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SO42−ZrO2 by incipient wetness impregnation: Influence of the sulfur concentration in the isomerization of n-butane
ELSEVIER
Applied Catalysis A: General 129 (1995) L151-L156
Letter
Preparation of Pt / SO: - -ZrO, by incipient wetness impregnation: Influence of the sulfur concentration in the isomerization of n-butane J.C. Yori, J.M. Parera * Institute de Investigaciones en Catdlisis y Petroquhica -INCAPE- Santiago de1 Ester0 2654, 3000 Santa Fe, Argentina Received 22 May 1995; revised 20 June 1995; accepted 20 June 1995
Abstract Pt/SOi--ZrO, was prepared by incipient wetness coimpregnation of platinum and SO:- on Zr( OH),. An active catalyst for the isomerization of n-butane was produced using 1 M H2S04. The sites responsible for the isomerization activity were those formed up to a sulfur concentration of 0.8 atom X nm- *. Keywords: n-Butane isomerization;
Platinum on sulfated zirconia; Sulphur concentration;
Incipient wetness
1. Introduction It has been found that the incorporation of sulfate ion onto ZrO, produced a great enhancement on its acidity [ 1,2] and on the capacity for activation of alkanes, particularly for isomerization. The additional incorporation of a metallic phase avoids the fast deactivation due to coking [ 3,4]. Ebitani et al. [ 4] suppose that the strong interaction with the support produces the loss of the typical properties of the metallic function. Other authors [ $61 suggest that platinum is in the reduced state, but covered with a sulfur layer, which is the cause of the inhibition of the dehydrogenating properties. Another topic in discussion is related to the nature and the origin of the acidity in SO:--ZrO,; Lewis and Bronsted sites are held responsible for the catalytic activity. The influence of the concentration of sulfur is also * Corresponding
author. Tel. ( + 54-42) 33858, Fax ( +54-42)
0926-860X/95/$09.50 SSDIO926-860X(95)00160-3
31068 and ( +54-42)
550944.
L152
J.C. Yori, JM. Parero/Applied
Catn1wi.r A: Gmercrl 129 (1995) L151-L156
in discussion [7-l 11. Besides, there are doubts related to the capacity of the catalyst to retain sulfur. Some authors point out that part of the activity drop could be due to a loss of sulfur [ 121, but Keogh et al. [ 131 do not find sulfur loss after a long period on stream for Pt/SO;--ZrO, fed with n-hexadecane. The most common method of preparation of this catalyst is by the percolation or dipping of Zr(OH), in a 0.5 M H2S04 solution, drying and impregnation with a H,Cl,Pt solution. This method needs considerable time, costly operations and has the danger of mutual displacement of H2S04 and H,Cl,Pt to the impregnating solution. In this work we use a different technique to incorporate together platinum and SOi- into Zr( OH),, the incipient wetness impregnation. This technique enables us to obtain materials with different SO:- concentrations by using different concentrations of acid in the impregnation. With these catalysts we try to clarify the role of the concentration of sulfur in the catalytic properties of Pt/SO i=ZrO,.
2. Experimental 2.1. Catalysts Zr( OH), was obtained by a technique already described [ 141. Even the addition of ammonia to zirconyl chloride produces the precipitation of a non-stoichiometric hydrated zirconium oxide, which we conventionally designate as Zr(OH),. The impregnating sulfate-platinum solution was prepared by dissolving an adequate amount of H,Cl,Pt . 6H20 (to obtain 0.25% Pt in the final catalyst) in a volume of H2S04 corresponding to the pore volume of Zr( OH), (0.4 ml/g) plus an excess of 10%. The concentration of the acid was varied up to 2.5 M, and the solids A-F were produced. The impregnating time was 3 h, then the solids were dried overnight at 110°C ground and sieved, and the 35-80 mesh fraction was used. After being loaded in the reactor for the catalytic test, the solid was calcined at 620°C for 3 h and then reduced in a hydrogen stream at 300°C for 1 h. Catalyst sulfur content was analyzed by combustion using a LECO CS-244 analyzer. 2.2. Materials H2S04 and H,Cl,Pt . 6H20 were Carlo Erba RPE. The gases were Matheson normal butane (99.5%), and pure air and hydrogen provided by AGA. 2.3. Catalytic test n-C, isomerization was used as a test reaction. It was performed at atmospheric pressure using a quartz fixed bed reactor of 0.39 in. internal diameter operated under isothermal conditions. The conditions of the catalytic test were: mass of
J.C. Yori, J.M. Parers/Applied
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Caialysis A: General 129 (1995) LlSl-L156
catalyst 0.5 g, temperature = 300°C WHSV = 1 h ~ ’ and molar ratio HJn-C, = 6. The test lasted four hours. The reaction products were analyzed using an on-line chromatograph with a FID detector and a 6 m long, l/8 in. diameter column packed with 25% dimethylsulfolane on chromosorb P. The n-C, conversion and the selectivity to i-C, were calculated from the chromatographic analysis.
