuum-dried for 3 days followed by heating at 70−80 °C. This procedure results in the formation of MgO precursor aerogels. Calcination of these aerogels at 500 °C gives nanocrystalline MgO aerogels. Typically, for any homogeneous reaction involving two or more reactants, the rate of mixing should be faster than the rate of reaction. This implies that for the case of NC-MgO, rapid mixing is necessary to achieve homogeneous nucleation and also better product quality with high surface area. Any extent of improper mixing can affect the scale-up of such a process similar to conventional batch-to-batch variations. Thus IJM is suited to avoid such variations. Hydrolysis of Mg(OCH3)2 can be controlled effectively to tune the gelation kinetics using IJM, and aerogels with high surface area can be obtained by controlled drying of wet gels, even without opting for supercritical drying as reported by Klabunde and coworkers.18,21 In view of the above Introduction, the manuscript is organized as follows: in the Experimental Section we present the design of the microjet−micromixer, the flow synthesis approach for synthesizing NC-MgO using this microjet− micromixer, and the drying method. This is followed by a discussion on the results obtained under different conditions and their relationship to the nature of the mixing zone. Upon analyzing the results to explore the relationship between the extent of mixing and the properties of the dry gel, we conclude the manuscript, giving some highlights on the scale-up aspects.

2. EXPERIMENTAL SECTION 2.1. Fabrication of Impinging Jet Micromixer. A microjet device was fabricated and assembled such that it comprises two micromachined segments attached on a backbone structure that allows us to change the angle between the microchannels in the same plane. A detailed schematic is shown in Figure 1A. The different components of the device are described subsequently. The microchannels of 0.3 mm diameter were machined in SS316 segments (6) (W ≈ 10 mm, H ≈ 10 mm, L ≈ 12 mm). The two segments (6) can be adjusted simultaneously to get an equal angular distance from the point of jet interaction (Figure 1B). The tension springs (4) were used to ensure that the two segments (6) are held at a fixed distance and at a fixed angle, which can be adjusted by using the rotation screw (5). The experimental setup is shown in Figure 1C. Two reactants are pumped in the individual segments using syringe pumps (Longer Precision Pumps Ltd., China). The injected fluids exit the segments at high velocity and intersect to yield a thin sheet of mixing zone followed by a thread (Figure 1D). The velocity of the jets was adjusted to obtain a stable mixing zone. The images of the mixing zone at different jet velocities and at different angles between the jets were recorded by using a high-speed camera (Red Lake, U.S.A.) at a frame rate of 500 fps. The images were analyzed using Image-Pro Plus (version 5.1) software. 2.2. Experimental Procedure. Mg(OCH3)2(6−10 wt %, Sigma-Aldrich), methanol (Merck Chemicals, Germany), 17377

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Figure 2. (A) High speed camera images of the mixing zone at different angle between the jets (θ = 70° to 120°). (B) Schematic of the impingement angle on the stability of mixing zone and its thickness. (C) Effect of jet impingement angle on the thickness of mixing zone (μm). (D) Effect of jet impingement angle on the aspect ratio of the mixing zone. At 140° the mixing zone was not stable and hence is not included in this analysis.

2.3. Characterization. The following techniques were used for characterizing the gel samples prepared under different conditions. (1) FT-IR spectra of the samples were recorded using Perkin-Elmer FT-IR spectrophotometer, in the range of 4000−450 cm−1 with a resolution of 4 cm−1. (2) X- ray diffraction patterns of the dried and calcinated samples were recorded on the PanalyticalXpert Pro instrument operated at 40 kV and 30 mA using Cu Kα radiation. X- ray diffraction patterns of the sample were recorded in the 2θ range of 10−80° with scan rate of 2.3°/min. (3) N2 adsorption and desorption isotherms of the samples were recorded using a Quantachrome-Autosorb instrument. The surface area of the sample was calculated by BET method using the obtained isotherm in the relative pressure range of 0.05−0.3. Pore size distribution was calculated by the BJH method using the desorption isotherm. Total pore volume was calculated at the maximum relative pressure value in the isotherm. (4) A small quantity of the sample was dispersed in ethanol and was coated onto a TEM grid and was allowed to dry. Transmission electron micrographs of the sample were

toluene (Merck Chemicals, Germany), and Milli-Q grade water were used for all the experiments. Solutions of Mg(OCH3)2 (0.4 M) and H2O (0.8 M) were prepared using methanol and toluene mixtures, such that the toluene-to-methanol volume ratio is maintained at 1.60. Two different glass syringes were filled with these solutions and syringe pumps (Longer Precision Pumps Ltd., China) were used to dispense the solutions from the syringes. Syringes were connected to the inlets of the IJM (Figure 1A) using glass-to-metal connectors (Figure 1C). Wet gel samples were synthesized at different flow rates and at different angles between the jets. Subsequently, the gel was collected in sample vials. Gels were allowed to age for 1 day, then were vacuum-dried at 90 °C for 12 h followed by calcination at 500 °C for 1 h. The dried gels were subjected to characterization. Synthesis of NC-MgO was also carried out in batch process by adopting the sol−gel process with some modifications.21 In brief, to the 0.8 M H2O solution, was added an equal amount of 0.4 M Mg(OCH3)2 solution (prepared in toluene− methanol mixtures) in a beaker at room temperature, and the solution turned to rigid gel within 30 s. The wet gel was dried as explained above to obtain NC-MgO. 17378

