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Sulfonic acid-functionalized phenylene-bridged periodic mesoporous organosilicas as catalyst materials
Applied Catalysis A: General 299 (2006) 193–201 www.elsevier.com/locate/apcata
Sulfonic acid-functionalized phenylene-bridged periodic mesoporous organosilicas as catalyst materials ´ rpa´d Molna´r * Bulcsu´ Ra´c, Pe´ter Hegyes, Peter Forgo, A Department of Organic Chemistry, University of Szeged, Do´m te´r 8, Szeged, Hungary Received 30 May 2005; received in revised form 12 October 2005; accepted 13 October 2005 Available online 28 November 2005
Abstract Three solid acids based on bridged periodic mesoporous organosilica structure (PMO) with a benzene ring as the rigid unit incorporated in the framework and functionalized with anchored sulfonic acid groups were synthesized. Samples were prepared by either sol–gel polymerization of 1,4-bis(triethoxysilyl)benzene (BTEB) or co-condensation of BTEB and 3-mercaptopropyltrimethoxysilane (MPTMS) in the presence of octadecyltrimethylammonium bromide surfactant. Physical characterization data (X-ray powder diffraction, nitrogen adsorption and desorption, and NMR spectroscopy) and acid–base titration indicate the formation of ordered structure and successful functionalization. Catalytic properties were studied in both gas-phase and liquid-phase reactions. The catalytic performance of the PMO-based samples in the isopropylation of phenol in the gas-phase, particularly their stability, exceeds markedly those of functionalized mesoporous ordered materials (MCM-41, HMS and SBA-15). Selectivities in the Fries rearrangement of phenyl acetate over the PMO-based catalysts differ significantly from that of the homogeneous reaction. The sample with benzenesulfonic acid surface functions exhibits higher activities and different selectivities in the dimerization of 2-phenylpropene and in the rearrangement–aromatization of ketoisophorone as compared to samples functionalized with propanesulfonic acid groups. # 2005 Elsevier B.V. All rights reserved. Keywords: Periodic mesoporous organosilicas; Benzene bridged; Benzenesulfonic acid; Propanesulfonic acid; Phenol isopropylation; Fries rearrangement; Shape selectivity
1. Introduction A new method the so-called micelle templated synthesis developed recently [1,2] allows the preparation of silica materials of ordered mesoporous structure. Important characteristics are the narrow pore size distribution, very high surface area and a large number of surface silanol groups. The method creates numerous possibilities to synthesize new compositions. These mesoporous hybrid organic–inorganic materials have important and unique properties and varied application possibilities. Ordered mesoporous silicas, however, are particularly well-suited for the use as catalysts. The catalytically active material may be immobilized by encapsulation or through complex bonding or ionic interaction with suitable surfaces. Appropriately selected functional groups may
* Corresponding author. Tel.: +36 62 544 277; fax: +36 62 544 200. ´ . Molna´r). E-mail address: [email protected] (A 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.10.026
also be covalently bound to the surface. Successful applications have been disclosed and reviewed [3,4]. A unique group of hybrid organic–inorganic materials are bridged polysilsesquioxanes [5–7], which are prepared by sol– gel polymerization of bis-trialkoxyorganosilane monomers and have an organic functionality as part of the solid network. The resulting amorphous materials, which may have porous structure, are excellent candidates for application as optical and electronic devices, ceramics, adsorbents and catalyst supports. However, applying a structure directing agent during the sol–gel synthesis, a periodic mesoporous organosilica (PMO) is formed with the organic moiety uniformly incorporated in the framework. The micelle templated synthesis, in this case, improves structural order resulting in materials with highly uniform channels, hexagonally ordered pores and a narrow pore size distribution. With carefully chosen bridging organic moieties the materials have structural rigidity. This and the possibility to control hydrophilic–hydrophobic properties are particularly useful in catalytic applications. The periodic surface structure is advantageous because it can enable
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structural orientation of guest molecules and enhance selectivity and activity in catalysis [8]. Furthermore, cocondensation of bis-trialkoxyorganosilanes monomers and trialkoxyorganosilanes with suitable functional groups allows easy access to functionalized periodic ordered mesoporous silicas. By carefully choosing the precursor molecules a wide variety of organic species can be incorporated inside the channel walls. The organic bridging units incorporated into PMOs are ethane [9–11], ethene [10], benzene [12–15], biphenyl [16,17] and thiophene [13]. Materials with functional groups such as amino [18] and sulfonic acid [14–16,19] with potential for catalytic applications were also synthesized and characterized. The first example of the catalytic application of bridged polysilsesquioxanes was disclosed in 2002 using a benzenesilica xerogel [20]. The catalyst samples functionalized with various amine moieties proved to be active in the Knoevenagel condensation and compared well with aminopropyl modified silicas. There exist recent examples of bridged POMs with anchored sulfonic acid groups. First, thiol-functionalized samples were prepared by co-condensation or post-synthesis modification (grafting) followed by oxidation of the thiol group by H2O2. Co-condensations were performed by reacting 3-mercaptopropyltrimethoxysilane (MPTMS) with 1,4-bis(triethoxysilyl)benzene (BTEB) or 1,2-bis(triethoxysilyl)ethane in the presence of ionic (octadecyltrimethylammonium chloride) [21] or neutral (polyether) [22] templates. The ethane-bridged catalyst tested in the isopropylation of phenol exhibited much higher catalytic activity than ZSM-5 and higher stability than MCM-41 modified with the same group [21]. In the other study [22], ethane-bridged silicas displayed higher activity in the formation of bisphenol-A than their benzene-bridged counterparts. Samples made by grafting, in turn, were more active than those prepared by co-condensation. This was attributed to the irregular distribution of active sites. Grafted materials are known to have the active sites on the surface and near the pore mouth thereby being more accessible for the reactants. In a recent paper, the synthesis of ethane-bridged samples with increased amounts of MPTMS incorporated were described [23]. The samples showed increased acidities, but porosity, structural order and oxidation efficiency decreased. The catalyst made with 40% MPTMS (acid capacity = 0.98 H+ mmol g 1) was tested in two probe reactions. It showed activities comparable to Nafion-H both in esterification and ester hydrolysis. Recently, Inagaki et al. described the synthesis of PMOs with benzene [14] and biphenyl bridges [16,17] functionalized with sulfonic acid groups. The method used was cocondensation carried out in basic medium in the presence of cationic surfactant. However, there is only a recent report for the catalytic application of these materials in esterification [24]. The PMO functionalized with benzenesulfonic acid groups as described in Ref. [14], however, has not been used in catalytic reactions. The aim of the present study is, therefore, to prepare and characterize ordered mesoporous benzene-silica materials
