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Solvent effect on the preparation of H3PW12O40 supported on alumina
Catalysis Today 107–108 (2005) 816–825 www.elsevier.com/locate/cattod
Solvent effect on the preparation of H3PW12O40 supported on alumina Edne´ia Caliman, Jose´ A. Dias *, Sı´lvia C.L. Dias, Alexandre G.S. Prado Universidade de Brası´lia, Instituto de Quı´mica, Laborato´rio de Cata´lise, Caixa Postal 04478, Brası´lia-DF 70904-970, Brazil Available online 19 August 2005
Abstract Preparation, structural characterization and solid acidity studies of H3PW12O40 (H3PW) supported (20, 40, 50 and 80 wt.%) on g-alumina have been performed. Impregnation of H3PW on g-alumina was performed by the evaporation technique using water (acidified with HCl), ethanol and acetonitrile as solvents. The presence of the Keggin anion on alumina surface is very dependent upon the preparation conditions and pH, as indicated by partial decomposition in water solution. High acid concentration in aqueous solutions is required to obtain intact the Keggin anion. Ethanol showed the formation of distinct species on the alumina surface. In contrast, acetonitrile has not displayed any significant decomposition of the Keggin anion. All these conclusions are based on FTIR, Raman, 31P MAS–NMR, XRD and SEM–EDX measurements. The presence of the Keggin structure on the alumina surface can be followed by those techniques, eliminating any doubt about collapse of the supported anion. Calorimetric and adsorption of pyridine interactions in cyclohexane slurry (Cal-ad method) showed that the catalyst H3PW/Al2O3 is a weaker acid than pure H3PW or supported on silica, but stronger than alumina. The enthalpies indicated partial neutralization of the most basic sites of alumina at the expense of the strongest protons of H3PW. # 2005 Elsevier B.V. All rights reserved. Keywords: 12-Tungstophosphoric acid; Alumina; Supported heteropolyacid; Acidity;
1. Introduction Applications of polyoxometalates (POMs) have largely been demonstrated on different areas, and some of the most relevant properties for technological applications rely on solubility, acidity and redox potentials [1–6]. These properties can be promptly designed by choosing the counter cation and the polyanion class at atomic and molecular level. Particularly in catalysis, promising possibilities of heterogenization for homogeneous systems, offer the great advantage of easier separation of the catalyst from the products in chemical reactions [7]. Heteropoly compounds with Keggin structure are the most studied class within POMs, because they possess relatively high thermal stability [8,9] and acidity [10]. A wide range of acid strength can be obtained by substitution * Corresponding author. Tel.: +55 61 307 2162; fax: +55 61 368 6901. E-mail address: [email protected] (J.A. Dias). URL: http://www.unb.br/iq/labpesq/qi/labcatalise.htm 0920-5861/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2005.07.102
31
P MAS–NMR
of the cation and the addenda atoms, which allow them to be used in several reactions in homogeneous and heterogeneous media [11–13]. For heterogeneous systems, it is possible to control solid acid strength by supporting POMs on different carriers [14]. Surface area enhancement, higher dispersion of acidic protons, heterogenization and acid strength control are some of the goals for preparing supported heteropolyacids (HPAs). The most common materials used as supports include silica gel (the most studied) [15], mesoporous silica [16–18], carbon [19] and to a lesser extent alumina [20–24], because it has been considered not suitable as support for HPAs [20]. This work deals with preparation, structural characterization and acidity measurements of H3PW12O40 (H3PW) supported on alumina. Structural analysis has used XRD, FTIR, Raman, 31P MAS–NMR and SEM–EDS microscopy to investigate the nature of Keggin anion on the alumina surface. The acidity was studied by calorimetry and adsorption measurements in liquid phase (Cal-ad method). The results are compared to pure H3PW and supported on silica gel.
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2. Experimental 2.1. Materials, preparation of supported H3PW and thermal treatments H3PW was purchased from Aldrich and recrystallised before the preparation of solution for impregnation. TG analysis showed 21 mol of water per mole of acid. Alumina used as support was acidic, Brockmann I (g-Al2O3), particles with 150 mesh and surface area of 155 m2 g1, obtained from Aldrich. Acetonitrile (99.5%), ethanol (96%) and concentrated HCl were purchased from VETECQuı´mica Fina Ltda., and were used without further purification. Cyclohexane (99.9%, VETEC) was purified by drying over 4A molecular sieves (Aldrich) for 24 h, and then distilled over P2O5 (Merck). Pyridine (99.9%, VETEC) was distilled over CaH2 (Merck). Dried solvents were stored in a container with 4A molecular sieves. The supported H3PW catalysts were prepared by the impregnation method. Aqueous solutions (in HCl 0.1 and 0.5 mol l1) and organic solvent solutions (acetonitrile and ethanol) of H3PW were prepared, with concentrations depending upon the loading required to the support (20, 40, 60 and 80 wt.% H3PW) using 10 ml of the solution per gram of alumina. The support was added to the solution producing slurry. The slurry was stirred, and evaporated at about 80 8C (65 8C for ethanol) until dry. Then, the solid was ground to fine particles (using agate gral and pestle) and dried in a fixed bed reactor under vacuum. All samples of supported H3PW/ Al2O3 were dried at 200 8C for 6 h under vacuum, before any measurement. For comparison, a mechanical mixture was prepared by mixing H3PW (20 wt.%) with Al2O3, grounded to finer particles and heated at 200 8C for 6 h. 2.2. Spectroscopy and microscopy data Infrared and Raman spectra were obtained with a BrukerEquinox 55 spectrophotometer. For FTIR, samples were mixed with dried KBr (Merck) forming pellets, and the spectra taken with 4 cm1 resolution and 256 scans. The signal was obtained with a DTGS detector. For Raman, pure samples packed into sample cups, were irradiated with laser (Nd-YAG) at 1064 nm and power of 126 mW, obtaining spectra with 2 cm1 resolution and 256 scans on a Bruker FRA 106/S module attached to the Equinox spectrometer. The signal was detected by a liquid N2 cooled Ge detector. All FTIR and Raman spectra were obtained on the samples previously calcined at 200 8C for 6 h, under ambient conditions at room temperature (25 8C). 31 P MAS–NMR spectra were acquired using a MAS probe of 7 mm (silicon nitride rotor with torlon cap) in a Varian 7.05 T Mercury Plus spectrometer. The following conditions were used: single pulse excitation with 8.0 ms duration, acquisition time of 0.1 ms, recycle delay of 10 s, no 1H decoupling, MAS rate at 5 kHz and minimum of 256 acquisitions. Signals were indirectly referenced to 85 wt.%
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phosphoric acid. The H3PW/Al2O3 samples were packed into the rotor inside a dry box with dry nitrogen. The XRD patterns were collected using a Rigaku D/Max2A/C with Cu Ka radiation at 40 kV and 20 mA. A 2u range from 2 to 508 was scanned at 28 min1. Scanning electron microscopy (SEM) coupled with energy dispersive spectrometer (EDS) was obtained in a LEO 440 (20 kV) with carbon coating. 2.3. Calorimetric and adsorption titrations For calcined samples of supported H3PW on alumina, a diluted pyridine solution in cyclohexane is added to a slurry of the solid in anhydrous cyclohexane, and each measurement of the heat evolved and the equilibrium amount of base in solution were determined in two independent experiments. Once both data are obtained, the calculations are performed using a non-linear least squares program with a Simplex routine for data minimization. The program uses a Langmuir-type equation with variable number of sites, set by the user, according to the equation: h X ¼ g
ni Ki ½B DHi 1 þ Ki ½B
(1)
Using this model, it is possible to calculate the number of each different type of site (ni), the equilibrium constant (Ki) and the enthalpy (DHi) from analysis of the sum of the heat evolved per gram of the solid (h/g) and the concentration of base in solution ([B]). Details of Cal-ad method have been published elsewhere [15]. Samples of supported H3PW (0.5 g) were weighed and transferred to an isothermal calorimetric cell, followed by the addition of 50 ml of anhydrous cyclohexane. A calibrated syringe (Hamilton, 5 ml) was filled with a known concentration of pyridine solution (e.g., 0.1 mol l1). All these operations were carried out in an inert atmosphere glove bag filled with dry nitrogen. Then, both systems (cell and syringe) were inserted into the calorimeter holder, which were immersed in a thermal bath regulated at 25 8C. The calorimeter (model ISC 4300 from Calorimetry Sciences Corporation) was connected to a computer, and the experiment setup was made using the software provided. The addition of pyridine solution was done in the incremental mode. A calibration curve was performed prior or after each titration to obtain the equivalent energy of the system. As was observed separately, the heat of dilution of the addition of pyridine to cyclohexane was negligible. The adsorption experiments (25 8C) were carried out using 0.5 g of solid weighed and added to a sealed three neck round bottom flask with 50 ml of cyclohexane. Both operations were conducted inside a glove box. Addition of pyridine solution was performed using an automatic burette from Metrohm (Dosimat 665). The added volumes were the same as for the calorimetric experiment, as required by Cal-ad method. After each addition, 1 ml sample of
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solution was removed from the flask and placed into a quartz cell of 1 cm path-length. One milliliter of cyclohexane was added back into the flask in order to keep the volume constant. The absorbance of pyridine was measured at 251 nm (Beckman DU-650 UV–vis spectrophotometer) to determine its equilibrium concentration in solution using an analytical curve. Since the amount of pyridine added was known, the amount of base adsorbed by the solid was calculated by the difference. Both calorimetric and adsorption experiments were checked for diffusion constrains. It was observed that for standing times above 3 min, there was no variation in the absorbance or the heat measured, and therefore the experiments were repeated (three times) under these optimized conditions. The Cal-ad method has been applied together with spectroscopic data to analyze acidity of various heteropolyacids such as H3PW12O40 [10], supported H3PW12O40 on silica [15] and cesium derivatives of H3PW [25]. In addition, other solids such as silica gel [26], mordenite [27], Y zeolite [28], ZSM-5 [29], TS-1 [30], sulfated zirconia [31], etc., are among the variety of important solid acids characterized by this method.
