- Email: [email protected]
ZrO2 superacid catalysts
ELSEVIER
MaterialsChemistryandPhysics 50 (1997) 15-19
Preparation and properties of ultrafine SOd2- /ZrO, superacid catal.ysts C.X. Ciao, Z. Gao * Deparbnent
of Chemistry,
Fudan
Univer&,
Shanghai
200433,
People’s
Republic
of Chino
Abstract Ultrafine SO,‘- /ZrO, superacid catalysts were prepared in a two-step synthesis by impregnating ultrafine crystalline ZrO, with sulfuric acid. Ultrafine crystalline zirconia was obtained by supercritical drying of an alcogel. The crystallization, crystal size, surface area, sulfur content, acid strength and catalytic activity for n-butane isomerization of sulfated amorphous and ultrafine crystalline zirconia were tested and compared. The catalyst prepared from ultrafine crystalline zirconia has smaller crystallite size, larger surface area, greater sulfur content and higher superacidity. Hence, it displays higher catalytic activity and stability for n-butane isomerization. Keywords:
Ultrafine
crystalline
zirconia;
Ultrafine
SO4 *-/Zr02
superacid;
1. Introduction Sulfated zirconia is an important type of solid superacid, which shows good activity for a variety of hydrocarbon conversion reactions, such as isomerization, alkylation and cracking [ l-31. The activity of this type of solid superacid catalyst depends on the preparation methods and conditions. The most common ways of preparing Sob’- /ZrO, are immersing amorphous zirconium hydroxide in a dilute solution of sulfuric acid or impregnating the amorphous zirconium hydroxide with a solution of ammonium sulfate followed by drying and calcining at elevated temperature. The zirconium hydroxide precursor, sulfur content and calcination temperature all may affect the catalytic performance of the prepared catalysts [ 4-61. Recently, a one-step synthesis for the production of Sod’- /ZrO, superacid catalysts has been developed [ 71. In the synthesis a zirconia-sulfate aerogel was prepared by the sol-gel method followed by supercritical drying. After calcination at 773 K for 2 h, the BET (Brunauer, Emmett and Teller) surface area of the zirconiasulfate aerogel containing 20 mol.% sulfate is around 155 m2 g- ‘, which is almost the same as that of the catalysts prepared in common ways [5,6]. The aerogel catalysts are catalytically active for n-butane isomerization at 553 K, but no comparison in activity between the aerogel catalysts and others has been made. Therefore, the advantages of the onestep synthesis method are yet unclear. We have noticed that in the one-step synthesis method a large excess of sulfuric acid was used. Although as high as * Correspondingauthor. 0254-0584/97/$17.000 PNS0254-0584(97)01S94-4
1997 Elsevier
Science S.A. All rights reserved
n-Butane
isomerization
20 mol.% of sulfate has been added into the aerogel, only a small amount of the sulfate may create superacidic active sites on the surface of the aerogel? because a large part of the sulfate has been lost during the amorphous-to-tetragonal crystallization of zirconia at high temperature. Hence, in this work a two-step synthesis method has been tried. Ultrafine crystalline ZrO, wasobtained by the sol-gel method followed by supercritical drying, and then immersed in a dilute H2S04 solution to prepare ultrafine SOd2- /Zr02 superacid catalyst. The phase transition, surface area, sulfurcontent, superacidity and catalytic activity of the ultrafine superacid catalyst prepared by the two-step synthesis method were investigated and compared with those of the catalyst prepared in the usual way.
