Heterogenization of Heteropolyacids: A General Discussion on the

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2546

Ind. Eng. Chem. Res. 1996, 35, 2546-2560

Heterogenization of Heteropolyacids: A General Discussion on the Preparation of Supported Acid Catalysts Yue Wu,* Xingkai Ye, Xiangguang Yang, Xinping Wang, Wenling Chu, and Yucai Hu Changchun Institute of Applied Chemistry, Academia Sinica Changchun, Jilin 130022, People’s Republic of China

Heteropolyacids (HPAs) possess both acidic and redox catalytic properties and held extensive promise of practical application. These type of compound display a great potential of specific synthesis reactions for replacing sulfuric acid to satisfy the requirements of environmental protection. Heterogenizing HPAs would not only make them more useful in liquid phase oxidation with oxygen and in acid-catalyzed reaction, as the catalyst is often difficult to separate from the reaction products, but also create favorable factors for realizing heterogenization of homogeneous reaction and even utilizing new technology of catalytic distillation. In this paper, different kinds of porous materials which are well characterized, including oxides such as Al2O3, SiO2, TiO2, diatomite, bentonite, and active carbon of different sources, were used as support for heterogenizing HPAs (in different media), and the obtained results, the intrinsic characters of supports which may influence both the nature of the interaction between HPAs and supports in the heterogenization and the activity in the catalytic reaction, are explored. It is expected that these can provide a referential model for preparing supported acid catalyst used in liquid phase. Introduction Heteropolyacids (HPAs) possess both acidic and redox catalytic properties and held extensive promise of practical application (Misono et al., 1990). At present, a major subclass of the heteropolyacid is Keggin type HPA with molecular formula XnM12O40(8-n)-. It consists of 12 coordinate ions (M) (tungsten or molybdenum) and oxygen ions arranged symmetrically around a central atom (X) which normally is phosphorus (P) or silicon (Si) (Figure 1). The counter ions can be either protons or metal cations or a mixture of these. In previous literatures reports, there are many summaries and comments concerning the structure, synthesis, characterization, and catalytic function of HPAs (Pope et al., 1991; Tsheneshkoba, 1991; Misono et al., 1992; Wu et al., 1985). The most important reason that HPAs are very often used as acid catalyst is the lower charge density on the surface of the spherical HPA molecules. As there is almost no charge localization, the protons are very mobile resulting in strong Bronsted acidity, some 100 times stronger than sulfuric acid when applied as a solid or in nonaqueous media. Further advantages of HPA catalysts are the low volatility, low corrosivity, and high activity and selectivity for several reactions, when compared to conventional mineral acids. These type of compound display a great potential of specific synthesis reaction for replacing sulfuric acid to satisty the requirements of environmental protection. A representative use of tungsten-based HPA as a catalyst to replace sulfuric acid is hydration of an alkene to an alcohol, which is a typical proton acid-catalyzed reaction (Misono et al., 1990). HPAs composed of molybdenum and vanadium, in contrast to the tungsten-based compounds, own a stronger redox property. These compounds can be used as catalysts for the production of methacrylic acid (Misono et al., 1990) and have been tested in many other oxidation reactions including the oxidation of methanol, * To whom all correspondence should be addressed.

Figure 1. Structure of H3PW12O40. (A) Heteropolyanion with the Keggin structure, PW12O403-. (B) An example of the secondary structure (a part), H3PW12O40‚6H2O. Each H5O2+ bridges four polyanions.

the oxidation of a ketone to an acid, and the Wacker oxidation of 1-alkenes to aldehydes and ketones. Therefore, HPAs have also become a type of redox catalysts with promise of wide application (Misono et al., 1992). Heterogenizing HPAs would not only make them more useful in liquid phase oxidation with oxygen and in acid-catalyzed reaction, as the catalyst is often difficult to separate from the reaction products, but also create favorable factors for realizing the heterogenization of a homogeneous reaction and even utilizing the new technology of catalytic distillation. At the same time, heterogenizing HPAs would simplify the technology so that it would make supported HPA catalyst widely applied. Certainly, the nature of supports apparently affects the structure and acidity, as well as the redox property of supported HPA, which is the problem to be resolved in heterogenization of HPAs. In this paper, besides normal metal oxides, such as SiO2, Al2O3, and TiO2, active carbon and an ionexchange resin have been employed. Sebulsky and Henke (1971) used SiO2, SiO2-Al2O3, and Al2O3 as supports of silicotungstic acid (SiW12) in an alkylation reaction and obtained results that the activity of the supported catalyst is strongly dependent upon SiO2

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Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 2547

content. A conclusion was thus drawn that SiO2 is the most suitable support for this reaction. Izumi and Urabe (1981) in their previous works have shown qualitatively that HPAs show a high affinity to active carbons. After impregnation of active carbon with a concentrated solution of phosphotungstic acid and consecutive drying, HPA could not be desorbed from the carbon support, although HPAs are highly soluble in water, methanol, and acetone. Prolonged extraction of the impregnated active carbon with methanol in a Soxhlet apparatus was reported not to be able to remove a measurable amount of HPA which is adsorbed in the micropores of the active carbon. In different solvents, Kolikov et al. (1989) researched in detail the adsorption process of SiW12 on silica, Al2O3, and active carbon. The adsorption of HPAs on the supports was explained by polarity of solvent and competitive adsorption as well as pore structure of the supports. It was found that the adsorption of PW12 on both SiO2 and Al2O3 is reversible, but on Al2O3, aluminum salt would be desorbed from the support surface. However, as far as the active carbon is concerned, the adsorption is partially irreversible. SEM study also pointed out that there are three kinds of adsorbed PW12 in light of the difference in amount, namely monolayered, multilayered, and isolated dispersed. More recently, Schwegler et al. (1992) have pointed out that heteropolyacid (HPAs) such as phosphotungstic acid (PW12) and silicotungstic acid (SiW12) could be adsorbed strongly on various active carbon. But the adsorption amount and strength might be different with respect to the nature of the active carbon and HPAs. On the basis of the adsorption isotherms measured, they tried to find out the nature of the interaction between HPAs and carbon surface. Although it is not sure which kind of bond can result in strong adsorption, an assumption can be made that this action may be due to electrostatic attraction. A recent NMR study illustrated that 12-molybdophosphoric acid (PMo12) can adsorb on alumina with its original structure keep intact (Chem et al., 1988). In case of SiO2-supported H4SiMo12O40, two kinds of heteropolyanion were detected. For lowloaded supported sample, there exist an intensive interaction between heteropolyanions and hydroxyl groups on the silica surface; however, for high-loaded supported sample, a structure approaching that of H4SiMo12O40 was formed (Thonvenot et al., 1991). In addition, the NMR data showed the interaction of supported H3PW12O40 with surface OH groups of silica (Mestkhin et al., 1990). It is reported in the literature (Thonvenot et al., 1991) that [SiMo12O40]4- anions could be reconstructed on the silica surface from silica and splitted molybdate or even MoO3 with the assistance of water. Active carbon, silica, and alumina are usual supports for catalysts; especially their use as a carrier for noble metals has been studied extensively (Zecchina et al., 1993). Oxide supports such as SiO2 and Al2O3 have a large surface area and a peculiar pore structure. In the process of adsorption, surface hydroxyl groups play a very important role in adsorbing different ions from solution. Generally speaking, basic solids, such as Al2O3 and MgO, have a trend of decomposing HPAs (Nowinsla et al., 1991). On the other hand, although SiO2supported HPAs are more stable, the interaction of HPA and the OH surface groups can bring about the decrease of acidity and redox property of HPAs (RocchiccioliDeltcheff et al., 1990).

