Energy & Fuels 1998, 12, 109-114
109
Activity and Regenerability of Sulfated Zirconia Superacid Catalysts in Isobutane/1-Butene Alkylation Debasish Das* and Dipak K. Chakrabarty Solid State Laboratory, Department of Chemistry, Indian Institute of Technology, Powai, Bombay 400 076, India Received May 29, 1997X
Alkylation of isobutane with 1-butene was carried out in the gas phase at atmospheric pressure over sulfate-promoted zirconia superacid catalysts prepared by two different procedure. The method of sulfation does not seem to much affect the surface area, superacidity, and crystalline phases present in the catalysts. However, sulfation by impregnation produces a catalyst with more acid sites in the weak and medium strength region. The catalysts showed high C8 selectivity comparable to liquid acid catalysts; however, use of 1-butene as olefin feed reduces the selectivity toward trimethylpentanes. Increase of olefin space velocity and reaction temperature increases olefin conversion but dimerization of olefin was favored over alkylation. Although the catalysts lost their activity within initial few minutes, they could be regenerated by heating in air. Activity of the regenerated catalysts is slightly lower, but the selectivity remains nearly unchanged.
Introduction Alkylation of isobutane with light olefins gives highly branched paraffins (C8, mainly trimethylpentanes) used for the production of reformulated gasolines. The high octane number, relatively low volatility, clean burning characteristics, and complete absence of sulfur and nitrogen makes alkylate an excellent blending component for reformulated gasoline. However, existing commercial alkylation processes involve the use of highly corrosive and hazardous concentrated hydrofluoric or sulfuric acid catalysts.1,2 While growing concern for environmental pollution restricts the use of such liquid acid catalysts, on the other hand, the demand for reformulated gasoline is increasing every year. In order to meet the increasing demands for alkylate, it is necessary to replace these hazardous liquid acid catalysts by safe and environment friendly solid acid catalysts. A number of solid acids have been tried as alkylation catalysts,3-8 among them large pore Y and beta zeolite and solid superacids like sulfated zirconia showed promising results. Solid superacids such as sulfated * Corresponding author. Present address: Chemistry Department, Catalysis Laboratory, Room No. 213B, National Taiwan University, No. 1, Roosevelt Road, Sec. 4, Taipei 106, Taiwan, R.O.C. Tel: (+88602) 369 0152, extn 111. Fax: (+886-02) 363 6359. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, December 1, 1997. (1) Scott, B. Hydrocarbon Process. 1992, Oct, 77 (2) Hoffman, H. L. Hydrocarbon Process. 1991, Feb, 37. (3) Kirsch, F. W.; Potts, J. D.; Barmby, D. S. J. Catal. 1972, 27, 142. (4) Minachev, K. M.; Mortinov, E. S.; Zen’kovsky, S. M.; Mostovoy, N. V.; Kononov, N. F. ACS Symp. Ser. 1977, 55, 89. (5) Chu, Y. F.; Chester, A. W. Zeolites 1986, 6, 195. (6) Weitkamp, J. Stud. Surf. Sci. Catal. 1980, 5, 65. (7) Liang, H.; Anthony, R. G. Prepr. 106th ACS Meeting, Div. Pet. Chem. Aug. 22-27, Chicago, IL 1993, 892. (8) Corma, A.; Juan-Rajadell, M. I.; Lo´pez Nieto, J. M.; Martı´nez, A.; Martı´nez, C. Appl. Catal. A 1994, 111, 175.
