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Thermal desorption of n-butylamine from a silica-alumina surface
Thermal Desorption of n-Butylamine from a Silica-Alumina Surface J. M. GUIL, J. E. HERRERO, AND A. RUIZ PANIEGO Instituto de Qu[mica-F[sica "'Rocasolano, "" CSIC, Serrano 119, Madrid-6, Spain
Received January 31, 1984; accepted April 30, 1984 Temperature-programmed desorption (TPD) of n-butylamine (n-ba) has been used as a tool to investigate the nature of the active sites on the surface of a silica-alumina gel. On dry samples, at low temperature and short times of adsorption, the TPD spectra show two peaks: the first peak corresponds to n-ba weakly held that desorbs around 150°C whereas the second one is composed of propylene and 1-butene formed during the TPD at temperatures about 400°C. As temperature or time of adsorption is increased, a slow conversion of the first into the second peak is observed. At increasing adsorption temperatures, an intermediate peak develops around 300°C. When water is preadsorbed, the height and shape of the first peak change slightly, the change is more pronounced in the last peak and is very important in the intermediate one. Conclusions about the nature o f the active sites on the surface are drawn from these experimental facts. Finally, apparent activation energies of desorption for the first two peaks are calculated. © 1984AcademicPress,Inc. INTRODUCTION
A number of methods have been applied for the determination of the number and strength of the acid centers that exist on the surface of oxide catalysts, particularly in the case of catalysts like silica-alumina, very commonly used in many reactions such as cracking, isomerization or polymerization of hydrocarbons where these centers play a prominent role (1, 2). Most of these studies are based on the interaction of the surface with a probe molecule, monitored by a suitable experimental technique. The probe nbutylamine, a strong base, is one of these useful molecules. The method of titration of acid centers with n-ba (3) can distinguish acid centers of different strength using various indicators but it cannot identify the nature of these centers, i.e., whether they are Brrnsted or Lewis acids. The same may be said of the adsorption of n-ba from the gas phase (4). Infrared spectroscopy (5) has made it possible to ascertain the presence on the surface of different species of n-ba molecules that have reacted with acid centers of different kinds. A drawback of this technique is its
nonquantitative character and the fact that it is difficult to make experiments at various temperatures. A useful technique is also the temperature-programmed desorption o f n-ba (6) on silica-alumina that gives spectra containing more than one peak, showing the existence of different species adsorbed on the surface. The present paper discusses the information about the surface of a silica-alumina gel obtained through the last technique using nbutylamine as the probe molecule. The influence of such a variable as temperature or state of hydration of the surface is also considered. EXPERIMENTAL
The silica-alumina used in this work was a commercial sample LA-3P of Ketjen, Amsterdam, in cylinder form, which contained 13.8% alumina by weight. This catalyst had been characterized previously (7), its BET area being 430 m 2 g-1 (area of a N2 molecule in the monolayer = 0.162 n m 2) and its pore volume 0.51 cm 3 g-l. The same catalyst sample, 0.69 g, was used in all the experi111
Journal of Colloid and Interface Science, Vol. 102,No. 1, November1984
0021-9797/84 $3.00 Copyright© 1984by AcademicPress,Inc. All fightsof reproductionin any form reserved.
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GUIL, HERRERO, AND RUIZ PANIEGO
ments, being activated prior to the adsorption measurements for 1 hr in oxygen at 500°C, followed by outgassing at the same temperature for about 16 hr. The n-ba purified by fractional distillation had a purity better than 99.8%. High purity helium was used as cartier gas. The apparatus and cell used in the T P D studies were similar to those described by Cvetanovic and Amenomiya (8). The thermal conductivity detector and the flow control system of a HewlettPackard 5710 gas chromatograph were used. Desorbed products were condensed at the detector exit for chromatographic analysis. A temperature programmer, Stanton Redcroft 681, allowed 10 different heating rates in the range 2-20°C min -l. Most of the experiments were carried out at a heating rate of 8°C min -l. Nevertheless, since a linear relationship between detector signal and heating rate was found, it was possible to compare spectra obtained at different values of the latter by only applying a proportionality factor. The activated sample was cooled down to the temperature of adsorption and n-ba was admitted into the cell in a measured dose. Unless otherwise stated, the standard dose was 2.2 X 10-4 mole at a pressure of 50 m m H g (1 m m H g = 133.3 Pa). At room temperature, this dose was completely taken up by the sample within the first 30 min leaving 0.4 X 1014 molecules cm -2 on the surface. Assuming for the area of the adsorbed n-ba molecule, if perpendicular to the surface, a value of 0.20 n m 2 (9), a coverage of 0.09 was attained for the standard dose; if the molecule adsorbed was lying flat on the surface, the molecular area would be 0.32 nm 2 (9) and the fraction covered would increase to 0.14. At higher temperatures some n-ba remained in the gas phase even after long equilibration times. Prior to the T P D experiments the sample was outgassed at adsorption temperature for 1 hr. Adsorption of water was carried out, in most cases, by exposing the sample to water vapor at room temperature for about 2 hr and pumping for 2 hr at the same temperature. In some cases, Journal of Colloid and Interface Science, Vol. 102, No. 1, November 1984
a measured dose of water o f 0.7 X was admitted into the cell.
10 -4
mole
RESULTS AND DISCUSSION The influence of several variables upon experimental results is now described and discussed.
Initial Coverage The T P D spectra obtained with different initial doses of n-ba are given in Fig. I. Curve e corresponds to the standard dose. Both adsorption and evacuation previous to the TPD were carded out at room temperature. With the smallest dose, only a peak (peak II) was observed appearing at about 400°C. As the initially adsorbed amount increased, another peak (peak I) gradually appeared at lower temperatures, around 150°C. Between them, a third peak seemed to be present. Peaks I and II grew as the initial coverage increased, the former without limitation whereas the latter apparently reached a saturation value. Similar observations for t-butylamine on several adsorbents have been reported previously (10). In the insert o f Fig. 1, the height of peak II is plotted against the amount initially adsorbed. The plot, in which results from other experiments were included, clearly demonstrated that peak II reached a saturation value. Chromatographic analysis showed peak I to correspond to n-ba; peak II was due to desorption of a mixture of propylene and 1-butene. When the adsorption and previous outgassing were performed at 150°C, the results plotted in Fig. 2 were obtained. The maxima of peak II were higher than in Fig. 1 in spite of the fact that no saturation value was yet attained as can be seen in the insert of Fig. 2, although the curve seemed to tend toward saturation. When previous outgassing was carried out at higher temperature, the net result was a decrease in the height o f the maxima of peak I and a shift toward higher temperatures. Peak II remained unaltered.
