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Al2O3-promoted sulfated zirconia catalysts for the isomerization of n-butane
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
2375
Al203-promoted sulfated zirconia catalysts for the isomerization of n-butane. R. Olindo', F. Pinna', G. Strukul', P. Canton b, P. Riellob, G. Cerrato ~ G. Meligrana~ and C. Morterrac
"Dip. Chimica and b Dip. Chimica Fisica, Universit~ Ca' Foscari di Venezia, Dorsoduro 2137, 30123 Venezia Italy ~Dip. Chimica IFM, Universit~ di Torino, Via Pietro Giuria 7, I-10125 Torino, Italy The catalytic activity in the isomerization of n-butane of a variety of sulfated zirconia catalysts promoted with different amounts of A1203 is reported together with a FTIR analysis of the surface chemical properties. 1. I N T R O D U C T I O N The isomerization reaction of normal paraffins to branched isomers is of great importance in the petroleum refining industry, due to the growing demand for high octane gasoline. Environmental constraints on the use of traditional catalysts have attracted considerable interest on strongly acidic solid materials, suitable for catalyzing hydrocarbon reactions. Sulfated zirconia (ZrO2/SO4) is known to catalyze the isomerization of light paraffins under mild conditions but a rapid deactivation of the catalyst has usually been observed. Za<)2/SO4 is normally prepared by sulfation of amorphous zirconia hydrate followed by calcination and the properties of the resultant ZrO2/SO4 depend strongly on the preparation procedure: i.e. the nature of the starting material, sulfation procedure, calcination temperature, etc.[ 1].The addition of small amounts of Pt and the presence of H 2 in the feed has been found to increase the activity and, above all, the stability of the ZrOJSO4 catalysts. Recently Gao et al [2] have reported that the addition of small amounts of A1203 to the ZrO2/SO4 systems gives rise to catalysts more active and very stable with time, if n-butane isomerization is performed at 250~ in presence of H 2. According to the authors these remarkable properties are due to an appropriate distribution of acid site strengths. In a previous work [3] we have reported the effect of the addition of alumina on the structure and microstructure of a sulfated zirconia system. In this contribution a study of the catalytic activity of the same materials in the isomerization of n-butane will be reported, together with a systematic in-situ IR characterization of the surface chemical properties.
2. EXPERIMENTAL 2.1 Preparation The catalysts (ZSAn) have been prepared by conventional coprecipitation method described in detail elsewhere [3]. Briefly a mixed solution of ZrOC12.8H20 and Al(NO3)3.9H20 was slowly added to a 10N NH4OH precipitating solution under vigorous mechanical stirring. The precipitate was washed with H20 and dried at l l0~ for 20h. Sulfation was carded out by incipient wemess impregnation with an aqueous (NH4)2SO4 solution. The samples were dried again and then calcined at 650~ for 3h under a dry air flow and stored in air. As a reference, plain ZrO2/SO4 (ZS sample) and A1203/SO4 (SA sample) have been prepared in the same way starting from the above precursors.
2376
2.2. Characterization The quantitative detemaination of A1 and sulfates was performed by AA spectroscopy and ion chromatography respectively. Nitrogen adsorption and desorption were measured with a Micromeritics AS AP 2010 apparatus. The samples were pretreated at 200~ for 3 h under vacuum (1.33 Pa). FTIR spectra were obtained at 2 cm 1 resolution with a Bruker IFS88 spectrophotometer. Samples, in the form of thin layer deposition on a pure Si platelet, were activated at the chosen temperature (4000(2) in a quartz cell connected to a vacuum line (f'mal P= lxl0 6 Ton). All adsorptions of gas test molecules were performed in a strictly in situ configuration, which avoids samples contamination and permits spectra ratioing and subtraction.
