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SiO2-supported dodecatungstophosphoric acid and Nafion-H prepared by ball-milling for catalytic application
Applied Catalysis A: General 282 (2005) 255–265 www.elsevier.com/locate/apcata
SiO2-supported dodecatungstophosphoric acid and Nafion-H prepared by ball-milling for catalytic application ´ rpa´d Molna´ra,* Bulcsu´ Ra´ca, Gabriele Mulasb, Aniko´ Csongra´dia, Katalin Lo´kia, A a
Department of Organic Chemistry, University of Szeged, Do´m te´r 8, Szeged, Hungary Dipartimento di Chimica, Universita´ di Sassari, Via Vienna 2, I-07100 Sassari, Italy
b
Received 18 November 2004; received in revised form 15 December 2004; accepted 15 December 2004 Available online 18 January 2005
Abstract Dodecatungstophosphoric acid (HPW) and Nafion-H were supported on SiO2 using the ball-milling technique. Samples were characterized by physical (BET surface, SEM, DSC, XRD, DRIFT and Raman spectroscopies) and chemical (acid–base titration) methods. Catalytic properties were studied in a range of chemical transformations, which can be induced by electrophilic catalysts including Friedel– Crafts reactions (alkylation of toluene with benzyl alcohol, acylation of anisole with acetic anhydride) and the dimerization of amethylstyrene. Milling conditions (stainless steel or polystyrene apparatus, milling time), loading of the active components, and the type of support were found to have profound effects on the physical characteristics and catalytic performance of the samples. Ball-milled HPWSiO2 samples prepared under appropriately selected conditions exhibit catalytic properties similar to catalysts made by impregnation. HPW in ball-milled samples, in turn, proved to be more resistant to extraction with polar solvents (hot methanol) and these catalysts display higher stability in recycling studies. Supported Nafion-H samples prepared by ball-milling show high activities in test reactions. The best catalyst exceeds SAC-13 in the selective synthesis of 1,1,3-trimethyl-3-phenylindane in the dimerization of 2-phenylpropene. Catalysts with high catalytic performance require the use of optimal milling time, because particle coalescence and the resulting decrease in BET surface area hinder the accessibility of the active sites. # 2005 Elsevier B.V. All rights reserved. Keywords: Dodecatungstophosphoric acid; Nafion-H; Ball-milling; Friedel–Crafts alkylation; Friedel–Crafts acylation; Dimerization of 2-phenylpropene; Catalyst recycling
1. Introduction The use of solid (heterogeneous) catalysts in organic synthesis and in the industrial manufacture of chemicals is of increasing importance since they provide green alternatives to homogeneous catalysts. This is particularly important in acid catalysis where solid acids can be the safe alternatives to hazardous and corrosive materials such as hydrofluoric acid, sulfuric acid and nitric acid. In addition to large-scale industrial use, heterogeneous catalysts have already found extensive application in fine chemical synthesis [1] and, among others, protection/deprotection processes in organic synthesis [2]. * Corresponding author. Tel.: +36 62 544277; fax: +36 62 544200. ´ . 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.2004.12.020
Heteropoly compounds (heteropoly acids and salts) and Nafion resins, that is perfluorinated resinsulfonic acid based ion-exchanged polymers, are two important classes of catalysts with highly acidic character. Heteropoly acids, when used in non-polar solvents, that is, when applied as bulk catalysts, are weak superacids. Similarly, Nafion resins exhibit an acidic character comparable to that of 100% sulfuric acid. Both heteropoly acids [3–7] and Nafion-H [8,9] have been widely used in organic synthesis. However, heteropoly acids and Nafion resins have a common disadvantage. The surface area of bulk heteropoly acids is only a few m2 g 1. Nafion resins have an even lower surface area (0.02 m2 g 1); moreover, most of the active sites are inaccessible to reacting molecules. As a result, the specific activities of these catalytic materials, namely the number of reacting molecules on an active site, are very low.
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Attempts have recently been made to overcome this problem by the preparation of silica nanocomposites by using sol–gel synthesis. New, highly active catalysts have been prepared by the immobilization of the acidic components into a silica matrix. The heteropoly acid–silica nanocomposites have BET surface areas of a few hundred m2 g 1 and exhibit higher specific activities and proved to be recyclable [10–12]. Harmer et al. reported a similar method to prepare Nafion–silica nanocomposite materials [13]. These were shown to have large surface area (150– 500 m2 g 1) and contain small (<100 nm) Nafion particles entrapped in a porous silica framework [13]. Outstanding catalytic performance was found in further studies [13–16] and demonstrated recently with a commercially available catalyst called Nafion-H SAC-13 [17–20]. Mechanical treatment, more specifically, high-energy ball-milling has long been known as a powerful and useful technique to fabricate solid materials with unique properties. It is most often applied in mechanical alloying [21] and has recently gained interest in the synthesis of catalyst materials. The method allows the preparation of unique catalyst materials characterized by high specific activity due to the presence of nano-size particles [22–29]. In fact, there is a recent example of the use of mechanical treatment in heteropoly acid chemistry. Vedrine et al. dispersed HPW on its neutral Cs salt by grinding to produce high surface area materials, which exhibited much higher catalytic activity for butane isomerization [30]. On the basis of our recent experiences in this field [31,32] we now used mechanochemical synthesis as an alternative procedure to fabricate dodecatungstophosphoric acid (HPW) and Nafion-H catalysts supported on SiO2.
2. Experimental 2.1.1. Materials All reagents and dodecatungstophosphoric acid (HPW) were purchased from Aldrich and had purities at least 98%. Nafion-H was a DuPont product and Nafion-H SAC-13 (Nafion-H content = 10–20%) was produced by Aldrich. High surface area silica gel (Fluka, Silica gel 100, 410 m2 g 1) and low surface area silicon dioxide (Aldrich, 325 mesh, 10 m2 g 1) were used as support materials. Toluene used in Friedel–Crafts alkylations was stored over sodium wire. 2.2. Sample preparation Two techniques, mechanochemical ball-milling and impregnation were applied in catalyst preparation. A mixture of the active catalytic material and SiO2 were milled for various times in either a stainless steel mill with two steel balls (1 g each) or in a polystyrene mill with three polystyrene balls (0.5 g each). In a typical run 5 g of
material mixture were used. All manipulations were performed under air atmosphere. Catalysts were also prepared by impregnation in order to compare the effect of the different synthesis methods. In a typical synthesis, 4.5 g of SiO2 was suspended in the solution of 0.5 g of HPW in 17 ml of distilled water. Water was slowly removed by means of a rotary evaporator at 298 K and the catalyst was dried in vacuo at 443 K for 4 h. The code X-a/b-Y is used to denote samples. X gives the nominal loading (in wt.%) of the active ingredient, ‘a’ indicates the type of support used (F: Fluka silica gel, A: Aldrich SiO2), ‘b’ shows the type of milling apparatus (SS: stainless steel, PS: polystyrene) or indicates impregnation (I), and Y gives milling time (h). 2.3. Sample characterization X-ray powder diffraction (XRD) data were acquired using a Siemens D500 apparatus. Data were collected in the angular range 10–1008 applying Cu Ka radiation. Thermal stability was followed by differential scanning calorimetry (DSC). Thermograms were recorded using a Perkin-Elmer DSC7 apparatus at a scanning rate of 208 min 1. SEM–EDX analyses were carried out using a Jeol JSM-5600LV microscope operating at 20 kV equipped with an Oxford Instrument 6587 microprobe. Powder morphology was studied by the secondary electrons emitted by the samples. Compositional and elemental investigations were performed by backscattered electrons and EDX analysis, respectively. Powder morphological measurements were carried out using a Jeol JSM-5600LV scanning electron microscope, operating at 20 kV analyzing the secondary electrons emitted by the samples. Sample holders were covered by a flat graphite surface over which powders were deposited. Nitrogen adsorption and desorption isotherms were measured at 77 K using a Quantachrome Nova 2000 system (pretreatment: 423 K, 1 h, in vacuo). Data were analyzed using the BJH model. Diffuse reflectance IR measurements were made by a BioRad DigiLab FTS 65A/896 spectrometer equipped with a DTGS detector. Spectra were recorded between 400 and 4000 cm 1 with resolution of 4 cm 1 using the KBr pellet technique (0.015 g of catalyst sample and 300 mg of KBr). Raman measurements were carried out on a BioRad DigiLab FT Raman spectrometer. Spectra were recorded between 0 and 4000 cm 1. Acid–base titrations as described in [33] were used to measure acid capacity that is the quantity of catalytically active ingredient of samples. In a typical measurement 0.5 g of solid was suspended in 50 ml of 0.1 M KCl. The suspension was stirred for 20 min and titrated with 0.2 M KOH in the presence of thymol blue. Soxhlet extraction was carried out in a specially designed extractor. An amount of 1.0 g of HPW-containing catalyst was extracted with 60.0 ml of methanol. Samples were taken at given intervals and the quantity of dissolved HPW was
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determined using an 8485A HP Diode Array UV spectrophotometer.
