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SBA-16 supported amino acid Schiff base complexes of molybdenum as new heterogeneous molybdenum catalysts
Colloids and Surfaces A 570 (2019) 347–353
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
SBA-16 supported amino acid Schiff base complexes of molybdenum as new heterogeneous molybdenum catalysts
T
Majid Masteri-Farahania, , Paria Eghbalib,c, Ertan Şahinc ⁎
a
Faculty of Chemistry, Kharazmi University, Tehran, Iran Department of Analytical Chemistry, Faculty of Pharmacy, Girne American University, 99428 Kyrenia, North Cyprus, Mersin 10, Turkey c Department of Chemistry, Faculty of Science, Atatürk University, Erzurum, Turkey b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Amino acid Epoxidation Molybdenum SBA-16 Schiff base
Immobilization of molybdenum complexes of amino acid Schiff bases within the SBA-16 nanocages produced new heterogeneous catalysts for the epoxidation of olefins. First, amino acid Schiff bases were obtained through the reaction of amino acids with salicylaldehyde. Then, complexation of the prepared amino acid Schiff bases with molybdenum (VI) produced the molybdenum complexes of amino acid Schiff bases. Immobilization of the molybdenum complexes into the SBA-16 nanocages followed by silylation with triethoxyoctylsilane gave the heterogenized molybdenum catalysts. The obtained catalysts were characterized with several physicochemical techniques. FT-IR and inductively coupled plasma optical emission (ICP-OES) spectroscopies approved the inclusion of molybdenum complexes within the SBA-16 nanocages. The results of X-ray diffraction (XRD) and nitrogen adsorption-desorption (BET method) analyses illustrated that surface properties of SBA-16 were maintained upon the inclusion of molybdenum complexes. The prepared catalysts exhibited good activities and excellent selectivities (> 99%) in the epoxidation of olefins with tert-butyl hydroperoxide (TBHP).
⁎
Corresponding author. E-mail address: [email protected] (M. Masteri-Farahani).
https://doi.org/10.1016/j.colsurfa.2019.03.047 Received 10 January 2019; Received in revised form 7 March 2019; Accepted 16 March 2019 Available online 18 March 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 570 (2019) 347–353
M. Masteri-Farahani, et al.
Scheme 1. Schematic illustration of the immobilization of molybdenum complexes of amino acid Schiff bases into the SBA-16 nanocages. Table 1 FT-IR and molybdenum content data of the prepared catalysts. Sample
υMoO2 (cm−1)
υC=N (cm−1)a
Mo content of fresh catalyst (mmol.g-1)
Mo content of 5th recovered catalyst (mmol.g-1)
Mo-salgly@SBA-16 Mo-salala@SBA-16 Mo-salphenala@SBA-16 Mo-salleu@SBA-16 Mo-salisoleu@SBA-16 Mo-salhis@SBA-16
912, 902, 905, 908, 911, 905,
1610 1613 1610 1610 1611 1608
0.07 0.05 0.04 0.06 0.04 0.05
0.06 0.05 0.03 0.04 0.03 0.05
a
938 937 939 935 942 945
(1636) (1634) (1634) (1635) (1637) (1635)
The numbers in parentheses show υC=N in free ligands.
recoverable catalysts [9–15]. Among the various solids utilized for immobilization of homogeneous catalysts, SBA-16 has been appeared as suitable support for the preparation of heterogenized catalysts due to its large and well-ordered mesopores and nanocages as well as high surface area [16–20]. Homogeneous and heterogenized homogeneous molybdenum (VI) catalysts have attracted great interests and there are several reports about their utilization in catalytic organic reactions [21–29]. On the other hand, transition metal complexes containing amino acid Schiff base ligands have been used extensively in catalytic reactions [30–34]. Nonetheless, the reports on the study of heterogenized catalysts containing transition metal complexes of amino acid Schiff bases is limited [35–37]. Specially, the immobilization of amino acid Schiff base complexes of molybdenum into the SBA-16 nanocages has not been reported. Here, amino acid Schiff base complexes of molybdenum were
1. Introduction Recently, increasing attention has been devoted to develop environmental friendly processes in all fields of chemistry. One of the concerns is the waste minimization by utilization of green reagents and efficient catalysts. In this regard, isolation of catalyst or products from the reaction mixture is one of the difficulties which has an important contribution to waste production in a commercial process. Thus, the utilization of easily recoverable catalysts has attracted great attention due to facile isolation and several efforts have been done for the preparation of such catalysts [1–8]. Heterogenized homogeneous catalysts play an important role in developing green processes especially in chemical industries. According to green chemistry concept, not only the high activity of the catalyst is important, but also the good recoverability is critical. Immobilization of homogeneous catalysts on solid inorganic materials is an appropriate method for developing such 348
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heptamolybdate (3.0 g) in H2O (10 ml), 4 ml of acetylacetone was added. A yellow solid started to precipitate after adjusting the pH of the mixture to 3.5 with addition of 10% HNO3. After 1.5 h, the obtained MoO2(acac)2 was separated by filtration, washed with H2O and ethanol, and dried in vacuum oven. Fourier transform infrared (FT-IR) spectra were recorded on PerkinElmer Spectrum RXI FT-IR spectrometer. Elemental analyses were carried out by a VARIAN VISTA-MPX ICP-AES spectrometer. X-ray diffraction (XRD) analyses were done on a SIEFERT XRD 3003 PTS diffractometer using Cu Kα radiation (λ = 0.154 nm). Thermogravimetric analyses were performed on a Perkin Elmer Diamond Thermogravimeter from room temperature to 700 °C using a rate of 10 °C/min in static air. Scanning electron microscopy (SEM) images were provided using ZEISS-DSM 960 A microscope with an attached camera. Transmission electron microscopy (TEM) analysis was carried out with a Philips EM 208S instrument using an accelerating voltage of 100 kV. Samples were prepared by placing droplets of a suspension of the sample in ethanol on a polymer microgrid supported on a Cu grid. Nitrogen adsorption-desorption analyses were done at liquid nitrogen temperature (−196 °C) using Quantachrome Nova 2200e, Version 7.11 Analyzer. Before the analysis the samples were outgassed under high vacuum at 150 °C. In order to evaluate the progress of the epoxidation reactions, samples were withdrawn in given times and analyzed using a gas chromatograph (HP, Agilent 6890 N) equipped with a capillary column (HP-5) and a FID detector. The molybdenum content of recycled catalysts was measured using atomic absorption spectroscopy after their digestion in diluted HF solution.
