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Immobilization of iron tetrasulfophthalocyanine on functionalized MCM-48 and MCM-41 mesoporous silicas: Catalysts for oxidation of styrene
Journal of Colloid and Interface Science 319 (2008) 199–205 www.elsevier.com/locate/jcis
Immobilization of iron tetrasulfophthalocyanine on functionalized MCM-48 and MCM-41 mesoporous silicas: Catalysts for oxidation of styrene Mahtab Pirouzmand, Mostafa M. Amini ∗ , Nasser Safari ∗ Department of Chemistry, Shahid Beheshti University, Tehran 1983963113, Iran Received 25 September 2007; accepted 18 November 2007 Available online 24 November 2007
Abstract Iron tetrasulfophthalocyanine (FePcS) has been anchored on the surface of functionalized MCM-48 and MCM-41 silicas by means of chemical bonding to aminosilane groups. The prepared materials, FePcS/NH2 -MCM-48 and FePcS/NH2 -MCM-41, were characterized by diffuse reflectance UV–vis and infrared (IR) spectroscopies, low-angle X-ray diffraction (XRD), and surface area analysis (BET). The tendencies of FePcS absorption on functionalized MCM-48 and MCM-41 were measured by UV–vis spectroscopy. The functionalized MCM-48 showed a larger amine to silica ratio than the functionalized MCM-41. Low-angle X-ray diffraction analysis showed that, by anchoring iron tetrasulfophthalocyanine into functionalized MCM-48 and MCM-41, the intensity of main reflection decreased. The catalytic activities of the supported iron tetrasulfophthalocyanine were examined by the oxidation of styrene in the presence of tert-butyl hydroperoxide. The FePcS/NH2 -MCM-48 showed higher activity and durability in the liquid-phase oxidation of styrene under mild condition compared with the FePcS/NH2 -MCM-41 and unsupported catalyst. © 2007 Published by Elsevier Inc. Keywords: MCM-48; MCM-41; Iron tetrasulfophthalocyanine; Styrene
1. Introduction Metal complexes of phthalocyanines with facile preparation methods characterized by large-scale, high thermal stability, high conversion, and selectivity have attracted great interest [1]. From the environment viewpoint, the metallophthalocyanines being able to activate oxygen, thereby rendering these the capability of oxidizing organic compounds, represent a remarkable property compared with common transition-metal based oxidants. For instance, the industrial application of manganese, iron, and cobalt phthalocyanines for aerobic oxidation of mercaptan impurities and sweetening crude petrochemicals, the Merox process, is well established [2]. In addition, these compounds can be used as catalysts for the laboratory-scale oxidation of phenols to quinones [3], alkanes to alcohols or ketones [4], alkenes to epoxides [5], and oxidative degradation of chlorinated waste [6]. However, the aggregation and low solubility * Corresponding authors. Fax: +98 21 22431663.
E-mail addresses: [email protected] (M.M. Amini), [email protected] (N. Safari). 0021-9797/$ – see front matter © 2007 Published by Elsevier Inc. doi:10.1016/j.jcis.2007.11.024
of metallophthalocyanines are the main drawbacks in their application as catalysts. This property is further enhanced by the introduction of sulfate or carboxylate substituents at the periphery as the substituents are known to give rise to water-soluble derivatives with consistent homogeneous catalytic activity. Alternatively, the metallophthalocyanines can be immobilized on various supports in order to function as heterogeneous catalysts. The latter approach, due to environmental aspects, ease of separation, and recovery and recycling of catalyst, becomes a highly innovative research field. A wide variety of support substances such as polymer substrate [7], zeolite [8], and activated carbon [9] has been used for the immobilization of metallophthalocyanines. Zeolite encapsulation which is achieved by in situ cyclotetramerization of phthalonitrile to form phthalocyanines within the zeolite pore is particularly interesting [10]. However, large and hindered reactant molecules cannot enter the zeolite narrow pores, and greatly limiting access to encapsulated metallophthalocyanine sites on the framework walls. Therefore, the use of large-pores materials for the encapsulation of metallophthalocyanines is beneficial. Since the synthesis of the M41S family of micelle-template silicas [11],
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MCM-41, one member of this family, has attracted interest as a support for the immobilization of various catalysts in general [12–14] and metallophthalocyanines in particular [15]. Copperand cobalt-perchlorophthalocyanine [16,17], iron, manganese, and cobalt-tetrasulfophthalocyanine have been successfully anchored onto the surface of MCM-41 and have been used as catalysts in the liquid phase oxidation of various organic compounds [18,19]. Although, MCM-41 with a large pore volume can accommodate metallophthalocyanines, its tendency to absorb metal tetrasulfophthalocyanine (MPcS) is low, and there is only very weak interaction between MPcS and the siliceous surface. Therefore, due to the high solubility of MPcS in water, physically adsorbed MPcS can be leached out easily from the surface of support, which is supposed to act as a heterogeneous catalyst, and the concept of catalyst recovery is lost. In this context, increasing the susceptibility of mesoporous silica for the adsorption and immobilization of various metallophthalocyanines by means of chemical bonding via functionalized silica appears to be the proper approach. This approach has been used successfully by several researchers to immobilize MPcS on functionalized Si-MCM-41 [17,19]. A review of the literature shows that all studies on the immobilization of MPcS are devoted mostly to functionalized MCM-41, in spite of the many advantages of MCM-48. In contrast to the twodimensional hexagonal array of pore channels of MCM-41, MCM-48 with its three-dimensional pore network and larger surface area is more accessible to guest molecule if the pores are wide enough [20,21]. Therefore, the pore-blocking problem can be avoided to a great extent using MCM-48. Furthermore, the narrow pore size distribution and the interwoven and cubic pore structure of MCM-48 [22] make it more favorable in a catalytic system from the mass transfer kinetics point of view [21]. Consequently, higher stability and conversion in catalytic systems with MCM-48 support are expected. Despite the advantage of MCM-48 over MCM-41, the synthesis of high quality MCM48 with controlled pore size is more challenging. In the present work, we have used functionalized MCM-48 and MCM-41 as support for grafting FePcS and, after complete characterization, used as catalysts for oxidation of styrene in biphasic systems. 2. Materials and methods 2.1. Materials All reagents were purchased from Merck or Fluka and used as received. Sodium salt of FePcS was prepared according to the method of Weber and Busch [23] and was converted to corresponding acid (FePcS) by ion exchange resin. IR (KBr, cm−1 ): 3430, 3182, 1773, 1718, 1636, 1461, 1402, 1328, 1187, 1144, 1108, 1050, 1029, 930, 832, 748, 702, 635, 550. 2.1.1. Preparation of MCM-41, MCM-48, and functionalized MCM-41 and MCM-48 The MCM-41 and MCM-48 were synthesized according to a previously reported procedure [24,25], and then functionalized by 3-aminopropyltriethoxy silane [26]. In a typical reaction, 1 g of MCM-41 was suspended in 50 ml toluene and the mixture
