Catalytic Conversion of Polyolefins into Liquid Fuels over MCM-41

Energy & Fuels 1997, 11, 1225-1231

1225

Catalytic Conversion of Polyolefins into Liquid Fuels over MCM-41: Comparison with ZSM-5 and Amorphous SiO2-Al2O3 J. Aguado,* J. L. Sotelo, D. P. Serrano, J. A. Calles, and J. M. Escola Chemical Engineering Department, Faculty of Chemistry, Complutense University of Madrid, 28040 Madrid, Spain Received April 8, 1997X

The catalytic degradation of both low- and high-density polyethylene (LDPE and HDPE) and polypropylene (PP) has been investigated using MCM-41, a mesoporous aluminosilicate recently discovered, as catalyst. The results obtained have been compared to those of ZSM-5 zeolite and amorphous silica-alumina. For all the studied plastics, MCM-41 has been found more active than the amorphous SiO2-Al2O3, as a consequence of the higher surface area and the uniform mesoporosity present in the former. Compared to ZSM-5, MCM-41 exhibits a lower activity for the degradation of linear and low branched polymers (HDPE and LDPE, respectively), which can be related to the higher strength of the zeolite acid sites. However, the opposite is observed for the cracking of highly substituted plastics such as PP due to the severe steric hindrances these molecules encounter to enter into the narrow pores of the zeolite, as confirmed by molecular simulation measurements. Moreover, for the cracking of LDPE, HDPE, and PP, the selectivities toward hydrocarbons in the range of gasolines and middle distillates obtained over MCM-41 are clearly higher than those of ZSM-5. Therefore, MCM-41 is a catalyst potentially interesting for the conversion of polyolefinic plastic wastes into liquid fuels.

Introduction The output of plastic materials has continuously increased during the past decades, leading to a parallel rise in the generation of plastic wastes. The public concern over the environmental impact caused by polymeric wastes, and the economic interest for a profitable use of such abundant source of chemicals and energy, are the main driving forces of the current research activities aimed at the development of plastic recycling processes. Melting of mechanically granulated plastic wastes in order to reuse them in molding applications and energy recovered by incineration are at present the most established alternatives for plastic recycling.1,2 However, the former can be only applied to pure and highquality wastes whereas incineration suffers from a strong social rejection due to possible contribution of the effluent gases to the air pollution. Chemical recycling of plastic wastes includes a number of processes and technologies directed toward their conversion into the raw monomers or valuable feedstocks, which are viewed as very promising alternatives.3 Most of the petrochemical companies are involved in a variety of research projects for the development of chemical routes of plastic recycling.4-6 For condensation polymers, such as polyesters, polyamides, and polyurethanes, the starting monomers can be * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, October 1, 1997. (1) Babinchak, S. CHEMTECH 1991, 21 (12), 728. (2) Rowatt, R. J. CHEMTECH 1993, 23 (1), 56. (3) Layman, P. Chem. Eng. News 1993, 71 (41), 11. (4) Hirota, T.; Fagan, F. N. Makromol. Chem., Macromol. Symp. 1992, 57, 161. (5) Miller, A. Environ. Sci. Technol. 1994, 28, 16A. (6) Layman, P. Chem. Eng. News 1994, 72 (13), 19.

S0887-0624(97)00055-8 CCC: $14.00

recovered by hydrolysis, methanolysis, and glycolysis. On the contrary, addition polymers such as polyolefins need stronger thermal or catalytic treatments to be converted back into hydrocarbon mixtures useful as fuels or petrochemical feedstocks.7 Thermal degradation of polyolefins is usually a low selective process, leading to a wide distribution of waxy products8 that must be upgraded by a subsequent catalytic reforming. A remarkable exception is the thermal cracking of polystyrene which yields approximately 70-90% of styrene monomer.9 The catalytic cracking by contacting directly the polyolefins with a catalyst, usually an acid solid, is an interesting alternative, which allows to increase the conversion and to obtain high-quality products. Different works have been published using mainly amorphous SiO2-Al2O3 and different types of zeolites as catalysts.10-15 However, due to the bulky nature of the plastic molecules, the cracking reaction is strongly controlled by the pore size of the catalysts. Amorphous SiO2-Al2O3 usually present a wide distribution of pore radius whereas zeolites are microporous materials with pores in the range 0.4-0.8 (7) Kastner, H.; Kaminsky, W. Hydrocarbon Process. 1995, May, 109. (8) Wampler, T. P. J. Anal. Appl. Pyrol. 1989, 15, 187. (9) Audisio, G.; Bertini, F. J. Anal. Pyrol. 1992, 24, 61. (10) Uemichi, Y.; Kashiwaya, Y.; Tsukidate, M.; Ayame, A.; Kanoh, H. Bull. Chem. Soc. Jpn. 1983, 56, 2768. (11) Vasile, C.; Onu, P.; Barboiu, V.; Sabliovschi, M.; Moroi, G. Acta Polym. 1985, 36 (10), 543. (12) Vasile, C.; Onu, P.; Barboiu, V.; Sabliovschi, M.; Moroi, G.; Ganju, D.; Florea, M. Acta Polym. 1988, 39 (6), 306. (13) Ishihara, Y.; Nanbu, H.; Ikemaru, T.; Takesue, T. Fuel 1990, 69, 978. (14) Audisio, G.; Bertini, F.; Beltrame, P. L.; Carniti, P. Makromol. Chem., Macromol. Symp. 1992, 57, 191. (15) Lin, R.; White, R. L. J. Appl. Polym. Sci. 1995, 58, 1151.

