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Iron-Modified Mesoporous Silica as Efficient Solid Lewis Acid Catalyst for the Mukaiyama Aldol Reaction Wan Xu, Thierry Ollevier, and Freddy Kleitz ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03485 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018
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ACS Catalysis
Iron-Modified Mesoporous Silica as Efficient Solid Lewis Acid Catalyst for the Mukaiyama Aldol Reaction Wan Xua, Thierry Ollevier*a, Freddy Kleitz*a, b a.
Département de Chimie, Université Laval, 1045 Avenue de la Médecine, Québec, QC G1V 0A6, Canada
b.
Department of Inorganic Chemistry Functional Materials, Faculty of Chemistry, University of Vienna, Währinger Straße 42, 1090 Vienna, Austria
ABSTRACT: Fe-MCM-41 and Fe-SBA-15, two different iron-containing mesoporous silicas were successfully synthesized by a straightforward and versatile method using iron acetylacetonate as a metal precursor. pH adjustment with ammonia during the synthesis was found to be an efficient way to improve the iron content. Physicochemical parameters of the ironcontaining mesoporous silicas were obtained by nitrogen physisorption measurements, and the coordination environment of iron elements was validated by UV vis diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy. The surface acidity was tested by using a series of Hammett indicators. To further distinguish the Lewis acid sites on the surface, pyridine adsorption FT-IR method was implemented. These prepared nanoporous catalysts were screened in the Mukaiyama aldol reaction as a model reaction catalyzed by a Lewis acid. The Lewis acid catalytic activity of the materials was finetuned, and the corresponding aldol products were obtained in good yield and selectivity. More importantly, the solid catalysts were very stable and could be reused at least nine times while maintaining the same catalytic activity.
KEYWORDS. Mesoporous silica, grafting, iron acetylacetonate, water-tolerant Lewis acid, Mukaiyama aldol reaction
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INTRODUCTION
Homogeneous Lewis acid catalysts, such as AlCl3 and TiCl4, are applied in the production of petroleum-derived chemicals.1,2 However, the use of these Lewis acids has some disadvantages. For example, strictly anhydrous conditions are necessary for homogeneous Lewis acids because of their easy decomposition or complete deactivation in water. Furthermore, higher amounts of catalysts are usually required for reactions catalyzed by conventional Lewis acid catalysts, which can generate large aqueous effluents during the post-synthesis workup process3 and difficulty in the recycling of the catalysts for repeated use. Therefore, it has become a target of great interest to design efficient heterogeneous catalysts with Lewis acid sites for solving these issues. The most prominent and early developed examples of heterogeneous catalysts comprising Lewis acid sites are microporous zeolites.4-6 In organic chemistry, particularly, Kumar found that TS-1 and Ti-E zeolites could be used as active solid catalysts in the Mukaiyama aldol reaction in the absence of water.7,8 However, low yields led to a speculation that the too small microporous environment in zeolites prevents the contact of the reactants with the Lewis acid sites, decreasing the ability for the chemical transformation. Thus, silica-based materials with larger pore sizes, such as ordered mesoporous MCM-41 and SBA-15 silicas,9,10 have been highlighted as promising alternatives. Besides,
their high surface area (800-1000 m2/g), uniform and tunable pore diameter (usually comprised between 2 and 15 nm), ease of surface-functionalization, and well-defined particle morphology make them potential heterogeneous catalysts for a larger variety of organic reactions.11-19 Since the discovery of the Mukaiyama aldol reaction 40 years ago, different types of catalysts including both homogeneous and heterogeneous catalysts have been studied.2026 In organic synthesis, the Mukaiyama aldol reaction is an essential way –‘ …Š‡•‘•‡Ž‡…–‹˜‡Ž› •›•–Š‡•‹œ‡ Ehydroxycarbonyl compounds.20,27,28 A few Mukaiyama aldol-type reactions in the presence of iron compounds have been described since the reaction is generally catalyzed by a Lewis acid.21,22,29,30 Also, using some metal triflates, such as Sc(OTf)3, as catalysts for Mukaiyama aldol reaction of aldehydes and silyl enol ethers, Kobayashi and coworkers showed their good catalytic reactivity and selectivity compared to metal halides.31 On the other hand, heterogeneous catalysts, such as Ti-containing mesoporous materials, also showed satisfactory yields as high as 98% in solvent-free systems, which was much higher than that of Ti-microporous zeolites.32 Furthermore, according to a study on mesoporous Lewis acids obtained by immobilizing
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Sc(OTf)3 on mesoporous silica functionalized with sodium-benzenesulfonate,33,34 for the Mukaiyama aldol reactions, the thus-prepared materials displayed a much higher reactivity than that of homogeneous catalysts, e.g., Sc(OTf)3. In comparison to other metals, iron-based catalysts are cheap, low in toxicity, stable, and easily accessible. Furthermore, the Lewis acidity of Fe(III) is another significant benefit for the use of this metal in catalysis. As an illustration on the applicability of iron in catalysis, a series of Fecontaining porous materials used as catalysts in different organic reactions have been disclosed. Dhakshinamoorthy and co-workers synthesized Fe-containing porous metalorganic-frameworks (MOFs), which were used as solid ‡™‹• ƒ…‹† …ƒ–ƒŽ›•–• ˆ‘” –Š‡ ‹•‘•‡”‹œƒ–‹‘• ‘ˆ D-pinene oxide into camphonelal and isopinocamphone in the absence of solvent.35 According to other studies, Fe-containing mesoporous silica shows promises as an efficient catalyst for several other organic reactions. For example, studies have demonstrated that Fe-containing mesoporous materials are very active catalysts for Friedel-Crafts alkylations,31 which are usually catalyzed by Lewis acids. Besides being less sensitive to water (water-tolerant Lewis acids), they demonstrate better recycling performance in contrast to usual Lewis acids.36-38 However, surprisingly, Fe-containing mesoporous silica has not yet been used as a Lewis acid catalyst for Mukaiyama aldol reactions. In the present study, in order to develop an efficient Lewis acid Fe-containing mesoporous material for the Mukaiyama aldol reaction, a series of Fe-MCM-41 (pore size 3-4 nm) and Fe-SBA-15 (pore size 7-8 nm) with various iron contents were prepared by a specific post-grafting methodology, inspired by our previous work on Ti-SBA-15 oxidation catalysts.11,39 MCM-41 and SBA-15 were chosen as two different silica supports, to probe pore size effects, and iron(III) acetylacetonate was used as the metal precursor in this procedure.39 The surface iron content and bulk iron content were analyzed using X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma mass spectrometry (ICP-MS). Attenuated total reflectance infrared (ATR-IR), powder X-ray diffraction (XRD), diffuse reflectance UV visible spectroscopy (DR-UV-vis), and N2 physisorption measurements were also applied for the characterization of the final materials. The Lewis acidity of the prepared materials was studied by the Hammett indicator method to substantiate the nature of the active sites in the materials. Pyridine sorption probed by FT-IR analysis was performed to assess the Lewis acid sites. The above prepared Lewis acid heterogeneous catalysts were then tested in the Mukaiyama aldol reaction of benzaldehyde with 1trimethylsiloxycyclohexene or 1-phenyl-1-(trimethylsiloxy)-propene at ambient temperature in an aqueous environment, obtaining high yield and selectivity, and preventing the oxidation of benzaldehyde into benzoic acid.
