1982
Ind. Eng. Chem. Res. 2010, 49, 1982–1985
Regeneration of Coked Al-Promoted Sulfated Zirconia Catalysts by High Pressure Hydrogen Ying-Chieh Yang†,‡ and Hung-Shan Weng*,† Department of Chemical Engineering, National Cheng Kung UniVersity, Tainan 701, Taiwan, and Refining & Manufacturing Research Institute, CPC Corporation, Taiwan, Chia-Yi 600, Taiwan
To avoid the inconvenience and danger caused by using air or oxygen as a regenerating gas for coked catalysts, this work proposes an effective regeneration method for the fouled Al-promoted sulfated zirconia (AL/SZ) catalyst. This work tries hydrogen for regenerating fouled catalysts; nevertheless, regaining activity is low when introducing atmospheric hydrogen as a regenerating gas. Fortunately, over 98% of original activity can be regained using high pressure hydrogen (2.1 MPa) at 250 °C within 8 h. The operation temperature for this new regeneration method is much lower than that for conventional methods using air or oxygen as a regenerating gas. This method does not affect the product distribution of n-C4 isomerization. 1. Introduction The skeletal isomerization of n-alkanes (C5-C7) and the alkylation of i-C4 with olefin in the petroleum industry have recently become more important for producing high octane number gasoline. The petroleum industry first used noble-metalcontained acid catalysts (e.g., Pt/chlorinated alumina catalyst) in industrial isomerization processes,1 but this kind of catalyst has environmental pollution and equipment corrosion problems. Some industrial processes apply solid acid catalysts, because they have similar function but none of the above problems. The sulfated zirconia (SZ) catalyst, one of the solid acid catalysts, attracted considerable attention when it was first disclosed in the early 60s2 because of its high activity even at low reaction temperature for n-butane isomerization. However, rapid deactivation seriously disadvantages the SZ catalyst and limits its use in commercial process, so many researchers attempt to elucidate the cause of deactivation and try to prevent it. Several factors may cause SZ catalyst deactivation: coke deposition on the catalyst surface,3,4 reduction of Zr4+ to Zr3+ by hydrocarbon reaction,5 sulfate group attenuation owing to reduction by hydrogen,6 change of sulfur charge,7 change of zirconia phase from tetragonal to monoclinic,8 and poisoning by water.9 Among these factors, coke deposition is the main cause of deactivation, so controlling and/ or reducing coke deposition rate are important for prolonging catalyst life. Fouled SZ catalyst activity can be restored by burning coke off at high temperature.3,10,11 However, the amount of sulfur species may decline during the burning process, resulting in decreased activity. Fortunately, Li and Gonzalez12 found that the coke on a used SZ catalyst could be selectively burned off by oxygen at low temperatures without loss of sulfur species, thereby restoring catalyst activity. In a study on n-C4 isomerization over mesoporous SZ, Risch and Wolf13 regenerated used catalysts under oxygen at 500 °C for 1 h and concluded that regeneration in oxygen must be followed by rehydrating the catalyst, to recover original activity. Except for removal by reaction with oxygen of air, the coke of deactivated catalysts can also be removed by reaction with * To whom correspondence should be addressed. Tel.: +886 6 2757575. Fax: +886 6 2344496. E-mail address: hsweng@ mail.ncku.edu.tw. † National Cheng Kung University. ‡ CPC Corporation.