© 1997 American Chemical Society

1226 Energy & Fuels, Vol. 11, No. 6, 1997

nm, which hinders the access of the polymer molecules into the zeolite channels and cavities. In fact, several authors have suggested that the initial steps of the polyolefin degradation over zeolites take place mainly on the acid sites located on the external surface of the zeolite crystals.10,16 The same mechanism seems to account for the reforming of heavy oil from waste plastics over zeolites.17 In this paper, following a previous work,18 we report the results obtained in the catalytic cracking of low- and high-density polyethylene (LDPE and HDPE, respectively) and polypropylene (PP) using MCM-41 as a catalyst to yield liquid fuels. This material is a mesoporous silicate/aluminosilicate and first prepared in 1992 in the presence of alkyltrimethylammonium surfactants.19,20 It is characterized by having uniform mesopores between 2 and 10 nm, whose size can be adjusted by changing the synthesis conditions. Therefore, MCM-41 is a potentially interesting catalyst for the conversion of bulky substrates. The catalytic activity and product distribution obtained with this material in the conversion of polyolefinic plastic into feedstocks are discussed in this work by comparison to those corresponding to amorphous SiO2-Al2O3 and ZSM-5 zeolite samples. Experimental Section Catalyst Preparation. The MCM-41 sample was synthesized according to a previously published procedure.20 Thereby, a solution with 4.42 g of Cab-O-Sil silica and 1.67 g of sodium hydroxide in 44 g of deionized water was added with stirring to a second one formed by 0.19 g of sodium aluminate, 7.48 g of cetyltrimethylammonium bromide (CTMABr), and 22 g of deionized water. The mixture so obtained was loaded into a Teflon-lined autoclave and kept at 120 °C for 2 days in static conditions. The synthesis product was separated by filtration, washed with deionized water, dried at 110 °C, and calcined at 550 °C in static air for 14 h. The acidic form of MCM-41 was obtained by three times repeated ion exchange with 1 M NH4Cl aqueous solution at room temperature for 1 h, followed by calcination at 550 °C. ZSM-5 zeolite was prepared from ethanol-containing gels at 170 °C for 24 h according to the method described elsewhere.21 After the synthesis, the zeolite sample was ionexchanged with a 0.6 M HCl aqueous solution and then calcined at 550 °C for 14 h. The amorphous silica-alumina was prepared by the solgel route following a two-step method. In the first one, 16 g of tetraethyl orthosilicate was hydrolyzed with 10 g of 0.2 M aqueous HCl at room temperature for 45 min. Once the initially two phase system became monophasic, a solution containing 0.523 g of aluminum isopropoxide in 7 g of isopropyl alcohol was added and the mixture was stirred for 10 min to complete the hydrolysis of the Si and Al alkoxides. In the second step, the gel point was reached by dropwise addition of 21 wt % aqueous ammonia solution. The cogel so obtained (16) Mordi, R. C.; Fields, R.; Dwyer, J. J. Chem. Soc., Chem. Commun. 1992, 374. (17) Songip, A. R.; Masuda, T.; Kuwahara, H.; Hashimoto, K. Appl. Catal. B: Environ. 1993, 2, 153. (18) Aguado, J.; Serrano, D. P.; Romero, M. D.; Escola, J. M. Chem. Commun. 1996, 725. (19) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (20) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (21) Uguina, M. A.; de Lucas, A.; Ruiz, F.; Serrano, D. P. Ind. Eng. Chem. Res. 1995, 34, 451.