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EXPERIMENTAL SECTION
MCM-41 silica support. Mesoporous silica MCM-41 was synthesized using tetraethylorthosilicate (TEOS, 98%, Aldrich) as the silica source and cetyltrimethylammonium bromide (CH3(CH2)15N(Br)-(CH3)3, CTAB, Sigma-Aldrich) as a structure-directing agent under basic aqueous conditions. A typical synthesis is as follows: 9.65 g of CTAB was first dissolved in 480 g of distilled water with vigorous stirring at 35 °C to form a homogeneous solution. 36.5 mL of NH4OH (28%, Anachemia) was subsequently added into the solution, and the temperature of the mixture was then reduced to 25 °C. After 15 min, 40 g of TEOS was added, and the mixture was left stirring for 2 h. The mixture was subsequently aged at 90 °C under static conditions and the aging time was 3 days. The obtained white precipitate was filtered off and washed with distilled water. After being dried at 100 °C for 24 h to remove the surfactant, mesoporous silica MCM-41 was obtained after calcination at 550 °C for 5 h.40, 41 SBA-15 silica support. The synthesis procedure of SBA15 in this study was reported by Choi et al.42 In a typical synthesis process, the amphiphilic block copolymer Pluronic P123 was adopted as the structure-directing agent and TEOS as the silica source. First, a homogeneous solution was obtained after dissolving 13.9 g of Pluronic P123 (Aldrich, Mw = 5800 g/mol) in a mixed solution of HCl (7.7 g, 37%, Anachemia) and distilled water (252 g) at 35 °C for 2 h. After addition of TEOS into the solution, the mixture was kept stirring at the same temperature for 24 h, followed by aging the mixture at 100 °C for 24 h. The precipitated white solid was held in a mixture containing concentrated HCl and ethanol (100%, Fisher Scientific) for 30 min and then recovered by filtration. The resulting mesoporous silica SBA-15 was obtained by heating the product at 550 °C for 5 h. Preparation of Fe-MCM-41 and Fe-SBA-15 catalysts. Impregnation of iron species into MCM-41 and SBA-15 with different molar ratios was performed by a simple postgrafting method reported earlier by our group39 with some modifications. The method used Fe(acac)3 as a metal precursor and calcined mesoporous silica MCM-41 or SBA-15 as silica supports. The procedure was as follows: the given amount of Fe(acac)3 and 100 mL of 1-propanol (certified ACS grade, Fisher Scientific) were mixed at 45 °C under vigorous stirring until a homogeneous solution was formed. 1 g of calcined mesoporous silica MCM-41 or SBA-15 was then added into the grafting solution and finely dispersed by stirring. The mixture was continuously stirred at the same temperature for 2 h. In some cases, after the addition of the silica, concentrated ammonia (28%) was used to modify the pH of the mixture to 10. The yellow colored product was then filtered, washed with 1-propanol, and dried at 100 °C in the air for 24 h. The resulting material was calcined at 550 °C for 3 h. The resulting Fe3+-deposited mesoporous silica samples with different iron contents are
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ACS Catalysis
designated as Fe-MCM-41 (X%), Fe-SBA-15 (X%), Fe-MCM41 (X%, pH = 10), and Fe-SBA-15 (X%, pH = 10) (X = initial iron molar % of the synthesis solution). Passivation of mesoporous silica and iron-modified mesoporous silica. A conventional silylating agent hexamethyldisilazane (HMDS, 99.9%, Aldrich) was used for the passivation of the surface silanol groups. Iron-modified mesoporous silica was firstly degassed at 150 °C in a vacuum oven overnight, and 0.5 g of the material was dispersed in 25 mL of dry hexanes, 1 mL of HMDS was successively added into the mixture under vigorous stirring. The mixture was stirred for 24 h at room temperature. The excess HMDS was removed by Soxhlet extraction refluxing for at least 6 runs at 60 °C using dichloromethane (100 mL, certified ACS grade, Fisher Scientific) as the solvent. The obtained passivated materials were then dried in a 60 °C oven for 12 h. The obtained materials were named as FeMCM-41-HDMS and Fe-SBA-15-HDMS.43
Catalyst characterization and testing Low-angle powder X-ray diffraction (XRD) patterns were obtained using a Rigaku Multiplex instrument with — D ”ƒ†‹ƒ–‹‘• (30 kV, 40 mA). Wide-angle X-ray diffraction (XRD) patterns of all samples were collected a Sie•‡•• ‘†‡Ž a\\\ †‹ˆˆ”ƒ…–‘•‡–‡” ™‹–Š — D ”ƒ†‹ƒ–‹‘• P = 0.15496 nm). The wide-angle XRD pattern of Fe2O3 as reference was acquired from the Powder Diffraction File 2 (PDF-2) database licensed by the International Center for Diffraction Data (ICDD). The nitrogen adsorption-desorption isotherms were measured at -196 °C with a Quantachrome Autosorb-1 sorption analyser (Quantachrome Instruments, USA). Prior to the measurement, non-passivated samples were outgassed under vacuum at 200 °C and passivated materials at 80 °C for at least 12 h. The specific surface area was calculated through Brunauer Emmett Teller (BET) equation using the obtained adsorption data at P/P0 values between 0.05 and 0.2. The amount