J. Phys. Chem. B 2000, 104, 10321-10328
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Superacidity and Catalytic Activity of Sulfated Zirconia Naonobu Katada,* Jun-ichi Endo, Kei-ichi Notsu, Naoko Yasunobu, Norihiro Naito, and Miki Niwa Department of Materials Science, Faculty of Engineering, Tottori UniVersity, 4-101 Koyama-cho Minami, Tottori 680-8552, Japan ReceiVed: June 20, 2000; In Final Form: August 9, 2000
The acidic property of sulfated zirconia, a so-called solid superacid catalyst, was precisely determined by ammonia temperature-programmed desorption with water vapor treatment and theoretical analysis. The desorption peak from the zirconia support was removed by water vapor treatment, and the generation of two types of acid sites was clarified. One kind of acid site is a Lewis type generated on a submonolayer species of the sulfate covering the surface; the surface concentration of the acid site was 0.5 atoms nm-2, and the adsorption heat of ammonia was ca. 200 kJ mol-1. This corresponds to -19 of the H0 function, demonstrating superacidity. The other type of acid sites generated by loading excess sulfate possessed a high concentration (maximum, 2 nm-2), an adsorption heat of ca. 160 kJ mol-1, an H0 of -12, and Brønsted nature. The former was active for the Friedel-Crafts-type alkylation of benzene with benzyl chloride in the liquid phase, and the latter was active for the skeletal isomerization of butane in gas phase.
Introduction Sulfated zirconia1,2 shows remarkable catalytic activity for reactions which are not catalyzed by conventional solid catalysts, e.g., the skeletal isomerization of alkane,2 the alkylation of alkane by alkene,3 the activation of ethane4 and methane,5 the low-temperature cracking of polyolefin,6 and the Friedel-Craftstype alkylation7 and acylation8 of hydrocarbon with an inert molecule such as carbon monoxide.9 Moreover, catalytic functions have been improved by loading additional components such as platinum,10 tungsten,11 iron, and manganese.12 Loading sulfate and other multivalent anions, e.g., tungstate13 and molybdate,14 on zirconia and basic metal oxides such as iron,15 titanium,16 and tin17,18 oxides also generates solid acid catalysts.19 A superacid is defined to be an acid stronger than 100% perchloric or sulfuric acid,20 and therefore, H0 < -11.9.21 Superacidity on sulfated zirconia has been claimed on the basis of its changing the color of a Hammet indicator at pKa < -11.9.2 The high catalytic activity has been attributed to the superacidity,22,23 and evidence for the catalysis of the superacid site for the skeletal isomerization of butane based on activation energy investigation has been shown.24 Moreover, it has been reported on the basis of deactivation experiments that a relatively strong acid site is responsible for the isomerization.25 Also, for the synthesis of fine chemicals using sulfated zirconia as a Friedel-Crafts-type catalyst, superacidity is considered to be important.26 Some NMR studies indicate strong acidity.27,28 Electron spin resonance (ESR) spectroscopy supports the fairly strong acidity also.29 However, there are papers which claim that the sulfated zirconia has no superacidity30,31 or that catalytic activity is not related to superacidity.32 The infrared (IR) and nuclear magnetic resonance (NMR) spectra of carbon monoxide and hydrocarbons adsorbed on the sulfated zirconia33 showed acidities weaker than * Corresponding author. Phone/fax: +81-857-31-5684. E-mail:
[email protected].
