J. Phys. Chem. 1986, 90, 4194-4196
4794
Mechanism of Acidity Generation on Sulfur-Promoted Metal Oxides Tuo Jin, Tsutomu Yamaguchi, and Kozo Tanabe* Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan (Received: February 26, I986)
The effect of sulfur promotion on the generation of strong acidity on ZrO,, TiO,, Fe2O3, Al2O3, SnO,, SiO,, and BizO, was examined by the isomerization of cyclopropane and by infrared spectroscopy. The acidity generated varied on the types of oxides. The highest promotion effect was found in the cases of ZrO,, TiO,, and Fe203, while a lower effect was observed for A1,03 and Sn02,and no effect was found for Si02and Bi2O3. Infrared spectroscopic observation indicates the presence of the surface sulfur complex having covalent SO double bonds on all of the sulfur-promotedoxides. An admission of pyridine resulted in a remarkable shift in an asymmetric stretching frequency of S=O bonds in the cases of Zr02, TiO,, and Fe203; however, a small shift was found on AI2O3,and no shift was found on SiOz and BiZO3. The magnitude of this shift was well-correlated with the extent of the promotion effect. The drastic shift suggests that a change of electronic structure in the surface sulfur complex takes place by the adsorption of basic molecules. The origin of the generation of the strong acidity by a sulfur promotion is the formation of the surface sulfur complex which has a covalent SO double bond on one hand and a strong tendency of losing its double-bond character or decreasing the bond order of SO by an electronic shift from a basic molecule adsorbed to the sulfur complex on the other.
Introduction An introduction of a small amount of sulfur compounds onto metal oxides enhanced the acidic properties of the oxides remarkably, regardless of the types of introduced sulfur compounds such as (NH4)2S04,SO2,SO,, and HzS, provided that the sulfur species are fully oxidized.' The structure of the acidic species has been proposed to be a surface complex which is comprised of a metal cation and a sulfate ion with two covalent S=O bonds.] The processes of the transformation of the introduced sulfur compounds to the acidic species were also proposed.'a2 Based on the proposed structure, the mechanism of the generation of the strong acidity is discussed in the present article.
TABLE I: Catalysts Used
Experimental Section The sulfur-promoted oxides used in this study are shown in Table I. The metal hydroxides were prepared by precipitation of aqueous solutions of ZrO(N03)2-2H20,Tic],, Fe(N0,)3*9H20, A12(S0,),.18H20, SnCI,, and BiCI, and an ethanol solution of Si(OC2H5), with ammonia-water, respectively, followed by washing with deionized water and drying at 100 O C for 24 h. The sulfur-promoted oxides were prepared by immersing the corresponding hydroxides in a solution of ammonium sulfate (AS), evaporating to dryness, and calcining at 500 "C (at 600 "C for AS/Zr02) for 2.5 h. Amounts of impregnated sulfur are shown in Table I. Amounts of pyridine and ammonia adsorbed were measured at 150 OC by using a Cahn 2000 electrobalance. Prior to the measurement, the samples were evacuated at 400 OC for 2 h. The BET surface area was measured by a nitrogen adsorption at liquid nitrogen temperature. The details of IR measurements and the reaction procedure of the isomerization of cyclopropane are described in our previous paper.' The reaction temperature was 100 O C .
TABLE 11: Catalytic Activity of Sulfur-Promoted Metal Oxides in Isomerization of Cyclopropane reaction rate, reaction rate,
Results Catalytic activities of various sulfur-promoted oxides in the isomerization of cyclopropane, which is a typical acid-catalyzed reaction, are shown in Table 11. Unpromoted oxides were completely inactive in the isomerization reaction. The seven catalysts tested here can be classified into three groups from the magnitude of the sulfur-promoting effect on the catalytic activity. AS/ZrO,, AS/Ti02, and AS/Fe20, showed high activity, while the activity ( 1 ) Yamaguchi, T.; Jin, T.; Tanabe, K. J . Phys. Chem. 1986, 90, 3148. (2) Jin, T.; Machida, M.; Yamaguchi. T.: Tanabe, K. Inorg. Chem. 1984, 23, 4396. (3) Nakamoto, K. Infrared Spectra of Inorganic and Coordination Com-
pounds: Wiley: New York. 1970.
