Hydrocracking and Hydroisomerization of High-Density Polyethylene

Energy & Fuels 1997, 11, 1219-1224

1219

Hydrocracking and Hydroisomerization of High-Density Polyethylene and Waste Plastic over Zeolite and Silica-Alumina-Supported Ni and Ni-Mo Sulfides Weibing Ding, Jing Liang, and Larry L. Anderson* Department of Chemical and Fuels Engineering, University of Utah, Salt Lake City, Utah 84112 Received March 26, 1997X

Conversion of plastic waste into transportation fuels over bifunctional catalysts was systematically studied. Previous work showed that some acid catalysts were active for degradation of pure polyolefins, but they were easily deactivated by nitrogen, sulfur, and impurities contained in actual postconsumer plastic waste. Ni and NiMo sulfides loaded on a hybrid support (HSiAl), a mixture of HZSM-5 and silica-alumina, were found to be effective for converting both pure high-density polyethylene and plastic waste to gasoline-range products. Hydrocracking reactions were carried out mostly at 375 °C, 1000 psig H2 (initial), for a reaction time of 1 h, though the effects of initial hydrogen pressure and reaction time were also examined. Ni/HSiAl had higher hydrocracking and hydroisomerization ability than did NiMo/HSiAl. The quality of liquid products obtained over Ni/HSiAl was comparable to that of a commercial premium gasoline. Moreover, being resistant to poisoning by N- and S-containing compounds, these catalysts could be regenerated simply by recalcination and resulfiding.

Introduction The United States is heavily dependent on liquid fuels, such as gasoline, diesel, and jet fuel. The present demand for these fuels far exceeds domestic petroleum production capacity, and over one-half of them is imported. Meanwhile, the rapidly increasing use of plastic products results in a severe waste plastic disposal problem.1-3 Converting waste plastic into liquid fuels would not only supplement U.S. energy supplies but could also mitigate environmental disposal problems.4,5 The hydrocracking process, which basically converts high-boiling molecules into more desirable lower molecular weight products with low olefin and high iso/ normal paraffin yields by simultaneous or sequential hydrogenation and carbon bond breaking, is not only an important process used in modern oil refineries but also one of the most promising processes for conversion of waste plastic. Dual functional catalysts, having both cracking and hydrogenation-dehydrogenation functions, are used for this process. The cracking function is realized by an acidic support with a high surface area, while the hydrogenation component is usually a metal, oxide, or sulfide of group VIII and/or group VIb. The acidic supports used in today’s modern petroleum industry are mostly silica-alumina and various zeolites.6 * To whom correspondence should be addressed. Phone/fax: (801) 581-5162. E-mail: [email protected]. X Abstract published in Advance ACS Abstracts, September 15, 1997. (1) D’Amico, E.; Roberts, M. Chem. Week 1995, October 4, 32. (2) Fouhy, K.; Kim, I.; Moore, S.; Culp, E. Chem. Eng. 1993, December, 30. (3) Layman, P. Chem. Eng. News 1994, March 28, 19. (4) Ding, W. Ph. D. Dissertation, University of Utah, March, 1997. (5) Taghiei, M. M.; Feng, Z.; Huggins, F. E.; Huffman, G. P. Energy Fuels 1994, 8, 1228.