of nitrogen adsorbed at P/P0 = 0.95 was used to estimate the total volume of the mesopores. Non-local density functional theory (NLDFT) method was applied to determine the pore size distributions. The selected NLDFT kernel considers sorption of N2 on silica at -196 °C, assuming the model of equilibrium isotherm based on the desorption branch and cylindrical pore geometry.44 All the data was extracted by using the Autosorb-1 1.55 software. Attenuated total reflectance infrared (ATR-IR) spectra were measured with a Nicolet Magna 850 Fourier transform spectrometer with a liquid nitrogen cooled narrow band mercury cadmium telluride (MCT) detector. Each spectrum was gathered from the acquisition of 128 scans at 4 cm 1 resolution varied from 4000 to 700 cm 1 using a Happ Genzel apodization. All samples were dried in vacuum oven at 80 °C overnight before the ATR-IR measurements. Diffuse-reflectance UV vis spectra were recorded
in the range of 200 nm-800 nm using a Varian Cary 500 spectrophotometer equipped with a Praying Mantis accessory. X-ray photoelectron spectroscopy (XPS) spectra were collected on a Kratos Axis Ultra electron spectrometer (UK), —•‹•‰ ƒ •‘•‘…Š”‘•ƒ–‹… Ž D -ray source (Al K = 1486.6 eV) at a power of 300 W and operated at a base pressure of 5×10 10 Torr. Charge compensation was performed using a low-energy electron beam perpendicular to the surface of the samples. Survey spectra used for determining the elemental composition were collected at a pass energy of 160 eV. The bulk iron contents of Fe-SBA-15 and FeMCM-41 were achieved by ICP measurement using a Varian 800 MS spectrophotometer. Scanning electron microscopy (SEM) images and Energy-dispersive X-ray spectroscopy (EDX) spectra were performed using a JEOL JSM840A instrument. Before the analysis, a small amount of the sample was dispersed on the aluminum sample holder, both side of which was coated with gold and palladium. High-resolution transmission electron microscopy (HRTEM) and EDX analysis were recorded on a Cs-corrected Titan G2 ETEM, which was operated at an accelerating voltage of 300 kV. Energy dispersive X-Ray mapping images (EDX) were obtained from a square area 15 × 15 nm of TEM images and the acquisition time was 1 s. For the analysis, the samples were crushed in the analytical reagent grade acetone solvent, and a few drops of the resulting suspension were placed and dried on a carbon film supported on 300 mesh grids. For the catalytic tests, the products were analyzed using 1H and 13C nuclear resonance spectroscopy (NMR; Varian Inova NMR AS400 spectrometer 300 MHz, 400 MHz, 500 MHz). Chloroform-d was used as an internal standard for 1 H Ë cä^b ’’• ƒ•† 13C NMR H Ë ccä^_ ’’• . The syn/anti aldol ratio was determined by the 1H NMR analysis of the crude product. High-resolution mass spectra (HRMS) were recorded on an Agilent 6210 ESI TOF (time-of-flight) mass spectrometer. Titration of the Lewis acid solids with Hammett indicators. Titration with different Hammett indicators was performed to determine the surface acidity of the prepared catalysts, according to protocols.45 In this method, about 0.1 g of dried solid was transferred into a glass vial, after the addition of benzene, three or five drops of a 0.1% freshly made solution of the indicators in benzene was added. According to the final color of the mixture, it could be readily determined if a catalyst was in its basic or acidic form to all indicators or had an H0 value lying between two adjacent indicators. Before the color tests, all the samples were dried and stored in a glass vial in a desiccator to avoid water adsorption, which was able to cause a shift to a lower acid strength and decrease the color intensity of the adsorbed indicators. In addition, the titration solutions were suspended in a quartz cuvette and were analyzed using Varian Cary 500 spectrophotometer equipped with a Praying Mantis accessory. The benzene solvent was used as a blank
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reference. All the catalysts here were tested with three different indicators. These Hammett indicators were chosen based on their pKa value, which is presented in Table 1 as well as the colors of the acidic and basic forms of the indicators. Table 1 Indicators used for acid strength measurements Indicators
Basic color
Acidic color
pKa
Dimethyl yellow
Yellow
Red
+3.3
Phenylazodiphenylphosphine
Yellow
Purple
+1.5
Dicinnamalacetone
Yellow
Red
-3.0
Pyridine adsorption FT-IR experiments. Prior to the tests, the iron-containing mesoporous silica was freshly degassed at 150 °C for 24 h to remove the physisorbed water. 50 mg of degassed Fe-MCM-41 and Fe-SBA-15 were sus-
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pended in 3 mL of 5% pyridine solution in dry hexanes. After stirring for 1 h, the solvent of the mixture was evaporated under reduced pressure, and the obtained solids were dried at 100 °C for 1 h afterward and finally exposed at room temperature for 16 h. The final gathered powders were analyzed using ATR-IR spectroscopy.
