T.E. Tshabalala, M.S. Scurrell / Catalysis Communications 72 (2015) 49–52
51
Table 2
by volatilization. This suggests that the conversion of hexane is depen-
dent on the reaction temperature and aromatic selectivity and on the
availability of dehydrogenolysis sites in the catalyst.
Effect of temperature n-hexane conversion on Ga/H-ZSM-5, Zn/H-ZSM-5 and Mo/H-ZSM-
5 catalysts containing 2 wt% metal at 1, 5 and 10 h on-stream.
Metal
Temperature °C
n-Hexane conversion
(%)
Aromatic selectivity
(%)
The effect of reaction temperature is most noticeable on benzene
and toluene yields. At 500 °C, toluene dominated, but at higher temper-
ature, the benzene yield increased, reaching maximum values of 18
and 12% for the gallium and zinc catalysts respectively at 550 °C. This in-
crease in benzene was compensated for a decreased C8 aromatics yield
(ethyl-benzene and xylenes). An increase in temperature inhibited al-
kylation and favored dealkylation. The low yields of o-xylene relative
to m,p-xylenes are due to the shape selective character of the zeolite.
The p-xylene contributes to increasing the yields of m,p-xylenes [30].
Higher benzene and slightly lower toluene yield are attributed to the
demethylation of toluene and C8s. The molybdenum catalyst showed a
similar effect to that seen with gallium and zinc catalysts but the yields
of for each aromatic compound were b9%. The yield of benzene in-
creased from 2 to 8% while toluene remained constant with an increase
in reaction temperature. In the case of C8s, a decrease in the yield was
observed from 500 to 550 °C and remained constant at 600 °C. The
m,p-xylenes were also dominant in the C8 aromatics.
1 h
5 h
10 h
1 h
5 h
10 h
Ga
Zn
500
550
600
500
550
600
500
550
600
87.4
99.8
98.7
75.5
99.1
99.9
84.4
67.1
73.0
70.0
99.4
81.3
77.6
90.4
79.9
77.4
68.1
63.3
62.2
97.1
59.1
67.2
74.9
53.9
72.4
59.5
47.7
32.2
54.5
43.2
44.9
56.9
46.5
25.2
28.7
34.8
41.7
50.8
39.6
44.5
34.8
24.1
24.9
34.9
33.6
36.2
49.4
27.5
43.5
18.7
7.0
23.6
30.0
27.2
Mo
The influence of temperature on the conversion of n-hexane over
Ga/H-ZSM-5, Zn/H-ZSM-5 and Mo/H-ZSM-5 catalysts was studied at
temperatures between 500 and 600 °C. Tables 2 and 3 present the re-
sults of the effect of temperature on the aromatization of n-hexane
taken after 1, 5 and 10 h on-stream.
For reaction at 550 °C (Table 2) high stability of gallium catalysts was
seen at conversions of 96–99% with aromatics selectivity N50%. At 600
°C a decrease in aromatic selectivity 43 to 27% was noted with increas-
ing tos. For zinc catalysts, the conversion of n-hexane reached 100% at
600 °C followed by a rapid decrease as the time increased at tempera-
tures of 550 or 600 °C, attributed to catalyst deactivation due to coke for-
mation and zinc volatilization due to high reaction temperatures [26].
This also led to a decrease in the aromatics selectivity from 56 to 18%
at 550 °C and from 46 to 7% at 600 °C. The conversion of n-hexane
was initially 61 and 73% with molybdenum catalysts with an increase
in aromatics selectivity from 25 to 35% with increase in temperature.
Higher conversion observed at 600 °C after 1 h on-stream can be attrib-
uted to the fact that the molybdenum species could be in the form of the
carbide at this temperature [27]. It was also shown that molybdenum
carbide species could only be formed at temperatures above 580 °C,
when butane was reacted over Mo/H-ZSM-5 catalysts [28]. n-Hexane
conversion decreased with increase in tos, especially at 600 °C, where
the conversion decreased from 73 to almost 48%. This decrease is due
to catalyst deactivation, attributed to coking. A decrease in aromatics se-
lectivity with tos at 500 and 600 °C was observed but at 550 °C a stable
aromatic selectivity was maintained. The decrease in the aromatic selec-
tivity can be attributed to the decrease in conversion of n-hexane and
one contributing factor is the change in the geometry of the zeolite
pores due to coke blockage [29]. One other reason is that presence of
molybdenum species favor cracking reaction over aromatization
(Fig. 2). It is worth noting that gallium and zinc catalysts showed better
catalytic conversion of hexane at 550 °C with increase in tos. However,
the difference in the aromatic selectivity with zinc catalyst with increase
in due to the decrease in dehydrogenolysis activity of the catalysts
which is attributed to decrease in zinc content in the catalysts cause
4. Conclusions
High aromatization activity associated with gallium and zinc metal is
due to dehydrogenation activity. Gallium and zinc provides an alterna-
tive pathway reaction for the aromatization of n-hexane which gives a
high conversion and a high selectivity to aromatic compounds. On the
other hand, addition of molybdenum to H-ZSM-5 gave different results.
Aromatic products from Ga/H-ZSM-5 and Zn/H-ZSM-5 dominated over
those formed by cracking. This showed that the dehydrogenation activ-
ity contributes to aromatization and hence these catalysts are more se-
lective to the formation of aromatics. Molybdenum-containing catalysts
were more selective towards cracked products. At 550 °C gallium and
zinc catalysts showed good activity giving 99% conversions after 1 h
on-stream and aromatic selectivities of 55 and 57% were attained, re-
spectively. The gallium catalyst showed good activity and stability
with increasing tos. Zinc catalysts show poor stability because of zinc
volatilization. The gallium-based catalyst is particularly worthy of fur-
ther investigation.
Acknowledgments
We thank the University of the Witwatersrand, University of South
Africa, National Research Foundation (NRF) and SASOL for their finan-
cial support.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
Table 3
The effect of reaction temperature on the aromatic product distribution (mainly BTX) of aromatization of n-hexane over Ga/H-ZSM-5, Zn/H-ZSM-5 and Mo/H-ZSM-5 zeolite catalysts at
500–600 °C.
Metal
Temperature °C
%Conversion
%Yield
Benzene
Toluene
Et-Benzene
o-Xylene
m,p-Xylene
Ga
500
550
600
500
550
600
500
550
600
87.4
96.4
90.4
75.5
92.2
92.8
61.2
58.0
58.4
6.6
18.3
16.4
6.9
12.0
13.7
2.0
11.3
14.4
12.0
7.7
10.1
8.4
5.7
6.2
6.1
2.0
2.7
1.7
7.1
4.9
4.6
1.6
0.9
0.9
2.5
1.8
1.5
3.5
2.8
2.3
1.4
0.6
0.9
5.9
6.0
5.3
5.8
4.8
1.9
4.7
2.9
2.9
Zn
Mo
6.8
7.9