zirconia has been used since long for low-temperature isomerization of lower linear alkanes.9-12 Recently, Corma et al.8,13 reported alkylation of isobutane with 2-butene over sulfated zirconia superacid catalysts in a continuous flow reactor. The influence of various process parameters of the activity and selectivity of the superacid catalysts has also been studied by them. It has been reported earlier that the catalytic behavior of sulfated zirconia is greatly influenced by the method of preparation and activation conditions.12,14 However, very few reports are known on the effect of preparation methods and activation conditions of sulfated zirconia catalysts in the alkylation of isobutane with an olefin. Recently, comparison of the activity, selectivity, and deactivation behavior of sulfated ZrO2, TiO2, and SnO2 catalysts in the alkylation of isobutane with 2-butene has been reported.15 The effect of the nature of sulfate source, i.e., sulfuric acid and ammonium sulfate, and calcination temperature on the activity and selectivity of the catalysts has been discussed. It was found that among the three oxide systems, sulfated zirconia has very high initial activity. However, all the solid acid catalysts tried so far had a very short life due to the rapid deactivation in the first few minutes of the reaction. It is clear that unless the catalyst life is extended or a suitable catalyst regeneration method is developed the successful commercial use of these catalysts will not be possible. (9) Hino, M.; Kobayashi, S.; Arata, K. J. Am. Chem. Soc. 1979, 101, 6439. (10) Arata, K.; Hino, M. React. Kinet. Catal. Lett. 1984, 25, 143. (11) Hino, M.; Arata, K. J. Chem. Soc., Chem. Commun. 1980, 851. (12) Arata, K. Adv. Catal. 1990, 37, 165. (13) Corma, A.; Martı´nez, A.; Martı´nez, C. J. Catal. 1994, 149, 52. (14) Chen, F. R.; Coudurier, G.; Joly, J. F.; Vedrine, J. C. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1991, 36, 878. (15) Corma, A.; Martı´nez, A.; Martı´nez, C. Appl. Catal. A 1996, 144, 249.
S0887-0624(97)00078-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/12/1998
110 Energy & Fuels, Vol. 12, No. 1, 1998
Das and Chakrabarty
However, no study has been carried out on the regenerability of sulfated zirconia catalysts in the alkylation of isobutane with an olefin. In this paper, we present the results on the activity, selectivity, and regenerability of sulfate promoted zirconia catalysts in isobutane alkylation with 1-butene. In most of the reported works, alkylation was carried out under pressure and in liquid phase conditions. We have carried out the reaction in the gas phase conditions to have a low concentration of the reactants in the reaction system so that the rate of deactivation of the catalyst is slower. As it has been reported earlier that the catalytic behavior of sulfated zirconia catalysts is greatly influenced by the method of preparation and activation conditions,12,14,15 two catalyst preparation methods were followed (both using sulfuric acid as the sulfate source) to check whether the method of acid addition has any influence on the final catalyst properties. Experimental Section Catalyst Preparation. Zirconium hydroxide was freshly precipitated by hydrolyzing a solution of zirconium oxychloride (98%, LOBA) by dropwise addition of aqueous ammonia with constant stirring until the pH was 9.3. The resulting gel was washed throughly with water until free from Cl- and NH4+ ions and then dried at 110 °C. Zirconium hydroxide thus obtained was then sulfated following two different procedures. In the first method, uncalcined zirconium hydroxide was treated with 0.5 M aqueous sulfuric acid solution (5 mL/g solid) and the slurry was stirred for 30 min. The solution was then decanted and the solid was dried at 120 °C. This catalyst is designated as SZO(I). In the other method, as described by Hino et al.,9-11 sulfation was carried by dropwise addition of 0.5 M sulfuric acid (10 mL/g solid) to the zirconium hydroxide powder held on a filter paper and then the solid material was dried at 110 °C. This catalyst is designated as SZO(D). Catalyst Characterization. The powder X-ray diffraction pattern of the zirconia samples was recorded on a Philips PW 1820 X-ray diffractometer (40 kV, 30 mA) using nickel-filtered Cu KR radiation. The surface area of the samples was obtained by nitrogen adsorption at 77 K on a Carlo Erba sorptomatic instrument. Sulfur content of the sulfated samples was determined by elemental analysis on a Fisons instrument. Thermogravimetric analysis of the dried samples was carried out in air atmosphere on a Du Pont 9900 thermal analyzer at a heating rate of 10 °C/min. The surface acidity of the catalysts was measured by temperature-programmed desorption (TPD) of ammonia on a home-built apparatus. For these experiments, 300 mg of sample was pretreated at 600 °C in air flow for 3 h followed by cooling to 100 °C in a helium flow. Ammonia was then adsorbed at 100 °C to saturation and any physisorbed ammonia was flushed out in helium flow. TPD spectra were recorded by heating the sample from 100 to 600 °C at a heating rate of 10 °C/min. The amount of ammonia desorbed was monitored using a precalibrated thermal conductivity detector. Reaction Studies. Alkylation of isobutane with 1-butene was carried out in a fixed bed down flow reactor at atmospheric pressure. Premixed isobutane and 1-butene (>99% purity, Matheson) in a molar ratio of 14:1 was fed from a gas cylinder. A high paraffin to olefin ratio was chosen to reduce the chance of olefin dimerization. The reactant gas and the products were quantitatively analyzed in a on-line gas chromatograph using FID and a 50 m CP-Sil PONA capillary column. Individual hydrocarbons were identified by using available standard compounds. Data at different times on stream were obtained
Figure 1. XRD patterns of different zirconia samples: (A) SZO(I), (B) SZO(D), and (C) unsulfated zirconia (m and t stand for monoclinic and tetragonal phases, respectively). Table 1. Some Physicochemical Properties of Sulfated Zirconia Catalysts surface area sulfur tetragonal sample (m2/g) (wt %) phase (%) SZO(I) SZO(D)
143 157
2.13 1.58
100 100
acidity (×10-6 mol NH3/g) Wb
MSb
VSb
total
52.94 40.47 10.47 103.92 39.69 22.97 9.42 72.08
a From XRD. b W ) weak, MS ) medium strong, VS ) very strong.