Figure 2. Metal containing all of the functional groups detected on the surface of active carbon. Ia: Carboxyl group which is removed at about 200 °C (occurs only in products that have been oxidized at 150-200 °C). Ib: Carboxyl group, removable only at about 325 °C. II: Carboxyl group that occurs as a lactone. III: Phenolic hydroxyl group. IV: Carbonyl group that reacts with the carboxyl group II to form the lactone (or lactol).

In contrast to other supports, active carbon has a high specific surface area and is stable in a wide range of pH. During the preparation of the active carbon, due to various modification methods, the extent of the oxidation could be different and the surface chemistry of carbon is more complicated than that of the oxides. Under extreme conditions, two types of active carbon could be obtained. The reduced form (hydrophilic), the L-type carbon (activated at 400 °C), mainly contains acidic groups and has more aromatic characteristics like hydroquinone, whereas the oxidized form (hydrophoblic, the H-carbon containing more basic groups) would be obtained at higher temperature (at 800 °C) and has a pronounced quinonoid structure and double bonds. Functional groups existing on the carbon surface may be divided into acidic and basic types. The main types of acidic group comprise carboxyl, hydroxyl, phenolic, and carbonyl basic groups whose nature is still not well known are ascribed to structure corresponding to those of pyrone-like O

O

,

O

O

or chromene groups O

H R

O , R

H

(Boehm, 1989) (Figure 2). However, the nature of adsorption of HPAs on the surface of active carbon is

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2548 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 Table 1. Surface Properties of Supports and Supported Heteropolyacid supported amount (mg/g B) B(1) B(2) B(3) B(4) B(1)/PW12 B(2)/PW12 B(3)/PW12 aB(4)/PW12 B(1)/SiW12 B(2)/SiW12 B(3)/SiW12 B(4)/SiW12 B(1)/PMo12 B(2)/PMo12 B(3)/PMo12 B(1)/SiMo12 B(2)/SiMo12 B(3)/SiMo12 a

497 453 498 485 482 478 500 101 120 150 122 100 131

specific area (m2/g)

surface acid strength (Ho)

surface acid amount (mmol/g)

NH3 adsorbed (TPD) (10-5 mol/g)

effective amount of heteropolyacid on support (mg/g B)

43.3 77.6 81.7 37.5 21.1 24.5 37.6 9.9 9.6 12.6 15.1 8.0 27.4 23.9 24.8 29.6 24.1 40.5

+1.5 ∼ -3.2 +1.5 ∼ -3.2 +1.5 ∼ -3.2 -5.6 ∼ -8.2 +1.5 ∼ -3.2 +1.5 ∼ -3.2 +1.5 ∼ -3.2 +5.6 ∼ -8.2 +1.5 ∼ -3.2 +1.5 ∼ -3.2 +1.5 ∼ -3.2 +5.6 ∼ -8.2 +1.5 ∼ -3.2 +1.5 ∼ -3.2 +1.5 ∼ -3.2 +1.5 ∼ -3.2 +1.5 ∼ -3.2 +1.5 ∼ -3.2

0.15 0.30 0.32 0.92 0.76 0.71 0.77 0.93 0.57 0.84 0.78 0.91 0.48 0.52 0.59 0.50 0.57 0.56

5.16 5.04 7.97 8.88 8.60 9.83 12.89 15.96 5.73 8.77 14.04 14.81 6.16 7.28 14.27 5.59 8.17 13.27

83.6 94.4 123.8 153.3 41.3 63.1 101.1 106.7 37.5 44.3 86.9 25.5 37.3 60.5

B(4) does not adsorb PMo12 and SiMo12.

Table 2. Physico-Chemical Properties of Active Carbons active carbon

surface area (m2/g)

pH

element analysis

no.

type

pretreatment

post-treatment

post-treatment

C

H

1 3 4 5 6

coconut shell hawnut necleon hickory necleon coal based fiber carbon

1418 949 772 1072 850

1431 963 782 1113

6.18 5.53 6.09 6.71 4.19

98.0 84.83 89.47 81.21 78.1

0.53 0.58 0.52 0.3 1.4

a

N 0.77 0.72 4.5

Oa

ash

0.4 11.5 7.49 0.2 14.4

1.10 2.32 1.80 19.3 1.64

By normolization.

still unclear. In addition, due to the speciality of pore structure of active carbon, there exists an argument whether this adsorption is physical (pore structure, surface area) or chemical (surface chemistry) in nature (Schwegler et al., 1992). But judging from the properties of the HPAs and surface chemistry of active carbon, it is more likely that the surface chemistry of the carbon determines the adsorption. As may be seen from the above discussion, the surface chemistry of carbon is very complicated. So far there are no systematical studies on the interaction of HPAs and these supports, including which surface group plays the main role and how the acidcatalyzed reactions especially in the liquid phases by the supported catalysts are performed, and so on. Up to date, there is little reported concerning the activity of the supported acid catalysts with the elution of HPAs in liquid phase. In recent years, we have used many porous materials, including oxides such as Al2O3, SiO2, TiO2, diatomite, bentonite, and active carbon of different sources which have been well characterized as supports, and have systematically studied the adsorption of HPAs on these supports in different media and the influence of various supports on catalytic activity of HPA in the test reaction: esterification of acetic acid and alcohols (ethanol and n-butanol). In this paper, on the basis of our works, we would like to try to explore the intrinsic characters of the supports which may influence both the nature of the interaction between HPAs and supports in the heterogenization and the activity in the catalytic reaction. It is expected that these explorations can provide a referential model for preparing supported acid catalyst used in liquid phase. Experimental 1. Materials and Reagents. (A) Heteropolyacids. The heteropolyacids employed, such as PW12,

