Energy Fuels 2010, 24, 3760–3763 Published on Web 02/09/2010
: DOI:10.1021/ef901351f
Study on the Oligomerization of Ethylene in Fluidized Catalytic Cracking (FCC) Dry Gas over Metal-Loaded HZSM-5 Catalysts† Xue Ding, Chunyi Li,* and Chaohe Yang State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266555, Shandong, China Received November 13, 2009. Revised Manuscript Received January 26, 2010
Oligomerization of ethylene in fluidized catalytic cracking (FCC) dry gas was investigated as a feasible route for the use of FCC dry gas. A series of metal-loaded HZSM-5 (MZSM-5) catalysts were prepared by the impregnation method. It was characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and N2 adsorption that the metal introduction did not change the structure of the HZSM-5 catalyst but had an influence on acidic properties, i.e., decreasing Br€ onsted acid sites and forming Lewis acid sites together with the blocking of channels. The catalytic performances of MZSM-5 catalysts for the oligmerization of ethylene in FCC dry gas were studied in a fixed-bed reactor. It was observed that the addition of metal to fresh HZSM-5 enhanced the olefin yield. Among all catalysts studied, MgZSM-5 exhibited the highest propylene and total olefin yields of 12.21 and 17.28%, respectively. Higher olefin yields were obtained over the steam-treated HZSM-5 catalyst when adding moderate N2 as a diluent gas. The catalytic performance of the steam-treated HZSM-5 catalyst was also investigated in a fluidized-bed reactor, and similar product distribution could be obtained in comparison to that in a fixed-bed reactor. The conversion of ethylene could reach 47.22% at 500 °C with a propylene yield of 14.32%.
responsible for ethylene dimerization are low-valent nickel species.5-12 Hulea and co-workers used a series of Ni-containing zeolite in ethylene oligomerization, and the best catalytic performance over NiY, NiMCM-41, and NiMCM-22 were 30 g gcatalyst-1 h-1 (50 °C), 63.2 g gcatalyst-1 h-1 (150 °C), and 46 g gcatalyst-1 h-1 (150 °C), respectively.13-15 The balance between acid and nickel ion sites and the porous structures of catalysts played a significant role in determining the activity and selectivity. NiMCM-22 exhibited negligible activity because of its low Ni2þ/acid sites ratio and microspores system.15 Heydenrych et al. reported the ethylene oligomerization in a stirred slurry reactor over NiII-silica-alumina at 3.5 MPa and the temperature in the range of 120-180 °C. It was observed that, during continuous runs lasting 900 h, the conversion was stabilized at over 90% with the reaction rate of above 11.5 g gcatalyst-1 h-1.16 Iwamoto and co-workers prepared NiMCM-41 by the template ion-exchange method to convert ethylene to propylene and butylenes in the presence of steam, and the one-path conversion of ethylene was 68%, with propylene selectivity of 48% at 400 °C. The reaction proceeded via a metathesis mechanism, i.e., the dimerization
1. Introduction Currently, the total fluidized catalytic cracking unit (FCCU) productivity in China is 130 megatons/year, with dry gas of 5.2 megatons/year containing ethylene of approximately 1.0 megaton/year.1 With rapid expansion of refining capacity, the yield of ethylene in fluidized catalytic cracking (FCC) dry gas tends to increase. The two-stage riser catalytic cracking technology developed by the China University of Petroleum has played an important role in maximizing propylene yield with ethylene content in dry gas basically above 50 wt %.2 From the viewpoint of energy conservation and emission reduction, it is important to take full advantage of the ethylene resource in FCC dry gas and make it a new economic growth point of FCCU. The direct oligomerization of ethylene in FCC dry gas to C3-C4 hydrocarbons would be a feasible route for the use of FCC dry gas. The heterogeneous ethylene oligomerization has been reported in early papers on solid phosphoric acid (SPA) catalysts.3 Because of corrosion problem, other types of catalysts have been studied to replace catalysts containing phosphoric acid.4 Catalysts comprising nickel compounds supported on oxides and zeolites were widely reported, and the active sites