hydrogen.14 Hydrogen improved the reaction stability of n-C4 isomerization over SZ because coke formation was effectively prevented in the presence of H2.15 Kim et al.16 found that coke content reduced in the presence of hydrogen for n-C4 isomerization over SZ or Pt/SZ catalysts. In a preliminary study on n-butane isomerization over Al/SZ catalysts, we also observed coke removal in a fouled catalyst by reaction with hydrogen. Thereafter, we further investigated the role of hydrogen in this reaction.17 Catalyst regeneration in commercial isomerization processes uses air or oxygen to burn coke off. Most researches focus on catalyst regeneration by air or oxygen, but few papers study regeneration by hydrogen. Unfortunately, recovery performance is still low when hydrogen is used as the regenerating gas.18 Considering the inconvenience and danger caused by using air or oxygen as a regenerating gas for coked catalysts in a reaction system containing hydrogen, the present work focuses on regeneration of fouled Al/SZ catalysts by hydrogen and proposes a new regeneration method for recovering fouled catalysts to original activity by high pressure hydrogen. 2. Experimental Section 2.1. Catalyst Preparation. The methods for preparing SZ are already described elsewhere.19,20 Hexadecyl trimethyl ammonium bromide (2.5 g, Aldrich) is dissolved in a solution of 115 g water and 22.4 g HCl (37 wt %), followed by 5.99 g of 70 wt % Zr(O-nPr)4 in added 1-propanol. After stirring for 30 min, (NH4) 2SO4 in 23.0 g water is introduced to the solution (the molar ratio of SO4/Zr is about 0.7). The solution is stirred for 1 h at room temperature and, then, transferred into a polypropylene bottle and heated at 100 °C for 3 days. The precipitate is filtered and washed with deionized water, ethanol, and deionized water consecutively, finally drying at 100 °C for 12 h. Aqueous alumina sulfate is introduced into the uncalcined sulfated-ZrO2 via the incipient wetness impregnation technique to obtain the Al-promoted SZ with nominal 3 mol % Al2O3, calculated on the weight basis of ZrO2. The slurry is stirred for 1 h and oven-dried at 100 °C for 12 h. Solid particles are then calcined for 5 h at 650 °C in static air, thus obtaining the Al/ SZ catalyst. 2.2. Characterization. Coke content of the used catalyst is determined by the Elementar Vario EL C, H, N-elemental analyzer.
10.1021/ie900338b 2010 American Chemical Society Published on Web 01/08/2010
Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010
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Table 1. Coke Contents (wt %) in the Catalysts Experienced Coking for Various on-Stream Timesa,b before and after Regeneration under Different Conditions on-stream time in step II condition
0.5 h
1h
4h
before regeneration regeneration by H2 at 250 °C, 1 atm for 20 h regeneration by H2 at 250 °C, 2.1 MPa for 8 h regeneration by air at 500 °C for 3 h
0.09 0.05
0.11 0.09
0.13 0.10
0.03
0.03
0.06
a
0.04 b
Coke content of fresh Al/SZ is 0.06 wt %. Isomerization of n-C4 was conducted at 250 °C for 4 h with H2 as a carrier gas (step I), then shifting to N2 for various times (step II).
Figure 1. Variation of n-butane conversion at 250 °C in the presence of hydrogen (step I) and nitrogen (step II).