Table 1 Surface characterization of HPW samples Material
Acid capacity (mmol H+ g 1)
Surface areaa (m2 g 1)
85-A/SS-15 85-A/PS-7.5 85-A/PS-15 85-A/PS-30 10-A/PS-15 10-F/PS-15 10-F/PS-30 10-F/PS-50 10-A/I 10-F/I 20-F/I 100-/PS-15 Bulk HPW
4.61 6.69 6.50 6.17 0.53 0.86 (0.39)b [0.20]c 0.56 0.55 0.91 0.75 (0.22)b [0.14]c 1.40b 8.32 8.36
0.8 4 2.4 0.5 3.0 296 264 210 1.3 300 289 – –
2.4. Catalytic reactions All reactions were carried out in a round bottom flask equipped with a reflux condenser and stirred magnetically. Temperatures were controlled with an accuracy of 3 K. Catalyst quantities were 0.05 g (for supported samples), 0.005 g (for bulk HPW samples), or 0.0075 g (for bulk Nafion). The support materials alone did not show any activity in the reactions studied. Friedel–Crafts alkylation was carried out by reacting toluene with benzyl alcohol [4 ml of toluene, 0.104 ml (0.001 mol) of benzyl alcohol] in the presence of decane as internal standard. Reaction temperature = 373 K for HPW samples and 384 K for Nafion samples. The products are a mixture of phenyl-ortho-tolylmethane and phenyl-paratolylmethane with low amounts of dibenzyl ether as byproduct. Friedel–Crafts acylation was performed by reacting anisole with acetic anhydride at 423 K [4.1 ml of anisole, 0.094 ml (0.001 mol) of acetic anhydride in the presence of catalyst and decane as internal standard]. The product is 4-methoxyacetophenone and a small amount of 2methoxyacetophenone (amounting less than 5%) is also formed. Dimerization of 2-phenylpropene (a-methylstyrene, AMS) was carried out by reacting AMS in toluene at 333 K [5.0 ml of toluene, 0.650 ml (0.005 mol) of AMS]. The products are 4-methyl-2,4-diphenylpent-1-ene and 4methyl-2,4-diphenylpent-2-ene, and the cyclic dimer 1,1,3trimethyl-3-phenylindane. Samples withdrawn at certain time intervals from the reaction mixtures were analyzed by capillary GC (Hewlett Packard 5980, 50 m HP1 column, flame ionization detector) and TLC. Product identification was performed by means of GC–MS (HP 5890 GC coupled with a HP 5970 mass selective detector), by NMR spectroscopy [Bruker DRX 500 spectrometer at 500 MHz, CDCl3 as solvent and tetramethylsilane (TMS) as an internal reference] and by comparison with authentic samples. All products are known compounds and gave satisfactory spectral data.
3. Results and discussion 3.1. Supported HPW materials 3.1.1. Sample characterization The results of acid–base titration and BET surface areas of catalyst samples prepared by various methods are collected in Table 1. Acid–base titrations give quantitative information of HPW content. Noteworthy is the decreased value found for 85-A/SS-15 the only sample prepared in a stainless steel mill.
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a Surface area of the supports used: Fluka silica gel (F) = 410 m2 g 1, Aldrich SiO2 (A) = 10 m2 g 1. b Values in parenthesis: after extraction. c Values in brackets: after 10 runs in recycling studies.
With respect to BET surface areas the values decrease with increasing milling time in all cases compared to the corresponding support. The decrease is particularly significant for 85-A/SS-15 the (sample prepared by a stainless steel mill). A similar decrease for samples made in a polystyrene mill is observed only after prolonged milling (85-A/PS-30). Less significant changes are found for samples prepared by using Fluka silica. These changes may result from the aggregation of particles [34] and the loss of porous silica structure. Scanning electron micrographs show a wide range of particles with various sizes (Fig. 1, upper left picture). After prolonged milling larger particles appear but there is no significant change in particle size distribution. This indicates that the decreasing BET surface areas are mainly due to the loss of porous silica structure. Impregnation has a similar although less pronounced effect. The decrease in surface area, in this case, can be attributed to the filling up the pores by HPW. It is noteworthy that according to EDX results the active phase is homogeneously distributed in the silica matrix (Fig. 1). XRD patterns of samples with high HPW loading prepared using a stainless steel or a polystyrene mill are shown in Fig. 2. All reflections characteristic of HPW appear in the XRD pattern of sample 85-A/PS-15, whereas sample 85-A/SS-15 is almost featureless indicating only the presence of W3O8. Differences can also be observed in DRIFT spectra of the three samples (not shown). There are four bands in the fingerprint region characteristic of pure HPW around 1083, 983, 893 and 810 cm 1 assigned to the antisymmetrical stretching vibration of P–O, W=O, W–O– Wcorner and W–O–Wedge, respectively [35–37]. These bands can be observed in sample 85-A/SS-15, however, they are shifted to lower wave numbers. It is clear from these comparisons that the heavy milling conditions induced by
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Fig. 1. SEM (upper left picture) and EDX results of sample 10-F/PS-15.
the stainless steel mill bring about the decomposition of the Keggin structure. Characteristic XRD features of supported HPW samples with lower loading prepared by milling in a polystyrene mill are a broad halo or a strong, sharp reflection at 2u = 268 attributed to amorphous silica (Fluka) and silicon dioxide (Aldrich), respectively. The peaks of HPW are not visible, which is due to the low concentration of HPW. This is consistent with recent observations [37] that characteristic peaks of silica-supported HPW do not appear below loadings of 20 wt.%. The presence of HPW, however, can be detected by DRIFT and Raman spectroscopies. Although the broad and intensive band of silica at around 1100 cm 1 overlaps the absorption band of HPW at 1083 cm 1 (stretching vibration of P–O), the other three bands characteristic of HPW can be
Fig. 2. XRD patterns of bulk HPW, 85-A/PS-15, and 85-A/SS-15. *, HPW (75-2125); &, W3O8 (81-2262); *, SiO2 (83-2466); *, sample holder. The numbers of the corresponding JCPDS card files are given in parentheses.
identified as broad peaks (Fig. 3). Two of the major intense peaks of HPW at 991 and 1007 cm 1 (996 and 1011 cm 1 [36,38]) can also be identified in the Raman spectrum of sample 10-F/PS-15 (Fig. 4). DSC characterization data are given for five samples in Figs. 5 and 6. The method provides information about the thermal stability of the samples with respect to the loss of water and decomposition. HPW is known to exhibit two endothermal transitions at around 373 K and at or above 473 K (Fig. 5) [39–42]. These correspond to the loss of 14 and 6 mol of water, respectively [40]. Mechanical treatment of pure HPW results in the disappearance of the lowtemperature peak, a shift of the high-temperature peak, and an additional endothermal transition at 850 K (Fig. 5, 100-/ PS-15). This shows that the mechanical treatment induces changes in the interaction between protonated water molecules and the Keggin anion. The endothermal region of sample 85-A/PS-15 with high HPW loading (Fig. 6) is
Fig. 3. DRIFT spectra of bulk HPW and sample 10-F/PS-15.
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259
Fig. 4. Raman spectra of bulk HPW and sample 10-F/PS-15.
practically identical with that of bulk HPW (Fig. 5). The only feature of samples with low HPW loading prepared by either impregnation (Fig. 6, 10-F/I) or milling (Fig. 6, 10-F/ PS-15), in turn, is a single endothermal signal indicating the desorption of water in a single step. An exothermal peak at high-temperature (around 873 K) indicates the decomposition of HPW to the constituent oxides. A comparison of the DSC traces of sample 85-A/SS15 and 85-A/PS-15 shows that the peak of decomposition is much smaller for the former (Fig. 6). This difference and the broad endothermal peak of sample 85-A/SS-15, again, testify to the fact that milling in stainless steel mill brings about a significant degree of decomposition of the Keggin structure. The exothermal peak of the other supported samples (10-F/I, 10-F/PS-15) probably shifted to higher temperature not detected in the temperature range of our study. 3.1.2. Catalytic applications The results of catalytic test reactions determined in Friedel–Crafts alkylation and Friedel–Crafts acylation are collected in Table 2. The experimental conditions applied in the ball-milling process were changed with the aim of finding the optimal parameters. As expected from the XRD and spectroscopic characterization data sample 85-A/SS-15 exhibits low activities in both reactions. Samples made in a polystyrene mill, in turn, show satisfactory catalytic activities. Fabrication of all other samples, therefore, was performed using the polystyrene mill. Comparison of data obtained with catalysts with different loadings and various milling times indicate that high
Fig. 6. DSC scans of selected supported HPW samples.
