Effect of time on alternative hydrocarbon production from waste PS with biodiesel of candlenut oil using catalytic cracking and Si/Al-Ceramic as catalyst

In the present study, the catalytic cracking process of polystyrene (PS) plastic waste and biodiesel of candlenut oil was carried out using Si/Al-ceramic catalyst with a mass ratio of 7:3 at 300 °C. This study aimed to investigate the effect of reaction time on the distribution of hydrocarbon fuel fractions produced. They are characterized by XRD, SEM-EDX, FTIR, and N2 gas adsorption-desorption. The products components of PS catalytic cracking were analyzed by gas chromatography-mass spectroscopy (GC-MS). The liquid fuel obtained was mixed with commercial fuel, Premium RON 88, and methyl tertiary butyl ether (MTBE) additive to determine its physicochemical properties and performance in a generator engine with gasoline-based fuel. The result showed that an increase in reaction time from 60 minutes to 120 minutes of PS cracking over Si/Al-Ceramic catalyst leads to the enrichment of long-chain hydrocarbon fraction (> C12) by 25.61% which is attributed to the formation of styrene dimer. The fuel has thermal efficiency of 26.49% with density of 0.7405 g/cm3, flashpoint of < -50 °C, calorific value of 9642.42 kcal/kg, and octane number of 92.2. These characteristics are in accordance with SNI 06-3506-1994 standard. Therefore, liquid fuel from catalytic cracking of polystyrene plastic wastes can be used as a potential alternative fuel in the gasoline range.


Introduction
Polystyrene (PS) is widely used for some purposes such as packaging, food container, etc.However, this broadly use generates lots of wastes and causes environmental pollution because of their non-biodegradable nature.Since polystyrene has high energy content [1], chemical and thermal degradation of PS are the most probable recycling approach to recover useful products like monomer and fine chemicals from waste plastic [2].The catalytic cracking of PS is more favorable as it can reduce the overall energy consumption and enhance selectivity [3], [4], [5].
A great variety of solid acidic catalysts such as zeolite, alumina, and silica-alumina at 350°C have been reported to significantly modify the selectivity of PS cracking process since the major products are benzene, ethylbenzene, and cumene [6].Moreover, reactivity of the catalyst is influenced by Si/Al ratio which determine the amount of acid site.MCM-41, a well-known mesoporous structure, has few acid site but offers an advantage in term of porosity as it pores lie in the mesoporous size [7].This disadvantage can be handled by metal impregnation on MCM-41 framework [8], [9].It was reported by [3], [4] that impregnation of Al metal into MCM-41 lead to an increase in the amount of the acid site and this give a potential material to be used as catalyst in a cracking reaction.In the previous study [10], catalytic cracking of Polypropylene (PP) plastic waste have been carried out using modified Al-MCM-41 and produced liquid fuel product at gasoline fraction with yield of 71.89%.
In this present study, we investigated the catalytic cracking process of polystyrene plastic waste over Al-MCM-41ceramic.The focus of this study will be placed on the effect of reaction time toward the distribution of liquid hydrocarbon products, especially for the production of long chain hydrocarbon fractions (> C12).In addition, the physiochemical properties of the resulting fuel oil including density, flash point, calorific value, and octane number are investigated and compared with SNI 06-3506-1994 gasoline oil quality and International Standard HIBER11Z Hibernia Petroleum Canada 2016.The performance of PS fuel blend in term of brake thermal efficiency analysis is also performed in gasoline set engine.

Material and methods
Polystyrene (PS) foam was used as feedstock in the catalytic cracking process to produce liquid oil.These raw materials were collected from the disposal site in Keputih, Surabaya.PS foam was prepared by cutting it into small pieces and pressed to reduce its large volume.Commercial Al-MCM-41 catalyst was used for these work.

Preparation of Catalyst
Al-MCM-41 catalysts were initially activated by putting it into oven at 80 °C for an hour.Unused spark plugs were washed, crushed, and sorted into ceramic parts.The activated Al-MCM-41 catalysts were then mixed with the spark plug ceramic powder with a mass ratio of catalyst by ceramic equals to 7:3.Catalyst blend was made into pellets with a diameter and thickness of 1 cm.

Catalyst Characterization
The acidity of the catalyst was measured by FTIR in which the sample has saturated steam of pyridine absorption.The N2 adsorption and desorption measurement of the prepared catalyst were measured by Quantachrome Autosorb iQ gas sorption analyzer at 77.35K.Calculation of the surface area was evaluated using Brunauer-Emmet-Teller (BET) method, while pore size distribution was estimated from the branch of the isotherm using Barret-Joyner-Halenda (BJH) method [11].
Scanning Electron Microscopy (SEM) was used to determine the surface morphology of the catalyst.In addition, Energy Dispersive X-Ray (EDX) analysis was associated to investigate the corresponding elements presented in the sample.The crystallinity and thermal stability of the catalyst were investigated by Wide-Angle X-Ray Diffraction (WAXD) and DTA measurements respectively.