Fig. 1. XRD patterns of pristine SBA-16 and catalysts. Table 2 Textural parameters of samples determined from XRD analysis. Sample
2θ (º)
d110 (Å)
Lattice parametera (Å)
SBA-16 Mo-salgly@SBA-16 Mo-salala@SBA-16 Mo-salphenala@SBA-16 Mo-salleu@SBA-16 Mo-salisoleu@SBA-16 Mo-salhis@SBA-16
1.35 1.30 1.30 1.25 1.30 1.25 1.25
65.4 67.9 67.9 70.6 67.9 70.6 70.6
92.5 96.0 96.0 99.8 96.0 99.8 99.8
a
2.2. Synthesis of the molybdenum complexes of amino acids Schiff bases The amino acid (glycine, alanine, phenylalanine, leucine, isoleucine, and histidine) Schiff base ligands were synthesized through the common procedure introduced for the synthesis of Schiff base ligands [40]. Typically, 4 mmol of the amino acid was added to a solution of salicylaldehyde (4 mmol) in ethanol (20 ml). The mixture was stirred for 4 h to achieve amino acid Schiff base ligand. Afterwards, 4 mmol of MoO2(acac)2 in 20 ml ethanol was added and the solution refluxed for 8 h to give the corresponding molybdenum Schiff base complex. The obtained complex was separated by filtration and dried.
Determined by equation: a0 = √2 d110.
synthesized and immobilized into the SBA-16 nanocages. In order to avoid from the leaching of molybdenum complex, the entrance size of mesopores were adjusted through the silylation reaction. The prepared heterogeneous catalysts were utilized in the epoxidation of olefins with tert-butyl hydroperoxide (TBHP). The advantages of achieved catalytic systems are the facile isolation of the heterogenized catalysts at the end of reactions besides the good activities, excellent selectivities, and high stabilities.
2.3. Preparation of the catalysts First, the SBA-16 was dried at 120 °C in vacuum oven overnight to remove the adsorbed water molecules. Then, to a suspension of SBA-16 (1 g) in 20 ml methanol, 4 mmol of the molybdenum Schiff base complex in 10 ml methanol was added and the mixture refluxed for 8 h under N2 atmosphere. After evaporating the methanol solvent, the product was dried at 70 °C for 4 h. The obtained solid was reacted with triethoxyoctylsilane to adjust the pore size of the SBA-16 support through the previously reported method [41]. The solid was dispersed in a solution of triethoxyoctylsilane (4 mmol) in toluene (15 ml). Then, 5 ml of pyridine was added and refluxed for 10 h under N2 atmosphere. The obtained catalyst was isolated by filtration and soxhlet washed with ethanol to eliminate the remained reagents. Eventually, the product was dried at 70 °C in vacuum oven overnight to remove the remained solvent.
2. Experimental 2.1. Materials and instrumentation Ttriblock co-polymer Pluronic F127 and tert-butylhydroperoxide (TBHP, 80% in di-tertiary butyl peroxide) were obtained from SigmaAldrich Company. Cyclooctene (95%) was purchased from Across Company. The other chemicals were provided from Merck Company and used without further purification. The purity of the compounds were given in supplementary information. Mesoporous SBA-16 material was prepared as followed [38]. To a solution of Pluronic F127 (0.32 g) and cetyl trimethylammonium bromide (CTAB, 0.037 g) in 35 ml of 2 M HCl, 1 g of TEOS was added and the mixture stirred under reflux condition firstly at 40 °C for 6 h and then at 80 °C for 6 h. The obtained solid was separated by filtration and dried at 80 °C. Finally, the solid was calcined at 550 °C for 6 h to remove the organic species. MoO2(acac)2 (acac = acetylacetonate ligand) was prepared according to reported method [39]. To a solution of ammonium
2.4. Catalytic epoxidation of olefins 4 mmol of olefin and 50 mg of catalyst was added to a flask containing 5 ml chloroform and after addition of TBHP (80% in di-tertiary butyl peroxide, 14.4 mmol), the mixture was refluxed under N2 atmosphere. In order to monitor the reaction progress, samples were withdrawn from the reaction mixture in specific times and analyzed with a gas chromatograph. In order to study the catalyst reusability, it was 349
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Fig. 2. Nitrogen adsorption isotherms and BJH pore size distributions of prepared catalysts.
