Energy Fuels 2010, 24, 4231–4240 Published on Web 07/23/2010
: DOI:10.1021/ef100227f
Catalytic Pyrolysis of Low-Density Polyethylene over Alumina-Supported Noble Metal Catalysts Nagi Insura, Jude A. Onwudili, and Paul T. Williams* Energy and Resources Research Institute The University of Leeds, Leeds, LS2 9JT, U.K. Received March 1, 2010. Revised Manuscript Received July 5, 2010
The effects of Pt/Al2O3 and Rh/Al2O3 catalysts on the pyrolysis of low-density polyethylene (LDPE) at a temperature of 425 °C and residence time of 1 h was investigated in a batch stainless steel autoclave reactor. Preliminary observations showed that neither catalyst lowered the degradation temperature of LDPE. Generally, the presence of either catalyst influenced the secondary reactions and improved oil component distribution. Compared to the results from the noncatalytic pyrolysis of LDPE under similar conditions, the catalysts appeared to have caused a slight decrease in the proportion of oil product in favor of coke formation, which increased with increasing catalyst loading. The presence of both catalysts altered the composition of gas products, for instance, both catalysts increased the saturated aliphatic gases to up to 90 wt % of total gas product while alkene gases reduced. It appeared that the catalytic activity of the alumina support toward dehydrogenation and aromatization was predominant. The condensation of the aromatic compounds on the catalyst surface led to coke formation. Deactivation of the catalyst occurred after first use due to carbon deposition. Reactivation of the aged catalyst restored most of their physical properties with differences in their catalytic activities compared to fresh catalysts.
Many research groups6-9 have extensively studied pyrolysis of plastic waste. A large number of studies have employed catalysts to improve the fuel quality of the oil product to give compositions similar to that of commercial fuel types.4,5,7 Incorporation of catalytic cracking usually produces higher yields of light hydrocarbon compounds, such as aromatics and cycloalkanes.7,8 Several studies have investigated the use of zeolite catalysts, such as HZSM-5, MCM, NH4Y, NaY, and FCC.5-11 Zeolite catalysts have a suitable geometry and high diffusion of reactants in the surface structure that make them selective toward formation of alkenes, cycloalkanes, and aromatics.5,10 The acidity of these catalysts enables them to favor hydrogen transfer due to the appropriate molecular size pore, pore distribution, and the presence of acidic active sites. The activity of the catalyst depends on the feasibility of the molecules to access and be adsorbed at the reactive sites, which is controlled by the pore size and the size of adsorbents and products.5 Aguado et al.4 investigated a zeolite catalyst for polyethylene cracking. It was reported that over n-HZSM5, 77 wt % of product was a gasoline fraction compared to only 56 wt % without the catalyst. Uemichi et al.11 employed a silica-alumina catalyst and reported that the proportions of aromatics and naphthalenes increased as a result of the acidic properties of the catalyst. The relative yield of C3-C5 fractions was more than 70 wt %, and this was attributed to the dominance of isomerization, aromatization, and hydrogen transfer reactions during the secondary reactions.
1. Introduction Fossil fuels are limited, nonrenewable resources, and continuous consumption will eventually lead to their decline. Waste organic materials may become an important component in the drive toward alternative energy and petrochemical resources. Thermochemical treatment of organic wastes via pyrolysis is one of the promising technologies for their conversion into useful products such as fuels and chemical feedstocks. Many studies have investigated the thermochemical treatment of wastes using pyrolysis to produce oil, gas, and char products, for example, from waste plastics, scrap tires, and municipal solid waste.1-3 These waste fractions have an organic polymeric structure that can be thermally broken down into smaller fragments. These fragments represent the precursor monomers or similar structures consisting of a wide range of hydrocarbons and their derivatives.1-4 The product chemistry depends on the chemical nature of the waste material and to some extent, the pyrolysis conditions. One of the main fractions of municipal solid waste is the plastic waste in which polyethylene is predominant.5,6 Diversion of waste plastics from the traditional waste disposal methods toward thermochemical recycling provides an alternative way for the recovery of valuable hydrocarbon contents and mitigates environmental pollution. *To whom correspondence should be addressed. Telephone: 44 1133432504. E-mail:
[email protected]. (1) Williams, P. T.; Williams, E. A. J. Inst. Energy 1998, 71, 81–93. (2) Williams, P. T.; Besler, S.; Taylor, D. T. J. Inst. Energy 1995, 68, 11–21. (3) Williams, P. T.; Besler, S. J. Inst. Energy 1992, 65, 192–200. (4) Aguado, J.; Serrano, D. P.; San Miguel, G.; Castro, M. C.; Madrid, S. J. Anal. Appl. Pyrolysis 2007, 791, 415–423. (5) Pinto, F.; Costa, P.; Gulyurtlu, I.; Cabrita, I. J. Anal. Appl. Pyrolysis 1999, 51, 57–71. (6) Serrano, D. P.; Aguado, J.; Escola, J. M.; Garagorri, E. Appl. Catal., B 2003, 44, 95–105. r 2010 American Chemical Society
(7) Bagri, R.; Williams, P. T. J. Anal. Appl. Pyrolysis 2002, 63, 29–41. (8) Williams, P. T.; Slaney, E. Res. Conserv. Recycl. 2007, 51, 754–769. (9) Kaminsky, W.; Predel, M.; Sadiki, A. Polym. Degrad. Stab. 2004, 85, 1045–1050. (10) Buekens, A. G.; Huang, H. Res. Conserv. Recycl. 1998, 23, 163– 181. (11) Uemichi, Y.; Ayame, A.; Kashiwaya, Y.; Kanoh, H. J. Chromatogr. 1983, 259, 69–77.
