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to move between different metal–acid sites, required for the dehy-
a bifunctional system. More difficult, is the balance between the
strength and number of these metal and acid sites in order to effec-
tuate these intermediate catalytic reactions. As demonstrated in
Section 3.1, both metal and Brønsted acid sites in the MoO2−x(OH)y
phase are expected to be in neighboring positions to each other.
So, n-octane molecules are not required to undergo the repeated
adsorption/desorption processes at distant sites. Also, the catalytic
processes in this case take place on the catalyst surface and not
limited by the pore sizes as is the case in noble metals deposited
on zeolites.
at 653 K as compared to 7.4% at 623 K is observed (Table 1). Similar
catalytic behavior is observed in the case of linear C3–C7 alka-
nes in which the concentration increases from 8.3% at 623 K to
40.4% at 653 K. The most noticeable increase in these hydrocrack-
ing products concerns C3–C5 species which indicates central C–C
bond cleavage of n-octane. The same situation is observed in the
case of iC4–iC5 isomers. On the basis of these results, it is possi-
ble to attribute the substantial increase in iC4–iC5 isomers at 653 K
to simple dissociation of i-C8 molecules. However, isomerization
is not excluded. The distribution of different isomerization and
hydrocracking catalytic products of n-octane molecules as a func-
tion of reaction temperature between 573 and 653 K is presented
in Fig. 5.
Aromatic compounds were not detected at 573 and 598 K
reaction temperatures. Xylenes (2.2%) were formed at 623 K and
increase to 7.9% at 653 K. Toluene, ethyl- and trimethyl benzene
were also formed at 653 K. The formation of toluene and trimethyl
benzene is most probably due to dimerization reaction in which the
total carbon atoms in both products are 16. Moreover, the forma-
tion of xylenes at 623 and 653 K reaction temperatures is attributed
to dehydrocyclization of methyl heptane.
Catalytic reactions products at a given reaction temperature on
well-defined catalyst surface are determined by thermodynamic
and kinetic factors. In the case of MoO2−x(OH)y catalyst, the surface
structure seems to be stable following the exposure of MoO3/TiO2
to hydrogen at 653 K for 12 h. Therefore, it is possible to go up
and down in changing the reaction temperatures between 573
and 653 K and reproduce the corresponding products distribution
at a given temperature. This is very important in a sense that
there is no surface modification such as sintering. Moreover, it
is concluded that 623 K reaction temperature corresponds to an
good conversion of n-octane was obtained. In addition, at this reac-
tion temperature, other branched iC4–iC7 products (7.7%) of high
octane numbers and less amount of aromatic compound (2.1%)
are obtained. Consequently, the research octane number (RON)
increased from −19 [38] for pure n-octane to 44.4 for branched
products.
The catalytic performances of the MoO2−x(OH)y catalyst for
n-octane will be studied as a function of reaction temperature,
hydrogen pressure and hydrogen flow rate, liquid hourly space
velocity LHSV and time on stream catalyst stability.
3.2.1. Effect of reaction temperature
The effect of reaction temperature on the catalytic reactions of n-
octane on MoO2−x(OH)y catalyst surface in the range of 573–653 K
was studied under the following conditions: 0.4 h−1 LHSV, H2/HC
(mol) = 30.2 and 5 bar pressure (Table 1). The conversion increases
from 23.3% at 573 K to 95.5% at 653 K, while the isomerization
selectivity decreases from 91.7% to 17.9% respectively. The best
performances of the catalyst toward n-octane take place at 623 K,
in similar way to what is observed in the case of n-hexane and
n-heptane, in which a conversion of 83% and isomerization selec-
tivity of 80.4% were obtained. The major isomerization products
in the reaction temperatures range of 573–623 K consist of mono-
as the reaction temperature increases. Beyond 623 K, a drastic
decrease in the isomerization products in favor of cracking prod-
ucts takes place. Low concentrations of aromatics were formed at
653 K (Table 1).
Analysis of different isomerization and to lesser extent crack-
ing and aromatic products as a function of reaction temperature is
presented as follows: mono-branched isomers are the most abun-
dant at reaction temperatures between 573 and 623 K. Of these,
3-methylheptane (3-MC7) is present in the highest concentration.
On the other hand, 2-MC7, which is favored thermodynamically,
is more abundant than the bulky 4-methylheptane (4-MC7). The
distribution of these mono-branched isomers over heterogeneous
acid catalysts following protonated cyclopropane (PCP) mechanism
is predicted to be of the order of 1/2/1 for 2-MC7/3-MC7/4-MC7
bution of 3-MC7 > 2-MC7 > 4-MC7 takes place, thus excluding the
PCP mechanism in this case. In fact, comparable mono-branched
isomers distribution is observed using Pt on different zeolite cat-
alytic systems [37]. As could be observed in Table 1, the relative
concentrations of mono-branched isomers decrease, in the order
tion decreases from 27.5% at 573 K to 19.6% at 623 K. The same
trend is observed in the case of 3-MC7 and 4-MC7. On the con-
trary, multi-branched i-C8 isomers relative concentrations do not
follow the same trend (Table 1). This is the case of 3-ethylhexane
(3-EC6) and ethyl-methyl pentane (EM-C5). In fact, the relative
concentration of dimethylhexanes (DMH) increases from 6.3% at
573 K to 16.9% at 623 K. The same trend is observed in the case of
2,2,3-trimethylpentane (2,2,3 TMC5).
Optimization of the experimental conditions for a mixture of
the different linear C5–C8 hydrocarbons relative concentrations
as present in light naphtha is under way. Interesting results were
obtained at 2 bar hydrogen pressure, LHSV value of 0.8 h−1, H2/HC
(mol) = 16 and 623 K reaction temperature.
3.2.2. Effect of hydrogen pressure
Considering catalytic results of n-octane on the bifunctional
MoO2−x(OH)y carried out at 623 K and 5 bar pressure, hydroiso-
merization to mono- and multi-branched i-C8 constitute the major
reactions products. Therefore, it is not expected to observe major
ing the hydrogen pressure on the basis of slight difference in the
number of moles between the products and reactant. In this work,
we have studied the catalytic reactions of n-octane by changing
the hydrogen pressure from 1 to 5 bar (Fig. 6). In the case of 1 bar
pressure, all the converted n-C8 molecules (75.1%) were trans-
formed to mono- and multi-branched (92.7%) i-C8 compounds.
Slight changes in conversion to 85.6% and isomerization selectivity
of 80% and an increase in the relative concentration of multi-
branched isomers were obtained at 5 bar pressure. Regardless of
the hydrogen pressure, hydroisomerization products maintain the
same order 3-MC7 > 2-MC7 > 4-MC7 and the relative concentra-
tions of 3-MC7/2-MC7 = 1.2, and 3-MC7/4-MC7 = 2.5. This is a clear
indication that hydroisomerization of n-octane follows the same
reaction mechanism.
The relative concentrations of isoalkanes (iC4–iC7) are very low
in the reaction temperatures between 573 and 623 K as compared
to i-C8 isomers. To note that these concentrations increase slightly
as a function of the reaction temperature in opposite to the i-C8
isomers. Substantial increase in (iC4–iC7) concentrations to 31.7%