acids with salicylaldehyde in ethanol according to the common method described in earlier reports [39]. The obtained amino acid Schiff base ligands were treated with molybdenum precursor to achieve the desired molybdenum complexes. The immobilization process comprised of first loading the obtained molybdenum Schiff base complex within the SBA16 nanocages. The size of pore entrances should become sufficiently small to enclose the molybdenum complexes in the SBA-16 nanocages and inhibit their leaching during the catalytic epoxidation. For adjusting the pore entrances, silylation with appropriate reagent should be performed. Hence, the obtained SBA-16 containing molybdenum complex was reacted with triethoxyoctylsilane to modify the pore entrance size without blocking them [41]. The obtained product was soxhlet washed with ethanol to eliminate the unreacted reagents. It is notable that FT-IR and ICP-OES analyses (not given here) showed that if the silylation of the pore entrances of pristine SBA-16 is done in the first stage, any effort to immobilize the molybdenum complexes into the SBA-16 nanocages is failed. FT-IR spectroscopy was exploited to provide evidences for the successful immobilization of molybdenum complexes into the SBA-16 nanocages and the next silylation reaction. The prepared materials exhibited two peaks at about 1700 cm−1 and 1630 cm−1 due to stretching vibrations of amino acid carboxyl groups and imine bonds, respectively. Moreover, some peaks were observed at around 1050 and 800 cm−1 owing to the asymmetric and symmetric stretching vibrations of Si-O-Si in the SBA-16 support. Also, two bands were appeared at about 2860 and 2960 cm−1 due to the C–H stretching vibrations of octyl groups in silylating reagents and amino acids. The appearance of two adjacent
Table 3 The textural parameters of materials determined from N2 sorption analysis. Sample
Average pore diameter (Å)
Total pore volume (ml. g−1)
SBET (m2. g−1)
SBA-16 Mo-salgly@SBA-16 Mo-salala@SBA-16 Mo-salphenala@SBA-16 Mo-salleu@SBA-16 Mo-salisoleu@SBA-16 Mo-salhis@SBA-16
38 35 32 30 32 31 30
0.31 0.27 0.22 0.12 0.20 0.18 0.15
705 453 395 206 401 356 344
isolated at the end of epoxidation reaction and then washed thoroughly with methanol and dried at 70 °C in vacuum oven overnight. The recovered catalyst was reused in the cyclooctene epoxidation in the same reaction conditions mentioned above. 3. Results and discussion 3.1. Preparation and characterization of the catalysts The immobilization of molybdenum complexes of amino acid Schiff bases within the SBA-16 nanocages was done through the strategy outlined in Scheme 1. First, the Schiff base ligands were prepared by reaction of amino 350
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Fig. 3. TGA curves of the prepared catalysts.
peaks in the range of 890-950 cm−1 in the FT-IR spectra of the catalysts indicates the existence of cis-MoO2 fragment in their structure (Table 1) [42]. The observation of these peaks approves the successful immobilization of the molybdenum complex and silylation reaction. ICP-OES analysis was exploited to determine the molybdenum content of freshly prepared catalysts (Table 1) which further approved the encapsulation of molybdenum complexes into the SBA-16 nanocages. X-ray diffraction (XRD) analysis verified the structure retention of SBA-16 in the course of immobilization of the molybdenum complexes into the SBA-16 nanocages. In the XRD patterns of the samples, a single peak is seen at 2θ = 1.2−1.5°, assigned to the (110) plane of the cubic unit cell (Fig. 1). The relative decrease in the intensity of this peak in the XRD patterns of catalysts with respect the SBA-6 indicates the incorporation of molybdenum complexes into the SBA-16 nanocages. Furthermore, the shift of the peaks to lower angles revealed the expansion of unit cell due to the immobilization of molybdenum complexes within the SBA-16 nanocages (Table 2). Nitrogen adsorption-desorption analysis was conducted to study the surface area and isotherms of the obtained materials after immobilization of the molybdenum complexes into the SBA-16 nanocages. As seen in Fig. 2, the obtained materials show type IV isotherms with hysteresis loops specifying the materials with cage-like mesopores [43]. The observation of hysteresis loops in the obtained isotherms is a result of capillary condensation of N2 gas within the nanocages. Upon the immobilization of molybdenum complex within the nanocages, the inflection points of isotherms shifted towards the lower
relative pressures and the adsorbed volume of N2 reduced due to the pore size reduction. Furthermore, the surface areas (SBET), pore volumes, and average pore diameters were decreased upon immobilization of the molybdenum complex (Table 3). These observations provide further evidences for immobilization of the molybdenum complexes into the SBA-16 nanocages. The sequence of increasing the surface area of the catalysts is as below: Mo-salgly@SBA-16 > Mo-salleu@SBA-16 > Mo-salala@SBA16 > Mo-salisoleu@SBA-16 > Mo-salhis@SBA-16 > Mo-salphenala@ SBA-16. The observed order in surface area is compatible with the steric requirements of the amino acid groups in the Schiff base ligand. Increasing the amino acid steric hindrance results in the decrease in surface area of the corresponding catalyst. Thermogravimetric analysis (TGA) of the prepared catalysts was done and the results are exhibited in Fig. 3. The TGA curves showed weight loss of 4–7 % on heating over the range 25–210 °C which can be assigned to the depletion of adsorbed water molecules. The next weight loss was occurred in the range of 250–600 °C due to the combustion of the organic species. The amount of weight loss was found to be compatible with the carbon content of the Schiff base ligands. With increasing the carbon content of the amino acid Schiff base, the corresponding weight loss in TGA curve is increased. 3.2. Catalytic study of the obtained catalysts The obtained catalysts were utilized in the catalytic epoxidation of 351
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Table 4 Epoxidation of olefins with TBHP in the presence of prepared catalysts. Catalyst
Run
olefin
Time (h)
Conversiona(%)
Epoxide (%)
Mo:substrate molar ratio
TOFb (h-1)
Mo-salala @SBA-16
1 3 4 5 6 8 9 10 11 13 14 15 16 18 19 20 21 23 24 25 26 28 29 30 31 32