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Figure 3. N2 sorption isotherms of the NC-MgO samples and inset shows the pore size distribution of the samples which were synthesized at different angle between the jets.

(see Figure SI-1, Supporting Information [[SI]). This indicates the presence of uniform pores in the sample.22 In the case of synthesis of NC-MgO in a microchannel reactor, the two reactants upon mixing yield a rigid gel almost instantaneously and thus clog the microchannel eventually. We, therefore, used an IJM12,13 for this purpose. In IJMs, mixing occurs outside the channels and the extent of mixing decides the homogeneity in the nucleation of the nanostructures. 3.1. Effect of Jet Impingement Angle (θJ). Upon impingement of two coplanar cylindrical jets, a mixing zone in the form of a thin sheet is formed where the shape and size of the liquid sheet are determined by the angle at which they impinge (Figure 1D).14,15 We designed our experimental setup such that the jet containing Mg(OCH3)2 in the methanol− toluene mixture impinges on a jet that contains water in a methanol−toluene mixture. The angle between the impinging jets could be changed over a wider range as per the requirement. Initially, the volumetric flow rate of the individual jets was maintained as 15 mL/min (per syringe) to obtain strong and stable jets followed by a thin mixing zone (Figure 2A). Increasing the impingement angle leads to thinning of the impingement zone that leads to unstable impingement resulting in instantaneous fragmentation of the fluid. A schematic of the observation is shown in Figure 2B. Quantitative observation of this effect can be seen in Figure 2C,D. Figure 3 shows N2sorption isotherms, and Table 1 shows the summary of the characterization details of the samples. There was a small but gradual increase in the BET surface area of the samples obtained as the angle of jet impingement varied from 70° to 120°. All the analytical characterizations (see Figure SI-2, SI for PXRD results and Figure 5 for TEM, SAED, and HRTEM images) confirm that the sample is nanocrystalline MgO. Among all the samples prepared at different angles, the maximum BET surface area (∼340 m2/g), was observed for the sample obtained by impinging the jets at 120°, and this value is nearly 2 times higher than the BET surface area of the batch sample. Upon increasing the impingement angle further from 120° to 140°, the BET surface area of the sample decreased from 340 to 228 m2/g. These changes in the surface area are attributed to the nature of mixing and the local homogeneity/

recorded using Technai-T20 transmission electron microscopy, operated at 300 kV.

3. RESULTS AND DISCUSSION Initially we carried out the synthesis of NC-MgO in batch method using the sol-gel process with some modifications. The Table 1. Details of the Surface Area and Pore Size Analysis Obtained for Samples Prepared at Different Angles between the Jets sample

angle between jets (deg)

BET surface area (m2 /g)

pore volume (mL/g)

crystallite size (nm) from XRD

1 2 3 4

70 90 120 140

299 321 339 228

0.69 0.72 0.77 0.59

4.7 5.5 4.7 5.8

Table 2. Different Parameters (refer to Figure 1C) of the Mixing Zone Determined at Different Angles between the Jets impinge. angle between jets (deg)

ave aspect ratio (-)

mixing zone - ave thickness (μm)

BET surface area (m2/g)

∼70 ∼90 ∼120 ∼140

2.1 1.8 1.5 −

14.3 13.4 8.7 −

299 320 340 228

Table 3. Details of the Surface Area and Pore Size Analysis for Different Flow Rates sample

total flow rate (mL/min)

Re

BET surface area (m2/g)

pore vol. (mL/g)

crystallite size (nm) from XRD

1 2 3

20 30 40

1040 1560 2080

327 322 250

0.73 0.85 0.59

4.9 4.6 4.7

BET surface area of the batch sample was determined to be ∼190 m2/g, with type H1, adsorption−desorption isotherm 17379

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Figure 4. N2 sorption isotherms of the NC-MgO sample and inset of (B) shows the pore size distribution of the samples which were synthesized at different flow rates, the angle between the jets was kept constant (120°).

