Cubic phenylene bridged mesoporous hybrids from allylorganosilane precursors and their applications in Friedel–Crafts acylation reaction

Microporous and Mesoporous Materials 101 (2007) 231–239 www.elsevier.com/locate/micromeso

Cubic phenylene bridged mesoporous hybrids from allylorganosilane precursors and their applications in Friedel–Crafts acylation reaction Mahendra P. Kapoor a,*, Yuuki Kasama a, Masaaki Yanagi a, Takuji Yokoyama a, Shinji Inagaki b, Toyoshi Shimada c, Hironobu Nanbu a, Lekh R. Juneja a a

Taiyo Kagaku Co. Ltd., Nano-Function Division, 1-3-1 Takaramachi, Yokkaichi, Mie, 510-0844, Japan b Toyota Central R&D Laboratories Inc., Nagakute, Aichi, 480-1192, Japan c Nara National College of Technology, 22 Yata-cho, Yamatokoriyama, Nara, 639-1080, Japan Received 26 July 2006; received in revised form 26 September 2006; accepted 2 October 2006 Available online 15 November 2006

Abstract The phenylene-bridged hybrid mesoporous silica material with well-defined cubic three-dimensional (Pm3n) symmetry was prepared using a allylorganosilane precursor 1,4-bis(triallylsilyl)benzene and cetyltrimethylammoniumchloride (C16TMACl) surfactant in acidic medium. Sulfonic acid functionalized derivatives of 3d-cubic phenylene-bridged hybrid mesoporous silica materials were prepared and found effective in Friedel–Crafts acylation reaction. The catalytic results were also compared to the sulfonic acid functionalized derivatives of phenylene-bridged mesoporous silica with 2d-hexagonal (P6mn) symmetry, sulfonated SBA-1 (Pm3n) mesoporous silica, and sulfonated phenyltrimethoxy silane (PTMS) grafted SBA-1 (Pm3n) mesoporous silica. The degree of sulfonation could be controlled using the different sulfonating agents as well as the time on sulfonation. It was revealed that 3d-cubic mesoporous hybrids could be functionalized with higher concentration of sulfonic acid moieties. Friedel–Crafts acylation reaction was performed over all sulfonated materials. Mesoporous materials derived from allylorganosilane precursors showed an excellent activity in Friedel–Crafts acylation of aromatic ether anisol using acetic anhydride as acylating agent.  2006 Elsevier Inc. All rights reserved. Keywords: Three-dimensional; Phenylene-bridged; Allylorganosilane; Mesoporous; Hybrids; Friedel–Crafts acylation; Catalysis

1. Introduction Friedel–Crafts alkylation and acylation reactions are fundamental and important processes in organic synthesis as well as in industrial chemistry [1]. While the alkylation reaction proceeds in the presence of a catalytic amount of Lewis acid, the acylation reaction generally requires more than a stoichiometric amount of Lewis acid due to the involvement of the Lewis acid sites by coordination to the corresponding aromatic ketone products. Acylation of aromatic compounds to prepare aromatic ketones is of *

Corresponding author. Tel.: +81 59 347 5410; fax: +81 59 347 5417. E-mail address: [email protected] (M.P. Kapoor).

1387-1811/$ - see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.10.004

commercial relevance and significance in several areas of fine chemical and pharmaceutical industries. The utility of polymer supported Lewis acid catalysts, microencapsulated Lewis acids, acidic halides, metal alkyls and alkoxides, metal triflate [2–10], 4-dialkylaminopyridines, clays, zeolites, Nafion-H, and yttria-zirconia, has been recognized [11–14]. Metal triflates were found with significant activity, however the very high cost and susceptibility to aqueous media of most of the metal triflates except lanthanide triflates [15,16], which are stable in aqueous media and recoverable from aqueous solution become major concerns for their industrial applications. The replacement of the conventional acylation catalysts has been the subject of many studies because of the safety,

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Fig. 1. Friedel–Crafts acylation over three-dimensional cubic (Pm3n) phenylene-bridged mesoporous silica.

