2276
Znd. Eng. Chem. Res. 1991,30, 2276-2279
Peculiar Pore Structure of the Coke Coating Formed on Pt-Sn/y-A1203 Catalystst Piotr Kirszensztejn,* Zenon Foltynowicz, and Leszek Wachowski Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60- 780 Poznan, Poland
Pt-Sn/y-A1203 catalysts for the conversion of hexane displayed propensity toward surface coking. The changes of catalyst porosity as a consequence of coke deposition are described. The carbonaceous deposit was found to significantly enlarge the cumulative pore volume V-, cumulative pore surface S,,, and specific surface area SBET. As a result of a comparison of the nitrogen isotherms as well as pore size distribution curves of the samples before and after coke deposition, coke was found to have its own porous structure which was developed on the basis of the catalyst porous system.
Introduction Carbon formation is an unavoidable side reaction in the catalytic processing of hydrocarbons (Butl and Petersen, 1988). A carbonaceous overlayer on the catalyst causes severe deactivation, which is the main drawback of the Pt/A1203reforming catalyst (Lieske et al., 1987; Oudar and Wise, 1985). This can be attributed to such factors as active-center blocking and/or partial pore mouth blocking by the deposited carbonaceous material (Butl and Petersen, 1988; Scaroni et al., 1985). In most papers devoted to investigation of catalyst texture changes as a consequence of coke deposition, coking is claimed to be responsible for a decrease in the specific surface (Appleby et al., 1962; Ramser and Hill, 1958; Bilbao et al., 1985), pore surface area, and pore volume (Bilbao et al., 1985; Langner, 1981; Stiegel, 1981; Evans et al., 1983). It is supposed that coke is mostly deposited in narrow pores, in particular in those of a diameter smaller than 50 A (Dejaifve et al., 1981; Beuther et al., 1980). Ramser and Hill (1958) reported that the formed coke was not uniformly distributed in the catalyst, because a decrease in pore volume was twice the volume of the deposited coke. The work of Przystajko et al. (1986) indicates that the coke has its own specific porosity independent of the texture of pores of the oxide matrix. The coke specific porosity has also been observed by Butterworth and Scaroni (1985) as well as Egiazorow et al. (1982). In our previous paper we described the effect of the preparation method on the porosity of Pt-Sn/y-Al2OB catalyst (Kirszensztejn et al., 1989). It was found that this catalyst undergoes surface coking during the reaction of hexane dehydrogenation. An investigation of the changes in surface area and porous pattern of Pt-Sn/y-A1203 catalyst (prepared in various ways) as a result of coke coating was the aim of this work. Experimental Section Description of Materials. Pt/y-A1203 (denoted as A1203) and its modification containing tin additives (different amounts) were prepared by three different preparation techniques according to the procedure described elsewhere (Kirszensztejn et al., 1989; Kirszensztejn, 1991a,b). According to the first preparation methods (denoted as the A series) tin was introduced (by impregnation) as acid solution (tin metal was dissolved in aqua regia); in the second method, tin was introduced as anhydrous SnCl, dissolved in acetone (denoted as B); and the final series, denoted as D, was prepared by the coprecipitation method where Sn(CH,COO), and A1(OC3H7)3 This paper is dedicated to the memory of Professor Herbert A. Laitinen.
ossa-5ss5/91/2630-2276$02.5010
Table I. Analysis of Nitrogen Adsorption Data: Cumulative Pore Volume (V,,), Cumulative Pore Surface and Mean Area (So,,), Specific Surface Area (SBm), Statistical Pore Radius (rav) atomic ratio V-, cm3 S, sample Pt:Sna g-l x IO-' m2 g-' SBm,m2 g-1 r,,, A Al,On A1;O;-C A4 A4C A8 A8-C B4 B4-C B8 B8-C D4 D4-C D8 D8-C
4.0 10.2 4.0 8.3 4.0 9.5 3.8 8.0 3.8 8.7 4.4 10.2 4.2 10.3
1:4 1:4 1:8 1:8 1:4 1:4 1:8 1:8 1:4 1:4 1:8 1:8
285 558 256 507 267 599 250 398 269 566 308 685 302 665
270 531 244 513 238 505 241 419 246 543 267 616 269 586
28 36 31 32 30 31 30 40 28 31 28 29 27 31
In all cases the content of Pt was constant and equal to 0.5% wfw.
