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[CTA]Fe/MCM-41: An efficient and reusable catalyst for green synthesis of xanthene derivatives
⁎Corresponding author. m.pirouzmand@tabrizu.ac.ir (Mahtab Pirouzmand)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Iron was incorporated into MCM-41 via direct synthesis and wet impregnation methods. Removal of the surfactant occluded in Fe/MCM-41 pores has been performed by using two procedures: solvent extraction and calcination. For comparison, a series of template-containing mesoporous Fe/MCM-41 were also synthesized without surfactant removal. The catalysts were examined in three component reaction to afford benzoxanthene derivative. A high yield of 91% was achieved over template-containing Fe/MCM-41 catalyst. This high activity is due to both lewis acid sites and ionic template. This catalyst could be easily recovered by filtration and reused without loss of its catalytic activity because the organic template does not leach during the reaction. It is also very efficient for the synthesis of a hydroxanthene derivative.
Keywords
Ionic template
Fe/MCM-41
Solid acid catalyst
Xanthene
Solvent free
1 Introduction
Xanthenes and benzoxanthenes are important categories of organic compounds. They have various pharmacological activities such as antibacterial (Omolo et al., 2011), antiviral (Ion et al., 1998), antiplasmodial (Azebaze et al., 2006), and anti-inflammatory properties (Poupelin et al., 1978), and have been utilized as antagonists for drug-resistant leukemia lines (Nguyen et al., 2009).
There are several methods for the synthesis of xanthene derivatives; these compounds are conventionally prepared using condensation of aromatic aldehydes and β-naphthol with dimedone in the presence of acid catalysts such as FeCl3·6H2O and [bmim][BF4] ionic liquid (Fan et al., 2005), Fe3+-montmorillonite (Song et al., 2007), silica-sulfonic acid (Mohammadi Ziarani et al., 2011), heteropolyacid supported MCM-41 (Karthikeyan and Pandurangan, 2009) and nanospherical mesoporous Lewis acid polymer Sc(OTf)2-NSMP (Zhang et al., 2014).
Recently, mesoporous molecular sieves (like MCM-41 and MCM-48), have attracted much interest in catalysis, due to the high surface area, well defined pore shape, narrow pore size distribution, and good thermal stability. These are particularly attractive for heterogeneous reactions of large organic molecules for which microporous zeolites cannot be used. These mesoporous material, are prepared by using ionic surfactants (Beck et al., 1992). Generally, the residual templates inside the pores are removed by calcination or extraction. As ionic surfactants contain an organic cation such as cetyltrimethylammonium (C19H42N+) and an inorganic anion, we hypothesized that could resemble ionic liquid. Herein, we report for the first time the preparation of two xanthene derivatives in the presence of template-containing Fe/MCM-41 as heterogeneous catalyst under solvent free conditions.
2 Materials and methods
2.1 Materials
Cetyltrimethylammonium bromide (CTAB) was purchased from Sigma-Aldrich Inc. and the other reagents were purchased from Merck company.
2.2 Methods
2.2.1 Catalyst preparation
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–
Direct Synthesis method (DS)
Fe/MCM-41 samples were synthesized according to standard literature procedure (Tantirungrotechai et al., 2011). In a typical non-hydrothermal synthesis, 2.74 mmol of CTAB was dissolved in 480 mL of NaOH aqueous solution (15.0 mM), followed by a drop wise addition of 22.4 mmol of tetraethylorthosilicate (TEOS). Then, a solid powder of iron nitrate (0.18 g) was slowly added. The mixture was vigorously stirred and heated to 80 °C for 2 h. Subsequently, the product powder was isolated by hot filtration, washed with deionized water and air-dried.
In some preparations, the cationic surfactant was removed either by hot solvent extraction or by calcination in air at 640 °C for 6 h. The solvent extraction was performed by stirring 1 g of the air-dried product with a 1 M HCl solution in ethanol (liquid: solid 300 mL/g) at 333 K for 24 h. Then the mesoporous material was collected by filtration and dried at room temperature. In order to compare the effect of the template, a series of as-synthesized molecular sieves, containing their organic template, were also tested for benzoxanthene synthesis without further modification. Moreover physical mixture of Fe/MCM-41 WI (0.1 g) and CTAB (0.02 g) was also used for synthesis reactions.
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–
Wet Impregnation method (WI)
0.18 g of Fe(NO3)3·6H2O was dissolved in 10 ml distilled water. 0.9 g calcined MCM-41 was added to the iron nitrate solution. After stirring for 3 h at room temperature, the solid was filtered from the solution and dried at 60 °C.