3. Results and discussion Table 1 shows the properties of the catalysts after the catalytic test. As was increases when increasing the expected, the sulfate content in Pt/SOi--ZrO, H,SO, concentration used in the impregnation, reaching a content of about 3.8% for 2.5 M H2S04. This catalyst has a specific surface area of 90 m’/g, which means a sulfur content of 2.7 atom per nm*. Our range of sulfate concentrations is similar Table I Properties of the catalyst samples after the catalytic test Catalyst
A (H,SO, B (H,SO, C (H,SO, D (HaSO, E (H$O, F (H>SO,
0.0 M) 0.25 M) 0.5 M) l.OM) 1.5 M) 2.5 M)
SO:(% W./W.)
Pt (% wt./wt.)
Sg (BET) (m/g)
0.00 0.35 0.60 1.24 2.70 3.80
0.25 0.25 0.25 0.25 0.25 0.25
30 65 17 91 89 90
t-&SO,
CONCENTRATION,
N
Fig. 1. Sulfur content of the catalyst samples as a function of H,SO., concentration of the impregnating solution. Curve 1,total sulfur introduced by incipient wetness impregnation. 2, sulfur aftercalcination at 620°C and reduction at 300°C. 3, sulfur after the catalytic test at 300°C.
L154
J.C. Yori, J.M. Parera /Applied
0
0.5 SULFUR
Catalysis A: General 129 (1995) L151-L156
10
I 1.5
CONCENTRATION,
I 20
’ 0 25
at/nId
Fig. 2. Conversion of n-C, and selectivity to i-C, as a function of catalyst sulfur content. Test conditions: 3OO”C, WHSV = I (g n-&/g cat. h), molar ratio H,/n-C, = 6, atmospheric pressure. A, B, C, D, E and F, catalysts of Table 1.
to the one obtained in the references using the most common preparation method, percolation or dipping in an excess of 0.5 M H2S04 solution. Table 1 shows that an increase in the HISO concentration produces an increase in the specific surface area until 1 M H2S04. Greater concentrations do not modify the specific surface area of the samples, which is about 90 m*/g. This surface is lower than those obtained using the methods of percolation or impregnation with excess H2S04 solution, whose values ranged 100 m2/g [3] to 114 m*/g [4]. The sulfur contents expressed per gram of Zr02 obtained for each H2S04 concentration are plotted in Fig. 1 as a function of this concentration for the following cases: (a) total amount incorporated during the incipient wetness impregnation; (b) remaining amount after the pretreatments (calcination and reduction) ; (c) remaining amount after the pretreatments and reaction. In all cases an important loss of sulfur during the calcination and reduction is observed while no or little loss is observed during the reaction. The conversion of n-C4 and the selectivity to i-C4 as a function of the concentration of surface sulfur atoms are plotted in Fig. 2. The conversion and selectivity values shown for each catalyst were constant throughout all the run. Zirconia possesses weak acid and basic properties [ 151 and has no capacity for n-C, isomerization (catal. A). The addition of SOi- promotes its acidity and isomerization activity. Two zones can be distinguished in the conversion curve: (a) For sulfur to concentrations of concentration lower than 1 atom X nrn- * (corresponding impregnating H2S04 solutions smaller than 1 M) the activity of the catalyst increases proportionally to the surface concentration of sulfur. (b) For higher sulfur concentrations, both activity and selectivity remain constant when the concentration
J.C. Yori, J.M. Parers/Applied
Catalysis A: General 129 (1995) L151-L156
L155
of sulfur is increased. According to Sarzanini et al. [ 161, we consider that all sulfates are at the surface of sulfated ZrO,. The following observations can be made: (i) From a practical point of view, a 1 M concentration of sulfuric acid is convenient in the preparation of Pt/SOi--ZrO, by incipient wetness coimpregnation of platinum and SO;-. (ii) There exists a limit of 1 S atom X nme2 in the concentration of active species with capacity for the isomerization of n-C,. Above this value, the sites generated do not seem to have isomerization capacity and should be in excess. There are different