113
n-BUTYLAMINE DESORPTION FROM SILICA-ALUMINA SURFACE
5--
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100
200
300
400
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FIo. 1. Dependence of TPD spectra on initial adsorbed amount: (a) 0.04; (b) 0.13; (c) 0.23; (d) 0.31; (e) 0.50; (£) 0.58 × l014 molecules cm -2. Adsorption at room temperature. Insert: detector response for peak II in arbitrary units vs initial adsorbed amount in molecules cm -2 × 10 -14.
All these experiments revealed the existence of at least two types of centers, as previously discussed by Takahashi et al. (6). Peak I consists of n-ba physisorbed or weakly che-
misorbed on centers with adsorption energies distributed over a wide range, being strongly affected by outgassing at higher temperatures. On the centers responsible for the formation II
Q
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LL-
10
~10 E
t-
el
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0
E
L 100
200
300
400 t l°C
FIG. 2. Dependence of TPD spectra on initial adsorbed amount: (a) 0.09; (b) 0.23; (c) 0.45 X 1014 molecules cm -2. Adsorption at 150°C. Insert: detector response for peak II in arbitrary units vs initial adsorbed amount in molecules cm -2 X 10 -14. Journal of Colloid and Interface Science, Vol. 102, No. 1, November
1984
114
GULL, HERRERO, AND RUIZ PANIEGO
of peak II, the n-ba, more strongly held, decomposed into propylene and 1-butene. From these results, it is not clear whether this decomposition occurred immediately after the adsorption or during the TPD as the sample temperature was raised to that corresponding to peak II. This latter possibility seems more likely; the former would mean that propylene and 1-butene remained adsorbed on the surface up to 400°C. Peri (11, 12) found that adsorbed 1-butene and even its polymerization products were removed from a silica-alumina surface at lower temperatures. On the other hand, Koubek et al. (13) reported that decomposition reactions of amines on alumina take place at higher temperatures.
Time and Temperature of Adsorption A slow conversion of peak I into peak II was clearly shown when longer adsorption times (Fig. 3) or higher adsorption temperatures (Fig. 4) were used. In this case, the most remarkable effect was the progressive appearance of a third peak between the other
two, with its maximum at about 300°C. Chromatographic analysis showed this peak to correspond also to n-ba. It must be pointed out that when outgassing, but not adsorption, took place at higher temperatures this peak did not appear. The effects just described can be interpreted with the aid of the scheme L
, II (C3/C4)
n-baga/
1
B
, III
Small doses of n-ba are strongly adsorbed on centers designated by L in the scheme. These centers can be identified as Lewis acid centers that accept the nitrogen lone electron pair to form a strong bond. Peri (11, 12, 14, 15) has demonstrated the existence of several types of such centers. The number of these L centers is limited and they are soon saturated so that eventually chemisorpfion upon less strong centers and physical adsorption
--3 d
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100
I
I
200
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I 400 t / ° C
FIG. 3. Influence of adsorption time at room temperature: (a) 15 rain; (b) 30 rain; (c) 125 rain; (d) 67
hr. Standarddose. Journal of Colloid and Interface Science, Vol. 102, No. 1, November 1984
115
n-BUTYLAMINE DESORPTION F R O M S I L I C A - A L U M I N A SURFACE
II
A
L
~I0
-
-
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I 100
J J
I 200
300
400
~IO C
FIG. 4. Adsorption temperature; (a) r.t.; (b) 100°C; (c) 150°C; (d) 200 °C. Standard dose.
become competitive (A). It is very probable that weak chemisorption occurs on Br6nstedtype acid centers formed by hydroxyl groups situated on Si or Al atoms on the surface. Morimoto et al. (5) identified by IR spectroscopy both types of acid centers in the adsorption of n-ba on silica-alumina; n-ba on Lewis acid sites was observed even after evacuation at 500°C, On the other hand they also showed the presence of more than two kinds of Br6nsted sites with different acid strength. The n-ba adsorbed on the stronger of these sites remained on the surface even when the system was outgassed at temperatures up to 300°C. During the T P D experiment, desorption of physically adsorbed n-ba overlapped that of the weakly chemisorbed probe on O H groups eventually to form peak I. The n-ba strongly chemisorbed on Lewis acid centers remained on the surface until higher temperatures were reached and then it decomposed to give propylene and 1-butene giving rise to peak II. Adsorbed pyridine, on the contrary, being a more stable species although a weaker
base, remains on the surface up to 500°C (5, 6). The slow conversion of the peak I into peak II must be ascribed to a surface process, since there was no n-ba in the gas phase (residual pressure was null). In order to explain it, a certain mobility, although reduced, of the OH groups throughout the surface must be postulated. In this way, an OH group on top of an A1 atom can be displaced out of its position leaving exposed the A1 atom that becomes a Lewis acid site on which a molecule of n-ba from a neighbor position on the surface can be more strongly adsorbed, the process being energetically favorable. Since the mobility of O H groups on the surface increases with temperature, a larger peak II was obtained in experiments at higher temperature. On the contrary, an increase in outgassing temperature did not modify peak II because outgassing produced a displacement toward the left side in the proposed scheme, balancing the effect of the larger mobility of O H groups just described. This effect explains also the fact that at small Journal of Colloid and Interface Science, Vol. 102, No. 1, November1984
116
GUIL, HERRERO, AND RUIZ PANIEGO
adsorption times a saturation seemed to have been reached at room temperature but not at 150°C under the same experimental conditions. Some evidence of O H mobility will be furnished below. Finally, peak III can be attributed to adsorption on more strongly acid hydroxyl groups. Rouxhet and co-workers (16-18) identified such groups (LF2) on the surface of silica-alumina. These LF2 centers are thought to be silanol groups adjoining an A1 atom and are only a small part of the surface hydroxyls. As already said, Morimoto et aL (5) also found strong Br/Snsted centers upon which n-ba remained adsorbed at temperatures up to 300°C. At low temperature there are only a few of such centers. The higher mobility of hydroxyls promoted by an increase in temperature increased the number of those centers upon which molecules of nba were adsorbed and later, during TPD, desorbed without decomposition. In order to arrive at a better understanding
of these phenomena a new series of experiments were performed with the silica-alumina surface modified by the presence of water. W a t e r on S i l i c a - A l u m i n a
