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Acidity of silica-substituted zirconia
Catalysis Today, 14 (1992) 189-194 Elsevier Science Publishers B.V., Amsterdam
189
Acidity of silica-substitutedzirconia S. Soled and 6. B. McVicker Exxon Research and EngineeringCompany, Route 22 East, Annandale, New
Jersey
08801
Abstract The acidity of zirconium-siliconoxides prepared by cogel and surfaceimpregnationtechniques are compared using 2-methylpent-2-eneisomerization as an acidity probe. Silicon substitutesinto zirconia over a wide stoichiometryrange. Generally, increasing silicon content increases the surface area and lowers the isoelectricpoint of the resulting zirconiumsilicon oxide phase. The relative number of acid sites obtained by comparing the rates of formation of the 3MP2 isomer increases with silicon content (up to -75X), whereas acid site density, as measured by the normalized rate, plateaus near "20% silicon addition and decreases at silicon loadings above '85%. Acid strengths,as measured by the ratio of methyl group to double bond shift, increaseswith increasingsilicon substitution peaking between 75-80% silicon addition. The acid strengths of zirconiumsilicon oxides bracket that of a typical reforming catalyst support, 0.9% Cl-A1203. Samples prepared by impregnatingsilica on the surface of zirconium hydroxide and calcining appear to have a higher relative surface concentrationof silica than bulk substitutedsamples.
1. INTRODUCTION Zirconia has the unusual ability to allow divalent, trivalent and tetravalentcation substitutions. For example, such transitionmetals as Fe, Co, Rh, Ru, Cu, and Cr can be substitutedinto zirconia (1,2). This property of zirconia results from its fluorite-relatedstructure allowing variable amounts of oxygen vacancies. Such vacancies are responsible for the high ionic conductivity of zirconia-basedsolid electrolyte materials (3). Zirconia is finding increasing use as a catalyst support, as for example, in CO hydrogenation (4) and n-paraffindehydrocyclization (5). Tanabe points out that zirconia possesses acidic, basic, oxidizing and reducing sites on its surface (6). Silicon substitutioninto divalent, trivalent and tetravalent oxides, such as magnesia, alumina, and titania will often enhance the acidity of the mixed oxide (7). The generation of high temperaturestability and enhanced acidity upon silicon substitution into the anatase polymorph of titania was recently described (8). Dzisko has shown that the acidity of silica gels increasesupon the addition of small amounts of zirconia (9). Yamaguchi et al. recently described zirconia microcrystals deposited on silica that retain an acid functionality,but lose their basic properties (10). Tanabe's empirical relationshipbetween the electronegativityof two component oxides in a
0920-5861/92/$05.00
0 1992 Elsevier Science Publishers B.V. All rights reserved.
mixed metal oxide suggests that zirconia-silica solid solutions would develop acidity (7). The present study explores differences between zirconia-silicacogel-derivedoxides and zirconia that has been surface-impregnatedwith silica. The iso~rization of Z-~thylpent-2-ene serves as a sensitivemodel compound reaction to monitor changes in the acid patterns of modified zirconia compounds (11). 2. EXPERINENTAL Bulk zirconia-silicasamples were prepared by hydrolyzing zirconium n-propoxide/tetramethyl orthosilicatemixtures with water (4 moles of water/mole (Zr+Si)) under constant stirring at room temperature. For the Zr,OSSi.g502and Si02 samples, the alkoxide was hydrolyzedwith a water/n-propanolmixture with a 4/1/l water/n-propanol/(Zr +Si) molar ratio. After thorough washing with warm water, the samples were calcined at 5OO'C for 3 hours. Surface impregnatedsamples were prepared by depositing silica onto a zirconium zirconium hydroxide surface and calcining to convert the hydroxide to zirconium oxide. Zirconium hydroxidewas precipitated by adding sufficient ammonium hydroxide to an aqueous solution of zirconyl nitrate to attain a pH of 10. The precipitatewas filtered,washed