surface area of the sample at different jet impingement angles. Since the impingement region for the impingement angle of 140° and beyond it no longer remains planar, it was difficult to measure the aspect ratio and average thickness of the same. Moreover, formation of several satellite drops was seen in almost the entire impingement area, indicating the unstable impinging zone. 3.2. Effect of Reynolds Number (Re). Apart from the dimensions of the impinging region, its structure and the residence time in the mixing zone also affects the extent of mixing in these domains. Since high surface area is achieved for an impingement angle of 120°, further studies to understand the effect of residence time and structure of mixing zone on the properties of dry gel were carried out at this angle. The jet Reynolds number was varied by changing the superficial jet velocity (u) while keeping the jet diameter (300 μm) and angle (θJ = 120°) constant. With increasing flow rate of the reactants (i.e., the jet velocity), the shape of the impingement zone (mixing zone) changed considerably thereby changing its thickness and also the residence time in the mixing zone. This would affect the surface area of the material significantly. Figure 4 shows the N2 adsorption − desorption isotherms of the samples synthesized at different flow rates. The flow rates for individual reactants (jet) were varied in the range of 10 to 20 mL/min (individual jet velocity ∼2.31 − 4.73 m/s), beyond which it was difficult to get stable mixing zone. Table 3 summarizes the analysis of the dried gel samples synthesized at different Re. For all the samples synthesized using the IJM, the isotherms are close to type IV which indicates the presence of micropores in addition to mesopores. Hysteresis of these samples is closer to H1 and this kind of hysteresis loop is generally observed from the porous materials which consist of agglomeration of uniform spheres of uniform size and shape or particles connected by a network of nearly cylindrical channels.22,23 TEM images of the high surface area sample are shown in Figure 5. Selective area electron diffraction (SAED) shows the polycrystalline nature of the sample and the corresponding

Figure 5. (A) TEM, and (B−C) HRTEM images of NC-MgO synthesized at flow rate of 30 mL/min with the angle between the jets 120° (D) SAED pattern of image (C).

inhomogeneity in the mixing zone. To analyze this aspect better, images of the mixing zone were captured by a highspeed camera (typical residence time in the mixing zone was in the range of 0.5−20 ms). Figure 2A displays the high-speed camera images of the mixing zones at different impingement angles between the jets (θJ). The thickness of the mixing zone and aspect ratio (a/b) of the mixing zone at different angles are displayed respectively in C and D of Figure 2. It can be noticed that a very thin mixing zone with the aspect ratio close to unity (i.e., a ≈ b) was obtained when θJ was 120°. Since the volume of the reagents pumped was constant and equal flow rates were maintained for both the jets, the enhancement in the mixing was purely due to a thinner mixing zone with larger area. This enhanced mixing helps to achieve uniformity in the concentration and thereby ensures a uniform and fast reaction rate (more homogeneous nucleation). Table 2 summarizes the comparison between thickness of the mixing zone and the BET 17380

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Figure 6. (A) Schematic of the jet impingement region. (B) High-speed camera images of the mixing zone at different flow rates at a fixed angle of impingement ∼120°. (C) Effect of flow rate on the thickness of the impingement region. (D) Effect of flow rate on the aspect ratio of the mixing zone.

region was small and the reaction did not finish in the highshear mixing zone. Thus, higher flow rates which do not lead to a stable mixing zone are actually not useful for this specific case. Thus, even though the thickness of the mixing zone was high when the individual flow rate of the jets was 10 mL/min, it did not have any adverse affect on the surface area of this sample. Thus, a combination of better mixing and sufficient residence time is needed to achieve higher surface area for the gel. While the present work is restricted in exploring the effect of different design and operating parameters on the final surface area of the material, the scale-up needs to be explored to produce it in large quantity. One of the ways can be to explore the impingement of two-dimensional (2D planar) jets rather than one-dimensional (1D) jets at a fixed angle. More studies on the hydrodynamics and mixing of planar jets and their possible application for the synthesis of nanogels are in progress. Issues such as the nature (shape and size) of the mixing zone, its stability, etc. studied experimentally and through the flow modeling of the impinging planar jets will be discussed separately.

planes were indexed on the diffraction pattern. The lattice plane seen in Figure 5B corresponds to the (200) plane of MgO. The ideal flow rate of the individual jets for obtaining high surface area of the materials was in the range 10−15 mL/min. When this flow rate increased from 15 to 20 mL/min, the surface area of the synthesized NC-MgO decreased from ∼325 to 250 m2/g. (We verified these observations a few times, and the results were reproducible within a range of ±4%.) With increasing liquid flow rate the mixing zone was found to deviate from planar topology (Figure 6A), that severely affects the surface area. The thickness and aspect ratio of the mixing zone at different flow rates are respectively shown in C and D of Figure 6. At a fixed flow rate the film thickness was found to have a slight variation around the mean value. This was observed because, while the flow rate was constant, rapid increase in the viscosity of the fluid in the mixing region showed an aftereffect on the fluid that would impinge, thereby leaving a fluctuating nature of the impingement region. This kind of historical effect was further verified from the observation of the aspect ratio. Also, no significant variation in the final surface area of the material was observed due to the transient variation in the film thickness or the aspect ratio. However, while, in general, the mean film thickness decreased with increasing flow rate, the final surface area actually decreased as the contact time for the reagents in the mixing