low cost and ease of recovery derived from heterogeneous catalysis. Surfactant-templated mesoporous silicas functionalized with alkylsulfonic acid groups onto the pore surface have been reported to be efficient solid acid catalysts for acid-base reactions [17,18]. In addition, mercapto-functional mesoporous silicas have received considerable attention as the anchored thiol group can be oxidized to provide sulfonic acid functionality for applications in solid acid catalysis [19]. Periodic mesoporous organic–inorganic hybrids were dicovered in the late 1990s [20–23] and have raised the general expectation that their efficient uses can be dimensionally expanded to a number of versatile applications as catalysts [24–26]. Till date several aspects of ethane-, phenylene-, biphenylene-, thiophene- and ferrocene-bridged hybrid mesoporous materials with two-dimensional hexagonal symmetries (P6mm) have been reported in the literature [27–38]. The controls in materials morphologies and structural symmetries were studied and the importance of three-dimensional structures of hybrid mesoporous materials was addressed. Three-dimensional structures could be more suitable for applications as catalysts or adsorbents as they do not suffer from mass-transfer limitations and therefore allow the easier diffusion of reactants [39]. The successful synthesis of ethane-hybrid mesoporous materials with cubic Pm3n space group symmetries with external decaoctahedral morphology was defined [40–43]. The synthesis of highly ordered three-dimensional periodic ethane mesoporous silica with cubic Fm3m symmetry, 3d-cubic of Im3m symmetry, and bicontinuous cubic of Ia3d symmetry are also reported [44–46]. Therefore, the most commonly used typical organic linker for cubic mesophase formation is ethane. However, due to limited chemical functionality of ethane group, the functionalization of ethane bridged mesoporous materials is difficult. Goto et al. presented the synthesis of mesoporous phenylene-silica materials with three-dimensional cage structures using hexadecyltrimethylammonium bromide, hexadecylpyridinium chloride and triblock copolymer F88 surfactants [47]. All materials showed spherical particle morphologies with diameters of 0.5 mm to 4 mm. Sulfonic acid functionalized periodic mesoporous hybrids are an emerging class of new materials that hold significant promise in catalysis. The framework hydrophobic properties of materials have been shown to be very reactive in esterification reactions [48–51] and condensation

of phenol with acetone to form Bisphenol A [52]. Sulfonic acid functionalized hybrid mesoporous materials could have better catalytic properties compared to triflate materials (Ho value 15.0) because of the comparatively lower negative Ho value, and could suppress potential competitive side reactions. Anticipating that somewhat mild acidic conditions might be the reason for the detrimental effect, we propose sulfonic acid functionalized phenylene-bridged mesoporous silicas with cubic symmetry as a replacement catalyst for the acylation reaction. In this paper, we report the synthesis of three-dimensional cubic mesophases of phenylene-bridged hybrids using phenylene-bridged allylorganosilane precursors via electrostatic templating pathways in acidic medium. The phenylene-bridged mesoporous silica with three-dimensional cubic symmetry clearly shows the cubic particle morphology. The sulfonic acid derivatives of the threedimensional cubic mesophase of the phenylene-bridged hybrids were also synthesized and examined for the Friedel–Crafts acylation (Fig. 1). The results were compared to the sulfonic acid functionalized derivatives of phenylene-bridged mesoporous silica with 2d-hexagonal (P6mn) symmetry derived using Brij-56 surfactant under acidic conditions [53], pure silica SBA-1 (Pm3n) mesoporous silica [54], and phenyltrimethoxysilane (PTMS) grafted SBA-1 mesoporous silica. The degree of sulfonation was controlled via employing the different sulfonating agents as well as time on the stream of sulfonation. The Friedel– Crafts acylation reaction products are of commercial importance in the fine chemicals industries, especially as a intermediate for the synthesis of fragrances and pharmaceuticals. 2. Experimental 2.1. Synthesis Stable allylorganosilane precursor, 1,4-bis(triallylsilyl)benzene was synthesized and purified by silica gel column chromatography under ambient conditions according to the modified procedure already reported in our earlier publications [55,56]. In a typical preparation of the threedimensional cubic mesophase preparation, 1,4-bis(triallylsilyl)benzene (2.65 mmol) was suspended in an aqueous acidic solution of C16TMACl (1.1 mmol in 60 g ionexchanged water) containing 20 g concentrated (36%)

