Aromatization of isobutane and isobutene over Mo 2C/ZSM-5 catalyst

Abstract

The reaction pathways of isobutane have been investigated on Mo 2C deposited on ZSM-5 and SiO 2. Particular attention was paid to the effects of the composition of ZSM-5 and to the influence of the Mo 2C loading. ZSM-5 itself catalyzed the reaction of isobutane well above 700 K. The conversion and the product distribution sensitively depended on the temperature and the composition of zeolite. At lower temperature the cracking and the dehydrogenation reactions were the dominant process, while at higher temperatures the formation of aromatics (benzene, xylene and toluene) came into prominence. At 773 K the conversion of isobutane was more than ∼90% on ZSM-5 with SiO 2/Al 2O 3 = 30, this value decayed to 4–5% on the sample with SiO 2/Al 2O 3 = 280. At the same time the selectivity of aromatics decreased from ∼32% to ∼0.1%. Deposition of Mo 2C markedly changed the catalytic performance of the ZSM-5 samples: the dehydrogenation and the aromatization processes became more dominant. In the dehydrogenation of isobutane, 1-butene, isobutene, trans-2-butene and cis-2-butene were produced. On all samples isobutene was produced with highest selectivities. From the extrapolation of selectivities to zero conversion we obtained that methane, propylene, butenes and hydrogen are the primary products on pure and Mo 2C-containing ZSM-5. Aromatics are generated in secondary processes, mainly in the oligomerization and aromatization of isobutenes. The promoting effect of Mo 2C is well exhibited in the case of SiO 2, which is practically an inactive support. The main reaction of isobutane on Mo 2C/SiO 2 is its dehydrogenation. Aromatics, xylene and benzene, were also produced with the selectivity of 5–16% at a conversion of 4–21%. Higher values (16% selectivity for aromatics at a conversion level of 30%) were obtained at 873 K in the course of the reduction and carburization of MoO 3/SiO 2 suggesting that the simultaneous presence of both the Mo–O and Mo–C species is the most favorable for the activation and aromatization of isobutane. Isobutene, the primary product of the dehydrogenation of isobutane, is very reactive on pure and Mo 2C-containing ZSM-5. High temperature favored the formation of xylene, while at lower temperature several C 6–C 8 hydrocarbons were produced. The effect of Mo 2C was less than in the case of the reaction of isobutane.