cyclooctene cyclohexene 1-hexene 1-octene cyclooctene cyclohexene 1-hexene 1-octene cyclooctene cyclohexene 1-hexene 1-octene cyclooctene cyclohexene 1-hexene 1-octene cyclooctene cyclohexene 1-hexene 1-octene cyclooctene cyclohexene 1-hexene 1-octene Cyclooctene Cyclooctene
8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 12 12
> 99 91 66 46 > 99 87 59 41 97 83 54 35 95 79 51 32 93 74 49 30 98 85 60 41 9 4
> > > > > > > > > > > > > > > > > > > > > > > > 41 51
1:1600
640
1:2000
890
1:1333
546
1:2000
760
1:1600
552
1:1142
451
– –
– –
Mo-salphenala @SBA-16 Mo-salleu @SBA-16 Mo-salisoleu @SBA-16 Mo-salhis @SBA-16 Mo-salgly @SBA-16 SBA-16 No catalyst
99 99 99 99 99 99 99 99 99 99 99 99 99 > 99 99 99 99 99 99 99 99 99 99 99 99
Reaction conditions: catalyst (100 mg), olefin (8 mmol), TBHP (14.4 mmol), refluxing CHCl3 (10 ml). a Determined using isooctane as internal standard. b Determined as mmol of product per mmol of molybdenum in catalyst per time (2 h). Table 5 Results of the epoxidation of cyclooctene with TBHP after isolation of the catalysts. Catalyst
Run
olefin
Time (h)
Conversiona(%)
Epoxide (%)
Mo-salala @SBA-16 Mo-salphenala @SBA-16 Mo-salleu @SBA-16 Mo-salisoleu @SBA-16 Mo-salhis @SBA-16 Mo-salgly @SBA-16
1
Cycloocteneb Cyclooctenec Cycloocteneb Cyclooctenec Cycloocteneb Cyclooctenec Cycloocteneb Cyclooctenec Cycloocteneb Cyclooctenec Cycloocteneb Cyclooctenec
2 12 2 12 2 12 2 12 2 12 2 12
80 82 89 92 82 86 76 79 69 70 79 81
> > > > > > > > > > > >
2 3 4 5 6
99 99 99 99 99 99 99 99 99 99 99 99
Reaction conditions: catalyst (100 mg), cyclooctene (8 mmol), TBHP (14.4 mmol), refluxing CHCl3 (10 ml). a Determined using isooctane as internal standard; b Catalytic activity instantly after isolation of catalyst (2 h); c Reaction progress after 10 h of catalyst isolation.
Fig. 4. Catalytic reusabilities of the catalysts in the cyclooctene epoxidation with TBHP. Reaction conditions: Recovered catalyst (100 mg), cyclooctene (8 mmol), TBHP (14.4 mmol), refluxing CHCl3 (10 ml).
some olefins with TBHP and the results were shown in Table 4. As can be seen in Table 4, the catalysts were active in the epoxidation of olefins with selectivities of 100% towards the epoxides. The results of Table 4 reveals that the TOF values increase as: Mosalphenala@SBA-16 > Mo-salisoleu@SBA-16 > Mo-salala@SBA16 > Mo-salhis@SBA-16 > Mo-salleu@SBA-16 > Mo-salgly@SBA-16. The observed sequence can be explained with taking into account the coordination of produced tert-butanol byproduct to the molybdenum active site of the catalyst which decreases the rate of epoxidation reaction. With increasing the electron density on the molybdenum site, the coordination of tert-butanol is retarded which results in the less deactivation of the catalyst. Furthermore, the more steric hindrance of the Schiff base ligand assists this effect. Thus, the catalyst possessing more electron donating phenyl alanine group shows
the highest catalytic activity. Another parameter influencing the conversion of olefins can be seen with inspecting the results of Table 4. The sequence of olefin reactivities towards the formation of epoxides is cyclooctene > cyclohexene > 1hexene > 1-octene. Our earlier works on the olefin epoxidations have indicated that two factors determine the olefin reactivities i.e. steric and electronic effects. It has been shown that the more electron density on the olefin double bond enhances the epoxidation reactivity [28,29,44]. Thus, olefins having inner C]C bonds such as cyclooctene and cyclohexene are more reactive with respect to the olefins with terminal C]C bonds such as 1-hexene and 1-octene. The lower reactivity of 1-octene with respect to 1-hexene can be explained by taking into account the steric hindrance of the larger group connected to C]C bond in the case of 1-octene in comparison with 1-hexene which make 352
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it difficult the approaching to the molybdenum center. Besides, further experiments were done without catalysts or with SBA-16 as catalyst which revealed very low conversion of cyclooctene after 12 h reaction. This observation obviously verifies the key role of the molybdenum center in the selective epoxidation of olefins. The catalysts stabilities were explored with further tests in which epoxidation of cyclooctene was permitted until 2 h and then the catalysts were separated by filtration. The filtrates were further reacted under the same conditions for further 10 h. In each case, the cyclooctene conversions were determined after 2 h and 12 h (Table 5). The results revealed that after separating the catalysts, the increase in the conversion of olefins is slightly increased. This observation clearly indicates that there is no leached molybdenum species in the solution and the catalysts are stable in the reaction conditions. Thus, the catalytic epoxidations is done heterogeneously in the presence of obtained catalysts. The catalysts reusabilities were also studied by recovering the catalysts and reusing in the cyclooctene epoxidation. As shown in Fig. 4, slight decreases in conversions were seen verifying the stability of the molybdenum species within the SBA-16 nanocages. The molybdenum content of the recovered catalysts after 5th catalytic reactions was measured by using ICP-OES analysis. As clearly seen in Table 1, the results further approve the catalysts stabilities against leaching to the solution.
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4. Conclusion In summary, an efficient process was introduced for designing new selective heterogeneous molybdenum catalysts for the olefin epoxidation with TBHP. SBA-16 was found to be a proper support for the immobilization of molybdenum amino acid Schiff base complexes. The results of physicochemical analyses approved the successful immobilization of molybdenum complexes within the SBA-16 nanocages. The catalysts showed highly activities and selectivities in the olefin epoxidation with TBHP. Also, the epoxidation reaction was entirely terminated by isolating the catalysts. The catalysts show heterogeneous character and could be readily isolated by filtration and reused five times with slight loss of their catalytic activities and selectivities. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfa.2019.03.047. References [1] D.R. Godhani, H.D. Nakum, D.K. Parmar, J.P. Mehta, N.C. Desai, Inorg. Chem.