with anchored propanesulfonic and benzenesulfonic acid groups and test their performance in various catalytic transformations. 2. Experimental All reagents were purchased from Aldrich and used without further purification. 1,4-Bis(triethoxysilyl)benzene (BTEB) was synthesized according to a literature method [25]. 2.1. Synthesis of catalyst materials Mesoporous sulfonic acid-functionalized benzene-silica [PMO(benzene)] was prepared from BTEB in the presence of octadecyltrimethylammonium bromide (ODTMA-Br) surfactant as described in Ref. [14]. Sulfonation was performed using chlorosulfonic acid (1.5 g of benzene-silica, 25 ml of dichloromethane, 5.2 ml of ClSO2OH) at reflux temperature (1 h). After cooling to room temperature an excess of acetic acid was added to neutralize unreacted ClSO2OH then the product was washed with water until all soluble acidic components were removed. Further washing was carried out with: (i) 100 ml of H2O:THF (1:1), (ii) 100 ml of THF, (iii) 100 ml diethyl ether. Finally, the product was dried in vacuo (373 K, 3 h) [sample designation: PMO(benzene)-SO3H]. Two other samples were synthesized by co-condensation from BTEB and 3-mercaptopropyltrimethoxysilane (MPTMS) as described in Ref. [15]. Molar ratios of BTEB:MPTMS were 3:1 or 1:2. Oxidation of thiol groups was performed with H2O2 in a methanol–water mixture (1 g of solid, 2.04 g of aqueous 35% H2O2 dissolved in three parts of methanol, 333 K, 24 h). The suspension was filtered, washed with H2O and EtOH. The wet material was resuspended in 1 wt% H2SO4 for 4 h, washed with water extensively and dried in vacuo (373 K, 24 h) [sample designation: PMO(benzene)-PrSO3H(co)25, PMO(benzene)PrSO3H(co)67). 2.2. Sample characterization X-ray powder diffraction (XRD) data were acquired on a Philips PW-1820/1830 diffractometer using Cu Ka radiation. The data were collected from 08 to 708 (2u) with resolution of 0.028. Nitrogen adsorption and desorption isotherms were measured at 77 K using Quantachrome Nova 2000 system. The data were analyzed using the BJH model. The desorption curve was used to calculate the pore size distribution and pore volume. Solid state NMR spectra were recorded on a Bruker Avance spectrometer operating at 11.7 T magnetic field (13C: 125.7 MHz, 29Si: 99.3 MHz). Samples were packed in a 4 mm diameter ZrO2 rotor and were spun at 5 kHz speed. 29Si CP-MAS experiments were acquired with the following parameter sets: 1H 908 pulse was 4.0 ms; cross polarization contact time was 5 ms; 1H decoupling 62.5 kHz, 5 s repetition delay was applied and a 403 ppm spectral region was recorded with a carrier frequency placed at 0 ppm, the 1H frequency was positioned to 4.7 ppm. 1 K tranzients were recorded in 4 K
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complex data-points resulting in 51.3 ms acquisition time. 13C CP-MAS experiments were acquired with the following parameter sets: 1H 908 pulse was 4.0 ms; cross polarization contact time was 2 ms; 1H decoupling 62.5 kHz, 2 s repetition delay was applied and a 300 ppm spectral region was recorded with a carrier frequency placed at 110 ppm, the 1H frequency was positioned to 4.7 ppm. 1 K tranzients were recorded in 2 K complex data-points resulting in 27.3 ms acquisition time. Lorentzian window function (with 50 Hz broadening factor) was applied prior to all Fourier transformations. Spectra were referenced to tetramethylsilane (d13C = 0 ppm, d29Si = 0 ppm). Acid capacities were determined by acid–base titrations as described in Ref. [26]. In a typical measurement 0.1 g of solid was suspended in 10 ml of 0.1 M KCl. The suspension was stirred for 20 min and titrated with 0.2 M KOH in the presence of phenolphthalein.
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ratio = 1:2) was fed into a glass reactor holding 0.1 g of catalyst at 423 K (feeding rate = 0.2 ml h 1). The Fries rearrangement: phenyl acetate/phenol = 1:60, phenyl acetate to active site ratio = 11:1, 423 K, 5 h. The dimerization of 2-phenylpropene (a-methylstyrene, AMS): 0.650 ml (5 mmol) of AMS in 5 ml of toluene was stirred at 333 K in the presence of 0.05 g of catalyst. The products are 4-methyl-2,4-diphenylpent-1-ene and 4-methyl2,4-diphenylpent-2-ene, and the cyclic dimer 1,1,3-trimethyl-3phenylindan. Rearrangement–aromatization of ketoisophorone (KIP): ketoisophorone (0.244 ml, 1.4 mmol) was added dropwise to a stirred mixture of acetic anhydride (0.413 ml, 4.3 mmol) and 50 mg of catalyst (Ac2O/KIP molar ratio = 3) under nitrogen. Reactions were run at 318 K. 3. Results and discussion
2.3. Catalytic test reactions 3.1. Catalyst characterization Reactions were carried out in a round bottom flask equipped with a reflux condenser and stirred magnetically. A flow system was used in the alkylation of phenol with isopropyl alcohol. Temperatures were controlled with an accuracy of 3 K. Benzene and toluene used in Friedel–Crafts alkylation were stored over sodium wire. Capillary GC (Hewlett Packard 5980, 50-m HP1 column) was used for analysis. Product identification was made by using GC–MS (HP 5890 GC + HP 5970 mass selective detector), NMR (Bruker DRX 500 spectrometer at 500 MHz, liquid-phase 1H spectra), and with comparison with authentic samples. Conversion values collected in tables were reproducible within 5%. Friedel–Crafts alkylations: (i) benzyl alcohol (0.104 ml, 0.001 mol) was reacted with benzene (4.19 ml) or toluene (5 ml) at 357 K in the presence of 0.1 g of catalyst and decane as internal standard with magnetic stirring. Competitive studies were performed with benzyl alcohol (0.104 ml, 0.001 mol) and a mixture of benzene (1.78 ml) and toluene (2.13 ml). (ii) Alkylation of phenol with isopropyl alcohol was performed in a flow system. A mixture of phenol and isopropyl alcohol (molar
The powder XRD patterns of the surfactant-free hybrid material PMO(benzene) have three peaks in the low-angle diffraction regime (2u < 108) with d spacing at 4.67, 2.77 and 1.41 nm with a lattice constant of 5.39 nm (Fig. 1, inset). These are assigned to two-dimensional hexagonal symmetry [15]. At scattering angles 2u = 10–408 three additional peaks appear (d = 0.76, 0.37 and 0.22 nm) indicating molecular scale periodicity of the wall with spacing of 0.76 nm. Two peaks at low scattering angles disappear upon sulfonation [Fig. 1, sample PMO(benzene)-SO3H]. Peaks at medium scattering angles, in turn, are clearly visible, hence the 0.76 nm periodic structure of the wall is retained. Both samples prepared by co-condensation [PMO(benzene)-PrSH(co)25 and PMO(benzene)-PrSH(co)67] have a single diffraction peak at low angle (4.7 nm, d = 5.85 nm) [Fig. 2, only PMO(benzene)-PrSH(co)25 is shown]. At medium scattering angles, both have peaks at 0.76, 0.4 and 0.25 nm assigned to a periodic structure. Upon oxidation, with hydrogen peroxide, the XRD patters do not change significantly (Fig. 2).