S0887-0624(97)00051-0 CCC: $14.00

A polymerization catalyst (TiCl3) and metal-promoted sulfated zirconia were successfully used for hydrocracking of pure polyolefins; however, these catalysts had negative effects on the degradation of commingled postconsumer plastic. Possibly they were poisoned by heteroatoms (N and S) and impurities contained in waste plastic.7-9 Transition metal sulfides are good candidates for the hydrocracking of waste plastic, since the hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) functions of these materials have been well recognized for several decades.10 Commercial catalysts usually adopt Ni, Mo, W, Co, or their combinations as the active metal sulfide component, whereas silica-alumina and zeolites, such as erionite, mordenite, Y, and ZSM-5,6,11-18 were used as supports. The activity of these catalysts was tested by measuring the hydrogenation of benzene, the hydrocracking and hydroisomerization of n-alkanes, HDN, and HDS activity.19-25 It was concluded that a flexible (6) Bhatia, S. Zeolite Catalytisis: Principles and Applications; CRC Press Inc.: Boca Raton, FL, 1990. (7) Ding, W.; Liang, J.; Anderson, L. L. Fuel Process. Technol., in press. (8) Venkatesh, K. R.; Hu, J.; Dogan, C.; Tierney, J. W.; Wender, I. Energy Fuels 1995, 9, 888. (9) Venkatesh, K. R.; Hu, J.; Wang, W.; Holder, G. D.; Tierney, J. W.; Wender, I. Energy Fuels 1996, 10, 1163-1170. (10) Katzer, J. R.; Sivasurbamanian, R. Catal. Rev. Sci. Eng. 1979, 20, 155. (11) Bolton, A. P.; Bujalski, R. L. J. Catal. 1971, 23, 331. (12) Chen, N. Y.; Gorring, R. L.; Ireland, H. R.; Stein, T. R. Oil Gas J. 1977, 75, 165. (13) Dohler, W. Proc. Int. Congr. Catal., 8th 1984, 3, 499. (14) Dufresne, P.; Marcilly, C. U.S. Patent 4738940, 1988. (15) Haynes, H. W., Jr.; Parcher, J. F.; Helmer, N. E. I&EC Proc. Des. Dev. 1983, 22, 401. (16) Ward, J. W. U.S. Patent 3926780, 1975. (17) Weitkamp, J.; Jacobs, P. A.; Martens, J. A. Appl. Catal. 1983, 8, 123. (18) Yan, T. Y. Ind. Eng. Chem. Res. 1990, 29 1995. (19) Leglise, J.; el Qotbi, M.; Goupil, J. M.; Cornet, D. Catal. Lett. 1991, 10, 103.

© 1997 American Chemical Society

1220 Energy & Fuels, Vol. 11, No. 6, 1997

selectivity combined with high activity of a bifunctional catalyst depended on the relative strengths of the metal sulfide and acidic functions. We have found that HZSM-5 was effective for the hydrocracking of high-density polyethylene (HDPE) and plastic waste,7 although the quality of the liquids produced was far below that of commercial gasoline possibly because of a lack of sufficient hydrogenation function. To successfully convert waste plastic into transportation fuels, a catalyst combining hydrocracking, hydrogenation, HDN, and HDS abilities is needed. In this study, a new hybrid support, which is a mixture of HZSM-5 and silica-alumina, was used as an acidic component, and NiMo or Ni sulfide was used for hydrogenation. In this way we attempted to achieve a better balance between the hydrogenation and acidic functions and, consequently, produce desired, clean, gasoline-like products. A commercial hydrocracking catalyst, KC-2600, obtained from Akzo Nobel Inc., was also used for this purpose, and the results were compared. Experimental Section High-density polyethylene (HDPE, average MW ca. 125 000; d ) 0.950 g/cm3; in bead form) was purchased from Aldrich Chemical Co. Commingled postconsumer plastic (CP#2), obtained from the American Plastics Council, was ground to -8 mesh. The detailed analyses of CP#2 have been reported elsewhere.26 Qualitatively, CP#2 consists of mostly highdensity polyethylene with small amounts of polypropylene, polystyrene, and other polymers as well as some impurities. The elemental analysis showed that CP#2 contained 0.65 wt % nitrogen and 0.01 wt % sulfur. A commercial premium gasoline was collected from a local retail gasoline station on December 24, 1996 in Salt Lake City, Utah. KC-2600, a commercial hydrocracking catalyst, was provided by Akzo Nobel Chemical, Inc. It may contain NiMo/ zeolite and/or NiMo/Al2O3. HZSM-5 and SiO2-Al2O3 (with 13% Al2O3 content) were purchased from United Catalysts Inc. and Aldrich Chemical Co., respectively. The average pore size and surface area of the SiO2-Al2O3 were 65 Å and 475 m2/g, respectively,27 while those of the HZSM-5 were 6.2 Å and greater than 200 m2/g, respectively.28 The HZSM-5 contained 10-40 wt % of a proprietary binder. The metal salts, nickel(II) nitrate hexahydrate and ammonium molybdate tetrahydrate, were obtained from Aldrich Chemical Co. and Fluka Company, respectively. The hybrid support, namely, HSiAl, was prepared by cogrinding a mixture of 4 parts by weight of SiO2-Al2O3 with 1 part by weight of HZSM-5. The support particles were collected under a 100 mesh Tyler standard sieve. Supported Ni catalysts were prepared by impregnation of Ni(NO3)2‚6H2O onto the support followed by drying in a vacuum oven at 110 °C for 12 h. The dried samples were then calcined in a muffle oven at 500 °C in air for 16 h. The resulting catalysts were stored in vials before sulfidation or used for further prepara(20) Vazquez, M. I.; Escardino, A.; Corma, A. Ind. Eng. Chem. Res. 1987, 26, 1495. (21) Welters, W. J. J.; Vorbeck, G.; Zandbergen, H. W.; de Haan, J. W.; de Beer, V. H. J.; van Santen, R. A. J. Catal. 1994, 150, 155. (22) Song, C. S.; Reddy, K. M. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1996, 41, 567. (23) van de Ven, L. J. M.; van Oers, E. M.; de Haan, J. W.; de Beer, V. H. J.; van Santen, R. A. J. Catal. 1996, 161, 819. (24) Cid, R.; Fierro, J. L. G.; Agudo, A. L. Zeolites 1990, 10, 95. (25) Kovacheva, P.; Davidova, N.; Novakova, J. Zeolites 1990, 11, 54. (26) Ding, W.; Liang, J.; Anderson, L. L. Fuel Process. Technol. 1996, 49, 49. (27) Aldrich Chemical Co. (28) United Catalysts Inc.

