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92
N. M. Al-Otaibi, G. Hutchings
Taubert et al. [18] studied the oxidative dehydrodimeriza-
tion of isobutene using Bi O and mixtures with different
subsequently analysed using off-line gas chromatography.
The gaseous products were analysed by using on-line gas
chromatography. The results of the two analyses were
combined and the isobutene conversion was determined on
the basis of inlet and exit concentrations of isobutene.
Additionally, the conversion was cross-checked against
products made and the carbon mass balance was typically
98–102%. During the short-timescale of these experiments
(90 min) no appreciable carbon deposition on the catalysts
was observed. Hence the data presented are for the initial
catalyst performance.
2
3
additives (Cr O , MoO , NH VO , SnO , and V O ). They
2 5
2
3
3
4
3
2
observed that the highest yields of 2,5-dimethyl-1,5-hexa-
diene yield were achieved with Bi O as the catalyst and
2
3
selectivities over 90% were observed. However, in the
absence of an acidic component the aromatization to form
p-xylene was not observed. In this paper we study the
aromatization of isobutene over a range of H-ZSM-5/oxide
composite catalysts in an attempt to identify an improved
catalyst. We have previously studied composite catalysts of
these types for methanol and propane conversion [19–21]
and we now extend these studies to the conversion of
isobutene.
3 Result and Discussion
3.1 Effect of Reaction Temperature
2
Experimental
The effect of reaction temperature on the conversion of
isobutene and the selectivity to aromatization products over
H-ZSM-5 was studied at three temperatures (300, 400 and
500 °C) and the results are shown in Fig. 1. The selectivity
for the isomerisation of the isobutene to other butenes, the
competing reaction for dimerisation and aromatization, was
steady at ca. 20% for 300 and 400 °C but decreased at
500 °C. It was found that when the reaction temperature
increased, both isobutene conversion and the selectivity to
aromatic components in the reaction products proportion-
ally increased, consequently, the yield of benzene, toluene
and xylenes (BTX) also increased. This is considered to be
due to the enhanced dimerization of isobutene and the
subsequent cracking of the higher alkanes and alkenes
which are the primary products, together with enhanced
aromatization as shown in Table 1. Of the aromatic com-
pounds formed, toluene was the dominant product at all
2
.1 Catalyst Preparation
Composite catalysts comprising metal oxides (Ga O ,CuO
2
3
and ZnO, Fluka) with zeolite H-ZSM-5 were prepared by
physically mixing the three metal oxides with H-ZSM-5(H-
ZSM-5, 30 Si/Al, Valfor). All materials were used as
powders (typically 20–50 l). Three different ratios (ca.
1
:1, 1:2, and 2:1 ratios by weight) of metal oxides with the
zeolite were prepared. The physical mixtures were obtained
by mixing the required amount of the metal oxides with
H-ZSM-5 and grinding for 3 min. The resultant powders
were pressed to form a pellet and crushed to required mesh
(
40–60 l). The catalysts along were analysed using X-ray
fluorescence to determine the elemental composition of the
catalysts. The ratios of metal oxide:zeolite determined
experimentally were: Ga O 34.5, 53.3, 69.3%; ZnO 32.1,
2
3
5
1.4, 65.3%; CuO 30.5, 49.5, 68.2%. Powder X-ray dif-
fraction confirmed that no loss in crystallinity of the zeolite
was observed before and after use as a catalyst, and fur-
thermore that the catalysts comprised mixtures of the oxi-
des as expected and no change was observed after use as a
catalyst for isobutene conversion as determined by powder
X-ray diffraction.
1
00
80
60
2
.2 Catalyst Testing
4
2
0
0
0
Catalyst testing was performed using a fixed bed stainless
00
steel reactor (316 stainless steel, i.d. 3/8 ) at atmospheric
pressure. The catalyst bed was held in the reactor by means
of glass wool. The catalyst bed was located in the iso-
thermal heating zone. The catalyst bed was heated by a
Carbolite tube furnace and the reaction temperature
was measured using a thermocouple located in the catalyst
bed. The reaction products were passed though a cold
trap (0 °C) to obtain the liquid products which were
300
400
500
Reaction temperature (°C)
Aromatics
C4H10
CH4
C4H8
C2H6
C5+
C2H4
C3H8
C3H6
Conversion
Fig. 1 Effect of reaction temperature on aromatization of isobutene
over H-ZSM-5 zeolite
1
23