Fig. 1. XRD patterns of PMO(benzene) (inset) and PMO(benzene)-SO3H.
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Fig. 2. XRD patterns of PMO(benzene)-PrSH(co)25 (A) and PMO(benzene)-PrSO3H(co)25 (B).
Table 1 Characterization of precursors and catalysts Catalyst
SBET (m2 g 1)
Pore diameter (nm)
Pore volume (cm3 g 1)
PMO(benzene) PMO(benzene)-SO3H PMO(benzene)-PrSH(co)25 PMO(benzene)-PrSO3H(co)25 PMO(benzene)-PrSH(co)67 PMO(benzene)-PrSO3H(co)67
1138 497 (471) 959 809 (851) 950 503
3.3 3.5 (3.6) 4.2 3.6 (1.6) 2.3 2.2
1.23 0.40 (0.47) 1.20 0.76 (0.66) 2.30 2.20
Evaluating these results, we can conclude that these mesoporous materials have the same periodic structure in the pore walls as reported previously [15,22]. Data with respect to textural properties (specific surface area, pore diameter and pore volume) are collected in Table 1. BET surface areas decrease by oxidation of the thiol function and, particularly, upon sulfonation of the benzene-silica parent structure. Nitrogen adsorption isotherms of all materials were
Acid capacity (mmol H+ g 1) 1.03 0.68
d100 47.0 47.1 47.1 48.3
1.29
of type IV, with an inflexion point (capillary condensation step) at p/p0 < 0.4 and 0.5 (as representative example, PMO(benzene)-PrSO3H(co)25 is shown in Fig. 3). Pore diameters change slightly on oxidation (Fig. 3, inset) but all functionalized materials have mesostructure with narrow pore diameter (Fig. 4). The presence of the organic functionalities of the solid samples was confirmed by 29Si and 13C NMR spectroscopies.
Fig. 3. Nitrogen isotherms (adsorption: shaded symbols, desorption: open symbols) and pore size distribution (inset) of surfactant-free PMO(benzene) materials made by co-condensation. Isotherms: PMO(benzene)-PrSO3H(co). Inset: open symbols, PMO(benzene)-PrSH(co); shaded symbols, PMO(benzene)-PrSO3H(co).
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Fig. 4. Pore size distribution of: (A) PMO(benzene)-SO3H and (B) PMO(benzene)-PrSO3H(co)25.
The cross polarized 29Si MAS spectrum of PMO(benzene) shows two intense peaks at 82.7 and 72.0 ppm which can be assigned to 2T and 3T sites for Si attached to phenylene (Fig. 5A) [15,24]. The absence of Q sites indicates that carbonsilicon bonds were not cleaved during synthesis. The signal for the bridging benzene ring appears at 134.3 ppm in the solid state cross polarized 13C-MAS spectrum (Fig. 5, spectrum B; PMO(benzene)-PrSO3H(co)67 is shown as representative example) [15,22,24]. Three intense signals can be assigned to the three carbon atoms of the propyl group (Fig. 5). The resonances appearing at 12.5 and 17.7 ppm belong to C1 attached to silicon and C2, respectively [15,21,22,24]. The signal at 54.5 ppm can be assigned to C3 bearing sulfonic acid
Fig. 5. (A)
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function [15,22,24]. The downfield shift of the signal from 22.1 ppm of carbon C3 bearing the SH function in sample PMO(benzene)-PrSH(co)67 testifies to the successful transformation upon oxidation to the sulfonic acid group. Additional signals (indicated by asterisk in Fig. 5B) appearing in the range between 30 and 40 ppm may belong to residual surfactant [22] and indicate incomplete oxidation of the thiol group [23]. Resonances at 16.9 and 56.4 ppm may be assigned to ethoxy groups formed during template removal [21,22]. The acid capacity of the sulfonic acid containing samples were measured by titration and data are given in Table 1. Sulfonation with chlorosulfonic acid is quite effective since it generates about the same concentration of acid sites as co-
Si CP-MAS spectrum of sample PMO(benzene) and (B)
13
C spectrum of PMO(benzene)-PrSO3H(co)67.
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condensation with an increased amount of MPTMS [compare PMO(benzene)-SO3H and PMO(benzene)-PrSO3H(co)67]. 3.2. Catalytic transformations First, the Friedel–Crafts alkylation of benzene and toluene with benzyl alcohol was studied to acquire information with respect to the general catalytic performance of the three catalyst materials. Data determined in both individual and competitive reactions are collected in Fig. 6. PMO(benzene)-SO3H with aromatic sulfonic acid functions shows the best performance whereas PMO(benzene)-PrSO3H(co)67 exhibits surprisingly low specific activities. Concerning both the activity of the three catalysts and the ktoluene/kbenzene ratios the results are very similar to those found over sulfonic acid modified mesoporous silicas [27]. Two other Friedel–Crafts-type reactions, the alkylation of phenol with isopropyl alcohol and the Fries rearrangement of phenyl acetate (Friedel–Crafts acylation) were also studied. PMO(benzene)-PrSO3H(co)25 exhibits the highest initial activity and gives the monoalkylated product with high selectivities in phenol alkylation (Table 2, entry 2). In longer runs, however, selectivities become very similar. Activities decrease slowly with increasing time-on-stream (Table 2; Fig. 7). Comparative studies were also performed with ordered mesoporous catalysts modified by propanesulfonic acid functions (Table 2, entries 4–6; Fig. 7). When initial conversions are considered, all three catalysts have a lower initial activity particularly HMS-PrSO3H(co). More importantly, they exhibit lower stability: activities measured at 14 h are well below those of the three PMO catalysts. This phenomenon has already been observed and attributed to the higher hydrothermal stability of the ethane-bridged PMO structure [21]. We have made an XRD study with the catalyst samples recovered after 14 h time-on-stream. Two characteristic X-ray
Fig. 6. Rate data and relative rates of Friedel–Crafts alkylation of benzene and toluene with benzyl alcohol (reaction time = 30 min). Shaded bars: rate data measured in individual reactions; empty bars: relative rates determined in competitive reactions.