activities can be achieved with samples of low HPW content provided an optimal milling time of 15 h is applied (Fig. 7). The best results were found over 10-F/PS-15, which displayed a particularly high initial activity in Friedel– Crafts alkylation (Fig. 7). When the catalytic performance of samples prepared by ball-milling and those made by impregnation are compared the former exhibit somewhat better properties. It is also seen, however, that bulk HPW and HPW, which underwent ballmilling without SiO2 (100-/PS-15) are equally good catalysts (Table 2). With respect to selectivities, the alkylation reaction gives a mixture of two isomeric alkylated products and benzyl ether. Such low selectivities are, in general, characteristic of Friedel–Crafts alkylations [43]. It is of interest to note, however that dialkylated products could not be found. Friedel–Crafts acylation of anisole, in turn, is a highly selective process yielding the para isomer with selectivities better than 95% over all catalyst samples. Table 2 Catalytic performance of HPW samples in Friedel–Crafts reactionsa Material
85-A/SS-15 85-A/PS-7.5 85-A/PS-15 85-A/PS-30 10-A/PS-15 10-F/PS-15 10-F/PS-30 10-F/PS-50 10-A/I 10-F/I 20-F/I 100-/PS-15 Bulk HPW a
Acylationc
30 min
120 min
4 97 85 72 27 100 24 1 96 30 65 100 100
18 100 99 99 47 100 26 2 100 99 99 100 100
(26/41/32)e (35/65/0) (34/63/3) (34/61/5) (36/59/5) (45/55/0) (33/67/0) (20/46/34) (44/56/0) (43/52/5) (44/55/1) (41/50/9) (40/51/9)
30 min
120 mind
22 94 85 50 21 75 63 33 100 87 100 100 100
51 100 100 91 43 100 70 41 100 100 100 100 100
Values indicate conversions in %. Alkylation of toluene with benzyl alcohol at 373 K. c Acylation of anisole with acetic anhydride at 423 K. d Product selectivities: 95% or higher para substitution. e Values in parenthesis indicate product selectivities: ortho/para/dibenzyl ether. b
Fig. 5. DSC scans of unsupported HPW samples.
Alkylationb
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Fig. 8. Loss of HPW as a function of time by hot methanol extraction. ~, 10-F/I; &, 20-F/I; *, 10-F/PS-15. Fig. 7. The effect of milling time on catalytic activities of HPW-SiO2 samples in Friedel–Crafts reactions. Circles: 10-F/PS-15, squares: 10-F/PS30, triangles: 10-F/PS-50. Filled symbols: Friedel–Crafts alkylation of toluene with benzyl alcohol. Open symbols: Friedel–Crafts acylation of anisole with acetic anhydride.
It is also worth making a comparison of the physical characterization data summarized in Table 1 and activities given in Table 2. When ball-milled samples with 10% HPW loading are considered activities determined at short reaction time in Friedel–Crafts reactions parallel both surface areas and acidities. For the two samples made by impregnation of Fluka silica with identical surface area activities in Friedel– Crafts alkylation correlate with acidities: 20-F/I with higher loading and higher acid capacity is more active than 10-F/I. The third sample (10-A/I) with a medium acid capacity, in turn, exhibits the highest activity. This must be due to the low specific surface area of Aldrich silica, which results in a high acid site density of this sample. Heteropoly acids either in the bulk or supported form suffer from decreasing activity in repeated applications. In gas–solid systems at high-temperature, this is due to the formation of carbonaceous deposits [5]. In liquid–solid applications this can be accounted for by the loss of the catalytically active material as a result of dissolution brought about by the high solubility of heteropoly acids in water and oxygen-containing organic solvents [44]. It is of interest, therefore, to compare the stability behavior of these new catalysts and samples prepared by impregnation. Three catalysts were chosen to carry out a test when the time dependence of HPW loss by treatment of hot methanol was determined as described in [45]. Results are shown in Fig. 8. Samples prepared by impregnation (10-F/I and 20-F/I) exhibit similar degrees of HPW loss after a 2 h extraction despite the different loadings. In contrast, sample 10-F/PS15, which proved to be the best ball-milled catalyst loses only half the amount of HPW compared to the other two samples. The amounts of HPW retained were also determined by titration. Data thus measured (Table 1, values in parenthesis) are in good agreement with those indicated in Fig. 8: the quantity of the active material retained in 10-F/I and the ball-milled sample 10-F/PS-15 can be estimated to be about 26 and 38%, respectively.
Catalytic performance of two of the above samples (10-F/ I and 10-F/PS-15) was tested using the recovered materials after the dissolution experiments. The activity changes in Friedel–Crafts alkylation are in agreement with the result of the above study. The ball-milled 10-F/PS-15 sample exhibits a lower initial activity but the final conversion is almost as high as that of the original sample (Fig. 9). As expected, a large drop of activity is found for 10-F/I. In contrast, the decrease in activities in Friedel–Crafts acylation is much more pronounced for both samples with the ball-milled catalyst displaying somewhat better activity (Fig. 10). This can be accounted for by the different nature of the two test reactions: it is known that Friedel–Crafts acylation usually requires more than catalytic amounts of the catalyst, which is due to the fact that in this case the product carbonyl compounds interact with the catalyst. It should be emphasized here, that it is an established fact that supported heteropoly acid catalysts usually require at least 10–20% loading to have satisfactory catalytic activities. At lower loadings they suffer decomposition and low acidity due to strong interactions with the support material [36,46]. Our ball-milled 10-F/PS-15 sample, in contrast, has about 4% of HPW after methanol extraction, and still exhibits good catalyst performance in Friedel–Crafts alkylation. Recycling studies were also carried out using the best catalyst sample prepared by ball-milling (10-F/PS-15).
Fig. 9. Time dependence of conversion in Friedel–Crafts alkylation. ~, 10F/I; *, 10-F/PS-15.
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Fig. 10. Time dependence of conversion of in Friedel–Crafts acylation. ~, 10-F/I; *, 10-F/PS-15. Solid lines: as-prepared catalysts, broken lines: after extraction with hot methanol.
261
Friedel–Crafts acylation of anisole [47–49]. However, all catalyst samples applied in these studies have high loadings (20–60%). In contrast, our best catalysts exhibit equally good catalyst performance with loadings as low as 10%. The problem of deactivation is also addressed in the above references and noted with other acidic catalysts, for example various zeolites [50] and sulfated zirconia [51]. This phenomenon is primarily due to the strong but reversible adsorption of products, which, however, does not appear to be significant in our case (see Table 2 and Fig. 10). A recent study reports on the stability of alumosilica SBA-15 catalysts in alkylation of toluene showing significant deactivation in repeated uses [52]. This, again, is in sharp contrast to the performance of sample 10-F/PS-15 in recycling studies (see Fig. 11). 3.2. Supported Nafion-H materials
Sample 10-F/I made by impregnation and two unsupported HPW samples (bulk HPW and 100-/PS-15) were also used for comparison. The reaction selected was Friedel–Crafts alkylation of toluene with benzyl alcohol. The process produces water as byproduct, which is able to dissolve the active catalytic ingredient. We argued, therefore, that this reaction is a sensitive test to evaluate stability and durability of the samples. As seen in Fig. 11 bulk HPW shows some activity drop already in run 5, whereas for the other three catalysts decreasing activities are observed only in run 7. Subsequent data acquired in further repeated uses clearly show that the performance of the ball-milled 10-F/PS-15 catalyst exceeds those of all other samples studied. A short analysis of relevant observations found in literature and their comparison with the present data with respect to both activities and stabilities underline the characteristics of our samples made by mechanical treatment. Emphasis may be put on acylation, which is of practical importance. Results with silica-supported heteropoly acid catalysts disclosed recently indicate that high activities and high para selectivities can be achieved in
3.2.1. Sample characterization The results of the acid–base titrations and BET surface area measurements of Nafion-H materials all containing 15% Nafion are collected in Table 3. These include two samples made with Aldrich SiO2 in a stainless steel mill applying two milling times (15-A/SS-15 and 15-A/SS-50) and five catalysts prepared by using Fluka silica gel in a polystyrene mill (15-F/PS-h). The corresponding data for SAC-13, a commercially available Nafion-H silica nanocomposite prepared by the sol–gel method, are also given. It is seen that the quantity of Nafion accessible increases with increasing milling time. BET surface areas, in contrast, decrease as duration of the milling process increases just as observed for HPW samples (see Table 1). It is known [34] that as particles become smaller agglomeration becomes more pronounced. The local high-energy conditions in the surface particles promoted by the mechanical treatment induce particle coalescence and a corresponding decreases in the BET surface. As found for supported HPW materials a milling time of 15 h appears to be optimal. Indeed, SEM micrographs of samples milled for various time intervals show significant differences (Fig. 12). The presence of polyhedral particles with very narrow diameter distribution (the maximum size is below 10 mm) is seen for sample 15-F/PS-5 (Fig. 12, left picture). The other Table 3 Surface characterization of Nafion-H samples
Fig. 11. Recycling studies in Friedel–Crafts alkylation of four selected HPW/SiO2 catalysts (reaction time: 2 h).