0022-3654/86/2090-4794$01.50/0
catalyst AS/ZrO,' AS/Ti02 AS/Fe20, AS/AI,O3 AS/SnO, AS/Si02 AS/Bi203
starting material ZrO(NO)3.2H20 TiCI, Fe(NO3),.9H,O A12(SO,),*18H,O SnCI,
Si(OC,H,), BiCI,
amt of S as SO,. w t % 8 2 2 4 2 2 5
"AS = ammonium sulfate.
catalyst ASIZrO?" ASjTi0,AS/Fe20, AS/A120,
pnol/(min g) 243 226
202
catalyst ASISnO, ASjSi0,AS/Bi,O,
pmol/(min g) 49 0 0
69
'AS = ammonium sulfate. TABLE 111: Surface Areas and Amounts of Pyridine and Ammonia Adsorbed
catalyst AS/Zr02" AS/Ti02 AS/Fe20, ASIA1201 AS/Sn02 AS/Si02 AS/Bi203
surface area,
pyridine ads,
ammonia ads,
m2/g
rnmo1i.g
mmol/g
120 60 40 230
0.21
0.26
0.16
0.17 0.07
80
0.10
250 4
0.06 0.26
0.36 0.18
0.02
0.14
0
0.01
"AS = ammonium sulfate. of AS/A1203and AS/SnO, was low compared to that of above catalysts. AS/Si02 and AS/Bi,O, were completely inactive. Table 111 shows the surface areas and the amounts of pyridine and ammonia adsorbed irreversibly at 150 OC. The amounts of pyridine and ammonia adsorbed, which indicate the amount of acidic sites, seem to roughly relate to the surface areas, whereas it is difficult to find a correlation between the catalytic activity and the surface areas or the amounts of base adsorbed. This suggests that the number of acidic sites is not an important factor in controlling the catalytic activity. Therefore, the differences in the catalytic activity should be attributed to the differences in the acidic strength of the individual catalysts.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 4795
Acidity Generation on S-Promoted Metal Oxides
TABLE I V Changes in Bond Order of SO and Partial Charge on Oxygen of SO by Pyridine Adsorption
SO stretching freq, cm-' before DV adsb after DY ads
catalvst AS/Zr02" AS/Ti02 AS/Fe203 AS/A1203 AS/Si02 AS/ Bi203
~
1390 1375 1375 1398 1410 1370
bond order before DY ads after DY ads
partial charge on oxygen before DV ads after DV ads
'
1.86 1.84 1.84 1.88 1.89 1.83
1339 1326 1329 1365 1410 1370
-0.14 -0.16 -0.16 -0.12 -0.10 -0.17
1.78 1.76 1.76 1.82 1.89 1.83
-0.22 -0.24 -0.24 -0.18 -0.10 -0.17
pki
"AS = ammonium sulfate. 'py = pyridine.
