Catalytic cracking reactions of polyethylene to light alkanes in ionic liquids

Green Context

The cracking of polyethylene is one of the most important plastic recycling processes. Several methods have been reported for this process including catalytic reactions although the wide range of products can be a problem. Here the novel use of ionic liquids for polyethylene cracking is described. Apart from the inherent advantages of using these involatile liquids (see for example the article on p. 1 of this issue from Rogers’ laboratory), the method is quite selective towards low molecular weight feedstocks—the best possible products from this cracking reaction.

JHC

Summary

The cracking of polyethylene to light alkanes can be achieved in ionic liquid systems, such as 1-ethyl-3-methylimidazolium chloride–aluminium(III) chloride. The major products of the reaction are C₃–C₅ gaseous alkanes (such as isobutane) and branched cyclic alkanes, all of which are useful feedstocks.

Introduction

The catalytic cracking of polyethylene has been used as a process for its recycling and conversion into other valuable products.¹ To date, several different methods have been employed, which include pyrolysis,² catalytic cracking with zeolites or mesoporous materials,^3–5 use of supported chloroaluminate(III) catalysts⁶ and reaction with supercritical water.⁷ Here we report a new method for the cracking of polyethylene, using chloroaluminate(III) ionic liquids, which gives a significantly different spectrum of hydrocarbon products.

Room temperature ionic liquids such as [emim]Cl–AlCl₃ (X = 0.67)† ([emim]⁺ = 1-ethyl-3-methylimidazolium cation, see Fig. 1),⁸ have been found to be promising solvents for a wide range of industrial applications from petrochemicals,⁹ bulk chemicals,^10,11 fine chemicals¹² and the treatment of nuclear waste.¹³

Fig. 1 Structure of the [emim]⁺ cation.

Results and discussion

We have found previously that chloroaluminate(III) ionic liquids were capable of causing isomerisation and cracking reactions of fatty acid derivatives,¹¹ and we considered that similar reactions might occur with high molecular weight alkanes, such as polyethylene. The cracking reactions of polyethylene were investigated by suspending powdered low density polyethylene (LDPE) or high density polyethylene (HDPE) samples in several ionic liquids (typical data are included in Table 1). An acidic co-catalyst was added, such as [emim][HCl₂]¹⁴ (1 mol%) or concentrated sulfuric acid (2 mol%), and the mixtures were stirred at 90–250 °C for 1–6 days. It should be noted that the addition of a proton source to an acidic chloroaluminate(III) ionic liquid results in Brønsted superacidity of the proton.^15,16 The product distributions appeared to be independent of the ionic liquid cation used (ionic liquids tested include 1-butylpyridinium ([C₄py]) chloride–AlCl₃ (X = 0.67), [emim]Cl–AlCl₃ (X = 0.67), 1-butyl-3-methylimidazolium ([bmim]) chloride–AlCl₃ (X = 0.67) and LiCl–AlCl₃ (X = 0.67, and T > 150 °C) for a given temperature and time. Also, the type of proton source made little difference to the outcome of the reaction. For example, concentrated H₂SO₄ gave similar results to [emim][HCl₂] or water for concentrations less than 2 mol%. During the reaction, a gas was evolved from the surface of the polyethylene, which was collected in a cold trap (−196 °C) and analysed by gas chromatography and by ¹H and ¹³C NMR spectroscopy in CDCl₃ (sealed tube). The volatile hydrocarbons isolated were all alkanes and had the chemical formula C_nH_{2n + 2} (see Table 2). The reaction temperature appears to affect the composition of the volatile products formed. Typical compositions are given in Table 2, for reactions carried out at 120 and 200 °C. At the higher temperature, a higher proportion of propane, and less pentanes were formed (Note: gas chromatographic analysis gave no evidence for the formation of methane or ethane in this reaction). It is also interesting to note that the amount of hexanes and heptanes formed was less than 2%. A plausible explanation for this shift to alkanes of lower molecular weight and the increased amount of n-butane as the temperature increases is as follows. These cracking reactions probably involve the formation of carbocationic species,¹⁷ as fragments are broken off from the polyethylene chain. This has a high activation energy barrier. This activation energy is lowered if the cations formed are tertiary cations, as is the case with [C(CH₃)₃]⁺ or [(CH₃)₂CC₂H₅]⁺. The formation of secondary cations (leading to propane or butane) has a higher activation energy and hence lower quantities are observed. As the reaction temperature is raised, it becomes easier to overcome the activation energy barrier to form the secondary carbocations and hence more of the products derived from secondary carbocations are observed. Products derived from primary cations (methane and ethane) are not observed.