(6) Sohn, J. R.; Cho, E. S. Appl. Catal., A 2005, 282, 147–154. (7) Sohn, J. R.; Park, W. C.; Kim, H. W. J. Catal. 2002, 209, 69–74. (8) Sohn, J. R.; Lee, S. H. Appl. Catal., A 2007, 321, 27–34. (9) Cai, T.; Zang, L.; Qi, A.; Wang, D.; Cao, D.; Li, L. Appl. Catal. 1991, 69, 1–13. (10) Ghosh, A. K.; Kevan, L. J. Phys. Chem. 1990, 94, 3117–3121. (11) Hartmann, M.; P€ oppl, A.; Kevan, L. J. Phys. Chem. 1996, 100, 9906–9910. (12) Elev, I. V.; Shelimov, B. N.; Kazansky, V. B. J. Catal. 1984, 89, 470–477. (13) Hulea, V.; Fajula, F. J. Catal. 2004, 225, 213–222. (14) Lallemand, M.; Finiels, A.; Fajula, F.; Hulea, V. Appl. Catal., A 2006, 301, 196–201. (15) Lallemand, M.; Rusu, O. A.; Dumitriu, E.; Finiels, A.; Fajula, F.; Hulea, V. Appl. Catal., A 2008, 338, 37–43. (16) Heydenrych, M. D.; Nicolaides, C. P.; Scurrel, M. S. J. Catal. 2001, 197, 49–57.
† This paper has been designated for the Asia Pacific Conference on Sustainable Energy and Environmental Technologies (APCSEET) special section. *To whom correspondence should be addressed. Telephone: þ860532-86981862. E-mail:
[email protected]. (1) Ding, X.; Geng, S.; Li, C.; Yang, C.; Wang, G. J. Nat. Gas Chem. 2009, 18, 156–160. (2) Li, C.; Yang, C.; Shan, H. Ind. Eng. Chem. Res. 2007, 46, 4914– 4920. (3) Wen, L.; Liu, Y.; Zhang, W.; Xu, Z.; Cui, S.; Ding, Y.; Wang, S. Chem. J. Chin. Univ. 1994, 15, 428–430. (4) Dutta, P.; Roy, S. C.; Nandi, L. N.; Samuel, P.; Pillai, S. M.; Bhat, B. D.; Ravindranathan, M. J. Mol. Catal. A: Chem. 2004, 223, 231–235. (5) Sohn, J. R.; Lim, J. S. Catal. Today 2006, 111, 403–411.
r 2010 American Chemical Society
3760
pubs.acs.org/EF
Energy Fuels 2010, 24, 3760–3763
: DOI:10.1021/ef901351f
Ding et al.
of ethylene on Ni to 1-butylene, the subsequent isomerization to 2-butylene on acid sites of MCM-41, and finally, the metathesis of 2-butylene with ethylene to propylene on Ni.17,18 However, the catalyst may not be stable enough in the presence of steam. The oligomerization of ethylene on proton-exchanged zeolite catalysts following the carbenium ion mechanism has also been widely investigated.19-23 Mobil has developed the Mobil olefin to gasoline/distillate (MOGD) process for the conversion of lighter olefins to fuel products, such as gasoline based on ZSM-5 zeolite.22,23 SAPO-34 exhibited high propylene selectivity of 73.3% at ethylene conversion of 71.2% and 450 °C, attributed to the shape selectivity effect of its micropores and modest acid strength; however, rapid deactivation was observed during 2 h runs.21,24 Thus far, few attempts have been made in the direct conversion of ethylene in FCC dry gas to C3-C4 olefins. Our previous work indicated that HZSM-5 was suitable for this reaction among several different types of catalysts in consideration of both activity and stability; moreover, the dimerization of ethylene proceeded via the Eley-Rideal mechanism.1,25,26 Metals have usually been used to modify zeolite property to enhance catalytic activity and selectivity because of the ability of accommodating acidity of zeolite and, thus, adjusting product distribution.19,27-29 In this paper, a series of MZSM-5 catalysts were prepared by the impregnation method and evaluated in a fixed-bed reactor for the oligomerization of ethylene in FCC dry gas; moreover, a reaction over the steam-treated HZSM-5 catalyst was also investigated in a fluidized-bed reactor. It is anticipated that this research will pave the way for future research in the use of ethylene in FCC dry gas.