from the samples collected in multiple sampling loops from the same continuous run. The sulfated zirconia catalysts were pelletized, crushed, and sieved to 60-80 µm particle size and activated in the reactor by calcining in air at 550 °C for 4 h. Air flow was then replaced by nitrogen and the catalyst temperature was lowered to the desired reaction temperature.
Results and Discussion XRD patterns of the calcined samples are shown in Figure 1. While sulfated zirconia samples show the presence of pure tetragonal phase only, the unsulfated zirconia sample showed the presence of both tetragonal and monoclinic phases. It is known that zirconia prepared by precipitation and calcination of zirconyl salts give a mixture of tetragonal and monoclinic phases and the relative amount of each phase depends upon the pH of precipitation.16 Precipitation at pH 9.3 or above gives the tetragonal phase as the major constituent. It was also reported that sulfate treatment stabilizes the tetragonal phase in zirconia.16,17 Table 1 shows the the method of sulfation does not have much influence on the surface area of the samples. Dropwise addition of sulfuric acid produced a catalyst with a marginally higher surface area. However, the sample prepared by impregnation contained more sulfur than that prepared by the other method. This is (16) Corma, A.; Forne´s, V.; Juan-Rajadell, M. I.; Lo´pez Nieto, J. M. Appl. Catal. A 1994, 116, 151. (17) Davis, B. H.; Keogh, R. A.; Srinivasan, R. Catal. Today 1994, 20, 219.
Sulfated Zirconia Superacid Catalysts
Figure 2. Weight loss curves as a percentage of initial weight for (a) unsulfated zirconia (s), (b) SZO(D) (-‚-) and (c) SZO(I) (- -) samples.
Figure 3. Temperature-programmed desorption patterns of the sulfated zirconia samples: (A) SZO(D); (B) SZO(I).
possibly due to the presence of some free sulfuric acid in this sample. Thermogravimetric curves of the samples are presented in Figure 2. The unsulfated zirconia sample showed a weight loss of about 26% in a single step. For sulfated zirconia samples two distinct weight loss regions are observed. The first weight loss of about 2225% occurs during heating to 600 °C and was due to the removal of water. The SZO(D) sample showed a little higher weight loss in this region. However, this is not unexpected as the weight loss in this region depends upon the drying conditions and the amount of SO42- added during sulfation.18 The second weight loss of about 6-8% was observed above 600 °C and is attributed to the evolution of oxides of sulfur.18 Temperature-programmed desorption spectra of the samples are shown in Figure 3. It can be seen from the figure that the two sulfated zirconia samples have similar acid site distribution pattern with three des(18) Srinivasan, R.; Keogh, R. A.; Milburn, D. R.; Davis, B. H. J. Catal. 1995, 153, 123.
