4
60
AFROUKHTEH-LANGAROUDI et al.
7
6
5
4
3
2
1
attributed to the fact that ethylene was formed via pri-
mary carbonium ions which requires high activation
energy [5]. Higher temperature fulfils the high activa-
tion energy requirement.
ZSM-5(300)
ZSM-5(50)
The data presented in Table 2 show that the selec-
tivity of ZSM-5 zeolite for propylene production using
any SiO /Al O molar ratio or any type of feed was
2
2
3
higher compared to ethylene and butenes. However,
ZSM-5(300) showed higher selectivity to propylene
compared to ZSM-5(50). Propylene selectivity of
about 30 and 28 wt % was obtained over ZSM-5(300)
for n-hexane and n-heptane cracking, respectively.
Maximum propylene to ethylene ratio of 2.7 and 2.48
was observed over ZSM-5(300) at 500°C for n-hexane
and n-heptane cracking, respectively.
0
100 200 300 400 500 600 700 800
Temperature, °C
Fig. 3. NH -TPD profiles of the ZSM-5 catalysts.
3
The distribution of products obtained from crack-
ing of n-hexane and n-heptane over both ZSM-5 zeo-
lites at different temperatures is plotted in Figs. 5
and 6. As shown in Figs. 5 and 6, the selectivity of eth-
ylene, dry gas and total olefins obeys an ascending
trend by increasing the reaction temperature for both
zeolites, while selectivity to alkanes decreased at high
temperatures. No considerable changes were observed
for butenes selectivity at different temperatures.
As opposed to other products, propylene follows a dif-
ferent trend over the two ZSM-5 zeolites. Under
ZSM-5(50), a linear increase was observed for propyl-
ene selectivity correlated with temperature whereas
under ZSM-5(300) there is no considerable difference
in terms of propylene selectivity at different tempera-
tures for both feedstocks.
amount of coke formed on the catalyst bed was insig-
nificant, so that there was no significant change in
appearance of catalyst.
Table 2 shows that temperature has a significant
influence on the behaviour of the catalysts. The con-
version of n-hexane and n-heptane at different reac-
tion temperatures are plotted in Fig. 4 showing an
ascending correlation with temperature over either
zeolites. Both trend lines have the same slope, yet the
conversion values for n-hexane are greater than n-hep-
tane regardless of SiO /Al O molar ratio. Despite this
2
2
3
similar pattern for both zeolites, ZSM-5(50) yielded
greater conversion values independent of feedstock
(
Fig. 4). The slope of the trend line between 550 to
6
00°C in case of ZSM-5(50) was higher than the slope
The selectivity for alkanes and total olefins
are summarized in Table 2. Regardless of feed type,
ZSM-5(50) is significantly selective for alkanes while
ZSM-5(300) was much more selective for light olefins.
This observation may be attributed to the less acidic
properties of ZSM-5(300) which reduces H-transfer
reactions [5, 18] inhibiting the conversion of olefin
products to alkanes leading to olefins selectivity of
the catalyst. ZSM-5(50) becomes less selective for
alkanes at high temperature leading to maximum
alkane selectivity 79.97 and 78.21 at 500°C for n-hex-
ane and n-heptane cracking, respectively. To further
understand, the alkanes to olefins ratios were calcu-
lated which was about 4.5, 3, and 2 under ZSM-5(50)
for both feedstocks at reaction temperatures 500, 550,
and 600°C, respectively while the ratio under
ZSM-5(300) is about 1 at all reaction temperatures.
The direct relationship between olefins selectivity and
conversion of n-hexane and n-heptane to the products
are shown in Fig 7.
between 500 to 550°C, while for ZSM-5(300) a reverse
trend was observed. In general, catalytic cracking
occurs at the acidic sites of zeolites and the conversion
depends on the acidity of the catalyst [21]. On the
other hand, acidic property of the zeolites highly
depends on SiO /Al O molar ratio [5, 18]. The higher
the Al content, the more acid sites become available
on the zeolite [19]. Therefore, lower conversion values
in case of ZSM-5(300) were expected. Overall, maxi-
mum conversion of 93.84 and 88.93 was achieved over
ZSM-5(50) at 600°C for n-hexane and n-heptane,
respectively.
2
2
3
It can be concluded that for both zeolites and both
feedstocks, the selectivity of ethylene increased at
higher reaction temperatures (Table 2). This can be
Table 1. NH -TPD results of ZSM-5 zeolites
3
–1
Desorption
peak area
NH (μmol g )
3
Catalyst
As it is shown, the olefins selectivity increased with
enhancing the conversion at 500, 550, and 600°C.
Therefore, temperature enhancement has desirable
influence on ZSM-5 performance at catalytic crack-
ing process. It causes increasing both conversion and
olefins selectivity. Consequently, the catalytic crack-
ing process over ZSM-5 at higher temperature is more
adsorbed
ZSM-5(300)
ZSM-5(50)
5966
7723
33.22
43.00
PETROLEUM CHEMISTRY
Vol. 58
No. 5
2018