SiW12, PMo12, and SiMo12, were prepared according to the literature (Tsigdions, 1975). By means of element analysis, infrared spectroscopy (IR), and thermogravimetric analysis (TG)-differential thermal analysis (DTA), it was confirmed that the samples obtained possess Keggin structure. Their corresponding amounts of crystallized water were also measured: the results obtained were in agreement with those in the literature. (B) Supports and Solid Acid Catalysts. Metal oxides used were commercially available. TiO2 (specific surface area is 8.4 m2/g) was purchased from Rongxing Chemical Factory, Shenyang. SiO2 (S ) 357 m2/g) was from Haiyang Chemical Factory, Qingdao. γ-Al2O3 (S ) 267 m2/g) was from Zibo Aluminum Factory, Shandong. HZSM-5(1) (Si/Al ) 50) and HZSM-5(2) (Si/Al ) 38) were provided by Zeolite Factory, Nankai University, Tianjin. Diatomite (Gaotai in Jilin) and bentonite (Jiutai, Jilin) were washed with water and then dried before use, and corresponding data are listed in Table 1, in which B1 indicates original bentonite, B2 indicates treatment by pure acetic acid, B3 indicates treatment by dilute sulfuric acid, and B4 indicates treatment by 2 N sulfuric acid. Active carbons used were treated with HCl and then washed with water followed by drying. Six sorts of active carbon were employed: coconut shell carbon (C1, C2, Guanghua Timber Mill, Beijing), hawnut necleon carbon (C3, Liaoyuan Active Carbon Factory, Jilin), nickory nut necleon carbon (C4, Tonghua Activated carbon Factory, Jilin), coal-based carbon (C5, Xinhua Timber Mill, Shanxi), and fiber carbon (C6, Carbon Fiber Factory, Shanghai). The corresponding data are listed in Table 2. Strong acidic ion-exchange resin was provided by Nankai University, Tianjin. All other chemical reagents used were analytical grade. 2. Characterization of Supports and Supported Acid Catalysts. (A) Surface area was measured by

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BET method. (B) Acid strength and surface acidity were measured with Hammett indictor and by pH titration, as well as NH3 temperature-programmed desorption (NH3-TPD) method, respetively. (C) For the adsorption isotherms, a 0.2 g sample of support was added into 5 mL of HPA solution of known concentration. The mixture was kept at room temperature (15 °C) for 3 h with continued shaking. After centrifugation, the supernatant was diluted quantitatively with distilled water. The concentration of HPA in the diluted supernatant was measured using a Specord UV-vis spectrophotometer (Carl Zeiss, Germany) at 240-270 nm. The amount of immobilized HPA was expressed as milligram of HPA per gram of support. (D) The amount of oxygen-containing surface groups on active carbon was determined by means of Boehm’s titration. The types of groups, such as carboxyl, lacton, phenol, and carbonyl, were confirmed by using X-ray photoelectron spectroscopy (XPS) (VG Scientific ESCALAB-MK) and infrared spectroscopy (IR) (Dio-Red, FI-IR, USA). (E) Pore structure of the active carbon was measured on an ASAP 2400 apparatus manufactured by Mark Company, America. Pore size distribution was estimated on the basis of the BJH equation. Total pore capacity was calculated through the adsorption amount corresponding to 0.98 of relative pressure. 3. Preparation of Supported Catalyst of HPA. (A) Impregnation. The supported catalysts were prepared by conventional wet impregnation: a given amount of supports was added into HPA aqueous solution. Subsequently, the sample was continuously shaken at room temperature for 4 h. Then, excess water was evaporated in a water bath. The wetted samples were dried at 120 °C for 24 h. (B) Adsorption. A weighed amount of support was placed in a 200 mL flask equipped with reflux condenser, and HPA aqueous solution was added. Then, the solution was heated to boiling for some time. The mixture was left overnight. After separating the liquid, the supported catalysts were dried at 120 °C, meanwhile the amount of residual HPA in the mother liquid was determined. 4. Activity of Catalysts and Products Analysis. (A) An amount of 50 mL of the mixed solution of acetic acid and alcohol, whose molar ratio is constant (1:1), and supported catalyst, containing an equal amount of HPA, was placed in a 100 mL three-necked flask equipped with a reflux condenser and a thermometer. The reaction mixture was heated to react under constant power supplies. One of the products (H2O) was collected in a reflux separator, and the amount of H2O produced in an interval of 10 min was recorded until no water flowed out (the theoretical amount of H2O produced is 6-6.1 mL). The activity of the catalysts was judged from the kinetic curve of produced H2O. When it was cooled to room temperature, liquid products were poured out, and the catalyst remaining in the reactor was employed for the next runs. The amount of immobilized HPA eluted in reaction solution was measured using Specord UV-vis spectrophotometer. (B) The conversion and selectivity to reactants were calculated by acid-value titration and gas chromatography (GC) made by Shanghai Analytic Apparatus Factory 102G-type equipped with a thermal conductivity detector and a column filled with chromosorb containing di-n-nonyl sebacate and operated at 110 °C. Hydrogen was used as the carrier gas, and the conversion of

Figure 3. Kinetic curves of H2O produced in esterification over various catalysts: (1) H2SO4, (2) SiW12, (3) ion-exchange resin, (4) Y-zeolite (Si/Al ) 2.6), (5) HZSM-5 (Si/Al ) 38).

Figure 4. Kinetic curves of H2O produced in esterification over SiW12 immobilized on different supports: (1) TiO2, (2) SiO2, (3) HZSM-5 (Si/Al ) 50), (4) diatomite, (5) HZSM-5 (Si/Al ) 38), (6) Y-zeolite, (7) Al2O3. Note: The amount of SiW12 eluted from supports is in the order of TiO2 > SiO2 > diatomite > HZSM-5 (Si/Al ) 50) > HZSM-5 (Si/Al ) 38) > Al2O3 > Y-zeolite.

alcohol was calculated using the external standard method. No byproducts were detected in the course of the experiments. Results 1. Metal Oxide. (A) Effect of Different Supports on the Catalytic Activity of HPA(SiW12) in the Reaction of Esterification. Figure 3 gives the kinetic curves of H2O produced in esterification obtained under the same conditions and over H2SO4, HPA, ion-exchange resin, and zeolite catalysts. It can be seen from the figure that the activity of HPA is nearly comparable with that of sulfuric acid. The activity of ion-exchange resin takes second place but is much higher than that of zeolites, whose activity is always lower at relatively low temperature ( SiO2 (344.0) > HZSM-5 (Si/Al ) 50) (172.0) > diatomite (270.3) > HZSM-5 (Si/Al ) 38) (141.3) > Y-zeolite (58.8) > Al2O3 (69.0). It has been found that a part of immobilized HPA could elute from the supported catalysts during the

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2550 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996

Figure 6. Adsorption of SiW12 on inorganic support.