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
SBA-16 supported amino acid Schiff base complexes of molybdenum as new heterogeneous molybdenum catalysts
T
Majid Masteri-Farahania, , Paria Eghbalib,c, Ertan Şahinc ⁎
a
Faculty of Chemistry, Kharazmi University, Tehran, Iran Department of Analytical Chemistry, Faculty of Pharmacy, Girne American University, 99428 Kyrenia, North Cyprus, Mersin 10, Turkey c Department of Chemistry, Faculty of Science, Atatürk University, Erzurum, Turkey b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Amino acid Epoxidation Molybdenum SBA-16 Schiff base
Immobilization of molybdenum complexes of amino acid Schiff bases within the SBA-16 nanocages produced new heterogeneous catalysts for the epoxidation of olefins. First, amino acid Schiff bases were obtained through the reaction of amino acids with salicylaldehyde. Then, complexation of the prepared amino acid Schiff bases with molybdenum (VI) produced the molybdenum complexes of amino acid Schiff bases. Immobilization of the molybdenum complexes into the SBA-16 nanocages followed by silylation with triethoxyoctylsilane gave the heterogenized molybdenum catalysts. The obtained catalysts were characterized with several physicochemical techniques. FT-IR and inductively coupled plasma optical emission (ICP-OES) spectroscopies approved the inclusion of molybdenum complexes within the SBA-16 nanocages. The results of X-ray diffraction (XRD) and nitrogen adsorption-desorption (BET method) analyses illustrated that surface properties of SBA-16 were maintained upon the inclusion of molybdenum complexes. The prepared catalysts exhibited good activities and excellent selectivities (> 99%) in the epoxidation of olefins with tert-butyl hydroperoxide (TBHP).
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Corresponding author. E-mail address: [email protected] (M. Masteri-Farahani).
https://doi.org/10.1016/j.colsurfa.2019.03.047 Received 10 January 2019; Received in revised form 7 March 2019; Accepted 16 March 2019 Available online 18 March 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Schematic illustration of the immobilization of molybdenum complexes of amino acid Schiff bases into the SBA-16 nanocages. Table 1 FT-IR and molybdenum content data of the prepared catalysts. Sample
υMoO2 (cm−1)
υC=N (cm−1)a
Mo content of fresh catalyst (mmol.g-1)
Mo content of 5th recovered catalyst (mmol.g-1)
Mo-salgly@SBA-16 Mo-salala@SBA-16 Mo-salphenala@SBA-16 Mo-salleu@SBA-16 Mo-salisoleu@SBA-16 Mo-salhis@SBA-16
912, 902, 905, 908, 911, 905,
1610 1613 1610 1610 1611 1608
0.07 0.05 0.04 0.06 0.04 0.05
0.06 0.05 0.03 0.04 0.03 0.05
a
938 937 939 935 942 945
(1636) (1634) (1634) (1635) (1637) (1635)
The numbers in parentheses show υC=N in free ligands.
recoverable catalysts [9–15]. Among the various solids utilized for immobilization of homogeneous catalysts, SBA-16 has been appeared as suitable support for the preparation of heterogenized catalysts due to its large and well-ordered mesopores and nanocages as well as high surface area [16–20]. Homogeneous and heterogenized homogeneous molybdenum (VI) catalysts have attracted great interests and there are several reports about their utilization in catalytic organic reactions [21–29]. On the other hand, transition metal complexes containing amino acid Schiff base ligands have been used extensively in catalytic reactions [30–34]. Nonetheless, the reports on the study of heterogenized catalysts containing transition metal complexes of amino acid Schiff bases is limited [35–37]. Specially, the immobilization of amino acid Schiff base complexes of molybdenum into the SBA-16 nanocages has not been reported. Here, amino acid Schiff base complexes of molybdenum were
1. Introduction Recently, increasing attention has been devoted to develop environmental friendly processes in all fields of chemistry. One of the concerns is the waste minimization by utilization of green reagents and efficient catalysts. In this regard, isolation of catalyst or products from the reaction mixture is one of the difficulties which has an important contribution to waste production in a commercial process. Thus, the utilization of easily recoverable catalysts has attracted great attention due to facile isolation and several efforts have been done for the preparation of such catalysts [1–8]. Heterogenized homogeneous catalysts play an important role in developing green processes especially in chemical industries. According to green chemistry concept, not only the high activity of the catalyst is important, but also the good recoverability is critical. Immobilization of homogeneous catalysts on solid inorganic materials is an appropriate method for developing such 348
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heptamolybdate (3.0 g) in H2O (10 ml), 4 ml of acetylacetone was added. A yellow solid started to precipitate after adjusting the pH of the mixture to 3.5 with addition of 10% HNO3. After 1.5 h, the obtained MoO2(acac)2 was separated by filtration, washed with H2O and ethanol, and dried in vacuum oven. Fourier transform infrared (FT-IR) spectra were recorded on PerkinElmer Spectrum RXI FT-IR spectrometer. Elemental analyses were carried out by a VARIAN VISTA-MPX ICP-AES spectrometer. X-ray diffraction (XRD) analyses were done on a SIEFERT XRD 3003 PTS diffractometer using Cu Kα radiation (λ = 0.154 nm). Thermogravimetric analyses were performed on a Perkin Elmer Diamond Thermogravimeter from room temperature to 700 °C using a rate of 10 °C/min in static air. Scanning electron microscopy (SEM) images were provided using ZEISS-DSM 960 A microscope with an attached camera. Transmission electron microscopy (TEM) analysis was carried out with a Philips EM 208S instrument using an accelerating voltage of 100 kV. Samples were prepared by placing droplets of a suspension of the sample in ethanol on a polymer microgrid supported on a Cu grid. Nitrogen adsorption-desorption analyses were done at liquid nitrogen temperature (−196 °C) using Quantachrome Nova 2200e, Version 7.11 Analyzer. Before the analysis the samples were outgassed under high vacuum at 150 °C. In order to evaluate the progress of the epoxidation reactions, samples were withdrawn in given times and analyzed using a gas chromatograph (HP, Agilent 6890 N) equipped with a capillary column (HP-5) and a FID detector. The molybdenum content of recycled catalysts was measured using atomic absorption spectroscopy after their digestion in diluted HF solution.