2. Experimental Zirconium oxychloride was dissolved in distilled water, and aqueous ammonia was added with stirring until pH= 9-10. The white zirconium hydroxide precipitate was filtered and washed thoroughly with distilled water followed by drying at 383 K for 24 h. Then it was ground into a fine powder ( > 100 mesh), which was designated as ZrO,(A). The ZrO,( A) powder was immersed in a 0.5 mol l- ’ H,SO, solution for 30 min, and then filtered and dried at 383 K for 24 h to form the sulfated sample S04’- /ZrO,( A). Zirconia hydrogel wasprepared from zirconium oxychloride and aqueous ammonia, and it was then exchanged with ethanol for 7-8 times with a hydrogel/ethanol volume ratio of 1.5 to form a zirconia alcogel. The alcogel was transferred
C.X. Miao,
16
Z. Gao /Materials
Chmisrty
to an autoclave, and then the autoclave was sealed and its temperature was slowly raised to 533 K and held for 1 h to remove the alcohol under supercritical conditions. After releasing the fluid, the remained alcohol wascompletely displaced by N,. The product obtained was a milk white semitransparent powder, which was designated as ZrO,( B). The 210,(B) powder was sulfated in the same way as ZrO,(A) to form SO,“-/ZrO,(B). X-ray powder diffraction measurements were perfotmed on a Rigaku D/MAX-IIA instrument with monochromatic
and Physics
50 (1991)
15-19
Cu Ka radiation, scan speed 16” mix-r-’ and scan range 5-70”. The specific surface area of the samples wasmeasured on a Micromeritics ASAP 2000 system under liquid N, temperature using N2 as the adsorbate. Transmission electron microscope (TEM) images of the samples were recorded on a H-700s TEM instrument. Chemical method was used for the detection of the sulfate content in the samples. The sulfate was turned into BaSO, and determined by gravimetric method. The acid strength of the samples was determined by a flow Hammett indicator method [8]. The superacidity and catalytic activity of the samples were characterized by measuring their activities forn-butane isomerization under static and flow conditions. In the former case, a static reactor system was used and the catalyst loading was 0.5 g. The amount of n-butane injected for each test was5 ml of gas (1.0X lo5 Pa, 293 K), and the reaction temperature was 308 K. In the latter case, a flow-type microreactor was used and the catalyst loading was 1.0 g. The reaction temperature was 423 K in N2 atmosphere and 523 Kin H2 atmosphere, and the weight hour space velocity (WHSV) ofbutane was 0.3 h-l. All the SO,“- /ZQ catalysts were pretreated in air at 723 K for 3 h before reaction. The reaction products were analyzed by an on-line gas chromatograph equipped with a flame ion detector.
3. Results and discussion
Fig. 1. TEM image of Z&,(B)
(original
magnification,
Fig. 2, TEM images of (a) ZrO,(B)
200 000 X ).
and (b) ZrO,(
The TEM image of ZrO,( B) supercritically dried at523 K is given in Fig. 1, showing that ultrafme crystallites with average size around 5 nm are formed. Fig. 2 gives the images of ZrO,(A) and ZrO,(B) after calcination at 923 K for 3 h.
A) after calcination
at 923 K (original
magnification,
100 000 X ).
C.X. Miao,
Fig. 3. TEM Table 1 Properties
2. Gao /Materials
images of (a) SOJ2-/ZrOZ(B)
Chemimy
and (b) S042- /ZrO,(A)
and Physics
17
50 (1997jmlS19
after calcination
at 923 K (original
magnification,
200 000 X )
of the samples
Sample
ZrOZ(B) ZrO,(A) SO,‘-/ZrOa(B) Sod’- /ZrO,(A)
Colour
milk white white white white
h-r
Average
Cm* g-‘)
Before calculation
After calculation
185.4 136.7 -
56.5 18.3 113.6 96.2
Ultrafine globular crystallites with average size of about 10 nm are formed in the case of ZrO,(B), while large agglomerates with average size of about 200 nm are formed from ZrO,( A). The TEM images of the sulfated samples after calcination at 923 K for 3 h are shown in Fig. 3. The suppression of crystallite growth and agglomeration in the presence of sulfate [ 91 is observed clearly. The average crystallite sizes of SOj2- /ZrO,( A) and SO4‘- /ZrO,( B) after calcination are only about 8 and 5 nm, respectively, which are obviously smaller than the crystalline sizes of ZrO,( A) and ZrO,( B) calcined at the same temperature. The BET surface area, average size and sulfur content of the samples calcined at 923 K were measured and listed in Table 1. Both S04’-/ZrO*(A) and S042-/ZrOl(B) take the form of ultrafine crystallites. However, S04’- /ZrO,( B) prepared from supercritically dried ZrO,(B) has smaller average crystallite size, larger BET surface area and higher sulfur content. XRD patterns of the non-sulfated and sulfated samples were recorded after calcination at different temperatures. At
673K, ZrO,(A) waspartiallycrystallizedintothetetragonal
size (nm)
so3 (wt.%)
10 200 5 8
3.6 3.3
and monoclinic structures, whereas ZrO,(B) was already partially crystallized into the tetragonal structure after supercritical drying at 533 K. Further heat treatment caused ZrO,(A) and ZrO,(B) to be more completely crystallized and to transform from the tetragonal phase to the monoclinic phase. The addition of sulfate retarded both the amorphousto-tetragonal crystallization and the tetragonal-to-monoclinic transformation of ZrO,( A) and ZrO,( B) . Heating to 923 K was required to fully crystallize the S04’-/Zr02(A) and S04’- /ZrO,( B) samples. All the XECD characteristic peaks of the crystalline ZrO,(B) and SO,*-/ZrO,(B) samples are lower and broader than those of the crystalline ZrO,( A) and Table 2 Acid strength Sample
S04’-/Zr02(B) S04*-/ZrOz(A) t
of the samples Ho - 12.7
- 13.2
- 13.8
- 14.5
- 16.0
t +
+ +
+ +
+ +
t +
= indicatorschangedcolour.