2.2.2 Catalyst characterization
XRD measurements were performed on a Philips-PW 17C diffractometer with Cu Kα radiation over the 2θ range 1–10°. The elemental chemical compositions of the samples were determined by EDX (VEGA TESCAN-LMU, Czech Republic) under vacuum mode for precise measurement of both light and heavy elements. FTIR spectra were recorded on a Bruker Tensor 27 instrument using KBr pressed powder discs. A Mettler Toledo thermogravimetry (TG/SDTA 851) was used for thermogravimetric analysis (TGA). About 10 mg of the sample was heated at 10 °C/min to 700 °C in air flow (100 ml/min).
2.2.3 Synthesis of 12-phenyl-9,9-dimethyl-8,9,10,12-tetrahydrobenzo[a]xanthene-11(11H)-ones (Compound 1)
A mixture of β-naphthol (0.144 g, 1 mmol), dimedone (0.140 g, 1 mmol), benzaldehyde (1 ml, 1 mmol) and Fe/MCM-41 (0.1 g), was stirred in an oil-bath (110 °C). After completion of the reaction, as monitored with TLC, the reaction mixture was cooled to room temperature; ethyl acetate (15 mL) was added to it, stirred for 10 min. Then the catalyst was separated by filtration (for separation of calcined catalysts, a magnet was put at the bottom of the reaction flask). Finally, ethyl acetate was evaporated under vacuum to give the crude product. The crude product was recrystallized from EtOH to yield pure xanthen-11-one derivatives.
All of the pure products were characterized by comparison of their physical (melting point) and spectral data (1H NMR) with those of authentic samples (Supplementary information).
2.2.4 Synthesis of 9-phenyl-3,3,6,6-tetramethyl-3,4,5,6,7,9-hexahydroxanthene-1(1H),8(2H)-dione (Compound 2)
This xanthene derivative was synthesized by the above mentioned procedure with 2 mmol dimedone without introducing of β-naphthol.
3 Results and discussion
Most attempts to introduce surface-bound active sites have centered on aqueous impregnation methods. Wet impregnation involves bringing the precursor solution into the pore space of the support. However, it has been found that catalysts prepared by this method have a limited reusability, since in this method, active species leach out from the support due to a weak interaction of metal ions with silica. To avoid this problem we used direct synthesis approach for incorporating the metal into the MCM-41, where an iron precursor salt is present in the MCM-41 synthesis gel.
The low angle XRD pattern of MCM-41 exhibits four well-resolved peaks with a very intense diffraction peak (1 0 0) at 2θ ≈ 2.5° and three peaks with lower intensity at higher degree (3–5°) which were indexed to the 1 1 0, 2 0 0, and 2 1 0 reflections of the hexagonal P6mm honeycomb lattice (Kresge et al., 1992). The XRD results suggested that the ordered mesoporous structure of as-synthesized MCM-41 was preserved after the introduction of iron species (Fig. 1a). Although after incorporation, Fe/MCM-41(WI) changed into more disordered structure in comparison with Fe/MCM-41(DS), as the whole metal precursor retained on the support (Fig. 1b).
X-ray powder diffraction of (a) Fe/MCM-41 (DS) and (b) Fe/MCM-41 (WI).
The elemental concentration on the catalyst is reported in Table 1. EDX results show that WI method can achieve a high metal loading, because there is no intermediate washing step involved in the wet impregnation approach, in contrast to the direct synthesis procedure. Furthermore, the solvent extraction method decreases metal loading because during this process, the iron ions leaches out of the support.
3.1 Influence of the template removing method
Two methods have been widely used to remove the organic templates molecules: calcination at 500–600 °C, in air or oxygen atmosphere, and extraction with a hot conventional solvent. In the present work, we employed both of these two techniques to remove the organic template molecule from the silicate lattice.
The removal of the template from as-synthesized MCM-41 was confirmed by Infrared spectroscopy. The nearly complete removal of the surfactant could be confirmed with the absence of peaks around 2970 and 2927 cm−1, corresponding to the organic template. Fig. 2 shows the FTIR spectra of template-containing Fe/MCM-41, solvent extracted and calcined Fe/MCM-41 samples. The [CTA]Fe/MCM-41 sample exhibits absorption bands around 2925 and 2854 cm−1, corresponding to C–H stretch vibrations of the surfactant molecules. Two weak peaks in this region are also observed in the spectrum of solvent extracted Fe/MCM-41 samples. In fact, solvent extraction method always leaves some traces of surfactant in contrast to the traditional calcination one which fully eliminates the template. The template containing sample also shows an absorption band at 1382 cm−1 that is absent in the calcined Fe/MCM-41. This could be attributed to the vibration of N–O of nitrate.![FT-IR spectrum of (a) [CTA] Fe/MCM-41, (b) extracted Fe/MCM-41 and (c) calcined Fe/MCM-41 prepared by direct synthesis.](/content/184/2017/10/8/img/10.1016_j.arabjc.2016.06.017-fig2.png)
FT-IR spectrum of (a) [CTA] Fe/MCM-41, (b) extracted Fe/MCM-41 and (c) calcined Fe/MCM-41 prepared by direct synthesis.