Table 2 Catalytic performance of various ordered mesoporous silica materials functionalized with sulfonic acid groups in the Friedel–Crafts alkylation of phenol with isopropyl alcohol Entry
Catalyst
Acid capacitya
TOFb
Mono/dic
1 2
PMO(benzene)-SO3H PMO(benzene)PrSO3H(co)25 PMO(benzene)PrSO3H(co)67 MCM41-PrSO3H(co) HMS-PrSO3H(co) SBA15-PrSO3H(co)
1.03 0.68
3.95 (2.95) 5.28 (3.29)
0.7 (0.6) 13.5 (1.3)
1.29
2.41 (1.67)
1.4 (0.4)
1.10 0.74 1.07
2.90 (0.58) 2.90 (1.29) 2.91 (1.26)
0.13 (0.19) 2.5 (1.4) 0.2 (0.2)
3 4 5 6 a
mmol H+ g 1. mmolproduct active site 1 h 1 determined at 2 and 14 h. c Ratio of monoisopropylated to diisopropylated products determined at 2 and 14 h, respectively. b
diffractograms are shown in Fig. 8. Whereas the original features of catalyst PMO(benzene)-PrSO3H(co)25 found before reaction are practically preserved (Fig. 8, spectrum A), the disappearance of the ordered mesoporous structure for sample MCM41-PrSO3H(co) is evident (Fig. 8, spectrum B). Herewith, a clear proof is presented to the higher stability of PMO-based catalyst materials in catalytic application. Other physical characterization data determined for two of the PMO catalysts and MCM41-PrSO3H(co) recovered after reaction provide further proof to the higher stability of the PMO(benzene)-based catalysts. These catalyst samples exhibit only negligible changes in BET surface, pore diameter and pore volume with a single exception: the pore diameter of PMO(benzene)-PrSO3H(co)25 decreases significantly (see data in parenthesis in Table 1). In sharp contrast, there is a drastic drop of the BET surface and pore volume of MCM41PrSO3H(co): the original values of 1145 m2 g 1 and 0.83 cm3 g 1 [27], respectively, change to 255 m2 g 1 and 0.31 cm3 g 1. The pore diameter, in turn, remains almost unchanged (3.9 nm versus 3.3 nm). The activity and selectivity of PMO catalysts in the Fries rearrangement of phenyl acetate is compared to that of a homogeneous reaction induced by para-toluenesulfonic acid
Fig. 7. Change in the activity of PMO catalysts and functionalized mesoporous silicas in the Friedel–Crafts alkylation of phenol with isopropyl alcohol. ~, PMO(benzene)-SO3H; &, PMO(benzene)-PrSO3H(co)25; *, PMO(benzene)PrSO3H(co)67; *, MCM41-PrSO3H(co); +, HMS-PrSO3H(co); ~, SBA15PrSO3H(co).
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Fig. 8. XRD patterns of PMO(benzene)-SO3H(co)25 (A) and MCM41-PrSO3H(co) (B) recovered after the use in the Friedel–Crafts alkylation of phenol with isopropyl alcohol.
( p-TsOH). Among the solid acids the two samples with higher acid capacities [PMO(benzene)-SO3H, PMO(benzene)-PrSO3H(co)67] exhibit higher activities (Fig. 9). It is quite surprising that acid strength does not appear to affect catalyst performance in this case. Regioselectivities, in turn, differ significantly. High ortho selectivity is characteristic in the homogeneous reaction (Fig. 9). Over the solid acids selectivities depend significantly on time-on-stream. At the beginning high ortho selectivity is observed, whereas the para isomer becomes the main product with increasing reaction time. Changes in selectivities in micro- and mesoporous systems on fresh and aged catalysts are not unprecedented and welldocumented for the skeletal isomerization of butenes to isobutylene [28,29]. High isobutylene selectivity was observed over the aged catalyst, which was attributed to a change from a bimolecular to monomolecular mechanism due to the buildup of polymeric and carbonaceous materials. Similar changes were achieved by removal of non-shape selective acid sites and lowering the number of acid sites [30]. It may be surmised that a
change in mechanism, namely, an intramolecular acylation to an intermolecular process may take place in our case. Further, some shape selective effect cannot be ruled out either, even though the initial pore diameter values are too high for this phenomenon. A strong indication for this is that the highest para/ortho ratio is found for PMO(benzene)-PrSO3H(co)67, which has the smallest pore diameter (Table 1). Poisoning the surface active sites and buildup of polymeric materials inside the pores severely affect catalyst selectivities, which may be a limiting factor as reported earlier [31,32]. In our case, however, strong adsorption of reactants/products may result in the formation of an aged catalyst with such improved properties. Catalysts were further tested in the dimerization of amethylstyrene (AMS) (Scheme 1) and in the rearrangement– aromatization of ketoisophorone (KIP) (Scheme 2). As seen in Fig. 10, isomeric pentenes are formed as the main products in the dimerization of AMS over catalysts modified with propanesulfonic acid. In contrast, catalyst PMO(benzene)-SO3H bearing the stronger arenesulfonic acid surface function yields more
Scheme 1. Dimerization of a-methylstyrene (AMS).
Fig. 9. Activities and selectivities in the Fries rearrangement of phenyl acetate (reaction time = 24 h).
Scheme 2. Rearrangement–aromatization of ketoisophorone (KIP).
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Fig. 10. Activities and selectivities in the dimerization of AMS (reaction time = 24 h). P1: 4-methyl-2,4-diphenylpent-1-ene; P2: 4-methyl-2,4-diphenylpent-2-ene; indan: 1,1,3-trimethyl-3-phenylindan.
active electrophilic catalysts in various organic transformations. 2. The catalytic performance of the PMO-based samples exceeds markedly those of functionalized mesoporous ordered materials (MCM-41, HMS and SBA-15). The increased stability of PMO samples in flow reactor studies (alkylation of phenol in the gas-phase) is attributed to the higher stability of the benzene-bridged framework under reaction conditions as indicated by physical characterization data of the used catalysts. 3. Selectivities in the Fries rearrangement of phenyl acetate over the PMO-based catalysts differ significantly from that of the homogeneous reaction. A change in the mechanism and changes in catalyst characteristics due to strong adsorption of reactants/products may result in the formation of an aged catalyst with such improved properties. 4. The sample with benzenesulfonic acid surface functions [PMO(benzene)-SO3H] exhibits higher activities and different selectivities in the dimerization of 2-phenylpropene and in the rearrangement–aromatization of ketoisophorone as compared to samples functionalized with propanesulfonic acid groups. Acknowledgment We are most grateful for financial supports from the Hungarian National Science Foundation (OTKA Grants T042603, TS044690 and M041532). References
Fig. 11. Activities and selectivities in the rearrangement–aromatization of ketoisophorone (reaction time = 24 h). enAc: enol monoacetate; HQAc: 2,3,5-trimethylhydroquinone diacetate.