those of alumina34 and alkaline zeolites.35 Even when the indicator method is used, the results suggest that the acid strength of sulfated zirconia is similar to that of 100% sulfuric acid.36 Quantum chemical calculation demonstrated that the acid strength of sulfated zirconia was similar to that of sulfuric acid37 and weaker than that of zeolite.38 A combination of NMR and IR techniques elucidated the complex property of the multiple types of acid sites.39 Thus there is serious controversy over the interpretation of the acidic property of sulfated zirconia, and this is further complicated by the nature of sulfated zirconia being variable according to the preparation method and conditions40 (especially the quality and concentration of surface hydroxyl groups40,43), as shown by the reports of various preparation techniques.41,42 Even a recent review could not determine whether sulfated zirconia is a superacid.44 Researchers point out the shortages in the analytical methods of acidity. Only a method using typical basesse.g., aminess as probes can determine the absolute acid strength, which is defined as the equilibrium constant of the reaction between acid and base, for example, the H0 function.21 Therefore, spectroscopic methods cannot directly determine the strength of acidity. On the other hand, the chemical nature of a solid surface may change with its environment, and so, the indicator method in solution may detect the property far from the working state of the catalyst. In addition, it is generally difficult to obtain accurate and quantitative results from the color of the indicator. From these points of view, a method using a gaseous base probe such as ammonia is promising. Temperature-programmed desorption (TPD) of ammonia45 is one promising method and has been applied to the sulfated zirconia catalyst.46-48 Corma et al. reported that the sulfated zirconia showed a desorption peak at quite a high temperature, 815 K,46 but it was soon pointed out that because their measurements were carried out using a thermal conductivity detector (TCD), the desorption of the sulfur compound could possibly have disturbed the spectrum.49 Through careful measurements, the desorption peak at
10.1021/jp002212o CCC: $19.00 © 2000 American Chemical Society Published on Web 10/13/2000
10322 J. Phys. Chem. B, Vol. 104, No. 44, 2000 a lower temperature49 and, in addition, the desorption peak on the zirconia support, which was usually large were observed.50 There is a hypothesis that the small difference in the peak temperature between zirconia and sulfated zirconia shows strong acidity.50 However, these small difference cannot explain the drastic change of catalytic activity due to the loading of sulfate. Moreover, the desorption temperature of the TPD spectrum does not show the acid strength directly.51 Because of these shortages, the ammonia TPD on these catalysts is not accepted widely. In addition, some authors reported a completely different TPD spectrum; Essayem et al. reported that no desorption of ammonia from their sulfated zirconia was observed up to 900 K.52 The TPD of pyridine has been applied to the sulfated zirconia,53,54 but it has the same problems. We have developed experimental and analytical methods to determine the quantity and strength of acid precisely from the TPD spectrum, mainly on zeolites. Water vapor treatment removed the unnecessary peak from weakly held ammonia to show the actual property of the acid sites.55,56 Theoretical analysis based on derived equations helped to determine the adsorption heat of ammonia from the TPD spectrum.57,58 These methods clarified the acidic properties of various zeolites.45 Our theory was confirmed to be applicable to a tungsta-zirconia catalyst, and in addition, water vapor treatment selectively removed the ammonia species on zirconia exposed surfaces but did not affect the species adsorbed on the tungsta. Thus, the acid quantity and strength of the tungsta-zirconia were first determined; on the monolayer of tungsta, one acid site with the ammonia adsorption heat of ca. 130 kJ mol-1 was generated with four to five atoms of tungsten.59 In the present study, we applied these methods to the sulfated zirconia catalysts to determine the acidic property on the basis of preliminary experiments.60 The benzaldehyde-ammonia titration (BAT) method,61,62 which measured the spreading of acidic components on basic metal oxide, was also applied to clarify the surface structure of the sulfate species. Moreover, the determined properties were related with catalytic activities for the skeletal isomerization of butane and Friedel-Crafts alkylation of benzene with benzyl chloride. Experimental Section Preparation of Zirconia and Sulfated Zirconia. Zirconium oxynitrate [ZrO(NO3)2] was solved into a nitric acid solution, and aqueous ammonia was slowly added under stirring to precipitate zirconium hydroxide until the pH reached ca. 11. The obtained