18
C.X. Mao,
Table 3 Reaction data for n-butane Sample
41.6
S042-/ZrOl(A)
40.1
Chemistry
and Physics 50 (1957)
15-19
isomerization
Static system k, (X 1O-1 h-l)
So,2-/Zro,(B)
Z. Gao/Materials
Flow system Reactant
Temperature (K)
n-C, Conversion
(%)
2min
1Omin
60 min
120 min
180 min
360 min
2400 min
NJC, =9 H,IC,=9 N2/C4 =9
423 523 423
20.9 39.5 20.0
10.3 34.3 10.4
6.8 18.8 2.1
5.7 18.1 -
4.8 16.7 -
4.2 15.2 -
14.6
H&=9
523
38.6
30.0
15.4
12.1
11.8
11.5
8.5
S0,2-/Zr0,(A) samples owing to their smaller average crystallite sizes. The crystallization of zirconia was reported by several authors [ 2-5,7, lo] to be necessary for the generation of superacidity. Although different structures of the active sulfate species on the surface of zirconia have been proposed [ 1l-141, it is generally believed that at higher temperatures after the crystallization of zirconia the sulfate ions are transformed to a more strongly covalent species with S=O bond orders close to two, and the electron withdrawing ability of these S=O bonds increases the acid strength of the zirconia surface. The acid strengths of SO,‘-/ZrO,(A) and SO,‘-/ ZrO,( B) after calcination at 923 K for 3 h were tested using a flow Hammett indicator method [S] and the results are shown in Table 2. Both samples are superacidic, i.e. their Ho is I - 16. In the previous literature [2,15], treatment of crystallized zrOz with sulfate ions was not effective in superacidity generation. Catalysts prepared by sulfating zirconia precursors having tetragonal and monoclinic crystal structures were inactive for n-butane isomerization. The success of this two-step synthesis method may be explained by the large difference in surface area between the ultrafine a;d ordinary crystalline zirconia. From Table 1 we can see that the supercritically dried ultrafine crystalline ZrO,(B) has a BET surface area of 185.4 m2 g- ‘, which is even higher than the value of 136.7 m2 g- ’ for the amorphous ZrO,(A). On the other hand, the surface area of a crystalline ZrO, prepared conventionally by calcining an amorphous zirconium hydroxide at high temperature is usually in the range of lo50 m* g-‘. The high surface area of the ultrafine crystallites undoubtedly facilitates the adsorption of sulfate ions and hence also the creation of superacidic sites on the oxide. The catalytic activities of Sob’-/ZrO,(A) and SO,‘- / ZrO,(B) solid superacid catalysts toward n-butane isomerization were tested under static and flow conditions. In our previous paper [ 161, it has been found that at 298-308 K under static reaction conditions the kinetics of n-butane isomerization on S04’- /ZrO, solid superacid catalysts follow the rate law of a first-order reversible reaction, and the rate constants of the catalysts correlate fairly well with the superacidity of the catalysts. Table 3 gives the rate constants of the two catalysts for n-butane isomerization reaction at 308 K. The rate constant kI of SO,‘- /ZrO,(B) is slightly higher
than that of S04’- /ZrO,(A) showing that there are probably more superacidic sites on the surface of S042-/Zr0,(B), which is consistent with the results of sulfur content measurements as seen in Table 1. Butane isomerization reaction on the two types of catalysts was also carried out in the presence of N, and H, at 423 and 523 K, respectively. Under these conditions, the isomerization selectivities of the catalysts were always above 95%. The reaction data are listed in Table 3. The initial activities of S0,2-/Zr0,(B) are again slightly higher than those of S0,2- /Zr02( A). All the catalysts deactivate quickly in the first hour of the reaction run owing to the formation of a carbonaceous deposit { 15,17,18 ] . In the presence of H2 the rate of deactivation is reduced, indicating that hydrogen inhibits the coke formation even in the absence of metals, such as Pt and Ni. T’he catalyst stability of SOb2- /ZrO,(B) is evidently higher than that of S04’- /ZrO,(A) . Since isomerization is a reaction less demanding of acid strength than coking, the increase in stability suggests that the two catalysts differ from each other not only in acid site concentration but also in acid site distribution.