M.P. Kapoor et al. / Microporous and Mesoporous Materials 101 (2007) 231–239

hydrochloric acid and stirred to promote hydrolysis at 40 C for 24 h followed by aging at ca. 92 C for a further 24 h under static conditions. The cubic mesophase of phenylene bridged mesoporous hybrids was filtered and washed thoroughly with copious amounts (>300 mL) of deionized water and dried under vacuum at ambient temperature. Highly ordered cubic phenylene-silica mesoporous material was finally collected after removal of the surfactant by solvent extraction. Typically, 1.0 g of an as-synthesized material was extracted with 150 mL of ethanol with 2.0 g of 36% HCl aqueous solution at 65 C for 6 h and then the solvent extraction process was repeated again for the complete removal of the surfactant. Phenylene-bridged mesoporous silica with 2D-hexagonal (P6mn) symmetry was derived using Brij-56 surfactant under acidic conditions according to the procedure reported elsewhere [53]. SBA-1 (Pm3n) mesoporous material was prepared as described in [54]. The phenylene grafted SBA1 mesoporous silica was also prepared. In a typical grafting procedure, phenyltrimethoxysilane (PTMS) (1 mL) was added dropwise into the previously dispersed mesoporous SBA-1 silica (0.5 g) in chloroform (50 mL). The suspension was stirred at room temperature for three days. The phenylene-grafted mesoporous SBA-1 was recovered after filtration and repeatedly washing with chloroform. The material was dried in air and under vacuum before use.

233

dispersed solution of mesoporous hybrid materials in concentrated sulfuric acid was stirred at 90 C for 6 h under a nitrogen environment. The dispersion was cooled at room temperature and diluted with a large volume of deionized water. The solids were filtered and repeatedly washed with deionized water. Finally, the solids were extracted and washed with hot (boiled) deionized water and dried under vacuum for complete dryness. The samples with varied extent of sulfonic acids were prepared by altering the time on sulfonation. The sample was also sulfonated for 1, 2, 4 h for comparison. Ar  H þ H2 SO4 ¼¼¼ Ar  SO2 OH 2.3.2. With fuming sulfuric acid The mesoporous phenylene silicas dried under evacuation were added to 25% SO3/H2SO4 solution. The dispersed solution was kept at 90 C for 6 h. The dispersion was cooled to room temperature, and added to a large amount of deionized water. The solid was filtered and repeatedly washed with deionized water. The soild was then suspended in hot boiling water for 30 min and washed with boiled water. Finally the solid was stirred in 4 M hydrocholoric acid aqueous solution for 20 h to protonate and form sulfonic acid moieties. Ar  H þ SO3 ðH2 SO4 Þ ¼¼¼ Ar  SO2 OH

2.2. Characterization methods Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku RINT-2200 V diffractometer with CuKa radiation (40 kV, 30 mA) from 1 to 10 2h, 0.01 step size, and 1 2h min1 scan speed. The porosimetry experiments (N2 adsorption isotherms) were performed on a BELSORP-18 sorptometer at 196 C. Prior to measurement, all samples were out gassed at 80 C at 103 Torr. The Brunauer–Emmett–Teller (BET) surface areas were calculated from the linear part of the BET plot. Pore size distributions (DFT) were determined from the adsorption branch of the isotherms. 29Si magic-angle spinning (MAS) and 13C cross polarization (CP) NMR spectra were recorded on a JEOL spectrometer using a 4 mm zirconia rotor and a sample spinning frequency of 4 kHz. The chemical shifts were referenced to tetramethylsilane (TMS) at 0 ppm. Thermo gravimetrical analysis (TGA/DTA) performed using (Seiko instruments) in air and nitrogen atmospheres were used to check the materials stability.

2.3.3. With chlorosulfonic acid In another method of sulfonation, the chlorosulfonic acid was used. Similar process was employed as mentioned above. Ar  H þ ClSO2 OH ¼¼¼¼ Ar  SO2 Cl ¼¼ ¼ Ar  SO2 OH 2.4. Estimation of the extent of sulfonation (Ion-exchange capacity) The acid content of the resultant mesoporous solids was estimated from the acid–base potentiometric titration curve. The sulfonated derivative of the mesoporous hybrid (100 mg) was immersed in 20 g of 2.0 M NaCl aqueous solutions. The resulting suspension was stirred at room temperature for 24 h until equilibrium was reached. The filtrate was titrated with 0.1 M NaOH to produce a titration curve.