2.3. Catalytic Reaction. The isomerization of n-butane to isobutane is carried out in a fixed-bed reactor and operated at atmospheric pressure. A 1 g portion of catalyst is loaded into the reactor and then pretreated in dry air for 3 h at 450 °C. Thereafter, the catalyst is cooled to 250 °C. Subsequently, the reaction starts with 250 °C using hydrogen as a carrier gas for 4 h (step I), then the carrier gas is shifted to nitrogen (step II) for 30 min to 4 h. The catalyst is deactivated in step II. After step II, n-butane is stopped and the nitrogen is replaced by hydrogen or air. The reaction temperature stays the same as isomerization reaction proceeds under atmosphere or 2.1 MPa for 4 to 30 h when hydrogen is used as regenerating gas. The reactor is heated at a rate of 5 °C/min to 500 °C when regenerating gas uses air, then the temperature is held at 500 °C for 3 h. After regeneration, reaction tests are performed again using the same procedure as described above. The weight hourly space velocity (WHSV) of n-butane is set at 0.62 h-1 and the n-butane/H2 ratio is 1/10 (v/v). Effluent gas analysis uses a gas chromatograph (Varian 3800) equipped with a 50 m Plot AL/M column and a flame ionization detector (FID). 3. Results and Discussion 3.1. Characters of Used Al/SZ Catalysts. Figure 1 shows that conversion is about 55% at the very beginning of the run over fresh Al/SZ catalyst, declining gradually and slightly to a relatively stable state after 4 h (step I). However, when the carrier gas changes from hydrogen to nitrogen (step II), conversion decreases very quickly. Table 1 shows coke contents in the fresh and used catalysts. A small amount of coke is found in the fresh catalyst. Coke deposit is very low when hydrogen is employed as a carrier gas in step I because H2 presence successfully inhibits coke formation in the reaction stream,21 while it increases very quickly in 60 min and then slows down after carrier gas changes from hydrogen to nitrogen. The speed of coke deposition seemingly concurs with catalytic activity loss.22 3.2. Catalyst Regeneration in Hydrogen and Air. (a) Catalyst Regeneration in Air. Coke on the decayed catalyst can react with hydrogen or air at different reaction temperatures and evolves different gases. In the present work, the used catalyst was regenerated by air at 500 °C for 3 h after the n-C4 isomerization reaction proceeded over Al/SZ via steps I and II at 250 °C (refer to Figure 1). The catalytic activity of the
Figure 2. Variation of n-butane conversion at 250 °C over the fresh catalyst and regenerated catalysts. Reaction condition for n-C4 isomerization over the fresh catalyst: in H2 for 4 h, then in N2 for 1 h. Regeneration condition: in air at 500 °C for 3 h. Reaction condition for n-C4 isomerization over the regenerated catalyst: in H2 at 250 °C for 4 h.
regenerated catalyst for n-C4 isomerization was tested at the same reaction condition as step I. Figure 2 shows the results. Regenerated catalyst activity is a little lower than that of the fresh catalyst, regaining ca. 97% of original catalytic activity. This result reveals that activity of the fouled Al/SZ catalyst cannot be fully regained by air. (b) Catalyst Regeneration in Atmospheric Hydrogen. The regeneration of used catalyst in atmospheric hydrogen was performed at 250 °C after step II. The experiment interrupted the n-C4 stream and changed carrier gas from nitrogen to hydrogen. This work studied duration effect of regeneration by reactivating the decayed catalyst in the presence of hydrogen at 250 °C for 20 and 30 h, respectively, maintaining reaction system pressure at atmosphere. The regenerated catalysts were subjected to characteristic analysis and activity test for n-C4 isomerization. The extent of coke removal under this regeneration condition seems low (Table 1). Only 67, 25, and 30% of total coke were removed from these three used catalysts, respectively. Figure 3 shows the variation of n-butane conversion over fresh and regenerated catalysts. The conversion over fresh catalyst decreases gradually while that over the regenerated catalyst increases gradually to a relatively stable level, though regenerated catalyst activity is still lower than that of the fresh catalyst. When the catalysts were regenerated for 20 and 30 h and were reused for n-butane isomerization with hydrogen as the carrier gas, although only about 43-46% of initial activity was regained at the very beginning, restoration approached 73-76% in only
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Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010
Figure 3. Variation of n-butane conversion at 250 °C over the fresh and regenerated catalysts. Reaction condition for n-C4 isomerization over the fresh catalyst: in H2 for 4 h, then in N2 for 1 h. Regeneration condition: in atmospheric hydrogen at 250 °C for 20 and 30 h. Reaction condition for n-C4 isomerization over the regenerated catalyst: in H2 at 250 °C for 4 h.
Figure 4. Coke contents of regenerated catalysts at different regeneration times. Reaction condition for n-C4 isomerization over the fresh catalyst: in H2 at 250 °C for 4 h, then in N2 for 1 h. Regeneration condition: in hydrogen of 2.1 MPa at 250 °C for various times.
