Characteristics of the copper phthalocyanines synthesized at various conditions under the classical and microwave processes

Synthetic Metals 141 (2004) 259–264

Characteristics of the copper phthalocyanines synthesized at various conditions under the classical and microwave processes K.S. Jung a , J.H. Kwon a , S.M. Son b , J.S. Shin c , G.D. Lee c , S.S. Park c,∗ b

a Phthalos Co., Pukyong Technocomplex, Pukyong National University, Pusan 608-739, South Korea Division of Image and Information Engineering, Pukyong National University, Pusan 608-739, South Korea c Division of Chemical Engineering, Pukyong National University, Pusan 608-739, South Korea

Received 4 April 2003; accepted 25 June 2003

Abstract Copper phthalocyanines were synthesized by the reaction of phthalic anhydride, urea, and copper(I) chloride at various temperatures and times under classical and microwave processes. The samples synthesized at various conditions were characterized by the means of chemical analysis, scanning electron microscopy (SEM), X-ray diffraction (XRD), BET, and particle size analysis. The best yield of 85.1 wt.% was obtained at 180 ◦ C for 4 h under the classical process and that of 88.2 wt.% was obtained at 170 ◦ C for 4 h under the microwave process. Comparison between the classical and microwave processes revealed that the non-thermal effects of microwaves were existed during reaction period because the reaction occurred at lower temperature in a very limited extension. © 2003 Elsevier B.V. All rights reserved. Keywords: Phthalocyanine; Microwave; Classical; Synthesis

1. Introduction In the recent years, it has been recognized that microwave processing has attracted potential as an alternative to classical processing because of the inherent advantages of microwave heating, which is selective, direct, rapid, internal, and controllable [1]. The microwave processing therefore has been applied in such varied fields as pulp drying, food cooking, organic synthesis, ceramic sintering, composite joining, chemical analyses, and waste treatment [2–4]. In many organic syntheses, it has been used in combination with solvent-free technique to improve organic synthesis, leading to shorter reaction-time, higher yield, clear product and easier work-up than classical processing [5]. It was reported that slower reacting systems in organic syntheses tended to show a greater effect under the microwave processing than faster reacting systems, compared to the classical processing. The acceleration of reactions by microwaves results from material–microwave interactions leading to thermal and non-thermal effects [6–8]. The thermal heating phenomenon associated with microwave irradiation in the

∗ Corresponding author. Tel.: +82-51-620-1688; fax: +82-51-620-1680. E-mail address: [email protected] (S.S. Park).

0379-6779/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0379-6779(03)00414-4

organic syntheses is mainly due to dipolar and interfacial polarization, although conduction losses can also be important at higher temperatures; when a molecule is irradiated with microwaves, it rotates to align itself with the applied field and consequently the molecule continually attempts to realign itself with changing field and energy is absorbed. The non-thermal effects associated with microwave irradiation may be mainly due to the increase of probability of molecular impacts, the decrease of activation energy and the intervention of localized microscopic high temperatures. Metal-free phthalocyanine and its metal complexes have been intensively investigated since the early-1930s and have been widely used in many fields of organic pigment, chemical sensor, electro-chromic display devices, photovoltaic cells, xerography, optical disk, catalysis, non-linear optics, etc. [9,10]. These versatile features have stimulated attempts on the syntheses of new phthalocyanine derivatives with objective of developing new materials containing improved or high functional characteristics. Phthalocyanine can be obtained by the classic template reactions starting from diverse precursors, such as phthalonitrile, dicyanobenzene, cyanobenzamide, phthalimide, phthalic acid, etc. in high-boiling non-aqueous solvents at elevated temperatures. Phthalocyanine forms metal complexes with various metal elements, which can be synthesized chemically from metal salts. For the production of a copper phthalocyanine, it has

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been most common to employ as an industrial method, which comprises heating phthalic acid or a phthalic acid derivative, and urea or urea derivatives and a copper salt in the presence of a catalyst in an inert organic solvent [11]. Despite the abundance of reported data on copper phthalocyanine preparation, there is no information in the available literature on the use of microwave processing to obtain the above products starting from urea and phthalic anhydride as the cheapest phthalocyanine precursors. The emergence of microwave technology in phthalic anhydride/urea process presents an excellent new option for the synthesis of phthaocyanine and its derivatives. In the present study, the possibility and effectiveness of synthesizing copper phthalocyanine from phthalic anhydride, urea and copper(I) chloride in the presence of a catalyst in an inert solvent under both classical and microwave processes was investigated by comparing reaction-times and reaction temperatures.