Aguado et al. was dried at 110 °C overnight and activated by calcination at 550 °C for 14 h. Catalyst Characterization. The chemical composition of the samples was determined by X-ray fluorescence (XRF) with a Philips PW 1404 spectrometer. X-ray diffraction patterns were collected with a Philips XPÄ ERT MPD diffractometer with Cu KR radiation and Ni filter. High-resolution 27Al magic angle spinning nuclear magnetic resonance (MAS-NMR) spectra of the MCM-41 samples were recorded at 104.26 MHz, using a Bruker MSL-400 spectrometer equipped with a Fourier transform unit. The spinning frequency was 4000 cps with time intervals of 5 s between successive accumulations. The measurements were carried out at room temperature with [Al(H2O)6]3+ as external standard reference, the accumulations being amounted to 2000 and 400 FIDs, respectively. The N2 adsorption-desorption isotherms were measured at 77 K on a Micromeritics ASAP 2010 instrument using standard adsorption techniques. The samples were previously outgassed by treatment at 200 °C for 5 h under vacuum. The surface area was calculated using the BET equation whereas the pore size distribution was determined by applying the BJH model with the Harkins and Jura equation assuming a cylindrical pore geometry. For ZSM-5 sample, the micropore volume and the external surface area were determined by means of the t-plot method. NH3 thermal programmed desorption (TPD) measurements were performed on a Micromeritics 2900 apparatus equipped with a thermal conductivity detector. The samples were first treated in an He stream at 560 °C and thereafter saturated with NH3 at 180 °C for 30 min. The physically adsorbed NH3 was removed by flowing He (50 mL(STP)/min) through the sample at 180 °C for 90 min. Finally, the ammonia TPD was carried out by increasing the temperature up to 550 °C with a heating rate of 15 °C/min, the NH3 concentration in the effluent being continuously monitorized. Catalytic Tests. The polyolefins used in this work were provided by REPSOL, and had the following features: LDPE (average molecular weight, Mw ) 416 000), HDPE (Mw ) 188 000), and PP (Mw ) 450 000, isotacticity index ) 93%). The thermal and catalytic cracking of these polyolefins were performed at 400 °C and atmospheric pressure in a batch reactor with continuous N2 flow (25 mL(STP)/min). In each experiment, 1.2 g of plastic was loaded into the reactor and mixed with the appropriate amount of catalyst. Previously, the catalysts were ground and sieved until obtained a particle size below 74 µm. The reactor was heated to the desired reaction temperature in 15 min, which was kept constant for a period of 30 min. During this time, the liquid and gaseous products coming out from the reactor were separated in a condenser and accumulated to determine their composition by gas chromatography. The gaseous products were analyzed with a Hewlett-Packard 5880 GC on a Porapak Q column, whereas the composition of the liquid products was determined with a Perkin-Elmer 8310 GC using a 25 m long BP-5 capillary column. Molecular Simulation. Molecular simulation techniques were applied to estimate the effective cross diameter of the polymeric chains and to determine whether oligomeric fragments of the polyolefins used in this work can be adsorbed and accommodated within the pore network of the ZSM-5 zeolite. The calculations were performed using the Cerius2 program, developed by BIOSYM/Molecular Simulations. The structure of oligomers with a chain length of 20 carbon atoms was generated with a molecular mechanics force field, universal force field (UFF), including a parametrized description of the full periodic table, which has been proved to reproduce the structure of many organic and inorganic compounds.22,23 The partial charges in the atoms of the molecules (22) Rappe´, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard III, W. A.; Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024.

Conversion of Polyolefins into Liquid Fuels over MCM-41

Energy & Fuels, Vol. 11, No. 6, 1997 1227

were calculated by a charge equilibration method,24 followed by an energy minimization process in order to determine the most stable configuration of the molecule. The effective diameters of the cross section in the oligomers were estimated as the distance between the atomic centers plus their van der Waals radii. This diameter was calculated at the points in the oligomeric chain having the maximum cross section. Molecular simulations oligomers adsorption on the ZSM-5 structure were performed at fixed pressure (101.33 kPa), temperature (673 K), and volume in the grand canonical ensemble by the Monte Carlo technique (GCMC).25 A number of 12-unit cells of MFI topology were taken as simulation box. The interactions adsorbate-adsorbent and adsorbate-adsorbate were modeled by Lennard-Jones 12-6 potential and the values of the corresponding parameters were taken from the literature.26 Partial charges were also placed in the zeolite atoms (Si +2; O -1) and Coulombic interactions were also taken into account. The average loading of the oligomers on the ZSM-5 unit considered was determined with 107 configurations.