4 h. The restoration rate during the isomerization process is far faster than that in the regeneration process by pure hydrogen. (c) Catalyst Regeneration in Hydrogen of 2.1 MPa. Although coke can react with atmospheric hydrogen; unfortunately, the extent of restoration in activity is low. Therefore, this work tested a higher hydrogen pressure. Hydrogen pressure was changed from atmosphere to 2.1 MPa, and other regeneration conditions were kept the same. The used catalyst for regeneration was that being proceeded in step II for 1 h. Figure 4 represents the coke contents of catalysts at different regeneration times in hydrogen of 2.1 MPa. Coke content quickly decreases with time, down to 0.03 wt % in 8 h. The percentage of coke removal improves considerably in 2.1 MPa compared to in atmospheric hydrogen. From the results of coke content, this research infers that catalytic activity can be nearly regained when the used catalyst regenerates with hydrogen of 2.1 MPa for 6 h. Figure 5 displays catalytic activity for isomerization reaction after regeneration. Catalyst activity regenerated for 4 h increases
Figure 5. Variation of n-butane conversion at 250 °C over the fresh and regenerated catalyst. Reaction condition for n-C4 isomerization over the fresh catalyst: in H2 for 4 h, then in N2 for 1 h. Regeneration condition: in hydrogen of 2.1 MPa at 250 °C for 4 and 8 h. Reaction condition for n-C4 isomerization over the regenerated catalyst: in H2 at 250 °C for 4 h.
Figure 6. Variation of n-butane conversion at 250 °C over the fresh and regenerated catalyst. Reaction condition for n-C4 isomerization over the fresh catalyst: in H2 for 4 h, then in N2 for 1 h. Regeneration condition: in hydrogen of 2.1 MPa at 250 °C for 20 h. Reaction condition for n-C4 isomerization over the regenerated catalyst: in H2 at 250 °C for 4 h.
gradually during isomerization reaction and becomes stable after 2 h, ca. 91% of original activity is regained. Coke content analysis also reveals that the catalyst regenerated for 4 h still has a small amount of residue coke, which is continually removed during isomerization. Catalyst activity regenerated for 8 h is higher than that for 4 h, and conversion maintains the same level, about 98% of steady-state conversion over the fresh catalyst, during isomerization reaction. Initial activity of the regenerated catalyst is slightly lower than that of the fresh catalyst because the catalyst with active sites of higher activity is less easily regenerated than that with active sites of lower activity.4 Catalyst activity can be almost fully restored when hydrogen of 2.1 MPa is employed as the regenerating gas within 8 h, while it can be recovered only about 46% in 30 h with atmospheric hydrogen. Figure 6 shows catalytic activity after regeneration for 20 h. No matter whether at initial or stable period, regenerated catalyst activity is the same as that of the fresh catalyst. In other words, catalytic activity is fully regenerated. Table 2 presents the product distribution and isomerization efficiency, defined as i-C4/
Ind. Eng. Chem. Res., Vol. 49, No. 4, 2010 Table 2. Product Distributions and Isomerization Efficiencies, Using Fresh and Regenerated Catalysts for n-C4 Isomerization product distribution (mol %)
C1
C2
C3
i-C4 n-C4 i-C5 n-C5
fresh catalyst 4 h regeneration 8 h regeneration 20 h regeneration
3.1 2.4 3.5 3.1
2.8 1.9 2.8 2.6
5.0 4.8 5.4 5.2
38.4 36.4 39.0 38.4
a
48.9 52.8 49.6 49.0
1.2 1.2 1.3 1.2
0.5 0.5 0.5 0.5
isomerization efficiencya 0.75 0.77 0.74 0.75
Isomerization efficiency: defined as i-C4/(C1 + C2 + C3 + i-C4 +
C5).