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RESULTS AND DISCUSSION
Synthesis and characterization of the materials. Both Fe-MCM-41 and Fe-SBA-15 materials were synthesized by a post grafting method using Fe(acac)3 as the iron source. There are several advantages of using Fe(acac)3 for the grafting procedure. In contrast to metal nitrides, chlorides or other metal precursors, which can easily generate an aggregation of the metal oxide forming large clusters, the relatively higher stability of acetylacetonate precursors can limit their hydrolysis and condensation.39,46 Furthermore, acetylacetonate complexes are less moisture-sensitive.47,48 There are two documented mechanisms of the interaction of acac compounds with the silica support surface according to
Figure 1 N2 adsorption desorption isotherms measured at 77 K (-196 °C) for Fe-MCM-41 (A) and Fe-SBA-15 (C), with various iron contents and the corresponding NLDFT pore size distributions for Fe-MCM-41 (B) calculated from the adsorption branch of the isotherm, and Fe-SBA-15 (D) calculated from the desorption branch of the isotherm.
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previous report.39 First, the Fe(acac)3 compound could be connected to the silica surface through hydrogen bonding. Second, the ligand exchange between the acac and silanol groups forming a covalent Fe O Si bond could also exist. Therefore, a high iron dispersion on the silica surface can be obtained using such Fe(acac)3 precursors. These previous results showed that this method was highly reproducible and could be fine-tuned by just changing the molar ratio of metal-to-silicon of the grafting gel or with the addition of ammonia during the synthesis procedure. In present study, the initial Fe/Si molar ratios of the grafting gel were 1%, 3%, 5%, 10% and 20%. Another batch of samples was synthesized using Fe(acac)3 as the precursor with the same iron concentrations, except for the addition of ammonia during the grafting process. The N2 adsorption-desorption isotherms and the respective pore size distributions of the prepared Fe-MCM-41 and Fe-SBA-15 samples are shown in Figure 1, and structural properties of all the prepared materials are compiled in Table 1. All the samples exhibited similar type IV isotherms which are the typical feature of a well-defined mesoporous structure.10 Compared to Fe-MCM-41, isotherms of FeSBA-15 showed a well-resolved H1 hysteresis loop with a steep increase in the adsorption volume within a relative pressure (P/P0) between 0.6 to 0.8, indicating the highly ordered mesostructure of the catalysts with large uniform cylindrical channels.49 Both of the pure mesoporous silica supports, MCM-41 and SBA-15, show high BET surface area (1168 and 1067 m2/g) and pore volume (1.0 and 1.4 cm3/g), respectively, which slightly decrease in the cases of FeMCM-41 (5%, 10% and 20%) and Fe-SBA-15 (5%, 10% and 20%), after the insertion of iron into the mesopores of the supports. Meanwhile, when the catalysts were synthesized through the grafting procedure performed at pH = 10, using the same iron concentration in the initial grafting gel, a significant reduction of the surface area, pore volume, and pore size was observed. Besides, the onset of the capillary
condensation of the isotherms for Fe-MCM-41 and Fe-SBA15 with ammonia treatment shifted towards lower relative pressures (0.2 0.3 and 0.6 0.7, respectively) demonstrating smaller average pore size of these samples compared to native mesoporous siliceous materials. A drastic decrease of pore volume was also observed in these samples. Such a phenomenon could be attributed to higher loading iron species both on the external surface and inside the channels of mesoporous silica. In order to investigate the location of the iron species grafted in the silica, the surface Fe/Si molar ratio and bulk Fe/Si molar ratio were measured by XPS and ICP-MS, respectively, and are summarized in Table 2. One can observe that all the Fe/Si molar ratio in the obtained materials are lower than the initial iron concentration, suggesting only a fraction of the iron precursors could be inserted into the silica matrix. It was assumed that the Fe/Si molar ratio measured by XPS should be close to the bulk Fe/Si molar ratio measured by ICP-MS, showing a uniform grafting of the iron precursor on the surface of silica. As shown in Table 2, only materials with lower Fe concentration in the initial gel showed a well-dispersed iron loading on the silica. Samples obtained with pH adjustment met a strong increase in Fe/Si molar ratio values. This increase in iron content on the silica might be attributed to nano-sized iron oxide particles on the external surface of silica when the iron loading was too high.39 It was postulated previously that the grafting of the iron acetylacetonate could depend on its coordination stability.39 For Fe(acac)3, the interaction between the precursor complex and the silanol groups is believed to take place through single-layer ligand exchange that will be limited by the number of the silanol groups available on the silica surface. Therefore, although 20 mol% of the precursor solution was used, both surface and bulk Fe/Si molar ratio of the resulting Fe-MCM-41 and Fe-SBA-15 seemed to reach a plateau. This plateau may be probably attributed
Table 2 Chemical composition and the structural properties of all the prepared materials SBETa
dpb,c
Vpd
(m2/g)
(nm)
(cm3/g)
MCM-41
1168
3.8
Fe-MCM-41 (1%)
970
Entry
Catalyst
1 2
Fe/Sie (mol%)
Fe/Sif (mol%)
1.0
0
0
3.9
0.8
1
0.4
3
Fe-MCM-41 (3%)
991
4.0
0.8
1
0.7
4
Fe-MCM-41 (5%)
1088
3.8
0.9
1
0.6
5
Fe-MCM-41 (5%, pH = 10)
683
3.5
0.5
13
4
6
Fe-MCM-41 (10%)
1071
3.8
0.9
2
0.7
7
Fe-MCM-41 (10%, pH = 10)
834
3.5
0.6
13
6
8
Fe-MCM-41 (20%)
1088
3.8
0.9
2.4
0.7
9
Fe-MCM-41 (20%, pH = 10)
758
3.5
0.5
15
10
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10
SBA-15
1067
8.5
1.4
0
0
11
Fe-SBA-15 (1%)
879
8.2
1.2
1
0.4
12
Fe-SBA-15 (3%)
836
8.2
1.1
1
0.8
13
Fe-SBA-15 (5%)
960
8.5
1.3
0.9
0.5
14
Fe-SBA-15 (5%, pH = 10)
466
8.2
0.9
7
3
15
Fe-SBA-15 (10%)
868
8.5
1.2
1.7
0.7
16
Fe-SBA-15 (10%, pH = 10)
521
8.2
0.9
11
7
17
Fe-SBA-15 (20%)
742
8.5
1.0
1.7
0.9
18
Fe-SBA-15 (20%, pH = 10)
563
8.2
0.9
15
10
a
SBET, specific surface areas calculated from the values of the relative pressure ranging from 0.05 to 0.20; Pore diameter calculated using NLDFT method for the materials with MCM-41 as a silica support (NLDFT adsorption conditions, i.e., adsorption branch); c Pore diameter calculated using NLDFT method for the materials with SBA-15 as a silica support (NLDFT equilibrium conditions, i.e., desorption branch); d Vp, adsorbed volume obtained at P/P = 0.95; 0 e measured by X-ray photoelectron spectroscopy (XPS); f measured by ICP-MS.
