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Adsorption of Wells–Dawson tungsten heteropolyacid on sol–gel alumina: Structural features and thermal stability
Journal of Colloid and Interface Science 292 (2005) 486–492 www.elsevier.com/locate/jcis
Adsorption of Wells–Dawson tungsten heteropolyacid on sol–gel alumina: Structural features and thermal stability A. Tarlani a , M. Abedini a , M. Khabaz b , M. Mohammadpour Amini b,∗ a Department of Chemistry, University of Tehran, Tehran, Iran b Department of Chemistry, Shahid Beheshti University, Tehran, Iran
Received 4 March 2005; accepted 26 May 2005 Available online 3 August 2005
Abstract The Wells–Dawson tungsten heteropolyacid, H6 P2 W18 O62 supported on sol–gel and non-sol–gel alumina has been investigated by infrared spectroscopy (IR), thermal analysis (TGA/DSC), and X-ray diffraction (XRD). X-ray diffraction indicates that the heteropolyacid primary structure in bulk form holds up to 350 ◦ C and by supporting it on the sol–gel alumina the thermal stability rose to 650 ◦ C. UV–vis spectroscopy showed that the sol–gel alumina has a higher tendency to adsorb Wells–Dawson tungsten heteropolyacid than the non-sol–gel alumina. The heteropolyacid showed higher interaction with the sol–gel alumina than with the non-sol–gel. Esterification of propanoic acid with hexanol in the presence of alumina-supported heteropolyacid revealed that the acidic character of the heteropolyacid remains active to some extent. 2005 Elsevier Inc. All rights reserved. Keywords: Heteropolyacid; Wells–Dawson; Sol–gel; Alumina; Adsorption
1. Introduction Research and development in the field of catalysis in the new millennium has been directed toward the preparation of catalysts with enhanced performance and less hazard to the environment [1–3]. Among many catalysts, heteropolyacids (HPAs) with variety of catalytic applications have been a focus of this attention [4–6]. Heteropolyacids have diverse structures and their constitutional elements can readily be varied and tuned for different catalytic applications in homogeneous and heterogeneous systems. Due to their practical applications, HPAs in heterogeneous form are more desirable and a significant number of them have been prepared for liquid- or gas-phase reactions by supporting Keggin- or Dawson-type HPAs on a wide variety of supports. Acidity of the HPAs for certain catalytic applications can be ad* Corresponding author. Fax: +98 21 2403041.
E-mail address: [email protected] (M.M. Amini). 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.05.090
justed by choosing suitable supports. Apparently, besides the surface area, particle size, pore structure, and distribution of protons of HPAs, which are usually referred to as the elements of tertiary structure, the nature of the support is very influential on catalytic activity. The tertiary structure actually is the HPA’s assembled structure and has a significant impact on the activity of solid HPAs, along with the primary and secondary structure of HPAs [5]. Realizing the higher advantage of heterogeneous systems, behavior and stability of Keggin-type HPAs on several supports have been investigated [7–10]. Stability and catalytic activity of supported HPAs depend on the type of HPA [11–13] in addition to the nature of the support [7,14,15]. Furthermore, recent studies show that the nature of supported species and also their stability are related to the conditions of catalyst preparation, including pH, concentration of impregnating solution, and HPA loading on support, along with the above-mentioned parameters [16,17]. Stability of 12tungstophosphoric and 12-molybdophosphoric acids on ba-
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sic support has been the subject of debate [18]. Misono believes that supports with basic sites on their surfaces can decompose heteropolyanions [19]. However, formation of aluminum salt rather than decomposition has been suggested for 12-molybdophosphoric acid and 12-tungstosilicate supported on alumina [20]. Interestingly, it seems that the latter is a very efficient catalyst for the conversion of methanol to hydrocarbon [21]. Furthermore, a recent study shows that the type of alumina is very influential on the stability of HPA [7]. The dimeric heteropolyanion is the dominant species on a moderately basic alumina, whereas the lacunary species is the dominant one on more basic alumina [7]. This is in good agreement with the solution stability scheme of 12-tungstophosphoric acid, which revealed that the dimeric and lacunary species exist at lower and higher hydroxyl concentrations, respectively [22]. Similar behavior has been disclosed for the 12-tungstophosphoric acid supported on MCM-41 molecular sieve, by formation of lacunary species due to basic impurities or impregnation in aqueous solution with pH above 2, whereas the Keggin structure is not stable [9]. The majority of research in the HPAs and heteropolyanion-supported systems have been carried out with Keggin type HPAs. By realizing that the Dawsontype HPA, H6 P2 W18 O62 , is more active than the Keggin counterpart for the gas phase reactions [23] and that the former is thermally less stable than the latter [24], stabilization of Dawson HPA on alumina support for high-temperature gas phase reactions and investigation of its behavior could be fruitful. In this work we report on the interaction of Dawson HPA, H6 P2 W18 O62 , with sol–gel derived and commercial alumina and demonstrate its thermal stability on alumina up to 650 ◦ C in comparison to the unsupported one, which converts to Keggin structure at 620 ◦ C.
2. Experimental 2.1. Materials Aluminum 2-butoxide was synthesized from aluminum and 2-butanol according to the general method for preparation of aluminum alkoxides. Commercial alumina (Merck) with surface area 190 m2 /g and sol–gel derived alumina with surface area 220 m2 /g were used as supports. The sol–gel derived alumina was prepared from aluminum 2-butoxide according to the previously reported procedure [25]. The potassium salt of the Wells–Dawson type heteropolyacid, K6 P2 W18 O62 , was prepared according to the procedure described by Randall [26] and then was converted to the corresponding acid, H6 P2 W18 O62 , by passing it through a Dowex-50W-X8 ion exchange column. In the final step water was removed under reduced pressure and the remaining solid was collected and dried at 120 ◦ C overnight. Infrared and NMR measurements confirm the purity of prepared Dawson HPA in this work. 31 P NMR, −12.7 ppm. Literature, −12.9 [27].