opinions regarding the nature and origin of the acid sites of SOi--ZrO,. Jin et al. [ 71 reported the existence of only a high Lewis acidity, while Komarov et al. [ 81 and Nascimento et al. [ 93 informed about the presence of both Lewis and Bronsted acidity. Bolis et al. [ lo] postulated the existence of two types of Lewis sites, the most abundant having low acid strength. Morterra et al. [ 1 l] found the existence of three types of surface sulfate in SO:--ZrO,, which were a function of the degree of surface coverage: (a) Species formed at low coverage values (lower than 0.8 S atom X nme2), which correspond to isolated surface SO:- groups located in crystallographic defective configurations (side terminations) ; these species originate from Lewis acid sites. (b) Species similar to the previous ones, formed up to coverage values near to 2 S atoms X nmp2, which correspond roughly to the completion of a half-monolayer. These species are located on the regular patches of low index crystal planes (top terminations of the scale-like particles); they produce Lewis and some Bronsted acid sites. (c) When the sulfur concentration becomes higher than an average half-monolayer, polynuclear surface sulfates appear, probably of the pyrosulfates ( S20;- ) type which are also mainly located on the regular patches of low-index crystal planes (top terminations), and originate from Bronsted sites. The formation of this pyrosulfate type species does not affect the species previously formed at coverages lower than 0.8 S atom X nmp2. An interpretation of Fig. 2 can be done taking into account the previous discussion. The concentration of sulfur controls the catalytic activity until all the available surface configurations that are able to produce isolated surface SO;- groups are occupied. This occupation generates Lewis type acidity and corresponds to a sulfur concentration of about 0.8 S atom X nmp2. The sites formed at higher coverages do not have capacity for isomerizing n-C,. It can be concluded that the active sites in n-C4 isomerization are Lewis ones, in agreement with the statement of Comelli et al. [ 121. These authors conclude that the Lewis sites are the catalytically active ones because upon hydration of the catalyst a drop in activity is produced which is recovered after calcination at 620°C. Similarly, Keogh et al. [ 171 demonstrated that the active sites for the conversion of n-hexadecane are not of the Bronsted type, because water has a poisoning effect.
J.C. Yori, J.M. Pam-a/Applied
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Catalysis A: General 129 (1995) LISI-L156
The low activity of sample B may be due to the fact that part of the acid added is initially consumed for the neutralization of the basicity of ZrO,. This sample shows a very low selectivity to i-C4, propane and ethane being mainly produced. These products come from disproportionation and cracking reactions, more demanding of acid strength. This would indicate that not all the active sites are energetically equal, and that the surface sulfate groups are located in the solid occupying sites of decreasing energy. When the concentration of SOi- is increased between 0.35 and 1.24% (samples B-D) the catalytic activity is proportionally increased with a slope of 0.014 reacted n-C, molecules per second and sulfur atom.
4. Conclusions The incipient wetness coimpregnation of Zr( OH), with H,S04 and H,Cl,Pt is a practical method of preparing Pt/SOz--ZrO,. The specific surface area and the catalytic activity increase with the increase in H2S04 concentration up to 1 M. For higher concentrations both properties remain constant. The sulfated Zr02 surface is heterogeneous. The isolated sulfate groups formed at sulfur concentrations lower than 0.8 S atomXnm-* are catalytically active for n-C, isomerization. The sites generated at higher concentrations are not active and do not affect the activity of the pre-existent sites.
