1.3.1 Effect of Catalyst Loading

The data regarding effects of prepared catalysts at 623 K and 10 % loading on HDPE degradation into liquid, gas, residue, and waxy products yield are presented in Fig. 2 and Table 2. It is well noted that AlSBA-15 catalyst showed good degrada­tion activity to produce light hydrocarbon liquids while HZSM-5 catalysts having greater microporous surface area (Si/Al ratio 80 and 14) produced higher amounts of gaseous products. It is further explained that the primary cracking reactions might have occurred on the external surface which was in contact with the poly­mer. Meanwhile, the smaller fragments which were products of the initial reaction were mainly cracked within the microporous surface of the catalysts. However, the amount of coke was found to be lower as compared to the other degraded products.

In a previous study, Hwang et al. (2002) reported that the strong acid sites could accelerate cracking and deactivation reactions which resulted in the higher yield of coke as observed in case of prepared catalysts HZSM-5 and acid-treated SBA-15 in this study. Significant production of solid wax compounds using SBA-15 was also observed, and this could be due to insufficient number of acid sites in the material. Large wax formation after thermal degradation at the

Table 2 HDPE degradation-based product yield (%) by different catalysts

bottom of reactor could cause difficulty in the collection of other products such as liquid and gases. However, Mastral et al. (2006) reported that HZSM-5 pos­sessed an excellent stability that could effectively prevent the formation of coke due to its particular structure. Based on these findings, it could be concluded that the pore size, acidity, and shape are the important parameters that can affect the degradation activity of zeolitic catalysts. Similar findings have been reported by Hernandez et al. (2006) who observed that catalytic degradation using HZSM-5 yielded higher gas products during HDPE degradation. In another study con­ducted by Mastral et al. (2006) who carried out degradation process at different temperatures in a fluidized bed reactor, mesoporous materials such as SBA-15 did not show adequate degradation results. These observations confirmed our present findings as presented in Fig. 2. According to Urquieta et al. (2002), higher acidity in zeolitic catalysts might result in higher gaseous yield and a reduction in liquid yield. Again, the results confirmed findings made in this study.

The gas fraction compositions after HDPE degradation using different catalysts are shown in Fig. 3. It is noted that the catalysts HZSM-5(80) and HZSM-5 (14) exhibited the highest fraction for carbon chain C4 (37.1 and 30.2 %, respectively) and the lowest for carbon chain C5 (4.4 and 1.4 %, respectively). Meanwhile, SBA-15 and AlSBA-15 catalysts yielded the highest fraction for C3 (47.2 %) and C4 products. It should be noted that C1 products were not detected in the GC analysis. Both SBA-15 and AlSBA-15 catalysts produced more significant amount of C5 gaseous products (12.3 and 11.6 %, respectively). In the case of the mixture of HZSM-5 (80) and AlSBA-15 as catalyst, the gas carbon chain distribution was more uniform.

Findings and degradation trends using these catalysts for liquid product yield are shown in Fig. 4. The catalysts exhibited higher liquid products in favor of C8-Ci2, followed by Ci3-Ci6 and Ci7-C20. HZSM-5 (80) and HZSM-5(14) had

36.4 and 34.7 % liquid products, respectively, for C8-C12. Meanwhile, AlSBA-15 catalyst had the highest percentage of liquid products for C8-C12 (40.9 %). The pro­portion of liquid products for C13-C16 was also found to increase (37.3 %) when AlSBA-15 was used as catalyst instead of HZSM-5 catalysts as in the earlier case.