Maryam Tashia,
Behnaz Shafieea,
Yoshie Sakamakib,
Ji-Yun Hub,
Zachary Heidrickb,
Ahmad R. Khosropour*ab and
M. Hassan Beyzavi
*b
aDepartment of Chemistry, University of Isfahan, 81746-73441, Isfahan, Iran. E-mail: arkhosropour@chem.ui.ac.ir
bDepartment of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701, USA. E-mail: beyzavi@uark.edu
First published on 13th November 2018
The combination of continuous flow technology with immobilizing of only 0.13 mol% of triflic acid (TfOH) on silica-encapsulated superparamagnetic iron oxide nanoparticles (SPIONs) under solvent-free conditions successfully provided a powerful, efficient, and eco-friendly route for the synthesis of plasticizers. The turnover frequency value in micro-flow conditions varied in the range of 948.7 to 7384.6 h−1 compared to 403.8 to 3099 h−1 for in-flask. This technique works efficiently, encouraging future applications of micro-flow nano-catalysis in green chemistry.
To this end, we prepared TfOH functionalized silica-encapsulated superparamagnetic iron oxide nanoparticles (TfOH@SPIONs) and tested their efficiency in synthesizing plasticizers with micro-flow techniques. Plasticizers are an essential ingredient for the producing of macromolecule materials used widely by polymer industries.30,31 A variety of modified catalytic processes have been performed; from traditional catalysts, such as sulfuric acid32 and titanate,33 to more modern ionic liquids34 for the synthesis of plasticizer esters. Although these methods are valuable, most of them require hours of reflux, utilizing expensive or corrosive catalysts, tedious work-up, difficult recovery, and easy deactivation with consumption of energy. Therefore, implementation of eco-friendly and straightforward chemical methods remains a particularly interesting task.
The aforementioned explanations and our recent investigations on the micro-flow nano-catalyst technology,35 encouraged us to develop the ultimate green synthesis, very practical, and straightforward protocol for the synthesis of plasticizer esters via the first combination of TfOH@SPION with micro-flow technique.
The FT-IR spectrum of TfOH@SPION shows absorption bands at 3411 cm−1 (O–H stretching vibration), 1377, 1178, 1040 cm−1 (OS
O and SO3− stretching bands), 646 cm−1 (Si–O), and 635–587 cm−1 (Fe–O), respectively (Fig. S1, ESI†). The XRD spectrum of the TfOH@SPION was similar to that of SPION which illustrated that no structural changes occurred during immobilization of TfOH (Fig. S2†). TEM images of TfOH@SPION revealed that it appears to have almost a cubic structure with the average size about 13–15 nm (Fig. S3†). The dc magnetic characterization of Fe3O4, the neat SPION nanoparticles, and TfOH@SPION were examined at room temperature in an external field range of ±10 kOe. In this investigation, due to the functionalization of SPIONs by TfOH, Ms (saturation magnetization) was found to be 10 emu g−1, which is considerably lower than 40 emu g−1 of the SPIONs (Fig. S4†).
After structural characterization of the catalyst, the catalytic activity of TfOH@SPION in the preliminary studies was evaluated through batch manner for the synthesis of a general commercial plasticizer, diisobutylphthalate (DIBP), as a model. For comparison, six frequently used catalysts as well as TfOH@SPION, were investigated under the same conditions. The template reaction was first run with stirring of phthalic anhydride (1 mmol) and 2-isobutanol (2 mmol) in the presence of 0.13 mol% various catalysts (based on active sites) at 100 °C for 200 min. As shown in Fig. 1, TfOH@SPION gave the highest yield (70%) among all the catalysts investigated. p-Toluenesulfonic acid (p-TSA, 50%) and Amberlyst 15 (35%) showed moderate yields, while n-butyltin dihydroxychloride, pentadecafluorooctanoic acid, Nafion-H and trichlorotriazine did not catalyze the reaction.
Next, the TfOH loading amount of TfOH@SPION was examined (Fig. 2). The yield of DIBP increases with higher loading TfOH of TfOH@SPION, which comes to the plateau of about 70% yield at a loading level of 40 mg (corresponds to 0.13 mol%). Notably, lower yields were obtained when the same reaction was carried out with lower amount of the catalyst (Fig. 2). Moreover, no reaction occurred in the absence of a catalyst and no valuable yield was obtained in the presence of SPION without loading of TfOH.
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Fig. 2 TfOH loading effect of TfOH@SPIONs for the reaction of phthalic acid (1.0 mmol) and 2-butanol (2.0 mmol) at 100 °C after 200 min. |
Given these promising results, we next tried to optimize the reaction conditions by changing the method from batch to continuous flow technology. Based on the optimized batch conditions, the effect of reaction temperature and flow rate on the reaction efficiency were further studied.