2.3 Catalytic measurements n-butane isomerization experiments were performed in a quartz flow microreactor (200 mm long, 8 mm i.d.) externally heated by an oven. The temperature was controlled with a thermocouple immersed in the catalytic bed. The reaction was carried out at atmospheric pressure at either 150~ or 250~ Prior to reaction, catalyst (0.5 g) underwent an activation step in dry air flow at 450~ for 1.5 h. The feed mixture (total flow 25 ml/min) consisted of nbutane and H 2 or He in the ratio 1:4 (GHSV=720 hl). Reactant and products were analyzed on line by gas chromatography. 3. R E S U L T S AND DISCUSSION
3.1 Catalytic activity In Table 1 is reported a summary of the analytical and morphological parameters of the AI promoted samples and the corresponding plain sulfated zirconia and alumina after calcination at 650. The presence of AI203 gives rise to a noticeable increase (about 30%) of BET surface area in comparison with ZS sample, but BET values remain practically constant in spite of the different amount of A1203 added. The sulfur content is similar in all samples and ranges between 2.3 and 3.5 SO4 groups/nm2 i.e. a little lower than 4, the calculated value for the full coverage at the monolayer [4]. Table 1 Analytical and morphological parameters of the catalysts Samples ZS ZSA1 ZSA2 ZSA3 ZSA5 ZSA9 ZSA15 SA
Al (mol.%) 0 0.7 1.4 2.7 4.5 7.3 12.4 100
BET (m2/g)
SO, 2 (wt%)
81 111 114 113 112 118 128 192
3.9 4.1 4.2 6.3 5.0 4.5 5.9 5.3
SO 2 groups/nm2 2.9 2.3 2.3 3.5 2.8 2.9 2.9 1.7
The activity of the catalysts has been tested in the isomerization of n-butane. SZ samples have been checked at 150~ in the absence of H 2. The profile obtained (conversion vs. time), here not reported for sake of brevity, is typical: after an initial fast decay the activity declines slowly as a consequence of the deactivation process suggested by the observation of minor amounts of some cracking products (propane <3% and methane <1%). In spite of this fast deactivation, ZSA3 sample is the most active one. In all cases the product mixture consisted of >94% i-butane, with only minor amounts of propane(<4%) and pentanes (<2%). The initial
2377 activity can be completely restored after calcining the samples in dry air at 450~ If the reaction is run at 250~ and in presence of He (Fig. 1), a promoting effect of AI203 on sulfated zirconia is evident in the samples containing an amount of A1203 lower than 5% mol. For the ZSA3 sample, a conversion of about 35% was reached at steady state (4h) and, most important, no deactivation was observed after 24h on stream. It should be remarked that the promotion is not due to an additive effect because the sulfated alumina sample (SA) is totally inactive and a mechanical mixture of ZS and SA gave the same conversion of the ZS sample.
40
7,
40
L
30
k-
A5 mech. mixture
2o
v
30 o
o :3 o
20 ~
%3
o
O
~2-
o lO
1o 0
5
10 AI20 3 (mol. %)
Fig. 1. n-butane conversion function of A1203 content.
15 as a
o 500
o 550 600 650 700 750 Calcination Temperature (~ ~
|
I
Fig. 2. Effect of calcination temperature on sulfate content and on n-butane conversion for ZSA3 sample.
The catalytic activity (expressed as n-butane conversion %) and the amount of sulfate, measured by ion chromatography, for sample ZSA3 are plotted in Fig. 2 versus the catalyst calcination temperature. The maximum conversion occurred at a calcination temperatm'e of 650~ while the sulfur contents shows a volcano shaped curve, as usually happens for the sulfated zirconia systems, with a maximum centered at about 650~ The curve describing the catalytic activity is very sharp, showing how critical is the catalysts activation procedure since for similar sulfur content the activity may change of three times. On the base of these results we have decided to calcine the samples at 650~ and perform the comparison of the catalytic activity previously shown in Fig.1. 3.2 FTIR data Systematic in-situ IR measurements have been carded out on several ZSAn catalysts in order to find out how the surface chemical properties of sulfated zirconia are modified by the introduction of A1203. The spectral analysis concerned: (i) nature and relative amount of both intrinsic surface species (i.e., OH groups) and introduced surface functionalities (i.e., sulfates); (ii) surface activity, as probed by the adsorption/desorption of suitable test molecules. Although several vacuum activation temperatures, between 25 ~ and 550~ were tested (in order to follow the evolution of the samples surface features with activation conditions), only systems activated at 400~ will be referred to in the following, as this temperature represents a suitable reference activation condition, quite close to that of the maximum catalytic activity and achieved in the standard dynamic activation step carded out before catalytic tests. The most significant spectral data, and the relevant interpretation, can be summarized as follows.