Results and Discussion Physicochemical Properties of the Catalysts. The different catalysts used in this work for the degradation of polyolefinic plastics were characterized by several techniques in order to gain information about the properties which are mainly responsible for their catalytic activity: porous structure, state of the Al atoms, and concentration and strength of the acid sites. This characterization was carried out in more detail on the MCM-41 sample, since this is a new and less studied material. XRD spectra of the as-synthesized MCM-41 catalyst exhibit a single strong reflection corresponding to the d100 spacing at 3.9 nm, which is typical of this kind of mesoporous materials.19 TG analysis in air of the assynthesized MCM-41 leads to a 49% weight loss between 200 and 550 °C due to the decomposition and removal of the surfactant molecules occluded within the pores. This value is in agreement with the data previously reported in the literature.27 The maximum of the major peak in the differential curve is located at ca. 250 °C, showing that the surfactant can be completely removed by calcination of the sample at 550 °C. After this thermal treatment, the position of the XRD reflection decreases by 0.35 nm due to a contraction of the structure. The N2 adsorption isotherm at 77 K and the pore size distribution of the MCM-41 are compared in Figure 1, a and b, respectively, to those of the amorphous SiO2Al2O3. The surface area and pore volume of the catalysts are given in Table 1. For the MCM-41 sample, most of the N2 adsorption takes place at relative pressures ranging between 0.2 and 0.6, with an inflection point at p/p0 ) 0.3 which is a typical feature of the uniform mesoporosity of this material.19 The high surface area of MCM41 is remarkable, while the pore size distribution shows the presence of well-defined pores with 2.9 nm diameter. In contrast, the surface (23) Casewit, C. J.; Colwell, K. S.; Rappe´, A. K. J. Am. Chem. Soc. 1992, 114, 10035. (24) Rappe´, A. K.; Goddard III, W. A. J. Phys. Chem. 1991, 95, 3358. (25) Allen, M. P.; Tildesley, D. J. Computer Simulations of Liquids; Oxford University Press Inc.: New York, 1996. (26) Yashonath, S.; Thomas, J. M.; Novak, A. K.; Cheetham, A. K. Nature 1988, 311, 601. (27) Chen, C. Y.; Li, H. X.; Davis, M. E. Microporous Mater. 1993, 2, 17.

Figure 1. N2 adsorption at 77 K on the MCM-41 and the amorphous SiO2-Al2O3 samples: (a, top) adsorption isotherms, (b, bottom) pore size distribution. Table 1. Properties of the Catalysts N2 adsorption (77 K) NH3 TPD (180-550 °C) Si/Al

SBET (m2/g)

VP (cm3/g)a

MCM-41 42.7 SiO2-Al2O3 35.6 ZSM-5 31.0

1327 261 362

1.51 0.97 0.18

catalyst

a

acidity Tmax (mmol/g) (°C) NH3/Al 0.30 0.24 0.52

314 302 470

0.79 0.53 1.00

Total pore volume measured at p/p0 ) 0.995.

area of the amorphous SiO2-Al2O3 sample is appreciably lower, exhibiting a very wide distribution of pore sizes. In fact, this material presents pores with sizes in the whole range between 4 and 110 nm, with three maxima around 10, 45, and 100 nm, denoting the irregularity of its pore structure. The Al content of the catalysts is also given in Table 1. The Si/Al ratio of the MCM-41 sample is higher than the one corresponding to the starting synthesis gel (Si/Al ) 30), showing the incorporation of the Al atoms into this structure is less favored than that of the Si species. An important matter is the state of the Al atoms in the MCM-41 sample since different authors have reported that, depending on the Al source used in the synthesis and/or the activation conditions, octahedral extraframework Al species can be formed.27-29 Figure 2 illustrates the 27Al MAS NMR spectra of both the as-synthesized and the activated MCM-41 samples.


Figure 2. 27Al MAS NMR spectra of the MCM-41 before and after activation.

Figure 3. NH3 TPD of the ZSM-5, MCM-41, and amorphous SiO2-Al2O3 samples.

In the first case, an unique peak with a chemical shift of 53 ppm is observed proving that all the Al atoms are tetrahedrally coordinated in the as-synthesized sample. However, after the activation of the sample by calcination at 550 °C to remove the surfactant, ion exchange of Na+ by NH4+, and final calcination at 550 °C to generate the acid form, a peak at 0 ppm appears, which denotes the presence of Al atoms with octahedral coordination, probably due to the formation of extraframework species. Nevertheless, the proportion of octahedral Al in the calcined protonic MCM-41 is estimated to be just around 5% from the corresponding peak area in the NMR spectra. The acidic properties of the catalysts were tested by NH3 TPD between 180 and 550 °C. The desorption curves obtained are compared in Figure 3, while the amount of adsorbed-desorbed ammonia and the position of the TPD maximum are shown in Table 1. It is interesting to note the similarity between the TPD (28) Janicke, M.; Kumar, D.; Stucky, G. D.; Chmelka, B. F. Stud. Surf. Sci. Catal. 1994, 84, 243. (29) Luan, Z.; Cheng, C. F.; Zhou, W.; Klinowski, J. J. Phys. Chem. 1995, 99, 1018.