solid was washed with water and calcined to prepare zirconia at 573 K for 4 h under atmospheric conditions. Sulfate was loaded by an impregnation method; 5 g of zirconia was suspended in aqueous solutions containing various amounts of sulfuric acid [shown by the weight ratio of SO42-/ZrO2 (wt %) in the following descriptions] with a constant volume of 100 cm3, followed by drying on a hotplate. After drying for ca. 2 h, the obtained solid was calcined at 923 K for 4 h under atmospheric conditions. The elemental analysis after calcination was carried out with a Horiba EMIA-610 FA analyzer. BET surface area was determined from the amount of nitrogen adsorbed at 77 K under 30.39 kPa of nitrogen (P/P0 ) 0.3). Benzaldehyde-Ammonia Titration. The coverage of the surface by sulfate species was measured by the BAT method.62 After the sample was pretreated at 673 K in flowing oxygen, 1 mm3 (ca. 9.9 µmol) of benzaldehyde was injected from the septum installed before the reactor at 523 K in helium flow. The eluted aldehyde was monitored by a gas chromatograph (GC) with a column of Silicone Grease 20%/Chlomosorb WAW
Katada et al. and a flame ionization detector (FID), and the injections were repeated several times until the adsorption was saturated. Then gaseous ammonia (10 cm3, 0.409 mmol) was injected from the septum to convert the adsorbed species into benzonitrile. Benzaldehyde forms a stable benzoate anion on a basic metal oxide such as zirconia in high surface concentrations but is not adsorbed on acidic material. Because the benzoate anion is almost completely converted into benzonitrile, the amount of yielded benzonitrile must reflect the amount of adsorbed benzoate species, and therefore, the coverage of the zirconia surface by acidic material is calculated as follows:
exposure ) nitrile yield per BET surface area (BN) on the catalyst/BN on zirconia (2.2 molecules nm-2) (1) coverage ) 1 - exposure
(2)
Temperature-Programmed Desorption. The TPD experiments were carried out according to the method in our paper45 after the evacuation of the catalyst at 873 K in a quartz cell designed by Amenomiya et al.,63 followed by the adsorption of ammonia and water vapor treatment at 373 K. The temperature was increased at the heating rate of 10 K min-1, and the desorbed materials were analyzed with a mass spectrometer (ULVAC UPM-ST-200P). The amount of ammonia was determined on the basis of the intensity of the fragment with m/e ) 16 because the parent peak with m/e ) 17 was influenced by the desorbed water. In the standard experiments, the catalyst amount was 0.1 g, the flow rate of the carrier helium was 1 cm3 STP s-1 (ca. 41 µmol s-1), and the pressure in the catalyst bed was 13.3 kPa, but these were varied in some experiments to obtain the relationship between the peak maximum temperature and experimental conditions; this relationship is necessary to calculate the adsorption heat of ammonia. Infrared Spectroscopy. The infrared (IR) spectrum was collected on a sample disk (10 mg, 1 cm in diameter) set in an in-situ cell. The sample disk was evacuated at 873 K for 1 h, followed by the adsorption of pyridine vapor (ca. 400 Pa) at 373 K for 30 min and further evacuation at 573 K for 1 h. Then the spectrum was collected with a JASCO FT/IR-5300 spectrometer. Catalytic Reaction. The skeletal isomerization of n-butane into isobutane (2-methylpropane) was carried out at 423 K through, the two experimental methods. In the pulse experiments, a pulse of n-butane (53 µmol) was injected onto the catalyst (0.25 g) pretreated at 873 K in 20 cm3 min-1 of flowing helium, and the products were analyzed with an FID-GC instrument with a column of VZ-7 (2 mm i.d., 4 m in length) operated at room temperature. The activity is shown through the yield of isobutane from the first pulse of n-butane. In the closed circular experiments, a mixture of n-butane (0.264 mmol) and helium (0.881 mmol) was introduced into the reactor (167 cm3) in which the catalyst (0.1 g) was set. The reaction rate was determined from the observed linear relationship between the isobutane concentration and reaction time at 0.5-3 h. All the experiments were carried out under differential conditions (conversion, 15 wt %. This relationship between the sulfur loading and the sulfuric acid content (concentration) in the impregnated solution agrees with the previous reports; Nascimento et al. reported that almost constant sulfur content at ca. 1-1.5 × 10-4 g m-2snamely, 2-3 atoms nm-2swas obtained from sulfuric acid solutions with various concentrations,64 and Faˇrcas¸ iu et al. found that approximately 4-5 µmol m-2snamely, 2.5-3 atoms
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Figure 3. Ammonia TPD spectra measured before (broken lines) and after (solid lines) water vapor treatment over sulfated zirconia impregnated from solutions with 0 (a), 1.9 (b), 3.7 (c), 10.7 (d), 15.4 (e), 18.0 (f), 21.0 (g), and 24.0 wt % (h) of SO42-.