Catalytic Cracking of Polystyrene
Catalytic cracking processes were conducted in a continuous reactor made of stainless steel.It consists of heating chamber, catalytic reforming chamber, cooling chamber, and collecting chamber as shown in Figure 1.In this work, 500 g of the compressed polystyrene foam was fed into heating chamber.A total of 9 mg of the pelleted catalyst was inserted into catalyst tube filled with glass wool.In addition, nitrogen gas was flowed into heating chamber for a minute to remove oxygen gas.The reactor was heated to 300°C and this temperature was maintained for 60 -120 minutes at atmospheric pressure.The feedstock was converted into vapors, which were condensed into liquid oil after passing through the cooling chamber and collected in the collecting chamber at the bottom.

Catalyst Characterization
The type of acidic side of the catalyst, the Bronsted and Lewis acidic side, was determined by FTIR spectroscopic analysis with pyridine as a probe molecule.The FTIR-pyridine spectra depicted in Figure 1 shows several peaks including peak at wave number 1548.89 cm -1 and 1446.66 cm -1 which correspond to Bronsted and Lewis acidic side respectively.
Low peaks around 1455 cm -1 and high peaks around 1620 cm -1 indicated that pyridine was adsorbed in the Lewis acidic side [12].On the other hand, Aini reported that peaks at wave number 1638 cm -1 and 1545 cm -1 was the stretching vibration characteristic of pyridinium ion (C5H5NH + ), in which pyridine binds to protons.The presence of pyridinium stretching peak proved that there were acidic Bronsted hydroxyl collection.The impregnation of Al metal ions into tetrahedral coordinated silica structures of MCM-41 produce Bronsted acidic side [13].The amount of each acidic side was given in Table 1.The result of N2 adsorption and desorption of the modified catalyst was compared to Al-MCM-41 and given in Figure 2.
The graph indicated isotherm type IV which corresponded to a typical graph of Al-MCM-41 catalyst [14].It revealed that the addition of ceramic with the mass ratio of Al MCM-41: ceramic (7:3) did not change the isotherm graph type of Al-MCM-41.In addition, modification of ceramic into Al-MCM-41 influenced the surface area of catalyst.The result of BET analysis in Table 4 showed that Al-MCM-41-ceramic had larger surface area than Al-MCM-41.The surface area increased by 14.42% as some site in the catalyst surface were occupied by ceramic and the pore volume increased by 8.42%.

Catalytic Cracking Process
Catalytic cracking process of PS in the presence of Al-MCM-41-ceramic catalyst was examined at 300 °C under N2 gas.This observation was conducted for 60 minutes and 120 minutes to investigate the effect of reaction time toward the production of hydrocarbon liquid fraction.PS thermal cracking was also conducted under the same condition for comparison.The chromatogram of the catalytic cracking products for 60 minutes and 120 minute reaction time were presented successively in Figure 8 (a) and (b), while for PS thermal cracking in (c).
The component of PS catalytic cracking product at reaction time 60 and 90 minutes was given in Figure 9.The major product of PS catalytic cracking with 60 minutes of exposure time were α-methyl styrene, benzocyclobutane, ethyl benzene, cumene, and toluene.Polystyrene undergoes two main reaction, a stepwise de-polymerization and polymer decomposition into oligomers followed by their consecutive cracking.Acid-catalyzed cracking mechanism could be categorized into carbenium nature.The most favorable reaction pathway included the attack of proton which associated with a Bronsted acidic site to the aromatic ring of PS since its phenyl group side was reactive enough toward electrophilic reagent.The carbocation further experienced β-scission followed by inter/intramolecular hydrogen transfer, resulting different products such as benzene, ethylbenzene, cumene, etc [16].Marczewski group's work revealed that the sequence of styrene dimers second reaction began from the attack of Brønsted acid sites on aromatic ring.The resulting carbenium ion underwent dealkylation and gave benzene and alkenylaromatic cation.The latter compound either reacts with a hydride anion gives phenylbutene or to phenylbutadiene, the coke precursor after proton detachment.Phenylbutene further experienced protonation either in aromatic ring which results in dealkylation or in the aliphatic chain which initiates further transformation.It was found that two main reactions occur: internal cyclization and side chain cracking.The former reaction results in indane and naphthalene derivatives while the latter produces toluene, ethylbenzene and styrene.Moreover, the proposed reaction mechanism of PS cracking over Al-MCM-41-ceramis in the present study was depicted in Figure 11.
Increase reaction time into 120 minute caused the formation of styrene dimers which contributed toward the enrichment of long chain hydrocarbon fractions (>C12).It was due to the re-polymerization process of α-methylstyrene.This reaction mechanism was supported by the fact that the amount of α-methylstyrene as the major product in shorter reaction time (60 minutes) reduced by 52.26% after 120 minute reaction proceeded and lead to the formation of styrene dimer.Styrene dimer formed including diphenylmethane, diphenylethane, diphenylpropane, 1,3-diphenylbuthene, 1,3diphenylbuthane, and 1,4-diphenyl-1,3-butadiene which identified from GC-MS analysis.
Similar PS degradation mechanism over acidic catalyst had been studied comprehensively [17].Marczewski group's work revealed that the sequence of styrene dimers second reaction began from the attack of Brønsted acid sites on aromatic ring.The resulting carbenium ion underwent dealkylation and gave benzene and alkenylaromatic cation.The latter compound either reacts with a hydride anion gives phenylbutene or to phenylbutadiene, the coke precursor after proton detachment.Phenylbutene further experienced protonation either in aromatic ring which results in dealkylation or in the aliphatic chain which initiates further transformation.It was found that two main reactions occur: internal cyclization and side chain cracking.The former reaction results in indane and naphthalene derivatives while the latter produces toluene, ethylbenzene and styrene.Moreover, the proposed reaction mechanism of PS cracking over Al-MCM-41-ceramis in the present study was depicted in Figure 11.
Increase in reaction time into 120 minute caused the formation of styrene dimers which contributed toward the enrichment of long chain hydrocarbon fractions (>C12).It was due to the re-polymerization process of α-methylstyrene.This reaction mechanism was supported by the fact that the amount of α-methylstyrene as the major product in shorter reaction time (60 minutes) reduced by 52.26% after 120 minute reaction proceeded and lead to the formation of styrene dimer.Styrene dimer formed including diphenylmethane, diphenylethane, diphenylpropane, 1,3-diphenylbuthene, 1,3diphenylbuthane, and 1,4-diphenyl-1,3-butadiene which identified from GC-MS analysis.