was stirred for 1 h and then 1.6 g of (C2 H5 O)3 Si(CH2 )3 NH2 was added and refluxed for 2 h. The white solid was removed from the solvent by filtration and was washed by toluene and chloroform and then dried at room temperature. Functionalized MCM-41 and MCM-48 are designated as NH2 -MCM-41 (1) and NH2 -MCM-48 (2), respectively. 2.1.2. Absorption studies and preparation of immobilized iron tetrasulfophthalocyanine samples To demonstrate that the functionalized MCM-41 and MCM48 silicas have a greater tendency to absorb FePcS and to determine the optimum contact time required for the mesoporous silicas to absorb FePcS from the impregnation solution, 0.1 g of each mesoporous silica, bare and functionalized, was suspended in 7.5 ml aqueous solution of FePcS with initial concentration of 4 mg/ml. Each suspension was stirred at room temperature and the contact time varied from 10 to 1440 min. Then, the solid was removed from the solution by centrifugation and the UV–vis spectra of solution were recorded. To further examine the graft quantity and its corresponding reproduction, absorption studies were repeated thrice and consistent data were obtained. Similar results were obtained when the experiments were carried out in larger scales. For the preparation of mesoporous silica-supported FePcS and investigation of their physiochemical properties and catalytic activities, 1.00 g of each functionalized mesoporous silica was suspended in aqueous solution of FePcS (40 mg FePcS in 20 ml for 4% loading). The mixture was stirred overnight and then a greenish solid was collected by filtration, washed with distilled water, and finally dried at room temperature. The elemental analyses of mesoporous silica-supported FePcS, FePcS/NH2 -MCM-41 (3), and FePcS/NH2 -MCM-48 (4), before and after anchoring of the FePcS are given in Table 1. The physically adsorbed water was removed from samples prior to elemental analysis by drying them under reduced pressure. Therefore, the given hydrogen analysis in Table 1 included the hydrogen of the surface hydroxyls of silicas and also occluded water hydrogen. Similar to the absorption studies, each experiment was repeated thrice and reproducible data were obtained. Moreover, similar graft quantities were obtained when the experiments were carried out in larger scales. For the leaching study of FePcS from functionalized mesoporous silicas, 0.01 g of each sample was suspended in 20 ml distilled water and stirred for several hours. The solid was then removed by filtration and the UV–vis spectrum of solution was recorded. Table 1 Elemental analyses of functionalized silica materials before and after FePcS loading Material
C (%)
H (%)
N (%)
Si (%)
Fe (%)
NH2 -MCM-41 NH2 -MCM-48 FePcS/NH2 -MCM-41 FePcS/NH2 -MCM-48
10.54 9.55 11.80 10.85
2.73 3.38 2.69 3.32
2.97 3.20 3.34 3.56
39.09 39.14 37.59 37.64
– – 0.24 0.24
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2.2. Methods Infrared spectra were performed (KBr pellets) on a Shimadzu model FT-IR 4600 spectrometer. The solution and diffuse reflectance (DR) UV–vis spectra in the range of 300– 800 nm were recorded on a Shimadzu 2100 spectrophotometer, and DR UV–vis spectra referenced to BaSO4 . X-ray diffraction patterns were obtained on a Philips-PW 17C diffractometer with CuKα radiation. Scans were performed from (2θ ) 2◦ to 10◦ by rate of 3◦ /min. Nitrogen gas adsorption, using ultrahigh purity nitrogen gas (99.999%), adsorbed volumetrically on samples at −196 ◦ C, led to their specific surface area evaluation. An automatic stainless-steel volumetric adsorption apparatus was used to make these measurements. Experimental details for the volumetric adsorption set up are given elsewhere [27]. Prior to adsorption calculations, the sample weight was corrected for any weight loss due to degassing or drying. Pore distributions for diameters in the range of 10–500 Å were calculated by the same equipment using Barrett–Joyner–Halenda (BJH) method.
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of the grafted materials confirmed the presence of amine groups on the surface of the mesoporous silicas. The results of carbon and nitrogen analyses of functionalized MCM-41 and MCM-48 materials are compiled in Table 1. The observed C to N ratios, 3.55 and 2.98 for NH2 -MCM41 (1) and NH2 -MCM-48 (2), respectively, were close to the theoretical value of 2.57. Furthermore, the amount of grafted amine (Table 2) shows that the amine loading in NH2 -MCM41 and NH2 -MCM-48 were 2.53 and 2.74 mmol amines/g of SiO2 , respectively. Those values are corresponded to 4.6 and 2.8 amines/nm2 in NH2 -MCM-41 and NH2 -MCM-48, respectively. The specific surface areas of bare silicas, functionalized silicas and the FePcS grafted on functionalized silicas are listed in Table 3. The data show that the surface area of MCM-41 and MCM-48 decreased significantly after modification with 3aminopropyltriethoxy silane. The decrease in surface areas can be attributed to the increase in agglomeration of silica particles and/or occupation of pores after modification. 3.2. Preparation of supported catalysts
2.3. Catalytic reactions The epoxidation of styrene with tert-butyl hydroperoxide was performed in a 25-ml round-bottom flask. In a typical experiment, the flask was charged with 1.25 ml of 0.02 M CH3 OH/H2 O solution of styrene (0.025 mmol), 2.5 ml mixture of water and methanol (1:1, v/v), and 22 mg FePcS/NH2 MCM-41 or FePcS/NH2 -MCM-48 (4% loading) as supported catalysts. To this mixture, a 1.25 ml of 0.02 M CH3 OH/H2 O solution of tert-butyl hydroperoxide (0.025 mmol) was added during 30 min in five portions. The reactant mixture was stirred vigorously for 6 h at room temperature and then the catalyst was removed by centrifugation and the organic phase was analyzed with a HPLC (Knauer K-105) equipped with a Eurosphere 100 C18 column. The activity of bare and functionalized MCM-41 and MCM-48 was also measured in the absence of FePcS. Conversion for blank test was below 2%.