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curves corresponding to the MCM-41 material and the amorphous SiO2-Al2O3 sample with desorption maximum at 314 and 302 °C, respectively, which are quite lower than the one observed in the ZSM-5 zeolite (470 °C). These results confirm that the acid sites in MCM41 are weaker than those of ZSM-5 and very close in strength to the acid sites of amorphous materials. In fact, it has been reported that, in spite of this ordered and regular porosity, the pore walls of MCM-41 are amorphous, without any crystalline order of the SiO4 and AlO4- tetrahedra.27 The ratio between the adsorbed-desorbed NH3 and the Al content varies greatly among these samples. For ZSM-5 this ratio is unity, indicating that all the Al atoms are associated to acid sites accessible to the ammonia molecules. The values of this ratio are 0.79 and 0.53 for MCM-41 and the amorphous SiO2-Al2O3 sample, respectively. These differences may be due to several reasons concerning the MCM-41 acidity: (a) presence of extraframework Al species, (b) existence of very weak acid sites that release the adsorbed NH3 below the starting temperature in the TPD experiment (180 °C), (c) different nature of the acid sites present in each sample, and (d) location of a part of the Al atoms within the pore walls, in positions inaccessible to the NH3 molecules. In this way, it has been recently reported that the acid sites present in MCM-41 materials are both Bro¨nsted and Lewis sites with a high proportion of the latter,30-32 whereas it is known that the acidity of ZSM-5 samples is mainly related to Bro¨nsted sites. The adsorption of ammonia on these two types of acid sites may proceed with a different stoichiometry and/or take place in a different temperature range, which could explain the observed variation in the ratio between the adsorbed NH3 and the Al content. The width of the pore wall in MCM-41 can be estimated from both XRD and N2 adsorption data. According to the XRD d100 spacing, the hexagonal unit cell parameter (a0 ) 2d100/x3) of the calcined MCM-41 sample is 4.1 nm which, after subtraction of the pore diameter, leads to a thickness of the pore walls of 1.2 nm. This result agrees well with values earlier reported in the literature for MCM-41 materials. Thus, Chen et al.33 have concluded that a thickness of 1.0 nm is enough to include between two and three monolayers of TO4 tetrahedra along the pore wall. Likewise, a model for MCM-41 has been developed by molecular dynamics simulation34 with wall thickness around 1 nm, showing the presence of several TO4 tetrahedra along the wall, the middle ones being not in contact with the pores. If it is assumed that the Al atoms are randomly distributed along the pore wall, it means that a percentage of them will not be accessible to the NH3 molecules. We think this is the major reason for the discrepancy observed between the NH3 TPD measurements and the Al content of MCM-41, in addition to the presence of around 5% of extraframework Al. In the case of the (30) Busio, M.; Ja¨nchen, J.; van Hooff, J. H. C. Microporous Mater. 1995, 5, 211. (31) Corma, A.; Grande, M. S.; Gonzalez-Alfaro, V.; Orchilles, A. V. J. Catal. 1996, 159, 375. (32) Mokaya, R.; Jones, W.; Luan, Z.; Alba, M. D.; Klinowski, J. Catal. Lett. 1996, 37, 113. (33) Chen, C. Y.; Burkett, S. L.; Li, H. X.; Davis, M. E. Microporous Mater. 1993, 2, 27. (34) Feuston, B. P.; Higgins, J. B. J. Phys. Chem. 1994, 98, 4459.



Figure 4. Carbon distribution in the products of the HDPE cracking (400 °C, 0.5 h, plastic/catalyst ) 18 w/w).

Figure 5. Carbon distribution in the products of the LDPE cracking (400 °C, 0.5 h, plastic/catalyst ) 18 w/w).