2378 (a) Surface hydration. The spectral region typical of OH stretching vibrations (3800-3000 cm q, see Fig. 3a) indicates that, for alumina loading up to about 3% (curve 2), the VOH profile is virtually unchanged with respect to that of a pure ZS system (curve 1). The spectruna presents two relatively sharp bands of quite different intensity, centred at-3760 cm" (w) and-3655 cm" (m) respectively. These bands are normally ascribed to terminal and bridged hydroxyls, respectively, substantially free from lateral interactions of the H-bonding type [5]. At higher A1203 loading (curve 3), the profile of free OH bands becomes less and less evident, while a broad tailing, extending down to -3200 cm l and due to OH species perturbed by H-bonding interactions, becomes more and more intense. These observations indicate that, above a certain concentration, the presence of alumina makes the ZSA system more and more difficult to dehydrate. This fact ought to be related to the increasingly amorphous nature of the mixed oxidic system, as shown by the results previously obtained [3] by an X-ray diffraction study performed on the same catalysts: from the diffraction patterns it was possible to evidence that the A1203 content influences the growth of zirconia crystallites, stabilizing crystallites of small dimensions and so the tetragonal form. For an alumina loading higher than 5% the zirconia particle dimensions are so small that the samples are completely amorphous. A hydroxide-like nature is so suggested for the fraction of the surface layer of the system becoming amorphous that is not covered by sulfates.
VS_ o
a
/
3650
0.2
Vs=~ I
101(~
I
0.5
~
i ~ 3655
---------~-...__L2
- ~__~- -=~
'
3900
I
3600
'
I
3300
' 1500 3000 1800 wavenumber (cm"1)
1200
900
Figure 3. IR spectra in the VOHregion (section a) and in the mid-IR spectral range of surface sulfates (section b) of ZS (curves 1), ZSA3 (curves 2), and ZSA9 (curves 3). (b) Surface sulfates. In the case of the unpromoted ZS, the spectral region typical of sulfate vibrations (1500850cm 1, see Fig. 3b, curve 1) presents a couple of well separated and relatively sharp bands centred at-1390 cm q (due to uncoupled v(S=O) stretching vibrational mode(s)) and at 10151025 cm l (due to v(S-O) stretching vibrational mode(s)). These bands are usually ascribed to isolated poly-dentate sulfate groups of highly covalent character, like in the case of organic sulfates [6]. For alumina loading up to about 3% (curve 2), the spectral profile of activated
23'/9 ZSAn systems does not present appreciable changes, r for a moderate increase of the absorbance intensity in the sub-range around 1200 cm ~ where the symmetrical stretching vibrations of coupled S=O oscillators arc usually rvportcxl to absorb. When the alumina loading reaches or exceeds some 5% (curve 3), the spectral profile of surface sulfates becomes severely and increasingly modified: the peaks typical of covalent poly-dentate sulfates become weaker, broader and less separated in fl~uency, while stronger and stronger spectral components grow in the spectral sub-range between 1300 and 1100 cm ~. In non-promoted ZS systems, features like these arc normally observed when the sulfate loading is appreciably above that of a statistical monolaycr (,~4 groups per nm r [7]) and/or in the case of non-calcined systems (i.e., when sulfates arc occupying all crystallographic terminations present in the surface layer, and strongly interact with one another). The gradual modification of the spectral features of sulfates observed with ZSAn systems is certainly not ascribable to an excess of sulfates coverage, as the analysis indicates that the overall concentration of sulfates remains virtually constant with alumina loading. Rather, the gradual loss of crystallinity [3] brought about by growing alumina loading is thought to be responsible for the observed changes: sulfates assume gradually the characteristics they exhibit on non-calcined ZS systems, where the strong reciprocal perturbation among sulfate groups is determined by an excess of sulfates as well as by the disordered (amorphous) nature of the precursor system. (c) Surface acidity: the ambient temperature (RT) adsorption of CO It has been long known that, on oxidic systems with no d-electrons, the reversible RT adsorption of CO is a suitable tool to reveal the strong Lewis acidity due to highly uncoordinated (cus) surface cationic sites [8]. In the case of activated plain ZS catalysts, CO uptake at RT yields a broad asymmetric band centred at-2195 cm I (m), due to the weak o--coordinative interaction of CO with highly uncoordinated Zr4+ surface centres, acting as Lewis sites of medium-high strength. The CO band visible at-2195 crn1 upon adsorption at RT on plain SZ catalysts represents the high-v fraction (approximately 20%) of a larger unresolved broad absorption apparently centred a t - 2 1 9 0 cm and observable upon CO uptake at temperatures as low