Study of selectivity of MoO3-catalyzed C6–C7 hydrocarbons hydroisomerization: Mechanistic insights into the formation of carbonaceous deposits on the catalyst surface

Hydroisomerization of several model hydrocarbons such as n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, cyclohexane, methylcyclopentane, benzene and toluene using H2-reduced MoO3 as catalyst is described here in order to rationalize the involved reactions and mechanisms of reactions. In addition, the reactions in this process, the hydrocracking and carbonaceous deposits were also rationalized. In the case of n-heptane the results for long time reaction (164 h) suggest that hydroisomerization and hydrocracking are not only taking place as consecutive reactions (n-heptane → monobranched products → multibranched products → hydrocracked products); but also as parallel reactions (n-heptane, monobranched or multibranched products → hydrocracked products). Likewise, these results showed that from initial stages, hydroisomerization of n-heptane mainly leads to C4, C3 and C2 hydrocracked products in a mole ratio of 1:2.2:2, respectively. It is interesting to point out that both i-butane and C1 are not formed from n-heptane under the experimental conditions of this work. In the case of 2-methylhexane hydroisomerization the mole ratio of C4, C3 and C2 is 1:0.6:0.006, and the C4 fraction is mainly formed by i- and n-butane (1.6:1 mol ratio). C1 was not observed in the 2-methylhexane hydroisomerization. On the other hand, the results obtained in the hydroisomerization of 3-methylhexane, 2,3-dimethylpentane, methylcyclopentane, cyclohexane and toluene indicate that the transformation proceeded without i-butane and C1, nevertheless n-butane is formed in noticeable amount. A quantitative balance between reactants and products of hydroisomerization indicated that each C4, C3 or C2 molecule is formed exclusively from one reactant molecule. That is, the hydrocracking of each reactant molecule generates two fragments: one corresponding to the C4, C3 or C2 molecule (from a carbenium ion) and the other corresponding to the olefinic molecule (propene, butenes, pentenes, cyclopentene). It is proposed that the formation of carbonaceous deposit occurs through a sequence of consecutive reactions consisting of oligomerization and/or alkylation, cyclization, cycloaddition, and rearrangement of the olefinic products formed during hydrocracking.