4. Conclusions A two-step synthesis method for the preparation of ultrafine Sod2 - IZrO, superacid catalysts has been developed. Ultrafine crystalline ZrO, obtained by supercritical drying of a zirconia alcogel adsorbed sulfuric acid. After calcination at high temperature the adsorbed sulfate species transformed to an activated state with LO bond orders close to two, which increased the acid strength of the zirconia surface. The superacidity and the catalytic activity and stability for n-butane isomerization of the catalysts synthesized in this way are higher than those of S0,2-/Zr0, catalysts prepared by impregnating ordinary amorphous zirconia with sulfuric acid.
References [ 11 M. Hino, S. Kobayashi and K. Arata, J. Am. Chem. Sot., 101 (1979) 6439. [ 21 K. Arata, Adv. Catal., 37 (1990) 165.
C.X. Miao,
Z. Gao /Materials
Chemistry
[3 ] T. Yamaguchi, Appl. Catal., 61 ( 1990) 1. [4] F.R. Chen, G. Coudurier, J.F. Joly and J.C. Vedrine, J. Caral., I43 (1993) 616. [5 ] C.X. Miao, J.M. Chen and 2. Gao, Chem. J. Chinese Univ., 16 ( 1995) 591. [ 61 D. Farcasiu and J.Q. Li, Appl. Catal., 128 (1995) 97. [7] D.A. Ward and E.I. Ko, J. Catal., I50 (1994) 18. [ 81 W.M. Hua, J.M. Chen and 2. Gao, Shiyou Huagong, 24 (1995) 385. [ 91 C.J. Norman, P.A. Goulding and I. McAlpine, Caral. Today, 20 ( 1994) 313. [ lo] Z. Gao, J.M. Chen and Y. Tang, Chem. J. Chinese Univ., 13 (1992) 1498. ~ ~~
and Physics 50 (1997)
1.5-19
19
Illj T. Jin, T. Yamaguchi and K. Tanabe, J. Phys. Chem., 90 (1986) 4794. 1121 T. Yamaguchi, T. Jin and K. Tanabe, J. Phys. Chem., 90 (1986) 3148. [ 131 M. Ben&cl, 0. Saw, J.C. Lavalley and B.A. Morrow, Muter. Chem. Phys., 19 (1988) 147. [ 141 M. Waqif, J. Bachelier, 0. Saur and J.C. Lavalley, J. Mol. Catal., 72 (1992) 147. [ 151 R.A. Comelli, C.R. Vera and J.M. Parera, J. Catal., I51 (1995) 96. [ 161Z. Gao, J.M. Chen, W.M. Hua and Y. Tang, Stud. Surf: Sci. Caral., 90 (1994) 507. 1171 J.C. Yori and J.M. Parera, Appl. Caral. A, 129 (1995) 83. Cl81 C.X. Miao, W.M. Hua and Z. Gao, Chinese J. Cut& 18 (1997) 13.