When the sample was brought into contact with a small amount of water at room temperature and then outgassed, the T P D spectrum shown in Fig. 5 (curve a) was obtained. Water was nonselectively adsorbed on centers covering a wide range of adsorption energies. Peri (14) also found, by IR spectroscopy, a nonselective adsorption of water on silicaalumina samples. When after adsorption of the same dose at room temperature the system was heated up to 200°C, kept at this temperature and then allowed to cool down again, curve b was obtained. It can be seen clearly that a rearrangement on the surface has taken place: some water has migrated and has been adsorbed preferentially on ten-
d
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FIG. 5. Influence of the presence of water: Curves a and b: adsorption of 0.13 × ]014 water molecules cm -2 at (a) r.t.; (b) 200°C. Curves c and d: adsorption of the standard dose of n-ha on dry sample at (c) r.t.; (d) 200°C. Curve c': adsorption at r.t. of the standard dose of n-ha on a sample with preadsorbed water (0.13 × 1014 molecules cm-2). Journal of Colloid and Interface Science, Vol. 102, No. 1, November 1984
n-BUTYLAMINE DESORPTION FROM SILICA-ALUMINA SURFACE
ters within a narrower energy range producing the small peak that can be observed around 325°C. High temperature favors the mobility of water on the surface but, even at 200°C, this mobility is restricted; a long period of time was required for the low and broad peak in curve b to appear. The adsorption of larger amounts of water, up to the solid wetting, yielded T P D spectra with broad peaks, whose maxima were attained at about 125°C, provided that outgassing of samples was carried out at room temperature. Long tails extended beyond 525°C. The shape of the curves depended strongly on initial conditions; the tails were specially sensitive to heating rates, showing that a nonequilibrium desorption process was involved. These results are in agreement with experimental facts firmly established some time ago: desorption of molecular water at relatively low temperature, followed by slow desorption of water formed by condensation of hydroxyl groups on the surface (19, 20).
117
TPD on Hydrated Surface (a) Partially hydrated surface. The presence of water modifies the T P D spectra of nba on silica-alumina. Thus, in Fig. 5, curve c corresponds to adsorption of n-ba at room temperature on silica-alumina evacuated at 500°C; in curve c' a small amount of water (the same as in curve a) had been previously adsorbed also at room temperature. It must be noted that the detector response for water was about one-half the response found for nba or for olefins. In Figs. 5 and 6 desorbed water was present in the entire spectrum, mixed with the other desorption products, and therefore the corresponding concentrations could no longer be calculated. For this reason, the plot ordinate is now given in arbitrary units of detector response. It can be seen that the presence of a small amount of water hardly produced any difference in peak I. The difference, however, was important in the intermediate peak III. This difference could not be accounted for by
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FIG. 6. Comparison of TPD spectra obtained from dry and wet samples: Adsorption of the standard dose of n-ba on a dry sample at (a) r.t.; (b) 100°C; (c) 150°C. Adsorption of the standard dose of n-ba on a sample saturated with water at (a') r.t.; (b~) 100°C; (c') 150°C. Journal of Colloid and Interface Science, Vol. t02, No. 1, N o v e m b e r 1984
118
GUIL, HERRERO, AND RUIZ PANIEGO
simultaneous desorption of water and n-ba in this temperature range: the detector response is much larger than the corresponding to the sum of n-ba and water (curve a) desorbed in this range. On the other hand, the increase of peak III was similar to that observed when the adsorption temperature was higher (Fig. 5, curves c' and d). Curve d is the same as in Fig. 4 (adsorption at 200°C). The similarity of both curves suggests an increase in the number of the corresponding active centers (probably the LF2 of Rouxhet and co-workers) brought about in one case by the greater mobility of surface hydroxyls and, in the other, by a greater number of them. Part of the adsorbed water was now converted into active hydroxyls, resulting in an increase in the amount of n-ba adsorbed upon them. The detector response was consequently increased. Chromatographic analysis showed that, also in this case, peak III was free from olefins. Peak II diminished and shifted to higher temperatures. As is known, the presence of water should result in a decrease in the number of Lewis sites which would be converted into Brfnsted acid centers. Since peak II was due, as stated above, to decomposition at high temperatures during the TPD of nba strongly adsorbed on Lewis centers, the peak should decrease. (b) Saturated surface. In Fig. 6, curves a, b, and c are the same as in Fig. 4 and corrrespond to adsorption of n-ba at different temperatures. Curves a', b', and c' were obtained at the same temperatures (room temperature, 100, and 150°C, respectively) but water was adsorbed up to surface saturation prior to the adsorption of n-ba. Comparison of both series of curves reveals the following features: (i) Peak I is larger in the experiments with preadsorbed water. In each series it decreases as adsorption (and outgassing) temperature increases. The explanation is straightforward: in peak I there was no limitation for adsorption; saturation was not reached under experimental conditions. Therefore, when water Journal of Colloid and Interface Science. Vol. 102, No. 1, November 1984
was preadsorbed, peak I was larger than when n-ba was adsorbed on a dry silicaalumina. Due surely to a competition of adsorbates, the peak was smaller than that corresponding to the addition of the single peaks of water and n-ba. Pumping at higher temperatures removed progressively larger portions of the weakly held species contributing to this peak. (ii) Peak III is much larger in the experiments using water. The explanation has been previously given: water increased the number of centers that produced peak III. In each series of experiments, peak III increased also with adsorption temperature, due to the higher mobility of hydroxyl groups that also tended to increase the number of such centers. (iii) In peak II the same effect was observed: the peak increased with the presence of water and with increasing temperature of adsorption. The latter has been previously explained. The former effect is now opposite to the decrease observed when small amounts of water were preadsorbed. This fact must be related to the already mentioned persistence of water on the surface of silica-alumina up to higher temperature. The conversion of Lewis into Brrnsted active centers and the competition of water and n-ba for the same sites must also complicate the interpretation of the experimental results. (iv) Finally, peak II shifted to higher temperatures when water was previously adsorbed (Figs. 5 and 6). Besides that, chromatographic analysis showed that, in this case, the mixture of olefins contained a larger proportion of 1butene. The same phenomenon had been already reported (6) when the alumina content of the sample was increased. In both cases, the effect must be produced by a slight modification in the nature of the active center due to an alteration of its environment.