as - 173~ The latter band is thought to be representative of the overall surface Lewis acidity of ZS catalysts [8]. On ZSAn systems, and for n up to 3, CO uptake at RT indicates that: (i) there is no formation of carbonyl-like bands at v > 2000 cm -~, meaning that A13+ ions possibly present in the surface layer do not possess tetrahedral coordination and/or do not reach a sufficient uncoordination to adsorb CO [9]; (ii) the carbonyl-like band at ~2195 cm 1 due to Zr4+cus sites maintains the spectral position, the peak intensity and half-band width virtually unchanged with respect to plain SZ. In other words, for alumina loading up to some 3%, the strong Lewis acidity of ZS catalysts is not modified to a spectroscopically appreciable extent. When the alumina loading exceeds ~5%, CO uptake at ambient tempe~_ture indicates that: (i) still no bands ascribable to the o-coordinative interaction of CO with AlrVc~ sites are observed; (ii) the band at --2195 crn~, due to Zr4+cJCO surface complexes, declines with alumina content and for n ~15 has virtually vanished. This implies that high-loaded ZSAn systems possess a fast decreasing (and eventually null) strong Lewis acidity of the type that is usually considered to participate in the overall catalytic process of n-alkanes isomerization. This may be determined by a gradually modified surface composition of the system becoming amorphous, and/or by the more difficult surface activation (dehydration) of the high-loaded system (as reported above). (d) Surface acidity: the RT adsorption/desorption of pyridine (py). It is well known that, on oxidic systems, py adsorption/desorption at mild temperatures ranging between RT and ~150~ can distinguish, at least in qualitative terms, between Lewis acidity (L) and Br~nstexl acidity 03).'_among the several absorption bands formed, characteristic bands are observed at ~1445 crn~ (analytical band for py-L species) and a t - 1 5 4 5 cm I (analytical band for py-B species). Recently it has been shown [10] that, in oxidic systems carrying surface anionic species (and the systems here represented have indeed appreciable fractions of the surface layer covered by sulfate groups), py uptake is somewhat misleading as far as the identification and dosing of Lewis acidity is concerned, whereas it seems to be adequate for the identification and possibly the titration of Brcnsted acidity. Unlike pure ZrO 2 (on which no BrOnsted acidity is ever present), ZS systems exhibit variable amounts of
2380 Brcnsted acid sites, whose amount closely depend on both sulfate loading and activation temperature. In particular, the ZS systems considered here present a moderate, but still appreciable, surface concentration of BrCnsted acidic protons that, together with the abundant Lewis acid sites mentioned above, are thought to participate actively in the acid/base step(s) of the catalytic processes occurring on ZS catalysts. For alumina loading up to some 5%, ZSAn catalysts adsorb py at RT yielding a py-B analytical mode at ~1545 crn1 virtually unchanged (in spectral shape, position, and relative intensity) with respect to plain ZS, whereas for loading above 5% the amount of BrCnsted acidity tends to increase somewhat. The spectral response given by the analytical mode of py-B on ZSAn does not say much as far as the strength of the relevant BrCnsted acid sites is concerned, as the spectral features of the 19b mode of pyridinium ions are almost completely insensitive to the actual proton-releasing activity of the proton acidic centres. On the other hand, the use of other probe molecules like, for instance, the adsorption of CO at-196~ (not described here for the sake of brevity), indicates that the proton-releasing strength of Br~nsted acid sites on ZSAn is not appreciably different from that of plain ZS. As for the quantitative aspect of the BrOnsted acidity of ZSAn systems, the moderate increase observed for n > 3 is somewhat expected, as the high alumina loading renders the system more difficult to dehydrate. But it is also recalled that, for n > 3, ZSAn systems become catalytically less and less active, consistently with the fact that, to a small increase of Bronsted acidity (if any), corresponds a sharp decrease of the Lewis acidity. Gao et all.[2] reported that the high isomerization activity and stability of the alumina promoted catalysts under H 2 at 250~ were due to an appropriate distribution of acid site strengths and to an enhanced number of acid sites with intermediate acid strength. From the result so far obtained in this study no appreciable change of the surface chemical properties for the introduction of A1203 has been detected. So more work is needed and is in progress to shed more light on the effect of the Al2Oy 4. A C K N O W L E D G M E N T S The financial support from "Ministero dell'Universit~ e della Ricerca Scientif'lca" (MURST 60% and project Cofin-98) is gratefully acknowledged. REFERENCES
1. 2. 3.