3. Results and discussion A preliminary study by TG–DTA showed that H3PW supported on alumina (prepared by impregnation in acetonitrile solution) was stable at the chosen calcination temperature. Derivative thermogravimetry (DTG) displayed a large peak starting at 230 8C (with a small maximum at about 250 8C) and ending at about 500 8C. This mass loss can be attributed to the transformation of the hexahydrated form of H3PW to the anhydride phase [9]. This water release is practically simultaneous to the partial decomposition of the Keggin structure. The final destruction of H3PW supported on Al2O3 occurs at about 580 8C (maximum at 550 8C in the DTG curve). Therefore, as determined formerly for pure H3PW [8], the thermal treatment conditions used for H3PW/Al2O3 are appropriated to obtain intact the supported Keggin structure without the formation of the anhydride phase. 3.1. FTIR spectra The supported H3PW samples were analyzed by FTIR in order to confirm the presence of the Keggin anion on Al2O3 surface. The structure of PW12O403 anion works as a model for this class of heteropolyacid, consisting of a PO4 tetrahedron surrounded by four W3O13 groups formed by edge-sharing octahedra. These groups, are connected to each other by corner-sharing oxygens [32], and this arrangement gives rise to four types of oxygen bands between 1200 and 700 cm1, a fingerprint region for these compounds. Fig. 1A shows the sample with 40 wt.% H3PW/Al2O3, prepared in aqueous solution with HCl 0.5 mol l1. The
Fig. 1. (A) FTIR spectra of: (a) 40 wt.% H3PW/Al2O3 (HCl 0.5 mol l1); (b) Al2O3; (c) H3PW; (d) subtraction (40 wt.% H3PW/Al2O3–Al2O3). (B) FTIR spectra of 40 wt.% H3PW/Al2O3 in: (a) HCl 0.1 mol l1; (b) HCl 0.5 mol l1; (c) ethanol; (d) CH3CN. Pure H3PW (e) is shown for reference.
typical bands for absorptions of P–O (1080 cm1), W = Ot (983 cm1), W–Oc–W (898 cm1) and W–Oe–W (797 cm1) are clearly displayed. These bands are also preserved on the supported samples with more than 40 wt.% H3PW. In order to make it clearer, a subtraction spectrum was produced (40 wt.% H3PW minus alumina), which has shown unequivocally the presence of the Keggin anion. In the samples with 20 wt.% H3PW/Al2O3, the existence of the Keggin anion is not evident, whatever is the solvent used. Actually, its spectrum is similar to that of alumina, which shows a high absorption band between 1000 and 600 cm1. Based upon the area of the Keggin anion (about 1.1 nm2) and the surface area of alumina (155 m2 g1), theoretically a loading of about 40 wt.% of H3PW saturates the surface. Therefore, loading H3PW bellow the monolayer leads to species that either are highly dispersed or are decomposed on the surface. In order to check these possibilities other experiments were performed. Fig. 1B shows the several preparations of 40 wt.% H3PW on alumina with different solvents. The results reveal that there is a clear correspondence between the solvent used for impregnation (pH when aqueous solutions are used) and the stability of Keggin anion. The impregnations with CH3CN, ethanol and water (HCl 0.5 mol l1) show the typical Keggin structure by the corresponding absorptions, although the absorption at 897 cm1 is much less intense for the ethanol preparation than the others. On the other hand, from aqueous
E. Caliman et al. / Catalysis Today 107–108 (2005) 816–825
solution with HCl 0.1 mol l1, there is a decomposition of the [PW12O40]3 anion according to the splitting of the band at 1080 cm1 into two others (1100 and 1053 cm1), and the displacement of a band from 983 to 960 cm1, which are closely found [33] in the spectrum of lacunary species of [PW11O39]7. This agrees with many studies related to increasing stability of Keggin anion in organic solvents, and hydrolysis reaction of H3PW when the pH is higher than 2 [32]. Nevertheless, the other bands are not displaced significantly (i.e., they are the typical Keggin absorptions, but broader), which may point to some intermediate form between the anhydride and the anhydrous H3PW [34]. 3.2. Raman spectra Raman spectra of H3PW supported on alumina (prepared in CH3CN) are displayed in Fig. 2A. Crystalline H3PW shows characteristic bands at 1100 cm1 (stretching vibration of P–O), 990 cm1 (stretching of W = O), 900 cm1 (bending of W–Oc–W) and 550 cm1 (bending of O–P–O). The sample with 20 wt.% H3PW/Al2O3 shows only a broad and low intensity band at about 990 cm1. For 40% H3PW, it is clear the presence of the most bands related to the Keggin vibrations. In order to make sure there was no decomposition of H3PW after deposition over alumina and further calcination, Raman spectra were taken for the sample with 40 wt.% H3PW after impregnation on Al2O3 (complete
Fig. 2. (A) Raman spectra of: (a) Al2O3; (b) 20; (c) 40; (d) 50 wt.% of H3PW/Al2O3; (e) H3PW. Supported samples were prepared in CH3CN. (B) Raman spectra of 40 wt.% H3PW/Al2O3 prepared in different solvents: (b) HCl 0.5 mol l1; (c) ethanol; (d) acetonitrile. WO3 (a) and H3PW (e) are added for reference.
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dryness of the sample), and after calcinations procedure (200 8C/vacuum/6 h). Both spectra were identical to that in Fig. 2A. Fig. 2B shows Raman spectra of supported samples (40 wt.%) prepared in the different solvents. Looking at the Raman spectrum of the ethanol impregnation, W = O vibration band is split up in two peaks at 997 and 991 cm1. This indicates that different species of H3PW are interacting with the alumina surface after impregnation. In addition, a Raman spectrum of WO3 was taken in order to check this possible decomposition product. It can be seen the WO3 spectrum is very different from the supported samples. Thus, it can be concluded that at 40 wt.% loading, H3PW structure is not decomposed to form WO3. At lower coverage (20–40 wt.% H3PW) there is probably a strong interaction of H3PW with alumina surface, which can give rise to surface species with distorted Keggin anion or fragmented [PW12O40]3 structure. 3.3. XRD and SEM–EDX analysis XRD patterns (Fig. 3A and B) were obtained on the samples with 20, 40 and 80 wt.% H3PW, prepared in aqueous solution with HCl 0.5 mol l1 and acetonitrile, respectively. Alumina displayed only a very low intensity signal without any distinct peak. Supported samples prepared in HCl 0.5 mol l1 presented only wide bands
Fig. 3. (A) XRD patterns of H3PW/Al2O3 prepared in H2O (HCl 0.5 mol l1) with: (b) 20; (c) 40; (d) 80 wt.% of H3PW. Pure Al2O3 (a) and H3PW (e) are included for reference. (B) XRD patterns of H3PW/Al2O3 prepared in CH3CN with: (b) 20; (c) 40; (d) 80 wt.% of H3PW. Pure Al2O3 (a) and H3PW (e) are included for reference.
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centered at 2u = 68 (20 wt.%) and at 2u = 7.58 (40 wt.%). For the sample with 80 wt.%, a characteristic pattern of hydrated H3PW was observed. Because of the FTIR and Raman spectra of the 40 wt.% samples (the theoretical monolayer) evidenced the Keggin structure, it can be inferred that H3PW is probably highly dispersed on the alumina. Crystals of H3PW supported on Al2O3 surface are deposited as separate entities, but not big enough or well formed to be detected by XRD. In contrast, supported samples prepared in acetonitrile showed isolated peaks coincident with crystalline H3PW on loadings as low as 20 wt.% (Fig. 3B). Thus, the preparation method using acetonitrile probably produced larger crystal domains on the alumina surface than the equivalent aqueous acidic impregnations. A mechanical mixture with 20 wt.% H3PW loading exhibited the powder pattern of the Keggin structure. However, compared to the supported sample prepared in acetonitrile, the peaks are much similar (line-width and intensity) to crystalline H3PW. Thus, when crystalline phases are detected on the surface, XRD can be used to distinguish supported H3PW/Al2O3 from mechanical mixtures, as observed for silica-supported systems [15].