$1
5e
i-
d 1365
1500
1300
1500
1300
1500
1300
1500
1300
1500
1300
1500
1300
cm-I Figure 2. Infrared spectra of oxides promoted by (NH4)2S04:a, evacuated at 400 OC; b-d, pyridine successively adsorbed at 150 OC. Wave number,
1
I
1600
I
I
1400
1200
I
1000
1
8 00
Wave number, cm-' Figure 1. Infrared spectra of AS/Zr02 before and after the pyridine
adsorption: solid line, evacuated at 500 O C for 2 h; broken line, pyridine adsorbed at 150 OC and evacuated at 150 OC. Infrared spectra of AS/Zr02 are shown in Figure 1. AS/Zr02 showed four IR bands at 1390, 1190, 1020, and 930 cm-' in the SO stretching frequency region. In our previous paper,' the former two and the latter two bands were assigned to the asymmetric and symmetric stretching frequencies of the O=S=O and 0-S-0 groups, respectively. The proposed structure as an acidic species is as follows'
showed high catalytic activity, about 33 cm-' for less active AS/A1203, and 0 for inactive AS/Si02 and AS/Bi203(Table IV). Thus, the shift of the stretching frequency of group I can be related to the catalytic activity: the larger the shift of the asymmetric stretching frequency of the S=O bond, the higher the catalytic activity. It is also worth mentioning that when pyridine was admitted stepwise on AS/Zr02, AS/Ti02, or AS/Fe203, each shift of the asymmetric stretching frequency of group I was kept constant with increasing amounts of pyridine adsorbed (Figure 2b-d), with only the intensities of the bands before and after the adsorption being changed. This fact strongly suggests that the extent of the shift of the stretching frequency of group I by the adsorption of pyridine is independent of the amount of pyridine adsorbed, and this represents the acidic strength of the catalysts, and that the catalytic activity depends principally on the acidic strength.
Discussion
where M represents the metal ion of oxides, and the presence of covalent SO double bonds characterizes this species. When pyridine was adsorbed on the sample, an asymmetric stretching frequency (1390 cm-I) of S=O in structure I' shifted drastically to a lower frequency (1339 cm-I, broken line in Figure 1). Since the absorption bands at 1610 and 1450 cm-' of pyridine indicate the presence of coordinated pyridine and no band at 1540 cm-' indicates the absence of pyridinium ion: the catalyst possesses solely Lewis acidity. A central cation in structure I' acts as a Lewis acid site. IR spectra of various samples are shown in Figure 2. The absorption of an asymmetric stretching frequency of SO double bonds in group I 0
0
\/ / \ I
was commonly found in the range of 1370-1410 cm-I; however, the frequency shift of this band by pyridine adsorption was different depending on the individual support oxides. The shift was about 50 cm-l for AS/Zr02, AS/Ti02, and AS/Fe203, which (4) Parry,
E. P. J . Catal. 1963, 2, 371.
The drastic shift of the IR band by a pyridine adsorption (Figures 1 and 2) indicates a strong interaction between an adsorbed pyridine molecule and the surface sulfur complex. To understand this interaction and to elucidate the electronic structure and the dynamic behavior of the acidic group, changes in the bond order of SO bonds and the partial charge on the oxygen of SO bonds will be discussed first. Relationship between Bond Order and Stretching Frequency of SO. Gillespie and Robinson5 have proposed an empirical relationship between the bond order, n, and the force constant, k , of SO.
n = 1.11 X 10-6k
+ 0.7
(1)
The eigenfrequency of a resonant oscillator,f, is represented as
-( --)
f = cu = 1
k
Ii2
27c where c, u, and I.L are the velocity of light, the wavenumber, and the reduced mass of the oscillator, respectively. From eq 1 and 2, the bond order of SO can be represented by a wavenumber directly n = 6.9741
X
+ 0.7
10-732
(5) Gillespie, R. J.; Robinson, E. A. Can. J . Chem. 1963,4 2 , 2074.
(3)
4796
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986
where r, is the average wavenumber of the asymmetric and symmetric stretching frequencies of the SO bond in group I.4 According to Bellamy et a1.,6 the relationship between asymmetric and symmetric stretching frequencies of group I can be written as
(4) where vSymand vas,,, represent the wavenumbers of symmetric and asymmetric stretching of group I, respectively. Combining eq 4 with Gillespie’s equation4 (5) where x is the number of oxygen atoms in a SO, group, we obtain the following equation by considering x = 2 in the case of the species having group I.