G.K. Chuah, Catal. Today, 49 (1999) 131. Z. Gao, Y. Xia, W. Hua and C. Miao, Topics Catal. 6 (1998) 101. R. Olindo, F. Pinna, G. Strukul, P. Canton, P. Riello, G. Cerrato and C. Morterra, Chem. Mater., submitted. 4. M. Waqif, J. Bachelier, O. Saur and J. C. LavaUey, J. Mol. Catal., 72 (1992) 127. 5. T. Yamaguchi, Y. Nakao and K. Tanabe, Bull. Chem. Soc. Japan, 51 (1978) 2482. 6. M. Bensitel, O. Saur, J.-C. Lavalley and B.A. Morrow, Mater. Chem. Phys., 19 (1988) 147. 7. P. Nascimento, C. Akratopoulou, M. Oszagyan, G. Coudurier, C. Travers, J.F. Joly and J. Vedrine, in New Frontiers in Catalysis, ed. L. Guzci, F. Solymosi and P. Tetenyi, Elsevier Science Publishers B.V., 1993, p. 1185. 8. C. Morterra, G. Cerrato and F. Pinna, Spectrochim. Acta, 55 (1999) 95. 9. C. Morterra and G. Magnacca, Catal. Today, 27 (1996) 497. 10. C. Morterra and G. Cerrato, Phys. Chem. Chem. Phys., 1 (1999) 2825.
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Al203-promoted sulfated zirconia catalysts for the isomerization of n-butane. R. Olindo', F. Pinna', G. Strukul', P. Canton b, P. Riellob, G. Cerrato ~ G. Meligrana~ and C. Morterrac
"Dip. Chimica and b Dip. Chimica Fisica, Universit~ Ca' Foscari di Venezia, Dorsoduro 2137, 30123 Venezia Italy ~Dip. Chimica IFM, Universit~ di Torino, Via Pietro Giuria 7, I-10125 Torino, Italy The catalytic activity in the isomerization of n-butane of a variety of sulfated zirconia catalysts promoted with different amounts of A1203 is reported together with a FTIR analysis of the surface chemical properties. 1. I N T R O D U C T I O N The isomerization reaction of normal paraffins to branched isomers is of great importance in the petroleum refining industry, due to the growing demand for high octane gasoline. Environmental constraints on the use of traditional catalysts have attracted considerable interest on strongly acidic solid materials, suitable for catalyzing hydrocarbon reactions. Sulfated zirconia (ZrO2/SO4) is known to catalyze the isomerization of light paraffins under mild conditions but a rapid deactivation of the catalyst has usually been observed. Za<)2/SO4 is normally prepared by sulfation of amorphous zirconia hydrate followed by calcination and the properties of the resultant ZrO2/SO4 depend strongly on the preparation procedure: i.e. the nature of the starting material, sulfation procedure, calcination temperature, etc.[ 1].The addition of small amounts of Pt and the presence of H 2 in the feed has been found to increase the activity and, above all, the stability of the ZrOJSO4 catalysts. Recently Gao et al [2] have reported that the addition of small amounts of A1203 to the ZrO2/SO4 systems gives rise to catalysts more active and very stable with time, if n-butane isomerization is performed at 250~ in presence of H 2. According to the authors these remarkable properties are due to an appropriate distribution of acid site strengths. In a previous work [3] we have reported the effect of the addition of alumina on the structure and microstructure of a sulfated zirconia system. In this contribution a study of the catalytic activity of the same materials in the isomerization of n-butane will be reported, together with a systematic in-situ IR characterization of the surface chemical properties.
2. EXPERIMENTAL 2.1 Preparation The catalysts (ZSAn) have been prepared by conventional coprecipitation method described in detail elsewhere [3]. Briefly a mixed solution of ZrOC12.8H20 and Al(NO3)3.9H20 was slowly added to a 10N NH4OH precipitating solution under vigorous mechanical stirring. The precipitate was washed with H20 and dried at l l0~ for 20h. Sulfation was carded out by incipient wemess impregnation with an aqueous (NH4)2SO4 solution. The samples were dried again and then calcined at 650~ for 3h under a dry air flow and stored in air. As a reference, plain ZrO2/SO4 (ZS sample) and A1203/SO4 (SA sample) have been prepared in the same way starting from the above precursors.
2376
2.2. Characterization The quantitative detemaination of A1 and sulfates was performed by AA spectroscopy and ion chromatography respectively. Nitrogen adsorption and desorption were measured with a Micromeritics AS AP 2010 apparatus. The samples were pretreated at 200~ for 3 h under vacuum (1.33 Pa). FTIR spectra were obtained at 2 cm 1 resolution with a Bruker IFS88 spectrophotometer. Samples, in the form of thin layer deposition on a pure Si platelet, were activated at the chosen temperature (4000(2) in a quartz cell connected to a vacuum line (f'mal P= lxl0 6 Ton). All adsorptions of gas test molecules were performed in a strictly in situ configuration, which avoids samples contamination and permits spectra ratioing and subtraction.