4231
pubs.acs.org/EF
Energy Fuels 2010, 24, 4231–4240
: DOI:10.1021/ef100227f
Insura et al. 12-14
Other groups have investigated metal catalysts. For example, Pt catalysts have been used widely in the petroleum industry as reforming catalysts where the naphtha fraction was converted to gasoline fuel. Rhodium is a valuable metal owing to its capability to catalyze hydrogenation, allylic substitution, and C-H activation in addition to other reactions.15 Uemichi et al.16 investigated a Pt catalyst as a reforming catalyst in three different forms, Pt/C, Pt/Al2O3, and 0.5% Pt/SiO2-Al2O3. The order of hydrogenation activity was Pt/C > Pt/Al2O3 >Pt/SiO2-Al2O3, which was different than the order of aromatization, in which, Pt/C > Pt/SiO2-AlO3 > Pt/Al2O3. These catalysts have a high selectivity to aromatic compounds. This activity may be attributed to the availability of both cracking and reforming reactions; in other words, it is because of the synergy of two different active sites. The same authors studied Pt/C and Fe/C in the catalytic decomposition of polypropylene.17 They reported that both catalysts resulted in an increase in aromatics yield, and that Pt metal was more active than Fe. It is well-known that the role of catalytic metallic sites is mainly to catalyze hydrogenation and dehydrogenation reactions.15 When supported or mixed with other materials such as the acidic catalysts including alumina and zeolite, they gain extra properties and behave like bifunctional catalysts. Subsequently the activities of these types of catalysts are not only manifested in hydrogenation/dehydrogenation reactions but also isomerization, cyclization, and aromatization. Ding et al.18 compared the effects of silica-alumina alone with those impregnated with Pt, Ni, Pd, or Fe catalysts in the conversion of a mixture of high-density polyethylene (HDPE) and coal at 430 °C for 1 h. The active metal sites caused hydrogenation, while silica-alumina sites led to hydrocracking. Metals combined with alumina/silica (acidic sites) showed more varied activity than metals alone.12 Ali et al.13 reported improvement in hydroconversion of n-alkane on Pt/Al2O3 catalysts in the presence of other noble metals or chlorine. In this paper, the effects of Pt and Rh catalysts, supported on alumina, have been investigated in relation to the catalytic pyrolysis of low-density polyethylene at 425 °C for 1 h using an autoclave reactor. The activities of each catalyst in the conversion as well as their selectivity toward the composition and distribution of the pyrolysis products have been investigated and compared with the noncatalytic pyrolysis of lowdensity polyethylene (LDPE) under similar conditions. Noncatalytic pyrolysis of LDPE at 425 °C produced oil composed of mainly straight chain alkanes and alkenes ranging from C5 to C40.19 The use of the catalysts in this paper was intended to alter the compositional quality of the pyrolysis oil product toward short chain components and aromatics with the aim of improving the fuel properties of the oil. In addition, the effect
of different catalyst loading as well as their deactivation and reactivation on the products distribution were studied. 2. Materials and Methods 2.1. Materials. Low-density polyethylene in the form of white pellets as well as pelletized Pt/Al2O3 and Rh/Al2O3 catalysts were purchased from Sigma-Aldrich, U.K. The supplier has characterized the properties of the catalyst listed here. The Pt/ Al2O3 catalyst has a particle size of 3.2 mm, BET surface area of 116.1 m2/g, pore size of 9.5 nm, and Pt content of 0.5 or 1%. Similarly, the Rh/Al2O3 has the following properties: particle size of 3.2 mm, BET surface area of 101.5 m2/g, pore size of 9.2 nm, and Rh content of 0.5%. The materials were used as received. 2.2. Pyrolysis Reactor. The reactor utilized in this work was a batch pressurized autoclave reactor. It was an a 300 mL Parr mini bench top reactor, type 4561m stirred pressure reactor made of T 316 stainless steel and was obtained from the Parr Instrument Co., Moline, IL. The reactor was heated using an external mantle type furnace. Between 1 and 4 g of each catalyst was used, and these amounts corresponded to noble metal loadings (Pt and Rh) of 0.005 or 0.02 g, respectively. The catalyst pellets were mixed with 10 g of the low-density polyethylene sample and loaded into the autoclave reactor. The weight of the reactor with the loaded sample including catalysts (where applicable) was taken. The pyrolysis reactor was sealed, purged with N2 (oxygen-free) gas for 10 min, and heated to 425 °C. At this temperature, each experiment was held for 1 h in each case. Since experimental work was carried out in a closed system, the evolution of hot gas reaction products increased the internal pressure of the reactor during the experiments. In an earlier study on the noncatalytic pyrolysis of LDPE,19 the products obtained at 400 °C consisted mainly of wax with small yields of hydrocarbon gases. In this work, preliminary investigations showed that in the presence of either Pt/Al2O3 or Rh/Al2O3 catalysts at 400 °C mainly wax was produced. This suggested that these catalysts did not decrease the temperature of primary conversion of low-density polyethylene in this reactor system. However, noncatalytic pyrolysis achieved complete degradation of LDPE at 425 °C, producing hydrocarbon gases and an oil, comprising a wide range of alkanes and alkenes, reaching up to C40.19 Therefore, a temperature of 425 °C was chosen for this catalytic study. At the end of the reaction, the operating pressure was recorded and the reactor rapidly cooled to room temperature. The pressure exerted by the produced gases on the cooled reactor was noted after which the gas product was sampled for analysis. After the gas was sampled, the reactor was opened and reweighed. The difference in weight of the reactor plus feed and the reactor plus its contents after discharging the gas was taken as the weight of gas produced. The weight of oil produced was obtained from the difference between the total weight of the reactor contents and the weight of solid residue after filtration. The weight of char was obtained by the difference in the weight of solid residue and the weight of fresh catalyst loaded into the reactor. Experiments were carried out in duplicate and averages reported. The experimental deviations on the data presented, in terms of products distribution, were within 3%, as shown in the results. 2.3. Gas Analysis. The gaseous product collected at the end of each experiment was analyzed by packed column gas chromatography using two separate gas chromatographs. C1 to C4 hydrocarbon gases were analyzed using a Varian 3380 gas chromatograph with a flame ionization detector, with a 80100 mesh Haysep column and nitrogen carrier gas. Permanent gases were analyzed by a second Varian 3380 GC with two separate columns using argon as the carrier gas. Hydrogen, oxygen, carbon monoxide, and nitrogen were analyzed on a 60-80 mesh molecular sieve column with argon carrier gas,
(12) Ding, W. B.; Tuntawiroon, W.; Liang, J.; Anderson, L. L. Fuel Process Technol. 1996, 49, 49–63. (13) Ali, A.-G. A.; Ali, L. I.; Aboul-Fotouha, S. M.; Aboul-Gheit, A. K. Appl. Catal., A 1998, 170, 285–296. (14) Casta~ no, P.; Pawelec, B.; Fierro, L. G.; Arandes, J .M.; Bilbao, J. Fuel 2007, 86, 2262–2274. (15) Modern Rhodium-Catalysed Organic Reactions; Evans, P. A., Ed.; Wiley-VCH: Weinheim, Germany, 2005. (16) Uemichi, Y.; Makino, Y.; Kanazuka, T. J. Anal. Appl. Pyrolysis 1989, 14, 331–44. (17) Uemichi, Y.; Makino, Y.; Kanazuka, T. J. Anal. Appl. Pyrolysis 1989, 16, 229–238. (18) Ding, W. B.; Liang, J.; Anderson, L. L. Fuel Process Technol. 1997, 51, 47–62. (19) Onwudili, J. A.; Insura, N.; Williams, P. T. J. Anal. Appl. Pyrolysis 2009, 86, 293–303.
4232
Energy Fuels 2010, 24, 4231–4240
: DOI:10.1021/ef100227f
Insura et al.
