2670
Znd. Eng. Chem. Res. 1992,31,2670-2674
Propane Dehydrogenation and Coke Formation on Chromia-Alumina Catalysts: Effect of Reductive Pretreatments Osvaldo F. Gorriz,+Vicente Cortes Corberin? and Jose L. G. Fierro*?* Znstituto de Znvestigaciones en Tecnologia Quimica, ZNTEQUZ, Casilla Correo 290,5700 San Luis, Argentina, and Znstituto de Catdlisis y Petroleoquimica, CSZC, Campus UAM Cantoblanco, 28049 Madrid, Spain
Propane dehydrogenation and coke formation over a series of alumina-supported chromia catalysts with different chromia contents were examined. X-ray photoelectron spectroscopy (XPS)showed that the calcined catalysts displayed Cr(VI) and Cr(III) in proportions which depended on the overall chromia content. Cr(VI) appeared to be reduced in the initial stagea of the dehydrogenationreaction, for which important changes in the selectivity to propene and in the coking rate were noted. These changes were mainly related to the hydrocarbon combustion by the oxygen released from Cr203and to the suppression of the strong acid sites by coking. Coke formation on CO-prereduced catalysta was fast and could be related to the level and stability of the conversion to propene. Both activity and selectivity of the catalysts were determined by the dispersion of Cr and by the effect of Cr content on the porous structure of the catalysts.
Introduction Dehydrogenation of light alkanes has a great industrial importance because it represents an alternative for obtaining alkenes for polymerization and other organic syntheses from low cost saturatad hydrocarbon feedstocks. Supported chromia catalysts have been used for many decades in alkane dehydrogenation (Kearby, 1955). As dehydrogenation thermodynamicsrequires high temperatures, usually above 800 K, for acceptable alkene yields to be obtained, highly stable carriers are of prime importance. Alumina is widely used since it is a refractory, high surface area oxide; however, it presents the difficulty of catalyzing side reactions, i.e., coking and cracking, leading to catalyst deactivation. Within this frame, there is a controversy in the literature on the influence of carbon deposita during dehydrogenation over chromiaalumina catalysta. While some authora argue on the existence of a correlation between catalyst deactivation and the amount of coke deposited (Noda et al., 1974; Toei et al., 1975; Dumez and Froment, 1976), some others claim that this correlation is not so clear (Heinemann, 1951; Shendrik et al., 1965; Boutry et al., 1967). In previous studies, Gorriz et al. (1989) have reported that the catalytic behavior of chromia-alumina catalysts prepared by impregnation show an initial period during which propene yield remains essentially unchanged, and then deactivation occurs. In this initial step of reaction, whose duration depends on the chromia content and the reaction conditions, several authors have found an activity maximum, which was attributed to redox effects of the chromium oxide phase produced in the course of the reaction (Shendrik et al., 1965; Boutry et al., 1967; Gorriz et al., 1989). A recent report (Gorriz et al., 1992) indicated that the initial activity of the catalysts can be related to the amount of Cr(VI) present in the catalyst before the reaction, while the conversion stability before deactivation is related to the dispersion of the chromium oxide phase. Also, the pronounced deactivation of the catalysts after this stability period has been attributed by some authors to the formation of Cr(I1) (Marcilly and Delmon, 1972), which almost coincides with the deposition of a limited amount of coke that could modify the surface texture of the catalyst. Accordingly, the present work was undertaken with the aim of analyzing the behavior of the catalysts in the ab+
INTEQUI.
* CSIC.
Table I. Catalyst Samples Characterirticr % ~ r z 0 3
0 1.5
3.0 6.0
4 (mZ/g) 145 137 138 140
9
(cm /g) 0.38 0.38 0.37 0.35
%
cr209 9.0 12.0 15.0
(=$a) 126 109 96
5
(cm /g) 0.32 0.30 0.28
sence of redox changes to correlate this behavior only to the coke formation. To reach this goal, the behaviors of oxidized and reduced catalysts for propane dehydrogenation were compared to observe the effect of oxidized chromium on the activity and coke formation. Also the catalysta reduction, the coke formation from propane and propene, and the effect of this coke on the catalyst texture were studied.