Iron tetrasulfophthalocyanine was prepared according to an earlier report [23]. Due to the higher catalytic activity of iron phthalocyanine among metal phthalocyanines, we have chosen that as a catalyst in our investigation. The immobilization of FePcS on the inner surface of the pores of NH2 -MCM-41 or NH2 -MCM-48 occurs by means of chemical bonding with the amine groups is shown in Scheme 1. As previously reported, a strong interaction of FePcS acidic protons with amine groups resulted in the retention of FePcS on the silica surface [18]. Absorption of FePcS on NH2 -MCM-41 and NH2 -MCM-48 was investigated by UV–vis spectroscopy. Fig. 1 shows the absorption spectrum of FePcS solution with concentration of 4 mg/ml before and after contact with MCM41 and MCM-48, and Figs. 2 and 3 show the absorption spectra of the above-mentioned FePcS solution before and after contact Table 3 Texture parameters for the mesoporous materials before and after grafting with amine and anchoring with FePcS
3. Results and discussion 3.1. Modification of mesoporous MCM-41 and MCM-48 All mesoporous silicas were grafted with propylamine group by treating with (EtO)3 SiCH2 CH2 CH2 NH2 in refluxing toluene [28]. The propylamine group attached to the mesoporous silica surface by condensation of silanol and ethoxy groups. The organosilane and amine group characteristic peaks (2930 and 1485, 1565 cm−1 , respectively) in the FT-IR spectra
Sample MCM-41 MCM-48 NH2 -MCM-41 NH2 -MCM-48 FePcS/NH2 -MCM-41 FePcS/NH2 -MCM-48
Total pore volume (ml/g)
BET-specific surface area (m2 /g) 1070 1238 331 584 116 148
0.52 0.75 0.21 0.28 0.059 0.076
Table 2 Chemical compositions of functionalized MCM-41 and MCM-48 Material
mmol amine/g SiO2
mmol FePcS/g SiO2
mol FePcS/mol amine
(1) NH2 -MCM-41 (2) NH2 -MCM-48 (3) FePcS/NH2 -MCM-41 (4) FePcS/NH2 -MCM-48
2.53 2.74 2.96 3.15
– – 0.056 0.056
– – 0.019 0.018
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Scheme 1. Immobilization of FePcS on the inner surface of functionalized mesoporous silica.
Fig. 1. UV–vis spectra of a FePcS solution before and after contact with MCM-41 and MCM-48 up to 24 h (curves are overlapped).
Fig. 3. UV–vis spectra of a FePcS solution (a) before and after contact with functionalized MCM-48 for (b) 10 min, (c) 30 min, and (d) 60 min and 24 h.
Fig. 2. UV–vis spectra of a FePcS solution (a) before and after contact with functionalized MCM-41 for (b) 10 min, (c) 30 min, and (d) 60 min and 24 h.
with NH2 -MCM-41 and NH2 -MCM-48, respectively. Apparently, mesoporous silicas, MCM-41 and MCM-48, before modification with the amine, showed no tendency for the absorption of FePcS. This was confirmed by the absence of FePcS characteristic peaks in the DR UV–vis and IR spectra of bare MCM-41 and MCM-48 which were impregnated with FePcS and then washed with water. The UV–vis spectra of FePcS solution after contact with NH2 -MCM-41 and NH2 -MCM-48, except for the intensity of peaks, were similar to the solution prior to contact, which revealed that the FePcS after contact remained intact in solution. The kinetics of absorption of FePcS on NH2 -MCM-41 and NH2 -MCM-48, shown in Fig. 4, has been investigated by UV– vis spectroscopy for an interval of 10–200 min using a solution
Fig. 4. Absorption of FePcS on (a) NH2 -MCM-41, (b) NH2 -MCM-48 as a function of time.
with concentration of 4 mg/ml of FePcS. Kinetic adsorption of FePcS on NH2 -MCM-41 in comparison with NH2 -MCM-48 was slower. The slower rate of absorption on the former can be attributed to the lower concentration of amine groups according to elemental analysis. However, because the reaction between acidic protons of FePcS and amine groups was fast, the effects of surface area, pore size, pore distribution of functionalized MCM-41 and MCM-48 on the rate of absorption could not be ruled out. The amounts of FePcS that absorbed on NH2 -MCM41 and NH2 -MCM-48 were 0.24 and 0.28 mg/m2 , respectively. Similar results were also obtained when the experiments were carried out in larger scales.
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Fig. 5. DR UV–vis spectra of (a) FePcS/NH2 -MCM-41, and (b) FePcS/ NH2 -MCM-48.
The DR UV–vis spectra of FePcS supported on functionalized MCM-41 and MCM-48 (Fig. 5) demonstrate clearly that the iron tetrasulfophthalocyanine anchored on supports and did not degrade. The above conclusion confirmed also by the infrared spectroscopy. Interestingly, the DR UV–vis spectrum of FePcS/NH2 -MCM-41 showed a broad band, which extended to the 630–690 nm region. Considering the general agreement in the literature that the dimeric and monomeric forms of FePcS in solution show absorption maximum at 630 and 680 nm, respectively [29] one can conclude that the FePcS absorbed on NH2 -MCM-48 is mostly in the form of µ-oxo-dimer whereas on the NH2 -MCM-41 is a mixture of dimeric and monomeric forms. Furthermore, the DR UV–vis spectra of FePcS/NH2 MCM-41 and FePcS/NH2 -MCM-48 are similar to the UV–vis spectrum of an aqueous solution of FePcS. This indicates that the orbital associated with the absorption bands are not involved in the bonding of FePcS to the surface of NH2 -MCM-41 and NH2 -MCM-48. Apparently, immobilization of FePcS on the surface of MCM-48 and MCM-41 occurs by means of chemical bonding with the amine groups. After immobilization of FePcS with functionalized NH2 MCM-41 and NH2 -MCM-48, the specific surface areas decreased further, indicating that the pores of functionalized mesoporous materials are being filled. Considering the same FePcS loading of NH2 -MCM-48 and NH2 -MCM-41, the larger loss of surface area of the NH2 -MCM-48 in comparison with NH2 -MCM-41 after anchoring FePcS is attributed to the different pore structure and the larger pore volume of the formerly mentioned material or to higher amine loading. Interestingly, none of the supported FePcS showed leaching after several hours of contact with the aqueous solution. Apparently, the strong interaction of NH2 -MCM-41 and NH2 -MCM-48 with FePcS makes them suitable heterogeneous catalysts. The low-angle X-ray diffraction patterns of MCM-41 (a), NH2 -MCM-41 (b), and FePcS/NH2 -MCM-41 (c) are shown in Fig. 6 and those of MCM-48 (a), NH2 -MCM-48 (b), and FePcS/NH2 -MCM-48 (c) are seen in Fig. 7. Apparently, by functionalizing MCM-41 and MCM-48 and eventually anchoring FePcS the intensities of reflections at 2θ = 2.4◦ are decreased significantly. The decrease in the intensity of reflec-
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Fig. 6. Low-angle XRD patterns of (a) calcined MCM-41, (b) NH2 -MCM-41, and (c) FePcS/NH2 -MCM-41.