cyclic dimer indan derivative. A similar observation has been made for sulfonic acid functionalized mesoporous silicas [27]. The general picture for the rearrangement–aromatization of KIP is similar. The two products of this reaction require fairly different acidic sites: HQAc results from a more demanding reaction, whereas enAc is formed in a simple acetylation process. Indeed, PMO(benzene)-SO3H is the most active and yields the highest amount of HQAc (Fig. 11). In addition to the lower activity, the other two catalysts produce enAc with high selectivities. 4. Conclusions 1. Three solid acid samples of periodic ordered bridged mesoporous structure with a benzene ring as the rigid unit incorporated in the framework and functionalized with anchored sulfonic acid groups have been found to act as
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Sulfonic acid-functionalized phenylene-bridged periodic mesoporous organosilicas as catalyst materials ´ rpa´d Molna´r * Bulcsu´ Ra´c, Pe´ter Hegyes, Peter Forgo, A Department of Organic Chemistry, University of Szeged, Do´m te´r 8, Szeged, Hungary Received 30 May 2005; received in revised form 12 October 2005; accepted 13 October 2005 Available online 28 November 2005
Abstract Three solid acids based on bridged periodic mesoporous organosilica structure (PMO) with a benzene ring as the rigid unit incorporated in the framework and functionalized with anchored sulfonic acid groups were synthesized. Samples were prepared by either sol–gel polymerization of 1,4-bis(triethoxysilyl)benzene (BTEB) or co-condensation of BTEB and 3-mercaptopropyltrimethoxysilane (MPTMS) in the presence of octadecyltrimethylammonium bromide surfactant. Physical characterization data (X-ray powder diffraction, nitrogen adsorption and desorption, and NMR spectroscopy) and acid–base titration indicate the formation of ordered structure and successful functionalization. Catalytic properties were studied in both gas-phase and liquid-phase reactions. The catalytic performance of the PMO-based samples in the isopropylation of phenol in the gas-phase, particularly their stability, exceeds markedly those of functionalized mesoporous ordered materials (MCM-41, HMS and SBA-15). Selectivities in the Fries rearrangement of phenyl acetate over the PMO-based catalysts differ significantly from that of the homogeneous reaction. The sample with benzenesulfonic acid surface functions exhibits higher activities and different selectivities in the dimerization of 2-phenylpropene and in the rearrangement–aromatization of ketoisophorone as compared to samples functionalized with propanesulfonic acid groups. # 2005 Elsevier B.V. All rights reserved. Keywords: Periodic mesoporous organosilicas; Benzene bridged; Benzenesulfonic acid; Propanesulfonic acid; Phenol isopropylation; Fries rearrangement; Shape selectivity
1. Introduction A new method the so-called micelle templated synthesis developed recently [1,2] allows the preparation of silica materials of ordered mesoporous structure. Important characteristics are the narrow pore size distribution, very high surface area and a large number of surface silanol groups. The method creates numerous possibilities to synthesize new compositions. These mesoporous hybrid organic–inorganic materials have important and unique properties and varied application possibilities. Ordered mesoporous silicas, however, are particularly well-suited for the use as catalysts. The catalytically active material may be immobilized by encapsulation or through complex bonding or ionic interaction with suitable surfaces. Appropriately selected functional groups may
* Corresponding author. Tel.: +36 62 544 277; fax: +36 62 544 200. ´ . Molna´r). E-mail address: [email protected] (A 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.10.026
also be covalently bound to the surface. Successful applications have been disclosed and reviewed [3,4]. A unique group of hybrid organic–inorganic materials are bridged polysilsesquioxanes [5–7], which are prepared by sol– gel polymerization of bis-trialkoxyorganosilane monomers and have an organic functionality as part of the solid network. The resulting amorphous materials, which may have porous structure, are excellent candidates for application as optical and electronic devices, ceramics, adsorbents and catalyst supports. However, applying a structure directing agent during the sol–gel synthesis, a periodic mesoporous organosilica (PMO) is formed with the organic moiety uniformly incorporated in the framework. The micelle templated synthesis, in this case, improves structural order resulting in materials with highly uniform channels, hexagonally ordered pores and a narrow pore size distribution. With carefully chosen bridging organic moieties the materials have structural rigidity. This and the possibility to control hydrophilic–hydrophobic properties are particularly useful in catalytic applications. The periodic surface structure is advantageous because it can enable
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structural orientation of guest molecules and enhance selectivity and activity in catalysis [8]. Furthermore, cocondensation of bis-trialkoxyorganosilanes monomers and trialkoxyorganosilanes with suitable functional groups allows easy access to functionalized periodic ordered mesoporous silicas. By carefully choosing the precursor molecules a wide variety of organic species can be incorporated inside the channel walls. The organic bridging units incorporated into PMOs are ethane [9–11], ethene [10], benzene [12–15], biphenyl [16,17] and thiophene [13]. Materials with functional groups such as amino [18] and sulfonic acid [14–16,19] with potential for catalytic applications were also synthesized and characterized. The first example of the catalytic application of bridged polysilsesquioxanes was disclosed in 2002 using a benzenesilica xerogel [20]. The catalyst samples functionalized with various amine moieties proved to be active in the Knoevenagel condensation and compared well with aminopropyl modified silicas. There exist recent examples of bridged POMs with anchored sulfonic acid groups. First, thiol-functionalized samples were prepared by co-condensation or post-synthesis modification (grafting) followed by oxidation of the thiol group by H2O2. Co-condensations were performed by reacting 3-mercaptopropyltrimethoxysilane (MPTMS) with 1,4-bis(triethoxysilyl)benzene (BTEB) or 1,2-bis(triethoxysilyl)ethane in the presence of ionic (octadecyltrimethylammonium chloride) [21] or neutral (polyether) [22] templates. The ethane-bridged catalyst tested in the isopropylation of phenol exhibited much higher catalytic activity than ZSM-5 and higher stability than MCM-41 modified with the same group [21]. In the other study [22], ethane-bridged silicas displayed higher activity in the formation of bisphenol-A than their benzene-bridged counterparts. Samples made by grafting, in turn, were more active than those prepared by co-condensation. This was attributed to the irregular distribution of active sites. Grafted materials are known to have the active sites on the surface and near the pore mouth thereby being more accessible for the reactants. In a recent paper, the synthesis of ethane-bridged samples with increased amounts of MPTMS incorporated were described [23]. The samples showed increased acidities, but porosity, structural order and oxidation efficiency decreased. The catalyst made with 40% MPTMS (acid capacity = 0.98 H+ mmol g 1) was tested in two probe reactions. It showed activities comparable to Nafion-H both in esterification and ester hydrolysis. Recently, Inagaki et al. described the synthesis of PMOs with benzene [14] and biphenyl bridges [16,17] functionalized with sulfonic acid groups. The method used was cocondensation carried out in basic medium in the presence of cationic surfactant. However, there is only a recent report for the catalytic application of these materials in esterification [24]. The PMO functionalized with benzenesulfonic acid groups as described in Ref. [14], however, has not been used in catalytic reactions. The aim of the present study is, therefore, to prepare and characterize ordered mesoporous benzene-silica materials