2. Experimental 2.1. Materials Phthalic anhydride and ammonium molybdate were purchased from Junsei Chemical Company. Urea was purchased from Katayama Chemical Company. Copper(I) chloride was purchased from Katayama Chemical Company. Alkyl benzene with a boiling point of 190 ◦ C was provided from Isu Chemical Company. 2.2. Conventional and microwave synthesis Alkyl benzene (100 g), urea (49 g), phthalic anhydride (42 g), CuCl (7 g), and ammonium molybdate (0.1 g) were charged together into a 250 ml reaction flask, which was fitted with a modified thermocouple, a reflux condenser, and a motor-driven stirrer. A heating mantle and a modified microwave synthetic unit were used as a classical and microwave heating source, respectively. The modified microwave synthetic unit includes: a microwave generator giving a variable power supply up to 600 W operating at 2.45 GHz; stainless steel shielded K-type thermocouple for temperature measurement; a mode stirrer for field uniformity; a programmable controller for feedback control; an exhaust fan for vent. It was gradually heated up to 120 ◦ C with heating rate of 2 ◦ C/min, and then heated to between 155 and 180 ◦ C with heating rate of 0.25 ◦ C/min. It was maintained in that temperature range for 0.5–4 h at the stirring speed of about 100 rpm. From reaction product thus obtained, solvent alkyl benzene was distilled off by a reduced pressure distillation. Then, residue was washed with methyl alcohol. After filtration, the cake was acid-treated for 1 h by H2 SO4 solution (100 ml, 0.02 M), alkali-treated for 1 h by NaOH solution (100 ml, 0.02 M), then washed with hot distilled water until the washing solution became neutral. After filtration, it was dried at 105 ◦ C over 24 h

in a dry oven, whereupon the yield of final product was obtained [12]. 2.3. Measurement of yield and purity The yield of copper phthalocyanine was calculated by the following equation: yield (%) =

Wf × 100 (WPA /MPA ) × (1/4) × MCuPc

where Wf is the amount of purified final product, WPA the amount of phthalic anhydride, MPA the molecular weight of phthalic anhydride and MCuPc is the molecular weight of copper phthalocyanine. To determine purity of sample, concentric H2 SO4 solution (10 ml) was added into 1.0 g of sample, and the sample was dissolved. Then, 50 ml of distilled water was added thereto, and the solution was stirred at 60 rpm for 1 h. After that, precipitate was collected by filtration, and it was washed with hot distilled water until the washing solution became neutral, then dried at 105 ◦ C over 24 h in a dry oven, whereupon purity was obtained [12]. 2.4. Characterization of CuPc The sample for X-ray diffraction (XRD) analysis was mounted on a sample holder with a large cavity and a fairly smooth surface was obtained by pressing the powder sample with a glass plate. XRD analyses were performed using a Rigaku RINT 2000 diffractometer with Ni-filtered Cu K␣ (35 kV, 25 mA). The scanning speed and step size were 0.08 and 0.02◦ /min, respectively. Microstructural characterizations of the samples were carried out using Jeol JSM-5400 scanning electron microscopy (SEM). The sample for particle size analyses (PSA) was prepared as an aqueous dispersion at a loading of 1% by ultrasonic dispersion at 30–35 ◦ C for 5 min. PSA analyses were conducted using LKY-1 type Micro-particle Diameter Analyzer equipped with a centrifugal settler. The surface area of the samples was measured using Quantachrome AS3B-Kr analyzer by BET nitrogen adsorption method after outgassing at 350 ◦ C for at least 1 h.