Table 1. Product Yields from the Pyrolysis of LDPE at 425 °C with 0.5% Pt/Al2O3 Catalyst product yield (wt %) 0.5% Pt/Al2O3 loading
running pressure (MPa)
oil
gas
coke/char
no catalyst 1 g (0.005 g of Pt) 2 g (0.01 g of Pt) 4 g (0.02 g of Pt) 4 g, calcined (0.02 g of Pt)
1.52 1.60 1.65 1.72 1.65
89.8 ( 0.72 88.1 ( 0.40 86.7 ( 0.44 84.5 ( 0.56 84.6 ( 0.63
10.2 ( 0.16 9.80 ( 0.25 10.0 ( 0.20 10.5 ( 0.24 9.90 ( 0.28
2.10 ( 0.02 3.30 ( 0.08 5.00 ( 0.03 5.45 ( 0.13
while a Haysep 80-100 mesh column was used to analyze for carbon dioxide. However, no oxygen, carbon monoxide, or carbon dioxide was present in the pyrolysis gas products of LDPE. The mass of gas produced was determined from the concentrations of individual gases and the gas pressure in the reactor and calculated using the ideal gas law, since the final pressure after reactor cooling was in the range of 1-5 bar. The errors in gas analysis are shown in the respective table of results. 2.4. Oil Analysis. The oil products derived from the degradation of the plastics were analyzed by capillary column gas chromatography with flame ionization detection (GC/FID) for quantification and also gas chromatography/mass spectrometry (GC/MS) to aid in compound identification. The GC/ FID system used was a Carlo Erba HRGC 5300 Mega Series GC equipped with an HP-5 (5% phenyl methyl silicone) capillary column. The carrier gas was helium. The temperature program that was 40 °C was held for 5 min, heated at 5 °C min-1 heating rate to 100 °C, increased to 280 °C at 4 °C min-1, and held for another 30 min at 280 °C. The analytical procedures have been detailed in a previous paper by the authors.19
Table 2. Influence of 0.5% Pt/Al2O3 Catalysts on Distribution of Gas Product fresh catalyst gas components (wt % gas)
calcined catalyst
no catalysts
1g
2g
4g
4g
methane ethane propane butane total alkanes
9.80 21.6 24.5 13.7 69.6
10.2 23.0 28.0 14.5 75.7
11.7 26.3 32.1 13. 6 83.7
14.7 26.2 32.6 16.4 89.8
13.0 28.3 33.7 14.1 89.1
ethene propene butene and butadiene total alkenes
4.90 18.6 8.43 32.0
2.70 14.0 6.60 23.3
1.95 8.76 4.67 15.4
0.64 4.98 3.16 8.78
0.33 5.43 3.26 9.02
hydrogen total gases
0.45 102
0.88 99.9
0.91 100
1.45 100
1.30 99.7
observed as coke. Furthermore, Pt-based catalysts supported on alumina are notably useful for dehydrogenation.16 This result suggests that the alumina in the catalyst in addition to the platinum metal could have promoted the formation of coke. Alumina, having acidic active sites, is known to promote precursory aromatization-type reactions such as cyclization, isomerization, and polymerization depending on the size of metal crystallites in the catalyst support.20 The formation and deposition of carbon on the Pt/Al2O3 catalyst surface has been reported by other studies during hydrocarbon cracking.16,20,21 There was a gradual decrease in oil yield as the catalyst loading increased, corresponding to enhanced coke yield. However, the gas yield appeared not to be greatly influenced by catalyst loading. 3.1.2. Gas Product Distribution. Table 2 shows the yield of gases from the catalytic pyrolysis of low-density polyethylene in relation to Pt/Al2O3 catalyst loading. The hydrogen yield increased in the presence of the catalyst and further increased as the catalyst loading was increased. In the absence of catalyst, the hydrogen yield was only 0.45 wt % of the total gas product weight but reached 1.45 wt % yield in the presence of 4 g of Pt catalyst. The increase of hydrogen yield was an indication of the possibility of dehydrogenation reactions, hence it can be inferred that the rate of dehydrogenation leading to the formation of coke on the catalyst surface increased with catalyst loading, as was the amount of alumina present. Barbier et al.20 has shown that metal surface poisoning in Pt/Al2O3 began from the first minute with saturation occurring after only 15 min during the pyrolysis of cyclopentane, resulting from stronger stabilization of cyclopentadiene on the catalyst support.
3. Results and Discussion 3.1. Influence of Pt/Al2O3 Catalyst. 3.1.1. Yield of Pyrolysis Products. Table 1 shows the yield of liquid, gas, and solid products derived from the catalytic pyrolysis of lowdensity polyethylene in the presence of Pt/Al2O3 catalyst. From Table 1, it is clear that coke formation on the catalyst and reactor walls was a major effect of the catalyst on the pyrolysis products from low-density polyethylene. For example, during the noncatalytic pyrolysis of the plastic, no char formation was observed at 425 °C; however, this was different in the presence of the Pt/Al2O3 catalyst. Even at low catalyst loadings, coking was also observed on the internal walls of the reactor. Generally, the coke formation in the presence of the catalyst increased to a maximum of 5 wt % with increasing catalyst loading of up to 4 g. Where 4 g of calcined Pt/Al2O3 was used, char product increased to 5.45 wt %. The formation of coke appeared to correspond to the decrease of unsaturated aliphatic gases in the gas product, when compared to noncatalytic work. Since hydrogen gas production was, in the first instance, a result of dehydrogenation, an increase in hydrogen gas as observed in this work would have corresponded to an increase in the yield of alkene gases. This was not the case as results showed that the yield of alkene gases decreased even as hydrogen gas increased (Table 2). Therefore, a lower yield of alkene gases in the presence of Pt/Al2O3 compared to noncatalytic work could be linked to their removal in the gas-phase due to their reaction on the surface of the catalyst to form coke. In addition, the formation of alkene gases may have been suppressed by the interaction between the catalyst and the precursors of alkene gases. Hence, the formation of coke may be attributed to the condensation of dehydrogenated (multiunsaturated) compounds, including alkene gases, on the catalyst surface to form high-molecular weight molecules
(20) Barbier, J.; Corro, G.; Zhang, Y.; Bournonville, J. P.; Franck, J. P. Appl. Catal. 1985, 13, 245–255. (21) Bocanegra, S. A.; Castro, A. A.; Guerrero-Ruiz, A.; Scelza, O. A.; de Miguel, S. R. Chem. Eng. J. 2006, 118, 161–166.
4233
Energy Fuels 2010, 24, 4231–4240
: DOI:10.1021/ef100227f
Insura et al.
Figure 1. Effect of the Pt/Al2O3 catalyst on oil product distribution.