nm-2sof sulfur was loaded by the impregnation of diluted sulfuric acid solution, whereas the loading increased by increasing the sulfuric acid concentration to greater than the threshold value.65 The surface structure was analyzed by the BAT method.62 The coverage by sulfate on zirconia was determined on the basis of the selective adsorption of benzaldehyde on a basic metal oxide such as zirconia, as shown in Figure 2. The coverage reached about 90% at 3 wt % of the sulfur content in the solution, where ca. 2.5 atoms nm-2 of the surface concentration of sulfate species was observed. Acidic Property. Figure 3 (broken line) shows the ammonia TPD spectrum measured by the conventional procedures without water vapor treatment. Both zirconia and the sulfur-loaded samples showed the large desorption peak at ca. 500 K. On the samples with the sulfur concentration of 2.2-2.6 atoms nm-2 (prepared from 2-15 wt % of the impregnation solution), an additional peak appeared at quite a high temperature, ca. 750 K. The water vapor treatment drastically changed the TPD spectra (Figure 3, solid lines) as follows: (1) on zirconia, the peak was almost diminished, and (2) on the samples with the sulfur concentration of 2.2-2.6 atoms nm-2 (prepared from 2-15 wt % of the impregnation solution), the peak at 750 K was not affected, while the peak at the low temperature was almost diminished. (3) On the sample with an excess amount of sulfate (>7 atoms nm-2), a peak was still observed at ca. 600 K. Thus, the water vapor treatment indicates the generation of two types of acid sites by loading sulfate. One is the very strong site showing a peak at 750 K, which appeared with 2-15 wt % of the impregnation solution, and the other is the weak acid site showing a peak at 600 K, which appeared with >15 wt %. As shown in Figure 4, the surface concentration of the stronger acid site is determined from the peak area to be about 0.5 nm-2. The weak acid showed a maximum concentration of ca. 2 nm-2 with 20 wt % SO42-/ZrO2 of the impregnated solution, and the higher loading decreased the acidity. To analyze the desorption peak according to our theory, we measured the TPD spectra with varying the W (catalyst amount, kg)/F (flow rate of carrier gas, m3 s-1) from 1 to 50 kg s m-3.60 The peak temperature increased with increasing W/F ratio, and an almost linear relationship was observed between the terms ln Tm - ln(A0W/F) and 1/Tm, where Tm is the peak temperature
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Figure 4. Acid concentration (b) and strength (2, adsorption heat of ammonia) of sulfated zirconia.
Figure 6. Relative intensity of pyridine species adsorbed on Brønsted acid sites in IR spectrum of pyridine adsorbed at 373 K, followed by evacuation at 573 K.
Figure 5. Plots of ln Tm - ln(A0W/F) against 1/Tm on sulfated zirconia impregnated from a solution with 10 wt % of SO42-.
and A0 is the amount of desorbed ammonia, as shown in Figure 5. The meaning of this linear relationship will be discussed below. The amounts of Brønsted and Lewis acid sites were estimated from the IR spectrum of adsorbed pyridine. The relative intensity of pyridine species adsorbed on Brønsted acid site was shown by the equation
IB IB + IL
(3)
where IB is the intensity of peak at 1545 cm-1, attributed to the pyridinium cation adsorbed on the Brønsted acid site and IL is the intensity at 1455 cm-1 of the pyridine molecule coordinated to the Lewis acid site. As shown in Figure 6, with 2-15 wt % of the sulfuric acid content of the impregnation solution, almost no Brønsted acidity was observed, while 10-40% of Brønsted acidity was observed with >20 wt %. In other words, Lewis acidity was predominant at a surface sulfur concentration of 2.5 atoms nm-2. This is in agreement with the findings of Nascimento et al. because they reported that the Lewis acid was predominant at sulfur contents of 90%, whereas the color of the catalyst became light-brown after the reaction,
Figure 7. Catalytic activity for the liquid-phase alkylation of benzene with benzyl chloride into diphenylmethane.
showing the formation of a small amount of polymerized species. The activity for the alkylation of benzene showed the maximum at 5-15 wt % of the SO42- loading, as shown in Figure 7. Excess sulfate loading suppressed the activity. Only isobutane was detected in the products of the reaction of n-butane at 423 K, while the color of the catalyst became black. As shown in Figure 8, the samples prepared from 0-15 wt % of the impregnated solution displayed low activity for the skeletal isomerization in both the pulse and closed circular experiments. The high activity was observed on the samples prepared with a large amount of sulfuric acid, >20 wt %. Discussion Structure. The elemental analysis showed a constant sulfur concentration of ca. 2.5 atoms nm-2, with 2-15 wt % of the sulfur content in the impregnated solution. This points out that a fraction of sulfate, whose concentration was 2.5 S nm-2, was tightly anchored the zirconia surface, while excess sulfate was eliminated probably during the high-temperature calcination. It is estimated from the covalent and van der Waals radii of the atoms that one SO4 tetrahedra occupies ca. 0.25 nm2 (Scheme 1). The concentration of sulfur on the surface saturated with this species is 1/0.25 ) 4 nm-2. On the other hand, the kinetic
Sulfated Zirconia
Figure 8. Catalytic activity for the skeletal isomerization of n-butane into isobutane (2-methylpropane), as measured by closed circular (0, solid line) and pulse (4, broken line) experiments.