Material
Acid capacity (mmol H+ g 1)
Surface area (m2 g 1)
Bulk Nafion 15-A/SS-15 15-A/SS-50 15-F/PS-5 15-F/PS-10 15-F/PS-15 15-F/PS-25 15-F/PS-50 SAC-13
0.90 0.24 0.65 0.42 0.45 0.62 0.68 0.65 0.67
n.d.a 21 16 313 310 277 230 11 682
a
Not determined.
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Fig. 12. SEM micrographs of sample 15-F/PS milled for various times.
Fig. 13. EDX results of sample 15-F/PS-50.
micrograph (sample 15-F/PS-50, Fig. 12, right picture) shows a completely different picture. As a result of the longer mechanical treatment, particles of medium size almost completely disappeared and powder coalescence resulted in the formation of much bigger, elongated grains with lengths larger than 80 mm. Furthermore, according to EDX results presented in Fig. 13, the active phase is not homogeneously distributed in the silica matrix, which is in contrast to what was observed for HPW samples. It is evident from these results that prolonged milling brings about the coalescence of Nafion particles. Most of the characteristic IR bands of Nafion-H are those of the fluorocarbon main chain (1100–1350, 805, 775, 630 cm 1) corresponding to various C–F vibrations [53]. In addition, bands due to the SO3 group (1210 cm 1) and the
Fig. 14. DRIFT spectra of SAC-13 and sample 15-F/PS-15.
C–O–C bond (980 cm 1) are also observed. Of these, bands at 1210, 1160, 800, and 630 cm 1 were observed in the spectrum of a 40% Nafion-H silica nanocomposite prepared by the sol–gel method [13]. Fig. 14 shows the spectra of SAC-13 and sample 15-F/PS-15. Due to the much lower Nafion content of these samples, the bands at 1210 and 1160 cm 1 appear as a shoulder of the strong and broad band of silica at around 1100 cm 1 (Si–O–Si stretching vibration). Two additional typical bands centered around 975 and 800 cm 1, however, are clearly visible. Peaks at 3340 and 3400 cm 1 are characteristic of adsorbed water. 3.2.2. Catalytic applications The results of catalytic test reactions determined in Friedel–Crafts alkylation and dimerization of 2-phenylpropene (AMS) are collected in Table 4. Catalytic activities in Friedel–Crafts alkylation show again that Aldrich SiO2 and the stainless steel mill are not ideal for catalyst synthesis. Samples fabricated in a polystyrene mill using Fluka silica exhibit varying initial activities. It is also interesting that initial activities of SAC13 are also very low. However, complete conversions can be attained at 60 min with all catalysts. Data with respect to the corresponding selectivities (Fig. 15), in turn, show only negligible differences with the two isomeric alkylated compounds as the main products. Dimerization of AMS is an important process to produce the open-chain and cyclic dimers 4-methyl-2,4-diphenylpent-1-ene and 1,1,3-trimethyl-3-phenylindane, respectively, which are applied in polymer production and in the electronic industry [54]. Studies have been made to
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Table 4 Catalytic performance of Nafion-H/SiO2 samplesa Material
Bulk Nafion 15-A/SS-15 15-A/SS-50 15-F/PS-5 15-F/PS-10 15-F/PS-15 15-F/PS-25 15-F/PS-50 SAC-13 a b c
Friedel–Crafts alkylationb
Dimerization of 2-phenylpropenec
15 min
30 min
60 min
30 min
6h
48 1 0 16 88 95 38 73 9
99 1 0 43 100 98 62 100 18
99 2 1 100 100 100 100 100 100
1 96 0 38 67 72 88 64 94
3 99 0 97 98 99 98 99 99
Values indicate conversions in %. Alkylation of toluene with benzyl alcohol at 384 K. Reaction in toluene at 333 K. Fig. 16. Selectivity in the dimerization of 2-phenylpropene over Nafion samples (data measured after 6 h).
test various catalysts and find appropriate reaction conditions for selective production of either of the two dimers [54–58]. As seen in Table 4, bulk Nafion-H exhibits very low reactivity. It is surprising that sample 15-A/SS-15, which was inactive in Friedel–Crafts alkylation shows very high activity in this reaction even at short reaction time. Wide changes in selectivities can be observed (Fig. 16). There are three important points to be emphasized. (i) The selectivity over the highly active 15-A/SS-15 catalyst is completely different than those of all other samples furnishing 4-methyl-2,4-diphenylpent-1-ene as the main product with high selectivity. (ii) 15-F/PS-5 exhibits the highest selectivity in the formation of the cyclic dimer 1,1,3trimethyl-3-phenylindane but selectivities decrease sharply with increasing milling time. Namely, selectivity of the formation of the cyclic dimer decreases, whereas that of 4methyl-2,4-diphenylpent-1-ene increases. (iii) The selectivity of 15-F/PS-5 exceeds even that of SAC-13. The activity data of the two test reactions show that at short milling time the acid capacity (the quantity of
accessible Nafion-H) measured by acid–base titration determines initial reactivities. At longer milling times, in turn, BET surface area is the determining factor to affect activities. As discussed, a longer milling process induces unfavorable changes, namely the coalescence (agglomeration) of Nafion particles. As a result, the accessibility of surface active sites by the reacting molecules is hindered. This may explain the contrasting behavior of 15-A/SS-15 and 15-F/PS-5 with respect to selectivity of AMS dimerization. It is known from literature studies that the substituted isomeric pentene dimers are intermediates in the formation of 1,1,3-trimethyl-3-phenylindane [54–56]. This can also be seen in the time-dependence of the reaction over the two samples exhibiting opposite selectivities (Figs. 17 and 18). The concentration of the 4-methyl-2,4-diphenylpent-1-ene isomer goes through a maximum over 15-F/PS-5, since it transforms into the indane derivative (Fig. 18). In contrast, the high initial concentration of 4-methyl-2,4diphenylpent-1-ene decreases only very slowly over 15-A/
Fig. 15. Selectivity in the Friedel–Crafts alkylation of toluene with benzyl alcohol over Nafion samples (data measured after 60 min).
Fig. 17. Time dependence of conversion and selectivities in the dimerization of 2-phenylpropene over 15-A/SS-15. *, conversion; &, 4-methyl-2,4diphenylpen-1-ene selectivity; ~, 4-methyl-2,4-diphenylpen-2-ene selectivity; *, 1,1,3-trimethyl-3-phenylindane selectivity.
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Hungarian National Science Foundation (OTKA grants T042603 and TS044690). This work was also funded by the Ministry of University in Italy, Rome (PRIN projects), and the University of Sassari. References
Fig. 18. Time dependence of conversion and selectivities in the dimerization of 2-phenylpropene over 15-F/PS-15. *, conversion; *, 1,1,3-trimethyl-3-phenylindane selectivity; &, 4-methyl-2,4-diphenylpen-1-ene selectivity; ~, 4-methyl-2,4-diphenylpen-2-ene selectivity.
SS-15 (Fig. 17). This indicates that it is not able to undergo cyclization, which is certainly due to the inaccessibility of the active sites to the large dimeric intermediate.
4. Conclusions High-energy ball-milling has been shown to be a promising alternative to sol–gel synthesis to prepare silica-supported heteropoly acid and Nafion-H materials for catalytic applications. A uniform distribution of HPW in the silica matrix is found for HPW-SiO2 samples, whereas particle coalescence is observed for Nafion samples after prolonged milling. Ball-milled HPW-SiO2 samples prepared under appropriately selected conditions exhibit catalytic properties similar to catalysts made by impregnation. HPW in ballmilled samples, in turn, proved to be more resistant to extraction with polar solvents (hot methanol) and these catalysts display higher stability in recycling studies. Supported Nafion-H samples prepared by ball-milling show high activities in test reactions. Sample 15-F/PS-5, the best catalyst, exceeds even SAC-13 in the selective synthesis of 1,1,3-trimethyl-3-phenylindane in the dimerization of 2phenylpropene. Catalysts with high catalytic performance require the use of optimal milling time, because particle coalescence and the resulting decrease in BET surface area hinder the accessibility of the active sites.