Table 1 shows that the total gas yield was relatively stable with increasing Pt/Al2O3 catalyst loading. The increase in the proportion of alkane gases may suggest that the alkene gases were not simply being converted via hydrogenation to alkanes but were probably being removed due to their interaction with the relevant active sites of the catalyst, subsequently leading to coke formation. Besides, carbon deposition on the catalyst hampered the accessibility of the Pt metal surface for hydrogenation catalysis. It was also possible that the adsorption and polymerization of intermediate species to form coke on the catalyst surface suppressed the reactions that would have led to the formation of alkene gases. At the same time, the cracking of larger molecules, including alkenes, apparently produced more alkane gases thereby keeping the total gas weight nearly constant. By studying carbon deposition of Pt/Al2O3 using cyclopentane as a coking agent (under pyrolysis), Barbier et al.22 suggested that the kinetic rate-determining step of coke deposition on the support was the dehydrogenation of cyclopentane, for which the catalytic activity depended on the metal surface. Hence, there was a proportional relationship between the amount of coke deposited on the catalyst and the partial pressure of cyclopentadiene, the dehydrogenation product, in the gas phase. Furthermore, the authors suggested that the polymerization and rearrangement of the gas-phase dehydrogenation product, cyclopentadiene on the catalyst support led to the formation of polyaromatic hydrocarbons, which condensed to form coke. This suggests the possibility of unsaturated gases forming coke on the surface of the catalyst. It has been reported that the condensation of PAHs on the catalyst surface is exhibited as coke deposits.7 Mostly, the proportion of all the alkane gas components increased in the presence of Pt catalysts. C2-C3 alkane gases were the dominant gas product, especially at higher catalyst loading. In the presence of 1 g of Pt/Al2O3, the yield of alkane gases was 75.7 wt % of the total gas product. The alkane yield increased to nearly 90 wt % of the total gas product yield as the catalyst loading was raised to 4 g. Similarly, calcined catalyst improved the yield of saturated gas. The increase in the alkane gas components may be advantageous, as it tended to raise the net calorific value of the gas product. The yield of total alkene gases was 32.0 wt % in the absence of
the Pt/Al2O3 catalyst then reduced to 23.3 wt % in the presence of 1 g of Pt/Al2O3 catalyst, further reducing to only 8.78 wt % in the presence of 4 g of catalyst. During the thermal noncatalytic pyrolysis of similar LDPE samples, ethane and propane were the predominant gases;19 however, levels of alkene gases were higher. Hence, the prevailing mechanistic function of the catalyst in this study appeared to be dehydrocylization and condensation of the alkene products to form coke.20 3.1.3. Oil Product. In Figure 1, it is clear that the selectivity of the pyrolysis process to produce short hydrocarbon chains with four carbon atoms in the oil increased in the presence of the fresh Pt/Al2O3 catalysts. However, the selectivity to produce short hydrocarbon chain compounds decreased with calcined catalyst. The total yield of cycloalkanes (cycloalkanes, identified as cyclohexane, methylcyclohexane, dimethylcyclohexane, and cyclooctane) decreased by 5% compared to experiments without the catalyst and further declined by 33% when the catalyst loading was doubled. This reduction in cycloalkane yield may be due to dehydrogenation reactions on the catalyst surface. The dehydrogenation reaction could convert these cycloalkanes to aromatic compounds. This suggestion is supported by the increasing yield of some aromatic compounds, such as toluene, benzene, and xylene in the oil product, compared to the noncatalytic process (Figure 2). However, the cycloalkane yield was up to 3.10 wt % of the oil product in the presence of calcined Pt/Al2O3 catalyst. Also, there was a slight increase in the proportion of aromatics in the oil. The distribution and yield of alkanes and alkenes in the product oil were highly influenced by the presence of the Pt/Al2O3 catalyst. In the presence of 2 g of Pt/Al2O3 catalyst, the total yield of n-alkanes improved to 51.3 wt %, which was more than 11% of that from thermal cracking. The improvement of alkanes yield continued with an increase in the load of Pt catalyst. In the presence of 4 g of catalyst, their yield was enhanced to nearly 60 wt %, an increment of 29% on the noncatalytic experiments. This increase in the proportion of alkanes corresponded to a gradual decrease in total oil product, as a result of enhanced char formation as earlier mentioned. The catalytic activity did seem to favor the removal of alkene, as shown in the final gas and liquid product, with increased catalyst loading. The decrease in the yield of alkenes corresponded to increased char formation, which rather suggested the mechanism of dehydrocyclization as reported by Uemechi et al.16 The percentage of n-alkenes
(22) Barbier, J.; Elassal, L.; Gnep, N. S.; Guisnet, M.; Molina, W.; Zhang, Y.; Bournonville, J. P.; Franck, J. P. Bull. Soc. Chim. Fr. 1984, 1, 245–249.
4234
Energy Fuels 2010, 24, 4231–4240
: DOI:10.1021/ef100227f
Insura et al.
Figure 2. Aromatic compounds found in the oil in relation to the Pt/Al2O3.
declined from 12.5 wt % during thermal pyrolysis to 8.87 wt % in the presence of 2 g of Pt/Al2O3 catalyst, which was nearly 30% less. The total concentration of alkenes in the oil product nearly halved when the amount of Pt/Al2O3 catalyst was doubled. Comparatively, the individual yield of hexene, heptene, and octene was 1.27, 1.30, and 1.35 wt % and decreased to 0.42, 0.47, and 0.49 wt %, respectively, in the presence of 4 g of the catalyst. From Figure 1, it is clear that the fresh Pt/Al2O3 catalyst produced an oil with a higher proportion of alkane compounds than the oil produced from noncatalytic work. At the same time, the proportions of alkenes and cycloalkanes decreased while aromatics remained largely stable. Although the catalytic activity of Pt metal for CdC bond hydrogenation in hydrocarbons has been reported by several studies,13,23-25 Pt activity may shift to dehydrogenation depending on the reaction conditions, metal active site dispersion, and metal active surface accessibility.20,23 The hydrogenation activity of Pt catalysts has been reported by several studies.24-30 However, many studies have used hydrogen gas as the reaction atmosphere, for example, hydrogen was used in the catalytic hydrogenation of benzene and cyclohexene in a tubular quartz reactor by Lu et al.28 on a PtNi/Al2O3 and PtCo/Al2O3 catalysts at low temperature (0-70 °C). The activity of metallic Pt sites in hydrocarbon reactions is due to their ability to break C;C, C;H, and H;H links, in addition to the high capacity for chemisorption of hydrogen.30 Ali13 reported on the activity of a 0.35% Pt/Al2O3 catalyst in the hydrogenation of benzene and toluene to cyclohexane and methyl-cyclohexane at 50170 °C, where activity increased at higher temperatures up to 250 °C. Chen et al.26 studied the hydrogenation of 1-hexene on Pt supported on a carbon molecular sieve