In order to check the crystalline formation and dispersion of H3PW over alumina, scanning electron micrographs (SEM) were taken on the aqueous impregnation products (Fig. 4). The tungsten mapping on alumina surface of samples with 20, 40 and 80 wt.% H3PW (Fig. 4d–f) shows that the polyacid is highly dispersed on the surface, with increasing tendency to agglomeration for higher loadings. The SEM pictures (Fig. 4a–c) display no resolved micro domains of H3PW. For the 20 wt.% H3PW/Al2O3, there is a distribution of crystallites with different sizes and the presence of some voids among them. Some roughness can be observed on the surface. For the 40 wt.% sample, the surface is smoother than the 20 wt.% H3PW/Al2O3. No crystallites on the surface could be distinguished, although no void on the surface is observed. The sample with 80 wt.% H3PW shows the presence of larger agglomerates than the 20 wt.%, and the surface is rougher than the sample with 40 wt.%. These results agree with the obtained XRD patterns, since no defined crystalline phase was observed for the samples with 20 and 40 wt.%. At 80 wt.% loadings, the formation of large crystallites gives rise to the observed pattern.
Fig. 4. Scanning electron micrographs of H3PW/Al2O3 prepared in HCl 0.5 mol l1 with: (a) 20; (b) 40; (c) 80 wt.% H3PW, and the respective (d–f) X-ray emission dot map obtained with X-ray fluorescence microprobe. The white dots represent the tungsten atoms.
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Fig. 4. (Continued ).
3.4.
31
P MAS–NMR spectra
Fig. 5A shows the 31P MAS–NMR spectra of the H3PW/ Al2O3 system, obtained by impregnation in water with HCl 0.1 mol l1, for different contents of polyacid. For low concentration of H3PW (20 wt.%) there is a wide single peak (line-width of 330 Hz) centered at 13.5 ppm, while hydrated H3PW shows a peak at 15.5 ppm (both referenced to 85 wt.% H3PO4). It has been discussed that 31P MAS–NMR signal of H3PW depends upon the hydration degree and can be varied from 10.0 to 16 ppm [10]. For the 40 wt.% H3PW/ Al2O3 sample, two peaks were observed: a higher one centered at 13.4 ppm and a smaller at 15.5 ppm. At higher concentrations (50–80 wt.%), the two peaks maintained the same positions, but the second (15.5 ppm) became more intense with the H3PW loading. These results indicate two kinds of supported H3PW. Type 1 consists of micro crystals of H3PW deposited on Al2O3, but without direct interaction with the surface. This species practically do not differ from pure hydrated H3PW (d 15.5 ppm). Type 2 consists of H3PW interacting with the surface acid sites from alumina (d = 13.5 ppm). Therefore, according to the coverage of H3PW on Al2O3 these two species are distributed on the supported samples. For amounts below 40 wt.%, there is a strong interaction of H3PW with alumina surface forming
species with distorted or decomposed Keggin structure characterized by d 13.5 ppm. Upon higher concentration (40–80 wt.%), alumina surface is saturated and H3PW crystals are agglomerated on surface (but without direct interaction), which is characterized by a growing second peak assigned to pure H3PW at d 15.5 ppm. Coupling these results with FTIR and Raman measurements, there is an indication that the Keggin anion may undergo some degree of decomposition. It may have produced a lacunary species, some intermediate anhydride or a fragmented phase. The lacunary species ([PW11O39]7) should present a signal at 10.4 ppm [32], which is different from the one obtained (13.5 ppm). The peak present at d = 13.5 ppm is also claimed [16,23] to be the formation of dimeric species ([P2W21O71]6 or [P2W18O62]6). Other study [34] points to the formation of some anhydride phase of H3PW. Therefore, the presence of one or more species from decomposition of the Keggin unit is established on these supported solids under the present preparation conditions, although the exact species cannot be specified at this point. A mechanical mixture of 40 wt.% H3PW with Al2O3 gives rise to a single peak at 15.6 ppm with approximately the same line-width of hydrated H3PW (75 Hz). Therefore, 31 P MAS–NMR is able to distinguish supported samples from mechanical mixtures of H3PW and alumina.
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Fig. 5B shows the NMR spectra of H3PW/Al2O3 prepared in CH3CN solution. All samples display a single peak centered at 14.6 ppm. These results indicate that there is no decomposition of H3PW prepared in this solvent, and the signal is equivalent to crystalline hydrated H3PW (15.5 ppm). Fig. 5C shows the spectra of 40 wt.% H3PW/Al2O3 prepared in different solvents. The samples prepared in ethanol and water (HCl 0.1 mol l1) exhibit two peaks. One peak is assigned to the hydrated Keggin anion (14.6 ppm) and the other at 11.8 ppm may be attributed to the [PW11O39]7 or another fragmented form of the polyanion, as mentioned before. It is clear from the spectrum of the system prepared in HCl 0.5 mol l1, which showed a single peak at 14.6 ppm, that pH is a major aspect in aqueous impregnation. The same isotropic shift for this sample (14.6 ppm) is observed for preparation in acetonitrile. 3.5. Calorimetric and adsorption analysis of supported H3PW/Al2O3 Based upon the spectroscopy and microscopy results, a preliminary calorimetric run for acidity was tested on the catalysts that unequivocally showed no decomposition. The main samples were those prepared in water at higher acidity (HCl 0.5 mol l1) and in acetonitrile. Samples with 20 and 40 wt.% loadings were tested, because although the theoretical monolayer is about 40 wt.%, studies of the 31P MAS–NMR relaxation for silica supported heteropolyacids [35] have demonstrated that the real monolayer is about half of that calculated. In order to verify acidity of the produced materials, calorimetric titrations with pyridine were performed, and calculated average enthalpies are presented in Table 1. These initial results show that the interaction of H3PW with Al2O3 is almost independent of heteropolyacid content from 20 to 40 wt.% (DH ffi 19 kcal mol1) for the strongest site for both solvent preparations. Therefore, considering the higher dispersion of sample loaded with 20 wt.% and the easy preparation of supported catalysts from aqueous solution, a full calorimetric and adsorption study (Cal-ad method) was carried out on that sample. Table 1 Results of calorimetric titrations (25 8C) of supported H3PW/Al2O3 (prepared in H2O acidified with HCl 0.5 mol l1 or CH3CN) with pyridine in cyclohexane slurry
Fig. 5. (A) 31P MAS–NMR spectra of: (a) hydrated H3PW Al2O3 with (b) 20; (c) 40; (d) 50 wt.% of H3PW. Samples HCl 0.1 mol l1. (B) 31P MAS–NMR spectra of hydrated supported H3PW on alumina prepared in acetonitrile. (C) NMR spectra of hydrated H3PW and 40 wt.% H3PW/Al2O3 different solvents.
and H3PW/ prepared in H3PW and 31 P MAS– prepared in
Samplea
H3PW (wt.%)
H+/g of solid (mmol)
DH (kcal mol1)b
20HPW-WA 40HPW-WA 20HPW-AC 40HPW-AC
20 40 20 40
0.202 0.404 0.202 0.404
20 18 17 19
WA = H2O with HCl 0.5 mol l1; AC = CH3CN. The enthalpy is an average based on the first three additions of pyridine in the titration, using the limiting reagent approximation. To convert the enthalpy values to kJ mol1 multiply by 4.184. a
b
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Fig. 6. Calorimetric and adsorption isotherms (25 1 8C) of 20 wt.% H3PW/Al2O3 (prepared in H2O with HCl 0.5 mol l1) reacted with pyridine in cyclohexane slurry.