Table 1. Hydrocarbon Composition of FCC Dry Gas composition
CH4
C2H6
C2H4
C3þ
vol % wt %
33.34 21.89
16.70 20.56
49.84 57.27
0.12 0.28
obtained using a volumetric adsorption apparatus (ASAP 2010, Micromeritics Co., Norcross, GA), with nitrogen as the adsorbate at -196 °C. 2.2. Catalytic Reaction. The catalytic reaction was carried out using a fixed-bed reactor of 7 mm inner diameter and a self-made fluidized-bed reactor of 15 mm inner diameter in a vertical furnace.26 FCC dry gas was supplied as a feedstock without further treatment. The typical hydrocarbon composition of dry gas was listed in Table 1. The catalyst was filled in the middle of the reactor with quartz sand placed above and below to arrest back mixing. The catalysts were heated from room temperature to the desired value under N2 stream before the reaction. Subsequently, the FCC dry gas was fed into the reactor through a mass flow meter, and the reaction of ethylene was tested. The product composition was analyzed by a GC3800 gas chromatograph with a flame ionization detector (FID). The ethylene conversion is defined as (where all percentages are in weight (wt), “in” denotes C2H4 in the feedstock, “out” denotes C2H4 in the products, and x and y denote the number of carbon and hydrogen atoms in molecule CxHy, respectively) wC2 H4in - wC2 H4out 100% conversion ¼ wC2 H4in yield ðCx Hy Þ ¼ wCx Hy out 100% In the fluidized-bed reactor, dry gas flowed from the bottom to the top of the reactor tube. Experimental data were obtained during 15 min of continuous services of the catalytic runs. 2.3. Thermodynamic Equilibrium Composition. The equilibrium conversion in the oligomerization of ethylene was calculated as a function of the temperature to determine the maximum conversion of ethylene and yield of olefins under conditions that only the conversion of ethylene to C3-C5 olefins was taken into account. The calculation was performed using MATLAB programs based on the minimization of the total Gibbs energy of the system. The results of the calculation are shown in Figure 1. The calculation indicated that ethylene showed a nearly complete conversion at the temperature lower than 400 °C, and the conversion decreased with an increasing temperature. The olefin yields at low temperature decreased in the order of C5= >C4= >C3=. With the increase in temperature, the C5= yield was decreased and C4= and C3= yields were increased and reached the maximum, respectively. In other words, the product molecular weight decreased with increasing temperature.
2. Experimental Section 2.1. Material Preparation and Characterization. HZSM-5 zeolite (38:1 Si/Al) was kindly provided by the Catalyst Plant of Nankai University. A series of MZSM-5 (see Table 2) with 5.0 wt % of different kinds of metals were prepared by the impregnation of HZSM-5 zeolite powder with a corresponding aqueous solution of M(NO3)x, followed by calcination at 700 °C for 1.5 h in air. The catalyst was prepared by mixing 30 wt % zeolite powder with 70 wt % silica sol as an inert supporter. The mixture obtained was calcined at 700 °C for 2 h and then milled into 80-180 mesh. HZSM-5 catalysts were steam-deactivated at 780 °C for 4 h to obtain steam-treated catalysts. X-ray diffractograms of catalysts were taken by a Rugaku D/Max RB diffractometer using Cu KR radiation operated at 40 kV and 40 mA with a scanning speed of 10°/min. Fourier transform infrared (FTIR) spectroscopy was employed to study the acidity, as reported elsewhere.1 The micropore area and volume were