2.3. Sulfonation procedures 2.5. Friedel–Crafts acylation procedure Different sulfonation procedures were employed. Prior to sulfonation, all samples studied were dried under evacuation at 103 Torr for 2 h. 2.3.1. With concentrated sulfuric acid The cubic mesophases of phenylene-bridged silicas were subsequently sulfonated using concentrated H2SO4. The

We used the sulfonated 3d-cubic mesoporous phenylene-silica and other sulfonic acid functionalized mesoporous materials for Friedel–Crafts acylation reaction, [15,16] an acid catalyzed carbon–carbon bond forming reaction. The reaction was carried out in a batch reaction system using normal vessels. In a typical procedure, to 3d-cubic

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sulfonated mesoporous phenylene-silica catalyst (400 mg) was added to a mixture of anisole (0.8 mol) and acetic anhydride (1.6 mmol). The mixture was stirred for 5 min at ambient temperature. Then LiClO4 (4.4 mol in 16 mL of CH3NO2) was added and the mixture was heated at 50 C with stirring and continuously stirred for another 6 h. After the reaction, the catalyst was filtered off and washed with CH3CN. The filtrate was mixed with CH2Cl2 (30 mL) and water (30 mL). The organic materials were separated, washed repeatedly with water and concentrated under reduced pressure using a rotary evaporator. Finally, the residue was purified via chromatography using silica gel to yield 4-methoxyphenyl methyl ketone (87.6%). The product structure was confirmed via 1H NMR and 13C NMR. The catalyst was recovered quantitatively by simple filtration and could be reused after washing and drying. The second and third runs with the recycled catalyst gave 81.3% and 73.8% yields, respectively. 1H NMR (CDCl3) d 2.59 (s, 3H), 3.91 (s, 3H), 6.97 (d, 2H), 7.98 (d, 2H) and 13C NMR (CDCl3) d 26.6, 55.2, 113.9, 130.6, 130.8, 163.4, 196.5. The recovered catalyst was activated by heating at 90 C under vacuum for 3 h and reused for acylation of a fresh portion of anisole. The recovered catalyst was used for one more consecutive acylation reaction of anisole. 3. Results and discussion 3.1. Materials The X-ray diffraction pattern of the surfactant free 3dcubic phenylene-bridged mesoporous hybrid showed three main peaks at (2 0 0), (2 1 0) and (2 1 1), which could be well fitted to the diffraction lines for a Pm3n-type cubic struc˚ . The d-spacing of the most ture with a unit cell ao = 85.9 A ˚ , while the d-spacings of intense peak (2 1 0) was 38.4 A ˚ and 35.1 A ˚ , respecthe (2 0 0) and (2 1 1) peaks were 42.7 A tively (Fig. 2). No noticeable shrinkage was observed after surfactant removal. Owing to the strong similarity between this diffractogram and that of SBA-1 (Pm3n) [54] and ethane bridged cubic hybrid silicas (Pm3n), [44–46], it is inferred that the structure of the present material was also identified as belonging to the same space group. Further, transmission electron microscopy (TEM; Fig. 3a) showing the orientation of the structure along the {2 4 1} direction

1200

Sulfonated 1000

Intensity, CPS

234

800 600

d210 d200

d211

400 200

Calcinated 0

1

2

3

4

5

2-theta, degree Fig. 2. X-Ray diffraction patterns for three dimensional cubic (Pm3n) phenylene-bridged mesoporous materials derived from 1,4-bis(triallylsilyl)benzene-recursor: (a) after extraction and (b) after sulfonation.

and scanning electron microscopy (SEM; Fig. 3b and c) results provide direct confirmation of the three-dimensional cage-type (Pm3n) symmetry and well-defined cubic particle morphology. Such external morphology is uncommon and could be obtained only via balanced self-assembly using allyl derivatives of the phenylene-bridged precursor [55–62]. The nitrogen adsorption–desorption isotherm was type IV, typical of periodic mesoporous materials (Fig. 4). The Brunauer–Emmett–Teller (BET) surface area, pore volume, and NLDFT pore diameter (N2 adsorption at 77 K on silica MCM type) were 686 m2 g1, 0.314 cm3 g1, and ˚ , respectively. The physico-chemical and textural 37.7 A properties of all materials of this study are presented in Table 1. Compositional and structural properties were confirmed from NMR and TGA/DTA analyses [54]. 29Si MAS NMR results exhibited two peaks at 80.9 and 72.4 ppm, attributable to T3 [SiC–(OSi)3], a fully condensed silicon, and T2 [SiC(OH)–(OSi)2], a partly hydrolyzed silica species. The absence of any signal due to SiO4 species at ca. 100 ppm confirms that all Si atoms are covalently connected to carbon atoms in the framework suggesting no Si–C bond cleavage occurred during template extraction. The results also revealed that the 3d-cubic mesoporous phenylene-silica exhibited a high degree of framework cross-linking. The 13C cross-polarization CP–MAS–NMR spectrum of the surfactant free mesoporous hybrid displays a resonance at 133.5 ppm due to carbons on the benzene

Fig. 3. (a) High-resolution TEM micrograph (b) SEM image confirming the cubic morphology and (c) enlarged SEM image of cubic particles for threedimensional cubic (Pm3n) phenylene-bridged mesoporous materials.