Table 1 The reaction of polyethylene in several ionic liquids

Entry	Polymer	Ionic liquid	T/°C	t/h	Yield (%)	Low-volatility to volatile product ratio
a 1 mol% of [emim][HCl2] used as co-catalyst.b Reaction heated to 90 °C for 72 h before being heated to 120 °C.c 2 mol% of concentrated sulfuric acid used as co-catalyst.d Some decomposition to carbon observed due to the high reaction temperature.
1	HDPE	[emim]Cl–AlCl3 (X = 0.67)^a	120^b	72	95	1.2
2	HDPE	[emim]Cl–AlCl3 (X = 0.67)^a	200	72	90	0.8
3	LDPE	[emim]Cl–AlCl3 (X = 0.67^a	120^b	72	68	0.8
4	LDPE	[C4py]Cl–AlCl3 (X = 0.67)^a	120^b	72	62	1.0
5	LDPE	[bmim]Cl–AlCl3 (X = 0.67)^a	120^b	72	60	1.0
6	LDPE	[bmim]Cl–AlCl3 (X = 0.67)^c	200	48	85	0.9
7	LDPE	LiCl–AlCl3 (X = 0.67)^c	250	24	95	0.6^d

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Table 2 Composition of the volatile hydrocarbons in the cracking of polyethylene in Table 1

Volatile alkane product	Composition at 120 °C for entry 1 (%)	Composition at 200 °C for entry 2 (%)	Composition at 200 °C for entry 6 (%)
Propane	10	35	30
2-Methylpropane	55	50	45
2-Methylbutane	20	5	5
Butane	5	10	15
Others	10	5	5

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The low-volatility liquid products of the reaction (resembling turpentine spirit in smell) floated on the surface of the ionic liquid and were collected by washing the ionic liquid with an inert solvent (such as cyclohexane). The amount of low-volatile products formed appeared to show a slight temperature dependence. At higher temperatures, more volatile products were formed, as would be expected, because of the increase in entropy due to the formation of gaseous products. Analysis of this low-volatility liquid by ¹H and ¹³C NMR spectroscopy showed that it was composed of a large number of alkanes. The spectra suggested that the alkanes possessed cyclic structures and that a considerable amount of branching had occurred. These products typically consisted of a large number of hydrocarbons that contain features such as cyclohexyl and cyclopentyl rings, methyl groups, attached to aliphatic rings, isopropyl groups and short CH₂ chains. Integration of the ¹H NMR spectra revealed that approximately 33% of the hydrogen atoms occurred in terminal methyl groups (mostly as doublets at 0.85 to 0.90 ppm), and the remainder were in methylene or methine groups. The low-volatile alkanes were further analysed by GCMS to identify individual components of the mixture. There is strong evidence here for the presence of numerous polycyclic aliphatic hydrocarbons, with several fused rings per molecule. The molecular weights of the products were all above 162, which corresponds to C₁₀H₁₈ (perhydronaphthalene). It is of interest to note that over 10 isomeric compounds with the formula C₁₇H₂₈ were detected. This formula corresponds to the aliphatic backbone of many steroids. Based upon the NMR and GCMS evidence, possible structures as shown in Fig. 2 would be typical of the compounds present.

Fig. 2 Proposed typical structures of low-volatility alkanes obtained.

An important feature of these ionic liquid reactions is that there is no evidence for significant formation of aromatic compounds or olefins. The only non-alkane product observed, usually at temperatues above 200 °C, was a black solid of high carbon content. The ionic liquid process contrasts with the cracking reactions carried out in the presence of supercritical water⁷ where a mixture of n-alkenes and n-alkanes are formed, or with zeolites^3,18,19 where a wide range of hydrocarbons are formed, including aromatics, alkenes and alkanes. The volatile compounds produced in the ionic liquid reactions have the formula C_nH_{2n + 2}. In order for the hydrogen balance to be maintained, in going from (CH₂)_n to C_nH_{2n + 2}, compounds must be formed that have a hydrogen to carbon ratio of less than 2. This is achieved in the reaction by the formation of cyclic and particularly fused cyclic alkanes as in Fig. 2. This type of behaviour has been observed in the reactions of methyl oleate¹¹ and methyl stearate²⁰ in [emim]Cl–AlCl₃ (X = 0.67), where oligomerised products (particularly the trimer of methyl oleate or methyl stearate) contain several carbocyclic rings.^11,20 An explanation as to why no alkenes are observed is that alkenes (e.g. methyl oleate) rapidly oligomerise in acidic chloroaluminate(III) ionic liquids.¹¹ So if alkenes are formed during the reaction, they would rapidly polymerise as soon as they are formed.