2.3 Catalytic measurements n-butane isomerization experiments were performed in a quartz flow microreactor (200 mm long, 8 mm i.d.) externally heated by an oven. The temperature was controlled with a thermocouple immersed in the catalytic bed. The reaction was carried out at atmospheric pressure at either 150~ or 250~ Prior to reaction, catalyst (0.5 g) underwent an activation step in dry air flow at 450~ for 1.5 h. The feed mixture (total flow 25 ml/min) consisted of nbutane and H 2 or He in the ratio 1:4 (GHSV=720 hl). Reactant and products were analyzed on line by gas chromatography. 3. R E S U L T S AND DISCUSSION
3.1 Catalytic activity In Table 1 is reported a summary of the analytical and morphological parameters of the AI promoted samples and the corresponding plain sulfated zirconia and alumina after calcination at 650. The presence of AI203 gives rise to a noticeable increase (about 30%) of BET surface area in comparison with ZS sample, but BET values remain practically constant in spite of the different amount of A1203 added. The sulfur content is similar in all samples and ranges between 2.3 and 3.5 SO4 groups/nm2 i.e. a little lower than 4, the calculated value for the full coverage at the monolayer [4]. Table 1 Analytical and morphological parameters of the catalysts Samples ZS ZSA1 ZSA2 ZSA3 ZSA5 ZSA9 ZSA15 SA
Al (mol.%) 0 0.7 1.4 2.7 4.5 7.3 12.4 100
BET (m2/g)
SO, 2 (wt%)
81 111 114 113 112 118 128 192
3.9 4.1 4.2 6.3 5.0 4.5 5.9 5.3
SO 2 groups/nm2 2.9 2.3 2.3 3.5 2.8 2.9 2.9 1.7
The activity of the catalysts has been tested in the isomerization of n-butane. SZ samples have been checked at 150~ in the absence of H 2. The profile obtained (conversion vs. time), here not reported for sake of brevity, is typical: after an initial fast decay the activity declines slowly as a consequence of the deactivation process suggested by the observation of minor amounts of some cracking products (propane <3% and methane <1%). In spite of this fast deactivation, ZSA3 sample is the most active one. In all cases the product mixture consisted of >94% i-butane, with only minor amounts of propane(<4%) and pentanes (<2%). The initial
2377 activity can be completely restored after calcining the samples in dry air at 450~ If the reaction is run at 250~ and in presence of He (Fig. 1), a promoting effect of AI203 on sulfated zirconia is evident in the samples containing an amount of A1203 lower than 5% mol. For the ZSA3 sample, a conversion of about 35% was reached at steady state (4h) and, most important, no deactivation was observed after 24h on stream. It should be remarked that the promotion is not due to an additive effect because the sulfated alumina sample (SA) is totally inactive and a mechanical mixture of ZS and SA gave the same conversion of the ZS sample.
40
7,
40
L
30
k-
A5 mech. mixture
2o
v
30 o
o :3 o
20 ~
%3
o
O
~2-
o lO
1o 0
5
10 AI20 3 (mol. %)
Fig. 1. n-butane conversion function of A1203 content.
15 as a
o 500
o 550 600 650 700 750 Calcination Temperature (~ ~
|
I
Fig. 2. Effect of calcination temperature on sulfate content and on n-butane conversion for ZSA3 sample.
The catalytic activity (expressed as n-butane conversion %) and the amount of sulfate, measured by ion chromatography, for sample ZSA3 are plotted in Fig. 2 versus the catalyst calcination temperature. The maximum conversion occurred at a calcination temperatm'e of 650~ while the sulfur contents shows a volcano shaped curve, as usually happens for the sulfated zirconia systems, with a maximum centered at about 650~ The curve describing the catalytic activity is very sharp, showing how critical is the catalysts activation procedure since for similar sulfur content the activity may change of three times. On the base of these results we have decided to calcine the samples at 650~ and perform the comparison of the catalytic activity previously shown in Fig.1. 3.2 FTIR data Systematic in-situ IR measurements have been carded out on several ZSAn catalysts in order to find out how the surface chemical properties of sulfated zirconia are modified by the introduction of A1203. The spectral analysis concerned: (i) nature and relative amount of both intrinsic surface species (i.e., OH groups) and introduced surface functionalities (i.e., sulfates); (ii) surface activity, as probed by the adsorption/desorption of suitable test molecules. Although several vacuum activation temperatures, between 25 ~ and 550~ were tested (in order to follow the evolution of the samples surface features with activation conditions), only systems activated at 400~ will be referred to in the following, as this temperature represents a suitable reference activation condition, quite close to that of the maximum catalytic activity and achieved in the standard dynamic activation step carded out before catalytic tests. The most significant spectral data, and the relevant interpretation, can be summarized as follows.