Ding et al. tion of NiMo catalysts. The procedure for impregnating Mo metal on the support was the same as the procedure stated above except that (NH4)6Mo7O24‚4H2O was used as metal precursor. The Ni metal loading was 2.6%, while Mo loading was 7.0%, if any. The calcined oxide catalysts were presulfided in a tubular flow reactor. The temperature profile was programmed as follows. The catalyst was heated from room temperature to 400 °C in about 20 min with helium flow (100 mL/min) and remained at that temperature for 1 h. Then gas flow was switched from helium to a mixture of hydrogen and hydrogen sulfide (H2:H2S ) 9:1, by volume; 50 mL/min) and maintained for 2 h followed by stabilization of the catalyst with helium flow at 400 °C for 1 h. After the catalyst was cooled to room temperature with helium flow it was transferred to a tubing reactor for degradation of plastics. Hydrocracking reactions of HDPE and CP#2 were carried out in a 27 mL tubing reactor at 375 °C for 0-60 min. Typically, 2 g of plastic and a calculated amount of presulfided catalyst were fed into the reactor, which was then closed, purged with nitrogen, and then pressurized with hydrogen to the desired initial pressure. The reactor was then immersed in a preheated fluidized sand bath and reached the desired reaction temperature within 3-4 min. The mixing of reactants and catalyst particles was achieved by horizontal shaking of the reactor at 160 rpm. Detailed reaction procedures and definitions of yields have been reported elsewhere.7 The gaseous products were collected and analyzed by GCFID (HP-5890II) using a column packed with HayeSep Q. The analyses of liquid products were carried out on a HewlettPackard 5890II gas chromatograph coupled to a HewlettPackard 5971A mass spectrometer (GC/MS). A 30 m DB-5 capillary column was used for the GC-MS analyses. The gaseous products consisted of mostly C1-C4 hydrocarbons with some C5 and small amounts of C6 compounds. The liquid products consisted of mainly C5-C12 compounds with small amounts of C13+ and C3-C4 compounds. The boiling point distribution of a commercial gasoline and liquid products obtained from hydrocracking of CP#2 was determined by simulated distillation according to ASTM D 2887-89. The analysis was performed on an HP-5890II gas chromatograph (FID), using a Petrocol B column (length of 6 ft and outside diameter of 1/8 in.). The amounts of carbon, nitrogen, and hydrogen in the liquid products obtained from hydrocracking of CP#2 were determined using a LECO CHN-600 elementary analyzer. Sulfur was determined with a LECO SC-132 sulfur analyzer. The detection limits for carbon, hydrogen, nitrogen, and sulfur were 0.01 wt %.