MaterialsChemistryandPhysics 50 (1997) 15-19
Preparation and properties of ultrafine SOd2- /ZrO, superacid catal.ysts C.X. Ciao, Z. Gao * Deparbnent
of Chemistry,
Fudan
Univer&,
Shanghai
200433,
People’s
Republic
of Chino
Abstract Ultrafine SO,‘- /ZrO, superacid catalysts were prepared in a two-step synthesis by impregnating ultrafine crystalline ZrO, with sulfuric acid. Ultrafine crystalline zirconia was obtained by supercritical drying of an alcogel. The crystallization, crystal size, surface area, sulfur content, acid strength and catalytic activity for n-butane isomerization of sulfated amorphous and ultrafine crystalline zirconia were tested and compared. The catalyst prepared from ultrafine crystalline zirconia has smaller crystallite size, larger surface area, greater sulfur content and higher superacidity. Hence, it displays higher catalytic activity and stability for n-butane isomerization. Keywords:
Ultrafine
crystalline
zirconia;
Ultrafine
SO4 *-/Zr02
superacid;
1. Introduction Sulfated zirconia is an important type of solid superacid, which shows good activity for a variety of hydrocarbon conversion reactions, such as isomerization, alkylation and cracking [ l-31. The activity of this type of solid superacid catalyst depends on the preparation methods and conditions. The most common ways of preparing Sob’- /ZrO, are immersing amorphous zirconium hydroxide in a dilute solution of sulfuric acid or impregnating the amorphous zirconium hydroxide with a solution of ammonium sulfate followed by drying and calcining at elevated temperature. The zirconium hydroxide precursor, sulfur content and calcination temperature all may affect the catalytic performance of the prepared catalysts [ 4-61. Recently, a one-step synthesis for the production of Sod’- /ZrO, superacid catalysts has been developed [ 71. In the synthesis a zirconia-sulfate aerogel was prepared by the sol-gel method followed by supercritical drying. After calcination at 773 K for 2 h, the BET (Brunauer, Emmett and Teller) surface area of the zirconiasulfate aerogel containing 20 mol.% sulfate is around 155 m2 g- ‘, which is almost the same as that of the catalysts prepared in common ways [5,6]. The aerogel catalysts are catalytically active for n-butane isomerization at 553 K, but no comparison in activity between the aerogel catalysts and others has been made. Therefore, the advantages of the onestep synthesis method are yet unclear. We have noticed that in the one-step synthesis method a large excess of sulfuric acid was used. Although as high as * Correspondingauthor. 0254-0584/97/$17.000 PNS0254-0584(97)01S94-4
1997 Elsevier
Science S.A. All rights reserved
n-Butane
isomerization
20 mol.% of sulfate has been added into the aerogel, only a small amount of the sulfate may create superacidic active sites on the surface of the aerogel? because a large part of the sulfate has been lost during the amorphous-to-tetragonal crystallization of zirconia at high temperature. Hence, in this work a two-step synthesis method has been tried. Ultrafine crystalline ZrO, wasobtained by the sol-gel method followed by supercritical drying, and then immersed in a dilute H2S04 solution to prepare ultrafine SOd2- /Zr02 superacid catalyst. The phase transition, surface area, sulfurcontent, superacidity and catalytic activity of the ultrafine superacid catalyst prepared by the two-step synthesis method were investigated and compared with those of the catalyst prepared in the usual way.
2. Experimental Zirconium oxychloride was dissolved in distilled water, and aqueous ammonia was added with stirring until pH= 9-10. The white zirconium hydroxide precipitate was filtered and washed thoroughly with distilled water followed by drying at 383 K for 24 h. Then it was ground into a fine powder ( > 100 mesh), which was designated as ZrO,(A). The ZrO,( A) powder was immersed in a 0.5 mol l- ’ H,SO, solution for 30 min, and then filtered and dried at 383 K for 24 h to form the sulfated sample S04’- /ZrO,( A). Zirconia hydrogel wasprepared from zirconium oxychloride and aqueous ammonia, and it was then exchanged with ethanol for 7-8 times with a hydrogel/ethanol volume ratio of 1.5 to form a zirconia alcogel. The alcogel was transferred
C.X. Miao,
16
Z. Gao /Materials
Chmisrty
to an autoclave, and then the autoclave was sealed and its temperature was slowly raised to 533 K and held for 1 h to remove the alcohol under supercritical conditions. After releasing the fluid, the remained alcohol wascompletely displaced by N,. The product obtained was a milk white semitransparent powder, which was designated as ZrO,( B). The 210,(B) powder was sulfated in the same way as ZrO,(A) to form SO,“-/ZrO,(B). X-ray powder diffraction measurements were perfotmed on a Rigaku D/MAX-IIA instrument with monochromatic
and Physics
50 (1991)
15-19
Cu Ka radiation, scan speed 16” mix-r-’ and scan range 5-70”. The specific surface area of the samples wasmeasured on a Micromeritics ASAP 2000 system under liquid N, temperature using N2 as the adsorbate. Transmission electron microscope (TEM) images of the samples were recorded on a H-700s TEM instrument. Chemical method was used for the detection of the sulfate content in the samples. The sulfate was turned into BaSO, and determined by gravimetric method. The acid strength of the samples was determined by a flow Hammett indicator method [8]. The superacidity and catalytic activity of the samples were characterized by measuring their activities forn-butane isomerization under static and flow conditions. In the former case, a static reactor system was used and the catalyst loading was 0.5 g. The amount of n-butane injected for each test was5 ml of gas (1.0X lo5 Pa, 293 K), and the reaction temperature was 308 K. In the latter case, a flow-type microreactor was used and the catalyst loading was 1.0 g. The reaction temperature was 423 K in N2 atmosphere and 523 Kin H2 atmosphere, and the weight hour space velocity (WHSV) ofbutane was 0.3 h-l. All the SO,“- /ZQ catalysts were pretreated in air at 723 K for 3 h before reaction. The reaction products were analyzed by an on-line gas chromatograph equipped with a flame ion detector.