4. CONCLUSION A continuous flow approach was developed for the synthesis of nanocrystalline MgO which has a great market potential among nanoproducts. A rapid sol-gel process is successfully trans17381

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formed to continuous flow process using an impinging jet micromixer. The angle between the jets and flow rates at which the reactants impinge were optimized to obtain maximum surface area of the sample. The role of the nature of mixing zone on the final surface area of the dry gel has been studied. The surface area of the sample, synthesized through continuous flow methods was comparable to market standards (Nanoscale Corp., U.S.A.) with high reproducibility and consistency. The method is quite generic and can be easily adapted to other materials synthesized by a sol-gel process.

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(11) https://www.imm-mainz.de/fileadmin/IMM-upload/FlyerKatalog_etc/Catalogue09_IJMM.pdf. (12) Demyanovich, R. J.; Bourne, J. R. Rapid micromixing by the impingement of thin liquid sheets. 1. A photographic study of the flow pattern. Ind. Eng. Chem. Res. 1989, 28 (6), 825−830. (13) Demyanovich, R. J.; Bourne, J. R. Rapid micromixing by the impingement of thin liquid sheets. 2. Mixing study. Ind. Eng. Chem. Res. 1989, 28 (6), 830−839. (14) Li, R.; Ashgriz, N. Characteristics of liquid sheets formed by two impinging jets. Phys. Fluids 2006, 18, 087104. (15) Jung, S.; Hoath, S. D.; Martin, G. D.; Hutchings, I. M. Atomization patterns produced by the oblique collision of two Newtonian liquid jets. Phys. Fluids 2010, 22, 042101. (16) Erni, P.; Elabbadi, A. Free impinging jet microreactors: controlling reactive flows via surface tension and fluid viscoelasticity. Langmuir 2013, 29 (25), 7812−7824. (17) Chintareddy, V. R.; Lakshmi Kantam, M. Recent developments on catalytic applications of nano-crystalline magnesium oxide. Catal. Surveys Asia 2011, 15 (2), 89−110. (18) Rajagopalan, S.; Koper, O.; Decker, S.; Klabunde, K. J. Nanocrystalline metal oxides as destructive adsorbents for organophosphorus compounds at ambient temperatures. Chem.Eur. J. 2002, 8 (11), 2602−2607. (19) http://en.wikipedia.org/wiki/NanoScale_Corporation (20) Diao, Y.; Walawender, W. P.; Sorensen, C. M.; Klabunde, K. J.; Ricker, T. Hydrolysis of magnesium methoxide. Effects of toluene on gel structure and gel chemistry. Chem. Mater. 2002, 14 (1), 362−368. (21) Ranjit, K. T.; Klabunde, K. J. Solvent effects in the hydrolysis of magnesium methoxide, and the production of nanocrystalline magnesium hydroxide. An aid in understanding the formation of porous inorganic materials. Chem. Mater. 2005, 17 (1), 65−73. (22) Leofanti, G.; Padovan, M.; Tozzola, G.; Venturelli, B. Surface area and pore texture of catalysts. Catal. Today 1998, 41 (1), 207−219. (23) Reichenauer, G.; Scherer, G. Nitrogen sorption in aerogels. J. Non-Cryst. Solids 2001, 285 (1), 167−174.

ASSOCIATED CONTENT

S Supporting Information *

(i) N2 sorption isotherm for the batch sample, (ii) PXRD of the NC-MgO synthesized in batch and continuous flow methods, (iii) FT-IR spectra of the samples synthesized in batch and continuous flow methods. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Fax: 9120-25902636. Telephone: 9120 25902013. E-mail: pl. [email protected]. *Fax: 9120-25902621. Telephone: 9120- 25902153. E-mail: aa. [email protected]. Notes

The authors declare the following competing financial interest(s): The authors have filed a disclosure of invention (No. 2012-INV-0107) at CSIR-NCL.

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ACKNOWLEDGMENTS D.V.R.K. thanks CSIR, New Delhi, for a fellowship, B.L.V.P. thanks the Center of Excellence on Microreactor Engineering of CSIR-NCL and A.A.K. thanks the Dept. of Science & Technology (SR/S3/CE/0032/2010) for the financial support for this work.

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REFERENCES

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