Air-Flow Impacting for Continuous, Highly Efficient ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Air-Flow Impacting for Continuous, Highly Efficient, Large-Scale Mechanochemical Synthesis: A Proof-of-Concept Study Bingyang Sun,† Yi He,*,‡ Rufang Peng,*,† Shijin Chu,† and Jin Zuo§ †

School of Materials Science and Engineering & State Key Laboratory Cultivation Base for Nonmetal Composite and Functional Materials, Southwest University of Science and Technology, Mianyang 621010, P. R. China ‡ School of National Defence Science & Technology, Southwest University of Science and Technology, Mianyang 621010, P. R. China § China Aerodynamics Research and Development Center, Mianyang 621010, P. R. China S Supporting Information *

ABSTRACT: We report a novel air-flow impacting method for mechanochemical synthesis. It is an alternative approach for conventional mechanochemical synthesis. The feasibility of this method was demonstrated via the preparation of three Schiff bases containing N,N′-bis(m-nitrobenzylidene)-p-phenylenediamine (compound 1), N,N′-bis(2-hydroxy-1-naphthylmethylene)-p-phenylenediamine (compound 2), and polymeric Schiff base as the model compounds. The as-prepared Schiff bases were characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction, differential thermal analysis, nuclear magnetic resonance, ultraviolet−visible (UV− vis) spectroscopy, and single crystal X-ray diffraction. All the results indicated that two bis-schiff bases were successfully synthesized after 3 min at the rate of 1.5 kg min−1. In addition, kinetic analysis was carried out to study the reaction mechanisms by detecting the UV−vis spectra of the products at different reaction times. It was found that the preparation of compound 1 belonged to the two-dimensional diffusion-controlled model, while the synthesis of compound 2 is the two-dimensional diffusion-controlled product growth following deceleratory nucleation. KEYWORDS: Air-flow impacting, p-Phenylenediamine, Mechanochemical synthesis, Schiff base, Kinetic analysis

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INTRODUCTION

large-scale production. Therefore, development of novel mechanochemical synthesis methods is highly desirable. Air-flow impacting is a common method for the reduction of particle size on the large scale. The comminution mechanism of air-flow impacting is completely different from conventional mechanical ball milling, hand grinding, and TSE.17−20 In airflow impacting, a gaseous medium is introduced, and impacting takes place by colliding particles at high speed. The particle’s speed may reach up to 300 m/s or more. Air-flow impacting exhibits various advantages,21 such as the absence of cross contamination in conventional mechanochemical synthesis methods, ability to fragmentate heat-sensitive substances, and large-scale preparation at several kg min−1 rates. In addition, it can achieve sustained response and is environmentally friendly as it avoids the use of various organic solvents. However, to the best of our knowledge, the application of this method in chemical synthesis has not been reported to date. In the present work, we demonstrate that air-flow impacting can be used for chemical preparation for the first time. Schiff base, as a model compound because it is often used in

Mechanochemical synthesis has drawn increasing attention from chemists and materials scientists, which provide efficient, clean, and high-yielding organic processes in modern synthetic chemistry.1−3 It is widely used for the preparation of various substances, including small organic molecules,4,5 pharmaceutical cocrystals,6 metal−organic frameworks, covalent organic frameworks,7 nanomaterials, nanocomposites, etc.8−15 In general, mechanochemical synthesis is realized according to mechanical ball milling and hand grinding. However, these methods suffer from many drawbacks. For example, the mechanical ball milling strategy will produce high temperature in the preparation process, which is not suitable to prepare heat-sensitive substances. It also may pollute the product by dropping a small fraction of abraded balls as a contaminant. Moreover, hand grinding approaches are carried out in the gram-scale quantities, which restrict their wide applications. To overcome these problems, Stuart’s group reported on twin screw extrusion (TSE) for mechanochemical synthesis.16 Various metal complexes have been synthesized using TSE. Although TSE can obtain products at kg h−1 rates, the materials need to be mixed together by hand before injection into the TSE. Furthermore, the kg h−1 rate is not sufficient to achieve © XXXX American Chemical Society

Received: November 29, 2015 Revised: January 27, 2016

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DOI: 10.1021/acssuschemeng.5b01579 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering conventional mechanochemical synthesis,22−24 is selected. Three Schiff bases (Schemes 1 and 2), N,N′-bis(m-nitroScheme 1. Synthesis of Two bis-Schiff Bases