Fig. 1. XRD patterns of pristine SBA-16 and catalysts. Table 2 Textural parameters of samples determined from XRD analysis. Sample
2θ (º)
d110 (Å)
Lattice parametera (Å)
SBA-16 Mo-salgly@SBA-16 Mo-salala@SBA-16 Mo-salphenala@SBA-16 Mo-salleu@SBA-16 Mo-salisoleu@SBA-16 Mo-salhis@SBA-16
1.35 1.30 1.30 1.25 1.30 1.25 1.25
65.4 67.9 67.9 70.6 67.9 70.6 70.6
92.5 96.0 96.0 99.8 96.0 99.8 99.8
a
2.2. Synthesis of the molybdenum complexes of amino acids Schiff bases The amino acid (glycine, alanine, phenylalanine, leucine, isoleucine, and histidine) Schiff base ligands were synthesized through the common procedure introduced for the synthesis of Schiff base ligands [40]. Typically, 4 mmol of the amino acid was added to a solution of salicylaldehyde (4 mmol) in ethanol (20 ml). The mixture was stirred for 4 h to achieve amino acid Schiff base ligand. Afterwards, 4 mmol of MoO2(acac)2 in 20 ml ethanol was added and the solution refluxed for 8 h to give the corresponding molybdenum Schiff base complex. The obtained complex was separated by filtration and dried.
Determined by equation: a0 = √2 d110.
synthesized and immobilized into the SBA-16 nanocages. In order to avoid from the leaching of molybdenum complex, the entrance size of mesopores were adjusted through the silylation reaction. The prepared heterogeneous catalysts were utilized in the epoxidation of olefins with tert-butyl hydroperoxide (TBHP). The advantages of achieved catalytic systems are the facile isolation of the heterogenized catalysts at the end of reactions besides the good activities, excellent selectivities, and high stabilities.
2.3. Preparation of the catalysts First, the SBA-16 was dried at 120 °C in vacuum oven overnight to remove the adsorbed water molecules. Then, to a suspension of SBA-16 (1 g) in 20 ml methanol, 4 mmol of the molybdenum Schiff base complex in 10 ml methanol was added and the mixture refluxed for 8 h under N2 atmosphere. After evaporating the methanol solvent, the product was dried at 70 °C for 4 h. The obtained solid was reacted with triethoxyoctylsilane to adjust the pore size of the SBA-16 support through the previously reported method [41]. The solid was dispersed in a solution of triethoxyoctylsilane (4 mmol) in toluene (15 ml). Then, 5 ml of pyridine was added and refluxed for 10 h under N2 atmosphere. The obtained catalyst was isolated by filtration and soxhlet washed with ethanol to eliminate the remained reagents. Eventually, the product was dried at 70 °C in vacuum oven overnight to remove the remained solvent.
2. Experimental 2.1. Materials and instrumentation Ttriblock co-polymer Pluronic F127 and tert-butylhydroperoxide (TBHP, 80% in di-tertiary butyl peroxide) were obtained from SigmaAldrich Company. Cyclooctene (95%) was purchased from Across Company. The other chemicals were provided from Merck Company and used without further purification. The purity of the compounds were given in supplementary information. Mesoporous SBA-16 material was prepared as followed [38]. To a solution of Pluronic F127 (0.32 g) and cetyl trimethylammonium bromide (CTAB, 0.037 g) in 35 ml of 2 M HCl, 1 g of TEOS was added and the mixture stirred under reflux condition firstly at 40 °C for 6 h and then at 80 °C for 6 h. The obtained solid was separated by filtration and dried at 80 °C. Finally, the solid was calcined at 550 °C for 6 h to remove the organic species. MoO2(acac)2 (acac = acetylacetonate ligand) was prepared according to reported method [39]. To a solution of ammonium
2.4. Catalytic epoxidation of olefins 4 mmol of olefin and 50 mg of catalyst was added to a flask containing 5 ml chloroform and after addition of TBHP (80% in di-tertiary butyl peroxide, 14.4 mmol), the mixture was refluxed under N2 atmosphere. In order to monitor the reaction progress, samples were withdrawn from the reaction mixture in specific times and analyzed with a gas chromatograph. In order to study the catalyst reusability, it was 349
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Fig. 2. Nitrogen adsorption isotherms and BJH pore size distributions of prepared catalysts.