(C1 + C2 + C3 + i-C4 + C5),22 over the fresh catalyst and regenerated catalysts. The product distributions over these three regenerated catalysts are almost the same as that over the fresh catalyst, so this regeneration method does not change isomerization efficiency over Al/SZ. Villegas et al.22 studied the regeneration for n-C4 isomerization over zeolites and found that isomerization efficiency increased after regeneration because the residual coke reduced the zeolite effective pore diameter, increasing shape selectivity to the monomolecular mechanism of n-C4 isomerization. 3.3. Industrial and Economic Feasibility. Industrial isomerization process or hydrodesulfurization process usually use hydrogen as carrier gas23 and air to regenerate used catalysts. Therefore, the operation process must be shutdown and, then, purged completely by nitrogen gas before regenerating the used catalyst to avoid the possibility of explosion. Only these two steps must take 2-3 days. Considering safety and runaway during regeneration, the oxygen content must be lower than 0.5 vol %, and regeneration temperature must be step by step before reaching to 400-450 °C. This regeneration method is inconvenient and danger. The new effective regeneration method by this work can directly be used after stopping feed because of regeneration gas being the same as carrier gas, and the regeneration pressure is similar to operation pressure, ca. 1.5-3.5 MPa.23 The new effective regeneration method can save much time comparing with conventional method using air or oxygen as a regenerating gas. Therefore, this new regeneration method has high feasibility and must be more economic than conventional methods using air or oxygen as a regenerating gas. 4. Conclusions When the Al/SZ catalyst is used to catalyze n-butane isomerization at 250 °C, its activity decays quickly in the absence of hydrogen due to coke deposition, but decays slowly in the presence of hydrogen. The deposited coke on the catalyst can be removed by reacting with hydrogen; therefore catalytic activity is more stable in the presence of hydrogen than in nitrogen. Although about 97% of original activity can be regained when air is employed as a regenerating gas; it is inconvenient and dangerous to use air in a reaction system containing hydrogen. The present work therefore tries hydrogen for regenerating fouled catalysts; however, introducing atmospheric hydrogen as a regenerating gas regains low initial activity. Fortunately, the catalyst activity can be almost fully restored (98%) using hydrogen of 2.1 MPa at 250 °C within 8 h. The operation temperature for this new regeneration method is much lower than that for conventional methods using air or oxygen as a regenerating gas. The regeneration method does not change the product distribution of n-C4 isomerization. This new regeneration method has high industrial feasibility and is more economical than conventional methods using air or oxygen as a regenerating gas.
1985
Acknowledgment The authors kindly acknowledge partial financial support for this research from the Chinese Petroleum Corporation. Literature Cited (1) Weyda, H.; Ko¨hler, E. Modern refining conceptssan update on naphtha-isomerization to modern gasoline manufacture. Catal. Today 2003, 81, 51. (2) Holm, V. C. F.; Bailey, G. C. Sulfate-treated zirconia-gel catalyst. US Patent No. 3032599, 1962. (3) Li, B.; Gonzalez, R. D. An in situ DRIFTS study of the deactivation and regeneration of sulfated zirconia. Catal. Today 1998, 46, 55. (4) Kim, S. Y.; Goodwin, J. G.; Galloway, D. n-Butane isomerization on sulfated zirconia:active site heterogeneity and deactivation. Catal. Today 2000, 63, 21. (5) Vera, C. R.; Pieck, C. L.; Shimizu, K.; Querini, C. A.; Parera, J. M. Coking of SO42--ZrO2 catalysts during isomerization of n-butane and its relation to the reaction mechanism. J. Catal. 