(583bu) Optimization of Si/Al Ratio, Fe and P Loadings for Higher Stability of P-Fe/HZSM-5 in Steam Catalytic Cracking

Light
olefins such as ethylene and propylene are the backbone of chemical industries.
Steam cracking because of emission of large amount of CO₂ and energy
consumption [1] is not the appropriate way to produce light olefins. So,
thermal catalytic cracking attracted attention. Among catalysts, HZSM-5 due to
acidity, hydrothermal stability and shape selectivity [2] is a suitable
catalyst. However, high acidity of HZSM-5 which results in secondary reactions
and consequently reduction of products yield is a serious problem. Modification
of HZSM-5 with various elements such as transition metals [3] and rare earth
elements [4] is an effective way to adjust the acidity in order to obtain high
light olefins yield.

On
the other hand, dealumination of HZSM-5 in the
presence of steam in high temperature of reaction is a serious drawbacks. Modification of HZSM-5 with phosphorous is an
appropriate alternative to overcome this problem [5].    

In
this study, we aim to increase light olefins yield and improve hydrothermal
stability of HZSM-5 by considering three factors including Fe, P and Si/Al
ratio. Therefore, a series of P-Fe/ZSM-5 was designed by statistical design
using Box-Behnken and impregnated by co-impregnation
method. The effect of each factor and their interactions were studied by
response surface methodology.

Thermal
catalytic cracking of naphtha was carried out for 20 h. The experimental
results demonstrated that steam cracking is the main reaction for ethylene
production [3] whereas thermal catalytic cracking is the major mechanism for
propylene production. So, initial propylene yield and reduction of propylene
during reaction time (the difference of propylene yield between the initial
time and after 20 h) were subjected to analysis of variance (ANOVA). According
to ANOVA analysis and their F values, factors A (Si/Al), B (Fe) and C (P) and
their interactions A×B, A×C and B×C affected both
initial propylene yield and propylene reduction, significantly. Fig.1 (a) and
(b) shows the contour plots of interactions of Fe and P on propylene yield and
propylene reduction. According to Fig. 1 (a), propylene yield increases with
the simultaneous movements of Fe and P up to a maximum point in which propylene
yield is 32.62 wt.%. The seame trend is observed for
propylene reduction up to a maximum point (propylene reduction=10.2 wt.%)

Figure
1.  Contour plots of A)
initial propylene yield, B) Propylene reduction as function of B (Fe wt.%) and C (P wt.%) at Si/Al=137.

Fig.
2 presents the NH₃-TPD of some catalysts. According to Fig. 2 and
experimental results, more strong acid sites causes more initial propylene
yield. According to TPR analysis, the redox
properties of catalysts improve by adding P and increasing Si/Al which results
in the speed of electron transfer and eventually increasing propylene yield.
According to NH₃-TPD and TPO, more number of weak acid sites results
in more amount of coke formation. According to TPO and XRD, dealumination
has a remarkable effect on catalytic instability. Fe(6)-P(2)/HZSM-5
with 1.86 wt.% of propylene reduction showed the highest catalytic stability.

Figure
2.  A)
NH₃-TPD of some catalysts, B) Weight loss with temperature of some
catalysts after 20 h.

References

1.      Yoshimura, Y., Kijima, N., Hayakawa, T., Murata, K.,
Suzuki, K., Mizukami, F.,

Matano, K., Konishi,
T., Oikawa, T., Saito, M., Shiojima, T., Shiozawa, K., Wakui, K.,

Sawada, G., Sato, K., Matsuo, S., and Yamaoka, N. Catalysis
Surveys from Japan. 4,

157 (2000).

2.      Zhao, G., Teng, J., Xie, Z., Jin, W., Yang, W., Chen, Q., and Tang, Y. J. Catal. 248, 29 (2007).

3.      Lu, J., Zhao, Z., Xu, C.,
Zhang, P., Duan, A. J. Catal.
Commun.
199?203, 7 (2006)

4.      Xiaoning, W., Zhen, Z., Chunming,
X., Aijun, D., Li, Z., Guiyuan.
J. J. Rare Earths. 321?328, 25 (2007).

5.     
Blasco, T., Corma, A.,
Martínez-Triguero, J. J. Catal. 267?277, 237 (2006)

6.      Teimouri Sendesi, S.M., Towfighi, J., Keyvanloo, K. J.
Catal. Commun. 114-118, 27 (2012).