Experimental Section Catalyst Preparation and Textural Analysis. Catalyst samples were prepared by impregnating a commercial y-alumina ALCOA F110, previously heated at 773 K in air for 7 h (specific surface area 150 m2/g, pore volume 0.38 cm3/g, and average pore radius 2 nm) with an aqueous solution of chromic acid, according to the pore filling method. The samples were then dried at 383 K for 7 h and calcined at 773 K for 30 h. The samples will be referred to hereafter as CA-x, where x denotes the chromia content of the sample, expressed as the weight percent of Cr203. Textural analysis of catalyst samples was done by means of adsorption-desorption of nitrogen at 77 K, using an Accu Sorb 2100 E equipment (Micromeritics). Chemical composition, specific surface areas, and pore volumes of the prepared samples are shown in Table I. X-ray Photoelectron Spectroscopy. X-ray photoelectron (XP)spectra of powdered catalysta were acquired by using a Leybold LHS 10 spectrometer, equipped with a magnesium anode (Mg Ka = 1253.6 eV) operated at 12 kV and 10 mA and hemispherical electron analyzer. Each spectral region of photoelectrons of interest was scanned a number of times in order to obtain good signal-bnoise ratios. The residual pressure inside the analysis chamber was kept at values below 7.5 X lo4 Torr. C Is, Al2p, and Cr 2p signals were recorded for each catalyst. All binding energies (BE) were referred to the adventitious C 1s line at 284.6 eV. Microgravimetry. Catalyst reduction by CO and extent of coke deposition on the reduced samples were con-
0888-5885/92/2631-2670$03.00/0@ 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 12, 1992 2671
I
I
I
5
10
1s
wt % Cr,O,
Figure 2. Dependence of the Cr 2p/M 2p XPS intensity ratio on the chromium content of the Catalysts.
I 0
6
10
16
20
26
Time (min)
585
575 BE (eV)
580
Figure 1. Cr 2 ~ 3core , ~ level spectra of fresh catalysts: (a) CA-1.5; (b) CA-3.0; (c) CA-6.0; (d) CA-9.0; (e) CA-12.0; (f) CA-15.0.
ducted by microgravimetryon a Cahn 2000 microbalance, operated with a sensitivity of 1pg for the reduction experiments and 10 pg for the coke deposition studies. In these experiments, samples were charged in the microbalance and heated at a rate of 4 K/min up to the reduction temperature in He flow. Reduction was made with a 42 mol % mixture of CO in He, at a total flow of 36 cm3/min, until constant weight was attained. After catalyst reduction, the system was purged to remove CO, and the study of the coke formation was started by feeding a gaseous mixture of 35 mol % of the hydrocarbon (propane or propene, according to the experimental program) in He at a total flow of 44 cm3(STP)/min. Catalytic Activity Tests. The activity of the catalyst samples for the dehydrogenation of propane was tested in a stainless steel, fixed-bed tubular reactor, at 873 K and atmospheric pressure, using a catalyst charge of 2 g and a gaseous mixture of 50 mol % propane in nitrogen at a total flow of 1cm3(STP)/s as a feed. The fed gases were carefully dried by passing them through a column filled with silica gel and molecular sieve and preheated before being introduced into the reactor. Prior to the run, the catalyst was heated up to the reaction temperature in flowing nitrogen. Reactant and effluent reaction products were analyzed by GC by using an activated alumina packed column and a flame ionization detector (FID). Propene and, in some cases, carbon oxides were the only volatile products detected. Conversion and selectivity to propene, expressed as mole percent, were calculated as the ratio of moles of converted propane to the moles of fed propane and the ratio of moles of formed propene to the moles of converted propane, respectively.