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2.2. Methods The UV–vis spectra of solutions in the range of 200– 400 nm were recorded on a Shimadzu 2100 spectrophotometer, using a quartz cell of optical path 10.0. Infrared spectra were recorded (KBr pellets) on a Shimadzu Model FT-IR 4600 spectrometer. The 31 P NMR spectrum of heteropolyacid was recorded in aqueous solution (vs 85% H3 PO4 ) using a Brucker 300 MHz spectrometer. Thermal analysis of the HPA and alumina-supported HPA were performed on a Rheometric Scientific STA-1500 with heating rate 10 ◦ C/min. X-ray diffraction patterns were obtained on a Philips-PW 17C diffractometer with CuKα radiation. Scans were performed from (2θ ) 4◦ –70◦ at a rate of 3◦ /min. The total acidity of catalysts was determined according to the literature procedure by titration with a standard solution of sodium hydroxide [44]. 2.3. Adsorption studies and preparation of alumina-supported HPA To determine the optimum contact time that is required for the alumina to adsorb HPA from impregnating solution, and also to see to what extent HPA can be loaded on the sol– gel and commercial alumina, 0.01 g sol–gel derived or commercial alumina was suspended in a 20-ml aqueous solution of HPA with initial concentration 0.25 mg/ml. The suspension was mixed at room temperature and the contact time varied from 30 to 1440 min. Then the solid was removed from the solution by centrifugation and UV–vis spectra of solutions were recorded. Alumina-supported HPA was prepared by impregnation. For impregnation, with 10% HPA loading, 0.5 g HPA was dissolved in 4 ml water and was added to 4.5 g commercial alumina. The suspension was mixed for 24 h at room temperature and then dried for 48 h at 75 ◦ C. Impregnation of sol–gel derived alumina with HPA and heat treatment was performed in a similar manner, but due to the absorption of a large amount of water by the sol–gel derived alumina, 15 ml water was used. Alumina-supported HPA samples with 30% HPA loading were prepared similarly. To remove physically adsorbed HPA, all alumina-supported HPA samples were washed with distilled water several times and dried at 100 ◦ C. 2.4. Catalytic reaction The catalytic acidity of alumina-supported HPA was examined in an acid catalyst reaction. In a typical reaction 2.0 g of catalyst with 30% loading was suspended in 150 ml of toluene containing 11.1 g (0.15 mol) propanoic acid and 30.6 g (0.300 mol) hexanol in a round-bottom flask, which was equipped with a Dean–Stark trap and condenser. The reaction mixture was refluxed for 3 h. Progress of the reaction was monitored by collected water in the Dean–Stark trap. After esterification, the catalyst was removed by filtration
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Fig. 1. Infrared spectra of H6 P2 W18 O62 ·nH2 O after heat treatment at (a) 100, (b) 300, (c) 600, and (d) 700 ◦ C.
and the ester was isolated by distillation as the sole product and its formation was confirmed by IR, NMR, and mass spectroscopes. Conversions were calculated from amount of collected water.
3. Results and discussion 3.1. Properties of bulk HPA The thermal behavior of H6 P2 W18 O62 has been investigated by infrared spectroscopy. Characteristic bands of unsupported HPA appear at 1090, 962, 914, and 778 cm−1 (Fig. 1a), which correspond to P–O, W=O, W–O–Was and W–O–Ws vibrations, respectively, and they are in good agreement with the previously reported values [27]. The HPA major bands remain unchanged upon heat treatment for 5 h at 300 ◦ C (Fig. 1b), which indicates that the Dawson primary structure, H6 P2 W18 O62 , remains intact up to 300 ◦ C. This temperature is slightly higher than the previously reported value (<300 ◦ C) [6], which could be due to the higher concentration of α-H6 P2 W18 O62 form in the present study.
When heat treatment temperature is increased above 300 ◦ C the bands shift slightly and become broad, but the Dawson structure characteristic bands remain favorably constant, which indicates that the HPA begins to lose its secondary structure (H6 P2 W18 O62 ·nH2 O) and probably some modification also occurs in the primary structure. The term primary structure of HPA refers only to the structure of the large polyanion and the term secondary structure refers to the three-dimensional arrangement of large polyanions, cations, and water of crystallization [5]. No significant change was observed in the IR spectra after heat treatment up to 600 ◦ C for 5 h (Fig. 1c), except broadening of peaks and decrease in their intensities. In fact, the thermal stability of H6 P2 W18 O62 has been a controversial subject; on the basis of FTIR results several authors reported that the Dawson HPA decomposes and loses its catalytic activity at 300 ◦ C [5]. However, Baronetti et al., with XRD and FTIR results, showed that the Dawson HPA keeps its primary structure and catalytic activity up to 600 ◦ C [28]. From the FTIR data of heat-treated HPA samples it is concluded that due to dehydration and modification, which take place in the secondary structure, and also possible disorder in the primary structure, a definite identification of primary structure above 350 ◦ C was uncertain. By taking into account that the hydration–dehydration process for the Dawson HPA is reversible [28], inconsistency in thermal stability of HPA in various reports is not surprising. We believe that the inconsistency simply arises from various heat-treatment details and the extreme hygroscopic nature of dehydrated HPA. Interestingly, this behavior also has been noticed to some extent in the XRD patterns of fresh and calcined samples at 200 and 300 ◦ C [28]. Furthermore, the IR spectrum of the sample calcined for 5 h at 650 ◦ C revealed peaks at 1084, 976, and 887 cm−1 , which suggest that a Keggin-type framework has been formed [29]. In this work, by heat treatment at 700 ◦ C for 5 h, the Keggin structure collapsed and typical bands of the WO3 –P2 O5 system in the region 1000–1200 cm−1 appeared in the IR spectrum (Fig. 1d) [30]. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were also employed to investigate the thermal behavior of Dawson HPA. The TGA curve of HPA (Fig. 2) shows about 9% weight loss upon heating to 350 ◦ C. This weight loss is accompanied by several endothermic peaks in the DSC curve, which indicates that the weight loss is due primarily to the release of about 24 mol of crystallization water at various stages in the range 75– 350 ◦ C. This is in good agreement with the IR data and shows that HPA retains its primary structure framework. The TGA curve of HPA shows about 2% weight loss above 350 ◦ C, which corresponds to the release of constitutional water, deprivation from primary structure, and transmutation to other species, according to XRD results (in situ). There is no significant weight loss above 400 ◦ C, and at 630 ◦ C, an exothermic peak appears in the DSC curve, which indicates that the intermediate compound converts to PW12 O38.5 , a Keggin-type species. Formation of a defect
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Fig. 2. DSC and TGA curves of H6 P2 W18 O62 ·nH2 O.