SCHEME 1: Estimation of Surface Area Occupied by an SO42- Aniona
a The S-O covalent bond length is assumed to be 0.143 nm, the O-S-O bond angle 109°, and the van der Waals radius of the oxygen atom 0.140 nm. The shaded area is ca. 0.25 nm2
SCHEME 2: Proposed Superacidic Species
diameter of molecule is widely estimated to be 21/6 times larger than the static diameter.66 The occupation area based on the kinetic diameter should be 0.25 × 21/3 ) 0.31 nm2, and hence, the concentration must be 3.2 nm-2. Because the observed concentration of 2.5 nm-2 was smaller but not so far from the estimated values of 3.2-4 nm-2, the surface is considered to be roughly covered by isolated species of sulfate. The BAT analysis evidenced that the surface was covered by the loaded material with 2.5 S nm-2. From IR spectroscopy, the species in Scheme 2 was proposed to reside on the sulfated metal oxide.22,67 This isolated structure is consistent with the present results, and therefore, we propose that the layer of this or similar species covers the zirconia surface. Hereafter, it is termed the “submonolayer” of the sulfate species. Probably because of the strong interaction between the acid (sulfuric acid) and the base (zirconia surface), the isolated submonolayer species is spontaneously formed. Such a spontaneous coverage68 has been found on V2O5,69 MoO3,70,71 WO3,59,72 SiO2,73,74 and GeO2,75 loaded on Al2O3, SnO2, TiO2, and ZrO2. In such a case, it is usually observed that covering the surface with these modifiers prevents thermal sintering of the support. We have already shown that the loaded material protected the particle like an egg shell.76 This is consistent with the highBET surface area of the samples covered by the sulfate
J. Phys. Chem. B, Vol. 104, No. 44, 2000 10325 submonolayer species, as shown in Figure 1. In summary, the submonolayer species of sulfate roughly covers the surface of the samples prepared from 2-15 wt % of the solution. On the other hand, the surface concentration of loaded sulfur again increased when the sulfuric acid content of the solution was increased to >15 wt %. From 15 to 20 wt %, a monolayerlike structure is possible because the surface concentration of sulfur is close to 10 atoms nm-2; it is possible to form a monolayer consisting of S-O-S bonds with such a concentration. However, with excess sulfuric acid, the formation of some zirconium sulfate salt is suggested from a concentration of sulfur as high as 40 atoms nm-2. Indeed, it was observed that a fraction of the zirconia solid was dissolved into the solution and recrystallized by removing water via heating when the sulfuric acid content in the impregnation solution was >20 wt %. It is suggested that some complicated reactions proceed by impregnating sulfuric acid at such a high concentration. TPD Spectrum. Diminishing the desorption peak on the zirconia support by water vapor treatment has been discussed on the bases of TPD and IR experiments.59 The Lewis acid site generated on the basic metal oxide by the dehydration of surface hydroxyl groups was probably inactivated by the rehydration of the surface through water vapor treatment. On the contrary, the amount and nature of the ammonia species (both coordinated molecule and ammonium cation) adsorbed on WO3/ZrO2 were unchanged by water vapor treatment. It is supposed that the adsorbed species due to the zirconia surface was selectively removed by water vapor treatment on the sulfated zirconia also. In other words, the acid site, which had been generated by loading sulfate and was responsible for the catalytic activities, was selectively detected. Hereafter, the acidic property of the sulfated zirconia will be analyzed on the basis of the water vapor treatment method. Acidic Property. On the catalysts covered by the submonolayer of the sulfate species (2-15 wt % SO42-/ZrO2), a fairly strong acid site with ca. 0.5 nm-2 of the surface concentration was observed, while excess sulfate diminished the strong acid site and generated a relatively weak but highly concentrated (maximum, 2 nm-2) acid site. Thus, the acidic property seems dependent on the structure suggested above. To determine the acid strength from the TPD spectrum, we will show what controls the TPD spectrum. The TPD experiments are classified as63 (1) being controlled by kinetics, i.e., the activation energy is high, (2) being controlled by equilibrium, i.e., re-adsorption of desorbed material freely occurs (3) or being controlled by diffusion. The theoretical equation of the second case is
