Investigation Of Isomerization Of N-Heptane Over Molybdenum Oxide Based Catalysts

As one of the main routes to produce high-octane gasoline blending components in the crude oil refining industry, alkanes isomerization has drawn much attention throughout the world because it can greatly improve the gasoline quality. For alkanes isomerization, one of the key issues is to develop catalysts with higher activity and higher stability, which has been widely pursued due to its importance both in academic research and in industrial utilization.The objective of our work is to investigate the isomerization of n-heptane over molybdenum oxide based catalysts. The effects of the preparation and reaction conditions on the conversion and isomerization selectivity of n-heptane were studied carefully. It has been found that the MoOx catalyst obtained by H2 reduction at 623 K in a period of 6 hours termed as [MoOx(6h)] exhibite a maximum catalytic activity in the reduction temperature ranging from 573 K to 773 K (the conversion of n-heptane reached 40%), and the catalytic activity of the catalyst reached the maximum value with the conversion of n-heptane being 69.7% at 598 K in the reaction temperature range of 523 K to 623K. XRD studies show that the active phase of MoOx catalyst for n-heptane isomerization is MoOxHy and MoO2, and the contribution of MoOxHy is more important for the n-heptane isomerization. It is also found that the MoOx catalyst obtained from H2 reduction has a the mesoporous characteristic, with its maximum pore diameter being at 41 A. To our best knowledge, this has never been reported in literature. The dynamics of n-heptane isomerization on the MoOx catalyst was investigated for the first time. In the reaction temperature ranged from 523 to 598 K, the apparent activation energy for the n-heptane isomerization and the cracking of n-heptane is 49.3 kJ/mol and 60.6 kJ/mol, respectively; At 573 K, the reaction order for H2 partial pressure on the MoOxcatalyst for the n-heptane isomerization is 0.35, and for n-heptane the order is 0.33. It has demonstrated that addiing of 2~5 wt% Ni to MoOa can markedly shorten the H2 reduction period which is necessary for the formation of active phase Ni-MoOx catalyzing n-heptane isomerization. The relative activity for n-heptane isomerization in terms of unit surface area of the catalyst can be enhanced largely by addition of Ni as well. On the MoOx catalyst obtained by H2 reduction at 623 K in a period of 6 hours, the conversion of n-heptane is 40%; and the conversion of n-heptane reaches to 45.3% on the 2 wt.%Ni-MoOx(6h) catalyst at thesame reaction conditions. The apparent activation energy on the Ni-MoOx(6h) catalyst obtained through the Arrhenius equation is 35.3 kJ/mol. It is found that the Ni-MoOx catalyst possesses a preferable oxygen and water vapor resistance compared to the MoOx catalyst; the action of Ni might be attributed to the promoted H2 activation, thus in favor of the reduction of MoOx catalyst. n-Heptane isomerization on the MoOx-SiO2 and MoOx/SiO2 catalyst was studied. The physical and chemical structure of MoOx-SiO2 and MoOx/SiO2 catalyst was characterized by means of SEM, N2 adsorption-desorption method, EDS and XRD. The results show that the 44.6wt%SiO2 in MoOx-SiO2 catalyst acts as a framework in which a large amount of bulk MoOx phase is enveloped to form typical mesoporous structure that is similar to that of bulk MoOx catalyst. 55.4wt%MoOx-SiO2 catalyst exhibits an improved mechanical strength in comparison to bulk MoOx catalyst and maintains the high catalytic activity and isomerization selectivity of bulk MoOx catalyst. MoOx/SiO2 catalyst displays a lower reaction specific activity than MoOx-SiO2 and bulk MoOx catalyst, which may be due to the interaction between the support and MoOs in MoOx/SiO2 catalyst.