Figure 5. NH3-TPD of different oxide supports.

reaction. The amount (in mg) of eluted HPA varied although the catalysts contained an equal amount of HPA (∼530 mg/g). As shown by the numerical value given in parentheses, except for Y-zeolite and diatomite, the variation of the amount of the eluted HPA and activity for different catalysts show the same trend, that is to say, the larger the amount of HPA eluted, the higher the activity. For instance the HPA immobilized on TiO2 was almost completely eluted, so it showed the highest activity. This result illustrates that the activity of the supported catalyst is derived not only from the HPA on supports but also from the HPA eluted out in reactants. It also illustrates that the amount of HPA possible to be firmly immobilized on support depends on the nature of the supports. (B) Surface Acidity of Supports. Results obtained by NH3-TPD corresponding to the surface acidity of the oxide supports are given in Figure 5a,b. It can be found that the surface acidic properties of supports, including acidity and acid strength, are quite different. For simple oxides and zeolites, it is obvious that there is a corresponding relation between acidic properties and amount of strongly immobilized HPA. For example, the surface of TiO2 having almost no acidity cannot adsorb HPA; compared to the surface of SiO2, the surface of Al2O3 has much more acidity and higher acid strength, and the amount of HPA strongly immobilized is also higher. The amount of HPA firmly immobilized on Y-zeolite is higher than that on HZSM-5 due to its higher acidity and acid strength. From the above discussion, it is noteworthy to say that, in agreement with the general trend of impregnation method, the amount and the strength of immobilized HPA on support are detemined mainly by the surface acidic property of supports. It is to say that the ionic interaction between the surface sites and the HPAs is dominative; however, the physical properties of the support are not important. (C) Adsorption Isotherms of HPAs on Various Oxide Supports. Adsorption isotherms of SiW12 on

various supports are given in Figure 6. It is obvious that a nice regularity is observed between the adsorption amount of HPA and the surface acid-basic property of supports. In the case when SiO2 was impregnated from 6.5 g/L up to 60.08 g/L, SiW12 was hardly adsorbed. For diatomite and TiO2, no adsorption of SiW12 was observed under the conditions of lower impregnating concentration. Opposite phenomenon happens with the impregnation concentration up to 30.0 g/L and 15.25 g/L, respectively, but even when the concentration increased to 60.0 g/L, the amount of adsorbed HPA on diatomite and TiO2 merely reached 15 and 25 mg/g, respectively. On the contrary, the adsorption of SiW12 on γ-Al2O3 is very remarkable; the adsorption amount reached 370 mg/g Al2O3 (60.0 g/L of impregnating concentration). It is clear from the above results that the amount of HPA possible to be strongly immobilized on supports is dependent on the interaction between HPA and the surface of supports. 2. Bentonite. (A) Adsorption of HPA on Modified Bentonite. The main composition of Na-based bentonite used is montmorillonite, consisting of layered aluminosilicate and water. Although bentonite contains about 20∼30% of Al2O3, it does not adsorb HPA from aqueous solution even when treated with acid. This is different from zeolites. But in acetic acid solution (pH ) 4.21), an opposite case could be observed. The surface properties of four modified bentonites and the amount of adsorbed HPA are presented in Table 1, showing that surface acid strength of B(4) treated by 2 N H2SO4 is higher than that of the others. The order of acidity measured by titration and NH3-TPD has the same trend: B(4) > B(3) > B(2) > B(1). The acid strength of supports after adsorbing HPA remains almost unchanged, but the acidity for B(1), B(2), and B(3) is obviously increased. The amount of immobilized HPA is increased with the acidity. It is noteworthy that the amount of immobilized HPA composed of molybdenum on various betonites is low in contrast to that of tungsten-based HPAs whose adsorption amount is large and adsorption isotherms fit well the Langmuir equation (see Figure 7). Particularly, B(4) with high acid strength does not adsorb, this distinction between Wand Mo-based HPAs could be explained by the softness of heteropolyanions, which according to the literature (Izumi et al., 1983) are as follows: SiW12 > PW12 > PMo12 > SiMo12. Species with larger softness would produce stable intermediates. According to the HSAB principle (Pearson, 1963), it seems that the acid on the surface of bentonites, both original and treated with acid, is also in the soft state. It is emphatically pointed out that the specific surface area of supported catalysts, especially that of B(4), is significantly decreased com-

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Figure 8. The activity of SiW12/bentonite for the esterification in liquid-solid phase and the amount of SiW12 eluted.

Figure 7. The plots of equilibrium adsorption amount X as a function of X/C over PW12 (a) and SiW12 (b): O B(1), b B(2), 4 B(3), 2 B(4).

pared with that of the support itself. It appears to be clear from the above discussion that there exists a strong dependence of the adsorption amount of HPA on both acid strength of the medium and surface acid strength of the support. The surface of supports has little effect on the adsorption of HPA with higher acid strength, i.e. W-based HPAs. The HPA amount adsorbed on different supports is nearly the same, whereas for HPAs whose acid strength is weaker, i.e. Mo-based HPAs, even no adsorption would be observed on supports with higher acid strength. (B) Activity and Immobilized Firmness of HPAs Supported on Bentonite in Esterification. It has been found that the HPA immobilized on the support could elute in the reaction medium in the liquid-solid phase operation. At the first reaction run, the amount of HPA eluted is very notable, but with the reaction runs, a parabolically decreasing trend is observed (see Figure 8). At the same time, the catalytic activity is also decreased with the reaction runs. After several runs, the eluation of HPA from the support was no longer observed and the catalytic activity remained unchanged. It could thus be illustrated that at the beginning of the reaction, there exist not only liquidsolid reaction (derived from immobilized HPA) but also liquid phase reaction (from eluted HPA). If the amount of HPA eluted at the last run could be negligible, then the reaction could be considered as a single liquid-solid phase one. If the catalytic activity decrease in the next run is ascribed to the elution of HPA, from the plot of