Results and Discussion Hydrocracking and Hydroisomerization of HDPE. The effects of catalysts (KC-2600, NiMo/HSiAl, and Ni/HSiAl) and the effects of different amounts of catalyst (weight ratio of catalyst-to-feed, 0.20 or 0.40) on yields and conversion of HDPE are shown in Figure 1. At 375 °C, virtually no thermal hydrocracking occurred. However, in the presence of 20% KC-2600, NiMo/HSiAl, or Ni/HSiAl, the conversion of HDPE reached 64.5%, 65.3%, and 81.7%, respectively. The liquid yields were higher than the corresponding gas yields. When the catalyst amount was increased to 40%, more than 99% conversion was obtained over either NiMo/HSiAl or Ni/HSiAl, and above 90% over KC-2600. Using more catalyst that the 40% value resulted in more gases being produced. When 40% hybrid support (HSiAl containing 20 wt % HZSM-5 and 80 wt % SiO2-Al2O3) was used as a catalyst for hydrocracking of HDPE under the same

Hydrocracking and Hydroisomerization

Energy & Fuels, Vol. 11, No. 6, 1997 1221

Figure 1. Results of degradation of HDPE in a 27 mL tubing reactor at 375 °C, 1000 psig H2 (initial), for a reaction time of 60 min with the indicated catalysts. Table 1. Composition of Products Obtained from Hydrocracking of HDPEa catalyst C1 C2 C3 iso-C4 n-C4 C5 C6 C7 C8 C9 C10 C11 C12 C13+

KC-2600

NiMo/HSiAl

Carbon Number Distribution, wt % 0.25 0.49 0.86 1.46 19.68 23.44 19.02 10.76 9.40 8.89 8.75 16.18 25.07 20.74 3.72 4.20 2.07 5.37 0.71 2.78 0.31 2.01 0.12 1.35 0.04 1.02 trace 0.60

Composition of Liquid Products, wt % n-paraffins 25.37 36.54 isoparaffins 68.96 52.42 cyclo-paraffins 2.87 8.09 olefins 1.38 0.07 aromatics 1.41 2.89

Ni/HSiAl 0.54 2.44 20.96 22.90 7.33 10.94 21.66 3.05 4.99 3.18 1.42 0.18 0.06 trace 16.23 67.66 9.24 0.27 6.60

a Reaction conditions: 27 mL tubing reactor, 375 °C, 1000 psig H2 (initial), reaction time of 60 min, catalyst:feed ) 0.4:1.0 by weight.

reaction conditions stated in Figure 1, 66% conversion was achieved. The metal function that is present in the metal-supported catalyst allows facile formation of olefins, which can then form the carbenium ions, which in turn undergo the chemistry that is observed. The hybrid support, on the other hand, does not contain the metal function, and therefore, the formation of carbenium ions is more difficult in this case. The rates would be much slower in the latter case, and hence, the conversion would be lower. For 20% catalyst loading, the addition of Mo onto Ni/ HSiAl decreased the overall rate of the hydrocracking reaction of HDPE. This nonsynergistic effect may be due to a poor distribution of metal sulfides on the support. Only part of single metal sulfide phases, like nickel sulfide, would be formed in the HZSM-5 pores, whereas the MoS2-supported nickel sulfide phase (NiMoS) is unlikely to be formed in the small HZSM-5 pores. Even if some NiMoS is formed on the external surface of the zeolite particles, its contribution to hydrocracking is still low because metal sulfide located on the external surface cannot prevent the deposition of coke in the channels of HZSM-5 and/or it may block some pore openings of HZSM-5. Therefore, more coke