catalyst. They found that the highest activity of hydrogenation was reached when the catalyst was diluted by HY zeolite, and it was credited to hydrogen spillover. Again Chen et al.27 attributed the hydrogenation of hexene, cyclohexane, and naphthalene on Pt/zeolite catalyst to hydrogen spillover from the metallic surface to the zeolite texture, facilitating the saturation reaction of unsaturated species. In contrast, under inert and high temperature conditions, the mechanism of catalytic activity involving Pt tends toward dehydrogenation, especially when supported on alumina. This could be as a result of the activation of the acidic sites of alumina by high temperature. For example, around 425-500 °C, Pt/Al2O3 catalyst showed high activity in the dehydrogenation of cyclohexane.31 At 530 °C, butane was dehydrogenated to butene, isobutene, and butadiene on a Na-doped Pt/Al2O3 in a continuous flow reactor.21 It was found that carbon deposition on the metallic surface was highest when sodium doping was absent, thereby suggesting that the presence of doping sodium reduced the acidity of alumina. The above indicate that the hydrogenation activity of Pt metal was suppressed by its inaccessibility due to coking catalyzed by the alumina support. Figure 1 shows that the total aromatic yield was not significantly affected by the presence of the Pt/Al2O3 catalyst. However, there were slight increases in the yield of some important individual aromatic compounds such as benzene, toluene, propyl benzene, and indane. This effect was attributed to the influence of acidic sites on the alumina which converted the cycloalkanes to aromatic compounds through dehydrogenation; however, carbon retention through the interaction of ;CdC; bonds with the alumina active sites appeared to be stronger, thus enabling condensation of unsaturated molecules, which could lead to coke formation. Uemechi et al.16 reported a 3-fold increase in aromatic compounds yield from PE cracked in the presence of Pt/ Al2O3. This discrepancy may have resulted from a number of factors such as contact times (20 min), reaction temperature, and the type of reactor used in their work. The 1 h reaction time used in this work in addition to the autoclave reactor system, which prevented the hot gases from escaping, may have promoted polymerization and condensation of the aromatic compounds on the catalyst surface to form coke,
(23) Fogler, H. S. In Elements of Chemical Reaction Engineering, 4th ed.; Amundson, R. N., Ed.; Prentice Hall PTR: Upper Saddle River, NJ, 2006. (24) Corma, A.; Martinez, A.; Martinez-Soria, V. J. Catal. 1997, 169, 480–489. (25) Fuente, M.; Pulgar, G.; Gonzalez, F.; Pesquera, C.; Blanco, C. Appl. Catal., A 2001, 208, 35–46. (26) Chen, H.; Yang, H.; Briker, Y.; Fairbridge, C.; Omotoso, O.; Ding, L.; Zheng, Y.; Ring, Z. Catal. Today 2007, 125, 256–262. (27) Chen, H.; Yang, H.; Omotoso, O.; Ding, L.; Briker, Y.; Zheng, Y.; Ring, Z. Appl. Catal., A 2009, 358, 103–109. (28) Lu, S.; Lonergan, W. W.; Bosco, J. P.; Wang, S.; Zhu, Y.; Xie, Y.; Chen, J. P. J. Catal. 2008, 259, 260–268. (29) Akdogan, Y.; Vogt, C.; Bauer, M.; Bertagonolli, H.; Giurgiu, L.; Rodur, E. Phys. Chem. Chem. Phys. 2008, 10, 2952–2963. (30) Dowden, D. A. The Reactions of Hydrocarbons on Multimetallic Catalysts; Royal Society of Chemistry: London, 1978.
(31) Ali, L. I.; Ali, A-G. A.; Aboul-Fotouha, S. M.; Aboul-Gheit, A. K. Appl. Catal., A 1999, 177, 99–110.
4235
Energy Fuels 2010, 24, 4231–4240
: DOI:10.1021/ef100227f
Insura et al.
Figure 3. Simulated distillation curves of the pyrolysis oils in relation to Pt/Al2O3 compared to commercial gas oil. Table 3. Product Yields from the Pyrolysis of LDPE at 425 °C with 0.5% Rh/Al2O3 Catalyst product yield (wt %) 0.5% Rh/Al2O3 loading
running pressure (MPa)
oil
gas
coke/char
no catalysts 1 g (0.005 g of Rh) 2 g (0.01 g of Rh) 4 g (0.02 g of Rh) 4 g calcined (0.02 g of Rh)
1.52 1.65 1.72 1.79 2.00
89.8 ( 0.72 85.4 ( 1.00 84.9 ( 0.43 80.5 ( 0.50 79.6 ( 0.75
10.2 ( 0.26 11.0 ( 0.15 11.3 ( 0.26 11.6 ( 0.20 12.7 ( 0.30
3.52 ( 0.04 3.82 ( 0.04 7.90 ( 0.11 7.32 ( 0.10
leading to a reduction in “available” aromatic compounds in the oil. In contrast, the presence of the calcined catalyst improved total aromatic yield to 13.4 wt %, which was more than the fraction of aromatics obtained from either the noncatalytic work or from experiments with the fresh catalysts at the same loading. Here most of aromatic compounds increased in yield, indicating that the calcined Pt/Al2O3 catalyzed aromatization. Calcination of Pt catalysts causes a change of the oxidation state of the metal component from metallic Pt (Pt0) active sites to oxide forms (Ptδþ),29 which are larger and as such occupy more of the catalyst surface. The proportion of alkane compounds in the oils greatly reduced when the same loading of calcined Pt catalyst was applied, thus indicating that more unsaturation was achieved in general, compared to noncalcined catalyst. Thus, calcination appeared to increase the activity of the catalyst for the formation of not only cycloalkanes and aromatics but also of char, possibly via enhanced dehydrocylization mechanism where polymerization of unsaturated hydrocarbons led to deposition of polyaromatic compounds during dehydrogenation reactions.20,21 The consequence of this effect was a reduction in the proportion of n-alkanes and C4 hydrocarbons in the oil product. Figure 3 shows simulated distillation curves for the boiling point range of the derived oils compared to that of standard gas oil fuel determined by gas chromatography with flame ionization detection. In general, the influence of the catalyst on the boiling point range distribution of the oils was not very significant. With the use of the gasoline fraction as an example, it can be seen that there was only a slight improvement in the presence of the Pt/Al2O3 catalyst and there was little or no effect of the calcined catalyst. For instance, the gasoline fraction was about 41 wt % in the absence of the catalyst and improved to 47.1 wt % in the presence of 2 g of fresh Pt/Al2O3. No significant difference was observed in the
proportion of gasoline fraction when the catalyst loading was 4 g for either fresh or calcined catalyst. 3.2. Influence of Rh/Al2O3 Catalyst. 3.2.1. Yield of Pyrolysis Products. Table 3 shows the product yields from the conversion of low-density polyethylene at 425 °C using different loadings of 0.5% Rh/Al2O3 catalyst. The data shows that the presence of the Rh/Al2O3 catalyst in the cracking of LDPE clearly affected the distribution of the products. There was a marginal increase in gas yield and a decrease in the oil yield in favor of the formation of coke deposited on the catalyst surfaces and to a lesser extent on the reactor walls. Similar to the results obtained when using Pt/Al2O3, coke formation increased with increasing catalyst loading and was generally higher than the corresponding yield obtained with a similar loading of Pt/Al2O3. 3.2.2. Gas Product Distribution. Table 4 shows the gas product distribution derived from the catalytic pyrolysis of low-density polyethylene at 425 °C in the autoclave reactor. Hydrogen gas production increased with increasing loadings of Rh/Al2O3 catalyst. However, the selectivity of Rh/Al2O3 catalyst for hydrogen production reduced in the presence of the calcined catalyst. The yield of alkane gases increased due mainly to the marked increase in methane concentration, which further increased as the catalyst loading was increased. During experiments without any catalyst, alkane gases dominated the gas product with a yield of 67.6 wt %, whereas alkene gas yield was 32.0%. Propane and ethane dominated the alkane gas product at 24.5 and 21.6 wt %, respectively, of the total gas produced, followed by the propene, 18.6 wt %. However, in the presence of 1 g of Rh/Al2O3 catalyst, methane was the main gas product at 22.8 wt %, followed by ethane and propane resulting in increasing alkane gas yield to 78.6 wt % of total gas product while the yield of alkene gases reduced to 20.6 wt %. Raising the catalyst loading to 2 g gave a methane yield of 30 wt % while the yields of both propane and ethane remained fairly stable. 4236
Energy Fuels 2010, 24, 4231–4240
: DOI:10.1021/ef100227f
Insura et al.