Fig. 7. Low-angle XRD patterns of (a) calcined MCM-48, (b) NH2 -MCM-48, and (c) FePcS/NH2 -MCM-48.
tions, which is well known in immobilization of various catalysts on mesoporous silicas, is attributed to the occupation of mesopore channels [30]. These results are in accordance with the decrease of surface areas by grafting amine groups on MCM-41 and MCM-48 and eventually anchoring with FePcS. 3.3. Preliminary catalytic properties of immobilized FeTSPc in oxidation of styrene The catalytic activities of supported catalysts have been tested in the oxidation of styrene by tert-butylhydroperoxide (TBHP) in a mixture of methanol and water at room temperature. The oxidation of styrene with TBHP in the presence of FePcS/NH2 -MCM-41 and FePcS/NH2 -MCM-48 resulted in the formation of benzaldehyde. The catalytic performances of FePcS in homogeneous and heterogeneous systems are compared in Table 4. Although styrene conversions in heterogeneous systems are comparable with the homogeneous ones, the homogeneous systems showed higher selectivity to benzoic acid, which is not a desirable product. Apparently, TBHP oxidized styrene deeper in homogeneous systems. Interestingly, in spite of the same loading of FePcS for FePcS/NH2 -MCM-41 and FePcS/NH2 -MCM-48, the former catalyst showed a notably lower conversion. The higher activity of FePcS/NH2 -MCM-48 in comparison with FePcS/NH2 MCM-41 can be attributed to the more efficient dispersion of
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Table 4 Oxidation of styrene in homogeneous and heterogeneous systems with FePcS Catalyst FePcS FePcS FePcS/NH2 -MCM-41 FePcS/NH2 -MCM-41 FePcS/NH2 -MCM-48 FePcS/NH2 -MCM-48
Reaction time (h)
Conversion (%)
Benzaldehyde
6 24 6 24 6 24
39.3 57.8 16.7 46.9 21.9 65.5
18.7 36.2 19.1 20.2 23.9 21.4
TOFa
Benzoic acid
Yield (%) (benzaldehyde)
47.3 36.7 0 0 0 0
7.3 20.9 3.2 9.5 5.2 14.0
19.7 28.9 8.3 23.5 10.9 32.7
Selectivity (%)
a TOF = turnover frequency (mol styrene converted/mol Fe).
the former. It is of interest to compare the activity of a homogeneous system with that of a heterogeneous system. The unsupported FePcS with 57.8% conversion after 24 h with turnover frequency (TOF) of 28.9 mol(styrene) mol(Fe)−1 h−1 mol was found to be more active than FePcS/NH2 -MCM-41 but lower than FePcS/NH2 -MCM-48 with a conversion of 65.5 and a turn TOF of 32.7 mol(styrene) mol(Fe)−1 h−1 mol. Thus, encapsulated FePcS in MCM-48 is a more effective catalyst than that in MCM-41, and unsupported catalyst. This can be attributed to the different pore structure of MCM-48 and possibly to higher concentration of µ-oxo-dimer form of FePcS, due to the internal framework structure of MCM-48. Although the higher activity of FePcS anchored into MCM-48 in dimeric form contradicts what is known to be a higher activity of monomeric species in a homogeneous system, a similar behavior also has been reported for the FePcS anchored into mesoporous and amorphous silicas [18]. Furthermore, FePcS/NH2 -MCM-48 provided easier access to guest molecules due to its three-dimensional pore structure resulting in higher activity. Alternatively, the higher activity of FePcS/NH2 -MCM-48 can be explained by its higher surface area and larger pore size in compassion with FePcS/NH2 MCM-41. It seems that the larger pore size of MCM-48 is more resistant to blocking and favors the adsorption of styrene and consequently shows higher activity. Finally, the higher activity of FePcS/NH2 -MCM-48 in comparison with FePcS/NH2 MCM-41 can be attributed to the presence of free amino groups. Enhancement of catalytic activity in the presence of an amine group has been reported previously [31]. Although oxidations of styrene by various catalysts were investigated extensively, only a limited number of them are with MPcS. Among the large numbers of metallophthalocyanines, only CuPc and its derivatives CuCl8 Pc, Cu(SO3 Na)4 Pc, and CuCl16 Pc were used as catalysts in the oxidation of styrene [16,32]. Interestingly, in spite of oxidation of styrene at 40 ◦ C with CuPc or its derivatives, either in form of neat or supported, the FePcS/NH2 -MCM-48 with 65.5% conversion showed higher superiority than the aforementioned catalysts [16]. In contrast, the FePcS/NH2 -MCM-41 with 46.9% conversion showed similar activity compared to CuCl16 Pc/NH2 MCM-41, which is the best catalyst among them [16,32]. Furthermore, the conversion of styrene oxidation by TBHP with supported catalysts in this study was significantly higher than oxidation by hydrogen peroxide with ZnF2 O4 [33], Fe2 O3 [33], and Fe-SBA-15 [34]. Finally, it is worth mentioning that these catalysts showed a much higher conversion of styrene and lower
selectivity to benzaldehyde compared with the iron-containing MCM-41 under our experimental conditions [13,35]. A blank test over MCM-48, MCM-41, and their functionalized materials under identical conditions showed only negligible conversions. Tert-butyl hydroperoxide in the absence of catalysts gave no reaction product. For the investigation of FePcS leaching into the solution, the supported catalysts were filtered out and the filtered solutions were analyzed for FePcS using a UV–vis spectrophotometer. Although no leaching of active species into the solution could be detected, the catalysts had lower activity on the second run. Furthermore, FePcS, after a 24 h use in the homogeneous system, lost its activity completely, whereas FePcS/NH2 -MCM-48 and FePcS/NH2 -MCM41 after 24 h showed some activities; interestingly the durability and activity of the former were higher than those of the latter. Apparently, immobilization of FePcS onto MCM-41 and MCM-48 pores improved its durability. The higher durability of immobilized FePcS than the unsupported FePcS catalyst in the oxidation of styrene could be attributed to the absence of free –SO3 H groups in heterogeneous systems. It seems that the stability and durability were associated with the acidic sites. In this case, the higher durability of FePcS/NH2 -MCM48 than FePcS/NH2 -MCM-41 in the oxidation of styrene could be attributed to the higher concentration of amine groups in the former and consequently lower numbers of acidic groups. Interestingly, degradation of MPcS to metal-free phthalocyanines in acidic solution was observed in the unsupported MPcS [36,37]. 4. Summary Iron tetrasulfophthalocyanine has been anchored onto functionalized MCM-48 and MCM-41 silicas and used as catalyst for the liquid oxidation of styrene. The BET, diffuse reflectance UV–vis, and liquid phase UV–vis analyses show that the behavior of these materials, because of differences in pore structure, varies to great extent. The functionalized MCM-48 absorbs twice more iron tetrasulfophthalocyanine (16%, w/w) than the functionalized MCM-41 (8%, w/w). The catalyst anchored on functionalized MCM-48 shows higher activity and durability in comparison with MCM-41 in the oxidation of styrene. Our future work is directed toward investigating the stability of catalysts in the aerobic oxidation of organic substrates.