Keggin framework was confirmed by XRD (in situ). Interestingly, dehydration of Keggin-type HPA at elevated temperature and formation of PW12 O38.5 have previously been reported [31,32]. Such behavior has also been observed for H3 PMo12 O40 and ascribed to the loss of lattice oxygen and formation of PMo12 O38.5 species [33]. The exothermic peak at 730 ◦ C is attributed to decomposition of HPA to phosphorus and tungsten oxides in accordance with XRD analysis. Formation of various species by heat treatment of HPA was followed by X-ray diffraction. Figs. 3a, 3b, 3c, and 3d show the X-ray diffraction patterns of bulk H6 P2 W18 O62 · nH2 O after heat treatment for 5 h at 100, 300, 500, and 650 ◦ C, respectively. In Fig. 3a the X-ray diffraction peaks with d space values of 12.92, 9.90, 9.56, 3.61, 3.52, 3.31, and 2.95, in Fig. 3b with values of 11.52, 9.19, 4.78, 4.45, 3.73, 3.51, 3.50, 3.07, 2.96, 2.82, 2.63, 2.48, 2.38, and 2.25, and in Fig. 3d with values of 4.45, 3.84, 3.73, 2.67, 2.65, 2.18, 1.87, and 1.68 match well to H6 P2 W18 O62 ·nH2 O, H6 P2 W18 O62 , and PW12 O38.5 , respectively [28,34], and the interpretation is in good agreement with the thermal analysis results, indicating that the HPA loses constitutional water upon heat treatment above 350 ◦ C. However, the XRD pattern of a sample obtained after heat treatment of HPA at 500 ◦ C was not associated with pure HPA. The major crystalline phase appeared to be a mixture and identification was uncertain, in contrast to a previous report [28]. Such uncertainty has also been reported for the potassium salt of HPA [35]. As mentioned above, the inconsistency may be due to the difference in heat treatment process and sample storage.
Fig. 3. XRD patterns of H6 P2 W18 O62 ·nH2 O after heat treatment at (a) 100, (b) 300, (c) 500, and (d) 650 ◦ C.
3.2. HPA adsorption on sol–gel and commercial alumina Adsorption of HPA on alumina was studied by UV– vis spectroscopy. Fig. 4 shows the absorption spectra of a HPA solution with concentration 0.25 mg/ml (a) after contact for 24 h with commercial (b) and sol–gel derived alumina (c). As can be seen, concentration of HPA in solution decreased significantly after contact with the sol–gel alumina compared to that in contact with commercial alumina.
Fig. 4. UV–vis spectra of a HPA solution (a) before, and after contact with (b) commercial and (c) sol–gel derived alumina.
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Fig. 5. Adsorption of HPA on sol–gel (a) and commercial alumina (b) as a function of time.
The amount of HPA that adsorbed on the sol–gel alumina is about 1.86 mg/m2 and on the commercial alumina about 0.84 mg/m2 . The higher tendency of sol–gel alumina to adsorb HPA in comparison to the commercial alumina is attributed in a great extent to the hydroxyl-rich surface of the sol–gel alumina rather than to its larger surface area. The strong interaction between HPA and the surface OH groups can be interpreted as transfer of acidic protons of the strong Wells–Dawson HPA to the OH groups of the alumina surface. The endowment of sol–gel derived alumina with a hydroxyl-rich surface was obvious from the IR spectrum of the freshly prepared sample. By taking into account that the interaction between HPA and support is mainly electrostatic in nature [9], a stronger interaction of HPA with the sol– gel alumina is expected. No attempt was made to quantify the number of hydroxyl groups on the aluminas. The final spectra of HPA solutions after contact with the two types of aluminas are qualitatively similar, which indicates that the HPA structure remained intact in solution. This conclusion was also confirmed by 31 P NMR of HPA solution before and after contact with aluminas. Kinetics of adsorption of HPA on sol–gel and commercial aluminas, as shown in Fig. 5, has been investigated by UV–vis spectroscopy for an interval of 10–750 min using a solution with a concentration of 0.25 mg HPA/ml. The concentration of adsorbed HPA on both aluminas after about 30 min remained constant; however, in the plateau of the adsorption curve versus time, the amount of HPA adsorbed on sol–gel alumina is more than twice the amount on commercial alumina. These results signify the important role of sol–gel base oxides in supported catalysis. 3.3. Heteropolyacid/alumina characterization The infrared spectrum of Dawson HPA supported on the sol–gel derived alumina with 10% loading (Fig. 6a) shows bands at 1072, 951, 899, and 783 cm−1 . Apparently, low loading resulted in a strong interaction of HPA with alumina and consequently a drastic shift of bands in the infrared spectrum. For the commercial alumina (Fig. 6b), bands appeared at 1095, 955, 906, and 817 cm−1 . These data indicated that the type of alumina is influential on the interaction
Fig. 6. Infrared spectrum of Dawson HPA supported on sol–gel derived alumina (a) and on commercial alumina (b) with 10% loading.
Fig. 7. DSC and TGA curves of H6 P2 W18 O62 ·nH2 O supported on sol–gel alumina with 10% loading.
of HPA and alumina and the type of interaction is different to some extent. Such drastic behavior of alumina type has been reported for the Keggin-type HPA supported on alumina, which is related to the acid–base character of alumina [7]. In this context, a different structural feature for the HPA species supported on the sol–gel derived alumina, which is known to be endowed with a hydroxyl-rich surface, is not unusual. This kind of interaction previously has been reported for the silica-supported Dawson HPA and attributed to the interaction of silanol groups of the support and the octahedral WO6 species [36]. Interestingly, recent study shows that this type of interaction leads to the formation of (SiOH2 )+ (H2 PW12 O40 )− [9,37], which corresponds to a Keggin-type HPA structure. Thermal analysis of HPA supported on sol–gel alumina with 10% loading, as shown in Fig. 7, exhibits no exo- or endothermic peaks above 200 ◦ C, but there is a steady decrease in weight. Similar behavior was observed for the sol–gel and commercial aluminas in higher loading. This type of thermal behavior has previously been observed in zeolites and other HPAs with Wells–Dawson structures [38].