and less conversion were obtained over NiMo/HSiAl. Similar results on zeolite Y-supported Ni and NiMo sulfides were observed by Welters et al.29 The products obtained from hydrocracking and hydroisomerization of HDPE over bifunctional catalysts are shown in Table 1. Gaseous products consisted of small amounts of C1 and C2 but large amounts of C3 and C4 with high iso/normal ratio of C4. Liquid products contained mostly normal and iso paraffins, some aromatics and cyclo-paraffins, with little olefins. The ratios of iso/normal paraffins were 1.43, 2.72, and 4.17 for liquids obtained from hydrocracking of HDPE over NiMo/HSiAl, KC-2600, and Ni/HSiAl, respectively. Liquid products obtained over NiMo/HSiAl contained more n-paraffins, less aromatics, and less olefins than those obtained over Ni/HSiAl. This indicated that the hydrogenation activity of NiMo/HSiAl was higher than that of Ni/HSiAl, implying that the NiMoS phase was more active in hydrogenation than nickel sulfide. Similar observations were also reported by Leglise et al.30 for stabilized HY zeolites as supports for Ni and NiMo catalysts. Hydroisomerization contributed to the high ratios of iso/normal paraffins. HZSM-5 and sulfided Ni/SiO2Al2O3 were reported to have hydroisomerization functions.7,31,32 Ni/HSiAl showed higher hydroisomerization ability than did NiMo/HSiAl and KC-2600, probably owing to its well-balanced hydrogenation ability and acidity. High yield of C3 hydrocarbons (ca. 20%) means cracking reactions may occur according to a pentacoordinated carbonium ion mechanism.32,33 Disproportionation reactions also occurred during hydrocracking by a mechanism involving an alkylation process in which an olefinic intermediate adds on to an adsorbed carbonium ion and then cracks to yield a fragment of higher carbon number and one of lower carbon number than the reactants.34 Excess butane may be attributed to disproportionation reactions. Production of excess butane in n-decane hydrocracking over sulfided Ni/SiO2Al2O3 was also reported by Langlois et al.31 The concentration of aromatics and benzene in gasoline have been limited by environmental regulations to less than 25% and 2% respectively, indicating a trend to reduce aromatics in reformulated gasoline in the near future.35 Compared with a commercial premium gasoline (15.44% n-paraffins, 59.09% iso-paraffins, 1.69% cyclo-paraffins, 0.94% olefins, and 20.24% aromatics), liquids obtained from hydrocracking of HDPE over KC2600, NiMo/HSiAl, and Ni/HSiAl contained much less aromatics, 1.41-6.6% (Table 1). A more environmentally acceptable gasoline-like product was obtained over Ni/HSiAl, which consisted of less aromatics (6.6%) and (29) Welter, W. J. J.; van der waerden, O. H.; de Beer, V. H. J.; van Santen, R. A. Ind. Eng. Chem. Res. 1995, 34, 1166. (30) Leglise, J.; Janin, A.; Lavalley, J. C.; Cornet, D. J. Catal. 1988, 114, 338. (31) Langlois, G. E.; Sullivan, R. F.; Egan, C. J. J. Phys. Chem. 1966, 70, 3666. (32) Abbot, J.; Wojciechowski, W. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 501. (33) Anders, G.; Burkhardt, I.; Illgen, U.; Schulz, I. W.; Scheve, J. Appl. Catal. 1990, 62, 281. (34) Holmstrom, A.; Sorrik, E. M. J. Chromatogr. 1970, 53, 95; J. Appl. Polym. Sci. 1974, 18, 761; J. Polym. Sci., Polym. Symp. 1976, 57, 33. (35) Bell, A. T.; Manzer, L. E.; Chen, N. Y.; Weekman, V. W.; Hegedus, L. L.; Pereira, C. J. Chem. Eng. Prog. 1995, February, 26.

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Figure 2. GC-MS analyses of liquid products obtained from hydrocracking of HDPE in a 27 mL tubing reactor at 375 °C, 1000 psig H2 (initial), for a reaction time of 1 h with 40% Ni/HSiAl.