The condition for the initial examinations, particularly the catalyst amount and substrate molar ratio, were selected according to our results obtained for the batch method. An initial study revealed that the residence time and the reaction temperature were crucial factors for this transformation. Periodic analyses of the reaction mixture illustrated the effect of the residence time (t: 0.5, 1, 1.5 and 2 h) and temperature (T: 70, 80, 90 and 100 °C) to the yield of the product (Fig. 3).
As shown in Fig. 3, combinatorial sixteen conditions from four temperatures (70, 80, 90, 100 °C) and four residence time (0.5, 1.0, 1.5, 2.0 h) were systematically examined. Generally, either increasing temperature, or prolonging residence time could increase the product yield. For example, at 70 °C, the yield was increased from 9 to 39, 48, 59% respectively with the residence time extended from 0.5 to 1.0, between 1.5 to 2.0 h, respectively. Similarly, keeping the same residence time of 0.5 h, the product yield was increased from 9 to 15, 20, 25% when temperature was elevated from 70 to 80, to 90, to 100 °C, respectively. These effects approach the maximum at 90 °C with residence time of 1.5 h, giving a yield of 74%, where further increase the temperature or the residence time didn't improve the yield significantly (only 1% higher). Therefore, a reaction temperature of 90 °C and a residence time of 1.5 h (corresponds to a flow rate of 0.4 mL h−1 in this study) were used as the optimal conditions for the following studies. Next, we evaluated the scope and generality of this procedure and the results are summarized in Table 1.
Entry | Product | Batch | Microreactor | ||||
---|---|---|---|---|---|---|---|
Time (min) | Yielda (%) | TOF (h−1) | Time (min) | Yielda (%) | TOF (h−1) | ||
a Isolated yields. | |||||||
1 | ![]() |
50 | 95 | 2192 | 20 | 96 | 5538 |
2 | ![]() |
35 | 94 | 3098 | 15 | 96 | 7384 |
3 | ![]() |
35 | 90 | 2967 | 15 | 92 | 7076 |
4 | ![]() |
215 | 94 | 504 | 95 | 96 | 1165 |
5 | ![]() |
160 | 80 | 576 | 70 | 82 | 1351 |
6 | ![]() |
200 | 70 | 403 | 90 | 75 | 948 |
7 | ![]() |
130 | 87 | 772 | 60 | 90 | 1730 |
8 | ![]() |
60 | 92 | 1769 | 25 | 95 | 4384 |
9 | ![]() |
180 | 89 | 570 | 80 | 92 | 1326 |
As shown in Table 1, the TfOH@SPION catalyst shows a broad substrate scope with yields ranging between 70–96%. In all cases, micro-flow technology-assisted reactions were superior to the traditional batch reactions in terms of yield and reaction time. The reaction time of microreactor was generally shortened more than half of that of the batch reactions, while still giving a slightly higher yield. The turnover frequencies (TOF) in micro-flow technology is 948.7–7384.6 h−1 compared to 403.8–3099 h−1 in traditional manner, which may be attributed to better dispersion of the nano-catalyst in a minimum volume in contrast to magnetically stirred reactions. Moreover, this protocol avoids the problems associated with catalyst handling, safety, and pollution. To the best of our knowledge, this is the first example of efficient synthesis of plasticizers using the combination of nano-catalyst with micro-flow technology.
It is noteworthy to mention that the reusability and recyclability of a catalyst has the positive effect on diminishing the waste and reducing of the production cost. In this study, TfOH@SPIONs could be conveniently recovered by applying an external magnetic field and the recovered catalyst were reused for at least six times without any substantial loss of its catalytic performance. As demonstrated in Fig. 4, the product yield of the seventh run for the synthesis of DIBP under flow chemistry, is 65% only slightly lower than 75% of the first run.
Finally, encouraged by the above results, the scale-up synthesis of DIBP was also investigated as the model reaction. We increased the scale of the reaction to 5 mmol, keeping the reaction stoichiometry intact. The reaction proceeded as expected and the product was obtained with similar yields (75%) to the small-scale reaction.
It is widely assumed that the reaction occurs via the esterification of carboxylic acid derivatives via activation of carbonyl group with acidic hydrogens on the nanoparticle surface (Fig. 5).
The catalyst was analyzed by FT-IR (Fourier transform infrared spectroscopy, Nicolet-Impact 400D spectrophotometer in KBr pellets and reported in cm−1), XRD (Powder X-ray diffraction, D8 Advance DVANCE with Davincl, Bruker), TEM (Philips CM30 unit operated at 150 kV) and VSM (vibrating sample magnetometer at Meghnatis Daghigh Kavir Co.).
Thin layer chromatography (TLC) was performed on UV-active aluminum-backed plates of silica gel (TLC Silica gel 60 F254). 1HNMR spectra were measured on a Bruker DPX 400 MHz spectrometer in CDCl3 with chemical shift (δ) given in ppm and coupling constants given in Hz. The sonication was performed in a UP 400S ultrasonic processor equipped with a 3 mm wide and 140 mm long probe.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra07838f |
This journal is © The Royal Society of Chemistry 2018 |