Polystyrene degradation studies using Cu supported catalysts

Highlights

• Cu supported catalysts were used for the degradation of WEPS.

• The Cu supported catalysts were reported with good catalytic performance.

• The catalysts increased the selectivity of useful hydrocarbons.

• 20% Cu-Al₂O₃ was found the best catalysts for WEPS degradation.

• 20% Cu-Al₂O₃ was compared with other catalysts and found economical.

Abstract

Waste management of plastics through conversion into valuable hydrocarbons is in high demand. The degradation of WEPS was investigated in a Pyrex batch reactor using Cu metal, Cu-Al₂O₃, Cu-Mmn and Cu-AC catalysts. Catalysts with the maximum products were characterized for BET surface area and SEM in comparison to their supporting materials. Cu supported catalysts were found to increase the selectivity of low molecular weight aromatic hydrocarbons as compared to Cu metal and the supports. Among the supported catalysts, 20% Cu-Al₂O₃ was found to have moderate activity and high selectivity to yield desirable low molecular weight aromatic hydrocarbons. The BET surface area increased only in the case of 20% Cu-Al₂O₃, whereas the SEM images of 20% Cu-Al₂O₃ reveal aggregated porous structures with apparent active Cu metal which provide high surface area for the degradation of WEPS. The yield of styrene monomer was 60.48%, toluene 10.98%, ethylbenzene 9.92% and methylstyrene was 3.66% using 20% Cu-Al₂O₃ catalyst.

Introduction

The increasing demand and growth of polymeric plastics is due to its low cost and desirable physical, mechanical and chemical properties specifically its easy moldable nature, high strength and inertness. These polymeric plastics are the sources of renewable energy, but after use, they are discarded in open space in huge quantities on a daily basis, causing environmental pollution and ruin the beauty of nature [1], [2], [3]. Polystyrene (PS) is a high demand commodity plastic and also among the huge discarded plastic wastes. PS ranks fourth in the world's consumption of plastics and is 10 wt.% of the total plastic waste [4], [5], [6]. PS has various applications ranging from households to industries. Like they are used in toys, coffee cups, kitchen appliances, insulating material, packing material and in the refrigerator industry [4], [6], [7].

PS and other plastics are non-biodegradable and remain in landfill sites for hundreds of years. The incineration of PS is the wastage of valuable carbon resources, which also produces toxic gases and therefore, this practice is prohibited [4], [8], [9], [10]. In the modern world tertiary recycling, especially thermocatalytic degradation is considered the best way to take advantage of these valuable carbon resources and decrease plastic waste by converting them into valuable hydrocarbons [3], [11], [12]. A rich literature is available on the thermal degradation of PS by using different additives, solvents and special types of reactors [4], [13], [14]. Selection or design of a catalyst for the efficient transformation of PS waste into more valuable products with the reduction of degradation temperature and heating time to an ultimate low cost method, is an important task [15], [16]. To achieve this job, numerous acidic and basic bulk heterogeneous catalysts have been employed for the degradation of PS [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. Currently, metal catalysts such as Cu and Fe [6], [12], [29], mixed catalysts such as silica–alumina [3], modified catalysts such as K₂O modified Si-MCM-41 [30] and impregnated catalysts such as Mg, Sn and Cd supported Al₂O₃ [31], [32] have gained acceleration for the degradation of waste plastics. However, a negligible amount of work is available using these catalysts for the degradation of PS. Precious and non-precious metals as catalyst have been applied widely both in labs and industries for different processes with significant results of catalytic activities and selectivity [33], [34], [35], [36] like iron (Fe) mesh has been used for the degradation of PS producing 80% liquids in a very short time [6] and a copper (Cu) coil reactor with the interaction of a microwave was used for the co-liquefaction of Makarwal coal and waste PS using high temperature and rapid degradation, achieving 66 wt.% liquid products [3]. Films of PS with metal powders (like Cu) were formed and degraded at 420 °C for 10 min in a Pyrex vessel as a reactor and about 77 wt.% liquid products were formed with 61% styrene monomer selectivity [29].

Heterogeneous catalysts preparation using the wet impregnation method is one of the most important technique, which affect the surface area, pore size and pore volume. It also exposes the metal for interaction with the reactants. For impregnation, mostly solid supports like activated charcoal (AC), natural clays, alumina (Al₂O₃), silica (SiO₂), titania (TiO₂), zirconia ((ZrO₂)) or zeolites (ZSM-41, MCM-41, etc.) have been used [37], [38]. Selecting and utilizing a proper support for the impregnation of an active center is also a very important factor, which affect the selectivity of products [39]. Al₂O₃ is widely used for the loading of active metals due to its large specific surface area, well defined pore size distributions, wide temperature range stability, and adequate dispersion of active phase [40], [41]. Our previous studies focusing on the effect of different metals impregnation over Al₂O₃ and its comparison has also been reported [32]. Carbon supports are desirable as they are stable in pH dependent media, easily recoverable and have resistance to high temperature. AC supports are suitable because of their porous nature and high surface area for the dispersion of active metal in catalysis [42], [43], [44]. In the field of catalysis, mineral clays have also gained much interest because of their high porosity, exchangeable cations and swellable properties. Among clays, montmorillonite (Mmn) has been used as a carrier for active metals in catalysis [45], [46], [47], [48].

The use of impregnated catalysts for the degradation of PS is a better option that can meet low cost, high activity and more product selectivity. Tae et al. [49] used acid treated halloysite catalysts for the degradation of PS and found a maximum liquid product yield of 90.20 wt.% with HH catalyst using 450 °C degradation temperature and 120 min reaction time yielding toluene 7.14%, ethylbenzene 8.36%, styrene monomer 58.82% and α-methylstyrene 7.19%. Chumbhale et al. [50] found that a HDM (147) catalyst with high activity using 360 °C degradation temperature, 90 min reaction time and 1:0.01 polymer to catalysts ratio yielding 59% liquid products with toluene, 5.72%, ethylbenzene 8.36%, styrene 67.71% and α-methylstyrene 11.53%. They also reported another method using modified HY catalysts [50] and found HY-700 the best for the degradation of PS using 375 °C temperature, 90 min reaction time and 1:0.01 polymer to catalysts ratio, producing 68 wt.% liquid product with toluene 7.20%, ethylbenzene 7.20%, styrene 66.60% and α-methylstyrene 9.30%. Xie et al. [30] reported the degradation of PS using a base modified silicon mesoporous molecular sieve and reported 9% K₂O/Si-MCM-41 with maximum activity and selectivity. The yield of liquid products with 9% K₂O/Si-MCM-41 catalysts was 85.67% and the yield of styrene monomer was 69.02%. These methods have investigated modified catalysts with good catalytic activity and high selectivity but they involve tedious procedures, high energy requirements and were much time consuming.

The current study investigates the degradation of WEPS by using novel Cu impregnated catalysts over different supports and their comparison for maximum activity and selectivity- by using more cost effective method.