2378 (a) Surface hydration. The spectral region typical of OH stretching vibrations (3800-3000 cm q, see Fig. 3a) indicates that, for alumina loading up to about 3% (curve 2), the VOH profile is virtually unchanged with respect to that of a pure ZS system (curve 1). The spectruna presents two relatively sharp bands of quite different intensity, centred at-3760 cm" (w) and-3655 cm" (m) respectively. These bands are normally ascribed to terminal and bridged hydroxyls, respectively, substantially free from lateral interactions of the H-bonding type [5]. At higher A1203 loading (curve 3), the profile of free OH bands becomes less and less evident, while a broad tailing, extending down to -3200 cm l and due to OH species perturbed by H-bonding interactions, becomes more and more intense. These observations indicate that, above a certain concentration, the presence of alumina makes the ZSA system more and more difficult to dehydrate. This fact ought to be related to the increasingly amorphous nature of the mixed oxidic system, as shown by the results previously obtained [3] by an X-ray diffraction study performed on the same catalysts: from the diffraction patterns it was possible to evidence that the A1203 content influences the growth of zirconia crystallites, stabilizing crystallites of small dimensions and so the tetragonal form. For an alumina loading higher than 5% the zirconia particle dimensions are so small that the samples are completely amorphous. A hydroxide-like nature is so suggested for the fraction of the surface layer of the system becoming amorphous that is not covered by sulfates.
VS_ o
a
/
3650
0.2
Vs=~ I
101(~
I
0.5
~
i ~ 3655
---------~-...__L2
- ~__~- -=~
'
3900
I
3600
'
I
3300
' 1500 3000 1800 wavenumber (cm"1)
1200
900
Figure 3. IR spectra in the VOHregion (section a) and in the mid-IR spectral range of surface sulfates (section b) of ZS (curves 1), ZSA3 (curves 2), and ZSA9 (curves 3). (b) Surface sulfates. In the case of the unpromoted ZS, the spectral region typical of sulfate vibrations (1500850cm 1, see Fig. 3b, curve 1) presents a couple of well separated and relatively sharp bands centred at-1390 cm q (due to uncoupled v(S=O) stretching vibrational mode(s)) and at 10151025 cm l (due to v(S-O) stretching vibrational mode(s)). These bands are usually ascribed to isolated poly-dentate sulfate groups of highly covalent character, like in the case of organic sulfates [6]. For alumina loading up to about 3% (curve 2), the spectral profile of activated
23'/9 ZSAn systems does not present appreciable changes, r for a moderate increase of the absorbance intensity in the sub-range around 1200 cm ~ where the symmetrical stretching vibrations of coupled S=O oscillators arc usually rvportcxl to absorb. When the alumina loading reaches or exceeds some 5% (curve 3), the spectral profile of surface sulfates becomes severely and increasingly modified: the peaks typical of covalent poly-dentate sulfates become weaker, broader and less separated in fl~uency, while stronger and stronger spectral components grow in the spectral sub-range between 1300 and 1100 cm ~. In non-promoted ZS systems, features like these arc normally observed when the sulfate loading is appreciably above that of a statistical monolaycr (,~4 groups per nm r [7]) and/or in the case of non-calcined systems (i.e., when sulfates arc occupying all crystallographic terminations present in the surface layer, and strongly interact with one another). The gradual modification of the spectral features of sulfates observed with ZSAn systems is certainly not ascribable to an excess of sulfates coverage, as the analysis indicates that the overall concentration of sulfates remains virtually constant with alumina loading. Rather, the gradual loss of crystallinity [3] brought about by growing alumina loading is thought to be responsible for the observed changes: sulfates assume gradually the characteristics they exhibit on non-calcined ZS systems, where the strong reciprocal perturbation among sulfate groups is determined by an excess of sulfates as well as by the disordered (amorphous) nature of the precursor system. (c) Surface acidity: the ambient temperature (RT) adsorption of CO It has been long known that, on oxidic systems with no d-electrons, the reversible RT adsorption of CO is a suitable tool to reveal the strong Lewis acidity due to highly uncoordinated (cus) surface cationic sites [8]. In the case of activated plain ZS catalysts, CO uptake at RT yields a broad asymmetric band centred at-2195 cm I (m), due to the weak o--coordinative interaction of CO with highly uncoordinated Zr4+ surface centres, acting as Lewis sites of medium-high strength. The CO band visible at-2195 crn1 upon adsorption at RT on plain SZ catalysts represents the high-v fraction (approximately 20%) of a larger unresolved broad absorption apparently centred a t - 2 1 9 0 cm and observable upon CO uptake at temperatures as low as - 173~ The latter