Table 11. Contribution (in percent) of Pores of a Given Cross Section to Cumulative Volume of Pores and the Carbonaceous Deposit' Der gram of Catalyst % of V,,
samDle 30-40 A1203 A1203-C A4 A4-C A8 A8-C B4 B4-C B8 B8-C D4 D4-C D8 D8-C a
7.8 5.2 7.2 7.3 7.2 7.7 8.7 4.4 10.1 8.2 7.4 9.8 8.9 8.5
at pore diam range (A)
40-60 60-100 20.2 36.7 15.0 35.0 20.9 42.7 21.4 33.1 18.2 35.5 17.4 37.0 19.5 35.3 12.4 26.5 18.5 32.9 17.7 35.0 24.6 48.7 21.7 41.0 26.0 43.1 24.6 37.9
100-200 24.3 41.7 27.3 33.6 29.0 29.6 33.3 37.6 28.9 29.4 18.3 23.3 17.8 27.4
200-300 3.1 3.1 1.9 4.7 3.3 3.7 3.2 16.1 4.1 5.2 1.0 1.5 1.5 1.6
mg of C l g of catal 0 1.78 0 0.82 0 1.55 0 0.82 0 1.58 0 0.72 0 0.82
After catalytic reaction (6 h) at atmospheric pressure.
were used as precursors. On such prepared samples (for all series) after their calcination (5 h, 823 K), Pt was introduced by the impregnation method (acetone solution of H2PtC&)in the amount of 0.5% w/w. The PkSn molar ratio and the sample notation are given in Table I. These samples were tested as catalysts under conditions similar to those of the reforming reaction. Catalytic tests were carried out at a temperature of 723 K for 6 h with hexane/hydrogen mixtures (1/6.5 v/v) at total atmospheric pressure. The total flow rate of gases in the feed was 1200/h gas hourly space velocity (GHSV), and the contact 0 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991 2277 n 7
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Figure 1. Nitrogen adsorption-desorption isotherms (a) and pore volume distribution (b) as a function of pore diameter for samples A1103 and Al*O&. 7
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Figure 3. Nitrogen adsorption-desorption isotherms (a) and pore volume distribution (b) as a function of pore diameter for samples A8 and A8-C.
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Figure 2. Nitrogen adsorption-desorption isotherms (a) and pore volume distribution (b) as a function of pore diameter for samples A4 and A4-C.
time was 1.6 s. After the reaction completion the amounts of coke deposited in the coked samples (denoted as AlzO&, A4-C, ...) were determined by elemental analysis (see Table 11). Determination of Pore Size Distribution. Low temperature (77 K) nitrogen adsorption measurements were carried out on a Sartorius microbalance (Gravimat, type 4133). Each adsorption isotherm was made of 28 measurement points. All the conditions were the same as for all precursor samples (Kirszensztejn et al., 1989) except that the coked samples were outgassed (vacuum of lo+ Torr) at 423 K. The pore size distributions were calculated from the adsorption branch of the isotherm by the C-I method (Cranston and Inkley, 1958). As a result of calthe culation, the data on cumulative pore volume V,, cumulative surface area S,,,, and the mean pore radius rav as well as the plots of the pore volume and the pore surface area distributions as functions of the pore diameter d , were obtained.
Results and Discussion The catalytic activity of the Pt-Sn/y-A1203 catalysts changes during the reaction as a consequence of the socalled “coke” formation on the catalyst surface. During the change of the catalyst activity (as a consequence of the coke formation), it is believed that the surface chemical nature and pore dimension may change. The results of pore size distribution determination clearly show that the pore structure of the catalysts has been developed by the coke deposition on their surfaces. Adsorption-desorption isotherms for nitrogen on coke coated samples and, for the sake of comparison, on their
0
50
100 150 200 250 300 Pore diameter [
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Figure 4. Nitrogen adsorption-desorption isotherms (a) and pore volume distribution (b) as a function of pore diameter for samples B4 and B4-C.
