The combined effects of microporous HZSM-5 (Si/Al = 80) catalyst and mesoporous acidic AlSBA-15 on the degradation of HDPE are presented by data in Table 6. It was observed that a conversion of about 100 % yield toward liquid, gaseous, and residue products without any formation of solid waxy compound was achieved. According to literature, these two catalysts constitute principally Lewis and Bronsted acid sites with variable surface areas and pore sizes (Ooi and Bhatia 2007).

However, results shown here are in accordance with the shape selectivity effect in microporous and mesoporous materials. It was associated with the narrow pores to access active sites. It was noted that for acidic enhancement SBA-15 catalyst, the larger pore size and channels allowed the formation of higher hydrocarbon products such as light liquid products. The high yield of gaseous products shown by HZSM-5 catalyst was mainly due to the slow diffusion of cracked products within the internal pores. The findings made in this work were consistent with those reported by pre­vious researchers. It has been reported that the diffusion rate of hydrocarbon mol­ecules could be retarded using microporous catalysts (Urquieta et al. 2002).

Overall, the combined effects of HZSM-5 (80) and AlSBA-15 catalysts gave rise to positive improvements as compared to the performance of the individual cata­lyst. For all three combinations of catalyst ratio, light liquid degradation product yields were higher than 20 wt. %. The optimum liquid yield was demonstrated by the HZSM-5 (80) to AlSBA-15 catalyst ratio of 1:2 (which was 26.5 wt. %). Although the increment of both catalysts increased the liquid yield as compared to the catalyst ratio of 1:1, further addition of AlSBA-15 could provide higher liquid yield while simultaneously suppressing the yield of gaseous product more effec­tively. Another advantage of applying the 1:2 catalyst loading ratio was that it also inhibited the solid coke formation on the catalyst. This could be explained from the theory that larger composition of HZSM-5 catalyst will contribute to higher coke

Fig. 14 Composition of gas products at 673 K using composite HZSM-5(80) and AlSBA-15 catalysts at different ratios

Fig. 15 Liquid degradation products at 673 K using composite HZSM-5 (80) andAlSBA-15 catalysts at different catalyst ratios

formation and more rapid deactivation. In this respect, the mesoporous catalyst is generally more effective than the microporous catalyst in suppressing the formation of large molecules and thus causing less carbon deposits (Urquieta et al. 2002).

As can be seen in Fig. 14, the gas products distribution was quite uniform. For the catalyst ratio of 1:1, the smallest compositions were the carbon chain of C1 (12.4 %) and C5 (12.7 %) while the highest were the carbon chain of C3 (28.9 %) and C4 (28.6 %). However, all of them were marginally different. For unequal cat­alyst mixing ratios such as 2:1 and 1:2, the uniformity of product distribution was not able to be maintained. For HZSM-5(80) to AlSBA-15 catalyst ratio of 2:1, the smallest compositions were the carbon chain of C1 and C5 (about 13.0 %) while the highest proportion was the carbon chain of C4 (31.7 %). On the other hand, for catalyst ratio of 1:2, increasing amount of AlSBA-15 composition resulted in lower production of C5 carbon chain (8.9 %) while showing higher composition in the carbon chain of C3 (31.4 %).

Meanwhile, the effect of varying catalyst mixing ratio on the liquid phase degradation products is illustrated in Fig. 15. Mixture of HZSM-5(80) and AlSBA-15
catalysts with catalyst loading ratio of 1:1 produced nearly 89 % of carbon chain range C8-C12, C13-C16, and C17-C20 from the overall liquid composition. Increasing the ratio of either HZSM-5(80) or AlSBA-15 amount in the catalyst mixture appar­ently shifted the product distribution toward heavier carbon chain. The effect of adding HZSM-5(80) was more dominant as it increased the carbon chain range of C17-C20 to 35.7 % while adding AlSBA-15 only increased the carbon chain range of C13—C16 to 28.4 %.

4 Conclusions

The results reported and discussed in the present work demonstrate that micro — and mesoporous materials show promising properties to be used as catalysts in the degradation of HDPE into gaseous and liquid hydrocarbon fuels at 350-500 °C. HZSM-5 (14), HZSM-5 (80), SBA-15, and AlSBA-15 were used under various operating conditions to obtained liquid biofuels. Mixture of HZSM-5 (80) and AlSBA-15 with ratio of 1:2 exhibits higher degradation activity to yield higher liquid biofuels with valuable gas product at 400 °C. In addition, significant HDPE conversions into liquid fuel with lower coke contents were achieved in a batch reactor over the HZSM-5 catalyst as compared to mesoporous silica catalyst SBA-15. The pore shape of zeolites was very important for deter­mining their activities and product selectivity in the degradation of polymeric materials because it influenced the degradation and deactivation rates simulta­neously. SBA-15 containing aluminum catalyst was of a potential interest in the cracking of heavier feedstock such as palm oil waste into biofuels. The weaker acid properties exhibited by the mesostructured catalysts, i. e., SBA-15 and Al-SBA-15 were responsible for their reduced gaseous product production capaci­ties. However, their presence coupled with the combination of HZSM-5 catalysts in the conversion of polyethylene materials promoted a substantial conversion of the original long-chain hydrocarbons into lighter liquid hydrocarbon products (up to 26.5 wt. %). Furthermore, larger pore dimensions exhibited by these sol­ids did not allow for any product selectivity, resulting in the possible formation of a wide range of branched hydrocarbon and alkyl derivative aromatic products. These results suggest that catalytic degradation of HDPE leads into higher liq­uid hydrocarbons yield at lower temperature. It is stated that such type of chemi­cal recycling, i. e., conversion of waste HDPE into hydrocarbon feedstock used as resource for biofuel has been recognized as an ideal approach and could sig­nificantly reduce the net cost of disposal. It is concluded that under appropriate reaction conditions, suitable catalysts such as HZSM-5 (80) and AlSBA-15 have the ability to control both the product yield and product distribution from HDPE degradation, potentially leading to a cheaper process with more valuable products such as biofuel.