with a dilute NH40H solution (at pH 10) to remove residual chloride, and dried at 110°C overnight. The silica was impregnatedonto the zirconium hydroxide to the point of incipient wetness with a tetraethylorthosili~atesolution in ethanol, and then placed along with a separate beaker of water in a closed jar for 48 hours to promote hydrolysis. Finally, the sample was calcined at 5OO'C for three hours. The Si surface loadings were calculated on an atomic percent basis, so that both the bulk and surface modified samples contain the same molar ratio of Si to Zr at the same nominal loading. X-ray diffractionspectra were used to identify the phases present. The 2-~thylpent-2-ene (2MP2) iso~rization test was carried out as described previously (11). Zeta potentialmeasurementswere made on a Matec 8050 electrokinetic instrument at particle suspensions of less than 1 volume percent. Sufficient 1N NaOH was initiallyadded to the suspensions to raise the pH to between 7 and 9 and the electrokineticparameters were measured during titrationwith 1N HCl. 3. RESULTS AN8 DISCUSSION the sunmnarizes compositions and surface areas of Table 1 zirconia-silicasamples, and the results are shown in figure 1. For pure Zr02 as well as for 1% silicon surface or bulk loadings, the monoclinic Zr02 edification is formed. For surface impregnatedsamples, a tetragonal Zr82 phase is stabilizedat a silicon concentrationof 5% and above. With bulk-substitution,the 5% substituted sample also appears tetragonal, but an x-ray amorphous phase forms at higher loadings. Several of the samples were calcined for an additionalsixteen hours at 5000C with no change in the observed phase. Surface areas of both the bulk and surface modified zirconium-siliconoxides increase with silicon levels. Surface areas of the zirconium-siliconoxides are essentiallyindependentof the preparation ~thod.
191
Table 1: Compositionsand surface areas of zirconia-silicacatalysts Composition
Surface Area(m2/gm)a
Phaseb
Bulk Substitutions Zr02
19
m
Zr.ggSi.0102
48
In
Zr.ggSi.0502
66
t
Zr.85Si.1502
128
a
Zr.75Si.2502
148
a
Zr.5Si.502
210
a
Zr.25Si.7502
346
a
Zr.15Si.8502
384
a
Zr.05Si.9502 Si02
324
a
558
a
Zr(OH)4 Zr(OH)q/lX Si
23
In
43
Ill
Zr(OH)4/5% Si
85
t
Zr(OH)4/15%Si
121
t
Zr(OH)4/25%Si
166
t
Surface Imoresnations
a All compositionscalcined at 500X, 3 hours b m: monoclinic; t:tetragonal;a: amorphous The isoelectricpoint of the host zirconia decreases with the substitution of silica as would be expected (12). Figure 2 illustrates the decrease in the isoelectric point upon substitutionof 25 and 75 percent silicon into Zr02. The effect of silicon substitutionon the acidity of the mixed oxides isomerization patterns of a was characterized by examining the 2-methylpent-2-ene(2MP2) feed. 2MP2 isomerizationis a useful probe reaction since the formation rates and rate ratios of the product hexene isomers reflect the acid site concentrationand strength. For example, weak solid acids easily catalyze the double bond migration during conversion of 2MP2 to 4-methylpent-2-ene(4MP2). Formationof 3-methylpent-2-ene (3MP2), which involves a skeletal methyl group shift, requires considerably higher acidity. For a homologous series of solid acids, differences in 3MP2 rates normalizedwith respect to surface area reflect the density of acid sites possessing strengths sufficient to catalyze the skeletal isomerization. Since skeletal isomerizationrates generally increase with
192
increasingacid strength,the ratio of methyl group migration rate to double bond shift rate should increasewith increasingacid strength. The use of rate ratios (e.g. 3MP2/4HP2) instead of individualconversion rates as a diagnostic probe of relative acid strengths is preferred since differences in acid site populationsare normalized. Using the above guidelines the 2MP2 isomerization pattern for zirconia-silica prepared by both bulk substitutionand surface impregnationprocedureswere evaluated. Reproducibility of test results was within 5%.
600 3004
e. zeta potentii
m”fg
I 1 +
bulk subrtitutiOn 0
&w‘aoB
400
/
i
I
I
60 1
(mvl
!