3. Results and discussion 3.1. Yield analyses In the previous investigation [13], we tested the thermal behavior of every reactants as well as reaction mixtures when they were exposed to 2.45 GHz microwave field. This preliminary step allowed determining adequate conditions of incident power and irradiation time. In this case, all materials were polar, excepted solvent alkyl benzene, and consequently highly subjected to couple with microwave radiations.

100

100

80

80

Yield (%)

Yield (%)

K.S. Jung et al. / Synthetic Metals 141 (2004) 259–264

60

60

40

40

20

0.5

(a)

261

155˚C

160˚C

170˚C

180˚C

1

2

20

0.5

4

Time (h)

(b)

155˚C

160˚C

170˚C

180˚C

1

2

4

Time (h)

Fig. 1. Product yield of the samples synthesized at various temperatures for various times under (a) classical and (b) microwave processes.

Fig. 1 shows the product yield of samples synthesized, as a function of reaction temperatures varying from 155 to 180 ◦ C for reaction-times varying from 0.5 to 4.0 h, under both classical and microwave processes. It indicated that the product yield increased with increasing the reaction temperature and reaction-time under both the processes. An enhancement of the reaction was observed clearly under the microwave process in the same range of reaction condition, compared to the classical process; the product yields were higher (up to ∼40%) in all reaction-time ranges under the microwave process than under the classical process. The reasons were that at the step at which the reaction intermediate was formed under the classical process, the viscosity of reaction mixtures increased to cause an insufficient mixing, non-uniformity in heat transfer and adhesion of the reaction mixture to a reactor wall. As a result, not only the operation for the reaction was hindered, but also the product yield was decreased. However, the viscosity of reaction mixtures decreased due to intensive internal heating of microwaves under the microwave process, the uni-

formity of the reaction mixtures increased, and the reaction proceeded smoothly to synthesize final product at high yields. Also, high yield, low reaction temperature and short reaction-time exhibited under the microwave process should be attributed to a significant increase in the reaction rate of reactive species. It meant that the microwave process can be expected to be more-efficient synthetic process for the synthesis of copper phthalocyanine by effecting homogeneous mixing and enhanced reaction rate of the reactive species due to the intense internal heating, together with the differential polarization effects [14]. 3.2. Microstructure analyses X-ray diffraction patterns of the samples synthesized at 155–180 ◦ C for 0.5–4 h under both classical and microwave processes were given in Figs. 2 and 3. XRD patterns of the samples synthesized at various temperatures for 2 h under the microwave process were almost same as those under the classical process. However, XRD patterns of the samples

Fig. 2. X-ray diffraction patterns of the samples synthesized at various temperatures for 2 h under (a) classical and (b) microwave processes.

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Fig. 3. X-ray diffraction patterns of the samples synthesized at 155 ◦ C for various times under (a) classical and (b) microwave processes.

synthesized for various times at 155 ◦ C under the microwave process were almost same as those under the classical process, but difference was only that the peak intensities of XRD patterns of samples synthesized under the microwave process were stronger than those under the classical process. Scanning electron microscopy pictures of the samples synthesized at 155–180 ◦ C for 0.5–4 h under the classical and microwave processes were given in Figs. 4 and 5. These SEM pictures clearly showed that all of the samples synthesized under both the processes were well crystallized with acicular shape. Many larger and thicker crystals, resulting in a larger particle size, existed in the samples synthesized under the classical process. However, the size of crystals was more uniform, respectively, under the microwave process. The SEM results shown in Fig. 5 were consistent well with the XRD results shown in Fig. 3. From the results of XRD and SEM analyses, crystal growth was accelerated by increasing reaction temperature and time under both classical and microwave processes. In the other hand, it was certainly found that the rate of

Table 1 The average particle size of the samples synthesized at various temperatures for 2 h under classical and microwave processes Process

Reaction temperature (◦ C) 155

Classical (␮m) Microwave (␮m)