Figure 3. Results of degradation of CP#2 in a 27 mL tubing reactor at 375 °C, 1000 psig H2 (initial), for a reaction time of 60 min with the indicated catalysts.

more isoparaffins (67.66%) than did the commercial gasoline. The GC-MS profile of liquid products obtained from the hydrocracking of HDPE over Ni/HSiAl is shown in Figure 2. Among the small amounts of aromatics produced, no benzene was detected. Hydrocracking and Hydroisomerization of CP#2. No catalyst was reported to be active for degradation of CP#2 (an actual plastic waste sample obtained from the American plastic Council) at temperatures below 400 °C, since CP#2 contained small amounts of nitrogen and sulfur as well as impurities that may poison some solid acids.7 Bifunctional catalysts, KC-2600, NiMo/ HSiAl, and Ni/HSiAl, were very active for converting CP#2 (Figure 3). At 375 °C, thermal conversion was minimal, while catalytic effects were significant. More than 99% conversion was obtained over 40% Ni/HSiAl and 40% NiMo/HSiAl. Gas yields increased and liquid yields decreased with increased catalyst loading. An interesting result was that liquid products obtained over each catalyst were clean and white or light yellow with a gasoline-like smell, while the liquids produced from thermal reactions (under H2) or catalytic reactions over HZSM-5 and TiCl3 were brown red with a strong

unpleasant smell.36 It is noteworthy that ca. 99% conversion was also reached with addition of only 20% Ni/HSiAl, whereas only about 84% conversion was obtained over 20% NiMo/HSiAl. The reason Ni/HSiAl is more active in hydrocracking than NiMo/HSiAl was discussed earlier in this paper. CP#2 should be easier to convert than HDPE, since it is a mixture of polyolefins with mostly HDPE, which was found to be harder to convert among the common polyolefins.7,37 This is true for thermal reactions.7 However, the conversion of CP#2 was always lower than that of HDPE over HZSM-5 or TiCl3 under the same reaction conditions,7 which indicates that nitrogen- and sulfur-containing compounds may have poisonous effects. Nevertheless, the conversion of CP#2 reached 99% whereas the conversion of HDPE was only 82% when 20% sulfided Ni/HSiAl was used. This means that this bifunctional catalyst is more resistant to nitrogenand sulfur-containing compounds and is suitable for hydrocracking of plastic waste. Figure 4 shows composition of liquids produced from the hydrocracking of CP#2 over all three catalysts. Isoparaffins are the major compounds with the ratio of iso/normal paraffins decreasing in the order Ni/HSiAl > KC-2600 > NiMo/HSiAl. Less aromatics and more n-paraffins were obtained over NiMo/HSiAl than over Ni/HSiAl. All these trends were the same as those observed for hydrocracking of HDPE. Figure 5 shows the results of simulated distillation of a commercial premium gasoline and liquid products obtained from hydrocracking of CP#2. The volatility of plastic waste-derived liquids was close to that of the commercial premium gasoline. Liquids produced over KC-2600 were lighter than those obtained over Ni/HSiAl and NiMo/HSiAl. All liquids contained about 90% lighter components (bp less than 216 °C). (36) Ding, W.; Liang, J.; Anderson, L. L. Prepr.sAm. Chem. Soc., Div. Pet. Chem., in press. (37) Anderson, L. L.; Tuntawiroon, W. Prepr.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38, 816.

Hydrocracking and Hydroisomerization

Figure 4. Composition of liquid products obtained from hydrocracking of CP#2 in a 27 mL tubing reactor at 375 °C, 1000 psig H2 (initial), catalyst:feed ) 0.4:1.0 (by weight), at a reaction time of 60 min.

Energy & Fuels, Vol. 11, No. 6, 1997 1223

Figure 7. Effect of initial hydrogen pressure on hydrocracking of CP#2 over Ni/HSiAl (reaction condtions: 27 mL tubing reactor, 375 °C, 60 min, catalyst:feed ) 0.2:1.0 by weight). Table 2. Effects of Initial Hydrogen Pressures on Composition of Liquid Products Obtained from Hydrocracking of CP#2a,b 250 psig 500 psig 750 psig 1000 psig CPGc n-paraffins isoparaffins cyclo-paraffins olefins aromatics

5.03 42.83 14.75 5.00 32.39

4.87 42.49 17.11 4.05 31.48

5.08 48.89 18.57 1.92 25.54

7.49 47.15 20.57 1.70 23.09

15.44 59.09 1.69 0.94 20.24

a Reaction conditions are listed in Figure 7. b Numbers in the table are in wt %. c Commercial premium gasoline.