References [ 1] [Z] [3] [4] [5] [6] [7] [ 81 191
[ IO] [ 1I] [ 121 [ 131 [ 141 [ 151 [ 161 [ 171
M. Hino and K. Arata, J. Chem. Sot. Chem. Commun., ( 1979) 1148. M. Hino and K. Arata, J. Chem. Sot., Chem. Commun., ( 1980) 851. J. Yori, M. D’Amato, G. Costa and J. Parera, J. Catal., 153 (1995) 218. K. Ebitani, J. Konishi and H. Hattori, J. Catal.. 130 ( 1991) 257. Z. Paal, M. Muhler and R. Schlogl, J. Catal., 143 ( 1993) 321. A. Dicko, X. Song, A. Adnot and A. Sayari, J. Catal., 150 ( 1994) 254. T. Jin, T. Yamaguchi and K. Tanabe, J. Phys. Chem., 90 ( 1986) 4794. V. Komarov and M. Sinilo, Kinet. Catal., 29 ( 1988) 701. P. Nascimento, K. Akratopoulou, M. Oszagyan, G. Coudurier, C. Travers, J. Joly and J. Vedrine, Stud. Surf. Sci. Catal., 75 (1993) 1185. V. Bolis, C. Morterra, M. Volante, L. Orio and B. Fubini, Langmuir, 6 ( 1990) 695. C. Morterra, G. Cerrato and V. Bolis, Catal. Today, 17 ( 1993) 505. R. Comelli, C. Vera and J. Parera, J. Catal., 151 ( 1995) 96. R. Keogh, D. Sparks and B. Davis, Stud. Surf. Sci. Catal., 88 ( 1994) 647. J. Yori, J. Luy and J. Parera, Appt. Catal., 46 ( 1989) 103. Z. Feng, W. Postula, A. Akgennan and R. Anthony, Ind. Eng. Chem. Res., 34 ( 1995) 78. C. Sarzanini, G. Sachero, F. Pinna, M. Signoretto, G. Cerrato and C. Morterra, J. Mater. Chem., 5 ( 1995) 353. R. Keogh, R. Srinivasan and B. Davis, J. Catal., 151 ( 1995) 292.
Applied Catalysis A: General 129 (1995) L151-L156
Letter
Preparation of Pt / SO: - -ZrO, by incipient wetness impregnation: Influence of the sulfur concentration in the isomerization of n-butane J.C. Yori, J.M. Parera * Institute de Investigaciones en Catdlisis y Petroquhica -INCAPE- Santiago de1 Ester0 2654, 3000 Santa Fe, Argentina Received 22 May 1995; revised 20 June 1995; accepted 20 June 1995
Abstract Pt/SOi--ZrO, was prepared by incipient wetness coimpregnation of platinum and SO:- on Zr( OH),. An active catalyst for the isomerization of n-butane was produced using 1 M H2S04. The sites responsible for the isomerization activity were those formed up to a sulfur concentration of 0.8 atom X nm- *. Keywords: n-Butane isomerization;
Platinum on sulfated zirconia; Sulphur concentration;
Incipient wetness
1. Introduction It has been found that the incorporation of sulfate ion onto ZrO, produced a great enhancement on its acidity [ 1,2] and on the capacity for activation of alkanes, particularly for isomerization. The additional incorporation of a metallic phase avoids the fast deactivation due to coking [ 3,4]. Ebitani et al. [ 4] suppose that the strong interaction with the support produces the loss of the typical properties of the metallic function. Other authors [ $61 suggest that platinum is in the reduced state, but covered with a sulfur layer, which is the cause of the inhibition of the dehydrogenating properties. Another topic in discussion is related to the nature and the origin of the acidity in SO:--ZrO,; Lewis and Bronsted sites are held responsible for the catalytic activity. The influence of the concentration of sulfur is also * Corresponding
author. Tel. ( + 54-42) 33858, Fax ( +54-42)
0926-860X/95/$09.50 SSDIO926-860X(95)00160-3
31068 and ( +54-42)
550944.