Acknowledgments We kindly acknowledge Dr. A. Canu and Dr. C. Deidda for technical assistance during SEM measurements and the ‘‘Centro Restauro-Soprintendenza Beni Archeologici, Sassari e Nuoro’’ where such measurements were carried out. We are most grateful for the financial support from the
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SiO2-supported dodecatungstophosphoric acid and Nafion-H prepared by ball-milling for catalytic application ´ rpa´d Molna´ra,* Bulcsu´ Ra´ca, Gabriele Mulasb, Aniko´ Csongra´dia, Katalin Lo´kia, A a
Department of Organic Chemistry, University of Szeged, Do´m te´r 8, Szeged, Hungary Dipartimento di Chimica, Universita´ di Sassari, Via Vienna 2, I-07100 Sassari, Italy
b
Received 18 November 2004; received in revised form 15 December 2004; accepted 15 December 2004 Available online 18 January 2005
Abstract Dodecatungstophosphoric acid (HPW) and Nafion-H were supported on SiO2 using the ball-milling technique. Samples were characterized by physical (BET surface, SEM, DSC, XRD, DRIFT and Raman spectroscopies) and chemical (acid–base titration) methods. Catalytic properties were studied in a range of chemical transformations, which can be induced by electrophilic catalysts including Friedel– Crafts reactions (alkylation of toluene with benzyl alcohol, acylation of anisole with acetic anhydride) and the dimerization of amethylstyrene. Milling conditions (stainless steel or polystyrene apparatus, milling time), loading of the active components, and the type of support were found to have profound effects on the physical characteristics and catalytic performance of the samples. Ball-milled HPWSiO2 samples prepared under appropriately selected conditions exhibit catalytic properties similar to catalysts made by impregnation. HPW in ball-milled samples, in turn, proved to be more resistant to extraction with polar solvents (hot methanol) and these catalysts display higher stability in recycling studies. Supported Nafion-H samples prepared by ball-milling show high activities in test reactions. The best catalyst exceeds SAC-13 in the selective synthesis of 1,1,3-trimethyl-3-phenylindane in the dimerization of 2-phenylpropene. Catalysts with high catalytic performance require the use of optimal milling time, because particle coalescence and the resulting decrease in BET surface area hinder the accessibility of the active sites. # 2005 Elsevier B.V. All rights reserved. Keywords: Dodecatungstophosphoric acid; Nafion-H; Ball-milling; Friedel–Crafts alkylation; Friedel–Crafts acylation; Dimerization of 2-phenylpropene; Catalyst recycling
1. Introduction The use of solid (heterogeneous) catalysts in organic synthesis and in the industrial manufacture of chemicals is of increasing importance since they provide green alternatives to homogeneous catalysts. This is particularly important in acid catalysis where solid acids can be the safe alternatives to hazardous and corrosive materials such as hydrofluoric acid, sulfuric acid and nitric acid. In addition to large-scale industrial use, heterogeneous catalysts have already found extensive application in fine chemical synthesis [1] and, among others, protection/deprotection processes in organic synthesis [2]. * Corresponding author. Tel.: +36 62 544277; fax: +36 62 544200. ´ . 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.2004.12.020
Heteropoly compounds (heteropoly acids and salts) and Nafion resins, that is perfluorinated resinsulfonic acid based ion-exchanged polymers, are two important classes of catalysts with highly acidic character. Heteropoly acids, when used in non-polar solvents, that is, when applied as bulk catalysts, are weak superacids. Similarly, Nafion resins exhibit an acidic character comparable to that of 100% sulfuric acid. Both heteropoly acids [3–7] and Nafion-H [8,9] have been widely used in organic synthesis. However, heteropoly acids and Nafion resins have a common disadvantage. The surface area of bulk heteropoly acids is only a few m2 g 1. Nafion resins have an even lower surface area (0.02 m2 g 1); moreover, most of the active sites are inaccessible to reacting molecules. As a result, the specific activities of these catalytic materials, namely the number of reacting molecules on an active site, are very low.
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Attempts have recently been made to overcome this problem by the preparation of silica nanocomposites by using sol–gel synthesis. New, highly active catalysts have been prepared by the immobilization of the acidic components into a silica matrix. The heteropoly acid–silica nanocomposites have BET surface areas of a few hundred m2 g 1 and exhibit higher specific activities and proved to be recyclable [10–12]. Harmer et al. reported a similar method to prepare Nafion–silica nanocomposite materials [13]. These were shown to have large surface area (150– 500 m2 g 1) and contain small (<100 nm) Nafion particles entrapped in a porous silica framework [13]. Outstanding catalytic performance was found in further studies [13–16] and demonstrated recently with a commercially available catalyst called Nafion-H SAC-13 [17–20]. Mechanical treatment, more specifically, high-energy ball-milling has long been known as a powerful and useful technique to fabricate solid materials with unique properties. It is most often applied in mechanical alloying [21] and has recently gained interest in the synthesis of catalyst materials. The method allows the preparation of unique catalyst materials characterized by high specific activity due to the presence of nano-size particles [22–29]. In fact, there is a recent example of the use of mechanical treatment in heteropoly acid chemistry. Vedrine et al. dispersed HPW on its neutral Cs salt by grinding to produce high surface area materials, which exhibited much higher catalytic activity for butane isomerization [30]. On the basis of our recent experiences in this field [31,32] we now used mechanochemical synthesis as an alternative procedure to fabricate dodecatungstophosphoric acid (HPW) and Nafion-H catalysts supported on SiO2.
2. Experimental 2.1.1. Materials All reagents and dodecatungstophosphoric acid (HPW) were purchased from Aldrich and had purities at least 98%. Nafion-H was a DuPont product and Nafion-H SAC-13 (Nafion-H content = 10–20%) was produced by Aldrich. High surface area silica gel (Fluka, Silica gel 100, 410 m2 g 1) and low surface area silicon dioxide (Aldrich, 325 mesh, 10 m2 g 1) were used as support materials. Toluene used in Friedel–Crafts alkylations was stored over sodium wire. 2.2. Sample preparation Two techniques, mechanochemical ball-milling and impregnation were applied in catalyst preparation. A mixture of the active catalytic material and SiO2 were milled for various times in either a stainless steel mill with two steel balls (1 g each) or in a polystyrene mill with three polystyrene balls (0.5 g each). In a typical run 5 g of
material mixture were used. All manipulations were performed under air atmosphere. Catalysts were also prepared by impregnation in order to compare the effect of the different synthesis methods. In a typical synthesis, 4.5 g of SiO2 was suspended in the solution of 0.5 g of HPW in 17 ml of distilled water. Water was slowly removed by means of a rotary evaporator at 298 K and the catalyst was dried in vacuo at 443 K for 4 h. The code X-a/b-Y is used to denote samples. X gives the nominal loading (in wt.%) of the active ingredient, ‘a’ indicates the type of support used (F: Fluka silica gel, A: Aldrich SiO2), ‘b’ shows the type of milling apparatus (SS: stainless steel, PS: polystyrene) or indicates impregnation (I), and Y gives milling time (h). 2.3. Sample characterization X-ray powder diffraction (XRD) data were acquired using a Siemens D500 apparatus. Data were collected in the angular range 10–1008 applying Cu Ka radiation. Thermal stability was followed by differential scanning calorimetry (DSC). Thermograms were recorded using a Perkin-Elmer DSC7 apparatus at a scanning rate of 208 min 1. SEM–EDX analyses were carried out using a Jeol JSM-5600LV microscope operating at 20 kV equipped with an Oxford Instrument 6587 microprobe. Powder morphology was studied by the secondary electrons emitted by the samples. Compositional and elemental investigations were performed by backscattered electrons and EDX analysis, respectively. Powder morphological measurements were carried out using a Jeol JSM-5600LV scanning electron microscope, operating at 20 kV analyzing the secondary electrons emitted by the samples. Sample holders were covered by a flat graphite surface over which powders were deposited. Nitrogen adsorption and desorption isotherms were measured at 77 K using a Quantachrome Nova 2000 system (pretreatment: 423 K, 1 h, in vacuo). Data were analyzed using the BJH model. Diffuse reflectance IR measurements were made by a BioRad DigiLab FTS 65A/896 spectrometer equipped with a DTGS detector. Spectra were recorded between 400 and 4000 cm 1 with resolution of 4 cm 1 using the KBr pellet technique (0.015 g of catalyst sample and 300 mg of KBr). Raman measurements were carried out on a BioRad DigiLab FT Raman spectrometer. Spectra were recorded between 0 and 4000 cm 1. Acid–base titrations as described in [33] were used to measure acid capacity that is the quantity of catalytically active ingredient of samples. In a typical measurement 0.5 g of solid was suspended in 50 ml of 0.1 M KCl. The suspension was stirred for 20 min and titrated with 0.2 M KOH in the presence of thymol blue. Soxhlet extraction was carried out in a specially designed extractor. An amount of 1.0 g of HPW-containing catalyst was extracted with 60.0 ml of methanol. Samples were taken at given intervals and the quantity of dissolved HPW was
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determined using an 8485A HP Diode Array UV spectrophotometer.
