Conversion of Polyethylene into Transportation Fuels by the

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Conversion of Polyethylene into Transportation Fuels by the Combination of Thermal Cracking and Catalytic Hydroreforming over Ni-Supported Hierarchical Beta Zeolite J. M. Escola,*,† J. Aguado,† D. P. Serrano,‡,§ L. Briones,† J. L. Díaz de Tuesta,† R. Calvo,∥ and E. Fernandez∥ †

Department of Chemical and Environmental Technology, and ‡Department of Chemical and Energy Technology, ESCET Rey Juan Carlos University, c/Tulipán s/n, 28933 Móstoles, Madrid, Spain § IMDEA Energy Institute, c/Ramón de la Sagra 3, 28935 Móstoles, Madrid, Spain ∥ URBASER, Avenida de Tenerife 4, 28703 San Sebastian de los Reyes, Madrid, Spain ABSTRACT: The oil obtained from the thermal cracking of low-density polyethylene (LDPE) is formed mainly by linear hydrocarbons with a high quantity of olefins, which hinders the possible application of this product in the formulation of transportation fuels. However, hydroreforming of this oil using bifunctional catalysts with high accessibility to the active sites would allow for the properties of the gasoline and diesel fractions to be significantly upgraded. This is the case of the catalyst employed here because it consists of hierarchical Beta zeolite (with a bimodal micro-mesoporosity) and containing 7 wt % Ni. The presence of nickel in the catalyst increased the share of gasolines with regard to the h-Beta support. The effect of the main variables involved in the hydroreforming process has been investigated and optimized, showing that the extent of hydrocracking is favored when increasing the temperature, the pressure, and the catalyst/feed ratio, leading to enhanced gasoline yields at the expense of heavy (C19−C40) and especially light (C13−C18) diesel fractions because of the faster diffusion of the latter. Ni/h-Beta proved to be an especially adequate catalyst for obtaining gasolines; therefore, a maximum in the selectivity toward gasoline (up to 68.7%) was found in the hydroreforming at 40 bar of hydrogen pressure. On the other hand, the values of the bromine index indicated that 80−100% of the olefins present in the raw oil were hydrogenated depending upon the reaction conditions. In addition, the Ni/h-Beta catalyst showed high activity for aromatization and, especially, hydroisomerization reactions. Thus, a 53% share of isoparaffins in the gasolines was obtained at long reaction times. The gasoline and diesel fractions obtained showed a high research octane number (RON) (>80) and cetane indexes above specifications (>70), respectively, which is indicative of their high quality as transportation fuels.

1. INTRODUCTION Hierarchical zeolites are characterized by the presence of a bimodal micro-/mesoporous pore size distribution that confers them very interesting properties. 1−3 The presence of mesopores gives rise to the appearance of a non-sterically hindered surface area and enhances mass-transfer rates, which leads toward higher activity in a variety of reactions, such as polyolefin cracking,4,5 olefin epoxidation,6,7 benzene alkylation,8 gasoil cracking,9 jasminaldehyde synthesis,10 etc. Additionally, the occurrence of shortened diffusion path lengths may change the product selectivity because the extent of the secondary reactions is markedly reduced.8,11 In this regard, a particularly interesting result is the observed higher resistance to deactivation observed over mesoporous zeolites.12,13 The secondary mesoporosity causes a faster diffusion of the coke precursors out of the zeolite, avoiding the appearance of micropore blocking phenomena that otherwise would deactivate the zeolite. On the other hand, the presence of mesopores in hierarchical zeolites enables the preparation of bifunctional catalysts with higher dispersion of the active phase, because the latter is expected to be located within the mesopores.14 In previous works of our group, hierarchical zeolites have been successfully applied in the polyolefin [low-density polyethylene (LDPE), high-density polyethylene (HDPE), © 2012 American Chemical Society