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Table 1 Esterification yields of propanoic acid with HPA in heterogeneous system Catalyst
Reaction Total acidity Conversion time (min) (mmol/g) (%)
H6 P2 W18 O62 /γ -Al2 O3 (sol–gel) 180 180 H6 P2 W18 O62 /γ -Al2 O3 γ -Al2 O3 (sol–gel) 180 180 γ -Al2 O3 None 180
Fig. 8. XRD pattern of H6 P2 W18 O62 ·nH2 O supported on sol–gel alumina after heat treatment at (a) 300 and (b) 650 ◦ C.
The X-ray diffraction patterns of HPA supported on the sol–gel and commercial alumina with 10–30% loading showed amorphous patterns. However, after calcination at 300 ◦ C, as shown for the sol–gel alumina with 10% loading (Fig. 8a), very weak and broad peaks developed. Similar patterns were observed for HPA supported on both aluminas up to 30% loading after heat treatment at 300 ◦ C. Such an amorphous pattern has previously been reported for the Keggin-type HPA on various supports with loading levels of as high as 50% and explained as the result of high dispersion of adsorbed species on the support and lack of large HPA particles on the support [7,9,39,40]. Interestingly, only when HPA loading increased to 30% on the sol–gel alumina and heat treatment was at 650 ◦ C for 5 h, XRD revealed a reasonably fine pattern, Fig. 8b, which corresponds to P2 W18 O59 species [41]. It is worth noting that the XRD pattern of HPA supported on the commercial alumina after heat treatment at 600 ◦ C for 5 h also matched well to P2 W18 O59 . The XRD pattern of HPA supported on the commercial alumina after heat treatment at the same temperature was similar. Apparently, thermal stability of Dawson HPA is enhanced to 650 ◦ C and primary structure preserved by supporting it on aluminas, whereas the bulk one transmutes to a mixture of unknown phases in the range of 350–500 ◦ C and to a species with Keggin skeleton above 650 ◦ C. Such higher thermal stability has been observed for the Keggin-type HPA supported on mesoporous silica and has been correlated to the loss of protons or acidic groups [8,9]. 3.4. Catalyst acidity It is generally accepted that the esterification reactions catalyzed by Brønsted acids and also the yields depend on concentration of either alcohol or acid. In the present study alcohol was used as excess reagent. In order to demonstrate that the catalytic activity of Brønsted acid of alumina-
0.21 0.09 0.02 Neutral –
30 22 15 11 0
supported HPA remains intact, we have used it as an acid catalyst in the esterification of propanoic acid with hexanol. Esterification yields in the presence of alumina-supported HPA and also pure sol–gel and commercial alumina are given in Table 1. Apparently, acidic sites of Wells–Dawson HPA supported on sol–gel and commercial alumina remain active to some extent for the acid catalyst reactions, the yield is relatively higher for the former than the latter, and both are more active than pure alumina. By taking into consideration that the surface areas of supported catalysts decrease where oxides are used as supports [42], one can conclude that the higher activity of alumina-supported HPA is due to the higher acidity rather than to alteration in the surface area of supported alumina. For further proof the total acidity of all samples were measured and the results are given in Table 1. Notably, the acidity of sol–gel alumina increases to a large extent and commercial alumina to a lesser extent when HPA is supported on them. The higher total acidity value of sol–gel alumina-supported HPA is reflected in the esterification yields. Furthermore, the higher catalytic activity of bare sol–gel alumina in comparison to the commercial one can be attributed to higher surface area of the former. Although esterifications of various alcohols by HPAs have been investigated extensively [43], only a limited number of them are HPAs with Wells–Dawson structure. In addition, the majority of investigations have been carried out in a homogeneous system, in contrast to the heterogeneous system in the present study. Furthermore, type of support, HPA loading, esterification conditions, and type of alcohol and acid used in various studies are different, which makes comparison among them difficult. Recent work showed that the esterification yields for the Keggin HPA supported on commercial alumina strongly depend on the type of acid and alcohol used when other parameters are kept constant [44]. Notably, the esterification yield of butanol with propanoic acid with a similar loading for the Keggin HPA supported on alumina is higher than the esterification yield in the present work [44]. Higher yield can be rationalized by the stronger acidic nature of the bulk Keggin HPA and/or the size of the reacting alcohol. Carbon-supported Keggin HPA also was used for the esterification of propanoic and acrylic acids by butanol [45,46]. Although carbon-supported HPA is a very efficient catalyst for esterification, its activity is lower than that of the unsupported HPA. Finally, it should be mentioned that the activity of HPA supported on alumina in a heterogeneous esterification reaction decreased significantly in com-
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parison to that in the homogeneous system [47], and this is consistent with the earlier report that the acidity of HPAs is reduced by supporting them on oxides [48].
4. Conclusion The thermal behavior of Wells–Dawson tungsten HPA supported on sol–gel and non-sol–gel aluminas was investigated. It was shown that the type of alumina is influential on the interaction of heteropolyacids/alumina, and the amount of adsorbed HPA is also different to a large extent. X-ray diffraction indicates that the primary structure of bulk HPA holds up to 350 ◦ C and by supporting it on sol–gel alumina the thermal stability rose to 650 ◦ C. The XRD results also show that the HPA is well dispersed on alumina and heattreatment temperatures have a marked effect on the X-ray diffraction patterns of supported HPA. The stronger interaction of HPA with sol–gel alumina is attributed to the fact that the sol–gel derived alumina is endowed with a hydroxyl-rich surface. By supporting HPA on aluminas, the total acidity of sol–gel alumina increases to a large extent and commercial alumina to a lesser extent. The higher value of the total acidity of sol–gel alumina-supported HPA is reflected in the esterification yield of propanoic acid with hexanol and indicates that the acidic sites of the supported HPA remain active to some extent. Investigation of catalytic activity of aluminasupported HPA in oxidation reactions is in progress. Acknowledgment The generous financial support of the Research Council of University of Tehran (Grant 514-4-589) is gratefully acknowledged.