M.P. Kapoor et al. / Microporous and Mesoporous Materials 101 (2007) 231–239

235

2.0

37.7 1.5

200

Ca Calcinated

Calcinated

150

DV

Nitrogen adsorbed, cc/g

250

1.0

27.0

100 Sulfonated

50 0 0.0

0.2

0.4

0.6

0.8

Sulfonated

0.5

0.0

1.0

0

Partial pressure, P/Po

20

40

60

80

100

Pore diameter, Å [DFT] Fig. 4. (a) Nitrogen adsorption–desorption isotherms {adsorption (d) and desorption (s)}, and (b) pore size distribution curves for three-dimensional cubic (Pm3n) phenylene-bridged mesoporous materials (a) after extraction and (b) after sulfonation.

SBET, (m2 g1)

PDDFT, ˚) (A

Pore volume, (cm3 g1)

SBA-1-(Pm3n) SBA-1-(Pm3n)-SO3H

1017 476

34.6 22.4

0.593 0.274

646 404

28.1 23.3

0.344 0.281

2d-hexagonal-(P6mm) 2d-hexagonal-(P6mm)SO3H

1043 489

41.5 30.6

0.941 0.461

3d-cubic-(Pm3n) 3d-cubic-(Pm3n)-SO3H

686 324

37.7 27.0

0.314 0.144

SBA-1-(Pm3n)-Ph (graft) SBA-1-(Pm3n)-Ph (graft)SO3H

DTA, μV

Materials

TG, μg

Table 1 Textural properties of phenylene-bridged mesoporous hybrids

Temperature, o C 6. 0

8 6 4 2

5. 0 0

DTA, μV

TG, μg

ring. This confirmed the phenylene moieties are covalently linked to silica, composed of SiO1.5–C6H4–SiO1.5 units. Thermogravimetric analysis of the 3d-cubic mesophase of phenylene silica hybrids confirmed that the materials are thermally stable up to 450 C in the airflow atmosphere (Fig. 5). Sulfonic acid derivatives of mesoporous hybrids were produced after sulfonation with different sulfonation agents. However, the sulfonation with concentrated sulfuric acid is recommended for the purpose of Friedel–Crafts acylation reaction. The materials retained their mesoporosity and parent nature (Fig. 2b). The pore volume, BET surface area, and DFT pore diameter were 0.144 cm3 g1, ˚ , respectively. The acidic amount 324 m2 g1 and 27.0 A of the sulfonic acid derivative of the cubic mesophase of mesoporous phenylene–silica estimated by the acid–base titration method was H+ = 0.973 mmol g1, which is about 1.8 times the acidic amount of the 2d-hexagonal mesoporous phenylene–silica (0.594 mmol g1) derived using Brij56 surfactant [53]. The acid amount of the sulfonic acid derivative of SBA-1 sulfonated under identical conditions was H+ = 0.319 mmol g1. Phenylene grafted SBA-1 was sulfonated under identical conditions and showed the improved acid amount of H+ = 0.864 mmol g1. The extent of acid amount was also found to be dependent on

5. 5

-2

4. 5

-4 4. 0 0

200

400

600

800

-6 1000

Temperature, o C Fig. 5. Theromogravimetric weight loss curves for three-dimensional cubic (Pm3n) phenylene-bridged mesoporous hybrids: (a) under air flow and (b) under nitrogen flow.

the time of sulfonation, however the concentration of sulfonic acid sites (acidity, mmol H+ m2) was altered significantly (Fig. 6). The textural properties and acid amount of sulfonated 3d-cubic phenylene mesoporous silicas with varied time on sulfonation are listed in Table 2. Therefore, three-dimensional cubic mesophase (Pm3n) of phenylene silica could be prepared under acid conditions using the 1,4-bis(triallylsilyl)benzene monomer precursor. Acidic medium is important because depending on the synthetic parameters both the rigidity of the precursor and the

M.P. Kapoor et al. / Microporous and Mesoporous Materials 101 (2007) 231–239 1.2

3

+

60 2

40 1

20 0

-2

80

0

0

1

2

3

4

5

-1

Acidity, (mmol H m )x10

Product yi eld, %

-3

4

1.0 0.8 0.6 0.4 0.2

+

100

Acid amount, (mmol H g )

236

0.0

6

Time on sulfonation, h

Fig. 6. (s) Ion-exchange capacity (acid amount), (h) concentration of active sites, and ( ) product yield as a function of time on sulfonation in Friedel–Crafts acylation reaction over three-dimensional cubic (Pm3n) phenylene-bridged mesoporous hybrids functionalized with concentrated sulfuric acid.