Figure 3. Influence of chromia content in the reduction of catalpta by C O 0 , CA-1.5; +, CA-3.0; CA-6.0; O , CA-9.0; X, CA-12.0; 0 , CA-15.0.
*,
Remults and Discussion Surface Characterization by XPS. The oxidation state of chromium and the chromia exposure at the support surface as a function of chromia contents were studied by XPS. Figure 1 shows the most intense Cr 2pSl2peak of catalysts with variable chromium contents. As can be seen, the Cr 2P3l2 profile is quite complicatedand markedly depends on chromia content. For the lowest Cr contents , two only a peak at BE ca.580.0 eV was o b s e ~ dhowever, other peaks, of almost the same intensity, at ca. 580.0 and 578.1 eV were clearly distinguished for the highest Cr contents. For intermediate chromia contents the peak at 580.0 eV also dominated, but a shoulder placed in the low BE side at ca. 578.1 eV can be discerned. From the BE values and in agreement with literature findings (Hardcastle and Wachs, 1988; Wachs and Hardcastle, 1988), it can be inferred that Cr(V1) species appear on catalyst surfaces for every explored composition, while Cr(II1) species become stable only at higher chromia contents. The relative exposure of the chromium oxide on the support surface was calculated by comparing the intensity of signals corresponding to the Cr 2p and Al 2p peaks. Figure 2 shows the dependence of IcrJIAIwith chromia content. This relation was almost linear on low chromia contents (below 6 mol %). Further increases in the chr~miacontenta produced a lower increase of this relation, indicating a poorer surface exposure of chromia. Reduction Pretreatments. Figure 3 shows the weight losses of different catalyst samples produced by CO reduction at 873 K as a function of the time of reaction. The maximum extent of reduction was the highest for the catalyst with the lowest chromia content (CA-1.51, and it decreased as the chromia content increased. Table I1 is a compilation of the values of the extent of reduction (Am), initial reduction rate ( r J , and the reduction degree (a),
2672 Ind. Eng. Chem. Res., Vol. 31, No. 12,1992 Table 11. Reduction of Catalysts by CO at 873 K Am(expt) [1@ ri [lo‘ g/(g of % CrzOa catalyst-min)] g/(g catalyst)] 1.5 3.0 6.0 9.0 12.0
5.52 4.02
5.03 3.06 3.34 4.82
15.0
a 0.497 0.175 0.109 0.038 0.033 0.021
2.35 1.82 2.08 1.09 1.26 1.09
I
Table 111. Coke Formation on Chromia-Alumina Catalysts at 873 K max coke loading (109 ri 110s g/(mT of catalyst).min] g/m2 of catalyst)a ICrzOs propane propene propane propene
1
c
a
0
2.0 11.0 14.0 32.1 32.9 41.3 48.0
0 1.5 3.0 6.0 9.0 12.0 15.0
12.3 48.5 63 76 99 105 120
0.80 0.75 0.73 0.83 0.78 0.84
2.13 (309) 2.12 (290) 2.12 (293) 2.01 (281) 2.13 (268) 2.25 (245) 2.05 (197)
(110) (104) (102) (105)
(85) (81)
Values in parentheses are expressed in mg/(g of catalyst). 1eo
*
.
*
0 0
.
SO
40
1 ,
120
;n
Time (min)
40
Figure 4. Coke deposition from propane onto catalysts prereduced by CO: 0 , CA-0.0; CA-1.5; CA-3.0; 0,CA-6.0; X, CA-9.0; 0 ,
+,
CA-12.0; A, CA-15.0.
*,
0 0
I
300
300
Coke (mg/g cat)
Figure 6. Influence of coke deposition in the surface areas of catalyst~. , CA-3.0 coked by propene; CA-15.0 coked from propene; CA-x (0 Iz 5 6.0) coked from propane; 0,CA-z (12.0 Iz I15.0) coked from propane.
1
+,
*,
1
0
200
100
I
100
60
160
R
200
Time ( m i n )
Figure 5. Coke deposition from propene onto catalysts prereduced by CO: 0 , CA-0.O; +, CA-1.5; CA-3.0; X,CA-15.0.