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Adsorption of Wells–Dawson tungsten heteropolyacid on sol–gel alumina: Structural features and thermal stability A. Tarlani a , M. Abedini a , M. Khabaz b , M. Mohammadpour Amini b,∗ a Department of Chemistry, University of Tehran, Tehran, Iran b Department of Chemistry, Shahid Beheshti University, Tehran, Iran
Received 4 March 2005; accepted 26 May 2005 Available online 3 August 2005
Abstract The Wells–Dawson tungsten heteropolyacid, H6 P2 W18 O62 supported on sol–gel and non-sol–gel alumina has been investigated by infrared spectroscopy (IR), thermal analysis (TGA/DSC), and X-ray diffraction (XRD). X-ray diffraction indicates that the heteropolyacid primary structure in bulk form holds up to 350 ◦ C and by supporting it on the sol–gel alumina the thermal stability rose to 650 ◦ C. UV–vis spectroscopy showed that the sol–gel alumina has a higher tendency to adsorb Wells–Dawson tungsten heteropolyacid than the non-sol–gel alumina. The heteropolyacid showed higher interaction with the sol–gel alumina than with the non-sol–gel. Esterification of propanoic acid with hexanol in the presence of alumina-supported heteropolyacid revealed that the acidic character of the heteropolyacid remains active to some extent. 2005 Elsevier Inc. All rights reserved. Keywords: Heteropolyacid; Wells–Dawson; Sol–gel; Alumina; Adsorption
1. Introduction Research and development in the field of catalysis in the new millennium has been directed toward the preparation of catalysts with enhanced performance and less hazard to the environment [1–3]. Among many catalysts, heteropolyacids (HPAs) with variety of catalytic applications have been a focus of this attention [4–6]. Heteropolyacids have diverse structures and their constitutional elements can readily be varied and tuned for different catalytic applications in homogeneous and heterogeneous systems. Due to their practical applications, HPAs in heterogeneous form are more desirable and a significant number of them have been prepared for liquid- or gas-phase reactions by supporting Keggin- or Dawson-type HPAs on a wide variety of supports. Acidity of the HPAs for certain catalytic applications can be ad* Corresponding author. Fax: +98 21 2403041.
E-mail address: [email protected] (M.M. Amini). 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.05.090
justed by choosing suitable supports. Apparently, besides the surface area, particle size, pore structure, and distribution of protons of HPAs, which are usually referred to as the elements of tertiary structure, the nature of the support is very influential on catalytic activity. The tertiary structure actually is the HPA’s assembled structure and has a significant impact on the activity of solid HPAs, along with the primary and secondary structure of HPAs [5]. Realizing the higher advantage of heterogeneous systems, behavior and stability of Keggin-type HPAs on several supports have been investigated [7–10]. Stability and catalytic activity of supported HPAs depend on the type of HPA [11–13] in addition to the nature of the support [7,14,15]. Furthermore, recent studies show that the nature of supported species and also their stability are related to the conditions of catalyst preparation, including pH, concentration of impregnating solution, and HPA loading on support, along with the above-mentioned parameters [16,17]. Stability of 12tungstophosphoric and 12-molybdophosphoric acids on ba-
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sic support has been the subject of debate [18]. Misono believes that supports with basic sites on their surfaces can decompose heteropolyanions [19]. However, formation of aluminum salt rather than decomposition has been suggested for 12-molybdophosphoric acid and 12-tungstosilicate supported on alumina [20]. Interestingly, it seems that the latter is a very efficient catalyst for the conversion of methanol to hydrocarbon [21]. Furthermore, a recent study shows that the type of alumina is very influential on the stability of HPA [7]. The dimeric heteropolyanion is the dominant species on a moderately basic alumina, whereas the lacunary species is the dominant one on more basic alumina [7]. This is in good agreement with the solution stability scheme of 12-tungstophosphoric acid, which revealed that the dimeric and lacunary species exist at lower and higher hydroxyl concentrations, respectively [22]. Similar behavior has been disclosed for the 12-tungstophosphoric acid supported on MCM-41 molecular sieve, by formation of lacunary species due to basic impurities or impregnation in aqueous solution with pH above 2, whereas the Keggin structure is not stable [9]. The majority of research in the HPAs and heteropolyanion-supported systems have been carried out with Keggin type HPAs. By realizing that the Dawsontype HPA, H6 P2 W18 O62 , is more active than the Keggin counterpart for the gas phase reactions [23] and that the former is thermally less stable than the latter [24], stabilization of Dawson HPA on alumina support for high-temperature gas phase reactions and investigation of its behavior could be fruitful. In this work we report on the interaction of Dawson HPA, H6 P2 W18 O62 , with sol–gel derived and commercial alumina and demonstrate its thermal stability on alumina up to 650 ◦ C in comparison to the unsupported one, which converts to Keggin structure at 620 ◦ C.
2. Experimental 2.1. Materials Aluminum 2-butoxide was synthesized from aluminum and 2-butanol according to the general method for preparation of aluminum alkoxides. Commercial alumina (Merck) with surface area 190 m2 /g and sol–gel derived alumina with surface area 220 m2 /g were used as supports. The sol–gel derived alumina was prepared from aluminum 2-butoxide according to the previously reported procedure [25]. The potassium salt of the Wells–Dawson type heteropolyacid, K6 P2 W18 O62 , was prepared according to the procedure described by Randall [26] and then was converted to the corresponding acid, H6 P2 W18 O62 , by passing it through a Dowex-50W-X8 ion exchange column. In the final step water was removed under reduced pressure and the remaining solid was collected and dried at 120 ◦ C overnight. Infrared and NMR measurements confirm the purity of prepared Dawson HPA in this work. 31 P NMR, −12.7 ppm. Literature, −12.9 [27].