L152
J.C. Yori, JM. Parero/Applied
Catn1wi.r A: Gmercrl 129 (1995) L151-L156
in discussion [7-l 11. Besides, there are doubts related to the capacity of the catalyst to retain sulfur. Some authors point out that part of the activity drop could be due to a loss of sulfur [ 121, but Keogh et al. [ 131 do not find sulfur loss after a long period on stream for Pt/SO;--ZrO, fed with n-hexadecane. The most common method of preparation of this catalyst is by the percolation or dipping of Zr(OH), in a 0.5 M H2S04 solution, drying and impregnation with a H,Cl,Pt solution. This method needs considerable time, costly operations and has the danger of mutual displacement of H2S04 and H,Cl,Pt to the impregnating solution. In this work we use a different technique to incorporate together platinum and SOi- into Zr( OH),, the incipient wetness impregnation. This technique enables us to obtain materials with different SO:- concentrations by using different concentrations of acid in the impregnation. With these catalysts we try to clarify the role of the concentration of sulfur in the catalytic properties of Pt/SO i=ZrO,.
2. Experimental 2.1. Catalysts Zr( OH), was obtained by a technique already described [ 141. Even the addition of ammonia to zirconyl chloride produces the precipitation of a non-stoichiometric hydrated zirconium oxide, which we conventionally designate as Zr(OH),. The impregnating sulfate-platinum solution was prepared by dissolving an adequate amount of H,Cl,Pt . 6H20 (to obtain 0.25% Pt in the final catalyst) in a volume of H2S04 corresponding to the pore volume of Zr( OH), (0.4 ml/g) plus an excess of 10%. The concentration of the acid was varied up to 2.5 M, and the solids A-F were produced. The impregnating time was 3 h, then the solids were dried overnight at 110°C ground and sieved, and the 35-80 mesh fraction was used. After being loaded in the reactor for the catalytic test, the solid was calcined at 620°C for 3 h and then reduced in a hydrogen stream at 300°C for 1 h. Catalyst sulfur content was analyzed by combustion using a LECO CS-244 analyzer. 2.2. Materials H2S04 and H,Cl,Pt . 6H20 were Carlo Erba RPE. The gases were Matheson normal butane (99.5%), and pure air and hydrogen provided by AGA. 2.3. Catalytic test n-C, isomerization was used as a test reaction. It was performed at atmospheric pressure using a quartz fixed bed reactor of 0.39 in. internal diameter operated under isothermal conditions. The conditions of the catalytic test were: mass of
J.C. Yori, J.M. Parers/Applied
L153
Caialysis A: General 129 (1995) LlSl-L156
catalyst 0.5 g, temperature = 300°C WHSV = 1 h ~ ’ and molar ratio HJn-C, = 6. The test lasted four hours. The reaction products were analyzed using an on-line chromatograph with a FID detector and a 6 m long, l/8 in. diameter column packed with 25% dimethylsulfolane on chromosorb P. The n-C, conversion and the selectivity to i-C, were calculated from the chromatographic analysis.
3. Results and discussion Table 1 shows the properties of the catalysts after the catalytic test. As was increases when increasing the expected, the sulfate content in Pt/SOi--ZrO, H,SO, concentration used in the impregnation, reaching a content of about 3.8% for 2.5 M H2S04. This catalyst has a specific surface area of 90 m’/g, which means a sulfur content of 2.7 atom per nm*. Our range of sulfate concentrations is similar Table I Properties of the catalyst samples after the catalytic test Catalyst
A (H,SO, B (H,SO, C (H,SO, D (HaSO, E (H$O, F (H>SO,
0.0 M) 0.25 M) 0.5 M) l.OM) 1.5 M) 2.5 M)
SO:(% W./W.)
Pt (% wt./wt.)
Sg (BET) (m/g)
0.00 0.35 0.60 1.24 2.70 3.80
0.25 0.25 0.25 0.25 0.25 0.25
30 65 17 91 89 90
t-&SO,
CONCENTRATION,
N
Fig. 1. Sulfur content of the catalyst samples as a function of H,SO., concentration of the impregnating solution. Curve 1,total sulfur introduced by incipient wetness impregnation. 2, sulfur aftercalcination at 620°C and reduction at 300°C. 3, sulfur after the catalytic test at 300°C.
