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Regarding the product yields, it is obvious that the thermal cracking of WCO gives the lowest yields of desirable products, i.e., gasoline (17.3 wt. %) and lower olefins (2.3 wt. %). This is because of lacking acidity; thus, most of the intermediate oxygenated compounds were not converted [6]. The presence of the catalysts generally increases the fractions of gaseous and gasoline hydrocarbons by the cracking of LCO and HCO compounds. However, there are pronounced differences in the product distribution because of the different acid site amount and porosity. With the lowest acid site amount and largest mesopores, Al-SBA-15 produces the most LCO (30.4 wt. %), but the least C2–C4 olefins (8.5 wt. %). In contrast, H-ZSM-5 having the highest acid site amount exhibits the largest fractions of gasoline and C2–C4 olefins (35.0 wt. % and 30.4 wt. % respectively). The superior selectivity toward gasoline over H-ZSM-5 is well documented [8,16]. With respect to Al-SBA-15 and H-ZSM-5, the fractions of gasoline and lower olefins obtained with ZSC-24 are ranking in between. However, the outstanding yield for C2–C4 olefins over H-ZSM-5 mainly results from the high conversion rather than from selectivity since the latter is lowest (73.5 wt. %) in this series. Remarkably, the highest selectivity to C2–C4 olefins has been achieved over ZSC-24 (93.2%), confirming the advantage of ZSC-24 over the reference catalysts. The lower selectivity to C2–C4 olefins over Al-SBA-15 in comparison with ZSC-24 can be attributed to the shape selectivity of microporous ZSM-5 domains in the latter. However, the low selectivity to C2–C4 olefins over H-ZSM-5 is probably due to its very high amount of acid sites (mostly located in micropores) and the diffusion constraints imposed by the small pore size of H-ZSM-5 [13]. This might lead to elongated residence times and undesired consecutive reactions of the olefins. Tago et al. [18,19] studied the effect of crystal sizes and acidic properties on the catalytic performance of H-ZSM-5 in the cracking of n-hexane at high temperature (550–650 ◦C). Reducing the crystal size to nano-scale led to rapid diffusion of light olefins out of the intercrystalline micropores of nanosized zeolites as result of the shortened diffusion path lengths, which helped avoid the further transformation of these olefins. This clearly indicates the detrimental effect of internal mass transfer limitation in H-ZSM-5. On the other hand, increasing the number of acid sites enhanced the conversion of n-hexane but decreased the selectivity to light olefins. They postulated that the higher amount of acid sites promoted hydrogen transfer reactions which consumed light olefins to produce gasoline aromatics and light paraffins. A similar behavior can be seen from Figure 2 wherein H-ZSM-5 displays higher concentrations of light paraffins and aromatic hydrocarbons than ZSC-24, confirming the acceleration of hydrogen transfer reactions over H-ZSM-5 due to its higher acid site amount. Thus, the remarkable selectivity to C2–C4 olefins over the ZSC-24 catalyst can be attributed to the improved molecular transport being well balanced with the medium acid site amount which 199PDF Image | Zeolite Catalysis
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