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2.2. Methods The UV–vis spectra of solutions in the range of 200– 400 nm were recorded on a Shimadzu 2100 spectrophotometer, using a quartz cell of optical path 10.0. Infrared spectra were recorded (KBr pellets) on a Shimadzu Model FT-IR 4600 spectrometer. The 31 P NMR spectrum of heteropolyacid was recorded in aqueous solution (vs 85% H3 PO4 ) using a Brucker 300 MHz spectrometer. Thermal analysis of the HPA and alumina-supported HPA were performed on a Rheometric Scientific STA-1500 with heating rate 10 ◦ C/min. X-ray diffraction patterns were obtained on a Philips-PW 17C diffractometer with CuKα radiation. Scans were performed from (2θ ) 4◦ –70◦ at a rate of 3◦ /min. The total acidity of catalysts was determined according to the literature procedure by titration with a standard solution of sodium hydroxide [44]. 2.3. Adsorption studies and preparation of alumina-supported HPA To determine the optimum contact time that is required for the alumina to adsorb HPA from impregnating solution, and also to see to what extent HPA can be loaded on the sol– gel and commercial alumina, 0.01 g sol–gel derived or commercial alumina was suspended in a 20-ml aqueous solution of HPA with initial concentration 0.25 mg/ml. The suspension was mixed at room temperature and the contact time varied from 30 to 1440 min. Then the solid was removed from the solution by centrifugation and UV–vis spectra of solutions were recorded. Alumina-supported HPA was prepared by impregnation. For impregnation, with 10% HPA loading, 0.5 g HPA was dissolved in 4 ml water and was added to 4.5 g commercial alumina. The suspension was mixed for 24 h at room temperature and then dried for 48 h at 75 ◦ C. Impregnation of sol–gel derived alumina with HPA and heat treatment was performed in a similar manner, but due to the absorption of a large amount of water by the sol–gel derived alumina, 15 ml water was used. Alumina-supported HPA samples with 30% HPA loading were prepared similarly. To remove physically adsorbed HPA, all alumina-supported HPA samples were washed with distilled water several times and dried at 100 ◦ C. 2.4. Catalytic reaction The catalytic acidity of alumina-supported HPA was examined in an acid catalyst reaction. In a typical reaction 2.0 g of catalyst with 30% loading was suspended in 150 ml of toluene containing 11.1 g (0.15 mol) propanoic acid and 30.6 g (0.300 mol) hexanol in a round-bottom flask, which was equipped with a Dean–Stark trap and condenser. The reaction mixture was refluxed for 3 h. Progress of the reaction was monitored by collected water in the Dean–Stark trap. After esterification, the catalyst was removed by filtration
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Fig. 1. Infrared spectra of H6 P2 W18 O62 ·nH2 O after heat treatment at (a) 100, (b) 300, (c) 600, and (d) 700 ◦ C.
and the ester was isolated by distillation as the sole product and its formation was confirmed by IR, NMR, and mass spectroscopes. Conversions were calculated from amount of collected water.
3. Results and discussion 3.1. Properties of bulk HPA The thermal behavior of H6 P2 W18 O62 has been investigated by infrared spectroscopy. Characteristic bands of unsupported HPA appear at 1090, 962, 914, and 778 cm−1 (Fig. 1a), which correspond to P–O, W=O, W–O–Was and W–O–Ws vibrations, respectively, and they are in good agreement with the previously reported values [27]. The HPA major bands remain unchanged upon heat treatment for 5 h at 300 ◦ C (Fig. 1b), which indicates that the Dawson primary structure, H6 P2 W18 O62 , remains intact up to 300 ◦ C. This temperature is slightly higher than the previously reported value (<300 ◦ C) [6], which could be due to the higher concentration of α-H6 P2 W18 O62 form in the present study.
When heat treatment temperature is increased above 300 ◦ C the bands shift slightly and become broad, but the Dawson structure characteristic bands remain favorably constant, which indicates that the HPA begins to lose its secondary structure (H6 P2 W18 O62 ·nH2 O) and probably some modification also occurs in the primary structure. The term primary structure of HPA refers only to the structure of the large polyanion and the term secondary structure refers to the three-dimensional arrangement of large polyanions, cations, and water of crystallization [5]. No significant change was observed in the IR spectra after heat treatment up to 600 ◦ C for 5 h (Fig. 1c), except broadening of peaks and decrease in their intensities. In fact, the thermal stability of H6 P2 W18 O62 has been a controversial subject; on the basis of FTIR results several authors reported that the Dawson HPA decomposes and loses its catalytic activity at 300 ◦ C [5]. However, Baronetti et al., with XRD and FTIR results, showed that the Dawson HPA keeps its primary structure and catalytic activity up to 600 ◦ C [28]. From the FTIR data of heat-treated HPA samples it is concluded that due to dehydration and modification, which take place in the secondary structure, and also possible disorder in the primary structure, a definite identification of primary structure above 350 ◦ C was uncertain. By taking into account that the hydration–dehydration process for the Dawson HPA is reversible [28], inconsistency in thermal stability of HPA in various reports is not surprising. We believe that the inconsistency simply arises from various heat-treatment details and the extreme hygroscopic nature of dehydrated HPA. Interestingly, this behavior also has been noticed to some extent in the XRD patterns of fresh and calcined samples at 200 and 300 ◦ C [28]. Furthermore, the IR spectrum of the sample calcined for 5 h at 650 ◦ C revealed peaks at 1084, 976, and 887 cm−1 , which suggest that a Keggin-type framework has been formed [29]. In this work, by heat treatment at 700 ◦ C for 5 h, the Keggin structure collapsed and typical bands of the WO3 –P2 O5 system in the region 1000–1200 cm−1 appeared in the IR spectrum (Fig. 1d) [30]. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were also employed to investigate the thermal behavior of Dawson HPA. The TGA curve of HPA (Fig. 2) shows about 9% weight loss upon heating to 350 ◦ C. This weight loss is accompanied by several endothermic peaks in the DSC curve, which indicates that the weight loss is due primarily to the release of about 24 mol of crystallization water at various stages in the range 75– 350 ◦ C. This is in good agreement with the IR data and shows that HPA retains its primary structure framework. The TGA curve of HPA shows about 2% weight loss above 350 ◦ C, which corresponds to the release of constitutional water, deprivation from primary structure, and transmutation to other species, according to XRD results (in situ). There is no significant weight loss above 400 ◦ C, and at 630 ◦ C, an exothermic peak appears in the DSC curve, which indicates that the intermediate compound converts to PW12 O38.5 , a Keggin-type species. Formation of a defect
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Fig. 2. DSC and TGA curves of H6 P2 W18 O62 ·nH2 O.
