| dc.description.abstract | The majority of the world’s gasoline is produced with the Fluid Catalytic Cracking (FCC) process, in which heavy gas oil (HGO) or vacuum gas oil (VGO) is cracked into smaller hydrocarbons. The FCC catalyst used for this cracking contains three components: clay, binder and zeolite. Though it is said that the zeolite is the active component in the FCC catalyst, a true understanding of the nature of all the acid sites in the FCC, and the complete cracking mechanism, is still lacking.
In this master thesis the true origin and structure of all the acid sites, present in the FCC catalyst, is elucidated. The acidity is determined of both the individual and mixed components, with respect to the FCC. The acidity was studied with pyridine and CO based FT-Infrared measurements. Additionally, 1H-NMR was employed to detect the different protons in the samples. The structure of the acid sites is investigated with 27Al MAS and 27Al MQ-MAS NMR spectroscopy. Correlations between the proximity of different surface hydroxyl groups and aluminium structures are obtained with {1H-27Al} CP and {27Al-1H} CP NMR spectroscopy.
In this research we show that the zeolite is indeed the component which contributes the most to the overall acidity of the FCC catalyst. Here we show that a higher aluminium content results in an increased concentration of Lewis acid sites and medium Brønsted acid sites. The increase in medium Brønsted acid sites was correlated to a decreased electronegativity of the tetrahedral aluminium species. The formation of the Lewis acid sites is associated with the formation of penta-coordinated Extra Framework Aluminium (EFAL) species. Furthermore, we show that the aluminium center of the aluminol groups and Brønsted acid sites can act as a Lewis acid site.
Additionally, we show that the binder is the component which contributes most to the acidity of the active matrix. The binder contains mostly Lewis acid sites, which suggests that the pre-cracking is Lewis acid catalysed. Furthermore, it is shown that the binder contains medium Brønsted acid sites which form due to the interaction between silica and alumina. The Brønsted acid sites were associated with the formation of a tetrahedrally coordinated aluminium species.
Moreover, a correlation between the OH bands in the infrared spectra are correlated to the proton peaks in the 1H-NMR spectra. We thereby show that 1H-NMR is a useful technique to analyse the change in the amount and acidity of the Brønsted acid sites.
Additionally, it was shown that heating of the FCC catalyst resulted in a structural collapse of the binder and clay. Furthermore, heating of the samples generated penta- coordinated type aluminium, which were therefore ascribed exclusively to EF aluminium species. Moreover, prolonged heating resulted in an increased amount of Lewis acid sites and a decreased amount of Brønsted acid sites. This was associated with an increased heterogeneity around the aluminium nuclei. | |