L154
J.C. Yori, J.M. Parera /Applied
0
0.5 SULFUR
Catalysis A: General 129 (1995) L151-L156
10
I 1.5
CONCENTRATION,
I 20
’ 0 25
at/nId
Fig. 2. Conversion of n-C, and selectivity to i-C, as a function of catalyst sulfur content. Test conditions: 3OO”C, WHSV = I (g n-&/g cat. h), molar ratio H,/n-C, = 6, atmospheric pressure. A, B, C, D, E and F, catalysts of Table 1.
to the one obtained in the references using the most common preparation method, percolation or dipping in an excess of 0.5 M H2S04 solution. Table 1 shows that an increase in the HISO concentration produces an increase in the specific surface area until 1 M H2S04. Greater concentrations do not modify the specific surface area of the samples, which is about 90 m*/g. This surface is lower than those obtained using the methods of percolation or impregnation with excess H2S04 solution, whose values ranged 100 m2/g [3] to 114 m*/g [4]. The sulfur contents expressed per gram of Zr02 obtained for each H2S04 concentration are plotted in Fig. 1 as a function of this concentration for the following cases: (a) total amount incorporated during the incipient wetness impregnation; (b) remaining amount after the pretreatments (calcination and reduction) ; (c) remaining amount after the pretreatments and reaction. In all cases an important loss of sulfur during the calcination and reduction is observed while no or little loss is observed during the reaction. The conversion of n-C4 and the selectivity to i-C4 as a function of the concentration of surface sulfur atoms are plotted in Fig. 2. The conversion and selectivity values shown for each catalyst were constant throughout all the run. Zirconia possesses weak acid and basic properties [ 151 and has no capacity for n-C, isomerization (catal. A). The addition of SOi- promotes its acidity and isomerization activity. Two zones can be distinguished in the conversion curve: (a) For sulfur to concentrations of concentration lower than 1 atom X nrn- * (corresponding impregnating H2S04 solutions smaller than 1 M) the activity of the catalyst increases proportionally to the surface concentration of sulfur. (b) For higher sulfur concentrations, both activity and selectivity remain constant when the concentration
J.C. Yori, J.M. Parers/Applied
Catalysis A: General 129 (1995) L151-L156
L155
of sulfur is increased. According to Sarzanini et al. [ 161, we consider that all sulfates are at the surface of sulfated ZrO,. The following observations can be made: (i) From a practical point of view, a 1 M concentration of sulfuric acid is convenient in the preparation of Pt/SOi--ZrO, by incipient wetness coimpregnation of platinum and SO;-. (ii) There exists a limit of 1 S atom X nme2 in the concentration of active species with capacity for the isomerization of n-C,. Above this value, the sites generated do not seem to have isomerization capacity and should be in excess. There are different opinions regarding the nature and origin of the acid sites of SOi--ZrO,. Jin et al. [ 71 reported the existence of only a high Lewis acidity, while Komarov et al. [ 81 and Nascimento et al. [ 93 informed about the presence of both Lewis and Bronsted acidity. Bolis et al. [ lo] postulated the existence of two types of Lewis sites, the most abundant having low acid strength. Morterra et al. [ 1 l] found the existence of three types of surface sulfate in SO:--ZrO,, which were a function of the degree of surface coverage: (a) Species formed at low coverage values (lower than 0.8 S atom X nme2), which correspond to isolated surface SO:- groups located in crystallographic defective configurations (side terminations) ; these species originate from Lewis acid sites. (b) Species similar to the previous ones, formed up to coverage values near to 2 S atoms X nmp2, which correspond roughly to the completion of a half-monolayer. These species are located on the regular patches of low index crystal planes (top terminations of the scale-like particles); they produce Lewis and some Bronsted acid sites. (c) When the sulfur concentration becomes higher than an average half-monolayer, polynuclear surface sulfates appear, probably of the pyrosulfates ( S20;- ) type which are also mainly located on the regular patches of low-index crystal planes (top terminations), and originate from Bronsted sites. The formation of this pyrosulfate type species does not affect the species previously formed at coverages lower than 0.8 S atom X nmp2. An interpretation of Fig. 2 can be done taking into account the previous discussion. The concentration of sulfur controls the catalytic activity until all the available surface configurations that are able to produce isolated surface SO;- groups are occupied. This occupation generates Lewis type acidity and corresponds to a sulfur concentration of about 0.8 S atom X nmp2. The sites formed at higher coverages do not have capacity for isomerizing n-C,. It can be concluded that the active sites in n-C4 isomerization are Lewis ones, in agreement with the statement of Comelli et al. [ 121. These authors conclude that the Lewis sites are the catalytically active ones because upon hydration of the catalyst a drop in activity is produced which is recovered after calcination at 620°C. Similarly, Keogh et al. [ 171 demonstrated that the active sites for the conversion of n-hexadecane are not of the Bronsted type, because water has a poisoning effect.