Figure 1. Schematic illustration of the reaction equipment (1, impacting chamber; 2, material conveying system; 3, circulation collecting system; 4, induced draft fan system; 5, sampling and collecting system; 6, feed port).

benzylidene)-p-phenylenediamine (compound 1), N,N′-bis(2hydroxy-1-naphthylmethylene)-p-phenylenediamine (compound 2), and polymeric Schiff base, were prepared by airflow impacting. All products were characterized by powder Xray diffraction (PXRD), differential thermal analysis (DTA), ultraviolet−visible spectroscopy (UV−vis), Fourier transform infrared (FT-IR) spectroscopy, 1H nuclear magnetic resonance (NMR) spectroscopy, and single crystal X-ray diffraction (SCXRD). Moreover, the kinetics of the reactions have been investigated via ex situ UV−vis absorption spectroscopy.

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and washed three times with ethanol. Then, the product was dried in a vacuum at 45 °C, and 0.83 kg of polymeric Schiff base was collected. The polymeric Schiff base was obtained in 85.6% yield. Characterization. PXRD experiments of the samples were performed on a Philips X’Pert Pro X-ray diffractometer (PANalytical, Holland) with Cu Ka1 radiation (λ = 0.15418 nm) at 80 mA and 40 kV. FT-IR spectra were measured on a Nicolet-5700 FTIR spectrometer (KBr pellet) in the range of 4000−400 cm−1. 1H NMR spectra were recorded on Bruker Avance 600 spectrometers with dimethyl sulfoxide (DMSO) as the solvent and tetramethylsilane as the internal reference. UV−vis spectra were recorded with a UV1800 spectrophotometer (Shimadzu, Japan) with ethanol and dichloromethane as the solvent. DTA curves were measured on a WCR-1B analyzer with a heating ramp of 10 °C/min under air flow. Single Crystal X-ray Diffraction. The single crystal size was 0.26 mm × 0.22 mm × 0.14 mm. A light red crystal of the title compound was mounted randomly on a glass fiber. The single-crystal X-ray diffraction data of compound 2 were collected with a Bruker Smart APEX II CCD diffractometer equipped with graphite-monochromatic Mo Kα radiation (λ = 0.71073 Å) via an ω scan mode in the range of 3.49° ≤ θ ≤ 27.52° at 113(2) K. The collected 14,066 diffraction points included an independent 2513 diffraction points [Rint = 0.0495]. The structure was solved by direct methods with the SHELXS-97 program.28

EXPERIMENTAL SECTION

Chemicals and Apparatus. p-Phenylenediamine, m-nitobenzaldehyde, 2-hydroxy-naphthaldehy, and 1,4-phthalaldehyde were purchased from Wuhan Hong Rui Kang Reagent Co., Ltd. (Hubei Province, China). All the solvents were obtained from Chengdu Kelong Chemical Reagents Company (Sichuan Province, China). Air-flow impacting equipment for chemical synthesis is shown in Figure 1 and Figure S1. Reactant particles are first transferred into the impacting chamber through the feed port. Afterward, the particles are accelerated to high velocities (300 m/s) by compressed air (1.5 MPa). During the process, violent collisions happened among the reactant particles, leading to the comminution of reactant particles and chemical reactions to form products. Finally, the products are collected through the circulation collecting system. Synthesis of Two bis-Schiff Bases. Two bis-Schiff bases were prepared as follows. p-Phenylenediamine (0.43 kg) and fragrant aldehyde (m-nitobenzaldehyde (1.21 kg) and 2-hydroxy-naphthaldehy (1.38 kg)) were mixed in a 1:2 molar ratio, and the mixture was transferred into an impacting chamber at the rate of 1.5 kg min−1 (90 kg h−1). Then, the materials were accelerated to supersonic by compressed air (1.5 MPa). The products were collected after different reaction times (20 s intervals). Here, 1.54 and 1.68 kg of the products were collected after 3 min, respectively. Then, the products were washed three times with ethanol, recrystallized from ethyl acetate, and dried in a vacuum at 45 °C. Compounds 1 and 2 were obtained in 89.8% and 82.5% yields, respectively. The space time yields for the synthesis of two bis-Schiff bases also have been calculated, yielding up to 216 × 104 kg per m3 per day. Synthesis of Polymeric Schiff Base. The polymeric Schiff base was prepared as follows. p-Phenylenediamine (0.43 kg) and 1,4phthalaldehyde (0.54 kg) were mixed in a 1:1 molar ratio, and the mixture was transferred into an impacting chamber at the rate of 1.5 kg min−1 (90 kg h−1). Then, the materials were accelerated to supersonic by compressed air (1.5 MPa). The products were collected after 5 min