Activation Energies Activation energies of desorption have been determined by the method described by Cvetanovic and Amenomiya (8). A first-order
n-BUTYLAMINE DESORPTION FROM SILICA-ALUMINA SURFACE desorption from an homogeneous surface, if the rate constant is given by the Arrhenius equation, can be expressed by 2 log TM -- log fl = E/2.303TM + B
(1)
where E stands for the activation energy of desorption, Ed, if readsorption on the sample is negligible or for the heat of adsorption, £xH, when readsorption occurs freely. Since weak adsorption of n-ba is probably non activated, both A H and Ea are identical. Equation [ 1] can be used to obtain E experimentally by varying the heating rate fl and measuring the corresponding temperature at the peak maximum, TM. From the slope of the plot: 2 log TM - log fl vs I/TM, values of E for peaks I and II were determined through experiments where adsorption of the standard dose was carded out at room temperature. Peak III, although clearly distinct under cetain conditions, is too broad to allow a correct estimation of the shift of its maxim u m with changes in ft. From the corresponding plots for peaks I and II (Fig. 7) values of 21 and 51 kcal mole -1, respectively, were obtained. The experimental points show considerable dispersion and, on the other hand, the theoretical basis of the model differ from the actual system under study. Thus, the above values are to be taken only as indicative. Notwithstanding this limitation, the value of 21 kcal mole -1 obtained for this first peak would be a plausible figure as an average desorption energy of n-ba adsorbed on the heterogeneous surface. The value of 51 kcal
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mole -~ for peak II corresponds to the activation energy for decomposition of the n-ba molecule, which otherwise would have remained attached to the centers L of the surface up to higher temperatures. This value is consistent with that of 52 kcal mole -1 given for the energy of deamination of diethylamine on silica-alumina (21). CONCLUSIONS Temperature-programmed desorption of nba has been found to be an useful technique for studying the nature of active acid centers on the surface of oxides. Previous conclusions about the existence of Lewis acid centers on the surface of silica-alumina are confirmed and extended. Convincing evidence is afforded for the increase of the number of the strongest Br6nsted acid centers, responsible for peak III, when the temperature of the adsorption was increased. Also, the slow conversion of peak I into peak II favored by an increase in temperature of adsorption is clearly demonstrated. The proposed explanation based on the mobility of O H groups on the surface seems to be the only possible one. The value of the activation energy of desorption obtained for peak II, very similar to the energy required for the decomposition of diethylamine, confirmed the idea that this decomposition was the determining step in the formation of peak II and that this step occurred during the TPD, when the temperature reached a certain value. The role played by preadsorbed water is clear as far as peak I and peak III are concerned. Particularly, the increase in strong Br6nsted acid sites brought about by the presence of preadsorbed water is clearly established. In contrast, the effect of water on peak II is more obscure. The increase in lbutene in the mixture of olefins was also found by Takahashi et al. (6) but has not yet received satisfactory explanation.
I 2.5 103/TH
FIG. 7. Dependence of peak temperature on heating rate.
REFERENCES 1. Tanabe, K., "Solid Acids and Bases." Academic Press, New York/London, 1970. Journal of Colloid and Interface Science, Vol. 102, No. 1, November 1984
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GUIL, HERRERO, AND RUIZ PANIEGO
2. Forni, L., CataL Rev. 8, 65 (1967). 3. Tamele, M. W., Discuss. Faraday Soc. 8, 270 (1950). 4. Zettlemoyer, A. C., and Chessick, J. J., J. Phys. Chem. 64, 1131 (1960). 5. Morimoto, T., Imai, J., and Nagao, M., J. Phys. Chem. 78, 704 (1974). 6. Takahashi, M., Iwasawa, Y., and Ogasawara, S., J. Catal. 45, 15 (1976). 7. Casquero Ruiz, J. D., Guil Pinto, J. M., and Ruiz Paniego, A., An. Quire. 77, 348 (1981). 8. Cvetanovic, R. J., and Amenomiya, Y., "Advances in Catalysis," Vol. 17, p. 103. Academic Press, New York, 1967. 9. McClellan, A. L., and Harnsberger, H. F., £ Colloid Interface Sci. 23, 577 (1967). 10. Mieville, R. L., and Meyers, B. L., J. CataL 74, 196 (1982). 11. Peri, J. B., in "Third Congress on Catalysis," Vol. II, p. 1100. North-Holland, Amsterdam, 1965.
Journal of Colloid and Interface Science, Vol. 102, No. 1, November 1984
12. Peri, J. B., J. Phys. Chem. 72, 2917 (1968). 13. Koubek, J., Volf, J., and Pasek, J., J. Catal. 38, 385 (1975). 14. Peri, J. B., J. Phys. Chem. 70, 3168 (1966) 15. Peri, J. B., J. Catal. 41, 227 (1976). 16. Rouxhet, P. G., and Sempels, R. E., J. Chem. Soc. Faraday Trans. 1 70, 2021 (1974). 17. Sempels, R. E., and Rouxhet, P. G., J. Colloid Interface Sci. 55, 263 (1976). 18. Scockart, P. O., and Rouxhet, P. G., J. Colloid Interface Sci. 86, 96 (1982). 19. Hair, M. L., "Infrared Spectroscopy in Surface Chemistry." Dekker, New York, 1967. 20. Morimoto, T., Nagao, M., and Imai, J., Bull. Chem. Soc. Jpn. 44, 1282 (1971). 21. Ebeid, M., and Pasek, J., Collect. Czech. Chem. Commun. 35, 2166 (1970).