The calorimetric and adsorption isotherms of 20 wt.% H3PW/Al2O3 (prepared in HCl 0.5 mol l1) reacted with pyridine are presented in Fig. 6. The Cal-ad data were analyzed assuming the presence of one, two and three different sites on the catalyst surface. The best results indicated a two-site model as the most representative, with lower standard deviations on the calculated parameters and residual sum of the squares in the same order of magnitude of the experimental error (based on the calorimetric measurements). Moreover, the choice of two sites is also supported by the FTIR spectrum of the adsorbed pyridine. The absorption region of pyridine adducts (1700– 1400 cm1) showed three peaks at 1540, 1488 and 1635 cm1. The first two absorptions are due to pyridinium ion while the third one has a contribution of coordinated pyridine [36]. It should be mentioned that this absorption band at 1635 cm1 is not totally conclusive because of contributions of pyridinium ion and moisture, since the
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spectrum was taken at ambient conditions. The strongest site present on H3PW/Al2O3 is Bro¨nsted and the weaker is Lewis-type. The enthalpy of interaction of 20 wt.% H3PW/Al2O3 with pyridine on the strongest site is DH1 = 22.6 kcal mol1. This value is lower than pure H3PW (DH1 = 32.7 kcal mol1) [10] but higher than pure g-Al2O3 (DH1 = 15.2 kcal mol1) [37]. Comparing with 25 wt.% H3PW/ SiO2 (DH1 = 27.9 kcal mol1) [15], it can be stated that a strong interaction occurs between H3PW and Al2O3 producing a solid acid that is weaker than pure H3PW or supported on silica. Table 2 shows the thermodynamic parameters for these solids. The dispersion of H3PW on alumina can be inferred from the number of the strongest acid sites (n1). Compared to H3PW (n1 = 0.08 mmol g1), the protons on 20 wt.% H3PW/Al2O3 (n1 = 0.154 mmol g1) are much more available. Based on the total amount of available protons on the 20 wt.% supported sample (0.202 mmol g1) about 76% were titrated with pyridine, compared to only 8 and 45% of H3PW and H3PW/SiO2, for the strongest sites, respectively. These results confirm our earlier spectroscopy and microscopy data, which showed that H3PW was highly dispersed on the alumina surface. This leads to more available protons for surface-type catalytic reactions, although the strength is much lower than both solids formerly mentioned. It is interesting to note that the strongest acid site for 20 wt.% H3PW/Al2O3 is similar to the second acid site of pure H3PW (vide Table 2). Both enthalpy (DH1 22 kcal mol1) and the amount of sites (n1 0.16 mmol g1) demonstrate that the strongest protons of H3PW are mostly tied to alumina, probably neutralizing basic sites present on its surface. Thus, only the weaker sites present on pure H3PW become available for catalytic reactions when it is supported on alumina, which may explain the lower activity of this catalyst compared to the similar supported on silica [24]. These arguments match very well when one compares the second site of the supported H3PW on alumina. About 76% of H3PW protons were neutralized by pyridine (site 1) and
Table 2 Cal-ad results (25 1 8C) for 20 wt.% H3PW/Al2O3 reacted with pyridine in cyclohexane slurry Parameter 1
n1 (mmol g ) K1 (mol1 l) DH1 (kcal mol1)b n2 (mmol g1) K2 (mol1 l) DH2 (kcal mol1)b
20 wt.% H3PW/Al2O3
H3PWc
25 wt.% H3PW/SiO2 d
0.152 0.007 (1.20 0.02) 10 5 22.6 0.2 0.20 0.04 (4.9 0.9) 10 3 12.0 1.1
0.079 0.005 (3.70 0.02) 105 32.7 0.3 0.16 0.05 (2.9 1.0) 103 19.6 4.8
0.102 0.005 (6.7 0.1) 105 27.9 0.7 0.310 0.061 (3.8 0.7) 104 10.1 2.9
Data of H3PW and 25 wt.% H3PW/SiO2 are included for comparisona. a The Cal-ad results for alumina [37] are: DH1 = 15.2 1.2; K1 = (3.31 0.04) 104; n1 = 0.16 0.01; DH2 = 6.7 2.9; K2 = (5.9 2.8) 102; n2 = 0.25 0.15. Units are the same as above. b To convert the enthalpy values to kJ mol1 multiply by 4.184. c Data from reference [10]. d Data from reference [15].
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the remaining 24% reacted with the most basic hydroxyl groups on alumina surface. The enthalpy for the second site of 20 wt.% H3PW/Al2O3 is 12 kcal mol1, and the amount of sites is n2 0.20 mmol g1. These quantities are about the same for the strongest sites on pure alumina (mostly Lewis-type, with DH1 = 15.2 kcal mol1 and n1 = 0.16 mmol g1) [37]. Therefore, the second site of 20 wt.% H3PW/Al2O3 is mostly the original acidic sites of pure alumina, which has not reacted with H3PW during the impregnation and further calcination process.
Acknowledgments The authors thank Dr. Edi Mendes Guimara˜es from Laborato´rio de Difrac¸a˜o de Raios-X (IG/UnB) for XRD measurements, Dr. Maria Jose´ A. Sales and Mr. Filipe A. Silva from Laborato´rio de Pesquisa em Fı´sico-Quı´mica de Polı´meros (IQ/UnB) for TG/DTG experiments and Dr. David P. Geeverghese for revising the manuscript. Also, we are grateful to CNPq for a scholarship to pursue a Doctorate degree (E.C.), and financial support given by IQ/UnB (FUNPE), FINATEC, FINEP/CTPetro (0663/00), FINEP/ CTInfra (0970/01) and FAP-DF/SCDT/CNPq.
4. Conclusions This study deals with preparation, characterization and acidity behavior of 12-tungstophosphoric acid (H3PW12O40) supported on g-alumina. The catalyst H3PW/Al2O3 was prepared using different solvents (water, ethanol and acetonitrile) by the impregnation-evaporation technique. The results clearly indicated the dependence of the Keggin structure stability on the surface with the hydrolysis of the anion. For aqueous solutions, high acid concentration is mandatory to avoid decomposition of the polyanion. It was showed by FTIR and Raman spectroscopy that the splitting of the bands related to P–O and W = O vibrations, respectively, are indicative of the formation of fragmented Keggin anion. Also, 31P MAS–NMR spectroscopy displayed two peaks in the spectra of decomposed samples. High dispersion of H3PW on the alumina surface was confirmed by XRD and SEM–EDX results. Although the preparation of supported catalyst in acetonitrile does not cause any destruction of the Keggin structure, higher dispersion is obtained in aqueous acid medium. Thus, the best preparation achieved was to disperse H3PW on alumina in aqueous solution with HCl 0.5 mol l1. The acidity of the samples with 20 and 40 wt.% of H3PW were studied by calorimetry for those, which did not show any decomposition (aqueous HCl 0.5 mol l1 and acetonitrile). It was observed that the interaction of H3PW is practically independent of the polyanion in that range of concentration. Thermodynamic data for the 20 wt.% H3PW/Al2O3 were obtained by the Calad method (calorimetry and adsorption of the solid with pyridine in ciclohexane slurry). A two-site model (supported by the results of pyridine adsorption analyzed by FTIR) gave DH1 = 22.6 kcal mol1, n1 = 0.15 mmol g1 and DH2 = 12.0 kcal mol1, n2 = 0.20 mmol g1, respectively, for the strongest site (Bro¨nsted) and the weaker site (Lewis). The catalyst H3PW/Al2O3 is a weaker acid than pure H3PW or supported on silica, but stronger than pure alumina. On the other hand, the dispersion of the protons on alumina surface is higher than silica, and more active surface protons became more available than pure H3PW, according to the n1 values. The neutralization of the strongest protons of pure H3PW occurred on the alumina surface, because of the reaction of the acid sites with the most basic sites of the support.