Performance of Fuel Blend
The percentage of thermal efficiency of the fuel blend increases with increasing load (watts) on the gasoline engine as shown in Figure 11.At the low loading value (watts), CBC and CBT fuel blend variation have higher thermal efficiency compared to the mixture of premium and MTBE (CB sample) as a standard.However, at a maximum load of 2438 watts, the thermal efficiency of the CBC and CBT fuel mixture decreases and both have lower than premium values.CBC sample possess the highest thermal efficiency of 26.49% at 1843 watt loading values.At the same loading value, the thermal efficiency for CBT and CBC samples is 25.49% and 25.55% respectively.This is due to an increase in the long chain hydrocarbon fraction (> C12) of 26.51% from the catalytic cracking of polystyrene which has an effect on the density of CBC fuel blend variation.The thermal efficiency of the fuel blend is influenced by its heating value, density, and viscosity [22].These three parameters greatly affect the amount of fuel which entering the carburettor and combustion process.

Conclusion
Several conclusions can be drawn from the present study as follows.Modification of ceramic into Al-MCM-41 catalyst with mass ratio of Al-MCM-41 : ceramic 7:3 can increase the Si/Al ratio up to 45% without changing the hexagonal mesoporous structural properties of Al-MCM-41.Reaction is one of the most important parameters that influence the selectivity of hydrocarbon liquid fraction in catalytic cracking process of polystyrene.Increase in reaction time from 60 minutes to 90 minutes of PS cracking over Al-MCM-41-ceramic catalyst leads to the enrichment of long chain hydrocarbon fraction (> C12) by 25.61% which associated with the formation of styrene dimer due to the repolymerization reaction in a batch system.The major styrene dimer obtained from 120 minutes reaction time including diphenylpropane, 1,3-diphenylbutene, and 1,3-diphenylbutane.Characteristics of liquid fuel blends produced are in accordance with SNI 06-3506-1994 with density value @ 15°C of 0.7405 g/cm 3 , flash point of < -50°C, calorific value of 9642.42 kcal/kg, and octane number of 92.2.Performance testing on gasoline engines shows that the thermal efficiency percent of the fuel blend increases with increasing load (watts).The CBC fuel mixture has the highest thermal efficiency percent of 26.49% at the loading value of 1843 watts, where this value is greater than the mixture of premium-MTBE fuel which is equal to 25.55%.

Figure 1
Figure 1 FTIR spectrogram of pyridine adsorption

Figure 8 Figure 9
Figure 8 Distribution of liquid hydrocarbon fraction resulting from PS catalytic cracking for reaction time (a).60 minutes (b).120 minutes

Figure 10
Figure 10 Thermal efficiency of fuel blend

Table 1
Result of FTIR analysis

Table 2
Result of BET analysis CatalystSurface Area (m 2 /g) Pore Volume (cc/g)