∆x (%) as a function of ∆m (mg), where ∆x and ∆m are the difference in conversions of reactants and the amounts of HPA eluted in nth and last runs, respectively, it can be found that there exist a interrelation between the activity decrease and the immobilized firmness of HPA on different supports (see Figures 9 and 10). Due to the difference in acid strength of HPA (different softness of heteropolyanions), the W- and Mocontaining HPAs on the same support have different trend in the variation of amount of eluted HPA and catalytic activity. For instance, when W-HPA was supported on B(1) and B(2) with weaker acidity, ∆m is smaller and ∆x is greater. This result indicates that the catalytic activity is mainly contributed by the eluted HPA (strong interaction between HPA and support). While using B(3) and B(4) with stronger acidity as supports, the contrary effect on reaction is probably due to immobilization of HPA (weak interaction betweeen HPA and support (Figure 9)). However, the results of Mo-HPA are contrary to the variation explained above (Figure 10). 3. Active Carbon. (A) Surface Acid-Base Properties and Adsorption for HPA in Aqueous Solution on Various Active Carbons. At first, we found in experimentals, similar to oxide supports, the catalytic activity of supported catalysts obtained by impregnating HPA(SiW12) on various active carbons has a good relation with the pH value of active carbons in water (see Figure 11). It is clear that the active carbon, which has a low pH value, i.e., a larger acidity, presents a higher activity. The physico-chemical properties of five sorts of active carbon of different sources are listed in Table 2. Their pore structure and texture are shown in Figure 12 and Figure 13. After they are treated by acid and washed with water, pH values of these sorts of carbon measured by acidbase titration are given in Figure 14. In Figure 15, the adsorption isotherms of SiW12 on acid-modified active carbons are shown. It has been proven that the micropores of the carbon (Figure 12) are not likely, to be important for the adsorption. But clearly there exists a relation between adsorption amount of HPA and pH value of the supports. The coal carbon with “basicity” makes the adsorbed amount of HPA the highest while the acidic fiber carbon can hardly adsorb HPA. It is significant that, regardless of HPA containing tungsten or molybdenum, the isotherms obtained for basic carbon (i.e., C1 and C5) fit the Freundlich equation well. Whereas, on acidic carbons (C3 and C4) Langmuir

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Figure 9. The relation between ∆m and ∆% for supported W-containing HPA catalyst.

Figure 10. The relation between ∆m and ∆% for supported Mo-containing HPA catalyst.

Figure 11. Kinetics of H2O produced in esterification over SiW12 supported on various active carbons: (1) C(3) (pH ) 2.73), (2) C(1) (pH ) 5.60), (3) C(5) (pH ) 6.07), (4) C(2) (pH ) 6.61), (5) C(4) (pH ) 9.65).

isotherms are observed. It can be concluded that the dependence of the adsorption amount of HPA on acidbasic properties of the support surface is further proved in the case of active carbons (cf. Figure 16). (B) Adsorption of HPA on Active Carbon in Acidic Medium. The results showed that the acidity of the medium has a profound influence on adsorption of HPA. In H2SO4, HCl, and H3PO4 solutions (3 N concentration), as far as SiW12 is concerned, the adsorption isotherms of basic carbons C(1) and C(5) (see Figure 17) are analogous to that in water and fit the Freundlich equation, the corresponding parameters of which are given in Table 3. In light of the results shown, it can be said that there is a good relation between Freundlich parameters and pH values. The lower the pH value is the higher the k value and adsorption strength are. On the contrary, although the adsorption amount of SiW12

Figure 12. A dv/log(D) desorption pore volume plot.

on acidic carbons, such as C(3) and C(4), in acid media, is higher than that in water, the isotherms are S-shaped (see Figure 18) indicating that the adsorption process is more complicated. It is interesting that the adsorption of SiW12 on different kinds of carbon in acetic acid solution all fit the Freundich equation (Figure 19). And it is found from the data tabulated in Table 4 that the isotherm parameters for the adsorption of SiW12 on basic carbons, i.e. C(1) and C(5), are lower than that on acidic carbons C(3) and C(4). This means that the adsorption of HPA on acidic carbon in acetic acid is stronger. The results obtained for PW12 and PMo12 appear to be similar with those described above; a different profile can be observed for SiMo12 adsorbed on basic carbons

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Figure 13. SEM picture of activated carbons studied.

Figure 14. pH titration curves for the modified active carbons.

(C1 and C5). The Freundlich parameters of their adsorption isotherms are inversely proportional to the pH value due to its weakest acid strength, although the adsorption amount of SiMo12 in 3 N H2SO4, HCl, and H3PO4 media is greater than that in water, and the adsorption on the acidic carbons (C3 and C4) is also consistent with that of SiW12. These results (Table 5) provide a further evidence that acidic properties of HPA itself are also responsible for the adsorption. We are interested in the adsorption isotherm in pure acetic acid medium, as shown in Figure 20. The adsorption amount is linearly increased with the concentration of HPA. The reason is that, compared to the heteropolyanion containing tungsten, Mo-HPA is less stable. SiMo12 which is easily degraded according to the

Figure 15. Adsorption isotherms of SiW12 on various carbons in aqueous solution at 15 °C.

following reaction is the only degraded product adsorbed (Tsigdions, 1969). pH ) 1.0-2.5

[SiMo12O40]4- 98 pH ) 4.2-5.4

[SiMo11O39]8- 98 MoO42-

+

+


Figure 16. NH3-TPD of SiW12/C.

Figure 17. (a) Adsorption isotherms of SiW12 on C(1) from acid aqueous media: H2SO4 (O); HCl (b); H3PO4 (2); dash line is adsorption from aqueous solution (0). (b) Adsorption isotherms of SiW12 on C(5) from aqueous media: H2SO4 (O); HCl (b); H3PO4 (2); dash line is adsorption from aqueous solution. Table 3. Freundlich Parameters of SiW12 on C(1) and C(5) C(1)

C(5)

mediun

pH

k

1/n

k

1/n

H2SO4 HCl H3PO4 H2O

0.19 0.64 0.73 2.21

144.7 129.1 77.8 58.9

0.257 0.294 0.277 0.295

289.8 278.7 226.9 94.6

0.194 0.210 0.228 0.154

This result showed that acidic properties of the medium have an important effect on the adsorption of HPA on active carbon. (C) Catalytic Activity and Firmness of Immobilized HPA on Active Carbon in Esterification. On all sorts of carbon, HPAs containing W had higher catalytic activity and better firmness of immobilized HPA. Similar to the case using bentonites as a support,

a small amount of HPA is also eluted in the liquid phase. It is also found that the difference of supports and HPAs can lead to a variation in the amount of eluted HPA. Using the same treating method as in the case of bentonite, curves of ∆x vs ∆m can also be obtained (see Figure 21). Different from the results obtained from bentonite, in the case of carbons, the variation of ∆x vs ∆m has the same trend for catalysts containing tungsten and molybdenum. The amount of eluted HPA increased with the decrease in acidity of active carbons, namely C5 < C1 < C4 < C3.As can be seen, the variation of ∆x vs ∆m for HPA containing tungsten on bentonite has a different trend from that containing molybdenum (cf. Figure 9 and Figure 10). The main reason seems to be related to the surface property of the supports. In the case of active carbon,