and polypropylene (PP)] cracking, showing quite higher activities than conventional zeolites because of the lower steric constraints and enhanced diffusion of the bulky plastic macromolecules.4,5,15 However, the obtained liquids contained high amounts of olefins, which are harmful for their usage in the formulation of fuels because they might cause the formation of gums in the car injectors and engines. Bifunctional catalysts are characterized by the presence of both acid and metal sites. The acid sites are usually provided by an aluminosilicate carrier (zeolites, Al-MCM-41, and amorphous SiO2−Al2O3), while the metal sites come from a supported metal, such as Pt, Pd, or Ni. These catalysts are widely applied in hydroreforming reactions, such as hydrocracking, hydroisomerization, hydrodesulfurization, and hydrodenitrification.16−20 In these reactions, the presence of mesopores has shown to increase the activity because of both the better dispersion of the metal phase21 and the improved diffusion of the bulky molecules.22 To decrease the olefins content, we have applied recently bifunctional hierarchical zeolites (Ni/h-ZSM-5 and Ni/h-Beta) for the hydroreforming of the oil coming from LDPE thermal Received: March 9, 2012 Published: May 31, 2012 3187

dx.doi.org/10.1021/ef300938r | Energy Fuels 2012, 26, 3187−3195

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both cases, the analyses were carried out after outgassing the samples at 350 °C under vacuum for 16 h. Brunauer−Emmett−Teller (BET) surface areas were determined using the relative pressure range of 0.05−0.16 in the nitrogen adsorption isotherm as the range of linearity. Argon adsorption isotherms were analyzed by applying a nonlocal density functional theory (NLDFT), assuming argon adsorption in cylindrical pores. Transmission electron micrographs (TEMs) were collected on a Phillips TECNAI 20 microscope equipped with a LaB6 filament under an accelerating voltage of 200 kV. Prior to the observation, the samples were dispersed in acetone, stirred in an ultrasonic bath, and finally deposited over a carbon-coated copper grid. The acid properties of the catalysts were determined by ammonia temperature-programmed desorption (TPD) in a Micromeritics AutoChem 2910 system using He as the carrier gas. Previously, the samples were outgassed by heating under a helium flow to 560 °C and keeping this temperature for 30 min. After cooling to 180 °C, an ammonia flow was passed through the sample for 30 min and, subsequently, the physisorbed ammonia was removed by flowing helium at 180 °C for 90 min. Chemisorbed ammonia was determined by increasing the temperature with a heating rate of 15 °C min−1 up to 550 °C. The ammonia concentration in the effluent helium stream was monitored with a thermal conductivity detector (TCD). Hydrogen temperature-programmed reduction (TPR) experiments were carried out in a Micromeritics AutoChem 2910 system. Initially, the samples were outgassed under Ar flow (35 N mL min−1), heating to 80 °C with a heating rate of 15 °C/min and holding for 30 min. The analysis was performed under a flow (35 N mL min−1) of 10% H2 in Ar, heating to 800 °C with a heating rate of 5 °C min−1 and holding this temperature for 10 min. Prior to the detector, the effluent gases were cooled in an isopropanol−liquid nitrogen trap at −80 °C. 2.4. LDPE Thermal Cracking. LDPE thermal cracking reactions were carried out in a stirred 100 mL stainless-steel autoclave reactor heated by a ceramic oven. In each experiment, 30 g of pure LDPE (Mw = 416 000, REPSOL) was thermally cracked at 400 °C for 90 min under 1.5 bar of nitrogen under a stirring speed of 700 rpm. These conditions were chosen because they led toward the lowest amount of gases ( 95), while the cetane index decreased slightly (∼70− 75) with regard to the starting feed (82.4). This is caused by the formation of aromatics and branched hydrocarbons during the hydroreforming, which diminishes the calculated cetane index of the diesel. However, this cetane index is clearly above the requested specification by the legislation (>51). These results indicate that the obtained liquid products can be used as valuable components for the formulation of gasoline and diesel transportation fuels. It is noteworthy to point out that the hydrogenation was achieved with a mild hydrogen pressure (20 bar) under cold charge (not flowing hydrogen). This is indicative of the high hydrogenation activity of this catalyst. From the three tested temperatures, we have chosen 310 °C as the most adequate for carrying out the hydroreforming because it gave rise to a remarkable selectivity toward gasoline (54%) with a high amount of isoparaffins, while the total content of aromatics in the fuels was suitable. The influence of the hydrogen pressure within the 5−40 bar range was investigated working at 310 °C for 45 min and using a catalyst/feed mass ratio of 1:30. Figure 7 shows the yields by groups obtained in these hydroreforming reactions. Seemingly, the increase in hydrogen pressure enhanced the extent of hydrocracking reactions toward gasoline, reaching the highest yield in this fraction (68.7%) at 40 bar of hydrogen. In contrast, the yield of gaseous hydrocarbons remained fairly low (around 6%), likely because of the moderate acid strength of the Ni/hBeta zeolite. The increase in gasoline took place at the expense of both light and heavy diesel fractions. Interestingly, the increase in hydrogen pressure was more effective than the temperature in augmenting the extent of heavy diesel cracking (around 9% drop at 40 bar of hydrogen). The enhancement of hydrogen partial pressure is supposed to decrease the average lifetime of the intermediate carbenium ions because of its quick hydrogenation. This fact leads to an enhanced availability of acid sites causing the observed higher extent of hydrocracking reactions. Table 4 shows the characterization data of the fuels obtained at different pressures. As expected, the bromine index of the liquid product decreased with the hydrogen pressure reaching complete hydrogenation of the olefins at 40 bar of