References [1] I.V. Kozhevnikov, Catalysis for Fine Chemical Syntheses—Catalysis by Polyoxometalates, vol. 2, John Wiley & Sons Ltd., Chichester, 2002. [2] M. Misono, Chem. Commun. (2001) 1141. [3] N. Mizuno, M. Misono, Chem. Rev. 98 (1998) 199. [4] T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 41 (1996) 113. [5] A. Corma, Chem. Rev. 95 (1995) 559. [6] C.L. Hill, C.M. Prosser-McCartha, Coord. Chem. Rev. 146 (1995) 407. [7] C. Trolliet, G. Coudurier, J.C. Ve´drine, Top. Catal. 15 (2001) 73. [8] R.S. Drago, J.A. Dias, T.O. Maier, J. Am. Chem. Soc. 119 (1997) 7702. [9] J.A. Dias, S.C.L. Dias, N.E. Kob, J. Chem. Soc. Dalton Trans. 3 (2001) 228. [10] J.A. Dias, J.P. Osegovic, R.S. Drago, J. Catal. 183 (1999) 83. [11] R. Neumann, Prog. Inorg. Chem. 47 (1998) 317. [12] T. Okuhara, Chem. Rev. 102 (2002) 3641. [13] Y. Izumi, K. Urabe, M. Onaka, Micropor. Mesopor. Mater. 21 (1998) 629. [14] J.B. Moffat, Metal–Oxygen Clusters—The Surface and Catalytic Properties of Heteropoly Oxometalates, Kluwer, New York, 2001. [15] J.A. Dias, E. Caliman, S.C.L. Dias, M. Paulo, A.T.C.P. de Souza, Catal. Today 85 (2003) 39. [16] I.V. Kozhevnikov, K.R. Klotststra, A. Sinnema, H.W. Zandbergen, H. van Bekkum, J. Mol. Catal. A 114 (1996) 287. [17] P.A. Jalil, M.A. Al-Daous, A.A. Al-Arfaj, A.M. Al-Amer, J. Beltramini, A.I. Barri, Appl. Catal. A 207 (2001) 159. [18] T. Blasco, A. Corma, A. Martı´nez, P. Martı´nez-Escolano, J. Catal. 177 (1998) 306. [19] F. Levebvre, P. Dupont, A. Auroux, React. Kinet. Catal. Lett. 55 (1995) 3. [20] Y. Izumi, R. Hasebe, K. Urabe, J. Catal. 84 (1983) 402. [21] K.M. Rao, R. Gobetto, A. Iannibello, A. Zecchina, J. Catal. 119 (1989) 512. [22] K. Nowinska, R. Fiedorow, J. Adamiec, J. Chem. Soc. Faraday Trans. 87 (1991) 749. [23] L.R. Pizzio, C.V. Ca´ceres, M.N. Blanco, Appl. Catal. A 167 (1998) 283. [24] L.R. Pizzio, P.G. Va´zquez, C.V. Ca´ceres, M.N. Blanco, Appl. Catal. A 256 (2003) 125. [25] J.A. Dias, E. Caliman, S.C.L. Dias, Micropor. Mesopor. Mater. 76 (2004) 221. [26] C.W. Chronister, R.S. Drago, J. Am. Chem. Soc. 115 (1993) 4793. [27] J.L. de Macedo, S.C.L. Dias, J.A. Dias, Micropor. Mesopor. Mater. 72 (2004) 119. [28] S.C.L. Dias, J.L. de Macedo, J.A. Dias, Phys. Chem. Chem. Phys. 5 (2003) 5574.
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Solvent effect on the preparation of H3PW12O40 supported on alumina Edne´ia Caliman, Jose´ A. Dias *, Sı´lvia C.L. Dias, Alexandre G.S. Prado Universidade de Brası´lia, Instituto de Quı´mica, Laborato´rio de Cata´lise, Caixa Postal 04478, Brası´lia-DF 70904-970, Brazil Available online 19 August 2005
Abstract Preparation, structural characterization and solid acidity studies of H3PW12O40 (H3PW) supported (20, 40, 50 and 80 wt.%) on g-alumina have been performed. Impregnation of H3PW on g-alumina was performed by the evaporation technique using water (acidified with HCl), ethanol and acetonitrile as solvents. The presence of the Keggin anion on alumina surface is very dependent upon the preparation conditions and pH, as indicated by partial decomposition in water solution. High acid concentration in aqueous solutions is required to obtain intact the Keggin anion. Ethanol showed the formation of distinct species on the alumina surface. In contrast, acetonitrile has not displayed any significant decomposition of the Keggin anion. All these conclusions are based on FTIR, Raman, 31P MAS–NMR, XRD and SEM–EDX measurements. The presence of the Keggin structure on the alumina surface can be followed by those techniques, eliminating any doubt about collapse of the supported anion. Calorimetric and adsorption of pyridine interactions in cyclohexane slurry (Cal-ad method) showed that the catalyst H3PW/Al2O3 is a weaker acid than pure H3PW or supported on silica, but stronger than alumina. The enthalpies indicated partial neutralization of the most basic sites of alumina at the expense of the strongest protons of H3PW. # 2005 Elsevier B.V. All rights reserved. Keywords: 12-Tungstophosphoric acid; Alumina; Supported heteropolyacid; Acidity;
1. Introduction Applications of polyoxometalates (POMs) have largely been demonstrated on different areas, and some of the most relevant properties for technological applications rely on solubility, acidity and redox potentials [1–6]. These properties can be promptly designed by choosing the counter cation and the polyanion class at atomic and molecular level. Particularly in catalysis, promising possibilities of heterogenization for homogeneous systems, offer the great advantage of easier separation of the catalyst from the products in chemical reactions [7]. Heteropoly compounds with Keggin structure are the most studied class within POMs, because they possess relatively high thermal stability [8,9] and acidity [10]. A wide range of acid strength can be obtained by substitution * Corresponding author. Tel.: +55 61 307 2162; fax: +55 61 368 6901. E-mail address: [email protected] (J.A. Dias). URL: http://www.unb.br/iq/labpesq/qi/labcatalise.htm 0920-5861/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2005.07.102
31
P MAS–NMR
of the cation and the addenda atoms, which allow them to be used in several reactions in homogeneous and heterogeneous media [11–13]. For heterogeneous systems, it is possible to control solid acid strength by supporting POMs on different carriers [14]. Surface area enhancement, higher dispersion of acidic protons, heterogenization and acid strength control are some of the goals for preparing supported heteropolyacids (HPAs). The most common materials used as supports include silica gel (the most studied) [15], mesoporous silica [16–18], carbon [19] and to a lesser extent alumina [20–24], because it has been considered not suitable as support for HPAs [20]. This work deals with preparation, structural characterization and acidity measurements of H3PW12O40 (H3PW) supported on alumina. Structural analysis has used XRD, FTIR, Raman, 31P MAS–NMR and SEM–EDS microscopy to investigate the nature of Keggin anion on the alumina surface. The acidity was studied by calorimetry and adsorption measurements in liquid phase (Cal-ad method). The results are compared to pure H3PW and supported on silica gel.
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2. Experimental 2.1. Materials, preparation of supported H3PW and thermal treatments H3PW was purchased from Aldrich and recrystallised before the preparation of solution for impregnation. TG analysis showed 21 mol of water per mole of acid. Alumina used as support was acidic, Brockmann I (g-Al2O3), particles with 150 mesh and surface area of 155 m2 g1, obtained from Aldrich. Acetonitrile (99.5%), ethanol (96%) and concentrated HCl were purchased from VETECQuı´mica Fina Ltda., and were used without further purification. Cyclohexane (99.9%, VETEC) was purified by drying over 4A molecular sieves (Aldrich) for 24 h, and then distilled over P2O5 (Merck). Pyridine (99.9%, VETEC) was distilled over CaH2 (Merck). Dried solvents were stored in a container with 4A molecular sieves. The supported H3PW catalysts were prepared by the impregnation method. Aqueous solutions (in HCl 0.1 and 0.5 mol l1) and organic solvent solutions (acetonitrile and ethanol) of H3PW were prepared, with concentrations depending upon the loading required to the support (20, 40, 60 and 80 wt.% H3PW) using 10 ml of the solution per gram of alumina. The support was added to the solution producing slurry. The slurry was stirred, and evaporated at about 80 8C (65 8C for ethanol) until dry. Then, the solid was ground to fine particles (using agate gral and pestle) and dried in a fixed bed reactor under vacuum. All samples of supported H3PW/ Al2O3 were dried at 200 8C for 6 h under vacuum, before any measurement. For comparison, a mechanical mixture was prepared by mixing H3PW (20 wt.%) with Al2O3, grounded to finer particles and heated at 200 8C for 6 h. 2.2. Spectroscopy and microscopy data Infrared and Raman spectra were obtained with a BrukerEquinox 55 spectrophotometer. For FTIR, samples were mixed with dried KBr (Merck) forming pellets, and the spectra taken with 4 cm1 resolution and 256 scans. The signal was obtained with a DTGS detector. For Raman, pure samples packed into sample cups, were irradiated with laser (Nd-YAG) at 1064 nm and power of 126 mW, obtaining spectra with 2 cm1 resolution and 256 scans on a Bruker FRA 106/S module attached to the Equinox spectrometer. The signal was detected by a liquid N2 cooled Ge detector. All FTIR and Raman spectra were obtained on the samples previously calcined at 200 8C for 6 h, under ambient conditions at room temperature (25 8C). 31 P MAS–NMR spectra were acquired using a MAS probe of 7 mm (silicon nitride rotor with torlon cap) in a Varian 7.05 T Mercury Plus spectrometer. The following conditions were used: single pulse excitation with 8.0 ms duration, acquisition time of 0.1 ms, recycle delay of 10 s, no 1H decoupling, MAS rate at 5 kHz and minimum of 256 acquisitions. Signals were indirectly referenced to 85 wt.%