+

+


Figure 18. (a) Adsorption isotherms of SiW12 on C(3) from acid aqueous media: H2SO4 (O); HCl (b); H3PO4 (2); dash line is adsorption from aqueous solution (0). (b) Adsorption isotherms of SiW12 on C(4) from aqueous media: H2SO4 (O); HCl (b); H3PO4 (2); dash line is adsorption from aqueous solution. Table 5. Freundlich Parameters of SiMo12 and PMo12 on C(1) and C(5) C(1) HPA SiMo12 PMo12

Figure 19. Adsorption isotherms of SiW12 on active carbons from pure acetic acid medium. Table 4. Freundlich Parameters of SiW12 on Active Carbons in CH3COOH active carbon

C(1)

C(3)

C(4)

C(5)

k 1/n

40.4 0.280

67.4 0.313

119.1 0.286

55.7 0.253

it refers to both acidic (C3 and C4) and “basic” (C1 and C5) surfaces, while for bentonite, only the acidic surface plays a role. On the basis of our studies, a catalyst consisting of HPA/C(5) which presents an excellent activity and stability in the esterification (ethanol and n-butanol with acetic acid) (see Figure 22a,b) has been developed. Discussion 1. Surface Hydroxyl (-OH) of Various Supports. On the basis of the obtained results, we can suggest that

C(5)

medium

pH value

k

1/n

k

1/n

H2SO4 HCl H3PO4 H2SO4 HCl H3PO4

0.10 0.51 0.88 0.10 0.45 0.73

101.9 134.7 137.2 240.8 219.2 139.8

0.293 0.302 0.187 0.307 0.390 0.432

115.7 138.4 158.7 97.3 184.5 126.9

0.402 0.378 0.363 0.565 0.377 0.432

there is a direct interaction between HPAs and the surface of supports. So it is reasonable to assume that the adsorption of HPAs on various supports could be ascribled to an acid-base reaction. On the surface of various supports studied, including zeolite, bentonite, and active carbon, besides exposed cation (including C) and oxygen ion, there is also a large amount of hydroxyl ions which own characteristics of a Bronsted acid and Bronsted base and which cover a wide range from “weaker” acid (SiO2) reacting as a “hard” acid to “stronger” base (Al2O3) reacting as “hard” base. The acidic hydroxyl reacts with a basic molecule with weaker basicity to form hydrogen-bonded OH‚‚‚B, whereas the protonated species O-‚‚‚BH+ would be obtained when reacting with a stronger base. On the other hand, the basic hydroxyl with acid (AH) might produce H2O. At present, many methods have been used to characterize the acid-basicity, for example the point of zero charge (PZC) or the point of equal charge (Gonzalez et al., 1994) etc. The most representative example is the intermediate electronegativity (Sint) calculated according to the principle of electronegativity equalization proposed by Sonderson (Mortier, 1978; Dwyer et al., 1983). The calculated values of intermediate electronegativity of a series of support with different SiO2/Al2O3 are shown in Figure 23. It may be found that the Sint value and acidity of the surface hydroxyl increased with the SiO2 content. This sequence is in agreement with that of the loaded amount of HPA on this series of support. In other words, the greater the acidity of support is the easier the removal of HPA is, i.e. the less the amount of immobilized HPA. The

+

+


Figure 20. (a) Adsorption isotherms of SiMo12 on active carbons from acetic acid medium. (b) Adsorption isotherms of SiMo12 on active c from pure acetic acid medium.

Figure 21. The plots of the change of conversion (∆%) for esterification against change of amount of HPA eluted (∆mg).

calculated Sint value of TiO2 (Sint ) 3.362) does not obey this rule, but the same conclusion could be drawn

by analysis of the heat of formation (Table 6) of the surface hydroxyl of simple oxides, and the acid strength

+

+


a

b

Figure 22. (a) Stability of the supported catalyst for esterification in liquid-solid phase and amount of HPA eluted. (b) Stability of the supported catalyst for esterification in gas-solid phase (a: n-butanol; b: ethanol).

Figure 23. Electronegativity (Sint) of supports with different SiO2/Al2O3 ratios. Table 6. Heat of Formation of Hydroxyl Group on Various Oxides (Morimoto, 1976) oxide

heat of formation (kJ/mol H2O)

TiO2 (anatas) TiO2 (rutile) SiO2 R-Al2O3 γ-Al2O3

110.8 78.1 67.6 66.8 40.3

as well as the acidity measured by NH3-TPD (Figure 5). The acid-basicity of hydroxyl on various oxides is given as follows: TiO2 > SiO2 > HZSM-5 (Si/Al ) 50)

> HZSM-5 (Si/Al ) 38) > Y-Zeolite > Al2O3, which is in agreement with the amount of HPA possible to be removed from the corresponding support. It is well known that the surface of active carbon studied contains some different chemical groups as shown in Figure 2, which have been systematically analyzed by Boehm titration, XPS, and IR (Tables 7 and 8 and Figures 24 and 25). It is found that the amount of the surface oxygen-containing groups depends on the source of active carbons. But there is a common trend, namely -OH > CdO > COOH (∼lactone). In addition, the total amount of oxygen-containing group on C(3) is much higher than that of the other carbons. Considering the basic carbonyl group, we can find that the relative amount of carbonyl on C5 is the highest, accounting for about 40% of the total surface groups. The total of acidic groups on surface of carbons has a certain relation with its PZNPC (ponit of zero net proton charge) (cf. Figure 26). And these results obtained by Boehm titration are in agreenment with that obtained by XPS investigation, namely -OH > CdO > COOH, meaning that the existence of a highest basic content of CdO on the surface of C(5) can be further confirmed. As shown in Figure 25, in the region of 2000∼1000 cm-1, beside the three broad bands at 1800∼1660,1600-1400, and 1300-1000 cm-1, there appears a very strong band at 1650 cm-1. As reported by Shizaki et al. (1981), the three broad bands are usually observed in IR spectra of carbon and they suggested that the band in the region of 1710-1760 cm-1 may be caused by a vibration of CdO in lactone or carboxyl groups. The absorption band occurring at 1557 cm-1 can be attributed to the chelating carbonyl structure (i.e. γ-chromone). It is believed that a band at ca. 1557 cm-1 originates from different species on active carbons, including CdO or CdC vibrations (Gezybeki et al., 1992). However, it is unclear if this band is related to either CdC or CdO vibrations or both of them. But the systematic increase with HNO3 modification favors the interpretation by CdO groups. Overlapping bands in the range of 10001300 cm-1 may be caused by different bonding oxygen related to stretching C-O vibrations of phenol, carboxyl, or ether, and so on. It is worth noting that the band at 1720 cm-1 of acidic active carbon (i.e. C3) is extremely strong. On the basis of the above discussion, it could be concluded that the presence of a hydroxyl on the carbon surface is no doubt; it could be formed on active carbon by many ways in acidic media. 2. Model of Interaction between HPA and Hydroxyl of Support. It is well known that heteropolyacid is an acid with higher acid strength. Heteropolyanion (HPAn), analogous to the anions such as SO42-, PO43-, and AsO43-, also belongs to the inorganic oxygencontaining anion. In view of the difference in softness or hardness of hydroxyl of various supports and HPAs, the reaction between heteropolyanion and hydroxyl on the support may proceed according to a two-step ligand exchange mechanism (Anderson et al., 1981) as follows (e.g. hard-hard reaction):