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-8-8C
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0
50
100 150 200 250 300 Pore diameter [
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Figure 5. Nitrogen adsorption-desorption isotherms (a) and pore volume distribution (b) as a function of pore diameter for samples B8 and B8-C.
precursors are given in Figures la-7a. The total amount of the adsorbed nitrogen is, surprisingly, much higher for coke-loaded samples than for the naked precursors. As we published previously (Kirszensztejn et al., 1989), samples without coke were characterized by the isotherm of approximately type IV (according to IUPAC classification; Singh et al., 1985). Coke deposition is responsible for changing the isotherm shapes. A t a higher relative pressure, ( p / p o ) ,the isotherms of coked samples are steeper than those of initial preparations and their shape is close to that of type I1 isotherms. A comparison of the shapes of the hysteresis loops indicates the change of the type of pores occurring in the coke-loaded samples. In the case
2278 Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991 14.
12. MEO 1 O . I
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0
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100 150 200 250 300 Pore diameter [
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Figure 6. Nitrogen adsorption-desorption isotherms (a) and pore volume distribution (b) as a function of pore diameter for samples D4 and D4-C.
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100 150 200 250 300 Pore diameter [ A ]
Figure 7. Nitrogen adsorption-desorption isotherms (a) and pore volume distribution (b) as a function of pore diameter for samples D8 and D8-C.
of reference Pt/yA1203(denoted further as A1203) the hysteresis loop shape is changed from that of type H2 to the mixed type, approximated by H1 plus H4 (coke coated). In this case the shoulder point moves from p / p o = 0.40 to 0.64. This indicates large pore formation during coke deposition on the A1203 surface. In the case of A and B series samples, coke formation leads also to a change of the pore shape. New hysteresis loops may be considered as appearing as a result of superposition of type H1 and H4 loops, which proves the complex character of the pore system. This means that narrow, slitlike pores are formed apart from tubular pores of variable radius. The nitrogen adsorption isotherm on coke-coated A series samples shows a hysteresis loop extended up to p / p o = 0.64 (Figure 2a) in comparison with p / p o = 0.45 for naked samples (samples A4-C and A4, respectively). This indicates the origin of larger pores. A similar tendency can be observed among the samples of series B, especially in the case of sample B4 with lower tin content (shoulder point a t p / p o = 0.53 for B4-C in comparison with p / p o = 0.34 for B4; Figure 4a). Samples with low tin addition are characterized by larger pores in the coke layer than samples with higher contents of tin, which is indicated by the position of the shoulder point at lower p / p o ,for example, 0.5 for sample AS-C (Figure 3a) (0.45 in the case of A8). In the case of coked samples of series D, the presence of coke causes the formation of narrow pores, which can be seen in the plots of the isotherms (Figures 6a and 7a) where the hysteresis loop begins at a lower value of p / p o (0.42 and 0.32) in comparison to the naked samples (0.49 and 0.36). This indicates small pore formation during coke
deposition on the catalyst surface. The results of the pore structure determination are summarized in Table I. The values of cumulative pore volume (V-), cumulative pore surface area (S-), specific surface area (SBm),and average pore radius (r,J for coked samples are compared with those for fresh catalysts. As can be easily noticed for the samples with coke deposited on their surface, these parameters take much higher values. Generally, cumulative pore volume is 2-2.5 times bigger for coked samples and practically independent of the supporting catalyst. Moreover, the cumulative pore surface as well as specific surface area is also approximately twice as high as the initial one. This indicates that the coke formed on the catalyst surface during the investigated catalytic process has its own porous structure. Samples A4 and B4, which undergo coking in the reforming process to a considerably lesser degree than other samples from these series (obtained by impregnation), are covered with coke characterized by the smallest parameters of porous structure. The values of average pore radius, listed in Table I, are not particularly representative for the changes in porous structure occurring during coke deposition, because of their statistical meaning. The discussed changes, characterized previously by the shift of shoulder points on adsorption isotherms, can be easily observed on the plots of pore size distribution, which are presented in Figures