Figure 7. Effect of the hydrogen pressure in the hydroreforming of the oil from LDPE thermal cracking over Ni/h-Beta (T = 310 °C; oil/ catalyst mass ratio = 30; and t = 45 min).

Table 4. Characterization Data of the Fuels Obtained at Different Hydrogen Pressures (T = 310 °C; t = 45 min; Catalyst/Feed Mass Ratio = 1:30; and N = 700 rpm) n-paraffins (wt %)a isoparaffins (wt %)a naphthenes (wt %)a aromatics (wt %)b bromine index a

feed

P = 5 bar

P = 10 bar

P = 20 bar

P = 40 bar

35.1

46.9

33.2

35.5

38.4

18.7

30.9

42.3

39.5

40.8

6.0

8.6

6.0

6.0

9.9

1.7

21.4

18.0

20.5

9.6

54.1

9.0

4.9

2.4

0

b

Determined by PIONA analysis in gasolines. Determined by HPLC in total fuels.

hydrogen. It is noteworthy to indicate that, even at the lower hydrogen pressure (5 bar), the catalyst is able to hydrogenate the olefins to a large extent (>80%), which is indicative of its high hydrogenation activity, ascribed to the good dispersion of the catalyst because of the presence of small size nickel particles. Likewise, at 40 bar of hydrogen pressure, no olefins were detected; therefore, these conditions allowed for the removal of them entirely from the fuels. At 5 bar of hydrogen pressure, the main products obtained in the gasoline fraction are normal paraffins (46.9%), while isoparaffins account for 30.9%. Hydroisomerization is enhanced at 10−20 bar, leading toward a decrease in the amount of n-paraffins to 33.2−35.5% in favor of the isoparaffins (39.5−42.3%). At 40 bar, the most remarkable result is a drop in the total aromatic content from 18.0−21.4% obtained for the other pressures to 9.6%. This fact indicates that higher hydrogen pressures suppress secondary reactions, such as aromatization and, presumably, coke formation. In this regard, metal and acid sites in the catalyst appear to be well-balanced to reduce these secondary reactions. Consequently, hydrogenation of the olefinic intermediates over the metal sites or, more likely, hydrogen spillover from them takes place in the reaction medium. Figure 8 shows the yields by groups obtained in the hydroreforming of the oil from LDPE thermal cracking using different catalyst/feed mass ratios at 310 °C under 20 bar of 3192