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Immobilization of iron tetrasulfophthalocyanine on functionalized MCM-48 and MCM-41 mesoporous silicas: Catalysts for oxidation of styrene Mahtab Pirouzmand, Mostafa M. Amini ∗ , Nasser Safari ∗ Department of Chemistry, Shahid Beheshti University, Tehran 1983963113, Iran Received 25 September 2007; accepted 18 November 2007 Available online 24 November 2007
Abstract Iron tetrasulfophthalocyanine (FePcS) has been anchored on the surface of functionalized MCM-48 and MCM-41 silicas by means of chemical bonding to aminosilane groups. The prepared materials, FePcS/NH2 -MCM-48 and FePcS/NH2 -MCM-41, were characterized by diffuse reflectance UV–vis and infrared (IR) spectroscopies, low-angle X-ray diffraction (XRD), and surface area analysis (BET). The tendencies of FePcS absorption on functionalized MCM-48 and MCM-41 were measured by UV–vis spectroscopy. The functionalized MCM-48 showed a larger amine to silica ratio than the functionalized MCM-41. Low-angle X-ray diffraction analysis showed that, by anchoring iron tetrasulfophthalocyanine into functionalized MCM-48 and MCM-41, the intensity of main reflection decreased. The catalytic activities of the supported iron tetrasulfophthalocyanine were examined by the oxidation of styrene in the presence of tert-butyl hydroperoxide. The FePcS/NH2 -MCM-48 showed higher activity and durability in the liquid-phase oxidation of styrene under mild condition compared with the FePcS/NH2 -MCM-41 and unsupported catalyst. © 2007 Published by Elsevier Inc. Keywords: MCM-48; MCM-41; Iron tetrasulfophthalocyanine; Styrene
1. Introduction Metal complexes of phthalocyanines with facile preparation methods characterized by large-scale, high thermal stability, high conversion, and selectivity have attracted great interest [1]. From the environment viewpoint, the metallophthalocyanines being able to activate oxygen, thereby rendering these the capability of oxidizing organic compounds, represent a remarkable property compared with common transition-metal based oxidants. For instance, the industrial application of manganese, iron, and cobalt phthalocyanines for aerobic oxidation of mercaptan impurities and sweetening crude petrochemicals, the Merox process, is well established [2]. In addition, these compounds can be used as catalysts for the laboratory-scale oxidation of phenols to quinones [3], alkanes to alcohols or ketones [4], alkenes to epoxides [5], and oxidative degradation of chlorinated waste [6]. However, the aggregation and low solubility * Corresponding authors. Fax: +98 21 22431663.
E-mail addresses: [email protected] (M.M. Amini), [email protected] (N. Safari). 0021-9797/$ – see front matter © 2007 Published by Elsevier Inc. doi:10.1016/j.jcis.2007.11.024
of metallophthalocyanines are the main drawbacks in their application as catalysts. This property is further enhanced by the introduction of sulfate or carboxylate substituents at the periphery as the substituents are known to give rise to water-soluble derivatives with consistent homogeneous catalytic activity. Alternatively, the metallophthalocyanines can be immobilized on various supports in order to function as heterogeneous catalysts. The latter approach, due to environmental aspects, ease of separation, and recovery and recycling of catalyst, becomes a highly innovative research field. A wide variety of support substances such as polymer substrate [7], zeolite [8], and activated carbon [9] has been used for the immobilization of metallophthalocyanines. Zeolite encapsulation which is achieved by in situ cyclotetramerization of phthalonitrile to form phthalocyanines within the zeolite pore is particularly interesting [10]. However, large and hindered reactant molecules cannot enter the zeolite narrow pores, and greatly limiting access to encapsulated metallophthalocyanine sites on the framework walls. Therefore, the use of large-pores materials for the encapsulation of metallophthalocyanines is beneficial. Since the synthesis of the M41S family of micelle-template silicas [11],
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MCM-41, one member of this family, has attracted interest as a support for the immobilization of various catalysts in general [12–14] and metallophthalocyanines in particular [15]. Copperand cobalt-perchlorophthalocyanine [16,17], iron, manganese, and cobalt-tetrasulfophthalocyanine have been successfully anchored onto the surface of MCM-41 and have been used as catalysts in the liquid phase oxidation of various organic compounds [18,19]. Although, MCM-41 with a large pore volume can accommodate metallophthalocyanines, its tendency to absorb metal tetrasulfophthalocyanine (MPcS) is low, and there is only very weak interaction between MPcS and the siliceous surface. Therefore, due to the high solubility of MPcS in water, physically adsorbed MPcS can be leached out easily from the surface of support, which is supposed to act as a heterogeneous catalyst, and the concept of catalyst recovery is lost. In this context, increasing the susceptibility of mesoporous silica for the adsorption and immobilization of various metallophthalocyanines by means of chemical bonding via functionalized silica appears to be the proper approach. This approach has been used successfully by several researchers to immobilize MPcS on functionalized Si-MCM-41 [17,19]. A review of the literature shows that all studies on the immobilization of MPcS are devoted mostly to functionalized MCM-41, in spite of the many advantages of MCM-48. In contrast to the twodimensional hexagonal array of pore channels of MCM-41, MCM-48 with its three-dimensional pore network and larger surface area is more accessible to guest molecule if the pores are wide enough [20,21]. Therefore, the pore-blocking problem can be avoided to a great extent using MCM-48. Furthermore, the narrow pore size distribution and the interwoven and cubic pore structure of MCM-48 [22] make it more favorable in a catalytic system from the mass transfer kinetics point of view [21]. Consequently, higher stability and conversion in catalytic systems with MCM-48 support are expected. Despite the advantage of MCM-48 over MCM-41, the synthesis of high quality MCM48 with controlled pore size is more challenging. In the present work, we have used functionalized MCM-48 and MCM-41 as support for grafting FePcS and, after complete characterization, used as catalysts for oxidation of styrene in biphasic systems. 2. Materials and methods 2.1. Materials All reagents were purchased from Merck or Fluka and used as received. Sodium salt of FePcS was prepared according to the method of Weber and Busch [23] and was converted to corresponding acid (FePcS) by ion exchange resin. IR (KBr, cm−1 ): 3430, 3182, 1773, 1718, 1636, 1461, 1402, 1328, 1187, 1144, 1108, 1050, 1029, 930, 832, 748, 702, 635, 550. 2.1.1. Preparation of MCM-41, MCM-48, and functionalized MCM-41 and MCM-48 The MCM-41 and MCM-48 were synthesized according to a previously reported procedure [24,25], and then functionalized by 3-aminopropyltriethoxy silane [26]. In a typical reaction, 1 g of MCM-41 was suspended in 50 ml toluene and the mixture