Table 3 Friedel–Crafts acylation using concentrated sulfuric acid functionalized three-dimensional cubic (Pm3n) phenylene-bridged mesoporous hybrids Materials

Acid amount (mmol H+/g)

Acidity (mmol H+/m2)

Product yielda (%)

SBA-1-(Pm3n)SO3H SBA-1-(Pm3n)-Ph (graft)-SO3H 2d-hexagonal(P6mm)-SO3H 3d-cubic -(Pm3n)SO3H 3d-cubic -(Pm3n)SO3H-R1 3d-cubic -(Pm3n)SO3H-R2 3d-cubic-(Pm3n)

0.319

0.67 · 103

26.7

0.864

2.14 · 103

60.6

0.594

1.22 · 103

36.1

0.973

3.01 · 103

87.6

0.901

-nd-

81.3

0.852

-nd-

73.8

–

–

<0.05

a

surface polarity could be controlled in the acid medium. The S+XI+route adopted in the acid conditions provided more flexibility in control of the anisotropic growth of morphological symmetries in contrast to the usual S+I route under basic conditions. The exerted charge matching forces between the cationically charged micelle surface (from quaternary ammonium surfactant) and cationically charged silsesquioxane precursors in very strongly acidic aqueous media leads to an interesting lowest energy surface particle cubic cage type macro-morphology. The second reason to facile formation of three-dimensional cubic mesostructure was altering the highly reactive alkoxy group in the phenylene-bridged precursor to an allyl group, which is unreactive towards silica and readily leaves the reaction mixture during hydrolysis. It is also believed that the reaction pathway for the deallylation of allylorganosilane precursors under acidic conditions in the presence of a cationic surfactant followed the established mechanism of protodesilylation of allylic silanes [55–58]. 3.2. Catalytic performance Among the solid acids, only HY and Hb have been commercially used for the acylation of anisole [63]. Rhodia introduced first industrial application of solid acid liquid phase acylation of anisole with acetic anhydride over HY and Hb in a recyclic fixed bed reactor. Both Lewis and Bronsted acid sites of the zeolites were active, but Lewis acid sites were more effective and selective for the acylation of aromatic ring. The reaction was performed at 160 C

4-Methoxyphenyl methyl ketone.

under 20 bar of N2 pressure to give 75% conversion with 98% selectivity of the product [64,65]. The process operates at high temperature and very high pressure and also needs a solvent for uniform mixing. We have designed a facile high yield Friedel–Crafts acylation process for the anisol with acetic anhydride operating at mild temperature. The results listed in Table 3 demonstrate the performance of sulfonic acid functionalized phenylene-bridged mesoporous silicas on Friedel–Crafts acylation. The reaction procedural details are provided in the Experimental section. The catalytic results demonstrate that sulfonic acid functionalized phenylene-bridged mesoporous silicas with cubic symmetry exhibit high activity in Friedel–Crafts acylation (yield 87.6%) compared to two dimensional analogous phenylene hybrid mesoporous silica (yield 36.1%) derived using oligomeric Brij-56 surfactant under acidic conditions [53]. While, sulfonated SBA-1 and sulfonated phenylene grafted SBA-1 exhibited the 26.7% and 60.6% product yields, respectively. All materials were sulfonated under identical conditions using concentrated sulfuric acid. The product yield was less than 0.05% showing practically no Friedel–Crafts acylation activity when phenylene-bridged mesoporous silicas with 3d-cubic symmetry were used without sulfuric acid functionalization. The results evidently explained that co-catalyst LiClO4 do not directly participate in the reaction and total activity is only attributable to sulfonic acid derivatives of these hybrid mesoporous materials. The estimated acid amounts in materials cannot fully explain the significant difference in the catalytic activity.