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RESULTS AND DISCUSSION Synthesis and Characterization of Two bis-Schiff Bases. Two bis-Schiff bases were prepared by air-flow impacting (Scheme 1). The structure of two bis-Schiff bases was confirmed by FT-IR spectroscopy, DTA, XRD, UV−vis spectroscopy, and 1H NMR spectroscopy. Figure 2 shows the FT-IR spectra of the raw materials, compound 1, and compound 2. As shown in Figure 2a, for p-phenylenediamine (p-pda), the peak at 3195−3371 cm−1 can be assigned to the N−H stretching vibration, and the peaks located at 1630 and 1518 cm−1 are attributable to the bending vibrations δ N−H and skeletal vibration of benzene. The FT-IR spectrum of mnitobenzaldehyde (m-nbd) shows the characteristic stretching bands of the carbonyl function of the aldehydes (−CHO) at 2870 (ν C−H stretching) and 1704 cm−1 (ν CO stretching), and the peak at 3060 cm−1 can be assigned to the aromatic carbon-H stretching (νCph−H). For pure compound 1, the

Scheme 2. Synthesis of Polymeric Schiff Base

B


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Figure 4. XRD patterns of (a) p-pda, m-nbd, pure compound 1, and the products at different reaction times and (b) p-pda, napht, pure compound 2, and the products at different reaction times. Characteristic reflections for compound 1 and compound 2 are indicated by “∗”.

Figure 2. FT-IR spectra of (a) p-pda, m-nbd, and pure compound 1 and (b) p-pda, napht, and pure compound 2.

disappearance of the carbonyl stretching bands of m-nbd and N−H stretching bands of p-pda indicated total consumption of the starting materials, and a new absorption peak appearing at 1622 cm−1 is attributed to the CN stretching vibration. Moreover, similar to those shown in Figure 2a, a new absorption peak appears at 1620 cm−1 corresponding to the CN stretching vibration in the FT-IR spectra, shown in Figure 2b (compound 2), which is the characteristic band of theSchiff base. In addition, the peak at 3460 cm−1 is ascribed to the hydroxyl (−OH) stretching vibration. Thus, the FT-IR spectra demonstrate that the two bis-Schiff bases have been successfully prepared. In order to identify the reaction process, DTA curves were measured (Figure 3). The p-pda shows two endothermic peaks

napht diffraction peaks were reduced as reaction time increased. These results suggested that the two bis-Schiff bases were obtained. The 1H NMR spectra also confirmed the successful synthesis of the two bis-Schiff bases. Figure 5a shows the 1H NMR spectrum of compound 1. The single peak at 8.77 ppm is attributed to the azomethine proton (−CHN−, denoted as b). The spectrum also exhibits a singlet at 8.92 ppm, multiplets at 8.37−8.42 ppm, and triplet at 7.82−7.88 ppm, which correspond to aromatic ring protons (denoted as c, d, f, and e). The single peak at 8.92 ppm is attributed to the aromatic ring protons of the para-disubstituted benzenes (denoted as c). Compound 2 exists in two tautomeric forms including enolimine and keto-amine (Scheme 3), which induces an intramolecular proton transfer from the hydroxyl oxygen to the imine nitrogen. In Figure 5b, it can be concluded that compound 2 mainly adopts the keto-amine form. The doublet at 15.87−15.85 ppm is assigned to the −NH proton. Pronounced downfield is due to the strong intramolecular hydrogen bonding. The signal of the hydrogen atom of the azomethine CH group is considered to be a valuable parameter in distinguishing between tautomeric forms. When the molecule exists in the enol-imino form, the −CHN− signal is a singlet. When the molecule is exist in the keto-amine form, this signal is split into a doublet due to the H−NH coupling. Thus, the signals at 7.79−7.84 ppm are ascribed to the doublet of the = CH−NH− proton, and the singlet of aromatic ring protons overlap. The doublets at 7.01−7.06, 7.92−7.97, 8.52− 8.57, and 9.71−9.76 ppm and multiplets at 7.34−7.40 and 7.54−7.60 ppm correspond to the aromatic ring protons (denoted as d, f, e, i, and g, h). The crystal and molecular structures of compound 2 were determined by single crystal X-ray diffraction (Figures 6 and 7). Single crystals of compound 2 were grown by slow evaporation of the chlorobenzene solution. Single crystal data was collected, solved, and refined in a P21/c monoclinic space group. Figure 6 shows the molecular structure of compound 2. The result was consistent with the 1H NMR spectra, and the molecule existed in the keto-amine form. As shown in Figure 7, the crystal structure shows the presence of an intramolecular N1−H1···O1 carboxyl hydrogen bond for 2-hydroxy-naphthaldehy on cocrystallizing with p-phenylenediamine, and they are connected via water molecules and separated via a bifurcation of the O2−H2···O1 hydrogen bond. The formation of crystalline water was due to the formation of water molecules in the process of the reaction, and compressed air can also contain certain water. The result also suggested that the bis-Schiff base was successfully synthesized.