Received January 31, 1984; accepted April 30, 1984 Temperature-programmed desorption (TPD) of n-butylamine (n-ba) has been used as a tool to investigate the nature of the active sites on the surface of a silica-alumina gel. On dry samples, at low temperature and short times of adsorption, the TPD spectra show two peaks: the first peak corresponds to n-ba weakly held that desorbs around 150°C whereas the second one is composed of propylene and 1-butene formed during the TPD at temperatures about 400°C. As temperature or time of adsorption is increased, a slow conversion of the first into the second peak is observed. At increasing adsorption temperatures, an intermediate peak develops around 300°C. When water is preadsorbed, the height and shape of the first peak change slightly, the change is more pronounced in the last peak and is very important in the intermediate one. Conclusions about the nature o f the active sites on the surface are drawn from these experimental facts. Finally, apparent activation energies of desorption for the first two peaks are calculated. © 1984AcademicPress,Inc. INTRODUCTION
A number of methods have been applied for the determination of the number and strength of the acid centers that exist on the surface of oxide catalysts, particularly in the case of catalysts like silica-alumina, very commonly used in many reactions such as cracking, isomerization or polymerization of hydrocarbons where these centers play a prominent role (1, 2). Most of these studies are based on the interaction of the surface with a probe molecule, monitored by a suitable experimental technique. The probe nbutylamine, a strong base, is one of these useful molecules. The method of titration of acid centers with n-ba (3) can distinguish acid centers of different strength using various indicators but it cannot identify the nature of these centers, i.e., whether they are Brrnsted or Lewis acids. The same may be said of the adsorption of n-ba from the gas phase (4). Infrared spectroscopy (5) has made it possible to ascertain the presence on the surface of different species of n-ba molecules that have reacted with acid centers of different kinds. A drawback of this technique is its
nonquantitative character and the fact that it is difficult to make experiments at various temperatures. A useful technique is also the temperature-programmed desorption o f n-ba (6) on silica-alumina that gives spectra containing more than one peak, showing the existence of different species adsorbed on the surface. The present paper discusses the information about the surface of a silica-alumina gel obtained through the last technique using nbutylamine as the probe molecule. The influence of such a variable as temperature or state of hydration of the surface is also considered. EXPERIMENTAL
The silica-alumina used in this work was a commercial sample LA-3P of Ketjen, Amsterdam, in cylinder form, which contained 13.8% alumina by weight. This catalyst had been characterized previously (7), its BET area being 430 m 2 g-1 (area of a N2 molecule in the monolayer = 0.162 n m 2) and its pore volume 0.51 cm 3 g-l. The same catalyst sample, 0.69 g, was used in all the experi111
Journal of Colloid and Interface Science, Vol. 102,No. 1, November1984
0021-9797/84 $3.00 Copyright© 1984by AcademicPress,Inc. All fightsof reproductionin any form reserved.
112
GUIL, HERRERO, AND RUIZ PANIEGO
ments, being activated prior to the adsorption measurements for 1 hr in oxygen at 500°C, followed by outgassing at the same temperature for about 16 hr. The n-ba purified by fractional distillation had a purity better than 99.8%. High purity helium was used as cartier gas. The apparatus and cell used in the T P D studies were similar to those described by Cvetanovic and Amenomiya (8). The thermal conductivity detector and the flow control system of a HewlettPackard 5710 gas chromatograph were used. Desorbed products were condensed at the detector exit for chromatographic analysis. A temperature programmer, Stanton Redcroft 681, allowed 10 different heating rates in the range 2-20°C min -l. Most of the experiments were carried out at a heating rate of 8°C min -l. Nevertheless, since a linear relationship between detector signal and heating rate was found, it was possible to compare spectra obtained at different values of the latter by only applying a proportionality factor. The activated sample was cooled down to the temperature of adsorption and n-ba was admitted into the cell in a measured dose. Unless otherwise stated, the standard dose was 2.2 X 10-4 mole at a pressure of 50 m m H g (1 m m H g = 133.3 Pa). At room temperature, this dose was completely taken up by the sample within the first 30 min leaving 0.4 X 1014 molecules cm -2 on the surface. Assuming for the area of the adsorbed n-ba molecule, if perpendicular to the surface, a value of 0.20 n m 2 (9), a coverage of 0.09 was attained for the standard dose; if the molecule adsorbed was lying flat on the surface, the molecular area would be 0.32 nm 2 (9) and the fraction covered would increase to 0.14. At higher temperatures some n-ba remained in the gas phase even after long equilibration times. Prior to the T P D experiments the sample was outgassed at adsorption temperature for 1 hr. Adsorption of water was carried out, in most cases, by exposing the sample to water vapor at room temperature for about 2 hr and pumping for 2 hr at the same temperature. In some cases, Journal of Colloid and Interface Science, Vol. 102, No. 1, November 1984
a measured dose of water o f 0.7 X was admitted into the cell.
10 -4
mole
RESULTS AND DISCUSSION The influence of several variables upon experimental results is now described and discussed.
Initial Coverage The T P D spectra obtained with different initial doses of n-ba are given in Fig. I. Curve e corresponds to the standard dose. Both adsorption and evacuation previous to the TPD were carded out at room temperature. With the smallest dose, only a peak (peak II) was observed appearing at about 400°C. As the initially adsorbed amount increased, another peak (peak I) gradually appeared at lower temperatures, around 150°C. Between them, a third peak seemed to be present. Peaks I and II grew as the initial coverage increased, the former without limitation whereas the latter apparently reached a saturation value. Similar observations for t-butylamine on several adsorbents have been reported previously (10). In the insert o f Fig. 1, the height of peak II is plotted against the amount initially adsorbed. The plot, in which results from other experiments were included, clearly demonstrated that peak II reached a saturation value. Chromatographic analysis showed peak I to correspond to n-ba; peak II was due to desorption of a mixture of propylene and 1-butene. When the adsorption and previous outgassing were performed at 150°C, the results plotted in Fig. 2 were obtained. The maxima of peak II were higher than in Fig. 1 in spite of the fact that no saturation value was yet attained as can be seen in the insert of Fig. 2, although the curve seemed to tend toward saturation. When previous outgassing was carried out at higher temperature, the net result was a decrease in the height o f the maxima of peak I and a shift toward higher temperatures. Peak II remained unaltered.