Table 1 Surface characterization of HPW samples Material
Acid capacity (mmol H+ g 1)
Surface areaa (m2 g 1)
85-A/SS-15 85-A/PS-7.5 85-A/PS-15 85-A/PS-30 10-A/PS-15 10-F/PS-15 10-F/PS-30 10-F/PS-50 10-A/I 10-F/I 20-F/I 100-/PS-15 Bulk HPW
4.61 6.69 6.50 6.17 0.53 0.86 (0.39)b [0.20]c 0.56 0.55 0.91 0.75 (0.22)b [0.14]c 1.40b 8.32 8.36
0.8 4 2.4 0.5 3.0 296 264 210 1.3 300 289 – –
2.4. Catalytic reactions All reactions were carried out in a round bottom flask equipped with a reflux condenser and stirred magnetically. Temperatures were controlled with an accuracy of 3 K. Catalyst quantities were 0.05 g (for supported samples), 0.005 g (for bulk HPW samples), or 0.0075 g (for bulk Nafion). The support materials alone did not show any activity in the reactions studied. Friedel–Crafts alkylation was carried out by reacting toluene with benzyl alcohol [4 ml of toluene, 0.104 ml (0.001 mol) of benzyl alcohol] in the presence of decane as internal standard. Reaction temperature = 373 K for HPW samples and 384 K for Nafion samples. The products are a mixture of phenyl-ortho-tolylmethane and phenyl-paratolylmethane with low amounts of dibenzyl ether as byproduct. Friedel–Crafts acylation was performed by reacting anisole with acetic anhydride at 423 K [4.1 ml of anisole, 0.094 ml (0.001 mol) of acetic anhydride in the presence of catalyst and decane as internal standard]. The product is 4-methoxyacetophenone and a small amount of 2methoxyacetophenone (amounting less than 5%) is also formed. Dimerization of 2-phenylpropene (a-methylstyrene, AMS) was carried out by reacting AMS in toluene at 333 K [5.0 ml of toluene, 0.650 ml (0.005 mol) of AMS]. The products are 4-methyl-2,4-diphenylpent-1-ene and 4methyl-2,4-diphenylpent-2-ene, and the cyclic dimer 1,1,3trimethyl-3-phenylindane. Samples withdrawn at certain time intervals from the reaction mixtures were analyzed by capillary GC (Hewlett Packard 5980, 50 m HP1 column, flame ionization detector) and TLC. Product identification was performed by means of GC–MS (HP 5890 GC coupled with a HP 5970 mass selective detector), by NMR spectroscopy [Bruker DRX 500 spectrometer at 500 MHz, CDCl3 as solvent and tetramethylsilane (TMS) as an internal reference] and by comparison with authentic samples. All products are known compounds and gave satisfactory spectral data.
3. Results and discussion 3.1. Supported HPW materials 3.1.1. Sample characterization The results of acid–base titration and BET surface areas of catalyst samples prepared by various methods are collected in Table 1. Acid–base titrations give quantitative information of HPW content. Noteworthy is the decreased value found for 85-A/SS-15 the only sample prepared in a stainless steel mill.
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a Surface area of the supports used: Fluka silica gel (F) = 410 m2 g 1, Aldrich SiO2 (A) = 10 m2 g 1. b Values in parenthesis: after extraction. c Values in brackets: after 10 runs in recycling studies.
With respect to BET surface areas the values decrease with increasing milling time in all cases compared to the corresponding support. The decrease is particularly significant for 85-A/SS-15 the (sample prepared by a stainless steel mill). A similar decrease for samples made in a polystyrene mill is observed only after prolonged milling (85-A/PS-30). Less significant changes are found for samples prepared by using Fluka silica. These changes may result from the aggregation of particles [34] and the loss of porous silica structure. Scanning electron micrographs show a wide range of particles with various sizes (Fig. 1, upper left picture). After prolonged milling larger particles appear but there is no significant change in particle size distribution. This indicates that the decreasing BET surface areas are mainly due to the loss of porous silica structure. Impregnation has a similar although less pronounced effect. The decrease in surface area, in this case, can be attributed to the filling up the pores by HPW. It is noteworthy that according to EDX results the active phase is homogeneously distributed in the silica matrix (Fig. 1). XRD patterns of samples with high HPW loading prepared using a stainless steel or a polystyrene mill are shown in Fig. 2. All reflections characteristic of HPW appear in the XRD pattern of sample 85-A/PS-15, whereas sample 85-A/SS-15 is almost featureless indicating only the presence of W3O8. Differences can also be observed in DRIFT spectra of the three samples (not shown). There are four bands in the fingerprint region characteristic of pure HPW around 1083, 983, 893 and 810 cm 1 assigned to the antisymmetrical stretching vibration of P–O, W=O, W–O– Wcorner and W–O–Wedge, respectively [35–37]. These bands can be observed in sample 85-A/SS-15, however, they are shifted to lower wave numbers. It is clear from these comparisons that the heavy milling conditions induced by
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Fig. 1. SEM (upper left picture) and EDX results of sample 10-F/PS-15.
the stainless steel mill bring about the decomposition of the Keggin structure. Characteristic XRD features of supported HPW samples with lower loading prepared by milling in a polystyrene mill are a broad halo or a strong, sharp reflection at 2u = 268 attributed to amorphous silica (Fluka) and silicon dioxide (Aldrich), respectively. The peaks of HPW are not visible, which is due to the low concentration of HPW. This is consistent with recent observations [37] that characteristic peaks of silica-supported HPW do not appear below loadings of 20 wt.%. The presence of HPW, however, can be detected by DRIFT and Raman spectroscopies. Although the broad and intensive band of silica at around 1100 cm 1 overlaps the absorption band of HPW at 1083 cm 1 (stretching vibration of P–O), the other three bands characteristic of HPW can be
Fig. 2. XRD patterns of bulk HPW, 85-A/PS-15, and 85-A/SS-15. *, HPW (75-2125); &, W3O8 (81-2262); *, SiO2 (83-2466); *, sample holder. The numbers of the corresponding JCPDS card files are given in parentheses.
identified as broad peaks (Fig. 3). Two of the major intense peaks of HPW at 991 and 1007 cm 1 (996 and 1011 cm 1 [36,38]) can also be identified in the Raman spectrum of sample 10-F/PS-15 (Fig. 4). DSC characterization data are given for five samples in Figs. 5 and 6. The method provides information about the thermal stability of the samples with respect to the loss of water and decomposition. HPW is known to exhibit two endothermal transitions at around 373 K and at or above 473 K (Fig. 5) [39–42]. These correspond to the loss of 14 and 6 mol of water, respectively [40]. Mechanical treatment of pure HPW results in the disappearance of the lowtemperature peak, a shift of the high-temperature peak, and an additional endothermal transition at 850 K (Fig. 5, 100-/ PS-15). This shows that the mechanical treatment induces changes in the interaction between protonated water molecules and the Keggin anion. The endothermal region of sample 85-A/PS-15 with high HPW loading (Fig. 6) is
Fig. 3. DRIFT spectra of bulk HPW and sample 10-F/PS-15.
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259
Fig. 4. Raman spectra of bulk HPW and sample 10-F/PS-15.
practically identical with that of bulk HPW (Fig. 5). The only feature of samples with low HPW loading prepared by either impregnation (Fig. 6, 10-F/I) or milling (Fig. 6, 10-F/ PS-15), in turn, is a single endothermal signal indicating the desorption of water in a single step. An exothermal peak at high-temperature (around 873 K) indicates the decomposition of HPW to the constituent oxides. A comparison of the DSC traces of sample 85-A/SS15 and 85-A/PS-15 shows that the peak of decomposition is much smaller for the former (Fig. 6). This difference and the broad endothermal peak of sample 85-A/SS-15, again, testify to the fact that milling in stainless steel mill brings about a significant degree of decomposition of the Keggin structure. The exothermal peak of the other supported samples (10-F/I, 10-F/PS-15) probably shifted to higher temperature not detected in the temperature range of our study. 3.1.2. Catalytic applications The results of catalytic test reactions determined in Friedel–Crafts alkylation and Friedel–Crafts acylation are collected in Table 2. The experimental conditions applied in the ball-milling process were changed with the aim of finding the optimal parameters. As expected from the XRD and spectroscopic characterization data sample 85-A/SS-15 exhibits low activities in both reactions. Samples made in a polystyrene mill, in turn, show satisfactory catalytic activities. Fabrication of all other samples, therefore, was performed using the polystyrene mill. Comparison of data obtained with catalysts with different loadings and various milling times indicate that high
Fig. 6. DSC scans of selected supported HPW samples.
