Degradation of High-Density Polyethylene into Liquid Fuels Using Microporous and Mesoporous Catalysts

Ahmad Zuhairi Abdullah, Shazia Sultana, Steven Lim and Mushtaq Ahmad

Abstract The potential application of acidic HZSM-5 and AlSBA-15 materials for catalytic degradation of high-density polyethylene (HDPE) into liquid hydrocarbon fuels was investigated using a tubular batch reactor. The reaction was carried out at various catalyst loadings between 5 and 15 % with 1:1, 2:1, and 3:1 HZSM-5 to AlSBA-15 ratios. The catalysts exhibited remarkable catalytic activity with conver­sions into liquid light hydrocarbons of up to 25 %. The gaseous product distribution showed a wide spectrum of hydrocarbons (Ci-C5) while the most predominant prod­ucts were C3 and C4 (47 and 40 %, respectively). Meanwhile, the liquid products were mostly in the range of C8-C25 depending on the reaction parameters and the amount produced decreased with increasing carbon number. Thus, catalytic degrada­tion of HDPE was a promising route for obtaining valuable fuels and petrochemicals from waste polymers. It required relatively low degradation temperatures to obtain liquid hydrocarbons with boiling points within that of gasoline range. At the same time, it could reduce the environmental problems caused by waste polymers.

Keywords High-density polyethylene • HZSM-5 • ALSBA-15 • Catalytic degradation • Hydrocarbon fuel

1 Introduction

In recent decades, plastic materials consumption has undergone a significant growth. According to an estimate, the global production and consumption of plas­tics have increased by about 10 % annually. At the same time, synthetic plastics

A. Z. Abdullah • S. Sultana (*) • S. Lim

School of Chemical Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia e-mail: shaziaflora@hotmail. com

M. Ahmad

Biofuel Laboratory, Quaid-i-Azam University, Islamabad 45320, Pakistan

A. Domingos Padula et al. (eds.), Liquid Biofuels: Emergence, Development and Prospects, Lecture Notes in Energy 27, DOI: 10.1007/978-1-4471-6482-1_13, © Springer-Verlag London 2014

recycling has gained considerable attention all over the world due to the serious environmental problems caused by waste plastics and resources needed for man­ufacturing of such huge quantity of plastic materials. There are several methods practiced currently for degradation and disposal of high-density polyethylene (HDPE) such as landfill, thermal degradation, and incineration. However, all of these methods have not gained social acceptance for disposal of the waste mate­rial (Lee et al. 2002). The disposal is also currently becoming legally restricted because of a decrease in landfill availability, cost increase, and strong pollution concerns such as emissions of various combustion products.

Recycling of plastics should be projected to minimize the pollution. It has lower energy demand to support the process to offer high energy conservation while at the same time enhancing the efficiency of the process. Plastics recycling technolo­gies have been historically divided into four general types. Primary and secondary recycling processes involve the processing of waste/scrap materials into a product with characteristics similar to and different from those of original product, respec­tively. Tertiary recycling involves the production of basic chemicals and fuels from plastics waste/scrap as part of the municipal waste stream or as a segregated waste. Meanwhile, quaternary recycling process retrieves the energy content of waste/ scrap plastics by burning or incineration. Although the primary and secondary recy­cling processes are the most common being applied in the society, tertiary recycling and quaternary recycling have been regarded as more sustainable (Garforth et al. 1997). In contrast to landfill, thermal, and incineration methods used for degrada­tion of HDPE, chemical recycling using catalysts has emerged as a potentially inter­esting alternative. It can achieve degradation of plastic wastes for conversion into a variety of useful products, mainly as liquid fuels and raw chemical feedstock (Lee et al. 2002). In this regards, catalytic degradation using materials such as zeolites is considered to be suitable due to their unique pore diameter. These materials seem to be especially useable as catalyst supports for waste polymer degradation.

Generally, zeolites are aluminosilicate members of the family of microporous solids known as ‘molecular sieves’ and an ‘open’ structure that can accommodate a wide variety of cations. Meanwhile, mesoporous silicates such as MCM-41 and SBA-15 are porous silicates with huge surface areas (normally >1,000 m2/g), large pore sizes (2 nm < size < 20 nm), and ordered arrays of cylindrical mesopores with very regular pore morphology (Garforth et al. 1997).

In recent years, several researchers have reported the synthesis of a new class of porous materials which are supposed to combine the properties of both zeolites and ordered mesoporous aluminosilicates (Trong et al. 2001). Some examples of mesoporous materials that have been investigated in the past include MCM-41, KFS-16, and SAPO-37 (Jalil 2002; Marcilla et al. 2002; Miskolczi et al. 2005; Sakata et al. 2002). In continuing the efforts to degrade the HDPE waste into useful products such as liquid fuel, this study has been initiated. The objective of the research is the use of a laboratory tubular batch reactor to study: (i) the potential application of HZSM-5 and AlSBA-15 as acidic microporous and mesoporous catalysts for degradation of HDPE, (ii) the activ­ity of these catalysts and their effects under various operating conditions on product dis­tribution and selectivity, and (iii) to enhance the potential benefit of catalytic polymer recycling for industrial application in the future.

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