*,
expressed as the ratio of the experimental weight loss to the theoretical value expected for the reduction of all the chromium present from Cr(VI) to Cr(lII). Observed weight losses in samples with the lower chromia contents were almost double those with the higher chromia contents. The calculated a values were in good agreement with the tendency observed by XPS. Coke Formation on Prereduced Catalysts. The kinetic curves of coke formation on chromia-alumina catalysts for a feed of propane and propene are displayed in Figurea 4 and 5, respectively. From these curves, the initial rate of coke formation has been calculated from the slope of the curves at zero time. Moreover, the extent of coke formation was taken from the “equilibrium”values measured at very long reaction times. Both parameters are compiled in Table 111. The initial rate of coke formation as well as the total amount of deposited coke for both propane and propene varied depending on the chromia content of the catalysts. In general, as the chromia content increased, the initial rate of coke formation increased, but conversely the extant of coke deposited per unit mass of catalyst decreased. However, when the final coke loadings were referred to the specific surface area of the catalysts, there was an almost constant value irrespective of chromium oxide content of the catalysts. Table 111 also evidences that initial rates of coke formation were much higher for propene than for
0
2
4
s
8
10
12
Pore Radius (nm)
Figure 7. Influence of coke deposition in the pore distribution of catalysts 0 , CA-3.0 with 5 (mg of coke)/(g of catalyst); 0 , CA-3.0 with 39.3 (mg of coke)/(g of catalyst); 0,CA-15.0 with 5 (mg of coke)/(g of catalyst);4 CA-15.0 with 30 (mg of coke)/(g of catalyst).
propane. This can be interpreted assuming that the alkene or surface species originating from it are the precursors of coke, as suggested by K h i g and T6Gnyi in their study of ethane dehydrogenation on u118upported CqO3 (K6nig and TBGnyi, 1976a,b). Note also that the initial coking rate with propene on the bare support surface (0% Cr203) was 1order of magnitude higher than with propane, and of the same order as compared with those observed in the lower chromia content catalyst samples (CA-1.5 and CA3.0) with propene. Although one should expect that initial rates should be determined by the conversion level of propane to propene, it is clear also that the chromia content plays a role, as a variation can be observed even when the propene concentration is kept constant in the feed, especially on the lower chromium contents samples. The participation of the alumina carrier (0% Cr2O3) to coking rates and extant of coke formation is not s u r p r i s i i . This arises in the acid sites present on its surface which induced a faster dehydrogenation for the propene than for propane. To avoid this side reaction, poisoning of the acid sites of alumina, very often with potassium, was a common
Ind. Eng. Chem. Res., Vol. 31, No. 12, 1992 2673
* 361
X'
*
* X
.
*'
x x
*
x
Ik'*' -20' 0
S
10
Time (min)
I
20
16
Figure 9. Influence of pretreatments in the evolution of coke formation from propene onto CA-15.0 0,unreduced, 0, prereduced by
co.
60 1
I100
t
I .-
I/--------
0 0
60
100
160
0 200
Time-on-Stream (min)
Figure 10. Influence of chromia content in the catalytic propane dehydrogenation over chromia-alumina catalpts prereduced by C O 0, CA-1.5; 0, CA-3.0; 0 , CA-6.0; A, CA-15.0. Full symbols conversion. Open symbols: Selectivity.