;
i
*
J
modlfloatlon *
300
~~
200
I
IOOo-
I 0%
, 20%
40%
60%
30%
100%
atomic percent silicon
Figure 1. Surface Areas of SilicaModified Zirconia
z
;)
D
,
0
*
PH
Figure 2. Zeta Potential Measurementof Silica-Substituted Zirconias
The curves presented in Figure 3 show that the isomerizationrates of 2MP2 to 3MP2 at a reaction temperaturesof 25O'C increase smoothly up to silicon contents of * 75-80% and and then fall off for the highest silicon substitutions. Figure 4 shows that the rate normalizedto surface area on the bulk substituted samples begins to reach a plateau at "20% silicon addition, and remains constant until decreasing at the highest silicon substitution. At the highest silicon concentrations, silicon-silicon interactionspredominate and the acid sites decrease. Furthermore, the curves shown in Figures 3 and 4 indicatethat the number and density of acid sites generated in zirconia-silicacompositionsis slightly higher on the surface-impregnatedsamples at equal silicon loadings. One would expect some silicon redistribution as the zirconium hydroxide rearranges to the oxide, but it appears that the cogel samples are more completely mixed, whereas the surface impregnated samples have a higher density of silicon near the surface. Changes in the relative acid strengths,as judged by the 3~P2/4MP2 rate ratio, for a series of zirconia-silicasamples prepared by both bulk substitutionand surface impregnation techniquesare summarized in Figure 5. The curves show that the acid strengthsof the zirconia-silicacompositions smoothly increase with increasingsilicon content up to 75-80% silicon and then decreases at higher silicon loading. The corresponding 3MP2/4MP2 ratio for a O.g% Cl/Al203 catalyst is included in Figure 5 for comparison. Thus zirconia-silica catalysts can be prepared with acidities
193
bracketingthat of chlorided-alumina. These strong acidities are consistent with the electronegativitydifferencesof Zr (1.5) and Si (1.8) (13).
2,6t-3MP2
formation
20%
0%
rate (mol/hr/gm)
602
4on
x 1000
100%
8Oli
OU
20%
t-2MP2
feed: 2soc; 1 hour into run
t-2MP2
Figure 3. t-3MP2 Rate: Bulk Substitutedvs. Surface-Modified ZrOp-SiO2
0%
202
40%
60%
802
100%
atomic percent silicon 2MP2
feed;
25OC,
40%
601
8Oli
1oou
atomic percent silicon
atomic percent silicon
1 hr into run
Figure 5. 3MP2/4MP2 Ratio for Bulk vs. Surface ImpregnatedZrOz-SiO2
feed: 26OC;
1 hour into run
Figure 4. t-3MP2 Normalized Rate: Bulk Substitutionvs SurfaceModified ZrOp-SiOp
194
The authors wish to thank Sal Miseo, John Ziemiak, Lenny Yacullo, and Joe Scanlon for their help in this study. 5. REFERENCES 1 2 3 4 5 6 7 8
Y.C. Zhang, R. Kershaw, K. Dwight, and A. Wold, 3. Sol. St. Chem. 72 (1988) 131-6. P. Wu, R. Kershaw, K. Dwight, and A. Wold, J. Mat. Sci. Lett. 6 (1987) 753. A.R. West, Solid State Chemistry and its Applications, John Wiley, Chichester and New York, 1984. T. Iizuka, Y. Tanaka, and K. Tanabe, J. Catal. 76 (1982) 1. M. Hino and K. Arata, J. Chem. Sot., Chem. Commun. (1987) 1355. K. Tanabe, Mater. Chem and Phys., 13 (1985) 347. K. Tanabe, Solid Acid and Base Catalysts, in Catalysis, Science and Technology, J.R. Anderson and M. Boudart (ed.), Springer-Verlag,Berlin, Heidelberg and New York, 2, 231,
(1981). 9 S. Soled, and G.B. McVicker, Prepr. ACS, Div of Petr.
Chem. 34(3) (1989) 645. 10 V.A. Dzisko, Proc. 3rd Intl. Congr. Catal. 1 (1964) 422. 11 T. Yamaguchi, T. Morita, T. Salama, and K. Tanabe, Catal. Lettr. 4, (1990) l-6. 12 G.M. Kramer, and G.B. McVicker, Acct. Chem. Res. 19 (1986) 78. 13 G.A. Parks, Chem. Rev. 65, (1965) 177. 14 W.B. Pearson, Crystal Chemistry and Physics of Metals and Alloys, Wiley-Interscience,N.Y. p. 70 (1972).