3. Results and Discussion 3.1. Oligomerization Properties of Ethylene in FCC Dry Gas over MZSM-5 Catalysts. The reaction was carried out over 0.5 g of MZSM-5 catalyst at 0.3 MPa, 550 °C, and a total flow rate of 100 mL/min in a fixed-bed reactor. The results are listed in Table 2. In comparison to fresh HZSM-5, the ethylene conversion showed a decrease of different degrees, while the olefin yield exhibited an improvement over the MZSM-5 catalyst. The MgZSM-5 catalyst exhibited the best propylene and total olefin yields of 12.21 and 17.28%, respectively. The propylene yield decreased in the order: Mg > Mn > Sr > Cr > Zr > Mo > HZSM-5 (steam treated). Figure 2 shows the diffractogram of HZSM-5 impregnated with magnesium. No significant difference was found between the diffractogram of HZSM-5 and the catalyst after impregnation with magnesium, indicating that the metal species were highly
(17) Iwamoto, M.; Kosugi, Y. J. Phys. Chem. C 2007, 111, 13–15. (18) Ikeda, K.; Kawamura, Y.; Yamanoto, T.; Iwamoto, M. Catal. Commun. 2008, 9, 106–110. (19) Mat, R.; Amin, N. A. S.; Ramli, Z.; Abu Bakar, W. A. J. Nat. Gas Chem. 2006, 15, 259–265. (20) Bessell, S.; Seddon, D. J. Catal. 1987, 105, 270–275. (21) Oikawa, H.; Shibata, Y.; Inazu, K.; Iwase, Y.; Murai, K.; Hyodo, S.; Kobayashi, G.; Bata, T. Appl. Catal., A 2006, 312, 181–185. (22) Chen, C. S. H.; Heights, B. U.S. Patent 4,520,221, 1985. (23) Chen, C. S. H.; Heights, B. U.S. Patent 4,658,079, 1987. (24) Li, J.; Qi, Y.; Liu, Z.; Liu, G.; Chang, F. Chin. J. Catal. 2008, 29, 660–664. (25) Ding, X.; Li, C.; Yang, C.; Shan, H. J. Chin. Univ. Pet. 2009, 33, 145–149. (26) Liu, F.; Li, C.; Ding, X.; You, X. J. Nat. Gas Chem. 2007, 16, 301– 307. (27) Zhang, J.; Zhang, Y.; Liang, S. J. Mol. Catal. 2004, 18, 357–360. (28) Wang, X.; Zhao, Z.; Xu, C.; Duan, A.; Zhang, L.; Jiang, G. J. Rare Earth 2007, 25, 321–328. (29) Zhu, G. Acta Pet. Sin. 1997, 13, 100–104.
3761
Energy Fuels 2010, 24, 3760–3763
: DOI:10.1021/ef901351f
Ding et al.
Table 2. Oligomerization of Ethylene in FCC Dry Gas over MZSM-5 Catalysts product yield (%) number
catalyst
conversion (%)
C1þC2
LPG
C3=
C3= þ C4=
C5þ
C3= þ C4=/LPG
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
MgZSM-5 SrZSM-5 TiZSM-5 ZrZSM-5 VZSM-5 CrZSM-5 MoZSM-5 MnZSM-5 FeZSM-5 CoZSM-5 NiZSM-5 CuZSM-5 AgZSM-5 ZnZSM-5 CdZSM-5 GaZSM-5 LaZSM-5 CeZSM-5 HZSM-5 (fresh) HZSM-5 (steam treated)
47.10 51.51 68.22 58.48 64.01 59.49 43.93 47.90 69.82 61.30 68.84 65.30 69.31 66.10 69.18 68.88 65.38 70.41 83.15 69.17
58.82 57.87 64.32 58.27 61.51 56.41 59.70 55.22 60.70 68.17 65.57 58.36 60.70 60.06 67.11 65.59 62.64 59.26 63.47 59.81
24.12 24.52 25.95 26.31 23.85 25.63 19.16 32.55 26.02 15.12 22.06 25.15 26.91 23.32 22.33 23.38 25.70 26.19 26.53 27.24
12.21 11.47 8.04 10.23 8.12 10.37 10.13 11.70 7.61 6.42 6.69 8.83 6.98 8.14 6.20 6.75 8.98 7.55 4.63 9.36
17.28 16.67 11.20 14.56 11.67 15.12 14.61 17.28 10.71 8.72 9.32 12.39 9.61 11.29 8.44 9.12 12.24 10.43 6.47 13.27
5.03 6.58 2.50 5.97 6.45 8.74 8.38 9.17 6.42 7.90 5.28 8.60 5.41 8.90 3.55 3.94 3.78 7.82 5.51 4.72
71.62 67.99 43.18 55.34 48.93 59.00 76.27 53.10 41.17 57.68 42.24 49.27 35.72 48.41 37.78 38.99 47.64 39.80 24.38 48.69
Figure 3. FTIR spectra of HZSM-5 and MgZSM-5.