with anchored propanesulfonic and benzenesulfonic acid groups and test their performance in various catalytic transformations. 2. Experimental All reagents were purchased from Aldrich and used without further purification. 1,4-Bis(triethoxysilyl)benzene (BTEB) was synthesized according to a literature method [25]. 2.1. Synthesis of catalyst materials Mesoporous sulfonic acid-functionalized benzene-silica [PMO(benzene)] was prepared from BTEB in the presence of octadecyltrimethylammonium bromide (ODTMA-Br) surfactant as described in Ref. [14]. Sulfonation was performed using chlorosulfonic acid (1.5 g of benzene-silica, 25 ml of dichloromethane, 5.2 ml of ClSO2OH) at reflux temperature (1 h). After cooling to room temperature an excess of acetic acid was added to neutralize unreacted ClSO2OH then the product was washed with water until all soluble acidic components were removed. Further washing was carried out with: (i) 100 ml of H2O:THF (1:1), (ii) 100 ml of THF, (iii) 100 ml diethyl ether. Finally, the product was dried in vacuo (373 K, 3 h) [sample designation: PMO(benzene)-SO3H]. Two other samples were synthesized by co-condensation from BTEB and 3-mercaptopropyltrimethoxysilane (MPTMS) as described in Ref. [15]. Molar ratios of BTEB:MPTMS were 3:1 or 1:2. Oxidation of thiol groups was performed with H2O2 in a methanol–water mixture (1 g of solid, 2.04 g of aqueous 35% H2O2 dissolved in three parts of methanol, 333 K, 24 h). The suspension was filtered, washed with H2O and EtOH. The wet material was resuspended in 1 wt% H2SO4 for 4 h, washed with water extensively and dried in vacuo (373 K, 24 h) [sample designation: PMO(benzene)-PrSO3H(co)25, PMO(benzene)PrSO3H(co)67). 2.2. Sample characterization X-ray powder diffraction (XRD) data were acquired on a Philips PW-1820/1830 diffractometer using Cu Ka radiation. The data were collected from 08 to 708 (2u) with resolution of 0.028. Nitrogen adsorption and desorption isotherms were measured at 77 K using Quantachrome Nova 2000 system. The data were analyzed using the BJH model. The desorption curve was used to calculate the pore size distribution and pore volume. Solid state NMR spectra were recorded on a Bruker Avance spectrometer operating at 11.7 T magnetic field (13C: 125.7 MHz, 29Si: 99.3 MHz). Samples were packed in a 4 mm diameter ZrO2 rotor and were spun at 5 kHz speed. 29Si CP-MAS experiments were acquired with the following parameter sets: 1H 908 pulse was 4.0 ms; cross polarization contact time was 5 ms; 1H decoupling 62.5 kHz, 5 s repetition delay was applied and a 403 ppm spectral region was recorded with a carrier frequency placed at 0 ppm, the 1H frequency was positioned to 4.7 ppm. 1 K tranzients were recorded in 4 K
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complex data-points resulting in 51.3 ms acquisition time. 13C CP-MAS experiments were acquired with the following parameter sets: 1H 908 pulse was 4.0 ms; cross polarization contact time was 2 ms; 1H decoupling 62.5 kHz, 2 s repetition delay was applied and a 300 ppm spectral region was recorded with a carrier frequency placed at 110 ppm, the 1H frequency was positioned to 4.7 ppm. 1 K tranzients were recorded in 2 K complex data-points resulting in 27.3 ms acquisition time. Lorentzian window function (with 50 Hz broadening factor) was applied prior to all Fourier transformations. Spectra were referenced to tetramethylsilane (d13C = 0 ppm, d29Si = 0 ppm). Acid capacities were determined by acid–base titrations as described in Ref. [26]. In a typical measurement 0.1 g of solid was suspended in 10 ml of 0.1 M KCl. The suspension was stirred for 20 min and titrated with 0.2 M KOH in the presence of phenolphthalein.
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ratio = 1:2) was fed into a glass reactor holding 0.1 g of catalyst at 423 K (feeding rate = 0.2 ml h 1). The Fries rearrangement: phenyl acetate/phenol = 1:60, phenyl acetate to active site ratio = 11:1, 423 K, 5 h. The dimerization of 2-phenylpropene (a-methylstyrene, AMS): 0.650 ml (5 mmol) of AMS in 5 ml of toluene was stirred at 333 K in the presence of 0.05 g of catalyst. The products are 4-methyl-2,4-diphenylpent-1-ene and 4-methyl2,4-diphenylpent-2-ene, and the cyclic dimer 1,1,3-trimethyl-3phenylindan. Rearrangement–aromatization of ketoisophorone (KIP): ketoisophorone (0.244 ml, 1.4 mmol) was added dropwise to a stirred mixture of acetic anhydride (0.413 ml, 4.3 mmol) and 50 mg of catalyst (Ac2O/KIP molar ratio = 3) under nitrogen. Reactions were run at 318 K. 3. Results and discussion
2.3. Catalytic test reactions 3.1. Catalyst characterization Reactions were carried out in a round bottom flask equipped with a reflux condenser and stirred magnetically. A flow system was used in the alkylation of phenol with isopropyl alcohol. Temperatures were controlled with an accuracy of 3 K. Benzene and toluene used in Friedel–Crafts alkylation were stored over sodium wire. Capillary GC (Hewlett Packard 5980, 50-m HP1 column) was used for analysis. Product identification was made by using GC–MS (HP 5890 GC + HP 5970 mass selective detector), NMR (Bruker DRX 500 spectrometer at 500 MHz, liquid-phase 1H spectra), and with comparison with authentic samples. Conversion values collected in tables were reproducible within 5%. Friedel–Crafts alkylations: (i) benzyl alcohol (0.104 ml, 0.001 mol) was reacted with benzene (4.19 ml) or toluene (5 ml) at 357 K in the presence of 0.1 g of catalyst and decane as internal standard with magnetic stirring. Competitive studies were performed with benzyl alcohol (0.104 ml, 0.001 mol) and a mixture of benzene (1.78 ml) and toluene (2.13 ml). (ii) Alkylation of phenol with isopropyl alcohol was performed in a flow system. A mixture of phenol and isopropyl alcohol (molar
The powder XRD patterns of the surfactant-free hybrid material PMO(benzene) have three peaks in the low-angle diffraction regime (2u < 108) with d spacing at 4.67, 2.77 and 1.41 nm with a lattice constant of 5.39 nm (Fig. 1, inset). These are assigned to two-dimensional hexagonal symmetry [15]. At scattering angles 2u = 10–408 three additional peaks appear (d = 0.76, 0.37 and 0.22 nm) indicating molecular scale periodicity of the wall with spacing of 0.76 nm. Two peaks at low scattering angles disappear upon sulfonation [Fig. 1, sample PMO(benzene)-SO3H]. Peaks at medium scattering angles, in turn, are clearly visible, hence the 0.76 nm periodic structure of the wall is retained. Both samples prepared by co-condensation [PMO(benzene)-PrSH(co)25 and PMO(benzene)-PrSH(co)67] have a single diffraction peak at low angle (4.7 nm, d = 5.85 nm) [Fig. 2, only PMO(benzene)-PrSH(co)25 is shown]. At medium scattering angles, both have peaks at 0.76, 0.4 and 0.25 nm assigned to a periodic structure. Upon oxidation, with hydrogen peroxide, the XRD patters do not change significantly (Fig. 2).