The oxidation state of chromium also affected the selectivity of the samples. So, the initial selectivity of the unreduced sample was much lower than that of the reduced ones, but it recovered quickly to similar levels with time on stream. Selectivity to propene increased with time on stream in all cases. In the unreduced sample this behavior was due to the initial formation of oxygenated compounds, produced in the reduction of Cr(V1) by the hydrocarbon, which disappeared as the reduction to Cr(lII) was completed. In reduced samples it implied suppression of the strong acidic centers by the quick deposition of coke in the initial stages of the reaction. This result excludes participation of Cr(V1) in dehydrogenation of propane. Several authors claim that the active site in alkane dehydrogenation is Cr(II) (Ashmawy,1980, Lug0 and Lunsford, 1985), while others propose that Cr(II1) sites are active (Marcilly and Delmon, 1972; Kiinig and TgGnyi, 1976a,b). The combined use of XPS, microgravimetric reduction, and activity data obtained in this study lead us to conclude that in the CrzO3/Al2O3catalysts the dehydrogenation activity is related to Cr(II1). The influence of oxidation state of chromium on coke formation has also been examined. Microgravimetric studies of coke deposition on one unreduced CA-15.0 sample and its CO-prereduced homologue showed that coke formed immediately when contacting propene with the reduced sample, but this formation was not observed on the unreduced sample until a certain time had elapsed (Figure9). In this time lapse,comparable to that observed in the reduction experiments with CO, a clear inhibition of coke formation was observed. After this initial period, the rate of coke formation attained values similar to those observed for the prereduced sample. In general, the catalytic behavior of samples prereduced by CO was found to depend on the chromia content
2674 Ind. Eng. Chem. Res., Vol. 31, No. 12, 1992
(Figure 10). The initial conversion increased with increasing chromia content up to a 6 w t %. A further increase of chromia content was accompanied by a much lower increase of the initial conversion. From a comparison of these data with those of XPS in Figure 2, it is clear that a parallelism exists between the initial propane conversion and the intensity ratio Icr/IAI. Finally, the increase of conversion with increasing chromia content can also explain the results of coke formation from propane. The increase of initial rates of coke formation with chromia content, in the region in which the XPS intensity ratio ICr/IAIprogressively increased, can be interpreted as due to the increase of exposed Cr(II1) sites responsible for the reaction. Upon the addition of larger chromia amounts, the number of exposed sites does not increase and hence the activity remains constant. This direct relationship between initial coke formation rate and propane conversion is clear evidence of the participation of the olefinic product in the formation of coke.
Conclusions Chromium in the oxidized state affected both catalytic activity and coke formation in the initial stages of the reaction time, while the reduction of the catalysts was produced. In these first stages substantial changes in the selectivity to olefiic product as well as an important inhibition of coke formation were observed. The changes of selectivity noted in the unreduced catalysts came from the transformation of the hydrocarbon into oxygenated products, mainly COz and water, and suppression of strong acid centers by coke deposition. Catalyst samplea prereduced by CO showed only selectivity changes determined by the suppression of acid sites. Coke formation onto prereduced catalysts was immediate and was related to the level and stability of the conversion to the olefinic product. Activity and stability of the catalysts studied here were determined by the Cr dispersion and by the effect of the chromia content on the porous structure of the catalyst, respectively. The catalytic stability of catalysts with high chromia content was lower because the chromia content affected the surface texture. The fact that the chromia content affected the porous structure was evidenced by the progressive decrease of both BET area and pore volume with increasing Cr content (cf. Table I). The final coke loading was related to the total surface area of the catalysta. Fresh catalysts having the higher micropore volume and the higher surface areas acquired, after the first coking, a porous structure close to that of catalysts with the higher chromia content, and from this moment the rates of coking for the fresh and partially coked catalyst with similar textural properties were essentially the same. Acknowledgment "his work was developed thanksto a cooperative project in the frame of the CONICET-CSICScientific Cooperation Program. Nomenclature dV/dR = pore size distribution I,,/I*, = ratio of intensity of XPS signals corresponding to the Cr 2p and AI 2p peaks, undimensional
Am = extent of reduction expressed aa experimental weight loss during reducing treatments, g/ (g of catalyst) ri = initial rate of reduction, g/(g of catalyst-min) S, = specific surface area, m2/g V, = specific pore volume, cm3/g cy = reduction degree, expressed as ratio of the experimental weight loss to the theoretical value expected for the reduction of all the chromium present from Cr(VI) to Cr(III), undimensional Registry No. ALCOA F110, 1344-28-1;propene, 115-07-1; chromia, 1308-38-9;propane, 74-98-6.
