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The use of synthetic zeolites in catalysis

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Zainuddin, Zainurlis (1995) The use of synthetic zeolites in catalysis. Research Master thesis, University of Tasmania.

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Abstract

This study reports on investigations into the use of synthetic zeolites in
catalysis in an attempt to design catalyst systems which have size and shape
selectivity. Zeolites have been widely used in petroleum refineries as acid
catalysts. This has led to the field of study known as "shape selective
catalysis", since the product distribution is limited to those molecules which
can pass through the zeolite channel. In this study, zeolites have been used
both as an acid catalyst for the cracking of hydrocarbons, and as a catalyst
support for a cationic palladium complex.
n-Octane and 2-methylheptane were used in cracking reactions over
zeolites HY, HZSM-5, and their combinations. The two hydrocarbons were
selected to discern the skeletal effect introduced by the structure of a molecule,
while the zeolites were chosen because of their common use in commercial
catalytic cracking.
Three ways of mixing the zeolites were employed, by which the
influence of each zeolite on a particular feedstock could be isolated:-
i. Placing the bed of zeolite HY prior to that of zeolite HZSM-5.
ii. Placing the bed of zeolite HZSM-5 prior to that of zeolite HY.
iii. Mixing the two zeolites thoroughly.
It was found that overall distributions of cracking products by carbon
number from reaction of either paraffin on combinations of HY and HZSM-5,
and from reaction of a mixture of the paraffins on either zeolite, can be
reasonably described by the addition of product distributions on individual
catalysts and from individual paraffins, weighted according to the relative
amounts present.
Ratios of branched to linear paraffins are much more strongly influenced
by catalyst type and feedstock than the corresponding ratios for olefinic
products. For reactions on catalyst mixtures, distributions of the total products
by carbon number correspond well to a summation o~ contributions on the
individual catalysts. However, a greater departure from prediction is seen for
individual distributions of paraffins, olefins and aromatics, as well as for ratios
of branched to linear paraffins, showing that hydrogen transfer processes and
isomerisation must occur. The addition of pentasil resulted in enrichment of the linear saturates at lower carbon number which is due to preferential cracking of linear paraffins over the branched isomers.
The use of zeolite as a catalyst support was investigated by immobilising
a cationic palladium complex, [Pd(2,2'-bipy)i]2+, in NaY zeolite.
Three immobilisation methods were investigated, from which a model for
location and distribution of the supported complex was proposed:-
i. Prepared from [Pd(2,2'-bipy)i](N03)2 complex which was exchanged
directly with the zeolite (method I).
ii. Synthesised in-situ from Pd(2,2'-bipy)Cl2 complex which was
exchanged with the zeolite in the presence of 2,2'-bipyridyl
(method II).
iii. Synthesised in-situ from [Pd(NH3)4]2+ which was exchanged with the
zeolite. Free 2,2'-bipyridyl was then added to the cation-exchanged
zeolite in solution to replace the ammine (method III).
UV-visible, atomic absorption, and raman spectroscopies, together with
microprobe and elemental analyses, were used to characterise the zeolite
supported complexes.
Attempts to directly ion-exchange [Pd(2,2'-bipy)i]2+ into the zeolite
resulted in a maximum palladium loading of 0.62 wt% compared with a
maximum loading of 8.97 wt% when [Pd(NH3)4]2+ was ion-exchanged into
the zeolite. Efforts to obtain higher loadings of [Pd(2,2'-bipy)i]2+ by in-situ
synthesis from Pd(2,2'-bipy)Cli and 2,2'-bipyridyl resulted in an increased
maximum palladium loading to 1.15 wt%. Although a maximum Pd loading of
8.97 wt% could be obtained by ion-exchanging [Pd(NH3)4]2+ into the zeolite,
it was found that the [Pd(NH3)4]2+ could only be partially converted to
[Pd(2,2'-bipy)i]2+. The inability to achieve 100% conversion using this method
is probably due to the steric constraints within the smaller zeolite cages.
The catalytic activity of the zeolite supported complexes was tested for
dimerisation of ethene. It was found that the product distribution resulting from
the supported complexes was similar to that obtained from the homogeneous
system.

Item Type: Thesis (Research Master)
Keywords: Zeolites, Catalysis, Catalytic cracking, Catalyst supports
Copyright Holders: The Author
Copyright Information:

Copyright 1995 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Includes bibliographical references. Thesis (M.Sc.)--University of Tasmania, 1996

Date Deposited: 04 Feb 2015 23:23
Last Modified: 24 Aug 2016 04:24
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