band is thought to be representative of the overall surface Lewis acidity of ZS catalysts [8]. On ZSAn systems, and for n up to 3, CO uptake at RT indicates that: (i) there is no formation of carbonyl-like bands at v > 2000 cm -~, meaning that A13+ ions possibly present in the surface layer do not possess tetrahedral coordination and/or do not reach a sufficient uncoordination to adsorb CO [9]; (ii) the carbonyl-like band at ~2195 cm 1 due to Zr4+cus sites maintains the spectral position, the peak intensity and half-band width virtually unchanged with respect to plain SZ. In other words, for alumina loading up to some 3%, the strong Lewis acidity of ZS catalysts is not modified to a spectroscopically appreciable extent. When the alumina loading exceeds ~5%, CO uptake at ambient tempe~_ture indicates that: (i) still no bands ascribable to the o-coordinative interaction of CO with AlrVc~ sites are observed; (ii) the band at --2195 crn~, due to Zr4+cJCO surface complexes, declines with alumina content and for n ~15 has virtually vanished. This implies that high-loaded ZSAn systems possess a fast decreasing (and eventually null) strong Lewis acidity of the type that is usually considered to participate in the overall catalytic process of n-alkanes isomerization. This may be determined by a gradually modified surface composition of the system becoming amorphous, and/or by the more difficult surface activation (dehydration) of the high-loaded system (as reported above). (d) Surface acidity: the RT adsorption/desorption of pyridine (py). It is well known that, on oxidic systems, py adsorption/desorption at mild temperatures ranging between RT and ~150~ can distinguish, at least in qualitative terms, between Lewis acidity (L) and Br~nstexl acidity 03).'_among the several absorption bands formed, characteristic bands are observed at ~1445 crn~ (analytical band for py-L species) and a t - 1 5 4 5 cm I (analytical band for py-B species). Recently it has been shown [10] that, in oxidic systems carrying surface anionic species (and the systems here represented have indeed appreciable fractions of the surface layer covered by sulfate groups), py uptake is somewhat misleading as far as the identification and dosing of Lewis acidity is concerned, whereas it seems to be adequate for the identification and possibly the titration of Brcnsted acidity. Unlike pure ZrO 2 (on which no BrOnsted acidity is ever present), ZS systems exhibit variable amounts of
2380 Brcnsted acid sites, whose amount closely depend on both sulfate loading and activation temperature. In particular, the ZS systems considered here present a moderate, but still appreciable, surface concentration of BrCnsted acidic protons that, together with the abundant Lewis acid sites mentioned above, are thought to participate actively in the acid/base step(s) of the catalytic processes occurring on ZS catalysts. For alumina loading up to some 5%, ZSAn catalysts adsorb py at RT yielding a py-B analytical mode at ~1545 crn1 virtually unchanged (in spectral shape, position, and relative intensity) with respect to plain ZS, whereas for loading above 5% the amount of BrCnsted acidity tends to increase somewhat. The spectral response given by the analytical mode of py-B on ZSAn does not say much as far as the strength of the relevant BrCnsted acid sites is concerned, as the spectral features of the 19b mode of pyridinium ions are almost completely insensitive to the actual proton-releasing activity of the proton acidic centres. On the other hand, the use of other probe molecules like, for instance, the adsorption of CO at-196~ (not described here for the sake of brevity), indicates that the proton-releasing strength of Br~nsted acid sites on ZSAn is not appreciably different from that of plain ZS. As for the quantitative aspect of the BrOnsted acidity of ZSAn systems, the moderate increase observed for n > 3 is somewhat expected, as the high alumina loading renders the system more difficult to dehydrate. But it is also recalled that, for n > 3, ZSAn systems become catalytically less and less active, consistently with the fact that, to a small increase of Bronsted acidity (if any), corresponds a sharp decrease of the Lewis acidity. Gao et all.[2] reported that the high isomerization activity and stability of the alumina promoted catalysts under H 2 at 250~ were due to an appropriate distribution of acid site strengths and to an enhanced number of acid sites with intermediate acid strength. From the result so far obtained in this study no appreciable change of the surface chemical properties for the introduction of A1203 has been detected. So more work is needed and is in progress to shed more light on the effect of the Al2Oy 4. A C K N O W L E D G M E N T S The financial support from "Ministero dell'Universit~ e della Ricerca Scientif'lca" (MURST 60% and project Cofin-98) is gratefully acknowledged. REFERENCES
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