was stirred for 1 h and then 1.6 g of (C2 H5 O)3 Si(CH2 )3 NH2 was added and refluxed for 2 h. The white solid was removed from the solvent by filtration and was washed by toluene and chloroform and then dried at room temperature. Functionalized MCM-41 and MCM-48 are designated as NH2 -MCM-41 (1) and NH2 -MCM-48 (2), respectively. 2.1.2. Absorption studies and preparation of immobilized iron tetrasulfophthalocyanine samples To demonstrate that the functionalized MCM-41 and MCM48 silicas have a greater tendency to absorb FePcS and to determine the optimum contact time required for the mesoporous silicas to absorb FePcS from the impregnation solution, 0.1 g of each mesoporous silica, bare and functionalized, was suspended in 7.5 ml aqueous solution of FePcS with initial concentration of 4 mg/ml. Each suspension was stirred at room temperature and the contact time varied from 10 to 1440 min. Then, the solid was removed from the solution by centrifugation and the UV–vis spectra of solution were recorded. To further examine the graft quantity and its corresponding reproduction, absorption studies were repeated thrice and consistent data were obtained. Similar results were obtained when the experiments were carried out in larger scales. For the preparation of mesoporous silica-supported FePcS and investigation of their physiochemical properties and catalytic activities, 1.00 g of each functionalized mesoporous silica was suspended in aqueous solution of FePcS (40 mg FePcS in 20 ml for 4% loading). The mixture was stirred overnight and then a greenish solid was collected by filtration, washed with distilled water, and finally dried at room temperature. The elemental analyses of mesoporous silica-supported FePcS, FePcS/NH2 -MCM-41 (3), and FePcS/NH2 -MCM-48 (4), before and after anchoring of the FePcS are given in Table 1. The physically adsorbed water was removed from samples prior to elemental analysis by drying them under reduced pressure. Therefore, the given hydrogen analysis in Table 1 included the hydrogen of the surface hydroxyls of silicas and also occluded water hydrogen. Similar to the absorption studies, each experiment was repeated thrice and reproducible data were obtained. Moreover, similar graft quantities were obtained when the experiments were carried out in larger scales. For the leaching study of FePcS from functionalized mesoporous silicas, 0.01 g of each sample was suspended in 20 ml distilled water and stirred for several hours. The solid was then removed by filtration and the UV–vis spectrum of solution was recorded. Table 1 Elemental analyses of functionalized silica materials before and after FePcS loading Material
C (%)
H (%)
N (%)
Si (%)
Fe (%)
NH2 -MCM-41 NH2 -MCM-48 FePcS/NH2 -MCM-41 FePcS/NH2 -MCM-48
10.54 9.55 11.80 10.85
2.73 3.38 2.69 3.32
2.97 3.20 3.34 3.56
39.09 39.14 37.59 37.64
– – 0.24 0.24
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2.2. Methods Infrared spectra were performed (KBr pellets) on a Shimadzu model FT-IR 4600 spectrometer. The solution and diffuse reflectance (DR) UV–vis spectra in the range of 300– 800 nm were recorded on a Shimadzu 2100 spectrophotometer, and DR UV–vis spectra referenced to BaSO4 . X-ray diffraction patterns were obtained on a Philips-PW 17C diffractometer with CuKα radiation. Scans were performed from (2θ ) 2◦ to 10◦ by rate of 3◦ /min. Nitrogen gas adsorption, using ultrahigh purity nitrogen gas (99.999%), adsorbed volumetrically on samples at −196 ◦ C, led to their specific surface area evaluation. An automatic stainless-steel volumetric adsorption apparatus was used to make these measurements. Experimental details for the volumetric adsorption set up are given elsewhere [27]. Prior to adsorption calculations, the sample weight was corrected for any weight loss due to degassing or drying. Pore distributions for diameters in the range of 10–500 Å were calculated by the same equipment using Barrett–Joyner–Halenda (BJH) method.
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of the grafted materials confirmed the presence of amine groups on the surface of the mesoporous silicas. The results of carbon and nitrogen analyses of functionalized MCM-41 and MCM-48 materials are compiled in Table 1. The observed C to N ratios, 3.55 and 2.98 for NH2 -MCM41 (1) and NH2 -MCM-48 (2), respectively, were close to the theoretical value of 2.57. Furthermore, the amount of grafted amine (Table 2) shows that the amine loading in NH2 -MCM41 and NH2 -MCM-48 were 2.53 and 2.74 mmol amines/g of SiO2 , respectively. Those values are corresponded to 4.6 and 2.8 amines/nm2 in NH2 -MCM-41 and NH2 -MCM-48, respectively. The specific surface areas of bare silicas, functionalized silicas and the FePcS grafted on functionalized silicas are listed in Table 3. The data show that the surface area of MCM-41 and MCM-48 decreased significantly after modification with 3aminopropyltriethoxy silane. The decrease in surface areas can be attributed to the increase in agglomeration of silica particles and/or occupation of pores after modification. 3.2. Preparation of supported catalysts
2.3. Catalytic reactions The epoxidation of styrene with tert-butyl hydroperoxide was performed in a 25-ml round-bottom flask. In a typical experiment, the flask was charged with 1.25 ml of 0.02 M CH3 OH/H2 O solution of styrene (0.025 mmol), 2.5 ml mixture of water and methanol (1:1, v/v), and 22 mg FePcS/NH2 MCM-41 or FePcS/NH2 -MCM-48 (4% loading) as supported catalysts. To this mixture, a 1.25 ml of 0.02 M CH3 OH/H2 O solution of tert-butyl hydroperoxide (0.025 mmol) was added during 30 min in five portions. The reactant mixture was stirred vigorously for 6 h at room temperature and then the catalyst was removed by centrifugation and the organic phase was analyzed with a HPLC (Knauer K-105) equipped with a Eurosphere 100 C18 column. The activity of bare and functionalized MCM-41 and MCM-48 was also measured in the absence of FePcS. Conversion for blank test was below 2%.