b
to the decreasing amount of available silanol groups on the silica surface as most of which have already reacted with the iron precursor. As it was previously observed in the case of acac-substituted titanium alkoxide precursor,46 the interactions between similar titanium precursors and the negatively charged silica surface Si O under basic conditions could be stronger than that of Si OH and SiOH2+ groups on the silica surface at lower pH.46 pH adjustment was confirmed to be an efficient way to improve metal loading. This conclusion was further proved here with Fe. Indeed, when the pH of the initial precursor solution gel was adjusted to 10 by adding concentrated ammonia, the iron loading substantially increased to 15%, compared to only 2.4% for the sample with no pH adjustment. Under basic conditions, decomplexation of Fe(acac)3 could occur, and the surface of silica becomes negatively charged. There are three possible iron cationic species produced from the hydrolysis of Fe(acac)3, [Fe(acac)2(H2O)2]+, [Fe(acac)(H2O)4]2+ and [Fe(H2O)6]3+,50 which were then available for the ligand exchange with the SiO on the silica surface. Also, the products of hydrolyzed Fe(acac)3 showed a low coordination stability, which probably leads to a multi-layer grafting, forming small iron oxide particles.39 Therefore, it could be expected that the pH adjustment method can increase the iron loading. Low-angle XRD measurements of Fe-MCM-41 and FeSBA-15 are shown in Figure S1 (SI. The diffractograms of FeMCM-41 and Fe-SBA-15 are consistent with that of their corresponding bare mesoporous silicas.51, 9 Both diffractograms showed three well-resolved peaks associated with (1 0 0), (1 1 0) and (2 0 0) reflections, which originated from the highly ordered hexagonal arranged (p6mm) mesopores. This observation indicated that the mesostructure of MCM-41 and SBA-15 was preserved after the insertion of the Fe species. However, the diffraction peaks shifted to
higher angle region, suggesting that the d(100) spacing and the hexagonal unit cell parameters a0, calculated by 2d(100) Ø_á †‡…”‡ƒ•‡† ™‹–Š –Š‡ ‹•…”‡ƒ•‡† ‹”‘• Ž‘ƒ†‹•‰ä Š‡ explanation suggested by Bouazizi52 was that a slight framework compaction could occur because of the electrostatic attraction between Fe and lattice O atoms. Besides, the diffraction peaks of Fe-MCM-41 (10%, pH = 10) laying in the range of 3.5-5 broadened, indicating a slight decrease of the structural regularity of the sample. This structural deformation can be ascribed to the fact that small iron oxide clusters or nanoparticles started to form in the mesopores or on the external surface of those catalysts upon pH adjustment.53 The mesoporous materials were also characterized by wide-angle XRD measurements, and the results are shown in Figure S2 (SI). No clear diffraction peaks were observed in the wide-angle XRD patterns for the samples synthesized neither with nor without pH adjustment. This phenomenon indicates the absence of iron oxide species with large crystal domain size. The presence of smaller crystals of size below the XRD detection limit (< 4-6 nm) however cannot be excluded.53 The absence of diffraction peaks may also be attributed to the amorphous nature of metal layers coated on the walls of the pores, even after calcination at high temperature. The high dispersion of iron species on the mesoporous surface could also be demonstrated by other methods. SEM images of Fe-MCM41 and Fe-SBA-15 (SI, Figure S3) showed no large crystal (FeO)n particles on the external surfaces of the materials, which is in agreement with the XRD results. Fe-MCM-41 (B and C) shown in Figure S3 exhibited spherical-like particles ƒ”‘—•† ] Q• in size as pure MCM-41 material (A) while FeSBA-15 (E and F) showed typical thread-like morphology as SBA-15 (D). The elemental composition of Fe-MCM-41 and Fe-SBA-15 was obtained by EDX analysis. No Fe signal
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solvents for organic reactions have been attracting increasing interests because of environmental concerns. Table 5 presents the catalytic behavior of Fe-MCM-41 (50 mg, Fe/Si = 20%, pH = 10) for the Mukaiyama aldol reaction of benzaldehyde (0.2 mmol) with 1-phenyl-1-(trimethylsiloxy)propene (0.4 mmol) conducted in various typical organic solvents.
Table 5 Effect of the solvent for Mukaiyama aldol reaction
Solvent
Yield (%)c
syn/anti
1a
H2O+SDS
86
90:10
2b
H2O+ethanol
90
79:21
3
THF
4
MeCN
5
CH2Cl2
6
DMC
Entry
18d
45:55
Reaction condition: benzaldehyde, 0.2 mmol; 1-phenyl-1-(trimethylsiloxy)-propene, 0.4 mmol; a water, 0.5 mL, SDS (11.5 mg); room temperature; time, 24 h. b Water 0.25 mL, ethanol 0.25 mL; room temperature; time, 24 h. c Yield of aldol product after purification by silica gel column chromatography; d conversion of benzaldehyde determined by 1H NMR. SDS, sodium dodecyl sulfate; DMC, dimethyl carbonate.