Table 2 Textural properties of three-dimensional cubic (Pm3n) phenylene-bridged mesoporous hybrids functionalized using concentrated sulfuric acid with varied sulfonation time and their performance in Friedel–Crafts acylation Sulfonation time (h)

SBET (m2 g1)

PDDFT ˚) (A

Pore volume (cm3 g1)

Acid amount (mmol H+/g)

Acidity (mmol H+/ m2)

Product yielda (%)

1 2 4 6

501 441 336 324

32.1 30.6 27.2 27.0

0.281 0.211 0.156 0.144

0.601 0.833 0.931 0.973

1.20 · 103 1.89 · 103 2.77 · 103 3.01 · 103

35.6 56.7 83.9 87.6

a

4-Methoxyphenyl methyl ketone.

M.P. Kapoor et al. / Microporous and Mesoporous Materials 101 (2007) 231–239

However, the concentrations of the sulfonic acid sites (acidity, mmol H+ m2) based on the surface area of the sulfonated materials are 3.01 x 103 and 1.22 · 103 for three-dimensional and two-dimensional sulfonic acid functionalized phenylene-bridged mesoporous silicas, respectively. While, in the case of sulfonated SBA-1 and sulfonated phenylene grafted SBA-1, the values of concentrations of the sulfonic acid sites were 0.67 · 103 and 2.14 · 103, respectively. Ability to anchor higher concentration of sulfonic acid sites and easier access of most of the available reaction sites in the event of the diffusion of the reactants and the products during the reaction process is one of the main reasons for the enhanced activity in the mesoporous material with 3d-cubic symmetry. Because, easier diffusion of the reactants in the three-dimensional cubic structure of the material also greatly reduces the mass transfer limitations and consequently gave better yields of the products. In the case of materials with 2d-hexagonal symmetry, structurally it was rather difficult to anchor the high concentration of sulfonic acid functionalities and it might also be possible that the reactant cannot reach some of the sulfonic acid sites buried deep in the walls of the materials. In contrast, most of the available reaction sites were easily accessible in 3d-cubic phenylene-bridged mesoporous silicas prepared with high concentration of sulfonic acid moieties. In addition, the acylation activity could also be largely affected by the surface hydrophilic/hydrophobic properties. The sulfonated SBA-1 materials gave a lower yield (26.7%) because of the low concentration of the sulfonic acid sites (0.67 · 103, mmol H+ m2) and least hydrophobicity. In contrast, the phenylene grafted sulfonated SBA-1 mesoporous materials are hydrophobic in nature and showed comparatively high yield (60.6%), owing to its three-dimensional open cage type structure, the yield being even better than highly hydrophobic sulfonic acid functionalized 2dhexagonal phenylene-bridged mesoporous silicas (36.1%), evidently showing the effect of hydrophobicity in acylation reaction. The exact reasons are not yet known, however it could possibly alter/facilitate the rate of diffusion of the reactants and the product in the pore system. The significant difference between the yields of sulfonic acid functionalized phenylene-bridged mesoporous silicas with cubic symmetry (Pm3n symmetry) and phenylene grafted sulfonated SBA-1 mesoporous materials also with 3d-cubic Pm3n symmetry can be attributed to the structural arrangement of both the materials. In the later case, the phenylene group protrudes into the walls of the channels instead of the built in walls of the channels. Also, the product yields were found to be varied with the extent of the sulfonic functionalization achieved by altering the time on sulfonation. After 1 h of sulfonation time, the product yield was only 35.6%, which was found to increase with increasing sulfonation time. No significant increase was observed after 4 h of sulfonation time. The product yield was 83.9%, which was increased to 87.6% after 6 h of sulfonation time. The trend of catalytic performance