Energy & Fuels, Vol. 12, No. 1, 1998 111
orption peaks at about 180, 360, and 590 °C. The first two broad peaks are assigned to sites with weak and medium-strong acidity, while the high-temperature peak was assigned to superacid sites.13 In order to ascertain that the high-temperature peak is due to ammonia only and not sulfur dioxide, the gaseous effluent was collected and analyzed separately. Catalysts pretreated at 600 °C when subjected to TPD did not show any sulfur dioxide in the effluent gases (although some sulfur dioxide evolution was noticed during the pretreatment process). Analysis of the effluent gases showed only ammonia to be present. Reproducible ammonia TPD pattern could be obtained on the same sample if the temperature of TPD does not exceed 620 °C. This shows that acidity remains unchanged upto 620 °C. However, if heating during TPD was continued beyond 620 °C, loss of acidity occurs with evolution of sulfur dioxide. The acid site distribution given in Table 1 shows that two samples have similar number of superacid sites but the SZO(I) sample has a greater number of acids sites of weak and medium strength. It was also observed that there is a direct relationship between the total acidity and the sulfur content of the catalysts. The SZO(I) sample containing more sulfur was found to have more total acidity than the SZO(D). However, the increase in sulfur content mainly increases the number of acid sites in the weak and medium strength region and has very little effect on the superacidic sites. Alkylation Reaction. The chemistry of alkylation reaction is complex and has been discussed extensively earlier.19 The simplest model of alkylation reaction consists of the main desired reaction between isoparaffin (P) and olefins (O) to produce alkylates (A), mainly trimethylpentanes (reaction 1) and undesired side reaction like olefin dimerization (reaction 2) to produce higher olefins (D). Therefore, in order to promote reaction 1 over reaction 2, a very high isoparaffin to olefin (P/O) ratio must be maintained.
P+OfA
(1)
O+OfD
(2)
In addition, other reactions like disproportionation, polymerization, cracking, and self-alkylation also occur, making the overall reaction more complex. Also, solid acid catalysts can be deactivated by blockage of the acidic sites by the adsorption of higher olefins (D) and also by coking. Effect of Time on Stream (TOS). It is known that the solid acid catalysts undergo rapid deactivation during the initial few minutes of alkylation due to coking of the acidic sites.6,8,13 Figure 4 shows the 1-butene conversion obtained at different time on stream with the two sulfated zirconia catalysts. It can be seen that in either case, the olefin conversion sharply drops from initial 100% to about 40% within 15 min. The drop during the initial 5-7 min was much higher in case of SZO(I). In contrast, during liquid-phase reaction the olefin conversion drops to about 20% within 7 min TOS.13 The product distributions at different TOS are given in Table 2. It was observed that, although 1-butene conversion was very high at initial TOS, the product mainly consists of C5-7 hydrocarbons and no
112 Energy & Fuels, Vol. 12, No. 1, 1998
Das and Chakrabarty
Figure 4. Effect of time on stream on 1-butene conversion for SZO(D) (9) and SZO(I) (0). Table 2. Olefin Conversion and Product Distribution as a Function of TOS (Temperature ) 32 °C, Pressure ) 1 atm, Olefin WHSV ) 1.03 h-1) time on stream (min) 3
5
7
9
11
13
15
SZO(I) butene conv (%) 100.0 100.0 69.4 62.2 55.6 37.6 C5+ distribution (wt %) C5-7 99.7 89.9 46.0 86.0 92.3 99.5 100.0 C8 0.3 10.0 53.5 13.8 7.7 0.5 0.0 C9+ 0.0 0.1 0.5 0.2 0.0 0.0 0.0 SZO(D) butene conv (%) 100.0 100.0 84.3 79.5 68.4 62.3 C5+ distribution (wt %) C5-7 100.0 87.2 46.4 24.6 6.4 0.0 C8 0.0 12.5 52.9 74.6 92.7 98.7 C9+ 0.0 0.3 0.7 0.8 0.9 1.3
42.5 0.0 99.0 1.0
trace of C8 hydrocarbons was found. As both alkylation and olefin oligomerization reaction would give C8 hydrocarbons, the formation of C5-7 hydrocarbons therefore suggests that the initial alkylation or oligomerization products undergo cracking on some acid sites of the catalysts. As these acid sites responsible for cracking get deactivated with time, the amount of C8 hydrocarbons in the product becomes appreciable. Further deactivation of the acid sites with time is responsible for the loss of activity of the catalysts. It may be mentioned here that unlike liquid phase reactions where the reaction is carried out under pressure, the cracking of initial products will be more favored in the present case. It is generally believed that the superacids sites are responsible for isoparaffin alkylation. Corma et al.,13 however, did not observe any relationship between superacidity and alkylation activity in the liquid-phase alkylation of isobutane with 2-butene. They proposed that the superacid sites of sulfated zirconia catalysts are not the only active sites responsible for isobutane alkylation and other acid sites also play an important role. Acid strength necessary to perform different reactions are proposed to be in the following order.15