189
Acidity of silica-substitutedzirconia S. Soled and 6. B. McVicker Exxon Research and EngineeringCompany, Route 22 East, Annandale, New
Jersey
08801
Abstract The acidity of zirconium-siliconoxides prepared by cogel and surfaceimpregnationtechniques are compared using 2-methylpent-2-eneisomerization as an acidity probe. Silicon substitutesinto zirconia over a wide stoichiometryrange. Generally, increasing silicon content increases the surface area and lowers the isoelectricpoint of the resulting zirconiumsilicon oxide phase. The relative number of acid sites obtained by comparing the rates of formation of the 3MP2 isomer increases with silicon content (up to -75X), whereas acid site density, as measured by the normalized rate, plateaus near "20% silicon addition and decreases at silicon loadings above '85%. Acid strengths,as measured by the ratio of methyl group to double bond shift, increaseswith increasingsilicon substitution peaking between 75-80% silicon addition. The acid strengths of zirconiumsilicon oxides bracket that of a typical reforming catalyst support, 0.9% Cl-A1203. Samples prepared by impregnatingsilica on the surface of zirconium hydroxide and calcining appear to have a higher relative surface concentrationof silica than bulk substitutedsamples.
1. INTRODUCTION Zirconia has the unusual ability to allow divalent, trivalent and tetravalentcation substitutions. For example, such transitionmetals as Fe, Co, Rh, Ru, Cu, and Cr can be substitutedinto zirconia (1,2). This property of zirconia results from its fluorite-relatedstructure allowing variable amounts of oxygen vacancies. Such vacancies are responsible for the high ionic conductivity of zirconia-basedsolid electrolyte materials (3). Zirconia is finding increasing use as a catalyst support, as for example, in CO hydrogenation (4) and n-paraffindehydrocyclization (5). Tanabe points out that zirconia possesses acidic, basic, oxidizing and reducing sites on its surface (6). Silicon substitutioninto divalent, trivalent and tetravalent oxides, such as magnesia, alumina, and titania will often enhance the acidity of the mixed oxide (7). The generation of high temperaturestability and enhanced acidity upon silicon substitution into the anatase polymorph of titania was recently described (8). Dzisko has shown that the acidity of silica gels increasesupon the addition of small amounts of zirconia (9). Yamaguchi et al. recently described zirconia microcrystals deposited on silica that retain an acid functionality,but lose their basic properties (10). Tanabe's empirical relationshipbetween the electronegativityof two component oxides in a
0920-5861/92/$05.00
0 1992 Elsevier Science Publishers B.V. All rights reserved.
mixed metal oxide suggests that zirconia-silica solid solutions would develop acidity (7). The present study explores differences between zirconia-silicacogel-derivedoxides and zirconia that has been surface-impregnatedwith silica. The iso~rization of Z-~thylpent-2-ene serves as a sensitivemodel compound reaction to monitor changes in the acid patterns of modified zirconia compounds (11). 2. EXPERINENTAL Bulk zirconia-silicasamples were prepared by hydrolyzing zirconium n-propoxide/tetramethyl orthosilicatemixtures with water (4 moles of water/mole (Zr+Si)) under constant stirring at room temperature. For the Zr,OSSi.g502and Si02 samples, the alkoxide was hydrolyzedwith a water/n-propanolmixture with a 4/1/l water/n-propanol/(Zr +Si) molar ratio. After thorough washing with warm water, the samples were calcined at 5OO'C for 3 hours. Surface impregnatedsamples were prepared by depositing silica onto a zirconium zirconium hydroxide surface and calcining to convert the hydroxide to zirconium oxide. Zirconium hydroxidewas precipitated by adding sufficient ammonium hydroxide to an aqueous solution of zirconyl nitrate to attain a pH of 10. The precipitatewas filtered,washed with a dilute NH40H solution (at pH 10) to remove residual chloride, and dried at 