Catalytic cracking of petroleum vacuum residue in supercritical water media: Impact of α-Fe2O3 in the form of free nanoparticles and silica-supported granules

Catalytic cracking of bottom product from the vacuum distillation unit, i.e., vacuum residue (VR) was investigated in supercritical water (SCW) over different forms of iron oxide nano-catalyst as free alpha-Fe2O3 nanoparticles and SiO2-supported alpha-Fe2O3 crystals. The effects of operational parameters including reaction temperature, reaction time, catalyst to oil weight ratio, and water to oil weight ratio were studied on the yield of maltene and coke produced from VR cracking as well as asphaltene conversion, in a batch autoclave reactor. SEM, XRD and TGA analyses were used to characterize the catalyst before and after the reaction. TGA and XRD images confirmed phase transformation of alpha-Fe2O3 to magnetite (Fe3O4) during reaction with free nanoparticles, and SEM images showed agglomeration of the nanoparticles significantly. It was resulted that silica-supported alpha-Fe2O3 catalyst was more stable and more effective than that of free nanoparticles in the process of VR cracking. The suggested mechanism of the reaction would be a cycle beginning with oxidative cracking of hydrocarbons by the lattice oxygen provided from reduction of alpha-Fe2O3 to Fe3O4 then releasing the active hydrogen by SCW gas shift reaction to hydrocracking VR and production of aliphatic compounds. At the same time, the reduced form of the catalyst (Fe3O4) would be converted to alpha-Fe2O3 by the oxygen atom released from SCW. (C) 2015 Elsevier Ltd. All rights reserved.