cracking > alkylation > olefin dimerization (oligomerization)
It can be seen from Table 2 that the selectivity for C8 fractions reached about 53% at 7 min TOS on both the catalysts. At higher TOS, the selectivity of the two catalysts, however, showed quite different behavior. In case of SZO(I), selectivity for C8 hydrocarbons decreased continuously after 7 min TOS and, at 15 min, the product consists of only the cracked products, whereas for SZO(D) the selectivity for C8 hydrocarbons continuously increased with TOS and, after 13 min, the product consists mostly the C8 fractions. Thus, when the catalysts were less deactivated during initial few minutes of reaction both showed almost similar product distribution and after 7 min TOS, the product pattern showed opposite behaviour. One reason for this may be that, since the total conversion has decreased with TOS, the available number of acid sites responsible for cracking are just sufficient to crack the alkylate formed on SZO(I) catalyst. It may be noted here that this catalyst has a much higher number of acid sites in the weak and medium strong region (Table 1) than the other catalyst. Also the SZO-I catalyst suffered more rapid deactivation after initial few minutes of reaction which is probably due to the greater number of strong and medium strong acid sites in this catalyst. Comparison with Liquid Acid Catalysts. A comparison of the composition of alkylate obtained with the commercial liquid acid catalysts and sulfated zirconia catalysts is given in Table 3. It can be seen that sulfated zirconia catalysts showed slightly higher cracking activity than the liquid acid catalysts as indicated by more C5-7 fractions in the product. Selectivity toward C8 hydrocarbon was comparable to that obtained with liquid acid catalysts but the amount of dimethylhexane (DMH) was found to be much higher on sulfated zirconia catalysts in the present study. On the other hand, the liquid-phase alkylation reaction on sulfated zirconia catalysts produced much higher fraction of trimethylpentanes (TMP’s) and less DMH’s.13 This may be related to the nature of the olefin feed used in the experiments. In the present work, 1-butene is used as the olefin feed whereas 2-butene was used in the other study. Primary reaction between the tert-butyl cation (generated from isobutane) and 1-butene or 2-butene will give dimethylhexane or trimethylpentane, respectively. However, both dimethylhexane and trimethylpentane are observed in the product indicating interconversion of 1-butene to 2-butene on sulfated zirconia catalysts. It is known that under alkylation reaction conditions interconversion of 1-butene and 2-butene occurs on the acidic catalysts.19 However, it appears that the interconversion of 1-butene to 2-butene does not go to completion or proceeds much more slowly resulting in higher amount of dimethylhexane in the product when 1-butene is used as olefin feed. Better alkylate quality is obtained when 2-butene is used as olefin feed. Dimethylhexane can also be formed by dimerization of olefin molecules. However, as in both ref 13 and the present work an almost similar paraffin olefin molar ratio was used, it indicates that the nature of the olefin feed makes the product composition different. Effect of Olefin Space Velocity (OSV). Figure 5 shows the effect of OSV on 1-butene conversion at 7 min (19) Corma, A.; Martı´nez, A. Catal. Rev. Sci. Eng. 1993, 35, 483.
Sulfated Zirconia Superacid Catalysts
Energy & Fuels, Vol. 12, No. 1, 1998 113
Table 3. Comparison of Alkylation Activity of Liquid Acid and Solid Acid Catalysts catalyst
H2SO4
HF
SZO(I)
SZO(D)
SZ
ref reaction temp (K) P/O (molar) olefin olefin space velocity (h-1) TOS (min) C5+ distribution (wt %) C5-7 C8 C9+ C8 distribution (wt %) TMP DMH octene TMP/DMH
13 283 7.2 2-butene
13
present work 305 14.0 1-butene 1.9 7
present work 305 14.0 1-butene 1.9 7
13 275 15.0 2-butene 2.0 4
7.5 88.1 4.4
2.7 93.2 4.1
8.0 92.0 0.0
18.3 81.7 0.0
18.1 74.5 7.4
90.8 9.2
91.8 8.2
9.9
11.2
62.6 27.4 10.0 2.3
65.0 27.3 7.7 2.4
94.5 3.6 1.9 26.2
2-butene
Table 4. Effect of Olefin WHSV on the Activity of Sulfated Zirconia Catalysts (Temperature ) 32 °C; TOS ) 7 min) SZO-I olefin WHSV
(h-1)
butene conversion (%) C5+ distribution (wt %) C5-7 C8 C9+ C8 distribution (wt %) TMP DMH DMH(ene)a TMP distribution (wt %) 2,2,4 2,2,3 2,3,4 2,3,3 TMP/DMH a
Figure 5. Effect of space velocity on 1-butene conversion at 7 min time on stream for SZO(D) (9) and SZO(I) (0).