acids with salicylaldehyde in ethanol according to the common method described in earlier reports [39]. The obtained amino acid Schiff base ligands were treated with molybdenum precursor to achieve the desired molybdenum complexes. The immobilization process comprised of first loading the obtained molybdenum Schiff base complex within the SBA16 nanocages. The size of pore entrances should become sufficiently small to enclose the molybdenum complexes in the SBA-16 nanocages and inhibit their leaching during the catalytic epoxidation. For adjusting the pore entrances, silylation with appropriate reagent should be performed. Hence, the obtained SBA-16 containing molybdenum complex was reacted with triethoxyoctylsilane to modify the pore entrance size without blocking them [41]. The obtained product was soxhlet washed with ethanol to eliminate the unreacted reagents. It is notable that FT-IR and ICP-OES analyses (not given here) showed that if the silylation of the pore entrances of pristine SBA-16 is done in the first stage, any effort to immobilize the molybdenum complexes into the SBA-16 nanocages is failed. FT-IR spectroscopy was exploited to provide evidences for the successful immobilization of molybdenum complexes into the SBA-16 nanocages and the next silylation reaction. The prepared materials exhibited two peaks at about 1700 cm−1 and 1630 cm−1 due to stretching vibrations of amino acid carboxyl groups and imine bonds, respectively. Moreover, some peaks were observed at around 1050 and 800 cm−1 owing to the asymmetric and symmetric stretching vibrations of Si-O-Si in the SBA-16 support. Also, two bands were appeared at about 2860 and 2960 cm−1 due to the C–H stretching vibrations of octyl groups in silylating reagents and amino acids. The appearance of two adjacent
Table 3 The textural parameters of materials determined from N2 sorption analysis. Sample
Average pore diameter (Å)
Total pore volume (ml. g−1)
SBET (m2. g−1)
SBA-16 Mo-salgly@SBA-16 Mo-salala@SBA-16 Mo-salphenala@SBA-16 Mo-salleu@SBA-16 Mo-salisoleu@SBA-16 Mo-salhis@SBA-16
38 35 32 30 32 31 30
0.31 0.27 0.22 0.12 0.20 0.18 0.15
705 453 395 206 401 356 344
isolated at the end of epoxidation reaction and then washed thoroughly with methanol and dried at 70 °C in vacuum oven overnight. The recovered catalyst was reused in the cyclooctene epoxidation in the same reaction conditions mentioned above. 3. Results and discussion 3.1. Preparation and characterization of the catalysts The immobilization of molybdenum complexes of amino acid Schiff bases within the SBA-16 nanocages was done through the strategy outlined in Scheme 1. First, the Schiff base ligands were prepared by reaction of amino 350
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Fig. 3. TGA curves of the prepared catalysts.
peaks in the range of 890-950 cm−1 in the FT-IR spectra of the catalysts indicates the existence of cis-MoO2 fragment in their structure (Table 1) [42]. The observation of these peaks approves the successful immobilization of the molybdenum complex and silylation reaction. ICP-OES analysis was exploited to determine the molybdenum content of freshly prepared catalysts (Table 1) which further approved the encapsulation of molybdenum complexes into the SBA-16 nanocages. X-ray diffraction (XRD) analysis verified the structure retention of SBA-16 in the course of immobilization of the molybdenum complexes into the SBA-16 nanocages. In the XRD patterns of the samples, a single peak is seen at 2θ = 1.2−1.5°, assigned to the (110) plane of the cubic unit cell (Fig. 1). The relative decrease in the intensity of this peak in the XRD patterns of catalysts with respect the SBA-6 indicates the incorporation of molybdenum complexes into the SBA-16 nanocages. Furthermore, the shift of the peaks to lower angles revealed the expansion of unit cell due to the immobilization of molybdenum complexes within the SBA-16 nanocages (Table 2). Nitrogen adsorption-desorption analysis was conducted to study the surface area and isotherms of the obtained materials after immobilization of the molybdenum complexes into the SBA-16 nanocages. As seen in Fig. 2, the obtained materials show type IV isotherms with hysteresis loops specifying the materials with cage-like mesopores [43]. The observation of hysteresis loops in the obtained isotherms is a result of capillary condensation of N2 gas within the nanocages. Upon the immobilization of molybdenum complex within the nanocages, the inflection points of isotherms shifted towards the lower
relative pressures and the adsorbed volume of N2 reduced due to the pore size reduction. Furthermore, the surface areas (SBET), pore volumes, and average pore diameters were decreased upon immobilization of the molybdenum complex (Table 3). These observations provide further evidences for immobilization of the molybdenum complexes into the SBA-16 nanocages. The sequence of increasing the surface area of the catalysts is as below: Mo-salgly@SBA-16 > Mo-salleu@SBA-16 > Mo-salala@SBA16 > Mo-salisoleu@SBA-16 > Mo-salhis@SBA-16 > Mo-salphenala@ SBA-16. The observed order in surface area is compatible with the steric requirements of the amino acid groups in the Schiff base ligand. Increasing the amino acid steric hindrance results in the decrease in surface area of the corresponding catalyst. Thermogravimetric analysis (TGA) of the prepared catalysts was done and the results are exhibited in Fig. 3. The TGA curves showed weight loss of 4–7 % on heating over the range 25–210 °C which can be assigned to the depletion of adsorbed water molecules. The next weight loss was occurred in the range of 250–600 °C due to the combustion of the organic species. The amount of weight loss was found to be compatible with the carbon content of the Schiff base ligands. With increasing the carbon content of the amino acid Schiff base, the corresponding weight loss in TGA curve is increased. 3.2. Catalytic study of the obtained catalysts The obtained catalysts were utilized in the catalytic epoxidation of 351
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Table 4 Epoxidation of olefins with TBHP in the presence of prepared catalysts. Catalyst
Run
olefin
Time (h)
Conversiona(%)
Epoxide (%)
Mo:substrate molar ratio
TOFb (h-1)
Mo-salala @SBA-16
1 3 4 5 6 8 9 10 11 13 14 15 16 18 19 20 21 23 24 25 26 28 29 30 31 32