Fig. 1. XRD patterns of PMO(benzene) (inset) and PMO(benzene)-SO3H.
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Fig. 2. XRD patterns of PMO(benzene)-PrSH(co)25 (A) and PMO(benzene)-PrSO3H(co)25 (B).
Table 1 Characterization of precursors and catalysts Catalyst
SBET (m2 g 1)
Pore diameter (nm)
Pore volume (cm3 g 1)
PMO(benzene) PMO(benzene)-SO3H PMO(benzene)-PrSH(co)25 PMO(benzene)-PrSO3H(co)25 PMO(benzene)-PrSH(co)67 PMO(benzene)-PrSO3H(co)67
1138 497 (471) 959 809 (851) 950 503
3.3 3.5 (3.6) 4.2 3.6 (1.6) 2.3 2.2
1.23 0.40 (0.47) 1.20 0.76 (0.66) 2.30 2.20
Evaluating these results, we can conclude that these mesoporous materials have the same periodic structure in the pore walls as reported previously [15,22]. Data with respect to textural properties (specific surface area, pore diameter and pore volume) are collected in Table 1. BET surface areas decrease by oxidation of the thiol function and, particularly, upon sulfonation of the benzene-silica parent structure. Nitrogen adsorption isotherms of all materials were
Acid capacity (mmol H+ g 1) 1.03 0.68
d100 47.0 47.1 47.1 48.3
1.29
of type IV, with an inflexion point (capillary condensation step) at p/p0 < 0.4 and 0.5 (as representative example, PMO(benzene)-PrSO3H(co)25 is shown in Fig. 3). Pore diameters change slightly on oxidation (Fig. 3, inset) but all functionalized materials have mesostructure with narrow pore diameter (Fig. 4). The presence of the organic functionalities of the solid samples was confirmed by 29Si and 13C NMR spectroscopies.
Fig. 3. Nitrogen isotherms (adsorption: shaded symbols, desorption: open symbols) and pore size distribution (inset) of surfactant-free PMO(benzene) materials made by co-condensation. Isotherms: PMO(benzene)-PrSO3H(co). Inset: open symbols, PMO(benzene)-PrSH(co); shaded symbols, PMO(benzene)-PrSO3H(co).
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Fig. 4. Pore size distribution of: (A) PMO(benzene)-SO3H and (B) PMO(benzene)-PrSO3H(co)25.
The cross polarized 29Si MAS spectrum of PMO(benzene) shows two intense peaks at 82.7 and 72.0 ppm which can be assigned to 2T and 3T sites for Si attached to phenylene (Fig. 5A) [15,24]. The absence of Q sites indicates that carbonsilicon bonds were not cleaved during synthesis. The signal for the bridging benzene ring appears at 134.3 ppm in the solid state cross polarized 13C-MAS spectrum (Fig. 5, spectrum B; PMO(benzene)-PrSO3H(co)67 is shown as representative example) [15,22,24]. Three intense signals can be assigned to the three carbon atoms of the propyl group (Fig. 5). The resonances appearing at 12.5 and 17.7 ppm belong to C1 attached to silicon and C2, respectively [15,21,22,24]. The signal at 54.5 ppm can be assigned to C3 bearing sulfonic acid
Fig. 5. (A)
29
function [15,22,24]. The downfield shift of the signal from 22.1 ppm of carbon C3 bearing the SH function in sample PMO(benzene)-PrSH(co)67 testifies to the successful transformation upon oxidation to the sulfonic acid group. Additional signals (indicated by asterisk in Fig. 5B) appearing in the range between 30 and 40 ppm may belong to residual surfactant [22] and indicate incomplete oxidation of the thiol group [23]. Resonances at 16.9 and 56.4 ppm may be assigned to ethoxy groups formed during template removal [21,22]. The acid capacity of the sulfonic acid containing samples were measured by titration and data are given in Table 1. Sulfonation with chlorosulfonic acid is quite effective since it generates about the same concentration of acid sites as co-
Si CP-MAS spectrum of sample PMO(benzene) and (B)
13
C spectrum of PMO(benzene)-PrSO3H(co)67.
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condensation with an increased amount of MPTMS [compare PMO(benzene)-SO3H and PMO(benzene)-PrSO3H(co)67]. 3.2. Catalytic transformations First, the Friedel–Crafts alkylation of benzene and toluene with benzyl alcohol was studied to acquire information with respect to the general catalytic performance of the three catalyst materials. Data determined in both individual and competitive reactions are collected in Fig. 6. PMO(benzene)-SO3H with aromatic sulfonic acid functions shows the best performance whereas PMO(benzene)-PrSO3H(co)67 exhibits surprisingly low specific activities. Concerning both the activity of the three catalysts and the ktoluene/kbenzene ratios the results are very similar to those found over sulfonic acid modified mesoporous silicas [27]. Two other Friedel–Crafts-type reactions, the alkylation of phenol with isopropyl alcohol and the Fries rearrangement of phenyl acetate (Friedel–Crafts acylation) were also studied. PMO(benzene)-PrSO3H(co)25 exhibits the highest initial activity and gives the monoalkylated product with high selectivities in phenol alkylation (Table 2, entry 2). In longer runs, however, selectivities become very similar. Activities decrease slowly with increasing time-on-stream (Table 2; Fig. 7). Comparative studies were also performed with ordered mesoporous catalysts modified by propanesulfonic acid functions (Table 2, entries 4–6; Fig. 7). When initial conversions are considered, all three catalysts have a lower initial activity particularly HMS-PrSO3H(co). More importantly, they exhibit lower stability: activities measured at 14 h are well below those of the three PMO catalysts. This phenomenon has already been observed and attributed to the higher hydrothermal stability of the ethane-bridged PMO structure [21]. We have made an XRD study with the catalyst samples recovered after 14 h time-on-stream. Two characteristic X-ray
Fig. 6. Rate data and relative rates of Friedel–Crafts alkylation of benzene and toluene with benzyl alcohol (reaction time = 30 min). Shaded bars: rate data measured in individual reactions; empty bars: relative rates determined in competitive reactions.