TOS. A sharp fall in olefin conversion was observed with increase in olefin space velocity as reported earlier by Corma et al.13 A similar fall in olefin conversion with increasing OSV was also reported for other solid acid catalysts like zeolite beta20 and MCM-22.21 The product compositions at different OSV is given in Table 4. For both the catalysts, the amount of cracked products and C9+ fractions were found to decrease with the increase in OSV but the amount of C8 hydrocarbons was increased. At lower OSV, olefin conversion was low and the number of strong acid sites responsible for cracking was sufficient to crack the alkylate formed into C5-7 fractions. At moderate OSV, deactivation of the strong acid sites sets in within 7 min time on stream and the product contained more of C8 fractions. Lower conversion for SZO-I catalyst was possibly due to the faster deactivation of the strong and medium strong acid sites. At even higher OSV, the catalysts were mostly deactivated at 7 min time on stream and the olefin conversion reached almost similar value for both the catalysts. The distributions of the C8 hydrocarbons are also given in Table 3. With increase in OSV, the amount of TMP’s in the product decreases but that of DMH increases as observed earlier by others.13 The reason (20) Corma, A.; Go´mez, V.; Martı´nez, C. Appl. Catal. A 1994, 119, 83. (21) Corma, A.; Martı´nez, A.; Martı´nez, C. Catal. Lett. 1994, 28, 187.
SZO-D
0.74
1.03
1.85
0.74
1.03
1.85
93.0
69.4
29.7
94.5
84.3
26.0
63.5 36.1 0.4
46.0 53.5 0.5
8.0 92.0 0.0
63.7 35.6 0.7
46.4 52.9 0.7
18.3 81.7 0.0
72.8 15.2 12.0
65.6 18.4 16.0
62.6 27.4 10.0
78.9 12.5 8.6
69.1 21.0 9.9
65.0 27.3 7.7
66.1 17.4 16.5
68.2 23.6 8.2
64.0 28.1 7.9
4.8
3.6
2.3
78.1 9.3 7.0 5.6 6.3
64.0 18.2 9.4 8.4 3.3
63.3 29.4 7.3 0.0 2.4
Dimethylhexene.
Table 5. Effect of Temperature on the Activity of Sulfated Zirconia Catalysts (Olefin WHSV ) 1.03 h-1, TOS ) 7 min) SZO(I)
SZO(D)
temp (°C)
32
50
32
50
butene conversion (%) C5+ distribution (wt %) C5-7 C8 C9+ C8 distribution (wt %) TMP DMH DMH(ene) TMP distribution (wt %) 2,2,4 2,2,3 2,3,4 2,3,3 TMP/DMH
69.4
91.0
84.3
94.7
46.0 53.5 0.5
97.9 2.1
46.4 52.9 0.7
17.7 79.9 2.4
65.6 18.4 16.0
64.9 25.1 10.0
69.1 21.0 9.9
70.2 23.3 6.5
68.2 23.6 8.2
47.6 17.2 21.9 13.3 2.6
64.0 18.2 9.4 8.4 3.3
30.2 27.6 21.6 20.6 3.0
3.6
is that the acid sites responsible for TMP formation are stronger than those responsible for DMH formation and at higher OSV, the strong acid sites are deactivated at a faster rate than the weak acid sites. The ratio of TMP/ DMH, which may be considered as the measure of alkylation vs isomerization activity, was found to decrease continuously at higher OSV. Among different TMP’s 2,2,4-trimethylpentane was found to be most abundant, followed by 2,2,3-TMP and other isomers. Effect of Reaction Temperature. Conversion of 1-butene and product distribution at 7 min TOS for
114 Energy & Fuels, Vol. 12, No. 1, 1998
Das and Chakrabarty
Table 6. Activity and Selectivity of Regenerated Sulfated Zirconia Catalysts (Olefin WHSV ) 1.03 h-1, TOS ) 7 min, Temperature ) 32 °C) SZO(I)
SZO(D)
no. of regenerations
0
1
2
3
0
1
2
3
butene conversion (%) C5+ distribution (wt %) C5-7 C8 C9+ C8 distribution (wt %) TMP DMH DMH(ene) TMP/DMH
69.4
70.2
68.2
61.8
84.3
84.0
80.1
75.8
46.0 53.5 0.5
45.3 54.2 0.5
48.4 50.8 0.8
47.0 51.9 1.1
46.4 52.9 0.7
45.4 54.1 0.5
48.0 51.1 0.9
45.8 53.3 0.9
65.6 18.4 16.0 3.6
68.2 18.8 13.0 3.6
70.0 21.5 8.5 3.3
71.8 21.1 7.1 3.4
69.1 21.0 9.9 3.3
72.4 18.9 8.7 3.8
70.3 21.2 8.5 3.3
74.1 22.4 3.5 3.3