3. Results and discussion
Fig. 1. TEM image of Z&,(B)
(original
magnification,
Fig. 2, TEM images of (a) ZrO,(B)
200 000 X ).
and (b) ZrO,(
The TEM image of ZrO,( B) supercritically dried at523 K is given in Fig. 1, showing that ultrafme crystallites with average size around 5 nm are formed. Fig. 2 gives the images of ZrO,(A) and ZrO,(B) after calcination at 923 K for 3 h.
A) after calcination
at 923 K (original
magnification,
100 000 X ).
C.X. Miao,
Fig. 3. TEM Table 1 Properties
2. Gao /Materials
images of (a) SOJ2-/ZrOZ(B)
Chemimy
and (b) S042- /ZrO,(A)
and Physics
17
50 (1997jmlS19
after calcination
at 923 K (original
magnification,
200 000 X )
of the samples
Sample
ZrOZ(B) ZrO,(A) SO,‘-/ZrOa(B) Sod’- /ZrO,(A)
Colour
milk white white white white
h-r
Average
Cm* g-‘)
Before calculation
After calculation
185.4 136.7 -
56.5 18.3 113.6 96.2
Ultrafine globular crystallites with average size of about 10 nm are formed in the case of ZrO,(B), while large agglomerates with average size of about 200 nm are formed from ZrO,( A). The TEM images of the sulfated samples after calcination at 923 K for 3 h are shown in Fig. 3. The suppression of crystallite growth and agglomeration in the presence of sulfate [ 91 is observed clearly. The average crystallite sizes of SOj2- /ZrO,( A) and SO4‘- /ZrO,( B) after calcination are only about 8 and 5 nm, respectively, which are obviously smaller than the crystalline sizes of ZrO,( A) and ZrO,( B) calcined at the same temperature. The BET surface area, average size and sulfur content of the samples calcined at 923 K were measured and listed in Table 1. Both S04’-/ZrO*(A) and S042-/ZrOl(B) take the form of ultrafine crystallites. However, S04’- /ZrO,( B) prepared from supercritically dried ZrO,(B) has smaller average crystallite size, larger BET surface area and higher sulfur content. XRD patterns of the non-sulfated and sulfated samples were recorded after calcination at different temperatures. At
673K, ZrO,(A) waspartiallycrystallizedintothetetragonal
size (nm)
so3 (wt.%)
10 200 5 8
3.6 3.3
and monoclinic structures, whereas ZrO,(B) was already partially crystallized into the tetragonal structure after supercritical drying at 533 K. Further heat treatment caused ZrO,(A) and ZrO,(B) to be more completely crystallized and to transform from the tetragonal phase to the monoclinic phase. The addition of sulfate retarded both the amorphousto-tetragonal crystallization and the tetragonal-to-monoclinic transformation of ZrO,( A) and ZrO,( B) . Heating to 923 K was required to fully crystallize the S04’-/Zr02(A) and S04’- /ZrO,( B) samples. All the XECD characteristic peaks of the crystalline ZrO,(B) and SO,*-/ZrO,(B) samples are lower and broader than those of the crystalline ZrO,( A) and Table 2 Acid strength Sample
S04’-/Zr02(B) S04*-/ZrOz(A) t
of the samples Ho - 12.7
- 13.2
- 13.8
- 14.5
- 16.0
t +
+ +
+ +
+ +
t +
= indicatorschangedcolour.