Table 2. Catalytic Cracking of HDPE (400 °C, 0.5 h, plastic/catalyst ) 18 w/w)

contrary, the mesopores present in MCM-41 and the amorphous SiO2-Al2O3 should favor the diffusion and subsequent reaction of polyethylene. However, the order of activities observed suggests that the major factor influencing the polyethylene cracking is the acidity rather than the pore size of the catalysts. The most active catalyst is the ZSM-5 zeolite, which presents the highest and strongest acidity. On the other hand, the superior activity of MCM-41 compared to the amorphous SiO2-Al2O3 can be mainly assigned to the higher surface area of the former, since both materials present similar acid sites. It is interesting to note that the differences in catalytic activity among the three samples are shortened when going from HDPE to LDPE. For the same reactions conditions, the catalytic cracking of LDPE proceeds faster than the degradation of HDPE. This effect is more evident when using MCM-41 and the amorphous SiO2-Al2O3 as catalysts and it is a consequence of the structure of these polymers. High-density polyethylene is typically formed by linear macromolecules, whereas low-density polyethylene is characterized by a certain degree of branching. The presence of tertiary carbons in LDPE provides favorable positions for the initiation of the polymer chain cracking since their activation, by hydride abstraction or by addition of a proton, does not require a very strong acidity. These reactions can be easily catalyzed by mild acid sites, such as those present in MCM-41 and amorphous SiO2-Al2O3 samples. Great differences are also observed among the catalysts regarding the product distribution. Over ZSM-5 zeolite the cracking of both HDPE and LDPE leads to a high proportion of gaseous hydrocarbons rich in olefins. In contrast, the main products obtained with MCM-41 and amorphous SiO2-Al2O3 are liquid fractions with boiling points in the range of gasolines (C5-C12) and middle distillates (C13-C22), suggesting that the product selectivities are well correlated with the pore size distribution of each catalyst. In this way, it must be pointed out that the upper limit observed for the product distribution corresponding to the polyethylene cracking over ZSM-5 (C12) is close to the largest molecules which are typically formed within its pore system. This fact suggests that over the ZSM-5 sample used in this work,

product selectivity (%) catalyst

conv C1-C4 C2-C4 olef C5-C12 C13-C22 C23-C40 (%) paraf

MCM-41 35.2 SiO2-Al2O3 9.2 ZSM-5 96.4

14.6 15.9 13.2

20.0 19.6 38.9

52.6 50.1 46.3

12.6 13.1 1.5

0.2 1.3 0.1

Table 3. Catalytic Cracking of LDPE (400 °C, 0.5 h, plastic/catalyst ) 18 w/w) product selectivity (%) catalyst


MCM-41 67.6 SiO2-Al2O3 34.4 ZSM-5 95.4

6.9 9.4 19.8

12.6 14.9 32.8

63.9 55.8 44.6

16.3 18.9 2.3

0.3 1.0 0.5

amorphous SiO2-Al2O3, its lower surface area must lead to a lower proportion of accessible Al atoms, explaining the decrease in the NH3/Al ratio compared to MCM-41. Catalytic Cracking of Polyethylene. The catalytic degradation of both high- and low-density polyethylene has been investigated at 400 °C in a batch reactor with a duration of the experiments of 30 min. Under these conditions, the conversions obtained in blank experiments without catalyst were almost negligible (less than 1%), showing that the thermal cracking of these polymers at 400 °C is very slow. In both thermal and catalytic tests, the plastic conversion values have been calculated by considering only those products having boiling points low enough to leave the reactor in the N2 stream, which accounts for hydrocarbons with a total number of carbons up to approximately 40. Heavier cracking products, remaining in the reactor, were not considered in calculating the plastic conversion. Tables 2 and 3 summarize the results obtained in the catalytic cracking of HDPE and LDPE over the different catalysts, while the product distributions per carbon atom number are shown in Figures 4 and 5, respectively. For both polymers, the highest activity is observed with the ZSM-5 zeolite with conversions close to 100%. This catalyst has pores with diameters around 0.55 nm, and hence it was expected that the access and intracrystalline diffusion of the bulky plastic molecules to the internal acid sites are strongly hindered. On the


Aguado et al.

Table 4. Catalytic Cracking of PP (400 °C, 0.5 h, plastic/ catalyst ) 36 w/w) product selectivity (%) catalyst