lb-7b. A comparison of the pore distribution curves for the initial sample, A1203 (Figwe lb), and for the sample after coking, A1203-C,proves that the catalyst surface has been covered with a coke layer showing a wide spectrum of pores. Table I1 presents the contribution of pores of different diameters to the value of V-, in percent. The formed coke contains mainly mesopores of diameters ranging from 100 to 200 A. In the initial Al2O3, they make up 24% of the pore volume while in the coked A1203-Ctheir contribution reaches 42%. The contribution of the pores of diameters ranging from 60 to 100 i\ is similar in both samples (37 and 35%, respectively). A sample of A4 catalyst after coking, A4-C, displays a polydispersive pore distribution. The contribution of the pores of 60-100-A diameter to V,, in A4-C is lower than in A4, whereas the contribution of wide pores of diameters from 100 to 200 A increases from 27 to 34% and those of diameters 200-300 A from 2 to 5 % . Similar relations have been observed for sample B4-C, Figure 4b, for which the contribution of pores of diameters 200-300 A increases even more, with respect to sample B4, from 3 to 16% and the contribution of pores of diameters smaller than 60 i\ decreases from 28 to 16%. As we can see from Table 11, the catalysts of all series containing tin in the amount corresponding to the ratio P t S n = 1:4 are less susceptible to coking. An increase in Sn content (samples with ratio Pt:Sn = 1:8)favors coke deposition (catalysts A8 and B8). However, it is worth noting that the depositing coke shows a pore distribution similar to that of the catalyst it is depositing on. In the case of catalysts A8 and B8 as well as their coked forms, Figures 3b and 5b, the pores of diameters from different ranges bring almost identical contributions to V,, Table 11. It can be concluded that in both cases the matrix effect of the catalyst plays a significant role. The catalysts of series D were obtained by the coprecipitation method and differ in nature from catalysts of series A and B prepared by the impregnation method. In the previous paper one of us reported (Kania et al., 1990) that the surface of some binary oxide catalyst on the basis of r-Al,O, became blocked by coke of a nonporous nature.
Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991 2279 In our case we observed coke to have a porous structure for catalysts where tin was introduced by both coprecipitation and impregnation method. According to data from Table I, for the catalysts of the series D we observed the greatest increase in Vcumand particularly in Scum and SBm, when compared with their values for A1203and the samples of the series A and B. Thus, the coke precipitating on the surface of D series catalysts increases the contribution of the pores with diameters of 100-200 A. This is evidenced by their much greater contribution (expressed in percent) to V,, when compared to that for the naked samples, Table 11. The contribution of pores of smaller diameter decreases, independently of the P t S n ratio. It is worth noting that the forming coke has a binary pore structure. As follows from the curves illustrating the pore volume distribution, Figures 6b and 7b, for the samples of this series, the coke under formation developes in the A1203 matrix and, additionally, a branch of pores with diameters ranging from 75 to 125 A develops, which is particularly visible for the sample D8-C, Figure 7b.
Conclusions As follows from the results of determination of porosity of Pt-Sn/y-A1203 coked in hexane dehydrogenation, the formed coke not only does not block the pore structure of the parent catalyst, but its pores have their own structure. The structure is, however, determined by the texture of the catalyst surface on which the coke is being formed. The parameters of the porous structure of coked catalyst, like those of initial forms, are affected by the Pt:Sn ratio and by the catalyst preparation method. If a preparation method based on coprecipitation is applied, then it is possible to obtain catalysts of the most developed specific surface and pore surface of both pure and coked sample. Thus, the addition of tin to A1203,unlike the addition of other admixtures, leads to the formation of coke of ita own pore structure on the catalyst surface. The obtained results are consistent with those reported in a group of papers (Fiedorow et al., 1981; Butl and Petersen, 1988;Egiazorow et al., 19821, where the formation of coke of its own porous structure has been indicated. However, to the best of our knowledge, a coke with so much developed surface has never been observed. Registry No. Pt, 1440-06-4; Sn, 7440-31-5; N2, 7721-37-9; carbon, 7440-44-0.
Literature Cited Appleby, W. G.; Gibson, J. W.; Good, G. M. Coke formation in catalytic cracking. Ind. Eng. Chem. Process Des. Deu. 1962, I , 102-110. Beuther, H.; Larson, 0. A.; Perotta, A. J. The mechanism of coke formation on catalyst. Stud. Surf. Sci. Catal. 1980, 6, 271.