Figure 3. DTA curves of (a) p-pda, m-nbd, pure compound 1, and product at 20 s and (b) p-pda, napht, pure compound 2, and product at 20 s.

at 142 and 168 °C (Figure 3a), which ascribe to the melting and volatility of p-pda, respectively. Similarly, m-nbd exhibits two endothermic peaks at 58 and 150 °C, which are attributed to the melting and volatility of m-nbd, respectively. The melting endothermic peak of p-pda disappeared, and a new melting endothermic peak was observed at 250 °C after 20 s. According to the DTA curve of compound 1, this new peak is the melting endothermic peak of compound 1, which proves that the reaction occurrs at 20 s. 2-Hydroxy-naphthaldehy (napht) shows two endothermic peaks at 76 and 163 °C (Figure 3b). The first endothermic peak could be ascribed to melting, and the latter indicates the volatile temperature of napht. As far as compound 2 is concerned, DTA curves also indicate that the reaction has occurred at 20 s (Figure 3b). These results also demonstrated the successful preparation of the two bis-Schiff bases. Figure 4 shows the XRD patterns of p-pda, m-nbd, napht, compound 1, compound 2, and the reaction products at different reaction times. As shown in Figure 4a and b, new diffraction peaks assigned to compound 1 and compound 2 appeared after 20 s, and the intensities of p-pda, m-nbd, and C


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Figure 5. 1H NMR spectra of (a) compound 1 and (b) compound 2.

Scheme 3. Tautomeric Structures of Enaminones: (a) enolimine, (b) keto-amine

Synthesis of Polymeric Schiff Base. The two bis-Schiff bases were successfully synthesized by air-flow impacting, and we next explored more mechanisms in the mechanosynthesis of the model polymeric Schiff base of 1,4-phthalaldehyde and pphenylenediamine. The studied and synthesized polymeric Schiff base in this work is shown in Scheme 2. This polymeric Schiff base is prepared from 1,4-phthalaldehyde and pphenylenediamine by air-flow impacting. Formation of the azomethine −CHN−R bond by a polycondensation reaction is confirmed by the appearance of the strong imine ν CN stretching band at 1613 cm−1 (Figure S2). The characteristic stretching bands of the carbonyl function of the aldehydes

Figure 7. Crystal structure of compound 2.

(−CHO) at 2871 cm−1(ν C−H stretching) and 1691 cm−1 (ν CO stretching) are faintly visible and correspond probably to end groups. The polymeric Schiff base was also characterized by

Figure 6. Molecular structure of compound 2. D


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Figure 8. (a) UV−vis spectra of p-pda, m-nbd, and pure compound 1. (b) UV−vis spectra of pure compound 1 at different reaction times. (c) UV− vis spectra of p-pda, napht, and pure compound 2. (d) UV−vis spectra of pure compound 2 at different reaction times.

Figure 9. UV−vis spectra at different mix times: (a) physical mixture of p-pda and m-nbd and (b) physical mixture of p-pda and napht.

contraction R2 and R3, Prout−Tompkins B1, and A2, A3, and A4 Avrami−Erofe’ev models.25−27 For compound 1, the experimental data was consistent with the D2 kinetic model (Figure S4), indicating that the mechanism of compound 1 belonged to the two-dimensional diffusion-controlled model. The fitting of nonlinearized data to the general Avrami−Erofe’ev equation (It/Imax= 1 − e−(kt)n, Figure S7), as well as the linearization of the data in the form of a Sharp−Hancock plot of (ln(−ln(1 − It/Imax)) vs ln t (Figure S6), also belonged to the D2 model. The Avrami−Erofe’ev equation is the most commonly employed method for describing solid-state reaction processes, and the Sharp− Hancock plot is the most facile way to check the authenticity of the Avrami−Erofe’ev model. The D2 model refers to diffusion and occurs radially through a cylindrical shell and with an increasing reaction zone. For compound 2, the data were consistent with the A4 kinetic model (Figures S5, S6, and S8), which is the two-dimensional diffusion-controlled product growth following deceleratory nucleation. Deceleratory nucleation suggests that the nucleation sites are not saturated at the start of the reaction but are activated one-by-one. Fitting the experimental data to the Avrami−Erofe’ev model enabled us to determine the rate constants of compound 1 and compound 2 as K1 = 0.018 s−1 and K2 = 0.01 s−1. This indicates