Iron tetrasulfophthalocyanine was prepared according to an earlier report [23]. Due to the higher catalytic activity of iron phthalocyanine among metal phthalocyanines, we have chosen that as a catalyst in our investigation. The immobilization of FePcS on the inner surface of the pores of NH2 -MCM-41 or NH2 -MCM-48 occurs by means of chemical bonding with the amine groups is shown in Scheme 1. As previously reported, a strong interaction of FePcS acidic protons with amine groups resulted in the retention of FePcS on the silica surface [18]. Absorption of FePcS on NH2 -MCM-41 and NH2 -MCM-48 was investigated by UV–vis spectroscopy. Fig. 1 shows the absorption spectrum of FePcS solution with concentration of 4 mg/ml before and after contact with MCM41 and MCM-48, and Figs. 2 and 3 show the absorption spectra of the above-mentioned FePcS solution before and after contact Table 3 Texture parameters for the mesoporous materials before and after grafting with amine and anchoring with FePcS
3. Results and discussion 3.1. Modification of mesoporous MCM-41 and MCM-48 All mesoporous silicas were grafted with propylamine group by treating with (EtO)3 SiCH2 CH2 CH2 NH2 in refluxing toluene [28]. The propylamine group attached to the mesoporous silica surface by condensation of silanol and ethoxy groups. The organosilane and amine group characteristic peaks (2930 and 1485, 1565 cm−1 , respectively) in the FT-IR spectra
Sample MCM-41 MCM-48 NH2 -MCM-41 NH2 -MCM-48 FePcS/NH2 -MCM-41 FePcS/NH2 -MCM-48
Total pore volume (ml/g)
BET-specific surface area (m2 /g) 1070 1238 331 584 116 148
0.52 0.75 0.21 0.28 0.059 0.076
Table 2 Chemical compositions of functionalized MCM-41 and MCM-48 Material
mmol amine/g SiO2
mmol FePcS/g SiO2
mol FePcS/mol amine
(1) NH2 -MCM-41 (2) NH2 -MCM-48 (3) FePcS/NH2 -MCM-41 (4) FePcS/NH2 -MCM-48
2.53 2.74 2.96 3.15
– – 0.056 0.056
– – 0.019 0.018
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Scheme 1. Immobilization of FePcS on the inner surface of functionalized mesoporous silica.
Fig. 1. UV–vis spectra of a FePcS solution before and after contact with MCM-41 and MCM-48 up to 24 h (curves are overlapped).
Fig. 3. UV–vis spectra of a FePcS solution (a) before and after contact with functionalized MCM-48 for (b) 10 min, (c) 30 min, and (d) 60 min and 24 h.
Fig. 2. UV–vis spectra of a FePcS solution (a) before and after contact with functionalized MCM-41 for (b) 10 min, (c) 30 min, and (d) 60 min and 24 h.
with NH2 -MCM-41 and NH2 -MCM-48, respectively. Apparently, mesoporous silicas, MCM-41 and MCM-48, before modification with the amine, showed no tendency for the absorption of FePcS. This was confirmed by the absence of FePcS characteristic peaks in the DR UV–vis and IR spectra of bare MCM-41 and MCM-48 which were impregnated with FePcS and then washed with water. The UV–vis spectra of FePcS solution after contact with NH2 -MCM-41 and NH2 -MCM-48, except for the intensity of peaks, were similar to the solution prior to contact, which revealed that the FePcS after contact remained intact in solution. The kinetics of absorption of FePcS on NH2 -MCM-41 and NH2 -MCM-48, shown in Fig. 4, has been investigated by UV– vis spectroscopy for an interval of 10–200 min using a solution
Fig. 4. Absorption of FePcS on (a) NH2 -MCM-41, (b) NH2 -MCM-48 as a function of time.
with concentration of 4 mg/ml of FePcS. Kinetic adsorption of FePcS on NH2 -MCM-41 in comparison with NH2 -MCM-48 was slower. The slower rate of absorption on the former can be attributed to the lower concentration of amine groups according to elemental analysis. However, because the reaction between acidic protons of FePcS and amine groups was fast, the effects of surface area, pore size, pore distribution of functionalized MCM-41 and MCM-48 on the rate of absorption could not be ruled out. The amounts of FePcS that absorbed on NH2 -MCM41 and NH2 -MCM-48 were 0.24 and 0.28 mg/m2 , respectively. Similar results were also obtained when the experiments were carried out in larger scales.
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Fig. 5. DR UV–vis spectra of (a) FePcS/NH2 -MCM-41, and (b) FePcS/ NH2 -MCM-48.
The DR UV–vis spectra of FePcS supported on functionalized MCM-41 and MCM-48 (Fig. 5) demonstrate clearly that the iron tetrasulfophthalocyanine anchored on supports and did not degrade. The above conclusion confirmed also by the infrared spectroscopy. Interestingly, the DR UV–vis spectrum of FePcS/NH2 -MCM-41 showed a broad band, which extended to the 630–690 nm region. Considering the general agreement in the literature that the dimeric and monomeric forms of FePcS in solution show absorption maximum at 630 and 680 nm, respectively [29] one can conclude that the FePcS absorbed on NH2 -MCM-48 is mostly in the form of µ-oxo-dimer whereas on the NH2 -MCM-41 is a mixture of dimeric and monomeric forms. Furthermore, the DR UV–vis spectra of FePcS/NH2 MCM-41 and FePcS/NH2 -MCM-48 are similar to the UV–vis spectrum of an aqueous solution of FePcS. This indicates that the orbital associated with the absorption bands are not involved in the bonding of FePcS to the surface of NH2 -MCM-41 and NH2 -MCM-48. Apparently, immobilization of FePcS on the surface of MCM-48 and MCM-41 occurs by means of chemical bonding with the amine groups. After immobilization of FePcS with functionalized NH2 MCM-41 and NH2 -MCM-48, the specific surface areas decreased further, indicating that the pores of functionalized mesoporous materials are being filled. Considering the same FePcS loading of NH2 -MCM-48 and NH2 -MCM-41, the larger loss of surface area of the NH2 -MCM-48 in comparison with NH2 -MCM-41 after anchoring FePcS is attributed to the different pore structure and the larger pore volume of the formerly mentioned material or to higher amine loading. Interestingly, none of the supported FePcS showed leaching after several hours of contact with the aqueous solution. Apparently, the strong interaction of NH2 -MCM-41 and NH2 -MCM-48 with FePcS makes them suitable heterogeneous catalysts. The low-angle X-ray diffraction patterns of MCM-41 (a), NH2 -MCM-41 (b), and FePcS/NH2 -MCM-41 (c) are shown in Fig. 6 and those of MCM-48 (a), NH2 -MCM-48 (b), and FePcS/NH2 -MCM-48 (c) are seen in Fig. 7. Apparently, by functionalizing MCM-41 and MCM-48 and eventually anchoring FePcS the intensities of reflections at 2θ = 2.4◦ are decreased significantly. The decrease in the intensity of reflec-
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Fig. 6. Low-angle XRD patterns of (a) calcined MCM-41, (b) NH2 -MCM-41, and (c) FePcS/NH2 -MCM-41.