237

along with acid amount and concentration of acidic sites are shown in Fig. 6. Further, the reusability of the materials was also monitored. After, the reaction, the catalyst was filtered off and recycled for further use (refer Experimental section). Slight loss of its original catalytic activity was observed after the first recycle (R1; yield 81.3%). On the second recycle (R2), the catalytic activity was further reduced and the product yield was 73.8%. No leaching of the reaction active sites during catalysis was observed, however it can be considered that during regeneration process some active sites may not have been fully recovered and resulted in the somewhat lower product yields on recycled use. This implies that the materials could be recovered easily by simple filtration and reused with minimal loss of activity. The product yields were considerably affected when fuming sulfuric acid (48.6%) or chlorosulfonic acid (30.7%) was used as a sulfonation agent for the functionalization of phenylene-bridged mesoporous silicas with cubic Pm3n symmetry. The sulfonic functionalization was prolonged for identical sulfonation temperature and time. Table 4 lists the catalytic performance of materials in Friedel–Crafts acylation reaction. The acid amount was quite lowered H+ = 0.629 mmol g1 and H+ = 0.407 mmol g1, when fuming sulfuric acid or chlorosulfonic acid were used, respectively. The possible explanation could be related to sulfonation mechanism. Sulfonic acid groups are introduced into phenylene rings by the electrophile sulfur trioxide (SO3 is a reactive species having partial positive charge on the sulfur atom) in sulfuric acid. The sulfonation is unique among the electrophilic aromatic substitution reactions because it is reversible, and upon heating a sulfonic acid in dilute sulfuric acid partially removes the sulfonate groups, which results in a lower extent of sulfonation. The results have also demonstrated that the extent of sulfonation was greater when concentrated sulfuric acid was employed. Therefore, sulfonic acid functionalized three-dimensional phenylene-bridged mesoporous silicas are reasonable strong protic acids for the acylation reaction and provide a more environmentally friendly form of acylation catalysis. The activity and selectivity of reactants diffused on the high surface area sulfonic acid functionalized

Table 4 Performance of three-dimensional cubic (Pm3n) phenylene-bridged mesoporous hybrids in Friedel–Crafts acylation functionalized using different sulfonation agents Sulfonation agent

Acid amount (mmol H+/g)

Acidity (mmol H+/m2)

Product yielda (%)

Concentrated sulfuric acid Fuming sulfuric acid Chlorosulfonic acid

0.973

3.01 · 103

87.6

0.629

1.63 · 103

48.6

0.407

1.13 · 103

30.7

a

4-Methoxyphenyl methyl ketone.

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M.P. Kapoor et al. / Microporous and Mesoporous Materials 101 (2007) 231–239

three-dimensional phenylene-bridged mesoporous silica improve as the effective surface area of the active reaction sites is easily available. It is true that Lewis acid sites are mostly active for Friedel–Crafts acylation, however the Friedel–Crafts reaction does also proceeds on the Bronsted acidic sites. Whereas, sulfonic acid functionalized phenylene bridged mesoporous silicas are rich in Bronsted acidic sites, however depending on the reaction parameters, the Bronsted and Lewis sites are inter-convertible, which is influenced by the structure (bonding angle), reaction enthalpy and coordination and surface changes on the particle. In the system studied, the initial density of Bronsted acidic sites could be high due to the removal of adsorbed water, however in the real time reaction conditions, the Bronsted acidic sites began to convert into Lewis sites, which are the prompt active reaction sites for the Friedel–Crafts acylation. Also catalytic results for the Friedel–Crafts acylation reaction clearly indicate a relationship between the catalytic activity and the structural properties. These mesoporous hybrid silicas also have good thermal and mechanical stability and the materials can be easily handled, are non-corrosive and less toxic. After the reaction, the catalytic materials can be separated from the reaction mixture through simple filtration and can be recycled and reused, and may be useful in industrial applications. The sulfonic acid functionalized three-dimensional phenylene-bridged mesoporous silica could have general applicability for acylation catalysis of a wide variety of reactants. 4. Conclusions In summary, we have presented a preparation of cubic mesophases of three-dimensional phenylene-bridged mesoporous hybrid silicas with Pm3n symmetry using allylorganosilane precursors and their functionalized derivatives using different sulfonating agents. The extent of sulfonic acid species onto the materials could be controlled via varying the time on sulfonation reaction. The 3d-cubic phenylene bridged mesoporous silica could be prepared with higher concentration of sulfonic acid site among the different sulfonated materials prepared under identical synthetic conditions. Friedel–Crafts acylation proceeds smoothly to produce an aromatic ketone (4-methoxyphenyl methyl ketone) in attractive yields. The catalytic results also suggest that the aforementioned materials may also be effective in many useful carbon–carbon bond formation reactions. The Friedel–Crafts acylation reaction products are of commercial importance in the fine chemicals industry, especially as intermediate for the synthesis of fragrances and pharmaceuticals. For example, the acylation of veratrole (1,2 dimethoxy benezene) produces acetoveratrone, which is a useful intermediate for the synthesis of papaverine (1(3,4-dimethoxybenzyl)-6,7-dimethoxyisoquinoline) an opiumalkaloids antispasmodic. Additionally, the materials can also find its applications as a solid fuel cell electrolyte material.

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