113
n-BUTYLAMINE DESORPTION FROM SILICA-ALUMINA SURFACE
5--
f ~
io
o
o°10
L O
U
O
0
£
100
200
300
400
t/°C
FIo. 1. Dependence of TPD spectra on initial adsorbed amount: (a) 0.04; (b) 0.13; (c) 0.23; (d) 0.31; (e) 0.50; (£) 0.58 × l014 molecules cm -2. Adsorption at room temperature. Insert: detector response for peak II in arbitrary units vs initial adsorbed amount in molecules cm -2 × 10 -14.
All these experiments revealed the existence of at least two types of centers, as previously discussed by Takahashi et al. (6). Peak I consists of n-ba physisorbed or weakly che-
misorbed on centers with adsorption energies distributed over a wide range, being strongly affected by outgassing at higher temperatures. On the centers responsible for the formation II
Q
t~ O r
LL-
10
~10 E
t-
el
0
eJ
C
O
0
0
0
E
L 100
200
300
400 t l°C
FIG. 2. Dependence of TPD spectra on initial adsorbed amount: (a) 0.09; (b) 0.23; (c) 0.45 X 1014 molecules cm -2. Adsorption at 150°C. Insert: detector response for peak II in arbitrary units vs initial adsorbed amount in molecules cm -2 X 10 -14. Journal of Colloid and Interface Science, Vol. 102, No. 1, November
1984
114
GULL, HERRERO, AND RUIZ PANIEGO
of peak II, the n-ba, more strongly held, decomposed into propylene and 1-butene. From these results, it is not clear whether this decomposition occurred immediately after the adsorption or during the TPD as the sample temperature was raised to that corresponding to peak II. This latter possibility seems more likely; the former would mean that propylene and 1-butene remained adsorbed on the surface up to 400°C. Peri (11, 12) found that adsorbed 1-butene and even its polymerization products were removed from a silica-alumina surface at lower temperatures. On the other hand, Koubek et al. (13) reported that decomposition reactions of amines on alumina take place at higher temperatures.
Time and Temperature of Adsorption A slow conversion of peak I into peak II was clearly shown when longer adsorption times (Fig. 3) or higher adsorption temperatures (Fig. 4) were used. In this case, the most remarkable effect was the progressive appearance of a third peak between the other
two, with its maximum at about 300°C. Chromatographic analysis showed this peak to correspond also to n-ba. It must be pointed out that when outgassing, but not adsorption, took place at higher temperatures this peak did not appear. The effects just described can be interpreted with the aid of the scheme L
, II (C3/C4)
n-baga/
1
B
, III
Small doses of n-ba are strongly adsorbed on centers designated by L in the scheme. These centers can be identified as Lewis acid centers that accept the nitrogen lone electron pair to form a strong bond. Peri (11, 12, 14, 15) has demonstrated the existence of several types of such centers. The number of these L centers is limited and they are soon saturated so that eventually chemisorpfion upon less strong centers and physical adsorption
--3 d
EIO--
I
100
I
I
200
300
I 400 t / ° C
FIG. 3. Influence of adsorption time at room temperature: (a) 15 rain; (b) 30 rain; (c) 125 rain; (d) 67
hr. Standarddose. Journal of Colloid and Interface Science, Vol. 102, No. 1, November 1984
115
n-BUTYLAMINE DESORPTION F R O M S I L I C A - A L U M I N A SURFACE
II
A
L
~I0
-
-
I o
I 100
J J
I 200
300
400
~IO C
FIG. 4. Adsorption temperature; (a) r.t.; (b) 100°C; (c) 150°C; (d) 200 °C. Standard dose.
become competitive (A). It is very probable that weak chemisorption occurs on Br6nstedtype acid centers formed by hydroxyl groups situated on Si or Al atoms on the surface. Morimoto et al. (5) identified by IR spectroscopy both types of acid centers in the adsorption of n-ba on silica-alumina; n-ba on Lewis acid sites was observed even after evacuation at 500°C, On the other hand they also showed the presence of more than two kinds of Br6nsted sites with different acid strength. The n-ba adsorbed on the stronger of these sites remained on the surface even when the system was outgassed at temperatures up to 300°C. During the T P D experiment, desorption of physically adsorbed n-ba overlapped that of the weakly chemisorbed probe on O H groups eventually to form peak I. The n-ba strongly chemisorbed on Lewis acid centers remained on the surface until higher temperatures were reached and then it decomposed to give propylene and 1-butene giving rise to peak II. Adsorbed pyridine, on the contrary, being a more stable species although a weaker
base, remains on the surface up to 500°C (5, 6). The slow conversion of the peak I into peak II must be ascribed to a surface process, since there was no n-ba in the gas phase (residual pressure was null). In order to explain it, a certain mobility, although reduced, of the OH groups throughout the surface must be postulated. In this way, an OH group on top of an A1 atom can be displaced out of its position leaving exposed the A1 atom that becomes a Lewis acid site on which a molecule of n-ba from a neighbor position on the surface can be more strongly adsorbed, the process being energetically favorable. Since the mobility of O H groups on the surface increases with temperature, a larger peak II was obtained in experiments at higher temperature. On the contrary, an increase in outgassing temperature did not modify peak II because outgassing produced a displacement toward the left side in the proposed scheme, balancing the effect of the larger mobility of O H groups just described. This effect explains also the fact that at small Journal of Colloid and Interface Science, Vol. 102, No. 1, November1984
116
GUIL, HERRERO, AND RUIZ PANIEGO
adsorption times a saturation seemed to have been reached at room temperature but not at 150°C under the same experimental conditions. Some evidence of O H mobility will be furnished below. Finally, peak III can be attributed to adsorption on more strongly acid hydroxyl groups. Rouxhet and co-workers (16-18) identified such groups (LF2) on the surface of silica-alumina. These LF2 centers are thought to be silanol groups adjoining an A1 atom and are only a small part of the surface hydroxyls. As already said, Morimoto et aL (5) also found strong Br/Snsted centers upon which n-ba remained adsorbed at temperatures up to 300°C. At low temperature there are only a few of such centers. The higher mobility of hydroxyls promoted by an increase in temperature increased the number of those centers upon which molecules of nba were adsorbed and later, during TPD, desorbed without decomposition. In order to arrive at a better understanding
of these phenomena a new series of experiments were performed with the silica-alumina surface modified by the presence of water. W a t e r on S i l i c a - A l u m i n a
When the sample was brought into contact with a small amount of water at room temperature and then outgassed, the T P D spectrum shown in Fig. 5 (curve a) was obtained. Water was nonselectively adsorbed on centers covering a wide range of adsorption energies. Peri (14) also found, by IR spectroscopy, a nonselective adsorption of water on silicaalumina samples. When after adsorption of the same dose at room temperature the system was heated up to 200°C, kept at this temperature and then allowed to cool down again, curve b was obtained. It can be seen clearly that a rearrangement on the surface has taken place: some water has migrated and has been adsorbed preferentially on ten-