J.C. Yori, J.M. Pam-a/Applied
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Catalysis A: General 129 (1995) LISI-L156
The low activity of sample B may be due to the fact that part of the acid added is initially consumed for the neutralization of the basicity of ZrO,. This sample shows a very low selectivity to i-C4, propane and ethane being mainly produced. These products come from disproportionation and cracking reactions, more demanding of acid strength. This would indicate that not all the active sites are energetically equal, and that the surface sulfate groups are located in the solid occupying sites of decreasing energy. When the concentration of SOi- is increased between 0.35 and 1.24% (samples B-D) the catalytic activity is proportionally increased with a slope of 0.014 reacted n-C, molecules per second and sulfur atom.
4. Conclusions The incipient wetness coimpregnation of Zr( OH), with H,S04 and H,Cl,Pt is a practical method of preparing Pt/SOz--ZrO,. The specific surface area and the catalytic activity increase with the increase in H2S04 concentration up to 1 M. For higher concentrations both properties remain constant. The sulfated Zr02 surface is heterogeneous. The isolated sulfate groups formed at sulfur concentrations lower than 0.8 S atomXnm-* are catalytically active for n-C, isomerization. The sites generated at higher concentrations are not active and do not affect the activity of the pre-existent sites.
References [ 1] [Z] [3] [4] [5] [6] [7] [ 81 191
[ IO] [ 1I] [ 121 [ 131 [ 141 [ 151 [ 161 [ 171
M. Hino and K. Arata, J. Chem. Sot. Chem. Commun., ( 1979) 1148. M. Hino and K. Arata, J. Chem. Sot., Chem. Commun., ( 1980) 851. J. Yori, M. D’Amato, G. Costa and J. Parera, J. Catal., 153 (1995) 218. K. Ebitani, J. Konishi and H. Hattori, J. Catal.. 130 ( 1991) 257. Z. Paal, M. Muhler and R. Schlogl, J. Catal., 143 ( 1993) 321. A. Dicko, X. Song, A. Adnot and A. Sayari, J. Catal., 150 ( 1994) 254. T. Jin, T. Yamaguchi and K. Tanabe, J. Phys. Chem., 90 ( 1986) 4794. V. Komarov and M. Sinilo, Kinet. Catal., 29 ( 1988) 701. P. Nascimento, K. Akratopoulou, M. Oszagyan, G. Coudurier, C. Travers, J. Joly and J. Vedrine, Stud. Surf. Sci. Catal., 75 (1993) 1185. V. Bolis, C. Morterra, M. Volante, L. Orio and B. Fubini, Langmuir, 6 ( 1990) 695. C. Morterra, G. Cerrato and V. Bolis, Catal. Today, 17 ( 1993) 505. R. Comelli, C. Vera and J. Parera, J. Catal., 151 ( 1995) 96. R. Keogh, D. Sparks and B. Davis, Stud. Surf. Sci. Catal., 88 ( 1994) 647. J. Yori, J. Luy and J. Parera, Appt. Catal., 46 ( 1989) 103. Z. Feng, W. Postula, A. Akgennan and R. Anthony, Ind. Eng. Chem. Res., 34 ( 1995) 78. C. Sarzanini, G. Sachero, F. Pinna, M. Signoretto, G. Cerrato and C. Morterra, J. Mater. Chem., 5 ( 1995) 353. R. Keogh, R. Srinivasan and B. Davis, J. Catal., 151 ( 1995) 292.