ln Tm - ln
A0W ∆H β(1 - θm)(∆H - RTm) + ln (4) ) F RTm ∆S 0 P exp R
( )
where Tm is the peak maximum temperature (K), A0 is the amount of adsorption (acid) site (mol kg-1), W is the amount of sample used (kg), F is the flow rate of carrier gas (m3 s-1), ∆H is the enthalpy change (adsorption heat, J mol-1), R is the gas constant (8.314 J K-1 mol-1), β is the heating rate (K s-1), θm is the coverage of adsorption site at the peak maximum, P0 is the pressure of the standard conditions (1.013 × 105 Pa), and ∆S is the entropy change (J K-1 mol-1).45 Here, the second term of the right-hand side is almost unchanged by varying the parameters within a practical range,57 and therefore, the term
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ln Tm - ln(A0W/F) must show the linear relationship against 1/Tm if the second case is available. The measurements of TPD spectra were performed by varying the W/F ratio, and the liner relationship was obtained as shown in Figure 5. This demonstrates that the present experiments are classified into the second case, and the desorption process is controlled by the equilibrium between gaseous and adsorbed ammonia
Because the neutralization of the solid acid by ammonia in an aqueous medium (eq 8) is -(eq 5) - (eq 7), its Gibbs energy change ∆G8° can be expressed by eq 9 if it is assumed that the solid acid can react with the same strength as an acid in an aqueous medium
A + NH3 (aq) f NH3-A
(8)
NH3-A a NH3 (g) + A
∆G08 ) -(∆H05 - T0∆S05) - ∆G07
(9)
(5)
where A is the solid acid. This phenomenon is commonly observed on various zeolites45,57 and tungsta-zirconia catalysts.59 Subsequently, the whole entropy change was calculated from the slope and intercept of Figure 5 to be 190 J K-1 mol-1. Because the entropy change due to gas mixing was calculated to be 85-105 J K-1 mol-1 from the pressures of ammonia and the carrier gas,58 the entropy change with respect to the elemental step of desorption, which must be the difference between the whole and mixing entropies, should be 85-105 J K-1 mol-1. This value is almost same as those observed for various zeolites45,57 and tungsta-loaded zirconia59 (95-100 J K-1 mol-1) and, moreover, for the entropy change of the vaporization of ammonia (97 J K-1 mol-1) and various liquids (80-110 J K-1 mol-1).77 These agreements suggest a common rule that controls the TPD process on solids; the entropy change of desorption and vaporization is mainly determined by the free-volume change, in agreement with Trouton’s rule.45 From these observations, we can hereafter determine the adsorption heat of ammonia from the TPD spectrum by applying the same method58 to zeolites and tungsta-zirconia, which assumes equilibrium control and the constant entropy change of 95 J K-1 mol-1. On the basis of this theory, the acid strength of sulfated zirconia catalyst was calculated as shown in Figure 4. In the region of the 2-15 wt % of the sulfate content in the impregnated solution where the submonolayer formed, a high adsorption heat of ammonia (ca. 200 kJ mol-1) was obtained. The adsorption heat of the weaker acid site generated with >15 wt % of the impregnated solution was ca. 160 kJ mol-1. These values can be compared with those on other solid acids. Even on the weaker acid site, the adsorption heat is higher than those on any aluminosilicate zeolites (ca. 145, 130, 120, and 110 kJ mol-1 on mordenite, ZSM-5,58 β-zeolites,78 and Y-zeolites,56,79 respectively), which are known as some of the strongest solid acids; the strength of the exceptionally strong (ca. 170 kJ mol-1) acid site ascribed to an extraframework cation55,80 is comparable to the strength of the weaker acid site on the sulfated zirconia. Next, we will derive the H0 scale from the adsorption heat of ammonia. On the assumption that changes in the standard enthalpy and entropy of the adsorption of ammonia (5) are ∆H05 and ∆S05, respectively, and ignoring their temperature dependence, we can express the standard Gibbs energy change of reaction (5), ∆G05, at T0 ) 298 K as
∆G05 ) ∆H05 - T0∆S05
(6)
On the other hand, the standard Gibbs energy changes for the formation of gaseous and aqueous ammonia are -16.65 and -26.50 kJ mol-1, respectively, and therefore, the Gibbs energy change of reaction (7), ∆G07, is 16.65 - 26.50 ) -9.85 kJ mol-1.81
NH3 (g) + aq f NH3 (aq)
(7)
The thermodynamic equilibrium constant K8 of reaction 8 at temperature T0 can be expressed as follows:
K8 )
aNH3-A
) e-∆G8/RT ) e(∆H5-T ∆S5+∆G7)/RT 0
0
aAaNH3(aq)
0
0
0
0
(10)