activities can be achieved with samples of low HPW content provided an optimal milling time of 15 h is applied (Fig. 7). The best results were found over 10-F/PS-15, which displayed a particularly high initial activity in Friedel– Crafts alkylation (Fig. 7). When the catalytic performance of samples prepared by ball-milling and those made by impregnation are compared the former exhibit somewhat better properties. It is also seen, however, that bulk HPW and HPW, which underwent ballmilling without SiO2 (100-/PS-15) are equally good catalysts (Table 2). With respect to selectivities, the alkylation reaction gives a mixture of two isomeric alkylated products and benzyl ether. Such low selectivities are, in general, characteristic of Friedel–Crafts alkylations [43]. It is of interest to note, however that dialkylated products could not be found. Friedel–Crafts acylation of anisole, in turn, is a highly selective process yielding the para isomer with selectivities better than 95% over all catalyst samples. Table 2 Catalytic performance of HPW samples in Friedel–Crafts reactionsa Material
85-A/SS-15 85-A/PS-7.5 85-A/PS-15 85-A/PS-30 10-A/PS-15 10-F/PS-15 10-F/PS-30 10-F/PS-50 10-A/I 10-F/I 20-F/I 100-/PS-15 Bulk HPW a
Acylationc
30 min
120 min
4 97 85 72 27 100 24 1 96 30 65 100 100
18 100 99 99 47 100 26 2 100 99 99 100 100
(26/41/32)e (35/65/0) (34/63/3) (34/61/5) (36/59/5) (45/55/0) (33/67/0) (20/46/34) (44/56/0) (43/52/5) (44/55/1) (41/50/9) (40/51/9)
30 min
120 mind
22 94 85 50 21 75 63 33 100 87 100 100 100
51 100 100 91 43 100 70 41 100 100 100 100 100
Values indicate conversions in %. Alkylation of toluene with benzyl alcohol at 373 K. c Acylation of anisole with acetic anhydride at 423 K. d Product selectivities: 95% or higher para substitution. e Values in parenthesis indicate product selectivities: ortho/para/dibenzyl ether. b
Fig. 5. DSC scans of unsupported HPW samples.
Alkylationb
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Fig. 8. Loss of HPW as a function of time by hot methanol extraction. ~, 10-F/I; &, 20-F/I; *, 10-F/PS-15. Fig. 7. The effect of milling time on catalytic activities of HPW-SiO2 samples in Friedel–Crafts reactions. Circles: 10-F/PS-15, squares: 10-F/PS30, triangles: 10-F/PS-50. Filled symbols: Friedel–Crafts alkylation of toluene with benzyl alcohol. Open symbols: Friedel–Crafts acylation of anisole with acetic anhydride.
It is also worth making a comparison of the physical characterization data summarized in Table 1 and activities given in Table 2. When ball-milled samples with 10% HPW loading are considered activities determined at short reaction time in Friedel–Crafts reactions parallel both surface areas and acidities. For the two samples made by impregnation of Fluka silica with identical surface area activities in Friedel– Crafts alkylation correlate with acidities: 20-F/I with higher loading and higher acid capacity is more active than 10-F/I. The third sample (10-A/I) with a medium acid capacity, in turn, exhibits the highest activity. This must be due to the low specific surface area of Aldrich silica, which results in a high acid site density of this sample. Heteropoly acids either in the bulk or supported form suffer from decreasing activity in repeated applications. In gas–solid systems at high-temperature, this is due to the formation of carbonaceous deposits [5]. In liquid–solid applications this can be accounted for by the loss of the catalytically active material as a result of dissolution brought about by the high solubility of heteropoly acids in water and oxygen-containing organic solvents [44]. It is of interest, therefore, to compare the stability behavior of these new catalysts and samples prepared by impregnation. Three catalysts were chosen to carry out a test when the time dependence of HPW loss by treatment of hot methanol was determined as described in [45]. Results are shown in Fig. 8. Samples prepared by impregnation (10-F/I and 20-F/I) exhibit similar degrees of HPW loss after a 2 h extraction despite the different loadings. In contrast, sample 10-F/PS15, which proved to be the best ball-milled catalyst loses only half the amount of HPW compared to the other two samples. The amounts of HPW retained were also determined by titration. Data thus measured (Table 1, values in parenthesis) are in good agreement with those indicated in Fig. 8: the quantity of the active material retained in 10-F/I and the ball-milled sample 10-F/PS-15 can be estimated to be about 26 and 38%, respectively.
Catalytic performance of two of the above samples (10-F/ I and 10-F/PS-15) was tested using the recovered materials after the dissolution experiments. The activity changes in Friedel–Crafts alkylation are in agreement with the result of the above study. The ball-milled 10-F/PS-15 sample exhibits a lower initial activity but the final conversion is almost as high as that of the original sample (Fig. 9). As expected, a large drop of activity is found for 10-F/I. In contrast, the decrease in activities in Friedel–Crafts acylation is much more pronounced for both samples with the ball-milled catalyst displaying somewhat better activity (Fig. 10). This can be accounted for by the different nature of the two test reactions: it is known that Friedel–Crafts acylation usually requires more than catalytic amounts of the catalyst, which is due to the fact that in this case the product carbonyl compounds interact with the catalyst. It should be emphasized here, that it is an established fact that supported heteropoly acid catalysts usually require at least 10–20% loading to have satisfactory catalytic activities. At lower loadings they suffer decomposition and low acidity due to strong interactions with the support material [36,46]. Our ball-milled 10-F/PS-15 sample, in contrast, has about 4% of HPW after methanol extraction, and still exhibits good catalyst performance in Friedel–Crafts alkylation. Recycling studies were also carried out using the best catalyst sample prepared by ball-milling (10-F/PS-15).
Fig. 9. Time dependence of conversion in Friedel–Crafts alkylation. ~, 10F/I; *, 10-F/PS-15.
B. Ra´c et al. / Applied Catalysis A: General 282 (2005) 255–265
Fig. 10. Time dependence of conversion of in Friedel–Crafts acylation. ~, 10-F/I; *, 10-F/PS-15. Solid lines: as-prepared catalysts, broken lines: after extraction with hot methanol.
261
Friedel–Crafts acylation of anisole [47–49]. However, all catalyst samples applied in these studies have high loadings (20–60%). In contrast, our best catalysts exhibit equally good catalyst performance with loadings as low as 10%. The problem of deactivation is also addressed in the above references and noted with other acidic catalysts, for example various zeolites [50] and sulfated zirconia [51]. This phenomenon is primarily due to the strong but reversible adsorption of products, which, however, does not appear to be significant in our case (see Table 2 and Fig. 10). A recent study reports on the stability of alumosilica SBA-15 catalysts in alkylation of toluene showing significant deactivation in repeated uses [52]. This, again, is in sharp contrast to the performance of sample 10-F/PS-15 in recycling studies (see Fig. 11). 3.2. Supported Nafion-H materials
Sample 10-F/I made by impregnation and two unsupported HPW samples (bulk HPW and 100-/PS-15) were also used for comparison. The reaction selected was Friedel–Crafts alkylation of toluene with benzyl alcohol. The process produces water as byproduct, which is able to dissolve the active catalytic ingredient. We argued, therefore, that this reaction is a sensitive test to evaluate stability and durability of the samples. As seen in Fig. 11 bulk HPW shows some activity drop already in run 5, whereas for the other three catalysts decreasing activities are observed only in run 7. Subsequent data acquired in further repeated uses clearly show that the performance of the ball-milled 10-F/PS-15 catalyst exceeds those of all other samples studied. A short analysis of relevant observations found in literature and their comparison with the present data with respect to both activities and stabilities underline the characteristics of our samples made by mechanical treatment. Emphasis may be put on acylation, which is of practical importance. Results with silica-supported heteropoly acid catalysts disclosed recently indicate that high activities and high para selectivities can be achieved in
3.2.1. Sample characterization The results of the acid–base titrations and BET surface area measurements of Nafion-H materials all containing 15% Nafion are collected in Table 3. These include two samples made with Aldrich SiO2 in a stainless steel mill applying two milling times (15-A/SS-15 and 15-A/SS-50) and five catalysts prepared by using Fluka silica gel in a polystyrene mill (15-F/PS-h). The corresponding data for SAC-13, a commercially available Nafion-H silica nanocomposite prepared by the sol–gel method, are also given. It is seen that the quantity of Nafion accessible increases with increasing milling time. BET surface areas, in contrast, decrease as duration of the milling process increases just as observed for HPW samples (see Table 1). It is known [34] that as particles become smaller agglomeration becomes more pronounced. The local high-energy conditions in the surface particles promoted by the mechanical treatment induce particle coalescence and a corresponding decreases in the BET surface. As found for supported HPW materials a milling time of 15 h appears to be optimal. Indeed, SEM micrographs of samples milled for various time intervals show significant differences (Fig. 12). The presence of polyhedral particles with very narrow diameter distribution (the maximum size is below 10 mm) is seen for sample 15-F/PS-5 (Fig. 12, left picture). The other Table 3 Surface characterization of Nafion-H samples
Fig. 11. Recycling studies in Friedel–Crafts alkylation of four selected HPW/SiO2 catalysts (reaction time: 2 h).