d
u
"~10 --
100
200
300
t,00
t/°C
FIG. 5. Influence of the presence of water: Curves a and b: adsorption of 0.13 × ]014 water molecules cm -2 at (a) r.t.; (b) 200°C. Curves c and d: adsorption of the standard dose of n-ha on dry sample at (c) r.t.; (d) 200°C. Curve c': adsorption at r.t. of the standard dose of n-ha on a sample with preadsorbed water (0.13 × 1014 molecules cm-2). Journal of Colloid and Interface Science, Vol. 102, No. 1, November 1984
n-BUTYLAMINE DESORPTION FROM SILICA-ALUMINA SURFACE
ters within a narrower energy range producing the small peak that can be observed around 325°C. High temperature favors the mobility of water on the surface but, even at 200°C, this mobility is restricted; a long period of time was required for the low and broad peak in curve b to appear. The adsorption of larger amounts of water, up to the solid wetting, yielded T P D spectra with broad peaks, whose maxima were attained at about 125°C, provided that outgassing of samples was carried out at room temperature. Long tails extended beyond 525°C. The shape of the curves depended strongly on initial conditions; the tails were specially sensitive to heating rates, showing that a nonequilibrium desorption process was involved. These results are in agreement with experimental facts firmly established some time ago: desorption of molecular water at relatively low temperature, followed by slow desorption of water formed by condensation of hydroxyl groups on the surface (19, 20).
117
TPD on Hydrated Surface (a) Partially hydrated surface. The presence of water modifies the T P D spectra of nba on silica-alumina. Thus, in Fig. 5, curve c corresponds to adsorption of n-ba at room temperature on silica-alumina evacuated at 500°C; in curve c' a small amount of water (the same as in curve a) had been previously adsorbed also at room temperature. It must be noted that the detector response for water was about one-half the response found for nba or for olefins. In Figs. 5 and 6 desorbed water was present in the entire spectrum, mixed with the other desorption products, and therefore the corresponding concentrations could no longer be calculated. For this reason, the plot ordinate is now given in arbitrary units of detector response. It can be seen that the presence of a small amount of water hardly produced any difference in peak I. The difference, however, was important in the intermediate peak III. This difference could not be accounted for by
15
I1
o;
I
P
O'
J
c
c' III
//
5
/,
'
!
'\
t
11 1/
/ /
"q
~
/~'~.~
/
~
rI i/
/
,
~,~_~
/I
Jl
//
~
Ii \ \
,','
'., \\
,"
~
--~_-,.-~
\
~\ \ \\
I
I
I
I
100
200
300
/,00
-t l°C
FIG. 6. Comparison of TPD spectra obtained from dry and wet samples: Adsorption of the standard dose of n-ba on a dry sample at (a) r.t.; (b) 100°C; (c) 150°C. Adsorption of the standard dose of n-ba on a sample saturated with water at (a') r.t.; (b~) 100°C; (c') 150°C. Journal of Colloid and Interface Science, Vol. t02, No. 1, N o v e m b e r 1984
118
GUIL, HERRERO, AND RUIZ PANIEGO
simultaneous desorption of water and n-ba in this temperature range: the detector response is much larger than the corresponding to the sum of n-ba and water (curve a) desorbed in this range. On the other hand, the increase of peak III was similar to that observed when the adsorption temperature was higher (Fig. 5, curves c' and d). Curve d is the same as in Fig. 4 (adsorption at 200°C). The similarity of both curves suggests an increase in the number of the corresponding active centers (probably the LF2 of Rouxhet and co-workers) brought about in one case by the greater mobility of surface hydroxyls and, in the other, by a greater number of them. Part of the adsorbed water was now converted into active hydroxyls, resulting in an increase in the amount of n-ba adsorbed upon them. The detector response was consequently increased. Chromatographic analysis showed that, also in this case, peak III was free from olefins. Peak II diminished and shifted to higher temperatures. As is known, the presence of water should result in a decrease in the number of Lewis sites which would be converted into Brfnsted acid centers. Since peak II was due, as stated above, to decomposition at high temperatures during the TPD of nba strongly adsorbed on Lewis centers, the peak should decrease. (b) Saturated surface. In Fig. 6, curves a, b, and c are the same as in Fig. 4 and corrrespond to adsorption of n-ba at different temperatures. Curves a', b', and c' were obtained at the same temperatures (room temperature, 100, and 150°C, respectively) but water was adsorbed up to surface saturation prior to the adsorption of n-ba. Comparison of both series of curves reveals the following features: (i) Peak I is larger in the experiments with preadsorbed water. In each series it decreases as adsorption (and outgassing) temperature increases. The explanation is straightforward: in peak I there was no limitation for adsorption; saturation was not reached under experimental conditions. Therefore, when water Journal of Colloid and Interface Science. Vol. 102, No. 1, November 1984
was preadsorbed, peak I was larger than when n-ba was adsorbed on a dry silicaalumina. Due surely to a competition of adsorbates, the peak was smaller than that corresponding to the addition of the single peaks of water and n-ba. Pumping at higher temperatures removed progressively larger portions of the weakly held species contributing to this peak. (ii) Peak III is much larger in the experiments using water. The explanation has been previously given: water increased the number of centers that produced peak III. In each series of experiments, peak III increased also with adsorption temperature, due to the higher mobility of hydroxyl groups that also tended to increase the number of such centers. (iii) In peak II the same effect was observed: the peak increased with the presence of water and with increasing temperature of adsorption. The latter has been previously explained. The former effect is now opposite to the decrease observed when small amounts of water were preadsorbed. This fact must be related to the already mentioned persistence of water on the surface of silica-alumina up to higher temperature. The conversion of Lewis into Brrnsted active centers and the competition of water and n-ba for the same sites must also complicate the interpretation of the experimental results. (iv) Finally, peak II shifted to higher temperatures when water was previously adsorbed (Figs. 5 and 6). Besides that, chromatographic analysis showed that, in this case, the mixture of olefins contained a larger proportion of 1butene. The same phenomenon had been already reported (6) when the alumina content of the sample was increased. In both cases, the effect must be produced by a slight modification in the nature of the active center due to an alteration of its environment.