Table 2 Catalytic performance of various ordered mesoporous silica materials functionalized with sulfonic acid groups in the Friedel–Crafts alkylation of phenol with isopropyl alcohol Entry
Catalyst
Acid capacitya
TOFb
Mono/dic
1 2
PMO(benzene)-SO3H PMO(benzene)PrSO3H(co)25 PMO(benzene)PrSO3H(co)67 MCM41-PrSO3H(co) HMS-PrSO3H(co) SBA15-PrSO3H(co)
1.03 0.68
3.95 (2.95) 5.28 (3.29)
0.7 (0.6) 13.5 (1.3)
1.29
2.41 (1.67)
1.4 (0.4)
1.10 0.74 1.07
2.90 (0.58) 2.90 (1.29) 2.91 (1.26)
0.13 (0.19) 2.5 (1.4) 0.2 (0.2)
3 4 5 6 a
mmol H+ g 1. mmolproduct active site 1 h 1 determined at 2 and 14 h. c Ratio of monoisopropylated to diisopropylated products determined at 2 and 14 h, respectively. b
diffractograms are shown in Fig. 8. Whereas the original features of catalyst PMO(benzene)-PrSO3H(co)25 found before reaction are practically preserved (Fig. 8, spectrum A), the disappearance of the ordered mesoporous structure for sample MCM41-PrSO3H(co) is evident (Fig. 8, spectrum B). Herewith, a clear proof is presented to the higher stability of PMO-based catalyst materials in catalytic application. Other physical characterization data determined for two of the PMO catalysts and MCM41-PrSO3H(co) recovered after reaction provide further proof to the higher stability of the PMO(benzene)-based catalysts. These catalyst samples exhibit only negligible changes in BET surface, pore diameter and pore volume with a single exception: the pore diameter of PMO(benzene)-PrSO3H(co)25 decreases significantly (see data in parenthesis in Table 1). In sharp contrast, there is a drastic drop of the BET surface and pore volume of MCM41PrSO3H(co): the original values of 1145 m2 g 1 and 0.83 cm3 g 1 [27], respectively, change to 255 m2 g 1 and 0.31 cm3 g 1. The pore diameter, in turn, remains almost unchanged (3.9 nm versus 3.3 nm). The activity and selectivity of PMO catalysts in the Fries rearrangement of phenyl acetate is compared to that of a homogeneous reaction induced by para-toluenesulfonic acid
Fig. 7. Change in the activity of PMO catalysts and functionalized mesoporous silicas in the Friedel–Crafts alkylation of phenol with isopropyl alcohol. ~, PMO(benzene)-SO3H; &, PMO(benzene)-PrSO3H(co)25; *, PMO(benzene)PrSO3H(co)67; *, MCM41-PrSO3H(co); +, HMS-PrSO3H(co); ~, SBA15PrSO3H(co).
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Fig. 8. XRD patterns of PMO(benzene)-SO3H(co)25 (A) and MCM41-PrSO3H(co) (B) recovered after the use in the Friedel–Crafts alkylation of phenol with isopropyl alcohol.
( p-TsOH). Among the solid acids the two samples with higher acid capacities [PMO(benzene)-SO3H, PMO(benzene)-PrSO3H(co)67] exhibit higher activities (Fig. 9). It is quite surprising that acid strength does not appear to affect catalyst performance in this case. Regioselectivities, in turn, differ significantly. High ortho selectivity is characteristic in the homogeneous reaction (Fig. 9). Over the solid acids selectivities depend significantly on time-on-stream. At the beginning high ortho selectivity is observed, whereas the para isomer becomes the main product with increasing reaction time. Changes in selectivities in micro- and mesoporous systems on fresh and aged catalysts are not unprecedented and welldocumented for the skeletal isomerization of butenes to isobutylene [28,29]. High isobutylene selectivity was observed over the aged catalyst, which was attributed to a change from a bimolecular to monomolecular mechanism due to the buildup of polymeric and carbonaceous materials. Similar changes were achieved by removal of non-shape selective acid sites and lowering the number of acid sites [30]. It may be surmised that a
change in mechanism, namely, an intramolecular acylation to an intermolecular process may take place in our case. Further, some shape selective effect cannot be ruled out either, even though the initial pore diameter values are too high for this phenomenon. A strong indication for this is that the highest para/ortho ratio is found for PMO(benzene)-PrSO3H(co)67, which has the smallest pore diameter (Table 1). Poisoning the surface active sites and buildup of polymeric materials inside the pores severely affect catalyst selectivities, which may be a limiting factor as reported earlier [31,32]. In our case, however, strong adsorption of reactants/products may result in the formation of an aged catalyst with such improved properties. Catalysts were further tested in the dimerization of amethylstyrene (AMS) (Scheme 1) and in the rearrangement– aromatization of ketoisophorone (KIP) (Scheme 2). As seen in Fig. 10, isomeric pentenes are formed as the main products in the dimerization of AMS over catalysts modified with propanesulfonic acid. In contrast, catalyst PMO(benzene)-SO3H bearing the stronger arenesulfonic acid surface function yields more
Scheme 1. Dimerization of a-methylstyrene (AMS).
Fig. 9. Activities and selectivities in the Fries rearrangement of phenyl acetate (reaction time = 24 h).
Scheme 2. Rearrangement–aromatization of ketoisophorone (KIP).
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Fig. 10. Activities and selectivities in the dimerization of AMS (reaction time = 24 h). P1: 4-methyl-2,4-diphenylpent-1-ene; P2: 4-methyl-2,4-diphenylpent-2-ene; indan: 1,1,3-trimethyl-3-phenylindan.
active electrophilic catalysts in various organic transformations. 2. The catalytic performance of the PMO-based samples exceeds markedly those of functionalized mesoporous ordered materials (MCM-41, HMS and SBA-15). The increased stability of PMO samples in flow reactor studies (alkylation of phenol in the gas-phase) is attributed to the higher stability of the benzene-bridged framework under reaction conditions as indicated by physical characterization data of the used catalysts. 3. Selectivities in the Fries rearrangement of phenyl acetate over the PMO-based catalysts differ significantly from that of the homogeneous reaction. A change in the mechanism and changes in catalyst characteristics due to strong adsorption of reactants/products may result in the formation of an aged catalyst with such improved properties. 4. The sample with benzenesulfonic acid surface functions [PMO(benzene)-SO3H] exhibits higher activities and different selectivities in the dimerization of 2-phenylpropene and in the rearrangement–aromatization of ketoisophorone as compared to samples functionalized with propanesulfonic acid groups. Acknowledgment We are most grateful for financial supports from the Hungarian National Science Foundation (OTKA Grants T042603, TS044690 and M041532). References
Fig. 11. Activities and selectivities in the rearrangement–aromatization of ketoisophorone (reaction time = 24 h). enAc: enol monoacetate; HQAc: 2,3,5-trimethylhydroquinone diacetate.
cyclic dimer indan derivative. A similar observation has been made for sulfonic acid functionalized mesoporous silicas [27]. The general picture for the rearrangement–aromatization of KIP is similar. The two products of this reaction require fairly different acidic sites: HQAc results from a more demanding reaction, whereas enAc is formed in a simple acetylation process. Indeed, PMO(benzene)-SO3H is the most active and yields the highest amount of HQAc (Fig. 11). In addition to the lower activity, the other two catalysts produce enAc with high selectivities. 4. Conclusions 1. Three solid acid samples of periodic ordered bridged mesoporous structure with a benzene ring as the rigid unit incorporated in the framework and functionalized with anchored sulfonic acid groups have been found to act as
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