18
C.X. Mao,
Table 3 Reaction data for n-butane Sample
41.6
S042-/ZrOl(A)
40.1
Chemistry
and Physics 50 (1957)
15-19
isomerization
Static system k, (X 1O-1 h-l)
So,2-/Zro,(B)
Z. Gao/Materials
Flow system Reactant
Temperature (K)
n-C, Conversion
(%)
2min
1Omin
60 min
120 min
180 min
360 min
2400 min
NJC, =9 H,IC,=9 N2/C4 =9
423 523 423
20.9 39.5 20.0
10.3 34.3 10.4
6.8 18.8 2.1
5.7 18.1 -
4.8 16.7 -
4.2 15.2 -
14.6
H&=9
523
38.6
30.0
15.4
12.1
11.8
11.5
8.5
S0,2-/Zr0,(A) samples owing to their smaller average crystallite sizes. The crystallization of zirconia was reported by several authors [ 2-5,7, lo] to be necessary for the generation of superacidity. Although different structures of the active sulfate species on the surface of zirconia have been proposed [ 1l-141, it is generally believed that at higher temperatures after the crystallization of zirconia the sulfate ions are transformed to a more strongly covalent species with S=O bond orders close to two, and the electron withdrawing ability of these S=O bonds increases the acid strength of the zirconia surface. The acid strengths of SO,‘-/ZrO,(A) and SO,‘-/ ZrO,( B) after calcination at 923 K for 3 h were tested using a flow Hammett indicator method [S] and the results are shown in Table 2. Both samples are superacidic, i.e. their Ho is I - 16. In the previous literature [2,15], treatment of crystallized zrOz with sulfate ions was not effective in superacidity generation. Catalysts prepared by sulfating zirconia precursors having tetragonal and monoclinic crystal structures were inactive for n-butane isomerization. The success of this two-step synthesis method may be explained by the large difference in surface area between the ultrafine a;d ordinary crystalline zirconia. From Table 1 we can see that the supercritically dried ultrafine crystalline ZrO,(B) has a BET surface area of 185.4 m2 g- ‘, which is even higher than the value of 136.7 m2 g- ’ for the amorphous ZrO,(A). On the other hand, the surface area of a crystalline ZrO, prepared conventionally by calcining an amorphous zirconium hydroxide at high temperature is usually in the range of lo50 m* g-‘. The high surface area of the ultrafine crystallites undoubtedly facilitates the adsorption of sulfate ions and hence also the creation of superacidic sites on the oxide. The catalytic activities of Sob’-/ZrO,(A) and SO,‘- / ZrO,(B) solid superacid catalysts toward n-butane isomerization were tested under static and flow conditions. In our previous paper [ 161, it has been found that at 298-308 K under static reaction conditions the kinetics of n-butane isomerization on S04’- /ZrO, solid superacid catalysts follow the rate law of a first-order reversible reaction, and the rate constants of the catalysts correlate fairly well with the superacidity of the catalysts. Table 3 gives the rate constants of the two catalysts for n-butane isomerization reaction at 308 K. The rate constant kI of SO,‘- /ZrO,(B) is slightly higher
than that of S04’- /ZrO,(A) showing that there are probably more superacidic sites on the surface of S042-/Zr0,(B), which is consistent with the results of sulfur content measurements as seen in Table 1. Butane isomerization reaction on the two types of catalysts was also carried out in the presence of N, and H, at 423 and 523 K, respectively. Under these conditions, the isomerization selectivities of the catalysts were always above 95%. The reaction data are listed in Table 3. The initial activities of S0,2-/Zr0,(B) are again slightly higher than those of S0,2- /Zr02( A). All the catalysts deactivate quickly in the first hour of the reaction run owing to the formation of a carbonaceous deposit { 15,17,18 ] . In the presence of H2 the rate of deactivation is reduced, indicating that hydrogen inhibits the coke formation even in the absence of metals, such as Pt and Ni. T’he catalyst stability of SOb2- /ZrO,(B) is evidently higher than that of S04’- /ZrO,(A) . Since isomerization is a reaction less demanding of acid strength than coking, the increase in stability suggests that the two catalysts differ from each other not only in acid site concentration but also in acid site distribution.
4. Conclusions A two-step synthesis method for the preparation of ultrafine Sod2 - IZrO, superacid catalysts has been developed. Ultrafine crystalline ZrO, obtained by supercritical drying of a zirconia alcogel adsorbed sulfuric acid. After calcination at high temperature the adsorbed sulfate species transformed to an activated state with LO bond orders close to two, which increased the acid strength of the zirconia surface. The superacidity and the catalytic activity and stability for n-butane isomerization of the catalysts synthesized in this way are higher than those of S0,2-/Zr0, catalysts prepared by impregnating ordinary amorphous zirconia with sulfuric acid.
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