cyclooctene cyclohexene 1-hexene 1-octene cyclooctene cyclohexene 1-hexene 1-octene cyclooctene cyclohexene 1-hexene 1-octene cyclooctene cyclohexene 1-hexene 1-octene cyclooctene cyclohexene 1-hexene 1-octene cyclooctene cyclohexene 1-hexene 1-octene Cyclooctene Cyclooctene
8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 12 12
> 99 91 66 46 > 99 87 59 41 97 83 54 35 95 79 51 32 93 74 49 30 98 85 60 41 9 4
> > > > > > > > > > > > > > > > > > > > > > > > 41 51
1:1600
640
1:2000
890
1:1333
546
1:2000
760
1:1600
552
1:1142
451
– –
– –
Mo-salphenala @SBA-16 Mo-salleu @SBA-16 Mo-salisoleu @SBA-16 Mo-salhis @SBA-16 Mo-salgly @SBA-16 SBA-16 No catalyst
99 99 99 99 99 99 99 99 99 99 99 99 99 > 99 99 99 99 99 99 99 99 99 99 99 99
Reaction conditions: catalyst (100 mg), olefin (8 mmol), TBHP (14.4 mmol), refluxing CHCl3 (10 ml). a Determined using isooctane as internal standard. b Determined as mmol of product per mmol of molybdenum in catalyst per time (2 h). Table 5 Results of the epoxidation of cyclooctene with TBHP after isolation of the catalysts. Catalyst
Run
olefin
Time (h)
Conversiona(%)
Epoxide (%)
Mo-salala @SBA-16 Mo-salphenala @SBA-16 Mo-salleu @SBA-16 Mo-salisoleu @SBA-16 Mo-salhis @SBA-16 Mo-salgly @SBA-16
1
Cycloocteneb Cyclooctenec Cycloocteneb Cyclooctenec Cycloocteneb Cyclooctenec Cycloocteneb Cyclooctenec Cycloocteneb Cyclooctenec Cycloocteneb Cyclooctenec
2 12 2 12 2 12 2 12 2 12 2 12
80 82 89 92 82 86 76 79 69 70 79 81
> > > > > > > > > > > >
2 3 4 5 6
99 99 99 99 99 99 99 99 99 99 99 99
Reaction conditions: catalyst (100 mg), cyclooctene (8 mmol), TBHP (14.4 mmol), refluxing CHCl3 (10 ml). a Determined using isooctane as internal standard; b Catalytic activity instantly after isolation of catalyst (2 h); c Reaction progress after 10 h of catalyst isolation.
Fig. 4. Catalytic reusabilities of the catalysts in the cyclooctene epoxidation with TBHP. Reaction conditions: Recovered catalyst (100 mg), cyclooctene (8 mmol), TBHP (14.4 mmol), refluxing CHCl3 (10 ml).
some olefins with TBHP and the results were shown in Table 4. As can be seen in Table 4, the catalysts were active in the epoxidation of olefins with selectivities of 100% towards the epoxides. The results of Table 4 reveals that the TOF values increase as: Mosalphenala@SBA-16 > Mo-salisoleu@SBA-16 > Mo-salala@SBA16 > Mo-salhis@SBA-16 > Mo-salleu@SBA-16 > Mo-salgly@SBA-16. The observed sequence can be explained with taking into account the coordination of produced tert-butanol byproduct to the molybdenum active site of the catalyst which decreases the rate of epoxidation reaction. With increasing the electron density on the molybdenum site, the coordination of tert-butanol is retarded which results in the less deactivation of the catalyst. Furthermore, the more steric hindrance of the Schiff base ligand assists this effect. Thus, the catalyst possessing more electron donating phenyl alanine group shows
the highest catalytic activity. Another parameter influencing the conversion of olefins can be seen with inspecting the results of Table 4. The sequence of olefin reactivities towards the formation of epoxides is cyclooctene > cyclohexene > 1hexene > 1-octene. Our earlier works on the olefin epoxidations have indicated that two factors determine the olefin reactivities i.e. steric and electronic effects. It has been shown that the more electron density on the olefin double bond enhances the epoxidation reactivity [28,29,44]. Thus, olefins having inner C]C bonds such as cyclooctene and cyclohexene are more reactive with respect to the olefins with terminal C]C bonds such as 1-hexene and 1-octene. The lower reactivity of 1-octene with respect to 1-hexene can be explained by taking into account the steric hindrance of the larger group connected to C]C bond in the case of 1-octene in comparison with 1-hexene which make 352
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it difficult the approaching to the molybdenum center. Besides, further experiments were done without catalysts or with SBA-16 as catalyst which revealed very low conversion of cyclooctene after 12 h reaction. This observation obviously verifies the key role of the molybdenum center in the selective epoxidation of olefins. The catalysts stabilities were explored with further tests in which epoxidation of cyclooctene was permitted until 2 h and then the catalysts were separated by filtration. The filtrates were further reacted under the same conditions for further 10 h. In each case, the cyclooctene conversions were determined after 2 h and 12 h (Table 5). The results revealed that after separating the catalysts, the increase in the conversion of olefins is slightly increased. This observation clearly indicates that there is no leached molybdenum species in the solution and the catalysts are stable in the reaction conditions. Thus, the catalytic epoxidations is done heterogeneously in the presence of obtained catalysts. The catalysts reusabilities were also studied by recovering the catalysts and reusing in the cyclooctene epoxidation. As shown in Fig. 4, slight decreases in conversions were seen verifying the stability of the molybdenum species within the SBA-16 nanocages. The molybdenum content of the recovered catalysts after 5th catalytic reactions was measured by using ICP-OES analysis. As clearly seen in Table 1, the results further approve the catalysts stabilities against leaching to the solution.
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4. Conclusion In summary, an efficient process was introduced for designing new selective heterogeneous molybdenum catalysts for the olefin epoxidation with TBHP. SBA-16 was found to be a proper support for the immobilization of molybdenum amino acid Schiff base complexes. The results of physicochemical analyses approved the successful immobilization of molybdenum complexes within the SBA-16 nanocages. The catalysts showed highly activities and selectivities in the olefin epoxidation with TBHP. Also, the epoxidation reaction was entirely terminated by isolating the catalysts. The catalysts show heterogeneous character and could be readily isolated by filtration and reused five times with slight loss of their catalytic activities and selectivities. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfa.2019.03.047. References [1] D.R. Godhani, H.D. Nakum, D.K. Parmar, J.P. Mehta, N.C. Desai, Inorg. Chem.
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