In our previous work, TGA analysis was also employed to determine quantitatively the amount of surfactant (Pirouzmand et al., 2015). The amount of template was estimated from the weight loss between 150 and 450 °C. These amounts were approximately 25%, 5% and lower than 0.5% for template-containing, extracted and calcined Ca/MCM-41, respectively. These data also confirmed that most of the template molecules had been released via calcination method.
3.2 Synthesis of xanthene derivatives
To compare the catalytic performance of prepared catalysts, one-pot multicomponent condensation between benzaldehyde and β-naphthol in the presence of dimedone was chosen as a model reaction (Table 2). At first the effect of catalyst amount on reaction rate was checked. As the minimum required amount of catalyst that showed the best efficiency (time and yield) was 0.1 g, we chose this amount for the synthesis reactions. It was expected that the reaction yield would increased with the iron content of catalyst, in accordance with an increase in the concentration of Lewis acidic centers. While the template containing Fe/MCM-41 exhibits superior catalytic activity in terms of yield (91%) and reaction time than both calcined catalyst and catalyst prepared by wet impregnation (Table 1, entry 1, 3, 4), it can be seen that the cationic surfactant CTAB accelerated the model reaction to afford the desired product in good yield. The higher activity of extracted Fe/MCM-41 with very low iron content, than calcined Fe/MCM-41 confirmed this effect (entry 2 & 3), because extracted catalyst still contained low template amount. On the other hand, metal free [CTA]MCM-41 showed lower activity in comparison with [CTA]Fe/MCM-41. It can be concluded that ionic surfactant intensifies the effect of Lewis acid sites.
Entry
Catalyst
Time (min)
Yield (%)b
1
[CTA]Fe/MCM41 DS
25
91
2
Extracted Fe/MCM-41 DS
35
81
3
Calcined Fe/MCM-41 DS
40
76
4
Fe/MCM-41 WI
47
55
5
[CTA]MCM-41
45
73
6
Calcined MCM-41
45
71
7
Fe/MCM-41 WI + CTA
40
71
To prove the complementary effect of ionic surfactant, the catalytic activity of prepared Fe/MCM-41 catalysts was also evaluated in the synthesis of a hydrobenzene derivative from dimedone and benzaldehyde (Table 3). Interestingly, template-containing Fe/MCM-41 exhibited high catalytic activity for the synthesis of 9-phenyl-3,3,6,6-tetramethyl-3,4,5,6,7,9-hexahydroxanthene-1(1H),8(2H)-dione (entry 1).
Entry
Catalyst
Time (min)
Yield (%)b
1
[CTA] Fe/MCM-41 DS
20
97
2
Extracted Fe/MCM-41DS
25
90
3
Calcined Fe/MCM-41 DS
30
85
4
Fe/MCM-41WI
35
70
5
[CTA]MCM-41
30
82
6
Calcined MCM-41
40
73
7
Fe/MCM-41 WI + CTA
30
79
The proposed mechanism for the synthesis of compounds 1 and 2 is shown in Scheme 1. Fe3+ acts as a Lewis acid to catalyze the nucleophilic attack on the carbonyl group of the aldehyde and in turn facilitates the formation of Knoevenagel intermediate A. In the next step, this intermediate promotes the attack of β-naphthol or dimedone via conjugate Michael type addition. Intramolecular condensation and the removal of a molecule of water result in the desired products.
Proposed mechanism for the synthesis of two xanthene derivatives.
Ionic surfactant enhances the rate of these reactions because the charged intermediates are present. Moreover it increases effective collisions between reactants. Generally ionic surfactants in hydrophobic substrates can self-assemble reverse clusters, which provide a large interface between the catalyst and reactant where the reaction takes place, so greatly enhancing the reaction rate. Also the formation of CTAB cluster would congregate catalytic species Fe3+ at the reaction interface, which could catalyze the reaction more efficiently.
3.3 Reusability of catalyst
One of the most important features of a solid catalyst is the ability to be recycled. Template-containing Fe/MCM-41 synthesized via direct approach ([CTA]Fe/MCM-41 DS) that displayed the best catalytic activity was studied further. After the first test, the used catalyst was filtered, washed with ethyl acetate to remove any unreacted precursor and organic products. Then the recovered catalyst was charged for the next run. Recyclability of the catalyst was assessed by using it for 5 cycles (Table 4).