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phosphoric acid. The H3PW/Al2O3 samples were packed into the rotor inside a dry box with dry nitrogen. The XRD patterns were collected using a Rigaku D/Max2A/C with Cu Ka radiation at 40 kV and 20 mA. A 2u range from 2 to 508 was scanned at 28 min1. Scanning electron microscopy (SEM) coupled with energy dispersive spectrometer (EDS) was obtained in a LEO 440 (20 kV) with carbon coating. 2.3. Calorimetric and adsorption titrations For calcined samples of supported H3PW on alumina, a diluted pyridine solution in cyclohexane is added to a slurry of the solid in anhydrous cyclohexane, and each measurement of the heat evolved and the equilibrium amount of base in solution were determined in two independent experiments. Once both data are obtained, the calculations are performed using a non-linear least squares program with a Simplex routine for data minimization. The program uses a Langmuir-type equation with variable number of sites, set by the user, according to the equation: h X ¼ g
ni Ki ½B DHi 1 þ Ki ½B
(1)
Using this model, it is possible to calculate the number of each different type of site (ni), the equilibrium constant (Ki) and the enthalpy (DHi) from analysis of the sum of the heat evolved per gram of the solid (h/g) and the concentration of base in solution ([B]). Details of Cal-ad method have been published elsewhere [15]. Samples of supported H3PW (0.5 g) were weighed and transferred to an isothermal calorimetric cell, followed by the addition of 50 ml of anhydrous cyclohexane. A calibrated syringe (Hamilton, 5 ml) was filled with a known concentration of pyridine solution (e.g., 0.1 mol l1). All these operations were carried out in an inert atmosphere glove bag filled with dry nitrogen. Then, both systems (cell and syringe) were inserted into the calorimeter holder, which were immersed in a thermal bath regulated at 25 8C. The calorimeter (model ISC 4300 from Calorimetry Sciences Corporation) was connected to a computer, and the experiment setup was made using the software provided. The addition of pyridine solution was done in the incremental mode. A calibration curve was performed prior or after each titration to obtain the equivalent energy of the system. As was observed separately, the heat of dilution of the addition of pyridine to cyclohexane was negligible. The adsorption experiments (25 8C) were carried out using 0.5 g of solid weighed and added to a sealed three neck round bottom flask with 50 ml of cyclohexane. Both operations were conducted inside a glove box. Addition of pyridine solution was performed using an automatic burette from Metrohm (Dosimat 665). The added volumes were the same as for the calorimetric experiment, as required by Cal-ad method. After each addition, 1 ml sample of
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solution was removed from the flask and placed into a quartz cell of 1 cm path-length. One milliliter of cyclohexane was added back into the flask in order to keep the volume constant. The absorbance of pyridine was measured at 251 nm (Beckman DU-650 UV–vis spectrophotometer) to determine its equilibrium concentration in solution using an analytical curve. Since the amount of pyridine added was known, the amount of base adsorbed by the solid was calculated by the difference. Both calorimetric and adsorption experiments were checked for diffusion constrains. It was observed that for standing times above 3 min, there was no variation in the absorbance or the heat measured, and therefore the experiments were repeated (three times) under these optimized conditions. The Cal-ad method has been applied together with spectroscopic data to analyze acidity of various heteropolyacids such as H3PW12O40 [10], supported H3PW12O40 on silica [15] and cesium derivatives of H3PW [25]. In addition, other solids such as silica gel [26], mordenite [27], Y zeolite [28], ZSM-5 [29], TS-1 [30], sulfated zirconia [31], etc., are among the variety of important solid acids characterized by this method.
3. Results and discussion A preliminary study by TG–DTA showed that H3PW supported on alumina (prepared by impregnation in acetonitrile solution) was stable at the chosen calcination temperature. Derivative thermogravimetry (DTG) displayed a large peak starting at 230 8C (with a small maximum at about 250 8C) and ending at about 500 8C. This mass loss can be attributed to the transformation of the hexahydrated form of H3PW to the anhydride phase [9]. This water release is practically simultaneous to the partial decomposition of the Keggin structure. The final destruction of H3PW supported on Al2O3 occurs at about 580 8C (maximum at 550 8C in the DTG curve). Therefore, as determined formerly for pure H3PW [8], the thermal treatment conditions used for H3PW/Al2O3 are appropriated to obtain intact the supported Keggin structure without the formation of the anhydride phase. 3.1. FTIR spectra The supported H3PW samples were analyzed by FTIR in order to confirm the presence of the Keggin anion on Al2O3 surface. The structure of PW12O403 anion works as a model for this class of heteropolyacid, consisting of a PO4 tetrahedron surrounded by four W3O13 groups formed by edge-sharing octahedra. These groups, are connected to each other by corner-sharing oxygens [32], and this arrangement gives rise to four types of oxygen bands between 1200 and 700 cm1, a fingerprint region for these compounds. Fig. 1A shows the sample with 40 wt.% H3PW/Al2O3, prepared in aqueous solution with HCl 0.5 mol l1. The
Fig. 1. (A) FTIR spectra of: (a) 40 wt.% H3PW/Al2O3 (HCl 0.5 mol l1); (b) Al2O3; (c) H3PW; (d) subtraction (40 wt.% H3PW/Al2O3–Al2O3). (B) FTIR spectra of 40 wt.% H3PW/Al2O3 in: (a) HCl 0.1 mol l1; (b) HCl 0.5 mol l1; (c) ethanol; (d) CH3CN. Pure H3PW (e) is shown for reference.
typical bands for absorptions of P–O (1080 cm1), W = Ot (983 cm1), W–Oc–W (898 cm1) and W–Oe–W (797 cm1) are clearly displayed. These bands are also preserved on the supported samples with more than 40 wt.% H3PW. In order to make it clearer, a subtraction spectrum was produced (40 wt.% H3PW minus alumina), which has shown unequivocally the presence of the Keggin anion. In the samples with 20 wt.% H3PW/Al2O3, the existence of the Keggin anion is not evident, whatever is the solvent used. Actually, its spectrum is similar to that of alumina, which shows a high absorption band between 1000 and 600 cm1. Based upon the area of the Keggin anion (about 1.1 nm2) and the surface area of alumina (155 m2 g1), theoretically a loading of about 40 wt.% of H3PW saturates the surface. Therefore, loading H3PW bellow the monolayer leads to species that either are highly dispersed or are decomposed on the surface. In order to check these possibilities other experiments were performed. Fig. 1B shows the several preparations of 40 wt.% H3PW on alumina with different solvents. The results reveal that there is a clear correspondence between the solvent used for impregnation (pH when aqueous solutions are used) and the stability of Keggin anion. The impregnations with CH3CN, ethanol and water (HCl 0.5 mol l1) show the typical Keggin structure by the corresponding absorptions, although the absorption at 897 cm1 is much less intense for the ethanol preparation than the others. On the other hand, from aqueous
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solution with HCl 0.1 mol l1, there is a decomposition of the [PW12O40]3 anion according to the splitting of the band at 1080 cm1 into two others (1100 and 1053 cm1), and the displacement of a band from 983 to 960 cm1, which are closely found [33] in the spectrum of lacunary species of [PW11O39]7. This agrees with many studies related to increasing stability of Keggin anion in organic solvents, and hydrolysis reaction of H3PW when the pH is higher than 2 [32]. Nevertheless, the other bands are not displaced significantly (i.e., they are the typical Keggin absorptions, but broader), which may point to some intermediate form between the anhydride and the anhydrous H3PW [34]. 3.2. Raman spectra Raman spectra of H3PW supported on alumina (prepared in CH3CN) are displayed in Fig. 2A. Crystalline H3PW shows characteristic bands at 1100 cm1 (stretching vibration of P–O), 990 cm1 (stretching of W = O), 900 cm1 (bending of W–Oc–W) and 550 cm1 (bending of O–P–O). The sample with 20 wt.% H3PW/Al2O3 shows only a broad and low intensity band at about 990 cm1. For 40% H3PW, it is clear the presence of the most bands related to the Keggin vibrations. In order to make sure there was no decomposition of H3PW after deposition over alumina and further calcination, Raman spectra were taken for the sample with 40 wt.% H3PW after impregnation on Al2O3 (complete
Fig. 2. (A) Raman spectra of: (a) Al2O3; (b) 20; (c) 40; (d) 50 wt.% of H3PW/Al2O3; (e) H3PW. Supported samples were prepared in CH3CN. (B) Raman spectra of 40 wt.% H3PW/Al2O3 prepared in different solvents: (b) HCl 0.5 mol l1; (c) ethanol; (d) acetonitrile. WO3 (a) and H3PW (e) are added for reference.
