Effect of the temperature on the catalytic pyrolysis using ZSM-5 with different acidity

Tableau 30 shows that with high Si/Al ratio, the effect of the temperature is more significant. Indeed, the yield of liquid increases from 29 to 44 wt% for ZSM-5(150) whereas for ZSM-5 (23) the amount of liquid is stable at around 40 wt.%. ZSM-5 (150) characterized by a low acidity cannot degrade the entire polymer at 400°C whereas for ZSM-5 (23), the highest acidity of the catalyst has led to degrade the entire polymer even at a low temperature.

Tableau 30 : pyrolysis yields for zeolitic experiments (%, by weight)

Liquid Wax Gas Residue/NDP*

Figure 53 : Distribution of the products for ZSM-5 (23) and ZSM-5 (150)

According to Figure 53 that shows the distribution of C3-C5 and C6-C12 contained in the oil obtained from ZSM-5(23) and ZSM-5(150) pyrolysis of PE it can be noted that for ZSM-5 (150), the amount of

0

ZSM-5 23 ZSM-5 150 ZSM-5 23 ZSM-5 150

wt-%

400 425 450 475

C

-C

C

-C

C3-C5 increased from 43 to 55 wt% by increasing the temperature at the opposite of ZSM-5 (23) for which the total amount of C3-C5 is stable at around 50% even when the temperature increases from 425 to 475°C. At low acidity, the effect of cracking to form lighter hydrocarbons is more pronounced.

Furthermore, for C6-C12 products, their amount decreases with the temperature from 57 to 45 wt-%

for ZSM-5 (150) whereas for ZSM-5 (23) it decreases from 56 wt-% at 400°C to stabilize at 50 wt-% for the other temperatures.

The distribution of alkanes, alkenes, aromatics and of cycles (including naphtenes and unsaturated cycle) is presented in Tableau 31 as a fonction of the temperature. The amount of aromatics for ZSM-5 (1ZSM-50) slightly increases from 20 wt-% at 400°C to 24% at 42ZSM-5°C and then it decreases to reach 16% at 475°C. On the other hand, the amount of cycles increases with the temperature from 47 at 400°C to 54%. Aromatics are generally formed either by transalkyladdition or by cyclization that can be favored for high acidity ZSM-5 [148]. In the case of ZSM-5 (150), high temperatures inhibited the formation of aromatics probably because its acidity is too low. It can be also seen that the highest yield of alkenes is observed at 475°C whereas the lowest yield of alkanes was reached at the same temperature. On the contrary, the high acidity of ZSM-5 (23) exhibits secondary reactions and especially the formation of aromatics where their amount increase with the temperature from 24 to 30 wt-%. Furthermore, Diels Alder reactions are favored at higher temperature [153] and this parameter will enhance the formation of aromatics at higher temperature for the ZSM-5 (23). According to the amount of cycles, it can be seen that their amount decreases when the amount of aromatics increases. This behavior is attributed to the decrease of the amount of C5 mainly composed of cycloalkanes to form aromatics.

Figure 54 presents the main aromatic pyrolysis products obtained versus temperature. It can be noted that increasing the temperature leads to a decrease of the amount of xylene, ethyl benzene and trimethylbenzene when ZSM-5 (150) is used while the amount of benzene is approximatively constant.

It can be supposed that the substituted aromatics can be cracked at high temperature to form benzene and small chain hydrocarbons. As the selectivity towards toluene also decreases, it is assumed that for ZSM-5 (150), the alkylation reaction is not favored at high temperature. Furthermore, the cyclization is not possible for this catalyst since there is not a large amount of acid sites. These two explanations can lead to the decrease of aromatics in the pyrolysis oil with the temperature.

In the case of ZSM-5 (23), the amount of substituted aromatics did not change within the temperature whereas the amount of BTX increases. This result can prove that the cyclization is faster than alkylation leading to increase the amount of aromatics. The same behavior was observed by Herna´ndez et al [76], who studied the catalytic flash pyrolysis of HDPE using HZSM-5 with Si/Al =22.2 and P. N. Sharratt et al [155] who also studied the catalytic pyrolysis of HDPE over HZSM-5 (Si/Al=17.5).

Tableau 31 : The distribution of alkanes, alkenes, cycled hydrcarbons and aromatics for ZSM-5 (23) and ZSM-5 (150)

Temperature Alkanes Alkenes Cycled Aromatics

ZSM-5 (23)

Figure 54 : Composition of aromatics for ZSM-5 (23) and ZSM-5 (150)

In this part, we showed that ZSM-5 (23) that present a high acidity has led to form large amount of aromatics that increases with the temperature. ZSM-5 (150) presenting lower acidity will favor the formation of light molecular weight hydrocarbons by inhibiting the formation of aromatics. The next part will discuss the recyclability of ZSM-5 (23) in order to evaluate the feasibility of several uses of the catalyst that will be an economical advantage in the industrial field.

IV.5. Study of the catalyst after pyrolysis and of its recovery

Before presenting the characterizations, it is crucial to observe visually the difference between fresh and spent catalysts. Figure 55 presents the visual appearance of ZSM-5 (23) before and after pyrolysis process at 450°C. As it can be seen, the fresh ZSM-5 has a white color whereas after pyrolysis, the catalyst has changed its color to brown. This is attributed to the presence of coke/char in the structure of the catalyst that leads to change both its visual appearance and properties as it will be presented in

0

400 425 450 475 400 425 450 475

ZSM-5 23 ZSM-5 150

Wt-%

Benzene Toluene Xylene

Ethylbenzene Trimethylbenzene m-ethylmethylbenzene Ʃ BTX

the next part. Coke is considered as a catalyst product whereas char is formed via thermal pyrolysis [160]. Many researchers attribute this nomination. Elordi et al [161] defined coke as the carbonaceous material deposited on the catalyst and Huber and Corma defined coke as the organic fraction that can be removed only by calcination [162]. However, according to Du et al, coke and char can be formed in the catalytic pyrolysis where char is formed on the surface of the catalyst whereas coke is present in the pores and can block the accessibility to the sites [160].

Figure 55 : Change color observation for ZSM-5 (23) before and after pyrolysis test