15.98 6.26

160 25.77 10.93

crystal growth increased at the condition of relatively lower temperature and shorter time under the microwave process, compared to the classical process. These phenomena may be related to non-thermal effect of microwaves in the stage of nucleation and crystal growth. The mechanism of enhanced kinetics in the stage of nucleation and crystal growth is not known in detail. It is, however, generally accepted that increase of probability of molecular impacts, the decrease of activation energy are provided through non-thermal effect of microwaves. Further investigation of the relationship between microwaves and crystal size in the process of crystal growth in the samples are now in progress. 3.3. Particle size and BET analyses The average particle sizes of the samples synthesized at 155–180 ◦ C for 0.5–4 h under both the classical and microwave processes were given in Tables 1 and 2. It showed that the ranges of average particle size were from 7.8 to 25.72 ␮m under the classical process and from 5.54 to Table 3 The surface area of the samples synthesized at various temperatures for 2 h under classical and microwave processes Reaction temperature (◦ C)

Process 170 23.85 14.05

180 25.72 16.17

(m2 /g)

Classical Microwave (m2 /g)

155

160

170

180

11.40 80.36

38.63 74.99

60.28 43.09

58.22 40.25

Table 2 The average particle size of the samples synthesized for various times at 155 ◦ C under classical and microwave processes

Table 4 The surface area of the samples synthesized for various times at 155 ◦ C under classical and microwave processes

Process

Process

Reaction-time (h) 0.5

Classical (␮m) Microwave (␮m)

7.8 5.54

1 15.89 5.97

2 15.98 6.26

Reaction-time (h)

4 20.24 12.54

(m2 /g)

Classical Microwave (m2 /g)

0.5

1

2

4

12.41 10.76

10.93 11.93

11.40 80.36

11.16 78.29

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Fig. 4. SEM micrographs of the samples synthesized at (a) 155 ◦ C, (b) 160 ◦ C, (c) 170 ◦ C, and (d) 180 ◦ C for 2 h under classical process and at (e) 155 ◦ C, (f) 160 ◦ C, (g) 170 ◦ C, and (h) 180 ◦ C for 2 h under microwave process.

16.17 ␮m under the microwave process, respectively. Also, it was founded that the average particle size increased with increasing reaction-time and temperature under both the processes. However, the average particle size was much smaller for the samples synthesized at same reaction condition under the microwave process, compared to the classical process. In addition, it was found that the particle size distributions of the samples synthesized under microwave

process were much narrower than those of the samples synthesized under the classical process. The surface area of the samples synthesized at 155–180 ◦ C for 0.5–4 h under both the classical and microwave processes were given in Tables 3 and 4. It showed that the ranges of surface area were from 11.40 to 60.28 m2 /g under the classical process and from 40.25 to 80.36 m2 /g under the microwave process, respectively. In general, the decreased

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Fig. 5. SEM micrographs of the samples synthesized at 155 ◦ C for (a) 0.5 h, (b) 2 h, and (c) 4 h under classical process and for (d) 0.5 h, (e) 2 h, and (f) 4 h under microwave process.

volume of the fine crystals with increasing reaction temperature considerably decreases the surface area of samples because fine crystals have a large surface area. However, it was founded that the surface area increased under the classical process with increasing reaction temperature, but decreased under the microwave process. These results may be related to non-thermal effect of microwaves in the stage of nucleation and crystal growth. 4. Conclusion The present results clearly showed that microwave irradiation leads to acceleration of crystallization of copper phthalocyanine, expecting that copper phthalocyanine can be prepared at lower temperature and shorter time than those required under classical process, under microwave process. The samples synthesized under the microwave process had small average size and narrow size distribution, which helped the pigments to be dispersed more stably in aqueous medium. The microwave process described in the present paper has proven to be quite effective due to its intense internal heating, compared to the classical process, for the synthesis of copper phthalocyanine from phthalic anhydride, urea and copper(I) chloride in the presence of a catalyst in an inert solvent.

Acknowledgements This work was supported by Korea Research Foundation Grant (KRF-2002-002-D00107).

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