Keggin framework was confirmed by XRD (in situ). Interestingly, dehydration of Keggin-type HPA at elevated temperature and formation of PW12 O38.5 have previously been reported [31,32]. Such behavior has also been observed for H3 PMo12 O40 and ascribed to the loss of lattice oxygen and formation of PMo12 O38.5 species [33]. The exothermic peak at 730 ◦ C is attributed to decomposition of HPA to phosphorus and tungsten oxides in accordance with XRD analysis. Formation of various species by heat treatment of HPA was followed by X-ray diffraction. Figs. 3a, 3b, 3c, and 3d show the X-ray diffraction patterns of bulk H6 P2 W18 O62 · nH2 O after heat treatment for 5 h at 100, 300, 500, and 650 ◦ C, respectively. In Fig. 3a the X-ray diffraction peaks with d space values of 12.92, 9.90, 9.56, 3.61, 3.52, 3.31, and 2.95, in Fig. 3b with values of 11.52, 9.19, 4.78, 4.45, 3.73, 3.51, 3.50, 3.07, 2.96, 2.82, 2.63, 2.48, 2.38, and 2.25, and in Fig. 3d with values of 4.45, 3.84, 3.73, 2.67, 2.65, 2.18, 1.87, and 1.68 match well to H6 P2 W18 O62 ·nH2 O, H6 P2 W18 O62 , and PW12 O38.5 , respectively [28,34], and the interpretation is in good agreement with the thermal analysis results, indicating that the HPA loses constitutional water upon heat treatment above 350 ◦ C. However, the XRD pattern of a sample obtained after heat treatment of HPA at 500 ◦ C was not associated with pure HPA. The major crystalline phase appeared to be a mixture and identification was uncertain, in contrast to a previous report [28]. Such uncertainty has also been reported for the potassium salt of HPA [35]. As mentioned above, the inconsistency may be due to the difference in heat treatment process and sample storage.
Fig. 3. XRD patterns of H6 P2 W18 O62 ·nH2 O after heat treatment at (a) 100, (b) 300, (c) 500, and (d) 650 ◦ C.
3.2. HPA adsorption on sol–gel and commercial alumina Adsorption of HPA on alumina was studied by UV– vis spectroscopy. Fig. 4 shows the absorption spectra of a HPA solution with concentration 0.25 mg/ml (a) after contact for 24 h with commercial (b) and sol–gel derived alumina (c). As can be seen, concentration of HPA in solution decreased significantly after contact with the sol–gel alumina compared to that in contact with commercial alumina.
Fig. 4. UV–vis spectra of a HPA solution (a) before, and after contact with (b) commercial and (c) sol–gel derived alumina.
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Fig. 5. Adsorption of HPA on sol–gel (a) and commercial alumina (b) as a function of time.
The amount of HPA that adsorbed on the sol–gel alumina is about 1.86 mg/m2 and on the commercial alumina about 0.84 mg/m2 . The higher tendency of sol–gel alumina to adsorb HPA in comparison to the commercial alumina is attributed in a great extent to the hydroxyl-rich surface of the sol–gel alumina rather than to its larger surface area. The strong interaction between HPA and the surface OH groups can be interpreted as transfer of acidic protons of the strong Wells–Dawson HPA to the OH groups of the alumina surface. The endowment of sol–gel derived alumina with a hydroxyl-rich surface was obvious from the IR spectrum of the freshly prepared sample. By taking into account that the interaction between HPA and support is mainly electrostatic in nature [9], a stronger interaction of HPA with the sol– gel alumina is expected. No attempt was made to quantify the number of hydroxyl groups on the aluminas. The final spectra of HPA solutions after contact with the two types of aluminas are qualitatively similar, which indicates that the HPA structure remained intact in solution. This conclusion was also confirmed by 31 P NMR of HPA solution before and after contact with aluminas. Kinetics of adsorption of HPA on sol–gel and commercial aluminas, as shown in Fig. 5, has been investigated by UV–vis spectroscopy for an interval of 10–750 min using a solution with a concentration of 0.25 mg HPA/ml. The concentration of adsorbed HPA on both aluminas after about 30 min remained constant; however, in the plateau of the adsorption curve versus time, the amount of HPA adsorbed on sol–gel alumina is more than twice the amount on commercial alumina. These results signify the important role of sol–gel base oxides in supported catalysis. 3.3. Heteropolyacid/alumina characterization The infrared spectrum of Dawson HPA supported on the sol–gel derived alumina with 10% loading (Fig. 6a) shows bands at 1072, 951, 899, and 783 cm−1 . Apparently, low loading resulted in a strong interaction of HPA with alumina and consequently a drastic shift of bands in the infrared spectrum. For the commercial alumina (Fig. 6b), bands appeared at 1095, 955, 906, and 817 cm−1 . These data indicated that the type of alumina is influential on the interaction
Fig. 6. Infrared spectrum of Dawson HPA supported on sol–gel derived alumina (a) and on commercial alumina (b) with 10% loading.
Fig. 7. DSC and TGA curves of H6 P2 W18 O62 ·nH2 O supported on sol–gel alumina with 10% loading.
of HPA and alumina and the type of interaction is different to some extent. Such drastic behavior of alumina type has been reported for the Keggin-type HPA supported on alumina, which is related to the acid–base character of alumina [7]. In this context, a different structural feature for the HPA species supported on the sol–gel derived alumina, which is known to be endowed with a hydroxyl-rich surface, is not unusual. This kind of interaction previously has been reported for the silica-supported Dawson HPA and attributed to the interaction of silanol groups of the support and the octahedral WO6 species [36]. Interestingly, recent study shows that this type of interaction leads to the formation of (SiOH2 )+ (H2 PW12 O40 )− [9,37], which corresponds to a Keggin-type HPA structure. Thermal analysis of HPA supported on sol–gel alumina with 10% loading, as shown in Fig. 7, exhibits no exo- or endothermic peaks above 200 ◦ C, but there is a steady decrease in weight. Similar behavior was observed for the sol–gel and commercial aluminas in higher loading. This type of thermal behavior has previously been observed in zeolites and other HPAs with Wells–Dawson structures [38].