M-OH(s) + H+(aq) f MOH2+(s)

(1a)

MOH2+(s) + (HPAn)-1(aq) f M(HPAn)(s) + H2O(l) (1b) where M and MOH(s) are metal ion (including C) and surface hydroxyl, respectively. The protonation step (eq

+

+

2558 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 Table 7. Surface Acidity of Modified Active Carbon active carbon C(1) C(3) C(4) C(5) C(6) a

I

surface oxygen-containing group (mequiv/g)a II III

0.30 0.38 0.24 0.34

0.23 0.31 0.20 0.33

0.08 0.20 0.06 0.07

IV

total

I | III | IV -II (mequiv/100 g)

0.04 0.18 0.08 0.05

0.65 1.07 0.58 0.79

190 450 180 130

PNZPC 6.18 5.53 6.09 6.71 4.19

I, II, III, and IV are phenol, carbonyl, carboxyl, lactone, respectively.

Table 8. XPS Data of Surface Component of Active Carbon active carbon

Eb (eV)

C1 C3 C4 C5 C6

284.2 284.6 284.2 284.1 284.6

C-H content (%) 74.0 67.8 72.0 62.0 58.7

Eb (eV) 285.5 286.7 285.5 285.6 286.7

OH content (%) 22.0 17.8 22.0 20.0 9.9

Eb (eV) 287.9 287.9 287.9 287.1 287.9

CdO content (%) 4.0 7.2 6.0 11.0 8.9

Eb (eV)

COOH content (%)

289.5

7.2

289.5 289.5

7.0 8.5

Figure 26. Relation between PZNPC and surface oxygen-containing groups on modified active carbons. Figure 24. X-ray photoelectron spectroscopy of the original active carbons.

The other acceptable mechanism is coordination of protonated surface hydroxyl with heteropolyanion in solution to form an outersphere surface complex (Anderson et al., 1981) instead of exchange reaction (e.g. hardsoft reaction):

MOH2+(s) + (HPAn)-1(aq) f MOH2+(HPAn)-1 (2)

Figure 25. Infrared spectra (IR) of the modified active carbons (C5**: treated with HNO3).

1a) is thought to render the surface hydroxyl more exchangeable; according to this mechanism, the heteropolyanion would be strongly adsorbed onto supports. It is selfevident that in this case HPA would lose its acidic catalytic property, an extreme example for Al2O3 with basic hydroxyl belongs to this type.

Owing to the difference in acid-base strength of the surface hydroxyl as well as HPA, the result of interaction of them would lead to the formation of active intermediates with different acid strength and immobilized firmness, such an active intermediate has acidity like HPA. The pH value of the medium and the acidity of HPA itself have an effect on its immobilized firmness. The models of the two mechanisms can be described as shown in Figure 27. Up to now, it is still very difficult to quantitatively deal with the character of adsorption of oxygen-containing anions by equilibrium model. This is because the adsorption of these anions on the surface hydroxyl is usually partly irreversible due to the amphoteric character of the surface hydroxyl group. In addition, because of the higher surface density of most anions, it is likely that the reason is related to interaction between the surface and dissolved anion as well as the interaction among adsorbed molecules in the horizontal direction. So under the ideal conditions, it is reasonable to

+

+


conditional constant can be defined as follows:

K)

[HPA] [MHPAn] [MHPAn] ) ) [MOH][HPA]tot [MOH][HPA] [HPA]tot [H+] Ks + (5) [H ] + Ka

because of

[M-OH] ) [MOH]tot - [MHPAn] - [MOH2+] [M-O-] If one assumes

[MOH2+] + [M-O-] ) 0 Figure 27. A schematic proposal of a hydroxyl surface showing association with surface hydroxyl (S): (a) exchange mechanism, (b) surface hydroxyl, (c) coordination mechanism, (R) inner-sphere complexes, (β) outer-sphere complexes, (d) the diffuse ion swarm.

when adsorption is proceeded according to the exchange mechanism, we have

[M-OH] ) [M-OH]tot - [MHPAn]

assume that the first step of dissociation of HPA in solvent should be considered as

Ka )

[H+][HPAn][HPA]

(1)

At the same time, the surface hydroxyl could also form several kinds of adsorption sites in solvent. Ka1

[MHPAn] ) K[M-OH][HPA]tot ) K{[MOH]tot [MHPAn]}[HPA]tot ) K[MOH]tot[HPA]tot K[HPA]tot[MHPAn] So the following expression is obtained

Ka2

M-OH2+ y\z MOH y\z MO- + H+ small

pH

r Ka1 )

Ka2 )

f

[MHPAn] )

large

[MOH][H+]

(2a)

[M-OH2+]

K[MOH]tot[HPA]tot 1 + K[HPA]tot

(8)

If the following expressions were considered

[M-OH]tot ) b, [MHPAn] ) q, [HPA]tot ) c

[MO-][H+] [MOH]

(2b) then eq (8) would become

When HPA is adsorbed, the adsorption states, in general, would be represented as follows:

MOH + HPA h [MOH2+‚‚‚HPAn-] h MHPAn + H2O Under the limited condition, according to the exchange mechanism, an expression

Ks )

(7)

Combining steps (5) and (7), the equation can be rearranged to give

Ka

HPA y\z H+ + [HPAn]with

(6)

[M-HPAn] [MOH][HPA]

(3)

is formulated. If suggested that

[HPA] [HPA] ) ) [HPA] + [HPAn] [HPA] + Ka[HPA]/[H+] [H+] [H+] + Ka

(4)

