Abstract

Light olefins and diolefins such as ethylene, propylene, butenes and 1,3-butadiene are considered as the backbone of the petrochemical industry. They are precursors of numerous plastic materials, synthetic fibers, and rubbers. In recent years, the thermo catalytic cracking (TCC) process has been developed in our lab with the objective to selectively produce light olefins, particularly ethylene and propylene, from liquid hydrocarbon feedstocks such as petroleum naphtha and gas oils. With the continuous decline of conventional oil reserves, heavy petroleum feedstocks become essential alternatives for commercial petroleum products. However, the preliminary catalytic results in the TCC of heavy feedstock have indicated insufficient on-stream long-term stability and a high selectivity to polyaromatic hydrocarbons, which are usually considered as precursors for coke. In this dissertation, hybrid catalysts have been developed and studied with the goal of resolving the problems of stability of catalyst activity and selectivity. Several active metal species (such as Ni, Re, Ru) have been loaded on the co-catalyst component surface. These metal species are able to produce very active hydrogen species, in virtue of its steam-reforming activity, and to spill them over to the acidic sites of the main catalyst component. These hydrogen species, once transferred (or "spilt over") onto the surface of the main cracking catalyst component, might interact with the reaction intermediate being adsorbed on the acidic sites. This resulted in a decreased formation of coke precursors, and consequently, catalyst deactivation was significantly retarded. The results obtained from cracking tests on both petroleum feedstocks and model molecules indicated that the spilt-over hydrogen species had significant effects on heavy hydrocarbon feedstocks, such as vacuum gas oil, and they could affect the reaction intermediates only when the latter were formed on the external surface of microporous ZSM-5 zeolite particles. Moreover, data of the most recent work show that it is necessary to choose the ZSM-5 zeolite (that is the cracking component of the hybrid catalyst) that has a high density of acid sites; however, its acid strength should be relatively mild in order to achieve a high total conversion and a high propylene/ethylene product ratio. These mild acid sites also lead to lower coke deposition and a lighter nature of coke thus improving the cleaning action of the hydrogen spilt-over species.