Table 4. Effect of 0.5% Rh/Al2O3 Catalyst on Gas Product Distribution gas components (wt % gas)
no catalyst
1 g of fresh Rh/Al2O3
2 g of fresh Rh/Al2O3
4 g of fresh Rh/Al2O3
4 g of calcined Rh/Al2O3
methane ethane propane butane total alkanes
9.80 21.6 24.5 13.7 69.6
22.8 21.0 21.9 12.8 78.6
29.7 18.9 20.7 10.8 80.0
45.7 16.4 17.2 10.3 89.7
47.2 17.3 17.3 10.2 92.1
ethene propene butene total alkenes
4.90 18.6 8.43 32.0
2.28 12.8 5.48 20.6
2.61 10.8 5.41 18.4
0.86 5.17 3.53 9.56
0.31 3.94 2.99 7.24
hydrogen total gases
0.45 102
0.82 100
1.17 99.6
1.21 101
0.87 100
Figure 4. Oil product distributions in the presence of the Rh/Al2O3 catalyst.
The yield of total alkanes increased to 80 wt % of the total gas while alkene gases decreased slightly. With a loading of 4 g of Rh/Al2O3 catalyst, the methane yield increased dramatically to 45.7 wt % whereas the propane and ethane yields were slightly decreased. A similar proportion of methane was obtained with 4 g of calcined Rh/Al2O3. Hence, with the 4 g loading of both the fresh and calcined catalysts, the proportion of alkane gases averaged over 90 wt %. This reduction in the yield of unsaturated hydrocarbon gases accompanied by a high increase in methane and total alkane yield reflected the possibility of demethylation and saturation reactions. Rh/Al2O3 catalysts can catalyze the demethylation of aromatics; the hydrogenolysis mechanism is known for Rh/Al2O3 catalyst reactions at similar and higher temperatures.30-32 The formation of methane was credited to hydrogenolysis reactions, in which case catalytic cracking was followed by saturation after a primary dehydrogenation step.30 3.2.3.. Oil Product. Figure 4 displays the distribution of compounds in the oil products obtained in the absence and presence of increasing Rh/Al2O3 catalyst produced from the catalytic pyrolysis of low-density polyethylene in the autoclave reactor at 425 °C. The oils from the catalytic work showed a marginal increase in the yield of n-alkanes and aromatics, with a reduction in the yield of n-alkenes compared to the oil from noncatalytic pyrolysis. The reduction in alkene content may be attributed to their conversion to aromatic compounds during cyclization and aromatization reactions, again an effect attributable to the alumina support.
In the presence of 2 g of 0.5% Rh/Al2O3, the yield of alkenes was reduced by 31% compared to noncatalytic work. It reduced further to 9.56 wt % when the catalyst loading was increased to 4 g. Similarly, the yield of alkenes reduced to 6.7 wt % when calcined 4 g Rh/Al2O3 was employed . In the absence of the Rh/Al2O3 catalyst, the oil product contained only 12 wt % aromatics, which then showed a small increase as the catalyst loading was increased reaching 14.7 wt % when the catalyst loading was 4 g and up to 16 wt % with 4 g of the calcined catalyst. This may imply that the calcined catalyst possessed more activity in dehydrogenation and aromatization, which may be a reflection of the effects of Rh oxides. The yield of each individual aromatic compound such as benzene, toluene, ethylbenzene, cyclohexane and methylcyclohexane (Figure 5) as well as xylene, cumene, Indane, indene, naphthalene and dimethyl naphthalene improved with increased catalyst loading. It appeared that in the presence of Rh/Al2O3, the reduction in n-alkene formation resulted in their conversion to aromatic compounds via dehydrogenation, cyclization and aromatization reactions, as evidenced by the improvement in the aromatic yield. However, it is clear that the degree of aromatization was higher using Rh/Al2O3 rather than Pt/Al2O3. Upon an increase in the amount of Rh/Al2O3 catalyst, the rate of dehydrogenation and aromatization reactions increased. The decreasing aliphatic yield and the increasing the yield of aromatics showed that the dehydrogenation reactions dominated the process due to dehydrocyclization with higher catalyst loading (both fresh and calcined). It has been suggested that during dehydrogenation, aromatization reactions resulted in the production of alkene carbonium ion intermediates, which were finally stabilized by aromatization.10
(32) Vidal, H.; Bernal, S.; Baker, R. T.; Cifredo, G. A.; Finol, D.; Rodrıguez-Izquierdo, J. M. Appl. Catal., A 2001, 208, 111–123.
4237
Energy Fuels 2010, 24, 4231–4240
: DOI:10.1021/ef100227f
Insura et al.