Run
1
2
3
4
5
Compound 1 yield (%)
91
91
90
89
86
Compound 2 yield (%)
97
96
96
94
91
EDX analysis was performed to estimate the iron content of the recovered catalyst after first and fifth run and showed the presence of Fe atomic % = 5.01 and 4.31 respectively. This indicates that negligible leaching of the iron from this support occurs under the reaction condition. The ([CTA]Fe/MCM-41 DS catalyst showed only minor deactivation (from 91% to 86%) with small loss of iron. So it has substantial stability under the used experimental conditions.
To determine the amount of organic template (CTA) remained in recovered [CTA]Fe/MCM-41 pores, thermogravimetric analysis experiment was performed. The TG curve of the as-synthesized Fe/MCM-41 DS is shown in Fig. 3a. Three distinct stages of weight loss were observed in template-containing Fe/MCM-41, Weight loss below 150 °C (due to desorption of water), 200–450 °C (due to decomposition of the template) and above 450 °C (due to water loss via condensation of silanol groups to form siloxane bonds). Weight loss of about 21% at temperatures between 200 and 450 °C is observed. This amount was approximately 20% for [CTA]Fe/MCM-41, after first and fifth run. So despite a weak interaction of CTA cations with silyloxy anions, they are not continuously leached because they are stabilized in the micelles as a consequence of a strong interaction of the non-polar tails (Martins et al., 2006; Martins and Cardoso, 2007). Therefore, [CTA]Fe/MCM-41 was found to be reusable catalyst, which is one of the main requirements for employing this catalyst to production of xanthenes derivatives.![TGA profile of (a) [CTA]Fe/MCM-41 DS, (b) after 1st run and (c) after 5th run.](/content/184/2017/10/8/img/10.1016_j.arabjc.2016.06.017-fig6.png)
TGA profile of (a) [CTA]Fe/MCM-41 DS, (b) after 1st run and (c) after 5th run.
3.4 Comparison with other catalysts present in literature
Table 5 and Table 6 compare efficiency of the present catalyst with the most active catalysts reported in the literature for synthesis of compounds 1 and 2, respectively. Table 5 shows that the activity of [CTA]Fe/MCM-41 is comparable to other catalysts. Furthermore, two reactions were catalyzed by ionic liquids (entry 3 and 4); however, there are certain concerns over the use of ionic liquids as green solvents because large amounts of organic solvent and energy are used in the preparations of these ionic liquids (Deetlefs and Seddon, 2010). The main advantage of our catalyst is it’s one-pot synthesis via a simple and cost effective approach. In addition, [CTA]Fe/MCM-41 is one of the most active catalysts for synthesis of compound 2 (Table 6).
Entry
Catalyst
Time (min)
Temperature (°C)
Yield (%)
Refs.
1
Fe3O4@MCM-41-SO3H
25
r.t.
90
Saadatjoo et al. (2013)
2
CuO nanoparticles
16
100
95
Chaudhary et al. (2014)
3
DSIMHSa
20
55
93
Shirini et al. (2014)
4
H2SO3 [Msim]BF4b
10
110
93
Zolfigol et al. (2012)
5
Silica sulfuric acid
60
80
83
Nazeruddin et al. (2011)
6
[CTA]Fe/MCM-41(DS)
25
110
91
This work
Entry
Catalyst
Time (min)
Temperature (°C)
Yield (%)
Refs.
1
DSIMHSa
3
55
95
Shirini et al. (2014)
2
H2SO3 [Msim]BF4b
10
70
96
Zolfigol et al. (2012)
2
FeCl3-rice husk
60
100
91
Shirini et al. (2013)
3
Fe3O4@SiO2-SO3H
4
110
97
Naeimi and Sadat Nazifi (2013)
4
HPWA/MCM-41
300
90
94
Karthikeyan and Pandurangan (2009)
5
Fe(HSO4)3
5
120
94
Shaterian et al. (2009)
6
[CTA]Fe/MCM-41(DS)
20
110
97
This work
4 Conclusions
Fe/MCM-41 catalysts have been synthesized by direct synthesis and wet impregnation methods. Then the ionic template was removed by two different methods: solvent extraction and calcination. The prepared samples were used as catalyst in the synthesis of two xanthene derivatives. The results showed that template-containing Fe/MCM-41, prepared by direct synthesis approach, was the most active catalyst for these processes, which could be attributed to the Lewis acid sites and ionic template effect. This catalyst was also able to be reused without significant loss of activity. This new strategy for xanthene derivatives preparation has two advantages: Firstly, it saves energy and time since the template does not have to be removed and there is no need to catalyst activation after recovery; moreover, it also reduces the pollutants released to the environment. The second advantage is that the purposed catalyst is synthesized by a one-pot approach via a non-hydrothermally simple method.
Acknowledgement
The authors thank Research Affairs of University of Tabriz for financial support.
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Appendix A
Supplementary material
Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.arabjc.2016.06.017.
Appendix A
Supplementary material
Supplementary data 1
Supplementary data 1