110°C overnight. The silica was impregnatedonto the zirconium hydroxide to the point of incipient wetness with a tetraethylorthosili~atesolution in ethanol, and then placed along with a separate beaker of water in a closed jar for 48 hours to promote hydrolysis. Finally, the sample was calcined at 5OO'C for three hours. The Si surface loadings were calculated on an atomic percent basis, so that both the bulk and surface modified samples contain the same molar ratio of Si to Zr at the same nominal loading. X-ray diffractionspectra were used to identify the phases present. The 2-~thylpent-2-ene (2MP2) iso~rization test was carried out as described previously (11). Zeta potentialmeasurementswere made on a Matec 8050 electrokinetic instrument at particle suspensions of less than 1 volume percent. Sufficient 1N NaOH was initiallyadded to the suspensions to raise the pH to between 7 and 9 and the electrokineticparameters were measured during titrationwith 1N HCl. 3. RESULTS AN8 DISCUSSION the sunmnarizes compositions and surface areas of Table 1 zirconia-silicasamples, and the results are shown in figure 1. For pure Zr02 as well as for 1% silicon surface or bulk loadings, the monoclinic Zr02 edification is formed. For surface impregnatedsamples, a tetragonal Zr82 phase is stabilizedat a silicon concentrationof 5% and above. With bulk-substitution,the 5% substituted sample also appears tetragonal, but an x-ray amorphous phase forms at higher loadings. Several of the samples were calcined for an additionalsixteen hours at 5000C with no change in the observed phase. Surface areas of both the bulk and surface modified zirconium-siliconoxides increase with silicon levels. Surface areas of the zirconium-siliconoxides are essentiallyindependentof the preparation ~thod.
191
Table 1: Compositionsand surface areas of zirconia-silicacatalysts Composition
Surface Area(m2/gm)a
Phaseb
Bulk Substitutions Zr02
19
m
Zr.ggSi.0102
48
In
Zr.ggSi.0502
66
t
Zr.85Si.1502
128
a
Zr.75Si.2502
148
a
Zr.5Si.502
210
a
Zr.25Si.7502
346
a
Zr.15Si.8502
384
a
Zr.05Si.9502 Si02
324
a
558
a
Zr(OH)4 Zr(OH)q/lX Si
23
In
43
Ill
Zr(OH)4/5% Si
85
t
Zr(OH)4/15%Si
121
t
Zr(OH)4/25%Si
166
t
Surface Imoresnations
a All compositionscalcined at 500X, 3 hours b m: monoclinic; t:tetragonal;a: amorphous The isoelectricpoint of the host zirconia decreases with the substitution of silica as would be expected (12). Figure 2 illustrates the decrease in the isoelectric point upon substitutionof 25 and 75 percent silicon into Zr02. The effect of silicon substitutionon the acidity of the mixed oxides isomerization patterns of a was characterized by examining the 2-methylpent-2-ene(2MP2) feed. 2MP2 isomerizationis a useful probe reaction since the formation rates and rate ratios of the product hexene isomers reflect the acid site concentrationand strength. For example, weak solid acids easily catalyze the double bond migration during conversion of 2MP2 to 4-methylpent-2-ene(4MP2). Formationof 3-methylpent-2-ene (3MP2), which involves a skeletal methyl group shift, requires considerably higher acidity. For a homologous series of solid acids, differences in 3MP2 rates normalizedwith respect to surface area reflect the density of acid sites possessing strengths sufficient to catalyze the skeletal isomerization. Since skeletal isomerizationrates generally increase with
192
increasingacid strength,the ratio of methyl group migration rate to double bond shift rate should increasewith increasingacid strength. The use of rate ratios (e.g. 3MP2/4HP2) instead of individualconversion rates as a diagnostic probe of relative acid strengths is preferred since differences in acid site populationsare normalized. Using the above guidelines the 2MP2 isomerization pattern for zirconia-silica prepared by both bulk substitutionand surface impregnationprocedureswere evaluated. Reproducibility of test results was within 5%.
600 3004
e. zeta potentii
m”fg
I 1 +
bulk subrtitutiOn 0
&w‘aoB
400
/
i
I
I
60 1
(mvl
!