Figure 1. Composition at thermodynamic equilibrium at various temperatures under 0.3 MPa.
It can be seen that strong Br€ onsted acid sites and weak Lewis acid sites are in the majority on fresh HZSM-5. The introduction of metal on HZSM-5 modified the acidic property. In comparison to fresh HZSM-5, the amount of Br€ onsted acid sites of MgZSM-5 decreased, whereas more Lewis acid sites were formed, especially for strong sites. Other MZSM-5 catalysts showed a similar trend (not shown). This is in agreement with the report by Mat and Wang that the effect is attributed to the coverage of Br€ onsted acid sites by metal introduction as well as the generation of new Lewis acid sites from the substitution of metal ion for protons of hydroxyl groups.19,31,32 The oligomerization of ethylene mainly proceeds over Br€ onsted acid sites of catalysts; hence, the ethylene conversion decreased because of the deposition of metal species over acid sites and the blocking of the channels.21 Moreover, the hydrogen-transfer reaction was also inhibited, leading to a higher olefin yield. The addition of metal to the HZSM-5 catalyst could enhance the olefin yield. However, in comparison to the MZSM-5 catalyst, the steam-treated HZSM-5 catalyst exhibited a higher olefin yield. A previous experiment indicated that adding inert gas into the reaction system favored the increase of the olefin yield.33 As shown in Figure 4, the activity of fresh HZSM-5 and steam-treated HZSM-5 was investigated at certain N2/dry gas ratios and different
Figure 2. XRD patterns of HZSM-5 and MgZSM-5.
dispersed on the surface of the catalyst and the HZSM-5 structure was not destroyed by the introduction of metal. All of the other MZSM-5 catalysts showed a similar pattern (not shown). The micropore areas and volumes of fresh HZSM-5 and MgZSM-5 obtained by the N2 adsorption-desorption method are 59.26 and 52.09 m2/g and 0.034 and 0.031 cm3/g, respectively. This indicated that the introduction of magnesium caused blocking of channels. The FTIR spectra of adsorbed pyridine on HZSM-5 and MgZSM-5 catalysts after degassing at 200 and 450 °C are shown in Figure 3, and absorption bands associated with the chemisorbed pyridine were observed at 1450 and 1540 cm-1, which correspond to Br€ onsted and Lewis acid sites, respectively.30
(31) Wang, J.; Kang, M.; Zhang, Z.; Wang, X. J. Nat. Gas Chem. 2002, 11, 43–50. (32) Li, Y.; Xie, W.; Yong, S. Appl. Catal., A 1997, 150, 231–242. (33) Ding, X.; Li, C.; Yang, C. Pet. Technol. Appl. 2009, 27, 209–212.
(30) Poncelet, G; Dubru, M. L. J. Catal. 1978, 52, 321–331.