different reaction temperatures are given in Table 5. It shows that there was a considerable increase in olefin conversion at 50 °C on both catalysts. However, further increase in reaction temperature resulted in drastic decline in conversion which is expected as the catalysts undergo more rapid deactivation at higher temperatures. As seen from Table 5, on increasing the reaction temperature to 50 °C there was faster poisoning of the stronger acid sites responsible for cracking and C8 hydrocarbons constitute the predominant fractions of the product. However, the composition of C8 hydrocarbons changed with increase in reaction temperature. Faster poisoning of strong acid sites (responsible for alkylation) than those of lower acid strength (active for olefin dimerization) results in increased dimethylhexane selectivity in the product. The higher DMH selectivity for SZO-I may be related to its higher number of weak acid sites. As a result, the TMP/DMH ratio decreases with the increase in reaction temperature. In spite of its decrease with increase in temperature, 2,2,4-TMP remains the major component in the product among the different isomers. The amount of 2,3,4- and 2,3,3-TMP isomers was also found to increase with reaction temperature. Catalyst Regeneration. It has been found that sulfate-promoted zirconia catalysts suffer rapid deactivation due to coking during the initial few minutes of reaction. In order to make the use of such catalysts meaningful, it is necessary to regenerate them successfully. Although there is no report on the reusability of these catalysts in the alkylation reactions, it has been shown recently that in n-butane isomerization the deactivated sulfated zirconia catalysts can be fully regenerated by oxidative treatment at 450-480 °C.22,23 We have found that both the catalysts used in the present work could be regenerated by passing air through the catalyst bed for 6 h at 450 °C. Table 6 shows that the catalysts can be recycled three times without any significant loss in C8 selectivity and only slight decrease in activity. The small decrease in activity was possibly due to a decrease in the number
active sites by loss of sulfate groups from the surface. Loss of surface sulfate groups during alkylation reaction was recently observed by Corma et al.15 It was suggested that the loss of sulfur is due to chemical interaction between butene molecules and some surface sulfate groups which promotes sulfate decomposition under alkylation reaction conditions.24 From the C8 distribution given in Table 6 it appears that the acid strength distribution of the acid sites slightly changes after catalyst regeneration. It shows that the amount of TMP and DMH increases marginally after regeneration, while the hydride transfer capacity of the catalysts was found to increase to some extent reducing the amount of DMH(ene) in the product. The TMP/DMH ratio was found to be almost unaffected by catalyst regeneration.
(22) Spielbauer, D.; Mekhemer, G. A. H.; Bosch, E.; Kno¨zinger, H. Catal. Lett. 1996, 36, 59. (23) Li, C.; Stair, P. C. Catal. Lett. 1996, 36, 119.
EF970078+
Conclusions The sulfation procedure was found to have negligible effect on the nature crystalline phases present and surface areas of the sulfated zirconia superacid catalysts. Impregnation with sulfuric acid gives a catalyst with higher acidity, mostly in the weak and medium strong region. However, the superacidic sites are found to be independent of the method of sulfate incorporation. Both catalysts showed high C8 selectivity comparable to liquid acid catalysts, but under gas-phase operating conditions using 1-butene as olefin feed the selectivity toward TMP is rather low, although the catalyst deactivation is much slower. Increase in olefin space velocity and reaction temperature was found to decrease the alkylation activity over olefin dimerization as indicated by the fall of the TMP/DMH ratio. Even if the catalysts lost their activity within the initial few minutes of reaction, they can be regenerated successfully by heating in air at 450 °C. Acknowledgment. This work has been supported by a research grant from the Centre for High Technology, Ministry of Petroleum, New Delhi. We thank Prof. C. S. Swamy, Chemistry Department, IIT Madras, for surface area measurements.
(24) Lee, J. S.; Yeom, M. H.; Park, D. S. J. Catal. 1990, 126, 361.