Figure 5. Aromatic compounds found in the oil in the absence and presence of the Rh/Al2O3 catalyst.
in the presence of fresh Rh/Al2O3. In addition to increasing the total yield of C5-C30 alkanes, Rh/Al2O3 improved the yield of short alkane chains compared with their yields in the absence of the catalyst. Thus, the catalyst helped to divert oil composition toward lower boiling point hydrocarbons. The gasoline-range fraction showed little or no improvement in the presence Rh/Al2O3. 3.3. Catalyst Deactivation and Reactivation. Catalyst deactivation tests were conducted by reusing the catalysts for further LDPE pyrolysis. In this section, “aged catalysts” refer to catalysts that have been previously used only once by the authors for the catalytic pyrolysis of LDPE. Similarly, “reactivated catalysts” refer to aged catalysts that were subjected to reactivation by heating in a muffle furnace at 600 °C for 2 h. For this test, 4 g each of the aged (used), reactivated, and fresh catalysts was employed. The results from the analysis of pyrolysis products obtained from the different sets of catalysts were then compared. For both Rh/ Al2O3 and Pt/Al2O3 catalysts, visual observation of the aged (used) catalysts, showed deposits of carbon on the surface of the catalyst and their color changed from gray to black. In addition, the weights of the aged catalysts increased by up to 20 wt % compared to when they had not been used (fresh catalysts). Table 5 shows a comparison of the properties of the aged, reactivated, and fresh catalysts. Results show how the surface area of each aged catalyst (Rh/Al2O3 and Pt/ Al2O3) reduced dramatically compared to the fresh Rh and Pt catalysts. For instance, the BET surface area of the fresh Pt/Al2O3 was 116.1 m2/g but this reduced to 8.1 m2/g after first use. Similarly, the surface area of Rh/Al2O3 fell from 101.5 to 21.7 m2/g after use. The reduction in surface area corresponded to the lowering of the pore volume to nearly zero for both aged catalysts. However, upon catalyst reactivation, the pore volume increased to nearly their original values. The surface areas of reactivated Rh/Al2O3 and Pt/Al2O3 catalysts recovered to 95.6 and 98.2 m2/g, respectively. Both calcined and reactivated catalysts (Rh/Al2O3 and Pt/Al2O3) showed identical characteristics compared to fresh catalysts, in relation to their physical properties. Table 6 shows the product distribution in relation to the activities of the aged and reactivated catalysts, respectively. Table 7 shows the comparison of oil compositions in the presence of fresh, aged, and reactivated Rh/Al2O3 and Pt/ Al2O3 catalysts. The results from both sets of aged catalysts showed different activities, indicating catalyst deactivation. For example, in the presence of the aged Pt/Al2O3 catalyst,
Scheme 1
The activity of Rh metal in hydrogenation/dehydrogenation reactions of hydrocarbons has been demonstrated by many studies13,23,31-34 The activity of the metallic site on the Rh often involves the adsorption of hydrocarbon species on metallic active sites and the subsequent cleavage of C-C and C-H, as well as the H-H bonds in the hydrogen molecule.30 Cleavage of H-H represents the first step in hydrogenolysis. Hydrogen species immediately adsorb on Rh sites because of its high capability to chemosorb hydrogen.30 The Rh-based catalyst was demonstrated to be the most active among the platinum group in hydrogenation of cyclohexene at low temperatures ranging from 50 to 150 °C, however, the catalytic activity was considerably reduced at higher temperature.34 In another study,13 Rh/Al2O3 showed high activity and selectivity in benzene and toluene hydrogenation at room temperature. At temperatures higher than 170 °C, the activity of the Rh/Al2O3 catalyst changed to dehydrogenation reactions, again possibly due to the thermal activation of the alumina active sites. The metallic catalytic activity could be expressed by Scheme 1. Hydrogenation reactions take place at low temperatures, up to 250 °C whereas dehydrogenation occurs at temperatures higher than 300 °C.30 Different precious metal catalysts have been investigated for the dehydrogenation of cyclohexane.31 Rh/Al2O3 catalyst showed high activity at temperatures up to 425 °C (second order among the other noble metal catalysts), however, this decreased gradually at higher temperature. This reduction was due to emergence of parasite reactions such as hydrogenolysis that reached 29% at 500 °C, and this behavior was credited to the d-bond character of Rh.31 Figure 6 shows the gas chromatography simulated distillation curves for the oil derived from the catalytic pyrolysis of polyethylene with the Rh/Al2O3 catalyst. The simulated distillation curves for the different oils appear identical, with minor differences. It shows that the hydrocarbon components with boiling points from 250 to 400 °C were enhanced (33) Huber, G. W.; Dumesic, J. A. Catal. Today 2006, 111, 119–132. (34) Aboul-Fotouh, S. M.; Aboul-Gheit, A. K. Appl. Catal., A 2001, 208, 55–61.
4238
Energy Fuels 2010, 24, 4231–4240
: DOI:10.1021/ef100227f
Insura et al.
Figure 6. Simulated distillation curves of the pyrolysis oils in relation to Rh/Al2O3 compared to commercial gas oil.
to that produced from the noncatalytic work. For instance, the yields of C4 hydrocarbons, cycloalkanes, and n-alkenes increased while the yield of n-alkanes and aromatics decreased in the presence of aged Rh/Al2O3 compared to the fresh catalyst. The increase in aromatics and the consequent decrease in alkanes with the aged Rh/Al2O3 catalyst suggested an additional catalytic activity involving possible cyclization and dehydrogenation of alkanes in the oil. This was also evident in the increase in the amount of hydrogen gas produced with the reactivated catalyst compared to either the fresh or aged catalyst The deactivation of noble metal catalysts by poisoning coke deposits, which block metal active sites, have been reported by many investigators.16,20-22 For instance, Uemichi et al.16 during pyrolysis of PE on Pt/C catalyst reported that a huge residue of coke affected the yield of liquid product negatively. Deactivation can be attributed to formation of multicyclic aromatics on the surface of the catalysts. Vidal et al.32 reported the deactivation of Rh/SiO2 during benzene hydrogenation after it showed high initial activity. Ali et al.13 noticed a decrease in the rate of hydrogenation of benzene and toluene on the Rh catalyst. They also suggested that the catalyst deactivation resulted from the deposition of inert species of products on the surface of the catalyst, thereby smothering and hiding the active metallic sites. However, pyrolysis of LDPE with the reactivated Pt/ Al2O3 catalysts gave product distributions as well as product compositions with more similarities to those of the calcined catalysts than to those from the fresh catalyst. A comparison of the composition of the oils from both the calcined and the reactivated Pt/Al2O3 catalysts showed a decrease in the n-alkane fractions. In the case of Rh/Al2O3, the product distribution obtained with the reactivated catalyst showed a significant difference from the fresh, calcined, and aged catalysts. Reactivated Rh/Al2O3 produced much less n-alkanes and much more n-alkenes than the fresh catalyst. The proportion of aromatics from the oil produced with the reactivated Rh/Al2O3 was significantly higher than that from the aged catalyst but not very significantly different compared to the fresh catalyst. Therefore, carbon deposits deactivated both Rh/Al2O3 and Pt/Al2O3 catalysts used in pyrolysis of LDPE after use. The aged catalysts were reactivated by heating in a muffle furnace to burnoff the deposited
Table 5. Comparison of the Textural Properties of Fresh, Aged, and Reactivated Catalyst catalyst
BET surface area m2/g
pore volume cm3/g
pore size, nm (width)
Rh/Al2O3 fresh aged reactivated calcined
101.5 21.7 95.6 98.2
0.23 0.05 0.24 0.24
9.22 9.80 10.1 9.83
Pt/Al2O3 fresh aged reactivated calcined
116.1 8.1 98.2 95.8
0.28 0.02 0.24 0.24
9.50 8.42 9.61 10.2
gas product yield reduced slightly. Other changes in oil and coke products yield were minor. However, very clear similarities existed between the distribution of products for the calcined (in Table 2) and reactivated Pt/Al2O3 catalysts. For the aged Rh/Al2O3 catalyst, the effect on product yield was different as the yields of coke and gas products reduced while oil product increased. Furthermore, alkene gas yield increased more than twice for the aged Pt/Al2O3 catalyst and by nearly 3 times for the aged Rh/Al2O3 catalyst compared to the fresh catalysts, respectively. Even though the gas product was still dominated by propane, ethane, and butane with the aged platinum catalyst, the total alkane yield reduced by a difference of 10 wt % compared with the fresh catalyst. Methane and propane dominated the gas product with the aged rhodium catalyst. The aged Rh/Al2O3 had a significant effect in the reduction of the proportion of methane formed. It can be seen that the percentage of methane decreased from 45.7 wt % of the total gas product with the fresh catalyst to only 21.7 wt % when aged Rh/Al2O3 was used. Table 7 shows that LDPE pyrolysis with reactivated Pt catalyst produced oil with composition similar to that produced under fresh Pt catalyst. The presence of reactivated Pt/Al2O3 enhanced the yield of n-alkanes, especially shorter chains, as well as aromatics while alkene yield reduced compared to the aged catalyst. The extent of recovery of the catalytic activity of the reactivated Pt/Al2O3 can be seen from Table 6, showing similarity of activity between the fresh and reactivated Pt catalysts. The compositional distribution of oil product in the presence of aged Rh catalyst was similar 4239
Energy Fuels 2010, 24, 4231–4240
: DOI:10.1021/ef100227f
Insura et al.