3762
Energy Fuels 2010, 24, 3760–3763
: DOI:10.1021/ef901351f
Ding et al. Table 4. Comparison between Fluidized- and Fixed-Bed Reactors
reactor conversion (%) product yield (%) C1þC2 LPG C5þ C3= C3= þ C4= C3=/LPG (%) C3= þ C4=/ LPG (%)
Figure 4. Olefin yields as a function of the ethylene conversion. Table 3. Effect of the Temperature on the Oligomerization of Ethylene in FCC Dry Gas in a Fluidized-Bed Reactor
conversion (%)
LPG
C3= þ C4=
C3 =
C3=/LPG (%)
350 400 450 500 550
55.27 66.58 57.13 47.22 24.14
23.61 28.59 31.50 30.80 28.24
14.24 16.51 19.88 21.21 22.05
6.14 8.46 12.36 14.32 15.83
25.99 29.61 39.23 46.48 56.03
fixed-bed reactor [550 °C, N2/ dry gas = 1:1 (vol)]
fixed-bed reactor (550 °C, no N2)
47.22
49.79
69.17
45.72 30.80 10.99 14.32 21.21 46.49 68.85
48.14 27.33 11.15 14.47 21.24 52.95 77.71
59.81 27.24 4.72 9.36 13.27 34.36 48.69
LPG increased with the temperature. The conversion of ethylene reached 47.22% at 500 °C, with the LPG yield of 30.80% and propylene content in LPG of 46.48%. Because of different contact conditions between catalysts and gas flow, the conversion of ethylene in Table 3 showed a reduction on the whole compared to fixed-bed reactor results.33 The product distribution of the fluidized-bed reactor at 500 °C and that of the fixed-bed reactor at 550 °C were listed in Table 4. With similar conversion of ethylene, the product in the fluidized-bed reactor at 500 °C was similar to that in the fixed-bed reactor at 550 °C and N2/dry gas ratio of 1:1 (vol) and no obvious differences in the yields of propylene and total olefins were observed. In other words, employing a fluidized-bed reactor could eliminate the need for N2 addition and a similar product distribution could be obtained under lower temperature compared to a fixed-bed reactor.
product yield (%) T (°C)
fluidizedbed reactor (500 °C)
temperatures and the results are compared to those of all MZSM-5 catalysts. It can be seen that a high activity always corresponds to a low olefin yield in each case. For similar conversion of ethylene, a higher olefin yield could be obtained with the addition of N2. A high olefin yield was achieved mainly in the conversion range of 30-60%. In this range, the property of MZSM-5 was similar to treated HZSM-5 without N2 addition and fresh HZSM-5 with a N2/dry gas ratio of 2:1 (vol). In all of the above cases, the optimum total olefin yield reached 21.24% over the steamtreated HZSM-5 catalyst at a N2/dry gas ratio of 1:1 (vol) and conversion of 49.79%. 3.2. Oligomerization in a Fluidized-Bed Reactor. Our previous work revealed that ethylene conversion could maintain above 95% during 20 h continuous runs over the HZSM-5 catalyst in a fixed-bed reactor at 400 °C under atmospheric pressure.25 However, coke deposition in a long-term run makes the fluidized-bed reactor a more practical choice. The effect of the reaction temperature on the oligomerization of ethylene in FCC dry gas over the steam-treated HZSM-5 catalyst in a fluidized-bed reactor was studied. Tests were conducted under atmospheric pressure, with a total flow rate of 400 mL/min and a catalyst weight of 10 g. The results are listed in Table 3. Unlike the calculated results above, the conversion of ethylene first reached a maximum value at 400 °C and then decreased with the increase of the temperature. The yield of liquefied petroleum gas (LPG) showed a similar trend. The yields of propylene and olefins and the propylene content in
4. Conclusions While the structure of HZSM-5 catalysts were not changed after the addition of 5.0 wt % metal, the presence of metal on HZSM-5 modified the acidic property, i.e., reducing Br€ onsted acid sites and increasing Lewis acid sites, as well as blocking catalyst channels. Loading of metal on fresh HZSM-5 improved the olefin yield. Among all MZSM-5 catalysts investigated, MgZSM-5 gave the highest propylene and total olefin yields of 12.21 and 17.28%, respectively. Steam treatment was employed as another approach to enhance olefin yields in the reaction. The total olefin yield of 21.24% was achieved over the steam-treated HZSM-5 catalyst at a N2/dry gas ratio of 1:1 (vol) in a fixed-bed reactor. However, the coking reaction restricts the practical application of the fixed-bed reactor and makes the fluidized-bed reactor more desirable. A similar product distribution could be obtained over the steam-treated HZSM-5 catalyst in a fluidized-bed reactor at relatively lower temperatures.
3763