Literature Cited Ashmawy, F. M. Surface composition and catalytic activity of chromia-alumina catalysts. J. Chem. SOC.,Faraday Trans. 1 1980,76, 2096. Boutry, P.;Montarnal, R.; Blejean, C. Influence of the reaction medium on the dehydrogenation activity of chromium oxide. Determination of the nature of catalytic surface sites. Bull. SOC. Chim. Fr. 1967,10,3690. de R m i , S.;Ferrati, G.;Fremiotti, S.; Cimino, A.; Indovina, V. Propane dehydrogenation on chromia/zirconia catalysts. Appl. Catal. 1992,81,113. Dumez, F. J.; Froment, G. F. Dehydrogenation of 1-butene into butadiene. Kinetics, catalyst cooking and reactor design. Ind. Eng. Chem. Process Des. Dev. 1976,15,291. Gorriz, 0.F.;Arrua, L. A.; Cadus, L. E.; Rivarola, J. B. P. Catalytic dehydrogenation of propane to propylene. Lat. Am. Appl. Res. 1989,19,31. Gorriz,0. F.; Cadus, L. E.; Rivarola, J. B. P. Catalytic dehydrogenation of propane: Deactivation by coke. To be published, 1992. Hardcastle, F. D.; Wachs, I. E. Raman spectroscopy of chromium oxide supported on Al2O3, Ti02 and Si02: A comparative study. J. Mol. Catal. 1988,46,173-186. Heinemann, H. Dehydrogenation of methylcyclopentadiene over chromia-alumina catalysts. Ind. Eng. Chem. 1951,43,2098. Hino, M.;Arata, K. Dehydrogenation of hexane to benzene over zirconia-supported chromia. J. Chem. SOC.,Chem. Commun. 1987,1355. Kearby, K. K. Catalytic dehydrogenation. In Catalysis; Emmett, P. H., Ed.; Reinhold: New York, 1955;Vol. 3,p 453. Kanig, P.; TBtBnyi, P. Kinetics of ethane dehydrogenation of achromium(II1) oxide catalysts. I. The rate determining step. Acta Chim. Acad. Sci. Hung. 1976a,89,123. Kanig, P.; TBtBnyi, P. Kinetics of ethane dehydrogenation of achromium(III) oxide catalysts. II. The rate equation. Acta Chim. Acad. Sci. Hung. 1976b,89,137. Lugo, H. J.; Lunsford, J. H. The dehydrogenation of ethane over chromium catalysts. J. Catal. 1985,91,155. Marcilly, Ch.; Delmon, B. The activity of true Crz03-A1203solid solutions in dehydrogenation. J. Catal. 1972,24,336. Noda, H.; Tone, S.; Otake, T. Kinetics of isopentane dehydrogenation on a chromia-aluminacatalyst with catalyst fouling. J. Chem. Eng. Jpn. 1974, 7,110. Shendrik, M. M.; Boreskov, G.K.; Kirilyuk, L. V. Variation of the activity of chromia-alumina Catalysts in the process of dehydrogenation of butane (in Russian). Kinet. i Katal. 1965,6, 313. Tanabe, K. Surface and catalytic properties of zirconia. Mater. Chem. Phys. 1985,13,347. Toei, R.; Nakanishi, K.; Yamada, K.; Okazaki, M. Catalytic fouling in the dehydrogenation of butane over chromia-aluminacatalysts. J. Chem. Eng. Jpn. 1975,8,131. Wachs, I. E.; Hardcastle, F. D. b a n spectroscopy of supportad metal oxides on Al2O3, TiOp and SiOz: A comparative study. Proceedings of the International Congress on Catalysis 9th, Calgary, 1988;Chemistry Institute of Canada: Calgary, 1988, Vol. 3,p 1449-1466.
Received for review June 1, 1992 Accepted September 25, 1992