Table 6. Effect of Catalyst Deactivation and Reactivation on Product Distribution 4 g of 0.5% Pt/Al2O3 products, wt %
4 g of 0.5% Rh/Al2O3
fresh
aged
reactivated
fresh
aged
reactivated
oil gas coke
84.5 ( 0.56 10.5 ( 0.24 5.00 ( 0.03
85.7 ( 0.30 9.10 ( 0.13 5.20 ( 0.13
87.0 ( 1.00 7.50 ( 0.11 5.50 ( 0.05
80.5 ( 0.50 11.6 ( 0.20 7.90 ( 0.20
87.0 ( 0.50 8.50 ( 0.08 4.50 ( 0.11
86.9 ( 0.60 8.50 ( 0.13 4.60 ( 0.03
hydrogen methane ethane propane butane total alkanes
1.45 14.7 26.2 32.6 16.4 89.8
1.33 10.2 21.0 27.8 20.0 79.0
0.81 21.7 16.8 21.2 14.1 73.9
1.98 33.7 17.4 21.0 12.7 84.8
ethene propene butene total alkenes
0.64 4.98 3.16 8.78
2.42 10.6 6.72 19.7
0.88 3.94 2.45 7.27
0.86 5.17 3.53 9.56
3.49 15.4 6.50 25.3
1.38 6.39 5.41 13.2
total gases
100
100
100
101
100
99.9
Gas Product Composition, wt/wt % 1.69 1.21 13.5 45.7 21.6 16.4 34.2 17.2 21.8 10.3 91.1 89.7
Table 7. Comparison of Oil Product Distributions in the Presence of Fresh, Aged, and Reactivated Rh/Al2O3 and Pt/Al2O3 Catalysts 4 g of 0.5% Pt/Al2O3 products, wt % oil C4 cycloalkanes n-alkanes n-alkenes aromatics
4 g of 0.5% Rh/Al2O3
no catalyst
fresh
aged
reactivated
fresh
aged
reactivated
0.12 2.69 46.2 12.5 12.0
1.14 1.82 59.6 5.39 11.7
0.86 3.01 43.2 9.10 11.7
0.46 2.65 45.4 6.86 13.7
0.10 2.78 48.3 7.10 14.7
0.42 3.02 45.1 11.3 10.6
0.91 4.38 35.4 12.6 16.1
carbon. The reactivated catalysts have physical properties similar to the fresh catalysts but much more closely to those of the calcined catalysts. This was not unexpected since the processes of calcination and reactivation were identical. Both reactivated catalysts, however, exhibited catalytic activities different from either fresh or calcined catalyst, especially in the case of Rh/Al2O3.
In addition, oil product obtained in the presence of Rh/ Al2O3 catalyst was similar to that from Pt/Al2O3 catalyst in enhancing the yield of alkanes and lowering the ratio of alkenes. Calcined Rh/Al2O3 and Pt/Al2O3 appeared to increase the proportion of aromatics in the oils compared to the fresh catalysts. In addition, oil product in the presence of both Rh/Al2O3 and Pt/Al2O3 catalysts was richer in short chain alkanes and aromatics, along with a lower ratio of alkenes compared to oil from noncatalytic pyrolysis. Although both catalysts appeared to have improved the fuel properties of the pyrolysis oil products, the formation of char/coke led to a significant decrease in the yield of oil. This could be as a result of the long residence time (1 h) used in this work. Apparently, some components of the oil were converted to coke/char owing to possible long contact times with the catalysts. The formation and subsequent condensation of aromatics on the catalysts’ surface resulted in higher coke/char yields. The activities of both catalysts increased with increasing loadings. The activity of the fresh Pt/Al2O3 catalyst appeared to be more suppressed by the alumina support than that of fresh Rh/ Al2O3. Both catalysts deactivated after first use but could be reactivated at 600 °C for 2 h in a muffle furnace. Reactivated Pt/Al2O3 showed a catalytic activity similar to its freshly calcined form, whereas reactivated Rh/Al2O3 showed a largely varied catalytic character.
4. Conclusions The catalytic activities of noble metals supported on alumina in the catalytic cracking of LDPE have been investigated in relation to the production of premium grade oils. These catalysts did not seem to decrease the temperature of LDPE degradation, but they may have helped in influencing the secondary reactions. The use of these catalysts led to coke formation at the operating temperature of 425 °C. Coke yield increased with catalyst loading. Consequently, oil product was slightly reduced in favor of char and hydrogen production. Both Rh/Al2O3 and Pt/Al2O3 catalysts gave gas product with a high ratio of alkanes reaching up to 90% of the total gas product. Methane was the dominant gas accounting for up to 46% of the total gas product weight for the Rh/Al2O3 catalyst, whereas ethane and propane dominated the gas products from the Pt/Al2O3 catalyst. It was possible that the proportion of alumina to the noble metals in the catalyst tilted the activity of the catalyst toward dehydrogenation, aromatization, and coke formation. However, it appeared that Rh metal catalyzed the formation of methane.
Acknowledgment. This work was supported by the U.K. Engineering and Physical Sciences Research Council under EPSRC Grant EP/D053110/1
4240