Material
Acid capacity (mmol H+ g 1)
Surface area (m2 g 1)
Bulk Nafion 15-A/SS-15 15-A/SS-50 15-F/PS-5 15-F/PS-10 15-F/PS-15 15-F/PS-25 15-F/PS-50 SAC-13
0.90 0.24 0.65 0.42 0.45 0.62 0.68 0.65 0.67
n.d.a 21 16 313 310 277 230 11 682
a
Not determined.
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Fig. 12. SEM micrographs of sample 15-F/PS milled for various times.
Fig. 13. EDX results of sample 15-F/PS-50.
micrograph (sample 15-F/PS-50, Fig. 12, right picture) shows a completely different picture. As a result of the longer mechanical treatment, particles of medium size almost completely disappeared and powder coalescence resulted in the formation of much bigger, elongated grains with lengths larger than 80 mm. Furthermore, according to EDX results presented in Fig. 13, the active phase is not homogeneously distributed in the silica matrix, which is in contrast to what was observed for HPW samples. It is evident from these results that prolonged milling brings about the coalescence of Nafion particles. Most of the characteristic IR bands of Nafion-H are those of the fluorocarbon main chain (1100–1350, 805, 775, 630 cm 1) corresponding to various C–F vibrations [53]. In addition, bands due to the SO3 group (1210 cm 1) and the
Fig. 14. DRIFT spectra of SAC-13 and sample 15-F/PS-15.
C–O–C bond (980 cm 1) are also observed. Of these, bands at 1210, 1160, 800, and 630 cm 1 were observed in the spectrum of a 40% Nafion-H silica nanocomposite prepared by the sol–gel method [13]. Fig. 14 shows the spectra of SAC-13 and sample 15-F/PS-15. Due to the much lower Nafion content of these samples, the bands at 1210 and 1160 cm 1 appear as a shoulder of the strong and broad band of silica at around 1100 cm 1 (Si–O–Si stretching vibration). Two additional typical bands centered around 975 and 800 cm 1, however, are clearly visible. Peaks at 3340 and 3400 cm 1 are characteristic of adsorbed water. 3.2.2. Catalytic applications The results of catalytic test reactions determined in Friedel–Crafts alkylation and dimerization of 2-phenylpropene (AMS) are collected in Table 4. Catalytic activities in Friedel–Crafts alkylation show again that Aldrich SiO2 and the stainless steel mill are not ideal for catalyst synthesis. Samples fabricated in a polystyrene mill using Fluka silica exhibit varying initial activities. It is also interesting that initial activities of SAC13 are also very low. However, complete conversions can be attained at 60 min with all catalysts. Data with respect to the corresponding selectivities (Fig. 15), in turn, show only negligible differences with the two isomeric alkylated compounds as the main products. Dimerization of AMS is an important process to produce the open-chain and cyclic dimers 4-methyl-2,4-diphenylpent-1-ene and 1,1,3-trimethyl-3-phenylindane, respectively, which are applied in polymer production and in the electronic industry [54]. Studies have been made to
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263
Table 4 Catalytic performance of Nafion-H/SiO2 samplesa Material
Bulk Nafion 15-A/SS-15 15-A/SS-50 15-F/PS-5 15-F/PS-10 15-F/PS-15 15-F/PS-25 15-F/PS-50 SAC-13 a b c
Friedel–Crafts alkylationb
Dimerization of 2-phenylpropenec
15 min
30 min
60 min
30 min
6h
48 1 0 16 88 95 38 73 9
99 1 0 43 100 98 62 100 18
99 2 1 100 100 100 100 100 100
1 96 0 38 67 72 88 64 94
3 99 0 97 98 99 98 99 99
Values indicate conversions in %. Alkylation of toluene with benzyl alcohol at 384 K. Reaction in toluene at 333 K. Fig. 16. Selectivity in the dimerization of 2-phenylpropene over Nafion samples (data measured after 6 h).
test various catalysts and find appropriate reaction conditions for selective production of either of the two dimers [54–58]. As seen in Table 4, bulk Nafion-H exhibits very low reactivity. It is surprising that sample 15-A/SS-15, which was inactive in Friedel–Crafts alkylation shows very high activity in this reaction even at short reaction time. Wide changes in selectivities can be observed (Fig. 16). There are three important points to be emphasized. (i) The selectivity over the highly active 15-A/SS-15 catalyst is completely different than those of all other samples furnishing 4-methyl-2,4-diphenylpent-1-ene as the main product with high selectivity. (ii) 15-F/PS-5 exhibits the highest selectivity in the formation of the cyclic dimer 1,1,3trimethyl-3-phenylindane but selectivities decrease sharply with increasing milling time. Namely, selectivity of the formation of the cyclic dimer decreases, whereas that of 4methyl-2,4-diphenylpent-1-ene increases. (iii) The selectivity of 15-F/PS-5 exceeds even that of SAC-13. The activity data of the two test reactions show that at short milling time the acid capacity (the quantity of
accessible Nafion-H) measured by acid–base titration determines initial reactivities. At longer milling times, in turn, BET surface area is the determining factor to affect activities. As discussed, a longer milling process induces unfavorable changes, namely the coalescence (agglomeration) of Nafion particles. As a result, the accessibility of surface active sites by the reacting molecules is hindered. This may explain the contrasting behavior of 15-A/SS-15 and 15-F/PS-5 with respect to selectivity of AMS dimerization. It is known from literature studies that the substituted isomeric pentene dimers are intermediates in the formation of 1,1,3-trimethyl-3-phenylindane [54–56]. This can also be seen in the time-dependence of the reaction over the two samples exhibiting opposite selectivities (Figs. 17 and 18). The concentration of the 4-methyl-2,4-diphenylpent-1-ene isomer goes through a maximum over 15-F/PS-5, since it transforms into the indane derivative (Fig. 18). In contrast, the high initial concentration of 4-methyl-2,4diphenylpent-1-ene decreases only very slowly over 15-A/
Fig. 15. Selectivity in the Friedel–Crafts alkylation of toluene with benzyl alcohol over Nafion samples (data measured after 60 min).
Fig. 17. Time dependence of conversion and selectivities in the dimerization of 2-phenylpropene over 15-A/SS-15. *, conversion; &, 4-methyl-2,4diphenylpen-1-ene selectivity; ~, 4-methyl-2,4-diphenylpen-2-ene selectivity; *, 1,1,3-trimethyl-3-phenylindane selectivity.
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Hungarian National Science Foundation (OTKA grants T042603 and TS044690). This work was also funded by the Ministry of University in Italy, Rome (PRIN projects), and the University of Sassari. References
Fig. 18. Time dependence of conversion and selectivities in the dimerization of 2-phenylpropene over 15-F/PS-15. *, conversion; *, 1,1,3-trimethyl-3-phenylindane selectivity; &, 4-methyl-2,4-diphenylpen-1-ene selectivity; ~, 4-methyl-2,4-diphenylpen-2-ene selectivity.
SS-15 (Fig. 17). This indicates that it is not able to undergo cyclization, which is certainly due to the inaccessibility of the active sites to the large dimeric intermediate.
4. Conclusions High-energy ball-milling has been shown to be a promising alternative to sol–gel synthesis to prepare silica-supported heteropoly acid and Nafion-H materials for catalytic applications. A uniform distribution of HPW in the silica matrix is found for HPW-SiO2 samples, whereas particle coalescence is observed for Nafion samples after prolonged milling. Ball-milled HPW-SiO2 samples prepared under appropriately selected conditions exhibit catalytic properties similar to catalysts made by impregnation. HPW in ballmilled samples, in turn, proved to be more resistant to extraction with polar solvents (hot methanol) and these catalysts display higher stability in recycling studies. Supported Nafion-H samples prepared by ball-milling show high activities in test reactions. Sample 15-F/PS-5, the best catalyst, exceeds even SAC-13 in the selective synthesis of 1,1,3-trimethyl-3-phenylindane in the dimerization of 2phenylpropene. Catalysts with high catalytic performance require the use of optimal milling time, because particle coalescence and the resulting decrease in BET surface area hinder the accessibility of the active sites.
Acknowledgments We kindly acknowledge Dr. A. Canu and Dr. C. Deidda for technical assistance during SEM measurements and the ‘‘Centro Restauro-Soprintendenza Beni Archeologici, Sassari e Nuoro’’ where such measurements were carried out. We are most grateful for the financial support from the
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