9
B = -vasym 10
+ 40
Thus, eq 3 is written as follows (7)
and hence, the bond order of SO can be calculated from the uaSym observed. Calculated bond orders before and after the pyridine adsorption are shown in Table IV. It is clear that the changes in the bond order by the pyridine adsorption are the highest in the most active catalysts. It is likely that the surface sulfur complexes in highly active catalysts have a strong tendency to reduce its bond order by the adsorption of basic molecules. Relationship between Bond Order of SO and Partial Charge on Oxygen of SO. Bond order reflects the electronic population of the valence electrons between two nuclei. There may be a quantitative relationship between the bond order and the electron distribution between two atoms bonded. If the two pairs of valence electrons of SO are completely located between S and 0, the bond order should be 2 as is shown by model A
s=o A
3-
s-0 B
Jin et al. since the replacement of one oxygen by two higher electronegative F atoms does not affect the bond order of SO. Thus, it is likely to assume that F does not influence the charge distribution on the three oxygens in the FS03- molecule. This indicates that a negative charge in FS03-is delocalized evenly onto three oxygens, and thus the partial charge on one oxygen should be -0.33. The bond order of FS03- was calculated to be 1.66. The estimated bond order of was 1.5, and the partial charge on oxygen was -0.5. The linear relationship Q=n-2
(8)
is obtained by using these values, where Q and n are the partial charge and the bond order, respectively. Change of Surface Sulfur Species by Pyridine Adsorption. The bond order and partial charge on oxygen in the SO bond of the samples are estimated by using eq 7 and 8, and the results are shown in Table IV. When pyridine was adsorbed on AS/Zr02, AS/Ti02, and AS/Fe20,, an asymmetric stretching frequency of SO shifted from 1375-1 390 to 1326-1 339 cm-I (Au = ca. 50 cm-I). This corresponds to a change of 0.08 in the bond order and of -0.08 in the partial charge on each oxygen. For AS/A1203, however, the changes in SO stretching frequency, bond order, and partial charge were 33 cm-I, 0.06, and -0.06, respectively. On the other hand, for AS/Si02 and AS/Bi203 these changes were zero. From these results, it is likely to conclude that the surface sulfur complex in the highly active catalysts or the highly acidic catalysts, which were obtained by an introduction of a sulfur compound to metal oxides, has a strong tendency of reducing the bond order of SO from a highly covalent double-bond character to a lesser double-bond character when a basic molecule is adsorbed on its central metal cation. The strong ability of a sulfur complex (I) to absorb electrons from a basic molecule is a driving force to generate highly acidic properties. The change of electronic structure caused by pyridine adsorption may be illustrated as follows, where the coordination number of a surface metal cation of a metal oxide was taken as 5.
s-oC
while if one of the electron pairs is localized on an 0 atom, the bond order becomes 1 as in model C. The bond order is between 1 and 2 when the electron pair is partially located around 0 as shown in model B. Changes in the bond order of SO can be calculated according to eq 3; however, little work has been done on the estimation of the partial charge on oxygen in the SO bond. Thus, we used the following procedure to determine the partial charge on oxygen. Calculation by eq 3 revealed the bond order of SOF, and S02F2 to be 2, and thus the partial charge on oxygen is 0. This also suggests that no electron shift from F to the SO, group occurs (6) Bellamy, L. J.: Williams, R. L. J. Chern. SOC.1957, 863.
The acid site is the metal cation (M) whose acidic strength can be strongly enhanced by the induction effect of S S O in the sulfur complex. Besides the strong inductive effect, three factors, namely valence, electronegativity, and coordination number of a metal cation of a metal oxide, are also considered to affect the acidic strength of a sulfur-promoted metal oxide. The strong effect of sulfur on acidity enhancement has been emphasized, and the mechanism of the effect was proposed in the present paper. Registry No. ZrOz, 1314-23-4; Ti02, 13463-67-7; Fe20,, 1309-37-1; AlzO,, 1344-28-1; S O 2 , 763 1-86-9; Bi203, 1304-76-3; (NHJ2S0,, 7783-20-2; S, 7704-34-9; pyridine, 110-86-1: cyclopentane, 75-19-4.