where ai shows the thermodynamic activity of component i. The activities of NH3-A and A are shown by θ (the coverage of acid site by ammonia) and 1 - θ, respectively. The activity of the aqueous ammonia can be shown by the concentration of ammonia [NH3] in units of mol dm-3. Hence, the eqs 11 and 12 are derived as 0 0 0 0 0 θ ) e(∆H5-T ∆S5+∆G7)/RT (1 - θ)[NH3]
(11)
0 0 0 0 0 θ ) (1 - θ)e(∆H5-T ∆S5+∆G7)/RT [NH3]
(12)
When an equilibrium (eq 13) between an acid AH and a small amount of base indicator B exists, the H0 function is defined as21
AH + B a A- + BH+
(13)
+
H0 ) pKa - log
[BH ] [B]
(14)
where Ka is the acid dissociation constant of the conjugated acid (BH+) of the base B, and pKa is -log Ka. Therefore, we can express the H0 of the solid acid as the eq 15 on the basis of the reaction between solid acid and a small amount of ammonia in an aqueous solution (eq 8)82
H0 ) pKNH4+ - log
θ [NH3]
(15)
where KNH4+ is the acid dissociation constant of ammonium ion in water. Substituting eq 12 into eq 15 derives
H0 ) pKNH4+ - log(1 - θ) -
∆H05 - T0∆S05 + ∆G07 RT0 ln 10
(16)
Because the amount of ammonia was assumed to be small, the term 1 - θ is close to 1. Substituting pKNH4+ ) 9.25,83 T0 ) 298 K, ∆S5° ) 95 J K-1 mol-1, ∆G7° ) -9.85 kJ mol-1, and R ) 8.314 J K-1 mol-1 into the eq 16 derives the relationship between the H0 function and the adsorption heat of ammonia (∆H5°, J mol-1)
H0 ) -1.75 × 10-4∆H5° + 15.9
(18)
Equation 17 is the relationship between the adsorption heat of ammonia and the H0 function on the solid acid where the entropy change of ammonia desorption is 95 J K-1 mol-1.
Sulfated Zirconia This theory suggests that the stronger acid site with 200 kJ mol-1 of the adsorption heat of ammonia corresponds to H0 ) -19.1. According to Conant and Hall,20 an acid stronger than 100% perchloric acid or sulfuric acid (H0 ) -11.9) is a superacid. Therefore, this type of acid site on the submonolayer species of sulfate loaded on zirconia is a superacid. This H0 value agrees well with the method of indication, which showed H0 < -16.04 on the sulfated zirconia calcined at 923 K.22 It is noteworthy that the number of this type of acid site (see Figure 4, ca. 0.5 nm-2) was much smaller than the number of the sulfate species (2.5 nm-2). Only a fraction of the submonolayer species seems to act as an acid site. As shown in Figure 6, the nature of this superacid site is mainly Lewis type. It is suggested that the isolated species in Scheme 1 is the Lewis-type superacid site, as previously proposed.19 On the other hand, the strength of the weaker acid site generated by excess sulfate corresponds to H0 ) -12, and hence, it is not a superacid but similar in strength to the 100% sulfuric acid. There are papers which claim that the acid strength of sulfated zirconia is similar to that of 100% sulfuric acid on the bases of the indicator method36 and the quantum chemical calculation.37 The surface concentration of this type of acid site was quite high, ca. 2 nm-2 (Figure 4), and its maximum was observed at 20 wt % of the sulfate content in the impregnated solution, i.e., with a sulfur surface concentration of ∼10 atoms nm-2. The high concentration of the acid site is similar to those observed on the multivalent anion-loaded oxide catalysts; ca. 1.5-2 nm-2 of Brønsted acid site was generated by loading the monolayers of tungsta on zirconia59 and molybdena on tin oxide84 with ca. 5 nm-2 of the loaded metal atom. Similar to these cases, it is suggested that the monolayerlike species of sulfate consisting of S, O, Zr, and H atoms generates Brønsted acidity. Catalytic Activity. As shown in Figure 7, the activity for the alkylation of benzene with benzyl chloride was generated by the impregnation of 5-15 wt % of sulfate. From these results, it is proposed that this Friedel-Crafts-type reaction is catalyzed by the Lewis-type superacid site on the submonolayer species of sulfate. In contrast to this simple conclusion, the explanation for the skeletal isomerization of butane is complex. In both of the pulse and closed circular experiments, which must show the initial and static activities, respectively, the activity was observed at >15 wt % of the sulfate content in the impregnated solution. Therefore, the superacidity may not be necessary for this reaction, at least at 423 K. With a high amount of sulfate, >15 wt %, Brønsted acidity was observed, and this type of acid site is probably one of the strongest Brønsted acids among solid acid catalysts. It is possible that (strong) Brønsted acidity is important