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dryness of the sample), and after calcinations procedure (200 8C/vacuum/6 h). Both spectra were identical to that in Fig. 2A. Fig. 2B shows Raman spectra of supported samples (40 wt.%) prepared in the different solvents. Looking at the Raman spectrum of the ethanol impregnation, W = O vibration band is split up in two peaks at 997 and 991 cm1. This indicates that different species of H3PW are interacting with the alumina surface after impregnation. In addition, a Raman spectrum of WO3 was taken in order to check this possible decomposition product. It can be seen the WO3 spectrum is very different from the supported samples. Thus, it can be concluded that at 40 wt.% loading, H3PW structure is not decomposed to form WO3. At lower coverage (20–40 wt.% H3PW) there is probably a strong interaction of H3PW with alumina surface, which can give rise to surface species with distorted Keggin anion or fragmented [PW12O40]3 structure. 3.3. XRD and SEM–EDX analysis XRD patterns (Fig. 3A and B) were obtained on the samples with 20, 40 and 80 wt.% H3PW, prepared in aqueous solution with HCl 0.5 mol l1 and acetonitrile, respectively. Alumina displayed only a very low intensity signal without any distinct peak. Supported samples prepared in HCl 0.5 mol l1 presented only wide bands
Fig. 3. (A) XRD patterns of H3PW/Al2O3 prepared in H2O (HCl 0.5 mol l1) with: (b) 20; (c) 40; (d) 80 wt.% of H3PW. Pure Al2O3 (a) and H3PW (e) are included for reference. (B) XRD patterns of H3PW/Al2O3 prepared in CH3CN with: (b) 20; (c) 40; (d) 80 wt.% of H3PW. Pure Al2O3 (a) and H3PW (e) are included for reference.
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centered at 2u = 68 (20 wt.%) and at 2u = 7.58 (40 wt.%). For the sample with 80 wt.%, a characteristic pattern of hydrated H3PW was observed. Because of the FTIR and Raman spectra of the 40 wt.% samples (the theoretical monolayer) evidenced the Keggin structure, it can be inferred that H3PW is probably highly dispersed on the alumina. Crystals of H3PW supported on Al2O3 surface are deposited as separate entities, but not big enough or well formed to be detected by XRD. In contrast, supported samples prepared in acetonitrile showed isolated peaks coincident with crystalline H3PW on loadings as low as 20 wt.% (Fig. 3B). Thus, the preparation method using acetonitrile probably produced larger crystal domains on the alumina surface than the equivalent aqueous acidic impregnations. A mechanical mixture with 20 wt.% H3PW loading exhibited the powder pattern of the Keggin structure. However, compared to the supported sample prepared in acetonitrile, the peaks are much similar (line-width and intensity) to crystalline H3PW. Thus, when crystalline phases are detected on the surface, XRD can be used to distinguish supported H3PW/Al2O3 from mechanical mixtures, as observed for silica-supported systems [15].
In order to check the crystalline formation and dispersion of H3PW over alumina, scanning electron micrographs (SEM) were taken on the aqueous impregnation products (Fig. 4). The tungsten mapping on alumina surface of samples with 20, 40 and 80 wt.% H3PW (Fig. 4d–f) shows that the polyacid is highly dispersed on the surface, with increasing tendency to agglomeration for higher loadings. The SEM pictures (Fig. 4a–c) display no resolved micro domains of H3PW. For the 20 wt.% H3PW/Al2O3, there is a distribution of crystallites with different sizes and the presence of some voids among them. Some roughness can be observed on the surface. For the 40 wt.% sample, the surface is smoother than the 20 wt.% H3PW/Al2O3. No crystallites on the surface could be distinguished, although no void on the surface is observed. The sample with 80 wt.% H3PW shows the presence of larger agglomerates than the 20 wt.%, and the surface is rougher than the sample with 40 wt.%. These results agree with the obtained XRD patterns, since no defined crystalline phase was observed for the samples with 20 and 40 wt.%. At 80 wt.% loadings, the formation of large crystallites gives rise to the observed pattern.
Fig. 4. Scanning electron micrographs of H3PW/Al2O3 prepared in HCl 0.5 mol l1 with: (a) 20; (b) 40; (c) 80 wt.% H3PW, and the respective (d–f) X-ray emission dot map obtained with X-ray fluorescence microprobe. The white dots represent the tungsten atoms.
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Fig. 4. (Continued ).
3.4.
31
P MAS–NMR spectra
Fig. 5A shows the 31P MAS–NMR spectra of the H3PW/ Al2O3 system, obtained by impregnation in water with HCl 0.1 mol l1, for different contents of polyacid. For low concentration of H3PW (20 wt.%) there is a wide single peak (line-width of 330 Hz) centered at 13.5 ppm, while hydrated H3PW shows a peak at 15.5 ppm (both referenced to 85 wt.% H3PO4). It has been discussed that 31P MAS–NMR signal of H3PW depends upon the hydration degree and can be varied from 10.0 to 16 ppm [10]. For the 40 wt.% H3PW/ Al2O3 sample, two peaks were observed: a higher one centered at 13.4 ppm and a smaller at 15.5 ppm. At higher concentrations (50–80 wt.%), the two peaks maintained the same positions, but the second (15.5 ppm) became more intense with the H3PW loading. These results indicate two kinds of supported H3PW. Type 1 consists of micro crystals of H3PW deposited on Al2O3, but without direct interaction with the surface. This species practically do not differ from pure hydrated H3PW (d 15.5 ppm). Type 2 consists of H3PW interacting with the surface acid sites from alumina (d = 13.5 ppm). Therefore, according to the coverage of H3PW on Al2O3 these two species are distributed on the supported samples. For amounts below 40 wt.%, there is a strong interaction of H3PW with alumina surface forming
species with distorted or decomposed Keggin structure characterized by d 13.5 ppm. Upon higher concentration (40–80 wt.%), alumina surface is saturated and H3PW crystals are agglomerated on surface (but without direct interaction), which is characterized by a growing second peak assigned to pure H3PW at d 15.5 ppm. Coupling these results with FTIR and Raman measurements, there is an indication that the Keggin anion may undergo some degree of decomposition. It may have produced a lacunary species, some intermediate anhydride or a fragmented phase. The lacunary species ([PW11O39]7) should present a signal at 10.4 ppm [32], which is different from the one obtained (13.5 ppm). The peak present at d = 13.5 ppm is also claimed [16,23] to be the formation of dimeric species ([P2W21O71]6 or [P2W18O62]6). Other study [34] points to the formation of some anhydride phase of H3PW. Therefore, the presence of one or more species from decomposition of the Keggin unit is established on these supported solids under the present preparation conditions, although the exact species cannot be specified at this point. A mechanical mixture of 40 wt.% H3PW with Al2O3 gives rise to a single peak at 15.6 ppm with approximately the same line-width of hydrated H3PW (75 Hz). Therefore, 31 P MAS–NMR is able to distinguish supported samples from mechanical mixtures of H3PW and alumina.
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Fig. 5B shows the NMR spectra of H3PW/Al2O3 prepared in CH3CN solution. All samples display a single peak centered at 14.6 ppm. These results indicate that there is no decomposition of H3PW prepared in this solvent, and the signal is equivalent to crystalline hydrated H3PW (15.5 ppm). Fig. 5C shows the spectra of 40 wt.% H3PW/Al2O3 prepared in different solvents. The samples prepared in ethanol and water (HCl 0.1 mol l1) exhibit two peaks. One peak is assigned to the hydrated Keggin anion (14.6 ppm) and the other at 11.8 ppm may be attributed to the [PW11O39]7 or another fragmented form of the polyanion, as mentioned before. It is clear from the spectrum of the system prepared in HCl 0.5 mol l1, which showed a single peak at 14.6 ppm, that pH is a major aspect in aqueous impregnation. The same isotropic shift for this sample (14.6 ppm) is observed for preparation in acetonitrile. 3.5. Calorimetric and adsorption analysis of supported H3PW/Al2O3 Based upon the spectroscopy and microscopy results, a preliminary calorimetric run for acidity was tested on the catalysts that unequivocally showed no decomposition. The main samples were those prepared in water at higher acidity (HCl 0.5 mol l1) and in acetonitrile. Samples with 20 and 40 wt.% loadings were tested, because although the theoretical monolayer is about 40 wt.%, studies of the 31P MAS–NMR relaxation for silica supported heteropolyacids [35] have demonstrated that the real monolayer is about half of that calculated. In order to verify acidity of the produced materials, calorimetric titrations with pyridine were performed, and calculated average enthalpies are presented in Table 1. These initial results show that the interaction of H3PW with Al2O3 is almost independent of heteropolyacid content from 20 to 40 wt.% (DH ffi 19 kcal mol1) for the strongest site for both solvent preparations. Therefore, considering the higher dispersion of sample loaded with 20 wt.% and the easy preparation of supported catalysts from aqueous solution, a full calorimetric and adsorption study (Cal-ad method) was carried out on that sample. Table 1 Results of calorimetric titrations (25 8C) of supported H3PW/Al2O3 (prepared in H2O acidified with HCl 0.5 mol l1 or CH3CN) with pyridine in cyclohexane slurry
Fig. 5. (A) 31P MAS–NMR spectra of: (a) hydrated H3PW Al2O3 with (b) 20; (c) 40; (d) 50 wt.% of H3PW. Samples HCl 0.1 mol l1. (B) 31P MAS–NMR spectra of hydrated supported H3PW on alumina prepared in acetonitrile. (C) NMR spectra of hydrated H3PW and 40 wt.% H3PW/Al2O3 different solvents.
and H3PW/ prepared in H3PW and 31 P MAS– prepared in
Samplea
H3PW (wt.%)
H+/g of solid (mmol)
DH (kcal mol1)b
20HPW-WA 40HPW-WA 20HPW-AC 40HPW-AC
20 40 20 40
0.202 0.404 0.202 0.404
20 18 17 19
WA = H2O with HCl 0.5 mol l1; AC = CH3CN. The enthalpy is an average based on the first three additions of pyridine in the titration, using the limiting reagent approximation. To convert the enthalpy values to kJ mol1 multiply by 4.184. a
b
E. Caliman et al. / Catalysis Today 107–108 (2005) 816–825
Fig. 6. Calorimetric and adsorption isotherms (25 1 8C) of 20 wt.% H3PW/Al2O3 (prepared in H2O with HCl 0.5 mol l1) reacted with pyridine in cyclohexane slurry.