MCM-41 99.2 SiO2-Al2O3 47.9 ZSM-5 11.3

3.9 4.0 22.3

10.7 9.8 27.0

65.0 51.5 50.0

20.0 34.1 0.7

0.4 0.6 0.0

the polyethylene cracking takes place mainly within the pores rather than on the external surface of the crystals. Two maxima are present in the product distribution per carbon atom number obtained with the three catalysts investigated. In the HDPE catalytic conversion, the first maximum is observed for C4 hydrocarbons over the three catalysts, whereas in the degradation of LDPE over MCM-41 and amorphous SiO2-Al2O3 it is a less pronounced maximum and displaced to C5 species. According to earlier works, the catalytic degradation of polyolefins proceeds through two main ionic mechanisms:11,15,35,36 (i) cracking at the end groups of the polymeric chains, and (ii) random cleavage of the polymer molecules at any bond in the chain. In both cases, the starting molecules may be intact polymer molecules or partially cracked polyolefins (oligomers). Simultaneously to the catalytic degradation, the thermal cracking of the plastic molecules into oligomers may also take place through a radical mechanism. C3-C5 hydrocarbons are the main products expected from the first catalytic pathway, which originates the first maximum present in the carbon distribution. The rest of the products observed can be formed by random cleavage of any bond in the chain or from the initial products of the end chain cracking through secondary reactions such as oligomerization, aromatization, etc. Thus, since the second maximum of the carbon distribution is observed in the range C6-C9, it is probably originated by the oligomerization of the C3-C5 olefins. Catalytic Cracking of Polypropylene. The catalytic conversion of polypropylene has been also investigated at 400 °C, as in the case of polyethylene. One significant difference between the thermal behaviour of these two plastics is that the thermal degradation of polypropylene is faster, as denoted by a certain conversion (around 8%) observed in experiments without catalyst after 30 min of reaction. The results obtained in the catalytic degradation of polypropylene are shown in Table 4 and Figure 6. In this case the order of activity is clearly different from that corresponding to polyethylenic plastics. Thus, the MCM-41 sample leads to almost 100% conversion whereas the activity obtained with the ZSM-5 zeolite is very close to that of the thermal cracking. Even the conversion with the amorphous SiO2-Al2O3 is quite higher than the observed with the zeolite in spite of the highest surface area and stronger acidity of the latter. Moreover, as has been commented for the thermal cracking, the polypropylene conversion over MCM-41 and amorphous SiO2-Al2O3 is quite faster than the degradation of both high- and low-density polyethylene. Note that these catalysts present PP conversions higher (35) Ivanova, S. R.; Gumerova, E. F.; Minsker, K. S.; Zaikov, G. E.; Berlin, A. A. Prog. Polym. Sci. 1990, 15, 193. (36) Ishihara, Y.; Nanbu, H.; Saido, K.; Ikemura, T.; Takesue, T. Bull. Chem. Soc. Jpn. 1991, 64, 3585.

Figure 6. Carbon distribution in the products of the PP cracking (400 °C, 0.5 h, plastic/catalyst ) 36 w/w).

than those observed for HDPE and LDPE in spite of using a plastic/catalyst ratio 2-fold higher. All these results are related and can be explained by the existence of a high proportion of tertiary carbons in the polypropylene chains, which are very reactive to undergo both thermal and catalytic cracking reactions. In fact, half of the carbon atoms in the polypropylene chains are tertiary due to the presence of side-chain methyl groups. An important conclusion can be derived from the practical absence of activity of the ZSM-5 sample compared to just the thermal cracking, since it can be assigned to steric and/or diffusion hindrances. The presence of the side-chain methyl groups increases the effective cross section of the polypropylene molecules compared to the polyethylenic chains, which may prevent their access to the active sites located within the zeolite pores. By molecular simulation on oligomeric units formed by 20 chain carbons, it has been possible to calculate the effective cross-section diameter of these molecules. We have taken as effective diameter the maximum atom distance measured perpendicularly to the longitudinal chain axis. The results obtained are as follows: 0.55 nm for the polyethylene oligomer, 0.64 nm for the isotactic polypropylene oligomer, and 0.73 nm for the syndiotactic polypropylene oligomer. Compared to the crystallographic pore size of the ZSM-5 zeolite (0.55 nm), those numbers show the difficulty for both types of polypropylene to enter and diffuse through the zeolite channels. Nevertheless, it has to be taken into account that neither the molecules nor the zeolite structure are completely rigid but they can be deformed to certain extension by thermal vibration, which explains that molecules somewhat larger than the crystallographic pore diameter may have access to the internal zeolite volume. In order to address this possibility, the adsorption of the polymer oligomers on the MFI structure at different temperatures has been predicted by molecular simulation. The results obtained show that the C20 polyethylene oligomer can be accommodated within the straight ZSM-5 channels, whereas the adsorption capacity for both polypropylene oligomers is zero, confirming the latter cannot enter the zeolite pores. At the same time, this result implies that the contribution to the plastic cracking of the acid sites present