The catalyst had no or weak catalytic activity when the reactions were carried out in polar (MeCN and THF) and non-polar (DMC and CH2Cl2) solvents, while the hydrophobic reactants can be easily dissolved in these solvents.70 On the contrary, the yield of the reaction carried out in water-SDS and water-ethanol systems reached up to 86% and 90%, respectively, suggesting they were both suitable media for the Mukaiyama aldol reaction, which was also evidenced by other researchers with different catalysts.71,72 The surfactant SDS was essential because the Mukaiyama aldol reaction involves hydrophobic molecules requiring water, and thus the yield of the product could be enhanced by the presence of SDS.70 Shintaku et al. reported that Ti-SBA-15 showed higher catalytic activity for the Mukaiyama aldol reaction conducted in water-THF solutions, while no catalytic activity in pure THF solvent was observed.70 As reported by Hatanaka et al., the transition state formed between the aldehyde activated by the Lewis acid and the silyl enol ether could be stabilized by water molecules, which could also contribute to the dissociation of trimethylsilyl groups, therefore, the reaction rate can be increased.73
Effect of the surfactant (SDS). Table S2 (SI) shows the effect of the surfactant for the Mukaiyama aldol reaction. It should be noted that both a water-SDS system and the water-ethanol system have previously been reported as effective media for Mukaiyama aldol reactions.23,74 Both SDS and ethanol were adopted as a surfactant or a dispersing agent, to enhance the solubility of the hydrophobic reactants in aqueous media. Fe-MCM-41 (10%, pH = 10) and FeSBA-15 (10%, pH = 10) were used as the catalysts under different conditions to compare the water-SDS and waterethanol system. As one can observe there were no noticeable differences in the final yields, therefore, both were suitable media for the Mukaiyama aldol reaction. However, a larger amount of the surfactant SDS, which was not environmentally friendly, was always required.74 Considering water-ethanol as a clean, environmentally benign medium and the surfactant-free aqueous system compared with the water-SDS system, all further experiments in this study were carried out at room temperature in water-ethanol. Effect of the iron content (Fe-MCM-41, Fe-SBA-15) on the reactivity. MCM-41 and SBA-15 with two different pore sizes were chosen to probe the effects of the pore size (MCM-41 and SBA-15) and the effects of iron content (Fe/Si = 1~20 mol%) of the catalysts for the Mukaiyama aldol reaction of both 1-phenyl-1-(trimethylsiloxy)-propene and 1-(trimethylsilyloxy)-cyclohexene with benzaldehyde were examined by using catalysts with various iron contents. As shown in Table 6, the lowest yields of 3-hydroxy-2-methyl1,3-diphenylpropan-1-one were obtained when using catalyst Fe-MCM-41 (1%, 3%) and Fe-SBA-15 (1%, 3%) with low iron content. It suggested that only a minimal amount of iron species was inserted into the silica supports when the iron concentration of the grafting gel was too low. These low yields were also in agreement with the weak acidity of these two catalysts, as demonstrated above in the surface acidity tests. When using catalysts with higher iron loading Fe-MCM-41 (5%) and Fe-SBA-15 (5%) exhibiting stronger acidity, the yield substantially increased to 85% and 90%. This increase in yields was due to the higher iron loading of the catalysts, indicating an increasing number of active Lewis acid sites in the catalysts. However, it seems that the catalytic activity reached a plateau when the Fe/Si molar ratio was 5%. The yield fluctuated around 90% when FeMCM-41 (10%, 20%) and Fe-SBA-15 (10%, 20%) were used as catalysts. Ammonia treatment was very efficient for increasing the iron loading, yet it could not improve the catalytic activity of the materials. Catalysts with ammonia treatment apparently possess higher iron loading but barely promote the yields. Similar results have been found in the Mukaiyama aldol reaction of 1-(trimethylsilyloxy)cyclohexene and benzaldehyde (Table 7). All the catalysts except Fe-MCM-41 (1%, 3%) and Fe-SBA-15 (1%, 3%) possess high catalytic activities, with a yield of product around 87%.
Table 6 Effect of the iron content in the Mukaiyama aldol reaction of 1-phenyl-1-(trimethylsiloxy)-propene with benzaldehyde
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Entry
Catalyst
Fe/Si (mol %)a
Yield (%)b
syn/anti
1
Fe-MCM-41 (1%)
0.4
50
77:23
2
Fe-MCM-41 (3%)
0.7
59
83:17
3
Fe-MCM-41 (5%)
0.6
85
70:30
4
Fe-MCM-41 (5%, pH = 10)
4
90
85:15
5
Fe-MCM-41 (10%)
0.7
82
62:38
6
Fe-MCM-41 (10%, pH = 10)
6
89
80:20
7
Fe-MCM-41 (20%)
0.7
90
70:30
8
Fe-MCM-41 (20%, pH = 10)
10
81
88:12
9
Fe- SBA-15 (1%)
0.4
50
75:25
10
Fe- SBA-15 (3%)
0.8
56
79:21
11
Fe-SBA-15 (5%)
0.5
90
71:29
12
Fe-SBA-15 (5, pH = 10)
3
70
86:14
13
Fe-SBA-15 (10)
0.7
86
80:20
14
Fe-SBA-15 (10, pH = 10)
7
86
80:20
15
Fe-SBA-15 (20)
0.9
90
70:30
16
Fe-SBA-15 (20, pH = 10)
10
93
79:21
Reaction conditions: benzaldehyde, 0.2 mmol; 1-phenyl-1-(trimethylsiloxy)-propene, 0.4 mmol; water 0.25 mL, ethanol 0.25 mL, room temperature; time, 24 h. a Measured by ICP-MS; b the yield of aldol product after purification by silica gel column chromatography.
However, no differences in the yield of reaction using catalysts exhibiting different pore sizes were observed. It indicates that the range of pore sizes from 4 nm to 9 nm has about the same effect on the Mukaiyama aldol reaction. The effects of pore size beyond this range need further investigation.
It can be postulated that iron-modified mesoporous silicas have a high catalytic activity for the Mukaiyama aldol reaction, owing to their highly dispersed active Lewis acid sites. Also, the adsorption and diffusion of the reactants could be favored by the mesoporous channels of the catalysts. The results showed a distinct increase in yield as the iron content was raised from 1% to 5%. Meanwhile, no significant difference in yield was observed for the catalysts with iron contents in the range of 5% to 20%. Therefore, these catalytic experiments can also lead to a conclusion that the active site density of catalysts with iron content of 5% was getting saturated, resulting in an unchanged catalytic activity of the materials with increased iron content.