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RON of the gasolines is higher than 80 and the cetane index is above 70. According to these data, seemingly the best choice to attain a high selectivity toward gasoline (∼55%) with a low production of gases (5%) is using a catalyst/feed ratio around 1:30 or 1:50. However, from both ratios, 1:30 seems the most adequate in terms of the high amount of isoparaffins found in gasolines (39.5%), as well as the suitable amount of aromatics (20.5%) attained in the total fuels. Another variable critical in the performance of the Ni/h-Beta catalyst is the reaction time. Figure 9 illustrates the yields by groups obtained in the

Figure 8. Influence of the catalyst/feed mass ratio in the hydroreforming of the oil from LDPE thermal cracking over Ni/h-Beta (T = 310 °C; PH2 = 20 bar; and t = 45 min).

hydrogen pressure for 45 min. The increase in the amount of catalyst led toward an enhancement in the extent of hydrocracking reactions, augmenting the share of gasoline and gases and decreasing those of diesel fractions especially when a 1:10 ratio was used. Hence, the share of gases and gasoline reached values of 14 and 62%, respectively, for the 1:10 mass ratio. Table 5 shows the characterization data

Figure 9. Effect of the reaction time in the hydroreforming of the oil from LDPE thermal cracking over the Ni/h-Beta catalyst (T = 310 °C; catalyst/feed ratio = 1:30; and PH2 = 20 bar).

Table 5. Characterization Data of the Fuels Obtained with Different Catalyst/Feed Ratios (P = 20 bar of Hydrogen; T = 310 °C; t = 45 min; and N = 700 rpm) n-paraffins (wt %)a isoparaffins (wt %)a naphthenes (wt %)a aromatics (wt %)b bromine index

feed

1:100

1:50

1:30

1:10

35.1 18.7 6.0 1.7 54.1

58.9 24.7 5.5 8.9 6.0

55.5 27.4 7.7 11.3 2.1

35.5 39.5 5.9 20.5 2.4

24.9 52.7 7.0 21.3 1.6

hydroreforming of the oil from the LDPE thermal cracking varying the reaction time within the range of 15−180 min at 310 °C under 20 bar of hydrogen pressure and using a catalyst/ feed mass ratio of 1:30. These results indicate that the share of gases augmented at longer reaction times (13.5% at 90 min), while the gasolines reached a maximum at 15 min (61%) and dropped later, remaining virtually constant along the time (∼55%). In contrast, the diesel fractions did not show any clear trend, varying the light diesel fraction within 22−26% and the heavy diesel fraction within 7−13%. Table 6 shows the characterization data of the fuels obtained after different reaction times. Seemingly, hydroisomerization, unlike hydrocracking, goes on with the time because the amount of isoparaffins increases, reaching 53.3% after 180 min of reaction, while n-paraffins drop continuously from 37.7 to 25.5%. This result bears out the previous finding about the good performance of Ni/h-Beta in promoting hydroisomerization reactions. A slight increase in naphthenes can also be observed after 90 and 180 min. On the other hand, the amount of aromatics remained rather stable with the time, reaching values in the range 17−20% after 45 min of reaction. The bromine index displayed a curious evolution because it decreased initially with the time, reaching a value of 2.4 at 45 min, increasing subsequently up to around 8−9. This fact is an indication of the higher difficulty of hydrogenating the olefins formed at long reaction times, probably because of the steric hindrance caused by its highly branched nature. In view of these data, it seems that the best reaction time is roughly 45 min because longer reaction times gave rise to an enhancement in the bromine index of the liquids.

a

Determined by PIONA analysis in gasolines. bDetermined by HPLC in total fuels.