;
i
*
J
modlfloatlon *
300
~~
200
I
IOOo-
I 0%
, 20%
40%
60%
30%
100%
atomic percent silicon
Figure 1. Surface Areas of SilicaModified Zirconia
z
;)
D
,
0
*
PH
Figure 2. Zeta Potential Measurementof Silica-Substituted Zirconias
The curves presented in Figure 3 show that the isomerizationrates of 2MP2 to 3MP2 at a reaction temperaturesof 25O'C increase smoothly up to silicon contents of * 75-80% and and then fall off for the highest silicon substitutions. Figure 4 shows that the rate normalizedto surface area on the bulk substituted samples begins to reach a plateau at "20% silicon addition, and remains constant until decreasing at the highest silicon substitution. At the highest silicon concentrations, silicon-silicon interactionspredominate and the acid sites decrease. Furthermore, the curves shown in Figures 3 and 4 indicatethat the number and density of acid sites generated in zirconia-silicacompositionsis slightly higher on the surface-impregnatedsamples at equal silicon loadings. One would expect some silicon redistribution as the zirconium hydroxide rearranges to the oxide, but it appears that the cogel samples are more completely mixed, whereas the surface impregnated samples have a higher density of silicon near the surface. Changes in the relative acid strengths,as judged by the 3~P2/4MP2 rate ratio, for a series of zirconia-silicasamples prepared by both bulk substitutionand surface impregnation techniquesare summarized in Figure 5. The curves show that the acid strengthsof the zirconia-silicacompositions smoothly increase with increasingsilicon content up to 75-80% silicon and then decreases at higher silicon loading. The corresponding 3MP2/4MP2 ratio for a O.g% Cl/Al203 catalyst is included in Figure 5 for comparison. Thus zirconia-silica catalysts can be prepared with acidities
193
bracketingthat of chlorided-alumina. These strong acidities are consistent with the electronegativitydifferencesof Zr (1.5) and Si (1.8) (13).
2,6t-3MP2
formation
20%
0%
rate (mol/hr/gm)
602
4on
x 1000
100%
8Oli
OU
20%
t-2MP2
feed: 2soc; 1 hour into run
t-2MP2
Figure 3. t-3MP2 Rate: Bulk Substitutedvs. Surface-Modified ZrOp-SiO2
0%
202
40%
60%
802
100%
atomic percent silicon 2MP2
feed;
25OC,
40%
601
8Oli
1oou
atomic percent silicon
atomic percent silicon
1 hr into run
Figure 5. 3MP2/4MP2 Ratio for Bulk vs. Surface ImpregnatedZrOz-SiO2
feed: 26OC;
1 hour into run
Figure 4. t-3MP2 Normalized Rate: Bulk Substitutionvs SurfaceModified ZrOp-SiOp
194
The authors wish to thank Sal Miseo, John Ziemiak, Lenny Yacullo, and Joe Scanlon for their help in this study. 5. REFERENCES 1 2 3 4 5 6 7 8
Y.C. Zhang, R. Kershaw, K. Dwight, and A. Wold, 3. Sol. St. Chem. 72 (1988) 131-6. P. Wu, R. Kershaw, K. Dwight, and A. Wold, J. Mat. Sci. Lett. 6 (1987) 753. A.R. West, Solid State Chemistry and its Applications, John Wiley, Chichester and New York, 1984. T. Iizuka, Y. Tanaka, and K. Tanabe, J. Catal. 76 (1982) 1. M. Hino and K. Arata, J. Chem. Sot., Chem. Commun. (1987) 1355. K. Tanabe, Mater. Chem and Phys., 13 (1985) 347. K. Tanabe, Solid Acid and Base Catalysts, in Catalysis, Science and Technology, J.R. Anderson and M. Boudart (ed.), Springer-Verlag,Berlin, Heidelberg and New York, 2, 231,
(1981). 9 S. Soled, and G.B. McVicker, Prepr. ACS, Div of Petr.
Chem. 34(3) (1989) 645. 10 V.A. Dzisko, Proc. 3rd Intl. Congr. Catal. 1 (1964) 422. 11 T. Yamaguchi, T. Morita, T. Salama, and K. Tanabe, Catal. Lettr. 4, (1990) l-6. 12 G.M. Kramer, and G.B. McVicker, Acct. Chem. Res. 19 (1986) 78. 13 G.A. Parks, Chem. Rev. 65, (1965) 177. 14 W.B. Pearson, Crystal Chemistry and Physics of Metals and Alloys, Wiley-Interscience,N.Y. p. 70 (1972).