In addition, to compare, FeCl3 was chosen as model homogeneous Lewis acid catalyst and also applied in the Mukaiyama aldol reactions. As shown in Table S3 (SI), surprisingly low yields of 27% and 42% were obtained when FeCl3 was used as catalyst, showing its distinctly lower catalytic activity and selectivity in aqueous media compared to the iron-modified mesoporous materials.
Table 7 Effect of the iron content for the Mukaiyama aldol reaction of 1-(trimethylsilyloxy)-cyclohexene with benzaldehyde Entry
Catalyst
Fe/Si (mol%)a
Yield (%)b
syn/anti
1
Fe-MCM-41 (1%)
0.4
38
80:20
2
Fe-MCM-41 (3)
0.7
49
78:22
3
Fe-MCM-41 (5%)
0.6
85
86:14
4
Fe-MCM-41 (5%, pH = 10)
4
78
78:22
5
Fe-MCM-41 (10%)
0.7
88
88:12
6
Fe-MCM-41 (10%, pH = 10)
6
84
78:22
7
Fe-MCM-41 (20%)
0.7
84
86:14
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Fe-MCM-41 (20%, pH = 10)
10
87
79:21
9
Fe-MCM-41 (1%)
0.4
29
77:23
10
Fe-MCM-41 (3%)
0.8
56
75:25
11
Fe-SBA-15 (5%)
0.5
85
88:12
12
Fe-SBA-15 (5%, pH = 10)
3
81
76:24
13
Fe-SBA-15 (10%)
0.7
81
78:22
14
Fe-SBA-15 (10%, pH = 10)
7
85
77:23
15
Fe-SBA-15 (20%)
0.9
85
80:20
16
Fe-SBA-15 (20, pH = 10)
10
82
75:25
Reaction conditions: benzaldehyde, 0.2 mmol; 1-(trimethylsilyloxy)cyclohexene, 0.4 mmol; water 0.25 mL, ethanol 0.25 ml, room temperature; time, 3 h. a measured by ICP-MS; b yield of aldol product after purification by silica gel column chromatography.
Substrate scope. To verify the versatility of our system, a series of aldehydes were tested with 1-phenyl-1-(trimethylsiloxy)-propene to explore the scope of the Fe-modified mesoporous materials (i.e., Fe-SBA-15 (5 % and 20 %)) in the catalytic Mukaiyama aldol reactions. As shown in Table 8, the corresponding aldol products were obtained in moderate to excellent yields in all cases. We first investigated the reaction of benzaldehyde with 1-phenyl-1-(trimethylsiloxy)-propene and excellent yields of 86 % and 90% were observed. Correspondingly, similar yields of coupling aldols were also achieved when 4-substituted benzaldehydes with either an electron-donating group (-OCH3, entries 3 and 4) or an electron-withdrawing group (-Cl, entries 5 and 6) were examined as substrates. Notably, high yields around 90% were obtained when the less reactive 4cyanobenzaldehyde, 4-nitrobenzaldehyde and 2-nitrobenzaldehyde were used as substrates (entries 7-12), although a reversed selectivity was observed, i.e., the anti-isomers were the major product. n-Butanal as a representative example of an aliphatic aldehyde showed lower reactivity than the benzaldehyde or benzaldehyde derivatives, while the aldol products were formed in 49% and 53% yields. However, a similar yield of 64 % was previously observed by the Ollevier group using the same aliphatic aldehyde (nbutanal) in pure water with an homogeneous catalysts.75 Table 8 Catalytic performances of Fe-SBA-15 (5%)(A) and Fe-SBA-15 (20%)(B) in Mukaiyama aldol reactions (substrate scope)
Entry
Cat.
1 2
Aldehyde
Rï
Yield (%)a
syn/anti
A
H
86
80:20
B
H
90
70:30
3
A
4-OMe
87
79:21
4
B
4-OMe
90
83:17
5
A
4-Cl
88
76:24
6
B
4-Cl
86
88:12
7
A
4-CN
90
34:66
8
B
4-CN
93
33:67
9
A
4-NO2
91
40:60
10
B
4-NO2
92
39:61
11
A
2-NO2
90
34:66
12
B
2-NO2
91
31:69
13
A
49
67:33
14
B
53
70:30
Reaction conditions: aldehyde, 0.2 mmol; 1-phenyl-1-(trimethylsiloxy)-propene, 0.4 mmol; water 0.25 mL, ethanol 0.25 mL, room temperature; time, 24 h. a Yield of aldol product after purification by silica gel column chromatography
Reusability experiment. One of the most significant advantages of heterogeneous Lewis acid catalysts is that they can normally be easily removed from the reaction medium and recycled. For reusability tests, 400 mg of Femodified mesoporous silica (e.g., Fe-SBA-15 (5%) and FeSBA-15 (20%)) were used as catalysts for the water-medium Mukaiyama aldol reactions between benzaldehyde and 1(trimethylsilyloxy)-cyclohexene, respectively. In each case, the catalyst was recollected by filtration, washed with ethanol and subsequently dried overnight at 150 • in a vacuum oven. The amount of the reactants was adjusted to the same scale according to the amount of catalyst. As presented in Table 9 and Table S4 (SI), the recycled catalysts could be reused for at least eight times without significant loss in the yield and no change in selectivity. It can thus be
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suggested that no elution of Fe species was taking place during the reactions and therefore, Fe-modified mesoporous silica can be viewed as highly stable, active and reusable heterogeneous catalysts for the Mukaiyama aldol reaction.
Reagents and conditions: benzaldehyde, 0.2 mmol; 1-(trimethylsilyloxy)cyclohexene, 0.4 mmol; water 0.25 mL, ethanol 0.25 mL, room temperature; time, 3 h. a Yield of aldol product after purification by silica gel column chromatography.