XRD (Figure S3), and the result was consistent with the reported cases.22 All the results indicated that the polymeric Schiff base can be synthesized successfully by air-flow impacting. Kinetic Analysis. To investigate the mechanisms and kinetics of the reactions, an ex situ UV−vis absorption spectrometry technique was used for studying the air-flow impacting preparation process. Owing to the different molecular structures of p-pda, m-nbd, napht, and the two bisSchiff bases, these compounds have different UV−vis absorption spectra. Compared with the raw materials, two new absorption peaks appear at 360 and 460 nm, contributing to the π−π* absorption peaks of the two bis-Schiff bases (Figure 8a and c). Thus, the characteristic absorption peaks of the two bis-Schiff bases can be used to monitor the reaction process. In order to monitor the reaction process, the products were collected after different reaction times. Figure 8b and d show the characteristic UV−vis absorption curves of the two bis-Schiff bases at different reaction times. We fitted the sigmoidal time dependence of the normalized two bis-Schiff bases absorption intensities (α = It/Imax, where It is the measured intensity at any point in time, and Imax is the maximum intensity vs time plot) to 10 solid-state reaction models: D1, D2, D3, and D4 diffusion models, geometrical E


ACS Sustainable Chemistry & Engineering that synthesis of compound 1 has higher efficiency than synthesis of compound 2. The reason for this might be that the presence of the strong electron-withdrawing group in m-nbd increases reactivity, while napht has large space resistance and decreases the reaction degree. To demonstrate that the Schiff base reaction cannot occur under the process of preparing solution for measuring UV−vis absorption spectra, UV−vis absorption spectra of the raw materials at different mix times were measured as shown in Figure 9. The characteristic absorption peak of the two bisSchiff bases at 360 and 460 nm did not appear until 10 h after the physical mixture of the raw materials. In general, the experimental process for measuring UV−vis absorption spectra can be accomplished within 3 h. Therefore, the possibility of the occurrence of the Schiff base reaction in solution can be excluded. Additionally, it is also important to note that all of the products have not been washed with ethanol before the UV− vis absorption spectra detection. According to these results, we can conclude that the solution did not cause the occurrence of the Schiff base reaction and will not affect the kinetic analysis.

ACKNOWLEDGMENTS

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REFERENCES

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CONCLUSIONS In summary, we demonstrated that air-flow impacting can be a novel and effective technique for chemical synthesis under solvent-free conditions for the first time. Two bis-Schiff bases as the model compounds were successfully synthesized to confirm the feasibility. Moreover, kinetic analysis was carried out to study the reaction mechanisms. The mechanisms and kinetics of the Schiff bases reactions have been investigated using the Avrami−Erofe’ev model, and the kinetic data suggest that the mechanisms of compound 1 and compound 2 were consistent with the D2 and A4 kinetic models, respectively. It should be noted that high throughput rates of 90 kg/h−1 were readily achieved and much greater rates of several hundred kg/h−1 should be possible by using larger scale equipment. The proposed chemical synthesis approach based on air-flow impacting has many advantages over conventional mechanochemical synthesis such as high efficiency, low cost, and large scale. Although this work is a proof-of-concept stage, we believe that air-flow impacting opens a new avenue for mechanochemical synthesis and holds great application potential in a variety of areas such as chemistry, material science, and pharmaceutical science. ASSOCIATED CONTENT

S Supporting Information *

SThe Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01579.

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This work was supported by the Major Program of the National Natural Science Foundation of China (51327804). The technology was supported by the Analytical and Testing Center of SWUST for performing XRD and 1H NMR characterizations.

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chematic diagram of the Laval nozzle, FT-IR spectrum, and XRD pattern of the polymeric Schiff base and spectra details of kinetic analysis. (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.H.). *E-mail: [email protected] (R.P.). Notes

The authors declare no competing financial interest. F


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