Figure 5. Boiling point distribution of a commercial premium gasoline and liquid products obtained from hydrocracking of CP#2 at 375 °C, 1000 psig H2 (initial), 60 min, with weight ratio of catalyst-to-feed 0.4:1.0.

Figure 6. Effect of reaction time on hydrocracking of CP#2 over Ni/HSiAl (reaction conditions: 27 mL tubing reactor, 375 °C, 1000 psig H2 (initial), catalyst:feed ) 0.2:1.0 by weight).

In terms of conversion and oil yield, 20% Ni/HSiAl was the best catalyst we have found so far. Reaction time had important effects on conversion and yields from hydrocracking of CP#2 (Figure 6). After 30 min, conversion and yields increased linearly with increasing reaction time, but the increase was not major compared with reaction times less than 30 min. The effects of initial hydrogen pressures are shown in Figure 7. In the range 250-750 psig, yields and conversion increased only slightly with increasing pressure. Approximately 99% conversion was obtained at 750 psig, whereas 85% conversion was obtained at 250 psig. When the pressure was greater than 750 psig, yields and conversion were not a function of pressure. The reason the reactions had to be carried out at higher pressures of hydrogen (g750 psig) was that the hydrogenation function associated with the Ni (or NiMo) sulfide was considerably weaker than that for other

catalysts such as Pt/HY hydroisomerization catalysts,38,39 although the latter may be poisoned by nitrogen and/or sulfur. Table 2 shows the effects of initial hydrogen pressure on the composition of liquid products obtained from hydrocracking of CP#2. Aromatics and olefins decreased and cyclo-paraffins increased with increasing initial hydrogen pressure. Generally, more n-paraffins and isoparaffins were obtained at higher pressures. This indicates that hydrogenation of olefins and aromatics may be favored at higher hydrogen pressures; therefore, correspondingly more n-paraffins and cyclo-paraffins were produced. Compared with the composition of a commercial premium gasoline, the liquid products obtained at 1000 psig H2 contained less n-paraffins, much more cyclo-paraffins, less isoparaffins, and slightly more aromatics. CP#2 contained 0.65% nitrogen and 0.01% sulfur.26 The liquids obtained from the hydrocracking of CP#2 at 1000 psig H2 (initial) over 20% Ni/HSiAl was subjected to elemental analyses; no nitrogen and sulfur was detected. This reflected that the sulfided Ni/HSiAl did have HDN function. Detailed compositional data on the products obtained from the hydrocracking of CP#2 over 20% Ni/HSiAl under 1000 psig H2 (initial) are listed in Table 3. The iso/normal paraffins ratios of C4, C5, C6, C7, C8, C9, and C11 are 4.61, 1.75, 10.04, 5.98, 12.85, 16.39, and 8.38, respectively. This means that Ni/HSiAl exhibits high hydroisomerization ability during hydrocracking of CP#2. On the other hand, hydrocracking is profound, since the amount of heavier hydrocarbons (gC13) is very low (ca. 0.14%), whereas C13+ hydrocarbons remained (38) Martin, A. A.; Chen, J. K.; John, V. T.; Dadybarjor, D. B. Ind. Eng. Chem. Res. 1989, 28, 1613. (39) Martens, J. A.; Jacobs, P. A.; Weitkamp, J. Appl. Catal. 1986, 20, 283.