The calorimetric and adsorption isotherms of 20 wt.% H3PW/Al2O3 (prepared in HCl 0.5 mol l1) reacted with pyridine are presented in Fig. 6. The Cal-ad data were analyzed assuming the presence of one, two and three different sites on the catalyst surface. The best results indicated a two-site model as the most representative, with lower standard deviations on the calculated parameters and residual sum of the squares in the same order of magnitude of the experimental error (based on the calorimetric measurements). Moreover, the choice of two sites is also supported by the FTIR spectrum of the adsorbed pyridine. The absorption region of pyridine adducts (1700– 1400 cm1) showed three peaks at 1540, 1488 and 1635 cm1. The first two absorptions are due to pyridinium ion while the third one has a contribution of coordinated pyridine [36]. It should be mentioned that this absorption band at 1635 cm1 is not totally conclusive because of contributions of pyridinium ion and moisture, since the
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spectrum was taken at ambient conditions. The strongest site present on H3PW/Al2O3 is Bro¨nsted and the weaker is Lewis-type. The enthalpy of interaction of 20 wt.% H3PW/Al2O3 with pyridine on the strongest site is DH1 = 22.6 kcal mol1. This value is lower than pure H3PW (DH1 = 32.7 kcal mol1) [10] but higher than pure g-Al2O3 (DH1 = 15.2 kcal mol1) [37]. Comparing with 25 wt.% H3PW/ SiO2 (DH1 = 27.9 kcal mol1) [15], it can be stated that a strong interaction occurs between H3PW and Al2O3 producing a solid acid that is weaker than pure H3PW or supported on silica. Table 2 shows the thermodynamic parameters for these solids. The dispersion of H3PW on alumina can be inferred from the number of the strongest acid sites (n1). Compared to H3PW (n1 = 0.08 mmol g1), the protons on 20 wt.% H3PW/Al2O3 (n1 = 0.154 mmol g1) are much more available. Based on the total amount of available protons on the 20 wt.% supported sample (0.202 mmol g1) about 76% were titrated with pyridine, compared to only 8 and 45% of H3PW and H3PW/SiO2, for the strongest sites, respectively. These results confirm our earlier spectroscopy and microscopy data, which showed that H3PW was highly dispersed on the alumina surface. This leads to more available protons for surface-type catalytic reactions, although the strength is much lower than both solids formerly mentioned. It is interesting to note that the strongest acid site for 20 wt.% H3PW/Al2O3 is similar to the second acid site of pure H3PW (vide Table 2). Both enthalpy (DH1 22 kcal mol1) and the amount of sites (n1 0.16 mmol g1) demonstrate that the strongest protons of H3PW are mostly tied to alumina, probably neutralizing basic sites present on its surface. Thus, only the weaker sites present on pure H3PW become available for catalytic reactions when it is supported on alumina, which may explain the lower activity of this catalyst compared to the similar supported on silica [24]. These arguments match very well when one compares the second site of the supported H3PW on alumina. About 76% of H3PW protons were neutralized by pyridine (site 1) and
Table 2 Cal-ad results (25 1 8C) for 20 wt.% H3PW/Al2O3 reacted with pyridine in cyclohexane slurry Parameter 1
n1 (mmol g ) K1 (mol1 l) DH1 (kcal mol1)b n2 (mmol g1) K2 (mol1 l) DH2 (kcal mol1)b
20 wt.% H3PW/Al2O3
H3PWc
25 wt.% H3PW/SiO2 d
0.152 0.007 (1.20 0.02) 10 5 22.6 0.2 0.20 0.04 (4.9 0.9) 10 3 12.0 1.1
0.079 0.005 (3.70 0.02) 105 32.7 0.3 0.16 0.05 (2.9 1.0) 103 19.6 4.8
0.102 0.005 (6.7 0.1) 105 27.9 0.7 0.310 0.061 (3.8 0.7) 104 10.1 2.9
Data of H3PW and 25 wt.% H3PW/SiO2 are included for comparisona. a The Cal-ad results for alumina [37] are: DH1 = 15.2 1.2; K1 = (3.31 0.04) 104; n1 = 0.16 0.01; DH2 = 6.7 2.9; K2 = (5.9 2.8) 102; n2 = 0.25 0.15. Units are the same as above. b To convert the enthalpy values to kJ mol1 multiply by 4.184. c Data from reference [10]. d Data from reference [15].
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the remaining 24% reacted with the most basic hydroxyl groups on alumina surface. The enthalpy for the second site of 20 wt.% H3PW/Al2O3 is 12 kcal mol1, and the amount of sites is n2 0.20 mmol g1. These quantities are about the same for the strongest sites on pure alumina (mostly Lewis-type, with DH1 = 15.2 kcal mol1 and n1 = 0.16 mmol g1) [37]. Therefore, the second site of 20 wt.% H3PW/Al2O3 is mostly the original acidic sites of pure alumina, which has not reacted with H3PW during the impregnation and further calcination process.
Acknowledgments The authors thank Dr. Edi Mendes Guimara˜es from Laborato´rio de Difrac¸a˜o de Raios-X (IG/UnB) for XRD measurements, Dr. Maria Jose´ A. Sales and Mr. Filipe A. Silva from Laborato´rio de Pesquisa em Fı´sico-Quı´mica de Polı´meros (IQ/UnB) for TG/DTG experiments and Dr. David P. Geeverghese for revising the manuscript. Also, we are grateful to CNPq for a scholarship to pursue a Doctorate degree (E.C.), and financial support given by IQ/UnB (FUNPE), FINATEC, FINEP/CTPetro (0663/00), FINEP/ CTInfra (0970/01) and FAP-DF/SCDT/CNPq.
4. Conclusions This study deals with preparation, characterization and acidity behavior of 12-tungstophosphoric acid (H3PW12O40) supported on g-alumina. The catalyst H3PW/Al2O3 was prepared using different solvents (water, ethanol and acetonitrile) by the impregnation-evaporation technique. The results clearly indicated the dependence of the Keggin structure stability on the surface with the hydrolysis of the anion. For aqueous solutions, high acid concentration is mandatory to avoid decomposition of the polyanion. It was showed by FTIR and Raman spectroscopy that the splitting of the bands related to P–O and W = O vibrations, respectively, are indicative of the formation of fragmented Keggin anion. Also, 31P MAS–NMR spectroscopy displayed two peaks in the spectra of decomposed samples. High dispersion of H3PW on the alumina surface was confirmed by XRD and SEM–EDX results. Although the preparation of supported catalyst in acetonitrile does not cause any destruction of the Keggin structure, higher dispersion is obtained in aqueous acid medium. Thus, the best preparation achieved was to disperse H3PW on alumina in aqueous solution with HCl 0.5 mol l1. The acidity of the samples with 20 and 40 wt.% of H3PW were studied by calorimetry for those, which did not show any decomposition (aqueous HCl 0.5 mol l1 and acetonitrile). It was observed that the interaction of H3PW is practically independent of the polyanion in that range of concentration. Thermodynamic data for the 20 wt.% H3PW/Al2O3 were obtained by the Calad method (calorimetry and adsorption of the solid with pyridine in ciclohexane slurry). A two-site model (supported by the results of pyridine adsorption analyzed by FTIR) gave DH1 = 22.6 kcal mol1, n1 = 0.15 mmol g1 and DH2 = 12.0 kcal mol1, n2 = 0.20 mmol g1, respectively, for the strongest site (Bro¨nsted) and the weaker site (Lewis). The catalyst H3PW/Al2O3 is a weaker acid than pure H3PW or supported on silica, but stronger than pure alumina. On the other hand, the dispersion of the protons on alumina surface is higher than silica, and more active surface protons became more available than pure H3PW, according to the n1 values. The neutralization of the strongest protons of pure H3PW occurred on the alumina surface, because of the reaction of the acid sites with the most basic sites of the support.
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