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Table 1 Esterification yields of propanoic acid with HPA in heterogeneous system Catalyst
Reaction Total acidity Conversion time (min) (mmol/g) (%)
H6 P2 W18 O62 /γ -Al2 O3 (sol–gel) 180 180 H6 P2 W18 O62 /γ -Al2 O3 γ -Al2 O3 (sol–gel) 180 180 γ -Al2 O3 None 180
Fig. 8. XRD pattern of H6 P2 W18 O62 ·nH2 O supported on sol–gel alumina after heat treatment at (a) 300 and (b) 650 ◦ C.
The X-ray diffraction patterns of HPA supported on the sol–gel and commercial alumina with 10–30% loading showed amorphous patterns. However, after calcination at 300 ◦ C, as shown for the sol–gel alumina with 10% loading (Fig. 8a), very weak and broad peaks developed. Similar patterns were observed for HPA supported on both aluminas up to 30% loading after heat treatment at 300 ◦ C. Such an amorphous pattern has previously been reported for the Keggin-type HPA on various supports with loading levels of as high as 50% and explained as the result of high dispersion of adsorbed species on the support and lack of large HPA particles on the support [7,9,39,40]. Interestingly, only when HPA loading increased to 30% on the sol–gel alumina and heat treatment was at 650 ◦ C for 5 h, XRD revealed a reasonably fine pattern, Fig. 8b, which corresponds to P2 W18 O59 species [41]. It is worth noting that the XRD pattern of HPA supported on the commercial alumina after heat treatment at 600 ◦ C for 5 h also matched well to P2 W18 O59 . The XRD pattern of HPA supported on the commercial alumina after heat treatment at the same temperature was similar. Apparently, thermal stability of Dawson HPA is enhanced to 650 ◦ C and primary structure preserved by supporting it on aluminas, whereas the bulk one transmutes to a mixture of unknown phases in the range of 350–500 ◦ C and to a species with Keggin skeleton above 650 ◦ C. Such higher thermal stability has been observed for the Keggin-type HPA supported on mesoporous silica and has been correlated to the loss of protons or acidic groups [8,9]. 3.4. Catalyst acidity It is generally accepted that the esterification reactions catalyzed by Brønsted acids and also the yields depend on concentration of either alcohol or acid. In the present study alcohol was used as excess reagent. In order to demonstrate that the catalytic activity of Brønsted acid of alumina-
0.21 0.09 0.02 Neutral –
30 22 15 11 0
supported HPA remains intact, we have used it as an acid catalyst in the esterification of propanoic acid with hexanol. Esterification yields in the presence of alumina-supported HPA and also pure sol–gel and commercial alumina are given in Table 1. Apparently, acidic sites of Wells–Dawson HPA supported on sol–gel and commercial alumina remain active to some extent for the acid catalyst reactions, the yield is relatively higher for the former than the latter, and both are more active than pure alumina. By taking into consideration that the surface areas of supported catalysts decrease where oxides are used as supports [42], one can conclude that the higher activity of alumina-supported HPA is due to the higher acidity rather than to alteration in the surface area of supported alumina. For further proof the total acidity of all samples were measured and the results are given in Table 1. Notably, the acidity of sol–gel alumina increases to a large extent and commercial alumina to a lesser extent when HPA is supported on them. The higher total acidity value of sol–gel alumina-supported HPA is reflected in the esterification yields. Furthermore, the higher catalytic activity of bare sol–gel alumina in comparison to the commercial one can be attributed to higher surface area of the former. Although esterifications of various alcohols by HPAs have been investigated extensively [43], only a limited number of them are HPAs with Wells–Dawson structure. In addition, the majority of investigations have been carried out in a homogeneous system, in contrast to the heterogeneous system in the present study. Furthermore, type of support, HPA loading, esterification conditions, and type of alcohol and acid used in various studies are different, which makes comparison among them difficult. Recent work showed that the esterification yields for the Keggin HPA supported on commercial alumina strongly depend on the type of acid and alcohol used when other parameters are kept constant [44]. Notably, the esterification yield of butanol with propanoic acid with a similar loading for the Keggin HPA supported on alumina is higher than the esterification yield in the present work [44]. Higher yield can be rationalized by the stronger acidic nature of the bulk Keggin HPA and/or the size of the reacting alcohol. Carbon-supported Keggin HPA also was used for the esterification of propanoic and acrylic acids by butanol [45,46]. Although carbon-supported HPA is a very efficient catalyst for esterification, its activity is lower than that of the unsupported HPA. Finally, it should be mentioned that the activity of HPA supported on alumina in a heterogeneous esterification reaction decreased significantly in com-
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parison to that in the homogeneous system [47], and this is consistent with the earlier report that the acidity of HPAs is reduced by supporting them on oxides [48].
4. Conclusion The thermal behavior of Wells–Dawson tungsten HPA supported on sol–gel and non-sol–gel aluminas was investigated. It was shown that the type of alumina is influential on the interaction of heteropolyacids/alumina, and the amount of adsorbed HPA is also different to a large extent. X-ray diffraction indicates that the primary structure of bulk HPA holds up to 350 ◦ C and by supporting it on sol–gel alumina the thermal stability rose to 650 ◦ C. The XRD results also show that the HPA is well dispersed on alumina and heattreatment temperatures have a marked effect on the X-ray diffraction patterns of supported HPA. The stronger interaction of HPA with sol–gel alumina is attributed to the fact that the sol–gel derived alumina is endowed with a hydroxyl-rich surface. By supporting HPA on aluminas, the total acidity of sol–gel alumina increases to a large extent and commercial alumina to a lesser extent. The higher value of the total acidity of sol–gel alumina-supported HPA is reflected in the esterification yield of propanoic acid with hexanol and indicates that the acidic sites of the supported HPA remain active to some extent. Investigation of catalytic activity of aluminasupported HPA in oxidation reactions is in progress. Acknowledgment The generous financial support of the Research Council of University of Tehran (Grant 514-4-589) is gratefully acknowledged.
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