on the external surface of the crystals in the ZSM-5 sample used in this work is negligible. Therefore, the high conversion obtained with this material in the HDPE and LDPE cracking takes place by penetration of polymer or oligomer molecules, at least up to a certain chain length, into the zeolite pores and subsequent reaction over the internal acid sites. This conclusion agrees with the above commented relationship between the largest compounds observed in the product distribution of HDPE and LDPE cracking over ZSM-5 and the maximum size of the molecules that are usually formed within the zeolite pores. On the contrary, if the initial cracking takes place on the zeolite external acid sites, a wider product distribution should be observed and the polypropylene cracking should proceed in a higher extension. Nevertheless, it has to be taken into account that the relative proportion and the catalytic contribution of the external surface of zeolites greatly depends on the zeolite crystal size. In our case, the ZSM-5 sample presents an average crystal size around 5 µm and, according to the t-plot method applied to the N2 adsorption isotherm, it has 7.1 m2/g of external surface area, which means that approximately just a 2% of the total surface area is external. It cannot be discarded that, if ZSM-5 samples with lower crystal sizes are used, the contribution of the external acidity to the polyolefins cracking becomes significant. Concerning the product distribution of the polypropylene cracking, the results show similar trends to those obtained in the polyethylene conversion. MCM41 and amorphous SiO2-Al2O3 lead mainly to liquid mixtures of hydrocarbons in the range of gasolines and middle distillates, with a higher proportion of C13-C22 products with the second catalyst, whereas C3-C5 hydrocarbons are the main products formed in the experiment carried out with the ZSM-5 zeolite. Likewise, two maxima at C5 and around C8-C9 are present in the carbon distribution corresponding to both MCM41 and amorphous SiO2-Al2O3 samples. In the last case, the first maximum is less pronounced while the distribution in the range C8-C18 is more uniform, which suggests a lower contribution for this catalyst of the cracking reactions taking place at the end of the chains. It can be interpreted as a consequence of the presence in the amorphous SiO2-Al2O3 of larger pores with irregular sizes and shapes which, besides the existence of a high proportion of highly reactive tertiary carbons, favors the random cracking at any position in the chain. On the contrary, the MCM-41 sample is formed by onedimensional and regular pores with diameters around 2.8 nm. Then, the plastic and/or oligomer molecules are probably accommodated in a linear configuration along the pores, exposing preferably the end of the chains to the active sites as they diffuse through the channels.

to the amorphous SiO2-Al2O3, which is interpreted in terms of its higher surface area and more regular mesoporosity. The differences in conversion among the catalysts are reduced when going from HDPE to LDPE due to the presence of a higher proportion of tertiary carbons in the second polymer, which favors the initiation of the chain cracking. On the other hand, the product distribution obtained with each catalyst is determined mainly by its pore size distribution. Over ZSM-5, almost 50% of the products from both HDPE and LDPE cracking are gaseous hydrocarbons (C2-C4) with a high proportion of olefins, whereas in the case of MCM-41 and amorphous SiO2-Al2O3 the degradation of these polymers leads mainly to liquid fractions with boiling points in the range of gasolines (C5-C12) and middle distillates (C13-C22). In the polypropylene degradation, the order of activity is strongly modified compared to the polyethylenic plastics. The highest activity is observed with the MCM-41 sample, while the conversion obtained with the ZSM-5 zeolite is very close to that of the thermal cracking. With MCM-41 and the amorphous SiO2Al2O3 the conversion of polypropylene is faster than in the case of HDPE and LDPE, which is related to the existence of a high proportion of tertiary carbons in the former. The product distribution of the polypropylene cracking over these catalysts is qualitatively similar to those of HDPE and LDPE with liquid hydrocarbon mixtures as predominant products. The plastic cracking over the ZSM-5 sample used in this work proceeds mainly within the zeolite pores rather than on the external surface, as denoted by the product distribution with hydrocarbons up to C12 obtained in the conversion of HDPE and LDPE and the absence of activity compared to the thermal cracking in the degradation of PP. For both isotactic and syndiotactic polypropylene, molecular simulation measurements indicate that the increase in their cross sections compared to polyethylene, derived from the presence of methyl side groups, hinders their access to the zeolite microporosity. Nevertheless, it cannot be discarded that, if ZSM-5 samples with small crystallites are used, the contribution of the external acid sites becomes significant. All these results show that MCM-41 is a material with promising catalytic properties for the conversion of polymeric wastes into liquid feedstocks. It presents a high surface area and, although its acidity is weaker than that of zeolites, the access of the polymer molecules to the active sites is not hindered as in the latter.

Conclusions In the catalytic cracking of HDPE and LDPE, the highest activity is observed with the ZSM-5 zeolite due to its stronger acidity. The conversions exhibited by the MCM-41 sample are superior than those corresponding

Acknowledgment. This work has been funded by the Comisio´n Interministerial de Ciencia y Tecnologı´a from Spain (Projects CICYT AMB-94/0681 and MAT95/2044E). The authors are also grateful to REPSOL S.A. for providing the plastic samples and to the Research Center of CEPSA in Madrid for the XRF measurements. EF970055V

Catalytic Conversion of Polyolefins into Liquid Fuels over MCM-41

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