Conclusion

The cracking of polyethylene to gaseous alkanes and low-volatile cyclic alkanes in chloroaluminate(III) ionic liquids provides an effective means of converting polyethylene into useful low molecular weight feedstock hydrocarbons. Yields as high as 95% with HDPE have been obtained. The reaction is thought to be superacid catalysed, as a proton source is needed.^15–17 Another attractive feature of this reaction is that the products are very easily separated from the ionic liquid by solvent extraction or other physical separation processes, enabling the ionic liquid to be reused in further reactions. A final point to note is that the rates of these reactions are highly dependant upon the surface area of the polyethylene in contact with the ionic liquid. This means that the polyethylene must be finely powdered for the reaction to succeed. Between 130 and 180 °C the polyethylene melts and the surface area in contact with the ionic liquid is reduced. Above 180 °C, efficient stirring must be employed to disperse the molten polyethylene in the ionic liquid.

Acknowledgements

We are indebted to the EPSRC and Royal Academy of Engineering for the award of a Clean Technology Fellowship (to K. R. S.).

References

1. L. Guterman, New Sci., 1998, 160, 24.
2. R. W. J. Westerhout, J. Waanders, J. A. M. Kuipers and W. P. vanSwaaij, Ind. Eng. Chem. Res., 1998, 37, 2293.
3. R. C. Mordi, R. Fields and J. Dwyer, J. Anal. Appl. Pyrolysis, 1994, 29, 45.
4. T. Isoda, T. Nakahara, K. Kusakabe and S. Morooka, Energy Fuels, 1998, 12, 1161.
5. J. Aguado, J. L. Sotelo, D. P. Serrano, J. A. Calles and J. M. Escola, Energy Fuels, 1997, 11, 1225.
6. R. S. Drago, S. C. Opetrosius and P. B. Kaufman, New J. Chem., 1994, 18, 937.
7. M. Watanabe, H. Hirakoso, S. Sawamoto, T. Adschiri and K. Arai, J. Supercritical Fluids, 1998, 13, 247.
8. K. R. Seddon, in Molten Salt Forum: Proceedings of 5th International Conference on Molten Salt Chemistry and Technology, ed. H. Wendt, 1998, vol. 5–6, pp. 53–62..
9. S. Einloft, H. Olivier and Y. Chauvin, US Pat., US 5550306, 1996..
10. A. K. Abdul-Sada, M. P. Atkins, B. Ellis, P. K. G. Hodgson, M. L. M. Morgan and K. R. Seddon, World Pat., WO 95 21806, 1995..
11. C. J. Adams, M. J. Earle, J. Hamill, C. M. Lok, G. Roberts and K. R. Seddon, World Pat., WO 98 07679, 1998..
12. A. Hirschauer and H. Olivier, Fr. Pat., FR 2757850, 1998..
13. M. Fields, W. R. Pitner, D. R. Rooney, K. R. Seddon and R. C. Thied, World Pat., WO 99 14160, 1999..
14. J. L. E. Campbell and K. E. Johnson, Inorg. Chem., 1993, 32, 3809.
15. G. P. Smith, A. S. Dworkin, R. M. Pagni and S. P. Zingg, J. Am. Chem. Soc., 1989, 111, 525.
16. J. L. E. Campbell and K. E. Johnson, J. Am. Chem. Soc., 1995, 117, 7791.
17. G. A. Olah, Friedel–Crafts Chemistry, Wiley-Interscience, New York, 1973..
18. J. Aguado, D. P. Serrano, M. D. Romero and J. M. Escola, Chem. Commun., 1996, 725.
19. R. W. J. Westerhout, J. A. M. Kuipers and W. P. M. vanSwaaij, Ind. Eng. Chem. Res., 1998, 37, 841.
20. C. J. Adams, M. J. Earle, J. Hamill, C. M. Lok, G. Roberts and K. R. Seddon, World Pat., WO 98 07680, 1998..

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Footnote

† The composition of a chloroaluminate(III) ionic liquid is best described by the apparent mole fraction of AlCl₃ {X(AlCl₃)} present. Ionic liquids with X(AlCl₃) < 0.5 contain an excess of Cl⁻ ions over [Al₂Cl₇]⁻ ions, and are called ‘basic’; those with X(AlCl₃) > 0.5 contain an excess of [Al₂Cl₇]⁻ ions over Cl⁻, and are called ‘acidic’; ionic liquids with X(AlCl₃) = 0.5 are called ‘neutral’.

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