From eq (3), under the condition of given pH value, a

q)

Kbc 1 + Kc

(9)

then,

q(1 + Kc) ) Kbc, Kd ) q/c ) Kb - Kq ) K(b - q) (10) Equation (10) is the typical Langmuir isotherm where Kd is the distribution coefficient which is the ratio of heteropolyanion adsorbed (q) and the concentration of HPA in solution (c), K is dependent on pH value under experimental conditions, and b is the concentration of surface hydroxyl. It is obvious that for different supports, HPAs as well as media K and b values are different. Therefore, when the adsorption follows the second mechanism, the system might undergo in a condition with uncertainty of b and K values. On the basis of our studies, the results referring to the interaction of HPAs with a surface hydroxyl group of different supports could

+

+


be interpreted in the sense of HSAB concept, which could be quatitatively llustrated by the following scheme:

Literature Cited Anderson, M. A., Rubin, A. J., Eds. Adsorption of Inorganics at Solid-Liquid Interface; Ann Arbor Science Publishers: Woburn, 1981. Boehm, H. P. In Structure and Reactivity of Surface; Morterra, C., Zecchina, A., CostaG., Eds.; Elsevier: Amsterdam, 1989; pp 145-157. Chem, W.; Luthra, N. P. NMR Study of the Adsorption of Phosphomolybdates on Alumina. J. Catal. 1988, 109, 163. Dwyer, J.; Fitch, F. R.; Nkang, E. E. Dependence of Zeolite Properties on Composition. Unifying Concepts. J. Phys. Chem. 1983, 87, 5402. Gezybeki, T.; Papp, H. Selective Catalytic Reduction of Nitric Oxide by Ammonis on Fe3+-Promoted Active Carbon. Appl. Catal. B Enviro. 1992, 1, 271. Gonzalez, R. D.; Miura, H. Preparation of SiO2 - and Al2O3supported Clusters of Pt Group Metals. Catal. Rev. Sci. Eng. 1994, 36 (1), 145. Izumi, Y.; Urabe, K. Catalysis of Heteropoly Acids Entrapped in Activated carbon. Chem. Lett. 1981, 113 (5), 663. Izumi, Y.; Matsuo, K.; Urabe, K. Efficient Homogeneous Acid Catalysis of Heteropoly Acid and Its Characterization Through Ether Cleavage Reaction. J. Mol. Catal. 1983, 18, 299. Ishizaki, C.; Marti, I. Surface Oxide Structures on a Commercial Activated Carbon. Carbon 1981, 19 (6), 409. Kolikov, S. M.; et al. Adsorption of Heteropoly Acid H4SiW12O40 by Porous Supports in Solvents. Izv. Akad. NAVK SSSR. Ser. Khim. 1989, 4, 763.

Mastikhin, V. M.; Kulikov, S. M.; Nosov, A. V.; et al. 1H and 31P MAS NMR Studies of Solid Heteropolyacids and H3PW12O40 Supported on SiO2. J. Mol. Catal. 1990, 60, 65. Misono, M.; Nojiri, N. Recent Progress in Catalytic Technology in Japan. Appl. Catal. 1990, 64 (1), 1. Misono, M.; Hashinoto, M. Future Opportunities in Organic Synthesis by Heteropoly Acid Catalysts. 1992, 34 (3), 152. Morimoto, T. Surface Hydroxyl Group of Metal Oxides. Shokubai, 1976, 18 (5), 107; Kagaku (Chemistry) 1976, 31, 61. Mortier, M. J. Zeolite Electronegativity Related to Physicochemical Properties. J. Catal. 1978, 55, 138. Nowinsla, K.; Fiedorow, R.; Adamiec, J. Catalytic Activity of Supported Heteropoly Acids for Reactions Requiring Strong Acid Centres. J. Chem. Soc. Faraday Trans. 1991, 87, 749. Pearson, R. G. Hard and Soft Acid and Bases. J. Am. Chem. Soc. 1963, 85, 3533. Pope, M. T.; Muller, A. Polyoxometalate Chemistry: An Old Field with New Dimensions in Several Disciplines. Angew. Chem., Int. Ed. Engl. 1991, 30, 34. Rocchiccioli-Deltcheff, C.; Amirouche, M.; Hewe, G.; Fournier, M.; Che, M.; Tatibonet, J. M. Structure and Catalytic Properties of Silica-Supported Polyoxomolybdates. J. Catal. 1990, 126, 591. Schwegler, M. A.; Vinke, P.; Van der Eijk, M.; Van Bekkum, H. Activated Carbon as a Support for Heteropolyanion Catalysts. Appl. Catal. A: General 1992, 80, 41-57. Sebulsky, R. T.; Henke, A. M. Alkylation of Benzene with Dodecene-1 Catalyzed by Supported Silicotungstic Acid. Ind. Eng. Chem. Process Des. Dev. 1971, 10 (2), 272. Thonvenot, R.; Rocchiccioli-Deltcheff, C.; Fournier, M. 31P NMR MAS Spin-Lattice Relaxation as a Dispersion Probe: An Easy Access to Active-site Concentration in Silica-Supported Dodicamolybdophosphoric Acid. J. Chem. Soc., Chem. Commun. 1991a, 1352. Thouvenot, R.; Fournier, M.; Rocchiccioli-Deltcheff, C. Catalysis by Polyoxometalates. J. Chem. Soc., Faraday Trans. 1991b, 87, 2829. Tsheneshkova, F. A. Heteropolyacids and Their Salts-New Perspective Catalysts for Petrochemical and Organic Synthesis. Petrochem. 1991, 31 (5), 579-591. Tsigdions, G. A. Heteropoly Compounds of Molybdenum and Tungsten. Climax Molybedenum Company Bulletin cdb-12d, November, 1969, 31. Tsigdinos, G. A. Preparation and Characterization of 12-Molybdophosphoric and 12-Molybdosilicic Acid and Their Metal Salts. Ind. Eng. Chem. Prod. R&D 1975, 13 (4), 267. Wu, Y.; Xu, B. Heteropolyacid (Salt)-A Versatile Catalyst. Huaxue Tongbao 1985, (4), 34. Zecchina, A.; Orean, C. O. Structure and Reactivity of Surface Species Obtained by Interaction of Organometallic Compounds with Oxidic Surfaces: IR Studies. Catal. Rev. Sci. Eng. 1993, 35 (2), 261.

Received for review July 28, 1995 Accepted May 13, 1996X IE950473S X Abstract published in Advance ACS Abstracts, July 1, 1996.

Heterogenization of Heteropolyacids: A General Discussion on the

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