Activation Energies Activation energies of desorption have been determined by the method described by Cvetanovic and Amenomiya (8). A first-order
n-BUTYLAMINE DESORPTION FROM SILICA-ALUMINA SURFACE desorption from an homogeneous surface, if the rate constant is given by the Arrhenius equation, can be expressed by 2 log TM -- log fl = E/2.303TM + B
(1)
where E stands for the activation energy of desorption, Ed, if readsorption on the sample is negligible or for the heat of adsorption, £xH, when readsorption occurs freely. Since weak adsorption of n-ba is probably non activated, both A H and Ea are identical. Equation [ 1] can be used to obtain E experimentally by varying the heating rate fl and measuring the corresponding temperature at the peak maximum, TM. From the slope of the plot: 2 log TM - log fl vs I/TM, values of E for peaks I and II were determined through experiments where adsorption of the standard dose was carded out at room temperature. Peak III, although clearly distinct under cetain conditions, is too broad to allow a correct estimation of the shift of its maxim u m with changes in ft. From the corresponding plots for peaks I and II (Fig. 7) values of 21 and 51 kcal mole -1, respectively, were obtained. The experimental points show considerable dispersion and, on the other hand, the theoretical basis of the model differ from the actual system under study. Thus, the above values are to be taken only as indicative. Notwithstanding this limitation, the value of 21 kcal mole -1 obtained for this first peak would be a plausible figure as an average desorption energy of n-ba adsorbed on the heterogeneous surface. The value of 51 kcal
~_7 oo,
" %//
I
I
o
o.
o
o
o
I 1.5
//
I
I
2.3
2./,
119
mole -~ for peak II corresponds to the activation energy for decomposition of the n-ba molecule, which otherwise would have remained attached to the centers L of the surface up to higher temperatures. This value is consistent with that of 52 kcal mole -1 given for the energy of deamination of diethylamine on silica-alumina (21). CONCLUSIONS Temperature-programmed desorption of nba has been found to be an useful technique for studying the nature of active acid centers on the surface of oxides. Previous conclusions about the existence of Lewis acid centers on the surface of silica-alumina are confirmed and extended. Convincing evidence is afforded for the increase of the number of the strongest Br6nsted acid centers, responsible for peak III, when the temperature of the adsorption was increased. Also, the slow conversion of peak I into peak II favored by an increase in temperature of adsorption is clearly demonstrated. The proposed explanation based on the mobility of O H groups on the surface seems to be the only possible one. The value of the activation energy of desorption obtained for peak II, very similar to the energy required for the decomposition of diethylamine, confirmed the idea that this decomposition was the determining step in the formation of peak II and that this step occurred during the TPD, when the temperature reached a certain value. The role played by preadsorbed water is clear as far as peak I and peak III are concerned. Particularly, the increase in strong Br6nsted acid sites brought about by the presence of preadsorbed water is clearly established. In contrast, the effect of water on peak II is more obscure. The increase in lbutene in the mixture of olefins was also found by Takahashi et al. (6) but has not yet received satisfactory explanation.
I 2.5 103/TH
FIG. 7. Dependence of peak temperature on heating rate.
REFERENCES 1. Tanabe, K., "Solid Acids and Bases." Academic Press, New York/London, 1970. Journal of Colloid and Interface Science, Vol. 102, No. 1, November 1984
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GUIL, HERRERO, AND RUIZ PANIEGO
2. Forni, L., CataL Rev. 8, 65 (1967). 3. Tamele, M. W., Discuss. Faraday Soc. 8, 270 (1950). 4. Zettlemoyer, A. C., and Chessick, J. J., J. Phys. Chem. 64, 1131 (1960). 5. Morimoto, T., Imai, J., and Nagao, M., J. Phys. Chem. 78, 704 (1974). 6. Takahashi, M., Iwasawa, Y., and Ogasawara, S., J. Catal. 45, 15 (1976). 7. Casquero Ruiz, J. D., Guil Pinto, J. M., and Ruiz Paniego, A., An. Quire. 77, 348 (1981). 8. Cvetanovic, R. J., and Amenomiya, Y., "Advances in Catalysis," Vol. 17, p. 103. Academic Press, New York, 1967. 9. McClellan, A. L., and Harnsberger, H. F., £ Colloid Interface Sci. 23, 577 (1967). 10. Mieville, R. L., and Meyers, B. L., J. CataL 74, 196 (1982). 11. Peri, J. B., in "Third Congress on Catalysis," Vol. II, p. 1100. North-Holland, Amsterdam, 1965.
Journal of Colloid and Interface Science, Vol. 102, No. 1, November 1984
12. Peri, J. B., J. Phys. Chem. 72, 2917 (1968). 13. Koubek, J., Volf, J., and Pasek, J., J. Catal. 38, 385 (1975). 14. Peri, J. B., J. Phys. Chem. 70, 3168 (1966) 15. Peri, J. B., J. Catal. 41, 227 (1976). 16. Rouxhet, P. G., and Sempels, R. E., J. Chem. Soc. Faraday Trans. 1 70, 2021 (1974). 17. Sempels, R. E., and Rouxhet, P. G., J. Colloid Interface Sci. 55, 263 (1976). 18. Scockart, P. O., and Rouxhet, P. G., J. Colloid Interface Sci. 86, 96 (1982). 19. Hair, M. L., "Infrared Spectroscopy in Surface Chemistry." Dekker, New York, 1967. 20. Morimoto, T., Nagao, M., and Imai, J., Bull. Chem. Soc. Jpn. 44, 1282 (1971). 21. Ebeid, M., and Pasek, J., Collect. Czech. Chem. Commun. 35, 2166 (1970).