Bilbao, J.; Aguavo, A. T.; Arandes, J. M. Coke deposition on silciaalumina catalysts in dehydration reactions. Ind. Eng. Chem. Process Des. Dev. 1985,24,531. Butl, J. B.; Petersen, E. E. Activation, Deactivation and Poisoning of Catalysts; Academic Press: San Diego, 1988; p 97. Butterworth, S. L.; Scaroni, A. W. Carbon-coated alumina as a catalysts support. Appl. Catal. 1985, 16, 375. Cranston, R. W.; Inkley, F. A. The Determination of Pore Structures from Nitrogen Adsorption Isotherms. Ado. Catal. 1957, 9, 143. Dejaifve, P.; Auroux, A,; Gravelle, P. C.; Vedrine, J. C.; Gabelica, 2.; Derouane, E. G. Methanol Conversion on Acidic ZSM-5, Offretite, and Mordenite Zeolites. A Comparative Study of the Formation and Stability of Coke Deposits. J. Catal. 1981, 70, 123. Egiazorow, J. G.; Cherches, B. K.; Krutko, N. P.; Potapova, L. L.; Korchow, 0. A. Effect of the modification of an indium oxide catalyst by potassium oxide on coke formation in the pyrolysis of hydrocarbon row material. Neftiechimija 1982,22,602. Evans, J. W.; Trimm, D. L.; Wainwright, M. S. Effect of coke formation on the reactions of dimethyl ether on acidic oxide catalysts. Ind Eng. Chem. Process Des. Dev. 1983,22, 242. Fiedorow, R. M. J.; Przystajko, W.; Sopa, M.; Dalla Lana, I. G. The nature and Catalytic Influence of Coke Formed on Alumina: Oxidative Dehydrogenation of Ethylbenzene. J. Catal. 1981,68, 33. Kania, W.; Jurczyk, K.; Foltynowicz, 2. Effect of coke deposition on the porous structure of some alumina modifications. Acta Chim. Hung. 1990, 127, 103-111. Kirszensztejn, P. A1203-Sn02systems as a support for metallic catalysts I. Preparation and structure. Mater. Chem. Phys. 1991a, 2, 117-128. Kirszensztejn, P. A1203-Sn02systems as a support for metallic catalysts 11. Porous structure. Mater. Chem. Phys. 1991b, 2, 129-139. Kirszensztejn, P.; Foltynowicz, 2.; Zielinski, St.; Hoflund, G. B. The Influence of Preparation Conditions on the Porous Structure of Pt-Sn/yA1203 Catalysts. Ann. Chim. Fr. 1989,14,449. Langner, B. E. Coke formation and deactivation of the catalyst in the reaction of propylene on calcinated NaNH,-Y. Ind. Eng. Chem. Process Des. Deu. 1981, 20, 326. Lieske, H.; Sarkany, A.; Volter, J. Hydrocarbon adsorption and coke formation on Pt/A1203 and Pt-Sn/A1203catalysts. Appl. Catal. 1987, 30, 69. Oudar, J. J.; Wise, E. Deactivation and Poisoning of Catalysts; Marcel Dekker: New York, 1985. Przystajko, W.; Fiedorow, R. M. J.; Dalla Lana, I. G. Coke-Catalyzed Ammoxidation of Toluene. Presented at the Tenth Canadian Symposium on Catalysis, Kingston, Ontario, 1986; paper no. 556. Ramser, J. N.; Hill, P. B. Physical structure of silica-alumina catalysts. Ind. Eng. Chem. 1958,50, 117-124. Scaroni, A. W.; Jenkins, R. G.; Walker, P. L., Jr. Coke deposition on cobalt-molybdenum/alumina and cobalt-molybdenum/carbon catalysts. Appl. Catal. 1985,14,173. Singh, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R.; Rouquerol, J.; Siemieniewska,T. Reporting physisorption data for gas/solid systems. Pure Appl. Chem. 1985,4, 603. Stiegel, G. J.; Polinski, L. M.; Tisher, R. E. Catalyst deactivation during coal liquefaction. The effect of catalyst diameter. Ind. Eng. Chem. Process Des. Dev. 1982,21, 477.
Receiued f o r reuiew October 22, 1990 Revised manuscript received May 9, 1991 Accepted May 23,1991