1999, 187, 39. (6) Ng, F. T. T.; Horva´t, N. Sulfur removal from ZrO2/SO42- during n-butane isomerization. Appl. Catal. 1995, 123, L197. (7) Dicko, A.; Song, X.; Adnot, A.; Sayari, A. Characterization of platinum on sulfated zirconia catalysts by temperature programmed reduction. J. Catal. 1994, 150, 254. (8) Li, C.; Stair, P. C. Ultraviolet Raman spectroscopy characterization of sulfated zirconia catalysts: fresh, deactivated and regenerated. Catal. Lett. 1996, 36, 119. (9) Morterra, C.; Cerrato, G.; Pinna, F.; Signoretto, M.; Strukul, G. On the acid-catalyzed isomerization of light paraffins over a ZrO2/SO4 system: The effect of hydration. J. Catal. 1994, 149, 181. (10) Lei, T.; Xu, J. S.; Tang, Y.; Hua, W. M.; Gao, Z. New solid superacid catalysts for n-butane isomerization: γ-Al2O3 or SiO2 supported sulfated zirconia. Appl. Catal. 2000, 192, 181. (11) Fo¨ttinger, K.; Halwax, E.; Vinek, H. Deactivation and regeneration of Pt containing sulfated zirconia and sulfated zirconia. Appl. Catal. 2006, 301, 115. (12) Li, B.; Gonzalez, R. D. TGA/FT-IR studies of the deactivation of sulfated zirconia catalysts. Appl. Catal. 1997, 165, 291. (13) Risch, M.; Wolf, E. E. n-Butane and n-pentane isomerization over mesoporous and conventional sulfated zirconia catalysts. Catal. Today 2000, 62, 255. (14) Trimm, D. L. Progress in catalyst deactiVation; Figneiredo, J. L., Ed.; Martinus Nijhoff: Hague, 1982; p 17. (15) Spielbauer, D.; Mekhemer, G. A. H.; Bosch, E.; Kno¨zinger, H. n-butane isomerization on sulfated zirconia. Deactivation and regeneration as studied by Raman, UV-VIS diffuse reflectance and ESR spectroscopy. Catal. Lett. 1996, 36, 59. (16) Kim, S. Y.; Goodwin, J. G.; Hammache, S.; Auroux, A.; Galloway, D. The impact of Pt and H2 on n-butane isomerization over sulfated zirconia: changes in intermediates coverage and reactivity. J. Catal. 2001, 201, 1. (17) Yang, Y. C.; Weng, H. S. The role of H2 in n-butane isomerization over Al-promoted sulfate zirconia catalyst. J. Mol. Catal. 2009, 304, 65. (18) Jong, S. J.; Pradhan, A. R.; Wu, J. F.; Tsai, T. C.; Liu, S. B. On the regeneration of coked H-ZSM-5 catalysts. J. Catal. 1998, 174, 210. (19) Ciesla, U.; Fro¨ba, M.; Stucky, G.; Schu¨th, F. Highly ordered porous zirconias from surfactant-controlled syntheses: zirconium oxide-sulfate and zirconium oxo phosphate. Chem. Mater. 1999, 11, 227. (20) Wang, J. H.; Mou, C. Y. Alumina-promoted mesoporous sulfated zirconia: A catalyst for n-butane isomerization. Appl. Catal. 2005, 286, 128. (21) Kondo, J. N.; Yang, S.; Zhu, Q.; Inagaki, S.; Domen, K. In situ infrared study of n-heptane isomerization over Pt/H-beta zeolites. J. Catal. 2007, 248, 53. (22) Villegas, J. I.; Kumara, N.; Heikkila¨, T.; Lehto, V. P.; Salmi, T.; Murzin, D. Yu. Isomerization of n-butane to isobutane over Pt-modified Beta and ZSM-5 zeolite catalysts: Catalyst deactivation and regeneration. Chem. Eng. J. 2006, 120, 83. (23) Weyda, H.; Ko¨hler, E. Modern refining conceptssan update on naphtha-isomerization to modern gasoline manufacture. Catal. Today 2003, 81, 51.
ReceiVed for reView March 1, 2009 ReVised manuscript receiVed December 19, 2009 Accepted December 21, 2009 IE900338B