corresponding to the fuels obtained using different catalyst/ feed ratios. The isoparaffin content in the gasolines increased with the catalyst amount, reaching a remarkable 52.7% with a catalyst/feed ratio of 1:10. At the same time, the n-paraffin content decreases from 58.9% with a catalyst content of 1:100 to 24.9% for a 1:10 ratio. These results indicate that Ni/h-Beta is a catalyst with a high activity toward hydroisomerization, which might be of interest for other kinds of processes in addition to the process studied here. On the other hand, the aromatic content also increased with the catalyst/feed ratio, although in this case a limit value around 20% appears to be attained for catalyst/feed ratios in the range of 1:30−1:10. The bromine index indicated that the hydrogenation of the fuels was quite significant even with the lowest catalyst/feed ratio (1:100), wherein 88% of the starting olefins was saturated. A trend to increase the olefin hydrogenation with the catalyst amount is observed, reaching at most 97% for a catalyst/feed ratio of 1:10. The quality of the fuels is fairly high, because the 3193

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Table 6. Characterization Data of the Fuels Obtained at Different Reaction Times (P = 20 bar of Hydrogen; T = 310 °C; Catalyst/Feed Mass Ratio = 1:30; and N = 700 rpm) n-paraffins (wt %)a isoparaffins (wt %)a naphthenes (wt %)a aromatics (wt %)b bromine index a

feed

t = 15 min

T = 45 min

t = 90 min

t = 180 min

35.1 18.7 6.0 1.7 54.1

37.7 35.2 5.3 12.4 3.4

35.5 39.5 5.9 20.5 2.4

33.5 40.3 9.0 17.9 9.0

25.5 53.3 9.8 17.5 7.8

Determined by PIONA analysis in gasolines. bDetermined by HPLC in total fuels.

Notes

4. CONCLUSION The influence of the main reaction variables (temperature, time, hydrogen partial pressure, and feed/catalyst mass ratio) over the product distribution obtained in the hydroreforming of the oil coming from LDPE thermal cracking was investigated over a Ni/h-Beta catalyst, with the goal of upgrading its quality as transportation fuels. This catalyst showed bifunctional properties with a bimodal micro-/mesopore pore size distribution, because of the hierarchical porosity of the zeolite, and containing 7% Ni. The characterization data indicated the presence of Ni particles inside the micropores as well as in the mesopores and over the external surface. The presence of nickel in the catalysts dramatically reduces the olefin content of the feed, avoiding oligomerization reactions and largely increasing the yield of gasolines in the products. The enhancement in the reaction temperature led toward increased hydrocracking because the amount of gasolines augmented from 250 to 350 °C. Light diesel (C13− C18) was cracked more deeply than heavy diesel (C19−C40), likely because of their faster diffusion into the zeolite micropores. Higher temperatures promoted the extension of aromatization reactions, especially when working above 300 °C. The increase in the hydrogen pressure up to 40 bar was very effective in hydrogenating olefins and also promoted the hydrocracking, giving rise to more gasolines, although it suppressed in a large proportion the formation of aromatic hydrocarbons. The increase in the catalyst/feed mass ratio from 1:100 to 1:10 enhanced the extent of hydrocracking with increasing yields of gasoline. The extent of hydroisomerization reactions increased with the reaction time, leading toward the highest isoparaffin yields after 180 min of reaction. From this study, it can be concluded that the Ni/h-Beta catalyst is especially appropriate for obtaining gasolines, with the most adequate conditions to maximize its selectivity (up to 68.7%) being the hydroreforming at 40 bar of hydrogen pressure. In terms of the amount and quality of the fuels obtained, the most adequate conditions are the following conditions: T = 310 °C; t = 45 min; catalyst/feed ratio = 1:30; and hydrogen pressure = 20 bar. On the other hand, regardless of the experimental conditions used, more than 80% of the olefins present in the raw oil were saturated during the hydroreforming treatment. In addition, the RON of the gasolines and cetane indexes of the diesel confirmed the good quality of the obtained fuels. Therefore, Ni/h-Beta constitutes a potential catalyst for the upgrading by hydroreforming of the hydrocarbons attained in the LDPE thermal cracking for its usage as transportation fuels.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Spanish Ministry of Science and Innovation (Project TRACE TRA2009-0111) and URBASER for their financial support to this research.

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