for this reaction.25 However, this explanation is not enough because a strong Brønsted acids such as mordenite or ZSM-5 zeolite never shows high activity for the skeletal isomerization; more precisely, these zeolites are known as cracking catalysts, but the selectivity for the isomerization is low. Therefore, another explanation is needed for the extremely high activity on the sulfated zirconia. One possible solution is that the copresence of Brønsted and Lewis acid sites, as observed, enhances the activity, and this is in agreement with the postulation that a pair of Brønsted and Lewis acid sites concertedly works as the active site.85 Another explanation is that the activity is much enhanced by the high surface concentration of the acid site at a maximum of ca. 2 nm-2. If the concerned mechanism is important, as above, multiple acid sites must access an intermediate molecule at once. A high
J. Phys. Chem. B, Vol. 104, No. 44, 2000 10327 surface concentration of acid sites such as >1.5 nm-2 is considered to allow access via the multiple acid sites. We have also observed that a Brønsted acid site with relatively high strength (130 kJ mol-1 of the adsorption heat of ammonia) on the tungsta monolayer was active for this reaction (the reaction temperature was 623 K) and that the surface concentration of acid site was high, ca. 1.5 nm-2.59 Because of the difficulty taking accurate measurements of surface acidity, no researcher has noticed the high density of the acid site on the sulfated zirconia, but this is possibly the reason of the high activity for the skeletal isomerization. Finally, we will discuss on the disagreement among the papers. The presence of the two or more kinds of acid sites on the sulfated zirconia in the present study is in agreement with the previous papers,30,39,86 and this must disturb the quantitative characterization of the acidic property. Moreover, the presence of the superacid site was indicated in relatively early works; for example, Hino and Arata et al., reported the generation of superacidity by pouring sulfuric acid solution into zirconia placed on a filter paper.2 On the basis of the present study, we conclude that only the submonolayer species of sulfate is supposed to be bonded on the zirconia surface through such a procedure. Recently, there has been a trend to optimize the preparation method of the catalyst to obtain an active catalyst for the skeletal isomerization.40-43 It is noteworthy that the two important papers which claim the weak acidity of sulfated zirconia stand on the commercially utilized sample.30,31 The optimized preparation method possibly results in the generation of a weaker Brønsted-type acid site but no superacidity, which was observed with excess sulfate loading (>15 wt %) in the present study. Conclusions (1) A submonolayer of sulfate, e.g., the isolated species in Scheme 1, is spontaneously formed by the impregnation of sulfuric acid on zirconia. (2) The water vapor treatment of ammonia TPD changed the spectrum, and this clarified the generation of acid sites by loading sulfate. (3) On the submonolayer species of sulfate, ca. 0.5 nm-2 of a Lewis-type superacid site with an ammonia adsorption heat of ca. 200 kJ mol-1 of and a H0 function of -19, was generated. This type of acid site catalyzes Friedel-Crafts type alkylation using benzyl chloride. (4) On the samples with excess sulfate loading, highly concentrated (maximum 2 nm-2) Brønsted acid sites with ca. 160 kJ mol-1 of the adsorption heat and -12 of the H0 function were observed. This type of acid site was active for the skeletal isomerization of butane at 423 K. References and Notes (1) Tanabe, K.; Itoh, M.; Morishige, K.; Hattori, H. In Preparation of Catalysts; Delmon, B., Jacobs, P. A., Poncelet, G., Eds.; Elsevier: Amsterdam, 1976; p 65. (2) Hino, M.; Kobayashi, S.; Arata, K. J. Am. Chem. Soc. 1979, 101, 6439. (3) Corma, A.; Martinez, A.; Martinez, C. J. Catal. 1994, 149, 52. (4) Cheung, T.-K.; Gates, B. C. J. Catal. 1997, 168, 522. (5) Kurosaka, T.; Matsuhashi, H.; Arata, K. J. Catal. 1998, 179, 28. (6) Negelein, D. L.; Lin, R.; White, R. L. J. Appl. Polym. Sci. 1998, 67, 341. (7) Hino, M.; Arata, K. Chem. Lett. 1981, 1671. (8) Tanabe, K.; Yamaguchi, T.; Akiyama, K.; Mitoh, A.; Iwabuchi, K.; Isogai, K. In Proceedings of the 8th International Catalysis Congress; Verlag Chemie: Weinheim, 1984; Vol. 5, p 601. (9) Clingenpeel, T. H.; Wessel, T. E.; Biaglow, A. I. J. Am. Chem. Soc. 1997, 119, 5469.
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