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Table 3. Carbon Number Distribution of Products Obtained from Hydrocracking of CP#2 over Ni/HSiAla

Table 4. Effects of Used Catalysta on Hydrocracking of HDPE and CP#2b

n-paraffins isoparaffins cyclo-paraffins olefins aromatics C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13+

0.54 2.87 8.09 3.42 2.82 1.34 0.60 0.47 0.18 0.08 trace

0.37 15.75 4.94 13.46 3.59 6.04 2.95 1.48 0.67 trace

0.44 0.21 4.72 4.02 2.84 0.82 0.28 trace

0.44 0.35 0.14 0.17 trace

1.55 2.07 3.66 3.53 2.44 1.72 0.14

a

Reaction conditions: 375 °C, 60 min, 1000 psig H2 (initial), catalyst:feed ) 0.2:1.0 by weight.

catalyst

gas yield, wt %

oil yield, wt %

conversion, wt %

fresh KC-2600, 40% used KC-2600, 40% fresh NiMo/HSiAl, 40% used NiMo/HSiAl, 40% fresh Ni/HSiAl, 40% used Ni/HSAl, 40%

HDPE Feed 57.2 56.2 54.1 51.4 57.6 56.1

32.8 34.6 45.2 43.8 42.0 43.4

90.0 90.8 99.3 95.2 99.6 99.5

fresh KC-2600, 40% used KC-2600, 40% fresh NiMo/HSiAl, 40% used NiMo/HSiAl, 40% fresh Ni/HSiAl, 20% used Ni/HSiAl, 20% fresh Ni/HSiAl, 40% used Ni/HSiAl, 40%

CP#2 Feed 48.1 44.0 52.0 47.9 34.4 35.6 51.8 51.3

42.0 44.9 47.1 46.5 64.8 64.0 47.7 48.4

90.1 88.9 99.1 94.4 99.2 99.6 99.5 99.7

a Used catalysts: recalcinated and resulfided catalysts. b Reaction conditions: 375 °C, 1000 psig H2 (initial), for a reaction time of 60 min.

Conclusions

Figure 8. Results of degradation of CP#2 in a 27 mL tubing reactor at 350 °C, 1000 psig H2 (initial), for a reaction time of 60 min with the indicated catalysts.

at 11.7% for the products obtained from the hydrocracking of CP#2 at 435 °C, 1000 psig H2 (initial), over 2% HZSM-5.7 Reaction temperature had a dramatic effect on conversion of CP#2. Figure 8 shows the results obtained from hydrocracking of CP#2 at 350 °C. Compared with the uncatalyzed reaction, addition of a catalyst improved conversion significantly. However, catalytic conversion at 350 °C was much lower than the corresponding results obtained at 375 °C (Figure 3). Effects of Used Catalysts. To test the effects of used catalysts (after reactions) on hydrocracking of HDPE and CP#2, the used catalysts were recalcinated and resulfided. The results of reactions over used catalysts are listed in Table 4. It is clear that conversion and yields obtained over used catalysts are nearly the same as those obtained over fresh catalysts. This suggests that Ni/HSiAl, NiMo/HSiAl, and KC-2600 can be regenerated simply by recalcination and resulfiding, showing possible feasibility for commercial purposes.

Bifunctional catalysts, sulfided Ni/HSiAl, NiMo/ HSiAl, and KC-2600, were very active for hydrocracking of HDPE and especially plastic waste at 375 °C. The three catalysts were resistant to nitrogen, sulfur, and impurities contained in plastic waste, and they may be regenerated simply by recalcination and resulfiding, indicating potential commercial use for hydrocracking of plastic waste to produce transportation fuels in a single-stage process. The hydrocracking ability of these catalysts was profound, since products obtained were mostly lighter hydrocarbons (eC13). The hydroisomerization ability of these catalysts was also significant and decreased in the order Ni/HSiAl > KC-2600 > NiMo/HSiAl. The higher hydrocracking and hydroisomerization ability of Ni/ HSiAl may be due to its metal sulfide-acid balance. No synergetic effects were observed when Mo was used as a promoter for Ni/HSiAl. Probably MoS2-supported nickel sulfide (NiMoS) could not form in the small HZSM-5 channels. The liquid products obtained from hydrocracking of HDPE over Ni/HSiAl and KC-2600 had better quality than did a commercial premium gasoline, i.e., contained more isoparaffins and less aromatics. Acknowledgment. We gratefully acknowledge the funding support from the U.S. Department of Energy through the Consortium for Fossil Fuel Liquefaction Science and the University of Utah. EF970051Q