Abstract
Mankind makes extensive use of crude oil to fuel its insatiable demands
for energy and hydrocarbon derivatives. The refining of crude oil is based on a
process known as cracking, where long-chain hydrocarbons are systematically
broken into smaller chain hydrocarbons known as fractions with each fraction
allowing for the production of a specific material. The maximum efficiency of
cracking can be achieved in the petroleum refining processes by controlling the
operating parameters of the units, and over the years many studies have attempted
to optimize the cracking conditions such as temperatures, pressures and the use of
a variety of catalysts to reach maximum productivity. Catalysts such as the Y-type
zeolite catalysts are often used because their acidity and thermal stability makes
them an ideal cracking catalyst; however the developments of enhanced catalytic
properties for zeolite-Y catalysts are essential to increase the production yields.
Optimization of the Y-type zeolite catalyst is the focus of this research and
accordingly the synthesis, characterization, modifications and catalysis have been
studied in depth. A review of the literature has shown that there are three main
techniques used to improve the zeolite properties following the synthesis process;
(Cation exchange, Dealumination and Desilication), since the crystalline structure
of a Y-type zeolite is prepared from an alkaline aluminosilicates gel. However, the
literature focuses mainly on the reaction variables used in the modifications. As
such this study focuses on the effects of treatment processes on the composition,
behaviour and catalytic properties of the synthesized Y-zeolite framework.
Laboratory experimental data has confirmed that a synthesis process using
24 h aging for crystal nuclei at 25 °C and 18 h crystallization time for crystal
growth at 100 °C produced the desired zeolite NaY morphology, and NaNH4Y
zeolite forms with various cation contents (3, 1.5 and 0.5 wt% Na+) were obtained
by subjecting the NaY form to a multi-stage ion exchange using 0.5 M NH4NO3 at
80 °C, while the HY form was obtained by the calcination of NH4Y form under
high temperature. Calcination temperatures above 450 °C were shown to indicate
a removal of the framework hydroxyl groups via dehydroxylation, which led to a
collapse of zeolite-Y structure, whereas raising the level of Na+ inside the zeolite
lattice throughout the calcination was led to a delay in the starting point of the
dehydroxylation region as confirmed via TG and DSC-analyses. This finding was
also used in the preparation of the USY form by steaming the HY form, as the
former is a traditional zeolite-Y form utilized in the refining units. It was found
that Na-ions hindered the extraction of Al-atoms from the Y-lattice thus reducing
the rate of dehydroxylation, and minimising rapid contraction of the unit cells and
Y-structure collapse, which helped make a rigid structure and a more resilient
lattice for steaming at high temperature. In addition, analyses data confirmed that
the extraction of EFAl-species from the USY-structures using an EDTA chemical
treatment led to an increase in the acidity of treated catalyst and the introduction
of mesopores. Lower lattice Si/Al ratio and larger porosity were also found using
the dealuminated-desilicated rather than the desilicated-dealuminated leaching
method in the treatment of both Y and USY structures via dislodgement of both Si
and Al-atoms in NaOH and HCl solution. Cracking was performed on deactivated
catalysts (450 °C) in a PFTR using nC7 in N2 at 325 – 425 °C and W/F = 22 – 44
g·h.mol-1, and confirmed that the tuned steaming/leaching conditions succeeded in
modifying the catalytic properties of the in house made catalysts, as they possess superior
performance when compared to the industrial catalysts typically used.