Table 9 Reusability of catalysts for Mukaiyama aldol reaction of 1-(trimethylsilyloxy)-cyclohexene with benzaldehyde over Fe-SBA-15 (20%) catalyst
„
Entry
Yield (%)a
syn/anti
1st
87
76:24
2nd
88
83:17
3rd
92
75:25
4th
93
77:23
5th
89
78:22
6th
91
79:21
7th
93
76:24
8th
93
82:18
9th
94
79:21
Reagents and conditions: benzaldehyde, 1 equiv.; 1-(trimethylsilyloxy)cyclohexene, 2 equiv.; water/ethanol = 1:1, room temperature; time, 3 h. a Yield of aldol product after purification by silica gel column chromatography.
Catalytic behavior of passivated Fe-MCM-41 and FeSBA-15. The catalytic activities of non-passivated materials and passivated materials were compared to verify a possible effect of the silanol groups present on the surface of silica for the activity (Table 10). High yield of the reaction could be obtained with the presence of all catalysts, among which the passivated materials showed a slightly better catalytic activity with the yield rising from 84 to 89% and from 85 to 92% for Fe-MCM-41 and Fe-SBA-15, respectively. It was expected that the silylating layer on the silica surface could provide strong surface hydrophobicity, which may facilitate the diffusion of the organic reactants, thus increasing possibility of contact between the hydrophobic reactants and the active Lewis acid sites, leading to a higher yield of the final product.33,70 Table 10 Catalytic test of Fe-MCM-41-HMDS (20%) and Fe-SBA-15-HMDS (20%) catalyst
CONCLUSION
To summarize, a versatile and easy synthesis of ironmodified mesoporous silica using acetylacetonate as a metal precursor was demonstrated. It was shown from XRD that no iron oxide species with large crystal domain size were formed during the synthesis. Furthermore, only a fraction of the iron precursor can be grafted on the silica, with cationic iron species preferentially dispersed on the surface of the silica support. The Lewis acidity of the prepared catalysts was explored by a quantitative titration method using various Hammet indicators. The results of pyridine adsorption FT-IR revealed that the iron-modified mesoporous materials exhibited a significant majority of Lewis acid sites compared to Brønsted sites. Finally, in the catalytic tests, the iron species deposited on mesoporous silica could function as highly active and selective sites for the Mukaiyama aldol reaction of various aldehydes with 1trimethylsilyloxycyclohexene and 1-phenyl-1-(trimethylsiloxy)-propene. The experimental procedure was straightforward, the ethanol-water solution used in this study was a clean solvent system and environmentally benign, no harmful organic solvents were used. The catalysts could be easily recovered and successively reused in the same reaction without loss of catalytic activity. Therefore, it is believed that iron-modified mesoporous silica is a highly active heterogeneous Lewis acid catalyst and could be further applied not only for the Mukaiyama aldol reaction but also for various environmentally-friendly chemical transformations in organic chemistry.
„ AUTHOR INFORMATION Corresponding Author *E-mails:
[email protected],
[email protected] ORCID Freddy Kleitz: 0000-0001-6769-4180 Thierry Ollevier: 0000-0002-6084-7954
Notes The authors declare no competing financial interest.
Author Contributions The manuscript was written through contributions of all authors. All authors have approved the final version of the manuscript.
Catalyst
Yield (%)a
syn/anti
Fe-MCM-41
84
86:14
Fe-MCM-41-HDMS
89
73:27
Funding Sources
Fe-SBA-15
85
80:20
Fe-SBA-15-HDMS
92
80:20
The authors thank the Fonds de recherche du Québec Nature et technologies (FRQNT), the Natural Sciences and Engineering Research Council of Canada (NSERC) and the
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Centre in Green Chemistry and Catalysis (CGCC) for financial support. F.K. also acknowledge the University of Vienna (Austria) for additional support.
(19) Zhang, F.; Liang, C.; Wu, X.; Li, H. Angew. Chem. Int. Ed. 2014, 53, 8498-8502. (20) Narasaka, K.; Soai, K.; Mukaiyama, T. Chem. Lett. 1974, 3, 12231224. (21) Kitanosono, T.; Ollevier, T.; Kobayashi, S. Chem. Asian J. 2013, 8, 3051-3062.
ASSOCIATED CONTENT Supporting Information Supporting Information including characterization of the catalysts (low-angle and wide-angle XRD patterns of the materials, SEM images as well as the EDX spectra, UV-vis diffuse reflectance spectra of the catalysts, and Lewis acidity studies) and experimental procedure for the catalytic tests (effect of the SDS surfactant, Mukaiyama aldol reaction using FeCl3 as catalyst, reusability tests and all the NMR spectra of the products) are available free of charge on the ACS Publications via the Internet at http://pubs.acs.org.
(22) Ollevier, T.; Plancq, B. Chem. Commun. 2012, 48, 2289-2291. (23) Kobayashi, S.; Nagayama, S.; Busujima, T. Tetrahedron 1999, 55, 8739-8746. (24) Kobayashi, S.; Manabe, K. Acc. Chem. Res. 2002, 35, 209-217. (25) Horike, S.; Dinca, M.; Tamaki, K.; Long, J. R. J. Am. Chem. Soc. 2008, 130, 5854-5855. (26) Zhang, F.; Wu, X.; Liang, C.; Li, X.; Wang, Z.; Li, H. Green Chem. 2014, 16, 3768-3777. (27) Narasaka, K.; Soai, K.; Aikawa, Y.; Mukaiyama, T. Bull. Chem. Soc. Jpn. 1976, 49, 779-783. (28) Saigo, K.; Osaki, M.; Mukaiyama, T. Chem. Lett. 1976, 5, 163-164.
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(29) Jankowska, J.; Mlynarski, J. J. Org. Chem. 2006, 71, 1317-1321.
ACKNOWLEDGMENT
The authors thank Prof. Ryong Ryoo and Mr. Jongho Han (KAIST and IBS, Daejeon, Republic of Korea) for their help with low-angle XRD and high-resolution transmission electron microscopy analyses.
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