Fig. 7. Low-angle XRD patterns of (a) calcined MCM-48, (b) NH2 -MCM-48, and (c) FePcS/NH2 -MCM-48.
tions, which is well known in immobilization of various catalysts on mesoporous silicas, is attributed to the occupation of mesopore channels [30]. These results are in accordance with the decrease of surface areas by grafting amine groups on MCM-41 and MCM-48 and eventually anchoring with FePcS. 3.3. Preliminary catalytic properties of immobilized FeTSPc in oxidation of styrene The catalytic activities of supported catalysts have been tested in the oxidation of styrene by tert-butylhydroperoxide (TBHP) in a mixture of methanol and water at room temperature. The oxidation of styrene with TBHP in the presence of FePcS/NH2 -MCM-41 and FePcS/NH2 -MCM-48 resulted in the formation of benzaldehyde. The catalytic performances of FePcS in homogeneous and heterogeneous systems are compared in Table 4. Although styrene conversions in heterogeneous systems are comparable with the homogeneous ones, the homogeneous systems showed higher selectivity to benzoic acid, which is not a desirable product. Apparently, TBHP oxidized styrene deeper in homogeneous systems. Interestingly, in spite of the same loading of FePcS for FePcS/NH2 -MCM-41 and FePcS/NH2 -MCM-48, the former catalyst showed a notably lower conversion. The higher activity of FePcS/NH2 -MCM-48 in comparison with FePcS/NH2 MCM-41 can be attributed to the more efficient dispersion of
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Table 4 Oxidation of styrene in homogeneous and heterogeneous systems with FePcS Catalyst FePcS FePcS FePcS/NH2 -MCM-41 FePcS/NH2 -MCM-41 FePcS/NH2 -MCM-48 FePcS/NH2 -MCM-48
Reaction time (h)
Conversion (%)
Benzaldehyde
6 24 6 24 6 24
39.3 57.8 16.7 46.9 21.9 65.5
18.7 36.2 19.1 20.2 23.9 21.4
TOFa
Benzoic acid
Yield (%) (benzaldehyde)
47.3 36.7 0 0 0 0
7.3 20.9 3.2 9.5 5.2 14.0
19.7 28.9 8.3 23.5 10.9 32.7
Selectivity (%)
a TOF = turnover frequency (mol styrene converted/mol Fe).
the former. It is of interest to compare the activity of a homogeneous system with that of a heterogeneous system. The unsupported FePcS with 57.8% conversion after 24 h with turnover frequency (TOF) of 28.9 mol(styrene) mol(Fe)−1 h−1 mol was found to be more active than FePcS/NH2 -MCM-41 but lower than FePcS/NH2 -MCM-48 with a conversion of 65.5 and a turn TOF of 32.7 mol(styrene) mol(Fe)−1 h−1 mol. Thus, encapsulated FePcS in MCM-48 is a more effective catalyst than that in MCM-41, and unsupported catalyst. This can be attributed to the different pore structure of MCM-48 and possibly to higher concentration of µ-oxo-dimer form of FePcS, due to the internal framework structure of MCM-48. Although the higher activity of FePcS anchored into MCM-48 in dimeric form contradicts what is known to be a higher activity of monomeric species in a homogeneous system, a similar behavior also has been reported for the FePcS anchored into mesoporous and amorphous silicas [18]. Furthermore, FePcS/NH2 -MCM-48 provided easier access to guest molecules due to its three-dimensional pore structure resulting in higher activity. Alternatively, the higher activity of FePcS/NH2 -MCM-48 can be explained by its higher surface area and larger pore size in compassion with FePcS/NH2 MCM-41. It seems that the larger pore size of MCM-48 is more resistant to blocking and favors the adsorption of styrene and consequently shows higher activity. Finally, the higher activity of FePcS/NH2 -MCM-48 in comparison with FePcS/NH2 MCM-41 can be attributed to the presence of free amino groups. Enhancement of catalytic activity in the presence of an amine group has been reported previously [31]. Although oxidations of styrene by various catalysts were investigated extensively, only a limited number of them are with MPcS. Among the large numbers of metallophthalocyanines, only CuPc and its derivatives CuCl8 Pc, Cu(SO3 Na)4 Pc, and CuCl16 Pc were used as catalysts in the oxidation of styrene [16,32]. Interestingly, in spite of oxidation of styrene at 40 ◦ C with CuPc or its derivatives, either in form of neat or supported, the FePcS/NH2 -MCM-48 with 65.5% conversion showed higher superiority than the aforementioned catalysts [16]. In contrast, the FePcS/NH2 -MCM-41 with 46.9% conversion showed similar activity compared to CuCl16 Pc/NH2 MCM-41, which is the best catalyst among them [16,32]. Furthermore, the conversion of styrene oxidation by TBHP with supported catalysts in this study was significantly higher than oxidation by hydrogen peroxide with ZnF2 O4 [33], Fe2 O3 [33], and Fe-SBA-15 [34]. Finally, it is worth mentioning that these catalysts showed a much higher conversion of styrene and lower
selectivity to benzaldehyde compared with the iron-containing MCM-41 under our experimental conditions [13,35]. A blank test over MCM-48, MCM-41, and their functionalized materials under identical conditions showed only negligible conversions. Tert-butyl hydroperoxide in the absence of catalysts gave no reaction product. For the investigation of FePcS leaching into the solution, the supported catalysts were filtered out and the filtered solutions were analyzed for FePcS using a UV–vis spectrophotometer. Although no leaching of active species into the solution could be detected, the catalysts had lower activity on the second run. Furthermore, FePcS, after a 24 h use in the homogeneous system, lost its activity completely, whereas FePcS/NH2 -MCM-48 and FePcS/NH2 -MCM41 after 24 h showed some activities; interestingly the durability and activity of the former were higher than those of the latter. Apparently, immobilization of FePcS onto MCM-41 and MCM-48 pores improved its durability. The higher durability of immobilized FePcS than the unsupported FePcS catalyst in the oxidation of styrene could be attributed to the absence of free –SO3 H groups in heterogeneous systems. It seems that the stability and durability were associated with the acidic sites. In this case, the higher durability of FePcS/NH2 -MCM48 than FePcS/NH2 -MCM-41 in the oxidation of styrene could be attributed to the higher concentration of amine groups in the former and consequently lower numbers of acidic groups. Interestingly, degradation of MPcS to metal-free phthalocyanines in acidic solution was observed in the unsupported MPcS [36,37]. 4. Summary Iron tetrasulfophthalocyanine has been anchored onto functionalized MCM-48 and MCM-41 silicas and used as catalyst for the liquid oxidation of styrene. The BET, diffuse reflectance UV–vis, and liquid phase UV–vis analyses show that the behavior of these materials, because of differences in pore structure, varies to great extent. The functionalized MCM-48 absorbs twice more iron tetrasulfophthalocyanine (16%, w/w) than the functionalized MCM-41 (8%, w/w). The catalyst anchored on functionalized MCM-48 shows higher activity and durability in comparison with MCM-41 in the oxidation of styrene. Our future work is directed toward investigating the stability of catalysts in the aerobic oxidation of organic substrates.
M. Pirouzmand et al. / Journal of Colloid and Interface Science 319 (2008) 199–205
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